Regulation of the cAMP signal transduction pathway by protein kinase C in rat submandibular cells.

Treatment of rat submandibular acinar cell extracts with the phorbol ester 12-O-tetradecanoylphorbol 13-acetate (TPA) caused the dose-dependent activation of protein kinase C (PKC), assessed by the phosphorylation of a novel and highly specific substrate. This effect was duplicated by a diacylglycerol, but not by the 4 alpha-phorbol ester 4 alpha-phorbol 12,13-didecanoate. The TPA elevation of PKC was blocked by the PKC inhibitors H-7 and sangivamycin. In intact cells, TPA caused the translocation of PKC from cytosol to membrane, consistent with its known mode of activation. The beta-adrenergic agonist, isoproterenol, stimulated cAMP levels which were significantly reduced by preactivation of PKC. This inhibitory PKC effect was reversed by H-7. When cAMP was stimulated at the post-receptor level, however, by forskolin, NaF or GTP[gamma S], PKC did not inhibit, but rather enhanced the cyclic nucleotide response. Since PKC phosphorylated an endogenous protein of 55 kDa, the size of the beta 1 receptor, these findings indicate that, as in other cell types, PKC can desensitize adenylate cyclase by direct phosphorylation of the beta receptor, but potentiate the cAMP response by a post-receptor mechanism. In mucin secretion studies in the model, TPA alone caused the cAMP-independent release of up to 44% total mucin, which was much less than additive with the isoproterenol response. When the cAMP-mucosecretory response was stimulated at the adenylate cyclase level by forskolin, however, the TPA + forskolin effects were additive. These findings on the modulation of cAMP by PKC indicate cross-talk regulation in the phosphoinositide-cAMP signal transduction pathways in submandibular acinar cells.