Bcl-xL Acts as an Inhibitor of IP3R Channels, Thereby Antagonizing Ca2+-Driven Apoptosis

Abstract 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.