PARG activity mediates intestinal injury induced by splanchnic artery occlusion and reperfusion

Selective small molecule PARG inhibitor causes replication fork stalling and cancer cell death

Poly(ADP-ribose)ylation (PARylation) by PAR polymerase 1 (PARP1) and PARylation removal by poly(ADP-ribose) glycohydrolase (PARG) critically regulate DNA damage responses; yet, conflicting reports obscure PARG biology and its impact on cancer cell resistance to PARP1 inhibitors. Here, we found that PARG expression is upregulated in many cancers. We employed chemical library screening to identify and optimize methylxanthine derivatives as selective bioavailable PARG inhibitors. Multiple crystal structures reveal how substituent positions on the methylxanthine core dictate binding modes and inducible-complementarity with a PARG-specific tyrosine clasp and arginine switch, supporting inhibitor specificity and a competitive inhibition mechanism. Cell-based assays show selective PARG inhibition and PARP1 hyperPARylation. Moreover, our PARG inhibitor sensitizes cells to radiation-induced DNA damage, suppresses replication fork progression and impedes cancer cell survival. In PARP inhibitor-resistant A172 glioblastoma cells, our PARG inhibitor shows comparable killing to Nedaplatin, providing further proof-of-concept that selectively inhibiting PARG can impair cancer cell survival.

Targeting poly(ADP-ribose) glycohydrolase to draw apoptosis codes in cancer

Poly(ADP-ribosyl)ation is a unique post-translational modification of proteins. The metabolism of poly(ADP-ribose) (PAR) is tightly regulated mainly by poly(ADP-ribose) polymerases (PARP) and poly(ADP-ribose) glycohydrolase (PARG). Accumulating evidence has suggested the biological functions of PAR metabolism in control of many cellular processes, such as cell proliferation, differentiation and death by remodeling chromatin structure and regulation of DNA transaction, including DNA repair, replication, recombination and transcription. However, the physiological roles of the catabolism of PAR catalyzed by PARG remain less understood than those of PAR synthesis by PARP. Noteworthy biochemical studies have revealed the importance of PAR catabolic pathway generating nuclear ATP via the coordinated actions of PARG and ADP-ribose pyrophosphorylase (ADPRPPL) for the driving of DNA repair and the maintenance of DNA replication apparatus while repairing DNA damage. Furthermore, genetic studies have shown the value of PARG as a therapeutic molecular target for PAR-mediated diseases, such as cancer, inflammation and many pathological conditions. In this review, we present the current knowledge of de-poly(ADP-ribosyl)ation catalyzed by PARG focusing on its role in DNA repair, replication and apoptosis. Furthermore, the induction of apoptosis code of DNA replication catastrophe by synthetic lethality of PARG inhibition and the recent progresses regarding the development of small molecule PARG inhibitors and their therapeutic potentials in cancer chemotherapy are highlighted in this review.

Effects of a new compound containing Palmitoylethanolamide and Baicalein in myocardial ischaemia/reperfusion injury in vivo

BACKGROUND

Myocardial ischemia/reperfusion (I/R) injury is the principal cause of death, happens after prolonged obstruction of the coronary arteries.  The first intervention to limit myocardial damage is directed to restoration of perfusion, to avoid inflammatory response and a significant oxidative stress triggered by infarction. Palmitoylethanolamide (PEA), is a well-known fatty acid amide-signaling molecule that possess an important anti-inflammatory and analgesic effects. PEA does not hold the ability to inhibit free radicals formation. Baicalein, a bioactive component isolated from a Chinese herbal medicine, has multiple pharmacological activities, such as a strong anti-oxidative effects.

PURPOSE

A combination of PEA and Baicalein could have beneficial effects on oxidative stress produced by inflammatory response.

STUDY DESIGN

In the present study we explored the effects of composite containing PEA and Baicalein in a model of myocardial I/R injury.

METHODS

Myocardial ischemia/reperfusion injury was induced by occlusion of the left anterior descending coronary artery for 30 min followed by 2 h of reperfusion. PEA-Baicalein (9:1), was administered (10 mg/kg) 5 min before the end of ischemia and 1 h after reperfusion.

RESULTS

In this study, we clearly demonstrated that PEA-Baicalein treatment decreases myocardial tissue injury, neutrophils infiltration, markers for mast cell activation expression as chymase and tryptase and pro-inflammatory cytokines production (TNF-α, IL-1β). Moreover, PEA-Baicalein treatment reduces stress oxidative and modulates Nf-kB and apoptosis pathways.

CONCLUSION

These results support the idea that the association between PEA and Baicalein should be a potent candidate for the treatment of myocardial I/R injury.

Targeting intestinal epithelial cell-programmed necrosis alleviates tissue injury after intestinal ischemia/reperfusion in rats

BACKGROUND

Intestinal dysfunction, especially acute pathologies linked to intestinal ischemia/reperfusion (I/R) injury, is profoundly affected by inflammation and improper execution of cell death. Few studies have examined the efficacy of combined strategies in regulated intestinal epithelial necrosis after intestinal I/R. Here, we evaluated the functional interaction between poly (adenosine diphosphate-ribose) polymerase 1 (PARP-1)-induced parthanatos and receptor-interacting protein 1/3 (RIP1/3) kinase-induced necroptosis in the pathophysiological course of acute ischemic intestinal injury.

METHODS

Anesthetized adult male Sprague-Dawley rats were subjected to superior mesenteric artery occlusion consisting of 1.5 h of ischemia and 6 h of reperfusion. The PARP-1-specific inhibitor PJ34 (10 mg/kg) and the RIP1-specific inhibitor Necrostatin-1 (1 mg/kg) were intraperitoneally administered 30 min before the induction of ischemia.

RESULTS

Intestinal I/R was found to result in PARP-1 activation and RIP1/3-mediated necrosome formation. PJ34 or Necrostatin-1 treatment significantly improved the mucosal injury, while the combined inhibition of PARP-1 and RIP1/3 conferred optimal protection of the intestine. Meanwhile, results from terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate-biotin nick end labeling assay showed a decrease in intestinal epithelial cell death. Interestingly, we further showed that PARP-1 might act as a downstream signaling molecule of RIP1 in the process of I/R-induced intestinal injury and that the RIP1/PARP-1-dependent cell death signaling pathway functioned independently of caspase 3 inhibition.

CONCLUSIONS

The results of our study provide a molecular basis for combination therapy that targets both pathways of regulated necrosis (parthanatos and necroptosis), to treat acute intestinal I/R-induced intestinal epithelial barrier disruption.

PARP-1 protects against colorectal tumor induction, but promotes inflammation-driven colorectal tumor progression

Colorectal cancer (CRC) is one of the most common tumor entities, which is causally linked to DNA repair defects and inflammatory bowel disease (IBD). Here, we studied the role of the DNA repair protein poly(ADP-ribose) polymerase-1 (PARP-1) in CRC. Tissue microarray analysis revealed PARP-1 overexpression in human CRC, correlating with disease progression. To elucidate its function in CRC, PARP-1 deficient (PARP-1^-/-) and wild-type animals (WT) were subjected to azoxymethane (AOM)/ dextran sodium sulfate (DSS)-induced colorectal carcinogenesis. Miniendoscopy showed significantly more tumors in WT than in PARP-1^-/- mice. Although the lack of PARP-1 moderately increased DNA damage, both genotypes exhibited comparable levels of AOM-induced autophagy and cell death. Interestingly, miniendoscopy revealed a higher AOM/DSS-triggered intestinal inflammation in WT animals, which was associated with increased levels of innate immune cells and proinflammatory cytokines. Tumors in WT animals were more aggressive, showing higher levels of STAT3 activation and cyclin D1 up-regulation. PARP-1^-/- animals were then crossed with O⁶-methylguanine-DNA methyltransferase (MGMT)-deficient animals hypersensitive to AOM. Intriguingly, PARP-1^-/-/MGMT^-/- double knockout (DKO) mice developed more, but much smaller tumors than MGMT^-/- animals. In contrast to MGMT-deficient mice, DKO animals showed strongly reduced AOM-dependent colonic cell death despite similar O⁶-methylguanine levels. Studies with PARP-1^-/- cells provided evidence for increased alkylation-induced DNA strand break formation when MGMT was inhibited, suggesting a role of PARP-1 in the response to O⁶-methylguanine adducts. Our findings reveal PARP-1 as a double-edged sword in colorectal carcinogenesis, which suppresses tumor initiation following DNA alkylation in a MGMT-dependent manner, but promotes inflammation-driven tumor progression.

Protective Effects of Ultramicronized Palmitoylethanolamide (PEA-um) in Myocardial Ischaemia and Reperfusion Injury in VIVO

Myocardial infarction is the leading cause of death, occurs after prolonged ischemia of the coronary arteries. Restore blood flow is the first intervention help against heart attack. However, reperfusion of the arteries leads to ischemia/reperfusion injury (I/R). The fatty acid amide palmitoylethanolamide (PEA) is an endogenous compound widely present in living organisms, with analgesic and anti-inflammatory properties. The present study evaluated the effect of ultramicronized palmitoylethanolamide (PEA-um) treatment on the inflammatory process associated with myocardial I/R. Myocardial ischemia reperfusion injury was induced by occlusion of the left anterior descending coronary artery for 30 min followed by 2 h of reperfusion. PEA-um, was administered (10 mg/kg) 15 min after ischemia and 1 h after reperfusion. In this study, we demonstrated that PEA-um treatment reduces myocardial tissue injury, neutrophil infiltration, adhesion molecules (ICAM-1, P-selectin) expression, proinflammatory cytokines (TNF-α, IL-1β) production, nitrotyrosine and PAR formation, nuclear factor kB expression, and apoptosis (Fas-L, Bcl-2) activation. In addition to study whether the protective effect of PEA-um on myocardial ischemia reperfusion injury is also related to the activation of PPAR-α, in a separate set of experiments it has been performed myocardial I/R in PPARα mice. Genetic ablation of peroxisome proliferator activated receptor (PPAR)-α in PPAR-αKO mice exacerbated Myocardial ischemia reperfusion injury when compared with PPAR-αWT mice. PEA-um induced cardioprotection in PPAR-α wild-type mice, but the same effect cannot be observed in PPAR-αKO mice. Our results have clearly shown a modulation of the inflammatory process, associated with myocardial ischemia reperfusion injury, following administration of PEA-um.

Functional Role of ADP-Ribosyl-Acceptor Hydrolase 3 in poly(ADP-Ribose) Polymerase-1 Response to Oxidative Stress

Poly-ADP-ribosylation has been proposed to be a reversible protein modification, participating in diverse cellular functions including DNA repair, chromatin remodeling, genetic stability, mitosis, and cell death. Poly-ADP-ribosylation is initiated by the transfer of the ADP-ribose moiety of NAD+ primarily to the carboxyl groups of glutamate and aspartate and amino group of lysine residues in target proteins, followed by elongation of poly(ADP-ribose) (PAR) chains via α-O-glycosidic (C- 1"-C-2') ribose-ribose bonds. PAR consists of polymers of ADP-ribose (up to 200 units) with branching via α-O-glycosidic (C-1"'-C-2") ribose-ribose bonds. Further, the pyrophosphate group of each ADP-ribose has two negative charges. Therefore, in proteins modified by PAR, a complex structure with negative charges may lead to dynamic changes of functions. PAR formation is catalyzed by poly(ADP-ribose) polymerases (PARPs) and terminated by several types of enzymes with PAR-degrading activities; poly(ADP-ribose) glycohydrolase (PARG), ADP-ribosylacceptor hydrolase (ARH) 3, ARH1, and macrodomain-containing proteins. PARG has been thought to be primarily responsible for PAR degradation. In 2006, ARH3 was cloned and identified as another type of PAR-degrading protein. Although PAR-degrading activity of ARH3 is less than that of PARG, different mechanisms of PAR recognition and the cellular localization of ARH3 appear to be responsible for unique cellular roles of ARH3 involving PAR. In the present review, we focused on our findings regarding structure, biological properties, and cellular functions of ARH3. In addition, we describe the current knowledge of poly-ADP-ribosylation and cell death pathways regulated PARP1, PARG, and ARH3.

Macrodomains: Structure, Function, Evolution, and Catalytic Activities

Recent developments indicate that macrodomains, an ancient and diverse protein domain family, are key players in the recognition, interpretation, and turnover of ADP-ribose (ADPr) signaling. Crucial to this is the ability of macrodomains to recognize ADPr either directly, in the form of a metabolic derivative, or as a modification covalently bound to proteins. Thus, macrodomains regulate a wide variety of cellular and organismal processes, including DNA damage repair, signal transduction, and immune response. Their importance is further indicated by the fact that dysregulation or mutation of a macrodomain is associated with several diseases, including cancer, developmental defects, and neurodegeneration. In this review, we summarize the current insights into macrodomain evolution and how this evolution influenced their structural and functional diversification. We highlight some aspects of macrodomain roles in pathobiology as well as their emerging potential as therapeutic targets.

Structure and function of the ARH family of ADP-ribosyl-acceptor hydrolases

ADP-ribosylation is a post-translational protein modification, in which ADP-ribose is transferred from nicotinamide adenine dinucleotide (NAD(+)) to specific acceptors, thereby altering their activities. The ADP-ribose transfer reactions are divided into mono- and poly-(ADP-ribosyl)ation. Cellular ADP-ribosylation levels are tightly regulated by enzymes that transfer ADP-ribose to acceptor proteins (e.g., ADP-ribosyltransferases, poly-(ADP-ribose) polymerases (PARP)) and those that cleave the linkage between ADP-ribose and acceptor (e.g., ADP-ribosyl-acceptor hydrolases (ARH), poly-(ADP-ribose) glycohydrolases (PARG)), thereby constituting an ADP-ribosylation cycle. This review summarizes current findings related to the ARH family of proteins. This family comprises three members (ARH1-3) with similar size (39kDa) and amino acid sequence. ARH1 catalyzes the hydrolysis of the N-glycosidic bond of mono-(ADP-ribosyl)ated arginine. ARH3 hydrolyzes poly-(ADP-ribose) (PAR) and O-acetyl-ADP-ribose. The different substrate specificities of ARH1 and ARH3 contribute to their unique roles in the cell. Based on a phenotype analysis of ARH1(-/-) and ARH3(-/-) mice, ARH1 is involved in the action by bacterial toxins as well as in tumorigenesis. ARH3 participates in the degradation of PAR that is synthesized by PARP1 in response to oxidative stress-induced DNA damage; this hydrolytic reaction suppresses PAR-mediated cell death, a pathway termed parthanatos.

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.

ADP-ribosyl-acceptor hydrolase 3 regulates poly (ADP-ribose) degradation and cell death during oxidative stress

Poly (ADP ribose) (PAR) formation catalyzed by PAR polymerase 1 in response to genotoxic stress mediates cell death due to necrosis and apoptosis. PAR glycohydrolase (PARG) has been thought to be the only enzyme responsible for hydrolysis of PAR in vivo. However, we show an alternative PAR-degradation pathway, resulting from action of ADP ribosyl-acceptor hydrolase (ARH) 3. PARG and ARH3, acting in tandem, regulate nuclear and cytoplasmic PAR degradation following hydrogen peroxide (H2O2) exposure. PAR is responsible for induction of parthanatos, a mechanism for caspase-independent cell death, triggered by apoptosis-inducing factor (AIF) release from mitochondria and its translocation to the nucleus, where it initiates DNA cleavage. PARG, by generating protein-free PAR from poly-ADP ribosylated protein, makes PAR translocation possible. A protective effect of ARH3 results from its lowering of PAR levels in the nucleus and the cytoplasm, thereby preventing release of AIF from mitochondria and its accumulation in the nucleus. Thus, PARG release of PAR attached to nuclear proteins, followed by ARH3 cleavage of PAR, is essential in regulating PAR-dependent AIF release from mitochondria and parthanatos.

The recognition and removal of cellular poly(ADP-ribose) signals

Poly(ADP-ribosyl)ation is involved in the regulation of a variety of cellular pathways, including, but not limited to, transcription, chromatin, DNA damage and other stress signalling. Similar to other tightly regulated post-translational modifications, poly(ADP-ribosyl)ation employs 'writers', 'readers' and 'erasers' to confer regulatory functions. The generation of poly(ADP-ribose) is catalyzed by poly(ADP-ribose) polymerase enzymes, which use NAD(+) as a cofactor to sequentially transfer ADP-ribose units generating long polymers, which, in turn, can affect protein function or serve as a recruitment platform for additional factors. Historically, research has focused on poly(ADP-ribose) generation pathways, with knowledge about PAR recognition and degradation lagging behind. Over recent years, several discoveries have significantly furthered our understanding of poly(ADP-ribose) recognition and, even more so, of poly(ADP-ribose) degradation. In this review, we summarize current knowledge about the protein modules recognizing poly(ADP-ribose) and discuss the newest developments on the complete reversibility of poly(ADP-ribosyl)ation.

Poly(ADP-ribose): PARadigms and PARadoxes

Poly(ADP-ribosyl)ation (PARylation) is a posttranslational protein modification (PTM) catalyzed by members of the poly(ADP-ribose) polymerase (PARP) enzyme family. PARPs use NAD(+) as substrate and upon cleaving off nicotinamide they transfer the ADP-ribosyl moiety covalently to suitable acceptor proteins and elongate the chain by adding further ADP-ribose units to create a branched polymer, termed poly(ADP-ribose) (PAR), which is rapidly degraded by poly(ADP-ribose) glycohydrolase (PARG) and ADP-ribosylhydrolase 3 (ARH3). In recent years several key discoveries changed the way we look at the biological roles and mode of operation of PARylation. These paradigm shifts include but are not limited to (1) a single PARP enzyme expanding to a PARP family; (2) DNA-break dependent activation extended to several other DNA dependent and independent PARP-activation mechanisms; (3) one molecular mechanism (covalent PARylation of target proteins) underlying the biological effect of PARPs is now complemented by several other mechanisms such as protein-protein interactions, PAR signaling, modulation of NAD(+) pools and (4) one principal biological role in DNA damage sensing expanded to numerous, diverse biological functions identifying PARP-1 as a real moonlighting protein. Here we review the most important paradigm shifts in PARylation research and also highlight some of the many controversial issues (or paradoxes) of the field such as (1) the mostly synergistic and not antagonistic biological effects of PARP-1 and PARG; (2) mitochondrial PARylation and PAR decomposition, (3) the cross-talk between PARylation and signaling pathways (protein kinases, phosphatases, calcium) and the (4) divergent roles of PARP/PARylation in longevity and in age-related diseases.

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.

Apocynin, a Plant-Derived Drug, Might Be Useful in the Treatment of Myocardial Ischemia Reperfusion Injury in Rat Hearts

Apocynin is a potent and selective inhibitor of the NADPH oxidase-dependent production of ROS by stimulated human PMNs. Apocynin was isolated by activity-guided isolation from Picrorhiza kurroa, and in the years following its discovery it has been used in many laboratories around the world. Reactive oxygen species (ROS) production by activated polymorphonuclear neutrophils (PMNs) plays an important role in many cardiovascular disease states, including myocardial ischemia reperfusion injury. The purpose of this study is to examine the beneficial effects of apocynin on myocardial ischemia reperfusion injury. Myocardial I/R injury was caused by clamping the left anterior descending (LAD) coronary artery for 20 min followed by release of the clamp allowing reperfusion for 1 h. Administration of apocynin i.p. (5mg/kg i.p. 10%DMSO) 15 min after ischemia significantly reduced the: 1) histological evidence of myocardial injury; 2) pro-inflammatory cytokines (TNF-α, IL-1β); 3) adhesion molecules (ICAM-1, P-Selectin); 4) nitrotyrosine formation; 5) NF-kB expression; 6) PAR formation; and 7) apoptosis (Bax, Bcl-2, Fas-L and tunel). Based on these findings we propose that apocynin would be useful in the treatment of various ischemia and reperfusion diseases.

The ups and downs of tannins as inhibitors of poly(ADP-ribose)glycohydrolase

DNA damage to cells activates nuclear poly(ADP-ribose)polymerases (PARPs) and the poly(ADP-ribose) (PAR) synthesized is rapidly cleaved into ADP-ribose (ADPR) by PAR glycohydrolase (PARG) action. Naturally appearing tannin-like molecules have been implicated in specific inhibition of the PARG enzyme. This review deals with the in vitro and in vivo effects of tannins on PAR metabolism and their downstream actions in DNA damage signaling.

PARP and PARG inhibitors--new therapeutic targets in cancer treatment

Today, the number of cancer patients throughout the world is increasing alarmingly and as per the World Health Organisation (WHO) data and statistics the prediction for the year 2020 will be 15 million new cases as compared to only 10 million cases in year 2000 leaving us dumbfounded. A lot of effort has been put in by researchers and scientists over decades to find drugs helpful in the treatment of cancers for the benefit of patients--the latest being the Poly ADP-ribose polymerase (PARP) and the Poly ADP-ribose glycohydrolase (PARG) inhibitors. This review highlights their mechanism of action under the rationale of their use and current development in the field of cancer.

Poly(ADP-ribose) catabolism triggers AMP-dependent mitochondrial energy failure

Upon massive DNA damage, hyperactivation of the nuclear enzyme poly(ADP-ribose) polymerase (PARP)-1 causes severe depletion of intracellular NAD and ATP pools as well as mitochondrial dysfunction. Thus far, the molecular mechanisms contributing to PARP-1-dependent impairment of mitochondrial functioning have not been identified. We found that degradation of the PARP-1 product poly(ADP-ribose) through the concerted actions of poly(ADP-ribose) glycohydrolase and NUDIX (nucleoside diphosphate-X) hydrolases leads to accumulation of AMP. The latter, in turn, inhibits the ADP/ATP translocator, prompting mitochondrial energy failure. For the first time, our findings identify NUDIX hydrolases as key enzymes involved in energy derangement during PARP-1 hyperactivity. Also, these data disclose unanticipated AMP-dependent impairment of mitochondrial exchange of adenine nucleotides, which can be of relevance to organelle functioning and disease pathogenesis.

Mono-galloyl glucose derivatives are potent poly(ADP-ribose) glycohydrolase (PARG) inhibitors and partially reduce PARP-1-dependent cell death

BACKGROUND AND PURPOSE

Maintenance of poly(ADP-ribose) (PAR) polymers at homoeostatic levels by PAR glycohydrolase (PARG) is central in cell functioning and survival. Yet the pharmacological relevance of PARG inhibitors is still debated. Gallotannin, a complex mixture of hydrolysable tannins from oak gall, inhibits PARG but which of its constituents is responsible for the inhibition and whether the pharmacodynamic properties are due to its antioxidant properties, has not yet been established.

EXPERIMENTAL APPROACH

A structure-activity relationship study was conducted on different natural and synthetic tannins/galloyl derivatives as potential PARG inhibitors, using a novel in vitro enzymic assay. Cytotoxicity was assayed in cultured HeLa cells.

KEY RESULTS

Mono-galloyl glucose compounds were potent inhibitors of PARG, with activities similar to that of ADP-(hydroxymethyl) pyrrolidinediol, the most potent PARG inhibitor yet identified. When tested on HeLa cells exposed to the PAR polymerase (PARP)-1-activating compound 1-methyl-3-nitro-1-nitrosoguanidine (MNNG), 3-galloyl glucose weakly inhibited PAR degradation. Conversely, the more lipophilic, 3-galloyl-1,2-O-isopropylidene glucose, despite being inactive on the pure enzyme, efficiently prolonged the half-life of the polymers in intact HeLa cells. Also, PARG inhibitors, but not radical scavengers, reduced, in part, cell death caused by MNNG.

CONCLUSIONS AND IMPLICATIONS

Taken together, our findings identify mono-galloyl glucose derivatives as potent PARG inhibitors, and emphasize the active function of this enzyme in cell death.

Poly(ADP-ribose) polymerase: a new therapeutic target?

PURPOSE OF REVIEW

To overview the emerging data in the literature showing the role of poly(ADP-ribose) polymerase (PARP) in the pathogenesis of critical illness.

RECENT FINDINGS

PARP, an abundant nuclear enzyme involved in DNA repair and transcriptional regulation, is now recognized as a key regulator of cell survival and cell death in response to noxious stimuli in various forms of cardiovascular collapse. PARP becomes activated in response to oxidative DNA damage and depletes cellular energy pools, thus leading to cellular dysfunction in various tissues. The activation of PARP may also induce various cell death processes, and promotes an inflammatory response. In circulatory shock PARP plays a crucial role both in the development of early cardiovascular dysfunction and in the delayed systemic inflammatory response syndrome with associated multiple organ failure. Inhibition of PARP activity is protective in various models of circulatory shock.

SUMMARY

A solid body of literature supports the view that PARP is an important target for therapeutic intervention in critical illness.

NAD+/NADH and NADP+/NADPH in cellular functions and cell death: regulation and biological consequences

Accumulating evidence has suggested that NAD (including NAD+ and NADH) and NADP (including NADP+ and NADPH) could belong to the fundamental common mediators of various biological processes, including energy metabolism, mitochondrial functions, calcium homeostasis, antioxidation/generation of oxidative stress, gene expression, immunological functions, aging, and cell death: First, it is established that NAD mediates energy metabolism and mitochondrial functions; second, NADPH is a key component in cellular antioxidation systems; and NADH-dependent reactive oxygen species (ROS) generation from mitochondria and NADPH oxidase-dependent ROS generation are two critical mechanisms of ROS generation; third, cyclic ADP-ribose and several other molecules that are generated from NAD and NADP could mediate calcium homeostasis; fourth, NAD and NADP modulate multiple key factors in cell death, such as mitochondrial permeability transition, energy state, poly(ADP-ribose) polymerase-1, and apoptosis-inducing factor; and fifth, NAD and NADP profoundly affect aging-influencing factors such as oxidative stress and mitochondrial activities, and NAD-dependent sirtuins also mediate the aging process. Moreover, many recent studies have suggested novel paradigms of NAD and NADP metabolism. Future investigation into the metabolism and biological functions of NAD and NADP may expose fundamental properties of life, and suggest new strategies for treating diseases and slowing the aging process.

Novel pharmacological approaches to the treatment of renal ischemia-reperfusion injury: a comprehensive review

Renal ischemia-reperfusion (I-R) contributes to the development of ischemic acute renal failure (ARF). Multi-factorial processes are involved in the development and progression of renal I-R injury with the generation of reactive oxygen species, nitric oxide and peroxynitrite, and the decline of antioxidant protection playing major roles, leading to dysfunction, injury, and death of the cells of the kidney. Renal inflammation, involving cytokine/adhesion molecule cascades with recruitment, activation, and diapedesis of circulating leukocytes is also implicated. Clinically, renal I-R occurs in a variety of medical and surgical settings and is responsible for the development of acute tubular necrosis (a characteristic feature of ischemic ARF), e.g., in renal transplantation where I-R of the kidney directly influences graft and patient survival. The cellular mechanisms involved in the development of renal I-R injury have been targeted by several pharmacological interventions. However, although showing promise in experimental models of renal I-R injury and ischemic ARF, they have not proved successful in the clinical setting (e.g., atrial natriuretic peptide, low-dose dopamine). This review highlights recent pharmacological developments, which have shown particular promise against experimental renal I-R injury and ischemic ARF, including novel antioxidants and antioxidant enzyme mimetics, nitric oxide and nitric oxide synthase inhibitors, erythropoietin, peroxisome-proliferator-activated receptor agonists, inhibitors of poly(ADP-ribose) polymerase, carbon monoxide-releasing molecules, statins, and adenosine. Novel approaches such as recent research involving combination therapies and the potential of non-pharmacological strategies are also considered.

Two small enzyme isoforms mediate mammalian mitochondrial poly(ADP-ribose) glycohydrolase (PARG) activity

Poly(ADP-ribose)glycohydrolase (PARG) is the major enzyme capable of rapidly hydrolyzing poly(ADP-ribose) (PAR) formed by the diverse members of the PARP enzyme family. This study presents an alternative splice mechanism by which two novel PARG protein isoforms of 60 kDa and 55 kDa are expressed from the human PARG gene, termed hPARG60 and hPARG55, respectively. Homologous forms were found in the mouse (mPARG63 and mPARG58) supporting the hypothesis that expression of small PARG isoforms is conserved among mammals. A PARG protein of approximately 60 kDa has been described for decades but with its genetic basis unknown, it was hypothesized to be a product of posttranslational cleavage of larger PARG isoforms. While this is not excluded entirely, isolation and expression of cDNA clones from different sources of RNA indicate that alternative splicing leads to expression of a catalytically active hPARG60 in multiple cell compartments. A second enzyme, hPARG55, that can be expressed through alternative translation initiation from hPARG60 transcripts is strictly targeted to the mitochondria. Functional studies of a mitochondrial targeting signal (MTS) in PARG exon IV suggest that hPARG60 may be capable of shuttling between nucleus and mitochondria, which would be in line with a proposed function of PAR in genotoxic stress-dependent, nuclear-mitochondrial crosstalk.

NAD+ and NADH in neuronal death

Neuronal death is a key pathological event in multiple neurological diseases. Increasing evidence has suggested that NAD+ and NADH mediate not only energy metabolism and mitochondrial functions, but also calcium homeostasis, aging, and cell death. This article is written to provide an overview about the information suggesting significant roles of NAD+ and NADH in neuronal death in certain neurological diseases. Our latest studies have suggested that intranasal administration with NAD+ can profoundly decrease ischemic brain damage. These observations suggest that NAD+ administration may be a novel therapeutic strategy for some neurological diseases.

Splanchnic ischemia and reperfusion injury is reduced by genetic or pharmacological inhibition of TNF-alpha

In the present study, we used TNF-alpha receptor 1 knockout (TNF-alphaR1KO) mice to evaluate a possible role of TNF-alpha on the pathogenesis of ischemia and reperfusion injury of the multivisceral organs. Ischemia and reperfusion injury was induced in mice by clamping the superior mesenteric artery and the celiac artery for 30 min, followed thereafter by reperfusion. Sixty minutes after reperfusion, animals were killed for histological examination and biochemical studies. Injured wild-type (WT) mice developed a significant increase of ileum TNF-alpha levels, myeloperoxidase activity, and marked histological injury and apoptosis. Ischemia and reperfusion injury of the multivisceral organs was also associated with a significant mortality. Reperfused ileum sections from injured WT mice showed positive staining for P-selectin, VCAM, ICAM-1, and E-selectin. The intensity and degree of P-selectin, E-selectin, VCAM, and ICAM-1 were reduced markedly in tissue sections from injured TNF-alphaR1KO mice. Ischemia and reperfusion-injured TNF-alphaR1KO mice also showed a significant reduction of neutrophil infiltration into the intestine, a reduction of apoptosis, an improved histological status of the intestine, and survival. In addition, we investigated the effect of Etanercept, a TNF-alpha soluble receptor construct, on ischemia and reperfusion injury of the multivisceral organs. Etanercept (5 mg/kg administered i.p. 5 min prior to reperfusion) significantly reduced the inflammatory response and the ileum injury. Taken together, our results clearly demonstrate that TNF-alpha plays an important role in the ischemia and reperfusion injury and put forward the hypothesis that modulation of TNF-alpha expression may represent a novel and possible strategy.

Poly(ADP-Ribose) Glycohydrolase Activity Mediates Post-Traumatic Inflammatory Reaction after Experimental Spinal Cord Trauma

The aim of the present study was to examine the role of poly-(ADP-ribose) glycohydrolase (PARG) on the modulation of the inflammatory response and tissue injury associated with neurotrauma. Spinal cord trauma was induced in wild-type (WT) mice by the application of vascular clips (force of 24 g) to the dura via a two-level T(6) to T(7) laminectomy. Spinal cord injury in WT mice resulted in severe trauma characterized by edema, neutrophil infiltration, and cytokine production followed by recruitment of other inflammatory cells, production of a range of inflammation mediators, tissue damage, apoptosis, and disease. The genetic disruption of the PARG gene in mice or the pharmacological inhibition of PARG with GPI 16552 [N-bis-(3-phenyl-propyl)9-oxo-fluorene-2,7-diamide] (40 mg/kg i.p. bolus), a novel and potent PARG inhibitor, significantly reduced the degree of spinal cord inflammation and tissue injury (histological score), neutrophil infiltration, cytokine production (tumor necrosis factor-alpha and interleukin-1beta), and apoptosis. In a separate experiment, we have clearly demonstrated that PARG inhibition significantly ameliorated the recovery of limb function. Taken together, our results indicate that PARG activity modulates the inflammatory response and tissue injury events associated with spinal cord trauma and participate in target organ damage under these conditions.

Poly(ADP-ribose) glycohydrolase silencing protects against H2O2-induced cell death

PAR [poly(ADP-ribose)] is a structural and regulatory component of multiprotein complexes in eukaryotic cells. PAR catabolism is accelerated under genotoxic stress conditions and this is largely attributable to the activity of a PARG (PAR glycohydrolase). To overcome the early embryonic lethality of parg-knockout mice and gain more insights into the biological functions of PARG, we used an RNA interference approach. We found that as little as 10% of PARG protein is sufficient to ensure basic cellular functions: PARG-silenced murine and human cells proliferated normally through several subculturing rounds and they were able to repair DNA damage induced by sublethal doses of H2O2. However, cell survival following treatment with higher concentrations of H2O2 (0.05-1 mM) was increased. In fact, PARG-silenced cells were more resistant than their wild-type counterparts to oxidant-induced apoptosis while exhibiting delayed PAR degradation and transient accumulation of ADP-ribose polymers longer than 15-mers at early stages of drug treatment. No difference was observed in response to the DNA alkylating agent N-methyl-N'-nitro-N-nitrosoguanidine, suggesting a specific involvement of PARG in the cellular response to oxidative DNA damage.

Post-treatment with the Ca2+–Mg2+-endonuclease inhibitor aurintricarboxylic acid prevents peroxynitrite-induced DNA damage and death of murine astrocytes

Oxidative stress plays critical roles in aging, cell death, and many diseases. Peroxynitrite is one of the major reactive oxygen species which mediates cell injury in a number of illnesses. It is of importance to identify the downstream events in peroxynitrite-initiated cell death cascade for preventing peroxynitrite toxicity. Ca(2+)-Mg(2+)-endonucleases have been suggested as the endonucleases that execute DNA fragmentation in several apoptotic cascades. In this study, we determined if astrocytes and neurons express the genes of Ca(2+)-Mg(2+)-endonucleases. We also tested our hypothesis that post-treatment with the Ca(2+)-Mg(2+)-endonuclease inhibitor aurintricarboxylic acid can decrease peroxynitrite-induced DNA damage and death of astrocytes. We found that both astrocytes and neurons express DNase I-like endonuclease-a major isoform of Ca(2+)-Mg(2+)-endonucleases. Treatment of astrocytes with aurintricarboxylic acid either before or after peroxynitrite exposures can profoundly decrease peroxynitrite-induced DNA damage and cell death. These results suggest that Ca(2+)-Mg(2+)-endonucleases may be a key downstream component in peroxynitrite-initiated cell death cascade in astrocytes and some other cell types, and aurintricarboxylic acid could be used to decrease peroxynitrite-induced DNA damage at delayed phases.

Poly(ADP-ribose) accumulation and enhancement of postischemic brain damage in 110-kDa poly(ADP-ribose) glycohydrolase null mice

Poly(ADP-ribose) (PAR) is a polymer synthesized by poly(ADP-ribose) polymerases (PARPs) and metabolized into free adenosine diphosphate (ADP)-ribose units by poly(ADP-ribose) glycohydrolase (PARG). Perturbations in PAR synthesis have been shown to play a key role in brain disorders including postischemic brain damage. A single parg gene but two PARG isoforms (110 and 60 kDa) have been detected in mouse cells. Complete suppression of parg gene causes early embryonic lethality, whereas mice selectively lacking the 110 kDa PARG isoform (PARG(110)(-/-)) develop normally. We used PARG(110)(-/-) mice to evaluate the importance of PAR catabolism to postischemic brain damage. Poly(ADP-ribose) contents were higher in the brain tissue of PARG(110)(-/-) than PARG(110)(+/+) mice, both under basal conditions and after PARP activation. Distal middle cerebral artery occlusion caused higher increase of brain PAR levels and larger infarct volumes in PARG(110)(-/-) mice than in wild-type counterparts. Of note, the brain of PARG(110)(-/-) mice showed reduced heat-shock protein (HSP)-70 and increased cyclooxygenase-2 expression under both control and ischemic conditions. No differences were detected in brain expression/activation of procaspase-3, PARP-1, Akt, HSP-25 and interleukin-1beta. Our findings show that PAR accumulation worsens ischemic brain injury, and highlight the therapeutic potential of strategies capable of maintaining PAR homeostasis.

Tyrphostin AG 126 reduces intestinal ischemia-reperfusion injury in the rat

In this study, we evaluated the effect of tyrphostin AG126, a tyrosine kinase inhibitor, in the splanchnic artery occlusion (SAO) shock mediated injury. SAO shock was induced in rats by clamping both the superior mesenteric artery and the celiac trunk for 45 min. After 1 h of reperfusion, SAO shocked rats developed a significant fall in mean arterial blood pressure. Ileum analysis revealed that SAO shock is characterized by a significant (P<0.01) induction in TNF-alpha and IL-1 ileum levels, while immunohistochemistry examination of necrotic ileum demonstrated a marked increase in the immunoreactivity in intracellular adhesion molecule (ICAM-1) and nitrotyrosine formation. A significant increase in myeloperoxidase activity (P<0.01) was also observed in rats subjected to ischemia-reperfusion injury. Tyrphostin AG126, given intraperitoneally 30 min before ischemia at the dose of 5 mg/kg, significantly improved mean arterial blood pressure, markedly reduced TNF-alpha and IL-1beta levels and the positive staining of ICAM-1 into the reperfused ileum. Tyrphostin AG126 significantly improved the histological status of the reperfused tissue. In conclusion, this study demonstrates that tyrphostin AG126 exerts multiple protective effects in splanchnic artery occlusion/reperfusion shock and suggests that this tyrosine kinase inhibitor may be a candidate for consideration as a therapeutic intervention for ischemia-reperfusion injury.

Poly(ADP-ribose) glycohydrolase inhibitor as chemosensitiser of malignant melanoma for temozolomide

Disruption of poly(ADP-ribose) polymerase (PARP) pathways by inhibitors of PARP catalytic domain has been shown to increase the anti-tumour activity of temozolomide (TMZ). Since PARP is inhibited by poly(ADP)ribosylation, herein we tested whether inhibition of poly(ADP-ribose) glycohydrolase (PARG) might enhance TMZ efficacy. The PARG inhibitor N-bis-(3-phenyl-propyl)9-oxo-fluorene-2,7-diamide (GPI 16552) was administered in combination with TMZ to mice injected subcutaneously or intracranially with B16 melanoma cells. The ability of treatment to reduce melanoma metastatic spreading and invasion of the extracellular matrix was also tested. The results indicated that combined treatment with GPI 16552 and TMZ significantly reduced melanoma growth, increased life-span of mice bearing tumour at the CNS site, and decreased the ability of melanoma cells to form lung metastases and to invade the extracellular matrix. In conclusion, PARG inhibition represents an alternative strategy to enhance TMZ efficacy against melanoma in peripheral as well as at CNS site.

Role of poly(ADP-ribose) glycohydrolase (PARG) in shock, ischemia and reperfusion

Poly(ADP-ribosyl)ation is regulated by the synthesizing enzyme poly(ADP-ribose) polymerase-1 (PARP-1) and the degrading enzyme poly(ADP-ribose) glycohydrolase (PARG). Homeostasis of poly(ADP-ribosyl)ation has been proposed to be an important regulator for pathogenesis in multi-cellular organisms. Although the role of PARP-1 in tissue damage, inflammation and ischemia has been extensively studied, the function of PARG in various cellular processes is largely unknown. Recent studies using chemical inhibitors of PARG and genetically engineered Drosophila and mouse models that carry a disrupted PARG gene have started to shed new light on the biological function of PARG in vivo. These animal models and cells isolated from them will be useful for further validation of PARG as a potential pharmaceutical target to intervene the pathogenesis induced by acute tissue injury, ischemia and inflammation.

Green tea polyphenol extract attenuates ischemia/reperfusion injury of the gut

Various studies have clearly demonstrated that green tea catechins possess potent antioxidative properties, and the preventive effects against various oxidative diseases have been reported. The aim of this study was to investigate the effect of green tea extract on the tissue injury caused by ischemia/reperfusion (I/R) of the gut. I/R injury of the intestine was caused by clamping both the superior mesenteric artery and the celiac trunk for 45 min followed by release of the clamp allowing reperfusion for 1 h or 4 h. This procedure results in splanchnic artery occlusion (SAO) shock. Rats subjected to SAO developed a significant fall in mean arterial blood pressure, and only 10% of the animals survived for the entire 4-h reperfusion period. Surviving animals were sacrificed for histological examination and biochemical studies. Rats subjected to SAO displayed a significant increase in tissue myeloperoxidase (MPO) activity and malondialdehyde (MDA) levels, significant increases in plasma tumor necrosis factor (TNF)-alpha levels and marked injury to the distal ileum. Increased immunoreactivity to nitrotyrosine was observed in the ileum of rats subjected to SAO. Staining of sections of the ileum obtained from SAO rats with anti-intercellular adhesion molecule (ICAM-1) antibody and with anti-P-selectin antibody resulted in diffuse staining. Administration of green tea extract (20 and 10 mg kg(-1) i.v.) 15 min prior to the onset of gut reperfusion significantly reduced in a dose-dependent manner the fall in mean arterial blood pressure, the mortality rate, infiltration of the reperfused intestine with polymorphonuclear neutrophils (MPO activity), lipid peroxidation (MDA levels), production of TNF-alpha, and histological evidence of gut injury. Administration of green tea extract also markedly reduced nitrotyrosine formation and the up-regulation of ICAM-1 and P-selectin during reperfusion. In order to clarify that green tea extract might be useful in the therapy of I/R injury, we also investigated the effect of green tea extract (20 mg kg(-1) i.v.) when administered 5 min after the onset of gut reperfusion. Similar to the pretreatment approach, the post-treatment also significantly reduced the gut injury induced by I/R. These results demonstrate that green tea extract significantly reduces I/R injury of the intestine.