The tumour promoters dieldrin and phenobarbital increase the frequency of c-Ha-ras wild-type, but not of c-Ha-ras mutated focal liver lesions in male C3H/He mice

Chronic administration of ethanol leads to an increased incidence of hepatocellular adenoma by promoting H-ras-mutated cells

This study used tissue samples from male B6C3F1 mice treated with ethanol in drinking water (0%, 2.5%, or 5%) for 4 or 104 weeks. We tested whether chronic alcohol drinking promotes oxidative stress in the liver and characterized the mutation profile of spontaneous and ethanol-induced tumors. We show that ethanol does not cause detectable oxidative stress in the liver at any time point and acts by promoting H-ras mutated cells.

Biological Basis of Differential Susceptibility to Hepatocarcinogenesis among Mouse Strains

There is a vast amount of literature related to mouse liver tumorigenesis generated over the past 60 years, not all of which has been captured here. The studies reported in this literature have generally been state of the art at the time they were carried out. A PubMed search on the topic "mouse liver tumors" covering the past 10 years yields over 7000 scientific papers. This review address several important topics related to the unresolved controversy regarding the relevance of mouse liver tumor responses observed in cancer bioassays. The inherent mouse strain differential sensitivities to hepatocarcinogenesis largely parallel the strain susceptibility to chemically induced liver neoplasia. The effects of phenobarbital and halogenated hydrocarbons in mouse hepatocarcinogenesis have been summarized because of recurring interest and numerous publications on these topics. No single simple paradigm fully explains differential mouse strain responses, which can vary more than 50-fold among inbred strains. In addition to inherent genetics, modifying factors including cell cycle balance, enzyme induction, DNA methylation, oncogenes and suppressor genes, diet, and intercellular communication influence susceptibility to spontaneous and induced mouse hepatocarcinogenesis. Comments are offered on the evaluation, interpretation, and relevance of mouse liver tumor responses in the context of cancer bioassays.

Levels of 4-aminobiphenyl-induced somatic H-ras mutation in mouse liver DNA correlate with potential for liver tumor development

The utility of liver H-ras codon 61 CAA to AAA mutant fraction as a biomarker of liver tumor development was investigated using neonatal male mice treated with 4-aminobiphenyl (4-ABP). Treatment with 0.1, 0.3, or 1.0 mumol 4-ABP produced dose-dependent increases in liver DNA adducts in B6C3F(1) and C57BL/6N mice. Eight months after treatment with 0.3 mumol 4-ABP or the DMSO vehicle, H-ras codon 61 CAA to AAA mutant fraction was measured in liver DNA samples (n = 12) by allele-specific competitive blocker-polymerase chain reaction (ACB-PCR). A significant increase in average mutant fraction was found in DNA of 4-ABP-treated mice, with an increase from 1.3 x 10(-5) (control) to 44.9 x 10(-5) (treated) in B6C3F(1) mice and from 1.4 x 10(-5) to 7.0 x 10(-5) in C57BL/6N mice. Compared with C57BL/6N mutant fractions, B6C3F(1) mutant fractions were more variable and included some particularly high mutant fractions, consistent with the more rapid development of liver foci expected in B6C3F(1) mouse liver. Twelve months after treatment, liver tumors developed in 79.2% of 4-ABP-treated and 22.2% of control B6C3F(1) mice; thus measurement of H-ras mutant fraction correlated with subsequent tumor development. This study demonstrates that ACB-PCR can directly measure background levels of somatic oncogene mutation and detect a carcinogen-induced increase in such mutation.

Modulation of liver tumorigenesis in Connexin32-deficient mouse

Connexin32 (Cx32) is the major gap junction forming protein in liver. Mice deficient in Cx32 demonstrate enhanced liver tumor formation, but are resistant to promotion of hepatocarcinogenesis by the model tumor promoter phenobarbital (PB). Here, we re-evaluate data on the number and sizes of glucose-6-phosphatase (G6Pase)-deficient liver lesions, both in Cx32-wildtype (WT) and Cx32-null male mice, obtained from two earlier experiments with similar protocols but paradoxical outcomes. In these experiments, enzyme-altered lesions were induced in mice of both strains by a single injection of N-nitrosodiethylamine (DEN) at age 6 weeks with a dose of 90 microg/g body weight (experiment 1) or at age 2 weeks with 10 microg/g body weight (experiment 2). Three weeks after DEN treatment groups of mice (sub-divided by Cx32 status) were also started on a PB-containing (0.05%) diet to test the responsiveness of the lesions to the tumor promoter. Additionally, for experiment 1, tumors were analyzed for the presence of Ha-ras and beta-catenin mutations. Based on the mutational analysis and the mathematical analysis of the G6Pase-deficient lesions, the two studies are consistent with the hypothesis of two types of lesions, 'late-type' lesions which are mainly characterized by beta-catenin mutations, and 'early-type' lesions that are frequently (but not exclusively) Ha-ras mutated. This concept affords an explanation as to the differential response seen in the two experiments with regard to Cx32 status and the role of PB as a tumor promoter (experiment 1) or inhibitor (as in experiment 2). Our findings also underscore the importance of the timing (6 weeks versus 2 weeks) of the genotoxic insult in relation to the developmental stage of the liver and the importance of clonal selection during tumor promotion.

Monograph: reassessment of human cancer risk of aldrin/dieldrin

In 1987, the US Environmental Protection Agency (EPA) classified aldrin and dieldrin as category B2 carcinogens, i.e. probable human carcinogens, based largely on the increase in liver tumors in mice fed either organochlorine insecticide. At that date, the relevant epidemiology was deemed inadequate to influence the cancer risk assessment. More time has now elapsed since early exposures of manufacturing workers to aldrin/dieldrin; therefore, updated epidemiological data possess more power to detect exposure-related differences in cancer risk and mortality. Also, recent experimental studies provide a plausible mode of action to explain the mouse specificity of dieldrin-induced hepatocarcinogenesis and call into question the relevance of this activity to human cancer risk. This monograph places this new information within the historic and current perspectives of human cancer risk assessment, including EPA's 1996 Proposed Guidelines for Carcinogen Risk Assessment. Updated epidemiological studies of manufacturing workers in which lifetime exposures to aldrin/dieldrin have been quantified do not indicate increased mortality or cancer risk. In fact, at the middle range of exposures, there is evidence of a decrease in both mortality from all causes and cancer. Recent experimental studies indicate that dieldrin-induced hepatocarcinogenesis in mice occurs through a nongenotoxic mode of action, in which the slow oxidative metabolism of dieldrin is accompanied by an increased production of reactive oxygen species, depletion of hepatic antioxidant defenses (particularly alpha-tocopherol), and peroxidation of liver lipids. Dieldrin-induced oxidative stress or its sequelae apparently result in modulation of gene expression that favors expansion of initiated mouse, but not rat, liver cells; thus, dieldrin acts as a nongenotoxic promoter/accelerator of background liver tumorigenesis in the mouse. Within the framework of EPA's Proposed Guidelines for Carcinogen Risk Assessment, it is proposed that the most appropriate cancer risk descriptor for aldrin/dieldrin, relating to the mouse liver tumor response, is 'not likely a human carcinogen', a descriptor consistent with the example of phenobarbital cited by EPA.

Frequency of ras mutations in liver neoplasms from B6C3F1 mice exposed to tetrafluoroethylene for two years

Tetrafluoroethylene (TFE) was evaluated for carcinogenicity in inhalation studies because of its high use in the production of Teflon. There was clear evidence of hepatocarcinogenic activity in B6C3F1 mice after 2 yr of TFE exposure. The present study was designed to characterize the mutation profiles of H- and K-ras oncogenes in liver neoplasms in mice after exposure to 0, 312, 625, or 1,250 ppm TFE. ras mutations were identified by restriction fragment length polymorphism, single-stranded conformation polymorphism analysis, and direct sequencing of polymerase chain reaction amplified-DNA isolated from frozen or paraffin-embedded liver neoplasms. A low frequency (15%, 9/59) of H-ras codon 61 mutations was detected in hepatocellular neoplasms when compared with the higher frequency (59% of this study and 56% of historical data) in spontaneously occurring liver neoplasms. There was no difference in the mutation frequency or spectrum among exposure groups or between benign and malignant hepatocellular neoplasms. K-ras mutations at codons 12, 13, and 61 and H-ras mutations at codon 117 were not detected in hepatocellular neoplasms. These data suggest that TFE-induced hepatocellular neoplasms are developed by pathways that are mostly independent of ras mutations. The ras mutation frequency and spectrum were similar to those of the structurally related chemical tetrachloroethylene.

Chemicals with carcinogenic activity in the rodent liver; mechanistic evaluation of human risk

A wide variety of chemicals, both naturally occurring and synthetic, have exhibited carcinogenic activity in rodent liver. Some are clearly DNA reactive whereas others produce only epigenetic effects. Hepatocarcinogens are categorized according to these properties and the characteristics of examples of both types are reviewed. DNA-reactive rodent hepatocarcinogens represent human cancer risk even at non-toxic exposures, whereas epigenetic agents pose either no risk because their effects are specific to rodents, or a risk only at high exposures at which they produce the same cellular effects in humans that are the basis for their carcinogenic activity in rodents.

Polymerase chain reaction and in situ hybridization: applications in toxicological pathology

Polymerase chain reaction (PCR) and in situ hybridization (ISH) have revolutionized the study of genes and gene expression, and many of these molecular biology advances will greatly impact research in toxicological pathology. PCR is one of the most powerful tools in molecular biology and involves primer-mediated enzymatic in vitro amplification of specific target DNA sequences. Recent innovative methods utilizing PCR technology have been developed to detect mutations in neoplastic and small subpopulations of cells, to study biomarkers of genetic susceptibility and genes involved with carcinogen metabolism, to estimate mutation frequencies, to find novel genes induced by chemical exposure, and to characterize gene expression. ISH provides data on individual cells rather than an average of total cellular populations and allows analysis for heterogeneity. When combined with PCR, the sensitivity of ISH is elevated, and single-copy DNA sequences, single-base mutations, or low copies of messenger RNA (mRNA) can potentially be detected within individual cells. Herein are reviewed ISH- and PCR-based techniques such as single-strand conformation polymorphism analysis to detect point mutations, allelotypic analysis for loss of heterozygosity, differential display of mRNA to characterize gene expression, quantitative reverse transcriptase polymerase chain reaction, and in situ polymerase chain reaction with emphasis on current or potential applications in toxicological pathology. These new and evolving techniques offer tremendous potential in providing new insights into the molecular basis of toxicity and carcinogenesis.

Hypomethylation of DNA: a nongenotoxic mechanism involved in tumor promotion

There is an abundant amount of information on the mechanisms of action of genotoxic chemicals that act as carcinogens and the role that mutations play in carcinogenesis. However, carcinogenesis is more than mutagenesis and many carcinogens are not mutagens. Thus, there is a need to consider nongenotoxic mechanisms that may be involved in carcinogenesis. In this paper, we review our working hypothesis that hypomethylation of DNA is an epigenetic, nongenotoxic mechanism that plays a role in tumor promotion by facilitating aberrant gene expression. The utility of employing experimental models that focus on relevant comparisons between sensitive and resistant strains of mice is emphasized. Additionally, aspects of DNA methylation in rodents and humans are compared and contrasted. We discuss hypomethylation of DNA as a secondary mechanism, that is expected to be threshold-exhibiting, and conclude by describing how this information may facilitate a rational approach towards risk assessment when dealing with nongenotoxic compounds that are carcinogenic in a bioassay.

Molecular aspects of chemical carcinogenesis: the roles of oncogenes and tumour suppressor genes

The observation that oncogenes are frequently activated in human tumours raises the question of whether these genes are involved in chemical carcinogenesis. H-ras activation is probably an initiating event in mouse skin and rat mammary gland systems. The H-ras oncogene is also important in mouse liver tumours; in mouse lung the K-ras gene is commonly activated. In both, the mutations observed are usually those predicted from the adduct-forming properties of the carcinogen. Among non-ras oncogenes, only raf and neu have been detected in experimental tumours. Tumour suppressor genes are frequently inactivated in human tumours. Searches for such phenomena in animal tumours have generally had disappointing results. p53 and Rb gene alterations are rarely observed in chemically-induced tumours. The reason may be that unknown tumour suppressor genes are involved in animal tumour development. Several novel genes have been identified using animal tumour susceptibility models. Thus, ras genes are important in chemical carcinogenesis, but as the methodology for studying other genes improves, their roles will be seen in perspective.

Role of mutations at codon 61 of the c-Ha-ras gene during diethylnitrosamine-induced hepatocarcinogenesis in C3H/He mice

Liver tumors of certain strains of mice frequently contain mutations at codon 61 of the c-Ha-ras gene. In our study, we investigated the significance of these mutations in the carcinogenic process. Male C3H/He mice received a single injection of diethylnitrosamine (DEN) on day 15 after birth, and groups of animals were killed at various time intervals between 11 and 52 wk after treatment. At the earlier time points (11-29 wk), we analyzed microdissected tissue from precancerous glucose-6-phosphatase-deficient liver lesions larger than approximately 200 microns in diameter, for the presence and pattern of c-Ha-ras codon 61 mutations. In parallel, the growth characteristics of these liver lesions were studied by pulse labeling with [3H]thymidine and by determining the size distribution of the lesions. At the later time points (42-52 wk after DEN treatment), liver tumors were dissected and also analyzed for the presence of c-Ha-ras mutations. We found mutations to be already present in some of the enzyme-altered liver lesions at weeks 11-29, suggesting that the mutations occurred early in the carcinogenic process. Whereas about 10% of the precancerous focal liver lesions showed mutations in the c-Ha-ras gene, the mutation frequency was increased to about 50% in the later-appearing hepatocellular adenomas and carcinomas, suggesting that c-Ha-ras codon 61 mutations may provide a selective advantage to the mutated cell clones.