New Insights into the Role and Regulation of Autophagy during Myocardial Ischaemia

Abstract

Macroautophagy (hereafter autophagy) is a catabolic process whereby long-lived proteins and damaged organelles, such as mitochondria, peroxisomes and endoplasmic reticulum, are sequestered by double membrane vesicles called autophagosomes, and delivered to lysosomes for degradation by hydrolases. The resulting macromolecules are then released back into the cytosol for reuse. Autophagy is regulated by autophagy genes (ATGs) encoding proteins involved in autophagosome formation and their fusion with lysosomes. Genetic screenings in yeast have identifiedmore than 30ATGs,most of which have mammalian homologues. A major signalling cascade regulating autophagy includes the mammalian target of rapamycin (mTOR) pathway, which phosphorylates Atg13 andULK1. Autophagy is easily detected under basal conditions in the heart, where it participates in homeostatic functions, such as turnover of proteins and organelles. However, autophagy is enhanced during fasting and other pathological conditions, including cardiac remodelling, heart failure and ischaemia/ reperfusion. The functional significance of autophagy in the heart during stress is complex. Although mild to modest activation of autophagy promotes survival of cardiomyocytes, excessive activation of autophagy could cause cell death. During the past few years, we have focused our scientific interest on the role of autophagy during cardiomyocyte energy deprivation, such as glucose starvation and ischaemia.We have studied the signalling mechanisms regulating autophagy in these conditions, and demonstrated that autophagy is activated and protective during energy stress. Yan et al. demonstrated that autophagy is significantly upregulated during chronic ischaemia (i.e. myocardial hibernation) in the pig heart, and that the level of autophagy is inversely correlated with that of apoptosis in the ischaemic area. In addition, Matsui et al. demonstrated that autophagy, which is activated during prolonged ischaemia through adenosine monophosphate-activated protein kinase (AMPK)-dependent mechanisms, is protective for the heart. Autophagymay compensate for the loss of energy through regeneration of amino acids and fatty acids, which can be used for adenosine triphosphate synthesis through the tricarboxylic acid cycle. Alternatively, autophagy removes damaged mitochondria, thereby reducing reactive oxygen species and inhibiting the release of pro-apoptotic factors, such as cytochrome C. Autophagy also eliminates protein aggregates, which accumulate during ischaemia and interfere with cellular functions. Amajor regulator of autophagy during ischaemia is AMPK, a well known sensor of energy depletion. AMPK activation during cardiomyocyte energy deprivation leads to inhibition of the mTOR pathway, resulting in the activation of autophagy. We have demonstrated recently that glycogen synthase kinase-3b (GSK-3b) is also activated during myocardial ischaemia, stimulating autophagy through inhibition of the mTOR pathway. Thus, both AMPK and GSK-3b act as energy sensors and inhibit mTOR during myocardial ischaemia. In order to clarify the functional significance of the mTOR inhibition during myocardial ischaemia, we investigated the involvement of Ras homologue enriched in brain (Rheb), a small guanosine triphosphate binding protein, which binds and selectively activates the multiprotein complex 1 of mTOR. Energy starvation activates AMPK andGSK-3b, which in turn stimulate tuberous sclerosis complex protein 2 (TSC2). Rheb is inhibited during energy stress through activation of TSC2. We have recently shown that mTOR is inhibited during nutrient starvation and ischaemia through inhibition of Rheb in cardiomyocytes. Furthermore, we demonstrated that forced activation ofRheb during ischaemia inhibits autophagy and enhances cell death. Our results, therefore, suggest that the inhibition of mTOR and the consequent activation of autophagy during myocardial ischaemia are protective. Furthermore, Rheb integrates the signals from the upstream kinases, namely AMPK and GSK-3b, thereby acting as a central regulator of the mTOR pathway and autophagy during cardiomyocyte ischaemia. INTERNATIONAL RESEARCH HIGHLIGHTS High Blood Press Cardiovasc Prev 2010; 17 (4): 235-236 1120-9879/10/0004-0235/$49.95/0