Isolation of a novel metabolizing system enriched in phase-II enzymes for short-term genotoxicity bioassays

On enzyme-based anticancer molecular dietary manipulations

Evidence from both epidemiological and experimental observations has fuelled the belief that the high consumption of fruits and vegetables rich in nutrients and phytochemicals may help prevent cancer and heart disease in humans. This concept has been drastically simplified from the dietary approaches to the use of single bioactive components both as a single supplement or in functional foods to manipulate xenobiotic metabolism. These procedures, which aim to induce mutagen/carcinogen detoxification or inhibit their bioactivation, fail to take into account the multiple and paradoxical biological outcomes of enzyme modulators that make their effects unpredictable. Here, we show that the idea that the physiological roles of specific catalysts may be easily manipulated by regular long-term administration of isolated nutrients and other chemicals derived from food plants is not viable. In contrast, we claim that the consumption of healthy diets is most likely to reduce mutagenesis and cancer risk, and that both research endeavours and dietary recommendations should be redirected away from single molecules to dietary patterns as a main strategy for public health policy.

Phenol-induced in vivo oxidative stress in skin: evidence for enhanced free radical generation, thiol oxidation, and antioxidant depletion

A variety of phenolic compounds are utilized in industry (e.g., for the production of phenol (PhOH)-formaldehyde resins, paints and lacquers, cosmetics, and pharmaceuticals). They can be toxic to skin, causing rash, dermal inflammation, contact dermatitis, depigmentation, and cancer promotion. The biochemical mechanisms for the dermal toxicity of phenolic compounds are not well understood. We hypothesized that topical PhOH exposure results in the generation of radicals, possibly via redox-cycling of phenoxyl radicals, which may be an important contributor to dermal toxicity via the stimulation of the induction and release of inflammatory mediators. To test this hypothesis, we (1) monitored in vivo the formation of PBN-spin-trapped radical adducts by ESR spectroscopy, (2) measured GSH, protein thiols, vitamin E, and total antioxidant reserves in the skin of B6C3F1 mice topically treated with PhOH, and (3) compared the responses with those produced by PhOH in mice with diminished levels of GSH. We found that dermal exposure to PhOH (3.5 mmol/kg, 100 microL on the shaved back, for 30 min) caused oxidation of GSH and protein thiols and decreased vitamin E and total antioxidant reserves in skin. The magnitude of the PhOH-induced generation of PBN-spin-trapped radical adducts in the skin of mice with diminished levels of GSH (pretreated with BCNU, an inhibitor of glutathione reductase, or BSO, an inhibitor of gamma-glutamylcysteine synthetase) was markedly higher compared to radical generation in mice treated with PhOH alone. Topical exposure to PhOH resulted in skin inflammation. Remarkably, this inflammatory response was accelerated in mice with a reduced level of GSH. Epidermal mouse cells exposed to phenolic compounds showed the induction of early inflammatory response mediators, such as prostaglandin E 2 and IL-1beta. Since dermal exposure to PhOH produced ESR-detectable PBN spin-trapped signals of lipid-derived radicals, we conclude that this PhOH-induced radical formation is involved in oxidative stress and dermal toxicity in vivo.

Evaluation of the ability of a battery of three in vitro genotoxicity tests to discriminate rodent carcinogens and non-carcinogens

One of the consequences of the low specificity of the in vitro mammalian cell genotoxicity assays reported in our previous paper [D. Kirkland, M. Aardema, L. Henderson, L. Muller, Evaluation of the ability of a battery of three in vitro genotoxicity tests to discriminate rodent carcinogens and non-carcinogens. I. Sensitivity, specificity and relative predictivity, Mutat. Res. 584 (2005) 1-256] is industry and regulatory agencies dealing with a large number of false-positive results during the safety assessment of new chemicals and drugs. Addressing positive results from in vitro genotoxicity assays to determine which are "false" requires extensive resources, including the conduct of additional animal studies. In order to reduce animal usage, and to conserve industry and regulatory agency resources, we thought it was important to raise the question as to whether the protocol requirements for a valid in vitro assay or the criteria for a positive result could be changed in order to increase specificity without a significant loss in sensitivity of these tests. We therefore analysed some results of the mouse lymphoma assay (MLA) and the chromosomal aberration (CA) test obtained for rodent carcinogens and non-carcinogens in more detail. For a number of chemicals that are positive only in either of these mammalian cell tests (i.e. negative in the Ames test) there was no correlation between rodent carcinogenicity and level of toxicity (we could not analyse this for the CA test as insufficient data were available in publications), magnitude of response or lowest effective positive concentration. On the basis of very limited in vitro and in vivo data, we could also find no correlation between the above parameters and formation of DNA adducts. Therefore, a change to the current criteria for required level of toxicity in the MLA, to limit positive calls to certain magnitudes of response, or to certain concentration ranges would not improve the specificity of the tests without significantly reducing the sensitivity. We also investigated a possible correlation between tumour profile (trans-species, trans-sex and multi-site versus single-species, single-sex and single-site) and pattern of genotoxicity results. Carcinogens showing the combination of trans-species, trans-sex and multi-site tumour profile were much more prevalent (70% more) in the group of chemicals giving positive results in all three in vitro assays than amongst those giving all negative results. However, single-species, single-sex, single-site carcinogens were not very prevalent even amongst those chemicals giving three negative results in vitro. Surprisingly, when mixed positive and negative results were compared, multi-site carcinogens were highly prevalent amongst chemicals giving only a single positive result in the battery of three in vitro tests. Finally we extended our relative predictivity (RP) calculations to combinations of positive and negative results in the genotoxicity battery. For two out of three tests positive, the RP for carcinogenicity was no higher than 1.0 and for 2/3 tests negative the RP for non-carcinogenicity was either zero (for Ames+MLA+MN) or 1.7 (for Ames+MLA+CA). Thus, all values were less than a meaningful RP of two, and indicate that it is not possible to predict outcome of the rodent carcinogenicity study when only 2/3 genotoxicity results are in agreement.

Carcinogenicity and chronic toxicity in mice and rats exposed by inhalation to para-dichlorobenzene for two years

Carcinogenicity and chronic toxicity of para-dichlorobenzene (p-DCB) were examined by exposing 50 BDF1 mice and 50 F344 rats of both sexes by inhalation to p-DCB vapor at a target concentration of 0 (control), 20, 75 or 300 ppm for 6 hr/day, 5 days/week and 2 years. Incidences of hepatocellular carcinomas, hepatoblastomas and hepatic histiocytic sarcomas in the 300 ppm-exposed male mice, and hepatocellular adenomas and carcinomas and hepatoblastomas in the 300 ppm-exposed female mice were increased. An increase in the incidences of most of those liver tumors was dose-related. No increase in tumor incidence was found in any p-DCB-exposed rat of either sex. Centrilobular hypertrophy of hepatocytes and papillary mineralization and pelvic urothelial hyperplasia of the kidney were noted in the 300 ppm-exposed male rats. Treatment- and age-related increases in incidences of the eosinophilic globules of the respiratory and olfactory epithelia in female rats and incidences of the respiratory metaplasia of the nasal gland epithelium in mice and rats and the olfactory epithelium in mice were noted. The nasal lesion was the most sensitive endpoint of chronic inhalation toxicity. Induction of the mouse hepatocarcinogenicity and lack of the rat nephrocarcinogenicity found in the present study were compared with the mouse liver tumors and the rat renal tumors reported by the NTP gavage study, and discussed in light of the estimated p-DCB uptake into the body through the inhalation and the oral administration.

Effect of inducers of DT-diaphorase on the haemolytic activity and nephrotoxicity of 2-amino-1,4-naphthoquinone in rats

Reduction of naphthoquinones by DT-diaphorase is often described as a detoxification reaction. This is true for some naphthoquinone derivatives, such as alkyl and di-alkyl naphthoquinones, but the situation with other substances, such as 2-hydroxy-1,4-naphthoquinone, is more complex. In the present study, the effect of several substances that are known to increase tissue activities of DT-diaphorase on the toxicity of 2-amino-1,4-naphthoquinone has been investigated. Like 2-hydroxy-1,4-naphthoquinone, the 2-amino-derivative was found to cause both haemolytic anaemia and renal tubular necrosis in rats. Again like 2-hydroxy-1,4-naphthoquinone, the severity of the haemolysis induced by the 2-amino derivative was increased in animals pre-treated with inducers of DT-diaphorase, but the degree of nephrotoxicity was decreased. With these substances, therefore, DT-diaphorase both activates and detoxifies the quinone, depending on the target organ. It is not possible to generalize with regard to the effects of modulation of tissue levels of DT-diaphorase on naphthoquinone toxicity in vivo, since this may change not only the severity of the toxic effects, but also the target organ specificity. In evaluating the possible therapeutic applications of such compounds, the possibility of toxic effects upon the blood and kidney must be borne in mind. In man, renal damage by compounds such as 2-hydroxy- and 2-amino-1,4-naphthoquinone may be a particular problem, because of the low level of DT-diaphorase in human liver.

Pitfalls of enzyme-based molecular anticancer dietary manipulations: food for thought

Dietary approaches to cancer chemoprevention increasingly have focused on single nutrients or phytochemicals to stimulate one or another enzymatic metabolizing system. These procedures, which aim to boost carcinogen detoxification or inhibit carcinogen bioactivation, fail to take into account the multiple and paradoxical biological outcomes of enzyme modulators that make their effects unpredictable. Here, we critically examine the scientific and medical evidence for the idea that the physiological roles of specific enzymes may be manipulated by regular, long-term administration of isolated nutrients and other chemicals derived from food plants. Instead, we argue that consumption of healthful diets is most likely to reduce mutagenesis and cancer risk, and that research efforts and dietary recommendations should be redirected away from single nutrients to emphasize the improvement of dietary patterns as a principal strategy for public health policy.

Effects of modulation of tissue activities of DT-diaphorase on the toxicity of 2,3-dimethyl-1,4-naphthoquinone to rats

The enzyme DT-diaphorase mediates the two-electron reduction of quinones to hydroquinones. It has previously been shown that the toxicity of 2-methyl-1,4-naphthoquinone to rats is decreased by pre-treatment of the animals with compounds that increase tissue levels of this enzyme. In contrast, the severity of the haemolytic anaemia induced in rats by 2-hydroxy-1,4-naphthoquinone was increased in animals with high levels of DT-diaphorase. In the present experiments, the effect of alterations in tissue diaphorase activities on the toxicity of a third naphthoquinone derivative, 2,3-dimethyl-1,4-naphthoquinone, has been investigated. This compound induced severe haemolysis and slight renal tubular necrosis in control rats. Pre-treatment of the animals with BHA, a potent inducer of DT-diaphorase, diminished the severity of the haemolysis induced by this compound and abolished its nephrotoxicity. Pre-treatment with dicoumarol, an inhibitor of this enzyme, caused only a slight increase in the haemolysis induced by 2,3-dimethyl-1,4-naphthoquinone, but provoked a massive increase in its nephrotoxicity. Modulation of DT-diaphorase activity in animals may therefore not only alter the severity of naphthoquinone toxicity, but also cause pronounced changes in the site of toxic action of these substances. The factors that may control whether induction of DT-diaphorase in animals will decrease or increase naphthoquinone toxicity are discussed.

Redox cycling of phenol induces oxidative stress in human epidermal keratinocytes

A variety of phenolic compounds are utilized for industrial production of phenol-formaldehyde resins, paints, lacquers, cosmetics, and pharmaceuticals. Skin exposure to industrial phenolics is known to cause skin rash, dermal inflammation, contact dermatitis, leucoderma, and cancer promotion. The biochemical mechanisms of cytotoxicity of phenolic compounds are not well understood. We hypothesized that enzymatic one-electron oxidation of phenolic compounds resulting in the generation of phenoxyl radicals may be an important contributor to the cytotoxic effects. Phenoxyl radicals are readily reduced by thiols, ascorbate, and other intracellular reductants (e.g., NADH, NADPH) regenerating the parent phenolic compound. Hence, phenolic compounds may undergo enzymatically driven redox-cycling thus causing oxidative stress. To test the hypothesis, we analyzed endogenous thiols, lipid peroxidation, and total antioxidant reserves in normal human keratinocytes exposed to phenol. Using a newly developed cis-parinaric acid-based procedure to assay site-specific oxidative stress in membrane phospholipids, we found that phenol at subtoxic concentrations (50 microM) caused oxidation of phosphatidylcholine and phosphatidylethanolamine (but not of phosphatidylserine) in keratinocytes. Phenol did not induce peroxidation of phospholipids in liposomes prepared from keratinocyte lipids labeled by cis-parinaric acid. Measurements with ThioGlo-1 showed that phenol depleted glutathione but did not produce thiyl radicals as evidenced by our high-performance liquid chromatography measurements of GS.-5, 5-dimethyl1pyrroline N-oxide nitrone. Additionally, phenol caused a significant decrease of protein SH groups. Luminol-enhanced chemiluminescence assay demonstrated a significant decrease in total antioxidant reserves of keratinocytes exposed to phenol. Incubation of ascorbate-preloaded keratinocytes with phenol produced an electron paramagnetic resonance-detectable signal of ascorbate radicals, suggesting that redox-cycling of one-electron oxidation products of phenol, its phenoxyl radicals, is involved in the oxidative effects. As no cytotoxicity was observed in keratinocytes exposed to 50 microM or 500 microM phenol, we conclude that phenol at subtoxic concentrations causes significant oxidative stress.

Effect of inducers of DT-diaphorase on the toxicity of 2-methyl- and 2-hydroxy-1,4-naphthoquinone to rats

It has previously been shown that rats pre-treated with butylated hydroxyanisole (BHA), a well-known inducer of the enzyme DT-diaphorase, are protected against the toxic effects of 2-methyl-1,4-naphthoquinone but are made more susceptible to the harmful action of 2-hydroxy-1,4-naphthoquinone. In the present experiments, the effects of BHA have been compared with those of other inducers of DT-diaphorase. Rats were dosed with BHA, butylated hydroxytoluene (BHT), ethoxyquin (EQ), dimethyl fumarate (DMF) or disulfiram (DIS) and then challenged with a toxic dose of the naphthoquinones. All the inducers protected against the haemolytic anaemia induced by 2-methyl-1,4-naphthoquinone in rats, with BHA, BHT and EQ being somewhat more effective than DMF and DIS. A similar order of activity was recorded in the relative ability of these substances to increase hepatic activities of DT-diaphorase, consistent with a role for this enzyme in facilitating conjugation and excretion of this naphthoquinone. In contrast, all the compounds increased the haemolytic activity of 2-hydroxy-1,4-naphthoquinone. DMF and DIS were significantly more effective in this regard than BHA, BHT and EQ. DMF and DIS also caused a much greater increase in levels of DT-diaphorase in the intestine, suggesting that 2-hydroxy-1,4-naphthoquinone is activated by this enzyme in the gut. BHA, BHT and EQ had no effect on the nephrotoxicity of 2-hydroxy-1,4-naphthoquinone, but the severity of the renal lesions was decreased in rats pre-treated with DMF and DIS. The results of the present experiments show that modulation of tissue levels of DT-diaphorase may not only alter the severity of naphthoquinone toxicity in vivo, but may also change the relative toxicity of these substances to different target organs.

Effect of butylated hydroxyanisole on the toxicity of 2-hydroxy-1,4-naphthoquinone to rats

It has previously been shown that rats pre-treated with butylated hydroxyanisole (BHA), a well-known inducer of the enzyme DT-diaphorase, are protected against the harmful effects of 2-methyl-1,4-naphthoquinone. This is consistent with a role for diaphorase in the detoxification of this quinone, but it is not known if increased tissue levels of this enzyme give protection against other naphthoquinone derivatives. In the present study, rats were dosed with BHA and then challenged with a toxic dose of 2-hydroxy-1,4-naphthoquinone, a substance that causes haemolytic anaemia and renal damage in vivo. Pre-treatment with BHA had no effect upon the nephrotoxicity of 2-hydroxy-1,4-naphthoquinone, but the severity of the haemolysis induced by this compound was increased in the animals given BHA. DT-Diaphorase is known to promote the redox cycling of 2-hydroxy-1,4-naphthoquinone in vitro, with concomitant formation of 'active oxygen' species. The results of the present experiment suggest that activation of 2-hydroxy-1,4-naphthoquinone by DT-diaphorase may also occur in vivo and show that increased tissue levels of DT-diaphorase are not always associated with naphthoquinone detoxification.

An evaluation of the carcinogenic hazard of 1,4-dichlorobenzene based on internationally recognized criteria

1,4-Dichlorobenzene (1,4-DCB) was shown to induce the formation of male rat renal tubule tumors and male and female mouse liver tumors when administered in a chronic bioassay. Since the original carcinogenicity findings, an extensive body of mechanistic information has been developed to elucidate the mode of action by which 1,4-DCB induces these effects and to evaluate the human relevance of the observed animal tumors. In addition, some regulatory and authoritative bodies (U.S. EPA and IARC) have developed rigorous scientific criteria for the amount and types of evidence needed to establish that a material causes kidney toxicity and tumors in male rats through a specific mechanism, alpha-2u-globulin nephropathy. This paper summarizes the mechanistic data developed for 1,4-DCB, which affords an understanding of the lack of human relevance of the male rat renal tubule tumors and mouse liver tumors; assesses that mechanistic data set utilizing the defined set of evaluation criteria formulated by U.S. EPA and IARC for alpha-2u-globulin nephropathy; and discusses the predictive power of mechanistic data developed to elucidate the mode of action of 1,4-DCB in inducing mouse liver tumors. Finally, there is a discussion of how some, but not all, regulatory and authoritative bodies have incorporated this substantial mechanistic data set for 1, 4-DCB into their cancer hazard evaluations and concluded that 1, 4-DCB presents little, if any, cancer hazard to humans.