Increased carcinogenic action of dimethylnitrosamine after prior administration of carbon tetrachloride

Evaluation of the carcinogenicity of carbon tetrachloride

Carbon tetrachloride (CCl4) has been extensively used and reported to produce toxicity, most notably involving the liver. Carbon tetrachloride metabolism involves CYP450-mediated bioactivation to trichloromethyl and trichloromethyl peroxy radicals, which are capable of macromolecular interaction with cell components including lipids and proteins. Radical interaction with lipids produces lipid peroxidation which can mediate cellular damage leading to cell death. Chronic exposure with CCl4 a rodent hepatic carcinogen with a mode of action (MOA) exhibits the following key events: 1) metabolic activation; 2) hepatocellular toxicity and cell death; 3) consequent regenerative increased cell proliferation; and 4) hepatocellular proliferative lesions (foci, adenomas, carcinomas). The induction of rodent hepatic tumors is dependent upon the dose (concentration and exposure duration) of CCl4, with tumors only occurring at cytotoxic exposure levels. Adrenal benign pheochromocytomas were also increased in mice at high CCl4 exposures; however, these tumors are not of relevant importance to human cancer risk. Few epidemiology studies that have been performed on CCl4, do not provide credible evidence of enhanced risk of occurrence of liver or adrenal cancers, but these studies have serious flaws limiting their usefulness for risk assessment. This manuscript summarizes the toxicity and carcinogenicity attributed to CCl4, specifically addressing MOA, dose-response, and human relevance.

Predicting Dose-Dependent Carcinogenicity of Chemical Mixtures Using a Novel Hybrid Neural Network Framework and Mathematical Approach

This study addresses the challenge of assessing the carcinogenic potential of hazardous chemical mixtures, such as per- and polyfluorinated substances (PFASs), which are known to contribute significantly to cancer development. Here, we propose a novel framework called HNN_MixCancer that utilizes a hybrid neural network (HNN) integrated into a machine-learning framework. This framework incorporates a mathematical model to simulate chemical mixtures, enabling the creation of classification models for binary (carcinogenic or noncarcinogenic) and multiclass classification (categorical carcinogenicity) and regression (carcinogenic potency). Through extensive experimentation, we demonstrate that our HNN model outperforms other methodologies, including random forest, bootstrap aggregating, adaptive boosting, support vector regressor, gradient boosting, kernel ridge, decision tree with AdaBoost, and KNeighbors, achieving a superior accuracy of 92.7% in binary classification. To address the limited availability of experimental data and enrich the training data, we generate an assumption-based virtual library of chemical mixtures using a known carcinogenic and noncarcinogenic single chemical for all the classification models. Remarkably, in this case, all methods achieve accuracies exceeding 98% for binary classification. In external validation tests, our HNN method achieves the highest accuracy of 80.5%. Furthermore, in multiclass classification, the HNN demonstrates an overall accuracy of 96.3%, outperforming RF, Bagging, and AdaBoost, which achieved 91.4%, 91.7%, and 80.2%, respectively. In regression models, HNN, RF, SVR, GB, KR, DT with AdaBoost, and KN achieved average R² values of 0.96, 0.90, 0.77, 0.94, 0.96, 0.96, and 0.97, respectively, showcasing their effectiveness in predicting the concentration at which a chemical mixture becomes carcinogenic. Our method exhibits exceptional predictive power in prioritizing carcinogenic chemical mixtures, even when relying on assumption-based mixtures. This capability is particularly valuable for toxicology studies that lack experimental data on the carcinogenicity and toxicity of chemical mixtures. To our knowledge, this study introduces the first method for predicting the carcinogenic potential of chemical mixtures. The HNN_MixCancer framework offers a novel alternative for dose-dependent carcinogen prediction. Ongoing efforts involve implementing the HNN method to predict mixture toxicity and expanding the application of HNN_MixCancer to include multiple mixtures such as PFAS mixtures and co-occurring chemicals.

Cocarcinogens and Tumor Promoters and Their Environmental Importance

In recent years a great deal of effort has been spent on uncovering new carcinogens and studying the mode of action of well-known carcinogens such as nitroso compounds, aromatic hydrocarbons and aromatic amines and their occurrence in air, water, foods and other environmental sources. It has been known for about thirty years that some materials, while non-carcinogenic, can enhance greatly the potency of low-levels of known environmental carcinogens. The first such material to be discovered was the tumor promotor, croton oil, derived from the seeds of a tropical plant, Croton tiglium L, indigenous to India and Sri Lanka. Researchers interested in chemical carcinogenesis have examined its unusual behavior for many years. The active principles of croton oil, the phorbol esters, were finally isolated and chemically characterized. Recently these phorbol esters have become widely available. Consequently, their effects have been examined in a wide range of biochemical and biological studies. The phorbol esters are not environmental factors in cancer causation since they occur in a toxic plant which is not used as a source of animal or human food. Other known tumor promoters are much less active, e.g., anthralin (1,8-dihydroxy-9-anthrone) and phenol. Thus, the phorbol esters, although valuable tools in studies on chemical carcinogenesis, are less relevant to environmental health and disease than another group of compounds known as cocarcinogens. Many cocarcinogens are ubiquitous environmental agents and only recently have these compounds been given more attention. It is expected that as more of these agents become known as environmental agents, new advances in cancer prevention will be made.

Rat Liver Foci Bioassay

This article is a review of the rat liver foci bioassay which is an initiation/promotion assay that utilizes preneoplastic foci of hepatocytes possessing altered enzymatic activity and altered cellular constituents as indicators of carcinogenesis. Rat liver tumor promoters that have been employed in the assay include: (1) a low level of 2-acetylaminofluorene in the diet, (2) phenobarbital in the diet and drinking water and (3) a choline deficient diet. Camma-glutamyl transpeptidase-positive, and adenosine triphosphatase, and glucose-6-phosphatase-defi-cient foci are used to indicate the initiation of cancer and are reviewed with respect to their relationship to cancer. Partial hepatectomy at the time of initiation increases the sensitivity of the assay to dialkylnitrosamines and chemicals which are otherwise negative in rat liver. The protocols being developed by the Health Effects Research Laboratory in Cincinnati include: (1) partial hepatectomy either 18 hours prior to or 14 days after administering the chemical under test, (2) phenobarbital promotion and (3) using gamma-glutamyl transpeptidase-positive foci as the indicator of carcinogenesis. The article also reviews the rat liver foci bioassay as a component of the Carcinogenesis Testing Matrix for tier II where it will be employed to confirm the carcinogenic hazard associated with environmental samples containing complex mixtures.

Mechanisms of liver disease: cross-talk between the NF-kappaB and JNK pathways

The liver plays a central role in the transformation and degradation of endogenous and exogenous chemicals, and in the removal of unwanted cells such as damaged, genetically mutated and virus-infected cells. Because of this function, the liver is susceptible to toxicity caused by the products generated during these natural occurrences. Hepatocyte death is the major feature of liver injury. In response to liver injury, specific intracellular processes are initiated to maintain liver integrity. Inflammatory cytokines including tumor necrosis factor (TNF)alpha and interleukin-6 (IL-6) are key mediators of these processes and activate different cellular response such as proliferation, survival and death. TNFalpha induces specific signaling pathways in hepatocytes that lead to activation of either pro-survival mediators or effectors of cell death. Whereas activation of transcription factor NF-kappaB promotes survival, c-Jun N-terminal kinases (JNKs) and caspases are strategic effectors of cell death in the TNFalpha-mediated signaling pathway. This review summarizes recent advances in the mechanisms of TNFalpha-induced hepatotoxicity and suggests that NF-kappaB plays a protective role against JNK-induced hepatocyte death. Identification of the mechanisms regulating interplay between the NF-kappaB and JNK pathways is required in the search for novel targets for the treatment of liver disease, including hepatitis and hepatocellular carcinoma.

Differential effects of acetaminophen pretreatment on hepatic aflatoxin B(1)-DNA binding, cellular proliferation, and aflatoxin B1-induced hepatic foci in rats and hamsters

Effects of acetaminophen (AAP) pretreatment on hepatic aflatoxin B1 (AFB(1))-DNA binding, cellular proliferation, and AFB(1)-induced glutathione S-transferase placental form (GST-P) positive hepatocytes and foci have been examined in young male rats and hamsters. Intraperitoneal (i.p.) dosing of 600mg AAP 3h before AFB(1) i.p. injection showed three-fold more AFB(1)-DNA binding in hamsters and 40% less binding in rats. Cell proliferation analyzed by bromodeoxyuridine incorporation was not significant (0.4-0.6%) 24-96h after AAP (600mg) treatment of rats; however, proliferation was stimulated and was maximum (11%) in hamsters at 72h after AAP treatment. Dosing of rats with AFB(1) alone at 0.5 or 2.5mg level gave an appreciable number of GST-P positive minifoci (two to nine cells) with a few foci larger than 100 microm; pretreatment with AAP (300 or 600mg) 48h before 0.5 or 2.5mg AFB(1) had no effect on the number and focal area of foci. In hamsters, 1 or 2mg AFB(1) alone yielded GST-P positive hepatocytes without any minifoci. Pretreatment with AAP (600mg) 48 or 72h before 1 or 2mg AFB(1) produced increases in both GST-P positive hepatocytes and minifoci. Thus, marked changes are observed after AAP pretreatment in hamsters compared to rats.

Effect of dietary restriction on partial hepatectomy-induced liver regeneration of aged F344 rats

Fourteen weeks-old male F344 rats maintained on a reduced caloric diet (60% of ad libitum (AL) food consumption) for 6 weeks or for 14 months did not affect the hepatic cell proliferation in terms of % S phase population, determined by evaluation of DNA synthesis in hepatocytes isolated from either young (5 months) or aged (18 months) rats. However, hepatic basal cellular DNA synthesis estimated by [3H]thymidine incorporation was reduced through acute dietary restriction (DR) in young rats, but increased in aged animals after 14 months restriction. Partial hepatectomy (PH) on aged rats stimulated hepatocyte regeneration and restored some aging-associated biochemical functions, such as drug metabolizing enzyme-dependent xenobiotic metabolic activation which was determined by measuring the formation of carcinogen-DNA adducts. Forty-eight hours after partial hepatectomy, the % of S phase population and the basal nuclear DNA synthesis of hepatocytes isolated from the partial hepatectomized DR-rats were 4- and 2.8-fold, respectively, greater than those of hepatocytes from AL-animals. DR reduced aflatoxin B1 (AFB1) metabolizing enzyme activity and decreased the AFB1-DNA adduct formation in young rats treated with AFB1. In aged AL-rats, the formation of AFB1-DNA adducts diminished to the same level as that of DR-groups and probably was due to the faster decline of drug metabolizing enzymes in aging AL-rats. However, 48 h after PH, the metabolic activation of AFB1 was restored in AL- and DR-groups which resulted in the increase of AFB1-DNA binding by 4.2 and 1.9-fold, respectively. During the liver regeneration of old PH-rats, DR inhibited the AFB1-DNA adduct formation after the PH-rats received a single dose of AFB1. DR increased benzo[a]pyrene (BaP) metabolic activation in both young and aged rats. Aging also decreased BaP-DNA adduct formation in both DR and AL-rats. The increase of BaP-DNA adduct formation in PH-groups was attributed to the restoration of BaP-metabolizing enzyme activity during liver regeneration. The PH-stimulated BaP-DNA adduct formation in AL- and DR-rats was 3.4- and 2.0-fold greater than control aged rats. Our results indicated that the stimulation of PH-induced liver regeneration by DR in aged animals may be attributed to the retardation of aging by DR and the retention of more active biochemical and enzymological functions in old DR-animals.

Simultaneous induction of liver cirrhosis and hepatocellular carcinomas in F-344 rats: establishment of a short hepatocarcinogenesis model

The relationship between liver cirrhosis and diethylnitrosamine (DEN)-induced hepatocarcinogenesis in male F-344 rats was studied. Hepatic cirrhosis was produced by combined administration of CCl4 (0.5 ml/kg b.w. by gavage, three times a week) and phenobarbital (PB) (0.05% in drinking water, continuously for 6 weeks), while the carcinogenic nitrosamine compound was given either preceding or following CCl4 + PB treatment at a single dose of 200 mg/kg b.w., ip. Liver lesions were evaluated histologically at the end of the 4th month. The established cirrhosis completely prevented the formation of hepatocellular carcinomas (HCCs), however, CCl4 + PB posttreatment resulted in a strong enhancing effect on DEN-hepatocarcinogenesis: 16 weeks after initiation severe cirrhosis and HCCs occurred simultaneously in more than 90% of the animals. Although the explanation of this highly accelerated carcinoma formation is not known at present, the authors hypothesize that the modulation of the drug-metabolizing enzyme system might play a central role in this profoundly altered host response.

Decreased hepatocarcinogenic effect of diethylnitrosamine in experimentally induced liver cirrhosis in rat: delay or inhibition?

The relationship between CCl4 or (CCl4 + phenobarbital)-induced liver cirrhosis and diethylnitrosamine (DEN) hepatocarcinogenesis in male F-344 rats was investigated. DEN given alone produced no liver lesions after 16 weeks, but 4/12 (33%) neoplastic nodules developed when nitrosamine was administered to rats with previously established cirrhosis. On the other hand, (CCl4 + phenobarbital) post-treatment had an even stronger effect, increasing the yield of neoplastic nodules to 100% (28/28). Since the exposure time of DEN was the same in all treated groups (4 months), the results indicate the decreased effectiveness of (CCl4 + phenobarbital) pretreatment on DEN hepatocarcinogenesis.

Effects of carbon tetrachloride, ethanol and acetaldehyde on diethylnitrosamine-induced hepatocarcinogenesis in rats

The modifying action of experimentally induced chronic liver injury on diethylnitrosamine (DEN) hepatocarcinogenesis was investigated using a minimal treatment protocol. A single dose of DEN (15 mg/kg b.w.) was administered as a carcinogen to 1-day-old Sprague-Dawley rats. From 3 weeks of age rats received repeated intraperitoneal injections of carbon tetrachloride (CCl4), or 10% ethanol or 5% acetaldehyde in the drinking water for 9 weeks. Combinations of CCl4 and ethanol or acetaldehyde were also tested. Morphology, immunohistochemistry for glutathione S-transferase-placental form, and incidence and quantity of preneoplastic lesions of the livers were studied. The chronic CCl4 administration produced complete or incomplete liver cirrhosis and exerted a strong promoting effect on the development of neoplastic nodules. Ethanol alone revealed no cirrhogenous or tumor-promoting effect, but enhanced both actions of CCl4. Acetaldehyde increased only the cirrhogenous effect of CCl4.

Modifying influence of prior treatment with toxic agents on induction of preneoplastic and neoplastic lesions in a medium-term multi-organ carcinogenesis bioassay

The modifying potential of prior administration of toxic agents was investigated in our multi-organ carcinogenesis model using male F344/DuCrj rats with the aim of assessing the link between tissue damage and initiation. Animals were administered one of four toxic agents for 8 wk, and then treated with N-diethylnitrosamine (DEN, 100 mg/kg body weight (b.w.), intraperitoneally (i.p.), single injection), N-methylnitrosourea (MNU, 20 mg/kg b.w., i.p., four times during wk 9 and 10), and dihydroxy-di-N-propylnitrosamine (DHPN, 0.1% in drinking water, during wk 11 and 12) for multi-organ carcinogenesis. All surviving rats were killed at the end of wk 36, and the major organs carefully examined for preneoplastic and neoplastic lesion development. Immunohistochemical demonstration of glutathione S-transferase placental form (GST-P) positive foci was also performed to facilitate quantitative assessment of liver lesion development. D-galactosamine (300 mg/kg b.w., i.p., once a week), a hepatotoxin, significantly inhibited the induction of GST-P positive foci, while 4,4'-diaminodiphenylmethane (DDPM, 0.1% in diet), a bile duct proliferator which is itself a hepatocarcinogen, possessed enhancing activity. DDPM, also a goitrogen, clearly inhibited the development of follicular cell tumors in the thyroid. Uracil (3.0% in diet), which is an inducer of papillomatosis in the urinary bladder, did not exert any enhancing potential on bladder carcinogenesis. Bleomycin (2 mg/kg b.w., i.p., twice a week), which is an alveolar epithelium injuring agent, also did not modify the induction of alveolar epithelium proliferative lesions. These results indicate that prior organ injury by toxic agents does not always act to enhance sensitivity to carcinogenesis.

Experimental induction of renal tumors

Renal tumors have been reported to be induced by natural products, chemical carcinogens, viruses, or radiation. Species or strain specificity and sex also appear to play significant roles in their development. In man, it is also likely that the heredity existence of other diseases, smoking, food habits, and irradiation may be etiological factors. It also appears that hormonal, chemical, and other environmental factors can play a role. Nephrotoxin modifies two-stage chemical carcinogenesis in rat kidney. Some nephrotoxins without carcinogenicity promote the development of renal tumors in rats pretreated with subcarcinogenic doses of chemical carcinogens. The importance of nephrotoxin in development of renal adenocarcinomas needs elucidation. Preneoplastic lesions in the kidneys can be recognized by histochemical methods with specific antibodies. It is hoped that further research will be continued, so that data obtained from experimental work will provide a better understanding of the etiology and induction of renal cancer in man.

Endogenous hepatic growth-modulating factors and effects of a choline-devoid diet and of phenobarbital on hepatocarcinogenesis in the rat

The activities of an endogenous inhibitor and of a stimulator of cell proliferation were assayed in the livers of sham-operated (SO) or partially hepatectomized (PH) adult rats; rats fed a choline-supplemented (CS) or a choline-devoid (CD) diet; the same diets followed by acute CCl4 intoxication; the same diets supplemented with phenobarbital (PHB); or a CD diet containing DL-ethionine (ETH). The inhibitor and the stimulator were semipurified by fractional ethanol precipitation of a liver cytosolic fraction, and their activities were assessed by means of bioassays in vitro. The livers of SO rats and of rats fed the CS diet contained only inhibitor activity. Following PH, a CD diet, or CCl4 intoxication the inhibitor activity was suppressed, and there was a simultaneous appearance of a stimulator activity. Thus, PH, a CD diet, and CCl4 intoxication cause similar cellular (loss and regeneration) and humoral-homeostatic changes in adult rat livers. We propose that these changes constitute a basic attribute of the mechanism whereby the three conditions affect similarly hepatocarcinogenesis in the rat, especially in the case of a CD diet, because the changes it induces are chronic rather than acute. PHB, another promoter of chemical hepatocarcinogenesis, affected neither the inhibitor nor the stimulator activity. Thus, PHB seems to be acting by a different mechanism than that of the other three agents. ETH did not modify the shift in the balance of the growth-modulating factors induced by a plain CD diet. This shift may account for the marked stimulation of carcinogen-induced oval cell proliferation exerted by a CD diet. The significance of these results is discussed in the context of known effects of a CD diet and of PHB on hepatocarcinogenesis in rats.

Quantitative correlation with carcinogenic potency of different short term tests

The following short term parameters evaluating essentially genotoxic effects were considered: liver DNA alkaline fragmentation (DFI), morphological transformation in hamster embryo cells (TPI), and mutagenicity in the Ames' test (MPI). The internal consistency of the carcinogenicity data (OPI) was rather high (r approximately or equal to 0.8). The correlation with OPI of DFI and MPI was statistically significant but rather modest, about 0.4-0.5. The best correlation between OPI and TPI was 0.65. This level of correlation was observed only when some kind of dose-response relationship for transformation could be established. When comparisons were attempted for exactly the same 27 compounds for all three tests, a general decrease in predictability was observed. This could be mainly due to problems of representation of the ideal population of chemicals using small samples. The more general problem of the quantitative approach to the predictability of short term tests was also discussed.

Effect of azathioprine and carbon tetrachloride on induction of hyperplastic liver nodule and hepatocellular carcinoma by diethylnitrosamine and N-2-fluorenylacetamide in rats

The enhancing or inhibitory action of the hepatotoxic agents, carbon tetrachloride (CCl4) and azathioprine (AZP), on the evolution of hyperplastic liver nodule (HN) and hepatocellular carcinoma (HCC) in diethylnitrosamine (DEN)- and N-2-fluorenylacetamide (FAA)-treated rats (control group) was tested. The area of gamma-glutamyl transpeptidase(gamma-GTP)-positive HN and/or foci in the eighth week was remarkably small in rats fed on a diet containing FAA and AZP (the AZP group), but was quite large in rats fed a diet containing FAA in addition to repeated CCl4 injections (the CCl4 group). HCC was first detected in the 21st week and the incidence of HCC within the 36 weeks of the experiment was very high in the CCl4 group. However, no tumor, including HCC, was detected in the AZP group during this observation period. No essential differences in the biochemical characteristics of HCC between the control group and the CCl4 group were observed with respect to several enzyme activities. The increased activity of liver aniline hydroxylase observed 12 hr after the administration of FAA, AZP, or DEN decreased when AZP was administered simultaneously with FAA to rats treated with DEN in advance. The mechanisms of the enhancing of inhibitory effect observed are discussed with special reference to the drug-drug interactions.

Promutagen activation in partially hepatectomized mice

Mutagenic activity of aflatoxin B1 (AFB1) was followed in the Ames Salmonella/microsome test using liver S-9 from control and partially hepatectomized (PH) mice. The S-9 preparations from the PH mice were capable of inducing significantly greater AFB1 mutagenic activity to strain TA100 than S-9 preparations from either unoperated control, sham-operated control, or Aroclor 1254-induced mice. Increased activity was observed with S-9 preparations made at both 42 h and 148 h following PH.

International Commission for Protection against Environmental Mutagens and Carcinogens. ICPEMC working paper 2/3: carcinogens and carcinogenesis enhancers

The concept that chemical agents may lead to enhancement of carcinogenesis, rather than to its complete induction, is explored to explain the inexact correlation between carcinogen prescreening tests and the results of whole animal bioassays. It is suggested that carcinogenesis-enhancing agents are non-genotoxic chemicals which are positive in animal carcinogenesis bioassays. The importance of understanding the mechanisms of action of carcinogenesis-enhancing agents is emphasized.

Histopathology of the rat liver following a single autoprotective dose of 3'-methyl-4-dimethylaminoazobenzene

Male inbred Leeds strain rats were given single intragastric doses of 1200 mg/kg of 3'-methyl-4-dimethylaminoazobenzene, autoprotective against liver necrosis and then sacrificed at intervals up to 14 months. About 40 per cent of the animals died, apparently as a result of pancreatic necrosis, between 9 and 13 days after treatment while the remainder survived until they were killed. The liver changes induced included an early intense ductular cell reaction, presistent hepatocellular abnormalities and at later stages, cholangiofibrosis and increasing numbers of parenchymal clear cells. Although no tumours arose as a result of treatment, the observed histopathological changes are similar to those seen during liver carcinogenesis induced by a variety of carcinogens and may be due specifically to the carcinogenic action of the azo dye.

Influence of repeated liver regeneration on hepatic carcinogenesis by diethylnitrosamine in mice

In mice given a single dose of diethylnitrosamine, a hepatonecrotic dose of carbon tetrachloride, 5 weeks after dosing with DEN and repeated 6 times at 4-weekly intervals, augmented the tumour yield in the livers. A single hepatonecrotic dose of CCl4 24 h before a single dose of DEN also increased the number of tumours produced. The effect of the repeated administration of CCl4 after the dose of DEN occurred in addition to, and was therefore independent of, the enhancing effect of a single dose of CCl4 before DEN. These results may be interpreted as implying (1) that the liver in the regenerative phase after a hepatonecrotic dose of CCl4 is more susceptible to an initiating action of DEN, i.e. produces more potential foci of tumour growth than in the normal liver and (2) that the repeated doses of CCl4 leading to repeated phases of regeneration, after the dose of DEN, provide a promoting stimulus.

Repeated partial hepatectomy as a promoting stimulus for carcinogenic response of liver to nitrosamines in rats

Partial hepatectomy 24 h before a single i.p. dose of dimethylnitrosamine, diethylnitrosamine or ethylmethylnitrosamine increased the carcinogenic response in the liver of rats as determined by the number of tumours and the number of "focal proliferations" produced. Secondly, in rats given a single i.p. dose of diethylnitrosamine, 3 partial hepatectomies 5, 10 and 15 weeks after dosing the animals increased the carcinogenic response in the liver. The stimulus of repeated partial hepatectomy therefore appears to act as a "promoting agent" for liver carcinogenesis, that is if the single dose of diethylnitrosamine is regarded as an "initiating agent" in the terms of the two-stage hypothesis.

Growth stimulation following serum transfer from carcinogen-treated donors to normal rats: a new aspect of early carcinogen actions

Especially in their early phase of action carcinogens are strong inhibitors of cell proliferation. This is an apparent contradiction to the promoting activity of oncogens in the process of carcinogenesis. Because of obvious similarities between restorative and neoplastic growth processess on the tissue level we have studied the possibility, whether such similarities do exist also with regard to stimulatory activities in the serum. To overcome the non-specific inhibitory effect of a carcinogen, the serum of 2-acetylaminofluorene (AAF) treated male Sprague-Dawley rats was transferred to normal recipients. The results demonstrate that the proliferation of the same tissues (hepatocytes, adrenocortical cells, thymocytes) as in liver regenerating animals were stimulated by the serum of the carcinogen-treated rats. Whether the observed short-term effect corresponds to the so-called promoting activity of the carcinogen(s) is discussed. Nature and origin of the humoral stimulator(s) are still unknown.

Influence of carbon tetrachloride on induction of tumours of the liver and kidneys in mice by netrosamines

Mice were given a single dose of nitrosolimethylamine, nitrosoliethylamine or nitrosomethylethylamine and the yield of tumours and related lesions in the livers determined 12 months later. A hepatonecrotic dose of CCl4 24 or 48 h before the nitrosamines, increased the yields of hepatocellular tumours and proliferative foci in the livers, whereas when given 60 h before the nitrosamines there was no significant effect.

The methylation of DNA in regenerating rat liver

There was more persistent methylation of liver DNA from rats given [14C]DMN if it was given during liver regeneration, either after partial hepatectomy or a dose of CCl4 compared to otherwise untreated rats. During liver regeneration, DMN metabolism was slowed making the active metabolites available for longer.

The effect of a protein-free diet, a sugar diet and of carbon tetrachloride administration on the toxicity and rate of metabolism of dimethylnitrosamine in different rat strains

A Porton and a hooded rat strain showed a raised LD50 for dimethylnitrosamine (DMN) when pre-conditioned on a protein-free and/or a sugar diet. Little or no such differential toxicity between normal and diet-fed animals was found for rats of the Wistar, BDIX and CFY strains. Pre-treatment with CCl4 did not alter significantly the toxicity of DMN in the Wistar strain. All 5 rat strains treated by diet or CCl4 administration metabolized DMN at a very much slower rate than did the controls, the rates for the different strains being quantitatively similar. It is concluded that the toxicity of DMN is not necessarily related to its rate of metabolism and that the effect of diet or CCl4 treatment of DMN toxicity is dependent on the strain of rat used.

Early investigations on the effect of methyl mercuric chloride upon DMN-acute hepatotoxicity

Acute toxicity induced by DMN was partially prevented by previously administering methyl mercuric chloride (MMC), a chemical inhibitor of the drug metabolizing enzyme system (DMES). We have studied the early changes occurring during the course of DMN-intoxication, namely disaggregation of polysomal profiles and necrosis, evaluated morphologically and by the release of S-GPT.

Methylated purines in the deoxyribonucleic acid of various Syrian-golden-hamster tissues after administration of a hepatocarcinogenic dose of dimethylnitrosamine

1. DNA was extracted from livers, kidneys and lungs of Syrian golden hamsters at various times (up to 96h) after injection of a hepatocarcinogenic dose of [14C]dimethylnitrosamine. Purine bases were released from the DNA by mild acid hydrolysis and separated by Sephadex G-10 chromatography. 2. At 7h after dimethylnitrosamine administration liver DNA was alkylated to the greatest extent, followed by that of lung and kidney, the values for which were 8 and 3% respectively of those for liver. 3. The O6-methylguanine/7-methylguanine ratios were initially the same in all three organs and in the liver DNA of rats under similar conditions of dose. 4. O6-Methylguanine was the most persistent alkylated purine in all three hamster tissues. There was evidence for excision of 7-methyl-guanine, the highest activity for this being present in the liver. 5. Detectable amounts of the minor products 3-methyladenine, 1-methyladenine, 3-methylguanine and 7-methyladenine were present in most hamster tissues, and their individual rates of loss from liver DNA were determined. 6. Ring-labelling of the normal purines in DNA was highest in the liver, followed closely by the lung (80% of that in liver) whereas the kidney had very low incorporation (3% of that in liver). 7. The results are discussed with respect to the hepatotoxicity of dimethylnitrosamine, the miscoding potential of the various alkylation products and the induction of liver tumours in hamsters.

Partial hepatectomy and toxicity of dimethyl-nitrosamine and carbon tetrachloride, in relation to the carcinogenic action of dimethylnitrosamine

The yield of tumours in the liver of rats was increased when dimethylnitrosamine was given 1, 6 or 12 h after partial hepatectomy and still further increased if it was given after an interval of 24-72 h. The increase was greater after two-thirds than after one-third hepatectomy. An increase in the number of kidney tumours was also found. Microsomal DMN-demethylase activity was depressed after partial hepatectomy for up to 6 days in mice and rats. The LD50 of DMN on the other hand was decreased for 3 days, after which it returned to normal. The extent of liver necrosis produced by DMN was increased at 6 and 24 h after partial hepatectomy but was within the usual range at longer intervals. These results suggest that prolonged exposure of the tissues to DMN after partial hepatectomy played a significant role in the development of liver tumours as well as those in the kidney, in addition to the role of regeneration of the liver, and that the relative roles were still to be elucidated.

Reduction of carbon tetrachloride toxicity by prior administration of a single small dose in mice and rats

Mice were given progressively smaller doses of carbon tetrachloride (CCl4) and at intervals later the LD50 of a second dose was determined. The LD50 was greater than in previously untreated mice as soon as 10 min after the first dose, increased to a maximum between 12 and 24 h that was maintained for about 3 or 4 days, after which the LD50 returned to normal by the 7th day. The maximum LD50 reached was dependent on the first dose, but even a minute first dose, 0-004 ml/kg, had a significant effect. The same phenomenon was confirmed in rats. Administration of promethazine before doseing with CCl4 did not have this effect, nor did an ether anaesthetic.

Diverse mechanisms of hepatocellular injuries due to chemicals: evidence in rats administered carbon tetrachloride or dimethylnitrosamine

Differences in acute hepatotoxicity of carbon tetrachloride (CCl4) and dimethylnitrosamine (DMN) have been tested in normal foetal, newborn and adult rats, foetal, newborn and adult rats pretreated with phenobarbitone and partially hepatectomized adult rats. While CCl4 is non-toxic to the foetal and newborn liver, DMN induces identical necrosis at all ages. Prior dosing with phenobarbitone augments CCl4 toxicity only in the adult and the newborn but the foetus continues to be resistant. Such pretreatment, on the other hand, significantly reduces the effects of DMN on liver in all animals. Partial hepatectomy makes the liver less susceptible to CCl4 and more so to DMN. Such diversities of hepatic response to the two toxins can be accounted for by the levels of the respective toxifying enzymes in the liver cell in different situations.

The different susceptibility of rat liver lobes to carbon tetrachloride and dimethylnitrosamine

The extent of liver damage in rats dosed with carbon tetrachloride (CCl₄) or dimethylnitrosamine (DMN), by intragastric or intraperitoneal injection, has been compared in the different lobes. The level of activity of DMN-demethylase in the lobes has also been measured as an index of the activity of the microsomal enzymes. DMN-demethylase activity was greater in the left and left median lobes than in the right and right median lobes. The extent of liver damage (disruption of the basophilic bodies and necrosis) was greater in the right than in the left lobes of animals dosed with CCl₄ but was greater in the left lobes of animals given DMN. The route of injection made no difference. The distribution of liver damage may be explained by the distribution of microsomal enzymes.