Bcl-xL acts as an inhibitor of IPR channels, thereby antagonizing Ca-driven apoptosis.

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