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

Adrenal steroid production is stimulated by adrenocorticotropin hormone act ivation of the cAMP-dependent protein kinase A (PKA) signaling pathway and subsequent induction of (S) under bar teroidogenic (A) under bar cute (R) u nder bar egulatory (StAR) protein expression. Herein we have compared StAR mRNA and protein levels in 8-Br-cAMP-treated mouse adrenocortical Y1 and th e derived PKA mutant Kin-8 cell lines to evaluate the PKA requirement in St AR 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 no t blocked in Kin-8 cells. Consistent with this data, a 2-fold increase in i ncorporation of [35(S)]methionine into StAR was also observed after 8-Br-cA MP treatment of both cell lines. Since StAR protein levels were not suffici ent to detect by Western analysis, these data indicate that PKA functions a t the post-translational level to regulate StAR expression and we propose t hat phosphorylation of StAR by PKA contributes to protein stability.