Src phosphorylates the insulin-like growth factor type I receptor on the autophosphorylation sites. Requirement for transformation by src

Impaired Shc, Ras, and MAPK activation but normal Akt activation in FL5.12 cells expressing an insulin-like growth factor I receptor mutated at tyrosines 1250 and 1251

The Y1250F/Y1251F mutant of the insulin-like growth factor I receptor (IGF-IR) has tyrosines 1250 and 1251 mutated to phenylalanines and is deficient in IGF-I-mediated suppression of apoptosis in FL5.12 lymphocytic cells. To address the mechanism of loss of function in this mutant we investigated signaling responses in FL5.12 cells overexpressing either a wild-type (WT) or Y1250F/Y1251F (mutant) IGF-IR. Cells expressing the mutant receptor were deficient in IGF-I-induced phosphorylation of the JNK pathway and had decreased ERK and p38 phosphorylation. IGF-I induced phosphorylation of Akt was comparable in WT and mutant expressing cells. The decreased activation of the mitogen-activated protein kinase (MAPK) pathways was accompanied by greatly decreased Ras activation in response to IGF-I. Although phosphorylation of Gab2 was similar in WT and mutant cell lines, phosphorylation of Shc on Tyr(313) in response to IGF-I was decreased in cells expressing the mutant receptor, as was recruitment of Grb2 and Ship to Shc. However, phosphorylation of Shc on Tyr(239), the Src phosphorylation site, was normal. A role for JNK in the survival of FL5.12 cells was supported by the observation that the JNK inhibitor SP600125 suppressed IGF-I-mediated protection from apoptosis. Altogether these data demonstrate that phosphorylation of Shc, and assembly of the Shc complex necessary for activation of Ras and the MAPK pathways are deficient in cells expressing the Y1250F/Y1251F mutant IGF-IR. This would explain the loss of IGF-I-mediated survival in FL5.12 cells expressing this mutant and may also explain why this mutant IGF-IR is deficient in functions associated with cellular transformation and cell migration in fibroblasts and epithelial tumor cells.

Light regulation of the insulin receptor in the retina

The peptide hormone insulin binds its cognate cell-surface receptors to activate a coordinated biochemical-signaling network and to induce intracellular events. The retina is an integral part of the central nervous system and is known to contain insulin receptors, although their function is unknown. This article, describes recent studies that link the photobleaching of rhodopsin to tyrosine phosphorylation of the insulin receptor and subsequent activation of phosphoinositide 3- kinase (PI3K). We recently found a light-dependent increase in tyrosine phosphorylation of the insulin receptor-beta-subunit (IR beta) and an increase in PI3K enzyme activity in isolated rod outer segments (ROS) and in anti-phosphotyrosine (PY) and anti-IR beta immunoprecipitates of retinal homogenates. The light effect, which was localized to photoreceptor neurons, is independent of insulin secretion. Our results suggest that light induces tyrosine phosphorylation of IR beta in outer-segment membranes, which leads to the binding of p85 through its N-terminal SH2 domain and the generation of PI-3,4,5-P3. We suggest that the physiological role of this process may be to provide neuroprotection of the retina against light damage by activating proteins that protect against stress-induced apoptosis. The studies linking PI3K activation through tyrosine phosphorylation of IR beta now provide physiological relevance for the presence of these receptors in the retina.

Muscle or fat? Rho bridges the GAP

Mesenchymal stem cells can differentiate into muscle or fat cells, depending on the exposure to growth factors. The Rho GTPase appears to play a crucial role in this decision of cellular fate, promoting myogenesis and inhibiting adipogenesis.

Epidermal growth factor-induced activation of the insulin-like growth factor I receptor in rat hepatocytes

Insulin-like growth factor I (IGF-I) plays a critical role in the induction of cell cycle progression and survival in many cell types. However, there is minimal IGF-I binding to hepatocytes, and a role for IGF-I in hepatocyte signaling has not been elucidated. The dynamics of IGF-I receptor (IGF-IR) activation were examined in freshly isolated rat hepatocytes. IGF-I did not activate the IGF-IR. However, des(1-3)IGF-I, which weakly binds IGF binding protein-3 (IGFBP-3), induced IGF-IR phosphorylation. IGFBP-3 surface coating was identified by confocal immunofluorescence microscopy. In contrast with the inactivity of IGF-I, epidermal growth factor (EGF) induced the tyrosine phosphorylation of the IGF-IR in parallel with EGF receptor phosphorylation. Transactivation of the IGF-IR by EGF was inhibited by tyrphostin I-Ome-AG538, a tyrosine kinase inhibitor with high specificity for the IGF-IR. Src kinase inhibitors pyrazolopyrimidine PP-1 and PP-2 inhibited transactivation of the IGF-IR by EGF. EGF stimulated the tyrosine phosphorylation of Src, and induced its association with the IGF-IR. EGF-induced phosphorylations of insulin-related substrate (IRS)-1, IRS-2, Akt, and p42/44 mitogen-activated protein kinases (MAPKs) were inhibited variably by I-Ome-AG538. In conclusion, the data show an EGF- and Src-mediated transactivation pathway for IGF-IR activation in hepatocytes, and indicate a role for the IGF-IR in hepatocyte intracellular signaling. The findings also show a role for IGFBP-3 in the inhibition of IGF-I signaling in hepatocytes.

Nuclear translocation of insulin receptor substrate-1 by oncogenes and Igf-I. Effect on ribosomal RNA synthesis

The insulin receptor substrate-1 (IRS-1) is one of the major substrates of both the insulin and IGF-I receptors and is generally localized in the cytosol/membrane fraction of the cell. We show here that a substantial fraction of IRS-1 is translocated to the nucleus in mouse embryo fibroblasts (MEF) expressing the simian virus 40 T antigen. Nuclear translocation of IRS-1 occurs also in MEF stimulated with IGF-I or in MEF expressing the oncogene v-src. Nuclear translocation of IRS-1 can be demonstrated by confocal microscopy, immunohistochemistry, or subcellular fractionation. An antibody to IRS-1 immunoprecipitates from nuclear fractions (but not from cytosolic fractions) the upstream binding factor, which is a key regulator of RNA polymerase I activity and ribosomal RNA (rRNA) synthesis. In agreement with this finding, in 32D murine hemopoietic cells, nuclear translocation of IRS-1 correlates with a markedly increased rRNA synthesis. Our experiments suggest that nuclear IRS-1 may play a specialized role in rRNA synthesis and/or processing.

In vivo regulation of phosphoinositide 3-kinase in retina through light-induced tyrosine phosphorylation of the insulin receptor beta-subunit

Recently, we have shown that phosphoinositide 3-kinase (PI3K) in bovine rod outer segment (ROS) is activated in vitro by tyrosine phosphorylation of the C-terminal tail of the insulin receptor (Rajala, R. V. S., and Anderson, R. E. (2001) Invest. Ophthal. Vis. Sci. 42, 3110-3117). In this study, we have investigated the in vivo mechanism of PI3K activation in the rodent retina and report the novel finding that light stimulates tyrosine phosphorylation of the beta-subunit of the insulin receptor (IRbeta) in ROS membranes, which leads to the association of PI3K enzyme activity with IRbeta. Retinas from light- or dark-adapted mice and rats were homogenized and immunoprecipitated with antibodies against phosphotyrosine, IRbeta, or the p85 regulatory subunit of PI3K, and PI3K activity was measured using PI-4,5-P(2) as substrate. We observed a light-dependent increase in tyrosine phosphorylation of IRbeta and an increase in PI3K enzyme activity in isolated ROS and in anti-phosphotyrosine and anti-IRbeta immunoprecipitates of retinal homogenates. The light effect was localized to photoreceptor neurons and is independent of insulin secretion. Our results suggest that light induces tyrosine phosphorylation of IRbeta in outer segment membranes, which leads to the binding of p85 through its N-terminal Src homology 2 domain and the generation of PI-3,4,5-P(3). We suggest that the physiological role of this process may be to provide neuroprotection of the retina against light damage by activating proteins that protect against stress-induced apoptosis.

Regulation of insulin-like growth factor type I (IGF-I) receptor kinase activity by protein tyrosine phosphatase 1B (PTP-1B) and enhanced IGF-I-mediated suppression of apoptosis and motility in PTP-1B-deficient fibroblasts

The insulin-like growth factor type I (IGF-I) receptor (IGF-IR), activated by its ligands IGF-I and IGF-II, can initiate several signal transduction pathways that mediate suppression of apoptosis, proliferation, differentiation, and transformation. Here we investigated the regulation of IGF-IR activation and function by protein tyrosine phosphatase 1B (PTP-1B). Coexpression of PTP-1B with a beta-chain construct of the IGF-IR (betaWT) inhibited IGF-IR kinase activity in fission yeast Schizosaccharomyces pombe, in COS cells, and in IGF-IR-deficient fibroblasts. In both spontaneously immortalized and simian virus 40 T antigen-transformed embryonic fibroblast cell lines derived from PTP-1B knockout mice, IGF-I induced higher levels of IGF-IR autophosphorylation and kinase activity than were induced in PTP-1B-expressing control cells. PTP-1B-deficient cells exhibited enhanced IGF-I-mediated protection from apoptosis in response to serum withdrawal or etoposide killing, as well as enhanced plating efficiency and IGF-I-mediated motility. Reexpression of PTP-1B in spontaneously immortalized fibroblasts resulted in decreased IGF-IR and AKT activation, as well as decreased protection from apoptosis and decreased motility. These findings demonstrate that PTP-1B can regulate IGF-IR kinase activity and function and that loss of PTP-1B can enhance IGF-I-mediated cell survival, growth, and motility in transformed cells.

Insulin-like growth factor 1 and a cytosolic tyrosine kinase activate chloride outward transport during maturation of hippocampal neurons

The development of hyperpolarizing inhibition is an important step in the maturation of neuronal networks. Hyperpolarizing inhibition requires Cl(-) outward transport that is accomplished by KCC2, a K(+)/Cl(-) cotransporter. We show that cultured hippocampal neurons initially contain an inactive form of the KCC2 protein, which becomes activated during subsequent maturation of the neurons. We also show that this process is accelerated by transient stimulation of IGF-1 receptors. Because the transporter can be rapidly activated by coapplication of IGF-1 and an Src kinase and can be deactivated by membrane-permeable protein tyrosine kinase inhibitors, we suggest that activation of K(+)/Cl(-) cotransporter function by endogenous protein tyrosine kinases mediates the developmental switch of GABAergic responses to hyperpolarizing inhibition.

Differential requirements of the MAP kinase and PI3 kinase signaling pathways in Src- versus insulin and IGF-1 receptors-induced growth and transformation of rat intestinal epithelial cells

There have been few studies on the specific signaling pathways involved in the transformation of epithelial cells by oncogenic protein tyrosine kinases. Here we investigate the requirement of MAP (MAPK) and phosphatidylinositol 3- (PI3K) kinases in the transformation of rat intestinal epithelial (RIE) cells by oncogenic forms of insulin receptor (gag-IR), insulin-like growth factor-1 receptor (gag-IGFR), and v-Src. MAPK is not significantly activated in cells transformed by gag-IR and gag-IGFR but is activated in v-Src transformed cells. Treatment with PD98059, a MEK inhibitor, at concentrations where MAPK activity was reduced below the basal level showed that MAPK is partially required for the monolayer growth of parental and transformed RIE cells. However, MAPK is not essential for the focus forming ability of the three oncogene-transformed cells. It is also not necessary for the colony forming ability of gag-IR- and gag-IGFR-, but is partially required for v-Src-transformed cells. PI3K is significantly activated in all three oncogene transformed RIE cells. LY294002, a PI3K inhibitor, potently inhibited monolayer growth of all three oncogene-transformed cells. However, at concentrations of LY294002 where activated forms of Akt, a downstream component of the PI3K pathway, were undetectable, colony and focus forming abilities of the v-Src-RIE cells were only slightly affected whereas those of gag-IR/IGFR-RIE cells were greatly inhibited. These results were confirmed using a different pharmacological inhibitor, wortmannin, and a dominant negative form of PI3K, Ap85. Similarly, rapamycin, known to inhibit p70S6 kinase, a downstream component of the PI3K-Akt pathway, also inhibited gag-IR/IGFR-induced, but not v-Src-induced, focus and colony formation. We conclude that the MAPK and PI3K signaling pathways are differentially required for transformation of RIE cells by oncogenic IR and IGFR versus Src and the pattern of requirements is different from that of fibroblast transformation.

Cooperative transformation of 32D cells by the combined expression of IRS-1 and V-Ha-Ras

32D cells expressing v-Ha-Ras fail to show a transformed phenotype. Since Ras requires an active IGF-1R for transformation of fibroblasts, we asked whether expression of IRS-1 or Shc (two of the major substrates of the IGF-1R) could co-operate with oncogenic Ras in transforming 32D cells. We find that IRS-1, but not Shc, in combination with v-Ha-Ras generates a fully transformed phenotype in 32D cells. 32D cells expressing both IRS-1 and v-Ha-Ras (32D/IRS1/Ras) survive and proliferate in the absence of IL-3, do not undergo granulocytic differentiation in the presence of G-CSF and form tumors in nu/nu and syngeneic mice. In contrast, 32D cells expressing singly IRS-1 or v-Ha-Ras exhibit only a block in differentiation capacity. Over-expression of Shc proteins, by itself, promotes differentiation of 32D cells. Concomitant expression of IRS-1 and v-Ha-Ras synergistically phosphorylates ERK-1 and ERK-2 whereas a MEK inhibitor rapidly induces death of 32D/IRS1/Ras transformed cells. Furthermore, transformed 32D/IRS1/Ras cells display high levels of PI3-K activation and undergo rapid apoptosis when exposed to PI3-K inhibitors. The data indicate that: (1) a fully transformed phenotype in 32D cells is generated when a block in differentiation (v-Ha-Ras) is coupled with another differentiation block (IRS-1); (2) PI3-K and MAPK activity are required for the survival of transformed cells; (3) the signals generated by IRS-1 and oncogenic Ras converge on ERK and PI3-K resulting in high levels of activation.

Potentiation of neuronal L calcium channels by IGF-1 requires phosphorylation of the alpha1 subunit on a specific tyrosine residue

Insulin-like growth factor 1 (IGF-1) rapidly potentiates N and L calcium channel currents in cerebellar granule neurons by an unknown mechanism. Here, we show that the L channel alpha1C subunit is tyrosine phosphorylated in response to IGF-1. Moreover, expression of kinase-dead c-Src in neurons or acute block of Src family kinases with a cell-permeable inhibitor specifically blocks L channel potentiation. Purified Src kinase phosphorylates tyrosine residue Y2122 of the C terminus of neuronal alpha1C in vitro, and c- and v-Src directly bind the C terminus. When expressed in neuroblastoma cells, point mutation of Y2122 prevents both tyrosine phosphorylation of alpha1C and IGF-1 potentiation. Our data provide a biochemical mechanism whereby phosphorylation of a single specific tyrosine residue rapidly modifies ion channel physiology.

Identification of Grb10 as a direct substrate for members of the Src tyrosine kinase family

Treatment of cells with insulin and protein tyrosine phosphatase inhibitors such as vanadate and pervanadate resulted in the tyrosine phosphorylation of Grb10, a Src homology 2 (SH2) and pleckstrin homology domain-containing adaptor protein which binds to a number of receptor tyrosine kinases including the insulin receptor (IR). Although Grb10 binds directly to the kinase domain of the IR, our data show that Grb10 is not a direct substrate for the IR tyrosine kinase. Consistent with this finding, Grb10 tyrosine phosphorylation in cells was inhibited by herbimycin A, a relatively specific inhibitor for members of the Src tyrosine kinase family, and by the expression of dominant negative Src or Fyn. In addition, Grb10 tyrosine phosphorylation was stimulated by expression of constitutively active Src or Fyn in cells and by incubation with purified Src or Fyn in vitro. The insulin stimulated or Src/Fyn-mediated tyrosine phosphorylation in vivo was significantly reduced when Grb10 tyrosine 67 was changed to glycine. This mutant form of Grb10 bound with higher affinity to the IR in cells than that of the wild-type protein, suggesting that tyrosine phosphorylation of Grb10 may normally negatively regulate its binding to the IR. Our data show that Grb10 is a new substrate for members of the Src tyrosine kinase family and that the tyrosine phosphorylation of the protein may play a potential role in cell signaling processes mediated by these kinases. Oncogene (2000).

Integrin-mediated RON growth factor receptor phosphorylation requires tyrosine kinase activity of both the receptor and c-Src

Cooperation between integrins and growth factor receptors plays an important role in the regulation of cell growth, differentiation, and survival. The function of growth factor receptor tyrosine kinases (RTKs) can be regulated by cell adhesion to extracellular matrix (ECM) even in the absence of ligand. We investigated the pathway involved in integrin-mediated RTK activation, using RON, the receptor for macrophage-stimulating protein. Adhesion of RON-expressing epithelial cells to ECM caused phosphorylation of RON, which depended on the kinase activity of both RON itself and c-Src. This conclusion is based on these observations: 1) ECM-induced RON phosphorylation was inhibited in cells expressing kinase-inactive c-Src; 2) active c-Src could phosphorylate immunoprecipitated RON from ECM-stimulated cells but not from unstimulated cells; and 3) ECM did not cause RON phosphorylation in cells expressing kinase-dead RON, nor could active c-Src phosphorylate RON immunoprecipitated from these cells. The data fit a pathway in which ECM-induced integrin aggregation causes both c-Src activation and RON oligomerization followed by RON kinase-dependent autophosphorylation; this results in RON becoming a target for activated c-Src, which phosphorylates additional tyrosines on RON. Integrin-induced epidermal growth factor receptor (EGFR) phosphorylation also depended on both EGFR and c-Src kinase activities. This sequence appears to be a general pathway for integrin-dependent growth factor RTK activation.

pp60(cSrc) is a caspase-3 substrate and Is essential for the transformed phenotype of A431 cells

The protein tyrosine kinase c-Src is a major signal transduction element in many growth factor receptor signals for proliferation and transformation. We showed recently that c-Src is a mediator of antiapoptotic signals through regulation of the antiapoptotic gene Bcl-X(L). A431 cells overexpress the EGF receptor (EGFR) and possess high Src activity. In A431 cells, Src is activated by the EGFR, and inhibition of the EGF receptor results in c-Src inhibition. In this study we show that (i) inhibition of the EGFR kinase or Src kinase by specific inhibitors results in growth inhibition and inhibition of colony formation in soft agar. The relative efficacies of the EGFR kinase inhibitor and of the Src kinase inhibitor are similar suggesting the major role src plays in the oncogenic signaling of EGFR in A431 cells. (ii) The Src kinase inhibitor PP1 sensitizes A431 cells to CDDP-induced apoptosis. (iii) CDDP induces caspase-3-dependent cleavage of the c-Src C-terminal portion and a concomitant reduction in Bcl-X(L) levels. We conclude that c-Src is an important antiapoptotic signaling molecule downstream of the EGF receptor that contributes to the transformed phenotype of A431 cells.

Tyrosine kinase signalling in breast cancer: insulin-like growth factors and their receptors in breast cancer

The insulin-like growth factor (IGF) system exerts pleiotropic effects on mammalian cells. This review focuses on type I IGF receptor (IGF1R)-mediated signal transduction and its relevance in breast cancer. Upon activation by the IGFs, IGF1R, a transmembrane tyrosine kinase receptor, undergoes autophosphorylation, and then binds and phosphorylates additional signaling molecules. These intermediates initiate a series of downstream signaling events that are involved in multiple physiologic processes for cells. Recent data demonstrate that the IGF receptor system actively interacts with the estrogen receptor and integrin receptor systems. Cross-talk among these pathways regulates breast cancer proliferation, protection from cell death, and metastasis. Better understanding of IGF biochemical signaling pathways is of utmost importance for developing therapies for breast cancer.

Inhibition of pp60c-Src reduces Bcl-XL expression and reverses the transformed phenotype of cells overexpressing EGF and HER-2 receptors

Tumors that overexpress HER-2/neu receptor or exhibit enhanced EGFR signaling have been reported to possess constitutively activated Src family kinases, especially pp60c-Src. High levels of pp60c-Src activity have also been reported for cell lines that overexpress the EGFR or the chimeric EGFR-HER-2 receptor. It has therefore been suggested that Src kinases may contribute significantly to the oncogenic phenotype of these cells and to the degree of malignancy of tumors that overexpress EGFR family receptors. In this study we show that the induced expression of c-SRC antisense RNA or the application of a selective Src kinase inhibitor induces growth arrest, programmed cell death and reverses the transformed properties of cells that overexpress EGFR or HER-2 receptors. We show that inhibition of Src kinase expression or activity results in the reduction of Stat3 tyrosine phosphorylation, decline of Bcl-XL expression, and induction of cell death. Using a construct in which the promoter of Bcl-X, which possesses putative Stat3 sites, is tethered to the luciferase reporter gene, we show that inhibition of Src activity or expression induces a decline in Bcl-X expression. We also show that the expression of activated Src induces activation of the Bcl-X promoter. This activation is inhibited by the expression of kinase dead Src or of Stat3beta, the dominant-negative form of Stat3. Taken together, these results support the hypothesis that Src positively regulates the transformed phenotype of cells overexpressing EGFR family kinases. Furthermore, these results also suggest that Src positively regulates Bcl-XL expression via Stat3 activation and thus acts not only as a potent mitogenic signaling element, but also as an anti-apoptotic signaling protein. The combination of both activities probably confers upon activated Src its oncogenic activity. Since Src kinase is activated in many tumors, pp60c-Src kinase inhibitors may prove useful as anti-cancer agents for many types of cancer.

Expression, purification, and characterization of the cytoplasmic domain of the human IGF-1 receptor using a baculovirus expression system

The cytoplasmatic domain of the beta-subunit of the human IGF-1 receptor (residues 929-1337) has been overexpressed in insect cells using the baculovirus expression system. Synthesis of the soluble protein (IGFK, M(r) 46 kDa) in Spodoptera frugiperda (Sf9) cells was detected 24 h after infection and maximal accumulation was achieved 40-48 h postinfection. Rapid purification to near homogeneity (>/=95% pure protein) was accomplished by sequential chromatography on Resource-Q and phenyl-Sepharose with a specific activity of 142 nmol/min/mg using poly[Glu:Tyr] as substrate. The purified IGFK showed a preference for Mn(2+) ions and a linear incorporation of (32)P from [gamma-(32)P]ATP over a 20-fold dilution of the protein and was stimulated 20-fold by the polycation poly-L-lysine. Interestingly, the kinase autophosphorylated on tyrosine and serine residues. In contrast, a kinase-negative mutant, IGFK-K1003A, did not undergo phosphorylation on tyrosine or serine residues, respectively, suggesting that IGF-1 receptor kinase is a dual specific kinase.

Akt-dependent and -independent survival signaling pathways utilized by insulin-like growth factor I

Protein kinase B (PKB)/Akt is implicated in survival signaling in a wide variety of cells including fibroblasts and epithelial and neuronal cells. We and others have described a linear survival signaling cascade used by insulinlike growth factor I (IGF-I) that consists of the IGF-I receptor, phosphoinositide 3-kinase (PI3 kinase), Akt, and Bad. Activation of this pathway can be sufficient to protect cells from apoptosis. However, previous work had not determined whether this pathway is invariably necessary for protection from apoptosis or whether there are alternative survival signaling pathways. In this communication, we report the existence of two survival signaling pathways, one dependent on PI3 kinase and Akt and the other independent of these enzymes. We found that survival signaling initiated by IGF-I treatment of Rat-1 cells could be blocked by overexpression of a dominant negative kinase-deficient Akt (K179A) as well as by wortmannin. This demonstrates a survival signaling pathway dependent on PI3 kinase and Akt. However, when IGF-I receptors were overexpressed in a Rat-1 background (RIG cells), an alternative pathway became apparent, in which survival mediated by IGF-I was no longer sensitive to wortmannin or to overexpression of dominant negative Akt, even though Akt activation and Bad phosphorylation were still wortmannin sensitive. Experiments with inhibitors of RNA synthesis showed that transcriptional activation is dispensable for this alternative PI3 kinase/Akt-independent survival signaling. These findings demonstrate the existence of a new survival signaling pathway independent of PI3 kinase, Akt, and new transcription and which is evident in fibroblasts overexpressing the IGF-I receptor.

N-Methyl-D-aspartate inhibits apoptosis through activation of phosphatidylinositol 3-kinase in cerebellar granule neurons. A role for insulin receptor substrate-1 in the neurotrophic action of n-methyl-D-aspartate and its inhibition by ethanol

Primary cultured rat cerebellar granule neurons underwent apoptosis when switched from medium containing 25 mM K+ to one containing 5 mM K+. N-methyl-D-aspartate (NMDA) protected granule neurons from apoptosis in medium containing 5 mM K+. Inhibition of apoptosis by NMDA was blocked by the phosphatidylinositol 3-kinase (PI 3-kinase) inhibitor LY294002, but it was unaffected by the mitogen-activated protein kinase kinase inhibitor PD 98059. The antiapoptotic action of NMDA was associated with an increase in the tyrosine phosphorylation of insulin receptor substrate 1 (IRS-1), an increase in the binding of the regulatory subunit of PI 3-kinase to IRS-1, and a stimulation of PI 3-kinase activity. In the absence of extracellular Ca2+, NMDA was unable to prevent apoptosis or to phosphorylate IRS-1 and activate PI 3-kinase. Significant inhibition of NMDA-mediated neuronal survival by ethanol (10-15%) was observed at 1 mM, and inhibition was half-maximal at 45-50 mM. Inhibition of neuronal survival by ethanol corresponded with a marked reduction in the capacity of NMDA to increase the concentration of intracellular Ca2+, phosphorylate IRS-1, and activate PI 3-kinase. These data demonstrate that the neurotrophic action of NMDA and its inhibition by ethanol are mediated by alterations in the activity of a PI 3-kinase-dependent antiapoptotic signaling pathway.

Full activation of the platelet-derived growth factor beta-receptor kinase involves multiple events

Activation of receptor tyrosine kinases is thought to involve ligand-induced dimerization, which promotes receptor transphosphorylation and thereby increases the receptor's phosphotransferase activity. We used two platelet-derived growth factor beta-receptor (beta-PDGFR) mutants to identify events that are required for full engagement (autophosphorylation and activation of the kinase activity) of the beta-PDGFR kinase. The F79/81 receptor (Tyr to Phe substitution at 579 and 581 in the juxtamembrane domain of the receptor) was capable of only very modest ligand-dependent autophosphorylation and also failed to associate with numerous SH2 domain-containing proteins. Furthermore, stimulation with platelet-derived growth factor (PDGF) did not increase the kinase activity of the F79/81 mutant toward exogenous substrates. However, the F79/81 receptor had basal kinase activity and could be artificially stimulated by incubation with ATP. Because the low kinase activity of the F857 mutant (Tyr to Phe substitution at 857 in the putative activation loop) could not be increased by incubation with ATP, failure to phosphorylate Tyr-857 may be the reason why the F79/81 mutant has low kinase activity. Surprisingly, the F857 mutant underwent efficient PDGF-dependent autophosphorylation. Thus the PDGF-dependent increase in the kinase activity of the receptor is not required for autophosphorylation. We conclude that full activation of the beta-PDGFR kinase requires at least two, apparently distinct events.

p125Fak focal adhesion kinase is a substrate for the insulin and insulin-like growth factor-I tyrosine kinase receptors

The focal adhesion kinase p125(Fak) is a widely expressed cytosolic tyrosine kinase, which is involved in integrin signaling and in signal transduction of a number of growth factors. In contrast to tyrosine kinase receptors such as the platelet-derived growth factor and the hepatocyte growth factor receptors, which induce p125(Fak) phosphorylation, insulin has been shown to promote its dephosphorylation. In this study, we compared p125(Fak) phosphorylation in insulin-stimulated cells maintained in suspension or in an adhesion state. We found that, in nonattached cells, insulin promotes p125(Fak) phosphorylation, whereas dephosphorylation occurred in attached cells. This was observed in Rat-1 fibroblasts overexpressing the insulin receptor, as well as in Hep G2 hepatocytes and in 3T3-L1 adipocytes expressing more natural levels of insulin receptors. Insulin-induced p125(Fak) phosphorylation correlated with an increase in paxillin phosphorylation, indicating that p125(Fak) kinase activity may be stimulated by insulin. Mixing of purified insulin or insulin-like growth factor-I (IGF-I) receptors with p125(Fak) resulted in an increase in p125(Fak) phosphorylation. Using a kinase-deficient p125(Fak) mutant, we found that this protein is a direct substrate of the insulin and IGF-I receptor tyrosine kinases. This view is supported by two additional findings. (i) A peptide corresponding to p125(Fak) sequence comprising amino acids 568-582, which contains tyrosines 576 and 577 of the kinase domain regulatory loop, is phosphorylated by the insulin receptor; and (ii) p125(Fak) phosphorylation by the insulin receptor is prevented by addition of this peptide. Finally, we observed that p125(Fak) phosphorylation by the receptor results in its activation. Our results show that the nature of the cross-talk between the insulin/IGF-I receptors and p125(Fak) is dependent on the cell architecture, and hence the interaction of the insulin/IGF-I signaling system with the integrin system will vary accordingly.

Novel recruitment of Shc, Grb2, and Sos by fibroblast growth factor receptor-1 in v-Src-transformed cells

In response to fibroblast growth factor (FGF), FGF receptor-1 (FGFR-1) (flg) becomes tyrosine phosphorylated and associates with phospholipase C gamma (PLC gamma) and a 90 kDa protein. We report here that in cells transformed by v-Src, FGFR-1 becomes phosphorylated on tyrosine; however, neither PLC gamma nor p90 was found to be associated with tyrosine-phosphorylated FGFR-1. Instead, there was a strong constitutive association of FGFR-1 with the adaptor proteins Shc and Grb2 and the Ras guanine nucleotide exchange factor Sos. Association with Shc and Grb2 and Sos was not observed in response to FGF. Suramin did not prevent either tyrosine phosphorylation or Shc/Grb2/Sos association, indicating a non-autocrine mechanism. Thus, in cells transformed by v-Src, tyrosine phosphorylation of FGFR-1 results not in the expected association with PLC gamma and p90, but rather in the recruitment of the Ras activating Shc/Grb2/Sos complex. These data suggest a mechanism for Ras activation by v-Src involving phosphorylation of novel tyrosine(s) on FGFR-1.

Cellular functions regulated by Src family kinases

Src family protein tyrosine kinases are activated following engagement of many different classes of cellular receptors and participate in signaling pathways that control a diverse spectrum of receptor-induced biological activities. While several of these kinases have evolved to play distinct roles in specific receptor pathways, there is considerable redundancy in the functions of these kinases, both with respect to the receptor pathways that activate these kinases and the downstream effectors that mediate their biological activities. This chapter reviews the evidence implicating Src family kinases in specific receptor pathways and describes the mechanisms leading to their activation, the targets that interact with these kinases, and the biological events that they regulate.

Insulin-like growth factor I receptor signaling in transformation by src oncogenes

R- cells, a line of mouse embryo fibroblasts with a targeted disruption of the insulin-like growth factor I (IGF-I) receptor genes, are refractory to transformation by several viral and cellular oncogenes. Using colony formation in soft agar as a measure of full transformation, we report here that R- cells can be transformed by v-src, although they still cannot be transformed by the activated c-src527 (mutation at tyrosine 527 to phenylalanine), which readily transforms mouse embryo cells with a wild-type number of IGF-I receptors (W cells). Although v-src is a more potent inducer of tyrosine phosphorylation than c-src527, the extent of phosphorylation of either insulin receptor substrate 1 or Shc, two of the major substrates of the IGF-I receptor, does not seem sufficiently different to explain the qualitative difference in soft agar growth. v-src, however, is considerably more efficient than c-src527 in its ability to tyrosyl phosphorylate, in R- cells, the focal adhesion kinase, Stat1, and p130cas. These results indicate that v-src, but not c-src527, can bypass the requirement for a functional IGF-I receptor in the full transformation of mouse embryo fibroblasts and suggest that qualitative and quantitative differences between the two oncogenes can be used to identify some of the signals relevant to the mechanism(s) of transformation.