Expression of Estrogen Receptor Alpha36 in the Hamster Ovary and Its Possible Regulation by Gonadotropins and Steroid Hormones.

Abstract

In the ovary, estrogen signaling is of utmost importance that is carried out by the estrogen receptor, member of nuclear receptor superfamily. In reproductive females, estrogen plays an essential role in the reproductive cyclicity and other ovarian functions. Here we report the expression of estrogen receptor α36 (ERα36), a splice variant of the estrogen receptor α (ERα), in the hamster ovary throughout the estrous cycle, and its possible regulation by follicle stimulating hormone (FSH), luteinizing hormone (LH), estradiol-17β (E) and progesterone (P). Hamster ovaries were collected from Day 1 (estrus, D1) though Day 4 (proestrus, D4) morning and D4 afternoon, and the expression pattern of ERα36 protein was examined by immunofluorescence and Western immunoblotting. Immunoblot data indicated that ERα36 expression declined by D4:0900h through D4:1600h compared to other days of the estrous cycle. Immunofluorescence results revealed that ERα36 was expressed in the plasma membrane of both follicular and interstitial cells in the ovary with highest level of expression in the granulosa cells; however, the immunostaining declined by D4:0900h through D4:1600h. No staining was observed in the cell nuclei. To determine the role of individual reproductive hormones on ovarian ERα36 expression, hypophysectomized hamsters were treated with FSH, LH, E or P on postoperative day 10. Immunoblot data revealed a significant decline in ERα36 protein in hypophysectomized hamsters compared to intact hamster on D1. FSH-treatment upregulated the receptor protein level to that of D1 hamsters. LH-treatment showed a similar increase. In contrast, a combined treatment of FSH and LH failed to raise the receptor protein level to that of the either hormone alone. E or P treatment upregulated ERα36 protein level, but P effect was stronger. Nevertheless, the increase in ERα36 expression was much less than that was induced by FSH and LH. No increase was observed after a combined treatment of E and P. The data were validated by immunofluorescence localization. These results indicate that the ERα splice variant, ERα36, is translated into protein in ovarian follicular cells, but the receptor protein is mostly localized in the plasma membrane in contrast to the classic ERα, which is located in the nucleus. Further, the induction of ERα mRNA splicing and the translation of the truncated receptor mRNA seems to be regulated by gonadotropins, such FSH or LH. Ovarian steroids may have a different regulatory role on ERα36 expression. Therefore, ERα36 may play a unique regulatory role in estrogen signaling during ovarian follicular development. (poster)