Nuclease sensitivity of estradiol-charged estrogen receptor binding sites in nuclei isolated from normal and neoplastic rat mammary tissues

The interaction of partially purified calf uterine estradiol-charged estrogen receptor (3H]ER) with rat nuclei was studied in vitro. We previously observed a significantly greater number of 3H]ER binding sites (at saturation) in nuclei of R3230AC mammary tumors from intact vs ovariectomized (ovex) rats with no difference in the affinity of 3H]ER binding for these nuclei. We now report on the nuclease sensitivity of 3H]ER binding sites in nuclei from these tumors and from normal rat tissues. Digestion of tumor nuclei with deoxyribonuclease I (DNase I) prior to incubation with 3H]ER in vitro resulted in a progressive loss of 3H]ER binding capacity, which was not accompanied by alterations in the affinity of 3H]ER for the nuclei (Kd = 1-3 nM). A significantly lower concentration (P less than 0.005) of DNase I eliminated 50% of the 3H]ER binding sites in nuclei of tumors from intact hosts (8 unit.min/ml) compared to tumors from ovex hosts (22 unit.min/ml). These results indicate that DNA regions capable of binding ER are more susceptible to DNase I digestion in tumors from intact rats than those from ovex hosts, suggesting that the endogenous hormonal milieu is responsible, at least in part, for maintenance of nuclease-sensitive DNA conformations in this hormone-responsive mammary tumor. The amount of DNase I required to eliminate 50% of 3H]ER binding to nuclei from lactating mammary gland, liver, and kidney ranged from 14 to 56 unit.min/ml. Therefore, accessibility of 3H]ER binding sites to nuclease digestion in normal rat tissue is generally less than that of R3230AC tumors.