Studies On The Role Of LXR In Myocardial Ischemia/Reperfusion Injury And Underlying Mechanisms

Objective: To determine the role of Liver X receptors(LXR) in myocardial ischemia-reperfusion injury(MI/R injury) and to investigate the underlying mechanisms involved.Methods: For the acute ischemia-reperfusion protocol, mice were randomly assigned to the following groups: sham, vehicle, 22(R)-HC(20 mg/kg), or GW3965(20 mg/kg) by intraperitoneal injection 15 minutes before reperfusion. Reperfusion commenced for 15 minutes(for phosphorylated protein assay), 3 hours(for apoptosis, inflammation, oxidation, and nitration assays), or 24 hours(for cardiac function and infarct size assay). To observe the long-term cardioprotective effect of LXR agonists, mice were randomly assigned to the following groups: sham, vehicle, 22(R)-HC(20 mg/kg), or GW3965(20 mg/kg) by intraperitoneal injection 15 minutes before reperfusion, and then daily following reperfusion for 4 weeks. LXR expression was detected by westert blot and quantitative PCR. Cardiomyocytes apoptosis was determined by TUNEL and Caspase-3 activity assay kit. Infarcted myocardial tissue and cardiac function was evaluated using biopsy staining, echocardiographic measurements and 18F-FDG Micro-PET/CT scanning. The mechanism of LXR cardioprotective effects in MI/R injury and the different functional roles of the LXR subtypes were determined by caspase activity, western blot, quantitative PCR, immunofluorescence, immunohistochemistry and Enzyme-linked immuno sorbent assay.Results: Both LXRα and LXRβ were detected in the mouse heart. LXRα, but not LXRb, was significantly upregulated after MI-R. Treatment with LXR agonists reduced myocardial infarction and improved contractile function after MI/R. Mechanistically, LXR activation differentially regulated cell-survival and cell-death kinases(increasing Akt and ERK phosphorylation and inhibiting p38 MAPK phosphorylation), inhibited endoplasmic reticulum stress(determined by CCAAT/enhancer-binding protein homologous protein and caspase-12), attenuated mitochondrial dysfunction(determined by mitochondrial cytochrome c release and caspase-9 activation), and reduced cardiomyocyte apoptosis. Furthermore, LXR activation inhibited MI/R-induced oxidative stress(determined by superoxide production and NADPH oxidase expression), nitrative stress(determined by peroxynitrite formation and inducible nitric oxide synthase expression), and inflammatory response(determined by macrophage infiltration and proinflammatory cytokine production) in ischemic-reperfused myocardium. LXRα-KO, but not LXRβ-KO, increased MI/R injury(greater apoptosis, larger infarct size, and poor cardiac function), exacerbated MI/R-induced oxidative/nitrative stress, increased inflammatory response, and enhanced p38 MAPK activation. Moreover, LXRa-KO, but not LXRb-KO, blocked the cardioprotective effects of LXR agonists.Conclusion: Our study provides the first evidence supporting LXRα as a novel cardiac protective receptor against MI/R injury via inhibition of both endoplasmic reticular stress and the mitochondrial apoptotic pathway. LXRα may potentially be an attractive molecular target for the treatment of ischemic heart disease.