Complete inhibition of poly(ADP-ribose) polymerase activity prevents the recovery of C3H10T1/2 cells from oxidative stress

PARP inhibitors as precision medicine for cancer treatment

Personalized or precision medicine is an emerging treatment approach tailored to individuals or certain groups of patients based on their unique characteristics. These types of therapies guided by biomarkers tend to be more effective than traditional approaches, especially in cancer. The inhibitor against poly (ADP-ribose) polymerase (PARP), olaparib (Lynparza, AstraZeneca), which was approved by the US Food and Drug Administration (FDA) in 2014, demonstrated efficacy specifically for ovarian cancer patients harboring mutations in BRCA genes, which encode proteins in DNA double-strand break repairs. However, the response to PARP inhibitors has been less encouraging in other cancer types that also carry defects in the BRCA genes. Thus, furthering our understanding of the underlying mechanism of PARP inhibitors and resistance is critical to improve their efficacy. In this review, we summarize the results of preclinical studies and the clinical application of PARP inhibitors, and discuss the future direction of PARP inhibitors as a potential marker-guided personalized medicine for cancer treatment.

Vitamin A depletion causes oxidative stress, mitochondrial dysfunction, and PARP-1-dependent energy deprivation

A significant unresolved question is how vitamin A deprivation causes, and why retinoic acid fails to reverse, immunodeficiency. When depleted of vitamin A, T cells undergo programmed cell death (PCD), which is enhanced by the natural competitor of retinol, anhydroretinol. PCD does not happen by apoptosis, despite the occurrence of shared early events, including mitochondrial membrane depolarization, permeability transition pore opening, and cytochrome c release. It also lacks caspase-3 activation, chromatin condensation, and endonuclease-mediated DNA degradation, hallmarks of apoptosis. PCD following vitamin A deprivation exhibits increased production of reactive oxygen species (ROS), drastic reductions in ATP and NAD(+) levels, and activation of poly-(ADP-ribose) polymerase (PARP) -1. These latter steps are causative because neutralizing ROS, imposing hypoxic conditions, or inhibiting PARP-1 by genetic or pharmacologic approaches prevents energy depletion and PCD. The data highlight a novel regulatory role of vitamin A in mitochondrial energy homeostasis.

Poly(ADP-ribosyl)ation in mammalian ageing

Poly(ADP-ribose) polymerases (PARPs) catalyze the post-translational modification of proteins with poly(ADP-ribose). Two PARP isoforms, PARP-1 and PARP-2, display catalytic activity by contact with DNA-strand breaks and are involved in DNA base-excision repair and other repair pathways. A body of correlative data suggests a link between DNA damage-induced poly(ADP-ribosyl)ation and mammalian longevity. Recent research on PARPs and poly(ADP-ribose) yielded several candidate mechanisms through which poly(ADP-ribosyl)ation might act as a factor that limits the rate of ageing.

Poly(ADP-ribose) glycohydrolase as a target for neuroprotective intervention: assessment of currently available pharmacological tools

Poly(ADP-ribose) glycohydrolase (PARG) is being considered as a therapeutic target for the prevention of neurodegeneration. Here, we assessed the pharmacological tools available for target validation. The tannic acid derivative gallotannin inhibited PARG in a cell-free assay but had no detectable effect on PARG function in intact cells. Its cytoprotective actions were associated rather with the radical-scavenging potential of the compound. In astrocytes exposed to high concentrations of the nonoxidative DNA-damaging agent N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), Poly(ADP-ribose) polymerase (PARP) inhibitors were fully protective, while gallotannin enhanced the damage. The compound N-bis-(3-phenyl-propyl)9-oxo-fluorene-2,7-diamide (GPI 16552), considered a potentially specific PARG inhibitor, had no effect in the different astrocyte death models compared with PARP inhibitors. In an in vitro PARG activity assay, the maximal inhibition that could be achieved with GPI 16552 was only 40% at a drug concentration of 80 microM. We conclude that neither GPI 16552 nor gallotannin are suitable for the evaluation of PARG in cellular death models, and that previous conclusions drawn from the use of these compounds should be interpreted with caution.

Enhanced telomere shortening in transformed lymphoblasts from patients with X linked dyskeratosis

AIM

Dyskeratosis congenita (DC) is characterised by the failure of those tissues that are rapidly dividing in the adult, particularly the skin, mucosae, and haemopoietic system. The X linked form of the disease is caused by mutations of the DKC1 gene, which encodes dyskerin, a protein that is necessary for the function of telomerase. Cultured DC lymphoblastoid cells are characterised by a reduced expansion of the cell population because of the progressive increase in apoptosis compared with the number of cell divisions. This report aimed to verify whether this is caused by a defect in telomerase function.

METHODS

Variations in telomere length over time were evaluated in two cultured lymphoblastoid cell lines derived from patients with X linked DC and control cells derived from a non-affected individual. In addition, the effect of inhibiting poly (ADP-ribose) polymerase (PARP), which is involved in the cellular response to excessive telomere shortening, was assessed. One DC cell line and the control cells were treated with the specific PARP inhibitor 1,5-dihydroxyquinoline (IQ).

RESULTS

In DC cells the increase in cell death was associated with progressive telomere shortening, and this was not seen in the control cells. Treatment with IQ delayed the increase of apoptosis in DC cells.

CONCLUSIONS

These observations indicate that the reduced expansion that characterises cultured cells obtained from patients with X linked DC is caused by premature telomere shortening.

1,5-isoquinolinediol increases the frequency of gene targeting by homologous recombination in mouse fibroblasts

Gene targeting is a technique that allows the introduction of predefined alterations into chromosomal DNA. It involves a homologous recombination reaction between the targeted genomic sequence and an exogenous targeting vector. In theory, gene targeting constitutes the ideal method of gene therapy for single gene disorders. In practice, gene targeting remains extremely inefficient for at least two reasons: very low frequency of homologous recombination in mammalian cells and high proficiency of the mammalian cells to randomly integrate the targeting vector by illegitimate recombination. One known method to improve the efficiency of gene targeting is inhibition of poly(ADP-ribose)polymerase (PARP). It has been shown that PARP inhibitors, such as 3-methoxybenzamide, could lower illegitimate recombination, thus increasing the ratio of gene targeting to random integration. However, the above inhibitors were reported to decrease the absolute frequency of gene targeting. Here we show that treatment of mouse Ltk cells with 1,5-isoquinolinediol, a recent generation PARP inhibitor, leads to an increase up to 8-fold in the absolute frequency of gene targeting in the correction of the mutation at the stable integrated HSV tk gene.

Activation of Poly(adenosine diphosphate–ribose) Polymerase in Mouse Tumors Treated by Photodynamic Therapy¶

Poly(adenosine diphosphate-ribose) polymerase (PARP) has recently been characterized as a key regulator of cell death-survival transcriptional programs associated with stress and inflammation. Possible participation of this enzyme in the response of tumors to photodynamic therapy (PDT) was investigated in this study. Immunohistochemical analysis of mouse FsaR tumors treated by PDT based on photosensitizers Photofrin or 5,10,15,20-tetra-(m-hydroxyphenyl)chlorine (mTHPC) revealed a strong positive staining for PARP product poly(ADP-ribose) at 30 min and 1 h after PDT, respectively, and even more intense positivity at 2 h after PDT with both photosensitizers. Flow cytometry-based examination showed the induction of poly-ADP-ribosylation in FsaR tumors at 30 min after PDT, with a trend for a further increase in the intensity by 2 h after PDT in both cancer cells and tumor-associated leukocytes. In FsaR cells treated in vitro by mTHPC-based PDT, flow cytometric analysis indicated that the activation of PARP concentrated in cells undergoing apoptosis and reached a maximum by 30 min after PDT. The administration of PARP inhibitors, 3-aminobenzamide or 1,5-isoquinolinediol, to FsaR tumor-bearing mice before PDT light treatment increased the resistance of these tumors to PDT. PARP appears to control the balance between apoptotic and necrotic cell death in PDT-treated tumors and regulate the progression of PDT-induced inflammatory or innate immune response.

The role of oxidative stress in mechanisms of metal-induced carcinogenesis

Metals are necessary for the normal functioning of cells and the survival of organisms. However, exposure to higher than the physiological levels of several metals may lead to tumor development. Although the exact molecular mechanism(s) of metal-induced carcinogenesis is not clear, a vast body of evidence indicates that metal-induced generation of reactive oxygen species (ROS) may play a central role in this process. Two main pathways of ROS-induced effects are discussed in this chapter: (i) increased DNA damage induced either directly or indirectly by impeding DNA repair, and (ii) modulation of nuclear transcriptional factor activities, such as NF-kappaB and AP-1, through mitogen-activated protein kinases signal transduction mechanisms.

Functions of poly(ADP-ribose) polymerase (PARP) in DNA repair, genomic integrity and cell death

Poly(ADP-ribose) polymerase (PARP) is responsible for post-translational modification of proteins in the response to numerous endogenous and environmental genotoxic agents. PARP and poly(ADP-ribosyl)ation are proposed to be important for the regulation of many cellular processes such as DNA repair, cell death, chromatin functions and genomic stability. Activation of PARP is one of the early DNA damage responses, among other DNA sensing molecules, such as DNA-PK, ATM and p53. The generation and characterization of PARP deficient mouse models have been instrumental in defining the biological role of the molecule and its involvement in the pathogenesis of various diseases including diabetes, stroke, Parkinson disease, general inflammation as well as tumorigenesis, and have, therefore, provided information for the development of pharmaceutical strategies for the treatment of diseases.

NMDA but not non-NMDA excitotoxicity is mediated by Poly(ADP-ribose) polymerase

Poly(ADP-ribose) polymerase (PARP-1), a nuclear enzyme that facilitates DNA repair, may be instrumental in acute neuronal cell death in a variety of insults including, cerebral ischemia, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced parkinsonism, and CNS trauma. Excitotoxicity is thought to underlie these and other toxic models of neuronal death. Different glutamate agonists may trigger different downstream pathways toward neurotoxicity. We examine the role of PARP-1 in NMDA- and non-NMDA-mediated excitotoxicity. NMDA and non-NMDA agonists were stereotactically delivered into the striatum of mice lacking PARP-1 and control mice in acute (48 hr) and chronic (3 week) toxicity paradigms. Mice lacking PARP-1 are highly resistant to the excitoxicity induced by NMDA but are as equally susceptible to AMPA excitotoxicity as wild-type mice. Restoring PARP-1 protein in mice lacking PARP-1 by viral transfection restored susceptibility to NMDA, supporting the requirement of PARP-1 in NMDA neurotoxicity. Furthermore, Western blot analyses demonstrate that PARP-1 is activated after NMDA delivery but not after AMPA administration. Consistent with the theory that nitric oxide (NO) and peroxynitrite are prominent in NMDA-induced neurotoxicity, PARP-1 was not activated in mice lacking the gene for neuronal NO synthase after NMDA administration. These results suggest a selective role of PARP-1 in glutamate excitoxicity, and strategies of inhibiting PARP-1 in NMDA-mediated neurotoxicity may offer substantial acute and chronic neuroprotection.

Biodistribution of 3,4-dihydro-5-[11C]methoxy-1(2H)-isoquinolinone, a potential PET tracer for poly(ADP-ribose) synthetase

Poly(adenosine diphosphate-ribose) synthetase (PARS) is a nuclear enzyme that is activated by deoxyribonucleic acid (DNA) strand breaks and participates in DNA repair. Excessive PARS activation, however, leads to cell death due to depletion of adenosine triphosphate (ATP). To evaluate whether it is possible to detect excessive activation of PARS with positron emission tomography (PET), we examined the pharmacokinetics of 3,4-dihydro-5-[(11)C]methoxy-1(2H)-isoquinolinone ([(11)C]MIQO), a potent poly(ADP-ribose) synthetase inhibitor, in the brain of rats and monkeys. Although the uptake of [(11)C]MIQO in the brain of normal rats was low, [(11)C]MIQO was rapidly incorporated into and then quickly washed out from the brain. The uptake of the radiotracer in the brain of normal monkeys was also low; however, [(11)C]MIQO gave a distribution image that differed from that of cerebral blood flow obtained by [(15)O]water-PET. No localization of [(11)C]MIQO in the brain of normal monkeys was observed. Low accumulation of some radioactivity was also observed in muscles surrounding the brain of monkeys, but did not seem to interfere with measurement of [(11)C]MIQO uptake in the brain with PET. Thus, detection of [(11)C]MIQO uptake with PET may be useful for detecting PARS activity in ischemic injury.

Brain ischemia and reperfusion: molecular mechanisms of neuronal injury

Brain ischemia and reperfusion engage multiple independently-fatal terminal pathways involving loss of membrane integrity in partitioning ions, progressive proteolysis, and inability to check these processes because of loss of general translation competence and reduced survival signal-transduction. Ischemia results in rapid loss of high-energy phosphate compounds and generalized depolarization, which induces release of glutamate and, in selectively vulnerable neurons (SVNs), opening of both voltage-dependent and glutamate-regulated calcium channels. This allows a large increase in cytosolic Ca(2+) associated with activation of mu-calpain, calcineurin, and phospholipases with consequent proteolysis of calpain substrates (including spectrin and eIF4G), activation of NOS and potentially of Bad, and accumulation of free arachidonic acid, which can induce depletion of Ca(2+) from the ER lumen. A kinase that shuts off translation initiation by phosphorylating the alpha-subunit of eukaryotic initiation factor-2 (eIF2alpha) is activated either by adenosine degradation products or depletion of ER lumenal Ca(2+). Early during reperfusion, oxidative metabolism of arachidonate causes a burst of excess oxygen radicals, iron is released from storage proteins by superoxide-mediated reduction, and NO is generated. These events result in peroxynitrite generation, inappropriate protein nitrosylation, and lipid peroxidation, which ultrastructurally appears to principally damage the plasmalemma of SVNs. The initial recovery of ATP supports very rapid eIF2alpha phosphorylation that in SVNs is prolonged and associated with a major reduction in protein synthesis. High catecholamine levels induced by the ischemic episode itself and/or drug administration down-regulate insulin secretion and induce inhibition of growth-factor receptor tyrosine kinase activity, effects associated with down-regulation of survival signal-transduction through the Ras pathway. Caspase activation occurs during the early hours of reperfusion following mitochondrial release of caspase 9 and cytochrome c. The SVNs find themselves with substantial membrane damage, calpain-mediated proteolytic degradation of eIF4G and cytoskeletal proteins, altered translation initiation mechanisms that substantially reduce total protein synthesis and impose major alterations in message selection, down-regulated survival signal-transduction, and caspase activation. This picture argues powerfully that, for therapy of brain ischemia and reperfusion, the concept of single drug intervention (which has characterized the approaches of basic research, the pharmaceutical industry, and clinical trials) cannot be effective. Although rigorous study of multi-drug protocols is very demanding, effective therapy is likely to require (1) peptide growth factors for early activation of survival-signaling pathways and recovery of translation competence, (2) inhibition of lipid peroxidation, (3) inhibition of calpain, and (4) caspase inhibition. Examination of such protocols will require not only characterization of functional and histopathologic outcome, but also study of biochemical markers of the injury processes to establish the role of each drug.

A cellular defense pathway regulating transcription through poly(ADP-ribosyl)ation in response to DNA damage

DNA damage is known to trigger key cellular defense pathways such as those involved in DNA repair. Here we provide evidence for a previously unrecognized pathway regulating transcription in response to DNA damage and show that this regulation is mediated by the abundant nuclear enzyme poly(ADP-ribose) polymerase. We found that poly(ADP-ribose) polymerase reduced the rate of transcription elongation by RNA polymerase II, suggesting that poly(ADP-ribose) polymerase negatively regulates transcription, possibly through the formation of poly(ADP-ribose) polymerase-RNA complexes. In damaged cells, poly(ADP-ribose) polymerase binds to DNA breaks and automodifies itself in the presence of NAD(+), resulting in poly(ADP-ribose) polymerase inactivation. We found that automodification of poly(ADP-ribose) polymerase in response to DNA damage resulted in the up-regulation of transcription, presumably because automodified poly(ADP-ribose) polymerase molecules were released from transcripts, thereby relieving the block on transcription. Because agents that damage DNA damage RNA as well, up-regulation of RNA synthesis in response to DNA damage may provide cells with a mechanism to compensate for the loss of damaged transcripts and may be critical for cell survival after exposure to DNA-damaging agents.

A fast signal-induced activation of Poly(ADP-ribose) polymerase: a novel downstream target of phospholipase c

We present the first evidence for a fast activation of the nuclear protein poly(ADP-ribose) polymerase (PARP) by signals evoked in the cell membrane, constituting a novel mode of signaling to the cell nucleus. PARP, an abundant, highly conserved, chromatin-bound protein found only in eukaryotes, exclusively catalyzes polyADP-ribosylation of DNA-binding proteins, thereby modulating their activity. Activation of PARP, reportedly induced by formation of DNA breaks, is involved in DNA transcription, replication, and repair. Our findings demonstrate an alternative mechanism: a fast activation of PARP, evoked by inositol 1,4,5,-trisphosphate-Ca(2+) mobilization, that does not involve DNA breaks. These findings identify PARP as a novel downstream target of phospholipase C, and unveil a novel fast signal-induced modification of DNA-binding proteins by polyADP-ribosylation.

Characterization of sPARP-1. An alternative product of PARP-1 gene with poly(ADP-ribose) polymerase activity independent of DNA strand breaks

Poly(ADP-ribose) polymerase-1 (PARP-1) is an abundant nuclear enzyme that catalyzes the synthesis of poly(ADP-ribose) (pADPr) from its substrate NAD(+) upon binding to DNA strand breaks. Poly(ADP-ribosyl)ation has been implicated in many cellular processes including replication, transcription, and the maintenance of genomic stability. However, studies with mice and cells lacking PARP-1 reveal a critical role for the enzyme in the maintenance of genomic integrity only. Recently, a significant level of poly(ADP-ribose) polymerase activity has been detected in fibroblasts derived from mice lacking PARP-1 following treatment with genotoxic agents (Shieh, W. M., Amé, J-C., Wilson, M. V., Wang, Z-Q., Koh, D. W., Jacobson, M. K., and Jacobson, E. L. (1998) J. Biol. Chem. 273, 30069-30072). We have isolated a cDNA that originates from PARP-1 (-/-) fibroblasts and encodes a polypeptide of 493 amino acid residues bearing poly(ADP-ribose) polymerase activity. This protein, that we named sPARP-1 for short poly(ADP-ribose) polymerase-1, has a calculated mass of 55.3 kDa and is identical in deduced amino acid sequence to the catalytic domain of PARP-1. Radiation hybrid analysis assigned the sPARP-1 gene to the chromosome 1H5-H6 in an immediate proximity to the known location of PARP-1 gene, indicating that sPARP-1 and PARP-1 are most probably products of the same gene. Active sPARP-1 is present in both PARP-1 (+/+) and PARP-1 (-/-) cells as demonstrated by activity-Western blotting and immunostaining using a specific antibody developed against sPARP-1. Like PARP-1, sPARP-1 is localized in the cell nucleus, uses NAD(+) as a substrate and is inhibited by nicotinamide analogues. sPARP-1 produces pADPr of similar length and structure to that of PARP-1. However, contrary to PARP-1, sPARP-1 does not require DNA strand breaks for its activation, although it is stimulated following genotoxic treatments.

Survival and proliferation of cells expressing caspase-uncleavable Poly(ADP-ribose) polymerase in response to death-inducing DNA damage by an alkylating agent

To determine whether caspase-3-induced cleavage of poly(ADP-ribose) polymerase (PARP), a DNA damage-sensitive enzyme, alters the balance between survival and death of the cells following DNA damage, we created stable cell lines that express either caspase-uncleavable mutant or wild type PARP in the background of PARP (-/-) fibroblasts. The survival and apoptotic responses of these cells were compared after exposure to N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), a DNA-damaging agent that activates PARP, or to tumor necrosis factor-alpha, which causes apoptosis without initial DNA damage. In response to MNNG, the cells with caspase-uncleavable PARP were very resistant to loss of viability or induction of apoptosis. Most significantly, approximately 25% of these cells survived and retained clonogenicity at a level of DNA damage that eliminated the cells with wild type PARP or PARP (-/-) cells. Expression of caspase-uncleavable PARP could not protect the cells from death induced by tumor necrosis factor, although there was a slower progression of apoptotic events in these cells. Therefore, one of the functions for cleavage of PARP during apoptosis induced by alkylating agents is to prevent survival of the extensively damaged cells.

Inhibition of poly(ADP-ribose)polymerase stimulates extrachromosomal homologous recombination in mouse Ltk-fibroblasts

Poly(ADP-ribose)polymerase (PARP) is an abundant nuclear enzyme activated by DNA breaks. PARP is generally believed to play a role in maintaining the integrity of the genome in eukaryote cells via anti-recombinogenic activity by preventing inappropriate homologous recombination reactions at DNA double-strand breaks. While inhibition of PARP reduces non-homologous recombination, at the same time it stimulates sister chromatid exchange and intrachromosomal homologous recombination. Here we report that the inhibition of PARP with 100 microg/ml (0.622 mM) 1,5-isoquinolinediol results in an average 4.6-fold increase in the frequency of extrachromosomal homologous recombination between two linearized plasmids carrying herpes simplex virus thymidine kinase genes inactivated by non-overlapping mutations, in mouse Ltk-fibroblasts. These results are in disagreement with the previously reported observation that PARP inhibition had no effect on extrachromosomal homologous recombination in Ltk-cells.

Cleavage of automodified poly(ADP-ribose) polymerase during apoptosis. Evidence for involvement of caspase-7

The abundant nuclear enzyme poly(ADP-ribose) polymerase (PARP) synthesizes poly(ADP-ribose) in response to DNA strand breaks. During almost all forms of apoptosis, PARP is cleaved by caspases, suggesting the crucial role of its inactivation. A few studies have also reported a stimulation of PARP during apoptosis. However, the role of PARP stimulation and cleavage during this cell death process remains poorly understood. Here, we measured the stimulation of endogenous poly(ADP-ribose) synthesis during VP-16-induced apoptosis in HL60 cells and found that PARP was cleaved by caspases at the time of its poly(ADP-ribosyl)ation. In vitro experiments showed that PARP cleavage by caspase-7, but not by caspase-3, was stimulated by its automodification by long and branched poly(ADP-ribose). Consistently, caspase-7 exhibited an affinity for poly(ADP-ribose), whereas caspase-3 did not. In addition, caspase-7 was activated and accumulated in the nucleus of HL60 cells in response to the VP-16 treatment. Furthermore, caspase-7 activation was concommitant with PARP cleavage in the caspase-3-deficient cell line MCF-7 in response to staurosporine treatment. These results strongly suggest that, in vivo, it is caspase-7 that is responsible for PARP cleavage and that poly(ADP-ribosyl)ation of PARP accelerates its proteolysis. Cleavage of the active form of caspase substrates could be a general feature of the apoptotic process, ensuring the rapid inactivation of stress signaling proteins.

Poly(ADP-ribosyl)ation reactions in the regulation of nuclear functions

Poly(ADP-ribosyl)ation is a post-translational modification of proteins. During this process, molecules of ADP-ribose are added successively on to acceptor proteins to form branched polymers. This modification is transient but very extensive in vivo, as polymer chains can reach more than 200 units on protein acceptors. The existence of the poly(ADP-ribose) polymer was first reported nearly 40 years ago. Since then, the importance of poly(ADP-ribose) synthesis has been established in many cellular processes. However, a clear and unified picture of the physiological role of poly(ADP-ribosyl)ation still remains to be established. The total dependence of poly(ADP-ribose) synthesis on DNA strand breaks strongly suggests that this post-translational modification is involved in the metabolism of nucleic acids. This view is also supported by the identification of direct protein-protein interactions involving poly(ADP-ribose) polymerase (113 kDa PARP), an enzyme catalysing the formation of poly(ADP-ribose), and key effectors of DNA repair, replication and transcription reactions. The presence of PARP in these multiprotein complexes, in addition to the actual poly(ADP-ribosyl)ation of some components of these complexes, clearly supports an important role for poly(ADP-ribosyl)ation reactions in DNA transactions. Accordingly, inhibition of poly(ADP-ribose) synthesis by any of several approaches and the analysis of PARP-deficient cells has revealed that the absence of poly(ADP-ribosyl)ation strongly affects DNA metabolism, most notably DNA repair. The recent identification of new poly(ADP-ribosyl)ating enzymes with distinct (non-standard) structures in eukaryotes and archaea has revealed a novel level of complexity in the regulation of poly(ADP-ribose) metabolism.

Nitric oxide in neurodegeneration

Nitric oxide (NO) is a unique biological messenger molecule which mediates diverse physiologic roles. NO mediates blood vessel relaxation by endothelium, immune activity of macrophages and neurotransmission of central and peripheral neurons. NO is produced from three NO Synthase (NOS) isoforms: Neuronal NOS (nNOS), endothelial NOS, and inducible NOS (iNOS). In the central nervous system, NO may play important roles in neurotransmitter release, neurotransmitter reuptake, neurodevelopment, synaptic plasticity, and regulation of gene expression. However, excessive production of NO following a pathologic insult can lead to neurotoxicity. NO plays a role in mediating neurotoxicity associated with a variety of neurologic disorders, including stroke, Parkinson's Disease, and HIV dementia.

Role of poly(ADP-ribose) synthetase in inflammation and ischaemia-reperfusion

Oxidative and nitrosative stress can trigger DNA strand breakage, which then activates the nuclear enzyme poly(ADP-ribose) synthetase (PARS). This enzyme has also been termed poly(ADP-ribose) polymerase (PARP) or poly(ADP-ribose) transferase (pADPRT). Rapid activation of the enzyme depletes the intracellular concentration of its substrate, nicotinamide adenine dinucleotide, thus slowing the rate of glycolysis, electron transport and subsequently ATP formation. This process can result in cell dysfunction and cell death. In this article, Csaba Szabó and Valina Dawson overview the impact of pharmacological inhibition or genetic inactivation of PARS on the course of oxidant-induced cell death in vitro, and in inflammation and reperfusion injury in vivo. A major trigger for DNA damage in pathophysiological conditions is peroxynitrite, a cytotoxic oxidant formed by the reaction between the free radicals nitric oxide and superoxide. The pharmacological inhibition of poly(ADP-ribose) synthetase is a novel approach for the experimental therapy of various forms of inflammation and shock, stroke, myocardial and intestinal ischaemia-reperfusion, and diabetes mellitus.

Stimulation of the DNA-dependent protein kinase by poly(ADP-ribose) polymerase

The DNA-dependent protein kinase (DNA-PK) is a heterotrimeric enzyme that binds to double-stranded DNA and is required for the rejoining of double-stranded DNA breaks in mammalian cells. It has been proposed that DNA-PK functions in this DNA repair pathway by binding to the ends of broken DNA molecules and phosphorylating proteins that bind to the damaged DNA ends. Another enzyme that binds to DNA strand breaks and may also function in the cellular response to DNA damage is the poly(ADP-ribose) polymerase (PARP). Here, we show that PARP can be phosphorylated by purified DNA-PK, and the catalytic subunit of DNA-PK is ADP-ribosylated by PARP. The protein kinase activity of DNA-PK can be stimulated by PARP in the presence of NAD+ in a reaction that is blocked by the PARP inhibitor 1, 5-dihydroxyisoquinoline. The stimulation of DNA-PK by PARP-mediated protein ADP-ribosylation occurs independent of the Ku70/80 complex. Taken together, these results show that PARP can modify the activity of DNA-PK in vitro and suggest that these enzymes may function coordinately in vivo in response to DNA damage.

PARP is important for genomic stability but dispensable in apoptosis

Mice lacking the gene encoding poly(ADP-ribosyl) transferase (PARP or ADPRT) display no phenotypic abnormalities, although aged mice are susceptible to epidermal hyperplasia and obesity in a mixed genetic background. Whereas embryonic fibroblasts lacking PARP exhibit normal DNA excision repair, they grow more slowly in vitro. Here we investigated the putative roles of PARP in cell proliferation, cell death, radiosensitivity, and DNA recombination, as well as chromosomal stability. We show that the proliferation deficiency in vitro and in vivo is most likely caused by a hypersensitive response to environmental stress. Although PARP is specifically cleaved during apoptosis, cells lacking this molecule apoptosed normally in response to treatment with anti-Fas, tumor neurosis factor alpha, gamma-irradiation, and dexamethasone, indicating that PARP is dispensable in apoptosis and that PARP-/- thymocytes are not hypersensitive to ionizing radiation. Furthermore, the capacity of mutant cells to carry out immunoglobulin class switching and V(D)J recombination is normal. Finally, primary PARP mutant fibroblasts and splenocytes exhibited an elevated frequency of spontaneous sister chromatid exchanges and elevated micronuclei formation after treatment with genotoxic agents, establishing an important role for PARP in the maintenance of genomic integrity.