Expression and hormonal regulation of the CCAAT enhancer binding protein-alpha during differentiation of rat ovarian follicles.

Follicular development involves both proliferation and differentiation of thecal and granulosa cells. The process is regulated by gonadotropins and paracrine and autocrine factors, including steroid hormones, presumably by the induction of different genes at specific time points. In the present study, the expression and distribution of the CCAAT enhancer-binding protein-alpha (C/EBP alpha) were studied in immature ovaries and in ovaries in which follicular growth and development were initiated with PMSG, whereas ovulation and luteal formation were induced by the injection of hCG. Ovaries were collected before and at different time points after PMSG (0, 6, 24, and 48 h) and hCG (0.25, 1, 3, 10, and 24 h) treatment for analyses of the contents of C/EBP alpha mRNA and protein and the cell-specific immunohistochemical localization of the protein. C/EBP alpha mRNA increased to maximal levels 24 h after PMSG treatment. The effect was specific for the ovary, as C/EBP alpha mRNA in the uterus did not change. C/EBP alpha mRNA decreased 10 h after hCG treatment and increased again in newly formed corpora lutea. Immunohistochemistry and immunoblotting demonstrated a similar increase in C/EBP alpha during follicular development. To examine the involvement of specific hormones in the regulation of C/EBP alpha, hypophysectomized immature rats were injected sequentially with estradiol and FSH. This treatment resulted in a substantial increase in C/EBP alpha mRNA and protein. These results demonstrate that C/EBP alpha is hormonally regulated in the ovary and suggest a role for C/EBP alpha during differentiation of ovarian cells and follicular development.