PROTEIN-KINASE-C POTENTIATION OF THE TYROSINE KINASE INHIBITOR-STIMULATED CYCLIC-GMP PRODUCTION IN RAT PINEALOCYTES

Inhibition of tyrosine kinase activities elevates cyclic GMP (cGMP) le vels in rat pinealocytes. Since protein kinase C (PKC) and intracellul ar Ca2+ both interact with the agonist-stimulated cGMP accumulation, i n this study their interactions with the tyrosine kinase inhibitor-med iated cGMP response were investigated. Two tyrosine kinase inhibitors, genistein and tyrphostin B42, increased basal cGMP accumulation conce ntration dose-dependently. This increase in cGMP accumulation was pote ntiated by 4 beta-phorbol 12-myristate 13-acetate (PMA), an activator of PKC, and blocked by calphostin C, a specific PKC inhibitor. The tyr osine kinase inhibitors had no effect on the in vitro or PMA-mediated translocation of PKC activity. However, when the phosphodiesterase was inhibited by isobutylmethylxanthine (IBMX), neither the tyrosine kina se inhibitors alone nor in combination with PMA had an effect on cGMP accumulation, suggesting that phosphodiesterase is a probable site of action of the inhibitors. In comparison, elevation of intracellular Ca 2+ by BayK 8644, ionomycin, or KCl inhibited the genistein- or tyrphos tin B42-mediated increase in cGMP accumulation. This inhibition persis ted in the presence of IBMX and was partly reversed by a Ca2+/calmodul in inhibitor. These results suggest that PKC modulates the rate of cGM P degradation through signalling pathways involving tyrosine phosphory lation. However, the inhibitory effect of the Ca2+-elevating agents on the tyrosine kinase inhibitor-stimulated cGMP accumulation appears to be independent of phosphodiesterase inhibition. Copyright (C) 1996 El sevier Science Inc.