Characterization of the hypothalamic-pituitary-gonadal axis in estrogen receptor (ER) null mice reveals hypergonadism and endocrine sex reversal in females lacking ERα but not ERβ

Abstract

To determine the role of each estrogen receptor (ER) form (ERα, ERβ) in mediating the estrogen actions necessary to maintain proper function of the hypothalamic-pituitary-gonadal axis, we have characterized the hypothalamic-pituitary-gonadal axis in female ER knockout (ERKO) mice. Evaluation of pituitary function included gene expression assays for Gnrhr, Cga, Lhb, Fshb, and Prl. Evaluation of ovarian steroidogenic capacity included gene expression assays for the components necessary for estradiol synthesis: i.e. Star, Cyp11a, Cyp17, Cyp19, Hsd3b1, and Hsd17b1. These data were corroborated by assessing plasma levels of the respective peptide and steroid hormones. αERKO and αβERKO females exhibited increased pituitary Cga and Lhb expression and increased plasma LH levels, whereas both were normal in βERKO. Pituitary Fshb expression and plasma FSH were normal in all three ERKOs. In the ovary, all three ERKOs exhibited normal expression of Star, Cyp11a, and Hsd3b1. In contrast, Cyp17 and Cyp19 expression were elevated in αERKO but normal in βERKO and αβERKO. Plasma steroid levels in each ERKO mirrored the steroidogenic enzyme expression, with only the αERKO exhibiting elevated androstenedione and estradiol. Elevated plasma testosterone in αERKO and αβERKO females was attributable to aberrant expression of Hsd17b3 in the ovary, representing a form of endocrine sex reversal, as this enzyme is unique to the testes. Enhanced steroidogenic capacity in αERKO ovaries was erased by treatment with a GnRH antagonist, indicating these phenotypes to be the indirect result of excess LH stimulation that follows the loss of ERα in the hypothalamic-pituitary axis. Overall, these findings indicate that ERα, but not ERβ, is indispensable to the negative-feedback effects of estradiol that maintain proper LH secretion from the pituitary. The subsequent hypergonadism is illustrated as increased Cyp17, Cyp19, Hsd17b1, and ectopic Hsd17b3 expression in the ovary.