Mutations in PKD1 are the most common cause of autosomal dominant polycystic kidney disease (ADPKD). The protein product of PKD1 (polycystin-1 (PC1)) is a large transmembrane protein with a short intracellular C terminus that interacts with numerous signaling molecules, including Galpha(12). Cyst formation in ADPKD results from numerous cellular defects, including abnormal cilia, changes in polarity, and dysregulated apoptosis and proliferation. Recently, we reported increased apoptosis in Madin-Darby canine kidney (MDCK) cells through Galpha(12) stimulation of JNK and degradation of the anti-apoptotic protein Bcl-2 (Yanamadala, V., Negoro, H., Gunaratnam, L., Kong, T., and Denker, B. M. (2007) J. Biol. Chem. 282, 24352-24363). Herein, we confirm this pathway in Galpha(12)-silenced MDCK cells and utilize MDCK cell lines harboring either overexpressed or silenced PC1 to demonstrate that PC1 expression levels determine activity of the JNK/Bcl-2 apoptosis pathway. PC1-overexpressing MDCK cells were resistant to thrombin/Galpha(12)-stimulated apoptosis, JNK activation, and Bcl-2 degradation. In contrast, PC1-silenced MDCK cells displayed enhanced thrombin-induced apoptosis, JNK activity, and Bcl-2 degradation. In pulldown experiments, PC1 bound to Galpha(12), but not the related Galpha(13) subunit, and thrombin-stimulated MDCK cells led to increased interaction of Galpha(12) with the PC1 C terminus. In transient transfection assays, a PC1 C-terminal mutant lacking the G protein-binding domain was uncoupled from PC1-inhibited apoptosis. PC1 expression levels may be increased or decreased in ADPKD, and these findings suggest a mechanism in which levels of PC1 expression modulate Galpha(12)/JNK-stimulated apoptosis. Taken together, these findings are consistent with a set point model in which PC1 expression levels regulate specific G protein signaling pathways important to cyst development.