Interaction of poly(ADP-ribose) polymerase 1 with apurinic/apyrimidinic sites within clustered DNA damage

AP endonuclease 1: Biological updates and advances in activity analysis

Apurinic/apyrimidinic endodeoxyribonuclease 1 (APE1, APEX1, REF1, HAP1) is an abasic site-specific endonuclease holding critical roles in numerous biological functions including base excision repair, the DNA damage response, redox regulation of transcription factors, RNA processing, and gene regulation. Pathologically, APE1 expression and function is linked with numerous human diseases including cancer, highlighting the importance of sensitive and quantitative assays to measure APE1 activity. Here, we summarize biochemical and biological roles for APE1 and expand on the discovery of APE1 inhibitors. Finally, we highlight the development of assays to monitor APE1 activity, detailing a recently improved and stabilized DNA Repair Molecular Beacon assay to analyze APE1 activity. The assay is amenable to analysis of purified protein, to measure changes in APE1 activity in cell lysates, to monitor human patient samples for defects in APE1 function, or the cellular and biochemical response to APE1 inhibitors.

Role of YB-1 in Regulation of Poly(ADP-Ribosylation) Catalyzed by Poly(ADP-Ribose) Polymerases

Poly(ADP-ribosyl)ation is a post-translational modification of proteins that performs an essential regulatory function in the cellular response to DNA damage. The key enzyme synthesizing poly(ADP-ribose) (PAR) in the cells is poly(ADP-ribose) polymerase 1 (PARP1). Understanding the mechanisms of the PARP1 activity regulation within the cells is necessary for development of the PARP1-targeted antitumor therapy. This review is devoted to the studies of the role of the RNA-binding protein YB-1 in the PARP1-catalyzed PARylation. The mechanisms of PARP1 activity stimulation by YB-1 protein can possibly be extended to other RNA-binding proteins involved in the maintenance of the genome stability.

The contribution of PARP1, PARP2 and poly(ADP-ribosyl)ation to base excision repair in the nucleosomal context

The regulation of repair processes including base excision repair (BER) in the presence of DNA damage is implemented by a cellular signal: poly(ADP-ribosyl)ation (PARylation), which is catalysed by PARP1 and PARP2. Despite ample studies, it is far from clear how BER is regulated by PARPs and how the roles are distributed between the PARPs. Here, we investigated the effects of PARP1, PARP2 and PARylation on activities of the main BER enzymes (APE1, DNA polymerase β [Polβ] and DNA ligase IIIα [LigIIIα]) in combination with BER scaffold protein XRCC1 in the nucleosomal context. We constructed nucleosome core particles with midward- or outward-oriented damage. It was concluded that in most cases, the presence of PARP1 leads to the suppression of the activities of APE1, Polβ and to a lesser extent LigIIIα. PARylation by PARP1 attenuated this effect to various degrees depending on the enzyme. PARP2 had an influence predominantly on the last stage of BER: DNA sealing. Nonetheless, PARylation by PARP2 led to Polβ inhibition and to significant stimulation of LigIIIα activities in a NAD⁺-dependent manner. On the basis of the obtained and literature data, we suggest a hypothetical model of the contribution of PARP1 and PARP2 to BER.

Functional Role of N-Terminal Extension of Human AP Endonuclease 1 In Coordination of Base Excision DNA Repair via Protein-Protein Interactions

Human apurinic/apyrimidinic endonuclease 1 (APE1) has multiple functions in base excision DNA repair (BER) and other cellular processes. Its eukaryote-specific N-terminal extension plays diverse regulatory roles in interaction with different partners. Here, we explored its involvement in interaction with canonical BER proteins. Using fluorescence based-techniques, we compared binding affinities of the full-length and N-terminally truncated forms of APE1 (APE1NΔ35 and APE1NΔ61) for functionally and structurally different DNA polymerase β (Polβ), X-ray repair cross-complementing protein 1 (XRCC1), and poly(adenosine diphosphate (ADP)-ribose) polymerase 1 (PARP1), in the absence and presence of model DNA intermediates. Influence of the N-terminal truncation on binding the AP site-containing DNA was additionally explored. These data suggest that the interaction domain for proteins is basically formed by the conserved catalytic core of APE1. The N-terminal extension being capable of dynamically interacting with the protein and DNA partners is mostly responsible for DNA-dependent modulation of protein–protein interactions. Polβ, XRCC1, and PARP1 were shown to more efficiently regulate the endonuclease activity of the full-length protein than that of APE1NΔ61, further suggesting contribution of the N-terminal extension to BER coordination. Our results advance the understanding of functional roles of eukaryote-specific protein extensions in highly coordinated BER processes.

DNA complexes with human apurinic/apyrimidinic endonuclease 1: structural insights revealed by pulsed dipolar EPR with orthogonal spin labeling

A DNA molecule is under continuous influence of endogenous and exogenous damaging factors, which produce a variety of DNA lesions. Apurinic/apyrimidinic sites (abasic or AP sites) are among the most common DNA lesions. In this work, we applied pulse dipolar electron paramagnetic resonance (EPR) spectroscopy in combination with molecular dynamics (MD) simulations to investigate in-depth conformational changes in DNA containing an AP site and in a complex of this DNA with AP endonuclease 1 (APE1). For this purpose, triarylmethyl (TAM)-based spin labels were attached to the 5' ends of an oligonucleotide duplex, and nitroxide spin labels were introduced into APE1. In this way, we created a system that enabled monitoring the conformational changes of the main APE1 substrate by EPR. In addition, we were able to trace substrate-to-product transformation in this system. The use of different (orthogonal) spin labels in the enzyme and in the DNA substrate has a crucial advantage allowing for detailed investigation of local damage and conformational changes in AP-DNA alone and in its complex with APE1.

Human apurinic/apyrimidinic endonuclease 1 is modified in vitro by poly(ADP-ribose) polymerase 1 under control of the structure of damaged DNA

Apurinic/apyrimidinic endonuclease 1 (APE1) is an essential multifunctional protein in mammals involved in base excision DNA repair (BER), regulation of gene expression and RNA metabolism. Its major enzymatic function is incision of AP sites. Poly(ADP-ribose) polymerase 1 (PARP1) modifies itself and target proteins with poly(ADP-ribose) (PAR), contributing to regulation of many processes. To understand molecular basis of functional cooperation between APE1 and PARP1 in BER, we examined PAR-binding activity and ADP-ribosylation of human APE1 in comparison with known targets of PARP1, using the full-length, N-terminally truncated and catalytically inactive forms of APE1. The protein binds preferentially large ADP-ribose polymers, being very similar to DNA polymerase β (Polβ) but contrasting with the scaffold XRCC1 protein. The interaction with PAR involves the universally conserved catalytic portion and the eukaryote-specific extension of APE1. The ADP-ribosylation of APE1 depends on the structure of PARP1-activating DNA, contrasting APE1 with Polβ and XRCC1. Relative levels of APE1 modification in the presence of different DNA substrates were found to correlate with affinities of the DNAs for APE1 and substrate activities in the enzymatic incision, suggesting the ADP-ribosylation to occur within the DNA-mediated ternary complex. This conclusion was confirmed by importance of the length of DNA region 3' to the AP site for the modification. Deletion of the N-terminal extension of APE1 produced no significant influence on both the ADP-ribosylation efficiency and hydrolytic stability of the modified protein, suggesting localization of target amino acids in the conserved catalytic portion. The most efficient ADP-ribosylation of the catalytically inactive APE1 mutant was shown to reduce the level of PARP1 automodification, suggesting possible role of APE1 in modulating PARP1 activity. Our data on primary role of DNA in controlling the PARP-catalysed modification provide new insights into mechanisms of protein targeting for ADP-ribosylation.

AP Endonuclease 1 as a Key Enzyme in Repair of Apurinic/Apyrimidinic Sites

Human apurinic/apyrimidinic endonuclease 1 (APE1) is one of the key participants in the DNA base excision repair system. APE1 hydrolyzes DNA adjacent to the 5'-end of an apurinic/apyrimidinic (AP) site to produce a nick with a 3'-hydroxyl group and a 5'-deoxyribose phosphate moiety. APE1 exhibits 3'-phosphodiesterase, 3'-5'-exonuclease, and 3'-phosphatase activities. APE1 was also identified as a redox factor (Ref-1). In this review, data on the role of APE1 in the DNA repair process and in other metabolic processes occurring in cells are analyzed as well as the interaction of this enzyme with DNA and other proteins participating in the repair system.

Identification of a conserved 5'-dRP lyase activity in bacterial DNA repair ligase D and its potential role in base excision repair

Bacillus subtilis is one of the bacterial members provided with a nonhomologous end joining (NHEJ) system constituted by the DNA-binding Ku homodimer that recruits the ATP-dependent DNA Ligase D (BsuLigD) to the double-stranded DNA breaks (DSBs) ends. BsuLigD has inherent polymerization and ligase activities that allow it to fill the short gaps that can arise after realignment of the broken ends and to seal the resulting nicks, contributing to genome stability during the stationary phase and germination of spores. Here we show that BsuLigD also has an intrinsic 5'-2-deoxyribose-5-phosphate (dRP) lyase activity located at the N-terminal ligase domain that in coordination with the polymerization and ligase activities allows efficient repairing of 2'-deoxyuridine-containing DNA in an in vitro reconstituted Base Excision Repair (BER) reaction. The requirement of a polymerization, a dRP removal and a final sealing step in BER, together with the joint participation of BsuLigD with the spore specific AP endonuclease in conferring spore resistance to ultrahigh vacuum desiccation suggest that BsuLigD could actively participate in this pathway. We demonstrate the presence of the dRP lyase activity also in the homolog protein from the distantly related bacterium Pseudomonas aeruginosa, allowing us to expand our results to other bacterial LigDs.

Poly(ADP-ribosyl)ation as a new posttranslational modification of YB-1

Multifunctional Y-box binding protein 1 (YB-1) is actively studied as one of the components of cellular response to genotoxic stress. However, the precise role of YB-1 in the process of DNA repair is still obscure. In the present work we report for the first time new posttranslational modification of YB-1 - poly(ADP-ribosyl)ation, catalyzed by one of the main regulatory enzymes of DNA repair - poly(ADP-ribose)polymerase 1 (PARP1) in the presence of model DNA substrate carrying multiple DNA lesions. Therefore, poly(ADP-ribosyl)ation of YB-1 catalyzed with PARP1, can be stimulated by damaged DNA. The observed property of YB-1 underlines its ability to participate in the DNA repair by its involvement in the regulatory cascades of DNA repair.

Design of a New Fluorescent Oligonucleotide-Based Assay for a Highly Specific Real-Time Detection of Apurinic/Apyrimidinic Site Cleavage by Tyrosyl-DNA Phosphodiesterase 1

Tyrosyl-DNA phosphodiesterase 1 (Tdp1) promotes catalytic scission of a phosphodiester bond between the 3'-end of DNA and the hydroxyl group of a tyrosine residue, as well as cleaving off a variety of other 3'-terminal phosphate-linked DNA substituents. We have shown recently that Tdp1 can initiate an apurinic/apyrimidinic (AP) site repair pathway that is independent from the one mediated by AP endonuclease 1 (APE1). Until recently, there was no method available of tracking the AP-site cleaving activity of Tdp1 by real-time fluorescence assay. In the present study we demonstrate a highly specific real-time detection of the AP-site cleaving activity of Tdp1 which allows one to distinguish it from the activity of APE1 by using a short hairpin oligonucleotide with a 1,12-dodecanediol loop, a 5'-fluorophore, and a 3'-quencher. Specific phosphodiesterase activity of Tdp1, which is usually able to remove quencher from the 3'-end of DNA, was suppressed in our approach by introducing a noncleavable phosphate group mimic between the 3'-end and the quencher. As a nondigestible 3'-phosphate analogue, we have used a new uncharged tetramethyl phosphoryl guanidine (Tmg) group, which is resistant to 3'-phosphodiesterase cleavage.

Interaction of PARP-2 with AP site containing DNA

In eukaryotes the stability of genome is provided by functioning of DNA repair systems. One of the main DNA repair pathways in eukaryotes is the base excision repair (BER). This system requires precise regulation for correct functioning. Two members of the PARP family - PARP-1 and PARP-2, which can be activated by DNA damage - are widely considered as regulators of DNA repair processes, including BER. In contrast to PARP-1, the role of PARP-2 in BER has not been extensively studied yet. Since AP site is one of the most frequent type of DNA damage and a key intermediate of BER at the stage preceding formation of DNA breaks, in this paper we focused on the characterization of PARP-2 interaction with AP site-containing DNAs. We demonstrated that PARP-2, like PARP-1, can interact with the intact AP site via Schiff base formation, in spite of crucial difference in the structure of the DNA binding domains of these PARPs. By cross-linking of PARPs to AP DNA, we determined that the N-terminal domains of both PARPs are involved in formation of cross-links with AP DNA. We have also confirmed that DNA binding by PARP-2, in contrast to PARP-1, is not modulated by autoPARylation. PARP-2, like PARP-1, can inhibit the activity of APE1 by binding to AP site, but, in contrast to PARP-1, this inhibitory influence is hardly regulated by PAR synthesis. At the same time, 5'-dRP lyase activity of both PARPs is comparable, although being much weaker than that of Pol β, which is considered as the main 5'-dRP lyase of the BER process.

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.

The mechanism of human tyrosyl-DNA phosphodiesterase 1 in the cleavage of AP site and its synthetic analogs

The mechanism of hydrolysis of the apurinic/apyrimidinic (AP) site and its synthetic analogs by using tyrosyl-DNA phosphodiesterase 1 (Tdp1) was analyzed. Tdp1 catalyzes the cleavage of AP site and the synthetic analog of the AP site, 3-hydroxy-2(hydroxymethyl)-tetrahydrofuran (THF), in DNA by hydrolysis of the phosphodiester bond between the substituent and 5' adjacent phosphate. The product of Tdp1 cleavage in the case of the AP site is unstable and is hydrolyzed with the formation of 3'- and 5'-margin phosphates. The following repair demands the ordered action of polynucleotide kinase phosphorylase, with XRCC1, DNA polymerase β, and DNA ligase. In the case of THF, Tdp1 generates break with the 5'-THF and the 3'-phosphate termini. Tdp1 is also able to effectively cleave non-nucleotide insertions in DNA, decanediol and diethyleneglycol moieties by the same mechanism as in the case of THF cleavage. The efficiency of Tdp1 catalyzed hydrolysis of AP-site analog correlates with the DNA helix distortion induced by the substituent. The following repair of 5'-THF and other AP-site analogs can be processed by the long-patch base excision repair pathway.

Pol β associated complex and base excision repair factors in mouse fibroblasts

During mammalian base excision repair (BER) of lesion-containing DNA, it is proposed that toxic strand-break intermediates generated throughout the pathway are sequestered and passed from one step to the next until repair is complete. This stepwise process is termed substrate channeling. A working model evaluated here is that a complex of BER factors may facilitate the BER process. FLAG-tagged DNA polymerase (pol) β was expressed in mouse fibroblasts carrying a deletion in the endogenous pol β gene, and the cell extract was subjected to an ‘affinity-capture’ procedure using anti-FLAG antibody. The pol β affinity-capture fraction (ACF) was found to contain several BER factors including polymerase-1, X-ray cross-complementing factor1-DNA ligase III and enzymes involved in processing 3′-blocked ends of BER intermediates, e.g. polynucleotide kinase and tyrosyl-DNA phosphodiesterase 1. In contrast, DNA glycosylases, apurinic/aprymidinic endonuclease 1 and flap endonuclease 1 and several other factors involved in BER were not present. Some of the BER factors in the pol β ACF were in a multi-protein complex as observed by sucrose gradient centrifugation. The pol β ACF was capable of substrate channeling for steps in vitro BER and was proficient in in vitro repair of substrates mimicking a 3′-blocked topoisomerase I covalent intermediate or an oxidative stress-induced 3′-blocked intermediate.

Poly(ADP-ribosyl)ation acts in the DNA demethylation of mouse primordial germ cells also with DNA damage-independent roles

Poly(ADP-ribosyl)ation regulates chromatin structure and transcription driving epigenetic events. In particular, Parp1 is able to directly influence DNA methylation patterns controlling transcription and activity of Dnmt1. Here, we show that ADP-ribose polymer levels and Parp1 expression are noticeably high in mouse primordial germ cells (PGCs) when the bulk of DNA demethylation occurs during germline epigenetic reprogramming in the embryo. Notably, Parp1 activity is stimulated in PGCs even before its participation in the DNA damage response associated with active DNA demethylation. We demonstrate that PARP inhibition impairs both genome-wide and locus-specific DNA methylation erasure in PGCs. Moreover, we evidence that impairment of PARP activity causes a significant reduction of expression of the gene coding for Tet1 hydroxylases involved in active DNA demethylation. Taken together these results demonstrate new and adjuvant roles of poly(ADP-ribosyl)ation during germline DNA demethylation and suggest its possible more general involvement in genome reprogramming.