Rho-associated kinase of chicken gizzard smooth muscle

Inhibition of smooth-muscle myosin-light-chain phosphatase by Ruthenium Red

Ruthenium Red (RuR) is widely used as an inhibitor of ryanodine receptor Ca(2+) release channels, but has additional effects such as the induction of Ca(2+) sensitization of contraction of permeabilized smooth muscles. To address the mechanism underlying this process, we examined the effects of RuR on contractility in permeabilized guinea-pig ileum and on the activity of myosin-light-chain phosphatase (MP). RuR increased the force at submaximal [Ca(2+)] (pCa 6.3) approx. 4-fold. This effect was not observed after thiophosphorylation of MP. RuR also seemed capable of preventing the thiophosphorylation of MP, suggesting a direct interaction of RuR with MP. Consistent with this possibility, smooth-muscle MP was inhibited by RuR in a concentration-dependent manner (IC(50) 23 microM). Exogenous calmodulin significantly increased RuR-induced contraction at pCa 6.3 but had little effect on contraction induced by microcystin at this [Ca(2+)]. Ca(2+)-independent contraction was induced by RuR (EC(50) 843 microM) and by microcystin (EC(50) 59 nM) but the maximal force induced by RuR was smaller than that induced by microcystin. The addition of 300 microM RuR enhanced the contraction induced by 30 nM microcystin but markedly decreased that induced by 1 microM microcystin. Such a dual action of RuR on microcystin-induced effects was not observed in experiments using purified MP. We conclude that the RuR-induced Ca(2+) sensitization of smooth-muscle contraction is due to the direct inhibition of MP by RuR.

Inhibition of myosin/moesin phosphatase by expression of the phosphoinhibitor protein CPI-17 alters microfilament organization and retards cell spreading

Cell migration and cytokinesis require reorganization of the cytoskeleton, involving phosphorylation and dephosphorylation of proteins such as myosin II and moesin. Myosin and moesin bind directly to a regulatory subunit of myosin/moesin phosphatase (MMP) that contains a protein type-1 phosphatase (PP1) catalytic subunit. Here we examined the role of MMP in cytoskeletal dynamics using a phosphorylation-dependent inhibitor protein specific for MMP, called CPI-17. Fibroblasts do not express CPI-17, making them a null background to study effects of expression. Wild type CPI-17 in rat embryo fibroblasts caused (1) abnormal accumulation of cortical F-actin fibers, distinct from the stress fibers induced by expression of active RhoA; (2) progressive contraction of cell area, leaving behind filamentous extensions that stained for F-actin and moesin, but not myosin; and (3) significantly retarded spreading of fibroblasts on fibronectin with elevated myosin II light chain phosphorylation. A phosphorylation site mutant CPI-17(T38A) and inhibitor-2 (Inh2), another PP1-specific inhibitor protein, served as controls and did not elicit these same responses when expressed at the same level as CPI-17. Inhibition of myosin light chain kinase by ML-9 prevented the abnormal accumulation of cortical microfilaments by CPI-17, but did not reverse shrinkage in area, whereas kinase inhibitors HA1077 and H7 prevented CPI-17-induced changes in microfilament distribution and cell contraction. These results highlight the physiological importance of myosin/moesin phosphatase regulation to dynamic remodeling of the cytoskeleton.

Phosphorylation of CPI-17, an inhibitory phosphoprotein of smooth muscle myosin phosphatase, by Rho-kinase

Phosphorylation of CPI-17 by Rho-associated kinase (Rho-kinase) and its effect on myosin phosphatase (MP) activity were investigated. CPI-17 was phosphorylated by Rho-kinase to 0.92 mol of P/mol of CPI-17 in vitro. The inhibitory phosphorylation site was Thr(38) (as reported previously) and was identified using a point mutant of CPI-17 and a phosphorylation state-specific antibody. Phosphorylation by Rho-kinase dramatically increased the inhibitory effect of CPI-17 on MP activity. Thus, CPI-17 as a substrate of Rho-kinase could be involved in the Ca(2+) sensitization of smooth muscle contraction as a downstream effector of Rho-kinase.

Myosin light chain phosphorylation-dependent modulation of volume-regulated anion channels in macrovascular endothelium

The Rho/Rho-associated kinase (ROK) pathway has been shown to modulate volume-regulated anion channels (VRAC) in cultured calf pulmonary artery endothelial (CPAE) cells. Since Rho/ROK can increase myosin light chain phosphorylation, we have now studied the effects of inhibitors of myosin light chain kinase (MLCK) or myosin light chain phosphatase (MLCP) on VRAC in CPAE. Application of ML-9, an MLCK inhibitor, inhibited VRAC, both when applied extracellularly or when dialyzed into the cell. A similar inhibitory effect was obtained by dialyzing the cells with AV25, a specific MLCK inhibitory peptide. Conversely, NIPP1(191-210), an MLCP inhibitory peptide, potentiated the activation of VRAC by a 25% hypotonic stimulus. These data indicate that activation of VRAC is modulated by MLC phosphorylation.

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

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.

Inhibition of Rho-associated kinase blocks agonist-induced Ca2+ sensitization of myosin phosphorylation and force in guinea-pig ileum

Ca2+ sensitization of smooth muscle contraction involves the small GTPase RhoA, inhibition of myosin light chain phosphatase (MLCP) and enhanced myosin regulatory light chain (LC20) phosphorylation. A potential effector of RhoA is Rho-associated kinase (ROK). The role of ROK in Ca2+ sensitization was investigated in guinea-pig ileum. Contraction of permeabilized muscle strips induced by GTPgammaS at pCa 6.5 was inhibited by the kinase inhibitors Y-27632, HA1077 and H-7 with IC50 values that correlated with the known Ki values for inhibition of ROK. GTPgammaS also increased LC20 phosphorylation and this was prevented by HA1077. Contraction and LC20 phosphorylation elicited at pCa 5.75 were, however, unaffected by HA1077. Pre-treatment of intact tissue strips with HA1077 abolished the tonic component of carbachol-induced contraction and the sustained elevation of LC20 phosphorylation, but had no effect on the transient or sustained increase in [Ca2+]i induced by carbachol. LC20 phosphorylation and contraction dynamics suggest that the ROK-mediated increase in LC20 phosphorylation is due to MLCP inhibition, not myosin light chain kinase activation. In the absence of Ca2+, GTPgammaS stimulated 35S incorporation from [35S]ATPgammaS into the myosin targeting subunit of MLCP (MYPT). The enhanced thiophosphorylation was inhibited by HA1077. No thiophosphorylation of LC20 was detected. These results indicate that ROK mediates agonist-induced increases in myosin phosphorylation and force by inhibiting MLCP activity through phosphorylation of MYPT. Under Ca2+-free conditions, ROK does not appear to phosphorylate LC20 in situ, in contrast to its ability to phosphorylate myosin in vitro. In particular, ROK activation is essential for the tonic phase of agonist-induced contraction.

Inhibitory phosphorylation site for Rho-associated kinase on smooth muscle myosin phosphatase

It is clear from several studies that myosin phosphatase (MP) can be inhibited via a pathway that involves RhoA. However, the mechanism of inhibition is not established. These studies were carried out to test the hypothesis that Rho-kinase (Rho-associated kinase) via phosphorylation of the myosin phosphatase target subunit 1 (MYPT1) inhibited MP activity and to identify relevant sites of phosphorylation. Phosphorylation by Rho-kinase inhibited MP activity and this reflected a decrease in V(max). Activity of MP with different substrates also was inhibited by phosphorylation. Two major sites of phosphorylation on MYPT1 were Thr(695) and Thr(850). Various point mutations were designed for these phosphorylation sites. Following thiophosphorylation by Rho-kinase and assays of phosphatase activity it was determined that Thr(695) was responsible for inhibition. A site- and phosphorylation-specific antibody was developed for the sequence flanking Thr(695) and this recognized only phosphorylated Thr(695) in both native and recombinant MYPT1. Using this antibody it was shown that stimulation of serum-starved Swiss 3T3 cells by lysophosphatidic acid, thought to activate RhoA pathways, induced an increase in Thr(695) phosphorylation on MYPT1 and this effect was blocked by a Rho-kinase inhibitor, Y-27632. In summary, these results offer strong support for a physiological role of Rho-kinase in regulation of MP activity.

Cellular redistribution of PKCalpha, rhoA, and ROKalpha following smooth muscle agonist stimulation

Efficient receptor-coupled activation of smooth muscle requires discrete coordination of many signal transducing events from the plasma membrane to the myofilaments. Recruitment of key factors to the plasma membrane is thought to be crucial for transduction of extracellular signals leading to contractility. We investigated, therefore, for the first time in intact differentiated smooth muscle cells, the distributions of three molecules important for receptor-coupled excitation: protein kinase Calpha (PKCalpha), rhoA, and rho kinase (ROK). We also directly confirmed, by single cell force measurements, carbachol-induced [Ca(2+)](i) sensitization of contractility. Laser scanning confocal immunofluorescent microscopy of central smooth muscle cell sections determined that, at rest, PKCalpha, rhoA, and ROKalpha were distributed predominantly throughout the cytosol. Muscarinic stimulation resulted in significant redistribution of each protein to the cell membrane. By digital image analysis, peripheral:cytosolic distributions of PKCalpha, rhoA, and ROKalpha were calculated as, respectively, 1.05 +/- 0.03 (8), 1.09 +/- 0.03 (5), and 1.26 +/- 0.04 (12) at rest, increasing significantly following stimulation to 2.09 +/- 0.22 (6), 2.02 +/- 0.12 (8), and 1.93 +/- 0.05 (10). It is proposed that this receptor-coupled recruitment to the cell periphery of the downstream signaling molecules PKCalpha, rhoA, and ROKalpha contributes to the efficacy of agonist-induced contractile activation of smooth muscle.

Regulation of Ca2+-independent smooth muscle contraction by alternative staurosporine-sensitive kinase

It is well known that inhibition of myosin phosphatase induces smooth muscle contraction in the absence of Ca2+. We characterized the kinase(s) which plays a role in Ca2+-independent, microcystin-LR-induced contraction in permeabilized smooth muscle of the rabbit portal vein. Assessments of various protein kinase inhibitors revealed this kinase(s) (1) was sensitive to staurosporine (1 microM), but resistant to other agents including wortmannin (10 microM), Y-27632 ((R)-(+)-trans-N-(4-pyridyl)-4-(1-aminoethyl)-cyclohexanecarboxamide+ ++, 100 microM). HA1077 (1-(5-isoquinolinylsulfonyl)-homopiperazine, 100 microM), H-7 (1-(5-isoquinolinylsulfonyl)-2-methylpiperazine, 100 microM), and calphostin C (100 microM), and (2) induced phosphorylation of 20 kDa myosin light chain at serine-19. We concluded that other kinases exist which phosphorylate myosin light chain at serine-19 and induce Ca2+-independent smooth muscle contraction, distinct from Rho-associated kinase, myosin light chain kinase, and protein kinase C.

The effects of the selective ROCK inhibitor, Y27632, on ET-1-induced hypertrophic response in neonatal rat cardiac myocytes--possible involvement of Rho/ROCK pathway in cardiac muscle cell hypertrophy

A small GTPase, Rho, participates in agonist-induced cytoskeletal organization and gene expression in many cell types including cardiac myocytes. However, little is known about the functions of Rho's downstream targets in cardiac myocytes. We examined the role of ROCK, a downstream target of Rho, in ET-1-induced hypertrophic response. Y27632, a selective ROCK inhibitor, inhibited ET-1-induced increases in natriuretic peptide production, cell size, protein synthesis, and myofibrillar organization. In addition, a dominant-negative mutant of p160ROCK suppressed ET-1-induced transcription of the BNP gene. These findings suggest that the Rho/ROCK pathway is an important component of ET-1-induced hypertrophic signals in cardiac myocytes.