Phorbol ester regulates stimulatory and inhibitory pathways of the hormone-sensitive adenylate cyclase system in rat reticulocytes

Abstract

Treatment of rat reticulocytes with tetradecanoyl phorbol acetate (TPA), a tumor-promoting phorbol ester which activates protein kinase C, resulted in an about 50% decrease in the stimulation of adenylate cyclase activity by a subsequent challenge with a ß-adrenoceptor agonist. This phenomenon mimics agonist-induced desensitization. This decline is due to a reduction in the V max of the adenylate cyclase system rather than to a change in affinity to the agonist. The ß-adrenoceptor number was not changed while the K D for an agonist but not for an antagonist was increased by TPA treatment. Prostaglandin E 1 (PGE 1) plus GTP, NaF plus AlCl 3, and guanylyl-5′-imidodiphosphate (GppNHp) regulated adenylate cyclase activity in a biphasic manner, i.e. stimulation at lower concentrations and inhibition at higher concentrations. The same treatment also caused a dose- and time-dependent reduction of the inhibitory phase of the PGE 1/GTP action but did not affect the inhibitory phase of GppNHp and NaF/AlCl 3 actions. Pertussis toxin (IAP) treatment caused a reduction of the inhibitory phase of PGE 1/GTP action similar to that caused by TPA treatment. No synergistic effect was observed when the cells were treated with TPA and IAP simultaneously. These results suggest that TPA treatment impairs the coupling between PGE 1 receptor and G i rather than enhances that between PGE 1 receptor and G s. Protein kinase C was involved in the regulation of hormone-sensitive adenylate cyclase, the ß-agonist-induced stimulatory pathway and the PGE 1-induced inhibitory pathway in rat reticulocytes, since other phorbol esters and diacylglycerol, which activate this kinase, caused the same response. Furthermore, 1-(5-isoquinolinylsulfonyl)-2-methyl-piperazine (H-7), an inhibitor of protein kinase C, prevented the response.