Control of mitochondrial outer membrane permeabilization and Bcl-xL levels by thioredoxin 2 in DT40 cells

Nrf2 Signaling Pathway: a Potential Therapeutic Target in Combating Oxidative Stress and Neurotoxicity in Chemotherapy-Induced Cognitive Impairment

Chemotherapy-induced cognitive impairment (CICI) is one of the major adverse effects of antineoplastic drugs, which decrease the quality of life in cancer survivors. Extensive experimental and clinical research suggests that chemotherapeutic drugs generate an enormous amount of reactive oxygen species (ROS), contributing to oxidative stress, neuroinflammation, blood-brain barrier (BBB) disruption, and neuronal death, eventually leading to CICI. Despite the progress in exploring different pathological mechanisms of CICI, effective treatment to prevent CICI progression has not been developed yet. Nrf2 is the principal transcription factor that regulates cellular redox balance and inflammation-related gene expression. Emerging evidence suggests that upregulation of Nrf2 and its target genes could suppress oxidative stress, and neuroinflammation, restore BBB integrity, and increase neurogenesis. This review discusses the role of Nrf2 in CICI, how it responds to oxidative stress, inflammation, neurotoxicity, and potential Nrf2 activators that could be used to enhance Nrf2 activation in CICI.

A metabolic perspective on nitric oxide function in melanoma

Nitric oxide (NO) generated from nitric oxide synthase (NOS) exerts a dichotomous effect in melanoma, suppressing or promoting tumor progression. This dichotomy is thought to depend on the intracellular NO concentration and the cell type in which it is generated. Due to its central role in the metabolism of multiple critical constituents involved in signaling and stress, it is crucial to explore NO's contribution to the metabolic dysfunction of melanoma. This review will discuss many known metabolites linked to NO production in melanoma. We discuss the synthesis of these metabolites, their role in biochemical pathways, and how they alter the biological processes observed in the melanoma tumor microenvironment. The metabolic pathways altered by NO and the corresponding metabolites reinforce its dual role in melanoma and support investigating this effect for potential avenues of therapeutic intervention.

Redox-dependent and independent effects of thioredoxin interacting protein

Thioredoxin interacting protein (TXNIP) is an important physiological inhibitor of the thioredoxin (TXN) redox system in cells. Regulation of TXNIP expression and/or activity not only plays an important role in redox regulation but also exerts redox-independent physiological effects that exhibit direct pathophysiological consequences including elevated inflammatory response, aberrant glucose metabolism, cellular senescence and apoptosis, cellular immunity, and tumorigenesis. This review provides a brief overview of the current knowledge concerning the redox-dependent and independent roles of TXNIP and its relevance to various disease states. The implications for the therapeutic targeting of TXNIP will also be discussed.

Mitochondrial redox and TCA cycle metabolite signaling in the heart

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

Mitochondrial TXN2 attenuates amyloidogenesis via selective inhibition of BACE1 expression

Thioredoxin-2 (TXN2) is a mitochondrial protein and represents one of the intrinsic antioxidant enzymes. It has long been recognized that mitochondrial dysfunction and oxidative stress contribute to the pathogenesis of Alzheimer's disease (AD). We hypothesized that mitochondrial TXN2 might play a role in AD-like pathology. In this study, we found that in SH-SY5Y and HEK cells stably express full-length human amyloid-β precursor protein (HEK-APP), TXN2 silencing or over-expression selectively increased or decreased the transcription of beta-site amyloid precursor protein cleaving enzyme 1 (BACE1), respectively, without altering the protein levels of others enzymes involved in the catalytic processing of APP. As a result, β-amyloid protein (Aβ) levels were significantly decreased by TXN2. In addition, in cells treated with 3-nitropropionic acid (3-NP) that is known to increase reactive oxygen species (ROS) and promote mitochondrial dysfunction, TXN2 silencing resulted in further enhancement of BACE1 protein levels, suggesting a role of TXN2 in ROS removal. The downstream signaling might involve NFκB, as TXN2 reduced the phosphorylation of p65 and IκBα; and p65 knockdown significantly attenuated TXN2-mediated regulation of BACE1. Concomitantly, the levels of cellular ROS, apoptosis-related proteins and cell viability were altered by TXN2 silencing or over-expression. In APPswe/PS1E9 mice, an animal model of AD, the cortical and hippocampal TXN2 protein levels were decreased at 12 months but not at 6 months, suggesting an age-dependent decline. Collectively, TXN2 regulated BACE1 expression and amyloidogenesis via cellular ROS and NFκB signaling. TXN2 might serve as a potential target especially for early intervention of AD.

In Vitro Antiplasmodium and Chloroquine Resistance Reversal Effects of Andrographolide

The emergence of drug-resistant strains of Plasmodium falciparum is the worst catastrophe that has ever confronted the dedicated efforts to eradicate malaria. This urged for searching other alternatives or sensitizers that reverse chloroquine resistance. In this experiment, the potential of andrographolide to inhibit plasmodial growth and reverse CQ resistance was tested in vitro using the SYBRE green-1-based drug sensitivity assay and isobologram technique, respectively. Its safety level toward mammalian cells was screened as well against Vero cells and RBCs using MTT-based drug sensitivity and RBC hemolysis assays, respectively. Its effect against hemozoin formation was screened using β-hematin formation and heme fractionation assays. Its molecular characters were determined using the conventional tests for the antioxidant effect measurement and the in silico molecular characterization using the online free chemi-informatic Molinspiration software. Results showed that andrographolide has a moderate antiplasmodium effect that does not entitle it to be a substituent for chloroquine. Furthermore, andrographolide ameliorated the sensitivity of the parasite to chloroquine. Besides, it showed an indirect inhibitory effect against hemozoin formation within the parasite and augmented the chloroquine-induced inhibition of hemozoin formation. The study suggests that its chloroquine resistance reversal effect may be due to inhibition of chloroquine accumulation or due to its impact on the biological activity of the parasite. Overall, this in vitro study is a clue for the reliability of andrographolide to be added with chloroquine for reversal of chloroquine resistance and tolerance, but further in vivo studies are recommended to confirm this notion. In spite of its prominent and safe in vitro and in vivo growth inhibitory effect and its in vitro chloroquine resistance reversing effect, it is inapplicable to implement it in malaria chemotherapy to substitute chloroquine or to reverse its resistance.

Doxorubicin and cisplatin induced cognitive impairment: The possible mechanisms and interventions

Chemotherapy has significantly increased the number of cancer survivors. However, chemotherapy itself carries various adverse effects that limit the efficacy of treatment and quality of life of the cancer patients. Most patients who have received chemotherapy report some cognitive deficit characterized by dysfunction in memory, learning, concentration, and reasoning. The phenomenon of cognitive decline developed from chemotherapy treatment is referred to as chemotherapy-induced cognitive impairment (CICI) or chemobrain. The two most common cancers occurring worldwide are lung and breast cancer. The predominant chemotherapeutic drugs used to treat lung and breast cancer are doxorubicin and cisplatin. There is evidence to suggest that both drugs potentially induce chemobrain. The evidence around the proposed pathogenesis of chemobrain caused by these two drugs is inconsistent. Understanding the underlying mechanisms involved in the development of chemobrain would aid in the prevention or treatment of the adverse effects of chemotherapy on brain. This review will summarize and discuss controversial findings and possible mechanisms involved in the development of chemobrain and the interventions which could limit it from in vitro, in vivo, and clinical studies.

Antioxidant Defence Systems and Oxidative Stress in Poultry Biology: An Update

Poultry in commercial settings are exposed to a range of stressors. A growing body of information clearly indicates that excess ROS/RNS production and oxidative stress are major detrimental consequences of the most common commercial stressors in poultry production. During evolution, antioxidant defence systems were developed in poultry to survive in an oxygenated atmosphere. They include a complex network of internally synthesised (e.g., antioxidant enzymes, (glutathione) GSH, (coenzyme Q) CoQ) and externally supplied (vitamin E, carotenoids, etc.) antioxidants. In fact, all antioxidants in the body work cooperatively as a team to maintain optimal redox balance in the cell/body. This balance is a key element in providing the necessary conditions for cell signalling, a vital process for regulation of the expression of various genes, stress adaptation and homeostasis maintenance in the body. Since ROS/RNS are considered to be important signalling molecules, their concentration is strictly regulated by the antioxidant defence network in conjunction with various transcription factors and vitagenes. In fact, activation of vitagenes via such transcription factors as Nrf2 leads to an additional synthesis of an array of protective molecules which can deal with increased ROS/RNS production. Therefore, it is a challenging task to develop a system of optimal antioxidant supplementation to help growing/productive birds maintain effective antioxidant defences and redox balance in the body. On the one hand, antioxidants, such as vitamin E, or minerals (e.g., Se, Mn, Cu and Zn) are a compulsory part of the commercial pre-mixes for poultry, and, in most cases, are adequate to meet the physiological requirements in these elements. On the other hand, due to the aforementioned commercially relevant stressors, there is a need for additional support for the antioxidant system in poultry. This new direction in improving antioxidant defences for poultry in stress conditions is related to an opportunity to activate a range of vitagenes (via Nrf2-related mechanisms: superoxide dismutase, SOD; heme oxygenase-1, HO-1; GSH and thioredoxin, or other mechanisms: Heat shock protein (HSP)/heat shock factor (HSP), sirtuins, etc.) to maximise internal AO protection and redox balance maintenance. Therefore, the development of vitagene-regulating nutritional supplements is on the agenda of many commercial companies worldwide.

Thioredoxin-Interacting Protein (TXNIP) in Cerebrovascular and Neurodegenerative Diseases: Regulation and Implication

Neurological diseases, including acute attacks (e.g., ischemic stroke) and chronic neurodegenerative diseases (e.g., Alzheimer's disease), have always been one of the leading cause of morbidity and mortality worldwide. These debilitating diseases represent an enormous disease burden, not only in terms of health suffering but also in economic costs. Although the clinical presentations differ for these diseases, a growing body of evidence suggests that oxidative stress and inflammatory responses in brain tissue significantly contribute to their pathology. However, therapies attempting to prevent oxidative damage or inhibiting inflammation have shown little success. Identification and targeting endogenous "upstream" mediators that normalize such processes will lead to improve therapeutic strategy of these diseases. Thioredoxin-interacting protein (TXNIP) is an endogenous inhibitor of the thioredoxin (TRX) system, a major cellular thiol-reducing and antioxidant system. TXNIP regulating redox/glucose-induced stress and inflammation, now is known to get upregulated in stroke and other brain diseases, and represents a promising therapeutic target. In particular, there is growing evidence that glucose strongly induces TXNIP in multiple cell types, suggesting possible physiological roles of TXNIP in glucose metabolism. Recently, a significant body of literature has supported an essential role of TXNIP in the activation of the NOD-like receptor protein (NLRP3)-inflammasome, a well-established multi-molecular protein complex and a pivotal mediator of sterile inflammation. Accordingly, TXNIP has been postulated to reside centrally in detecting cellular damage and mediating inflammatory responses to tissue injury. The majority of recent studies have shown that pharmacological inhibition or genetic deletion of TXNIP is neuroprotective and able to reduce detrimental aspects of pathology following cerebrovascular and neurodegenerative diseases. Conspicuously, the mainstream of the emerging evidences is highlighting TXNIP link to damaging signals in endothelial cells. Thereby, here, we keep the trend to present the accumulative data on CNS diseases dealing with vascular integrity. This review aims to summarize evidence supporting the significant contribution of regulatory mechanisms of TXNIP with the development of brain diseases, explore pharmacological strategies of targeting TXNIP, and outline obstacles to be considered for efficient clinical translation.

Detoxification of Mitochondrial Oxidants and Apoptotic Signaling Are Facilitated by Thioredoxin-2 and Peroxiredoxin-3 during Hyperoxic Injury

Mitochondria play a fundamental role in the regulation of cell death during accumulation of oxidants. High concentrations of atmospheric oxygen (hyperoxia), used clinically to treat tissue hypoxia in premature newborns, is known to elicit oxidative stress and mitochondrial injury to pulmonary epithelial cells. A consequence of oxidative stress in mitochondria is the accumulation of peroxides which are detoxified by the dedicated mitochondrial thioredoxin system. This system is comprised of the oxidoreductase activities of peroxiredoxin-3 (Prx3), thioredoxin-2 (Trx2), and thioredoxin reductase-2 (TrxR2). The goal of this study was to understand the role of the mitochondrial thioredoxin system and mitochondrial injuries during hyperoxic exposure. Flow analysis of the redox-sensitive, mitochondrial-specific fluorophore, MitoSOX, indicated increased levels of mitochondrial oxidant formation in human adenocarcinoma cells cultured in 95% oxygen. Increased expression of Trx2 and TrxR2 in response to hyperoxia were not attributable to changes in mitochondrial mass, suggesting that hyperoxic upregulation of mitochondrial thioredoxins prevents accumulation of oxidized Prx3. Mitochondrial oxidoreductase activities were modulated through pharmacological inhibition of TrxR2 with auranofin and genetically through shRNA knockdown of Trx2 and Prx3. Diminished Trx2 and Prx3 expression was associated with accumulation of mitochondrial superoxide; however, only shRNA knockdown of Trx2 increased susceptibility to hyperoxic cell death and increased phosphorylation of apoptosis signal-regulating kinase-1 (ASK1). In conclusion, the mitochondrial thioredoxin system regulates hyperoxic-mediated death of pulmonary epithelial cells through detoxification of oxidants and regulation of redox-dependent apoptotic signaling.

Mitochondrial Redox Signaling and Tumor Progression

Cancer cell can reprogram their energy production by switching mitochondrial oxidative phosphorylation to glycolysis. However, mitochondria play multiple roles in cancer cells, including redox regulation, reactive oxygen species (ROS) generation, and apoptotic signaling. Moreover, these mitochondrial roles are integrated via multiple interconnected metabolic and redox sensitive pathways. Interestingly, mitochondrial redox proteins biphasically regulate tumor progression depending on cellular ROS levels. Low level of ROS functions as signaling messengers promoting cancer cell proliferation and cancer invasion. However, anti-cancer drug-initiated stress signaling could induce excessive ROS, which is detrimental to cancer cells. Mitochondrial redox proteins could scavenger basal ROS and function as “tumor suppressors” or prevent excessive ROS to act as “tumor promoter”. Paradoxically, excessive ROS often also induce DNA mutations and/or promotes tumor metastasis at various stages of cancer progression. Targeting redox-sensitive pathways and transcriptional factors in the appropriate context offers great promise for cancer prevention and therapy. However, the therapeutics should be cancer-type and stage-dependent.

Thioredoxin-2 Modulates Neuronal Programmed Cell Death in the Embryonic Chick Spinal Cord in Basal and Target-Deprived Conditions

Thioredoxin-2 (Trx2) is a mitochondrial protein using a dithiol active site to reduce protein disulfides. In addition to the cytoprotective function of this enzyme, several studies have highlighted the implication of Trx2 in cellular signaling events. In particular, growing evidence points to such roles of redox enzymes in developmental processes taking place in the central nervous system. Here, we investigate the potential implication of Trx2 in embryonic development of chick spinal cord. To this end, we first studied the distribution of the enzyme in this tissue and report strong expression of Trx2 in chick embryo post-mitotic neurons at E4.5 and in motor neurons at E6.5. Using in ovo electroporation, we go on to highlight a cytoprotective effect of Trx2 on the programmed cell death (PCD) of neurons during spinal cord development and in a novel cultured spinal cord explant model. These findings suggest an implication of Trx2 in the modulation of developmental PCD of neurons during embryonic development of the spinal cord, possibly through redox regulation mechanisms.

Mitochondrial thioredoxin reductase inhibition, selenium status, and Nrf-2 activation are determinant factors modulating the toxicity of mercury compounds

The thioredoxin system has essential functions in the maintenance of cellular redox homeostasis in the cytosol, nucleus, and mitochondria. Thioredoxin (Trx) and thioredoxin reductase (TrxR) are targets for mercury compounds in vitro and in vivo. This study aimed at understanding mechanistically how the mitochondrial and cytosolic thioredoxin systems were affected by mercurials, including the regulation of TrxR transcription. The effects of coexposure to selenite and mercurials on the thioredoxin system were also addressed. Results in HepG2 cells showed that TrxR1 expression was enhanced by Hg(2+), whereas exposure to MeHg decreased expression. Selenite exposure also increased the expression of TrxR1 and resulted in higher specific activity. Coexposure to 2 µM selenite and up to 5 µM Hg(2+) increased even further TrxR1 expression. This synergistic effect was not verified for MeHg, because TrxR1 expression and activity were reduced. Analysis of Nrf-2 translocation to the nucleus and TrxR mRNA suggests that induction of TrxR1 transcription was slower upon exposure to MeHg in comparison to Hg(2+). Subcellular fractions showed that MeHg affected the activity of the thioredoxin system equally in the mitochondria and cytosol, whereas Hg(2+) inhibited primarily the activity of TrxR2. The expression of TrxR2 was not upregulated by any treatment. These results show important differences between the mechanisms of toxicity of Hg(2+) and MeHg and stress the narrow range of selenite concentrations capable of antagonizing mercury toxicity. The results also highlight the relevance of the mitochondrial thioredoxin system (TrxR2 and Trx2) in the development of mercury toxicity.

Thioredoxin/Txnip: redoxisome, as a redox switch for the pathogenesis of diseases

During the past few decades, it has been widely recognized that Reduction-Oxidation (redox) responses occurring at the intra- and extra-cellular levels are one of most important biological phenomena and dysregulated redox responses are involved in the initiation and progression of multiple diseases. Thioredoxin1 (Trx1) and Thioredoxin2 (Trx2), mainly located in the cytoplasm and mitochondria, respectively, are ubiquitously expressed in variety of cells and control cellular reactive oxygen species by reducing the disulfides into thiol groups. Thioredoxin interacting protein (Txnip/thioredoxin binding protein-2/vitamin D3 upregulated protein) directly binds to Trx1 and Trx2 (Trx) and inhibit the reducing activity of Trx through their disulfide exchange. Recent studies have revealed that Trx1 and Txnip are involved in some critical redox-dependent signal pathways including NLRP-3 inflammasome activation in a redox-dependent manner. Therefore, Trx/Txnip, a redox-sensitive signaling complex is a regulator of cellular redox status and has emerged as a key component in the link between redox regulation and the pathogenesis of diseases. Here, we review the novel functional concept of the redox-related protein complex, named “Redoxisome,” consisting of Trx/Txnip, as a critical regulator for intra- and extra-cellular redox signaling, involved in the pathogenesis of various diseases such as cancer, autoimmune disease, and diabetes.

Thioredoxin-interacting protein and myocardial mitochondrial function in ischemia-reperfusion injury

Cellular metabolism and reactive oxygen species (ROS) formation are interrelated processes in mitochondria and are implicated in a variety of human diseases including ischemic heart disease. During ischemia, mitochondrial respiration rates fall. Though seemingly paradoxical, reduced respiration has been observed to be cardioprotective due in part to reduced generation of ROS. Enhanced myocardial glucose uptake is considered beneficial for the myocardium under stress, as glucose is the primary substrate to support anaerobic metabolism. Thus, inhibition of mitochondrial respiration and uncoupling oxidative phosphorylation can protect the myocardium from irreversible ischemic damage. Growing evidence now positions the TXNIP/thioredoxin system at a nodal point linking pathways of antioxidant defense, cell survival, and energy metabolism. This emerging picture reveals TXNIP's function as a regulator of glucose homeostasis and may prove central to regulation of mitochondrial function during ischemia. In this review, we summarize how TXNIP and its binding partner thioredoxin act as regulators of mitochondrial metabolism. While the precise mechanism remains incompletely defined, the TXNIP-thioredoxin interaction has the potential to affect signaling that regulates mitochondrial bioenergetics and respiratory function with potential cardioprotection against ischemic injury.

Molecular mechanisms of retinal ganglion cell degeneration in glaucoma and future prospects for cell body and axonal protection

Glaucoma, which affects more than 70 million people worldwide, is a heterogeneous group of disorders with a resultant common denominator; optic neuropathy, eventually leading to irreversible blindness. The clinical manifestations of primary open-angle glaucoma (POAG), the most common subtype of glaucoma, include excavation of the optic disc and progressive loss of visual field. Axonal degeneration of retinal ganglion cells (RGCs) and apoptotic death of their cell bodies are observed in glaucoma, in which the reduction of intraocular pressure (IOP) is known to slow progression of the disease. A pattern of localized retinal nerve fiber layer (RNFL) defects in glaucoma patients indicates that axonal degeneration may precede RGC body death in this condition. The mechanisms of degeneration of neuronal cell bodies and their axons may differ. In this review, we addressed the molecular mechanisms of cell body death and axonal degeneration in glaucoma and proposed axonal protection in addition to cell body protection. The concept of axonal protection may become a new therapeutic strategy to prevent further axonal degeneration or revive dying axons in patients with preperimetric glaucoma. Further study will be needed to clarify whether the combination therapy of axonal protection and cell body protection will have greater protective effects in early or progressive glaucomatous optic neuropathy (GON).

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.

Mitochondrial thiols in the regulation of cell death pathways

SIGNIFICANCE

Regulation of mitochondrial H(2)O(2) homeostasis and its involvement in the regulation of redox-sensitive signaling and transcriptional pathways is the consequence of the concerted activities of the mitochondrial energy- and redox systems.

RECENT ADVANCES

The energy component of this mitochondrial energy-redox axis entails the formation of reducing equivalents and their flow through the respiratory chain with the consequent electron leak to generate [Formula: see text] and H(2)O(2). The mitochondrial redox component entails the thiol-based antioxidant system, largely accounted for by glutathione- and thioredoxin-based systems that support the activities of glutathione peroxidases, peroxiredoxins, and methionine sulfoxide reductase. The ultimate reductant for these systems is NADPH: mitochondrial sources of NADPH are the nicotinamide nucleotide transhydrogenase, isocitrate dehydrogenase-2, and malic enzyme. NADPH also supports the glutaredoxin activity that regulates the extent of S-glutathionylation of mitochondrial proteins in response to altered redox status.

CRITICAL ISSUES

The integrated network of these mitochondrial thiols constitute a regulatory device involved in the maintenance of steady-state levels of H(2)O(2), mitochondrial and cellular redox and metabolic homeostasis, as well as the modulation of cytosolic redox-sensitive signaling; disturbances of this regulatory device affects transcription, growth, and ultimately influences cell survival/death.

FUTURE DIRECTIONS

The modulation of key mitochondrial thiol proteins, which participate in redox signaling, maintenance of the bioenergetic machinery, oxidative stress responses, and cell death programming, provides a pivotal direction in developing new therapies towards the prevention and treatment of several diseases.

The effects of chromium(VI) on the thioredoxin system: implications for redox regulation

Hexavalent chromium [Cr(VI)] compounds are highly redox active and have long been recognized as potent cytotoxins and carcinogens. The intracellular reduction of Cr(VI) generates reactive Cr intermediates, which are themselves strong oxidants, as well as superoxide, hydrogen peroxide, and hydroxyl radical. These probably contribute to the oxidative damage and effects on redox-sensitive transcription factors that have been reported. However, the identification of events that initiate these signaling changes has been elusive. More recent studies show that Cr(VI) causes irreversible inhibition of thioredoxin reductase (TrxR) and oxidation of thioredoxin (Trx) and peroxiredoxin (Prx). Mitochondrial Trx2/Prx3 are more sensitive to Cr(VI) treatment than cytosolic Trx1/Prx1, although both compartments show thiol oxidation with higher doses or longer treatments. Thiol redox proteomics demonstrate that Trx2, Prx3, and Trx1 are among the most sensitive proteins in cells to Cr(VI) treatment. Their oxidation could therefore represent initiating events that have widespread implications for protein thiol redox control and for multiple aspects of redox signaling. This review summarizes the effects of Cr(VI) on the TrxR/Trx system and how these events could influence a number of downstream redox signaling systems that are influenced by Cr(VI) exposure. Some of the signaling events discussed include the activation of apoptosis signal regulating kinase and MAP kinases (p38 and JNK) and the modulation of a number of redox-sensitive transcription factors including AP-1, NF-κB, p53, and Nrf2.

HIV-1 Vpr triggers mitochondrial destruction by impairing Mfn2-mediated ER-mitochondria interaction

Human immunodeficiency virus 1 (HIV-1) viral protein R (Vpr) has been shown to induce host cell death by increasing the permeability of mitochondrial outer membrane (MOM). The mechanism underlying the damage to the mitochondria by Vpr, however, is not clearly illustrated. In this study, Vpr that is introduced, via transient transfection or lentivirus infection, into the human embryonic kidney cell line HEK293, human CD4⁺ T lymphoblast cell line SupT1, or human primary CD4⁺ T cells serves as the model system to study the molecular mechanism of Vpr-mediated HIV-1 pathogenesis. The results show that Vpr injures MOM and causes a loss in membrane potential (MMP) by posttranscriptionally reducing the expression of mitofusin 2 (Mfn2) via VprBP-DDB1-CUL4A ubiquitin ligase complex, gradually weakening MOM, and increasing mitochondrial deformation. Vpr also markedly decreases cytoplasmic levels of dynamin-related protein 1 (DRP1) and increases bulging in mitochondria-associated membranes (MAM), the specific regions of endoplasmic reticulum (ER) which form physical contacts with the mitochondria. Overexpression of Mfn2 and DRP1 significantly decreased the loss of MMP and apoptotic cell death caused by Vpr. Furthermore, by employing time-lapse confocal fluorescence microscopy, we identify the transport of Vpr protein from the ER, via MAM to the mitochondria. Taken together, our results suggest that Vpr-mediated cellular damage may occur on an alternative protein transport pathway from the ER, via MAM to the mitochondria, which are modulated by Mfn2 and DRP1.

Serum manganese superoxide dismutase and thioredoxin are potential prognostic markers for hepatitis C virus-related hepatocellular carcinoma

AIM: To evaluate the clinical significance of oxidative stress markers in patients with hepatitis C virus (HCV)-related hepatocellular carcinoma (HCC).

METHODS: Sixty-four consecutive patients who were admitted to Kagoshima University Medical and Dental Hospital were enrolled in this retrospective study. All patients had chronic liver disease (CLD) due to infection with HCV. Thirty patients with HCV-related HCC, 34 with HCV-related CLD without HCC (non-HCC), and 20 healthy volunteers (HVs) were enrolled. Possible associations between serum manganese superoxide dismutase (MnSOD) and thioredoxin (TRX) levels and clinical parameters or patient prognosis were analyzed over a mean follow-up period of 31.7 mo.

RESULTS: The serum MnSOD levels were significantly higher in patients with HCV-related HCC than in patients without HCC (P = 0.03) or HVs (P < 0.001). Similarly, serum TRX levels were also significantly higher in patients with HCV-related HCC than in patients without HCC (P = 0.04) or HVs (P < 0.01). However, serum levels of MnSOD and TRX were not correlated in patients with HCC. Among patients with HCC, the overall survival rate (OSR) was lower in patients with MnSOD levels ≥ 110 ng/mL than in patients with levels < 110 ng/mL (P = 0.01), and the OSR tended to be lower in patients with TRX levels < 80 ng/mL (P = 0.05). In addition, patient prognosis with HCC was poorest with serum MnSOD levels ≥ 110 ng/mL and serum TRX levels < 80 ng/mL. Furthermore, a multivariate analysis using a Cox proportional hazard model and serum levels of five factors (MnSOD, prothrombin time, serum albumin, serum α-fetoprotein (AFP), and serum des-γ-carboxy prothrombin) revealed that MnSOD levels ≥ 110 ng/mL (risk ratio: 4.12, 95% confidential interval: 1.22-13.88, P = 0.02) and AFP levels ≥ 40 ng/mL (risk ratio: 6.75; 95% confidential interval: 1.70-26.85, P < 0.01) were independent risk factors associated with a poor patient prognosis.

CONCLUSION: Serum MnSOD and TRX levels are potential clinical biomarkers that predict patient prognosis in HCV-related HCC.

Identification of a redox-modulatory interaction between uncoupling protein 3 and thioredoxin 2 in the mitochondrial intermembrane space

Uncoupling protein 3 (UCP3) is a member of the mitochondrial solute carrier superfamily that is enriched in skeletal muscle and controls mitochondrial reactive oxygen species (ROS) production, but the mechanisms underlying this function are unclear.

AIMS

The goal of this work focused on the identification of mechanisms underlying UCP3 functions.

RESULTS

Here we report that the N-terminal, intermembrane space (IMS)-localized hydrophilic domain of mouse UCP3 interacts with the N-terminal mitochondrial targeting signal of thioredoxin 2 (Trx2), a mitochondrial thiol reductase. Cellular immunoprecipitation and in vitro pull-down assays show that the UCP3-Trx2 complex forms directly, and that the Trx2 N-terminus is both necessary and sufficient to confer UCP3 binding. Mutation studies show that neither a catalytically inactivated Trx2 mutant, nor a mutant Trx2 bearing the N-terminal targeting sequence of cytochrome c oxidase (COXMTS-Trx2) bind UCP3. Biochemical analyses using permeabilized mitochondria, and live cell experiments using bimolecular fluorescence complementation show that the UCP3-Trx2 complex forms specifically in the IMS. Finally, studies in C2C12 myocytes stably overexpressing UCP3 (2.5-fold) and subjected to Trx2 knockdown show that Trx2 is required for the UCP3-dependent mitigation of complex III-driven mitochondrial ROS generation. UCP3 expression was increased in mice fed a high fat diet, leading to increased localization of Trx2 to the IMS. UCP3 overexpression also increased expression of the glucose transporter GLUT4 in a Trx2-dependent fashion.

INNOVATION

This is the first report of a mitochondrial protein-protein interaction with UCP3 and the first demonstration that UCP3 binds directly, and in cells and tissues with mitochondrial thioredoxin 2.

CONCLUSION

These studies identify a novel UCP3-Trx2 complex, a novel submitochondrial localization of Trx2, and a mechanism underlying UCP3-regulated mitochondrial ROS production.

Thioredoxin 1-mediated post-translational modifications: reduction, transnitrosylation, denitrosylation, and related proteomics methodologies

Despite the significance of redox post-translational modifications (PTMs) in regulating diverse signal transduction pathways, the enzymatic systems that catalyze reversible and specific oxidative or reductive modifications have yet to be firmly established. Thioredoxin 1 (Trx1) is a conserved antioxidant protein that is well known for its disulfide reductase activity. Interestingly, Trx1 is also able to transnitrosylate or denitrosylate (defined as processes to transfer or remove a nitric oxide entity to/from substrates) specific proteins. An intricate redox regulatory mechanism has recently been uncovered that accounts for the ability of Trx1 to catalyze these different redox PTMs. In this review, we will summarize the available evidence in support of Trx1 as a specific disulfide reductase, and denitrosylation and transnitrosylation agent, as well as the biological significance of the diverse array of Trx1-regulated pathways and processes under different physiological contexts. The dramatic progress in redox proteomics techniques has enabled the identification of an increasing number of proteins, including peroxiredoxin 1, whose disulfide bond formation and nitrosylation status are regulated by Trx1. This review will also summarize the advancements of redox proteomics techniques for the identification of the protein targets of Trx1-mediated PTMs. Collectively, these studies have shed light on the mechanisms that regulate Trx1-mediated reduction, transnitrosylation, and denitrosylation of specific target proteins, solidifying the role of Trx1 as a master regulator of redox signal transduction.

Thioredoxins 1 and 2 protect retinal ganglion cells from pharmacologically induced oxidative stress, optic nerve transection and ocular hypertension

Oxidative damage has been implicated in retinal ganglion cell (RGC) death after optic nerve transection (ONT) and during glaucomatous neuropathy. Here, we analyzed the expression and cell protective role of thioredoxins (TRX), key regulators of the cellular redox state, in RGCs damaged by pharmacologically induced oxidative stress, ONT and elevated intraocular pressure (IOP). The endogenous level of thioredoxin-1 (TRX1) and thioredoxin-2 (TRX2) in RGCs after axotomy and in RGC-5 cells after glutamate/buthionine sulfoximine (BSO) treatment showed upregulation of TRX2, whereas no significant change was observed in TRX1 expression. The increased level TRX-interacting protein (TXNIP) in the retinas was observed 2 and 5 weeks after IOP elevation. TRX1 level was decreased at 2 weeks and more prominently at 5 weeks after IOP increase. No change in TRX2 levels in response to IOP change was observed. Overexpression of TRX1 and TRX2 in RGC-5 treated with glutamate/BSO increased the cell survival by 2- and 3-fold 24 and 48 h after treatment, respectively. Overexpression of these proteins in the retina increased the survival of RGCs by 35 and 135% 7 and 14 days after ONT, respectively. In hypertensive eyes, RGC loss was approximately 27% 5 weeks after IOP elevation compared to control. TRX1 and TRX2 overexpression preserved approximately 45 and 37% of RGCs, respectively, that were destined to die due to IOP increase.

In situ kinetic trapping reveals a fingerprint of reversible protein thiol oxidation in the mitochondrial matrix

Reactive oxygen species (ROS) are released at the mitochondrial inner membrane by the electron transport chain (ETC). Increasing evidence suggests that mitochondrial H2O2 acts as a signaling molecule and participates in the (feedback) regulation of mitochondrial activity and turnover. It seems likely that key mitochondrial components contain redox-sensitive thiols that help to adapt protein function to changes in electron flow. However, the identity of most redox-regulated mitochondrial proteins remains to be defined. Thioredoxin 2 (Trx2) is the major protein-thiol-reducing oxidoreductase in the mitochondrial matrix. We used in situ mechanism-based kinetic trapping to identify disulfide-exchange interactions of Trx2 within functional mitochondria of intact cells. Mass spectrometry successfully identified known and suspected Trx2 target proteins and, in addition, revealed a set of new candidate target proteins. Our results suggest that the mitochondrial protein biosynthesis machinery is a major target of ETC-derived ROS. In particular, we identified mitochondrial methionyl-tRNA synthetase (mtMetRS) as one of the most prominent Trx2 target proteins. We show that an increase in ETC-derived oxidants leads to an increase in mtMetRS oxidation in intact cells. In conclusion, we find that in situ kinetic trapping provides starting points for future functional studies of intramitochondrial redox regulation.

Disruption of the mitochondrial thioredoxin system as a cell death mechanism of cationic triphenylmethanes

Alterations in mitochondrial structure and function are a hallmark of cancer cells compared to normal cells and thus targeting mitochondria has emerged as an novel approach to cancer therapy. The mitochondrial thioredoxin 2 (Trx2) system is critical for cell viability, but its role in cancer biology is not well understood. Recently some cationic triphenylmethanes such as brilliant green (BG) and gentian violet were shown to have antitumor and antiangiogenic activity with unknown mechanisms. Here we demonstrate that BG killed cells at nanomolar concentrations and targeted mitochondrial Trx2, which was oxidized and degraded. HeLa cells were more sensitive to BG than fibroblasts. In HeLa cells, Trx2 down-regulation by siRNA resulted in increased sensitivity to BG, whereas for fibroblasts, the same treatments had no effect. BG was observed to accumulate in mitochondria and cause a rapid and dramatic decrease in mitochondrial Trx2 protein. With a redox Western blot method, we found that treatment with BG caused oxidation of both Trx1 and Trx2, followed by release of cytochrome c and apoptosis-inducing factor from the mitochondria into the cytosol. Moreover, this treatment resulted in an elevation of the mRNA level of Lon protease, a protein quality control enzyme in the mitochondrial matrix, suggesting that the oxidized Trx2 may be degraded by Lon protease.

Thiol redox transitions by thioredoxin and thioredoxin-binding protein-2 in cell signaling

The cellular thiol redox state is a crucial mediator of metabolic, signaling and transcriptional processes in cells, and an exquisite balance between the oxidizing and reducing states is essential for the normal function and survival of cells. Reactive oxygen species (ROS) are widely known to function as a kind of second messenger for intracellular signaling and to modulate the thiol redox state. Thiol reduction is mainly controlled by the thioredoxin (TRX) system and glutathione (GSH) systems as scavengers of ROS and regulators of the protein redox states. The thioredoxin system is composed of several related molecules interacting through the cysteine residues at the active site, including thioredoxin, thioredoxin-2, a mitochondrial thioredoxin family, and transmembrane thioredoxin-related protein (TMX), an endoplasmic reticulum (ER)-specific thioredoxin family. Thioredoxin couples with thioredoxin-dependent peroxidases (peroxiredoxin) to scavenge hydrogen peroxide. In addition, thioredoxin does not simply act only as a scavenger of ROS but also as an important regulator of oxidative stress response through protein-protein interaction. The interaction of thioredoxin and thioredoxin-binding proteins such as thioredoxin-binding protein-2 (TBP-2, also called as Txnip or VDUP1), apoptosis signal kinase (ASK-1), redox factor 1 (Ref-1), Forkhead box class O 4 (FoxO4), and nod-like receptor proteins (NLRPs) suggested unconventional functions of thioredoxin and a novel mechanism of redox regulation. Here, we introduce the central mechanism of thiol redox transition in cell signaling regulated by thioredoxin and related molecules.

Highly pathogenic H5N1 avian influenza virus induces extracellular Ca2+ influx, leading to apoptosis in avian cells

In this study, we show that the highly pathogenic H5N1 avian influenza virus (AIV) (A/crow/Kyoto/53/04 and A/chicken/Egypt/CL6/07) induced apoptosis in duck embryonic fibroblasts (DEF). In contrast, apoptosis was reduced among cells infected with low-pathogenic AIVs (A/duck/HK/342/78 [H5N2], A/duck/HK/820/80 [H5N3], A/wigeon/Osaka/1/01 [H7N7], and A/turkey/Wisconsin/1/66 [H9N2]). Thus, we investigated the molecular mechanisms of apoptosis induced by H5N1-AIV infection. Caspase-dependent and -independent pathways contributed to the cytopathic effects. We further showed that, in the induction of apoptosis, the hemagglutinin of H5N1-AIV played a major role and its cleavage sequence was not critical. We also observed outer membrane permeabilization and loss of the transmembrane potential of the mitochondria of infected DEF, indicating that mitochondrial dysfunction was caused by the H5N1-AIV infection. We then analyzed Ca(2+) dynamics in the infected cells and demonstrated an increase in the concentration of Ca(2+) in the cytosol ([Ca(2+)](i)) and mitochondria ([Ca(2+)](m)) after H5N1-AIV infection. Regardless, gene expression important for regulating Ca(2+) efflux from the endoplasmic reticulum did not significantly change after H5N1-AIV infection. These results suggest that extracellular Ca(2+) may enter H5N1-AIV-infected cells. Indeed, EGTA, which chelates extracellular free Ca(2+), significantly reduced the [Ca(2+)](i), [Ca(2+)](m), and apoptosis induced by H5N1-AIV infection. In conclusion, we identified a novel mechanism for influenza A virus-mediated cell death, which involved the acceleration of extracellular Ca(2+) influx, leading to mitochondrial dysfunction and apoptosis. These findings may be useful for understanding the pathogenesis of H5N1-AIV in avian species as well as the impact of Ca(2+) homeostasis on influenza A virus infection.

A novel approach using DNA-repair-deficient chicken DT40 cell lines for screening and characterizing the genotoxicity of environmental contaminants

BACKGROUND

Many bacterial or mammalian cell-based test systems, such as the Ames test, chromosomal aberration assays, or gene mutation assays, are commonly used in developed countries to detect the genotoxicity of industrial chemicals. However, the specificity is generally limited and the sensitivity is not sufficiently high. In addition, most assays cannot provide information on mechanisms of genotoxicity of a given chemical.

OBJECTIVES

We aimed to establish a sensitive and fast screening method that is also capable of characterizing mechanisms of genotoxicity.

METHODS

We developed a novel bioassay employing gene-disrupted clones of the chicken DT40 B-lymphocyte line, which are designed to be deficient in several specific DNA repair pathways. Genotoxic chemicals can delay cellular proliferation in DNA-repair-deficient clones more significantly than in wild-type cells by interfering with DNA replication, thereby inducing DNA damage. In addition, we verified the validity of this assay by analyzing the genotoxicity of gamma-rays, ultraviolet (UV) light, and sodium metaarsenite (NaAsO(2)). We also characterized DNA lesions induced by NaAsO(2).

RESULTS

Genotoxicity of given stressors was successfully screened based on a comparison of proliferation kinetics between wild-type and DNA-repair-deficient mutants in 48 hr. We also found that NaAsO(2) apparently induces at least two types of damage: chromosomal breaks and UV photoproduct-like DNA lesions.

CONCLUSION

This bioassay is a reliable and sensitive screening tool for environmental mutagens as well as for further characterizing the nature of detected genotoxicity.

Prep1 directly regulates the intrinsic apoptotic pathway by controlling Bcl-XL levels

The Prep1 homeodomain transcription factor is essential in embryonic development. Prep1 hypomorphic mutant mouse (Prep1(i/i)) embryos (embryonic day 9.5) display an increased terminal deoxynucleotidyltransferase-mediated dUTP-biotin nick end labeling reaction compared to wild-type (WT) littermates. Prep1(i/i) mouse embryo fibroblasts (MEFs) show an increased basal level of annexin V binding activity, reduction of the mitochondrial-membrane potential, and increased caspase 9 and 3 activation, indicating increased apoptosis. Prep1(i/i) MEFs also respond faster than WT MEFs to genotoxic stress, indicating increased activation of the intrinsic apoptotic pathways. We did not observe an increase in p53 or an abnormal p53 response to apoptotic stimuli. However, hypomorphic MEFs have decreased endogenous levels of antiapoptotic Bcl-X(L) mRNA and protein, and Bcl-x overexpression rescues the defect of Prep1(i/i) MEFs. Using transient transfections and chromatin immunoprecipitation, we identified the Bcl-x promoter as a novel target of Prep1. Thus, Prep1 directly controls mitochondrial homeostasis (and the apoptotic potential) by modulating Bcl-x gene expression.

Thioredoxin 2 haploinsufficiency in mice results in impaired mitochondrial function and increased oxidative stress

The mitochondrial form of thioredoxin, thioredoxin 2 (Txn2), plays an important role in redox control and protection against ROS-induced mitochondrial damage. To evaluate the effect of reduced levels of Txn2 in vivo, we measured oxidative damage and mitochondrial function using mice heterozygous for the Txn2 gene (Txn2(+/-)). The Txn2(+/-) mice showed approximately 50% decrease in Trx-2 protein expression in all tissues without upregulating the other major components of the antioxidant defense system. Reduced levels of Txn2 resulted in decreased mitochondrial function as shown by reduced ATP production by isolated mitochondria and reduced activity of electron transport chain complexes (ETCs). Mitochondria isolated from Txn2(+/-) mice also showed increased ROS production compared to wild type mice. The Txn2(+/-) mice showed increased oxidative damage to nuclear DNA, lipids, and proteins in liver. In addition, we observed an increase in apoptosis in liver from Txn2(+/-) mice compared with wild type mice after diquat treatment. Our results suggest that Txn2 plays an important role in protecting the mitochondria against oxidative stress and in sensitizing the cells to ROS-induced apoptosis.

Cellular functions and transcriptional regulation of a third thioredoxin from Schizosaccharomyces pombe

The structural gene encoding a third thioredoxin (Trx) homologue, TRX3, of the fission yeast Schizosaccharomyces pombe was characterized and its regulation was studied. The determined DNA sequence encoded a putative 290 amino acid sequence of Trx with a molecular mass of 31 889 Da. The TRX3 mRNA level was increased in S. pombe cells harboring plasmid pTRX3, suggesting that the cloned TRX3 gene was functional. Yeast cultures harbouring plasmid pTRX3 exhibited shorter generation times and higher survival on solid minimal media plates incorporating mercury chloride (0.01 mmol/L) or hydrogen peroxide (1 mmol/L) compared with control cultures. Yeast cells containing extra copies of TRX3, but not TRX1 and TRX2, gave rise to lower reactive oxygen species levels than control cells. Oxidative stress owing to hydrogen peroxide and menadione enhanced the synthesis of β-galactosidase from the TRX3–lacZ fusion gene in Pap1-positive cells but not in Pap1-negative cells. The TRX3 mRNA level was increased by oxidative stress only in Pap1-positive cells. Basal expression of the TRX3 gene also depended on Pap1. We concluded that S. pombe TRX3 is linked with yeast growth and oxidative stress response, with its expression being regulated by oxidative stress in a Pap1-dependent manner.

Redox regulation of human thioredoxin network

Oxidative stresses are largely mediated by intracellular protein oxidations by reactive oxygen species (ROS). Host cells are equipped with antioxidants that scavenge ROS. The cellular reduction/oxidation (redox) balance is maintained by ROS and antioxidants. Accumulating evidence suggests that the redox balance plays an important role in cellular signaling through the redox modification of cysteine residues in various important components of the signal transduction pathway. Thioredoxin (TRX) is a small protein playing important roles in cellular responses, including cell growth, cell cycle, gene expression, and apoptosis, to maintain the redox circumstance. Moreover, many recent papers have shown that the redox regulation by TRX is deeply involved in the pathogenesis of various oxidative stress-associated disorders. This review focuses on TRX and its related molecules, and discusses the role of TRX-dependent redox regulation in oxidative stress-induced signal transduction.