Mutation of the SRC gene in endometrial carcinoma

The (Patho)Biology of SRC Kinase in Platelets and Megakaryocytes

Proto-oncogene tyrosine-protein kinase SRC (SRC), as other members of the SRC family kinases (SFK), plays an important role in regulating signal transduction by different cell surface receptors after changes in the cellular environment. Here, we reviewed the role of SRC in platelets and megakaryocytes (MK). In platelets, inactive closed SRC is coupled to the β subunit of integrin α_IIbβ₃ while upon fibrinogen binding during platelet activation, α_IIbβ₃-mediated outside-in signaling is initiated by activation of SRC. Active open SRC now further stimulates many downstream effectors via tyrosine phosphorylation of enzymes, adaptors, and especially cytoskeletal components. Functional platelet studies using SRC knockout mice or broad spectrum SFK inhibitors pointed out that SRC mediates their spreading on fibrinogen. On the other hand, an activating pathological SRC missense variant E527K in humans that causes bleeding inhibits collagen-induced platelet activation while stimulating platelet spreading. The role of SRC in megakaryopoiesis is much less studied. SRC knockout mice have a normal platelet count though studies with SFK inhibitors point out that SRC could interfere with MK polyploidization and proplatelet formation but these inhibitors are not specific. Patients with the SRC E527K variant have thrombocytopenia due to hyperactive SRC that inhibits proplatelet formation after increased spreading of MK on fibrinogen and enhanced formation of podosomes. Studies in humans have contributed significantly to our understanding of SRC signaling in platelets and MK.

Loss of desmoglein-2 promotes gallbladder carcinoma progression and resistance to EGFR-targeted therapy through Src kinase activation

Gallbladder carcinoma (GBC) exhibits poor prognosis due to local recurrence, metastasis, and resistance to targeted therapies. Using clinicopathological analyses of GBC patients along with molecular in vitro and tumor in vivo analysis of GBC cells, we showed that reduction of Dsg2 expression was highly associated with higher T stage, more perineural, and lymphatic invasion. Dsg2-depleted GBC cells exhibited significantly enhanced proliferation, migration, and invasiveness in vitro and tumor growth and metastasis in vivo through Src-mediated signaling activation. Interestingly, Dsg2 binding inhibited Src activation, whereas its loss activated cSrc-mediated EGFR plasma membrane clearance and cytoplasmic localization, which was associated with acquired EGFR-targeted therapy resistance and decreased overall survival. Inhibition of Src activity by dasatinib enhanced therapeutic response to anti-EGFR therapy. Dsg2 status can help stratify predicted patient response to anti-EGFR therapy and Src inhibition could be a promising strategy to improve the clinical efficacy of EGFR-targeted therapy.

c-Src and EGFR Inhibition in Molecular Cancer Therapy: What Else Can We Improve?

The proto-oncogene c-Src is a non-receptor tyrosine kinase playing a key role in many cellular pathways, including cell survival, migration and proliferation. c-Src de-regulation has been observed in several cancer types, making it an appealing target for drug discovery efforts. Recent evidence emphasizes its crucial role not only in promoting oncogenic traits, but also in the acquisition and maintenance of cancer resistance to various chemotherapeutic or molecular target drugs. c-Src modulates epidermal growth factor receptor (EGFR) activation and amplifies its downstream oncogenic signals. In this review, we report several studies supporting c-Src kinase role in the intricate mechanisms of resistance to EGFR tyrosine kinase inhibitors (TKIs). We further highlighted pre- and clinical progresses of combined treatment strategies made in recent years. Several pre-clinical data have encouraged the use of c-Src inhibitors in combination with EGFR inhibitors. However, clinical trials provided controversial outcomes in some cancer types. Despite c-Src inhibitors showed good tolerability in cancer patients, no incontrovertible and consistent clinical responses were recorded, supporting the idea that a better selection of patients is needed to improve clinical outcome. Currently, the identification of biological markers predictive of therapy response and the accurate molecular screening of cancer patients aimed to gain most clinical benefits become decisive and mandatory.

Dasatinib Treatment Increases Sensitivity to c-Met Inhibition in Triple-Negative Breast Cancer Cells

In pre-clinical studies, triple-negative breast cancer (TNBC) cells have demonstrated sensitivity to the multi-targeted kinase inhibitor dasatinib; however, clinical trials with single-agent dasatinib showed limited efficacy in unselected populations of breast cancer, including TNBC. To study potential mechanisms of resistance to dasatinib in TNBC, we established a cell line model of acquired dasatinib resistance (231-DasB). Following an approximately three-month exposure to incrementally increasing concentrations of dasatinib (200 nM to 500 nM) dasatinib, 231-DasB cells were resistant to the agent with a dasatinib IC50 value greater than 5 μM compared to 0.04 ± 0.001 µM in the parental MDA-MB-231 cells. 231-DasB cells also showed resistance (2.2-fold) to the Src kinase inhibitor PD180970. Treatment of 231-DasB cells with dasatinib did not inhibit phosphorylation of Src kinase. The 231-DasB cells also had significantly increased levels of p-Met compared to the parental MDA-MB-231 cells, as measured by luminex, and resistant cells demonstrated a significant increase in sensitivity to the c-Met inhibitor, CpdA, with an IC50 value of 1.4 ± 0.5 µM compared to an IC50 of 6.8 ± 0.2 µM in the parental MDA-MB-231 cells. Treatment with CpdA decreased p-Met and p-Src in both 231-DasB and MDA-MB-231 cells. Combined treatment with dasatinib and CpdA significantly inhibited the growth of MDA-MB-231 parental cells and prevented the emergence of dasatinib resistance. If these in vitro findings can be extrapolated to human cancer treatment, combined treatment with dasatinib and a c-Met inhibitor may block the development of acquired resistance and improve response rates to dasatinib treatment in TNBC.

Aberrant ERBB4-SRC Signaling as a Hallmark of Group 4 Medulloblastoma Revealed by Integrative Phosphoproteomic Profiling

The current consensus recognizes four main medulloblastoma subgroups (wingless, Sonic hedgehog, group 3 and group 4). While medulloblastoma subgroups have been characterized extensively at the (epi-)genomic and transcriptomic levels, the proteome and phosphoproteome landscape remain to be comprehensively elucidated. Using quantitative (phospho)-proteomics in primary human medulloblastomas, we unravel distinct posttranscriptional regulation leading to highly divergent oncogenic signaling and kinase activity profiles in groups 3 and 4 medulloblastomas. Specifically, proteomic and phosphoproteomic analyses identify aberrant ERBB4-SRC signaling in group 4. Hence, enforced expression of an activated SRC combined with p53 inactivation induces murine tumors that resemble group 4 medulloblastoma. Therefore, our integrative proteogenomics approach unveils an oncogenic pathway and potential therapeutic vulnerability in the most common medulloblastoma subgroup.

Oncodomains: A protein domain-centric framework for analyzing rare variants in tumor samples

The fight against cancer is hindered by its highly heterogeneous nature. Genome-wide sequencing studies have shown that individual malignancies contain many mutations that range from those commonly found in tumor genomes to rare somatic variants present only in a small fraction of lesions. Such rare somatic variants dominate the landscape of genomic mutations in cancer, yet efforts to correlate somatic mutations found in one or few individuals with functional roles have been largely unsuccessful. Traditional methods for identifying somatic variants that drive cancer are 'gene-centric' in that they consider only somatic variants within a particular gene and make no comparison to other similar genes in the same family that may play a similar role in cancer. In this work, we present oncodomain hotspots, a new 'domain-centric' method for identifying clusters of somatic mutations across entire gene families using protein domain models. Our analysis confirms that our approach creates a framework for leveraging structural and functional information encapsulated by protein domains into the analysis of somatic variants in cancer, enabling the assessment of even rare somatic variants by comparison to similar genes. Our results reveal a vast landscape of somatic variants that act at the level of domain families altering pathways known to be involved with cancer such as protein phosphorylation, signaling, gene regulation, and cell metabolism. Due to oncodomain hotspots' unique ability to assess rare variants, we expect our method to become an important tool for the analysis of sequenced tumor genomes, complementing existing methods.

Tyrosine Phosphorylation of Caspase-8 Abrogates Its Apoptotic Activity and Promotes Activation of c-Src

Src family tyrosine kinases (SFKs) phosphorylate caspase-8A at tyrosine (Y) 397 resulting in suppression of apoptosis. In addition, the phosphorylation of caspase-8A at other sites including Y465 has been implicated in the regulation of caspase-8 activity. However, the functional consequences of these modifications on caspase-8 processing/activity have not been elucidated. Moreover, various Src substrates are known to act as potent Src regulators, but no such role has been explored for caspase-8. We asked whether the newly identified caspase-8 phosphorylation sites might regulate caspase-8 activation and conversely, whether caspase-8 phosphorylation might affect Src activity. Here we show that Src phosphorylates caspase-8A at multiple tyrosine sites; of these, we have focused on Y397 within the linker region and Y465 within the p12 subunit of caspase-8A. We show that phosphomimetic mutation of caspase-8A at Y465 prevents its cleavage and the subsequent activation of caspase-3 and suppresses apoptosis. Furthermore, simultaneous phosphomimetic mutation of caspase-8A at Y397 and Y465 promotes the phosphorylation of c-Src at Y416 and increases c-Src activity. Finally, we demonstrate that caspase-8 activity prevents its own tyrosine phosphorylation by Src. Together these data reveal that dual phosphorylation converts caspase-8 from a pro-apoptotic to a pro-survival mediator. Specifically, tyrosine phosphorylation by Src renders caspase-8 uncleavable and thereby inactive, and at the same time converts it to a Src activator. This novel dynamic interplay between Src and caspase-8 likely acts as a potent signal-integrating switch directing the cell towards apoptosis or survival.

Loss of AF-6/afadin induces cell invasion, suppresses the formation of glandular structures and might be a predictive marker of resistance to chemotherapy in endometrial cancer

Background

AF-6/afadin plays an important role in the formation of adherence junctions. In breast and colon cancer, loss of AF-6/afadin induces cell migration and cell invasion. We aimed to elucidate the role of AF-6/afadin in human endometrial cancer.

Methods

Morphology and AF-6/afadin expression in endometrial cancer cell lines was investigated by 3-dimensional culture. We used Matrigel invasion assay to demonstrate AF-6/afadin knockdown induced invasive capability. Cell proliferation assay was performed to estimate chemoresistance to doxorubicin, paclitaxel and cisplatin induced by AF-6/afadin knockdown. The associations between AF-6/afadin expression and clinicopathological status were determined by immunohistochemical analysis in endometrial cancer tissues. Informed consent was obtained from all patients before the study.

Results

The majority of cell clumps in 3-dimensional cultures of Ishikawa cells that strongly expressed AF-6/afadin showed round gland-like structures. In contrast, the cell clumps in 3-dimensional cultures of HEC1A and AN3CA cells—both weakly expressing AF-6/afadin—showed irregular gland-like structures and disorganized colonies with no gland-like structures, respectively. AF-6/afadin knockdown resulted in reduced number of gland-like structures in 3-dimensional cultures and enhancement of cell invasion and phosphorylation of ERK1/2 and Src in the highly AF-6/afadin-expressing endometrial cancer cell line. Inhibitors of MAPK/ERK kinase (MEK) (U0126) and Src (SU6656) suppressed the AF-6/afadin knockdown-induced invasive capability. AF-6/afadin knockdown induced chemoresistance to doxorubicin, paclitaxel and cisplatin in Ishikawa cells, not in HEC1A. Immunohistochemical analysis showed that AF-6/afadin expression was significantly associated with myometrial invasion and high histological grade.

Conclusions

AF-6/afadin regulates cell morphology and invasiveness. Invasive capability is partly regulated through the ERK and Src pathway. The inhibitors to these pathways might be molecular-targeted drugs which suppress myometrial invasion in endometrial cancer. AF-6/afadin could be a useful selection marker for fertility-sparing therapy for patients with atypical hyperplasia or grade 1 endometrioid adenocarcinoma with no myometrial invasion. AF-6/afadin knockdown induced chemoresistance especially to cisplatin. Therefore, loss of AF-6/afadin might be a predictive marker of chemoresistance to cisplatin.

Critical amino acid residues in proteins: a BioMart integration of Reactome protein annotations with PRIDE mass spectrometry data and COSMIC somatic mutations

The reversible phosphorylation of serine, threonine and tyrosine hydroxyl groups is an especially prominent form of post-translational modification (PTM) of proteins. It plays critical roles in the regulation of diverse processes, and mutations that directly or indirectly affect these phosphorylation events have been associated with many cancers and other pathologies. Here, we describe the development of a new BioMart tool that gathers data from three different biological resources to provide the user with an integrated view of phosphorylation events associated with a human protein of interest, the complexes of which the protein (modified or not) is a part, the reactions in which the protein and its complexes participate and the somatic mutations that might be expected to perturb those functions. The three resources used are the Reactome, PRIDE and COSMIC databases. The Reactome knowledgebase contains annotations of phosphorylated human proteins linked to the reactions in which they are phosphorylated and dephosphorylated, to the complexes of which they are parts and to the reactions in which the phosphorylated proteins participate as substrates, catalysts and regulators. The PRIDE database holds extensive mass spectrometry data from which protein phosphorylation patterns can be inferred, and the COSMIC database holds records of somatic mutations found in human cancer cells. This tool supports both flexible, user-specified queries and standard (‘canned’) queries to retrieve frequently used combinations of data for user-specified proteins and reactions. We demonstrate using the Wnt signaling pathway and the human c-SRC protein how the tool can be used to place somatic mutation data into a functional perspective by changing critical residues involved in pathway modulation, and where available, check for mass spectrometry evidence in PRIDE supporting identification of the critical residue.

Database URL:http://www.reactome.org/cgi-bin/mart

Src kinase: a therapeutic opportunity in endocrine-responsive and resistant breast cancer

The intracellular kinase, Src, interacts with a diverse array of signaling elements, including the estrogen receptor to regulate breast cancer progression. Recent evidence has also implicated Src in mediating the response of breast cancer to endocrine agents and in the acquisition of antihormone resistance, a significant limiting factor to the clinical effectiveness of systemic endocrine therapy. A number of pharmacological inhibitors of Src kinase have been developed that are effective at suppressing breast cancer growth and invasion in vitro and inhibiting disease spread in vivo. Significantly, there appears to be added benefit when these agents are given in combination with anti-estrogens in endocrine-sensitive and -resistant models. These new findings suggest that Src inhibitors might have therapeutic value in breast cancer patients to improve endocrine response and circumvent resistance.

Targeted inhibition of SRC kinase signaling attenuates pancreatic tumorigenesis

Elevated Src expression correlates with malignant potential and metastatic disease in many tumors including pancreatic cancer. We sought to characterize the molecular effects of Src kinase inhibition with dasatinib (BMS-354825), a novel, multitargeted kinase inhibitor that targets Src family kinases in pancreatic ductal adenocarcinoma (PDA). We identified sensitive and resistant PDA cell lines to dasatinib treatment and tested the molecular effects of Src inhibition in vitro and in vivo. We show for the first time that cellular localization of Src expression affects survival in patients with PDA. Pancreatic tumors with increased membranous expression of Src resulted in decreased survival compared with tumors that had increased cytoplasmic Src expression. Src kinase inhibition with dasatinib markedly inhibits cell proliferation, migration, invasion, cell cycle progression and anchorage-independent growth, and stimulates apoptosis. This was accompanied by decreased phosphorylation of Src, focal adhesion kinase, paxillin, AKT, signal transducers and activators of transcription 3 (STAT3), extracellular signal-regulated kinase, and mitogen-activated protein kinase (MAPK), as well as decreased cyclin D1 expression in a time- and concentration-dependent manner. Furthermore, small interfering RNA to Src results in a significant decrease in cell proliferation, invasion, and migration of pancreatic cancer cells. Dasatinib treatment also inhibits in vivo pancreatic tumor growth. Mechanisms of resistance to Src inhibition seem to be related to a lack of inhibition of STAT3 and MAPK signaling. These results establish a mechanistic rationale for Src inhibition with dasatinib as a therapeutic target in the treatment of pancreatic cancer and identify potential biomarkers of resistance to Src inhibition.

Drosophila Abelson kinase mediates cell invasion and proliferation through two distinct MAPK pathways

The Abelson (Abl) family of non-receptor tyrosine kinases plays important role in cell morphogenesis, motility and proliferation. Although the function of Abl has been extensively studied in leukemia, its role in epithelial cell invasion remains obscure. Using the Drosophila wing epithelium as an in-vivo model system, we demonstrate that overexpression (activation) of Drosophila Abl (dAbl) causes loss of epithelial apical/basal cell polarity and secretion of matrix metalloproteinases, resulting in a cellular invasion and apoptosis. Our in vivo data indicate that dAbl acts downstream of the Src kinases, which are known regulators of cell adhesion and invasion. Downstream of dAbl, Rac GTPases activate two distinct MAPK pathways: JNK signaling (required for cell invasion and apoptosis) and ERK signaling (inducing cell proliferation). Activated Abl also increases the activity of Src members through a positive feedback loop leading to signal amplification. Thus targeting Src-Abl, using available dual inhibitors, could be of therapeutic importance in tumor cell metastasis.

The role of Src in solid tumors

The proto-oncogene c-Src (Src) encodes a nonreceptor tyrosine kinase whose expression and activity are correlated with advanced malignancy and poor prognosis in a variety of human cancers. Nine additional enzymes with homology to Src have been identified and collectively are referred to as Src family kinases (SFKs). Together, SFKs represent the largest family of nonreceptor tyrosine kinases and interact directly with receptor tyrosine kinases, G-protein-coupled receptors, steroid receptors, signal transducers and activators of transcription, and molecules involved in cell adhesion and migration. These interactions lead to a diverse array of biological functions including proliferation, cell growth, differentiation, cell shape, motility, migration, angiogenesis, and survival. Studies investigating mutational activation of Src in human cancers suggest that this may be a rare event and that wild-type Src is weakly oncogenic. Thus, the role of Src in the development and progression of human cancer remains unclear. Recently, it was suggested that increased SFK protein levels and, more importantly, SFK tyrosine kinase activity are linked to cancer progression and metastatic disease by facilitating the action of other signaling proteins. This accumulating body of evidence indicates that SFKs may represent a promising therapeutic target for the treatment of solid tumors. This review discusses the role of SFKs in solid tumors and the recent therapeutic advances aimed at targeting this family of tyrosine kinases in cancer.

Restriction of Src activity by Cullin-5

Src is a nonreceptor tyrosine kinase that coordinates responses to diverse soluble and adhesive signaling molecules and regulates cell proliferation, survival, differentiation and migration. Normally, Src activity is tightly regulated, and Src-catalyzed phosphorylation is counterbalanced by phosphotyrosine phosphatases. However, deregulated mutant Src causes malignant transformation when highly expressed. Src transformation is dose dependent, but it has been unclear how much mutant Src, compared with endogenous Src, is required for transformation. Here, we show that transformation requires high-level overexpression of mutant src mRNA, in part because active Src protein is degraded by ubiquitin-mediated proteolysis. We show that active but not inactive Src protein is downregulated depending on the putative tumor suppressor and E3 ubiquitin ligase component, Cullin-5 (Cul5). Cul5 removal synergizes with physiological levels of mutant src mRNA to increase protein tyrosine phosphorylation, induce morphological transformation, and deregulate growth. Cul5 also represses Src-induced tumorigenesis and regulates Src signaling in normal cells. These results suggest that, when Src is activated by mutation or physiological mechanisms, its effects are limited by Cul5, which downregulates active Src and its phosphorylated substrates. These findings demonstrate the importance of a new mechanism that downregulates Src signaling in cells.

Src kinase induces tumor formation in the c-SRC C57BL/6 mouse

Src kinase has been linked as a causative agent in the progression of a number of cancers including colon, breast, lung and melanoma. Src protein and activity levels are increased in colorectal cancer and liver metastases arising secondary to colon cancer. However, although Src protein is increased in colon cancer as early as the adenomatous polyp stage, a role for Src in carcinogenesis has not been established. We developed the c-SRC transgenic mouse in the C57BL/6 strain to address the issue of carcinogenesis in cells with high levels of Src expression. The transgene was constructed with the human c-SRC gene downstream of the mouse metallothionein promoter to create zinc inducible gene expression. In these C57BL/6 mice, Src protein was increased in a number of tissues both with and without zinc induction. No additional carcinogenic agent was administered. After 20 months, mice were assessed for tumor development in the liver and GI tract, as well as other organs. Of the mice with the transgene, 15% developed tumors in the liver while no tumors were detected in wild type C57BL/6 mice. A further study was conducted by crossing c-SRC C57BL/6 mice with p21 nullizygous mice to determine the effect of oncogene expression combined with inactivation of the tumor suppressor gene, p21. Addition of the c-SRC transgene to the p21-/- background increased tumor formation almost 3-fold, while it increased metastasis 6-fold. The data from our study show, for the first time, that Src kinase may play a role in carcinogenesis.

Combined inhibition of c-Src and epidermal growth factor receptor abrogates growth and invasion of head and neck squamous cell carcinoma

PURPOSE

Increased expression and/or activation of epidermal growth factor receptor (EGFR) is associated with tumor progression and poor prognosis in many cancers, including head and neck squamous cell carcinoma (HNSCC). Src family kinases, including c-Src, mediate a variety of intracellular or extracellular signals that contribute to tumor formation and progression. This study was undertaken to elucidate the role of c-Src in the growth and invasion of HNSCC and to determine the effects of combined targeting of EGFR and Src kinases in HNSCC cell lines.

EXPERIMENTAL DESIGN

HNSCC cells were engineered to stably express a dominant-active form of c-Src and investigated in cell growth and invasion assays. The biochemical effects of combined treatment with the Src inhibitor AZD0530, a potent, orally active Src inhibitor with Bcr/Abl activity, and the EGFR kinase inhibitor gefitinib were examined, as well as the consequences of dual Src/EGFR targeting on the growth and invasion of a panel of HNSCC cell lines.

RESULTS

HNSCC cells expressing dominant-active c-Src showed increased growth and invasion compared with vector-transfected controls. Combined treatment with AZD0530 and gefitinib resulted in greater inhibition of HNSCC cell growth and invasion compared with either agent alone.

CONCLUSIONS

These results suggest that increased expression and activation of c-Src promotes HNSCC progression where combined targeting of EGFR and c-Src may be an efficacious treatment approach.

The role of Src in prostate cancer

The Src family kinases (SFKs) are the largest family of nonreceptor protein tyrosine kinases and are responsible for signal transduction during many cellular activities, including differentiation, adhesion, and migration. Aberrant Src/SFK activity has been widely implicated in cancer development. Several lines of evidence indicate a role for SFKs in the development of prostate cancer, e.g. SFK overexpression in prostate cancer cell lines and tissues and reduced cancer cell proliferation, invasion, and migration following Src inhibition. In particular, Src may be involved in androgen-independent growth during advanced stages of disease. Src signaling is also a key pathway during normal and dysregulated bone functioning, and bone metastases are responsible for substantial morbidity in advanced prostate cancer. Src/SFK inhibition therefore represents a potentially useful therapeutic strategy for patients with various stages of prostate cancer. To date, four Src inhibitors have reached clinical trials. Of these, the broadest range of in vitro prostate cancer data are available for dasatinib, which inhibits several SFKs as well as other tyrosine kinases. Src inhibitors may be specifically evaluated in prostate cancer clinical trials in the near future.

Integrin αvβ3 Controls Activity and Oncogenic Potential of Primed c-Src

Increased activity of the proto-oncogene c-Src and elevated levels of integrin alpha(v)beta(3) are found in melanomas and multiple carcinomas. Regulation of c-Src involves "priming" through disruption of intramolecular interactions followed by "activation" through phosphorylation in the kinase domain. Interactions with overexpressed receptor tyrosine kinases or mutations in the SRC gene can induce priming of c-Src in cancer. Here, we show that alpha(v)beta(3) promotes activation of primed c-Src, causing enhanced phosphorylation of established Src substrates, survival, proliferation, and tumor growth. The beta(3) cytoplasmic tail is required and sufficient for integrin-mediated stimulation of all these events through a mechanism that is independent of beta(3) tyrosine phosphorylation. Instead, experiments using Src variants containing the v-Src Src homology 3 (SH3) domain and using mutant beta(3) subunits indicate that a functional interaction of the beta(3) cytoplasmic tail with the c-Src SH3 domain is required. These findings delineate a novel integrin-controlled oncogenic signaling cascade and suggest that the interaction of alpha(v)beta(3) with c-Src may represent a novel target for therapeutic intervention.

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.

Adhesion signaling by a novel mitotic substrate of src kinases

Src kinases are activated and relocalize to the cytoplasm during mitosis, but their mitotic function has remained elusive. We describe here a novel mitotic substrate of src kinases. Trask (transmembrane and associated with src kinases) is a 140 kDa type I transmembrane glycoprotein unrelated to currently known protein families. Src kinases phosphorylate Trask in vitro and mediate its mitotic hyperphosphorylation in vivo. Trask associates with both yes and src, is localized to the cell membrane during interphase, and undergoes cytoplasmic relocalization during mitosis. Overexpression of Trask leads to cell rounding and a loss of adhesion phenotype. Consistent with a function in cell adhesion, Trask interacts with a number of adhesion and matrix proteins including cadherins, syndecans, and the membrane-type serine protease 1 (MT-SP1), and is proteolytically cleaved by MT-SP1. Trask is unique among cell adhesion molecules in that it is under cell cycle regulation and thus links src kinases with the mitotic regulation of cell adhesion. This suggests a potential pathway by which hyperactive src kinases in tumors can deregulate adhesion signaling and mediate the metastatic phenotype.

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.

c-Src and cooperating partners in human cancer

The proto-oncogene c-src is rarely mutated in human cancers, and when overexpressed in normal cells is non- or weakly oncogenic. These observations have raised doubts about the involvement of c-src in the etiology of human tumors. However, recent studies have shown that c-Src, a non-receptor tyrosine kinase, exhibits elevated protein levels and activity in numerous types of human cancers. Furthermore, it has been found to be a critical component of multiple signaling pathways that regulate proliferation, survival, metastasis, and angiogenesis. Because of its important role in these oncogenic processes, it represents a therapeutic target ripe for exploitation.

Src family protein-tyrosine kinases alter the function of PTEN to regulate phosphatidylinositol 3-kinase/AKT cascades

Src family protein-tyrosine kinases, which play an important role in signal integration, have been implicated in tumorigenesis in multiple lineages, including breast cancer. We demonstrate, herein, that Src kinases regulate the phosphatidylinositol 3-kinase (PI3K) signaling cascade via altering the function of the PTEN tumor suppressor. Overexpression of activated Src protein-tyrosine kinases in PTEN-deficient breast cancer cells does not alter AKT phosphorylation, an indicator of signal transduction through the PI3K pathway. However, in the presence of functional PTEN, Src reverses the activity of PTEN, resulting in an increase in AKT phosphorylation. Activated Src reduces the ability of PTEN to dephosphorylate phosphatidylinositols in micelles and promotes AKT translocation to cellular plasma membranes but does not alter PTEN activity toward water-soluble phosphatidylinositols. Thus, Src may alter the capacity of the PTEN C2 domain to bind cellular membranes rather than directly interfering with PTEN enzymatic activity. Tyrosine phosphorylation of PTEN is increased in breast cancer cells treated with pervanadate, suggesting that PTEN contains sites for tyrosine phosphorylation. Src kinase inhibitors markedly decreased pervanadate-mediated tyrosine phosphorylation of PTEN. Further, expression of activated Src results in marked tyrosine phosphorylation of PTEN. SHP-1, a SH2 domain-containing protein-tyrosine phosphatase, selectively binds and dephosphorylates PTEN in Src transfected cells. Both Src inhibitors and SHP-1 overexpression reverse Src-induced loss of PTEN function. Coexpression of PTEN with activated Src reduces the stability of PTEN. Taken together, the data indicate that activated Src inhibits PTEN function leading to alterations in signaling through the PI3K/AKT pathway.

Constitutive activation of the SRC family kinase Hck results in spontaneous pulmonary inflammation and an enhanced innate immune response

To identify the physiological role of Hck, a functionally redundant member of the Src family of tyrosine kinases expressed in myelomonocytic cells, we generated Hck(F/F) "knock-in" mice which carry a targeted tyrosine (Y) to phenylalanine (F) substitution of the COOH-terminal, negative regulatory Y(499)-residue in the Hck protein. Unlike their Hck(-/-) "loss-of-function" counterparts, Hck(F/F) "gain-of-function" mice spontaneously acquired a lung pathology characterized by extensive eosinophilic and mononuclear cell infiltration within the lung parenchyma, alveolar airspaces, and around blood vessels, as well as marked epithelial mucus metaplasia in conducting airways. Lungs from Hck(F/F) mice showed areas of mild emphysema and pulmonary fibrosis, which together with inflammation resulted in altered lung function and respiratory distress in aging mice. When challenged transnasally with lipopolysaccharide (LPS), Hck(F/F) mice displayed an exaggerated pulmonary innate immune response, characterized by excessive release of matrix metalloproteinases and tumor necrosis factor (TNF)alpha. Similarly, Hck(F/F) mice were highly sensitive to endotoxemia after systemic administration of LPS, and macrophages and neutrophils derived from Hck(F/F) mice exhibited enhanced effector functions in vitro (e.g., nitric oxide and TNFalpha production, chemotaxis, and degranulation). Based on the demonstrated functional association of Hck with leukocyte integrins, we propose that constitutive activation of Hck may mimic adhesion-dependent priming of leukocytes. Thus, our observations collectively suggest an enhanced innate immune response in Hck(F/F) mice thereby skewing innate immunity from a reversible physiological host defense response to one causing irreversible tissue damage.