Novel mechanism for regulation of epidermal growth factor receptor endocytosis revealed by protein kinase A inhibition

Molecular Insights of Cholestasis in MDR2 Knockout Murine Liver Organoids

MDR3 (multidrug resistance 3) deficiency in humans (MDR2 in mice) causes progressive familial intrahepatic cholestasis type 3 (PFIC3). PFIC3 is a lethal disease characterized by an early onset of intrahepatic cholestasis progressing to liver cirrhosis, a preneoplastic condition, putting individuals at risk of hepatocellular carcinoma (HCC). Hepatocyte-like organoids from MDR2-deficient mice (MDR2KO) were used in this work to study the molecular alterations caused by the deficiency of this transporter. Proteomic analysis by mass spectrometry allowed characterization of 279 proteins that were differentially expressed in MDR2KO compared with wild-type organoids. Functional enrichment analysis indicated alterations in three main cellular functions: (1) interaction with the extracellular matrix, (2) remodeling intermediary metabolism, and (3) cell proliferation and differentiation. The affected cellular processes were validated by orthogonal molecular biology techniques. Our results point to molecular mechanisms associated with PFIC3 that may drive the progression to liver cirrhosis and HCC and suggest proteins and cellular processes that could be targeted for the development of early detection strategies for these severe liver diseases.

Phosphatidic acid-PKA signaling regulates p38 and ERK1/2 functions in ligand-independent EGFR endocytosis

Ligand-independent epidermal growth factor receptor (EGFR) endocytosis is inducible by a variety of stress conditions converging upon p38 kinase. A less known pathway involves phosphatidic acid (PA) signaling toward the activation of type 4 phosphodiesterases (PDE4) that decrease cAMP levels and protein kinase A (PKA) activity. This PA/PDE4/PKA pathway is triggered with propranolol used to inhibit PA hydrolysis and induces clathrin-dependent and clathrin-independent endocytosis, followed by reversible accumulation of EGFR in recycling endosomes. Here we give further evidence of this signaling pathway using biosensors of PA, cAMP, and PKA in live cells and then show that it activates p38 and ERK1/2 downstream the PKA inhibition. Clathrin-silencing and IN/SUR experiments involved the activity of p38 in the clathrin-dependent route, while ERK1/2 mediates clathrin-independent EGFR endocytosis. The PA/PDE4/PKA pathway selectively increases the EGFR endocytic rate without affecting LDLR and TfR constitute endocytosis. This selectiveness is probably because of EGFR phosphorylation, as detected in Th1046/1047 and Ser669 residues. The EGFR accumulates at perinuclear recycling endosomes colocalizing with TfR, fluorescent transferrin, and Rab11, while a small proportion distributes to Alix-endosomes. A non-selective recycling arrest includes LDLR and TfR in a reversible manner. The PA/PDE4/PKA pathway involving both p38 and ERK1/2 expands the possibilities of EGFR transmodulation and interference in cancer.

D-Propranolol Impairs EGFR Trafficking and Destabilizes Mutant p53 Counteracting AKT Signaling and Tumor Malignancy

Simple Summary

Cancer progression is frequently driven by altered functions of EGFR belonging to the tyrosine-kinase family of growth factor receptors and by the transcription factor p53, which is called the “genome guardian”. We report that D-Propranolol, previously used for other purposes in human patients, has antitumor effects involving a redistribution of cell surface EGFR to intracellular compartments and degradation of gain-of-function mutants of p53 (GOF-mutp53). These effects can be seen in cancer cell lines expressing EGFR and GOF-mutp53 and are reproduced in vivo, reducing tumor growth and prolonging survival of xenografted mice. D-Propranolol is proposed as a prototype drug for a new strategy against highly aggressive EGFR- and mutp53-expressing tumors.

Abstract

Cancer therapy may be improved by the simultaneous interference of two or more oncogenic pathways contributing to tumor progression and aggressiveness, such as EGFR and p53. Tumor cells expressing gain-of-function (GOF) mutants of p53 (mutp53) are usually resistant to EGFR inhibitors and display invasive migration and AKT-mediated survival associated with enhanced EGFR recycling. D-Propranolol (D-Prop), the non-beta blocker enantiomer of propranolol, was previously shown to induce EGFR internalization through a PKA inhibitory pathway that blocks the recycling of the receptor. Here, we first show that D-Prop decreases the levels of EGFR at the surface of GOF mutp53 cells, relocating the receptor towards recycling endosomes, both in the absence of ligand and during stimulation with high concentrations of EGF or TGF-α. D-Prop also inactivates AKT signaling and reduces the invasive migration and viability of these mutp53 cells. Unexpectedly, mutp53 protein, which is stabilized by interaction with the chaperone HSP90 and mediates cell oncogenic addiction, becomes destabilized after D-Prop treatment. HSP90 phosphorylation by PKA and its interaction with mutp53 are decreased by D-Prop, releasing mutp53 towards proteasomal degradation. Furthermore, a single daily dose of D-Prop reproduces most of these effects in xenografts of aggressive gallbladder cancerous G-415 cells expressing GOF R282W mutp53, resulting in reduced tumor growth and extended mice survival. D-Prop then emerges as an old drug endowed with a novel therapeutic potential against EGFR- and mutp53-driven tumor traits that are common to a large variety of cancers.

Discovery of a novel EGFR ligand DPBA that degrades EGFR and suppresses EGFR-positive NSCLC growth

Epidermal growth factor receptor (EGFR) activation plays a pivotal role in EGFR-driven non-small cell lung cancer (NSCLC) and is considered as a key target of molecular targeted therapy. EGFR tyrosine kinase inhibitors (TKIs) have been canonically used in NSCLC treatment. However, prevalent innate and acquired resistances and EGFR kinase-independent pro-survival properties limit the clinical efficacy of EGFR TKIs. Therefore, the discovery of novel EGFR degraders is a promising approach towards improving therapeutic efficacy and overcoming drug resistance. Here, we identified a 23-hydroxybetulinic acid derivative, namely DPBA, as a novel EGFR small-molecule ligand. It exerted potent in vitro and in vivo anticancer activity in both EGFR wild type and mutant NSCLC by degrading EGFR. Mechanistic studies disclosed that DPBA binds to the EGFR extracellular domain at sites differing from those of EGF and EGFR. DPBA did not induce EGFR dimerization, phosphorylation, and ubiquitination, but it significantly promoted EGFR degradation and repressed downstream survival pathways. Further analyses showed that DPBA induced clathrin-independent EGFR endocytosis mediated by flotillin-dependent lipid rafts and unaffected by EGFR TKIs. Activation of the early and late endosome markers rab5 and rab7 but not the recycling endosome marker rab11 was involved in DPBA-induced EGFR lysosomal degradation. The present study offers a new EGFR ligand for EGFR pharmacological degradation and proposes it as a potential treatment for EGFR-positive NSCLC, particularly NSCLC with innate or acquired EGFR TKI resistance. DPBA can also serve as a chemical probe in the studies on EGFR trafficking and degradation.

Processing Temporal Growth Factor Patterns by an Epidermal Growth Factor Receptor Network Dynamically Established in Space

The proto-oncogenic epidermal growth factor (EGF) receptor (EGFR) is a tyrosine kinase whose sensitivity and response to growth factor signals that vary over time and space determine cellular behavior within a developing tissue. The molecular reorganization of the receptors on the plasma membrane and the enzyme-kinetic mechanisms of phosphorylation are key determinants that couple growth factor binding to EGFR signaling. To enable signal initiation and termination while simultaneously accounting for suppression of aberrant signaling, a coordinated coupling of EGFR kinase and protein tyrosine phosphatase activity is established through space by vesicular dynamics. The dynamical operation mode of this network enables not only time-varying growth factor sensing but also adaptation of the response depending on cellular context. By connecting spatially coupled enzymatic kinase/phosphatase processes and the corresponding dynamical systems description of the EGFR network, we elaborate on the general principles necessary for processing complex growth factor signals.

Lipocalin-2 Regulates Epidermal Growth Factor Receptor Intracellular Trafficking

Epidermal growth factor receptor (EGFR) activation and lipocalin-2 (Lcn2) expression are frequently observed in the same pathological contexts, such as cancers or chronic kidney disease (CKD). However, the significance of this association is unknown. Here, we describe the role of Lcn2 in regulating EGFR trafficking. We show that Lcn2 increases EGFR cell surface abundance and is required for transforming growth factor α (TGF-α)-induced EGFR recycling to the plasma membrane and sustained activation. Lcn2 binds to the intracellular domain of EGFR in late endosomal compartments and inhibits its lysosomal degradation. Consistently, Lcn2 enhances EGFR-induced cell migration after TGF-α stimulation. In vivo, Lcn2 gene inactivation prevents EGFR recycling to the plasma membrane in an experimental model of CKD. Remarkably, this is associated with a dramatic decrease of renal lesions. Together, our data identify Lcn2 as a key mediator of EGFR trafficking processes. Hence, therapeutic inhibition of Lcn2 may counteract the deleterious effect of EGFR activation.

A Genome-scale CRISPR Screen Identifies the ERBB and mTOR Signaling Networks as Key Determinants of Response to PI3K Inhibition in Pancreatic Cancer

KRAS mutation is a key driver of pancreatic cancer and PI3K pathway activity is an additional requirement for Kras-induced tumorigenesis. Clinical trials of PI3K pathway inhibitors in pancreatic cancer have shown limited responses. Understanding the molecular basis for this lack of efficacy may direct future treatment strategies with emerging PI3K inhibitors. We sought new therapeutic approaches that synergize with PI3K inhibitors through pooled CRISPR modifier genetic screening and a drug combination screen. ERBB family receptor tyrosine kinase signaling and mTOR signaling were key modifiers of sensitivity to alpelisib and pictilisib. Inhibition of the ERBB family or mTOR was synergistic with PI3K inhibition in spheroid, stromal cocultures. Near-complete loss of ribosomal S6 phosphorylation was associated with synergy. Genetic alterations in the ERBB-PI3K signaling axis were associated with decreased survival of patients with pancreatic cancer. Suppression of the PI3K/mTOR axis is potentiated by dual PI3K and ERBB family or mTOR inhibition. Surprisingly, despite the presence of oncogenic KRAS, thought to bestow independence from receptor tyrosine kinase signaling, inhibition of the ERBB family blocks downstream pathway activation and synergizes with PI3K inhibitors. Further exploration of these therapeutic combinations is warranted for the treatment of pancreatic cancer.

Design and evaluation of engineered protein biosensors for live-cell imaging of EGFR phosphorylation

Live-cell fluorescence microscopy is broadly applied to study the dynamics of receptor-mediated cell signaling, but the availability of intracellular biosensors is limited. A biosensor based on the tandem SH2 domains from phospholipase C-γ1 (PLCγ1), tSH2-WT, has been used to measure phosphorylation of the epidermal growth factor receptor (EGFR). Here, we found that tSH2-WT lacked specificity for phosphorylated EGFR, consistent with the known promiscuity of SH2 domains. Further, EGF-stimulated membrane recruitment of tSH2-WT differed qualitatively from the expected kinetics of EGFR phosphorylation. Analysis of a mathematical model suggested, and experiments confirmed, that the high avidity of tSH2-WT resulted in saturation of its target and interference with EGFR endocytosis. To overcome the apparent target specificity and saturation issues, we implemented two protein engineering strategies. In the first approach, we screened a combinatorial library generated by random mutagenesis of the C-terminal SH2 domain (cSH2) of PLCγ1 and isolated a mutant form (mSH2) with enhanced specificity for phosphorylated Tyr992 (pTyr992) of EGFR. A biosensor based on mSH2 closely reported the kinetics of EGFR phosphorylation but retained cross-reactivity similar to tSH2-WT. In the second approach, we isolated a pTyr992-binding protein (SPY992) from a combinatorial library generated by mutagenesis of the Sso7d protein scaffold. Compared to tSH2-WT and mSH2, SPY992 exhibited superior performance as a specific, moderate-affinity biosensor. We extended this approach to isolate a biosensor for EGFR pTyr1148 (SPY1148). This approach of integrating theoretical considerations with protein engineering strategies can be generalized to design and evaluate suitable biosensors for various phospho-specific targets.

Galectin-8 induces partial epithelial-mesenchymal transition with invasive tumorigenic capabilities involving a FAK/EGFR/proteasome pathway in Madin-Darby canine kidney cells

Epithelial cells can acquire invasive and tumorigenic capabilities through epithelial–mesenchymal-transition (EMT). The glycan-binding protein galectin-8 (Gal-8) activates selective β1-integrins involved in EMT and is overexpressed by certain carcinomas. Here we show that Gal-8 overexpression or exogenous addition promotes proliferation, migration, and invasion in nontumoral Madin–Darby canine kidney (MDCK) cells, involving focal-adhesion kinase (FAK)-mediated transactivation of the epidermal growth factor receptor (EGFR), likely triggered by α5β1integrin binding. Under subconfluent conditions, Gal-8–overexpressing MDCK cells (MDCK-Gal-8^H) display hallmarks of EMT, including decreased E-cadherin and up-regulated expression of vimentin, fibronectin, and Snail, as well as increased β-catenin activity. Changes related to migration/invasion included higher expression of α5β1 integrin, extracellular matrix-degrading MMP13 and urokinase plasminogen activator/urokinase plasminogen activator receptor (uPA/uPAR) protease systems. Gal-8–stimulated FAK/EGFR pathway leads to proteasome overactivity characteristic of cancer cells. Yet MDCK-Gal-8^H cells still develop apical/basolateral polarity reverting EMT markers and proteasome activity under confluence. This is due to the opposite segregation of Gal-8 secretion (apical) and β1-integrins distribution (basolateral). Strikingly, MDCK-Gal-8^H cells acquired tumorigenic potential, as reflected in anchorage-independent growth in soft agar and tumor generation in immunodeficient NSG mice. Therefore, Gal-8 can promote oncogenic-like transformation of epithelial cells through partial and reversible EMT, accompanied by higher proliferation, migration/invasion, and tumorigenic properties.

Stress-Induced EGFR Trafficking: Mechanisms, Functions, and Therapeutic Implications

Epidermal growth factor receptor (EGFR) has fundamental roles in normal physiology and cancer, making it a rational target for cancer therapy. Surprisingly, however, inhibitors that target canonical, ligand-stimulated EGFR signaling have proven to be largely ineffective in treating many EGFR-dependent cancers. Recent evidence indicates that both intrinsic and therapy-induced cellular stress triggers robust, noncanonical pathways of ligand-independent EGFR trafficking and signaling, which provides cancer cells with a survival advantage and resistance to therapeutics. Here, we review the mechanistic regulation of noncanonical EGFR trafficking and signaling, and the pathological and therapeutic stresses that activate it. We also discuss the implications of this pathway in clinical treatment of EGFR-overexpressing cancers.

Differential Targeting of SLC30A10/ZnT10 Heterodimers to Endolysosomal Compartments Modulates EGF-Induced MEK/ERK1/2 Activity

The solute carrier 30A (SLC30A) family of zinc exporters transports zinc into the lumen of intracellular organelles in order to prevent zinc toxicity. We reported that formation of tyrosine dimers is required for ZnT3 (zinc transporter 3) zinc transport activity and targeting to synaptic-like microvesicles (SLMVs) in PC12 cells and the formation of ZnT3/ZnT10 heterodimers. Here, we focused on ZnT10 to determine the role of heterodimerization in the sorting of ZnTs in the endolysosomal pathway. Using cell fractionation, immunoprecipitation and immunofluorescence approaches, we found that ZnT10 resides in transferrin receptor and Rab5-positive endosomes and forms covalent heterodimers and oligomers with ZnT2, ZnT3 and ZnT4. The interaction of ZnT10 with ZnT3, mediated by dityrosine bonds, was unable to target ZnT10 into SLMVs in vitro or into synaptic vesicles isolated from mouse brain in vivo. However, ZnT3/ZnT10 heterodimers regulate epidermal growth factor receptor (EGF-R) signaling by increasing the phosphorylation of mitogen-activated protein kinase kinase (MEK) and extracellular signal-regulated kinase (ERK1/2), but not EGF-R, C-Raf or Akt phosphorylation in response to EGF. Further, mutation of tyrosine 4 in ZnT10 reduced ZnT3/ZnT10 dityrosine-mediated heterodimerization and zinc transport, as well as MEK and ERK1/2 phosphorylation, which were also reduced by the zinc chelator TPEN. Phosphorylation of these kinases is likely to occur in the cytosol as no differences in phosphorylation were observed in membrane fractions of control and ZnT3/ZnT10-expressing cells. We propose that ZnT10 plays a role in signal transduction, which is mediated by homo and heterodimerization with other ZnTs.

Cadherin-11 endocytosis through binding to clathrin promotes cadherin-11-mediated migration in prostate cancer cells

Cadherin-11 (Cad11) cell adhesion molecule plays a role in prostate cancer cell migration. Because disassembly of adhesion complexes through endocytosis of adhesion proteins has been shown to play a role in cell migration, we examined whether Cad11 endocytosis plays a role in Cad11-mediated migration. The mechanism by which Cad11 is internalized is unknown. Using a GST pulldown assay, we found that clathrin binds to the Cad11 cytoplasmic domain but not to that of E-cadherin. Using deletion analysis, we identified a unique sequence motif, VFEEE, in the Cad11 membrane proximal region (amino acid residues 11-15) that binds to clathrin. Endocytosis assays using K(+)-depletion buffer showed that Cad11 internalization is clathrin dependent. Proximity ligation assays showed that Cad11 colocalizes with clathrin, and immunofluorescence assays showed that Cad11 localizes in vesicles that stain for the early endosomal marker Rab5. Deletion of the VFEEE sequence from the Cad11 cytoplasmic domain (Cad11-cla-Δ5) leads to inhibition of Cad11 internalization and reduces Cad11-mediated cell migration in C4-2B and PC3-mm2 prostate cancer cells. These observations suggest that clathrin-mediated internalization of Cad11 regulates surface trafficking of Cad11 and that dynamic turnover of Cad11 regulates the migratory function of Cad11 in prostate cancer cells.

Protein kinase A activity is necessary for fission and fusion of Golgi to endoplasmic reticulum retrograde tubules

It is becoming increasingly accepted that together with vesicles, tubules play a major role in the transfer of cargo between different cellular compartments. In contrast to our understanding of the molecular mechanisms of vesicular transport, little is known about tubular transport. How signal transduction molecules regulate these two modes of membrane transport processes is also poorly understood. In this study we investigated whether protein kinase A (PKA) activity regulates the retrograde, tubular transport of Golgi matrix proteins from the Golgi to the endoplasmic reticulum (ER). We found that Golgi-to-ER retrograde transport of the Golgi matrix proteins giantin, GM130, GRASP55, GRASP65, and p115 was impaired in the presence of PKA inhibitors. In addition, we unexpectedly found accumulation of tubules containing both Golgi matrix proteins and resident Golgi transmembrane proteins. These tubules were still attached to the Golgi and were highly dynamic. Our data suggest that both fission and fusion of retrograde tubules are mechanisms regulated by PKA activity.

Key role of phosphodiesterase 4A (PDE4A) in autophagy triggered by yessotoxin

Understanding the mechanism of action of the yessotoxin (YTX) is crucial since this drug has potential pharmacological effects in allergic processes, tumor proliferation and neurodegenerative diseases. It has been described that YTX activates apoptosis after 24h of treatment, while after 48 h of incubation with the toxin a decrease in cell viability corresponding to cellular differentiation or non-apoptotic cell death was observed. In this paper, these processes were extensively studied by using the erythroleukemia K-562 cell line. On one hand, events of K-562 cell differentiation into erythrocytes after YTX treatment were studied using hemin as positive control of cell differentiation. Cell differentiation was studied through the cyclic nucleotide response element binding (phospho-CREB) and the transferrin receptor (TfR) expression. On the other hand, using rapamycin as positive control, autophagic hallmarks, as non-apoptotic cell death, were studied after toxin exposure. In this case, the mechanistic target of rapamycin (mTOR) and light chain 3B (LC3B) levels were measured to check autophagy activation. The results showed that cell differentiation was not occurring after 48 h of toxin incubation while at this time the autophagy was triggered. Furthermore after 24h of toxin treatment none of these processes were activated. In addition, the role of the type 4A phosphodiesterase (PDE4A), the intracellular target of YTX, was checked. PDE4A-silencing experiments showed different regulation steps of PDE4A in the autophagic processes triggered either by traditional compounds or YTX. In summary, after 48 h YTX treatment PDE4A-dependent autophagy, as non-apoptotic programmed cell death, is activated.

A kinase inhibitor screen reveals protein kinase C-dependent endocytic recycling of ErbB2 in breast cancer cells

ErbB2 overexpression drives oncogenesis in 20-30% cases of breast cancer. Oncogenic potential of ErbB2 is linked to inefficient endocytic traffic into lysosomes and preferential recycling. However, regulation of ErbB2 recycling is incompletely understood. We used a high-content immunofluorescence imaging-based kinase inhibitor screen on SKBR-3 breast cancer cells to identify kinases whose inhibition alters the clearance of cell surface ErbB2 induced by Hsp90 inhibitor 17-AAG. Less ErbB2 clearance was observed with broad-spectrum PKC inhibitor Ro 31-8220. A similar effect was observed with Go 6976, a selective inhibitor of classical Ca(2+)-dependent PKCs (α, β1, βII, and γ). PKC activation by PMA promoted surface ErbB2 clearance but without degradation, and ErbB2 was observed to move into a juxtanuclear compartment where it colocalized with PKC-α and PKC-δ together with the endocytic recycling regulator Arf6. PKC-α knockdown impaired the juxtanuclear localization of ErbB2. ErbB2 transit to the recycling compartment was also impaired upon PKC-δ knockdown. PMA-induced Erk phosphorylation was reduced by ErbB2 inhibitor lapatinib, as well as by knockdown of PKC-δ but not that of PKC-α. Our results suggest that activation of PKC-α and -δ mediates a novel positive feedback loop by promoting ErbB2 entry into the endocytic recycling compartment, consistent with reported positive roles for these PKCs in ErbB2-mediated tumorigenesis. As the endocytic recycling compartment/pericentrion has emerged as a PKC-dependent signaling hub for G-protein-coupled receptors, our findings raise the possibility that oncogenesis by ErbB2 involves previously unexplored PKC-dependent endosomal signaling.

Epidermal growth factor receptor endocytic traffic perturbation by phosphatidate phosphohydrolase inhibition: new strategy against cancer

Epidermal growth factor receptor (EGFR) exaggerated (oncogenic) function is currently targeted in cancer treatment with drugs that block receptor ligand binding or tyrosine kinase activity. Because endocytic trafficking is a crucial regulator of EGFR function, its pharmacological perturbation might provide a new anti-tumoral strategy. Inhibition of phosphatidic acid (PA) phosphohydrolase (PAP) activity has been shown to trigger PA signaling towards type 4 phosphodiesterase (PDE4) activation and protein kinase A inhibition, leading to internalization of empty/inactive EGFR. Here, we used propranolol, its l- and d- isomers and desipramine as PAP inhibitors to further explore the effects of PAP inhibition on EGFR endocytic trafficking and its consequences on EGFR-dependent cancer cell line models. PAP inhibition not only made EGFR inaccessible to stimuli but also prolonged the signaling lifetime of ligand-activated EGFR in recycling endosomes. Strikingly, such endocytic perturbations applied in acute/intermittent PAP inhibitor treatments selectively impaired cell proliferation/viability sustained by an exaggerated EGFR function. Phospholipase D inhibition with FIPI (5-fluoro-2-indolyl des-chlorohalopemide) and PDE4 inhibition with rolipram abrogated both the anti-tumoral and endocytic effects of PAP inhibition. Prolonged treatments with a low concentration of PAP inhibitors, although without detectable endocytic effects, still counteracted cell proliferation, induced apoptosis and decreased anchorage-independent growth of cells bearing EGFR oncogenic influences. Overall, our results show that PAP inhibitors can counteract EGFR oncogenic traits, including receptor overexpression or activating mutations resistant to current tyrosine kinase inhibitors, perturbing EGFR endocytic trafficking and perhaps other as yet unknown processes, depending on treatment conditions. This puts PAP activity forward as a new suitable target against EGFR-driven malignancy.

Correlation of EGFR, pEGFR and p16INK4 expressions and high risk HPV infection in HIV/AIDS-related squamous cell carcinoma of conjunctiva

Background

Squamous cell carcinoma of conjunctiva has increased tenfold in the era of HIV/AIDS. The disease pattern has also changed in Africa, affecting young persons, with peak age-specific incidence of 30-39 years, similar to that of Kaposi sarcoma, a well known HIV/AIDS defining neoplasm. In addition, the disease has assumed more aggressive clinical course. The contributing role of exposure to high risk HPV in the development of SCCC is still emerging.

Objective

The present study aimed to investigate if immunohistochemical expressions of EGFR, pEGFR and p16, could predict infection with high risk HPV in HIV-related SCCC.

Methods

FFPE tissue blocks of fifty-eight cases diagnosed on hematoxylin and eosin with SCCC between 2005-2011, and subsequently confirmed from medical records to be HIV positive at the department of human pathology, UoN/KNH, were used for the study. Immunohistochemistry was performed to assess the expressions of p16INK4A, EGFR and pEGFR. This was followed with semi-nested PCR based detection and sequencing of HPV genotypes. The sequences were compared with the GenBank database, and data analyzed for significant statistical correlations using SPSS 16.0. Ethical approval to conduct the study was obtained from KNH-ERC.

Results

Out of the fifty-eight cases of SCCC analyzed, twenty-nine (50%) had well differentiated (grade 1), twenty one (36.2%) moderately differentiated (grade 2) while eight (13.8%) had poorly differentiated (grade 3) tumours. Immunohistochemistry assay was done in all the fifty eight studied cases, of which thirty nine cases (67.2%) were positive for p16INK4A staining, forty eight cases (82.8%) for EGFR and fifty one cases (87.9%) showed positivity for p-EGFR. HPV DNA was detected in 4 out of 40 SCCC cases (10%) in which PCR was performed, with HPV16 being the only HPV sub-type detected. Significant statistical association was found between HPV detection and p16INK4 (p=0.000, at 99% C.I) and EGFR (p=0.028, at 95% C.I) expressions, but not pEGFR. In addition, the expressions of these biomarkers did not show any significant association with tumor grades.

Conclusion

This study points to an association of high risk HPV with over expressions of p16INK4A and EGFR proteins in AIDS-associated SCCC.

Effects of membrane trafficking on signaling by receptor tyrosine kinases

The intracellular trafficking machinery contributes to the spatial and temporal control of signaling by receptor tyrosine kinases (RTKs). The primary role in this process is played by endocytic trafficking, which regulates the localization of RTKs and their downstream effectors, as well as the duration and the extent of their activity. The key regulatory points along the endocytic pathway are internalization of RTKs from the plasma membrane, their sorting to degradation or recycling, and their residence in various endosomal compartments. Here I will review factors and mechanisms that modulate RTK signaling by (1) affecting receptor internalization, (2) regulating the balance between degradation and recycling of RTK, and (3) compartmentalization of signals in endosomes and other organelles. Cumulatively, these mechanisms illustrate a multilayered control of RTK signaling exerted by the trafficking machinery.

Sonic Hedgehog modulates EGFR dependent proliferation of neural stem cells during late mouse embryogenesis through EGFR transactivation

Sonic Hedgehog (Shh/GLI) and EGFR signaling pathways modulate Neural Stem Cell (NSC) proliferation. How these signals cooperate is therefore critical for understanding normal brain development and function. Here we report a novel acute effect of Shh signaling on EGFR function. We show that during late neocortex development, Shh mediates the activation of the ERK1/2 signaling pathway in Radial Glial cells (RGC) through EGFR transactivation. This process is dependent on metalloprotease activity and accounts for almost 50% of the EGFR-dependent mitogenic response of late NSCs. Furthermore, in HeLa cancer cells, a well-known model for studying the EGFR receptor function, Shh also induces cell proliferation involving EGFR activation, as reflected by EGFR internalization and ERK1/2 phosphorylation. These findings may have important implications for understanding the mechanisms that regulate NSC proliferation during neurogenesis and may lead to novel approaches to the treatment of tumors.

Calpains promote α2β1 integrin turnover in nonrecycling integrin pathway

A novel virus- and integrin clustering–specific pathway diverts integrin from its normal endo/exocytic traffic to a nonrecycling degradative endosomal route. Clustering of α2β1 integrin causes redistribution of the integrin to perinuclear endosomes, leading to enhanced integrin turnover promoted by calpains.

Collagen receptor integrins recycle between the plasma membrane and endosomes and facilitate formation and turnover of focal adhesions. In contrast, clustering of α2β1 integrin with antibodies or the human pathogen echovirus 1 (EV1) causes redistribution of α2 integrin to perinuclear multivesicular bodies, α2-MVBs. We show here that the internalized clustered α2 integrin remains in α2-MVBs and is not recycled back to the plasma membrane. Instead, receptor clustering and internalization lead to an accelerated down-regulation of α2β1 integrin compared to the slow turnover of unclustered α2 integrin. EV1 infection or integrin degradation is not associated with proteasomal or autophagosomal processes and shows no significant association with lysosomal pathway. In contrast, degradation is dependent on calpains, such that it is blocked by calpain inhibitors. We show that active calpain is present in α2-MVBs, internalized clustered α2β1 integrin coprecipitates with calpain-1, and calpain enzymes can degrade α2β1 integrin. In conclusion, we identified a novel virus- and clustering-specific pathway that diverts α2β1 integrin from its normal endo/exocytic traffic to a nonrecycling, calpain-dependent degradative endosomal route.

Overexpression of phospho mutant forms of transglutaminase 2 downregulates epidermal growth factor receptor

Simultaneous upregulation of transglutaminase 2 (TG2) and epidermal growth factor receptor (EGFR) have been reported in a number of systems. Moreover, TG2 has been identified as a downstream target gene for EGF/EGFR. However, it is not known whether the relationship between EGFR and TG2 is only one-way or collaborative. Using embryonic fibroblasts derived from TG2 null mice (MEF(tg2-/-)), co-overexpressing native TG2 and EGFR, here we report that TG2 differentially regulates EGFR protein in the presence and absence of EGF. In the absence of EGF, TG2 facilitates EGFR downregulation whereas in the presence of EGF, TG2 has opposite effect on EGFR and facilitates Akt phosphorylation. TG2 mediated ligand-independent downregulation of EGFR was not observed in MEF(tg2-/-) cells overexpressing Ser212Ala phospho mutant form of TG2 suggesting a role of TG2 phosphorylation in this process. However, similar to native TG2, Ser212Ala-TG2 mutant was also able to attenuate ligand-dependent down regulation of EGFR in MEF(tg2-/-) cells. Interestingly, overexpression of Ser216Ala-TG2 mutant led to downregulation of EGFR in MEF(tg2-/-) cells irrespective of the ligand. These results were further confirmed in breast cancer cells expressing high levels of EGFR. Collectively, data presented here suggests that the relationship between EGFR and TG2 is collaborative and phosphorylation of TG2 play a key role in it. Phospho mutant forms of TG2 reported in this study may be utilized as a part of a novel strategy to downregulate EGFR in cancers with enhanced EGFR signaling.

Macropinocytosis inhibitors and Arf6 regulate ErbB3 nuclear localization in prostate cancer cells

ErbB3 is a transmembrane tyrosine kinase receptor among the epidermal growth factor receptor (EGFR) family that plays an important role in prostate cancer (PCa) progression. We previously demonstrated that ErbB3 is located in the nucleus of PCa cell lines and PCa tissues. It also was observed that ErbB3 nuclear localization could discriminate between benign and malignant prostate tissues as well as between hormone sensitive and hormone-refractory PCa. Several studies have suggested a role for clathrin-mediated endosomal sorting in the nuclear localization of EGFR and ErbB2 but the mechanisms by which ErbB receptors escape recycling or degradation are not well known. Consequently to determine the role of endocytosis in the nuclear localization of ErbB3, different endocytotic inhibitors and specific si-RNAs were used to discriminate between clathrin-dependent and clathrin-independent pathways. We found that clathrin, caveolin, and membrane domains are not required for endocytosis-mediated nuclear localization of ErbB3 in PCa cells and we provide evidence that amiloride, a macropinocytosis inhibitor, and the ADP-ribosylation factor 6 (Arf6) are implicated in the compartimentalization of ErbB3. In conclusion, evidence for an endocytosis-based mechanism in the nuclear localization of ErbB3 in PCa cells is proposed. These results may help elucidate new therapeutic avenues in PCa that target nuclear ErbB3, which may participate in the progression and aggressivity of the disease.

Phosphatidic acid induces ligand-independent epidermal growth factor receptor endocytic traffic through PDE4 activation

Endocytic traffic can control cell surface versus intracellular distribution of empty/inactive EGFR, an thus its accessibility to external stimuli, through a pathway involving down regulation of PKA activity mediated by PA signaling towards PDE4. This novel control mechanism can trans-modulate EGFR function by heterologous stimuli of PLD.

Endocytosis modulates EGFR function by compartmentalizing and attenuating or enhancing its ligand-induced signaling. Here we show that it can also control the cell surface versus intracellular distribution of empty/inactive EGFR. Our previous observation that PKA inhibitors induce EGFR internalization prompted us to test phosphatidic acid (PA) generated by phospholipase D (PLD) as an endogenous down-regulator of PKA activity, which activates rolipram-sensitive type 4 phosphodiesterases (PDE4) that degrade cAMP. We found that inhibition of PA hydrolysis by propranolol, in the absence of ligand, provokes internalization of inactive (neither tyrosine-phosphorylated nor ubiquitinated) EGFR, accompanied by a transient increase in PA levels and PDE4s activity. This EGFR internalization is mimicked by PA micelles and is strongly counteracted by PLD2 silencing, rolipram or forskolin treatment, and PKA overexpression. Accelerated EGFR endocytosis seems to be mediated by clathrin-dependent and -independent pathways, leading to receptor accumulation in juxtanuclear recycling endosomes, also due to a decreased recycling. Internalized EGFR can remain intracellular without degradation for several hours or return rapidly to the cell surface upon discontinuation of the stimulus. This novel regulatory mechanism of EGFR, also novel function of signaling PA, can transmodulate receptor accessibility in response to heterologous stimuli.

MOR23 promotes muscle regeneration and regulates cell adhesion and migration

Odorant receptors (ORs) in the olfactory epithelium bind to volatile small molecules leading to the perception of smell. ORs are expressed in many tissues but their functions are largely unknown. We show multiple ORs display distinct mRNA expression patterns during myogenesis in vitro and muscle regeneration in vivo. Mouse OR23 (MOR23) expression is induced during muscle regeneration when muscle cells are extensively fusing and plays a key role in regulating migration and adhesion of muscle cells in vitro, two processes common during tissue repair. A soluble ligand for MOR23 is secreted by muscle cells in vitro and muscle tissue in vivo. MOR23 is necessary for proper skeletal muscle regeneration as loss of MOR23 leads to increased myofiber branching, commonly associated with muscular dystrophy. Together these data identify a functional role for an OR outside of the nose and suggest a larger role for ORs during tissue repair.

Galectin-8 induces apoptosis in Jurkat T cells by phosphatidic acid-mediated ERK1/2 activation supported by protein kinase A down-regulation

Galectins have been implicated in T cell homeostasis playing complementary pro-apoptotic roles. Here we show that galectin-8 (Gal-8) is a potent pro-apoptotic agent in Jurkat T cells inducing a complex phospholipase D/phosphatidic acid signaling pathway that has not been reported for any galectin before. Gal-8 increases phosphatidic signaling, which enhances the activity of both ERK1/2 and type 4 phosphodiesterases (PDE4), with a subsequent decrease in basal protein kinase A activity. Strikingly, rolipram inhibition of PDE4 decreases ERK1/2 activity. Thus Gal-8-induced PDE4 activation releases a negative influence of cAMP/protein kinase A on ERK1/2. The resulting strong ERK1/2 activation leads to expression of the death factor Fas ligand and caspase-mediated apoptosis. Several conditions that decrease ERK1/2 activity also decrease apoptosis, such as anti-Fas ligand blocking antibodies. In addition, experiments with freshly isolated human peripheral blood mononuclear cells, previously stimulated with anti-CD3 and anti-CD28, show that Gal-8 is pro-apoptotic on activated T cells, most likely on a subpopulation of them. Anti-Gal-8 autoantibodies from patients with systemic lupus erythematosus block the apoptotic effect of Gal-8. These results implicate Gal-8 as a novel T cell suppressive factor, which can be counterbalanced by function-blocking autoantibodies in autoimmunity.

Ovarian Cancer and Genetic Susceptibility: Association of A61G Polymorphism in the EGF Gene

Growth factors play an essential role in regulating cellular proliferation, and lack of control is characteristic of malignant development. The epidermal growth factor ( EGF) gene codifies a growth factor that binds to the EGF receptor (EGFR), which is involved in activating pathways that promote cellular proliferation, survival, migration and differentiation. The purpose of this study was to appraise the association between EGF gene A61G polymorphism with ovarian cancer susceptibility. A total of 564 DNA samples were analysed from 175 women with ovarian cancer and 389 women without cancer, through PCR-RFLP. We found a decreased risk for developing ovarian cancer in GG carriers compared to AA carriers (OR = 0.46, CI = 0.25–0.83, P = 0.010). The seemingly protective role in GG carriers was observed in women under 53 years of age (OR = 0.38, CI = 0.16–0.86, P = 0.011) and in patients diagnosed with advanced stage disease (OR = 0.38, CI = 0.18–0.81, P = 0.012). Allelic comparison evidenced similar results, with decreased risk for G allele. We further observed a linear trend for G allele in cancer risk. Moreover, we analysed the influence of genotypes in the time to onset of the disease and observed that GG carriers had ovarian cancer later than AA carriers ( P = 0.035). We hypothesize that this polymorphism confers protection for ovarian cancer development.

p120-catenin inhibits VE-cadherin internalization through a Rho-independent mechanism

p120-catenin is a cytoplasmic binding partner of cadherins and functions as a set point for cadherin expression by preventing cadherin endocytosis, and degradation. p120 is known to regulate cell motility and invasiveness by inhibiting RhoA activity. However, the relationship between these functions of p120 is not understood. Here, we provide evidence that p120 functions as part of a plasma membrane retention mechanism for VE-cadherin by preventing the recruitment of VE-cadherin into membrane domains enriched in components of the endocytic machinery, including clathrin and the adaptor complex AP-2. The mechanism by which p120 regulates VE-cadherin entry into endocytic compartments is dependent on p120's interaction with the cadherin juxtamembrane domain, but occurs independently of p120's prevention of Rho GTPase activity. These findings clarify the mechanism for p120's function in stabilizing VE-cadherin at the plasma membrane and demonstrate a novel role for p120 in modulating the availability of cadherins for entry into a clathrin-dependent endocytic pathway.

Intracellular mechanisms underlying the nicotinic enhancement of LTP in the rat dentate gyrus

We have previously shown that activation of nicotinic acetylcholine receptors (nAChRs) enhanced long-term potentiation (LTP) in the rat dentate gyrus in vitro via activation of alpha7 nAChR. In the present studies, mechanisms underlying the acute and chronic nicotinic enhancement of LTP were examined. In particular, the involvement of activation of intracellular kinases was examined using selective kinase antagonists, and the effects of enhancing cholinergic function with positive allosteric modulators of the alpha7 nAChR and with acetylcholinesterase (AChE) inhibitors were also investigated. Activation of extracellular signal-regulated kinase (ERK) and cAMP-dependent protein kinase (PKA) was found to be involved in the induction of the acute nicotinic enhancement of LTP, although not control LTP. In contrast, activation of the tyrosine kinase Src, Ca(2+)-calmodulin-dependent protein kinase II, Janus kinase 2 and p38 mitogen-activated protein kinase was not involved in the acute nicotinic enhancement of LTP, although Src activation was necessary for control LTP. Moreover, activation of phosphoinositide 3-kinase was involved in the acute nicotinic enhancement of LTP to a much lesser extent than in control LTP. Chronic nicotine enhancement of LTP was found to be dependent on PKA, ERK and Src kinases. Acute nicotinic enhancement of LTP was occluded by chronic nicotine treatment. The positive allosteric modulator PNU-120596 was found to strongly reduce the threshold for nicotinic enhancement of LTP, an affect mediated via the alpha7 nAChR as it was blocked by the selective antagonist methyllycaconitine. The AChE inhibitors tacrine and physostigmine enhanced control LTP.

c-jun-NH2JNK mediates invasive potential and EGFR activation by regulating the expression of HB-EGF in a urokinase-stimulated pathway

In this study, we demonstrated that tyrosine phosphorylation of EGFR and the autocrine expression of uPA and HB-EGF depend on the activity of c-jun amino-terminal kinase (JNK) in human prostatic DU-145 cells. These cells overexpress EGFR and produce a high amount of uPA. Treatment with either SP600125, a specific chemical inhibitor of JNK, or the expression of a dominant-negative JNK form inhibited autocrine production of uPA and HB-EGF, which block EGFR phosphorylation and mitigates invasive capacity. Our data provided evidence that in DU-145 cells, the maintenance of the activation level of EGFR, which determines the cellular invasive potential, operates through an autocrine loop involving the JNK-dependent production of uPA and HB-EGF activity. Moreover, we found that exogenously added uPA stimulates autocrine production of HB-EGF, and that blocking HB-EGF activity curbed DU-145 cell invasive potential.

cAMP-dependent protein kinase A and the dynamics of epithelial cell surface domains: moving membranes to keep in shape

Cyclic adenosine monophosphate (cAMP) and cAMP-dependent protein kinase A (PKA) are evolutionary conserved molecules with a well-established position in the complex network of signal transduction pathways. cAMP/PKA-mediated signaling pathways are implicated in many biological processes that cooperate in organ development including the motility, survival, proliferation and differentiation of epithelial cells. Cell surface polarity, here defined as the anisotropic organisation of cellular membranes, is a critical parameter for most of these processes. Changes in the activity of cAMP/PKA elicit a variety of effects on intracellular membrane dynamics, including membrane sorting and trafficking. One of the most intriguing aspects of cAMP/PKA signaling is its evolutionary conserved abundance on the one hand and its precise spatial-temporal actions on the other. Here, we review recent developments with regard to the role of cAMP/PKA in the regulation of intracellular membrane trafficking in relation to the dynamics of epithelial surface domains.

Cholesterol Depletion and Genistein as Tools to Promote F508delCFTR Retention at the Plasma Membrane

BACKGROUND/AIMS

F508delCFTR-, but not wtCFTR-, expressing fibroblasts resemble Niemann Pick type C cells in the massive intracellular accumulation of free cholesterol. The recruitment and activation of F508delCFTR by cholesterol depletion was studied.

METHODS

Filipin staining, forskolin-stimulated anion efflux and FITC-dextran uptake were studied in control cells and fibroblasts treated with 2-hydroxypropyl beta-cyclodextrin phosphatidylcholine large unilamellar vesicles to deplete cellular free cholesterol.

RESULTS

Treatment of F508delCFTR-, but not wtCFTR-, expressing fibroblasts with 2-hydroxypropyl beta-cyclodextrin resulted in a reduction in cellular cholesterol and a potentiation of the forskolin-induced anion efflux. In addition, forskolin also promoted a massive increase in the rate of endocytosis in F508delCFTR fibroblasts, which was absent in genistein- or cyclodextrin-treated cultures.

CONCLUSION

The results not only suggest that reducing cellular cholesterol may serve as pharmacotherapeutic tool in the treatment of cystic fibrosis but also reveal a novel mechanism for genistein regulation of F508delCFTR, i.e. retention by inhibition of endocytosis.

Epidermal growth factor receptor levels are reduced in mice with targeted disruption of the protein kinase A catalytic subunit

BACKGROUND

Epidermal Growth Factor Receptor (EGFR) is a key target molecule in current treatment of several neoplastic diseases. Hence, in order to develop and improve current drugs targeting EGFR signalling, an accurate understanding of how this signalling pathway is regulated is required. It has recently been demonstrated that inhibition of cAMP-dependent protein kinase (PKA) induces a ligand-independent internalization of EGFR. Cyclic-AMP-dependent protein kinase consists of a regulatory dimer bound to two catalytic subunits.

RESULTS

We have investigated the effect on EGFR levels after ablating the two catalytic subunits, Calpha and Cbeta in two different models. The first model used targeted disruption of either Calpha or Cbeta in mice whereas the second model used Calpha and Cbeta RNA interference in HeLa cells. In both models we observed a significant reduction of EGFR expression at the protein but not mRNA level.

CONCLUSION

Our results suggest that PKA may represent a target that when manipulated can maintain EGFR protein levels at the single cell level as well as in intact animals.

Nucleotide P2Y1 receptor regulates EGF receptor mitogenic signaling and expression in epithelial cells

Epidermal growth factor receptor (EGFR) function is transregulated by a variety of stimuli, including agonists of certain G-protein-coupled receptors (GPCRs). One of the most ubiquitous GPCRs is the P2Y(1) receptor (P2RY1, hereafter referred to as P2Y(1)R) for extracellular nucleotides, mainly ADP. Here, we show in tumoral HeLa cells and normal FRT epithelial cells that P2Y(1)R broadcasts mitogenic signals by transactivating the EGFR. The pathway involves PKC, Src and cell surface metalloproteases. Stimulation of P2Y(1)R for as little as 15-60 minutes triggers mitogenesis, mirroring the half-life of extracellular ADP. Apyrase degradation of extracellular nucleotides and drug inhibition of P2Y(1)R, both reduced basal cell proliferation of HeLa and FRT cells, but not MDCK cells, which do not express P2Y(1)R. Thus, cell-released nucleotides constitute strong mitogenic stimuli, which act via P2Y(1)R. Strikingly, MDCK cells ectopically expressing P2Y(1)R display a highly proliferative phenotype that depends on EGFR activity associated with an increased level of EGFR, thus disclosing a novel aspect of GPCR-mediated regulation of EGFR function. These results highlight a role of P2Y(1)R in EGFR-dependent epithelial cell proliferation. P2Y(1)R could potentially mediate both trophic stimuli of basally released nucleotides and first-line mitogenic stimulation upon tissue damage. It could also contribute to carcinogenesis and serve as target for antitumor therapies.

Antiribosomal-P autoantibodies from psychiatric lupus target a novel neuronal surface protein causing calcium influx and apoptosis

The interesting observation was made 20 years ago that psychotic manifestations in patients with systemic lupus erythematosus are associated with the production of antiribosomal-P protein (anti-P) autoantibodies. Since then, the pathogenic role of anti-P antibodies has attracted considerable attention, giving rise to long-term controversies as evidence has either contradicted or confirmed their clinical association with lupus psychosis. Furthermore, a plausible mechanism supporting an anti-P-mediated neuronal dysfunction is still lacking. We show that anti-P antibodies recognize a new integral membrane protein of the neuronal cell surface. In the brain, this neuronal surface P antigen (NSPA) is preferentially distributed in areas involved in memory, cognition, and emotion. When added to brain cellular cultures, anti-P antibodies caused a rapid and sustained increase in calcium influx in neurons, resulting in apoptotic cell death. In contrast, astrocytes, which do not express NSPA, were not affected. Injection of anti-P antibodies into the brain of living rats also triggered neuronal death by apoptosis. These results demonstrate a neuropathogenic potential of anti-P antibodies and contribute a mechanistic basis for psychiatric lupus. They also provide a molecular target for future exploration of this and other psychiatric diseases.

The subcellular localization of the Niemann-Pick Type C proteins depends on the adaptor complex AP-3

Niemann-Pick Type C (NP-C) disease, caused by mutations in either human NPC1 (hNPC1) or human NPC2 (hNPC2), is characterized by the accumulation of unesterified cholesterol in late endosomes. Although it is known that the NP-C proteins are targeted to late endosomal/lysosomal compartments, their delivery mechanisms have not been fully elucidated. To identify mechanisms regulating NP-C protein localization, we used Saccharomyces cerevisiae, which expresses functional homologs of both NP-C proteins - scNcr1p and scNpc2p. Targeting of scNcr1p to the vacuole was perturbed in AP-3-deficient yeast cells, whereas the delivery of scNpc2p was affected by deficiencies in either AP-3 or GGA. We focused on the role of the AP-3 pathway in the targeting of the mammalian NP-C proteins. We found that, although mouse NPC1 (mNPC1) and hNPC2 co-localize with AP-3 to a similar extent in fibroblasts, hNPC2 preferentially co-localizes with AP-1. Importantly, the targeting of both mammalian NPC1 and NPC2 is dependent on AP-3. Moreover, and consistent with the NP-C proteins playing a role in cholesterol metabolism, AP-3-deficient cells have reduced levels of cholesterol. These results provide information about how the NP-C proteins are targeted to their sites of action and illustrate the possibility that defective sorting of the NP-C proteins along the endocytic route can alter cellular cholesterol.

BLOC-1 complex deficiency alters the targeting of adaptor protein complex-3 cargoes

Mutational analyses have revealed many genes that are required for proper biogenesis of lysosomes and lysosome-related organelles. The proteins encoded by these genes assemble into five distinct complexes (AP-3, BLOC-1-3, and HOPS) that either sort membrane proteins or interact with SNAREs. Several of these seemingly distinct complexes cause similar phenotypic defects when they are rendered defective by mutation, but the underlying cellular mechanism is not understood. Here, we show that the BLOC-1 complex resides on microvesicles that also contain AP-3 subunits and membrane proteins that are known AP-3 cargoes. Mouse mutants that cause BLOC-1 or AP-3 deficiencies affected the targeting of LAMP1, phosphatidylinositol-4-kinase type II alpha, and VAMP7-TI. VAMP7-TI is an R-SNARE involved in vesicle fusion with late endosomes/lysosomes, and its cellular levels were selectively decreased in cells that were either AP-3- or BLOC-1-deficient. Furthermore, BLOC-1 deficiency selectively altered the subcellular distribution of VAMP7-TI cognate SNAREs. These results indicate that the BLOC-1 and AP-3 protein complexes affect the targeting of SNARE and non-SNARE AP-3 cargoes and suggest a function of the BLOC-1 complex in membrane protein sorting.

Aquaporin 2 (AQP2) and vasopressin type 2 receptor (V2R) endocytosis in kidney epithelial cells: AQP2 is located in 'endocytosis-resistant' membrane domains after vasopressin treatment

BACKGROUND INFORMATION

Aquaporin 2 (AQP2) plays an important, VP (vasopressin)-regulated role in water reabsorption by the kidney. The amount of AQP2 expressed at the surface of principal cells results from an equilibrium between the AQP2 in intracellular vesicles and the AQP2 on the plasma membrane. VP shifts the equilibrium in favour of the plasma membrane and this allows osmotic equilibration to occur between the collecting duct lumen and the interstitial space. Membrane accumulation of AQP2 could result from a VP-induced increase in exocytosis, a decrease in endocytosis, or both. In the present study, we further investigated AQP2 accumulation at the cell surface, and compared it with V2R (VP type 2 receptor) trafficking using cells that express epitope-tagged AQP2 and V2R.

RESULTS

Endocytosis of V2R and of AQP2 are independent events that can be separated temporally and spatially. The burst of endocytosis seen after VP addition to target cells, when AQP2 accumulates at the cell surface, is primarily due to internalization of the V2R. Increased endocytosis is not induced by forskolin, which also induces membrane accumulation of AQP2 by direct stimulation of adenylate cyclase. This indicates that cAMP elevation is not the primary cause of the initial, VP-induced endocytic process. After VP exposure, AQP2 is not located in endosomes with internalized V2R. Instead, it remains at the cell surface in 'endocytosis-resistant' membrane domains, visualized by confocal imaging. After VP washout, AQP2 is progressively internalized with the fluid-phase marker FITC-dextran, indicating that VP washout releases an endocytotic block that maintains AQP2 at the cell surface. Finally, polarized application of VP to filter-grown cells shows that apical VP can induce basolateral endocytosis and V2R down-regulation, and vice versa.

CONCLUSIONS

After VP stimulation of renal epithelial cells, AQP2 accumulates at the cell surface, while the V2R is actively internalized. This endocytotic block may involve a reduced capacity of phosphorylated AQP2 to interact with components of the endocytotic machinery. In addition, a complex cross-talk exists between the apical and basolateral plasma-membrane domains with respect to endocytosis and V2R down-regulation. This may be of physiological significance in down-regulating the VP response in the kidney in vivo.

Syndecan-1 expression in epithelial cells is induced by transforming growth factor beta through a PKA-dependent pathway

Syndecans comprise a major family of cell surface heparan sulfate proteoglycans (HSPGs). Syndecans bind and modulate a wide variety of biological molecules through their heparan sulfate (HS) moiety. Although all syndecans contain the ligand binding HS chains, they likely perform specific functions in vivo because their temporal and spatial expression patterns are different. However, how syndecan expression is regulated has yet to be clearly defined. In this study, we examined how syndecan-1 expression is regulated in epithelial cells. Our results showed that among several bioactive agents tested, only forskolin and three isoforms of TGFbeta (TGFbeta1-TGFbeta3) significantly induced syndecan-1, but not syndecan-4, expression on various epithelial cells. Steady-state syndecan-1 mRNA was not increased by TGFbeta treatment and cycloheximide did not inhibit syndecan-1 induction by TGFbeta, indicating that TGFbeta induces syndecan-1 in a post-translational manner. However, TGFbeta induction of syndecan-1 was inhibited by transient expression of a dominant-negative construct of protein kinase A (PKA) and by specific inhibitors of PKA. Further (i) syndecan-1 cytoplasmic domains were Ser-phosphorylated when cells were treated with TGFbeta and this was inhibited by a PKA inhibitor, (ii) PKA was co-immunoprecipitated from cell lysates by anti-syndecan-1 antibodies, (iii) PKA phosphorylated recombinant syndecan-1 cytoplasmic domains in vitro, and (iv) expression of a syndecan-1 construct with its invariant Ser(286) replaced with a Gly was not induced by TGFbeta. Together, these findings define a regulatory mechanism where TGFbeta signals through PKA to phosphorylate the syndecan-1 cytoplasmic domain and increases syndecan-1 expression on epithelial cells.

Effect of the anti-receptor ligand-blocking 225 monoclonal antibody on EGF receptor endocytosis and sorting

The anti-receptor antibody, 225 mAb, is known to block binding of ligand to the epidermal growth factor receptor (EGFR). However, the effect of this neutralizing antibody on EGFR endocytosis, trafficking and degradation remains unclear. Here, we demonstrate that endocytosis of (125)I-225 mAb occurs, albeit with a slower rate than that of EGF. Using pulse chase assays, we show that internalized (125)I-225 mAb is recycled to the surface much more efficiently than internalized (125)I-EGF. Also, we found that internalization of (125)I-225 mAb, in contrast to that of EGF, is independent of receptor tyrosine kinase activity, as evidenced by its insensitivity to AG1478, a specific EGFR tyrosine kinase inhibitor. Analysis of the levels of cell surface and total EGFR showed that treatment with 225 mAb results in a 30-40% decrease in surface EGFR and a relatively slow downregulation of total EGFR. Taken together, these data indicate that 225 mAb induces internalization and downregulation of EGFR via a mechanism distinct from that underlying EGF-induced EGFR internalization and downregulation.

Lack of tyrosine 320 impairs spontaneous endocytosis and enhances release of HLA-B27 molecules

Several lines of evidence suggest that endocytosis of MHC class I molecules requires conserved motifs within the cytoplasmic domain. In this study, we show, in the C58 rat thymoma cell line transfected with HLA-B27 molecules, that replacement of the highly conserved tyrosine (Tyr320) in the cytoplasmic domain of HLA-B27 does not hamper cell surface expression of beta2-microglobulin H chain heterodimers or formation of misfolded molecules. However, Tyr320 replacement markedly impairs spontaneous endocytosis of HLA-B27. Although wild-type molecules are mostly internalized via endosomal compartments, Tyr320-mutated molecules remain at the plasma membrane in which partial colocalization with endogenous transferrin receptors can be observed, also impairing their endocytosis. Finally, we show that Tyr320 substitution enhances release of cleaved forms of HLA-B27 from the cell surface. These studies show for the first time that Tyr320 is most likely part of a cytoplasmic sorting motif involved in spontaneous endocytosis and shedding of MHC class I molecules.

Dopamine D3 receptors regulate GABAA receptor function through a phospho-dependent endocytosis mechanism in nucleus accumbens

The dopamine D3 receptor, which is highly enriched in nucleus accumbens (NAc), has been suggested to play an important role in reinforcement and reward. To understand the potential cellular mechanism underlying D3 receptor functions, we examined the effect of D3 receptor activation on GABAA receptor (GABAAR)-mediated current and inhibitory synaptic transmission in medium spiny neurons of NAc. Application of PD128907 [(4aR,10bR)-3,4a,4,10b-tetrahydro-4-propyl-2H,5H-[1]benzopyrano-[4,3-b]-1,4-oxazin-9-ol hydrochloride], a specific D3 receptor agonist, caused a significant reduction of GABAAR current in acutely dissociated NAc neurons and miniature IPSC amplitude in NAc slices. This effect was blocked by dialysis with a dynamin inhibitory peptide, which prevents the clathrin/activator protein 2 (AP2)-mediated GABAA receptor endocytosis. In addition, the D3 effect on GABAAR current was prevented by agents that manipulate protein kinase A (PKA) activity. Infusion of a peptide derived from GABAAR beta subunits, which contains an atypical binding motif for the clathrin AP2 adaptor complex and the major PKA phosphorylation sites and binds with high affinity to AP2 only when dephosphorylated, diminished the D3 regulation of IPSC amplitude. The phosphorylated equivalent of the peptide was without effect. Moreover, PD128907 increased GABAAR internalization and reduced the surface expression of GABAA receptor beta subunits in NAc slices, which was prevented by dynamin inhibitory peptide or cAMP treatment. Together, our results suggest that D3 receptor activation suppresses the efficacy of inhibitory synaptic transmission in NAc by increasing the phospho-dependent endocytosis of GABAA receptors.

Galectin-8 binds specific β1 integrins and induces polarized spreading highlighted by asymmetric lamellipodia in Jurkat T cells

Integrin-mediated encounters of T cells with extracellular cues lead these cells to adhere to a variety of substrates and acquire a spread phenotype needed for their tissue incursions. We studied the effects of galectin-8 (Gal-8), a beta-galactoside binding lectin, on Jurkat T cells. Immobilized Gal-8 bound alpha1beta1, alpha3beta1 and alpha5beta1 but not alpha2beta1 and alpha4beta1 and adhered these cells with similar kinetics to immobilized fibronectin (FN). Function-blocking experiments with monoclonal anti-integrin antibodies suggested that alpha5beta1 is the main mediator of cell adhesion to this lectin. Gal-8, but not FN, induced extensive cell spreading frequently leading to a polarized phenotype characterized by an asymmetric lamellipodial protrusion. These morphological changes involved actin cytoskeletal rearrangements controlled by PI3K, Rac-1 and ERK1/2 activity. Gal-8-induced Rac-1 activation and binding to alpha1 and alpha5 integrins have not been described in any other cellular system. Strikingly, Gal-8 was also a strong stimulus on Jurkat cells in suspension, triggering ERK1/2 activation that in most adherent cells is instead dependent on cell attachment. In addition, we found that patients with systemic lupus erythematosus (SLE), a prototypic autoimmune disorder, produce Gal-8 autoantibodies that impede both its binding to integrins and cell adhesion. These are the first function-blocking autoantibodies reported for a member of the galectin family. These results indicate that Gal-8 constitutes a novel extracellular stimulus for T cells, able to bind specific beta1 integrins and to trigger signaling pathways conducive to cell spreading. Gal-8 could modulate a wide range of T cell-driven immune processes that eventually become altered in autoimmune disorders.

ApoER2 is endocytosed by a clathrin-mediated process involving the adaptor protein Dab2 independent of its Rafts' association

The apolipoprotein E receptor 2 (apoER2) is a member of the low-density lipoprotein receptor family which binds ligands such as reelin, apolipoprotein E and apolipoprotein J/clusterin and has been shown to play roles in neuronal migration during development and in male fertility. The function of apoER2 mainly depends on cellular signaling triggered by ligand binding. Although the receptor is internalized, the mechanism and functional significance of its endocytic trafficking remain unclear. Apolipoprotein E receptor 2 partitions into lipid rafts and interacts with caveolin-1, a feature that could modulate its endocytic behavior. Recent evidence also suggested that apoER2 might be endocytosed by a pathway independent of clathrin. Here, we show that despite a raft association, apoER2 internalization depends on its cytoplasmic FxNPXY motif that is similar to canonical motifs for clathrin-mediated endocytosis. This motif mediates receptor binding to the adaptor protein Dab2, which can interact directly with clathrin. Several inhibitory conditions of clathrin-mediated endocytosis, including expression of the dominant negative forms of eps15 and Dab2, decreased apoER2 internalization. In contrast, treatment with the drug nystatin, which blocks the caveolar/raft internalization pathway, has no effect on the receptor's endocytosis. Neither the transmembrane nor the proline-rich insert of the cytoplasmic domain, which has been previously reported to exclude the receptor from the clathrin-mediated pathway, altered apoER2 endocytic activity. These studies indicate that apoER2 internalizes through a clathrin-mediated pathway and that its association with caveolar and noncaveolar rafts does not determine its endocytosis.

GMx33 associates with the trans-Golgi matrix in a dynamic manner and sorts within tubules exiting the Golgi

The trans-Golgi matrix consists of a group of proteins dynamically associated with the trans-Golgi and thought to be involved in anterograde and retrograde Golgi traffic, as well as interactions with the cytoskeleton and maintenance of the Golgi structure. GMx33 is localized to the cytoplasmic face of the trans-Golgi and is also present in a large cytoplasmic pool. Here we demonstrate that GMx33 is dynamically associated with the trans-Golgi matrix, associating and dissociating with the Golgi in seconds. GMx33 can be locked onto the trans-Golgi matrix by GTPgammaS, indicating that its association is regulated in a GTP-dependent manner like several other Golgi matrix proteins. Using live-cell imaging we show that GMx33 exits the Golgi associated with tubules and within these tubules GMx33 segregates from transmembrane proteins followed by fragmentation of the tubules into smaller tubules and vesicles. Within vesicles produced by an in vitro budding reaction, GMx33 remains segregated in a matrixlike tail region that sometimes contains Golgin-245. This trans-matrix often links a few vesicles together. Together these data suggest that GMx33 is a member of the trans-Golgi matrix and offer clues regarding the role of the trans-Golgi matrix in sorting and exit from the Golgi.