Phosphorylation of the regulatory subunit of smooth muscle protein phosphatase 1M at Thr850 induces its dissociation from myosin

Targeting cerebrovascular Rho-kinase in stroke

BACKGROUND

Rho and Rho-associated kinase (ROCK) play pivotal roles in pathogenesis of vascular diseases including stroke. ROCK is expressed in all cell types relevant to stroke, and regulates a range of physiological processes.

OBJECTIVE

To provide an overview of ROCK as an experimental therapeutic target in cerebral ischemia, and the translational opportunities and obstacles in the prophylaxis and treatment of stroke.

METHODS

Relevant literature was reviewed.

RESULTS

ROCK activity is upregulated in chronic vascular risk factors such as diabetes, hyperlipidemia and hypertension, and more acutely by cerebral ischemia. ROCK activation is predicted to increase the risk of cerebral ischemia, and worsen the ischemic tissue outcome and functional recovery. Evidence suggests that ROCK inhibition is protective in models of cerebral ischemia. The benefit is mediated through multiple mechanisms.

CONCLUSION

ROCK is a promising therapeutic target in all stages of stroke.

ROCK isoform regulation of myosin phosphatase and contractility in vascular smooth muscle cells

Abnormal vascular smooth muscle cell (VSMC) contraction plays an important role in vascular diseases. The RhoA/ROCK signaling pathway is now well recognized to mediate vascular smooth muscle contraction in response to vasoconstrictors by inhibiting myosin phosphatase (MLCP) activity and increasing myosin light chain phosphorylation. Two ROCK isoforms, ROCK1 and ROCK2, are expressed in many tissues, yet the isoform-specific roles of ROCK1 and ROCK2 in vascular smooth muscle and the mechanism of ROCK-mediated regulation of MLCP are not well understood. In this study, ROCK2, but not ROCK1, bound directly to the myosin binding subunit of MLCP, yet both ROCK isoforms regulated MLCP and myosin light chain phosphorylation. Despite that both ROCK1 and ROCK2 regulated MLCP, the ROCK isoforms had distinct and opposing effects on VSMC morphology and ROCK2, but not ROCK1, had a predominant role in VSMC contractility. These data support that although the ROCK isoforms both regulate MLCP and myosin light chain phosphorylation through different mechanisms, they have distinct roles in VSMC function.

Thromboxane A2-induced bi-directional regulation of cerebral arterial tone

Myosin light chain phosphatase plays a critical role in modulating smooth muscle contraction in response to a variety of physiologic stimuli. A downstream target of the RhoA/Rho-kinase and nitric oxide (NO)/cGMP/cyclic GMP-dependent kinase (cGKI) pathways, myosin light chain phosphatase activity reflects the sum of both calcium sensitization and desensitization pathways through phosphorylation and dephosphorylation of the myosin phosphatase targeting subunit (MYPT1). As cerebral blood flow is highly spatio-temporally modulated under normal physiologic conditions, severe perturbations in normal cerebral blood flow, such as in cerebral vasospasm, can induce neurological deficits. In nonpermeabilized cerebral vessels stimulated with U-46619, a stable mimetic of endogenous thromboxane A2 implicated in the etiology of cerebral vasospasm, we observed significant increases in contractile force, RhoA activation, regulatory light chain phosphorylation, as well as phosphorylation of MYPT1 at Thr-696, Thr-853, and surprisingly Ser-695. Inhibition of nitric oxide signaling completely abrogated basal MYPT1 Ser-695 phosphorylation and significantly increased and potentiated U-46619-induced MYPT1 Thr-853 phosphorylation and contractile force, indicating that NO/cGMP/cGKI signaling maintains basal vascular tone through active inhibition of calcium sensitization. Surprisingly, a fall in Ser-695 phosphorylation did not result in an increase in phosphorylation of the Thr-696 site. Although activation of cGKI with exogenous cyclic nucleotides inhibited thromboxane A2-induced MYPT1 membrane association, RhoA activation, contractile force, and regulatory light chain phosphorylation, the anticipated decreases in MYPT1 phosphorylation at Thr-696/Thr-853 were not observed, indicating that the vasorelaxant effects of cGKI are not through dephosphorylation of MYPT1. Thus, thromboxane A2 signaling within the intact cerebral vasculature induces "buffered" vasoconstrictions, in which both the RhoA/Rho-kinase calcium-sensitizing and the NO/cGMP/cGKI calcium-desensitizing pathways are activated.

Stimulated calcium entry and constitutive RhoA kinase activity cause stretch-induced detrusor contraction

Urinary bladder wall muscle (i.e., detrusor smooth muscle; DSM) contracts in response to a quick-stretch, but this response is neither fully characterized, nor completely understood at the subcellular level. Strips of rabbit DSM were quick-stretched (5 ms) and held isometric for 10 s to measure the resulting peak quick-stretch contractile response (PQSR). The ability of selective Ca(2+) channel blockers and kinase inhibitors to alter the PQSR was measured, and the phosphorylation levels of myosin light chain (MLC) and myosin phosphatase targeting regulatory subunit (MYPT1) were recorded. DSM responded to a quick-stretch with a biphasic response consisting of an initial contraction peaking at 0.24+/-0.02-fold the maximum KCl-induced contraction (F(o)) by 1.48+/-0.17 s (PQSR) before falling to a weaker tonic (10 s) level (0.12+/-0.03-fold F(o)). The PQSR was dependent on the rate and degree of muscle stretch, displayed a refractory period, and was converted to a sustained response in the presence of muscarinic receptor stimulation. The PQSR was inhibited by nifedipine, 2-aminoethoxydiphenyl borate (2-APB), 100 microM gadolinium and Y-27632, but not by atropine, 10 microM gadolinium, LOE-908, cyclopiazonic acid, or GF-109203X. Y-27632 and nifedipine abolished the increase in MLC phosphorylation induced by a quick-stretch. Y-27632, but not nifedipine, inhibited basal MYPT1 phosphorylation, and a quick-stretch failed to increase phosphorylation of this rhoA kinase (ROCK) substrate above the basal level. These data support the hypothesis that constitutive ROCK activity is required for a quick-stretch to activate Ca(2+) entry and cause a myogenic contraction of DSM.

Role of src-family kinases in hypoxic vasoconstriction of rat pulmonary artery

Aims

We investigated the role of src-family kinases (srcFKs) in hypoxic pulmonary vasoconstriction (HPV) and how this relates to Rho-kinase-mediated Ca²⁺ sensitization and changes in intracellular Ca²⁺ concentration ([Ca²⁺]_i).

Methods and results

Intra-pulmonary arteries (IPAs) were obtained from male Wistar rats. HPV was induced in myograph-mounted IPAs. Auto-phosphorylation of srcFKs and phosphorylation of the regulatory subunit of myosin phosphatase (MYPT-1) and myosin light-chain (MLC₂₀) in response to hypoxia were determined by western blotting. Translocation of Rho-kinase and effects of siRNA knockdown of src and fyn were examined in cultured pulmonary artery smooth muscle cells (PASMCs). [Ca²⁺]_i was estimated in Fura-PE3-loaded IPA. HPV was inhibited by two blockers of srcFKs, SU6656 and PP2. Hypoxia enhanced phosphorylation of three srcFK proteins at Tyr-416 (60, 59, and 54 kDa, corresponding to src, fyn, and yes, respectively) and enhanced srcFK-dependent tyrosine phosphorylation of multiple target proteins. Hypoxia caused a complex, time-dependent enhancement of MYPT-1 and MLC₂₀ phosphorylation, both in the absence and presence of pre-constriction. The sustained component of this enhancement was blocked by SU6656 and the Rho-kinase inhibitor Y27632. In PASMCs, hypoxia caused translocation of Rho-kinase from the nucleus to the cytoplasm, and this was prevented by anti-src siRNA and to a lesser extent by anti-fyn siRNA. The biphasic increases in [Ca²⁺]_i that accompany HPV were also inhibited by PP2.

Conclusion

Hypoxia activates srcFKs and triggers protein tyrosine phosphorylation in IPA. Hypoxia-mediated Rho-kinase activation, Ca²⁺ sensitization, and [Ca²⁺]_i responses are depressed by srcFK inhibitors and/or siRNA knockdown, suggesting a central role of srcFKs in HPV.

Assembly of MYPT1 with protein phosphatase-1 in fibroblasts redirects localization and reorganizes the actin cytoskeleton

Dephosphorylation of actin-binding proteins by a specialized form of protein Ser/Thr phosphatase type-1 (PP1) regulates smooth muscle contraction and morphology and motility of nonmuscle cells. This myosin and ezrin/radixin/moesin (ERM)-targeted phosphatase comprises the delta isoform PP1 catalytic subunit plus a primary regulatory subunit called myosin phosphatase targeting (MYPT1). We reconstructed myosin/ERM phosphatase in living rat embryo fibroblasts (REF52 cells) by transient expression of epitope-tagged MYPT1 (myc-MYPT1) plus HA-tagged PP1. Unexpectedly, wild-type myc-MYPT1 expressed alone accumulated predominantly in the nucleus, as visualized by immunofluorescent microscopy, whereas if coexpressed with HA-PP1, it was localized in the cytosol and deposited on cytoskeleton myofilaments. The F38A mutation of MYPT1 that eliminates PP1 binding gave nuclear localization of myc-MYPT1, even when coexpressed with HA-PP1. Thus, expression of both subunits was necessary to form myosin/ERM phosphatase in situ and mediate myofilament localization. The results indicate there is little endogenous PP1 available for interaction or interchange with ectopic regulatory subunits in living cells. We concluded that myosin binding by the C-terminal domain of MYPT1 is not sufficient to override nuclear import in fibroblasts, but the binding of PP1 to myc-MYPT1 neutralizes nuclear import. Full-length myc-MYPT1 plus HA-PP1 induced only subtle changes in organization of the actin cytoskeleton, however coexpression of myc-MYPT1(1-300) with HA-PP1 dispersed stress fibers without major alteration in morphology and myc-MYPT1(1-498) disrupted the cytoskeleton and produced radically extended cells that appeared like neurons. Based on these responses, we conclude that the MYPT1 C-terminus functions as an auto-inhibitory domain, and a central domain in MYPT1 can mediate extensive reorganization of the actin cytoskeleton.

The pro-apoptotic protein Par-4 facilitates vascular contractility by cytoskeletal targeting of ZIPK

Par-4 (prostate apoptosis response 4) is a pro-apoptotic protein and tumour suppressor that was originally identified as a gene product up-regulated during apoptosis in prostate cancer cells. Here, we show, for the first time, that Par-4 is expressed and co-localizes with the actin filament bundles in vascular smooth muscle. Furthermore, we demonstrate that targeting of ZIPK to the actin filaments, as observed upon PGF-2α stimulation, is inhibited by the presence of a cell permeant Par-4 decoy peptide. The same decoy peptide also significantly inhibits PGF-2α induced contractions of smooth muscle tissue. Moreover, knockdown of Par-4 using antisense morpholino nucleotides results in significantly reduced contractility, and myosin light chain and myosin phosphatase target subunit phosphorylation. These results indicate that Par-4 facilitates contraction by targeting ZIPK to the vicinity of its substrates, myosin light chain and MYPT, which are located on the actin filaments. These results identify Par-4 as a novel regulator of myosin light chain phosphorylation in differentiated, contractile vascular smooth muscle.

The actin cytoskeleton in cancer cell motility

Cancer cell metastasis is a multi-stage process involving invasion into surrounding tissue, intravasation, transit in the blood or lymph, extravasation, and growth at a new site. Many of these steps require cell motility, which is driven by cycles of actin polymerization, cell adhesion and acto-myosin contraction. These processes have been studied in cancer cells in vitro for many years, often with seemingly contradictory results. The challenge now is to understand how the multitude of in vitro observations relates to the movement of cancer cells in living tumour tissue. In this review we will concentrate on actin protrusion and acto-myosin contraction. We will begin by presenting some general principles summarizing the widely-accepted mechanisms for the co-ordinated regulation of actin polymerization and contraction. We will then discuss more recent studies that investigate how experimental manipulation of actin dynamics affects cancer cell invasion in complex environments and in vivo.

Contractile forces in tumor cell migration

Cancer is a deadly disease primarily because of the ability of tumor cells to spread from the primary tumor, to invade into the connective tissue, and to form metastases at distant sites. In contrast to cell migration on a planar surface where large cell tractions and contractile forces are not essential, tractions and forces are thought to be crucial for overcoming the resistance and steric hindrance of a dense three-dimensional connective tissue matrix. In this review, we describe recently developed biophysical tools, including 2-D and 3-D traction microscopy to measure contractile forces of cells. We discuss evidence indicating that tumor cell invasiveness is associated with increased contractile force generation.

Mechanisms of Rho kinase regulation of vascular reactivity following hemorrhagic shock in rats

Our previous research showed that Rho kinase took part in the regulation of vascular hyporeactivity after shock. The objective of the present study was to investigate its mechanism. With isolated superior mesenteric artery (SMA) from hemorrhagic shock rats, we studied the relationship of Rho kinase regulating vascular reactivity to calcium sensitivity and myosin light chain phosphatase (MLCP) and myosin light chain kinase (MLCK). The vascular reactivity and calcium sensitivity of SMA were observed by measuring the contraction initiated by accumulative norepinephrine (NE) and calcium under depolarizing condition (120 mM K(+)) with an isolated organ perfusion system. Hypoxia-treated vascular smooth muscle cells (VSMCs) were used to study the effects of Rho kinase on the activity of MLCP and MLCK and the phosphorylation of 20-kDa myosin light chain (MLC(20)). Myosin light chain (20 kDa) phosphorylation of VSMC in mesenteric artery was detected by immunoprecipitation and Western blotting. The activity of MLCP and MLCK was assayed by enzymatic catalysis. The contractile response of VSMC was measured by the ratio of accumulative infiltration of fluorescent isothiocyanate-conjugated bovine serum albumin through transwell. The results indicated that the vascular reactivity and calcium sensitivity of SMA to NE and calcium following hemorrhagic shock and the contractile response of VSMC to NE following hypoxia were significantly decreased. Angiotensin II (Ang-II), the Rho kinase stimulator, significantly improved hypoxia or hemorrhagic shock-induced decrease of vascular reactivity and calcium sensitivity. These effects of Ang-II on vascular reactivity were abolished by Y-27632, the specific Rho kinase inhibitor. Calyculin A, the MLCP inhibitor, further enhanced Ang-II-induced increase of calcium sensitivity, but ML-9, the MLCK inhibitor, had no effect. Further studies showed Ang-II reversed the hypoxia-induced increase of MLCP activity and increased the hypoxia-induced decrease of MLC(20) phosphorylation in VSMC. It was suggested that Rho kinase played an important role in the regulation of vascular reactivity after hemorrhagic shock. The mechanisms may be related to its calcium sensitivity regulation. Rho kinase up-regulates calcium sensitivity of VSMC possibly through inhibiting the activity of MLCP and increasing the phosphorylation of MLC(20).

Protein kinase substrate identification on functional protein arrays

Background

Over the last decade, kinases have emerged as attractive therapeutic targets for a number of different diseases, and numerous high throughput screening efforts in the pharmaceutical community are directed towards discovery of compounds that regulate kinase function. The emerging utility of systems biology approaches has necessitated the development of multiplex tools suitable for proteomic-scale experiments to replace lower throughput technologies such as mass spectroscopy for the study of protein phosphorylation. Recently, a new approach for identifying substrates of protein kinases has applied the miniaturized format of functional protein arrays to characterize phosphorylation for thousands of candidate protein substrates in a single experiment. This method involves the addition of protein kinases in solution to arrays of immobilized proteins to identify substrates using highly sensitive radioactive detection and hit identification algorithms.

Results

To date, the factors required for optimal performance of protein array-based kinase substrate identification have not been described. In the current study, we have carried out a detailed characterization of the protein array-based method for kinase substrate identification, including an examination of the effects of time, buffer compositions, and protein concentration on the results. The protein array approach was compared to standard solution-based assays for assessing substrate phosphorylation, and a correlation of greater than 80% was observed. The results presented here demonstrate how novel substrates for protein kinases can be quickly identified from arrays containing thousands of human proteins to provide new clues to protein kinase function. In addition, a pooling-deconvolution strategy was developed and applied that enhances characterization of specific kinase-substrate relationships and decreases reagent consumption.

Conclusion

Functional protein microarrays are an important new tool that enables multiplex analysis of protein phosphorylation, and thus can be utilized to identify novel kinase substrates. Integrating this technology with a systems biology approach to cell signalling will help uncover new layers in our understanding of this essential class of enzymes.

Actin stress fibres

Animal cell movement is effected through a combination of protrusive and contractile events. Non-muscle cells contain stress fibres - bundles of actomyosin that are the major mediators of cell contraction and that can be compared to the highly organised actomyosin arrays of muscle cells. Recent studies have defined regulatory mechanisms that control stress fibre formation, placing the ROCK protein kinase at the centre of a complex signalling network controlling actomyosin contractility and stress fibre assembly. As we uncover the details of stress fibre construction, it is becoming clear that different categories of stress fibres exist. Some of these structures are less suited for cell motility and more suited to static contraction. In keeping with this, many specialised contractile cell types use stress fibres to remodel tissues and extracellular matrix.

Quantitative high-throughput cell-based assays for inhibitors of ROCK kinases

The serine/threonine kinases ROCK1 and ROCK2 are direct targets of activated rho GTPases, and aberrant rho/ROCK signaling has been implicated in a number of human diseases. We have developed novel methods for high-throughput assays of ROCK inhibitors that provide for quantitative evaluation of the ability of small molecules to inhibit the function of ROCK kinases in intact cells. Conditions for extraction of known phosphorylated substrates of ROCK were identified, and the involvement of ROCK in phosphorylation of these substrates was evaluated using small interfering RNA (siRNA). Of the potential substrates tested, MYPT1 was identified as a substrate whose phosphorylation was reduced markedly in the combined absence of ROCK1 and ROCK2 proteins, and ELISA methods were developed to allow quantitative measurement of the degree of phosphorylation of MYPT1 at residue T853 in cells grown in 96-well plates. These methods are amenable to high-throughput assays for identification of ROCK inhibitors within libraries of small molecules and can be used to compare compound potencies to prioritize compounds of interest for additional evaluation. These methods should be useful in drug discovery efforts directed toward identifying potent ROCK inhibitors for potential treatment of cancer, hypertension, or other diseases involving rho/ROCK signaling.

A method for measuring Rho kinase activity in tissues and cells

The Rho-associated kinases (ROCKs) can regulate cell shape and function by modulating the actin cytoskeleton. ROCKs are serine-threonine protein kinases that can phosphorylate adducin, ezrin-radixin-moesin proteins, LIM kinase, and myosin light chain phosphatase. In the cardiovascular system, the RhoA/ROCK pathway has been implicated in angiogenesis, atherosclerosis, cerebral and coronary vasospasm, cerebral ischemia, hypertension, myocardial hypertrophy, and neointima formation after vascular injury. ROCKs consist of two isoforms: ROCK1 and ROCK2. They share overall 65% homology in their amino acid sequence and 92% homology in their amino kinase domains. However, these two isoforms have different subcellular localizations and exert biologically different functions. In particular, ROCK1 appears to be more important for immunological functions, whereas ROCK2 is more important for endothelial and vascular smooth muscle function. Thus, the ability to measure ROCK activity in tissues and cells would be important for understanding mechanisms underlying cardiovascular disease. This chapter describes a method for measuring ROCK activity in peripheral blood, tissues, and cells.

Myosin phosphatase target subunit: Many roles in cell function

Phosphorylation of myosin II is important in many aspects of cell function and involves a myosin kinase, e.g. myosin light chain kinase, and a myosin phosphatase (MP). MP is regulated by the myosin phosphatase target subunit (MYPT1). The domain structure, properties, and genetic analyses of MYPT1 and its isoforms are outlined. MYPT1 binds the catalytic subunit of type 1 phosphatase, delta isoform, and also acts as an interactive platform for many other proteins. A key reaction for MP is with phosphorylated myosin II and the first process shown to be regulated by MP was contractile activity of smooth muscle. In cell division and cell migration myosin II phosphorylation also plays a critical role and these are discussed. However, based on the wide range of partners for MYPT1 it is likely that MP is implicated with substrates other than myosin II. Open questions are whether the diverse functions of MP reflect different cellular locations and/or specific roles for the MYPT1 isoforms.

Rho kinase (ROCK) inhibitors

The Rho kinase (ROCK) isoforms, ROCK1 and ROCK2, were initially discovered as downstream targets of the small GTP-binding protein Rho. Because ROCKs mediate various important cellular functions such as cell shape, motility, secretion, proliferation, and gene expression, it is likely that this pathway will intersect with other signaling pathways known to contribute to cardiovascular disease. Indeed, ROCKs have already been implicated in the regulation of vascular tone, proliferation, inflammation, and oxidative stress. However, it is not entirely clear how ROCKs are regulated, what some of their downstream targets are, and whether ROCK1 and ROCK2 mediate different cellular functions. Clinically, inhibition of ROCK pathway is believed to contribute to some of the cardiovascular benefits of statin therapy that are independent of lipid lowering (ie, pleiotropic effects). To what extent ROCK activity is inhibited in patients on statin therapy is not known, but it may have important clinical implications. Indeed, several pharmaceutical companies are already actively engaged in the development of ROCK inhibitors as the next generation of therapeutic agents for cardiovascular disease because evidence from animal studies suggests the potential involvement of ROCK in hypertension and atherosclerosis.

Flavone inhibits vascular contraction by decreasing phosphorylation of the myosin phosphatase target subunit

1. Flavonoids modulate vascular tone through an endothelium-dependent or -independent mechanism. Although a few mechanisms for endothelium-independent relaxation have been suggested, such as interference with protein kinase C or cAMP or cGMP phosphodiesterase, the inhibition of Ca(2+) release from intracellular stores or Ca(2+) influx from extracellular fluids, the mode of action of flavonoids remains elusive. 2. We hypothesized that treatment with flavone inhibits vascular smooth muscle contraction by decreasing the phosphorylation of the myosin phosphatase target subunit (MYPT1). 3. Rat aortic rings were denuded of endothelium, mounted in organ baths and contracted with U46619, a thromboxane A(2) analogue. 4. Flavone dose-dependently inhibited the U46619-induced contractile response and myosin light chain (MLC(20)) phosphorylation. At 10(-7) mol/L, U46619 induced vascular contraction with the concomitant phosphorylation of MYPT1 at Thr855, but not at Thr697. Incubation with flavone (100 or 300 micromol/L) for 30 min attenuated the phosphorylation of MYPT1(Thr855), but not MYPT1(Thr697). 5. It is concluded that treatment with flavone inhibits vascular smooth muscle contraction by decreasing the phosphorylation of the MYPT1. These results suggest that flavone causes endothelium-independent relaxation through, at least in part, the inhibition of p160 Rho-associated coiled-coil-containing protein kinase (ROCK) signalling.

Interaction between src family kinases and rho-kinase in agonist-induced Ca2+-sensitization of rat pulmonary artery

Aims

We investigated the role of src family kinases (srcFK) in agonist-mediated Ca²⁺-sensitization in pulmonary artery and whether this involves interaction with the rho/rho-kinase pathway.

Methods and results

Intra-pulmonary arteries (IPAs) and cultured pulmonary artery smooth muscle cells (PASMC) were obtained from rat. Expression of srcFK was determined at the mRNA and protein levels. Ca²⁺-sensitization was induced by prostaglandin F_2α (PGF_2α) in α-toxin-permeabilized IPAs. Phosphorylation of the regulatory subunit of myosin phosphatase (MYPT-1) and of myosin light-chain-20 (MLC₂₀) and translocation of rho-kinase in response to PGF_2α were also determined. Nine srcFK were expressed at the mRNA level, including src, fyn, and yes, and PGF_2α enhanced phosphorylation of three srcFK proteins at tyr-416. In α-toxin-permeabilized IPAs, PGF_2α enhanced the Ca²⁺-induced contraction (pCa 6.9) approximately three-fold. This enhancement was inhibited by the srcFK blockers SU6656 and PP2 and by the rho-kinase inhibitor Y27632. Y27632, but not SU6656 or PP2, also inhibited the underlying pCa 6.9 contraction. PGF_2α enhanced phosphorylation of MYPT-1 at thr-697 and thr-855 and of MLC₂₀ at ser-19. This enhancement, but not the underlying basal phosphorylation, was inhibited by SU6656. Y27632 suppressed both basal and PGF_2α-mediated phosphorylation. The effects of SU6656 and Y27632, on both contraction and MYPT-1 and MLC₂₀ phosphorylation, were not additive. PGF_2α triggered translocation of rho-kinase in PASMC, and this was inhibited by SU6656.

Conclusions

srcFK are activated by PGF_2α in the rat pulmonary artery and may contribute to Ca²⁺-sensitization and contraction via rho-kinase translocation and phosphorylation of MYPT-1.

Identification of a novel substrate for TNFalpha-induced kinase NUAK2

TNFalpha has multiple important cellular functions both in normal cells and in tumor cells. To explore the role of TNFalpha, we identified NUAK family, SNF1-like kinase 2 (NUAK2), as a TNFalpha-induced kinase by gene chip analysis. NUAK2 is known to be induced by various cellular stresses and involved in cell mortality, however, its substrate has never been identified. We developed original protocol of de novo screening for kinase substrates using an in vitro kinase assay and high performance liquid chromatography (HPLC). Using this procedure, we identified myosin phosphatase target subunit 1 (MYPT1) as a specific substrate for NUAK2. MYPT1 was phosphorylated at another site(s) by NUAK2, other than known Rho-kinase phosphorylation sites (Thr696 or Thr853) responsible for inhibition of myosin phosphatase activity. These data suggests different phosphorylation and regulation of MYPT1 activity by NUAK2.

cGMP-dependent relaxation of smooth muscle is coupled with the change in the phosphorylation of myosin phosphatase

Nitric oxide/cGMP pathway induces vasodilatation, yet the underlying mechanism is obscure. In the present study, we studied the mechanism of cGMP-induced relaxation of the smooth muscle contractile apparatus using permeabilized rabbit femoral arterial smooth muscle. 8-Br-cGMP-induced relaxation was accompanied with a decrease in myosin light chain (MLC) phosphorylation. MLC phosphatase (MLCP) activity, once decreased by agonist-stimulation, recovered to the resting level on addition of 8-Br-cGMP. Because MLCP activity is regulated by the phosphorylation of a MLCP-specific inhibitor, CPI17 at Thr38 and MBS (myosin binding subunit of MLCP) at Thr696, we examined the effect of 8-Br-cGMP on the phosphorylation of these MLCP modulators. Whereas CPI17 phosphorylation was unchanged after addition of 8-Br-cGMP, MBS phosphorylation at Thr696 was significantly decreased by 8-Br-cGMP. We found that 8-Br-cGMP markedly increased MBS phosphorylation at Ser695 in the fiber pretreated with phenylephrine. MBS phosphorylation of Thr696 phosphorylated MBS at Ser695 partially resumed MLCP activity inhibited by Thr696 phosphorylation. Whereas Ser695 phosphorylation was markedly increased, the extent of diphosphorylated MBS at Ser695 and Thr696 in fibers was unchanged after cGMP-stimulation. We found that MBS phosphatase activity in arteries for both diphosphorylated MBS and monophosphorylated MBS at Thr696 significantly increased by 8-Br-cGMP, whereas MBS kinase activity was unchanged. These results suggest that the phosphorylation at Ser640 induced by cGMP shifted the equilibrium of the Thr641 phosphorylation toward dephosphorylation, thus increasing MLCP activity. This results in the decrease in MLC phosphorylation and smooth muscle relaxation.

Phosphorylation of TIMAP by glycogen synthase kinase-3beta activates its associated protein phosphatase 1

TIMAP (TGF-beta1 inhibited, membrane-associated protein) is a prenylated, endothelial cell-predominant protein phosphatase 1 (PP1c) regulatory subunit that localizes to the plasma membrane of filopodia. Here, we determined whether phosphorylation regulates TIMAP-associated PP1c function. Phosphorylation of TIMAP was observed in cells metabolically labeled with [32P]orthophosphate and was reduced by inhibitors of protein kinase A (PKA) and glycogen synthase kinase-3 (GSK-3). In cell-free assays, immunopurified TIMAP was phosphorylated by PKA and, after PKA priming, by GSK-3beta. Site-specific Ser to Ala substitution identified amino acid residues Ser333/Ser337 as the likely PKA/GSK-3beta phosphorylation site. Substitution of Ala for Val and Phe in the KVSF motif of TIMAP (TIMAPV64A/F66A) abolished PP1c binding and TIMAP-associated PP1c activity. TIMAPV64A/F66A was hyper-phosphorylated in cells, indicating that TIMAP-associated PP1c auto-dephosphorylates TIMAP. Constitutively active GSK-3beta stimulated phosphorylation of TIMAPV64A/F66A, but not wild-type TIMAP, suggesting that the PKA/GSK-3beta site may be subject to dephosphorylation by TIMAP-associated PP1c. Substitution of Asp or Glu for Ser at amino acid residues 333 and 337 to mimic phosphorylation reduced the PP1c association with TIMAP. Conversely, GSK-3 inhibitors augmented PP1c association with TIMAP-PP1c in cells. The 333/337 phosphomimic mutations also increased TIMAP-associated PP1c activity in vitro and against the non-integrin laminin receptor 1 in cells. Finally, TIMAP mutants with reduced PP1c activity strongly stimulated endothelial cell filopodia formation, an effect mimicked by the GSK-3 inhibitor LiCl. We conclude that TIMAP is a target for PKA-primed GSK-3beta-mediated phosphorylation. This phosphorylation controls TIMAP association and activity of PP1c, in turn regulating extension of filopodia in endothelial cells.

The regulation of smooth muscle contractility by zipper-interacting protein kinase

Smooth muscle contractility is mainly regulated by phosphorylation of the 20 kDa myosin light chains (LC20), a process that is controlled by the opposing activities of myosin light chain kinase (MLCK) and myosin light chain phosphatase (MLCP). Recently, intensive research has revealed that various protein kinase networks including Rho-kinase, integrin-linked kinase, zipper-interacting protein kinase (ZIPK), and protein kinase C (PKC) are involved in the regulation of LC20 phosphorylation and have important roles in modulating smooth muscle contractile responses to Ca2+ (i.e., Ca2+ sensitization and Ca2+ desensitization). Here, we review the general background and structure of ZIPK and summarize our current understanding of its involvement in a number of cell processes including cell death (apoptosis), cell motility, and smooth muscle contraction. ZIPK has been found to induce the diphosphorylation of LC20 at Ser-19 and Thr-18 in a Ca2+-independent manner and to regulate MLCP activity directly through its phosphorylation of the myosin-targeting subunit of MLCP or indirectly through its phosphorylation of the PKC-potentiated inhibitory protein of MLCP. Future investigations of ZIPK function in smooth muscle will undoubtably focus on determining the mechanisms that regulate its cellular activity, including the identification of upstream signaling pathways, the characterization of autoinhibitory domains and regulatory phosphorylation sites, and the development of specific inhibitor compounds.

Current topics in the regulatory mechanism underlying the Ca2+ sensitization of the contractile apparatus in vascular smooth muscle

The Ca2+ signal is the primary determinant of the contraction of the vascular smooth muscle. However, the alteration of the Ca2+ sensitivity of the contractile apparatus also plays an essential role. The regulation of the myosin light chain phosphatase (MLCP) activity is considered to be the most important mechanism underlying the regulation of Ca2+ sensitivity. The investigations during the last 15 years have identified many proteins that participate in the regulation of the MLCP activity. Recently, the Ca2+ signal has also been shown to cross-talk with the mechanisms regulating the Ca2+ sensitivity. Consequently, Rho kinase, protein kinase C, CPI-17, and MYPT1 have all been suggested to play a physiologically important role in the regulation of the MLCP activity. We are now close to elucidating the major rules regulating the MLCP activity and the Ca2+ sensitivity during vascular contractions. This article will give an overview of the current understanding of the biochemical basis for the regulation of the MLCP activity, while also discussing their functional roles from a physiological point of view. I hope this article will help to develop new pharmacological strategies for the prevention and treatment of the pathological vasoconstriction often seen in vascular diseases.

Heat shock augments myosin phosphatase target-subunit phosphorylation

Our previous study demonstrated that heat shock augmented vascular contraction. In the present study, we hypothesized that heat shock augments myosin phosphatase target-subunit (MYPT1) phosphorylation resulting in augmented vascular contraction. Endothelium-denuded rat aortic rings were mounted in organ baths, exposed to heat shock (42 degrees C for 45 min), and subjected to contraction 4 h after the heat shock followed by Western blot analysis for MLC(20) (the 20 kDa light chains of myosin II) or MYPT1. The contractile responses in both control and heat shock-treated aorta were inhibited by Y27632, an inhibitor of Rho-kinase. The level of the MLC(20) and MYPT1(Thr855) phosphorylation in response to KCl was higher in heat shock-treated aorta than that in timed-control. The increased MYPT1(Thr855) phosphorylation was inhibited by Y27632 (1.0 microM) in parallel with inhibition of MLC(20) phosphorylation and vascular contraction. These results indicate that heat shock augments MYPT1 phosphorylation resulting in augmented vascular contraction.

ROCK1 phosphorylates and activates zipper-interacting protein kinase

Zipper-interacting protein kinase (ZIPK) regulates Ca(2+)-independent phosphorylation of both smooth muscle (to regulate contraction) and non-muscle myosin (to regulate non-apoptotic cell death) through either phosphorylation and inhibition of myosin phosphatase, the myosin phosphatase inhibitor CPI17, or direct phosphorylation of myosin light chain. ZIPK is regulated by multisite phosphorylation. Phosphorylation at least three sites Thr-180, Thr-225, and Thr-265 has been shown to be essential for full activity, whereas phosphorylation at Thr-299 regulates its intracellular localization. Herein we utilized an unbiased proteomics screen of smooth muscle extracts with synthetic peptides derived from the sequence of the regulatory phosphorylation sites of the enzyme to identify the protein kinases that might regulate ZIPK activity in vivo. Discrete kinase activities toward Thr-265 and Thr-299 were defined and identified by mass spectrometry as Rho kinase 1 (ROCK1). In vitro, ROCK1 showed a high degree of substrate specificity toward native ZIPK, both stoichiometrically phosphorylating the enzyme at Thr-265 and Thr-299 as well as bringing about activation. In HeLa cells, coexpression of ZIPK with ROCK1 altered the ROCK-induced phenotype of focused stress fiber pattern to a Rho-like phenotype of parallel stress fiber pattern. This effect was also dependent upon phosphorylation at Thr-265. Our findings provide a new regulatory pathway in smooth muscle and non-muscle cells whereby ROCK1 phosphorylates and regulates ZIP kinase.

Regulation of myosin II dynamics by phosphorylation and dephosphorylation of its light chain in epithelial cells

Nonmuscle myosin II, an actin-based motor protein, plays an essential role in actin cytoskeleton organization and cellular motility. Although phosphorylation of its regulatory light chain (MRLC) is known to be involved in myosin II filament assembly and motor activity in vitro, it remains unclear exactly how MRLC phosphorylation regulates myosin II dynamics in vivo. We established clones of Madin Darby canine kidney II epithelial cells expressing MRLC-enhanced green fluorescent protein or its mutants. Time-lapse imaging revealed that both phosphorylation and dephosphorylation are required for proper dynamics of myosin II. Inhibitors affecting myosin phosphorylation and MRLC mutants indicated that monophosphorylation of MRLC is required and sufficient for maintenance of stress fibers. Diphosphorylated MRLC stabilized myosin II filaments and was distributed locally in regions of stress fibers where contraction occurs, suggesting that diphosphorylation is involved in the spatial regulation of myosin II assembly and contraction. We further found that myosin phosphatase or Zipper-interacting protein kinase localizes to stress fibers depending on the activity of myosin II ATPase.

Sphingosine 1-phosphate causes airway hyper-reactivity by rho-mediated myosin phosphatase inactivation

In the present study, we investigated whether extracellular sphingosine 1-phosphate (S1P) is involved in airway hyper-reactivity in bronchial asthma. The effects of S1P on the response to methacholine was examined in the fura-2-loaded strips of guinea pig tracheal smooth muscle using simultaneous recording of the isometric tension and the ratio of fluorescence intensities at 340 and 380 nm (F(340)/F(380)). A 15-min pretreatment with S1P (>100 nM) markedly enhanced methacholine-induced contraction without elevating F(340)/F(380). This effect of S1P was suppressed in the presence of Y-27632 [(R)-(+)-trans-N-(4-pyridyl)-4-(1-aminoethyl)-cyclohexane-carboxamide], a selective inhibitor of Rho-kinase, in a concentration-dependent manner. Moreover, pretreatment with pertussis toxin caused an inhibition in S1P-induced hyper-reactivity to methacholine in a time- and concentration-dependent manner. In contrast, although S1P-induced Ca(2+) mobilization was attenuated by SKF96365 and verapamil, the subsequent response to methacholine was unaffected. A 15-min pretreatment with lower concentrations of S1P (<100 nM), which is clinically attainable, did not increase methacholine-induced contraction. However, when the incubation was lengthened to 6 h, S1P (<100 nM) enhanced the subsequent response to methacholine. Next, application of S1P to cultured human bronchial smooth muscle cells increased the proportion of active RhoA (GTP-RhoA) and phosphorylation of myosin phosphatase target subunit 1 (MYPT1). This phosphorylation of MYPT1 was significantly inhibited by application of Y-27632 and by pretreatment with pertussis toxin. Our findings demonstrate that exposure of airway smooth muscle to S1P results in airway hyper-reactivity mediated by Ca(2+) sensitization via inactivation of myosin phosphatase, which links G(i) and RhoA/Rho-kinase processes.

PITK, a PP1 targeting subunit that modulates the phosphorylation of the transcriptional regulator hnRNP K

Protein phosphatase-1 (PP1), through interactions with substrate targeting subunits, plays critical roles in the regulation of numerous cellular processes. Herein, we describe a newly identified regulatory subunit (PITK; Phosphatase Interactor Targeting K protein) that specifically targets the catalytic subunit of PP1 to nuclear foci to selectively bind and dephosphorylate the transcriptional regulator heterogeneous nuclear ribonucleoprotein K (hnRNP K) at a regulatory S284 site. Additionally, PITK is phosphorylated in vivo at S1013 and S1017, residues that flank or reside within the PP1C-binding motif, and this phosphorylation negatively regulates the binding of the phosphatase to PITK. A mutant variant, S1013,1017A-PITK, when expressed in intact cells, exhibited an increase in native PP1 binding and elicited a more profound dephosphorylation of hnRNPK at S284. A global analysis of transcription by Affymetrix microarray revealed that the expression of PITK resulted in the altered expression of 47 genes, including a marked induction of MEK5 (>14-fold, p<0.007). Additionally, the effects of PITK and S1013,1017A-PITK on transcription could be modulated by the co-expression of hnRNP K. Taken together, our findings provide a putative mechanism by which transcriptional activity of hnRNP K can be discretely controlled through the regulation of PP1 activity.

PHI-1 interacts with the catalytic subunit of myosin light chain phosphatase to produce a Ca(2+) independent increase in MLC(20) phosphorylation and force in avian smooth muscle

In avian smooth muscles, GTPgammaS produces a Rho kinase mediated increase in PHI-1 phosphorylation and force, but whether this correlation is causal is unknown. We examined the effect of phosphorylated PHI-1 (P-PHI-1) on force and myosin light chain (MLC(20)) phosphorylation at a constant [Ca(2+)]. P-PHI-1, but not PHI-1, increased MLC(20) phosphorylation and force, and phosphorylation of PHI-1 increased the interaction of PHI-1 with PP1c. Microcystin induced a dose-dependent reduction in the binding of PHI-1 to PP1c. These results suggest PHI-1 inhibits myosin light chain phosphatase by interacting with the active site of PP1c to produce a Ca(2+) independent increase in MLC(20) phosphorylation and force.

Rho-kinase-mediated regulation of receptor-agonist-stimulated smooth muscle contraction

Rho kinase was shown to regulate smooth muscle contraction through modulating myosin phosphatase (MLCP) activity, but the in vivo mechanism remains to be clarified. This study examined the effects of Rho kinase inhibition on the phosphorylation time course of MLCP subunit MYPT1 at Thr697 and Thr855 and MLCP inhibitory protein CPI-17 at Thr38 and on actin polymerization during the contraction of rat tail artery (RTA) smooth muscle. Rho kinase inhibitor Y27632 suppressed force activated by alpha(1)-adrenergic agonist phenylephrine or thromboxane A(2) analog U46619 with concomitant decreases in MLC(20) phosphorylation. Phenylephrine and U46619 significantly increased MYPT1(Thr855) phosphorylation that was eliminated by Y27632 pretreatment, whereas MYPT1(Thr697) phosphorylation was not stimulated. Phenylephrine increased CPI-17(Thr38) phosphorylation that was not inhibited by Y27632 but was abolished by a protein kinase C inhibitor Ro 31-8220; in contrast, U46619 did not stimulate CPI-17 phosphorylation. Both agonists increased actin polymerization that was diminished by Y27632 under phenylephrine but not U46619 activation. These results demonstrated a temporal correlation between MYPT1(Thr855) phosphorylation, MLC(20) phosphorylation, and contraction in a Rho-kinase-dependent manner for both phenylephrine and U46619 stimulation, suggesting that Rho kinase regulates MLCP activity through MYPT1(Thr855) phosphorylation during RTA smooth muscle contraction. Furthermore, Rho kinase regulates actin polymerization activated by alpha(1)-adrenoceptors but is less significant in thromboxane receptor stimulation.

ROCKs as therapeutic targets in cardiovascular diseases

There is growing evidence that Rho-kinases (ROCKs), the immediate downstream targets of the small guanosine triphosphate-binding protein Rho, may contribute to cardiovascular disease. ROCKs play a central role in diverse cellular functions such as smooth muscle contraction, stress fiber formation and cell migration and proliferation. Overactivity of ROCKs is observed in cerebral ischemia, coronary vasospasm, hypertension, vascular inflammation, arteriosclerosis and atherosclerosis. ROCKs, therefore, may be an important and still relatively unexplored therapeutic target in cardiovascular disease. Recent experimental and clinical studies using ROCK inhibitors such as Y-27632 and fasudil have revealed a critical role of ROCKs in embryonic development, inflammation and oncogenesis. This review will focus on the potential role of ROCKs in cellular functions and discuss the prospects of ROCK inhibitors as emerging therapy for cardiovascular diseases.

Effects of calpain and Rho-kinase inhibitors on the acrosome reaction and motility of fowl spermatozoa in vitro

At the avian body temperature of 40 degrees C, intact fowl spermatozoa require Ca(2+) for the initiation of motility and a combination of both Ca(2+) and homogenized inner perivitelline layer (IPVL) together to induce the acrosome reaction. Within the range of 1-100 micromol/l, neither PD 150606 (a Ca(2+)-dependent calpain inhibitor) nor Y-27632 (an inhibitor of Ca(2+)-dependent Rho-kinase) were able to inhibit the acrosome reaction induced by the presence of Ca(2+) and IPVL. However, PD 150606, although not Y-27632, was able to inhibit sperm motility initiated by Ca(2+), as well as motility initiated by calyculin A -- a specific inhibitor of protein phosphatases, which also initiates sperm motility at 40 degrees C. The addition of PD 150606 did not reduce the ATP concentrations of intact spermatozoa, nor the motility of demembranated spermatozoa. Immunoblot analysis of sperm extract using a polyclonal antibody against calpain 12 revealed a cross-reacting protein of approximately 80 kDa. These results suggest that Rho-kinase is not involved in the regulation of the acrosome reaction or of motility in fowl spermatozoa. In contrast, calpain appears to be involved in the regulation of flagellar movement, but not izn that of the acrosome reaction. Furthermore, it seems that endogenous calpain is present in the cytoplasmic matrix and/or the plasma membrane, but not retained in the axoneme and/or accessory cytoskeletal components.

Agonist- and depolarization-induced signals for myosin light chain phosphorylation and force generation of cultured vascular smooth muscle cells

Phosphorylation of myosin light chain (MLC) and contraction of differentiated smooth muscle cells in vascular walls are regulated by Ca2+ -dependent activation of MLC kinase, and by Rho-kinase- or protein-kinases-C-dependent inhibition of MLC phosphatase (MLCP). We examined regulatory pathways for MLC kinase and MLCP in cultured vascular smooth muscle cells (VSMCs), and for isometric force generation of VSMCs reconstituted in collagen fibers. Protein levels of RhoA, Rho-kinase and MYPT1 (a regulatory subunit of MLCP) were upregulated in cultured VSMCs, whereas a MLCP inhibitor protein, CPI-17, was downregulated. Endothelin-1 evoked a steady rise in levels of Ca2+, MLC phosphorylation and the contractile force of VSMCs, whereas angiotensin-II induced transient signals. Also, Thr853 phosphorylation of MYPT1 occurred in response to stimuli, but neither agonist induced phosphorylation of MYPT1 at Thr696. Unlike fresh aortic tissues, removal of Ca2+ or addition of voltage-dependent Ca2+ -channel blocker did not inhibit contractions of reconstituted VSMC fibers induced by agonists or even high concentrations of extracellular K+ ions. Inhibitors of Ins(1,4,5)P3-receptor and Rho-kinase antagonized agonist-induced or high-K+ -induced contraction in both reconstituted fibers and fresh tissues. These results indicate that both Ins(1,4,5)P3-induced Ca2+ release and Rho-kinase-induced MYPT1 phosphorylation at Thr853 play pivotal roles in MLC phosphorylation of cultured VSMCs where either Ca2+ -influx or CPI-17-MLCP signaling is downregulated.

Physiological role of ROCKs in the cardiovascular system

Rho-associated kinases (ROCKs), the immediate downstream targets of RhoA, are ubiquitously expressed serine-threonine protein kinases that are involved in diverse cellular functions, including smooth muscle contraction, actin cytoskeleton organization, cell adhesion and motility, and gene expression. Recent studies have shown that ROCKs may play a pivotal role in cardiovascular diseases such as vasospastic angina, ischemic stroke, and heart failure. Indeed, inhibition of ROCKs by statins or other selective inhibitors leads to the upregulation and activation of endothelial nitric oxide synthase (eNOS) and reduction of vascular inflammation and atherosclerosis. Thus inhibition of ROCKs may contribute to some of the cholesterol-independent beneficial effects of statin therapy. Currently, two ROCK isoforms have been identified, ROCK1 and ROCK2. Because ROCK inhibitors are nonselective with respect to ROCK1 and ROCK2 and also, in some cases, may be nonspecific with respect to other ROCK-related kinases such as myristolated alanine-rich C kinase substrate (MARCKS), protein kinase A, and protein kinase C, the precise role of ROCKs in cardiovascular disease remains unknown. However, with the recent development of ROCK1- and ROCK2-knockout mice, further dissection of ROCK signaling pathways is now possible. Herein we review what is known about the physiological role of ROCKs in the cardiovascular system and speculate about how inhibition of ROCKs could provide cardiovascular benefits.

Convergence of Ca2+-desensitizing mechanisms activated by forskolin and phenylephrine pretreatment, but not 8-bromo-cGMP

Contractile stimuli can sensitize myosin to Ca2+ by activating RhoA kinase (ROK) and PKC that inhibit myosin light chain phosphatase (MLCP) activity. Relaxant stimuli, acting through PKA and PKG (cyclic nucleotide-dependent protein kinases), and pretreatment with contractile agents such as phenylephrine (PE), can desensitize myosin to Ca2+. It is unknown precisely how these stimuli cause Ca2+ desensitization. To test the hypothesis that PKA, PKG, and PE pretreatment signaling systems converge to cause relaxation by inhibition of ROK in intact, isolated tissues, we examined the effects of forskolin (FSK; PKA activation), 8-bromo-cGMP (8br-cGMP; PKG activation), and PE pretreatment on KCl-induced force maintenance in rabbit arteries, a response nearly completely dependent on ROK activation. PE pretreatment and agents activating PKA and PKG caused Ca2+ desensitization by inhibiting KCl-induced tonic force and MLC phosphorylation without inhibiting intracellular [Ca2+]. At pCa 5 in beta-escin-permeabilized muscle, FSK and 8b-cGMP accelerated the relaxation rate when tissues were returned to pCa 9, suggesting that both agents can elevate MLCP activity. However, a component of the Ca2+ desensitization attributed to PKG activation in intact tissues appeared to involve a MLC phosphorylation-independent component. Inhibition of KCl-induced tonic force by the ROK inhibitor, Y-27632, and by PE pretreatment, were synergistically potentiated by 8b-cGMP, but not FSK. FSK and PE pretreatment, but not 8b-cGMP, inhibited the KCl-induced increase in site-specific myosin phosphatase target protein-1 phosphorylation at Thr853. These data support the hypothesis that PKA and PE pretreatment converge on a common Ca2+-desensitization pathway, but that PKG can act by a mechanism different from that activated by PKA and PE pretreatment.

RhoA/Rho-kinase in erectile tissue: mechanisms of disease and therapeutic insights

Penile erection is a complicated event involving the regulation of corpus cavernosal smooth muscle tone. Recently, the small monomeric G-protein RhoA and its downstream effector Rho-kinase have been proposed to be important players for mediating vasoconstriction in the penis. RhoA/Rho-kinase increases MLC (myosin light chain) phosphorylation through inhibition of MLCP (MLC phosphatase) thereby increasing Ca2+ sensitivity. This review will outline the RhoA/Rho-kinase signalling pathway, including the upstream regulators, guanine nucleotide exchange factors, GDP dissociation inhibitors and GTPase-activating proteins. We also summarize the current knowledge about the physiological roles of RhoA/Rho-kinase in both male and female erectile tissues and its aberrations contributing to erectile dysfunction in several disease states. Understanding the RhoA/Rho-kinase signalling pathway in the regulation of erection is important for the development of therapeutic interventions for erectile dysfunction.

RhoA- and PKC-alpha-mediated phosphorylation of MYPT and its association with HSP27 in colonic smooth muscle cells

Agonist-induced activation of the RhoA/Rho kinase (ROCK) pathway results in inhibition of myosin phosphatase and maintenance of myosin light chain (MLC20) phosphorylation. We have shown that RhoA/ROCKII translocates and associates with heat shock protein (HSP)27 in the particulate fraction. We hypothesize that inhibition of the 130-kDa regulatory myosin-binding subunit (MYPT) requires its association with HSP27 in the particulate fraction. Furthermore, it is not certain whether regulation of MYPT by CPI-17 or by ROCKII is due to cross talk between RhoA and PKC-alpha. Presently, we examined the cross talk between RhoA and PKC-alpha in the regulation of MYPT phosphorylation in rabbit colon smooth muscle cells. Acetylcholine induced 1) sustained phosphorylation of PKC-alpha, CPI-17, and MYPT; 2) an increase in the association of phospho-MYPT with HSP27 in the particulate fraction; 3) a decrease in myosin phosphatase activity (66.21+/-3.52 and 42.19+/-3.85% nM/ml lysate at 30 s and 4 min); and 4) an increase in PKC activity (298.12+/-46.60% and 290.59+/-22.07% at 30 s and 4 min). Inhibition of RhoA/ROCKII by Y-27632 inhibited phosphorylation of MYPT and its association with HSP27. Both Y27632 and a negative dominant construct of RhoA inhibited phosphorylation of MYPT and CPI-17. Inhibition of PKCs or calphostin C or selective inhibition of PKC-alpha by negative dominant constructs inhibited phosphorylation of MYPT and CPI-17. The results suggest that 1) acetylcholine induces activation of both RhoA and/or PKC-alpha pathways, suggesting cross talk between RhoA and PKC-alpha resulting in phosphorylation of MYPT, inhibition of myosin phosphatase activity, and maintenance of MLC phosphorylation; and 2) phosphorylated MYPT is associated with HSP27 and translocated to the particulate fraction, suggesting a scaffolding role for HSP27 in mediating the association of the complex MYPT/RhoA-ROCKII. Thus both pathways (PKC and RhoA) converge on the regulation of myosin phosphatase activities and modulate sustained phosphorylation of MLC20.

ZIP kinase, a key regulator of myosin protein phosphatase 1

Two major physiological roles have been defined for zipper interacting protein kinase (ZIPK), regulation of apoptosis in non-muscle cells and regulation of Ca(2+) sensitization in smooth muscle. Although much attention has focused on the role of ZIPK in the regulation of apoptotic events, its roles in smooth muscle are likely to have equal if not greater physiological relevance. We first identified ZIPK as a major protein kinase controlling the phosphorylation of myosin phosphatase (SMPP-1M) and the inhibitor protein CPI17 in smooth muscle. Phosphorylation of SMPP-1M and CPI17 by ZIPK inhibits phosphatase activity towards myosin and causes profound Ca(2+) sensitization and contraction in smooth muscle. ZIPK will also directly phosphorylate both muscle and non-muscle myosin. The highly selective actions of ZIPK in the control of myosin phosphorylation potentially make the enzyme an ideal candidate for the development of novel therapeutics to treat smooth muscle related disorders such as hypertension or asthma.

Regulation of pulmonary arterial myosin phosphatase activity in neonatal circulatory transition and in hypoxic pulmonary hypertension: a role for CPI-17

Neonatal circulatory transition is dependent upon tightly regulated pulmonary circuit relaxation. Persistent pulmonary hypertension of the newborn (PPHN) is characterized by pulmonary arterial myocyte relaxation failure. We examined the effect of short course (72 hour) in vivo normobaric hypoxia in newborn swine on smooth muscle contractile enzyme activity and regulatory phosphoprotein abundance, in tissue homogenates of 2nd to 4th generation pulmonary arteries. Myosin light chain kinase (MLCK) and phosphatase (MLCP) protein contents were unchanged in hypoxic pulmonary arteries compared to controls. MLCP activity increased in normoxic animals from birth to day 3. This was ablated by hypoxia; phosphatase activity, measured as in vitro myosin light chain dephosphorylation, was decreased significantly (P < 0.005) in the hypoxic group. Inhibitory site phosphorylations of MLCP myosin binding subunit at threonines 696 and 850 were similar in both hypoxic and normoxic subjects, suggesting that downregulation of MLCP in hypoxia does not involve this pathway. However, content of regulatory protein CPI-17 (protein kinase C-related phosphatase inhibitor) increased from birth in hypoxic subjects (P < 0.05); active (phosphorylated) CPI-17 protein abundance declined after birth in normals, but increased in hypoxic arteries (P < 0.05). This corresponded with the decrease in phosphatase activity. We speculate that CPI-17 may play a role in myosin phosphatase upregulation during neonatal circulatory transition, and in hypoxic inhibition of pulmonary phosphatase activity in PPHN.

Regulation of myosin light chain phosphatase and pulmonary arterial relaxation

Neonatal circulatory transition is dependent upon tightly regulated pulmonary circuit relaxation. Persistent pulmonary hypertension (PPHN), a rapidly progressive disease of pulmonary arterial vasospasm and remodelling, may be characterized by pulmonary arterial myocyte relaxation failure. A key regulator of vascular tone is myocyte calcium sensitivity, determined by the relative stoichiometry of myosin light chain phosphorylation and dephosphorylation. We have recently reported downregulation of myosin light chain phosphatase activity in a hypoxic model of neonatal pulmonary hypertension. This review examines the recognized pathways of regulation governing myosin light chain phosphatase activity, including targeting subunit isoform switching, targeting unit phosphorylation and catalytic site inhibition. In light of the reviewed literature, further speculation is proposed on the potential contributions of these mechanisms to the pathophysiology of the perinatal pulmonary arterial relaxation defect in PPHN.Key words: smooth muscle, pulmonary hypertension, myosin light chain phosphatase, CPI-17, MYPT, review.

PHI-1 induced enhancement of myosin phosphorylation in chicken smooth muscle

Herein, we provide evidence that in chicken smooth muscle, G-protein stimulation by a Rho-kinase pathway leads to an increase in myosin light chain phosphorylation. Additionally, G-protein stimulation did not increase MYPT1 phosphorylation at Thr695 or Thr850, and CPI-17, was not expressed in chicken smooth muscle. However, PHI-1 was present in chicken smooth muscle tissues. Both agonist and GTP(gamma)S stimulation result in an increase in PHI-1 phosphorylation, which is inhibited by inhibitors to both Rho-kinase (Y-27632) and (PKC) GF109203x. These data suggest that PHI-1 may act as a CPI-17 analog in chicken smooth muscle and inhibit myosin phosphatase activity during G-protein stimulation to produce Ca2+ sensitization.

Thromboxane A2-induced contraction of rat caudal arterial smooth muscle involves activation of Ca2+ entry and Ca2+ sensitization: Rho-associated kinase-mediated phosphorylation of MYPT1 at Thr-855, but not Thr-697

The signal transduction pathway whereby the TxA2 (thromboxane A2) mimetic U-46619 activates vascular smooth muscle contraction was investigated in de-endothelialized rat caudal artery. U-46619-evoked contraction was inhibited by the TP receptor (TxA2 receptor) antagonist SQ-29548, the ROK (Rho-associated kinase) inhibitors Y-27632 and H-1152, the MLCK (myosin light-chain kinase) inhibitors ML-7, ML-9 and wortmannin, the voltagegated Ca2+-channel blocker nicardipine, and removal of extracellular Ca2+; the protein kinase C inhibitor GF109203x had no effect. U-46619 elicited Ca2+ sensitization in a-toxin-permeabilized tissue. U-46619 induced activation of the small GTPase RhoA, consistent with the involvement of ROK. Two downstream targets of ROK were investigated: CPI-17 [protein kinase C-potentiated inhibitory protein for PP1 (protein phosphatase type 1) of 17 kDa], a myosin light-chain phosphatase inhibitor, was not phosphorylated at the functional site (Thr-38); phosphorylation of MYPT1 (myosin-targeting subunit of myosin light-chain phosphatase) was significantly increased at Thr-855, but not Thr-697. U-46619-evoked contraction correlated with phosphorylation of the 20 kDa light chains of myosin. We conclude that: (i) U-46619 induces contraction via activation of the Ca2+/calmodulin/MLCK pathway and of the RhoA/ROK pathway; (ii) Thr-855 of MYPT1 is phosphorylated by ROK at rest and in response to U-46619 stimulation; (iii) Thr-697 of MYPT1 is phosphorylated by a kinase other than ROK under resting conditions, and is not increased in response to U-46619 treatment; and (iv) neither ROK nor protein kinase C phosphorylates CPI-17 in this vascular smooth muscle in response to U-46619.

The Rho kinases I and II regulate different aspects of myosin II activity

The homologous mammalian rho kinases (ROCK I and II) are assumed to be functionally redundant, based largely on kinase construct overexpression. As downstream effectors of Rho GTPases, their major substrates are myosin light chain and myosin phosphatase. Both kinases are implicated in microfilament bundle assembly and smooth muscle contractility. Here, analysis of fibroblast adhesion to fibronectin revealed that although ROCK II was more abundant, its activity was always lower than ROCK I. Specific reduction of ROCK I by siRNA resulted in loss of stress fibers and focal adhesions, despite persistent ROCK II and guanine triphosphate-bound RhoA. In contrast, the microfilament cytoskeleton was enhanced by ROCK II down-regulation. Phagocytic uptake of fibronectin-coated beads was strongly down-regulated in ROCK II-depleted cells but not those lacking ROCK I. These effects originated in part from distinct lipid-binding preferences of ROCK pleckstrin homology domains. ROCK II bound phosphatidylinositol 3,4,5P(3) and was sensitive to its levels, properties not shared by ROCK I. Therefore, endogenous ROCKs are distinctly regulated and in turn are involved with different myosin compartments.

Regulation of myosin II during cytokinesis in higher eukaryotes

Cellular myosin II is the principal motor responsible for cytokinesis. In higher eukaryotes, phosphorylation of the regulatory light chain (MLC) of myosin II is a primary means of activating myosin II and is known to be crucial for the execution of cell division. Because signals transmitted by the mitotic spindle coordinate key spatial and temporal aspects of cytokinesis, such signals should ultimately function to activate myosin II. Thus, it follows that identification of regulatory factors involved in MLC phosphorylation should elucidate the nature of spindle-derived regulatory signals and lead to a model for how they control cytokinesis. However, the identity of these upstream molecules remains elusive. This review (which is part of the Cytokinesis series) summarizes current views of the regulatory pathway controlling MLC phosphorylation and features four candidate molecules that are likely immediate upstream myosin regulators. I discuss proposed functions for MLCK, ROCK, citron kinase and myosin phosphatase during cytokinesis and consider the possibility of a link between these molecules and the signals transmitted by the mitotic spindle.

Fibronectin's central cell-binding domain supports focal adhesion formation and Rho signal transduction

Fibroblast adhesion to fibronectin (FN) induces formation of focal adhesions (FAs), structures that have significant effect on cell migration and signaling. FA formation requires actomyosin-based contractility that is regulated by Rho-dependent myosin light chain (MLC) phosphorylation. Previous studies indicated that the FN central cell-binding (and integrin-binding) domain (CBD) is insufficient for FA formation and that the major heparin-binding domain (HepII) facilitates FA formation in a Rho-dependent manner. We describe here conditions under which FN CBD alone is sufficient for FA formation in both human dermal fibroblasts and the FN-null murine fibroblasts. CBD-mediated FA formation is dependent on its surface adsorption and the adhesion activity of the cells. Attachment of FN-null fibroblasts to CBD elicits the same biphasic regulation of Rho activity as seen on intact FN, whereas adhesion to HepII alone does not activate Rho. Activation of Rho requires high levels of integrin occupancy. However, FN or CBD may induce FAs without increased activation of Rho (i.e. the basal level of GTP-Rho induces sufficient phospho-MLC for FA assembly under this condition). In contrast, adhesion to HepII alone does not sustain MLC phosphorylation. Pulse stimulation of cells on CBD or HepII with lysophosphatidic acid elevates Rho GTP loading to the same level, but the lysophosphatidic acid-stimulated MLC phosphorylation is significantly lower in cells on HepII than on CBD. Coating HepII with suboptimal concentrations of CBD induces FAs without increased activation of Rho. Therefore, FN CBD can support FA formation and generate contraction by activating Rho or by facilitating Rho downstream signaling.

Rho-kinase mediates hyperglycemia-induced plasminogen activator inhibitor-1 expression in vascular endothelial cells

BACKGROUND

Elevated levels of plasminogen activator inhibitor-1 (PAI-1) are associated with myocardial infarction and stroke, especially in patients with diabetes. The induction of PAI-1 expression by hyperglycemia involves oxidative stress and protein kinase C (PKC). However, the mechanism by which hyperglycemia increases PAI-1 expression is unknown.

METHODS AND RESULTS

Compared with normoglycemia, exposure of human endothelial cells to hyperglycemia, but not mannitol, increased Rho-kinase activity in a time- and concentration-dependent manner. This increase was inhibited by a PKC inhibitor, GF109203X, and antioxidants N-acetylcysteine (NAC) and reduced form of glutathione (GSH). This correlated with inhibition of hyperglycemia-induced PAI-1 expression by GF109203X, NAC, and GSH. Hyperglycemia-increased PAI-1 mRNA and protein levels were inhibited by Rho-kinase inhibitors hydroxyfasudil and Y27632 and by a dominant-negative mutant of Rho-kinase. The mechanism for this inhibition occurs at the level of gene transcription because Rho-kinase inhibitors repress hyperglycemia-stimulated PAI-1 promoter activity without affecting mRNA stability. Hyperglycemia failed to stimulate Rho-kinase activity and PAI-1 expression in heterozygous ROCK I-knockout murine endothelial cells.

CONCLUSIONS

Hyperglycemia stimulates Rho-kinase activity via PKC- and oxidative stress-dependent pathways, leading to increased PAI-1 gene transcription. These results suggest that inhibition of ROCK I may be a novel therapeutic target for preventing thromboembolic complications of diabetes and cardiovascular disease.

Activation of Rho-associated kinase during augmented contraction of the basilar artery to serotonin after subarachnoid hemorrhage

Delayed cerebral vasospasm after subarachnoid hemorrhage (SAH) may be due, in part, to altered regulation of arterial smooth muscle contraction. Contraction of cerebral arteries to serotonin is augmented after experimental SAH. We hypothesized that activation of Rho-associated kinase (Rho kinase) contributes to augmented contraction of cerebral arteries to serotonin after SAH. Autologous arterial blood (SAH) or artificial cerebrospinal fluid (control) was injected into the cisterna magna of anesthetized rabbits. At 2 days after injection, the basilar artery was excised and isometric contraction of arterial rings was recorded. Maximum contraction of the basilar artery to serotonin was augmented about fourfold in SAH compared with control rabbits ( P < 0.01). Contraction to histamine was similar in the two groups. Fasudil hydrochloride (3 μmol/l), an inhibitor of Rho kinase, markedly attenuated serotonin-induced contraction. Fasudil had little effect on contractions induced by histamine or phorbol 12,13-dibutyrate. In addition, phosphorylation of myosin phosphatase, a major target of Rho kinase in regulation of smooth muscle contraction, in the basilar artery was examined by Western blotting. In basilar arteries of SAH, but not control, rabbits, serotonin increased phosphorylation of myosin phosphatase about twofold at Thr853 of the myosin-targeting subunit. These results suggest that enhanced activation of Rho kinase contributes to augmented contraction of the basilar artery to serotonin after SAH.

RhoA-Rho kinase pathway mediates thrombin- and U-46619-induced phosphorylation of a myosin phosphatase inhibitor, CPI-17, in vascular smooth muscle cells

Protein kinase C-potentiated phosphatase inhibitor of 17 kDa (CPI-17) mediates some agonist-induced smooth muscle contraction by suppressing the myosin phosphatase in a phosphorylation-dependent manner. The physiologically relevant kinases that phosphorylate CPI-17 remain to be identified. Several previous studies have shown that some agonist-induced CPI-17 phosphorylation in smooth muscle tissues was attenuated by the Rho kinase (ROCK) inhibitor Y-27632, suggesting that ROCK is involved in agonist-induced CPI-17 phosphorylation. However, Y-27632 has recently been found to inhibit protein kinase C (PKC)-delta, a well-recognized CPI-17 kinase. Thus the role of ROCK in agonist-induced CPI-17 phosphorylation remains uncertain. The present study was designed to address this important issue. We selectively activated the RhoA pathway using inducible adenovirus-mediated expression of a constitutively active mutant RhoA (V14RhoA) in primary cultured rabbit aortic vascular smooth muscle cells (VSMCs). V14RhoA caused expression level-dependent CPI-17 phosphorylation at Thr38 as well as myosin phosphatase phosphorylation at Thr853. Importantly, we have shown that V14RhoA-induced CPI-17 phosphorylation was not affected by the PKC inhibitor GF109203X but was abolished by Y-27632, suggesting that ROCK but not PKC was involved. Furthermore, we have shown that the contractile agonists thrombin and U-46619 induced CPI-17 phosphorylation in VSMCs. Similarly to V14RhoA-induced CPI-17 phosphorylation, thrombin-induced CPI-17 phosphorylation was not affected by inhibition of PKC with GF109203X, but it was blocked by inhibition of RhoA with adenovirus-mediated expression of exoenzyme C3 as well as by Y-27632. Taken together, our present data provide the first clear evidence indicating that ROCK is responsible for thrombin- and U-46619-induced CPI-17 phosphorylation in primary cultured VSMCs.