POTENTIAL MECHANISM OF ESTROGEN-MEDIATED DECREASE IN BONE-FORMATION -ESTROGEN INCREASES PRODUCTION OF INHIBITORY INSULIN-LIKE GROWTH FACTOR-BINDING PROTEIN-LC

Using a recently developed human osteoblastic cell line (hFOB/ER9) wit h high levels (similar to 4,000 per nucleus) of estrogen receptors and the characteristic phenotype of the mature osteoblast, we tested the hypothesis that estrogen decreases bone formation by inhibiting the ac tion of the insulin-like growth factor (IGF) paracrine/autocrine syste m. IGF-II, the predominant IGF produced by osteoblastic cells, was mea surable in hFOB/ER9-conditioned medium (similar to 10 ng/mL) and its l evel did not change significantly after treatment with 17 beta-estradi ol (E(2)) or anti-estrogens, Treatment with E(2) at 0.1-100 nM decreas ed [H-3]thymidine uptake to 53% of control (p < 0.001) in a dose-depen dent fashion. The predominant IGF-binding proteins (IGFBPs) produced b y hFOB/ER9 and by normal trabecular osteoblasts are IGFBP-3 and IGFBP- 4, of which IGFBP-4 is consistently inhibitory of IGF action. Treatmen t with E(2) at 0.01-10 nM for 48 h increased IGFBP-4 mRNA to 346% +/- 90% (mean +/- SE) of control (p < 0.05) and IGFBP-4 protein to 278% +/ - 75% of control (p < 0.01) in a dose-dependent fashion but did not al ter IGFBP-3 mRNA or protein. E(2) treatment also attenuated IGF-depend ent, IGFBP-4 specific proteolysis to similar to 50% of control. ICI 18 2,780, a pure anti-estrogen, completely blocked E(2)-mediated decrease s in cell proliferation and increases in levels of IGFBP-4 mRNA and pr otein. Treatment of the hFOB/ER9 cells with recombinant human IGFBP-4 (200 ng/mL) decreased cell proliferation to 55% of control (p < 0.01). Thus, E(2) acts on osteoblastic cells to increase availability of inh ibitory IGFBP-4, by both increasing its production and decreasing its degradation, which may oppose the mitogenic effect of the IGFs on oste oblastic cells. This action may mediate, at least in part, the decreas es in bone formation that are observed after estrogen treatment in viv o.