The formation of mutagenic derivatives of benzo[a]pyrene by peroxidising fatty acids

Environmental Air Pollutants Affecting Skin Functions with Systemic Implications

The increase in air pollution worldwide represents an environmental risk factor that has global implications for the health of humans worldwide. The skin of billions of people is exposed to a mixture of harmful air pollutants, which can affect its physiology and are responsible for cutaneous damage. Some polycyclic aromatic hydrocarbons are photoreactive and could be activated by ultraviolet radiation (UVR). Therefore, such UVR exposure would enhance their deleterious effects on the skin. Air pollution also affects vitamin D synthesis by reducing UVB radiation, which is essential for the production of vitamin D3, tachysterol, and lumisterol derivatives. Ambient air pollutants, photopollution, blue-light pollution, and cigarette smoke compromise cutaneous structural integrity, can interact with human skin microbiota, and trigger or exacerbate a range of skin diseases through various mechanisms. Generally, air pollution elicits an oxidative stress response on the skin that can activate the inflammatory responses. The aryl hydrocarbon receptor (AhR) can act as a sensor for small molecules such as air pollutants and plays a crucial role in responses to (photo)pollution. On the other hand, targeting AhR/Nrf2 is emerging as a novel treatment option for air pollutants that induce or exacerbate inflammatory skin diseases. Therefore, AhR with downstream regulatory pathways would represent a crucial signaling system regulating the skin phenotype in a Yin and Yang fashion defined by the chemical nature of the activating factor and the cellular and tissue context.

Benzo[a]pyrene—Environmental Occurrence, Human Exposure, and Mechanisms of Toxicity

Benzo[a]pyrene (B[a]P) is the main representative of polycyclic aromatic hydrocarbons (PAHs), and has been repeatedly found in the air, surface water, soil, and sediments. It is present in cigarette smoke as well as in food products, especially when smoked and grilled. Human exposure to B[a]P is therefore common. Research shows growing evidence concerning toxic effects induced by this substance. This xenobiotic is metabolized by cytochrome P450 (CYP P450) to carcinogenic metabolite: 7β,8α-dihydroxy-9α,10α-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (BPDE), which creates DNA adducts, causing mutations and malignant transformations. Moreover, B[a]P is epigenotoxic, neurotoxic, and teratogenic, and exhibits pro-oxidative potential and causes impairment of animals’ fertility. CYP P450 is strongly involved in B[a]P metabolism, and it is simultaneously expressed as a result of the association of B[a]P with aromatic hydrocarbon receptor (AhR), playing an essential role in the cancerogenic potential of various xenobiotics. In turn, polymorphism of CYP P450 genes determines the sensitivity of the organism to B[a]P. It was also observed that B[a]P facilitates the multiplication of viruses, which may be an additional problem with the widespread COVID-19 pandemic. Based on publications mainly from 2017 to 2022, this paper presents the occurrence of B[a]P in various environmental compartments and human surroundings, shows the exposure of humans to this substance, and describes the mechanisms of its toxicity.

Molecular Mechanisms of Action of Selected Substances Involved in the Reduction of Benzo[a]pyrene-Induced Oxidative Stress

Benzo[a]pyrene (BaP) is a polycyclic aromatic hydrocarbon (PAH) primarily formed by burning of fossil fuels, wood and other organic materials. BaP as group I carcinogen shows mutagenic and carcinogenic effects. One of the important mechanisms of action of (BaP) is its free radical activity, the effect of which is the induction of oxidative stress in cells. BaP induces oxidative stress through the production of reactive oxygen species (ROS), disturbances of the activity of antioxidant enzymes, and the reduction of the level of non-enzymatic antioxidants as well as of cytokine production. Chemical compounds, such as vitamin E, curcumin, quercetin, catechin, cyanidin, kuromanin, berberine, resveratrol, baicalein, myricetin, catechin hydrate, hesperetin, rhaponticin, as well as taurine, atorvastatin, diallyl sulfide, and those contained in green and white tea, lower the oxidative stress induced by BaP. They regulate the expression of genes involved in oxidative stress and inflammation, and therefore can reduce the level of ROS. These substances remove ROS and reduce the level of lipid and protein peroxidation, reduce formation of adducts with DNA, increase the level of enzymatic and non-enzymatic antioxidants and reduce the level of pro-inflammatory cytokines. BaP can undergo chemical modification in the living cells, which results in more reactive metabolites formation. Some of protective substances have the ability to reduce BaP metabolism, and in particular reduce the induction of cytochrome (CYP P450), which reduces the formation of oxidative metabolites, and therefore decreases ROS production. The aim of this review is to discuss the oxidative properties of BaP, and describe protective activities of selected chemicals against BaP activity based on of the latest publications.

Fatty acid radical formation in rats administered oxidized fatty acids: in vivo spin trapping investigation

We report in vivo evidence for fatty acid-derived free radical metabolite formation in bile of rats dosed with spin traps and oxidized polyunsaturated fatty acids (PUFA). When rats were dosed with the spin trap 5,5-dimethyl-1-pyrroline N-oxide (DMPO) and oxidized PUFA, the DMPO thiyl radical adduct was formed due to a reaction between oxidized PUFA and/or its metabolites with biliary glutathione. In vitro experiments were performed to determine the conditions necessary for the elimination of radical adduct formation by ex vivo reactions. Fatty acid-derived radical adducts of alpha-(4-pyridyl-1-oxide)-N-tert-butylnitrone (4-POBN) were detected in vivo in bile samples collected into a mixture of iodoacetamide, desferrioxamine, and glutathione peroxidase. Upon the administration of oxidized 13C-algal fatty acids and 4-POBN, the EPR spectrum of the radical adducts present in the bile exhibited hyperfine couplings due to 13C. Our data demonstrate that the carbon-centered radical adducts observed in in vivo experiments are unequivocally derived from oxidized PUFA. This in vivo evidence for PUFA-derived free radical formation supports the proposal that processes involving free radicals may be the molecular basis for the previously described cytotoxicity of dietary oxidized PUFA.

Routes of formation and toxic consequences of lipid oxidation products in foods

Lipid oxidation in foods is initiated by free radical and/or singlet oxygen mechanisms which generate a series of autocatalytic free radical reactions. These autoxidation reactions lead to the breakdown of lipid and to the formation of a wide array of oxidation products. The nature and proportion of these products can vary widely between foods and depend on the composition of the food as well as numerous environmental factors. The toxicological significance of lipid oxidation in foods is complicated by interactions of secondary lipid oxidation products with other food components. These interactions could either form complexes that limit the bioavailability of lipid breakdown products or can lead to the formation of toxic products derived from non-lipid sources. A lack of gross pathological consequences has generally been observed in animals fed oxidized fats. On the other hand, secondary products of lipid autoxidation can be absorbed and may cause an increase in oxidative stress and deleterious changes in lipoprotein and platelet metabolism. The presence of reactive lipid oxidation products in foods needs more systematic research in terms of complexities of food component interactions and the metabolic processing of these compounds.

Lack of modifying effects of linolic acid hydroperoxides and their secondary oxidative products on combined 7,12-dimethylbenz[a]anthracene and 1,2-dimethylhydrazine-initiated mammary gland, ear duct and colon carcinogenesis in female Sprague-Dawley rats

Effects of hydroperoxides, autoxidation products of linolic acid (HPO) and secondary oxidative products of HPO (SOP) (5% each in diet) were examined in female Sprague-Dawley rats. HPO and SOP administration was carried out during or subsequent to two injections of dimethylhydrazine (DMH) (40 mg/kg body wt s.c.), and a single i.g. dose of 7,12-dimethylbenz[a]anthracene (DMBA) (50 mg/kg body wt). No significant differences in the incidences of tumors in the mammary gland, colon, ear duct and hematopoietic system associated with HPO or SOP treatment were evident, during or after carcinogen exposure. The present results therefore indicate that the environmental contaminants, HPO and SOP, lack any potential for modification of mammary gland or colon carcinogenesis under the conditions of the investigation.

Mutagenic activation of xenobiotics by plant enzymes

Genetic evidence has indicated that plants can activate certain xenobiotics to mutagens, but biochemical evidence is as yet scarce. Nevertheless, plant microsomal enzymes and peroxidases have been shown to form reactive intermediates, the best studied examples being 2-aminofluorene, benzo[a]pyrene and pentachlorophenol. The latter two xenobiotics are converted to quinoid derivatives which are, in principle, able to redox cycle and generate active oxygen species. In analogy to results obtained in mammalian systems, covalent binding of reactive intermediates to DNA as well as fragmentation of DNA, are proposed as major mechanisms of action of mutagenic plant metabolites.

A role for dietary lipids and antioxidants in the activation of carcinogens

The ways in which dietary polyunsaturated fats and antioxidants affect the balance between activation and detoxification of environmental precarcinogens is discussed, with particular reference to the polycyclic aromatic hydrocarbon benzo(a)pyrene. The structure and composition of membranes and their susceptibility to peroxidation is dependent on the polyunsaturated fatty acid (PUFA) content of the cell and its antioxidant status, both of which are determined to a large degree by dietary intake of these compounds. An increase in the PUFA content of membranes stimulates the oxidation of precarcinogens to reactive intermediates by affecting the configuration and induction of membrane-bound enzymes (e.g., the mixed-function oxidase system and epoxide hydratase); providing increased availability of substrates (hydroperoxides) for peroxidases that cooxidise carcinogens (e.g., prostaglandin synthetase and P-450 peroxidase); and increasing the likelihood of direct activation reactions between peroxyl radicals and precarcinogens. Antioxidants, on the other hand, protect against lipid peroxidation, scavenge oxygen-derived free radicals and reactive carcinogenic species. In addition some synthetic antioxidants exert specific effects on enzymes, which results in increased detoxification and reduced rates of activation. The balance between dietary polyunsaturated fats, antioxidants and the initiation of carcinogenesis is discussed in relation to animal models of chemical carcinogenesis and the epidemiology of human cancer.

Co-oxidation of xenobiotics: lipid peroxyl derivatives as mediators of metabolism

Systems which carry out peroxyl-dependent oxidations can serve as activation systems for carcinogenic compounds. Some function via classical peroxidase reactions in which an enzyme-derived oxidant performs the electron abstraction from or oxygen donation to the oxidizable substrate. This mechanism applies to the peroxidative activation of aromatic amines and of the phenolic compound diethylstilbestrol. These classical peroxidase reactions may be initiated by hydrogen peroxide or by organic peroxides, including lipid hydroperoxides. A different mechanism is involved in the oxygenation of polycyclic aromatic hydrocarbons and of aflatoxin B1. In these cases the oxidant is a peroxyl radical, and the reaction occurs by the direct, non-enzymatic interaction of the peroxyl radical and the oxidizable substrate. Most peroxyl radicals in biological systems are lipid-derived. The key reaction which distinguishes the peroxyl radical-dependent oxidations from the classical peroxidase reactions is the ability of the former to epoxidize activated carbon-carbon double bonds. The epoxidation of benzo[a]pyrene derivatives has been studied extensively in subcellular and whole cell and tissue systems, and is discussed as a model for this class of reaction. Determining the generality of this activation path and its role in vivo present the major questions to be answered in regard to the importance of these reactions in chemical carcinogenesis.

Quinone formation from carcinogenic benzo[a]pyrene mediated by lipid peroxidation in phosphatidylcholine liposomes

The behavior of benzo[a]pyrene (B[a]P) during peroxidation of phosphatidylcholine (PC) liposomes initiated by an azo compound was investigated to examine the mechanism of quinone formation from carcinogenic B[a]P mediated by nonenzymatic lipid peroxidation occurring in vivo. B[a]P had a retarding effect on the peroxidation of polyunsaturated fatty acid moiety of PC. The major oxidation products which accumulated in the peroxidized liposomes were B[a]P 1,6-, 3,6-, and 6,12-quinone. Antioxidants acting as scavengers of chain-propagating lipid peroxy radicals effectively prevented not only lipid peroxidation but also B[a]P oxidation in the liposomal suspension. PC hydroperoxides, the primary products of PC oxidation, did not react with B[a]P in the absence of the azo compound, indicating that lipid peroxy radicals, not lipid hydroperoxides, are responsible for the formation of these quinones. The experiments using 18O2 gas and 18O-labeled methyl linoleate hydroperoxides demonstrated that B[a]P quinones are formed by incorporating molecular oxygen and their origin is partly due to the lipid peroxy radical. The mechanism proposed for the formation of B[a]P quinones mediated by peroxidation of membrane lipids involves a direct attack of the lipid peroxy radical on B[a]P and subsequent autocatalytic oxidation. Weak carcinogenic and noncarcinogenic pentacyclic aromatic hydrocarbons showed little reactivity to the lipid peroxy radical in the liposomes. Thus, the facility of the peroxidative attack on B[a]P may be related to the powerful carcinogenic activity of this substance.

Enhanced lipid peroxidation and lysosomal enzyme activity in the lungs of rats with prolonged pulmonary deposition of crocidolite asbestos

The interaction of UICC crocidolite asbestos with biological membranes in vivo was studied in rats after a single intratracheal dose of a suspension of 20 mg of fibres per rat. Development of lung fibrosis (increased level of hydroxyproline, a collagen index together with corresponding pathomorphological alteration) confirmed the penetration of crocidolite fibres into the lungs in the course of seven months exposure. The pulmonary deposition of crocidolite affected the lysosomal membranes of lung cells as manifested by (1) enhanced lipid peroxidation with (2) stimulation (release) of activity of beta-glucuronidase and cathepsin D. Enhanced lipid peroxidation and activity of beta-glucuronidase may contribute to the delayed, carcinogenic effects of crocidolite asbestos.

Inhibition of glutathione S-transferase P type-positive foci development by linolic acid hydroperoxides and their secondary oxidative products in a rat in vivo mid-term test for liver carcinogens

The effects of linolic acid hydroperoxides (product A) and secondary oxidative products of product A (product B), were examined in an in vivo mid-term test for hepatocarcinogens or hepatopromoters in rats. The number of placental type of glutathione S-transferase (GST-P)-positive foci of the liver was significantly reduced in rats given diethylnitrosamine (DEN) followed by products A (4.64 +/- 1.09, P less than 0.05) or B (3.62 +/- 1.65, P less than 0.01) as compared to the controls given carcinogen alone (6.31 +/- 2.82). The area of GST-P positive foci was also significantly reduced in rats given DEN followed by product B (0.30 +/- 0.21, P less than 0.05) as compared to the controls (0.47 +/- 0.23). These results suggest that linolic acid hydroperoxides or their secondary oxidative products are not hepatocarcinogens and rather may possess inhibitory potential for liver carcinogenesis.

The oxidation of benzo[a]pyrene mediated by lipid peroxidation in irradiated synthetic diets

The effect of gamma-irradiation (1000-4000 Gy) on the formation of lipid peroxides and on the oxidation of the environmental carcinogen benzo[a]pyrene (BP) has been studied in mixtures of starch/fat and BP which were used as models for natural foods. When mixtures containing polyunsaturated fats (mackerel oil and cod-liver oil which contain relatively large proportions of C20:5 and C22:6) were exposed to gamma-irradiation, large concentrations of lipid peroxide were formed and a concomitant oxidation of BP to mutagenic and toxic BP quinones took place. The rate of BP oxidation was closely related to the extent of peroxidation of the lipids in the starch mixtures and was dependent on the dose of gamma-irradiation and the presence of air. Mackerel oil also underwent peroxidation during the storage of both irradiated and unirradiated starch/mackerel oil/BP mixtures and this resulted in a significant oxidation of the BP present in these samples. Antioxidants such as vitamin E and BHA inhibited both lipid peroxidation and BP oxidation resulting from gamma-irradiation. These results demonstrate that the species generated during the peroxidation of unsaturated fats in foodstuffs can react with polycyclic aromatic hydrocarbons such as BP and convert them into active mutagenic and toxic products. This has important toxicological implications, particularly as the consumption of polyunsaturated fat in the Western world is increasing and gamma-irradiation may soon be widely used for food sterilization.

The dependence of the rate of BP metabolism in the rat small intestinal mucosa on the composition of the dietary fat

We studied the effects that dietary fat has on the capacity of preparations of rat small intestinal mucosal cells to metabolize benzo[a]pyrene (BP) in vitro and on the composition of fatty acids in the endoplasmic reticulum of the intestinal mucosa. When rats were fed diets containing different types of fat, there were significant changes in the incorporation of fatty acids into the endoplasmic reticulum of the mucosal cells of the small intestine: the proportions of polyunsaturated fatty acids in the endoplasmic reticulum reflected the amounts of these fatty acids in the dietary fat. The rate of BP oxidation in the intestinal mucosa was dependent on the amount and composition of the dietary fat, but the range and proportions of the metabolites produced were not affected. Dietary C18:2 was particularly important in elevating the rate of BP oxidation, but dietary C20:5 and C22:6 also effectively increased the rate of BP oxidation. The rate of BP oxidation in the small intestine of rats fed different diets was positively correlated with the proportion of polyunsaturated fatty acids in the endoplasmic reticulum of the mucosal cells.

Cancer and free radicals

It is now clear that free radical intermediates often are involved in the activation of many types of procarcinogens and promutagens to their active forms as well as in the binding of these activated species to DNA. In this chapter, a general introduction to free radical chemistry is presented, with some discussion of radical lifetimes and reactivities. Potential biological targets of radical attack include lipids, proteins, and nucleic acids, and the reactions of all three of these target molecules with radicals are discussed. Finally, the evidence linking free radical reactions with chemical carcinogenesis is reviewed. A mechanistic scheme that divides the mechanisms for activating procarcinogens into 5 types is suggested; of these, 3 types of mechanisms involve free radicals, either in the activation of the carcinogen or in its binding to DNA or both. It also is suggested that a "reverse binding" can occur in which radicals produced on the DNA backbone attack and bond to unactivated substrates, rather than activated substrates (such as radicals) attacking unactivated DNA. It is known that systems that produce superoxide can lead to the production of hydroxyl radicals and that these HO. radicals form radical sites on DNA; thus, reverse binding could occur when any species that can add to a free radical is in the vicinity of the radical-damaged DNA.