Abstract

Primary cultures of chromaffin cells from bovine adrenal medulla were used as a model to evaluate the ability of 8-Br cyclic AMP (8-Br cAMP) to induce tyrosine hydroxylase (TH) and to study the role of cAMP-dependent protein kinase (cAPK) in this induction. This cell preparation maintains a constant level of cyclic nucleotides, catecholamines and related enzyme activities for about four weeks. Exposure of the cells for 5 hr to 8-Br cAMP produces, 48 hr later, a dose-related increase in the TH activity; 8-Br cGMP fails to modify TH. The increase in TH activity caused by 8-Br cAMP is due to an increase of the V_max and is preceded by an activation of cytosol cAPK associated with a decrease of the total cytosol cAPK. A sustained increase in nuclear phosphorylation begins 8 to 12 hr after 8-Br cAMP application. The delayed increase in TH activity induced by 8-Br cAMP is blocked by actinomycin D, cycloheximide, colchicine and vinblastine. The reduction of the TH induction by colchicine and vinblastine (1 nM) is observed only when the inhibitors of the microtubular protein polymerization were added 4 to 12 hr after the incubation with 8-Br cAMP. The addition of colchicine 15 hr after 8-Br cAMP fails to inhibit TH induction. This blockade of TH induction is associated with an inhibition of the increase in nuclear phosphorylation, but is not associated with an inhibition of protein synthesis. The increase of endogenous cAMP and the induction of TH was also produced by cholera toxin. These results suggest that the increase of TH elicited by 8-Br cAMP is mediated by the translocation of cAPK subunits from cytosol to the nuclei and that this translocation requires the function of the microtubular network.