Feedback regulation mediated by Bcl-2 and DARPP-32 regulates inositol 1,4,5-trisphosphate receptor phosphorylation and promotes cell survival

Abstract

Bcl-2 interacts with the inositol 1,4,5-trisphosphate receptor (InsP 3R) and thus prevents InsP3-induced Ca2+ elevation that induces apoptosis. Here we report that Bcl-2 binds dopamine- and cAMP-regulated phosphoprotein of 32 kDa (DARPP-32), a protein kinase A (PKA)-activated and calcineurin (CaN)-deactivated inhibitor of protein phosphatase 1 (PP1). Bcl-2 docks DARPP-32 and CaN in a complex on the InsP 3R, creating a negative feedback loop that prevents exaggerated Ca2+ release by decreasing PKA-mediated InsP3R phosphorylation. T-cell activation increases PKA activity, phosphorylating both the InsP3R and DARPP-32. Phosphorylated DARPP-32 inhibits PP1, enhancing InsP3R phosphorylation and Ca2+ release. Elevated Ca2+ activates CaN, which dephosphorylates DARPP-32 to dampen Ca2+ release by eliminating PP1 inhibition to enable it to dephosphorylate the InsP3R. Knocking down either Bd-2 or DARPP-32 abrogates this feedback mechanism, resulting in increased Ca2+ elevation and apoptosis. This feedback mechanism appears to be exploited by high levels of Bcl-2 in chronic lymphocytic leukemia cells, repressing B-cell receptor-induced Ca2+ elevation and apoptosis.