Role of the androgen receptor in skeletal homeostasis: The androgen resistant testicular feminized male mouse model

The role of androgen receptor-mediated androgen action on bone was investigated in testicular feminized male (Tfm) mice. Cortical bone was found to be unresponsive to testosterone (T) in orchidectomized Tfm mice, whereas cortical thickness as well as trabecular BMD and structure were fully maintained by T in the corresponding Tabby control mice. These data show an essential role for androgen receptor-mediated androgen action in periosteal bone formation. Introduction: Androgens can affect the male skeleton both directly - through activation of the androgen receptor (AR) - and indirectly - through stimulation of estrogen receptors after aromatization. We assessed the importance of AR-mediated androgen action on bone in a mouse model of androgen resistance. Materials and Methods: Eight-week-old androgen-resistant testicular feminized male (Tfm) and Tabby control mice were orchidectomized (ORX) and treated for 4 weeks with a slow-release testosterone (T) pellet (delivering 167 mug/day) or a placebo pellet. A comprehensive analysis of the skeletal effects of androgen deficiency and replacement was performed using histomorphornetry, QCT, and biochemical assessment of bone turnover. Results: As expected, T increased trabecular BNID, volume, number, and width in ORX Tabby mice. In ORX Tfm mice, however, T had less effect on trabecular BMD and no effect on trabecular bone structure. T action on trabecular bone was associated with opposite changes in bone turnover: trabecular and endocortical bone turnover and serum levels of osteocalcin were all reduced by T in ORX Tabby mice, but not in ORX Tfm mice. T also increased cortical thickness (+ 16%), area, and density in ORX Tabby mice, but not in Tfm mice, resulting in greater bone strength in the Tabby control strain. The positive effects of T on cortical bone reflected a stimulatory effect on periosteal bone formation (+ 137%), which was again absent in Tfm mice. Conclusions: These data show that, in male mice, AR-mediated T action is essential for periosteal bone formation and contributes to trabecular bone maintenance.