Heterogeneity of Estrogen Binding Sites in Mouse Mammary Cancer

Abstract

Heterogeneity of binding sites was studied in a transplantable ovarian dependent mouse mammary tumor (MXT-3590). Saturation analysis of cytoplasmic or crude nuclear fractions revealed 2 binding components: type I which conforms to the classically described estrogen receptor and type II which has a lower affinity for estradiol but a greater capacity than type I sites. Exposure of cytosol to charcoal partially removes bound 3H-estradiol from type II sites but not from type I sites. Type II sites are specific for estrogens and do not translocate from the cytoplasmic to the nuclear compartment. Although type II sites undergo dissociation on prelabeled sucrose density gradients, they are readily demonstrable by postlabeling sucrose density gradient fractions and hydroxylapatite adsorption. The technique of postlabeling is recommended for the sucrose gradient analysis of type I and II sites. Saturation assays should be preformed over a wide range of 3H-estradiol concentrations (0.1-120 nM) for proper evaluation of both sites. These considerations may contribute to more accurate predictions about the response of breast cancers to endocrine therapies.