Dephosphorylation Of Light Chain Research Articles

Myosin light chain dephosphorylation by ppp1r12c promotes atrial hypocontractility in atrial fibrillation

Abstract Background Atrial fibrillation (AF), the most common sustained cardiac arrhythmia, increases thromboembolic stroke risk five-fold. Although atrial hypocontractility contributes to stroke risk in AF, the molecular mechanisms reducing myofilament contractile function remain unknown. We tested the hypothesis that increased expression of PPP1R12C, the PP1 regulatory subunit targeting atrial myosin light chain 2 (MLC2a), causes hypophosphorylation of MLC2a and results in atrial hypocontractility. Methods Right atrial appendage tissues were isolated from human AF patients versus sinus rhythm (SR) controls. Western blots, co-immunoprecipitation, and phosphorylation studies were performed to examine how the PP1c-PPP1R12C interaction causes MLC2a de-phosphorylation. In vitro studies of pharmacologic MRCK inhibitor (BDP5290) in atrial HL-1 cells were performed to evaluate PP1 holoenzyme activity on MLC2a. Cardiac-specific lentiviral PPP1R12C overexpression was performed in mice to evaluate atrial remodeling with atrial cell shortening assays, echocardiography, and AF inducibility with EP studies. Results In human patients with AF, PPP1R12C expression was increased two-fold versus SR controls (P=0.02, n=12,12 in each group) with > 40% reduction in MLC2a phosphorylation (P=0.000001, n=12,12 in each group). PPP1R12C-PP1c binding and PPP1R12C-MLC2a binding were significantly increased in AF (P=0.03 and 0.007 respectively, n=8,8 in each group). In vitro studies utilizing drug BDP5290, which inhibits T560-PPP1R12C phosphorylation, demonstrated increased PPP1R12C binding with both PP1c and MLC2a, and dephosphorylation of MLC2a. Lenti-12C mice demonstrated a 150% increase in LA size versus controls (P=5.0x10-6, n=12,8,12), with reduced atrial strain and atrial ejection fraction. Pacing-induced AF in Lenti-12C mice was significantly higher than controls (P=0.02 and 0.04 respectively, n= 6,6,5). Conclusions AF patients exhibit increased levels of PPP1R12C protein compared to controls. PPP1R12C overexpression in mice increases PP1c targeting to MLC2a and causes MLC2a dephosphorylation, which reduces atrial contractility and increases AF inducibility. These findings suggest that PP1 regulation of sarcomere function at MLC2a is a key regulator of atrial contractility in AF.

Abstract EC.02: Myosin Light Chain Dephosphorylation By Ppp1r12c Promotes Atria Hypocontractility In Atrial Fibrillation

Background: Atrial fibrillation (AF), the most common sustained cardiac arrhythmia, increases thromboembolic stroke risk five-fold. Although atrial hypocontractility contributes to stroke risk in AF, the molecular mechanisms reducing myofilament contractile function remain unknown. We tested the hypothesis that increased expression of PPP1R12C, the PP1 regulatory subunit targeting atrial myosin light chain 2 (MLC2a), causes hypophosphorylation of MLC2a and results in atrial hypocontractility. Methods: Right atrial appendage tissues were isolated from human AF patients versus sinus rhythm (SR) controls. Western blots, co-immunoprecipitation, and phosphorylation studies were performed to examine how the PP1c-PPP1R12C interaction causes MLC2a de-phosphorylation. In vitro studies of pharmacologic MRCK inhibitor (BDP5290) in atrial HL-1 cells were performed to evaluate PP1 holoenzyme activity on MLC2a. Cardiac-specific lentiviral PPP1R12C overexpression was performed in mice to evaluate atrial remodeling with atrial cell shortening assays, echocardiography, and AF inducibility with EP studies. Results: In human patients with AF, PPP1R12C expression was increased two-fold versus SR controls with > 40% reduction in MLC2a phosphorylation. 12C-PP1c binding and PPP1R12C-MLC2a binding were significantly increased in AF. In vitro studies utilizing drug BDP5290, which inhibits T560-PPP1R12C phosphorylation, demonstrated increased PPP1R12C binding with both PP1c and MLC2a, and dephosphorylation of MLC2a. Lenti-12C mice demonstrated a 150% increase in LA size versus controls, with reduced atrial strain and atrial ejection fraction. Pacing-induced AF in Lenti-12C mice was significantly higher than controls. Conclusions: AF patients exhibit increased levels of PPP1R12C protein compared to controls. PPP1R12C overexpression in mice increases PP1c targeting to MLC2a and causes MLC2a dephosphorylation, which reduces atrial contractility and increases AF inducibility. These findings suggest that PP1 regulation of sarcomere function at MLC2a is a key regulator of atrial contractility in AF.

67-kDa laminin receptor mediates oolonghomobisflavan B-induced cell growth inhibition in melanoma

BackgroundOolonghomobisflavans are unique polyphenols found in oolong teas. Oolonghomobisflavan B (OHBFB), a dimer of (–)-epigallocatechin-3-O-gallate (EGCG), is an active compound found in green tea. PurposeOHBFB has been reported to exert an inhibitory effect on lipase enzyme activity. However, little is known regarding its intercellular signaling induction effect. Further, there are no reports describing the anti-cancer effects of OHBFB. MethodsThe effect of OFBFB on B16 melanoma cells was evaluated by cell counting, and its mechanisms were determined by western blot analysis with or without protein phosphatase 2A (PP2A) inhibitor treatment. Intracellular cyclic adenosine monophosphate (cAMP) levels were evaluated by time-resolved fluorescence resonance energy transfer analysis. Quartz crystal microbalance (QCM) analysis was performed to assess the binding of OHBFB to 67LR. ResultsCell growth assay and western blot analyses showed that OHBFB inhibited melanoma cell growth, followed by myosin phosphatase target subunit 1 (MYPT1) and myosin regulatory light chain (MRLC) dephosphorylation via protein phosphatase 2A (PP2A)-dependent mechanisms. These effects are mediated by intracellular cAMP- and protein kinase A (PKA) A-dependent mechanisms. QCM analysis identified the 67-kDa laminin receptor (67LR) as an OHBFB receptor with a Kd of 3.7 µM. We also demonstrated for the first time that OHBFB intake suppresses tumor growth in vivo. ConclusionsTaken together, these results indicate that the cAMP/PKA/PP2A/MYPT1/MRLC pathway is a key mediator of melanoma cell growth inhibition following OHBFB binding to 67LR and that OHBFB suppresses tumor growth in vivo.

Abstract 13258: Myosin Light Chain Dephosphorylation by Ppp1r12c Promotes Atrial Hypocontractility in Atrial Fibrillation

Introduction: Atrial fibrillation (AF) is the most common sustained arrhythmia, with an estimated prevalence in the U.S. of 6.1 million . AF increases the risk of a thromboembolic stroke in five-fold. Although atrial hypocontractility contributes to stroke risk in AF, the molecular mechanisms reducing myofilament contractile function in AF remains unknown. We have recently identified protein phosphatase 1 subunit 12c (PPP1R12C) as a key molecule targeting myosin light chain phosphorylation in AF. Objective We hypothesize that the overexpression of PPP1R12C causes hypophosphorylation of atrial myosin light chain 2 (MLC2a), thereby decreasing atrial contractility in AF. Methods and Results Left and right atrial appendage tissues were isolated from AF patients versus sinus rhythm (SR). To evaluate the role of the PP1c-PPP1R12C interaction in MLC2a de-phosphorylation, we utilized Western blots, co-immunoprecipitation, and phosphorylation assays. In patients with AF, PPP1R12C expression was increased 3.5-fold versus SR controls with an 88% reduction in MLC2a phosphorylation. PPP1R12CPP1c binding and PPP1R12C-MLC2a binding were significantly increased in AF. In vitro studies of either pharmacologic (BDP5290) or genetic (T560A) PPP1R12C activation demonstrated increased PPP1R12C binding with both PP1c and MLC2a, and dephosphorylation of MLC2a. Additionally, to evaluate the role of PPP1R12C expression in cardiac function, mice with lentiviral cardiac-specific overexpression of PPP1R12C (Lenti-12C) were evaluated for atrial contractility using echocardiography, versus wild-type and Lenti-controls. Lenti-12C mice demonstrated a 150% increase in left atrium size versus controls, with reduced atrial strain and atrial ejection fraction. Also, programmed electrical stimulation was performed to evaluate AF inducibility in vivo. Pacing-induced AF in Lenti-12C mice was significantly higher than controls. Conclusion The overexpression of PPP1R12C increases PP1c targeting to MLC2a and provokes dephosphorylation, associated with a reduction in atrial contractility and increase in AF inducibility. All these discoveries suggest that PP1 regulation of sarcomere function at MLC2a is a main regulator of atrial contractility in AF.

Investigating the effects of mutations in non-muscle myosin 2A LMM by circular dichroism and electron microscopy analysis

Non-muscle myosin 2A (NM2A) is a ubiquitously-expressed myosin important for organizing cellular actin filaments. Upon regulatory light chain dephosphorylation, NM2A forms a compact, inactive (‘shutdown’) state, termed 10S, in which the filament-forming tail interacts with the myosin heads. Over 80 mutations in the NM2A heavy chain-encoding gene (MYH9) have been described, collectively causing an autosomal-dominant disorder. Two-thirds of these mutations occur in the coiled-coil tail, composed of the proximal subfragment-2 and distal light meromyosin (LMM).

Optogenetic relaxation of actomyosin contractility uncovers mechanistic roles of cortical tension during cytokinesis

Actomyosin contractility generated cooperatively by nonmuscle myosin II and actin filaments plays essential roles in a wide range of biological processes, such as cell motility, cytokinesis, and tissue morphogenesis. However, subcellular dynamics of actomyosin contractility underlying such processes remains elusive. Here, we demonstrate an optogenetic method to induce relaxation of actomyosin contractility at the subcellular level. The system, named OptoMYPT, combines a protein phosphatase 1c (PP1c)-binding domain of MYPT1 with an optogenetic dimerizer, so that it allows light-dependent recruitment of endogenous PP1c to the plasma membrane. Blue-light illumination is sufficient to induce dephosphorylation of myosin regulatory light chains and a decrease in actomyosin contractile force in mammalian cells and Xenopus embryos. The OptoMYPT system is further employed to understand the mechanics of actomyosin-based cortical tension and contractile ring tension during cytokinesis. We find that the relaxation of cortical tension at both poles by OptoMYPT accelerated the furrow ingression rate, revealing that the cortical tension substantially antagonizes constriction of the cleavage furrow. Based on these results, the OptoMYPT system provides opportunities to understand cellular and tissue mechanics.

Abstract P780: Myosin Light Chain Dephosphorylation by Ppp1r12c Promotes Atrial Hypocontractility in Atrial Fibrillation

Atrial fibrillation (AF) is the most common sustained arrhythmia, with an estimated prevalence in the U.S.of 6.1 million. AF increases the risk of a thromboembolic stroke in five-fold. Although atrial hypocontractility contributes to stroke risk in AF, the molecular mechanisms reducing myofilament contractile function in AF remains unknown. We have recently identified protein phosphatase 1 subunit 12c (PPP1R12C) as a key molecule targeting myosin light-chain phosphorylation in AF. Objective: We hypothesize that the overexpression of PPP1R12C causes hypophosphorylation of atrial myosin light-chain 2 (MLC2a), thereby decreasing atrial contractility in AF. Methods and Results: Left and right atrial appendage tissues were isolated from AF patients versus sinus rhythm (SR). To evaluate the role of the PP1c-PPP1R12C interaction in MLC2a de-phosphorylation, we utilized Western blots, co-immunoprecipitation, and phosphorylation assays. In patients with AF, PPP1R12C expression was increased 3.5-fold versus SR controls with an 88% reduction in MLC2a phosphorylation. PPP1R12C-PP1c binding and PPP1R12C-MLC2a binding were significantly increased in AF. In vitro studies of either pharmacologic (BDP5290) or genetic (T560A), PPP1R12C activation demonstrated increased PPP1R12C binding with both PP1c and MLC2a, and dephosphorylation of MLC2a. Additionally, to evaluate the role of PPP1R12C expression in cardiac function, mice with lentiviral cardiac-specific overexpression of PPP1R12C (Lenti-12C) were evaluated for atrial contractility using echocardiography, versus wild-type and Lenti-controls. Lenti-12C mice demonstrated a 150% increase in left atrium size versus controls, with reduced atrial strain and atrial ejection fraction. Also, programmed electrical stimulation was performed to evaluate AF inducibility in vivo. Pacing-induced AF in Lenti-12C mice was significantly higher than controls. Conclusion: The overexpression of PPP1R12C increases PP1c targeting to MLC2a and provokes dephosphorylation, associated with a reduction in atrial contractility and an increase in AF inducibility. All these discoveries suggest that PP1 regulation of sarcomere function at MLC2a is a main regulator of atrial contractility in AF.

MYOSIN LIGHT CHAIN DEPHOSPHORYLATION BY PPP1R12C PROMOTES ATRIAL HYPOCONTRACTILITY IN ATRIAL FIBRILLATION

Atrial fibrillation (AF) increases stroke risk five-fold. Atrial hypocontractility from atrial myosin light chain (MLC2a) dephosphorylation contributes to stroke risk in AF. Recent proteomic data has shown increased protein phosphatase 1 subunit 12C (PPP1R12C) targeting to MLC2a in AF. However, it

Abstract WMP39: Protein Phosphatase 1 Regulatory Subunit 12C Contributes to Atrial Myosin Light Chain Dephosphorylation in Atrial Fibrillation

Background: Atrial fibrillation (AF) increases stroke risk five-fold. Atrial hypocontractility from atrial myosin light chain (MLC2a) dephosphorylation contributes to stroke risk in AF. Recent proteomic data has shown increased protein phosphatase 1 subunit 12C (PPP1R12C) targeting to MLC2a in AF. However, it is unclear whether PPP1R12C causes MLC2a dephosphorylation in AF. Objective: Determine whether increased PPP1R12C expression causes MLC2a dephosphorylation and increases AF risk. Methods: Western blots and co-IPs were performed to evaluate the relationship among PPP1R12C, PP1c and MLC2a in human atrial tissues (AF vs SR). Mice with either a knockout (KO) or lentiviral (LV) cardiac overexpression of PPP1R12C were evaluated with invasive EP studies for AF inducibility vs WT controls. Results: In human AF, PPP1R12C was increased 4-fold ( P <0.005, n=6) with an 88% reduction in S-19-MLC2a phosphorylation ( P <0.05, n=4). PPP1R12C-PP1c and PPP1R12C-MLC2a binding was increased 2-fold in AF ( P <0.05, n=6). AF burden in LV-12C mice increased nearly tenfold vs. KO and WT mice ( P <0.05, n=6). Conclusion: In human AF, increased PPP1R12C expression is associated with reduced P-MLC2a through enhanced binding with the PP1c catalytic subunit. This dephosphorylation is a likely contributor to atrial hypocontractility and stroke risk in AF. Additionally, increased PPP1R12C expression in mice increases AF risk. Future studies will examine the effects of increased PPP1R12C expression upon atrial contractile function in mice.

Dephosphorylation of myosin regulatory light chain modulates actin–myosin interaction adverse to meat tenderness

SummaryThe objective of this study was to investigate the relationship between the dephosphorylation of myosin regulatory light chain and actin–myosin interaction after muscle homogenate was treated with alkaline phosphatase and phosphatase inhibitor. The myosin regulatory light chain was significantly dephosphorylated by alkaline phosphatase after incubation. Among different groups, dephosphorylated myosin regulatory light chain to a much greater extent leads to a lower actomyosin dissociation degree, thereby raising the actomyosin ATPase activity, whereas it exhibits a higher actomyosin dissociation degree and a lower actomyosin ATPase activity when the myosin regulatory light chain was in a low dephosphorylated state. The data suggest that the dephosphorylation of myosin regulatory light chain modulates actomyosin dissociation (the number of myosin interact with actin) negatively and has a positive influence on actomyosin ATPase activity (the interacting force between myosin and actin).

Subcellular Spatial Control of Non-Muscle Myosin 2 Redistribution and Stress Fiber Strain by Molecular Tattoo

The cellular distribution of the motor protein non-muscle myosin 2 (NM2) leads to different forms of intracellular strain driving cell motility, cytokinesis and morphogenetic processes including axonal growth and retraction. However, it remains elusive how these cellular processes are governed by the dynamic changes in NM2 localization and supramolecular assembly. To address this problem, we determined the effect of different types of NM2 inhibition on the dynamics of load-bearing stress fibers and unloaded inner cytoplasmatic NM2 structures in live HeLa cells. We followed NM2 redistribution via FRAP, applied also in combination with our recently developed optopharmacological tool, Molecular Tattoo, which allows subcellular confinement of drug effects via 2-photon induced photocrosslinking to targets. We found that the Rho-kinase inhibitor, Y-27632, dramatically accelerates NM2 redistribution and induces stress fiber dissolution, due to NM2 filament disassembly resulting from myosin light chain dephosphorylation. When NM2 was inhibited by para-nitroblebbistatin (pNBleb) or locally by tattooed azidoblebbistatin, in the stress fibers a significant acceleration and suppression of NM2 redistribution was detected at moderate and high inhibitor concentrations, respectively. The observed effects were local and specific for load-bearing peripheral stress fibers, implying the role of mechanical load in NM2 redistribution. Furthermore, in these tests stress fibers remained intact, contrary to that seen upon Rho-kinase inhibition. These results highlight that variations in the localization and/or pharmacological mechanism of NM2 inhibition produce distinct effects on intracellular strain and cellular morphogenesis.Supported by Hungarian Research and Innovation Fund (VKSZ_14-1-2015-0052).

Compressive stress induces dephosphorylation of the myosin regulatory light chain via RhoA phosphorylation by the adenylyl cyclase/protein kinase A signaling pathway.

Mechanical stress that arises due to deformation of the extracellular matrix (ECM) either stretches or compresses cells. The cellular response to stretching has been actively studied. For example, stretching induces phosphorylation of the myosin regulatory light chain (MRLC) via the RhoA/RhoA-associated protein kinase (ROCK) pathway, resulting in increased cellular tension. In contrast, the effects of compressive stress on cellular functions are not fully resolved. The mechanisms for sensing and differentially responding to stretching and compressive stress are not known. To address these questions, we investigated whether phosphorylation levels of MRLC were affected by compressive stress. Contrary to the response in stretching cells, MRLC was dephosphorylated 5 min after cells were subjected to compressive stress. Compressive loading induced activation of myosin phosphatase mediated via the dephosphorylation of myosin phosphatase targeting subunit 1 (Thr853). Because myosin phosphatase targeting subunit 1 (Thr853) is phosphorylated only by ROCK, compressive loading may have induced inactivation of ROCK. However, GTP-bound RhoA (active form) increased in response to compressive stress. The compression-induced activation of RhoA and inactivation of its effector ROCK are contradictory. This inconsistency was due to phosphorylation of RhoA (Ser188) that reduced affinity of RhoA to ROCK. Treatment with the inhibitor of protein kinase A that phosphorylates RhoA (Ser188) induced suppression of compression-stimulated MRLC dephosphorylation. Incidentally, stretching induced phosphorylation of MRLC, but did not affect phosphorylation levels of RhoA (Ser188). Together, our results suggest that RhoA phosphorylation is an important process for MRLC dephosphorylation by compressive loading, and for distinguishing between stretching and compressing cells.

Protein Phosphatase 1 β Paralogs Encode the Zebrafish Myosin Phosphatase Catalytic Subunit

BackgroundThe myosin phosphatase is a highly conserved regulator of actomyosin contractility. Zebrafish has emerged as an ideal model system to study the in vivo role of myosin phosphatase in controlling cell contractility, cell movement and epithelial biology. Most work in zebrafish has focused on the regulatory subunit of the myosin phosphatase called Mypt1. In this work, we examined the critical role of Protein Phosphatase 1, PP1, the catalytic subunit of the myosin phosphatase.Methodology/Principal FindingsWe observed that in zebrafish two paralogous genes encoding PP1β, called ppp1cba and ppp1cbb, are both broadly expressed during early development. Furthermore, we found that both gene products interact with Mypt1 and assemble an active myosin phosphatase complex. In addition, expression of this complex results in dephosphorylation of the myosin regulatory light chain and large scale rearrangements of the actin cytoskeleton. Morpholino knock-down of ppp1cba and ppp1cbb results in severe defects in morphogenetic cell movements during gastrulation through loss of myosin phosphatase function.Conclusions/SignificanceOur work demonstrates that zebrafish have two genes encoding PP1β, both of which can interact with Mypt1 and assemble an active myosin phosphatase. In addition, both genes are required for convergence and extension during gastrulation and correct dosage of the protein products is required.

P41 Hydrogen sulfide induces myosin light chain de-phosphorylation and smooth muscle relaxation in small intrapulmonary airways

Hydrogen sulfide (H 2 S) relaxes airway smooth muscle and it has been suggested to have a protective role during asthma and other obstructive lung diseases. However, the mechanism of H 2 S-induced airway relaxation is unknown. Smooth muscle cell (SMC) contraction depends on the phosphorylation state of the regulatory myosin light chain (rMLC) which is determined by the relative activities of the myosin light chain kinase (MLCK) and myosin light chain phosphatase (MLCP). Changes in intracellular Ca 2+ concentration in SMCs regulate MLCK activity whereas Ca 2+ independent mechanisms regulate MLCP activity. In an accompanying abstract, we report that H 2 S inhibits intracellular Ca 2+ release through InsP 3 receptors and agonist-induced intracellular Ca 2+ oscillations in airway SMCs. Here we investigated the effect of H 2 S on rMLC phosphorylation in small intrapulmonary airways. We hypothesize that H 2 S-induced airway relaxation is accompanied by rMLC de-phosphorylation that results from a decrease in MLCK activity due to the inhibition of agonist-induced Ca 2+ oscillations. We used mouse lung slices in combination with phase-contrast- and confocal- video microscopy to measure contraction and Ca 2+ signaling in SMCs of small airways and used Western blot analysis to measure protein phosphorylation in airways micro-isolated from lung slices. Methacholine (MCh, 0.3 μM) induced strong and sustained airway contraction. This response was accompanied by the generation of intracellular Ca 2+ oscillations in airway SMCs that persisted in time and by a sustained increase in rMLC phosphorylation. Subsequent addition of 100 μM Na 2 S (a H 2 S donor) resulted in a strong and sustained airway relaxation. This relaxation was accompanied by a decrease in the frequency and amplitude of Ca 2+ oscillations and by rMLC de-phosphorylation. Washout of H 2 S in the continuous presence of MCh resulted in the increase in Ca 2+ oscillation frequency and amplitude, rMLC phosphorylation, and airway re-contraction. Our results suggest H 2 S induces relaxation of small intrapulmonary airways by inhibiting agonist-induced Ca 2+ oscillations which in turn induces rMLC de-phosphorylation in airway SMCs.

Abstract 308: Papaverine Inhibits Force of Human Saphenous Vein Through Actin Depolymerization and Myosin Light Chain Dephosphorylation

Background Papaverine has been used to combat vasospasm in human saphenous veins (HSV) as an alternative to high pressure distension techniques. Papaverine acts as a nonspecific inhibitor of phosphodiesterases, increasing both cGMP and cAMP. We hypothesized that the mechanism by which papaverine reduces force through regulation of actin depolymerization and myosin light chain dephosphorylation. Methods HSV was pretreated with 1 mM papaverine followed by 5 μM norepinephrine. Force and intracellular [Ca 2+ ] i levels were concurrently measured using a Fluoroplex. Tissue was snap frozen for biochemical analyses. Results Papaverine completely inhibited norepinephrine-induced force development and intracellular [Ca 2+ ] i . Papaverine pretreatment led to increases in the phosphorylation of heat shock-related protein 20 (HSPB6), dephosphorylation of myosin light chain, and depolymerization of actin. Pretreament of papaverine led to an association of HSPB6 with 14-3-3 eta as determined by immunoprecipitation and mass spectrometry analysis. The interaction of HSPB6 with 14-3-3 leads to dissociation and activation of cofilin, an actin depolymerizing protein. Conclusions These results suggest that papaverine-induced force inhibition of HSV involves decreases in intracellular [Ca 2+ ] i by inhibiting the calcium regulatory pathways, leading to the dephosphorylation of myosin light chain. Additionally papaverine induces the phosphorylation of HSPB6, which leads to depolymerization of actin. Papaverine inhibits HSV spasm via multiple molecular mechanisms.

Irradiation-tolerant lung cancer cells acquire invasive ability dependent on dephosphorylation of the myosin regulatory light chain

Radiotherapy is one of the major treatment modalities for malignancies. However, cells surviving irradiation often display high levels of invasiveness. This study shows that irradiation-tolerant lung adenocarcinoma demonstrates high invasive capability depending on dephosphorylation of the myosin regulatory light chain (MRLC). In a collagen gel overlay condition, low-invasive subclones of lung adenocarcinoma (A549P-3) showed a round morphology and diphosphorylation of MRLC. In contrast, irradiation-tolerant A549P-3 cells (A549P-3IR) displayed high invasiveness and a lower level of MRLC diphosphorylation. In addition, inhibition of MRLC phosphatase activity decreased the invasive activity. These findings suggest that A549P-3IR cells acquire high invasiveness through MRLC dephosphorylation.

Stable Double-Headed Structure is Essential not only for the Inhibited State, but also for the Fully Activated State of Smooth Muscle Myosin

It is generally accepted that dephosphorylation of regulatory light chain (RLC) induces the interactions between the two heads of smooth muscle myosin (Sm) and between the head and the tail, thus inhibiting the motor activity, and phosphorylation of RLC interrupts above interactions, thus reversing the inhibition and stimulating the motor activity to maximal value. Thus it is predicted that Sm subfragment-1 (S1) containing only one head without the tail is fully active. However, no solid evident so far supports this prediction, although early studies showed that S1 produced by limited protease treatment is partially active. Here we produced a number of Sm truncations with various length of the tail. The stability of double-headed structure of Sm is dependent on the length of the coiled-coil tail. The Sm truncations with coiled-coil longer than 214 amino acids form stable double-headed structure (stable HMM), those with coiled-coil shorter than 179 amino acids form unstable double headed structure (unstable HMM), and that without coiled-coil, i.e. S1, is completely single-headed. Phosphorylation of RLC regulates the motor activity of stable HMM completely, regulates that of unstable HMM partially, and does not regulate that of S1. Unexpectedly, the actin-activated ATPase activity of S1, either unphosphorylated or phosphorylated, is higher than that of unphosphorylated stable HMM but less than 10% of that of phosphorylated stable HMM. The actin-activated ATPase activities of unphosphorylated Sm truncations increase with the shortening of the coiled-coil tail, and that of phosphorylated Sm truncations decrease with the shortening of the coiled-coil tail. These results indicate that the stable double-headed structure is critical not only for the inhibited state of unphosphorylated Sm, but also for the fully activated state of phosphorylated Sm.

Oxidized LDL/CD36 interaction induces loss of cell polarity and inhibits macrophage locomotion.

Cell polarization is essential for migration and the exploratory function of leukocytes. However, the mechanism by which cells maintain polarity or how cells revert to the immobilized state by gaining cellular symmetry is not clear. Previously we showed that interaction between oxidized low-density lipoprotein (oxLDL) and CD36 inhibits macrophage migration; in the current study we tested the hypothesis that oxLDL/CD36-induced inhibition of migration is the result of intracellular signals that regulate cell polarity. Live cell imaging of macrophages showed that oxLDL actuated retraction of macrophage front end lamellipodia and induced loss of cell polarity. Cd36 null and macrophages null for Vav, a guanine nucleotide exchange factor (GEF), did not show this effect. These findings were caused by Rac-mediated inhibition of nonmuscle myosin II, a cell polarity determinant. OxLDL induced dephosphorylation of myosin regulatory light chain (MRLC) by increasing the activity of Rac. Six-thioguanine triphosphate (6-thio-GTP), which inhibits Vav-mediated activation of Rac, abrogated the effect of oxLDL. Activation of the Vav-Rac-myosin II pathway by oxidant stress may induce trapping of macrophages at sites of chronic inflammation such as atherosclerotic plaque.

LDL-Induced Impairment of Human Vascular Smooth Muscle Cells Repair Function Is Reversed by HMG-CoA Reductase Inhibition

Growing human atherosclerotic plaques show a progressive loss of vascular smooth muscle cells (VSMC) becoming soft and vulnerable. Lipid loaded-VSMC show impaired vascular repair function and motility due to changes in cytoskeleton proteins involved in cell-migration. Clinical benefits of statins reducing coronary events have been related to repopulation of vulnerable plaques with VSMC. Here, we investigated whether HMG-CoA reductase inhibition with rosuvastatin can reverse the effects induced by atherogenic concentrations of LDL either in the native (nLDL) form or modified by aggregation (agLDL) on human VSMC motility. Using a model of wound repair, we showed that treatment of human coronary VSMC with rosuvastatin significantly prevented (and reversed) the inhibitory effect of nLDL and agLDL in the repair of the cell depleted areas. In addition, rosuvastatin significantly abolished the agLDL-induced dephosphorylation of myosin regulatory light chain as demonstrated by 2DE-electrophoresis and mass spectrometry. Besides, confocal microscopy showed that rosuvastatin enhances actin-cytoskeleton reorganization during lipid-loaded-VSMC attachment and spreading. The effects of rosuvastatin on actin-cytoskeleton dynamics and cell migration were dependent on ROCK-signalling. Furthermore, rosuvastatin caused a significant increase in RhoA-GTP in the cytosol of VSMC. Taken together, our study demonstrated that inhibition of HMG-CoA reductase restores the migratory capacity and repair function of VSMC that is impaired by native and aggregated LDL. This mechanism may contribute to the stabilization of lipid-rich atherosclerotic plaques afforded by statins.

Molecular Mechanism of Telokin-mediated Disinhibition of Myosin Light Chain Phosphatase and cAMP/cGMP-induced Relaxation of Gastrointestinal Smooth Muscle

Phospho-telokin is a target of elevated cyclic nucleotide concentrations that lead to relaxation of gastrointestinal and some vascular smooth muscles (SM). Here, we demonstrate that in telokin-null SM, both Ca(2+)-activated contraction and Ca(2+) sensitization of force induced by a GST-MYPT1(654-880) fragment inhibiting myosin light chain phosphatase were antagonized by the addition of recombinant S13D telokin, without changing the inhibitory phosphorylation status of endogenous MYPT1 (the regulatory subunit of myosin light chain phosphatase) at Thr-696/Thr-853 or activity of Rho kinase. Cyclic nucleotide-induced relaxation of force in telokin-null ileum muscle was reduced but not correlated with a change in MYPT1 phosphorylation. The 40% inhibited activity of phosphorylated MYPT1 in telokin-null ileum homogenates was restored to nonphosphorylated MYPT1 levels by addition of S13D telokin. Using the GST-MYPT1 fragment as a ligand and SM homogenates from WT and telokin KO mice as a source of endogenous proteins, we found that only in the presence of endogenous telokin, thiophospho-GST-MYPT1 co-precipitated with phospho-20-kDa myosin regulatory light chain 20 and PP1. Surface plasmon resonance studies showed that S13D telokin bound to full-length phospho-MYPT1. Results of a protein ligation assay also supported interaction of endogenous phosphorylated MYPT1 with telokin in SM cells. We conclude that the mechanism of action of phospho-telokin is not through modulation of the MYPT1 phosphorylation status but rather it contributes to cyclic nucleotide-induced relaxation of SM by interacting with and activating the inhibited full-length phospho-MYPT1/PP1 through facilitating its binding to phosphomyosin and thus accelerating 20-kDa myosin regulatory light chain dephosphorylation.