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Am J Physiol Cell Physiol 260: C259-C265, 1991;
0363-6143/91 $5.00
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AJP - Cell Physiology, Vol 260, Issue 2 C259-C265, Copyright © 1991 by American Physiological Society

ARTICLES

Modulation of Gs activity by phorbol myristate acetate in rat hepatocytes

S. M. Hernandez-Sotomayor, M. Macias-Silva, C. C. Malbon and J. A. Garcia-Sainz
Instituto de Fisiologia Celular, Universidad Nacional Autonoma de Mexico, Mexico D.F.

Activation of protein kinase C promotes heterologous desensitization of hepatic adenylate cyclase. The basis for this desensitization was explored by use of a strategy with several independent approaches. Although not influencing the amount of forskolin-stimulated adenylate cyclase activity (catalyst), treatment with phorbol 12-myristate 13-acetate (PMA) decreased adenylate cyclase activation in response to either sodium fluoride or guanylyl imidodiphosphate [Gpp(NH)p]. Adenosine 3',5'-cyclic monophosphate (cAMP) accumulation in cholera toxin-treated hepatocytes and both the basal and GTP-stimulated adenylate cyclase activity of membranes from toxin-treated cells displayed a marked reduction in response to PMA. The ability of cholate extracts of hepatocyte membranes to reconstitute beta-adrenergic-stimulated adenylate cyclase activity of membrane of S49 mouse lymphoma cyc- cells was reduced by treatment with PMA. Cholera toxin-catalyzed labeling of Gs alpha-subunits was likewise diminished by phorbol ester treatment. Immunoblots of membranes from control or PMA-treated hepatocytes showed no difference in the amount of Gs alpha. Immunoprecipitation studies failed to detect phosphorylation of this G protein alpha-subunit. The data demonstrate that PMA induces an alteration in the functional status of Gs without altering the amount of this transmembrane signaling element. The alteration in Gs function may play a significant role in heterologous desensitization.


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