Structural gender differences in bone mass - characterized by wider but not thicker bones - are generally attributed to opposing sex steroid actions in men and women. Recent findings have redefined the traditional concept of sex hormones as the main regulators of skeletal sexual dimorphism. GH-IGF1 action is likely to be the most important determinant of sex differences in bone mass. Estrogens limit periosteal bone expansion but stimulate endosteal bone apposition in females, whereas androgens stimulate radial bone expansion in males. Androgens not only act directly on bone through the androgen receptor (AR) but also activate estrogen receptor-α or -β (ERα or ERβ) following aromatization into estrogens. Both the AR and ERα pathways are needed to optimize radial cortical bone expansion, whereas AR signaling alone is the dominant pathway for normal male trabecular bone development. Estrogen/ERα-mediated effects in males may - at least partly - depend on interaction with IGF1. In addition, sex hormones and their receptors have an impact on the mechanical sensitivity of the growing skeleton. AR and ERβ signaling may limit the osteogenic response to loading in males and females respectively, while ERα may stimulate the response of bone to mechanical stimulation in the female skeleton. Overall, current evidence suggests that skeletal sexual dimorphism is not just the end result of differences in sex steroid secretion between the sexes, but depends on gender differences in GH-IGF1 and mechanical sensitivity to loading as well.

Androgens have well-established anabolic actions on skeletal muscle, although the direct effects of the androgen receptor (AR) in muscle remain unclear. We generated satellite cell-specific AR-knockout (satARKO) mice in which the AR is selectively ablated in satellite cells, the muscle precursor cells. Total-limb maximal grip strength is decreased by 7% in satARKO mice, with soleus muscles containing ∼10% more type I fibers and 10% less type IIa fibers than the corresponding control littermates. The weight of the perineal levator ani muscle is markedly reduced (-52%). Thus, muscle AR is involved in fiber-type distribution and force production of the limb muscles, while it is a major determinant of the perineal muscle mass. Surprisingly, myostatin (Mstn), a strong inhibitor of skeletal muscle growth, is one of the most androgen-responsive genes (6-fold reduction in satARKO) through direct transcription activation by the AR. Consequently, muscle hypertrophy in response to androgens is augmented in Mstn-knockout mice. Our finding that androgens induce Mstn signaling to restrain their own anabolic actions has implications for the treatment of muscle wasting disorders.-Dubois, V., Laurent, M. R., Sinnesael, M., Cielen, N., Helsen, C., Clinckemalie, L., Spans, L., Gayan-Ramirez, G., Deldicque, L., Hespel, P., Carmeliet, G., Vanderschueren, D., and Claessens, F. A satellite cell-specific knockout of the androgen receptor reveals myostatin as a direct androgen target in skeletal muscle.

Androgens play a key role in the maintenance of male skeletal integrity. The regulation of this integrity by androgen receptor (AR) signaling has been mainly attributed to osteoblasts. Although osteocytes have emerged as key regulators of bone remodeling, the influence of sex steroids on these cells has been poorly studied. We aimed to investigate the role of AR signaling, specifically in osteocytes using the Cre/LoxP system in male mice (driven by dentin matrix protein 1 [ocy-ARKOs]). Osteocyte fractions of control (AR(ex2)/Y) and ocy-ARKO (ARflox(ex2)/Y; DMP1-cre) mice isolated through sequential collagenase digestion showed increasing AR expression toward the mature osteocyte fraction of control males compared with the more immature fractions, whereas this was reduced by >80% in ocy-ARKO osteocytes. The skeletal phenotype of mutant mice was further assessed by histomorphometry and quantitative micro-computed tomography at 12 and 32 weeks of age. Ocy-ARKOs had significantly lower trabecular bone volume and number in femora and tibias at 32 weeks as well as decreased trabecular number in the L 5 vertebra at 12 weeks. Biomechanical testing showed that ocy-ARKO femora were also stiffer and required a lower ultimate force to induce failure at 32 weeks. However, femoral cortical structure was not significantly different at any time point. The absence of AR in osteocyte also did not appear to affect trabecular bone formation nor its response to mechanical loading. In conclusion, selective inactivation of the AR in osteocytes of male mice accelerates age-related deterioration of skeletal integrity. These findings provide evidence for a direct role of androgens in the maintenance of trabecular bone through actions of the AR in osteocytes. ß

Bone is an endocrine tissue expressing androgen and estrogen receptors as well as steroid metabolizing enzymes. The bioactivity of circulating sex steroids is modulated by sex hormone-binding globulin and local conversion in bone tissue, for example, from testosterone (T) to estradiol (E2) by aromatase, or to dihydrotestosterone by 5α-reductase enzymes. Our understanding of the structural basis for gender differences in bone strength has advanced considerably over recent years due to increasing use of (high resolution) peripheral computed tomography. These microarchitectural insights form the basis to understand sex steroid influences on male peak bone mass and turnover in cortical vs trabecular bone. Recent studies using Cre/LoxP technology have further refined our mechanistic insights from global knockout mice into the direct contributions of sex steroids and their respective nuclear receptors in osteoblasts, osteoclasts, osteocytes, and other cells to male osteoporosis. At the same time, these studies have reinforced the notion that androgen and estrogen deficiency have both direct and pleiotropic effects via interaction with, for example, insulin-like growth factor 1, inflammation, oxidative stress, central nervous system control of bone metabolism, adaptation to mechanical loading, etc., This review will summarize recent advances on these issues in the field of sex steroid actions in male bone homeostasis.

Testosterone is an important hormone for both bone gain and maintenance in men. Hypogonadal men have accelerated bone turnover and increased fracture risk. In these men, administration of testosterone inhibits bone resorption and maintains bone mass. Testosterone, however, is converted into estradiol via aromatization in many tissues including male bone. The importance of estrogen receptor alpha activation as well of aromatization of androgens into estrogens was highlighted by a number of cases of men suffering from an inactivating mutation in the estrogen receptor alpha or in the aromatase enzyme. All these men typically had low bone mass, high bone turnover and open epiphyses. In line with these findings, cohort studies have confirmed that estradiol contributes to the maintenance of bone mass after reaching peak bone mass, with an association between estradiol and fractures in elderly men. Recent studies in knock-out mice have increased our understanding of the role of androgens and estrogens in different bone compartments. Estrogen receptor activation, but not androgen receptor activation, is involved in the regulation of male longitudinal appendicular skeletal growth in mice. Both the androgen and the estrogen receptor can independently mediate the cancellous bone-sparing effects of sex steroids in male mice. Selective KO studies of the androgen receptor in osteoblasts in male mice suggest that the osteoblast in the target cell for androgen receptor mediated maintenance of trabecular bone volume and coordination of bone matrix synthesis and mineralization. Taken together, both human and animal studies suggest that testosterone has a dual mode of action on different bone surfaces with involvement of both the androgen and estrogen receptor.

Testosterone - in contrast with estrogen - is not uniformly associated with fracture risk in men. However, androgen receptor mediated action is clearly important for trabecular bone maintenance in male mice whereas both estrogens and androgens regulate cortical bone growth. The osteoblast and osteocyte appear to be involved in such androgen receptor mediated action on bone in male mice. Studies in mice also showed an unexpected interaction between osteocalcin and testosterone production in males and, vice versa, between ovarian production of follicle-stimulating hormone with testosterone and potentially bone formation.

Androgens are well known to enhance exercise-induced muscle hypertrophy; however, whether androgens also influence bone's adaptive response to mechanical loading remains unclear. We studied the adaptive osteogenic response to unilateral in vivo mechanical loading of tibia in adult male mice in both a long- and a short-term experimental set-up. Mice were divided into four groups: sham operated, orchidectomized (ORX), T (ORX+T), or nonaromatizable dihydrotestosterone (ORX+DHT) replacement. Significant interactions between androgen status and osteogenic response to mechanical loading were observed. Cortical thickness increased by T (0.14 vs 0.11 mm sham, P<.05) and DHT (0.17 vs 0.11 mm sham, P<.05). However, T partially (+36%) and DHT completely (+10%) failed to exhibit the loading-related increase observed in sham (+107%) and ORX (+131%, all P<.05) mice. ORX decreased periosteal bone formation, which was restored to sham levels by T and DHT. However, both androgens completely suppressed the loading-related increase in periosteal bone formation. Short-term loading decreased the number of sclerostin-positive osteocytes in sham, whereas in control fibulas, ORX decreased and T increased the number of sclerostin-positive osteocytes. Loading no longer down-regulated sclerostin in the ORX or T groups. In conclusion, both T and DHT suppress the osteogenic response to mechanical loading.

Androgen deficiency or androgen receptor knockout (ARKO) causes high-turnover osteopenia, but the target cells for this effect remain unclear. To examine whether AR in osteoclasts directly suppresses bone resorption, we crossed AR-floxed with cathepsin K-Cre mice. Osteoclast-specific ARKO (ocl-ARKO) mice showed no changes neither in osteoclast surface nor in bone microarchitecture nor in the response to orchidectomy and androgen replacement, indicating that the AR in osteoclasts is not critical for bone maintenance. In line with the lack of a bone phenotype, the levels of AR were very low in osteoclast-enriched cultures derived from bone marrow (BM) and undetectable in osteoclasts generated from spleen precursors. Since tibiae of ubiquitous ARKO mice displayed increased osteoclast counts, the role of AR was further explored using cell cultures from these animals. Osteoclast generation and activity in vitro were similar between ARKO and wildtype control (WT) mice. In co-culture experiments, BM stromal cells (BMSCs) were essential for the suppressive action of AR on osteoclastogenesis and osteoclast activity. Stimulation with 1,25(OH)2 vitamin D3 increased Rankl and decreased Tnfsf11 (osteoprotegerin, Opg) gene expression in BMSCs more than in osteoblasts. This increase in the Rankl/Opg ratio following 1,25(OH)2D3 stimulation was lower, not higher, in ARKO mice. Runx2 expression in BMSCs was however higher in ARKO vs. WT, suggesting that ARKO mice may more readily commit osteoprogenitor cells to osteoblastogenesis. In conclusion, the AR does not seem to suppress bone resorption through direct actions in osteoclasts. BMSCs may however represent an alternative AR target in the BM milieu.