The effects of estrogen on human breast carcinomas serially transplanted into nude mice

The effect and mechanisms of 17 beta-estradiol (E2) on breast cancer cells were studied in vivo and in vitro, using 5 human breast carcinomas serially transplanted into nude mice. These carcinoma strains consisted of 4 estrogen receptor (ER) positive tumors and 1 ER negative tumor. Mice bearing these tumors were treated with an intramuscular injection of E2 at a dosage of 50 mg/kg and the tumor doubling time (Td) was calculated in days. The tumor growth was significantly stimulated by E2 in 3 out of the 4 ER positive tumors, the Td of the E2 treated groups being 17.6 days for MCF-7 (control: -17.8 days), 12.8 days for R-27 (control: -12.5 days approximately 14.5 days) and 10.4 days for Br-10 (control: 14.5 days), however, in the T-61 tumor, the growth was inhibited by E2 in a dose dependent manner. In the case of the ER-negative MX-1 tumor, the tumor cell growth was not affected by E2. Discrepancies between the effects of E2 on ER-positive tumors were further analyzed by examining the steroid hormone receptor status and conducting in vitro growth studies. In vitro clonogenic cell assay reproduced the antitumor activity of E2, indicating that E2 directly inhibits part of the cell growth of T-61 tumors. The above results suggest that this experimental system provides a useful tool for analyzing the mechanism of estrogen in breast cancer and that the clonogenic assay using ER positive specimens can help to identify breast cancers sensitive to estrogen therapy.