Subcellular localization of APE1/Ref-1 in human hepatocellular carcinoma: possible prognostic significance

Role of APE1 in hepatocellular carcinoma and its prospects as a target in clinical settings (Review)

In recent years, the incidence of liver cancer has increased annually. However, current medical treatments for liver cancer are limited, and most patients have a high risk of recurrence after surgery. Therefore, the discovery and development of novel treatment targets for liver cancer is urgently needed. Apurinic/apyrimidinic endonuclease 1 (APE1) is a protein that has a DNA repair function and serves an important role in various physiological processes, including reduction-oxidation, cell proliferation and differentiation. The expression levels of APE1 are abnormally elevated in liver cancer cells, as ectopic expression of the APE1 gene has been reported, in addition to other abnormal signs, such as cell proliferation and migration. Therefore, it could be suggested that APE1 is an important indicator of hepatocellular carcinogenesis. APE1 may be used as a therapeutic target for tumors and proposed targeted therapy against abnormal APE1 expression could potentially inhibit the progression of tumors. The present review aimed to introduce the important role of APE1 in the physiological processes of tumor cells and the feasibility of using APE1 as a potential therapeutic target, providing a novel direction for the clinical treatment of liver cancer.

APE1 inhibition enhances ferroptotic cell death and contributes to hepatocellular carcinoma therapy

Ferroptosis, a regulated form of cell death triggered by iron-dependent lipid peroxidation, has emerged as a promising therapeutic strategy for cancer treatment, particularly in hepatocellular carcinoma (HCC). However, the mechanisms underlying the regulation of ferroptosis in HCC remain to be unclear. In this study, we have identified a novel regulatory pathway of ferroptosis involving the inhibition of Apurinic/apyrimidinic endonuclease 1 (APE1), a key enzyme with dual functions in DNA repair and redox regulation. Our findings demonstrate that inhibition of APE1 leads to the accumulation of lipid peroxidation and enhances ferroptosis in HCC. At the molecular level, the inhibition of APE1 enhances ferroptosis which relies on the redox activity of APE1 through the regulation of the NRF2/SLC7A11/GPX4 axis. We have identified that both genetic and chemical inhibition of APE1 increases AKT oxidation, resulting in an impairment of AKT phosphorylation and activation, which leads to the dephosphorylation and activation of GSK3β, facilitating the subsequent ubiquitin-proteasome-dependent degradation of NRF2. Consequently, the downregulation of NRF2 suppresses SLC7A11 and GPX4 expression, triggering ferroptosis in HCC cells and providing a potential therapeutic approach for ferroptosis-based therapy in HCC. Overall, our study uncovers a novel role and mechanism of APE1 in the regulation of ferroptosis and highlights the potential of targeting APE1 as a promising therapeutic strategy for HCC and other cancers.

Revisiting Two Decades of Research Focused on Targeting APE1 for Cancer Therapy: The Pros and Cons

APE1 is an essential endodeoxyribonuclease of the base excision repair pathway that maintains genome stability. It was identified as a pivotal factor favoring tumor progression and chemoresistance through the control of gene expression by a redox-based mechanism. APE1 is overexpressed and serum-secreted in different cancers, representing a prognostic and predictive factor and a promising non-invasive biomarker. Strategies directly targeting APE1 functions led to the identification of inhibitors showing potential therapeutic value, some of which are currently in clinical trials. Interestingly, evidence indicates novel roles of APE1 in RNA metabolism that are still not fully understood, including its activity in processing damaged RNA in chemoresistant phenotypes, regulating onco-miRNA maturation, and oxidized RNA decay. Recent data point out a control role for APE1 in the expression and sorting of onco-miRNAs within secreted extracellular vesicles. This review is focused on giving a portrait of the pros and cons of the last two decades of research aiming at the identification of inhibitors of the redox or DNA-repair functions of APE1 for the definition of novel targeted therapies for cancer. We will discuss the new perspectives in cancer therapy emerging from the unexpected finding of the APE1 role in miRNA processing for personalized therapy.

APE1 promotes non-homologous end joining by initiating DNA double-strand break formation and decreasing ubiquitination of artemis following oxidative genotoxic stress

BACKGROUND

Apurinic/apyrimidinic endonuclease 1 (APE1) imparts radio-resistance by repairing isolated lesions via the base excision repair (BER) pathway, but whether and how it is involved in the formation and/or repair of DSBs remains mostly unknown.

METHODS

Immunoblotting, fluorescent immunostaining, and the Comet assay were used to investigate the effect of APE1 on temporal DSB formation. Chromatin extraction, 53BP1 foci and co-immunoprecipitation, and rescue assays were used to evaluate non-homologous end joining (NHEJ) repair and APE1 effects. Colony formation, micronuclei measurements, flow cytometry, and xenograft models were used to examine the effect of APE1 expression on survival and synergistic lethality. Immunohistochemistry was used to detect APE1 and Artemis expression in cervical tumor tissues.

RESULTS

APE1 is upregulated in cervical tumor tissue compared to paired peri-tumor, and elevated APE1 expression is associated with radio-resistance. APE1 mediates resistance to oxidative genotoxic stress by activating NHEJ repair. APE1, via its endonuclease activity, initiates clustered lesion conversion to DSBs (within 1 h), promoting the activation of the DNA-dependent protein kinase catalytic subunit (DNA-PK_cs), a key kinase in the DNA damage response (DDR) and NHEJ pathway. APE1 then participates in NHEJ repair directly by interacting with DNA- PK_cs. Additionally, APE1 promotes NHEJ activity by decreasing the ubiquitination and degradation of Artemis, a nuclease with a critical role in the NHEJ pathway. Overall, APE1 deficiency leads to DSB accumulation at a late phase following oxidative stress (after 24 h), which also triggers activation of Ataxia-telangiectasia mutated (ATM), another key kinase of the DDR. Inhibition of ATM activity significantly promotes synergistic lethality with oxidative stress in APE1-deficient cells and tumors.

CONCLUSION

APE1 promotes NHEJ repair by temporally regulating DBS formation and repair following oxidative stress. This knowledge provides new insights into the design of combinatorial therapies and indicates the timing of administration and maintenance of DDR inhibitors for overcoming radio-resistance.

α-synucleinopathy exerts sex-dimorphic effects on the multipurpose DNA repair/redox protein APE1 in mice and humans

Lewy body disorders are characterized by oxidative damage to DNA and inclusions rich in aggregated forms of α-synuclein. Among other roles, apurinic/apyrimidinic endonuclease 1 (APE1) repairs oxidative DNA damage, and APE1 polymorphisms have been linked to cases of Lewy body disorders. However, the link between APE1 and α-synuclein is unexplored. We report that knockdown or inhibition of APE1 amplified inclusion formation in primary hippocampal cultures challenged with preformed α-synuclein fibrils. Fibril infusions into the mouse olfactory bulb/anterior olfactory nucleus (OB/AON) elicited a modest decrease in APE1 expression in the brains of male mice but an increase in females. Similarly, men with Lewy body disorders displayed lower APE1 expression in the OB and amygdala compared to women. Preformed fibril infusions of the mouse OB/AON induced more robust base excision repair of DNA lesions in females than males. No fibril-mediated loss of APE1 expression was observed in male mice when the antioxidant N-acetylcysteine was added to their diet. These findings reveal a potential sex-biased link between α-synucleinopathy and APE1 in mice and humans. Further studies are warranted to determine how this multifunctional protein modifies α-synuclein inclusions and, conversely, how α-synucleinopathy and biological sex interact to modify APE1.

Apurinic/apyrimidinic endonuclease 1 is associated with poor prognosis after curative resection followed by adjuvant chemotherapy in patients with stage III colon cancer

Purpose

Apurinic/apyrimidinic endonuclease 1 (APE1) is a key enzyme involved in the base excision repair pathway. It also has redox activity and maintains various transcription factors in an active reduced state. APE1 may be associated with chemoresistance. In the present study, we first investigated the expression level of APE1 protein and its correlation with oncologic outcomes of oxaliplatin-based chemotherapy in patients with stage III colon cancer. Further, we investigated the effects of human APE1 siRNA on the sensitivity of oxaliplatin in SNU-C2A colon cancer cells.

Methods

Tissue specimens from tumor and normal colon of 33 patients with stage III colon cancer were obtained from 2006 to 2009. The patients received at least eight cycles of oxaliplatin-based chemotherapy. APE1 expression was analyzed by immunohistochemistry and Western blotting using a cultured SNU-C2A cell line. Cell viability and apoptosis were determined by Cell Counting Kit-8 and caspase-3 cleavage using Western blotting.

Results

All the colon cancer tissues showed APE1 staining in the nucleus, whereas all the normal colon tissues were negative for APE1 staining in the cytoplasm. The group with a higher expression of APE1 demonstrated poorer prognosis than the group with low expression (P=0.026 for overall survival and P=0.021 for disease-free survival). Treatment with oxaliplatin resulted in a dose-dependent increase in APE1 expression in SNU-C2A cells. APE1 siRNA significantly enhanced oxaliplatin-induced growth inhibition, and also increased oxaliplatin-induced apoptosis in SNU-C2A cells.

Conclusion

APE1 could be considered a prognostic factor in colon cancer patients treated with oxaliplatin-based chemotherapy.

Oxidative stress-mediated epigenetic regulation by G-quadruplexes

Many cancer-associated genes are regulated by guanine (G)-rich sequences that are capable of refolding from the canonical duplex structure to an intrastrand G-quadruplex. These same sequences are sensitive to oxidative damage that is repaired by the base excision repair glycosylases OGG1 and NEIL1–3. We describe studies indicating that oxidation of a guanosine base in a gene promoter G-quadruplex can lead to up- and downregulation of gene expression that is location dependent and involves the base excision repair pathway in which the first intermediate, an apurinic (AP) site, plays a key role mediated by AP endonuclease 1 (APE1/REF1). The nuclease activity of APE1 is paused at a G-quadruplex, while the REF1 capacity of this protein engages activating transcription factors such as HIF-1α, AP-1 and p53. The mechanism has been probed by in vitro biophysical studies, whole-genome approaches and reporter plasmids in cellulo. Replacement of promoter elements by a G-quadruplex sequence usually led to upregulation, but depending on the strand and precise location, examples of downregulation were also found. The impact of oxidative stress-mediated lesions in the G-rich sequence enhanced the effect, whether it was positive or negative.

New perspectives in cancer biology from a study of canonical and non-canonical functions of base excision repair proteins with a focus on early steps

Alterations of DNA repair enzymes and consequential triggering of aberrant DNA damage response (DDR) pathways are thought to play a pivotal role in genomic instabilities associated with cancer development, and are further thought to be important predictive biomarkers for therapy using the synthetic lethality paradigm. However, novel unpredicted perspectives are emerging from the identification of several non-canonical roles of DNA repair enzymes, particularly in gene expression regulation, by different molecular mechanisms, such as (i) non-coding RNA regulation of tumour suppressors, (ii) epigenetic and transcriptional regulation of genes involved in genotoxic responses and (iii) paracrine effects of secreted DNA repair enzymes triggering the cell senescence phenotype. The base excision repair (BER) pathway, canonically involved in the repair of non-distorting DNA lesions generated by oxidative stress, ionising radiation, alkylation damage and spontaneous or enzymatic deamination of nucleotide bases, represents a paradigm for the multifaceted roles of complex DDR in human cells. This review will focus on what is known about the canonical and non-canonical functions of BER enzymes related to cancer development, highlighting novel opportunities to understand the biology of cancer and representing future perspectives for designing new anticancer strategies. We will specifically focus on APE1 as an example of a pleiotropic and multifunctional BER protein.

High nuclear expression of APE1 correlates with unfavorable prognosis and promotes tumor growth in hepatocellular carcinoma

APE1 is a multifunctional protein that plays important roles in cancer development. However, the association between APE1 expression and the clinicopathological parameters of HCC patients has not been fully characterized. In this study, bioinformatics analysis of APE1 was performed in several databases, including the TCGA, GeneCard, Human Protein Atlas and Ualcan databases. The relationship between APE1 mRNA expression and several attributes of liver cancer patients in TCGA was investigated. Then, the protein expression of APE1 was detected by IHC analysis in 95 HCC samples and the association between APE1 expression and the clinicopathological parameters of HCC patients was explored. GSEA-KEGG analysis was performed to predict the potential signaling pathways that associated with APE1 expression. Then the siRNA-mediated knockdown model of APE1 was constructed in HCC cell line to further detect the detailed function of APE1 in HCC development in vitro and in vivo. The results of the bioinformatics analysis showed that APE1 expression was primarily located in the cell nucleus. APE1 mRNA expression was substantially correlated with pathological grade and T status in TCGA database. Elevated APE1 expression was observed in HCC samples and was associated with unfavorable survival time in liver cancer patients. IHC data demonstrated that the nuclear expression of APE1 in HCC tissues was significantly higher than that in noncancerous tissues. The expression level of the APE1 protein in HCC was strongly associated with tumor diameter and overall survival. Survival analysis indicated that APE1 nuclear expression is an independent prognostic marker for the overall survival of HCC patients. GSEA-KEGG results confirmed that APE1 associated with the base excision repair signaling pathway. The data of phenotypic experiments indicated that APE1 remarkably promoted tumor growth both in HCC cells and xenografts. The findings firstly imply that nuclear expression of APE1 is a valuable prognostic marker for HCC. APE1 significantly facilitate HCC development and targeting APE1 may be a promising strategy for HCC treatment.

APE1/Ref-1 - One Target with Multiple Indications: Emerging Aspects and New Directions

In the realm of DNA repair, base excision repair (BER) protein, APE1/Ref-1 (Apurinic/Apyrimidinic Endonuclease 1/Redox Effector - 1, also called APE1) has been studied for decades. However, over the past decade, APE1 has been established as a key player in reduction-oxidation (redox) signaling. In the review by Caston et al. (The multifunctional APE1 DNA repair-redox signaling protein as a drug target in human disease), multiple roles of APE1 in cancer and other diseases are summarized. In this Review, we aim to expand on the contributions of APE1 to various diseases and its effect on disease progression. In the scope of cancer, more recent roles for APE1 have been identified in cancer cell metabolism, as well as chemotherapy-induced peripheral neuropathy (CIPN) and inflammation. Outside of cancer, APE1 signaling may be a critical factor in inflammatory bowel disease (IBD) and is also an emergent area of investigation in retinal ocular diseases. The ability of APE1 to regulate multiple transcription factors (TFs) and therefore multiple pathways that have implications outside of cancer, makes it a particularly unique and enticing target. We discuss APE1 redox inhibitors as a means of studying and potentially combating these diseases. Lastly, we examine the role of APE1 in RNA metabolism. Overall, this article builds on our previous review to elaborate on the roles and conceivable regulation of important pathways by APE1 in multiple diseases.

Mitochondrial apurinic/apyrimidinic endonuclease 1 enhances mtDNA repair contributing to cell proliferation and mitochondrial integrity in early stages of hepatocellular carcinoma

BACKGROUND

Hepatocellular carcinoma (HCC) is the leading cause of primary liver cancers. Surveillance of individuals at specific risk of developing HCC, early diagnostic markers, and new therapeutic approaches are essential to obtain a reduction in disease-related mortality. Apurinic/apyrimidinic endonuclease 1 (APE1) expression levels and its cytoplasmic localization have been reported to correlate with a lower degree of differentiation and shorter survival rate. The aim of this study is to fully investigate, for the first time, the role of the mitochondrial form of APE1 in HCC.

METHODS

As a study model, we analyzed samples from a cohort of patients diagnosed with HCC who underwent surgical resection. Mitochondrial APE1 content, expression levels of the mitochondrial import protein Mia40, and mtDNA damage of tumor tissue and distal non-tumor liver of each patient were analyzed. In parallel, we generated a stable HeLa clone for inducible silencing of endogenous APE1 and re-expression of the recombinant shRNA resistant mitochondrially targeted APE1 form (MTS-APE1). We evaluated mtDNA damage, cell growth, and mitochondrial respiration.

RESULTS

APE1's cytoplasmic positivity in Grades 1 and 2 HCC patients showed a significantly higher expression of mitochondrial APE1, which accounted for lower levels of mtDNA damage observed in the tumor tissue with respect to the distal area. In the contrast, the cytoplasmic positivity in Grade 3 was not associated with APE1's mitochondrial accumulation even when accounting for the higher number of mtDNA lesions measured. Loss of APE1 expression negatively affected mitochondrial respiration, cell viability, and proliferation as well as levels of mtDNA damage. Remarkably, the phenotype was efficiently rescued in MTS-APE1 clone, where APE1 is present only within the mitochondrial matrix.

CONCLUSIONS

Our study confirms the prominent role of the mitochondrial form of APE1 in the early stages of HCC development and the relevance of the non-nuclear fraction of APE1 in the disease progression. We have also confirmed overexpression of Mia40 and the role of the MIA pathway in the APE1 import process. Based on our data, inhibition of the APE1 transport by blocking the MIA pathway could represent a new therapeutic approach for reducing mitochondrial metabolism by preventing the efficient repair of mtDNA.

Sorafenib Resistance in Hepatocellular Carcinoma: The Relevance of Genetic Heterogeneity

Despite advances in biomedicine, the incidence and the mortality of hepatocellular carcinoma (HCC) remain high. The majority of HCC cases are diagnosed in later stages leading to the less than optimal outcome of the treatments. Molecular targeted therapy with sorafenib, a dual-target inhibitor targeting the serine-threonine kinase Raf and the tyrosine kinases VEGFR/PDGFR, is at present the main treatment for advanced-stage HCC, either in a single or combinatory regimen. However, it was observed in a large number of patients that its effectiveness is hampered by drug resistance. HCC is highly heterogeneous, within the tumor and among individuals, and this influences disease progression, classification, prognosis, and naturally cellular susceptibility to drug resistance. This review aims to provide an insight on how HCC heterogeneity influences the different primary mechanisms of chemoresistance against sorafenib including reduced drug intake, enhanced drug efflux, intracellular drug metabolism, alteration of molecular targets, activation/inactivation of signaling pathways, changes in the DNA repair machinery, and negative balance between apoptosis and survival of the cancer cells. The diverse variants, mutations, and polymorphisms in molecules and their association with drug response can be a helpful tool in treatment decision making. Accordingly, the existence of heterogeneous biomarkers in the tumor must be considered to strengthen multi-target strategies in patient-tailored treatment.

Base Excision Repair in the Immune System: Small DNA Lesions With Big Consequences

The integrity of the genome is under constant threat of environmental and endogenous agents that cause DNA damage. Endogenous damage is particularly pervasive, occurring at an estimated rate of 10,000–30,000 per cell/per day, and mostly involves chemical DNA base lesions caused by oxidation, depurination, alkylation, and deamination. The base excision repair (BER) pathway is primary responsible for removing and repairing these small base lesions that would otherwise lead to mutations or DNA breaks during replication. Next to preventing DNA mutations and damage, the BER pathway is also involved in mutagenic processes in B cells during immunoglobulin (Ig) class switch recombination (CSR) and somatic hypermutation (SHM), which are instigated by uracil (U) lesions derived from activation-induced cytidine deaminase (AID) activity. BER is required for the processing of AID-induced lesions into DNA double strand breaks (DSB) that are required for CSR, and is of pivotal importance for determining the mutagenic outcome of uracil lesions during SHM. Although uracils are generally efficiently repaired by error-free BER, this process is surprisingly error-prone at the Ig loci in proliferating B cells. Breakdown of this high-fidelity process outside of the Ig loci has been linked to mutations observed in B-cell tumors and DNA breaks and chromosomal translocations in activated B cells. Next to its role in preventing cancer, BER has also been implicated in immune tolerance. Several defects in BER components have been associated with autoimmune diseases, and animal models have shown that BER defects can cause autoimmunity in a B-cell intrinsic and extrinsic fashion. In this review we discuss the contribution of BER to genomic integrity in the context of immune receptor diversification, cancer and autoimmune diseases.

Graphene Quantum Dot-Based Nanocomposites for Diagnosing Cancer Biomarker APE1 in Living Cells

As an essential DNA repair enzyme, apurinic/apyrimidinic endonuclease 1 (APE1) is overexpressed in most human cancers and is identified as a cancer diagnostic and predictive biomarker for cancer risk assessment, diagnosis, prognosis, and prediction of treatment efficacy. Despite its importance in cancer, however, it is still a significant challenge nowadays to sense abundance variation and monitor enzymatic activity of this biomarker in living cells. Here, we report our construction of biocompatible functional nanocomposites, which are a combination of meticulously designed unimolecular DNA and fine-sized graphene quantum dots. Upon utilization of these nanocomposites as diagnostic probes, massive accumulation of fluorescence signal in living cells can be triggered by merely a small amount of cellular APE1 through repeated cycles of enzymatic catalysis. Most critically, our delicate structural designs assure that these graphene quantum dot-based nanocomposites are capable of sensing cancer biomarker APE1 in identical type of cells under different cell conditions and can be applied to multiple cancerous cells in a highly sensitive and specific manners. This work not only brings about new methods for cytology-based cancer screening but also lays down a general principle for fabricating diagnostic probes that target other endogenous biomarkers in living cells.

Cytoplasmic APE1 promotes resistance response in osteosarcoma patients with cisplatin treatment

Chemotherapy resistance has become a hold back and major clinical challenge in osteosarcoma cancer. The alteration and subcellular distribution of apurinic/apyrimidinic endonuclease 1 (APE1) has been reported to be involved in chemotherapy resistance in many cancers. Here, we report that the cytoplasmic distribution of APE1 plays a key role in the sensitivity of combination platinum chemotherapy in osteosarcoma. Interestingly, the prevalence of cisplatin-induced DNA damage and apoptosis in low cytoplasmic APE1 osteosarcoma cell lines was higher than in high expression of cytoplasmic APE1 cell lines. Overexpression of cytoplasmic APE1 protected the osteosarcoma cells from CDDP-induced apoptosis. In addition, clinical data also show that the level of cytoplasmic APE1 was negatively associated with sensitivity to combination chemotherapy of cisplatin in osteosarcoma patients. Our findings suggest that cytoplasmic APE1 plays a significant role in chemotherapy resistance. This role is a supplement to the extranuclear function of APE1, and cytoplasmic APE1 expression level could be a promising predictor of platinum treatment prognosis for osteosarcoma patients.

Surveillance of Tumour Development: The Relationship Between Tumour-Associated RNAs and Ribonucleases

Tumour progression is accompanied by rapid cell proliferation, loss of differentiation, the reprogramming of energy metabolism, loss of adhesion, escape of immune surveillance, induction of angiogenesis, and metastasis. Both coding and regulatory RNAs expressed by tumour cells and circulating in the blood are involved in all stages of tumour progression. Among the important tumour-associated RNAs are intracellular coding RNAs that determine the routes of metabolic pathways, cell cycle control, angiogenesis, adhesion, apoptosis and pathways responsible for transformation, and intracellular and extracellular non-coding RNAs involved in regulation of the expression of their proto-oncogenic and oncosuppressing mRNAs. Considering the diversity/variability of biological functions of RNAs, it becomes evident that extracellular RNAs represent important regulators of cell-to-cell communication and intracellular cascades that maintain cell proliferation and differentiation. In connection with the elucidation of such an important role for RNA, a surge in interest in RNA-degrading enzymes has increased. Natural ribonucleases (RNases) participate in various cellular processes including miRNA biogenesis, RNA decay and degradation that has determined their principal role in the sustention of RNA homeostasis in cells. Findings were obtained on the contribution of some endogenous ribonucleases in the maintenance of normal cell RNA homeostasis, which thus prevents cell transformation. These findings directed attention to exogenous ribonucleases as tools to compensate for the malfunction of endogenous ones. Recently a number of proteins with ribonuclease activity were discovered whose intracellular function remains unknown. Thus, the comprehensive investigation of physiological roles of RNases is still required. In this review we focused on the control mechanisms of cell transformation by endogenous ribonucleases, and the possibility of replacing malfunctioning enzymes with exogenous ones.

APEX1 Expression as a Potential Diagnostic Biomarker of Clear Cell Renal Cell Carcinoma and Hepatobiliary Carcinomas

Apurinic/apyrimidinic endonuclease 1/redox effector factor 1 (APEX1) has been known to play key roles in DNA repair, the regulation of diverse transcriptional activity, and cellular responses to redox activity. This study aimed to examine serum APEX1 (s-APEX1) expression as a possible screening biomarker for clear cell renal cell carcinoma (ccRCC), hepatocellular carcinoma (HCC), and proximal and distal cholangiocarcinoma (CC). A total of 216 frozen serum samples were collected from 39 healthy control cases, 32 patients with ≥58 copies/mL of hepatitis B viral DNA (HBV DNA (+)), 40 ccRCC cases, 59 HCC cases, and 46 CC cases. The serum samples were examined for s-APEX1 concentration by enzyme-linked immunosorbent assay. The association of APEX1 expression with clinicopathological characteristics was also studied by immunohistochemical staining in 106 ccRCC, 131 HCC, and 32 intrahepatic CC cases. The median s-APEX1 concentrations of the HCC, CC, ccRCC, healthy control, and HBV DNA (+) groups were 0.294, 0.710, 0.474, 0.038, and 2.384 ng/mL, respectively (p < 0.001). Univariate and multivariate analyses revealed that increased cytoplasmic APEX1 expression led to a shorter disease-free survival period in HCC and CC cases. We suggest that the s-APEX1 level could be a potential diagnostic biomarker of ccRCC, HCC, and CC. Additionally, cytoplasmic APEX1 expression in cancer cells could be used to predict relapses in patients with HCC or CC.

Probing and regulating the activity of cellular enzymes by using DNA tetrahedron nanostructures

Given the essential role of apurinic/apyrimidinic endonuclease (APE1) in gene repair and cancer progression, we report a novel approach for probing and regulating cellular APE1 activity by using DNA tetrahedrons.

Given the essential role of apurinic/apyrimidinic endonuclease (APE1) in gene repair and cancer progression, we report a novel approach for probing and regulating cellular APE1 activity by using DNA tetrahedrons. The tetrahedron with an AP site-containing antenna exhibits high sensitivity and specificity to APE1. It is suitable for APE1 in vitro detection (detection limit 5 pM) and cellular fluorescence imaging without any auxiliary transfection reagents, which discriminates the APE1 expression level of cancer cells and normal cells. In contrast, the tetrahedron with an AP site on its scaffold exhibits high binding affinity to APE1 but limits enzymatic catalysis making this nanostructure an APE1 inhibitor with an IC₅₀ of 14.8 nM. It suppresses the APE1 activity in living cells and sensitizes cancer cells to anticancer drugs. We also demonstrate that the APE1 probe and inhibitor can be switched allosterically via stand displacement, which holds potential for reversible inhibition of APE1. Our approach provides a new way for fabricating enzyme probes and regulators and new insights into enzyme–substrate interactions, and it can be expanded to regulate other nucleic acid related enzymes.

Serum AP-endonuclease 1 (sAPE1) as novel biomarker for hepatocellular carcinoma

Late diagnosis for Hepatocellular Carcinoma (HCC) remains one of the leading causes for the high mortality rate. The apurinic/apyrimidinic endonuclease 1 (APE1), an essential member of the base excision DNA repair (BER) pathway, contributes to cell response to oxidative stress and has other non-repair activities. In this study, we evaluate the role of serum APE1 (sAPE1) as a new diagnostic biomarker and we investigate the biological role for extracellular APE1 in HCC. sAPE1 level was quantified in 99 HCC patients, 50 non-HCC cirrhotic and 100 healthy controls. The expression level was significantly high in HCC (75.8 [67.3–87.9] pg/mL) compared to cirrhosis (29.8 [18.3–36.5] pg/mL] and controls (10.8 [7.5–13.2] pg/mL) (p < 0.001). The sAPE1 level corresponded with its protein expression in HCC tissue. sAPE1 had high diagnostic accuracy to differentiate HCC from cirrhotic (AUC = 0.87, sensitivity 88%, specificity 71%, cut-off of 36.3 pg/mL) and healthy subjects (AUC 0.98, sensibility 98% and specificity 83%, cut-off of 19.0 pg/mL). Recombinant APE1, exogenously added to JHH6 cells, significantly promotes IL-6 and IL-8 expression, suggesting a role of sAPE1 as a paracrine pro-inflammatory molecule, which may modulate the inflammatory status in cancer microenvironment. We described herein, for the first time to our knowledge, that sAPE1 might be considered as a promising diagnostic biomarker for HCC.

C3HeB/FeJ Mice mimic many aspects of gene expression and pathobiological features of human hepatocellular carcinoma

Hepatocellular carcinoma (HCC) remains a deadly cancer, underscoring the need for relevant preclinical models. Male C3HeB/FeJ mice model spontaneous HCC with some hepatocarcinogenesis susceptibility loci corresponding to syntenic regions of human chromosomes altered in HCC. We tested other properties of C3HeB/FeJ tumors for similarity to human HCC. C3HeB/FeJ tumors were grossly visible at 4 months of age, with prevalence and size increasing until about 11 months of age. Histologic features shared with human HCC include hepatosteatosis, tumor progression from dysplasia to poorly differentiated, vascular invasion, and trabecular, oncocytic, vacuolar, and clear cell variants. More tumor cells displayed cytoplasmic APE1 staining versus normal liver. Ultrasound effectively detected and monitored tumors, with 85.7% sensitivity. Over 5000 genes were differentially expressed based on the GSE62232 and GSE63898 human HCC datasets. Of these, 158 and 198 genes, respectively, were also differentially expressed in C3HeB/FeJ. Common cancer pathways, cell cycle, p53 signaling and other molecular aspects, were shared between human and mouse differentially expressed genes. We established eigengenes that distinguish HCC from normal liver in the C3HeB/FeJ model and a subset of human HCC. These features extend the relevance and improve the utility of the C3HeB/FeJ line for HCC studies.

Dysregulated Expression and Subcellular Localization of Base Excision Repair (BER) Pathway Enzymes in Gallbladder Cancer

Base excision repair (BER) pathway is one of the repair systems that has an impact on radiotherapy and chemotherapy for cancer patients. The molecular pathogenesis of gallbladder cancer is not known extensively. In the present study we investigated whether the expression of AP endonuclease 1 (APE1) and DNA polymerase β (DNA pol β), key enzymes of BER pathway has any clinical significance with gallbladder carcinogenesis. 41 gallbladder cancer, 27 chronic cholecystitis, and 3 normal gallbladder specimens were analyzed for the expression of APE1 and DNA polymerase β by western blotting, and subcellular localization studied by immunohistochemistry. The enzymatic activity of APE1 was also studied. The correlations with expression of the above proteins with clinical-pathological characteristics of gallbladder cancer patients were analyzed. The integrated density value ratio (relative expression) of total APE1 (37 kDa + 35 kDa variant) analyzed in the three groups of tissues, was 0.76±0.03 in normal gallbladder, 0.91±0.08 in chronic cholecystitis, and 1.12±0.05 in gallbladder cancer. APE1 was found to be up-regulated in 80% of gallbladder carcinoma samples (P = 0.01). A positive trend of APE1 expression with tumor stage and lymph node positivity was observed. The enzymatic activity of APE1 was found higher in gallbladder cancer samples in comparison with chronic cholecystitis. The integrated density value ratio of DNA polymerase β for normal gallbladder, chronic cholecystitis and gallbladder cancer tissue samples were 0.46±0.03, 0.7±0.06 and 1.33±0.1, respectively. DNA polymerase β was found to be upregulated in almost all gallbladder carcinoma samples (P =0.0001), and its expression was negatively correlated with age (P=0.02). DNA polymerase β expression showed a positive trend with tumor stage and nuclear differentiation of gallbladder cancer. It may be concluded that alteration of these BER pathway proteins may be the causal factors for carcinogenesis of gallbladder, and has targeted therapeutic potential.

GADD45α sensitizes cervical cancer cells to radiotherapy via increasing cytoplasmic APE1 level

Radioresistance remains a major clinical challenge in cervical cancer therapy. However, the mechanism for the development of radioresistance in cervical cancer is unclear. Herein, we determined that growth arrest and DNA-damage-inducible protein 45α (GADD45α) is decreased in radioresistant cervical cancer compared to radiosensitive cancer both in vitro and in vivo. In addition, silencing GADD45α prevents cervical cancer cells from undergoing radiation-induced DNA damage, cell cycle arrest, and apoptosis. More importantly, our data show that the overexpression of GADD45α significantly enhances the radiosensitivity of radioresistant cervical cancer cells. These data show that GADD45α decreases the cytoplasmic distribution of APE1, thereby enhancing the radiosensitivity of cervical cancer cells. Furthermore, we show that GADD45α inhibits the production of nitric oxide (NO), a nuclear APE1 export stimulator, by suppressing both endothelial NO synthase (eNOS) and inducible NO synthase (iNOS) in cervical cancer cells. In conclusion, our findings suggest that decreased GADD45α expression significantly contributes to the development of radioresistance and that ectopic expression of GADD45α sensitizes cervical cancer cells to radiotherapy. GADD45α inhibits the NO-regulated cytoplasmic localization of APE1 through inhibiting eNOS and iNOS, thereby enhancing the radiosensitivity of cervical cancer cells.

Bu-Fei decoction and modified Bu-Fei decoction inhibit the growth of non-small cell lung cancer, possibly via inhibition of apurinic/apyrimidinic endonuclease 1

Human apurinic/apyrimidinic endonuclease 1 (APE1) is a ubiquitous multifunctional protein, which possesses DNA repair and redox activities. High levels of APE1 are associated with chemo‑ and radioresistance, and poor prognosis in various types of cancer, including non‑small cell lung cancer (NSCLC). Bu‑Fei decoction (BFD) is a traditional Chinese herbal formula, which is believed to supplement Qi, clear away heat and nourish the lungs. BFD and modified Bu‑Fei decoction (MBFD) have been used in China to treat patients with lung cancer. The present study aimed to evaluate the potential antitumor effects of BFD and MBFD on NSCLC in vitro and in vivo. In addition, the possible contribution of APE1 was examined. MTT assay was used to investigated the anti-tumor activity of BFD and MBFD on H1975 and H292 NSCLC cell lines. The DNA damage of cells in the control and the experimental groups was detected using comet assay. The in vivo anti-tumor effects of BFD and MBFD were evaluated in a NSCLC tumor nude mouse xenograft model. Polymerase chain reaction (PCR), reverse transcription‑quantitative PCR (RT‑qPCR) analysis and western blot analysis were applied to analyze the mRNA and protein expression levels of APE1 in H1975 and H292 cells, so as to the xenograft tumor tissues. The concentration of APE1 in mice plasma was determined using enzyme linked immunosorbent assay (ELISA). In vitro, BFD and MBFD inhibited the growth of cultured H1975 and H292 NSCLC cells. The results of a comet assay revealed that BFD and MBFD increased DNA damage. Furthermore, the expression levels of APE1 were decreased in response to BFD and MBFD at the mRNA and protein levels. In mice carrying NSCLC xenografts, BFD and MBFD inhibited tumor growth and decreased APE1 expression. In addition, in normal human lung bronchial epithelial BEAS‑2B cells, the half maximal inhibitory concentrations of BFD and MBFD were much higher compared with in NSCLC cells, and they had no effect on DNA damage. These results suggested that BFD and MBFD may inhibit the growth of NSCLC, possibly by inhibiting APE1 expression.

Lead facilitates foci formation in a Balb/c-3T3 two-step cell transformation model: role of Ape1 function

Several possible mechanisms have been examined to gain an understanding on the carcinogenic properties of lead, which include among others, mitogenesis, alteration of gene expression, oxidative damage, and inhibition of DNA repair. The aim of the present study was to explore if low concentrations of lead, relevant for human exposure, interfere with Ape1 function, a base excision repair enzyme, and its role in cell transformation in Balb/c-3T3. Lead acetate 5 and 30 μM induced APE1 mRNA and upregulation of protein expression. This increase in mRNA expression is consistent throughout the chronic exposure. Additionally, we also found an impaired function of Ape1 through molecular beacon-based assay. To evaluate the impact of lead on foci formation, a Balb/c-3T3 two-step transformation model was used. Balb/c-3T3 cells were pretreated 1 week with low concentrations of lead before induction of transformation with n-methyl-n-nitrosoguanidine (MNNG) (0.5 μg/mL) and 12-O-tetradecanoylphorbol-13-acetate (TPA) (0.1 μg/mL) (a classical two-step protocol). Morphological cell transformation increased in response to lead pretreatment that was paralleled with an increase in Ape1 mRNA and protein overexpression and an impairment of Ape1 activity and correlating with foci number. In addition, we found that lead pretreatment and MNNG (transformation initiator) increased DNA damage, determined by comet assay. Our data suggest that low lead concentrations (5, 30 μM) could play a facilitating role in cellular transformation, probably through the impaired function of housekeeping genes such as Ape1, leading to DNA damage accumulation and chromosomal instability, one of the most important hallmarks of cancer induced by chronic exposures.

Paradoxical induction of growth arrest and apoptosis by EGF via the up-regulation of PTEN by activating Redox factor-1/Egr-1 in human lung cancer cells

Epidermal growth factor (EGF) signaling promotes cell proliferation and survival in several types of cancer. Here, however, we showed that EGF inhibits proliferation and promotes programmed cell death in non-small cell lung cancer (NSCLC) cells. In A549 cells, EGF increased redox factor-1 (Ref-1) expression and the association of Ref-1 with zinc finger-containing transcriptional regulator (EGR1) via activation of p22phox, RAC1, and an NADPH oxidase subunit. EGF increased p22phox and RAC1 expression through activation of purinergic receptors (P2Y). Elevated Ref-1/EGR1 levels increased phosphatase and tensin homolog (PTEN) levels, leading to inhibition of the Akt pathway. EGF-induced PTEN upregulation increased apoptosis and autophagy-induced damage in A549 cells, whereas Ref-1 knockdown blocked EGF-induced PTEN upregulation in an NADPH oxidase p22phox subunit-independent manner. In addition, p22phox knockdown restored EGF-induced effects, implying that changes in P2Y activity caused by EGF, which activates NADPH oxidase via RAC1, influenced Ref-1-mediated redox regulation. Finally, EGF similarly attenuated cell proliferation and promoted autophagy and apoptosis in vivo in a xenograft model using A549 cells. These findings reveal that EGF-induced redox signaling is linked to Ref-1-induced death in NSCLC cells.

APE1/Ref-1 redox-specific inhibition decreases survivin protein levels and induces cell cycle arrest in prostate cancer cells

A key feature of prostate cancer progression is the induction and activation of survival proteins, including the Inhibitor of Apoptosis (IAP) family member survivin. Apurinic/apyrimidinic endonuclease 1/redox effector factor 1 (APE1/Ref-1) is a multifunctional protein that is essential in activating oncogenic transcription factors. Because APE1/Ref-1 is expressed and elevated in prostate cancer, we sought to characterize APE1/Ref-1 expression and activity in human prostate cancer cell lines and determine the effect of selective reduction-oxidation (redox) function inhibition on prostate cancer cells in vitro and in vivo. Due to the role of oncogenic transcriptional activators NFĸB and STAT3 in survivin protein expression, and APE1/Ref-1 redox activity regulating their transcriptional activity, we assessed selective inhibition of APE1/Ref-1's redox function as a novel method to halt prostate cancer cell growth and survival. Our study demonstrates that survivin and APE1/Ref-1 are significantly higher in human prostate cancer specimens compared to noncancerous controls and that APE1/Ref-1 redox-specific inhibition with small molecule inhibitor, APX3330 and a second-generation inhibitor, APX2009, decreases prostate cancer cell proliferation and induces cell cycle arrest. Inhibition of APE1/Ref-1 redox function significantly reduced NFĸB transcriptional activity, survivin mRNA and survivin protein levels. These data indicate that APE1/Ref-1 is a key regulator of survivin and a potentially viable target in prostate cancer.

The expression of APE1 in triple-negative breast cancer and its effect on drug sensitivity of olaparib

Triple-negative breast cancer is a kind of breast cancer with poor prognosis and special biological behavior, which lacked endocrine therapy and targeted therapy. We investigate the effect of human APE1 (apurinic/apyrimidyl endonuclease 1), a rate-limiting enzyme of base excision repair, on the prognosis in triple-negative breast cancer and drug sensitivity of olaparib. The expression of APE1 was detected by immunohistochemistry in the triple-negative breast cancer tissues and its effect on survival of triple-negative breast cancer patients was followed. To find whether APE1 effect the drug sensitivity in triple-negative breast cancer cells, the APE1-knockout HCC1937 cell line (triple-negative breast cancer cell line) was established by CRISPR/Cas9 system. Then, we use the wild-type and knockout one to test the drug sensitivity of olaparib. The expression of APE1 in triple-negative breast cancer tissues was significantly higher than that in the adjacent tissues (85.6% vs 14.4%) and its expression was related to tumor size (p < 0.05). We also found that it is an independent prognostic factor in patients with triple-negative breast cancer (overall survival, p = 0.01). In vitro assay, the half maximal inhibitory concentration of olaparib in HCC1937-APE1-KO was significantly increased (17.22 vs 91.85 μM) compared to the wild type. The growth curve showed that olaparib had a stronger lethality on HCC1937 compared to HCC1937- APE1-KO (p < 0.05 on day 3). HCC1937 resulted in more mitotic G2/M arrest and increased apoptosis rate after treatment with 40 μM of olaparib, while HCC1937-APE1-KO did not change significantly. When HCC1937 was treated with different concentrations of olaparib, it was found that APE1 expression decreased more significantly at 15 μM of olaparib was. In HCC1937-APE1-KO, the expression of endogenous poly (ADP-ribose) polymerase 1 was also less than that of HCC1937. These results suggested that the expression of APE1 was an important basis for the maintenance of poly (ADP-ribose) polymerase 1, and the deletion of APE1 may be related to the resistance of olaparib.

APE1 overexpression is associated with poor survival in patients with solid tumors: a meta-analysis

APE1 is known as a key mediator of DNA damage repair pathways, and its clinical significance in different types of cancer is well studied. Herein, we performed a meta-analysis to determine the association of APE1 expression and survival in different types of solid cancer. We searched all eligible publications in PubMed, Web of Science and Embase platforms from inception to January 2017 and found 15 relevant manuscripts. Overall survival (OS), 12- and 36-month survival rates, and hazard ratios (HRs) were extracted and analyzed. Heterogeneity and publication bias were also assessed. A subgroup analysis of the different subcellular locations of APE1 was also conducted. Patients with higher APE1 levels demonstrated lower 12- and 36-month survival rates than those with low APE1 levels (HR 2.00, 95% CI 1.33–3.00, P = 0.0009; HR 1.84, 95% CI 1.19–2.84, P = 0.006). Importantly, the pooled analysis showed that high levels of APE1 predict shorter OS (HR 1.44, 95% CI 1.13–1.83, P = 0.003). Subgroup analysis revealed that both nuclear and cytoplasmic expression levels of APE1 are important indicators of poor prognosis in solid tumors.

The Relationship Between Single-Nucleotide Polymorphisms, the Expression of DNA Damage Response Genes, and Hepatocellular Carcinoma in a Polish Population

The molecular mechanism of hepatocellular carcinoma (HCC) is related to DNA damage caused by oxidative stress products induced by hepatitis B virus (HBV) or C (HCV) infection and exposure to environmental pollutants. Single-nucleotide polymorphisms (SNPs) of DNA damage response (DDR) genes may influence individual susceptibility to environmental risk factors and affect DNA repair efficacy, which, in turn, can influence the risk of HCC. The study evaluates a panel of 15 SNPs in 11 DDR genes (XRCC1, XRCC3, XPD, MUTYH, LIG1, LIG3, hOGG1, PARP1, NFIL1, FEN1, and APEX1) in 65 HCC patients, 50 HBV- and 50 HCV-infected non-cancerous patients, and 50 healthy controls. It also estimates the mRNA expression of nine DDR genes in cancerous and adjacent healthy liver tissues. Two of the investigated polymorphisms (rs1052133 and rs13181) were associated with HCC risk. For all investigated genes, the level of mRNA was significantly lower in HCC cancer tissue than in non-cancerous liver tissue. Seven of the investigated polymorphisms were statistically related to gene expression in cancer tissues. The disruption of DDR genes may be responsible for hepatocellular transformation in HCV-infected patients.

Exploiting the Ref-1-APE1 node in cancer signaling and other diseases: from bench to clinic

Reduction-oxidation factor 1-apurinic/apyrimidinic endonuclease (Ref-1/APE1) is a critical node in tumor cells, both as a redox regulator of transcription factor activation and as part of the DNA damage response. As a redox signaling protein, Ref-1/APE1 enhances the transcriptional activity of STAT3, HIF-1α, nuclear factor kappa B, and other transcription factors to promote growth, migration, and survival in tumor cells as well as inflammation and angiogenesis in the tumor microenvironment. Ref-1/APE1 is activated in a variety of cancers, including prostate, colon, pancreatic, ovarian, lung and leukemias, leading to increased aggressiveness. Transcription factors downstream of Ref-1/APE1 are key contributors to many cancers, and Ref-1/APE1 redox signaling inhibition slows growth and progression in a number of tumor types. Ref-1/APE1 inhibition is also highly effective when paired with other drugs, including standard-of-care therapies and therapies targeting pathways affected by Ref-1/APE1 redox signaling. Additionally, Ref-1/APE1 plays a role in a variety of other indications, such as retinopathy, inflammation, and neuropathy. In this review, we discuss the functional consequences of activation of the Ref-1/APE1 node in cancer and other diseases, as well as potential therapies targeting Ref-1/APE1 and related pathways in relevant diseases. APX3330, a novel oral anticancer agent and the first drug to target Ref-1/APE1 for cancer is entering clinical trials and will be explored in various cancers and other diseases bringing bench discoveries to the clinic.

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.

Transcriptional Up-Regulation of APE1/Ref-1 in Hepatic Tumor: Role in Hepatocytes Resistance to Oxidative Stress and Apoptosis

Objective

Human Hepatocellular Carcinoma (HCC) is the fifth most frequent neoplasm worldwide and the most serious complication of long-standing chronic liver diseases (CLD). Its development is associated with chronic inflammation and sustained oxidative stress. Deregulation of apurinic apyrimidinic endonuclease 1/redox effector factor 1 (APE1/Ref-1), a master regulator of cellular response to oxidative stress, has been associated with poor prognosis in several cancers including HCC.

Design

In the present study we investigated the APE1/Ref-1 mRNA levels in cirrhotic and HCC tissues obtained during HCC resection. The possible protective role of APE1/Ref-1 against oxidative stress and apoptosis was evaluated in vitro in immortalized human hepatocytes (IHH) over-expressing APE1/Ref-1.

Results

APE1/Ref-1 was up-regulated in HCC, regulation occurring at the transcriptional level. APE1/Ref-1 mRNA content increased with the progression of liver disease with the transcriptional up-regulation present in cirrhosis significantly increased in HCC. The up-regulation was higher in the less differentiated cancers. In vitro, over-expression of APE1/Ref-1 in normal hepatocytes conferred cell protection against oxidative stress and it was associated with BAX inhibition and escape from apoptosis.

Conclusion

APE1/Ref-1 is up-regulated in HCC and this over-expression correlates with cancer aggressiveness. The up-regulation occurs at the transcriptional level and it is present in the earliest phases of hepatocarcinogenesis. The APE-1/Ref-1 over-expression is associated with hepatocyte survival and inhibits BAX activation and apoptosis. These data suggest a possible role of APE1/Ref-1 over-expression both in hepatocyte survival and HCC development calling attention to this molecule as a promising marker for HCC diagnosis and treatment.

Association of DNA Repair Gene APE1 Asp148Glu Polymorphism with Breast Cancer Risk

OBJECTIVE

The aim of this study was to investigate the role of APE1 Asp148Glu polymorphism in breast cancer progression in Saudi population.

METHODS

We examined the genetic variations (rs1130409) in the DNA base excision repair gene APE1 at codon 148 (Asp148Glu) and its association with breast cancer risk using genotypic assays and in silico structural as well as functional predictions. In silico structural analysis was performed with Asp148Glu allele and compared with the predicted native protein structure. The wild and mutant 3D structures of APE1 were compared and analyzed using solvent accessibility models for protein stability confirmation.

RESULTS

Genotypic analysis of APE1 (rs1130409) showed statistically significant association of Asp148Glu with elevated susceptibility to breast cancer. The in silico analysis results indicated that the nsSNP Asp148Glu may cause changes in the protein structure and is associated with breast cancer risk.

CONCLUSION

Taken together, this is the first report that established that Asp148Glu variant has structural and functional effect on the APE1 and may play an important role in breast cancer progression in Saudi population.

Mitochondrial translocation of APE1 relies on the MIA pathway

APE1 is a multifunctional protein with a fundamental role in repairing nuclear and mitochondrial DNA lesions caused by oxidative and alkylating agents. Unfortunately, comprehensions of the mechanisms regulating APE1 intracellular trafficking are still fragmentary and contrasting. Recent data demonstrate that APE1 interacts with the mitochondrial import and assembly protein Mia40 suggesting the involvement of a redox-assisted mechanism, dependent on the disulfide transfer system, to be responsible of APE1 trafficking into the mitochondria. The MIA pathway is an import machinery that uses a redox system for cysteine enriched proteins to drive them in this compartment. It is composed by two main proteins: Mia40 is the oxidoreductase that catalyzes the formation of the disulfide bonds in the substrate, while ALR reoxidizes Mia40 after the import. In this study, we demonstrated that: (i) APE1 and Mia40 interact through disulfide bond formation; and (ii) Mia40 expression levels directly affect APE1's mitochondrial translocation and, consequently, play a role in the maintenance of mitochondrial DNA integrity. In summary, our data strongly support the hypothesis of a redox-assisted mechanism, dependent on Mia40, in controlling APE1 translocation into the mitochondrial inner membrane space and thus highlight the role of this protein transport pathway in the maintenance of mitochondrial DNA stability and cell survival.

Inhibitors of the apurinic/apyrimidinic endonuclease 1 (APE1)/nucleophosmin (NPM1) interaction that display anti-tumor properties

The apurinic/apyrimidinic endonuclease 1 (APE1) is a protein central to the base excision DNA repair pathway and operates in the modulation of gene expression through redox-dependent and independent mechanisms. Aberrant expression and localization of APE1 in tumors are recurrent hallmarks of aggressiveness and resistance to therapy. We identified and characterized the molecular association between APE1 and nucleophosmin (NPM1), a multifunctional protein involved in the preservation of genome stability and rRNA maturation. This protein-protein interaction modulates subcellular localization and endonuclease activity of APE1. Moreover, we reported a correlation between APE1 and NPM1 expression levels in ovarian cancer, with NPM1 overexpression being a marker of poor prognosis. These observations suggest that tumors that display an augmented APE1/NPM1 association may exhibit increased aggressiveness and resistance. Therefore, targeting the APE1/NPM1 interaction might represent an innovative strategy for the development of anticancer drugs, as tumor cells relying on higher levels of APE1 and NPM1 for proliferation and survival may be more sensitive than untransformed cells. We set up a chemiluminescence-based high-throughput screening assay in order to find small molecules able to interfere with the APE1/NPM1 interaction. This screening led to the identification of a set of bioactive compounds that impair the APE1/NPM1 association in living cells. Interestingly, some of these molecules display anti-proliferative activity and sensitize cells to therapeutically relevant genotoxins. Given the prognostic significance of APE1 and NPM1, these compounds might prove effective in the treatment of tumors that show abundant levels of both proteins, such as ovarian or hepatic carcinomas.

DNA repair and redox activities and inhibitors of apurinic/apyrimidinic endonuclease 1/redox effector factor 1 (APE1/Ref-1): a comparative analysis and their scope and limitations toward anticancer drug development

The apurinic/apyrimidinic endonuclease 1/redox effector factor 1 (APE1/Ref-1) is a multifunctional enzyme involved in DNA repair and activation of transcription factors through its redox function. The evolutionarily conserved C- and N-termini are involved in these functions independently. It is also reported that the activity of APE1/Ref-1 abruptly increases several-fold in various human cancers. The control over the outcomes of these two functions is emerging as a new strategy to combine enhanced DNA damage and chemotherapy in order to tackle the major hurdle of increased cancer cell growth and proliferation. Studies have targeted these two domains individually for the design and development of inhibitors for APE1/Ref-1. Here, we have made, for the first time, an attempt at a comparative analysis of APE1/Ref-1 inhibitors that target both DNA repair and redox activities simultaneously. We further discuss their scope and limitations with respect to the development of potential anticancer agents.

APE1/Ref-1 as an emerging therapeutic target for various human diseases: phytochemical modulation of its functions

Apurinic/apyrimidinic endonuclease 1 (APE1) is a multifunctional enzyme involved in the base excision repair (BER) pathway, which repairs oxidative base damage caused by endogenous and exogenous agents. APE1 acts as a reductive activator of many transcription factors (TFs) and has also been named redox effector factor 1, Ref-1. For example, APE1 activates activator protein-1, nuclear factor kappa B, hypoxia-inducible factor 1α, paired box gene 8, signal transducer activator of transcription 3 and p53, which are involved in apoptosis, inflammation, angiogenesis and survival pathways. APE1/Ref-1 maintains cellular homeostasis (redox) via the activation of TFs that regulate various physiological processes and that crosstalk with redox balancing agents (for example, thioredoxin, catalase and superoxide dismutase) by controlling levels of reactive oxygen and nitrogen species. The efficiency of APE1/Ref-1's function(s) depends on pairwise interaction with participant protein(s), the functions regulated by APE1/Ref-1 include the BER pathway, TFs, energy metabolism, cytoskeletal elements and stress-dependent responses. Thus, APE1/Ref-1 acts as a ‘hub-protein' that controls pathways that are important for cell survival. In this review, we will discuss APE1/Ref-1's versatile nature in various human etiologies, including neurodegeneration, cancer, cardiovascular and other diseases that have been linked with alterations in the expression, subcellular localization and activities of APE/Ref-1. APE1/Ref-1 can be targeted for therapeutic intervention using natural plant products that modulate the expression and functions of APE1/Ref-1. In addition, studies focusing on translational applications based on APE1/Ref-1-mediated therapeutic interventions are discussed.

Induction of apurinic endonuclease 1 overexpression by endoplasmic reticulum stress in hepatoma cells

Hepatocellular carcinoma (HCC) is one of the most common malignancies worldwide with poor prognosis due to resistance to conventional chemotherapy and limited efficacy of radiotherapy. Previous studies have noted the induction of endoplasmic reticulum stress or apurinic endonuclease 1 (APE1) expression in many tumors. Therefore, the aim of this study was to investigate the relationship between endoplasmic reticulum (ER stress) and APE1 in hepatocellular carcinoma. Here we investigate the expression of APE1 during ER stress in HepG2 and Huh-7 cell lines. Tunicamycin or brefeldin A, two ER stress inducers, increased APE1 and GRP78, an ER stress marker, expression in HepG2 and Huh-7 cells. Induction of APE1 expression was observed through transcription level in response to ER stress. APE1 nuclear localization during ER stress was determined using immunofluorescence assays in HepG2 cells. Furthermore, expression of Hepatitis B virus pre-S2∆ large mutant surface protein (pre-S2∆), an ER stress-induced protein, also increased GRP78 and APE1 expression in the normal hepatocyte NeHepLxHT cell line. Similarly, tumor samples showed higher expression of APE1 in ER stress-correlated liver cancer tissue in vivo. Our results demonstrate that ER stress and HBV pre-S2∆ increased APE1 expression, which may play an important role in resistance to chemotherapeutic agents or tumor development. Therefore, these data provide an important chemotherapeutic strategy in ER stress and HBV pre-S2∆-associated tumors.

Prognostic value of human apurinic/apyrimidinic endonuclease 1 (APE1) expression in breast cancer

Human apurinic/apyrimidinic endonuclease 1 (APE1) is an essential protein for DNA base excision repair (BER) and redox regulation. The ability of cancer cells to recognize DNA damage and initiate DNA repair is an important mechanism for therapeutic resistance. Several recent studies have suggested that APE1 expression levels and/or subcellular dysregulation may be used to indicate the sensitivity of tumors to radiotherapy or chemotherapy. In this study, we assessed the prognostic significance of APE1 and differences in APE1 expression levels according to breast cancer molecular subtypes. We analyzed formalin-fixed, paraffin-embedded tumor tissue sections from 243 cases diagnosed as invasive breast cancer at Ewha Womans University Medical Center between January 2003 and December 2008. Immunohistochemistry was performed and the nuclear level of APE1 was scored by taking into account the percentage of positive cells. Medical records were reviewed to investigate clinicopathologic characteristics. We found that nuclear APE1 high-level expression (proportion ≥50%) in breast cancer showed a tendency towards unfavorable prognosis regarding disease-free survival (p = 0.093). However, there was no significant difference in overall survival between low and high-level expression groups (p = 0.294). Interestingly, within the Ki-67 low-level expression group, APE1 low-level expression was significantly associated with poor overall survival (p = 0.007). A significant positive correlation was observed between APE1 nuclear expression and estrogen receptor status (75.7% vs. 59.7%, p = 0.022). Also, the luminal A subtype was the most commonly observed breast cancer subtype in the APE1 high-level expression group (61.6% vs. 45.2%, p = 0.000). This study suggests that APE1 expression may be associated with breast cancer prognosis. In particular, its role as a prognostic factor would be significant for breast cancers with a low Ki-67 proliferation index. It is proposed that nuclear APE1 may be a novel target in breast cancer with a low proliferation rate to obtain better outcome.

Expression and prognostic significance of APE1/Ref-1 and NPM1 proteins in high-grade ovarian serous cancer

OBJECTIVES

To correlate the expression profile of human apurinic endonuclease/redox factor 1 (APE1/Ref-1) with that of nucleolar/nucleoplasmic protein nucleophosmin 1 (NPM1) in association with the aggressiveness and progression of high-grade ovarian serous cancer.

METHODS

Retrospective study analyzing a tissue microarray of 73 women affected by high-grade ovarian serous cancer. Protein expression was assessed by immunohistochemistry on primary tumor masses and synchronous peritoneal metastases if present.

RESULTS

APE1/Ref-1 and NPM1 showed a significant correlation in ovarian serous cancer. Patients with a poorer outcome showed a significant overexpression of nuclear NPM1 protein. A Cox proportional hazards multivariate regression model revealed NPM1 expression to be independently significant for overall survival in high-grade ovarian serous cancers after correcting for stage, age, cytoreduction completeness, and platinum resistance.

CONCLUSIONS

APE1/Ref-1 interacts with NPM1 to control the DNA damage repair system, and it is likely that this interaction plays a defining role in high-grade ovarian serous carcinoma. A high NPM1 immunohistochemical expression was independently correlated with a shorter survival period and thus appears to be an important prognostic factor.

Human apurinic/apyrimidinic endonuclease 1

SIGNIFICANCE

Human apurinic/apyrimidinic endonuclease 1 (APE1, also known as REF-1) was isolated based on its ability to cleave at AP sites in DNA or activate the DNA binding activity of certain transcription factors. We review herein topics related to this multi-functional DNA repair and stress-response protein.

RECENT ADVANCES

APE1 displays homology to Escherichia coli exonuclease III and is a member of the divalent metal-dependent α/β fold-containing phosphoesterase superfamily of enzymes. APE1 has acquired distinct active site and loop elements that dictate substrate selectivity, and a unique N-terminus which at minimum imparts nuclear targeting and interaction specificity. Additional activities ascribed to APE1 include 3'-5' exonuclease, 3'-repair diesterase, nucleotide incision repair, damaged or site-specific RNA cleavage, and multiple transcription regulatory roles.

CRITICAL ISSUES

APE1 is essential for mouse embryogenesis and contributes to cell viability in a genetic background-dependent manner. Haploinsufficient APE1(+/-) mice exhibit reduced survival, increased cancer formation, and cellular/tissue hyper-sensitivity to oxidative stress, supporting the notion that impaired APE1 function associates with disease susceptibility. Although abnormal APE1 expression/localization has been seen in cancer and neuropathologies, and impaired-function variants have been described, a causal link between an APE1 defect and human disease remains elusive.

FUTURE DIRECTIONS

Ongoing efforts aim at delineating the biological role(s) of the different APE1 activities, as well as the regulatory mechanisms for its intra-cellular distribution and participation in diverse molecular pathways. The determination of whether APE1 defects contribute to human disease, particularly pathologies that involve oxidative stress, and whether APE1 small-molecule regulators have clinical utility, is central to future investigations.

Apurinic/apyrimidinic endonuclease-1 is associated with angiogenesis and VEGF production via upregulation of COX-2 expression in esophageal cancer tissues

Apurinic/apyrimidinic endonuclease-1 (APE-1) is a key enzyme responsible for DNA base excision repair and is also a multifunctional protein such as redox effector for several transcriptional factors. Our study was designed to investigate APE-1 expression and to study its interaction with cyclooxygenase (COX)-2 expression and VEGF production in the esophageal cancer. The expression of APE-1, COX-2, monocyte chemoattractant protein (MCP)-1, CC-chemokine receptor (CCR)2, and VEGF were evaluated by immunohistochemistry in 65 human esophageal squamous cell carcinoma (ESCC) tissues. Real-time PCR and Western blotting were performed to detect mRNA and protein expression of APE-1 and p-signal transducer and activator of transcription 3 (STAT3) expression in MCP-1-stimulated ESCC cell lines (KYSE 220 and EC-GI-10). siRNA for APE-1 was treated to determine the role of APE-1 in the regulation of COX-2 expression, VEGF production, and antiapoptotic effect against cisplatin. In human ESCC tissues, nuclear localization of APE-1 was observed in 92.3% (60/65) of all tissues. There was a significant relationship (P = 0.029, R = 0.49) between nuclear APE-1 and cytoplasmic COX-2 expression levels in the esophageal cancer tissues. In KYSE 220 and EC-GI-10 cells, MCP-1 stimulation significantly increased mRNA and protein expression of APE-1. Treatment with siRNA for APE-1 significantly inhibited p-STAT3 expression levels in MCP-1-stimulated cells. Furthermore, treatment of siRNA for APE-1 significantly reduced COX-2 expression and VEGF production in MCP-1-stimulated esophageal cancer cell lines. Treatment with APE-1 siRNA significantly increased apoptotic levels in cisplatin-incubated KYSE 220 and EC-GI-10 cells. We concluded that APE-1 is overexpressed and associated with COX-2 expression and VEGF production in esophageal cancer tissues.

APE1/Ref-1 enhances DNA binding activity of mutant p53 in a redox-dependent manner

Apurinic/apyrimidinic endonuclease 1/redox factor-1 (APE1/Ref-1) is a dual function protein; in addition to its DNA repair activity, it can stimulate DNA binding activity of numerous transcription factors as a reduction-oxidation (redox) factor. APE1/Ref-1 has been found to be a potent activator of wild-type p53 (wtp53) DNA binding in vitro and in vivo. Although p53 is mutated in most types of human cancer including hepatocellular carcinoma (HCC), little is known about whether APE1/Ref-1 can regulate mutant p53 (mutp53). Herein, we reported the increased APE1/Ref-1 protein and accumulation of mutp53 in HCC by immunohistochemistry. Of note, it was observed that APE1/Ref-1 high-expression and mutp53 expression were associated with carcinogenesis and progression of HCC. To determine whether APE1/Ref-1 regulates DNA binding of mutp53, we performed electromobility shift assays (EMSAs) and quantitative chromatin immunoprecipitation (ChIP) assays in HCC cell lines. In contrast to sequence-specific and DNA structure-dependent binding of wtp53, reduced mutp53 efficiently bound to nonlinear DNA, but not to linear DNA. Notably, overexpression of APE1/Ref-1 resulted in increased DNA binding activity of mutp53, while downregulation of APE1/Ref-1 caused a marked decrease of mutp53 DNA binding. In addition, APE1/Ref-1 could not potentiate the accumulation of p21 mRNA and protein in mutp53 cells. These data indicate that APE1/Ref-1 can stimulate mutp53 DNA binding in a redox-dependent manner.

Differential effects of methoxyamine on doxorubicin cytotoxicity and genotoxicity in MDA-MB-231 human breast cancer cells

Pharmacological inhibition of DNA repair is a promising approach to increase the effectiveness of anticancer drugs. The chemotherapeutic drug doxorubicin (Dox) may act, in part, by causing oxidative DNA damage. The base excision repair (BER) pathway effects the repair of many DNA lesions induced by reactive oxygen species (ROS). Methoxyamine (MX) is an indirect inhibitor of apurinic/apyrimidinic endonuclease 1 (APE1), a multifunctional BER protein. We have evaluated the effects of MX on the cytotoxicity and genotoxicity of Dox in MDA-MB-231 metastatic breast cancer cells. MX has little effects on the viability and proliferation of Dox-treated cells. However, as assessed by the cytokinesis-block micronucleus assay (CBMN), MX caused a significant 1.4-fold increase (P<0.05) in the frequency of micronucleated binucleated cells induced by Dox, and also altered the distribution of the numbers of micronuclei. The fluorescence probe dihydroethidium (DHE) indicated little production of ROS by Dox. Overall, our results suggest differential outcomes for the inhibition of APE1 activity in breast cancer cells exposed to Dox, with a sensitizing effect observed for genotoxicity but not for cytotoxicity.

Specific inhibition of the redox activity of ape1/ref-1 by e3330 blocks tnf-α-induced activation of IL-8 production in liver cancer cell lines

APE1/Ref-1 is a main regulator of cellular response to oxidative stress via DNA-repair function and co-activating activity on the NF-κB transcription factor. APE1 is central in controlling the oxidative stress-based inflammatory processes through modulation of cytokines expression and its overexpression is responsible for the onset of chemoresistance in different tumors including hepatic cancer. We examined the functional role of APE1 overexpression during hepatic cell damage related to fatty acid accumulation and the role of the redox function of APE1 in the inflammatory process. HepG2 cells were stably transfected with functional and non-functional APE1 encoding plasmids and the protective effect of APE1 overexpression toward genotoxic compounds or FAs accumulation, was tested. JHH6 cells were stimulated with TNF-α in the presence or absence of E3330, an APE1 redox inhibitor. IL-8 promoter activity was assessed by a luciferase reporter assay, gene expression by Real-Time PCR and cytokines (IL-6, IL-8, IL-12) levels measured by ELISA. APE1 over-expression did not prevent cytotoxicity induced by lipid accumulation. E3330 treatment prevented the functional activation of NF-κB via the alteration of APE1 subcellular trafficking and reduced IL-6 and IL-8 expression induced by TNF-α and FAs accumulation through blockage of the redox-mediated activation of NF-κB. APE1 overexpression observed in hepatic cancer cells may reflect an adaptive response to cell damage and may be responsible for further cell resistance to chemotherapy and for the onset of inflammatory response. The efficacy of the inhibition of APE1 redox activity in blocking TNF-α and FAs induced inflammatory response opens new perspectives for treatment of inflammatory-based liver diseases.

Colon cancer progression is driven by APEX1-mediated upregulation of Jagged

Aberrant expression of apurinic-apyrimidinic endonuclease-1 (APEX1) has been reported in numerous human solid tumors and is positively correlated with cancer progression; however, the role of APEX1 in tumor progression is poorly defined. Here, we show that APEX1 contributes to aggressive colon cancer behavior and functions as an upstream activator in the Jagged1/Notch signaling pathway. APEX1 overexpression or knockdown in human colon cancer cell lines induced profound changes in malignant properties such as cell proliferation, anchorage-independent growth, migration, invasion, and angiogenesis in vitro and in tumor formation and metastasis in mouse xenograft models. These oncogenic effects of APEX1 were mediated by the upregulation of Jagged1, a major Notch ligand. Furthermore, APEX1 expression was associated with Jagged1 in various colon cancer cell lines and in tissues from colon cancer patients. This finding identifies APEX1 as a positive regulator of Jagged1/Notch activity and suggests that it is a potential therapeutic target in colon cancers that exhibit high levels of Jagged1/Notch signaling.