A variety of
antiestrogens can be shown to antagonize
estrogen action in animal model systems. Several of these compounds are useful in the management of metastatic human
breast cancer. To further elucidate their mechanism of action, we studied several of these compounds using human
breast cancer cell lines maintained in long-term tissue culture as a model system.
Antiestrogens including
tamoxifen (NSC-180973;
ICI-46474),
nafoxidine.
CI-628, and
clomiphene citrate inhibit macromolecular synthesis below control levels in two human breast cell lines. This effect is limited to cell lines which contain
estrogen receptors. Simultaneous addition of as little as 1000-fold less
estradiol prevents
antiestrogen effects. Sequential addition of
estrogen for up to 48 hours to cells incubated in
antiestrogen reverses inhibition. If cells are continued in
antiestrogen alone for more than about 3 days, inhibitory effects become irreversible. The cells detach from the surface of the culture vessel and are no longer viable.
Tamoxifen competes with 3H-estradiol for specific receptor sites but with about a 100-fold lower apparent affinity. Direct binding of 3H-tamoxifen and 3H-estradiol to duplicate cytoplasmic extracts reveals equivalent numbers of binding sites but a 20-fold lower affinity for the
antiestrogen. There is reasonable agreement between concentrations of
tamoxifen which bind to receptor and concentrations which inhibit cells.