INSULIN SIGNALING AND ITS REGULATION OF SYSTEM-A AMINO-ACID-UPTAKE INCULTURED RAT VASCULAR SMOOTH-MUSCLE CELLS

Hyperinsulinemia has been recognized as an independent risk factor for atherosclerosis. However, its exact mechanisms are still unclear. In our previous work, we showed that 10 nmol/L insulin stimulated neither mitogen-activated protein kinase (MAP kinase) activity nor [H-3]thymi dine incorporation but did stimulate S6 kinase through the specific in sulin receptors in cultured rat vascular smooth muscle cells (VSMCs). In this study, we observed that greater than or equal to 1 nmol/L insu lin stimulated tyrosine phosphorylation of insulin receptor substrate- 1 (IRS-1) and activated IRS-1-dependent phosphatidylinositol 3'-kinase (PI 3'-kinase) and p70 S6 kinase (p70(S6K)) but not MAP kinase (extra cellular signal-regulated kinase 2) and p90 S6 kinase (p90(RSK)). Howe ver, 10 nmol/L insulin-like growth factor I stimulated all these pathw ays. Finally, 10 nmol/L insulin stimulated alpha-amino-isobutyric acid (AIB) uptake, and wortmannin (100 nmol/L) completely inhibited insuli n-stimulated AIB uptake, whereas rapamycin (20 nmol/L) had no such eff ect. Furthermore, cycloheximide (10 mu g/mL) completely inhibited insu lin-stimulated AIB uptake, but actinomycin D (5 mu g/mL) failed to inh ibit this. Thus, we reached the following conclusions: (1) Insulin (1 nmol/L) induced phosphorylation of IRS-1 and activated the PI 3'-kinas e and p70(S6K) pathways in VSMCs, even though 10 nmol/L insulin did no t significantly stimulate MAP kinase or p90(RSK). (2) Stimulation of A IB uptake by insulin was regulated at the translational level via wort mannin-sensitive pathways but not p70(S6K) pathways.