Protein Modifications Regulate the Role of 14-3-3γ Adaptor Protein in cAMP-induced Steroidogenesis in MA-10 Leydig Cells

The 14-3-3 protein family comprises adaptors and scaffolds that regulate intracellular signaling pathways. The 14-3-3 gamma isoform is a negative regulator of steroidogenesis that is hormonally induced and transiently functions at the initiation of steroidogenesis by delaying maximal steroidogenesis in MA-10 mouse tumor Leydig cells. Treatment of MA-10 cells with the cAMP analog 8-bromo-cAMP (8-Br-cAMP), which stimulates steroidogenesis, triggers the interaction of 14-3-3 gamma with the steroidogenic acute regulatory protein (STAR) in the cytosol, limiting STAR activity to basal levels. Over time, this interaction ceases, allowing for a 2-fold induction in STAR activity and maximal increase in the rate of steroid formation. The 14-3-3 gamma/ STAR pattern of interaction was found to be opposite that of the 14-3-3 gamma homodimerization pattern. Phosphorylation and acetylation of 14-3-3 gamma showed similar patterns to homodimerization and STAR binding, respectively. 14-3-3 gamma Ser(58) phosphorylation and 14-3-3 gamma Lys(49) acetylation were blocked using trans-activator of HIV transcription factor 1 peptides coupled to 14-3-3 gamma sequences containing Ser(58) or Lys(49). Blocking either one of these modifications further induced 8-Br-cAMP-induced steroidogenesis while reducing lipid storage, suggesting that the stored cholesterol is used for steroid formation. Taken together, these results indicate that Ser(58) phosphorylation and Lys(49) acetylation of 14-3-3 gamma occur in a coordinated time-dependent manner to regulate 14-3-3 gamma homodimerization. 14-3-3 gamma Ser(58) phosphorylation is required for STAR interactions under control conditions, and 14-3-3 gamma Lys(49) acetylation is important for the cAMP-dependent induction of these interactions.