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

Abstract

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

Estrogen receptor-α in osteocytes is important for trabecular bone formation in male mice

“…The main finding in the present study is that ERα in osteocytes is important for trabecular bone formation in male mice. In this study we used the Dmp1-Cre mouse model, which is a widely used and well-described mouse model for deleting gene expression in osteocytes (25,26). We confirmed the specificity of the Dmp1-Cre mouse model using ROSA26-Cre reporter mice, demonstrating specific recombination in osteocytes.…”

“…Previous studies have shown that the Dmp1-Cre mouse strain has the capacity to specifically inactivate loxP flanked genes in osteocytes expressing CRE-recombinase selectively in osteocytes under the control of a 10-kb promoter fragment of the osteocytic Dmp1 marker gene (25,26). To validate that the Dmp1-Cre mouse strain has the capacity to specifically recombine DNA in osteocytes postnatally, we mated Dmp1-Cre mice with ROSA26-Cre reporter mice.…”

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

“…However, even small changes, effected over a long period of time, can lead to an altered phenotype, such as the decreased trabecular bone volume observed in 6-month-old female mice. In the recent study by Sinnesael et al, 21 osteocytic AR expression was found to be pivotal for the maintenance of trabecular, but not cortical, bone volume in male mice upon aging. 21 Our study shows that the AR is also involved in the maintenance of trabecular bone volume of female mice.…”

“…In the recent study by Sinnesael et al, 21 osteocytic AR expression was found to be pivotal for the maintenance of trabecular, but not cortical, bone volume in male mice upon aging. 21 Our study shows that the AR is also involved in the maintenance of trabecular bone volume of female mice. This result has to be viewed in the light that although low levels of testosterone have been reported in fetal 23 and neonate 24 female mice, the serum testosterone concentration in adult female mice is actually 10-fold higher than the estradiol concentration, 25 which suggests that testosterone can act as a ligand, driving AR-mediated effects in the trabecular bone maintenance of female mice.…”

“…Some results have indicated a reduction in cortical bone thickness but no change in periosteal circumference when the osteocalcin promoter was used to target the exon 3 of the AR gene, 19 whereas no effect on the cortical bone was observed in another study utilizing the type I collagen 2.3 promoter. 20 In a recent study by Sinnesael et al, 21 osteocytic AR was shown to be crucial for the maintenance of trabecular bone volume in male mice. The role of AR in the regulation of the female skeleton is currently incompletely characterized.…”