Encyclopedia
Please note this is a comparison between Version 1 by Yang Xiang and Version 2 by Peter Tang.
As the organ executing gas exchange and directly facing the external environment, the lungs are challenged continuously by various stimuli, causing the disequilibration of redox homeostasis and leading to pulmonary diseases. The breakdown of oxidants/antioxidants system happens when the overproduction of free radicals results in an excess over the limitation of cleaning capability, which could lead to the oxidative modification of macromolecules including nucleic acids. The most common type of oxidative base, 8-oxoG, is considered the marker of DNA oxidative damage. The appearance of 8-oxoG could lead to base mismatch and its accumulation might end up as tumorigenesis. The base 8-oxoG was corrected by base excision repair initiated by 8-oxoguanine DNA glycosylase-1 (OGG1), which recognizes 8-oxoG from the genome and excises it from the DNA double strand, generating an AP site for further processing. Aside from its function in DNA damage repairment, it has been reported that OGG1 takes part in the regulation of gene expression, derived from its DNA binding characteristic, and showed impacts on inflammation.
- oxidative damage
- OGG1
- pulmonary diseases
1. Introduction
Redox reaction exists widely in the manifold biochemical processes of the human body, rendering regular physiological function. Redox regulation could be achieved via controlling the activity of enzymes and transcriptional factors. Oxidative modification of the protein is the essential mechanism of redox signaling; moreover, sensing of the redox state is also part of redox regulation [1][2][1,2].
Redox homeostasis is generally crucial for life and is more of a dynamic balance. Free radicals, mainly reactive oxygen species (ROS) and reactive nitrogen species (RNS), produced under physiological conditions are detoxified and scavenged by antioxidants, thus maintaining at a moderate level [3]. Exposure to endogenous and exogenous stimulation, such as chemicals, radiation and microbiome, would disequilibrate the oxidant/antioxidant system, leading to the excessive production of free radicals to a level exceeding the antioxidant capacity, thus resulting in oxidative stress.
The lungs are one of the organs directly in contact with the external environment and are often stimulated by external factors. Being the first interface between the internal environment and the outside world, airway epithelium is the primary target of inhaling gas or particles, which has important physiological functions in the process of sensing an injury signal, innate immune defense and regulating inflammatory response [4]. It’s reasonable to stress the impact oxidative stress has on the development of pulmonary diseases.
Oxidative stress pervades all principal levels, causing damages ranging from oxidative DNA damage to protein oxidation. Under physiological conditions, ROS could act as signaling factors in certain circumstances, regulating gene expression and affecting biological processes, such as proliferation, migration and angiogenesis. While excessively produced ROS could attack macromolecules such as proteins, lipids and nucleic acids, the accumulation of oxidative modification might turn into disease. With its broad involvement, oxidative stress was universally acknowledged as taking part in the genesis of pathological processes, such as tumor growth [5], metastasis [6], inflammation and fibrosis [7], to which a series of diseases, such as Parkinson’s syndrome [8][9][8,9], Alzheimer’s disease [10][11][10,11], aging [12], cancer [13][14][15][13,14,15], obesity [16][17][16,17], cardiovascular diseases [18], pulmonary fibrosis [19], inflammatory [20], rheumatoid arthritis [21] and so on, are related (Figure 1).
Figure 1. After oxidative stress attack, 8-oxoG is the most common form of bases which could mismatch with Adenine. The Adenine mismatched with 8-oxoG would match with Thymine after the normal DNA replication cycle, generating the transformation from GC-TA. Accumulation of such a transformation could lead to pathological processes, such as fibrosis, tumor growth, metastasis and inflammation. (drawn by Figdraw).
2. The Role of OGG1 in DNA Oxidative Modification
Oxidative stress can exacerbate airway inflammatory responses by which asthma, chronic obstructive pulmonary disease (COPD) and other invasive pulmonary diseases are accompanied [22][23][22,23]. Free radicals and reactive oxygen species can attenuate the mucosal function of organs, increase endothelial permeability, reduce endothelial adhesion and affect the reconstruction of extracellular matrix. In addition to the direct action of oxidants, the oxidative stress response can also aggravate the inflammatory response through the following mechanisms: (1). Oxidants can weaken the deformation ability of neutrophils, which causes the retention and activation of neutrophils in pulmonary microcirculation [24]; (2). Oxidative stress activates transcription factor nuclear factor κB (NF-κB) and activator protein 1 (AP-1), which regulate the release of inflammatory mediators, aggravating inflammation [25]; (3). Oxidants can promote the expression of adhesion molecules on the surface of neutrophils. Neutrophils recruited in the lungs release active oxygen and protein amines after activation, causing tissue damage at the site of inflammation [26]. Currently, most studies focus on changes in protein function or abnormal lipid metabolism under oxidative stress, whereas the mechanism of DNA oxidative modification on inflammatory response is not clear.
In the course of evolution, organisms evolved complex repair pathways that maintain genome integrity and accuracy, such as base excision repair (BER), nucleotide excision repair (NER), mismatch repair (MMR), homologous recombination (HR) and non-homologous termination (NHEJ) [27][28][29][30][31][27,28,29,30,31]. When facing stimulation, DNA oxidative damage could still accumulate despite the existence of multiple repair pathways, causing gene mutations or cell death [32][33][32,33].
DNA is oxidized to produce oxidative damage leading to modified bases (oxidized bases), chain break (DNA single and double chain break) and chain cross [34]. DNA oxidative modification has site specificity. Among all four bases—cytosine, guanine, adenine and thymine, the lowest redox potential makes guanine (G) the most fragile base for oxidative stress and 7, 8-dihydro-8-oxoguanine (8-oxoG) the most common DNA oxidative damage; thus, the appearance and accumulation of 8-oxoG in DNA sequence was also considered as the biomarker of DNA oxidative damage. As is mentioned above, 8-oxoG accumulation is usually closely related to many physiological processes, and due to the stacking of π-π bonds and the interaction of electron orbitals, the ionization energy of 5’ terminal guanine in continuous guanine is reduced, so that the 5’ ends containing multiple adjacent guanines, for example, 5’G < 5’GG < 5’GGG, could be the site of 8-oxoG preferentially [35]. The DNA double helix is not affected by 8-oxoG, but it can mismatch with adenine (A) and appear GC→TA mutation in DNA replication [36].
In eukaryotes, this DNA oxidative modification can be identified by the DNA glycosylation enzyme, which can repair 8-oxoG by means of base excision repair (BER) [37]. As the initiator of BER process, OGG1 plays a pivotal role in the removal of 8-oxoG and formamidopyrimidine (Fapy-G), the ring-opened guanine [37].
The base excision repair process could be divided into two sections, the AP sites formation and the resolution of AP sites, executed by multiple enzymes. DNA glycosylation enzymes cut off the N-glycosyl bonds by hydrolase activity and remove specific damage bases to produce base-free sites (AP sites), which are further treated with endonuclease and ligase. Finally, the complete repair of the original DNA sequence is realized. There are five DNA ribozymes specifically recognizing oxidized bases in mammalian cells. They are divided into two families: the Nth family including OGG1 and NTH1, and Nei family including NEIL1, NEIL2 and NEIL3 [38]. These two families are named after homologous proteins in bacteria—endonuclease III (Nth) and endonuclease VIII (Nei) [39][40][41][39,40,41], and each have different mechanisms of de-base lyase reaction. OGG1 and NTH1 cut the DNA chain by β-lyase activity, producing 3’ dRP and 5’ P ends, whereas NEIL1/2 have βδ lyase activity, thus creating 3’ P and 5’ P in the chain gap [42].
The mechanism of NEIL3 catalysis needs further study. So far, 8-hydroxyguanine DNA glucosidase 1 (OGG1) is a DNA repair enzyme found mainly in mammals that identifies and cleaves 8-oxoG on genomic DNA. OGG1 corrects the occurrence of 8-oxoG through a series of complex and subtle repair pathways to maintain genome accuracy. In the process, OGG1 binds to DNA containing damaged bases and bends the DNA strands, from which 8-oxoG is exposed and encapsulated in an OGG1 highly conserved catalytic active pocket, which thus cuts off the N-glycoside bond [43][44][45][43,44,45]. Among them, the binding ability of the OGG1 active pocket to 8-oxoG is about 105 times higher than that of peripheral pocket to guanine, which means the damaged base first enters the secondary center after OGG1 has been extruded into a double helix and then inserts itself into the active pocket (Figure 2a) [37][46][47][48][37,46,47,48]. This explains why OGG1 can accurately distinguish between highly similar 8-oxoG and G. The mechanism of OGG1 repair of 8-oxoG is summarized as follows: OGG1 removes the N-glycoside bond of the damaged base through the above recognition process, acting on a sugar-phosphate backbone to form an apurinic/apyrimidinic (AP)-site(s) (Figure 2b). Then, using its own weak AP lyase, OGG1 shows β-elimination thereby forming 5’ P (phosphate) and 3’ d RP (unsaturated hydroxyl aldehyde) ends. Because the 3’ end residue does not make a normal connection, OGG1 recruits apurinic/apyrimidinic endonuclease 1 (APE1) to clean up the 3’ end and get 3’-OH ends. At this point, DNA polymerase β inserts the correct nucleotide through the polymerization and DNA ligase III, and DNA ligase I connects the DNA chains, completing the base excision repair initiated by the OGG1 [42].
Figure 2. The recognition and excision of 8-oxoG. (a) The lesion-specific binding pocket of OGG1 allows the recognition of the extrahelical flipped 8-oxoG, and excises it from DNA strand (drawn by Figdraw). (b) OGG1 acts on a sugar-phosphate backbone to form AP-sites, generating 5’ P ends and 3’ d RP ends.
3. The Roles of Base Excision Repair Enzyme OGG1 in Gene Expression
Guanine is readily oxidized, thereby disrupting the integrity of the genome. From an evolutionary perspective, vertebrate genomes contain more guanine than other organisms. OxiDIP-Seq of human MCF10A cells shows that 8-oxoG is significantly enriched in the intergenic region and the intron region of gene body, and most of the 8-oxoG are enriched in protein coding DNA [49]. In total, 72% of human promoter regions are spread with higher GC base pairs [50], including gene-coding pro-inflammatory factors, proto-oncogenes and growth factors. Some conserved sequences recognized by transcription factors are also enriched with guanine. For example, transcription factor specificity protein 1 (SP-1) recognizes GC-rich sequences 5’-GGGGCGGGG -3’, and κB sequences identified by NF-κB have a 5’-GGGRNYYYCC-3’ pattern, of which 5’ ends are distributed with continuous guanine [51]. In recent years, there is growing evidence that 8-oxoG and its specific repair protein OGG1 have epigenetic regulatory effects on gene transcription. Some researchers found that Ogg1-/-mice shows significant 8-oxoG accumulation; however, it does not lead to cancer or affect life span and embryo development. Surprisingly, the inflammatory response in Ogg1-/-mice was also slighter [52], indicating that OGG1 function is closely related to promoting inflammatory cell recruitment and promoting cytokine expression.
Experimental evidence was widely reported in recent years. Researchers found an abnormal increase in 8-oxoG accumulation and a high spontaneous rate of lung cancer in the genome of Ogg1-/-mice [52]. The reduced immune response of Ogg1-/-mice models to endotoxic shock, diabetes and hypersensitivity manifested as decreased neutrophil infiltration and lower expressions of Th1 and Th2 cytokines [53]. Moreover, inflammation of the Ogg1-/-mice stomach was slighter after H. pylori infection when comparing to wildtype. Similarly, in an asthma model, the cell infiltration in the airway, perivascular and alveoli and the secretion of Th1, Th2 and Th17 cytokines of Ogg1-/-mice were slighter [54].
The small molecule inhibitor of OGG1,TH5487 is an effective selective active site binding agent, which inhibits the activity of OGG1 DNA glycosylase, so OGG1 cannot insert itself into the DNA chain and join the 8-oxoG, and also reduces the DNA occupation of NF-κB and the level of TNFα-induced neutrophil recruitment and various cytokines in human bronchial epithelial cells and in mice models [55]. In 2019, Pan et al. [56] showed that in lung inflammation challenged by ozone, levels of 8-oxoG in genomes gradually ascend. HE staining showed that with the prolongation of ozone stress, more inflammatory cells infiltrated airways around the alveoli, and other manifestations such as airway stenosis, partial alveolar rupture and enlarged alveolar cavity arose. After five days of ozone stress, 8-oxoG accumulated in the airway epithelium and showed an increased oxidation level of protein. The OGG1 inhibitor TH5487 treatment significantly improved above phenomenon and manifested as reduced infiltration of inflammatory cells in the airways, surrounding alveoli and the complete alveolar structure. There are also other studies reporting small molecular inhibitors designed for OGG1, which provide a bright outlook for clinical anti-inflammatory treatment [57].
The aforementioned studies suggest that OGG1 has immunomodulatory functions, which are closely related to promoting inflammatory cell recruitment and cytokine expression. Studying the role of 8-oxoG and its repair enzyme OGG1 in pulmonary inflammation plays an important role in elucidating the mechanism of pulmonary inflammation and developing new therapeutic targets for chronic pulmonary inflammation from the perspective of oxidative stress.
