Potentiation of Mitogenic Activity of Platelet-Derived Growth Factor by Physiological Concentrations of Insulin via the MAP Kinase Cascade in Rat A10 Vascular Smooth Muscle Cells

Hyperinsulinemia has been shown to beassociated with diabetic angiopathy. Migrationand proliferation of vascular smooth musclecells (VSMC) are the processes required for thedevelopment of atherosclerosis. In this study,we attempted to determine whether insulinaffects mitogenic signaling induced by plateletderivedgrowth factor (PDGF) in a rat VSMCcell line (A10 cells). PDGF stimulated DNAsynthesis which was totally dependent on Ras,because transfection of dominant negative Rasresulted in complete loss of PDGF-stimulatedDNA synthesis. Initiation of DNA synthesiswas preceded by activation of Raf-1, MEK andMAP kinases (Erk 1 and Erk2). Treatment ofthe cells with PD98059, an inhibitor of MAPKkinase (MEK) attenuated but did not abolishPDGF-stimulated DNA synthesis, suggestingthat MAPK is required but not essential forDNA synthesis. PDGF also stimulated phosphorylationof protein kinase B (Akt/PKB) andp70 S6Kinase (p70S6K) in a wortmannin-sensitivemanner. Rapamycin, an inhibitor ofp70S6K, markedly suppressed DNA synthesis.Low concentrations of insulin (1-10 nmol/l)alone showed little mitogenic activity and nosignificant effect on MAPK activity. However,the presence of insulin enhanced both DNAsynthesis and MAPK activation by PDGF. Theenhancing effect of insulin was not seen in cellstreated with PD98059. Insulin was withouteffect on PDGF-stimulated activations of protein kinase B (Akt/PKB) and p70S6K. We concludethat insulin, at pathophysiologically relevantconcentrations, potentiates the PDGFstimulatedDNA synthesis, at least in part, bypotentiating activation of the MAPK cascade.These results are consistent with the notionthat hyperinsulinemia is a risk factor for thedevelopment of atherosclerosis.