in the Short-Term and Long-Term Absence of Estrogens Receptor Content of MCF-7 Human Breast Cancer Cells Grown Proliferation , Hormonal Responsiveness , and Estrogen Updated

We have examined the effect of short-term and long-term growth in the absence of estrogens on the proliferation rate and estrogen and antiestrogen responsiveness of MCF-7 human breast cancer cells. The removal of phenol red, the pH indicator in tissue culture medium that is weakly estrogenic (Y. Berthois et al., Proc. Nati. Acad. Sci. USA, 83: 2496-2500, 1986), immediately slows the cell proliferation rate, and MCF-7 cells grown in phenol red-free medium with charcoal dextrantreated serum for periods up to 1 mo maintain this reduced rate of cell proliferation. In these short-term phenol red-withdrawn cells, estradici stimulates proliferation markedly and reproducibly, and antiestrogens inhibit estrogen-stimulated proliferation. Antiestrogens by themselves appear as partial agonists/antagonists; at low concentrations they stim ulate proliferation weakly, but they show no stimulation at the high concentrations where they fully inhibit estrogen-stimulated proliferation. In contrast to the short-term phenol red-withdrawn cells, cells maintained for several months (5 to 6 mo) in the apparently complete absence of estrogens (no phenol red, with charcoal dextran-treated calf serum) show a markedly increased basal rate of cell proliferation; estradiol is unable to increase this rate of proliferation further, but antiestrogens are able to decrease proliferation. This change in growth pattern is associated with a 3-fold increase in cellular estrogen receptor levels. Despite their differ ing basal growth rates, cells grown in either the short-term (<1 mo) or long-term (>6 mo) absence of estrogens both have progesterone receptor levels that are very low and, in both cases, estradiol increases progester one receptor levels markedly. Thus, under long-term estrogen-free con ditions, there is a dissociation between the stimulation of cell proliferation and of specific protein synthesis (progesterone receptor) by estrogen. The increase in the cell proliferation rate observed in cells grown in the longterm absence of estrogen may reflect altered regulation of growth factor production or altered sensitivity to growth factors in the medium or produced by the cells themselves. Hence, these breast cancer cells adapt significantly to long-term growth in estrogen-free conditions, an obser vation that may be relevant to understanding the growth of hormoneresponsive human breast cancers in vivo.