Neoplastic transformation and DNA damage of mouse mammary epithelial cells by N-methyl-N'-nitrosourea in organ culture.

An appropriate in vitro system was used to study the effect of a direct-acting carcinogen on the transformation of mammary epithelial cells in the organ culture of the whole mammary gland in vitro. Studies were done to determine the ability of N-methyl-N'-nitrosourea (MNU) to transform the mammary cells in organ culture. Mouse mammary glands were treated with single or multiple doses of MNU during various periods of the culture. To assay for neoplastic transformation potential of MNU on mammary cells, mammary glands were dissociated and the cells were injected into the parenchyma-free inguinal mammary fat pad of syngeneic virgin female host mice. Palpable tumors were observed in injected glands of 23% of the mice after 3-4 months and an additional 31% showed serially transplantable hyperplastic alveolar nodules (HANs). Histopathologic examination of the tissues showed that the tumors were mammary adenocarcinoma. All tumors and hyperplasias were secondarily transplanted into syngeneic animals, resulting in tumors and hyperplasias of similar histopathology. In addition, DNA damage of the epithelial cells in organ culture caused by MNU was also measurable using the new nick translation assay. The most extensive DNA damage occurred when the glands were treated on day 4 and day 5 of the mammogenic culture period. These results demonstrate that the mouse mammary epithelial cells are susceptible to the carcinogenic action of the direct-acting carcinogen MNU and that the whole mammary gland-culture system offers an appropriate in vitro model for studying the mechanism of carcinogenesis induced by MNU.

Fate of 7,12-dimethylbenz(a)anthracene in the mouse mammary gland during initiation and promotion stages of carcinogenesis in vitro.

Metabolic conversion and distribution of the products of 7,12-dimethylbenz(a)anthracene (DMBA) were analyzed in the mouse mammary cell transformation model in organ culture. In order to determine the levels of uptake of DMBA, the glands were exposed to the transforming dosage of the procarcinogen (7.8 microM, 20 microCi/ml) for 24 h, and the level of uptake was determined to be 8 x 10(4) cpm/mg of tissue. Subsequently, the glands were incubated in DMBA-free medium, and distribution of the radioactivity in DNA and in the acid-insoluble materials was measured. Data showed that, in addition to the 24-h DMBA treatment period, the initiation stage extends for another 3 h when the incubation is continued in DMBA-free medium. A saturation level of uptake of [3H]DMBA into the whole gland was observed at 12 h, while DMBA was continually metabolized with the products being bound to DNA and to the acid-insoluble fractions throughout the entire incubation period with or without DMBA. The three major adducts identified were anti-DMBA-3,4-diol-1,2-epoxide (DMBADE):deoxyguanosine, syn-DMBADE:deoxyadenosine, and anti-DMBADE:deoxyadenosine. Qualitatively the adduct profiles remained similar. However, with the additional 3-h incubation in DMBA-free medium, the three major DMBA-DNA adducts increased slightly by 6.5 to 7.5%. Thus the total 27-h time period can be considered as the duration of the initiation stage in the DMBA-induced carcinogenesis of mouse mammary cells in organ culture. Therefore the subsequent 6-h incubation in DMBA-free medium may be considered as within the promotional stage, and at the same time period the levels of the three DNA adducts decreased significantly by 67.5 to 84.1% (P less than 0.001).