Thioredoxin reductase inactivation as a pivotal mechanism of ifosfamide in cancer therapy

Pan-cancer analysis of cuproptosis-promoting gene signature from multiple perspectives

Cuproptosis is a newly discovered cell death form with a unique mechanism. Seven genes have been identified to facilitate the process. To explore the roles of cuproptosis in different cancers, we first used Gene Expression Profiling, Interactive Analysis, version 2, and cBioPortal to analyze expression, prognosis and mutation conditions in different cancers from The Cancer Genome Atlas (TCGA). Then, we conducted single sample gene set enrichment analysis to combine the signature of the cuproptosis-promoting genes for all TCGA cancers. Moreover, we performed a survival analysis to explore if cuproptosis-score could independently influence clinical outcomes. Next, we compared pathway enrichment, immune infiltration, gene set activity and gene mutation between different cuproptosis-score groups. Finally, based on the intersected genes from difference analysis and weighted gene co-expression network analysis, consensus clustering and Least Absolute Shrinkage and Selection Operator Cox regression were performed and nomograms were constructed. Cuproptosis-score was associated with a favorable prognosis in eight TCGA cancers. Cancer-associated fibroblasts, B cells, neutrophils and mast cells were generally less abundant, and ferroptosis activity was higher in high cuproptosis-score groups. The novel classifications could differentiate patients' overall survival, and the risk models could effectively predict patients' outcomes in kidney, renal clear cell carcinoma, liver hepatocellular carcinoma, mesothelioma and stomach adenocarcinoma. Cuproptosis activity was closely related to the prognosis of several cancers. Its effects on the immune microenvironment and its relationship with other cell death modes, especially ferroptosis, may become the focus of further research.

Personalized medicine of non-gene-specific chemotherapies for non-small cell lung cancer

Non-small cell lung cancer is recognized as the deadliest cancer across the globe. In some areas, it is more common in women than even breast and cervical cancer. Its rise, vaulted by smoking habits and increasing air pollution, has garnered much attention and resource in the medical field. The first lung cancer treatments were developed more than half a century ago. Unfortunately, many of the earlier chemotherapies often did more harm than good, especially when they were used to treat genetically unsuitable patients. With the introduction of personalized medicine, physicians are increasingly aware of when, how, and in whom, to use certain anti-cancer agents. Drugs such as tyrosine kinase inhibitors, anaplastic lymphoma kinase inhibitors, and monoclonal antibodies possess limited utility because they target specific oncogenic mutations, but other drugs that target mechanisms universal to all cancers do not. In this review, we discuss many of these non-oncogene-targeting anti-cancer agents including DNA replication inhibitors (i.e., alkylating agents and topoisomerase inhibitors) and cytoskeletal function inhibitors to highlight their application in the setting of personalized medicine as well as their limitations and resistance factors.

Role of Selenoproteins in Redox Regulation of Signaling and the Antioxidant System: A Review

Selenium is a vital trace element present as selenocysteine (Sec) in proteins that are, thus, known as selenoproteins. Humans have 25 selenoproteins, most of which are functionally characterized as oxidoreductases, where the Sec residue plays a catalytic role in redox regulation and antioxidant activity. Glutathione peroxidase plays a pivotal role in scavenging and inactivating hydrogen and lipid peroxides, whereas thioredoxin reductase reduces oxidized thioredoxins as well as non-disulfide substrates, such as lipid hydroperoxides and hydrogen peroxide. Selenoprotein R protects the cell against oxidative damage by reducing methionine-R-sulfoxide back to methionine. Selenoprotein O regulates redox homeostasis with catalytic activity of protein AMPylation. Moreover, endoplasmic reticulum (ER) membrane selenoproteins (SelI, K, N, S, and Sel15) are involved in ER membrane stress regulation. Selenoproteins containing the CXXU motif (SelH, M, T, V, and W) are putative oxidoreductases that participate in various cellular processes depending on redox regulation. Herein, we review the recent studies on the role of selenoproteins in redox regulation and their physiological functions in humans, as well as their role in various diseases.

Effects of Mammalian Thioredoxin Reductase Inhibitors

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

Selenium nanoparticles act as an intestinal p53 inhibitor mitigating chemotherapy-induced diarrhea in mice

Selenium, at high-dose levels approaching its toxicity, protects tissues from dose-limiting toxicities of many cancer chemotherapeutics without compromising their therapeutic effects on tumors, there by allowing the delivery of higher chemotherapeutic doses to achieve increased cure rate. In this regard, selenium nanoparticles (SeNPs), which show the lowest toxicity among extensively investigated selenium compounds including methylselenocysteine and selenomethionine, are more promising for application. The key issue remains to be resolved is whether low-toxicity SeNPs possess a selective protective mechanism. p53 or p53-regulated thrombospondin-1 has each been confirmed to be an appropriate target for therapeutic suppression to reduce side effects of anticancer therapy. The present study demonstrated that SeNPs transiently suppressed the expression of many intestinal p53-associated genes in healthy mice. SeNPs did not interfere with tumor-suppressive effect of nedaplatin, a cisplatin analogue; however, effectively reduced nedaplatin-evoked diarrhea. Nedaplatin-induced diarrhea was associated with activation of intestinal p53 and high expression of intestinal thrombospondin-1. The preventive effect of SeNPs on nedaplatin-induced diarrhea was correlated with a powerful concomitant suppression of p53 and thrombospondin-1. Moreover, the high-dose SeNPs used in the present study did not suppress growth nor caused liver and kidney injuries as well as alterations of hematological parameters in healthy mice. Overall, the present study reveals that chemotherapeutic selectivity conferred by SeNPs involves a dual suppression of two well-documented targets, the p53 and thrombospondin-1, providing mechanistic and pharmacologic insights on low-toxicity SeNPs as a potential chemoprotectant for mitigating chemotherapy-induced diarrhea.

Colletotrichum gloeosporioides- Contaminated Tea Infusion Blocks Lipids Reduction and Induces Kidney Damage in Mice

When the homogenate of fresh tea tree leaves was fermented to produce black tea beverage, the Colletotrichum gloeosporioides (main pathogen or endophyte of Camellia sinensis) may be mixed into the fermentation liquor. However, it was unclear whether C. gloeosporioides-contaminated tea beverage would damage human health. Therefore, we investigated the changes of functional components and the influences on mice. C. gloeosporioides was added to the green tea infusion. After cultivation of 48 h, tea polyphenols, caffeine, and L-theanine decreased by 31.0, 26.2, and 8.3%, respectively. The contaminated tea infusion showed brown stain, and produced a group of toxic materials named phthalic acid esters. The animal study showed that green tea without contamination significantly decreased levels of alanine aminotransferase, triglycerides, free fatty acids, low-density lipoprotein, and increased insulin level compared with obese mice. On the contrary, contaminated tea lost the effects on these indicators. Furthermore, the urea nitrogen and serum creatinine levels significantly increased in the contaminated tea-drinking mice. Altogether, our results indicate that C. gloeosporioides contamination can reduce the amount of functional components of green tea. Therefore, it inhibits some health-care function of lipid-lowering. In addition, the toxic components in contaminated tea infusion might induce renal damage.

Targeting the Selenoprotein Thioredoxin Reductase 1 for Anticancer Therapy

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

Study of small-cell lung cancer cell-based sensor and its applications in chemotherapy effects rapid evaluation for anticancer drugs

Small cell lung cancer (SCLC) is a smoking-related cancer disease. Despite improvement in clinical survival, SCLC outcome remains extremely poor. Cisplatin (DDP) is the first-line chemotherapy drug for SCLC, but the choice of second-line chemotherapy drugs is not clear. In this paper, a SCLC cell-based sensor was proposed, and its applications in chemotherapy effects rapid evaluation for anticancer drugs were investigated. SCLC cell lines lung adenocarcinoma cell (LTEP-P) and DDP-resistant lung adenocarcinoma cell (LTEP-P/DDP-1.0) are cultured on carbon screen-printed electrode (CSPE) to fabricate integrated cell-based sensor. Several chemotherapy anticancer drugs, including cisplatin, ifosmamide, gemcitabine, paclitaxel, docetaxel, vinorelbine, etoposide, camptothecin, and topotecan, are selected as experimental chemicals. 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) tests are conducted to evaluate chemotherapy drug effects on LTEP-P and LTEP-P/DDP-1.0 cell lines. Electrical cell-substrate impedance sensing (ECIS) responses to anti-tumor chemicals are measured and processed by double-layered cascaded stochastic resonance (DCSR). Cisplatin solutions in different concentrations measurement results demonstrate that LTEP-P cell-based sensor presents quantitative analysis abilities for cisplatin and topotecan. Cisplatin and its mixtures can also be discriminated. Results demonstrate that LTEP-P cell-based sensor sensitively evaluates chemotherapy drugs' apoptosis function to SCLC cells. LTEP-P/DDP-1.0 cell-based sensor responses demonstrate that gemcitabine, vinorelbine, and camptothecin are ideal second-line drugs for clinical post-cisplatin therapy than other drugs according to MTT test results. This work provides a novel way for SCLC second-line clinical chemotherapy drug screening.

Reversal of cisplatin resistance in human gastric cancer cells by a wogonin-conjugated Pt(IV) prodrug via attenuating Casein Kinase 2-mediated Nuclear Factor-κB pathways

Pt(IV) prodrugs, with two additional coordination sites in contrast to Pt(II) drugs, have been actively studied nowadays, for they can perform well in enhancing the accumulation and retention of the corresponding Pt(II) drugs in cancer cells. Our designed Pt(II) drug, DN604, was recently found to exhibit significant anticancer activity and low toxicity, while, wogonin, a naturally O-methylated flavones, has been widely investigated for its tumor therapeutic potential. Thus, two Pt(IV)-based prodrugs were derived by addition of a wogonin unit to the axial position of DN604 and its analogue DN603 via a linker group. In vitro cytotoxicity assay indicated that the resulting compound 8 not only inherited the genotoxicity of DN604 on gastric cancer cells, but also obtained the COX inhibitory property arising from wogonin. Further studies revealed that compound 8 caused the accumulation of ROS production and decreased the mitochondrial membrane potential (ΔΨm). The CK2α kinase activity assay, ChIP and luciferase assays showed that CK2 plays an important role in the blockade of compound 8 on activated NF-κB survival pathways, which were established for sensitivity of cancer cells to platinum drugs. Similarly in vivo, in nude mice with SGC-7901/cDDP xenografts, compound 8 improved the effectiveness of DN604 via reversing tumor resistance and maintaining low toxicity. Overall, compound 8 is a promising Pt(IV) prodrug, which could be used to promote the anticancer activity of its counterpart Pt(II) species and reverse drug resistance via attenuating CK2-mediated NF-κB pathways during platinum-based chemotherapies.

Selenoproteins in Tumorigenesis and Cancer Progression

Selenium is a micronutrient essential to human health and has long been associated with cancer prevention. Functionally, these effects are thought to be mediated by a class of selenium-containing proteins known as selenoproteins. Indeed, many selenoproteins have antioxidant activity which can attenuate cancer development by minimizing oxidative insult and resultant DNA damage. However, oxidative stress is increasingly being recognized for its "double-edged sword" effect in tumorigenesis, whereby it can mediate both negative and positive effects on tumor growth depending on the cellular context. In addition to their roles in redox homeostasis, recent work has also implicated selenoproteins in key oncogenic and tumor-suppressive pathways. Together, these data suggest that the overall contribution of selenoproteins to tumorigenesis is complicated and may be affected by a variety of factors. In this review, we discuss what is currently known about selenoproteins in tumorigenesis with a focus on their contextual roles in cancer development, growth, and progression.

Serum thioredoxin reductase is highly increased in mice with hepatocellular carcinoma and its activity is restrained by several mechanisms

Increased thioredoxin reductase (TrxR) levels in serum were recently identified as possible prognostic markers for human prostate cancer or hepatocellular carcinoma. We had earlier shown that serum levels of TrxR protein are very low in healthy mice, but can in close correlation to alanine aminotransferase (ALT) increase more than 200-fold upon chemically induced liver damage. We also found that enzymatic TrxR activity in serum is counteracted by a yet unidentified oxidase activity in serum. In the present study we found that mice carrying H22 hepatocellular carcinoma tumors present highly increased levels of TrxR in serum, similarly to that reported in human patients. In this case ALT levels did not parallel those of TrxR. We also discovered here that the TrxR-antagonistic oxidase activity in serum is due to the presence of quiescin Q6 sulfhydryl oxidase 1 (QSOX1). We furthermore found that the chemotherapeutic agents cisplatin or auranofin, when given systemically to H22 tumor bearing mice, can further inhibit TrxR activities in serum. The TrxR serum activity was also inhibited by endogenous electrophilic inhibitors, found to increase in tumor-bearing mice and to include protoporphyrin IX (PpIX) and 4-hydroxynonenal (HNE). Thus, hepatocellular carcinoma triggers high levels of serum TrxR that are not paralleled by ALT, and TrxR enzyme activity in serum is counteracted by several different mechanisms. The physiological role of TrxR in serum, if any, as well as its potential value as a prognostic marker for tumor progression, needs to be studied further.

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.

HQS-3, a newly synthesized flavonoid, possesses potent anti-tumor effect in vivo and in vitro

HQS-3 is a newly baicalein derivative with a benzene substitution. We investigated the anticancer effect of HQS-3 in vivo and in vitro. HQS-3 significantly decreased tumor growth in mice inoculated with Heps and HepG2 cells; and had little influence on the state and weight of animals. After treatment with 20 mg/kg HQS-3, the inhibitory rate of tumor weight in mice inoculated with Heps and HepG2 cells were 63.62% and 68.03%, respectively. Meanwhile, HQS-3 inhibited the viability of various kinds of tumor cells with IC50 values in the range of 22.98-54.32 μM after 48 h treatment measured by MTT-assay. HQS-3 remarkably inhibited viability of hepatoma cells in a concentration- and time-dependent manner and induced apoptosis in HepG2 cells by DAPI staining and Annexin V/PI double staining. The apoptosis-induction effect of HQS-3 was attributed to its ability to modulate the activity of caspase-9, caspase-3 and PARP. Moreover, the expression of bax protein was increased while the bcl-2 protein was decreased, leading to an increase in Bax/Bcl-2 ratio. The accumulation of ROS induced by HQS-3 in HepG2 cells was also observed. The further results suggested that HQS-3 induced mitochondrial-mediated apoptosis by increasing ROS level and inhibiting the expression of anti-oxidative protein SOD2. HQS-3 exerted anti-tumor activity both in vitro and in vivo via inducing tumor cells apoptosis, and these results suggested that it deserves further investigation as a novel chemotherapy for human tumors.

Growth inhibition of pancreatic cancer cells by histone deacetylase inhibitor belinostat through suppression of multiple pathways including HIF, NFkB, and mTOR signaling in vitro and in vivo

Pancreatic ductal adenocarcinoma is a devastating disease with few therapeutic options. Histone deacetylase inhibitors are a novel therapeutic approach to cancer treatment; and two new pan-histone deacetylase inhibitors (HDACi), belinostat and panobinostat, are undergoing clinical trials for advanced hematologic malignancies, non-small cell lung cancers and advanced ovarian epithelial cancers. We found that belinostat and panobinostat potently inhibited, in a dose-dependent manner, the growth of six (AsPc1, BxPc3, Panc0327, Panc0403, Panc1005, MiaPaCa2) of 14 human pancreatic cancer cell lines. Belinostat increased the percentage of apoptotic pancreatic cancer cells and caused prominent G2 /M growth arrest of most pancreatic cancer cells. Belinostat prominently inhibited PI3K-mTOR-4EBP1 signaling with a 50% suppression of phorphorylated 4EBP1 (AsPc1, BxPc3, Panc0327, Panc1005 cells). Surprisingly, belinostat profoundly blocked hypoxia signaling including the suppression of hypoxia response element reporter activity; as well as an approximately 10-fold decreased transcriptional expression of VEGF, adrenomedullin, and HIF1α at 1% compared to 20% O2 . Treatment with this HDACi decreased levels of thioredoxin mRNA associated with increased levels of its endogenous inhibitor thioredoxin binding protein-2. Also, belinostat alone and synergistically with gemcitabine significantly (P = 0.0044) decreased the size of human pancreatic tumors grown in immunodeficiency mice. Taken together, HDACi decreases growth, increases apoptosis, and is associated with blocking the AKT/mTOR pathway. Surprisingly, it blocked hypoxic growth related signals. Our studies of belinostat suggest it may be an effective drug for the treatment of pancreatic cancers when used in combination with other drugs such as gemcitabine.

A new mechanism of action for the anticancer drug mitomycin C: mechanism-based inhibition of thioredoxin reductase

Mitomycin C (MMC) is a chemotherapeutic drug that requires an enzymatic bioreduction to exert its biological effects. Upon reduction, MMC is converted into a highly reactive bis-electrophilic intermediate that alkylates cellular nucleophiles. Alkylation of DNA is the most favored mechanism of action for MMC, but other modes of action, such as redox cycling and inhibition of rRNA, may also contribute to the biological action of the drug. In this work, we show that thioredoxin reductase (TrxR) is also a cellular target for MMC. We show that MMC inhibits TrxR in vitro, using purified enzyme, and in vivo, using cancer cell cultures. The inactivation presents distinctive parameters of mechanism-based inhibitors: it is time- and concentration-dependent and irreversible. Additionally, spectroscopic experiments (UV, circular dichroism) show that the inactivated enzyme contains a mitosene chromophore. On the basis of kinetic and spectroscopic data, we propose a chemical mechanism for the inactivation of the enzyme that starts with a reduction of the quinone ring of MMC by the selenolthiol active site of TrxR and a subsequent alkylation of the active site by the activated drug. We also report that MMC inactivates TrxR in cancer cell cultures and that this inhibition correlates directly with the cytotoxicity of the drug, indicating that inhibition of TrxR may play a major role in the biological mode of action of the drug.

Selenoproteins in bladder cancer

Selenoproteins with genetically encoded selenium (Se) are very important in response to oxidative stress, redox balance and regulation of various metabolic and developmental processes. Although increased circulating Se has been associated with 33% risk reduction of bladder cancer, there are little data on selenoprotein expression at the protein and genetic level from both human and animal studies. Data from the Mammalian Gene Collection (MGC) Project clearly showed that highest mRNA expression in human urinary epithelium for TRXR1 (thioredoxin reductase 1), GPX1 (glutathione peroxidase 1), SEP15 (15 kDa selenoprotein), SELT (selenoprotein T) and SEPW1 (selenoprotein W1). Although bladder tumor has been characterized by increased Se, GPX and TRXR activity, circulating Se and GPX was interestingly decreased in these cancer patients. As such, selenoprotein expression in urinary epithelium may be involved in bladder cancer (development, progression and recurrence) and may play a significant role in chemotherapeutic intervention. Despite these findings, the role of selenoproteins in bladder cancer has rarely been investigated and the significance of selenoproteins in normal and malignant uroepithelium remains poorly understood.

Thioredoxins and glutaredoxins: unifying elements in redox biology

Since their discovery as a substrate for ribonucleotide reductase (RNR), the role of thioredoxin (Trx) and glutaredoxin (Grx) has been largely extended through their regulatory function. Both proteins act by changing the structure and activity of a broad spectrum of target proteins, typically by modifying redox status. Trx and Grx are members of families with multiple and partially redundant genes. The number of genes clearly increased with the appearance of multicellular organisms, in part because of new types of Trx and Grx with orthologs throughout the animal and plant kingdoms. The function of Trx and Grx also broadened as cells achieved increased complexity, especially in the regulation arena. In view of these progressive changes, the ubiquitous distribution of Trx and the wide occurrence of Grx enable these proteins to serve as indicators of the evolutionary history of redox regulation. In so doing, they add a unifying element that links the diverse forms of life to one another in an uninterrupted continuum. It is anticipated that future research will embellish this continuum and further elucidate the properties of these proteins and their impact on biology. The new information will be important not only to our understanding of the role of Trx and Grx in fundamental cell processes but also to future societal benefits as the proteins find new applications in a range of fields.

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

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

Thioredoxin reductase-1 knock down does not result in thioredoxin-1 oxidation

The active site of thioredoxin-1 (Trx1) is oxidized in cells with increased reactive oxygen species (ROS) and is reduced by thioredoxin reductase-1 (TrxR1). The purpose of the present study was to determine the extent to which the redox state of Trx1 is sensitive to changes in these opposing reactions. Trx1 redox state and ROS generation were measured in cells exposed to the TrxR1 inhibitors aurothioglucose (ATG) and monomethylarsonous acid (MMA(III)) and in cells depleted of TrxR1 activity by siRNA knock down. The results showed that all three treatments inhibited TrxR1 activity to similar extents (90% inhibition), but that only MMA(III) exposure resulted in oxidation of Trx1. Similarly, ROS levels were elevated in response to MMA(III), but not in response to ATG or TrxR1 siRNA. Therefore, TrxR1 inhibition alone was not sufficient to oxidize Trx1, suggesting that Trx1-independent pathways should be considered when evaluating pharmacological and toxicological mechanisms involving TrxR1 inhibition.