INTRODUCTION OF THE THIOREDOXIN SYSTEM
The thioredoxin system is composed of thioredoxin (Trx), thioredoxin reductase (TR) and reduced nicotinamide adenine dinucleotide phosphate (NADPH). Trx and TR are present in many tumor patient tissue and blood samples. The catalytic process of the Trx system is shown in Fig. 1 .
The pKa (−5.9) value of selenocysteine (SEC) is much lower than that of cysteine (−8.5). The Trx system is an NADPH-dependent reductase system that includes the Trx1 (cytoplasmic), Trx2 (mitochondrial), and Trx3 (testicular specific) subtypes (Powis and Kirkpatrick2007; Roman et al. 2020; Xinastle-Castillo and Landa 2022). The selenoprotein TR is the only reducing agent of Trx and has three isoforms (TR1, TR2, and TR3) in mammals (Powis and Kirkpatrick 2007). The structure of TR2 is similar to that of TR1 as both contain a C-terminal SEC active region (Sun et al. 1999) and can enter mitochondria to produce mature proteins.
Thioredoxin (Trx)
Trx is an important antioxidant molecule. Laurent et al. were the first to obtain Trx in E. coli in 1964. Trx1 can be combined with peroxidase and H2O2 to reduce oxidized proteins. Mice in which Trx1 and Trx2 are knocked out experience embryo death. Trx plays an outstanding role in redox reactions (Zhang et al. 2017). Trx was originally found to be used as electron donor for RNA reductase in E. coli (Laurent et al. 1964). The electron donor of RNA reductase is composed of α helix and β folded connections, and it is stably high in protein (Aguado-Llera et al. 2011; Gasdaska et al. 1994). Trx can be reduced by TR in the presence of NADPH to form oxygen in the oxidation state (Zhang et al. 2020). Trx, acting as a growth factor, interacts with TXNIP to perform the following roles: (1) regulating the immune function (Bjorklund et al. 2022); (2) scavenging intracellular reactive oxygen species (ROS), regulating other enzyme activities and reducing oxidative stress; (3) maintaining normal intracellular redox homeostasis (Zhang et al. 2020) and reducing Gpx3 (Bjornstedt et al. 1994); (4) scavenging singlet oxygen and hydroxyl radicals through ROS (Yi et al. 2019) and regulating p53 and Nrf2 gene expression (Maulik and Das 2008); (5) regulating apoptotic protein expression through the nuclear factor kappa B (NF-κB) signaling pathway (Schenk et al. 1994); and (6) regulating apoptosis through ASK1 (Liu and Min 2002). The function of Trx is shown in Fig. 2 .
Thioredoxin reductase (TR)
TR is a selenium enzyme that is closely related to lung cancer. TR can remove ROS in vivo and balance cells and tissues in the inner and lateral positions. After cell canceration, the expression of TR1 is increased, promoting cell growth and metastasis.
Structure and type of TR
Macromolecule TR mainly exists in higher eukaryotes, with a molecular weight of 55 kDa (Williams et al. 2000). TRl, TR2 and TR3 are mainly distributed in the cytoplasm, mitochondria and testes, respectively (Liu et al. 2023). Flavin adenine exists in TR1 and TR2 dinucleotide (FAD) binding and action sites, and TR3 contains the CVNVGC active site region. The end contains a relatively conserved active site, namely, Cys-Val-Asn-Val-GLy-Cys, which has a redox catalytic active site. The C-terminus contains Gly-Cys-Sec-Gly, and the binding domain of NADPH and FAD. They are active sites. The two regions are connected by a folded chain; the action site changes under catalytic conditions, the binding domain rotates 66° from NADPH to FAD, and the substrate is exposed (Lennon et al. 1999).
Biological function of TR
SEC is present in TR (Tuladhar et al. 2019), which is an important site for its enzyme activity (Zhong et al. 2000). TR can act on oxidized Trx to reduce Trx (Zhong et al. 2000), and NADPH is oxidized to NADP+ (Zhang et al. 2018). After Trx is reduced, the disulfide bonds of other proteins are reduced, thus exerting biological activity. In addition, TR can also reduce antioxidants, protein disulfide isomerase (PDI) (Che et al. 2020; Ogata et al. 2022), lipid peroxides and H2O2 (Ghneim et al. 2022). TR plays an antioxidant role through the catalytic regeneration of coenzyme Q10 (Preci et al. 2021). TR1 is present in tissue cytoplasm. The human TR gene is located on chromosomes 12Q23–Q24.1 (Gasdaska et al. 1996) and contains 3826 bp base cDNA (Gasdaska et al. 1995) encoding a total of 497 amino acid sequences (Jan et al. 2014).
Catalytic mechanism of TR
The SEC insertion sequence is required when SEC binds to the polypeptide chain of selenoprotein. The combined action of the selenocysteine insertion sequence (SECIS) element, SECIS binding protein 2 (SBP2), and other related compounds in the cell is needed. The binding of SEC greatly affects the activity of TR (Papp et al. 2007), and insufficient synthesis of SEC components changes TR storage (Lu et al. 2009). TR has unique properties in mammals (Liu and Min 2002), while TR is a small endogenous dimer flavin protein (molecular weight: approximately 33.9 kDa) in bacteria (Kreimer et al. 1997).
TR regulates cell growth and apoptosis
Enzyme inactivation of TR1 can lead to early embryo death (Jakupoglu et al. 2005), and this suggests that TRl is an extremely important substance in embryonic development. Nrf2 can regulate TR1 and affect cell proliferation (Gao et al. 2020), and TR is the only reducing agent that can reduce Trx. Therefore, through Trx or its own specific substrate, TR can play its redox role. The function of the TR system is shown in Fig. 3 .
Relationship between the thioredoxin system and diseases
There are many diseases related to the Trx system, including tumors, acquired immunodeficiency syndrome (AIDS), parasitic infections, rheumatoid arthritis (RA), hypertension, myocarditis and neurodegenerative diseases.
Acquired immunodeficiency syndrome (AIDS)
TR levels are high in cancer cells (Gencheva and Arner 2022). The expression of TR is increased in leukemia, solid tumors and lymphatic carcinoma (Campbell et al. 2021). At the early stages of a tumor, Trx protects against cancer. Trx was found to promote the expression of VEGF (Welsh et al. 2002). The development of AIDS is related to oxidative damage in the body. The synthesis of the endogenous antioxidant selenoprotein TR1 is decreased, and glutathione peroxidase and small-molecule selenium compounds are increased.
Parasitic infections and rheumatoid arthritis (RA)
A study found that Plasmodium falciparum contains TR1 (Krnajski et al. 2002). TR and its related Trx and glutenin were also found in Schistosoma japonicum. The original enzymes (Sun et al. 2001) Trx and TR are significantly increased in the synovial fluid and tissues of RA patients, but not in plasma (Maurice et al. 1999). The gold-containing compounds aurioglucosamine and ranonophen are clinically used to treat RA. They are highly effective inhibitors of TR (Smith et al. 1999).
Hypertension and myocarditis
Angiotensin Ⅱ can increase the activity of NADPH oxidase, cause mitochondrial function damage, produce ROS, and cause hyperemia and pressure disease (de Cavanagh et al. 2009). TR2 mainly exists in mitochondria. Matsushima found that overexpression of TR2 results in good prognosis after myocardial injury in rats (Matsushima et al. 2006). In transgenic rats with high expression of TR2, ROS production and the expression of NADPH oxidase decrease. TR2 can maintain the normal function of endothelial cells (Kameritsch et al. 2021). Moreover, TR gene silencing in the heart increases the probability of oxidative stress and myocardial hypertrophy (Li et al. 2017). The TR activity in the myocardium of pigs fed with low selenium decreases (Sun et al. 2020). The expression of Trx in myocarditis is increased, and motiflrin can reduce myocardial injury (Yuan et al. 2003).
Neurodegenerative diseases
The human brain needs oxygen, oxidative polyunsaturated fatty acids and cholesterol. It is more susceptible to free radical damage than other tissues are, and oxidative damage is likely to occur with increasing age. Therefore, oxidative damage is considered to be a major factor in neurodegenerative diseases in the elderly population (Singh et al. 2019). The Trx system is abundant in nerve cells and axons. Trx and TR are mainly located in the cytoplasm and result in oxidative damage. They are closely associated with neurodegenerative diseases. Oxidative damage is associated with damage to the endoplasmic reticulum and mitochondria, which can induce neurosis, apoptosis and protein misfolding. The level of Trxl in the brains of AD patients is low, and P-amyloid peptide accumulates in large quantities (Lovell et al. 2000). Meanwhile, overexpression of amyloid peptides can reduce the expression and activity of TR, thus reducing the ability of Trxl to reduce substrates (Lamoke et al. 2012). These studies suggest that the loss of Trx or TR function is likely to result in neuronal decline (Seyfried and Wullner 2007).
Alzheimer's disease, Parkinson's disease and Huntington's disease are the major neurodegenerative diseases. The human brain requires oxygen and is prone to oxidative damage, which is likely to occur with age. Therefore, oxidative damage is considered to be the main factor for the occurrence of neurodegenerative diseases in the elderly (Maiuri et al. 2019). Trx and TR are mainly located in the cytoplasm and closely associated with neurodegenerative diseases. Oxidative damage is related to damage to the endoplasmic reticulum and mitochondria, and induces neuronal apoptosis and protein misfolding. The level of Trx1 in the brains of patients with Alzheimer's disease is low, and amyloid β protein (Aβ) accumulates massively (Lovell et al. 2000). Overexpression of Trx1 protects cells against cytotoxicity induced by Aβ. Animal experiments have shown that overexpression of Aβ reduces the expression and activity of TR, resulting in the lack of substrate reduction ability of Trx1 (Lamoke et al. 2012). The results of previous studies indicate that the loss of Trx or TR function is likely to cause a decline in neuronal bias (Seyfried and Wullner 2007), but the specific mechanism is still unclear. It has been reported that TR and Trx exert protective effects against neurotoxicity induced by Aβ (Lovell et al. 2000). Retinal neurotoxicity mediated by Aβ includes damage to the Trx system and reduction in TR activity (Lamoke et al. 2012). In addition, Kudin et al. (Kudin et al. 2012) confirmed that the Trx2 system plays an important role in H2O2 detoxification in the hippocampus of rats. These findings highlight TR as an important source of reductive energy in the brain. TR can maintain the intracellular reduction state, and remove lipid peroxides and H2O2 under the action of NADPH (Lewin et al. 2001). Lovell et al. (Lovell et al. 2000) co-cultured Aβ with hippocampal nerve cells and found that the survival rate of nerve cells is low when Aβ is co-cultured with hippocampal nerve cells for 12 h. However, the survival rates of Aβ, TR and hippocampal neurons increased after co-culturing for 12 h, suggesting that TR has a protective effect on Aβ-induced neurocytotoxicity. The relationship between the Trx system and diseases is shown in Fig. 4 .
SEVERAL COMMONLY USED INHIBITORS OF TR
Over the last decade, many inhibitors have been developed that target the Trx system. Herein, several commonly used TR inhibitors are listed. (1) Platinum compounds include oxaliplatin, cisplatin, carboplatin and ginovine (Marzano et al. 2007). (2) Arsenic trioxide is an irreversible inhibitor of TR (Park 2020). (3) Motexafin gadolinium (MGd) is a porphyrin compound whose target molecule is TR1 in the cytoplasm (Hashemy et al. 2006). MGd can block the reducing activity of TR1 and generate peroxide (Liu et al. 2009). (4) Nitroaromatic compounds. the most typical of which is 1-chlorophyll-2,4-dinitrobenzene, are also widely used inhibitors for TR. (5) Flavonoids and turmeric compounds, which have long been used in tumor therapy, are mediated by the TR system. Turmeric compounds can inhibit TR. Flavonoids have an inhibitory effect on thioredoxin A1 of Corynebacterium pseudotuberculosis (Eberle et al. 2018). The inhibitors of TR are shown in Fig. 5 .
CONCLUSION
In summary, the Trx system is an important antioxidant reduction system in vivo that is closely related to cell proliferation, differentiation and death, and is associated with tumors, neurodegenerative diseases, rheumatoid arthritis, hypertension, myocarditis and other diseases. The Trx system can remove excessive ROS generated in the body and keep the cell in balance. Intervention of the Trx system is an important target of drugs.