In the C3H mouse mammary
adenocarcinoma,
estradiol cannot induce the
progesterone receptor, and the
tumor growth rate is not decreased by
ovariectomy. To find an explanation for this
estrogen resistance, we have compared the
estrogen receptor (ER) from this
tumor to the ER of uterus and of the mammary
tumors induced in rats by dimethylbenz(a)
anthracene. Since the ER concentration of the C3H
tumor is low (congruent to 20 fmol/mg
protein), we have used iodoestradiol of high specific activity to label the receptor. Several criteria of ER activation were studied. The dissociation rates of
estradiol with or without
sodium molybdate were similar in all tissues. In
metrizamide isopycnic gradients, ER from rat uterus and C3H
tumor had a similar density, both in the presence or absence of
DNA. The binding of ER to
DNA-cellulose was analyzed by incubating to equilibrium a constant amount of ER with a variable amount of
DNA, the
cellulose concentration being kept constant. The saturation data were plotted according to the method of Scatchard. The apparent affinity for
DNA of the cytosol ER was similar for the rat dimethylbenz(a)
anthracene tumors and the uterus (Kd congruent to 10 microM) but was significantly higher for the C3H
tumor ER (Kd congruent to 2.3 microM). Neither the substitution of
estradiol by iodoestradiol, nor the difference in cytosol
protein and ER concentrations, nor the nonspecific
steroid binding to
DNA-cellulose could explain this result. This difference was confirmed when using
DNA-
agarose or soluble
DNA in
sucrose gradients. Finally, the
salt concentrations necessary to elute ER from
DNA-cellulose columns were 0.20 and 0.28 M for uterine and C3H
tumor ER, respectively. To conclude, the C3H
tumor has a low content of ER which appears to have a higher affinity for
DNA than the ER of
estrogen-responsive tissue. We suggest that the reason for the inefficiency of ER in the C3H
tumor may be related to its increased affinity for nonspecific
DNA sites.