Cellular levels of 8-oxoguanine in either DNA or the nucleotide pool play pivotal roles in carcinogenesis and survival of cancer cells

The m6A methylation perturbs the Hoogsteen pairing-guided incorporation of an oxidized nucleotide

This study describes the structural implications and properties of m⁶A in reducing the incorporation of an oxidized nucleotide into DNA.

Natural nucleic acid bases can form Watson–Crick (WC) or Hoogsteen (HG) base pairs. Importantly, 8-oxo-2′-deoxyguanosine (8-oxo-dG) in DNA or 8-oxo-dG 5′-triphosphate (8-oxo-dGTP) favors a syn conformation because of the steric repulsion between O8 and O4′ of the deoxyribose ring. 8-oxo-dGTP can be incorporated into DNA opposite the templating adenine (A) using HG pairing as the dominant mechanism. Both RNA and DNA can be methylated at the N6 position of A to form N⁶-methyladenine (m⁶A). It has been found that certain viral infections may trigger an increase in the production of both 8-oxo-dGTP and m⁶A. The current study aims to systematically explore the effects of m⁶A methylation on HG base pairs and the consequent nucleotide incorporation. Our thermodynamic melting study shows that the m⁶A·8-oxo-dG is significantly less stable than the A·8-oxo-dG base pair in the paired region of a DNA duplex. Moreover, we have used pre-steady-state kinetics to examine the incorporation of 8-oxo-dGTP opposite m⁶A relative to A by a variety of reverse transcriptase (RT) enzymes and DNA polymerase (DNA pol) enzymes such as the human immunodeficiency virus type 1 (HIV-1) RT and human DNA pol β. The results demonstrate that all of these enzymes incorporate 8-oxo-dGTP less efficiently opposite m⁶A relative to A. Considering the steric bulk of the purine–purine pair between 8-oxo-dG and A, m⁶A methylation may affect the HG pairing to a great extent. Hence, it will be unfavorable to incorporate 8-oxo-dGTP into the growing strand opposite m⁶A. Moreover, the impeded incorporation of 8-oxo-dGTP opposite m⁶A has been extended to determine m⁶A at pre-defined positions in human rRNA. Our study may provide new insights into the roles of m⁶A in reducing the mutagenic potential of cellular 8-oxo-dGTP.

Neuronal P2X7 receptor-induced reactive oxygen species production contributes to nociceptive behavior in mice

ATP can activate a variety of pathways through P2 purinoreceptors, leading to neuroprotection and pathology in the CNS. Among all P2X receptors, the P2X7 receptor (P2X7R) is a well-defined therapeutic target for inflammatory and neuropathic pain. Activation of P2X7R can generate reactive oxygen species (ROS) in macrophages and microglia. However, the role of ROS in P2X7R–induced pain remains unexplored. Here, we investigated the downstream effects of neuronal P2X7R activation in the spinal cord. We found that ATP induces ROS production in spinal cord dorsal horn neurons, an effect eliminated by ROS scavenger N-tert-butyl-α-phenylnitrone (PBN) and P2X7R antagonist A438079. A similar effect was observed with a P2X7R agonist, BzATP, and was attenuated by a NADPH oxidase inhibitor apocynin. Intrathecal administration of BzATP resulted in ROS production in the spinal cord and oxidative DNA damage in dorsal horn neurons. BzATP also induced robust biphasic spontaneous nociceptive behavior. Pre-treatment with A438079 abolished all BzATP-induced nociceptive behaviors, while ROS scavengers dose-dependently attenuated the secondary response. Here, we provide evidence that neuronal P2X7R activation leads to ROS production and subsequent nociceptive pain in mice. Together, the data indicate that P2X7R-induced ROS play a critical role in the P2X7R signaling pathway of the CNS.

MTH1 as a nucleotide pool sanitizing enzyme: Friend or foe?

8-Oxo-7,8-dihydroguanine (GO) can originate as 8-oxo-7,8-dihydro-2'-deoxyguanosine 5'-triphosphate (8-oxo-dGTP), an oxidized form of dGTP in the nucleotide pool, or by direct oxidation of guanine base in DNA. Accumulation of GO in cellular genomes can result in mutagenesis or programmed cell death, and is thus minimized by the actions of MutT homolog-1 (MTH1) with 8-oxo-dGTPase, OGG1 with GO DNA glycosylase and MutY homolog (MUTYH) with adenine DNA glycosylase. Studies on Mth1/Ogg1/Mutyh-triple knockout mice demonstrated that the defense systems efficiently minimize GO accumulation in cellular genomes, and thus maintain low incidences of spontaneous mutagenesis and tumorigenesis. Mth1/Ogg1-double knockout mice increased GO accumulation in the genome, but exhibited little susceptibility to spontaneous tumorigenesis, thus revealing that accumulation of GO in cellular genomes induces MUTYH-dependent cell death. Cancer cells are exposed to high oxidative stress levels and accumulate a high level of 8-oxo-dGTP in their nucleotide pools; cancer cells consequently express increased levels of MTH1 to eliminate 8-oxo-dGTP, indicating that increased expression of MTH1 in cancer cells may be detrimental for cancer patients. Mth1/Ogg1-double knockout mice are highly vulnerable to neurodegeneration under oxidative conditions, while transgenic expression of human MTH1 efficiently prevents neurodegeneration by avoiding GO accumulation in mitochondrial genomes of neurons and/or nuclear genomes of microglia, indicating that increased expression of MTH1 may be beneficial for neuronal tissues.

Aberrant base excision repair pathway of oxidatively damaged DNA: Implications for degenerative diseases

In cellular organisms composition of DNA is constrained to only four nucleobases A, G, T and C, except for minor DNA base modifications such as methylation which serves for defence against foreign DNA or gene expression regulation. Interestingly, this severe evolutionary constraint among other things demands DNA repair systems to discriminate between regular and modified bases. DNA glycosylases specifically recognize and excise damaged bases among vast majority of regular bases in the base excision repair (BER) pathway. However, the mismatched base pairs in DNA can occur from a spontaneous conversion of 5-methylcytosine to thymine and DNA polymerase errors during replication. To counteract these mutagenic threats to genome stability, cells evolved special DNA repair systems that target the non-damaged DNA strand in a duplex to remove mismatched regular DNA bases. Mismatch-specific adenine- and thymine-DNA glycosylases (MutY/MUTYH and TDG/MBD4, respectively) initiated BER and mismatch repair (MMR) pathways can recognize and remove normal DNA bases in mismatched DNA duplexes. Importantly, in DNA repair deficient cells bacterial MutY, human TDG and mammalian MMR can act in the aberrant manner: MutY and TDG removes adenine and thymine opposite misincorporated 8-oxoguanine and damaged adenine, respectively, whereas MMR removes thymine opposite to O⁶-methylguanine. These unusual activities lead either to mutations or futile DNA repair, thus indicating that the DNA repair pathways which target non-damaged DNA strand can act in aberrant manner and introduce genome instability in the presence of unrepaired DNA lesions. Evidences accumulated showing that in addition to the accumulation of oxidatively damaged DNA in cells, the aberrant DNA repair can also contribute to cancer, brain disorders and premature senescence. For example, the aberrant BER and MMR pathways for oxidized guanine residues can lead to trinucleotide expansion that underlies Huntington's disease, a severe hereditary neurodegenerative syndrome. This review summarises the present knowledge about the aberrant DNA repair pathways for oxidized base modifications and their possible role in age-related diseases.

Repair of 8-oxoG:A mismatches by the MUTYH glycosylase: Mechanism, metals and medicine

Reactive oxygen and nitrogen species (RONS) may infringe on the passing of pristine genetic information by inducing DNA inter- and intra-strand crosslinks, protein-DNA crosslinks, and chemical alterations to the sugar or base moieties of DNA. 8-Oxo-7,8-dihydroguanine (8-oxoG) is one of the most prevalent DNA lesions formed by RONS and is repaired through the base excision repair (BER) pathway involving the DNA repair glycosylases OGG1 and MUTYH in eukaryotes. MUTYH removes adenine (A) from 8-oxoG:A mispairs, thus mitigating the potential of G:C to T:A transversion mutations from occurring in the genome. The paramount role of MUTYH in guarding the genome is well established in the etiology of a colorectal cancer predisposition syndrome involving variants of MUTYH, referred to as MUTYH-associated polyposis (MAP). In this review, we highlight recent advances in understanding how MUTYH structure and related function participate in the manifestation of human disease such as MAP. Here we focus on the importance of MUTYH's metal cofactor sites, including a recently discovered "Zinc linchpin" motif, as well as updates to the catalytic mechanism. Finally, we touch on the insight gleaned from studies with MAP-associated MUTYH variants and recent advances in understanding the multifaceted roles of MUTYH in the cell, both in the prevention of mutagenesis and tumorigenesis.

Repair of 8-oxo-7,8-dihydroguanine in prokaryotic and eukaryotic cells: Properties and biological roles of the Fpg and OGG1 DNA N-glycosylases

Oxidatively damaged DNA results from the attack of sugar and base moieties by reactive oxygen species (ROS), which are formed as byproducts of normal cell metabolism and during exposure to endogenous or exogenous chemical or physical agents. Guanine, having the lowest redox potential, is the DNA base the most susceptible to oxidation, yielding products such as 8-oxo-7,8-dihydroguanine (8-oxoG) and 2-6-diamino-4-hydroxy-5-formamidopyrimidine (FapyG). In DNA, 8-oxoG was shown to be mutagenic yielding GC to TA transversions upon incorporation of dAMP opposite this lesion by replicative DNA polymerases. In prokaryotic and eukaryotic cells, 8-oxoG is primarily repaired by the base excision repair pathway (BER) initiated by a DNA N-glycosylase, Fpg and OGG1, respectively. In Escherichia coli, Fpg cooperates with MutY and MutT to prevent 8-oxoG-induced mutations, the "GO-repair system". In Saccharomyces cerevisiae, OGG1 cooperates with nucleotide excision repair (NER), mismatch repair (MMR), post-replication repair (PRR) and DNA polymerase η to prevent mutagenesis. Human and mouse cells mobilize all these pathways using OGG1, MUTYH (MutY-homolog also known as MYH), MTH1 (MutT-homolog also known as NUDT1), NER, MMR, NEILs and DNA polymerases η and λ, to prevent 8-oxoG-induced mutations. In fact, mice deficient in both OGG1 and MUTYH develop cancer in different organs at adult age, which points to the critical impact of 8-oxoG repair on genetic stability in mammals. In this review, we will focus on Fpg and OGG1 proteins, their biochemical and structural properties as well as their biological roles. Other DNA N-glycosylases able to release 8-oxoG from damaged DNA in various organisms will be discussed. Finally, we will report on the role of OGG1 in human disease and the possible use of 8-oxoG DNA N-glycosylases as therapeutic targets.

Association of MTH1 expression with the tumor malignant potential and poor prognosis in patients with resected lung cancer

OBJECTIVES

The oxidized purine nucleoside triphosphatase, mutT homolog 1 (MTH1), physiologically sanitizes 8-oxo-dGTP in the nucleotide pool. Previous studies indicated that MTH1 is associated with tumor proliferation and invasion in non-small cell lung cancer (NSCLC) cell lines; however, the role of MTH1 in patients with NSCLC remains unclear.

MATERIALS AND METHODS

Two patient cohorts that underwent surgery for NSCLC in our institution were investigated retrospectively. In one cohort consisting of 197 patients, the associations between MTH1 expression and clinicopathological factors or prognosis were analyzed. In another cohort consisting of 41 patients, the relationship between MTH1 expression in the tumors and serum oxidative stress levels (evaluated by the diacron-reactive oxygen metabolites [d-ROMs] test) or antioxidant capacity in the patients (evaluated by the biological antioxidant potential (BAP) test) was analyzed. A total of 238 patients were assessed for MTH1 protein levels using immunohistochemistry.

RESULTS

Among the 197 patients in the former cohort, 111 (56.3%) exhibited high MTH1 expression, while 86 (43.7%) exhibited low MTH1 expression. Male sex, smoking habit of ≥20 pack-years, squamous cell carcinoma, pathological stage ≥ II, tumor diameter ≥30mm, lymph node metastases, pleural invasion, lymphatic permeation and vascular infiltration were significantly associated with high MTH1 expression (p<0.05). The high MTH1 expression group had a significantly worse prognosis than that of the low MTH1 expression group (5-year overall survival: 81.6% vs. 92.3%, p=0.0011; 5-year disease-free survival: 55.0% vs. 83.7%, p=0.0002). d-ROMs and BAP test values were significantly higher in the high than in the low MTH1 expression group (p<0.05).

CONCLUSION

This study showed that MTH1 protein expression was closely related to factors associated with a high malignant potential and poor patient survival. MTH1 may be a novel therapeutic target for NSCLC.

Autoantibody biomarkers for the detection of serous ovarian cancer

Objective The purpose of this study was to identify a panel of novel serum tumor antigen-associated autoantibody (TAAb) biomarkers for the diagnosis of high-grade serous ovarian cancer.

METHODS

To detect TAAb we probed high-density programmable protein microarrays (NAPPA) containing 10,247 antigens with sera from patients with serous ovarian cancer (n=30 cases/30 healthy controls) and measured bound IgG. We identified 735 promising tumor antigens and evaluated these with an independent set of serous ovarian cancer sera (n=30 cases/30 benign disease controls/30 healthy controls). Thirty-nine potential tumor autoantigens were identified and evaluated using an orthogonal programmable ELISA platform against a total of 153 sera samples (n=63 cases/30 benign disease controls/60 healthy controls). Sensitivities at 95% specificity were calculated and a classifier for the detection of high-grade serous ovarian cancer was constructed.

RESULTS

We identified 11-TAAbs (ICAM3, CTAG2, p53, STYXL1, PVR, POMC, NUDT11, TRIM39, UHMK1, KSR1, and NXF3) that distinguished high-grade serous ovarian cancer cases from healthy controls with a combined 45% sensitivity at 98% specificity.

CONCLUSION

These are potential circulating biomarkers for the detection of serous ovarian cancer, and warrant confirmation in larger clinical cohorts.

The anti/syn conformation of 8-oxo-7,8-dihydro-2'-deoxyguanosine is modulated by Bacillus subtilis PolX active site residues His255 and Asn263. Efficient processing of damaged 3'-ends

8-oxo-7,8-dihydro-2'-deoxyguanosine (8oxodG) is a major lesion resulting from oxidative stress and found in both DNA and dNTP pools. Such a lesion is usually removed from DNA by the Base Excision Repair (BER), a universally conserved DNA repair pathway. 8oxodG usually adopts the favored and promutagenic syn-conformation at the active site of DNA polymerases, allowing the base to hydrogen bonding with adenine during DNA synthesis. Here, we study the structural determinants that affect the glycosidic torsion-angle of 8oxodGTP at the catalytic active site of the family X DNA polymerase from Bacillus subtilis (PolXBs). We show that, unlike most DNA polymerases, PolXBs exhibits a similar efficiency to stabilize the anti and syn conformation of 8oxodGTP at the catalytic site. Kinetic analyses indicate that at least two conserved residues of the nucleotide binding pocket play opposite roles in the anti/syn conformation selectivity, Asn263 and His255 that favor incorporation of 8oxodGMP opposite dA and dC, respectively. In addition, the presence in PolXBs of Mn²⁺-dependent 3'-phosphatase and 3'-phosphodiesterase activities is also shown. Those activities rely on the catalytic center of the C-terminal Polymerase and Histidinol Phosphatase (PHP) domain of PolXBs and, together with its 3'-5' exonuclease activity allows the enzyme to resume gap-filling after processing of damaged 3' termini.

Expression and function of MutT homolog 1 in distinct subtypes of breast cancer

Human MutT homolog 1 (MTH1) detoxifies the oxidized DNA precursor 8-oxo-2′-deoxyguanosine-5′-triphosphate and serves a tumor suppressive role in distinct types of cancer. In the present study, the expression of MTH1 was examined in various subtypes of breast cancer, and the effect of its suppression on breast cancer growth was characterized in vitro and in vivo. MTH1 mRNA and protein levels were assessed using the reverse transcription-quantitative polymerase chain reaction and immunohistochemistry. The effect of MTH1 expression on the proliferation of breast cancer cells was investigated in vitro using Cell Counting Kit-8 and colony formation assays, and in vivo using breast cancer cell line xenografts in mice. The toxicity of the MTH1 inhibitor TH588 was investigated in nude mice. A marked increase in MTH1 protein and mRNA levels was demonstrated in breast cancer tissues compared with the non-cancerous control. However, no apparent differences in MTH1 expression were observed between distinct molecular subtypes of breast cancer. MTH1 overexpression was demonstrated to be independent of patient age, tumor size and lymph node metastasis. Inhibition of MTH1 decreased cancer cell viability and the clonogenic potential of cancer cells in a dose-dependent manner. These results were confirmed by decreased in vivo proliferation of MCF7, MDA-MB-231 and MDA-MB-453 cancer cell lines, representing distinct subtypes of breast cancer. Although inhibition of MTH1 activity decreased xenograft growth in mice, no major adverse effects of TH588 were detected on the basis of blood biochemistry, and liver and kidney function. The results of the present study suggested that MTH1 is overexpressed in the majority of breast cancers, independent of the molecular identity and clinicopathological features of the tumor, including patient age, tumor size and lymph node metastasis. Inhibition of MTH1 activity suppressed the growth of three subtypes of breast cancer, including luminal, basal-like and human epidermal growth factor receptor 2-positive, in vitro and in vivo. Treatment with the MTH1 inhibitor appears to be safe; however, further studies are required prior to the clinical use of MTH1 inhibitors.

Chenodeoxycholic Acid Derivative HS-1200 Inhibits Hepatocarcinogenesis and Improves Liver Function in Diethylnitrosamine-Exposed Rats by Downregulating MTH1

Aim. To investigate the effects of HS-1200 on liver tumorigenesis and liver function in a diethylnitrosamine- (DEN-) induced hepatocellular carcinoma (HCC) rat model. Methods. Rats were randomly assigned into five groups: control, HS-1200, HCC, HCC + low dose HS-1200, and HCC + high dose HS-1200 groups. Rat HCC model was established by intraperitoneal injection of DEN. And rats were given HS-1200 by daily oral gavage. After 20 weeks, we examined animal body weight, liver weight, liver pathological changes, serum levels of AST, ALT, and AFP, and mutT homologue gene 1 (MTH1) in liver tissue. Results. Oral gavage of HS-1200 significantly increased animal body weight and decreased liver weight as well as liver coefficient in HCC rats (P < 0.05 versus HCC group). Moreover, oral administration of HS-1200 suppressed tumorigenesis, attenuated pathological changes in liver tissues, and decreased serum levels of AST, ALT, and AFP in HCC rats (P < 0.05 versus HCC group). In addition, the mRNA level of MTH1 was upregulated in the liver tissues of HCC rats (P < 0.05 versus control group), which was reversed by HS-1200 treatment in a dose-dependent manner (P < 0.05 versus HCC group). Conclusions. HS-1200 inhibits hepatocarcinogenesis and improves liver function maybe by inducing downregulation of MTH1.

Structural and Kinetic Studies of the Human Nudix Hydrolase MTH1 Reveal the Mechanism for Its Broad Substrate Specificity

The human MutT homolog 1 (hMTH1, human NUDT1) hydrolyzes oxidatively damaged nucleoside triphosphates and is the main enzyme responsible for nucleotide sanitization. hMTH1 recently has received attention as an anticancer target because hMTH1 blockade leads to accumulation of oxidized nucleotides in the cell, resulting in mutations and death of cancer cells. Unlike Escherichia coli MutT, which shows high substrate specificity for 8-oxoguanine nucleotides, hMTH1 has broad substrate specificity for oxidized nucleotides, including 8-oxo-dGTP and 2-oxo-dATP. However, the reason for this broad substrate specificity remains unclear. Here, we determined crystal structures of hMTH1 in complex with 8-oxo-dGTP or 2-oxo-dATP at neutral pH. These structures based on high quality data showed that the base moieties of two substrates are located on the similar but not the same position in the substrate binding pocket and adopt a different hydrogen-bonding pattern, and both triphosphate moieties bind to the hMTH1 Nudix motif (i.e. the hydrolase motif) similarly and align for the hydrolysis reaction. We also performed kinetic assays on the substrate-binding Asp-120 mutants (D120N and D120A), and determined their crystal structures in complex with the substrates. Analyses of bond lengths with high-resolution X-ray data and the relationship between the structure and enzymatic activity revealed that hMTH1 recognizes the different oxidized nucleotides via an exchange of the protonation state at two neighboring aspartate residues (Asp-119 and Asp-120) in its substrate binding pocket. To our knowledge, this mechanism of broad substrate recognition by enzymes has not been reported previously and may have relevance for anticancer drug development strategies targeting hMTH1.

Prognostic impact of MutT homolog-1 expression on esophageal squamous cell carcinoma

MutT homolog‐1 (MTH1) is a pyrophosphatase that acts on oxidized nucleotides and hydrolyzes 8‐oxo‐2’‐deoxyguanosine triphosphate in deoxynucleoside triphosphate pool to prevent its incorporation into nuclear and mitochondrial DNA, result in reduce cytotoxicity in tumor cells. MTH1 is overexpressed in various cancers and is considered as a therapeutic target. Environmental factors such as cigarette smoking and alcohol consumption are critical risk factors for the development and progression of esophageal squamous cell carcinoma (ESCC), suggesting that oxidative stress contributes to the pathogenesis of ESCC. We examined the expression of MTH1 and the accumulation of 8‐oxo‐2’‐deoxyguanosine (8‐oxo‐dG) in 84 patients with ESCC who underwent curative resection without neoadjuvant therapy. MTH1 mRNA level was quantified by performing quantitative reverse transcription‐PCR. Immunohistochemical analysis of paraffin‐embedded cancer tissues was performed to determine MTH1 protein expression and 8‐oxo‐dG accumulation. MTH1 mRNA expression was higher in cancerous tissues than in the corresponding normal epithelium (P < 0.0001). Immunohistochemical analysis showed that high MTH1 expression was significantly associated with deeper tumor invasion and venous invasion, advanced cancer stage, and poor overall survival (P = 0.0021) and disease‐specific survival (P = 0.0013) compared with low MTH1 expression. Furthermore, high MTH1 expression was an independent predictor of poor disease‐specific survival (P = 0.0121). In contrast, 8‐oxo‐dG accumulation was not associated with any clinicopathological factor and poor prognosis. These results suggest that MTH1 overexpression is a predictor of ESCC progression and poor prognosis and that MTH1 can serve as a therapeutic target for treating patients with ESCC.

Mutations induced by 8-hydroxyguanine (8-oxo-7,8-dihydroguanine), a representative oxidized base, in mammalian cells

Guanine oxidation occurs in both DNA and the cellular nucleotide pool, and one of the major products is 8-hydroxyguanine (8-oxo-7,8-dihydroguanine). The mutagenic potentials of this oxidized base have been examined in various experimental systems. In this review, we summarize the mutagenicity of the base in mammalian cells. We also describe the effects of specialized DNA polymerases, DNA repair proteins, and nucleotide pool sanitization enzymes.

Human mitochondrial transcriptional factor A breaks the mitochondria-mediated vicious cycle in Alzheimer's disease

In the mitochondria-mediated vicious cycle of Alzheimer’s disease (AD), intracellular amyloid β (Aβ) induces mitochondrial dysfunction and reactive oxygen species, which further accelerate Aβ accumulation. This vicious cycle is thought to play a pivotal role in the development of AD, although the molecular mechanism remains unclear. Here, we examined the effects of human mitochondrial transcriptional factor A (hTFAM) on the pathology of a mouse model of AD (3xTg-AD), because TFAM is known to protect mitochondria from oxidative stress through maintenance of mitochondrial DNA (mtDNA). Expression of hTFAM significantly improved cognitive function, reducing accumulation of both 8-oxoguanine, an oxidized form of guanine, in mtDNA and intracellular Aβ in 3xTg-AD mice and increasing expression of transthyretin, known to inhibit Aβ aggregation. Next, we found that AD model neurons derived from human induced pluripotent stem cells carrying a mutant PSEN1_(P117L) gene, exhibited mitochondrial dysfunction, accumulation of 8-oxoguanine and single-strand breaks in mtDNA, and impaired neuritogenesis with a decreased expression of transthyretin, which is known to be downregulated by oxidative stress. Extracellular treatment with recombinant hTFAM effectively suppressed these deleterious outcomes. Moreover, the treatment increased expression of transthyretin, accompanied by reduction of intracellular Aβ. These results provide new insights into potential novel therapeutic targets.

MUTYH promotes oxidative microglial activation and inherited retinal degeneration

Oxidative stress is implicated in various neurodegenerative disorders, including retinitis pigmentosa (RP), an inherited disease that causes blindness. The biological and cellular mechanisms by which oxidative stress mediates neuronal cell death are largely unknown. In a mouse model of RP (rd10 mice), we show that oxidative DNA damage activates microglia through MutY homolog-mediated (MUYTH-mediated) base excision repair (BER), thereby exacerbating retinal inflammation and degeneration. In the early stage of retinal degeneration, oxidative DNA damage accumulated in the microglia and caused single-strand breaks (SSBs) and poly(ADP-ribose) polymerase activation. In contrast, Mutyh deficiency in rd10 mice prevented SSB formation in microglia, which in turn suppressed microglial activation and photoreceptor cell death. Moreover, Mutyh-deficient primary microglial cells attenuated the polarization to the inflammatory and cytotoxic phenotype under oxidative stress. Thus, MUTYH-mediated BER in oxidative microglial activation may be a novel target to dampen the disease progression in RP and other neurodegenerative disorders that are associated with oxidative stress.

Excessive Reactive Oxygen Species and Exotic DNA Lesions as an Exploitable Liability

Although the terms "excessive reactive oxygen species (ROS)" and "oxidative stress" are widely used, the implications of oxidative stress are often misunderstood. ROS are not a single species but a variety of compounds, each with unique biochemical properties and abilities to react with biomolecules. ROS cause activation of growth signals through thiol oxidation and may lead to DNA damage at elevated levels. In this review, we first discuss a conceptual framework for the interplay of ROS and antioxidants. This review then describes ROS signaling using FLT3-mediated growth signaling as an example. We then focus on ROS-mediated DNA damage. High concentrations of ROS result in various DNA lesions, including 8-oxo-7,8-dihydro-guanine, oxazolone, DNA-protein cross-links, and hydantoins, that have unique biological impacts. Here we delve into the biochemistry of nine well-characterized DNA lesions. Within each lesion, the types of repair mechanisms, the mutations induced, and their effects on transcription and replication are discussed. Finally, this review will discuss biochemically inspired implications for cancer therapy. Several teams have put forward designs to harness the excessive ROS and the burdened DNA repair systems of tumor cells for treating cancer. We discuss inhibition of the antioxidant system, the targeting of DNA repair, and ROS-activated prodrugs.

Birth of MTH1 as a therapeutic target for glioblastoma: MTH1 is indispensable for gliomatumorigenesis

Malignant glioma is the most common primary tumor of the central nervous system. Chemotherapy and radiotherapy are the most common therapeutic approaches in glioma therapy. Both processes mainly kill cancer cells through generating high Reactive Oxygen Species (ROS) and lead to oxidative DNA damage. However, tumor resistance to ROS is always a challenge for cancer treatment. Human Mut T homolog 1 (MTH1, also known as NUDT1) is regarded as a protector of nucleotides against oxidization. Recent reports have verified that overexpression of MTH1 could remove oxidized dNTP pools. Here, we find that MTH1 is overexpressed both at mRNA and protein levels in GBM. MTH1 silencing inhibits colony formation; tumor spheres formation and xenograft tumor growth, and more importantly, the viability of glioma cells is significantly decreased in H2O2 after MTH1 was knocked down in glioma. PI staining show that H2O2 cause more glioma cell death after MTH1 silencing. So we speculate that overexpression of MTH1 is crucial for glioma survival, suppression of its expression can inhibit cancer cell survival in vitro and in vivo, MTH1 may be a potential target for human glioma therapy in future.

Proteomic profiling of small-molecule inhibitors reveals dispensability of MTH1 for cancer cell survival

Since recent publications suggested that the survival of cancer cells depends on MTH1 to avoid incorporation of oxidized nucleotides into the cellular DNA, MTH1 has attracted attention as a potential cancer therapeutic target. In this study, we identified new purine-based MTH1 inhibitors by chemical array screening. However, although the MTH1 inhibitors identified in this study targeted cellular MTH1, they exhibited only weak cytotoxicity against cancer cells compared to recently reported first-in-class inhibitors. We performed proteomic profiling to investigate the modes of action by which chemically distinct MTH1 inhibitors induce cancer cell death, and found mechanistic differences among the first-in-class MTH1 inhibitors. In particular, we identified tubulin as the primary target of TH287 and TH588 responsible for the antitumor effects despite the nanomolar MTH1-inhibitory activity in vitro. Furthermore, overexpression of MTH1 did not rescue cells from MTH1 inhibitor–induced cell death, and siRNA-mediated knockdown of MTH1 did not suppress cancer cell growth. Taken together, we conclude that the cytotoxicity of MTH1 inhibitors is attributable to off-target effects and that MTH1 is not essential for cancer cell survival.

Induction of ROS Overload by Alantolactone Prompts Oxidative DNA Damage and Apoptosis in Colorectal Cancer Cells

Cancer cells typically display higher than normal levels of reactive oxygen species (ROS), which may promote cancer development and progression but may also render the cancer cells more vulnerable to further ROS insult. Indeed, many of the current anticancer therapeutics kill cancer cells via induction of oxidative stress, though they target both cancer and normal cells. Recently, alantolactone (ATL), a natural sesquiterpene lactone, has been shown to induce apoptosis by increasing ROS levels specifically in cancer cells; however, the molecular mechanisms linking ROS overproduction to apoptosis remain unclear. Here we show that the ATL-induced ROS overload in human SW480 and SW1116 colorectal cancer cells was followed by a prominent accumulation of cellular oxidized guanine (8-oxoG) and immediate increase in the number of DNA strand breaks, indicating that increased ROS resulted in extensive oxidative DNA damage. Consequently, the G₁/S-CDK suppresser CDKN1B (p21) and pro-apoptotic proteins Bax and activated caspase-3 were upregulated, while anti-apoptotic Bcl-2 was downregulated, which were followed by cell cycle arrest at G₁ and marked apoptosis in ATL-treated cancer but not non-cancer cells. These results suggest that the ATL-induced ROS overload triggers cell death through induction of massive oxidative DNA damage and subsequent activation of the intrinsic apoptosis pathway.

8-Oxoguanine accumulation in mitochondrial DNA causes mitochondrial dysfunction and impairs neuritogenesis in cultured adult mouse cortical neurons under oxidative conditions

Oxidative stress and mitochondrial dysfunction are implicated in aging-related neurodegenerative disorders. 8-Oxoguanine (8-oxoG), a common oxidised base lesion, is often highly accumulated in brains from patients with neurodegenerative disorders. MTH1 hydrolyses 8-oxo-2'-deoxyguanosine triphosphate (8-oxo-dGTP) to 8-oxo-dGMP and pyrophosphate in nucleotide pools, while OGG1 excises 8-oxoG paired with cytosine in DNA, thereby minimising the accumulation of 8-oxoG in DNA. Mth1/Ogg1-double knockout (TO-DKO) mice are highly susceptible to neurodegeneration under oxidative conditions and show increased accumulation of 8-oxoG in mitochondrial DNA (mtDNA) in neurons, suggesting that 8-oxoG accumulation in mtDNA causes mitochondrial dysfunction. Here, we evaluated the contribution of MTH1 and OGG1 to the prevention of mitochondrial dysfunction during neuritogenesis in vitro. We isolated cortical neurons from adult wild-type and TO-DKO mice and maintained them with or without antioxidants for 2 to 5 days and then examined neuritogenesis. In the presence of antioxidants, both TO-DKO and wild-type neurons exhibited efficient neurite extension and arborisation. However, in the absence of antioxidants, the accumulation of 8-oxoG in mtDNA of TO-DKO neurons was increased resulting in mitochondrial dysfunction. Cells also exhibited poor neurite outgrowth with decreased complexity of neuritic arborisation, indicating that MTH1 and OGG1 are essential for neuritogenesis under oxidative conditions.

Inhibitory Effect of 8-Halogenated 7-Deaza-2'-deoxyguanosine Triphosphates on Human 8-Oxo-2'-deoxyguanosine Triphosphatase, hMTH1, Activities

hMTH1 (8-oxo-2'-deoxyguanine triphosphatase) hydrolyzes oxidized nucleoside triphosphates; its presence is non-essential for survival of normal cells but is required for survival of cancer cells. In this study, 8-halogenated-7-deaza-2'-deoxyguanosine triphosphate (8-halogenated-7-deazadGTP) derivatives were synthesized. Interestingly, these triphosphates were poor substrates for hMTH1, but exhibited strong competitive inhibition against hMTH1 at nanomolar levels. This inhibitory effect is attributed to slower rate of hydrolysis, possibly arising from enzyme structural changes, specifically different stacking interactions with 8-halogenated-7-deazadGTP. This is the first example of using nucleotide derivatives to inhibit hMTH1, thus demonstrating their potential as antitumor agents.

Identification of potent and selective MTH1 inhibitors

Structure based design of a novel class of aminopyrimidine MTH1 (MutT homolog 1) inhibitors is described. Optimization led to identification of IACS-4759 (compound 5), a sub-nanomolar inhibitor of MTH1 with excellent cell permeability and good metabolic stability in microsomes. This compound robustly inhibited MTH1 activity in cells and proved to be an excellent tool for interrogation of the utility of MTH1 inhibition in the context of oncology.

Oxidative stress and metabolic disorders: Pathogenesis and therapeutic strategies

Increased body weight and metabolic disorder including insulin resistance, type 2 diabetes and cardiovascular complications together constitute metabolic syndrome. The pathogenesis of metabolic syndrome involves multitude of factors. A number of studies however indicate, with some conformity, that oxidative stress along with chronic inflammatory condition pave the way for the development of metabolic diseases. Oxidative stress, a state of lost balance between the oxidative and anti-oxidative systems of the cells and tissues, results in the over production of oxidative free radicals and reactive oxygen species (ROS). Excessive ROS generated could attack the cellular proteins, lipids and nucleic acids leading to cellular dysfunction including loss of energy metabolism, altered cell signalling and cell cycle control, genetic mutations, altered cellular transport mechanisms and overall decreased biological activity, immune activation and inflammation. In addition, nutritional stress such as that caused by high fat high carbohydrate diet also promotes oxidative stress as evident by increased lipid peroxidation products, protein carbonylation, and decreased antioxidant system and reduced glutathione (GSH) levels. These changes lead to initiation of pathogenic milieu and development of several chronic diseases. Studies suggest that in obese person oxidative stress and chronic inflammation are the important underlying factors that lead to development of pathologies such as carcinogenesis, obesity, diabetes, and cardiovascular diseases through altered cellular and nuclear mechanisms, including impaired DNA damage repair and cell cycle regulation. Here we discuss the aspects of metabolic disorders-induced oxidative stress in major pathological conditions and strategies for their prevention and therapy.

Echinacoside induces apoptotic cancer cell death by inhibiting the nucleotide pool sanitizing enzyme MTH1

Inhibition of the nucleotide pool sanitizing enzyme MTH1 causes extensive oxidative DNA damages and apoptosis in cancer cells and hence may be used as an anticancer strategy. As natural products have been a rich source of medicinal chemicals, in the present study, we used the MTH1-catalyzed enzymatic reaction as a high-throughput in vitro screening assay to search for natural compounds capable of inhibiting MTH1. Echinacoside, a compound derived from the medicinal plants Cistanche and Echinacea, effectively inhibited the catalytic activity of MTH1 in an in vitro assay. Treatment of various human cancer cell lines with Echinacoside resulted in a significant increase in the cellular level of oxidized guanine (8-oxoguanine), while cellular reactive oxygen species level remained unchanged, indicating that Echinacoside also inhibited the activity of cellular MTH1. Consequently, Echinacoside treatment induced an immediate and dramatic increase in DNA damage markers and upregulation of the G1/S-CDK inhibitor p21, which were followed by marked apoptotic cell death and cell cycle arrest in cancer but not in noncancer cells. Taken together, these studies identified a natural compound as an MTH1 inhibitor and suggest that natural products can be an important source of anticancer agents.

The Impact of Non-Enzymatic Reactions and Enzyme Promiscuity on Cellular Metabolism during (Oxidative) Stress Conditions

Cellular metabolism assembles in a structurally highly conserved, but functionally dynamic system, known as the metabolic network. This network involves highly active, enzyme-catalyzed metabolic pathways that provide the building blocks for cell growth. In parallel, however, chemical reactivity of metabolites and unspecific enzyme function give rise to a number of side products that are not part of canonical metabolic pathways. It is increasingly acknowledged that these molecules are important for the evolution of metabolism, affect metabolic efficiency, and that they play a potential role in human disease—age-related disorders and cancer in particular. In this review we discuss the impact of oxidative and other cellular stressors on the formation of metabolic side products, which originate as a consequence of: (i) chemical reactivity or modification of regular metabolites; (ii) through modifications in substrate specificity of damaged enzymes; and (iii) through altered metabolic flux that protects cells in stress conditions. In particular, oxidative and heat stress conditions are causative of metabolite and enzymatic damage and thus promote the non-canonical metabolic activity of the cells through an increased repertoire of side products. On the basis of selected examples, we discuss the consequences of non-canonical metabolic reactivity on evolution, function and repair of the metabolic network.

Crystal structure, biochemical and cellular activities demonstrate separate functions of MTH1 and MTH2

Deregulated redox metabolism in cancer leads to oxidative damage to cellular components including deoxyribonucleoside triphosphates (dNTPs). Targeting dNTP pool sanitizing enzymes, such as MTH1, is a highly promising anticancer strategy. The MTH2 protein, known as NUDT15, is described as the second human homologue of bacterial MutT with 8-oxo-dGTPase activity. We present the first NUDT15 crystal structure and demonstrate that NUDT15 prefers other nucleotide substrates over 8-oxo-dGTP. Key structural features are identified that explain different substrate preferences for NUDT15 and MTH1. We find that depletion of NUDT15 has no effect on incorporation of 8-oxo-dGTP into DNA and does not impact cancer cell survival in cell lines tested. NUDT17 and NUDT18 were also profiled and found to have far less activity than MTH1 against oxidized nucleotides. We show that NUDT15 is not a biologically relevant 8-oxo-dGTPase, and that MTH1 is the most prominent sanitizer of the cellular dNTP pool known to date.

Echinacoside Induces Apoptosis in Human SW480 Colorectal Cancer Cells by Induction of Oxidative DNA Damages

Echinacoside is a natural compound with potent reactive oxygen species (ROS)-scavenging and anti-oxidative bioactivities, which protect cells from oxidative damages. As cancer cells are often under intense oxidative stress, we therefore tested if Echinacoside treatment would promote cancer development. Surprisingly, we found that Echinacoside significantly inhibited the growth and proliferation of a panel of cancer cell lines. Treatment of the human SW480 cancer cells with Echinacoside resulted in marked apoptosis and cell cycle arrest, together with a significant increase in active caspase 3 and cleaved PARP, and upregulation of the G1/S-CDK blocker CDKN1B (p21). Interestingly, immunocytochemistry examination of drug-treated cancer cells revealed that Echinacoside caused a significant increase of intracellular oxidized guanine, 8-oxoG, and dramatic upregulation of the double-strand DNA break (DSB)-binding protein 53BP1, suggesting that Echinacoside induced cell cycle arrest and apoptosis in SW480 cancer cells via induction of oxidative DNA damages. These results establish Echinacoside as a novel chemical scaffold for development of anticancer drugs.

Epigenetic modifications in DNA could mimic oxidative DNA damage: A double-edged sword

Methylation of cytosine at the C5 position (5mC) represents an epigenetic modification that plays a fundamental role in embryonic development, transcriptional regulation, and other processes. It can also be a mutational hotspot at CpG dinucleotides as a result of spontaneous hydrolytic deamination of 5mC to thymine. The resulting G · T mismatch pair is recognized by thymine DNA glycosylase (TDG) and revereted to a G · C pair. Recent studies have shown that 5mC is consecutively catalyzed into 5-hydroxymethylcytosine (5hmC), 5-formylcytosine (5fC), and 5-carboxylcytosine (5caC) by a DNA dioxygenase from the ten-eleven translocation (TET) family. Two oxidative cytosine derivatives, 5fC and 5caC, are eliminated by TDG during active DNA demethylation. Therefore, TDG has versatile roles in epigenetic regulation to control the gene expression as well as the DNA repair pathway to prevent mutagenesis. 5fC and 5caC serve as intermediate products of active DNA demethylation and also behave as DNA damages that threaten genomic integrity. Here, we discuss the potential functions of 5mC oxidative derivatives in epigenetic modification and DNA damage.