poly(ADP-ribose) Glycohydrolase Activity Research Articles

PARG-deficient tumor cells have an increased dependence on EXO1/FEN1-mediated DNA repair

Targeting poly(ADP-ribose) glycohydrolase (PARG) is currently explored as a therapeutic approach to treat various cancer types, but we have a poor understanding of the specific genetic vulnerabilities that would make cancer cells susceptible to such a tailored therapy. Moreover, the identification of such vulnerabilities is of interest for targeting BRCA2;p53-deficient tumors that have acquired resistance to poly(ADP-ribose) polymerase inhibitors (PARPi) through loss of PARG expression. Here, by performing whole-genome CRISPR/Cas9 drop-out screens, we identify various genes involved in DNA repair to be essential for the survival of PARG;BRCA2;p53-deficient cells. In particular, our findings reveal EXO1 and FEN1 as major synthetic lethal interactors of PARG loss. We provide evidence for compromised replication fork progression, DNA single-strand break repair, and Okazaki fragment processing in PARG;BRCA2;p53-deficient cells, alterations that exacerbate the effects of EXO1/FEN1 inhibition and become lethal in this context. Since this sensitivity is dependent on BRCA2 defects, we propose to target EXO1/FEN1 in PARPi-resistant tumors that have lost PARG activity. Moreover, EXO1/FEN1 targeting may be a useful strategy for enhancing the effect of PARG inhibitors in homologous recombination-deficient tumors.

Abstract LB_A19: Genome-wide CRISPR/Cas9 screens for the identification of targeted vulnerabilties in PARP inhibitor-resistant PARG;BRCA2-deficient tumor cells: A focus on EXO1/FEN1-mediated DNA repair

Abstract Targeting poly(ADP-ribose) glycohydrolase (PARG) is currently explored as a therapeutic approach to treat various cancer types, but we have a poor understanding of the specific genetic vulnerabilities that would make cancer cells susceptible to such a tailored therapy. Moreover, the identification of such vulnerabilities is of interest to target BRCA2;p53-deficient tumors that have acquired resistance to poly(ADP-ribose) polymerase inhibitors (PARPi), through loss of PARG expression. Here, by performing whole genome CRISPR/Cas9 drop-out screens we identify various genes involved in DNA repair to be essential for the survival of PARG;BRCA2;p53-deficient cells. In particular, our findings reveal EXO1 and FEN1 as major synthetic lethal interactors to PARG loss. We provide evidence that PARG;BRCA2;p53-deficient cells exhibit compromised replication fork progression, DNA single-strand break repair and Okazaki fragment processing, alterations that exacerbate and become lethal upon EXO1/FEN1 inhibition. Since this sensitivity is dependent on BRCA2 defects, we propose EXO1/FEN1 targeting in PARPi-resistant tumors that have lost PARG activity. Moreover, EXO1/FEN1 targeting may be a useful strategy to enhance the effect of PARG inhibitors in HR-deficient tumors. Citation Format: Christina Andronikou, Kamila Burdova, Diego Dibitetto, Cor Lieftink, Roderick L. Beijersbergen, Hana Hanzlikova, Jos Jonkers, Sven Rottenberg. Genome-wide CRISPR/Cas9 screens for the identification of targeted vulnerabilties in PARP inhibitor-resistant PARG;BRCA2-deficient tumor cells: A focus on EXO1/FEN1-mediated DNA repair [abstract]. In: Proceedings of the AACR-NCI-EORTC Virtual International Conference on Molecular Targets and Cancer Therapeutics; 2023 Oct 11-15; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2023;22(12 Suppl):Abstract nr LB_A19.

The ADP-ribose hydrolase NUDT5 is important for DNA repair.

DNA damage leads to rapid synthesis of poly(ADP-ribose) (pADPr), which is important for damage signaling and repair. pADPr chains are removed by poly(ADP-ribose) glycohydrolase (PARG), releasing free mono(ADP-ribose) (mADPr). Here, we show that the NUDIX hydrolase NUDT5, which can hydrolyze mADPr to ribose-5-phosphate and either AMP or ATP, is recruited to damage sites through interaction with PARG. NUDT5 does not regulate PARP or PARG activity. Instead, loss of NUDT5 reduces basal cellular ATP levels and exacerbates the decrease in cellular ATP that occurs during DNA repair. Further, loss of NUDT5 activity impairs RAD51 recruitment, attenuates the phosphorylation of key DNA-repair proteins, and reduces both H2A.Z exchange at damage sites and repair by homologous recombination. The ability of NUDT5 to hydrolyze mADPr, and/or regulate cellular ATP, may therefore be important for efficient DNA repair. Targeting NUDT5 to disrupt PAR/mADPr and energy metabolism may be an effective anti-cancer strategy.

Poly(ADP-ribosyl)ating pathway regulates development from stem cell niche to longevity control.

The regulation of poly(ADP-ribose) polymerase, the enzyme responsible for the synthesis of homopolymer ADP-ribose chains on nuclear proteins, has been extensively studied over the last decades for its involvement in tumorigenesis processes. However, the regulation of poly(ADP-ribose) glycohydrolase (PARG), the enzyme responsible for removing this posttranslational modification, has attracted little attention. Here we identified that PARG activity is partly regulated by two phosphorylation sites, ph1 and ph2, in Drosophila We showed that the disruption of these sites affects the germline stem-cells maintenance/differentiation balance as well as embryonic and larval development, but also the synchronization of egg production with the availability of a calorically sufficient food source. Moreover, these PARG phosphorylation sites play an essential role in the control of fly survivability from larvae to adults. We also showed that PARG is phosphorylated by casein kinase 2 and that this phosphorylation seems to protect PARG protein against degradation in vivo. Taken together, these results suggest that the regulation of PARG protein activity plays a crucial role in the control of several developmental processes.

Selective monitoring of the protein-free ADP-ribose released by ADP-ribosylation reversal enzymes.

ADP-ribosylation is a key post-translational modification that regulates a wide variety of cellular stress responses. The ADP-ribosylation cycle is maintained by writers and erasers. For example, poly(ADP-ribosyl)ation cycles consist of two predominant enzymes, poly(ADP-ribose) polymerases (PARPs) and poly(ADP-ribose) glycohydrolase (PARG). However, historically, mechanisms of erasers of ADP-ribosylations have been understudied, primarily due to the lack of quantitative tools to selectively monitor specific activities of different ADP-ribosylation reversal enzymes. Here, we developed a new NUDT5-coupled AMP-Glo (NCAG) assay to specifically monitor the protein-free ADP-ribose released by ADP-ribosylation reversal enzymes. We found that NUDT5 selectively cleaves protein-free ADP-ribose, but not protein-bound poly- and mono-ADP-ribosylations, protein-free poly(ADP-ribose) chains, or NAD+. As a proof-of-concept, we successfully measured the kinetic parameters for the exo-glycohydrolase activity of PARG, which releases monomeric ADP-ribose, and monitored activities of site-specific mono-ADP-ribosyl-acceptor hydrolases, such as ARH3 and TARG1. This NCAG assay can be used as a general platform to study the mechanisms of diverse ADP-ribosylation reversal enzymes that release protein-free ADP-ribose as a product. Furthermore, this assay provides a useful tool to identify small-molecule probes targeting ADP-ribosylation metabolism and to quantify ADP-ribose concentrations in cells.

PO-496 Loss of PARG drives PARP inhibitor resistance in BRCA2-deficient mouse mammary tumours

IntroductionInhibitors of poly(ADP-ribose) polymerase (PARP) have recently entered the clinic for the treatment of homologous recombination (HR)-deficient cancers. Despite the success of this approach, resistance to PARP inhibitors (PARPi) is a clinical hurdle, and we poorly understand how cancer cells escape the deadly effects of PARPi without restoring the HR pathway.Material and methodsTo tackle this question, we generated matched PARPi-sensitive and -resistant Brca2-mutated mouse mammary tumours. By combining next-generation sequencing with functional genetic screens, we identified loss of poly(ADP-ribose) glycohydrolase (PARG) as a major resistance mechanism.Results and discussionsWe demonstrate that PARG depletion restores PAR formation, rescues controlled DNA replication fork progression and promotes the recruitment of downstream DNA repair factors. The potential relevance of PARG in clinical PARPi resistance is underscored by the presence of PARG-negative clones in a subset of human triple-negative breast and serous ovarian cancers. Importantly, acquisition However, the gain of PARPi resistance comes at a cost, as PARG inactivation results in new vulnerabilities that can be exploited therapeutically.ConclusionWe conclude that loss of PARG should be assessed as a potential cause of clinical PARPi resistance. In this case, measurement of PARG activity should further improve clinical decision making for patients with tumours that lack homology-directed DNA repair.

Abstract LB-A13: Targeting poly-ADP ribose glycohydrolase: pursuing a synthetic lethal therapeutic strategy to treat all pancreatic cancers

Abstract This study lays the groundwork to target poly-ADP ribose glycohydrolase (PARG), a DNA repair enzyme that counteracts PARP-1 activity and is upregulated in response to PARP inhibitor (PARPi) treatment. The goal of this study is to optimize PARPi-based therapies for all pancreatic cancer patients, irrespective of their DNA repair status (i.e., homologous repair (HR) deficiency). Metastatic pancreatic ductal adenocarcinoma (PDA) is a lethal disease with an average survival of less than one year, which highlights the need for novel treatment regimens. Though PARPi therapy offers a personalized strategy in a subset of tumors with HR defects, initially responsive patients typically become resistant to therapy. Therefore, identifying and combating PARPi resistance mechanisms is a rising concern. We have previously shown that PARG is a critical player in mediating resistance; therefore we evaluated PARG as a candidate target in PDA. Importantly, we demonstrated that the pro-oncogenic protein, HuR (ELAVL1), upregulates PARG specifically in tumor microenvironment niches (e.g., drug exposure), thus providing a unique, cancer-specific therapeutic window for targeting PARG. We evaluated a series of potent first-in-class, cell-active small molecule PARG inhibitors; PDDX-01, PDDX02 and PDDX-04 in pre-clinical models of PDA. These PARGi effectively inhibited PARG activity, caused persistence of PAR polymers, increased accumulation of γH2AX foci and inhibited short and long-term growth of PDA cell lines. Similar to PARPi agents, PARG inhibition was also shown to be synthetic lethal; DNA damage repair (DDR)- deficient cell lines were more sensitive than the DDR- proficient PDA cell lines. This was further validated in isogenic colorectal cell lines with varying DNA repair functionality; DLD1 lines with BRCA2 (+/+, +/-, -/-) and RKO lines with FANCC (+/+, +/-, -/-) modulation. Using chromatin tethering assays, these PARGis effectively trapped PARP1 on DNA, alone as well as combination with the PARPi olaparib, and a DNA damaging agent oxaliplatin. We also developed and characterized a PARG inhibition model in vivo in a DNA- repair proficient PDA line, MIA PaCa2. The doxycycline-inducible shPARG knockdown was validated for PARG inhibition, PAR accumulation and increased PARPi sensitivity in vitro. Complementing xenograft experiments were performed wherein 3 million cells were injected per flank in nude female athymic mice. At an average tumor volume of 50mm3, respective groups were fed DOX- chow to induce PARG knockdown and treated with olaparib intraperitoneally at 50mg/kg five times a week. PARG inhibition by doxycycline induction significantly decreased tumor volumes (50% decrease, p-value 0.0165), which was further enhanced with olaparib treatment (70% decrease, p- value 0.0004), when compared with control arms. Taken together, these studies help define and break a potent PARPi resistance mechanism and also validate pursuing PARG as a valuable “druggable” target in both HR-proficient and deficient PDA cells. Citation Format: Saswati N. Chand, AnnJosette Ramirez, Aditi Jain, Avinoam Nevlar, Cinthya Yabar-Lowder, Joseph A. Cozzitorto, Dominic I. James, Ian D. Waddell, Charles J. Yeo, Michael J. Pishvaian, Jordan M. Winter, Jonathan R. Brody. Targeting poly-ADP ribose glycohydrolase: pursuing a synthetic lethal therapeutic strategy to treat all pancreatic cancers [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2017 Oct 26-30; Philadelphia, PA. Philadelphia (PA): AACR; Mol Cancer Ther 2018;17(1 Suppl):Abstract nr LB-A13.

From the Cover: ROS-Induced Store-Operated Ca2+ Entry Coupled to PARP-1 Hyperactivation Is Independent of PARG Activity in Necrotic Cell Death.

2,3,5-tris(Glutathion-S-yl)hydroquinone, a potent nephrotoxic and nephrocarcinogenic metabolite of benzene and hydroquinone, generates reactive oxygen species (ROS) causing DNA strand breaks and the subsequent activation of DNA repair enzymes, including poly(ADP-ribose) polymerase (PARP)-1. Under robust oxidative DNA damage, PARP-1 is hyperactivated, resulting in the depletion of NAD+ and ATP with accompanying elevations in intracellular calcium concentrations (iCa2+), and ultimately necrotic cell death. The role of Ca2+ during PARP-dependent necrotic cell death remains unclear. We therefore sought to determine the relationship between Ca2+ and PARP-1 during ROS-induced necrotic cell death in human renal proximal tubule epithelial cells (HK-2). Our experiments suggest that store-operated Ca2+ channel (SOC) entry contributes to the coupling of PARP-1 activation to increases in iCa2+ during ROS-induced cell death. Poly(ADP-ribose)glycohydrolase (PARG), which catalyzes the degradation of PARs to yield free ADP-ribose (ADPR), is known to activate Ca2+ channels such as TRPM2. However, siRNA knockdown of PARG did not restore cell viability, indicating that free ADPR is not responsible for SOC activation in HK-2 cells. The data indicate that PARP-1 and iCa2+ are coupled through activation of SOC mediated Ca2+ entry in an apparently ADPR-independent fashion; alternative PAR-mediated signaling likely contributes to PARP-dependent necrotic cell death, perhaps via PAR-mediated signaling proteins that regulate iCa2+ homeostasis.

Abstract 4808: Identification of a novel compound, MO2455, that induces poly(ADP-ribose) (PAR) accumulation and inhibits the growth of cancer cells in vitro and in vivo

Abstract Poly (ADP-ribose) chain (PAR) formed by poly (ADP-ribose) polymerase (PARP) is catabolized mainly by poly (ADP-ribose) glycohydrolase (PARG). PARG is emerging as a therapeutic target for cancer, because its inhibition leads to cell death in some kinds of cancer cell lines. In order to obtain small molecules that effectively inhibit PARG protein, an in-house collection of ∼10,000 small molecules were screened for their ability to accumulate the PAR in A549 cells at 5 μM. We identified ∼100 hit compounds. Among them, MO2282 and MO2455 induced the most significant accumulation of PAR. MO2455 is an analogue of MO2282 with improved water-solubility. MO2282 and MO2455 were evaluated in vitro for their ability to inhibit the catalytic activity of PARG. MO2282 and MO2455 showed modest inhibition activities against recombinant rat PARG. They also inhibited the growth of various kinds of cancel cell lines in vitro at an IC50 value of 0.05 - 3.0 μM and showed different spectrum of antitumor activity from conventional anticancer drugs, CDDP, ADM, PTX, and CPT-11. Next, we investigated the growth-inhibitory effect of MO2455 on A549 cells in a xenotransplanted model. A significant anti-tumor activity was observed in mice when treated with MO2455 at doses of 25 mg/kg every two days. Although treatment-related body weight loss was observed in mice treated with MO2455, body weight recovered by day 8. In conclusion, the present data suggest that MO2455 has potential as a cancer drug with different mechanisms of action from conventional anti-cancer drugs. Citation Format: Tatsu Shimoyama, Takeshi Sawada, Mari Akimoto, Yuka Sasaki, Hiroaki Fujimori, Yoshinobu Ishikawa, Tadashi Okawara, Tetsumi Irie, Takeji Takamura, Kenji Matsuno, Kengo Inoue, Mitsuko Masutani, Fumiaki Koizumi. Identification of a novel compound, MO2455, that induces poly(ADP-ribose) (PAR) accumulation and inhibits the growth of cancer cells in vitro and in vivo. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 4808.

Abstract 1995: HuR dependent inhibition of PARG enhances PARP inhibitor therapy for DNA repair proficient and deficient pancreatic cancer cells

Abstract Introduction: Despite our deep understanding of genetic drivers of the disease, pancreatic ductal adenocarcinoma (PDA) continues to be associated with dismal survival rates. Targeting the DNA repair machinery has emerged as a promising therapeutic strategy to treat pancreatic cancer patients carrying DNA damage repair (DDR) mutations. Such mutations promote tumorigenesis, but also paradoxically render tumor cells particularly susceptible to platinum-based agents and PARP inhibitors (PARPi). However, despite promising preclinical and clinical results, early data demonstrate that eventually most tumors, regardless of DDR status, become resistant to PARPi-therapies. The mRNA-binding protein HuR, predominantly expressed in the nucleus, translocates to the cytoplasm upon tumor-associated stress where it post-transcriptionally regulates select mRNA cargo, resulting in resistance to DNA damaging agents in a harsh tumor microenvironment. Here, we sought to evaluate the role of HuR in regulating PARPi efficacy. Results: In response to cellular stress induced by IC50 dosages of a panel of PARPis such as Veliparib, Olaparib, Rucaparib, Talazoparib and Niraparib, nuclear localized HuR undergoes cytoplasmic translocation. Silencing of HuR via siRNA, CRISPR and a DOX-inducible system resulted in significant decrease in long and short- term PDA cell survival, irrespective of DDR status. To complement and validate in vitro findings, we employed a heterotropic mouse xenograft model using Mia.sh290 cells wherein DOX induction significantly reduced HuR expression. Olaparib mediated PARP inhibition (50mg/kg, 5 days a week) combined with DOX-induced HuR silencing resulted in significant reduction in tumor volumes, compared to Olaparib alone or DOX alone. Mechanistically, we demonstrate that the pro-survival protein HuR facilitates PDA cells to recover from PARPi insult by, in part, regulating poly ADP ribose glycohydrolase (PARG), the major enzyme responsible for hydrolyzing poly-ADP ribose (PAR) polymers, on chromatin and associated proteins. HuR binds to two 41- 43bp long sites in the 3’ untranslated region (3’UTR) of PARG, increasing its mRNA stability and protein expression. Increased PARG activity, further validated via exogenous overexpression, promotes DNA repair efficiency and increases PDA cell survival. Functional analysis indicate that such inhibition of HuR and/or PARG significantly enhances PARPi sensitivity in PDA cells, via increased accumulation of DNA damage γH2AX foci, preventing efficient removal of PAR polymers, and enhancing detrimental trapping of PARP1 on chromatin. Conclusions: Taken together, our results indicate that HuR- mediated upregulation of PARG acts as a universal pro-survival mechanism and HuR inhibition could significantly potentiate PARPi therapy in PDA, irrespective of DNA repair status. Citation Format: Saswati N. Chand, Mahsa Zarei, Akshay R. Kamath, Matthew J. Schiewer, Carmella Romeo, Joseph A. Cozzitorto, Nicole Meisner- Kober, Eric Londin, Isidore Rigoutsos, Karen Knudsen, John M. Pascal, Charles J. Yeo, Jordan M. Winter, Jonathan R. Brody. HuR dependent inhibition of PARG enhances PARP inhibitor therapy for DNA repair proficient and deficient pancreatic cancer cells. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 1995.

Abstract C39: First-in-class inhibitors of the putatively undruggable DNA repair target Poly(ADP-ribose) glycohydrolase (PARG)

Abstract Poly(ADP-ribose) glycohydrolase (PARG) is the only enzyme known to catalyse hydrolysis of the O-glycosidic linkages of ADP-ribose polymers, thereby reversing the effects of poly(ADP-ribose) polymerases (PARPs). PARG depletion, using RNAi, results in several effects such as PAR chain persistence, progression of single- to double-strand DNA lesions and NAD+ depletion. Given these findings, inhibition of PARG with a small molecule agent offers a potential opportunity to interfere with DNA repair mechanisms and induce cell death in those cells with increased susceptibility to DNA damage, such as tumour cells. Previous efforts to develop small molecule inhibitors of PARG activity have generally been hampered by poor physicochemical properties, off-target pharmacology and a lack of cell permeability, leading some to suggest that PARG may be undruggable. In contrast, we have now developed a series of first-in-class PARG inhibitors which display drug-like properties and attractive pharmacokinetic parameters. These compounds have proved to be useful biological tool compounds. Moreover, displaying selective activity in both biochemical and, more importantly, cellular assays of PARG function, these derivatives have allowed an exploration of the phenotypes resulting from reversible, pharmacological PARG inhibition in both in vitro cell panels and in vivo models. Furthermore, our initial bioinformatic analysis suggests that deficiency of a known tumour suppressor confers sensitivity to PARG inhibition, suggesting patient populations that will potentially benefit from PARGi therapies. Citation Format: Bohdan Waszkowycz, Dominic James, Ben Acton, Emma Fairweather, Sam Fritzl, Niall Hamilton, Nicola Hamilton, Sarah Holt, James Hitchen, Colin Hutton, Stuart Jones, Allan Jordan, Alison McGonagle, Daniel Mould, Helen Small, Kate Smith, Alexandra Stowell, Ian D. Waddell, Donald Ogilvie. First-in-class inhibitors of the putatively undruggable DNA repair target Poly(ADP-ribose) glycohydrolase (PARG). [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2015 Nov 5-9; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2015;14(12 Suppl 2):Abstract nr C39.

Abstract 2745: Pharmacological characterisation of cell active inhibitors of Poly(ADP-ribose) glycohydrolase (PARG)

Abstract Poly(ADP-ribose) glycohydrolase (PARG) is the only enzyme known to catalyse hydrolysis of the O-glycosidic linkages of ADP-ribose polymers, thereby reversing the effects of poly(ADP-ribose) polymerases. Total PARG deficiency leads to cell death whilst PARG depletion, using RNAi, leads to pleiotropic effects such as PAR chain persistence, progression of single- to double-strand DNA lesions and NAD+ depletion. Whilst efforts to develop small molecule inhibitors of PARG activity have generally been hampered by poor physiochemical properties, off-target pharmacology and a lack of cell permeability, we have now developed a series of PARG inhibitors which have proved to be useful biological tool compounds. Displaying selective activity in both biochemical and, more importantly, cellular assays of PARG function, these derivatives have allowed an exploration of the phenotypes resulting from reversible, pharmacological PARG inhibition. Citation Format: Dominic James, Allan Jordan, Nicola Hamilton, Alison McGonagle, Kate Smith, Alexandra Stowell, Ian Waddell, Bohdan Waszkowycz, Donald Ogilvie. Pharmacological characterisation of cell active inhibitors of Poly(ADP-ribose) glycohydrolase (PARG). [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 2745. doi:10.1158/1538-7445.AM2014-2745

Crystallographic and biochemical analysis of the mouse poly(ADP-ribose) glycohydrolase.

Protein poly(ADP-ribosyl)ation (PARylation) regulates a number of important cellular processes. Poly(ADP-ribose) glycohydrolase (PARG) is the primary enzyme responsible for hydrolyzing the poly(ADP-ribose) (PAR) polymer in vivo. Here we report crystal structures of the mouse PARG (mPARG) catalytic domain, its complexes with ADP-ribose (ADPr) and a PARG inhibitor ADP-HPD, as well as four PARG catalytic residues mutants. With these structures and biochemical analysis of 20 mPARG mutants, we provide a structural basis for understanding how the PAR polymer is recognized and hydrolyzed by mPARG. The structures and activity complementation experiment also suggest how the N-terminal flexible peptide preceding the PARG catalytic domain may regulate the enzymatic activity of PARG. This study contributes to our understanding of PARG catalytic and regulatory mechanisms as well as the rational design of PARG inhibitors.

Mitigation of gamma-radiation induced abasic sites in genomic DNA by dietary nicotinamide supplementation: Metabolic up-regulation of NAD+ biosynthesis

The search for non-toxic radio-protective drugs has yielded many potential agents but most of these compounds have certain amount of toxicity. The objective of the present study was to investigate dietary nicotinamide enrichment dependent adaptive response to potential cytotoxic effect of 60Co γ-radiation. To elucidate the possible underlying mechanism(s), male Swiss mice were maintained on control diet (CD) and nicotinamide supplemented diet (NSD). After 6 weeks of CD and NSD dietary regimen, we exposed the animals to γ-radiation (2, 4 and 6Gy) and investigated the profile of downstream metabolites and activities of enzymes involved in NAD+ biosynthesis. Increased activities of nicotinamide phosphoribosyltransferase (NAMPT) and nicotinamide mononucleotide adenylyltransferase (NMNAT) were observed up to 48h post-irradiation in NSD fed irradiated mice. Concomitant with increase in liver NAMPT and NMNAT activities, NAD+ levels were replenished in NSD fed and irradiated animals. However, NAMPT and NMNAT-mediated NAD+ biosynthesis and ATP levels were severely compromised in liver of CD fed irradiated mice. Another major finding of these studies revealed that under γ-radiation stress, dietary nicotinamide supplementation might induce higher and long-lasting poly(ADP)-ribose polymerase 1 (PARP1) and poly(ADP-ribose) glycohydrolase (PARG) activities in NSD fed animals compared to CD fed animals. To investigate liver DNA damage, number of apurinic/apyrimidinic sites (AP sites) and level of 8-hydroxy-2′-deoxyguanosine (8-oxo-dG) residues were quantified. A significant increase in liver DNA AP sites and 8-oxo-dG levels with concomitant increase in caspase-3 was observed in CD fed and irradiated animals compared to NSD fed and irradiated mice. In conclusion present studies show that under γ-radiation stress conditions, dietary nicotinamide supplementation restores DNA excision repair activity via prolonged activation of PARP1 and PARG activities. Present results clearly indicated that hepatic NAD+ replenishment might be a novel and potent approach to reduce radiation injury.

Structures of the Human Poly (ADP-Ribose) Glycohydrolase Catalytic Domain Confirm Catalytic Mechanism and Explain Inhibition by ADP-HPD Derivatives

Poly(ADP-ribose) glycohydrolase (PARG) is the only enzyme known to catalyse hydrolysis of the O-glycosidic linkages of ADP-ribose polymers, thereby reversing the effects of poly(ADP-ribose) polymerases. PARG deficiency leads to cell death whilst PARG depletion causes sensitisation to certain DNA damaging agents, implicating PARG as a potential therapeutic target in several disease areas. Efforts to develop small molecule inhibitors of PARG activity have until recently been hampered by a lack of structural information on PARG. We have used a combination of bio-informatic and experimental approaches to engineer a crystallisable, catalytically active fragment of human PARG (hPARG). Here, we present high-resolution structures of the catalytic domain of hPARG in unliganded form and in complex with three inhibitors: ADP-ribose (ADPR), adenosine 5′-diphosphate (hydroxymethyl)pyrrolidinediol (ADP-HPD) and 8-n-octyl-amino-ADP-HPD. Our structures confirm conservation of overall fold amongst mammalian PARG glycohydrolase domains, whilst revealing additional flexible regions in the catalytic site. These new structures rationalise a body of published mutational data and the reported structure-activity relationship for ADP-HPD based PARG inhibitors. In addition, we have developed and used biochemical, isothermal titration calorimetry and surface plasmon resonance assays to characterise the binding of inhibitors to our PARG protein, thus providing a starting point for the design of new inhibitors.

Altered Expression of the Poly(ADP-Ribosyl)ation Enzymes in Uveal Melanoma and Regulation ofPARGGene Expression by the Transcription Factor ERM

Poly(ADP-ribosyl)ation is a reversible post-translational modification that requires the contribution of the enzymes poly(ADP-ribose) polymerase-1 (PARP-1) and poly(ADP-ribose) glycohydrolase (PARG). Our study explores expression and activity of PARP-1 and PARG in uveal melanoma cell lines with varying tumorigenic properties. Gene profiling on microarrays was conducted using RNA prepared from the uveal melanoma cell lines T97, T98, T108, and T115. The activity of PARP-1 and PARG was monitored by enzymatic assays, whereas their expression was measured by Western blot and PCR. The PARG promoter was analyzed using promoter deletions and site-specific mutagenesis in transfection analyses. The transcription factors binding the PARG promoter were studied by electrophoretic mobility shift assay (EMSA) analyses. Suppression of PARP-1 and PARG expression was performed in T97 and T115 cells by RNAi, and their tumorigenic properties monitored by injections into athymic mice. Expression of PARP-1 was found to vary considerably between uveal melanoma cell lines with distinctive tumorigenic properties in vivo. Sp1 and the ETS protein ERM were shown to bind to the PARG gene promoter to ensure basal transcription in uveal melanoma. Importantly, suppression of PARG gene expression in T97 and T115 cells increased their capacity to form tumors in athymic mice, whereas suppression of PARP-1 significantly reduced or almost entirely abolished tumor formation. Our results suggest that while overexpression of PARP-1 may confer a proliferative advantage to aggressive uveal melanoma tumors, PARG may, on the other hand, support a tumor suppressor function in vivo.

Inhibition of poly(ADP-ribose) glycohydrolase (PARG) specifically kills BRCA2-deficient tumor cells

Poly(ADP-ribose) glycohydrolase (PARG), removes poly(ADP-ribose) subunits from proteins that have previously been modified by poly(ADP-ribose) polymerse. This ensures that modification is transient, and it is suggested that removal of poly(ADP-ribose) is essential for some types of DNA repair. Here we show increased γH2AX foci formation and increased homologous recombination when PARG is inhibited. These effects are reduced when replication is inhibited, suggesting that in the absence of PARG activity, replication forks collapse, and homologous recombination is induced for repair. Consistent with this, we show that cells deficient in the homologous recombination protein BRCA2 are sensitive to PARG depletion or inhibition. These data raise the exciting possibility that PARG inhibitors may be used to specifically kill BRCA2 and other homologous recombination-deficient tumors.

PARG: A Macrodomain in Disguise

Our understanding of poly-ADP-ribosylation as a posttranslational modification was limited by the lack of structural information on poly-ADP-ribose (PAR) hydrolysing enzymes. A recent study in Nature (Slade etal., 2011) reports the structure of PAR glycohydrolase (PARG), revealing unexpected similarity to the ubiquitous ADP-ribose-binding macrodomains.

Discovery and Structure–Activity Relationships of Modified Salicylanilides as Cell Permeable Inhibitors of Poly(ADP-ribose) Glycohydrolase (PARG)

The metabolism of poly(ADP-ribose) (PAR) in response to DNA strand breaks, which involves the concerted activities of poly(ADP-ribose) polymerases (PARPs) and poly(ADP-ribose) glycohydrolase (PARG), modulates cell recovery or cell death depending upon the level of DNA damage. While PARP inhibitors show high promise in clinical trials because of their low toxicity and selectivity for BRCA related cancers, evaluation of the therapeutic potential of PARG is limited by the lack of well-validated cell permeable inhibitors. In this study, target-related affinity profiling (TRAP), an alternative to high-throughput screening, was used to identify a number of druglike compounds from several chemical classes that demonstrated PARG inhibition in the low-micromolar range. A number of analogues of one of the most active chemotypes were synthesized to explore the structure-activity relationship (SAR) for that series. This led to the discovery of a putative pharmacophore for PARG inhibition that contains a modified salicylanilide structure. Interestingly, these compounds also inhibit PARP-1, indicating strong homology in the active sites of PARG and PARP-1 and raising a new challenge for development of PARG specific inhibitors. The cellular activity of a lead inhibitor was demonstrated by the inhibition of both PARP and PARG activity in squamous cell carcinoma cells, although preferential inhibition of PARG relative to PARP was observed. The ability of inhibitors to modulate PAR metabolism via simultaneous effects on PARPs and PARG may represent a new approach for therapeutic development.

Poly(ADP-Ribose) Metabolism in Brain and Its Role in Ischemia Pathology

The biological roles of poly(ADP-ribose) polymers (PAR) and poly(ADP-ribosyl)ation of proteins in the central nervous system are diverse. The homeostasis of PAR orchestrated by poly(ADP-ribose) polymerase-1 (PARP-1) and poly(ADP-ribose) glycohydrolase (PARG) is crucial for cell physiology and pathology. Both enzymes are ubiquitously distributed in neurons and glia; however, they are segregated at the subcellular level. PARP-1 serves as a "nick sensor" for single- or double-stranded breaks in DNA and is involved in long and short patch base-excision repair, while PARG breaks down PAR. The stimulation of PARP-1 and PAR formation can activate proinflammatory transcription factors, including nuclear factor kappa B. However, hyperactivation of PARP-1 can result in depletion of NAD/ATP, and in PAR-dependent mitochondrial pore formation leading to release of apoptosis inducing factor and cell death. The role of PAR as a death signaling molecule in brain ischemia-reperfusion and inflammation as well as the effect of gender and aging is presented in this review. Modulating the PAR level through pharmacological or genetic intervention on PARP-1/PARG activity and gene expression should be a valuable way for neuroprotective strategy.