Does Exposure to Trichloroethene in Low Doses Constitute a Cancer Risk to Humans?

Although chronic exposure to high doses of trichloroethene causes tumors of the lung, liver, and kidney in experimental animals, the epidemiology data in humans exposed to trichloroethene as a whole fail to support a causal association between trichloroethene exposure and cancers of the lungs, liver, or kidneys in humans at environmentally relevant concentrations. Environmentally relevant concentrations of trichloroethene are defined as 50 ppb (50 µg/L) in water or 5 ppb (27 µg/m³) in air. Tumor induction by trichloroethene in rodents exposed to very high doses over their whole lifespan has been observed in the kidney of rats and in the lung and liver of mice. Mechanistic data demonstrate that species-specific processes are involved in the carcinogenicity associated with chronic trichloroethene exposure in rodents. Based on these data and the results of recent well-conducted epidemiology studies, it can be concluded that humans exposed to trichloroethene at environmentally relevant concentrations are not at an increased risk for developing cancer.     

Trichloroethylene: Using New Information To Improve the Cancer Characterization

The views expressed in this paper are those of the authors and do not necessarily reflect the views or policies of the U.S. Environmental Protection Agency. Assessments of TCE's potential to cause cancer in humans have had to address issues concerning the strength of the human evidence and the relevance of the animal tumors to humans. The epidemiological database now includes analyses of multiple studies and molecular information. A recent analysis strongly suggests that TCE may induce cancer in humans at multiple sites, including kidney, liver, and lympho-hematopoietic cancer. Molecular analyses have found mutations of the von Hippel-Lindau tumor suppressor gene in renal tumors of TCE-exposed individuals. The animal bioassays have been followed up with mechanistic studies that provide insight into TCE's possible modes of action at each tumor site. This information suggests that TCE may act through mechanisms that can be relevant to human cancers. The mechanistic information can also be used to identify risk factors that may make some people more susceptible to TCE's adverse effects, allowing a fuller characterization of TCE's cancer potential in different groups of people.     

Genetic toxicology of 1,1,2-trichloroethylene

1,1,2-Trichloroethylene (TCE) is a widely used halogenated solvent, produced in hundreds of millions of kg each year for industrial purposes. Occupational and environmental exposure of human populations to TCE has been reported in industrialized areas. Long-term carcinogenicity studies in rodents demonstrate that exposure to high doses of TCE results in the induction of liver and lung tumors in the mouse, and tumors of the kidney and the testis in the rat. An indirect mechanism, based on the stimulation of liver peroxisome proliferation by TCE metabolites, was proposed to explain species differences in TCE hepatocarcinogenicity. Mutagenicity studies indicate that TCE is weakly active both in vitro, where liver microsomes produce electrophilic TCE metabolites, and also in vivo in mouse bone marrow, where high rates of micronuclei, but no structural chromosome aberrations, are found. Among TCE metabolites, trichloroacetic acid was reported to be carcinogenic to mouse liver. Furthermore, both trichloroacetic acid and chloral hydrate were found to be genotoxic in vivo, inducing structural and numerical chromosome abnormalities, respectively.     

Cadmium carcinogenesis

Cadmium is a heavy metal of considerable environmental and occupational concern. Cadmium compounds are classified as human carcinogens by several regulatory agencies. The most convincing data that cadmium is carcinogenic in humans comes from studies indicating occupational cadmium exposure is associated with lung cancer. Cadmium exposure has also been linked to human prostate and renal cancer, although this linkage is weaker than for lung cancer. Other target sites of cadmium carcinogenesis in humans, such as liver, pancreas and stomach, are considered equivocal. In animals, cadmium effectively induces cancers at multiple sites and by various routes. Cadmium inhalation in rats induces pulmonary adenocarcinomas, in accord with its role in human lung cancer. Cadmium can induce tumors and/or preneoplastic lesions within the rat prostate after ingestion or injection. At relatively high doses, cadmium induces benign testicular tumors in rats, but these appear to be due to early toxic lesions and loss of testicular function, rather than from a specific carcinogenic effect of cadmium. Like many other metals, cadmium salts will induce mesenchymal tumors at the site of subcutaneous (s.c.) or intramuscular (i.m.) injections, but the human relevance of these is dubious. Other targets of cadmium in rodents include the liver, adrenal, pancreas, pituitary, and hematopoietic system. With the exception of testicular tumors in rodents, the mechanisms of cadmium carcinogenesis are poorly defined. Cadmium can cause any number of molecular lesions that would be relevant to oncogenesis in various cellular model systems. Most studies indicate cadmium is poorly mutagenic and probably acts through indirect or epigenetic mechanisms, potentially including aberrant activation of oncogenes and suppression of apoptosis.     

Cadmium carcinogenesis in review

Cadmium is an inorganic toxicant of great environmental and occupational concern which was classified as a human carcinogen in 1993. Occupational cadmium exposure is associated with lung cancer in humans. Cadmium exposure has also, on occasion, been linked to human prostate cancer. The epidemiological data linking cadmium and pulmonary cancer are much stronger than for prostatic cancer. Other target sites for cadmium carcinogenesis in humans (liver, kidney, stomach) are considered equivocal. In rodents, cadmium causes tumors at several sites and by various routes. Cadmium inhalation in rats results in pulmonary adenocarcinomas, supporting a role in human lung cancer. Prostate tumors and preneoplastic proliferative lesions can be induced in rats after cadmium ingestion or injection. Prostatic carcinogenesis in rats occurs only at cadmium doses below those that induce chronic degeneration and dysfunction of the testes, a well-known effect of cadmium, confirming the androgen dependency of prostate tumors. Other targets of cadmium in rodents include the testes, adrenals, injection sites, and hematopoietic system. Various treatments can modify cadmium carcinogenesis including supplemental zinc, which prevents cadmium-induced injection site and testicular tumors while facilitating prostatic tumors. Cadmium is poorly mutagenic and probably acts through indirect mechanisms, although the precise mechanisms remain unknown.     

A review of the genotoxicity of ethylbenzene

Ethylbenzene is an important industrial chemical that has recently been classified as a possible human carcinogen (IARC class 2B). It induces tumours in rats and mice, but neither the relevance of these tumours to humans nor their mechanism of induction is clear. Considering the carcinogenic potential of ethylbenzene, it is of interest to determine whether there is sufficient data to characterize its mode of action as either genotoxic or non-genotoxic. A review of the currently available genotoxicity data is assessed. Ethylbenzene is not a bacterial mutagen, does not induce gene conversion or mutations in yeast and does not induce sister chromatid exchanges in CHO cells. Ethylbenzene is not clastogenic in CHO or rat liver cell lines but was reported to induce micronuclei in SHE cells in vitro. No evidence for genotoxicity has been seen in humans exposed to relatively high levels of ethylbenzene. Mouse lymphoma gene mutation studies produced a mixed series of responses that have proved difficult to interpret. An increase in morphological transformation of SHE cells was also found. Results from a more relevant series of in vivo genotoxicity studies, including acute and sub-chronic micronucleus tests and the mouse liver UDS assay, indicate a lack of in vivo genotoxic activity. The composite set of results from both in vitro and in vivo tests known to assess direct damage to DNA have been predominantly negative in the absence of excessive toxicity. The available data from the standard battery of genotoxicity assays do not support a genotoxic mechanism for ethylbenzene-induced kidney, liver or lung tumors in rats and mice.     

Trichloroethylene and Human Cancer

Evidence to suggest that trichloroethylene may be a human carcinogen comes mainly from two small epidemiological studies with supporting evidence from human toxicity and genotoxicity studies and from rodent cancer bioassays. Careful analysis of these data reveal marked inconsistencies between the data, differences in the conclusions drawn by various authorities reviewing the same data and, for certain key human studies, a complete absence of exposure data. Much of the rodent cancer data may be dismissed as not indicative of a human hazard because the tumors result from either peroxisome proliferation, or as a consequence of exceptionally high metabolic rates that are found uniquely in mouse tissues, or because the tumors only occur in the presence of overt toxicity. A common mechanism invoked to account for the development of renal tumors in both rats and humans is unproven, there is contrary evidence to suggest that this mechanism does not result in renal cancer, and alternative mechanisms have been proposed. Overall, the uncertainty and lack of consistency throughout the trichloroethylene studies, whether they are in humans, in animals, or in tests in vitro, lead to the conclusion that it would be wholly inappropriate to classify trichloroethylene as a human carcinogen.     

Trichloroethylene Carcinogenic Potency

Mention of tradename, specific equipment, or proprietary product does not constitute a guarantee or warranty by the State of California or imply approval to the exclusion of other products. The conclusions reached herein represent those of the author and do not necessarily represent those of the State of California. Current U.S. regulatory policy regarding the carcinogenic potential of trichloroethylene (TCE) in humans is conflicting. Weight-of-evidence considerations show either no or inconsistent associations between chronic TCE exposure and excess human cancer risk. Occupational exposure limits are so great (538,000 µg/m³) that daily absorbed doses are greater than the rodent no observed adverse effect level (NOAEL). Using the regulatory strength-of-evidence approach, recommended ambient air concentration limits (1.1 µg/m³) are derived using the default linearized multistage model. The latter assumes a radiomimetic mode of action, a highly questionable assumption in light of published TCE mechanism of action data. Given the current state of U.S. regulatory policy on this material, credible analyses of the data are needed from neutral nongovernmental organizations.     

Trichloroethylene in the Environment: Public Health Concerns

The Agency for Toxic Substances and Disease Registry (ATSDR) prepares toxi-cological profiles for hazardous substances found at waste sites and elsewhere in the environment. In 1997 the agency updated its toxicological profile for trichloroethylene and included new and expanded information on the health effects associated with exposures to trichloroethylene. Several endpoints of concern are described in the profile. However, in this paper only results from studies reporting developmental and carcinogenic effects from trichloroethylene exposures in human and experimental animal studies are summarized and evaluated. Based on its assessment of the available studies and limitations in the reported findings, ATSDR has determined there is limited but suggestive evidence that developmental effects may be a concern for some persons exposed to TCE in drinking water. Moreover, developmental effects may be the most sensitive of all non-cancer health effects associated with trichloroethylene exposures. Significant questions remain about the likely mode(s) of action for TCE-induced carcinogenesis in humans and the basis for differences in pharmacokinetics handling of TCE across animal strains and sex. However, on the basis of animal data and the suggestive, yet inconclusive, human data available, ATSDR has determined that cancer should be an effect of concern for people exposed to TCE in the environment. ATSDR agrees that the available literature supports the premise that TCE is “reasonably anticipated to be a human carcinogen” as defined by the U.S. National Toxicology Program.     

Banding carcinogenic risks in developed countries: A procedural basis for qualitative assessment

Readily achieved comparative assessment of carcinogenic risks consequent upon environmental exposures may increase understanding and contribute to cancer prevention. Procedures for hazard identification and quantitative risk assessment are established, but limited when addressing novel exposures to previously known carcinogens or any exposure to agents having only suspected carcinogenic activity. To complement other means of data evaluation, a procedure for qualitative assessment of carcinogenic risk is described. This involves categorizing the relevant carcinogen and circumstances under which exposure occurs. The categories for carcinogens are those used for hazard identification and involve whether the agent is (1) a recognized carcinogen for humans; (2) probably or (3) possibly carcinogenic for humans; (4) characterized by inadequate evidence of carcinogenicity; or (5) lacking carcinogenicity. Exposure is categorized by whether it is one which (1) establishes the agent as a recognized carcinogen; (2) is taken into account in establishing carcinogenicity status; (3) is distinct from those providing clearest evidence of carcinogenicity; (4) is not characterized in relation to carcinogenicity; or (5) involves an exposure in which absence of carcinogenic outcome is observed. These two categories of evidence allow the risk inherent in a situation to be banded as indicative of a proven, likely, inferred, unknown or unlikely carcinogenic outcome, and further characterized using sub-bands. The procedure has been applied to about fifty situations. For recognized carcinogens, including asbestos and polycyclic aromatic hydrocarbons, risks consequent upon occupational exposure, the impact of point source pollution, residence near contaminated sites and general environmental exposure are allocated across the proven band and a likely sub-band. For solvents, pesticides and other compounds having less clearly established carcinogenicity, impact on residents living near a production site, or near earlier related industrial activity is allocated to certain inferred sub-bands. Unknown carcinogenic outcome, which identifies exposure to an agent with inadequate evidence of carcinogenicity rather than being indicative of equivocal or negative data in any context, indicates both the impact of certain pollutants and user-exposure to some consumer products. Situations allocated to the unlikely risk band principally involve certain consumer products. Overall, such risk assessment may be of greatest worth in focusing community attention on proven causes of cancer and associated preventive measures.     

The Weight of Evidence Does Not Support the Listing of Styrene as “Reasonably Anticipated to be a Human Carcinogen” in NTP's Twelfth Report on Carcinogens

Styrene was listed as “reasonably anticipated to be a human carcinogen” in the twelfth edition of the National Toxicology Program's Report on Carcinogens based on what we contend are erroneous findings of limited evidence of carcinogenicity in humans, sufficient evidence of carcinogenicity in experimental animals, and supporting mechanistic data. The epidemiology studies show no consistent increased incidence of, or mortality from, any type of cancer. In animal studies, increased incidence rates of mostly benign tumors have been observed only in certain strains of one species (mice) and at one tissue site (lung). The lack of concordance of tumor incidence and tumor type among animals (even within the same species) and humans indicates that there has been no particular cancer consistently observed among all available studies. The only plausible mechanism for styrene-induced carcinogenesis—a non-genotoxic mode of action that is specific to the mouse lung—is not relevant to humans. As a whole, the evidence does not support the characterization of styrene as “reasonably anticipated to be a human carcinogen,” and styrene should not be listed in the Report on Carcinogens.     

Oncogene and tumor suppressor gene alterations in nasal tumors

The development of nasal tumors in humans and rodents is likely mediated through the accumulation of genetic alterations in genes that regulate cell proliferation, cell death and differentiation (oncogenes and tumor suppressor genes). By examination of the relationship between genetic alterations that are known to occur in human cancers with those induced in rodent tumors with defined carcinogenic exposures, biologically relevant mechanistic linkages of molecular events leading to tumors in rodents and humans can be established. Molecular genetic studies on nasal squamous cell carcinomas (SCC) in rats thus far have indicated the presence of oncogenes unrelated to the ras oncogene family and that p53 mutation occurs at a high frequency among the nasal SCC examined. The finding of p53 mutations in rat nasal SCC and the high prevalence of p53 mutations among human SCC, indicates that a common molecular alteration is shared between rodent and human SCC.     

Identification of differentially expressed genes in the livers of chronically i-As-treated hamsters

Inorganic arsenic (i-As) is a human carcinogen causing skin, lung, urinary bladder, liver and kidney tumors. Chronic exposure to this naturally occurring contaminant, mainly via drinking water, is a significant worldwide environmental health concern. To explore the molecular mechanisms of arsenic hepatic injury, a differential display polymerase chain reaction (DD-PCR) screening was undertaken to identify genes with distinct expression patterns between the liver of low i-As-exposed and control animals. Golden Syrian hamsters (5-6 weeks of age) received drinking water containing 15 mg i-As/L as sodium arsenite, or unaltered water for 18 weeks. The in vivo MN test was carried out, and the frequency of micronucleated reticulocytes (MN-RETs) was scored as a measure of exposure and As-related genotoxic/carcinogenic risk. A total of 68 differentially expressed bands were identified in our initial screen, 41 of which could be assigned to specific genes. Differential level of expression of a selected number of genes was verified using real-time RT-PCR with gene-specific primers. Arsenic-altered gene expression included genes related to stress response, cellular metabolism, cell cycle regulation, telomere maintenance, cell-cell communication and signal transduction. Significant differences of MN-RET were found between treated (8.70 ± 0.02 MN/1000RETs) and control (2.5 ± 0.70 MN/1000RETs) groups (P<0.001), demonstrating both the exposure and the i-As genotoxic/carcinogenic risk. Overall, this paper reveals some possible networks involved in hepatic arsenic-related genotoxicity, carcinogenesis and diabetogenesis. Additional studies to explore further the potential implications of each candidate gene are of especial interest. The present work opens the door to new prospects for the study of i-As mechanisms taking place in the liver under chronic settings.     

Low-dose arsenic-mediated metabolic shift is associated with activation of Polo-like kinase 1 (Plk1)

Arsenic is a well-established human carcinogen associated with cancers of the skin, liver, lung, kidney, and bladder. Although numerous carcinogenic pathways have been proposed, the molecular mechanisms underlying arsenic-associated cancer etiology are still elusive. The cellular responses to arsenic exposure are dose dependent. It was recently shown that low-dose arsenic leads to a metabolic shift from mitochondrial respiration to aerobic glycolysis via inactivation of tumor suppressor p53 and activation of NF-κB. However, how inactivation of p53, activation of NF-κB, and metabolic change are coordinated in response to low-dose arsenic exposure is still not completely understood. Polo-like kinase 1 (Plk1) is a well- documented regulator in many cell cycle-related events. Herein, we showed that low-dose arsenic leads to elevation of Plk1 in an NF-κB-dependent manner and that elevation of Plk1 contributes to the metabolic change from oxidative phosphorylation to glycolysis via activation of the PI3K/AKT/mTOR pathway. Furthermore, we showed that inhibition/depletion of Plk1 reverses low-dose arsenic-associated phenotypes, including enhanced cell proliferation, activation of the PI3K/AKT/mTOR pathway, and increased glycolysis. Finally, inhibition of the PI3K/AKT/mTOR pathway also antagonizes the enhanced glycolytic influx due to low-dose arsenic exposure. Our studies support the notion that Plk1 likely plays a critical role in cellular responses to low-dose arsenic.     

Genotoxicity of heat-processed foods

Gene-environment interactions include exposure to genotoxic compounds from our diet and it is no doubt, that humans are regularly exposed to e.g. food toxicants, not least from cooked foods. This paper reviews briefly four classes of cooked food toxicants, e.g. acrylamide, heterocyclic amines, nitrosamines and polyaromatic hydrocarbons. Many of these compounds have been recognised for decades also as environmental pollutants. In addition cigarette smokers and some occupational workers are exposed to them. Their occurrence, formation, metabolic activation, genotoxicity and human cancer risk are briefly presented along with figures on estimated exposure. Several lines of evidence indicate that cooking conditions and dietary habits can contribute to human cancer risk through the ingestion of genotoxic compounds from heat-processed foods. Such compounds cause different types of DNA damage: nucleotide alterations and gross chromosomal aberrations. Most genotoxic compounds begin their action at the DNA level by forming carcinogen-DNA adducts, which result from the covalent binding of a carcinogen or part of a carcinogen to a nucleotide. The genotoxic and carcinogenic potential of these cooked food toxicants have been evaluated regularly by the International Agency for Research on Cancer (IARC), which has come to the conclusion that several of these food-borne toxicants present in cooked foods are possibly (2A) or probably (2B) carcinogenic to humans, based on both high-dose, long-term animal studies and in vitro and in vivo genotoxicity tests. Yet, there is insufficient scientific evidence that these genotoxic compounds really cause human cancer, and no limits have been set for their presence in cooked foods. However, the competent authorities in most Western countries recommend minimising their occurrence, therefore this aspect is also included in this review.     

Animal carcinogenicity studies: 1. Poor human predictivity

The regulation of human exposure to potentially carcinogenic chemicals constitutes society's most important use of animal carcinogenicity data. Environmental contaminants of greatest concern within the USA are listed in the Environmental Protection Agency's (EPA's) Integrated Risk Information System (IRIS) chemicals database. However, of the 160 IRIS chemicals lacking even limited human exposure data but possessing animal data that had received a human carcinogenicity assessment by 1 January 2004, we found that in most cases (58.1%; 93/160), the EPA considered animal carcinogenicity data inadequate to support a classification of probable human carcinogen or non-carcinogen. For the 128 chemicals with human or animal data also assessed by the World Health Organisation's International Agency for Research on Cancer (IARC), human carcinogenicity classifications were compatible with EPA classifications only for those 17 having at least limited human data (p = 0.5896). For those 111 primarily reliant on animal data, the EPA was much more likely than the IARC to assign carcinogenicity classifications indicative of greater human risk (p < 0.0001). The IARC is a leading international authority on carcinogenicity assessments, and its significantly different human carcinogenicity classifications of identical chemicals indicate that: 1) in the absence of significant human data, the EPA is over-reliant on animal carcinogenicity data; 2) as a result, the EPA tends to over-predict carcinogenic risk; and 3) the true predictivity for human carcinogenicity of animal data is even poorer than is indicated by EPA figures alone. The EPA policy of erroneously assuming that tumours in animals are indicative of human carcinogenicity is implicated as a primary cause of these errors.     

Do cocarcinogenic effects of ELF electromagnetic fields require repeated long‐term interaction with carcinogens? Characteristics of positive studies using the DMBA breast cancer model in rats

The carcinogenic or cocarcinogenic potential of extremely low frequency (ELF; 50 or 60 Hz) magnetic fields (MFs) has been evaluated worldwide in diverse animal model systems. Though most results have been negative, weakly positive or equivocal results have been reported in several cancer models, including the rat DMBA (7,12-dimethylbenz[a]anthracene) model of mammary cancer. Based on the experimental conditions used in studies in which cocarcinogenic effects of ELF MF were found, it was recently proposed that MF exposure may potentiate the effects of known carcinogens only when the animals are exposed to both MF and carcinogen during an extended period of tumor development, i.e., when the carcinogen is given repeatedly during MF exposure. This review summarizes a series of experiments from our group, showing cocarcinogenic MF effects in the DMBA breast cancer model in rats, to test whether the above proposal is confirmed by existing data. Flux densities of 50 or 100 microT significantly increased the growth of mammary tumors, independent of whether DMBA was given in a single administration or repeatedly over a prolonged period. Thus, these data do not substantiate the hypothesis requiring repeated doses of DMBA during MF exposure. Instead, several other aspects of study design and experimental factors are identified that seem to be critical for the detection of cocarcinogenic effects of MF exposure in the rat DMBA mammary cancer model. These include the rat subline used, the dose of DMBA, the duration of MF exposure, the flux density, the background (sham control) tumor incidence, and the location of mammary tumors in the mammary gland complex. These and other experimental aspects may explain why some laboratories did not detect cocarcinogenic MF effects in the DMBA model. We hope that direct comparison of MF bioeffects in different rat sublines and further evaluation of other experimental differences between studies on MF exposure in the DMBA model will eventually determine which genetic and environmental factors are critical for potential carcinogenic or cocarcinogenic effects of ELF MF exposure.     

Aflatoxin B1-induced TP53 mutational pattern in normal human cells using the FASAY (Functional Analysis of Separated Alleles in Yeast)

Mutations in the TP53 gene are the most common alterations in human tumours. In hepatocellular carcinoma (HCC) related to exposure to aflatoxin B1, a specific G>T transversion in codon 249 is classically described as a hot spot. However, AFB1 is suspected to be a potent carcinogen in tissues other than the liver. By using the FASAY functional assay in yeast, the present study aimed at depicting the mutational pattern of TP53 in normal human fibroblasts after in vitro exposure to AFB1. Molecular analysis of mutants revealed that codon 245 was the main hot spot, whereas no mutations were found in codon 249. The locations of mutations within GG and GC/CG sequences are well in accordance with AFB1-adduct location data. In our assay with normal human fibroblasts, AFB1 mainly induced G>A transitions, followed by G>T and A>G mutations. This suggests that G>T transversions at codon 249 were likely the result of a selection bias in human HCC rather than a true fingerprint of AFB1 adducts. Indeed, a comparison of the mutation pattern with that found in human HCC excluding codon 249 reveals that the two spectra are quite similar. Furthermore, the similarity between our in vitro spectrum with that identified in AFB1-induced lung tumours in mice suggests that AFB1 may be a potent lung carcinogen in humans.     

A Review of the Epidemiology of Trichloroethylene and Kidney Cancer

The occupational epidemiological studies of trichloroethylene (TCE) exposure and kidney cancer are reviewed. Seven occupational cohort studies, conducted in the U.S., Finland, and Sweden involving over 130,000 workers, do not report statistically increased risks of kidney cancer among TCE-exposed workers. These studies were based on well-defined cohorts and exposure assessments involving urine biomonitoring or some type of job exposure matrix. In contrast, two German studies reported eight- to eleven-fold increased risks for renal cancer among TCE-exposed workers. However, numerous methodological and analytical shortcomings severely limit any interpretation of the German studies. We conclude that the more reliable epidemiologic data do not support a causal relationship between kidney cancer and TCE exposure.