Endoplasmic reticulum calcium pool depletion-induced apoptosis is coupled with activation of the death receptor 5 pathway

Thapsigargin (TG), by inducing perturbations in cellular Ca2+ homeostasis, has been shown to induce apoptosis. The molecular mechanisms of Ca2+ perturbation-induced apoptosis are not fully understood. In this study, we demonstrate for the first time that TG-mediated perturbations in Ca2+ homeostasis are coupled with activation of the death receptor 5 (DR5)-dependent apoptotic pathway in human cancer cells. TG selectively upregulated DR5 but had no effect on the expression 4 the other TRAIL receptor, DR4. TG also upregulated the expression of the DR5 ligand TRAIL (tumor necrosis factor-related apoptosis inducing ligand), albeit in a cell-type specific manner. TG-induced apoptosis has been shown to be associated with activation of the mitochondrial pathway. We found that TG upregulation of DR5 and TRAIL was coupled with caspase 8 activation and Bid cleavage, suggesting that the TG-regulated DR5 pathway could be linked to the mitochondrial pathway. TG enhanced not only DR5 mRNA stability but also increased induction of the DR5 genomic promoter-reporter gene. The TG-induced increase in DR5 expression appeared to occur as a consequence of TG-induced endoplasmic reticulum (ER) Ca2+ pool depletion. Thus, we report our novel findings that ER Ca2+ pool depletion-induced apoptotic signals are mediated, at least in part, via a DR5-dependent apoptotic pathway and there appears to be a cross-talk between the death receptor and mitochondrial pathways.