Elevated Src Family Kinase (SFK) activity is associated with tumour invasion and metastasis. The SFK inhibitor saracatinib (AZD0530) is currently in Phase II trials in patients including those with colorectal cancer (CRC), where links between SFK activity and poor prognosis are particularly striking. Saracatinib is likely to be used clinically in combination regimens, specifically with 5FU and oxaliplatin in CRC. The aim of this study was to determine the impact of saracatinib on oxaliplatin and 5FU efficacy in CRC cells. Saracatinib did not modulate 5FU efficacy but antagonized oxaliplatin in a schedule specific manner through reduced oxaliplatin uptake via an SFK independent mechanism. Saracatinib resembles the pharmacophore of known organic cation transporter (OCT) inhibitors and reduced oxaliplatin efficacy maximally in cells over-expressing OCT2. These data suggest that oxaliplatin uptake in CRC is attenuated by saracatinib via inhibition of OCT2, a potential consideration for the clinical development of this SFK inhibitor.
saracatinib/AZD0530; Src Family Kinase; Colorectal Cancer; Oxaliplatin Uptake; Organic Cation Transporters
Src family kinases (SFK) are implicated in the development of some colorectal cancers (CRC). One SFK member, Lck, is not detectable in normal colonic epithelium, but becomes aberrantly expressed in a subset of CRCs. Although SFK have been extensively studied in fibroblasts and different types of immune cells, their physical and functional targets in many epithelial cancers remain poorly characterised.
64 CRC cell lines were tested for expression of Lck. SW620 CRC cells, which express high levels of Lck and also contain high basal levels of tyrosine phosphorylated (pY) proteins, were then analysed to identify novel SFK targets. Since SH2 domains of SFK are known to often bind substrates after phosphorylation by the kinase domain, the LckSH2 was compared with 14 other SH2s for suitability as affinity chromatography reagent. Mass spectrometric analyses of LckSH2-purified pY proteins subsequently identified several proteins readily known as SFK kinase substrates, including cortactin, Tom1L1 (SRCASM), GIT1, vimentin and AFAP1L2 (XB130). Additional proteins previously reported as substrates of other tyrosine kinase were also detected, including the EGF and PDGF receptor target Odin. Odin was further analysed and found to contain substantially less pY upon inhibition of SFK activity in SW620 cells, indicating that it is a formerly unknown SFK target in CRC cells.
Rapid identification of known and novel SFK targets in CRC cells is feasible with SH2 domain affinity chromatography. The elucidation of new SFK targets like Odin in epithelial cancer cells is expected to lead to novel insight into cancer cell signalling mechanisms and may also serve to indicate new biomarkers for monitoring tumor cell responses to drug treatments.
Compartmentalization of Src tyrosine kinases (SFK) plays an important role in signal transduction induced by a number of extracellular stimuli. For example, Src mitogenic signaling induced by platelet-derived growth factor (PDGF) is initiated in cholesterol-enriched microdomain caveolae. How this Src subcellular localization is regulated is largely unknown. Here we show that the Tom1L1-clathrin heavy chain (CHC) complex negatively regulates the level of SFK in caveolae needed for the induction of DNA synthesis. Tom1L1 is both an interactor and a substrate of SFK. Intriguingly, it stimulates Src activity without promoting mitogenic signaling. We found that, upon association with CHC, Tom1L1 reduced the level of SFK in caveolae, thereby preventing its association with the PDGF receptor, which is required for the induction of mitogenesis. Similarly, the Tom1L1-CHC complex reduced also the level of oncogenic Src in cholesterol-enriched microdomains, thus affecting both its capacity to induce DNA synthesis and cell transformation. Conversely, Tom1L1, when not associated with CHC, accumulated in caveolae and promoted Src-driven DNA synthesis. We concluded that the Tom1L1-CHC complex defines a novel mechanism involved in negative regulation of mitogenic and transforming signals, by modulating SFK partitioning at the plasma membrane.
Compartmentalization of Src tyrosine kinases (SFK) plays an important role for signal transduction induced by a number of extracellular stimuli. For example, Src mitogenic signaling induced by the growth factor Platelet-Derived Growth Factor (PDGF) is initiated in cholesterol-enriched microdomains caveolae. How this Src sub-cellular localization is regulated is largely unknown. Here we show that the Tom1L1-Clathrin Heavy Chain (CHC) complex negatively regulates the level of SFK in caveolae needed for the induction of DNA synthesis. Tom1L1 is both an interactor and a substrate of SFK. Intriguingly, it stimulates Src activity without promoting mitogenic signaling. We found that, upon association with CHC, Tom1L1 reduced the level of SFK in caveolae, thereby preventing its association with the PDGF receptor, which is required for the induction of mitogenesis. Similarly, the Tom1L1-CHC complex reduced also the level of oncogenic Src in cholesterol-enriched microdomains, thus affecting both its capacity to induce DNA synthesis and cell transformation. Conversely, Tom1L1, when not associated with CHC, accumulated in caveolae and promoted Src-driven DNA synthesis. We concluded that the Tom1L1-CHC complex defines a novel mechanism involved in negative regulation of mitogenic and transforming signals, by modulating SFK partitioning at the plasma membrane.
Animals; Caveolae; enzymology; Cell Membrane; enzymology; Cell Transformation; Neoplastic; Clathrin Heavy Chains; metabolism; DNA; biosynthesis; Hela Cells; Humans; Mice; Multiprotein Complexes; metabolism; NIH 3T3 Cells; Protein Transport; Proto-Oncogene Proteins pp60(c-src); metabolism; Receptors; Platelet-Derived Growth Factor; metabolism; src-Family Kinases; chemistry; metabolism
The proto-oncogene c-Src (Src) is a nonreceptor tyrosine kinase whose expression and activity is 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 the 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 this may be a rare event and wild-type Src is weakly oncogenic. Thus, the role of Src in the development and progression of human cancer has remained unclear. Recently, it has been suggested that increased SFK protein levels and, more importantly, SFK tyrosine kinase activity is 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.
c-Src; solid tumors; Src family kinases; molecular inhibitors
Src family kinases (SFKs) are frequently over-expressed and/or activated in human cancers, and play key roles in cancer cell invasion, metastasis, proliferation, survival and angiogenesis. Allosteric activation of SFKs occurs through well-defined post-translational mechanisms, however the SFK member Fyn is over-expressed in multiple human cancers (prostate, melanoma, pancreatic, glioma, chronic myelogenous leukemia) and the mechanism of increased Fyn expression is unclear. Since activation of Ras oncogenes is a common oncogenic event leading to the activation of multiple effector pathways, we explored if Ras could induce Fyn expression. Retroviral transduction of the human keratinocyte cell line HaCaT with oncogenic H-Ras dramatically up-regulated Fyn mRNA (>100-fold, p<0.001), protein, and kinase activity without affecting Src levels or activity. Activation of Akt, but not MAPK or EGFR, was necessary and sufficient for induction of Fyn by H-Ras. Expression of active Fyn was sufficient to increase HaCaT cell migration and invasion, and the enhanced migration and invasion induced by H-Ras could be significantly blocked (70% reduction, p<0.01) by knockdown of Fyn with a specific siRNA or inhibition of SFKs with PP2. In addition, expression of Fyn in MDA-MB-231 breast cancer cells was dependent on PI3K activity and was involved in their invasive phenotype. Thus, the Ras/PI3K/Akt pathway can account for Fyn over-expression in cancers, and Fyn is a critical mediator of the Ras-stimulated invasive cell phenotype. These results support the development of therapeutic strategies targeting Akt/Fyn pathway to block migration and invasion of tumor cells.
Src-Famiy Kinases; Proto-Oncogene Proteins c-Fyn; Proto-Oncogene Proteins c-Akt; Genes; ras; Tumor Cell Invasion; Squamous Cell Carcinoma
Activating mutations in the Kit receptor tyrosine kinase are associated with Gastrointestinal Stromal Tumor (GIST). Imatinib inhibits Kit and is front-line therapy for GIST. However, imatinib most often elicits a partial response or stable disease and most GIST patients who initially respond to imatinib eventually acquire resistance. Thus improved treatment strategies for GIST are needed. We investigated the role of Src Family kinases (SFKs) in tumorigenesis in a mouse model of human GIST. Whereas the SFKs Src and Lyn were active in GIST, surprisingly imatinib treatment stimulated their phosphorylation/activation. We show that integrin signaling activates FAK and consequently SFKs in GIST and that imatinib enhanced integrin signaling implies a role for the extracellular matrix and integrin signaling in tumor maintenance and imatinib resistance. Dasatinib, an inhibitor of SFKs and Kit, inhibited SFKs and FAK activation in GIST but also inhibited Kit and Kit-dependent downstream signaling pathways including PI3-kinase, MAPK but not STAT signaling. While dasatinib and imatinib alone both produced a minimal histo-pathological response, combination therapy improved it leading to increased necrosis in GIST. These results highlight the importance of SFK and STAT signaling in GIST and suggest that stimulation of integrin signaling by imatinib may limit its clinical efficacy.
Vascular endothelial growth factor (VEGF) is the predominant pro-angiogenic cytokine in human malignancy, and its expression correlates with disease recurrence and poor outcomes in patients with colorectal cancer. Recently, expression of vascular endothelial growth factor receptors (VEGFRs) has been observed on tumours of epithelial origin, including those arising in the colon, but the molecular mechanisms governing potential VEGF-driven biologic functioning in these tumours are not well characterised. In this report, we investigated the role of Src family kinases (SFKs) in VEGF-mediated signalling in human colorectal carcinoma (CRC) cell lines. Vascular endothelial growth factor specifically activated SFKs in HT29 and KM12L4 CRC cell lines. Further, VEGF stimulation resulted in enhanced cellular migration, which was effectively blocked by pharmacologic inhibition of VEGFR-1 or Src kinase. Correspondingly, migration studies using siRNA clones with reduced Src expression confirmed the requirement for Src in VEGF-induced migration in these cells. Furthermore, VEGF treatment enhanced VEGFR-1/SFK complex formation and increased tyrosine phosphorylation of focal adhesion kinase, p130 cas and paxillin. Finally, we demonstrate that VEGF-induced migration is not due, at least in part, to VEGF acting as a mitogen. These results suggest that VEGFR-1 promotes migration of tumour cells through a Src-dependent pathway linked to activation of focal adhesion components that regulate this process.
VEGFR-1; VEGF; Src kinase; colorectal cancer
In subtypes and late stages of leukemias driven by the tyrosine kinase fusion protein Bcr-Abl, Src signaling critically contributes to the leukemic phenotype. We performed global tyrosine phosphoprofiling using quantitative mass spectrometry of Bcr-Abl transformed cells in which the activities of the Src family kinases (SFKs) were perturbed to build a detailed context-dependent network of cancer signaling. Perturbation of the SFKs Lyn and Hck with genetics or inhibitors revealed Bcr-Abl downstream phosphorylation events either mediated by or independent of SFKs. We identified multiple negative feedback mechanisms within the network of signaling events affected by Bcr-Abl and SFKs, and found that Bcr-Abl attenuated these inhibitory mechanisms. The Csk binding protein Pag1 (also known as Cbp) and the tyrosine phosphatase Ptpn18 both mediated negative feedback to SFKs. We observed Bcr-Abl-mediated phosphorylation of the phosphatase Shp2 (Ptpn11) and this may contribute to the suppression of these negative feedback mechanisms to promote Bcr-Abl-activated SFK signaling. Csk and a kinase-deficient Csk mutant both produced similar globally repressive signaling consequences, suggesting a critical role for the adaptor protein function of Csk in its inhibition of Bcr-Abl and SFK signaling. The identified Bcr-Abl-activated SFK regulatory mechanisms are candidates for dysregulation during leukemia progression and acquisition of SFK-mediated drug resistance.
Spatial regulation is an important feature of signal specificity elicited by cytoplasmic tyrosine kinases of the Src family (SRC family protein tyrosine kinases [SFK]). Cholesterol-enriched membrane domains, such as caveolae, regulate association of SFK with the platelet-derived growth factor receptor (PDGFR), which is needed for kinase activation and mitogenic signaling. PAG, a ubiquitously expressed member of the transmembrane adaptor protein family, is known to negatively regulate SFK signaling though binding to Csk. We report that PAG modulates PDGFR levels in caveolae and SFK mitogenic signaling through a Csk-independent mechanism. Regulation of SFK mitogenic activity by PAG requires the first N-terminal 97 aa (PAG-N), which include the extracellular and transmembrane domains, palmitoylation sites, and a short cytoplasmic sequence. We also show that PAG-N increases ganglioside GM1 levels at the cell surface and, thus, displaces PDGFR from caveolae, a process that requires the ganglioside-specific sialidase Neu-3. In conclusion, PAG regulates PDGFR membrane partitioning and SFK mitogenic signaling by modulating GM1 levels within caveolae independently from Csk.
Src family kinases (SFKs) are signaling enzymes that have long been recognized to regulate critical cellular processes such as proliferation, survival, migration, and metastasis. Recently, considerable work has elucidated mechanisms by which SFKs regulate normal and pathologic processes in vascular biology, including endothelial cell proliferation and permeability. Further, when inappropriately activated, SFKs promote pathologic inflammatory processes and tumor metastasis, in part through their effects on the regulation of endothelial monolayer permeability. In this review, we discuss the roles of aberrantly activated SFKs in mediating endothelial permeability in the context of inflammatory states and tumor cell metastasis. We further summarize recent efforts to translate Src-specific inhibitors into therapy for systemic inflammatory conditions and numerous solid organ cancers.
Src family kinases; Inhibitors; Tumor progression; Metastasis; Inflammation
Locoregional and distant recurrence remains common and usually fatal for patients with advanced head and neck squamous cell carcinoma (HNSCC). One promising molecular target in HNSCC is the Src family kinases (SFKs). SFKs can affect cellular proliferation and survival by activating the signal transducer and activator of transcription (STAT) family of transcription factors, especially STAT3. Surprisingly, sustained SFK inhibition resulted in only transient inhibition of STAT3. We investigated the mechanism underlying STAT3 activation and its biological importance. Specific c-Src knockdown with small interfering RNA (siRNA) resulted in STAT3 activation demonstrating specificity, which was inhibited by JAK (TYK2 and JAK2) depletion with siRNA. Sustained SFK inhibition also resulted in recovered JAK-STAT3 binding and JAK kinase activity after an initial reduction, although JAK phosphorylation paradoxically decreased. To determine the biologic significance of STAT3 activation, we combined specific STAT3 depletion with a pharmacologic SFK inhibitor and observed increased cell cycle arrest and apoptosis. Likewise, the addition of STAT3- or JAK-specific siRNA to c-Src–depleted cells enhanced cytotoxicity relative to cells incubated with c-Src siRNA alone. These results demonstrate that reactivation of STAT3 after sustained, specific c-Src inhibition is mediated through altered JAK-STAT3 binding and JAK kinase activity and that this compensatory pathway allows for cancer cell survival and proliferation despite durable c-Src inhibition. To our knowledge, this novel feedback pathway has never been described previously. Given that pharmacologic SFK inhibitors are currently being evaluated in clinical trials, these results have potential clinical implications for cancer therapy.
Src; STAT3; JAK; head and neck cancer
Activation of NMDA receptors (NMDAR) is associated with divergent downstream signaling leading to neuronal survival or death that may be regulated in part by whether the receptor is located synaptically or extrasynaptically. Distinct activation of the MAP kinases ERK and p38 by synaptic and extrasynaptic NMDAR is one of the mechanisms underlying these differences. We have recently shown that the Src family kinases (SFKs) play an important role in neonatal hypoxic-ischemic brain injury by regulating NMDAR phosphorylation. In this study, we characterized the distribution of NMDAR, SFKs and MAP kinases in synaptic and extrasynaptic membrane locations in the postnatal day 7 and adult mouse cortex. We found that the NMDAR, SFKs and phospho-NR2B were predominantly at synapses, whereas striatal-enriched protein tyrosine phosphatase (STEP) and its substrates ERK and p38 were much more concentrated extrasynaptically. NR1/NR2B was the main subunit at extrasynaptic membrane with concomitant NR2B phosphorylation at tyrosine (Y) 1336 in the immature brain. STEP expression increased, while p38 decreased with development in the extrasynaptic membrane. These results suggest that SFKs and STEP are poised to differentially regulate NMDAR-mediated signaling pathways due to their distinct subcellular localization, and thus may contribute to the age-specific differences seen in vulnerability, pathology and consequences of hypoxic–ischemic brain injury.
neonatal; p38; ERK; STEP; NR2B; phosphorylation
Background and aim
CD24 expression is associated with human colorectal cancer (CRC). Our previous data indicated that CD24 promoted the proliferation and invasion of colorectal cancer cells through the activation of ERK1/2. Since Src family kinases are frequently deregulated in CRC and closely related to the MAPK signaling pathway, we investigated the impact of Lyn, an important member of SFKs, on CD24-induced ERK1/2 activation in CRC.
Methods and Results
The interaction of CD24 and Lyn was identified by co-immunoprecipitation (Co-IP) and ectopic expression of CD24-induced Lyn activation. Inhibition of Lyn activation by phosphatase PP2 in SW480CD24cells abrogated CD24-induced invasion. The results of the Co-IP and immunofluorescence assay revealed that overexpression of CD24 enhanced the interaction of Lyn and ERK1/2 and induced the nuclear translocation of Lyn. However, inhibition of Lyn activity attenuated CD24-induced ERK1/2 activation, and depletion of CD24 disrupted Lyn-ERK1/2 interaction. Immunohistochemistry analysis for 202 cases of CRC showed that the expression of both CD24 and Lyn was positively correlated with tumor grade, stage, lymph node and distant metastasis. Patients with lower expression of CD24 or Lyn had a higher survival rate. The Cox multivariate analysis showed that CD24 expression, but not Lyn expression, was an independent prognostic factor of CRC.
Our results suggest that Lyn is involved in CD24-induced ERK1/2 activation in CRC. The expression of CD24 is associated with activation of Lyn and ERK1/2, which might be a novel mechanism related to CD24-mediated regulation of CRC development.
CD24; Lyn; ERK1/2; Colorectal cancer
Multiple SRC-family kinases (SFKs) are commonly activated in carcinoma and appear to have a role in metastasis through incompletely understood mechanisms. Recent studies have shown that CDCP1 (CUB (complement C1r/C1s, Uegf, Bmp1) Domain-Containing Protein-1) is a transmembrane protein and an SRC substrate potentially involved in metastasis. Here we show that increased SFK and CDCP1 tyrosine phosphorylation is, surprisingly, associated with a decrease in FAK phosphorylation. This appears to be true in human tumors as shown by our correlation analysis of a mass spectrometric data set of affinity-purified phosphotyrosine peptides obtained from normal and cancer lung tissue samples. Induction of tyrosine phosphorylation of CDCP1 in cell culture, including by a mAb that binds to its extracellular domain, promoted changes in SFK and FAK tyrosine phosphorylation, as well as in PKC™, a protein known to associate with CDCP1, and these changes are accompanied by increases in adhesion and motility. Thus, signaling events that accompany the CDCP1 tyrosine phosphorylation observed in cell lines and human lung tumors may explain how the CDCP1/SFK complex regulates motility and adhesion.
signaling; mass spectrometry; SRC; adhesion; motility; metastasis
A fundamental issue in cell biology is how signals are transmitted across membranes. A variety of transmembrane receptors, including multichain immune recognition receptors, lack catalytic activity and require Src family kinases (SFKs) for signal transduction. However, many receptors only bind and activate SFKs after ligand-induced receptor dimerization. This presents a conundrum: How do SFKs sense the dimerization of receptors to which they are not already bound? Most proposals to resolve this enigma invoke additional players, such as lipid rafts or receptor conformational changes. Here we used simple thermodynamics to show that SFK activation is a natural outcome of clustering of receptors with SFK phosphorylation sites, provided that there is phosphorylation-dependent receptor-SFK association and an SFK bound to one receptor can phosphorylate the second receptor or its associated SFK in a dimer. A simple system of receptor, SFK and an unregulated protein tyrosine phosphatase (PTP) can account for ligand-induced changes in phosphorylation observed in cells. We suggest that a core signaling system comprising a receptor with SFK phosphorylation sites, an SFK and an unregulated PTP provides a robust mechanism for transmembrane signal transduction. Other events that regulate signaling in specific cases may have evolved for fine-tuning of this basic mechanism.
Ephrin; C-type lectin; Nephrin; CDCP1; Reelin; Dab1; Multichain immune recognition receptors; T cell receptor; B cell receptor; FcεRI
Src-family tyrosine kinases (SFKs) play critical roles in regulating cellular proliferation in epithelial cells, and SFK activity is increased in many human carcinomas. Src-activating and signaling molecule (Srcasm) is a novel SFK substrate that downregulates SFK activityand promotes keratinocyte differentiation. Srcasm has also been to shown to function as an anti-oncogene in the epidermis and its levels are decreased in cutaneous squamous cell carcinoma (SCC). The purpose of this study is to determine if Srcasm levels are decreased in esophageal SCC (ESCC) compared with unremarkable mucosa. Given that Srcasm functions as an anti-onocogene in squamous epithelium, we hypothesized that Srcasm levels should be decreased in esophageal SCCs compared to unremarkable mucosa. To evaluate this hypothesis, we performed protein immunohistochemistry for Srcasm on nine unremarkable esophageal mucosal specimens and twelve esophageal SCCs. Our results show that Srcasm protein staining levels are decreased in esophageal SCC compared to unremarkable mucosa. These data show that the pattern of Srcasm staining inversely correlates with ESCC formation and is consistent with the hypothesis that Srcasm may function as an anti-oncogene in esophageal squamous mucosa.
Src-activating and signaling molecule; Src-family tyrosine kinases; esophageal squamous cell carcinoma
Src-family tyrosine kinases (SFKs) are important regulators of epithelial cell growth and differentiation. Characterization of cellular mechanisms that regulate SFK activity will provide insights into the pathogenesis of diseases associated with increased SFK activity. Keratin 14-Fyn (K14) transgenic mice were derived to characterize the effect of Fyn on epidermal growth and differentiation in vivo. The epidermis of K14-Fyn mice is thickened, manifests prominent scale, and exhibits features consistent with hyperproliferation. Increased epidermal Fyn levels correlate with activation of p44/42 MAP kinases, STAT-3, and PDK-1; key signaling molecules that promote epithelial cell growth.
The Src-activating and signaling molecule (Srcasm) is a substrate of SFKs that becomes tyrosine phosphorylated downstream of the EGF receptor. In vitro, increased Srcasm levels promote activation of endogenous Fyn and keratinocyte differentiation. To study the in vivo effect of Srcasm upon Fyn, double transgenic lines were derived. K14-Fyn/Srcasm transgenic mice did not manifest the hyperproliferative phenotype. In contrast, K14-Fyn/Srcasm-P transgenic mice, that express a non-phosphorylatable Srcasm mutant, maintain the hyperproliferative phenotype. Resolution of the hyper-proliferative phenotype correlated with reduced Fyn levels in vivo in three experimental systems: transgenic mice, primary keratinocytes, and cell lines. Biochemical studies reveal that Srcasm-dependent Fyn downregulation requires Fyn kinase activity, phosphorylation of Srcasm, and the SrcasmGAT domain. Therefore, Srcasm is a novel regulator of Fyn promoting kinase downregulation in a phosphorylation-dependent manner. Srcasm may act as a molecular ‘rheostat’ for activated SFKs, and cellular levels of Srcasm may be important for regulating epithelial hyperproliferation associated with increased SFK activity.
Src family kinases (SFK) are commonly deregulated in cancer cells. Among other functions SFK are critical for cellular migration and invasion. SFK inhibitors are being studied as targeted cancer drugs, but there are no biomarkers for non-invasive assessment of SFK inhibition. The aim of this study was to evaluate whether imaging of αVβ3 integrin activity with PET and [64Cu]DOTA–cyclo-Arg-Gly-Asp-Phe-Lys ([64Cu]DOTA–c(RGDfK)) can be used for monitoring response to the SFK inhibitor dasatinib.
SCID mice bearing U87MG xenografts were gavaged daily over 72 hours with 72 or 95 mg/kg dasatinib, or vehicle. Tumor uptake of [64Cu]DOTA–c(RGDfK) was measured by small animal PET. In parallel, fluorodeoxyglucose (FDG) scans were performed to assess tumor metabolism in response to dasatinib treatment.
Dasatinib significantly (p<0.0001) reduced [64Cu]DOTA–c(RGDfK) uptake by up to 59% in U87MG xenografts (2.10±0.14% ID/g in the 95 mg/kg group, 3.12±0.18% ID/g in the 72 mg/kg group, versus 5.08±0.80% ID/g in controls). In contrast, tumor FDG uptake showed no significant reduction with dasatinib therapy (8.13±0.45% ID/g in treated vs 10.39±1.04% ID/g in controls, p=0.170). Histologically, tumors were viable at the time of the follow-up PET scan, but showed inhibition of focal adhesion kinase. Continued dasatinib treatment resulted in a significant inhibition of tumor growth (tumor size on day 10 of therapy 21.13±2.60 mm2 in treated animals vs 122.50±17.68 mm2 in controls, p=0.001).
[64Cu]DOTA–c(RGDfK) may provide a sensitive means of monitoring tumor response to SFK inhibition in αVβ3 expressing cancers early in the course of therapy.
RGD peptide; αVβ3; PET imaging; dasatinib
The Src family of protein-tyrosine kinases (SFK) play important roles in mitogenesis and morphological changes induced by growth factors. The involved substrates are, however, ill defined. Using an antiphosphotyrosine antibody to screen tyrosine-phosphorylated cDNA expression library, we have identified Tom1L1, an adaptor protein of the Tom1 family and a novel substrate and activator of the SFK. Surprisingly, we found that Tom1L1 does not promote DNA synthesis induced by Src. Furthermore, we report that Tom1L1 negatively regulates SFK mitogenic signaling induced by platelet-derived growth factor (PDGF) through modulation of SFK-receptor association: (i) Tom1L1 inhibits DNA synthesis induced by PDGF; (ii) inhibition is overcome by c-myc expression or p53 inactivation, two regulators of SFK mitogenic function; (iii) Src or Fyn coexpression overrides Tom1L1 mitogenic activity; (iv) overexpression of the adaptor reduces Src association with the receptor; and (v) protein inactivation potentiates receptor complex formation, allowing increased SFK activation and DNA synthesis. However, Tom1L1 affects neither DNA synthesis induced by the constitutively active allele SrcY527F nor SFK-regulated actin assembly induced by PDGF. Finally, overexpressed Tom1 and Tom1L2 also associate with Src and affected mitogenic signaling in agreement with some redundancy among members of the Tom1 family. We concluded that Tom1L1 defines a novel mechanism for regulation of SFK mitogenic signaling induced by growth factors.
During the process of tumor progression and clinical treatments, tumor cells are exposed to oxidative stress. Tumor cells are frequently resistant to such stress by producing antiapoptotic signaling, including activation of Src family kinases (SFKs), although the molecular mechanism is not clear. In an attempt to identify the SFK-binding proteins selectively phosphorylated in gastric scirrhous carcinoma, we identified an uncharacterized protein, C9orf10. Here we report that C9orf10 (designated Ossa for oxidative stress-associated Src activator) is a novel RNA-binding protein that guards cancer cells from oxidative stress-induced apoptosis by activation of SFKs. Exposure to oxidative stress such as UV irradiation induces the association of Ossa/C9orf10 with regulatory domains of SFKs, which activates these kinases and causes marked tyrosine phosphorylation of C9orf10 in turn. Tyrosine-phosphorylated Ossa recruits p85 subunits of phosphatidylinositol 3-kinase (PI3-kinase) and behaves as a scaffolding protein for PI3-kinase and SFKs, which activates the Akt-mediated antiapoptotic pathway. On the other hand, the carboxyl terminus of Ossa has a distinct function that directly binds RNAs such as insulin-like growth factor II (IGF-II) mRNA and promotes the extracellular secretion of IGF-II. Our findings indicate that Ossa is a dual-functional protein and might be a novel therapeutic target which modulates the sensitivity of tumors to oxidative stress.
Betel quid (BQ)-chewing oral cancer is a prevalent disease in many countries of Southeast Asia. Yet, the precise disease mechanism remains largely unknown. Here, we show that BQ extract-induced cell motility in three oral cancer cells (Ca9-22, SAS, and SCC9) presumably involves the Src family kinases (SFKs). Besides, BQ extract can markedly induce cell migration of wild type mouse embryonic fibroblasts (MEFs) but not MEFs lacking three SFK members, namely, Src, Yes, and Fyn, indicating the requirement of SFKs for BQ-induced cell motility. Betel quid extract can also elevate cellular SFK activities because phosphorylation of tyrosine 416 at the catalytic domain is increased, which in turn promotes phosphorylation of an in vitro substrate, enolase. Furthermore, we identified that areca nut, a major component of BQ, is the key factor accounting for BQ-induced cell migration and invasion through SFKs-mediated signaling pathways. Immunohistochemistry revealed that, particularly in BQ-chewing cases, the activity of SFKs was significantly higher in tumor-adjacent mucosa than that in solid tumor areas (P < .01). These results suggest a possible role of SFKs in tumor-host interface and thus in early tumor invasion in vivo. Consistent with this is the observation that activation of SFKs is colocalized with invasive tumor fronts in oral squamous cell carcinoma. Together, we conclude that SFKs may represent a potential biomarker of invasion and therapeutic target in BQ-induced oral cancer.
The nonreceptor protein tyrosine kinase Src plays a crucial role in the signal transduction pathways involved in cell division, motility, adhesion, and survival in both normal and cancer cells. Although the Src family kinases (SFKs) are activated in various types of cancers, the exact mechanisms through which they contribute to the progression of individual tumors remain to be defined. The activation of Src in human cancers may occur through a variety of mechanisms that include domain interaction and structural remodeling in response to various activators or upstream kinases and phosphatastes. Because of Src's prominent roles in invasion and tumor progression, epithelial-to-mesenchymal transition, angiogenesis, and the development of metastasis, Src is a promising target for cancer therapy. Several small molecule inhibitors of Src are currently being investigated in clinical trials. In this article, we will summarize the mechanisms regulating Src kinase activity in normal and cancer cells and discuss the status of Src inhibitor development against various types of cancers.
Platelets play a fundamental role in hemostasis and thrombosis. They are also involved in pathologic conditions resulting from blocked blood vessels, including myocardial infarction and ischemic stroke. Platelet adhesion, activation, and aggregation at sites of vascular injury are regulated by a diverse repertoire of tyrosine kinase–linked and G protein–coupled receptors. Src family kinases (SFKs) play a central role in initiating and propagating signaling from several platelet surface receptors; however, the underlying mechanism of how SFK activity is regulated in platelets remains unclear. CD148 is the only receptor-like protein tyrosine phosphatase identified in platelets to date. In the present study, we show that mutant mice lacking CD148 exhibited a bleeding tendency and defective arterial thrombosis. Basal SFK activity was found to be markedly reduced in CD148-deficient platelets, resulting in a global hyporesponsiveness to agonists that signal through SFKs, including collagen and fibrinogen. G protein–coupled receptor responses to thrombin and other agonists were also marginally reduced. These results highlight CD148 as a global regulator of platelet activation and a novel antithrombotic drug target.
Src-family Kinases (SFKs) participate in the regulation of proliferation, differentiation, apoptosis, autophagy, adhesion, migration, invasion and angiogenesis in normal and cancer cells. Abnormal expression of SFKs has been documented in cancers that arise in breast, colon, ovary, melanocyte, gastric mucosa, head and neck, pancreas, lung and brain. Targeting SFKs in cancer cells has been shown to be a promising therapeutic strategy in solid tumors, particularly in ovarian, colon and breast cancers. Paclitaxel is one of most widely used chemotherapeutic agents for the management of ovarian, breast, lung and head and neck cancers. As a microtubule-stabilizing agent, paclitaxel possesses both mitosis-dependent and mitosis-independent activities against cancer cells. A variety of mechanisms such as deregulation of P-glycoprotein, alteration of tubulin isotypes, alteration of microtubule-regulatory proteins, deregulation of apoptotic signaling pathways, mutation of tubulins and overexpression of copper transporters have been implicated in the development of primary or secondary resistance to paclitaxel. By affecting cancer cell survival, proliferation, autophagy, microtubule stability, motility, and/or angiogenesis, SFKs interact with mechanisms that regulate paclitaxel sensitivity. Inhibition of SFKs can potentiate the anti-tumor activity of paclitaxel by enhancing apoptosis, autophagy and microtubule stability. Based on pre-clinical observations, administration of SFK inhibitors in combination with paclitaxel could improve treatment for ovarian, breast, lung and head and neck cancers. Identification and validation of predictive biomarkers could also permit personalization of the therapy.
Src; SFK; paclitaxel; dasatinib; ovarian cancer; drug sensitivity; resistance