Differential expression of the estrogen receptors α and β in the rat corpus luteum of pregnancy: Regulation by prolactin and placental lactogens

Estradiol, together with PRL and placental lactogens, regulates steroidogenesis and cell hypertrophy in the rat corpus luteum of pregnancy. Although binding experiments have demonstrated the presence of estrogen- binding sites, no evidence exists as to whether the rat corpus luteum of pregnancy expresses the estrogen receptor (ER) genes. In this investigation, we have analyzed the expression of the two ER genes (ERα and ERβ) (by RT- PCR and in situ hybridization) in the rat corpus luteum, studied their developmental changes throughout pregnancy, and investigated the regulation of ERα and ERβ messenger RNA (mRNA) expression by PRL and placental lactogens. The RT-PCR studies showed that both ER mRNA species (ERα and ERβ) are coexpressed in the rat corpus luteum during pregnancy. Whereas ERα mRNA increased from early pregnancy, reached a maximum at midpregnancy, and had a remarkable decline before parturition; ERβ mRNA remained constant throughout pregnancy, with a significant decline at parturition. Examination of ERα and ERβ mRNA expression at the cellular level, by in situ hybridization, showed ERα expressed in both follicles and corpus luteum, with maximal expression at midpregnancy. In parallel with the RT-PCR studies, ERβ mRNA was similarly expressed throughout pregnancy in the corpus luteum, but it was less abundant when compared with small and growing follicles. Western blot analysis revealed two ER immunoreactive proteins in the nuclear fraction obtained from pregnant rat corpus luteum: a 67-kDa moiety, highly expressed at midpregnancy but barely detectable in early and late gestation; and a 61-kDa form that remained developmentally unchanged. Hypophysectomy, performed early in pregnancy, induced a sharp decline in ERα mRNA expression but a less-marked reduction in ERβ mRNA levels. PRL treatment reverted the inhibition induced by hypophysectomy in both receptor subtypes. When primary luteinized cells were used to test the effect of PRL, rat placental lactogen I, and rat placental lactogen II on the expression of ERα and ERβ mRNA, all these lactogenic hormones stimulated both ER mRNA species in a dose-dependent manner. The regulation of ER mRNA expression was further evaluated in a luteal cell line, termed GG-CL, which apparently expresses only the ERβ mRNA species. Culture of the GG-CL cells, in the presence of PRL, resulted in a dose-related up-regulation of ERβ mRNA expression. In addition, PRL treatment enhanced the binding activity of GG-CL cell nuclear proteins 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 estrogen target gene in the corpus luteum of the pregnant rat.