Human breast cancer: biologically active estrogen receptor in the absence of estrogen?

The human breast cancer cell line MCF-7 does not require estrogen for growth, but paradoxically its growth is inhibited by antiestrogens. Our results show that, unlike normal target cells, MCF-7 cells carry most of their estrogen receptors in their nuclei even when these receptors are not charged with estrogens. The receptors for androgen and for progesterone, on the other hand, are localized in the cytoplasm as usual. Therefore, it is possible that the growth of these abnormal cells is stimulated by estrogen receptor in spite of the absence of the hormone and that the binding of antiestrogen molecules antagonize this stimulation.