COTRANSFECTION WITH PROTEIN-KINASE-D CONFERS PHORBOL-ESTER-MEDIATED INHIBITION ON GLUCAGON-STIMULATED CAMP ACCUMULATION IN COS CELLS TRANSFECTED TO OVEREXPRESS GLUCAGON RECEPTORS

Glucagon elicited a profound increase in the intracellular cAMP concen tration of COS-7 cells which had been transiently transfected with a c DNA encoding the rat glucagon receptor and under conditions where cAMP phosphodiesterase activity was fully inhibited. This was achieved in a dose-dependent fashion with an EC50 of 1.8 +/- 0.4 nM glucagon. In c ontrast with previous observations made using hepatocytes [Heyworth, W hetton, Kinsella and Houslay (1984) FEBS Lett. 170, 38-42], treatment of transfected COS-7 cells with PMA did not inhibit the ability of glu cagon to increase intracellular cAMP levels. PMA-mediated inhibition w as not conferred by treatment with okadaic acid, nor by co-transfectin g cells with cDNAs encoding various protein kinase C isoforms (PKC-alp ha, PKC-beta II and PKC-epsilon) or with the PMA-activated G-protein-r ecptor kinases GRK2 and GRK3. In contrast, PMA induced the marked inhi bition of glucagon-stimulated cAMP production in COS-7 cells that had been co-transfected with a cDNA encoding protein kinase D (PKD). Such inhibition was not due to an action on the catalytic unit of adenylate cyclase, as forskolin-stimulated cAMP production was unchanged by PMA treatment of COS cells that had been co-transfected with both the glu cagon receptor and PKD. PKD transcripts were detected in RNA isolated from hepatocytes but not from COS-7 cells. Transcripts for GRK2 were p resent in hepatocytes but not in COS cells, whereas transcripts for GR K3 were not found in either cell type. It is suggested that PKD may pl ay a role in the regulation of glucagon-stimulated adenylate cyclase.