Bcl ⁻2 regulates store-operated Ca ²⁺ entry to modulate ER stress-induced apoptosis

Abstract

Ca ²⁺ plays a significant role in linking the induction of apoptosis. The key anti-apoptotic protein, Bcl-2, has been reported to regulate the movement of Ca ²⁺ across the ER membrane, but the exact effect of Bcl-2 on Ca ²⁺ levels remains controversial. Store-operated Ca ²⁺ entry (SOCE), a major mode of Ca ²⁺ uptake in non-excitable cells, is activated by depletion of Ca ²⁺ in the ER. Depletion of Ca ²⁺ in the ER causes translocation of the SOC channel activator, STIM1, to the plasma membrane. Thereafter, STIM1 binds to Orai1 or/and TRPC1 channels, forcing them to open and thereby allow Ca ²⁺ entry. In addition, several anti-cancer drugs have been reported to induce apoptosis of cancer cells via the SOCE pathway. However, the detailed mechanism underlying the regulation of SOCE by Bcl-2 is not well understood. In this study, a three-amino acid mutation within the Bcl-2 BH1 domain was generated to verify the role of Bcl-2 in Ca ²⁺ handling during ER stress. The subcellular localization of the Bcl-2 mutant (mt) is similar to that in the wild-type Bcl-2 (WT) in the ER and mitochondria. We found that mt enhanced thapsigargin and tunicamycin-induced apoptosis through ER stress-mediated apoptosis but not through the death receptor- and mitochondria-dependent apoptosis, while WT prevented thapsigargin- and tunicamycin-induced apoptosis. In addition, mt depleted Ca ²⁺ in the ER lumen and also increased the expression of SOCE-related molecules. Therefore, a massive Ca ²⁺ influx via SOCE contributed to caspase activation and apoptosis. Furthermore, inhibiting SOCE or chelating either extracellular or intracellular Ca ²⁺ inhibited mt-mediated apoptosis. In brief, our results explored the critical role of Bcl-2 in Ca ²⁺ homeostasis and the modulation of ER stress.