Signal transduction of receptor-mediated antiproliferative action of melatonin on human prostate epithelial cells involves dual activation of Gα_s and Gα_q proteins

Melatonin has been shown to inhibit the proliferation of malignant and transformed human prostate epithelial cells by transcriptional up-regulation of p27^Kip1 expression via MTNR1A receptor-mediated activation of protein kinase A (PKA) and protein kinase C (PKC) in parallel. Given that melatonin MTNR1A receptor is a G protein-coupled receptor, this study was conducted to identify the specific G proteins that mediate the antiproliferative action of melatonin on human prostate epithelial cells. In 22Rv1 and RWPE-1 cells, knockdown of either Gα_s or Gα_q, but not Gα_i2 expression by RNA interference, abrogated the effects of melatonin on p27^Kip1 and cell proliferation. Conversely, cellular overexpression of activated mutants of Gα_s and Gα_q in 22Rv1 and RWPE-1 cells mimicked the effects of melatonin on prostate epithelial cell antiproliferation by increasing p27^Kip1 expression through downstream activation of PKA and PKC in parallel. Moreover, melatonin or 2-iodomelatonin induced elevation of adenosine-3′,5′- cyclic monophosphate (cAMP) in 22Rv1 and RWPE-1 cells. The effects of 2-iodomelatonin on cAMP were blocked by the nonselective MTNR1A/MTNR1B receptor antagonist luzindole but were not affected by the selective MTNR1B receptor antagonist 4-phenyl-2-propionamidotetraline (4-P-PDOT). Furthermore, knockdown of Gα_s mitigated the stimulatory effects of 2-iodomelatonin on cAMP. Collectively, the data demonstrated, for the first time, functional coupling of MTNR1A receptor to Gα_s in cancerous or transformed human cells expressing endogenous melatonin receptors. Our results also showed that dual activation of Gα_s and Gα_q proteins is involved in the signal transduction of MTNR1A receptor-mediated antiproliferative action of melatonin on human prostate epithelial cells.