G_q protein-induced apoptosis is mediated by AKT kinase inhibition that leads to protein kinase C-induced c-Jun N-terminal kinase activation

G_q protein-coupled receptors (G_qPCRs) regulate various cellular processes, including mainly proliferation and differentiation. In a previous study we found that in prostate cancer cells, the G_qPCR of gonadotropin-releasing hormone (GnRH) induces apoptosis by reducing the PKC-dependen tAKT activity and elevating JNK phosphorylation. Because it was thought that G_qPCRs mainly induce activation of AKT, we first undertook to examine how general this phenomenon is. In a screen of 21 cell lines we found that PKC activation results in the reduction of AKT activity, which correlates nicely with JNK activation and in some cases with apoptosis. To understand further the signaling pathways involved in this stimulation, we studied in detail SVOG-4O and αT3-1 cells. We found that prostaglandin F2α and GnRH agonist (GnRH-a) indeed induce significant Gα_q and PKC-dependent apoptosis in these cells. This is mediated by two signaling branches downstream of PKC, which converge at the level of MLK3 upstream of JNK. One branch consists of c-Src activation of the JNK cascade, and the second involves reduction of AKT activity that alleviates its inhibitory effect on MLK3 to allow the flow of the c-Src signal to JNK. At the MAPKK level, we found that the signal is transmitted by MKK7 and not MKK4. Our results present a general mechanism that mediates a G_qPCR- induced, death receptor-independent, apoptosis in physiological, as well as cancer-related systems.