An immunoblot assay for cysteine oxidation by reactive oxygen species allows detection of novel thioprotective efficacy of black tea extracts

INTRODUCTION

While cysteine thiol groups help to maintain the redox status of many proteins, they can be very susceptible to damaging oxidants. Despite broad interest in their antioxidant properties, whether tea polyphenols protect against protein thiol damage of this kind is unclear. This study sought to develop a simple immunoassay for use in screening tea extracts and other antioxidants for thioprotective efficacy at protein thiol groups.

METHODS

Fresh aqueous extracts were prepared from commercially sourced green, white, black and red teas. Traut's reagent (2-iminothiolane) was used to prepare surface-thiolated bovine serum albumin for use as assay substrate in the protein oxidation assay. Oxidative damage was induced during a 15 min incubation with hydrogen peroxide (H₂O₂) in the presence of tea extracts and reference antioxidants. The substrate protein was then derivatised with dimedone before samples were loaded onto a nitrocellulose membrane housed within a Slot-Blot apparatus. After blocking nonspecific protein binding a commercially available antibody was used to detect dimedone-labelled groups.

RESULTS

While the total phenol content of tea extracts typically correlated with their activity in lipid peroxidation and galvinoxyl radical-trapping assays, the former did not fully predict their abilities to suppress H₂O₂-induced cysteine oxidation, with black tea extracts displaying greater activity than the other teas and an apparent ability to reverse pre-existing cysteine oxidation. Among the model antioxidants tested, quercetin displayed a heightened ability to suppress cysteine oxidation.

DISCUSSION

This slot-blot immunoassay is a convenient method that facilitates standardised comparisons between the thioprotective properties of structurally- and constitutively-diverse antioxidants.

Carbonyl scavengers as pharmacotherapies in degenerative disease: Hydralazine repurposing and challenges in clinical translation

During cellular metabolism, spontaneous oxidative damage to unsaturated lipids generates many electrophilic carbonyl compounds that readily attack cell macromolecules, forming adducts that are potential drivers of tissue dysfunction. Since such damage is heightened in many degenerative conditions, researchers have assessed the efficacy of nucleophilic carbonyl-trapping drugs in animal models of such disorders, anticipating that they will protect tissues by intercepting toxic lipid-derived electrophiles (LDEs) within cells. This Commentary explores recent animal evidence for carbonyl scavenger efficacy in two disparate yet significant conditions known to involve LDE production, namely spinal cord injury (SCI) and alcoholic liver disease (ALD). Primary emphasis is placed on studies that utilised hydralazine, a clinically-approved "broad-spectrum" scavenger known to trap multiple LDEs. In addition to reviewing recent studies of hydralazine efficacy in animal SCI and ALD models, the Commentary reviews new insights concerning novel lifespan- and healthspan-extending properties of hydralazine obtained during studies in model invertebrate organisms, since the mechanisms involved seem of likely benefit during the treatment of degenerative disease. Finally, noting that human translation of the histoprotective properties of hydralazine have been limited, the final section of the Commentary will address two obstacles that hamper clinical translation of LDE-trapping therapies while also suggesting potential strategies for overcoming these problems.

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Influenza A infection attenuates relaxation responses of mouse tracheal smooth muscle evoked by acrolein

The airway epithelium is an important source of relaxant mediators, and damage to the epithelium caused by respiratory tract viruses may contribute to airway hyperreactivity. The aim of this study was to determine whether influenza A-induced epithelial damage would modulate relaxation responses evoked by acrolein, a toxic and prevalent component of smoke. Male BALB/c mice were inoculated intranasally with influenza A/PR-8/34 (VIRUS-infected) or allantoic fluid (SHAM-infected). On day 4 post-inoculation, isometric tension recording studies were conducted on carbachol pre-contracted tracheal segments isolated from VIRUS and SHAM mice. Relaxant responses to acrolein (30 μM) were markedly smaller in VIRUS segments compared to SHAM segments (2 ± 1% relaxation vs. 28 ± 5%, n=14, p<0.01). Similarly, relaxation responses of VIRUS segments to the neuropeptide substance P (SP) were greatly attenuated (1 ± 1% vs. 47 ± 6% evoked by 1 nM SP, n=14, p<0.001). Consistent with epithelial damage, PGE2 release in response to both acrolein and SP were reduced in VIRUS segments (>35% reduction, n=6, p<0.01), as determined using ELISA. In contrast, exogenous PGE2 was 2.8-fold more potent in VIRUS relative to SHAM segments (-log EC50 7.82 ± 0.14 vs. 7.38 ± 0.05, n=7, p<0.01) whilst responses of VIRUS segments to the β-adrenoceptor agonist isoprenaline were similar to SHAM segments. In conclusion, relaxation responses evoked by acrolein were profoundly diminished in tracheal segments isolated from influenza A-infected mice. The mechanism through which influenza A infection attenuates this response appears to involve reduced production of PGE2 in response to SP due to epithelial cell loss, and may provide insight into the airway hyperreactivity observed with influenza A infection.

Airborne acrolein induces keratin-8 (Ser-73) hyperphosphorylation and intermediate filament ubiquitination in bronchiolar lung cell monolayers

The combustion product acrolein is a key mediator of pulmonary edema in victims of smoke inhalation injury. Since studying acrolein toxicity in conventional in vitro systems is complicated by reactivity with nucleophilic culture media constituents, we explored an exposure system which delivers airborne acrolein directly to lung cell monolayers at the air-liquid interface. Calu-3 lung adenocarcinoma cells were maintained on membrane inserts such that the basal surface was bathed in nucleophile-free media while the upper surface remained in contact with acrolein-containing air. Cells were exposed to airborne acrolein for 30 min before they were allowed to recover in fresh media, with cell sampling at defined time points to allow evaluation of toxicity and protein damage. After prior exposure to acrolein, cell ATP levels remained close to controls for 4h but decreased in an exposure-dependent manner by 24h. A loss of transepithelial electrical resistance and increased permeability to fluorescein isothiocyanate-labeled dextran preceded ATP loss. Use of antibody arrays to monitor protein expression in exposed monolayers identified strong upregulation of phospho-keratin-8 (Ser(73)) as an early consequence of acrolein exposure. These changes were accompanied by chemical damage to keratin-8 and other intermediate filament family members, while acrolein exposure also resulted in controlled ubiquitination of high mass proteins within the intermediate filament extracts. These findings confirm the usefulness of systems allowing delivery of airborne smoke constituents to lung cell monolayers during studies of the molecular basis for acute smoke intoxication injury.

Acrolein relaxes mouse isolated tracheal smooth muscle via a TRPA1-dependent mechanism

Airway sensory C-fibres express TRPA1 channels which have recently been identified as a key chemosensory receptor for acrolein, a toxic and highly prevalent component of smoke. TRPA1 likely plays an intermediary role in eliciting a range of effects induced by acrolein including cough and neurogenic inflammation. Currently, it is not known whether acrolein-induced activation of TRPA1 produces other airway effects including relaxation of mouse airway smooth muscle. The aims of this study were to examine the effects of acrolein on airway smooth muscle tone in mouse isolated trachea, and to characterise the cellular and molecular mechanisms underpinning the effects of acrolein. Isometric tension recording studies were conducted on mouse isolated tracheal segments to characterise acrolein-induced relaxation responses. Release of the relaxant PGE₂ was measured by EIA to examine its role in the response. Use of selective antagonists/inhibitors permitted pharmacological characterisation of the molecular and cellular mechanisms underlying this relaxation response. Acrolein induced dose-dependent relaxation responses in mouse isolated tracheal segments. Importantly, these relaxation responses were significantly inhibited by the TRPA1 antagonists AP-18 and HC-030031, an NK₁ receptor antagonist RP-67580, and the EP₂ receptor antagonist PF-04418948, whilst completely abolished by the non-selective COX inhibitor indomethacin. Acrolein also caused rapid PGE₂ release which was suppressed by HC-030031. In summary, acrolein induced a novel bronchodilator response in mouse airways. Pharmacologic studies indicate that acrolein-induced relaxation likely involves interplay between TRPA1-expressing airway sensory C-fibres, NK₁ receptor-expressing epithelial cells, and EP₂-receptor expressing airway smooth muscle cells.

Chaperone heat shock protein 90 mobilization and hydralazine cytoprotection against acrolein-induced carbonyl stress

Toxic carbonyls such as acrolein participate in many degenerative diseases. Although the nucleophilic vasodilatory drug hydralazine readily traps such species under "test-tube" conditions, whether these reactions adequately explain its efficacy in animal models of carbonyl-mediated disease is uncertain. We have previously shown that hydralazine attacks carbonyl-adducted proteins in an "adduct-trapping" reaction that appears to take precedence over direct "carbonyl-sequestering" reactions, but how this reaction conferred cytoprotection was unclear. This study explored the possibility that by increasing the bulkiness of acrolein-adducted proteins, adduct-trapping might alter the redistribution of chaperones to damaged cytoskeletal proteins that are known targets for acrolein. Using A549 lung adenocarcinoma cells, the levels of chaperones heat shock protein (Hsp) 40, Hsp70, Hsp90, and Hsp110 were measured in intermediate filament extracts prepared after a 3-h exposure to acrolein. Exposure to acrolein alone modestly increased the levels of all four chaperones. Coexposure to hydralazine (10-100 μM) strongly suppressed cell ATP loss while producing strong adduct-trapping in intermediate filaments. Most strikingly, hydralazine selectively boosted the levels of cytoskeletal-associated Hsp90, including a high-mass species that was sensitive to the Hsp90 inhibitor 17-N-allylamino-17-demethoxygeldanamycin. Biochemical fractionation of acrolein- and hydralazine-treated cells revealed that hydralazine likely promoted Hsp90 migration from cytosol into other subcellular compartments. A role for Hsp90 mobilization in cytoprotection was confirmed by the finding that brief heat shock treatment suppressed acute acrolein toxicity in A549 cells. Taken together, these findings suggest that by increasing the steric bulk of carbonyl-adducted proteins, adduct-trapping drugs trigger the intracellular mobilization of the key molecular chaperone Hsp90.

Intermediate filament carbonylation during acute acrolein toxicity in A549 lung cells: functional consequences, chaperone redistribution, and protection by bisulfite

Extensive protein carbonylation accompanies cellular exposure to acrolein, a ubiquitous smoke constituent implicated in life-threatening pulmonary edema in fire victims, a condition involving rapid erosion of the "watertight" properties of respiratory epithelium. Since the identities of lung epithelial proteins that sustain carbonylation by acrolein are unknown, we sought to identify significant targets in subcellular fractions from A549 cells after 30 min exposure to either subtoxic or acutely toxic acrolein concentrations (60 or 360 fmol acrolein/cell). The lower concentration mainly modified cytosolic proteins while the higher concentration also damaged nuclear, membrane, and cytoskeletal proteins. The multifunctional intermediate filament proteins vimentin, keratin-18, keratin-7 and keratin-8, were conspicuous targets. Consistent with their mechanical functions, a loss of cellular adhesive strength accompanied adduction of the two most abundant intermediate filaments in A549 cells, keratins-8 and -18. Acrolein also elicited redistribution of several chaperones (Hsp40, -70, -90, and -110) to intermediate filament fractions, suggesting chaperone-mediated autophagy contributes to the triage of acrolein-adducted proteins. The carbonyl scavenger bisulfite suppressed acrolein toxicity, intermediate filament adduction, vimentin cross-linking, Hsp90 redistribution, and loss of cellular adhesive strength, while also suppressing vimentin hyperphosphorylation. These novel observations identify intermediate filaments as key targets for the reactive smoke constituent acrolein.

Toxicity of smoke extracts towards A549 lung cells: role of acrolein and suppression by carbonyl scavengers

The noxious 3-carbon electrophile acrolein forms on combustion of diverse organic matter including synthetic polymers such as polyethylene. While known to play a key role in smoke inhalation injury (SII), the molecular basis for the pulmonary toxicity of high dose acrolein-containing smoke is unclear. As a result, drug interventions in SII are poorly directed against pathogenetic smoke toxicants such as acrolein. The first aim of this study was to confirm a role for acrolein in the acute toxicity of smoke extracts towards A549 lung cells by monitoring adduction of known acrolein targets and the expression of acrolein-inducible genes. A second aim was to evaluate carbonyl scavengers for their abilities to protect cell targets and block smoke extract toxicity. Extracts were prepared by bubbling smoke released by smouldering polyethylene through a buffered saline-trap. Acrolein levels in the extracts were estimated via HPLC after derivatisation with 2,4-dinitrophenylhydrazine. Extracts were highly toxic towards A549 cells, eliciting greater ATP depletion than an equivalent concentration of acrolein alone. The toxicity was accompanied by pronounced carbonylation of several cytoskeletal targets, namely vimentin and keratins-7, -8 and -18. Western blotting revealed that polyethylene combustion products also upregulated several acrolein-responsive protein markers, including GADD45beta, NQO1, HMOX, Hsp70, Nur77 and Egr1. Several carbonyl scavengers (bisulfite, d-penicillamine, hydralazine and 1-hydrazinoisoquinoline) strongly attenuated smoke extract toxicity, with bisulfite suppressing both the adduction and cross-linking of intermediate filament targets. Bisulfite also suppressed the cytotoxicity of smoke extracts when detected using real-time monitoring of cellular impedance. These findings confirm a key role for acrolein in smoke cytotoxicity and suggest drugs that block acrolein toxicity deserve further investigation as possible interventions against SII.

Carbonyl-scavenging drugs & protection against carbonyl stress-associated cell injury

This mini-review highlights the chemical and cytoprotective properties of various hydralazine analogues that block the induction of cell death by acrolein, a highly toxic contributor to "carbonyl stress" during a diverse range of human diseases. Recent work on the action of hydralazine against various carbonyl-mediated pathologies is also reviewed.

Potentialities and pitfalls accompanying chemico-pharmacological strategies against endogenous electrophiles and carbonyl stress

The use of powerful analytical technologies to detect endogenous carbonyls formed as byproducts of oxidative cell injury has revealed that these species contribute to many human diseases. As electrophiles, they are attacked by reactive centers in cell macromolecules to form adducts, the levels of which serve as useful biomarkers of oxidative cell injury. Because the pathobiological significance of such damage is often unclear, the possibility of using low molecular weight drugs as exploratory sacrificial nucleophiles to intercept reactive carbonyls within cells and tissues is appealing. This perspective highlights the potential benefits of using carbonyl scavengers to evaluate the significance of endogenous carbonyls in particular diseases but also canvasses a number of challenges confronting this therapeutic strategy. Chief among the latter is the task of confirming that carbonyl sequestration underlies any suppression of disease symptoms elicited by these multipotent reagents, an issue needing clarification if these compounds are to command consideration as drug interventions in humans. Other problems include adverse consequences of reactions between carbonyl scavengers and important endogenous carbonyls (e.g., neurotoxicity due to pyridoxal depletion), as well as the potential for drugs to form ternary complexes with carbonylated cell proteins, raising the prospect of immunotoxicological outcomes. Strategies for moving carbonyl sequestering reagents from the laboratory bench to a clinical testing environment are discussed within the context of the search for new treatments for spinal cord injury, one of the most debilitating medical conditions sustainable by humans. This condition seems an appropriate test case for assessing carbonyl sequestering drugs given growing evidence for noxious carbonyls in the wave of neuronal cell death that follows traumatic injury to the spinal cord.

Intermolecular protein cross-linking during acrolein toxicity: efficacy of carbonyl scavengers as inhibitors of heat shock protein-90 cross-linking in A549 cells

The smoke-borne electrophile acrolein reacts extensively with proteins, forming carbonyl-retaining Michael adducts that may be attacked by adjacent protein nucleophiles to form cross-links. Because little information is available concerning the extent of intermolecular protein cross-linking during acrolein toxicity in cells, we used an antibody against a known target for toxic carbonyls, the chaperone protein Hsp90, to detect the formation of high-mass protein complexes in acrolein-exposed A549 cells. A 3 h exposure to acrolein (0 to 200 microM) resulted in concentration-dependent formation of a single high-mass band (approx. 180 kDa). This species was detected in cells exposed to just 50 microM acrolein, a concentration that did not elicit acute cell death as assessed by measurements of cell ATP levels. The formation of cross-linked Hsp90 coincided with a rapid loss of carbonyl adducts within cells that had been subjected to a brief "pulse" exposure to a subtoxic concentration of acrolein, suggesting Michael adducts are short-lived within cells due in part to consumption during reactions with protein nucleophiles. Cross-linked Hsp90 persisted following an overnight recovery incubation, suggesting the cellular ability to repair or degrade these species is limited. Two known carbonyl scavengers, hydralazine and bisulfite, strongly protected against the ATP depletion accompanying acrolein exposure, but only the latter suppressed protein adduction and Hsp90 cross-linking. As previously shown for hydralazine, mass spectrometry studies using a model peptide indicated that bisulfite traps carbonyl groups possessed by Michael addition adducts, and such adduct-trapping reactivity appeared to contribute to the blockade of Hsp90 cross-linking in acrolein-preloaded cells. Collectively, these findings establish that formation of stable intermolecular protein cross-links accompanies exposure to acrolein. Future clarification of the chemistry underlying this damage may provide novel biomarkers of acrolein exposure.

Carboxylic Acid Drug-Induced DNA Nicking in HEK293 Cells Expressing Human UDP-Glucuronosyltransferases: Role of Acyl Glucuronide Metabolites and Glycation Pathways

Glucuronidation of carboxylic-acid-containing drugs can yield reactive acyl (ester-linked) glucuronide metabolites that are able to modify endogenous macromolecules. Previous research has shown that several carboxylic acid drugs are genotoxic in isolated mouse hepatocytes, and that DNA damage is prevented by the glucuronidation inhibitor, borneol. Whether these species induce comparable genetic damage in human cells is unknown. In this study, we investigated the mechanisms of clofibric acid-induced genotoxicity in HEK293 cells expressing the human UDP-glucuronosyltransferases UGT1A3, UGT1A9, or UGT2B7, and screened three other carboxylic acid drugs for UGT-dependent genotoxicity. DNA damage was detected using the alkaline version of the comet assay. HEK293 cells were incubated for 18 h with vehicle (2.5 mM NaOH), 0.1-2.5 mM clofibric acid or 0.1-1.0 mM benoxaprofen, bezafibrate, or probenecid. To identify mechanisms underlying any observed genotoxicity, we treated UGT2B7 transfectants with 10 mM aminoguanidine, 1 mM borneol, or 2 mM desferrioxamine mesylate prior to co-incubation with 1 mM clofibric acid for 18 h. Compared to vehicle, clofibric acid, benoxaprofen, and probenecid produced significant DNA damage in all three UGT-transfected HEK293 cell lines, detectable from the lowest concentration tested. Bezafibrate caused DNA damage only at higher concentrations (1.0 mM) in UGT2B7- and UGT1A9-, but not UGT1A3-transfected cells. No drug-induced DNA damage was detected in untransfected cells, consistent with the limited glucuronidation capacity of these cells. The glycation/glycoxidation inhibitor aminoguanidine and the glucuronidation inhibitor borneol significantly decreased clofibric-acid-mediated DNA damage in UGT2B7 transfected cells by 73.5 and 94.8%, respectively. The inhibitor of transition-metal-catalyzed oxidation, desferrioxamine mesylate, had no significant effect on DNA damage. This study demonstrates the substrate-dependent role of human UGTs in the bioactivation of carboxylic acid drugs to genotoxic acyl glucuronide metabolites that are able to damage nuclear DNA via glycation and/or glycoxidation mechanisms.

Inactivation of glutathione peroxidase activity contributes to UV-induced squamous cell carcinoma formation

Cutaneous squamous cell carcinomas (CSCC) are a common malignancy of keratinocytes that arise in sites of the skin exposed to excessive UV radiation. In the present study, we show that human SCC cell lines, preneoplastic solar keratoses (SK), and CSCC are associated with perturbations in glutathione peroxidase (GPX) activity and peroxide levels. Specifically, we found that two of three SKs and four of five CSCCs, in vivo, were associated with decreased GPX activity and all SKs and CSCCs were associated with an elevated peroxide burden. Given the association of decreased GPX activity with CSCC, we examined the basis for the GPX deficiency in the CSCCs. Our data indicated that GPX was inactivated by a post-translational mechanism and that GPX could be inactivated by increases in intracellular peroxide levels. We next tested whether the decreased peroxidase activity coupled with an elevated peroxidative burden might contribute to CSCC formation in vivo. This was tested in Gpx1(-/-) and Gpx2(-/-) mice exposed to solar-simulated UV radiation. These studies showed that Gpx2 deficiency predisposed mice to UV-induced CSCC formation. These results suggest that inactivation of GPX2 in human skin may be an early event in UV-induced SCC formation.

Michael addition of acrolein to lysinyl and N-terminal residues of a model peptide: targets for cytoprotective hydrazino drugs

The antihypertensive drug hydralazine blocks acrolein-mediated toxicity by trapping both free aldehyde- and acrolein-adducted proteins, with the latter property more closely related to cytoprotection in cellular models. Here we report the identification of products from 'protein adduct-trapping' reactions using electrospray ionisation mass spectrometry (ESI-MS). Reaction of a 13-residue peptide containing a single lysine with acrolein for 30 min generated ions corresponding to mono- and bis-Michael-adducted peptides. An ion corresponding to a cyclic species formed from bis-adducted lysine was conspicuous at later times (60, 180 min). Tandem mass spectrometric (MS/MS) analysis revealed Michael adduction also occurred on the N-terminus, with a novel N-terminal (3-formyl-3,4-dehydropiperidino) species formed on this residue. Addition of hydralazine to acrolein-adducted peptides generated a diverse range of hydrazones that were also characterised by MS/MS analysis. The results confirm that mass spectrometry is a powerful tool for characterising the reactions of noxious electrophiles with biological macromolecules.

Bioactivation of Carboxylic Acid Compounds by UDP-Glucuronosyltransferases to DNA-Damaging Intermediates: Role of Glycoxidation and Oxidative Stress in Genotoxicity

Nonenzymatic modification of proteins by acyl glucuronides is well documented; however, little is known about their potential to damage DNA. We have previously reported that clofibric acid undergoes glucuronidation-dependent bioactivation to DNA-damaging species in cultured mouse hepatocytes. The aim of this study was to investigate the mechanisms underlying such DNA damage, and to screen chemically diverse carboxylic acid drugs for their DNA-damaging potential in glucuronidation proficient murine hepatocytes. Cells were incubated with each aglycone for 18 h, followed by assessment of compound cytotoxicity using the MTT assay and evaluation of DNA damage using the Comet assay. Relative cytotoxic potencies were ketoprofen > diclofenac, benoxaprofen, nafenopin >> gemfibrozil, probenecid > bezafibrate > clofibric acid. At a noncytotoxic (0.1 mM) concentration, only benoxaprofen, nafenopin, clofibric acid, and probenecid significantly increased Comet moments (P < 0.05 Kruskal-Wallis). Clofibric acid and probenecid exhibited the greatest DNA-damaging potency, producing significant DNA damage at 0.01 mM concentrations. The two drugs produced maximal increases in Comet moment of 4.51 x and 2.57 x control, respectively. The glucuronidation inhibitor borneol (1 mM) abolished the induction of DNA damage by 0.5 mM concentrations of clofibric acid and probenecid. In an in vitro cell-free system, clofibric acid glucuronide was 10 x more potent than glucuronic acid in causing DNA strand-nicking, although both compounds showed similar rates of autoxidation to generate hydroxyl radicals. In cultured hepatocytes, the glycation inhibitor, aminoguanidine, and the iron chelator, desferrioxamine mesylate, inhibited DNA damage by clofibric acid, whereas the free radical scavengers Trolox and butylated hydroxytoluene, and the superoxide dismutase mimetic bis-3,5-diisopropylsalicylate had no effect. In conclusion, clinically relevant concentrations of two structurally unrelated carboxylic acids, probenecid and clofibric acid, induced DNA damage in isolated hepatocytes via glucuronidation- dependent pathways. These findings suggest acyl glucuronides are able to access and damage nuclear DNA via iron-catalyzed glycation/glycoxidative processes.

Differences in lysine adduction by acrolein and methyl vinyl ketone: implications for cytotoxicity in cultured hepatocytes

Acrolein is a highly toxic environmental pollutant that readily alkylates the epsilon-amino group of lysine residues in proteins. In model systems, such chemistry involves sequential addition of two acrolein molecules to a given nitrogen, forming bis-Michael-adducted species that undergo aldol condensation and dehydration to form Nepsilon-(3-formyl-3,4-dehydropiperidino)lysine. Whether this ability to form cyclic adducts participates in the toxicity of acrolein is unknown. To address this issue, we compared the chemistry of protein adduction by acrolein to that of its close structural analogue methyl vinyl ketone, expecting that the alpha-methyl group would hinder the intramolecular cyclization of any bis-adducted species formed by methyl vinyl ketone. Both acrolein and methyl vinyl ketone displayed comparable protein carbonylating activity during in vitro studies with the model protein bovine serum albumin, confirming the alpha,beta,-unsaturated bond of both compounds is an efficient Michael acceptor for protein nucleophiles. However, differences in adduction chemistry became apparent during the use of electrospray ionization-MS to monitor reaction products in a lysine-containing peptide after modification by each compound. For example, although a Schiff base adduct was detected following reaction of the peptide with acrolein, an analogous species was not formed by methyl vinyl ketone. Furthermore, while ions corresponding to mono- and bis-Michael adducts were detected at the N-terminus and lysine residues following peptide modification by both carbonyls, only acrolein modification generated ions attributable to cyclic adducts. Despite these differences in adduction chemistry, in mouse hepatocytes, the two compounds exhibited very comparable abilities to induce rapid, concentration-dependent cell death as well as protein carbonylation. These findings suggest that the acute toxicity of short-chain alpha,beta-unsaturated carbonyl compounds involves their ability to form acyclic Michael addition adducts rather than Schiff conjugates or heterocyclic adducts.

Hydralazine Inhibits Rapid Acrolein-Induced Protein Oligomerization: Role of Aldehyde Scavenging and Adduct Trapping in Cross-Link Blocking and Cytoprotection

Hydralazine strongly suppresses the toxicity of acrolein, a reactive aldehyde that contributes to numerous health disorders. At least two mechanisms may underlie the cytoprotection, both of which involve the nucleophilic hydrazine possessed by hydralazine. Under the simplest scenario, hydralazine directly scavenges free acrolein, decreasing intracellular acrolein availability and thereby suppressing macromolecular adduction. In a second "adduct-trapping" mechanism, the drug forms hydrazones with acrolein-derived Michael adducts in cell proteins, preventing secondary reactions of adducted proteins that may trigger cell death. To identify the most important mechanism, we explored these two pathways in mouse hepatocytes poisoned with the acrolein precursor allyl alcohol. Intense concentration-dependent adduct-trapping in cell proteins accompanied the suppression of toxicity by hydralazine. However, protective concentrations of hydralazine did not alter extracellular free acrolein levels, cellular glutathione loss, or protein carbonylation, suggesting that the cytoprotection is not due to minimization of intracellular acrolein availability. To explore ways whereby adduct-trapping might confer cytoprotection, the effect of hydralazine on acrolein-induced protein cross-linking was examined. Using bovine pancreas ribonuclease A as a model protein, acrolein caused rapid time- and concentration-dependent cross-linking, with dimerized protein detectable within 45 min of commencing protein modification. Lysine adduction in monomeric protein preceded the appearance of oligomers, whereas reductive methylation of protein amine groups abolished both adduction and oligomerization. Hydralazine inhibited cross-linking if added 30 min after commencing acrolein exposure but was ineffective if added after a 90-min delay. Adduct-trapping closely accompanied the inhibition of cross-linking by hydralazine. These findings suggest that cross-link blocking may contribute to hydralazine cytoprotection.

Reactivity with Tris(hydroxymethyl)aminomethane confounds immunodetection of acrolein-adducted proteins

The toxic alpha,beta-unsaturated aldehyde acrolein readily attacks proteins, generating adducts at cysteine, histidine, and lysine residues. In this study, rabbit antiserum was raised against acrolein-modified keyhole limpet hemocyanin in the expectation that it would allow immunodetection of adducted proteins in biological samples. Using slot-blot and enzyme-linked immunosorbent assays, the antiserum detected acrolein-modified protein with high sensitivity and specificity. Adduct immunodetection was strongly inhibited by acrolein-modified polylysine but not polyhistidine. Efforts to develop a Western blotting method for detecting adducted proteins in cell lysates were hampered by irreproducible outcomes, evidently due to adduct instability during SDS-PAGE. Indeed, adducts generated via brief exposure of a model protein to acrolein displayed pH- and concentration-dependent instability to tris(hydroxymethyl)aminomethane (Tris), a nucleophilic buffer used in protein electrophoresis. The effect was most striking when Tris solutions were buffered to pH 8.0 and higher. In contrast, adducts formed during extended exposure to acrolein (> or =60 min) were completely stable to Tris. The time dependence of susceptibility raised the possibility that Tris interfered with specific steps in lysine modification, which involves stepwise Michael addition of two molecules of acrolein to the same residue, followed by condensation and dehydration to form a heterocyclic adduct, N(epsilon)-(3-formyl-3,4-dehydropiperidino)lysine. We hypothesize that carbonyl-retaining Michael adducts may react with Tris by forming imines with the primary amine of the buffer. Consistent with this idea, triethanolamine, a tertiary amine buffer unable to form imines, had no effect on acrolein-adducted protein. These effects of Tris may explain difficulties in the detection of acrolein-adducted proteins during conventional Western blotting procedures.

Optimisation of the comet genotoxicity assay in freshly isolated murine hepatocytes: detection of strong in vitro DNA damaging properties for styrene

While the comet assay is used to detect DNA damage in isolated cells following exposure to chemicals in vitro, few publications report the use of the procedure in liver cells isolated from mice. Our initial efforts to use the assay to assess DNA damage in mouse hepatocytes maintained on collagen-coated dishes were hampered by high levels of baseline damage in controls, which appeared to result from mechanical damage sustained during the dislodgement of adherent cells in the early stages of the assay protocol. Here we describe an efficient version of the comet assay in cultured mouse hepatocytes that involves careful recovery of cells using a "scraping" buffer supplemented with 10% high purity grade DMSO. Use of this buffer strongly diminished the frequency of false positives. Using the industrial reagent styrene as a positive control in the optimised procedure, non-cytotoxic concentrations of this substance (2.5-10 mM) significantly increased mean comet tail length, area, and moment. Co-incubation with the CYP inhibitor SKF-525A strongly attenuated these effects of styrene. Collectively, these findings confirm this method is highly suitable for the detection of DNA damage by bioactivation-dependent compounds in freshly isolated mouse hepatocytes.

Strong protein adduct trapping accompanies abolition of acrolein-mediated hepatotoxicity by hydralazine in mice

Acrolein is a highly reactive alpha,beta-unsaturated aldehyde that readily alkylates nucleophilic centers in cell macromolecules. Typically, such reactions proceed via Michael addition chemistry, forming adducts that retain an electrophilic carbonyl group. Since these species participate in secondary deleterious reactions, we hypothesize that inactivation of carbonyl adducts may attenuate acrolein toxicity. Indeed, we recently established that the nucleophilic antihypertensive drug hydralazine readily "traps" acrolein adducts in cell proteins and strongly suppresses acrolein-mediated toxicity in isolated hepatocytes. This work sought to determine whether hydralazine prevents the in vivo hepatotoxicity of the acrolein precursor allyl alcohol in whole mice and whether adduct trapping accompanies any such hepatoprotection. Mice received allyl alcohol alone or in conjunction with several doses of hydralazine. Four hours later, mice were sacrificed to allow for the determination of liver enzymes in plasma as markers of hepatic injury, whereas livers were assessed for glutathione and hydralazine-stabilized protein adducts. Hydralazine afforded strong, dose-dependent protection against the increases in plasma marker enzymes but not the hepatic glutathione depletion produced by allyl alcohol. Western blotting revealed intense, dose-dependent adduct trapping by hydralazine in numerous liver proteins over a broad 26- to 200-kDA mass range. In keeping with these findings, immunohistochemical analysis of liver slices indicated diffuse, extranuclear adduct trapping by hydralazine that was uniformly distributed across the liver lobule, with partial localization in parenchymal cell membranes. These findings concur with our hypothesis that hydralazine readily inactivates reactive carbonyl-retaining protein adducts formed by acrolein, thereby preventing secondary reactions that trigger cellular death.

Protein Adduct-Trapping by Hydrazinophthalazine Drugs: Mechanisms of Cytoprotection Against Acrolein-Mediated Toxicity

Acrolein is a highly toxic aldehyde involved in a number of diseases as well as drug-induced toxicities. Its pronounced toxicity reflects the readiness with which it forms adducts in proteins and DNA. As a bifunctional electrophile, initial reactions between acrolein and protein generate adducts containing an electrophilic center that can participate in secondary deleterious reactions (e.g., cross-linking). We hypothesize that inactivation of these reactive protein adducts with nucleophilic drugs may counteract acrolein toxicity. Because we previously observed that 1-hydrazinophthalazine (hydralazine) strongly diminishes the toxicity of the acrolein precursor allyl alcohol, we explored the possibility that hydralazine targets reactive acrolein adducts in proteins. We report that hydralazine abolished the immunoreactivity of an acrolein-modified model protein (bovine serum albumin), but only if the drug was added to the protein within 30 min of commencing modification by acrolein. The ability of a range of carbonyl-trapping drugs to interfere with "early" events in protein modification strongly correlated with their protective potencies against allyl alcohol toxicity in hepatocytes. In mass spectrometry studies using a model lysine-containing peptide, hydralazine rapidly formed hydrazones with Michael adducts generated by acrolein. Using an antibody raised against such ternary drug-acrolein-protein complexes in Western blotting experiments, clear adduct-trapping was evident in acrolein-preloaded hepatocytes exposed to cytoprotective concentrations of hydralazine ranging from 2 to 50 microM. These novel findings begin to reveal the molecular mechanisms whereby hydralazine functions as an efficient "protein adduct-trapping" drug.

Reactivity of hydrazinophthalazine drugs with the lipid peroxidation products acrolein and crotonaldehyde

The nucleophilic drug hydralazine strongly inhibits cell toxicity mediated by acrolein, a short chain 2-alkenal formed during lipid peroxidation. We here report the chemistry of acrolein-trapping by hydralazine, and show that together with its structural analogue dihydralazine, it also readily traps crotonaldehyde. Isolable reaction products included (1E)-acrylaldehyde phthalazin-1-ylhydrazone (E-APH), (1Z)-acrylaldehyde phthalazin-1-ylhydrazone (Z-APH), (1E,2E)-but-2-enal phthalazin-1-ylhydrazone (E-BPH) and (1Z,2E)-but-2-enal phthalazin-1-ylhydrazone (Z-BPH). Concentration-dependent formation of (1E)-acrylaldehyde phthalazin-1-ylhydrazone was observed in the culture media of cells co-exposed to hydralazine and the acrolein precursor allyl alcohol. These aldehyde-sequestering properties of hydrazinophthalazine drugs may contribute to the protection they provide against 2-alkenal-mediated toxicity.

Cylindrospermopsin-induced protein synthesis inhibition and its dissociation from acute toxicity in mouse hepatocytes

The toxicology of the cyanobacterial alkaloid cylindrospermopsin (CYN), a potent inhibitor of protein synthesis, appears complex and is not well understood. In exposed mice the liver is the main target for the toxic effects of CYN. In this study primary mouse hepatocyte cultures were used to investigate the mechanisms involved in CYN toxicity. The results show that 1-5 microM CYN caused significant concentration-dependent cytotoxicity (52%-82% cell death) at 18 h. Protein synthesis inhibition was a sensitive, early indicator of cellular responses to CYN. Following removal of the toxin, the inhibition of protein synthesis could not be reversed, showing behavior similar to that of the irreversible inhibitor emetine. In contrast to the LDH leakage, protein synthesis was maximally inhibited by 0.5 microM CYN. No protein synthesis occurred over 4-18 h at or above this concentration. Inhibition of cytochrome P450 (CYP450) activity with 50 microM proadifen or 50 microM ketoconazole diminished the toxicity of CYN but not the effects on protein synthesis. These findings imply a dissociation of the two events and implicate the involvement of CYP450-derived metabolites in the toxicity process, but not in the impairment of protein synthesis. Thus, the total abolition of protein synthesis may exaggerate the metabolite effects but cannot be considered a primary cause of cell death in hepatocytes over an acute time frame. In cell types deficient in CYP450 enzymes, protein synthesis inhibition may play a more crucial role in the development of cytotoxicity.

UDP-glucuronosyltransferase-dependent bioactivation of clofibric acid to a DNA-damaging intermediate in mouse hepatocytes

Glucuronidation of a number of carboxyl-containing drugs generates reactive acyl glucuronide metabolites. These electrophilic species alkylate cell proteins and may be implicated in the pathogenesis of a number of toxic syndromes seen in patients receiving the parent aglycones. Whether acyl glucuronides also attack nuclear DNA is unknown, although the acyl glucuronide formed from clofibric acid was recently found to decrease the transfection efficiency of phage DNA and generate strand breaks in plasmid DNA in vitro. To determine if such a DNA damage occurs within a cellular environment, the comet assay (i.e. single-cell gel electrophoresis) was used to detect DNA lesions in the nuclear genome of isolated mouse hepatocytes cultured with clofibric acid. Overnight exposure to 50 microM and higher concentrations of clofibric acid produced concentration-dependent increases in the comet areas of hepatocyte nuclei, with 1 mM clofibrate producing a 3.6-fold elevation over controls. These effects closely coincided with culture medium concentrations of the glucuronide metabolite formed from clofibric acid, 1-O-beta-clofibryl glucuronide. Consistent with a role for glucuronidation in the DNA damage observed, the glucuronidation inhibitor borneol diminished glucuronide formation from 100 microM clofibrate by 98% and returned comet areas to baseline levels. Collectively, these results suggest that the acyl glucuronide formed from clofibric acid is capable of migrating from its site of formation within the endoplasmic reticulum to generate strand nicks in nuclear DNA.

Aldehyde-sequestering drugs: tools for studying protein damage by lipid peroxidation products

Elevated levels of reactive alpha,beta-unsaturated aldehydes (e.g. malondialdehyde, 4-hydroxynonenal and acrolein) in the affected tissues of various degenerative conditions suggest these substances are active propagators of the disease process. One experimental approach to attenuating damage by these intermediates employs 'aldehyde-sequestering drugs' as sacrificial nucleophiles, thereby sparing cell macromolecules and perhaps slowing disease progression. Drugs with demonstrated trapping activity toward lipid-derived aldehydes include various amine compounds such as aminoguanidine, carnosine and pyridoxamine. We have focused on identifying scavengers of acrolein, perhaps the most toxic aldehyde formed during lipid peroxidation cascades. Various phthalazine compounds (hydralazine and dihydralazine) were found to trap acrolein readily, forming hydrazone derivatives in a rapid Schiff-type reaction. These compounds strongly protect against acrolein-mediated toxicity in isolated hepatocytes.

Oxidative bioactivation of crotyl alcohol to the toxic endogenous aldehyde crotonaldehyde: association of protein carbonylation with toxicity in mouse hepatocytes

Recent confirmation that the toxic unsaturated aldehyde crotonaldehyde (CA) contributes to protein damage during lipid peroxidation confers interest on the molecular actions of this substance. However, since a plethora of structurally related aldehydes form during membrane oxidation, clarifying the toxicological significance of individual products (e.g., CA) is challenging. To facilitate study of the mechanisms underlying CA toxicity, we explored the possibility that it can be formed enzymatically from an unsaturated precursor, crotyl alcohol. This is analogous to the way allyl alcohol is converted in vivo to its toxic oxidation product, acrolein. In kinetic studies, we found that crotyl alcohol was readily oxidized by equine liver alcohol dehydrogenase, with electrospray-mass spectrometry confirming that CA was the main product formed. Moreover, in mouse hepatocytes, crotyl alcohol produced marked time- and concentration-dependent cell killing as well as pronounced glutathione depletion. Both cytotoxicity and glutathione loss were abolished by the alcohol dehydrogenase inhibitor 4-methylpyrazole, indicating an oxidation product mediated these effects. In keeping with expectations that carbonyl-retaining Michael addition adducts would feature prominently during protein modification by CA, exposure to crotyl alcohol resulted in marked carbonylation of a wide range of cell proteins, an effect that was also abolished by 4-methylpyrazole. Damage to a subset of small proteins (e.g., 29, 32, 33 kDa) closely correlated with the severity of cell death. Collectively, these results demonstrate that crotyl alcohol is a useful tool for studying the biochemical and molecular events accompanying intracellular CA formation.

Extensive protein carbonylation precedes acrolein-mediated cell death in mouse hepatocytes

Allyl alcohol hepatotoxicity is mediated by an alcohol dehydrogenase-derived biotranformation product, acrolein. This highly reactive alpha,beta-unsaturated aldehyde readily alkylates model proteins in vitro, forming, among other products, Michael addition adducts that possess a free carbonyl group. Whether such damage accompanies acrolein-mediated toxicity in cells is unknown. In this work we established that allyl alcohol toxicity in mouse hepatocytes involves extensive carbonylation of a wide range of proteins, and that the severity of such damage to a subset of 18-50 kDa proteins closely correlated with the degree of cell death. In addition to abolishing cytotoxicity and glutathione depletion, the alcohol dehydrogenase inhibitor 4-methyl pyrazole strongly attenuated protein carbonylation. Conversely, cyanamide, an aldehyde dehydrogenase inhibitor, enhanced cytotoxicity and protein carbonylation. Since protein carbonylation clearly preceded the loss of membrane integrity, it may be associated with the toxic process leading to cell death.

Cell-free protein synthesis inhibition assay for the cyanobacterial toxin cylindrospermopsin

The cyanobacterial toxin cylindrospermopsin (CYN) is known to be a potent inhibitor of protein synthesis. This paper describes the use of a rabbit reticulocyte lysate translation system as a protein synthesis inhibition assay for CYN. A dose response curve for protein synthesis inhibition by CYN was constructed and was modeled to a sigmoidal dose response curve with variable slope (R2 = 0.98). In this assay, CYN has an IC50 of 120 nM [95% confidence limits (Cl) = 111-130 nM] with a detection limit in the region of 50 nM in the assay solution. Application of the assay allows quantification of toxin samples within the range 0.5-3.0 microM (200-1200 micrograms/L) CYN. To assess the usefulness of this assay, a range of toxic and nontoxic Cylindrospermopsis raciborskii extracts, including both laboratory strains and environmental samples, were assayed by protein synthesis inhibition. These CYN quantifications were then compared to quantifications obtained by high performance liquid chromatography (HPLC) and HPLC-tandem mass spectrometry (HPLCMS-MS). The results demonstrate that the protein synthesis inhibition assay correlates well with both HPLCMS-MS (r2 = 0.99) and HPLC (r2 = 0.97) quantifications. We conclude that this is an accurate and rapid assay for the measurement of cylindrospermopsin in cyanobacterial extracts.

Clofibrate pretreatment in mice confers resistance against hepatic lipid peroxidation

Pretreatment with peroxisome proliferators protects mice against various hepatotoxicants. Since our previous work suggested that the hepatoprotection may involve an increased ability to cope with oxidative stress, the present work directly addressed this possibility. Several observations indicated a heightened defense against oxidative stress accompanies the hepatoprotection produced by clofibrate. Firstly, the carbonyl content of hepatic proteins from clofibrate-pretreated mice was 40% lower than those from vehicle-treated controls. Secondly, liver homogenates from clofibrate-pretreated mice produced less thiobarbituric acid reactive substances upon incubation under aerobic conditions or exposure to ferrous sulfate. This effect was not due to lower levels of peroxidation-prone polyunsaturated fatty acids in clofibrate-treated livers. Thirdly, in vitro experiments indicated that the antioxidant factor in liver homogenates from clofibrate-pretreated mice was not glutathione. Rather, since it was inactivated by proteases and heat treatment, we concluded that a protein is involved. Collectively, our results suggest that a resistance to lipid peroxidation develops in mouse liver during exposure to clofibrate. The identity of the putative antioxidant protein and its contribution to the protection against liver toxicity observed in this and other laboratories awaits future investigation.

The antihypertensive hydralazine is an efficient scavenger of acrolein

Recent work indicates the highly toxic alpha,beta-unsaturated aldehyde acrolein is formed during the peroxidation of polyunsaturated lipids, raising the possibility that it functions as a 'toxicological second messenger' during oxidative cell injury. Acrolein reacts rapidly with proteins, forming adducts that retain carbonyl groups. Damage by this route may thus contribute to the burden of carbonylated proteins in tissues. This work evaluated several amine compounds with known aldehyde-scavenging properties for their ability to attenuate protein carbonylation by acrolein. The compounds tested were: (i) the glycoxidation inhibitors, aminoguanidine and carnosine; (ii) the antihypertensive, hydralazine; and (iii) the classic carbonyl reagent, methoxyamine. Each compound attenuated carbonylation of a model protein, bovine serum albumin, during reactions with acrolein at neutral pH and 37 degrees C. However, the most efficient agent was hydralazine, which strongly suppressed carbonylation under these conditions. Study of the rate of reaction between acrolein and the various amines in a protein-free buffered system buttressed these findings, since hydralazine reacted with acrolein at rates 2-3 times faster than its reaction with the other scavengers. Hydralazine also protected isolated mouse hepatocytes against cell killing by allyl alcohol, which undergoes in situ alcohol dehydrogenase-catalysed conversion to acrolein.

Clofibrate-induced in vitro hepatoprotection against acetaminophen is not due to altered glutathione homeostasis

Prior induction of peroxisome proliferation protects mice against the in vivo hepatotoxicity of acetaminophen and various other bioactivation-dependent toxicants. The mechanisms underlying such chemoresistance are poorly understood, although they have been suggested to involve alterations in glutathione homeostasis. To clarify the role of glutathione in this phenomenon, we isolated hepatocytes from mice in which hepatic peroxisome proliferation had been induced with clofibrate. The cells were incubated with a range of acetaminophen concentrations and the extent of cell killing after up to 8 h was assessed by measuring lactate dehydrogenase leakage from the cells. Hepatocytes from clofibrate-pretreated mice were much less susceptible to acetaminophen than cells from vehicle-treated controls. However, the extent of glutathione depletion during exposure to acetaminophen was similar in both cell types, as were rates of excretion of the product of glutathione-mediated detoxication of acetaminophen's quinoneimine metabolite, 3-glutathionyl-acetaminophen. The glutathione-replenishing ability of clofibrate-pretreated cells after a brief exposure to diethyl maleate also resembled that of control cells. More importantly, prior depletion of glutathione by diethyl maleate did not abolish the resistance of clofibrate-pretreated cells to acetaminophen. Taken together, these findings indicate that although glutathione-dependent pathways may contribute to hepatoprotection during peroxisome proliferation, the resistance phenomenon is not due exclusively to this mechanism.

Role of cytochrome P450 3A in the metabolism of mefloquine in human and animal hepatocytes

We studied mefloquine metabolism in cells and microsomes isolated from human and animal (monkey, dog, rat) livers. In both hepatocytes and microsomes, mefloquine underwent conversion to two major metabolites, carboxymefloquine and hydroxymefloquine. In human cells and microsomes these metabolites only were formed, as already demonstrated in vivo, while in other species several unidentified metabolites were also detected. After a 48 hr incubation with human and rat hepatocytes, metabolites accounted for 55-65% of the initial drug concentration, whereas in monkey and dog hepatocytes, mefloquine was entirely metabolized after 15 and 39 hrs, respectively. The consumption of mefloquine was less extensive in microsomes, and unchanged drug represented 60% (monkey) to 85-100% (human, dog, rat) of the total radioactivity after 5 hr incubations. The involvement of the cytochrome P450 3A subfamily in mefloquine biotransformation was suggested by several lines of evidence. Firstly, mefloquine metabolism was strongly increased in hepatic microsomes from dexamethasone-pretreated rats, and also in human and rat hepatocytes after prior treatment with a cytochrome P450 3A inducer. Secondly, mefloquine biotransformation in rifampycin-induced human hepatocytes was inhibited in a concentration-dependent manner by the cytochrome P450 3A inhibitor ketoconazole and thirdly, a strong correlation was found between erythromycin-N-demethylase activity (mediated by cytochrome P450 3A) and mefloquine metabolism in human microsomes (r=0.81, P < 0.05, N=13). Collectively, these findings concerning the role of cytochrome P450 3A in mefloquine metabolism may have important in vivo consequences especially with regard to the choice of agents used in multidrug antimalarial regimens.

Internal hazards: baseline DNA damage by endogenous products of normal metabolism

Recent improvements in the ability to detect chemically modified bases in DNA have revealed that not only does the genetic material incur damage by foreign chemicals, but that it also sustains injury by reactive products of normal physiological processes. This review summarises current understanding of the DNA-damaging potential of various substances of endogenous origin, including oxidants, lipid peroxidation products, alkylating agents, estrogens, chlorinating agents, reactive nitrogen species, and certain intermediates of various metabolic pathways. The strengths and weaknesses of the existing database for DNA damage by each class of substance are discussed, as are future strategies for resolving the difficult question of whether endogenous chemicals are significant contributors to spontaneous mutagenesis and cancer development in vivo.

Mutations at G:C base pairs predominate after replication of peroxyl radical-damaged pSP189 plasmids in human cells

The mutagenicity of peroxyl radicals, important participants in lipid peroxidation cascades, was investigated using a plasmid-based mutational assay system. Double-stranded pSP189 plasmids were incubated with a range of concentrations of the water-soluble peroxyl radical generator 2,2'-azobis(2-amidinopropane) hydrochloride (AAPH). Following replication in human Ad293 cells, the plasmids were screened for supF mutations in indicator bacteria. Exposure to peroxyl radicals caused strand nicking and a decrease in transfection efficiency, which was accompanied by a significant increase in supF mutants. Each of these effects was abolished in the presence of the water-soluble vitamin E analogue Trolox. Automated sequencing of 76 AAPH-induced mutant plasmids revealed that substitutions at G:C base pairs were the most common changes, accounting for 85.5% of all identified mutations. Of these, most comprised G:C-->T:A transversions (53.5%), with lesser contributions by G:C-->A:T transitions (23.9%) and G:C-->C:G transversions (22.5%). Collectively, these data confirm our previous findings concerning the spectrum of mutations produced upon bacterial replication of peroxyl radical-damaged phage DNA and extend them by showing that such damage has mutagenic consequences during replication in more complex eukaryotic systems.

Genotoxic lipid peroxidation products: their DNA damaging properties and role in formation of endogenous DNA adducts

The peroxidation of polyunsaturated lipids generates a range of substances that possess DNA damaging potential. This includes lipid hydroperoxides and various species that contain unpaired electrons, such as the alkoxyl and peroxyl radicals. In addition, a range of genotoxic carbonyl-containing compounds are formed, such as malondialdehyde, various 4-hydroxy-2-alkenals such as 4-hydroxynonenal and a number of 2-alkenals. It has previously been assumed that the antioxidants and electrophile scavenging enzymes existing in mammalian cells effectively protect the genetic material against these substances. However, thanks to recent analytical advances in the detection of low levels of DNA adducts, it is now evident that DNA adducts formed from a range of lipid peroxidation products are abundant in both rodent and human genomes. This suggests that the cellular defence system is not 100% efficient and that a proportion of endogenously produced lipid peroxidation products escape detoxification and cause DNA damage. This review surveys the genotoxic properties of the major classes of lipid peroxidation products, focusing on their chemistry of DNA adduction, the mutagenic properties of such damage and the evidence that it occurs in intact biological systems. Furthermore, avenues of future research that will clarify the significance of such damage to spontaneous mutagenesis and carcinogenesis are proposed and discussed.

Genotoxicity of acyl glucuronide metabolites formed from clofibric acid and gemfibrozil: a novel role for phase-II-mediated bioactivation in the hepatocarcinogenicity of the parent aglycones?

Glucuronides formed from carboxylate-containing xenobiotics are more chemically reactive than most Phase II conjugates. However, while they have been shown to form protein adducts, their reactions with DNA have received little attention. We thus used the M13 forward mutational assay to assess the genotoxicity of acyl glucuronides formed from two widely used fibrate hypolipidemics, clofibric acid and gemfibrozil. Single-stranded M13mp19 bacteriophage DNA was incubated in pH 7.4 buffer for 16 h in the presence of 0, 1, 2.5, and 5 mM concentrations of each glucuronide as well as the respective aglycones. The modified DNA was then transfected into SOS-induced competent Escherichia coli JM105 cells and the transfection efficiency was determined after phage growth overnight at 37 degrees C. Significantly, both acyl glucuronides, but not the aglycones, caused a concentration-dependent decrease in the transfection efficiency of the DNA, with a greater than 80% decrease in phage survival produced by the 5 mM concentrations of the glucuronides. No increase in lacZa mutations accompanied the loss of phage survival. We propose that these genotoxic effects involve reactions with nucleophilic centers in DNA via a Schiff base mechanism that is analogous to the glycosylation of DNA by endogenous sugars. Since strand nicking is known to accompany such damage, we also analyzed glucuronide-treated pSP189 plasmids for strand breakages via agarose gel electrophoresis. Both clofibric acid and gemfibrozil glucuronides produced significant concentration-related strand nicking and exhibited over 10-fold greater reactivity than the endogenous glycosylating agent, glucose 6-phosphate. On the basis of these findings, the possibility that this novel bioactivation route participates in the carcinogenicity of the fibrate hypolipidemics deserves investigation.

Formation of novel C1-oxidised abasic sites in alkylperoxyl radical-damaged plasmid DNA

We have recently shown that peroxyl radicals react with DNA to form alkali-labile sites. To further characterise these lesions, we studied their susceptibility to digestion by repair endonucleases that recognise different types of abasic sites. We found that peroxyl radical-damaged pSP189 plasmids were resistant to cleavage by T4 endonuclease V, an enzyme that incises DNA at "regular" and C4-oxidised abasic residues. In contrast, the DNA was digested by exonuclease III, an enzyme that recognises "regular" and C1-oxidised abasic sites. The presence of Trolox during exposure to peroxyl radicals reduced subsequent DNA cleavage by exonuclease III, while prior incubation of damaged plasmids with methoxyamine potentiated digestion by this enzyme. These findings suggest that peroxyl radical-induced DNA damage involves the generation of novel C1-oxidised deoxyribose residues.

Role of G-->T transversions in the mutagenicity of alkylperoxyl radicals: induction of alkali-labile sites in bacteriophage M13mp19

The mutagenicity of peroxyl radicals, ubiquitous products of lipid peroxidation, was assessed using an in vitro M13 forward mutational assay. Single-stranded M13mp19 plasmids were incubated with a range of concentrations of the azo initiator 2,2'-azobis(2-amidinopropane) hydrochloride, and then transfected into competent, SOS-induced Escherichia coli JM105 cells. Incubation with peroxyl radicals produced a concentration-dependent decrease in phage survival, with a 500 microM concentration of the azo initiator reducing the transfection efficiency by more than 90% while inducing a corresponding 6-fold increase in lacZ alpha mutation frequencies. Peroxyl radical-induced mutagenesis was completely prevented by the peroxyl radical scavenger Trolox. Automated DNA sequence analysis of the lacZ alpha gene of 100 peroxyl radical-induced mutants revealed that the most frequent sequence changes were base pair substitutions (92/95), with G-->T transversions predominating (73/92). Alkaline treatment prior to transfection diminished the mutagenicity of damaged plasmids to a level resembling that of unmodified DNA. While abasic sites might account for the sensitivity to alkaline cleavage, the possibility that unidentified nonabasic alkaline-labile lesions also contribute to peroxyl radical mutagenesis cannot be excluded. Collectively, these findings raise the possibility that DNA damage caused by a major class of endogenous radicals contributes to one of the most common spontaneous mutational events, the G-->T transversion.

Diminished susceptibility to proteolysis after protein modification by the lipid peroxidation product malondialdehyde: inhibitory role for crosslinked and noncrosslinked adducted proteins

The lipid peroxidation product malondialdehyde forms adducts with proteins that are detected during routine assays for protein carbonylation. To test whether this damage alters the susceptibility of a protein to proteolysis, we treated bovine serum albumin with various concentrations of malondialdehyde and examined its susceptibility to digestion by alpha-chymotrypsin. In keeping with findings concerning the consequences of protein damage by other carbonyl products of lipid peroxidation, we found that malondialdehyde-modified protein was resistant to proteolysis. Since significant protein crosslinking occurred during modification with malondialdehyde, we investigated the possibility that crosslinked proteins were acting as proteolytic inhibitors. Malondialdehyde-modified proteins were resolved into crosslinked and noncrosslinked forms and the effectiveness of both species as proteolytic antagonists was examined. While both forms of malondialdehyde-adducted proteins were more potent proteolytic inhibitors than unmodified albumin, there were no significant differences in inhibitory potency between crosslinked and noncrosslinked proteins. Our findings suggest that malondialdehyde-modification produces protease-resistant proteins without an obligatory role for crosslinking.

Introduction of carbonyl groups into proteins by the lipid peroxidation product, malondialdehyde

Incubation of model proteins with the toxic lipid peroxidation product malondialdehyde resulted in a time- and concentration-dependent increase in carbonyl contents. Carbonyl groups were detected either spectrophotometrically or immunochemically after derivatization with 2,4-dinitrophenylhydrazine. Although significant adduction occurred when modifications were performed at pH 7.0, carbonyl formation was most extensive when modifications were carried out at pH 4.0 or pH 5.0. Similarly, formation of intermolecular crosslinks was most extensive when reactions were carried out under mildly acidic conditions. Our results raise the possibility that malondialdehyde adducts contribute to the carbonyl content of proteins recovered from mammalian tissues.

Abasic sites stimulate double-stranded DNA cleavage mediated by topoisomerase II. DNA lesions as endogenous topoisomerase II poisons

Several clinically relevant anticancer drugs induce genomic mutations and cell death by increasing topoisomerase II-mediated DNA breakage. To determine whether endogenous DNA damage also affects this cleavage event, the effects of abasic sites (the most commonly formed spontaneous DNA lesion) on topoisomerase II activity were investigated. The presence of 3 abasic sites/plasmid stimulated enzyme-mediated DNA breakage > 6-fold, primarily by enhancing the forward rate of cleavage. This corresponds to a potency that is > 2000-fold higher than that of the anticancer drug, etoposide. These findings suggest that abasic sites represent endogenous topoisomerase II poisons and imply that anticancer drugs mimic the cleavage-enhancing actions of naturally occurring DNA lesions.