Synergistic targeting of TrxR1 and ATM/AKT pathway in human colon cancer cells

Thioredoxin reductase 1 (TrxR1) has emerged as a promising target for cancer therapy. In our previous research, we discovered several new TrxR1 inhibitors and found that they all have excellent anti-tumor activity. At the same time, we found these TrxR1 inhibitors all lead to an increase in AKT phosphorylation in cancer cells, but the detailed role of AKT phosphorylation in TrxR1 inhibitor-mediated cell death remains unclear. In this study, we identified the combination of AKT and TrxR1 inhibitor displayed a strong synergistic effect in colon cancer cells. Furthermore, we demonstrated that the synergistic effect of auranofin (TrxR1 inhibitor) and MK-2206 (AKT inhibitor) was caused by ROS accumulation. Importantly, we found that ATM inhibitor KU-55933 can block the increase of AKT phosphorylation caused by auranofin, and exhibited a synergistic effect with auranofin. Taken together, our study demonstrated that the activation of ATM/AKT pathway is a compensatory mechanism to cope with ROS accumulation induced by TrxR1 inhibitor, and synergistic targeting of TrxR1 and ATM/AKT pathway is a promising strategy for treating colon cancer.

Jolkinolide B targets thioredoxin and glutathione systems to induce ROS-mediated paraptosis and apoptosis in bladder cancer cells

Bladder cancer is a clinically heterogeneous disease with a poor prognosis. In the current study, anti-proliferation assay of a Euphorbiaceae diterpenoid library led to the identification of an anti-bladder cancer agent Jolkinolide B (JB). JB showed significant cytotoxicity against a panel of bladder cancer cell lines and suppressed the growth of cisplatin (CDDP)-resistant bladder cancer xenografts in single or combination treatments. Mechanistic study revealed that, besides inducing mitogen-activated protein kinase (MAPK)-related apoptosis, JB could trigger the paraptosis via activation of reactive oxygen species (ROS)-mediated endoplasmic reticulum (ER) stress and extracellular signal-regulated kinase (ERK) pathway. The excessive production of ROS could be induced by JB via inhibition of thioredoxin reductase 1 (TrxR1) and depletion of glutathione (GSH). Collectively, JB that targets thioredoxin and GSH systems to induce two distinct cell death modes may serve as a promising candidate in future anti-bladder cancer drug development.

Berberine accelerated wound healing by restoring TrxR1/JNK in diabetes

The high disability, mortality and morbidity of diabetic ulcers make it urgent to explore effective strategies for diabetic wound repair. TrxR1 plays a vital role in regulating redox homeostasis in various pathologies. In the present study, the effect of berberine (BBR) on diabetic wounds was investigated in streptozotocin (STZ)-induced diabetic rats and a high glucose (HG)-induced cell model, and the mechanism of BBR on TrxR1 was elucidated. BBR treatment remarkably accelerated wound healing and enhanced extracellular matrix (ECM) synthesis and significantly inhibited HG-induced HaCaT cell damage. Further analysis indicated that BBR activated TrxR1, suppressed its downstream JNK signaling, thereby inhibiting oxidative stress and apoptosis, promoted cell proliferation, down-regulated matrix metalloproteinase (MMP) 9 (MMP9) and up-regulated transforming growth factor-β1 (TGF-β1) and tissue inhibitors of MMP 1 (TIMP1), resulting in accelerated wound healing. Importantly, the enhancement of BBR on wound repair was further abolished by TrxR1 inhibitor. Moreover, in diabetic wounds induced by a combination of STZ injection and high-fat diet, BBR significantly increased wound closure rate and TrxR1 expression, and this was reversed by TrxR1 inhibitor. These data indicated that topical BBR treatment accelerated diabetic wound healing by activating TrxR1. Targeting TrxR1 may be a novel, effective strategy for restoring redox homeostasis and promoting diabetic wound healing.

Design, synthesis, and biological evaluation of a novel indoleamine 2,3-dioxigenase 1 (IDO1) and thioredoxin reductase (TrxR) dual inhibitor

Targeting the Trp-Kyn pathway is an attractive approach for cancer immunotherapy. Thioredoxin reductase (TrxR) enzymes are reactive oxygen species (ROS) modulators that are involved in the tumor cell growth and survival processes. The 4-phenylimidazole scaffold is well-established as useful for indoleamine 2,3-dioxygenase 1 (IDO1) inhibition, while piperlongumine (PL) and its derivatives have been reported to be inhibitors of TrxR. To take advantage of both immunotherapy and TrxR inhibition, we designed a first-generation dual IDO1 and TrxR inhibitor (ZC0101) using the structural combination of 4-phenylimidazole and PL scaffolds. ZC0101 exhibited better dual inhibition against IDO1 and TrxR in vitro and in cell enzyme assays than the uncombined forms of 4-phenylimidazole and PL. It also showed antiproliferative activity in various cancer cell lines, and a selective killing effect between normal and cancer cells. Furthermore, ZC0101 effectively induced apoptosis and ROS accumulation in cancer cells. Knockdown of TrxR1 and IDO1 expression induced cellular enzyme inhibition and ROS accumulation effects during ZC0101 treatment, but only reduced TrxR1 expression was able to improve ZC0101's antiproliferation effect. This proof-of-concept study provides a novel strategy for cancer treatment. ZC0101 represents a promising lead compound for the development of novel antitumor agents that can also be used as a valuable probe to clarify the relationships and mechanisms of cancer immunotherapy and ROS modulators.

Piperlonguminine and Piperine Analogues as TrxR Inhibitors that Promote ROS and Autophagy and Regulate p38 and Akt/mTOR Signaling

The natural products piperlongumine and piperine have been shown to inhibit cancer cell proliferation through elevation of reactive oxidative species (ROS) and eventually cell death, but only have modest cytotoxic potencies. A series of 14 novel phenylallylidenecyclohexenone analogues based on piperlongumine and piperine therefore were designed and synthesized, and their pharmacological properties were evaluated. Most of the compounds produced antiproliferative activities against five human cancer cells with IC50 values lower than those of piperlongumine and piperine. Among these, compound 9m exerted the most potent antiproliferative activity against drug-resistant Bel-7402/5-FU human liver cancer 5-FU resistant cells (IC50 = 0.8 μM), which was approximately 10-fold lower than piperlongumine (IC50 = 8.4 μM). Further, 9m showed considerably lower cytotoxicity against LO2 human normal liver epithelial cells compared to Bel-7402/5-FU. Mechanistically, compound 9m inhibited thioredoxin reductase (TrxR) activity, increased ROS levels, reduced mitochondrial transmembrane potential (MTP), and induced autophagy in Bel-7402/5-FU cells via regulation of autophagy-related proteins LC3, p62, and beclin-1. Finally, 9m activated significantly the p38 signaling pathways and suppressed the Akt/mTOR signaling pathways. In conclusion, 9m could be a promising candidate for the treatment of drug-resistant cancer cells and, as such, warrants further investigation.

Irreversible TrxR1 inhibitors block STAT3 activity and induce cancer cell death

Because of its key role in cancer development and progression, STAT3 has become an attractive target for developing new cancer therapeutics. While several STAT3 inhibitors have progressed to advanced stages of development, their underlying biology and mechanisms of action are often more complex than would be expected from specific binding to STAT3. Here, we have identified and optimized a series of compounds that block STAT3-dependent luciferase expression with nanomolar potency. Unexpectedly, our lead compounds did not bind to cellular STAT3 but to another prominent anticancer drug target, TrxR1. We further identified that TrxR1 inhibition induced Prx2 and STAT3 oxidation, which subsequently blocked STAT3-dependent transcription. Moreover, previously identified inhibitors of STAT3 were also found to inhibit TrxR1, and likewise, established TrxR1 inhibitors block STAT3-dependent transcriptional activity. These results provide new insights into the complexities of STAT3 redox regulation while highlighting a novel mechanism to block aberrant STAT3 signaling in cancer cells.

Curcuminoid WZ26, a TrxR1 inhibitor, effectively inhibits colon cancer cell growth and enhances cisplatin-induced cell death through the induction of ROS

Colon cancer is one of the leading causes of cancer-related deaths. Chemotherapy has improved survival in patients with colon cancer, but has a narrow therapeutic window due to its toxicity. Therefore, novel therapies for colon cancer are urgently needed. We previously developed a curcumin analog WZ26 as an anti-cancer agent in pre-clinical evaluation. In the present study, we further explored the mechanism and target of WZ26 in colon cancer cells. Our results show that WZ26 targets thioredoxin reductase 1 (TrxR1) and increases cellular reactive oxygen species (ROS) levels, which results in the activation of JNK signaling pathway in human colon cancer cells. Furthermore, we found that WZ26 significantly enhances cisplatin-induced cell growth inhibition in colon cancer cells. WZ26 combined with cisplatin markedly increases the accumulation of ROS, and thereby induces DNA damage and activation of JNK signaling pathway. Pretreatment with antioxidant N-acetyl-l-cysteine (NAC) significantly abrogates the combined treatment-induced ROS generation, DNA damage and cell death. In addition, the activation of JNK signaling pathway prompted by WZ26 and cisplatin was also reversed by NAC pretreatment. In vivo, WZ26 combined with cisplatin significantly inhibits tumor growth in a colon cancer xenograft model. Remarkably, WZ26 attenuates the body weight loss evoked by cisplatin treatment. This study discloses a previously unrecognized mechanism underlying the biological activity of WZ26, and reveals that WZ26 and cisplatin combinational treatment might potentially become a more effective regimen in colon cancer therapy.

The antimetastatic effect and underlying mechanisms of thioredoxin reductase inhibitor ethaselen

Treating cancer metastasis is of vital importance to prolong patients' survival. Thioredoxin reductase (TrxR) is overexpressed in many cancer types and has been recognized as an anti-cancer target. The organoselenium compound ethaselen (BBSKE) has been proved to be a TrxR inhibitor and inhibit various types of tumor growth. However, whether BBSKE could inhibit tumor metastasis remains unclear. In this study, we aim to explore the antimetastatic effect of BBSKE and underlying mechanisms. BBSKE was found to dose-dependently suppress migration and invasion of MCF-7 and LoVo cells in vitro. The underlying mechanisms may include inhibition of TrxR activity, elevation of reactive oxygen species (ROS), decrease of EGFR activation and HER2 expression. Besides, the epithelial to mesenchymal transition process and expression of CD44, MMP-9, VEGFR2 and PD-L1 were also abrogated. Decreased migration and invasion, lower expression levels of EGFR, HER2, N-cadherin, CD44, MMP-9, VEGFR2 and PD-L1 were also observed in TrxR1-knockdown MCF-7 and LoVo cells. In the mouse breast cancer 4T1 model, BBSKE not only inhibited progression of primary tumor, but also suppressed formation of metastatic lung nodules and liver micro-metastases, indicating that BBSKE could effectively abolish tumor metastasis. In conclusion, our findings show that BBSKE is able to inhibit migration and invasion of cancer cells in vitro and in vivo, and may be used to prevent and treat metastasis.

Selenium supplementation of lung epithelial cells enhances nuclear factor E2-related factor 2 (Nrf2) activation following thioredoxin reductase inhibition

The trace element selenium (Se) contributes to redox signaling, antioxidant defense, and immune responses in critically ill neonatal and adult patients. Se is required for the synthesis and function of selenoenzymes including thioredoxin (Trx) reductase-1 (TXNRD1) and glutathione peroxidases (GPx). We have previously identified TXNRD1, primarily expressed by airway epithelia, as a promising therapeutic target to prevent lung injury, likely via nuclear factor E2-related factor 2 (Nrf2)-dependent mechanisms. The present studies utilized the TXNRD1 inhibitor auranofin (AFN) to test the hypothesis that Se positively influences Nrf2 activation and selenoenzyme responses in lung epithelial cells. Murine transformed Club cells (mtCCs) were supplemented with 0, 10, 25, or 100 nM Na2SeO3 to create a range of Se conditions and were cultured in the presence or absence of 0.5 μM AFN. TXNRD1 and GPX2 protein expression and enzymatic activity were significantly greater upon Se supplementation (p < 0.05). AFN treatment (0.5 μM AFN for 1 h) significantly inhibited TXNRD1 but not GPx activity (p < 0.001). Recovery of TXNRD1 activity following AFN treatment was significantly enhanced by Se supplementation (p < 0.041). Finally, AFN-induced Nrf2 transcriptional activation was significantly greater in mtCCs supplemented in 25 or 100 nM Na2SeO3 when compared to non-supplemented controls (p < 0.05). Our novel studies indicate that Se levels positively influence Nrf2 activation and selenoenzyme responses following TXNRD1 inhibition. These data suggest that Se status significantly influences physiologic responses to TXNRD1 inhibitors. In conclusion, correction of clinical Se deficiency, if present, will be necessary for optimal therapeutic effectiveness of TXNRD1 inhibitors in the prevention of lung disease.

The thioredoxin reductase inhibitor auranofin induces heme oxygenase-1 in lung epithelial cells via Nrf2-dependent mechanisms

Thioredoxin reductase-1 (TXNRD1) inhibition effectively activates nuclear factor (erythroid-derived 2)-like 2 (Nrf2) responses and attenuates lung injury in acute respiratory distress syndrome (ARDS) and bronchopulmonary dysplasia (BPD) models. Upon TXNRD1 inhibition, heme oxygenase-1 (HO-1) is disproportionally increased compared with Nrf2 target NADPH quinone oxidoreductase-1 (Nqo1). HO-1 has been investigated as a potential therapeutic target in both ARDS and BPD. TXNRD1 is predominantly expressed in airway epithelial cells; however, the mechanism of HO-1 induction by TXNRD1 inhibitors is unknown. We tested the hypothesis that TXNRD1 inhibition induces HO-1 via Nrf2-dependent mechanisms. Wild-type (WT), Nrf2KO1.3, and Nrf2KO2.2 cells were morphologically indistinguishable, indicating that Nrf2 can be deleted from murine-transformed club cells (mtCCs) using CRISPR/Cas9 gene editing. Hemin, a Nrf2-independent HO-1-inducing agent, significantly increased HO-1 expression in WT, Nrf2KO1.3, and Nrf2KO2.2. Auranofin (AFN) (0.5 µM) inhibited TXNRD1 activity by 50% and increased Nqo1 and Hmox1 mRNA levels by 6- and 24-fold, respectively, in WT cells. Despite similar levels of TXNRD1 inhibition, Nqo1 mRNA levels were not different between control and AFN-treated Nrf2KO1.3 and Nrf2KO2.2. AFN slightly increased Hmox1 mRNA levels in Nrf2KO1.3 and Nrf2KO2.2 cells compared with controls. AFN failed to increase HO-1 protein in Nrf2KO1.3 and Nrf2KO2.2 compared with a 36-fold increase in WT mtCCs. Our data indicate that Nrf2 is the primary mechanism by which TXNRD1 inhibitors increase HO-1 in lung epithelia. Future studies will use ARDS and BPD models to define the role of HO-1 in attenuation of lung injury by TXNRD1 inhibitors.

Irreversible inhibition of cytosolic thioredoxin reductase 1 as a mechanistic basis for anticancer therapy

Cancer cells adapt to their inherently increased oxidative stress through activation of the glutathione (GSH) and thioredoxin (TXN) systems. Inhibition of both of these systems effectively kills cancer cells, but such broad inhibition of antioxidant activity also kills normal cells, which is highly unwanted in a clinical setting. We therefore evaluated targeting of the TXN pathway alone and, more specifically, selective inhibition of the cytosolic selenocysteine-containing enzyme TXN reductase 1 (TXNRD1). TXNRD1 inhibitors were discovered in a large screening effort and displayed increased specificity compared to pan-TXNRD inhibitors, such as auranofin, that also inhibit the mitochondrial enzyme TXNRD2 and additional targets. For our lead compounds, TXNRD1 inhibition correlated with cancer cell cytotoxicity, and inhibitor-triggered conversion of TXNRD1 from an antioxidant to a pro-oxidant enzyme correlated with corresponding increases in cellular production of H2O2 In mice, the most specific TXNRD1 inhibitor, here described as TXNRD1 inhibitor 1 (TRi-1), impaired growth and viability of human tumor xenografts and syngeneic mouse tumors while having little mitochondrial toxicity and being better tolerated than auranofin. These results display the therapeutic anticancer potential of irreversibly targeting cytosolic TXNRD1 using small molecules and present potent and selective TXNRD1 inhibitors. Given the pronounced up-regulation of TXNRD1 in several metastatic malignancies, it seems worthwhile to further explore the potential benefit of specific irreversible TXNRD1 inhibitors for anticancer therapy.

Preclinical anti-cancer activity and multiple mechanisms of action of a cationic silver complex bearing N-heterocyclic carbene ligands

Organometallic complexes offer the prospect of targeting multiple pathways that are important in cancer biology. Here, the preclinical activity and mechanism(s) of action of a silver-bis(N-heterocyclic carbine) complex (Ag8) were evaluated. Ag8 induced DNA damage via several mechanisms including topoisomerase I/II and thioredoxin reductase inhibition and induction of reactive oxygen species. DNA damage induction was consistent with cytotoxicity observed against proliferating cells and Ag8 induced cell death by apoptosis. Ag8 also inhibited DNA repair enzyme PARP1, showed preferential activity against cisplatin resistant A2780 cells and potentiated the activity of temozolomide. Ag8 was substantially less active against non-proliferating non-cancer cells and selectively inhibited glycolysis in cancer cells. Ag8 also induced significant anti-tumour effects against cells implanted intraperitoneally in hollow fibres but lacked activity against hollow fibres implanted subcutaneously. Thus, Ag8 targets multiple pathways of importance in cancer biology, is less active against non-cancer cells and shows activity in vivo in a loco-regional setting.

Brevetoxin-2, is a unique inhibitor of the C-terminal redox center of mammalian thioredoxin reductase-1

Karenia brevis, the Florida red tide dinoflagellate produces a suite of neurotoxins known as the brevetoxins. The most abundant of the brevetoxins PbTx-2, was found to inhibit the thioredoxin-thioredoxin reductase system, whereas the PbTx-3 has no effect on this system. On the other hand, PbTx-2 activates the reduction of small disulfides such as 5,5'-dithio-bis-(2-nitrobenzoic acid) by thioredoxin reductase. PbTx-2 has an α, β-unsaturated aldehyde moiety which functions as an efficient electrophile and selenocysteine conjugates are readily formed. PbTx-2 blocks the inhibition of TrxR by the inhibitor curcumin, whereas curcumin blocks PbTx-2 activation of TrxR. It is proposed that the mechanism of inhibition of thioredoxin reduction is via the formation of a Michael adduct between selenocysteine and the α, β-unsaturated aldehyde moiety of PbTx-2. PbTx-2 had no effect on the rates of reactions catalyzed by related enzymes such as glutathione reductase, glutathione peroxidase or glutaredoxin.

Perturbation of redox balance after thioredoxin reductase deficiency interrupts autophagy-lysosomal degradation pathway and enhances cell death in nutritionally stressed SH-SY5Y cells

Oxidative damage and aggregation of cellular proteins is a hallmark of neuronal cell death after neurotrauma and chronic neurodegenerative conditions. Autophagy and ubiquitin protease system are involved in degradation of protein aggregates, and interruption of their function is linked to apoptotic cell death in these diseases. Oxidative modification of cysteine groups in key molecular proteins has been linked to modification of cellular systems and cell death in these conditions. Glutathione and thioredoxin systems provide reducing protons that can effectively reverse protein modifications and promote cell survival. The central role of Thioredoxin in inhibition of apoptosis is well identified. Additionally, its involvement in initiation of autophagy has been suggested recently. We therefore aimed to investigate the involvement of Thioredoxin system in autophagy-apoptosis processes. A model of serum deprivation in SH-SY5Y was used that is associated with autophagy and apoptosis. Using pharmacological and RNA-editing technology we show that Thioredoxin reductase deficiency in this model enhances oxidative stress and interrupts the early protective autophagy and promotes apoptosis. This was associated with decreased protein-degradation in lysosomes due to altered lysosomal acidification and accumulation of autophagosomes as well as impairment in proteasome pathway. We further confirmed that the extent of oxidative stress is a determining factor in autophagy- apoptosis interplay, as upregulation of cellular reducing capacity by N-acetylcysteine prevented impairment in autophagy and proteasome systems thus promoted cell viability. Our study provides evidence that excessive oxidative stress inhibits protein degradation systems and affects the final stages of autophagy by inhibiting autolysosome maturation: a novel mechanistic link between protein aggregation and conversion of autophagy to apoptosis that can be applicable to neurodegenerative diseases.

Inhibitory nitrosylation of mammalian thioredoxin reductase 1: Molecular characterization and evidence for its functional role in cellular nitroso-redox imbalance

Mammalian thioredoxin 1 (Trx1) and the selenoprotein Trx reductase 1 (TrxR1) are key cellular enzymes that function coordinately in thiol-based redox regulation and signaling. Recent studies have revealed that the Trx1/TrxR1 system has an S-nitrosothiol reductase (denitrosylase) activity through which it can regulate nitric oxide-related cellular processes. In this study we revealed that TrxR1 is itself susceptible to nitrosylation, characterized the underlying mechanism, and explored its functional significance. We found that nitrosothiol or nitric oxide donating agents rapidly and effectively inhibited the activity of recombinant or endogenous TrxR1. In particular, the NADPH-reduced TrxR1 was partially and reversibly inhibited upon exposure to low concentrations (<10μM) of S-nitrosocysteine (CysNO) and markedly and continuously inhibited at higher doses. Concurrently, TrxR1 very efficiently reduced low, but not high, levels of CysNO. Biochemical and mass spectrometric analyses indicated that its active site selenocysteine residue renders TrxR1 highly susceptible to nitrosylation-mediated inhibition, and revealed both thiol and selenol modifications at the two redox active centers of the enzyme. Studies in HeLa cancer cells demonstrated that endogenous TrxR1 is sensitive to nitrosylation-dependent inactivation and pointed to an important role for glutathione in reversing or preventing this process. Notably, depletion of cellular glutathione with l-buthionine-sulfoximine synergized with nitrosating agents in promoting sustained nitrosylation and inactivation of TrxR1, events that were accompanied by significant oxidation of Trx1 and extensive cell death. Collectively, these findings expand our knowledge of the role and regulation of the mammalian Trx system in relation to cellular nitroso-redox imbalance. The observations raise the possibility of exploiting the nitrosylation susceptibility of TrxR1 for killing tumor cells.

Piperlongumine as a direct TrxR1 inhibitor with suppressive activity against gastric cancer

Piperlongumine (PL), a natural alkaloid isolated from the fruit of long pepper, is known to selectively kill tumor cells while sparing their normal counterparts. However, the cellular target and potent anticancer efficacy of PL in numerous types of human cancer cells have not been fully defined. We report here that PL may interact with the thioredoxin reductase 1 (TrxR1), an important selenocysteine (Sec)-containing antioxidant enzyme, to induce reactive oxygen species (ROS)-mediated apoptosis in human gastric cancer cells. By inhibiting TrxR1 activity and increasing intracellular ROS levels, PL induces a lethal endoplasmic reticulum stress and mitochondrial dysfunction in human gastric cancer cells. Importantly, knockdown of TrxR1 sensitizes cells to PL treatment, and PL displays synergistic lethality with GSH inhibitors (BSO and Erastin) against gastric cancer cells. In vivo, PL treatment markedly reduces the TrxR1 activity and tumor cell burden. Remarkably, TrxR1 was significantly overexpressed in gastric cancer cell lines and human gastric cancer tissues. Targeting TrxR1 with PL thus discloses a previously unrecognized mechanism underlying the biological activity of PL and provides an in-depth insight into the action of PL in the treatment of gastric cancer.

The Cellular Thioredoxin-1/Thioredoxin Reductase-1 Driven Oxidoreduction Represents a Chemotherapeutic Target for HIV-1 Entry Inhibition

Background

The entry of HIV into its host cell is an interesting target for chemotherapeutic intervention in the life-cycle of the virus. During entry, reduction of disulfide bridges in the viral envelope glycoprotein gp120 by cellular oxidoreductases is crucial. The cellular thioredoxin reductase-1 plays an important role in this oxidoreduction process by recycling electrons to thioredoxin-1. Therefore, thioredoxin reductase-1 inhibitors may inhibit gp120 reduction during HIV-1 entry. In this present study, tellurium-based thioredoxin reductase-1 inhibitors were investigated as potential inhibitors of HIV entry.

Results

The organotellurium compounds inhibited HIV-1 and HIV-2 replication in cell culture at low micromolar concentrations by targeting an early event in the viral infection cycle. Time-of-drug-addition studies pointed to virus entry as the drug target, more specifically: the organotellurium compound TE-2 showed a profile similar or close to that of the fusion inhibitor enfuvirtide (T-20). Surface plasmon resonance-based interaction studies revealed that the compounds do not directly interact with the HIV envelope glycoproteins gp120 and gp41, nor with soluble CD4, but instead, dose-dependently bind to thioredoxin reductase-1. By inhibiting the thioredoxin-1/thioredoxin reductase-1-directed oxidoreduction of gp120, the organotellurium compounds prevent conformational changes in the viral glycoprotein which are necessary during viral entry.

Conclusion

Our findings revealed that thioredoxin-1/thioredoxin reductase-1 acts as a cellular target for the inhibition of HIV entry.

Oxidation of structural cysteine residues in thioredoxin 1 by aromatic arsenicals enhances cancer cell cytotoxicity caused by the inhibition of thioredoxin reductase 1

Thioredoxin systems, composed of thioredoxin reductase (TrxR), thioredoxin (Trx) and NADPH, play important roles in maintaining cellular redox homeostasis and redox signaling. Recently the cytosolic Trx1 system has been shown to be a cellular target of arsenic containing compounds. To elucidate the relationship of the structure of arsenic compounds with their ability of inhibiting TrxR1 and Trx1, and cytotoxicity, we have investigated the reaction of Trx1 system with seven arsenic trithiolates: As(Cys)3, As(GS)3, As(Penicillamine)3, As(Mercaptoethanesulfonate)3, As(Mercaptopurine)3, As(2-mercaptopyridine)3 and As(2-mercaptopyridine N-oxide)3. The cytotoxicity of these arsenicals was consistent with their ability to inhibit TrxR1 in vitro and in cells. Unlike other arsenicals, As(Mercaptopurine)3 which did not show inhibitory effects on TrxR1 had very weak cytotoxicity, indicating that TrxR1 is a reliable drug target for arsenicals. Moreover, the two aromatic compounds As(2-mercaptopyridine)3 and As(2-mercaptopyridine N-oxide)3 showed stronger cytotoxicity than the others. As(2-mercaptopyridine)3 which selectively oxidized two structural cysteines (Cys62 and Cys69) in Trx1 showed mild improvement in cytotoxicity. As(2-mercaptopyridine N-oxide)3 oxidized all the Cys residues in Trx1, exhibiting the strongest cytotoxicity. Oxidation of Trx1 by As(2-mercaptopyridine)3 and As(2-mercaptopyridine N-oxide)3 affected electron transfer from NADPH and TrxR1 to peroxiredoxin 1 (Prx1), which could result in the reactive oxygen species elevation and trigger cell death process. These results suggest that oxidation of structural cysteine residues in Trx1 by aromatic group in TrxR1-targeting drugs may sensitize tumor cells to cell death, providing a novel approach to regulate cellular redox signaling and also a basis for rational design of new anticancer agents.

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.

APR-246/PRIMA-1MET inhibits thioredoxin reductase 1 and converts the enzyme to a dedicated NADPH oxidase

The low-molecular-weight compound APR-246 (PRIMA-1(MET)) restores wild-type conformation and function to mutant p53, and triggers apoptosis in tumor cells. We show here that APR-246 also targets the selenoprotein thioredoxin reductase 1 (TrxR1), a key regulator of cellular redox balance. APR-246 inhibited both recombinant TrxR1 in vitro and TrxR1 in cells. A Sec-to-Cys mutant of TrxR1 was not inhibited by APR-246, suggesting targeting of the selenocysteine residue in wild-type TrxR1. Preheated APR-246 and its conversion product methylene quinuclidinone (MQ) were much more efficient TrxR1 inhibitors than APR-246 itself, indicating that MQ is the active compound responsible for TrxR1 enzyme inhibition. TrxR1 inhibited by MQ was still functional as a pro-oxidant NADPH oxidase. Knockdown of TrxR1 caused a partial and reproducible attenuation of APR-246-induced tumor cell death independently of p53 status. Cellular TrxR1 activity was also inhibited by APR-246 irrespective of p53 status. We show that APR-246 can directly affect cellular redox status via targeting of TrxR1. Our findings provide an explanation for the previously observed effects of APR-246 on tumor cells lacking mutant p53.

Ethaselen: a potent mammalian thioredoxin reductase 1 inhibitor and novel organoselenium anticancer agent

Mammalian thioredoxin reductase 1 (TrxR1) is considered to be an important anticancer drug target and to be involved in both carcinogenesis and cancer progression. Here, we report that ethaselen, a novel organoselenium compound with anticancer activity, specifically binds to the unique selenocysteine-cysteine redox pair in the C-terminal active site of mammalian TrxR1. Ethaselen was found to be a potent inhibitor rather than an efficient substrate of mammalian TrxR1. It effectively inhibits wild-type mammalian TrxR1 at submicromolar concentrations with an initial mixed-type inhibition pattern. By using recombinant human TrxR1 variants and human glutathione reductase, we prove that ethaselen specifically targets the C-terminal but not the N-terminal active site of mammalian TrxR1. In A549 human lung cancer cells, ethaselen significantly suppresses cell viability in parallel with direct inhibition of TrxR1 activity. It does not, however, alter either the disulfide-reduction capability of thioredoxin or the activity of glutathione reductase. As a downstream effect of TrxR1 inactivation, ethaselen causes a dose-dependent thioredoxin oxidation and enhances the levels of cellular reactive oxygen species in A549 cells. Thus, we propose ethaselen as the first selenium-containing inhibitor of mammalian TrxR1 and provide evidence that selenium compounds can act as anticancer agents based on mammalian TrxR1 inhibition.

Inhibition of thioredoxin reductase 1 by porphyrins and other small molecules identified by a high-throughput screening assay

The selenoprotein thioredoxin reductase 1 (TrxR1) has in recent years been identified as a promising anticancer drug target. A high-throughput assay for discovery of novel compounds targeting the enzyme is therefore warranted. Herein, we describe a single-enzyme, dual-purpose assay for simultaneous identification of inhibitors and substrates of TrxR1. Using this assay to screen the LOPAC¹²⁸⁰ compound collection we identified several known inhibitors of TrxR1, thus validating the assay, as well as several compounds hitherto unknown to target the enzyme. These included rottlerin (previously reported as a PKCδ inhibitor and mitochondrial uncoupler) and the heme precursor protoporphyrin IX (PpIX). We found that PpIX was a potent competitive inhibitor of TrxR1, with a K(i)=2.7 μM with regard to Trx1, and in the absence of Trx1 displayed time-dependent irreversible inhibition with an apparent second-order rate constant (k(inact)) of (0.73 ± 0.07) × 10⁻³ μM⁻¹ min⁻¹. Exogenously delivered PpIX was cytotoxic, inhibited A549 cell proliferation, and was found to also inhibit cellular TrxR activity. Hemin and the ferrochelatase inhibitor NMPP also inhibited TrxR1 and showed cytotoxicity, but less potently compared to PpIX. We conclude that rottlerin-induced cellular effects may involve targeting of TrxR1. The unexpected finding of PpIX as a TrxR1 inhibitor suggests that such inhibition may contribute to symptoms associated with conditions of abnormally high PpIX levels, such as reduced ferrochelatase activity seen in erythropoietic protoporphyria. Finally, additional inhibitors of TrxR1 may be discovered and further characterized based upon the new high-throughput TrxR1 assay presented here.

Substrate and inhibitor specificities differ between human cytosolic and mitochondrial thioredoxin reductases: Implications for development of specific inhibitors

The cytosolic and mitochondrial thioredoxin reductases (TrxR1 and TrxR2) and thioredoxins (Trx1 and Trx2) are key components of the mammalian thioredoxin system, which is important for antioxidant defense and redox regulation of cell function. TrxR1 and TrxR2 are selenoproteins generally considered to have comparable properties, but to be functionally separated by their different compartments. To compare their properties we expressed recombinant human TrxR1 and TrxR2 and determined their substrate specificities and inhibition by metal compounds. TrxR2 preferred its endogenous substrate Trx2 over Trx1, whereas TrxR1 efficiently reduced both Trx1 and Trx2. TrxR2 displayed strikingly lower activity with dithionitrobenzoic acid (DTNB), lipoamide, and the quinone substrate juglone compared to TrxR1, and TrxR2 could not reduce lipoic acid. However, Sec-deficient two-amino-acid-truncated TrxR2 was almost as efficient as full-length TrxR2 in the reduction of DTNB. We found that the gold(I) compound auranofin efficiently inhibited both full-length TrxR1 and TrxR2 and truncated TrxR2. In contrast, some newly synthesized gold(I) compounds and cisplatin inhibited only full-length TrxR1 or TrxR2 and not truncated TrxR2. Surprisingly, one gold(I) compound, [Au(d2pype)(2)]Cl, was a better inhibitor of TrxR1, whereas another, [(iPr(2)Im)(2)Au]Cl, mainly inhibited TrxR2. These compounds also inhibited TrxR activity in the cytoplasm and mitochondria of cells, but their cytotoxicity was not always dependent on the proapoptotic proteins Bax and Bak. In conclusion, this study reveals significant differences between human TrxR1 and TrxR2 in substrate specificity and metal compound inhibition in vitro and in cells, which may be exploited for development of specific TrxR1- or TrxR2-targeting drugs.

Thiol-metabolizing proteins and endothelial redox state: differential modulation of eNOS and biopterin pathways

The intracellular redox state is stringently maintained by thiol-based antioxidants to establish a balance for the physiological and pathophysiological roles of reactive oxygen species. The relative contributions of the thioredoxin (Trx) and glutathione/glutaredoxin systems to intracellular redox balance are incompletely understood, as are the consequences of altered thiol metabolism on endothelial nitric oxide (NO) synthase (eNOS) and NO-dependent pathways in the endothelium. We designed duplex small interfering RNA (siRNA) constructs to specifically "knock down" the expression of three key thiol-metabolizing enzymes in cultured aortic endothelial cells. Transfection of siRNA constructs targeting glutathione reductase (GR), cytosolic Trx reductase (TrxR1), or mitochondrial Trx reductase (TrxR2) significantly decreased the intracellular reduced glutathione-to-oxidized glutathione ratio. siRNA-mediated knockdown of either GR, TrxR1, or TrxR2 markedly suppressed VEGF-induced NO production (measured by an electrochemical NO sensor) and also blocked eNOS enzyme activity (using the [(3)H]arginine/[(3)H]citrulline assay). Pretreatment of endothelial cells with N,N'-bis(2-chloroethyl)-N-nitrosourea, an inhibitor of GR and TrxR, significantly decreased VEGF-induced NO production. siRNA-mediated TrxR2 knockdown led to a marked increase in hydrogen peroxide (H(2)O(2)) production in endothelial cells. In contrast, knockdown of GR or TrxR1 only slightly increased H(2)O(2) production. Supplementation of endothelial cells with tetrahydrobiopterin prevented the increase in H(2)O(2) generation seen with siRNA-mediated knockdown of GR. These studies show that the differential regulation of thiol-metabolizing proteins leads to critical changes in oxidative and nitrosative stress pathways. Greater understanding of the differential regulation of thiol-metabolizing proteins may lead to the development of new pharmacological targets for diseases associated with oxidative stress in the vascular wall.