Carcinogenicity of diethylnitrosamine in newborn, infant, and adult mice.

Modifying effects of age, sex, and mouse strain on diethylnitrosamine (DEN) carcinogenesis have been investigated in C57BL/6Jx C3HeB/FeJ F1 (B6C3F1) and C3HeB/FeJxA/J F1 (C3AF1) hybrid mice. Animals each received four IP injections of 1.5 or 3.0 micrograms DEN/g body weight. The first injections were administered on days 1, 15, or 42 of life. Subsequent treatments were delivered at 3-, 6-, and 6-day intervals, respectively. Mice were kept under observation for the remaining life-span. DEN treatment induced tumors in liver, lungs, and forestomach in descending order of frequency. The majority of the induced liver tumors were hepatocellular carcinomas. Animals treated as newborns and infants developed significantly more liver tumors than animals that were treated as young adults. Newborn and infant females developed liver tumors at a later age (B6C3F1) and with a lower incidence (C3AF1) than similarly treated males. The B6C3F1 mice developed more hepatocellular carcinomas and a higher rate of pulmonary metastases than the C3AF1 mice. In contrast, C3AF1 mice developed lung tumors with a higher incidence and multiplicity than B6C3F1 hybrids. Forestomach tumors were observed also with a slightly but significantly higher incidence in C3AF1 mice.

Kinetics of diethylnitrosamine hepatocarcinogenesis in the infant mouse.

Kinetics of hepatocarcinogenesis was evaluated in 15-day-old male C57BL/6J X C3HeB/FeJ F1 mice using a nontoxic carcinogenic dose range of diethylnitrosamine (DEN). The carcinogen was injected i.p. once, and the animals were killed according to the protocol. Two studies were carried out sequentially over a period of 4 years. In the first study, groups of mice were treated with 0.625, 1.25, 2.5, and 5.0 micrograms of DEN per g of body weight, and subgroups of eight mice were killed at 10-week intervals, the first at 10 weeks following carcinogenic treatment. The dose-response relationship, transformation probabilities, and the dose versus time to 50% incidence of the early (basophilic foci) and later appearing focal and nodular hepatocellular lesions were evaluated. In the second study, groups of mice were treated with 0.312, 0.625, 1.25, 2.5, and 5.0 micrograms of DEN per g of body weight, and subgroups of 8 to 20 animals were killed at 10, 16, 20, 24, 30, 34, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, and 110 weeks after carcinogenic treatment. The numbers of induced basophilic foci and hepatocellular carcinomas per number of liver cells at risk (transformation probabilities) were used to evaluate dose-response, time-response, and time-dose kinetics. In both studies, the kinetics of hepatocarcinogenesis was evaluated from data plotted on the double logarithmic scale. Regardless of the dose used, DEN induced four distinct morphological entities: basophilic (glucose-6-phosphatase deficient) foci; hyperplastic nodules; hepatocellular adenomas; and hepatocellular carcinomas in all animals. The first study demonstrated a positive dose-response relationship and constancy (k) of the product of single dose (d) and the time to 50% (t50%) incidence (d . tn50% = k) for each of the four morphological entities. The numerical value of the power of time (n) increased from 2.6 to 2.7, 3.4, and 5.7 for the above four lesions, respectively. The second study showed first-order kinetics regarding the induction of basophilic foci and hepatocellular carcinomas. The transformation probability of development of basophilic foci was up to three orders of magnitude greater than that observed for development of hepatocellular carcinomas, suggesting a qualitative difference between these two types of hits. The time-response kinetics showed that the development of basophilic foci and carcinomas was related to the time factor by powers of 2 and 4 for these two lesions, respectively. The time-dose relationship to a fixed number of lesions per number of liver cells at carcinogenic risk showed a negative slope with an n value of 2 for the induction of basophilic foci (d . t28/10(7) = k) and an n value of 4 for the induction of hepatocellular carcinomas (d . t40.08/10(7) = k). The data indicated that at least two critical events are needed for the induction of basophilic foci and at least four events are required for the induction of hepatocellular carcinomas...

Neoplastic response of mouse tissues during perinatal age periods and its significance in chemical carcinogenesis.

A series of studies pertaining to perinatal carcinogenesis have been reviewed. Their main objective was development and definition of a sensitivity biologic model for carcinogenicity screening. Data were summarized on factors modifying the carcinogenic response of various tissues following transplacental, neonatal-infant, or adult exposure of (B57BL/6J X C3HeB/FeJ)F1 mice to a single administration of ENU. In addition, tumor response of mice treated during specific perinatal age periods with DEN, BP, aflatoxin B1, benzidine . 2HCl, DDT, dieldrin, and safrole were analyzed. The results revealed that the age of the animals at the time of carcinogenic exposure has been the most effective modulator of carcinogenesis in liver, lung, stomach, ovary, and lymphoreticular tissues. Infancy proved to be the most susceptible period to carcinogenesis as demonstrated by a great variety of tissues that responded to treatment and the incidence of tumors which developed. Depending on the nature of carcinogen, variation in organ sites undergoing carcinogenesis was considerable, apparently due to difference in their enzymatic competence to activate and metabolize the agent. Thus a single treatment with ENU, a spontaneously activated type of procarcinogen, induced 59 primary types of tumors in 22 tissues. In contrast, treatment of infants by procarcinogens requiring enzymatic activation led to development of tumors only at a limited number of tissue sites. However, regardless of the type of carcinogen used, the liver consistently responded with development of tumors. Detailed morphologic and biologic evaluations of the induced liver tumors demonstrated in addition to the benign neoplastic variety, the presence of the frank malignant tumors. The character of tumors was dependent not only on carcinogenicity of the agent used but also on the age of mice at the time of carcinogenic treatment. Perinatally induced primary liver tumors showed greater tendency to metastasize and were more readily transplantable into an isogeneic host than those induced at later age periods. Data showed the advantage of prenatal and/or postnatal treatment in combination of life-long exposures to test agents as a more sensitive bioassay system in comparison with solely postweaning treatment. Because the early age period is the most sensitivity life phase to carcinogenesis, it appears to be a good model for prescreening various potential carcinogens, especially when only small amounts of test substances are available. The importance of the proper relationship of such bioassay to the other test systems regarding assessment of potential human risk has been emphasized.

Morphology and metastatic nature of induced hepatic nodular lesions in C57BL x C3H F1 mice.

The metastatic capabilities of well-defined nodular hepatic lesions induced by benzo(a)pyrene, ethylnitrosourea, benzidine.2HCl, and diethylnitrosamine were evaluated. Coded liver and lung tissues from 1264 treated C57BL/6J x C3HeB/FeJ F1 mice were assessed independently for the presence of primary nodular lesions and metastases, respectively. Primary lesions were classified according to their size, cell morphology, and growth patterns into hyperplastic, adenomatous, and trabecular nodules. None of the 126 mice bearing hyperplastic nodules had pulmonary metastases. Four of 291 (1.4%) mice with adenomatous nodular lesions showed metastases. In contrast, of the 733 mice bearing the trabecular type of nodular lesions alone or in combination with other lesions 266 (36%) showed pulmonary metastases. The pulmonary metastases were first detected in mice dying between 51 and 60 weeks of age (5%). This rate increased as a function of age at death, reaching an incidence of 51% in mice surviving more than 81 weeks. It was concluded that nodules showing trabecular and the more anaplastic solid sheet type of growths represented bona fide hepatocellular carcinomas in the mouse.

Prenatal multicarcinogenesis by ethylnitrosourea in mice.

The Role of prenatal age, sex, and the maternal background upon the incidence, multiplicity, and spectrum of tumors induced by ethylnitrosourea has been studied in the offspring of reciprocal hybrids of the same genotype. The first generation (F1) offspring of C57BL/6J X C3HeB/FeJ and the reciprocal hybrids were observed throughout their life-span for tumor development following single i.p. injections of ethylnitrosourea (60 microng/g) given to pregnant mothers at 12, 14, 16, or 18 days of gestation. Animals exposed to ethylnitrosourea survived on the average for only 90 weeks. They developed by that age tumors in lungs, livers, ovaries, nervous system, and forestomach. Control mice killed at 90 weeks were essentially free of tumors. The fetal age at the time of administration of the carcinogen was one of the most significant modulators of tumor development in lung, liver, ovaries, and nervous system. The sex of the animals influenced the rate of development of liver tumors, whereas maternal background affected the multiplicity of lung tumors.

Conditions modifying development of tumors in mice at various sites by benzo(a)pyrene.

The modifying roles of age, sex, and strain of mice on the incidence, multiplicity, and spectrum of tumors induced by benzo(a)pyrene have been investigated. The first-generation (F1) hybrids of C57BL/6J X C3HeB/FeJ and C3HEB/Fej X A/J mice of both sexes were given single i.p. injections (75 or 150 mug/g) of benzo(a)pyrene at 1, 15, or 42 days of age. Experimental animals were allowed to live their life-spans, while animals in control groups were killed at 52, 90, 142, or 170 weeks of age. Animals treated with benzo(a)pyrene died, in general, by the 100th week of age due to development of liver, lung, stomach and lymphoreticular tumors. Few of the control animals died during that same observational period. The age of mice at the time of exposure to the carcinogen modified development of tumors at all the sites. The sex of animals influenced the development of liver and lymphoreticular tumors. The C3HeB/FeJ X A/J F1 hybrids developed lung tumors more readily than did the C57BL/6J X C3HeB/FeJ F1 mice, which had significantly more liver tumors and neoplasms of the lymphoreticular system than the former strain. No strain difference was observed in regard to tumors at other sites. Higher doses of benzo(a)pyrene were more effective in inducing lung, liver, and stomach tumors. In addition, 5 cases of pancreatic ductal adenoma and adenocarcinoma were observed in carcinogen-treated mice.

Factors modulating benzidine carcinogenicity bioassay.

An integrated series of studies was presented in which several factors were assessed as to their capability to influence the outcome of carcinogenicity of benzidine dihydrochloride in mice. In all studies C57BL/6J X C3HeB/FeJ F1 mice of both sexes were utilized. Animals were either 6 or 1 week of age at the beginning of carcinogenic treatment. Six-week-old mice were exposed to p.o. administration of carcinogen delivered either in food (50 or 100 ppm daily) or by stomach intubation at equivalent dose levels at twice-weekly intervals. In addition, a 150-ppm dose level in food was administered for 39, 54, or 84 weeks. A limited 3-week, daily intubation of benzidine (30 or 100 mug/mouse) was also explored in 1- and 6-week-old mice. Animals were killed in all studies at 90 weeks of age, at which time their tumor incidence was evaluated. Depending upon experimental conditions, benzidine treatment effected development of liver tumors, lung adenomas. Harderian gland cystadenomas, and lymphoreticular neoplasms. Continuous feeding of adult mice for 84 weeks at three dose levels of benzidine resulted in development of liver tumors with a positive dose-response relationship in both sexes. The analysis of data revealed a greater susceptibility of females than of males (94% versus 44% at 150 ppm). Twice-weekly administration of benzidine by stomach intubation was shown to be less hepatocarcinogenic than continuous feeding of equivalent amounts. In the series in which male mice were fed food containing 150 ppm of benzidine for only 34 or 54 weeks, in contrast to the above 84-week schedule, a negative relationship was observed between the incidence of liver tumors and the duration of treatment. Daily administration of 30 mug of benzidine to infants by stomach intubation for a 3-week period significantly enhanced development of liver tumors only in males (66%). Introduction of 150 ppm of benzidine into food offered to mother and offspring from delivery to weaning led to development of liver tumors in 95% of male mice and in 5% of females. No liver tumors developed following similar 3-week treatment of 6-week-old adults.

Factors influencing augmentation and/or acceleration of lymphoreticular tumors in mice by benzo(a)pyrene treatment.

The response of lymphoreticular tissues to a single i.p. injection of benzo(a)pyrene was studied in the first generation of C57BL/6J X C3HeB/FeJ F1 and C3HeB/FeJ X A/J F1 mice. Groups of 1-, 15-, and 42-day-old animals of both sexes received 75 or 150 mug of the carcinogen per g body weight. After a period of approximately 90 weeks, a high incidence (up to 43%) of reticulum cell sarcomas was observed in C57BL/6J X C3HeB/FeJ F1 mice treated with benzo(a)pyrene at 40 days of age. Animals treated with carcinogen at younger ages had a lower incidence of reticulum cell sarcomas. These sarcomas showed marked cellular pleomorphism and were classified into histiocytic, epitheloid-nodular, reticulocytic, and fibrocytic forms according to the predominant cell type. Lymphomas of thymic and extrathymic lymphoid origin and leukemia of granulocytic type were seen in a descending order of frequency. Control animals of either strain that were killed at 90 weeks of age were basically free of lymphoreticular tumors, while those kept under observation up to 170 weeks developed these tumors in 24% (C57BL X C3H F1) and 10% (C3H X A/J F1), respectively. Studies revealed that the augmentation and/or acceleration of development of the lymphoreticular neoplasms and specifically reticulum cell sarcomas by benzo(a)pyrene was dependent upon the strain and sex of mice used and the age at which the animals were exposed to carcinogen.