Newest findings on the oldest oncogene; how activated src does it

Palladin is a regulator of actin filament bundles at the ectoplasmic specialization in adult rat testes

In rat testes, the ectoplasmic specialization (ES) at the Sertoli-Sertoli and Sertoli-spermatid interface known as the basal ES at the blood-testis barrier and the apical ES in the adluminal compartment, respectively, is a testis-specific adherens junction. The remarkable ultrastructural feature of the ES is the actin filament bundles that sandwiched in between the cisternae of endoplasmic reticulum and apposing plasma membranes. Although these actin filament bundles undergo extensive reorganization to switch between their bundled and debundled state to facilitate blood-testis barrier restructuring and spermatid adhesion/transport, the regulatory molecules underlying these events remain unknown. Herein we report findings of an actin filament cross-linking/bundling protein palladin, which displayed restrictive spatiotemporal expression at the apical and the basal ES during the epithelial cycle. Palladin structurally interacted and colocalized with Eps8 (epidermal growth factor receptor pathway substrate 8, an actin barbed end capping and bundling protein) and Arp3 (actin related protein 3, which together with Arp2 form the Arp2/3 complex to induce branched actin nucleation, converting bundled actin filaments to an unbundled/branched network), illustrating its role in regulating actin filament bundle dynamics at the ES. A knockdown of palladin in Sertoli cells in vitro with an established tight junction (TJ)-permeability barrier was found to disrupt the TJ function, which was associated with a disorganization of actin filaments that affected protein distribution at the TJ. Its knockdown in vivo also perturbed F-actin organization that led to a loss of spermatid polarity and adhesion, causing defects in spermatid transport and spermiation. In summary, palladin is an actin filament regulator at the ES.

Distinct roles for N-Cadherin linked c-Src and fyn kinases in lens development

BACKGROUND

Src family tyrosine kinases (SFKs) are often coincidently expressed but few studies have dissected their individual functions in the same cell during development. Using the classical embryonic lens as our model, we investigated SFK signaling in the regulation of both differentiation initiation and morphogenesis, and the distinct functions of c-Src and Fyn in these processes.

RESULTS

Blocking SFK activity with the highly specific inhibitor PP1 induced initiation of the lens differentiation program but blocked lens fiber cell elongation and organization into mini lens-like structures called lentoids. These dichotomous roles for SFK signaling were discovered to reflect distinct functions of c-Src and Fyn and their differentiation-state-specific recruitment to and action at N-cadherin junctions. c-Src was highly associated with the nascent N-cadherin junctions of undifferentiated lens epithelial cells. Its siRNA knockdown promoted N-cadherin junctional maturation, blocked proliferation, and induced lens cell differentiation. In contrast, Fyn was recruited to mature N-cadherin junctions of differentiating lens cells and siRNA knockdown suppressed differentiation-specific gene expression and blocked morphogenesis.

CONCLUSIONS

Through inhibition of N-cadherin junction maturation, c-Src promotes lens epithelial cell proliferation and the maintenance of the lens epithelial cell undifferentiated state, while Fyn, signaling downstream of mature N-cadherin junctions, promotes lens fiber cell morphogenesis.

The effect of surface topography on cell shape and early ERK1/2 signaling in macrophages; linkage with FAK and Src

Implant surface topography can modulate macrophage behavior during wound healing by the production of proinflammatory cytokines. This study investigated the activation of FAK, Src, and ERK1/2 signaling intermediates of the proinflammatory ERK1/2 pathway in RAW 264.7 macrophages in response to polished (P), coarse-grit-blasted (B), acid etched (E), and grit-blasted and etched (SLA) surface topographies. In addition, the effects of these topographies on cell spreading, vinculin organization, and viability were determined. Macrophages on the SLA surface changed from predominantly well-spread cells to ones with a more spherical morphology over time. In contrast, macrophages on the P surface changed from being predominantly spherical cells to well spread. The morphological changes were associated with changes in the distribution of vinculin. The overall patterns of the pFAK, pSrc, pERK1/2 levels as well as pERK1/2 nuclear translocation associated with cell shape with greater activation being seen with a more spread morphology. These results suggest that surface topography differentially activates signaling pathways that affect cell function and raise the possibility that topographies can be designed to optimize desired cell responses.

Targeting Src family kinases inhibits bevacizumab-induced glioma cell invasion

Anti-VEGF antibody therapy with bevacizumab provides significant clinical benefit in patients with recurrent glioblastoma multiforme (GBM). Unfortunately, progression on bevacizumab therapy is often associated with a diffuse disease recurrence pattern, which limits subsequent therapeutic options. Therefore, there is an urgent need to understand bevacizumab's influence on glioma biology and block it's actions towards cell invasion.

To explore the mechanism(s) of GBM cell invasion we have examined a panel of serially transplanted human GBM lines grown either in short-term culture, as xenografts in mouse flank, or injected orthotopically in mouse brain. Using an orthotopic xenograft model that exhibits increased invasiveness upon bevacizumab treatment, we also tested the effect of dasatinib, a broad spectrum SFK inhibitor, on bevacizumab-induced invasion.

We show that 1) activation of Src family kinases (SFKs) is common in GBM, 2) the relative invasiveness of 17 serially transplanted GBM xenografts correlates strongly with p120 catenin phosphorylation at Y228, a Src kinase site, and 3) SFK activation assessed immunohistochemically in orthotopic xenografts, as well as the phosphorylation of downstream substrates occurs specifically at the invasive tumor edge. Further, we show that SFK signaling is markedly elevated at the invasive tumor front upon bevacizumab administration, and that dasatinib treatment effectively blocked the increased invasion induced by bevacizumab.

Our data are consistent with the hypothesis that the increased invasiveness associated with anti-VEGF therapy is due to increased SFK signaling, and support testing the combination of dasatinib with bevacizumab in the clinic.

A truncated fragment of Src protein kinase generated by calpain-mediated cleavage is a mediator of neuronal death in excitotoxicity

Excitotoxicity resulting from overstimulation of glutamate receptors is a major cause of neuronal death in cerebral ischemic stroke. The overstimulated ionotropic glutamate receptors exert their neurotoxic effects in part by overactivation of calpains, which induce neuronal death by catalyzing limited proteolysis of specific cellular proteins. Here, we report that in cultured cortical neurons and in vivo in a rat model of focal ischemic stroke, the tyrosine kinase Src is cleaved by calpains at a site in the N-terminal unique domain. This generates a truncated Src fragment of ~52 kDa, which we localized predominantly to the cytosol. A cell membrane-permeable fusion peptide derived from the unique domain of Src prevents calpain from cleaving Src in neurons and protects against excitotoxic neuronal death. To explore the role of the truncated Src fragment in neuronal death, we expressed a recombinant truncated Src fragment in cultured neurons and examined how it affects neuronal survival. Expression of this fragment, which lacks the myristoylation motif and unique domain, was sufficient to induce neuronal death. Furthermore, inactivation of the prosurvival kinase Akt is a key step in its neurotoxic signaling pathway. Because Src maintains neuronal survival, our results implicate calpain cleavage as a molecular switch converting Src from a promoter of cell survival to a mediator of neuronal death in excitotoxicity. Besides unveiling a new pathological action of Src, our discovery of the neurotoxic action of the truncated Src fragment suggests new therapeutic strategies with the potential to minimize brain damage in ischemic stroke.

Dasatinib inhibits primary melanoma cell proliferation through morphology-dependent disruption of Src-ERK signaling

New strategies for the treatment of advanced melanoma are urgently required. The RAS/RAF/MAPK pathway and c-Src are deregulated in the majority of malignant melanomas, suggesting that they may interact functionally and are involved in the development and progression of the malignancy. Preclinical studies have demonstrated variable inhibition of melanoma cell growth by dasatinib in vitro. Src may act through different downstream signaling pathways. In the present study, we demonstrate that dasatinib induces changes in cell morphology, characterized by an arborized and contracted appearance, and accompanied by a reduction in cell proliferation in primary melanoma cells. This morphological change is demonstrated to be associated with the inhibition of nuclear translocation of activated ERK1/2. Together, these results indicate that Src may promote cell proliferation through the activation of the ERK signaling pathway in melanoma oncogenesis.

Folic acid inhibits endothelial cell migration through inhibiting the RhoA activity mediated by activating the folic acid receptor/cSrc/p190RhoGAP-signaling pathway

Previously, our in vivo studies demonstrated that folic acid (FA) could inhibit angiogenesis and in vitro studies showed that FA reduced vascular endothelial cell proliferation through activating the cSrc/ERK-2/NFκB/p53 pathway mediated by FA receptor (FR). Here, we further examined the effect of FA on endothelial cell migration. Our results showed that FA (10 μM) inhibited the formation of lamellipodia, migration and capillary-like tube formation of human umbilical venous endothelial cells (HUVEC). These inhibition effects induced by FA treatment were not due to reduction of cell survival and cell adhesion on the collagen-coated plate. Treatment of HUVEC with FA (10 μM) increased the activity of cSrc and p190RhoGAP and decreased the activity of RhoA. Over-expression of the constitutively active RhoA construct (RhoA V14) prevented the FA-induced inhibition of migration and capillary-like tube formation in HUVEC. However, these preventive effects were abolished by pretreatment of HUVEC with a ROCK inhibitor, Y27632. Pretreatment with a cSrc inhibitor, PP2, prevented the FA-induced activation of p190GAP, reduction of the RhoA activity and migration inhibition in HUVEC. Moreover, pre-transfection with p190RhoGAP siRNA abolished the FA-induced reduction in the RhoA activity and migration inhibition in HUVEC. Taken together, our results suggest that FA might inhibit endothelial cell migration through inhibiting the RhoA activity mediated by activating the FR/cSrc/p190RhoGAP-signaling pathway. These findings further support the anti-angiogenic activity of FA.

Regulation of the SRC family kinases by Csk

The non-receptor tyrosine kinase Csk serves as an indispensable negative regulator of the Src family tyrosine kinases (SFKs) by specifically phosphorylating the negative regulatory site of SFKs, thereby suppressing their oncogenic potential. Csk is primarily regulated through its SH2 domain, which is required for membrane translocation of Csk via binding to scaffold proteins such as Cbp/PAG1. The binding of scaffolds to the SH2 domain can also upregulate Csk kinase activity. These regulatory features have been elucidated by analyses of Csk structure at the atomic levels. Although Csk itself may not be mutated in human cancers, perturbation of the regulatory system consisting of Csk, Cbp/PAG1, or other scaffolds, and certain tyrosine phosphatases may explain the upregulation of SFKs frequently observed in human cancers. This review focuses on the molecular bases for the function, structure, and regulation of Csk as a unique regulatory tyrosine kinase for SFKs.

Analysis of SRC oncogenic signaling in colorectal cancer by stable isotope labeling with heavy amino acids in mouse xenografts

The non-receptor tyrosine kinase SRC is frequently deregulated in human colorectal cancer (CRC), and SRC increased activity has been associated with poor clinical outcomes. In nude mice engrafted with human CRC cells, SRC over-expression favors tumor growth and is accompanied by a robust increase in tyrosine phosphorylation in tumor cells. How SRC contributes to this tumorigenic process is largely unknown. We analyzed SRC oncogenic signaling in these tumors by means of a novel quantitative proteomic analysis. This method is based on stable isotope labeling with amino acids of xenograft tumors by the addition of [(13)C(6)]-lysine into mouse food. An incorporation level greater than 88% was obtained in xenograft tumors after 30 days of the heavy lysine diet. Quantitative phosphoproteomic analysis of these tumors allowed the identification of 61 proteins that exhibited a significant increase in tyrosine phosphorylation and/or association with tyrosine phosphorylated proteins upon SRC expression. These mainly included molecules implicated in vesicular trafficking and signaling and RNA binding proteins. Most of these proteins were specific targets of SRC signaling in vivo, as they were not identified by analysis via stable isotope labeling by amino acids in cell culture (SILAC) of the same CRC cells in culture. This suggests that oncogenic signaling induced by SRC in tumors significantly differs from that induced by SRC in cell culture. We next confirmed this notion experimentally with the example of the vesicular trafficking protein and SRC substrate TOM1L1. We found that whereas TOM1L1 depletion only slightly affected SRC-induced proliferation of CRC cells in vitro, it drastically decreased tumor growth in xenografted nude mice. We thus concluded that this vesicular trafficking protein plays an important role in SRC-induced tumor growth. Overall, these data show that SILAC analysis in mouse xenografts is a valuable approach for deciphering tyrosine kinase oncogenic signaling in vivo.

A novel 7-bromoindirubin with potent anticancer activity suppresses survival of human melanoma cells associated with inhibition of STAT3 and Akt signaling

STAT3 and Akt signaling have been validated as potential molecular targets for treatment of cancers including melanoma. These small molecule inhibitors of STAT3 or Akt signaling are promising for developing anti-melanoma therapeutic agents. MLS-2438, a novel 7-bromoindirubin, a derivative of the natural product indirubin, was synthesized with a bromo-group at the 7-position on one indole ring and a hydrophilic group at the 3'-position on the other indole ring. We tested the anticancer activity of MLS-2438 and investigated its mechanism of action in human melanoma cell lines. Here, we show that MLS-2438 inhibits viability and induces apoptosis of human melanoma cells associated with inhibition of STAT3 and Akt signaling. Several pro-apoptotic Bcl-2 family proteins are involved in the MLS-2438 mediated apoptosis. MLS-2438 inhibits Src kinase activity in vitro and phosphorylation of JAK2, Src, STAT3 and Akt in cultured cancer cells. In contrast to the decreased phosphorylation levels of JAK2, Src, STAT3 and Akt, phosphorylation levels of the MAPK (Erk1/2) signaling protein were not reduced in cells treated with MLS-2438. These results demonstrate that MLS-2438, a novel natural product derivative, is a Src inhibitor and potentially regulates kinase activity of JAK2 and Akt in cancer cells. Importantly, MLS-2438 suppressed tumor growth with low toxicity in a mouse xenograft model of human melanoma. Our findings support further development of MLS-2438 as a potential small-molecule therapeutic agent that targets both STAT3 and Akt signaling in human melanoma cells.

GAB2--a scaffolding protein in cancer

Adaptor or scaffolding proteins mediate protein-protein interactions that drive the formation of protein complexes. Grb2-associated binding protein 2 (GAB2) scaffolding protein is an intermediary molecule that links plasma membrane receptor signaling including receptor tyrosine kinases with the downstream effectors, such as protein tyrosine phosphatase, nonreceptor type 11 (SHP2), p85 subunit of phosphoinositide-3 kinase (PI3-K), phospholipase C-gamma 1 (PLC-γ), v-crk sarcoma virus CT10 (CRK), Src homology 2 domain containing transforming protein 1 (SHC), and SH2 containing inositol phosphatase (SHIP). Although, well described in signal transduction, its role in cancer has recently been emerging especially in leukemia, breast and ovarian cancer, and melanoma. GAB2 is essential for two major signal transduction pathways in cancer, the PI3-K-AKT and extracellular signal-regulated kinase (ERK) signaling pathways, and thus regulates a number of key cellular processes. This review focuses on structure and function of GAB2, its regulatory proteins, emerging role in cancer, and potential as a therapeutic target.

The desmosomal armadillo protein plakoglobin regulates prostate cancer cell adhesion and motility through vitronectin-dependent Src signaling

Plakoglobin (PG) is an armadillo protein that associates with both classic and desmosomal cadherins, but is primarily concentrated in mature desmosomes in epithelia. While reduced levels of PG have been reported in localized and hormone refractory prostate tumors, the functional significance of these changes is unknown. Here we report that PG expression is reduced in samples of a prostate tumor tissue array and inversely correlated with advancing tumor potential in 7 PCa cell lines. Ectopically expressed PG enhanced intercellular adhesive strength, and attenuated the motility and invasion of aggressive cell lines, whereas silencing PG in less tumorigenic cells had the opposite effect. PG also regulated cell-substrate adhesion and motility through extracellular matrix (ECM)-dependent inhibition of Src kinase, suggesting that PG’s effects were not due solely to increased intercellular adhesion. PG silencing resulted in elevated levels of the ECM protein vitronectin (VN), and exposing PG-expressing cells to VN induced Src activity. Furthermore, increased VN levels and Src activation correlated with diminished expression of PG in patient tissues. Thus, PG may inhibit Src by keeping VN low. Our results suggest that loss of intercellular adhesion due to reduced PG expression might be exacerbated by activation of Src through a PG-dependent mechanism. Furthermore, PG down-regulation during PCa progression could contribute to the known VN-dependent promotion of PCa invasion and metastasis, demonstrating a novel functional interaction between desmosomal cell-cell adhesion and cell-substrate adhesion signaling axes in prostate cancer.

Spatiotemporal regulation of Src and its substrates at invadosomes

In the past decade, substantial progress has been made in understanding how Src family kinases regulate the formation and function of invadosomes. Invadosomes are organized actin-rich structures that contain an F-actin core surrounded by an adhesive ring and mediate invasive migration. Src kinases orchestrate, either directly or indirectly, each phase of the invadosome life cycle including invadosome assembly, maturation and matrix degradation and disassembly. Complex arrays of Src effector proteins are involved at different stages of invadosome maturation and their spatiotemporal activity must be tightly regulated to achieve effective invasive migration. In this review, we highlight some recent progress and the challenges of understanding how Src is regulated temporally and spatially to orchestrate the dynamics of invadosomes and mediate cell invasion.

Adhesion-mediated cytoskeletal remodeling is controlled by the direct scaffolding of Src from FAK complexes to lipid rafts by SSeCKS/AKAP12

Metastatic cell migration and invasion are regulated by altered adhesion-mediated signaling to the actin-based cytoskeleton via activated Src-FAK complexes. SSeCKS (the rodent orthologue of human Gravin/AKAP12), whose expression is downregulated by oncogenic Src and in many human cancers, antagonizes oncogenic Src pathways including those driving neovascularization at metastatic sites, metastatic cell motility and invasiveness. This is likely manifested through its function as a scaffolder of F-actin and signaling proteins such as cyclins, calmodulin, protein kinase (PK) C and PKA. Here, we show that in contrast to its ability to inhibit haptotaxis, SSeCKS increased prostate cancer cell adhesion to fibronectin (FN) and type I collagen in a FAK-dependent manner, correlating with a relative increase in FAK^poY397 levels. In contrast, SSeCKS suppressed adhesion-induced Src activation (Src^poY416) and phosphorylation of FAK at Y925, a known Src substrate site. SSeCKS also induced increased cell spreading, cell flattening, integrin β1 clustering and formation of mature focal adhesion plaques. An in silico analysis identified a Src-binding domain on SSeCKS (a.a.153–166) that is homologous to the Src binding domain of Caveolin-1, and this region is required for SSeCKS-Src interaction, for SSeCKS-enhanced Src activity and sequestration to lipid rafts, and for SSeCKS-enhanced adhesion of MAT-LyLu and CWR22Rv1 prostate cancer cells. Our data suggest a model in which SSeCKS suppresses oncogenic motility by sequestering Src to caveolin-rich lipid rafts, thereby disengaging Src from FAK-associated adhesion and signaling complexes.

Ubiquitin links to cytoskeletal dynamics, cell adhesion and migration

Post-translational modifications are used by cells to link additional information to proteins. Most modifications are subtle and concern small moieties such as a phosphate group or a lipid. In contrast, protein ubiquitylation entails the covalent attachment of a full-length protein such as ubiquitin. The protein ubiquitylation machinery is remarkably complex, comprising more than 15 Ubls (ubiquitin-like proteins) and several hundreds of ubiquitin-conjugating enzymes. Ubiquitin is best known for its role as a tag that induces protein destruction either by the proteasome or through targeting to lysosomes. However, addition of one or more Ubls also affects vesicular traffic, protein-protein interactions and signal transduction. It is by now well established that ubiquitylation is a component of most, if not all, cellular signalling pathways. Owing to its abundance in controlling cellular functions, ubiquitylation is also of key relevance to human pathologies, including cancer and inflammation. In the present review, we focus on its role in the control of cell adhesion, polarity and directional migration. It will become clear that protein modification by Ubls occurs at every level from the receptors at the plasma membrane down to cytoskeletal components such as actin, with differential consequences for the pathway's final output. Since ubiquitylation is fast as well as reversible, it represents a bona fide signalling event, which is used to fine-tune a cell's responses to receptor agonists.

Signalling pathways passing Src in pancreatic endocrine tumours: relevance for possible combined targeted therapies

The most frequent molecular abnormalities in pancreatic endocrine tumours (PETs) are mutations of the MEN1 gene, deregulation of the PI3K/AKT/mTOR signalling pathway and overactivation of growth factors and their receptors, such as the VEGF. On this basis, everolimus (Afinitor®; Novartis) and sunitinib (Sutent®; Pfizer) have both been approved by the FDA for the treatment of progressive, unresectable, locally advanced or metastatic PETs. However, molecular or surrogate markers able to predict the response of PET patients to treatment with these drugs are not available, and cancer cells treated with targeted therapies might develop escape pathways that evoke pro-survival feedback responses. The existence of cross-talk between different molecular pathways in PETs has been poorly investigated. In the present review, we present data supporting an important role for Src family kinases (SFKs) in PETs, together with the recent observation of a novel role for SFK in modulating the mTOR pathway activity. Of note, while treatment with everolimus triggered the activation of a survival response dependent on PI3K/AKT signalling in vitro, the simultaneous inhibition of SFKs blocked the activation of this unwanted escape signal. These studies might set the ground for the investigation of combined treatment of PETs with SFK and mTOR inhibitors.

Microenvironmental control of the breast cancer cell cycle

The mammary gland is one of the best-studied examples of an organ whose structure and function are influenced by reciprocal signaling and communication between cells and their microenvironment. The mammary epithelial cell (MEC) microenvironment includes stromal cells and extracellular matrix (ECM). Abundant evidence shows that the ECM and growth factors co-operate to regulate cell cycle progression, and that the ECM is altered in breast tumors. In particular, mammographically dense breast tissue is a significant risk factor for developing breast carcinomas. Dense breast tissue is associated with increased stromal collagen and epithelial cell content. In this article, we overview recent studies addressing the effects of ECM composition on the breast cancer cell cycle. Although the normal breast ECM keeps the MEC cycle in check, the ECM remodeling associated with breast cancer positively regulates the MEC cycle. ECM effects on the downstream biochemical and mechanosignaling pathways in both normal and tumorigenic MECs will be reviewed.

The Tumor Suppressor, p190RhoGAP, Differentially Initiates Apoptosis and Confers Docetaxel Sensitivity to Breast Cancer Cells

p190RhoGAP (p190) is a negative regulator of RhoGTPases and a putative tumor suppressor, whose mechanism of tumor suppression is poorly defined. Ectopic expression of p190 induces various morphological phenotypes, including multinucleation, dendrite-like formation, and chromatin condensation, suggesting an involvement in apoptosis. We examined the possibility that p190 can function as a tumor suppressor by regulating induction of apoptosis. We show that the predominant phenotype of p190 overexpression in a variety of cell lines is apoptosis, which is mediated through p190's regulation of Rho and caspases. The secondary phenotypes, multinucleation and dendrite-like formation, are determined by transformation status, not cell lineage, and appear to be intermediate phenotypes in the p190-induced apoptotic pathway. Finally, we show that p190 levels can regulate the apoptotic response of breast cancer cell lines to docetaxel through its regulation of Rho. Together, these findings suggest that one mechanism by which p190 can mediate its tumor-suppressive function is through regulation of Rho-activated cell death pathways and that this function can be exploited to optimize the action of cytoskeletal-based chemotherapeutics, such as the taxanes.

Necdin, a negative growth regulator, is a novel STAT3 target gene down-regulated in human cancer

Cytokine and growth factor signaling pathways involving STAT3 are frequently constitutively activated in many human primary tumors, and are known for the transcriptional role they play in controlling cell growth and cell cycle progression. However, the extent of STAT3's reach on transcriptional control of the genome as a whole remains an important question. We predicted that this persistent STAT3 signaling affects a wide variety of cellular functions, many of which still remain to be characterized. We took a broad approach to identify novel STAT3 regulated genes by examining changes in the genome-wide gene expression profile by microarray, using cells expressing constitutively-activated STAT3. Using computational analysis, we were able to define the gene expression profiles of cells containing activated STAT3 and identify candidate target genes with a wide range of biological functions. Among these genes we identified Necdin, a negative growth regulator, as a novel STAT3 target gene, whose expression is down-regulated at the mRNA and protein levels when STAT3 is constitutively active. This repression is STAT3 dependent, since inhibition of STAT3 using siRNA restores Necdin expression. A STAT3 DNA-binding site was identified in the Necdin promoter and both EMSA and chromatin immunoprecipitation confirm binding of STAT3 to this region. Necdin expression has previously been shown to be down-regulated in a melanoma and a drug-resistant ovarian cancer cell line. Further analysis of Necdin expression demonstrated repression in a STAT3-dependent manner in human melanoma, prostate and breast cancer cell lines. These results suggest that STAT3 coordinates expression of genes involved in multiple metabolic and biosynthetic pathways, integrating signals that lead to global transcriptional changes and oncogenesis. STAT3 may exert its oncogenic effect by up-regulating transcription of genes involved in promoting growth and proliferation, but also by down-regulating expression of negative regulators of the same cellular processes, such as Necdin.

The anti-cancer effects of (-)-epigallocatechin-3-gallate on the signaling pathways associated with membrane receptors in MCF-7 cells

(-)-Epigallocatechin-3-gallate (EGCg) has been implicated in cancer chemo-prevention in studies using many different kinds of cancer cells. The present study measured cell viability, osteopontin (OPN) secretion, fatty acid synthase (FAS) expression, and cytosolic Ca(2+) and verified the anti-cancer activities of EGCg in MCF-7 human breast cancer cells. EGCg-induced apoptosis was evidenced by nuclear condensation, increased protein levels of activated caspase-3, down-regulation of gelsolin and tropomyosin-4 (Tm-4), and up-regulation of tropomyosin-1(Tm-1). By disrupting adherens junction formation, EGCg caused accumulation of extra-nuclear β-catenin aggregates in the cytosol and alterations of the protein content and mRNA expression of E-cadherin and β-catenin, but not N-cadherin, in MCF-7 cells. To identify the putative mechanisms underlying the EGCg signaling pathways, EGFP (enhanced green fluorescence protein) was ectopically expressed in MCF-7 cells. This allowed us to monitor the EGCg-induced fluorescence changes associated with the effects of Triton X-100 (to remove plasma membrane) or the addition of laminin, anti-laminin receptor (LR) antibody, epidermal growth factor (EGF), and genistein on the cells. Our results indicated that EGCg acts via the signaling pathways associated with cell membrane to suppress cell proliferation, provoke apoptosis, and disturb cell-cell adhesion in MCF-7 cells. The altered events include the EGFR, LR, FAS, intracellular Ca(2+) , OPN secretion, caspace-3, gelsolin, Tm-4, Tm-1, and adherens junction proteins, E-cadherin and β-catenin.

Integrins in cell migration

Integrin-based adhesion has served as a model for studying the central role of adhesion in migration. In this article, we outline modes of migration, both integrin-dependent and -independent in vitro and in vivo. We next discuss the roles of adhesion contacts as signaling centers and linkages between the ECM and actin that allows adhesions to serve as traction sites. This includes signaling complexes that regulate migration and the interplay among adhesion, signaling, and pliability of the substratum. Finally, we address mechanisms of adhesion assembly and disassembly and the role of adhesion in cellular polarity.

Inhibitors of Src and focal adhesion kinase promote endocrine specification: impact on the derivation of β-cells from human pluripotent stem cells

Stepwise approaches for the derivation of β-cells from human embryonic stem cells have been described. However, low levels of endocrine specification limit the final yield of insulin-producing β-cells. In this study, we show that the pyrrolo-pyrimidine Src family kinase (SFK) inhibitor PP2 effectively promotes the endocrine specification of human embryonic stem cell derivatives based on its capacity to induce the expression of proendocrine transcription factors (NGN3, NEUROD1, NKX2.2, and PAX4) and to significantly increase the final yield of insulin-positive cells. We further demonstrate that PP2 inhibits the activation of focal adhesion kinase (FAK), and selective inhibition of this kinase is also sufficient to induce early endocrine commitment based on increased expression of NGN3, NEUROD1, and NKX2.2. Additional studies using dominant negative constructs and isolated human fetal pancreata suggest that c-Src is at least partially responsible for inhibiting early endocrine specification. Mechanistically, we propose that inhibition of SFK/FAK signaling can promote endocrine specification by limiting activation of the TGFβR/Smad2/3 pathway. Moreover, we show that inhibition of SFK/FAK signaling suppresses cell growth, increases the expression of the β-cell-associated cyclin-dependent kinase inhibitor p57kip2, and simultaneously suppresses the expression of Id1 and Id2. This study has important implications for the derivation of β-cells for the cell-based therapy of diabetes and sheds new light on the signaling events that regulate early endocrine specification.

Integrin signaling, cell survival, and anoikis: distinctions, differences, and differentiation

Cell survival and apoptosis implicate an increasing complexity of players and signaling pathways which regulate not only the decision-making process of surviving (or dying), but as well the execution of cell death proper. The same complex nature applies to anoikis, a form of caspase-dependent apoptosis that is largely regulated by integrin-mediated, cell-extracellular matrix interactions. Not surprisingly, the regulation of cell survival, apoptosis, and anoikis furthermore implicates additional mechanistic distinctions according to the specific tissue, cell type, and species. Incidentally, studies in recent years have unearthed yet another layer of complexity in the regulation of these cell processes, namely, the implication of cell differentiation state-specific mechanisms. Further analyses of such differentiation state-distinct mechanisms, either under normal or physiopathological contexts, should increase our understanding of diseases which implicate a deregulation of integrin function, cell survival, and anoikis.

The SRC-associated protein CUB Domain-Containing Protein-1 regulates adhesion and motility

Multiple SRC-family kinases (SFKs) are commonly activated in carcinoma and appear to have a role in metastasis through incompletely understood mechanisms. Recent studies have shown that CDCP1 (CUB (complement C1r/C1s, Uegf, Bmp1) Domain-Containing Protein-1) is a transmembrane protein and an SRC substrate potentially involved in metastasis. Here we show that increased SFK and CDCP1 tyrosine phosphorylation is, surprisingly, associated with a decrease in FAK phosphorylation. This appears to be true in human tumors as shown by our correlation analysis of a mass spectrometric data set of affinity-purified phosphotyrosine peptides obtained from normal and cancer lung tissue samples. Induction of tyrosine phosphorylation of CDCP1 in cell culture, including by a mAb that binds to its extracellular domain, promoted changes in SFK and FAK tyrosine phosphorylation, as well as in PKC(TM), a protein known to associate with CDCP1, and these changes are accompanied by increases in adhesion and motility. Thus, signaling events that accompany the CDCP1 tyrosine phosphorylation observed in cell lines and human lung tumors may explain how the CDCP1/SFK complex regulates motility and adhesion.

Suppression of E-cadherin function drives the early stages of Ras-induced squamous cell carcinoma through upregulation of FAK and Src

Advanced stages of epithelial carcinogenesis involve the loss of intercellular adhesion, but it remains unclear how proteins that regulate alterations in cell-cell and cell-matrix adhesion are deregulated to promote the early stages of cancer development. To address this, a three-dimensional human tissue model that mimics the incipient stages of Squamous Cell Carcinoma (SCC) was used to study how E-cadherin suppression promotes tumor progression in Ras-expressing human keratinocytes. We found that E-cadherin suppression triggered elevated mRNA and protein expression levels of Focal Adhesion Kinase (FAK), and increased FAK and Src activities above the level seen in Ras-expressing E-cadherin-competent keratinocytes. sh-RNA-mediated depletion of FAK and Src restored E-cadherin expression levels by increasing its stability in the membrane, and blocked tumor cell invasion in tissues. Surface transplantation of these tissues to mice resulted in reversion of the tumor phenotype to low-grade tumor islands in contrast to control tissues that manifested an aggressive, high-grade SCC. These findings suggest that the tumor-promoting effect of E-cadherin suppression, a common event in SCC development, is exacerbated by enhanced E-cadherin degradation induced by elevated FAK and Src activities. Furthermore, they imply that targeting FAK or Src in human epithelial cells with neoplastic potential may inhibit the early stages of SCC.

Rack1 promotes epithelial cell-cell adhesion by regulating E-cadherin endocytosis

E-cadherin and its cytoplasmic partners, catenins, mediate epithelial cell-cell adhesion. Disruption of this adhesion allows cancer cells to invade and metastasize. Aberrant activation of the Src tyrosine kinase disrupts cell-cell contacts through an E-cadherin/catenin-dependent mechanism. Previously we showed that Rack1 regulates the growth of colon cells by suppressing Src activity at G(1) and mitotic checkpoints, and in the intrinsic apoptotic and Akt cell survival pathways. Here we show that Rack1, partly by inhibiting Src, promotes cell-cell adhesion and reduces the invasive potential of colon cancer cells. Rack1 stabilizes E-cadherin and catenins at cell-cell contacts by inhibiting the Src phosphorylation of E-cadherin, the ubiquitination of E-cadherin by the E3 ligase Hakai and the endocytosis of E-cadherin. Upon depletion and restoration of extracellular calcium, Rack1 facilitates the re-assembly of E-cadherin-containing cell-cell contacts. Rack1 also blocks HGF-induced endocytosis of E-cadherin, disruption of cell-cell contacts and cell scatter. Our results uncover a novel function of Rack1 in maintaining the junctional homeostasis of intestinal epithelial cells by regulation of the Src- and growth factor-induced endocytosis of E-cadherin.

Steps in prostate cancer progression that lead to bone metastasis

Prostate cancer is a complex disease in which metastasis to the bone is the main cause of death. Initial stages of metastasis are generally similar to those for most solid tumors; however, the mechanisms that underlie the homing of prostate tumor cells to the bone are not completely understood. Prostate cancer bone metastasis is also a microenvironment-driven disease, involving bidirectional interactions between the tumor and the bone microenvironment. In this review, we discuss the current understanding of the biologic processes and regulatory factors involved in the metastasis of prostate cancer cells, and their specific properties that promote growth in bone. Although many of these processes still need to be fully elucidated, a better understanding of the complex tumor/microenvironment interplay is slowly leading to more effective therapies for patients with prostate cancer bone metastases.

The guanine nucleotide exchange factor Arhgef5 plays crucial roles in Src-induced podosome formation

Podosomes and invadopodia are actin-rich membrane protrusions that play a crucial role in cell adhesion and migration, and extracellular matrix remodeling in normal and cancer cells. The formation of podosomes and invadopodia is promoted by upregulation of some oncogenic molecules and is closely related to the invasive potential of cancer cells. However, the molecular mechanisms underlying the podosome and invadopodium formation still remain unclear. Here, we show that a guanine nucleotide exchange factor (GEF) for Rho family GTPases (Arhgef5) is crucial for Src-induced podosome formation. Using an inducible system for Src activation, we found that Src-induced podosome formation depends upon the Src SH3 domain, and identified Arhgef5 as a Src SH3-binding protein. RNA interference (RNAi)-mediated depletion of Arhgef5 caused robust inhibition of Src-dependent podosome formation. Overexpression of Arhgef5 promoted actin stress fiber remodeling through activating RhoA, and the activation of RhoA or Cdc42 was required for Src-induced podosome formation. Arhgef5 was tyrosine-phosphorylated by Src and bound to Src to positively regulate its activity. Furthermore, the pleckstrin homology (PH) domain of Arhgef5 was required for podosome formation, and Arhgef5 formed a ternary complex with Src and phosphoinositide 3-kinase when Src and/or Arhgef5 were upregulated. These findings provide novel insights into the molecular mechanisms of podosome and invadopodium formation induced by Src upregulation.

Learning about the functions of NME/NM23: lessons from knockout mice to silencing strategies

The human NME gene family (also known as NM23) comprises ten genes that are involved in diverse physiological and pathological processes including proliferation, differentiation, development, ciliary functions, and metastasis. For the moment, only the NME1, NME2, and NME7 genes have been inactivated in transgenic knockout mice, as well as a double NME1-NME2 gene knockout. Mice lacking NME1 or NME2 grow to adulthood without health problems, although NME1 (-/-) mice have modest growth retardation. Double knockout NME1 (-/-)-NME2 (-/-) mice, by contrast, are highly hypotrophic and die at birth from profound anemia due to impaired erythroblast development. Evidence for a metastasis suppressor function of NME1 in vivo comes from crossing NME1 (-/-) mice with mice prone to develop hepatocellular carcinoma; the double transgenic mice present a higher incidence of lung metastases. Silencing of NME1 by siRNA interference has confirmed this function by conferring a "metastatic phenotype" on non-invasive human epithelial cancer cell lines. This function is specific to NME1 and is not observed when the NME2 is silenced. The data indicate that NME1 loss is causally involved at the early stages of the metastatic cascade. NME2 (-/-) mice and NME2 silencing experiments reveal a specific role of NME2 in activation of heterotrimeric G proteins and of KCa3.1 channel in T cells, pointing to a role of NME2 as a histidine phosphotransferase. Regarding NME7, consistent with its expression in axonemal structures, NME7 (-/-) mice present lesions similar to primary ciliary dyskinesia. This review summarizes the recent data obtained by knockout and silencing of NME/NM23 genes that provide mechanistic insights into their respective roles in physiology and pathology.

Probing the role of the linker in ferrocene-ATP conjugates: monitoring protein kinase catalyzed phosphorylations electrochemically

The synthesis and electrochemical properties of ferrocene conjugates are presented for the purpose of investigating adenosine 5'-[γ-ferrocenoylalkyl] triphosphate (1 a-4 a, ferrocene (Fc)-ATP) as co-substrates for phosphorylation reactions. Compounds 1 a-4 a were synthesized, purified by HPLC, and characterized by NMR spectroscopy and mass spectrometry. In solution, all Fc-ATP bioconjugates exhibit a reversible one-electron redox process with a half-wave potential (E(1/2)) in the 390-430 mV range, peak separations (ΔE(p)) in the 40-70 mV range, and the peak current ratio (i(pa)/i(pc)) near unity. The peptide-modified surface Glu-Gly-Ile-Tyr-Asp-Val-Pro was used to study the sarcoma-related protein (Src) kinase activity by employing the Fc-ATP bioconjugates as co-substrates. Subsequent kinase-catalyzed transfer of the γ-Fc-phosphate group to the tyrosine residues of the surface-bound peptides was characterized by a formal potential (E°) ≈390 mV (vs. Ag/AgCl). The Fc-coverage, estimated by time-of-flight secondary-ion mass spectrometry (TOF-SIMS) and cyclic voltammetry (CV), suggested validity of Fc-ATP conjugates as kinase co-substrates. Depending on the length of the alkyl spacer of the Fc-ATP conjugate, different current densities were obtained, pointing to a direct correlation between the two. Molecular modeling revealed that the structural constraint imposed by the short alkyl spacer (1 a) causes a steric congestion and negatively affects the outcome of phosphorylation reaction. An optimal analytical response was obtained with the Fc-ATP conjugates with linker lengths longer than six CH(2) groups.

c-Src inactivation reduces renal epithelial cell-matrix adhesion, proliferation, and cyst formation

c-Src is a non-receptor tyrosine kinase whose activity is induced by phosphorylation at Y418 and translocation from the cytoplasm to the cell membrane. Increased activity of c-Src has been associated with cell proliferation, matrix adhesion, motility, and apoptosis in tumors. Immunohistochemistry suggested that activated (pY(418))-Src activity is increased in cyst-lining autosomal dominant polycystic kidney disease (ADPKD) epithelial cells in human and mouse ADPKD. Western blot analysis showed that SKI-606 (Wyeth) is a specific inhibitor of pY(418)-Src without demonstrable effects on epidermal growth factor receptor or ErbB2 activity in renal epithelia. In vitro studies on mouse inner medullary collecting duct (mIMCD) cells and human ADPKD cyst-lining epithelial cells showed that SKI-606 inhibited epithelial cell proliferation over a 24-h time frame. In addition, SKI-606 treatment caused a striking statistically significant decrease in adhesion of mIMCD and human ADPKD to extracellular collagen matrix. Retained viability of unattached cells was consistent with a primary effect on epithelial cell anchorage dependence mediated by the loss of extracellular matrix (ECM)-attachment due to α(2)β(1)-integrin function. SKI-606-mediated attenuation of the human ADPKD hyperproliferative and hyper-ECM-adhesive epithelial cell phenotype in vitro was paralleled by retardation of the renal cystic phenotype of Pkd1 orthologous ADPKD heterozygous mice in vivo. This suggests that SKI-606 has dual effects on cystic epithelial cell proliferation and ECM adhesion and may have therapeutic potential for ADPKD patients.

Metastasis suppressor genes at the interface between the environment and tumor cell growth

The molecular mechanisms and genetic programs required for cancer metastasis are sometimes overlapping, but components are clearly distinct from those promoting growth of a primary tumor. Every sequential, rate-limiting step in the sequence of events leading to metastasis requires coordinated expression of multiple genes, necessary signaling events, and favorable environmental conditions or the ability to escape negative selection pressures. Metastasis suppressors are molecules that inhibit the process of metastasis without preventing growth of the primary tumor. The cellular processes regulated by metastasis suppressors are diverse and function at every step in the metastatic cascade. As we gain knowledge into the molecular mechanisms of metastasis suppressors and cofactors with which they interact, we learn more about the process, including appreciation that some are potential targets for therapy of metastasis, the most lethal aspect of cancer. Until now, metastasis suppressors have been described largely by their function. With greater appreciation of their biochemical mechanisms of action, the importance of context is increasingly recognized especially since tumor cells exist in myriad microenvironments. In this chapter, we assemble the evidence that selected molecules are indeed suppressors of metastasis, collate the data defining the biochemical mechanisms of action, and glean insights regarding how metastasis suppressors regulate tumor cell communication to-from microenvironments.

Role for transcription factor TFII-I in the suppression of SSeCKS/Gravin/Akap12 transcription by Src

The SSeCKS/Gravin/AKAP12 gene, encoding a kinase scaffolding protein with metastasis-suppressing activity, is transcriptionally downregulated in Src-transformed cells through the recruitment of HDAC1 to a Src-responsive proximal promoter site charged with Sp1, Sp3 and USF1. However, the ectopic expression of these proteins formed a suppressive complex in Src-transformed but not in parental NIH3T3 cells, suggesting the involvement of additional repressor factors. Transcription factor II-I (TFII-I) [general transcription factor 2i (Gtf2i)] was identified by mass spectrometry as being associated with the SSeCKS promoter complex in NIH3T3/Src cells, and moreover, the Src-induced tyrosine phosphorylation of TFII-I significantly increased its binding to the SSeCKS proximal promoter. siRNA-mediated knockdown of TFII-I or the expression of TFII-I(Y248/249F) caused the derepression of SSeCKS in NIH3T3/Src cells. Taken with previous data showing that the tyrosine phosphorylation of TFII-I facilitates its nuclear translocation, these data suggest that Src-family kinase-mediated phosphorylation converts a portion of TFII-I into a transcriptional repressor.

Cdk5 targets active Src for ubiquitin-dependent degradation by phosphorylating Src(S75)

The non-receptor tyrosine kinase Src is a critical regulator of cytoskeletal contraction, cell adhesion, and migration. In normal cells, Src activity is stringently controlled by Csk-dependent phosphorylation of Src(Y530), and by Cullin-5-dependent ubiquitinylation, which affects active Src(pY419) exclusively, leading to its degradation by the proteosome. Previous work has shown that Src activity is also limited by Cdk5, a proline-directed kinase, which has been shown to phosphorylate Src(S75). Here we show that this phosphorylation promotes the ubiquitin-dependent degradation of Src, thus restricting the availability of active Src. We demonstrate that Src(S75) phosphorylation occurs in vivo in epithelial cells, and like ubiquitinylation, is associated only with active Src. Preventing Cdk5-dependent phosphorylation of Src(S75), by site-specific mutation of S75 or by Cdk5 inhibition or suppression, increases Src(Y419) phosphorylation and kinase activity, resulting in Src-dependent cytoskeletal changes. In transfected cells, ubiquitinylation of Src(S75A) is about 35% that of wild-type Src-V5, and its half-life is approximately 2.5-fold greater. Cdk5 suppression leads to a comparable decrease in the ubiquitinylation of endogenous Src and a similar increase in Src stability. Together, these findings demonstrate that Cdk5-dependent phosphorylation of Src(S75) is a physiologically significant mechanism of regulating intracellular Src activity.

Extra-nuclear activation of progesterone receptor in regulating arterial smooth muscle cell migration

We previously showed that progesterone (P4) inhibits the proliferation of rat aortic smooth muscle cells (RASMC). Here, we further demonstrate that P4 at physiologic levels (5-500 nM) concentration-dependently inhibited migration of cultured RASMC. The effect is blocked by pretreatment with progesterone receptor (PR) antagonist, RU486. The P4-induced RASMC migration inhibition was through RhoA inactivation induced by cSrc-enhanced RhoA degradation. The P4-induced increases of phosphorylated Src (pSrc) and PR-pSrc complex in RASMC were observed mainly in the membrane fraction. Pre-treatment with a cSrc inhibitor (PP2) or cSrc antisense oligonucleotides prevented the P4-induced decreases of the protein levels of RhoA, phosphorylated FAK (p-FAK) and paxillin phosphorylaton and migration inhibition in RASMC. These findings expend our knowledge of the basis of P4's effect on vascular smooth muscle cell migration and highlight novel pathways of signaling transduction of P4 through PR-mediated nongenomic mechanisms.

Location, location, location: new insights into O-GalNAc protein glycosylation

O-GalNAc glycosylation of proteins confers essential structural, protective and signaling roles in eumetazoans. Addition of O-glycans onto proteins is an extremely complex process that regulates both sites of attachment and the types of oligosaccharides added. Twenty distinct polypeptide GalNAc-transferases (GalNAc-Ts) initiate O-glycosylation and fine-tuning their expression provides a mechanism for regulating this action. Recently, a new mode of regulation has emerged where activation of Src kinase selectively redistributes Golgi-localized GalNAc-Ts to the ER. This relocalization results in a strong increase in the density of O-glycan decoration. In this review, we discuss how different mechanisms can regulate the number and the types of O-glycans decorating proteins. In addition, we speculate how Src-dependent relocation of GalNAc-Ts could play an important role in cancerous cellular transformation.

FAK phosphorylation at Tyr-925 regulates cross-talk between focal adhesion turnover and cell protrusion

FAK plays a key role in the regulation of cell migration. The authors show that the phosphorylation status of FAK at Tyr-925 is involved in FA turnover, formation of FAs, and increase in cell edge protrusion, together with activation of the p130^CAS/Rac1 signaling pathway.

 Cell migration is a highly complex process that requires the coordinated formation of membrane protrusion and focal adhesions (FAs). Focal adhesion kinase (FAK), a major signaling component of FAs, is involved in the disassembly process of FAs through phosphorylation and dephosphorylation of its tyrosine residues, but the role of such phosphorylations in nascent FA formation and turnover near the cell front and in cell protrusion is less well understood. In the present study, we demonstrate that, depending on the phosphorylation status of Tyr-925 residue, FAK modulates cell migration via two specific mechanisms. FAK^−/− mouse embryonic fibroblasts (MEFs) expressing nonphosphorylatable Y925F-FAK show increased interactions between FAK and unphosphorylated paxillin, which lead to FA stabilization and thus decreased FA turnover and reduced cell migration. Conversely, MEFs expressing phosphomimetic Y925E-FAK display unchanged FA disassembly rates, show increase in phosphorylated paxillin in FAs, and exhibit increased formation of nascent FAs at the cell leading edges. Moreover, Y925E-FAK cells present enhanced cell protrusion together with activation of the p130^CAS/Dock180/Rac1 signaling pathway. Together, our results demonstrate that phosphorylation of FAK at Tyr-925 is required for FAK-mediated cell migration and cell protrusion.

Src kinase mediates productive endocytic sorting of reovirus during cell entry

Reovirus cell entry is initiated by viral attachment to cell surface glycans and junctional adhesion molecule A. Following receptor engagement, reovirus is internalized into cells by receptor-mediated endocytosis using a process dependent on β1 integrin. Endocytosed virions undergo stepwise disassembly catalyzed by cathepsin proteases, followed by endosomal membrane penetration and delivery of transcriptionally active core particles into the cytoplasm. Cellular factors that mediate reovirus endocytosis are poorly defined. We found that both genistein, a broad-spectrum tyrosine kinase inhibitor, and PP2, a specific Src-family kinase inhibitor, diminish reovirus infectivity by blocking a cell entry step. Although neither inhibitor impedes internalization of reovirus virions, both inhibitors target virions to lysosomes. Reovirus colocalizes with Src during cell entry, and reovirus infection induces phosphorylation of Src at the activation residue, tyrosine 416. Diminished Src expression by RNA interference reduces reovirus infectivity, suggesting that Src is required for efficient reovirus entry. Collectively, these data provide evidence that Src kinase is an important mediator of signaling events that regulate the appropriate sorting of reovirus particles in the endocytic pathway for disassembly and cell entry.

Actin filament associated protein mediates c-Src related SRE/AP-1 transcriptional activation

AFAP is an adaptor protein involved in cytoskeletal organization and intracellular signaling. AFAP binds and activates c-Src; however, the downstream signals of this interaction remain unknown. Here we show that co-expression of AFAP and c-Src induce transcriptional activation of SRE and AP-1 in a c-Src activity dependent fashion. Structural-functional studies suggest that the proline-rich motif in the N-terminus of AFAP is critical for c-Src activation, and subsequent SRE/AP-1 transactivation and the actin-binding domain in the AFAP C-terminus is negatively involved in the regulation of AFAP/c-Src mediated SRE/AP-1 transactivation. Selective deletion of this domain enhances transactivation of SRE. We conclude that in addition to its role in the regulation of cytoskeletal structures, AFAP may also be involved in the c-Src related transcriptional activities.

Loss of tyrosine phosphatase-dependent inhibition promotes activation of tyrosine kinase c-Src in detached pancreatic cells

Despite an intense focus on novel therapeutic strategies, pancreatic adenocarcinoma remains one of the deadliest human malignancies. The frequent and rapid mortality associated with pancreatic cancer may be attributed to several factors, including late diagnosis, rapid tumor invasion into surrounding tissues, and formation of distant metastases. Both local invasion and metastasis require disruption of tumor cell contacts with the extracellular matrix. Detachment of normal cells from the extracellular matrix leads to a form of programmed cell death termed anoikis. Pancreatic cancer cells avert anoikis by activation of signaling pathways that allow for adhesion-independent survival. In the present studies, cellular signaling pathways activated in detached pancreatic cancer cells were examined. We demonstrate a rapid and robust activation of Src kinase in detached pancreatic cancer cells, relative to adherent. Src autophosphorylation rapidly returned to baseline levels upon reattachment to tissue culture plastic, in the presence or absence of specific extracellular matrix proteins. Treatment of pancreatic cancer cells with tyrosine phosphatase inhibitors increased steady-state Src autophosphorylation in adherent cells and abrogated the detachment-induced increase in Src autophosphorylation. Src was found to co-immunoprecipitate with the Src homology 2 (SH2) domain containing protein tyrosine phosphatase (SHP-2) in pancreatic cancer cells, suggesting that SHP-2 may participate in regulation of Src autophosphorylation in adherent cells. Src family kinase (SFK) dependent increases in Akt and Jun N-terminal kinase (JNK) phosphorylation were observed in detached cells, indicating the potential for Src-dependent activation of survival and stress pathways in pancreatic cancer cells that have detached from the extracellular matrix.

Mammary gland ECM remodeling, stiffness, and mechanosignaling in normal development and tumor progression

Cells of the mammary gland are in intimate contact with other cells and with the extracellular matrix (ECM), both of which provide not only a biochemical context, but a mechanical context as well. Cell-mediated contraction allows cells to sense the stiffness of their microenvironment, and respond with appropriate mechanosignaling events that regulate gene expression and differentiation. ECM composition and organization are tightly regulated throughout development of the mammary gland, resulting in corresponding regulation of the mechanical environment and proper tissue architecture. Mechanical regulation is also at play during breast carcinoma progression, as changes in ECM deposition, composition, and organization accompany breast carcinoma. These changes result in stiffer matrices that activate mechanosignaling pathways and thereby induce cell proliferation, facilitate local tumor cell invasion, and promote progression. Thus, understanding the role of forces in the mammary gland is crucial to understanding both normal developmental and pathological processes.

Phosphorylation of Trask by Src kinases inhibits integrin clustering and functions in exclusion with focal adhesion signaling

Trask is a recently described transmembrane substrate of Src kinases whose expression and phosphorylation has been correlated with the biology of some cancers. Little is known about the molecular functions of Trask, although its phosphorylation has been associated with cell adhesion. We have studied the effects of Trask phosphorylation on cell adhesion, integrin activation, clustering, and focal adhesion signaling. The small hairpin RNA (shRNA) knockdown of Trask results in increased cell adhesiveness and a failure to properly inactivate focal adhesion signaling, even in the unanchored state. On the contrary, the experimentally induced phosphorylation of Trask results in the inhibition of cell adhesion and inhibition of focal adhesion signaling. This is mediated through the inhibition of integrin clustering without affecting integrin affinity state or ligand binding activity. Furthermore, Trask signaling and focal adhesion signaling inactivate each other and signal in exclusion with each other, constituting a switch that underlies cell anchorage state. These data provide considerable insight into how Trask functions to regulate cell adhesion and reveal a novel pathway through which Src kinases can oppose integrin-mediated cell adhesion.

The transmembrane adaptor Cbp/PAG1 controls the malignant potential of human non-small cell lung cancers that have c-src upregulation

The tyrosine kinase c-Src is upregulated in various human cancers, although the precise regulatory mechanism underlying this upregulation is unclear. We previously reported that a transmembrane adaptor Csk-binding protein (Cbp; PAG1) plays an important role in controlling the cell transformation that is induced by the activation of c-Src. To elucidate the in vivo role of Cbp, we examined the function of Cbp in lung cancer cell lines and tissues. In this study, we found that Cbp was markedly downregulated in human non-small cell lung cancer (NSCLC) cells. The ectopic expression of Cbp suppressed the anchorage-independent growth of the NSCLC cell lines (A549 and Lu99) that had upregulated c-Src, whereas the Cbp expression had little effect on other NSCLC cell lines (PC9 and Lu65) that express normal levels of c-Src. The expression of Cbp suppressed the kinase activity of c-Src in A549 cells by recruiting c-Src and its negative regulator, C-terminal Src kinase (Csk), to lipid rafts. The treatment with Src inhibitors, such as PP2, dasatinib, and saracatinib, also suppressed the growth of A549 cells. Furthermore, Cbp expression attenuated the ability of A549 cells to form tumors in nude mice, invade in vitro, and metastasize in vivo. In addition, we found a significant inverse correlation between the level of Cbp expression and the extent of lymph node metastasis in human lung cancers. These results indicate that Cbp is required for the Csk-mediated inactivation of c-Src and may control the promotion of malignancy in NSCLC tumors that are characterized by c-Src upregulation.

Phosphorylation of Ser 21 in Fyn regulates its kinase activity, focal adhesion targeting, and is required for cell migration

The tyrosine kinase Fyn is a member of the Src family kinases which are important in many integrin-mediated cellular processes including cell adhesion and migration. Fyn has multiple phosphorylation sites which can affect its kinase activity. Among these phosphorylation sites, the serine 21 (S21) residue of Fyn is a protein kinase A (PKA) recognition site within an RxxS motif of the amino terminal SH4 domain of Fyn. In addition, S21 is critical for Fyn kinase-linked cellular signaling. Mutation of S21A blocks PKA phosphorylation of Fyn and alters its tyrosine kinase activity. Expression of Fyn S21A in cells lacking Src family kinases (SYF cell) led to decreased tyrosine phosphorylation of focal adhesion kinase resulting in reduced focal adhesion targeting, which slowed lamellipodia dynamics and thus cell migration. These changes in cell motility were reflected by the fact that cells expressing Fyn S21A were severely deficient in their ability to assemble and disassemble focal adhesions. Taken together, our findings indicate that phosphorylation of S21 within the pPKA recognition site (RxxS motif) of Fyn regulates its tyrosine kinase activity and controls focal adhesion targeting, and that this residue of Fyn is critical for transduction of signals arising from cell-extracellular matrix interactions.

Distinct interactions between c-Src and c-Met in mediating resistance to c-Src inhibition in head and neck cancer

PURPOSE

c-Src inhibition in cancer cells leads to an abrogation of invasion but a variable effect on apoptosis. The pathways downstream of c-Src promoting survival are not well characterized. Because cancer therapy that both decreases invasion and induces significant apoptosis would be ideal, we sought to characterize the mechanisms of resistance to c-Src inhibition.

EXPERIMENTAL DESIGN

c-Src was inhibited in a panel of oral cancer cell lines and subsequent survival and signaling measured. The interactions between c-Src and c-Met were evaluated using immunoprecitation and an in vitro kinase assay. Cytotoxicity was measured and the Chou-Talalay combination index calculated. An orthotopic model of oral cancer was used to assess the effects of c-Met and c-Src inhibitors.

RESULTS

Inhibition of c-Src resulted in c-Met inhibition in sensitive cells lines, but not in resistant cell lines. Isolated c-Met was a c-Src substrate in both sensitive and resistant cells, but there was no interaction of c-Src and c-Met in intact resistant cells. To examine the biological consequences of this mechanism, we demonstrated synergistic cytotoxicity, enhanced apoptosis, and decreased tumor size with the combination of c-Src and c-Met inhibitors.

CONCLUSIONS

Sustained c-Met activation can mediate resistance to c-Src inhibition. These data suggest that the differences between c-Met and c-Src signaling in sensitive and resistant cells are due to distinct factors promoting or inhibiting interactions, respectively, rather than to intrinsic structural changes in c-Src or c-Met. The synergistic cytotoxic effects of c-Src and c-Met inhibition may be important for the treatment of head and neck cancers.

Plakoglobin regulates cell motility through Rho- and fibronectin-dependent Src signaling

We previously showed that the cell-cell junction protein plakoglobin (PG) not only suppresses motility of keratinocytes in contact with each other, but also, unexpectedly, of single cells. Here we show that PG deficiency results in extracellular matrix (ECM)-dependent disruption of mature focal adhesions and cortical actin organization. Plating PG⁻/⁻ cells onto ECM deposited by PG+/⁻ cells partially restored normal cell morphology and inhibited PG⁻/⁻ cell motility. In over 70 adhesion molecules whose expression we previously showed to be altered in PG⁻/⁻ cells, a substantial decrease in fibronectin (FN) in PG⁻/⁻ cells stood out. Re-introduction of PG into PG⁻/⁻ cells restored FN expression, and keratinocyte motility was reversed by plating PG⁻/⁻ cells onto FN. Somewhat surprisingly, based on previously reported roles for PG in regulating gene transcription, PG-null cells exhibited an increase, not a decrease, in FN promoter activity. Instead, PG was required for maintenance of FN mRNA stability. PG⁻/⁻ cells exhibited an increase in activated Src, one of the kinases controlled by FN, a phenotype reversed by plating PG⁻/⁻ cells on ECM deposited by PG+/⁻ keratinocytes. PG⁻/⁻ cells also exhibited Src-independent activation of the small GTPases Rac1 and RhoA. Both Src and RhoA inhibition attenuated PG⁻/⁻ keratinocyte motility. We propose a novel role for PG in regulating cell motility through distinct ECM-Src and RhoGTPase-dependent pathways, influenced in part by PG-dependent regulation of FN mRNA stability.