Inhibin gene expression in the ovary is stimulated by FSH, which uses cAMP as an intracellular second messenger. To examine further the transcriptional regulation of the α inhibin gene by FSH and cAMP, we have isolated and characterized a genomic clone that contains the entire rat α inhibin gene. Sequence analysis of the α inhibin promoter region revealed several potential cAMP response elements (CREs) and transcription factor AP2-binding sites that might mediate cAMP regulation. To determine the functional importance of these sequences, fusion genes including the α inhibin 5′ flanking region linked to a luciferase reporter gene were transiently transfected into primary granulosa cells isolated from immature rats. These fusion genes were both expressed and regulated by the adenylyl cyclase activator forskolin in transfected granulosa cells. Analysis of a series of 5′ deletion mutants indicated that a construct containing as little as 170 basepairs up-stream of the α inhibin start site, which includes a single imperfect CRE and no AP2 sites, was regulated by forskolin. DNAse footprinting was used to demonstrate that bacterially expressed CRE-binding protein (CREB) binds to this CRE located 122 basepairs up-stream of the α inhibin gene transcriptional start site. To investigate further the role of this CRE in α inhibin gene expression, site-specific mutagenesis of the CRE was performed. The α inhibin promoter containing a mutated CRE was not regulated by forskolin in granulosa cells and did not bind the CREB protein. Interestingly, mutation of the CRE also substantially reduced basal expression of the α inhibin promoter. Lastly, a gel mobility shift assay was used to examine CRE-binding proteins from granulosa cell extracts. Granulosa cells contain a protein that specifically interacts with CRE-containing oligonucleotides or with the α inhibin promoter and that is recognized by antibodies against the CREB protein. Our results suggest that CREB or related transcription factors play an important role in both basal and cAMP-regulated expression of the α inhibin gene in ovarian granulosa cells.
ASJC Scopus subject areas
- Molecular Biology