The effects of estradiol are modulated in a tissue-specific manner in mice with inducible inactivation of ERα after sexual maturation

Mouse models with lifelong inactivation of estrogen receptor-α (ERα) show that ERα is the main mediator of estrogenic effects in bone, thymus, uterus, and fat. However, ERα inactivation early in life may cause developmental effects that confound the adult phenotypes. To address the specific role of adult ERα expression for estrogenic effects in bone and other nonskeletal tissues, we established a tamoxifen-inducible ERα-inactivated model by crossing CAGG-Cre-ER and ERα flox/flox mice. Tamoxifen-induced ERα inactivation after sexual maturation substantially reduced ERα mRNA levels in cortical bone, trabecular bone, thymus, uterus, gonadal fat, and hypothalamus, in CAGG-Cre-ERα flox/flox (inducible ERα KO ) compared with ERα flox/flox (control) mice. 17β-estradiol (E2) treatment increased trabecular bone volume fraction (BV/TV), cortical bone area, and uterine weight, while it reduced thymus weight and fat mass in ovariectomized control mice. The estrogenic responses were substantially reduced in inducible ERα KO mice compared with control mice on BV/TV (−67%), uterine weight (−94%), thymus weight (−70%), and gonadal fat mass (−94%). In contrast, the estrogenic response on cortical bone area was unaffected in inducible ERα KO compared with control mice. In conclusion, using an inducible ERα KO model, not confounded by lack of ERα during development, we demonstrate that ERα expression in sexually mature female mice is required for normal E2 responses in most, but not all, tissues. The finding that cortical, but not trabecular bone, responds normally to E2 treatment in inducible ERα KO mice strengthens the idea of cortical and trabecular bone being regulated by estrogen via different mechanisms.