Chromatin, SF-1, and CtBP structural and post-translational modifications induced by ACTH/cAMP accelerate CYP17 transcription rate
Dammer, Eric B.
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CYP17 is an ACTH/cAMP inducible gene in the human adrenal cortex encoding a cytochrome P450 enzyme with sterol 17α-hydroxylase activity and 17,20 lyase activity essential for biosynthesis of cortisol and androgens. Studies carried out during the past decade have shown that acclerated transcription of inducible eukaryotic genes involves sequential chromatin modifications by cooperative promoter-specific transcription factors and the class of proteins called transcriptional coregulators. In the present work, we aimed to first identify important chromatin modifications and chromatin modifying complexes at the CYP17 transcription start site and nearby steroidogenic factor-1 (SF-1) binding site. Then, we asked what modifications to SF-1 occur during the interaction of this nuclear receptor with the CYP17 promoter, and what their function may be. Finally, we asked how ACTH/cAMP signaling affects SF-1-containing chromatin-modifying complexes during the early phase of transcriptional induction of CYP17. Results from chromatin immunoprecipitation (ChIP) and mammalian two hybrid experiments identified complexes including one comprised of SF-1, steroid receptor coactivator-1 (SRC-1), and the histone acetyltransferase general control nonderepressed 5 (GCN5) as cAMP-inducible, but sensitive to the SF-1 antagonist sphingosine, and able to act in stimulating CYP17 transcription. Moreover, ATPases on the promoter coincided with manipulation of nucleosome histone H2 dimer content. Next, we found that SF-1 phosphorylation by glycogen synthase kinase 3beta (GSK3beta), reciprocal dephosphorylation by phosphatase(s), and acetylation by GCN5 at nearby sites at the ligand binding pocket opening were required for efficient CYP17 transcription. This leads us to propose that ligand binding to SF-1 is controlled by these post-translational modifications. Finally, we determined that the corepressors E1A C-terminal binding proteins (CtBP) 1 and 2 are protein kinase A (PKA) targets and are sensitive to PKA-dependent NADH accumulation. These effects of PKA activation by ACTH/cAMP in adrenal cortex cells enforce CYP17 transcription concomitant with dimerization of CtBP1 and CtBP2.