Gq-Induced Apoptosis is Mediated by AKT Inhibition That Leads to PKC-Induced JNK Activation

Abstract

Background/Aims: Gq protein-coupled receptors (GqPCRs) regulate various cellular processes including mainly proliferation and differentiation. In a previous study, we found that in prostate cancer cells, the GqPCR of GnRH induces apoptosis by reducing the PKC-dependent AKT activity and elevating JNK phosphorylation. Since it was thought that GqPCR induces mainly activation of AKT, we undertook to examine how general is this phenomenon and understand its signaling. Methods: We used various cells to follow the phosphorylation of signaling components using western blotting. Results: In a screen of 21 cell lines, we found that PKC activation results in the reduction of AKT activity, which correlates nicely to JNK activation and in some cases to apoptosis. To further understand the signaling pathways involved in this stimulation, we studied in detail the SVOG-4O and αT3-1 cells. We found that PGF2α and GnRH agonist (GnRH-a) indeed induce significant Gq- 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 on 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. Conclusion: Our results present a general mechanism that mediates a GqPCR-induced, death receptors-independent, apoptosis in physiological, as well as cancer-related systems.

Synergistic cytoprotection by co-treatment with dexamethasone and rapamycin against proinflammatory cytokine-induced alveolar epithelial cell injury

“…Because a selective JNK inhibitor, SP600125, inhibited cytokine-induced cytotoxicity in a dose-dependent manner, JNK/c-Jun pathway can be a pharmacological target in alleviating inflammation-induced alveolar epithelial injury. It is well established that JNK/c-Jun pathway is strongly associated with apoptosis [29, 30]. In fact, because co-treatment with dexamethasone and rapamycin augmented the inhibition of c-Jun phosphorylation, the synergistic cytoprotective effect would be, at least partially, derived from the inhibition of JNK/c-Jun pathway.…”

“…Augmentation of Akt activity by co-treatment with dexamethasone and rapamycin is also a very promising observation. Akt is well known as a primary molecule in survival signal pathways [29, 31], so it is probable that Akt also contributes to cell survival under cytokine stimulation. To clarify this point, we performed an additional experiment, wherein we checked whether the inhibition of Akt using a PI-3K inhibitor, LY294002, could cancel cytoprotection induced by dexamethasone and rapamycin.…”

Dissecting the Hormonal Signaling Landscape in Castration-Resistant Prostate Cancer

“…GnRH-R activation was further reported to be associated with proapoptotic effects [ 175 , 176 ]. In particular, in CRPC cells, GnRH agonists were shown to trigger apoptosis-related molecular events through the down-regulation of the PI3K/AKT signaling pathway, leading to the activation of the downstream JNK kinase, and MEK/ERK kinase activity [ 177 , 178 ].…”

“…In line with these observations, cleaved caspase-8 and -3, but not -9, and increased expression and phosphorylation of p53, were reported to increase in primary cell cultures from human PCa samples, supporting the idea that the extrinsic (but not intrinsic) apoptosis pathway is involved in the antitumor activity of GnRH agonists [ 171 , 173 , 180 , 181 ]. The different cell context-dependent biological effects of GnRH agonists at the pituitary vs. PCa cell level was suggested to be related to a transient vs. sustained activation of the intracellular signaling pathway (PKC/MAPK) in gonadotropes vs. cancer cells [ 176 , 182 ].…”

“…Moreover, in CRPC cells, GnRH agonists induce a sustained activation of the PKC/MAPK (p38MAPK and JNK) signaling cascade [ 182 ]. In particular, a c-Src-mediated signal and a reduction of AKT activity were found to be involved in the activation of the MLK3/JNK axis [ 176 ].…”