Relative Impact of Androgen and Estrogen Receptor Activation in the Effects of Androgens on Trabecular and Cortical Bone in Growing Male Mice: A Study in the Androgen Receptor Knockout Mouse Model

Abstract

The relative importance of AR and ER activation has been studied in pubertal male AR knockout and WT mice after orchidectomy and androgen replacement therapy, either with or without an aromatase inhibitor. AR activation dominates normal trabecular bone development and cortical bone modeling in male mice. Moreover, optimal periosteal bone expansion is only observed in the presence of both AR and ER activation.Introduction: Androgen receptor (AR)-mediated androgen action has traditionally been considered a key determinant of male skeletal growth. Increasing evidence, however, suggests that estrogens are also essential for normal male bone growth. Therefore, the relative importance of AR-mediated and estrogen receptor (ER)-mediated androgen action after aromatization remains to be clarified. Materials and Methods: Trabecular and cortical bone was studied in intact or orchidectomized pubertal AR knockout (ARKO) and male wildtype (WT) mice, with or without replacement therapy (3-8 weeks of age). Nonaromatizable (dihydrotestosterone [DHT]) and aromatizable (testosterone [T]) androgens and T plus an aromatase inhibitor (anastrazole) were administered to orchidectomized ARKO and WT mice. Trabecular and cortical bone modeling were evaluated by static and dynamic histomorphometry, respectively. Results: AR inactivation or orchidectomy induced a similar degree of trabecular bone loss (−68% and −71%, respectively). Both DHT and T prevented orchidectomy-induced bone loss in WT mice but not in ARKO mice. Administration of an aromatase inhibitor did not affect T action on trabecular bone. AR inactivation and orchidectomy had similar negative effects on cortical thickness (−13% and −8%, respectively) and periosteal bone formation (−50% and −26%, respectively). In orchidectomized WT mice, both DHT and T were found to stimulate periosteal bone formation and, as a result, to increase cortical thickness. In contrast, the periosteum of ARKO mice remained unresponsive to either DHT or T. Interestingly, administration of an aromatase inhibitor partly reduced T action on periosteal bone formation in orchidectomized WT mice (−34% versus orchidectomized WT mice on T), but not in ARKO mice. This effect was associated with a significant decrease in serum IGF-I (−21% versus orchidectomized WT mice on T). Conclusions: These findings suggest a major role for AR activation in normal development of trabecular bone and periosteal bone growth in male mice. Moreover, optimal stimulation of periosteal growth is only obtained in the presence of both AR and ER activation.

Androgen receptor (AR) in osteocytes is important for the maintenance of male skeletal integrity: Evidence from targeted AR disruption in mouse osteocytes

“…Treatments with aromatizable as well as nonaromatizable androgens are able to prevent ORX-induced decrease of trabecular and cortical bone mass in ORX mice but not in ARKO male mice. (5) These observations provide further evidence for a major role for AR activation in normal trabecular bone development and periosteal bone growth in male mice. It is presently unclear, however, what the target cells are for ARmediated actions on bone.…”

“…(12,13) The absence of a major cortical phenotype across a number of similar mice models contrasts with the significant cortical phenotype characterized by reduction of periosteal expansion observed in the ubiquitous ARKO model. (5) Puberty appears to be a critical period for the development of skeletal sexual dimorphism as characterized by wider and longer bones in male. During puberty, androgens have been shown to enhance cortical bone development in male mice.…”

“…(27) The AR appears to be critically involved in this effect because both nonaromatizable androgens and aromatizable androgens enhance skeletal development in animals with a functional AR. (5) Although a functional AR in a number of nonosteocyte bone cells, such as immature periosteal bone cells or osteoclasts, may be partly responsible for AR-mediated androgen action during puberty, it should be noted that AR expression is much lower in these cells than in osteocytes. (28) Estrogen and estrogen receptor alpha activation after aromatization of androgens play a role in cortical bone development as well, as illustrated by the cortical phenotype in AR-ERa double-knockout mice.…”

The mutual dependence between bone and gonads

“…Although skeletal size and volume are similar in prepubertal girls and boys (Kelly et al 1990, Seeman 2001, Vanderschueren et al 2004, Kirmani et al 2009, Callewaert et al 2010a, the sexual dimorphism in bone growth becomes apparent during puberty, at which time men reach higher peak bone mass (Clarke & Khosla 2010). This skeletal sexual dimorphism is mainly due to a stimulatory androgen action on periosteal bone formation in men, whereas an inhibitory estrogen-related action occurs in women (Bertelloni et al 1995, Finkelstein et al 1996, Katznelson et al 1996, Seeman 2001, Venken et al 2006, Kirmani et al 2009). Since an excess of androgen in women is associated with higher bone mineral density, there is evidence that androgens also affect peak bone mass in women (Buchanan et al 1988, Zborowski et al 2000, Wei et al 2010.…”

Inactivation of the androgen receptor in bone-forming cells leads to trabecular bone loss in adult female mice

“…9,12 A full AR knockout has been shown to exhibit increased bone turnover and reduced trabecular and cortical bone volume in male mice. [13][14][15][16] Further elimination of estrogen receptor a (ERa) caused an additional reduction in cortical bone mass. 14 In another full AR knockout model, the bone growth and composition of knockout female mice were indistinguishable from those of wild-type (WT) mice at 8 weeks of age, 17 suggesting that the AR is not important during the early development or rapid growth phase of the female skeleton.…”