Role of Age and Other Factors Influencing the Oncogenic Effect of Chemical Carcinogens in Mice

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Abstract
Present experiments have been carried out to elucidate the role of some factors influencing the oncogenic effect of chemical carcinogens in newborn animals. Mice of Swiss, C3Hf/Gs, and C57BL strains were injected at birth, subcutaneously or in the thymus region, with either 20-methylcholanthrene (MC) or 9,10-dymethyl-1,2-benzanthracene (DMBA). The most frequent neoplastic lesions observed in the various experimental groups were: pulmonary adenomas, malignant lymphomas, subcutaneous sarcomas, and hepatomas. The incidence of different tumors varied according to the strain characteristics, route of administration and type of carcinogen. Susceptibility to lung carcinogenesis appeared to be highest for Swiss mice, intermediate for C3Hf/Gs and low for C57BL mice, receiving MC or DMBA, in accordance with the known data oh spontaneous incidence of these tumors in the same strains. DMBA was found more effective as leukemogenic agent than MC, when tested in Swiss and C57BL strains. The incidence of malignant lymphomas obtained by direct injection of either carcinogens in the thymic region was much larger than that observed in mice treated subcutaneously with even higher doses of chemicals. Sarcomas occurred exclusively at the site of injection and only in animals receiving carcinogens by subcutaenous route, MC being more effective than DMBA. As regard to development of hepatomas, high incidence of these tumors was observed only in male mice of C3Hf/Gs strain treated with DMBA. On the basis of present results and of those obtained with other chemical agents, the possible mechanisms underlying the higher susceptibility of newborn versus adult animals to chemical as well as viral carcinogenesis are briefly discussed.