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

Role of reactive carbonyls and superoxide radicals in protein damage by cigarette smoke extracts: Comparison of Heat-not-Burn e-cigarettes to conventional cigarettes

Oxidative protein damage involving carbonylation of respiratory tract proteins typically accompanies exposure to tobacco smoke. Such damage can arise via multiple mechanisms, including direct amino acid oxidation by reactive oxygen species or protein adduction by electrophilic aldehydes. This study investigated the relative importance of these pathways during exposure of a model protein to fresh cigarette emission extracts. Briefly, protein carbonyl adducts were estimated in bovine serum albumin following incubation in buffered solutions with whole cigarette emissions extracts prepared from either a single 1R6F research cigarette or a single "Heat-not-Burn" e-cigarette. Although both extracts caused concentration-dependent protein carbonylation, conventional cigarette extracts produced higher adduct yields than e-cigarette extracts. Superoxide radical generation by conventional and e-cigarette emissions was assessed by monitoring nitro blue tetrazolium reduction and was considerably lower in extracts made from "Heat-Not-Burn" e-cigarettes. The superoxide dismutase/catalase mimic EUK-134 strongly suppressed radical production by whole smoke extracts from conventional cigarettes, however, it did not diminish protein carbonyl adduction when incubating smoke extracts with the model protein. In contrast, edaravone, a neuroprotective drug with strong carbonyl-trapping properties, strongly suppressed protein damage without inhibiting superoxide formation. Although these findings require extension to appropriate cell-based and in vivo systems, they suggest reactive aldehydes in tobacco smoke make greater contributions to oxidative protein damage than smoke phase radicals.

Genome-wide analysis of ALDH gene family in jujube and identification of ZjALDH3F3 for its important role in high-temperature tolerance

Aldehyde dehydrogenases (ALDHs) are NAD(P)-dependent enzymes that oxidize aliphatic and aromatic aldehydes. They play crucial roles in various biological processes and plant responses to stress. The impact of high temperatures on jujube quality and yield has been well documented. Nevertheless, the involvement of ALDHs in the response to heat stress remains poorly understood. This study aimed to identify ZjALDHs in the jujube genome (Ziziphus jujuba var. spinosa) and conducted in silico analyses. Phylogenetic analyses indicated that ALDHs in plants, including jujube, can be divided into ten families, and members from the same family share conserved gene and protein structures. Quantitative real-time PCR (qRT-PCR) and β-glucuronidase (GUS) histochemical staining were used to analyze the expression patterns of ZjALDHs in response to elevated temperatures. We identified a ZjALDH (ZjALDH3F3) gene displaying a significant upregulation and down-regulation, respectively in heat-resistant (HR) and heat-sensitive (HS) jujube in response to heat treatments. Such specific responses are probably attributed to the different heat-responsive cis-elements of ZjALDH3F3 in HR and HS jujubes. ZjALDH3F3 over-expressed in tobacco increased heat tolerance, as evidenced by the reduced accumulation of reactive oxygen species (ROS) and elevated activity of antioxidant enzymes. The qRT-PCR results indicated that the expression of antioxidant enzymes, abscisic acid (ABA), and stress-responsive genes was enhanced in transgenic tobacco. This study sheds novel light on the function of ZjALDHs in heat resistance of jujube.

Deletion of GSTA4-4 results in increased mitochondrial post-translational modification of proteins by reactive aldehydes following chronic ethanol consumption in mice

Chronic alcohol consumption induces hepatic oxidative stress resulting in production of highly reactive electrophilic α/β-unsaturated aldehydes that have the potential to modify proteins. A primary mechanism of reactive aldehyde detoxification by hepatocytes is through GSTA4-driven enzymatic conjugation with GSH. Given reports that oxidative stress initiates GSTA4 translocation to the mitochondria, we hypothesized that increased hepatocellular damage in ethanol (EtOH)-fed GSTA4^−/− mice is due to enhanced mitochondrial protein modification by reactive aldehydes. Chronic ingestion of EtOH increased hepatic protein carbonylation in GSTA4^−/− mice as evidenced by increased 4-HNE and MDA immunostaining in the hepatic periportal region. Using mass spectrometric analysis of biotin hydrazide conjugated carbonylated proteins, a total of 829 proteins were identified in microsomal, cytosolic and mitochondrial fractions. Of these, 417 were novel to EtOH models. Focusing on mitochondrial fractions, 1.61-fold more carbonylated proteins were identified in EtOH-fed GSTA4^−/− mice compared to their respective WT mice ingesting EtOH. Bioinformatic KEGG pathway analysis of carbonylated proteins from the mitochondrial fractions revealed an increased propensity for modification of proteins regulating oxidative phosphorylation, glucose, fatty acid, glutathione and amino acid metabolic processes in GSTA4^−/− mice. Additional analysis revealed sites of reactive aldehyde protein modification on 26 novel peptides/proteins isolated from either SV/GSTA4^−/− PF or EtOH fed mice. Among the peptides/proteins identified, ACSL, ACOX2, MTP, and THIKB contribute to regulation of fatty acid metabolism and ARG1, ARLY, and OAT, which regulate nitrogen and ammonia metabolism having direct relevance to ethanol-induced liver injury. These data define a role for GSTA4-4 in buffering hepatic oxidative stress associated with chronic alcohol consumption and that this GST isoform plays an important role in protecting against carbonylation of mitochondrial proteins.

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We demonstrate increased mitochondrial carbonylation in GSTA4-4 KO mice chronically fed EtOH.

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Using LC-MS we identify 829 total carbonylated proteins (417 novel to murine ALD).

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Pathway analysis revealed a propensity for adduction of fatty acid metabolic and electron transport proteins.

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Using MS/MS, 26 novel adducted peptides were identified.

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Reactive aldehyde modification of proteins contributes to pathogenesis of ALD.

Sex-, Species-, and Tissue-Specific Metabolism of Empagliflozin in Male Mouse Kidney Forms an Unstable Hemiacetal Metabolite (M466/2) That Degrades to 4-Hydroxycrotonaldehyde, a Reactive and Cytotoxic Species

Following oral administration of empagliflozin (1000 mg/kg/day) to male and female CD-1 mice for 2 years, renal tubular injury was identified in male mice. Renal injury was not detected in male mice (≤300 mg/kg/day), in female mice (1000 mg/kg/day), or in male or female Han Wistar rats (700 mg/kg/day). Using transfected HEK293 cells and Xenopus oocytes, empagliflozin was found to be a substrate of various mouse and rat organic anion transporters (oat/Oat) and organic anion transporting polypeptide (oatp/Oatp) transporters: mouse oat3, rat Oat3, mouse oatp1a1, and rat Oatp1a1. However, using isolated kidney slices from male and female mice and rats, no sex-based difference in the extent of uptake of empagliflozin occurred. Metabolism studies using hepatic and renal microsomes from male and female mice, rats, and humans revealed a hemiacetal metabolite of empagliflozin (M466/2), predominantly formed in male mouse kidney microsomes. Formation of M466/2 in male mouse kidney microsomes was 31-fold higher compared to that in female mouse kidney microsomes and was ∼29- and ∼20-fold higher compared to that in male and female mouse liver microsomes, respectively. M466/2 is unstable and degrades to form a phenol metabolite (M380/1) and 4-hydroxycrotonaldehyde (4-OH CTA). Formed 4-OH CTA was trapped by reduced GSH, and the structure of the GSH adduct was confirmed by mass spectrometry. Stoichiometric formation of M380/1 from M466/2 was observed (93-96% at 24 h); however, formation of 4-OH CTA was considerably lower (∼17.5% at 40 h), which is consistent with 4-OH CTA being a highly reactive species. These data represent a highly selective tissue-, species-, and sex-specific lesion in male CD-1 mice associated with a cytotoxic metabolite product, 4-OH CTA. In humans, glucuronidation of empagliflozin is the most prevalent metabolic pathway, and oxidation is a minor pathway. Thus, renal toxicity due to the formation of 4-OH CTA from empagliflozin is not expected in humans.

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.

Aldo-keto reductase family 1 B10 gene silencing results in growth inhibition of colorectal cancer cells: Implication for cancer intervention

Aldo-keto reductase family 1 B10 (AKR1B10), a member of aldo-keto reductase superfamily, is overexpressed in human hepatocellular carcinoma, lung squamous cell carcinoma and lung adenocarcinoma. Our previous study had demonstrated that the ectopic expression of AKR1B10 in 293T cells promotes cell proliferation. To evaluate its potential as a target for cancer intervention, in the current study we knocked down AKR1B10 expression in HCT-8 cells derived from a colorectal carcinoma, using chemically synthesized small interfering RNA (siRNA). The siRNA 1, targeted to encoding region, downregulated AKR1B10 expression by more than 60%, and siRNA 2, targeted to 3' untranslational region, reduced AKR1B10 expression by more than 95%. AKR1B10 silencing resulted in approximately a 50% decrease in cell growth rate and nearly 40% suppression of DNA synthesis. More importantly, AKR1B10 downregulation significantly reduced focus formation rate and colony size in semisolid culture, indicating the critical role of AKR1B10 in HCT-8 cell proliferation. Recombinant AKR1B10 protein showed strong enzymatic activity to acrolein and crotonaldehyde, with K(m) = 110.1 +/- 12.2 microM and V(max) = 3,122.0 +/- 64.7 nmol/mg protein/min for acrolein and K(m) = 86.7 +/- 14.3 microM and V(max) = 2,647.5 +/- 132.2 nmol/mg protein/min for crotonaldehyde. AKR1B10 downregulation enhanced the susceptibility of HCT-8 cells to acrolein (25 microM) and crotonaldehyde (50 microM), resulting in rapid oncotic cell death characterized with lactate dehydrogenase efflux and annexin-V staining. These results suggest that AKR1B10 may regulate cell proliferation and cellular response to additional carbonyl stress, thus being a potential target for cancer intervention.

The biochemistry of drug metabolism--an introduction: Part 2. Redox reactions and their enzymes

This review continues a general presentation of the metabolism of drugs and other xenobiotics started in a recent issue of Chemistry & Biodiversity. This Part 2 presents the numerous oxidoreductases involved, their nomenclature, relevant biochemical properties, catalytic mechanisms, and the very diverse reactions they catalyze. Many medicinally, environmentally, and toxicologically relevant examples are presented and discussed. Cytochromes P450 occupy a majority of the pages of Part 2, but a large number of relevant oxidoreductases are also considered, e.g., flavin-containing monooxygenases, amine oxidases, molybdenum hydroxylases, peroxidases, and the innumerable dehydrogenases/reductases.

Crotonaldehyde accumulates in glial cells of Alzheimer's disease brain

Several studies have documented the involvement of oxidative stress represented by lipid peroxidation in the pathogenesis of Alzheimer's disease (AD). To test whether the highly reactive carbonyl crotonaldehyde (CRA), generated during lipid peroxidation, is involved in AD, we performed an immunohistochemical analysis in AD and age-matched control hippocampi using a specific antibody against protein-bound CRA (P-CRA). In the AD cases, P-CRA immunoreactivity was preferentially localized in reactive astrocytes and microglia around senile plaques (SPs) and those present in the neuropil, while it was weakly detectable in neurons and neurofibrillary tangles. P-CRA immunoreactivity was also localized in all portions of diffuse SPs and the dystrophic neurites of neuritic and classical SPs, but was undetectable in amyloid cores. Age-matched controls showed P-CRA immunoreactivity only very weakly in neurons. In contrast to P-CRA, immunoreactivities for protein-bound acrolein and 4-hydroxy-2-nonenal were mainly localized to neurons and rarely seen in glial cells. Our results suggest that increased oxidative stress and CRA formation in glial cells is implicated in the disease processes of AD.

4-Hydroxymethyl-1,6,8-trimethylfuro[2,3-h]quinolin-2(1H)-one Induces Mitochondrial Dysfunction and Apoptosis upon Its Intracellular Oxidation

We investigated the mechanism of cell death induced by a furoquinolinone derivative, namely, 4-hydroxymethyl-1,6,8-trimethylfuro[2,3-h]quinolin-2(1H)-one (HOFQ), in the dark. Mitochondrial depolarization was found to be a causative event in HOFQ-induced apoptosis that was blunted either by replacing the 4-hydroxymethyl group with a methyl one, or by 4-methylpyrazole, an inhibitor of alcohol dehydrogenase (ADH). In vitro enzymatic assay demonstrated that HOFQ is a substrate of ADH. In isolated mitochondria HOFQ was without effect, whereas in the presence of ADH and NAD(+) it caused the opening of the permeability transition pore, indicating that HOFQ-oxidized products affect mitochondrial function directly. Finally, an analogue bearing the formyl group at the C-4 position mimicked all the effects exerted by HOFQ. In conclusion, these results suggest that the direct action on mitochondria of HOFQ-oxidized products are responsible for their cytotoxicity, which might be exacerbated, but hardly determined, by photodynamic action and/or binding to DNA.