PLATELET-DERIVED GROWTH-FACTOR INHIBITS INSULIN STIMULATION OF INSULIN-RECEPTOR SUBSTRATE-1-ASSOCIATED PHOSPHATIDYLINOSITOL 3-KINASE IN 3T3-L1 ADIPOCYTES WITHOUT AFFECTING GLUCOSE-TRANSPORT

Phosphatidylinositol 3-kinase (PI3K) activation is necessary for insul in-responsive glucose transporter (GLUT4) translocation and glucose tr ansport. Insulin and platelet-derived growth factor (PDGF) stimulate P I3K activity in 3T3-L1 adipocytes, but only insulin is capable of stim ulating GLUT4 translocation and glucose transport. We found that PDGF causes serine/threonine phosphorylation of insulin receptor substrate 1 (IRS-1) in 3T3-L1 cells, measured by altered mobility on SDS-polyacr ylamide gel, and this leads to a decrease in insulin-stimulated tyrosi ne phosphorylation of IRS-1. The PI3K inhibitors wortmannin and LY2940 02 inhibit the PDGF-induced phosphorylation of IRS-1, whereas the MEK inhibitor PD98059 was without a major effect. PDGF pretreatment for 60 -90 min led to a marked 80-90% reduction in insulin stimulatable phosp hotyrosine and IRS-1-associated PI3K activity. We examined the functio nal consequences of this decrease in IRS-1-associated PI3K activity. I nterestingly, insulin stimulation of GLUT4 translocation and glucose t ransport was unaffected by 60-90 min of PDGF preincubation. Furthermor e, insulin activation of Akt and p70(s6kinase), kinases downstream of PI3K, was unaffected by PDGF pretreatment, Wortmannin was capable of b locking these insulin actions following PDGF pretreatment, suggesting that PI3K was still necessary for these effects. In conclusion, 1) PDG F causes serine/threonine phosphorylation of IRS-1, and PI3K, or a kin ase downstream of PI3K, mediates this phosphorylation, 2) This PDGF-in duced phosphorylation of IRS-1 leads to a significant decrease in insu lin-stimulated PI3K activity. 3) PDGF has no effect on insulin stimula tion of Akt, p70(s6kinase), GLUT4 translocation, or glucose transport. 4) This suggests the existence of an IRS-1-independent pathway leadin g to the activation of PI3K, Akt, and p70(s6kinase); GLUT4 translocati on; and glucose transport.