Increased poly(ADP-ribosyl)ation in intact cells by cisplatin treatment

Determining the Potential of DNA Damage Response (DDR) Inhibitors in Cervical Cancer Therapy

Cisplatin-based chemo-radiotherapy (CRT) is the standard treatment for advanced cervical cancer (CC) but the response rate is poor (46-72%) and cisplatin is nephrotoxic. Therefore, better treatment of CC is urgently needed. We have directly compared, for the first time, the cytotoxicity of four DDR inhibitors (rucaparib/PARPi, VE-821/ATRi, PF-477736/CHK1i and MK-1775/WEE1i) as single agents, and in combination with cisplatin and radiotherapy (RT) in a panel of CC cells. All inhibitors alone caused concentration-dependent cytotoxicity. Low ATM and DNA-PKcs levels were associated with greater VE-821 cytotoxicity. Cisplatin induced ATR, CHK1 and WEE1 activity in all of the cell lines. Cisplatin only activated PARP in S-phase cells, but RT activated PARP in the entire population. Rucaparib was the most potent radiosensitiser and VE-821 was the most potent chemosensitiser. VE-821, PF-47736 and MK-1775 attenuated cisplatin-induced S-phase arrest but tended to increase G2 phase accumulation. In mice, cisplatin-induced acute kidney injury was associated with oxidative stress and PARP activation and was prevented by rucaparib. Therefore, while all inhibitors investigated may increase the efficacy of CRT, the greatest clinical potential of rucaparib may be in limiting kidney damage, which is dose-limiting.

Metabolic interaction between amino acid deprivation and cisplatin synergistically reduces phosphoribosyl-pyrophosphate and augments cisplatin cytotoxicity

Cisplatin is a mainstay of cancer chemotherapy. It forms DNA adducts, thereby activating poly(ADP-ribose) polymerases (PARPs) to initiate DNA repair. The PARP substrate NAD⁺ is synthesized from 5-phosphoribose-1-pyrophosphate (PRPP), and we found that treating cells for 6 h with cisplatin reduced intracellular PRPP availability. The decrease in PRPP was likely from (1) increased PRPP consumption, because cisplatin increased protein PARylation and PARP1 shRNA knock-down returned PRPP towards normal, and (2) decreased intracellular phosphate, which down-regulated PRPP synthetase activity. Depriving cells of a single essential amino acid decreased PRPP synthetase activity with a half-life of ~ 8 h, and combining cisplatin and amino acid deprivation synergistically reduced intracellular PRPP. PRPP is a rate-limiting substrate for purine nucleotide synthesis, and cisplatin inhibited de novo purine synthesis and DNA synthesis, with amino acid deprivation augmenting cisplatin’s effects. Amino acid deprivation enhanced cisplatin’s cytotoxicity, increasing cellular apoptosis and DNA strand breaks in vitro, and intermittent deprivation of lysine combined with a sub-therapeutic dose of cisplatin inhibited growth of ectopic hepatomas in mice. Augmentation of cisplatin’s biochemical and cytotoxic effects by amino acid deprivation suggest that intermittent deprivation of an essential amino acid could allow dose reduction of cisplatin; this could reduce the drug’s side effects, and allow its use in cisplatin-resistant tumors.

RNA splicing alteration in the response to platinum chemotherapy in ovarian cancer: A possible biomarker and therapeutic target

Since its discovery, alternative splicing has been recognized as a powerful way for a cell to amplify the genetic information and for a living organism to adapt, evolve, and survive. We now know that a very high number of genes are regulated by alternative splicing and that alterations of splicing have been observed in different types of human diseases, including cancer. Here, we review the accumulating knowledge that links the regulation of alternative splicing to the response to chemotherapy, focusing our attention on ovarian cancer and platinum-based treatments. Moreover, we discuss how expanding information could be exploited to identify new possible biomarkers of platinum response, to better select patients, and/or to design new therapies able to overcome platinum resistance.

PARP Inhibitor PJ34 Protects Mitochondria and Induces DNA-Damage Mediated Apoptosis in Combination With Cisplatin or Temozolomide in B16F10 Melanoma Cells

PARP-1 inhibition has recently been employed in both mono- and combination therapies in various malignancies including melanoma with both promising and contradicting results reported. Although deeper understanding of the underlying molecular mechanisms may help improving clinical modalities, the complex cellular effects of PARP inhibitors make disentangling of the mechanisms involved in combination therapies difficult. Here, we used two cytostatic agents used in melanoma therapies in combination with PARP inhibition to have an insight into cellular events using the B16F10 melanoma model. We found that, when used in combination with cisplatin or temozolomide, pharmacologic blockade of PARP-1 by PJ34 augmented the DNA-damaging and cytotoxic effects of both alkylating compounds. Interestingly, however, this synergism unfolds relatively slowly and is preceded by molecular events that are traditionally believed to support cell survival including the stabilization of mitochondrial membrane potential and morphology. Our data indicate that the PARP inhibitor PJ34 has, apparently, opposing effects on the mitochondrial structure and cell survival. While, initially, it stimulates mitochondrial fusion and hyperpolarization, hallmarks of mitochondrial protection, it enhances the cytotoxic effects of alkylating agents at later stages. These findings may contribute to the optimization of PARP inhibitor-based antineoplastic modalities.

Phase I/Ib study of olaparib and carboplatin in heavily pretreated recurrent high-grade serous ovarian cancer at low genetic risk

Purpose: To investigate maximum tolerated dose (MTD), activity and predictive biomarkers of olaparib with carboplatin in BRCA wild-type (BRCAwt) high grade serous ovarian carcinoma (HGSOC) patients. Methods: A 3+3 dose escalation study examined olaparib capsules (400 mg twice daily [BID], days 1-7) with carboplatin (AUC3-5 on day 1) every 21 days for 8 cycles, followed by olaparib 400 mg BID maintenance. Blood and tumor biopsy samples were collected pre- and on-treatment in the expansion cohort for PAR levels and proteomic endpoints. Results: 30 patients (median 7 prior regimens [2-12], 63% (19/30) platinum-resistant) were enrolled. Dose-limiting toxicity was thrombocytopenia/neutropenia, and infection with carboplatin AUC5 (2/6 patients). MTD was olaparib 400 mg BID + carboplatin AUC4. Grade 3/4 adverse events (>10%) included neutropenia (23%), thrombocytopenia (20%), and anemia (13%). Five of 25 (20%) evaluable patients had partial response (PR; median 4.5 months [3.3-9.5]). Clinical benefit rate (PR + stable disease ≥4 months) was 64% (16/25). A greater decrease in tissue PAR levels was seen in the clinical benefit group versus no benefit (median normalized linear change -1.84 [-3.39- -0.28] vs 0.51 [-0.27- 1.29], p = 0.001) and a DNA repair score by proteomics did not correlate with response. Conclusions: The olaparib and carboplatin combination is tolerable and has clinical benefit in subsets of heavily pretreated BRCAwt HGSOC, independent of platinum sensitivity.

Double-barreled gun: Combination of PARP inhibitor with conventional chemotherapy

DNA repair pathways are evolutionarily conserved molecular mechanisms that maintain the integrity of genomic DNA. In cancer therapies, the integrity and activity of DNA repair pathways predict therapy resistance and disease outcome. Members of the poly (ADP-ribose) polymerase (PARP) family initiate and organize the biologic process of DNA repair, which counteracts many types of chemotherapies. Since the first development in approximately 3 decades ago, PARP inhibitors have greatly changed the concept of cancer therapy, leading to encouraging improvements in tumor suppression and disease outcomes. Here we summaries both pre-clinical and clinical findings of PARP inhibitors applications, particularly for combination therapies.

Common Chemical Inductors of Replication Stress: Focus on Cell-Based Studies

DNA replication is a highly demanding process regarding the energy and material supply and must be precisely regulated, involving multiple cellular feedbacks. The slowing down or stalling of DNA synthesis and/or replication forks is referred to as replication stress (RS). Owing to the complexity and requirements of replication, a plethora of factors may interfere and challenge the genome stability, cell survival or affect the whole organism. This review outlines chemical compounds that are known inducers of RS and commonly used in laboratory research. These compounds act on replication by direct interaction with DNA causing DNA crosslinks and bulky lesions (cisplatin), chemical interference with the metabolism of deoxyribonucleotide triphosphates (hydroxyurea), direct inhibition of the activity of replicative DNA polymerases (aphidicolin) and interference with enzymes dealing with topological DNA stress (camptothecin, etoposide). As a variety of mechanisms can induce RS, the responses of mammalian cells also vary. Here, we review the activity and mechanism of action of these compounds based on recent knowledge, accompanied by examples of induced phenotypes, cellular readouts and commonly used doses.

An assay to measure poly(ADP ribose) glycohydrolase (PARG) activity in cells

After a DNA damage signal multiple polymers of ADP ribose attached to poly(ADP) ribose (PAR) polymerases (PARPs) are broken down by the enzyme poly(ADP) ribose glycohydrolase (PARG). Inhibition of PARG leads to a failure of DNA repair and small molecule inhibition of PARG has been a goal for many years. To determine whether biochemical inhibitors of PARG are active in cells we have designed an immunofluorescence assay to detect nuclear PAR after DNA damage. This 384-well assay is suitable for medium throughput high-content screening and can detect cell-permeable inhibitors of PARG from nM to µM potency. In addition, the assay has been shown to work in murine cells and in a variety of human cancer cells. Furthermore, the assay is suitable for detecting the DNA damage response induced by treatment with temozolomide and methylmethane sulfonate (MMS). Lastly, the assay has been shown to be robust over a period of several years.

An assay to measure poly(ADP ribose) glycohydrolase (PARG) activity in cells

After a DNA damage signal multiple polymers of ADP ribose attached to poly(ADP) ribose (PAR) polymerases (PARPs) are broken down by the enzyme poly(ADP) ribose glycohydrolase (PARG). Inhibition of PARG leads to a failure of DNA repair and small molecule inhibition of PARG has been a goal for many years. To determine whether biochemical inhibitors of PARG are active in cells we have designed an immunofluorescence assay to detect nuclear PAR after DNA damage. This 384-well assay is suitable for medium throughput high-content screening and can detect cell-permeable inhibitors of PARG from nM to µM potency. In addition, the assay has been shown to work in murine cells and in a variety of human cancer cells. Furthermore, the assay is suitable for detecting the DNA damage response induced by treatment with temozolomide and methylmethane sulfonate (MMS). Lastly, the assay has been shown to be robust over a period of several years.

PARP Inhibitor PJ34 Suppresses Osteogenic Differentiation in Mouse Mesenchymal Stem Cells by Modulating BMP-2 Signaling Pathway

Poly(ADP-ribosyl)ation is known to be involved in a variety of cellular processes, such as DNA repair, cell death, telomere regulation, genomic stability and cell differentiation by poly(ADP-ribose) polymerase (PARP). While PARP inhibitors are presently under clinical investigation for cancer therapy, little is known about their side effects. However, PARP involvement in mesenchymal stem cell (MSC) differentiation potentiates MSC-related side effects arising from PARP inhibition. In this study, effects of PARP inhibitors on MSCs were examined. MSCs demonstrated suppressed osteogenic differentiation after 1 µM PJ34 treatment without cytotoxicity, while differentiation of MSCs into chondrocytes or adipocytes was unaffected. PJ34 suppressed mRNA induction of osteogenic markers, such as Runx2, Osterix, Bone Morphogenetic Protein-2, Osteocalcin, bone sialoprotein, and Osteopontin, and protein levels of Bone Morphogenetic Protein-2, Osterix and Osteocalcin. PJ34 treatment also inhibited transcription factor regulators such as Smad1, Smad4, Smad5 and Smad8. Extracellular mineralized matrix formation was also diminished. These results strongly suggest that PARP inhibitors are capable of suppressing osteogenic differentiation and poly(ADP-ribosyl)ation may play a physiological role in this process through regulation of BMP-2 signaling. Therefore, PARP inhibition may potentially attenuate osteogenic metabolism, implicating cautious use of PARP inhibitors for cancer treatments and monitoring of patient bone metabolism levels.

Sulforaphane inhibits damage-induced poly (ADP-ribosyl)ation via direct interaction of its cellular metabolites with PARP-1

SCOPE

The isothiocyanate sulforaphane, a major breakdown product of the broccoli glucosinolate glucoraphanin, has frequently been proposed to exert anticarcinogenic properties. Potential underlying mechanisms include a zinc release from Kelch-like ECH-associated protein 1 followed by the induction of detoxifying enzymes. This suggests that sulforaphane may also interfere with other zinc-binding proteins, e.g. those essential for DNA repair. Therefore, we explored the impact of sulforaphane on poly (ADP-ribose)polymerase-1 (PARP-1), poly (ADP-ribosyl)ation (PARylation), and DNA single-strand break repair (SSBR) in cell culture.

METHODS AND RESULTS

Immunofluorescence analyses showed that sulforaphane diminished H2 O2 -induced PARylation in HeLa S3 cells starting from 15 μM despite increased lesion induction under these conditions. Subcellular experiments quantifying the damage-induced incorporation of (32) P-ADP-ribose by PARP-1 displayed no direct impact of sulforaphane itself, but cellular metabolites, namely the glutathione conjugates of sulforaphane and its interconversion product erucin, reduced PARP-1 activity concentration dependently. Interestingly, this sulforaphane metabolite-induced PARP-1 inhibition was prevented by thiol compounds. PARP-1 is a stimulating factor for DNA SSBR-rate and we further demonstrated that 25 μM sulforaphane also delayed the rejoining of H2 O2 -induced DNA strand breaks, although this might be partly due to increased lesion frequencies.

CONCLUSION

Sulforaphane interferes with damage-induced PARylation and SSBR, which implies a sulforaphane-dependent impairment of genomic stability.

A Clickable Aminooxy Probe for Monitoring Cellular ADP-Ribosylation

ADP-ribosylation is essential for cell function, yet there is a dearth of methods for detecting this post-translational modification in cells. Here, we describe a clickable aminooxy alkyne (AO-alkyne) probe that can detect cellular ADP-ribosylation on acidic amino acids following Cu-catalyzed conjugation to an azide-containing reporter. Using AO-alkyne, we show that PARP10 and PARP11 are auto-ADP-ribosylated in cells. We also demonstrate that AO-alkyne can be used to monitor stimulus-induced ADP-ribosylation in cells. Functional studies using AO-alkyne support a previously unknown mechanism for ADP-ribosylation on acidic amino acids, wherein a glutamate or aspartate at the initial C1'-position of ADP-ribose transfers to the C2' position. This new mechanism for ADP-ribosylation has important implications for how glutamyl/aspartyl-ADP-ribose is recognized by proteins in cells.

The role of DNA repair pathways in cisplatin resistant lung cancer

Platinum chemotherapeutic agents such as cisplatin are currently used in the treatment of various malignancies such as lung cancer. However, their efficacy is significantly hindered by the development of resistance during treatment. While a number of factors have been reported that contribute to the onset of this resistance phenotype, alterations in the DNA repair capacity of damaged cells is now recognised as an important factor in mediating this phenomenon. The mode of action of cisplatin has been linked to its ability to crosslink purine bases on the DNA, thereby interfering with DNA repair mechanisms and inducing DNA damage. Following DNA damage, cells respond by activating a DNA-damage response that either leads to repair of the lesion by the cell thereby promoting resistance to the drug, or cell death via activation of the apoptotic response. Therefore, DNA repair is a vital target to improving cancer therapy and reduce the resistance of tumour cells to DNA damaging agents currently used in the treatment of cancer patients. To date, despite the numerous findings that differential expression of components of the various DNA repair pathways correlate with response to cisplatin, translation of such findings in the clinical setting are still warranted. The identification of alterations in specific proteins and pathways that contribute to these unique DNA repair pathways in cisplatin resistant cancer cells may potentially lead to a renewed interest in the development of rational novel therapies for cisplatin resistant cancers, in particular, lung cancer.

Effect of p53 activity on the sensitivity of human glioblastoma cells to PARP-1 inhibitor in combination with topoisomerase I inhibitor or radiation

Poly (ADP-Ribose) polymerase-1 (PARP-1) is involved in the DNA repairing system by sensing and signaling the presence of DNA damage. Inhibition of PARP-1 is tested in combination with DNA damaging agents such as topoisomerase I inhibitors or ionizing radiations (RT) for the treatment of glioblastoma (GBM). Disruption of p53, widely prevalent in GBMs, plays a major role in DNA repairing system. The current study investigates whether p53 activity has an effect on the sensitivity of human GBM cells to PARP-1 inhibitors in combination with topoisomerase I inhibitor topotecan (TPT) and/or RT. Human GBM cell lines carrying a different functional status of p53 were treated with PARP-1 inhibitor NU1025, in combination with TPT and/or RT. Cytotoxic effects were examined by analyzing the antiproliferative activity, the cell cycle perturbations, and the DNA damage induced by combined treatments. PARP inhibition enhanced the antiproliferative activity, the cell cycle perturbations and the DNA damage induced by both TPT or RT in GBM cells. These effects were influenced by the p53 activity: cells carrying an active p53 were more sensitive to the combination of PARP inhibitor and RT, while cells carrying an inactive p53 displayed a higher sensitivity to the combination of PARP inhibitor and TPT. Our study suggests that p53 activity influences the differential sensitivity of GBM cells to combined treatments of TPT, RT, and PARP inhibitors. © 2014 International Society for Advancement of Cytometry.

ATM alters the otherwise robust chromatin mobility at sites of DNA double-strand breaks (DSBs) in human cells

Ionizing radiation induces DNA double strand breaks (DSBs) which can lead to the formation of chromosome rearrangements through error prone repair. In mammalian cells the positional stability of chromatin contributes to the maintenance of genome integrity. DSBs exhibit only a small, submicron scale diffusive mobility, but a slight increase in the mobility of chromatin domains by the induction of DSBs might influence repair fidelity and the formation of translocations. The radiation-induced local DNA decondensation in the vicinity of DSBs is one factor potentially enhancing the mobility of DSB-containing chromatin domains. Therefore in this study we focus on the influence of different chromatin modifying proteins, known to be activated by the DNA damage response, on the mobility of DSBs. IRIF (ionizing radiation induced foci) in U2OS cells stably expressing 53BP1-GFP were used as a surrogate marker of DSBs. Low angle charged particle irradiation, known to trigger a pronounced DNA decondensation, was used for the defined induction of linear tracks of IRIF. Our results show that movement of IRIF is independent of the investigated chromatin modifying proteins like ACF1 or PARP1 and PARG. Also depletion of proteins that tether DNA strands like MRE11 and cohesin did not alter IRIF dynamics significantly. Inhibition of ATM, a key component of DNA damage response signaling, resulted in a pronounced confinement of DSB mobility, which might be attributed to a diminished radiation induced decondensation. This confinement following ATM inhibition was confirmed using X-rays, proving that this effect is not restricted to densely ionizing radiation. In conclusion, repair sites of DSBs exhibit a limited mobility on a small spatial scale that is mainly unaffected by depletion of single remodeling or DNA tethering proteins. However, it relies on functional ATM kinase which is considered to influence the chromatin structure after irradiation.

Therapeutic applications of PARP inhibitors: anticancer therapy and beyond

The aim of this article is to describe the current and potential clinical translation of pharmacological inhibitors of poly(ADP-ribose) polymerase (PARP) for the therapy of various diseases. The first section of the present review summarizes the available preclinical and clinical data with PARP inhibitors in various forms of cancer. In this context, the role of PARP in single-strand DNA break repair is relevant, leading to replication-associated lesions that cannot be repaired if homologous recombination repair (HRR) is defective, and the synthetic lethality of PARP inhibitors in HRR-defective cancer. HRR defects are classically associated with BRCA1 and 2 mutations associated with familial breast and ovarian cancer, but there may be many other causes of HRR defects. Thus, PARP inhibitors may be the drugs of choice for BRCA mutant breast and ovarian cancers, and extend beyond these tumors if appropriate biomarkers can be developed to identify HRR defects. Multiple lines of preclinical data demonstrate that PARP inhibition increases cytotoxicity and tumor growth delay in combination with temozolomide, topoisomerase inhibitors and ionizing radiation. Both single agent and combination clinical trials are underway. The final part of the first section of the present review summarizes the current status of the various PARP inhibitors that are in various stages of clinical development. The second section of the present review summarizes the role of PARP in selected non-oncologic indications. In a number of severe, acute diseases (such as stroke, neurotrauma, circulatory shock and acute myocardial infarction) the clinical translatability of PARP inhibition is supported by multiple lines of preclinical data, as well as observational data demonstrating PARP activation in human tissue samples. In these disease indications, PARP overactivation due to oxidative and nitrative stress drives cell necrosis and pro-inflammatory gene expression, which contributes to disease pathology. Accordingly, multiple lines of preclinical data indicate the efficacy of PARP inhibitors to preserve viable tissue and to down-regulate inflammatory responses. As the clinical trials with PARP inhibitors in various forms of cancer progress, it is hoped that a second line of clinical investigations, aimed at testing of PARP inhibitors for various non-oncologic indications, will be initiated, as well.

The Role of PARP Inhibitors in the Treatment of Gynecologic Malignancies

Gynecologic malignancies annually account for over 91,000 new cancer cases and approximately 28,000 deaths in the United States. Although there have been advancements in cytotoxic chemotherapies, there has not been significant improvement in overall survival in these patients. While targeted therapies have shown some benefit in many solid tumors, further development of these agents is needed for the treatment of gynecologic malignancies. Poly(ADP-ribose) polymerase (PARP) catalyzes the polyADP-ribosylation of proteins involved in DNA repair. Inhibitors of PARP were originally developed for cancers with homologous recombination deficiencies, such as those harboring mutations in BRCA1 or BRCA2 genes. However, pre-clinical research and clinical trials have suggested that the activity of PARP inhibitors is not limited to those with BRCA mutations. PARP inhibitors may have activity in cancers deficient in other DNA repair genes, signaling pathways that mitigate DNA repair, or in combination with DNA-damaging agents independent of DNA repair dysfunction. Currently there are seven different PARP inhibitors in clinical development for cancer. While there has been promising clinical activity for some of these agents, there are still significant unanswered questions regarding their use. Going forward, specific questions that must be answered include timing of therapy, use in combination with cytotoxic agents or as single-agent maintenance therapy, and whether there is a predictive biomarker that can be used with PARP inhibition. Even with large strides in the treatment of many gynecologic malignancies in recent years, it is imperative that we develop newer agents and methods to identify patients that may benefit from these compounds. The focus of this review will be on pre-clinical data, current clinical trials, and the future of PARP inhibitors in the treatment of ovarian, endometrial, and cervical cancer.

PARP inhibitors: its role in treatment of cancer

PARP is an important protein in DNA repair pathways especially the base excision repair (BER). BER is involved in DNA repair of single strand breaks (SSBs). If BER is impaired, inhibiting poly(ADP-ribose) polymerase (PARP), SSBs accumulate and become double stand breaks (DSBs). The cells with increasing number of DSBs become more dependent on other repair pathways, mainly the homologous recombination (HR) and the nonhomologous end joining. Patients with defective HR, like BRCA-deficient cell lines, are even more susceptible to impairment of the BER pathway. Inhibitors of PARP preferentially kill cancer cells in BRCA-mutation cancer cell lines over normal cells. Also, PARP inhibitors increase cytotoxicity by inhibiting repair in the presence of chemotherapies that induces SSBs. These two principles have been tested clinically. Over the last few years, excitement over this class of agents has escalated due to reported activity as single agent in BRCA1- or BRCA2-associated ovarian or breast cancers, and in combination with chemotherapy in triple negative breast cancer. This review covers the current results of clinical trials testing those two principles. It also evaluates future directions for the field of PARP inhibitor development.

Human monocytes undergo excessive apoptosis following temozolomide activating the ATM/ATR pathway while dendritic cells and macrophages are resistant

Immunodeficiency is a severe therapy-limiting side effect of anticancer chemotherapy resulting from sensitivity of immunocompetent cells to DNA damaging agents. A central role in the immune system is played by monocytes that differentiate into macrophages and dendritic cells (DCs). In this study we compared human monocytes isolated from peripheral blood and cytokine matured macrophages and DCs derived from them and assessed the mechanism of toxicity of the DNA methylating anticancer drug temozolomide (TMZ) in these cell populations. We observed that monocytes, but not DCs and macrophages, were highly sensitive to the killing effect of TMZ. Studies on DNA damage and repair revealed that the initial DNA incision was efficient in monocytes while the re-ligation step of base excision repair (BER) can not be accomplished, resulting in an accumulation of DNA single-strand breaks (SSBs). Furthermore, monocytes accumulated DNA double-strand breaks (DSBs) following TMZ treatment, while DCs and macrophages were able to repair DSBs. Monocytes lack the DNA repair proteins XRCC1, ligase IIIα and PARP-1 whose expression is restored during differentiation into macrophages and DCs following treatment with GM-CSF and GM-CSF plus IL-4, respectively. These proteins play a key role both in BER and DSB repair by B-NHEJ, which explains the accumulation of DNA breaks in monocytes following TMZ treatment. Although TMZ provoked an upregulation of XRCC1 and ligase IIIα, BER was not enhanced likely because PARP-1 was not upregulated. Accordingly, inhibition of PARP-1 did not sensitize monocytes, but monocyte-derived DCs in which strong PARP activation was observed. TMZ induced in monocytes the DNA damage response pathways ATM-Chk2 and ATR-Chk1 resulting in p53 activation. Finally, upon activation of the Fas-receptor and the mitochondrial pathway apoptosis was executed in a caspase-dependent manner. The downregulation of DNA repair in monocytes, resulting in their selective killing by TMZ, might impact on the immune response during cancer chemotherapy.

Requirements to assess feasibility of phase 0 trials during major abdominal surgery: variability of PARP activity

PURPOSE

The aim of this study was to evaluate the feasibility of phase 0 trials in the setting of a routine surgical procedure. Logistic considerations, tissue sampling and tissue handling, and variability of a biomarker during surgery, in here PARP, were evaluated.

EXPERIMENTAL DESIGN

Patients with highly suspicious or proven diagnosis of advanced ovarian cancer, planned for debulking surgery were asked to allow sequential tumor biopsies during surgery. Biopsies were frozen immediately and PARP activity was measured subsequently.

RESULTS

Baseline biopsies were obtained from eight patients after a median time of 88 minutes (minimum of 50 to maximum of 123 minutes). Second and third biopsies were obtained after a median of 60 (32-96) and 101 (79-130) minutes, respectively. Mean tumor load was 44% (5%-100%), with a cellular viability of 98% (85%-100%). Median baseline PARP activity was 1035 pg/mL (range, 429-2663 pg/mL). The observed interpatient variability at baseline was large: SD was 0.59 after natural logarithm transformation.

CONCLUSIONS

Conducting phase 0 trials during surgery seems to be feasible in terms of logistic considerations. In preparation of a phase 0 trial during surgery, a feasibility study like this should be conducted to rule out major interactions of the surgical intervention with respect to the targeted biomarker.

Novel targeted therapies in head and neck cancer

INTRODUCTION

Molecularly targeted therapy, with the potential for increased selectivity and fewer adverse effects, hold promise in the treatment of HNSCC.

AREAS COVERED

Targeted agents for HNSCC expected to improve the effectiveness of current therapy including HER family, Src-family kinase, cell cycle, MET, AKT, HDAC, PARP, COX inhibitors and antiangiogenesis.

EXPERT OPINION

Epidermal growth factor receptor inhibitors are established in HNSCC and the need now is to find biomarkers for sensitivity to better select patients. Moreover, other pathway inhibitors hold significant promise and are being tested in clinical trials. Angiogenesis inhibition is likely to yield only modest efficacy alone but may augment existing standards. Lastly, one clinical arena where targeted therapies may find secure purchase is in the adjuvant or prevention setting where minimal or preneoplastic disease can be affected by inhibition of a single or few targets.

6-OHDA generated ROS induces DNA damage and p53- and PUMA-dependent cell death

BACKGROUND

Parkinson's disease (PD) is characterized by the selective loss of dopaminergic neurons in the substantia nigra (SN), resulting in tremor, rigidity, and bradykinesia. Although the etiology is unknown, insight into the disease process comes from the dopamine (DA) derivative, 6-hydroxydopamine (6-OHDA), which produces PD-like symptoms. Studies show that 6-OHDA activates stress pathways, such as the unfolded protein response (UPR), triggers mitochondrial release of cytochrome-c, and activates caspases, such as caspase-3. Because the BH3-only protein, Puma (p53-upregulated mediator of apoptosis), is activated in response to UPR, it is thought to be a link between cell stress and apoptosis.

RESULTS

To test the hypothesis that Puma serves such a role in 6-OHDA-mediated cell death, we compared the response of dopaminergic neurons from wild-type and Puma-null mice to 6-OHDA. Results indicate that Puma is required for 6-OHDA-induced cell death, in primary dissociated midbrain cultures as well as in vivo. In these cultures, 6-OHDA-induced DNA damage and p53 were required for 6-OHDA-induced cell death. In contrast, while 6-OHDA led to upregulation of UPR markers, loss of ATF3 did not protect against 6-OHDA.

CONCLUSIONS

Together, our results indicate that 6-OHDA-induced upregulation of Puma and cell death are independent of UPR. Instead, p53 and DNA damage repair pathways mediate 6-OHDA-induced toxicity.

Leukocyte elastase inhibitor: a new regulator of PARP-1

Poly(ADP-ribose) polymerase-1 (PARP-1) uses NAD(+) as a substrate to form ADP-ribose. During apoptosis, caspases cleave PARP-1 to avoid excessive NAD consumption. Because PARP-1 is a key regulator of the activity of DNases involved in caspase-dependent apoptosis, its cleavage is required to promote DNA degradation. To explore the situation in caspase-independent cell death, we investigated the effect of PARP-1 on the acid endonuclease leukocyte elastase inhibitor (LEI)-derived DNase II (L-DNase II). We found for the first time an association between PARP-1 and LEI/L-DNase II. Unexpectedly, we observed that LEI influenced the automodification of PARP-1.

Dual role of poly(ADP-ribose) glycohydrolase in the regulation of cell death in oxidatively stressed A549 cells

Activation of poly(ADP-ribose) polymerase-1 (PARP1) has been shown to mediate cell death induced by genotoxic stimuli. The role of poly(ADP-ribose) glycohydrolase (PARG), the enzyme responsible for polymer degradation, has been largely unexplored in the regulation of cell death. Using lentiviral gene silencing we generated A549 lung adenocarcinoma cell lines with stably suppressed PARG and PARP1 expression (shPARG and shPARP1 cell lines, respectively) and determined parameters of apoptotic and necrotic cell death following hydrogen peroxide exposure. shPARG cells accumulated large amounts of poly(ADP-ribosyl)ated proteins and exhibited reduced PARP activation. Hydrogen peroxide-induced cell death is regulated by PARG in a dual fashion. Whereas the shPARG cell line (similarly to shPARP1 cells) was resistant to the necrotic effect of high concentrations of hydrogen peroxide, these cells exhibited stronger apoptotic response. Both shPARP1 and especially shPARG cells displayed a delayed repair of DNA breaks and exhibited reduced clonogenic survival following hydrogen peroxide treatment. Translocation of apoptosis-inducing factor could not be observed, but cells could be saved by methyl pyruvate and alpha-ketoglutarate, indicating that energy failure may mediate cytotoxicity in our model. These data indicate that PARG is a survival factor at mild oxidative damage but contributes to the apoptosis-necrosis switch in severely damaged cells.

Dual role of poly(ADP-ribose) glycohydrolase in the regulation of cell death in oxidatively stressed A549 cells

Activation of poly(ADP-ribose) polymerase-1 (PARP1) has been shown to mediate cell death induced by genotoxic stimuli. The role of poly(ADP-ribose) glycohydrolase (PARG), the enzyme responsible for polymer degradation, has been largely unexplored in the regulation of cell death. Using lentiviral gene silencing we generated A549 lung adenocarcinoma cell lines with stably suppressed PARG and PARP1 expression (shPARG and shPARP1 cell lines, respectively) and determined parameters of apoptotic and necrotic cell death following hydrogen peroxide exposure. shPARG cells accumulated large amounts of poly(ADP-ribosyl)ated proteins and exhibited reduced PARP activation. Hydrogen peroxide-induced cell death is regulated by PARG in a dual fashion. Whereas the shPARG cell line (similarly to shPARP1 cells) was resistant to the necrotic effect of high concentrations of hydrogen peroxide, these cells exhibited stronger apoptotic response. Both shPARP1 and especially shPARG cells displayed a delayed repair of DNA breaks and exhibited reduced clonogenic survival following hydrogen peroxide treatment. Translocation of apoptosis-inducing factor could not be observed, but cells could be saved by methyl pyruvate and alpha-ketoglutarate, indicating that energy failure may mediate cytotoxicity in our model. These data indicate that PARG is a survival factor at mild oxidative damage but contributes to the apoptosis-necrosis switch in severely damaged cells.

Poly(ADP-ribose) polymerase-1 activity facilitates the dissociation of nuclear proteins from platinum-modified DNA

The affinity of the poly(ADP-ribose) polymerase-1 (PARP-1) for platinum-damaged DNA was first discovered during photo-cross-linking experiments using the photoactive compound Pt-BP6 [J. Am. Chem. Soc.2004, 126, 6536-6537], an analogue of the anticancer drug cis-diamminedichloroplatinum(II), cisplatin. Although PARP inhibitors sensitize cancer cells to cisplatin, there are conflicting reports in the literature about their efficacy. In order to improve our understanding of the mechanism by which PARP inhibition might potentiate the cell-killing ability of cisplatin, and to shed light on the source of the discrepancy among different laboratories, we have in the present study probed the influence of three PARP inhibitors in four types of cancer cells, cervical (HeLa), testicular (NTera2), pancreatic (BxPC3), and osteosarcoma (U2OS), on the results of Pt-BP6 photo-cross-linking experiments and cytotoxicity assays. We find that the activity of PARP proteins following exposure to platinum-modified DNA results in the dissociation of DNA-bound proteins. PARP inhibitors were able to sensitize some, but not all, of the cell lines to cisplatin. This cell line-dependence and the potential consequences of PARP-initiated protein removal from platinum-DNA lesions are discussed. Control experiments revealed that NTera2 cells are especially sensitive to PARP inhibition.

Impact of copper on the induction and repair of oxidative DNA damage, poly(ADP-ribosyl)ation and PARP-1 activity

Copper is an essential trace element involved, among other functions, in enzymatic antioxidative defense systems. However, nonprotein bound copper ions have been shown to generate reactive oxygen species. To gain insight into the discrepancy between the protective properties of copper on the one hand and its toxicity on the other hand, we examined the genotoxic effects of CuSO(4) in cultured human cells. Here we report that copper, at cytotoxic concentrations, induces oxidative DNA base modifications and DNA strand breaks. However, at lower noncytotoxic concentrations, copper inhibits the repair of oxidative DNA damage induced by visible light. As a first mechanistic hint, inhibition of H(2)O(2)-induced poly(ADP-ribosyl)ation was identified in cultured cells and further experiments demonstrated a strong inhibition of the activity of isolated poly(ADP-ribose)polymerase-1 (PARP-1) by copper. Bioavailability studies of copper showed a dose-dependent uptake in cells and pointed out the relevance of the applied concentrations. Taken together, the results indicate that copper, under conditions of either disturbed homeostasis or overload due to high exposure, exerts defined genotoxic effects. Hence, a balance needs to be maintained to ensure sufficient uptake and to prevent overload.

Differential effects of tannic acid on cisplatin induced nephrotoxicity in rats

Cisplatin is a widely used antineoplastic drug. Major drawback of cisplatin therapy is its nephrotoxicity. The objective of this study was to check the effect of tannic acid on cisplatin induced nephrotoxicity. Post-treatment of tannic acid prevents cisplatin (5mg/kg) induced nephrotoxicity and decreases poly(ADP-ribose) polymerase cleavage, phosphorylation of p38 and hypoacetylation of histone H4. In contrast, co-treatment of tannic acid potentiates the nephrotoxicity. Comparative nephrotoxicity studies show that co-treatment of tannic acid with reduced dose of cisplatin (1.5mg/kg) developed almost similar nephrotoxicity. MALDI protein profiling of plasma samples provides indirect evidence that tannic acid co-treatment increases bioavailability of cisplatin.

Lactate down-regulates cellular poly(ADP-ribose) formation in cultured human skin fibroblasts

BACKGROUND

Polyadenosine diphosphate-ribose (poly(ADP-ribose)) is a nuclear polymer which is derived from nicotinamide adenine dinucleotide (NAD(+)) catalysed by poly(ADP-ribose) polymerase 1 (PARP-1). Aside from the well known role of poly(ADP-ribosyl)ation (pADPR) in DNA repair, pADPR is also involved in other cellular processes such as apoptosis and gene expression. However, the factors that regulate the level of pADPR are not fully elucidated. In view of the fact that healing wounds contain high concentrations of lactate (10-15 mM) and exogenous lactate reduce the NAD(+) pool in cultured fibroblasts, we propose that high lactate lowers the level of nuclear pADPR.

MATERIALS AND METHODS

Neonatal human dermal fibroblasts (NHDF) were plated to subconfluence and allowed to adhere. Cells were treated with 15 mM l-lactate and pADPR production was assessed by immunofluorescence analysis using 10H antibody. Difference in pADPR production was determined by calculation of positively stained cells compared to total cell numbers. Inhibition of PARP activity was tested by treatment with 100 microM 3-aminobenzamide (3-AB). Specificity of the lactate effect on pADPR synthesis was verified by using the analogue d-lactate. The contents of nicotinamide adenine dinucleotide (NAD(+)) and its reduced form (NADH) in lactated and non-lactated cell cultures were quantified by the enzymatic cyclic assay.

RESULTS

We found that exogenous l-lactate (15 mM) can significantly depress pADPR content in cultured fibroblasts. PARP-1 activity was inhibited by 3-AB and analogue d-lactate showed no effect on pADPR synthesis. NAD(+)/NADH ratio was significantly lowered in lactated compared to non-lactated cell culture.

CONCLUSIONS

Exogenous l-lactate (15 mM) can depress pADPR content in cultured fibroblasts. In view of the fact that healing wounds contain such high concentrations of lactate, we propose that down regulation of pADPR is associated with elevated tissue repair via pADPR dependent gene expression. This observation is important in understanding the stimulation of lactate-mediated protein expression during wound healing.

Impact of arsenite and its methylated metabolites on PARP-1 activity, PARP-1 gene expression and poly(ADP-ribosyl)ation in cultured human cells

The underlying mechanisms of arsenic carcinogenicity are still not fully understood. Mechanisms currently discussed include the induction of oxidative DNA damage and the interference with DNA repair pathways. Still unclear is the role of biomethylation, which has long been considered to be one major detoxification process. Methylated arsenicals have recently been shown to interfere with DNA repair in cellular and subcellular systems, but up to now no DNA repair protein has been identified being particular sensitive towards methylated arsenicals in cultured cells. Here we report that the trivalent methylated metabolites MMA(III) and DMA(III) inhibit poly(ADP-ribosyl)ation in cultured human HeLa S3 cells at concentrations as low as 1nM, thereby showing for the first time an inactivation of an enzymatic reaction related to DNA repair by the trivalent methylated arsenicals at very low environmentally relevant concentrations. In contrast the pentavalent metabolites MMA(V) and DMA(V) showed no such effects up to high micromolar concentrations. All investigated arsenicals did not alter gene expression of PARP-1. However, all trivalent arsenicals were able to inhibit the activity of isolated PARP-1, indicating that the observed decrease in poly(ADP-ribosyl)ation in cultures human cells, predominantly mediated by PARP-1, is likely due to changes in the activity of PARP-1. Since poly(ADP-ribosyl)ation plays a major role in DNA repair, cell cycle control and thus in the maintenance of genomic stability, these findings could in part explain DNA repair inhibition and the genotoxic and carcinogenic effects of arsenic.

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.

Gallotannin inhibits the expression of chemokines and inflammatory cytokines in A549 cells

Tannins are plant-derived water-soluble polyphenols with wide-ranging biological activities. The mechanisms underlying the anti-inflammatory effect of tannins are not fully understood and may be the result of inhibition of poly(ADP-ribose) (PAR) glycohydrolase (PARG), the main catabolic enzyme of PAR metabolism. Therefore, we set out to investigate the mechanism of the anti-inflammatory effect of gallotannin (GT) in A549 cells with special regard to the role of poly(ADP-ribosyl)ation. Using an inflammation-focused low-density array and reverse transcription-polymerase chain reaction, we found that GT suppressed the expression of most cytokines and chemokines in cytokine-stimulated A549 cells, whereas the PARP inhibitor PJ-34 only inhibited few transcripts. Activation of the transcription factors, nuclear factor kappaB (NF-kappaB) and activator protein 1 (AP-1), was blocked by GT, whereas PJ-34 only suppressed NF-kappaB activation but not AP-1 activation. GT also inhibited IkappaB phosphorylation and nuclear translocation of NF-kappaB, but PJ-34 had no effect on these upstream events. In the AP-1 pathway, GT treatment, even in the absence of cytokines, caused maximal phosphorylation of c-Jun N-terminal kinase and c-Jun. GT also caused a low-level phosphorylation of p38, extracellular signal-regulated kinases 1 and 2, activating transcription factor2, and cAMP-response element-binding protein but inhibited cytokine-induced phosphorylation of these kinases and transcription factors. GT inhibited protein phosphatases 1 and 2A, which may explain the increased phosphorylation of mitogen-activated protein kinase and their substrates. GT exerted potent antioxidant effect but failed to cause PAR accumulation. In summary, the potent inhibitory effects of GT on the transcription of cytokine and chemokine genes are probably not related to PARG inhibition. Inhibition of AP-1 activation and upstream signaling events may be responsible for the effects of GT.

Nuclear poly(ADP-ribose) polymerase-1 rapidly triggers mitochondrial dysfunction

To obtain further information on time course and mechanisms of cell death after poly(ADP-ribose) polymerase-1 (PARP-1) hyperactivation, we used HeLa cells exposed for 1 h to the DNA alkylating agent N-methyl-N'-nitro-N-nitrosoguanidine. This treatment activated PARP-1 and caused a rapid drop of cellular NAD(H) and ATP contents, culminating 8-12 h later in cell death. PARP-1 antagonists fully prevented nucleotide depletion and death. Interestingly, in the early 60 min after challenge with N-methyl-N'-nitro-N-nitrosoguanidine, mitochondrial membrane potential and superoxide production significantly increased, whereas cellular ADP contents decreased. Again, these events were prevented by PARP-1 inhibitors, suggesting that PARP-1 hyperactivity leads to mitochondrial state 4 respiration. Mitochondrial membrane potential collapsed at later time points (3 h), when mitochondria released apoptosis-inducing factor and cytochrome c. Using immunocytochemistry and targeted luciferase transfection, we found that, despite an exclusive localization of PARP-1 and poly(ADP-ribose) in the nucleus, ATP levels first decreased in mitochondria and then in the cytoplasm of cells undergoing PARP-1 activation. PARP-1 inhibitors rescued ATP (but not NAD(H) levels) in cells undergoing hyper-poly(ADP-ribosyl)ation. Glycolysis played a central role in the energy recovery, whereas mitochondria consumed ATP in the early recovery phase and produced ATP in the late phase after PARP-1 inhibition, further indicating that nuclear poly(ADP-ribosyl)ation rapidly modulates mitochondrial functioning. Together, our data provide evidence for rapid nucleus-mitochondria cross-talk during hyper-poly(ADP-ribosyl)ation-dependent cell death.

Cytoprotective effect of gallotannin in oxidatively stressed HaCaT keratinocytes: the role of poly(ADP-ribose) metabolism

Oxidative stress-induced cytotoxicity is mediated in part by accelerated poly-ADP ribosylation. Peroxynitrite and hydrogen peroxide cause DNA breakage triggering the activation of the DNA nick sensor enzyme poly(ADP-ribose) polymerase-1 (PARP-1). Overactivation of PARP-1 leads to cell dysfunction and cell death mainly due to depletion of NAD(+) (the substrate of PARP-1) and ATP. PARP-1 attaches most ADP-ribose residues onto itself, leading to downregulation of enzyme activity. Here, we have investigated the role of poly(ADP-ribose) glycohydrolase (PARG), the poly(ADP-ribose)-catabolyzing enzyme in oxidative stress-induced cytotoxicity in HaCaT cells. We have found that inhibition of PARG by gallotannin (GT) (50 microM) provided significant cytoprotection to peroxynitrite- or hydrogen peroxide-treated HaCaT cells, as assessed by lactate dehydrogenase release and propidium iodide uptake (parameters of necrotic cell death) as well as caspase activation (apoptotic parameter). GT pretreatment has also inhibited the depletion of cellular NAD(+) pools in hydrogen peroxide- or peroxynitrite-treated HaCaT cells. GT caused the accumulation of poly(ADP-ribose) and concomitant inhibition in cellular PARP activity in oxidatively stressed cells. Therefore, PARG is likely to contribute to maintaining the active state of PARP-1 by continuously removing inhibitory ADP-ribose residues from PARP-1.

Poly(ADP-ribose) polymerase-1-mediated cell death in astrocytes requires NAD+ depletion and mitochondrial permeability transition

Extensive activation of poly(ADP-ribose) polymerase-1 (PARP-1) by DNA damage is a major cause of caspase-independent cell death in ischemia and inflammation. Here we show that NAD(+) depletion and mitochondrial permeability transition (MPT) are sequential and necessary steps in PARP-1-mediated cell death. Cultured mouse astrocytes were treated with the cytotoxic concentrations of N-methyl-N'-nitro-N-nitrosoguanidine or 3-morpholinosydnonimine to induce DNA damage and PARP-1 activation. The resulting cell death was preceded by NAD(+) depletion, mitochondrial membrane depolarization, and MPT. Sub-micromolar concentrations of cyclosporin A blocked MPT and cell death, suggesting that MPT is a necessary step linking PARP-1 activation to cell death. In astrocytes, extracellular NAD(+) can raise intracellular NAD(+) concentrations. To determine whether NAD(+) depletion is necessary for PARP-1-induced MPT, NAD(+) was restored to near-normal levels after PARP-1 activation. Restoration of NAD(+) enabled the recovery of mitochondrial membrane potential and blocked both MPT and cell death. Furthermore, both cyclosporin A and NAD(+) blocked translocation of the apoptosis-inducing factor from mitochondria to nuclei, a step previously shown necessary for PARP-1-induced cell death. These results suggest that NAD(+) depletion and MPT are necessary intermediary steps linking PARP-1 activation to AIF translocation and cell death.

Poly(ADP-ribose) polymerase-1 is a survival factor for radiation-exposed intestinal epithelial stem cells in vivo

Poly(ADP-ribose) polymerase-1 (PARP-1) is a key enzyme mediating the cellular response to DNA strand breaks. It plays a critical role in genomic stability and survival of proliferating cells in culture undergoing DNA damage. Intestinal epithelium is the most proliferative tissue in the mammalian body and its stem cells show extreme sensitivity to low-level genotoxic stress. We investigated the role of PARP-1 in the in vivo damage response of intestinal stem cells in crypts of PARP-1-/- and control mice following whole-body gamma-irradiation (1 Gy). In the PARP-1-/- mice there was a significant delay during the first 6 h in the transient p53 accumulation in stem cells whereas an increased number of cells were positive for p21(CIP1/WAF1). Either no or only marginal differences were noted in MDM2 expression, apoptosis, induction of or recovery from mitotic blockage, or inhibition of DNA synthesis. We further observed a dose-dependent reduction in crypt survival measured at 4 days post-irradiation in control mice, and this crypt-killing effect was significantly potentiated in PARP-1-/- mice. Our results thus establish that PARP-1 acts as a survival factor for intestinal stem cells in vivo and suggest a functional link with early p53 and p21(CIP1/WAF1) responses.

Role of poly(ADP-ribose)polymerase in cisplatin-induced injury in LLC-PK1 cells

Acute renal failure is a dose-limiting factor during cisplatin chemotherapy. We have previously shown in rats that the hydroxyl radical scavenger edaravone reverses cisplatin-induced in vivo renal damage. In the present study, the role of poly(ADP-ribose) polymerase (PARP) in cisplatin nephrotoxicity was investigated in porcine tubular cells LLC-PK1. Cell injury was elicited by transient exposure to 500 microM cisplatin for 1 h or continuous exposure to 30 microM cisplatin for 24 h. Various hydroxyl radical scavengers reversed cell damage in a transient but not permanent model. The cell injury seemed to be necrosis and apoptosis in transient and permanent models, respectively, as assessed by TUNEL method and Annexin V stain. PARP inhibitors such as 3-aminobenzamide and benzamide inhibited cell damage in transient but not permanent model. PARP-dependent cell injury was also observed after transient exposure to hydroxyl radical-generating solution. We demonstrated for the first time the activation of PARP in renal tubular cells by transient cisplatin exposure, as determined by immunofluorescent stain with anti-poly(ADP-ribose) antibody. Moreover, ATP was depleted by transient exposure to cisplatin or hydroxyl radical, both of which were reversed by PARP inhibitors. These findings suggest that hydroxyl radical generation followed by PARP activation contributes to the necrotic cell injury caused by a transient exposure to cisplatin.

Ischemic preconditioning by caspase cleavage of poly(ADP-ribose) polymerase-1

A transient, sublethal ischemic interval confers resistance to a subsequent, otherwise lethal ischemic insult, in a process termed ischemic preconditioning. Poly(ADP-ribose) polymerase-1 (PARP-1) normally functions in DNA repair, but extensive PARP-1 activation is a major cause of ischemic cell death. Because PARP-1 can be cleaved and inactivated by caspases, we investigated the possibility that caspase cleavage of PARP-1 could contribute to ischemic preconditioning. Murine cortical cultures were treated with glucose deprivation combined with 0.5 mm 2-deoxyglucose and 5 mm azide ("chemical ischemia") to model the reversible energy failure that occurs during transient ischemia in vivo. Cortical cultures preconditioned with 15 min of chemical ischemia showed increased resistance to subsequent, longer periods of chemical ischemia. These cultures were also more resistant to the PARP-1 activating agent, N-methyl-N'-nitro-N-nitrosoguanidine, suggesting reduced capacity for PARP-1 activation after preconditioning. Immunostaining for the 89 kDa PARP-1 cleavage fragment and for poly(ADP-ribose) formation confirmed that PARP-1 was cleaved and PARP-1 activity was attenuated in the preconditioned neurons. Preconditioning also produced an increase in activated caspase-3 peptide and an increase in caspase-3 activity in the cortical cultures. A cause-effect relationship between caspase activation, PARP-1 cleavage, and ischemic preconditioning was supported by studies using the caspase inhibitor Ac-Asp-Glu-Val-Asp-aldehyde (DEVD-CHO). Cultures treated with DEVD-CHO after preconditioning showed reduced PARP-1 cleavage and reduced resistance to subsequent ischemia. These findings suggest a novel interaction between the caspase- and PARP-1-mediated cell death pathways in which sublethal caspase activation leads to PARP-1 cleavage, thereby increasing resistance to subsequent ischemic stress.

Very low concentrations of arsenite suppress poly(ADP-ribosyl)ation in mammalian cells

Arsenite is a naturally occurring environmental pollutant of major concern, since adverse health effects including cancer of skin and internal organs have been attributed to chronic arsenic exposure especially via drinking water. Arsenite is not a significant inducer of point mutations but exerts clastogenic activities and interferes with various DNA repair systems at concentrations in the low micromolar range. Nevertheless, no single DNA repair protein exquisitely sensitive to arsenic has been identified. Here we report that poly(ADP-ribosyl)ation, which is predominantly mediated by poly(ADP-ribose) polymerase-1 (PARP-1), is inhibited at concentrations as low as 10 nM in cultured HeLa cells, closely matching arsenic concentrations in blood and urine of the general population. Since poly(ADP-ribosyl)ation is an immediate cellular response to DNA damage, playing a major role in DNA base excision repair and the maintenance of genomic stability, its inhibition by arsenite may add to the risk of cancer formation under low-exposure conditions.

Induction of poly(ADP-ribosyl)ation and DNA damage in human peripheral lymphocytes after treatment with (-)-epigallocatechin-gallate

With regard to a future use of tea polyphenols in intervention trials with individuals at high cancer risk, the effects of the tea ingredient (-)-epigallocatechin gallate (EGCG) on poly(ADP-ribose) (PAR) levels and on DNA damage were investigated in human lymphocytes. A dose- and time-dependent elevation of both PAR formation as assessed by quantitative immunofluorescence analysis and DNA damage as assessed by the comet assay were observed after treatment with EGCG at 20, 40 and 80 microM for 10-240 min. Maximum levels of PAR formation and of DNA damage were observed after 10 min at all concentrations tested. Increased PAR levels were still detectable by 240 min in the 40 and 80 microM groups. At the lowest concentration, which is near the physiological peak values found after tea ingestion, PAR formation was not correlated with DNA damage. Here, EGCG led to pronounced PAR levels, whereas the comet assay was almost negative. In contrast, such marked differences in time course and extent of both genotoxicity and PAR formation following EGCG treatment were not detected after gamma-irradiation. Our results suggest that the known chemopreventive effects of EGCG, the main constituent of tea, may be partly attributed to an induction of PAR formation.

Tricarboxylic acid cycle substrates prevent PARP-mediated death of neurons and astrocytes

The DNA repair enzyme, poly(ADP-ribose) polymerase-1 (PARP1), contributes to cell death during ischemia/reperfusion when extensively activated by DNA damage. The cell death resulting from PARP1 activation is linked to NAD+ depletion and energy failure, but the intervening steps are not well understood. Because glycolysis requires cytosolic NAD+, the authors tested whether PARP1 activation impairs glycolytic flux and whether substrates that bypass glycolysis can rescue cells after PARP1 activation. PARP1 was activated in mouse cortical astrocyte and astrocyte-neuron cocultures with the DNA alkylating agent, N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). Studies using the 2-deoxyglucose method confirmed that glycolytic flux was reduced by more than 90% in MNNG-treated cultures. The addition of 5 mmol/L of alpha-ketoglutarate, 5 mmol/L pyruvate, or other mitochondrial substrates to the cultures after MNNG treatment reduced cell death from approximately 70% to near basal levels, while PARP inhibitors and excess glucose had negligible effects. The mitochondrial substrates significantly reduced cell death, with delivery delayed up to 2 hours after MNNG washout. The findings suggest that impaired glycolytic flux is an important factor contributing to PARP1-mediated cell death. Delivery of alternative substrates may be a promising strategy for delayed treatment of PARP1-mediated cell death in ischemia and other disorders.

In-situ analysis of cellular poly(ADP-ribose) production in scrapie-infected mouse neuroblastoma cells

Transmissible spongiform encephalopathies (TSEs), also called prion diseases, are characterized by formation of the disease-associated isoform of prion protein (PrP(Sc)), which arises from a normal isoform termed PrP(c) by a post-translational conversion process occurring in an autocatalytic fashion. Oxidative stress has been proposed as a pathogenetic mechanism in TSEs and increased lipid peroxidation has recently been described in prion-infected cell cultures, suggesting an intrinsic link between the presence of prions and oxidative stress. We investigated if poly(ADP-ribose) formation can be detected in cultured cells upon prion infection, as this NAD+-consuming and DNA strand break-activated nuclear enzymatic reaction has the potential to cause rapid and lethal NAD+ depletion in cells under severe oxidative stress. Poly(ADP-ribose) production was analysed by immunofluorescence in freshly scrapie-infected Neuro2a-D11 mouse neuroblastoma cells, which had been confirmed by immunocytochemistry to produce PrP(Sc), and in uninfected controls. No spontaneous poly(ADP-ribose) specific signals were observed in infected or in uninfected cells, while both cell types readily reacted to H2O2 treatment with poly(ADP-ribose) synthesis in a dose-dependent manner, with no obvious difference in staining intensity at any dose tested. In summary, our data reveal that replication of scrapie agent in neuroblastoma cells can proceed without detectable stimulation of the cellular poly(ADP-ribosyl)ation system.