Class II phosphoinositide 3-kinase α-isoform regulates Rho, myosin phosphatase and contraction in vascular smooth muscle

We demonstrated previously that membrane depolarization and excitatory receptor agonists such as noradrenaline induce Ca(2+)- dependent Rho activation in VSM (vascular smooth muscle), resulting in MP (myosin phosphatase) inhibition through the mechanisms involving Rho kinase-mediated phosphorylation of its regulatory subunit MYPT1. In the present study, we show in de-endothelialized VSM strips that the PI3K (phosphoinositide 3-kinase) inhibitors LY294002 and wortmannin inhibited KCI membrane depolarization- and noradrenaline-induced Rho activation and MYPT1 phosphorylation, with concomitant inhibition of MLC (20-kDa myosin light chain) phosphorylation and contraction. LY294002 also augmented de-phosphorylation of MLC and resultantly relaxation in KCI-contracted VSM, whereas LY294002 was much less effective or ineffective under the conditions in which MP was inhibited by either a phosphatase inhibitor or a phorbol ester in Rho-independent manners. VSM express at least four PI3K isoforms, including the class I enzymes p110 alpha and p110 beta and the class II enzymes PI3K-C2 alpha and -C2 beta. The dose-response relationships of PI3K-inhibitor-induced inhibition of Rho, MLC phosphorylation and contraction were similar to that of PI3K-C2 alpha inhibition, but not to that of the class I PI3K inhibition. Moreover, KCl and noradrenaline induced stimulation of PI3K-C2 alpha in a Ca(2+)-dependent manner, but not of p110 alpha or p110 beta. Down-regulation of PI3K-C2 alpha expression by siRNA (small interfering RNA) inhibited contraction and phosphorylation of MYPT1 and MLC in VSM cells. Finally, intravenous wortmannin infusion induced sustained hypotension in rats, with inhibition of PI3K-C2 alpha activity, GTP-loading of Rho and MYPT1 phosphorylation in the artery. These results indicate the novel role of PI3K-C2 alpha in Ca(2+)-dependent Rho-mediated negative control of MP and thus VSM contraction.