Transplacental initiation of liver, lung, neurogenic, and connective tissue tumors by N-nitroso compounds in mice.
Epidemiological studies have implicated nitroso compounds as possible causative agents for human childhood cancers, including those of neurogenic origin. Published evidence from animal models, which is reviewed in this report, indicates that capacity for metabolic activation of nitrosamines is limited in rodent fetuses and that nitrosamines are correspondingly weak transplacental carcinogens. The C3H mouse fetus, however, has both moderate capability for activation of N-nitrosodimethylamine (NDMA) and proven susceptibility to transplacental causation of neurogenic tumors by a nitrosourea. We tested whether NDMA could act as a transplacental carcinogen in the C3H mouse, and whether it or N-nitrosodiethylamine (NDEA) would initiate neurogenic tumors. N-Nitrosoethylurea (NEU) served as positive control. C3H/HeNCr MTV- pregnant mice were treated ip on Gestation Day 16 or 19 with NDMA (0.1 mmol, 7.4 mg/kg, maximum nonfetotoxic dose), NDEA (0.5 mmol, 51 mg/kg), or NEU (0.4 mmol, 47 mg/kg). NDMA had significant transplacental carcinogenic effects, resulting in an increase in percentage female offspring with hepatocellular carcinomas and in average number of liver tumors after treatment on either gestational day, compared with controls. In the males there was a significant increase in numbers of liver tumors and carcinomas following Day 19 exposure. An increase in incidence of histiocytic and undifferentiated sarcomas was also of statistical significance. There was no change in number of pulmonary tumors. One intracranial schwannoma resulted. NDEA had no effect when given on Gestation Day 16, but caused a significant increase in liver and lung tumor numbers in both sexes when treatment was on Day 19. NEU induced the expected high incidence of lung tumors, significantly increased liver tumor incidence in females (Day 19 exposure), and produced schwannomas in 14 and 35% of the offspring after Days 16 or 19 treatment, respectively. The results show that NDMA at even a low dose had significant transplacental carcinogenic effects, including one schwannoma, which was most unlikely to have occurred spontaneously. However, this single neurogenic tumor contrasts with the absence of similar neoplasms in mice exposed transplacentally to NDEA, in view of the generally greater efficiency of ethylating agents as carcinogens for the nervous system in rodents. These data thus neither conclusively support nor refute the hypothesis that nitrosamines may initiate neurogenic tumors in fetuses.