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Human Thioredoxin Reductase 1

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Thioredoxin reductase (TrXR) is a ubiquitous enzyme which reduces the thioredoxin protein by a disulfide oxidoreductase activity ^[1]. This enzyme belongs to the flavoprotein family which needs cofactors to catalyze the NADPH dependent reaction. NADPH cofactor allows electrons transmission during the reaction via FAD from enzyme to oxidized protein. The thioredoxin system is thus composed of thioredoxin reductase, NADPH and thioredoxin with the following reaction:

TrxR1 belongs to one of the two forms of mammalian TrxR enzymes mostly present in cytosol contrary to TrxR2 which is only mitochondrial. TrxR1 is heterogeneous protein which is present in most tissues and she is specific of the small thioredoxin 1 protein (Trx1) ^[2]. Its capacity to reduce oxidized Trx1 is important to maintain the active site of Trx1. The redox activity of Trx1 reduced is the key of its biological activity ^[3]. TrxR1 can thus regulate Trx1 activities by its NADPH dependent reduction specificity.

Contents 1 Structure 2 Catalytic mechanism 3 Biological function 3.1 A central role for oxidative stress 3.2 Regulation of signal transduction pathways 3.3 And its impact on immune system 4 Diseases 4.1 Cancer 4.2 Cardiovascular diseases 4.3 Neuronal diseases 4.4 Impact on immune response 5 References

Structure

The gene of hTrxR1 is located on the 12q23-q24.1 chromosome. TrxR are homodimeric proteins. Each monomer has a molecular mass of 54,6 kDa.

There is no post-translational glycosylation in the human enzyme.

They are flavoproteins because they are selenium containing pyridine nucleotide-disulfide oxido-reductases proteins. They are also members of seleno proteins because of the C-ter redox catalytic site (“Cys-Val-Asn-Val-Gly-Cys”) which is conserved.

Each monomer contains:

• FAD prosthetic group containing the catalytic site

• NADPH binding site

• Active site containing a redox active disulphide.

The broad specificity of the protein is due to the second redox active site.

Catalytic mechanism

Se-containing TrxR has a catalytic action on the reduction reaction of Trx. The FAD domain which contain the active site (catalytic site) and the binding site of NADPH are close to the ring of FAD. That allows the electrons to go from NADPH to the substrate through the ring of FAD and the disulphide active site. There is no big conformational change in the enzyme to do that. When there is an excess of NADPH, hTrxR forms a stable charge transfer complex. In hTrxR there is an additional redox-active site which is not part of the conserved active site.

The action mechanism of Se-containing TrxR is supposed to be like that:

• The NADPH fixes to the TrxR NADPH binding site and the electrons go from NADPH to the conserved catalytic site through FAD to reduce the disulphide bond of TrxR. So at this step the catalytic site is in its oxidized form.

• Then there is the oxidation of the Cys-X-X-Cys site and the reduction of the C-terminal Se-containing site thanks to a Thiol-Disulfide exchange.

• The reduced C-terminal site goes away transferring electrons to a substrate.

One suggestion for the catalytic mechanism of hTrxR is that the C-terminal conserved site is flexible which allows the Se-containing part to bring the reducing equivalents to the substrate.

The selenium (Se) is essential for the activity of TrxR because it increase the TrxR activity. The increase of TrxR activity seems due to the incorporation of Se into TrxR which is associated to the increase of the specific activity of the enzyme. Se incorporation into the enzyme is not linked to an increase in protein synthesis.

Biological function

Biological functions of TrxR1 depend directly of thioredoxin activities ^[4]. Trx1 can interact with many partners in different cellular compartments. Its biological function is cellular localisation dependent. Reductase activity of Trx1 can regulate cell growth or apoptosis for example but in the nucleus, Trx can bind to different transcription factors. The key role of Trx is its capacity to defence against oxidative damage.

A central role for oxidative stress

Oxidative stress is the balance between oxidative and reducing species and several mediators can alternate this redox potential. Most of the time, pro-oxidative species derived from di-oxygen (O2) or nitrogen monoxide (NO) in high concentration cause oxidative damages. Reactive species have at list signaling functions and their increased levels are stimuli for cells. This response engages cells into a program to change their characteristics: as in differentiation or apoptosis.

Reductase activity of Trx1 can neutralize the reactive species to equilibrate the oxidative drift by mediating the reduction of proteins involved in scavenging reactive oxygen species (ROS) ^[5]. Trx1 is an important hydrogen donor to ribonucleotide reductase which has an intracellular antioxidant activity. Trx acts thus as an important regulator of the oxidative stress.

Regulation of signal transduction pathways

Some cellular pathways are affected by increase of the levels of oxidizing species below those inducing damage. Several of these pathways rely on transcriptional responses by activation of redox-sensitive transcription factors such as p53, AP-1 or NF-κB in cytoplasm ^{[6] [7]}. These transcription factors are then activated by Trx in nucleus. This enzyme over-expressed can thus bind redox-sensitive transcription factors and activates them. That leads to modulate their DNA-binding activity on the promoter region of several genes. Transcription factors regulate in this way expression of genes which leads to cellular activation and regulates apoptosis ^[8]. For example, the tumour suppressor protein p53 stimulates reporter gene expression involved in cellular function such as mitosis or apoptosis. He is the guardian of the genome in prevent mutations by inducing expression of various genes as redox related genes, apoptosis related genes and many other ^[9]. In addition, Trx can also regulate the transcription factor NF-κB which is involved in the control of several processes as cell growth, immune response or even inflammation ^[10].

And its impact on immune system

Thioredoxin was first identified as a cytokine like factor in virus transformed cells ^[11]. Indeed, the Trx protein allows reduce NF-κB by its binding to this transcription factor. NF-κB factor can be thus a redox sensitive factor by regulation of gene expression of cytokines or other immune response genes. Trx allows directly regulation of pro-inflammatory cytokines expression and demonstrates several anti-inflammatory effects ^[12].

Diseases

Cancer

Studies showed that Thioredoxine reducatase 1 (TrxR1) is overexpressed (10 times than the normal) in many cancer cells. Even if the direct link between TrxR1 and cancer induction is not clear, there is evidence that TrxR1 regulates DNA replication. Reduction in TrxR1 activity results in blockage of cells in phase S. Indeed, decrease in TrxR1 concentration leads to decrease in reduced-Trx (activated form) which normally acts as an electron donor for the Ribonucleotide reductase (the enzyme that syntehises most deoxyribonucleotides from ribonucleotides). Some TrxR1 isoforms are thought to guide actin and tubulin polymerization. Inactive-form Trx are also unable to inhibit apoptosis, as unable to inhibit the Apoptosis Signal-regulating Kinase 1 (ASK1), also known as Mitogen-Activated Protein Kinase Kinase Kinase 5 (MAP3K5). Furthermore, some studies showed that the Thioredoxin system, cooperating with the redox factor-1 (APE/Ref-1) regulates basal activity of p53. TrxR1 is then a good target for cancer treatement. Recent studied showed that arsenic trioxide AS2O3 or ATO inhibits TrxR1 in its redox state in presence of NADPH and is an approved anti-cancer chemotherapeutic drug commercially named Trisenox®. It is used to treat acute promyelocytic leukemia, multiple myeloma, chronic myelogenous leukemia, and acute myelogenous leukemia.

Cardiovascular diseases

Thioredoxin Reductase 1 (TrxR1) has important protection functions against oxidative stress due to its antioxidant properties. Studies showed that downregulation of TrxR1 inducing Trx downregulation leads to cardiomyocyte injuries. TrxR, indirectly via TrX, plays a protective role in myocardial ischemia/reperfusion. TrxR1, part of the Thioredoxin system is suggested to play a role in cellular defense against oxidized LDL and arteriosclerosis development. TrxR could also be involved in hypertension and diabetes as anormal levels of thioredoxin is found in hypertensive and diabetic patients serum compared to normal patients.

Neuronal diseases

Thioredoxin Reductase 1 (TrxR1) could be related to Parkinston’s Disease as TrxR1 expression decreased in the substantia nigra pars compacta of the Parkinson's disease mouse model, suggesting that TrxR1 may play a protective antioxidant role preventing neurodegeneration.

Although the direct involvement of Thioredoxin Reductase 1 is not clear in Alzheimer disease, studies showed that patients diagnosed with Alzheimer have a higher level of Thioredoxin Reductase 1 (TrxR1) in their brain.

Impact on immune response

Thioredoxin Reductase 1 (TrxR1) plays an important role in repressing HIV-1 protease by negatively regulating the HIV-1 transcriptional activator in human macrophages.

Studies suggested that TrxR1, by regulating ROS generation, regulate the redox-dependent signal transduction and may confer protection against inflammatory processes induced by influenza virus.

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References

1. ↑ Mustacich D, Powis G. Thioredoxin reductase. Biochem J. 2000 Feb 15;346 Pt 1:1-8.
2. ↑ Jurado J, Prieto-Alamo MJ, Madrid-Rísquez J, Pueyo C. Absolute gene expression patterns of thioredoxin and glutaredoxin redox systems in mouse. J Biol Chem. 2003 Nov 14;278(46):45546-54. Epub 2003 Sep 3.
3. ↑ Holmgren A, Björnstedt M. Thioredoxin and thioredoxin reductase. Methods Enzymol. 1995;252:199-208.
4. ↑ Oblong JE, Berggren M, Gasdaska PY, Powis G. Site-directed mutagenesis of active site cysteines in human thioredoxin produces competitive inhibitors of human thioredoxin reductase and elimination of mitogenic properties of thioredoxin. J Biol Chem. 1994 Apr 22;269(16):11714-20.
5. ↑ Powis G, Montfort WR. Properties and biological activities of thioredoxins. Annu Rev Pharmacol Toxicol. 2001;41:261-95.
6. ↑ Ueno M, Masutani H, Arai RJ, Yamauchi A, Hirota K, Sakai T, Inamoto T, Yamaoka Y, Yodoi J, Nikaido T. Thioredoxin-dependent redox regulation of p53-mediated p21 activation. J Biol Chem. 1999 Dec 10;274(50):35809-15.
7. ↑ Freemerman AJ, Gallegos A, Powis G. Nuclear factor kappaB transactivation is increased but is not involved in the proliferative effects of thioredoxin overexpression in MCF-7 breast cancer cells. Cancer Res. 1999 Aug 15;59(16):4090-4.
8. ↑ Cassidy PB, Edes K, Nelson CC, Parsawar K, Fitzpatrick FA, Moos PJ. Thioredoxin reductase is required for the inactivation of tumor suppressor p53 and for apoptosis induced by endogenous electrophiles. Carcinogenesis. 2006 Dec;27(12):2538-49. Epub 2006 Jun 15.
9. ↑ Polyak K, Xia Y, Zweier JL, Kinzler KW, Vogelstein B. A model for p53-induced apoptosis. Nature. 1997 Sep 18;389(6648):300-5.
10. ↑ Kabe Y, Ando K, Hirao S, Yoshida M, Handa H. Redox regulation of NF-kappaB activation: distinct redox regulation between the cytoplasm and the nucleus. Antioxid Redox Signal. 2005 Mar-Apr;7(3-4):395-403.
11. ↑ Nakamura H. Extracellular functions of thioredoxin. Novartis Found Symp. 2008;291:184-92; discussion 192-5, 221-4.
12. ↑ Bertini R, Howard OM, Dong HF, Oppenheim JJ, Bizzarri C, Sergi R, Caselli G, Pagliei S, Romines B, Wilshire JA, Mengozzi M, Nakamura H, Yodoi J, Pekkari K, Gurunath R, Holmgren A, Herzenberg LA, Herzenberg LA, Ghezzi P. Thioredoxin, a redox enzyme released in infection and inflammation, is a unique chemoattractant for neutrophils, monocytes, and T cells. J Exp Med. 1999 Jun 7;189(11):1783-9.