Gα12 stimulates apoptosis in epithelial cells through JNK1-mediated Bcl-2 degradation and up-regulation of IκBα

Apoptosis is an essential mechanism for the maintenance of somatic tissues, and when dysregulated can lead to numerous pathological conditions. G proteins regulate apoptosis in addition to other cellular functions, but the roles of specific G proteins in apoptosis signaling are not well characterized. G alpha(12) stimulates protein phosphatase 2A (PP2A), a serine/threonine phosphatase that modulates essential signaling pathways, including apoptosis. Herein, we examined whether G alpha(12) regulates apoptosis in epithelial cells. Inducible expression of G alpha(12) or constitutively active (QL)alpha(12) in Madin-Darby canine kidney cells led to increased apoptosis with expression of QL alpha(12), but not G alpha(12). Inducing QL alpha(12) led to degradation of the anti-apoptotic protein Bcl-2 (via the proteasome pathway), increased JNK activity, and up-regulated I kappa B alpha protein levels, a potent stimulator of apoptosis. Furthermore, the QL alpha(12)-stimulated activation of JNK was blocked by inhibiting PP2A. To characterize endogenous G alpha(12) signaling pathways, non-transfected MDCK-II and HEK293 cells were stimulated with thrombin. Thrombin activated endogenous G alpha(12) (confirmed by GST-tetratricopeptide repeat (TPR) pull-downs) and stimulated apoptosis in both cell types. The mechanisms of thrombin-stimulated apoptosis through endogenous G alpha(12) were nearly identical to the mechanisms identified in QL alpha(12)-MDCK cells and included loss of Bcl-2, JNK activation, and up-regulation of I kappa B alpha. Knockdown of the PP2A catalytic subunit in HEK293 cells inhibited thrombin-stimulated apoptosis, prevented JNK activation, and blocked Bcl-2 degradation. In summary, G alpha(12) has a major role in regulating epithelial cell apoptosis through PP2A and JNK activation leading to loss of Bcl-2 protein expression. Targeting these pathways in vivo may lead to new therapeutic strategies for a variety of disease processes.