A renaissance for SRC

Copper(II) and manganese(III) complexes of N'-[(2-hydroxy phenyl) carbonothioyl] pyridine-2-carbohydrazide: novel therapeutic agents for cancer

c-Src is a non-receptor tyrosine kinase which plays a significant role in the growth mediated signaling pathway impacting cellular proliferation, differentiation, mobility, survival and transformation. Myristoylation of pp60(c-src) leads to its membrane association and activation, a process catalyzed by N-myristoyltransferase (NMT). We have shown earlier increased NMT activity in the early stages of colon cancer. A novel sulfur nitrogen donor ligand and its Cu(II) and Mn(III) complexes have been prepared and characterized using various physicochemical analyses. These Cu(II) and Mn(III) complexes showed cytotoxicity against the colon cancer cell line HT29. The IC(50) for Cu(II) and Mn(III) complexes were 12.2 and 16.1 microM, respectively. HT29 cells treated with Cu(II) and Mn(III) complexes induced apoptosis and inhibited endogenous NMT activity. Furthermore, they induced higher levels of hsc70 and inhibited the expression of c-Src. Inhibition of endogenous NMT activity by metal complexes was demonstrated for the first time. This study also suggested that NMT activity is crucial for cell survival and demonstrated that cessation in activity results in apoptosis. These metal complexes may prove to be novel therapeutic agents for cancer targeting NMT.

Csk-deficient boundary cells are eliminated from normal Drosophila epithelia by exclusion, migration, and apoptosis

The construction and maintenance of normal epithelia relies on local signals that guide cells into their proper niches and remove unwanted cells. Failure to execute this process properly may result in aberrant development or diseases, including cancer and associated metastasis. Here, we show that local environment influences the behavior of dCsk-deficient cells. Broad loss of dCsk led to enlarged and mispatterned tissues due to overproliferation, a block in apoptosis, and decreased cadherin-mediated adhesion. Loss of dCsk in discrete patches led to a different outcome: epithelial exclusion, invasive migration, and apoptotic death. These latter phenotypes required sharp differences in dCsk activity between neighbors; dE-cadherin, P120-catenin, Rho1, JNK, and MMP2 mediated this signal. Together, our data demonstrate how the cellular microenvironment plays a central role in determining the outcome of altered dCsk activity, and reveal a role for P120-catenin in a mechanism that protects epithelial integrity by removing abnormal cells.

Chemical rescue of a mutant enzyme in living cells

The restoration of catalytic activity to mutant enzymes by small molecules is well established for in vitro systems. Here, we show that the protein tyrosine kinase Src arginine-388-->alanine (R388A) mutant can be rescued in live cells with the use of the small molecule imidazole. Cellular rescue of a viral Src homolog was rapid and reversible and conferred predicted oncogenic properties. Using chemical rescue in combination with mass spectrometry, we confirmed six known Src kinase substrates and identified several new protein targets. Chemical rescue data suggest that cellular Src is active under basal conditions. Rescue of R388A cellular Src provided insights into the mitogen-activated protein kinase pathway. This chemical rescue approach will likely have many applications in cell signaling.

AP23846, a novel and highly potent Src family kinase inhibitor, reduces vascular endothelial growth factor and interleukin-8 expression in human solid tumor cell lines and abrogates downstream angiogenic processes

c-Src is frequently activated in human malignancies, including colon, breast, and pancreatic carcinomas. Several recent studies have shown that activation of Src family kinases leads to tumor progression and metastasis by increasing cellular migration and invasion, promoting cell growth and survival, and deregulating expression of proangiogenic molecules. Therefore, selective inhibitors of Src are being developed for cancer therapy. In this study, we characterize the biological effects of the novel ATP-based Src family kinase inhibitor, AP23846, in tumor cells with high Src activity. As a lead compound, AP23846 is a potent c-Src kinase inhibitor (IC50 approximately 0.5 nmol/L in vitro, approximately 10-fold more potent than PP2, the most widely used commercially available Src family kinase inhibitor). At concentrations of 1 micromol/L, AP23846 led to complete Src inhibition for 48 hours in cells. No cytotoxicity was observed under these conditions, although proliferation rates were slower. Therefore, this was an excellent inhibitor to examine Src-regulated signaling pathways in tumor cells. AP23846 reduced cellular migration, vascular endothelial growth factor, and interleukin-8 in a dose-dependent fashion in pancreatic adenocarcinoma cells grown in vitro. Correspondingly, cell culture supernatants from L3.6pl pancreatic adenocarcinoma cells pretreated with AP23846 failed to promote migration of hepatic endothelial cells in vitro and failed to support angiogenesis into gel foams implanted s.c. in mice in vivo. These results suggest that Src inhibitors affect biological properties of tumor progression and may be useful as cancer therapeutic agents in more advanced disease.

Dasatinib (BMS-354825) tyrosine kinase inhibitor suppresses invasion and induces cell cycle arrest and apoptosis of head and neck squamous cell carcinoma and non-small cell lung cancer cells

PURPOSE

Epithelial tumors, including non-small cell lung cancer (NSCLC) and head and neck squamous cell carcinoma (HNSCC), present clinical challenges. One potential target for systemic therapy is Src family nonreceptor tyrosine kinases, which are overexpressed in these tumors and induce pleiotropic effects, including increased proliferation, enhanced survival, stimulation of angiogenesis, and changes in motility. Dasatinib (BMS-354825), an ATP-competitive, small molecule tyrosine kinase inhibitor, suppresses the activity of these kinases at subnanomolar concentrations. Therefore, we tested the antitumor effects of this inhibitor in vitro to determine whether in vivo analyses were warranted.

EXPERIMENTAL DESIGN

The antitumor effects of dasatinib on HNSCC and NSCLC cells were evaluated using assays to measure cell cycle progression, apoptosis, migration, and invasion. Western blotting was used to monitor its effects on cell signaling.

RESULTS

Dasatinib inhibited migration and invasion in all cell lines and induced cell cycle arrest (blocking the G1-S transition) and apoptosis in some lines. The effects on migration and invasion correlated with the inhibition of Src and downstream mediators of adhesion [e.g., focal adhesion kinase (FAK), p130, and paxillin], and the cell cycle effects and apoptosis correlated with the induction of p27 and the dephosphorylation of Rb. Dasatinib also induced morphologic changes that were consistent with an upstream role for Src in regulating focal adhesion complexes.

CONCLUSIONS

This study showed that Src inhibition in HNSCC and NSCLC has antitumor effects in vitro. This suggests that dasatinib would have therapeutic activity against these tumors. Clinical studies in these tumor types are warranted.

Anandamide inhibits adhesion and migration of breast cancer cells

The endocannabinoid system regulates cell proliferation in human breast cancer cells. We reasoned that stimulation of cannabinoid CB1 receptors could induce a non-invasive phenotype in breast metastatic cells. In a model of metastatic spreading in vivo, the metabolically stable anandamide analogue, 2-methyl-2'-F-anandamide (Met-F-AEA), significantly reduced the number and dimension of metastatic nodes, this effect being antagonized by the selective CB1 antagonist SR141716A. In MDA-MB-231 cells, a highly invasive human breast cancer cell line, and in TSA-E1 cells, a murine breast cancer cell line, Met-F-AEA inhibited adhesion and migration on type IV collagen in vitro without modifying integrin expression: both these effects were antagonized by SR141716A. In order to understand the molecular mechanism involved in these processes, we analyzed the phosphorylation of FAK and Src, two tyrosine kinases involved in migration and adhesion. In Met-F-AEA-treated cells, we observed a decreased tyrosine phosphorylation of both FAK and Src, this effect being attenuated by SR141716A. We propose that CB1 receptor agonists inhibit tumor cell invasion and metastasis by modulating FAK phosphorylation, and that CB1 receptor activation might represent a novel therapeutic strategy to slow down the growth of breast carcinoma and to inhibit its metastatic diffusion in vivo.

Upregulation and activation of PKC alpha by ErbB2 through Src promotes breast cancer cell invasion that can be blocked by combined treatment with PKC alpha and Src inhibitors

Although ErbB2 is known to enhance breast cancer metastasis, the signaling events responsible for this remain elusive. Alpha-isozyme of protein kinase C (PKCalpha), which is involved in cancer development and progression, has been suggested to be activated by ErbB2 without direct evidence. In addition, the roles of PKCalpha in ErbB2-mediated cancer cell malignancy have not been clearly identified. In this study, we investigated whether ErbB2 can activate PKCalpha and determined what role PKCalpha plays in ErbB2-mediated breast cancer cell invasion. We expressed wild-type and mutant ErbB2 with altered signaling capacities in MDA-MB-435 breast cancer cells and revealed that overexpression or activation of ErbB2 in MDA-MB-435 cells upregulated and activated PKCalpha and that downregulation of ErbB2 by small-interfering RNA decreased the expression and activity of PKCalpha in BT474 breast cancer cells. These in vitro results were supported by data from breast cancer patient samples. In 150 breast cancer tumor samples, ErbB2-overexpressing tumors showed significantly higher positive rates of PKCalpha membrane immunohistochemistry staining than that of ErbB2-low-expressing tumors. Mechanistically, we found that PKCalpha is co-immunoprecipitated with Src and PKCalpha expression and activity can be decreased by Src inhibitor PP2 and by the expression of a dominant-negative mutant of Src. Moreover, ErbB2-mediated upregulation of urokinase-type plasminogen activator receptor (uPAR) is reduced by either the PKCalpha inhibitor Go6976 or the Src inhibitor PP2, and the combination of Go6976 with PP2 is superior to either agent alone in suppressing uPAR expression and cell invasion. These results demonstrate that PKCalpha is critical for ErbB2-mediated cancer cell invasion and provide valuable insights for current and future PKCalpha and Src inhibitor clinical trials.

Bcr interferes with beta-catenin-Tcf1 interaction

The beta-catenin/Tcf complex is a downstream effector of the Wnt signalling pathway. It is a transcription complex, which activates gene expression and contributes to proliferation and tumor progression. Tcf1 in complex with beta-catenin is able to activate beta-catenin-dependent gene expression. We demonstrate that expressed Bcr is able to bind the transcription factor Tcf1 to disrupt the Tcf1/beta-catenin complex. Phosphorylation of Bcr by the tyrosine kinase pp60(src) can lead to dissociation of the transcriptionally inactive Bcr/Tcf1 complex. Thus two independent mechanisms may regulate Tcf/beta-catenin-mediated transcription via Bcr: binding to beta-catenin as we have previously shown and to Tcf1 as shown here.

Role of c-Src and focal adhesion kinase in progression and metastasis of estrogen receptor-positive breast cancer

The non-receptor tyrosine kinases c-Src and focal adhesion kinase (Fak) mediate signal transduction pathways that regulate cell proliferation, survival, invasion, and metastasis. Here, we investigated whether c-Src and Fak are activated during progression of hormone-dependent breast cancer. Maximally active c-Src was overexpressed in a subset of tamoxifen-resistant variants and in metastases of recurrent hormone-treated breast cancer. Active Fak was also frequently observed in these tumors. We also show that estrogen receptor (ER) can bind to Fak and that estrogen can modulate Fak autophosphorylation supporting a cross-talk between these two pathways. Inhibition of c-Src activity blocked proliferation of all tamoxifen-resistant variants, suggesting that inhibitors of c-Src-Fak activity may delay or prevent progression and metastasis of ER-positive tumors. These studies also raise the possibility that fully active forms of c-Src and Fak in breast tumors may be biomarkers to predict tamoxifen resistance and/or risk of recurrence in ER-positive breast cancer.

Src oncogene activates MMP-2 expression via the ERK/Sp1 pathway

Previous studies demonstrated that activation of Src oncogene increased matrix metalloproteinase-2 (MMP-2) expression in various types of cells. In this study, we elucidated the underlying mechanism of Src-induced MMP-2. We first used murine fibroblast cell line C3H10T1/2 and its v-Src transfectant IV5 to address this issue. RT-PCR and promoter activity assay indicated that Src stimulated MMP-2 via transcriptional activation. Transfection of constitutively active Src into C3H10T1/2 cells also stimulated MMP-2 mRNA expression. Deletion and mutation analysis indicated that the Sp1 site located within the -91/-84 region of human MMP-2 promoter is the major responsive element for Src. Electrophoresis mobility shift assays showed that Src enhanced the binding of Sp1 to this consensus site to stimulate MMP-2 gene expression. We next investigated the signaling pathway that mediated the effect of Src on MMP-2. Our data showed that extracellular signal-regulated kinase (ERK) pathway inhibitor PD98059, but not the c-Jun N-terminal kinase (JNK) inhibitor SP600125, p38 kinase inhibitor SB203580, and PI3K inhibitor wortmannin, attenuated Src-induced MMP-2 promoter activity. These inhibitors did not show significant effect on basal MMP-2 promoter activity in C3H10T1/2 cells. In addition, the dominant negative mutant of ERK-2 suppressed the activation of MMP-2 by Src. Treatment of PD98059 or an Src specific inhibitor PP1 reduced Sp1 DNA binding activity in IV5 cells. Taken together, our results strongly suggest that Src induces MMP-2 expression via transcription activation and the ERK/Sp1 signaling pathway is involved in this process.

Involvement of Src family kinases in N-cadherin phosphorylation and beta-catenin dissociation during transendothelial migration of melanoma cells

N-cadherin is recruited to the heterotypic contact during transendothelial migration of melanoma cells in a coculture system with tumor cells seeded on top of a monolayer of endothelial cells. However, beta-catenin dissociates from N-cadherin and redistributes to the nucleus of transmigrating melanoma cells to activate gene transcription. In this report, we demonstrate that Src becomes activated at the heterotypic contact between the transmigrating melanoma cell and neighboring endothelial cells. Src activation shows close temporal correlation with tyrosine phosphorylation of N-cadherin. Expression of a dominant-negative Src in melanoma cells blocks N-cadherin phosphorylation, beta-catenin dissociation, and nuclear translocation in transmigrating cells, consistent with the involvement of Src family kinases. In in vitro binding assays, Src-mediated phosphorylation of the N-cadherin cytoplasmic domain results in a significant reduction in beta-catenin binding. Although five phospho-tyrosine residues can be identified on the N-cadherin cytoplasmic domain by mass spectrometry, site-specific mutagenesis indicates that Tyr-860 is the critical amino acid involved in beta-catenin binding. Overexpression of N-cadherin carrying the Y860F mutation inhibits the transmigration of transfected cells across the endothelium. Together, the data suggest a novel role for tyrosine phosphorylation of N-cadherin by Src family kinases in the regulation of beta-catenin association during transendothelial migration of melanoma cells.

Action of the Src family kinase inhibitor, dasatinib (BMS-354825), on human prostate cancer cells

Src family kinases (SFK) are currently being investigated as targets for treatment strategies in various cancers. The novel SFK/Abl inhibitor, dasatinib (BMS-354825), is a promising therapeutic agent with oral bioavailability. Dasatinib has been shown to inhibit growth of Bcr-Abl-dependent chronic myeloid leukemia xenografts in nude mice. Dasatinib also has been shown to have activity against cultured human prostate and breast cancer cells. However, the molecular mechanism by which dasatinib acts on epithelial tumor cells remains unknown. In this study, we show that dasatinib blocks the kinase activities of the SFKs, Lyn, and Src, in human prostate cancer cells at low nanomolar concentrations. Moreover, focal adhesion kinase and Crk-associated substrate (p130(CAS)) signaling downstream of SFKs are also inhibited at similar concentrations of dasatinib. Consistent with inhibition of these signaling pathways, dasatinib suppresses cell adhesion, migration, and invasion of prostate cancer cells at low nanomolar concentrations. Therefore, dasatinib has potential as a therapeutic agent for metastatic prostate cancers harboring activated SFK and focal adhesion kinase signaling.

Molecular requirements for epithelial-mesenchymal transition during tumor progression

Epithelial-mesenchymal transitions (EMTs) occur as key steps during embryonic morphogenesis, and are now implicated in the progression of primary tumors towards metastases. Recent advances have fostered a more detailed understanding of molecular mechanisms and networks governing EMT in tumor progression. Besides TGFbeta and RTK/Ras signaling, autocrine factors and Wnt-, Notch-, Hedgehog- and NF-kappaB-dependent pathways were found to contribute to EMT. Repression of E-cadherin by transcriptional regulators such as Snail or Twist emerges as one critical step driving EMT, and this stage is currently being molecularly linked with many of the new players. Increasing evidence suggests that EMT plays a specific role in the migration of cells from a primary tumor into the circulation and may provide a rationale for developing more effective cancer therapies.

Elevated Src activity promotes cellular invasion and motility in tamoxifen resistant breast cancer cells

Src kinase plays a central role in growth factor signalling, regulating a diverse array of cellular functions including proliferation, migration and invasion. Recent studies have demonstrated that Src activity is frequently elevated in human tumours and correlates with disease stage. We have previously demonstrated that, upon acquisition of tamoxifen resistance, MCF7 cells display increased epidermal growth factor receptor (EGFR) activation and a more aggressive phenotype in vitro. Since tumours exhibiting elevated EGFR signalling may possess elevated levels of Src activity, we wished to investigate the role of Src in our MCF7 model of endocrine resistance. Src kinase activity was significantly elevated in tamoxifen-resistant (TamR) cells in comparison to wild type MCF7 cells. This increase was not due to elevated Src protein or gene expression. Treatment of TamR cells with the novel Src inhibitor, AZD0530, significantly reduced the amount of activated Src detectable in both cell types whilst having no effect on total Src levels. AZD0530 significantly suppressed the motile and invasive nature of TamR cells in vitro, reduced basal levels of activated focal adhesion kinase (FAK) and paxillin and promoted elongation of focal adhesions. Furthermore, the use of this compound in conjunction with the EGFR inhibitor, gefitinib, was markedly additive towards inhibition of TamR cell motility and invasion. These observations suggest that Src plays a pivotal role in mediating the motile and invasive phenotype observed in endocrine-resistant breast cancer cells. The use of Src inhibitors in conjunction with EGFR inhibitors such as gefitinib may provide an effective method with which to prevent cancer progression and metastasis.

Malignant transformation of an epithelial cell by v-Src via tv-a-mediated retroviral infection: A new cell model for studying carcinogenesis

Most human cancers are of epithelial origin, but many cell culture models for the study of cancer-causing genes use fibroblasts. In addition, efficient delivery and stable expression of foreign genes into non-transformed cell lines are often difficult. To address both questions, we here established a non-transformed rat kidney epithelial RK3E cell line that constitutively expresses tv-a (receptor for subgroup A avian leukosis virus, ALV) for delivery of foreign genes via avian retroviral infection. This cell line (RK3E/tv-a) allows efficient and stable expression of either single or multiple foreign genes. Furthermore, tv-a-mediated delivery of various oncogenes (v-src, H-ras, myc or akt) leads to malignant transformation. v-src-transformed cells exhibited classical cancerous phenotypes in vitro, and induced tumor formation and lung metastasis upon injecting into immunodeficient mice. Expression profiles of downstream molecular effectors (E-cadherin, beta-catenin, cyclin D1, Myc, VEGF, MMP-2, and MMP-9) in these cells correlate with characteristics of cancerous phenotypes. This new cell model serves as a useful tool to study cancer-causing genes in epithelial cell type.

Crystal structures of active SRC kinase domain complexes

c-Src was the first proto-oncoprotein to be identified, and has become the focus of many drug discovery programs. Src structures of a major inactive form have shown how the protein kinase is rigidified by several interdomain interactions; active configurations of Src are generated by release from this "assembled" or "bundled" form. Despite the importance of Src as a drug target, there is relatively little structural information available regarding the presumably more flexible active forms. Here we report three crystal structures of a dimeric active c-Src kinase domain, in an apo and two ligand complexed forms, with resolutions ranging from 2.9A to 1.95A. The structures show how the kinase domain, in the absence of the rigidifying interdomain interactions of the inactivation state, adopts a more open and flexible conformation. The ATP site inhibitor CGP77675 binds to the protein kinase with canonical hinge hydrogen bonds and also to the c-Src specific threonine 340. In contrast to purvalanol B binding in CDK2, purvalanol A binds in c-Src with a conformational change in a more open ATP pocket.

Inhibition of Src kinase activity by 7-[(2,4-dichloro-5-methoxyphenyl)amino]-2-heteroaryl-thieno[3,2-b]pyridine-6-carbonitriles

7-[(2,4-Dichloro-5-methoxyphenyl)amino]thieno[3,2-b]pyridine-6-carbonitriles with various heteroaryl groups at C-2 are inhibitors of Src kinase activity. Of these new analogs, compounds substituted at C-2 by a 3,5-furan or a 2,5-pyridine had the best activity in the Src enzyme and cell assays.

Crk-associated substrate tyrosine phosphorylation sites are critical for invasion and metastasis of SRC-transformed cells

Crk-associated substrate (CAS, p130Cas) is a major tyrosine phosphorylated protein in cells transformed by v-crk and v-src oncogenes. We recently reported that reexpression of CAS in CAS-deficient mouse embryo fibroblasts transformed by oncogenic Src promoted an invasive phenotype associated with enhanced cell migration through Matrigel, organization of actin into large podosome ring and belt structures, activation of matrix metalloproteinase-2, and elevated tyrosine phosphorylation of the focal adhesion proteins FAK and paxillin. We have now extended these studies to examine the mechanism by which CAS achieves these changes and to evaluate the potential role for CAS in promoting in vivo tumor growth and metastasis. Whereas the presence or absence of CAS did not alter the primary growth of subcutaneous-injected Src-transformed mouse embryo fibroblasts, CAS expression was required to promote lung metastasis following removal of the primary tumor. The substrate domain YxxP tyrosines, the major sites of CAS phosphorylation by Src that mediate interactions with Crk, were found to be critical for promoting both invasive and metastatic properties of the cells. The ability of CAS to promote Matrigel invasion, formation of large podosome structures, and tyrosine phosphorylation of Src substrates, including FAK, paxillin, and cortactin, was also strictly dependent on the YxxP tyrosines. In contrast, matrix metalloproteinase-2 activation was most dependent on the CAS SH3 domain, whereas the substrate domain YxxP sites also contributed to this property. Thus multiple CAS-mediated signaling events are implicated in promoting invasive and metastatic properties of Src-transformed cells.

Physical and Functional Association of c-Src and Adhesion and Degranulation Promoting Adaptor Protein (ADAP) in Osteoclastogenesis in Vitro

c-Src plays a crucial role in osteoclastogenesis. In this study, we searched for c-Src-binding proteins using a combination of pull-down assays and mass spectrometric analysis, and identified the association of adhesion and degranulation promoting adaptor protein (ADAP) with c-Src in RAW264 cells and osteoclast precursors prepared from bone marrow cells. The kinase activity and the SH2 domain of c-Src were required for this association and Tyr807 in the extreme carboxyl terminus of ADAP was identified as a major recognition site. ADAP was found to be expressed in cells at the prefusion stage and localized mainly in the leading edge of lamellipodia and in pseudopodia. Tyrosine phosphorylation of ADAP was induced in an integrin-dependent manner, and the level was Src kinase-dependent. ADAP-knockdown RAW264 cells showed retarded migration and formed few multinucleated cells. Cas, known to be phosphorylated by c-Src, was identified as a major tyrosine-phosphorylated protein in differentiating RAW264 cells and the phosphorylation appeared to be decreased in ADAP-knockdown cells. ADAP thus may play an important role as a partner of c-Src for cell migration and progression to the multinucleated cell stage in osteoclastogenesis in vitro.

Tyrosine phosphorylation of paxillin affects the metastatic potential of human osteosarcoma

To acquire information on signal alteration corresponding to the changes in metastatic potential, we analysed protein tyrosine phosphorylation of low- and high-metastatic human osteosarcoma HuO9 sublines, which were recently established as the first metastatic model of human osteosarcoma. Tyrosine phosphorylation of proteins around 60, 70, and 120-130 kDa was enhanced in high-metastatic sublines. Among these proteins, the protein around 70 kDa, which was most remarkably phosphorylated, was identified as paxillin, a scaffold protein in integrin signaling. Activity of Src family kinase correlated well with metastatic potential, and a Src family kinase inhibitor, PP2, not only abolished tyrosine phosphorylation of paxillin but also impaired the motility of high-metastatic sublines. The expression of paxillin was also elevated in high-metastatic sublines, and knocking down of paxillin expression by RNAi method resulted in attenuated motility of high-metastatic cells. We also demonstrated that the phosphorylated form of paxillin is essential for the migration-promoting effect in human osteosarcoma. These findings suggest that enhanced activity of Src family kinases and overexpression of paxillin synergistically contribute to the high metastatic potential of human osteosarcoma through the hyperphosphorylation of paxillin.

Signal transduction through substance P receptor in human glioblastoma cells: roles for Src and PKCdelta

Substance P receptor (SPR), a G protein-coupled receptor (GPCR), is found in human glioblastomas, and has been implicated in their growth. Consistent with a role for SPR in cell growth, activation of SPR in U373 MG human glioblastoma cells leads to the phosphorylation of mitogen-activated protein kinases [extracellular signal-regulated kinase 1 and 2 (ERK1/2)] and stimulation of cell proliferation. The purpose of the present study was to elucidate the pathway through which these actions occur. Using either the epidermal growth factor receptor (EGFR) kinase inhibitor, AG 1478, or a small-interfering RNA (siRNA) directed against human EGFR, we found that transactivation of EGFR by SPR is only marginally involved in SP-dependent ERK1/2 phosphorylation. Src, however, is shown to be a major component of SPR signaling because the Src kinase inhibitor, PP2, and a kinase-dead Src mutant both inhibit SP-dependent ERK1/2 phosphorylation. We also report that SPR stimulates the phosphorylation of protein kinase Cdelta(PKCdelta), and that this stimulation is blocked by PP2. SP-dependent ERK1/2 phosphorylation is also blocked by rottlerin, a PKCdelta inhibitor, and the calcium scavenger, BAPTA/AM. Finally, rottlerin and PP2 were both found to inhibit the growth of several glioblastoma cell lines, underscoring the potential of these agents to block glioblastoma growth.

Bone morphogenetic protein receptor type II C-terminus interacts with c-Src: implication for a role in pulmonary arterial hypertension

Mutations of bone morphogenetic protein receptor type II (BMPR-II) have been associated with familial and idiopathic pulmonary arterial hypertension (PAH). BMPR-II is a member of the transforming growth factor-beta receptor superfamily. It consists of extracellular, transmembrane, and kinase domains, and a unique C-terminus with mostly unknown function. However, a number of PAH-causing mutations are predicted to truncate the C-terminus, suggesting that this domain plays an important role in the homeostasis of pulmonary vessels. In this study, we sought to elucidate the functional role of this C-terminus by seeking its interacting partners. Using yeast two-hybrid screening, we identified c-Src tyrosine kinase as a binding partner of this C-terminus. In vitro co-immunoprecipitation confirmed their interaction. Mutations truncating the C-terminus disrupted their interaction, while missense mutation within kinase domain reduced their interaction. In addition, BMPR-II and c-Src tyrosine kinase colocalized within intracellular aggregates when overexpressed in HEK293 cells. Moreover, mutations truncating the C-terminus disrupted their colocalization, whereas missense mutation within kinase domain had no effect on their colocalization. Furthermore, BMP ligand stimulation decreased c-Src-activating phosphorylation at Tyrosine 418 in pulmonary smooth muscle cells in both time- and concentration-dependent manners. Mutations that truncated the C-terminus abolished this response. Taken together, these results suggest a model in which proliferative effect of c-Src by vasoactive molecules is balanced by opposing effect of BMP signaling in basal state, and the loss of this balance due to BMPR2 mutations leads to increased c-Src activity and subsequently cell growth.

c-Src: bridging the gap between phosphorylation- and acidification-induced gap junction channel closure

Gap junctions are a unique type of intercellular junction that mediate the direct exchange of small molecules between neighboring cells and play critical roles in the normal function of numerous organs. Mutations in the connexin proteins that make up gap junctions have been implicated in numerous human skin and neurosensory disorders. The ability of gap junctions to transmit molecules between cells is regulated by intracellular pH, the phosphorylation state of connexin, and the interaction of connexin with other cellular proteins. This Perspective focuses on the novel and complex events initiated by intracellular acidification resulting from tissue ischemia or hypoxia that lead to the interruption of intercellular communication between astrocytes. These events include alterations in connexin43 (Cx43) phosphorylation, disruption of beta-actin binding to Cx43, and the induced interaction of Cx43 with the c-Src tyrosine kinase, extracellular signal-regulated kinase 1 and 2, and mitogen-activated protein kinase phosphatase 1.

Down-regulation of the tumor suppressor gene C-terminal Src kinase: An early event during premalignant colonic epithelial hyperproliferation

Hyperproliferation of the premalignant epithelium is critical for colonic carcinogenesis; however the mechanisms remain largely unexplored. We report herein that prior to occurrence of neoplastic lesions in the azoxymethane-rat model of colon carcinogenesis; the tumor suppressor gene C-terminal Src kinase (Csk) was down-regulated with a concomitant increase in Src activity. Furthermore, pharmacological or genetic (RNA interference) inhibition of Csk resulted in increased proliferation in colon cancer cell lines through the mitogen-activated protein kinase dependent pathway. Thus, we demonstrate, for the first time, that Csk suppression is an important early event in colorectal cancer pathogenesis.

Differentiation of normal and cancer cells induced by sulfhydryl reduction: biochemical and molecular mechanisms

We examined the morphological, biochemical and molecular outcome of a nonspecific sulfhydryl reduction in cells, obtained by supplementation of N-acetyl-L-cysteine (NAC) in a 0.1-10 mM concentration range. In human normal primary keratinocytes and in colon and ovary carcinoma cells we obtained evidences for: (i) a dose-dependent inhibition of proliferation without toxicity or apoptosis; (ii) a transition from a proliferative mesenchymal morphology to cell-specific differentiated structures; (iii) a noticeable increase in cell-cell and cell-substratum junctions; (iv) a relocation of the oncogenic beta-catenin at the cell-cell junctions; (v) inhibition of microtubules aggregation; (vi) upregulation of differentiation-related genes including p53, heat shock protein 27 gene, N-myc downstream-regulated gene 1, E-cadherin, and downregulation of cyclooxygenase-2; (vii) inhibition of c-Src tyrosine kinase. In conclusion, a thiol reduction devoid of toxicity as that operated by NAC apparently leads to terminal differentiation of normal and cancer cells through a pleiade of converging mechanisms, many of which are targets of the recently developed differentiation therapy.

Tyrosine phosphorylation in mitochondria: a new frontier in mitochondrial signaling

Mitochondria are multifunctional organelles that participate in a range of cellular processes such as energy production, proliferation, death, and senescence. The involvement of mitochondria in such distinct aspects of cell life requires the existence of an integrated system of signals that enter and exit the organelle according to the diverse needs of the cell. The recent discovery of several protein kinases and phosphatases that localize partially or predominantly inside mitochondria opens new perspectives into the regulation of these signals. This review focuses on tyrosine phosphorylation in mitochondria. A description of the protein tyrosine kinases and phosphatases which regulate this process along with the mitochondrial tyrosine-phosphorylated proteins identified to date is followed by a discussion of the possible involvement of tyrosine phosphorylation in mitochondrial signaling and future perspectives for developments in this emerging field.

Src-mediated Tyrosine Phosphorylation of p47 in Hyperoxia-induced Activation of NADPH Oxidase and Generation of Reactive Oxygen Species in Lung Endothelial Cells

Superoxide (O(2)(-)) production by nonphagocytes, similar to phagocytes, is by activation of the NADPH oxidase multicomponent system. Although activation of neutrophil NADPH oxidase involves extensive serine phosphorylation of p47(phox), the role of tyrosine phosphorylation of p47(phox) in NADPH oxidase-dependent O(2)(-) production is unclear. We have shown recently that hyperoxia-induced NADPH oxidase activation in human pulmonary artery endothelial cells (HPAECs) is regulated by mitogen-activated protein kinase signal transduction. Here we provided evidence on the role of nonreceptor tyrosine kinase, Src, in hyperoxia-induced tyrosine phosphorylation of p47(phox) and NADPH oxidase activation in HPAECs. Exposure of HPAECs to hyperoxia for 1 h resulted in increased O(2)(-) and reactive oxygen species (ROS) production and enhanced tyrosine phosphorylation of Src as determined by Western blotting with phospho-Src antibodies. Pretreatment of HPAECs with the Src kinase inhibitor PP2 (1 mum) or transient expression of a dominant-negative mutant of Src attenuated hyperoxia-induced tyrosine phosphorylation of Src and ROS production. Furthermore, exposure of cells to hyperoxia enhanced tyrosine phosphorylation of p47(phox) and its translocation to cell peripheries that were attenuated by PP2. In vitro, Src phosphorylated recombinant p47(phox) in a time-dependent manner. Src immunoprecipitates of cell lysates from control cells revealed the presence of immunodetectable p47(phox) and p67(phox), suggesting the association of oxidase components with Src under basal conditions. Moreover, exposure of HPAECs to hyperoxia for 1 h enhanced the association of p47(phox), but not p67(phox), with Src. These results indicated that Src-dependent tyrosine phosphorylation of p47(phox) regulates hyperoxia-induced NADPH oxidase activation and ROS production in HPAECs.

c-src activating mutation analysis in Chinese patients with colorectal cancer

AIM: To investigate the occurrence of cellular src (c-src) activating mutation at codon 531 in colorectal cancer patients from Chinese mainland.

METHODS: Polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) assay followed by sequencing and single-strand conformation polymor-phism analysis were carried out to screen 110 samples of primary colorectal cancer and 20 colorectal liver metastases.

RESULTS: Only one sample showed PCR-RFLP-positive results and carried somatic codon 531 mutations. No additional mutation of c-src exon 12 was found.

CONCLUSION: c-src codon 531 mutation in colorectal cancer is not the cause of c-src activation.

ErbB2 promotes Src synthesis and stability: novel mechanisms of Src activation that confer breast cancer metastasis

Activation of Src kinase plays important roles in the development of many neoplasias. Most of the previous Src studies focused on the deregulation of Src kinase activity. The deregulated Src protein synthesis and stability in mediating malignant phenotypes of cancer cells, however, have been neglected. While investigating the signal transduction pathways contributing to ErbB2-mediated metastasis, we found that ErbB2-activated breast cancer cells that had higher metastatic potentials also had increased Src activity compared with ErbB2 low-expressing cells. The increased Src activity in ErbB2-activated cells paralleled higher Src protein levels, whereas Src RNA levels were not significantly altered. Our studies revealed two novel mechanisms that are involved in Src protein up-regulation and activation by ErbB2: (a) ErbB2 increased Src translation through activation of the Akt/mammalian target of rapamycin/4E-BP1 pathway and (b) ErbB2 increased Src stability most likely through the inhibition of the calpain protease. Furthermore, inhibition of Src activity by a Src-specific inhibitor, PP2, or a Src dominant-negative mutant dramatically reduced ErbB2-mediated cancer cell invasion in vitro and metastasis in an experimental metastasis animal model. Together, activation of ErbB2 and downstream signaling pathways can lead to increased Src protein synthesis and decreased Src protein degradation resulting in Src up-regulation and activation, which play critical roles in ErbB2-mediated breast cancer invasion and metastasis.

Iterative microarray and RNA interference-based interrogation of the SRC-induced invasive phenotype

Src kinase has long been recognized as a factor in the progression of colorectal cancer and seems to play a specific role in the development of the metastatic phenotype. In spite of numerous studies conducted to elucidate the exact role of Src in cancer progression, downstream targets of Src remain poorly understood. Gene expression profiling has permitted the identification of large sets of genes that may be functionally interrelated but it is often unclear as to which molecular pathways they belong. Here we have developed an iterative approach to experimentally reconstruct a network of gene activity regulated by Src and contributing to the invasive phenotype. Our strategy uses a combination of phenotypic anchoring of gene expression profiles and loss-of-function screening by way of RNA-mediated interference. Using a panel of human colon cancer cell lines exhibiting differential Src-specific activity and invasivity, we identify the first two levels of gene transcription responsible for the invasive phenotype, where first-tier genes are controlled by Src activity and the second-tier genes are under the influence of the first tier. Specifically, perturbation of first-tier gene activity by either pharmacologic inhibition of Src or RNA-mediated interference-directed knockdown leads to a loss of invasivity and decline of second-tier gene activity. The targeting of first-tier genes may be bypassed altogether because knockdown of second-tier genes led to a similar loss of invasive potential. In this manner, numerous members of a "transcriptional cascade" pathway for metastatic activity have been identified and functionally validated.

Indirubin derivatives inhibit Stat3 signaling and induce apoptosis in human cancer cells

Stat3 protein has an important role in oncogenesis and is a promising anticancer target. Indirubin, the active component of a traditional Chinese herbal medicine, has been shown previously to inhibit cyclin-dependent kinases, resulting in cell cycle arrest. Here, we show that the indirubin derivatives E564, E728, and E804 potently block constitutive Stat3 signaling in human breast and prostate cancer cells. In addition, E804 directly inhibits Src kinase activity (IC(50) = 0.43 microM) in an in vitro kinase assay. Levels of tyrosyl phosphorylation of c-Src are also reduced in cultured cells 30 min after E804 treatment. Tyrosyl phosphorylation of Stat3, which is known to be phosphorylated by c-Src, was decreased, and constitutive Stat3 DNA binding-activity was suppressed in cells 30 min after E804 treatment. The antiapoptotic proteins Mcl-1 and Survivin, which are encoded in target genes of Stat3, were down-regulated by indirubin derivatives, followed by induction of apoptosis. These results demonstrate that E804 directly blocks the Src-Stat3 signaling pathway, suggesting that the antitumor activity of indirubin compounds is at least partially due to inhibition of this pathway.

SRC family kinases in cell volume regulation

SRC family kinases are a group of nine cytoplasmic protein tyrosine kinases essential for many cell functions. Some appear to be ubiquitously expressed, whereas others are highly tissue specific. The ability of members of the SRC family to influence ion transport has been recognized for several years. Mounting evidence suggests a broad role for SRC family kinases in the cell response to both hypertonic and hypotonic stress, and in the ensuing regulatory volume increase or decrease. In addition, members of this tyrosine kinase family participate in the mechanotransduction that accompanies cell membrane deformation. Finally, at least one SRC family member operates in concert with the p38 MAPK to regulate tonicity-dependent gene transcription.

Distinct and opposite roles for SH2 and SH3 domains of v-src in embryo survival and hemangiosarcoma formation

The cellular proto-oncogene c-src is thought to be involved in formation, progression, and metastasis of a variety of tumor cell types, although its exact role during tumor cell genesis is not well defined. v-src, the viral oncogene counterpart of c-src, causes metastatic sarcomas, hemorrhagic disease, and hemangiosarcomas in chicken embryos and, thus, can be used as a constitutively activated form of src for experimentally-induced tumorigenesis. Here, we used retroviral vectors to express wild-type v-src or SH2 or SH3 domain-deleted forms (DeltaSH2 or DeltaSH3) to determine if different pathogenic effects resulted. Vectors were injected into early chick embryo midbrain ventricles and embryos were sacrificed at various ages up to embryonic day (E) 18. Retroviral expression of all forms of v-src resulted in transformation of pial connective tissue cells into large, rounded abnormal-appearing cells. Surprisingly, all forms of v-src were lethal. The v-src retrovirus was lethal and killed most embryos by E15 with the development of hemangiosarcomas over the injection site between E10-E12. The DeltaSH3 retrovirus was the most deadly, killing most embryos by E12, however, it never resulted in hemangiosarcoma formation. The DeltaSH2 retrovirus injected embryos survived longer than v-src or DeltaSH3 embryos, and some of these embryos also developed large hemangiosarcomas over the injection site between E13 and E18. These results demonstrate that the src SH2 domain is required to be fully lethal, whereas the presence of the SH3 domain attenuated lethality. Furthermore, the formation of hemangiosarcomas absolutely required the presence of the src SH3 domain and to some extent required the SH2 domain. This implicates distinct and opposite roles for SH2 and SH3 domains of src and their cellular binding partners in tumorigenesis and hemorrhagic disease.

Further studies on ethenyl and ethynyl-4-phenylamino-3-quinolinecarbonitriles: identification of a subnanomolar Src kinase inhibitor

Several new ethynyl- and ethenyl-4-phenylamino-3-quinolinecarbonitriles were synthesized and tested for Src inhibition. Derivatives bearing an ethenyl or ethynyl substituent at C-6 showed decreased Src inhibitory activity. Incorporation of an ethenylpyridine N-oxide group at C-7 provided 20b, a 0.6 nM inhibitor of Src enzymatic activity with excellent cellular potency.

Selective regulation of tumor necrosis factor-induced Erk signaling by Src family kinases and the T cell protein tyrosine phosphatase

The proinflammatory cytokine tumor necrosis factor (TNF) modulates cellular responses through the mitogen-activated protein kinase (MAPK) and nuclear factor-kappaB (NF-kappaB) signaling pathways, but the molecular mechanisms underlying MAPK activation are unknown. T cell protein tyrosine phosphatase (TCPTP) is essential for hematopoietic development and negatively regulates inflammatory responses. Using TCPTP-deficient fibroblasts, we show here that TCPTP regulates TNF-induced MAPK but not NF-kappaB signaling. TCPTP interacted with the adaptor protein TRAF2, and dephosphorylated and inactivated Src tyrosine kinases to suppress downstream signaling through extracellular signal-regulated kinases and production of interleukin 6. These results link TCPTP and Src tyrosine kinases to the selective regulation of TNF-induced MAPK signaling and identify a previously unknown mechanism for modulating inflammatory responses mediated by TNF.

Intracellular signalling and cancer: complex pathways lead to multiple targets

Normal cells proliferate, die and differentiate as and when they should for the proper functioning of any particular tissue type. These processes are governed by a complex series of intracellular pathways that have many internal checkpoints and safety nets. These ensuring a fine, but tight, balance on overall tissue growth and distribution. A series of key aberrations, resulting in the disruption of these intracellular pathways, can lead to the development of a malignancy. The nature of these alterations is often not only tumour-specific, but also different between individuals with the same tumour type. As a result, these pathways have to be carefully dissected and functionally assessed to identify valid targets with therapeutic potential in a wide range of tumour types.

The road to Src

More than a quarter of a century has elapsed since the identification of the c-src proto-oncogene. During that period, we have learned that cancer arises as the result of mutations in proto-oncogenes and tumor suppressor genes, and we are now seeing the first fruits of these discoveries, in the form of targeted therapies directed against activated tyrosine kinases such as Bcr-Abl, c-Kit and the EGF receptor. But the discovery of the c-src proto-oncogene was in turn based on decades of study on an avian RNA tumor virus, Rous sarcoma virus (RSV). Here I review the work that led up to the identification of the RSV transforming gene and its protein product, and how this information in turn led to the discovery of cellular Src.

Src Regulates Actin Dynamics and Invasion of Malignant Glial Cells in Three Dimensions

Malignant glioma is the major brain tumor in adults and has a poor prognosis. The failure to control invasive cell subpopulations may be the key reason for local glioma recurrence after radical tumor resection and may contribute substantially to the failure of the other treatment modalities such as radiation therapy and chemotherapy. As a model for this invasion, we have implanted spheroids from a human glioma cell line (U251) in three-dimensional collagen type I matrices, which these cells readily invade. We first observed that the Src family kinase-specific pharmacologic inhibitors PP2 and SU6656 significantly inhibited the invasion of the cells in this assay. We confirmed this result by showing that expression of two inhibitors of Src family function, dominant-negative-Src and CSK, also suppressed glioma cell invasion. To characterize this effect at the level of the cytoskeleton, we used fluorescent time-lapse microscopy on U251 cells stably expressing a YFP-actin construct and observed a rapid change in actin dynamics following addition of PP2 in both two-dimensional and three-dimensional cultures. In monolayer cultures, PP2 caused the disappearance of peripheral membrane ruffles within minutes. In three-dimensional cultures, PP2 induced the loss of actin bursting at the leading tip of the invadopodium. The inhibition of Src family activity is thus a potential therapeutic approach to treat highly invasive malignant glioma.