Relationships between Mammary Estrogen Receptor and Estrogenic Sensitivity. II. Binding of Cytoplasmic Receptor to Chromatin*

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Abstract
Mammary glands from nulliparous mice are responsive to estradiol, whereas mammary glands from lactating mice are unresponsive, despite the presence of high concentrations of estrogen receptors. This study examined the relation between mammary estrogenic sensitivity and ability of mammary estrogen receptors to bind to intact chromatin as well as to partially deproteinized chromatin. Mammary chromatin was prepared from nulliparous and lactating mice, linked covalently to cellulose and deproteinized sequentially by 0-8 M guanidine chloride (Gdn .cntdot. HCl). The binding of receptors to these various chromatin preparations was determined using partially purified [3H]estradiol-receptor complexes ([3H]ER) obtained by fractionation on diethylaminoethyl cellulose columns. The binding pattern of [3H]ER from nulliparous mice to chromatin fractions from either nulliparous or lactating mice revealed maximal binding activity with chromatin previously extracted with 4-6 M Gdn .cntdot. HCl. Binding was of high affinity [dissociation constant (Kd) 3.6 .times. 10-10 M], saturable and steroid receptor and species specific. However, mammary [3H]ER preparations from lactating mice bound poorly to intact chromatin as well as to the Gdn .cntdot. HCl extracted chromatin fractions isolated from either mammary gland of nulliparous or lactating mice. In mixing experiments the estrogen receptor preparation from lactating mice decreased substantially the binding activity of [3H]ER from nulliparous mice to chromatin suggesting the presence of an inhibiting factor. Thus, these studies reveal that the unresponsiveness of lactating mammary glands to estradiol coexists with the inability of estrogen receptors from lactating mice to interact with specific high affinity sites on mammary chromatin and also that this impeded interaction of estrogen receptors with chromatin may be due to some inhibitor(s) present in the cytosol of lactating mammary glands.