Cellular existence of estrogen receptor in C3H mouse mammary tumor.

Estrogen receptor (ER) in the C3H mouse mammary tumor was characterized in the cytosol and determined in the nuclear KCl extract and the nuclear KCl unextractable fraction (non-extract). In the cytosol, the estradiol (E2)-ER complex was sedimented in the 8S region by 5-20% linear sucrose gradient centrifugation. A steroid specificity study showed the presence of estrogen-specific binder with a dissociation constant (Kd) of approximately 2 nM. ER levels in the cytosol, the nuclear KCl extract, and the suspension of nuclear KCl unextractable fraction were determined by Scatchard analyses, using the dextran-coated charcoal (DCC) adsorption method for the former two and the sedimentation method for the latter. Although ER was present in the nuclear fractions, it seems more plausible that cytosol ER contributes to nuclear translocation of the estrogen-ER complex. The results suggest the presence of an intranuclear mechanism for the estrogen action.