Phorbol ester-induced augmentation and inhibition of epinephrine-stimulated adenylate cyclase in S49 lymphoma cells

Abstract

The effects of 4-beta phorbol 12-myristate 13-acetate (PMA) on hormone and forskolin-stimulated adenylate cyclase were evaluated in S49 lymphoma cells. Treatment of wild type (WT) S49 cells with PMA caused stimulation, inhibition or had no effect on epinephrine stimulation of cAMP accumulation. The effect observed was dependent on the length of PMA treatment, the concentration of PMA and the concentration of hormone (or forskolin) used to stimulate cAMP accumulation. Longer treatment times with PMA and higher PMA concentrations favored the inhibitory effects. Pretreating WT with 0.5 microM PMA for 18 min caused an increase in the EC50 and maximal levels for epinephrine stimulation of cAMP accumulation. Thus inhibition was seen at relatively low epinephrine concentrations and augmentation with high concentrations. The inhibitory effects of PMA on epinephrine-stimulated adenylate cyclase activity were observed only at low free Mg++ concentrations (0.75 mM). The effects of PMA on PGE1-stimulated cAMP accumulation were similar to those observed for epinephrine. In S49 WT cells 100 nM PMA augmented 5 microM forskolin-stimulated cAMP accumulation; however with 100 microM forskolin, PMA effects were minimal. PMA also attenuated Gi-mediated Gpp(NH)p inhibition of forskolin-stimulated adenylate cyclase in both WT and cyc- membranes, resembling the effects of pertussis toxin. The effects of various phorbol analogues on epinephrine-stimulated cAMP accumulation were as follows: 4 beta-phorbol 12,13-didecanoate had similar effects to PMA, 4 alpha-phorbol 12,13-didecanoate had no effects and 1-oleoyl, 2-acetylglycerol augmented epinephrine-stimulated cAMP accumulation at concentrations greater than or equal to 5 microM. Our results are consistent with a dual mechanism of PMA action on adenylate cyclase involving protein kinase C-mediated phosphorylation of Gi and of the beta-adrenergic receptor, the former leading to augmentation and the latter to inhibition of hormone-stimulated adenylate cyclase.