Different mechanisms are involved in cAMP-mediated induction of mRNAs for subunits of cAMP-dependent protein kinases

Abstract

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 (RIα, RIIα, RIIβ, and Cα) 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 RIIβ 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 RIIβ, was also seen at the level of transcription. Maximal transcription rate for c-fos, RIα, and Ca were observed after 30 min, whereas that for Rllβ was increasing during the 2-h period examined. Transcriptional activation of the RIα gene also appeared faster than that for RIIβ. 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 RIα (2-fold). In contrast, the inductions of RIIβ, RIIα, and Cα 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 RIα 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 Ca and the RII subunits.