Bcl-2 suppresses Ca release through inositol 1,4,5-trisphosphate receptors and inhibits Ca uptake by mitochondria without affecting ER calcium store content

Cell survival is promoted by the oncoprotein Bcl-2. Previous studies have established that one of the pro-survival actions of Bcl-2 is to reduce cellular fluxes of Ca(2+) within cells. In particular, Bcl-2 has been demonstrated to inhibit the release of Ca(2+) from the endoplasmic reticulum. However, the mechanism by which Bcl-2 causes reduced Ca(2+) release is unclear. In the accompanying paper C.J. Hanson, M.D. Bootman, C.W. Distelhorst, T. Maraldi, H.L. Roderick, The cellular concentration of Bcl-2 determines its pro- or anti-apoptotic effect, Cell Calcium (2008)], we described that only stable expression of Bcl-2 allowed it to work in a pro-survival manner whereas transient expression did not. In this study, we have employed HEK-293 cells that stably express Bcl-2, and which are, therefore, protected from pro-apoptotic stimuli, to examine the effect of Bcl-2 on Ca(2+) homeostasis and signalling. We observed that Bcl-2 expression decreased the Ca(2+) responses of cells induced by application of submaximal agonist concentrations. Whereas, decreasing endogenous Bcl-2 concentration using siRNA potentiated Ca(2+) responses. Furthermore, we found that Bcl-2 expression reduced mitochondrial Ca(2+) uptake by raising the threshold cytosolic Ca(2+) concentration required to activate sequestration. Using a number of different assays, we did not find any evidence for reduction of endoplasmic reticulum luminal Ca(2+) in our Bcl-2-expressing cells. Indeed, we observed that Bcl-2 served to preserve the content of the agonist-sensitive Ca(2+) pool. Endogenous Bcl-2 was found to interact with inositol 1,4,5-trisphosphate receptors (InsP(3)Rs) in our cells, and to modify the profile of InsP(3)R expression. Our data suggest that the presence of Bcl-2 in the proteome of cells has multiple effects on agonist-mediated Ca(2+) signals, and can abrogate responses to submaximal levels of stimulation through direct control of InsP(3)Rs.