Mutagenic potential of binary and complex mixtures using different enzyme induction systems

Dioxin suppresses benzo[a]pyrene-induced mutations and DNA adduct formation through cytochrome P450 1A1 induction and (±)-anti-benzo[a]pyrene-7,8-diol-9,10-epoxide inactivation in human hepatoma cells

Benzo[a]pyrene (BaP) is metabolically activated by cytochrome P450 enzymes, and forms DNA adduct leading to mutations. Cytochrome P450 1A1 plays a central role in this activation step, and this enzyme is strongly induced by chemical agents that bind to the aryl hydrocarbon receptor (AhR), which is also known as a dioxin receptor. 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD), a potent AhR ligand has not been shown to form any DNA adduct, but has a possibility to aggravate the toxicity of precarcinogenic polycyclic hydrocarbons through the induction of metabolic enzymes. We treated human hepatoma cells (HepG2) with TCDD, and subsequently exposed them to BaP to elucidate the synergistic effects on mutations. Surprisingly, mutant frequency induced by BaP at the hypoxanthine-guanine phosphribosyltransferase (HPRT) locus was decreased by pretreatment with TCDD. In correlation with decrease in the mutant frequencies, BaP-DNA adduct formation was also decreased by TCDD pretreatment. This suppressive effect of TCDD was more potent when the cells were exposed to (±)-anti-benzo[a]pyrene-7,8-diol-9,10-epoxide (BPDE), a reactive metabolic intermediate of BaP. Among the enzymes catalyzing BaP oxidation and conjugation, cytochrome P450 1A1, 1A2, 3A4 and UDP-glucuronosyltransferase 1A1 mRNAs were induced by the exposure to TCDD. In cytochrome P450 1A1-deficient murine cells and cytochrome P450 1A1-uninducible human cells, TCDD could not suppress BPDE-DNA adduct formation. Further experiments using "Tet-On" cytochrome P450 1A1-overexpressing cells and a recombinant cytochrome P450 1A1 enzyme demonstrated that this is the key enzyme involved in the biotransformation of BaP, that is, both production and inactivation of BPDE. We conclude that TCDD-induced cytochrome P450 catalyzes the metabolism of BPDE to as yet-unidentified products that are not apparently DNA-reactive, thereby reducing mutations in hepatoma cells.

Pentachlorophenol Poisoning

Despite being banned in many countries and having its use severely restricted in others, pentachlorophenol (PCP) remains an important pesticide from a toxicological perspective. It is a stable and persistent compound. In humans it is readily absorbed by ingestion and inhalation but is less well absorbed dermally. Its distribution is limited, its metabolism extensive and it is eliminated only slowly. Assessment of the toxicity of PCP is confounded by the presence of contaminants known to cause effects identical to those attributed to PCP. However, severe exposure by any route may result in an acute and occasionally fatal illness that bears all the hallmarks of being mediated by uncoupling of oxidative phosphorylation. Tachycardia, tachypnoea, sweating, altered consciousness, hyperthermia, convulsions and early onset of marked rigor (if death occurs) are the most notable features. Pulmonary oedema, intravascular haemolysis, pancreatitis, jaundice and acute renal failure have been reported. There is no antidote and no adequate data to support the use of repeat-dose oral cholestyramine, forced diuresis or urine alkalinisation as effective methods of enhancing PCP elimination in poisoned humans. Supportive care and vigorous management of hyperthermia should produce a satisfactory outcome. Chronic occupational exposure to PCP may produce a syndrome similar to acute systemic poisoning, together with conjunctivitis and irritation of the upper respiratory and oral mucosae. Long-term exposure has also been reported to result in chronic fatigue or neuropsychiatric features in combination with skin infections (including chloracne), chronic respiratory symptoms, neuralgic pains in the legs, and impaired fertility and hypothyroidism secondary to endocrine disruption. PCP is a weak mutagen but the available data for humans are insufficient to classify it more strongly than as a probable carcinogen.

Methods for the spiral Salmonella mutagenicity assay including specialized applications

An automated approach to bacterial mutagenicity testing - the spiral Salmonella assay - was developed to simplify testing and to reduce the labor and materials required to generate dose-responsive mutagenicity information. This document provides the reader with an overview of the spiral assay and a discussion of its application for examining the mutagenic potential of pure compounds, complex environmental mixtures, and interactive effects. Guidelines for performing a routine spiral assay are presented, and alternative test methods intended to overcome a variety of technical difficulties (such as restricted sample availability, sample viscosity or volatility, etc.) are recommended. Methods for the computerized analysis of data and the interpretation of results are discussed.

Mutagenicity of TCDD in Big Blue transgenic rats

The compound 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is a significant environmental contaminant resulting from such industrial processes as pulp and paper production. TCDD is a suspected human carcinogen and its ability to induce cancer in laboratory rodents is well documented. Its mechanism of tumor initiation, however, is not well understood and in vitro mutagenicity studies have yielded inconsistent results. In this study, Big Blue lacI transgenic rats were used to assess the mutagenicity of TCDD in both male and female animals. After 6 weeks of exposure to 2 microg/kg TCDD neither an increase in mutation frequency nor any change in mutation spectrum was observed in either male or female animals.