DIFFERENT MECHANISMS ARE INVOLVED IN CAMP-MEDIATED INDUCTION OF MESSENGER-RNAS FOR SUBUNITS OF CAMP-DEPENDENT PROTEIN-KINASES

The present study addresses possible mechanisms through which cAMP mediates its effects on mRNA levels for the subunits of protein kinase A(PKA) and the cellular protooncogene, c-fos. Messenger RNAs for the PKA subunits (RI-alpha, RII-alpha, RII-beta, and C-alpha) were regulated by cAMP with similar kinetics in Sertoli cells. However, effects of cAMP on the PKA mRNAs were slow compared to a well characterized cAMP responsive gene, c-fos. The magnitude of stimulation was dramatically different between the various PKA subunits, in that RII-beta mRNA increased more than 50-fold while the mRNAs for the other subunits were induced only two to four times. Separation of nuclear and cytoplasmic RNA demonstrated that mRNAs for PKA subunits were stimulated to the same extent in these two cellular compartments. The more rapid induction of c-fos mRNA by cAMP, compared to the mRNA for RII-beta, was also seen at the level of transcription. Maximal transcription rate for c-fos, RI-alpha, and C-alpha were observed after 30 min, whereas that for RII-beta was increasing during the 2-h period examined. Transcriptional activation of the RI-alpha gene also appeared faster than that for RII-beta. When Sertoli cells were incubated with 8-(4-chlorophenylthio) cAMP and cycloheximide, a potent inhibitor of protein synthesis, we observed a super-induction of the mRNAs for c-fos (10-fold) and RI-alpha (2-fold). In contrast, the inductions of RII-beta, RII-alpha, and C-alpha were almost completely inhibited by cycloheximide. This was also found in nuclear RNA. These results indicate that different factors are involved in cAMP-mediated induction of mRNAs for the various PKA subunits. In the case of RI-alpha it seems to be a direct effect of cAMP on stable factors, whereas proteins with either rapid turnover or factors induced by cAMP are involved in the stimulation of C-alpha and the RII subunits.