Post-translational regulation of steroidogenic acute regulatory protein by cAMP-dependent protein kinase A.

Adrenal steroid production is stimulated by adrenocorticotropin hormone activation of the cAMP-dependent protein kinase A (PKA) signaling pathway and subsequent induction of (S) under bar teroidogenic (A) under bar cute (R) under bar egulatory (StAR) protein expression. Herein we have compared StAR mRNA and protein levels in 8-Br-cAMP-treated mouse adrenocortical Y1 and the derived PKA mutant Kin-8 cell lines to evaluate the PKA requirement in StAR expression. StAR mRNA was induced by 8-Br-cAMP-treatment of both Y1 and Kin-8 cells with maximal expression levels in Kin-g cells approximately 50% of that observed in Y1 cells. StAR protein levels, as detected by Western analysis, were concomitantly increased in Y1 cells but were not detected in the Kin-8 cells. StAR mRNA colocalized with the active polysome fractions in both 8-Br-cAMP-treated Y1 and Kin-8 cells, indicating translation was not blocked in Kin-8 cells. Consistent with this data, a 2-fold increase in incorporation of [35(S)]methionine into StAR was also observed after 8-Br-cAMP treatment of both cell lines. Since StAR protein levels were not sufficient to detect by Western analysis, these data indicate that PKA functions at the post-translational level to regulate StAR expression and we propose that phosphorylation of StAR by PKA contributes to protein stability.