Cross talk among tyrosine kinase receptors in PC12 cells: desensitization of mitogenic epidermal growth factor receptors by the neurotrophic factors, nerve growth factor and basic fibroblast growth factor.

We have studied the effects of nerve growth factor (NGF) and basic fibroblast growth factor (bFGF) on epidermal growth factor (EGF) binding to PC12 cells. We show that NGF and bFGF rapidly induce a reduction in 125I-EGF binding to PC12 cells in a dose-dependent manner. This decrease amounts to 50% for NGF and 35% for bFGF. Both factors appear to act through a protein kinase C(PKC)-independent pathway, because their effect persists in PKC-downregulated PC12 cells. Scatchard analysis indicates that NGF and bFGF decrease the number of high affinity EGF binding sites. In addition to their effect on EGF binding, NGF and bFGF activate in intact PC12 cells one or several serine/threonine kinases leading to EGF receptor threonine phosphorylation. Using an in vitro phosphorylation system, we show that NGF- or bFGF-activated extracellular regulated kinase 1 (ERK1) is able to phosphorylate a kinase-deficient EGF receptor. Phosphoamino acid analysis indicates that this phosphorylation occurs mainly on threonine residues. Furthermore, two comparable phosphopeptides are observed in the EGF receptor, phosphorylated either in vivo after NGF treatment or in a cell-free system by NGF-activated ERK1. Finally, a good correlation was found between the time courses of ERK1 activation and 125I-EGF binding inhibition after NGF or bFGF treatment. In conclusion, in PC12 cells the NGF- and bFGF-stimulated ERK1 appears to be involved in the induction of the threonine phosphorylation of the EGF receptor and the decrease in the number of high affinity EGF binding sites.

Gastrin induces phosphorylation of eIF4E binding protein 1 and translation initiation of ornithine decarboxylase mRNA.

Gastrin via its G-protein coupled specific receptor induces transcription of c-fos and c-jun genes through a ras-MAPK pathway. Ornithine Decarboxylase (ODC), a growth regulated proto-oncogene, was chosen to investigate gastrin effects on translation initiation of mRNAs exhibiting a 5'UnTranslated Region (5'UTR) responsible for translation repression in quiescent cells. In AR4-2J tumoral cells, we first demonstrated that gastrin increases ODC mRNA translation. Transient transfections with various CAT chimeric constructs suggested a direct involvement of the 5'UTR in this observation. Translation of this group of mRNAs is enhanced by the availability of the cap-binding protein (eIF4E) that is increased after phosphorylation of its specific binding protein eIF4E-BP1. We found that AR4-2J cells over-expressed eIF4E protein which was not modulated by gastrin treatment. Rapamycin which inhibits 4E-BP1 phosphorylation, completely prevents gastrin-mediated increase of ODC translation indicating that 4E-BP1 could be involved in regulating ODC translation. Implication of 4E-BP1 in mediating gastrin effects is corroborated by the capacity of the ligand to affect 4E-BP1 phosphorylation. These results indicate that gastrin enhances ornithine decarboxylase mRNA translation through a rapamycin sensitive pathway and provide the first evidence in the control of 4E-BP1 phosphorylation after occupancy of a G protein-coupled receptor.

Progastrin a new pro-angiogenic factor in colorectal cancer.

Angiogenesis is essential in tumor progression and metastatic process, and increased angiogenesis has been associated with poor prognosis and relapse of colorectal cancer (CRC). VEGF has become the main target of anti-angiogenic therapy. However, most patients relapse after an initial response or present a resistance to the treatment. Identification of new pro-angiogenic factors may help to improve anti-angiogenic therapy. In this study, we demonstrated that the pro-hormone progastrin (PG), over-expressed in CRC, recognized as a growth factor, is a potent pro-angiogenic factor. In transgenic mice and human colorectal HPs producing high levels of PG, we correlated PG overexpression with an increased vascularization. In vitro, exogenous PG and conditioned media (CM) from CRC cells producing PG increased endothelial cell proliferation and migration. We also showed that treatment with exogenous PG can increase the ability of endothelial cells to form capillary-like structures. Moreover, we demonstrated that PG enhanced endothelial permeability. The finding that PG stimulated the phosphorylation of vascular endothelial (VE)-cadherin, p125-FAK, paxillin and induced actin remodelling was consistent with a role of these components in PG-stimulated endothelial cell migration and permeability. The pro-angiogenic effects observed with CM were significantly inhibited when CRC cells expressed a PG shRNA. In vivo, we found an important decrease in tumor growth and neovascularization when the CRC cells expressing the PG shRNA were xenografted in mice or in the chick chorioallantoic membrane model. We also observed an increase in the coverage of blood vessels by pericytes and a decrease in endothelial permeability when PG expression was blocked. Our results demonstrate that PG is a new pro-angiogenic factor in CRC and an attractive therapeutic target.

Gastrin stimulates the formation of a p60Src/p125FAK complex upstream of the phosphatidylinositol 3-kinase signaling pathway.

The molecular events whereby gastrin occupancy of G/CCK(B) receptors leads to phosphatidylinositol (PI) 3-kinase activation have been examined. We report here that this peptide promotes the association between two non-receptor tyrosine kinases, p60Src and p125FAK, and elicits a parallel increase in tyrosine phosphorylation and activity of both kinases. Gastrin-induced PI 3-kinase activity was coprecipitated with p60Src and p125FAK and was inhibited by herbimycin A, the selective Src inhibitor PP-2 or cytochalasin D, which disrupts the actin cytoskeleton and prevents p125FAK activity. These results indicate, for the first time, that a p60Src/p125FAK complex acts upstream of the gastrin-stimulated PI 3-kinase pathway.

Gastrin induces tyrosine phosphorylation of Shc proteins and their association with the Grb2/Sos complex.

Gastrin/CCKB G protein-coupled receptors have been shown to mediate proliferative effects of their endogenous ligands. In the present study, we examined the signal transduction mechanisms linked to the G/CCKB receptor occupancy. We report here that gastrin stimulates MAP kinase activation in a dose- and time-dependent manner, a pathway known to play a key role in cell proliferation. We also characterized the molecular events, upstream of p21-Ras, that may link the MAP kinase pathway to G/CCKB receptors. Gastrin induced a rapid and transient increase in tyrosine phosphorylation of several proteins including the 2 isoforms (46 and 52 kDa) of the adaptor protein Shc. Phosphorylated Shc subsequently associated with a complex that includes Grb2 and the p21-Ras activator, Sos. Our results also indicate that Sos becomes phosphorylated in response to gastrin as shown by a reduction in electrophoretic mobility of the protein. Tyrosine phosphorylation of Shc and subsequent complex formation with Grb2 and Sos appear to be a common mechanism by which tyrosine kinase receptors and the G/CCKB G protein-coupled receptor stimulate the Ras-dependent MAP kinase pathway.

Gastrin-induced DNA synthesis requires p38-MAPK activation via PKC/Ca(2+) and Src-dependent mechanisms.

We present evidence that gastrin, binding to a G protein-coupled receptor, activates the p38-mitogen-activated protein kinase (MAPK) pathway. Blockage of protein kinase C (PKC) by GF109203X, depletion of intracellular calcium by thapsigargin or inhibition of Src family kinases by PP2 prevented p38-MAPK activation and the Src kinase activity stimulated by gastrin. Inhibition of the PI 3-kinase by wortmannin or LY294002 did not affect these responses. In addition, the p38-MAPK inhibitor, SB203580, repressed gastrin-induced [(3)H]thymidine incorporation, indicating a major role of p38-MAPK in the growth-promoting effect of gastrin. Our results demonstrate that gastrin-induced DNA synthesis requires p38-MAPK activation through mechanisms that involve calcium mobilization, PKC and Src family kinases.

The phosphoinositide 3-kinase/Akt pathway is activated by daunorubicin in human acute myeloid leukemia cell lines.

Daunorubicin induces apoptosis in myeloid leukemia cells by activation of neutral sphingomyelinase and ceramide generation occurring 4-10 min after daunorubicin addition. We show here that daunorubicin is able to increase the phosphoinositide 3-kinase activity and enhance intracellular phosphoinositide 3-kinase lipid products prior to ceramide generation. Daunorubicin activates Akt, a downstream phosphoinositide 3-kinase effector. Interestingly, the phosphoinositide 3-kinase inhibitors wortmannin and LY294002 accelerate daunorubicin-induced apoptosis in U937 cells. The phosphoinositide 3-kinase/Akt pathway has been involved in cell survival following serum deprivation, tumor necrosis factor alpha, anti-Fas and UV radiations. Our results suggest that anti-tumor agents such as daunorubicin may also activate anti-apoptotic signals that could contribute to drug resistance.

Gastrin stimulates tyrosine phosphorylation of insulin receptor substrate 1 and its association with Grb2 and the phosphatidylinositol 3-kinase.

The growth-promoting effects of gastrin on normal and neoplastic gastrointestinal tissues have been shown to be mediated by the gastrin/CCKB receptor, which belongs to the family of G protein-coupled receptors. However, the downstream signaling pathways activated by gastrin are not well characterized. In the present study, we demonstrate that gastrin stimulates tyrosine phosphorylation of insulin receptor substrate 1 (IRS-1), the major cytoplasmic substrate of the insulin receptor. The gastrin-induced phosphorylation of IRS-1 was rapid and transient, occurring within 30 s of treatment and diminishing thereafter. IRS-1 binds several proteins containing Src homology 2 domains through its multiple tyrosine phosphorylation sites. Following gastrin stimulation, we observed a time- and dose-dependent association of IRS-1 with the p85 regulatory subunit of phosphatidylinositol 3-kinase (PI 3-kinase). In addition, activation of PI 3-kinase was detected in anti-IRS-1 immunoprecipitates from gastrin-treated cells, suggesting that tyrosine phosphorylation of IRS-1, which leads to the rapid recruitment of p85, might be one mechanism used by gastrin to activate PI 3-kinase. We have previously reported that tyrosine phosphorylation of Shc and its association with the Grb2-Sos complex may contribute to the activation of the mitogen-activated protein kinase pathway by gastrin. We report here that Grb2 also interacts with tyrosine-phosphorylated IRS-1 in response to gastrin. Taken together, our results suggest that IRS-1 may serve as a converging target in the signaling pathways stimulated by receptors that belong to different families, such as the gastrin/CCKB G protein-coupled receptor and the insulin receptor.

Tyrosine phosphorylation of insulin receptor substrate-1 and activation of the PI-3-kinase pathway by glycine-extended gastrin precursors.

Glycine-extended gastrin precursors (G-Gly) were considered as processing intermediates devoid of biological activity. However, we have recently identified selective receptors for G-Gly which mediate the proliferative effects of this precursor. Little is known about the signaling pathways activated by G-Gly. In the present study, we demonstrate that PI-3-kinase is rapidly and transiently activated by G-Gly. We also observed a rapid increase in the tyrosine phosphorylation of IRS-1 and an activation of the PI-3-kinase in anti-IRS-1 immunoprecipitates, suggesting that PI-3-kinase may be activated by association with tyrosine phosphorylated IRS-1. We also demonstrated that gastrin precursors activate the serine/threonine kinase, p70 kDa S6 kinase (p70S6K), through a wortmannin sensitive pathway.

The insulin and insulin-like growth factor-I receptor substrate IRS-1 associates with and activates phosphatidylinositol 3-kinase in vitro.

Phosphatidylinositol 3-kinase (PtdIns-3-kinase) is thought to participate in the transductional cascade used by several tyrosine kinase receptors including the insulin-like growth factor (IGF)-I receptor and the insulin receptor. The major insulin receptor cellular substrate IRS-1 (pp185) has been proposed as a possible link between the insulin receptor and PtdIns-3-kinase. In this study we show that both insulin and IGF-I treatment of murine fibroblasts transfected with insulin or IGF-I receptors increase PtdIns-3-kinase activity immunoprecipitated with an antibody directed against the 85-kDa subunit of PtdIns-3-kinase. Whereas only a small amount of PtdIns-3-kinase is found associated with the insulin and IGF-I receptor, a considerable PtdIns-3-kinase activity is immunoprecipitated by an antibody raised against IRS-1. Additionally, insulin and IGF-I stimulation of murine fibroblasts expressing insulin or IGF-I receptors induce tyrosine phosphorylation of IRS-1 and its association with PtdIns-3-kinase. Since IRS-1 seems to be the connection between PtdIns-3-kinase and insulin or IGF-I receptor, we used reconstitution experiments to characterize the implication of IRS-1 in the activation of PtdIns-3-kinase. We show that immunoaffinity-purified IRS-1 can be phosphorylated by ligand-stimulated insulin and IGF-I receptors and that this phosphorylation allows the association of IRS-1 with PtdIns-3-kinase. The interaction between PtdIns-3-kinase and IRS-1 phosphorylated by the insulin or the IGF-I receptor results in the activation of PtdIns-3-kinase. In conclusion, our results demonstrate that IRS-1 is a key component in the signal transduction pathway of PtdIns-3-kinase activation induced by insulin and IGF-I.

Tyrosine kinase activity of a chimeric insulin-like-growth-factor-1 receptor containing the insulin receptor C-terminal domain. Comparison with the tyrosine kinase activities of the insulin and insulin-like-growth-factor-1 receptors using a cell-free system.

In a previous study, we showed that a chimeric insulin-like-growth-factor-1 (IGF-1) receptor, with the beta subunit C-terminal part of the insulin receptor was more efficient in stimulating glycogen synthesis and p44mapk activity compared to the wild-type IFG-1 receptor [Tartare, S., Mothe, I., Kowalski-Chauvel, A., Breittmayer, J.-P., Ballotti, R. & Van Obberghen, E. (1994) J. Biol. Chem. 269, 11449-11455]. These data indicate that the receptor C-terminal domain plays an important role in the transmission of biological effects. To understand the molecular basis of the differences in receptor specificity, we studied the characteristics of insulin, IGF-1 and chimeric receptor tyrosine kinase activities in a cell-free system. We found that, compared to wild-type insulin and IGF-1 receptors, the chimeric receptor showed a decrease in (a) autophosphorylation, (b) tyrosine kinase activity towards insulin receptor substrate-1 and the insulin receptor-(1142-1158)-peptide, and (c) the ability to activate phosphatidylinositol 3-kinase. However, for all the effects measured in a cell-free system, the chimeric receptor displayed an increased response to IGF-1 compared to the native IGF-1 receptor. Concerning the cation dependence of the tyrosine kinase activity, we showed that, at 10 mM Mg2+, the ligand-stimulated phosphorylation of poly(Glu80Tyr20) by both insulin receptor and chimeric receptor was increased by Mn2+. Conversely at 50 mM Mg2+, the chimeric receptor behaved like the IGF-1 receptor, since the presence of Mn2+ decreased the stimulatory effect of IGF-1 on their kinase activity. Furthermore, the Km of the chimeric receptor for ATP was increased compared to the wild-type receptors. These data demonstrate that the replacement of the C-terminal tail of the IGF-1 receptor by that of the insulin receptor has changed the receptor characteristics studied in a cell-free system. Our findings indicate that the C-terminal domain of the insulin receptor beta subunit plays a key role in regulation of the tyrosine kinase activity. The fine-tuning of the tyrosine kinase by the C-terminal tail could participate in the receptor specificity.

Insulin stimulates phosphatidylinositol-3-kinase activity in rat adipocytes.

Phosphatidylinositol (PtdIns) 3-kinase is thought to participate in the signal transduction pathways initiated by the activation of receptor tyrosine kinases including the insulin receptor. To approach the physiological relevance of this enzyme in insulin signaling, we studied the activation of PtdIns-3-kinase in adipocytes, a major insulin target tissue for glucose transport and utilisation. To analyze possible interactions of the enzyme with cellular proteins, immunoprecipitations with the following antibodies were performed: (a) anti-phosphotyrosine antibodies, (b) two antibodies to the 85-kDa subunit of PtdIns-3-kinase (p85) and (c) an antibody to the 185-kDa major insulin receptor substrate (p185). We show that in cell extracts from adipocytes exposed to insulin, and after immunoprecipitation with an anti-phosphotyrosine antibody and an antibody to p85, we are able to detect a PtdIns-3-kinase activity stimulated by the hormone. Similarly, after immunoprecipitation with an antibody to p185, an increase in the PtdIns-3-kinase activity could be demonstrated. Taken together these results suggest that, upon insulin stimulation of fat cells, PtdIns-3-kinase itself is tyrosine phosphorylated and/or associated with an insulin receptor substrate, such as p185, which could function as a link between the insulin receptor and PtdIns-3-kinase. The PtdIns-3-kinase was activated within 1 min of exposure to insulin, and the half-maximal effect was reached at the same concentration, i.e. 3 nM, as for stimulation of the insulin receptor kinase. Subcellular fractionation showed that PtdIns-3-kinase activity was found both in the membranes and in the cytosol. Further, immunoprecipitation with an antibody to p85, which possesses the capacity to activate PtdIns-3-kinase, suggests that the presence of the enzyme in the membrane may be due to an insulin-induced recruitment of the PtdIns-3-kinase from the cytosol to the membrane. Finally, we used isoproterenol, which exerts antagonistic effects on insulin action. This drug was found to inhibit both the PtdIns-3-kinase and the insulin receptor activation by insulin, suggesting that the activation of the PtdIns-3-kinase was closely regulated by the insulin receptor tyrosine kinase. The occurrence of an insulin-stimulated PtdIns-3-kinase in adipocytes leads us to propose that this enzyme might be implicated in the generation of metabolic responses induced by insulin.

Src-family tyrosine kinases in activation of ERK-1 and p85/p110-phosphatidylinositol 3-kinase by G/CCKB receptors.

We have analyzed in Chinese hamster ovary cells the upstream mediators by which the G protein-coupled receptor, gastrin/CCKB, activates the extracellular-regulated kinases (ERKs) and p85/p110-phosphatidylinositol 3-kinase (PI 3-kinase) pathways. Overexpression of an inhibitory mutant of Shc completely blocked gastrin-stimulated Shc.Grb2 complex formation but partially inhibited ERK-1 activation by this peptide. Expression of Csk, which inactivates Src-family kinases, totally inhibited gastrin-induced Src-like activity detected in anti-Src and anti-Shc precipitates but diminished by 50% Shc phosphorylation and ERK-1 activation. We observed a rapid tyrosine phosphorylation of insulin receptor substrate-1 (IRS-1) and an increase in Src-like kinase activity in anti-IRS-1 immunoprecipitates from gastrin-stimulated cells, suggesting that IRS-1 may be a direct substrate of Src. This hypothesis was supported by the inhibition of gastrin-induced Src. IRS-1 complex formation and IRS-1 phosphorylation in Csk-transfected cells. In addition, the increase in PI 3-kinase activity measured in anti-p85 or anti-IRS-1 precipitates following gastrin stimulation was abolished by Csk. Our results demonstrate the existence of two mechanisms in gastrin-mediated ERKs activation. One requires Shc phosphorylation by Src-family kinases, and the other one is independent of these two proteins. They also indicate that tyrosine phosphorylation of IRS-1 by Src-family kinases could lead to the recruitment and the activation of the p85/p110-PI 3-kinase in response to gastrin.

Ca2+ and protein kinase C-dependent mechanisms involved in gastrin-induced Shc/Grb2 complex formation and P44-mitogen-activated protein kinase activation.

The proliferative effects of gastrin on normal and neoplastic gastro-intestinal tissues have been shown to be mediated by the gastrin/CCKB (G/CCKB) G-protein-coupled receptors. We have recently reported that gastrin stimulates the tyrosine phosphorylation of Shc proteins and their subsequent association with the Grb2/Sos complex, leading to mitogen-activated protein kinase (MAPK) activation, a pathway known to play an important role in cell proliferation. We undertook the present study to characterize the signalling pathways used by this receptor to mediate the activation of the Shc/Grb2 complex. Since G/CCKB receptor occupancy leads to the activation of the phospholipase C (PLC)/protein kinase C (PKC) pathway, we examined whether PKC stimulation and Ca2+ mobilization contribute to the phosphorylation of Shc proteins and their association with Grb2 in response to gastrin. Our results indicate that Shc proteins are tyrosine phosphorylated and associate with Grb2 in response to phorbol esters, suggesting that activation of PKC is a potential signalling pathway leading to activation of the Shc/Grb2 complex. Inhibition of PKC by GF109203X completely blocked the effect of PMA on Shc tyrosine phosphorylation and its subsequent association with Grb2, but had a partial inhibitory effect on the response to gastrin. Depletion of the intracellular Ca2+ pools by treatment with thapsigargin blocked the increase in intracellular free calcium concentration induced by gastrin and diminished the ability of the peptide to stimulate Shc phosphorylation and recruitment of Grb2. In addition, removal of extracellular Ca2+ partially inhibited the effect of gastrin on Shc phosphorylation as well as its association with Grb2, indicating that the effects of gastrin are also mediated by Ca2+-dependent mechanisms. Furthermore, we show that blockage of the two major early signals generated by activation of PLC, which induced the activation of the Shc/Grb2 complex, also blocked gastrin-induced MAPK activation.

Wortmannin-sensitive activation of p70S6-kinase and MAP-kinase by the G protein-coupled receptor, G/CCKB.

The gastrin/CCKB (G/CCKB) G protein-coupled receptor has been shown to mediate the proliferative effects of gastrin on normal and neoplastic gastro-intestinal tissues. In the present study, we examined the signal transduction mechanisms coupled to this receptor. We report here that phosphorylation and activity of the p70S6K, whose major substrate is the ribosomal S6 protein, are enhanced in response to gastrin. These effects were completely reversed by a commonly used PI-3-kinase inhibitor, wortmannin, suggesting that p70S6K may be a downstream target of PI-3-kinase in a signaling cascade induced by gastrin. In addition, blocking PI-3-kinase activity by wortmannin partially decreased gastrin-induced MAPK activation (42% +/- 3) as well as the tyrosine phosphorylation of She (50% +/- 6), an upstream regulator of the Ras-dependent MAPK pathway. These results indicate that at least two signaling pathways lead to MAPK activation by gastrin, only one of which is sensitive to PI-3-kinase inhibitors.

Signal transduction by a chimeric insulin-like growth factor-1 (IGF-1) receptor having the carboxyl-terminal domain of the insulin receptor.

Insulin-like growth factor-1 receptors (IGF-1R) and insulin receptors (IR) are closely related tyrosine kinases. However, the IR plays a major role in metabolism control, whereas the IGF-1R is mainly involved in growth and differentiation. With these observations in mind, we wished to define the regions of IR and IGF-1R responsible for generation of biological specificity. We constructed a chimeric IGF-1R in which the carboxyl-terminal domain was replaced by that of IR. This receptor (IGF/CTIR) was expressed in NIH3T3 cells, and we compared its biological activity with that of wild-type receptors. The IGF/CTIR was fully functional regarding kinase activity and biological properties. Comparison of insulin and IGF-1 effects on IR and IGF-1R cells, respectively, indicated that the IR is more efficient in stimulating glycogen synthesis and p44mapk activity than is the IGF-1R. Interestingly, in IGF/CTIR16 cells expressing only 250,000 receptors glycogen synthesis was better stimulated than in IGF-1R cells with 600,000 receptors. Similarly, p44mapk activation was slightly higher in IGF/CTIR16 cells than in IGF-1R cells. These results suggest that the carboxyl-terminal domain of IR is more tightly coupled to the stimulation of glycogen synthesis and to the p44mapk pathway than is that of IGF-1R. We propose that this domain plays a crucial role in the transmission of biological effects and could account, at least in part, for receptor specificity.

Mutation of Asn-391 within the conserved NPXXY motif of the cholecystokinin B receptor abolishes Gq protein activation without affecting its association with the receptor.

Among the most conserved regions in the G-protein-coupled receptors is the (N/D)PX(2-3)Y motif of the seventh transmembrane domain (X represents any amino acid). The mutation of the Asn/Asp residue of this motif in different G-protein-coupled receptors was shown to affect the activation of either adenylyl cyclase or phospholipase C. We have mutated the Asn residue (Asn-391) of the NPXXY motif in the CCKBR to Ala and determined the effects of the mutation on binding, signaling, and G-proteins coupling after expression of the mutated receptor in COS cells. The mutated receptor displayed similar expression levels and high affinity CCK binding compared with the wild type CCKBR. However, unlike the wild type CCKBR, the mutated receptor was completely unable to mediate activation of either phospholipase C and protein kinase C-dependent and -independent mitogen-activated protein kinase pathways, indicating an essential role of Asn-391 in CCKBR signaling. Coimmunoprecipitation experiments allowed us to show that the inactive mutant retains an intact capacity to form stable complexes with G(q)alpha subunits in response to CCK. These results indicate that the formation of high affinity CCK-receptor-G(q) protein complexes is not sufficient to activate G(q) and suggest that Asn-391 is specifically involved in G(q) proteins activation.