Palmitoylation regulates the intracellular trafficking and stability of c-Met

c-Met is a receptor tyrosine kinase whose activity can promote both mitogenic and motogenic phenotypes involved in tissue development and cancer progression. Herein, we report the first evidence that c-Met is palmitoylated and that palmitoylation facilitates its trafficking and stability. Inhibition of palmitoylation reduced the expression of c-Met in multiple cancer cell lines post-transcriptionally. Using surface biotinylation, confocal microscopy, and metabolic labeling we determined that inhibition of palmitoylation reduces the stability of newly synthesized c-Met and causes accumulation at the Golgi. Acyl-biotin exchange and click chemistry-based palmitate labeling indicated the c-Met β-chain is palmitoylated, and site-directed mutagenesis revealed two likely cysteine palmitoylation sites. Moreover, by monitoring palmitoylation kinetics during the biosynthesis and trafficking of c-Met, we revealed that stable palmitoylation occurs in the endoplasmic reticulum prior to cleavage of the 170 kDa c-Met precursor to the mature 140 kDa form. Our data suggest palmitoylation is required for egress from the Golgi for transport to the plasma membrane. These findings introduce palmitoylation as a critical modification of c-Met, providing a novel therapeutic target for c-Met-driven cancers.

The Arf-like GTPase Arl8b is essential for three-dimensional invasive growth of prostate cancer in vitro and xenograft formation and growth in vivo

Cancer is a multistep process that requires cells to respond appropriately to the tumor microenvironment, both in early proliferative stages and in later invasive disease. Arl8b is a lysosome localized Arf-like GTPase that controls the spatial distribution of lysosomes via recruitment of kinesin motors. Common features of the tumor microenvironment such as acidic extracellular pH and various growthfactors stimulate lysosome trafficking to the cell periphery (anterograde), which is critical for tumor invasion by facilitating the release of lysosomal proteases to promote matrix remodeling. Herein we report for the first time that Arl8b regulates anterograde lysosome trafficking in response to hepatocyte growth factor, epidermal growth factor, and acidic extracellular pH. Depletion of Arl8b results in juxtanuclear lysosome aggregation, and this effect corresponds with both diminished invasive growth and proteolytic extracellular matrix degradation in a three-dimensional model of prostate cancer. Strikingly, we found that depletion of Arl8b abolishes the ability of prostate cancer cells to establish subcutaneous xenografts in mice. We present evidence that Arl8b facilitates lipid hydrolysis to maintain efficient metabolism for a proliferative capacity in low nutrient environments, suggesting a likely explanation for the complete inability of Arl8b-depleted tumor cells to grow in vivo. In conclusion, we have identified two mechanisms by which Arl8b regulates cancer progression: 1) through lysosome positioning and protease release leading to an invasive phenotype and 2) through control of lipid metabolism to support cellular proliferation. These novel roles highlight that Arl8b is a potential target for the development of novel anti-cancer therapeutics.

The oleocanthal-based homovanillyl sinapate as a novel c-Met inhibitor

The hepatocyte growth factor (HGF)/mesenchymal-epithelial transition factor (c-Met) signaling axis has gained considerable attention as an attractive molecular target for therapeutic blockade of cancer. Inspired by the chemical structure of S (-)-oleocanthal, a natural secoiridoid from extra-virgin olive oil with documented anticancer activity against c-Met-dependent malignancies, the research presented herein reports on the discovery of the novel olive-derived homovanillyl sinapate (HVS) as a promising c-Met inhibitor. HVS was distinguished for its remarkable potency against wild-type c-Met and its oncogenic variant in cell-free assays and confirmed by in silico docking studies. Furthermore, HVS substantially impaired the c-Met-mediated growth across a broad spectrum of breast cancer cells, while similar treatment doses had no effect on the non-tumorigenic mammary epithelial cell growth. In addition, HVS caused a dose-dependent inhibition of HGF-induced, but not epidermal growth factor (EGF)-induced, cell scattering in addition to HGF-mediated migration, invasion, and 3-dimensional (3D) proliferation of tumor cell spheroids. HVS treatment effects were mediated via inhibition of ligand-mediated c-Met activation and its downstream mitogenic signaling and blocking molecular mediators involved in cellular motility across different cellular contexts. An interesting feature of HVS is its good selectivity for c-Met and Abelson murine leukemia viral oncogene homolog 1 (ABL1) when profiled against a panel of kinases. Docking studies revealed interactions likely to impart high dual affinity for both ABL1 and c-Met kinases. HVS markedly reduced tumor growth, showed excellent pharmacodynamics, and suppressed cell proliferation and microvessel density in an orthotopic model of triple negative breast cancer. Collectively, the present findings suggested that the oleocanthal-based HVS is a promising c-Met inhibitor lead entity with excellent therapeutic potential to control malignancies with aberrant c-Met activity.

Repurposed drug screen identifies cardiac glycosides as inhibitors of TGF-β-induced cancer-associated fibroblast differentiation

The tumor microenvironment, primarily composed of myofibroblasts, directly influences the progression of solid tumors. Through secretion of growth factors, extracellular matrix deposition, and contractile mechanotransduction, myofibroblasts, or cancer-associated fibroblasts (CAFs), support angiogenesis and cancer cell invasion and metastasis. The differentiation of fibroblasts to CAFs is primarily induced by TGF-β from cancer cells. To discover agents capable of blocking CAF differentiation, we developed a high content immunofluorescence-based assay to screen repurposed chemical libraries utilizing fibronectin expression as an initial CAF marker. Screening of the Prestwick chemical library and NIH Clinical Collection repurposed drug library, totaling over 1700 compounds, identified cardiac glycosides as particularly potent CAF blocking agents. Cardiac glycosides are traditionally used to regulate intracellular calcium by inhibiting the Na+/K+ ATPase to control cardiac contractility. Herein, we report that multiple cardiac glycoside compounds, including digoxin, are able to inhibit TGF-β-induced fibronectin expression at low nanomolar concentrations without undesirable cell toxicity. We found this inhibition to hold true for multiple fibroblast cell lines. Using real-time qPCR, we determined that digoxin prevented induction of multiple CAF markers. Furthermore, we report that digoxin is able to prevent TGF-β-induced fibroblast contraction of extracellular matrix, a major phenotypic consequence of CAF differentiation. Assessing the mechanism of inhibition, we found digoxin reduced SMAD promoter activity downstream of TGF-β, and we provide data that the effect is through inhibition of its known target, the Na+/K+ ATPase. These findings support a critical role for calcium signaling during CAF differentiation and highlight a novel, repurposable modality for cancer therapy.

Lysosome trafficking is necessary for EGF-driven invasion and is regulated by p38 MAPK and Na+/H+ exchangers

Background

Tumor invasion through a basement membrane is one of the earliest steps in metastasis, and growth factors, such as Epidermal Growth Factor (EGF) and Hepatocyte Growth Factor (HGF), stimulate this process in a majority of solid tumors. Basement membrane breakdown is one of the hallmarks of invasion; therefore, tumor cells secrete a variety of proteases to aid in this process, including lysosomal proteases. Previous studies demonstrated that peripheral lysosome distribution coincides with the release of lysosomal cathepsins.

Methods

Immunofluorescence microscopy, western blot, and 2D and 3D cell culture techniques were performed to evaluate the effects of EGF on lysosome trafficking and cell motility and invasion.

Results

EGF-mediated lysosome trafficking, protease secretion, and invasion is regulated by the activity of p38 mitogen activated protein kinase (MAPK) and sodium hydrogen exchangers (NHEs). Interestingly, EGF stimulates anterograde lysosome trafficking through a different mechanism than previously reported for HGF, suggesting that there are redundant signaling pathways that control lysosome positioning and trafficking in tumor cells.

Conclusions

These data suggest that EGF stimulation induces peripheral (anterograde) lysosome trafficking, which is critical for EGF-mediated invasion and protease release, through the activation of p38 MAPK and NHEs. Taken together, this report demonstrates that anterograde lysosome trafficking is necessary for EGF-mediated tumor invasion and begins to characterize the molecular mechanisms required for EGF-stimulated lysosome trafficking.

Electronic supplementary material

The online version of this article (10.1186/s12885-017-3660-3) contains supplementary material, which is available to authorized users.

Zinc finger E-box binding homeobox-1 (Zeb1) drives anterograde lysosome trafficking and tumor cell invasion via upregulation of Na+/H+ Exchanger-1 (NHE1)

Tumor cell invasion through the extracellular matrix is facilitated by the secretion of lysosome-associated proteases. As a common mechanism for secretion, lysosomes must first traffic to the cell periphery (anterograde trafficking), consistent with invasive cells often containing lysosomes closer to the plasma membrane compared to non-invasive cells. Epithelial to mesenchymal transition (EMT) is a transcriptionally driven program that promotes an invasive phenotype, and Zeb1 is one transcription factor that activates the mesenchymal gene expression program. The role of lysosome trafficking in EMT-driven invasion has not been previously investigated. We found that cells with increased levels of Zeb1 displayed lysosomes located closer to the cell periphery and demonstrated increased protease secretion and invasion in 3-dimensional (3D) cultures compared to their epithelial counterparts. Additionally, preventing anterograde lysosome trafficking via pharmacological inhibition of Na+/H+ exchanger 1 (NHE1) or shRNA depletion of ADP-ribosylation like protein 8b (Arl8b) reversed the invasive phenotype of mesenchymal cells, thus supporting a role for lysosome positioning in EMT-mediated tumor cell invasion. Immunoblot revealed that expression of Na+/H+ exchanger 1 correlated with Zeb1 expression. Furthermore, we found that the transcription factor Zeb1 binds to the Na+/H+ exchanger 1 promoter, suggesting that Zeb1 directly controls Na+/H+ transcription. Collectively, these results provide insight into a novel mechanism regulating Na+/H+ exchanger 1 expression and support a role for anterograde lysosome trafficking in Zeb1-driven cancer progression. © 2016 Wiley Periodicals, Inc.

Novel c-Met inhibitory olive secoiridoid semisynthetic analogs for the control of invasive breast cancer

Dysregulated receptor tyrosine kinase c-Met and its ligand HGF is valid and attractive molecular target for therapeutic blockade in cancer. Inspired by the chemical structure of the naturally occurring olive secoiridoid (-)-oleocanthal (1) and its documented anticancer activity against c-Met-dependent malignancies, a previous study reported tyrosol sinapate (4) as a c-Met inhibitor hit. This study reports additional semisynthetic optimization and SAR of 4 to improve its selective activity against c-Met-dependent breast cancer by increasing its capacity to inhibit c-Met phosphorylation. Forty-three compounds (5-47) were synthesized, among which the novel analog homovanillyl sinapate (HVS-16) was distinguished for its remarkable activity. HVS-16 substantially impaired c-Met-mediated proliferation, migration, and invasion across human breast cancer cell lines in two- and three-dimensional culture systems, while similar treatment doses were found to have effect neither on the non-tumorigenic human mammary epithelial cell growth nor on the c-Met independent breast cancer cell viability. HVS-16 showed a dose-dependent inhibition of ligand-mediated c-Met activation in human breast cancer cells. Docking studies revealed that HVS-16 fits very well inside c-Met crystal structures, satisfying critical interactions at the ATP binding site. This study identified important structural pharmacophoric features in HVS-16 and correlated its postulated binding pose with c-Met kinase assay data that would guide future olive secoiridoid bioisostere lead design. Results presented herein suggest HVS-16 as a promising c-Met inhibitor validated hit with potential to control invasive breast malignancies with aberrant c-Met activity.

Isothiocyanatostilbenes as novel c-Met inhibitors

The hepatocyte growth factor receptor (HGFR or c-Met) is a driver of multiple cancer subtypes. While there are several c-Met inhibitors in development, few have been approved for clinical use, warranting the need for continued research and development of c-Met targeting therapeutic modalities. The research presented here demonstrates a particular class of compounds known as isothiocyanatostilbenes can act as c-Met inhibitors in multiple cancer cell lines. Specifically, we found that 4,4′-Diisothiocyanatostilbene-2,2′-disulfonic acid (DIDS) and 4,4′-Diisothiocyanatodihydrostilbene-2,2′-disulfonic acid (H2DIDS) had c-Met inhibitory effective doses in the low micromolar range while 4-acetamido-4′-isothiocyanatostilbene-2,2′-disulfonic acid (SITS) and 4,4′-dinitrostilbene-2, 2′-disulfonic acid (DNDS) exhibited IC₅₀s 100 to 1000 fold higher. These compounds displayed much greater selectivity for inhibiting c-Met activation compared to similar receptor tyrosine kinases. In addition, DIDS and H2DIDS reduced hepatocyte growth factor (HGF)-induced, but not epidermal growth factor (EGF)-induced, cell scattering, wound healing, and 3-dimensional (3D) proliferation of tumor cell spheroids. In-cell and cell-free assays suggested that DIDS and H2DIDS can inhibit and reverse c-Met phosphorylation, similar to SU11274. Additional data demonstrated that DIDS is tolerable in vivo. These data provide preliminary support for future studies examining DIDS, H2DIDS, and derivatives as potential c-Met therapeutics.

Characterization of an Oncolytic Adenovirus Vector Constructed to Target the cMet Receptor

The cMet receptor is a homodimer with tyrosine kinase activity. Upon stimulation with its ligand, hepatocyte growth factor (HGF), the receptor mediates wide physiologic actions. The HGF-cMet signaling pathway is dysregulated in many cancers, which makes cMet an important target for novel therapeutic interventions. Oncolytic adenoviruses (Ads) have been used for the past three decades as a promising therapeutic approach for a wide array of neoplastic diseases. To date, achieving cancer-specific replication of oncolytic Ads has been accomplished by either viral genome deletions or by incorporating tumor selective promoters. To achieve novel specificity of oncolytic Ad infection of cancer cells that overexpress cMet, we inserted the HGF NK2 sequence, corresponding to a competitive antagonist of HGF binding to the cMet receptor, into the Ad serotype 5 (Ad5) fiber gene. The resulting vector, Ad5-pIX-RFP-FF/NK2, was rescued, amplified in HEK293 cells, and characterized. Binding specificity and viral infectivity were tested in various cancer cell lines that express varying levels of cMet and hCAR (the Ad5 receptor). We found that Ad5-pIX-RFP-FF/NK2 demonstrated binding specificity to the cMet receptor. In addition, there was enhanced viral infectivity and virus replication compared with a non-targeted Ad vector. Although NK2 weakly induces cMet receptor activation, our results showed no receptor phosphorylation in the context of an oncolytic Ad virus. In summary, these results suggest that an oncolytic Ad retargeted to the cMet receptor is a promising vector for developing a novel cancer therapeutic agent.

Lipin-1 contributes to modified low-density lipoprotein-elicited macrophage pro-inflammatory responses

Atherosclerosis is a chronic inflammatory disease of large and medium-sized arteries and the underlying cause of cardiovascular disease, a major cause of mortality worldwide. The over-accumulation of modified cholesterol-containing low-density lipoproteins (e.g. oxLDL) in the artery wall and the subsequent recruitment and activation of macrophages contributes to the development of atherosclerosis. The excessive uptake of modified-LDL by macrophages leads to a lipid-laden "foamy" phenotype and pro-inflammatory cytokine production. Modified-LDLs promote foam cell formation in part by stimulating de novo lipid biosynthesis. However, it is unknown if lipid biosynthesis directly regulates foam cell pro-inflammatory mediator production. Lipin-1, a phosphatidate phosphohydrolase required for the generation of diacylglycerol during glycerolipid synthesis has recently been demonstrated to contribute to bacterial-induced pro-inflammatory responses by macrophages. In this study we present evidence demonstrating the presence of lipin-1 within macrophages in human atherosclerotic plaques. Additionally, reducing lipin-1 levels in macrophages significantly inhibits both modified-LDL-induced foam cell formation in vitro, as observed by smaller/fewer intracellular lipid inclusions, and ablates modified-LDL-elicited production of the pro-atherogenic mediators tumor necrosis factor-α, interleukin-6, and prostaglandin E2. These findings demonstrate a critical role for lipin-1 in the regulation of macrophage inflammatory responses to modified-LDL. These data begin to link the processes of foam cell formation and pro-inflammatory cytokine production within macrophages.

PEGylated γ-tocotrienol isomer of vitamin E: Synthesis, characterization, in vitro cytotoxicity, and oral bioavailability

Vitamin E refers to a family of eight isomers divided into two subgroups, tocopherols and the therapeutically active tocotrienols (T3). The PEGylated α-tocopherol isomer of vitamin E (vitamin E TPGS) has been extensively investigated for its solubilizing capacity as a nonionic surfactant in various drug delivery systems. Limited information, however, is available about the PEG conjugates of the tocotrienol isomers of vitamin E. In this study two PEGylated γ-T3 variants with mPEG molecular weights of 350 (γ-T3PGS 350) and 1000 (γ-T3PGS 1000) were synthesized by a two-step reaction procedure and characterized by (1)H NMR, HPLC, and mass spectroscopy. The physical properties of their self-assemblies in water were characterized by zeta, CMC, and size analysis. Similar physical properties were found between the PEGylated T3 and vitamin E TPGS. PEGylated T3 were also found to retain the in vitro cytotoxic activity of the free T3 against the MCF-7 and the triple-negative MDA-MB-231 breast cancer cells. PEGylated γ-T3 also increased the oral bioavailability of γ-T3 by threefolds when compared to the bioavailability of γ-T3 formulated into a self-emulsified drug delivery system. No significant differences in biological activity were found between the PEG 350 and 100 conjugates. Results from this study suggest that PEGylation of γ-T3 represents a viable platform for the oral and parenteral delivery of γ-T3 for potential use in the prevention of breast cancer.

The histone deacetylase inhibitor cambinol prevents acidic pHe-induced anterograde lysosome trafficking independently of sirtuin activity

Common features of the solid tumor microenvironment, such as acidic extracellular pH and growth factors, are known to induce the redistribution of lysosomes from a perinuclear region to a position near the plasma membrane. Lysosome/plasma membrane juxtaposition facilitates invasion by allowing for the release of lysosomal proteases, including cathepsin B, which contribute to matrix degradation. In this study we identified the sirtuin 1/sirtuin 2 (SIRT1/2) inhibitor cambinol acts as a drug that inhibits lysosome redistribution and tumor invasion. Treatment of cells with cambinol resulted in a juxtanuclear lysosome aggregation (JLA) similar to that seen upon treatment with the PPARγ agonist, troglitazone (Tro). Like Tro, cambinol required the activity of ERK1/2 in order to induce this lysosome clustering phenotype. However, cambinol did not require the activity of Rab7, suggesting that this drug causes JLA by a mechanism different from what is known for Tro. Additionally, cambinol-induced JLA was not a result of autophagy induction. Further investigation revealed that cambinol triggered JLA independently of its activity as a SIRT1/2 inhibitor, suggesting that this drug could have effects in addition to SIRT1/2 inhibition that could be developed into a novel anti-cancer therapy.

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Cambinol prevents acidic pH_e-induced anterograde lysosome trafficking.

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Cambinol-mediated lysosome aggregation is not dependent on sirtuin activity.

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ERK1/2 activity is necessary for cambinol-driven juxtanuclear lysosome aggregation.

Abstract 382: Lipin-1 Links Pro-inflammatory Responses and Foam Cell Formation by Oxidized-Low Density Lipoprotein-Elicited Macrophages

Atherosclerosis is a chronic inflammatory disease of large and medium-sized arteries and one of the underlying causes of cardiovascular disease (CVD). Macrophages participate decisively in the development and promotion of atherosclerosis. Macrophages infiltrate the arterial intima to ingest modified low density lipoproteins (e.g. oxLDLs) via scavenger receptors. The scavenging of oxLDLs results in foam cell formation due to enhanced lipid droplet biogenesis. These foam cells eventually release pro-inflammatory cytokines that promote atherosclerosis. However, it is currently unknown whether there is a link between lipid droplet biogenesis and pro-inflammatory cytokine production in macrophages that scavenge oxLDL. Lipin-1, a phosphatidate phosphohydrolase enzyme, partially contributes to macrophage pro-inflammatory cytokine production following stimulation with bacteria. Lipin-1 is also required for lipid droplet biogenesis in macrophages. Finally, we observed lipin-1 protein within macrophages from human atherosclerotic plaques. Thus, we hypothesized that lipid droplet biogenesis, via lipin-1 activity, directly contributes to foam cell pro-inflammatory cytokine production. To test this hypothesis we compared lipid droplet biogenesis and pro-inflammatory cytokine responses of oxLDL-stimulated wild type and lipin-1-depleted macrophages. Depletion of lipin-1 inhibited oxLDL-induced foam cell generation by reducing lipid droplet number, area, and staining intensity. There were no differences in scavenger receptor expression or uptake of oxLDL between wild type and lipin-1-depleted cells. In addition, depletion of lipin-1 also ablated oxLDL-elicited production of the pro-atherogenic cytokines tumor necrosis factor-α and interleukin-6. These findings demonstrate a critical role for lipin-1 in the regulation of macrophage inflammatory responses to oxLDL. Furthermore, these data begin to link foam cell formation, via lipid droplet biogenesis, and pro-inflammatory cytokine production within oxLDL stimulated macrophages. Thus, our studies suggest that lipid droplet biogenesis may be an ideal therapeutic target to inhibit inflammation associated with atherosclerosis to treat CVD.

The polyphenols (-)-epigallocatechin-3-gallate and luteolin synergistically inhibit TGF-β-induced myofibroblast phenotypes through RhoA and ERK inhibition

The presence of reactive stroma, predominantly composed of myofibroblasts, is directly associated with and drives prostate cancer progression. We have previously shown that (−)-Epigallocatechin-3-gallate (EGCG), in the form of Polyphenon E, significantly decreases serum levels of HGF and VEGF in prostate cancer patients. Given that HGF and VEGF are secreted from surrounding tumor myofibroblasts, these observations suggested that EGCG may inhibit prostate cancer-associated myofibroblast differentiation. Herein, we demonstrate that micromolar combinations of EGCG and a second polyphenol, luteolin, synergistically inhibit TGF-β-induced myofibroblast phenotypes in prostate fibroblast cell lines, as observed primarily by potentiation of fibronectin expression. Functionally, EGCG and luteolin inhibited TGF-β-induced extracellular matrix contraction, an enhancer of tumor cell invasion. EGCG and luteolin inhibited downstream TGF-β-induced signaling, including activation of ERK and AKT, respectively, but mechanistically, only ERK appeared to be necessary for TGF-β-induced fibronectin expression. Furthermore, neither EGCG nor luteolin affected Smad signaling or nuclear translocation. Rho signaling was found to be necessary for TGF-β-induced fibronectin expression and EGCG and luteolin each reduced RhoA activation. Finally, EGCG and luteolin were shown to reverse TGF-β-induced fibronectin expression, implicating that these natural compounds may be useful not only in preventing but also in treating already activated myofibroblasts and the diseases they cause, including cancer. The ability of EGCG and luteolin to synergistically target myofibroblasts suggests that combined clinical use of these compounds could prevent or reverse cancer progression through targeting the tumor microenvironment, in addition to the tumor itself.

Supporting a role for the GTPase Rab7 in prostate cancer progression

Invasion and subsequent metastasis is the major cause of death from most cancers including prostate cancer. Herein we report on the potential tumor suppressive properties of Rab7, a GTPase that regulates trafficking of lysosomes. The movement of lysosomes to the cell surface in response to environmental cues increases the secretion of proteinases and cell invasion. We determined that Troglitazone and other members of the Thiazolidinedione family inhibit cell-surface directed lysosome trafficking and cathepsin B secretion through a Rab7-dependent mechanism. Moreover, Rab7 shRNA expressing cells were found to be more invasive in vitro and in vivo. Increased invasiveness was accompanied by elevated expression of the c-Met receptor and prolonged downstream signaling, thereby supporting a role for Rab7 as a mediator of signaling down-regulation. Taken together, these results suggested that Rab7 acts as a negative regulator of prostate tumor growth and invasion, providing further evidence for its potential as a tumor suppressor.

The sirtuins promote Dishevelled-1 scaffolding of TIAM1, Rac activation and cell migration

Rac1-GTPases serve as intermediary cellular switches which conduct transient and constitutive signals from upstream cues, including those from Ras oncoproteins. While the sirtuin1 (SIRT1) deacetylase is overexpressed in several human cancers and has recently been linked to cancer cell motility as a context-dependent regulator of multiple pathways, its role in Rac1 activation has not been reported. Likewise, SIRT2 has been demonstrated to be upregulated in some cancers; however, studies have also reported its role in tumor suppression. Here, we demonstrate that SIRT1 and SIRT2 positively regulate the levels of Rac1-GTP and the activity of T-cell lymphoma invasion and metastasis 1 (TIAM1), a Rac guanine nucleotide exchange factor (GEF). Transient inhibition of SIRT1 and SIRT2 resulted in increased acetylation of TIAM1 whereas chronic SIRT2 knockdown resulted in enhanced acetylation of TIAM1. SIRT1 regulates Dishevelled (DVL) protein levels in cancer cells and DVL along with TIAM1 are known to augment Rac activation; however, SIRT1 or 2 have not been previously linked with TIAM1. We found that diminished sirtuin activity led to the disruption of the DVL1-TIAM1 interaction. We hence propose a model for Rac activation where SIRT1/2 positively modulate the DVL/TIAM1/Rac axis and promote sustained pathway activation.

Abstract A86: SIRT1 regulates Rac1-GTPase activation via the guanine nucleotide exchange factor TIAM1

SIRT1 deacetylase is overexpressed in different types of cancers and we and a few others have described a role for SIRT1 in regulating cancer cell migration. Rho GTPases are classic regulators of cell motility and are known to aberrantly enhance cancer cell migration in response to several anomalous cues. We hypothesized that one of mechanisms employed by SIRT1 to regulate cell motility is through regulation of Rac1-GTPase activation.

To address this we utilized two cancer cell lines, CFPAC-1 pancreatic and MDA-MB-231 breast cancer cells, as our model. A significant decrease was observed in active Rac1 (GTP-bound Rac1) levels, in a GST-PAK-binding domain pull-down assay, upon inhibition of SIRT1 by small molecule inhibitors or knockdown of SIRT1 by shRNA as compared to vehicle-treated or non-targeting shRNA transfected cells. Subsequent analysis revealed that SIRT1 co-immunoprecipitated with the Rac-guanine nucleotide exchange factor (GEF), TIAM1. To test whether TIAM1 is acetylated and if SIRT1 regulates its deacetylation we performed antibody-based pull-down of acetylated lysines from whole cell protein extracts and observed TIAM1 acetylation. Inhibition of deacetylase activity by small molecule inhibitors or overexpression of acetyltransferases caused an enhancement of acetylated TIAM1 signal. While a change in TIAM1 localization was not observed with SIRT1 knockdown, its ability to activate Rac1 was compromised.

Several studies have demonstrated SIRT1's pro-tumorigenic potential. In this study we elucidate a mechanism by which SIRT1 regulates Rac-GTPase activation via TIAM1 acetylation. Controlling the “switch-mechanism” that the small GTPases utilize for inducing downstream activity can be crucial from a drug-development perspective. Our findings not only provide a new mechanism for regulation of Rac-GTPase activity but also emphasize SIRT1's potential as a targetable molecule in cancer.

This work is supported by CA155223 and the graduate stipend for Madhurima Saxena is provided by a Carroll-Feist Pre-doctoral Fellowship awarded by the Feist-Weiller Cancer Center at LSU Health Shreveport, Shreveport, LA.

Citation Format: Madhurima Saxena, Samantha S. Dykes, Allison E. Wang, James A. Cardelli2,3, Kevin Pruitt1,3. SIRT1 regulates Rac1-GTPase activation via the guanine nucleotide exchange factor TIAM1. [abstract]. In: Proceedings of the AACR Special Conference on Tumor Invasion and Metastasis; Jan 20-23, 2013; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2013;73(3 Suppl):Abstract nr A86.

Abstract A83: c-Met requires palmitoylation for proper stability and trafficking in cancer cells

The influence of growth factor receptors on cancer progression as both early promoters as well as drivers of late-stage invasion and metastasis has been thoroughly studied over the last several decades. Overexpression and activating mutations of several receptor tyrosine kinases (RTKs) are commonly detected in most cancer types. Despite this, there still remains a great deal not fully understood about RTK expression and regulation that may reveal novel strategies for therapeutic targeting. In particular, c-Met is an RTK whose downstream signal transduction can promote both mitogenic and motogenic phenotypes in cancer cells and whose expression is correlated with poor prognosis and resistance to therapy. c-Met is activated by either autocrine or paracrine ligand stimulation. Alternatively, receptor overexpression allows for ligand-independent dimerization and therefore constitutive activation.

In addition, multiple reports have identified the enzyme fatty acid synthase (FASN) as being commonly overexpressed in prostate cancer, and that this aberrant expression is an early event that becomes more pronounced with aggressive androgen-independent and metastatic disease. FASN is the sole enzyme responsible for de novo synthesis of the 16-carbon saturated fatty acid palmitate. In cancer, de novo lipids are more selectively partitioned into lipid rafts as phospholipids as well as utilized for post-translational acyl-modifications of signaling proteins. Previous findings have led us to identify a novel mechanism by which FASN activity regulates c-Met expression. Our work has determined that inhibition or shRNA knockdown of FASN results in a post-translational downregulation of already synthesized c-Met protein. This downregulation is prevented by the addition of exogenous palmitate.

Based on these findings we have subsequently acquired convincing data that the c-Met receptor tyrosine kinase is palmitoylated and that this palmitoylation regulates its stability. Inhibition of palmitoylation reduces the expression of c-Met in multiple cancer cell lines. This protein loss occurs post-transcriptionally and is associated with accumulation of c-Met in Golgi compartments. Using inhibitors to a number of internalization pathways, as well as surface biotinylation studies, confocal microscopy, and metabolic-ortholog labeling we determined that inhibition of palmitoylation reduces the stability of newly synthesized, c-Met as opposed to inducing internalization and degradation. Moreover, both an acyl-biotin exchange technique and a click-chemistry based palmitate-labeling protocol suggest c-Met itself is palmitoylated. Observing palmitoylation kinetics has provided evidence that c-Met is palmitoylated in the ER prior to cleavage of the 170kd c-Met precursor into its mature 140kd form. Taken together, these findings suggest inhibition of palmitoylation or FASN activity could be a novel target for preventing invasion and metastasis driven by c-Met overexpression.

Citation Format: David T. Coleman, James A. Cardelli. c-Met requires palmitoylation for proper stability and trafficking in cancer cells. [abstract]. In: Proceedings of the AACR Special Conference on Tumor Invasion and Metastasis; Jan 20-23, 2013; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2013;73(3 Suppl):Abstract nr A83.

Abstract A73: Evaluating tissue biomarker effects of an oral green tea extract, polyphenon E, using reverse phase protein array in women with operable breast cancer

Background: Numerous epidemiologic studies and experimental data support potential anti-tumor effects of green tea and its main component, epigallocatechin-3-gallate (EGCG), in breast cancer. However, there is limited data on the effects of tea catechins on breast cancer in human intervention trials. The purpose of this study is to evaluate tumor proteomic changes after short-term pre-surgical administration of an oral green tea extract, Polyphenon E (Poly E), in women with operable breast cancer using reverse phase protein array (RPPA).

Methods: This is a phase II single-arm open-label trial of oral Poly E 800 mg daily for 2-4 weeks in women with histologically-confirmed breast cancer on core biopsy who were scheduled for surgical resection. Formalin-fixed paraffin-embedded (FFPE) tumor tissue from the diagnostic core biopsy (pre-treatment) and surgical resection (post-treatment) were analyzed for expression of the Ki-67 proliferation index, estrogen receptor (ER), progesterone receptor (PR), and HER2 by immunohistochemistry (IHC). Protein was extracted for RPPA analysis of 161 proteins, including components of the PI3K/AKT and MAPK pathways. Women were matched by age, breast cancer stage, ER/PR/HER2 status, and time interval between breast biopsy and surgery to untreated historical controls. Paired t-test was used to calculate changes in protein markers before and after Poly E treatment and 2-sample t-test to compare biomarker changes in the treatment and no treatment groups. All statistical analyses were 2-sided and performed using SAS version 9.1.

Results: From Feb 2008 to Sept 2009, 25 women were enrolled and 21 were evaluable. Median age: 50 years (range, 33-71); White/Hispanic/Black (%): 44/48/7; Stage 0/I/II/III (%): 11/48/30/11; hormone receptor +/- (%): 85/15. Mean duration on Poly E was 20 days (range, 13-36). We demonstrated significant correlations between RPPA and IHC for Ki-67 (0.46, P<0.0001), ER (0.45, P=0.0017), PR (0.46, P=0.0014), and a trend for HER2 (0.28, P=0.0923). Poly E treatment did not cause a significant decrease in Ki-67 compared to untreated controls (mean absolute change, -0.5% vs. +2.6%, P=0.83). In the Poly E-treated group, the RPPA results showed significant modulation of apoptosis and PI3K/AKT pathway proteins (P<0.05). After Bonferroni correction to adjust for multiple comparisons (P<0.00031), 11 markers remained statistically significant comparing change from baseline with Poly E treatment: up-regulation of MEK1, JNK2, and p38-MAPK; down-regulation of CDK4, HER2-pY1248, MAPK-pT202-Y204, MIG-6, mTOR-pS2448, PRAS40-pT246, Src-pY416, and Scr-pY527. Compared to untreated controls, the Poly E group had significant changes from baseline in 10 proteins: up-regulation of IRS1, p38-MAPK, Notch1, and YAP; down-regulation of ERCC1, MIG-6, p90RSK-pT359-S363, PRAS40-pT246, Smad3, and Src-pY416 (P<0.05).

Conclusions: Short-term administration of Poly E did not significantly decrease proliferation in breast tumor tissue; however, our RPPA data suggests that Poly E may act on alternative signaling pathways. The changes we observed in CDK4, HER2, Src, MAPK, and JNK expression are consistent with preclinical studies of EGCG. This is one of the first human intervention trials to demonstrate the biologic effects of Poly E on growth factor signaling pathways in breast cancer.

Citation Format: Kimberly A. Ho, Davida Kornreich, James A. Cardelli, Jerry McLarty, Dawn L. Hershman, Matthew Maurer, Kevin Kalinsky, Bret Taback, Hanina Hibshoosh, Tao Su, Susan F. Refice, Katherine D. Crew. Evaluating tissue biomarker effects of an oral green tea extract, polyphenon E, using reverse phase protein array in women with operable breast cancer. [abstract]. In: Proceedings of the Eleventh Annual AACR International Conference on Frontiers in Cancer Prevention Research; 2012 Oct 16-19; Anaheim, CA. Philadelphia (PA): AACR; Cancer Prev Res 2012;5(11 Suppl):Abstract nr A73.

Host interferon-γ inducible protein contributes to Brucella survival

Brucella spp. are highly adapted intracellular pathogens of mammals that cause chronic infections while surving and replicating in host monocytes and macrophages. Although monocytes are normally susceptible to infection, pretreatment with pro-inflammatory cytokine interferon-γ (IFN-γ) activates cellular defense mechanisms that increase intracellular killing of Brucella and prevents bacterial replication. We examined the contribution of the IFN-γ inducible GTPase, LRG-47, to B. abortus 2308 infection in in vitro and in vivo murine models. Infecting non-activated macrophages from LRG-47(-/-) mice revealed that loss of this host protein negatively effected the intracellular survival and replication of IgG opsonized B. abortus. In contrast, survival and replication of non-opsonized B. abortus was the same in both C57/B6 and LRG-47(-/-) peritoneal macrophages. Following IFN-γ activation of LRG-47(-/-) monocytes, IgG opsonized B. abortus survived better than non-opsonized bacteria. The differential fate of opsonized and non-opsonized B. abortus was only observed in macrophages collected from LRG-47(-/-) mice. Given the specific nature of the relationship between this host protein and the mechanism of Brucella internalization, LRG-47(-/-) mice were infected with B. abortus to assess whether the loss of the lrg47 protein would affect the ability of the bacteria to colonize or persist within the host. B. abortus were able to establish and maintain similar numbers of bacteria in both C57/B6 mice and LRG-47(-/-) through 3 weeks post intraperitoneal infection. By 9 weeks p.i. fewer B. abortus were recovered from LRG-47(-/-) mice than controls, suggesting that the host protein has a positive role in maintaining long term persistence of the bacteria within the host. These observations demonstrating a positive role for a host IFN-γ induced protein defense protein has yet to be reported. These results provide interesting insight into the complex interaction between Brucella and their host.

Abstract 1251: A role for lactate dehydrogenase in the c-Met/HGF signaling axis

Tumor cells often display a glycolytic phenotype, even in the presence of oxygen; a phenomenon known as the Warburg effect. The ability of cancerous cells to perform aerobic glycolysis is unique when compared to non-cancerous cells, and therefore, presents a possible therapeutic target for elimination of cancer cells that rely on aerobic glycolysis. Activation of the receptor tyrosine kinase, c-Met, by hepatocyte growth factor (HGF) is a major contributing event in the progression of prostate cancer (PCa). Signaling through this receptor leads to increased cell motility, proliferation, invasion and metastasis. Studies have shown that targeting this signaling axis can greatly reduce the number of bone metastases that arise from primary PCa tumors. Oxamate, a lactate dehydrogenase (LDH) inhibitor, and 2-deoxyglucose (2-DG) are known glycolytic inhibitors. Our studies have found that oxamate reduced c-Met activation and HGF-induced cell motility, while 2-DG had no effect. In order to confirm that oxamate was targeting LDH-A or LDH-B, we created LDH-A, LDH-B, and LDH-A/B knockdown (KD) DU145 cells. Although all of the clones used in the experiments had greater than 70% KD, we were unable to recapitulate the effects seen with oxamate. We believe that this could be due to incomplete KD. In support of this, use of a specific LDH-A (πFly-21) inhibitor significantly reduced c-Met activation and HGF-induced cell motility. Together, these data support a role for LDH-A in facilitating HGF-induced c-Met activation. Because oxamate is a direct LDH inhibitor, we hypothesized that oxamate would have a greater reduction in lactate (LA) export from cells than 2-DG; however, both, oxamate and 2-DG reduced LA production by approximately 75% in DU145 prostate cancer cells. Therefore, the ability of oxamate to reduce c-Met activation is not apparently due to a decrease in LA export. However, it is possible that LA import via monocarboxylate transporter 1 (MCT1) activity may be important for c-Met activation and downstream signaling events. In support of this, KD of MCT1 resulted in reduced c-Met activation and blocked HGF-induced cell motility. These data indicate a potential connection between LDH activity/lactate import and the c-Met signaling axis, suggesting that inhibition of these processes may be a new mechanism for preventing c-Met driven metastatic events.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 1251. doi:1538-7445.AM2012-1251

Abstract 1216: c-Met protein expression is regulated by palmitoylation in prostate cancer cells

Prostate cancer (PCa) is the most common noncutaneous neoplasia diagnosed in men and represents roughly 10% of cancer-associated mortalities. The lethal phenotype of PCa is primarily characterized by progression of tumor cells to androgen-independence and metastasis. Organ-confined PCa is often curable with surgery and/or radiation therapy, however, as the disease becomes metastatic, the likelihood of survival becomes minimal. Key influences in the progression to metastasis are growth factor receptors including the receptor tyrosine kinase c-Met. Numerous bodies of evidence strongly link aberrant c-Met signaling with causation and/or progression of prostate cancer. Overexpression of c-Met can lead to activation in a ligand-independent manner and has been identified as a common mechanism of resistance to RTK-targeted therapy. In addition, multiple reports have identified the enzyme fatty acid synthase (FASN) as being commonly overexpressed in prostate cancer, and that this aberrant expression is an early event that becomes more pronounced with aggressive androgen-independent and metastatic disease. FASN is the sole enzyme responsible for de novo synthesis of the 16-carbon saturated fatty acid palmitate. In cancer, de novo lipids are more selectively partitioned into lipid rafts as phospholipids as well as utilized for post-translational acyl-modifications of signaling proteins. Previous findings have led us to identify a novel mechanism by which FASN activity regulates c-Met expression. Our work has determined that inhibition or shRNA knockdown of FASN results in a post-translational downregulation of already synthesized c-Met protein. This downregulation is prevented by the addition of exogenous palmitate. Here we provide evidence that inhibition of palmitoylation, using the palmitate analog 2-bromopalmitate, lowers total c-Met levels. This protein loss occurs post-transcriptionally and is associated with accumulation of c-Met in Golgi compartments. Using inhibitors to a number of internalization pathways, as well as surface biotinylation studies, our findings suggest the inhibition of palmitoylation reduces the stability of newly synthesized c-Met as opposed to inducing internalization and degradation. Moreover, both an acyl-biotin exchange technique and a click-chemistry based palmitate-labeling protocol have provided evidence suggesting c-Met itself is palmitoylated. From these data, we hypothesize that c-Met requires palmitoylation along a biosynthetic route to promote its stability and trafficking to the cell surface. In the absence of palmitoylation, c-Met accumulates within the Golgi and is disposed of through a yet to be determined degradation pathway. Our findings potentially reveal a novel mechanism for restricting the expression of c-Met as a means of preventing prostate cancer invasion and metastasis.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 1216. doi:1538-7445.AM2012-1216

The HGF-met signaling axis: emerging themes and targets of inhibition

The Met tyrosine kinase receptor is the only known receptor for hepatocyte growth factor (HGF). Downstream Met signaling is essential for embryonic development; however, aberrant Met signaling promotes tumor progression by facilitating cell proliferation, survival, migration, invasion, and metastasis. Tumor cell invasion is considered an important step in distant metastatic foci formation. Several recent reviews have focused on the pleiotropic effects of Met signaling in both tumor cells and in the surrounding stromal cells. This review will summarize the currently described mechanisms driving Met induced tumor cell progression and invasion, the role played by cells in the tumor stroma, and therapeutic approaches to block receptor activity. In addition, this review will also highlight two new areas of development: 1) attenuation of Met signaling via multiple mechanisms of action targeting tumor cells and cells in the surrounding stroma using plant-derived polyphenols and 2) the induction by HGF of atypical lysosome trafficking, leading to increased protease secretion and tumor cell invasion. These new areas of research will help to uncover novel therapeutic targets to block the HGF/Met signaling axis to slow cancer progression.

Helicobacter pylori induces cancer cell motility independent of the c-Met receptor

Background:

The hepatocyte growth factor (HGF) receptor, c-Met, is strongly implicated in late-stage cancer progression and poor patient prognosis. The stomach pathogen, Helicobacter pylori (H. pylori), was recently proposed to stimulate c-Met phosphorylation dependent upon interaction of c-Met with the bacterial CagA protein required for H. pylori-induced cancer cell motility and invasion.

Materials and Methods:

In this report, we employed short hairpin RNA (shRNA), western blot analysis using antibodies recognizing phosphorylation at discrete c-Met residues, and immunofluorescence microscopy to investigate the CagA-c-Met interaction.

Results:

The data showed that shRNA-mediated c-Met knockdown did not reduce H. pylori-induced cell motility, suggesting that c-Met was not required for motility. Surprisingly, c-Met knockdown did not reduce the level of an H. pylori-induced protein recognized by a phospho-c-Met antibody. This 125 kD protein was 10 kD smaller than c-Met, suggesting that H. pylori did not phosphorylate c-Met but cross-reacted with another protein. This hypothesis was confirmed when c-Met phosphorylation inhibitors did not lower the levels of the bacteria-induced 125 kD protein, and c-Met immunoprecipitation (IP) did not detect this 125 kD protein from H. pylori-treated lysates. This protein was identified as a product of antibody cross reactivity with phosphorylated CagA. We also confirmed that CagA interacts with c-Met, but this interaction may have caused previous authors to misinterpret phosphorylated CagA as c-Met phosphorylation. Finally, pretreatment with the proteasomal inhibitor, lactacystin, caused prolonged HGF-induced c-Met phosphorylation and facilitated a CagA-negative H. pylori to stimulate AGS cell motility, suggesting that sustained c-Met phosphorylation compensates for the loss of CagA-dependent signaling.

Conclusions:

These data demonstrate that H. pylori stimulates cancer cell motility independent of the c-Met receptor. We further hypothesize that although H. pylori does not target c-Met, the bacteria may still utilize c-Met effector signaling to stimulate CagA-independent cancer cell motility, which may provide a further mechanism of H. pylori-dependent gastric cancer progression.

Abstract 2914: A role for glycolysis in the c-Met/HGF signaling axis

Mitochondrial respiration is the most efficient form of energy production in the cell; however, in the absence of oxygen, such as within a solid tumor mass, cells rely on glycolysis as the main source of energy. Under hypoxic conditions the end product of glycolysis, pyruvate, is reduced to lactate by lactate dehydrogenase (LDH), which is then exported out of the cell. This reduction is essential to regenerate NAD+ consumed by the earlier steps of glycolysis. Recent evidence suggests that hepatocyte growth factor (HGF), the only known ligand for the c-Met receptor, causes an increase in glycolysis. This led our laboratory to hypothesize that glycolysis may be required for HGF-induced phenotypic changes in cancer cells. HGF signaling through the c-Met receptor induces an epithelial-mesenchymal transition (EMT), including loss of cell-cell adhesions through down-regulation of E-cadherin expression, as well as increased motility and cell scattering characteristic of the initiating steps leading to cancer cell invasion and metastasis.

Using DU145 prostate cancer cells, we have found that inhibitors of glycolysis (2-DG) and lactate production via LDH activity (oxamate), were able to inhibit HGF-induced cell scattering. Using a wound healing assay, we observed that oxamate was able to prevent HGF-induced cell migration, suggesting a role for glycolysis in cell motility. Additionally, lactic acid was able to rescue cell scattering and wound healing blocked by oxamate, but not by 2-DG. Lactic acid measurements showed that oxamate prevented an increase in glycolysis stimulated by HGF. Measurement of ATP levels also suggested that 2-DG was able to block cell motility due to a reduction in ATP, while oxamate appeared to work through a yet undetermined method. Western blot analysis indicated that oxamate caused a decrease in HGF-induced phospho-Met and downstream phospho-Akt levels. Using H1993 lung cancer cells with constitutively active c-Met, we were able to determine that oxamate deactivated c-Met, while not affecting total c-Met levels. The ability of oxamate to reduce phosphorylation levels appeared to be Met specific based on its inability to block phosphorylation of other activated receptors. Oxamate not only inhibits LDH, but also aspartate aminotransferase (AAT), an enzyme involved in the malate-aspartate shuttle. Our data suggest, however, that the ability of oxamate to inhibit HGF-induced signaling and cell scattering is not through its ability to inhibit AAT.

Taken together, our data suggest the c-Met/HGF signaling axis is dependent on glycolysis and perhaps lactic acid levels. Our long-term goals are to elucidate the specific mechanism for this dependence as a means of defining novel targets for preventing c-Met-mediated cancer cell invasion and metastasis.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 2914. doi:10.1158/1538-7445.AM2011-2914

The polyphenol epigallocatechin-3-gallate affects lipid rafts to block activation of the c-Met receptor in prostate cancer cells

The HGF/c-Met pathway is an important regulator of signaling pathways responsible for invasion and metastasis of most human cancers, including prostate cancer. Exposure of DU145 prostate tumor cells to HGF stimulates the PI3-kinase and MAPK pathways, leading to increased scattering, motility, and invasion, which was prevented by the addition of EGCG. EGCG acted at the level of preventing phosphorylation of tyrosines 1234/1235 in the kinase domain of the c-Met receptor without effecting dimerization. HGF-induced changes were independent of the formation of reactive oxygen species, suggesting that EGCG functioned independent of its antioxidant ability. ECG, another tea polyphenol, was as effective as EGCG, while EGC and EC were less effective. EGCG added up to 4 h after the addition of HGF still blocked cell scattering and reduced the HGF-induced phosphorylation of c-Met, Akt, and Erk, suggesting that EGCG could act both by preventing activation of c-Met by HGF and by attenuating the activity of pathways already induced by HGF. HGF did not activate the MAPK and PI3-K pathways in cells treated with methyl-beta-cyclodextrin (mCD) to remove cholesterol. Furthermore, subcellular fractionation approaches demonstrated that only phosphorylated c-Met accumulated in Triton X-100 membrane insoluble fractions, supporting a role for lipid rafts in regulating c-Met signaling. Finally, EGCG treatment inhibited DiIC16 incorporation into membrane lipid ordered domains, and cholesterol partially inhibited the EGCG effects on signaling. Together, these results suggest that green tea polyphenols with the R1 galloyl group prevent activation of the c-Met receptor by altering the structure or function of lipid rafts.

HGF-induced invasion by prostate tumor cells requires anterograde lysosome trafficking and activity of Na+-H+ exchangers

Hepatocyte growth factor (HGF) is found in tumor microenvironments, and interaction with its tyrosine kinase receptor Met triggers cell invasion and metastasis. It was previously shown that acidic extracellular pH stimulated peripheral lysosome trafficking, resulting in increased cathepsin B secretion and tumor cell invasion, which was dependent upon sodium-proton exchanger (NHE) activity. We now demonstrate that HGF induced the trafficking of lysosomes to the cell periphery, independent of HGF-induced epithelial-mesenchymal transition. HGF-induced anterograde lysosome trafficking depended upon the PI3K pathway, microtubules and RhoA, resulting in increased cathepsin B secretion and invasion by the cells. HGF-induced NHE activity via increased net acid production, and inhibition of NHE activity with 5-(N-ethyl-N-isopropyl)-amiloride (EIPA), or a combination of the NHE1-specific drug cariporide and the NHE3-specific drug s3226 prevented HGF-induced anterograde trafficking and induced retrograde trafficking in HGF-overexpressing cells. EIPA treatment reduced cathepsin B secretion and HGF-induced invasion by the tumor cells. Lysosomes were located more peripherally in Rab7-shRNA-expressing cells and these cells were more invasive than control cells. Overexpression of the Rab7 effector protein, RILP, resulted in a juxtanuclear location of lysosomes and reduced HGF-induced invasion. Together, these results suggest that the location of lysosomes is an inherently important aspect of invasion by tumor cells.

Thiazolidinediones induce Rab7-RILP-MAPK-dependent juxtanuclear lysosome aggregation and reduce tumor cell invasion

Acidic extracellular pH (pHe) has been shown to stimulate peripheral lysosome trafficking, resulting in cathepsin B secretion and tumor invasion. In addition, inhibitors of sodium-proton exchangers (NHE) such as EIPA, cariporide and s3226, as well as the non-specific NHE inhibitor, troglitazone (Tro), blocked these changes. In this paper, we report a differential ability of the thiazolidinedione (TZD) family of compounds to induce a time-dependent retrograde aggregation of lysosomes over the microtubule-organizing center (MTOC) in tumor cells exposed to acidic pHe. This trafficking event depended on microtubules and the MAP-Kinase pathway, but was independent of Rho GTPase activity. Expression of shRNA implicated Rab7 in this process, and subcellular fractionation revealed that levels of Rab7, RILP and Erk1/2 were increased on lysosomes purified from cells treated with Tro. In addition, DN-RILP overexpression studies indicated that this Rab7 effector also played a role in TZD-induced retrograde trafficking. Tro was able to prevent acidic pHe-induced cell invasion. Finally, DU145 prostate tumor cells stably over-expressing WT-RILP, a condition where lysosomes aggregate to the MTOC in the absence of Tro, did not invade in response to acidic pHe, suggesting that the regulation of lysosome trafficking is an inherently important aspect of tumor cell invasion.

Na+/H+ exchangers and RhoA regulate acidic extracellular pH-induced lysosome trafficking in prostate cancer cells

Acidic extracellular pH (pHe) is a common feature of the tumor microenvironment and has been implicated in tumor invasion through the induction of protease secretion.Since lysosomes constitute the major storehouse of cellular proteases, the trafficking of lysosomes to the cell periphery may be required in order to secrete proteases. We demonstrate that a pHe of 6.4-6.8 induced the trafficking of lysosomes to membrane protrusions in the cell periphery. This trafficking event depended upon the PI3K pathway, the GTPase RhoA and sodium-proton exchange activity, resulting in lysosomal exocytosis. Acidic pHe induced a cytoplasmic acidification (although cytoplasmic acidification was not sufficient for acidic pHe-induced lysosome trafficking and exocytosis) and inhibition of NHE activity with the amiloride derivative, EIPA or the anti-diabetic agent troglitazone prevented lysosome trafficking to the cell periphery. Interestingly, using the more specific NHE1 and NHE3 inhibitors, cariporide and s3226 respectively, we show that multiple NHE isoforms are involved in acidic pHe-induced lysosome trafficking and exocytosis. Moreover, in cells expressing NHE1 shRNA, although basal NHE activity was decreased, lysosomes still underwent acidic pHe-induced trafficking,suggesting compensation by other NHE family members.Together these data implicate proton exchangers, especially NHE1 and NHE3, in acidic pHe-induced lysosome trafficking and exocytosis.

The green tea polyphenol EGCG potentiates the antiproliferative activity of c-Met and epidermal growth factor receptor inhibitors in non-small cell lung cancer cells

PURPOSE

Activation of the c-Met and epidermal growth factor receptors (EGFR) promotes the growth and survival of non-small cell lung cancer (NSCLC). Specific receptor antagonists have shown efficacy in the clinic, but tumors often become resistant to these therapies. We investigated the ability of (-)-epigallocatechin-3-gallate (EGCG) to inhibit cell proliferation, and c-Met receptor and EGFR kinase activation in several NSCLC cell lines.

EXPERIMENTAL DESIGN

NSCLC cell lines with variable sensitivity to the EGFR antagonist erlotinib were studied. Cell growth was evaluated using proliferation and colony formation assays. Kinase activation was assessed via Western blot analysis. Experiments were conducted with EGCG, the EGFR antagonist erlotinib, and the c-Met inhibitor SU11274. The antagonists were also tested in a xenograft model using SCID mice.

RESULTS

EGCG inhibited cell proliferation in erlotinib-sensitive and -resistant cell lines, including those with c-Met overexpression, and acquired resistance to erlotinib. The combination of erlotinib and EGCG resulted in greater inhibition of cell proliferation and colony formation than either agent alone. EGCG also completely inhibited ligand-induced c-Met phosphorylation and partially inhibited EGFR phosphorylation. The triple combination of EGCG/erlotinib/SU11274 resulted in a greater inhibition of proliferation than EGCG with erlotinib. Finally, the combination of EGCG and erlotinib significantly slowed the growth rate of H460 xenografts.

CONCLUSION

EGCG is a potent inhibitor of cell proliferation, independent of EGFR inhibition, in several NSCLC cell lines, including those resistant to both EGFR kinase inhibitors and those overexpressing c-Met. Therefore, EGCG might be a useful agent to study as an adjunct to other anticancer agents.

Layer-by-Layer-Coated Gelatin Nanoparticles as a Vehicle for Delivery of Natural Polyphenols

Natural polyphenols with previously demonstrated anticancer potential, epigallocatechin gallate (EGCG), tannic acid, curcumin, and theaflavin, were encased into gelatin-based 200 nm nanoparticles consisting of a soft gel-like interior with or without a surrounding LbL shell of polyelectrolytes (polystyrene sulfonate/polyallylamine hydrochloride, polyglutamic acid/poly-l-lysine, dextran sulfate/protamine sulfate, carboxymethyl cellulose/gelatin, type A) assembled using the layer-by-layer technique. The characteristics of polyphenol loading and factors affecting their release from the nanocapsules were investigated. Nanoparticle-encapsulated EGCG retained its biological activity and blocked hepatocyte growth factor (HGF)-induced intracellular signaling in the breast cancer cell line MBA-MD-231 as potently as free EGCG.

Tea polyphenols decrease serum levels of prostate-specific antigen, hepatocyte growth factor, and vascular endothelial growth factor in prostate cancer patients and inhibit production of hepatocyte growth factor and vascular endothelial growth factor in vitro

The purpose of this study was to determine the effects of short-term supplementation with the active compounds in green tea on serum biomarkers in patients with prostate cancer. Twenty-six men with positive prostate biopsies and scheduled for radical prostatectomy were given daily doses of Polyphenon E, which contained 800 mg of (-)-epigallocatechin-3-gallate (EGCG) and lesser amounts of (-)-epicatechin, (-)-epigallocatechin, and (-)-epicatechin-3-gallate (a total of 1.3 g of tea polyphenols), until time of radical prostatectomy. Serum was collected before initiation of the drug study and on the day of prostatectomy. Serum biomarkers hepatocyte growth factor (HGF), vascular endothelial growth factor (VEGF), insulin-like growth factor (IGF)-I, IGF binding protein-3 (IGFBP-3), and prostate-specific antigen (PSA) were analyzed by ELISA. Toxicity was monitored primarily through liver function enzymes. Changes in serum components were analyzed statistically using the Wilcoxon signed rank test. Cancer-associated fibroblasts were treated with EGCG, and HGF and VEGF protein and mRNA levels were measured. HGF, VEGF, PSA, IGF-I, IGFBP-3, and the IGF-I/IGFBP-3 ratio decreased significantly during the study. All of the liver function tests also decreased, five of them significantly: total protein, albumin, aspartate aminotransferase, alkaline phosphatase, and amylase. The decrease in HGF and VEGF was confirmed in prostate cancer-associated fibroblasts in vitro. Our results show a significant reduction in serum levels of PSA, HGF, and VEGF in men with prostate cancer after brief treatment with EGCG (Polyphenon E), with no elevation of liver enzymes. These findings support a potential role for Polyphenon E in the treatment or prevention of prostate cancer.

Inhibition of fatty acid synthase by luteolin post-transcriptionally down-regulates c-Met expression independent of proteosomal/lysosomal degradation

The hepatocyte growth factor (HGF)/c-Met signaling pathway is involved in the progression of several cancers and associated with increased tumor invasion and metastatic potential. We determined previously that the polyphenol epigallocatechin-3-gallate inhibited HGF-induced c-Met phosphorylation in a variety of tumor cell lines in part by disrupting lipid rafts. Fatty acid synthase (FASN) is implicated in cancer progression and may regulate lipid raft function. We therefore examined the effects of luteolin, a potent FASN inhibitor, on c-Met signaling. Luteolin blocked HGF-induced c-Met phosphorylation and scattering of DU145 prostate cancer cells, but inhibition required at least a 4 h preincubation time. Western blot analysis indicated that inhibition of HGF-induced scattering by luteolin occurred coincident with reduction of total c-Met protein in DU145 cells. In addition, luteolin-induced c-Met down-regulation was mimicked by a pharmacologic inhibitor of FASN, C75, or short hairpin RNA knockdown of FASN. Consistent with a role for FASN, loss of c-Met in cells treated with C75 or luteolin was prevented by exogenous addition of palmitate. Luteolin-induced loss of c-Met primarily occurred at a post-transcriptional level and involved cell surface internalization but did not involve translation inhibition, nor was it dependent on the activity of the 26S proteosome or acidic lysosomes. Taken together, our study shows a novel connection between FASN activity and c-Met protein expression and suggests that luteolin could act as a novel HGF/c-Met inhibitor by reducing expression of this receptor.

The beta1 integrin activates JNK independent of CagA, and JNK activation is required for Helicobacter pylori CagA+-induced motility of gastric cancer cells

The Helicobacter pylori CagA protein is translocated into gastric epithelial cells through a type IV secretion system (TFSS), and published studies suggest CagA is critical for H. pylori-associated carcinogenesis. CagA is thought to be necessary and sufficient to induce the motogenic response observed in response to CagA+ strains, as CagA interacts with proteins involved in adhesion and motility. We report that H. pylori strain 60190 stimulated AGS cell motility through a CagA- and TFSS-dependent mechanism, because strains 60190DeltacagA or 60190DeltacagE (TFSS-defective) did not increase motility. The JNK pathway is critical for H. pylori-dependent cell motility, as inhibition using SP600125 (JNK1/2/3 inhibitor) or a JNK2/3-specific inhibitor blocked motility. JNK mediates H. pylori-induced cell motility by activating paxillin, because JNK inhibition blocked paxillinTyr-118 phosphorylation, and paxillin expression knockdown completely abrogated bacteria-induced motility. Furthermore, JNK and paxillinTyr-118 were activated by 60190DeltacagA but not 60190DeltacagE, demonstrating CagA-independent signaling critical for cell motility. A beta1 integrin-blocking antibody significantly inhibited JNK and paxillinTyr-118 phosphorylation and cell scattering, demonstrating that CagA-independent signaling required for cell motility occurs through beta1. The requirement of both Src and focal adhesion kinase for signaling and motility further suggests the importance of integrin signaling in H. pylori-induced cell motility. Finally, we show that JNK activation occurs independent of known upstream kinases and signaling molecules, including Nod1, Cdc42, Rac1, MKK4, and MKK7, which demonstrates novel signaling leading to JNK activation. We report for the first time that H. pylori mediates CagA-independent signaling that promotes cell motility through the beta1 integrin pathway.

Rapamycin inhibits cell motility by suppression of mTOR-mediated S6K1 and 4E-BP1 pathways

Rapamycin, an inhibitor of the mammalian target of rapamycin (mTOR), inhibits tumor cell motility. However, the underlying mechanism is poorly understood. Here, we show that rapamycin inhibited type I insulin-like growth factor (IGF-I)-stimulated motility of a panel of cell lines. Expression of a rapamycin-resistant mutant of mTOR (mTORrr) prevented rapamycin inhibition of cell motility. However, cells expressing a kinase-dead mTORrr remained sensitive to rapamycin. Downregulation of raptor or rictor by RNA interference (RNAi) decreased cell motility. However, only downregulation of raptor mimicked the effect of rapamycin, inhibiting phosphorylation of S6 kinase 1 (S6K1) and 4E-BP1. Cells infected with an adenovirus expressing constitutively active and rapamycin-resistant mutant of p70 S6K1, but not with an adenovirus expressing wild-type S6K1, or a control virus, conferred to resistance to rapamycin. Further, IGF-I failed to stimulate motility of the cells, in which S6K1 was downregulated by RNAi. Moreover, downregulation of eukaryotic initiation factor 4E (eIF4E)-binding protein 1 (4E-BP1) by RNAi-attenuated rapamycin inhibition of cell motility. In contrast, expression of constitutively active 4E-BP1 dramatically inhibited IGF-I-stimulated cell motility. The results indicate that both S6K1 and 4E-BP1 pathways, regulated by TORC1, are required for cell motility. Rapamycin inhibits IGF-I-stimulated cell motility, through suppression of both S6K1 and 4E-BP1/eIF4E-signaling pathways, as a consequence of inhibition of mTOR kinase activity.

The green tea catechins, (-)-Epigallocatechin-3-gallate (EGCG) and (-)-Epicatechin-3-gallate (ECG), inhibit HGF/Met signaling in immortalized and tumorigenic breast epithelial cells

The hepatocyte growth factor (HGF) receptor, Met, is a strong prognostic indicator of breast cancer patient outcome and survival, suggesting that therapies targeting Met may have beneficial outcomes in the clinic. (-)-Epigallocatechin-3-gallate (EGCG), a catechin found in green tea, has been recognized as a potential therapeutic agent. We assessed the ability of EGCG to inhibit HGF signaling in the immortalized, nontumorigenic breast cell line, MCF10A, and the invasive breast carcinoma cell line, MDA-MB-231. HGF treatment in both cell lines induced rapid, sustained activation of Met, ERK and AKT. Pretreatment of cells with concentrations of EGCG as low as 0.3 microM inhibited HGF-induced Met phosphorylation and downstream activation of AKT and ERK. Treatment with 5.0 microM EGCG blocked the ability of HGF to induce cell motility and invasion. We assessed the ability of alternative green tea catechins to inhibit HGF-induced signaling and motility. (-)-Epicatechin-3-gallate (ECG) functioned similar to EGCG by completely blocking HGF-induced signaling as low as 0.6 microM and motility at 5 microM in MCF10A cells; whereas, (-)-epicatechin (EC) was unable to inhibit HGF-induced events at any concentration tested. (-)-Epigallocatechin (EGC), however, completely repressed HGF-induced AKT and ERK phosphorylation at concentrations of 10 and 20 microM, but was incapable of blocking Met activation. Despite these observations, EGC did inhibit HGF-induced motility in MCF10A cells at 10 microM. These observations suggest that the R1 galloyl and the R2 hydroxyl groups are important in mediating the green tea catechins' inhibitory effect towards HGF/Met signaling. These combined in vitro studies reveal the possible benefits of green tea polyphenols as cancer therapeutic agents to inhibit Met signaling and potentially block invasive cancer growth.

GLUT1CBP(TIP2/GIPC1) interactions with GLUT1 and myosin VI: evidence supporting an adapter function for GLUT1CBP

We identified a novel interaction between myosin VI and the GLUT1 transporter binding protein GLUT1CBP(GIPC1) and first proposed that as an adapter molecule it might function to couple vesicle-bound proteins to myosin VI movement. This study refines the model by identifying two myosin VI binding domains in the GIPC1 C terminus, assigning respective oligomerization and myosin VI binding functions to separate N- and C-terminal domains, and defining a central region in the myosin VI tail that binds GIPC1. Data further supporting the model demonstrate that 1) myosin VI and GIPC1 interactions do not require a mediating protein; 2) the myosin VI binding domain in GIPC1 is necessary for intracellular interactions of GIPC1 with myosin VI and recruitment of overexpressed myosin VI to membrane structures, but not for the association of GIPC1 with such structures; 3) GIPC1/myosin VI complexes coordinately move within cellular extensions of the cell in an actin-dependent and microtubule-independent manner; and 4) blocking either GIPC1 interactions with myosin VI or GLUT1 interactions with GIPC1 disrupts normal GLUT1 trafficking in polarized epithelial cells, leading to a reduction in the level of GLUT1 in the plasma membrane and concomitant accumulation in internal membrane structures.

Opsonized virulent Brucella abortus replicates within nonacidic, endoplasmic reticulum-negative, LAMP-1-positive phagosomes in human monocytes

Cells in the Brucella spp. are intracellular pathogens that survive and replicate within host monocytes. Brucella maintains persistent infections in animals despite the production of high levels of anti-Brucella-specific antibodies. To determine the effect of antibody opsonization on the ability of Brucella to establish itself within monocytes, the intracellular trafficking of virulent Brucella abortus 2308 and attenuated hfq and bacA mutants was followed in the human monocytic cell line THP-1. Early trafficking events of B. abortus 2308-containing phagosomes (BCP) were indistinguishable from those seen for control particles (heat-killed B. abortus 2308, live Escherichia coli HB101, or latex beads). All phagosomes transiently communicated the early-endosomal compartment and rapidly matured into LAMP-1(+), cathepsin D(+), and acidic phagosomes. By 2 h postinfection, however, the number of cathepsin D(+) BCP was significantly lower for live B. abortus 2308-infected cells than for either Brucella mutant strains or control particles. B. abortus 2308 persisted within these cathepsin D(-), LAMP-1(+), and acidic vesicles; however, at the onset of intracellular replication, the numbers of acidic B. abortus 2308 BCP decreased while remaining cathepsin D(-) and LAMP-1(+). In contrast to B. abortus 2308, the isogenic hfq and bacA mutants remained in acidic, LAMP-1(+) phagosomes and failed to initiate intracellular replication. Notably, markers specific for the host endoplasmic reticulum were absent from the BCPs throughout the course of the infection. Thus, opsonized B. abortus in human monocytes survives within phagosomes that remain in the endosomal pathway and replication of virulent B. abortus 2308 within these vesicles corresponds with an increase in intraphagosomal pH.

Adaptation of the Brucellae to their intracellular niche

Members of the bacterial genus Brucella are facultative intracellular pathogens that reside predominantly within membrane-bound compartments within two host cell types, macrophages and placental trophoblasts. Within macrophages, the brucellae route themselves to an intracellular compartment that is favourable for survival and replication, and they also appear to be well-adapted from a physiological standpoint to withstand the environmental conditions encountered during prolonged residence in this intracellular niche. Much less is known about the interactions of the Brucella with placental trophoblasts, but experimental evidence suggests that these bacteria use an iron acquisition system to support extensive intracellular replication within these host cells that is not required for survival and replication in host macrophages. Thus, it appears that the brucellae rely upon the products of distinct subsets of genes to adapt successfully to the environmental conditions encountered within the two cell types within which they reside in their mammalian hosts.

RacB regulates cytoskeletal function in Dictyostelium spp

Thus far, 14 homologues of mammalian Rac proteins have been identified in Dictyostelium. It is unclear whether each of these genes has a unique function or to what extent they play redundant roles in actin cytoskeletal organization. To investigate the specific function of RacB, we have conditionally expressed wild-type (WT-RacB), dominant negative (N17-RacB), and constitutively activated (V12-RacB) versions of the protein. On induction, cells expressing V12-RacB stopped growing, detached from the surface, and formed numerous spherical surface protrusions while cells overexpressing WT-RacB became flattened on the surface. In contrast, cells overexpressing N17-RacB did not show any significant morphological abnormalities. The surface protrusions seen in V12-RacB cells appear to be actin-driven protrusions because they were enriched in F-actin and were inhibitable by cytochalasin A treatment. The protrusions in V12-RacB cells did not require myosin II activity, which distinguishes them from blebs formed by wild-type cells under stress. Finally, we examined the functional consequences of expression of wild-type and mutant RacB. Phagocytosis, endocytosis, and fluid phase efflux rates were reduced in all cell lines expressing RacB proteins but the greatest decrease was observed for cells expressing V12-RacB. From these results, we conclude that like other members of the Rho family, RacB induces polymerization of actin but the consequences of activation appear to be different from other Dictyostelium Rac proteins so far investigated, resulting in different morphological and functional changes in cells.

Truncation of herpes simplex virus type 2 glycoprotein B increases its cell surface expression and activity in cell-cell fusion, but these properties are unrelated

Formation of small polykaryons by cell-cell fusion is characteristic of herpes simplex virus (HSV) lesions, but the great majority of viruses isolated from such lesions produce only limited cell fusion in tissue culture. Because of this, HSV laboratory strains that produce extensive cell fusion (syncytium formation) in culture are regarded as variants or mutants. Furthermore, the rarity of clinical isolates able to produce syncytia in culture suggests that extensive cell fusion is deleterious in vivo. Mutations that confer a syncytial phenotype can then be regarded as bypassing a mechanism that normally limits cell fusion. Determination of how these mutations, some of which are in the cytoplasmic tail of glycoprotein B (gB), lead to syncytium formation will likely reveal how fusion is controlled. Here we show the following. (i) Truncation of the cytoplasmic tail of HSV type 2 gB (gB-2) by a minimum of 25 residues or a maximum of 49 residues produces a syncytial phenotype. (ii) Truncation by 20 to 49 residues increases cell fusion when gB-2 is coexpressed with only gD-2, gH-2, and gL-2. (iii) Truncation by 25 or more residues removes a potential endocytosis motif and increases gB-2 cell surface expression. (iv) Mutation of this motif increases gB-2 cell surface expression but does not increase fusogenic activity, whereas mutation of another potential endocytosis motif does not increase surface expression but does increase fusogenic activity. Therefore, syncytial mutations in the cytoplasmic tail of gB-2 do not act by increasing cell surface levels of the protein.

p110-related PI 3-kinases regulate phagosome-phagosome fusion and phagosomal pH through a PKB/Akt dependent pathway in Dictyostelium

The Dictyostelium p110-related PI 3-kinases, PIK1 and PIK2, regulate the endosomal pathway and the actin cytoskeleton, but do not significantly regulate internalization of particles in D. discoideum. Bacteria internalized into (Δ)ddpik1/ddpik2 cells or cells treated with PI 3-kinase inhibitors remained intact as single particles in phagosomes with closely associated membranes after 2 hours of internalization, while in control cells, bacteria appeared degraded in multi-particle spacious phagosomes. Addition of LY294002 to control cells, after 60 minutes of chase, blocked formation of spacious phagosomes, suggesting PI 3-kinases acted late to regulate spacious phagosome formation. Phagosomes purified from control and drug treated cells contained equivalent levels of lysosomal proteins, including the proton pump complex, and were acidic, but in drug treated cells and (Δ)ddpik1/ddpik2 cells phagosomal pH was significantly more acidic during maturation than the pH of control phagosomes. Inhibition of phagosomal maturation by LY294002 was overcome by increasing phagosomal pH with NH(4)Cl, suggesting that an increase in pH might trigger homotypic phagosome fusion. A pkbA null cell line (PKB/Akt) reproduced the phenotype described for cells treated with PI 3-kinase inhibitors and (Δ)ddpik1/ddpik2 cells. We propose that PI 3-kinases, through a PKB/Akt dependent pathway, directly regulate homotypic fusion of single particle containing phagosomes to form multi-particle, spacious phagosomes, possibly through the regulation of phagosomal pH.

Inactivation of lmpA, encoding a LIMPII-related endosomal protein, suppresses the internalization and endosomal trafficking defects in profilin-null mutants

Profilin is a key phosphoinositide and actin-binding protein connecting and coordinating changes in signal transduction pathways with alterations in the actin cytoskeleton. Using biochemical assays and microscopic approaches, we demonstrate that profilin-null cells are defective in macropinocytosis, fluid phase efflux, and secretion of lysosomal enzymes but are unexpectedly more efficient in phagocytosis than wild-type cells. Disruption of the lmpA gene encoding a protein (DdLIMP) belonging to the CD36/LIMPII family suppressed, to different degrees, most of the profilin-minus defects, including the increase in F-actin, but did not rescue the secretion defect. Immunofluorescence microscopy indicated that DdLIMP, which is also capable of binding phosphoinositides, was associated with macropinosomes but was not detected in the plasma membrane. Also, inactivation of the lmpA gene in wild-type strains resulted in defects in macropinocytosis and fluid phase efflux but not in phagocytosis. These results suggest an important role for profilin in regulating the internalization of fluid and particles and the movement of material along the endosomal pathway; they also demonstrate a functional interaction between profilin and DdLIMP that may connect phosphoinositide-based signaling through the actin cytoskeleton with endolysosomal membrane trafficking events.

Intracellular growth of Legionella pneumophila in Dictyostelium discoideum, a system for genetic analysis of host-pathogen interactions

Conditions were established in which Legionella pneumophila, an intracellular bacterial pathogen, could replicate within the unicellular organism Dictyostelium discoideum. By several criteria, L. pneumophila grew by the same mechanism within D. discoideum as it does in amoebae and macrophages. Bacteria grew within membrane-bound vesicles associated with rough endoplasmic reticulum, and L. pneumophila dot/icm mutants, blocked for growth in macrophages and amoebae, also did not grow in D. discoideum. Internalized L. pneumophila avoided degradation by D. discoideum and showed evidence of reduced fusion with endocytic compartments. The ability of L. pneumophila to grow within D. discoideum depended on the growth state of the cells. D. discoideum grown as adherent monolayers was susceptible to L. pneumophila infection and to contact-dependent cytotoxicity during high-multiplicity infections, whereas D. discoideum grown in suspension was relatively resistant to cytotoxicity and did not support intracellular growth. Some known D. discoideum mutants were examined for their effect on growth of L. pneumophila. The coronin mutant and the myoA/B double myosin I mutant were more permissive than wild-type strains for intracellular growth. Growth of L. pneumophila in a G(beta) mutant was slightly reduced compared to the parent strain. This work demonstrates the usefulness of the L. pneumophila-D. discoideum system for genetic analysis of host-pathogen interactions.

Early and late endosomal compartments of Entamoeba histolytica are enriched in cysteine proteases, acid phosphatase and several Ras-related Rab GTPases

Pure populations of early and late endosomes of Entamoeba histolytica were isolated by magnetic fractionation and characterized. It was shown that these vesicles were enriched in acid phosphatase and cysteine protease activities. An important virulence factor, a 27-kDa cysteine protease, was also enriched in early and late endosomes of E. histolytica. These data suggest that E. histolytica hydrolases reside in compartments that are part of or communicate with the endosomal pathway. To begin to identify the role of Rab GTPases in E. histolytica, an oligonucleotide approach was employed to screen an E. histolytica cDNA library for genes encoding Rab-like proteins. cDNAs encoding a Rab11-like protein (EhRab11) and a novel Rab protein (EhRabA) were isolated and characterized. The EhRab11 cDNA predicts a polypeptide of at least 206 amino acids with a molecular mass of at least 23.2 kDa. Phylogenetic analysis and alignment of EhRab11 with other Rab proteins demonstrated that EhRab11 shared significant homology at the amino acid level with Rab11-like proteins from a number of other eukaryotes, suggesting that EhRab11 is a Rab11 homolog for E. histolytica. The EhRabA clone predicts a polypeptide of 219 amino acids with a molecular mass of at least 24.5 kDa. EhRabA shared only limited homology at the amino acid level with other Rab proteins, suggesting that it is a novel member of this family of GTP-binding proteins. Finally, Western blot analysis demonstrated that EhRab11 and a previously described Rab7-like GTPase from E. histolytica was enriched in magnetically purified endosomal compartments of this organism.

Dictyostelium lysosomal proteins with different sugar modifications sort to functionally distinct compartments

Many Dictyostelium lysosomal enzymes contain mannose-6-phosphate (Man-6-P) in their N-linked oligosaccharide chains. We have now characterized a new group of lysosomal proteins that contain N-acetylglucosamine-1-phosphate (GlcNAc-1-P) linked to serine residues. GlcNAc-1-P-containing proteins, which include papain-like cysteine proteinases, cofractionate with the lysosomal markers and are in functional vesicles of the endosomal/lysosomal pathway. Immunoblots probed with reagents specific for each carbohydrate modification indicate that the lysosomal proteins are modified either by Man-6-P or GlcNAc-1-P, but not by both. Confocal microscopy shows that the two sets of proteins reside in physically and functionally distinct compartments. Vesicles with GlcNAc-1-P fuse with nascent bacteria-loaded phagosomes less than 3 minutes after ingestion, while those with Man-6-P do not participate in bacterial digestion until about 15 minutes after phagocytosis. Even though both types of vesicles fuse with phagosomes, GlcNAc-1-P- and Man-6-P-bearing proteins rarely colocalize. Since both lysosomal enzymes and their bound carbohydrate modifications are stable in lysosomes, a targeting or retrieval mechanism based on these carbohydrate modifications probably establishes and/or maintains segregation.

Phagosomal proteins of Dictyostelium discoideum

In recognizing food particles. Dictyostelium cell-surface molecules initiate cytoskeletal rearrangements that result in phagosome formation. After feeding D. discoideum cells latex beads, early phagosomes were isolated on sucrose step gradients. Protein analyses of these vesicles showed that they contained glycoproteins and surface-labeled species corresponding to integral plasma membrane proteins. Cytoskeletal proteins also were associated with phagosomes, including myosin II, actin and a 30 kDa-actin bundling protein. As seen by the acridine orange fluorescence of vesicles containing bacteria, phagosomes were acidified rapidly by a vacuolar H(+)-ATPase that was detected by immunoblotting. Except for the loss of cytoskeletal proteins, few other changes over time were noted in the protein profiles of phagosomes, suggesting that phagosome maturation was incomplete. The indigestibility of the beads possibly inhibited further endocytic processing, which has been observed by others. Since nascent phagosomes contained molecules of both the cytoskeleton and plasma membrane, they will be useful in studies aimed at identifying specific protein associations occurring between membrane proteins and the cytoskeleton during phagocytosis.

Cloning and characterization of a Dictyostelium discoideum cDNA encoding a protein related to the medium chain subunit of clathrin-associated adaptor complexes

We describe the cloning and characterization of a cDNA, DdApm1, encoding a putative medium chain subunit of a clathrin-associated protein (adaptor or assembly protein [AP]) complex in Dictyostelium discoideum. The DdApm1 clone is predicted to encode a polypeptide of 439 amino acids (aa) with a molecular mass of 49.9 kDa. The predicted translation product (DdApm1p) shares at least 51.7% identity and 76.3% similarity with the medium chain subunits of plasma membrane (mb)-associated clathrin AP complexes from rat and Caenorhabditis elegans. The deduced aa sequence also demonstrates significant but lesser homology to a number of medium chain subunits of Golgi-associated clathrin AP complexes. Since DdApm1p demonstrates significantly greater homology to plasma mb-associated clathrin AP complex medium chains than to their Golgi-associated counterparts, we suggest that DdApm1p may be a medium chain subunit of an AP complex involved in clathrin function at the plasma mb of D. discoideum. Southern blot analysis indicated that DdApm1 gene defines a single copy gene in the D. discoideum genome. Northern blot analysis of RNA purified at different times during growth and development demonstrated that the DdApm1 gene is expressed at relatively constant levels throughout the life cycle of the organism. DdApm1 is the first reported full-length cDNA encoding a subunit of an AP complex in D. discoideum, and thus provides the first evidence for the existence of AP complexes in this organism.

Involvement of the vacuolar proton-translocating ATPase in multiple steps of the endo-lysosomal system and in the contractile vacuole system of Dictyostelium discoideum

We have investigated the effects of Concanamycin A (CMA), a specific inhibitor of vacuolar type H(+)-ATPases, on acidification and function of the endo-lysosomal and contractile vacuole (CV) systems of D. discoideum. This drug inhibited acidification and increased the pH of endo-lysosomal vesicles both in vivo and in vitro in a dose dependent manner. Treatment also inhibited endocytosis and exocytosis of fluid phase, and phagocytosis of latex beads. This report also confirms our previous conclusions (Cardelli et al. (1989) J. Biol. Chem. 264, 3454–3463) that maintenance of acidic pH in lumenal compartments is required for efficient processing and targeting of a lysosomal enzyme, alpha-mannosidase. CMA treatment compromised the function of the contractile vacuole complex as amoebae exposed to a hypo-osmotic environment in the presence of CMA, swelled rapidly and ruptured. Fluorescence microscopy revealed that CMA treatment induced gross morphological changes in D. discoideum cells, characterized by the formation of large intracellular vacuoles containing fluid phase. The reticular membranes of the CV system were also no longer as apparent in drug treated cells. Finally, this is the first report describing cells that can adapt in the presence of CMA; in nutrient medium, D. discoideum overcame the effects of CMA after one hour of drug treatment even in the absence of protein synthesis. Upon adaptation to CMA, normal sized endo-lysosomal vesicles reappeared, endo-lysosomal pH decreased, and the rate of endocytosis, exocytosis and phagocytosis returned to normal. This study demonstrates that the V-H(+)-ATPase plays an important role in maintaining the integrity and function of the endo-lysosomal and CV systems and that D. discoideum can compensate for the loss of a functional V-H(+)-ATPase.

Examination of the endosomal and lysosomal pathways in Dictyostelium discoideum myosin I mutants

The role of myosin Is in endosomal trafficking and the lysosomal system was investigated in a Dictyostelium discoideum myosin I double mutant myoB-/C-, that has been previously shown to exhibit defects in fluid-phase endocytosis during growth in suspension culture (Novak et al., 1995). Various properties of the endosomal pathway in the myoB-/C- double mutant as well as in the myoB- and myoC- single mutants, including intravesicular pH, and intracellular retention time and exocytosis of a fluid phase marker, were found to be indistinguishable from wild-type parental cells. The intimate connection between the contractile vacuole complex and the endocytic pathway in Dictyostelium, and the localization of a myosin I to the contractile vacuole in Acanthamoeba, led us to also examine the structure and function of this organelle in the three myosin I mutants. No alteration in contractile vacuole structure or function was observed in the myoB-, myoC- or myoB-/C- cell lines. The transport, processing, and localization of a lysosomal enzyme, alpha-mannosidase, were also unaltered in all three mutants. However, the myoB- and myoB-/C- cell lines, but not the myoC- cell line, were found to oversecrete the lysosomal enzymes alpha-mannosidase and acid phosphatase, during growth and starvation. None of the mutants oversecreted proteins following the constitutive secretory pathway. Two additional myosin I mutants, myoA- and myoA-/B-, were also found to oversecrete the lysosomally localized enzymes alpha-mannosidase and acid phosphatase. Taken together, these results suggest that these myosins do not play a role in the intracellular movement of vesicles, but that they may participate in controlling events that occur at the actin-rich cortical region of the cell. While no direct evidence has been found for the association of myosin Is with lysosomes, we predict that the integrity of the lysosomal system is tied to the fidelity of the actin cortex, and changes in cortical organization could influence lysosomal-related membrane events such as internalization or transit of vesicles to the cell surface.