Liver carcinogenesis is not a predicted outcome of chemically induced hepatocyte proliferation

Reactive metabolites in the biotransformation of molecules containing a furan ring

Many xenobiotics containing a furan ring are toxic and/or carcinogenic. The harmful effects of these compounds require furan ring oxidation. This reaction generates an electrophilic intermediate. Depending on the furan ring substituents, the intermediate is either an epoxide or a cis-enedione with more ring substitution favoring epoxide formation. Either intermediate reacts with cellular nucleophiles such as protein or DNA to trigger toxicities. The reactivity of the metabolite determines which cellular nucleophiles are targeted. The toxicity of a particular furan is also influenced by the presence of competing metabolic pathways or efficient detoxification routes. GSH plays an important role in modulating the harmful effects of this class of compound by reacting with the reactive metabolite. However, this may not represent a detoxification step in all cases.

Clarifying carcinogenicity of ethylbenzene

Ethylbenzene has been evaluated for carcinogenic activity in Fischer rats and B6C3F1 mice exposed by inhalation (Chan et al., 1998; Chan, 1999) and in Sprague-Dawley rats after oral exposure (Maltoni et al., 1985,1997). Bioassay findings are summarized below to expand on those not stated clearly or completely in Saghir et al. (2010). Overall in these three studies animals exposed to ethylbenzene had increased tumors in rats for kidneys, testes, head (including rare neuroesthesioepitheliomas), and total malignant tumors, whilst in mice tumor incidences were increased in the lung and liver (Huff, 2002). Thus ethylbenzene was carcinogenic by two exposure routes to both sexes of two species of rodents, two strains of rats, and one strain of mice, causing collectively tumors in five different target organs and a composite of "total malignant" tumors.

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.

Chronic Microcystin Exposure Induces Hepatocyte Proliferation with Increased Expression of Mitotic and Cyclin-associated Genes in P53-deficient Mice

Homozygous p53 deficient knockout mice were used to assess the role of p53 in tumor promotion by the protein phosphatase inhibitor and hepatic tumor promoter microcystin-LR (MCLR). More than 50% of human cancers bear mutations in the p53 gene, and in particular, p53 tumor suppressor gene mutations have been shown to play a major role in hepatocarcinogenesis. Trp53 homozygous (inactivated p53) and age-matched wild-type control mice were assigned to vehicle or MCLR-treated groups. MCLR or saline was administered daily for up to 28 days. RNA from the 28-day study was hybridized onto Mouse Genome GeneChip arrays. Selected RNA from 28 days and earlier time points was also processed for quantitative polymerase chain reaction (PCR). Livers from the 28-day, Trp53-deficient, MCLR group displayed greater hyperplastic and dysplastic changes morphologically and increases in Ki-67 and phosphohistone H3 (mitotic marker) immunoreactivity. Gene-expression analysis revealed significant increases in expression of cell-cycle regulation and cellular proliferation genes in the MCLR-treated, p53-deficient mutant mice compared to controls. These data suggest that regulation of the cell cycle by p53 is important in preventing the proliferative response associated with chronic, sublethal microcystin exposure, and therefore, conclude that p53 plays an important role in MCLR-induced tumor promotion.

Benzene-induced cancers: abridged history and occupational health impact

Benzene-induced cancer in humans was first reported in the late 1920s. Carcinogenesis findings in animals were not reported conclusively until 1979. Industry exploited this "discrepancy" to discredit the use of animal bioassays as surrogates for human exposure experience. The cardinal reason for the delay between first recognizing leukemia in humans and sought-after neoplasia in animals centers on poor design and conduct of experimental studies. The first evidence of carcinogenicity in animals manifested as malignant tumors of the zymbal glands (sebaceous glands in the ear canal) of rats, and industry attempted to discount this as being irrelevant to humans, as this organ is vestigial and not present per se in humans. Nonetheless, shortly thereafter benzene was shown to be carcinogenic to multiple organ sites in both sexes of multiple strains and multiple species of laboratory animals exposed via various routes. This paper presents a condensed history of the benzene bioassay story with mention of benzene-associated human cancers.

An autoradiographic study of cellular proliferaton, DNA synthesis and cell cycle variability in the rat liver caused by phenobarbital-induced oxidative stress: the protective role of melatonin

The protective effect of melatonin against phenobarbital-induced oxidative stress in the rat liver was measured based on lipid peroxidation levels (malondialedyde and 4-hydroxyalkenals). Cellular proliferation, DNA synthesis and cell cycle duration were quantitated by the incorporation of (3)H-thymidine, detected by autoradiography, into newly synthesized DNA. Two experiments were carried out in this study, each on four equal-sized groups of male rats (control, melatonin [10 mg/kg], phenobabital [20 mg/kg] and phenobarbital plus melatonin). Experiment I was designed to study the proliferative activity and rate of DNA synthesis, and measure the levels of lipid peroxidation, while experiment II was for cell cycle time determination. Relative to the controls, the phenobarbital-treated rats showed a significant increase (P < 0.01) in the lipid peroxidation levels (30.7%), labelling index (69.4%) and rate of DNA synthesis (37.8%), and a decrease in the cell cycle time. Administering melatonin to the phenobarbital-treated rats significantly reduced (P < 0.01) the lipid peroxidation levels (23.5%), labelling index (38.2%) and rate of DNA synthesis (29.0%), and increased the cell cycle time. These results seem to indicate that the stimulatory effect of phenobarbital on the oxidized lipids, proliferative activity, kinetics of DNA synthesis and cell cycle time alteration in the liver may be one of the mechanisms by which the non-genotoxic mitogen induces its carcinogenic action. Furthermore, melatonin displayed powerful protection against the toxic effect of phenobarbital.

Assessment of hepatocellular damage and hematological alterations in mice chronically fed p-dimethyl aminoazobenzene and phenobarbital

Two sets of mice (Mus musculus) were chronically fed 0.06% p-dimethylaminoazobenzene (p-DAB) and 0.05% Phenobarbital (PB) for 90 and 120 days, respectively, and several cell biological and hematological parameters were studied against normal diet fed controls. The cell biological studies included: (i) matrix metalloproteinase (MMP) and reduced glutathione content (GSH), and (ii) ultra-structural changes in liver through scanning (SEM) and transmission (TEM) electron microscopies. Further, changes in some other parameters like blood glucose level, cholesterol and hemoglobin contents, serum cortisol concentration and rate of viability of lymphocytes were also recorded. The serum hormonal levels of estradiol and testosterone were also measured in view of the observation that mice subjected to chronic feeding of p-DAB and PB had dramatically reduced reproductive abilities. All results clearly indicated that the chronic feeding of the carcinogens induced considerable toxicity and palpable hepato-cellular injuries along with some other changes during the carcinogenetic process in liver.

Differential display in rat livers treated for 13 weeks with phenobarbital implicates a role for metabolic and oxidative stress in nongenotoxic carcinogenicity

Hepatic enzyme inducers such as phenobarbital are often nongenotoxic rodent hepatocarcinogens. Currently, nongenotoxic hepatocarcinogens can only be definitively identified through costly and extensive long-term, repeat-dose studies (e.g., 2-year rodent carcinogenicity assays). Although liver tumors caused by these compounds are often not found to be relevant to human health, the mechanism(s) by which they cause carcinogenesis are not well understood. Toxicogenomic technologies represent a new approach to understanding the molecular bases of toxicological liabilities such asnongenotoxic carcinogenicity early in the drug discovery/development process. Microarrays have been used to identify mechanistic molecular markers of nongenotoxic rodent hepatocarcinogenesis in short-term, repeat-dose preclinical safety studies. However, the initial "noise" of early adaptive changes may confound mechanistic interpretation of transcription profiling data from short-term studies, and the molecular processes triggered by treatment with a xenobiotic agent are likely to change over the course of long-term treatment. Here, we describe the use of a differential display technology to understand the molecular mechanisms related to 13 weeks of dosing with the prototype rodent nongenotoxic hepatocarcinogen, phenobarbital. These findings implicate a continuing role for oxidative stress in nongenotoxic carcinogenicity.An Excel data file containing raw data is available in full at http://taylorandfrancis.metapress.com/openurl.asp?genre=journal&issn=0192-6233. Click on the issue link for 33(1), then select this article. A download option appears at the bottom of this abstract. The file contains raw data for all gene changes detected by AFLP, including novel genes and genes of unknown function; sequences of detected genes; and animal body and liver weight ratios. In order to access the full article online, you must either have an individual subscription or a member subscription accessed through www.toxpath.org.

Assessment of the genotoxic and cytotoxic potential of an anti-epileptic drug, phenobarbital, in mice: a time course study

To examine if chronic oral administration of phenobarbital (PB), a widely used anti-epileptic drug, has any genotoxic and cytotoxic potential in mice, a mammalian model, cytogenetic assays through several endpoints such as chromosome aberrations, induction of micronuclei, mitotic index of bone marrow cells, sperm-head anomaly in testis and enzymatic assays of several toxicity marker enzymes have been conducted by use of standard techniques. Mice of both treated (chronically receiving an oral dose of PB at 1.2 mg/kg bw) and control (without receiving PB) groups were sacrificed at 7, 15, 30, 60, 90 and 120 days for the study with all the above-mentioned protocols. Further, total protein profiles in liver of both control and treated mice were analyzed through the SDS-PAGE technique at day 60. The results of all these studies, when compared with controls, showed that PB has both genotoxic and cytotoxic potential in apparently increasing intensity at longer periods of chronic feeding in mice, which would warrant due consideration in its long-term use on human subjects.

Relevance of animal carcinogenesis findings to human cancer predictions and prevention

Use of laboratory animals to identify carcinogenic potential of chemicals, mixtures, and other agents has a modern history of greater than 40 years from which much useful scientific and public health information can be derived. While laboratory animals differ from humans in some respects that may affect responses to hazardous exposures, use of such models is based on experimental evidence indicating that there are more genetic, genomic, physiological, biochemical, and metabolic similarities than differences among mammalian species. Issues of concordance of responses between rodent species and between rodents and humans as well as repeatability and site-specificity are important considerations in evaluating laboratory animal carcinogenicity results. Variables in experimental design such as animal strain, diet, route of exposure, and study, duration as well as single-site versus multisite carcinogenic responses all influence interpretation and intelligent use of study data. Similarities and differences in site-specific laboratory animal and corresponding human cancers should also be considered in study evaluation. Recent attempts to explore genetically engineered mice and to humanize the mouse for more relevant identification of carcinogen hazard identification have yielded mixed results. In the end we are confronted by the realization that virtually all animal cancer models are useful but imperfect surrogates for humans. Assuming the percentage of chemicals currently in commerce that are estimated to be potent animal or human carcinogens is quite low, the task of identifying agents with significant carcinogenic potential is daunting and important. The biological conundrum of scientific debate regarding the relevance of carcinogenicity studies in laboratory animals is likely to continue. Nonetheless public health considerations must take precedence when deciding human safety issues.

Absence of carcinogenic activity in Fischer rats and B6C3F1 mice following 103-week inhalation exposures to toluene

Toluene, methylbenzene, is used to back-blend gasoline, as a chemical intermediate, and as a solvent; more than 7 million tonnes are produced each year in the United States. Following 14-15-week toxicity studies to estimate appropriate exposure concentrations for the carcinogenesis bioassays, toxicology and carcinogenesis studies of toluene (>99% pure) were conducted by whole-body inhalation exposures of F344/N rats and B6C3F1 mice of each sex for 15 months or two years. Toluene levels were 0 (chamber controls), 600, and 1,200 ppm for rats and 0, 120, 600, and 1200 ppm for mice. Exposures were 6.5 hr/day 5 days/wk. Genetic toxicology studies using Salmonella typhimurium, mouse L5178Y lymphoma cells, and Chinese hamster ovary cells were negative. No chemically related neoplasm was found in male rats, and one nasal, two kidney, and two forestomach neoplasms observed in female rats were considered not to be associated with the toluene exposure. For mice, no biologically important increase was observed for any nonneoplastic or neoplastic lesion. Studies by others had reported carcinogenicity of toluene, especially for total malignant tumors.

1,4-Dichlorobenzene-Induced liver tumors in the mouse: evaluation of the role of chlorohydroquinones

1,4-Dichlorobenzene (1,4-DCB) is a wide-spread environmental contaminant and well-described hepatotoxicant for rats and mice. The prolonged oral or inhalation exposure to 1,4-DCB is associated with an increased frequency of hepatic tumors in mice, but not in rats. Evidence is lacking of direct genotoxicity with 1,4-DCB or its metabolites, and no generally accepted mechanism has been found to account for the increased numbers of 1,4-DCB-induced hepatic tumors in mice. No information is available on the carcinogenic effects of 1,4-DCB in humans. Here we consider evidence that the biotransformation of 1,4-DCB to substituted hydroquinone species contributes to hepatic adenoma and carcinoma formation in mouse liver. This phenomenon has implications for human carcinogenesis.

Epigenetic mechanisms of chemical carcinogenesis

Chemically induced cancer is a multi-step process involving damage to the genome initially followed by clonal expansion of the DNA damaged cell eventually leading to a neoplasm. Chemical carcinogens have been shown to impact at all of the stages of the tumorigenesis process. It has become apparent that chemical and physical agents that induce cancer may do so through several different cellular and molecular mechanisms. Epigenetic (nongenotoxic) chemical carcinogens are those agents that function to induce tumor formation by mechanisms exclusive of direct modification or damage to DNA. These agents appear to modulate cell growth and cell death and exhibit dose response relationships between exposure and tumor formation. The exact and/or exclusive mechanisms by which these agents function have not been established, however, changes in cell growth regulation and gene expression are important to tumor formation. This review focuses on several potential mechanisms and cellular processes that may be involved in nongenotoxic chemical carcinogenesis.

Long-term chemical carcinogenesis bioassays predict human cancer hazards. Issues, controversies, and uncertainties

Long-term carcinogenesis bioassays are the most valued and predictive means for identifying potential carcinogenic hazards of various agents to humans. Agents may be chemicals, chemical mixtures, multiple chemicals, combinations of chemicals, residues and contaminants, commercial products and formulations, and various exposure circumstances. Life-styles, dietary factors, and occupational exposure circumstances are very difficult, but not totally impossible, to evaluate experimentally. Historically, the first chemical bioassay took place in the early part of this century: Yamagiwa and Ichikawa in 1915, showed that coal tar applied experimentally to rabbit ears caused skin carcinomas. Since then, nearly 1500-2000 bioassays of one sort or another have been carried out. Importantly, however, some of these bioassays must be considered inadequate for judging the absence of carcinogenicity, since there were various limitations on the way they were performed: too few animals, too short a duration, too low exposure concentrations, too limited pathology, as examples. Thus, each bioassay must be critically evaluated, especially those reported to be negative, because "false negatives" are certainly more hazardous to human health than are "false positives". Likewise, one must be careful not to discount bioassay results simply because a target organ in rodents may not have a direct counterpart in humans (e.g., Zymbal glands), or because an organ site in rodents may not be a major site of cancers in humans (e.g., mouse liver). The design and conduct of a bioassay is not simple, however, and one must be fully aware of possible pitfalls as well as viable and often necessary alternatives. Similarly, evaluating results and interpreting findings must be approached with the utmost objectivity and consistency. These and other select issues, controversies, and uncertainties possibly encountered in long-term bioassays are covered in this paper. One fact remains abundantly clear: for every known human carcinogen that has been tested adequately in laboratory animals, the findings of carcinogenicity are concordant.

Apoptosis and cell proliferation in rat hepatocytes induced by barbiturates

To examine the effect on cell population in hepatocytes of phenobarbital (PB) and other barbiturates, PB, allobarbital (ALB), barbital sodium (BS) and barbituric acid (BA) were given orally to male rats for 7 consecutive days. Although there was no apparent change in non-promoting BA, hepatomegaly was induced by PB, BS and ALB, which are promoters of hepatocarcinogenesis. In PB- and BS-treated livers, hepatomegaly was attributable to hepatocyte proliferation and enzyme induction. In ALB-treated liver, it was attributable to enzyme induction. The level of cell proliferation was reduced to less than the control values following withdrawal of PB, ALB and BS. It seemed that the degree of suppression of cell proliferation following withdrawal of these compounds correlated to the degree of cell proliferation (PB>BS>ALB) during treatment. In PB-treated liver, apoptosis was induced during treatment, serving to eliminate the excess of hepatocytes. This suggests that short-term administration of PB neither induced suppression of apoptosis nor disturbed homeostasis of hepatocyte populations.

Short- and intermediate-term carcinogenicity testing--a review. Part 1: the prototypes mouse skin tumour assay and rat liver focus assay

Carcinogenicity testing is by far the most expensive and time-consuming study type of toxicology. For many years, the lifetime exposure with the maximum tolerated dose in two rodent species has been the gold standard of carcinogenicity testing of pharmaceuticals. Major change was introduced by the Fourth International Conference on Harmonization in July 1997; a chronic rodent bioassay in one species and a short-term carcinogenicity assay are regarded as sufficient for registration. Such requirements provide the opportunity to redirect the vast resources previously spent on the lifetime study in the second species. Numerous experimental protocols for short- and intermediate-term carcinogenicity testing in many target tissues have been available for years. The first part of this review describes the basic principles of short- and intermediate-term carcinogenicity testing using the examples of the widely used mouse skin tumour assay and the rat liver foci assay. In the context of these experimental models, the discrimination and quantification of initiating and promoting activity and the use of preneoplastic lesions as endpoints in carcinogenicity testing are described. The review includes the limitations of the models with regard to the extrapolation from effects observed in animal experiments to a potential exposure of humans.

Apparent increased risk of leukemia in their highest category of exposure to tetrachloroethylene (PCE) in drinking water

The potential human carcinogenicity of PCE has been the subject of study for many years, yet the largest and most recent occupational studies have not reported any increased risk of leukemia in PCE-exposed groups, let alone a risk of the magnitude suggested by Aschengrau et al. The EPA's own Science Advisory Board concluded in 1991 that PCE is a chemical "for which there is no compelling evidence of human cancer risk, accompanied by animal data of carcinogenicity whose extrapolation to humans is ambiguous." Given this background, it is not plausible that a leukemia risk of the magnitude reported by Aschengrau et al. should exist but not have been found among highly exposed occupational groups. Aschengrau et al. could contribute to our understanding of this inconsistency by presenting the additional data analysis that I have suggested.

Mechanisms, chemical carcinogenesis, and risk assessment: cell proliferation and cancer

Mechanisms of carcinogenesis--and in particular chemically associated carcinogenicity--have attracted considerable scientific and public attention in the last decade. Much insight has been gained that will lead to more reasoned and better prevention, intervention, and treatment for the reduction of environmentally caused cancers. However, there seems to be an exaggerated tendency to embrace "mechanisms" not yet fully characterized, completely tested, unequivocally proven, and consensus accepted. More than 100 agents and exposure circumstances have been identified as causally or strongly associated with human cancers; for many the evidence was discovered first in experimental animals. More chemicals have been uncovered as carcinogenic in experimental animals, with as yet no or little available information in exposed human populations. Additional and expanded mechanistic and epidemiological studies should further elucidate the relevance of these agents to adverse human health effects, including cancers. Claims are being posed that certain chemical-specific "mechanisms" in experimental systems are irrelevant to humans, and thus chemicals thought to be aberrantly carcinogenic in animals would present no cancer hazard to exposed humans. Nonetheless before undeniable proof becomes available, we must continue to proceed with sensitive and responsible caution. This commentary offers a central and personal view of one such mechanism: cell proliferation and cancer.

Implications for the involvement of the immune system in parasite-associated cancers

Biological factors can be carcinogenic risk factors in humans and in animals. Numerous theories have been developed to explain the causal link between biologically-associated disease and the ensuing neoplasia. In this paper we discuss the merits of one of these theories, the possible association between the mammalian inflammatory response and cancer.