Existence of receptors bound to endogenous estradiol in breast cancers of premenopausal and postmenopausal women.

The amounts of free and occupied estrogen receptors were determined in cytosols and cellular (cytoplasmic + nuclear) extracts of 23 human breast cancers. In vivo undersaturation of the receptors was observed in all tumors. A positive correlation was found between the degree of saturation and the concentration of circulating estrogens in premenopausal women. The saturation of the sites was higher in cellular extracts than in cytosols, pointing to the existence of receptor-estradiol complexes bound to nuclei. Our results suggest that the absolute number of occupied receptors depends upon the level of available cytoplasmic receptors as well as upon the level of tissue estradiol.