Disulfide reductase systems in liver

Glyphosate and aminomethylphosphonic acid impact on redox status and biotransformation in fish and the mitigating effects of diet supplementation

Fish reared under seminatural conditions can be challenged by exposure to herbicides. Farming facilities relying on the surrounding area's water quality can be affected by glyphosate and aminomethylphosphonic acid (AMPA) contamination. This review summarizes findings on how glyphosate and AMPA in the amounts registered in surface waterbodies affect redox status and biotransformation in fish and covers the aspect of diet supplementation for oxidative stress relief. Environmentally relevant concentrations of glyphosate and AMPA can alter the transcription and catalytic activities of antioxidant enzymes, decrease the content of reduced glutathione, and increase the accumulation of lipid peroxidation products, all of which are signs of a redox imbalance. Glyphosate has been shown to affect complex I in the mitochondrial respiratory chain and dysregulate iron transport-related genes, causing redox disturbance. Relatively high but environmentally realistic glyphosate concentrations can initiate the induction of cytochrome P450 biotransformation enzymes, alter the regulation of ABC exporters, and cause the inhibition of the redox-sensitive Nrf2 signaling pathway. Studies on reducing herbicide toxicity through dietary supplementation are a promising area of research. Natural functional supplements have been proven to have great potential for mitigating glyphosate-induced oxidative stress and thereby improving fish health, which in turn means maintaining productivity in fish farms that use natural water. However, data on the effects of AMPA on fish are scarce, and studies on the alleviation of its toxicity in fish are lacking. Considering the variety of AMPA contamination routes, one cannot underestimate the need for further research.

Unresolved questions regarding cellular cysteine sources and their possible relationships to ferroptosis

Cysteine is required for synthesis of glutathione (GSH), coenzyme A, other sulfur-containing metabolites, and most proteins. In most cells, cysteine comes from extracellular disulfide sources including cystine, glutathione-disulfide, and peptides. The thioredoxin reductase-1 (TrxR1)- or glutathione-disulfide reductase (GSR)-driven enzymatic systems can fuel cystine reduction via thioredoxins, glutaredoxins, or other thioredoxin-fold proteins. Free cystine enters cells thorough the cystine-glutamate antiporter, xCT, but systemically, plasma glutathione-disulfide might predominate as a cystine source. Erastin, inhibiting both xCT and voltage-dependent anion channels, induces ferroptotic cell death, so named because this type of cell death is antagonized by iron-chelators. Many cancer cells seem to be predisposed to ferroptosis, which has been proposed as a targetable cancer liability. Ferroptosis is associated with lipid peroxidation and loss of either glutathione peroxidase-4 (GPX4) or ferroptosis suppressor protein-1 (FSP1), which each prevent accumulation of lipid peroxides. It has been suggested that an xCT inhibition-induced cellular cysteine-deficiency lowers GSH levels, starving GPX4 for reducing power and allowing membrane lipid peroxides to accumulate, thereby causing ferroptosis. Aspects of ferroptosis are however not fully understood and need to be further scrutinized, for example that neither disruption of GSH synthesis, loss of GSH, nor disruption of glutathione disulfide reductase (GSR), triggers ferroptosis in animal models. Here we reevaluate the relationships between Erastin, xCT, GPX4, cellular cysteine and GSH, RSL3 or ML162, and ferroptosis. We conclude that, whereas both Cys and ferroptosis are potential liabilities in cancer, their relationship to each other remains insufficiently understood.

Formation of EGCG oxidation self-assembled nanoparticles and their antioxidant activity in vitro and hepatic REDOX regulation activity in vivo

(-)-Epigallocatechin-3-gallate (EGCG) is a major polyphenol in tea and exerts several health-promoting effects. It easily autoxidizes into complex polymers and becomes deactivated due to the presence of multiple phenolic hydroxyl structures. Nonetheless, the morphology and biological activity of complex EGCG polymers are yet to be clarified. The present study demonstrated that EGCG autoxidation self-assembled nanoparticles (ENPs) exhibit antioxidant activity in vitro and hepatic REDOX homeostasis regulation activity in vivo. Also, the formation of ENPs during the EGCG autoxidation process was based on the intermolecular interaction forces that maintain the stability of the nanoparticles. Similar to EGCG, ENPs are scavengers of reactive oxygen species and hydroxyl radicals in vitro and also regulate hepatic REDOX activity through liver redox enzymes, including thioredoxin reductase (TrxR), thioredoxin (Trx), glutathione reductase (GR), glutaredoxin (Grx), and glutathione S-transferase (GST) in vivo. Moreover, ENPs activate the NRF2 antioxidant-responsive element pathway, exerting a detoxification effect at high doses. Unlike EGCG, ENPs do not cause liver damage at low doses and also maintain liver biosafety at high doses through self-assembly, forming large particles, which is supported by the unchanged levels of liver damage biomarkers, including serum alanine aminotransferase (ALT), aspartate aminotransferase (AST), liver γ-phosphorylated histone 2AX (γ-H2AX), and P53-related genes (Thbs, MDM2, P53, and Bax). Collectively, these findings revealed that ENPs, with adequate biosafety and regulation of hepatic redox activity in vivo, may serve as substitutes with significant potential for antioxidant applications or as food additives to overcome the instability and liver toxicity of EGCG.

The role of thioredoxin and glutathione systems in arsenic-induced liver injury in rats under glutathione depletion

Antioxidant systems like thioredoxin (Trx) and glutaredoxin (Grx) maintain oxidative stress balance. These systems have cross-talk supported by some in vitro studies. We investigated the underlying mechanisms of arsenic-induced liver injury in glutathione-deficient rats and whether there was any cross-talk between the Trx and Grx systems. The rats in arsenic-treated groups were administered with sodium arsenite (10, 20 mg/kg b w/d) for four weeks. In buthionine sulfoximine (BSO, an inhibitor of GSH) and 20 mg/kg arsenic combined groups, rats were injected with 2 mmol/kg BSO intraperitoneally twice per week. BSO exacerbated arsenic-induced liver injury by increasing arsenic accumulation in urine, serum, and liver while decreasing glutathione activity and resulting in upregulated mRNA expression of the Trx system and downregulation of Grx mRNA expression. The impact of Trx lasted longer than that of the Grx. The Trx system remained highly expressed, while GSH, Grx1, and Grx2 levels were decreased. The inhibitory effect of only BSO treatment on Grx1 and Grx2 was not pronounced. However, the combined impact of arsenic and BSO upregulated Trx expression, primarily related to further reduction of GSH. As a result, the suppressed Grxs were protected by the upregulated Trxs, which serve as a backup antioxidant defense system in the liver.

Redox Biology and Liver Fibrosis

Hepatic fibrosis is a complex process that develops in chronic liver diseases. Even though the initiation and progression of fibrosis rely on the underlying etiology, mutual mechanisms can be recognized and targeted for therapeutic purposes. Irrespective of the primary cause of liver disease, persistent damage to parenchymal cells triggers the overproduction of reactive species, with the consequent disruption of redox balance. Reactive species are important mediators for the homeostasis of both hepatocytes and non-parenchymal liver cells. Indeed, other than acting as cytotoxic agents, reactive species are able to modulate specific signaling pathways that may be relevant to hepatic fibrogenesis. After a brief introduction to redox biology and the mechanisms of fibrogenesis, this review aims to summarize the current evidence of the involvement of redox-dependent pathways in liver fibrosis and focuses on possible therapeutic targets.

Simultaneous DHA and organic selenium production by Schizochytrium sp.: a theoretical basis

Docosahexaenoic acid (DHA) and selenium (Se) are nutrients that confer several health benefits to both humans and animals. Widespread use of DHA in milk powder and health products requires large-scale mass production via Schizochytrium sp., while Se intended for human consumption is produced as organic Se via yeast. However, producing these nutrients on an industrial scale is constrained by various factors. We found that supplementing Schizochytrium sp. with Na2SeO3 (0.5 mg/L) improves its biomass and DHA production and also provides organic Se. De novo assembled transcriptome and biochemical indicators showed that Na2SeO3 promotes forming acetyl coenzyme A and L-cysteine via the glycerol kinase and cysteine synthase pathways, promoting DHA synthesis through the polyketide synthase pathway. However, high doses of Na2SeO3 (5 mg/L) limited the biomass of Schizochytrium sp. and DHA content. This study provided a theoretical basis for the simultaneous production of organic Se and DHA via Schizochytrium sp.

The Duality of Arsenic Metabolism: Impact on Human Health

Arsenic is a naturally occurring hazardous element that is environmentally ubiquitous in various chemical forms. Upon exposure, the human body initiates an elimination pathway of progressive methylation into relatively less bioreactive and more easily excretable pentavalent methylated forms. Given its association with decreasing the internal burden of arsenic with ensuing attenuation of its related toxicities, biomethylation has been applauded for decades as a pure route of arsenic detoxification. However, the emergence of detectable trivalent species with profound toxicity has opened a long-standing debate regarding whether arsenic methylation is a detoxifying or bioactivating mechanism. In this review, we approach the topic of arsenic metabolism from both perspectives to create a complete picture of its potential role in the mitigation or aggravation of various arsenic-related pathologies.

Pathways Linking Nicotinamide Adenine Dinucleotide Phosphate Production to Endoplasmic Reticulum Protein Oxidation and Stress

The endoplasmic reticulum (ER) lumen is highly oxidizing compared to other subcellular compartments, and maintaining the appropriate levels of oxidizing and reducing equivalents is essential to ER function. Both protein oxidation itself and other essential ER processes, such as the degradation of misfolded proteins and the sequestration of cellular calcium, are tuned to the ER redox state. Simultaneously, nutrients are oxidized in the cytosol and mitochondria to power ATP generation, reductive biosynthesis, and defense against reactive oxygen species. These parallel needs for protein oxidation in the ER and nutrient oxidation in the cytosol and mitochondria raise the possibility that the two processes compete for electron acceptors, even though they occur in separate cellular compartments. A key molecule central to both processes is NADPH, which is produced by reduction of NADP+ during nutrient catabolism and which in turn drives the reduction of components such as glutathione and thioredoxin that influence the redox potential in the ER lumen. For this reason, NADPH might serve as a mediator linking metabolic activity to ER homeostasis and stress, and represent a novel form of mitochondria-to-ER communication. In this review, we discuss oxidative protein folding in the ER, NADPH generation by the major pathways that mediate it, and ER-localized systems that can link the two processes to connect ER function to metabolic activity.

The Aging Liver: Redox Biology and Liver Regeneration

Significance: During aging, excessive production of reactive species in the liver leads to redox imbalance with consequent oxidative damage and impaired organ homeostasis. Nevertheless, slight amounts of reactive species may modulate several transcription factors, acting as second messengers and regulating specific signaling pathways. These redox-dependent alterations may impact the age-associated decline in liver regeneration. Recent Advances: In the last few decades, relevant findings related to redox alterations in the aging liver were investigated. Consistently, recent research broadened understanding of redox modifications and signaling related to liver regeneration. Other than reporting the effect of oxidative stress, epigenetic and post-translational modifications, as well as modulation of specific redox-sensitive cellular signaling, were described. Among them, the present review focuses on Wnt/β-catenin, the nuclear factor (erythroid-derived 2)-like 2 (NRF2), members of the Forkhead box O (FoxO) family, and the p53 tumor suppressor. Critical Issues: Even though alteration in redox homeostasis occurs both in aging and in impaired liver regeneration, the associative mechanisms are not clearly defined. Of note, antioxidants are not effective in slowing hepatic senescence, and do not clearly improve liver repopulation after hepatectomy or transplant in humans. Future Directions: Further investigations are needed to define mutual redox-dependent molecular pathways involved both in aging and in the decline of liver regeneration. Preclinical studies aimed at the characterization of these pathways would define possible therapeutic targets for human trials. Antioxid. Redox Signal. 35, 832-847.

[The effect of prenatal stress on antioxidant glutathion-assosiated enzymes activity in subcellular fractions of rat's liver]

The activity of glutathione-associated antioxidant enzymes in subcellular fractions (cytosolic, mitochondrial, and cell nucleus fractions) was investigated in the liver of adult male Wistar rats born after prenatal stress was. Two groups of animals were studied in the experiment: (1) control group included - animals was born by intact mothers, and (2) prenatal stress group included animals whose mothers were subjected to immobilization stress in high-light conditions from the 15th to the 19th day of pregnancy. The activity of glutathione peroxidase (EC 1.11.1.9) in prenatally stressed animals decreased in the fractions of nuclei and mitochondria compared to the control group, while the activity of glutathione reductase (EC 1.8.1.7.) increased in the same subcellular fractions. The activity of glutathione transferase (EC 2.5.1.18) in prenatally stressed rats reduced in the cytosol and mitochondrial fractions as compared to control group. Redistribution of the antioxidant enzyme activity in the cytosol, the fraction of nuclei and the mitochondrial fraction of liver tissue may contribute to the formation of the pathological phenotype of prenatally stressed offspring.

Homozygous methylentetrahydrofolate reductase C667T genotype anticipates age at venous thromboembolism by one decade

The aim of the study was to compare age at first venous thromboembolism (VTE), plasma homocysteine and activated partial thromboplastin time ratio (aPTTr) amongst unprovoked VTE patients with the methylentetrahydrofolate reductase (MTHFR) C667T genotypes, and to identify predictors of age at first VTE, of plasma homocysteine and of the aPTTr; to evaluate whether heterozygous or homozygous prothrombin (PT) G20210A mutation lowered the age at first VTE when associated with MTHFR TT. Retrospective cohort study on 259 MTHFR TT, 76 MTHFR TC and 64 MTHFR CC participants with unprovoked VTE; each participant contributed age, sex, age at VTE, history of dyslipidaemia, hypertension, smoking, homocysteine (measured by enzyme immunoassay) and aPTTr (measured by standard coagulation assay). Age at first VTE was lower in MTHFR TT than MTHFR TC and CC (41 ± 14 vs. 50 ± 16 vs. 51 ± 12 years, respectively, P < 0.0001); plasma homocysteine was higher in MTHFR TT than in the other groups (22 ± 21 vs. 12 ± 11.6 vs. 10 ± 3.3 μmol/l, respectively, P = 0.0005) whilst aPTTr was not different. MTHFR TT independently predicted age at first VTE (P = 0.001), plasma homocysteine (P < 0.0001) alongside sex (P = 0.0007), age and smoking (P = 0.03 for both). Compound MTHFR TT with PT GA or AA had no lowering effect on age at first VTE compared with MTHFR TT alone (41 ± 13 vs. 41 ± 14 years). Plasma homocysteine inversely related to aPTTr in the MTHFR TT group (P = 0.003). MTHFR TT anticipates age at first VTE by an average of 10 years compared with MTHFR TC and CC genotypes.

Supplemental Ascorbate Diminishes DNA Damage Yet Depletes Glutathione and Increases Acute Liver Failure in a Mouse Model of Hepatic Antioxidant System Disruption

Cellular oxidants are primarily managed by the thioredoxin reductase-1 (TrxR1)- and glutathione reductase (Gsr)-driven antioxidant systems. In mice having hepatocyte-specific co-disruption of TrxR1 and Gsr (TrxR1/Gsr-null livers), methionine catabolism sustains hepatic levels of reduced glutathione (GSH). Although most mice with TrxR1/Gsr-null livers exhibit long-term survival, ~25% die from spontaneous liver failure between 4- and 7-weeks of age. Here we tested whether liver failure was ameliorated by ascorbate supplementation. Following ascorbate, dehydroascorbate, or mock treatment, we assessed survival, liver histology, or hepatic redox markers including GSH and GSSG, redox enzyme activities, and oxidative damage markers. Unexpectedly, rather than providing protection, ascorbate (5 mg/mL, drinking water) increased the death-rate to 43%. In adults, ascorbate (4 mg/g × 3 days i.p.) caused hepatocyte necrosis and loss of hepatic GSH in TrxR1/Gsr-null livers but not in wildtype controls. Dehydroascorbate (0.3 mg/g i.p.) also depleted hepatic GSH in TrxR1/Gsr-null livers, whereas GSH levels were not significantly affected by either treatment in wildtype livers. Curiously, however, despite depleting GSH, ascorbate treatment diminished basal DNA damage and oxidative stress markers in TrxR1/Gsr-null livers. This suggests that, although ascorbate supplementation can prevent oxidative damage, it also can deplete GSH and compromise already stressed livers.

Sulfur Metabolism Under Stress

Significance: In humans, imbalances in the reduction-oxidation (redox) status of cells are associated with many pathological states. In addition, many therapeutics and prophylactics used as interventions for diverse pathologies either directly modulate oxidant levels or otherwise influence endogenous cellular redox systems. Recent Advances: The cellular machineries that maintain redox homeostasis or that function within antioxidant defense systems rely heavily on the regulated reactivities of sulfur atoms either within or derived from the amino acids cysteine and methionine. Recent advances have substantially advanced our understanding of the complex and essential chemistry of biological sulfur-containing molecules. Critical Issues: The redox machineries that maintain cellular homeostasis under diverse stresses can consume large amounts of energy to generate reducing power and/or large amounts of sulfur-containing nutrients to replenish or sustain intracellular stores. By understanding the metabolic pathways underlying these responses, one can better predict how to protect cells from specific stresses. Future Directions: Here, we summarize the current state of knowledge about the impacts of different stresses on cellular metabolism of sulfur-containing molecules. This analysis suggests that there remains more to be learned about how cells use sulfur chemistry to respond to stresses, which could in turn lead to advances in therapeutic interventions for some exposures or conditions.

NAD+ metabolism: pathophysiologic mechanisms and therapeutic potential

Nicotinamide adenine dinucleotide (NAD+) and its metabolites function as critical regulators to maintain physiologic processes, enabling the plastic cells to adapt to environmental changes including nutrient perturbation, genotoxic factors, circadian disorder, infection, inflammation and xenobiotics. These effects are mainly achieved by the driving effect of NAD+ on metabolic pathways as enzyme cofactors transferring hydrogen in oxidation-reduction reactions. Besides, multiple NAD+-dependent enzymes are involved in physiology either by post-synthesis chemical modification of DNA, RNA and proteins, or releasing second messenger cyclic ADP-ribose (cADPR) and NAADP+. Prolonged disequilibrium of NAD+ metabolism disturbs the physiological functions, resulting in diseases including metabolic diseases, cancer, aging and neurodegeneration disorder. In this review, we summarize recent advances in our understanding of the molecular mechanisms of NAD+-regulated physiological responses to stresses, the contribution of NAD+ deficiency to various diseases via manipulating cellular communication networks and the potential new avenues for therapeutic intervention.

NADPH and Glutathione Redox Link TCA Cycle Activity to Endoplasmic Reticulum Homeostasis

Many metabolic diseases disrupt endoplasmic reticulum (ER) homeostasis, but little is known about how metabolic activity is communicated to the ER. Here, we show in hepatocytes and other metabolically active cells that decreasing the availability of substrate for the tricarboxylic acid (TCA) cycle diminished NADPH production, elevated glutathione oxidation, led to altered oxidative maturation of ER client proteins, and attenuated ER stress. This attenuation was prevented when glutathione oxidation was disfavored. ER stress was also alleviated by inhibiting either TCA-dependent NADPH production or Glutathione Reductase. Conversely, stimulating TCA activity increased NADPH production, glutathione reduction, and ER stress. Validating these findings, deletion of the Mitochondrial Pyruvate Carrier-which is known to decrease TCA cycle activity and protect the liver from steatohepatitis-also diminished NADPH, elevated glutathione oxidation, and alleviated ER stress. Together, our results demonstrate a novel pathway by which mitochondrial metabolic activity is communicated to the ER through the relay of redox metabolites.

The Role of Mitochondria in Sex-Dependent Differences in Hepatic Steatosis and Oxidative Stress in Response to Cafeteria Diet-Induced Obesity in Mice

Female mice fed a cafeteria diet (FCaf) develop higher liver steatosis and oxidative stress than males (MCaf) as a consequence of unresolved ER stress. Here, we investigated whether mitochondria play a role in this sex difference. The isolated mitochondria from FCaf showed more signs of oxidative stress than those of MCaf, correlated with a reduced content of GSH, increased amount of reactive oxygen species (ROS), and lower activities of enzymes involved in ROS neutralisation. Mitochondria from FCaf and MCaf livers exhibited lower rates of succinate-driven state III respiration and reduced ATPase activity in intact coupled mitochondria compared to their controls fed a standard diet (FC and MC), with no differences between the sexes. Fatty acid oxidation in mitochondria and peroxisomes was higher in MCaf and FCaf compared to their respective controls. In the intact perfused liver, there was no difference between sex or diet regarding the fatty acid oxidation rate. These results indicated that cafeteria diet did not affect mitochondrial energy metabolism, even in FCaf livers, which have higher steatosis and cellular oxidative stress. Nevertheless, the increase in mitochondrial ROS generation associated with a decrease in the antioxidant defence capacity, probably contributes to inducing or reinforcing the ER stress in FCaf livers.

Disulfide reductase systems in liver

Intermediary metabolism and detoxification place high demands on the disulfide reductase systems in most hepatocyte subcellular compartments. Biosynthetic, metabolic, cytoprotective and signalling activities in the cytosol; regulation of transcription in nuclei; respiration in mitochondria; and protein folding in endoplasmic reticulum all require resident disulfide reductase activities. In the cytosol, two NADPH-dependent enzymes, glutathione reductase and thioredoxin reductase, as well as a recently identified NADPH-independent system that uses catabolism of methionine to maintain pools of reduced glutathione, supply disulfide reducing power. However the necessary discontinuity between the cytosol and the interior of organelles restricts the ability of the cytosolic systems to support needs in other compartments. Maintenance of molecular- and charge-gradients across the inner-mitochondrial membrane, which is needed for oxidative phosphorylation, mandates that the matrix maintain an autonomous set of NADPH-dependent disulfide reductase systems. Elsewhere, complex mechanisms mediate the transfer of cytosolic reducing power into specific compartments. The redox needs in each compartment also differ, with the lumen of the endoplasmic reticulum, the mitochondrial inter-membrane space and some signalling proteins in the cytosol each requiring different levels of protein oxidation. Here, we present an overview of the current understanding of the disulfide reductase systems in major subcellular compartments of hepatocytes, integrating knowledge obtained from direct analyses on liver with inferences from other model systems. Additionally, we discuss relevant advances in the expanding field of redox signalling. LINKED ARTICLES: This article is part of a themed section on Chemical Biology of Reactive Sulfur Species. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.4/issuetoc.

Highlighted mechanistic aspects in the chemical biology of reactive sulfur species

LINKED ARTICLES

This article is part of a themed section on Chemical Biology of Reactive Sulfur Species. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.4/issuetoc.