DIFFERENTIAL EXPRESSION OF THE ESTROGEN-RECEPTOR-ALPHA AND ESTROGEN-RECEPTOR-BETA IN THE RAT CORPUS-LUTEUM OF PREGNANCY - REGULATION BY PROLACTIN AND PLACENTAL LACTOGENS

Estradiol, together with PRL and placental lactogens, regulates steroi dogenesis and cell hypertrophy in the rat corpus luteum of pregnancy. Although binding experiments have demonstrated the presence of estroge n-binding sites, no evidence exists as to whether; the rat corpus lute um of pregnancy expresses the estrogen receptor (ER) genes. In this in vestigation, we have analyzed the expression of the two ER genes (ER a lpha and ER beta) (by RT-PCR and in situ hybridization) in the rat cor pus luteum, studied their developmental changes throughout pregnancy, and investigated the regulation of ER alpha and ER beta messenger RNA (mRNA) expression by PRL and placental lactogens. The RT-PCR studies s howed that both ER mRNA species (ER alpha and ER beta) are coexpressed in the rat corpus luteum during pregnancy. Whereas ER alpha mRNA incr eased from early pregnancy, reached a maximum at midpregnancy, and had a remarkable decline before parturition; ER beta mRNA remained consta nt throughout pregnancy, with a significant decline at parturition. Ex amination of ER alpha and ER beta mRNA expression at the cellular leve l, by in situ hybridization, showed ER alpha expressed in both follicl es and corpus luteum, with maximal expression at midpregnancy. In para llel with the RT-PCR studies, ER beta mRNA was similarly expressed thr oughout pregnancy in the corpus luteum, but it was less abundant when compared with small and growing follicles. western blot analysis revea led two ER immunoreactive proteins in the nuclear fraction obtained fr om pregnant rat corpus luteum: a 67-kDa moiety, highly expressed at mi dpregnancy; but barely detectable in early and late gestation; and a 6 1-kDa form that remained developmentally unchanged. Hypophysectomy, pe rformed early in pregnancy, induced a sharp decline in ER alpha mRNA e xpression but a less-marked reduction in ER beta mRNA levels. PRL trea tment reverted the inhibition induced by hypophysectomy in both recept or subtypes. When primary luteinized cells were used to test the effec t of PRL, rat placental lactogen I, and rat placental lactogen II on t he expression of ER alpha and ER beta mRNA, all these lactogenic hormo nes stimulated both ER mRNA species in a dose-dependent manner. The re gulation of ER mRNA expression was further evaluated in a luteal cell line, termed GG-CL, which apparently expresses only the ER beta mRNA s pecies. Culture of the GG-CL cells, in the presence of PRL, resulted i n a dose-related up-regulation of ER beta mRNA expression. In addition , PRL treatment enhanced the binding activity of GG-CL cell nuclear pr oteins to a classical estrogen response element. Furthermore, in these cells, estradiol treatment induced a dose-dependent up-regulation of the mRNA encoding protein kinase C delta isoform, a well-known estroge n target gene in the corpus luteum of the pregnant rat.