Cardioprotection by Endoplasmic Reticulum Stress–Induced Autophagy

Abstract

This study tested the hypothesis that the induction of autophagy by producing therapeutic amounts of endoplasmic reticulum (ER) stress in the heart before an ischemic insult would ameliorate/reduce subsequent lethal myocardial ischemic/reperfusion (I/R) injury (similar to ischemic preconditioning). A dose-response study with both tunicamycin and thapsigargin was performed to determine the optimal dose (0.3 mg/kg) for inducing autophagy for cardioprotection. The Sprague-Dawley rats weighing between 250 and 300 g were randomly assigned into five groups: normal control (injected with saline only), high (3 mg/kg), and low (0.3 mg/kg) doses of tunicamycin or thapsigargin, respectively. After 48 h, the rats were subjected to an isolated working heart preparation: 30 min ischemia followed by 2 h of reperfusion with continuous left ventricular function monitoring. At the end, the hearts were subjected to either measurement of infarct size or cardiomyocyte apoptosis. Some hearts (from different sets of experiments) were used for transmission electron microscopy (TEM), confocal microscopy, or Western blot analysis. Tunicamycin and thapsigargin, irrespective of the dose, induced sufficient ER stress, as evidenced by the increased phosphorylation or activation of eIF2α and PERK. Such ER stress potentiated autophagy in the heart, as evidenced by an increase in LC3-II, beclin-1, and Atg5. This was also supported by TEM, clearly showing the production of autophagosomes, and by confocal microscopy, showing upregulation of eIF2α and beclin-1. The autophagy produced with lower doses of tunicamycin and thapsigargin in the face of myocardial I/R resulted in reduction of the I/R injury, as evidenced by improved left ventricular function and reduced myocardial infarct size and cardiomyocyte apoptosis. In concert, an induction of GRP78 and activation of Akt and Bcl-2 occurred. The higher doses conversely were detrimental for the heart and were associated with induction of CHOP and downregulation of Akt. The results thus display the proof of concept that induction of autophagy by ER stress (therapeutic amount) before ischemia (similar to ischemic preconditioning) could reduce subsequent lethal ischemic reperfusion injury.

The endoplasmic reticulum stress inhibitor salubrinal inhibits the activation of autophagy and neuroprotection induced by brain ischemic preconditioning

“…Several lines of evidence demonstrated that ER stress contributes to autophagy activation [25][26][27][28] . An essential step in ER stress-induced autophagosome formation is the phosphorylation of PERK/eIF2α.…”

“…If ER stress is mild, cells will express an adaptive response through autophagy and will survive during subsequent lethal ischemia. If ER stress is overwhelming, however, the cells will die through ER stress-related apoptosis [26] . Caspase-12, a murine protein that is associated with the ER membrane, is involved in ER stress-dependent apoptosis.…”

“…Furthermore, preconditioning-induced autophagy was able to inhibit excessive ER related-apoptosis in lethal ischemia, suggesting a correlation between ER stress and autophagy during preconditioning [24] . Furthermore, mounting evidence has shown that ER stress contributes to the activation of autophagy [25][26][27][28] . We thus speculate that ischemic preconditioning might induce mild ER stress to initiate the autophagic pathway.…”

Redox-Sensitive Endoplasmic Reticulum Stress and Autophagy at Rostral Ventrolateral Medulla Contribute to Hypertension in Spontaneously Hypertensive Rats

“…For example, autophagy produced with lower doses of ER stress inducers results in protection against ischemic/reperfusion injury in heart. 47 Nonetheless, under chronic ER stress, an excessive and uncontrolled autophagic activation can lead to the depletion of essential molecules and organelles, which triggers autophagic cell death. This form of cell death has recently been shown to play an important role in the development of ER stress-associated cardiovascular diseases.…”

Application and interpretation of current autophagy inhibitors and activators

“…ER stress enhances autophagy by negatively regulating the AKT/TSC/mTOR pathway [27] . ER stress inducers such as brefeldin A, thapsigargin and tunicamycin increase the formation of autophagic vesicles with the expression of Beclin and LC-3 (microtubule-associated protein1 light chain 3) II, two autophagic markers [28,29] . However, conflicting data have also been reported.…”