Mammary carcinoma induced in male Sprague-Dawley rats by multiple intragastric intubations of 7,12-dimethylbenz-(a)anthracene showed ovary-independent growth but contained estrogen receptors (ER) and progesterone receptors (Yoshida, Yoshida, Fukunishi, Sato, Okamoto, and Matsumoto, Cancer Res., 42: 2434–2439, 1982). Transplantable carcinoma (MT6) was obtained from dimethylbenz(a)anthracene-induced mammary carcinoma and then maintained in male rats. MT6 tumors with ER grew equally well in males, females, gonadectomized males, males given injections of bromocryptine (1 mg/day) or lisuride hydrogen maleate (50 µg/day), and gonadectomized males receiving smaller doses of 17β-estradiol (1 to 100 µg/2 days). However, the growth of MT6 was inhibited markedly by injection of a very large amount of 17β-estradiol (1 mg/2 days). Although transplanted MT6 tumors were ductal carcinoma with cribriform pattern with ER, tumors recurring after injection with 1 mg 17β-estradiol were found to be spindle cell carcinoma without ER, which could grow equally well in recipients treated with or without 1 mg 17β-estradiol. These observations suggest that the growth of ovary-independent MT6 tumors with ER and progesterone receptors is inhibited only by pharmacological doses of estrogens and that the loss of growth-inhibiting effect of pharmacological doses of estrogen on MT6 tumors occurs during high-dose estrogen treatment.


Supported by a Grant-in-Aid for Cancer Research from the Ministry of Education, Science, and Culture, Japan.

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