Bradykinin and phorbol 12-myristate 13-acetate stimulate adenylate cyclase activity in serum-depleted cultured airway smooth muscle via a protein kinase C (PKC)-dependent pathway. The probable target is the type II adenylate cyclase, which can integrate coincident signals from both PKC and Gs. Therefore, activation of Gs (by cholera-toxin pre-treatment) amplified the bradykinin-stimulated cyclic AMP signal and concurrently attenuated the partial activation of extracellular-signal-regulated kinase-2 (ERK-2) by bradykinin. We have previously demonstrated 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 activation [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 coincident signal detection by adenylate cyclase may be an important physiological route for the modulation of early mitogenic signalling. Furthermore, the direct inhibition of adenylate cyclase activity enables bradykinin to induce DNA synthesis, indicating that the PKC-dependent activation of adenylate cyclase limits entry of cells into the cell cycle. These studies suggest that the mitogenicity of an agonist may be governed, in part, by its ability to stimulate an inhibitory cyclic AMP signal pathway in the cell. The activation of adenylate cyclase by PKC appears to be downstream of phospholipase D. However, in cells that were maintained in growth serum (i.e. were not growth-arrested), bradykinin was unable to elicit a PKC-stimulated cyclic AMP response. The lesion in the signal-response coupling was not at the level of either the receptor or phospholipase D, which remain functionally operative and suggests modification occurs at either PKC or adenylate cyclase itself. These studies are discussed with respect to the cell signal regulation of mitogenesis in airway smooth muscle.
Protein kinase C isoenzymes: a review of their structure, regulation and role in regulating airways smooth muscle tone and mitogenesis