Estrogen Receptor-α Dependency of Estrogen’s Stimulatory Action on Cancellous Bone Formation in Male Mice

Abstract

We examined whether estrogen receptor (ER)α is required for estrogen to stimulate cancellous bone formation in long bones of male mice. 17β-Estradiol (E2) was administered to ERα−/− male mice or wild-type (WT) littermate controls at 40, 400, or 4000 μg/kg by daily sc injection for 28 d and histomorphometric analysis performed at the distal femoral metaphysis. In WT mice, treatment with E2 (40 μg/kg·d) increased the proportion of cancellous bone surfaces undergoing mineralization and stimulated mineral apposition rate. In addition, higher doses of E2 induced the formation of new cancellous bone formation surfaces in WT mice. In contrast, E2 had little effect on any of these parameters in ERα−/− mice. Immunohistochemistry was subsequently performed using an ERα-specific C-terminal polyclonal antibody. In WT mice, ERα was expressed both by cancellous osteoblasts and a significant proportion of mononuclear bone marrow cells. Immunoreactivity was also observed in cancellous osteoblasts of ERα−/− mice, resulting from expression of the activation function-1-deficient 46-kDa ERα isoform previously reported to be expressed in normal osteoblasts and bones of ERα−/− mice. Taken together, our results suggest that estrogen stimulates bone formation in mouse long bones via a mechanism that requires the presence of full-length ERα possessing activation function-1.