Bcl-xL acts as an inhibitor of IP(3)R channels, thereby antagonizing Ca(2+)-driven apoptosis

Anti-apoptotic Bcl-2-family members not only act at mitochondria but also at the endoplasmic reticulum, where they impact Ca(2+) dynamics by controlling IP(3) receptor (IP(3)R) function. Current models propose distinct roles for Bcl-2 vs. Bcl-xL, with Bcl-2 inhibiting IP(3)Rs and preventing pro-apoptotic Ca(2+) release and Bcl-xL sensitizing IP(3)Rs to low [IP(3)] and promoting pro-survival Ca(2+) oscillations. We here demonstrate that Bcl-xL too inhibits IP(3)R-mediated Ca(2+) release by interacting with the same IP(3)R regions as Bcl-2. Via in silico superposition, we previously found that the residue K87 of Bcl-xL spatially resembled K17 of Bcl-2, a residue critical for Bcl-2’s IP(3)R-inhibitory properties. Mutagenesis of K87 in Bcl-xL impaired its binding to IP(3)R and abrogated Bcl-xL’s inhibitory effect on IP(3)Rs. Single-channel recordings demonstrate that purified Bcl-xL, but not Bcl-xL(K87D), suppressed IP(3)R single-channel openings stimulated by sub-maximal and threshold [IP(3)]. Moreover, we demonstrate that Bcl-xL-mediated inhibition of IP(3)Rs contributes to its anti-apoptotic properties against Ca(2+)-driven apoptosis. Staurosporine (STS) elicits long-lasting Ca(2+) elevations in wild-type but not in IP(3)R-knockout HeLa cells, sensitizing the former to STS treatment. Overexpression of Bcl-xL in wild-type HeLa cells suppressed STS-induced Ca(2+) signals and cell death, while Bcl-xL(K87D) was much less effective in doing so. In the absence of IP(3)Rs, Bcl-xL and Bcl-xL(K87D) were equally effective in suppressing STS-induced cell death. Finally, we demonstrate that endogenous Bcl-xL also suppress IP(3)R activity in MDA-MB-231 breast cancer cells, whereby Bcl-xL knockdown augmented IP(3)R-mediated Ca(2+) release and increased the sensitivity towards STS, without altering the ER Ca(2+) content. Hence, this study challenges the current paradigm of divergent functions for Bcl-2 and Bcl-xL in Ca(2+)-signaling modulation and reveals that, similarly to Bcl-2, Bcl-xL inhibits IP(3)R-mediated Ca(2+) release and IP(3)R-driven cell death. Our work further underpins that IP(3)R inhibition is an integral part of Bcl-xL’s anti-apoptotic function.