ADENYLATE-CYCLASE, CYCLIC-AMP AND EXTRACELLULAR-SIGNAL-REGULATED KINASE-2 IN AIRWAY SMOOTH-MUSCLE - MODULATION BY PROTEIN-KINASE-C AND GROWTH SERUM

Bradykinin and phorbol 12-myristate 13-acetate stimulate adenylate cyc lase activity in serum-depleted cultured airway smooth muscle via a pr otein kinase C (PKC)-dependent pathway. The probable target is the typ e II adenylate cyclase, which can integrate coincident signals from bo th PKC and G(s). Therefore, activation of G(s) (by cholera-toxin pre-t reatment) amplified the bradykinin-stimulated cyclic AMP signal and co ncurrently attenuated the partial activation of extracellular-signal-r egulated kinase-2 (ERK-2) by bradykinin. We have previously demonstrat ed that, in order to induce full activation of ERK-2 with bradykinin, it is necessary to obliterate PKC-stimulated cyclic AMP formation. We concluded that the cyclic AMP signal limits the magnitude, of ERK-2 ac tivation [Pyne, Moughal, Stevens, Tolan and Pyne (1994) Biochem.J. 304 , 611-616]. The present study indicates that the bradykinin-stimulated ERK-2-pathway is entirely cyclic AMP-sensitive, and suggests that coi ncident signal detection by adenylate cyclase may be an important phys iological route for the modulation of early mitogenic signalling. Furt hermore, the direct inhibition of adenylate cyclase activity enables b radykinin to induce DNA synthesis, indicating that the PKC-dependent a ctivation of adenylate cyclase limits entry of cells into the cell cyc le. These studies suggest that the mitogenicity of an agonist may be g overned, in part, by its ability to stimulate an inhibitory cyclic AMP signal pathway in the cell. The activation of adenylate cyclase by PK C appears to be downstream of phospholipase D. However, in cells that were maintained in growth serum (i.e. were not growth-arrested), brady kinin was unable to elicit a PKC-stimulated cyclic AMP response. The l esion in the signal-response coupling was not at the level of either t he receptor or phospholipase D, which remain functionally operative an d suggests modification occurs at either PKC or adenylate cyclase itse lf. These studies are discussed with respect to the cell signal regula tion of mitogenesis in airway smooth muscle.