Estrogen regulation of human osteoblast function is determined by the stage of differentiation and the estrogen receptor isoform

Although osteoblasts have been shown to respond to estrogens and express both isoforms of the estrogen receptor (ERα and ERβ), the role each isoform plays in osteoblast cell function and differentiation is unknown. The two ER isoforms are known to differentially regulate estrogen-inducible promoter-reporter gene constructs, but their individual effects on endogenous gene expression in osteoblasts have not been reported. We compared the effects of 171β-estradiol (E) and tamoxifen (TAM) on gene expression and matrix formation during the differentiation of human osteoblast cell lines stably expressing either ERα (hFOB/ERα9) or ERβ (hFOB/ERβ6). Expression of the appropriate ER isoform in these cells was confirmed by northern and western blotting and the responses to E in the hFOB/ERβ6 line were abolished by-an ERβ-specific inhibitor. The data demonstrate that (1) in both the hFOB/ER cell lines, certain responses to E or TAM (including alkaline phosphatase, IL-6 and IL-11 production) are more pronounced at the late mineralization stage of differentiation compared to earlier Stages, (2) E exerted a greater regulation of bone nodule formation and matrix protein/cytokine production in the ERα cells than in ERβ cells, and (3) the regulated expression of select genes differed between the ERα and ERβ cells. TAM had no effect on nodule formation in either cell line and was a less potent regulator of gene/protein expression than E. Thus, both the ER isoform and the stage of differentiation appear to influence the response of osteoblast cells to E and TAM.