Protein APE1 Research Articles

DNA Damage Accelerates G-Quadruplex Folding in a Duplex-G-Quadruplex-Duplex Context.

Molecular details for the impact of DNA damage on folding of potential G-quadruplex sequences (PQSs) to noncanonical DNA structures involved in gene regulation are poorly understood. Here, the effects of DNA base damage and strand breaks on PQS folding kinetics were studied in the context of the VEGF promoter sequence embedded between two DNA duplex anchors, termed a duplex-G-quadruplex-duplex (DGD) motif. This DGD scaffold imposes constraints on the PQS folding process that more closely mimic those found in genomic DNA. Folding kinetics were monitored by circular dichroism (CD) to find folding half-lives ranging from 2 s to 12 min depending on the DNA damage type and sequence position. The presence of Mg2+ ions and G-quadruplex (G4)-binding protein APE1 facilitated the folding reactions. A strand break placing all four G runs required for G4 formation on one side of the break accelerated the folding rate by >150-fold compared to the undamaged sequence. Combined 1D 1H NMR and CD analyses confirmed that isothermal folding of the VEGF-DGD constructs yielded spectral signatures that suggest the formation of G4 motifs and demonstrated a folding dependency on the nature and location of DNA damage. Importantly, the PQS folding half-lives measured are relevant to replication, transcription, and DNA repair time frames.

Methionine sulfoxide reductase 2 regulates Cvt autophagic pathway by altering the stability of Atg19 and Ape1 in Saccharomyces cerevisiae

The reversible oxidation of methionine plays a crucial role in redox regulation of proteins. Methionine oxidation in proteins causes major structural modifications that can destabilize and abrogate their function. The highly conserved methionine sulfoxide reductases protect proteins from oxidative damage by reducing their oxidized methionines, thus restoring their stability and function. Deletion or mutation in conserved methionine sulfoxide reductases leads to aging and several human neurological disorders and also reduces yeast growth on nonfermentable carbon sources. Despite their importance in human health, limited information about their physiological substrates in humans and yeast is available. For the first time, we show that Mxr2 interacts invivo with two core proteins of the cytoplasm to vacuole targeting (Cvt) autophagy pathway, Atg19, and Ape1 in Saccharomyces cerevisiae. Deletion of MXR2 induces instability and early turnover of immature Ape1 and Atg19 proteins and reduces the leucine aminopeptidase activity of Ape1 without affecting the maturation process of Ape1. Additonally, Mxr2 interacts with the immature Ape1, dependent on Met17 present within the propeptide of Ape1 as a single substitution mutation of Met17 to Leu abolishes this interaction. Importantly, Ape1 M17L mutant protein resists oxidative stress-induced degradation in WT and mxr2Δ cells. By identifying Atg19 and Ape1 as cytosolic substrates of Mxr2, our study maps the hitherto unexplored connection between Mxr2 and the Cvt autophagy pathway and sheds light on Mxr2-dependent oxidative regulation of the Cvt pathway.

Abstract B053: Applying a new drug discovery platform for the discovery and development of inhibitors of DDR proteins APE1 and Pol eta

Abstract Quickly generating reliable hits from library screening and rapidly converting them into lead compounds, which can be verified in functional assays, is critical in early preclinical drug discovery. The expedited validation of “druggability” of targets and identification of modulators to advance to preclinical studies can significantly increase the success of drug discovery and development. Our SaXPy (“SAR by X-ray Poses Quickly”) platform, which is applicable to any X-ray crystallography-enabled drug target, couples the established methods of protein X-ray crystallography and fragment-based drug discovery (FBDD) with advanced computational and medicinal chemistry to deliver small molecule modulators or targeted protein degradation ligands in a short timeframe. Our approach, especially for elusive or “undruggable” targets, allows (i) hit generation; (ii) mapping of protein-ligand interactions; (iii) assessment of target ligandability; (iv) discovery of novel and potential allosteric binding sites; and (v) hit-to-lead progression. These advances inform chemical tractability, downstream biology, and intellectual property. We describe here the application of SaXPy in the discovery and development of DNA Damage Response (DDR) inhibitors against DNA polymerase eta (Pol 𝝶 or POLH) and apurinic/apyrimidinic endonuclease 1 (APE1 or APEX1). Notably, our studies resulted in the first crystal structures of these targets bound to small molecules and the discovery of novel, multiple binding sites, which we could quickly progress from hit-to-lead development and demonstrate functional activity in bioassays. Citation Format: Debanu Das, Matthew Duncton, Ashley Deacon, David Wilson III. Applying a new drug discovery platform for the discovery and development of inhibitors of DDR proteins APE1 and Pol eta [abstract]. In: Proceedings of the AACR-NCI-EORTC Virtual International Conference on Molecular Targets and Cancer Therapeutics; 2023 Oct 11-15; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2023;22(12 Suppl):Abstract nr B053.

Two aminopeptidase I homologs convergently contribute to pathobiology of fungal entomopathogen Beauveria bassiana via divergent physiology-dependent autophagy pathways for vacuolar targeting

IntroductionIn yeast, the cytoplasm-to-vacuole targeting (Cvt) pathway acts as a biosynthetic autophagy-related process, in which vacuolar targeting of hydrolase is mediated by the machineries involved in the selective autophagy. However, the mechanistic insights into vacuolar targeting of hydrolases through the selective autophagy pathway still remain enigmatic in filamentous fungi. ObjectivesOur study aims to investigate the mechanisms involved in vacuolar targeting of hydrolases in filamentous fungi. MethodsThe filamentous entomopathogenic fungus Beauveria bassiana was used as a representative of filamentous fungi. We identified the homologs of yeast aminopeptidase I (Ape1) in B. bassiana by bioinformatic analyses and characterized their physiological roles by gene function analyses. Pathways for vacuolar targeting of hydrolases were investigated via molecular trafficking analyses. ResultsB. bassiana has two homologs of yeast aminopeptidase I (Ape1) which are designated as BbApe1A and BbApe1B. The two homologs of yeast Ape1 contribute to starvation tolerance, development, and virulence in B. bassiana. Significantly, BbNbr1 acts as a selective autophagy receptor to mediate the vacuolar targeting of the two Ape1 proteins, in which BbApe1B interacts with BbNbr1 also directly interacting with BbAtg8, and BbApe1A has an additional requirement of the scaffold protein BbAtg11 that interacts with BbNbr1 and BbAtg8. Protein processing occurs at both terminuses of BbApe1A and only at carboxyl terminus of BbApe1B, which is also dependent on the autophagy-related proteins. Together, the functions and translocation processes of the two Ape1 proteins are associated with autophagy in fungal lifecycle. ConclusionThis study reveals the functions and translocation processes for vacuolar hydrolases in the insect-pathogenic fungi and improves our understandings of the Nbr1-mediated vacuolar targeting pathway in the filamentous fungi.

Metal coordination micelles for anti-cancer treatment by gene-editing and phototherapy

CRISPR-Cas9 is a central focus of the emerging field of gene editing and photodynamic therapy (PDT) is a clinical-stage ablation modality combining photosensitizers with light irradiation. But metal coordination biomaterials for the applications of both have rarely been investigated. Herein, Chlorin-e6 (Ce6) Manganese (Mn) coordination micelles loaded with Cas9, termed Ce6-Mn-Cas9, were developed for augmented combination anti-cancer treatment. Manganese played multiple roles to facilitate Cas9 and single guide RNA (sgRNA) ribonucleoprotein (RNP) delivery, Fenton-like effect, and enhanced endonuclease activity of RNP. Histidine (His)-tagged RNP could be coordinated to Ce6 encapsulated in Pluronic F127 (F127) micelles by simple admixture. Triggered by ATP and endolysosomal acidic pH, Ce6-Mn-Cas9 released Cas9 without altering protein structure or function. Dual guide RNAs were designed to target the antioxidant regulator MTH1 and the DNA repair protein APE1, resulting in increased oxygen and enhanced PDT effect. In a murine tumor model, Ce6-Mn-Cas9 inhibited tumor growth with the combination therapy of PDT and gene editing. Taken together, Ce6-Mn-Cas9 represents a new biomaterial with a high degree of versatility to enable photo- and gene-therapy approaches.

Genome-Wide Binding Analysis of DNA Repair Protein APE1 in Tumor Cells by ChIP-Seq.

The base excision repair (BER) is the primary damage repair pathway for repairing most of the endogenous DNA damage including oxidative base lesions, apurinic/apyrimidinic (AP) sites, and single-strand breaks (SSBs) in the genome. Repair of these damages in cells relies on sequential recruitment and coordinated actions of multiple DNA repair enzymes, which include DNA glycosylases (such as OGG1), AP-endonucleases (APE1), DNA polymerases, and DNA ligases. APE1 plays a key role in the BER pathway by repairing the AP sites and SSBs in the genome. Several methods have been developed to generate a map of endogenous AP sites or SSBs in the genome and the binding of DNA repair proteins. In this chapter, we describe detailed approaches to map genome-wide occupancy or enrichment of APE1 in human cells using chromatin immunoprecipitation followed by next-generation sequencing (ChIP-seq). Further, we discuss standard bioinformatics approaches for analyzing ChIP-seq data to identify APE1 enrichment or binding peaks in the genome.

Inhibition of human APE1 and MTH1 DNA repair proteins by dextran-coated γ-Fe2O3 ultrasmall superparamagnetic iron oxide nanoparticles.

Aim: To quantitatively evaluate the inhibition of human DNA repair proteins APE1 and MTH1 by dextran-coated γ-Fe2O3 ultrasmall superparamagnetic iron oxide nanoparticles (dUSPIONs). Materials & methods: Liquid chromatography-tandem mass spectrometry with isotope-dilution was used to measure the expression levels of APE1 and MTH1 in MCL-5 cells exposed to increasing doses of dUSPIONs. The expression levels of APE1 and MTH1 were measured in cytoplasmic and nuclear fractions of cell extracts. Results: APE1 and MTH1 expression was significantly inhibited in both cell fractions at the highest dUSPION dose. The expression of MTH1 was linearly inhibited across the full dUSPION dose range in both fractions. Conclusion: These findings warrant further studies to characterize the capacity of dUSPIONs to inhibit other DNA repair proteins in vitro and in vivo.

Sirt3 Regulates Response to Oxidative Stress by Interacting with BER Proteins in Colorectal Cancer.

The oxidative damages are well-recognized factors in the pathogenesis of colorectal cancer (CRC). Increased levels of reactive oxygen species (ROS) can lead to oxidative DNA damage, which, if unrepaired, can be an underlying cause of cancerogenic transformation. To defend against these threats, cells have developed a range of defense mechanisms. One of the most important protection mechanisms is DNA repair systems, both nuclear and mitochondrial. Sirt3 is a mitochondrial protein involved in regulating NEIL1, NEIL2, MUTYH, APE1, and LIG3 proteins, which are involved in DNA repair, including mitochondrial repair through mtBER (mitochondrial Base Excision Repair). In this work, we show that NEIL1, NEIL2, MUTYH, APE1, and LIG3 are regulated by Sirt3 through deacetylation, and moreover, Sirt3 is directly involved in physical interaction with MUTYH, NEIL1, and APE1, which indicates the controlling role of Sirt3 over the mtBER mechanism. Also, if the cells deprived of Sirt3 are exposed to oxidative stress, altered levels of those proteins can be observed, which supports the theory of the regulatory role of Sirt3. Finally, to fully confirm the role of Sirt3 in DNA repair, we examined its role in apoptosis and found the impact of this protein on cell survival rate. Using the knowledge obtained in the course of conducted experiments, we postulate consideration of Sirt3 as a target in the rising vulnerability of cancer cells during therapy and therefore increasing the effectiveness of cancer treatment.

Molecular Mechanisms Regulating the DNA Repair Protein APE1: A Focus on Its Flexible N-Terminal Tail Domain.

APE1 (DNA (apurinic/apyrimidinic site) endonuclease 1) is a key enzyme of one of the major DNA repair routes, the BER (base excision repair) pathway. APE1 fulfils additional functions, acting as a redox regulator of transcription factors and taking part in RNA metabolism. The mechanisms regulating APE1 are still being deciphered. Structurally, human APE1 consists of a well-characterized globular catalytic domain responsible for its endonuclease activity, preceded by a conformationally flexible N-terminal extension, acquired along evolution. This N-terminal tail appears to play a prominent role in the modulation of APE1 and probably in BER coordination. Thus, it is primarily involved in mediating APE1 localization, post-translational modifications, and protein–protein interactions, with all three factors jointly contributing to regulate the enzyme. In this review, recent insights on the regulatory role of the N-terminal region in several aspects of APE1 function are covered. In particular, interaction of this region with nucleophosmin (NPM1) might modulate certain APE1 activities, representing a paradigmatic example of the interconnection between various regulatory factors.

Multiple myeloma-derived exosomes inhibit osteoblastic differentiation and improve IL-6 secretion of BMSCs from multiple myeloma

Bone marrow stromal cells (BMSCs) play a critical role in multiple myeloma (MM) pathogenesis by cell contact, and secretion of cytokines, growth factors and extracellular vesicles. Exosomes are secreted by...

Sirt3 regulates the level of mitochondrial DNA repair activity through deacetylation of NEIL1, NEIL2, OGG1, MUTYH, APE1 and LIG3 in colorectal cancer.

Colorectal cancer (CRC) is one of the most common malignant tumors. One of the factors increasing the risk of its occurrence may be the reduced efficiency of repairing DNA damage, both nuclear and mitochondrial. The main mechanism for repairing oxidative damage is the BER system (in mitochondria mtBER), whose key proteins NEIL1, NEIL2, OGG1, MUTYH, APE1 and LIG3 obtain full efficiency only at the appropriate level of acetylation. Sirtuin 3 is a key protein for mitochondrial homeostasis, regulating a number of metabolic processes related mainly to the control of the level of reactive oxygen species. Because Sirt3 possesses acetylase activity, it can modulate the level of activity of mtBER proteins by their deacetylation. The conducted study showed that the tested proteins NEIL1, NEIL2, OGG1, MUTYH, APE1 and LIG3 are the substrate for the enzymatic deacetylation activity of Sirt3, which may lead to modulation of the risk of CRC, and in cancer cells may be a potential therapeutic target enhancing the action of cytostatic drugs.

APE1 and NPM1 protect cancer cells from platinum compounds cytotoxicity and their expression pattern has a prognostic value in TNBC

BackgroundTriple negative breast cancer (TNBC) is a breast cancer subgroup characterized by a lack of hormone receptors’ expression and no HER2 overexpression. These molecular features both drastically reduce treatment options and confer poor prognosis. Platinum (Pt)-salts are being investigated as a new therapeutic strategy. The base excision repair (BER) pathway is important for resistance to Pt-based therapies. Overexpression of APE1, a pivotal enzyme of the BER pathway, as well as the expression of NPM1, a functional regulator of APE1, are associated with poor outcome and resistance to Pt-based therapies.MethodsWe evaluated the role of NPM1, APE1 and altered NPM1/APE1 interaction in the response to Pt-salts treatment in different cell lines: APE1 knockout (KO) cells, NPM1 KO cells, cell line models having an altered APE1/NPM1 interaction and HCC70 and HCC1937 TNBC cell lines, having different levels of APE1/NPM1. We evaluated the TNBC cells response to new chemotherapeutic small molecules targeting the endonuclease activity of APE1 or the APE1/NPM1 interaction, in combination with Pt-salts treatments. Expression levels’ correlation between APE1 and NPM1 and their impact on prognosis was analyzed in a cohort of TNBC patients through immunohistochemistry. Bioinformatics analysis, using TCGA datasets, was performed to predict a molecular signature of cancers based on APE1 and NPM1 expression.ResultsAPE1 and NPM1, and their interaction as well, protect from the cytotoxicity induced by Pt-salts treatment. HCC1937 cells, having higher levels of APE1/NPM1 proteins, are more resistant to Pt-salts treatment compared to the HCC70 cells. A sensitization effect by APE1 inhibitors to Pt-compounds was observed. The association of NPM1/APE1 with cancer gene signatures highlighted alterations concerning cell-cycle dependent proteins.ConclusionsAPE1 and NPM1 protect cancer cells from Pt-compounds cytotoxicity, suggesting a possible improvement of the activity of Pt-based therapy for TNBC, using the NPM1 and APE1 proteins as secondary therapeutic targets. Based on positive or negative correlation with APE1 and NPM1 gene expression levels, we finally propose several TNBC gene signatures that should deserve further attention for their potential impact on TNBC precision medicine approaches.

DNA repair protein APE1 is involved in host response during pneumococcal meningitis and its expression can be modulated by vitamin B6

BackgroundThe production of reactive oxygen species (ROS) during pneumococcal meningitis (PM) leads to severe DNA damage in the neurons and is the major cause of cell death during infection. Hence, the use of antioxidants as adjuvant therapy has been investigated. Previous studies have demonstrated the possible participation of apurinic/apyrimidinic endonuclease (APE1) during PM. The aims of this study were to investigate the APE1 expression in the cortical and hippocampal tissues of infant Wistar rats infected with Streptococcus pneumoniae and its association with cell death and understand the role of vitamin B6 (vitB6) as a protective factor against cell death.MethodsAPE1 expression and oxidative stress markers were analyzed at two-time points, 20 and 24 h post infection (p.i.), in the cortex (CX) and hippocampus (HC) of rats supplemented with vitB6. Statistical analyses were performed by the nonparametric Kruskal–Wallis test using Dunn’s post test.ResultsOur results showed high protein levels of APE1 in CX and HC of infected rats. In the CX, at 20 h p.i., vitB6 supplementation led to the reduction of expression of APE1 and apoptosis-inducing factor, while no significant changes in the transcript levels of caspase-3 were observed. Furthermore, levels of carbonyl content and glutamate in the CX were reduced by vitB6 supplementation at the same time point of 20 h p.i.. Since our data showed a significant effect of vitB6 on the CX at 20 h p.i. rather than that at 24 h p.i., we evaluated the effect of administering a second dose of vitB6 at 18 h p.i. and sacrifice at 24 h p.i.. Reduction in the oxidative stress and APE1 levels were observed, although the latter was not significant. Although the levels of APE1 was not significantly changed in the HC with vitB6 adjuvant therapy, vitB6 supplementation prevented the formation of the truncated form of APE1 (34 kDa) that is associated with apoptosis.ConclusionsOur data suggest that PM affects APE1 expression, which can be modulated by vitB6. Additionally, vitB6 contributes to the reduction of glutamate and ROS levels. Besides the potential to reduce cell death and oxidative stress during neuroinflammation, vitB6 showed enhanced effect on the CX than on the HC during PM.

Curcumin revitalizes Amyloid beta (25-35)-induced and organophosphate pesticides pestered neurotoxicity in SH-SY5Y and IMR-32 cells via activation of APE1 and Nrf2.

Amyloid beta (Aβ) peptide deposition is the primary cause of neurodegeneration in Alzheimer's disease (AD) pathogenesis. Several reports point towards the role of pesticides in the AD pathogenesis, especially organophosphate pesticides (OPPs). Monocrotophos (MCP) and Chlorpyrifos (CP) are the most widely used OPPs. In this study, the role of MCP and CP in augmenting the Aβ-induced oxidative stress associated with the neurodegeneration in AD has been assessed in human neuroblastoma IMR-32 and SH-SY5Y cell lines. From the cell survival assay, it was observed that MCP and CP reduced cell survival both dose- and time-dependently. Nitro blue tetrazolium (NBT) based assay for determination of intracellular reactive oxygen species (ROS) demonstrated that Aβ(25-35), MCP or CP produce significant oxidative stress alone or synergistically in IMR-32 and SH-SY5Y cells, while pretreatment of curcumin reduced ROS levels significantly in all treatment combinations. In this study, we also demonstrate that treatment of Aβ(25-35) and MCP upregulated inducible nitric oxide synthase (iNOS/NOS2) whereas, no change was observed in neuronal nitric oxide synthase (nNOS/NOS1), but down-regulation of the nuclear factor erythroid 2-related factor 2 (Nrf2) level was observed. While curcumin pretreatment resulted in upregulation of iNOS and Nrf2 proteins. Also, the expression of key DNA repair enzymes APE1, DNA polymerase beta (Pol β), and PARP1 were found to be downregulated upon treatment with MCP, Aβ(25-35) and their combinations at 24h and 48h time points. In this study, pretreatment of curcumin to the SH-SY5Y cells enhanced the expression of DNA repair enzymes APE1, pol β, and PARP1 enzymes to counter the oxidative DNA base damage via base excision repair (BER) pathway, and also activated the antioxidant element (ARE) via Nrf2 upregulation. Furthermore, the immunofluorescent confocal imaging studies in SH-SY5Y and IMR-32 cells treated with Aβ(25-35) and MCP-mediated oxidative stress and their combinations at different time periods suggesting for cross-talk between the two proteins APE1 and Nrf2. The APE1's association with Nrf2 might be associated with the redox function of APE1 that might be directly regulating the ARE-mediated neuronal survival mechanisms.

Abstract 1414: Chromatin remodeling histone chaperone FACT complex modulates AP site damage repair in chromatin and sensitizes cancer cells to chemotherapeutic drugs

Abstract Recognition and repair of DNA lesions in the genome are critical for maintaining genomic stability and reducing the generation of mutations that lead to cancer development. The most frequently formed DNA lesion in the genome is the apurinic/apyrimidinic (AP) site which is mutagenic and blocks transcription and replication. The primary enzyme to repair AP sites in mammalian cells is the AP endonuclease (APE1), which often overexpresses in diverse cancer types and its overexpression is associated with patients’ resistance to chemotherapeutic drugs. The repair of AP sites by APE1 through the Base Excision Repair (BER) pathway is extensively investigated in vitro. However, how AP site is recognized by APE1 in the context of highly complex nucleosome structure in chromatin is unknown. Because DNA is packaged tightly in nucleosome, the ability of repair proteins to access sites of DNA damage and facilitates repair of the damage requires chromatin remodeling activities. Here, we show that APE1 interacts with chromatin remodeling histone chaperone complex, FACT (facilitates of chromatin transcription) via its N-terminal domain in cells. By immunoprecipitation of endogenous APE1 from chromatin fraction and separation of protein bands in SDS-PAGE followed by MALDI-TOF-TOF analysis, we have identified both subunits (SPT16 & SSRP1) of FACT as the prominent APE1 interacting partners. Subsequently, we confirmed the interaction of APE1 with FACT complex by Co-IP and immunofluorescence analysis and found that both SPT16 and SSRP1 interact with APE1 in the nucleus and in chromatin. Interestingly, we found rapid eviction of histones with concomitant degradation of FACT complex upon induction of DNA damages. Downregulation of FACT complex abrogates the nucleosome eviction, the recruitment of repair protein APE1 in chromatin and the repair of AP sites, demonstrating the functional importance of nucleosomes disruption in BER pathway and identifying chromatin remodeling protein required for the process. Notably, knock-down of FACT showed increased sensitivity of cancer cells to many chemotherapeutic drugs. We also found that FACT down-regulated cells have much higher accumulation of AP site damages in the genome compared to control cells. Our study revealed a key role of nucleosome remodeling complex FACT in DNA damage repair in BER pathway. This study also suggests that histone chaperone complex could be a potential target for enhancing sensitivity of tumor cells to chemotherapy. Citation Format: Heyu Song, Shrabasti Roychoudhury, Dan Feng, Kishor Bhakat. Chromatin remodeling histone chaperone FACT complex modulates AP site damage repair in chromatin and sensitizes cancer cells to chemotherapeutic drugs [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 1414. doi:10.1158/1538-7445.AM2017-1414

DNA base excision repair proteins APE-1 and XRCC-1 are overexpressed in oral tongue squamous cell carcinoma.

DNA repair systems play a critical role in protecting the human genome from damage caused by carcinogens. Modifications in DNA repair genes may be responsible for tumor development and resistance of malignant cells to chemotherapeutic agents. The major pathway for oxidative DNA damage repair is the base excision repair pathway. This study aimed to assess the immunoexpression of DNA repair proteins APE-1 and XRCC-1 and its association with clinical, histologic, and survival parameters in oral tongue squamous cell carcinoma, to investigate a possible role for those proteins in tumor behavior. The expression of APE-1 and XRCC-1 was evaluated by immunohistochemistry in 82 cases of oral tongue squamous cell carcinoma. Histopathological grading was performed for each case. Pearson's chi-square and Fisher's exact tests were used to determine the association between protein expressions and clinicopathological features of tumors, whereas Kaplan-Meier curves and Cox regression were used to analyze disease-specific and disease-free survival. Statistical significance was set at P ≤ 0.05. APE-1 was highly expressed in the nucleus and cytoplasm in 64.6% of cases, and XRCC-1 showed overexpression only in the nucleus in 61% of cases. High expression of XRCC-1 was significantly associated with tumors at early clinical stages (I and II, P < 0.01) and nodal status (P = 0.03). Both proteins were not associated with other clinical parameters, histopathological grading, or survival. DNA base excision repair proteins APE-1 and XRCC-1 are upregulated in oral tongue squamous cell carcinoma, and XRCC-1 expression is associated with better clinical staging and nodal status.

Aqueous Extract of Smokeless Tobacco (gutkha) Deregulates Tumor Suppressor and DNA Repair Response in a Murine Model of Smokeless Tobacco Use.

The effect of smokeless tobacco (gutkha) was investigated by treating male and female Swiss Albino mice with an aqueous extract of smokeless tobacco (AEST). AEST was administered at a dose of 25 mg kg-1 body weight per day for different time periods (6, 12, 16, and 24 weeks), and control animals were provided only drinking water without AEST for the same period. Control and AEST-treated mice were observed for different oxidative stress parameters, nitric oxide (NO) release, and myeloperoxidase (MPO) release, and they were evaluated for alterations in tumor suppressor and DNA repair responses in the liver and spleen. Both male and female mice treated with AEST showed significant increase in lipid peroxidation, protein carbonylation, and NO and MPO release in the liver and spleen compared to age- and gender-matched controls. The significant decline in tumor suppressor p53 protein levels, likely mediated by concomitantly upregulated levels of Mdm2, was observed. We also observed a significant decline in the levels of DNA repair proteins Brca2 and Ape-1 compared to the respective controls. Thus, AEST induces oxidative stress, inflammation, and significantly lowers tumor suppressor and DNA repair responses. These factors may work in conjunction to increase the risk for certain diseases, including cancer.

A threshold of endogenous stress is required to engage cellular response to protect against mutagenesis.

Endogenous stress represents a major source of genome instability, but is in essence difficult to apprehend. Incorporation of labeled radionuclides into DNA constitutes a tractable model to analyze cellular responses to endogenous attacks. Here we show that incorporation of [3H]thymidine into CHO cells generates oxidative-induced mutagenesis, but, with a peak at low doses. Proteomic analysis showed that the cellular response differs between low and high levels of endogenous stress. In particular, these results confirmed the involvement of proteins implicated in redox homeostasis and DNA damage signaling pathways. Induced-mutagenesis was abolished by the anti-oxidant N-acetyl cysteine and plateaued, at high doses, upon exposure to L-buthionine sulfoximine, which represses cellular detoxification. The [3H]thymidine-induced mutation spectrum revealed mostly base substitutions, exhibiting a signature specific for low doses (GC > CG and AT > CG). Consistently, the enzymatic activity of the base excision repair protein APE-1 is induced at only medium or high doses. Collectively, the data reveal that a threshold of endogenous stress must be reached to trigger cellular detoxification and DNA repair programs; below this threshold, the consequences of endogenous stress escape cellular surveillance, leading to high levels of mutagenesis. Therefore, low doses of endogenous local stress can jeopardize genome integrity more efficiently than higher doses.

XPC deficiency is related to APE1 and OGG1 expression and function

Oxidative DNA damage is considered to be a major cause of neurodegeneration and internal tumors observed in syndromes that result from nucleotide excision repair (NER) deficiencies, such as Xeroderma Pigmentosum (XP) and Cockayne Syndrome (CS). Recent evidence has shown that NER aids in removing oxidized DNA damage and may interact with base excision repair (BER) enzymes. Here, we investigated APE1 and OGG1 expression, localization and activity after oxidative stress in XPC-deficient cells. The endogenous APE1 and OGG1 mRNA levels were lower in XPC-deficient fibroblasts. However, XPC-deficient cells did not show hypersensitivity to oxidative stress compared with NER-proficient cells. To confirm the impact of an XPC deficiency in regulating APE1 and OGG1 expression and activity, we established an XPC-complemented cell line. Although the XPC complementation was only partial and transient, the transfected cells exhibited greater OGG1 expression and activity compared with XPC-deficient cells. However, the APE1 expression and activity did not significantly change. Furthermore, we observed a physical interaction between the XPC and APE1 proteins. Together, the results indicate that the responses of XPC-deficient cells under oxidative stress may not only be associated with NER deficiency per se but may also include new XPC functions in regulating BER proteins.

APE1, the DNA base excision repair protein, regulates the removal of platinum adducts in sensory neuronal cultures by NER

Peripheral neuropathy is one of the major side effects of treatment with the anticancer drug, cisplatin. One proposed mechanism for this neurotoxicity is the formation of platinum adducts in sensory neurons that could contribute to DNA damage. Although this damage is largely repaired by nuclear excision repair (NER), our previous findings suggest that augmenting the base excision repair pathway (BER) by overexpressing the repair protein APE1 protects sensory neurons from cisplatin-induced neurotoxicity. The question remains whether APE1 contributes to the ability of the NER pathway to repair platinum-damage in neuronal cells. To examine this, we manipulated APE1 expression in sensory neuronal cultures and measured Pt-removal after exposure to cisplatin. When neuronal cultures were treated with increasing concentrations of cisplatin for two or three hours, there was a concentration-dependent increase in Pt-damage that peaked at four hours and returned to near baseline levels after 24h. In cultures where APE1 expression was reduced by ∼80% using siRNA directed at APE1, there was a significant inhibition of Pt-removal over eight hours which was reversed by overexpressing APE1 using a lentiviral construct for human wtAPE1. Overexpressing a mutant APE1 (C65 APE1), which only has DNA repair activity, but not its other significant redox-signaling function, mimicked the effects of wtAPE1. Overexpressing DNA repair activity mutant APE1 (226+177APE1), with only redox activity was ineffective suggesting it is the DNA repair function of APE1 and not its redox-signaling, that restores the Pt-damage removal. Together, these data provide the first evidence that a critical BER enzyme, APE1, helps regulate the NER pathway in the repair of cisplatin damage in sensory neurons.