Free radical formation in murine skin treated with tumour promoting organic peroxides

Electron paramagnetic resonance and spin trapping to detect free radicals from allergenic hydroperoxides in contact with the skin: from the molecule to the tissue

A major research topic consists of revealing the contribution of radical-mediated reactions in dermatological diseases related to xenobiotic-induced stress, to succeed risk assessment procedures protecting producers and consumers. Allergic contact dermatitis is the clinically relevant consequence of skin sensitization, one of the most critical occupational and environmental health issues related to xenobiotics exposure. The first key event identified for the skin sensitization process to a chemical is its aptitude to react with epidermal proteins and form antigenic structures that will further trigger the immune response. Many chemical sensitizers are suspected to react through mechanisms involving radical intermediates. This review focuses on recent progress we have accomplished over the last few years studying radical intermediates derived from skin sensitizing chemicals by electron paramagnetic resonance in combination with the spin trapping technique. Our work is carried out "from the molecule", performing studies in solution, "to the tissue", by the development of a methodology on a reconstructed human epidermis model, very close in terms of histology and metabolic/enzymatic activity to real human epidermis, that can be used as suitable biological tissue model. The benefits are to test chemicals under conditions close to human use and real-life sensitization exposures and benefit from the 3D microenvironment. This article is protected by copyright. All rights reserved.

Mechanistic Insights on Skin Sensitization to Linalool Hydroperoxides: EPR Evidence on Radical Intermediates Formation in Reconstructed Human Epidermis and 13C NMR Reactivity Studies with Thiol Residues

Linalool is one of the most commonly used fragrance terpenes in consumer products. While pure linalool is considered as non-allergenic because it has a very low skin sensitization potential, its autoxidation on air leads to allylic hydroperoxides that have been shown to be major skin sensitizers. These hydroperoxides have the potential to form antigens via radical mechanisms. In order to obtain in-depth insights of such reactivity, we first investigated the formation of free radicals derived from linalool hydroperoxides in situ in a model of human reconstructed epidermis by electron paramagnetic resonance combined with spin trapping. The formation of carbon- and oxygen-centered radical species derived from the hydroperoxides was especially evidenced in an epidermis model, mimicking human skin and thus closer to what may happen in vivo. To further investigate these results, we synthesized linalool hydroperoxides containing a ¹³C-substitution at positions precursor of carbon radicals to elucidate if one of these positions could react with cysteine, its thiol chemical function being one of the most labile groups prone to react through radical mechanisms. Reactions were followed by mono- and bidimensional ¹³C NMR. We validated that carbon radicals derived from allylic hydrogen abstraction by the initially formed alkoxyl radical and/or from its β-scission can alter directly the lateral chain of cysteine forming adducts via radical processes. Such results provide an original vision on the mechanisms likely involved in the reaction with thiol groups that might be present in the skin environment. Consequently, the present findings are a step ahead toward the understanding of protein binding processes to allergenic allylic hydroperoxides of linalool through the involvement of free radical species and thus of their sensitizing potential.

Reactivity and Product Analysis of a Pair of Cumyloxyl and tert-Butoxyl Radicals Generated in Photolysis of tert-Butyl Cumyl Peroxide

Alkoxyl radicals play important roles in various fields of chemistry. Understanding their reactivity is essential to applying their chemistry for industrial and biological purposes. Hydrogen-atom transfer and C-C β-scission reactions have been reported from alkoxyl radicals. The ratios of these two processes were investigated using cumyloxyl (CumO^•) and tert-butoxyl radicals (t-BuO^•), respectively. However, the products generated from the pair of radicals have not been investigated in detail. In this study, CumO^• and t-BuO^• were simultaneously generated from the photolysis of tert-butyl cumyl peroxide to understand the chemical behavior of the pair of radicals by analyzing the products and their distribution. Electron paramagnetic resonance and/or transient absorption spectroscopy analyses of radicals, including CumO^• and t-BuO^•, provide more information about the radicals generated during the photolysis of tert-butyl cumyl peroxide. Furthermore, the photoproducts of (3-(tert-butylperoxy)pentane-3-yl)benzene demonstrated that the ether products were formed in in-cage reactions. The triplet-sensitized reaction induced by acetophenone, which is produced from CumO^•, clarified that the spin state did not affect the product distribution.

Formation of methyl radicals derived from cumene hydroperoxide in reconstructed human epidermis: an EPR spin trapping confirmation by using 13C-substitution

Dermal exposure to cumene hydroperoxide (CumOOH) during manufacturing processes is a toxicological issue for the industry. Its genotoxicity, mutagenic action, ability to promote skin tumour, capacity to induce epidermal hyperplasia, and aptitude to induce allergic and irritant skin contact dermatitis are well known. These toxic effects appear to be mediated through the activation to free radical species such as hydroxyl, alkoxyl, and alkyl radicals characterised basically by electron paramagnetic resonance (EPR) and spin-trapping (ST) techniques. To be a skin sensitiser CumOOH needs to covalently bind to skin proteins in the epidermis to form the antigenic entity triggering the immunotoxic reaction. Cleavage of the O-O bond allows formation of unstable CumO^•/CumOO^• radicals rearranging to longer half-life specific carbon-centred radicals R^• proposed to be at the origin of the antigen formation. Nevertheless, it is not still clear which R^• is precisely formed in the epidermis and thus involved in the sensitisation process. The aim of this work was to elucidate in conditions closer to real-life sensitisation which specific R^• are formed in a 3D reconstructed human epidermis (RHE) model by using ¹³C-substituted CumOOH at carbon positions precursors of potentially reactive radicals and EPR-ST. We demonstrated that most probably methyl radicals derived from β-scission of CumO^• radicals occur in RHE through a one-electron reductive pathway suggesting that these could be involved in the antigen formation inducing skin sensitisation. We also describe a coupling between nitroxide radicals and β position ¹³C atoms that could be of an added value to the very few examples existing for the coupling of radicals with ¹³C atoms.

Dermal Exposure to Cumene Hydroperoxide: Assessing Its Toxic Relevance and Oxidant Potential

Cumene hydroperoxide (CHP) is a high production volume chemical that is used to generate phenol and acetone. Dermal exposure to CHP was hypothesized to result in systemic tissue toxicity, production of free radicals, and consequent decrease in plasma antioxidant levels. To evaluate the hypothesis and characterize the toxicity of CHP, male and female B6C3F1/N mice and F344/N rats were exposed to varying doses of CHP applied topically for 14 or 90 days. No significant changes in survival or body weight of mice and rats were observed following 14 days of exposure. However, 90 days of CHP exposure at the high dose (12 mg/kg) triggered a significant decrease (-15%) in the body weight of the male rat group only. Irritation of the skin was observed at the site of application and was characterized by inflammation and epidermal hyperplasia. In treated animals, histology of liver tissue, free radical generation, and antioxidant levels in blood plasma were not significantly changed as compared to the corresponding controls. Consistent with the lack of systemic damage, no increase in micronucleated erythrocytes was seen in peripheral blood. In conclusion, topical CHP application caused skin damage only at the application site and did not cause systemic tissue impairment.

Determination of Wavelength-Specific UV Protection Factors of Sunscreens in Intact Skin by EPR Measurement of UV-Induced Reactive Melanin Radical

There remains an unmet need for skin tissue-based assays for the measurement of the UVA protection and efficacy of sunscreens. Here we describe development of a novel electron paramagnetic resonance assay that uses the photogeneration of reactive melanin radical as a measure of UV light penetration to melanocytes in situ in skin. We have used areas of focal melanocytic hyperplasia in the skin of Monodelphis domestica to model the human nevus. We show that we are able to use this assay to determine the monochromatic protection factors (mPF) of research and commercial sunscreens at specific narrow wavebands of UVB, UVA and blue visible light. Both commercial sunscreens, a sun protection factor (SPF) 4 and an SPF 30 product, had mPFs in the UVB range that correlated well with their claimed SPF. However, their mPF in the UVA ranges were only about one-third of claimed SPF. This technique can be used to design and assay sunscreens with optimally balanced UVA and UVB protection.

UV causation of melanoma in Xiphophorus is dominated by melanin photosensitized oxidant production

Controversy continues both as to which wavelengths of sunlight cause melanoma and the mechanisms by which these different wavelengths act. Direct absorption of UVB by DNA is central in albino animal models, but melanin-pigmented models have shown major contributions by wavelengths longer than UVB that are thought to be mediated by photosensitized oxidant production. The only model for which the action spectrum of melanoma causation is known is a genetically melanoma-susceptible specific cross of Xiphophorus fish. We used electron paramagnetic resonance to quantitatively detect the UV induction of reactive melanin radicals in situ in the melanin-containing cells in the skin of this model and derived the action spectrum for melanin-photosensitized oxidant production (Phi(ox)). This action spectrum was identical to that for melanoma induction (Phi(mel)). These results confirm the hypothesis that melanin-photosensitized radical production is the major causative step of melanoma in this model and demonstrate that the wavelengths and mechanisms of melanoma causation in different models are dependent on the presence of melanin. This approach should be applicable to humans, thus providing an accurate surrogate for Phi(mel) for prevention studies.

Cytotoxicity and genotoxicity induced by the photochemical alkoxyl radical source N-tert-butoxypyridine-2-thione in L5178Y mouse lymphoma cells under UVA irradiation

The cell-damaging effects of N-tert-butoxypyridine-2-thione (tBuOPT), which generates alkoxyl and thiyl radicals on photolysis, have been investigated in L5178Y mouse lymphoma cells. The UVA irradiation of 2.5 microM tBuOPT inhibits strongly cell growth and cell viability, causes pronounced membrane damage, and induces micronuclei. Without irradiation, tBuOPT does not cause any cell damage at 2.5 microM concentration. The phototoxicity of tBuOPT is effectively inhibited by the radical scavenger glutathione, while the photogenotoxicity (micronuclei induction) is not affected by this strong hydrogen-atom donor. Thus, for the cytotoxicity and genotoxicity different reactive species seems to be responsible. The cytotoxicity is presumably caused by oxyl radicals, which are derived from tert-butoxyl radicals generated by photocleavage of tBuOPT, while in the genotoxicity the less reactive pyridyl-2-thiyl radicals appear to play a role. These results demonstrate that N-alkoxypyridinethiones are useful photochemical sources of oxyl and thiyl radicals to elucidate biological effects caused by these free radicals.

Quantitative Measurements of Oxidative Stress in Mouse Skin Induced by X-Ray Irradiation

To find efficient methods to evaluate oxidative stress in mouse skin caused by X-ray irradiation, several markers and methodologies were examined. Hairless mice were irradiated with 50 Gy X-rays and skin homogenates or skin strips were prepared. Lipid peroxidation was measured using the skin homogenate as the level of thiobarbituric acid reactive substances. The level of lipid peroxidation increased with time after irradiation and was twice that of the control at 78 h. ESR spectra of skin strips showed a clear signal for the ascorbyl radical, which increased with time after irradiation in a manner similar to that of lipid peroxidation. To measure levels of glutathione (GSH) and its oxidized forms (GSSG) simultaneously, two HPLC methods, sample derivatization with 1-fluoro-2,4-dinitrobenzene and detection with a UV detector (method A) and no derivatization and detection with an electrochemical detector (method B), were compared and the latter was found to be better. No significant change was observed within 24 h after irradiation in the levels of GSH and GSSG measured by method B. The GSH/GSSG ratio may be a less sensitive parameter for the evaluation of acute oxidative stress caused by X-ray irradiation in the skin. Monitoring the ascorbyl radical seems to be a good way to evaluate oxidative stress in skin in vivo.

Free radical scavenging abilities of flavonoids as mechanism of protection against mutagenicity induced by tert-butyl hydroperoxide or cumene hydroperoxide in Salmonella typhimurium TA102

Mutagenicity induced by tert-butyl hydroperoxide (BHP) or cumene hydroperoxide (CHP) in Salmonella typhimurium TA102 was effectively reduced by flavonols with 3',4'-hydroxyl groups such as fisetin, quercetin, rutin, isoquercitrin, hyperoxide, myricetin, myricitrin, robinetin, and to a lesser extent also by morin and kaempferol (ID50=0.25-1.05 micromol per plate). With the exception of isorhamnetin, rhamnetin, morin, and kaempferol, closely similar results were obtained with both peroxides. Hydrogenation of the double bond between carbons 2 and 3 (dihydroquercetin, dihydrorobinetin) as well as the additional elimination of the carbonyl function at carbon 4 (catechins) resulted in a loss of antimutagenicity with the notable exception of catechin itself. Again, all flavones and flavanones tested were inactive except luteolin, luteolin-7-glucoside, diosmetin, and naringenin. The typical radical scavenger butylated hydroxytoluene also showed strong antimutagenicity against CHP (ID50=5.4 micromol per plate) and BHP (ID50=11.4 micromol per plate). Other lipophilic scavengers such as alpha-tocopherol and N,N'-diphenyl-1,4-phenylenediamine exerted only moderate effects, the hydrophilic scavenger trolox was inactive. The metal chelating agent 1,10-phenanthroline strongly reduced mutagenicities induced by CHP and BHP (ID50=2.75 and 2.5 micromol per plate) at low concentrations but induced mutagenic activities at higher concentrations. The iron chelator deferoxamine mesylate, however, was less effective in both respects. The copper chelator neocuproine effectively inhibited mutagenicity induced by BHP (ID50=39.7 micromol per plate) and CHP (ID50=25.9 micrommol per plate), the iron chelator 2,2'-dipyridyl was less potent (ID50=6.25 mmol per plate against BHP, 0.42 mmol per plate against CHP). In the absence of BHP and CHP, yet not in the presence of these hydroperoxides, quercetin, rutin, catechin, epicatechin, and naringenin induced strong mutagenic activities in S. typhimurium TA102. Radical scavenging activities of flavonoids against peroxyl radicals generated from 2,2'-azo-bis(2-amidinopropane)dihydrochloride (AAPH) as measured in the haemolysis test, confirmed that in general flavonoids with di- or trihydroxy hydroxyl functions especially in positions 3', 4', 5' are effective radical scavengers. In this test system, however, luteolin was the most potent compound, followed by epicatechin and eriodictyol. Again, isorhamnetin was a potent inhibitor of lysis of red blood cells despite the presence of a 3'-OCH3 function. Radical scavenging activities of flavonoids against the stable radical 2,2-diphenyl-1-picrylhydrazyl (DPPH) in principle obeyed the rules outlined above. Flavanones, tamarixetin, and rhamnetin, however, were only weakly active against DPPH, while isorhamnetin was again a potent compound. From these results we conclude that in the Salmonella/reversion assay with strain TA102 antimutagenic activities of flavonoids against the peroxide mutagens CHP and BHP are mainly caused by radical scavenging effects.

In vitro and in vivo assessment of the irritation potential of different spin traps in human skin

No clinical data are available on the acute cutaneous toxicity of spin traps which are frequently used in combination with the electron paramagnetic resonance (EPR) technique for detection of free radicals and reactive oxygen/nitrogen species. The purpose of this study was to evaluate the acute dermatotoxicity of the following spin traps in human skin: C-phenyl-N-tert.-butyl nitrone (PBN), C-(4-pyridinyl-N-oxide)-N-tert.-butylnitrone (POBN), 5, 5-dimethyl-l-pyrroline-N-oxide(DMPO), 5 diethoxyphosphoryl-5-methyl-l-pyrroline-N-oxide (DEPMPO), diethyldithiocarbamate (DDC) and N-methyl-D-glucamine dithiocarbamate (MGD). The corrosivity of the test substances was first assessed in human skin in vitro by measurement of transcutaneous electrical resistance (TER). In this assay all spin traps were non-corrosive at 500 mM concentration. Subsequently cutaneous irritation of the spin traps was determined at different concentrations (50, 250 and 500 mM) in human skin according to a routine four h human patch test in comparison to the standardized irritant sodium laurylsulfate (SLS, 20%). The response was evaluated clinically as well as by a biophysical method analyzing transepidermal water loss (TEWL). PBN and DEPMPO caused a transient and weak inflammatory reaction at 500 mM in four of 17 and in two of 17 volunteers, respectively. DMPO, POBN, DDC, MGID, and the iron complexes of DDC and MGD were clinically non-irritant at all concentrations tested and no delayed-acute inflammatory reactions were observed. However, the TEWL values were significantly increased by all spin traps except DMPO at 500 mM, indicating disturbed epidermal barrier function. We conclude that the spin traps investigated have a low potential to cause acute skin toxicity and may be used safely for in vivo EPR studies in human skin.

DNA methylation by tert-butyl hydroperoxide-iron (II): a role for the transition metal ion in the production of DNA base adducts

Metabolic degradation of both endogenous and exogenous peroxides is associated with the etiology of several diseases including cancer. Tert-butyl hydroperoxide (TBHP) has been widely employed as a model compound to study the cytotoxicity and promoting effects of organic peroxides. Recently, we reported that incubations of TBHP with iron (II) and calf thymus DNA led to generation of high yields of methyl radicals and to DNA methylation. Interestingly, DNA was methylated to products expected from both free radical and ionic mechanisms such as 8-methylguanine (C8-MeGua) and 7-methylguanine (N7-MeGua), respectively. To elucidate the mechanisms by which methyl radicals can produce different types of DNA adducts, we examined the effects of transition metal ions (iron (II), iron (III) and copper (I)) and metal ion chelators (ethylenediamine-N,N,N",N"-tetraacetate (EDTA) and desferal) on the nature and the yields of the DNA adducts produced during TBHP decomposition. The results led us to propose that a direct methyl radical attack on DNA guanine residues produces C8-MeGua whereas N7-MeGua and 3-methyladenine (N3-MeAde) are likely to be produced by attack of nucleophilic DNA centers on methyl radical generated in situ by the assistance of transition metal ions bound to DNA.

Promotion by Helicobacter hepaticus-induced hepatitis of hepatic tumors initiated by N-nitrosodimethylamine in male A/JCr mice

A new murine Helicobacter species, Helicobacter hepaticus, infects the livers of mice, causing a progressive chronic active hepatitis culminating in hepatocellular tumors. To examine the role of chronic H. hepaticus infection in carcinogenesis, H. hepaticus-infected male infant mice of A/JCr strain were given a single i.p. dose of N-nitrosodimethylamine (NDMA). Noninfected A/J mice similarly treated with NDMA served as controls. The effect of hepatitis induced by H. hepaticus was studied for 64 wk. At 31-36 wk, the incidence of hepatocellular adenomas in infected mice was significantly higher than in noninfected mice (82 vs 52%; p = 0.05). The multiplicity of hepatocellular tumors was also significantly higher in infected mice compared to noninfected mice (3.2 +/- 0.09 vs 0.09 +/- 0.2; p = 0.03). At 51-64 wk, many (10/18) infected mice developed hepatocellular carcinomas while only 2 of 19 control mice developed such tumors (p = 0.005). Overexpression of cyclin D was observed in hepatocytes as well as adenomas induced by NDMA in H. hepaticus-infected mice, suggesting its role in inflammation, abnormal cell growth, and early neoplasia. High molecular weight keratins were highly expressed in hyperplastic oval cells in hepatitis and in liver tumors in mice with hepatitis, establishing a reliable marker for oval cells in formalin-fixed, paraffin-embedded tissue. Thus, chronic H. hepaticus infection significantly stimulated cyclin D expression, accelerated the development of liver tumors, increased the multiplicity of such lesions, and enhanced the progression of benign to malignant tumors.

EPR detection of free radicals in UV-irradiated skin: mouse versus human

Ultraviolet radiation produces free radicals in Skh-1 mouse skin, contributing to photoaging and carcinogenesis. If a mouse model is a general indicator of free radical processes in human skin photobiology, then radical production observed in mouse and human skin should be directly comparative. In this work we show that UV radiation (lambda > 300 nm, 14 microW/cm2 UVB; 3.5 mW/cm2 UVA) increases the ascorbate free radical (Asc.-) electron paramagnetic resonance (EPR) signal in both Skh-1 mouse skin (45%) and human facial skin biopsies (340%). Visible light (lambda > 400 nm; 0.23 mW/cm2 UVA) also increased the Asc.- signal in human skin samples (45%) but did not increase baseline mouse Asc.-, indicating that human skin is more susceptible to free radical formation and that a chromophore for visible light may be present. Using EPR spin-trapping techniques, UV radiation produced spin adducts consistent with trapping lipid alkyl radicals in mouse skin (alpha-[4-pyridyl 1-oxide]-N-tert-butyl nitrone/alkyl radical adduct; aN = 15.56 G and aH = 2.70 G) and lipid alkoxyl radicals in human skin (5,5-dimethylpyrroline-1-oxide/alkoxyl radical adduct; aN = 14.54 G and aH = 16.0 G). Topical application of the iron chelator Desferal to human skin significantly decreases these radicals (approximately 50%), indicating a role for iron in lipid peroxidation; Desferal has previously been shown to decrease radical production in mouse skin. This work supports the use of the Skh-1 mouse as a predictive tool for free radical formation in human skin. These results provide the first direct evidence for UV radiation-induced free radical formation at near physiological temperatures in human skin and suggest that iron chelators may be useful as photoprotective agents.

EPR studies on the effects of complexation of heme by hemopexin upon its reactions with organic peroxides

Hemopexin, a heme-binding serum glycoprotein, is thought to play an important role in the prevention of oxidative damage that may be catalysed by free heme. Through the use of EPR techniques, the generation of free radicals from organic hydroperoxides by heme and heme-hemopexin complexes, and the concomitant formation of high oxidation-state iron species has been studied; these species are implicated as causative agents in processes such as cardiovascular disease and carcinogenesis. From the rates of production of these species from both n-alkyl and branched hydroperoxides, it has been inferred that the dramatic reduction in the yield of oxidising species generated by heme upon its complexation with hemopexin arises from steric hindrance of the access of hydroperoxide to the bound heme.