THE PURE ANTIESTROGEN ICI-182,780 BINDS TO A HIGH-AFFINITY SITE DISTINCT FROM THE ESTROGEN-RECEPTOR

Both estrogen receptor-positive (ER(+)), tamoxifen-sensitive (5-21) an d tamoxifen-resistant (5-23) subclones of the parental MCF-7 breast ca ncer cell line were used in competitive ligand binding studies involvi ng either [H-3]ICI 182,780 or 4-OH-[H-3]tamoxifen (4OHT) displacement by unlabelled estradiol (E(2)) or the antiestrogens (AE) 4OHT and ICI 182,780. Neither radioligand was displaced significantly by E(2) over a range of concentrations; binding was predominantly inhibited by the corresponding radio-inert ligand. Scatchard analysis of the data revea led that the binding capacities of both cell lines for ICI 182,780 wer e approximately 7-fold greater than the previously determined number o f ER sites per cell, with the affinity being an order of magnitude les s than that of E(2) for ER. No difference was found between the TAM-se nsitive and -resistant cells in their binding of either AE. When cells were preincubated with either E(2), TAM or 4OHT at a high, fixed conc entration to block the ER or AE binding sites (AEBS), respectively, di splaceable binding of [H-3]ICI 182,780 was still observed, indicating binding at a site other than the classical ER or previously described AEBS. Our results suggest that there is a specific, saturable and rela tively high-affinity binding site for ICI 182,780 in MCF 5-21 and MCF 5-23 breast cancer cells. However, the physiological relevance of this binding site requires further clarification because in cell growth as says, E(2) (at 1/10 the dose of ICI 182,780) overcame the inhibitory e ffect of the antiestrogen in both of the cell lines. (C) 1995 Wiley-Li ss, Inc.