Osteoblast differentiation influences androgen and estrogen receptor-alpha and -beta expression

Abstract

Significant levels of estrogen and androgens circulate in men and women, and both play an important role in bone metabolism. While it is well established that either estrogen or androgen replacement therapy is effective at ameliorating bone loss associated with hypogonadism, recent evidence nevertheless suggests that estrogen and androgens have distinct molecular actions on the skeleton. In this study, we have employed normal rat calvarial osteoblast cultures to characterize relative expression profiles of estrogen (ER and ER ) and androgen receptors (AR) during osteoblast differentiation. Normal osteoblast cultures can proceed through in vitro differentiation with distinct stages of proliferation, matrix maturation and mineralization in the appropriate differentiation medium containing ascorbic acid. Expression profiles of AR, ER and ER in primary cultures during osteoblast differentiation were characterized both by semi-quantitative relative RT-PCR and by Western analysis. In cultures induced to differentiate by growth in the presence of ascorbic acid, the expression profile for each receptor was unique during the course of differentiation. ER levels were elevated during matrix maturation and then declined during mineralization. ER expression was relatively constant throughout differentiation, exhibiting more constitutive expression. In contrast, AR levels were lowest during proliferation, and then increased throughout differentiation with highest levels in the most mature mineralizing cultures. Since steroid hormone action is generally mediated by specific cognate receptors, these results suggest that androgen actions may target cells during the mineralization stage of osteoblast differentiation, while estrogen action through either receptor isoform is more likely to affect osteoblasts earlier during matrix maturation. Interestingly, sex steroid receptor expression profiles did not exhibit the same patterns of regulation if osteoblast cultures were grown without ascorbic acid in medium that did not support extracellular matrix deposition. Thus, sex steroids may distinctly influence skeletal health by differential modulation of function during osteoblast differentiation.

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

“…Interestingly, the highest expression of AR has been observed in osteocytes. (15) Osteocytes are not only the most abundant cells within bone, but they also play a key role in the regulation of bone remodeling as well as in its response to mechanical loading. (16) Moreover, osteocytes express a number of potentially important genes such as receptor activator of NF-kB ligand (RANKL), which are involved in bone remodeling and which might be androgen regulated.…”

Expression of an estrogen receptor agonist in differentiating osteoblast cultures

“…In this regard, osteoblasts from fetal rat bone express little functional sex steroid receptor but acquire the ability to drive complex direct and indirect responses after transgenic receptor expression (3,12,20,22,23). Still, some studies suggest that endogenous estrogen receptor ␣ (ER␣) levels increase during late-stage osteoblast differentiation (24,25). To address this further, we investigated whether osteoblasts acquire a functional sex steroid response during differentiation in vitro.…”

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

“…The expression of the AR has been reported to be higher in cortical bone osteocytes compared with those in the trabecular bone. 6 Further, the removal of the DNA-bindingdependent activity of the AR in osteoblasts and osteocytes using the osteocalcin promoter 19 has been shown to lead to a reduction in cortical bone thickness, but not perimeter among male mice. However, in the study by Notini et al, 20 in which an artificial 2.3-kb-long collagen-1 promoter was used to induce the Cre-mediated deletion of exon 3 of the AR gene in osteoblasts before mineralization, no changes in the cortical bone thickness could be detected, either.…”

“…1,2 Reduction of serum androgen levels in aging men or in patients treated with anti-androgen therapy has been shown to be associated with the reduced bone mineral density (BMD) of the skeleton. 3 In mice, the androgen receptor (AR) is expressed in osteoblasts, osteocytes [4][5][6] and osteoclasts. 7 Androgens have been suggested to regulate the periosteal expansion of long bones and to inhibit endosteal bone growth.…”

“…7 Androgens have been suggested to regulate the periosteal expansion of long bones and to inhibit endosteal bone growth. 6,8 The inhibition of endosteal bone growth has been considered to be due to androgen-mediated osteoblast apoptosis. 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.…”