Signal transduction by G‐proteins, Rho‐kinase and protein phosphatase to smooth muscle and non‐muscle myosin II

Abstract

We here review mechanisms that can regulate the activity of myosin II, in smooth muscle and non‐muscle cells, by modulating the Ca2+ sensitivity of myosin regulatory light chain (RLC) phosphorylation. The major mechanism of Ca2+ sensitization of smooth muscle contraction and non‐muscle cell motility is through inhibition of the smooth muscle myosin phosphatase (MLCP) that dephosphorylates the RLC in smooth muscle and non‐muscle. The active, GTP‐bound form of the small GTPase RhoA activates a serine/threonine kinase, Rho‐kinase, that phosphorylates the regulatory subunit of MLCP and inhibits phosphatase activity. G‐protein‐coupled release of arachidonic acid may also contribute to inhibition of MLCP acting, at least in part, through the Rho/Rho‐kinase pathway. Protein kinase C(s) activated by phorbol esters and diacylglycerol can also inhibit MLCP by phosphorylating and thereby activating CPI‐17, an inhibitor of its catalytic subunit; this mechanism is independent of the Rho/Rho‐kinase pathway and plays only a minor, transient role in the G‐protein‐coupled mechanism of Ca2+ sensitization. Ca2+ sensitization by the Rho/Rho‐kinase pathway contributes to the tonic phase of agonist‐induced contraction in smooth muscle, and abnormally increased activation of myosin II by this mechanism is thought to play a role in diseases such as high blood pressure and cancer cell metastasis.

Dependence of metastatic cancer cell invasion on MLCK-catalyzed phosphorylation of myosin regulatory light chain

“…3,4 MLCKcatalysed phosphorylation of MRLC is essential for the stimulation of myosin ATPase function and is involved in the control of various processes in normal cells such as contraction, motility, secretion, cytokinesis, and cytoskeletal remodeling. 2,3 Recently, it has been shown that MRLC phosphorylation also plays significant roles in malignant cells by regulating invasiveness and motility …”

“…Phosphorylation by MLCK on thr 18 and ser 19 has been shown to be critical for the activation of myosin ATPase activity and the contractile functions in smooth and nonmuscle cells. 3,4 Here, we report that MLCK-catalysed MRLC phosphorylation is also critical for the invasiveness of metastatic cancer cells. Inhibition of invasion was principally caused by impairment of cell motility, while adhesion to matrix and secretion of gelatinases were unaffected.…”

Contribution of cellular contractility to spatial and temporal variations in cellular stiffness

“…The phosphorylation of the myosin regulatory light chain (RLC) is essential for the formation of stress fibers originating from the interactions between myosin II and F-actin. It is known that the phosphorylation of RLC is complementarily regulated by two signaling pathways that involve (i) myosin light chain kinase (MLCK) and (ii) myosin light chain phosphatase (MLCP) [35][36][37]. LPA activates small GTPase RhoA via G-protein Each petri dish on which the fibroblasts were plated was filled with DMEM without FBS, and then was incubated for one night before the experiments were carried out.…”

“…Fig. 2 shows a temporal variation in the topographic and stiffness images of a fibroblast stimulated with 5 µM LPA, which served to increase cellular contractility [35][36][37]. The topographic images ( Figs.…”

PDE5 inhibitors for LUTS

“…This is thought to occur through regulatory mechanisms that involve calcium sensitizing. 59,60 Rho A, a G-protein, and its target Rho-kinase have been found to be involved with a-adrenergic (NE) and endothelin-1-stimulated smooth muscle contraction. This occurs through RK inhibition of MLC phosphatase.…”