Induction of cell membrane protrusions by the N-terminal glutaredoxin domain of a rare splice variant of human thioredoxin reductase 1

Selenoprotein: Potential Player in Redox Regulation in Chlamydomonas reinhardtii

Selenium (Se) is an essential micro-element for many organisms, including Chlamydomonas reinhardtii, and is required in trace amounts. It is obtained from the 21st amino acid selenocysteine (Sec, U), genetically encoded by the UGA codon. Proteins containing Sec are known as selenoproteins. In eukaryotes, selenoproteins are present in animals and algae, whereas fungi and higher plants lack them. The human genome contains 25 selenoproteins, most of which are involved in antioxidant defense activity, redox regulation, and redox signaling. In algae, 42 selenoprotein families were identified using various bioinformatics approaches, out of which C. reinhardtii is known to have 10 selenoprotein genes. However, the role of selenoproteins in Chlamydomonas is yet to be reported. Chlamydomonas selenoproteins contain conserved domains such as CVNVGC and GCUG, in the case of thioredoxin reductase, and CXXU in other selenoproteins. Interestingly, Sec amino acid residue is present in a catalytically active domain in Chlamydomonas selenoproteins, similar to human selenoproteins. Based on catalytical active sites and conserved domains present in Chlamydomonas selenoproteins, we suggest that Chlamydomonas selenoproteins could have a role in redox regulation and defense by acting as antioxidants in various physiological conditions.

Biochemical and structural characterizations of thioredoxin reductase selenoproteins of the parasitic filarial nematodes Brugia malayi and Onchocerca volvulus

Enzymes in the thiol redox systems of microbial pathogens are promising targets for drug development. In this study we characterized the thioredoxin reductase (TrxR) selenoproteins from Brugia malayi and Onchocerca volvulus, filarial nematode parasites and causative agents of lymphatic filariasis and onchocerciasis, respectively. The two filarial enzymes showed similar turnover numbers and affinities for different thioredoxin (Trx) proteins, but with a clear preference for the autologous Trx. Human TrxR1 (hTrxR1) had a high and similar specific activity versus the human and filarial Trxs, suggesting that, in vivo, hTrxR1 could possibly be the reducing agent of parasite Trxs once they are released into the host. Both filarial TrxRs were efficiently inhibited by auranofin and by a recently described inhibitor of human TrxR1 (TRi-1), but not as efficiently by the alternative compound TRi-2. The enzyme from B. malayi was structurally characterized also in complex with NADPH and auranofin, producing the first crystallographic structure of a nematode TrxR. The protein represents an unusual fusion of a mammalian-type TrxR protein architecture with an N-terminal glutaredoxin-like (Grx) domain lacking typical Grx motifs. Unlike thioredoxin glutathione reductases (TGRs) found in platyhelminths and mammals, which are also Grx-TrxR domain fusion proteins, the TrxRs from the filarial nematodes lacked glutathione disulfide reductase and Grx activities. The structural determinations revealed that the Grx domain of TrxR from B. malayi contains a cysteine (C22), conserved in TrxRs from clade IIIc nematodes, that directly interacts with the C-terminal cysteine-selenocysteine motif of the homo-dimeric subunit. Interestingly, despite this finding we found that altering C22 by mutation to serine did not affect enzyme catalysis. Thus, although the function of the Grx domain in these filarial TrxRs remains to be determined, the results obtained provide insights on key properties of this important family of selenoprotein flavoenzymes that are potential drug targets for treatment of filariasis.

Effects of Mammalian Thioredoxin Reductase Inhibitors

The mammalian thioredoxin system is driven by NADPH through the activities of isoforms of the selenoprotein thioredoxin reductase (TXNRD, TrxR), which in turn help to keep thioredoxins (TXN, Trx) and further downstream targets reduced. Due to a wide range of functions in antioxidant defense, cell proliferation, and redox signaling, strong cellular aberrations are seen upon the targeting of TrxR enzymes by inhibitors. However, such inhibition can nonetheless have rather unexpected consequences. Accumulating data suggest that inhibition of TrxR in normal cells typically yields a paradoxical effect of increased antioxidant defense, with metabolic pathway reprogramming, increased cellular proliferation, and altered cellular differentiation patterns. Conversely, inhibition of TrxR in cancer cells can yield excessive levels of reactive oxygen species (ROS) resulting in cell death and thus anticancer efficacy. The observed increases in antioxidant capacity upon inhibition of TrxR in normal cells are in part dependent upon activation of the Nrf2 transcription factor, while exaggerated ROS levels in cancer cells can be explained by a non-oncogene addiction of cancer cells to TrxR1 due to their increased endogenous production of ROS. These separate consequences of TrxR inhibition can be utilized therapeutically. Importantly, however, a thorough knowledge of the molecular mechanisms underlying effects triggered by TrxR inhibition is crucial for the understanding of therapy outcomes after use of such inhibitors. The mammalian thioredoxin system is driven by thioredoxin reductases (TXNRD, TrxR), which keeps thioredoxins (TXN, Trx) and further downstream targets reduced. In normal cells, inhibition of TrxR yields a paradoxical effect of increased antioxidant defense upon activation of the Nrf2 transcription factor. In cancer cells, however, inhibition of TrxR yields excessive reactive oxygen species (ROS) levels resulting in cell death and thus anticancer efficacy, which can be explained by a non-oncogene addiction of cancer cells to TrxR1 due to their increased endogenous production of ROS. These separate consequences of TrxR inhibition can be utilized therapeutically.

Implications of the mitochondrial interactome of mammalian thioredoxin 2 for normal cellular function and disease

Multiple thioredoxin isoforms exist in all living cells. To explore the possible functions of mammalian mitochondrial thioredoxin 2 (Trx2), an interactome of mouse Trx2 was initially created using (i) a monothiol mouse Trx2 species for capturing protein partners from different organs and (ii) yeast two hybrid screens on human liver and rat brain cDNA libraries. The resulting interactome consisted of 195 proteins (Trx2 included) plus the mitochondrial 16S RNA. 48 of these proteins were classified as mitochondrial (MitoCarta2.0 human inventory). In a second step, the mouse interactome was combined with the current four-membered mitochondrial sub-network of human Trx2 (BioGRID) to give a 53-membered human Trx2 mitochondrial interactome (52 interactor proteins plus the mitochondrial 16S RNA). Although thioredoxins are thiol-employing disulfide oxidoreductases, approximately half of the detected interactions were not due to covalent disulfide bonds. This finding reinstates the extended role of thioredoxins as moderators of protein function by specific non-covalent, protein-protein interactions. Analysis of the mitochondrial interactome suggested that human Trx2 was involved potentially in mitochondrial integrity, formation of iron sulfur clusters, detoxification of aldehydes, mitoribosome assembly and protein synthesis, protein folding, ADP ribosylation, amino acid and lipid metabolism, glycolysis, the TCA cycle and the electron transport chain. The oxidoreductase functions of Trx2 were verified by its detected interactions with mitochondrial peroxiredoxins and methionine sulfoxide reductase. Parkinson's disease, triosephosphate isomerase deficiency, combined oxidative phosphorylation deficiency, and lactate dehydrogenase b deficiency are some of the diseases where the proposed mitochondrial network of Trx2 may be implicated.

Modulation of the transcriptomic profile of the R2C tumor Leydig cell line by the adipose tissue derived hormone leptin

Leptin is an important adipose derived hormone being secreted by adipocytes and involved in appetite regulation. Disruption of normal plasma levels of leptin has been associated with the pathogenesis of obesity. It is now well established that obesity is associated to an increased risk of cancer development, including testicular cancers. Recently, we have shown that high levels of leptin have inhibitory effects on cAMP-dependent steroidogenic genes expression in MA-10 Leydig cells. Hence, we examined if leptin could alter the transcriptome of the constitutively steroidogenic rat tumor Leydig cell line R2C. These cells were treated with high levels of leptin (1000 ng/ml) for 4 h, followed by mRNA extraction and RNA-Seq analysis. Interestingly, leptin had no effect on steroidogenic gene expression or pathways promoting tumorigenesis of this cell line. Genes being upregulated or downregulated by leptin were enriched in biological processes that were non-relevant to Leydig cell function. Surprisingly, lepr isoforms were weakly expressed in R2C cells and may explain in part the low levels of leptin's response for tumor Leydig cells. Hence, the current findings suggest that acute treatments of tumor Leydig cells with high levels of leptin have negligible effects on transcriptomics and rather supports that leptin would have a central effect on the hypothalamus to influence testicular function.

Mitochondrial reactive oxygen species regulate the induction of CD8+ T cells by plasmacytoid dendritic cells

Cross-presentation allows exogenous antigen presentation in association with major histocompatibility complex class I molecules, a process crucial for the priming of CD8⁺ T-cell responses against viruses and tumors. By contrast to conventional dendritic cells (cDC), which cross-present antigens in the steady state, plasmacytoid dendritic cells (pDC) acquire this ability only after stimulation by Toll-like receptor (TLR) ligands. The intracellular pathways accounting for this functional difference are still unknown. Here we show that the induction of cross-presentation by pDCs is regulated by mitochondria through a reactive oxygen species (ROS)-dependent mechanism, involving pH alkalization and antigen protection. The reduction of mitochondrial ROS production dramatically decreases the cross-presentation capacity of pDCs, leading to a strong reduction of their capacity to trigger CD8⁺ T-cell responses. Our results demonstrate the importance of mitochondrial metabolism in pDC biology, particularly for the induction of adaptive immune responses.

The A to Z of modulated cell patterning by mammalian thioredoxin reductases

Mammalian thioredoxin reductases (TrxRs) are selenocysteine-containing proteins (selenoproteins) that propel a large number of functions through reduction of several substrates including the active site disulfide of thioredoxins (Trxs). Well-known enzymatic systems that in turn are supported by Trxs and TrxRs include deoxyribonucleotide synthesis through ribonucleotide reductase, antioxidant defense through peroxiredoxins and methionine sulfoxide reductases, and redox modulation of a number of transcription factors. Although these functions may be essential for cells due to crucial roles in maintenance of cell viability and proliferation, findings during the last decade reveal that mammals have major redundancy in their cellular reductive systems. The synthesis of glutathione (GSH) and reductive functions of GSH-dependent pathways typically act in parallel with Trx-dependent pathways, with only one of these systems often being sufficient to support viability. Importantly, this does not imply that a modulation of the Trx system will remain without consequences, even when GSH-dependent pathways remain functional. As suggested by several recent findings, the Trx system in general and the TrxRs in particular, function as key regulators of signaling pathways. In this review article we will discuss findings that collectively suggest that modulation in mammalian systems of cytosolic TrxR1 (TXNRD1) or mitochondrial TrxR2 (TXNRD2) influence cell patterning and cellular stress responses. Effects of lower activities include increased adipogenesis, insulin responsiveness, glycogen accumulation, hyperproliferation, and distorted embryonic development, while increased activities correlate with decreased proliferation and extended lifespan, as well as worse cancer prognosis. The molecular mechanisms that underlie these diverse effects, involving regulation of protein phosphorylation cascades and of key transcription factors that guide cellular differentiation pathways, will be discussed. We conclude that the selenium-dependent oxidoreductases TrxR1 and TrxR2 should be considered as key components of signaling pathways that control cell differentiation and cellular stress responses.

Quantitative proteomic analysis of HeLa cells in response to biocompatible Fe2C@C nanoparticles: 16O/18O-labelling & HPLC-ESI-orbit-trap profiling approach

The effective detection of molecular biomarkers, such as proteins, lipids, carbohydrates, and pathogens, in a living body is a huge challenge in the field of nanomedicine. Here, we have investigated the comparative quantitative proteomics analysis of the molecular response of HeLa cells to biocompatible Fe2C@C nanoparticles (NPs) using 16O/18O isotopic labelling of the cell culture. The relative binding efficiency of proteins to Fe2C@C NPs was calculated. HPLC-ESI-orbit-trap analysis found 51 differentially expressed proteins, out of which 23 were over-expressed and 28 down-regulated. This study showed that Fe2C@C NPs alter the expression of the proteins involved in endocytosis, cell-cycle regulation, and cell membrane protrusion. Further, the quantification and validation of the mass spectrometry (MS) results was successfully confirmed by western blot analysis of cytochrome C. The change in the expression of proteins can be useful for early stage disease diagnoses and the development of tailored therapeutic strategies. This study is the first large-scale characterization of low abundance proteins on Fe2C@C NPs, providing the biochemical basis for the assessment of the suitability of magnetic NPs as biomedical markers and emerging functional probes.

Silencing thioredoxin1 exacerbates damage of astrocytes exposed to OGD/R by aggravating apoptosis through the Actin-Ras2-cAMP-PKA pathway

PURPOSE OF THE STUDY

Induction of endogenous antioxidants is one of the key molecular mechanisms of cell resistance to hypoxia/ischemia. Thioredoxin1 (Trx1) is a small multifunctional ubiquitous antioxidant with redox-active dithiol and plays an important role in cell apoptosis through mitochondrial apoptosis pathways. The specific role of Trx1 in ischemia-reperfusion induced astrocyte apoptosis, however, remains unclear.

MATERIALS AND METHODS

In this study, we investigated the effect of Trx1 on apoptosis of astrocyte using an in vitro oxygen-glucose deprivation/reoxygenation (OGD/R) model which mimics ischemic/reperfusion conditions in vivo. The astrocytes prepared from newborn Sprague-Dawley rats were exposed to OGD for 4 h followed by reoxygenation for 24 h. Next, 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) was used to assess cell viability while cell damage was assessed by lactate dehydrogenase (LDH).

RESULTS

We found that OGD/R increased cell death as well as the expression of Trx1 and that the interference of Trx1 further aggravated astrocyte damage under OGD/R condition. Furthermore, we detected an increase in the intracellular expressions of Ras2, cAMP, and PKA under OGD/R condition, which paralleled cell injury.

CONCLUSIONS

Notably, the deletion of Trx1 exacerbated astrocyte apoptosis via the Ras2-cAMP-PKA signaling pathway. We concluded that Trx1 protects astrocytes against apoptotic injury induced by OGD/R, and this protective effect may be partly related to the Ras2-cAMP-PKA signaling pathway.

Targeting the Selenoprotein Thioredoxin Reductase 1 for Anticancer Therapy

The cytosolic selenoprotein thioredoxin reductase 1 (TrxR1, encoded in human by TXNRD1) is implied to have several different roles in relation to cancer. Its physiologic functions may protect normal cells from carcinogenesis, but may also promote cancer progression if carcinogenesis nonetheless occurs. With distinct links to Nrf2 signaling, ribonucleotide reductase-dependent production of deoxyribonucleotides and its support of several antioxidant systems counteracting oxidative stress, the metabolic pathways regulated, and affected by TrxR1, are altogether of crucial importance in cancer. These pathways and causal relationships are at the same time highly intricate. In spite of the complexity in the cellular redox networks, several observations discussed in this chapter suggest that specific targeting of TrxR1 may be promising as a mechanistic principle for anticancer therapy.

Activated leukocyte cell adhesion molecule (ALCAM) is a marker of recurrence and promotes cell migration, invasion, and metastasis in early-stage endometrioid endometrial cancer

Endometrial cancer is the most common gynaecological cancer in western countries, being the most common subtype of endometrioid tumours. Most patients are diagnosed at an early stage and present an excellent prognosis. However, a number of those continue to suffer recurrence, without means of identification by risk classification systems. Thus, finding a reliable marker to predict recurrence becomes an important unmet clinical issue. ALCAM is a cell-cell adhesion molecule and member of the immunoglobulin superfamily that has been associated with the genesis of many cancers. Here, we first determined the value of ALCAM as a marker of recurrence in endometrioid endometrial cancer by conducting a retrospective multicentre study of 174 primary tumours. In early-stage patients (N = 134), recurrence-free survival was poorer in patients with ALCAM-positive compared to ALCAM-negative tumours (HR 4.237; 95% CI 1.01-17.76). This difference was more significant in patients with early-stage moderately-poorly differentiated tumours (HR 9.259; 95% CI 2.12-53.47). In multivariate analysis, ALCAM positivity was an independent prognostic factor in early-stage disease (HR 6.027; 95% CI 1.41-25.74). Then we demonstrated in vitro a role for ALCAM in cell migration and invasion by using a loss-of-function model in two endometrial cancer cell lines. ALCAM depletion resulted in a reduced primary tumour size and reduced metastatic local spread in an orthotopic murine model. Gene expression analysis of ALCAM-depleted cell lines pointed to motility, invasiveness, cellular assembly, and organization as the most deregulated functions. Finally, we assessed some of the downstream effector genes that are involved in ALCAM-mediated cell migration; specifically FLNB, TXNRD1, and LAMC2 were validated at the mRNA and protein level. In conclusion, our results highlight the potential of ALCAM as a recurrent biomarker in early-stage endometrioid endometrial cancer and point to ALCAM as an important molecule in endometrial cancer dissemination by regulating cell migration, invasion, and metastasis. Copyright © 2016 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Thioredoxin superfamily and its effects on cardiac physiology and pathology

A precise control of oxidation/reduction of protein thiols is essential for intact cardiac physiology. Irreversible oxidative modifications have been proposed to play a role in the pathogenesis of cardiovascular diseases. An imbalance of redox homeostasis with diminution of antioxidant capacities predisposes the heart to oxidant injury. There is growing interest in endoplasmic reticulum (ER) stress in the cardiovascular field, since perturbation of redox homeostasis in the ER is sufficient to cause ER stress. Because a number of human diseases are related to altered redox homeostasis and defects in protein folding, many research efforts have been devoted in recent years to understanding the structure and enzymatic properties of the thioredoxin superfamily. The thioredoxin superfamily has been well documented as thiol oxidoreductases to exert a role in various cell signaling pathways. The redox properties of the thioredoxin motif account for the different functions of several members of the thioredoxin superfamily. While thioredoxin and glutaredoxin primarily act as antioxidants by reducing protein disulfides and mixed disulfide, another member of the superfamily, protein disulfide isomerase (PDI), can act as an oxidant by forming intrachain disulfide bonds that contribute to proper protein folding. Increasing evidence suggests a pivotal role of PDI in the survival pathway that promotes cardiomyocyte survival and leads to more favorable cardiac remodeling. Thus, the thiol redox state is important for cellular redox signaling and survival pathway in the heart. This review summarizes the key features of major members of the thioredoxin superfamily directly involved in cardiac physiology and pathology.

Inhibition of type I insulin-like growth factor receptor tyrosine kinase by picropodophyllin induces apoptosis and cell cycle arrest in T lymphoblastic leukemia/lymphoma

It has been recently shown that the type I insulin-like growth factor receptor (IGF-IR) contributes significantly to the survival of T lymphoblastic leukemia/lymphoma (T-LBL) cells, and it was therefore suggested that IGF-IR could represent a legitimate therapeutic target in this aggressive disease. Picropodophyllin (PPP) is a potent, selective inhibitor of IGF-IR that is currently used with notable success in clinical trials that include patients with aggressive types of epithelial tumors. In the present study, we tested the effects of PPP on Jurkat and Molt-3 cells; two prototype T-LBL cell lines. Our results demonstrate that PPP efficiently induced apoptotic cell death and cell cycle arrest of these two cells. These effects were attributable to alterations of downstream target proteins. By using proteomic analysis, seven different proteins were found to be affected by PPP treatment of Jurkat cells. These proteins are involved in various aspects of cellular metabolism, cytoskeleton organization and signal transduction pathways. The results suggest that PPP affects multiple signaling molecules and inhibits fundamental pathways that control cell growth and survival. Our study also provides novel evidence that PPP could be potentially utilized for the treatment of aggressive T-LBL.

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.

Identification of a dithiol-disulfide switch in collapsin response mediator protein 2 (CRMP2) that is toggled in a model of neuronal differentiation

Vertebrate-specific glutaredoxin 2 (Grx2) is expressed in at least two isoforms, mitochondrial Grx2a and cytosolic Grx2c. We have previously shown that cytosolic Grx2 is essential for embryonic development of the brain. In particular, we identified collapsin response mediator protein 2 (CRMP2/DPYSL2), a mediator of the semaphorin-plexin signaling pathway, as redox-regulated target of Grx2c and demonstrated that this regulation is required for normal axonal outgrowth. In this study, we demonstrate the molecular mechanism of this regulation, a specific and reversible intermolecular Cys-504-Cys-504 dithiol-disulfide switch in homotetrameric CRMP2. This switch determines two conformations of the quaternary CRMP2 complex that controls axonal outgrowth and thus neuronal development.

Thioredoxin-mediated redox regulation of resistance to endocrine therapy in breast cancer

Resistance to endocrine therapy in breast carcinogenesis due to the redox regulation of the signal transduction system by reactive oxygen species (ROS) is the subject of this review article. Both antiestrogens and aromatase inhibitors are thought to prevent cancer through modulating the estrogen receptor function, but other mechanisms cannot be ruled out as these compounds also block metabolism and redox cycling of estrogen and are free radical scavengers. Endocrine therapeutic agents, such as, tamoxifen and other antiestrogens, and the aromatase inhibitor, exemestane, are capable of producing ROS. Aggressive breast cancer cells have high oxidative stress and chronic treatment with exemestane, fulvestrant or tamoxifen may add additional ROS stress. Breast cancer cells receiving long-term antiestrogen treatment appear to adapt to this increased persistent level of ROS. This, in turn, may lead to the disruption of reversible redox signaling that involves redox-sensitive phosphatases, protein kinases, such as, ERK and AKT, and transcription factors, such as, AP-1, NRF-1 and NF-κB. Thioredoxin modulates the expression of estrogen responsive genes through modulating the production of H2O2 in breast cancer cells. Overexpressing thioredoxine reductase 2 and reducing oxidized thioredoxin restores tamoxifen sensitivity to previously resistant breast cancer cells. In summary, it appears that resistance to endocrine therapy may be mediated, in part, by ROS-mediated dysregulation of both estrogen-dependent and estrogen-independent redox-sensitive signaling pathways. Further studies are needed to define the mechanism of action of thioredoxin modifiers, and their effect on the redox regulation that contributes to restoring the antiestrogen-mediated signal transduction system and growth inhibitory action.

The rare TXNRD1_v3 ("v3") splice variant of human thioredoxin reductase 1 protein is targeted to membrane rafts by N-acylation and induces filopodia independently of its redox active site integrity

The human selenoprotein thioredoxin reductase 1 (TrxR1), encoded by the TXNRD1 gene, is a key player in redox regulation. Alternative splicing generates several TrxR1 variants, one of which is v3 that carries an atypical N-terminal glutaredoxin domain. When overexpressed, v3 associates with membranes and triggers formation of filopodia. Here we found that membrane targeting of v3 is mediated by myristoylation and palmitoylation of its N-terminal MGC motif, through which v3 specifically targets membrane rafts. This was suggested by its localization in cholera toxin subunit B-stained membrane areas and also shown using lipid fractionation experiments. Utilizing site-directed mutant variants, we also found that v3-mediated generation of filopodia is independent of the Cys residues in its redox active site, but dependent upon its membrane raft targeting. These results identify v3 as an intricately regulated protein that expands TXNRD1-derived protein functions to the membrane raft compartment.

Integrating constitutive gene expression and chemoactivity: mining the NCI60 anticancer screen

Studies into the genetic origins of tumor cell chemoactivity pose significant challenges to bioinformatic mining efforts. Connections between measures of gene expression and chemoactivity have the potential to identify clinical biomarkers of compound response, cellular pathways important to efficacy and potential toxicities; all vital to anticancer drug development. An investigation has been conducted that jointly explores tumor-cell constitutive NCI60 gene expression profiles and small-molecule NCI60 growth inhibition chemoactivity profiles, viewed from novel applications of self-organizing maps (SOMs) and pathway-centric analyses of gene expressions, to identify subsets of over- and under-expressed pathway genes that discriminate chemo-sensitive and chemo-insensitive tumor cell types. Linear Discriminant Analysis (LDA) is used to quantify the accuracy of discriminating genes to predict tumor cell chemoactivity. LDA results find 15% higher prediction accuracies, using ∼30% fewer genes, for pathway-derived discriminating genes when compared to genes derived using conventional gene expression-chemoactivity correlations. The proposed pathway-centric data mining procedure was used to derive discriminating genes for ten well-known compounds. Discriminating genes were further evaluated using gene set enrichment analysis (GSEA) to reveal a cellular genetic landscape, comprised of small numbers of key over and under expressed on- and off-target pathway genes, as important for a compound’s tumor cell chemoactivity. Literature-based validations are provided as support for chemo-important pathways derived from this procedure. Qualitatively similar results are found when using gene expression measurements derived from different microarray platforms. The data used in this analysis is available at http://pubchem.ncbi.nlm.nih.gov/andhttp://www.ncbi.nlm.nih.gov/projects/geo (GPL96, GSE32474).

Thioredoxin reductase linked to cytoskeleton by focal adhesion kinase reverses actin S-nitrosylation and restores neutrophil β(2) integrin function

The investigation goal was to identify mechanisms for reversal of actin S-nitrosylation in neutrophils after exposure to high oxygen partial pressures. Prior work has shown that hyperoxia causes S-nitrosylated actin (SNO-actin) formation, which mediates β(2) integrin dysfunction, and these changes can be reversed by formylmethionylleucylphenylalanine or 8-bromo-cyclic GMP. Herein we show that thioredoxin reductase (TrxR) is responsible for actin denitrosylation. Approximately 80% of cellular TrxR is localized to the cytosol, divided between the G-actin and short filamentous actin (sF-actin) fractions based on Triton solubility of cell lysates. TrxR linkage to sF-actin requires focal adhesion kinase (FAK) based on immunoprecipitation studies. S-Nitrosylation accelerates actin filament turnover (by mechanisms described previously (Thom, S. R., Bhopale, V. M., Yang, M., Bogush, M., Huang, S., and Milovanova, T. (2011) Neutrophil β(2) integrin inhibition by enhanced interactions of vasodilator stimulated phosphoprotein with S-nitrosylated actin. J. Biol. Chem. 286, 32854-32865), which causes FAK to disassociate from sF-actin. TrxR subsequently dissociates from FAK, and the physical separation from actin impedes denitrosylation. If SNO-actin is photochemically reduced with UV light or if actin filament turnover is impeded by incubations with cytochalasin D, latrunculin B, 8-bromo-cGMP, or formylmethionylleucylphenylalanine, FAK and TrxR reassociate with sF-actin and cause SNO-actin removal. FAK-TrxR association can also be demonstrated using isolated enzymes in ex vivo preparations. Uniquely, the FAK kinase domain is the site of TrxR linkage. We conclude that through its scaffold function, FAK influences TrxR activity and actin S-nitrosylation.

Composition and evolution of the vertebrate and mammalian selenoproteomes

Background

Selenium is an essential trace element in mammals due to its presence in proteins in the form of selenocysteine (Sec). Human genome codes for 25 Sec-containing protein genes, and mouse and rat genomes for 24.

Methodology/Principal Findings

We characterized the selenoproteomes of 44 sequenced vertebrates by applying gene prediction and phylogenetic reconstruction methods, supplemented with the analyses of gene structures, alternative splicing isoforms, untranslated regions, SECIS elements, and pseudogenes. In total, we detected 45 selenoprotein subfamilies. 28 of them were found in mammals, and 41 in bony fishes. We define the ancestral vertebrate (28 proteins) and mammalian (25 proteins) selenoproteomes, and describe how they evolved along lineages through gene duplication (20 events), gene loss (10 events) and replacement of Sec with cysteine (12 events). We show that an intronless selenophosphate synthetase 2 gene evolved in early mammals and replaced functionally the original multiexon gene in placental mammals, whereas both genes remain in marsupials. Mammalian thioredoxin reductase 1 and thioredoxin-glutathione reductase evolved from an ancestral glutaredoxin-domain containing enzyme, still present in fish. Selenoprotein V and GPx6 evolved specifically in placental mammals from duplications of SelW and GPx3, respectively, and GPx6 lost Sec several times independently. Bony fishes were characterized by duplications of several selenoprotein families (GPx1, GPx3, GPx4, Dio3, MsrB1, SelJ, SelO, SelT, SelU1, and SelW2). Finally, we report identification of new isoforms for several selenoproteins and describe unusually conserved selenoprotein pseudogenes.

Conclusions/Significance

This analysis represents the first comprehensive survey of the vertebrate and mammal selenoproteomes, and depicts their evolution along lineages. It also provides a wealth of information on these selenoproteins and their forms.

Vertebrate-specific glutaredoxin is essential for brain development

Cellular functions and survival are dependent on a tightly controlled redox potential. Currently, an increasing amount of data supports the concept of local changes in the redox environment and specific redox signaling events controlling cell function. Specific protein thiol groups are the major targets of redox signaling and regulation. Thioredoxins and glutaredoxins catalyze reversible thiol-disulfide exchange reactions and are primary regulators of the protein thiol redox state. Here, we demonstrate that embryonic brain development depends on the enzymatic activity of glutaredoxin 2. Zebrafish with silenced expression of glutaredoxin 2 lost virtually all types of neurons by apoptotic cell death and the ability to develop an axonal scaffold. As demonstrated in zebrafish and in a human cellular model for neuronal differentiation, glutaredoxin 2 controls axonal outgrowth via thiol redox regulation of collapsin response mediator protein 2, a central component of the semaphorin pathway. This study provides an example of a specific thiol redox regulation essential for vertebrate embryonic development.

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.

Thioredoxin and glutathione systems differ in parasitic and free-living platyhelminths

Background

The thioredoxin and/or glutathione pathways occur in all organisms. They provide electrons for deoxyribonucleotide synthesis, function as antioxidant defenses, in detoxification, Fe/S biogenesis and participate in a variety of cellular processes. In contrast to their mammalian hosts, platyhelminth (flatworm) parasites studied so far, lack conventional thioredoxin and glutathione systems. Instead, they possess a linked thioredoxin-glutathione system with the selenocysteine-containing enzyme thioredoxin glutathione reductase (TGR) as the single redox hub that controls the overall redox homeostasis. TGR has been recently validated as a drug target for schistosomiasis and new drug leads targeting TGR have recently been identified for these platyhelminth infections that affect more than 200 million people and for which a single drug is currently available. Little is known regarding the genomic structure of flatworm TGRs, the expression of TGR variants and whether the absence of conventional thioredoxin and glutathione systems is a signature of the entire platyhelminth phylum.

Results

We examine platyhelminth genomes and transcriptomes and find that all platyhelminth parasites (from classes Cestoda and Trematoda) conform to a biochemical scenario involving, exclusively, a selenium-dependent linked thioredoxin-glutathione system having TGR as a central redox hub. In contrast, the free-living platyhelminth Schmidtea mediterranea (Class Turbellaria) possesses conventional and linked thioredoxin and glutathione systems. We identify TGR variants in Schistosoma spp. derived from a single gene, and demonstrate their expression. We also provide experimental evidence that alternative initiation of transcription and alternative transcript processing contribute to the generation of TGR variants in platyhelminth parasites.

Conclusions

Our results indicate that thioredoxin and glutathione pathways differ in parasitic and free-living flatworms and that canonical enzymes were specifically lost in the parasitic lineage. Platyhelminth parasites possess a unique and simplified redox system for diverse essential processes, and thus TGR is an excellent drug target for platyhelminth infections. Inhibition of the central redox wire hub would lead to overall disruption of redox homeostasis and disable DNA synthesis.

Differentially expressed alternatively spliced genes in malignant pleural mesothelioma identified using massively parallel transcriptome sequencing

Background

Analyses of Expressed Sequence Tags (ESTs) databases suggest that most human genes have multiple alternative splice variants. The alternative splicing of pre-mRNA is tightly regulated during development and in different tissue types. Changes in splicing patterns have been described in disease states. Recently, we used whole-transcriptome shotgun pryrosequencing to characterize 4 malignant pleural mesothelioma (MPM) tumors, 1 lung adenocarcinoma and 1 normal lung. We hypothesized that alternative splicing profiles might be detected in the sequencing data for the expressed genes in these samples.

Methods

We developed a software pipeline to map the transcriptome read sequences of the 4 MPM samples and 1 normal lung sample onto known exon junction sequences in the comprehensive AceView database of expressed sequences and to count how many reads map to each junction. 13,274,187 transcriptome reads generated by the Roche/454 sequencing platform for 5 samples were compared with 151,486 exon junctions from the AceView database. The exon junction expression index (EJEI) was calculated for each exon junction in each sample to measure the differential expression of alternative splicing events. Top ten exon junctions with the largest EJEI difference between the 4 mesothelioma and the normal lung sample were then examined for differential expression using Quantitative Real Time PCR (qRT-PCR) in the 5 sequenced samples. Two of the differentially expressed exon junctions (ACTG2.aAug05 and CDK4.aAug05) were further examined with qRT-PCR in additional 18 MPM and 18 normal lung specimens.

Results

We found 70,953 exon junctions covered by at least one sequence read in at least one of the 5 samples. All 10 identified most differentially expressed exon junctions were validated as present by RT-PCR, and 8 were differentially expressed exactly as predicted by the sequence analysis. The differential expression of the AceView exon junctions for the ACTG2 and CDK4 genes were also observed to be statistically significant in an additional 18 MPM and 18 normal lung samples examined using qRT-PCR. The differential expression of these two junctions was shown to successfully classify these mesothelioma and normal lung specimens with high sensitivity (89% and 78%, respectively).

Conclusion

Whole-transcriptome shotgun sequencing, combined with a downstream bioinformatics pipeline, provides powerful tools for the identification of differentially expressed exon junctions resulting from alternative splice variants. The alternatively spliced genes discovered in the study could serve as useful diagnostic markers as well as potential therapeutic targets for MPM.

Proteome changes in human bronchoalveolar cells following styrene exposure indicate involvement of oxidative stress in the molecular-response mechanism

Styrene is a volatile organic compound that is widely used as an intermediate in many industrial settings. There are known adverse health effects at environmentally significant concentrations, but little is known about the molecular effect of exposure to styrene at sub-acute toxic concentrations. We exposed human lung epithelial cells, at a wide range of concentrations (1 mg/m(3)-10 g/m(3)), to styrene and analyzed the effects on the proteome level by 2-DE, where 1380 proteins spots were detected and 266 were identified unambiguously by MS. A set of 16 protein spots were found to be significantly altered due to exposure to styrene at environmentally significant concentrations of 1-10 mg/m(3) (0.2-2.3 ppm). Among these, superoxide dismutase as well as biliverdin reductase A could be correlated with the molecular pathway of oxidative stress, while eukaryotic translation initiation factor 5A-1, ezrin, lamin B2 and voltage-dependent anion channel 2 have been reported to be involved in apoptosis. Treatment with styrene also caused the formation of styrene oxide-protein adducts, specifically for thioredoxin reductase 1. These results underline the relevance of oxidative stress as a primary molecular response mechanism of lung epithelial cells to styrene exposure at indoor-relevant concentrations.

CUG start codon generates thioredoxin/glutathione reductase isoforms in mouse testes

Mammalian cytosolic and mitochondrial thioredoxin reductases are essential selenocysteine-containing enzymes that control thioredoxin functions. Thioredoxin/glutathione reductase (TGR) is a third member of this enzyme family. It has an additional glutaredoxin domain and shows highest expression in testes. Herein, we found that human and several other mammalian TGR genes lack any AUG codons that could function in translation initiation. Although mouse and rat TGRs have such codons, we detected protein sequences upstream of them by immunoblot assays and direct proteomic analyses. Further gene engineering and expression analyses demonstrated that a CUG codon, located upstream of the sequences previously thought to initiate translation, is the actual start codon in mouse TGR. The use of this codon relies on the Kozak consensus sequence and ribosome-scanning mechanism. However, CUG serves as an inefficient start codon that allows downstream initiation, thus generating two isoforms of the enzyme in vivo and in vitro. The use of CUG evolved in mammalian TGRs, and in some of these organisms, GUG is used instead. The newly discovered longer TGR form shows cytosolic localization in cultured cells and is expressed in spermatids in mouse testes. This study shows that CUG codon is used as an inefficient start codon to generate protein isoforms in mouse.

High levels of thioredoxin reductase 1 modulate drug-specific cytotoxic efficacy

The selenoprotein thioredoxin reductase 1 (TrxR1) is currently recognized as a plausible anticancer drug target. Here we analyzed the effects of TrxR1 targeting in the human A549 lung carcinoma cell line, having a very high basal TrxR1 expression. We determined the total cellular TrxR activity to be 271.4 +/- 39.5 nmol min(-1) per milligram of total protein, which by far exceeded the total thioredoxin activity (39.2 +/- 3.5 nmol min(-1) per milligram of total protein). Knocking down TrxR1 by approx 90% using siRNA gave only a slight effect on cell growth, irrespective of concurrent glutathione depletion (> or = 98% decrease), and no increase in cell death or distorted cell cycle phase distributions. This apparent lack of phenotype could probably be explained by Trx functions being maintained by the remaining TrxR1 activity. TrxR1 knockdown nonetheless yielded drug-specific modulation of cytotoxic efficacy in response to various chemotherapeutic agents. No changes in response upon exposure to auranofin or juglone were seen after TrxR1 knockdown, whereas sensitivity to 1-chloro-2,4-dinitrobenzene or menadione became markedly increased. In contrast, a virtually complete resistance to cisplatin using concentrations up to 20 microM appeared upon TrxR1 knockdown. The results suggest that high overexpression of TrxR has an impact not necessarily linked to Trx function that nonetheless modulates drug-specific cytotoxic responses.

The human thioredoxin reductase-1 splice variant TXNRD1_v3 is an atypical inducer of cytoplasmic filaments and cell membrane filopodia

Thioredoxin reductases are important selenoproteins maintaining cellular redox balance and regulating several redox dependent processes in apoptosis, cell proliferation and differentiation. Specific functions of dedicated splice variants may add further complexity to the functions of these proteins. We show here that a splice variant of human thioredoxin reductase 1, TXNRD1_v3, forms both dynamic cytoplasmic filaments and provokes instantaneous formation of dynamic cell membrane protrusions identified as filopodia. Using truncated versions of the protein we found that both the cytoplasmic filaments and the filopodia formation were exclusively dependent on the glutaredoxin domain of the protein. Interestingly, actin polymerization was required for filopodia formation triggered by TXNRD1_v3, but not for generation of cytoplasmic filaments. We conclude that the glutaredoxin domain of TXNRD1_v3 is an atypical regulator of the cell cytoskeleton that potently induces formation of highly ordered cytoplasmic filaments and cell membrane filopodia.

The human selenoproteome: recent insights into functions and regulation

Selenium (Se) is a nutritional trace mineral essential for various aspects of human health that exerts its effects mainly through its incorporation into selenoproteins as the amino acid, selenocysteine. Twenty-five selenoprotein genes have been identified in humans and several selenoproteins are broadly classified as antioxidant enzymes. As progress is made on characterizing the individual members of this protein family, however, it is becoming clear that their properties and functions are quite diverse. This review summarizes recent insights into properties of individual selenoproteins such as tissue distribution, subcellular localization, and regulation of expression. Also discussed are potential roles the different selenoproteins play in human health and disease.

Transcriptional regulation of mammalian selenoprotein expression

BACKGROUND

Selenoproteins contain the twenty-first amino acid, selenocysteine, and are involved in cellular defenses against oxidative damage, important metabolic and developmental pathways, and responses to environmental challenges. Elucidating the mechanisms regulating selenoprotein expression at the transcriptional level is a key to understanding how these mechanisms are called into play to respond to the changing environment.

METHODS

This review summarizes published studies on transcriptional regulation of selenoprotein genes, focused primarily on genes whose encoded protein functions are at least partially understood. This is followed by in silico analysis of predicted regulatory elements in selenoprotein genes, including those in the aforementioned category as well as the genes whose functions are not known.

RESULTS

Our findings reveal regulatory pathways common to many selenoprotein genes, including several involved in stress-responses. In addition, tissue-specific regulatory factors are implicated in regulating many selenoprotein genes.

CONCLUSIONS

These studies provide new insights into how selenoprotein genes respond to environmental and other challenges, and the roles these proteins play in allowing cells to adapt to these changes.

GENERAL SIGNIFICANCE

Elucidating the regulatory mechanisms affecting selenoprotein expression is essential for understanding their roles in human diseases, and for developing diagnostic and potential therapeutic approaches to address dysregulation of members of this gene family.

Focus on mammalian thioredoxin reductases--important selenoproteins with versatile functions

Thioredoxin systems, involving redox active thioredoxins and thioredoxin reductases, sustain a number of important thioredoxin-dependent pathways. These redox active proteins support several processes crucial for cell function, cell proliferation, antioxidant defense and redox-regulated signaling cascades. Mammalian thioredoxin reductases are selenium-containing flavoprotein oxidoreductases, dependent upon a selenocysteine residue for reduction of the active site disulfide in thioredoxins. Their activity is required for normal thioredoxin function. The mammalian thioredoxin reductases also display surprisingly multifaceted properties and functions beyond thioredoxin reduction. Expressed from three separate genes (in human named TXNRD1, TXNRD2 and TXNRD3), the thioredoxin reductases can each reduce a number of different types of substrates in different cellular compartments. Their expression patterns involve intriguingly complex transcriptional mechanisms resulting in several splice variants, encoding a number of protein variants likely to have specialized functions in a cell- and tissue-type restricted manner. The thioredoxin reductases are also targeted by a number of drugs and compounds having an impact on cell function and promoting oxidative stress, some of which are used in treatment of rheumatoid arthritis, cancer or other diseases. However, potential specific or essential roles for different forms of human or mouse thioredoxin reductases in health or disease are still rather unclear, although it is known that at least the murine Txnrd1 and Txnrd2 genes are essential for normal development during embryogenesis. This review is a survey of current knowledge of mammalian thioredoxin reductase function and expression, with a focus on human and mouse and a discussion of the striking complexity of these proteins. Several yet open questions regarding their regulation and roles in different cells or tissues are emphasized. It is concluded that the intriguingly complex regulation and function of mammalian thioredoxin reductases within the cellular context and in intact mammals strongly suggests that their functions are highly fi ne-tuned with the many pathways involving thioredoxins and thioredoxin-related proteins. These selenoproteins furthermore propagate many functions beyond a reduction of thioredoxins. Aberrant regulation of thioredoxin reductases, or a particular dependence upon these enzymes in diseased cells, may underlie their presumed therapeutic importance as enzymatic targets using electrophilic drugs. These reductases are also likely to mediate several of the effects on health and disease that are linked to different levels of nutritional selenium intake. The thioredoxin reductases and their splice variants may be pivotal components of diverse cellular signaling pathways, having importance in several redox-related aspects of health and disease. Clearly, a detailed understanding of mammalian thioredoxin reductases is necessary for a full comprehension of the thioredoxin system and of selenium dependent processes in mammals.