Immunolocalization of thioredoxin and glutaredoxin in mammalian hypophysis

Elevated antioxidant defence in the brain of deep-diving pinnipeds

While foraging, marine mammals undertake repetitive diving bouts. When the animal surfaces, reperfusion makes oxygen readily available for the electron transport chain, which leads to increased production of reactive oxygen species and risk of oxidative damage. In blood and several tissues, such as heart, lung, muscle and kidney, marine mammals generally exhibit an elevated antioxidant defence. However, the brain, whose functional integrity is critical to survival, has received little attention. We previously observed an enhanced expression of several antioxidant genes in cortical neurons of hooded seals (Cystophora cristata). Here, we studied antioxidant gene expression and enzymatic activity in the visual cortex, cerebellum and hippocampus of harp seals (Pagophilus groenlandicus) and hooded seals. Moreover, we tested several genes for positive selection. We found that antioxidants in the first line of defence, such as superoxide dismutase (SOD), glutathione peroxidase (GPX) and glutathione (GSH) were constitutively enhanced in the seal brain compared to mice (Mus musculus), whereas the glutaredoxin and thioredoxin systems were not. Possibly, the activity of the latter systems is stress-induced rather than constitutively elevated. Further, some, but not all members, of the glutathione-s-transferase (GST) family appear more highly expressed. We found no signatures of positive selection, indicating that sequence and function of the studied antioxidants are conserved in pinnipeds.

Glutaredoxin: Discovery, redox defense and much more

Glutaredoxin, Grx, is a small protein containing an active site cysteine pair and was discovered in 1976 by Arne Holmgren. The Grx system, comprised of Grx, glutathione, glutathione reductase, and NADPH, was first described as an electron donor for Ribonucleotide Reductase but, from the first discovery in E.coli, the Grx family has impressively grown, particularly in the last two decades. Several isoforms have been described in different organisms (from bacteria to humans) and with different functions. The unique characteristic of Grxs is their ability to catalyse glutathione-dependent redox regulation via glutathionylation, the conjugation of glutathione to a substrate, and its reverse reaction, deglutathionylation. Grxs have also recently been enrolled in iron sulphur cluster formation. These functions have been implied in various physiological and pathological conditions, from immune defense to neurodegeneration and cancer development thus making Grx a possible drug target. This review aims to give an overview on Grxs, starting by a phylogenetic analysis of vertebrate Grxs, followed by an analysis of the mechanisms of action, the specific characteristics of the different human isoforms and a discussion on aspects related to human physiology and diseases.

Mitochondrial redox and TCA cycle metabolite signaling in the heart

Mitochondria are essential signaling organelles that regulate a broad range of cellular processes and thereby heart function. Multiple mechanisms participate in the communication between mitochondria and the nucleus that maintain cardiomyocyte homeostasis, including mitochondrial reactive oxygen species (ROS) and metabolic shifts in TCA cycle metabolite availability. An increased rate of ROS generation can cause irreversible damage to the cell and proposed to be a leading cause of many pathologies, including accelerated aging and heart disease. Myocardial impairments are also characterised by specific coordinated metabolic changes and dysregulated inflammatory responses. Hence, the mitochondrial respiratory chain is an important mediator between health and disease in the heart. This review will first outline the sources of ROS in the heart, mitochondrial metabolite dynamics, and provide an overview of their implications for heart disease. In addition, we will concentrate our discussion around current cardioprotective strategies relevant to mitochondrial ROS. Thorough understanding of mitochondrial signaling and the complex interplay with vital signaling pathways in the heart might allow us to develop novel therapeutic approaches to cardiovascular disease.

Knockout of PRDX6 induces mitochondrial dysfunction and cell cycle arrest at G2/M in HepG2 hepatocarcinoma cells

Peroxiredoxin 6 (PRDX6) has been associated with tumor progression and cancer metastasis. Its acting on phospholipid hydroperoxides and its phospholipase-A2 activity are unique among the peroxiredoxin family and add complexity to its action mechanisms. As a first step towards the study of PRDX6 involvement in cancer, we have constructed a human hepatocarcinoma HepG2^PRDX6-/- cell line using the CRISPR/Cas9 technique and have characterized the cellular response to lack of PRDX6.

Applying quantitative global and redox proteomics, flow cytometry, in vivo extracellular flow analysis, Western blot and electron microscopy, we have detected diminished respiratory capacity, downregulation of mitochondrial proteins and altered mitochondrial morphology. Autophagic vesicles were abundant while the unfolded protein response (UPR), HIF1A and NRF2 transcription factors were not activated, despite increased levels of p62/SQSTM1 and reactive oxygen species (ROS). Insulin receptor (INSR), 3-phosphoinositide-dependent protein kinase 1 (PDPK1), uptake of glucose and hexokinase-2 (HK2) decreased markedly while nucleotide biosynthesis, lipogenesis and synthesis of long chain polyunsaturated fatty acids (LC-PUFA) increased. 254 Cys-peptides belonging to 202 proteins underwent significant redox changes. PRDX6 knockout had an antiproliferative effect due to cell cycle arrest at G2/M transition, without signs of apoptosis.

Loss of PLA2 may affect the levels of specific lipids altering lipid signaling pathways, while loss of peroxidase activity could induce redox changes at critical sensitive cysteine residues in key proteins. Oxidation of specific cysteines in Proliferating Cell Nuclear Antigen (PCNA) could interfere with entry into mitosis. The GSH/Glutaredoxin system was downregulated likely contributing to these redox changes. Altogether the data demonstrate that loss of PRDX6 slows down cell division and alters metabolism and mitochondrial function, so that cell survival depends on glycolysis to lactate for ATP production and on AMPK-independent autophagy to obtain building blocks for biosynthesis. PRDX6 is an important link in the chain of elements connecting redox homeostasis and proliferation.

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A CRISPR-Cas9 based PRDX6 KO human cell line is characterized for the first time.

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Loss of PRDX6 causes mitochondrial dysfunction, autophagy and slow growth rate.

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Glucose uptake and HK2 decrease; nucleotide biosynthesis and lipogenesis increase.

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Oxidation of PCNA Cys residues could be responsible for cell cycle arrest at G2/M.

Determination of glutaredoxin enzyme activity and protein S-glutathionylation using fluorescent eosin-glutathione

Glutaredoxins catalyze glutathione-dependent disulfide oxidoreductions, particularly reduction of glutathione (GSH)-protein mixed disulfides. Mammalian glutaredoxins are present in the cytosol/nucleus as Grx1 or in mitochondria as Grx2a. Here we describe di-eosin-glutathione disulfide (Di-E-GSSG) as a new tool to study glutaredoxin (Grx) activity. Di-E-GSSG has almost no fluorescence in its disulfide form due to self-quenching, whereas the reduced form (E-GSH) has a large fluorescence emission at 545 nm after excitation at 520 nm. Di-E-GSSG was a very poor substrate for glutathione reductase, but we discovered that the molecule was an excellent substrate for glutaredoxin in a coupled assay system with GSH, nicotinamide adenine dinucleotide phosphate (NADPH), and glutathione reductase or with lipoamide, NADH, and lipoamide dehydrogenase. In addition, Di-E-GSSG was used to glutathionylate the free SH group of bovine serum albumin (BSA), yielding eosin-glutathionylated BSA (E-GS-BSA) readily observed in ultraviolet (UV) light. E-GS-BSA also displayed a quenched fluorescence, and its Grx-catalyzed reduction could be followed by the formation of E-GSH by fluorescence emission using microtiter plates. This way of measuring Grx activity provided an ultrasensitive method that detected Grx1 and Grx2 at picomolar levels. Human Grx1 was readily quantified in 40 μl of plasma and determined to be 680 ± 208 pM in healthy controls.

Thioredoxins, glutaredoxins, and peroxiredoxins--molecular mechanisms and health significance: from cofactors to antioxidants to redox signaling

Thioredoxins (Trxs), glutaredoxins (Grxs), and peroxiredoxins (Prxs) have been characterized as electron donors, guards of the intracellular redox state, and "antioxidants". Today, these redox catalysts are increasingly recognized for their specific role in redox signaling. The number of publications published on the functions of these proteins continues to increase exponentially. The field is experiencing an exciting transformation, from looking at a general redox homeostasis and the pathological oxidative stress model to realizing redox changes as a part of localized, rapid, specific, and reversible redox-regulated signaling events. This review summarizes the almost 50 years of research on these proteins, focusing primarily on data from vertebrates and mammals. The role of Trx fold proteins in redox signaling is discussed by looking at reaction mechanisms, reversible oxidative post-translational modifications of proteins, and characterized interaction partners. On the basis of this analysis, the specific regulatory functions are exemplified for the cellular processes of apoptosis, proliferation, and iron metabolism. The importance of Trxs, Grxs, and Prxs for human health is addressed in the second part of this review, that is, their potential impact and functions in different cell types, tissues, and various pathological conditions.

Human Protein Atlas of redox systems - what can be learnt?

BACKGROUND

High-throughput screening projects are popular approaches to yield a vast amount of information amenable for database mining and "hypothesis generation". The keys to success for these approaches depend upon the quality of primary data, choice of algorithms for data analyses, solidity in data annotations and the general usefulness of the results. A large initiative aimed at mapping the expression of all human proteins is the Human Protein Atlas (www.proteinatlas.org), encompassing immunohistochemical analyses of human tissues utilizing antibodies raised against a large number of human proteins. Here, we wished to probe what could be learnt from this atlas using a manual in-depth analysis of the results regarding the expression of key proteins in the human glutathione and thioredoxin systems.

METHODS

The freely available on-line data of immunohistochemical analyses for selected human redox proteins within the Human Protein Atlas were here analyzed, provided that reasonably solid data existed for the antibodies that were employed. This included tissue expression data for thioredoxin 1 (Trx1), Trx2, thioredoxin reductase 1 (TrxR1), TrxR2, glutathione reductase (GR), glucose 6-phosphate dehydrogenase (G6PD), γ-glutamyl cysteinyl synthase (gGCS) and the six peroxiredoxins Prx1 to Prx6. The data were further complemented with a screen using a polyclonal peptide antibody raised against the unique glutaredoxin domain of TXNRD1_v3 ("v3"). The results from fifteen major tissues and organs are presented (lung, kidney, liver, lymph node, testis, prostate, ovary, breast, pancreas, cerebellum, hippocampus, cerebral cortex, skin, skeletal muscle and heart muscle) and discussed considering earlier findings described in the literature.

RESULTS

Staining patterns proved to be highly variable and often unexpected both in terms of tissues analyzed and the individual target proteins. Among the analyzed tissues, only macrophages of the lung, tubular cells of the kidney, lymphoid cells of lymph nodes, Leydig cells in the testis, glandular cells of the prostate and exocrine glandular cells of the pancreas, showed positive staining with all of the fourteen antibodies that were analyzed. Among these antibodies, those against Trx1, TrxR2 and G6PD showed the most restricted staining across different tissues, while others including the antibodies against Trx2, TrxR1, GR, Prx3, Prx4 and Prx6 gave strong staining in most tissues. Staining for v3 was strong in many cells and tissues, which was unexpected considering previous results mapping transcripts for this protein. No obvious co-variation in staining across tissues could be noted when comparing any two of the analyzed antibodies. Staining for G6PD was weak in most tissues, except for cells of the seminiferous ducts in testis and follicular cells of the ovary, where G6PD staining was strong.

CONCLUSIONS

Results from high-throughput screening projects such as the Human Protein Atlas must be taken with caution and need to be duly confirmed by thorough in-depth follow-up studies. The varying staining intensities comparing tissues as seen here for most of the analyzed antibodies nonetheless suggest that the overall profile of the human redox systems may vary significantly between different cell types and between different tissues.

GENERAL SIGNIFICANCE

The Human Protein Atlas data suggest that the individual proteins of the human thioredoxin and glutathione systems may be strikingly tissue- and cell type-specific in terms of expression levels, but we also conclude that these type of high-throughput results should be taken with significant caution and must be duly verified using subsequent focused and detailed hypothesis-guided follow-up studies. This article is part of a Special Issue entitled Human and Murine Redox Protein Atlases.

Glutaredoxin-1 mediates NADPH-dependent stimulation of calcium-dependent insulin secretion

Nicotinamide adenine dinucleotide phosphate (NADPH) enhances Ca(2+)-induced exocytosis in pancreatic beta-cells, an effect suggested to involve the cytosolic redox protein glutaredoxin-1 (GRX-1). We here detail the role of GRX-1 in NADPH-stimulated beta-cell exocytosis and glucose-stimulated insulin secretion. Silencing of GRX-1 by RNA interference reduced glucose-stimulated insulin secretion in both clonal INS-1 832/13 cells and primary rat islets. GRX-1 silencing did not affect cell viability or the intracellular redox environment, suggesting that GRX-1 regulates the exocytotic machinery by a local action. By contrast, knockdown of the related protein thioredoxin-1 (TRX-1) was ineffective. Confocal immunocytochemistry revealed that GRX-1 locates to the cell periphery, whereas TRX-1 expression is uniform. These data suggest that the distinct subcellular localizations of TRX-1 and GRX-1 result in differences in substrate specificities and actions on insulin secretion. Single-cell exocytosis was likewise suppressed by GRX-1 knockdown in both rat beta-cells and clonal 832/13 cells, whereas after overexpression exocytosis increased by approximately 40%. Intracellular addition of NADPH (0.1 mm) stimulated Ca(2+)-evoked exocytosis in both cell types. Interestingly, the stimulatory action of NADPH on the exocytotic machinery coincided with an approximately 30% inhibition in whole-cell Ca(2+) currents. After GRX-1 silencing, NADPH failed to amplify insulin release but still inhibited Ca(2+) currents in 832/13 cells. In conclusion, NADPH stimulates the exocytotic machinery in pancreatic beta-cells. This effect is mediated by the NADPH acceptor protein GRX-1 by a local redox reaction that accelerates beta-cell exocytosis and, in turn, insulin secretion.

Redox control of exocytosis: regulatory role of NADPH, thioredoxin, and glutaredoxin

Cellular redox state is an important metabolic variable, influencing many aspects of cell function like growth, apoptosis, and reductive biosynthesis. In this report, we identify NADPH as a candidate signaling molecule for exocytosis in neuroendocrine cells. In pancreatic beta-cells, glucose acutely raised the NADPH-to-NADP+ ratio and stimulated insulin release in parallel. Furthermore, intracellular addition of NADPH directly stimulated exocytosis of insulin granules. Effects of NADPH on exocytosis are proposed to be mediated by the redox proteins glutaredoxin (GRX) and thioredoxin (TRX) on the basis of the following evidence: 1) Expression of GRX mRNA is very high in beta-cells compared with other studied tissues, and GRX protein expression is high in islets and in brain; 2) GRX and TRX are localized in distinct microdomains in the cytosol of beta-cells; and 3) microinjection of recombinant GRX potentiated effects of NADPH on exocytosis, whereas TRX antagonized the NADPH effect. We propose that the NADPH/GRX/TRX redox regulation mediates a novel signaling pathway of nutrient-induced insulin secretion.

The thioredoxin system?From science to clinic

The thioredoxin system-formed by thioredoxin reductase and its characteristic substrate thioredoxin-is an important constituent of the intracellular redox milieu. Interactions with many different metabolic pathways such as DNA-synthesis, selenium metabolism, and the antioxidative network as well as significant species differences render this system an attractive target for chemotherapeutic approaches in many fields of medicine-ranging from infectious diseases to cancer therapy. In this review we will present and evaluate the preclinical and clinical results available today. Current trends in drug development are emphasized.

Involvement of thioredoxin in the regulation of growth hormone secretion in rat pituitary cell cultures

We report here an examination of the effect of thioredoxin (TRX) on the secretion of growth hormone (GH) from rat anterior pituitary cells in vitro. Treatment of rat pituitary cells with growth hormone-releasing factor (GRF), but not GH, led to a significant increase in intracellular TRX protein levels. GRF, recombinant human TRX (rhTRX), and a combination thereof were all shown to induce immediate GH secretion from pituitary cells, as evidenced by perifusion experiments. RhTRX, but not other reducing agents such as beta-mercaptoethanol and N-acetyl-L-cysteine, augmented GRF-stimulated and -unstimulated GH secretion from rat pituitary cells in a dose-dependent manner. RhTRX did not significantly affect the GH mRNA expression of pituitary cells stimulated in the presence or absence of GRF. In addition, rhTRX-augmented GH secretion was not significantly affected by the presence of cycloheximide. Collectively, these findings suggest that TRX is induced by stimulation with GRF and plays a regulatory role in GH secretion from rat anterior pituitary cells by enhancing the secretion of stored GH, rather than by the synthesis of GH.

Glutaredoxin protects cerebellar granule neurons from dopamine-induced apoptosis by dual activation of the ras-phosphoinositide 3-kinase and jun n-terminal kinase pathways

Glutaredoxin 2 (Grx2) from Escherichia coli protects cerebellar neurons from dopamine-induced apoptosis via nuclear factor kappa B (NF-kappaB) activation, which is mediated by the expression of redox factor-1 (Ref-1). An analysis of the mechanisms underlying Grx2 protective activity revealed dual activation of signal transduction pathways. Grx2 significantly activated the Ras/phosphoinositide 3-kinase/Akt/NF-kappaB cascade in parallel to the Jun N-terminal kinase (JNK)/AP1 cascade. Dopamine, in comparison, down-regulated both pathways. Treatment of neurons with Ref-1 antisense oligonucleotide reduced the ability of Grx2 to activate Akt and AP-1 but had no effect on the phosphorylation of JNK1/2, suggesting that Akt/NF-kappaB and AP-1 are regulated by Ref-1. Exposure of the neurons to JNK1/2 antisense oligonucleotide in the presence of Grx2 significantly reduced AP-1 and NF-kappaB DNA binding activities and abolished Grx2 protection. These results demonstrate that dual activation of Ras/phosphoinositide 3-kinase and AP-1 cascades, which are mediated by Ref-1, is an essential component of the Grx2 mechanism of action.

Immunohistochemical localization of thioredoxin and glutaredoxin in mouse embryos and fetuses

Although oxygen is essential for promoting energy metabolism and for enhancing cell proliferation, early mouse embryos are very sensitive to high oxygen concentration. Because the tissue-specificity and sequential changes of the expression of antioxidative enzymes in rodent embryos have not been investigated systematically, we examined the ontogenesis of thioredoxin (TRX) and glutaredoxin (GRX) in mouse embryos and fetuses by using immunohistochemical methods. These compounds were found to be localized in most tissues examined, with some tissue specificity and temporal sequence. In many tissues, both TRX and GRX began to be expressed at embryonic day 11 (E11) or E13 and appeared to increase later in development, but the heart and great vessels of E8.5 embryos were already positive for their immunoreactivity. The stage at which the antioxidative enzymes begin to be expressed seems to coincide with the stage at which rodent embryos acquire the capacity of aerobic energy metabolism. Although TRX and GRX were co-localized in many tissues and showed similar sequential changes of expression, their expression patterns were different in the fetal cartilage, suggesting that they may play different roles in endochondral ossification. Their immunoreactivity was not homogeneous in the liver and the epithelium of uriniferous tubules, probably because their expression is associated with the proliferating and metabolic activities of the cell, as suggested by previous investigators. These results suggest that TRX and GRX play some tissue-specific roles in mammalian morphogenesis as well as general roles as antioxidant enzymes.

Characterization of mammalian thioredoxin reductase, thioredoxin and glutaredoxin by immunochemical methods

Specific polyclonal antibodies towards the oxidized form of bovine thioredoxin reductase (TR) have been obtained in rabbits, and purified. The antigenicity was lost upon reduction of TR by NADPH indicating a large conformational change upon reduction of the redox-active disulfide in the enzyme. The antibodies did not cross-react with other bovine NADPH-dependent dehydrogenases. No reactivity was observed with TR from bacteria, yeast or rat and only a slight reaction was obtained with TR from horse. Immunoaffinity purified anti-thioredoxin and anti-glutaredoxin antibodies were used to develop competitive indirect ELISA assays that were validated giving very good linearity, reproducibility, sensitivity and parallelism. The glutaredoxin (Grx) immunoassay is the first quantitative method described to measure the protein. When applied to a battery of calf tissues the contents of Grx varied from 7 to 120 micrograms per gram of fresh tissue. Skeletal and heart muscles gave the lowest values and spleen and salivary glands the highest. However, skeletal muscle showed the highest gluthathione-hydroxyethyl disulfide oxidoreductase specific activity.

Purification from placenta, amino acid sequence, structure comparisons and cDNA cloning of human glutaredoxin

Glutaredoxin is generally a glutathione-dependent hydrogen donor for ribonucleotide reductase and also catalyses general glutathione (GSH)-disulfide-oxidoreduction reactions in the presence of NADPH and glutathione reductase. A Glutaredoxin from human placenta was purified to homogeneity, as judged by SDS/PAGE and IEF (12 kDa). Purification was monitored by the activity with hydroxyethyl disulfide as a substrate. Values of pI for glutaredoxin were obtained by IEF; the pI of the protein shifted from 7.3 in its fully reduced state to 9.0 in the oxidized state after treatment with excess hydroxyethyl disulfide. The glutaredoxin preparation showed GSH-dependent hydrogen-donor activity with recombinant mouse ribonucleotide reductase, it exhibited dehydroascorbate reductase activity as well as hydroxyethyl-disulfide-reducing activity. The amino acid sequence (residues 3-104) of glutaredoxin was determined by peptide sequencing and residues 1, 2 and 105 by cDNA sequence analysis. The glutaredoxin sequence comprised the classical active site for glutaredoxins -Cys22-Pro-Tyr-Cys25- and three additional half-cystine residues; two of these in positions 78 and 82. The sequence was similar to other known mammalian glutaredoxins (about 80% identities), with important differences such as one additional Cys residue (Cys7) and no Met residue. The sequence of human glutaredoxin was compared to that of Escherichia coli glutaredoxin with known three-dimensional structure in solution to identify conserved residues and predict a structure from alignment. In particular the GSH-binding site of glutaredoxin was conserved between all molecules. A cDNA that encodes the entire glutaredoxin gene (grx) and flanking sequences was isolated from a human spleen cDNA library. The nucleotide sequence of this cDNA (0.8 kb) was determined, including the complete grx gene.

Topological relationships between porcine anterior pituitary hormones and the thioredoxin and glutaredoxin systems

Thioredoxin (TRX) and glutaredoxin (GRX) had previously been localized in folliculo-stellatae (FS) cells and in only a fraction of glandular cells of the anterior pituitary (Padilla et al., 1992). Here we report on a double immunolabelling study carried out to determine the correlation between the type of secretory cell and the presence of TRX or GRX. TRX and GRX levels were under the detection limits in gonadotropes, thyrotropes and corticotropes. A considerable proportion of lactotropes contained TRX or GRX; a higher proportion of somatotropes contained TRX and all of them were GRX-positive. The secretory cell types more frequently detected in the epithelium of well-defined follicles lined by TRX-positive FS cells were somatotropes and lactotropes followed by corticotropes; gonadotropes and thyrotropes were scarce in these structures. Regarding the biological functions of glutaredoxin and thioredoxin, these results show that involvement in the processing of secretory proteins is not a general property of these two thiol-disulfide oxidoreductases, not even specifically in the case of cysteine-rich secretory proteins. On the other hand, another type of functional specificity perhaps related to the heterogeneous response of the endocrine cells is discussed.

Purification and properties of bovine thioredoxin system

Using a variety of chromatographic techniques, a crude extract from bovine liver was fractionated to obtain pure preparations of thioredoxin reductase, thioredoxin, glutaredoxin and glutathione reductase with good yields. The turbidimetric assay of thioredoxin with insulin as the disulfide substrate was optimized; by incorporation of the lag time (tau) into the calculations, linearity was maintained for a wider range of thioredoxin concentrations, and a distinction could be made between reduced and non-reduced forms. Subunit composition and molecular mass, absorption spectrum and kinetic parameters of thioredoxin reductase were similar to those of other mammalian thioredoxin reductases. By chromatofocusing, two peaks of activity were detected at pH 5.5 and 5.8. Structural changes undergone by the thioredoxin molecule upon oxido-reduction were detected by isoelectric focusing, with a shift of 0.1 pH unit of its pI, and by analytical anion exchange chromatography, with a conspicuous shift of its retention time. These two methods also revealed the presence of a form of thioredoxin not undergoing the above mentioned redox-mediated structural shifts that accounted for > 75% of the total activity.