A comparative study of estrogen receptors a and b in the female reproductive tract

Abstract

Estrogens have a pronounced influence on the function of the female reproductive system. The regulation of biological processes by estrogens is mediated via specific intracellular receptors (ERs), ERalpha and ERbeta, which function as regulators of transcriptional activity. In order to further understand the role and action of estrogens in general and the relative importance of each ER subtype in particular, a study on the role of ERbeta and its relationship to ERalpha in the reproductive system has been performed. A specific aim was to study ERbeta in the process of cervical ripening in humans. This study has a comparative design, using rats and cynomolgus macaques as animal models for comparison to humans.

Distinct expression pattems were evident for the two ERs in different compartments of the female reproductive tract of the rat. ERalpha was the dominating subtype in uterus, oviduct, and cervix/vagina, with the distribution varying in stroma and epithelium during the estrous cycle. Low levels ERalpha mRNA and protein were observed in ovarian stroma cells, while ERbeta was strongly expressed in the granulosa cells. ERbeta mRNA and protein were present in the different compartments of the uterus and cervix/vagina, but only sparsely expressed in the oviduct.

Administration of progesterone (P4) for 24 hours to ovariectomized (OVX) rats was associated with a reduction of ERalpha levels in the luminal epithelium of the endometrium, but not in the glands, which suggests that functional responses to P4 in relation to ERalpha are different between luminal and glandular epithelium. E2 or P4 treatment did not affect the expression of ERbeta in the endometrium.

Long-term treatment (35 months) of OVX cynomolgus macaques with estrogen and/or progestin, or tamoxifen, resulted in a modulation of ERalpha protein levels in the endometrium, more pronounced in the superficial than in the basal layer. Treatment with progestin alone or combined with estrogen down-regulated ERalpha in the superficial glands, whereas both ERalpha and ERbeta were increased after progestin treatment in the superficial stroma. Tamoxifen administration increased the levels of ERalpha, which may be one reason behind the formation of endometrial hyperplasia, a common side-effect from tamoxifen treatment. The ratio of ERalpha/ERbeta after different treatments might determine the estrogen response and indicate the rate of proliferation.

ERbeta mRNA and protein levels were low in the cervix from non-pregnant women, but increased at term pregnancy. After parturition ERbeta expression decreased again to the levels seen in non-pregnant women. In contrast to ERbeta, ERalpha expression was high in the cervix from, nonpregnant and term pregnant women, but significantly decreased after parturition. This suggests that a switch from ERalpha to ERbeta dominance occurs in the cervix at term pregnancy, possibly to block the activity of labor-associated genes. The distribution of ERbeta in neutrophils and endothelial cells of blood vessels and the co-localization of ERbeta and leukocyte markers CD45 and CD68 suggest a possible direct effect of estrogens on leukocytes via ERbeta. In conclusion, short-term steroid treatment of OVX rats and long-term treatment of OVX monkeys influences ERalpha expression more than ERbeta in the uterus/endometrium. The upregulation of ERbeta in cervical stroma at term pregnancy indicates a role of this ER subtype at parturition. Estrogens might affect cervical ripening also via ERbeta present in the invading leukocytes. The results from this study demonstrate that the multifunctional activities of estrogens and progestins are mediated by a complex regulation of ERalpha and ERbeta in the female reproductive tract.