Cellular death linked to irreversible stress in the sarcoplasmic reticulum: The effect of inhibiting Ca2+–ATPase or protein glycosylation in the myocardiac cell model H9c2

Experimental sarcoplasmic reticulum damage induced by 3 μM thapsigargin or 1 μg/ml tunicamycin provoked viability loss of the cell population in approximately 72 h. Release of cytochrome c from mitochondria was an early event and Bax translocation to the mitochondria preceded or was simultaneous with cytochrome c release. The release of cytochrome c was not related with mitochondria depolarization or caspase activation. Irreversible stress in the sarcoplasmic reticulum, detected by the early activation of caspase 12, was functionally linked to the mitochondrial apoptotic pathway. Caspase 3 processing was blocked by cells preincubation with a selective inhibitor of either caspase 9 or caspase 8 whereas caspase 8 activation was inhibited by a selective caspase 9 inhibitor. This was consistent with the involvement of caspase 8 in a positive feedback loop leading to amplify the caspase cascade. Caspase inhibition did not protect against cell death indicating the existence of alternative caspase-independent mechanisms.