Thioredoxin interacting protein: redox dependent and independent regulatory mechanisms

Thioredoxin-Interacting Protein (TXNIP) Suppresses Expression of Glutamine Synthetase by Inducing Oxidative Stress in Retinal Muller Glia Under Diabetic Conditions

BACKGROUND Diabetic retinopathy (DR) is a progressive neurodegenerative disease with early-stage symptoms such as dysfunction of Muller cells, which leads to ganglion cell death. Its pathogenesis is probably associated with oxidative stress and a recently discovered protein, thioredoxin-interacting protein (TXNIP). MATERIAL AND METHODS To explore the role of TXNIP in DR, we cultured Muller cells under diabetic conditions, and then used immunohistochemistry, Western blot, and RT-PCR to detect the expression level of TXNIP under diabetic conditions. We demonstrated the expression level of glutamine synthetase (GS) when TXNIP was inhibited. To explore the potential pathway of TXNIP-induced cell damage in DR, we confirmed the role of IL-1β under diabetic conditions. RESULTS Diabetes induces TXNIP expressions at mRNA levels, but shows the opposite effect on GS. IL-1β plays an important role in this pathway. Azaserine effectively increased the expression of GS via attenuating the expression of TXNIP. CONCLUSIONS This study demonstrates the role of TXNIP and its mechanism in DR, provides a possible treatment for DR, and lays a new theoretical foundation for the clinical treatment of DR and other diabetic microvascular changes.

Relationship between thioredoxin and thioredoxin-binding protein in patients with gestational diabetes mellitus

OBJECTIVE

This study examined the clinical and biological significance of thioredoxin (Trx) and thioredoxin-binding protein (TrxBP), which are redox-active proteins that control multiple biological functions, in gestational diabetes.

METHODS

We measured serum concentrations of Trx, TrxBP, insulin and other blood parameters, as well as insulin resistance and glucose tolerance in pregnant women with or without gestational dieabetes mellitus (GDM) (34/34) at the early second trimester.

RESULTS

Contrary to diabetes patients, serum TrxBP levels were lower in women with GDM than healthy pregnant controls. The serum insulin concentrations were higher in GDM, but the difference was not statistically significant. Furthermore, the intracellular redox potential ratio (Trx/TrxBP) of GDM patients was higher than that of the control group.

CONCLUSION

During pregnancy, the mother is potentially subjected to glucotoxicity as well as oxidative stress (OS) to help the foetus absorb more nutrients. Our results suggest that the Trx/TrxBP system may mediate a compensating mechanism. Reduced TrxBP levels and consequent enhanced Trx activity may alleviate OS and protect the foetus from hypoglycaemia. We hypothesise that the decrease in TrxBP levels is not a consequence of GDM, but rather is an instance of the active functional role of TrxBP in maternal development, unifying redox regulation and glucose metabolism.

Epigenome-wide association study identifies TXNIP gene associated with type 2 diabetes mellitus and sustained hyperglycemia

Type 2 diabetes mellitus (DM) is an established risk factor for a wide range of vascular diseases, including ischemic stroke (IS). Glycated hemoglobin A1c (HbA1c), a marker for average blood glucose levels over the previous 12 weeks, is used as a measure of glycemic control and also as a diagnostic criterion for diabetes (HbA1c levels ≥ 6.5%). Epigenetic mechanisms, such as DNA methylation, may be associated with aging processes and with modulation of the risk of various pathologies, such as DM. Specifically, DNA methylation could be one of the mechanisms mediating the relation between DM and environmental exposures. Our goal was to identify new CpG methylation sites associated with DM. We performed a genome-wide methylation study in whole-blood DNA from an IS patient cohorts. Illumina HumanMethylation450 BeadChip array was used to measure DNA methylation in CpG sites. All statistical analyses were adjusted for sex, age, hyperlipidemia, body mass index (BMI), smoking habit and cell count. Findings were replicated in two independent cohorts, an IS cohort and a population-based cohort, using the same array. In the discovery phase (N = 355), we identified a CpG site, cg19693031 (located in the TXNIP gene) that was associated with DM (P = 1.17 × 10(-12)); this CpG was replicated in two independent cohorts (N = 167 and N = 645). Methylation of TXNIP was inversely and intensely associated with HbA1c levels (P = 7.3 × 10(-16)), specifically related to diabetic patients with poor control of glucose levels. We identified an association between the TXNIP gene and DM through epigenetic mechanisms, related to sustained hyperglycemia levels (HbA1c ≥ 7%).

Chloroplastic thioredoxin m functions as a major regulator of Calvin cycle enzymes during photosynthesis in vivo

Thioredoxins (Trxs) regulate the activity of various chloroplastic proteins in a light-dependent manner. Five types of Trxs function in different physiological processes in the chloroplast of Arabidopsis thaliana. Previous in vitro experiments have suggested that the f-type Trx (Trx f) is the main redox regulator of chloroplast enzymes, including Calvin cycle enzymes. To investigate the in vivo contribution of each Trx isoform to the redox regulatory system, we first quantified the protein concentration of each Trx isoform in the chloroplast stroma. The m-type Trx (Trx m), which consists of four isoforms, was the most abundant type. Next, we analyzed several Arabidopsis Trx-m-deficient mutants to elucidate the physiological role of Trx m in vivo. Deficiency of Trx m impaired plant growth and decreased the CO2 assimilation rate. We also determined the redox state of Trx target enzymes to examine their photo-reduction, which is essential for enzyme activation. In the Trx-m-deficient mutants, the reduction level of fructose-1,6-bisphosphatase and sedoheptulose-1,7-bisphosphatase was lower than that in the wild type. Inconsistently with the historical view, our in vivo study suggested that Trx m plays a more important role than Trx f in the activation of Calvin cycle enzymes.

The structural basis for the negative regulation of thioredoxin by thioredoxin-interacting protein

The redox-dependent inhibition of thioredoxin (TRX) by thioredoxin-interacting protein (TXNIP) plays a pivotal role in various cancers and metabolic syndromes. However, the molecular mechanism of this regulation is largely unknown. Here, we present the crystal structure of the TRX-TXNIP complex and demonstrate that the inhibition of TRX by TXNIP is mediated by an intermolecular disulphide interaction resulting from a novel disulphide bond-switching mechanism. Upon binding to TRX, TXNIP undergoes a structural rearrangement that involves switching of a head-to-tail interprotomer Cys63-Cys247 disulphide between TXNIP molecules to an interdomain Cys63-Cys190 disulphide, and the formation of a de novo intermolecular TXNIP Cys247-TRX Cys32 disulphide. This disulphide-switching event unexpectedly results in a domain arrangement of TXNIP that is entirely different from those of other arrestin family proteins. We further show that the intermolecular disulphide bond between TRX and TXNIP dissociates in the presence of high concentrations of reactive oxygen species. This study provides insight into TRX and TXNIP-dependent cellular regulation.

First molecular characterisation and expression analysis of a teleost thioredoxin-interacting protein (TXNIP) gene from rock bream (Oplegnathus fasciatus)

Thioredoxin-interacting protein (TXNIP) is an important regulator of glucose metabolism that functions by inhibiting cellular glucose uptake. The full-length rock bream (Oplegnathus fasciatus) TXNIP (RbTXNIP) cDNA (2499 bp) contains an open reading frame of 1188 bp encoding 396 amino acids. Furthermore, multiple alignments showed that the arrestin domain was well conserved among the other TXNIP sequences tested. RbTXNIP was predicted to contain a PxxP and PPxY motif. Phylogenetic analysis indicated that RbTXNIP is most closely related to Fugu rubripes TXNIP. RbTXNIP was expressed significantly in the RBC, intestine, and spleen. RbTXNIP mRNA expression was also examined in several tissues under conditions of bacterial and viral challenge. Generally, all tissues examined from fish infected with Streptococcus iniae, Edwardsiella tarda and red sea bream iridovirus (RSIV) showed significant downregulation in RbTXNIP expression compared to controls. However, RbTXNIP expression showed significant upregulation in the spleen and kidney after injection of recombinant rock bream TRx1 protein. These findings provide a molecular foundation for functional studies and applications in teleosts.

The downregulation of thioredoxin accelerated Neuro2a cell apoptosis induced by advanced glycation end product via activating several pathways

Thioredoxin (Trx), a 12 kDa protein, has different functions in different cellular environments, playing important anti-oxidative and anti-apoptotic roles and regulating the expression of transcription factors. Advanced glycation end products (AGEs) are a heterogeneous group of irreversible adducts from glucose-protein condensation reactions and are considered crucial to the development of diabetic nephropathy, retinopathy, neurodegeneration and atherosclerosis. The aim of this study was to use a Trx inhibitor to investigate the effects and mechanism of Trx down-regulation on AGE-induced Neuro2a cell apoptosis. Neuro2a cells were cultured in vitro and treated with different conditions. The apoptosis and proliferation of Neuro2a cells were detected using flow cytometry, DNA-Ladder and CCK8 assays. Rho 123 was used to detect the mitochondrial membrane potential. ROS generation and caspase3 activity were detected using a DCFH-DA probe and micro-plate reader. Western blotting and real-time PCR were used to detect the expression of proteins and genes. We found that the down-regulation of thioredoxin could accelerate AGE-induced apoptosis in Neuro2a cells. A possible underlying mechanism is that the down-regulation of thioredoxin stimulated the up-regulation of ASK1, p-JNK, PTEN, and Txnip, as well as the down-regulation of p-AKT, ultimately increasing ROS levels and caspase3 activity.

RNA-seq-based transcriptome profiling reveals differential gene expression in the lungs of Sprague-Dawley rats during early-phase acute hypobaric hypoxia

Individuals subjected to hypobaric hypoxia at high altitudes may exhibit differential physiological responses in terms of susceptibility and tolerance to the development of hypoxia-related disorders. We studied early-phase gene expression in the lungs of Sprague-Dawley rats exhibiting such differential physiological responses after exposure to acute hypobaric hypoxia for 1 h at a simulated altitude of 9144 m. RNA-seq transcriptome profiling of lung tissues revealed differential gene expression in tolerant and susceptible groups, subsequently validated by qRT-PCR for ten selected differentially expressed genes. The gene expression pattern indicated hypometabolism and negative regulation of vasoconstriction in all groups except susceptible rats, coupled with altered MAPK, p53 and JAK-STAT signaling. Upregulation of early-phase response genes including Dusp1 (dual specificity phosphatase), Cdkn1a (cyclin-dependent kinase inhibitor 1A), Txnip (thioredoxin-interacting protein), Rgs1 (regulator of G-protein signaling 1) and Rgs2 (regulator of G-protein signaling 2) in susceptible rats indicated a progression toward growth arrest and apoptosis. Enhanced expression of cell adhesion molecules, wound healing and repair bioprocesses was observed in tolerant males. Upregulated Kcnj15 (potassium inwardly rectifying channel subfamily j membrane 15) and Vsig4 (V-set and Ig domain containing 4) variants in tolerant females suggested adaptation to hypoxia possibly by fluid reabsorption to avoid edematous conditions and suppression of T cell proliferation to avoid acute lung inflammation. Our study might help in understanding the molecular-physiological mechanisms associated with progressive damage in the lung tissues of susceptible and tissue-protective measures in tolerant rats during acute hypobaric hypoxia.

MTA2 enhances colony formation and tumor growth of gastric cancer cells through IL-11

Background

We have preliminarily reported MTA2 expression in gastric cancer and its biological functions by using knockdown cell models, while the molecular mechanisms of MTA2 in regulating malignant behaviors are still unclear.

Methods

MTA2 overexpression models were established by transfection assay in gastric cancer cells BGC-823 and MKN28. Cell proliferation assay, colony formation in soft agar, wound-healing assay and transwell migration assay were performed with MTA2 overexpression and negative control (NC) cells. Subcutaneous xenografts and pulmonary metastasis models by BGC-823/MTA2 and BGC-823/NC cells were used to observe the capacity of growth and metastasis in vivo. Differential gene expression in MTA2 knockdown and overexpression cells was analyzed by microarrays. IL-11, which demonstrated as differential expression in microarray, was detected by real-time PCR, western blot, ELISA and immunohistochemistry staining. Recombinant human IL-11 (rhIL-11) was administrated in cell proliferation and colony formation as rescue assay.

Results

The numbers of colonies in soft agar were significantly more in BGC-823/MTA2 and MKN28/MTA2 cells, comparing with those in their NC cells. Capabilities of cell proliferation, wound-healing and cell migration were not significantly changed in MTA2 overexpression cells. The sizes of subcutaneous xenografts and pulmonary metastases of BGC-832/MTA2 cells were significantly larger than those in BGC-823/NC group. Differential expression of IL-11 was identified by genome expression microarray both in MTA2 knockdown and overexpression cells. IL-11 expression was elevated in BGC-823/MTA2 cells, whereas reduced in SGC-7901/shMTA2 cells. Administration of rhIL-11 recovered colony formation capacity of SGC-7901/shMTA2 cells.

Conclusions

MTA2 overexpression enhances colony formation and tumor growth of gastric cancer cells, but not plays important role in cancer cell migration and metastasis. IL-11 is one of the downstream effectors of MTA2 in regulating gastric cancer cells growth.

Electronic supplementary material

The online version of this article (doi:10.1186/s12885-015-1366-y) contains supplementary material, which is available to authorized users.

Role of inflammasome activation in the pathophysiology of vascular diseases of the neurovascular unit

SIGNIFICANCE

Inflammation is the standard double-edged defense mechanism that aims at protecting the human physiological homeostasis from devastating threats. Both acute and chronic inflammation have been implicated in the occurrence and progression of vascular diseases. Interference with components of the immune system to improve patient outcome after ischemic injury has been uniformly unsuccessful. There is a need for a deeper understanding of the innate immune response to injury in order to modulate, rather than to block inflammation and improve the outcome for vascular diseases.

RECENT ADVANCES

Nucleotide-binding oligomerization domain-like receptors or NOD-like receptor proteins (NLRPs) can be activated by sterile and microbial inflammation. NLR family plays a major role in activating the inflammasome.

CRITICAL ISSUES

The aim of this work is to review recent findings that provided insights into key inflammatory mechanisms and define the place of the inflammasome, a multi-protein complex involved in instigating inflammation in neurovascular diseases, including retinopathy, neurodegenerative diseases, and stroke.

FUTURE DIRECTIONS

The significant contribution of NLRP-inflammasome activation to vascular disease of the neurovascular unit in the brain and retina suggests that therapeutic strategies focused on specific targeting of inflammasome components could significantly improve the outcomes of these diseases.

Thioredoxin-interacting protein: pathophysiology and emerging pharmacotherapeutics in cardiovascular disease and diabetes

The thioredoxin system, which consists of thioredoxin (Trx), nicotinamide adenine dinucleotide phosphate (NADPH) and thioredoxin reductase (TrxR), has emerged as a major anti-oxidant involved in the maintenance of cellular physiology and survival. Dysregulation in this system has been associated with metabolic, cardiovascular, and malignant disorders. Thioredoxin-interacting protein (TXNIP), also known as vitamin D-upregulated protein or thioredoxin-binding-protein-2, functions as a physiological inhibitor of Trx, and pathological suppression of Trx by TXNIP has been demonstrated in diabetes and cardiovascular diseases. Furthermore, TXNIP effects are partially Trx-independent; these include direct activation of inflammation and inhibition of glucose uptake. Many of the effects of TXNIP are initiated by its dissociation from intra-nuclear binding with Trx or other SH-containing proteins: these effects include its migration to cytoplasm, modulating stress responses in mitochondria and endoplasmic reticulum, and also potentially activating apoptotic pathways. TXNIP also interacts with the nitric oxide (NO) signaling system, with apparent suppression of NO effect. TXNIP production is modulated by redox stress, glucose levels, hypoxia and several inflammatory activators. In recent studies, it has been shown that therapeutic agents including insulin, metformin, angiotensin converting enzyme inhibitors and calcium channel blockers reduce TXNIP expression, although it is uncertain to what extent TXNIP suppression contributes to their clinical efficacy. This review addresses the role of TXNIP in health and in cardiovascular and metabolic disorders. Finally, the potential advantages (and disadvantages) of pharmacological suppression of TXNIP in cardiovascular disease and diabetes are summarized.

Comparative proteomic analyses of urine from rat urothelial carcinoma chemically induced by exposure to N-butyl-N-(4-hydroxybutyl)-nitrosamine

Bladder cancer is estimated to be the ninth most common malignancy with a high rate of recurrence and progression despite therapy, early diagnosis being crucial for timely intervention.

Identification of gene signatures for prednisolone-induced metabolic dysfunction in collagen-induced arthritic mice

BACKGROUND

Prednisolone is a potent anti-inflammatory glucocorticoid (GC) but chronic use is hampered by metabolic side effects. Little is known about the long-term effects of GCs on gene-expression in vivo during inflammation.

AIM

Identify gene signatures underlying prednisolone-induced metabolic side effects in a complex in vivo inflammatory setting after long-term treatment.

MATERIALS & METHODS

We performed whole-genome expression profiling in liver and muscle from arthritic and nonarthritic mice treated with several doses of prednisolone for 3 weeks and used text-mining to link gene signatures to metabolic pathways.

RESULTS

Prednisolone-induced gene signatures were highly tissue specific. We identified a short-list of genes significantly affected by both prednisolone and inflammation in liver and involved in glucose and fatty acid metabolism. For several of these genes the association with GCs is novel.

CONCLUSION

The identified gene signatures may provide useful starting points for the development of GCs with a better safety profile.

Effects of vitamin D3 stimulation of thioredoxin-interacting protein in hepatocellular carcinoma

AIM

Thioredoxin-interacting protein (TXNIP) promotes oxidative stress by inactivating thioredoxin (TXN). This protein is involved in diverse disease processes, including insulin resistance, atherosclerosis and carcinogenesis. The aim of the present study was to measure the expression and function of TXNIP in in vitro models of liver disease, as well as in primary human hepatocellular carcinoma (HCC) tissue specimens. In addition, we wanted to determine the effects of vitamin D3-induced TXNIP stimulation in HCC-derived cell lines.

METHODS

TXNIP expression was measured by quantitative reverse transcription polymerase chain reaction and western blots. TXNIP expression was stimulated by vitamin D exposure and by transfection. Cell proliferation, apoptosis and reactive oxygen species were determined by standard assays.

RESULTS

TXNIP expression levels were low in HCC cell lines, and vitamin D3 stimulated TXNIP expression in vitro. In HCC cells transfected with a TXNIP expression vector or treated with exogenous vitamin D3, there was a reduction in cell proliferation and an increase in apoptosis. Cells expressing TXNIP were markedly susceptible to oxidative injury induced by cobalt chloride or bacterial lipopolysaccharide. TXNIP expression was reduced or absent in a majority of primary human HCC specimens relative to matching, non-cancerous liver tissue.

CONCLUSION

TXNIP expression is low or absent in human HCC specimens and HCC-derived cell lines. Vitamin D3 stimulates TXNIP expression, resulting in diminished proliferation and enhanced apoptosis. Liver cells expressing TXNIP are primed for oxidative injury. These findings suggest that stimulation of TXNIP expression, by factors such as vitamin D3, may attenuate carcinogenesis in patients with chronic liver disease.

Bacterial fucose-rich polysaccharide stabilizes MAPK-mediated Nrf2/Keap1 signaling by directly scavenging reactive oxygen species during hydrogen peroxide-induced apoptosis of human lung fibroblast cells

Continuous free radical assault upsets cellular homeostasis and dysregulates associated signaling pathways to promote stress-induced cell death. In spite of the continuous development and implementation of effective therapeutic strategies, limitations in treatments for stress-induced toxicities remain. The purpose of the present study was to determine the potential therapeutic efficacy of bacterial fucose polysaccharides against hydrogen peroxide (H2O2)-induced stress in human lung fibroblast (WI38) cells and to understand the associated molecular mechanisms. In two different fermentation processes, Bacillus megaterium RB-05 biosynthesized two non-identical fucose polysaccharides; of these, the polysaccharide having a high-fucose content (∼ 42%) conferred the maximum free radical scavenging efficiency in vitro. Structural characterizations of the purified polysaccharides were performed using HPLC, GC-MS, and (1)H/(13)C/2D-COSY NMR. H2O2 (300 µM) insult to WI38 cells showed anti-proliferative effects by inducing intracellular reactive oxygen species (ROS) and by disrupting mitochondrial membrane permeability, followed by apoptosis. The polysaccharide (250 µg/mL) attenuated the cell death process by directly scavenging intracellular ROS rather than activating endogenous antioxidant enzymes. This process encompasses inhibition of caspase-9/3/7, a decrease in the ratio of Bax/Bcl2, relocalization of translocated Bax and cytochrome c, upregulation of anti-apoptotic members of the Bcl2 family and a decrease in the phosphorylation of MAPKs (mitogen activated protein kinases). Furthermore, cellular homeostasis was re-established via stabilization of MAPK-mediated Nrf2/Keap1 signaling and transcription of downstream cytoprotective genes. This molecular study uniquely introduces a fucose-rich bacterial polysaccharide as a potential inhibitor of H2O2-induced stress and toxicities.

The thioredoxin system in neonatal lung disease

SIGNIFICANCE

Fetal lung development takes place in hypoxia meaning that premature birth is hyperoxia for the prematurely born infant. The most common respiratory morbidity afflicting premature infants is bronchopulmonary dysplasia (BPD). Pathophysiologically, BPD represents the impact of injury, including O2 toxicity, to the immature developing lung that causes arrested lung development.

RECENT ADVANCES

The thioredoxin (Trx) system, which is predominantly expressed in pulmonary epithelia in the newborn lung, acts as an antioxidant system; however, it is increasingly recognized as a key redox regulator of signal transduction and gene expression via thiol-disulfide exchange reactions.

CRITICAL ISSUES

This review focuses on the contribution of Trx family proteins toward normal and aberrant lung development, in particular, the roles of the Trx system in hyperoxic responses of alveolar epithelial cells, aberrant lung development in animal models of BPD, O2-dependent signaling processes, and possible therapeutic efficacy in preventing O2-mediated lung injury.

FUTURE DIRECTIONS

The significant contribution of the Trx system toward redox regulation of key developmental pathways necessary for proper lung development suggests that therapeutic strategies focused on preserving pulmonary Trx function could significantly improve the outcomes of prematurely born human infants.

Deletion of thioredoxin-interacting protein preserves retinal neuronal function by preventing inflammation and vascular injury

BACKGROUND AND PURPOSE

Retinal neurodegeneration is an early and critical event in several diseases associated with blindness. Clinically, therapies that target neurodegeneration fail. We aimed to elucidate the multiple roles by which thioredoxin-interacting protein (TXNIP) contributes to initial and sustained retinal neurodegeneration.

EXPERIMENTAL APPROACH

Neurotoxicity was induced by intravitreal injection of NMDA into wild-type (WT) and TXNIP-knockout (TKO) mice. The expression of apoptotic and inflammatory markers was assessed by immunohistochemistry, elisa and Western blot. Microvascular degeneration was assessed by periodic acid-Schiff and haematoxylin staining and retinal function by electroretinogram.

KEY RESULTS

NMDA induced early (1 day) and significant retinal PARP activation, a threefold increase in TUNEL-positive nuclei and 40% neuronal loss in ganglion cell layer (GCL); and vascular permeability in WT but not TKO mice. NMDA induced glial activation, expression of TNF-α and IL-1β that co-localized with Müller cells in WT but not TKO mice. In parallel, NMDA triggered the expression of NOD-like receptor protein (NLRP3), activation of caspase-1, and release of IL-1β and TNF-α in primary WT but not TKO Müller cultures. After 14 days, NMDA induced 1.9-fold microvascular degeneration, 60% neuronal loss in GCL and increased TUNEL-labelled cells in the GCL and inner nuclear layer in WT but not TKO mice. Electroretinogram analysis showed more significant reductions in b-wave amplitudes in WT than in TKO mice.

CONCLUSION AND IMPLICATIONS

Targeting TXNIP expression prevented early retinal ganglion cell death, glial activation, retinal inflammation and secondary neuro/microvascular degeneration and preserved retinal function. TXNIP is a promising new therapeutic target for retinal neurodegenerative diseases.

Sestrin2 decreases renal oxidative stress, lowers blood pressure, and mediates dopamine D2 receptor-induced inhibition of reactive oxygen species production

The dopamine D2 receptor (D2R) decreases renal reactive oxygen species (ROS) production and regulates blood pressure, in part, via positive regulation of paraoxonase 2. Sestrin2, a highly conserved antioxidant protein, regulates intracellular ROS level by regenerating hyperoxidized peroxiredoxins. We hypothesized that sestrin2 may be involved in preventing excessive renal ROS production and thus contribute to the maintenance of normal blood pressure. Moreover, the D2R may decrease ROS production, in part, through the regulation of sestrin2. Renal sestrin2 expression was lower (-62±13%) in D2R(-/-) than in D2R(+/+) mice. Silencing D2R in human renal proximal tubule cells decreased sestrin2 expression (-53±3%) and increased hyperoxidized peroxiredoxins (2.9-fold). Stimulation of D2R in renal proximal tubule cells increased sestrin2 expression (1.6-fold), decreased hyperoxidized peroxiredoxins (-61±3%), and reduced ROS production (-31±4%). Silencing sestrin2 in renal proximal tubule cells increased hyperoxidized peroxiredoxins (2.1-fold) and ROS production (1.3-fold). Silencing sestrin2 also abolished D2R-induced decrease in peroxiredoxin hyperoxidation and partially prevented the inhibitory effect of D2R stimulation on ROS production. Silencing paraoxonase 2 increased sestrin2 ubiquitinylation (2.8-fold), decreased sestrin2 expression (-30±3%), and increased ROS production (1.3-fold), peroxiredoxin hyperoxidation (2.9-fold), and lipid peroxidation (2.3-fold), and blocked the increase in sestrin2 that occurs with D2R stimulation. In vivo renal selective silencing of sestrin2 by the renal subcapsular infusion of sestrin2 small interfering RNA (3 μg/day; 7 days) in mice increased renal oxidative stress (1.3-fold) and blood pressure. These results suggest that the D2R, via paraoxonase 2 and sestrin2, keeps normal renal redox balance, which contributes to the maintenance of normal blood pressure.

Apoptosis of NOD.H2 h4 thyrocytes by low concentrations of iodide is associated with impaired control of oxidative stress

BACKGROUND

Enhanced iodide intake in NOD.H2(h4) mice accelerates the incidence and severity of spontaneous autoimmune thyroiditis (SAT) via an unknown mechanism. A plausible hypothesis is that iodide-induced apoptosis of thyrocytes can create imbalances in antigenic load and/or disruption of immunoregulatory mechanisms that facilitate activation of autoreactive T cells in cervical lymph nodes draining the thyroid.

METHODS

We examined whether NOD.H2(h4) thyrocytes, exposed to low NaI concentrations in vitro, are more susceptible to apoptosis compared to thyrocytes from CBA/J mice, which are resistant to iodide-accelerated SAT (ISAT). We also looked, at the transcriptional level, for differential activation of genes involved in apoptosis or oxidative stress pathways that may account for potential differences in iodide-mediated apoptosis between NOD.H2(h4) and CBA/J thyrocytes.

RESULTS

We report that NOD.H2(h4) thyrocytes, cultured for 24 h at very low (4-8 μM) concentrations of NaI, exhibit high levels (40-55%) of apoptosis, as assessed microscopically following staining with fluorescent caspase inhibitors. Similar treatment of thyrocytes from CBA/J mice, which are resistant to ISAT, yielded significantly lower (10-20%) apoptotic rates. Expression analysis by real-time polymerase chain reaction using arrays of apoptosis- and oxidative stress-related genes showed that NaI intake upregulates the expression of 22 genes involved in ROS metabolism and/or antioxidant function in CBA/J thyrocytes, whereas only two of these genes were upregulated in NOD.H2(h4) thyrocytes. Among the set of overexpressed genes were those encoding thyroid peroxidase (Tpo; 5.77-fold), glutathione peroxidases (Gpx2, Gpx4, Gpx7; 2.03-3.14-fold), peroxiredoxins (Prdx1, Prdx2, Prdx5; 2.27-2.97-fold), superoxide dismutase 1 (Sod1; 3.57-fold), thioredoxin 1 (Txn1; 2.13-fold), and the uncoupling proteins 2 and 3 (Ucp2, Ucp3; 2.01-2.15-fold).

CONCLUSIONS

The results demonstrate that an impaired control of oxidative stress mechanisms is associated with the observed high susceptibility of NOD.H2(h4) thyrocytes to NaI-mediated apoptosis, and suggest a contributing factor for the development of ISAT in this strain.

Molecular mechanisms of curcumin on diabetes-induced endothelial dysfunctions: Txnip, ICAM-1, and NOX2 expressions

We aim to investigate the effects of curcumin on preventing diabetes-induced vascular inflammation in association with its actions on Txnip, ICAM-1, and NOX2 enzyme expressions. Male Wistar rats were divided into four groups: control (CON), diabetic (DM; streptozotocin (STZ), i.v. 55 mg/kg BW), control-treated with curcumin (CONCUR; 300 mg/kg BW), and diabetes treated with curcumin (DMCUR; 300 mg/kg BW). 12th week after STZ injection, iris blood perfusion, leukocyte adhesion, Txnip, p47phox, and malondialdehyde (MDA) levels were determined by using laser Doppler, intravital fluorescent confocal microscopy, Western Blot analysis, and TBAR assay, respectively. The iris blood perfusion of DM and DMCUR was decreased significantly compared to CON and CONCUR (P < 0.001). Plasma glucose and HbA1c of DM and DMCUR were increased significantly compared to CON and CONCUR (P < 0.001). Leukocyte adhesion, ICAM-1, p47phox expression, and MDA levels in DM were increased significantly compared to CON, CONCUR, and DMCUR (P < 0.05). Txnip expression in DM and DMCUR was significantly higher than CON and CONCUR (P < 0.05). From Pearson's analysis, the correlation between the plasma MDA level and the endothelial functions was significant. It suggested that curcumin could ameliorate diabetic vascular inflammation by decreasing ROS overproduction, reducing leukocyte-endothelium interaction, and inhibiting ICAM-1 and NOX2 expression.

Minireview: Thioredoxin-interacting protein: regulation and function in the pancreatic β-cell

Pancreatic β-cells are responsible for insulin production, and loss of functional β-cell mass is now recognized as a critical step in the pathogenesis of both type 1 and type 2 diabetes. However, the factors controlling the life and death of the pancreatic β-cell have only started to be elucidated. Discovered as the top glucose-induced gene in a human islet microarray study 12 years ago, thioredoxin-interacting protein (TXNIP) has now emerged as such a key player in pancreatic β-cell biology. Since then, β-cell expression of TXNIP has been found to be tightly regulated by multiple factors and to be dramatically increased in diabetic islets. Elevated TXNIP levels induce β-cell apoptosis, whereas TXNIP deficiency protects against type 1 and type 2 diabetes by promoting β-cell survival. TXNIP interacts with and inhibits thioredoxin and thereby controls the cellular redox state, but it also belongs to the α-arrestin family of proteins and regulates a variety of metabolic processes. Most recently, TXNIP has been discovered to control β-cell microRNA expression, β-cell function, and insulin production. In this review, the current state of knowledge regarding regulation and function of TXNIP in the pancreatic β-cell and the implications for drug development are discussed.

Thioredoxin-interacting protein is a biomechanical regulator of Src activity: key role in endothelial cell stress fiber formation

RATIONALE

Fluid shear stress differentially regulates endothelial cell stress fiber formation with decreased stress fibers in areas of disturbed flow compared with steady flow areas. Importantly, stress fibers are critical for several endothelial cell functions including cell shape, mechano-signal transduction, and endothelial cell-cell junction integrity. A key mediator of steady flow-induced stress fiber formation is Src that regulates downstream signaling mediators such as phosphorylation of cortactin, activity of focal adhesion kinase, and small GTPases.

OBJECTIVE

Previously, we showed that thioredoxin-interacting protein (TXNIP, also VDUP1 [vitamin D upregulated protein 1] and TBP-2 [thioredoxin binding protein 2]) was regulated by fluid shear stress; TXNIP expression was increased in disturbed flow compared with steady flow areas. Although TXNIP was originally characterized for its role in redox and metabolic cellular functions, recent reports show important scaffold functions related to its α-arrestin structure. Based on these findings, we hypothesized that TXNIP acts as a biomechanical sensor that regulates Src kinase activity and stress fiber formation.

METHODS AND RESULTS

Using en face immunohistochemistry of the aorta and cultured endothelial cells, we show inverse relationship between TXNIP expression and Src activity. Specifically, steady flow increased Src activity and stress fiber formation, whereas it decreased TXNIP expression. In contrast, disturbed flow had opposite effects. We studied the role of TXNIP in regulating Src homology phosphatase-2 plasma membrane localization and vascular endothelial cadherin binding because Src homology phosphatase-2 indirectly regulates dephosphorylation of Src tyrosine 527 that inhibits Src activity. Using immunohistochemistry and immunoprecipitation, we found that TXNIP prevented Src homology phosphatase-2-vascular endothelial cadherin interaction.

CONCLUSIONS

In summary, these data characterize a fluid shear stress-mediated mechanism for stress fiber formation that involves a TXNIP-dependent vascular endothelial cadherin-Src homology phosphatase-2-Src pathway.

A critical role for thioredoxin-interacting protein in diabetes-related impairment of angiogenesis

Impaired angiogenesis in ischemic tissue is a hallmark of diabetes. Thioredoxin-interacting protein (TXNIP) is an exquisitely glucose-sensitive gene that is overexpressed in diabetes. As TXNIP modulates the activity of the key angiogenic cytokine vascular endothelial growth factor (VEGF), we hypothesized that hyperglycemia-induced dysregulation of TXNIP may play a role in the pathogenesis of impaired angiogenesis in diabetes. In the current study, we report that high glucose-mediated overexpression of TXNIP induces a widespread impairment in endothelial cell (EC) function and survival by reducing VEGF production and sensitivity to VEGF action, findings that are rescued by silencing TXNIP with small interfering RNA. High glucose-induced EC dysfunction was recapitulated in normal glucose conditions by overexpressing either TXNIP or a TXNIP C247S mutant unable to bind thioredoxin, suggesting that TXNIP effects are largely independent of thioredoxin activity. In streptozotocin-induced diabetic mice, TXNIP knockdown to nondiabetic levels rescued diabetes-related impairment of angiogenesis, arteriogenesis, blood flow, and functional recovery in an ischemic hindlimb. These findings were associated with in vivo restoration of VEGF production to nondiabetic levels. These data implicate a critical role for TXNIP in diabetes-related impairment of ischemia-mediated angiogenesis and identify TXNIP as a potential therapeutic target for the vascular complications of diabetes.

Targeted metabolomics connects thioredoxin-interacting protein (TXNIP) to mitochondrial fuel selection and regulation of specific oxidoreductase enzymes in skeletal muscle

Thioredoxin-interacting protein (TXNIP) is an α-arrestin family member involved in redox sensing and metabolic control. Growing evidence links TXNIP to mitochondrial function, but the molecular nature of this relationship has remained poorly defined. Herein, we employed targeted metabolomics and comprehensive bioenergetic analyses to evaluate oxidative metabolism and respiratory kinetics in mouse models of total body (TKO) and skeletal muscle-specific (TXNIP(SKM-/-)) Txnip deficiency. Compared with littermate controls, both TKO and TXNIP(SKM-/-) mice had reduced exercise tolerance in association with muscle-specific impairments in substrate oxidation. Oxidative insufficiencies in TXNIP null muscles were not due to perturbations in mitochondrial mass, the electron transport chain, or emission of reactive oxygen species. Instead, metabolic profiling analyses led to the discovery that TXNIP deficiency causes marked deficits in enzymes required for catabolism of branched chain amino acids, ketones, and lactate, along with more modest reductions in enzymes of β-oxidation and the tricarboxylic acid cycle. The decrements in enzyme activity were accompanied by comparable deficits in protein abundance without changes in mRNA expression, implying dysregulation of protein synthesis or stability. Considering that TXNIP expression increases in response to starvation, diabetes, and exercise, these findings point to a novel role for TXNIP in coordinating mitochondrial fuel switching in response to nutrient availability.

The thioredoxin antioxidant system

The thioredoxin (Trx) system, which is composed of NADPH, thioredoxin reductase (TrxR), and thioredoxin, is a key antioxidant system in defense against oxidative stress through its disulfide reductase activity regulating protein dithiol/disulfide balance. The Trx system provides the electrons to thiol-dependent peroxidases (peroxiredoxins) to remove reactive oxygen and nitrogen species with a fast reaction rate. Trx antioxidant functions are also shown by involvement in DNA and protein repair by reducing ribonucleotide reductase, methionine sulfoxide reductases, and regulating the activity of many redox-sensitive transcription factors. Moreover, Trx systems play critical roles in the immune response, virus infection, and cell death via interaction with thioredoxin-interacting protein. In mammalian cells, the cytosolic and mitochondrial Trx systems, in which TrxRs are high molecular weight selenoenzymes, together with the glutathione-glutaredoxin (Grx) system (NADPH, glutathione reductase, GSH, and Grx) control the cellular redox environment. Recently mammalian thioredoxin and glutathione systems have been found to be able to provide the electrons crossly and to serve as a backup system for each other. In contrast, bacteria TrxRs are low molecular weight enzymes with a structure and reaction mechanism distinct from mammalian TrxR. Many bacterial species possess specific thiol-dependent antioxidant systems, and the significance of the Trx system in the defense against oxidative stress is different. Particularly, the absence of a GSH-Grx system in some pathogenic bacteria such as Helicobacter pylori, Mycobacterium tuberculosis, and Staphylococcus aureus makes the bacterial Trx system essential for survival under oxidative stress. This provides an opportunity to kill these bacteria by targeting the TrxR-Trx system.

IL-22 activates oxidant signaling in pulmonary vascular smooth muscle cells

Reactive oxygen species (ROS) mediate cell-signaling processes in response to various ligands and play important roles in the pathogenesis of cardiovascular diseases. The present study reports that interleukin-22 (IL-22) elicits signal transduction in vascular smooth muscle cells (SMCs) through a ROS-dependent mechanism. We find that pulmonary artery SMCs express IL-22 receptor alpha 1 and that IL-22 activates STAT3 through this receptor. IL-22-induced signaling is found to be mediated by NADPH oxidase, as indicated by the observations that the inhibition and siRNA knock-down of this enzyme inhibit IL-22 signaling. IL-22 triggers the oxidative modifications of proteins through protein carbonylation and protein glutathionylation. Mass spectrometry identified some proteins that are carbonylated in response to IL-22 stimulation, including α-enolase, heat shock cognate 71kDa protein, mitochondrial 60kDa heat shock protein, and cytoplasmic 2 actin and determined that α-tubulin is glutathionylated. Protein glutathionylation and STAT3 phosphorylation are enhanced by the siRNA knock-down of glutaredoxin, while IL-22-mediated STAT3 phosphorylation is suppressed by knocking down thioredoxin interacting protein, an inhibitor of thioredoxin. IL-22 is also found to promote the growth of SMCs via NADPH oxidase. In rats, pulmonary hypertension is found to be associated with increased smooth muscle IL-22 expression. These results show that IL-22 promotes the growth of pulmonary vascular SMCs via a signaling mechanism that involves NADPH oxidase-dependent oxidation.

Txnip contributes to impaired glucose tolerance by upregulating the expression of genes involved in hepatic gluconeogenesis in mice

AIMS/HYPOTHESIS

Thioredoxin-interacting protein (TXNIP) is upregulated in the hyperglycaemic state and represses glucose uptake, resulting in imbalanced glucose homeostasis. In this study, we propose a mechanism of how TXNIP impairs hepatic glucose tolerance at the transcriptional level.

METHODS

We administered adenoviral Txnip (Ad-Txnip) to normal mice and performed intraperitoneal glucose tolerance tests (IPGTT), insulin tolerance tests (ITT) and pyruvate tolerance tests (PTT). After Ad-Txnip administration, the expression of genes involved in glucose metabolism, including G6pc and Gck, was analysed using quantitative real-time PCR and western blot. To understand the increased G6pc expression in liver resulting from Txnip overexpression, we performed pull-down assays for TXNIP and small heterodimer partner (SHP). Luciferase reporter assays and chromatin immunoprecipitation using the Txnip promoter were performed to elucidate the interrelationship between carbohydrate response element-binding protein (ChREBP) and transcription factor E3 (TFE3) in the regulation of Txnip expression.

RESULTS

Overabundance of TXNIP resulted in impaired glucose, insulin and pyruvate tolerance in normal mice. Ad-Txnip transduction upregulated G6pc expression and caused a decrease in Gck levels in the liver of normal mice and primary hepatocytes. TXNIP increased G6pc expression by forming a complex with SHP, which is known to be a negative modulator of gluconeogenesis. Txnip expression in mouse models of diabetes was decreased by Ad-Tfe3 administration, suggesting that TFE3 may play a negative role through competition with ChREBP at the E-box of the Txnip promoter.

CONCLUSIONS/INTERPRETATION

We demonstrated that TXNIP impairs glucose and insulin tolerance in mice by upregulating G6pc through interaction with SHP.

The thioredoxin system as a therapeutic target in human health and disease

The thioredoxin (Trx) system comprises Trx, truncated Trx (Trx-80), Trx reductase, and NADPH, besides a natural Trx inhibitor, the thioredoxin-interacting protein (TXNIP). This system is essential for maintaining the balance of the cellular redox status, and it is involved in the regulation of redox signaling. It is also pivotal for growth promotion, neuroprotection, inflammatory modulation, antiapoptosis, immune function, and atherosclerosis. As an ubiquitous and multifunctional protein, Trx is expressed in all forms of life, executing its function through its antioxidative, protein-reducing, and signal-transducing activities. In this review, the biological properties of the Trx system are highlighted, and its implications in several human diseases are discussed, including cardiovascular diseases, heart failure, stroke, inflammation, metabolic syndrome, neurodegenerative diseases, arthritis, and cancer. The last chapter addresses the emerging therapeutic approaches targeting the Trx system in human diseases.

Reciprocal regulation of NO signaling and TXNIP expression in humans: impact of aging and ramipril therapy

BACKGROUND

Impaired tissue responsiveness to nitric oxide (NO) occurs in many cardiovascular diseases as well as with advanced age and is a correlate of poor outcomes. This phenomenon results from oxidative stress, with NO "scavenging" and dysfunction of soluble guanylate cyclase (sGC). Thioredoxin-interacting protein (TXNIP) is a major intracellular regulator of inflammatory activation and redox stress, but its interactions with NO/sGC are poorly understood. We have now evaluated the relationship between platelet TXNIP expression and function of the NO/sGC axis in subjects of varying age and during therapy with ramipril.

METHODS & RESULTS

Young (n=42) and aging (n=49) subjects underwent evaluation of platelet TXNIP content. Aging subjects additionally had measurements of platelet NO responsiveness and routine biochemistry. Platelet TXNIP content was greater (376±33 units) in the aging compared to younger subjects (289±13 units; p<0.05). In the aging subjects there was a significant negative correlation (r=-0.50, p<0.001) between platelet TXNIP content and NO responsiveness. In a separate cohort of 15 subjects two week treatment with ramipril, which reversed platelet NO resistance and potentiated sGC activity, also decreased platelet TXNIP content by 40% (p=0.011).

CONCLUSIONS

Platelet TXNIP content increases with aging, varies inversely with responsiveness to NO, and diminishes rapidly following treatment with ramipril. These data suggest that TXNIP-induced oxidative stress may be a critical modulator of tissue resistance to NO, a fundamental basis for cardiovascular disease. Analogously suppression of TXNIP expression can potentially be utilized as an index of restoration of cardiovascular homeostasis.

The interactions of oxidative stress and inflammation with vascular dysfunction in ageing: the vascular health triad

Oxidative stress and inflammation are increased with advancing age. Evidence suggests that oxidative stress and inflammation both lead to impaired vascular function. There is also evidence to suggest that inflammation may cause an increase in radical production leading to enhanced oxidative stress. In addition, oxidative stress may cause an increase in inflammation; however, the interactions between these factors are not fully understood. In this review, we propose the vascular health triad, which draws associations and interactions between oxidative stress and inflammation seen in ageing, and the consequences for vascular function. We review evidence suggesting that exercise may ameliorate the age-related decline in vascular function, through reductions in both oxidative stress and inflammation.

Structure of the N-terminal domain of human thioredoxin-interacting protein

Thioredoxin-interacting protein (TXNIP) is one of the six known α-arrestins and has recently received considerable attention owing to its involvement in redox signalling and metabolism. Various stress stimuli such as high glucose, heat shock, UV, H2O2 and mechanical stress among others robustly induce the expression of TXNIP, resulting in the sequestration and inactivation of thioredoxin, which in turn leads to cellular oxidative stress. While TXNIP is the only α-arrestin known to bind thioredoxin, TXNIP and two other α-arrestins, Arrdc4 and Arrdc3, have been implicated in metabolism. Furthermore, owing to its roles in the pathologies of diabetes and cardiovascular disease, TXNIP is considered to be a promising drug target. Based on their amino-acid sequences, TXNIP and the other α-arrestins are remotely related to β-arrestins. Here, the crystal structure of the N-terminal domain of TXNIP is reported. It provides the first structural information on any of the α-arrestins and reveals that although TXNIP adopts a β-arrestin fold as predicted, it is structurally more similar to Vps26 proteins than to β-arrestins, while sharing below 15% pairwise sequence identity with either.

Identification and immunological characterization of thioredoxin transmembrane-related protein from Clonorchis sinensis

Thioredoxin transmembrane related protein (TMX), a member of thioredoxin superfamily, is localized to the endoplasmic reticulum and possesses a thioredoxin-like domain that plays an important role as an oxidoreductase. The functions of TMX in Clonorchis sinensis remain to be elucidated. In this study, we cloned and characterized a novel TMX of C. sinensis (CsTMX). The CsTMX cDNA sequence contained a 414-nucleotide open-reading frame encoding a protein of 137 amino acids. A thioredoxin domain was found in the position of aa21-117 and contained the putative active-site motif Cys-Pro-Ala-Cys. BLASTx analysis showed that CsTMX shared 39-57% amino acid identities with TMX of other organisms. Quantitative RT-PCR analysis demonstrated that CsTMX was differentially transcribed, with the highest level of expression in the adult worm stage and the lowest expression in egg stage. In addition, immunofluorescence assay showed CsTMX was localized in the tegument, vitelline gland, intestine, and intrauterine eggs of adult worm. Besides, immunoblot assay revealed that the recombinant CsTMX (rCsTMX) could be recognized by the sera from rats infected with C. sinensis and the sera from rats immunized by excretory-secretory products. Furthermore, analysis of the antibody isotype profile revealed that rats subcutaneously immunized with rCsTMX developed rCsTMX-specific antibody, which is dominance of IgG2a in sera. Meanwhile, production of IFN-γ was elevated strongly in the supernatants of spleen cell. The results collectively indicated that CsTMX might play an important role in the host-parasite interaction, as well as CsTMX probably involved in immunoregulation of host by inducing Th1-type dominated immune response in rats.

Thioredoxin-interacting protein mediates sustained VEGFR2 signaling in endothelial cells required for angiogenesis

OBJECTIVE

Thioredoxin-interacting protein (TXNIP) is an α-arrestin protein whose function is important for the regulation of vascular endothelial growth factor receptor 2 (VEGFR2) signaling and endothelial cell survival. Because VEGFR2 is critical for angiogenesis, we explored the role of TXNIP in VEGF-induced angiogenesis.

APPROACH AND RESULTS

TXNIP knockdown inhibited VEGF-induced endothelial cell tube formation and proliferation in cultured human umbilical vein endothelial cell. To elucidate the mechanism by which TXNIP altered VEGFR2 signaling in human umbilical vein endothelial cell, we studied phosphorylation of VEGFR2, phospholipase C gamma-1 (PLCγ1), endothelial NO synthase, and Akt (known as protein kinase B). TXNIP knockdown significantly decreased phosphorylation of VEGFR2 and PLCγ1 at times >5 minutes, but phosphorylation was unchanged at 2 minutes, as was Akt and endothelial NO synthase phosphorylation. Cell-surface biotinylation assay showed that TXNIP knockdown significantly attenuated VEGFR2 internalization. These results suggested that TXNIP was required for sustained VEGFR2 signaling, which is mediated largely by internalized VEGFR2. Rab5 knockdown to inhibit the trafficking and fusion of early endosomes significantly blocked VEGF-induced VEGFR2 internalization and phosphorylation of VEGFR2 and PLCγ1. Immunofluorescence and coimmunoprecipitation showed that TXNIP was part of a complex that included Rab5 and VEGFR2. Finally, TXNIP knockdown prevented the association of VEGFR2 and Rab5.

CONCLUSIONS

Our results show that TXNIP is essential for VEGFR2 internalization in Rab5 positive endosomes, which is required for endothelial cell growth and angiogenesis.

Acute hyperglycaemia enhances oxidative stress and aggravates myocardial ischaemia/reperfusion injury: role of thioredoxin-interacting protein

Hyperglycaemia during acute myocardial infarction is common and associated with increased mortality. Thioredoxin-interacting protein (Txnip) is a modulator of cellular redox state and contributes to cell apoptosis. This study aimed to investigate whether or not hyperglycaemia enhances Txnip expression in myocardial ischaemia/reperfusion (MI/R) and consequently exacerbates MI/R injury. Rats were subjected to 30 min. of left coronary artery ligation followed by 4 hrs of reperfusion and treated with saline or high glucose (HG, 500 g/l, 4 ml/kg/h intravenously). In vitro study was performed on cultured rat cardiomyocytes subjected to simulated ischaemia/reperfusion (SI/R) and incubated with HG (25 mM) or normal glucose (5.6 mM) medium. In vivo HG infusion during MI/R significantly impaired cardiac function, aggravated myocardial injury and increased cardiac oxidative stress. Meanwhile, Txnip expression was enhanced whereas thioredoxin activity was inhibited following HG treatment in ischaemia/reperfusion (I/R) hearts. In addition, HG activated p38 MAPK and inhibited Akt in I/R hearts. In cultured cardiomyocytes subjected to SI/R, HG incubation stimulated Txnip expression and reduced thioredoxin activity. Overexpression of Txnip enhanced HG-induced superoxide generation and aggravated cardiomyocyte apoptosis, whereas Txnip RNAi significantly blunted the deleterious effects of HG. Moreover, inhibition of p38 MAPK or activation of Akt markedly blocked HG-induced Txnip expression in I/R cardiomyocytes. Most importantly, intramyocardial injection of Txnip siRNA markedly decreased Txnip expression and alleviated MI/R injury in HG-treated rats. Hyperglycaemia enhances myocardial Txnip expression, possibly through reciprocally modulating p38 MAPK and Akt activation, leading to aggravated oxidative stress and subsequently, amplification of cardiac injury following MI/R.

Thioredoxin system in cell death progression

SIGNIFICANCE

The thioredoxin (Trx) system, comprising nicotinamide adenine dinucleotide phosphate, Trx reductase (TrxR), and Trx, is critical for maintaining cellular redox balance and antioxidant function, including control of oxidative stress and cell death.

RECENT ADVANCES

Here, we focus on the research progress that is involved in the regulation of apoptosis by Trx systems. In mammalian cells, cytosolic Trx1 and mitochondrial Trx2 systems are the major disulfide reductases supplying electrons to enzymes for cell proliferation and viability. The reduced/dithiol form of Trxs binds to apoptosis signal-regulating kinase 1 (ASK1) and inhibits its activity to prevent stress- and cytokine-induced apoptosis. When Trx is oxidized, it dissociates from ASK1 and apoptosis is stimulated. The binding of Trx by its inhibitor Trx interacting protein (TXNIP) also contributes to the apoptosis process by removing Trx from ASK1. TrxRs are large homodimeric selenoproteins with an overall structure which is similar to that of glutathione reductase, and contain an active site GCUG in the C-terminus.

CRITICAL ISSUES AND FUTURE DIRECTIONS

In the regulation of cell death processes, Trx redox state and TrxR activities are key factors that determine the cell fate. The high reactivity of Sec in TrxRs and its accessible location make TrxR enzymes emerge as targets for pharmaceutic drugs. TrxR inactivation by covalent modification does not only change the redox state and activity of Trx, but may also convert TrxR into a reactive oxygen species generator. Numerous electrophilic compounds including some environmental toxins and pharmaceutical drugs inhibit TrxR. We have classified these compounds into four types and propose some useful principles to understand the reaction mechanism of the TrxR inhibition by these compounds.

Redox homeostasis in pancreatic β cells

We reviewed mechanisms that determine reactive oxygen species (redox) homeostasis, redox information signaling and metabolic/regulatory function of autocrine insulin signaling in pancreatic β cells, and consequences of oxidative stress and dysregulation of redox/information signaling for their dysfunction. We emphasize the role of mitochondrion in β cell molecular physiology and pathology, including the antioxidant role of mitochondrial uncoupling protein UCP2. Since in pancreatic β cells pyruvate cannot be easily diverted towards lactate dehydrogenase for lactate formation, the respiration and oxidative phosphorylation intensity are governed by the availability of glucose, leading to a certain ATP/ADP ratio, whereas in other cell types, cell demand dictates respiration/metabolism rates. Moreover, we examine the possibility that type 2 diabetes mellitus might be considered as an inevitable result of progressive self-accelerating oxidative stress and concomitantly dysregulated information signaling in peripheral tissues as well as in pancreatic β cells. It is because the redox signaling is inherent to the insulin receptor signaling mechanism and its impairment leads to the oxidative and nitrosative stress. Also emerging concepts, admiting participation of redox signaling even in glucose sensing and insulin release in pancreatic β cells, fit in this view. For example, NADPH has been firmly established to be a modulator of glucose-stimulated insulin release.

New insight into the mechanisms underlying the function of the incretin hormone glucagon-like peptide-1 in pancreatic β-cells: the involvement of the Wnt signaling pathway effector β-catenin

During the past two decades, the exploration of function of two incretin hormones, namely glucagon-like peptide-1 (GLP-1) and gastric inhibitory peptide (GIP), has led to the development of two categories of novel therapeutic agents for diabetes and its complications, known as GLP-1 receptor (GLP-1R) agonists and DPP-IV inhibitors. Mechanisms underlying the function of GLP-1, however, still need to be further explored. GLP-1 not only functions as an incretin hormone in stimulating insulin secretion in response to nutritional, hormonal and neuronal stimulations, but also acts as an "insulin-like" factor in β-cell and extra-pancreatic organs. In addition to these insulinotropic and insulinomimetic effects, GLP-1 was shown to exert its protective effect in β-cell by repressing the expression of TxNIP, a mediator of glucolipotoxicity. A number of recent studies have shown that the Wnt signaling pathway effector, the bipartite transcription factor β-catenin/TCF, controls not only the production of GLP-1, but also the function of GLP-1. Furthermore, previously assumed "degradation" products of GLP-1(7-36)amide, including GLP-1(9-36)amide and GLP-1(28-36)amide, have been shown to exert beneficial effect in pancreas and extra-pancreatic tissues or cell lineages. Here we summarized our current knowledge on the metabolic, proliferative and protective effects of GLP-1(7-36)amide and its cleavage fragments, mainly focusing on pancreatic β-cells and the involvement of the Wnt signaling pathway effector β-catenin.

Glucose sensing by ChREBP/MondoA-Mlx transcription factors

The paralogous transcription factors ChREBP and MondoA, together with their common binding partner Mlx, have emerged as key mediators of intracellular glucose sensing. By regulating target genes involved in glycolysis and lipogenesis, they mediate metabolic adaptation to changing glucose levels. As disturbed glucose homeostasis plays a central role in human metabolic diseases and as cancer cells often display altered glucose metabolism, better understanding of cellular glucose sensing will likely uncover new therapeutic opportunities. Here we review the regulation, function and evolutionary conservation of the ChREBP/MondoA-Mlx glucose sensing system and discuss possible directions for future research.

Thioredoxin-interacting protein mediates nuclear-to-plasma membrane communication: role in vascular endothelial growth factor 2 signaling

OBJECTIVE

Thioredoxin-interacting protein (TXNIP) and poly-ADP-ribose polymerase 1 (PARP1) are both regulated by changes in cellular reduction-oxidation (redox) state and localize to the nucleus basally in human umbilical vein endothelial cells (HUVEC). Previously we showed a novel mechanism for PARP1 inhibition-mediated HUVEC survival through activation of vascular endothelial growth factor receptor 2 (VEGFR2) signaling in response to stress-induced apoptosis. In addition, we showed TXNIP translocation to the plasma membrane (PM) and activation of VEGFR2 in response to physiological stimuli. Because TXNIP is an α-arrestin that regulates VEGFR2 signaling, we hypothesized that PARP1 regulates TXNIP localization and function that might affect HUVEC stress-induced apoptosis.

METHODS AND RESULTS

HUVEC treated with 10 μmol/L PARP1 inhibitor (PJ34) were protected from TNF (10 ng/mL) or H(2)O(2) (300 μmol/L) mediated cell death. HUVEC transfected with TXNIP siRNA lost the protective effect of PARP1 inhibition, suggesting a protective role for TXNIP. Using immunofluorescence, cell fractionation analysis, and plasma membrane sheet assay, TXNIP was shown to translocate to the plasma membrane after PARP1 inhibition. TXNIP translocation was associated with activation of VEGFR2 signaling. Functionally, TXNIP-PARP1 interaction was decreased on PJ34 treatment, suggesting PARP1 as a novel regulator of TXNIP localization and function.

CONCLUSIONS

These findings demonstrate a novel regulatory mechanism of TXNIP by PARP1 to mediate activation of plasma membrane signaling and cell survival.

Redox redux: protecting the ischemic myocardium

Cardiac ischemia-reperfusion (I-R) injury occurs upon prompt restoration of blood flow to the ischemic myocardium after an acute myocardial infarction. Interestingly, many of the features of I-R injury are related to impaired mitochondrial signaling and mitochondrial dysfunction. Restoring cardiac energy bioavailability and reduction-oxidation (redox) signaling are therefore important in recovery after I-R injury. In this issue of the JCI, Yoshioka and colleagues describe an important and unexpected role for thioredoxin-interacting protein (TXNIP) in the control of mitochondrial respiration and cell energy metabolism. Their findings could open the door for development of TXNIP-targeted therapeutic approaches for the treatment of cardiac I-R injury.