Cross-talk between the Platelet-derived Growth Factor and the Insulin Signaling Pathways in 3T3-L1 Adipocytes

Phosphatidylinositol (PI) 3-kinase is activated by various growth factors such as PDGF (platelet-derived growth factor) and insulin. The aim of the present study was to determine whether PDGF could modulate insulin activation of PI 3-kinase in 3T3-L1 adipocytes. When cells were preincubated for 5–15 min with PDGF, PI 3-kinase activity associated to insulin receptor substrate 1 (IRS 1) in response to insulin was decreased, due to reduced association of the PI 3-kinase p85 subunit with IRS 1. In addition, following this PDGF pretreatment, the tyrosine phosphorylation of IRS 1 in response to insulin and its electrophoretic mobility were diminished. The change in the mobility of IRS 1 could be attributed to PDGF-induced serine/threonine phosphorylation of the protein which was partly inhibited by PI 3-kinase inhibitors. By contrast, epidermal growth factor, which does not stimulate PI 3-kinase, had no effect on the association of PI 3-kinase with IRS 1 in response to insulin. This series of results indicates that the PDGF-induced serine/threonine phosphorylation of IRS 1 could be due to activation of PI 3-kinase pathway. Furthermore, this phosphorylation of IRS 1 is associated with a decrease in its tyrosine phosphorylation by insulin and in its association with the p85 subunit of PI 3-kinase. This study suggests that a cross-talk exists between the different pathways stimulated by PDGF and insulin in intact cells. Phosphatidylinositol (PI) 3-kinase is activated by various growth factors such as PDGF (platelet-derived growth factor) and insulin. The aim of the present study was to determine whether PDGF could modulate insulin activation of PI 3-kinase in 3T3-L1 adipocytes. When cells were preincubated for 5–15 min with PDGF, PI 3-kinase activity associated to insulin receptor substrate 1 (IRS 1) in response to insulin was decreased, due to reduced association of the PI 3-kinase p85 subunit with IRS 1. In addition, following this PDGF pretreatment, the tyrosine phosphorylation of IRS 1 in response to insulin and its electrophoretic mobility were diminished. The change in the mobility of IRS 1 could be attributed to PDGF-induced serine/threonine phosphorylation of the protein which was partly inhibited by PI 3-kinase inhibitors. By contrast, epidermal growth factor, which does not stimulate PI 3-kinase, had no effect on the association of PI 3-kinase with IRS 1 in response to insulin. This series of results indicates that the PDGF-induced serine/threonine phosphorylation of IRS 1 could be due to activation of PI 3-kinase pathway. Furthermore, this phosphorylation of IRS 1 is associated with a decrease in its tyrosine phosphorylation by insulin and in its association with the p85 subunit of PI 3-kinase. This study suggests that a cross-talk exists between the different pathways stimulated by PDGF and insulin in intact cells. Phosphatidylinositol (PI) 3-kinase 1The abbreviations used are: PI 3-kinase, phosphatidylinositol 3-kinase; PDGF, platelet-derived growth factor; EGF, epidermal growth factor; DMEM, Dulbecco's modified Eagle's medium; PVDF, polyvinylidene difluoride; IRS 1, insulin receptor substrate 1; PAGE, polyacrylamide gel electrophoresis. is a common element of the signaling pathway of a large number of tyrosine kinase receptors. PI 3-kinase is a heterodimer consisting of an 85-kDa regulatory subunit (p85) containing one Src homology 3 (SH3) domain and two Src homology 2 (SH2) domains (1Escobedo J.A. Navankasattusas S. Kavanaugh W.M. Milfay D. Fried V.A. Williams L.T. Cell. 1991; 65: 75-82Abstract Full Text PDF PubMed Scopus (375) Google Scholar, 2Otsu M. Hiles I. Gout I. Fry M.J. Ruiz-Larrea F. Panayotou G. Thompson A. Dhand R. Hsuan J. Totty N. Smith A.D. Morgan S.J. Courtneidge S.A. Parker P.J. Waterfield M.D. Cell. 1991; 65: 91-104Abstract Full Text PDF PubMed Scopus (541) Google Scholar, 3Skolnik E.Y. Margolis B. Mohammadi M. Lowenstein E. Fischer R. Drepps A. Ullrich A. Schlessinger J. Cell. 1991; 65: 83-90Abstract Full Text PDF PubMed Scopus (439) Google Scholar) and an 110-kDa catalytic subunit (4Hiles I.D. Otsu M. Volinia S. Fry M.J. Goud I. Dhand R. Panayotou G. Ruiz-Larrea F. Thompson A. Totty N.F. Hsuan J.J. Courtneidge S.A. Parker P.J. Waterfield M.D. Cell. 1992; 70: 419-429Abstract Full Text PDF PubMed Scopus (540) Google Scholar). The catalytic subunit phosphorylates inositol lipids at the D-3 position of the inositol ring and has been shown to possess a serine kinase activity (5Dhand R. Hiles I. Panayotou G. Roche S. Fry M.J. Gout I. Totty N.F. Truong O. Vicendo P. Yonezawa K. Kasuga M. Courtneidge S.A. Waterfield M.D. EMBO J. 1994; 13: 522-533Crossref PubMed Scopus (415) Google Scholar). By contrast, the p85 regulatory subunit functions as an adaptor which, via its SH2 domains, links PI 3-kinase to tyrosine-phosphorylated proteins such as autophosphorylated tyrosine kinase receptors (6Kapeller R. Cantley L.C. BioEssays. 1994; 16: 565-576Crossref PubMed Scopus (553) Google Scholar). This association leads to the stimulation of the kinase activities of the p110 subunit (6Kapeller R. Cantley L.C. BioEssays. 1994; 16: 565-576Crossref PubMed Scopus (553) Google Scholar, 7Dhand R. Hara K. Hiles I. Bax B. Gout I. Panayotou G. Fry M.J. Yonezawa K. Kasuga M. Waterfield M.D. EMBO J. 1994; 13: 511-521Crossref PubMed Scopus (295) Google Scholar). Since PI 3-kinase is activated by a large range of peptide growth factors, this enzyme activity appears to be implicated in various cellular responses including promotion of cell growth, regulation of cell differentiation, and metabolism (for review see Ref. 6Kapeller R. Cantley L.C. BioEssays. 1994; 16: 565-576Crossref PubMed Scopus (553) Google Scholar). Despite this, each growth factor triggers distinct and specific biological responses in each particular cell type. In the present study, we looked at the effects of a prior stimulation by platelet-derived growth factor (PDGF) on the further ability of insulin to activate PI 3-kinase. We took advantage of the 3T3-L1 adipocytes where PI 3-kinase can be activated by both insulin and PDGF but not by EGF (8Ricort J.-M. Tanti J.-F. Van Obberghen E. Le Marchand-Brustel Y. Eur. J. Biochem. 1996; 239: 17-22Crossref PubMed Scopus (85) Google Scholar, 9Navé B.T. Haigh R.J. Hayward A.C. Siddle K. Shepherd P.R. Biochem. J. 1996; 318: 55-60Crossref PubMed Scopus (129) Google Scholar). When PDGF stimulates tyrosine phosphorylation of its receptor, the p85 regulatory subunit of PI 3-kinase associates to phosphorylated Tyr-Xaa-Xaa-Met motifs of the PDGF receptor (10Fantl W.J. Escobedo J.A. Martin G.A. Turck C.W. Del Rosario M. McCormick F. Williams L.T. Cell. 1992; 69: 413-423Abstract Full Text PDF PubMed Scopus (473) Google Scholar, 11Kashishian A. Kazlauskas A. Cooper J.A. EMBO J. 1992; 11: 1373-1382Crossref PubMed Scopus (201) Google Scholar). In contrast, insulin activates the tyrosine kinase activity of its receptor that subsequently phosphorylates insulin receptor substrate 1 (IRS 1) on YMXM motifs allowing binding of p85 to IRS 1 and PI 3-kinase activation (12Myers Jr., M.G. Backer J.M. Sun X.J. Shoelson S. Hu P. Schlessinger J. Yoakim M. Schauffhausen B. White M.F. Proc. Natl. Acad. Sci. U. S. A. 1992; 89: 10350-10354Crossref PubMed Scopus (385) Google Scholar, 13Backer J.M. Myers Jr., M.G. Shoelson S.E. Chin D.J. Sun X.-J. Miralpeix M. Hu P. Margolis B. Skolnik E.Y. Schlessinger J. White M.F. EMBO J. 1992; 11: 3469-3479Crossref PubMed Scopus (820) Google Scholar, 14Yonezawa K. Ueda H. Hara K. Nishida K. Ando A. Chavanieu A. Matsuba H. Shii K. Yokono K. Fukui Y. Calas B. Grigorescu F. Dhand R. Gout I. Otsu M. Waterfield M.D. Kasuga M. J. Biol. Chem. 1992; 267: 25958-25966Abstract Full Text PDF PubMed Google Scholar). Our data show that pretreatment with PDGF decreases the ability of insulin to phosphorylate IRS 1 on tyrosine residues and consequently decreases both the amount of the p85 subunit and the PI 3-kinase activity associated with IRS 1. These results demonstrate that, in 3T3-L1 adipocytes, a cross-talk exists between the pathways stimulated by PDGF and insulin. This phenomenon could play a role in the regulation of the cell responses to growth factors and may explain the modulation of the cellular responses to different stimuli. Antibodies to IRS 1 were obtained from a rabbit injected with a peptide corresponding to the sequence comprising amino acids 489–507 of the protein and used at 1/100 dilution for immunoblotting. Antibodies used for immunoblotting of the p85 subunit of the PI 3-kinase and of phosphotyrosine-containing proteins were from UBI (Lake Placid, NY). Antibodies to phosphotyrosine used in immunoprecipitation assays were obtained after injection of a rabbit with phosphotyrosine coupled to bovine immunoglobulins. Insulin was a gift from Lilly (Paris, France). PDGF-BB was from Pepro Tech. Inc. (Rocky Hill, NJ). Bovine serum albumin was from Intergen Co. (Purchase, NY). All other biochemicals were from Sigma or Serva (Heidelberg, Germany). Radiochemicals were from ICN Pharmaceuticals (Orsay, France). 3T3-L1 fibroblasts were cultured in DMEM, 10% fetal calf serum and induced to differentiate into adipocytes as described (8Ricort J.-M. Tanti J.-F. Van Obberghen E. Le Marchand-Brustel Y. Eur. J. Biochem. 1996; 239: 17-22Crossref PubMed Scopus (85) Google Scholar). 3T3-L1 adipocytes were used between 8 and 12 days after initiation of differentiation, when more than 95% of the cells exhibited an adipocyte-like phenotype. Sixteen hours before each experiment, the cells were changed to serum-free DMEM supplemented with 0.5% (w/v) bovine serum albumin. 3T3-L1 adipocytes were pretreated at 37 °C without or with PDGF (50 ng/ml) or EGF (100 nm) for different periods. Then the cells were stimulated or not for 5 min with insulin (100 nm). The cells were solubilized for 40 min at 4 °C in 700 μl of buffer A (20 mm Tris, pH 7.4, 137 mm NaCl, 100 mm NaF, 10 mm EDTA, 2 mmNa3VO4, 10 mm pyrophosphate, 1 mm PMSF, 100 units/ml aprotinin) containing 1% Nonidet P-40 (v/v). Lysates were centrifuged for 10 min at 13,000 ×g. Supernatants were incubated for 2 h at 4 °C with antibodies to IRS 1 or to phosphotyrosine coupled to protein A-Sepharose beads. Immune pellets were washed twice with each of the three following buffers: (a) phosphate-buffered saline containing 1% Nonidet P-40 and 200 μmNa3VO4; (b) 100 mm Tris, pH 7.4, 0.5 m LiCl, 200 μmNa3VO4, and (c) 10 mmTris, pH 7.4, 100 mm NaCl, 1 mm EDTA, 200 μm Na3VO4. Bead-associated PI 3-kinase activity was assayed from the phosphorylation of PI in the presence of [γ-32P]ATP as described previously (15Heydrick S.J. Jullien D. Gautier N. Tanti J.-F. Giorgetti S. Van Obberghen E. Le Marchand-Brustel Y. J. Clin. Invest. 1993; 91: 1358-1366Crossref PubMed Scopus (118) Google Scholar). The reaction products were separated by thin layer chromatography. Quantification was performed after autoradiography by Cerenkov counting of the corresponding to phosphatidylinositol were pretreated with PDGF (50 ng/ml) for different and stimulated with insulin (100 nm) for 5 When cells were with 100 or with μm for min prior to PDGF Then the cells were solubilized in buffer A containing 1% Nonidet were incubated for 2 h with antibodies to IRS 1 on protein were washed as described and were with for 10 and proteins by with a were to a polyvinylidene The was incubated with buffer bovine serum for 2 h at and at 4 °C with antibodies to IRS 1, to the p85 subunit of PI 3-kinase, or to were washed three min in phosphate-buffered saline containing 1% Nonidet When antibodies to phosphotyrosine were a further was performed for 1 h at with rabbit immunoglobulins. were incubated for 1 h at A and washed as were to 3T3-L1 adipocytes in DMEM supplemented with 0.5% bovine serum albumin (w/v) were for h at 37 °C with When cells were with 100 for min prior to a or stimulation with Then the cells were stimulated or not for 5 min with 100 insulin. were washed with buffer A and solubilized for 40 min at 4 °C in μl of buffer A supplemented with 1% Nonidet P-40 (v/v). were centrifuged at 13,000 for 10 Supernatants were for 2 h at 4 °C with antibodies to IRS 1 on protein The pellets were washed as described with for 10 and separated on a The gel was and We the of PI 3-kinase activation by PDGF in 3T3-L1 adipocytes. were incubated for various with PI 3-kinase activity was in obtained with antibodies to phosphotyrosine PDGF stimulated the PI 3-kinase activity between 5 and min of Then the activity to This of PI 3-kinase activation the tyrosine phosphorylation of the PDGF receptor 1) that PI 3-kinase activity in was due to the association of the enzyme with the tyrosine-phosphorylated PDGF receptors. By contrast, EGF not change in the PI 3-kinase activity in 3T3-L1 adipocytes but was to activate protein kinase not of PDGF on the association of p85 with IRS 1 and on tyrosine phosphorylation of IRS 1. were incubated without or with PDGF for the were without or with 100 insulin for 5 were to antibodies to IRS 1. were separated by to and with antibodies to the p85 subunit of the PI 3-kinase phosphotyrosine or to IRS 1 as described in of different of PDGF on PI 3-kinase activity associated to phosphotyrosine-containing 3T3-L1 adipocytes were incubated with or without PDGF for the before and immunoprecipitation with antibodies to PI 3-kinase was performed on the pellets as described Quantification of the PI 3-kinase was performed by to the activity after 5 min of PDGF stimulation the S.E. of three cells were stimulated with PDGF for the were solubilized and proteins were separated on phosphotyrosine-containing proteins were as described The the results obtained in one in which been Since PI 3-kinase associates to activated PDGF we whether a pretreatment of the cells by PDGF could with a activation of PI 3-kinase by insulin. this 3T3-L1 adipocytes were stimulated with PDGF for different before a insulin PI 3-kinase activity was in obtained with antibodies to IRS 1 Insulin PI 3-kinase activity associated to IRS 1 in different When the cells were pretreated with PDGF for 5–15 min before insulin PI 3-kinase activity associated to IRS 1 was after min of PDGF was at min of PDGF pretreatment and PI 3-kinase activity by after min from the results with PDGF, EGF pretreatment of the cells not the of PI 3-kinase activity associated to IRS 1 in response to a insulin The that the PI 3-kinase activity associated with IRS 1 following insulin stimulation is by a PDGF We to determine whether a of the amount of the enzyme associated with IRS 1 or an of the PI 3-kinase activity 3T3-L1 adipocytes were pretreated with PDGF for different before insulin stimulation were solubilized and with antibodies to IRS 1. The proteins were by and to The PI 3-kinase associated with IRS 1 was by antibodies to its p85 subunit In the of PDGF pretreatment, insulin induced the association of the PI 3-kinase p85 subunit with IRS 1. The pretreatment with PDGF for min reduced the amount of the p85 associated to IRS 1. In the tyrosine phosphorylation of proteins was with an to The of the of its receptor was not by PDGF not By contrast, a PDGF pretreatment induced a decrease in the tyrosine phosphorylation of IRS 1 3 be that PDGF not tyrosine phosphorylation of IRS 1 not shown and Ref. J.-M. Tanti J.-F. Van Obberghen E. Le Marchand-Brustel Y. Eur. J. Biochem. 1996; 239: 17-22Crossref PubMed Scopus (85) Google Scholar). The decrease in IRS 1 tyrosine phosphorylation was not due to a in the amount of IRS 1 present in the pellets was in that PDGF pretreatment modified the cellular amount of IRS 1 the ability of the to IRS 1 3 PDGF induced an in the of IRS 1, which was to that induced by insulin 3 whether the IRS 1 electrophoretic in cells was due to a change in its phosphorylation 3T3-L1 adipocytes were with were stimulated for or min without or with PDGF (50 before insulin (100 5 the of the the proteins were solubilized and to immunoprecipitation with antibodies to IRS 1. proteins were by by autoradiography A of IRS 1 phosphorylation was in insulin and PDGF induced the of into IRS 1 and Since insulin but not PDGF not shown and Ref. J.-M. Tanti J.-F. Van Obberghen E. Le Marchand-Brustel Y. Eur. J. Biochem. 1996; 239: 17-22Crossref PubMed Scopus (85) Google induced the tyrosine phosphorylation of IRS 1, results that the phosphorylation on tyrosine residues in the presence of insulin and on serine/threonine residues following PDGF Furthermore, the induced by insulin and PDGF were not an insulin stimulation after a PDGF pretreatment not into IRS 1. These results that PDGF could the phosphorylation of IRS 1 in intact cells on serine/threonine residues and its ability to be tyrosine-phosphorylated by insulin and to the p85 subunit of PI 3-kinase. We looked at the that PI 3-kinase was in the phosphorylation of IRS 1 in response to PI 3-kinase is activated in response to PDGF J.-M. Tanti J.-F. Van Obberghen E. Le Marchand-Brustel Y. Eur. J. Biochem. 1996; 239: 17-22Crossref PubMed Scopus (85) Google B.T. Haigh R.J. Hayward A.C. Siddle K. Shepherd P.R. Biochem. J. 1996; 318: 55-60Crossref PubMed Scopus (129) Google and and PI 3-kinase a serine kinase activity IRS 1 in J.-F. Van Obberghen E. Le Marchand-Brustel Y. Biochem. J. 1994; PubMed Scopus Google Scholar, K. J. Biol. Chem. 1994; Full Text PDF PubMed Google Scholar, Biochem. PubMed Scopus Google Scholar). were incubated with two of the with 100 or μm for min prior to with PDGF or insulin IRS 1 tyrosine phosphorylation was by immunoblotting with antibodies to phosphotyrosine as described of the cells with the tyrosine phosphorylation of IRS 1 induced by insulin as Biochem. 1996; PubMed Scopus Google was without with and the effect of PDGF on the tyrosine phosphorylation of IRS 1 induced by and the electrophoretic mobility induced by PDGF When cells were with as described by the serine/threonine phosphorylation induced by PDGF of on the PDGF-induced phosphorylation of IRS 1. 3T3-L1 adipocytes were for h with were for min without or with with 100 and incubated for min with or without PDGF as in to were and with antibodies IRS 1, and pellets were for as described in to A is shown in the Quantification was performed with and the S.E. of three shown in the The present study was to at a modulation of insulin by growth factors, at the of PI 3-kinase. PI 3-kinase is in signaling pathways Cell. 1991; PubMed Scopus Google such as activated by insulin and of this enzyme results from the association of the p85 subunit of the PI 3-kinase to tyrosine-phosphorylated IRS 1 in response to insulin (12Myers Jr., M.G. Backer J.M. Sun X.J. Shoelson S. Hu P. Schlessinger J. Yoakim M. Schauffhausen B. White M.F. Proc. Natl. Acad. Sci. U. S. A. 1992; 89: 10350-10354Crossref PubMed Scopus (385) Google Scholar, 13Backer J.M. Myers Jr., M.G. Shoelson S.E. Chin D.J. Sun X.-J. Miralpeix M. Hu P. Margolis B. Skolnik E.Y. Schlessinger J. White M.F. EMBO J. 1992; 11: 3469-3479Crossref PubMed Scopus (820) Google Scholar, 14Yonezawa K. Ueda H. Hara K. Nishida K. Ando A. Chavanieu A. Matsuba H. Shii K. Yokono K. Fukui Y. Calas B. Grigorescu F. Dhand R. Gout I. Otsu M. Waterfield M.D. Kasuga M. J. Biol. Chem. 1992; 267: 25958-25966Abstract Full Text PDF PubMed Google Scholar, S. R. A. S. Van Obberghen E. J. Biol. Chem. 1993; Full Text PDF PubMed Google Scholar) or to tyrosine-phosphorylated PDGF receptor in response to PDGF (10Fantl W.J. Escobedo J.A. Martin G.A. Turck C.W. Del Rosario M. McCormick F. Williams L.T. Cell. 1992; 69: 413-423Abstract Full Text PDF PubMed Scopus (473) Google Scholar, 11Kashishian A. Kazlauskas A. Cooper J.A. EMBO J. 1992; 11: 1373-1382Crossref PubMed Scopus (201) Google Scholar). we advantage of 3T3-L1 adipocytes, which PDGF, and EGF to show that a pretreatment of the cells with PDGF, but not with EGF, the PI 3-kinase activity and the amount of p85 subunit associated with IRS 1 in response to a insulin This effect was after a PDGF in the tyrosine phosphorylation of IRS 1 was and this could explain the amount of p85 subunit associated with IRS 1 in response to insulin. The decrease in IRS 1 tyrosine phosphorylation was not due to an of insulin receptor tyrosine kinase activity PDGF not the of the By contrast, PDGF the electrophoretic mobility of IRS 1, the of a phosphorylation of IRS 1 following PDGF Since PDGF not tyrosine phosphorylation of IRS 1 not shown and Ref. J.-M. Tanti J.-F. Van Obberghen E. Le Marchand-Brustel Y. Eur. J. Biochem. 1996; 239: 17-22Crossref PubMed Scopus (85) Google the can from an in the serine/threonine phosphorylation of IRS 1 following PDGF These results of J.-F. Van Obberghen E. Le Marchand-Brustel Y. J. Biol. Chem. 1994; Full Text PDF PubMed Google Scholar) or H. R. R. A. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar, P. A. R. White M.F. 1996; PubMed Scopus Google Scholar) which IRS 1 serine/threonine phosphorylation and decrease IRS 1 tyrosine is to that the stimulation of IRS 1 serine/threonine phosphorylation induced by PDGF was for the of IRS 1 tyrosine The by which the serine/threonine phosphorylation of IRS 1 could modulate its tyrosine phosphorylation is not In the of was that the phosphorylated of IRS 1 could as an of the insulin receptor tyrosine kinase activity P. A. R. White M.F. 1996; PubMed Scopus Google Scholar). This in the present study insulin receptor was not by a PDGF pretreatment not When cells were with the tyrosine kinase activity of the insulin receptor was not but the of IRS 1 had a ability to with the insulin receptor J.-F. Van Obberghen E. Le Marchand-Brustel Y. J. Biol. Chem. 1994; Full Text PDF PubMed Google Scholar). has been that the phosphorylation of the serine residues of IRS 1, in the of the tyrosine residues in the binding of PI 3-kinase, could decrease the association between PI 3-kinase and IRS 1 I. Van Obberghen E. J. Biol. Chem. 1996; Full Text Full Text PDF PubMed Scopus Google Scholar). a could be in the effect we in cells. on the of in the serine/threonine phosphorylation of IRS 1. kinase M.J. Myers Jr., M.G. S. White M.J. J. Biol. Chem. 1993; Full Text PDF PubMed Google Scholar) and the serine kinase activity of PI 3-kinase J.-F. Van Obberghen E. Le Marchand-Brustel Y. Biochem. J. 1994; PubMed Scopus Google Scholar, K. J. Biol. Chem. 1994; Full Text PDF PubMed Google Scholar, Biochem. PubMed Scopus Google Scholar) been shown to phosphorylate IRS 1 in A series of data for a role of PI 3-kinase in the phosphorylation of IRS 1 following PDGF EGF, which is to activate PI 3-kinase in 3T3-L1 adipocytes but which activates other signaling pathways such as the protein kinase kinase does not the PI 3-kinase activity associated with IRS 1. the serine/threonine phosphorylation of IRS 1 induced by PDGF is partly inhibited by two of PI 3-kinase as by the of the mobility of IRS 1 or by the cell These data that PI 3-kinase can a serine/threonine phosphorylation of IRS 1 not in in obtained from cells J.-F. Van Obberghen E. Le Marchand-Brustel Y. Biochem. J. 1994; PubMed Scopus Google Scholar, K. J. Biol. Chem. 1994; Full Text PDF PubMed Google Scholar, Biochem. PubMed Scopus Google Scholar) but in cells stimulated by is that the phosphorylation of IRS 1 after a PDGF results from the activation of serine activated by the PI 3-kinase pathway. serine/threonine of PI 3-kinase, such as the protein kinase P. M. PubMed Scopus Google Scholar, A.D. EMBO J. PubMed Scopus Google Scholar, P.J. PubMed Scopus Google Scholar) and protein kinase and K. R. S. N. J. K. Fukui Y. K. Kazlauskas A. S. EMBO J. 1996; PubMed Scopus Google Scholar, E. I. J. J. Cell. 1993; Full Text PDF PubMed Scopus Google Scholar) activated by kinase of the PI 3-kinase could in the phosphorylation of IRS 1 the PDGF-induced phosphorylation of IRS 1 was not by to the enzyme implicated in the serine/threonine phosphorylation of IRS 1. The decrease in PI 3-kinase activity associated with IRS 1 was the of the reduced IRS 1 tyrosine phosphorylation at of PDGF other could be in such an has been shown that the p85 subunit tyrosine-phosphorylated after a PDGF stimulation W.M. A. Escobedo J.A. Williams L.T. Cell. Biol. 1992; PubMed Scopus Google Scholar, J. Dhand R. Waterfield M.D. J. Biol. Chem. 1996; Full Text Full Text PDF PubMed Scopus Google Scholar) and that this tyrosine phosphorylation its ability to with proteins such as the activated PDGF an is in we such a tyrosine phosphorylation of p85 in cell is that the activated PDGF receptor a large amount of p85 subunit following PDGF stimulation by a the amount of p85 to with IRS 1 after insulin a of PI 3-kinase from the PDGF receptors IRS 1 following insulin stimulation in a study R. W.J. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar) performed with cells by of insulin receptors the PDGF receptors were The present series of has been performed in 3T3-L1 adipocytes, the insulin and PDGF receptors. Since has been shown in cells that of the of associated with the PDGF receptor J. Dhand R. Waterfield M.D. J. Biol. Chem. 1996; Full Text Full Text PDF PubMed Scopus Google this is not to explain is PI 3-kinase activity associated with IRS 1 to after min of PDGF This serine/threonine phosphorylation of IRS 1 by PDGF was for at with a decrease in the tyrosine phosphorylation of IRS 1 and of its association with the p85 The for this is Our results show that between the different growth factors and signaling common to such as the PI 3-kinase, the of different of the various The of could in or The of such between the different signaling pathways be to the of such We for and G. for and M. for