MiR-34c promotes hepatic stellate cell activation and Liver Fibrogenesis by suppressing ACSL1 expression.

Normally, there are multiple microRNAs involved in the pathogenesis of liver fibrosis. In our work, we aimed at identifying the role of miR-34c in the hepatic stellate cell (HSC) activation and liver fibrosis and its potential mechanism. Our results have shown that during natural activation of HSC, the level of miR-34c was increased significantly whereas acyl-CoA synthetase long-chain family member-1(ACSL1), which is a key enzyme can affect fatty acid(FA) synthesis, was decreased. A double fluorescence reporter assay further confirmed that ACSL1 is a direct target gene of miR-34c. Moreover, the inhibition of miR-34C can attenuate the synthesis of collagen in HSC-T6. In our rescue assay, ACSL1 expression was 1.49-fold higher compared to normal control cells which were transfected with the miR-34c inhibitor in a stable low expression ACSL1 cell line. While at the same time, α-SMA and Col1α expression decreased by 18.22% and 2.58%, respectively. Moreover, we performed an in vivo model using dimethylnitrosamine (DMN) in conjunction with the miR-34c agomir, combined with the treatment of DMN and the miR-34c agomir can increase liver fibrosis. Meanwhile, the degree of hepatic fibrosis was increased and lipid droplets reduced dramatically in rats and HSC-T6 cell treated with miR-34c mimics alone compared to untreated groups. Our results indicate that miR-34c plays an essential role in liver fibrosis by targeting ACSL1 closely associated with lipid droplets, and it might be used as a potential therapeutic target.

C3H/He Mice as an Incompatible Cholangiocarcinoma Model by Clonorchis sinensis, Dicyclanil and N-Nitrosodimethylamine.

Clonorchis sinensis is a Group-I bio-carcinogen, associated with cholangiocarcinoma (CCA). The hamster is the only experimental model of C. sinensis-mediated CCA, but we oblige another animal model. The present study intended to develop a C. sinensis (Cs) mediated CCA model using C3H/He mice, co-stimulated with N-nitrosodimethyl-amine (NDMA) and dicyclanil (DC). The mice were divided into 8 groups with different combinations of Cs, NDMA, and DC. Six months later the mice were sacrificed and subjected to gross and histopathological examination. The body weights were significantly reduced among the groups treated with 2 or more agents (eg. Cs+NDMA, Cs+DC, NDMA+DC, and Cs+NDMA+DC). In contrast, liver weight percentages to body weight were increased in above groups by 4.1% to 4.7%. A Change of the spleen weight was observed only in Cs+NDMA group. Though C. sinensis infection is evident from hyperplastic changes, only 1 worm was recovered. T wo mice, 1 from Cs and the other from Cs+DC group, showed mass forming lesions; 1 (281.2 mm(3)) from the Cs group was a hepatocellular adenoma and the other (280.6 mm(3)) from the Cs+DC group was a cystic mass (peliosis). Higher prevalence of gray-white nodules was observed in Cs group (42.9%) followed by Cs+NDMA+DC group (21.4%). The mice of the Cs+NDMA+DC group showed hyper-proliferation of the bile duct with fibrotic changes. No characteristic change for CCA was recognized in any of the groups. In conclusion, C3H/He mice produce no CCA but extensive fibrosis when they are challenged by Cs, NDMA, and DC together.

Dietary N-nitroso compounds and risk of colorectal cancer: a case-control study in Newfoundland and Labrador and Ontario, Canada.

Several N-nitroso compounds (NOC) have been shown to be carcinogenic in a variety of laboratory animals, but evidence of their carcinogenicity in humans is lacking. We aimed to examine the association between NOC intake and colorectal cancer (CRC) risk and possible effect modification by vitamins C and E and protein in a large case-control study carried out in Newfoundland and Labrador and Ontario, Canada. A total of 1760 case patients with pathologically confirmed adenocarcinoma and 2481 population controls were asked to complete a self-administered FFQ to evaluate their dietary intakes 1 year before diagnosis (for cases) or interview (for controls). Adjusted OR and 95 % CI were calculated across the quintiles of NOC (measured by N-nitrosodimethylamine (NDMA)) intake and relevant food items using unconditional logistic regression. NDMA intake was found to be associated with a higher risk of CRC (highest v. lowest quintiles: OR 1·42, 95 % CI 1·03, 1·96; P for trend = 0·005), specifically for rectal carcinoma (OR 1·61, 95 % CI 1·11, 2·35; P for trend = 0·01). CRC risk also increased with the consumption of NDMA-containing meats when the highest tertile was compared with the lowest tertile (OR 1·47, 95 % CI 1·03, 2·10; P for trend = 0·20). There was evidence of effect modification between dietary vitamin E and NDMA. Individuals with high NDMA and low vitamin E intakes had a significantly increased risk than those with both low NDMA and low vitamin E intakes (OR 3·01, 95 % CI 1·43, 6·51; P for interaction = 0·017). The present results support the hypothesis that NOC intake may be positively associated with CRC risk in humans. Vitamin E, which inhibits nitrosation, could modify the effect of NDMA on CRC risk.

Exploring the uncertainties in cancer risk assessment using the integrated probabilistic risk assessment (IPRA) approach.

Current methods for cancer risk assessment result in single values, without any quantitative information on the uncertainties in these values. Therefore, single risk values could easily be overinterpreted. In this study, we discuss a full probabilistic cancer risk assessment approach in which all the generally recognized uncertainties in both exposure and hazard assessment are quantitatively characterized and probabilistically evaluated, resulting in a confidence interval for the final risk estimate. The methodology is applied to three example chemicals (aflatoxin, N-nitrosodimethylamine, and methyleugenol). These examples illustrate that the uncertainty in a cancer risk estimate may be huge, making single value estimates of cancer risk meaningless. Further, a risk based on linear extrapolation tends to be lower than the upper 95% confidence limit of a probabilistic risk estimate, and in that sense it is not conservative. Our conceptual analysis showed that there are two possible basic approaches for cancer risk assessment, depending on the interpretation of the dose-incidence data measured in animals. However, it remains unclear which of the two interpretations is the more adequate one, adding an additional uncertainty to the already huge confidence intervals for cancer risk estimates.

Indirect effect of a turmeric diet: enhanced bile duct proliferation in Syrian hamsters with a combination of partial obstruction by Opisthorchis viverrini infection and inflammation by N-nitrosodimethylamine administration.

The present study revealed the indirect effect of a turmeric (TUR) diet on the histopathological changes and proliferating cell nuclear antigen staining in Syrian hamsters with partial obstruction by liver fluke (Opisthorchis viverrini) infection and inflammation by N-nitrosodimethylamine (NDMA) administration. The result of the analysis of histopathological changes shows that a TUR diet has an anti-inflammatory property in the case of a single condition of NDMA administration or O. viverrini infection, as has been reported previously. Unfortunately, an adverse indirect effect of TUR was observed in the combination of infection with O. viverrini and administration of NDMA, with a 30-50% increase in new bile duct formation, correlated with an increase in proliferating cell nuclear antigen. Our present result suggests that the properties of curcumin are anti-inflammation and antioxidant including enhancing biliary contraction and bile flow. Thus, a combination of factors (treated with O. viverrini, NDMA, and TUR diet) result in an increasing bile duct proliferation which may cause from biliary homeostasis.

Relationship between the proliferative capability of hepatocytes and the intrahepatic expression of hepatocyte growth factor and c-Met in the course of cirrhosis development in rats.

Liver cirrhosis is the fatal end stage of various chronic liver diseases. One of the most important causes of liver cirrhosis appears to be an impaired proliferative capability of hepatocytes caused by continuous hepatic damage. Hepatocyte growth factor (HGF) and its specific receptor, c-Met, play a pivotal role in hepatocyte proliferation. However, the relationship between the proliferative capability of hepatocytes and the intrahepatic expression of HGF and c-Met during the course of cirrhosis development has not been studied fully. In the present study, liver cirrhosis was produced in rats by intraperitoneally administering dimethylnitrosamine (DMN) and intrahepatic expression levels of HGF and c-Met were quantitatively estimated using a real-time reverse transcription-polymerase chain reaction (RT-PCR) method. Histological examination of liver sections and biochemical estimation of serum levels of transaminase revealed that the degree of hepatocyte destruction was elevated gradually during cirrhosis development in the DMN-induced rat cirrhosis model. The proliferative capability of hepatocytes estimated immunohistochemically by proliferating cell nuclear antigen staining was markedly increased at an early stage of cirrhosis development. However, it was gradually decreased thereafter and suppressed substantially at the time of cirrhosis manifestation. Intrahepatic HGF expression was also increased significantly during the course of cirrhosis development but decreased significantly at the time of cirrhosis manifestation. Conversely, there was a tendency for the intrahepatic expression of c-Met to decrease from an early stage of cirrhosis development and intrahepatic c-Met expression was decreased significantly at the time of cirrhosis manifestation. These results suggest that the highly proliferative capability of hepatocytes at an early stage of cirrhosis development is induced by increased intrahepatic HGF expression. However, both HGF and c-Met expression levels in the liver are decreased significantly there-after. Accordingly, the proliferative capability of hepatocytes is severely impaired and extracellular matrix components are deposited to retrieve space lost by the destruction of hepatic parenchyma, resulting in the establishment of liver cirrhosis.

Production of kidney tumors in rats with low dose of dimethylnitrosamine after partial hepatectomy.

The occurrence of kidney tumors in inbred Wistar rats as a result of dimethylnitrosamine (DMN) administration at different intervals after partial hepatectomy was studied. The schedule for DMN administration was determined on the basis of the levels of liver dimethylnitrosamine demethylase (DMN-d) at different intervals after the operation. DMN-d was 62% of the control values at 24 hours. 72% at 72 hours, and 96% at 92 hours after the operation. At these intervals a single dose of DMN (10 mg/kg body wt) was given to partially hepatectomized animals and to untreated controls. At termination, when the animals were 94 weeks old, no kidney tumors were found in the control animals, whereas 23 of 54 animals (43%) that had been given injections of DMN 24 hours after partial hepatectomy developed kidney tumors. Kidney tumor incidence was 28 or 12%, respectively, when the carcinogen was administered 72 of 92 hours after the operation. The kidney tumor incidence and the activity of liver DMN-d were inversely related.

Lung tumorigenesis in mice after chronic exposure in early life to a low dose of dimethylnitrosamine.

Strain A female mice were exposed to 10 ppb dimethylnitrosamine (DMN) in their drinking water for 4 weeks before mating. Treatment was continued through pregnancy and lactation and after weaning until the progeny were 22 weeks old. The incidence of primary lung tumors among the treated progeny (23%) was significantly higher (P less than 0.021) than that among controls (8%). The effect of the DMN was greatest among the males: 32% had lung tumors, compared with 4% of the control males (P less than 0.016). The DMN-exposed females also had a higher lung tumor incidence than did the controls, but the difference was not of statistical significance. These results demonstrate carcinogenicity of DMN at a dose approaching amounts possibly encountered by the human population as a result of environmental exposure.

Statistical aspects of extrapolation of dichotomous dose-response data.

A mathematical model based on the hypothesis that carcinogenesis is a multistage process was proposed for the statistical problem of extrapolating the results of bioassay experiments performed at high dose levels of the carcinogenic risk at lower dose levels. This general multistage model had extrapolation characteristic similar to those of the more specific models that are commonly used; a graphic technique based on statistical likelihood procedures provided a measure of the uncertainty in this extrapolation problem. The examples used were 1) exposure to DES and mammary carcinoma and 2) exposure to DMN and liver carcinoma.

Kupffer cell sarcoma in rats after exposure to small doses of dimethylnitrosamine and N-2-acetylaminofluorene during hepatic regeneration.

During experiments to study the evolution of hepatocellular carcinoma in adult male Wistar rats by exposing regenerating livers to the action of small doses of dimethylnitrosamine (DMN) or N-2-acetylaminofluorene (AAF), several primary sarcomas of the liver were incidentally observed. The morphology and behavior of the tumors suggest their origin from Kupffer's cells. Kupffer cell sarcomas occurred more frequently when 70% hepatectomy was used as the regenerative stimulus. None of the 36 animals treated with AAF alone and 2 of the 38 rats given DMN only had this tumor.

A reliable, efficient, microinjection apparatus and methodology for the in vivo exposure of rainbow trout and salmon embryos to chemical carcinogens.

A modular apparatus and technique for the injection of salmonid fish embryos with chemical carcinogens are described. A key feature of the methodology is the relative ease of routine through-the-eggshell injection, into the yolk sac of living salmonid fish embryos, inside the "eyed-stage" egg. The procedure is sufficiently rapid that 2 persons working as a team can give injections to 200 embryos per hour. The injection per se induces low mortality, i.e., optimal net survival rates (controls given an injection of dimethyl sulfoxide vs. those not given an injection) in the range of 70-90%. Because only small amounts of chemical are handled in relatively dilute form, the exposure method poses low risks to both the experimentalist and the environment. Preliminary results in a test of 4 carcinogens that differed widely in their structures and requirements for metabolic activation indicated that hepatocellular neoplasms were induced in rainbow trout (Salmo gairdneri) in response to 100 ng aflatoxin B1 (CAS: 1162-65-8)/egg, 1 microgram N-methyl-N'-nitro-N-nitrosoguanidine (CAS: 70-25-7)/egg, and 10 micrograms benzo[a]pyrene (CAS: 50-32-8)/egg. Nine months after exposure, liver neoplasms were observed in 25, 21, and 9%, respectively, of the rainbow trout, but no neoplasms were observed in rainbow trout exposed to 100 micrograms dimethylnitrosamine (CAS: 62-75-9)/egg. Liver neoplasms were also induced in 17% of coho salmon (Oncorhynchus kisutch) given an injection as embryos of 90 ng aflatoxin B1/egg or 5 micrograms N-methyl-N'-nitro-N-nitrosoguanidine/egg.

Morphologic character of transforming renal cell cultures derived from Wistar rats given dimethylnitrosamine.

The morphologic character of kidney cells during serial subculture following isolation from Wistar rats treated several hours to 1 week previously with a carcinogenic dose of dimethylnitrosamine (DMN; 60 mg/kg body weight following protein deprivation) was compared with the appearance of cultures derived from normal control rats. Apart from early signs of cell toxicity, cultures from DMN-treated rats appeared similar to those from untreated rats for the first four passages. Control cells underwent senescence usually by subculture 4, whereas the test cultures survived to express morphologic transformation (usually at subculture 5) as dense macroscopic colonies of piled up cells. In 18 of the 20 test cultures, the cell populations that persisted in continuous culture following expression of morphologic transformation were exclusively mesenchymal, closely resembling DMN-induced renal mesenchymal tumor cells in continuous culture. In the remaining 2 test cultures from DMN-treated rats, a persisting population of abnormal epithelium was present in addition to morphologically transformed mesenchymal cells. The occurrence of populations of altered mesenchymal and epithelial cells characterized by prolonged survival in vitro following isolation from rats shortly after treatment with a carcinogenic dose of DMN was believed to be related to the long-term induction in the rat kidney of a high incidence of mesenchymal tumors and a lower incidence of cortical epithelial tumors by the same dose schedule.

Intake of volatile nitrosamines from consumption of alcohols.

Volatile nitrosamines were determined in alcoholic drinks during epidemiologic studies on the relationship between esophageal cancer incidence and alcohol consumption in Normandy, France. Nitrosodimethylamine (NDMA) was found commonly in most alcoholic drinks tested, with the exception of wine. The average level, about 2 micrograms/liter in beers, was higher than that for other drinks; the range was 0.2--8.6 micrograms/liter. Traces of nitrosodiethylamine (NDEA) were also detected in spirits and ciders. No significant increases in levels were found after nitrosation. Calculation of daily intake in the study region showed that the main intake of volatile nitrosamine is from NDMA in beer. The intake of NDEA through consumption of cider is about one-third that of NDMA from all sources.

Selective induction of intestinal tumors in rats by methyl(acetoxymethyl)nitrosamine, an ester of the presumed reactive metabolite of dimethylnitrosamine.

Methyl(acetoxymethyl)nitrosamine (DMN-OAc) was synthesized and tested for toxicity and carcinogenicity in rats to test the hypothesis that alpha-hydroxylation is required for metabolic activation of dimethylnitrosamine (DMN) to a reactive, proximate carcinogen. The acute median lethal doses (LD50) of DMN-OAc and DMN injected ip into 5-week-old male Sprague-Dawley (Charles River (CD) rats were determined to be 0.19 and 0.59 mmole/kg body weight or 25 mg DMN-OAc/kg and 44 mg DMN/kg body weight, respectively. Single ip injections of one-half the LD50 DMN-OAc (13 mg/kg body weight) in 5-week-old rats of both sexes resulted in a high incidence of epithelial tumors of the intestinal tract. Mean survival times for rats with intestinal tumors were 353 days for males and 433 days for females. Tumors were rarely found at other sites. DMN at equivalent toxic (one-half the LD50, 22 mg/kg) and molar (= one-sixth LD50, 7.0 mg/kg) dose levels, yielded (as expected) tumors of kidneys, lungs, and occasionally other organs, but at a much lower incidence. The finding of the potent carcinogenicity of DMN-OAc supported the postulate that alpha-hydroxylation of DMN in vivo generates a proximate carcinogen.

Alkylation of rat liver DNA by dimethylnitrosamine: effect of dosage on O6-methylguanine levels.

Alkylation of liver DNA was studied following administration to Sprague-Dawley rats of doses of dimethylnitrosamine (DMN) varying from 0.25 to 20 mg/kg body weight. Measurements were made of the amounts of O6-methylguanine and 7-methylguanine present in liver DNA at 4 and 24 hours after treatment with the carcinogen. There was a linear relationship between 7-methylguanine levels and dose of the nitrosamine at both of these times. In contrast, the corresponding levels of O6-methylguanine were not directly proportional to dosage but were less than expected, particularly at low doses below 2.5 mg/kg. This discrepancy was significant at 4 hours, but was even more marked at 24 hours. Only doses above 4 mg/kg at the 4-hour time point gave rise to a 0.11 ratio of alkylation of guanine at the O6-position to that at the 7-position. This ratio was that expected for the initial interaction of the alkylating species derived from DMN with DNA. Evidence was obtained to support the hypothesis that these results were due to an enzymatic removal of O6-methylguanine from liver DNA, which occurred much more efficiently at lower initial levels of alkylation. Repeated daily injections of DMN up to 11 days alos gave rise to O6-methylguanine levels that were not proportional to dosage but were relatively greater at higher dose levels. The significance of these findings in the induction of liver cancer by feeding or repeated injection of DMN was explored.

Carcinogenicity and acute toxicity of dimethylnitrosamine in rainbow trout (Salmo gairdneri).

Four-week-old rainbow trout (Salmo gairdneri) were fed diets containing 0, 3, 50, 200, 400, and 800 ppm dimethylnitrosamine (DMN) for 52 weeks. At the end of 52 weeks, the fish were fed a control diet without DMN for an additional 26 weeks. Samples were taken at 26, 52, and 78 weeks to determine tumor incidence. A dose-related carcinogenic response was established from these results, and an equation was derived to relate the level of the carcinogen to the hepatocellular carcinoma incidence. From a published dose-response study that used outbred Porton rats, a second equation was derived for comparison. Rats and trout were approximately equal in their sensitivity to DMN carcinogenesis. The median lethal dose after ip injection of DMN was 1,770 mg/kg body weight in rainbow trout. Relative to the range of 15-50 mg/kg body weight reported for several mammalian species, trout were resistant to the acute toxicity of DMN.

Pharmacokinetics of N-nitrosodimethylamine in beagles.

The pharmacokinetics of N-nitrosodimethylamine (NDMA) has been studied in beagles. Four male beagles were given 0.5- and 1.0-mg/kg doses of NDMA i.v. and 1.0- and 5.0-mg/kg doses p.o., and at appropriate times after dosing blood samples were drawn and the concentration of NDMA was measured. The experiments were separated by at least 1 week. Following a bolus i.v. dose, the concentration of NDMA in blood declined biphasically with a mean distribution half-life of 19 min and a mean elimination half-life of 73 min. The areas under the blood concentration versus time curves were proportional to the dose indicating that the pharmacokinetics in this dose range were first order. The mean systemic clearance was 43.3 ml/min/kg, the volume of distribution at steady state was 1.9 liters/kg and the mean residence time was 45 min. The clearance of NDMA in the dog was entirely metabolic because no NDMA could be detected in urine after i.v. dosing. The areas under the curve and maximum concentration in blood after the two p.o. doses were not proportional to dose. The evidence suggests that the pharmacokinetics of the 1.0-mg/kg dose were first order, but at the 5.0-mg/kg dose the metabolism of NDMA was saturated. The bioavailability of the lower p.o. dose (i.e., the fraction of the dose that reached the systemic circulation) averaged 93%. The high bioavailability was unexpected since, in the rat, the bioavailability of NDMA is only about 10%, and the systemic clearance in the dog exceeds hepatic blood flow. These data suggest that a substantial fraction of the systemic clearance is extrahepatic and that the pharmacokinetics of NDMA in higher species may be quite different from that observed in rodents.

Metabolic denitrosation of N-nitrosodimethylamine in vivo in the rat.

Enzymatic denitrosation is a potentially inactivating metabolic route that has been shown to convert carcinogenic N-nitrosodimethylamine (NDMA) to methylamine (MA) in vitro. To investigate its quantitative course in vivo, groups of 8-week-old male Fischer rats have been given small (8-15 mumol/kg) p.o. or i.v. bolus doses of 14C-labeled NDMA and the subsequent formation of radioactive MA has been monitored by high performance liquid chromatographic analysis of serially collected blood samples from each individual. Adjusting the [14C]MA fluxes observed for the previously measured rates at which MA is itself eliminated from the system after intragastric administration, denitrosation was calculated to represent a rather uniform 21.3 +/- 1.3% (SE) of total NDMA elimination in the four animals studied. By contrast, repetition of the experiment with fully deuterated NDMA (NDMA-d6) revealed a significantly wider variance in the results (39.8 +/- 8.9%). An alternative calculation using values for elimination of i.v. doses of MA and its trideuteromethyl analogue gave an even larger difference for MA formation between NDMA and NDMA-d6, the estimated extents of in vivo denitrosation in this case being 14.5 +/- 0.9% and 48.3 +/- 10.8%, respectively. The results indicate that denitrosation is a major metabolic pathway for NDMA elimination and suggest that deuteration of the carcinogen induces a shift in its metabolism toward increasing denitrosation at the expense of the competing activation pathway. Consequently, denitrosation may be the previously undefined in vivo metabolic route, the existence of which was suggested by the findings that deuteration of NDMA lowered its hepatocarcinogenicity and liver DNA alkylating ability in rats.

Enhanced repair of O6-methylguanine DNA adducts in the liver of transgenic mice expressing the ada gene.

The capacity to repair O6-methylguanine-DNA adducts was measured in the liver of transgenic mice expressing a chimeric gene consisting of the inducible P-enolpyruvate carboxykinase (GTP) promoter linked to the bacterial O6-alkylguanine-DNA alkyltransferase (ada) gene. Under induced conditions, total hepatic alkyltransferase reached 32.8 +/- 4.2 (SE) fmol/micrograms DNA compared to 7.8 +/- 1.1 fmol/micrograms DNA in nontransgenic mice. Administration of methylnitrosourea or nitrosodimethylamine to both groups of mice produced O6-methylguanine-DNA adducts which resulted in repair-mediated depletion of total hepatic alkyltransferase in a dose-dependent fashion. In nontransgenic mice, depletion of hepatic alkyltransferase occurred at lower doses of carcinogen, and recovery of alkyltransferase activity occurred later than in ada+ transgenic mice. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of residual alkyltransferase activity after methylating agent exposure indicated that the bacterial as well as endogenous mammalian alkyltransferases were functioning as DNA repair proteins in hepatocytes in vivo. Analysis of O6-methylguanine- and N7-methylguanine-DNA adducts in the liver of transgenic and nontransgenic mice after treatment with one dose of 50 mg/kg methylnitrosourea i.p. revealed that transgenic mice repaired in situ O6-methylguanine-DNA adducts approximately 3 times faster than nontransgenic mice, commensurate with the increase in alkyltransferase activity. Thus, ada+ transgenic mice treated with methylnitrosourea have lower levels of persistent mutagenic O6-methylguanine adducts than ada- nontransgenic mice. Hepatic expression of bacterial alkyltransferase appears to protect mice from the DNA-damaging effects of N-nitroso compounds in vivo.

Carcinogenic effects of sequential administration of two nitrosamines in Fischer 344 rats.

The carcinogenic effects of sequential treatment of female F344 rats with two nitrosamines were studied. The animals received either methylethylnitrosamine (NMEA), a strong liver carcinogen, N-nitrosomethylaniline (NMA), a moderately strong esophageal carcinogen, or N-nitrosopyrrolidine, (NPyr), a weaker liver carcinogen. The sequentially treated groups were given NMEA followed by NMA and vice versa, NPyr followed by NMEA and vice versa. The dose and duration for each chemical in the sequentially treated groups were identical for the individual treatments. The animals were allowed to die or were killed when moribund. The animals surviving longer than 110 weeks were sacrificed. The NMEA-NPyr and NPyr-NMEA groups had a tumor spectrum characteristic for NMEA alone (a mixture of hepatic carcinomas and sarcomas with extensive metastases to the lungs). The survival was reduced in the NMEA-NPyr group compared to the NMEA alone group. The time to death of the NMA-NMEA group was not affected by the NMA treatment, but many of the animals had esophageal neoplasms. The NMEA-NMA group survival was reduced when compared to the NMEA alone group but the tumor spectrum was dominated by NMEA. The data indicate that when the target organ is the same, the effect of two nitrosamines is additive with the stronger carcinogen dominating the tumor spectrum. When the target organs are different, the initial exposure influences the tumor spectrum, although the treatment with the second nitrosamine enhances the tumorigenicity of the initial nitrosamine.