Naoki Isaka

Inhibitory Phosphorylation Site for Rho-associated Kinase on Smooth Muscle Myosin Phosphatase

It is clear from several studies that myosin phosphatase (MP) can be inhibited via a pathway that involves RhoA. However, the mechanism of inhibition is not established. These studies were carried out to test the hypothesis that Rho-kinase (Rho-associated kinase) via phosphorylation of the myosin phosphatase target subunit 1 (MYPT1) inhibited MP activity and to identify relevant sites of phosphorylation. Phosphorylation by Rho-kinase inhibited MP activity and this reflected a decrease in V max. Activity of MP with different substrates also was inhibited by phosphorylation. Two major sites of phosphorylation on MYPT1 were Thr695 and Thr850. Various point mutations were designed for these phosphorylation sites. Following thiophosphorylation by Rho-kinase and assays of phosphatase activity it was determined that Thr695 was responsible for inhibition. A site- and phosphorylation-specific antibody was developed for the sequence flanking Thr695 and this recognized only phosphorylated Thr695 in both native and recombinant MYPT1. Using this antibody it was shown that stimulation of serum-starved Swiss 3T3 cells by lysophosphatidic acid, thought to activate RhoA pathways, induced an increase in Thr695 phosphorylation on MYPT1 and this effect was blocked by a Rho-kinase inhibitor, Y-27632. In summary, these results offer strong support for a physiological role of Rho-kinase in regulation of MP activity.

Activation of RhoA and Inhibition of Myosin Phosphatase as Important Components in Hypertension in Vascular Smooth Muscle

Abstract— Two mechanisms are proposed to account for the inhibition of myosin phosphatase (MP) involved in Ca2+ sensitization of vascular muscle, ie, phosphorylation of either MYPT1, a target subunit of MP or CPI-17, an inhibitory phosphoprotein. In cultured vascular aorta smooth muscle cells (VSMCs), stimulation with angiotensin II activated RhoA, and this was blocked by pretreatment with 8-bromo-cGMP. VSMCs stimulated by angiotensin II, endothelin-1, or U-46619 significantly increased the phosphorylation levels of both MYPT1 (at Thr696) and CPI-17 (at Thr38). The angiotensin II-induced phosphorylation of MYPT1 was completely blocked by 8-bromo-cGMP or Y-27632 (a Rho-kinase inhibitor), but not by GF109203X (a PKC inhibitor). In contrast, phosphorylation of CPI-17 was inhibited only by GF109203X. Y-27632 dramatically corrected the hypertension in N&ohgr;-nitro-l-arginine methyl ester (L-NAME)-treated rats, and this hypertension also was sensitive to isosorbide mononitrate. The level of the active form of RhoA was significantly higher in aortas from L-NAME-treated rats. Expression of RhoA, Rho-kinase, MYPT1, CPI-17, and myosin light chain kinase were not significantly different in aortas from L-NAME-treated and control rats. Activation of RhoA without changes in levels of other signaling molecules were observed in three other rat models of hypertension, ie, stroke-prone spontaneously hypertensive rats, renal hypertensive rats, and DOCA-salt rats. These results suggest that independent of the cause of hypertension, a common point in downstream signaling and a critical component of hypertension is activation of RhoA and subsequent activation of Rho-kinase.

Rho-associated Kinase of Chicken Gizzard Smooth Muscle

Rho-associated kinase (Rho-kinase) from chicken gizzard smooth muscle was purified to apparent homogeneity (160 kDa on SDS-polyacrylamide gel electrophoresis) and identified as the ROKα isoform. Several substrates were phosphorylated. Rates with myosin phosphatase target subunit 1 (MYPT1), myosin, and the 20-kDa myosin light chain were higher than other substrates. Thiophosphorylation of MYPT1 inhibited myosin phosphatase activity. Phosphorylation of myosin at serine 19 increased actin-activated Mg+-ATPase activity, i.e. similar to myosin light chain kinase. Myosin phosphorylation was increased at higher ionic strengths, possibly by formation of 6 S myosin. Phosphorylation of the isolated light chain and myosin phosphatase was decreased by increasing ionic strength. Rho-kinase was stimulated 1.5–2-fold by guanosine 5′-O-3-(thio)triphosphate·RhoA, whereas limited tryptic hydrolysis caused a 5–6-fold activation, independent of RhoA. Several kinase inhibitors were screened and most effective were Y-27632, staurosporine, and H-89. Several lipids caused slight activation of Rho-kinase, but arachidonic acid (30–50 μm) induced a 5–6-fold activation, independent of RhoA. These results suggest that Rho-kinase of smooth muscle may be involved in the contractile process via phosphorylation of MYPT1 and myosin. Activation by arachidonic acid presents a possible regulatory mechanism for Rho-kinase.

Rho/Rho-Kinase Pathway Contributes to C-Reactive Protein–Induced Plasminogen Activator Inhibitor-1 Expression in Endothelial Cells

Objective—Rho/Rho-kinase pathway plays pivotal roles in cardiovascular diseases including arteriosclerosis and hypertension. Recently it has become evident that C-reactive protein (CRP), a powerful marker for cardiovascular events, has direct proatherothrombotic effects on vascular cells. However, its molecular mechanism has not been fully investigated. We examined the involvement of Rho/Rho-kinase signaling in CRP-induced plasminogen activator inhibitor-1 (PAI-1) expression in bovine aortic endothelial cells (BAECs). Methods and Results—PAI-1 expression was determined by Western blotting. RhoA activation was determined by an affinity pull-down assay using Rho-binding fragment of rhotekin. NF-&kgr;B activity was determined using the luciferase reporter gene. Incubation of BAECs with human recombinant CRP (≥25 &mgr;g/mL) induced a significant increase in PAI-1 expression. Stimulation of BAECs with CRP significantly increased RhoA activation. Pretreatment with TAT-C3 (a membrane-permeable RhoA inhibitor) and Y-27632 (Rho-kinase inhibitor) significantly inhibited CRP-induced PAI-1 expression. NF-&kgr;B activity was markedly enhanced by CRP and pretreatment with Y-27632 inhibited its activation. Parthenolide, SN50, and BAY 11-7082 (NF-&kgr;B inhibitors) significantly blocked CRP-mediated PAI-1 expression. Conclusions—These data suggested that CRP activates Rho/Rho-kinase signaling, which in turn activates NF-&kgr;B activity, resulting in PAI-1 expression in BAEC. These observations provide evidence for the possible involvement of Rho/Rho-kinase signaling in CRP-induced atherothrombogenesis.

The Negative Inotropic Effect of β3-Adrenoceptor Stimulation in the Beating Guinea Pig Heart

Although beta3-adrenoceptors (ARs) have been extensively characterized in brown and white adipocytes, their actions in the beating heart are unclear. We examined the effects of a beta3-AR agonist, BRL37344, on cardiac function and calcium transients in Langendorff-perfused guinea pig hearts by simultaneously measuring left ventricular (LV) pressure and Ca2+-dependent indo-1 fluorescence. BRL37344 induced a dose-dependent negative inotropic effect at concentrations from 10(-11) to 10(-8) M. Maximally, LV developed pressure decreased to 80+/-2%, +dP/dt to 81+/-2%. and -dP/dt to 81+/-3% of their respective control values (p < 0.01). The amplitude of the Ca2+ transient also decreased (to 92+/-3% of the control level; p < 0.01). The BRL37344 dose-response curve was not altered by nadolol (10(-5) M), a potent beta1- and beta2-AR antagonist, but completely suppressed by bupranolol (10(-6) M), a potent beta1-, beta2- and beta3-AR antagonist. To assess the potential role of a nitric oxide synthase (NOS) pathway, we determined whether the NOS inhibitor, N(G)-nitro-L-arginine methyl ester (L-NAME), modified the contractile response to BRL37344. L-NAME (10(-7) and 10(-4) M) attenuated the negative inotropic effects on LV developed pressure by 35 and 50%, suggesting that beta3-AR stimulation induces a negative inotropic effect on guinea pig hearts partly through a decrease in the Ca2+ transient and partly by the NOS pathway.

Effects of the Phosphorylation of Myosin Phosphatase by Cyclic GMP-dependent Protein Kinase

Cyclic GMP-dependent protein kinase (PKG) phosphorylated, in vitro, the large (MYPT1) and small (M20) regulatory subunits of myosin phosphatase (MP) with maximum stoichiometries of 1.8 and 0.6 mol of phosphate/mol subunit, respectively. The phosphorylation of these subunits by PKG did not affect the phosphatase activity towards the 20 kDa myosin light chain. However, phosphorylation of the MP holoenzyme decreased the binding of MP to phospholipid. The phosphorylation of the serine residue of the C-terminal part of MYPT1 was crucial for these interactions. These results suggest that the phosphorylation of MP by PKG is not a direct mechanism in activating MP activity, and that other indirect mechanisms, including the interaction between MP and phospholipids, might be candidates for Ca2+ desensitization via cGMP in smooth muscle.

Prognostic significance of early diagnosis in acute pulmonary thromboembolism with circulatory failure

Abstract. Despite the advances in our understanding of venous thromboembolic disease, the prevalence of pulmonary thromboembolism (PTE) at autopsy has not changed over 3 decades. When patients survive long enough to have a diagnosis of massive PTE and start receiving treatment, the outlook is considered to be moderately good. However, the diagnosis is often difficult to obtain and is frequently missed. We hypothesize that mortality of acute PTE is reduced by early diagnosis. Eighty-five patients with acute PTE with circulatory failure who survived 1 h after the onset were divided into two groups: the early Dx group consisted of the patients whose disease was diagnosed as acute PTE within 24 h of the onset, and the Late Dx group included patients whose disease was not diagnosed within 24 h of onset, or died without clinical diagnosis between 1 and 24 h after the onset. Overall mortality was significantly low in the Early Dx group compared with that of the Late Dx group (21.6% vs 67.6%, P < 0.0001). Multiple logistic regression analysis demonstrated that a reduction in in-hospital mortality was associated with early diagnosis (odds ratio for in-hospital death, 0.094; 95% confidence interval, 0.03–0.33). The results of our study suggested that early diagnosis might favorably affect the in-hospital clinical outcome of hemodynamically unstable patients with acute PTE.

The relationship between aortic augmentation index and pulse wave velocity: an invasive study.

Objectives The aortic augmentation index (AI) and aortic pulse wave velocity (PWV) are known to be indicators of arterial stiffness. However, it is not clear whether aortic AI and PWV reflect aortic stiffness in similar ways. We investigated the relationship between aortic AI and PWV by measuring them directly using a catheter technique. Design and methods Forty-one patients, aged 34–79 years, were studied during diagnostic cardiac catheterization. Aortic pressures were measured using a catheter-tip manometer at two points, one in the ascending aorta and one 40 cm distally in the descending aorta. Aortic AI was defined as the difference between early and late pressure peaks divided by the pulse pressure of the ascending aorta. Aortic PWV was calculated as the distance between the two measuring sites divided by the transit time. We also examined the effects of vasodilatation on AI and PWV by the intra-aortic administration of nitroglycerin in 15 patients. Results AI was significantly related to age, systolic aortic pressure, heart rate, left ventricular ejection time, and height. Aortic PWV showed an association only with age and systolic aortic pressure. There was no significant relationship between aortic AI and PWV (r = 0.28, NS). Nitroglycerin also produced different effects on aortic AI and PWV: aortic AI was significantly decreased (−0.17, P < 0.01) after nitroglycerin, but PWV remained unchanged (+0.4 m/s, NS). Conclusions Aortic AI and PWV cannot be used interchangeably as an index of arterial stiffness. AI may not be a true indicator of arterial stiffness, but an index of wave reflection including PWV.

Increased tissue factor pathway inhibitor in patients with acute myocardial infarction.

We examined hemostatic abnormalities in 23 patients with acute myocardial infarction (AMI), 10 with pulmonary embolism (PE), and 10 with deep vein thrombosis (DVT). At the onset of AMI, plasma levels of tissue‐type plasminogen activator (t‐PA), PA inhibitor‐I (PAI‐I), fibrin‐D‐dimer, thrombin‐antithrombin complex (TAT), and plasmin‐plasmin inhibitor complex (PPIC) were significantly increased. Both the plasma total TFPI and free‐TFPI levels in the AMI patients were significantly higher than those in the healthy volunteers, PE patients, and DVT patients. There was no significant difference in total TFPI or free‐TFPI among patients with PE, those with DVT, and healthy volunteers. One hour after percutaneous transluminal coronary angioplasty (PTCA) in the AMI group, the total TFPI level was further increased, and it was significantly reduced 24 hr after PTCA, to a level similar to that in healthy volunteers. Free‐TFPI showed a pattern similar to that of total TFPI. The ratio of free‐TFPI/total TFPI was highest 1 hr after PTCA. Increased TFPI in AMI patients might be released from ischemic tissues. Am. J. Hematol. 55:183–187, 1997.

Circadian rhythm of blood pressure is transformed from a dipper to a non-dipper pattern in shift workers with hypertension

Shift workers make great use of health care services because they are associated with increased cardiovascular morbidity and mortality. Whether the circadian rhythm of blood pressure rapidly adapts to shift work is controversial. It is unknown if shift work has adverse effects on blood pressure in patients with hypertension. To evaluate the effects of shift work, we examined 12 male shift workers with untreated hypertension aged 53.6 ± 2.5 years. Twenty-four hour ambulatory blood pressure monitoring was performed three times as follows: the last day of a 4-day period of day shifts (09.00 to 21.00), the first day of a 4-day period of night shifts (21.00 to 09.00), and the fourth day of night shifts (21.00 to 09.00). Blood pressure at night-time dropped significantly in the day-shift workers, showing a dipper pattern. Average differences in blood pressure in the sleep-wake cycle were decreased by 8.5% at the beginning of night shift work showing a non-dipper pattern. After 4 days the pattern was completely reversed to a dipper pattern. The results indicate that the circadian blood pressure pattern is changed from a dipper to a non-dipper pattern on the first day of the night shift and reverses to a dipper pattern within a few days. We suggest that night shift work may have unfavourable effects on blood pressure in patients with hypertension.