Estrogen Receptor-alpha Dependency of Estrogen's Stimulatory Action on Cancellous Bone Formation in Male Mice

We examined whether estrogen receptor (ER) alpha is required for estrogen to stimulate cancellous bone formation in long bones of male mice. 17beta-Estradiol (E-2) was administered to ERalpha(-/-) male mice or wild-type (WT) littermate controls at 40, 400, or 4000 mug/kg by daily sc injection for 28 d and histomorphometric analysis performed at the distal femoral metaphysis. In WT mice, treatment with E-2 (40 mug/kg.d) increased the proportion of cancellous bone surfaces undergoing mineralization and stimulated mineral apposition rate. In addition, higher doses of E-2 induced the formation of new cancellous bone formation surfaces in WT mice. In contrast, E-2 had little effect on any of these parameters in ERalpha(-/-) mice. Immunohistochemistry was subsequently performed using an ERalpha-specific C-terminal polyclonal antibody. In WT mice, ERalpha was expressed both by cancellous osteoblasts and a significant proportion of mononuclear bone marrow cells. Immunoreactivity was also observed in cancellous osteoblasts of ERalpha(-/-) mice, resulting from expression of the activation function-1-deficient 46-kDa ERalpha isoform previously reported to be expressed in normal osteoblasts and bones of ERalpha(-/-) 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 ERalpha possessing activation function-1.