In vivo and in vitro mutagenicity studies of a possible carcinogen, trichloroethylene, and its two stabilizers, epichlorohydrin and 1,2-epoxybutane

Renal toxicity and carcinogenicity of trichloroethylene: key results, mechanisms, and controversies

The discussion on renal carcinogenicity of trichloroethylene addresses epidemiological, mechanistic, and metabolic aspects. After trichloroethylene exposure of rats, renal cell tumors were found increased in males, and an increased incidence of interstitial cell tumors of the testes was reported. Studies on the metabolism of trichloroethylene in rodents and in humans support the role of bioactivation reactions for the development of tumors following exposure to trichloroethylene. Epidemiological cohort studies addressing the carcinogenicity of trichloroethylene with respect to the renal or urothelial target sites have been conducted, and no clear evidence for an elevated renal or urinary tract cancer risk in trichloroethylene-exposed groups was visible in exposed populations. However, a cohort study of 169 male workers having been exposed to unusually high levels of trichloroethylene in Germany within the period between 1956 and 1975 supported a nephrocarcinogenic effect of trichloroethylene in humans. The results of this study were discussed in the literature with considerable reserve; criticism was based mainly on the choice of the study group, which had been recruited from personnel of a company in which a cluster of four renal tumors was observed previously. Hence, a further case-control study was conducted in the same region. This study confirmed the results of the previous cohort study, supporting the concept of involvement of prolonged and high-dose trichloroethylene exposures in the development of renal cell cancer. Further investigations on patients with renal cell carcinoma and with histories of high trichloroethylene exposures, on the basis of excretion of marker proteins in the urine, pointed to toxic damage to the proximal renal tubules by trichloroethylene. The hypothesis of implication of a glutathione transferase-dependent bioactivating pathway of trichloroethylene, established in experimental animals, seems at least also plausible for humans. Apparently, the occurrence of renal cell carcinomas in man follows high-dose exposures to trichloroethylene that are also accompanied by damage to tubular renal cells. Development of renal cell carcinomas has been related to mutations in the vonHippel-Lindau (VHL) tumor suppressor gene. Renal cell carcinoma tissues of persons with histories of prolonged high-dose exposure to trichloroethylene were investigated for the occurrence of mutations of the vonHippel-Lindau (VHL) tumor suppressor gene. VHL gene mutations were found in the majority of renal cell tumors associated with high-level exposure to trichloroethylene. A specific mutational hot spot at the VHL nucleotide 454 was addressed as a unique mutation pattern of the VHL tumor suppressor gene. A synopsis of all experimental, clinical, and epidemiological data suggests that reactive metabolites of trichloroethylene, with likely involvement of dichlorovinyl-cysteine (DCVC), exert a genotoxic effect on the proximal tubule of the human kidney. This constitutes a tumor-initiating process of genotoxic nature, the initial genotoxic effect apparently being linked with mutational changes in the VHL tumor suppressor gene. However, there is compelling evidence that the full development of a malignant tumor requires continued promotional stimuli. Repetitive episodes of high peak exposures to trichloroethylene over a prolonged period of time apparently led to nephrotoxicity, visualized by the excretion of tubular marker proteins in the urine. This critical process of development of tubular damage by trichloroethylene must follow a "conventional" dose-dependence, implying a practical threshold. This view is much corroborated by the fact that the occurrence of human renal cell cancer is obviously confined to cases of unusually high trichloroethylene exposures in the past, with special characteristics of very high and repetitive peak exposures. Current instruments of regulation should be adjusted to allow adequate consideration of su

Genetic toxicology of epichlorohydrin: a review

Epichlorohydrin (ECH) is one of the more commercially important aliphatic epoxides used extensively as an industrial intermediate, a laboratory reagent, and as an insecticide. It is a volatile, colourless liquid with an ethereal odour. It behaves as an alkylating agent. Reports have shown it to cause the respiratory and dermal toxicity in animals and humans. It has also been reported to be carcinogenic in experimental models. Thus, the wide-spread use of this aliphatic epoxide is of great concern in human health problem. The purpose of this paper is to critically review and update the mutagenic and clastogenic effects of ECH based on available literature.

Mechanistically-based human hazard assessment of peroxisome proliferator-induced hepatocarcinogenesis

In this review we have evaluated the relationship between peroxisome proliferation and hepatocarcinogenesis. To do so, we identified all chemicals known to produce peroxisome proliferation and selected those for which there are data (on peroxisome proliferation and hepatocarcinogenesis) which meet certain criteria chosen to facilitate comparison of these phenomena. The summarised data and definition of the methodology used has been collected in appendices. These comparisons enabled us to evaluate the relationship between these phenomena using reliable data. As there is a good correlation between them, we further explored the mechanisms of action that have been proposed (direct genotoxic activity, production of hydrogen peroxide, cell proliferation and receptor activation). The relationship between these events in other species, including humans, was also reviewed and finally an overview of the assessment of human hazard is presented in section IX. Some of the first chemicals which were shown to produce peroxisome proliferation were also hepatocarcinogens whose carcinogenicity could not be readily explained by genotoxic activity. This raised the suggestion that the unusual phenomenon of peroxisome proliferation was intricately linked to the carcinogenic activity of these agents. Three questions have exercised the attention of regulatory, industrial and academic toxicology since then; are chemicals which elicit peroxisome proliferation in the liver actually a coherent class of chemical carcinogens?; does the early biological phenomenon of peroxisome proliferation have real predictive value for and mechanistic association with rodent carcinogenesis?; and what hazard/risk do these agents pose to humans that may be exposed to them? Whether peroxisome proliferators are indeed a discrete class of rodent carcinogens would appear to be the single, most important question. If so, then the assumptions and procedures relevant to human hazard and risk assessment should be applied to the class and should be essentially generic; if not, each chemical should be considered independently. Our critical analysis of the published data for over 70 agents which have been shown to possess intrinsic ability to induce peroxisome proliferation in the livers of rodents has led to the conclusion that there exists a strong correlation between peroxisome proliferation as n early effect in the liver and hepatocarcinogenicity in chronic exposure studies. An almost perfect correlation was observed between the induction of peroxisomes in the rodent liver and the eventual appearance of tumours following chronic exposure The few exceptions to this were largely explainable (section II).(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Nondisjunction induced in mouse spermatogenesis by chloral hydrate, a metabolite of trichloroethylene

The effects of chloral hydrate (CH), an in vivo metabolite of trichloroethylene, have been evaluated by cytogenetic observations of mouse secondary spermatocytes after ip treatment with 82.7, 165.4, or 413.5 mg/kg bw. Hyper-haploid metaphases have been scored to determine whether previous observations in various nonmammalian organisms about an effect of this drug on the mitotic spindle could be confirmed in mice. At each dose, the frequencies of hyper-haploid cells have been estimated to assess the response of pachytene, preleptotene, premeiotic, and staminal gonial cells. Significant increases above the control value have been observed particularly after treatment of actively dividing gonial cells, confirming the results obtained with the same batch of the drug in a parallel collaborative investigation with Aspergillus nidulans. Thus: a) chloral hydrate has been shown to be effective in inducing nondisjunction in a mammalian system; b) a prevalent action on the mitotic spindle has been confirmed and quantified; and c) the usefulness of parallel investigations with different methods is stressed, particularly to collect information about the mechanisms of induction of nondisjunction events.

Mutagenic action of structurally related alkene oxides on Schizosaccharomyces pombe: the influence, 'in vitro', of mouse-liver metabolizing system

A series of aliphatic epoxides were tested for their ability to induce forward mutations in the yeast Schizosaccharomyces pombe. For all the compounds under study, a linear dose-response relationship was found, and the ranking of the relative specific activity was: epichlorohydrin greater than ethylene oxide greater than glycidol greater than 1,2-epoxybutane greater than 1,1,1-trichloropropylene oxide greater than propylene oxide greater than 2,3-epoxybutane. The influence of the metabolic conversion of the epoxides by the mouse-liver S9 fraction was also investigated. In such conditions, except for the 2,3-epoxybutane, the genetic activity of epoxides seems to be reduced.