Oxidative stress impairs insulin but not platelet-derived growth factor signalling in 3T3-L1 adipocytes

Activation of phosphatidylinositol 3-kinase (PI 3-kinase) is a common event in both insulin and platelet-derived growth factor (PDGF) signalling, but only insulin activates this enzyme in the high-speed pellet (HSP), and induces GLUT4 translocation. Recently, we have demonstrated that exposure of 3T3-L1 adipocytes to oxidative stress impairs insulin-stimulated GLUT4 translocation and glucose transport, associated with impaired PI 3-kinase translocation and activation in the HSP [Tirosh, Potashnik, Bashan and Rudich (1999) J. Biol. Chem. 274, 10595–10602]. In this study the effect of a 2h exposure to ≈ 30µM H₂O₂ on insulin versus PDGF-BB signalling and metabolic effects was compared. PDGF-stimulated p85-associated PI 3-kinase activity in total cell lysates, as well as co-precipitation of the PDGF receptor, were unaffected by oxidative stress. Additionally, the increase in p85 association with the plasma-membrane lawns by PDGF remained intact following oxidation, whereas the insulin effect was decreased. PDGF significantly increased protein kinase B (PKB) activity in early differentiated cells, and that of p70 S6-kinase in both early and fully differentiated 3T3-L1 adipocytes. Following oxidation the effect of PDGF on PKB and p70 S6-kinase activation remained intact, whereas significant inhibition of insulin-stimulated activation of those enzymes was observed. In accordance, in both early and fully differentiated cells, oxidative stress completely blunted insulin- but not PDGF-stimulated protein synthesis. In conclusion, oxidative stress impairs insulin, but not PDGF, signalling and metabolic actions in both early and fully differentiated 3T3-L1 adipocytes. This emphasizes compartment-specific activation of PI 3-kinase as an oxidation-sensitive step specifically leading to insulin resistance.