v-Src suppresses SHPS-1 expression via the Ras-MAP kinase pathway to promote the oncogenic growth of cells

Role of c-Src in Carcinogenesis and Drug Resistance

The aberrant transformation of normal cells into cancer cells, known as carcinogenesis, is a complex process involving numerous genetic and molecular alterations in response to innate and environmental stimuli. The Src family kinases (SFK) are key components of signaling pathways implicated in carcinogenesis, with c-Src and its oncogenic counterpart v-Src often playing a significant role. The discovery of c-Src represents a compelling narrative highlighting groundbreaking discoveries and valuable insights into the molecular mechanisms underlying carcinogenesis. Upon oncogenic activation, c-Src activates multiple downstream signaling pathways, including the PI3K-AKT pathway, the Ras-MAPK pathway, the JAK-STAT3 pathway, and the FAK/Paxillin pathway, which are important for cell proliferation, survival, migration, invasion, metastasis, and drug resistance. In this review, we delve into the discovery of c-Src and v-Src, the structure of c-Src, and the molecular mechanisms that activate c-Src. We also focus on the various signaling pathways that c-Src employs to promote oncogenesis and resistance to chemotherapy drugs as well as molecularly targeted agents.

SIRPB1 promotes prostate cancer cell proliferation via Akt activation

BACKGROUND

Signal regulatory protein β1 (SIRPB1) is a signal regulatory protein member of the immunoglobulin superfamily and is capable of modulating receptor tyrosine kinase-coupled signaling. Copy number variations at the SIRPB1 locus were previously reported to associate with prostate cancer aggressiveness in patients, however, the role of SIRPB1 in prostate carcinogenesis is unknown.

METHODS

Fluorescence in situ hybridization and laser-capture microdissection coupled with quantitative polymerase chain reaction was utilized to determine SIRPB1 gene amplification and messenger RNA expression in prostate cancer specimens. The effect of knockdown of SIRPB1 by RNA interference in PC3 prostate cancer cells on cell growth in colony formation assays and cell mobility in wound-healing, transwell assays, and cell cycle analysis was determined. Overexpression of SIPRB1 in C4-2 prostate cancer cells on cell migration, invasion, colony formation and cell cycle progression and tumor take rate in xenografts was also determined. Western blot assay of potential downstream SIRPB1 pathways was also performed.

RESULTS

SIRPB1 gene amplification was detected in up to 37.5% of prostate cancer specimens based on in silico analysis of several publicly available datasets. SIRPB1 gene amplification and overexpression were detected in prostate cancer specimens. The knockdown of SIRPB1 significantly suppressed cell growth in colony formation assays and cell mobility. SIRPB1 knockdown also induced cell cycle arrest during the G₀ /G₁ phase and enhancement of apoptosis. Conversely, overexpression of SIPRB1 in C4-2 prostate cancer cells significantly enhanced cell migration, invasion, colony formation, and cell cycle progression and increased C4-2 xenograft tumor take rate in nude mice. Finally, this study presented evidence for SIRPB1 regulation of Akt phosphorylation and showed that Akt inhibition could abolish SIRPB1 stimulation of prostate cancer cell proliferation.

CONCLUSIONS

These results suggest that SIRPB1 is a potential oncogene capable of activating Akt signaling to stimulate prostate cancer proliferation and could be a biomarker for patients at risk of developing aggressive prostate cancer.

Global tyrosine kinome profiling of human thyroid tumors identifies Src as a promising target for invasive cancers

BACKGROUND

Novel therapies are needed for the treatment of invasive thyroid cancers. Aberrant activation of tyrosine kinases plays an important role in thyroid oncogenesis. Because current targeted therapies are biased toward a small subset of tyrosine kinases, we conducted a study to reveal novel therapeutic targets for thyroid cancer using a bead-based, high-throughput system.

METHODS

Thyroid tumors and matched normal tissues were harvested from twenty-six patients in the operating room. Protein lysates were analyzed using the Luminex immunosandwich, a bead-based kinase phosphorylation assay. Data was analyzed using GenePattern 3.0 software and clustered according to histology, demographic factors, and tumor status regarding capsular invasion, size, lymphovascular invasion, and extrathyroidal extension. Survival and invasion assays were performed to determine the effect of Src inhibition in papillary thyroid cancer (PTC) cells.

RESULTS

Tyrosine kinome profiling demonstrated upregulation of nine tyrosine kinases in tumors relative to matched normal thyroid tissue: EGFR, PTK6, BTK, HCK, ABL1, TNK1, GRB2, ERK, and SRC. Supervised clustering of well-differentiated tumors by histology, gender, age, or size did not reveal significant differences in tyrosine kinase activity. However, supervised clustering by the presence of invasive disease showed increased Src activity in invasive tumors relative to non-invasive tumors (60% v. 0%, p<0.05). In vitro, we found that Src inhibition in PTC cells decreased cell invasion and proliferation.

CONCLUSION

Global kinome analysis enables the discovery of novel targets for thyroid cancer therapy. Further investigation of Src targeted therapy for advanced thyroid cancer is warranted.

Recovery of anoikis in Src-transformed cells and human breast carcinoma cells by restoration of the SIRP α1/SHP-2 signaling system

Src kinase dysregulation contributes to cancer progression but mechanistic understanding for this contribution remains incomplete. Signal regulatory protein α1 (SIRPα1) is a tumor suppressor that is constitutively suppressed in v-Src-transformed cells, where restoration of SIRPα1 expression inhibits anchorage-independent growth. In this study, we investigated the role of the protein tyrosine phosphatase-2 (SHP-2) in SIRPα1 activity. SHP-2 suppression resulted in a blockade of SIRPα1-mediated inhibition of anchorage-independent growth. Notably, we found that SIRPα1 did not act in v-Src-transformed cells by triggering cell growth arrest but by eliciting a suspension-selective apoptosis (anoikis), and that SHP-2 was required for this effect. Furthermore, we found that SHP-2 was crucial for recovery of stress fiber and focal contact formation by SIRPα1 in v-Src-transformed cells. Finally, we found that SIRPα1/SHP-2 signaling regulates anoikis in human breast carcinoma cells with activated c-Src. Taken together, our findings define SHP-2 as an essential component of tumor suppression and anoikis mediated by SIRPα1 in human breast carcinoma cells as well as in v-Src-transformed cells.

Dissection of DEN-induced platelet proteome changes reveals the progressively dys-regulated pathways indicative of hepatocarcinogenesis

Due to the lack of precise markers indicative of its occurrence, progression, and malignant stages, hepatocellular carcinoma (HCC) is currently associated with high mortality. Given the fact that thrombocytopenia is associated with chronic liver diseases, and the multifunctional nature of platelets we reason that phenotype-specific platelets could be the systemic barometer for hepato-carcinogenesis. The mass spectrometry (MS)-based proteomic efforts to discover novel biomarkers in plasma or serum are largely compromised by a few of the overwhelmingly abundant proteins that comprise over 95% of the total protein mass of plasma or sera. Platelets however are free of these MS signal-suppressing proteins. On the basis of a HCC animal model where diethyl nitrosamine (DEN) administration on male rats specifically induces HCC, by using a multiplex quantitative proteomic approach, we profiled the phase-to-phase proteome changes in a series of viable phenotype-specific platelets along with the DEN-induced progressive liver transformation. The platelet proteome was found highly responsive to each physiological stage of liver inflammation or pathogenesis. Using data-dependent bioinformatics network analysis, we found that certain pathway modules involved in immune response, tissue wound repair, apoptosis, cell proliferation, and catabolism and metabolism were differentially regulated, which were uncovered by the DEN-induced differential expression of the corresponding pathway components. The phase-specific presentations of these pathways suggested that the DEN-induced progression of immune suppression and apoptosis resistance is dynamically coordinated in the platelets. These novel platelet signatures are interconnected in the dynamic networks along with HCC progression and could be identified noninvasively for HCC prognosis and early diagnosis.

Phosphorylation of histone H3 at Ser10: its role in cell transformation by v-Src

We found that transformation by v-src constitutively activated phosphorylation of histone H3 at Ser10 in a transformation-specific manner. While nontransforming mutant of v-src did not activate H3 phosphorylation, H3 phosphorylation in cells expressing temperature-sensitive mutant of v-src was temperature-dependent. Inhibition of Ras signaling by Gap1m, a GTPase-activation protein for Ras, or S17N Ras, a dominant negative form of Ras, substantially suppressed the Ser10 phosphorylation of H3. Similarly, treatment of cells with manumycin A, a potent inhibitor of Ras-falnesyl transferase, clearly suppressed the H3 phosphorylation. In contrast, inhibition of STAT3 signaling or PI3K signaling did not perturb H3 phosphorylation. We found, however, inhibition of MEK or MSK1 markedly suppressed H3 phosphorylation. In addition, inhibition of MSK1 expression by its siRNA substantially suppressed H3 phosphorylation and anchorage-independent growth of transformed cells. Taken together, our results strongly suggest the importance of MSK1 and H3 phosphorylation in cell transformation by v-Src.

Nek2 as an effective target for inhibition of tumorigenic growth and peritoneal dissemination of cholangiocarcinoma

We investigated the role of Nek2, a member of the serine/threonine kinase family, Nek, in the tumorigenic growth of cholangiocarcinoma cells. Expression of Nek2 is elevated in cholangiocarcinoma in a tumor-specific manner as compared with that of normal fibroblast cells. Expression of exogenous Nek2 did not perturb the growth of cholangiocarcinoma cells, whereas suppression of the Nek2 expression with siRNA resulted in the inhibition of cell proliferation and induced cell death. In xenograft-nude mouse model, s.c. injection of Nek2 siRNA around the tumor nodules resulted in reduction of tumor size as compared with those of control siRNA injection. In peritoneal dissemination model, Nek2 siRNA-treated mice showed statistically longer survival periods in comparison with those of the control siRNA-treated mice. Taken together, our data indicate a pivotal role of Nek2 in tumorigenic growth of cholangiocarcinoma.

A role for SHPS-1/SIRPalpha in Concanavalin A-dependent production of MMP-9

SHPS-1/SIRPalpha1 is a transmembrane glycoprotein that belongs to the immunoglobulin (Ig) super family. In the present study, we show that SHPS-1 strongly associates with Concanavalin A (Con A), a plant lectin obtained from jack beans. Further studies with SHPS-1 mutants reveal that the extracellular domain of SHPS-1 containing the Ig sequence is responsible for its association with Con A. Con A treatment induces cross-linking and multimerization of the SHPS-1 protein in the plasma membrane, accompanied by its tyrosine phosphorylation and recruitment of SHP-2. In contrast, Ricinus communis agglutinin (RCA), another lectin obtained from castor bean, does not bind or activate tyrosine phosphorylation of SHPS-1. Moreover, Con A activates Akt in a SHP-2-dependent manner. Treatment of mouse embryonic fibroblasts (MEFs) with Con A induces secretion of matrix metalloproteinase (MMP)-9, a phenomenon that is inhibited in cells expressing YF mutant of SHPS-1, a dominant negative form of Akt or in cells pre-treated with an Akt inhibitor, LY294002 or extracellular-signal regulated kinase (Erk) inhibitor, U0126. In addition, expression of the YF mutant of SHPS-1 inhibits Con A-dependent activation of Akt and Erk kinases. Taken together, our results suggest that SHPS-1 is a receptor for Con A that mediates Con A-dependent MMP-9 secretion through SHP-2-promoted activation of both Akt and Erk pathways.

SIRPalpha1 and SIRPalpha2: their role as tumor suppressors in breast carcinoma cells

We have previously reported that expression of SIRPalpha1/SHPS-1 was strongly suppressed in v-Src-transformed cells and its forced expression resulted in the suppression of anchorage-independent growth of the cells [K. Machida, S. Matsuda, K. Yamaki, T. Senga, A.A. Thant, H. Kurata, K. Miyazaki, K. Hayashi, T. Okuda, T. Kitamura, T. Hayakawa, M. Hamaguchi, v-Src suppresses SHPS-1 expression via the Ras-MAP kinase pathway to promote the oncogenic growth of cells, Oncogene 19 (2000) 1710-1718]. We examined the effect of human SIRPalpha1 expression in breast cancer cell lines, Hs578T and MCF7, and compared with the effect of SIRPalpha2 expression in Hs578T. Forced expression of either SIRPalpha1 or SIRPalpha2 did not perturb the growth of Hs578T in a conventional attached condition. Their expression, however, enforced the actin stress fiber formation and induced activation of Rho, but not Rac, in Hs578T cells. Moreover, forced expression of either SIRPalpha1 or SIRPalpha2 displayed distinct suppressive effect on the anchorage-independent growth of Hs578T cells. Similarly, forced expression of SIRPalpha1 in MCF7 specifically suppressed the anchorage-independent growth of the cells. Taken together, our results strongly suggest the function of SIRPalpha1 and 2 as type II tumor suppressors for human breast carcinoma.

Annexin II binds to SHP2 and this interaction is regulated by HSP70 levels

The protein tyrosine phosphatase SHP2 is a positive effector of EGFR signaling. To improve our understanding of SHP2's function, we searched for additional binding proteins of SHP2. We found that Annexin II is an SHP2-binding protein. Physical interactions of SHP2 with Annexin II were confirmed in vivo. Furthermore, binding of SHP2 with Annexin II was regulated somewhat by EGF treatment and the extracellular Ca2+ chelator, EGTA. Previously, we reported that HSP70 levels can influence the binding of SHP2 with EGFR. Interestingly, increased HSP70 levels also inhibited the binding of SHP2 with Annexin II after EGF treatment in vivo. In addition, immunostaining experiments indicated that a fraction of SHP2 and Annexin II co-localized in the cell membrane region after EGF treatment. Our findings indicate that Annexin II is binding partner of SHP2 and the binding of SHP2 with Annexin II is affected by EGF stimulation, extracellular calcium levels, and the levels of HSP70.

Transcriptional Regulation of Signal Regulatory Protein α1 Inhibitory Receptors by Epidermal Growth Factor Receptor Signaling

Signal regulatory protein (SIRP) alpha1 is a membrane glycoprotein and a member of the SIRP receptor family. These transmembrane receptors have been shown to exert negative effects on signal transduction by receptor tyrosine kinases via immunoreceptor tyrosine-based inhibitory motifs in the carboxyl domain. Previous work has demonstrated that SIRPs negatively regulate many signaling pathways leading to reduction in tumor migration, survival, and cell transformation. Thus, modulation of SIRP expression levels or activity could be of great significance in the field of cancer therapy. The aim of the present study was to determine the factors that regulate levels of SIRPalpha1 in human glioblastoma cells that frequently overexpress the epidermal growth factor receptor (EGFR) because SIRPs have been shown to negatively regulate EGFR signaling. Northern blot analysis and immunoprecipitation assays showed variable expression levels of endogenous SIRPalpha transcripts in nine well-characterized glioblastoma cell lines. We examined SIRPalpha1 regulation in U87MG and U373MG cells in comparison with clonal derivatives that express a truncated form of erbB2, which negatively regulates EGFR signaling by inducing the formation of nonfunctional heterodimeric complexes. Mutant erbB2-expressing cells contained more SIRPalpha1 mRNA when compared with the parental cells in presence or absence of serum. Similarly, immunoprecipitation assays showed increased SIRPalpha1 protein levels in erbB-inhibited cells when compared with parental cells. Messenger RNA stability assays revealed that the increased mRNA levels in EGFR-inhibited cells were due to an induction of transcription. Consistent with this finding, expression of the erbB2 mutant receptor up-regulated SIRPalpha1 promoter activity in all cell lines tested. Interestingly, pharmacological inhibition of the kinase activities of EGFR, erbB2, and src and activation of mitogen-activated protein kinase, but not phosphatidylinositol 3'-kinase, significantly up-regulated SIRPalpha1 promoter activity. Based on these observations, we hypothesize that down-modulation of EGFR signaling leads to transcriptional up-regulation of the inhibitory SIRPalpha1 gene. These data may be important in the application of erbB-inhibitory strategies and for design of therapies for the treatment of glial tumors and other epithelial malignancies.

Resistance of B16 Melanoma Cells to CD47-induced Negative Regulation of Motility as a Result of Aberrant N-Glycosylation of SHPS-1

The adhesion receptor SHPS-1 activates the protein-tyrosine-phosphatase SHP-2 and thereby promotes integrin-mediated reorganization of the cytoskeleton. SHPS-1 also contributes to cell-cell communication through association with CD47. Although functional alteration of SHPS-1 is implicated in cellular transformation, the role of the CD47-SHPS-1 interaction in carcinogenesis has been unclear. A soluble SHPS-1 ligand (CD47-Fc) has now been shown to bind to Melan-a non-tumorigenic melanocytes but not to syngeneic B16F10 melanoma cells. Treatment of B16F10 cells with 1-deoxymannojirimycin, which prevents N-glycan processing, restored the ability of SHPS-1 derived from these cells to bind CD47-Fc in vitro, indicating that aberrant N-glycosylation of SHPS-1 impairs CD47 binding in B16F10 cells. CD47-Fc inhibited the migration of Melan-a cells but not that of B16F10 cells. However, a monoclonal antibody that reacts with SHPS-1 on both Melan-a and B16F10 cells inhibited the migration of both cell types similarly. CD47 binding induced proteasome-mediated degradation of SHPS-1 in a tyrosine phosphorylation-independent manner. Furthermore, overexpression of SHPS-1 reduced the level of tyrosine phosphorylation of focal adhesion kinase, and this effect was reversed by CD47 binding. These results suggest that CD47 binds to and thereby down-regulates SHPS-1 on adjacent cells, resulting in inhibition of cell motility. Resistance to this inhibitory mechanism may contribute to the highly metastatic potential of B16 melanoma.

Expression and activation of signal regulatory protein alpha on astrocytomas

High-grade astrocytomas and glioblastomas are usually unresectable because they extensively invade surrounding brain tissue. Here, we report the expression and function of a receptor on many astrocytomas that may alter both the proliferative and invasive potential of these tumors. Signal regulatory protein (SIRP) alpha1 is an immunoglobulin superfamily transmembrane glycoprotein that is normally expressed in subsets of myeloid and neuronal cells. Transfection of many cell types with SIRPalpha1, including glioblastomas, has been shown to inhibit their proliferation in response to a range of growth factors. Furthermore, the expression of a murine SIRPalpha1 mutant has been shown to enhance cell adhesion and initial cell spreading but to inhibit cell extension and movement. The extracellular portion of SIRPalpha1 binds CD47 (integrin-associated protein), although this interaction is not required for integrin-mediated activation of SIRPalpha1. On phosphorylation, SIRPalpha1 recruits the tyrosine phosphatases SHP-1 and SHP-2, which are important in its functions. Although SHP-1 is uniquely expressed on hematopoietic cells, SHP-2 is ubiquitously expressed, so that SIRPalpha1 has the potential to function in many cell types, including astrocytomas. Because SIRPalpha1 regulates cell functions that may contribute to the malignancy of these tumors, we examined the expression of SIRPs in astrocytoma cell lines by flow cytometry using a monoclonal antibody against all SIRPs. Screening of nine cell lines revealed clear cell surface expression of SIRPs on five cell lines, whereas Northern blotting for SIRPalpha transcripts showed mRNA present in eight of nine cell lines. All nine cell lines expressed the ligand for SIRPalpha1, CD47. To further examine the expression and function of SIRPs, we studied the SF126 and U373MG astrocytoma cell lines, both of which express SIRPs, in greater detail. SIRP transcripts in these cells are identical in sequence to SIRPalpha1. The expressed deglycosylated protein is the same size as SIRPalpha1, but in the astrocytoma cells, it is underglycosylated compared with SIRPalpha1 produced in transfected Chinese hamster ovary cells. It is nonetheless still capable of binding soluble CD47. Moreover, SIRPalpha1 in each of the two cell lines recruited SHP-2 on phosphorylation, and SIRPalpha1 phosphorylation in cultured cells is CD47 dependent. Finally, examination of frozen sections from 10 primary brain tumor biopsies by immunohistochemistry revealed expression of SIRPs on seven of the specimens, some of which expressed high levels of SIRPs. Most of the tumors also expressed CD47. This is the first demonstration that astrocytomas can express SIRPalpha. Given the known role of SIRPalpha in regulating cell adhesion and responses to mitogenic growth factors, the expression of SIRPalpha1 on astrocytomas may be of considerable importance in brain tumor biology, and it offers the potential of a new avenue for therapeutic intervention.

Ubiquitination-mediated regulation of biosynthesis of the adhesion receptor SHPS-1 in response to endoplasmic reticulum stress

Misfolding of proteins during endoplasmic reticulum (ER) stress results in the formation of cytotoxic aggregates. The ER-associated degradation pathway counteracts such aggregation through the elimination of misfolded proteins by the ubiquitin-proteasome system. We now show that SHP substrate-1 (SHPS-1), a transmembrane glycoprotein that regulates cytoskeletal reorganization and cell-cell communication, is a physiological substrate for the Skp1-Cullin1-NFB42-Rbx1 (SCF(NFB42)) E3 ubiquitin ligase, a proposed mediator of ER-associated degradation. SCF(NFB42) mediated the polyubiquitination of immature SHPS-1 and its degradation by the proteasome. Ectopic expression of NFB42 both suppressed the formation of aggresome-like structures and the phosphorylation of the translational regulator eIF2alpha induced by overproduction of SHPS-1 as well as increased the amount of mature SHPS-1 at the cell surface. An NFB42 mutant lacking the F box domain had no such effects. Our results suggest that SCF(NFB42) regulates SHPS-1 biosynthesis in response to ER stress.

A role for SHPS-1/SIRPalpha1 in IL-1beta- and TNFalpha-dependent signaling

We investigated the role of SHPS-1/SIRPalpha1 in IL-1beta- and TNFalpha-dependent signaling that leads to the activation of Erk 1/2 and Akt. Treatment of Balb3T3 cells with IL-1beta or TNFalpha activated tyrosine phosphorylation of SHPS-1, its association with SHP-2 and the phosphorylation of Erk 1/2 and Akt. PP1, a specific inhibitor for the Src family protein tyrosine kinases, strongly inhibited tyrosine phosphorylation of SHPS-1 and complex formation of SHPS-1 with SHP-2 by IL-1beta. In addition, PP1 substantially inhibited the IL-2beta- and TNFalpha-dependent activation of Erk 1/2 and Akt. Exogenous expression of either SHPS-1 mutants that lack SHP-2 binding function or a dominant negative mutant of SHP-2 markedly inhibited the activation of Erk 1/2 and Akt by IL-1beta, whereas wild type SHPS-1 did not. Moreover, IL-1beta-stimulation induced association of SHPS-1 with IL-1RAcP, a second subunit of IL-1 receptor, whereas expression of SHPS-1 mutant that lack SHP-2 binding function clearly blocked the association and tyrosine phosphorylation of endogenous SHPS-1. Taken together, our results strongly suggest that activation of Erk 1/2 and Akt by proinflammatory cytokines requires tyrosine phosphorylation of SHPS-1 and subsequent association of SHPS-1 with SHP-2.

SHPS-1, a multifunctional transmembrane glycoprotein

Src homology 2 (SH2) domain-containing protein tyrosine phosphatase substrate 1 (SHPS-1) is a member of the signal regulatory protein (SIRP) family. The amino-terminal immunoglobulin-like domain of SHPS-1 is necessary for interaction with CD47, a ligand for SHPS-1, which plays an important role in cell-cell interaction. The intracellular region of SHPS-1, on the other hand, may act as a scaffold protein, binding to various adapter proteins. Interestingly, increasing evidence has shown that SHPS-1 is involved in various biological phenomena, including suppression of anchorage-independent cell growth, negative regulation of immune cells, self-recognition of red blood cells, mediation of macrophage multinucleation, skeletal muscle differentiation, entrainment of circadian clock, neuronal survival and synaptogenesis. Recent progress has been made in attributing these novel exciting functions. Here we discuss how this interesting molecule works and consider its true role in biology.

Hyaluronan activates cell motility of v-Src-transformed cells via Ras-mitogen-activated protein kinase and phosphoinositide 3-kinase-Akt in a tumor-specific manner

We investigated the production of hyaluronan (HA) and its effect on cell motility in cells expressing the v-src mutants. Transformation of 3Y1 by v-src virtually activated HA secretion, whereas G2A v-src, a nonmyristoylated form of v-src defective in cell transformation, had no effect. In cells expressing the temperature-sensitive mutant of v-Src, HA secretion was temperature dependent. In addition, HA as small as 1 nM, on the other side, activated cell motility in a tumor-specific manner. HA treatment strongly activated the motility of v-Src-transformed 3Y1, whereas it showed no effect on 3Y1- and 3Y1-expressing G2A v-src. HA-dependent cell locomotion was strongly blocked by either expression of dominant-negative Ras or treatment with a Ras farnesyltransferase inhibitor. Similarly, both the MEK1 inhibitor and the kinase inhibitor clearly inhibited HA-dependent cell locomotion. In contrast, cells transformed with an active MEK1 did not respond to the HA. Finally, an anti-CD44-neutralizing antibody could block the activation of cell motility by HA as well as the HA-dependent phosphorylation of mitogen-activated protein kinase and Akt. Taken together, these results suggest that simultaneous activation of the Ras-mitogen-activated protein kinase pathway and the phosphoinositide 3-kinase pathway by the HA-CD44 interaction is required for the activation of HA-dependent cell locomotion in v-Src-transformed cells.

Role of Src expression and activation in human cancer

Since the original identification of a transmissible agent responsible for the development of tumors in chickens, now known to be a retrovirus encoding the v-src gene, significant progress has been made in defining the potential functions of its human homolog, SRC. The product of the human SRC gene, c-Src, is found to be over-expressed and highly activated in a wide variety of human cancers. The relationship between Src activation and cancer progression appears to be significant. Moreover, Src may have an influence on the development of the metastatic phenotype. This review discusses the data supporting a role for c-Src as a critical component of the signal transduction pathways that control cancer cell development and growth, and provides the rationale for targeting Src in drug discovery efforts.