Microsomal binding sites for nonsteroidal anti-estrogens in MCF 7 human mammary carcinoma cells. Demonstration of high affinity and narrow specificity for basic ether derivatives of triphenylethylene

Abstract

In the presence of estradiol, at a concentration sufficient to saturate the estrogen receptor, the antiestrogenic and anti-tumor agent tamoxifen was bound to a high affinity (KD = 0.97 +/- 0.15 nM at 4 degrees C) saturable binding site (141,300 +/- 20,100 sites/cell) in MCF 7 human mammary carcinoma cells. The distribution of this site between cytosol, mitochondrial, microsomal (heavy and light), and nuclear fractions paralleled that of NADPH-cytochrome c reductase, an enzyme marker for endoplasmic reticulum. The interaction between tamoxifen and the high affinity site was influenced by changes in pH, ionic strength, and temperature. The kinetic rate constants k+1 and k-1 showed strong temperature dependence, but KD was unaffected by changes in temperature. Competition studies employing analogs of the anit-estrogens tamoxifen, clomiphene, and CI 628 revealed narrow specificity for triphenylethylene derivatives with basic ether side chains.