Growth Stimulation of T47d Human Breast Cancer Cells by the Anti- Progestin Ru486

Our laboratory has previously reported that physiological levels of progestins stimulate growth of the human breast cancer cell line T47D. We have also determined that the antiprogestin RU486 inhibits this stimulation. Here we present the unexpected finding that RU486 alone also stimulates growth of these breast cancer cells. Thus, RU486 exhibits both antagonist and agonist-like activity, in a manner somewhat reminiscent of the antiestrogen tamoxifen. To our knowledge, this is the first report of stimulation of breast cancer growth by RU486.