The mechanism of cell cycle regulation by v-Src

CircHAS2 activates CCNE2 to promote cell proliferation and sensitizes the response of colorectal cancer to anlotinib

BACKGROUND

Tyrosine kinase inhibitors (TKIs) are crucial in the targeted treatment of advanced colorectal cancer (CRC). Anlotinib, a multi-target TKI, has previously been demonstrated to offer therapeutic benefits in previous studies. Circular RNAs (circRNAs) have been implicated in CRC progression and their unique structural stability serves as promising biomarkers. The detailed molecular mechanisms and specific biomarkers related to circRNAs in the era of targeted therapies, however, remain obscure.

METHODS

The whole transcriptome RNA sequencing and function experiments were conducted to identify candidate anlotinib-regulated circRNAs, whose mechanism was confirmed by molecular biology experiments. CircHAS2 was profiled in a library of patient-derived CRC organoids (n = 22) and patient-derived CRC tumors in mice. Furthermore, a prospective phase II clinical study of 14 advanced CRC patients with anlotinib-based therapy was commenced to verify drug sensitivity (ClinicalTrials.gov identifier: NCT05262335).

RESULTS

Anlotinib inhibits tumor growth in vitro and in vivo by downregulating circHAS2. CircHAS2 modulates CCNE2 activation by acting as a sponge for miR-1244, and binding to USP10 to facilitate p53 nuclear export as well as degradation. In parallel, circHAS2 serves as a potent biomarker predictive of anlotinib sensitivity, both in patient-derived organoids and xenograft models. Moreover, the efficacy of anlotinib inclusion into the treatment regimen yields meaningful clinical responses in patients with high levels of circHAS2. Our findings offer a promising targeted strategy for approximately 52.9% of advanced CRC patients who have high circHAS2 levels.

CONCLUSIONS

CircHAS2 promotes cell proliferation via the miR-1244/CCNE2 and USP10/p53/CCNE2 bidirectional axes. Patient-derived organoids and xenograft models are employed to validate the sensitivity to anlotinib. Furthermore, our preliminary Phase II clinical study, involving advanced CRC patients treated with anlotinib, confirmed circHAS2 as a potential sensitivity marker.

V-Src delocalizes Aurora B by suppressing Aurora B kinase activity during monopolar cytokinesis

c-Src tyrosine kinase plays roles in a wide range of signaling events and its increased activity is frequently observed in a variety of epithelial and non-epithelial cancers. v-Src, an oncogene first identified in the Rous sarcoma virus, is an oncogenic version of c-Src and has constitutively active tyrosine kinase activity. We previously showed that v-Src induces Aurora B delocalization, resulting in cytokinesis failure and binucleated cell formation. In the present study, we explored the mechanism underlying v-Src-induced Aurora B delocalization. Treatment with the Eg5 inhibitor (+)-S-trityl-L-cysteine (STLC) arrested cells in a prometaphase-like state with a monopolar spindle; upon further inhibition of cyclin-dependent kinase (CDK1) by RO-3306, cells underwent monopolar cytokinesis with bleb-like protrusions. Aurora B was localized to the protruding furrow region or the polarized plasma membrane 30 min after RO-3306 addition, whereas inducible v-Src expression caused Aurora B delocalization in cells undergoing monopolar cytokinesis. Delocalization was similarly observed in monopolar cytokinesis induced by inhibiting Mps1, instead of CDK1, in the STLC-arrested mitotic cells. Importantly, western blotting analysis and in vitro kinase assay revealed that v-Src decreased the levels of Aurora B autophosphorylation and its kinase activity. Furthermore, like v-Src, treatment with the Aurora B inhibitor ZM447439 also caused Aurora B delocalization at concentrations that partially inhibited Aurora B autophosphorylation. Given that phosphorylation of Aurora B by v-Src was not observed, these results suggest that v-Src causes Aurora B delocalization by indirectly suppressing Aurora B kinase activity.

Anti-colorectal cancer of Ardisia gigantifolia Stapf. and targets prediction via network pharmacology and molecular docking study

BACKGROUND

Ardisia gigantifolia Stapf. (AGS), a Chinese folk medicine widely grows in the south of China and several studies reported that AGS could inhibit the proliferation of breast cancer, liver cancer, and bladder cancer cell lines. However, little is known about its anti-colorectal cancer (CRC) efficiency.

METHODS

In the present study, a combination of MTT assay, network pharmacological analysis, bioinformatics, molecular docking, and molecular dynamics simulation study was used to investigate the active ingredients, and targets of AGS against CRC, as well as the potential mechanism.

RESULTS

MTT assay showed that three kinds of fractions from AGS, including the n-butanol extract (NBAGS), ethyl acetate fraction (EAAGS), and petroleum ether fraction (PEAGS) significantly inhibited the proliferation of CRC cells, with the IC₅₀ values of 197.24, 264.85, 15.45 µg/mL on HCT116 cells, and 523.6, 323.59, 150.31 µg/mL on SW620 cells, respectively. Eleven active ingredients, including, 11-O-galloylbergenin, 11-O-protocatechuoylbergenin, 11-O-syringylbergenin, ardisiacrispin B, bergenin, epicatechin-3-gallate, gallic acid, quercetin, stigmasterol, stigmasterol-3-o-β-D-glucopyranoside were identified. A total of 173 targets related to the bioactive components and 21,572 targets related to CRC were picked out through database searching. Based on the crossover targets of AGS and CRC, a protein-protein interaction network was built up by the String database, from which it was concluded that the core targets would be SRC, MAPK1, ESR1, HSP90AA1, MAPK8. Besides, GO analysis showed that the numbers of biological process, cellular component, and molecular function of AGS against CRC were 1079, 44, and 132, respectively, and KEGG pathway enrichment indicated that 96 signaling pathways in all would probably be involved in AGS against CRC, among which MAPK signaling pathway, lipid, and atherosclerosis, proteoglycans in cancer, prostate cancer, adherens junction would probably be the major pathways. The docking study verified that AGS had multiple ingredients and multiple targets against CRC. Molecular dynamics (MD) simulation analysis showed that the binding would be stable via forming hydrogen bonds.

CONCLUSION

Our study showed that AGS had good anti-CRC potency with the characteristics of multi-ingredients, -targets, and -signaling pathways.

From Structure Modification to Drug Launch: A Systematic Review of the Ongoing Development of Cyclin-Dependent Kinase Inhibitors for Multiple Cancer Therapy

Herein, we discuss more than 50 cyclin-dependent kinase (CDK) inhibitors that have been approved or have undergone clinical trials and their therapeutic application in multiple cancers. This review discusses the design strategies, structure-activity relationships, and efficacy performances of these selective or nonselective CDK inhibitors. The theoretical basis of early broad-spectrum CDK inhibitors is similar to the scope of chemotherapy, but because their toxicity is greater than the benefit, there is no clinical therapeutic window. The notion that selective CDK inhibitors have a safer therapeutic potential than pan-CDK inhibitors has been widely recognized during the research process. Four CDK4/6 inhibitors have been approved for the treatment of breast cancer or for prophylactic administration during chemotherapy to protect bone marrow and immune system function. Furthermore, the emerging strategies in the field of CDK inhibitors are summarized briefly, and CDKs continue to be widely pursued as emerging anticancer drug targets for drug discovery.

A Model for the Signal Initiation Complex Between Arrestin-3 and the Src Family Kinase Fgr

Arrestins regulate a wide range of signaling events, most notably when bound to active G protein-coupled receptors (GPCRs). Among the known effectors recruited by GPCR-bound arrestins are Src family kinases, which regulate cellular growth and proliferation. Here, we focus on arrestin-3 interactions with Fgr kinase, a member of the Src family. Previous reports demonstrated that Fgr exhibits high constitutive activity, but can be further activated by both arrestin-dependent and arrestin-independent pathways. We report that arrestin-3 modulates Fgr activity with a hallmark bell-shaped concentration-dependence, consistent with a role as a signaling scaffold. We further demonstrate using NMR spectroscopy that a polyproline motif within arrestin-3 interacts directly with the SH3 domain of Fgr. To provide a framework for this interaction, we determined the crystal structure of the Fgr SH3 domain at 1.9 Å resolution and developed a model for the GPCR-arrestin-3-Fgr complex that is supported by mutagenesis. This model suggests that Fgr interacts with arrestin-3 at multiple sites and is consistent with the locations of disease-associated Fgr mutations. Collectively, these studies provide a structural framework for arrestin-dependent activation of Fgr.

EPS8 phosphorylation by Src modulates its oncogenic functions

Background

EPS8 is a scaffolding protein that regulates proliferation, actin dynamics and receptor trafficking. Its expression is increased in cancer, enhancing mitogenesis, migration and tumorigenesis. Src phosphorylates EPS8 at four tyrosine residues, although the function is unknown. Here we investigated the pro-oncogenic role of EPS8 tyrosine phosphorylation at Src target sites in HNSCC.

Methods

Plasmids expressing EPS8 Src-mediated phosphorylation site mutants (Y485F, Y525F, Y602F, Y774F and all four combined [FFFF]) were expressed in cells containing a normal endogenous level of EPS8. In addition, cells were treated with dasatinib to inhibit Src activity. EPS8 downstream targets were evaluated by western blotting. Wound closure, proliferation, immunofluorescence and tumorgenicity assays were used to investigate the impact of phenylalanine mutations on EPS8 biological functions.

Results

FOXM1, AURKA, and AURKB were decreased in cells expressing FFFF- and Y602F-EPS8 mutants, while cells harbouring the Y485F-, Y525F- and Y774F-EPS8 mutants showed no differences compared to controls. Consistent with this, dasatinib decreased the expression of EPS8 targets. Moreover, Y602F- and FFFF-EPS8 mutants reduced mitogenesis and motility. Strikingly though, FFFF- or Y602F-EPS8 mutants actually promoted tumorigenicity compared with control cells.

Conclusions

Phosphorylation of EPS8 at Y602 is crucial for signalling to the cell cycle and may provide insight to explain reduced efficacy of dasatinib treatment.

Blockade of HMGB1 signaling pathway by ethyl pyruvate inhibits tumor growth in diffuse large B-cell lymphoma

High mobility group box 1 (HMGB1) protein in the tumor microenvironment actively contributes to tumor progression but its role in diffuse large B-cell lymphoma (DLBCL) is unknown. The aim of this study was to determine the mechanism by which HMGB1 promotes tumor growth in DLBCL and whether blockade of HMGB1 signaling pathway could inhibit tumorigenesis. We report that HMGB1 promotes proliferation of DLBCL cells by activation of AKT, extracellular signal-regulated kinases 1/2 (ERK1/2), signal transducer and activator of transcription 3 (STAT3) and SRC Proto-Oncogene, Non-Receptor Tyrosine Kinase (Src). Ethyl pyruvate (EP), an anti-inflammatory agent, inhibits HMGB1 active release from DLBCL cells and significantly inhibited proliferation of DLBCL cells in vitro. Treatment with EP significantly prevented and inhibited tumor growth in vivo and prolonged DLBCL-bearing mice survival. EP significantly downregulated HMGB1 expression and phosphorylation of Src and ERK1/2 in mice lymphoma tissue. EP induced accumulation of the cell cycle inhibitor p27 but downregulated expression of cyclin-dependent kinase 2 (CDK2). Increased nuclear translocation of p27 interacted with CDK2 and cyclin A, which led to blockade of cell cycle progression at the G1 to S phase transition. In conclusion, we demonstrated for the first time that blockade of HMGB1-mediated signaling pathway by EP effectively inhibited DLBCL tumorigenesis and disease progression.

Increased PTPRA expression leads to poor prognosis through c-Src activation and G1 phase progression in squamous cell lung cancer

PTPRA is reported to be involved in cancer development and progression through activating the Src family kinase (SFK) signaling pathways, however, the roles of PTPRA in the squamous cell lung cancer (SCC) development are unclear. The purpose of this study was to clarify the clinical relevance and biological roles of PTPRA in SCC. We found that PTPRA was upregulated in squamous cell lung cancer compared to matched normal tissues at the mRNA (N=20, P=0.004) and protein expression levels (N=75, P<0.001). Notably, high mRNA level of PTPRA was significantly correlated with poorer prognosis in 675 SCC patients from the Kaplan-Meier plotter database. With 75 cases, we found that PTPRA protein expression was significantly correlated with tumor size (P=0.002), lymph node metastasis (P=0.008), depth of tumor invasion (P<0.001) and clinical stage (P<0.001). The Kaplan-Meier plot suggested that high expression of PTPRA had poorer overall survival in SCC patients (P=0.009). Multivariate Cox regression analysis suggested that PTPRA expression was an independent prognostic factor in SCC patients. In the cellular models, PTPRA promotes SCC cell proliferation through modulating Src activation as well as cell cycle progression. In conclusion, higher PTPRA level was associated with worse prognosis of SCC patients and PTPRA could promote the cell cycle progression through stimulating the c-Src signaling pathways.

v-Src-induced nuclear localization of YAP is involved in multipolar spindle formation in tetraploid cells

The protein-tyrosine kinase, c-Src, is involved in a variety of signaling events, including cell division. We have reported that v-Src, which is a mutant variant of the cellular proto-oncogene, c-Src, causes delocalization of Aurora B kinase, resulting in a furrow regression in cytokinesis and the generation of multinucleated cells. However, the effect of v-Src on mitotic spindle formation is unknown. Here we show that v-Src-expressing HCT116 and NIH3T3 cells undergo abnormal cell division, in which cells separate into more than two cells. Upon v-Src expression, the proportion of multinucleated cells is increased in a time-dependent manner. Flow cytometry analysis revealed that v-Src increases the number of cells having a ≥4N DNA content. Microscopic analysis showed that v-Src induces the formation of multipolar spindles with excess centrosomes. These results suggest that v-Src induces multipolar spindle formation by generating multinucleated cells. Tetraploidy activates the tetraploidy checkpoint, leading to a cell cycle arrest of tetraploid cells at the G1 phase, in which the nuclear exclusion of the transcription co-activator YAP plays a critical role. In multinucleated cells that are induced by cytochalasin B and the Plk1 inhibitor, YAP is excluded from the nucleus. However, v-Src prevents this nuclear exclusion of YAP through a decrease in the phosphorylation of YAP at Ser127 in multinucleated cells. Furthermore, v-Src decreases the expression level of p53, which also plays a critical role in the cell cycle arrest of tetraploid cells. These results suggest that v-Src promotes abnormal spindle formation in at least two ways: generation of multinucleated cells and a weakening of the tetraploidy checkpoint.

v-Src Causes Chromosome Bridges in a Caffeine-Sensitive Manner by Generating DNA Damage

An increase in Src activity is commonly observed in epithelial cancers. Aberrant activation of the kinase activity is associated with malignant progression. However, the mechanisms that underlie the Src-induced malignant progression of cancer are not completely understood. We show here that v-Src, an oncogene that was first identified from a Rous sarcoma virus and a mutant variant of c-Src, leads to an increase in the number of anaphase and telophase cells having chromosome bridges. v-Src increases the number of γH2AX foci, and this increase is inhibited by treatment with PP2, a Src kinase inhibitor. v-Src induces the phosphorylation of KAP1 at Ser824, Chk2 at Thr68, and Chk1 at Ser345, suggesting the activation of the ATM/ATR pathway. Caffeine decreases the number of cells having chromosome bridges at a concentration incapable of inhibiting Chk1 phosphorylation at Ser345. These results suggest that v-Src induces chromosome bridges via generation of DNA damage and the subsequent DNA damage response, possibly by homologous recombination. A chromosome bridge gives rise to the accumulation of DNA damage directly through chromosome breakage and indirectly through cytokinesis failure-induced multinucleation. We propose that v-Src-induced chromosome bridge formation is one of the causes of the v-Src-induced malignant progression of cancer cells.

Enhanced Ca2+Entry and Tyrosine Phosphorylation Mediate Nanostructure-Induced Endothelial Proliferation

Nanostructured substrates have been recognized to initiate transcriptional programs promoting cell proliferation. Specifically β-catenin has been identified as transcriptional regulator, activated by adhesion to nanostructures. We set out to identify processes responsible for nanostructure-induced endothelial β-catenin signaling. Transmission electron microscopy (TEM) of cell contacts to differently sized polyethylene terephthalate (PET) surface structures (ripples with 250 to 300 nm and walls with 1.5 μm periodicity) revealed different patterns of cell-substrate interactions. Cell adhesion to ripples occurred exclusively on ripple peaks, while cells were attached to walls continuously. The Src kinase inhibitor PP2 was active only in cells grown on ripples, while the Abl inhibitors dasatinib and imatinib suppressed β-catenin translocation on both structures. Moreover, Gd³⁺ sensitive Ca²⁺ entry was observed in response to mechanical stimulation or Ca²⁺ store depletion exclusively in cells grown on ripples. Both PP2 and Gd³⁺ suppressed β-catenin nuclear translocation along with proliferation in cells grown on ripples but not on walls. Our results suggest that adhesion of endothelial cells to ripple structured PET induces highly specific, interface topology-dependent changes in cellular signalling, characterized by promotion of Gd³⁺ -sensitive Ca²⁺ entry and Src/Abl activation. We propose that these signaling events are crucially involved in nanostructure-induced promotion of cell proliferation.

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.

Identification of a Src tyrosine kinase/SIAH2 E3 ubiquitin ligase pathway that regulates C/EBPδ expression and contributes to transformation of breast tumor cells

The transcription factor CCAAT/enhancer-binding protein delta (C/EBPδ, CEBPD) is a tumor suppressor that is downregulated during breast cancer progression but may also promote metastasis. Here, we have investigated the mechanism(s) regulating C/EBPδ expression and its role in human breast cancer cells. We describe a novel pathway by which the tyrosine kinase Src downregulates C/EBPδ through the SIAH2 E3 ubiquitin ligase. Src phosphorylates SIAH2 in vitro and leads to tyrosine phosphorylation and activation of SIAH2 in breast tumor cell lines. SIAH2 interacts with C/EBPδ, but not C/EBPβ, and promotes its polyubiquitination and proteasomal degradation. Src/SIAH2-mediated inhibition of C/EBPδ expression supports elevated cyclin D1 levels, phosphorylation of retinoblastoma protein (Rb), motility, invasive properties, and survival of transformed cells. Pharmacological inhibition of Src family kinases by SKI-606 (bosutinib) induces C/EBPδ expression in an SIAH2-dependent manner, which is necessary for "therapeutic" responses to SKI-606 in vitro. Ectopic expression of degradation-resistant mutants of C/EBPδ, which do not interact with SIAH2 and/or cannot be polyubiquitinated, prevents full transformation of MCF-10A cells by activated Src (Src truncated at amino acid 531 [Src-531]) in vitro. These data reveal that C/EBPδ expression can be regulated at the protein level by oncogenic Src kinase signals through SIAH2, thus contributing to breast epithelial cell transformation.

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.

Targeting SRC in glioblastoma tumors and brain metastases: rationale and preclinical studies

Glioblastoma (GBM) is an extremely aggressive, infiltrative tumor with a poor prognosis. The regulatory approval of bevacizumab for recurrent GBM has confirmed that molecularly targeted agents have potential for GBM treatment. Preclinical data showing that SRC and SRC-family kinases (SFKs) mediate intracellular signaling pathways controlling key biologic/oncogenic processes provide a strong rationale for investigating SRC/SFK inhibitors, e.g., dasatinib, in GBM and clinical studies are underway. The activity of these agents against solid tumors suggests that they may also be useful in treating brain metastases. This article reviews the potential for using SRC/SFK inhibitors to treat GBM and brain metastases.

The tumor suppressor functions of p27(kip1) include control of the mesenchymal/amoeboid transition

In many human cancers, p27 downregulation correlates with a worse prognosis, suggesting that p27 levels could represent an important determinant in cell transformation and cancer development. Using a mouse model system based on v-src-induced transformation, we show here that p27 absence is always linked to a more aggressive phenotype. When cultured in three-dimensional contexts, v-src-transformed p27-null fibroblasts undergo a morphological switch from an elongated to a rounded cell shape, accompanied by amoeboid-like morphology and motility. Importantly, the acquisition of the amoeboid motility is associated with a greater ability to move and colonize distant sites in vivo. The reintroduction of different p27 mutants in v-src-transformed p27-null cells demonstrates that the control of cell proliferation and motility represents two distinct functions of p27, both necessary for it to fully act as a tumor suppressor. Thus, we highlight here a new p27 function in driving cell plasticity that is associated with its C-terminal portion and does not depend on the control of cyclin-dependent kinase activity.

The tyrosine kinase inhibitor AMN107 (Nilotinib) exhibits off-target effects in lymphoblastic cell lines

The aminopyrimidine inhibitor AMN107 (Nilotinib) was rationally designed to antagonize the aberrant tyrosine kinase activity of Bcr-Abl-positive cells. We here evaluated, whether AMN107 is also able to induce apoptosis in Bcr-Abl-negative cells of lymphatic origin. The B-cell lines DOHH-2 and WSU-NHL and the T-cell lines Jurkat and HUT78 were incubated with increasing amounts of AMN107 corresponding to clinically achievable dosages. Subsequently, induced molecular changes were assessed by FACS analysis, Western blot, and enzyme activity assays. Although AMN107 exhibited only a minor apoptosis-inducing effect in the T-cell lines, it exerted a considerable, dose-dependent cytotoxicity in the B-cell lines. Using selective caspase-inhibitors, we show that apoptosis in responder cell lines critically relies on activation of caspase-6 and caspase-9. Cell lines sensitive and resistant towards AMN107 can be discriminated by their differential expression of Src-kinases. Although the AMN107-sensitive cell lines DOHH-2 and WSU-NHL exhibited low or no expression of the Src-kinases Lck, phosphorylated Lck, and Yes with a concomitant high expression of Hck, Lyn, and phosphorylated Lyn, the expression pattern of these kinases was inverse in the AMN107-resistant T-cell lines. In conclusion, this is the first report providing evidence that activity of AMN107 is not restricted to Bcr-Abl, c-Kit, or PDGFR-positive cells, but also extends to lymphatic cell lines of B-cell origin.

PSM/SH2B1 splice variants: critical role in src catalytic activation and the resulting STAT3s-mediated mitogenic response

A role of PSM/SH2B1 had been shown in mitogenesis and extending to phenotypic cell transformation, however, the underlying molecular mechanism remained to be established. Here, four alternative PSM splice variants and individual functional protein domains were compared for their role in the regulation of Src activity. We found that elevated cellular levels of PSM variants resulted in phenotypic cell transformation and potentiated cell proliferation and survival in response to serum withdrawal. PSM variant activity presented a consistent signature pattern for any tested response of highest activity observed for gamma, followed by delta, alpha, and beta with decreasing activity. PSM-potentiated cell proliferation was sensitive to Src inhibitor herbimycin and PSM and Src were found in the same immune complex. PSM variants were substrates of the Src Tyr kinase and potentiated Src catalytic activity by increasing the V(max) and decreasing the K(m) for ATP with the signature pattern of variant activity. Dominant-negative PSM peptide mimetics including the SH2 or PH domains inhibited Src catalytic activity as well as Src-mediated phenotypic cell transformation. Activation of major Src substrate STAT3 was similarly potentiated by the PSM variants in a Src-dependent fashion or inhibited by PSM domain-specific peptide mimetics. Expression of a dominant-negative STAT3 mutant blocked PSM variant-mediated phenotypic cell transformation. Our results implicate an essential role of the PSM variants in the activation of the Src kinase and the resulting mitogenic response--extending to phenotypic cell transformation and involving the established Src substrate STAT3.

Dual targeting of Src and ER prevents acquired antihormone resistance in breast cancer cells

Acquired resistance to endocrine therapies presents a major obstacle to the successful treatment of breast cancer patients. Previously, we have shown that acquisition of resistance to tamoxifen in breast cancer cells is accompanied by an elevation in Src kinase activity which promotes an aggressive, invasive phenotype in vitro. Here, we have explored the potential therapeutic effects of combining Src inhibition with anti-oestrogen treatment on the development of endocrine insensitivity in breast cancer cells. Treatment of MCF7 and T47D cells with tamoxifen alone resulted in an initial growth inhibitory phase followed by the eventual development of tamoxifen resistance together with an elevation of Src kinase activity, which was central to their increased invasive capacity. Chronic exposure of both cell types to the Src inhibitor, AZD0530, as a monotherapy resulted in outgrowth of AZD0530-resistant cells, in which Src kinase activity remained suppressed as did their in vitro invasive nature. Treatment of both MCF7 and T47D cells with AZD0530 in combination with tamoxifen resulted in a reduction of Src activity together with inhibition of focal adhesion kinase phosphorylation and a complete abrogation of their in vitro invasive behaviour. Furthermore, combination therapy significantly suppressed expression of cyclinD1 and c-myc and prevented cell proliferation and the subsequent emergence of a resistant phenotype, with total cell loss occurring by 12 weeks. These data demonstrate that pharmacological targeting of Src kinase, in conjunction with antihormone therapies, effectively prevents antihormone resistance in breast cancer cells in vitro and suggests a potential novel therapeutic benefit of Src kinase inhibitors clinically.

Suppression of v-Src transformation by andrographolide via degradation of the v-Src protein and attenuation of the Erk signaling pathway

Elevated expression and aberrant activation of the src oncogene are strongly associated with cancer initiation and progression, thereby making Src a promising molecular target for anti-cancer therapy. Through drug screening using a temperature-inducible v-Src-transformed epithelial cell line, we found that andrographolide could suppress v-Src-induced transformation and down-regulate v-Src protein expression. In addition, actin cable dissolution and E-cadherin down-regulation, features of transformed phenotype, are perturbed by andrographolide. Moreover, andrographolide promoted v-Src degradation via a ubiquitin-dependent manner. Although andrographolide treatment altered the tyrosine phosphorylation pattern in v-Src-expressing cells, it did not directly affect the kinase activity of v-Src. Both the Erk and phosphatidylinositol 3-kinase signaling pathways were strongly inhibited in andrographolide-treated v-Src cells. However, only MKK inhibitors (PD98059 and U0126) were able to cause a non-transformed morphology similar to that of andrographolide-treated v-Src cells. Moreover, overexpression of constitutively active MKK1 in v-Src cells blocked andrographolide-mediated morphological inhibition. Interestingly, andrographolide treatment could also reduce the protein level of the c-Src truncation mutant (Src531), an Src mutant originally identified from human colon cancer cells. In summary, we demonstrated that andrographolide antagonized v-Src action through promotion of v-Src protein degradation. Furthermore, attenuation of the Erk1/2 signaling pathway is essential for andrographolide-mediated inhibition of v-Src transformation. Our results demonstrate that andrographolide can act as a v-Src inhibitor and reveal a novel action mechanism of andrographolide.

EphB/ephrin-B interaction mediates adult stem cell attachment, spreading, and migration: implications for dental tissue repair

Human adult dental pulp stem cells (DPSCs) reside predominantly within the perivascular niche of dental pulp and are thought to originate from migrating neural crest cells during development. The Eph family of receptor tyrosine kinases and their ligands, the ephrin molecules, play an essential role in the migration of neural crest cells during development and stem cell niche maintenance. The present study examined the expression and function of the B-subclass Eph/ephrin molecules on DPSCs. Multiple receptors were primarily identified on DPSCs within the perivascular niche, whereas ephrin-B1 and ephrin-B3 were expressed by the surrounding pulp tissue. EphB/ephrin-B bidirectional signaling inhibited cell attachment and spreading, predominately via the mitogen-activated protein kinase (MAPK) pathway for forward signaling and phosphorylation of Src family tyrosine kinases via reverse ephrin-B signaling. DPSC migration was restricted through unidirectional ephrin-B1-activated EphB forward signaling, primarily signaling through the MAPK pathway. Furthermore, we observed that ephrin-B1 was downregulated in diseased adult teeth compared with paired uninjured controls. Collectively, these studies suggest that EphB/ephrin-B molecules play a role in restricting DPSC attachment and migration to maintain DPSCs within their stem cell niche under steady-state conditions. These results may have implications for dental pulp development and regeneration.

p27 phosphorylation by Src regulates inhibition of cyclin E-Cdk2

The kinase inhibitor p27Kip1 regulates the G1 cell cycle phase. Here, we present data indicating that the oncogenic kinase Src regulates p27 stability through phosphorylation of p27 at tyrosine 74 and tyrosine 88. Src inhibitors increase cellular p27 stability, and Src overexpression accelerates p27 proteolysis. Src-phosphorylated p27 is shown to inhibit cyclin E-Cdk2 poorly in vitro, and Src transfection reduces p27-cyclin E-Cdk2 complexes. Our data indicate that phosphorylation by Src impairs the Cdk2 inhibitory action of p27 and reduces its steady-state binding to cyclin E-Cdk2 to facilitate cyclin E-Cdk2-dependent p27 proteolysis. Furthermore, we find that Src-activated breast cancer lines show reduced p27 and observe a correlation between Src activation and reduced nuclear p27 in 482 primary human breast cancers. Importantly, we report that in tamoxifen-resistant breast cancer cell lines, Src inhibition can increase p27 levels and restore tamoxifen sensitivity. These data provide a new rationale for Src inhibitors in cancer therapy.

Increases in c-Src expression level and activity do not promote the growth of human colorectal carcinoma cells in vitro and in vivo

The levels and activity of c-Src in colorectal cancer cells increase steadily during the course of colorectal carcinogenesis and are most highly elevated in advanced metastatic disease. However, the effects of increases in c-Src activity on the proliferation of colorectal cancer cells during early and late stages of tumorigenesis remain elusive. To study the consequences of increases in c-Src levels and activity on the growth of colorectal cancer cells in later stages of colorectal carcinogenesis, we developed human colorectal cancer cell lines in which c-Src levels and activity could be inducibly increased by a tightly controlled expression of wild-type c-Src or of the constitutively active mutant of c-Src, c-SrcY527F. Src induction activated multiple signaling pathways (often associated with a proliferative response) but promoted neither cell proliferation in vitro nor tumor growth in a xenograft model in vivo. These results indicate that, in more advanced stages of colorectal carcinogenesis, increases in c-Src levels and activity are likely to have functions other than the direct promotion of tumor growth.

Identification and functional signature of genes regulated by structurally different ABL kinase inhibitors

Dasatinib is an ATP-competitive, multi-targeted SRC and ABL kinase inhibitor that can bind BCR-ABL in both the active and inactive conformations. From a clinical standpoint, dasatinib is particularly attractive because it has been shown to induce hematologic and cytogenetic responses in imatinib-resistant chronic myeloid leukemia patients. The fact because the combination of imatinib and dasatinib shows the additive/synergistic growth inhibition on wild-type p210 BCR-ABL-expressing cells, we reasoned that these ABL kinase inhibitors might induce the different molecular pathways. To address this question, we used DNA microarrays to identify genes whose transcription was altered by imatinib and dasatinib. K562 cells were cultured with imatinib or dasatinib for 16 h, and gene expression data were obtained from three independent microarray hybridizations. Almost all of the imatinib- and dasatinib-responsive genes appeared to be similarly increased or decreased in K562 cells; however, small subsets of genes were identified as selectively altered expression by either imatinib or dasatinib. The distinct genes that are selectively modulated by dasatinib are cyclin-dependent kinase 2 (CDK2) and CDK8, which had a maximal reduction of <5-fold in microarray screen. To assess the functional importance of dasatinib regulated genes, we used RNA interference to determine whether reduction of CDK2 and CDK8 affected the growth inhibition. K562 and TF-1BCR-ABL cells, pretreated with CDK2 or CDK8 small interfering RNA, showed additive growth inhibition with imatinib, but not with dasatinib. These findings demonstrate that the additive/synergistic growth inhibition by imatinib and dasatinib may be mediated in part by CDK2 and CDK8.

Peptide modulators of Src activity in G1 regulate entry into S phase and proliferation of NIH 3T3 cells

Cascades of kinases and phosphatases are regulated by selective protein-protein interactions that are essential for signal transduction. Peptide modulators of these interactions have been used to dissect the function of individual components of the signaling cascade, without relying on either the over- or underexpression of proteins. Previously, we identified RACK1 as an endogenous substrate, binding partner and inhibitor of Src tyrosine kinases. Here, we utilized cell-permeable peptides that selectively disrupt or enhance the interaction of RACK1 and Src to further examine the function of RACK1. Our results provide direct physiologic evidence that RACK1 regulates growth of NIH3T3 cells by suppressing the activity of Src and other cell cycle regulators in G1, and delaying entry into S phase. They also demonstrate the potential for using peptide modulators of Src activity as a tool for regulating cell growth, and for designing new strategies for cancer therapy that target specific protein-protein interactions.

v-Src tyrosine phosphorylation of connexin43: regulation of gap junction communication and effects on cell transformation

The oncogenic tyrosine kinase, v-Src, phosphorylates connexin43 (Cx43) on Y247 and Y265 and inhibits Cx43 gap junctional communication (GJC), the process of intercellular exchange of ions and metabolites. To test the role of a negative charge on Cx43 induced by tyrosine phosphorylation, we expressed Cx43 with glutamic acid substitutions at Y247 or Y265. The Cx43Y247E or Cx43Y265E channels were functional in Cx43 knockout fibroblasts, indicating that introducing a negative charge on Cx43 was not likely the mechanism for v-Src disruption of GJC. Cells coexpressing v-Src and the triple serine to alanine mutant, Cx43S255/279/282A, confirmed that mitogen-activated protein (MAP) kinase phosphorylation of Cx43 was not required for v-Src-induced disruption of GJC and that tyrosine phosphorylation was sufficient. In addition, v-Src cells containing v-Src-resistant gap junctions, Cx43Y247/265F, displayed properties of cell migration, adhesion, and proliferation similar to Cx43wt/v-Src cells, suggesting that Cx43 tyrosine phosphorylation and disruption of GJC are not involved in these transformed cell properties.

RACK1 inhibits colonic cell growth by regulating Src activity at cell cycle checkpoints

Previously, we showed that Src tyrosine kinases are activated early in the development of human colon cancer and are suppressed as intestinal cells differentiate. We identified RACK1 as an endogenous substrate, binding partner and inhibitor of Src. Here we show (by overexpressing RACK1, depleting Src or RACK1 and utilizing cell-permeable peptides that perturb RACK1's interaction with Src) that RACK1 regulates growth of colon cells by suppressing Src activity at G(1) and mitotic checkpoints, and consequently delaying cell cycle progression. Activated Src rescues RACK1-inhibited growth of HT-29 cells. Conversely, inhibiting Src abolishes growth promoted by RACK1 depletion in normal cells. Two potential mechanisms whereby RACK1 regulates mitotic exit are identified: suppression of Src-mediated Sam68 phosphorylation and maintenance of the cyclin-dependent kinase (CDK) 1-cyclin B complex in an active state. Our results reveal novel mechanisms of cell cycle control in G(1) and mitosis of colon cells. The significance of this work lies in the discovery of a mechanism by which the growth of colon cancer cells can be slowed, by RACK1 suppression of an oncogenic kinase at critical cell cycle checkpoints. Small molecules that mimic RACK1 function may provide a powerful new approach to the treatment of colon cancer.

Src tyrosine kinase as a chemotherapeutic target: is there a clinical case?

Src tyrosine kinase was the first protooncogene described. It has been found to be overexpressed and activated in a large number of different cancers. Cellular Src has been shown to activate a number of different effectors that are involved in different aspects of cancer biology such as metastasis, cell cycle regulation and cell survival. Despite this, Src inhibitors have not entered the regular arsenal of chemotherapeutics. This article reviews some of the biology, rationale, in vitro and in vivo preclinical evidence, and some very early clinical trials demonstrating efficacy of Src inhibitors.

Activated Src abrogates the Myc requirement for the G0/G1 transition but not for the G1/S transition

The transcription factor Myc plays a central role in the control of cellular proliferation. Myc expression is induced by growth factors in a pathway mediated by cellular Src (c-Src), but it is not clear whether Myc induction or activity is required for malignant transformation by activated Src. We introduced v-Src into a c-myc(-/-) derivative of Rat-1 fibroblasts and into 3T9 mouse fibroblasts harboring a conditionally excisable c-myc allele. Expression of activated viral Src in Myc-deficient cells led to loss of actin stress fibers and surface fibronectin, indicating that Myc is dispensable for v-Src-induced morphological transformation. However, v-Src failed to rescue the proliferative defect resulting from the loss of Myc. In Myc-deficient cells, despite its inability to overcome this proliferation block, v-Src was able to regulate the expression of certain Myc transcriptional targets and induce the expression of active cyclin D/Cdk4 and Cdk6 complexes; it also induced the phosphorylation of Rb, albeit at reduced levels. In contrast, however, in the absence of Myc, the level of Cdk2 kinase activity was drastically reduced. This reduction in Cdk2 activity was associated with a decrease in the expression of Cdk7, Cdc25A, and cyclin A. Coexpression of Cdk2 plus cyclin E and/or cyclin A rescued the G1/S block and allowed the cells to enter mitosis. These results indicate that in the absence of Myc, v-Src can activate early G1 cell cycle regulators but fails to activate regulators of the late G1/S transition.

Active Src elevates the expression of beta-catenin by enhancement of cap-dependent translation

The proto-oncogene pp60(c-Src) (c-Src) is activated in many types of cancer and contributes to the transformed phenotype of the tumor, although its role is not yet fully understood. Here we report that active Src elevates the levels of beta-catenin by enhancing cap-dependent translation. Src induces phosphorylation of the eukaryotic initiation factor 4E via the Ras/Raf/ERK pathway and the phosphorylation of its inhibitor 4E-BP1 via the PI3K/mTOR pathway. Activated Src enhances the accumulation of nuclear beta-catenin and enhances its transcriptional activity, elevating target genes such as cyclin D1. This novel activation of the Wnt pathway by Src most probably contributes to the oncogenic phenotype of cancer cells.

Induction of Id1 and Id3 by latent membrane protein 1 of Epstein-Barr virus and regulation of p27/Kip and cyclin-dependent kinase 2 in rodent fibroblast transformation

Latent membrane protein 1 (LMP1), the Epstein-Barr virus oncoprotein, activates NF-kappaB, phosphatidylinositol 3-kinase, mitogen-activated protein kinase, and c-Jun N-terminal kinase signaling. To determine global transcriptional changes induced by LMP1 in epithelial cells, genomic analysis of C33A cells stably expressing LMP1 was performed. Relatively few genes were induced by LMP1. Expression of two members of the Id (inhibitor of differentiation) family of proteins, Id1 and Id3, was induced in the presence of LMP1 and confirmed by mRNA and protein in C33A and Rat-1 cells. In Rat-1 foci transformed by LMP1, Id1 protein was also increased. Id proteins are known negative regulators of E-box proteins that positively regulate p16 and potentially other cyclin-dependent kinase inhibitors (cdki's). In LMP1-expressing Rat-1 cells, cdki p27 was specifically downregulated. Decreased p27 was correlated with increased levels of Cdk2 and increased levels of phosphorylated retinoblastoma protein. This study describes new properties of LMP1 that likely contribute to transformation and oncogenesis.

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

Oncogenic forms of the non-receptor tyrosine kinase Src alter cell structure, in particular the actin cytoskeleton and the adhesion networks that control cell migration, and also transmit signals that regulate proliferation and cell survival. Recent work indicates that they do so by influencing the RhoA-ROCK pathway that controls contractile actin filament assembly, the STAT family of transcription factors needed for transformation, and the Cbl ubiquitin ligase that controls Src protein levels. These studies also shed light on the role of focal adhesion kinase (FAK) downstream of v-Src and other signalling pathways in controlling migration, invasion and survival of transformed cells. Src directly phosphorylates integrins and can also modulate R-Ras activity. Moreover, it stimulates the E-cadherin regulator Hakai, interacts with and phosphorylates the novel podosome-linked adaptor protein Fish, and progressively phosphorylates the gap junction component connexion 43. A recurring theme is the identification of novel and important Src substrates that mediate key biological events associated with transformation.

The thyroid hormone receptor is a suppressor of ras-mediated transcription, proliferation, and transformation

The thyroid hormone triiodothyronine (T3) has a profound effect on growth, differentiation, and metabolism in higher organisms. Here we demonstrate that T3 inhibits ras-induced proliferation in neuroblastoma cells and blocks induction of cyclin D1 expression by the oncogene. The hormone, at physiological concentrations, strongly antagonizes the transcriptional response mediated by the Ras/mitogen-activated protein kinase/ribosomal-S6 subunit kinase (Rsk) signaling pathway in cells expressing thyroid hormone receptors (TRs). T3 blocks the response to the oncogenic forms of the three ras isoforms (H-, K-, and N-ras) and both TRalpha and TRbeta can mediate this action. The main target for induction of cyclin D1 transcription by oncogenic ras in neuroblastoma cells is a cyclic AMP response element (CRE) located in proximal promoter sequences, and T3 represses the transcriptional activity of b-Zip transcription factors such as CREB (CRE-binding protein) or ATF-2 (activation transcription factor 2) that are direct targets of Rsk2 and bind to this sequence. The hormone also blocks fibroblast transformation by oncogenic ras when TR is expressed. Furthermore, TRs act as suppressors of tumor formation by the oncogene in vivo in nude mice. The TRbeta isoform has stronger antitransforming properties than the alpha isoform and can inhibit tumorigenesis even in hypothyroid mice. These results show the existence of a previously unrecognized transcriptional cross talk between the TRs and the ras oncogene which influences relevant processes such as cell proliferation, transformation, or tumorigenesis.

RACK1 regulates G1/S progression by suppressing Src kinase activity

Cancer genes exert their greatest influence on the cell cycle by targeting regulators of a critical checkpoint in late G(1). Once cells pass this checkpoint, they are fated to replicate DNA and divide. Cancer cells subvert controls at work at this restriction point and remain in cycle. Previously, we showed that RACK1 inhibits the oncogenic Src tyrosine kinase and NIH 3T3 cell growth. RACK1 inhibits cell growth, in part, by prolonging G(0)/G(1). Here we show that RACK1 overexpression induces a partial G(1) arrest by suppressing Src activity at the G(1) checkpoint. RACK1 works through Src to inhibit Vav2, Rho GTPases, Stat3, and Myc. Consequently, cyclin D1 and cyclin-dependent kinases 4 and 2 (CDK4 and CDK2, respectively) are suppressed, CDK inhibitor p27 and retinoblastoma protein are activated, E2F1 is sequestered, and G(1)/S progression is delayed. Conversely, downregulation of RACK1 by short interference RNA activates Src-mediated signaling, induces Myc and cyclin D1, and accelerates G(1)/S progression. RACK1 suppresses Src- but not mitogen-activated protein kinase-dependent platelet-derived growth factor signaling. We also show that Stat3 is required for Rac1 induction of Myc. Our results reveal a novel mechanism of cell cycle control in late G(1) that works via an endogenous inhibitor of the Src kinase.

Constitutively active Src facilitates NGF-induced phosphorylation of TrkA and causes enhancement of the MAPK signaling in SK-N-MC cells

Here we investigated a biological association of constitutively active Src with TrkA in SK-N-MC human neuroblastoma cells. Activation of TrkA and extracellular signal-regulated kinase (ERK) by nerve growth factor (NGF) was inhibited by pretreatment with PP2, an inhibitor of Src family kinases. Moreover, NGF-induced phosphorylation of TrkA and ERK was also attenuated by the transfection with a dominant-negative src construct. On the other hand, the transfection with a constitutively active src construct enhanced these phosphorylations. In addition, we showed that active Src phosphorylates TrkA directly in vitro, and that Src associates with TrkA through Grb2 after NGF stimulation. These results suggest that constitutively active Src that associates with TrkA through Grb2 after NGF stimulation participates in TrkA phosphorylation and in turn enhances the mitogen-activated protein kinase signaling in SK-N-MC cells.

Cyclin E and cyclin A are likely targets of Src for PDGF-induced DNA synthesis in fibroblasts

How tyrosine kinases of the Src family regulate platelet-derived growth factor (PDGF)-induced DNA synthesis remains elusive. Here we show that the E1A antigen of adenovirus 5 overrides the G1 block elicited by the kinase-inactive mutant SrcK(-). This was dependent upon the CR2 region of E1A that upregulated cyclin E and cyclin A and inactivated the pocket protein pRb. E1A rescue was independent of pRb. Expression of SrcK(-) in fibroblasts prevented PDGF-induced expression of cyclins E and A. This effect was overcome by E1A. Constitutive expression of cyclins E and A, but not D1, restored mitogenesis that was inhibited by SrcK(-). We conclude that both cyclin E and cyclin A are likely targets of Src mediating PDGF-induced DNA synthesis.

Transition between proliferation and differentiation for lens epithelial cells is regulated by Src family kinases

As in many cell types, lens cells must withdraw from the cell cycle before they initiate their differentiation. The involvement of Src family kinases (SFKs) in this key initiating event in cell differentiation was examined in lens epithelial cell cultures. SFK activity was suppressed with the specific inhibitor PP1. This induced expression of the cyclin-dependent kinase (CDK) inhibitors p27 and p57 and suppressed lens epithelial cell proliferation. Therefore, inhibition of SFK activity created conditions permissive for undifferentiated lens epithelial cells to withdraw from the cell cycle. Growth of the lens epithelial cell cultures in the presence of PP1 induced expression of filensin and CP49, lens differentiation-specific intermediate filament proteins, providing evidence that suppression of SFK activity also promoted the initiation of lens cell differentiation. The mechanism by which PP1 signaled cell cycle withdrawal and commitment to differentiation was shown to involve induction of N-cadherin cell-cell junction assembly and reorganization of the actin cytoskeleton from stress fibers to cortical filaments. This result was supported by the compaction of the epithelial monolayer in response to PP1, a morphogenetic change that we have previously shown to be dependent on N-cadherin function and a hallmark of the commencement of the lens differentiation program in culture. The results presented in this study suggest that the decision of lens epithelial cells to withdraw from the cell cycle and initiate differentiation requires inhibition of SFKs and the formation of N-cadherin cell-cell junctions.

Src in cancer: deregulation and consequences for cell behaviour

Considerable evidence now implicates elevated expression and/or activity of Src in cancer development. In cells, endogenous Src is switched from an inactive to an active state by a variety of mechanisms that simultaneously relieve constraints on the kinase and protein-interacting Src homology (SH) domains. As a result, Src is translocated to the cell periphery, often to sites of cell adhesion, where myristylation mediates attachment to the inner surface of the plasma membrane. From these peripheral sites, Src's catalytic activity initiates intracellular signal transduction pathways that influence cell growth and adhesion strength, the latter contributing to control of cell migration. De-regulation in cancer cells may therefore enhance tumour growth and/or stimulate migratory or invasive potential in cells that would normally be relatively non-motile. Evidence now exists to suggest that Src may also influence the life or death decisions that cells make during many biological processes. Thus, Src modulation in cancer cells can alter cell responses that are often perturbed in cancer. Consequently, there is optimism that drugs which inhibit Src's kinase activity, or the activity of its downstream effectors, might have profound effects on cancer cell behaviour and be useful therapeutic agents.

v-Src's hold over actin and cell adhesions

The oncoprotein v-Src and its cellular homologue (c-Src) are tyrosine kinases that modulate the actin cytoskeleton and cell adhesions. Through the concerted action of their protein-interaction and kinase domains, they are targeted to cell matrix integrin adhesions or cadherin-dependent junctions between epithelial cells, where they phosphorylate substrates that induce adhesion turnover and actin re-modelling. Recent experiments have defined some of the key targets and effector pathways that mediate the pleiotropic oncogenic effects of v-Src.