Expression of human MutT homologue (hMTH1) protein in primary non-small-cell lung carcinomas and histologically normal surrounding tissue

Hypoxia-Targeted Immunotherapy with PD-1 Blockade in Head and Neck Cancer

Intratumoral hypoxia is associated with tumor progression, aggressiveness, and therapeutic resistance in several cancers. Hypoxia causes cancer cells to experience replication stress, thereby activating DNA damage and repair pathways. MutT homologue-1 (MTH1, also known as NUDT1), a member of the Nudix family, maintains the genomic integrity and viability of tumor cells in the hypoxic tumor microenvironment. Although hypoxia is associated with poor prognosis and can cause therapeutic resistance by regulating the microenvironment, it has not been considered a treatable target in cancer. This study aimed to investigate whether hypoxia-induced MTH1 is a useful target for immunotherapy and whether hypoxic conditions influence the antitumor activity of immune cells. Our results showed that MTH1 expression was elevated under hypoxic conditions in head and neck cancer cell lines. Furthermore, we identified a novel MTH1-targeting epitope peptide that can activate peptide-specific CD4+ helper T cells with cytotoxic activity. The proliferation and cytotoxic activity of T cells were maintained under hypoxic conditions, and PD-1 blockade further augmented the cytotoxicity. These results indicate that MTH1-targeted immunotherapy combined with checkpoint blockade can be an effective strategy for the treatment of hypoxic tumors.

Exploring MTH1 inhibitory potential of Thymoquinone and Baicalin for therapeutic targeting of breast cancer

Cancers frequently have increased ROS levels due to disrupted redox balance, leading to oxidative DNA and protein damage, mutations, and apoptosis. The MTH1 protein plays a crucial role by sanitizing the oxidized dNTP pools. Hence, cancer cells rely on MTH1 to prevent the integration of oxidized dNTPs into DNA, preventing DNA damage and allowing cancer cell proliferation. We have discovered Thymoquinone (TQ) and Baicalin (BC) as inhibitors of MTH1 using combined docking and MD simulation approaches complemented by experimental validations via assessing binding affinity and enzyme inhibition. Docking and MD simulations studies revealed an efficient binding of TQ and BC to the active site pocket of the MTH1, and the resultant complexes are appreciably stable. Fluorescence measurements estimated a strong binding affinity of TQ and BC with Ka 3.4 ×106 and 1.0 ×105, respectively. Treating breast cancer cells with TQ and BC significantly inhibited the growth and proliferation (IC50 values 28.3 µM and 34.8 µM) and induced apoptosis. TQ and BC increased the ROS production in MCF7 cells, imposing substantial oxidative stress on cancer cells and leading to cell death. Finally, TQ and BC are proven strong MTH1 inhibitors, offering promising prospects for anti-cancer therapy.

Mitotic MTH1 Inhibitors in Treatment of Cancer

The DNA damage response (DDR) protein MTH1 is sanitising the oxidized dNTP pool and preventing incorporation of oxidative damage into DNA and has an emerging role in mitosis. It is a stress-induced protein and often found to be overexpressed in cancer. Mitotic MTH1 inhibitors arrest cells in mitosis and result in incorporation of oxidative damage into DNA and selective killing of cancer cells. Here, I discuss the leading mitotic MTH1 inhibitor TH1579 (OXC-101, karonudib), now being evaluated in clinical trials, and describe its dual effect on mitosis and incorporation of oxidative DNA damage in cancer cells. I describe why MTH1 inhibitors that solely inhibits the enzyme activity fail to kill cancer cells and discuss if MTH1 is a valid target for cancer treatment. I discuss emerging roles of MTH1 in regulating tubulin polymerisation and mitosis and the necessity of developing the basic science insights along with translational efforts. I also give a perspective on how edgetic perturbation is making target validation difficult in the DDR field.

Targeting the DNA damage response and repair in cancer through nucleotide metabolism

The exploitation of the DNA damage response and DNA repair proficiency of cancer cells is an important anticancer strategy. The replication and repair of DNA are dependent upon the supply of deoxynucleoside triphosphate (dNTP) building blocks, which are produced and maintained by nucleotide metabolic pathways. Enzymes within these pathways can be promising targets to selectively induce toxic DNA lesions in cancer cells. These same pathways also activate antimetabolites, an important group of chemotherapies that disrupt both nucleotide and DNA metabolism to induce DNA damage in cancer cells. Thus, dNTP metabolic enzymes can also be targeted to refine the use of these chemotherapeutics, many of which remain standard of care in common cancers. In this review article, we will discuss both these approaches exemplified by the enzymes MTH1, MTHFD2 and SAMHD1. © 2022 The Authors. Molecular Oncology published by John Wiley & Sons Ltd on behalf of Federation of European Biochemical Societies.

Specific 8-oxo-dGTPase activity of MTH1 (NUDT1) protein as a quantitative marker and prognostic factor in human colorectal cancer

The MTH1 (NUDT1) gene, because it is frequently upregulated in many types of human cancers, has been considered a general marker of carcinogenesis for over two decades. The MTH1 protein hydrolyzes the oxidized mutagenic DNA precursor, 8-oxo-7,8-dihydro-2'-deoxyguanosine 5'-triphosphate (8-oxo-dGTP), to the corresponding 5'-monophosphate and inorganic pyrophosphate. This prevents its incorporation into DNA by DNA polymerases and protects cells from the accumulation of 8-oxo-dGTP-induced point mutations. Elevated MTH1 mRNA and protein in many types of human cancer indicate a worse prognosis. However, the enzymatic activity of MTH1 has remained largely uninvestigated in this context. Therefore, we have set out to determine the specific 8-oxo-dGTPase activity of MTH1 in 57 pairs of human colorectal cancers (CRC) and adjacent cancer-free tissues (CFCF). The goal was to ascertain the potential for measuring this enzymatic activity as a way to differentiate cancerous from non-cancerous specimens of the intestine, as well as defining its capabilities as a prognostic value for disease-free survival. We found that 79% of CRC tumors exhibited a higher MTH1 activity than did CFCF, with a significant 1.6-fold increase in overall median value (p < 1E-6). The 8-oxo-dGTPase in both tissues was proportional to the corresponding levels of MTH1 protein, as assayed by Western blotting. Activity higher than the ROC-optimized threshold (AUC = 0.71) indicated cancerous tissue, with a 54% sensitivity and an 83% specificity. Postoperative fate followed for up to 100 months showed that higher 8-oxo-dGTPase, in either the CFCF or the CRC tumor, clearly lowered the probability of a relapse-free survival, although borderline statistical significance (p < 0.05) was crossed only for the CFCF.

HIF2α promotes tumour growth in clear cell renal cell carcinoma by increasing the expression of NUDT1 to reduce oxidative stress

BACKGROUND

The key role of hypoxia-inducible factor 2alpha (HIF2α) in the process of renal cancer has been confirmed. In the field of tumour research, oxidative stress is also considered to be an important influencing factor. However, the relationship and biological benefits of oxidative stress and HIF2α in ccRCC remain unclear. This research attempts to explore the effect of oxidative stress on the cancer-promoting effect of HIF2α in ccRCC and reveal its mechanism of action.

METHODS

The bioinformatics analysis for ccRCC is based on whole transcriptome sequencing and TCGA database. The detection of the expression level of related molecules is realised by western blot and PCR. The expression of Nucleoside diphosphate-linked moiety X-type motif 1 (NUDT1) was knocked down by lentiviral infection technology. The functional role of NUDT1 were further investigated by CCK8 assays, transwell assays and cell oxidative stress indicator detection. The exploration of related molecular mechanisms is realised by Luciferase assays and Chromatin immunoprecipitation (ChIP) assays.

RESULTS

Molecular screening based on knockdown HIF2α sequencing data and oxidative stress related data sets showed that NUDT1 is considered to be an important molecule for the interaction of HIF2α with oxidative stress. Subsequent experimental results showed that NUDT1 can cooperate with HIF2α to promote the progression of ccRCC. And this biological effect was found to be caused by the oxidative stress regulated by NUDT1. Mechanistically, HIF2α transcription activates the expression of NUDT1, thereby inhibiting oxidative stress and promoting the progression of ccRCC.

CONCLUSIONS

This research clarified a novel mechanism by which HIF2α stabilises sirtuin 3 (SIRT3) through direct transcriptional activation of NUDT1, thereby inhibiting oxidative stress to promote the development of ccRCC. It provided the possibility for the selection of new therapeutic targets for ccRCC and the study of combination medication regimens.

Inhibitor development of MTH1 via high-throughput screening with fragment based library and MTH1 substrate binding cavity

MutT Homolog 1 (MTH1) has been proven to hydrolyze oxidized nucleotide triphosphates during DNA repair. It can prevent the incorporation of wrong nucleotides during DNA replication and mitigate cell apoptosis. In a cancer cell, abundant reactive oxygen species can lead to substantial DNA damage and DNA mutations by base-pairing mismatch. MTH1 could eliminate oxidized dNTP and prevent cancer cells from entering cell death. Therefore, inhibition of MTH1 activity is considered to be an anti-cancer therapeutic target. In this study, high-throughput screening techniques were combined with a fragment-based library containing 2,313 compounds, which were used to screen for lead compounds with MTH1 inhibitor activity. Four compounds with MTH1 inhibitor ability were selected, and compound MI0639 was found to have the highest effective inhibition. To discover the selectivity and specificity of this action, several derivatives based on the MTH1 and MI0639 complex structure were synthesized. We compared 14 complex structures of MTH1 and the various compounds in combination with enzymatic inhibition and thermodynamic analysis. Nanomolar-range IC₅₀ inhibition abilities by enzyme kinetics and K_d values by thermodynamic analysis were obtained for two compounds, named MI1020 and MI1024. Based on structural information and compound optimization, we aim to provide a strategy for the development of MTH1 inhibitors with high selectivity and specificity.

Radiolabeled 6-(2, 3-Dichlorophenyl)-N4-methylpyrimidine-2, 4-diamine (TH287): A Potential Radiotracer for Measuring and Imaging MTH1

MTH1 (MutT homolog 1) or NUDT1 (Nudix Hydrolase 1), also known as oxidized purine nucleoside triphosphatase, has potential as a biomarker for monitoring cancer progression and quantifying target engagement for relevant therapies. In this study, we validate one MTH1 inhibitor TH287 as a PET MTH1 radiotracer. TH287 was radiolabeled with tritium and the binding of [³H]TH287 to MTH1 was evaluated in live glioblastoma cells (U251MG) through saturation and competitive binding assays, together with in vitro enzymatic assays. Furthermore, TH287 was radiolabeled with carbon-11 for in vivo microPET studies. Saturation binding assays show that [³H]TH287 has a dissociation constant (K_d) of 1.97 ± 0.18 nM, B_max of 2676 ± 122 fmol/mg protein for U251MG cells, and n_H of 0.98 ± 0.02. Competitive binding assays show that TH287 (K_i: 3.04 ± 0.14 nM) has a higher affinity for MTH1 in U251MG cells compared to another well studied MTH1 inhibitor: (S)-crizotinib (K_i: 153.90 ± 20.48 nM). In vitro enzymatic assays show that TH287 has an IC₅₀ of 2.2 nM in inhibiting MTH1 hydrolase activity and a K_i of 1.3 nM from kinetics assays, these results are consistent with our radioligand binding assays. Furthermore, MicroPET imaging shows that [¹¹C]TH287 gets into the brain with rapid clearance from the brain, kidney, and heart. The results presented here indicate that radiolabeled TH287 has favorable properties to be a useful tool for measuring MTH1 in vitro and for further evaluation for in vivo PET imaging MTH1 of brain tumors and other central nervous system disorders.

Antitumour activity of TH1579, a novel MTH1 inhibitor, against castration-resistant prostate cancer

Castration-resistant prostate cancer (CRPC) treatment still remains difficult. The aim of the present study was to determine the antitumour efficacy of the MutT homolog 1 (MTH1) inhibitor, TH1579, against castration-resistant prostate cancer. PC-3 and DU-145 prostate cancer cells were treated with different concentrations of TH1579. C4-2 cells with or without androgen receptor (AR) were also treated with TH1579 to assess AR function. Cell survival, 8-oxo-dG levels and DNA damage were measured using cell viability assays, western blotting, immunofluorescence analysis and flow cytometry. TH1579 inhibited CRPC cell proliferation in a dose-dependent manner. The viabilities of PC-3 and DU-145 cells treated with 1 µM of TH1579 were 28.6 and 24.1%, respectively. The viabilities of C4-2 cells with and without AR treated with 1 µM TH1579 were 10.6 and 19.0%, respectively. Moreover, TH1579 treatment increased 8-oxo-dG levels, as well as the number of 53BP1 and γH2A.X foci, resulting in increased DNA double-strand breakage and apoptosis in PC-3 and DU-145 cells. The findings of the present study demonstrated that TH1579 exerted strong antitumour effects on CRPC cells, and may therefore be used as a potential therapeutic agent for the clinical treatment of CRPC.

The high expression of MTH1 and NUDT5 promotes tumor metastasis and indicates a poor prognosis in patients with non-small-cell lung cancer

MutT Homolog 1 (MTH1) is a mammalian 8-oxodGTPase for sanitizing oxidative damage to the nucleotide pool. Nudix type 5 (NUDT5) also sanitizes 8-oxodGDP in the nucleotide pool. The role of MTH1 and NUDT5 in non-small-cell lung cancer (NSCLC) progression and metastasis remains unclear. In the present study, we reported that MTH1 and NUDT5 were upregulated in NSCLC cell lines and tissues, and higher levels of MTH1 or NUDT5 were associated with tumor metastasis and a poor prognosis in patients with NSCLC. Their suppression also restrained tumor growth and lung metastasis in vivo and significantly inhibited NSCLC cell migration, invasion, cell proliferation and cell cycle progression while promoting apoptosis in vitro. The opposite effects were observed in vitro following MTH1 or NUDT5 rescue. In addition, the upregulation of MTH1 or NUDT5 enhanced the MAPK pathway and PI3K/AKT activity. Furthermore, MTH1 and NUDT5 induce epithelial-mesenchymal transition both in vitro and in vivo. These results highlight the essential role of MTH1 and NUDT5 in NSCLC tumor tumorigenesis and metastasis as well as their functions as valuable markers of the NSCLC prognosis and potential therapeutic targets.

The high expression of MTH1 and NUDT5 predict a poor survival and are associated with malignancy of esophageal squamous cell carcinoma

Background

MTH1 and NUDT5 effectively degrade nucleotides containing 8-oxoguanine. MTH1 and NUDT5 have been linked to the malignancy of multiple cancers. However, their functions in tumor growth and metastasis in esophageal squamous carcinoma (ESCC) remain obscure. Our present study aims to explore their prognostic value in ESCC and investigate their function in MTH1 or NUDT5-knockout tumor cells.

Methods

MTH1 and NUDT5 protein expression in ESCC adjacent normal tissues and tumor tissues was examined by immunohistochemistry staining. Kaplan–Meier curves were used to assess the association between their expression and overall survival (OS) in ESCC patients. Univariate and Multivariate Cox regression analyses were generated to determine the correlation between these protein expression and OS of ESCC patients. Protein expression in ESCC cell lines were measured by Western blotting. To explore the potential effects of the MTH1 and NUDT5 protein in ESCC, cell models with MTH1 or NUDT5 depletion were established. CCK-8, cell cycle, Western blotting, migration and invasion assays were performed.

Results

Our present study demonstrated that the levels of MTH1 and NUDT5 were upregulated in ESCC cell lines and ESCC tissues, the expression of MTH1 and NUDT5 in ESCC tissues was significantly higher than in adjacent non-tumorous, and higher levels of MTH1 and NUDT5 predicted a worse prognosis in patients with ESCC. MTH1 and NUDT5 are novel biomarkers of the progression of ESCC and a poor prognosis. We also found for the first time that the high expression of NUDT5 independently predicted lower OS in patients with ESCC (hazard ratio (HR) 1.751; 95% confidence interval (CI) [1.056–2.903]; p = 0.030). In addition, the depletion of MTH1 and NUDT5 strongly suppressed the proliferation of ESCC cells and significantly delayed the G1 phase of the cell cycle. Furthermore, we found that MTH1 and NUDT5 silencing inhibited epithelial–mesenchymal transition mainly by the MAPK/MEK/ERK dependent pathway, which in turn significantly decreased the cell migration and invasion of ESCC cells. Our results suggested that the overexpression of MTH1 and NUDT5 is probably involved in the tumor development and poor prognosis of ESCC.

The Existence of MTH1-independent 8-oxodGTPase Activity in Cancer Cells as a Compensatory Mechanism against On-target Effects of MTH1 Inhibitors

Investigations into the human 8-oxodGTPase, MutT Homolog 1 (MTH1), have risen sharply since the first-in-class MTH1 inhibitors were reported to be highly tumoricidal. However, MTH1 as a cancer therapeutic target is currently controversial because subsequently developed inhibitors did not exhibit similar cytotoxic effects. Here, we provide the first direct evidence for MTH1-independent 8-oxodGTPase function in human cancer cells and human tumors, using a novel ATP-releasing guanine-oxidized (ARGO) chemical probe. Our studies show that this functionally redundant 8-oxodGTPase activity is not decreased by five different published MTH1-targeting small molecules or by MTH1 depletion. Significantly, while only the two first-in-class inhibitors, TH588 and TH287, reduced cancer cell viability, all five inhibitors evaluated in our studies decreased 8-oxodGTPase activity to a similar extent. Thus, the reported efficacy of the first-in-class MTH1 inhibitors does not arise from their inhibition of MTH1-specific 8-oxodGTPase activity. Comparison of DNA strand breaks, genomic 8-oxoguanine incorporation, or alterations in cellular oxidative state by TH287 versus the noncytotoxic inhibitor, IACS-4759, contradict that the cytotoxicity of the former results solely from increased levels of oxidatively damaged genomic DNA. Thus, our findings indicate that mechanisms unrelated to oxidative stress or DNA damage likely underlie the reported efficacy of the first-in-class inhibitors. Our study suggests that MTH1 functional redundancy, existing to different extents in all cancer lines and human tumors evaluated in our study, is a thus far undefined factor which is likely to be critical in understanding the importance of MTH1 and its clinical targeting in cancer.

Mechanisms of MTH1 inhibition-induced DNA strand breaks: The slippery slope from the oxidized nucleotide pool to genotoxic damage

Unlike normal tissues, tumor cells possess a propensity for genomic instability, resulting from elevated oxidant levels produced by oncogenic signaling and aberrant cellular metabolism. Thus, targeting mechanisms that protect cancer cells from the tumor-inhibitory consequences of their redox imbalance and spontaneous DNA-damaging events is expected to have broad-spectrum efficacy and a high therapeutic index. One critical mechanism for tumor cell protection from oxidant stress is the hydrolysis of oxidized nucleotides. Human MutT homolog 1 (MTH1), the mammalian nudix (nucleoside diphosphate X) pyrophosphatase (NUDT1), protects tumor cells from oxidative stress-induced genomic DNA damage by cleansing the nucleotide pool of oxidized purine nucleotides. Depletion or pharmacologic inhibition of MTH1 results in genomic DNA strand breaks in many cancer cells. However, the mechanisms underlying how oxidized nucleotides, thought mainly to be mutagenic rather than genotoxic, induce DNA strand breaks are largely unknown. Given the recent therapeutic interest in targeting MTH1, a better understanding of such mechanisms is crucial to its successful translation into the clinic and in identifying the molecular contexts under which its inhibition is likely to be beneficial. Here we provide a comprehensive perspective on MTH1 function and its importance in protecting genome integrity, in the context of tumor-associated oxidative stress and the mechanisms that likely lead to irreparable DNA strand breaks as a result of MTH1 inhibition.

Crystal Structure and Substrate Specificity of the 8-oxo-dGTP Hydrolase NUDT1 from Arabidopsis thaliana

Arabidopsis thaliana NUDT1 (AtNUDT1) belongs to the Nudix family of proteins, which have a diverse range of substrates, including oxidized nucleotides such as 8-oxo-dGTP. The hydrolysis of oxidized dNTPs is highly important as it prevents their incorporation into DNA, thus preventing mutations and DNA damage. AtNUDT1 is the sole Nudix enzyme from A. thaliana shown to have activity against 8-oxo-dGTP. We present the structure of AtNUDT1 in complex with 8-oxo-dGTP. Structural comparison with bacterial and human homologues reveals a conserved overall fold. Analysis of the 8-oxo-dGTP binding mode shows that the residues Asn76 and Ser89 interact with the O8 atom of the substrate, a feature not observed in structures of protein homologues solved to date. Kinetic analysis of wild-type and mutant AtNUDT1 confirmed that these active site residues influence 8-oxo-dGTP hydrolysis. A recent study showed that AtNUDT1 is also able to hydrolyze terpene compounds. The diversity of reactions catalyzed by AtNUDT1 suggests that this Nudix enzyme from higher plants has evolved in a manner distinct to those from other organisms.

hMTH1 is required for maintaining migration and invasion potential of human thyroid cancer cells

Cancer cells, including thyroid cancer cells, suffer from oxidative stress damaging multiple cellular targets, such as DNA and the nucleotide pool. The human MutT homologue 1 (hMTH1) controls the oxidative DNA damage load by sanitizing the nucleotide pool from the oxidized DNA precursor, 8-oxodGTP. It has previously been shown that hMTH1 is essential for cancer cell proliferation and survival, therefore hMTH1 inhibition has been proposed as a novel anticancer therapeutic strategy. Here we show that thyroid cancer cells respond to siRNA mediated hMTH1 depletion with increased DNA damage load and moderately reduced proliferation rates, but without detectable apoptosis, cell-cycle arrest or senescence. Importantly, however, hMTH1 depletion significantly reduced migration and invasion potential of the thyroid cancer cells. Accordingly, our results allow us to propose that hMTH1 may be a therapeutic target in thyroid malignancy, especially for controlling metastasis.

MTH1 deficiency selectively increases non-cytotoxic oxidative DNA damage in lung cancer cells: more bad news than good?

Background

Targeted therapies are based on exploiting cancer-cell-specific genetic features or phenotypic traits to selectively kill cancer cells while leaving normal cells unaffected. Oxidative stress is a cancer hallmark phenotype. Given that free nucleotide pools are particularly vulnerable to oxidation, the nucleotide pool sanitising enzyme, MTH1, is potentially conditionally essential in cancer cells. However, findings from previous MTH1 studies have been contradictory, meaning the relevance of MTH1 in cancer is still to be determined. Here we ascertained the role of MTH1 specifically in lung cancer cell maintenance, and the potential of MTH1 inhibition as a targeted therapy strategy to improve lung cancer treatments.

Methods

Using siRNA-mediated knockdown or small-molecule inhibition, we tested the genotoxic and cytotoxic effects of MTH1 deficiency on H23 (p53-mutated), H522 (p53-mutated) and A549 (wildtype p53) non-small cell lung cancer cell lines relative to normal MRC-5 lung fibroblasts. We also assessed if MTH1 inhibition augments current therapies.

Results

MTH1 knockdown increased levels of oxidatively damaged DNA and DNA damage signaling alterations in all lung cancer cell lines but not normal fibroblasts, despite no detectable differences in reactive oxygen species levels between any cell lines. Furthermore, MTH1 knockdown reduced H23 cell proliferation. However, unexpectedly, it did not induce apoptosis in any cell line or enhance the effects of gemcitabine, cisplatin or radiation in combination treatments. Contrastingly, TH287 and TH588 MTH1 inhibitors induced apoptosis in H23 and H522 cells, but only increased oxidative DNA damage levels in H23, indicating that they kill cells independently of DNA oxidation and seemingly via MTH1-distinct mechanisms.

Conclusions

MTH1 has a NSCLC-specific p53-independent role for suppressing DNA oxidation and genomic instability, though surprisingly the basis of this may not be reactive-oxygen-species-associated oxidative stress. Despite this, overall our cell viability data indicates that targeting MTH1 will likely not be an across-the-board effective NSCLC therapeutic strategy; rather it induces non-cytotoxic DNA damage that could promote cancer heterogeneity and evolution.

Electronic supplementary material

The online version of this article (10.1186/s12885-018-4332-7) contains supplementary material, which is available to authorized users.

TH588, an MTH1 inhibitor, enhances phenethyl isothiocyanate-induced growth inhibition in pancreatic cancer cells

Chemotherapy and radiotherapy are the most common approaches in cancer therapy. They may kill cancer cells through the generation of high levels of reactive oxygen species (ROS), which leads to oxidative DNA damage. However, tumor resistance to ROS is a problem in cancer therapy. MTH1 sanitizes oxidized dNTP pools to prevent the incorporation of damaged bases during DNA replication. Although MTH1 is non-essential in normal cells, cancer cells require MTH1 activity to avoid the incorporation of oxidized dNTPs, which would result in DNA damage and cell death. By targeting a redox-adaptation mechanism, MTH1 inhibition represents a novel therapeutic strategy against cancer. However, recent reports have indicated that growth inhibition by MTH1 inhibitors may be due to off-target cytotoxic effects. TH588, one of the first-in-class MTH1 inhibitors, kills cancer cells by an off-target effect. However, a low concentration of TH588 may effectively inhibit MTH1 activity without inhibiting cell proliferation. Phenethyl isothiocyanate (PEITC) is a dietary anticarcinogenic compound and an inducer of ROS. In the present study, it has been demonstrated that combined treatment with PEITC and TH588 effectively inhibited the growth of pancreatic cancer MIAPaCa-2 and Panc-1 cells. The antioxidant N-acetylcysteine negated this synergistic growth inhibition. PEITC and TH588 cooperatively induced the formation of 8-oxo-deoxyguanine in nuclei and pH2AX foci, a marker of DNA damage. However, the combined effects are not associated with MTH1 mRNA expression in several cancer cell lines, suggesting that the possibility of an off-target effect of TH588 cannot be eliminated. These results suggest that the combination of PEITC and TH588 has potential as a novel therapeutic strategy against pancreatic cancer.

MutT-related proteins are novel progression and prognostic markers for colorectal cancer

Background

MutT-related proteins, including MTH1, MTH2, MTH3 and NUDT5, can effectively degrade 8-oxoGua-containing nucleotides. The MTH1 expression is elevated in many types of human tumors and MTH1 overexpression correlates with the tumor pathological stage and poor prognosis. However, the expression of other MutT-related proteins in human cancers remains unknown. The present study systematically investigated the expression of MTH1, MTH2, MTH3 and NUDT5 in human colorectal cancer to establish its clinical significance.

Methods

Amounts of MutT-related mRNA and protein in CRC cell lines were assessed by qRT-PCR and Western blotting, respectively. Furthermore, the MutT-related protein expression was evaluated by immunohistochemical staining of tissue microarrays containing 87 paired CRC tissues and by Western blotting of 44 CRC tissue samples. Finally, the effect of knockdown of MutT-related proteins on CRC cell proliferation was investigated.

Results

The expression of MTH1, MTH2, MTH3 and NUDT5 was significantly higher in CRC cells and CRC tissues than normal cells and tissues, and this phenomenon was significantly associated with AJCC stage and lymph node metastasis of CRC specimens. CRC patients with high expression of MTH1, MTH2 or NUDT5 had an extremely poor overall survival after surgical resection. Notably, NUDT5 was an independent prognostic factor of CRC patients. We found that knockdown of MutT-related proteins inhibited CRC cell proliferation.

Conclusions

We showed for the first time that MutT-related proteins play an important role in CRC progression and prognosis. Further investigations are needed to elucidate the role of these proteins in CRC progression and their potential use for therapeutic targets.

The role of miR-485-5p/NUDT1 axis in gastric cancer

Background

Cancers can survive the oxidative conditions by upregulating nucleoside diphosphate linked moiety X-type motif 1 (NUDT1). However, the mechanisms underlying gastric carcinogenesis and the dys-regulation of NUDT1 in gastric cancer (GC) remain unknown. Our study aimed to explore the role of NUDT1 and its regulatory pathway by miR-485-5p in GC.

Methods

Gastric cancer tissues and paired noncancerous tissue samples were collected, and the expression level of NUDT1 and miR-485-5p were detected. Two cohorts from The Cancer Genome Atlas (TCGA) database and another cohort from the Tianjin Medical University Cancer Institute and Hospital were further analyzed. Luciferase assays were performed, and the effects of the miR-485-5p/NUDT1 axis on GC cells and normal gastric cells were determined by subsequent experiments.

Results

We found that the expression of miR-485-5p was clearly repressed in GC tissues, while NUDT1 expression level was dramatically increased. The overexpression of NUDT1 correlated closely with an increase in invasive depth and a decrease in survival in GC patients. MiR-485-5p could directly bind to the 3′UTR of NUDT1 mRNA and induce its degradation, thus down-regulate its expression. The miR-485-5p/NUDT1 axis could lead to the changes of 8-oxo-dG in GC cells. And the increased expression of NUDT1 resulting from the downregulation of miR-485-5p could accelerate cell proliferation and metastasis in GC. However, the growth and migration of normal gastric cells did not depend on the protection of NUDT1, while the overexpression of NUDT1 could promote malignant transition in normal gastric cells.

Conclusions

MiR-485-5p acts as a tumor suppressor by targeting NUDT1 in GC. The miR-485-5p/NUDT1 axis is involved in the processes of cell growth and cell motility and plays a key role in the tumorigenesis of GC.

Electronic supplementary material

The online version of this article (doi:10.1186/s12935-017-0462-2) contains supplementary material, which is available to authorized users.

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.

MTH1 as a Chemotherapeutic Target: The Elephant in the Room

Many tumors sustain elevated levels of reactive oxygen species (ROS), which drive oncogenic signaling. However, ROS can also trigger anti-tumor responses, such as cell death or senescence, through induction of oxidative stress and concomitant DNA damage. To circumvent the adverse consequences of elevated ROS levels, many tumors develop adaptive responses, such as enhanced redox-protective or oxidatively-generated damage repair pathways. Targeting these enhanced oxidative stress-protective mechanisms is likely to be both therapeutically effective and highly specific to cancer, as normal cells are less reliant on such mechanisms. In this review, we discuss one such stress-protective protein human MutT Homolog1 (MTH1), an enzyme that eliminates 8-oxo-7,8-dihydro-2’-deoxyguanosine triphosphate (8-oxodGTP) through its pyrophosphatase activity, and is found to be elevated in many cancers. Our studies, and subsequently those of others, identified MTH1 inhibition as an effective tumor-suppressive strategy. However, recent studies with the first wave of MTH1 inhibitors have produced conflicting results regarding their cytotoxicity in cancer cells and have led to questions regarding the validity of MTH1 as a chemotherapeutic target. To address the proverbial "elephant in the room" as to whether MTH1 is a bona fide chemotherapeutic target, we provide an overview of MTH1 function in the context of tumor biology, summarize the current literature on MTH1 inhibitors, and discuss the molecular contexts likely required for its efficacy as a therapeutic target.

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.

A Chimeric ATP-Linked Nucleotide Enables Luminescence Signaling of Damage Surveillance by MTH1, a Cancer Target

The enzyme MTH1 cleanses the cellular nucleotide pool of oxidatively damaged 8-oxo-dGTP, preventing mutagenesis by this nucleotide. The enzyme is considered a promising therapeutic target; however, methods to measure its activity are indirect and laborious and have low sensitivity. Here we describe a novel ATP-linked chimeric nucleotide (ARGO) that enables luminescence signaling of the enzymatic reaction, greatly simplifying the measurement of MTH1 activity. We show that the reporting system can be used to identify inhibitors of MTH1, and we use it to quantify enzyme activity in eight cell lines and in colorectal tumor tissue. The ARGO reporter is likely to have considerable utility in the study of the biology of MTH1 and potentially in analyzing patient samples during clinical testing.

The role of microRNA in metastatic processes of non-small cell lung carcinoma

BACKGROUND

MicroRNAs are small non-coding one-stranded RNA molecules that play an important role in the post-transcriptional regulation of genes. Bioinformatic predictions indicate that each miRNA can regulate hundreds of target genes. MicroRNA expression can be associated with various cellular processes leading to the metastasis of malignant tumours including non-small cell lung carcinoma. This review summarizes current knowledge on the role of microRNAs in NSCLC metastasis to the brain and lymph nodes.

METHODS

A search of the NCBI/PubMed database for publications on expression levels and the mechanisms of microRNA action in NSCLC metastasis.

RESULTS AND CONCLUSION

Dysregulation of microRNAs in NSCLC can be associated with brain and lymph node metastasis. There are differences in microRNA expression profiling between NSCLC with and without metastases but it is currently not possible to reliably predict the site of metastasis in NSCLC. Based on data from RNAmicroarrays, bioinformatics analysis is able to predict the target genes of highlighted microRNAs, providing us with complex information about cancer cell features such as enhanced proliferation, migration and invasion. Such microRNAs may then be knocked-down using siRNAs or substituted with miRNA mimics. RNA microarray profiling may thus be a useful tool to select up- or down-regulated microRNAs. A number of authors suggest that microRNAs could serve as biomarkers and therapeutic targets in the treatment of NSCLC metastasis.

Hypoxia-inducible factor-1 modulates upregulation of mutT homolog-1 in colorectal cancer

AIM: To investigate the roles and interactions of mutT homolog (MTH)-1 and hypoxia-inducible factor (HIF)-1α in human colorectal cancer (CRC).

METHODS: The expression and distribution of HIF-1α and MTH-1 proteins were detected in human CRC tissues by immunohistochemistry and quantitative real-time polymerase chain reaction (qRT-PCR). SW480 and HT-29 cells were exposed to normoxia or hypoxia. Protein and mRNA levels of HIF-1α and MTH-1 were analyzed by western blotting and qRT-PCR, respectively. In order to determine the effect of HIF-1α on the expression of MTH-1 and the amount of 8-oxo-deoxyguanosine triphosphate (dGTP) in SW480 and HT-29 cells, HIF-1α was silenced with small interfering RNA (siRNA). Growth studies were conducted on cells with HIF-1α inhibition using a xenograft tumor model. Finally, MTH-1 protein was detected by western blotting in vivo.

RESULTS: High MTH-1 mRNA expression was detected in 64.2% of cases (54/84), and this was significantly correlated with tumor stage (P = 0.023) and size (P = 0.043). HIF-1α protein expression was correlated significantly with MTH-1 expression (R = 0.640; P < 0.01) in human CRC tissues. Hypoxic stress induced mRNA and protein expression of MTH-1 in SW480 and HT-29 cells. Inhibition of HIF-1α by siRNA decreased the expression of MTH-1 and led to the accumulation of 8-oxo-dGTP in SW480 and HT-29 cells. In the in vivo xenograft tumor model, expression of MTH-1 was decreased in the HIF-1α siRNA group, and the tumor volume was much smaller than that in the mock siRNA group.

CONCLUSION: MTH-1 expression in CRC cells was upregulated via HIF-1α in response to hypoxic stress, emphasizing the crucial role of HIF-1α-induced MTH-1 in tumor growth.

Influence of Chirality of Crizotinib on Its MTH1 Protein Inhibitory Activity: Insight from Molecular Dynamics Simulations and Binding Free Energy Calculations

As a promising target for the treatment of lung cancer, the MutT Homolog 1 (MTH1) protein can be inhibited by crizotinib. A recent work shows that the inhibitory potency of (S)-crizotinib against MTH1 is about 20 times over that of (R)-crizotinib. But the detailed molecular mechanism remains unclear. In this study, molecular dynamics (MD) simulations and free energy calculations were used to elucidate the mechanism about the effect of chirality of crizotinib on the inhibitory activity against MTH1. The binding free energy of (S)-crizotinib predicted by the Molecular Mechanics/Generalized Born Surface Area (MM/GBSA) and Adaptive biasing force (ABF) methodologies is much lower than that of (R)-crizotinib, which is consistent with the experimental data. The analysis of the individual energy terms suggests that the van der Waals interactions are important for distinguishing the binding of (S)-crizotinib and (R)-crizotinib. The binding free energy decomposition analysis illustrated that residues Tyr7, Phe27, Phe72 and Trp117 were important for the selective binding of (S)-crizotinib to MTH1. The adaptive biasing force (ABF) method was further employed to elucidate the unbinding process of (S)-crizotinib and (R)-crizotinib from the binding pocket of MTH1. ABF simulation results suggest that the reaction coordinates of the (S)-crizotinib from the binding pocket is different from (R)-crizotinib. The results from our study can reveal the details about the effect of chirality on the inhibition activity of crizotinib to MTH1 and provide valuable information for the design of more potent inhibitors.

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.

Status of the Parkinson's disease gene family expression in non-small-cell lung cancer

Background

The purpose of this study is to detect the Parkinson’s disease gene family mRNA relative expression in the non-small-cell lung cancer (NSCLC) tumor tissue and analyze the association between tumor characteristics and the Parkinson’s disease gene family.

Methods

Tumor tissue and tumor-adjacent tissue of 114 NSCLC patients were collected and SYBR quantitative analysis was used to detect the relative expression level of nine Parkinson’s disease gene mRNAs. Then, paired sample test, two-sided Student’s t-test, or two-sided Wilcoxon rank sum test was performed to analyze the mRNA relative expression level of nine Parkinson’s disease gene mRNAs in different gender, tumor histology, and tumor stage.

Results

Overexpression in the tumors was detected in 46/114 (40.35 %) PARK1/4, 74/114 (64.91 %) PARK2, 104/114 (91.23 %) PARK5, 95/114 (83.33 %) PARK6, 80/114 (70.18 %) PARK7, 55/114 (48.25 %) PARK8, 100/114 (87.72 %) PARK9, 55/114 (48.25 %) PARK15, and 99/114 (86.84 %) glucocerebrosidase (GBA). Five genes PARK5 (91.23 %), PARK6 (83.33 %), PARK7 (70.18 %), PARK9 (87.72 %), and GBA (86.84 %) were supposed to be overexpressed in the lung tumor tissues compared with tumor-adjacent tissues. There was no significant difference in PARK1/4, PARK2, PARK5, PARK9, and GBA mRNA expression by different tumor stage, whereas, PARK6, PARK7, PARK8, and PARK15 mRNA expression were found to have significant difference in the comparison of different tumor stages. The expression of PARK6 (P = 0.01, P = 0.03) and PARK15 (P < 0.001, P < 0.001) were significantly higher in stages I and II when compared with stage III, respectively. NSCLC patients in stage I showed the higher expression PARK7 compared to the patients in stage II (P = 0.003).

Conclusions

The high expression of PARK6, PARK7, and PARK15 might lead to the occurrence of a primary NSCLC tumor, and the tumor with a decreasing expression of these three genes tends to be stages II and III. The results of our study indicate that the Parkinson’s disease gene family may be a potential marker for the prediction of NSCLC.

Electronic supplementary material

The online version of this article (doi:10.1186/s12957-015-0646-y) contains supplementary material, which is available to authorized users.

Investigation of micronucleus induction in MTH1 knockdown cells exposed to UVA, UVB or UVC

The longer wave parts of UVR can increase the production of reactive oxygen species (ROS) which can oxidize nucleotides in the DNA or in the nucleotide pool leading to mutations. Oxidized bases in the DNA are repaired mainly by the DNA base excision repair system and incorporation of oxidized nucleotides into newly synthesized DNA can be prevented by the enzyme MTH1. Here we hypothesize that the formation of several oxidized base damages (from pool and DNA) in close proximity, would cause a high number of base excision repair events, leading to DNA double strand breaks (DSB) and therefore giving rise to cytogenetic damage. If this hypothesis is true, cells with low levels of MTH1 will show higher cytogenetic damage after the longer wave parts of UVR. We analyzed micronuclei induction (MN) as an endpoint for cytogenetic damage in the human lymphoblastoid cell line, TK6, with a normal and a reduced level of MTH1 exposed to UVR. The results indicate a higher level of micronuclei at all incubation times after exposure to the longer wave parts of UVR. There is no significant difference between wildtype and MTH1-knockdown TK6 cells, indicating that MTH1 has no protective role in UVR-induced cytogenetic damage. This indicates that DSBs induced by UV arise from damage forms by direct interaction of UV or ROS with the DNA rather than through oxidation of dNTP.

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

8-Oxoguanine, a major oxidized base lesion formed by reactive oxygen species, causes G to T transversion mutations or leads to cell death in mammals if it accumulates in DNA. 8-Oxoguanine can originate as 8-oxo-dGTP, formed in the nucleotide pool, or by direct oxidation of the DNA guanine base. MTH1, also known as NUDT1, with 8-oxo-dGTP hydrolyzing activity, 8-oxoguanine DNA glycosylase (OGG1) an 8-oxoG DNA glycosylase, and MutY homolog (MUTYH) with adenine DNA glycosylase activity, minimize the accumulation of 8-oxoG in DNA; deficiencies in these enzymes increase spontaneous and induced tumorigenesis susceptibility. However, different tissue types have different tumorigenesis susceptibilities. These can be reversed by combined deficiencies in the defense systems, because cell death induced by accumulation of 8-oxoG in DNA is dependent on MUTYH, which can be suppressed by MTH1 and OGG1. In cancer cells encountering high oxidative stress levels, a high level of 8-oxo-dGTP accumulates in the nucleotide pool, and cells therefore express increased levels of MTH1 in order to eliminate 8-oxo-dGTP. Suppression of MTH1 may be an efficient strategy for killing cancer cells; however, because MTH1 and OGG1 protect normal tissues from oxidative-stress-induced cell death, it is important that MTH1 inhibition does not increase the risk of healthy tissue degeneration.

MutT Homolog 1 (MTH1) maintains multiple KRAS-driven pro-malignant pathways

Oncogenic RAS promotes production of reactive oxygen species (ROS), which mediate pro-malignant signaling but can also trigger DNA damage-induced tumor suppression. Thus RAS-driven tumor cells require redox-protective mechanisms to mitigate the damaging aspects of ROS. Here, we show that MutT Homolog 1 (MTH1), the mammalian 8-oxodGTPase that sanitizes oxidative damage in the nucleotide pool, is important for maintaining several KRAS-driven pro-malignant traits in a nonsmall cell lung carcinoma (NSCLC) model. MTH1 suppression in KRAS-mutant NSCLC cells impairs proliferation and xenograft tumor formation. Furthermore, MTH1 levels modulate KRAS-induced transformation of immortalized lung epithelial cells. MTH1 expression is upregulated by oncogenic KRAS and correlates positively with high KRAS levels in NSCLC human tumors. At a molecular level, in p53-competent KRAS-mutant cells, MTH1 loss provokes DNA damage and induction of oncogene-induced senescence. In p53-nonfunctional KRAS-mutant cells, MTH1 suppression does not produce DNA damage but reduces proliferation and leads to an adaptive decrease in KRAS expression levels. Thus, MTH1 not only enables evasion of oxidative DNA damage and its consequences, but can also function as a molecular rheostat for maintaining oncogene expression at optimal levels. Accordingly, our results indicate MTH1 is a novel and critical component of oncogenic KRAS-associated malignancy and its inhibition is likely to yield significant tumor-suppressive outcomes in KRAS-driven tumors.

Oxidative Stress and DNA Damage in Human Gastric Carcinoma: 8-Oxo-7'8-dihydro-2'-deoxyguanosine (8-oxo-dG) as a Possible Tumor Marker

We characterized the oxidative stress (OS) status by the levels of reduced/oxidized glutathione (GSH/GSSG), malondialdehyde (MDA) and the mutagenic base 8-oxo-7′8-dihydro-2′-deoxyguanosine (8-oxo-dG) in human gastric carcinoma (HGC) samples and compared the results with normal tissue from the same patients. We also analyzed 8-oxo-dG in peripheral mononuclear cells (PMNC) and urine from healthy control subjects and in affected patients in the basal state and one, three, six, nine and twelve months after tumor resection. The levels of DNA repair enzyme mRNA expression (hOGG1, RAD51, MUYTH and MTH1) were determined in tumor specimens and compared with normal mucosa. Tumor specimens exhibited increased levels of MDA and 8-oxo-dG compared with normal gastric tissue. GSH levels were also increased, while GSSG levels remained stable. DNA repair enzyme mRNA expression was induced in the tumor tissues. Levels of 8-oxo-dG were significantly elevated in both urine and PMNC of gastric cancer patients compared with healthy controls. After gastrectomy, the levels of the damaged base in urine and PMNC decreased progressively to values close to those found in the healthy population. The high levels of 8-oxo-dG in urine may be related to the increased induction of DNA repair activity in tumor tissue, and the changes observed after tumor resection support its potential use as a tumor marker.

Lycopene inhibits DNA damage and reduces hMTH1 mRNA expression in the liver of Mongolian gerbils treated with ferric nitrilotriacetate

Epidemiologic studies suggest that lycopene may reduce the risk of certain cancers, but the underlying mechanisms are unclear. We hereby examined in vivo anticarcinogenic effects of supplemental lycopene on hepatic DNA damage (Comet formation and levels of 8-oxo-2'-deoxyguanosine, 8-oxo-dG) and human MutT homologue (hMTH1) mRNA expression in Mongolian gerbils injected i.p. with ferric nitrilotriacetate (Fe/NTA). Gerbils were pre-treated with two dose (10 and 20 mg/kg BW) of lycopene on alternate morning for 10 d. Six hours after the last lycopene supplementation, the gerbils received Fe/NTA (0.16 mmol/10 mL/kg BW) and were killed 3 h later. We found that the livers of Fe/NTA-treated animals exhibited a 5-fold increase in Comet formation (expressed as tail moment), a 2.6-fold increase in 8-oxo-dG levels and a significant increase in hMTH1 mRNA expression. Supplemental lycopene completely inhibited Comet formation (P<0.001) and significantly suppressed 8-oxo-dG levels and hMTH1 expression in a dose-dependent manner. The expression levels of hMTH1 mRNA are highly correlated with hepatic levels of 8-oxo-dG and tail moment, suggesting that hMTH1 gene expression represents a molecular marker of oxidative DNA damage. These results demonstrate the potential of lycopene as a promising chemopreventive agent against hepatotumorigenesis.

Oxidation in the nucleotide pool, the DNA damage response and cellular senescence: Defective bricks build a defective house

Activation of persistent DNA damage response (DDR) signaling is associated with the induction of a permanent proliferative arrest known as cellular senescence, a phenomenon intrinsically linked to both tissue aging as well as tumor suppression. The DNA damage observed in senescent cells has been attributed to elevated levels of reactive oxygen species (ROS), failing DNA damage repair processes, and/or oncogenic activation. It is not clear how labile molecules such as ROS are able to damage chromatin-bound DNA to a sufficient extent to invoke persistent DNA damage and DDR signaling. Recent evidence suggests that the nucleotide pool is a significant target for oxidants and that oxidized nucleotides, once incorporated into genomic DNA, can lead to the induction of a DNA strand break-associated DDR that triggers senescence in normal cells and in cells sustaining oncogene activation. Evasion of this DDR and resulting senescence is a key step in tumor progression. This review will explore the role of oxidation in the nucleotide pool as a major effector of oxidative stress-induced genotoxic damage and DDR in the context of cellular senescence and tumorigenic transformation.

Combustion products of 1,3-butadiene inhibit catalase activity and induce expression of oxidative DNA damage repair enzymes in human bronchial epithelial cells

1,3-Butadiene, an important petrochemical, is commonly burned off when excess amounts need to be destroyed. This combustion process produces butadiene soot (BDS), which is composed of a complex mixture of polycyclic aromatic hydrocarbons in particulates ranging in size from <1 microm to 1 mm. An organic extract of BDS is both cytotoxic and genotoxic to normal human bronchial epithelial (NHBE) cells. Based on the oxidizing potential of BDS, we hypothesized that an organic extract of this particulate matter would (1) cause enzyme inactivation due to protein amino acid oxidation and (2) induce oxidative DNA damage in NHBE cells. Thus, our aims were to determine the effect of butadiene soot ethanol extract (BSEE) on both enzyme activity and the expression of proteins involved in the repair of oxidative DNA damage. Catalase was found to be sensitive to BDS as catalase activity was potently diminished in the presence of BSEE. Using Western analysis, both the alpha isoform of human 8-oxoguanine DNA glycosylase (alpha-hOGG1) and human apurinic/apyrimidinic endonuclease (APE-1) were shown to be significantly overexpressed as compared to untreated controls after exposure of NHBE cells to BSEE. Our results indicate that BSEE is capable of effectively inactivating the antioxidant enzyme catalase, presumably via oxidation of protein amino acids. The presence of oxidized biomolecules may partially explain the extranuclear fluorescence that is detected when NHBE cells are treated with an organic extract of BDS. Overexpression of both alpha-hOGG1 and APE-1 proteins following treatment of NHBE cells with BSEE suggests that this mixture causes oxidative DNA damage.

DNA damage, superoxide, and mutant K-ras in human lung adenocarcinoma cells

DNA single-strand breaks (quantitative comet assay) were assessed to indicate ongoing genetic instability in a panel of human lung adenocarcinoma cell lines. Of these, 19/20 showed more DNA damage than a nontransformed cell line from human peripheral lung epithelium, HPL1D. DNA damage was significantly greater in those derived from pleural effusates vs those from lymph node metastases. DNA strand breaks correlated positively with superoxide (nitroblue tetrazolium reduction assay), and negatively with amount of OGG1, a repair enzyme for oxidative DNA damage. Levels of CuZn superoxide dismutase varied moderately among the lines and did not correlate with other parameters. A role for mutant K-ras through generation of reactive oxygen species was examined. Cells with mutant K-ras had significantly lower amounts of manganese superoxide dismutase (MnSOD) vs those with wild-type K-ras, but MnSOD protein correlated positively with superoxide levels. In a subset of cell lines with similar levels of MnSOD, comparable to those in HPL1D cells, K-ras activity correlated positively with levels of both superoxide and DNA strand breaks. These results suggest that persistent DNA damage in some lung adenocarcinoma cells may be caused by superoxide resulting from mutant K-ras activity, and that OGG1 is important for prevention of this damage.

AtNUDX1, an 8-oxo-7,8-dihydro-2'-deoxyguanosine 5'-triphosphate pyrophosphohydrolase, is responsible for eliminating oxidized nucleotides in Arabidopsis

Cellular DNA, RNA and their precursor nucleotides are at high risk of being oxidized by reactive oxygen species. An oxidized base, 8-oxo-7,8-dihydro-2'-(deoxy)guanosine, can pair with both adenine and cytosine, and thus would cause both replicational and translational errors. Previously, we have reported that an Arabidopsis Nudix hydrolase, AtNUDX1, acts to hydrolyze an oxidized deoxyribonucleotide, 8-oxo-7,8-dihydro-2'-deoxyguanosine 5'-triphosphate (8-oxo-dGTP). Here we showed that 8-oxo-dGTP pyrophosphohydrolase activity is not exhibited by any other Arabidopsis Nudix hydrolase. AtNUDX1 acted on an oxidized ribonucleotide, 8-oxo-GTP, with high affinity (K(m) 28.1 microM). In a transcriptional mutational analysis using the lacZ reporter gene, the phenotypic suppression of the lacZ amber mutation in a mutT-deficient Escherichia coli strain caused by the misincorporation of 8-oxo-GTP into the mRNA was significantly diminished by expression of AtNUDX1. These findings suggest that AtNUDX1 prevents transcriptional errors in vivo. A confocal microscopic analysis using a green fluorescent protein (GFP) fusion protein demonstrated that AtNUDX1 is distributed in the cytosol, where the main pool of nucleotides in the cells exists. The level of 8-oxo-guanosine in genomic DNA was significantly increased in knockout nudx1 plants compared with wild-type plants under normal and oxidative stress (3 microM paraquat) conditions. The results obtained here indicate that AtNUDX1 functions in cellular defense against oxidative DNA and RNA damage through the sanitization of their precursor pools in the cytosol in Arabidopsis cells.

Measurement of 7,8-dihydro-8-oxo-2'-deoxyguanosine metabolism in MCF-7 cells at low concentrations using accelerator mass spectrometry

Growing evidence suggests that oxidative damage to cells generates mutagenic 7,8-dihydro-8-oxo-2'-deoxyguanosine (8-oxodG), which may initiate diseases related to aging and carcinogenesis. Kinetic measurement of 8-oxodG metabolism and repair in cells has been hampered by poor assay sensitivity and by difficulty characterizing the flux of oxidized nucleotides through the relevant metabolic pathways. We report here the development of a sensitive and quantitative approach to characterizing the kinetics and metabolic sources of 8-oxodG in MCF-7 human breast cancer cells by accelerator mass spectrometry. We observed that [(14)C]8-oxodG at medium concentrations of up to 2 pmol/ml was taken up by MCF-7 cells, phosphorylated to mono-, di-, and triphosphate derivatives, and incorporated into DNA. Oxidative stress caused by exposure of the cells to 17beta-estradiol resulted in a reduction in the rate of [(14)C]8-oxodG incorporation into DNA and an increase in the ratio of 8-oxodG monophosphate (8-oxodGMP) to 8-oxodG triphosphate (8-oxodGTP) in the nucleotide pool. 17beta-Estradiol-induced oxidative stress up-regulated the nucleotide pool cleansing enzyme MTH1 and possibly other Nudix-related pyrophosphohydrolases. These data support the conclusion that 8-oxodGTP is formed in the nucleotide pool by both 8-oxodG metabolism and endogenous reactive oxygen species. The metabolism of 8-oxodG to 8-oxodGTP, followed by incorporation into DNA is a mechanism by which the cellular presence of this oxidized nucleoside can lead to mutations.

Base excision repair modulation as a risk factor for human cancers

Oxidative DNA damage and DNA repair mediate the development of several human pathologies, including cancer. The major pathway for oxidative DNA damage repair is base excision repair (BER). Functional assays performed in blood leukocytes of cancer patients and matched controls show that specific BER pathways are decreased in cancer patients, and may be risk factors. These include 8-oxoguanine (8-oxoG) repair in lung and head and neck cancer patients and repair of lipid peroxidation (LPO) induced 1,N(6)-ethenoadenine (epsilonA) in lung cancer patients. Decrease of excision of LPO-induced DNA damage, epsilonA and 3,N(4)-ethenocytosine (epsilonC) was observed in blood leukocytes of patients developing lung adenocarcinoma, specific histological type of cancer related to inflammation and healing of scars. BER proteins activity depends on gene polymorphism, interactions between BER system partners and post-translational modifications. Polymorphisms of DNA glycosylases may change their enzymatic activities, and some polymorphisms increase the risk of inflammation-related cancers, colorectal, lung and other types. Polymorphisms of BER platform protein, XRCC1 are connected with increased risk of tobacco-related cancers. BER efficiency may also be changed by reactive oxygen species and some diet components, which induce transcription of several glycosylases as well as a major human AP-endonuclease, APE1. BER is also changed in tumors in comparison to unaffected surrounding tissues, and this change may be due to transcription stimulation, post-translational modification of BER enzymes as well as protein-protein interactions. Modulation of BER enzymes activities may be, then, an important factor determining the risk of cancer and also may participate in cancer development.

Contribution of hMTH1 to the maintenance of 8-oxoguanine levels in lung DNA of non-small-cell lung cancer patients

BACKGROUND

The level of 8-oxoguanine (8-oxoG), a general marker of oxidative DNA damage, in DNA is the result of both an equilibrium between the rates of its formation and removal from DNA by DNA repair enzymes and the removal of 8-oxodGTP from the cellular nucleotide pool by hydrolysis to 8-oxodGMP, preventing its incorporation into DNA. To determine the contribution of each component to the level of 8-oxoG in DNA, we compared 8-oxoG-excising activity (encoded by hOGG1), 8-oxodGTPase activity (encoded by hMTH1), and 8-oxoG levels in DNA from tumors and surrounding normal lung tissues from non-small-cell lung cancer patients.

METHODS

We measured the level of 8-oxoG in DNA of 47 patients by high-performance liquid chromatography/electrochemical detection (HPLC/ECD), hOGG1 activity in tissue extracts of 56 patients by the nicking assay using an oligodeoxynucleotide containing a single 8-oxoG, and hMTH1 activity in tissue extracts of 33 patients by HPLC/UV detection. All statistical tests were two-sided.

RESULTS

The 8-oxoG level was lower in tumor DNA than in DNA from normal lung tissue (geometric mean: 5.81 versus 10.18 8-oxoG/10(6) G, geometric mean of difference = 1.75; P<.001). The hOGG1 activity was also lower in tumor than in normal lung tissue (geometric mean: 8.76 versus 20.91 pmol/h/mg protein, geometric mean of difference = 2.39; P<.001), whereas the hMTH1 activity was higher in tumor than in normal lung tissue (geometric mean: 28.79 versus 8.94 nmol/h/mg protein, geometric mean of difference = 0.31; P<.001). The activity of hMTH1 was three orders of magnitude higher than that of hOGG1 (nanomoles versus picomoles per hour per milligram of protein, respectively).

CONCLUSIONS

Several different components contribute to the maintenance of 8-oxoG levels in human DNA, with the greatest contributor being the removal of 8-oxodGTP from the cellular nucleotide pool by hMTH1.

The defense mechanisms in mammalian cells against oxidative damage in nucleic acids and their involvement in the suppression of mutagenesis and cell death

To counteract oxidative damage in nucleic acids, mammalian cells are equipped with several defense mechanisms. We herein review that MTH1, MUTYH and OGG1 play important roles in mammalian cells avoiding an accumulation of oxidative DNA damage, both in the nuclear and mitochondrial genomes, thereby suppressing carcinogenesis and cell death. MTH1 efficiently hydrolyzes oxidized purine nucleoside triphosphates, such as 8-oxo-dGTP, 8-oxo-dATP and 2-hydroxy (OH)-dATP, to the monophosphates, thus avoiding the incorporation of such oxidized nucleotides into the nuclear and mitochondrial genomes. OGG1 excises 8-oxoG in DNA as a DNA glycosylase and thus minimizes the accumulation of 8-oxoG in the cellular genomes. MUTYH excises adenine opposite 8-oxoG, and thus suppresses 8-oxoG-induced mutagenesis. MUTYH also possesses a 2-OH-A DNA glycosylase activity for excising 2-OH-A incorporated into the cellular genomes. Increased susceptibilities to spontaneous carcinogenesis of the liver, lung or intestine were observed in MTH1-, OGG1- and MUTYH-null mice, respectively. The increased occurrence of lung tumors in OGG1-null mice was abolished by the concomitant disruption of the Mth1 gene, indicating that an increased accumulation of 8-oxoG and/or 2-OH-A might cause cell death. Furthermore, these defense mechanisms also likely play an important role in neuroprotection.

Cellular 8-oxo-7,8-dihydro-2'-deoxyguanosine 5'-triphosphate pyrophosphohydrolase activity of human and mouse MTH1 proteins does not depend on the proliferation rate

Mammalian MTH1 proteins, homologs of Escherichia coli MutT, are enzymes decomposing 8-oxo-7,8-dihydro-2'-deoxyguanosine 5'-triphosphate (8-oxo-dGTP) to 8-oxo-7,8-dihydro-2'-deoxyguanosine 5'-monophosphate and inorganic pyrophosphate. They play an antimutagenic role by preventing the incorporation of promutagenic 8-oxo-dGTP into DNA. MTH1 gene expression is higher in some physiological types of mammalian cells and in numerous cancer cells, but the mechanism of that upregulation still remains unclear. It has been hypothesized that MTH1 expression might be associated with a proliferation rate of the cells. Therefore, we tested this hypothesis by comparing the functional levels of MTH1 gene expression measured as the 8-oxo-dGTPase activity of its protein products in normal mouse livers and hepatectomized regenerating livers. Although the proliferation rate of the hepatocytes in the regenerating livers was much higher than that in control livers, as confirmed by immunohistochemical assay of proliferating cell nuclear antigen, the 8-oxo-dGTPase activity was not different. In a second approach, we used 57 lines of human cancer cells in which 8-oxo-dGTPase activity was measured and confronted with cell population doubling time. No significant correlations between 8-oxo-dGTPase activity and proliferation rate were observed within groups of six leukemia, eight melanoma, nine lung, seven colon, six central nervous system, six ovarian, eight renal, and seven breast cancer cell lines. Thus, we conclude that the MTH1 expression manifested as the 8-oxo-dGTPase activity of its protein products in mammalian cells is not associated with proliferation rate. Our results will help in further testing of the hypothesis that MTH1 overexpression may be a specific marker of carcinogenesis and/or oxidative stress.

Biological Relevance of Adduct Detection to the Chemoprevention of Cancer

Adducts arise from the chemical modification of bases in DNA or amino acids in proteins by toxic chemicals. Many chemicals known to be carcinogenic in humans have been shown to form adducts or to cause oxidative damage to genomic DNA in model systems. Biomarkers of carcinogenesis reflect biological events that take place between exposure to external or endogenous carcinogens and the subsequent development of cancer. Therapeutic intervention for the purpose of cancer chemoprevention may modify these biomarkers. In this article, the potential efficacy of DNA adducts as biomarkers of carcinogenesis and chemoprevention is discussed using criteria defined for phases of biomarker development. The sensitivity of adduct detection in histologically normal tissue offers opportunities for the early detection of carcinogenesis. Extensive evidence for aflatoxin B(1) adducts as biomarkers of risk and progression of hepatic carcinogenesis and for oxidative DNA adducts as biomarkers of the development of prostate carcinogenesis is reviewed together with the clinical trials measuring these adducts as biomarkers of the efficacy of chemoprevention. Favorable modification of oxidative DNA adducts by dietary intervention and chemoprevention has been demonstrated in preclinical and clinical studies. Protein adducts and DNA adducts in blood constituents or urine may act as useful surrogates for the target organ. Additional information regarding reliability, reproducibility, specificity, and confounding variables are required at the clinical level to validate adducts as suitable biomarkers of chemoprevention. "We do not administer antihypertensive drugs to patients in clinical trials without checking their blood pressure, so why should we give antioxidants without checking that they have decreased oxidant status.

Oxidative damage is increased in human liver tissue adjacent to hepatocellular carcinoma

Accumulation of genetic alterations in hepatocarcinogenesis is closely associated with chronic inflammatory liver disease. 8-oxo-2'-deoxyguanosine (8-oxo-dG), the major promutagenic DNA adduct caused by reactive oxygen species (ROS), leads to G:C --> T:A transversions. These lesions can be enzymatically repaired mainly by human MutT homolog 1 (hMTH1), human 8-oxo-guanine DNA glycosylase (hOGG1) and human MutY homolog (hMYH). The aim of this study was to evaluate the extent of oxidative damage and its dependence on the cellular antioxidative capacity and the expression of specific DNA repair enzymes in tumor (tu) and corresponding adjacent nontumor (ntu) liver tissue of 23 patients with histologically confirmed hepatocellular carcinoma. 8-oxo-dG levels, as detected by high-pressure liquid chromatography with electrochemical detection, were significantly (P =.003) elevated in ntu tissue (median, 129 fmol/microg DNA) as compared to tu tissue (median, 52 fmol/microg DNA), and were closely associated with inflammatory infiltration. In ntu tissue, the hepatic iron concentration and malondialdehyde levels were significantly (P =.001) higher as compared to tu tissue. Glutathione content, glutathione peroxidase activity and manganese superoxide dismutase messenger RNA (mRNA) expression did not show statistical differences between ntu and tu tissue. Real-time reverse transcription polymerase chain reaction revealed in tu tissue significantly (P =.014) higher hMTH1 mRNA expression compared to ntu tissue. In contrast, hMYH mRNA expression was significantly (P <.05) higher in ntu tissue. No difference in hOGG1 mRNA expression was seen between tu and ntu. In conclusion, these data suggest that ROS generated by chronic inflammation contribute to human hepatocarcinogenesis. The role of DNA repair enzymes appears to be of reactive rather than causative manner.