Shibata Keita, Sakai Hiroyasu, Huang Qian, Kamata Hirotoshi, Chiba Yoshihiko, Misawa Miwa, Ikebe Reiko, Ikebe Mitsuo
Department of Microbiology and Physiological Systems, University of Massachusetts Medical School, Worcester, Massachusetts.
J Cell Physiol. 2015 Jun;230(6):1352-64. doi: 10.1002/jcp.24878.
Phosphorylation of regulatory light chain (MLC) activates myosin II, which enables it to promote contractile and motile activities of cells. We report here a novel signaling mechanism that activates MLC phosphorylation and smooth muscle contraction. Contractile agonists activated Rac1, and Rac1 inhibition diminished agonist-induced MLC phosphorylation, thus inhibiting smooth muscle contraction. Rac1 inhibits the activity of MLC phosphatase (MLCP) but not that of MLC kinase, through a phosphatase that targets MYPT1 (a regulatory subunit of MLCP) and CPI-17 (a MLCP specific inhibitor) rather than through the RhoA-Rho dependent kinase (ROCK) pathway. Rac1 inhibition decreased the activity of protein kinase C (PKC), which also contributes to the change in CPI-17 phosphorylation. We propose that activation of Rac1 increases the activity of PKC, which increases the phosphorylation of CPI-17 and MYPT1 by inhibiting the phosphatase that targets these proteins, thereby decreasing the activity of MLCP and increasing phosphorylation of MLC. Our results suggest that Rac1 coordinates with RhoA to increase MLC phosphorylation by inactivation of CPI-17/MYPT1 phosphatase, which decreases MLCP activity thus promoting MLC phosphorylation and cell contraction.
调节性轻链（MLC）的磷酸化激活肌球蛋白II，使其能够促进细胞的收缩和运动活性。我们在此报告一种激活MLC磷酸化和平滑肌收缩的新信号机制。收缩激动剂激活Rac1，而Rac1抑制可减少激动剂诱导的MLC磷酸化，从而抑制平滑肌收缩。Rac1通过一种靶向MYPT1（MLCP的调节亚基）和CPI-17（MLCP特异性抑制剂）的磷酸酶来抑制MLC磷酸酶（MLCP）的活性，而不是通过RhoA-Rho依赖性激酶（ROCK）途径抑制MLC激酶的活性。Rac1抑制降低了蛋白激酶C（PKC）的活性，这也导致了CPI-17磷酸化的变化。我们提出，Rac1的激活增加了PKC的活性，PKC通过抑制靶向这些蛋白的磷酸酶来增加CPI-17和MYPT1的磷酸化，从而降低MLCP的活性并增加MLC的磷酸化。我们的结果表明，Rac1与RhoA协同作用，通过使CPI-17/MYPT1磷酸酶失活来增加MLC磷酸化，这降低了MLCP活性，从而促进MLC磷酸化和细胞收缩。
