Rac1 Modulates Agonist-Induced Smooth Muscle Contraction

Abstract

Smooth muscle is distributed in many organs, and plays a critical role in maintaining blood pressure in vasculature and airflow in airway and malfunction of the smooth muscle causes severe health problems. Smooth muscle contraction is primarily regulated by myosin light chain (MLC) phosphorylation, which is regulated by both Ca2+ dependent and Ca2+ independent pathways. Recent studies have revealed that MLC phosphatase (MLCP) plays a key role in the latter mechanism. MLCP activity is regulated by the phosphorylation of MYPT1, a myosin binding regulatory subunit of MLCP, and CPI-17, a MLCP specific inhibitor protein. Here we found novel signaling mechanism of controlling smooth muscle contraction induced by agonists. We found that agonist stimulation significantly activates Rac1 activity in smooth muscle, and the inhibition of Rac markedly decreases agonist induced force of arterial smooth muscle. Moreover, introduction of active Rac1 activates smooth muscle contraction, while dominant-negative Rac1 attenuates the agonist-induced contraction. Rac1 dependent change in contraction is accompanied by the change in MLC phosphorylation, MYPT1 phosphorylation and CPI-17 hosphorylation. Rac inhibitors activated MLCP activity, but not MLCK activity, suggesting that Rac1 dependent change in MLC phosphorylation is due to the change in MLCP activity. Rac inhibitors did not change RhoA and ROCK activities, unlike RhoA pathway. Interestingly, Rac1 inhibition induced the change in MYPT1/CPI-17 phosphatase activity. The result suggests that Rac1 dependent change in MYPT1/CPI-17 phosphorylation is due to the change in the phosphatase activity rather than the kinase activites that phopshorylate MYPT1 and CPI-17 such as ROCK. These results suggest that agonist induced change in MLCP activity in smooth muscle is concertedly regulated by RhoA pathway that modulates ROCK and Rac1 pathway that regulates MYPT1/CPI-17 phosphatases.