Effects of gonadal and adrenal androgens in a novel androgen-responsive human osteoblastic cell line

While androgens have important skeletal effects, the mechanism(s) of androgen action on bone remain unclear. Current osteoblast models to study androgen effects have several limitations, including the presence of heterogeneous cell populations. In this study, we examined the effects of androgens on the proliferation and differentiation of a novel human fetal osteoblastic cell line (hFOB/AR-6) that expresses a mature osteoblast phenotype and a physiological number (~4,000/nucleus) of androgen receptors (AR). Treatment with 5α-dihydrotestosterone (5α-DHT) inhibited the proliferation of hFOB/AR-6 cells in a dose-dependent fashion, while it had no effect on the proliferation of hFOB cells, which express low levels of AR (<200/nucleus). In hFOB/AR-6 cells, co-treatment with the specific AR antagonist, hydroxyflutamide abolished 5α-DHT-induced growth inhibition. Steady-state levels of transforming growth factor-β₁ (TGF-β₁) and TGF- β-induced early gene (TIEG) mRNA decreased after treatment of hFOB/AR-6 cells with 5α-DHT, suggesting a role for the TGF-β₁-TIEG pathway in mediating 5α-DHT-induced growth inhibition of hFOB/AR-6 cells. In support of this, co-treatment of hFOB/AR-6 cells with TGF-β₁ (40 pg/ml) reversed the 5α-DHT-induced growth inhibition, whereas TGF-β₁ alone at this dose had no effect on hFOB/AR-6 cell proliferation. Furthermore, treatment of hFOB/AR-6 cells with 5α-DHT and testosterone (10^-8 M) inhibited basal and 1,25- (OH)₂D₃-induced alkaline phosphatase (ALP) activity and type I collagen synthesis without affecting osteocalcin production. Thus, in this fetal osteoblast cell line expressing a physiological number of AR, androgens decrease proliferation and the expression of markers associated with osteoblast differentiation. These studies suggest that the potential anabolic effect of androgens on bone may not be mediated at the level of the mature osteoblast.