Protein kinases induce isoproterenol desensitization of beta-adrenoceptor-coupled adenylate cyclase system: significance of receptor occupancy

Difference in sensitivity to alkaline phosphatase treatment between rat reticulocyte membranes in which beta-adrenoceptor desensitization was induced by isoproterenol, dibutyryl cAMP and phorbol ester

The effect of alkaline phosphatase (3.1.3.1) on desensitization of beta-adrenoceptor-responsive adenylate cyclase and the role of phosphorylation in desensitization were examined. Treatment of rat reticulocytes with isoproterenol, dibutyryl cAMP and tetradecanoyl phorbol acetate (TPA) caused the desensitization of beta-adrenoceptor-coupled adenylate cyclase. When the membranes from dibutyryl cAMP- and TPA-desensitized cells were incubated with alkaline phosphatase for 60 min at 30 degrees C, pH 8.0, the desensitization of isoproterenol-stimulated adenylate cyclase was markedly attenuated in both preparations. When the membranes from isoproterenol-desensitized cells were treated with alkaline phosphatase under the same conditions, the attenuation of the desensitization of alkaline phosphatase was less than in the case of treatment with dibutyryl cAMP or TPA. In other words, isoproterenol-induced desensitization was more resistant to alkaline phosphatase treatment. Isoproterenol- and dibutyryl cAMP-induced desensitization of NaF-stimulated adenylate cyclase were also attenuated by alkaline phosphatase treatment. Although the stability of the Gs-catalytic unit complex of adenylate cyclase was reduced by isoproterenol treatment, the reduction of stability was also decreased by alkaline phosphatase treatment.

Reduction in the stability of the Gs-catalytic unit complex of adenylate cyclase in isoproterenol-induced heterologous desensitization

Treatment of rat reticulocytes with isoproterenol resulted in the heterologous desensitization of adenylate cyclase, causing a reduction in NaF/AlCl3- and guanylyl 5'-imidodiphosphate (Gpp(NH)p)-stimulated activities as well as in activity stimulated by beta-adrenoceptor agonists. Desensitization was also induced by dibutyryl cyclic AMP and 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine (H-7), an inhibitor of cyclic AMP-dependent protein kinase (protein kinase A), prevented the isoproterenol-induced desensitization, suggesting the involvement of protein kinase A in the desensitization. Gs in the desensitized cell membrane was activated by treatment with Gpp(NH)p or NaF/AlCl3 in the presence of Mg2+. The activated state, the Gs-C complex, was gradually shifted to the basal state, i.e. the Gs-C complex was dissociated by removal of Mg2+. The rate constant of the dissociation was increased in the desensitized cell membranes (0.074 min-1) as compared with that in the control cell membranes (0.022 min-1). The half life of the Gs-C complex, calculated from the rate constant, was decreased during the process of desensitization. The results indicate that reduction in the stability of the Gs-C complex is related to heterologous desensitization. Agents which increase the level of cyclic AMP in the cell also reduced the stability of the complex. The potency of such an effect was in the following order; isoproterenol approximately NaF/AlCl3 (k = 0.071 min-1) greater than forskolin (0.039 min-1) greater than Mn2+ approximately dibutyryl cyclic AMP (0.030 min-1). The former two activate Gs and form Gs-C complex, while the latter two cause intracellular cyclic AMP accumulation without activation of Gs. It may be deduced from this order that the reduction of the stability depends not only on the increase in the cyclic AMP level but also on the level of Gs-C complex formed.

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

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 beta-adrenoceptor agonist. This phenomenon mimics agonist-induced desensitization. This decline is due to a reduction in the Vmax of the adenylate cyclase system rather than to a change in affinity to the agonist. The beta-adrenoceptor number was not changed while the KD for an agonist but not for an antagonist was increased by TPA treatment. Prostaglandin E1 (PGE1) plus GTP, NaF plus AlCl3, 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 PGE1/GTP action but did not affect the inhibitory phase of GppNHp and NaF/AlCl3 actions. Pertussis toxin (IAP) treatment caused a reduction of the inhibitory phase of PGE1/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 PGE1 receptor and Gi rather than enhances that between PGE1 receptor and Gs. Protein kinase C was involved in the regulation of hormone-sensitive adenylate cyclase, the beta-agonist-induced stimulatory pathway and the PGE1-induced inhibitory pathway in rat reticulocytes, since other phorbol esters and diacylglycerol, which activate this kinase, caused the same response.(ABSTRACT TRUNCATED AT 250 WORDS)