Signal transduction in smooth muscle - Selected contribution: Insulin utilizes NO/cGMP pathway to activate myosin phosphatase via Rho inhibition in vascular smooth muscle

Our laboratory has recently demonstrated that insulin induces relaxation of vascular smooth muscle cells (VSMCs) by activating myosin-bound phosphatase (MBP) and by inhibiting Rho kinase (Begum N, Duddy N, Sandu OA, Reinzie J, and Ragolia L. Mol Endocrinol 14: 1365-1376, 2000). In this study, we tested the hypothesis that insulin via the nitric oxide (NO)/cGMP pathway may inactivate Rho, resulting in a decrease in phosphorylation of the myosin-bound subunit (MBSThr695) of MBP and in its activation. Treatment of confluent serum-starved VSMCs with insulin prevented thrombin-induced increases in membrane-associated RhoA, Rho kinase activation, and site-specific phosphorylation of MBSThr695 of MBP and caused MBP activation. Preexposure to N-G-monomethyl-L-arginine, a NO synthase inhibitor, and R-p-8-(4-chlorophenylthio) cGMP, a cGMP antagonist, attenuated insulin's inhibitory effect on Rho translocation and restored thrombin-mediated Rho kinase activation and site-specific MBSThr695 phosphorylation, resulting in MBP inactivation. In contrast, 8-bromo-cGMP, a cGMP agonist, mimicked insulin's inhibitory effects by abolishing thrombin-mediated Rho signaling and promoted dephosphorylation of MBSThr695. Furthermore, expression of a dominant-negative RhoA decreased basal as well as thrombin-induced MBSThr695 phosphorylation and caused insulin activation of MBP. Collectively, these results indicate that insulin inhibits Rho signaling by decreasing RhoA translocation via the NO/cGMP signaling pathway to cause MBP activation via site-specific dephosphorylation of its regulatory subunit MBS.