Targeting of androgen receptor in bone reveals a lack of androgen anabolic action and inhibition of osteogenesis

Abstract

Androgens are anabolic hormones that affect many tissues, including bone. However, an anabolic effect of androgen treatment on bone in eugonadal subjects has not been observed and clinical trials have been disappointing. The androgen receptor (AR) mediates biological responses to androgens. In bone tissue, both AR and the estrogen receptor (ER) are expressed. Since androgens can be converted into estrogen, the specific role of the AR in maintenance of skeletal homoeostasis remains controversial. The goal of this study was to use skeletally targeted overexpression of AR in differentiated osteoblasts as a means of elucidating the specific role(s) for AR transactivation in the mature bone compartment. Transgenic mice overexpressing AR under the control of the 2.3-kb α1 (I)-collagen promoter fragment showed no difference in body composition, testosterone, or 17β-estradiol levels. However, transgenic males have reduced serum osteocalcin, CTx and TRAPC5b levels, and a bone phenotype was observed. In cortical bone, high-resolution micro-computed tomography revealed no difference in periosteal perimeter but a significant reduction in cortical bone area due to an enlarged marrow cavity. Endocortical bone formation rate was also significantly inhibited. Biomechanical analyses showed decreased whole bone strength and quality, with significant reductions in all parameters tested. Trabecular morphology was altered, with increased bone volume comprised of more trabeculae that were closer together but not thicker. Expression of genes involved in bone formation and bone resorption was significantly reduced. The consequences of androgen action are compartment-specific; anabolic effects are exhibited exclusively at periosteal surfaces, but in mature osteoblasts androgens inhibited osteogenesis with detrimental effects on matrix quality, bone fragility and whole bone strength. Thus, the present data demonstrate that enhanced androgen signaling targeted to bone results in low bone turnover and inhibition of bone formation by differentiated osteoblasts. These results indicate that direct androgen action in mature osteoblasts is not anabolic, and raise concerns regarding anabolic steroid abuse in the developing skeleton or high-dose treatment in eugonadal adults.

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

“…Between 12 and 32 weeks of age, mice apparently also do not experience significant cortical bone loss, in contrast to the decline in trabecular bone. (27,33) Despite the apparent preservation of cortical integrity, a lower ultimate force was needed for breaking ocy-ARKO bones compared with controls at 32 weeks of age. In line with our findings, previous models inactivating the AR at earlier stages of the osteoblast did not observe a significant difference in cortical structure, but mechanical properties were not documented.…”

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

“…This outcome differs from the results reported by Chiang et al, 19 who observed a small reduction in the femoral cortical bone thickness in mice in which the exon 3 of the AR gene was deleted by similar osteocalcin promoter-activated Cre recombination. Taken together, the osteoblast targeted models by Notini et al, 20 Chiang et al 19 and us as well as the overexpression models by Wiren et al, 9,10 indicate that osteoblast precursors, but not mature osteoblasts or osteocytes, are the cells implementing the AR-mediated effects on periosteal apposition.…”

“…8 Overexpression of the AR in premature osteoblasts of male mice using the 3.6 kb type I collagen promoter has been shown to lead to an increase in periosteal bone formation and a reduction in endosteal bone formation, 9 whereas use of the 2.3 kb type I collagen promoter, expressed in mature osteoblasts, only reduced endosteal bone formation with no effect on the periosteum. 10 These different effects can be attributed, in addition to different stages of osteoblast maturation at which these promoters are active, to various patterns of their expression in bone tissue. The 2.3 kb type I collagen promoter is strongly expressed in endosteal osteoblasts and osteocytes, but not in the periosteum, 10,11 whereas the 3.6 kb type I collagen promoter has been shown to be active in the periosteum.…”

“…10 These different effects can be attributed, in addition to different stages of osteoblast maturation at which these promoters are active, to various patterns of their expression in bone tissue. The 2.3 kb type I collagen promoter is strongly expressed in endosteal osteoblasts and osteocytes, but not in the periosteum, 10,11 whereas the 3.6 kb type I collagen promoter has been shown to be active in the periosteum. 9,12 A full AR knockout has been shown to exhibit increased bone turnover and reduced trabecular and cortical bone volume in male mice.…”

Identification of gene pathways altered by deletion of the androgen receptor specifically in mineralizing osteoblasts and osteocytes in mice

“…In contrast, overexpression of the AR in osteoblasts from late in the matrix development and maturation stage using the 2 . 3 kb Col1a1 promoter has no effect on bone size as measured by periosteal circumference (Wiren et al 2008). Both AR transgenic models do, however, have a common phenotype of reduced formation at endocortical surfaces and increased trabecular bone volume as a result of reduced bone turnover (Wiren et al 2004(Wiren et al , 2008.…”