Difficulty of mode of action determination for trichloroethylene: An example of complex interactions of metabolites and other chemical exposures

Residential cancer cluster investigation nearby a Superfund Study Area with trichloroethylene contamination

PURPOSE

Trichloroethylene (TCE) is an industrial solvent associated with liver cancer, kidney cancer, and non-Hodgkin's lymphoma (NHL). It is unclear whether an excess of TCE-associated cancers have occurred surrounding the Middlefield-Ellis-Whisman Superfund site in Mountain View, California. We conducted a population-based cancer cluster investigation comparing the incidence of NHL, liver, and kidney cancers in the neighborhood of interest to the incidence among residents in the surrounding four-county region.

METHODS

Case counts and address information were obtained using routinely collected data from the Greater Bay Area Cancer Registry, part of the Surveillance, Epidemiology, and End Results program. Population denominators were obtained from the 1990, 2000, and 2010 US censuses. Standardized incidence ratios (SIRs) with two-sided 99 % confidence intervals (CIs) were calculated for time intervals surrounding the US Censuses.

RESULTS

There were no statistically significant differences between the neighborhood of interest and the larger region for cancers of the liver or kidney. A statistically significant elevation was observed for NHL during one of the three time periods evaluated (1996-2005: SIR = 1.8, 99 % CI 1.1-2.8). No statistically significant NHL elevation existed in the earlier 1988-1995 (SIR = 1.3, 99 % CI 0.5-2.6) or later 2006-2011 (SIR = 1.3, 99 % CI 0.6-2.4) periods.

CONCLUSION

There is no evidence of an increased incidence of liver or kidney cancer, and there is a lack of evidence of a consistent, sustained, or more recent elevation in NHL occurrence in this neighborhood. This evaluation included existing cancer registry data, which cannot speak to specific exposures incurred by past or current residents of this neighborhood.

Peripheral blood counts in workers exposed to synthetic fibres

Acrylonitrile is an intermediary with possible adverse health effects in the synthesis of organic products, such as acrylic fibres. This investigation was undertaken to determine the possible changes in the peripheral blood counts in workers of a polyacrylic fibres plant. The study involved 218 workers exposed to acrylonitrile at low doses and a control group of 200 unexposed workers. The chosen subjects underwent blood tests in order to check their haematological parameters. There were no statistically significant differences between the two groups in terms of the red blood cells, haemoglobin and total number of leukocytes. An increase in the neutrophils associated with a reduction of lymphocytes, both statistically significant, was observed. The authors hypothesized that the neutrophils are influenced by the exposure to acrylonitrile at low doses.

Human health effects of trichloroethylene: key findings and scientific issues

BACKGROUND

In support of the Integrated Risk Information System (IRIS), the U.S. Environmental Protection Agency (EPA) completed a toxicological review of trichloroethylene (TCE) in September 2011, which was the result of an effort spanning > 20 years.

OBJECTIVES

We summarized the key findings and scientific issues regarding the human health effects of TCE in the U.S. EPA's toxicological review.

METHODS

In this assessment we synthesized and characterized thousands of epidemiologic, experimental animal, and mechanistic studies, and addressed several key scientific issues through modeling of TCE toxicokinetics, meta-analyses of epidemiologic studies, and analyses of mechanistic data.

DISCUSSION

Toxicokinetic modeling aided in characterizing the toxicological role of the complex metabolism and multiple metabolites of TCE. Meta-analyses of the epidemiologic data strongly supported the conclusions that TCE causes kidney cancer in humans and that TCE may also cause liver cancer and non-Hodgkin lymphoma. Mechanistic analyses support a key role for mutagenicity in TCE-induced kidney carcinogenicity. Recent evidence from studies in both humans and experimental animals point to the involvement of TCE exposure in autoimmune disease and hypersensitivity. Recent avian and in vitro mechanistic studies provided biological plausibility that TCE plays a role in developmental cardiac toxicity, the subject of substantial debate due to mixed results from epidemiologic and rodent studies.

CONCLUSIONS

TCE is carcinogenic to humans by all routes of exposure and poses a potential human health hazard for noncancer toxicity to the central nervous system, kidney, liver, immune system, male reproductive system, and the developing embryo/fetus.

The epidemiology and risk factors for renal cancer

BACKGROUND

Renal cancer is a frequently occurring malignancy with over 270,000 new cases diagnosed and it being responsible for 110,000 deaths annually on a global basis. Incidence rates have gradually increased whilst mortality rates are starting to plateau.

OBJECTIVE

To review epidemiology and risk factors for renal cancer.

METHODS

The current data is based on a thorough review of available original and review articles on epidemiology and risk factors for renal cancer with a systemic literature search utilising Medline.

RESULTS

The prevalence of associated risk factors such as genetic susceptibility, smoking, hypertension and obesity are changing and could account for the changes in incidence whilst the role of diet and occupational exposure to carcinogens requires further investigation.

CONCLUSION

Despite the evidence of various associated risk factors, further work is required from well designed studies to gain a greater understanding of the etiology of renal cancer.

Quantification of perchloroethylene residues in dry-cleaned fabrics

We have used a novel gas chromatography/mass spectrometry (GC/MS)-based approach to quantify perchloroethylene (PCE) residues in dry-cleaned fabrics. Residual PCE was extracted from fabric samples with methanol and concentration was calculated by the gas chromatographic peak area, standardized using PCE calibration data. Extracts examined were from samples of 100% wool, polyester, cotton, or silk, which were dry cleaned from one to six times in seven different Northern Virginia dry-cleaning establishments. Additional experiments were conducted to investigate the kinetics of PCE release in the extraction solvent and to the open air. We found that polyester, cotton, and wool retained ≥ µM levels of PCE, that these levels increased in successive dry-cleaning cycles, and that PCE is slowly volatilized from these fabrics under ambient room air conditions. We found that silk does not retain appreciable PCE. Measured differences across dry-cleaning establishments and fabric type suggest more vigorous monitoring of PCE residues may be warranted. Environ. Toxicol. Chem. 2011;30:2481-2487. © 2011 SETAC.

Evaluating pharmacokinetic and pharmacodynamic interactions with computational models in supporting cumulative risk assessment

Simultaneous or sequential exposure to multiple chemicals may cause interactions in the pharmacokinetics (PK) and/or pharmacodynamics (PD) of the individual chemicals. Such interactions can cause modification of the internal or target dose/response of one chemical in the mixture by other chemical(s), resulting in a change in the toxicity from that predicted from the summation of the effects of the single chemicals using dose additivity. In such cases, conducting quantitative cumulative risk assessment for chemicals present as a mixture is difficult. The uncertainties that arise from PK interactions can be addressed by developing physiologically based pharmacokinetic (PBPK) models to describe the disposition of chemical mixtures. Further, PK models can be developed to describe mechanisms of action and tissue responses. In this article, PBPK/PD modeling efforts conducted to investigate chemical interactions at the PK and PD levels are reviewed to demonstrate the use of this predictive modeling framework in assessing health risks associated with exposures to complex chemical mixtures.

Epidemiology and risk factors for kidney cancer

After more than two decades of rising rates, in recent years the total kidney cancer incidence worldwide has shown signs of stabilizing, or even decreasing. In adults, kidney cancer consists of renal cell carcinoma (RCC), the predominant form, and renal transitional cell carcinoma (RTCC); these types primarily arise in the renal parenchyma and renal pelvis, respectively. Although temporal trends by kidney cancer type are not well established worldwide, incidence of RCC in the US has continued to rise, mainly for early-stage tumors, while that of RTCC has declined, and total kidney cancer mortality rates have leveled. Stabilization of kidney cancer mortality rates has also been reported in Europe. These trends are consistent with reports of increasing incidental diagnoses and a downward shift in tumor stage and size in clinical series. The changing prevalence of known risk factors for RCC, including cigarette smoking, obesity, and hypertension, is also likely to affect incidence trends, although their relative impact may differ between populations. Accumulating evidence suggests an etiologic role in RCC for physical activity, alcohol consumption, occupational exposure to trichloroethylene, and high parity among women, but further research is needed into the potential causal effects of these factors. Genetic factors and their interaction with environmental exposures are believed to influence risk of developing RCC, but a limited number of studies using candidate-gene approaches have not produced conclusive results. Large consortium efforts employing genome-wide scanning technology are underway, which hold promise for novel discoveries in renal carcinogenesis.

Occupational trichloroethylene exposure and renal carcinoma risk: evidence of genetic susceptibility by reductive metabolism gene variants

Trichloroethylene (TCE) is a suspected renal carcinogen. TCE-associated renal genotoxicity occurs predominantly through glutathione S-transferase (GST) conjugation and bioactivation by renal cysteine beta-lyase (CCBL1). We conducted a case-control study in Central Europe (1,097 cases and 1,476 controls) specifically designed to assess risk associated with occupational exposure to TCE through analysis of detailed job histories. All jobs were coded for organic/chlorinated solvent and TCE exposure (ever/never) as well as the frequency and intensity of exposure based on detailed occupational questionnaires, specialized questionnaires, and expert assessments. Increased risk was observed among subjects ever TCE exposed [odds ratio (OR) = 1.63; 95% confidence interval (95% CI), 1.04-2.54]. Exposure-response trends were observed among subjects above and below the median exposure [average intensity (OR = 1.38; 95% CI, 0.81-2.35; OR = 2.34; 95% CI, 1.05-5.21; P(trend) = 0.02)]. A significant association was found among TCE-exposed subjects with at least one intact GSTT1 allele (active genotype; OR = 1.88; 95% CI, 1.06-3.33) but not among subjects with two deleted alleles (null genotype; OR = 0.93; 95% CI, 0.35-2.44; P(interaction) = 0.18). Similar associations for all exposure metrics including average intensity were observed among GSTT1-active subjects (OR = 1.56; 95% CI, 0.79-3.10; OR = 2.77; 95% CI, 1.01-7.58; P(trend) = 0.02) but not among GSTT1 nulls (OR = 0.81; 95% CI, 0.24-2.72; OR = 1.16; 95% CI, 0.27-5.04; P(trend) = 1.00; P(interaction) = 0.34). Further evidence of heterogeneity was seen among TCE-exposed subjects with >or=1 minor allele of several CCBL1-tagging single nucleotide polymorphisms: rs2293968, rs2280841, rs2259043, and rs941960. These findings provide the strongest evidence to date that TCE exposure is associated with increased renal cancer risk, particularly among individuals carrying polymorphisms in genes that are important in the reductive metabolism of this chemical, and provides biological plausibility of the association in humans.

A reexamination of the PPAR-alpha activation mode of action as a basis for assessing human cancer risks of environmental contaminants

BACKGROUND

Diverse environmental contaminants, including the plasticizer di(2-ethylhexyl)phthalate (DEHP), are hepatocarcinogenic peroxisome proliferators in rodents. Peroxisome proliferator-activated receptor-alpha (PPAR-alpha) activation and its sequelae have been proposed to constitute a mode of action (MOA) for hepatocarcinogenesis by such agents as a sole causative factor. Further, based on a hypothesized lower sensitivity of humans to this MOA, prior reviews have concluded that rodent hepatocarcinogenesis by PPAR-alpha agonists is irrelevant to human carcinogenic risk.

DATA SYNTHESIS

Herein, we review recent studies that experimentally challenge the PPAR-alpha activation MOA hypothesis, providing evidence that DEHP is hepatocarcinogenic in PPAR-alpha-null mice and that the MOA but not hepatocarcinogenesis is evoked by PPAR-alpha activation in a transgenic mouse model. We further examine whether relative potency for PPAR-alpha activation or other steps in the MOA correlates with tumorigenic potency. In addition, for most PPAR-alpha agonists of environmental concern, available data are insufficient to characterize relative human sensitivity to this rodent MOA or to induction of hepatocarcinogenesis.

CONCLUSIONS

Our review and analyses raise questions about the hypothesized PPAR-alpha activation MOA as a sole explanation for rodent hepatocarcinogenesis by PPAR-alpha agonists and therefore its utility as a primary basis for assessing human carcinogenic risk from the diverse compounds that activate PPAR-alpha. These findings have broad implications for how MOA hypotheses are developed, tested, and applied in human health risk assessment. We discuss alternatives to the current approaches to these key aspects of mechanistic data evaluation.

Occupation and renal cell cancer in Central and Eastern Europe

OBJECTIVE

Central and Eastern Europe has among the highest rates of renal cell cancer worldwide. Few studies have been conducted in these areas to investigate the possible role of occupational exposures in renal cell cancer aetiology. The purpose of this study was to examine the association of renal cell cancer with employment in specific occupations and industries.

METHODS

From 1999 to 2003, we conducted a hospital-based case-control study in seven areas of the Czech Republic, Poland, Romania and Russia. A detailed occupational history was collected from renal cell cancer cases and controls, together with information on potential confounders. Odds ratios (ORs) and 95% CI of cancer risk were calculated for having ever been employed in selected jobs and industries, with follow-up analyses examining duration of employment.

RESULTS

A total of 992 histologically confirmed incident renal cell cancer cases and 1459 controls were included in the analysis. An increased risk of renal cell cancer was observed for workers in agricultural labour and animal husbandry (OR 1.43; 95% CI 1.05 to 1.93), particularly among women employed as general farm workers (OR 2.73; 95% CI 1.05 to 7.13). Risk gradients for agricultural work increased with longer employment. An overall increased risk of renal cell cancer was seen among architects and engineers (OR 1.89; 95% CI 1.35 to 2.65), and mechanical engineers (OR 1.71; 95% CI 1.03 to 2.84).

CONCLUSIONS

Our data suggest an association between renal cell cancer and agricultural work, particularly among female workers.

Pharmacokinetic analysis of trichloroethylene metabolism in male B6C3F1 mice: Formation and disposition of trichloroacetic acid, dichloroacetic acid, S-(1,2-dichlorovinyl)glutathione and S-(1,2-dichlorovinyl)-L-cysteine

Trichloroethylene (TCE) is a well-known carcinogen in rodents and concerns exist regarding its potential carcinogenicity in humans. Oxidative metabolites of TCE, such as dichloroacetic acid (DCA) and trichloroacetic acid (TCA), are thought to be hepatotoxic and carcinogenic in mice. The reactive products of glutathione conjugation, such as S-(1,2-dichlorovinyl)-L-cysteine (DCVC), and S-(1,2-dichlorovinyl) glutathione (DCVG), are associated with renal toxicity in rats. Recently, we developed a new analytical method for simultaneous assessment of these TCE metabolites in small-volume biological samples. Since important gaps remain in our understanding of the pharmacokinetics of TCE and its metabolites, we studied a time-course of DCA, TCA, DCVG and DCVG formation and elimination after a single oral dose of 2100 mg/kg TCE in male B6C3F1 mice. Based on systemic concentration-time data, we constructed multi-compartment models to explore the kinetic properties of the formation and disposition of TCE metabolites, as well as the source of DCA formation. We conclude that TCE-oxide is the most likely source of DCA. According to the best-fit model, bioavailability of oral TCE was approximately 74%, and the half-life and clearance of each metabolite in the mouse were as follows: DCA: 0.6 h, 0.081 ml/h; TCA: 12 h, 3.80 ml/h; DCVG: 1.4 h, 16.8 ml/h; DCVC: 1.2 h, 176 ml/h. In B6C3F1 mice, oxidative metabolites are formed in much greater quantities (approximately 3600 fold difference) than glutathione-conjugative metabolites. In addition, DCA is produced to a very limited extent relative to TCA, while most of DCVG is converted into DCVC. These pharmacokinetic studies provide insight into the kinetic properties of four key biomarkers of TCE toxicity in the mouse, representing novel information that can be used in risk assessment.

Evaluation of the role of peroxisome proliferator-activated receptor alpha (PPARalpha) in mouse liver tumor induction by trichloroethylene and metabolites

Trichloroethylene (TCE) is an industrial solvent and a widespread environmental contaminant. Induction of liver cancer in mice by TCE is thought to be mediated by two metabolites, dichloroacetate (DCA) and trichloroacetate (TCA), both of which are themselves mouse liver carcinogens. TCE, TCA, and DCA are relatively weak peroxisome proliferators (PP), a group of rodent hepatocarcinogens that activate a nuclear receptor, PP-activated receptor alpha (PPARalpha. The objective of this review is to assess the weight of evidence (WOE) that PPARalpha is or is not mechanistically involved in mouse liver tumor induction by TCE and metabolites. Based on similarities of TCE and TCA to typical PP, including dose-response characteristics showing PPARalpha-dependent responses coincident with liver tumor induction and abolishment of TCE and TCA effects in PPARalpha-null mice, the WOE supports the hypothesis that PPARalpha plays a dominant role in TCE- and TCA-induced hepatocarcinogenesis. Data indicates that the MOA for DCA tumor induction is PPARalpha-independent. Uncertainties remain regarding the genesis of the TCE-induced tumors. In contrast to the TCA-induced tumors, which have molecular features similar to those induced by typical PP, there is evidence, albeit weak, that TCE tumors arise by a mode of action (MOA) different from that of TCA tumors, based largely on dissimilarities in molecular markers found in TCE versus TCA-induced tumors. In summary, the WOE indicates that TCA-induced liver tumors arise by a PPARalpha-dependent MOA. Although the TCE MOA is likely dominated by a PPARalpha-dependent contribution from TCA, the contribution of a PPARalpha-independent MOA from DCA cannot be ruled out.

Genetic signature for human risk assessment: lessons from trichloroethylene

Trichloroethylene (TCE), an organic solvent commonly used for metal degreasing and as a chemical additive, is a significant environmental contaminant that poses health concerns in humans. The US Environmental Protection Agency (EPA) is currently revising the 2001 TCE human risk assessment draft. The next draft is expected to be ready in 2008. TCE metabolites are detectable in humans and carry varying potencies for induction of cancers in animals. Genomic mechanisms have been explored in animals and humans to link TCE to carcinogenesis. DNA analysis provides an opportunity for detection of unique genetic alterations representing a signature of TCE exposure. These alterations can arise from genotoxic and nongenotoxic pathways at multiple points throughout tumorigenesis. Although fixation of alterations may require several stages of selection and modification, the spectra can be specific to TCE. Only a fraction of these alterations eventually lead to tumor formation and some contribute to tumor progression. Genetic events in two major TCE target organs are reviewed, including the VHL gene in kidney, and the Ras gene and genome-wide hypomethylation in liver. Attempts to identify a genetic signature of TCE exposure are challenged by inconsistent findings, lack of evidence of promutagenic lesions, biological relevance of specific genomic changes, and likelihood of coexposures. For human risk assessment, genome-wide screening is useful and is possible with the development of new DNA-sequencing technologies. Genetic screening for preneoplastic and tumor tissues from high-risk population is proposed to exclude the noise of passenger mutations and genetic polymorphisms.