CCAAT/enhancer-binding proteins regulate expression of the human steroidogenic acute regulatory protein (StAR) gene

Two putative CCAAT/enhancer-binding protein (C/EBP) response elements were identified in the proximal promoter of the human steroidogenic acute regulatory protein (StAR) gene, which encodes a key protein-regulating steroid hormone synthesis. Expression of C/EBP alpha and -beta increased StAR promoter activity in COS-l and HepG2 cells. Cotransfection of C/EBP alpha or -beta and steroidogenic factor 1, a transcription factor required for cAMP regulation of StAR expression, into COS-1 augmented 8-bromoadenosine 3':5'-cyclic monophosphate (8-Br-cAMP)-stimulated promoter activity. When the putative C/EBP response elements were mutated, individually or together, a pronounced decline in basal StAR promoter activity in human granulosa-lutein cells resulted, but the fold stimulation of promoter activity by 8-Br-cAMP was unaffected. Recombinant C/EBP alpha and -beta bound to the two identified sequences but not the mutated elements. Human granulosa-lutein cell nuclear extracts also bound these elements but not the mutated sequences. An antibody to C/EBP beta, but not C/EBP alpha, supershifted the nuclear protein complex associated with the more distal element. The complex formed by nuclear extracts with the proximal element was not supershifted by either antibody. Western blot analysis revealed the presence of C/EBPa and C/EBP beta in human granulosalutein cell nuclear extracts. C/EBP beta levels were up-regulated 3-fold by 8-Br-cAMP treatment. Our studies demonstrate a role for C/EBP beta as well as yet to be identified proteins, which can bind to C/EBP response elements, in the regulation of StAR gene expression and suggest a mechanism by which C/EBP beta participates in the cAMP regulation of StAR gene transcription.