Regenerative hyperplasia is not required for liver tumor induction in female B6C3F1 mice exposed to trihalomethanes

The Generation of ROS by Exposure to Trihalomethanes Promotes the IκBα/NF-κB/p65 Complex Dissociation in Human Lung Fibroblast

Background: Disinfection by-products used to obtain drinking water, including halomethanes (HMs) such as CH2Cl2, CHCl3, and BrCHCl2, induce cytotoxicity and hyperproliferation in human lung fibroblasts (MRC-5). Enzymes such as superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GPx) modulate these damages through their biotransformation processes, potentially generating toxic metabolites. However, the role of the oxidative stress response in cellular hyperproliferation, modulated by nuclear factor-kappa B (NF-κB), remains unclear. Methods: In this study, MRC-5 cells were treated with these compounds to evaluate reactive oxygen species (ROS) production, lipid peroxidation, phospho-NF-κB/p65 (Ser536) levels, and the activities of SOD, CAT, and GPx. Additionally, the interactions between HMs and ROS with the IκBα/NF-κB/p65 complex were analyzed using molecular docking. Results: Correlation analysis among biomarkers revealed positive relationships between pro-oxidant damage and antioxidant responses, particularly in cells treated with CH2Cl2 and BrCHCl2. Conversely, negative relationships were observed between ROS levels and NF-κB/p65 levels in cells treated with CH2Cl2 and CHCl3. The estimated relative free energy of binding using thermodynamic integration with the p65 subunit of NF-κB was -3.3 kcal/mol for BrCHCl2, -3.5 kcal/mol for both CHCl3 and O2•, and -3.6 kcal/mol for H2O2. Conclusions: Chloride and bromide atoms were found in close contact with IPT domain residues, particularly in the RHD region involved in DNA binding. Ser281 is located within this domain, facilitating the phosphorylation of this protein. Similarly, both ROS interacted with the IPT domain in the RHD region, with H2O2 forming a side-chain oxygen interaction with Leu280 adjacent to the phosphorylation site of p65. However, the negative correlation between ROS and phospho-NF-κB/p65 suggests that steric hindrance by ROS on the C-terminal domain of NF-κB/p65 may play a role in the antioxidant response.

Trihalomethanes in global drinking water: Distributions, risk assessments, and attributable disease burden of bladder cancer

This study aimed to assess the global spatiotemporal variations of trihalomethanes (THMs) in drinking water, evaluate their cancer and non-cancer risks, and THM-attributable bladder cancer burden. THM concentrations in drinking water around fifty years on a global scale were integrated. Health risks were assessed using Monte Carlo simulations and attributable bladder cancer burden was estimated by comparative risk assessment methodology. The results showed that global mean THM concentrations in drinking water significantly decreased from 78.37 μg/L (1973-1983) to 51.99 μg/L (1984-2004) and to 21.90 μg/L (after 2004). The lifestage-integrative cancer risk and hazard index of THMs through all exposure pathways were acceptable with the average level of 6.45 × 10-5 and 7.63 × 10-2, respectively. The global attributable disability adjusted of life years (DALYs) and the age-standardized DALYs rate (ASDR) dropped by 16% and 56% from 1990-1994 to 2015-2019, respectively. A big decline in the attributable ASDR was observed in the United Kingdom (62%) and the United States (27%), while China experienced a nearly 3-fold increase due to the expanded water supply coverage and increased life expectancy. However, China also benefited from the spread of chlorination, which helped reduce nearly 90% of unsafe-water-caused mortality from 1998 to 2018.

Charting Uncertainty in Science-Policy Discourses: The Construction of the Chlorinated Drinking-Water Issue and Cancer

Drinking-water guidelines remain an ongoing issue within Canada, as elsewhere. Given recent epidemiological evidence concerning chlorinated disinfection byproducts in drinking water and cancer outcomes, some branches within Health Canada have been undertaking an extensive review of the issue. This paper examines what impact contested scientific authorities, as filtered through a regulatory agency, may have on the policymaking process in the setting of Canadian drinking-water guidelines. Using an agenda-setting framework, we rely on a textual analysis of a Health Canada expert panel report and a position paper written to accompany the panel report; Canadian print media translations of scientific evidence; and in-depth interviews with scientists (from the academy, industry, and government) and other interested stakeholders [for example, chlorine and water industry, and environmental nongovernmental organizations]. Through this analysis we reconstruct a discourse which suggests government-science in policy, rather than policymakers, is primarily and presently driving the issue. The issue itself appears to remain a debate which is largely over the strength of the scientific evidence from regulatory and public-health scientists (for example, in Health Canada), and their colleague research scientists (for example, leading researchers in the field). Although we argue that it is primarily cancer that drives the science-policy agenda, with respect to chlorinated drinking water, it is possible that reproductive effects are likely to be central to the debate for controlling chlorine use in the future.

Generation of oxyradicals (O2. and H2O2), mitochondrial activity and induction of apoptosis of PBMC of Cyprinus carpio carpio treated in vivo with halomethanes and with recombinant HSP60 kDa and with LPS of Klebsiella pneumoniae

Halomethanes (HM) can be immunotoxic in mammals; however, in the fish immune system HM effects are unknown. In the current study, we evaluated the mitochondrial activity (MA) by MTT, induction of apoptosis by SubG0 technique and quantified serum ROS concentration (O2. and H2O2) and ROS production in PBMC of Cyprinus carpio carpio treated i.p. with CH2Cl2, CHCl3 and BrCHCl2 (0.004-40.0 mg/kg) for 96 h. Positive controls were recombinant heat shock protein of 60 kDa (rHSP60 kDa) of Klebsiella pneumoniae and its LPS. In addition, for in vitro PBMC cultures, two culture media and two sources of sera were tested. Both positive controls increased the MA more than 4-fold as well as the production of O2. (26-fold) and H2O2 (5-fold) compared to their controls. HM induced different effects on MA, ROS production and an induction of apoptosis, depending on the chlorination patterns and the dose; however, a systemic damage prevails. To fish treated with CH2Cl2, the apoptosis was related with serum ROS concentration and with MA. In contrast, in fish dosed with CHCl3 relationships were not found, deducing a systemic damage. However, in fish treated with BrCHCl2, serum O2. concentration and in vitro ROS generation performed by PBMC were involved in the induction of apoptosis of these cells but not with MA suggesting also immunotoxic effects. The current study demonstrated that HMs are immunomodulators increasing an acute inflammatory response and that rHSP60kDA of K. pneumoniae and its LPS are appropriate antigens to assess the immune response of C. c. carpio.

Pro-oxidant and antioxidant responses in the liver and kidney of wild Goodea gracilis and their relation with halomethanes bioactivation

In mammals, it has been shown that halomethanes (HM) are bioactivated by enzymes such as CYP 2E1 and the theta isoform of GST to produce reactive metabolites. However, in fish, little information is available, although HM can form autochthonously in aquatic environments. This study assessed the effect of HM in dusky splitfin (Goodea gracilis) from three lakes of the Valley of Mexico by analysing specific HM biomarkers as well as a broad range of biomarkers. The concentration of HM was a function of its half-life (higher in deep waters), while its precursors and solar radiation are secondary factors that determine its concentration. The kidney showed higher basal metabolism than the liver, probably because of its function as a haematopoietic and filtration organ. Using integrated biological response version 2 (IBRv2), it was found that the hepatic and renal O₂· content is a pro-oxidant force capable of inducing oxidative stress (ROOH, TBARS and RC=O). Early damage was found to be dependent on low concentrations of HM in Major Lake, whereas late damage was observed in fish exposed to higher concentrations of HM in Zumpango Lake and Ancient Lake. The activities of enzymes involved in antioxidant defence seemed to be inefficient. The quantitative assessment of biomarkers (ANOVA) and the estimate of parameter A obtained from IBRv2 provided different information. However, the data support the greater predictive power of IBRv2, but it requires a series of interrelated biomarkers to infer these possibilities. G. gracilis presents marked patterns of adaptation, which are dependant on the HM concentrations in environmental mixtures, although the response is complex and many toxicants could induce similar responses.

Halomethane-induced cytotoxicity and cell proliferation in human lung MRC-5 fibroblasts and NL20-TA epithelial cells

Halomethanes (HMs) can be formed during the chlorination process to obtain drinking water. In liver cells, HMs had been shown to be mutagenic and carcinogenic; however, their bioactivation by CYP 2E1 and GSTT1 is required. Although inhalation is the most common pathway of exposure, reports on the toxic effects induced by HMs in human lung are contradictory. The aim of this study was therefore to evaluate in vitro cytotoxicity and cell proliferation induced by CH(2)Cl(2), CHCl(3) and BrCHCl(2) in human lung NL20-TA epithelial cells and MRC-5 fibroblasts, and their relationship with CYP 2E1 and GSTT1 activity. High concentrations of these HMs induced cytotoxicity, particularly in cells treated with BrCHCl(2). Low concentrations of BrCHCl(2) stimulated hyperproliferation of fibroblasts, the most probable consequence of which is regenerative proliferation related to collagen induction. Fibroblasts exposed to BrCHCl(2) exhibited low levels of CYP 2E1 activity suggesting that released bromine is able to alter this activity by affecting the active site or auto regulating the activity itself. GSTT1 was up to ten times more active than CYP 2E1 in both cell lines, indicating that potential lung damage is due to formation of pro-carcinogens such as formaldehyde.

Evidence of bioactivation of halomethanes and its relation to oxidative stress response in Chirostoma riojai, an endangered fish from a polluted lake in Mexico

Halomethanes (HMs) are produced autochthonously in water bodies through the action of ultraviolet light in the presence of HM precursors, such as dissolved organic carbon and halogens. In mammals, toxic effects induced by HMs are diverse and include oxidative stress, which is also induced by divalent and polyvalent metals; however, in fish little information is available on HM metabolism and its possible consequences at the population level. In the present study, high CYP 2E1 and GST theta-like activities were found in viscera of the Toluca silverside Chirostoma riojai from Lake Zumpango (LZ; central Mexico). Formaldehyde, one of the HM metabolites, was correlated with CYP 2E1 activity and also induced lipid peroxidation in viscera. Hepatic CYP 2E1 activity was correlated with GST theta-like activity, suggesting the coupling of both pathways of HM bioactivation and its consequent oxidative damage. Sediment metals, among others, were also responsible for oxidative stress, particularly iron, lead, arsenic and manganese. However, under normal environmental conditions, the antioxidant enzymes of this species sustain catalysis adapted to oxidative stress. Findings suggest that this fish species apparently has mechanisms of adaptation and recovery that enable it to confront toxic agents of natural origin, such as metals and other substances formed through natural processes, e.g., HMs. This has allowed C. riojai to colonize LZ despite the high sensitivity of this species to xenobiotics of anthropogenic origin.

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.

A preliminary operational classification system for nonmutagenic modes of action for carcinogenesis

This article proposes a system of categories for nonmutagenic modes of action for carcinogenesis. The classification is of modes of action rather than individual carcinogens, because the same compound can affect carcinogenesis in more than one way. Basically, we categorize modes of action as: (1) co-initiation (facilitating the original mutagenic changes in stem and progenitor cells that start the cancer process) (e.g. induction of activating enzymes for other carcinogens); (2) promotion (enhancing the relative growth vs differentiation/death of initiated clones (e.g. inhibition of growth-suppressing cell-cell communication); (3) progression (enhancing the growth, malignancy, or spread of already developed tumors) (e.g. suppression of immune surveillance, hormonally mediated growth stimulation for tumors with appropriate receptors by estrogens); and (4) multiphase (e.g., "epigenetic" silencing of tumor suppressor genes). A priori, agents that act at relatively early stages in the process are expected to manifest greater relative susceptibility in early life, whereas agents that act via later stage modes will tend to show greater susceptibility for exposures later in life.

A Framework for Human Relevance Analysis of Information on Carcinogenic Modes of Action

The human relevance framework (HRF) outlines a four-part process, beginning with data on the mode of action (MOA) in laboratory animals, for evaluating the human relevance of animal tumors. Drawing on U.S. EPA and IPCS proposals for animal MOA analysis, the HRF expands those analyses to include a systematic evaluation of comparability, or lack of comparability, between the postulated animal MOA and related information from human data sources. The HRF evolved through a series of case studies representing several different MOAs. HRF analyses produced divergent outcomes, some leading to complete risk assessment and others discontinuing the process, according to the data available from animal and human sources. Two case examples call for complete risk assessments. One is the default: When data are insufficient to confidently postulate a MOA for test animals, the animal tumor data are presumed to be relevant for risk assessment and a complete risk assessment is necessary. The other is the product of a data-based finding that the animal MOA is relevant to humans. For the specific MOA and endpoint combinations studied for this article, full risk assessments are necessary for potentially relevant MOAs involving cytotoxicity and cell proliferation in animals and humans (Case Study 6, chloroform) and formation of urinary-tract calculi (Case Study 7, melamine). In other circumstances, when data-based findings for the chemical and endpoint combination studied indicate that the tumor-related animal MOA is unlikely to have a human counterpart, there is little reason to continue the risk assessment for that combination. Similarly, when qualitative considerations identify MOAs specific to the test species or quantitative considerations indicate that the animal MOA is unlikely to occur in humans, such hazard findings are generally conclusive and further risk assessment is not necessary for the endpoint-MOA combination under study. Case examples include a tumor-related protein specific to test animals (Case Study 3, d-limonene), the tumor consequences of hormone suppression typical of laboratory animals but not humans (Case Study 4, atrazine), and chemical-related enhanced hormone clearance rates in animals relative to humans (Case Study 5, phenobarbital). The human relevance analysis is highly specific for the chemical-MOA-tissue-endpoint combination under analysis in any particular case: different tissues, different endpoints, or alternative MOAs for a given chemical may result in different human relevance findings. By providing a systematic approach to using MOA data, the HRF offers a new tool for the scientific community's overall effort to enhance the predictive power, reliability and transparency of cancer risk assessment.

Risk characterization and exposure assessment in arseniasis-endemic areas of Taiwan

This paper examines the age-specific human health risks exposed to inorganic arsenic through arsenic-contaminated farmed fish/shrimp and groundwater consumptions in arseniasis-endemic areas of blackfoot disease (BFD)-endemic area and Lanyang Plain in Taiwan, based on an probabilistic integrated risk assessment framework. We employ an age-dependent predictive physiologically-based pharmacokinetic model to account for arsenic concentrations in target organs. We reconstruct age-specific dose-response profiles for arsenicosis and arsenic-induced cancers by best fitting a pharmacodynamics-based three-parameter Hill equation model to published epidemiological data from West Bengal and Taiwan. The predicted median arsenic concentrations in age group-specific skin, lung, and bladder ranged from 2.24-5.70, 3.76-9.46, and 5.11-20.71 micro g g(-1) in BFD-endemic area, whereas 4.98-12.04, 8.23-19.92, and 11.07-43.45 micro g g(-1) in Lanyang Plain, respectively. Risk analysis indicates that consumption of arsenic-contaminated farmed fish/shrimp and groundwater in arseniasis-endemic areas may increase threat to prevalence of arsenicosis for all age groups, whereas adults may undergo potential risks of arsenic-induced skin, lung and bladder cancers. We show that peoples in Lanyang Plain are more readily associated with higher morbidities for arsenicosis and skin cancer as well as fatalities for lung and bladder cancers than that of peoples in BFD-endemic area. Here we report the first case in which theoretical human health risks for consuming As-contaminated farmed fish/shrimp and groundwater in the arseniasis-endemic areas are alarming under a conservative condition based on a probabilistic risk assessment framework.

Uremic toxins: a new focus on an old subject

The uremic syndrome is characterized by an accumulation of uremic toxins due to inadequate kidney function. The European Uremic Toxin (EUTox) Work Group has listed 90 compounds considered to be uremic toxins. Sixty-eight have a molecular weight less than 500 Da, 12 exceed 12,000 Da, and 10 have a molecular weight between 500 and 12,000 Da. Twenty-five solutes (28%) are protein bound. The kinetics of urea removal is not representative of other molecules such as protein-bound solutes or the middle molecules, making Kt/V misleading. Clearances of urea, even in well-dialyzed patients, amount to only one-sixth of physiological clearance. In contrast to native kidney function, the removal of uremic toxins in dialysis is achieved by a one-step membrane-based process and is intermittent. The resulting sawtooth plasma concentrations of uremic toxins contrast with the continuous function of native kidneys, which provides constant solute clearances and mass removal rates. Our increasing knowledge of uremic toxins will help guide future treatment strategies to remove them.

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.

Utilization of juvenile animal studies to determine the human effects and risks of environmental toxicants during postnatal developmental stages

BACKGROUND

Toxicology studies utilizing animals and in vitro cellular or tissue preparations have been used to study the toxic effects and mechanism of action of drugs and chemicals and to determine the effective and safe dose of drugs in humans and the risk of toxicity from chemical exposures. Testing in animals could be improved if animal dosing using the mg/kg basis was abandoned and drugs and chemicals were administered to compare the effects of pharmacokinetically and toxicokinetically equivalent serum levels in the animal model and human. Because alert physicians or epidemiology studies, not animal studies, have discovered most human teratogens and toxicities in children, animal studies play a minor role in discovering teratogens and agents that are deleterious to infants and children. In vitro studies play even a less important role, although they are helpful in describing the cellular or tissue effects of the drugs or chemicals and their mechanism of action. One cannot determine the magnitude of human risks from in vitro studies when they are the only source of toxicology data.

METHODS

Toxicology studies on adult animals is carried out by pharmaceutical companies, chemical companies, the Food and Drug Administration (FDA), many laboratories at the National Institutes of Health, and scientific investigators in laboratories throughout the world. Although there is a vast amount of animal toxicology studies carried out on pregnant animals and adult animals, there is a paucity of animal studies utilizing newborn, infant, and juvenile animals. This deficiency is compounded by the fact that there are very few toxicology studies carried out in children. That is one reason why pregnant women and children are referred to as "therapeutic orphans."

RESULTS

When animal studies are carried out with newborn and developing animals, the results demonstrate that generalizations are less applicable and less predictable than the toxicology studies in pregnant animals. Although many studies show that infants and developing animals may have difficulty in metabolizing drugs and are more vulnerable to the toxic effects of environmental chemicals, there are exceptions that indicate that infants and developing animals may be less vulnerable and more resilient to some drugs and chemicals. In other words, the generalization indicating that developing animals are always more sensitive to environmental toxicants is not valid. For animal toxicology studies to be useful, animal studies have to utilize modern concepts of pharmacokinetics and toxicokinetics, as well as "mechanism of action" (MOA) studies to determine whether animal data can be utilized for determining human risk. One example is the inability to determine carcinogenic risks in humans for some drugs and chemicals that produce tumors in rodents, When the oncogenesis is the result of peroxisome proliferation, a reaction that is of diminished importance in humans.

CONCLUSIONS

Scientists can utilize animal studies to study the toxicokinetic and toxicodynamic aspects of drugs and environmental toxicants. But they have to be carried out with the most modern techniques and interpreted with the highest level of scholarship and objectivity. Threshold exposures, no-adverse-effect level (NOAEL) exposures, and toxic effects can be determined in animals, but have to be interpreted with caution when applying them to the human. Adult problems in growth, endocrine dysfunction, neurobehavioral abnormalities, and oncogenesis may be related to exposures to drugs, chemicals, and physical agents during development and may be fruitful areas for investigation. Maximum permissible exposures have to be based on data, not on generalizations that are applied to all drugs and chemicals. Epidemiology studies are still the best methodology for determining the human risk and the effects of environmental toxicants. Carrying out these focused studies in developing humans will be difficult. Animal studies may be our only alternative for answering many questions with regard to specific postnatal developmental vulnerabilities.

Low water-soluble uremic toxins

The uremic syndrome is the result of the retention of solutes, which under normal conditions are cleared by the healthy kidneys. Uremic retention products are arbitrarily subdivided according to their molecular weight. Low-molecular-weight molecules are characterized by a molecular weight below 500 D. The purpose of the present publication is to review the main water soluble, nonprotein bound uremic retention solutes, together with their main toxic effects. We will consecutively discuss creatinine, glomerulopressin, the guanidines, the methylamines, myo-inositol, oxalate, phenylacetyl-glutamine, phosphate, the polyamines, pseudouridine, the purines, the trihalomethanes, and urea per se.

Experimental cancer studies of chlorinated by-products

Chlorinated drinking water contains a number of different by-products formed during the chlorination process from organic matter. The carcinogenicity of only a fraction of them have been evaluated in experimental animals. The focus has been on compounds and groups of compounds that are most abundant in chlorinated drinking water or the in vitro toxicity data have suggested genotoxic potential. From trihalomethanes, chloroform causes liver tumors in mice and female rats and renal tumors in male mice and rats. Tumor formation by chloroform is strongly associated with cytotoxicity and regenerative cell proliferation in tissues and that has been considered to be one determinant of its carcinogenicity. From halogenic acetic acids, dichloroacetic acid (DCA) and trichlotoacetic acid (TCA) are hepatocarcinogenic in mice and DCA in male rats. Their genotoxicity is equivocal and nongenotoxic mechanisms, such as peroxisome proliferation and hypomethylation of DNA in the liver, likely contribute to tumor development. From chlorinated furanones (CHFs), 3-chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone (MX) is a multisite carcinogen in rats (e.g. in thyroid glands and liver) and it has caused DNA damage in vivo. MX may be a complete carcinogen because it also has promoter properties in vitro. Chlorinated drinking water may also contain brominated by-products providing the raw water contains bromide. At least some of them (bromodichloromethane, bromoform) have been shown to be carcinogenic in laboratory animals. Altogether, although several by-products have been shown to have carcinogenic potential in laboratory animals, it not yet possible to state which compounds or groups of by-products cause the cancer risk in chlorinated drinking water. The cellular mechanisms of their effects and these effects at low concentrations are still poorly understood. The few studies with mixtures of these by-products suggest that the mixture effects may be complex and unpredictable (inhibitory, additive, synergistic).

Chemicals studied and evaluated in long-term carcinogenesis bioassays by both the Ramazzini Foundation and the National Toxicology Program: in tribute to Cesare Maltoni and David Rall

The Ramazzini Foundation (RF) in Bentivoglio, Italy and the National Toxicology Program (NTP) in Research Triangle Park, North Carolina have carried out several hundred chemical carcinogenesis bioassays: 200 by RF and 500 by NTP. Of these, 21 have been evaluated by both laboratories. The 14 chemicals for which both laboratories have designed, conducted, and reported bioassay results are: acrylonitrile, benzene, chlorine, diesel fuel, ethylbenzene, methylene chloride (dichloromethane), propylene, styrene, styrene oxide, toluene, trichloroethylene, trichlorofluoromethane, vinylidene chloride, and xylenes. The other seven chemicals (two are fibers) were evaluated by both laboratories, but results have not yet been published. Results of these 14 interlaboratory studies were compared both to explore consistency of carcinogenic responses and to identify possible factors that may reveal reasons for any differences observed. Individual carcinogenesis results from each laboratory were duplicated and complementary. Of the 14 chemicals compared, 11 (80%) were either carcinogenic (9 chemicals) or noncarcinogenic (2 chemicals) in both studies. Eight of the paired chemicals had at least one carcinogenic target site in common. The other three were carcinogenic in one laboratory but not in the other. Possible explanations for these differences include dose, method of administration, duration of follow-up, and whether or not total tumors are counted. The collaboration between these two pioneering bioassay laboratory programs contributes greatly to our understanding of chemical carcinogenesis and results in better protection of workers and the general population from chemical diseases, especially cancers.

Chloroform: exposure estimation, hazard characterization, and exposure-response analysis

Chloroform has been assessed as a Priority Substance under the Canadian Environmental Protection Act. The general population in Canada is exposed to chloroform principally through inhalation of indoor air, particularly during showering, and through ingestion of tap water. Data on concentrations of chloroform in various media were sufficient to serve as the basis for development of deterministic and probabilistic estimates of exposure for the general population in Canada. On the basis of data acquired principally in studies in experimental animals, chloroform causes hepatic and renal tumors in mice and renal tumors in rats. The weight of evidence indicates that chloroform is likely carcinogenic only at concentrations that induce the obligatory precursor lesions of cytotoxicity and proliferative regenerative response. Since this cytotoxicity is primarily related to rates of formation of reactive, oxidative metabolites, dose response has been characterized in the context of rates of formation of reactive metabolites in the target tissue. Results presented here are from a "hybrid" physiologically based pharmacokinetic (PBPK) animal model that was revised to permit its extension to humans. The relevant measure of exposure response, namely, the mean rate of metabolism in humans associated with a 5% increase in tumor risk (TC05), was estimated on the basis of this PBPK model and compared with tissue dose measures resulting from 24-h multimedia exposure scenarios for Canadians based on midpoint and 95th percentiles for concentrations in outdoor air, indoor air, air in the shower compartment, air in the bathroom after showering, tap water, and food. Nonneoplastic effects observed most consistently at lowest concentrations or doses following repeated exposures of rats and mice to chloroform are cytotoxicity and regenerative proliferation. As for cancer, target organs are the liver and kidney. In addition, chloroform has induced nasal lesions in rats and mice exposed by both inhalation and ingestion at lowest concentrations or doses. The mean rate of metabolism associated with a 5% increase in fatty cysts estimated on the basis of the PBPK model was compared with tissue dose measures resulting from the scenarios already described, and lowest concentrations reported to induce cellular proliferation in the nasal cavities of rats and mice were compared directly with midpoint and 95th percentile estimates of concentrations of chloroform in indoor air in Canada. The degree of confidence in the underlying database and uncertainties in estimates of exposure and in characterization of hazard and dose response are delineated.

Metabolism and cytotoxicity of aflatoxin b1 in cytochrome p-450-expressing human lung cells

The mycotoxin aflatoxin B(1) (AFB(1)) is a hepatocarcinogen in many animal models and probably a human carcinogen. Besides being a dietary carcinogen, AFB(1) has been detected in dusts generated in the processing and transportation of AFB(1)-contaminated products. Inhalation of grain dusts contaminated with AFB(1) may be a risk factor in human lung cancer. Aflatoxin B(1) requires cytochrome P-450 (CYP)-mediated activation to form cytotoxic and DNA-reactive intermediates, and this activation in human liver is mediated by the CYP 1A2 and 3A4 isoforms. Which isoforms are important in AFB(1) activation in human lung is not well understood. To investigate whether these CYPs can activate AFB(1) at low, environmentally relevant concentrations in human lung cells, SV40 immortalized human bronchial epithelial cells (BEAS-2B) that were transfected with cDNA for CYPs 3A4 (B3A4) or 1A2 (B-CMV1A2) were used. B-CMV1A2 cultured in 15 nM AFB(1) produced the AFB(1)-glutathione conjugate (AFB(1)-GSH) and aflatoxin M(1) (AFM(1)), while B3A4 cells produced only aflatoxin Q(1) (AFQ(1)) at 0.15 microM AFB(1). Nontransfected BEAS-2B cells produced no metabolites, even at 1.5 mM AFB(1). Microsomes prepared from B-CMV1A2 and B3A4 cells activated AFB(1) to AFB(1) 8,9-epoxide (AFBO), while those from BEAS-2B cells did not produce AFBO. Cytosol from all three cell types was ineffective at glutathione S-transferase (GST)-mediated trapping of enzymatically generated AFB(1) 8,9-epoxide. B-CMV1A2 cells were 100-fold more sensitive to AFB(1) compared to B3A4 cells, and were 6000-fold more sensitive than control BEAS-2B cells. Western immunoblots confirmed that only B-CMV1A2 cells expressed CYP 1A2 protein, while CYP 3A4 was only in B3A4 cells. B-CMV1A2 cells were the most sensitive to AFB(1), followed by B3A4 cells. CYP 3A4, which has been predicted to activate AFB(1) primarily at higher AFB(1) concentrations, was also responsible for significant AFB(1) toxicity at low concentrations. These data indicate that human lung cells expressing these CYP isoforms are capable of activating AFB(1), even at environmentally relevant concentrations.

Carcinogenicity of bromodichloromethane administered in drinking water to Male F344/N Rats and B6C3F1 mice

A life-time exposure study was conducted to assess the carcinogenicity of bromodichloromethane (BDCM) administered in the drinking water to male F344/N rats and B6C3F(1) mice. In mouse, the calculated mean daily BDCM concentrations (measured concentrations corrected for on-cage loss of chemical) were 0.06, 0.28 and 0.49 g/l. Time-weighted water consumption of 135, 97, and 89 ml/kg/day resulted in mean daily doses of 8.1, 27.2, and 43.4 mg BDCM/kg/day. No changes in feed consumption, final body weight, or survival were observed. Kidney weights were significantly depressed at 27.2 and 43.4 mg BDCM/kg/day. There was no increase in neoplasia in the liver, kidney, spleen, testis, bladder, sections along the alimentary tract, excised lesions, or at any other organ site. In rat, the corrected mean daily BDCM concentrations were 0.06, 0.33, and 0.62 g/l. Time-weighted water consumption of 65, 63, and 59 ml/kg/day yielded 3.9, 20.6 and 36.3 mg BDCM/kg/day. No alterations in feed consumption, body weight gain, and survival were seen. Kidney weight was significantly depressed in the 36.3-mg/kg/day treatment group. There was a significantly enhanced prevalence and multiplicity of hepatocellular adenomas at 3.9 mg BDCM/kg/day (15.5% and 0.16/animal vs. 2.2% and 0.02/animal for the control). Hepatocellular carcinomas increased from 2.2% and 0.02/animal for the control and 3.9 mg BDCM/kg/day to 8.3% and 0.10/animal at 20.6 mg BDCM/kg/day. The combined neoplasms were enhanced at 3.9 and 20.6 mg BDCM/kg/day. Liver neoplasia was depressed to the control value at 36.3 mg BDCM/kg. The prevalence of basophilic and clear cell, but not eosinophilic cells, altered foci of cells declined with increasing dose. BDCM did not increase cancer in the large bowel, renal tubules, or in any of the other tissues examined. Renal tubular hyperplasia was observed at 36.3 mg BDCM/kg (15.8% vs. 8.7% for the control group). Under the conditions of the study, BDCM in the drinking water was not carcinogenic in the male B6C3F(1) mouse, but was carcinogenic in the male F344/N rat based on an increased hepatocellular neoplasia.

Chronic toxicity of bromodichloromethane to the Japanese medaka (Oryzias latipes)

Japanese medaka (Oryzias latipes) were continually exposed in a flow-through diluter system for 9 months to measured bromodichloromethane (BDCM) concentrations of 0.018, 0.143, or 1.424 mg/L. Parameters evaluated were hepatocarcinogenicity, hepatocellular proliferation, hematology, and intrahepatic BDCM concentration. BDCM was not hepatocarcinogenic to medaka at the concentrations tested. Chronic toxicity was evidenced at 6 and 9 months by statistically significant (alpha = 0.05) levels of gallbladder lesions and bile duct abnormalities in medaka treated with 1.424 mg/L BDCM. Hepatocellular proliferation was assessed after 1, 4, and 20 days of BDCM exposure. Treatment-related increases or decreases in cellular proliferation were not observed at any time point. Hematocrit, leukocrit, cell viability, and cell counts of treated fish after 9 months of BDCM exposure were not significantly different from control fish. Intrahepatic concentrations were evaluated by gas chromatography after 9 months of BDCM exposure. Fish livers from all three BDCM treatments had detectable amounts of BDCM, with median intrahepatic concentrations of 1.02, 2.89, and 21.25 mg BDCM/kg fish liver in the low, middle, and high concentrations, respectively. Medaka chronic toxicity effects of statistically significant gallbladder and bile duct abnormalities occurred at 1.424 mg/L BDCM, well above median drinking water levels.

Dose-response analyses of experimental cancer data

Dose-response analysis provides a powerful tool to determine causality from experimental cancer data, estimate low-dose risk, and evaluate mechanistic hypotheses. However, the interpretation of cancer dose-response data can be influenced by how the dose and response terms are characterized. Using the poly-3 quantal response method to adjust for the extensive and early development of lethal lymphomas in butadiene-exposed mice provided a means of obtaining a better representation of dose-response relationships for late-developing tumors induced by this chemical. Fitting a Weibull model to survival-adjusted tumor data for chloroprene and butadiene indicated similar carcinogenic potencies for these chemicals in mice. In conjunction with the rodent toxicity and carcinogenicity studies conducted by the National Toxicology Program, toxicokinetic studies are performed to characterize relationships between exposure and tissue concentrations of parent compound and metabolites. A physiologically based pharmacokinetic model (PBPK) of butadiene dosimetry indicated that differences in carcinogenic response between rats and mice are not simply due to differences in tissue concentrations of epoxybutene, a mutagenic metabolic intermediate. Thus, factors beyond tissue dosimetry of this metabolite must be important in butadiene-induced carcinogenesis. A PBPK model for isoprene indicated that blood concentrations of isoprene epoxides are a better indicator of kidney cancer risk than are measurements of isoprene-exposure concentrations. An evaluation of dose-response relationships for cytotoxicity, regenerative hyperplasia, and tumor induction by trihalomethanes indicates that for this family of chemicals, cell proliferation is not a reliable predictor of tumor response.

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.

Comparative characterization of CHCl(3) metabolism and toxicokinetics in rodent strains differently susceptible to chloroform-induced carcinogenicity

A comparative kinetic study in B6C3F1 mice, Osborne-Mendel (OM) and Sprague-Dawley (SD) rats has been undertaken with the major aim to determine the extent of covalent binding of chloroform reactive metabolites produced in vivo through oxidative and/or reductive metabolism in the target organs of chloroform carcinogenicity. Some additional kinetic observations of chloroform biotransformation were also collected comparatively. Expiration of [14C]-CO(2) showed that chloroform metabolism went to saturation in all tested rodent strains. In the B6C3F1 mouse maximal rates of approximately 135 µmol [14C]-CO(2)/kg b.w./h were reached at a dose of approximately 150 mg/kg, while in the two rat strains saturation occurred at a dose of approximately 60 mg/kg, with a maximal rate of approximately 40 µmol [14C]-CO(2)/kg b.w./h. At doses of 150-180 mg/kg b.w., limited differences were found in the distribution and elimination of [14C]-chloroform in the liver and kidney. Species differences have been found in the kinetics of alkali-extractable radioactivity in the blood. The levels of adducts of electrophilic intermediates with the polar heads (PH) of phospholipids (PL) showed a limited variability accross the rodents tested and did not correlate with the species and organ susceptibility to chloroform carcinogenicity. The levels of adducts of radical intermediates with the fatty acyl chains (FC) of PL were much lower than the PH adducts in all the samples analyzed; at the carcinogenicity bioassay doses, statistically significant levels of hepatic FC adducts were present only in the B6C3F1 mouse, where chloroform is hepatocarcinogenic. The observations in the rat kidney were suggestive of the formation of electrophilic reactive metabolites, presumably different from phosgene and associated with an initial chloroform reduction.

A comprehensive approach for integration of toxicity and cancer risk assessments

Experimental observations and theoretical considerations indicate a dose threshold for most chemically induced noncancer toxic effects below which the increased risk of toxicity is zero. Thus, the historical approach for minimizing risk from toxic chemicals has been to experimentally determine a no-observed-adverse-effect-level (NOAEL) and then to apply safety or uncertainty factors to estimate a dose not expected to produce that toxic effect in humans. In contrast, for radiation and chemically induced cancer, it has been assumed that all agents operate by a genotoxic mode of action and that some risk can be assigned to even vanishingly small doses. Accordingly, risk assessments for carcinogens have commonly been based on the assumption that the tumor dose-response curve at low doses is linear and passes through the origin. Mode of action is defined as a fundamental obligatory step in the induction of toxicity or cancer. It is now clear that tumor induction can arise in a variety of ways including not only a DNA-reactive genotoxic mode of action, but also non-DNA-reactive nongenotoxic-cytotoxic and nongenotoxic-mitogenic modes of action. Initial risk assessment approaches that recognized this distinction identified a chemical carcinogen as either genotoxic or nongenotoxic, with no middle ground. The realization that there is a continuum whereby different chemicals can act by a combination of modes of action and the recent explosion of research into molecular mechanisms of carcinogenesis indicate that all relevant information should be integrated into the risk assessment process on a case by case basis. A comprehensive approach to risk assessment demands that default assumptions be replaced with an integrated understanding of the rate-limiting steps in the induction of toxicity or cancer along with quantitative measures of the shapes of those dose-response curves. The examples of more contemporary risk assessments are presented for chloroform and vinyl acetate.

Drinking water disinfection byproducts: review and approach to toxicity evaluation

There is widespread potential for human exposure to disinfection byproducts (DBPs) in drinking water because everyone drinks, bathes, cooks, and cleans with water. The need for clean and safe water led the U.S. Congress to pass the Safe Drinking Water Act more than 20 years ago in 1974. In 1976, chloroform, a trihalomethane (THM) and a principal DBP, was shown to be carcinogenic in rodents. This prompted the U.S. Environmental Protection Agency (U.S. EPA) in 1979 to develop a drinking water rule that would provide guidance on the levels of THMs allowed in drinking water. Further concern was raised by epidemiology studies suggesting a weak association between the consumption of chlorinated drinking water and the occurrence of bladder, colon, and rectal cancer. In 1992 the U.S. EPA initiated a negotiated rulemaking to evaluate the need for additional controls for microbial pathogens and DBPs. The goal was to develop an approach that would reduce the level of exposure from disinfectants and DBPs without undermining the control of microbial pathogens. The product of these deliberations was a proposed stage 1 DBP rule. It was agreed that additional information was necessary on how to optimize the use of disinfectants while maintaining control of pathogens before further controls to reduce exposure beyond stage 1 were warranted. In response to this need, the U.S. EPA developed a 5-year research plan to support the development of the longer term rules to control microbial pathogens and DBPs. A considerable body of toxicologic data has been developed on DBPs that occur in the drinking water, but the main emphasis has been on THMs. Given the complexity of the problem and the need for additional data to support the drinking water DBP rules, the U.S. EPA, the National Institute of Environmental Health Sciences, and the U.S. Army are working together to develop a comprehensive biologic and mechanistic DBP database. Selected DBPs will be tested using 2-year toxicity and carcinogenicity studies in standard rodent models; transgenic mouse models and small fish models; in vitro mechanistic and toxicokinetic studies; and reproductive, immunotoxicity, and developmental studies. The goal is to create a toxicity database that reflects a wide range of DBPs resulting from different disinfection practices. This paper describes the approach developed by these agencies to provide the information needed to make scientifically based regulatory decisions.