Estrogen receptor-beta mRNA expression in rat ovary: down-regulation by gonadotropins

Abstract

We have examined the expression and regulation of the two estrogen receptor (ER alpha and ER beta) genes in the rat ovary, using Northern blotting, RT-PCR, and in situ hybridization histochemistry. Northern blotting results show that the ovary expresses both ER alpha and ER beta genes as single (approximately 6.5-kb) and multiple (ranging from approximately 1.0-kb to approximately 10.0-kb) transcripts, respectively. ER alpha mRNA is expressed at a level lower than ER beta mRNA in immature rat ovaries. This relationship appears unchanged between sexually mature adult rats and immature rats. In sexually mature adult rats undergoing endogenous hormonal changes, whole ovarian content of ER beta mRNA, as determined by RT-PCR, remained more or less constant with the exception of the evening of proestrus when ER beta mRNA levels were decreased. Examination of ER beta mRNA expression at the cellular level, by in situ hybridization, showed that ER beta mRNA is expressed preferentially in granulosa cells of small, growing, and preovulatory follicles, although weak expression of ER beta mRNA was observed in a subset of corpora lutea, and that the decrease in ER beta mRNA during proestrous evening is attributable, at least in part, to down-regulation of ER beta mRNA in the preovulatory follicles. This type of expression and regulation was not typical for ER alpha mRNA in the ovary. Although whole ovarian content of ER alpha mRNA was clearly detected by RT-PCR, no apparent modulation of ER alpha mRNA levels was observed during the estrous cycle. Examination of ER alpha mRNA expression at the cellular level, by in situ hybridization, showed that ER alpha mRNA is expressed at a low level throughout the ovary with no particular cellular localization. To further examine the potential role of the preovulatory pituitary gonadotropins in regulating ER beta mRNA expression in the ovary, we used immature rats treated with gonadotropins. In rats undergoing exogenous hormonal challenges, whole ovarian content of ER beta mRNA, as determined by RT-PCR, remained more or less unchanged after an injection of PMSG. In contrast, a subsequent injection of human CG (hCG) resulted in a substantial decrease in whole ovarian content of ER beta mRNA. In situ hybridization for ER beta mRNA shows that small, growing, and preovulatory follicles express ER beta mRNA in the granulosa cells. The preovulatory follicles contain ER beta mRNA at a level lower than that observed for small and growing follicles. In addition, there is an abrupt decrease in ER beta mRNA expression in the preovulatory follicles after hCG injection. The inhibitory effect of hCG on ER beta mRNA expression was also observed in cultured granulosa cells. Moreover, agents stimulating LH/CG receptor-associated intracellular signaling pathways (forskolin and a phorbol ester) readily mimicked the effect of hCG in down-regulating ER beta mRNA in cultured granulosa cells. Taken together, our results demonstrate that 1) the ovary expresses both ER alpha and ER beta genes, although ER beta is the predominant form of estrogen receptor in the ovary, 2) ER beta mRNA is localized predominantly to the granulosa cells of small, growing, and preovulatory follicles, and 3) the preovulatory LH surge down-regulates ER beta mRNA. These results clearly implicate the physiological importance of ER beta in female reproductive functions.