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

Mechanical stress increases RhoA activation in airway smooth muscle cells

Cultured airway smooth muscle cells subjected to cyclic strain respond with increased cytoskeletal organization and contractility resembling effects described with RhoA activation. To test the hypothesis that strain increases cell cytoskeletal organization through RhoA, cells were subjected to strain in the presence of known activators or inhibitors of RhoA. Ten percent cyclic deformational strain (serum-free conditions) increased F-actin staining (152% over control), and this effect was enhanced by serum or lysophosphatidic acid (180%), but decreased (68%) with Clostridium botulinum toxin inhibition of RhoA or with the Rho kinase inhibitor Y27632 (67%). When cells expressing the dominant negative N17-RhoA isoform were subjected to strain, F-actin staining was disorganized and cells failed to elongate or migrate relative to strain direction. When cells expressing a green fluorescent protein (GFP)-RhoA fusion protein were subjected to strain, GFP showed up to 25% greater cell membrane staining than control cells. Finally, strain caused a 4-fold increase in RhoA activation (Rhotekin binding assay), and a 3-fold increase myosin phosphatase phosphorylation that was inhibited by Y27632. We conclude that mechanical stress activates RhoA, an event that may increase airway smooth muscle contractility.

Ca2+-dependent and Ca2+-independent regulation of smooth muscle contraction

An increase in the cytosolic Ca2+ concentration is a prerequisite in activation of contractile activity of smooth muscle. The shape of the Ca2+-signal is determined by spatial distribution and kinetics of Ca2+-binding sites in the cell. The increase in cytosolic Ca2+ activates myosin light chain kinase (MLCK) which in turn phosphorylates the regulatory light chains of myosin II. This Ca2+-dependent MLC20 phosphorylation is modulated in a Ca2+-independent manner by inhibiting the constitutive active myosin light chain phosphatase mediated by the monomeric GTPase Rho and the Rho-associated kinase as well as protein kinase C or by increasing its activity through cGMP. Furthermore, the activity of MLCK may be decreased due to phosphorylation by CaM kinase II and perhaps p21 activated protein kinase. Hence, smooth muscle tone appears to be regulated by a network of activating and inactivating intracellular signaling cascades which not only show a temporal but also a spatial activation pattern.

Microtubule-disruption-induced and chemotactic-peptide-induced migration of human neutrophils: implications for differential sets of signalling pathways

Neutrophil granulocytes rely on a functional actin network for directed migration. Microtubule disassembly does not impair receptor-linked chemotaxis, instead it induces development of polarity and chemokinesis in neutrophils concomitant with polarized distribution of alpha-actinin and F-actin. Cells stimulated with colchicine, which disassembles microtubules, migrate with a speed comparable to cells exposed to chemotactic peptide. We investigated signalling pathways involved in colchicine-induced neutrophil polarization and migration. Colchicine-induced development of polarity was insensitive to treatment with pertussis toxin, in contrast to chemotactic-peptide-induced shape changes, which were completely abolished by this treatment. Thus, colchicine does not appear to act via activating heterotrimeric G(i) proteins. Colchicine does also not seem to act via phosphatidylinositol 3-kinase, as it failed to induce phosphorylation of its downstream target Akt and the potent phosphatidylinositol 3-kinase inhibitor wortmannin failed to inhibit colchicine-induced shape changes. By contrast, wortmannin significantly reduced chemotactic-peptide-induced shape changes. However, the Rho-kinase inhibitor Y-27632 (10 micro M) inhibited colchicine-induced development of polarity by 95+/-3% (n=5) and chemokinesis by 76+/-9% (n=3), which suggests that the Rho-Rho-kinase pathway has a crucial role in polarity and migration. Indeed, treatment of cells with colchicine induced a significant increase in membrane-bound Rho-kinase II, which is indicative of activation of this protein. This membrane translocation could be prevented by taxol, which stabilizes microtubules. Colchicine also induced a marked increase in myosin light chain phosphorylation, which could be suppressed by Y-27632 and by taxol. In summary, we provide evidence that microtubule disassembly induces in neutrophils a selective activation of Rho-kinase, bypassing activation of heterotrimeric Gi proteins and phosphatidylinositol 3-kinase. This process is sufficient for induction of chemokinesis and mediates increased phosphorylation of myosin light chain and accumulation of F-actin and alpha-actinin in the leading edge.

The role of RhoA and Rho-associated kinase in vascular smooth muscle contraction

A variety of contractile agonists trigger activation of the small GTPase RhoA. An important target of activated RhoA in smooth muscle is Rho-associated kinase (ROK), one of the downstream targets that is the myosin binding subunit (MYPT1) of myosin light chain phosphatase (MLCP). Phosphorylation of MYPT1 at T695 by activated ROK results in a decrease in phosphatase activity of MLCP and an increase in myosin light chain (LC(20)) phosphorylation catalyzed by Ca(2)(+)/calmodulin-dependent myosin light chain kinase and/or a distinct Ca(2)(+)-independent kinase. LC(20) phosphorylation in turn triggers cross-bridge cycling and force development. ROK also phosphorylates the cytosolic protein CPI-17 (at T38), which thereby becomes a potent inhibitor of MLCP. The RhoA/ROK pathway has been implicated in the tonic phase of force maintenance in response to various agonists, with no evident role in the phasic response, suggesting this pathway as a potential target for antihypertensive therapy. Indeed, ROK inhibitors restore normal blood pressure in several rat hypertensive models.

Inhibition of rho-associated kinase reduces MLC20 phosphorylation and contractility of intact myometrium and attenuates agonist-induced Ca2+ sensitization of force of permeabilized rat myometrium

The role of rhoA/rho-associated kinase (ROK) signaling pathways in agonist-induced contraction of the rat myometrium was investigated. We measured the [Ca(2+)](i)-force relationship, phosphorylation of myosin regulatory light chains (MLC(20)) in intact tissue and the Ca(2+)-sensitization of force in permeabilized myometrial cells of rat. In measurements of the relationship between [Ca(2+)](i) and tension in intact tissue, Y-27632, a ROK inhibitor, significantly attenuated the carbachol-induced contraction without changing [Ca (2+)](i). Phosphorylation of MLC(20) was increased by carbachol and this increased phosphorylation was blocked by treatment of tissue with Y-27632. In tension measurements of single hyperpermeable cells, carbachol evoked sustained contraction at constant pCa 6.7 and these agonist-induced contractions were decreased by treatment with Y-27632. These results suggest that activation of a ROK-mediated signaling pathway(s) plays an important role in agonist-induced alterations in MLC(20) phosphorylation and force of rat myometrium.

Functional interplay between angiotensin II and nitric oxide: cyclic GMP as a key mediator

Angiotensin II (Ang II) and nitric oxide (NO) signaling pathways mutually regulate each other by multiple mechanisms. Ang II regulates the expression of NO synthase and NO production, whereas NO downregulates the Ang II type I (AT1) receptor. In addition, downstream effectors of Ang II and NO signaling pathways also interact with each other. A feedback mechanism between Ang II and NO is critical for normal vascular structure and function. Imbalance of Ang II and NO has been implicated in the pathophysiology of many vascular diseases. In this review, we focus on the diverse ways in which Ang II and NO interact and the importance of the balance between the signaling pathways activated by these mediators.

Involvement of Src family protein tyrosine kinases in Ca(2+) sensitization of coronary artery contraction mediated by a sphingosylphosphorylcholine-Rho-kinase pathway

We recently reported that sphingosylphosphorylcholine (SPC) is a novel messenger for Rho-kinase-mediated Ca(2+) sensitization of vascular smooth muscle (VSM) contraction. Subcellular localization and kinase activity of Src family protein kinases (SrcPTKs), except for c-Src, is controlled by a reversible S-palmitoylation, an event inhibited by eicosapentaenoic acid (EPA). We examined the possible involvement of SrcPTKs in SPC-induced Ca(2+) sensitization and effects of EPA. We used porcine coronary VSM and rat aortic VSM cells (VSMCs) in primary culture. An SrcPTKs inhibitor, PP1, and EPA inhibited SPC-induced contraction, concentration-dependently, without affecting [Ca(2+)](i) levels and the Ca(2+)-dependent contraction induced by high K(+) depolarization. A digitized immunocytochemical analysis in VSMCs revealed that SPC induced translocation of Fyn, but not of c-Src, from the cytosol to the cell membrane, an event abolished by EPA. Translocation of Rho-kinase from the cytosol to the cell membrane by SPC was also inhibited by EPA and PP1. The SPC-induced activation of SrcPTKs was blocked by EPA and PP1, but not by Y27632, an Rho-kinase inhibitor. Rho-kinase-dependent phosphorylation of myosin phosphatase induced by SPC was inhibited by EPA, PP1, and Y27632. Translocation and activation of SrcPTKs, including Fyn, play an important role in Ca(2+) sensitization of VSM contractions mediated by a SPC-Rho-kinase pathway.

Conventional protein kinase C mediates phorbol-dibutyrate-induced cytoskeletal remodeling in a7r5 smooth muscle cells

Phorbol dibutyrate (PDBu) induced the formation of podosome-like structures together with partial disassembly of actin stress fibers in A7r5 smooth muscle cells. These podosomes contained alpha-actinin, F-actin, and vinculin and exhibit a tubular, column-like structure arising perpendicularly from the bottom of PDBu-treated cells. The conventional protein kinase C (PKC) antagonist, GO6976, inhibited PDBu-induced cytoskeletal remodeling at 0.1 microM, whereas the novel PKC antagonist, rottlerin, was ineffective at 10 microM. PDBu induced the translocation of the conventional PKC-alpha but not the novel PKC-delta to the sites of podosome formation in A7r5 cells. Although partial disassembly of actin stress fibers was observed in both Y-27632- and PDBu-treated cells, focal adhesions were much reduced in number and size only in Y-27632-treated cells. Furthermore, PDBu restored focal adhesions in Y-27632-treated cells. Live video fluorescence microscopy of alpha-actinin GFP revealed a lag phase of about 20 min prior to the rapid formation and dynamic reorganization of podosomes during PDBu treatment. These findings suggest that conventional PKCs mediate PDBu-induced formation of dynamic podosome-like structures in A7r5 cells, and Rho-kinase is unlikely to be the underlying mechanism. The podosome columns could represent molecular scaffolds where PKC-alpha phosphorylates regulatory proteins necessary for Ca(2+) sensitization in smooth muscle cells.

Differential association and localization of myosin phosphatase subunits during agonist-induced signal transduction in smooth muscle

It has been known for some time that agonist-induced contractions of vascular smooth muscle are often associated with a sensitization of the contractile apparatus to intracellular Ca2+. One mechanism that has been suggested to explain Ca2+ sensitization is inhibition of myosin phosphatase activity. In the present study, we tested the hypothesis that differential localization of the phosphatase might be associated with its inhibition. Quantitative confocal microscopy of freshly dissociated, fully contractile smooth muscle cells was used in parallel with measurements of myosin light chain and myosin phosphatase phosphorylation. The results indicate that, in the smooth muscle cells, the catalytic and targeting subunits of the phosphatase are dissociated from each other in an agonist-specific manner and that the dissociation is accompanied by a slower rate of myosin phosphorylation. Targeting of myosin phosphatase to the cell membrane precedes the dissociation of subunits and is associated with phosphorylation of the targeting subunit at a Rho-associated kinase (ROK) phosphorylation site. The phosphorylation and membrane translocation of the targeting subunit are inhibited by a ROK inhibitor. This dissociation of subunits may provide a mechanism for the decreased phosphatase activity of phosphorylated myosin phosphatase.

Human and rabbit cavernosal smooth muscle cells express Rho-kinase

Rho-kinase is an enzyme involved in the Ca2+-sensitizing pathway in smooth muscle cells. Inhibition of this enzyme has been recently demonstrated to elicit penile erection by relaxing cavernosal smooth muscle. We aimed to investigate the presence and activity of Rho-kinase in human cavernosal smooth muscle. Primary culture of smooth muscle cells from human and rabbit penile corpus cavernosum was developed, and cells showed characteristic myocyte morphology and alpha-actin immunoreactivity. The presence of Rho-kinase was demonstrated by indirect immunofluorescence and Western blotting. A specific inhibitor of Rho-kinase, Y-27632, inhibited in a concentration-dependent manner the kinase activity of the protein immunoprecipitated with anti-Rho-kinase antibody. These results demonstrate for the first time expression and activity of Rho-kinase in human penile cavernosal smooth muscle cells and suggest that these cells can provide a cellular model for the study of enzymes involved in Ca2+-sensitizing pathways.

Invited review: regulation of myosin phosphorylation in smooth muscle

Phosphorylation of the regulatory light chains of myosin II (rMLC) by the Ca(2+)/calmodulin-dependent myosin light-chain kinase (MLCK) and dephosphorylation by a type 1 phosphatase (MLCP), which is targeted to myosin by a regulatory subunit (MYPT1), are the predominant mechanisms of regulation of smooth muscle tone. The activities of both enzymes are modulated by several protein kinases. MLCK is inhibited by the Ca(2+)/calmodulin-dependent protein kinase II, whereas the activity of MLCP is increased by cGMP and perhaps also cAMP-dependent protein kinases. In either case, this results in a decrease in the Ca(2+) sensitivity of rMLC phosphorylation and force production. The activity of MLCP is inhibited by Rho-associated kinase, one of the effectors of the monomeric GTPase Rho, and protein kinase C, leading to an increase in Ca(2+) sensitivity. Hence, smooth muscle tone appears to be regulated by a network of activating and inactivating intracellular signaling cascades.

Regulation by GDI of RhoA/Rho-kinase-induced Ca2+ sensitization of smooth muscle myosin II

We characterized the role of guanine nucleotide dissociation inhibitor (GDI) in RhoA/Rho-kinase-mediated Ca2+ sensitization of smooth muscle. Endogenous contents (approximately 2-4 microM) of RhoA and RhoGDI were near stoichiometric, whereas a supraphysiological GDI concentration was required to relax Ca2+ sensitization of force by GTP and guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS). GDI also inhibited Ca2+ sensitization by GTP. G14V RhoA, by alpha-adrenergic and muscarinic agonists, and extracted RhoA from membranes. GTPgammaS translocated Rho-kinase to a Triton X-114-extractable membrane fraction. GTP. G14V RhoA complexed with GDI also induced Ca2+ sensitization, probably through in vivo dissociation of GTP. RhoA from the complex, because it was reversed by addition of excess GDI. GDI did not inhibit Ca2+ sensitization by phorbol ester. Constitutively active Cdc42 and Rac1 inhibited Ca2+ sensitization by GTP. G14V RhoA. We conclude that 1) the most likely in vivo function of GDI is to prevent perpetual "recycling" of GDP. RhoA to GTP. RhoA; 2) nucleotide exchange (GTP for GDP) on complexed GDP. RhoA/GDI can precede translocation of RhoA to the membrane; 3) activation of Rho-kinase exposes a hydrophobic domain; and 4) Cdc42 and Rac1 can inhibit Ca2+ sensitization by activated GTP. RhoA.

Augmented acetylcholine-induced translocation of RhoA in bronchial smooth muscle from antigen-induced airway hyperresponsive rats

Acetylcholine (ACh)-induced translocation of RhoA in bronchial smooth muscle of repeatedly antigen-challenged rats that have a marked airway hyperresponsiveness (AHR) was examined. ACh induced time- and concentration-dependent translocation of RhoA to the plasma membrane, indicating an activation of RhoA in bronchial smooth muscle. The level of ACh-induced RhoA translocation was further increased markedly in the AHR group as compared to that in the control group. It is suggested that the augmented activation of RhoA observed in the hyperresponsive bronchial smooth muscle might be responsible for the enhanced ACh-induced Ca(2+) sensitization of bronchial smooth muscle contraction associated with AHR.

Smooth muscle excitation-contraction coupling: a role for caveolae and caveolins?

Agonist stimulation of smooth muscle contractility involves integration of many signal-transducing events from the plasma membrane to myofilaments in the cytoplasm. Recent evidence suggests an important role for membranous invaginations termed caveolae, and their integral protein components caveolins, in the coordination of extracellular contractile stimuli and intracellular effectors in smooth muscle.

Microtubule depolymerization facilitates contraction of vascular smooth muscle via increased activation of RhoA/Rho-kinase

The microtubule network is in a dynamic equilibrium between free and polymerized tubulin, with depolymerization resulting in increased cellular contractility (1-4). Originally, microtubule depolymerization was thought to facilitate contractile responses via the release of an internal, mechanical opposition to contraction. However, recent evidence suggests that depolymerization may also lead to the enhanced activity of various intracellular signaling proteins. The precise signaling pathway by which microtubule depolymerization facilitates vascular smooth muscle contraction is unknown. In non-vascular cells, depolymerization initiates stress fiber formation via increased activity of the small G-protein, RhoA (5-7). The role of this signaling candidate in a calcium-sensitizing contractile pathway is well established. We and others have found it tempting to speculate that RhoA mediates a contractile pathway enhanced by microtubule depolymerization. We further hypothesize the involvement of microtubule depolymerization (via RhoA and Rho-kinase) in the regulation of vascular smooth muscle contraction, with evidence of potential augmentations of this pathway contributing to the increased vasoconstrictor sensitivity seen in various hypertensive animal models.

Inhibition of PKCalpha and rhoA translocation in differentiated smooth muscle by a caveolin scaffolding domain peptide

Receptor-coupled contraction of smooth muscle involves recruitment to the plasma membrane of downstream effector molecules PKCalpha and rhoA but the mechanism of this signal integration is unclear. Caveolins, the principal structural proteins of caveolar plasma membrane invaginations, have been implicated in the organization and regulation of many signal transducing molecules. Thus, using laser scanning confocal immunofluorescent microscopy, we tested the hypothesis that caveolin is involved in smooth muscle signaling by investigating caveolin isoform expression and localization, together with the effect of a peptide inhibitor of caveolin function, in intact differentiated smooth muscle cells. All three main caveolin isoforms were identified in uterine, stomach, and ileal smooth muscles and assumed a predominantly plasma membranous localization in myometrial cells. Cytoplasmic introduction of a peptide corresponding to the caveolin-1 scaffolding domain-an essential region for caveolin interaction with signaling molecules--significantly inhibited agonist-induced translocation of both PKCalpha and rhoA. Translocation was unimpaired by a scrambled peptide and was unaltered in sham-treated cells. The membranous localization of caveolins, and direct inhibition of receptor-coupled PKCalpha and rhoA translocation by the caveolin-1 scaffolding domain, supports the concept that caveolins can regulate the integration of extracellular contractile stimuli and downstream intracellular effectors in smooth muscle.

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.