Structure of the Alpha-Inhibin Gene and its Regulation in the Ruminant Gonad: Inverse Relationship to Oxytocin Gene Expression1

The genes for the alpha subunit of inhibin and for the nonapeptide hormone oxytocin are both expressed in the granulosa cells of the ruminant follicle as well as in the Sertoli cells of the ruminant testis. Northern hybridization of mRNA from both ovary and testis indicate that in both gonads the expression of the two genes is inversely regulated. In the luteinizing granulosa cells, in vitro as in vivo, the alpha-inhibin gene is down-regulated when the oxytocin gene is up-regulated. In the Sertoli cells of the bull and sheep testis, the situation is similar, with the alpha-inhibin gene being up-regulated in the prepubertal gonad and down-regulated concomitantly with an up-regulation of the oxytocin gene in early puberty. The gene for the bovine alpha-inhibin subunit was cloned and characterized. Assessment of transcriptional initiation by primer extension and ribonuclease protection assays showed that several different sites were used in both granulosa cells and testis. Transient transfection of primary bovine granulosa cells with alpha-inhibin/luciferase gene constructs indicated that a major promoter element resided in the region -178 to -245 respective to the methionine start codon of translation, a region that contains a cAMP response element. The ability of forskolin to up-regulate the transcription of transfected gene constructs also depended on the integrity of this region. In contrast, transfection of TM4 cells led to transcriptional initiation from an unusual site in the alpha-inhibin gene and to a lack of forskolin regulation. Comparison of the alpha-inhibin and oxytocin genes indicates that although both can be up-regulated by FSH or by forskolin within the same cells, different mechanisms of signal transduction are involved to explain the temporal differences in expression. Together the results indicate that a differentiation step occurring in Sertoli cells at early puberty and in granulosa cells at luteinization involves comparable regulation of genes through the sequential action of different cAMP-linked transcription factors.