HETEROGENEITY OF BINDING-SITES AND BIOEFFECTS OF RALOXIFENE ON THE HUMAN LEUKEMIC-CELL LINE FLG-29.1

The benzothiophene divarative raloxifene is known to mimic estrogen in human bone remodeling. To investigate the ''in vitro'' properties of raloxifene on osteoclast precursors, the human leukemic cell Line FLG 29.1, which differentiates toward the osteoclastic phenotype, was exam ined for raloxifene binding and for evidence of its bioeffects. Scatch ard and Hill analysis of binding data with the tritiated raloxifene de monstrated the presence of two classes of binding sites in both nuclea r and cytosol fractions with Kd values of similar to 1 nM and similar to 5 nM, respectively. In addition, analysis of binding data using tri tiated 17 beta E-2 as ligand at high concentrations (10 - 40 nM) and e ither unlabeled 17 beta E-2 or raloxifene as competitors gave similar results demonstrating the presence of type II EBS in these cells. Pico molar concentrations of raloxifene significantly (p<0.05) inhibited ce ll proliferation. Moreover, the compound at nanomolar concentrations i nduced a significant dose- and time-dependent increase of progesterone receptor, and activated apoptotic cell death. These findings clearly demonstrate that raloxifene acts as an estrogen agonist in FLG; 29.1 c ells, acting through the estrogen receptor and, possibly, via multiple cooperative binding component(s). (C) 1997 Academic Press.