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

Imaging Molecular Targets and Metabolic Pathways in Breast Cancer for Improved Clinical Management: Current Practice and Future Perspectives

Breast cancer is the most frequently diagnosed cancer and leading cause of cancer-related deaths worldwide. Timely decision-making that enables implementation of the most appropriate therapy or therapies is essential for achieving the best clinical outcomes in breast cancer. While clinicopathologic characteristics and immunohistochemistry have traditionally been used in decision-making, these clinical and laboratory parameters may be difficult to ascertain or be equivocal due to tumor heterogeneity. Tumor heterogeneity is described as a phenomenon characterized by spatial or temporal phenotypic variations in tumor characteristics. Spatial variations occur within tumor lesions or between lesions at a single time point while temporal variations are seen as tumor lesions evolve with time. Due to limitations associated with immunohistochemistry (which requires invasive biopsies), whole-body molecular imaging tools such as standard-of-care [18F]FDG and [18F]FES PET/CT are indispensable in addressing this conundrum. Despite their proven utility, these standard-of-care imaging methods are often unable to image a myriad of other molecular pathways associated with breast cancer. This has stimulated interest in the development of novel radiopharmaceuticals targeting other molecular pathways and processes. In this review, we discuss validated and potential roles of these standard-of-care and novel molecular approaches. These approaches' relationships with patient clinicopathologic and immunohistochemical characteristics as well as their influence on patient management will be discussed in greater detail. This paper will also introduce and discuss the potential utility of novel PARP inhibitor-based radiopharmaceuticals as non-invasive biomarkers of PARP expression/upregulation.

Combined use of 177 Lu-DOTATATE peptide receptor radionuclide therapy and fluzoparib for treatment of well-differentiated neuroendocrine tumors: A preclinical study

Peptide receptor radionuclide therapy (¹⁷⁷ Lu-DOTATATE) causes DNA strand breaks and has been validated for well-differentiated neuroendocrine tumor treatment. Poly-(ADP-ribose)-polymerase inhibitors have also been used for malignant tumors with deficient DNA repair. We aimed to determine whether the poly-(ADP-ribose)-polymerase inhibitor fluzoparib could enhance the anti-tumor effects of ¹⁷⁷ Lu-DOTATATE in neuroendocrine tumor cells and xenografts. The neuroendocrine characteristics of NCI-H727 bronchial carcinoid cells were evaluated by immunofluorescence staining. The synergistic effects of fluzoparib and ¹⁷⁷ Lu-DOTATATE were evaluated by cell proliferation and flow cytometry assays. Tumor response and the side effects of combination therapy were also assessed in xenograft mice treated with ⁷⁷ Lu-DOTATATE and fluzoparib alone or in combination. Somatostatin receptors were specifically expressed in NCI-H727 cells and tumor xenografts. ¹⁷⁷ Lu-DOTATATE (22.20 MBq mL^-1 ) and fluzoparib (50 µm) inhibited cell proliferation by 16.6% and 35.6%, respectively, compared to 73.2% in cells treated with their combination. Tumor cell proliferation was significantly suppressed by ¹⁷⁷ Lu-DOTATATE (22.20 MBq mL^-1 , 4.4-fold) and fluzoparib (50 µm, 2.1-fold). ¹⁷⁷ Lu-DOTATATE caused cell cycle arrest mainly at G1 phase, whereas fluzoparib caused arrest at G2/M phase, and combined treatment with both agents caused cell cycle arrest at G1 phase, similar to ¹⁷⁷ Lu-DOTATATE alone. The volume of tumor xenografts was reduced by 18.6% in mice receiving combined treatment, compared to 4.9% and 11.4% in mice treated with ¹⁷⁷ Lu-DOTATATE or fluzoparib alone. Fluzoparib can potentiate the anti-tumor effect of ¹⁷⁷ Lu-DOTATATE in NCI-H727 cells in a synergistic manner by arresting the cell cycle at G1 phase. Further preclinical and clinical studies are warranted to validate these findings.

Cell death modulation by transient receptor potential melastatin channels TRPM2 and TRPM7 and their underlying molecular mechanisms

Transient receptor potential melastatin (TRPM) channels are members of the transient receptor potential (TRP) channels, a family of evolutionarily conserved integral membrane proteins. TRPM channels are nonselective cation channels, mediating the influx of various ions including Ca²⁺, Na⁺ and Zn²⁺. The function of TRPM channels is vital for cell proliferation, cell development and cell death. Cell death is a key procedure during embryonic development, organism homeostasis, aging and disease. The category of cell death modalities, beyond the traditionally defined concepts of necrosis, autophagy, and apoptosis, were extended with the discovery of pyroptosis, necroptosis and ferroptosis. As upstream signaling regulators of cell death, TRPM channels have been involved inrelevant pathologies. In this review, we introduced several cell death modalities, then summarized the contribution of TRPM channels (especially TRPM2 and TRPM7) to different cell death modalities and discussed the underlying regulatory mechanisms. Our work highlighted the possibility of TRPM channels as potential therapeutic targets in cell death-related diseases.

Analysis of knockout mutants reveals non-redundant functions of poly(ADP-ribose)polymerase isoforms in Arabidopsis

The enzyme poly(ADP-ribose)polymerase (PARP) has a dual function being involved both in the poly(ADP-ribosyl)ation and being a constituent of the NAD(+) salvage pathway. To date most studies, both in plant and non-plant systems, have focused on the signaling role of PARP in poly(ADP-ribosyl)ation rather than any role that can be ascribed to its metabolic function. In order to address this question we here used a combination of expression, transcript and protein localization studies of all three PARP isoforms of Arabidopsis alongside physiological analysis of the corresponding mutants. Our analyses indicated that whilst all isoforms of PARP were localized to the nucleus they are also present in non-nuclear locations with parp1 and parp3 also localised in the cytosol, and parp2 also present in the mitochondria. We next isolated and characterized insertional knockout mutants of all three isoforms confirming a complete knockout in the full length transcript levels of the target genes as well as a reduced total leaf NAD hydrolase activity in the two isoforms (PARP1, PARP2) that are highly expressed in leaves. Physiological evaluation of the mutant lines revealed that they displayed distinctive metabolic and root growth characteristics albeit unaltered leaf morphology under optimal growth conditions. We therefore conclude that the PARP isoforms play non-redundant non-nuclear metabolic roles and that their function is highly important in rapidly growing tissues such as the shoot apical meristem, roots and seeds.

The use of Olaparib (AZD2281) potentiates SN-38 cytotoxicity in colon cancer cells by indirect inhibition of Rad51-mediated repair of DNA double-strand breaks

Potent application of topoisomerase I inhibitor plus PARP inhibitor has been suggested to be an effective strategy for cancer therapy. Reportedly, mismatch repair (MMR)-deficient colon cancer cells are sensitive to topoisomerase I inhibitor, presumably due to microsatellite instability (MSI) of the MRE11 locus. We examined the synergy of SN-38, an active metabolite of irinotecan, in combination with the PARP inhibitor olaparib in colon cancer cells showing different MMR status, such as MSI or microsatellite stable (MSS) phenotype. Treatment with SN-38 and olaparib in combination almost halved the IC50 of SN-38 for a broad spectrum of colon cancer cells independent of the MMR status. Furthermore, olaparib potentiated S-phase-specific double-strand DNA breaks (DSB) induced by SN-38, which is followed by Rad51 recruitment. siRNA-mediated knockdown of Rad51, but not Mre11 or Rad50, increased the sensitivity to olaparib and/or SN-38 treatment in colon cancer cells. In vivo study using mouse xenograft demonstrated that olaparib was effective to potentiate the antitumor effect of irinotecan. In conclusion, olaparib shows a synergistic effect in colon cancer cells in combination with SN-38 or irinotecan, potentiated by the Rad51-mediated HR pathway, irrespective of the Mre11-mediated failure of the MRN complex. These results may contribute to future clinical trials using PARP inhibitor plus topoisomerase I inhibitor in combination. Furthermore, the synergistic effect comprising topoisomerase I-mediated DNA breakage-reunion reaction, PARP and Rad51-mediated HR pathway suggests the triple synthetic lethal pathways contribute to this event and are applicable as a potential target for future chemotherapy.

The Sound of Silence: RNAi in Poly (ADP-Ribose) Research

Poly(ADP-ribosyl)-ation is a nonprotein posttranslational modification of proteins and plays an integral part in cell physiology and pathology. The metabolism of poly(ADP-ribose) (PAR) is regulated by its synthesis by poly(ADP-ribose) polymerases (PARPs) and on the catabolic side by poly(ADP-ribose) glycohydrolase (PARG). PARPs convert NAD+ molecules into PAR chains that interact covalently or noncovalently with target proteins and thereby modify their structure and functions. PAR synthesis is activated when PARP1 and PARP2 bind to DNA breaks and these two enzymes account for almost all PAR formation after genotoxic stress. PARG cleaves PAR molecules into free PAR and finally ADP-ribose (ADPR) moieties, both acting as messengers in cellular stress signaling. In this review, we discuss the potential of RNAi to manipulate the levels of PARPs and PARG, and consequently those of PAR and ADPR, and compare the results with those obtained after genetic or chemical disruption.

Cytokine-mediated β-cell damage in PARP-1-deficient islets

Poly(ADP)-ribose polymerase (PARP) is an abundant nuclear protein that is activated by DNA damage; once active, it modifies nuclear proteins through attachment of poly(ADP)-ribose units derived from β-nicotinamide adenine dinucleotide (NAD(+)). In mice, the deletion of PARP-1 attenuates tissue injury in a number of animal models of human disease, including streptozotocin-induced diabetes. Also, inflammatory cell signaling and inflammatory gene expression are attenuated in macrophages isolated from endotoxin-treated PARP-1-deficient mice. In this study, the effects of PARP-1 deletion on cytokine-mediated β-cell damage and macrophage activation were evaluated. There are no defects in inflammatory mediator signaling or inflammatory gene expression in macrophages and islets isolated from PARP-1-deficient mice. While PARP-1 deficiency protects islets against cytokine-induced islet cell death as measured by biochemical assays of membrane polarization, the genetic absence of PARP-1 does not effect cytokine-induced inhibition of insulin secretion or cytokine-induced DNA damage in islets. While PARP-1 deficiency appears to provide protection from cell death, it fails to provide protection against the inhibitory actions of cytokines on insulin secretion or the damaging actions on islet DNA integrity.

Cell death and autophagy under oxidative stress: roles of poly(ADP-Ribose) polymerases and Ca(2+)

On the cellular level, oxidative stress may cause various responses, including autophagy and cell death. All of these outcomes involve disturbed Ca(2+) signaling. Here we show that the nuclear enzymes poly(ADP-ribose) polymerase 1 (PARP1) and PARP2 control cytosolic Ca(2+) shifts from extracellular and intracellular sources associated with autophagy or cell death. The different Ca(2+) signals arise from the transient receptor potential melastatin 2 (TRPM2) channels located in the cellular and lysosomal membranes. They induce specific stress kinase responses of canonical autophagy and cell death pathways. Autophagy is under the control of PARP1, which operates as an autophagy suppressor after oxidative stress. Cell death is activated downstream of extracellular signal-regulated kinase 1/2 (ERK1/2) and AKT, whereas cell survival correlates with the phosphorylation of p38, stress-activated protein kinase/Jun amino-terminal kinase (SAPK/JNK), and cyclic AMP response element-binding protein (CREB) with its activating transcription factor (ATF-1). Our results highlight an important role for PARP1 and PARP2 in the epigenetic control of cell death and autophagy pathways.

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.

Poly(ADP-ribose)glycohydrolase is an upstream regulator of Ca2+ fluxes in oxidative cell death

Oxidative DNA damage to cells activates poly(ADP-ribose)polymerase-1 (PARP-1) and the poly(ADP-ribose) formed is rapidly degraded to ADP-ribose by poly(ADP-ribose)glycohydrolase (PARG). Here we show that PARP-1 and PARG control extracellular Ca²⁺ fluxes through melastatin-like transient receptor potential 2 channels (TRPM2) in a cell death signaling pathway. TRPM2 activation accounts for essentially the entire Ca²⁺ influx into the cytosol, activating caspases and causing the translocation of apoptosis inducing factor (AIF) from the inner mitochondrial membrane to the nucleus followed by cell death. Abrogation of PARP-1 or PARG function disrupts these signals and reduces cell death. ADP-ribose-loading of cells induces Ca²⁺ fluxes in the absence of oxidative damage, suggesting that ADP-ribose is the key metabolite of the PARP-1/PARG system regulating TRPM2. We conclude that PARP-1/PARG control a cell death signal pathway that operates between five different cell compartments and communicates via three types of chemical messengers: a nucleotide, a cation, and proteins.

Alkylation DNA damage in combination with PARP inhibition results in formation of S-phase-dependent double-strand breaks

The combination of poly(ADP-ribose)polymerase (PARP) inhibitors and alkylating agents is currently being investigated in cancer therapy clinical trials. However, the DNA lesions producing the synergistic cell killing effect in tumors are not fully understood. Treatment of human and mouse fibroblasts with the monofunctional DNA methylating agent methyl methanesulfonate (MMS) in the presence of a PARP inhibitor has been shown to trigger a cell cycle checkpoint response. Among other changes, this DNA damage response to combination treatment includes activation of ATM/Chk2 and phosphorylation of histone H2A.X. These changes are consistent with DNA double-strand break (DSB) formation during the response, but the measurement of DSBs has not been addressed. Such DSB evaluation is important in understanding this DNA damage response because events other than DSB formation are known to lead to ATM/Chk2 activation and H2A.X phosphorylation. Here, we examined the structural integrity of genomic DNA after the combined treatment of cells with MMS and a PARP inhibitor, i.e., exposure to a sub-lethal dose of MMS in the presence of the PARP inhibitor 4-amino-1,8-napthalimide (4-AN). We used pulsed field gel electrophoresis (PFGE) for measurement of DSBs in both human and mouse embryonic fibroblasts, and flow cytometry to follow the phosphorylated form of H2A.X (gamma-H2A.X). The results indicate that DSBs are formed with the combination treatment, but not following treatment with either agent alone. Our data also show that formation of gamma-H2A.X correlates with PARP-1-expressing cells in S-phase of the cell cycle. The observations support the model that persistence of PARP-1 at base excision repair intermediates, as cells move into S-phase, leads to DSBs and the attendant checkpoint responses.

PARP inhibition: PARP1 and beyond

Recent findings have thrust poly(ADP-ribose) polymerases (PARPs) into the limelight as potential chemotherapeutic targets. To provide a framework for understanding these recent observations, we review what is known about the structures and functions of the family of PARP enzymes, and then outline a series of questions that should be addressed to guide the rational development of PARP inhibitors as anticancer agents.

Oocyte numbers in the mouse increase after treatment with 5-aminoisoquinolinone: a potent inhibitor of poly(ADP-ribosyl)ation

Poly(ADP-ribosyl)ation is a posttranslational protein modification carried out by a family of enzymes referred to as poly(ADP-ribose) polymerases (PARPs). It has been proposed that the broad nuclear distribution of PARPs may allow them to modulate gene expression in addition to their more accepted role as DNA repair mediators. The role of poly(ADP-ribosyl)ation during oogenesis and folliculogenesis is unknown. Here we found that when 3- to 4-wk-old mice were injected with 5-amninoisoquinolinone, a water soluble inhibitor of poly(ADP-ribosyl)ation, it leads to considerably increased oocyte numbers and a dramatic increase in primordial follicle numbers. Furthermore, we show that inhibition of poly(ADP-ribosyl)ation leads to an increased expression of specific genes and pathways in mouse ovaries, in particular, transforming growth factor superfamily members. Our results demonstrate that poly(ADP-ribosyl)ation, is important in oogenesis and folliculogenesis, and it may have a differential role in regulating gene expression, DNA repair, and apoptosis. The novel function of poly(ADP-ribosyl)ation in oogenesis and folliculogenesis sheds light on the alternative role that DNA repair mediators may play in cellular development and differentiation.

PARP-3 is a mono-ADP-ribosylase that activates PARP-1 in the absence of DNA

The PARP-3 protein is closely related to the PARP-1 and PARP-2 proteins, which are involved in DNA repair and genome maintenance. Here, we characterized the biochemical properties of human PARP-3. PARP-3 is able to ADP-ribosylate itself as well as histone H1, a previously unknown substrate for PARP-3. PARP-3 is not activated upon binding to DNA and is a mono-ADP-ribosylase, in contrast to PARP-1 and PARP-2. PARP-3 interacts with PARP-1 and activates PARP-1 in the absence of DNA, resulting in synthesis of polymers of ADP-ribose. The N-terminal WGR domain of PARP-3 is involved in this activation. The functional interaction between PARP-3 and PARP-1 suggests that it may have a role in DNA repair. However, here we report that PARP-3 small interfering RNA-depleted cells are not sensitive to the topoisomerase I poison camptothecin, inducing DNA single-strand breaks, and repair these lesions as efficiently as wild-type cells. Altogether, these results suggest that the interaction between PARP-1 and PARP-3 is unrelated to DNA single-strand break repair.

Poly (ADP-ribose) polymerases (PARPs) 1-3 regulate astrocyte activation

Besides their traditional role in maintaining CNS homeostasis, astrocytes also participate in innate immune responses. Indeed, we have previously demonstrated that astrocytes are capable of recognizing bacterial pathogens such as Staphylococcus aureus, a common etiologic agent of CNS infections, and respond with the robust production of numerous proinflammatory mediators. Suppression of Poly (ADP-ribose) polymerase-1 (PARP-1), a DNA repair enzyme, has been shown to attenuate inflammatory responses in several cell types including mixed glial cultures. However, a role for PARP-1 in regulating innate immune responses in purified astrocytes and the potential for multiple PARP family members to cooperatively regulate astrocyte activation has not yet been examined. The synthetic PARP-1 inhibitor PJ-34 attenuated the production of several proinflammatory mediators by astrocytes in response to S. aureus stimulation including nitric oxide, interleukin-1 beta, tumor necrosis factor-alpha, and CCL2. The release of all four mediators was partially reduced in PARP-1 knockout (KO) astrocytes compared to wild-type cells. The residual inflammatory mediator expression detected in PARP-1 KO astrocytes was further blocked with PJ-34, suggesting either non-specific effects of the drug or actions on alternative PARP isoforms. Reduction in PARP-2 or PARP-3 expression by siRNA knock down revealed that these isoforms also contributed to inflammatory mediator regulation in response to S. aureus. Interestingly, the combined targeting of either PARP-1/PARP-2 or PARP-2/PARP-3 attenuated astrocyte inflammatory responses more effectively compared to knock down of either PARP alone, suggesting cooperativity between PARP isoforms. Collectively, these findings suggest that PARPs influence the extent of S. aureus-induced astrocyte activation.

Nuclear ADP-ribosylation reactions in mammalian cells: where are we today and where are we going?

Since poly-ADP ribose was discovered over 40 years ago, there has been significant progress in research into the biology of mono- and poly-ADP-ribosylation reactions. During the last decade, it became clear that ADP-ribosylation reactions play important roles in a wide range of physiological and pathophysiological processes, including inter- and intracellular signaling, transcriptional regulation, DNA repair pathways and maintenance of genomic stability, telomere dynamics, cell differentiation and proliferation, and necrosis and apoptosis. ADP-ribosylation reactions are phylogenetically ancient and can be classified into four major groups: mono-ADP-ribosylation, poly-ADP-ribosylation, ADP-ribose cyclization, and formation of O-acetyl-ADP-ribose. In the human genome, more than 30 different genes coding for enzymes associated with distinct ADP-ribosylation activities have been identified. This review highlights the recent advances in the rapidly growing field of nuclear mono-ADP-ribosylation and poly-ADP-ribosylation reactions and the distinct ADP-ribosylating enzyme families involved in these processes, including the proposed family of novel poly-ADP-ribose polymerase-like mono-ADP-ribose transferases and the potential mono-ADP-ribosylation activities of the sirtuin family of NAD(+)-dependent histone deacetylases. A special focus is placed on the known roles of distinct mono- and poly-ADP-ribosylation reactions in physiological processes, such as mitosis, cellular differentiation and proliferation, telomere dynamics, and aging, as well as "programmed necrosis" (i.e., high-mobility-group protein B1 release) and apoptosis (i.e., apoptosis-inducing factor shuttling). The proposed molecular mechanisms involved in these processes, such as signaling, chromatin modification (i.e., "histone code"), and remodeling of chromatin structure (i.e., DNA damage response, transcriptional regulation, and insulator function), are described. A potential cross talk between nuclear ADP-ribosylation processes and other NAD(+)-dependent pathways is discussed.

Down-modulation of poly(ADP-ribose) polymerase-1 (PARP-1) in human TUR leukemia cells restores transcriptional responsiveness for differentiation and cell cycle arrest

Aggressive tumor developing human TUR myeloid leukemia cells continued cell cycle progression in the presence of the differentiation-inducing phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA). Similar results were obtained after stable transfection of TUR cells with the pTracer control vector (pTracer TUR cells). In contrast, TUR transfectants containing a constitutively active poly(ADP-ribose) polymerase-1 (PARP-1) gene fragment in antisense orientation within the pTracer vector (asPARP TUR cells) demonstrated increasing cell attachment and differentiation after TPA treatment. Moreover, asPARP TUR cells ceased to divide upon TPA stimulation. Cell cycle analysis revealed a predominant G0/G1 arrest and a partial G2/M arrest in TPA-treated asPARP TUR cells, whereas little if any population was detectable in S phase. Microarray gene expression analysis exhibited a significant down-regulation of cell cycle genes in phorbol ester-stimulated asPARP TUR and markedly elevated levels of differentiation-associated factors in contrast to TPA-incubated wild-type TUR cells. Whereas PARP-1 can associate with the 20S proteasome in leukemia cells, a significant reduction of this proteolytic activity was observed in asPARP TUR cells. Conversely, protein levels of manganese superoxide dismutase and the matrix metalloproteinases MMP-1 and MMP-9 were progressively increased in TPA-treated asPARP TUR cells, respectively. These findings underscore an important function of PARP-1 in human leukemia cells to connect cell cycle progression and control of differentiation.

Biphasic changes in the levels of poly(ADP-ribose) polymerase-1 and caspase 3 in the immature brain following hypoxia-ischemia

Poly(ADP-ribose) polymerase-1 (PARP-1) is a DNA repair-associated enzyme that has multiple roles in cell death. This study examined the involvement of PARP-1 in ischemic brain injury in the 7-day old rat, 0.5-48 h after unilateral carotid artery ligation and 2 h of 7.8% oxygen. This experimental paradigm produced a mild to moderate injury; 40-67% of animals in the ligated groups had histological evidence of neuronal death. Ipsilateral cortical injury was seen at all survival times, while mild contralateral cortical injury was seen only at the 1h survival time. Hippocampal injury was delayed relative to the cortex and did not show a biphasic pattern. Immunohistochemical staining for PARP showed bilateral increased staining as early as 1 h post-hypoxia. PARP staining at early time periods was most intense in layer V of cortex, but did not demonstrate a pattern of cell clusters or columns. Ipsilateral PARP-1 levels quantified by western blotting showed a biphasic pattern of elevation with peaks at 0.5 and 12 h post-hypoxia. Contralateral PARP-1 levels were also elevated at 0.5 and 24 h. PARP activity as determined by immunoreactivity for poly(ADP-ribose) (PAR) was increased ipsilaterally at 0.5, 2 and 12 h survival times. Cortical caspase 3-activity was increased ipsilaterally at 6, 12, and 24 h and contralaterally at 0.5, 1, 2 and 6 h post-hypoxia. There are three main findings in this study. First, changes in the distribution and amount of cell death correlate well with measured PARP-1 levels after hypoxia-ischemia, and both display biphasic characteristics. Second, there are significant early, transient morphological and biochemical changes in the contralateral cortex after neonatal hypoxia-ischemia due to unilateral permanent occlusion of a carotid artery followed by 2 h of systemic hypoxia. Third, variability in the responses of individual pups to hypoxia-ischemia suggests the presence of unidentified confounding factors.

Beneficial effects of PJ34 and INO-1001, two novel water-soluble poly(ADP-ribose) polymerase inhibitors, on the consequences of traumatic brain injury in rat

Traumatic brain injury produces peroxynitrite, a powerful oxidant which triggers DNA strand breaks, leading to the activation of poly(ADP-ribose)polymerase-1 (PARP-1). We previously demonstrated that 3-aminobenzamide, a PARP inhibitor, is neuroprotective in a model of traumatic brain injury induced by fluid percussion in rat, suggesting that PARP-1 could be a therapeutic target. In order to confirm this hypothesis, we investigated the effects of PJ34 and INO-1001, two PARP inhibitors from structural classes other than benzamide, on the post-traumatic consequences. Pre- and post-treatments with PJ34 (30 mg/kg/day) and INO-1001 (10 mg/kg/day) decrease the neurological deficit at 3 days post-injury and this deficit is still reduced at 7 days. These neurological recovery-promoting effects are associated with the inhibition of PARP-1 activation caused by trauma, as demonstrated by abolishment of immunostaining of poly(ADP-ribose). Thus, the present work strengthens strongly the concept that PARP-1 inhibition may be a suitable approach for the treatment of brain trauma.

Co-localization of poly(ADPR)polymerase 1 (PARP-1) poly(ADPR)polymerase 2 (PARP-2) and related proteins in rat testis nuclear matrix defined by chemical cross-linking

Poly(ADPR)polymerase 1 and 2 (PARP-1, PARP-2) are nuclear enzymes which function is based on specific interactions with DNA and nuclear proteins. PARPs targets include proteins involved in DNA replication, repair, and transcription and their function can be modulated either by protein-protein interaction with native PARP-1 and by non-covalent interaction with poly(ADP-ribose) (pADPR) linked to the auto-modified PARP-1. Moreover, the association of pADPR and PARP-1 with the nuclear matrix (NM) has been reported, based on the poly(ADP-ribosyl)ation of nuclear matrix proteins (NMPs). In the present article, by the use of DNA and protein cross-linking reactions, by cis-diamminedichloroplatinum II (cDDP) and sodium tetrathionate (NaTT) respectively, we present more evidences about the association of PARP-1, PARP-2, and PARPs related proteins with the NM. Our findings confirmed that NM could be seen as a fraction greatly enriched in transcription factors (i.e., C/EBP-beta) and enzymes (DNA Topo II, DNA PK) that co-localize with PARP-1 and -2 at the matrix associated regions (MARs) of chromatin. Moreover, pADPR contributes to PARP-1 localization at the NM, showing that PARP(s) activity co-operates to the functions of this nuclear fraction.

Apoptosis-inducing factor substitutes for caspase executioners in NMDA-triggered excitotoxic neuronal death

The profound neuroprotection observed in poly(ADP-ribose) polymerase-1 (PARP-1) null mice to ischemic and excitotoxic injury positions PARP-1 as a major mediator of neuronal cell death. We report here that apoptosis-inducing factor (AIF) mediates PARP-1-dependent glutamate excitotoxicity in a caspase-independent manner after translocation from the mitochondria to the nucleus. In primary murine cortical cultures, neurotoxic NMDA exposure triggers AIF translocation, mitochondrial membrane depolarization, and phosphatidyl serine exposure on the cell surface, which precedes cytochrome c release and caspase activation. NMDA neurotoxicity is not affected by broad-spectrum caspase inhibitors, but it is prevented by Bcl-2 overexpression and a neutralizing antibody to AIF. These results link PARP-1 activation with AIF translocation in NMDA-triggered excitotoxic neuronal death and provide a paradigm in which AIF can substitute for caspase executioners.

Poly(ADP-ribosyl)ation regulates CTCF-dependent chromatin insulation

Chromatin insulators demarcate expression domains by blocking the cis effects of enhancers or silencers in a position-dependent manner. We show that the chromatin insulator protein CTCF carries a post-translational modification: poly(ADP-ribosyl)ation. Chromatin immunoprecipitation analysis showed that a poly(ADP-ribosyl)ation mark, which exclusively segregates with the maternal allele of the insulator domain in the H19 imprinting control region, requires the bases that are essential for interaction with CTCF. Chromatin immunoprecipitation-on-chip analysis documented that the link between CTCF and poly(ADP-ribosyl)ation extended to more than 140 mouse CTCF target sites. An insulator trap assay showed that the insulator function of most of these CTCF target sites is sensitive to 3-aminobenzamide, an inhibitor of poly(ADP-ribose) polymerase activity. We suggest that poly(ADP-ribosyl)ation imparts chromatin insulator properties to CTCF at both imprinted and nonimprinted loci, which has implications for the regulation of expression domains and their demise in pathological lesions.

Individual expression of poliovirus 2Apro and 3Cpro induces activation of caspase-3 and PARP cleavage in HeLa cells

The expression of individual viral genes enables the study of their effects on cellular functions. Our group previously generated stable HeLa cell lines that efficiently express poliovirus proteases 2A (clone 2A7d) and 3C (clone 3C7) under the control of tetracycline [Virology 266 (2000a) 352; J. Virol. 74 (2000b) 2383]. Upon induction of these proteases, the cells undergo drastic morphological alterations and eventually die. The present paper characterizes, in detail, the cellular and molecular events that lead to cell death in these lines. Several signs of apoptosis were observed in both 2A7d- and 3C7-induced cells, such as nuclear fragmentation, DNA breakdown (as determined by TUNEL), and phosphatidylserine translocation. Protease 2A induces the cleavage of poly-ADP-ribose-polymerase (PARP). This is blocked by the caspase-3 inhibitor DEVD in both 2A7d-On and 3C7-On cells suggesting that this enzyme might account for PARP cleavage in both cell lines. The results indicate that both poliovirus proteases induce apoptosis by mechanisms involving caspase activation, although the kinetics of apoptosis differs.

Inhibition of poly(ADP-ribose) polymerase prevents allergen-induced asthma-like reaction in sensitized Guinea pigs

Poly(ADP-ribose) polymerase (PARP) plays an important role in tissue injury in conditions associated with oxidative stress and inflammation. Because asthma is a chronic inflammatory disorder of the airways, we designed the present experimental study to evaluate the effects of PARP inhibition on allergen-induced asthma-like reaction in ovalbumin-sensitized guinea pigs. Cough and dyspnea in response to ovalbumin aerosol were absent in naive guinea pigs, whereas they became severe in the sensitized animals. In the latter ones, ovalbumin aerosol also induced a rapid increase in PARP activity, bronchiolar constriction, pulmonary air space inflation, mast cell degranulation, poly(ADP-ribose) and nitrotyrosine immunostaining, myeloperoxidase activity, and malondialdehyde in lung tissue, as well as a rise in the amounts of nitrites and tumor necrosis factor-alpha in bronchoalveolar lavage fluid. Pretreatment with the PARP inhibitors 3-aminobenzamide (10 mg/kg b.wt.) or 5-aminoisoquinolinone (0.5 mg/kg b.wt.) given i.p. 3 h before ovalbumin challenge significantly reduced the severity of cough and the occurrence of dyspnea and delayed the onset of respiratory abnormalities. Both PARP inhibitors were also able to prevent the above morphological and biochemical changes of lung tissue or bronchoalveolar lavage fluid induced by ovalbumin challenge. Conversely, p-aminobenzoic acid, the inactive analog of 3-aminobenzamide, had no effects.

Poly(ADP-ribose) and carcinogenesis

Poly(ADP-ribose) and poly(ADP-ribose) polymerase (PARP) were discovered about 40 years ago, but their significance was not well elucidated until recently. In the early stage of the history of PARP, the presence of antibodies in the sera of human patients with lupus erythematosus indicated its natural occurrence. PARP, as well as the degrading enzyme, poly(ADP-ribose) glycohydrolase (PARG), are present in most eukaryotes except for yeasts. Studies that used inhibitors of PARP indicated the involvement of PARP and poly(ADP-ribose) in DNA damage repair, and eventually PARP was purified and the gene was cloned. Molecular analysis then revealed various functional domains, such as the one for binding to strand breaks of DNA. Parp-1-deficient and Parg-deficient cells showed, in general, enhanced sensitivity to the lethal effects of ionizing radiation and alkylating agents. Parp-1 knockout mouse embryonic stem cells developed into teratocarcinoma-like tumors when injected subcutaneously into nude mice, these tumors featuring giant cells similar to syncytiotrophoblastic giant cells with hyperploidy. Parp-1 was also found in centrosomes, suggesting that poly(ADP-ribose) and PARP-1 are functionally involved in the maintenance of chromatin structure and the equal distribution of chromosomes into daughter cells. Intriguing findings on the real biological significance continue to be generated, with new light shed on mechanisms of carcinogenesis and pointing to novel cancer treatments. Highlights during the last four decades of studies by laboratories focusing on poly(ADP-ribose)/PARP, including our own, are condensed and summarized in this review.

FGF2-mediated upregulation of urokinase-type plasminogen activator expression requires a MAP-kinase dependent activation of poly(ADP-ribose) polymerase

Poly(ADP-ribosyl)ation is a post-translational modification of protein occurring in the nucleus by poly(ADP-ribose) polymerase enzyme activity. The main role of poly(ADP-ribose) polymerase system as "nick sensor" and DNA breaks repair is based on its activation via DNA strand breaks. Furthermore, poly(ADP-ribose) polymerase modifies the binding to DNA of several transcriptional factors by poly(ADP-ribosyl)ation, thereby regulating also transcriptional gene expression. We have analyzed whether poly(ADP-ribose) polymerase activity is involved in basic fibroblast growth factor (FGF2)-mediated upregulation of urokinase-type plasminogen activator (uPA) mRNA. We demonstrated that specific inhibition of poly(ADP-ribose) polymerase activity via 3-aminobenzamide (3ABA) or NAD+ deprivation prevents FGF2-mediated uPA mRNA over-expression and cell-associated plasminogen activator (PA) production in GM7373 endothelial cell line. We verified that FGF2 stimulates poly(ADP-ribose) polymerase activity by a DNA strand breaks-independent manner which involves a mitogen-activated protein kinases (MAPK)-dependent pathway, as confirmed by using PD98059 inhibitor and anisomycin stimulation. Poly(ADP-ribose) polymerase involved in this mechanism is mainly the 60 kDa molecular mass isoform, that presents an increase in serine phosphorylation in the presence of FGF2.

Involvement of poly ADP ribosyl polymerase-1 in acute but not chronic zinc toxicity

We have previously suggested that zinc-induced neuronal death may be mediated in part by inhibition of the glycolytic enzyme glyceraldehyde-3-phosphate dehydrogenase (GAPDH), secondary to depletion of the essential cosubstrate NAD+. Given convergent evidence implicating the NAD+-catabolizing enzyme, poly ADP ribosyl polymerase (PARP) in mediating ATP depletion and neuronal death after excitotoxic and ischemic insults, we tested the specific hypothesis that the neuronal death induced by exposure to toxic levels of extracellular zinc might be partly mediated by PARP. PARP was activated in cultured mouse cortical astrocytes after a toxic acute Zn2+ exposure (350 microm Zn2+ for 15 min), but not in cortical neurons or glia after exposure to a toxic chronic Zn2+ exposure (40 microm Zn2+ for 1-4 h), an exposure sufficient to deplete NAD+ and ATP levels. Furthermore, the neurotoxicity induced by acute, but not chronic, Zn2+ exposure was reduced in mixed neuronal-glial cultures prepared from mutant mice lacking the PARP gene. These data suggest PARP activation may contribute to more fulminant forms of Zn2+-induced neuronal death.

Deleterious poly(ADP-ribose)polymerase-1 pathway activation in traumatic brain injury in rat

Traumatic brain injury produces nitric oxide and reactive oxygen species. Peroxynitrite, resulting from the combination of nitric oxide and superoxide anions, triggers DNA strand breaks, leading to the activation of poly(ADP-ribose)polymerase-1. As excessive activation of this enzyme induces cell death, we examined the production of nitrosative stress, the activation of poly(ADP-ribose)polymerase-1, and the role of this enzyme in the outcomes of traumatic brain injury produced by fluid percussion in rats. Immunohistochemistry showed that 3-nitrotyrosine, an indicator of nitrosative stress, and poly(ADP-ribose), a marker of poly(ADP-ribose)polymerase-1 activation, were present as early as 30 min post-injury, and that persisted for 72 h. The poly(ADP-ribose)polymerase inhibitor, 3-aminobenzamide, at 10 and 30 mg/kg, significantly improved the neurological deficit, with a 60% reduction in the brain lesion volume and inhibition of poly(ADP-ribose)polymerase-1 activation. Thus, poly(ADP-ribose)polymerase-1 is involved in the neurological consequences of traumatic brain injury and may be a promising therapeutic target in clinical treatment of acute brain trauma.

Is poly(ADP-ribose) polymerase involved in bovine placental retention?

Poly(ADP-ribose) polymerase (PARP) is the enzyme which utilises NAD to synthesise poly(ADP-ribose) polymers. This process appears in response to DNA lesions. Oxidative stress, which might be involved in bovine placental retention, is the reason for oxidative DNA injury. In this mini-review, the relationship between PARP activity and bovine placental retention is discussed. The results of our experiments on PARP activity in placental tissues showed that the enzyme of 113 kDa and its cleavage products were present in retained as well as released fetal membranes. Western blotting technique showed different intensities in the staining of bands which might suggest different activities of the enzyme.

Poly(ADPR) polymerase-1 and poly(ADPR) glycohydrolase level and distribution in differentiating rat germinal cells

Poly(ADP-ribose)polymerase (PARP-1) and poly(ADP-ribose)glycohydrolase (PARG) are responsible for the transient poly(ADP-ribosyl)ation of proteins in eukaryotic cells. This biochemical reaction plays an active role in DNA replication and repair, transcription, cell differentiation and death. The aim of this study was to investigate the levels and the sub-cellular distribution of such enzymes in rat germinal cells at different stages of differentiation, i.e. in primary spermatocytes and round spermatids, representing meiotic and post-meiotic cells, respectively. The determination of the level of PARP-1 mRNA and protein revealed its higher expression in primary spermatocytes, thus implying that PARP-1 is one of the meiotic genes whose expression is requested at the pachytene phase of the meiosis. We also demonstrated that rat germinal cells contain both the forms of PARG (i.e. of 110 and 60 kDa) so far described in somatic cells. In our experimental system, the large PARG was present and active mainly in the nuclear fraction of primary spermatocytes, whereas round spermatids showed a higher level of the 60 kDa PARG in the post-nuclear fraction. Collectively, our data show a different expression level of PARP-1 and a different endocellular distribution of PARG and suggest a role for the poly(ADP-ribose) turnover in distinct pathways in meiotic and post-meiotic germinal cells.

PARP-1, a determinant of cell survival in response to DNA damage

Poly(ADP-ribose) polymerase-1 (PARP-1) plays a primary role in the process of poly(ADP-ribosyl)ation. This posttranslational modification of nuclear proteins is activated in response to DNA damage. Having been studied for more than 30 years, PARP-1 is now known to be implicated in several crucial cellular processes: DNA replication, transcription, DNA repair, apoptosis, and genome stability. In this review, we focus on recent findings suggesting that PARP-1 participates in DNA damage signaling in cell death. Of clinical relevance is its role in cancer therapy, irradiation, and chemotherapy, all of which may cause DNA damage and overactivate PARP-1, resulting in inflammation caused by necrosis. Therefore, we will discuss how inhibition of PARP-1 may enhance the efficiency of cancer therapy.

Poly(ADP-ribose) polymerase-1 is a positive regulator of the p53-mediated G1 arrest response following ionizing radiation

Poly(ADP-ribose) polymerase-1 (PARP-1) and the p53 tumor suppressor protein are both involved in the cellular response to genotoxic stress. Upon binding to the site of DNA strand breakage, PARP-1 is activated, leading to rapid and transient poly(ADP-ribosyl)ation of nuclear proteins using NAD+ as substrate. To investigate the role of PARP-1 in the p53 response to ionizing radiation in human cells, PARP-1 function was disrupted in wild-type p53 expressing MCF-7 and BJ/TERT cells using two strategies: chemical inhibition with 1,5-dihydroxyisoquinoline, and trans-dominant inhibition by overexpression of the PARP-1 DNA-binding domain. Although a number of proteins can catalyze poly(ADP-ribosyl)ation in addition to PARP-1, we show that PARP-1 is the only detectable active species in BJ/TERT and MCF-7 cells. 1,5-Dihydroxyisoquinoline treatment prior to ionizing radiation delayed and attenuated the induction of two p53-responsive genes, p21 and mdm-2, and led to suppression of the p53-mediated G1-arrest response in MCF-7 and BJ/TERT cells. Trans-dominant inhibition of PARP-1 by overexpression of the PARP-1 DNA-binding domain in MCF-7 cells also led to a delay and attenuation in p21 induction and suppression of the p53-mediated G1 arrest response to ionizing radiation. Hence, inhibition of endogenous PARP-1 function suppresses the transactivation function of p53 in response to ionizing radiation. This study establishes PARP-1 as a critical regulator of the p53 response to DNA damage.

Poly(ADP-ribose) polymerase-1 regulates activation of activator protein-1 in murine fibroblasts

Poly(ADP-ribose) polymerase (PARP)-1 is activated in response to DNA injury in the nucleus of eukaryotic cells and has been implicated in cell dysfunction in inflammation. We investigated the role of PARP-1 on the AP-1 pathway, which is involved in the signal transduction of the inflammatory process. In murine wild-type fibroblasts, oxidative challenge by peroxynitrite and hydrogen peroxide or immunological challenge by IL-1 and 20% FCS induced phosphorylation of the mitogen-activated protein kinase kinase-4, activation of c-Jun N-terminal kinase (JNK), and DNA binding of AP-1. In comparative experiments, peroxynitrite induced DNA binding of heat shock factor-1. Pretreatment of wild-type cells with 5-iodo-6-amino-1,2-benzopyrone, a PARP-1 inhibitor, inhibited JNK activation and DNA binding of AP-1. In parallel experiments in PARP-1-deficient fibroblasts, DNA binding of AP-1 was completely abolished. Activation of JNK was significantly elevated at basal condition, but it exhibited a lesser increase after oxidative or immunological challenge than in wild-type fibroblasts. Nuclear content of phosphorylated mitogen-activated protein kinase kinase-4 was observed in PARP-1-deficient cells after peroxynitrite challenge only. Western blotting analysis for AP-1 subunits indicated that c-Fos was similarly expressed in wild-type and PARP-1-deficient cells. Phosphorylated c-Jun was expressed after oxidative or immunological challenge, but not in basal condition, in wild-type cells; however, it was significantly elevated at basal condition and further enhanced after oxidative or immunological challenge in PARP-1-deficient cells. No DNA binding of heat shock factor-1 was observed in PARP-1-deficient cells. These data demonstrate that PARP-1 plays a pivotal role in the modulation of transcription.

The genes pme-1 and pme-2 encode two poly(ADP-ribose) polymerases in Caenorhabditis elegans

Poly(ADP-ribose) polymerases (PARPs) are an expanding, well-conserved family of enzymes found in many metazoan species, including plants. The enzyme catalyses poly(ADP-ribosyl)ation, a post-translational modification that is important in DNA repair and programmed cell death. In the present study, we report the finding of an endogenous source of poly(ADP-ribosyl)ation in total extracts of the nematode Caenorhabditis elegans. Two cDNAs encoding highly similar proteins to human PARP-1 (huPARP-1) and huPARP-2 are described, and we propose to name the corresponding enzymes poly(ADP-ribose) metabolism enzyme 1 (PME-1) and PME-2 respectively. PME-1 (108 kDa) shares 31% identity with huPARP-1 and has an overall structure similar to other PARP-1 subfamily members. It contains sequences having considerable similarity to zinc-finger motifs I and II, as well as with the catalytic domain of huPARP-1. PME-2 (61 kDa) has structural similarities with the catalytic domain of PARPs in general and shares 24% identity with huPARP-2. Recombinant PME-1 and PME-2 display PARP activity, which may partially account for the similar activity found in the worm. A partial duplication of the pme-1 gene with pseudogene-like features was found in the nematode genome. Messenger RNA for pme-1 are 5'-tagged with splice leader 1, whereas those for pme - 2 are tagged with splice leader 2, suggesting an operon-like expression for pme - 2. The expression pattern of pme-1 and pme-2 is also developmentally regulated. Together, these results show that PARP-1 and -2 are conserved in evolution and must have important functions in multicellular organisms. We propose using C. elegans as a model to understand better the functions of these enzymes.

Functional characterization of the poly(ADP-ribose) polymerase activity of tankyrase 1, a potential regulator of telomere length

Poly(ADP-ribose) polymerases (PARPs) comprise a growing family of enzymes known to be involved in genotoxic signaling and metabolic regulation. One of the latest family members, tankyrase 1, was shown to be involved in maintenance of telomere integrity. Here we expressed full-length tankyrase 1 and a fragment, termed T-PARP, spanning the poly(ADP-ribose) polymerase domain and characterized the enzymatic properties of the two proteins. Both, tankyrase 1 and T-PARP catalyze an auto poly(ADP-ribosyl)ation reaction with comparable catalytic activity. In contrast, (ADP-ribosyl)ation of TRF1, a previously described substrate, is strongly performed only by the full-length enzyme but not by T-PARP. Characterization of the poly(ADP-ribose) products reveals that tankyrase 1 synthesizes polymers with an average chain length of 20 units and no detectable branching of the polymers. Finally, we show that the catalytic efficiency of tankyrase 1, as expressed by the k(cat)/K(m) value, is approximately 150-fold lower compared to the basal activity of the poly(ADP-ribose) polymerase, PARP 1.

Poly(ADP-ribose) polymerase: killer or conspirator? The 'suicide hypothesis' revisited

Poly(ADP-ribose) polymerase 1 (PARP-1) is an abundant nuclear enzyme involved in DNA repair. The therapeutic efficacy of drugs that inhibit PARP-1 in various disorders underscores the active role of PARP-1 in cell death. Although it is well established that excessive DNA damage causes PARP-1 hyperactivation, which leads to cell death by energy failure, a new mechanistic perspective is emerging following the identification of various PARPs that exhibit different features and subcellular distributions. Studies demonstrating the significant role of PARP-1 in the regulation of gene transcription have further increased the intricacy of poly(ADP-ribosyl)ation in the control of cell homeostasis and challenge the notion that energy collapse is the sole mechanism by which poly(ADP-ribose) formation contributes to cell death. The hypothesis that PARPs might regulate cell fate as essential modulators of death and survival transcriptional programs will be discussed with particular focus on the regulation of transcription factors such as nuclear factor kappaB and p53. (An animation depicting the involvement of PARP-1 in the 'suicide hypothesis' is available at http://archive.bmn.com/supp/tips/tips2303a.html)

Activation and caspase-mediated inhibition of PARP: a molecular switch between fibroblast necrosis and apoptosis in death receptor signaling

Death ligands not only induce apoptosis but can also trigger necrosis with distinct biochemical and morphological features. We recently showed that in L929 cells CD95 ligation induces apoptosis, whereas TNF elicits necrosis. Treatment with anti-CD95 resulted in typical apoptosis characterized by caspase activation and DNA fragmentation. These events were barely induced by TNF, although TNF triggered cell death to a similar extent as CD95. Surprisingly, whereas the caspase inhibitor zVAD prevented CD95-mediated apoptosis, it potentiated TNF-induced necrosis. Cotreatment with TNF and zVAD was characterized by ATP depletion and accelerated necrosis. To investigate the mechanisms underlying TNF-induced cell death and its potentiation by zVAD, we examined the role of poly(ADP-ribose)polymerase-1 (PARP-1). TNF but not CD95 mediated PARP activation, whereas a PARP inhibitor suppressed TNF-induced necrosis and the sensitizing effect of zVAD. In addition, fibroblasts expressing a noncleavable PARP-1 mutant were more sensitive to TNF than wild-type cells. Our results indicate that TNF induces PARP activation leading to ATP depletion and subsequent necrosis. In contrast, in CD95-mediated apoptosis caspases cause PARP-1 cleavage and thereby maintain ATP levels. Because ATP is required for apoptosis, we suggest that PARP-1 cleavage functions as a molecular switch between apoptotic and necrotic modes of death receptor-induced cell death.

Potential clinical applications of poly(ADP-ribose) polymerase (PARP) inhibitors

Poly(ADP-ribose) polymerases (PARPs) are defined as cell signaling enzymes that catalyze the transfer of ADP-ribose units from NAD(+)to a number of acceptor proteins. PARP-1, the best characterized member of the PARP family, that presently includes six members, is an abundant nuclear enzyme implicated in cellular responses to DNA injury provoked by genotoxic stress (oxygen radicals, ionizing radiations and monofunctional alkylating agents). Due to its involvement either in DNA repair or in cell death, PARP-1 is regarded as a double-edged regulator of cellular functions. In fact, when the DNA damage is moderate, PARP-1 participates in the DNA repair process. Conversely, in the case of massive DNA injury, elevated PARP-1 activation leads to rapid NAD(+)/ATP consumption and cell death by necrosis. Excessive PARP-1 activity has been implicated in the pathogenesis of numerous clinical conditions such as stroke, myocardial infarction, shock, diabetes and neurodegenerative disorders. PARP-1 could therefore be considered as a potential target for the development of pharmacological strategies to enhance the antitumor efficacy of radio- and chemotherapy or to treat a number of clinical conditions characterized by oxidative or NO-induced stress and consequent PARP-1 activation. Moreover, the discovery of novel functions for the multiple members of the PARP family might lead in the future to additional clinical indications for PARP inhibitors.

Poly(ADP-ribose) polymerase: a guardian angel protecting the genome and suppressing tumorigenesis

Poly(ADP-ribosyl)ation is an immediate cellular response to DNA damage generated either exogenously or endogenously. This post-translational modification is catalyzed by poly(ADP-ribose) polymerase (PARP, PARP-1, EC 2.4.2.30). It is proposed that this protein plays a multifunctional role in many cellular processes, including DNA repair, recombination, cell proliferation and death, as well as genomic stability. Chemical inhibitors of the enzyme, dominant negative or null mutations of PARP-1 cause a high degree of genomic instability in cells. Inhibition of PARP activity by chemical inhibitors renders mice or rats susceptible to carcinogenic agents in various tumor models, indicating a role for PARP-1 in suppressing tumorigenesis. Despite the above observations, PARP-1 knockout mice are generally not prone to the development of tumors. An enhanced tumor development was observed, however, when the PARP-1 null mutation was introduced into severely compromised immune-deficient mice (a mutation in DNA-dependent protein kinase) or mice lacking other DNA repair or chromosomal guardian molecules, such as p53 or Ku80. These studies indicate that PARP-1 functions as a cofactor to suppress tumorigenesis via its role in stabilization of the genome, and/or by interacting with other DNA strand break-sensing molecules. Studies using PARP-1 mutants and chemical inhibitors have started to shed light on the role of this protein and of the specific protein post-translational modification in the control of genomic stability and hence its involvement in cancer.

Poly(ADP-ribose) polymerase cleavage during apoptosis: when and where?

Poly(ADP-ribose) polymerase-1 (PARP-1) plays the active role of "nick sensor" during DNA repair and apoptosis, when it synthesizes ADP-ribose from NAD(+) in the presence of DNA strand breaks. Moreover, PARP-1 becomes a target of apoptotic caspases, which originate two proteolytic fragments of 89 and 24 kDa. The precise relationship between PARP-1 activation and degradation during apoptosis is still a matter of debate. In human Hep-2 cells driven to apoptosis by actinomycin D, we have monitored PARP-1 activity by the mAb 10H, which is specific for the ADP-ribose polymers, and we have observed that poly(ADP-ribose) synthesis is a very early response to the apoptotic stimulus. The analysis of the presence and fate of the p89 proteolytic fragment revealed that PARP-1 proteolysis by caspases is concomitant with poly(ADP-ribose) synthesis and that p89 migrates from the nucleus into the cytoplasm in late apoptotic cells with advanced nuclear fragmentation.

Physiology and pathophysiology of poly(ADP-ribosyl)ation

One of the immediate eukaryotic cellular responses to DNA breakage is the covalent post-translational modification of nuclear proteins with poly(ADP-ribose) from NAD+ as precursor, mostly catalysed by poly(ADP-ribose) polymerase-1 (PARP-1). Recently several other polypeptides have been shown to catalyse poly(ADP-ribose) formation. Poly(ADP-ribosyl)ation is involved in a variety of physiological and pathophysiological phenomena. Physiological functions include its participation in DNA-base excision repair, DNA-damage signalling, regulation of genomic stability, and regulation of transcription and proteasomal function, supporting the previously observed correlation of cellular poly(ADP-ribosyl)ation capacity with mammalian life. The pathophysiology effects are mediated through PARP-1 overactivity, which can cause cell suicide by NAD+ depletion. It is apparent that the latter effect underlies the pathogenesis of a wide range of disease states including type-1 diabetes, ischaemic infarcts in various organs, and septic or haemorrhagic shock. Therefore pharmacological modulation of poly(ADP-ribosyl)ation may prove to be an exciting option for various highly prevalent, disabling and even lethal diseases.

Importance of poly(ADP-ribose) glycohydrolase in the control of poly(ADP-ribose) metabolism

Poly(ADP-ribosyl)ation is a posttranslational modification that alters the functions of the acceptor proteins and is catalyzed by the poly(ADP-ribose) polymerase (PARP) family of enzymes. Following DNA damage, activated poly(ADP-ribose) polymerase-1 (PARP-1) catalyzes the elongation and branching of poly(ADP-ribose) (pADPr) covalently attached to nuclear target proteins. Although the biological role of poly(ADP-ribosyl)ation has not yet been defined, it has been implicated in many important cellular processes such as DNA repair and replication, modulation of chromatin structure, and apoptosis. The transient nature and modulation of poly(ADP-ribosyl)ation depend on the activity of a unique cytoplasmic enzyme called poly(ADP-ribose) glycohydrolase which hydrolyzes pADPr bound to acceptor proteins in free ADP-ribose residues. While the PARP homologues have been recently reviewed, there are relatively scarce data about PARG in the literature. Here we summarize the latest advances in the PARG field, addressing the question of its putative nucleo-cytoplasmic shuttling that could enable the tight regulation of pADPr metabolism. This would contribute to the elucidation of the biological significance of poly(ADP-ribosyl)ation.

Photoaffinity labeling of mouse fibroblast enzymes by a base excision repair intermediate. Evidence for the role of poly(ADP-ribose) polymerase-1 in DNA repair

To examine the interaction of mammalian base excision repair (BER) enzymes with DNA intermediates formed during BER, we used a novel photoaffinity labeling probe and mouse embryonic fibroblast cellular extracts. The probe was formed in situ, using an end-labeled oligonucleotide containing a synthetic abasic site; this site was incised by apurinic/apyrimidinic endonuclease creating a nick with 3'-hydroxyl and 5'-reduced sugar phosphate groups at the margins, and then a dNMP carrying a photoreactive adduct was added to the 3'-hydroxyl group. With near-UV light (312 nm) exposure of the extract/probe mixture, six proteins were strongly labeled. Four of these include poly(ADP-ribose) polymerase-1 (PARP-1) and the BER participants flap endonuclease-1, DNA polymerase beta, and apurinic/apyrimidinic endonuclease. The amount of the probe cross-linked to PARP-1 was greater than that cross-linked to the other proteins. The specificity of PARP-1 labeling was examined using various competitor oligonucleotides and DNA probes with alternate structures. PARP-1 labeling was stronger with a DNA representing a BER intermediate than with a nick in double-stranded DNA. These results indicate that proteins interacting preferentially with a photoreactive BER intermediate can be selected from the crude cellular extract.

The world according to PARP

An immediate cellular response to DNA damage is the synthesis of poly(ADP-ribose) by the enzyme poly(ADP-ribose) polymerase (PARP). This nuclear enzyme and the unique post-translational modification it catalyzes have long been considered to function exclusively in cellular surveillance of genotoxic stress. The recent identification of multiple members of a PARP family might force a revision of this concept. The novel primary structures and subcellular localizations for some of these PARPs suggests new and unexpected roles for poly(ADP-ribosyl)ation in telomere replication and cellular transport.

Rat germinal cells require PARP for repair of DNA damage induced by gamma-irradiation and H2O2 treatment

The ability of rat germinal cells to recover from genotoxic stress has been investigated using isolated populations of primary spermatocytes and round spermatids. Using a comet assay at pH 10.0 to assess single strand breakage (SSB) in DNA, it was found that a high level of damage was induced by 5 Gy gamma-irradiation and acute exposure to 50 microM H2O2. This damage was effectively repaired during a subsequent recovery period of 1-3 hours culture in vitro but repair was significantly delayed in the presence of the poly(ADP-ribose)polymerase (PARP) inhibitor 3-aminobenzamide (3-ABA). Immunofluorescence detection of PARP with specific antibodies localised the protein to discrete foci within the nucleus of both spermatocytes and spermatids. Poly(ADP-ribose) (pADPR) could also be detected in spermatid nuclei following gamma-irradiation or H2O2 treatment. Moreover, PARP activation occurs both in spermatocytes and spermatids left to recover after both genotoxic stresses. The NO donors, 3-morpholino-sydnonimine (SIN-1) and S-nitrosoglutathione (SNOG), caused significant SSBs in both spermatocytes and spermatids. The effects of SIN-1 could be prevented by exogenous catalase (CAT), but not superoxide dismutase (SOD), in the cell suspensions. SNOG-induced SSBs were insensitive to both CAT and SOD. It is concluded that DNA in spermatocytes and spermatids is sensitive to damage by gamma-irradiation and H2O2 and that efficient repair of SSBs requires PARP activity.

Poly(APD-ribosyl)ation, a DNA damage-driven protein modification and regulator of genomic instability

Activation of poly(ADP-ribose) polymerase-1 (PARP-1) is an immediate cellular reaction to DNA strand breakage as induced by alkylating agents, ionizing radiation or oxidants. The resulting formation of protein-coupled poly(ADP-ribose) facilitates survival of proliferating cells under conditions of DNA damage, probably via its contribution to DNA base-excision repair. Furthermore, based on recent results there is a role emerging for PARP-1 as a negative regulator of genomic instability in cells under genotoxic stress. Regarding possible applications for clinical cancer therapy with DNA-damaging agents, it appears that both inhibition and up-regulation of the poly(ADP-ribosyl)ation response in the malignant cells to be eradicated are promising strategies to improve the outcome of such therapy, albeit for different reasons.

Inhibition of poly(ADP-ribose) polymerase activity is insufficient to induce tetraploidy

Poly(ADP-ribose) polymerase (PARP) knockout mice are resistant to murine models of human diseases such as cerebral and myocardial ischemia, traumatic brain injury, diabetes, Parkinsonism, endotoxic shock and arthritis, implicating PARP in the pathogenesis of these diseases. Potent selective PARP inhibitors are therefore being evaluated as novel therapeutic agents in the treatment of these diseases. Inhibition or depletion of PARP, however, increases genomic instability in cells exposed to genotoxic agents. We recently demonstrated the presence of a genomically unstable tetraploid population in PARP(-/-) fibroblasts and its loss after stable transfection with PARP cDNA. To elucidate whether the genomic instability is attributable to PARP deficiency or lack of PARP activity, we investigated the effects of PARP inhibition on development of tetraploidy. Immortalized wild-type and PARP(-/-) fibroblasts were exposed for 3 weeks to 20 microM GPI 6150 (1,11b-dihydro-[2H:]benzopyrano[4,3,2-de]isoquinolin-3-one), a novel small molecule specific competitive inhibitor of PARP (K(i) = 60 nM) and one of the most potent PARP inhibitors to date (IC(50) = 0.15 microM). Although GPI 6150 initially decreased cell growth in wild-type cells, there was no effect on cell growth or viability after 24 h. GPI 6150 inhibited endogenous PARP activity in wild-type cells by approximately 91%, to about the residual levels in PARP(-/-) cells. Flow cytometric analysis of unsynchronized wild-type cells exposed for 3 weeks to GPI 6150 did not induce the development of tetraploidy, suggesting that, aside from its catalytic function, PARP may play other essential roles in the maintenance of genomic stability.