Distinct Roles of Autophagy in the Heart During Ischemia and Reperfusion

Abstract

Abstract-Autophagy is an intracellular bulk degradation process for proteins and organelles. In the heart, autophagy is stimulated by myocardial ischemia. However, the causative role of autophagy in the survival of cardiac myocytes and the underlying signaling mechanisms are poorly understood. Glucose deprivation (GD), which mimics myocardial ischemia, induces autophagy in cultured cardiac myocytes. Survival of cardiac myocytes was decreased by 3-methyladenine, an inhibitor of autophagy, suggesting that autophagy is protective against GD in cardiac myocytes. GD-induced autophagy coincided with activation of AMP-activated protein kinase (AMPK) and inactivation of mTOR (mammalian target of rapamycin). Inhibition of AMPK by adenine 9-␤-D-arabinofuranoside or dominant negative AMPK significantly reduced GD-induced autophagy, whereas stimulation of autophagy by rapamycin failed to cause an additive effect on GD-induced autophagy, suggesting that activation of AMPK and inhibition of mTOR mediate GD-induced autophagy. Autophagy was also induced by ischemia and further enhanced by reperfusion in the mouse heart, in vivo. Autophagy resulting from ischemia was accompanied by activation of AMPK and was inhibited by dominant negative AMPK. In contrast, autophagy during reperfusion was accompanied by upregulation of Beclin 1 but not by activation of AMPK. Induction of autophagy and cardiac injury during the reperfusion phase was significantly attenuated in beclin 1 ϩ/Ϫ mice. These results suggest that, in the heart, ischemia stimulates autophagy through an AMPK-dependent mechanism, whereas ischemia/reperfusion stimulates autophagy through a Beclin 1-dependent but AMPK-independent mechanism. Furthermore, autophagy plays distinct roles during ischemia and reperfusion: autophagy may be protective during ischemia, whereas it may be detrimental during reperfusion. (Circ Res. 2007;100:914-922.) Key Words: autophagy Ⅲ AMP-activated protein kinase (AMPK) Ⅲ beclin 1 Ⅲ ischemia/reperfusion A utophagy is an intracellular bulk degradation process whereby cytosolic, long-lived proteins and organelles are degraded and recycled. 1 Autophagy occurs at basal levels but can be further induced by stresses, such as nutrient depletion. 2 Autolysosomal degradation of membrane lipids and proteins generates free fatty acids and amino acids, which can be reused to maintain mitochondrial ATP production and protein synthesis, and promote cell survival. Disruption of this pathway prevents cell survival in diverse organisms. 2 Interestingly, autophagy also promotes programmed cell death in some circumstances. 3,4 Thus, autophagy has a dual role in cell survival, although, in cardiac myocytes, it remains to be elucidated whether autophagy is required for survival, and is thereby salutary, or whether it mediates cell death, and is detrimental during pathologically relevant stresses, such as ischemia and reperfusion.The mTOR (mammalian target of rapamycin) pathway is a key regulator of cell growth and proliferation and integrates signals regarding nutrie...

MicroRNA‐21 protects against cardiac hypoxia/reoxygenation injury by inhibiting excessive autophagy in H9c2 cells via the Akt/mTOR pathway

“…However, stimulated autophagy has been implicated in several types of cardiomyopathy 25. Autophagy is anti‐apoptotic and contributes to cellular recovery during brief myocardial ischaemia, but the subsequent reperfusion could cause excessive autophagy, which is considered cytotoxic and termed as class II programmed or autophagic cell death 6, 7, 8, 26. Our previous studies showed that inhibiting excessive autophagy could significantly decrease the infarct size and improve cardiac function after I/R‐induced myocardial injury 27, 28.…”

“…Enhanced levels of autophagy have been observed in the heart during both ischaemia and reperfusion 4, 5, 6. Studies have reported that autophagy has a dual, opposite role in the heart, depending on the stimulus 7.…”

“…Studies have reported that autophagy has a dual, opposite role in the heart, depending on the stimulus 7. Strategies that enhance autophagy can promote survival in response to milder stress, such as brief hypoxia and low levels of oxidative stress 6, 8, whereas severe stress, such as prolonged hypoxia or subsequent reperfusion, results in excessive autophagy, which may cause cell death by triggering excessive self‐digestion of essential proteins and organelles 9, 10. The manipulation of autophagy may represent a potential future therapeutic target to protect against ischaemia/reperfusion (I/R)‐induced cardiomyocyte death and preserve cardiac function.…”

Metformin Improves Neurologic Outcome Via AMP‐Activated Protein Kinase–Mediated Autophagy Activation in a Rat Model of Cardiac Arrest and Resuscitation

“…Emerging evidence indicates that AMPK can be activated by metformin in the brain, exerting a neuroprotective role in response to energy depletion through maintaining cellular energy homeostasis,24, 25, 26 promoting mitochondrial biogenesis,27 and increasing brain‐derived neurotrophic factor expression to promote neuronal survival 28. Recently, autophagy has been proposed as a downstream target of AMPK, and AMPK‐induced autophagy activation protects against ischemic injury to peripheral tissue injury29, 30, 31 as well as ischemic brain injury 32. These studies have prompted us to hypothesize that metformin pretreatment may prime the brain to better stand ischemic damage resulting from asphyxial CA and cardiopulmonary resuscitation (CA/CPR) through promoting AMPK‐mediated autophagy activation.…”

β‐Guanidinopropionic acid extends the lifespan of Drosophila melanogaster via an AMP‐activated protein kinase‐dependent increase in autophagy

“…Experiments in mammals have demonstrated that AMPK controls autophagy through mammalian target of rapamycin (mTOR) and Atg1/ULK1 signalings, which augments the quality of cellular housekeeping (Alers et al ., 2012). Previous studies indicate that the activation of AMPK is able to induce autophagy and subsequently confer to the protective effects against ischemic injury in the heart (Matsui et al ., 2007), kidney (Wang et al ., 2013), liver (Zaouali et al ., 2013), and muscular tissues (Pauly et al ., 2012). In the present study, we investigated the effects of β‐GPA in dietary supplements on lifespan of Drosophila and the underlying mechanisms.…”