Expression of Estrogen Receptor-β Protein in Rodent Ovary

Estrogen is an essential hormone for the LH surge and ovulation. The primary source of estrogen is from ovarian granulosa cells and in rats, estrogen, in turn, increases granulosa cell number and enhances FSH-stimulated gene expression in these cells. Thus, rat granulosa cells both respond to and synthesize estrogen. To further elucidate the mechanisms mediating the actions of estrogen in granulosa cells, we have identified and characterized the estrogen receptor-β (ER-β) subtype in rodent granulosa cells. ER-β protein was localized to the nuclei of rat granulosa cells in preantral and antral follicles by immunocytochemistry, coincident with the location of ER-β messenger RNA (mRNA). Immunoprecipitation and Western blot analysis using ER-β specific antisera demonstrated a protein of approximately 60 kDa in granulosa cells prepared from PMSG-primed immature mice and estrogen-treated immature rats. Extracts from granulosa cells specifically bound an estrogen response element and the complex was recognized by antisera to ER-β. A synthetic steroid estrogen radioligand,[ 125I]-17α-iodovinyl-11β-methoxyestradiol ([125I]-VME2), bound to cytosolic granulosa cell preparations with high affinity (estimated KD value of 401 ± 83 pm, and Bmax value of 102± 9 fmol/mg protein). ER-β protein levels rapidly declined following hCG treatment consistent with the reported decrease in binding activity and ER-β mRNA levels by high levels of gonadotropins. Overall, we have demonstrated that 1) ER-β protein is the dominant estrogen receptor subtype present in rodent granulosa cells, 2) this receptor is functional, and 3) it is regulated by ovulatory doses of gonadotropins. Thus, ER-β is likely to be a mediator of estrogen action in rodent granulosa cells during follicular development.