C/EBPβ (CCAAT/enhancer-binding protein β) mediates progesterone production through transcriptional regulation in co-operation with SF-1 (steroidogenic factor-1).

The transcription factor SF-1 (steroidogenic factor-1) is a master regulator of steroidogenesis. Previously, we have found that SF-1 induces the differentiation of mesenchymal stem cells into steroidogenic cells. To elucidate the molecular mechanisms of SF-1-mediated functions, we attempted to identify protein components of the SF-1 nuclear protein complex in differentiated cells. SF-1 immunoaffinity chromatography followed by MS/MS analysis was performed, and 24 proteins were identified. Among these proteins, we focused on C/EBPβ (CCAAT/enhancer-binding protein β), which is an essential transcription factor for ovulation and luteinization, as the transcriptional mechanisms of C/EBPβ working together with SF-1 are poorly understood. C/EBPβ knockdown attenuated cAMP-induced progesterone production in granulosa tumour-derived KGN cells by altering STAR (steroidogenic acute regulatory protein), CYP11A1 (cytochrome P450, family 11, subfamily A, polypeptide 1) and HSD3B2 (hydroxy-δ-5-steroid dehydrogenase, 3β- and steroid δ-isomerase 2) expression. EMSA and ChIP assays revealed novel C/EBPβ-binding sites in the upstream regions of the HSD3B2 and CYP11A1 genes. These interactions were enhanced by cAMP stimulation. Luciferase assays showed that C/EBPβ-responsive regions were found in each promoter and C/EBPβ is involved in the cAMP-induced transcriptional activity of these genes together with SF-1. These results indicate that C/EBPβ is an important mediator of progesterone production by working together with SF-1, especially under tropic hormone-stimulated conditions.