PI3-Kinase Controls Smooth Muscle Contraction Via Regulation Of MLCP Activity

Abstract

We demonstrated for the first time that PI3-kinase plays a role in the regulation of smooth muscle contraction by controlling MLCP. The inhibition of PI3-kinase markedly inhibited Ca2+-induced contraction and GTPγS induced Ca2+ sensitization of α-toxin permeabilized vascular smooth muscle as well as K+-induced contraction of intact vascular smooth muscle. The contractile inhibition was accompanied by the decrease in MLC phosphorylation and MBS phosphorylation at Thr696 and Thr853, which are responsible for the inhibition of MLCP activity. On the other hand, the inhibition of PI3-kinase had no effect on MLCK activity. These results suggest that PI3-kinase is involved in the regulation of MLCP, thus regulating MLC phosphorylation. An Akt specific inhibitor, SH-6, had no effect on the contraction, suggesting that Akt, one of the major down-stream effecter of the PI3-kinase pathway is not involved in this mechanism. MBS phosphorylation at Thr853, a Rho kinase specific site, was decreased by the inhibition of PI3-kinase even at rest, when Rho kinase is not activated. These results suggest that PI3-kinase does not influence the MBS kinases, such as Rho kinase. In fact, we found that the PI3-kinase inhibition activated MBS phosphatase activity. Furthermore, we found that PI3-kinase inhibition increased MBS phsophorylation at the PKG site, suggesting the activation of PKG pathway. Since the activation of the cGMP/PKG pathway decreases MLC phosphorylation by activating MBS phosphatase (Nakamura et al., 2007), our results suggest that PI3-kinase regulates smooth muscle contraction by modulating the PKG pathway.