Inositol 1,4,5-trisphosphate receptor (IP3R)-mediated release of calcium from endoplasmic reticulum (ER) stores has been implicated in various apoptotic processes. Although the antiapoptotic protein Bcl-2 (found in both mitochondrial and ER membranes) can quell oscillations in intracellular calcium, the mechanisms through which Bcl-2 affects calcium signaling--and the precise role of ER Bcl-2--have remained unclear. Chen et al. found that WEHI7.2 T cells transfected with either human Bcl-2 or a Bcl-2 mutant targeted to the ER showed an attenuated increase in cytosolic calcium in response to stimulation with an antibody to CD3. Bcl-2 also inhibited the increase in cytosolic calcium produced by a membrane-permeant IP3 ester but had no effect on calcium release after SERCA (sarcoendoplasmic reticulum Ca2+-ATPase) inhibition with thapsigargin nor did it affect calcium concentration in the ER lumen (measured with Fura-2FF). The affinity of IP3 for the IP3R in microsomes isolated from cells expressing Bcl-2 was lower than that in microsomes from cells that did not express Bcl-2; however, Bcl-2 inhibited calcium release from the ER even in permeabilized cells exposed to saturating concentrations of IP3. Adding purified Bcl-2 to IP3Rs incorporated into planar lipid bilayers decreased their open probability in the presence of IP3. Western analysis combined with either blue native gel electrophoresis of ER membranes from WEHI7.2 cells expressing Bcl-2 or with coimmunoprecipitation using transfected WEHI7.2 cells or a T cell line that expressed endogenous Bcl-2 indicated that Bcl-2 and IP3R formed a complex in vivo. Thus, ER Bcl-2 appears to play a role in regulating IP3R-mediated calcium release from the ER.
R. Chen, I. Valencia, F. Zhong, K. S. McColl, H. L. Roderick, M. D. Bootman, M. J. Berridge, S. J. Conway, A. B. Holmes, G. A. Mignery, P. Velez, C. W. Distelhorst, Bcl-2 functionally interacts with inositol 1,4,5-trisphosphate receptors to regulate calcium release from the ER in response to inositol 1,4,5-trisphosphate. J. Cell Biol. 166, 193-203 (2004). [Abstract] [Full Text]