Whether and how insulin counteracts the cytotoxic effects of hypoxia and improves cardiomyocyte viability remains unclear. To achieve this aim, cultured neonatal rat cardiomyocytes pretreated with vehicle or 1 μM insulin were exposed to either normoxic or hypoxia environment for up to 24 h. Cell viability was monitored and cellular apoptosis as well as necrosis, indexes of autophagy, endoplasmic reticular (ER) stress, and expressions of specific relevant mediators of the signaling pathways of autophagy were also assessed. Hypoxia impaired cell viability, induced autophagy, triggered apoptosis, activated ER stress pathway-associated apoptotic responses along with downstream pro-apoptotic transcriptional factor C/EBP homologous protein (CHOP), and increased apoptosis of myocardial cells. On the other hand, insulin pretreatment effectively ameliorated autophagy via PI3-K/Akt signaling pathway, suppressed ER stress, and prevented hypoxia-induced cellular apoptosis. In an ex vivo study, isolated rat hearts were pre-treated in some cases with insulin and subjected to proximal left coronary artery ligation to induce acute myocardial ischemia. Coronary ligation-induced acute ischemia upregulated glucose-related protein 78 (GRP78) and triggered cellular apoptosis in the jeopardized myocardium. Conversely, insulin pretreatment suppressed these hypoxia-related cytotoxic events and reduced myocardial infarct size by up to 15.2%. In conclusion, hypoxia impedes cell viability through triggering autophagy, ER stress and apoptosis, whereas insulin pretreatment effectively prevents these cytotoxic actions of hypoxia, preserves myocardial cell viability and reduces myocardial infarct size. These results indicated the cytoprotective mechanism of insulin against the insult of hypoxia may justify insulin as a therapeutic option for patients with acute myocardial infarction.
Keywords: Autophagy; Cardiomyocyte; Hypoxia; Infarction; Insulin.
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