Stable transfectants of smooth muscle cell line lacking the expression of myosin light chain kinase and their characterization with respect to the actomyosin system

Hemin-Induced Endothelial Dysfunction and Endothelial to Mesenchymal Transition in the Pathogenesis of Pulmonary Hypertension Due to Chronic Hemolysis

Pulmonary hypertension in sickle cell disease is an independent predictor of mortality, yet the pathogenesis of pulmonary vascular disease in chronic hemolytic disorders remains incompletely understood and treatment options are limited primarily to supportive care. The release of extracellular hemoglobin has been implicated in the development of pulmonary hypertension, and in this study we explored the direct effects of hemin, the oxidized moiety of heme, on the pulmonary artery endothelium. We found that low dose hemin exposure leads to significantly increased endothelial cell proliferation, migration, and cytokine release as markers of endothelial dysfunction. Protein expression changes in our pulmonary artery endothelial cells showed upregulation of mesenchymal markers after hemin treatment in conjunction with a decrease in endothelial markers. Endothelial to mesenchymal transition (EndoMT) resulting from hemin exposure was further confirmed by showing upregulation of the transcription factors SNAI1 and SLUG, known to regulate EndoMT. Lastly, given the endothelial dysfunction and phenotypic transition observed, the endothelial cytoskeleton was considered a potential novel target. Inhibiting myosin light chain kinase, to prevent phosphorylation of myosin light chain and cytoskeletal contraction, attenuated hemin-induced endothelial hyper-proliferation, migration, and cytokine release. The findings in this study implicate hemin as a key inducer of endothelial dysfunction through EndoMT, which may play an important role in pulmonary vascular remodeling during the development of pulmonary hypertension in chronic hemolytic states.

Melatonin inhibits the migration of human lung adenocarcinoma A549 cell lines involving JNK/MAPK pathway

Objective

Melatonin, an indolamine produced and secreted predominately by the pineal gland, exhibits a variety of physiological functions, possesses antioxidant and antitumor properties. But, the mechanisms for the anti-cancer effects are unknown. The present study explored the effects of melatonin on the migration of human lung adenocarcinoma A549 cells and its mechanism.

Methods

MTT assay was employed to measure the viability of A549 cells treated with different concentrations of melatonin. The effect of melatonin on the migration of A549 cells was analyzed by wound healing assay. Occludin location was observed by immunofluorescence. The expression of occludin, osteopontin (OPN), myosin light chain kinase (MLCK) and phosphorylation of myosin light chain (MLC), JNK were detected by western blots.

Results

After A549 cells were treated with melatonin, the viability and migration of the cells were inhibited significantly. The relative migration rate of A549 cells treated with melatonin was only about 20% at 24 h. The expression level of OPN, MLCK and phosphorylation of MLC of A549 cells were reduced, while the expression of occludin was conversely elevated, and occludin located on the cell surface was obviously increased. The phosphorylation status of JNK in A549 cells was also reduced when cells were treated by melatonin.

Conclusions

Melatonin significantly inhibits the migration of A549 cells, and this may be associated with the down-regulation of the expression of OPN, MLCK, phosphorylation of MLC, and up-regulation of the expression of occludin involving JNK/MAPK pathway.

Recombinant human CXCL8(3-72)K11R/G31P regulates smooth muscle cell proliferation and migration through blockage of interleukin-8 receptor

Atherosclerosis is a chronic inflammatory disease with multiple contributing factors. Hyperlipidemia is one of the major independent risks, and interleukin-8 (IL-8), as an inflammatory factor, plays an important role in the development of atherosclerosis. The aims of the study were to examine the therapeutic efficacy of G31P, an antagonist of IL-8 receptor, with a mouse model of hyperlipidemia and the potential mechanisms of G31P through the vascular smooth muscle cell (VSMC) proliferation and migration in a cell line. In vivo study: Male BALB/c mice were fed a high-fat diet for 6 months. G31P was injected subcutaneously. Blood keratinocyte chemoattractant, lipid profile, and aorta expression of inflammatory factors, matrix metalloproteinases, MMP2 and MMP9 were investigated. In vitro study: A7R5 cells were treated with IL-8 with/without G31P. Cell proliferation and migration were investigated. G31P significantly suppressed the hyperlipidermia-induced abnormal lipid profile and increased IL-8, proinflammatory factor, MMP2 and MMP9 expression. G31P also inhibited VSMC proliferation and migration both in vitro and in vivo. These findings indicate the potential therapeutic effects of G31P in preventing the development of atherosclerosis by antagonizing IL-8 receptor and decreasing the biologic activity of IL-8.

A novel all-trans retinoid acid derivatives inhibits the migration of breast cancer cell lines MDA-MB-231 via myosin light chain kinase involving p38-MAPK pathway

OBJECTIVE

To explore the effect and its probable mechanism of a synthetic retinoid 4-amino-2-tri-fluoromethyl-phenyl ester (ATPR) on the migration of human breast cancer MDA-MB-231 cells.

METHODS

MTT assay was performed to measure the proliferation of MDA-MB-231 cells treated with different concentrations of all-trans retinoic acid (ATRA) and ATPR. The effect of ATPR and ML-7, a selective inhibitor of myosin light chain kinase (MLCK), and SB203580, an inhibitor of p38, on the migration of MDA-MB-231 cells were analyzed by wound healing assay. The expression of MLCK and phosphorylation of myosin light chain (MLC), ERK, JNK, p38 proteins were detected by western blot

RESULTS

After the cells were treated by ATRA and ATPR, the proliferation and migration of breast cancer MDA-MB-231 cells were inhibited significantly. The IC 50 of ATRA and ATPR is 34.08 μmol/l and 18.06 μmol/l respectively. The relative migration rate of MDA-MB-231 cells treated with ATPR reached 50% at 48 h while the ATRA group is over 90%. The relative migration rate of ML-7 group and SB group had significant decrease compared with control group. The expression level of MLCK and phosphorylation of MLC of breast cancer cells was reduced when the cells were treated by ATPR with 48 h, the phosphorylation of ERK, JNK and p38 in breast cancer also reduced when cells were treated by ATPR with 2 h. In addition, ML-7 (50 μmol/l) could inhibit the phosphorylation of p38 and SB (50 μmol/l) could inhibit the expression of MLCK and phosphorylation of MLC.

CONCLUSIONS

ATPR had a better inhibition on the proliferation and the migration of breast cancer MDA-MB-231 cells than ATRA, and its probable mechanism was associated with the down regulation of expression of MLCK and phosphorylation of MLC protein involving p38-MAPK pathway.

ROCK1 induces ERK nuclear translocation in PDGF-BB-stimulated migration of rat vascular smooth muscle cells

It has been known that Rho-associated protein kinase (ROCK) signaling regulates the migration of vascular smooth muscle cells (VSMCs). However, the isoform-specific roles of ROCK and its underlying mechanism in VSMC migration are not well understood. The current study thus aimed to investigate the roles of ROCK1/2 and their relationship to the MAPK signaling pathway in platelet-derived growth factor (PDGF)-induced rat aorta VSMC migration by manipulating ROCK gene expression. The results revealed that ROCK1 small interfering ribonucleic acid (siRNA) rather than ROCK2 siRNA decreased PDGF-BB-generated VSMC migration, and upregulation of ROCK1 expression via transfection of constructed pEGFP-C1/ROCK1 plasmid further increased the migration of PDGF-BB-treated VSMCs. In PDGF-treated VSMCs, ROCK1 siRNA did not affect the phosphorylation levels of ERK and p38 in the cytoplasm, but decreased the level of ERK phosphorylation in the nucleus. These findings demonstrate that activated ROCK1 can promote VSMC migration through facilitating phosphorylation and nuclear translocation of ERK protein.

MLCK-independent phosphorylation of MLC20 and its regulation by MAP kinase pathway in human bladder smooth muscle cells

Myosins are a superfamily of actin-based molecular motor proteins, which hydrolyze ATP and generate various forms of eukaryotic motility and muscle contraction. Myosin light chain 20 (MLC20) is small ring around the neck region of heavy chain of myosins. Phosphorylation of MLC20 is thought to play a key role in regulation of smooth muscle contraction. Calcium- and calmodulin-dependent myosin light chain kinase (MLCK) is considered the primary regulator of MLC20 phosphorylation. However, several observations in smooth muscle contraction cannot be explained by the mode of phosphorylation. By performing a series of experiments in vitro and in vivo, we report here MLCK-independent MLC20 phosphorylation. Gene expression study reveals that expression of MLCK in smooth muscles is inconsistent with MLC20 phosphorylation at Ser19. None of inactivating calmodulin/MLCK, depriving of calcium and silencing MLCK expression by siRNA blocks effectively the phosphorylation of MLC20 at Ser19. In addition, by overexpressing active human MAP (mitogen-activated protein)-ERK kinase kinase-1 (MEKK1) and blocking its downstream messengers, we have demonstrated a new regulatory system of MLC phosphorylation via MEKK1, which downregulates Ser19 phosphorylation of MLC20 through its downstream molecules, p38, JNK, and ERK in human bladder smooth muscle cells.

PKC plays an important mediated effect in arginine vasopressin induced restoration of vascular responsiveness and calcium sensitization following hemorrhagic shock in rats

The present study investigated the mediated effect of protein kinase C (PKC) in arginine vasopressin (AVP)-induced restoration of vascular responsiveness and calcium sensitization following hemorrhagic shock. Using both isolated superior mesenteric artery from hemorrhagic shock rats and hypoxia-treated vascular smooth muscle cell (VSMC), we investigated the roles of PKC-alpha, delta and epsilon isoforms in AVP-induced restoration of vascular reactivity and calcium sensitivity. Meanwhile, effects of their specific inhibitors on the activity of myosin light chain phosphatase (MLCP), myosin light chain kinase (MLCK), and the phosphorylation of myosin light chain (MLC(20)) in VSMC were observed. The results indicated that AVP improved the reactivity of superior mesenteric artery and VSMC to norepinephrine and calcium following hemorrhagic shock and hypoxia. PKC-alpha inhibitor and PKC-epsilon inhibitory peptide antagonized these effects of AVP, while PKC-delta inhibitor only partially antagonized these effects of AVP. AVP up-regulated the expression of PKC-alpha and epsilon in the particulate fractions of hypoxia-treated VSMC with the decrease of the activity of MLCP and the increase of the phosphorylation of MLC(20). These effects of AVP were inhibited by PKC-alpha inhibitor and PKC-epsilon inhibitory peptide, but not by the PKC-delta inhibitor. The results suggested that PKC plays an important role in AVP-induced restoration of vascular reactivity and calcium sensitivity following hemorrhagic shock. PKC-alpha and epsilon may be the main isoforms involved in this process and play effect via MLC(20) phosphorylation dependent mechanism, while PKC-delta may be partially involved in AVP action by other mechanisms.

Nonkinase activity of MLCK in elongated filopodia formation and chemotaxis of vascular smooth muscle cells toward sphingosylphosphorylcholine

The actin-myosin interaction of vascular smooth muscle cells (VSMCs) is regulated by myosin light chain kinase (MLCK), which is a fusion protein of the central catalytic domain with the N-terminal actin-binding and C-terminal myosin-binding domains. In addition to the regulatory role of kinase activity mediated by the catalytic domain, nonkinase activity that derives from both terminals is able to exert a regulatory role as reviewed by Nakamura et al. (32). We previously showed that nonkinase activity mediated the filopodia upon the stimulation by sphingosylphosphorylcholine (SPC) (25). To explore the regulatory role of nonkinase activity in chemotaxis, we constructed VSMCs where the expression of MLCK was totally abolished by using a lentivirus-mediated RNAi system. We hypothesized that the MLCK-downregulated VSMCs were unable to form filopodia and to migrate upon SPC stimulation and confirmed the hypothesis. We further constructed a kinase-inactive mutant from bovine cDNA coding wild-type (WT) MLCK by mutating the ATP-binding sites located in the catalytic domain, followed by confirming the presence (absence) of the kinase activity of WT (kinase-inactive mutant). We transfected WT and the mutant into MLCK-downregulated VSMCs. We expected that the transfected VSMCs will recover the ability to induce filopodia and chemotaxis toward SPC and found both constructs rescued the ability. Because they share the actin- and myosin-binding domains, we concluded nonkinase activity plays a major role for SPC-induced migration.

Myosin light-chain kinase contributes to the proliferation and migration of breast cancer cells through cross-talk with activated ERK1/2

Myosin light-chain kinase (MLCK) plays a crucial role in the cell migration and tumor metastasis. Herein, we investigated the signaling pathways involved in MLCK using ML-7, a specific inhibitor of MLCK, in breast cancer cell proliferation and migration. Our data showed that reduction of MLCK in breast cancer cells mediated by 20 microM ML-7 was able to depress the cell proliferation and migration using two parallel cell lines (MCF-7 and LM-MCF/MDA-MB-231) with different metastatic abilities through reciprocal cross-talk with activated ERK1/2, in which both phosphorylated myosin light chain (p-MLC) and cascades of beta-catenin, cyclin D1, survivin, and c-Myc serve as essential downstream effectors.

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).

Blebbistatin inhibits the chemotaxis of vascular smooth muscle cells by disrupting the myosin II-actin interaction

Blebbistatin is a myosin II-specific inhibitor. However, the mechanism and tissue specificity of the drug are not well understood. Blebbistatin blocked the chemotaxis of vascular smooth muscle cells (VSMCs) toward sphingosylphosphorylcholine (IC(50) = 26.1 +/- 0.2 and 27.5 +/- 0.5 microM for GbaSM-4 and A7r5 cells, respectively) and platelet-derived growth factor BB (IC(50) = 32.3 +/- 0.9 and 31.6 +/- 1.3 muM for GbaSM-4 and A7r5 cells, respectively) at similar concentrations. Immunofluorescence and fluorescent resonance energy transfer analysis indicated a blebbistatin-induced disruption of the actin-myosin interaction in VSMCs. Subsequent experiments indicated that blebbistatin inhibited the Mg(2+)-ATPase activity of the unphosphorylated (IC(50) = 12.6 +/- 1.6 and 4.3 +/- 0.5 microM for gizzard and bovine stomach, respectively) and phosphorylated (IC(50) = 15.0 +/- 0.6 microM for gizzard) forms of purified smooth muscle myosin II, suggesting a direct effect on myosin II motor activity. It was further observed that the Mg(2+)-ATPase activities of gizzard myosin II fragments, heavy meromyosin (IC(50) = 14.4 +/- 1.6 microM) and subfragment 1 (IC(50) = 5.5 +/- 0.4 microM), were also inhibited by blebbistatin. Assay by in vitro motility indicated that the inhibitory effect of blebbistatin was reversible. Electron-microscopic evaluation showed that blebbistatin induced a distinct conformational change (i.e., swelling) of the myosin II head. The results suggest that the site of blebbistatin action is within the S1 portion of smooth muscle myosin II.

Role of non-kinase activity of myosin light-chain kinase in regulating smooth muscle contraction, a review dedicated to Dr. Setsuro Ebashi

Myosin light-chain kinase (MLCK) of smooth muscle consists of an actin-binding domain at the N-terminal, the catalytic domain in the central portion, and the myosin-binding domain at the C-terminal. The kinase activity is mediated by the catalytic domain that phosphorylates the myosin light-chain of 20kDa (MLC20), activating smooth muscle myosin to interact with actin. Although the regulatory role of the kinase activity is well established, the role of non-kinase activity derived from actin-binding and myosin-binding domains remains unknown. This review is dedicated to Dr. Setsuro Ebashi, who devoted himself to elucidating the non-kinase activity of MLCK after establishing calcium regulation through troponin in skeletal and cardiac muscles. He proposed that the actin-myosin interaction of smooth muscle could be activated by the non-kinase activity of MLCK, a mechanism that is quite independent of MLC20 phosphorylation. The authors will extend his proposal for the role of non-kinase activity. In this review, we express MLCK and its fragments as recombinant proteins to examine their effects on the actin-myosin interaction in vitro. We also down-regulate MLCK in the cultured smooth muscle cells, and propose that MLC20 phosphorylation is not obligatory for the smooth muscle to contract.

Intracellular signal transduction for migration and actin remodeling in vascular smooth muscle cells after sphingosylphosphorylcholine stimulation

Molecular mechanisms underlying migration of vascular smooth muscle cells (VSMCs) toward sphingosylphosphorylcholine (SPC) were analyzed in light of the hypothesis that remodeling of the actin cytoskeleton should be involved. After SPC stimulation, mitogen-activated protein kinases (MAPKs), including p38 MAPK (p38) and p42/44 MAPK (p42/44), were found to be phosphorylated. Migration of cells toward SPC was reduced in the presence of SB-203580, an inhibitor of p38, but not PD-98059, an inhibitor of p42/44. Pertussis toxin (PTX), a Giprotein inhibitor, induced an inhibitory effect on p38 phosphorylation and VSMC migration. Myosin light chain (MLC) phosphorylation occurred after SPC stimulation with or without pretreatment with SB-203580 or PTX. The MLC kinase inhibitor ML-7 and the Rho kinase inhibitor Y-27632 inhibited MLC phosphorylation but only partially inhibited SPC-directed migration. Complete inhibition was achieved with the addition of SB-203580. After SPC stimulation, the actin cytoskeleton formed thick bundles of actin filaments around the periphery of cells, and the cells were surrounded by elongated filopodia, i.e., magunapodia. The peripheral actin bundles consisted of α- and β-actin, but magunapodia consisted exclusively of β-actin. Such a remodeling of actin was reversed by addition of SB-203580 and PTX, but not ML-7 or Y-27632. Taken together, our biochemical and morphological data confirmed the regulation of actin remodeling and suggest that VSMCs migrate toward SPC, not only by an MLC phosphorylation-dependent pathway, but also by an MLC phosphorylation-independent pathway.

Rho-kinase-mediated Ca2+-independent contraction in rat embryo fibroblasts

Thus far, determining the relative contribution of Ca2+/calmodulin-dependent myosin light chain kinase (MLCK) and Ca2+-independent Rho-kinase pathways to myosin II activation and contraction has been difficult. In this study, we characterize the role of Rho-kinase in a rat embryo fibroblast cell line (REF-52), which contains no detectable MLCK. No endogenous MLCK could be detected in REF-52 cells by either Western or Northern blot analysis. In the presence or absence of Ca2+, thrombin or lysophosphatidic acid (LPA) increased RhoA activity and Rhokinase activity, correlating with isometric tension development and myosin II regulatory light chain (RLC) phosphorylation. Resting tension is associated with a basal phosphorylation of 0.31 +/- 0.02 mol PO4/mol RLC, whereas upon LPA or thrombin treatment myosin II RLC phosphorylation increases to 1.08 +/- 0.05 and 0.82 +/- 0.05 mol PO4/mol RLC, respectively, within 2.5 min. Ca2+ chelation has minimal effect on the kinetics and magnitude of isometric tension development and RLC phosphorylation. Treatment of REF-52 cells with the Rho-kinase-specific inhibitor Y-27632 abolished thrombin- and LPA-stimulated contraction and RLC phosphorylation. These results suggest that Rho-kinase is sufficient to activate myosin II motor activity and contraction in REF-52 cells.

Nicotinic Acetylcholine Receptor α7 Subunit Mediates Migration of Vascular Smooth Muscle Cells Toward Nicotine

GbaSM-4 cells, vascular smooth muscle cells (VSMCs) derived from brain basilar arteries, were shown to migrate toward d-nicotine by augmenting the actin cytoskeleton in their cell bodies and lamellipodia, and expression of nicotinic acetylcholine receptor (alpha7-nAChR) was detected in GbaSM-4 cells. Their chemotaxis was antagonized by an alpha7-nAChR antagonist of methyllycaconitine. It was also antagonized by inhibiting myosin light chain (MLC) kinase and by down-regulating MLC kinase. However, the changes in MLC phosphorylation were not associated with the nicotine treatment, suggesting the involvement of non-kinase activity of MLC kinase as reviewed by Gao et al. (IUBMB Life. 2001;51:337). This plot may work to induce arteriosclerosis during cigarette smoking.

Distribution and expression of non-muscle myosin light chain kinase in rabbit livers

AIM: To study the distribution and expression of non-muscle myosin light chain kinase (nmMLCK) in rabbit livers.

METHODS: Human nmMLCK N-terminal cDNA was amplified by polymerase chain reaction (PCR) and was inserted into pBKcmv to construct expression vectors. The recombinant plasmid was transformed into XL1-blue. Expression protein was induced by IPTG and then purified by SDS-PAGE and electroelution, which was used to prepare the polycolonal antibody to detect the distribution and expression of nmMLCK in rabbit livers with immunofluorescene techniques.

RESULTS: The polyclonal antibody was prepared, by which nmMLCK expression was detected and distributed mainly in peripheral hepatocytes.

CONCLUSION: nmMLCK can express in hepatocytes peripherally, and may play certain roles in the regulation of hepatic functions.

LFA-1-induced T cell migration on ICAM-1 involves regulation of MLCK-mediated attachment and ROCK-dependent detachment

This study analyzes signaling events initiated through binding of the leukocyte integrin LFA-1 to ICAM-1, which leads to T cell attachment, polarization and random migration. These events are critically dependent on dynamic changes in the acto-myosin cytoskeleton under the regulation of myosin light chain kinase and ROCK (Rho kinase). A key finding is that the activity of these two kinases is spatially segregated. Myosin light chain kinase (MLCK) must operate at the leading edge of the T cell because blocking its activity causes the polarized T cell to retract from the front of the cell. These activities are mirrored by inhibiting calmodulin, the activator of MLCK. In contrast inhibition of ROCK (and RhoA) has the effect of preventing detachment of the T cell trailing edge, showing that this kinase operates at the rear of the cell. This compartmentalized activity of the two kinases is reflected in their localization within the T cell. Myosin light chain kinase is concentrated at the leading edge, overlapping F-actin, whereas ROCK is more widely distributed in the trailing edge of the T cell. Thus these two kinases perform two different functions in the migrating T cell, with myosin light chain kinase activity important for attachment and movement at the leading edge and ROCK activity required for the detachment of the trailing edge. These two actomyosin-dependent processes operate coordinately to cause forward migration of a T cell.

Mutation analysis of the non-muscle myosin light chain kinase (MLCK) deletion constructs on CV1 fibroblast contractile activity and proliferation

Smooth muscle myosin light chain kinase (MLCK) is a multifunctional molecule composed of an N-terminal actin binding domain, a central kinase domain, and C-terminal calmodulin- and myosin-binding domains. We previously cloned and characterized a novel MLCK isoform from endothelial cells (EC MLCK) consisting of 1,914 amino acids displaying a higher molecular weight (210 kDa) and a novel-amino-terminal stretch of 922 amino acids not shared by the smooth muscle isoform (smMLCK, 150 kDa). To further define the role of specific EC MLCK motifs in endothelial and non-muscle cells, we constructed two epitope-tagged EC MLCK deletion mutants in mammalian expression vectors lacking either the C-terminal auto-inhibitory and calmodulin-binding domain (EC MLCK1745) or the ATP-binding site (EC MLCKATPdel). Expression of EC MLCK1745 in CV1 fibroblasts showed increased basal actin stress fiber formation, which was markedly enhanced after tumor necrosis factor (TNF-alpha) or thrombin treatment. Distribution of EC MLCK1745 was largely confined to stress fibers, cortical actin filaments, and focal adhesion contacts, and co-localized with myosin light chains (MLCs) diphosphorylated on Ser(19) and Thr(18). In contrast, immunofluorescence staining demonstrated that EC MLCKATPdel abolished thrombin- and TNFalpha-induced stress fiber formation and MLC phosphorylation, suggesting this kinase-dead mutant functions as a dominant-negative MLCK construct, thereby confirming the role of EC MLCK in stress fiber formation. Finally, we compared the serum-stimulated growth rate of mutant MLCK-transfected fibroblasts to sham controls, and found EC MLCK1745 to augment thymidine incorporation whereas EC MLCKATPdel reduced CV1 growth rates. These data demonstrate the necessary role for MLCK in driving the contractile apparatus via MLC phosphorylation, which can alter fibroblast growth and contractility.

Mechanisms underlying the hydrogen peroxide-induced, endothelium-independent relaxation of the norepinephrine-contraction in guinea-pig aorta

The mechanisms underlying the hydrogen peroxide-induced relaxation of the norepinephrine-contraction were studied by measuring isometric force, myosin light chain (MLC(20)) phosphorylation and cyclic GMP in endothelium-denuded muscle from the guinea-pig aorta. Norepinephrine (5.2+/-1.3 microM) produced a phasic, followed by a tonic contraction. Hydrogen peroxide (10 and 100 microM), glyceryl trinitrate (30 and 300 nM) and 8-bromo cyclic GMP (30 and 100 microM) did not change the basal tone, but reduced the norepinephrine-induced contraction. Phosphorylation of MLC(20) (percentage of phosphorylated to total MLC(20)) was increased 1 min (5.9+/-1.0% vs. 35.9+/-4.9%) and, to a lesser extent, 20 min (3.7+/-1.7% vs. 13.9+/-1.6%) after the addition of norepinephrine. Hydrogen peroxide (100 microM) did not modify basal MLC(20) phosphorylation, but reduced the increase in MLC(20) phosphorylation induced by 1-min exposure to norepinephrine (20.9+/-4.1%). Its effect was abolished by catalase. When the tissue was incubated for 20 min with norepinephrine in the presence of hydrogen peroxide, norepinephrine-induced MLC(20) phosphorylation was not changed (13.6+/-1.5%), as compared to that in the absence of hydrogen peroxide. Hydrogen peroxide relaxed norepinephrine-stimulated aortas in a concentration-dependent fashion with EC(50) values of 5.9+/-0.2 microM. The relaxation was inhibited by soluble guanylate cyclase inhibitors and increased by an inhibitor of cyclic GMP-selective phosphodiesterase. In aorta precontracted with norepinephrine, hydrogen peroxide (100 microM) relaxed the tissue by 89+/-11% and almost doubled tissue concentrations of cyclic GMP, whereas sodium nitroprusside (1 microM) relaxed the tissue by 100% and increased cyclic GMP concentrations 30-fold. It is suggested that the inhibitory effects of hydrogen peroxide on the norepinephrine-induced phasic and sustained contractions are explained by a decrease in MLC(20) phosphorylation and by an alteration in MLC(20) phosphorylation-independent mechanisms, respectively. The effects of hydrogen peroxide were in part mediated by cyclic GMP.

Role of the short isoform of myosin light chain kinase in the contraction of cultured smooth muscle cells as examined by its down-regulation

GbaSM-4 cells, smooth muscle cells derived from brain basilar artery, which express both 210-kDa long and 130-kDa short isoforms of myosin light chain kinase (MLCK), were infected with an adenovirus vector carrying a 1.4-kb catalytic portion of MLCK-cDNA in an antisense orientation. Western blot analysis showed that the expression of short MLCK was depressed without affecting long MLCK expression. The contraction of the down-regulated cells was measured by the cell-populated collagen-fiber method. The tension development after stimulation with norepinephrine or was depressed. The additional infection of the down-regulated cells with the adenovirus construct containing the same insert in a sense direction rescued not only the short MLCK expression but also contraction, confirming the physiological role of short MLCK in the contraction. To examine the role of long MLCK in the residual contraction persisting in the short MLCK-deficient cells, long MLCK was further down-regulated by increasing the multiplicity of infection of the antisense construct. The additional down-regulation of long MLCK expression, however, did not alter the residual contraction, ruling out the involvement of long MLCK in the contractile activity. Further, in the cells where short MLCK was down-regulated specifically, the extent of phosphorylation of 20-kDa myosin light chain (MLC20) after the agonist stimulation was not affected. This finding suggests that there are additional factors to MLC20 phosphorylation that contribute to regulate smooth muscle contraction.

The tumor vascular targeting agent combretastatin A-4-phosphate induces reorganization of the actin cytoskeleton and early membrane blebbing in human endothelial cells

Combretastatin A-4-phosphate (CA-4-P) is a tubulin-binding compound currently in clinical trial as a tumor vascular-targeting agent. In endothelial cells, CA-4-P is known to cause microtubule depolymerization, but little is known about its subsequent effects on cell morphology and function. Here, we demonstrate that within minutes of endothelial cell exposure to CA-4-P, myosin light chain (MLC) was phosphorylated, leading to actinomyosin contractility, assembly of actin stress fibers, and formation of focal adhesions. These cytoskeletal alterations appeared to be a consequence of Rho activation, as they were abolished by either the Rho inhibitor C3 exoenzyme or Rho-kinase inhibitor Y-27632. In response to CA-4-P, some cells rapidly assumed a blebbing morphology in which F-actin accumulated around surface blebs, stress fibers misassembled into a spherical network surrounding the cytoplasm, and focal adhesions appeared malformed. Blebbing was associated with decreased cell viability and could be inhibited by Rho/Rho-kinase inhibitors or by blocking the CA-4-P-mediated activation of stress-activated protein kinase-2/p38. The extracellular-regulated kinases 1 and 2 (ERK-1/2) were shown to protect against blebbing since blebbing was attenuated on ERK-1/2 stimulation and was up-regulated by specific inhibition of ERK-1/2 activation. The use of MLC kinase (MLCK) and myosin adenosine triphosphatase inhibitors led us to propose a role for MLCK and myosin activity independent of MLC phosphorylation in regulating the blebbing process. CA-4-P-mediated contractility and blebbing were associated with a Rho-dependent increase in monolayer permeability to dextrans, suggesting that such functional changes may be important in the rapid response of the tumor endothelium to CA-4-P in vivo.

A fluorescent resonant energy transfer-based biosensor reveals transient and regional myosin light chain kinase activation in lamella and cleavage furrows

Approaches with high spatial and temporal resolution are required to understand the regulation of nonmuscle myosin II in vivo. Using fluorescence resonance energy transfer we have produced a novel biosensor allowing simultaneous determination of myosin light chain kinase (MLCK) localization and its [Ca2+]4/calmodulin-binding state in living cells. We observe transient recruitment of diffuse MLCK to stress fibers and its in situ activation before contraction. MLCK is highly active in the lamella of migrating cells, but not at the retracting tail. This unexpected result highlights a potential role for MLCK-mediated myosin contractility in the lamella as a driving force for migration. During cytokinesis, MLCK was enriched at the spindle equator during late metaphase, and was maximally activated just before cleavage furrow constriction. As furrow contraction was completed, active MLCK was redistributed to the poles of the daughter cells. These results show MLCK is a myosin regulator in the lamella and contractile ring, and pinpoints sites where myosin function may be mediated by other kinases.

[Targeting of myosin light chain kinase in vascular smooth muscle cells, and its implication for drug discovery]

We constructed a plasmid vector to have a 1.4-kb insert of myosin light chain kinase (MLCK) cDNA in an antisense direction to express antisense mRNA. The construct was then transfected to SM3, a cell line from vascular smooth muscle cells (VSMCs), producing a few stable transformants. We explained the methods in detail. The down-regulation of MLCK expression in the transfectants was confirmed by both Northern and Western blots. The control showed chemotactic motility to the platelet derived growth factor. However, the transfectants did not show chemotactic motility, indicating the essential role of MLCK in the motility. It was discussed that down-regulation of MLCK expression could be utilized to discover the drug for arteriosclerosis which prevents the proliferating VSMCs from forming arterial plaques.

Rho-Rho kinase is involved in smooth muscle cell migration through myosin light chain phosphorylation-dependent and independent pathways

Although Rho, a small GTPase, has been demonstrated to play an important role in the smooth muscle contraction and relaxation, little is known about the involvement of Rho protein in smooth muscle cell (SMC) migration. In this study the role of Rho-Rho kinase pathway was examined in SMC migration induced by platelet-derived growth factor (PDGF) and lysophosphatidic acid (LPA). C3 transferase, a specific inhibitor of Rho, blocked SMC migration induced by PDGF and LPA. Y-27632, a specific inhibitor of Rho kinase, a direct target molecule of Rho, inhibited PDGF and LPA-induced SMC migration in a concentration dependent manner. Although rapid increase in myosin light chain (MLC) phosphorylation in SMC treated with LPA was observed, no enhanced MLC phosphorylation was detected in response to PDGF. Y-27632 suppressed LPA-induced as well as basal level of MLC phosphorylation. ML-9, a specific inhibitor of myosin light chain kinase (MLCK), inhibited PDGF and LPA-induced SMC migration without the suppression of MLC phosphorylation at 5 min incubation, suggesting that MLCK may contribute to SMC migration via mechanism other than MLC phosphorylation. These results suggest that Rho-Rho kinase pathway is implicated in SMC migration and that different signaling pathways downstream of Rho-Rho kinase may be involved in LPA and PDGF-induced SMC migration. MLC phosphorylation via Rho-Rho kinase pathway appears to be implicated in LPA-dependent SMC migration. Whereas PDGF-mediated SMC migration is independent of increased MLC phosphorylation and other target molecules downstream of Rho-Rho kinase seem to be involved.

Contribution of small GTPase Rho and its target protein rock in a murine model of lung fibrosis

Excess fibroblasts and inflammatory cells may play an important role in the pathogenesis of idiopathic pulmonary fibrosis (IPF). The small GTPase, Rho, and its target protein, Rho-associated coiled-coil-forming protein kinase (ROCK), have been recognized to be major regulators of cell locomotion mediated by reorganization of the actin cytoskelton. Activated ROCK inhibits myosin phosphatase, and this in turn induces phosphorylation of the myosin light chain (MLC). To determine the mechanisms underlying the deterioration process of IPF, we investigated the effect of Y-27632, a selective ROCK inhibitor, in a murine model of bleomycin (BLM)-induced lung fibrosis. The Aschcroft score and hydroxyproline content of the BLM-treated mouse lung decreased in response to Y-27632 treatment. The number of broncoalveolar cells was decreased by Y-27632, and migration of macrophages, neutrophils, and fibroblasts in vitro was inhibited by Y-27632 regardless of various stimuli. Although expression of ROCK-II mRNA in the lung homogenates of the BLM-treated mice was increased approximately 9-fold, expression of ROCK-II protein showed only a slight tendency to increase. BLM elevated MLC phosphorylation levels, and Y-27632 inhibited BLM response. These findings indicate that the Rho/ROCK-mediated pathway plays an important role in IPF, and that blocking of this pathway leads to inhibition of IPF development.

Localization and activity of myosin light chain kinase isoforms during the cell cycle

Phosphorylation on Ser 19 of the myosin II regulatory light chain by myosin light chain kinase (MLCK) regulates actomyosin contractility in smooth muscle and vertebrate nonmuscle cells. The smooth/nonmuscle MLCK gene locus produces two kinases, a high molecular weight isoform (long MLCK) and a low molecular weight isoform (short MLCK), that are differentially expressed in smooth and nonmuscle tissues. To study the relative localization of the MLCK isoforms in cultured nonmuscle cells and to determine the spatial and temporal dynamics of MLCK localization during mitosis, we constructed green fluorescent protein fusions of the long and short MLCKs. In interphase cells, localization of the long MLCK to stress fibers is mediated by five DXRXXL motifs, which span the junction of the NH(2)-terminal extension and the short MLCK. In contrast, localization of the long MLCK to the cleavage furrow in dividing cells requires the five DXRXXL motifs as well as additional amino acid sequences present in the NH(2)-terminal extension. Thus, it appears that nonmuscle cells utilize different mechanisms for targeting the long MLCK to actomyosin structures during interphase and mitosis. Further studies have shown that the long MLCK has twofold lower kinase activity in early mitosis than in interphase or in the early stages of postmitotic spreading. These findings suggest a model in which MLCK and the myosin II phosphatase (Totsukawa, G., Y. Yamakita, S. Yamashiro, H. Hosoya, D.J. Hartshorne, and F. Matsumura. 1999. J. Cell Biol. 144:735-744) act cooperatively to regulate the level of Ser 19-phosphorylated myosin II during mitosis and initiate cytokinesis through the activation of myosin II motor activity.