Although our laboratory has reported that normal human osteoblast-like (hOB) cells contain estrogen receptors, we have failed to find major effects of 17β-estradiol (E2) on modulation of proliferation of bone matrix protein production by hOB cells. Because the major effect of E2in vivo is to decrease bone resorption and because transforming growth factor-β (TGF-β) has been reported to decrease osteoclast-mediated bone resorption, we have tested the hypothesis that the effect of E2on osteoclast activity is, at least in part, indirectly mediated by enhancing production of TGF-β by osteoblasts. We therefore have extended our studies to examine possible TGF-β gene expression including the modulation of the release of TGF-β by E2 in near homogenous populations of hOB cells. TGF-β protein production was measured using growth inhibition of CCL-64 cells and verified by blocking effects with anti-TGF-β antibodies. TGF-β1messenger RNA (mRNA) steady state levels were assessed by northern blot analysis and quantitated by densitome-tric measurement using 18S ribosomal RNA as a reference. There was an E2dose-dependent increase in TGF-β protein production within 24 h of challenge with E2. Northern blots from these cells demonstrated a dose-dependent increase in steady state mRNA levels of TGF-β1within 6 h of treatment. PTH was also a potent stimulator of TGF-β protein and message levels in a dose-dependent manner. Interestingly, coincubation of equimolar concentrations of E2and PTH (10−8m) abrogated the stimulation of TGF-β1mRNA and protein. Decreasing the relative concentration of PTH in this coincubation with E2increased TGF-β1mRNA and protein levels. These data support the fact that E2modulates TGF-β production in osteoblasts. In this manner TGF-β may mediate E2inhibition of osteoclast activity.
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