Antiestrogen ICI 164,384 reduces cellular estrogen receptor content by increasing its turnover.

The ability of estrogens to stimulate the transcriptional activity of the estrogen receptor can be inhibited by a diverse range of estrogen antagonists. Here we show that the antiestrogen ICI 164,384, N-(n-butyl)-11-[3,17 beta-dihydroxy-estra-1,3,5(10)-trien-7 alpha-yl]N-methylundecanamide, rapidly reduces the levels of receptor protein transiently expressed in cells without affecting receptor mRNA abundance. The reduction in the levels of receptor protein is dose dependent, reversible by estradiol, and mediated by the hormone-binding domain of the receptor. Pulse-chase experiments indicate that the half-life of the receptor is reduced from approximately 5 hr in the presence of estradiol to less than 1 hr by ICI 164,384. A similar reduction in estrogen receptor levels is demonstrated in human breast cancer cells treated with ICI 164,384. We discuss the possibility that the increased turnover of the receptor might be a consequence of impaired receptor dimerization.