This gene is a member of the NAD(P)H dehydrogenase (quinone) family and encodes a cytoplasmic 2-electron reductase. This FAD-binding protein forms homodimers and reduces quinones to hydroquinones. This enzyme facilitates the two electron reduction of quinone to hydroquinone. NQO1-mediated two electron reduction of quinone to hydroquinone thereby indirectly prevents the one electron reduction of quinone to the semiquinone free radical.[10]
The ubiquitin-independent p53 degradation pathway is regulated by NQO1. NQO1 stabilizes p53, protecting it from degradation. Individuals with decreased NQO1 expression/activity have reduced p53 stability, which may lead to resistance to drugs such as chemotherapeutics.[11]
Detoxification
Quinonoid compounds generate reactive oxygen species (ROS) via redox cycling mechanisms and arylating nucleophiles. NQO1 removes quinone from biological systems through detoxification reaction:
NAD(P)H + a quinone → NAD(P)+ + a hydroquinone.
This reaction oxidises the substrate without the formation of damaging semiquinone and oxygen free radical species. The localization of NQO1 in epithelial and endothelial tissues of mice, rats and humans indicates their importance in detoxifying agent, since their location facilitates exposure to compounds entering the body.
Vitamin K metabolism
The enzyme is also involved in biosynthetic processes such as the vitamin K-dependent gamma-carboxylation of glutamate residues in prothrombin synthesis.[12] NQO1 catalyzes the reduction of vitamin K1, K2 and K3 into their hydroquinone form, but it only has a high affinity for Vitamin K3. Vitamin K hydroquinone serves as a cofactor for vitamin K γ‐carboxylase that catalyzes γ‐carboxylation of specific glutamic acid residues in Gla‐factors/proteins (Gla domain) leading to their activation and participation in blood clotting and bone metabolism. Vitamin K is used as radiation sensitizer or in mixtures with other chemotherapeutic drugs to treat several types of cancer. ROS generated in redox cycling contributes to anticancer activity of vitamin K. NQO1 competes with enzymes that redox cycle vitamin K to formation of semiquinone and ROS. NQO1is therefore able to detoxify vitamin K3 and protect cells against oxidative stress.[13]
Bioactivation of antitumor agents
Several anti-tumor agents such as mitosenes, indolequinones, aziridinylbenzoquinones and β-lapachone have been designed be bioactivated by NQO1 from various prodrugs. The high levels of NQO1 expression in many human solid tumors compared to normal tissue ensures their selective activation within tumor cells.[14][15]
Reduction of endogenous quinones
NQO1 plays a role in ubiquinone and vitamin E quinone metabolism. These quinones protect cellular membranes from peroxidative injury in their reduced state. Furthermore, reduced forms of ubiquinone and vitamin E quinone have been shown to possess antioxidant properties that are superior to their non-reduced forms.[16]
Polymorphisms
P187S
One widespread single-nucleotide polymorphism of the NQO1 gene (NQO1*2), found homozygous in 4% to 20% of different populations, has found to be connected with different forms of cancer and a lowered efficiency of some chemotherapeutics like mitomycin C. This single nucleotide polymorphism leads to a proline serine exchange on position 187. NAD(P)H dehydrogenase [quinone] 1 P187S has been shown to have a lowered activity and stability. Crystallographic and nuclear magnetic resonance data show that the reason for this different behaviour is found in a flexible C-terminus of the protein leading to a destabilization of the whole protein.[17] Recent pharmacological research suggests feasibility of genotype-directed redox chemotherapeutic intervention targeting NQO1*2 breast cancer.[18]
A comprehensive meta-analysis showed an association between overall cancer risk and P187S.[19]
R139W
One further single nucleotide polymorphism, found homozygous in 0% to 5% of different ethnic population, is leading to an amino acid exchange on position 139 from arginine to tryptophane.[20] Furthermore, an alternative RNA splicing site is created leading to a loss of the quinone binding site.[21]
The variant protein of NQO1*3 has similar stability as its wild-type counterpart. The variation between the two is substrate specific and it has reduced activity for some substrates.[22] It has been recently shown that the NQO1*3 polymorphism may also lead to reduced NQO1 protein expression.[11]
External (via chemicals) and internal (stress response or caloric restriction) induction of NQO1 is mediated solely through the Keap1/Nrf2/ARE. Keap1 acts as the sensor which loses its ability to target Nrf2 for degradation upon exposure to the inducers. Nrf2 is consequently stabilized and accumulated in the nucleus upon which it binds to the AREs and initiates expression of cytoprotective genes including NQO1.[24]
p53 and p73
p53 and p73 are tumor suppressor proteins and their degradation is tightly regulated by ubiquitination. Recently it was shown that their degradation can also occur via an ubiquitin-independent process;[25] NQO1 blocks p53 and p73 degradation in the presence of NADH and protects them from 20S proteasomal degradation. This protein-protein interaction between p53 and NQO1 was non-catalytic.[26]
Ornithine decarboxylase
Ornithine decarboxylase (ODC), is a labile protein that is the first rate-limiting enzyme in polyamine biosynthesis. Its degradation is regulated by antizyme that is induced by polyamine production. NQO1 has been shown to stabilize the degradation of ODC by binding to it and protecting it from 20S proteasomal degradation.
Clinical significance
Mutations in this gene have been associated with tardive dyskinesia (TD), an increased risk of hematotoxicity after exposure to benzene, and susceptibility to various forms of cancer. Altered expression of this protein has been seen in many tumors and is also associated with Alzheimer's disease (AD).[10]
Benzene toxicity
Benzene poisoning can increase risk of hematological cancers and other disorders. The mechanism of benzene metabolism and how it affects toxicity has not been completely understood. A general observation is that there is high variation in the extent of damage due to benzene poisoning. A possible explanation is the accumulation of phenols and hydroquinone in the target organ—the bone marrow—and subsequent oxidation of these metabolites to reactive quinone metabolites via a number of possible pathways.[11] A case-control study conducted in China showed that patients with two copies of the NQO1 C609T (NQO1*2 polymorphism) mutation had a 7.6-fold increased risk of benzene poisoning compared to those who carried one or two wild-type NQO1 alleles.[27]
Alzheimer's disease
Oxidative stress has been linked to onset of Alzheimer's disease (AD)[28] Since the NQO1*2 polymorphism affects the NQO1 activity and hence increase in oxidative stress, it has been postulated that this might increase the susceptibility of affected subjects for developing AD. A study conducted with a Chinese population consisting of 104 LOAD patients and 128 control patients disproved this hypothesis.[29]
Cancer
Meta-analyses have been performed to examine the association between NQO1 polymorphism and increased cancer risk.[19] The results from some of these analyses have been summarized in the table below:
^ abcRoss D, Siegel D (2004). "NAD(P)H:quinone oxidoreductase 1 (NQO1, DT-diaphorase), functions and pharmacogenetics". Quinones and Quinone Enzymes, Part B. Methods in Enzymology. Vol. 382. pp. 115–44. doi:10.1016/S0076-6879(04)82008-1. ISBN978-0-12-182786-1. PMID15047100.
^Pan SS, Forrest GL, Akman SA, Hu LT (Jan 1995). "NAD(P)H:quinone oxidoreductase expression and mitomycin C resistance developed by human colon cancer HCT 116 cells". Cancer Research. 55 (2): 330–5. PMID7812966.
^Rothman N, Smith MT, Hayes RB, Traver RD, Hoener B, Campleman S, Li GL, Dosemeci M, Linet M, Zhang L, Xi L, Wacholder S, Lu W, Meyer KB, Titenko-Holland N, Stewart JT, Yin S, Ross D (Jul 1997). "Benzene poisoning, a risk factor for hematological malignancy, is associated with the NQO1 609C-->T mutation and rapid fractional excretion of chlorzoxazone". Cancer Research. 57 (14): 2839–42. PMID9230185.
^Miranda S, Opazo C, Larrondo LF, Muñoz FJ, Ruiz F, Leighton F, Inestrosa NC (Dec 2000). "The role of oxidative stress in the toxicity induced by amyloid beta-peptide in Alzheimer's disease". Progress in Neurobiology. 62 (6): 633–48. doi:10.1016/S0301-0082(00)00015-0. hdl:10533/172411. PMID10880853. S2CID53185151.
^Wang B, Jin F, Xie Y, Tang Y, Kan R, Zheng C, Yang Z, Wang L (Dec 2006). "Association analysis of NAD(P)H:quinone oxidoreductase gene 609 C/T polymorphism with Alzheimer's disease". Neuroscience Letters. 409 (3): 179–81. doi:10.1016/j.neulet.2006.09.042. PMID17027152. S2CID19068892.
^Li C, Zhou Y (Jun 2014). "Association between NQO1 C609T polymorphism and acute lymphoblastic leukemia risk: evidence from an updated meta-analysis based on 17 case-control studies". Journal of Cancer Research and Clinical Oncology. 140 (6): 873–81. doi:10.1007/s00432-014-1595-5. PMID24488035. S2CID33710993.
^Gong M, Yi Q, Wang W (Oct 2013). "Association between NQO1 C609T polymorphism and bladder cancer susceptibility: a systemic review and meta-analysis". Tumour Biology. 34 (5): 2551–6. doi:10.1007/s13277-013-0799-7. PMID23749485. S2CID18272815.
^Yang FY, Guan QK, Cui YH, Zhao ZQ, Rao W, Xi Z (Sep 2012). "NAD(P)H quinone oxidoreductase 1 (NQO1) genetic C609T polymorphism is associated with the risk of digestive tract cancer: a meta-analysis based on 21 case-control studies". European Journal of Cancer Prevention. 21 (5): 432–41. doi:10.1097/CEJ.0b013e32834f7514. PMID22387672. S2CID41837215.
Jaiswal AK (1991). "Human NAD(P)H:quinone oxidoreductase (NQO1) gene structure and induction by dioxin". Biochemistry. 30 (44): 10647–53. doi:10.1021/bi00108a007. PMID1657151.
Traver RD, Horikoshi T, Danenberg KD, Stadlbauer TH, Danenberg PV, Ross D, Gibson NW (1992). "NAD(P)H:quinone oxidoreductase gene expression in human colon carcinoma cells: characterization of a mutation which modulates DT-diaphorase activity and mitomycin sensitivity". Cancer Res. 52 (4): 797–802. PMID1737339.
Chen LZ, Harris PC, Apostolou S, Baker E, Holman K, Lane SA, Nancarrow JK, Whitmore SA, Stallings RL, Hildebrand CE (1991). "A refined physical map of the long arm of human chromosome 16". Genomics. 10 (2): 308–12. doi:10.1016/0888-7543(91)90313-4. PMID2071140.
Maruyama K, Sugano S (1994). "Oligo-capping: a simple method to replace the cap structure of eukaryotic mRNAs with oligoribonucleotides". Gene. 138 (1–2): 171–4. doi:10.1016/0378-1119(94)90802-8. PMID8125298.
Rothman N, Smith MT, Hayes RB, Traver RD, Hoener B, Campleman S, Li GL, Dosemeci M, Linet M, Zhang L, Xi L, Wacholder S, Lu W, Meyer KB, Titenko-Holland N, Stewart JT, Yin S, Ross D (1997). "Benzene poisoning, a risk factor for hematological malignancy, is associated with the NQO1 609C-->T mutation and rapid fractional excretion of chlorzoxazone". Cancer Res. 57 (14): 2839–42. PMID9230185.
Suzuki Y, Yoshitomo-Nakagawa K, Maruyama K, Suyama A, Sugano S (1997). "Construction and characterization of a full length-enriched and a 5'-end-enriched cDNA library". Gene. 200 (1–2): 149–56. doi:10.1016/S0378-1119(97)00411-3. PMID9373149.
Smiley JF, Levey AI, Mesulam MM (1998). "Infracortical interstitial cells concurrently expressing m2-muscarinic receptors, acetylcholinesterase and nicotinamide adenine dinucleotide phosphate-diaphorase in the human and monkey cerebral cortex". Neuroscience. 84 (3): 755–69. doi:10.1016/S0306-4522(97)00524-1. PMID9579781. S2CID25807845.
Eliasson M, Boström M, DePierre JW (1999). "Levels and subcellular distributions of detoxifying enzymes in the ovarian corpus luteum of the pregnant and non-pregnant pig". Biochem. Pharmacol. 58 (8): 1287–92. doi:10.1016/S0006-2952(99)00185-9. PMID10487530.
Skelly JV, Sanderson MR, Suter DA, Baumann U, Read MA, Gregory DS, Bennett M, Hobbs SM, Neidle S (1999). "Crystal structure of human DT-diaphorase: a model for interaction with the cytotoxic prodrug 5-(aziridin-1-yl)-2,4-dinitrobenzamide (CB1954)". J. Med. Chem. 42 (21): 4325–30. doi:10.1021/jm991060m. PMID10543876.
Harada S, Fujii C, Hayashi A, Ohkoshi N (2001). "An association between idiopathic Parkinson's disease and polymorphisms of phase II detoxification enzymes: glutathione S-transferase M1 and quinone oxidoreductase 1 and 2". Biochem. Biophys. Res. Commun. 288 (4): 887–92. doi:10.1006/bbrc.2001.5868. PMID11688992.
Siegel D, Ryder J, Ross D (2001). "NAD(P)H: quinone oxidoreductase 1 expression in human bone marrow endothelial cells". Toxicol. Lett. 125 (1–3): 93–8. doi:10.1016/S0378-4274(01)00426-X. PMID11701227.
Winski SL, Koutalos Y, Bentley DL, Ross D (2002). "Subcellular localization of NAD(P)H:quinone oxidoreductase 1 in human cancer cells". Cancer Res. 62 (5): 1420–4. PMID11888914.
Begleiter A, Lange L (2002). "Lack of NQO1 induction in human tumor cells is not due to changes in the promoter region of the gene". Int. J. Oncol. 20 (4): 835–8. doi:10.3892/ijo.20.4.835. PMID11894133.