Melatonin-induced suppression of PC12 cell growth is mediated by its Gi coupled transmembrane receptors

The effects of pertussis toxin, an uncoupler of Gi protein from adenylate c yclase, and luzindole, a competitive inhibitor of melatonin receptor bindin g, were examined for their ability to inhibit melatonin-induced suppression of PC12 cell growth. Both agents inhibited the melatonin response suggesti ng that melatonin may be acting through one of its Gi coupled cell surface receptors. This is confirmed by Western blots demonstrating the presence of MT1 receptors in PC12 cells. Coupling of the Gi protein to these receptors is demonstrated by failure of melatonin to suppress cell growth in PKA def icient A126-1B2-1 mutant PC12 cells. Similarly, melatonin failed to prevent cell proliferation when cells were incubated in the presence of the PKA in hibitor, Rp-cAMP. Retinoic acid and dexamethasone, agents known to effect P C12 cell growth and/or differentiation, displayed differential effects on t he actions of melatonin. In the presence of melatonin and low concentration s of retinoic acid (100 nM), PC12 cell proliferation was stimulated compare d to that seen with either agent alone, whereas no increase in cell prolife ration was observed when higher concentrations of retinoic acid (100 muM) w ere used. The effects of dexamethasone on suppression of PC12 cell growth w ere additive with that of melatonin whereas, 1,25-dihydroxyvitamin D-3 (IC5 0=10 nM), which by itself had no effect on PC12 cell growth, was found to i nhibit the melatonin response. This study demonstrates that inhibition of P C12 cell growth, at physiological concentrations of melatonin, is mediated by cAMP-dependent cell surface receptors and this response is altered by ot her growth factors known to effect PC12 cell proliferation and differentiat ion. (C) 2001 Elsevier Science BY All rights reserved.