Endothelin-l modulates insulin signaling through phosphatidylinositol 3-kinase pathway in vascular smooth muscle cells

Diminished insulin action in the vasculature may contribute to the developm ent of cardiovascular diseases in diabetes. me have studied insulin's effec ts on the phosphatidylinositol (PI) 3-kinase pathway in arterial smooth mus cle cells (SMCs) and its inhibition by endothelin (ET)-1, a potent vasoacti ve hormone reported to be elevated in insulin resistance and other vascular diseases. ET-1 increased the level of serine phosphorylation of insulin re ceptor beta subunit but increased both tyrosine and serine phosphorylation of insulin receptor substrate (IRS)-2. Pretreatment of cells with ET-1 (10 nmol/l) inhibited insulin-stimulated PI 3-kinase activity associated with I RS-2 by 50-60% and inhibited the association of p85 subunit of PI 3-kinase to IRS-2. The inhibition of insulin-stimulated PI 3-kinase activity by ET-1 was prevented by BQ-123, a selective ET, receptor antagonist, but was not affected by pertussis toxin. Treatment of cells with phorbol 12-myristate 1 3-acetate, an activator of protein kinase C (PKC), reduced both insulin-sti mulated PI 3-kinase activity by 57% and the association of IRS-2 to the p85 subunit of PI 3-kinase by 40%, whereas GF109203X, a specific inhibitor of PKC, partially prevented the inhibitory effect of ET-1 on insulin-induced P I 3-kinase activity. These results suggested that ET-1 could interfere with insulin signaling in SMCs by both PKC-dependent and -independent pathways.