Apparent rat strain-related sensitivity to phorbol promotion of mammary carcinogenesis

It has been reported that twice-weekly i.p. injections of 4 mg phorbol for 10 weeks, after a single feeding of 6 mg dimethylbenz(a)anthracene (DMBA) in female Wistar rats, led to a significant augmentation of mammary adenocarcinoma incidence and of lymphatic leukemia incidence as compared to 6 mg DMBA alone. In an experiment reported here, in female Sprague-Dawley rats, using the same doses of DMBA and phorbol and the same injection schedule, phorbol given after DMBA did not augment mammary adenocarcinoma incidence or lymphatic leukemia incidence as compared to DMBA given alone. It thus appears that there is a strain-related sensitivity between Wistar and Sprague-Dawley rats with regard to the promoting activity of phorbol when phorbol treatment follows DMBA treatment, and mammary adenocarcinoma incidence and lymphatic leukemia incidence are studied. Further, in Sprague-Dawley rats, phorbol did not promote mammary fibroadenoma incidence in DMBA-treated rats, mammary adenocarcinoma incidence in procarbazine-treated rats, and mammary adenocarcinoma incidence or mammary fibroadenoma incidence in X-ray-treated rats. DMBA and procarbazine, with or without phorbol, tended to induce more mammary neoplasms in the anterior (thoracic) than in the posterior (abdominal) mammary glands. X-irradiation tended to induce mammary neoplasms in approximately equal numbers in the anterior and posterior mammary glands. It was suggested that regional differences in chemically induced mammary carcinogenesis were due to a difference in the transport and delivery of the chemical carcinogens to the regions rather than a difference in the amount of mammary gland tissue in the regions. An analysis of the numbers of Sprague-Dawley rats that developed either no mammary neoplasms, or only mammary adenocarcinomas, or only mammary fibroadenomas, or both mammary adenocarcinomas and mammary fibroadenomas in response to DMBA, procarbazine, and X-ray, suggested that the development of a mammary adenocarcinoma or the development of a mammary fibroadenoma are independent processes.