Mechanisms of Inhibin Alpha Subunit Gene Regulation in the Rat and Human Ovary

Spatiotemporal gene expression is achieved by the cooperation of transcription factors, chromatin-modifying enzymes, and epigenetic modifications. In the ovary, precise gene regulation is critical during the periovulatory period when the luteinizing hormone (LH) surge triggers ovulation and luteinization. Aberrant gene expression during this time period has pathological effects on the reproductive cycle, as is the case with misexpression of the inhibin alpha subunit gene. The inhibin alpha gene is rapidly repressed in response to the LH surge. In the rat, repression is maintained in the corpus luteum, whereas in the human, expression rebounds during the luteal phase. The overall goal of this thesis was to explore the transient and epigenetic mechanisms that contribute to inhibin alpha gene expression in the rat and determine whether they are conserved in the human.;I characterize the NR4A receptors as LH-induced repressors of inhibin alpha expression. I show that instead of binding directly to the DNA, the receptors are recruited to the promoter by physical interaction with GATA-4. I also examine epigenetic contributions to inhibin alpha regulation at three phases in the rat: high expression in the preovulatory follicle, acute repression in the ovulatory follicle, and permanent repression in the corpus luteum. I find that promoter DNA methylation is increased only in the corpus luteum, but the repressive histone modifications H3K9 and H3K27 trimethylation increase on the promoter in the ovulatory follicle and corpus luteum, while active H3K4 trimethylation decreases. Therefore, repressive changes in histone marks coincide with the initial repression of inhibin alpha expression before DNA methylation occurs after luteinization. Lastly, I use primary human granulosa-lutein cells to characterize expression of transcription factors that may play a role in regulating inhibin alpha expression. I also identify two potential differences in inhibin alpha regulation between rat and human luteal cells. CREB occupies the promoter in human granulosa-lutein cells whereas CREB is blocked by CpG methylation in the rat. Additionally, DNA methylation decreases as human cells luteinize, in opposition to the increased DNA methylation after luteinization in the rat. Therefore, inhibin alpha gene regulation involves transient and epigenetic mechanisms to achieve cell- and species-specific expression.