Preinduced molecular chaperones in the endoplasmic reticulum protect cardiomyocytes from lethal injury.
Although molecular chaperones in the endoplasmic reticulum (ER) are known to be involved in folding and assembly of glycosylated proteins, it is unclear whether preinduced ER chaperones can protect cardiomyocytes from lethal injury. In this study we used tunicamycin, an inhibitor of N-linked glycosylation in the ER, to preinduce ER chaperones in H9c2 cardiomyocytes and we tested the cytoprotective role of preinduced ER chaperones in the cardiomyocytes. Expression of GRP78 at both protein and mRNA levels was markedly increased in cardiomyocytes pretreated with tunicamycin, when compared to non-treatment controls. Following prolonged ATP depletion or oxidative stress, which was used to simulate cardiac ischemia and reperfusion injury, respectively, the release of lactate dehydrogenase (LDH) from tunicamycin-pretreated cardiomyocytes was significantly lower than from non-pretreated cardiomycocytes. Tunicamycin-pretreated cardiomyocytes showed significantly higher Ca2+ release into cytoplasm than controls when treated with both caffeine and thapsigargin, indicating higher storage of Ca2+ in the ER. After oxidative stress, cytosolic Ca2+ levels were maintained relatively stable in tunicamycin-pretreated cardiomyocytes, when compared to control cardiomyocytes. These observations suggest that preinduced ER chaperones protect cardiomyocytes from lethal injury, at least in part, by preventing an increase in cytosolic Ca2+.