Protein disulfide isomerase and glutathione are alternative substrates in the one Cys catalytic cycle of glutathione peroxidase 7

Key Proteins for Regeneration in A. mexicanum: Transcriptomic Insights From Aged and Juvenile Limbs

The axolotl, known for its remarkable regenerative abilities, is an excellent model for studying regenerative therapies. Nevertheless, the precise molecular mechanisms governing its regenerative potential remain uncertain. In this study, we collected samples from axolotls of different ages, including 8-year-old individuals and 8-month-old juveniles, obtaining their blastemas 10 days after amputation. Subsequently, we conducted a transcriptomic analysis comparing our samples to a set of previously published experiments. Our analysis unveiled a distinctive transcriptional response in the blastema, characterized by differential gene expression associated with processes such as bone and tissue remodeling, transcriptional regulation, angiogenesis, and intercellular communication. To gain deeper insights, we compared these findings with those from aged axolotls that showed no signs of regeneration 10 days after amputation. We identified four genes-FSTL1, ADAMTS17, GPX7, and CTHRC1-that showed higher expression in regenerating tissue compared to aged axolotls. Further scrutiny, including structural and homology analysis, revealed that these genes are conserved across vertebrate species. Our discoveries point to a group of proteins relevant to tissue regeneration, with their conservation in vertebrates suggesting critical roles in development. These findings also propose a novel gene set involved in axolotl regeneration, laying a promising foundation for future investigations across vertebrates.

Viral Infections and the Glutathione Peroxidase Family: Mechanisms of Disease Development

Significance: The glutathione peroxidase (GPx) family is recognized for its essential function in maintaining cellular redox balance and countering the overproduction of reactive oxygen species (ROS), a process intricately linked to the progression of various diseases including those spurred by viral infections. The modulation of GPx activity by viruses presents a critical juncture in disease pathogenesis, influencing cellular responses and the trajectory of infection-induced diseases. Recent Advances: Cutting-edge research has unveiled the GPx family's dynamic role in modulating viral pathogenesis. Notably, GPX4's pivotal function in regulating ferroptosis presents a novel avenue for the antiviral therapy. The discovery that selenium, an essential micronutrient for GPx activity, possesses antiviral properties has propelled us toward rethinking traditional treatment modalities. Critical Issues: Deciphering the intricate relationship between viral infections and GPx family members is paramount. Viral invasion can precipitate significant alterations in GPx function, influencing disease outcomes. The multifaceted nature of GPx activity during viral infections suggests that a deeper understanding of these interactions could yield novel insights into disease mechanisms, diagnostics, prognostics, and even chemotherapeutic resistance. Future Directions: This review aims to synthesize current knowledge on the impact of viral infections on GPx activity and expression and identify key advances. By elucidating the mechanisms through which GPx family members intersect with viral pathogenesis, we propose to uncover innovative therapeutic strategies that leverage the antioxidant properties of GPx to combat viral infections. The exploration of GPx as a therapeutic target and biomarker holds promise for the development of next-generation antiviral therapies. Antioxid. Redox Signal. 00, 000-000.

Glutathione-Dependent Pathways in Cancer Cells

The most abundant tripeptide-glutathione (GSH)-and the major GSH-related enzymes-glutathione peroxidases (GPxs) and glutathione S-transferases (GSTs)-are highly significant in the regulation of tumor cell viability, initiation of tumor development, its progression, and drug resistance. The high level of GSH synthesis in different cancer types depends not only on the increasing expression of the key enzymes of the γ-glutamyl cycle but also on the changes in transport velocity of its precursor amino acids. The ability of GPxs to reduce hydroperoxides is used for cellular viability, and each member of the GPx family has a different mechanism of action and site for maintaining redox balance. GSTs not only catalyze the conjugation of GSH to electrophilic substances and the reduction of organic hydroperoxides but also take part in the regulation of cellular signaling pathways. By catalyzing the S-glutathionylation of key target proteins, GSTs are involved in the regulation of major cellular processes, including metabolism (e.g., glycolysis and the PPP), signal transduction, transcription regulation, and the development of resistance to anticancer drugs. In this review, recent findings in GSH synthesis, the roles and functions of GPxs, and GST isoforms in cancer development are discussed, along with the search for GST and GPx inhibitors for cancer treatment.

Oxytosis/Ferroptosis in Neurodegeneration: the Underlying Role of Master Regulator Glutathione Peroxidase 4 (GPX4)

Oxytosis/ferroptosis is an iron-dependent oxidative form of cell death triggered by lethal accumulation of phospholipid hydroperoxides (PLOOHs) in membranes. Failure of the intricate PLOOH repair system is a principle cause of ferroptotic cell death. Glutathione peroxidase 4 (GPX4) is distinctly vital for converting PLOOHs in membranes to non-toxic alcohols. As such, GPX4 is known as the master regulator of oxytosis/ferroptosis. Ferroptosis has been implicated in a number of disorders such as neurodegenerative diseases (amyotrophic lateral sclerosis (ALS), Alzheimer's disease (AD), Parkinson's disease (PD), and Huntington's disease (HD), etc.), ischemia/reperfusion injury, and kidney degeneration. Reduced function of GPX4 is frequently observed in degenerative disorders. In this study, we examine how diminished GPX4 function may be a critical event in triggering oxytosis/ferroptosis to perpetuate or initiate the neurodegenerative diseases and assess the possible therapeutic importance of oxytosis/ferroptosis in neurodegenerative disorders. These discoveries are important for advancing our understanding of neurodegenerative diseases because oxytosis/ferroptosis may provide a new target to slow the course of the disease.

Biological and Catalytic Properties of Selenoproteins

Selenocysteine is a catalytic residue at the active site of all selenoenzymes in bacteria and mammals, and it is incorporated into the polypeptide backbone by a co-translational process that relies on the recoding of a UGA termination codon into a serine/selenocysteine codon. The best-characterized selenoproteins from mammalian species and bacteria are discussed with emphasis on their biological function and catalytic mechanisms. A total of 25 genes coding for selenoproteins have been identified in the genome of mammals. Unlike the selenoenzymes of anaerobic bacteria, most mammalian selenoenzymes work as antioxidants and as redox regulators of cell metabolism and functions. Selenoprotein P contains several selenocysteine residues and serves as a selenocysteine reservoir for other selenoproteins in mammals. Although extensively studied, glutathione peroxidases are incompletely understood in terms of local and time-dependent distribution, and regulatory functions. Selenoenzymes take advantage of the nucleophilic reactivity of the selenolate form of selenocysteine. It is used with peroxides and their by-products such as disulfides and sulfoxides, but also with iodine in iodinated phenolic substrates. This results in the formation of Se-X bonds (X = O, S, N, or I) from which a selenenylsulfide intermediate is invariably produced. The initial selenolate group is then recycled by thiol addition. In bacterial glycine reductase and D-proline reductase, an unusual catalytic rupture of selenium-carbon bonds is observed. The exchange of selenium for sulfur in selenoproteins, and information obtained from model reactions, suggest that a generic advantage of selenium compared with sulfur relies on faster kinetics and better reversibility of its oxidation reactions.

Research progress of glutathione peroxidase family (GPX) in redoxidation

Maintaining the balance of a cell's redox function is key to determining cell fate. In the critical redox system of mammalian cells, glutathione peroxidase (GPX) is the most prominent family of proteins with a multifaceted function that affects almost all cellular processes. A total of eight members of the GPX family are currently found, namely GPX1-GPX8. They have long been used as antioxidant enzymes to play an important role in combating oxidative stress and maintaining redox balance. However, each member of the GPX family has a different mechanism of action and site of action in maintaining redox balance. GPX1-4 and GPX6 use selenocysteine as the active center to catalyze the reduction of H₂O₂ or organic hydroperoxides to water or corresponding alcohols, thereby reducing their toxicity and maintaining redox balance. In addition to reducing H₂O₂ and small molecule hydroperoxides, GPX4 is also capable of reducing complex lipid compounds. It is the only enzyme in the GPX family that directly reduces and destroys lipid hydroperoxides. The active sites of GPX5 and GPX7-GPX8 do not contain selenium cysteine (Secys), but instead, have cysteine residues (Cys) as their active sites. GPX5 is mainly expressed in epididymal tissue and plays a role in protecting sperm from oxidative stress. Both enzymes, GPX7 and GPX8, are located in the endoplasmic reticulum and are necessary enzymes involved in the oxidative folding of endoplasmic reticulum proteins, and GPX8 also plays an important role in the regulation of Ca²⁺ in the endoplasmic reticulum. With an in-depth understanding of the role of the GPX family members in health and disease development, redox balance has become the functional core of GPX family, in order to further clarify the expression and regulatory mechanism of each member in the redox process, we reviewed GPX family members separately.

A white paper on Phospholipid Hydroperoxide Glutathione Peroxidase (GPx4) forty years later

The purification of a protein inhibiting lipid peroxidation led to the discovery of the selenoperoxidase GPx4 forty years ago. Thus, the evidence of the enzymatic activity was reached after identifying the biological effect and unambiguously defined the relationship between the biological function and the enzymatic activity. In the syllogism where GPx4 inhibits lipid peroxidation and its inhibition is lethal, cell death is operated by lipid peroxidation. Based on this rationale, this form of cell death emerged as regulated iron-enforced oxygen toxicity and was named ferroptosis in 2012. In the last decades, we learned that reduction of lipid hydroperoxides is indispensable and, in cooperation with prooxidant systems, controls the critical steady state of lipid peroxidation. This concept defined the GPx4 reaction as both the target for possible anti-cancer therapy and if insufficient, as cause of degenerative diseases. We know the reaction mechanism, but the details of the interaction at the membrane cytosol interface are still poorly defined. We know the gene structure, but the knowledge about expression control is still limited. The same holds true for post-transcriptional modifications. Reverse genetics indicate that GPx4 has a role in inflammation, immunity, and differentiation, but the observations emerging from these studies need a more specifically addressed biochemical evidence. Finally, the role of GPx4 in spermatogenesis disclosed an area unconnected to lipid peroxidation. In its mitochondrial and nuclear form, the peroxidase catalyzes the oxidation of protein thiols in two specific aspects of sperm maturation: stabilization of the mid-piece and chromatin compaction. Thus, although available evidence converges to the notion that GPx4 activity is vital due to the inhibition of lipid peroxidation, it is reasonable to foresee other unknown aspects of the GPx4 reaction to be disclosed.

A Bioinformatic Analysis: The Overexpression and Prognostic Potential of GPX7 in Lower-Grade Glioma

Purpose

Glutathione peroxidase-7 (GPX7) is a newly discovered non-selenium-containing protein with glutathione peroxidase activity, which mainly protects the organism from oxidative damage and is very important for basic biology studies. This study aims to reveal the expression pattern of GPX7 and its prognosis potential from a pan-cancer perspective.

Methods

Expression levels of GPX7 in human tumor tissues and normal tissues were evaluated using Human Protein Atlas (HPA), the Cancer Genome Atlas (TCGA), Genotype-Tissue Expression (GTEx) and UALCAN databases. The prognostic potential of GPX7 for 33 TCGA tumors was evaluated by Kaplan–Meier analysis and Cox regression analysis. Subsequently, the Chinese Glioma Genome Atlas (CGGA) dataset was used to further verify the expression of GPX7 and its prognostic potential in glioma. We explored the correlation between GPX7 and immune infiltration, tumor mutational burden (TMB) and microsatellite instability (MSI). Furthermore, a nomogram lower-grade glioma (LGG) was constructed to verify the prognostic outcome of patients. Finally, the relationship between GPX7 and treatment regimens for LGG was also explored.

Results

GPX7 was overexpressed in multiple tumors. Elevated expression of GPX7 was associated with poor prognosis of LGG patients (OS hazard ratio (HR) = 1.044, P < 0.0001; DFS HR = 1.035, P < 0.0001; PFS HR = 1.045, P < 0.0001). GPX7 was proved to be an independent prognostic factor of LGG through univariate and multivariate Cox analysis. The nomogram confirmed a better predictability (Concordance index (C-index): 0.845; 95% CI, 0.825–0.865). GPX7 was positively correlated with TMB in LGG. GPX7 expression was negatively correlated with half-maximal inhibitory concentration (IC50) of temozolomide (TMZ) (\documentclass[12pt]{minimal} \usepackage{wasysym} \usepackage[substack]{amsmath} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage[mathscr]{eucal} \usepackage{mathrsfs} \DeclareFontFamily{T1}{linotext}{} \DeclareFontShape{T1}{linotext}{m}{n} {linotext }{} \DeclareSymbolFont{linotext}{T1}{linotext}{m}{n} \DeclareSymbolFontAlphabet{\mathLINOTEXT}{linotext} \begin{document} $$\mathord{\buildrel{\lower3pt\hbox{$\scriptscriptstyle\frown$}}\over \rho } $$ \end{document}_spearman= −0.59, P =1.3e-48).

Conclusion

GPX7 was upregulated in multiple tumors, and it was a potential prognostic biomarker in LGG. High-expressed GPX7 can predict the sensitivity of TMZ in LGG patients.

GPX7 Is Targeted by miR-29b and GPX7 Knockdown Enhances Ferroptosis Induced by Erastin in Glioma

Background

Glioma is a lethal primary tumor of central nervous system. Ferroptosis is a newly identified form of necrotic cell death. Triggering ferroptosis has shown potential to eliminate aggressive tumors. GPX7, a member of glutathione peroxidase family (GPXs), has been described to participate in oxidative stress and tumorigenesis. However, the biological functions of GPX7 in glioma are still unknown.

Methods

Bioinformatics method was used to assess the prognostic role of GPX7 in glioma. CCK8, wound healing, transwell and cell apoptosis assays were performed to explore the functions of GPX7 in glioma cells. In vivo experiment was also conducted to confirm in vitro findings. Ferroptosis-related assays were carried out to investigate the association between GPX7 and ferroptosis in glioma.

Results

GPX7 was aberrantly expressed in glioma and higher expression of GPX7 was correlated with adverse outcomes. GPX7 silencing enhanced ferroptosis-related oxidative stress in glioma cells and the loss of GXP7 sensitized glioma to ferroptosis induced by erastin. Furthermore, we found that miR-29b directly suppressed GPX7 expression post-transcriptionally. Reconstitution of miR-29b enhanced erastin sensitivity, partly via GPX7 suppression.

Conclusions

Our study clarified the prognostic role of GPX7 in glioma and preliminarily revealed the role of GPX7 in ferroptosis, which may be conducive to the exploration of therapeutic targets of glioma.

ROS as Regulators of Cellular Processes in Melanoma

In this review, we examine the multiple roles of ROS in the pathogenesis of melanoma, focusing on signal transduction and regulation of gene expression. In recent years, different studies have analyzed the dual role of ROS in regulating the redox system, with both negative and positive consequences on human health, depending on cell concentration of these agents. High ROS levels can result from an altered balance between oxidant generation and intracellular antioxidant activity and can produce harmful effects. In contrast, low amounts of ROS are considered beneficial, since they trigger signaling pathways involved in physiological activities and programmed cell death, with protective effects against melanoma. Here, we examine these beneficial roles, which could have interesting implications in melanoma treatment.

Synergistic Effects of Dietary Selenomethionine and Vitamin C on the Immunity, Antioxidant Status, and Intestinal Microbiota in Sea Cucumber (Apostichopus japonicus)

A 30-day feeding trial was carried out to investigate the interactive effects of dietary selenium (selenomethionine) and vitamin C (Vc) in Apostichopus japonicus. Two selenium (0 and 5 mg/kg) and three vitamin C (0, 5000, and 10,000 mg/kg) combined groups of feed were formulated (Designated as LSeLVc, LSeMVc, LSeHVc, HSeLVc, HSeMVc and HSeHVc, respectively) and fed the sea cucumbers. Our results showed no significant effects on the growth-related parameters in sea cucumber (P > 0.05). Furthermore, the reciprocal action between Se and Vc had significant (P < 0.05) effects on Se accumulation in the respiratory tree and intestines. Also, the lysozyme, glutathione peroxidase activity, and the relative expression levels such as LZM, GPX, Hsp70, and Hsp90 in different tissues were significantly increased in the group of sea cucumber fed diet with 5 mg Se in combination with 5000 mg Vc compared with the control group (P < 0.05). However, MDA and H2O2 contents in the body wall were significantly reduced in the HSeHVc group (P < 0.05). In addition, analysis of intestinal flora revealed that Haloferula abundance was highest in the LSeMVc group than other treatment groups, and Vibrio abundance was decreased with combined Se and Vc supplement. Finally, the species diversity of the gut microbial community of sea cucumber in HSeMVc group was lower than those in other treatment groups. The results showed that the interaction of selenium and vitamin C had positive effects on improving the immune status, antioxidant capacity, and digestive ability of A. japonicus.

Oxidative stress drives divergent evolution of the glutathione peroxidase (GPX) gene family in mammals

The molecular basis for adaptations to extreme environments can now be understood by interrogating the ever-increasing number of sequenced genomes. Mammals such as cetaceans, bats, and highland species can protect themselves from oxidative stress, a disruption in the balance of reactive oxygen species, which results in oxidative injury and cell damage. Here, we consider the evolution of the glutathione peroxidase (GPX) family of antioxidant enzymes by interrogating publicly available genome data from 70 mammalian species from all major clades. We identified 8 GPX subclasses ubiquitous to all mammalian groups. Mammalian GPX gene families resolved into the GPX4/7/8 and GPX1/2/3/5/6 groups and are characterized by several instances of gene duplication and loss, indicating a dynamic process of gene birth and death in mammals. Seven of the eight GPX subfamilies (all but GPX7) were under positive selection, with the residues under selection located at or close to active sites or at the dimer interface. We also reveal evidence of a correlation between ecological niches (e.g. high oxidative stress) and the divergent selection and gene copy number of GPX subclasses. Notably, a convergent expansion of GPX1 was observed in several independent lineages of mammals under oxidative stress and may be important for avoiding oxidative damage. Collectively, this study suggests that the GPX gene family has shaped the adaption of mammals to stressful environments.

Emerging roles for non-selenium containing ER-resident glutathione peroxidases in cell signaling and disease

Maintenance of cellular redox control is pivotal for normal cellular functions and cell fate decisions including cell death. Among the key cellular redox systems in mammals, the glutathione peroxidase (GPX) family of proteins is the largest conferring multifaceted functions and affecting virtually all cellular processes. The endoplasmic reticulum (ER)-resident GPXs, designated as GPX7 and GPX8, are the most recently added members of this family of enzymes. Recent studies have provided exciting insights how both enzymes support critical processes of the ER including oxidative protein folding, maintenance of ER redox control by eliminating H₂O₂, and preventing palmitic acid-induced lipotoxicity. Consequently, numerous pathological conditions, such as neurodegeneration, cancer and metabolic diseases have been linked with altered GPX7 and GPX8 expression. Studies in mice have demonstrated that loss of GPX7 leads to increased differentiation of preadipocytes, increased tumorigenesis and shortened lifespan. By contrast, GPX8 deficiency in mice results in enhanced caspase-4/11 activation and increased endotoxic shock in colitis model. With the increasing recognition that both types of enzymes are dysregulated in various tumor entities in man, we deem a review of the emerging roles played by GPX7 and GPX8 in health and disease development timely and appropriate.

A novel soybean protein disulphide isomerase family protein possesses dithiol oxidation activity: identification and characterization of GmPDIL6

Secretory and membrane proteins synthesized in the endoplasmic reticulum (ER) are folded with intramolecular disulphide bonds, viz. oxidative folding, catalysed by the protein disulphide isomerase (PDI) family proteins. Here, we identified a novel soybean PDI family protein, GmPDIL6. GmPDIL6 has a single thioredoxin-domain with a putative N-terminal signal peptide and an active centre (CKHC). Recombinant GmPDIL6 forms various oligomers binding iron. Oligomers with or without iron binding and monomers exhibited a dithiol oxidase activity level comparable to those of other soybean PDI family proteins. However, they displayed no disulphide reductase and extremely low oxidative refolding activity. Interestingly, GmPDIL6 was mainly expressed in the cotyledon during synthesis of seed storage proteins and GmPDIL6 mRNA was up-regulated under ER stress. GmPDIL6 may play a role in the formation of disulphide bonds in nascent proteins for oxidative folding in the ER.

Improved production of Humira antibody in the genetically engineered Escherichia coli SHuffle, by co-expression of human PDI-GPx7 fusions

Abstract

Microbial production of antibodies offers the promise of cheap, fast, and efficient production of antibodies at an industrial scale. Limiting this capacity in prokaryotes is the absence of the post-translational machinery, present in dedicated antibody producing eukaryotic cell lines, such as B cells. There has been few and limited success in producing full-length, correctly folded, and assembled IgG in the cytoplasm of prokaryotic cell lines. One such success was achieved by utilizing the genetically engineered Escherichia coli strain SHuffle with an oxidative cytoplasm. Due to the genetic disruption of reductive pathways, SHuffle cells are under constant oxidative stress, including increased levels of hydrogen peroxide (H₂O₂). The oxidizing capacity of H₂O₂ was linked to improved disulfide bond formation, by expressing a fusion of two endoplasmic reticulum-resident proteins, the thiol peroxidase GPx7 and the protein disulfide isomerase, PDI. In concert, these proteins mediate disulfide transfer from H₂O₂ to target proteins via PDI-Gpx7 fusions. The potential of this new strain was tested with Humira, a blockbuster antibody usually produced in eukaryotic cells. Expression results demonstrate that the new engineered SHuffle strain (SHuffle2) could produce Humira IgG four-fold better than the parental strain, both in shake-flask and in high-density fermentation. These preliminary studies guide the field in genetically engineering eukaryotic redox pathways in prokaryotes for the production of complex macromolecules.

Key points

• A eukaryotic redox pathway was engineered into the E. coli strain SHuffle in order to improve the yield of the blockbuster antibody Humira.

• The best peroxidase-PDI fusion was selected using bioinformatics and in vivo studies.

• Improved yields of Humira were demonstrated at shake-flask and high-density fermenters.

Electronic supplementary material

The online version of this article (10.1007/s00253-020-10920-5) contains supplementary material, which is available to authorized users.

Regulatory Phenomena in the Glutathione Peroxidase Superfamily

Significance: The selenium-containing Glutathione peroxidases (GPxs)1-4 protect against oxidative challenge, inhibit inflammation and oxidant-induced regulated cell death. Recent Advances: GPx1 and GPx4 dampen phosphorylation cascades predominantly via prevention of inactivation of phosphatases by H₂O₂ or lipid hydroperoxides. GPx2 regulates the balance between regeneration and apoptotic cell shedding in the intestine. It inhibits inflammation-induced carcinogenesis in the gut but promotes growth of established cancers. GPx3 deficiency facilitates platelet aggregation likely via disinhibition of thromboxane biosynthesis. It is also considered a tumor suppressor. GPx4 is expressed in three different forms. The cytosolic form proved to inhibit interleukin-1-driven nuclear factor κB activation and leukotriene biosynthesis. Moreover, it is a key regulator of ferroptosis, because it reduces hydroperoxy groups of complex lipids and silences lipoxygenases. By alternate substrate use, the nuclear form contributes to chromatin compaction. Mitochondrial GPx4 forms the mitochondrial sheath of spermatozoa and, thus, guarantees male fertility. Out of the less characterized GPxs, the cysteine-containing GPx7 and GPx8 are unique in contributing to oxidative protein folding in the endoplasmic reticulum by reacting with protein isomerase as an alternate substrate. A yeast 2-Cysteine glutathione peroxidase equipped with CP and CR was reported to sense H₂O₂ for inducing an adaptive response. Critical Issues: Most of the findings compiled are derived from tissue culture and/or animal studies only. Their impact on human physiology is sometimes questionable. Future Directions: The expression of individual GPxs and GPx-dependent regulatory phenomena are to be further investigated, in particular in respect to human health.

Characterization of the endoplasmic reticulum-resident peroxidases GPx7 and GPx8 shows the higher oxidative activity of GPx7 and its linkage to oxidative protein folding

Oxidative protein folding occurs primarily in the mammalian endoplasmic reticulum, enabled by a diverse network comprising more than 20 members of the protein disulfide isomerase (PDI) family and more than five PDI oxidases. Although the canonical disulfide bond formation pathway involving Ero1α and PDI has been well-studied so far, the physiological roles of the newly identified PDI oxidases, glutathione peroxidase-7 (GPx7) and -8 (GPx8), are only poorly understood. We here demonstrated that human GPx7 has much higher reactivity with H₂O₂ and hence greater PDI oxidation activity than human GPx8. The high reactivity of GPx7 is due to the presence of a catalytic tetrad at the redox-active site, which stabilizes the sulfenylated species generated upon the reaction with H₂O₂ Although it was previously postulated that GPx7 catalysis involved a highly reactive peroxidatic cysteine that can be sulfenylated by H₂O₂, we revealed that a resolving cysteine instead regulates the PDI oxidation activity of GPx7. We also determined that GPx7 formed complexes preferentially with PDI and P5 in H₂O₂-treated cells. Altogether, these results suggest that human GPx7 functions as an H₂O₂-dependent PDI oxidase in cells, whereas PDI oxidation may not be the central physiological role of human GPx8.

Mechanistic study of PDIA1-catalyzed TFF3 dimerization during sepsis

AIMS

Trefoil factor 3 (TFF3) is a gut mucosal protective molecule that is secreted by intestinal goblet cells. The dimeric structure of TFF3 enables it to function in intestinal mucosal repair and to maintain its own stability. Protein disulfide isomerase a1 (PDIA1) can directly catalyze the formation, isomerization and reduction of disulfide bonds in proteins and may play an important role in the formation of TFF3 dimer. In this study, we focused on the specific molecular mechanism of TFF3 dimerization by PDIA1 and the changes during sepsis.

METHODS

We examined the changes of PDIA1 and TFF3 in sepsis rats and cell models and used a variety of experimental techniques to investigate the specific molecular mechanism of PDIA1-catalyzed TFF3 dimerization.

KEY FINDINGS

We found that PDIA1 can directly catalyze the dimerization of TFF3. Our MD model proposed that two TFF3 monomers form hydrogen bonds with the region b' of PDIA1 through two stepwise reactions. Furthermore, we propose that the Cys24-Cys27 active site at the region a' of PDIA1 mediates disulfide bond formation between the Cys79 residues of each of the two TFF3 monomers via deprotonation and nucleophilic attack. During sepsis, PDIA1 is downregulated and the excessive release of nitric oxide (NO) promoted PDIA1 nitrosylation. This modification reduced PDIA1 activity, which resulted in the corresponding decrease of TFF3 dimerization and compromised TFF3 dimer function.

SIGNIFICANCE

Our study revealed a novel mechanism for the inhibition of intestinal mucosal repair during sepsis and revealed novel targets for the prevention and treatment of sepsis.

Lack of glutathione peroxidase-8 in the ER impacts on lipid composition of HeLa cells microsomal membranes

GPx8 is a glutathione peroxidase homolog inserted in the membranes of endoplasmic reticulum (ER), where it seemingly plays a role in controlling redox status by preventing the spill of H₂O₂. We addressed the impact of GPx8 silencing on the lipidome of microsomal membranes, using stably GPx8-silenced HeLa cells. The two cell lines were clearly separated by Principal Component Analysis (PCA) and Partial Least Square Discriminant analysis (PLS-DA) of lipidome. Considering in detail the individual lipid classes, we observed that unsaturated glycerophospholipids (GPL) decreased, while only in phosphatidylinositols (PI) a substitution of monounsaturated fatty acids (MUFA) for polyunsaturated fatty acids (PUFA) was observed. Among sphingolipids (SL), ceramides (CER) decreased while sphingomyelins (SM) and neutral glycophingolipids (nGSL) increased. Here, in addition, longer chains than in controls in the amide fatty acid were present. The increase up to four folds of the CER (d18:1; c24:0) containing three hexose units, was the most remarkable species increasing in the differential lipidome of siGPx8 cells. Quantitative RT-PCR complied with lipidomic analysis specifically showing an increased expression of: i) acyl-CoA synthetase 5 (ACSL5); ii) CER synthase 2 and 4; iii) CER transporter (CERT); iv) UDP-glucosyl transferase (UDP-GlcT), associated to a decreased expression of UDP-galactosyl transferase (UDP-GalT). A role of the unfolded protein response (UPR) and the spliced form of the transcription factor XBP1 on the transcriptional changes of GPx8 silenced cells was ruled-out. Similarly, also the involvement of Nrf2 and NF-κB. Altogether our results indicate that GPx8-silencing of HeLa yields a membrane depleted by about 24% of polyunsaturated GPL and a corresponding increase of saturated or monounsaturated SM and specific nGSL. This is tentatively interpreted as an adaptive mechanism leading to an increased resistance to radical oxidations. Moreover, the marked shift of fatty acid composition of PI emerges as a possibly relevant issue in respect to the impact of GPx8 on signaling pathways.

Catalysis of Peroxide Reduction by Fast Reacting Protein Thiols

Life on Earth evolved in the presence of hydrogen peroxide, and other peroxides also emerged before and with the rise of aerobic metabolism. They were considered only as toxic byproducts for many years. Nowadays, peroxides are also regarded as metabolic products that play essential physiological cellular roles. Organisms have developed efficient mechanisms to metabolize peroxides, mostly based on two kinds of redox chemistry, catalases/peroxidases that depend on the heme prosthetic group to afford peroxide reduction and thiol-based peroxidases that support their redox activities on specialized fast reacting cysteine/selenocysteine (Cys/Sec) residues. Among the last group, glutathione peroxidases (GPxs) and peroxiredoxins (Prxs) are the most widespread and abundant families, and they are the leitmotif of this review. After presenting the properties and roles of different peroxides in biology, we discuss the chemical mechanisms of peroxide reduction by low molecular weight thiols, Prxs, GPxs, and other thiol-based peroxidases. Special attention is paid to the catalytic properties of Prxs and also to the importance and comparative outlook of the properties of Sec and its role in GPxs. To finish, we describe and discuss the current views on the activities of thiol-based peroxidases in peroxide-mediated redox signaling processes.

Oxidative protein folding in the plant endoplasmic reticulum

For most of the proteins synthesized in the endoplasmic reticulum (ER), disulfide bond formation accompanies protein folding in a process called oxidative folding. Oxidative folding is catalyzed by a number of enzymes, including the family of protein disulfide isomerases (PDIs), as well as other proteins that supply oxidizing equivalents to PDI family proteins, like ER oxidoreductin 1 (Ero1). Oxidative protein folding in the ER is a basic vital function, and understanding its molecular mechanism is critical for the application of plants as protein production tools. Here, I review the recent research and progress related to the enzymes involved in oxidative folding in the plant ER. Firstly, nine groups of plant PDI family proteins are introduced. Next, the enzymatic properties of plant Ero1 are described. Finally, the cooperative folding by multiple PDI family proteins and Ero1 is described.

Oxidative protein folding: state-of-the-art and current avenues of research in plants

Contents Summary 1230 I. Introduction 1230 II. Formation and isomerization of disulfides in the ER and the Golgi apparatus 1231 III. The disulfide relay in the mitochondrial intermembrane space: why are plants different? 1236 IV. Disulfide bond formation on luminal proteins in thylakoids 1240 V. Conclusion 1242 Acknowledgements 1242 References 1242 SUMMARY: Disulfide bonds are post-translational modifications crucial for the structure and function of thousands of proteins. Their formation and isomerization, referred to as oxidative folding, require specific protein machineries found in oxidizing subcellular compartments, namely the endoplasmic reticulum and the associated endomembrane system, the intermembrane space of mitochondria and the thylakoid lumen of chloroplasts. At least one protein component is required for transferring electrons from substrate proteins to an acceptor that is usually molecular oxygen. For oxidation reactions, incoming reduced substrates are oxidized by thiol-oxidoreductase proteins (or domains in case of chimeric proteins), which are usually themselves oxidized by a single thiol oxidase, the enzyme generating disulfide bonds de novo. By contrast, the description of the molecular actors and pathways involved in proofreading and isomerization of misfolded proteins, which require a tightly controlled redox balance, lags behind. Herein we provide a general overview of the knowledge acquired on the systems responsible for oxidative protein folding in photosynthetic organisms, highlighting their particularities compared to other eukaryotes. Current research challenges are discussed including the importance and specificity of these oxidation systems in the context of the existence of reducing systems in the same compartments.

Redox Signaling by Reactive Electrophiles and Oxidants

The concept of cell signaling in the context of nonenzyme-assisted protein modifications by reactive electrophilic and oxidative species, broadly known as redox signaling, is a uniquely complex topic that has been approached from numerous different and multidisciplinary angles. Our Review reflects on five aspects critical for understanding how nature harnesses these noncanonical post-translational modifications to coordinate distinct cellular activities: (1) specific players and their generation, (2) physicochemical properties, (3) mechanisms of action, (4) methods of interrogation, and (5) functional roles in health and disease. Emphasis is primarily placed on the latest progress in the field, but several aspects of classical work likely forgotten/lost are also recollected. For researchers with interests in getting into the field, our Review is anticipated to function as a primer. For the expert, we aim to stimulate thought and discussion about fundamentals of redox signaling mechanisms and nuances of specificity/selectivity and timing in this sophisticated yet fascinating arena at the crossroads of chemistry and biology.

GPx4, Lipid Peroxidation, and Cell Death: Discoveries, Rediscoveries, and Open Issues

SIGNIFICANCE

Iron-dependent lipid peroxidation is a complex oxidative process where phospholipid hydroperoxides (PLOOH) are produced in membranes and finally transformed into a series of decomposition products, some of which are endowed with biological activity. It is specifically prevented by glutathione peroxidase 4 (GPx4), the selenoenzyme that reduces PLOOH by glutathione (GSH). PLOOH is both a product and the major initiator of peroxidative chain reactions, as well as an activator of lipoxygenases. α-Tocopherol both specifically breaks peroxidative chain propagation and inhibits lipoxygenases. Thus, GPx4, GSH, and α-tocopherol are integrated in a concerted anti-peroxidant mechanism. Recent Advances: Ferroptosis has been recently identified as a cell death subroutine that is specifically activated by missing GPx4 activity and inhibited by iron chelation or α-tocopherol supplementation. Ferroptosis induction may underlie spontaneous human diseases, such as major neurodegeneration and neuroinflammation, causing an excessive cell death. The basic mechanism of ferroptosis, therefore, fits the features of activation of lipid peroxidation.

CRITICAL ISSUES

Still lacking are convincing proofs that lipoxygenases are involved in ferroptosis. Also, unknown are the molecules eventually killing cells and the mechanisms underlying the drop of the cellular anti-peroxidant capacity.

FUTURE DIRECTIONS

Molecular events and mechanisms of ferroptosis to be unraveled and validated on animal models are GPx4 inactivation, role of GSH concentration, increased iron availability, and membrane structure and composition. This is expected to drive drug discovery that is aimed at halting cell death in degenerative diseases or boosting it in cancer cells. Antioxid. Redox Signal. 29, 61-74.

Glutathione peroxidase 4-catalyzed reduction of lipid hydroperoxides in membranes: The polar head of membrane phospholipids binds the enzyme and addresses the fatty acid hydroperoxide group toward the redox center

GPx4 is a monomeric glutathione peroxidase, unique in reducing the hydroperoxide group (-OOH) of fatty acids esterified in membrane phospholipids. This reaction inhibits lipid peroxidation and accounts for enzyme's vital role. Here we investigated the interaction of GPx4 with membrane phospholipids. A cationic surface near the GPx4 catalytic center interacts with phospholipid polar heads. Accordingly, SPR analysis indicates cardiolipin as the phospholipid with maximal affinity to GPx4. Consistent with the electrostatic nature of the interaction, KCl increases the K_D. Molecular dynamic (MD) simulation shows that a -OOH posed in the core of the membrane as 13 - or 9 -OOH of tetra-linoleoyl cardiolipin or 15 -OOH stearoyl-arachidonoyl-phosphaphatidylcholine moves to the lipid-water interface. Thereby, the -OOH groups are addressed toward the GPx4 redox center. In this pose, however, the catalytic site facing the membrane would be inaccessible to GSH, but the consecutive redox processes facilitate access of GSH, which further primes undocking of the enzyme, because GSH competes for the binding residues implicated in docking. During the final phase of the catalytic cycle, while GSSG is produced, GPx4 is disconnected from the membrane. The observation that GSH depletion in cells induces GPx4 translocation to the membrane, is in agreement with this concept.

ER-resident antioxidative GPx7 and GPx8 enzyme isoforms protect insulin-secreting INS-1E β-cells against lipotoxicity by improving the ER antioxidative capacity

Increased circulating levels of saturated fatty acids (FFAs) and glucose are considered to be major mediators of β-cell dysfunction and death in T2DM. Although it has been proposed that endoplasmic reticulum (ER) and oxidative stress play a crucial role in gluco/lipotoxicity, their interplay and relative contribution to β-cell dysfunction and apoptosis has not been fully elucidated. In addition it is still unclear how palmitate - the physiologically most abundant long-chain saturated FFA - elicits ER stress and which immediate signals commit β-cells to apoptosis. To study the underlying mechanisms of palmitate-mediated ER stress and β-cell toxicity, we exploited the observation that the recently described ER-resident GPx7 and GPx8 are not expressed in rat β-cells. Expression of GPx7 or GPx8 attenuated FFAs-mediated H₂O₂ generation, ER stress, and apoptosis induction. These results could be confirmed by a H₂O₂-specific inactivating ER catalase, indicating that accumulation of H₂O₂ in the ER lumen is critical in FFA-induced ER stress. Furthermore, neither the expression of GPx7 nor of GPx8 increased insulin content or facilitated disulfide bond formation in insulin-secreting INS-1E cells. Hence, reduction of H₂O₂ by ER-GPx isoforms is not rate-limiting in oxidative protein folding in rat β-cells. These data suggest that FFA-mediated ER stress is partially dependent on oxidative stress and selective expression of GPx7 or GPx8 improves the ER antioxidative capacity of rat β-cells without compromising insulin production and the oxidative protein folding machinery.

A glutathione peroxidase from Antarctic psychrotrophic bacterium Pseudoalteromonas sp. ANT506: Cloning and heterologous expression of the gene and characterization of recombinant enzyme

A glutathione peroxidase (GPx) gene, designated as PsGPx, was cloned from Antarctic psychrotrophic bacterium Pseudoalteromonas sp. ANT506 and expressed in Escherichia coli. The full-length PsGPx contained a 585-bp encoding 194 amino acids with predicted molecular masses of approx. 21.7 kDa. Multiple sequence alignments revealed that PsGPx belonged to the thioredoxin-like superfamily. PsGPx was heterologously overexpressed in E. coli, purified and characterized. The maximum catalytic temperature and pH value for recombinant PsGPx (rPsGPx) were 30°C and pH 9.0, respectively. rPsGPx retained 45% of the maximum activity at 0°C and exhibited high thermolability with a half-life of approx. 40 min at 40°C. In addition, the enzymatic activity of rPsGPx was still manifested under 3 M NaCl. The K_m and V_max values of the recombinant enzyme using GSH and H₂O₂ as substrates were 1.73 mM and 16.28 nmol/mL/min versus 2.46 mM and 21.50 nmol/mL/min, respectively.

Glutathione peroxidase-like enzymes cover five distinct cell compartments and membrane surfaces in Arabidopsis thaliana

Glutathione peroxidase-like enzymes (GPXLs) constitute a family of eight peroxidases in Arabidopsis thaliana. In contrast to the eponymous selenocysteine glutathione peroxidases in mammalian cells that use glutathione as electron donor, GPXLs rely on cysteine instead of selenocysteine for activity and depend on the thioredoxin system for reduction. Although plant GPXLs have been implicated in important agronomic traits such as drought tolerance, photooxidative tolerance and immune responses, there remain major ambiguities regarding their subcellular localization. Because their site of action is a prerequisite for an understanding of their function, we investigated the localization of all eight GPXLs in stable Arabidopsis lines expressing N-terminal and C-terminal fusions with redox-sensitive green fluorescent protein 2 (roGFP2) using confocal microscopy. GPXL1 and GPXL7 were found in plastids, while GPXL2 and GPXL8 are cytosolic nuclear. The N-terminal target peptide of GPXL6 is sufficient to direct roGFP2 into mitochondria. Interestingly, GPXL3, GPXL4 and GPXL5 all appear to be membrane bound. GPXL3 was found exclusively in the secretory pathway where it is anchored by a single N-terminal transmembrane domain. GPXL4 and GPXL5 are anchored to the plasma membrane. Presence of an N-terminal myristoylation motif and genetic disruption of membrane association through targeted mutagenesis point to myristoylation as essential for membrane localization.

Protein disulfide isomerases: Redox connections in and out of the endoplasmic reticulum

Protein disulfide isomerases are thiol oxidoreductase chaperones from thioredoxin superfamily. As redox folding catalysts from the endoplasmic reticulum (ER), their roles in ER-related redox homeostasis and signaling are well-studied. PDIA1 exerts thiol oxidation/reduction and isomerization, plus chaperone effects. Also, substantial evidence indicates that PDIs regulate thiol-disulfide switches in other cell locations such as cell surface and possibly cytosol. Subcellular PDI translocation routes remain unclear and seem Golgi-independent. The list of signaling and structural proteins reportedly regulated by PDIs keeps growing, via thiol switches involving oxidation, reduction and isomerization, S-(de)nytrosylation, (de)glutathyonylation and protein oligomerization. PDIA1 is required for agonist-triggered Nox NADPH oxidase activation and cell migration in vascular cells and macrophages, while PDIA1-dependent cytoskeletal regulation appears a converging pathway. Extracellularly, PDIs crucially regulate thiol redox signaling of thrombosis/platelet activation, e.g., integrins, and PDIA1 supports expansive caliber remodeling during injury repair via matrix/cytoskeletal organization. Some proteins display regulatory PDI-like motifs. PDI effects are orchestrated by expression levels or post-translational modifications. PDI is redox-sensitive, although probably not a mass-effect redox sensor due to kinetic constraints. Rather, the "all-in-one" organization of its peculiar redox/chaperone properties likely provide PDIs with precision and versatility in redox signaling, making them promising therapeutic targets.

Alkyl chain modulated cytotoxicity and antioxidant activity of bioinspired amphiphilic selenolanes

A series of amphiphilic conjugates of dihydroxy selenolane (DHS) and monoamine selenolane (MAS), which we had previously reported to inhibit lipid peroxidation and assist the oxidative protein folding reaction respectively in cell free systems, were evaluated for cytotoxicity, associated mechanisms and antioxidant effects in cells. Our results indicated that a fatty acid/alkyl group of variable chain lengths (C6-14) as a lipophilic moiety of the DHS/MAS conjugates not only improved their ability to incorporate within the plasma membrane of cells but also modulated their cytotoxicity. In the concentration range of 1-50 μM, C6 conjugates were non-toxic whereas the long chain (≥C8) conjugates showed significant cytotoxicity. The induction of toxicity investigated by the changes in membrane leakage, fluidity, mitochondrial membrane potential and annexin-V-propidium iodide (PI) staining by using flow cytometry revealed plasma membrane disintegration and subsequent induction of necrosis as the major mechanism. Further, the conjugates of DHS and MAS also showed differential as well as nonlinear tendency in cytotoxicity with respect to chain lengths and this effect was attributed to their self-aggregation properties. Compared with the parent compounds, C6 conjugates not only exhibited better antioxidant activity in terms of the induction of selenoproteins such as glutathione peroxidase 1 (GPx1), GPx4 and thioredoxin reductase 1 (TrxR1) but also protected cells from the AAPH induced oxidative stress. In conclusion, the present study suggests the importance of hydrophilic-lipophilic balance (HLB) in fine tuning the toxicity and activity of bioinspired amphiphilic antioxidants.

Glutathione Is the Resolving Thiol for Thioredoxin Peroxidase Activity of 1-Cys Peroxiredoxin Without Being Consumed During the Catalytic Cycle

AIMS

A three-step catalytic cycle is common to all peroxiredoxins (Prxs), despite structural and kinetic differences. The second step in 1-Cys type Prxs is a matter of debate since they lack an additional cysteine to play the resolving role, as happens with the 2-Cys Prxs. The aim of this study was to elucidate the role of glutathione (GSH) in the thioredoxin-dependent peroxidase activity of Saccharomyces cerevisiae mitochondrial Prx1p, a 1-Cys type Prx.

RESULTS

The peroxidatic Cys91 residue of two Prx1p peptides can be linked by a disulfide, which can be reduced by thioredoxin and by GSH (Km=6.1 μM). GSH forms a mixed disulfide with the peroxidatic cysteine spontaneously in vitro and in vivo. Mitochondrial Trx3p deglutathionylates Prx1p without formation of GSSG so that GSH is not consumed in the process. The structural unit of native Prx1p is a dimer whose subunits are not covalently linked, but a hexameric assembly of three disulfide-bound dimers can also be formed.

INNOVATION

GSH is presented as a protective cofactor of Prx1p, which is not consumed during the peroxidase reaction, but provides a robust mechanism as the resolving cysteine and efficiently prevents Prx1p overoxidation. GSH exerts these roles at concentrations well below those commonly considered necessary for its antioxidant and redox buffering functions.

CONCLUSION

A 1-Cys peroxide scavenging mechanism operates in yeast mitochondria involving an autonomous glutathione molecule and the thioredoxin system, which could have universal validity. Prx1p is fairly well protected from overoxidation, questioning its role in a floodgate mechanism for H2O2 signaling.

Selenocysteine oxidation in glutathione peroxidase catalysis: an MS-supported quantum mechanics study

Glutathione peroxidases (GPxs) are enzymes working with either selenium or sulfur catalysis. They adopted diverse functions ranging from detoxification of H(2)O(2) to redox signaling and differentiation. The relative stability of the selenoenzymes, however, remained enigmatic in view of the postulated involvement of a highly unstable selenenic acid form during catalysis. Nevertheless, density functional theory calculations obtained with a representative active site model verify the mechanistic concept of GPx catalysis and underscore its efficiency. However, they also allow that the selenenic acid, in the absence of the reducing substrate, reacts with a nitrogen in the active site. MS/MS analysis of oxidized rat GPx4 complies with the predicted structure, an 8-membered ring, in which selenium is bound as selenenylamide to the protein backbone. The intermediate can be re-integrated into the canonical GPx cycle by glutathione, whereas, under denaturing conditions, its selenium moiety undergoes β-cleavage with formation of a dehydro-alanine residue. The selenenylamide bypass prevents destruction of the redox center due to over-oxidation of the selenium or its elimination and likely allows fine-tuning of GPx activity or alternate substrate reactions for regulatory purposes.

Composition of the redox environment of the endoplasmic reticulum and sources of hydrogen peroxide

The endoplasmic reticulum (ER) is a metabolically active organelle, which has a central role in proteostasis by translating, modifying, folding, and occasionally degrading secretory and membrane proteins. The lumen of the ER represents a separate compartment of the eukaryotic cell, with a characteristic proteome and metabolome. Although the redox metabolome and proteome of the compartment have not been holistically explored, it is evident that proper redox conditions are necessary for the functioning of many luminal pathways. These redox conditions are defined by local oxidoreductases and the membrane transport of electron donors and acceptors. The main electron carriers of the compartment are identical with those of the other organelles: glutathione, pyridine and flavin nucleotides, ascorbate, and others. However, their composition, concentration, and redox state in the ER lumen can be different from those observed in other compartments. The terminal oxidases of oxidative protein folding generate and maintain an "oxidative environment" by oxidizing protein thiols and producing hydrogen peroxide. ER-specific mechanisms reutilize hydrogen peroxide as an electron acceptor of oxidative folding. These mechanisms, together with membrane and kinetic barriers, guarantee that redox systems in the reduced or oxidized state can be present simultaneously in the lumen. The present knowledge on the in vivo conditions of ER redox is rather limited; development of new genetically encoded targetable sensors for the measurement of the luminal state of redox systems other than thiol/disulfide will contribute to a better understanding of ER redox homeostasis.

Understanding mammalian glutathione peroxidase 7 in the light of its homologs

The glutathione peroxidase homologs (GPxs) efficiently reduce hydroperoxides using electrons from glutathione (GSH), thioredoxin (Trx), or protein disulfide isomerase (PDI). Trx is preferentially used by the GPxs of the majority of bacteria, invertebrates, plants, and fungi. GSH or PDI, instead, is preferentially used by vertebrate GPxs that operate by Sec or Cys catalysis, respectively. Mammalian GPx7 and GPx8 are unique homologs that contain a peroxidatic Cys (CP). Being reduced by PDI and located within the endoplasmic reticulum (ER), these enzymes have been involved in oxidative protein folding. Kinetic analysis indicates that oxidation of PDI by recombinant GPx7 occurs at a much faster rate than that of GSH. Nonetheless, activity measurement suggests that, at physiological concentrations, a competition between these two substrates takes place, with the rate of PDI oxidation by GPx7 controlled by the concentration of GSH, whereas the GSSG produced in the competing reaction contributes to the ER redox buffer. A mechanism has been proposed for GPx7 involving two Cys residues, in which an intramolecular disulfide of the CP is formed with an alleged resolving Cys (CR) located in the strongly conserved FPCNQ motif (C86 in humans), a noncanonical position in GPxs. Kinetic measurements and comparison with the other thiol peroxidases containing a functional CR suggest that a resolving function of C86 in the catalytic cycle is very unlikely. We propose that GPx7 is catalytically active as a 1-Cys-GPx, in which CP both reduces H2O2 and oxidizes PDI, and that the CP-C86 disulfide has instead the role of stabilizing the oxidized peroxidase in the absence of the reducing substrate.

Effects of in ovo injection with selenium on immune and antioxidant responses during experimental necrotic enteritis in broiler chickens

This study was conducted to investigate the effects of in ovo injection of Se on modulating the immune system and antioxidant responses in broiler chickens with experimental necrotic enteritis. Broiler eggs were injected at 18 d of embryo age with either 100 μL of PBS alone or sodium selenite (Na2SeO3) in PBS, providing 0 (SS0), 10 (SS10), or 20 (SS20) μg of Se/egg. At 14 d posthatch, PBS-treated and uninfected chickens were kept as the control group, whereas the remaining chickens were orally infected with 1.0 × 10(4) sporulated oocysts of Eimeria maxima (SS0, SS10, SS20). At 18 d posthatch, E. maxima-infected chickens were orally infected with 1.0 × 10(9) cfu of Clostridium perfringens. Infected control SS0 group showed significantly decreased BW compared with the uninfected control. However, SS20 group showed significantly increased BW compared with the infected control SS0 group, whereas the BW were similar among uninfected control and infected SS10 and SS20 groups. The SS10 group showed significantly lower intestinal lesions compared with the SS0 group, and oocyst production was decreased in both SS10 and SS20 groups. Serum malondialdehyde level and catalase activity were also decreased in both SS10 and SS20 groups, whereas the superoxide dismutase level was significantly lower in the SS10 group compared with the SS0 group. The SS20 group showed significantly higher levels of transcripts for IL-1β and IL-6 in intestine, and SS10 and SS20 groups had higher levels of transcripts for IL-8 and inducible nitric oxide synthase expression and decreased glutathione peroxidase 7 mRNA levels compared with the SS0 group. The SS10 and SS20 groups also showed increased serum antibody levels to C. perfringens α-toxin and NetB toxin compared with the SS0 group. These collective results suggest that the injection of Se into the amniotic cavity of developing eggs may be beneficial for enhancing immune and antioxidant responses in the hatched chickens exposed to the necrotic enteritis-causing pathogens.

Biochemical and pathological studies on peroxidases -an updated review

Peroxidases represent a family of isoenzymes actively involved in oxidizing reactive oxygen species, innate immunity, hormone biosynthesis and pathogenesis of several diseases. Different types of peroxidases have organ, tissues, cellular and sub-cellular level of specificities in their function. Different diseases lead to varied expressions of peroxidases based on several mechanisms proposed. Several researches are going on to understand its deficiency, over-expression and malfunction in relation with different diseases. Some common diseases of mankind like cancer, cardiovascular diseases and diabetes directly or indirectly involve the role of peroxidases. So the status of peroxidase levels may also function as a marker of different diseases. Although many types of diseases in human beings have a strong correlation with tissue specific peroxidases, the clear role of these oxido-reductases is not yet fully understood. Here we are focusing on the role of peroxidases in relations with different diseases occurring due to oxidative stress.

GPx8 peroxidase prevents leakage of H2O2 from the endoplasmic reticulum

Unbalanced endoplasmic reticulum (ER) homeostasis (ER stress) leads to increased generation of reactive oxygen species (ROS). Disulfide-bond formation in the ER by Ero1 family oxidases produces hydrogen peroxide (H2O2) and thereby constitutes one potential source of ER-stress-induced ROS. However, we demonstrate that Ero1α-derived H2O2 is rapidly cleared by glutathione peroxidase (GPx) 8. In 293 cells, GPx8 and reduced/activated forms of Ero1α co-reside in the rough ER subdomain. Loss of GPx8 causes ER stress, leakage of Ero1α-derived H2O2 to the cytosol, and cell death. In contrast, peroxiredoxin (Prx) IV, another H2O2-detoxifying rough ER enzyme, does not protect from Ero1α-mediated toxicity, as is currently proposed. Only when Ero1α-catalyzed H2O2 production is artificially maximized can PrxIV participate in its reduction. We conclude that the peroxidase activity of the described Ero1α-GPx8 complex prevents diffusion of Ero1α-derived H2O2 within and out of the rough ER. Along with the induction of GPX8 in ER-stressed cells, these findings question a ubiquitous role of Ero1α as a producer of cytoplasmic ROS under ER stress.

An overview of mechanisms of redox signaling

A principal characteristic of redox signaling is that it involves an oxidation-reduction reaction or covalent adduct formation between the sensor signaling protein and second messenger. Non-redox signaling may involve alteration of the second messenger as in hydrolysis of GTP by G proteins, modification of the signaling protein as in farnesylation, or simple non-covalent binding of an agonist or second messenger. The chemistry of redox signaling is reviewed here. Specifically we have described how among the so-called reactive oxygen species, only hydroperoxides clearly fit the role of a second messenger. Consideration of reaction kinetics and cellular location strongly suggests that for hydroperoxides, particular protein cysteines are the targets and that the requirements for redox signaling is that these cysteines are in microenvironments in which the cysteine is ionized to the thiolate, and a proton can be donated to form a leaving group. The chemistry described here is the same as occurs in the cysteine and selenocysteine peroxidases that are generally considered the primary defense against oxidative stress. But, these same enzymes can also act as the sensors and transducer for signaling. Conditions that would allow specific signaling by peroxynitrite and superoxide are also defined. Signaling by other electrophiles, which includes lipid peroxidation products, quinones formed from polyphenols and other metabolites also involves reaction with specific protein thiolates. Again, kinetics and location are the primary determinants that provide specificity required for physiological signaling although enzymatic catalysis is not likely involved. This article is part of a Special Issue entitled "Redox Signalling in the Cardiovascular System".

Glutathione peroxidase 7 utilizes hydrogen peroxide generated by Ero1α to promote oxidative protein folding

AIMS

Ero1 flavoproteins catalyze oxidative folding in the endoplasmic reticulum (ER), consuming oxygen and generating hydrogen peroxide (H2O2). The ER-localized glutathione peroxidase 7 (GPx7) shows protein disulfide isomerase (PDI)-dependent peroxidase activity in vitro. Our work aims at identifying the physiological role of GPx7 in the Ero1α/PDI oxidative folding pathway and at dissecting the reaction mechanisms of GPx7.

RESULTS

Our data show that GPx7 can utilize Ero1α-produced H2O2 to accelerate oxidative folding of substrates both in vitro and in vivo. H2O2 oxidizes Cys57 of GPx7 to sulfenic acid, which can be resolved by Cys86 to form an intramolecular disulfide bond. Both the disulfide form and sulfenic acid form of GPx7 can oxidize PDI for catalyzing oxidative folding. GPx7 prefers to interact with the a domain of PDI, and intramolecular cooperation between the two redox-active sites of PDI increases the activity of the Ero1α/GPx7/PDI triad.

INNOVATION

Our in vitro and in vivo evidence provides mechanistic insights into how cells consume potentially harmful H2O2 while optimizing oxidative protein folding via the Ero1α/GPx7/PDI triad. Cys57 can promote PDI oxidation in two ways, and Cys86 emerges as a novel noncanonical resolving cysteine.

CONCLUSION

GPx7 promotes oxidative protein folding, directly utilizing Ero1α-generated H2O2 in the early secretory compartment. Thus, the Ero1α/GPx7/PDI triad generates two disulfide bonds and two H2O molecules at the expense of a single O2 molecule.

Destroy and exploit: catalyzed removal of hydroperoxides from the endoplasmic reticulum

Peroxidases are enzymes that reduce hydroperoxide substrates. In many cases, hydroperoxide reduction is coupled to the formation of a disulfide bond, which is transferred onto specific acceptor molecules, the so-called reducing substrates. As such, peroxidases control the spatiotemporal distribution of diffusible second messengers such as hydrogen peroxide (H₂O₂) and generate new disulfides. Members of two families of peroxidases, peroxiredoxins (Prxs) and glutathione peroxidases (GPxs), reside in different subcellular compartments or are secreted from cells. This review discusses the properties and physiological roles of PrxIV, GPx7, and GPx8 in the endoplasmic reticulum (ER) of higher eukaryotic cells where H₂O₂ and—possibly—lipid hydroperoxides are regularly produced. Different peroxide sources and reducing substrates for ER peroxidases are critically evaluated. Peroxidase-catalyzed detoxification of hydroperoxides coupled to the productive use of disulfides, for instance, in the ER-associated process of oxidative protein folding, appears to emerge as a common theme. Nonetheless, in vitro and in vivo studies have demonstrated that individual peroxidases serve specific, nonoverlapping roles in ER physiology.