|Year : 2018 | Volume
| Issue : 2 | Page : 67-78
Apurinic/apyrimidinic endonuclease 1 performs multiple roles in controlling the outcome of cancer cells toward radiation and chemotherapeutic agents
Dindial Ramotar1, Alain Nepveu2
1 Department of Medicine, Research Center, Rosemont Hospital, University of Montreal, Montreal, Quebec, Canada
2 Goodman Cancer Research Centre, McGill University; Department of Medicine and Oncology, McGill University, Montreal, Quebec, Canada
|Date of Web Publication||22-May-2018|
Dr. Dindial Ramotar
Department of Medicine, Research Center, Rosemont Hospital, University of Montreal, Montreal, Quebec
Source of Support: None, Conflict of Interest: None
Many endogenous and exogenous sources produce reactive oxygen species such as superoxide radical anions and hydrogen peroxide that are converted to the highly reactive form, hydroxyl radical. It is this latter species that can damage several macromolecules in the cells, in particular, the DNA to produce a variety of DNA lesions. These DNA lesions include oxidatively damaged purine and pyrimidine bases, as well as single-strand and double-strand breaks. These unrepaired DNA lesions lead to base substitutions, deletions, insertions, and rearrangements of the chromosome, ultimately altering the stability of the genome. Maintaining the integrity of the genome is essential to prevent various diseases such as several types of cancers. There are several DNA repair pathways including base-excision repair (BER), nucleotide-excision repair, mismatch repair, homologous recombination, and nonhomologous end joining that operate in the human cells to prevent genomic instability. Each of these DNA repair pathways consists of multiple enzymes that execute specific function (s). This review focuses on a key enzyme apurinic/apyrimidinic endonuclease 1 (APE1) that belongs to the BER pathway that plays a pivotal role in the removal of modified DNA bases. We provide an overview of the multifaceted roles performed by APE1, which also serves as a redox factor and referred to as redox effector factor 1 (Ref-1) or APE1/Ref-1. In addition, we discuss more recent findings whereby (i) peroxiredoxin 1 controls the redox activity of APE1 and (ii) CUT-like homeobox 1 protein, a transcription factor that binds to DNA and stimulates the DNA repair activities of APE1 to confer resistance to radio- and chemotherapy.
Keywords: Apurinic/apyrimidinic endonuclease 1 DNA repair and redox factor, oxidative DNA damage, peroxiredoxin 1, reactive oxygen species, DNA-binding factor CUT-like homeobox 1
|How to cite this article:|
Ramotar D, Nepveu A. Apurinic/apyrimidinic endonuclease 1 performs multiple roles in controlling the outcome of cancer cells toward radiation and chemotherapeutic agents. J Radiat Cancer Res 2018;9:67-78
|How to cite this URL:|
Ramotar D, Nepveu A. Apurinic/apyrimidinic endonuclease 1 performs multiple roles in controlling the outcome of cancer cells toward radiation and chemotherapeutic agents. J Radiat Cancer Res [serial online] 2018 [cited 2018 Aug 21];9:67-78. Available from: http://www.journalrcr.org/text.asp?2018/9/2/67/232986
| Introduction|| |
Reactive oxygen species (ROS) such as superoxide radical anions (O2“-), hydrogen peroxide (H2O2), and hydroxyl radical (OH“) are produced endogenously during metabolism, as well as upon exposure to many exogenous sources such as radiation., ROS induce oxidative damages to membranes, proteins, and the cellular DNA, leading to various diseases including several types of cancers, e.g., gastric carcinomas.,, Of relevance to this review is ROS-induced oxidative base damage to DNA and the mechanisms that are involved in removing these lesions to prevent genomic instability, which could lead to various diseases., The oxidatively damaged bases are removed and replaced by normal bases via the base-excision DNA repair (BER) pathway [Figure 1]. In the BER pathway, a DNA glycosylase recognizes and cleaves the N-glycosidic bond between the oxidatively modified base and the sugar moiety to produce a C1′ hydrolyzed abasic sugar [Figure 1]. The abasic site is incised by an apurinic/apyrimidinic (AP) endonuclease leaving a 3′-hydroxyl group and a 5′-deoxyribose phosphate (dRP) [left side of [Figure 1]. DNA polymerase β is recruited to insert the correct nucleotide and at the same time uses its β-lyase (a 5′-deoxyribose phosphodiesterase) activity to remove the dRP [Figure 1]. The resulting nick is sealed by a DNA ligase in a set of reactions that constitutes the BER pathway [Figure 1].,, While key components and auxiliary factors of the BER pathway are essential to maintain the stability of the genome, alterations of these factors are in part responsible for the invasiveness and radioresistance observed by some cancer types.
|Figure 1: Illustration of the base-excision DNA repair pathway in human cells. The damaged bases are removed by mono- - or bi-functional DNA glycosylases. Monofunctional DNA glycosylases leave an apurinic/apyrimidinic site, which is then cleaved by an apurinic/apyrimidinic endonuclease 1 to leave a 3′-hydroxyl group for DNA repair synthesis and 5′-deoxyribose phosphate. DNA polymerase β is recruited to insert a single nucleotide (short-patch DNA repair) while simultaneously removing the 5′-deoxyribose phosphate by an inherent β-lyase activity. The 5′-dRP can also be removed if DNA synthesis occurs via the long-patch DNA repair that uses either DNA polymerase d, e, or β to insert more than one nucleotide. The resulting displaced 5′-dRP resembles a flap-structure that is removed by the FEN1 endonuclease. Following nucleotide (s) insertion by DNA polymerase, the resulting nick is sealed by DNA ligase. If the damaged base is removed by a bi-functional DNA glycosylase, it leaves a nicked DNA containing either a 3′-a, β unsaturated aldehyde or 3′-phosphate (P) that can be removed by apurinic/apyrimidinic endonuclease 1. polynucleotide kinase can also remove the 3′-phosphate. CUT-like homeobo × 1 uses its CUT domains to activate apurinic/apyrimidinic endonuclease 1 apurinic/apyrimidinic endonuclease activity. Depicted also are the structures of two base lesions, deamination of cytosine to form uracil and oxidation of thymine to form 5-hydroxymethyluracil. The structure of the alkylating anticancer drug temozolomide, which creates methylated purines, is shown|
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It is noteworthy that besides the BER pathway, there are several other DNA repair pathways that can process damaged DNA and these include nucleotide-excision repair, mismatch repair, and homologous and nonhomologous repair. These pathways recognize distinct DNA lesions and use different sets of proteins to repair the damaged DNA, which are covered in many excellent reviews.
Types of DNA lesions induced by ionizing radiation
Ionizing radiation-induced free radical formation can generate a broad spectrum of oxidatively modified DNA base lesions that block replication and transcription. These modified bases include 5-hydroxymethyluracil [Figure 1] and 8-hydroxyguanine, as well as imidazole ring opening of adenine and guanine to yield 4,6-diamino-5-formamidopyrimidine and 2,6-diamino-4-hydroxy-5-formamidopyrimidine, respectively.,,, Ionizing radiation can also cause multiple types of lesions to the sugar moiety, leading to ring opening of the sugar without breaking the DNA strand and others where the ring-opened sugar leads to DNA single-strand breaks possessing blocked 3′-termini such as 3′-phosphoglycolate. In addition to the base and sugar damage, ionizing radiation can also generate DNA single- and double-strand breaks, as well as clustered DNA lesions., It is noteworthy that a major product of ROS-induced lipid peroxidation is malondialdehyde, which can form adducts with adenine, guanine, and cytosine. The ratio of these various DNA lesions is dependent on the oxygen and antioxidant content, as well as the DNA repair capacity of a given cell type.
DNA glycosylases involved in removing oxidatively modified bases
Human cells have 11 DNA glycosylases that are classified into two categories: (i) monofunctional and (ii) bifunctional. The monofunctional DNA glycosylases act by removing only the modified base to create an AP site [Figure 1]. These enzymes include uracil DNA glycosylase (UNG1) that removes uracil and methylpurine-DNA glycosylase that removes alkylated DNA bases such as 3-methyladenine [Figure 1]. The bifunctional DNA glycosylases act by removing the damaged base and at the same time use its endowed AP lyase activity to cleave the resulting AP site by either a β-elimination or β,δ-elimination reaction to produce a single-strand break terminated with 3′-α, β unsaturated aldehyde (polyunsaturated aldehyde [PUA]) or 3′-phosphate (P) [Figure 1]. Some of the bifunctional DNA glycosylases include NTH1 ( Escherichia More Details coli endonuclease III-like 1), NEIL1 and 2 (E. coli Nei endonuclease VIII-like 1), and OGG1 (8-oxoguanine DNA glycosylase) that remove 8-hydroxyguanine, 5-hydroxymethyluracil, and oxidized pyrimidines, respectively [Figure 1]. Following the actions of mono- and bi-functional DNA glycosylases on modified bases, the respective intermediates that are generated, AP sites, and DNA strand breaks terminated with blocked 3′-ends are considered secondary genotoxic lesions and must be further processed [Figure 1]. These 3′-blocking groups such as 3′-phosphate are processed by either APE1 or polynucleotide kinase, as well as involving poly (ADP) ribose polymerase [Figure 1]. The rest of this review is devoted to highlight the role of the AP endonuclease and redox factor, APE1, in cancer biology, focusing primarily on its function in regulating inflammatory and anticancer drug responses.
Apurinic/apyrimidinic endonuclease 1 is both a DNA repair enzyme and a redox transcriptional activator
The APE1/Ref-1 is an essential multifunctional 37 kDa protein involved in the repair of damaged DNA and the regulation of transcription., It is an abundant protein (100,000–1,000,000 molecules per cell) that functions as a monomer and contains three independent functional domains. The first domain encompasses 33–35 amino acid residues of the N-terminal, which is an unstructured sequence that is involved in protein–protein interaction, modulating the catalytic activity of the protein on abasic DNA and binding and metabolizing RNA [Figure 2]. The second domain of the protein, which spans the region between residues 35 and 127, is responsible for redox activity, and the third domain that spans from amino acid 127 onward executes the DNA repair function [Figure 2].
|Figure 2: Linear structure of apurinic/apyrimidinic endonuclease 1 highlighting the active domains. Light green, protein–protein interaction domain; Cyan, the redox domain harboring the redox active cysteine, C65; and Red, the domain carrying the DNA repair functions|
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Apurinic/apyrimidinic endonuclease 1 DNA repair roles
In DNA repair, APE1 uses its AP endonuclease activity to hydrolyze the 5′-phosphodiester bond at AP sites that are left by monofunctional DNA glycosylases to create a 3′-hydroxyl group for DNA repair synthesis by DNA polymerases [Figure 1]. APE1 also possesses a 3′-diesterase activity that can act to remove a variety of blocked 3′-termini at DNA strand breaks such as 3′-phosphoglycolate and 3′-phosphate, which are created by bifunctional DNA glycosylases as well as directly by ionizing radiation [Figure 1].,,, It can also remove these blocked 3′-termini at DNA strand breaks by the aid of a 3′-to 5′-exonuclease activity, although it is believed that this activity may have a specialized function to facilitate removal of misincorporated dNTPs by DNA polymerases during DNA repair synthesis.,,, In addition to the AP endonuclease, 3′-diesterase, and the 3′-to 5′-exonuclease activities, APE1 is endowed with a major nucleotide-incision repair (NIR) endonuclease activity that cleaves duplex DNA 5′ next to oxidative base damage that is generated, e.g., by ionizing radiation, and these lesions include 5,6-dihydro-2′-deoxyuridine, 5,6-dihydrothymidine, 5-hydroxy-2′-deoxyuridine, 5-hydroxy-2′-deoxycitidine, and alpha-2′-deoxynucleosides (αdA, αdT, and αdC).,,,,
It is noteworthy that some of the naturally occurring oxidized base lesions may be beneficial for cells and exerting epigenetic control as in the case of 8-oxo-7,8-dihydroguanine (8-oxoG). Both 8-oxoG lesions and the 8-oxoG DNA glycosylase OGG1 have been shown to be present on some promoters to regulate the activity of transcription factors (e.g., Hif1α, STAT1, nuclear factor-κB [NF-κB], and MYC) that control expression of genes involved in cancer development, such as vascular endothelial growth factor.,, Since APE1 possesses the NIR endonuclease activity, it is not known whether this activity would similarly control the repair of key oxidized base lesions to preserve gene regulation.
Although APE1 is essential, it is possible to derive cells with reduced level of its expression and monitor their responses toward DNA-damaging agents. Indeed, cells downregulated for APE1 display many phenotypes including sensitivity to ionizing radiation, slower growth rate due to checkpoint arrest caused by the spontaneous accumulation of unrepaired DNA lesions, stalling of transcription, and induced levels of apoptosis. It is the vital function of APE1 in maintaining DNA repair triggered the hunt for inhibitors of APE1 with the intention of using these molecules to sensitize cancer cells to ionizing radiation and genotoxic chemotherapeutic agents.
While the cleavage of AP sites in vitro by APE1 is well established, the molecular details of how it engages with the lesion in vivo have only been recently studied. Several lysine residues, Lys6, 7, 27, 31, 32, and 35, on the N-terminal of APE1, can be acetylated and both Lys6 and 7 are essential for survival.,,, APE1 becomes acetylated following its binding to AP sites in the chromatin, and the resulting modified enzyme remains associated with the chromatin throughout the cell cycle. The acetylation of APE1 causes a conformational change that enhances the AP endonuclease activity of APE1, as well as its interaction with downstream BER proteins such as DNA polymerase β and causing the stimulation of its dRP lyase activity [Figure 1]., APE1 lacking acetylation on Lys6 and Lys7 causes telomere fusion and mitotic defects, likely a consequence of unrepaired oxidative DNA lesions in the telomere leading to DNA strand breaks that engage in telomere fusion.
It is noteworthy that other types of modification also occur on APE1, e.g., oxidation, ubiquitination, and phosphorylation., The latter occurs on APE1 at Thr232 by a cyclin-dependent kinase and thus inhibits its AP endonuclease activity, resulting in the accumulation of DNA damage and promoting neuronal cell death.
Apurinic/apyrimidinic endonuclease 1 redox roles
Besides its DNA repair activities, APE1 directly or indirectly regulates transcription. For example, APE1 can stimulate the DNA-binding activities of various oxidized transcription factors including AP-1, NF-κB, Myb, p53, hypoxia-inducible factor-1, and Pax proteins, which are involved in regulating cell growth, differentiation, survival, and death.,,,,, APE1 performs these functions using its redox cysteine residue Cys65 [Figure 2] to reduce the transcription factors, thereby facilitating their binding onto the promoter of target genes. APE1 has six additional cysteines, and Cys93 and Cys99 may also have limited redox roles. APE1 in contact with H2O2 forms a single disulfide bond between Cys65 and Cys138 within 5 min. In addition, the S-nitrosating agent, S-nitrosoglutathion that serves as a donor of nitric oxide can stimulate nuclear export of APE1 by mediating S-nitrosation of residues Cys93 and Cys310. Nonetheless, none of the seven cysteines has a direct role in APE1 ability to repair damaged DNA.
A recent study showed that APE1 can negatively regulate the function of the nuclear factor erythroid-related factor 2 (NRF2), which has recently emerged to be a vital transcription factor in the defense against oxidative stress. Inhibition of the redox function of APE1 potently activates NRF2 target genes, underscoring the importance of eliminating the threat posed by ROS. This is in contrast to the phenotypes caused by downregulating the level of APE1 that results in the inhibition of proliferation, induction of apoptosis, and telomere shortening due in part to the accumulation of unrepaired DNA lesions.,,
Apurinic/apyrimidinic endonuclease 1 can be secreted to curtail inflammation
Besides the movement of APE1 from the cytoplasm to the nucleus in response to oxidative stress, it can also be secreted from tumor necrosis factor-alpha (TNF-α)-stimulated cells to perform a role in blocking inflammation. During vascular inflammatory responses, macrophages release the TNF-α, which binds to the TNF-α receptor TNFR1, a member of the TNF receptor superfamily characterized as a group of cytokine receptors that possess the ability to bind TNFs via an extracellular cysteine-rich domain that forms disulfide bonds to allow recognition of its ligand TNF-α., Binding of TNF-α to TNFR1 results in the production of ROS and the activation of the transcription factor NF-κB. The activated NF-κB moves to the nucleus to activate the transcription of genes involved in the pathogenesis of inflammatory lesions, including cytokines, chemokines, and adhesion molecules such as the vascular cell adhesion molecule-1. The presence of extracellular-secreted APE1/Ref-1, which can be induced by the histone deacetylase inhibitor trichostatin A, serves to reduce TNFR1 activity by causing a conformational change via thiol-disulfide exchange. This conformational change prevents the binding of TNF-α to the TNFR1 and inhibits the inflammatory signaling that leads to generation of ROS and activation of NF-κB. Thus, APE1 when secreted can serve as an endogenous inhibitor of vascular inflammation and likely a predictor of chemotherapeutic efficacy using serum APE1 autoantibodies.
Apurinic/apyrimidinic endonuclease 1 controls miRNA processing and its interactome is linked to cancer RNA metabolism
APE1 is also involved in RNA metabolism, whereby it is required to process primary miRNA through its association with the DROSHA-processing complex during responses to DNA-damaging agents. APE1 was shown to bind to more than 1000 different RNA molecules in cancer cells and many include noncoding RNAs and primary miRNA. For example, APE1 can process miR-221/222 for regulating the expression of the tumor suppressor PTEN. It uses its N-terminal 33 amino acid residues to bind structured RNA molecules in vitro and cleave single-stranded RNA-bearing oxidized bases and AP sites that would compromise translation activity.,,,,,,, This same N-terminal region of APE1 [Figure 2] is involved in multiple functions including an indispensable role in NIR endonuclease activity, modulation of protein–protein interactions, and the redox regulation of transcription factors., As would be expected, APE1 deficiency alters the expression of hundreds of genes that are related to cancer cell proliferation, invasion, and chemoresistance.
In short, for APE1 to execute its role in DNA repair and gene regulation, there must be regulatory mechanisms that switch on/off-tune and fine-tune the different APE1 activities and these include (i) alteration in APE1 redox state, (ii) translocation of APE1 from the cytoplasm to the nucleus, and (iii) modulation of APE1 by posttranslational modifications.,,,,, Besides these regulatory mechanisms, APE1 is known to exist in complexes with other proteins, and thus, modulation of its partners within the interactome could also influence APE1 accessibility to the DNA. Nearly 60 proteins are believed to interact directly or indirectly with APE1 using different approaches with a single affinity tag; however, the functional implications of APE1 interaction with each individual protein partner are still not known.,
Apurinic/apyrimidinic endonuclease 1 redox activity is controlled by a new partner, peroxiredoxin 1
We used a stringent tandem affinity approach to investigate the APE1 interactome under physiological conditions and when cells are challenged with the oxidant H2O2. We reported a novel APE1 interacting partner, peroxiredoxin 1 (PRDX1) which is a member of the peroxiredoxin family that acts as a peroxidase to decompose H2O2 to water [Figure 3]. PRDX1 also has a second function, which is to serve as a chaperone to protect proteins such as Phosphatase And Tensin Homolog (PTEN) from oxidative damage.,, To explore the functional relevance of the interaction between APE1 and PRDX1, we used short hairpin RNA to knockdown the expression of PRDX1 and monitor several characteristics of APE1. PRDX1 downregulation did not interfere with APE1 expression level, its stability, or AP endonuclease activity. In fact, it has been shown that PRDX1 null cells did not alter the activity levels of three additional DNA repair enzymes of the BER pathway that include UNG1, 8-oxoguanine DNA glycosylase, and endonuclease III homolog I. Thus, these observations eliminate the possibility that PRDX1 serves as a chaperone to protect the APE1 functional levels. However, a striking observation was captured by indirect immunofluorescence staining and confirmed by FACS analysis demonstrating that APE1 became more readily detectable in the cells knockdown for PRDX1. It seems likely that the downregulation of PRDX1 might liberate APE1.
|Figure 3: Peroxiredoxin 1 interacts with apurinic/apyrimidinic endonuclease 1 and sequesters its redox function. It is believed that the physiological role of peroxiredoxin 1 is to decompose the low levels of hydrogen peroxide that is constantly produced in the cells. In this process, peroxiredoxin 1 becomes oxidized and forms a complex with apurinic/apyrimidinic endonuclease 1 so that it can be regenerated to the reduced form. This cycling reaction sequesters apurinic/apyrimidinic endonuclease 1, and therefore, it is not available to reduce nuclear factor κB, which in its reduced state binds to cognate promoters and activate gene expression such as interleukin-8. In the absence of peroxiredoxin 1, apurinic/apyrimidinic endonuclease 1 is liberated causing activation of interleukin-8 via nuclear factor κB|
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A floodgate hypothesis has been proposed for the function of PRDX1 in combating oxidative stress. In this simple model, it has been proposed that under the normal physiological redox state of the cell (perhaps low concentration of H2O2), the monomeric form of PRDX1 that possesses peroxidase activity deals with the burden of endogenously produced H2O2 as part of a first-line defense that keeps NF-κB from being activated [Figure 3]. Once PRDX1 expression is diminished, the resulting elevated level of endogenous-free radicals causes the activation of NF-κB. The activated NF-κB then translocates to the nucleus leading to the induction of its target genes including interleukin-8 (IL-8) [Figure 3]; a proinflammatory chemokine IL-8 that is linked to cancer migration, invasion, and metastasis.,,,,,, Blocking the NF-κB translocation step with a known specific inhibitor, RO106, prevented the induction of IL-8 in PRDX1 knockdown cells. Further, binding of NF-κB to the promoter element of target genes such as IL-8 requires that its redox-sensitive cysteine, Cys62, must be reduced. It is believed that in the absence of PRDX1, APE1 is poised to maintain NF-κB in the reduced form such that it activates IL-8 expression. This is a likely possibility as APE1 has been shown to reduce NF-κB. A model [Figure 3] has been proposed whereby downregulation of PRDX1 leads to the activation of IL-8 in an APE1-dependent manner. Consistent with this model, APE1 knockdown blocks the high-level expression of IL-8 in PRDX1 knockdown cells.
It would appear that PRDX1 interaction with APE1 provides a novel regulatory scheme, whereby APE1 redox function is engaged in a cycle of maintaining a pool of reduced PRDX1 to detoxify endogenously accumulated H2O2 [Figure 3]. In this manner, APE1 is not fully available to reduce transcription factors and activate gene expression. Disrupting this balance, for example, depleting PRDX1, is expected to augment proinflammatory response leading to cancer invasion and metastasis. In fact, it has been shown that mice lacking PRDX1 are viable but develop several types of diseases including a high incidence of lymphomas and hepatocellular carcinomas. Tissues from prdx1-/- null mice display higher levels of ROS that correlate with significant accumulation of ROS-induced DNA base lesions as detected by liquid- and gas-chromatography/mass spectroscopy. These oxidative DNA lesions include the cyclic nucleosides (5′R, 5′S)-cyclo-2′-deoxyadenosine and (5′R, 5′S)-cyclo-2′-deoxyguanosine, which must be removed from the genome; otherwise, they block transcription and replication. The accumulation of these oxidative DNA lesions in prdx1-/- tissues implies that PRDX1 is involved in maintaining genomic integrity.
PRDX1 can be viewed as sequestering APE1 from activating IL-8 [Figure 3]. As such, PRDX1 can be considered to have a novel role as an anti-inflammatory protein that functions by blocking APE1 from activating IL-8. It is noteworthy that a recent study documented that another member of the peroxiredoxin family, PRDX2 transfers oxidizing equivalents onto the STAT3 transcriptional activator to form PRDX2-STAT3 disulfide-linked conjugates. This relay blocks STAT3 function in transcriptional activation but rescued upon PRDX2 depletion. This is similar to the control exerted on IL-8 expression by APE1-PRDX1 but which becomes inducible upon PRDX1 depletion.
Because APE1 is an essential DNA repair enzyme, sequestering even a fraction of APE1 as a conjugate would have deleterious consequences on the genome, unless APE1-PRDX1 performs a more efficient role in DNA repair. There are important clues hinting that the APE1-PRDX1 conjugate might act to safeguard the genome and prevent genetic alterations. First, PRDX1, and not PRDX2, can functionally substitute for its yeast counterpart Tsa1 in preventing instability of the yeast genome. Second, PRDX1 is associated with the telomeres and believed to protect this region of the chromosomes from oxidative damage. Third, APE1 can incise that many of the same oxidative lesions found accumulated in the tissues of prdx1-/- null mice, as well as a range of other oxidative DNA lesions.,,,,, Fourth, fibroblasts of pdrx1-/- mice exhibit increased levels of ROS, as well as elevated frequency in loss-of-heterozygosity, a likely consequence of increased damage to the genome by ROS.,, And fifth, prdx1-/- null mice, which are viable (unlike ape1-/- null which are nonviable), develop several diseases including a high incidence of lymphomas and hepatocellular carcinomas that correlate with increased levels of oxidative damage to the DNA.,,
Apurinic/apyrimidinic endonuclease 1 is activated by CUT-like homeobox 1 and confers resistance to ionizing radiation and temozolomide
Our interest in the CUT-like homeobox 1 (CUX1) protein emanated from several independent observations demonstrating that it functions as an auxiliary factor that stimulates at least two DNA repair enzymes of the BER pathway, the OGG1 DNA glycosylase and APE1.,,, The CUX1 gene resides in a chromosomal region, 7q22, that is frequently and highly amplified in glioblastomas as assessed by the genome-wide copy number analysis performed by The Cancer Genome Atlas More Details. Many other human tumors exhibit high CUX1 gene copy number, and the increased CUX1 expression is associated with tumor progression and shorter patient survival.,, Transgenic mice over-expressing the p200 CUX1 protein in the mammary epithelial cells develop mammary tumors, and strikingly approximately half of these tumors harbor a spontaneous point mutation activating the Kras oncogene. The cooperation between RAS and CUX1 was confirmed using lentiviral infections in the lung. It is well established that RAS oncogenes fail to transform primary cells and instead cause their senescence. Previous studies revealed that activation of the RAS pathway leads to heightened production of ROS, which cause oxidative DNA damage and ultimately senescence.,,, This has been documented in tissue culture, transgenic mouse models, and human premalignant lesions.,,,,,, Cancer cells can reduce ROS levels by increasing the expression of antioxidants.,, Alternatively, cancer cells can adapt to the elevated ROS by increasing their capacity to repair oxidative DNA damage.,, CUX1 was shown to accelerate the repair of oxidative DNA damage, thereby preventing RAS-induced senescence in primary cells and enabling proliferation in spite of the presence of elevated ROS levels. In agreement with these findings, CUX1 knockdown causes synthetic lethality in human tumor cell lines in which the RAS pathway is activated as a consequence of an activating mutation in either KRAS, HRAS, BRAF, or the EGFR gene., A genome-wide RNAi screen designed to hunt for genes that when downregulated would cause synthetic lethality with KRAS revealed that, in addition to CUX1, four genes involved in distinct steps of the BER pathway (NEIL2, XRCC1, POLβ, and LIG3) supporting the notion that RAS-transformed cells are acutely dependent on the efficient repair of oxidative DNA damage.
The abundant full-length CUX1 protein, often called p200 CUX1, contains four conserved DNA-binding domains including a CUT homeodomain and three CUT domains, C1, C2, and C3 which are referred to as Cut repeats. p200 CUX1 can be proteolytically processed to the shorter p110 CUX1 form that functions as a transcription factor.,, However, the native p200 CUX1 plays a direct role in DNA repair through its three CUT domains. In response to DNA damage, such as laser-induced oxidative damage, p200 CUX1 rapidly accumulates on the DNA within seconds indicative of a direct role in DNA repair. Single-cell gel electrophoresis (comet assay) performed in various conditions showed that repair of oxidized bases is delayed or accelerated following CUX1 knockdown or overexpression, respectively., Strikingly, mouse embryonic fibroblasts (MEFs) derived from Cux1-/- knockout mice rapidly undergo senescence when exposed to 20% oxygen, while they proliferate normally in lower oxygen (3%), suggesting that CUX1 is needed to repair oxygen-induced oxidative damage.In vitro DNA repair assays with both radioactively labeled and fluorophore-based probes containing 8-oxo-guanine lesions established that CUT domains stimulate the glycosylase and AP/lyase activities of the OGG1 DNA glycosylase.,,,, A small recombinant CUX1 protein bearing only two CUT domains, C1 and C2 (C1–C2), and devoid of transcriptional potential is rapidly recruited to DNA damage. This C1–C2 fragment is sufficient to accelerate the repair of oxidative DNA damage,,, confer resistance to ionizing radiation, and rescue the inability of Cux1-/- MEFs to proliferate in 20% oxygen.
In a separate study, CUT domains were also shown to stimulate the AP endonuclease activity of APE1 using in vitro DNA repair assays [Figure 4].In vivo CUX1 knockdown caused a decrease in APE1 activity, which correlated with an increase in the number of abasic sites in genomic DNA. In contrast, ectopic expression of the native p200 CUX1 protein or the small C1-C2 recombinant protein increased APE1 activity and reduced the number of abasic sites in genomic DNA. Since CUX1 is expressed at high levels in most glioblastomas, it was necessary to test whether varying CUX1 level in these tumor cells would affect their sensitivity toward the standard-of-care treatment regimen that includes ionizing radiation and the chemotherapeutic agent temozolomide. Temozolomide is a mono-alkylating agent, which causes DNA damage by methylating guanine to produce N 7-methylguanine and O 6-methylguanine adducts, at least 70 and 5%, respectively, as well as methylating adenine to produce nearly 8% of N 3-methyladenine as adducts. Repair of these alkylated base lesions such as N 7-methylguanine or N 3-methyladenine is initiated by the N-methylpurine-DNA glycosylase, which produces an AP site that can be cleaved by APE1 followed by DNA repair synthesis using the BER pathway [Figure 1].,CUX1 knockdown increased the sensitivity of glioblastoma cells to temozolomide, whereas overexpression of CUX1 or the small C1–C2 protein increased resistance to the anticancer drug. Likewise, the resistance of glioblastoma cells to the combined treatment of ionizing radiation and temozolomide was reduced by CUX1 knockdown but increased by overexpression of CUX1 or the small C1–C2 protein. The impact of CUX1 on the resistance to radiation was independently demonstrated in tumor cell lines from breast, lung, and colorectal cancers.
|Figure 4: Depiction of the fluorescently labeled stem-loop substrate used for monitoring apurinic/apyrimidinic endonuclease 1 apurinic/apyrimidinic endonuclease activity. The stem-loop deoxyoligonucleotide substrate contains a single synthetic apurinic/apyrimidinic site tetrahydrofuran (THF) opposite adenine (THF: A) at the six position from the 5′-end bearing 6-carboxyfluorescein (green dot) and the quencher Dabcyl (black dot) on the 3′-end. The enzymatic incision of the substrate by APE1 releases a fluorescently labeled 5-mer product (red dot) that can be detected by a fluorometer (excitation/emission 490/520 nm)|
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It is noteworthy that APE1 expression is also elevated in many cancers including gliomas, medulloblastomas, and primitive neuroectodermal tumors., The higher APE1 activity was shown to contribute to the resistance of the brain cancer cells to temozolomide, while diminishing APE1 activity can increase sensitivity of cancer cells toward this drug.,,,,,,,, Likewise, APE1 is overexpressed in ovarian cancer and its downregulation can block ovarian cancer cell and tumor growth., Similarly, APE1 is upregulated in human pancreatic cancer cells and inhibition of its redox activity blocked the migration and proliferation of these cells. In light of these findings, significant efforts have been devoted to developing inhibitors toward APE1 to sensitize cells to anticancer drugs. Unfortunately, as APE1 is an essential protein, any drug that inhibits its activity is likely to exhibit a rather small therapeutic window. In contrast, CUX1 is a nonessential protein that is required only in situ ation of oxidative stress as caused by altered metabolism in RAS-driven cancer cells or by ionizing radiation during radiotherapy. The newer findings that CUX1 has an impact on the resistance of cancer cells toward chemotherapy identify the CUT domains of CUX1 as interesting therapeutic targets.
Inhibitors of the apurinic/apyrimidinic endonuclease and redox activity of apurinic/apyrimidinic endonuclease 1
Several inhibitors target the two main functional domains, DNA repair and redox, of APE1. Methoxyamine is a nonspecific indirect inhibitor of APE1, which binds irreversibly to AP sites in DNA to form an adduct that prevents AP endonucleases such as E. coli exonuclease III and endonuclease IV from cleaving the AP site lesion., Methoxyamine is currently in phase II trial (NCT02395692) in combination with standard temozolomide chemotherapy for treating relapsed glioblastoma patients. Another compound under phase II trial (NCT02014545) is lucanthone that has been shown to inhibit APE1 DNA repair activity in cellular extracts and increased the cell killing effect of temozolomide. Lucanthone easily crosses the blood–brain barrier and the phase II trial is to explore its ability to sensitize patients who are given whole-brain radiation therapy due to metastatic brain tumors that derived from nonsmall cell lung cancer. The inhibitor AR03 (2, 4, 9-trimethylbenzo[b] [1,8]-naphthyridin-5-amine) has been shown to inhibit APE1 ability to cleave AP sites in glioblastoma cells, as well as in in vitro systems., This inhibitor also can potentiate the cytotoxicity of temozolomide in glioblastoma cells.,
A number of compounds have been shown to alter the redox function of APE1 and are potential inhibitors that can sensitize cancer cells to radiation therapy. The predominant isoflavones, genistein and daidzein of soybean, can suppress tumor cell growth and potentiate radiation-induced cell killing in vitro.,, These isoflavones are believed to block APE1 ability to activate NF-κB binding to its promoters. Tanshinone IIA, a Chinese herbal compound, is a promising bioactive inhibitor that can block the redox function of APE1 with a dissociation constant in the subnanomolar range (0.88 nM) and cause an increase in the cytoxicity of ionizing radiation and known chemotherapeutics., Finally, the quinone derivative (2E)-2-( [4,5-dimethoxy-2-methyl-3,6-dioxo -1,4-cyclohexadien-1-yl] methylene)-undecanoic acid or referred to as E3330 was first shown to inhibit NF-κB activity in nuclear extracts and soon after discovered to be targeting APE1 redox function, and not the DNA repair activities, with binding constant of 1.6 nM.,,, Despite these progresses, more efforts are still needed to find effective inhibitors of APE1 functions that can be safely used in combination therapy to treat various types of cancers.
The association of APE1-PRDX1 safeguards APE1 from reducing transcription factors such as NF-κB and activating superfluous gene expression, which otherwise could trigger cancer invasion and metastasis. Because APE1 is in contact with several other factors raises the question how does this enzyme becomes available to repair DNA lesions in a tightly compacted chromatin. One possible explanation is that CUX1 recruitment to damaged DNA may serve to facilitate the turnover of APE1. It is believe that the mechanistic understanding of the APE1-CUX1-mediated repair of DNA lesions should lead to the development of new drugs capable of blocking the activating role of CUX1 on APE1, thereby sensitizing cells to genotoxic anticancer drugs.
Financial support and sponsorship
This work was supported by grants from the Natural Science and Engineering Research Council of Canada to D. R. (RGPIN/202432-2012) and the Cancer Research Society to A. N. grant #702996, respectively.
Conflicts of interest
There are no conflicts of interest.
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