Estrogenic and androgenic regulation of human osteoblast-like cells is mediated by specific steroid receptors.

Abstract

The effectiveness of estrogen replacement therapy in the prevention of postmenopausal osteoporosis has led to its current widespread use throughout the United States and much of Western Europe, and recently, clinical correlations between circulating androgen levels and structural bone integrity have been presented. Nevertheless, the biochemical mechanism through which estrogens and androgens act to protect and maintain bone has remained unclear. One possibility is that these hormones directly modulate the activity of cells responsible for bone formation. Therefore, studies were conducted to examine the effects of sex steroids on human osteoblast-like cells. In the first set of experiments, a finite human cell line was established from trabecular bone explants obtained from a 48 year-old woman. These cells, designated BG688, were characterized as osteoblast-like in phenotype using several independent criteria. In addition to classical osteoblast markers, BG688 cells also possess approximately 2400 high affinity (K(d) = 0.45nM) 17-β estradiol (E₂) binding sites per cell. The binding of E₂ to a subset of these sites was specific. BG688 cells were also shown to respond to a physiological concentration (10nM) of E₂, which elicits pleiotropic changes in several mRNA levels including a 2-fold increase in the steady state concentration of α₁(I)-procollagen mRNA. These results indicate that human osteoblast-like cells respond to E₂ via a receptor mediated mechanism, but that, unlike the reproductive tissues, osteoblasts are a less sensitive target. In the second series of experiments, the effects of androgenic hormones on the osteoblast-like, human osteosarcoma cell line, HOS TE85 were evaluated. Employing radiolabelled dihydrotestosterone (DHT), 2800 saturable, high-affinity (K(d) = 0.66nM) androgen binding sites were detected per HOS TE85 cell. Androgen binding was specific. The expression of androgen receptors in HOS TE85 cells was further substantiated by Northern analysis. Physiological concentrations of DHT and testosterone decreased HOS TE85 cell proliferation. This finding suggests that androgens may also play a role in osteoblast differentiation. In support of this hypothesis, treatment with testosterone enhanced the abundance of both α₁ (I)-procollagen mRNA and transforming growth factor- β mRNA. The non-aromatizable androgen DHT also elicited an increase in the steady state concentration of α₁(I)-procollagen mRNA. The findings presented herein are significant within the field of bone cell biology in that they demonstrate that osteoblasts are a target cell for the action of sex steroids, via their cognate, high-affinity receptors. These results also have important implications within the broader context of bone pathophysiology in that they suggest a direct modulation of bone forming and bone remodeling activity by sex steroids.