Involvement of botulinum C3-sensitive GTP-binding proteins in alpha 1-adrenoceptor subtypes mediating Ca(2+)-sensitization

Regulation of smooth muscle contraction by small GTPases

Next to changes in cytosolic [Ca(2+)], members of the Rho subfamily of small GTPases, in particular Rho and its effector Rho kinase, also known as ROK or ROCK, emerged as key regulators of smooth muscle function in health and disease. In this review, we will focus on the regulation of the contractile machinery by Rho/ROK signaling and its interaction with PKC and cyclic nucleotide signaling. We will briefly discuss the emerging evidence that remodeling of cortical actin is necessary for contraction.

The Rho-A/Rho-kinase pathway is up-regulated but remains inhibited by cyclic guanosine monophosphate-dependent mechanisms during endotoxemia in small mesenteric arteries

OBJECTIVE

We investigated whether a reduced activity in the Rho-A/Rho-kinase pathway could be involved in the impaired vascular reactivity observed in septic shock.

DESIGN

Ex vivo animal study.

SETTING

University research laboratory.

SUBJECTS

Male Wistar rats.

INTERVENTIONS

Rats received an intraperitoneal injection of lipopolysaccharide (LPS, 10 mg/kg) either 6 or 24 hours before the onset of our experiments. The effects of Y-27632 (a Rho-kinase inhibitor) were assessed in first-order mesenteric rings taken from these animals using wire myograph. The expression of Rho-A, Rho-kinases I and II, and the total and phosphorylated myosin phosphatase targeting subunit 1 (MYPT1) were assessed by Western blotting.

MEASUREMENTS AND MAIN RESULTS

The EC50 to Y-27632 was reduced from 2.10 microM (1.22-3.66 microM) (control) to 0.21 microM (0.09-0.44 microM), and 9.54 (0.82-110.30) nM in LPS-treated groups 6 and 24 hours, respectively. The increased potency of Y-27632 was partially reversed by endothelium removal at both 6 and 24 hours. Incubation of Nomega-nitro-l-arginine methyl ester hydrochloride or 1400W (a nonselective and an inducible nitric oxide synthase inhibitor, respectively) normalized the responses to Y-27632 seen 6 hours after LPS. However, 1400W had no effect, whereas Nomega-nitro-l-arginine methyl ester hydrochloride caused a partial reduction in the enhanced potency of Y-27632 found 24 hours after LPS. The soluble guanylate cyclase inhibitor oxadiazolo[4,3-alpha]quinoxalin-1-one was able to bring the Y-27632 response back to normal both 6 and 24 hours after LPS. Rho-A, Rho-kinase I, Rho-kinase II, and MYPT1 were increased in mesenteric arteries from endotoxemic rats, but the phosphorylated MYPT1 was significantly reduced. However, incubation with oxadiazolo[4,3-alpha]quinoxalin-1-one circumvented the inhibition of MYPT1 phosphorylation found in preparations from LPS-treated animals.

CONCLUSIONS

Our findings revealed an impaired Rho-A/Rho-kinase-mediated phosphorylation of MYPT1 in vessels from endotoxemic animals in a cyclic guanosine monophosphate-dependent manner, suggesting that changes in mechanisms involved in calcium sensitization play a pivotal role in cardiovascular changes observed in septic shock.

Rho kinase polymorphism influences blood pressure and systemic vascular resistance in human twins: role of heredity

The Rho/Rho kinase (ROCK) pathway is implicated in experimental hypertension. We, therefore, explored the role of ROCK2 genetic variation in human blood pressure (BP) regulation, exploiting the advantages of a human twin sample to probe heritability. The focus of this work is the common nonsynonymous variant at ROCK2: Thr431Asn. Cardiovascular and autonomic traits displayed substantial heritability (from approximately 33% to 71%; P<0.05). The Asn/Asn genotype (compared with Asn/Thr or Thr/Thr) was associated with greater resting systolic (P<0.001), diastolic (P<0.0001), and mean BP (P<0.0001); allelic variation at ROCK2 accounted for up to approximately 5% of BP variation (P<0.0001). Systemic vascular resistance was higher in Asn/Asn individuals (P=0.049), whereas cardiac output, large artery compliance, and vasoactive hormone secretion were not different. Coupling of the renin-angiotensin system to systemic resistance and BP was diminished in Asn/Asn homozygotes, suggesting genetic pleiotropy of Thr431Asn, confirmed by bivariate genetic analyses. The Asn/Asn genotype also predicted higher BP after environmental (cold) stress. The rise in heart rate after cold was less pronounced in Asn/Asn individuals, consistent with intact baroreceptor function, and baroreceptor slope was not influenced by genotype. Common genetic variation (Thr431Asn) at ROCK2 predicts increased BP, systemic vascular resistance (although not large artery compliance), and resistance in response to the endogenous renin-angiotensin system, indicating a resistance vessel-based effect on elevated BP. The results suggest that common variation in ROCK2 exerts systemic resistance-mediated changes in BP, documenting a novel mechanism for human circulatory control, and suggesting new possibilities for diagnostic profiling and treatment of subjects at risk of developing hypertension.

Fasudil hydrochloride hydrate, a Rho-kinase (ROCK) inhibitor, suppresses collagen production and enhances collagenase activity in hepatic stellate cells

BACKGROUND/AIMS

The Rho-ROCK signaling pathways play an important role in the activation of hepatic stellate cells (HSCs). We investigated the effects of fasudil hydrochloride hydrate (fasudil), a Rho-kinase (ROCK) inhibitor, on cell growth, collagen production, and collagenase activity in HSCs.

METHODS

Rat HSCs and human HSC-derived TWNT-4 cells were cultured for studies on stress fiber formation and alpha-smooth muscle actin (alpha-SMA) expression. Proliferation was measured by BrdU incorporation, and apoptosis by TUNEL assay. The phosphorylation states of the MAP kinases (MAPKs), extra cellular signal -regulated kinase 1/2 (ERK1/2), c-jun kinase (JNK), and p38 were evaluated by western blot analysis. Type I collagen, matrix metalloproteinase-1 (MMP-1) and tissue inhibitor of matrix metalloproteinase-1 (TIMP-1) production and gene expression were evaluated by ELISA and real-time PCR, respectively. Collagenase activity (active MMP-1) was also evaluated.

RESULTS

Fasudil (100 microM) inhibited cell spreading, the formation of stress fibers, and expression of alpha-SMA with concomitant suppression of cell growth, although it did not induce apoptosis. Fasudil inhibited phosphorylation of ERK1/2, JNK, and p38. Treatment with fasudil suppressed the production and transcription of collagen and TIMP, stimulated the production and transcription of MMP-1, and enhanced collagenase activity.

CONCLUSION

These findings demonstrated that fasudil not only suppresses proliferation and collagen production but also increases collagenase activity.

Epigallocatechin-3-gallate, a green-tea polyphenol, suppresses Rho signaling in TWNT-4 human hepatic stellate cells

Epigallocatechin-3-gallate (EGCG), a major constituent of the polyphenoids in green tea, has been reported to possess a wide range of biologic activities, including antifibrogenesis. Activated hepatic stellate cells (HSCs) are central to hepatic fibrosis, and Rho (a small GTPase)-signaling pathways have been implicated in the activation and proliferation of HSCs. In this study, we investigated the effect of EGCG on Rho-signaling pathways in activated human HSC-derived TWNT-4 cells. EGCG inhibited stress-fiber formation, an indicator of Rho activation, and changed the distribution of alpha-smooth-muscle actin. These inhibitory effects of EGCG were restored by overexpression of constitutively active Rho. A pull-down assay revealed that activated Rho (GTP-bound state) was strongly inhibited by ECGC and accompanied by suppressed phosphorylation of focal adhesion kinase, which is a regulator of Rho-signaling pathways. 5-Bromo-2'-deoxy-uridine incorporation demonstrated that ECGC (100 micromol/L suppressed cell growth by 80%, and terminal deoxynucleotidyl transferase viotin-deoxyruidine triphosphate nick end-labeling revealed that EGCG (100 micromol/L) caused apoptosis in half of the total cells. EGCG also strongly inhibited lysophoaphatidic acid (an activator of Rho) and induced phosphorylation of mitogen-activated protein kinases (Erk1/2, c-jun kinase, and p38). These findings demonstrate that EGCG regulates the structure and growth of HSCs by way of Rho-signaling pathways and suggest that EGCG has therapeutic potential in the setting of liver fibrosis.

Sphingosine 1-phosphate enhances portal pressure in isolated perfused liver via S1P2 with Rho activation

Although structural changes are most important to determine vascular resistance in portal hypertension, vasoactive mediators also contribute to its regulation. Hepatic stellate cells (HSCs) are assumed to play a role in modulating intrahepatic vascular resistance based on their residence in the space of Disse and capacity to contract. Because sphingosine 1-phosphate (S1P) has been shown to stimulate HSC contractility, we wondered if S1P could regulate portal pressure. S1P at 0.5-5 microM increased portal pressure in isolated rat perfused liver. This effect was abrogated in the presence of a binding antagonist for S1P2, JTE-013. Perfusion of isolated rat liver with 5 microM S1P increased Rho activity in the liver, and co-perfusion with JTE-013 cancelled S1P-induced Rho activation. Because S1P is present in human plasma at approximately 0.2 microM, S1P might readily regulate portal vascular tone in physiological and pathological status. The antagonist for S1P2 merits consideration for treatment of portal hypertension.

Mechanisms involved in carbachol-induced Ca(2+) sensitization of contractile elements in rat proximal and distal colon

1. Mechanisms involved in Ca(2+) sensitization of contractile elements induced by the activation of muscarinic receptors in membrane-permeabilized preparations of the rat proximal and distal colon were studied. 2. In alpha-toxin-permeabilized preparations from the rat proximal and distal colon, Ca(2+) induced a rapid phasic and subsequent tonic component. After Ca(2+)-induced contraction reached a plateau, guanosine 5'-triphosphate (GTP) and carbachol (CCh) in the presence of GTP further contracted preparations of both the proximal and distal colon (Ca(2+) sensitization). Y-27632, a rho-kinase inhibitor, inhibited GTP plus CCh-induced Ca(2+) sensitization more significantly in the proximal colon than in the distal colon. 3. Y-27632 at 10 microm had no effect on Ca(2+)-induced contraction or slightly inhibited phorbol-12,13-dibutyrate-induced Ca(2+) sensitization in either proximal or distal colon. Chelerythrine, a protein kinase C inhibitor, inhibited GTP plus CCh-induced Ca(2+) sensitization in the distal colon, but not in the proximal colon. The component of Ca(2+) sensitization that persisted after the chelerythrine treatment was completely inhibited by Y-27632. 4. In beta-escin-permeabilized preparations of the proximal colon, C3 exoenzyme completely inhibited GTP plus CCh-induced Ca(2+) sensitization, but PKC(19-31) did not. In the distal colon, C3 exoenzyme abolished GTP-induced Ca(2+) sensitization. It inhibited CCh-induced sensitization by 50 % and the remaining component was inhibited by PKC(19-31). 5. These results suggest that both protein kinase C and rho pathways in parallel mediate the Ca(2+) sensitization coupled to activation of muscarinic receptors in the rat distal colon, whereas the rho pathway alone mediates this action in the proximal colon.

Effect of changes in pH on wall tension in isolated rat pulmonary artery: role of the RhoA/Rho-kinase pathway

Pulmonary arteries (PA) are resistant to the vasodilator effects of extracellular acidosis in systemic vessels; the mechanism underlying this difference between systemic and pulmonary circulations has not been elucidated. We hypothesized that RhoA/Rho-kinase-mediated Ca2+ sensitization pathway played a greater role in tension development in pulmonary than in systemic vascular smooth muscle and that this pathway was insensitive to acidosis. In arterial rings contracted with the alpha1-agonist phenylephrine (PE), the Rho-kinase inhibitor Y-27632 (< or =3 microM) induced greater relaxation in precontracted PA rings than in aortic rings. In PA rings stimulated by PE, the activation of RhoA was greater than in aorta. Normocapnic acidosis (NA) induced a smaller relaxation in precontracted PA than in aorta. However, in the presence of nifedipine and thapsigargin, when PE-induced contraction was predominantly mediated by Rho-kinase, the relaxant effect of NA was reduced and similar in both vessel types. Furthermore, in the presence of Y-27632, NA induced a greater relaxation in both PA and aorta, which was similar in both vessels. Finally, in alpha-toxin-permeabilized smooth muscle, PE-induced contraction at constant Ca2+ activity was inhibited by Y-27632 and unaffected by acidosis. These results indicate that Ca2+ sensitization induced by the RhoA/Rho-kinase pathway played a greater role in agonist-induced vascular smooth muscle contraction in PA than in aorta and that tension mediated by this pathway was insensitive to acidosis. The predominant role of the RhoA/Rho-kinase pathway in the pulmonary vasculature may account for the resistance of this circulation to the vasodilator effect of acidosis observed in the systemic circulation.

Agonist-induced changes in the phosphorylation of the myosin- binding subunit of myosin light chain phosphatase and CPI17, two regulatory factors of myosin light chain phosphatase, in smooth muscle

The inhibition of myosin light chain phosphatase (MLCP) enhances smooth muscle contraction at a constant [Ca2+]. There are two components, myosin-binding subunit of MLCP (MBS) and CPI17, thought to be responsible for the inhibition of MLCP by external stimuli. The phosphorylation of MBS at Thr-641 and of CPI17 at Thr-38 inhibits the MLCP activity in vitro. Here we determined the changes in the phosphorylation of MBS and CPI17 after agonist stimulation in intact as well as permeabilized smooth muscle strips using phosphorylation-site-specific antibodies as probes. The CPI17 phosphorylation transiently increased after agonist stimulation in both alpha-toxin skinned and intact fibres. The time course of the increase in CPI17 phosphorylation after stimulation correlated with the increase in myosin regulatory light chain (MLC) phosphorylation. The increase in CPI17 phosphorylation was significantly diminished by Y27632, a Rho kinase inhibitor, and GF109203x, a protein kinase C inhibitor, suggesting that both the protein kinase C and Rho kinase pathways influence the change in CPI17 phosphorylation. On the other hand, a significant level of MBS phosphorylation at Thr-641, an inhibitory site, was observed in the resting state for both skinned and intact fibres and the agonist stimulation did not significantly alter the MBS phosphorylation level at Thr-641. While the removal of the agonist markedly decreased MLC phosphorylation and induced relaxation, the phosphorylation of MBS was unchanged, while CPI17 phosphorylation markedly diminished. These results strongly suggest that the phosphorylation of CPI17 plays a more significant role in the agonist-induced increase in myosin phosphorylation and contraction of smooth muscle than MBS phosphorylation in the Ca2+-independent activation mechanism of smooth muscle contraction.

Increased expression and activity of RhoA are associated with increased DNA synthesis and reduced p27(Kip1) expression in the vasculature of hypertensive rats

We have previously shown that the function of the small G protein Rho is required for vascular smooth muscle cell proliferation and migration. We hypothesized that changes in Rho or Rho signaling might contribute to enhanced vascular proliferative responses associated with hypertension. Western blot analysis revealed that total RhoA expression was approximately 2-fold higher in aortas, tail arteries, and aortic smooth muscle cells (ASMCs) obtained from adult male spontaneously hypertensive rats (SHR) compared with those from Wistar Kyoto rats (WKY). An increase in active GTP-bound RhoA was detected in aortic homogenates by affinity precipitation with the RhoA effector rhotekin and by examining RhoA-[(35)S]GTPgammaS binding. RhoA protein and activity were also increased in vessels from rats treated with N-nitro-L-arginine methyl ester to increase blood pressure. Thrombin-stimulated RhoA activation was also significantly greater in ASMCs from SHR. As a functional correlate of these changes in Rho signaling, thrombin-stimulated DNA synthesis was enhanced in tail arteries and ASMCs from SHR. Expression of the cyclin-dependent kinase inhibitor p27(Kip1) was decreased by two thirds in SHR, and this decrease was mimicked in ASMCs by expression of a constitutively active (GTPase-deficient) mutant of RhoA. Wortmannin (10 nmol/L) fully inhibited the decrease in p27(Kip1) induced by RhoA, and a membrane-targeted catalytic subunit of phosphatidylinositol-3 kinase (PI3K [p110(CAAX)]) decreased p27(Kip1) expression, suggesting that RhoA signals through PI3K. These data provide evidence that RhoA brings about changes in DNA synthesis through reduced expression of p27(Kip1), mediated in part via PI3K, and suggest that increases in RhoA expression and activity contribute to the enhanced vascular responsiveness observed in hypertension.

A selective ROCK inhibitor, Y27632, prevents dimethylnitrosamine-induced hepatic fibrosis in rats

BACKGROUND

p160ROCK is a direct Rho target which mediates Rho-induced assembly of focal adhesions and stress fibers. We previously reported that Rho signaling pathways are involved in the activation of hepatic stellate cells (HSC) in vitro. The aim of the present study was to test the hypothesis that an inhibitor specific for p160ROCK (Y27632) could prevent experimental hepatic fibrosis induced by dimethylnitrosamine (DMN) in rats.

METHODS

Y27632 was given orally at 30 mg/kg daily for 4 weeks after the first injection of DMN. The degree of fibrosis was evaluated by image analysis and also by measurements of collagen and hydroxyproline content in the liver. The expression of alpha-smooth muscle actin (alpha-SMA) in the liver and in the primary cultured HSC was also evaluated. Semi-quantitative RT-PCR was performed to evaluate the expression of type I collagen mRNA in the liver.

RESULTS

Y27632 treatment significantly decreased the occurrence of DMN-induced hepatic fibrosis and reduced the collagen and hydroxyproline content and alpha-SMA expression in the liver. The expression of alpha-SMA in HSC was also suppressed in vitro.

CONCLUSIONS

These findings indicate that inhibitors of the Rho-ROCK pathway might be useful therapeutically in hepatic fibrosis.

Alpha-adrenoceptor agonists produce Ca2+ oscillations in isolated rat aorta: role of protein kinase C

We investigated the relationship between tension development and the cytosolic free Ca2+ level ([Ca2+]i) in responses to norepinephrine (NE) and selective alpha2-adrenoceptor agonist, UK14,304 of the endothelium-denuded rat aorta loaded with fura PE-3. NE (3 x 10(-8) M) evoked a rapid increase in [Ca2+]i followed by slight decreasing to a steady state level and produced a contraction. After the NE-induced increase in [Ca2+]i had reached a maximum, the [Ca2+]i showed persistent oscillations. The Ca2+ oscillations were superimposed on the sustained increase in [Ca2+]i. UK14,304 (3 x 10(-6) M) also evoked an increase in [Ca2+]i and produced a contraction. However, the UK14,304-induced effect on [Ca2+]i was characterized by pronounced oscillations, and the amplitude of the sustained increase in [Ca2+]i was less than that seen with NE. Protein kinase C inhibitor, Ro31-8220 (3 x 10(-6) M) and verapamil (10(-5) M) abolished both NE and UK14,304-evoked Ca2+ oscillations. UK14,304-induced contractions were also strongly inhibited by Ro31-8220 and verapamil. However, NE induced contractions were partly inhibited by these inhibitors. The sustained increases in [Ca2+]i evoked NE and UK14,304 were not significantly inhibited by Ro31-8220 and verapamil. These results suggest that NE and UK14,304 produce Ca2+ oscillations during sustained contractions in rat aorta. The alpha2 adrenoceptor agonist, UK14,304-induced sustained contraction and Ca2+ oscillations may be due to PKC activation and opening of voltage-dependent L type Ca2+ channels.

The role of Rho in G protein-coupled receptor signal transduction

Low molecular weight G proteins of the Rho subfamily are regulators of actin cytoskeletal organization. In contrast to the heterotrimeric G proteins, the small GTPases are not directly activated through ligand binding to G protein-coupled receptors (GPCRs). However, a subset of GPCRs, including those for lysophosphatidic acid and thrombin, induce stress fibers, focal adhesions, and cell rounding through Rho-dependent pathways. C3 exoenzyme has been a useful tool for demonstrating Rho involvement in these and other responses, including Ca2+ sensitization of smooth muscle contraction, cell migration, transformation, and serum response element-mediated gene expression. Most of the GPCRs that induce Rho-dependent responses can activate Gq, but this is not a sufficient signal. Recent data demonstrate that G alpha 12/13 can induce Rho-dependent responses. Furthermore, G alpha 12/13 can bind and activate Rho-specific guanine nucleotide exchange factors, providing a mechanism by which GPCRs that couple to G alpha 12/13 could activate Rho and its downstream responses.

Characteristics of Ca2+ oscillations in ileal longitudinal muscle cells of guinea pig

We studied the mechanisms and characteristics of the spontaneously evoked intracellular Ca2+ changes (Ca2+ oscillations) in ileal longitudinal smooth muscle from guinea pig. Two-dimensional images of Ca2+ oscillations were obtained at 33-ms intervals with a Ca2+-sensitive fluorescence probe, fluo-3 using the intensified CCD camera. Nicardipine (10-7 M) significantly decreased the maximum level of fluorescence intensity of the Ca2+ oscillations, inhibited the frequency of the oscillations and tended to decrease the basal level of fluorescence intensity. However, tetrodotoxin (3 x 10-7 M) did not affect these oscillations. Phorbol 12,13-dibutyrate (10-7 M) significantly increased the maximum level of fluorescence intensity and the frequency of Ca2+ oscillations, and it changed them to steady and chronometric Ca2+ oscillations. Cyclopiazonic acid (3 x 10-5 M) also significantly increased the frequency of Ca2+ oscillations. Acetylcholine (10-8 M) increased the basal and maximum level of fluorescence intensity and the frequency of Ca2+ oscillations, and accelerated their onset. The increase of basal level of fluorescence intensity was then decreased by cyclopiazonic acid treatment. These results suggest that the augmentation of Ca2+ oscillations is mainly due to the activation of L-type Ca2+ channels, which is modulated by protein kinase C, and that the emptying of intracellular Ca2+ stores may activate the Ca2+ oscillations mediated through the increase of Ca2+ influx in ileal smooth muscle of guinea pig.

M3 muscarinic acetylcholine receptors regulate cytoplasmic myosin by a process involving RhoA and requiring conventional protein kinase C isoforms

Although muscarinic acetylcholine receptors (mAChR) regulate the activity of smooth muscle myosin, the effects of mAChR activation on cytoplasmic myosin have not been characterized. We found that activation of transfected human M3 mAChR induces the phosphorylation of myosin light chains (MLC) and the formation of myosin-containing stress fibers in Chinese hamster ovary (CHO-m3) cells. Direct activation of protein kinase C (PKC) with phorbol 12-myristate 13-acetate (PMA) also induces myosin light chain phosphorylation and myosin reorganization in CHO-m3 cells. Conventional (alpha), novel (delta), and atypical (iota) PKC isoforms are activated by mAChR stimulation or PMA treatment in CHO-m3 cells, as indicated by PKC translocation or degradation. mAChR-mediated myosin reorganization is abolished by inhibiting conventional PKC isoforms with Go6976 (IC50 = 0.4 microM), calphostin C (IC50 = 2.4 microM), or chelerythrine (IC50 = 8.0 microM). Stable expression of dominant negative RhoAAsn-19 diminishes, but does not abolish, mAChR-mediated myosin reorganization in the CHO-m3 cells. Similarly, mAChR-mediated myosin reorganization is diminished, but not abolished, in CHO-m3 cells which are multi-nucleate due to inactivation of Rho with C3 exoenzyme. Expression of dominant negative RhoAAsn-19 or inactivation of RhoA with C3 exoenzyme does not affect PMA-induced myosin reorganization. These findings indicate that the PKC-mediated pathway of myosin reorganization (induced either by M3 mAChR activation or PMA treatment) can continue to operate even when RhoA activity is diminished in CHO-m3 cells. Conventional PKC isoforms and RhoA may participate in separate but parallel pathways induced by M3 mAChR activation to regulate cytoplasmic myosin. Changes in cytoplasmic myosin elicited by M3 mAChR activation may contribute to the unique ability of these receptors to regulate cell morphology, adhesion, and proliferation.

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.

Rho and Rho kinase mediate thrombin-stimulated vascular smooth muscle cell DNA synthesis and migration

Aberrant regulation of smooth muscle cell proliferation and migration is associated with the pathophysiology of vascular disorders such as hypertension, atherosclerosis, restenosis, and graft rejection. To elucidate molecular mechanisms that regulate proliferation and migration of vascular smooth muscle cells, we determined whether signaling through the small G protein Rho is involved in thrombin- and phenylephrine-stimulated proliferation and migration of rat aortic smooth muscle cells (RASMCs). Thrombin and the thrombin peptide SFLLRNP stimulated DNA synthesis of RASMCs as measured by [3H]thymidine incorporation. Both ligands also increased cell migration as measured by the Boyden chamber method. L-Phenylephrine failed to induce either of these responses but increased inositol phosphate accumulation and mitogen-activated protein kinase activation in these cells, which indicated that the cells were responsive to alpha1-adrenergic stimulation. The C3 exoenzyme, which ADP-ribosylates and inactivates Rho, fully inhibited both thrombin-stimulated proliferation and migration but had no effect on inositol phosphate accumulation. In addition, Y-27632, an inhibitor of the Rho effector p160ROCK/Rho kinase, decreased thrombin-stimulated DNA synthesis and migration. To directly examine Rho activation, Rho-[35S]GTPgammaS binding was measured. The addition of the thrombin peptide SFLLRNP, but not phenylephrine, to RASMC lysates resulted in a significant increase in Rho-[35S]GTPgammaS binding. Thrombin and SFLLRNP, but not phenylephrine, also increased membrane-associated Rho in intact RASMCs, consistent with selective activation of Rho by thrombin. These results indicate that thrombin activates Rho in RASMCs and establish Rho as a critical mediator of thrombin receptor effects on DNA synthesis and cell migration in these cells.

Tyrosine phosphorylation as a convergent pathway of heterotrimeric G protein- and rho protein-mediated Ca2+ sensitization of smooth muscle of rabbit mesenteric artery

1. The aim of this study was to determine whether different signal transduction mechanisms underlie the Ca2+ sensitizing effects of guanosine 5'-O-(3-thiotriphosphate) (GTP(gamma)S) and receptor agonists on beta-escin-skinned smooth muscle of rabbit mesenteric artery. 2. In the homogenate of the beta-escin-skinned arterial strip, C3 exoenzyme of Clostridium botulinum catalyzed the [32P]-ADP-ribosylation of only one protein that had the same molecular mass as the protein detected in Western blots with anti-rho p21 antibody. Pretreatment of preparations with C3 resulted in great inhibition of GTP(gamma)S-induced Ca2+ sensitization, although the effect of GTP(gamma)S at higher concentrations (> or = 30 microM) was not completely blocked by this treatment. In contrast, the enhancement by phenylephrine and histamine, in the presence of guanosine 5'-triphosphate, of the Ca2+-induced contraction was not affected by C3 pretreatment. 3. The protein kinase C (PKC) inhibitors calphostin C and staurosporine completely eliminated the enhancement by phorbol ester 12,13-dibutyrate of the Ca2+-induced contraction. However, these PKC inhibitors had no effect on GTP(gamma)S- and receptor agonist-induced Ca2+ sensitization. 4. The tyrosine kinase inhibitors genistein and tyrphostin 25 caused an irreversible and complete block of the enhancement by GTP(gamma)S of the Ca2+-induced contraction without affecting this Ca2+ contraction. The inactive genistein analogue daidzein did not modify the effect of GTP(gamma)S. The Ca2+ sensitizing effects of phenylephrine and histamine were also blocked by these tyrosine kinase inhibitors. 5. These results suggest that rho p21 predominantly mediates GTP(gamma)S-induced Ca2+ sensitization of beta-escin-skinned smooth muscle of rabbit mesenteric artery, while the Ca2+ sensitizing actions of heterotrimeric G protein-coupled receptor agonists do not involve this small G protein. However, it seems that tyrosine phosphorylation, but not PKC activation, plays an important role in both of the rho p21 protein- and heterotrimeric G protein-mediated Ca2+ sensitization mechanisms.

Involvement of small GTPases in the regulation of smooth muscle contraction

Neurohumoral stimulation of smooth muscle leads to an increased responsiveness of the myofilaments to Ca2+. This review provides a summary of the data that suggest that the signalling from the membrane-bound serpentine receptors to the contractile apparatus leading to the increase in Ca(2+)-sensitivity requires the activation of the Ras-related low molecular mass GTPase Rho. In smooth muscle permeabilized with alpha-toxin or beta-escin, the increase in force elicited by different agonists at fixed [Ca2+] (Ca(2+)-sensitization) can be inhibited by bacterial toxins (EDIN, and exoenzyme C3) which ADP-ribosylate and inactivate Rho proteins. Moreover, the agonist-induced increase in Ca(2+)-sensitivity can be mimicked by constitutively active recombinant Rho proteins. The physiological relevance of this mechanism is suggested by the fact that toxins that are internalized into intact cells (toxin B from C. difficile and a chimeric toxin (DC3B) consisting of C3 and the (non-catalytic) B fragment of diphteria toxin (inhibit the tonic phase of an agonist-induced contraction. Toxin B inhibits contraction without affecting the intracellular Ca(2+)-transient determined with fura-2. However, it inhibits phosphorylation of the regulatory light chains of myosin (MLC). Rho has been suggested to activate a Rho-associated kinase which in turn phosphorylates the myosin binding subunit of the myosin light chain phosphatase. This would lead to an increase in phosphorylation of MLC and hence of force at constant Ca2+. The Ca(2+)-sensitizing effect of agonists is also inhibited by tyrosine kinase inhibitors. This suggests the possibility that in smooth muscle, like in non-muscle cells, there is a cross-talk between Rho and tyrosine kinases.

Regulation of Ca2+ sensitization by PKC and rho proteins in ovine cerebral arteries: effects of artery size and age

G protein-regulated Ca2+ sensitivity of vascular contractile proteins plays an important role in cerebrovascular reactivity. The present study examines the intracellular mechanisms that govern G protein-regulated Ca2+ sensitivity in cerebral arteries of different size and age. We studied beta-escin-permeabilized segments of common carotid, basilar, and middle cerebral arteries from nonpregnant adult and near-term fetal sheep. Activation of protein kinase C (PKC) by (-)-indolactam V or a phorbol ester produced receptor-independent increases in Ca2+ sensitivity. Such increases were more marked in immature arteries and were inversely correlated with artery size in both mature and immature arteries. However, inhibitors of PKC did not significantly affect increases in Ca2+ sensitivity in responses to either serotonin (5-hydroxytryptamine, 5-HT) or guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS). Alternatively, deactivation of rho p21, a small G protein associated with Rho kinase, by exotoxin C3 fully prevented increases in Ca2+ sensitivity in responses to 5-HT or GTPgammaS in both adult and fetal arteries of all types. Neither inhibitors of PKC nor exotoxin C3 altered baseline Ca2+ sensitivity. We conclude that patterns of receptor- and/or G protein-mediated modulation of Ca2+ sensitivity are dependent on an intracellular pathway that involves activation of small G proteins and Rho kinase. In contrast, PKC has little, if any, role in agonist-induced Ca2+ sensitization under the present experimental conditions.

Regional differences in alpha1-adrenoceptor subtypes and mechanisms in rabbit arteries

Contractility mediated through alpha1-adrenoceptor subtypes and the maximum binding site (Bmax value) and the dissociation constant (Kd value) for [125I]HEAT ([125I]iodo-2-(beta-(4-hydroxyphenyl)ethylaminomethyl)tetralone) were determined in the following rabbit arteries: thoracic and abdominal aorta, mesenteric, renal and iliac arteries, and the alpha1-adrenoceptor subtypes mediating contractile mechanisms in vascular smooth muscle were studied. The pD2 values for norepinephrine differed considerably among the arteries in the presence of nicardipine (10(-5) M), while the pA2 values for 5-methylurapidil against norepinephrine were identical at low affinity in all the arteries used. In Ca2+-free physiological saline solution (Ca2+-free PSS), the pA2 values for 5-methylurapidil were also similar except for the renal artery, in which there were no stable contractions. In normal PSS, the concentration-response curves for norepinephrine with chloroethylclonidine-pretreatment were shifted to the right (pD2 values of 5.58, 5.70, 5.74, 5.98 and 6.38 for thoracic and abdominal aorta, mesenteric, renal and iliac arteries, respectively). In the [125I]HEAT binding study using membrane preparations obtained from chloroethylclonidine-treated strips, the Bmax values (33.2-105.2 fmol/mg protein) for [125I]HEAT varied considerably among arteries, while the Kd values (0.20-0.26 nM) were identical. The logarithm of Bmax values is proportional to the pD2 values for norepinephrine (slope=0.69, r=0.961). These observations suggest that the regional differences in potency (pD2 value) of the alpha1-adrenoceptor agonist, norepinephrine, are a result of the differences in population and density of alpha1-adrenoceptor subtypes in rabbit arteries.

Possible involvement of the novel CPI-17 protein in protein kinase C signal transduction of rabbit arterial smooth muscle

1. CPI-17 has recently been identified as a novel protein in vascular smooth muscle. In vitro , its phosphorylation and thiophosphorylation by protein kinase C (PKC) specifically inhibits the type 1 class of protein phosphatases, including myosin light chain (MLC) phosphatase. 2. Both of the phosphorylated CPI-17 states dose-dependently potentiated submaximal contractions at constant [Ca2+] in beta-escin-permeabilized and Triton X-100-demembranated arterial smooth muscle, but produced no effect in intact and less intensely permeabilized (alpha-toxin) tissue. Thiophosphorylated CPI-17 (tp-CPI) induced large contractions even under Ca2+-free conditions and decreased Ca2+ EC50 by more than an order of magnitude. Unphosphorylated CPI-17 produced minimal but significant effects. 3. tp-CPI substantially increased the steady-state MLC phosphorylation to Ca2+ ratios in beta-escin preparations. 4. tp-CPI affected the kinetics of contraction and relaxation and of MLC phosphorylation and dephosphorylation in such a manner that indicates its major physiological effect is to inhibit MLC phosphatase. 5. Results from use of specific inhibitors in concurrence with tp-CPI repudiate the involvement of general G proteins, rho A or PKC itself in the Ca2+ sensitization by tp-CPI. 6. Our results indicate that phosphorylation of CPI-17 by PKC stimulates binding of CPI-17 to and subsequent inhibition of MLC phosphatase. This implies that CPI-17 accounts largely for the heretofore unknown signalling pathway between PKC and inhibited MLC phosphatase.

Angiotensin II activates RhoA in cardiac myocytes: a critical role of RhoA in angiotensin II-induced premyofibril formation

The organization of actin into striated fibers (myofibrils) is one of the major features of cardiac hypertrophy. However, its signal transduction mechanism is not well understood. Although Rho-family small G proteins have been implicated in actin organization in many cell types, it is not fully elucidated whether Rho mediates the organization of actin fibers by hypertrophic stimuli in cardiac myocytes. Therefore, we examined (1) whether Rho is activated by the hypertrophic stimulus, angiotensin II (Ang II), and (2) whether Rho mediates the Ang II-induced organization of actin fibers in cultured neonatal rat cardiac myocytes. Treatment of myocytes with Ang II caused a rapid formation of both striated (mature myofibrils) and nonstriated (premyofibrils) actin fibers within 30 minutes, as determined by phalloidin stainings of the polymerized actin and troponin T stainings. Immunoblot analyses and immunostainings have indicated that cardiac myocytes express RhoA, but RhoB is undetectable. In the control state, RhoA was observed predominantly in the cytosolic fraction, but it was translocated in part to the particulate fraction in response to Ang II, consistent with activation of RhoA by Ang II. Incubation of myocytes with exoenzyme C3 for 48 hours completely ADP-ribosylated Rho in vivo. The C3 treatment abolished formation of premyofibrils induced by Ang II, suggesting that Ang II causes premyofibril formation via a Rho-dependent mechanism. The Ang II-induced mature myofibril formation was only partly abolished by C3. Expression of constitutively active RhoA (V14RhoA) caused the formation of premyofibrils but not mature myofibrils. The C3 treatment inhibited Ang II-induced atrial natriuretic factor induction, whereas it had no effect on c-fos induction. These results indicate that RhoA is activated by Ang II and mediates the Ang II-induced formation of premyofibrils and induction of a subset of genes. Distinct signaling mechanisms seem to be responsible for striated mature myofibril formation by Ang II.

Protein kinase C mediates increase of Ca2+ sensitivity for contraction by cholinoceptor partial agonist in ileal longitudinal muscle of guinea pig

1. Experiments were designed to study the roles of protein kinase C in carbachol- and pilocarpine-induced contraction and the increase in cytosolic Ca2+ concentration ([Ca2+]i) in guinea pig ileal longitudinal muscle. 2. The protein kinase C inhibitors, GF 109203X (10 microM), calphostin C (10 microM) and H-7 (10 microM), reduced the maximum of the concentration response curve produced by pilocarpine more effectively than that produced by carbachol. 3. The slopes of the regression lines between [Ca2+]i and tension development for pilocarpine and carbachol in tissues treated with GF 109203X were significantly gentler than those for untreated tissues. 4. The protein kinase C alpha- and beta 1 selective inhibitor Goe 6976 (1 microM) decreased both [Ca2+]i and contraction, but did not affect the slopes of the regression lines for pilocarpine and carbachol. 5. These results suggest that protein kinase C (both n- and/or a-type) plays an important role in the increase of Ca2+ sensitivity of the contractile element, and that pilocarpine mainly activates the protein kinase C-dependent pathways for contractile mechanisms in guinea pig ileal longitudinal muscle.

Clostridium difficile toxin B inhibits carbachol-induced force and myosin light chain phosphorylation in guinea-pig smooth muscle: role of Rho proteins

1. Clostridium difficile toxin B glucosylates the Ras-related low molecular mass GTPases of the Rho subfamily thereby inactivating them. In the present report, toxin B was applied as a tool to test whether Rho proteins participate in the carbachol-induced increase in the Ca2+ sensitivity of force and myosin light chain (MLC) phosphorylation in intact intestinal smooth muscle. 2. Small strips of the longitudinal muscle of guinea-pig small intestine were incubated in toxin B (40 ng ml-1) overnight. Carbachol-induced force and intracellular [Ca2+], and, in a separate series, force and MLC phosphorylation, were determined. 3. Carbachol induced a biphasic contraction: an initial rapid increase in force (peak 1) followed by a partial relaxation and a second delayed increase in force (peak 2). The peak of the Ca2+ signal measured with fura-2 preceded peak 1 of force and then declined to a lower suprabasal steady-state level. Peak 2 was not associated with a significant increase in [Ca2+]. Toxin B nearly completely inhibited peak 2 while peak 1 was not significantly inhibited. Toxin B had no effect on the Ca2+ transient. 4. In control strips, MLC phosphorylation at peak 2 was 27.7% which was significantly higher than the resting value (18.6%). The inhibition of the second, delayed, rise in force induced by toxin B was associated with complete inhibition of the increase in MLC phosphorylation. The resting MLC phosphorylation was not significantly different from that of the control strips. 5. The initial increase in MLC phosphorylation determined 3 s after exposure to carbachol was 54% in the control strips. Toxin B also inhibited this initial phosphorylation peak despite the fact that the Ca2+ transient and the initial increase in force were not inhibited by toxin B. This suggests that Rho proteins play an important role in setting the balance between MLC phosphorylation and dephosphorylation reactions even at high levels of intracellular Ca2+. 6. These findings are consistent with the hypothesis that the delayed rise in force elicited by carbachol is due to an increase in the Ca2+ sensitivity of MLC phosphorylation mediated by Rho proteins.

Inhibition of RhoA translocation and calcium sensitization by in vivo ADP-ribosylation with the chimeric toxin DC3B

Pretreatment of intact rabbit portal vein smooth muscle with the chimeric toxin DC3B (10(-6) M, 48 h; ; ) ADP-ribosylated endogenous RhoA, including cytosolic RhoA complexed with rhoGDI, and inhibited the tonic phase of phenylephrine-induced contraction and the Ca2+-sensitization of force by phenylephrine, endothelin and guanosine triphosphate (GTP)gammaS, but did not inhibit Ca2+-sensitization by phorbol dibutyrate. DC3B also inhibited GTPgammaS-induced translocation of cytosolic RhoA () to the membrane fraction. In DC3B-treated muscles the small fraction of membrane-associated RhoA could be immunoprecipitated, even after exposure to GTPgammaS, which prevents immunoprecipitation of non-ADP-ribosylated RhoA. Dissociation of cytosolic RhoA-rhoGDI complexes with SDS restored the immunoprecipitability and ADP ribosylatability of RhoA, indicating that both the ADP-ribosylation site (Asn 41) and RhoA insert loop (Wei et al., 1997) are masked by rhoGDI and that the long axes of the two proteins are in parallel in the heterodimer. We conclude that RhoA plays a significant role in G-protein-, but not protein kinase C-mediated, Ca2+ sensitization and that ADP ribosylation inhibits in vivo the Ca2+-sensitizing effect of RhoA by interfering with its binding to a membrane-associated effector.

Receptor-dependent G protein-mediated Ca2+ sensitization in canine airway smooth muscle

To determine the mechanisms of receptor-dependent Ca2+ sensitization in airway smooth muscle, canine tracheal smooth muscle (CTSM) was permeabilized with alpha-toxin or beta-escin. Although the effects of 5-hydroxytryptamine (100 microM), histamine (100 microM), and the thromboxane A2 analogue U-46619 (100 microM) were negligible, carbachol (100 microM) and endothelin-1 (ET-1, 1 microM) evoked additional contractions of 47.0 +/- 5.90% and 25.0 +/- 5.37% (n = 6) at pCa 6.7 with GTP (3 microM) (normalized to the maximum contraction at pCa 4.5) in alpha-toxin-permeabilized CTSM. GDP-beta-S (1 mM) reversed the carbachol and ET-1 responses completely. GTP-gamma-S (30 microM) and 4 beta-phorbol 12,13-dibutyrate (PDBu, 3 microM) increased the Ca2+ sensitivity (median effective pCa) of contraction by 1.8- and 4.4-fold, respectively (n = 4-11, P < 0.05). The effects of saturating concentrations of GTP-gamma-S and PDBu were additive. A synthetic peptide (T2) corresponding to the actin-binding site of calponin caused a dose-dependent contraction of beta-escin permeabilized CTSM, with the peak effect (25 +/- 4%, n = 4) at 1200 microM, PDBu (3 microM) caused contraction of the T2 peptide-treated CTSM. In conclusion, Ca2+ sensitization of CTSM depends on receptor type and is mediated by G proteins and protein kinase C whose effects are additive, with a partial contribution by calponin.

Focal adhesions, contractility, and signaling

Focal adhesions are sites of tight adhesion to the underlying extracellular matrix developed by cells in culture. They provided a structural link between the actin cytoskeleton and the extracellular matrix and are regions of signal transduction that relate to growth control. The assembly of focal adhesions is regulated by the GTP-binding protein Rho. Rho stimulates contractility which, in cells that are tightly adherent to the substrate, generates isometric tension. In turn, this leads to the bundling of actin filaments and the aggregation of integrins (extracellular matrix receptors) in the plane of the membrane. The aggregation of integrins activates the focal adhesion kinase and leads to the assembly of a multicomponent signaling complex.

Role of Rho proteins in carbachol-induced contractions in intact and permeabilized guinea-pig intestinal smooth muscle

1. The aim of this study was to determine whether the low molecular mass GTPase RhoA or related proteins are involved in carbachol- and high-K(+)-induced contractions in intact intestinal smooth muscle as well as the carbachol-induced increase in Ca2+ sensitivity of the myofilaments in permeabilized preparations. 2. The carbachol-induced increase in the Ca2+ sensitivity of force production in beta-escin-permeabilized intestinal smooth muscle was enhanced in preparations that were loaded with the constitutively active mutant of RhoA, Val14RhoA, and was inhibited by exoenzyme C3 from Clostridium botulinum, which ADP-ribosylates and inactivates small GTPases of the Rho family. The effect of C3 on Ca2+ sensitivity in the absence of the agonist was negligible, while the maximal Ca(2+)-activated force was inhibited by about 20%. 3. Inhibition of carbachol-induced force was associated with an increase in ADP-ribosylation of a protein band with a molecular mass of approximately 22 kDa, corresponding to Rho, and was partially reversed in the presence of Ile41RhoA, which is not a substrate for C3. Val14RhoA did not restore carbachol-induced Ca2+ sensitization in C3-treated smooth muscle. 4. In intact intestinal smooth muscle, toxin B from Clostridium difficile, which monoglucosylates members of the Rho family, inhibited high-K(+)-induced contractions and the initial phasic response to carbachol by about 30%. The delayed contractile response to carbachol was completely inhibited. 5. In smooth muscle preparations that were permeabilized with beta-escin after treatment with toxin B, carbachol-and GTP gamma S-induced Ca2+ sensitization was significantly inhibited. 6. These findings are consistent with a role for Rho or Rho-like proteins in agonist-induced increase in Ca2+ sensitivity of force production in intact and permeabilized intestinal smooth muscle.

Inhibitory effect of phorbol 12,13-dibutyrate on norepinephrine-induced contraction in rabbit iris dilator muscle

The hypothesis that the increase in Ca2+ sensitivity on norepinephrine-induced contraction of smooth muscles and also the decrease of the norepinephrine-induced sustained level of intracellular Ca2+ concentration are produced by the activation of protein kinase C was tested. Phorbol 12,13-dibutyrate (PDB; 10(-6) M) relaxed the norepinephrine-induced sustained contraction in a concentration-dependent manner. On pretreatment with PDB a transient contraction was produced by the application of norepinephrine, but the sustained contraction was significantly reduced. The sustained elevations of intracellular Ca2+ concentration ([Ca2+]i) and the contraction induced by norepinephrine in fura-2-loaded preparations were decreased by the application of PDB. These inhibitory effects were antagonized by potent protein kinase inhibitors, 2-(1-(3-dimethylaminopropyl)-indol-3-yl)-3-(-indol-3-yl)-maleimide (GF 109203X) (10 (-6) M) and 1-(5-isoquinolinesulfonyl)-2-methylpiperazine dihydrochloride (H-7) (10 (-6) M), but were not affected by a protein kinase A/G inhibitor, N-(2-cinnamylaminoethyl)-5-isoquinolinesulfonamide (H-88) (10(-6) M). The slope of the regression line for norepinephrine for [Ca2+]i and tension was significantly steeper than those obtained with high K+. Also, on pretreatment with PDB the Ca2+ sensitivity of the K(+)-induced contraction was decreased, but the Ca2+ sensitivity of norepinephrine-induced contraction tended to be increased. These observations indicate that PDB induces a decrease of [Ca2+]i on Ca2+ mobility and an increase of Ca2+ sensitivity on contraction of smooth muscle through the activation of protein kinase C.

MCI-826 is a potent and selective antagonist of peptide leukotrienes (p-LTs) and has characteristics distinctive from those of FPL 55712

Antagonistic effects of a newly synthesized compound, (E)-2,2-diethyl-3'-[2-[2-(4-isopropyl)thiazolyl]ethenyl]succinanilic+ ++ acid sodium salt (MCI-826) on the contraction of the isolated guinea pig trachea and human bronchus induced by various agonists including peptide leukotrienes (p-LTs), histamine, acetylcholine (ACh), prostaglandin (PG) D2 and others were investigated and compared with the effects of a p-LT antagonist, FPL 55712, in some experiments. MCI-826 potently antagonized LTD4- and LTE4-induced contractions at extremely low concentrations in the isolated guinea pig trachea with pA2 values of 8.3 and 8.9, respectively, on a molar basis. These values indicated that MCI-826 is over 100 times stronger than FPL 55712. Similarly, MCI-826 at 10(-8) g/ml (2.4 x 10(-8) M) markedly antagonized LTD4-induced contractions of the isolated human bronchus. Although FPL 55712 fairly inhibited the 10(-9) g/ml LTC4-induced contraction of the isolated guinea pig trachea, MCI-826 had little effect on the contraction at high concentrations like 3 x 10(-6) g/ml (7.1 x 10(-6) M). MCI-826 modestly affected the other agonist-induced contractions and the resting tonus of the isolated guinea pig trachea at 10(-6) g/ml (2.4 x 10(-6) M) or higher concentrations, but FPL 55712 caused fair inhibition of some of those contractions and gradually lowered the resting tonus with time. These results indicate that MCI-826 is a highly potent and selective antagonist of LTD4 and LTE4 and can be a useful tool for biological and pharmacological experiments on p-LTs.