Yoko Yano

Cardioprotective Role of AT2 Receptor in Postinfarction Left Ventricular Remodeling

Abstract—The aim of this study was to determine the role of the AT2 receptor (AT2R) in left ventricular (LV) remodeling after myocardial infarction (MI). The left anterior descending arteries were ligated in AT2R gene knockout (Agtr2-) and wild-type (Agtr2+) mice. The LV remodeling was evaluated by echocardiography and histology over a period of 2 weeks after MI. The infarct sizes in hearts excised from Agtr2+ and Agtr2- mice on day 1 were similar. The mortality rate of Agtr2- mice (62.9%) on day 14 after MI was significantly (P <0.05) higher than that of Agtr2+ mice (39.7%). Accordingly, LV/body weight ratios (3.7±0.2 versus 3.0±0.1 on day 14) and LV end-diastolic (4.8±0.3 versus 3.9±0.4 mm on day 7) and end-systolic (4.4±0.3 versus 3.2±0.6 mm on day 7) dimensions evaluated by echocardiography were significantly greater in Agtr2- than in Agtr2+ mice. The rates of ventricular arrhythmia, rates of cardiac rupture, and blood pressures in the 2 strains were similar after MI. Myocyte cross-sectional areas were increased after MI, but the magnitudes were similar in Agtr2+ and Agtr2- mice, indicating the greater increases in LV dimensions and weight in Agtr2- mice are due to elongation of myocyte length and/or an increase in the interstitial weight (including vasculatures, infiltrated cells, and interstitial fluid). Interstitial fibrosis in remote myocardium was not evident in either strain. These results indicate AT2R plays a significant role in the protection against early development of LV dilation, thereby reducing the early mortality rate after MI.

Genetic ablation of the transcription repressor Bach1 leads to myocardial protection against ischemia/reperfusion in mice

Bach1 is a transcriptional repressor of heme oxygenase‐1 gene (Hmox‐1) and β‐globin gene. Heme oxygenase (HO)‐1 is an inducible cytoprotective enzyme that degrades pro‐oxidant heme to carbon monoxide (CO) and biliverdin/bilirubin, which are thought to mediate anti‐inflammatory and anti‐oxidant actions of HO‐1. In the present study, we investigated the role of Bach1 in tissue protection against myocardial ischemia/reperfusion (I/R) injury in vivo using mice lacking the Bach1 gene (Bach1−/−) and wild‐type (Bach1+/+) mice. In Bach1−/− mice, myocardial expression of HO‐1 protein was constitutively up‐regulated by 3.4‐fold compared to that in Bach1+/+ mice. While myocardial I/R induced HO‐1 protein in ischemic myocytes in both strains of mice, the extent of induction was significantly greater in Bach1−/− mice than in Bach1+/+ mice. Myocardial infarction was markedly reduced in size by 48.4% in Bach1−/− mice. Pretreatment of Bach1−/− mice with zinc‐protoporphyrin, an inhibitor of HO activity, abolished the infarction‐reducing effect of Bach1 disruption, indicating that reduction in the infarct size was mediated, at least in part, by HO‐1 activity. Thus, Bach1 plays a pivotal role in setting the levels of both constitutive and inducible expression of HO‐1 in the myocardium. Bach1 inactivation during I/R appears to be a key mechanism controlling the activation level of cytoprotective program involving HO‐1.

Myocardial Protection Against Pressure Overload in Mice Lacking Bach1, a Transcriptional Repressor of Heme Oxygenase-1

Bach1 is a stress-responsive transcriptional factor that is thought to control the expression levels of cytoprotective factors, including heme-oxygenase (HO)-1. In the present study, we investigated the roles of Bach1 in the development of left ventricular (LV) hypertrophy and remodeling induced by transverse aortic constriction (TAC) in vivo using Bach1 gene-deficient (Bach1−/−) mice. TAC for 3 weeks in wild-type control (Bach1+/+) mice produced LV hypertrophy and remodeling manifested by increased heart weight, histological findings showing increased myocyte cross-sectional area (CSA) and interstitial fibrosis (picro Sirius red staining), reexpressions of ANP, BNP, and &bgr;MHC genes, and echocardiographic findings showing wall thickening, LV dilatation, and reduced LV contraction. Deletion of Bach1 caused significant reductions in heart weight (by 16%), CSA (by 36%), tissue collagen content (by 38%), and gene expression levels of ANP (by 75%), BNP (by 45%), and &bgr;MHC (by 74%). Echocardiography revealed reduced LV dimension and ameliorated LV contractile function. Deletion of Bach1 in the LV caused marked upregulation of HO-1 protein accompanied by elevated HO activity in both basal or TAC-stimulated conditions. Treatment of Bach1−/− mice with tin-protoporphyrin, an inhibitor of HO, abolished the antihypertrophic and antiremodeling effects of Bach1 gene ablation. These results suggest that deletion of Bach1 caused upregulation of cytoprotective HO-1, thereby inhibiting TAC-induced LV hypertrophy and remodeling, at least in part, through activation of HO. Bach1 repressively controls myocardial HO-1 expression both in basal and stressed conditions, inhibition of Bach1 may be a novel therapeutic strategy to protect the myocardium from pressure overload.

Beneficial Effect of T-Type Calcium Channel Blockers on Endothelial Function in Patients with Essential Hypertension

Endothelial function is impaired in essential hypertension. T-type but not L-type voltage-gated Ca2+ channels were detected in the vascular endothelium. The purpose of the present study was to clarify the role of T-type Ca2+ channels in endothelial function. We studied flow-mediated vasodilation (FMD) and sublingual nitroglycerin (NTG)-induced vasodilation in the brachial artery. Forty patients with essential hypertension were randomly assigned to treatment with efonidipine, a T- and L-type Ca2+ channel blocker, or with nifedipine, an L-type Ca2+ channel blocker. Twenty healthy normotensive individuals were included as a control group. In patients with essential hypertension, FMD was attenuated and NTG was similar that of compared to healthy controls. After 12 weeks, the decrease in mean blood pressure in the efonidipine and nifedipine groups were similar. The endothelial function index, a ratio of FMD/NTG, was significantly increased by efonidipine (73±24 to 94±20%) but unchanged by nifedipine. Urinary excretion 8-hydroxy-2′-deoxyguanosine (8-OHdG) and serum malondialdehyde-modified low-density lipoprotein (LDL) were decreased by efonidipine but unchanged by nifedipine. These results suggest that a T-type Ca2+ channel blocker, but not an L-type Ca2+ channel blocker, may improve vascular endothelial dysfunction in patients with essential hypertension via a reduction in oxidative stress.

Change in bilirubin level following acute myocardial infarction is an index for heme oxygenase activation.

Objectives: Heme oxygenase 1 (HO-1) is rapidly induced by stress, degrading pro-oxidant heme into carbon monoxide, bilirubin, and free iron (Fe). Induction of HO-1 is an important defense mechanism against tissue injury. Here, we tested the hypothesis that HO-1 is activated in the myocardium after acute myocardial infarction (AMI) in humans. Methods: Changes in the HO-1 activity after AMI were analyzed by measuring serum levels of bilirubin and Fe. Blood samples were collected in patients with AMI (n = 41) serially after the interventional therapy and compared with non-AMI subjects (n = 18). HO-1 protein levels were measured in a sample of AMI patients (n = 12). Results: In AMI patients, but not in non-AMI subjects, serum levels of bilirubin (1.57 fold, P < 0.001) and Fe (1.35 fold, P < 0.01) were transiently elevated, both levels peaking 18–21 hours after the start of sampling. The peak changes in the levels of bilirubin and Fe in AMI patients were significantly correlated with each other. Furthermore, the serum HO-1 protein level was elevated, and its change was significantly correlated with the change in bilirubin level (r = 0.82, P < 0.005). Those with a high bilirubin response (peak levels >0.5 mg/dL) had richer collateral flow into the ischemic myocardium. Conclusions: These results suggest that heme oxygenase (HO) was activated following AMI, and it was detectable in the serum. Our data provide the first evidence of HO-1 induction following stress in humans. The change in bilirubin level may be a novel index for high collateral flow formation following AMI.

Immunohistochemical distributions of the tissue kallikrein-kinin system in ischemic and non-ischemic mouse heart.

Summary: Kinins have been shown to play a cardioprotective role during myocardial ischemia. However, the localization of each of the components of the kallikrein‐kinin system in the heart has not been determined in a cell type‐specific manner. Recently, mK1 has been identified as the major tissue kallikrein with the strongest bradykinin‐forming activity among the products of the mouse tissue kallikrein gene superfamily. In the study presented here, we investigated the localizations of mK1, kininogen and bradykinin B2 receptors (B2Rs) in ischemic and non‐ischemic left ventricles by immunohistochemistry. Kininogen, which contains bradykinin as a surface epitope, was detected by an anti‐bradykinin antibody. Changes in the amounts of mK1 and B2R were evaluated by Western blot analysis. Myocardial ischemia was induced by ligation of the left anterior descending coronary artery for 60 min followed by reperfusion for 24 h. mK1 and B2Rs were most abundantly expressed in the vascular endothelium and, to a lesser extent, in fibroblasts. No immunohistochemical signal of these molecules was detected in myocytes. Kininogen was localized in the vascular endothelium and the smooth muscle layer. Myocardial ischemia, although it had no effect on the localization of these molecules, increased the amounts of mK1 and B2R. We have obtained immunohistochemical evidence that all components of the tissue kallikrein‐kinin system are present in the mouse heart. The coronary artery is the major site of kallikrein‐kinin activity both in ischemic and non‐ischemic hearts.