Direct stimulation of poly(ADP ribose) polymerase in permeabilized cells by double-stranded DNA oligomers

The Development of Rucaparib/Rubraca®: A Story of the Synergy Between Science and Serendipity

The poly(ADP-ribose) polymerase (PARP) inhibitor, Rubraca®, was given its first accelerated approval for BRCA-mutated ovarian cancer by the FDA at the end of 2016, and further approval by the FDA, EMA and NICE followed. Scientists at Newcastle University initiated the early stages, and several collaborations with scientists in academia and the pharmaceutical industry enabled its final development to the approval stage. Although originally considered as a chemo- or radiosensitiser, its current application is as a single agent exploiting tumour-specific defects in DNA repair. As well as involving intellectual and physical effort, there have been a series of fortuitous occurrences and coincidences of timing that ensured its success. This review describes the history of the relationship between science and serendipity that brought us to the current position.

Strategies Employed for the Development of PARP Inhibitors

This chapter describes the approaches taken in the development of the first PARP inhibitor to enter clinical trial, rucaparib (now called Rubraca), in 2003. We describe the general principles of crystal-based drug design, the purification and crystallization of the PARP-1 catalytic domain and how this was used to develop highly potent PARP inhibitors, based on the nicotinamide pharmacophore. Several methods have been used to determine the inhibitory potency in cell-free and whole cell assays, each described with reference to its advantages and disadvantages.

Strategies employed for the development of PARP inhibitors

This chapter describes the approaches taken in the development of the first PARP inhibitor to enter into clinical trial, AG-014699. We describe the general principles of crystal-based drug design, the purification, and crystallization of the PARP-1 catalytic domain, and how this approach was used to develop highly potent PARP inhibitors based on the nicotinamide pharmacophore. Several methods have been used to determine the inhibitory potency of designed inhibitors in cell-free and whole cell assays; each is described with reference to its advantages and disadvantages.

Rapamycin inhibits poly(ADP-ribosyl)ation in intact cells

Rapamycin is an immunosuppressive drug, which inhibits the mammalian target of rapamycin (mTOR) kinase activity inducing changes in cell proliferation. Synthesis of poly(ADP-ribose) (PAR) is an immediate cellular response to genotoxic stress catalyzed mostly by poly(ADP-ribose) polymerase 1 (PARP-1), which is also controlled by signaling pathways. Therefore, we investigated whether rapamycin affects PAR production. Strikingly, rapamycin inhibited PAR synthesis in living fibroblasts in a dose-dependent manner as monitored by immunofluorescence. PARP-1 activity was then assayed in vitro, revealing that down-regulation of cellular PAR production by rapamycin was apparently not due to competitive PARP-1 inhibition. Further studies showed that rapamycin did not influence the cellular NAD pool and the activation of PARP-1 in extracts of pretreated fibroblasts. Collectively, our data suggest that inhibition of cellular PAR synthesis by rapamycin is mediated by formation of a detergent-sensitive complex in living cells, and that rapamycin may have a potential as therapeutic PARP inhibitor.

Ionizing radiation-induced NF-kappaB activation requires PARP-1 function to confer radioresistance

Recent reports implicate poly(ADP-ribose) polymerase-1 (PARP-1) in the activation of nuclear factor kappaB (NF-kappaB). We investigated the role of PARP-1 in the NF-kappaB signalling cascade induced by ionizing radiation (IR). AG14361, a potent PARP-1 inhibitor, was used in two breast cancer cell lines expressing different levels of constitutively activated NF-kappaB, as well as mouse embryonic fibroblasts (MEFs) proficient or deficient for PARP-1 or NF-kappaB p65. In the breast cancer cell lines, AG14361 had no effect on IR-induced degradation of IkappaBalpha or nuclear translocation of p50 or p65. However, AG14361 inhibited IR-induced NF-kappaB-dependent transcription of a luciferase reporter gene. Similarly, in PARP-1(-/-) MEFs, IR-induced nuclear translocation of p50 and p65 was normal, but kappaB binding and transcriptional activation did not occur. AG14361 sensitized both breast cancer cell lines to IR-induced cell killing, inhibited IR-induced XIAP expression and increased caspase-3 activity. However, AG14361 failed to increase IR-induced caspase activity when p65 was knocked down by siRNA. Consistent with this, AG14361 sensitized p65(+/+) but not p65(-/-) MEFs to IR. We conclude that PARP-1 activity is essential in the upstream regulation of IR-induced NF-kappaB activation. These data indicate that potentiation of IR-induced cytotoxicity by AG14361 is mediated solely by inhibition of NF-kappaB activation.

Effect of zinc on cellular poly(ADP-ribosyl)ation capacity

Poly(ADP-ribosyl)ation is a posttranslational protein modification, which is catalyzed by poly(ADP-ribose) polymerase-1 (PARP-1) and plays a role in DNA repair and maintenance of genomic stability. A decrease in cellular poly(ADP-ribosyl)ation has been implicated in the aging process. As PARP-1 is a zinc finger protein its decreased function might be related to age-related zinc deficiency. To test this hypothesis we assessed cellular poly(ADP-ribosyl)ation capacity in 29 donors from Greece, Italy and Poland as function of age and nutritional zinc status. Our results reveal a positive correlation between cellular poly(ADP-ribosyl)ation capacity and zinc status in human peripheral blood mononuclear cells (PBMC) (p<0.05). We could also confirm a decrease of PARP-1 activity with donor age, highlighting the role of poly(ADP-ribosyl)ation in the aging process. The results demonstrate that zinc supplementation in elderly people can increase the cellular poly(ADP-ribosyl)ation capacity of their PBMC. We speculate that this may help maintain integrity and stability of the genome more efficiently and thus contribute to an extension of healthspan.

Flow-cytometric assessment of cellular poly(ADP-ribosyl)ation capacity in peripheral blood lymphocytes

Background

Poly(ADP-ribosyl)ation is a posttranslational modification of nuclear proteins catalysed by poly(ADP-ribose) polymerases (PARPs), using NAD⁺ as a substrate. Activation of PARP-1 is in immediate response to DNA damage generated by endogenous and exogenous damaging agents. It has been implicated in several crucial cellular processes including DNA repair and maintenance of genomic stability, which are both intimately linked with the ageing process. The measurement of cellular poly(ADP-ribosyl)ation capacity, defined as the amount of poly(ADP-ribose) produced under maximal stimulation, is therefore relevant for research on ageing, as well as for a variety of other scientific questions.

Results

This paper reports a new, robust protocol for the measurement of cellular poly(ADP-ribosyl)ation capacity in PBMC or Jurkat T-cells using flow cytometry, based on a previously established immuno-dot-blot assay. In order to validate the new assay, we determined the dose-response curve of 3-aminobenzamide, a well-known competitive PARP inhibitor, and we derived an IC₅₀ that is very close to the published value. When testing a set of PBMC samples taken from fifteen healthy young human donors, we could confirm the presence of a substantial interindividual variation, as previously observed using a radiometric assay.

Conclusion

The methodology described in this paper should be generally useful for the determination of cellular poly(ADP-ribosyl)ation capacity in a wide variety of settings, especially for the comparison of large sets of samples, such as population studies. In contrast to previously published radiometric or immuno-dot-blot assays, the new FACS-based method allows (i) selective analysis of mononuclear cells by gating and (ii) detection of a possible heterogeneity in poly(ADP-ribosyl)ation capacity between cells of the same type.

Temozolomide Pharmacodynamics in Patients with Metastatic Melanoma: DNA Damage and Activity of Repair EnzymesO6-Alkylguanine Alkyltransferase and Poly(ADP-Ribose) Polymerase-1

PURPOSE

Temozolomide, a DNA methylating agent used to treat melanoma, induces DNA damage, which is repaired by O6-alkylguanine alkyltransferase (ATase) and poly(ADP-ribose) polymerase-1 (PARP-1)-dependent base excision repair. The current study was done to define the effect of temozolomide on DNA integrity and relevant repair enzymes as a prelude to a phase I trial of the combination of temozolomide with a PARP inhibitor.

EXPERIMENTAL DESIGN

Temozolomide (200 mg/m2 oral administration) was given to 12 patients with metastatic malignant melanoma. Peripheral blood lymphocytes (PBL) were analyzed for PARP activity, DNA single-strand breakage, ATase levels, and DNA methylation. PARP activity was also measured in tumor biopsies from 9 of 12 patients and in PBLs from healthy volunteers.

RESULTS

Temozolomide pharmacokinetics were consistent with previous reports. Temozolomide therapy caused a substantial and sustained elevation of N7-methylguanine levels, a modest and sustained reduction in ATase activity, and a modest and transient increase in DNA strand breaks and PARP activity in PBLs. PARP-1 activity in tumor homogenates was variable (828 +/- 599 pmol PAR monomer/mg protein) and was not consistently affected by temozolomide treatment.

CONCLUSIONS

The effect of temozolomide reported here are consistent with those documented in previous studies with temozolomide and similar drug, dacarbazine, demonstrating that a representative patient population was investigated. Furthermore, PARP activity was not inhibited by temozolomide treatment and this newly validated pharmacodynamic assay is therefore suitable for use in a proof-of-principle phase I trial a PARP-1 inhibitor in combination with temozolomide.

Specific killing of BRCA2-deficient tumours with inhibitors of poly(ADP-ribose) polymerase

Poly(ADP-ribose) polymerase (PARP1) facilitates DNA repair by binding to DNA breaks and attracting DNA repair proteins to the site of damage. Nevertheless, PARP1-/- mice are viable, fertile and do not develop early onset tumours. Here, we show that PARP inhibitors trigger gamma-H2AX and RAD51 foci formation. We propose that, in the absence of PARP1, spontaneous single-strand breaks collapse replication forks and trigger homologous recombination for repair. Furthermore, we show that BRCA2-deficient cells, as a result of their deficiency in homologous recombination, are acutely sensitive to PARP inhibitors, presumably because resultant collapsed replication forks are no longer repaired. Thus, PARP1 activity is essential in homologous recombination-deficient BRCA2 mutant cells. We exploit this requirement in order to kill BRCA2-deficient tumours by PARP inhibition alone. Treatment with PARP inhibitors is likely to be highly tumour specific, because only the tumours (which are BRCA2-/-) in BRCA2+/- patients are defective in homologous recombination. The use of an inhibitor of a DNA repair enzyme alone to selectively kill a tumour, in the absence of an exogenous DNA-damaging agent, represents a new concept in cancer treatment.

Anticancer chemosensitization and radiosensitization by the novel poly(ADP-ribose) polymerase-1 inhibitor AG14361

BACKGROUND

Poly(ADP-ribose) polymerase-1 (PARP-1) facilitates the repair of DNA strand breaks. Inhibiting PARP-1 increases the cytotoxicity of DNA-damaging chemotherapy and radiation therapy in vitro. Because classical PARP-1 inhibitors have limited clinical utility, we investigated whether AG14361, a novel potent PARP-1 inhibitor (inhibition constant <5 nM), enhances the effects of chemotherapy and radiation therapy in human cancer cell cultures and xenografts.

METHODS

The effect of AG14361 on the antitumor activity of the DNA alkylating agent temozolomide, topoisomerase I poisons topotecan or irinotecan, or x-irradiation or gamma-radiation was investigated in human cancer cell lines A549, LoVo, and SW620 by proliferation and survival assays and in xenografts in mice by tumor volume determination. The specificity of AG14361 for PARP-1 was investigated by microarray analysis and by antiproliferation and acute toxicity assays in PARP-1-/- and PARP-1+/+ cells and mice. After intraperitoneal administration, the concentration of AG14361 was determined in mouse plasma and tissues, and its effect on PARP-1 activity was determined in tumor homogenates. All statistical tests were two-sided.

RESULTS

AG14361 at 0.4 micro M did not affect cancer cell gene expression or growth, but it did increase the antiproliferative activity of temozolomide (e.g., in LoVo cells by 5.5-fold, 95% confidence interval [CI] = 4.9-fold to 5.9-fold; P =.004) and topotecan (e.g., in LoVo cells by 1.6-fold, 95% CI = 1.3-fold to 1.9-fold; P =.002) and inhibited recovery from potentially lethal gamma-radiation damage in LoVo cells by 73% (95% CI = 48% to 98%). In vivo, nontoxic doses of AG14361 increased the delay of LoVo xenograft growth induced by irinotecan, x-irradiation, or temozolomide by two- to threefold. The combination of AG14361 and temozolomide caused complete regression of SW620 xenograft tumors. AG14361 was retained in xenografts in which PARP-1 activity was inhibited by more than 75% for at least 4 hours.

CONCLUSION

AG14361 is, to our knowledge, the first high-potency PARP-1 inhibitor with the specificity and in vivo activity to enhance chemotherapy and radiation therapy of human cancer.

Arsenic toxicity and potential mechanisms of action

Exposure to the metalloid arsenic is a daily occurrence because of its environmental pervasiveness. Arsenic, which is found in several different chemical forms and oxidation states, causes acute and chronic adverse health effects, including cancer. The metabolism of arsenic has an important role in its toxicity. The metabolism involves reduction to a trivalent state and oxidative methylation to a pentavalent state. The trivalent arsenicals, including those methylated, have more potent toxic properties than the pentavalent arsenicals. The exact mechanism of the action of arsenic is not known, but several hypotheses have been proposed. At a biochemical level, inorganic arsenic in the pentavalent state may replace phosphate in several reactions. In the trivalent state, inorganic and organic (methylated) arsenic may react with critical thiols in proteins and inhibit their activity. Regarding cancer, potential mechanisms include genotoxicity, altered DNA methylation, oxidative stress, altered cell proliferation, co-carcinogenesis, and tumor promotion. A better understanding of the mechanism(s) of action of arsenic will make a more confident determination of the risks associated with exposure to this chemical.

Inhibition of homodimerization of poly(ADP-ribose) polymerase by its C-terminal cleavage products produced during apoptosis

The biochemical role of the C-terminal fragment of poly(ADP-ribose) polymerase (PARP) was investigated in HeLa cells undergoing UV-mediated apoptosis. During the course of apoptosis, the C-terminal cleavage product of PARP interacted with intact PARP and down-regulated PARP activity by blocking the homodimerization of PARP. The basic leucine zipper motif in the auto-modification domain of the C-terminal fragment of PARP represented the site of association, and Leu(405) was critical to the ability of the basic leucine zipper motif to associate with intact PARP. The expression of the C-terminal fragment of PARP stimulated UV-mediated apoptosis. These results suggest that the C-terminal cleavage product of PARP produced during apoptosis blocks the homodimerization of PARP and inhibits the cellular PARP activity. The inhibition of the cellular PARP activity might prevent cellular NAD(+) depletion and stimulate apoptosis by maintaining the basal cellular energy level required for the completion of apoptosis.

DNA-binding activity of the N-terminal cleavage product of poly(ADP-ribose) polymerase is required for UV mediated apoptosis

The role of the N-terminal cleavage product of poly(ADP-ribose) polymerase (PARP) on UV mediated apoptosis was investigated in cultured HeLa cells. Ultrastructural analysis of cells expressing caspase-resistant PARP (PARP(D214A)) revealed the typical features of necrosis following UV treatment. However, cells co-expressing PARP(D214A) with the N-terminal fragment of PARP containing the DNA-binding domain underwent apoptosis instead of necrosis. In this study, we have demonstrated that the DNA-binding activity of the N-terminal fragment of PARP is important for the execution of apoptosis. Point mutations were introduced in the DNA-binding sites of the N-terminal fragment. Cells co-expressing PARP(D214A) with the mutated N-terminal fragments neither stimulated apoptosis nor prevented necrosis in response to UV irradiation. The present study proposes that the DNA-binding activity of the N-terminal fragment of PARP in UV treated cells prevents cellular ATP depletion, a mechanism by which necrotic cell death is triggered.

Elevation of apoptotic potential by anoxia hyperoxia shift in NIH3T3 cells

Apoptosis has been hypothesized to be mediated through the induction of free radicals via oxidative pathway. In this study, we demonstrated the induction of cellular apoptosis by anoxia-hyperoxia shift, but not by anoxia or hyperoxia alone in NIH3T3 cells. The decrement of ROS by anoxia thus appears to be an essential early event leading to apoptosis. G1 arrest was detected in anoxia-treated cells, and postanoxic oxygen recovery could reverse this effect, and induce apoptosis. On analysis of the binding activity of AP-1, we found biphasic induction of binding ability in cells undergoing anoxia-hyperoxia shift. In the early stage of anoxia, a transitional increase of AP-1 binding activity was detected, which was reduced to the minimal levels after 24 h of anoxia. During the period of postanoxic hyperoxia treatment, the binding activity of AP-1 was reinduced and increased remarkably with time up to 24 h. These results were in accordance with the expressions of c-jun and c-fos proteins. Enhancement of poly(ADP-ribosyl)ation activities, especially ADP-ribosylation of histone H1 was detected in post-anoxic hyperoxia-treated cells, and cleavage of PARP and activation of caspase 3 were also observed in post-anoxic hyperoxia (recovery) treated cells, but not in anoxia-treated cells. We propose that the differential induction of c-jun/c-fos (AP-1) gene expressions and sequential activation of PARP activity are essential in anoxia/hyperoxia-induced apoptosis.

In situ staining for poly(ADP-ribose) polymerase activity using an NAD analogue

Poly(ADP-ribose) polymerase (PARP) is a highly abundant nuclear enzyme which metabolizes NAD, in response to DNA strand breakage, to produce chains of poly(ADP-ribose) attached to nuclear proteins. PARP activation has been implicated in ischemia/reperfusion injury, but its biological significance is not fully understood. We have modified an existing in situ method for detection of PARP activity by using an NAD analogue in which adenine is modified by an "etheno" (vinyl) bridge. Etheno-NAD serves as a PARP substrate in an initial enzymatic reaction; a specific antibody to ethenoadenosine is then used in an immunohistochemical reaction to detect the production of modified poly(ADP-ribose). The method produces strong and specific labeling of nuclei in which PARP has been activated, i.e., those in which DNA strand breaks have been produced, and the results can be analyzed by microscopy, flow cytometry, or colorimetry. The method is applicable to cultured cells in several formats and to frozen tissue sections. The particular characteristics of the new method may assist in future in situ studies of PARP activation.

Inhibition of poly(ADP-ribose) polymerase by arsenite

Inorganic arsenic is considered a human carcinogen based principally on epidemiological evidence. Unlike most initiating chemicals, arsenic is inactive or extremely weak in its ability to directly induce gene mutations. Arsenite has been shown, however, to enhance mutagenicity when present with other agents such as UV radiation. Synergistic potentiation of chromosomal damage has been shown with co-treatment with DNA-crosslinking agents. Arsenite at low concentrations is known to be highly selective in reacting with closely spaced (vicinal) dithiol groups in proteins. Poly(ADP-ribose) polymerase (PARP) is known to contain such vicinal dithiol groups. Stimulation of PARP is an immediate response of eukaryotic cells to DNA strand breaks and has been implicated in DNA repair. The effect of treatment with sodium arsenite on PARP activity was assessed as follows: Molt-3 cells (a human T-cell lymphoma-derived cell line) in culture were treated for 24 h with concentrations of sodium arsenite ranging from 2.5 up to 25 microM. Speciation of inorganic arsenic and cell viability were determined. Cell cycle kinetics were measured by flow cytometry. Poly(ADP-ribose) synthesis was assayed using a palindromic decameric deoxynucleotide to stimulate enzyme activity. Results show that arsenite decreases PARP activity in a dose-dependent manner with an approximately 50% decrease in enzyme activity at 10 microM arsenite and 80% viability. The percent of cells in S-phase increases with increasing concentration of arsenite. These results provide further indication that arsenite may potentiate genetic damage through reaction with dithiols in DNA repair proteins such as PARP, perhaps resulting in interference with normal repair function.

Low nicotinamide mononucleotide adenylyltransferase activity in a tiazofurin-resistant cell line: effects on NAD metabolism and DNA repair

Poly(ADP-ribose) polymerase (PADPRP), which uses NAD to synthesize ADP-ribose polymers, is activated by DNA strand breaks and mediates cellular responses to DNA damage. The consequences of low cellular NAD levels in a cell line deficient in nicotinamide mononucleotide adenylyltransferase (NMNAT), an enzyme essential for NAD biosynthesis, were investigated by assessing NAD metabolism and DNA repair after treatment with alkylating agents. A tiazofurin-resistant L1210 cell line (TZR) was isolated. NAD levels were approximately 5933 and 3375 pmol mg(-1) protein for parental (wild type, WT) and TZR cells respectively, and NMNAT levels were reduced by > 95%. TZR cells were more sensitive to temozolomide (TM) and 1-methyl-3-nitro-1-nitroso-guanidine (MNNG), particularly at concentrations that caused > 50% NAD depletion. TM and MNNG treatment decreased NAD levels in both cell lines, but took longer to return to control levels in TZR cells. For example, MNNG (5 microM), depleted NAD levels at 6 h to approximately 4512 (WT) and 1442 (TZR) pmol mg(-1) protein; however, NAD levels had returned to control levels by 8 h in WT cells, but were not restored by 16 h in TZR cells. Both cell lines were equisensitive to the growth-inhibitory effects of NU1025 per se (IC50 370 microM). Co-exposure of the cell lines to TM (100 microM) with increasing concentrations of NU1025 led to a synergistic enhancement of cytotoxicity, with IC50 values for NU1025 decreasing to 17 +/- 4 microM (TZR) and 37 +/- 6 microM (WT). A similar enhanced sensitivity to NU1025 (approximately 2.7-fold) was obtained when TZR cells were co-exposed to MNNG + NU1025. TM-induced DNA strand breaks were increased by co-incubation with NU1025, and again the TZR cell line showed increased sensitivity to NU1025. There were no significant changes in NMNAT activity in response to MNNG treatment over 24 h, either in the presence or in the absence of NU1025. These data demonstrate that modest decreases in cellular NAD levels can sensitize cells to alkylating agents and PADPRP inhibitors.

Poly(ADP-ribosyl)ation of histone H1 correlates with internucleosomal DNA fragmentation during apoptosis

The biochemical role of poly(ADP-ribosyl)ation on internucleosomal DNA fragmentation associated with apoptosis was investigated in HL 60 human premyelocytic leukemia cells. It was found that UV light and chemotherapeutic drugs including adriamycin, mitomycin C, and cisplatin increased poly(ADP-ribosyl)ation of nuclear proteins, particularly histone H1. A poly(ADP-ribose) polymerase inhibitor, 3-aminobenzamide, prevented both internucleosomal DNA fragmentation and histone H1 poly(ADP-ribosyl)ation in cells treated with the apoptosis inducers. When nuclear chromatin was made accessible to the exogenous nuclease in a permeabilized cell system, chromatin of UV-treated cells was more susceptible to micrococcal nuclease than the chromatin of control cells. Suppression of histone H1 poly(ADP-ribosyl)ation by 3-aminobenzamide reduced the micrococcal nuclease digestibility of internucleosomal chromatin in UV-treated cells. These results suggest that the poly(ADP-ribosyl)ation of histone H1 correlates with the internucleosomal DNA fragmentation during apoptosis mediated by DNA damaging agents. This suggestion is supported by the finding that xeroderma pigmentosum cells which are defective in introducing incision at the site of DNA damage, failed to induce DNA fragmentation as well as histone H1 poly(ADP-ribosyl)ation after UV irradiation. We propose that poly(ADP-ribosyl)ation of histone H1 protein in the early stage of apoptosis facilitates internucleosomal DNA fragmentation by increasing the susceptibility of chromatin to cellular endonuclease.

Potentiation of temozolomide-induced cytotoxicity: a comparative study of the biological effects of poly(ADP-ribose) polymerase inhibitors

Four poly(ADP-ribose) polymerase (PADPRP) inhibitors [3-aminobenzamide, benzamide, 3,4-dihydro-5-methoxyisoquinolin-1(2H)-one (PD 128763) and 8-hydroxy-2-methylquinazolin-4(3H)-one (NU1025)] were compared with respect to their effects on a number of biological end points. The following parameters were assessed: their ability to inhibit the enzyme in permeabilised L1210 cells; their ability to potentiate the cytotoxicity of temozolomide (including the cytotoxicity of the compounds per se); their ability to increase net levels of temozolomide-induced DNA strand breaks and inhibit temozolomide-induced NAD depletion. PD 128763 and NU1025 were equipotent as PADPRP inhibitors, and 40- and 50-fold more potent than benzamide and 3-aminobenzamide respectively. All the compounds acted in a concentration-dependent manner to potentiate the cytotoxicity and increase DNA strand break levels in cells treated with temozolomide. There was an excellent correlation between the potency of the compounds as PADPRP inhibitors and their effects on cell survival and DNA repair. Temozolomide treatment caused a decrease in cellular NAD levels, and this was abolished by the PADPRP inhibitors. In conclusion, the new generation of PADPRP inhibitors are at least 50-fold more effective than 3-aminobenzamide as chemopotentiators, and can be used at micromolar rather than millimolar concentrations in intact cells.

Heat shock induces resistance in rat pancreatic islet cells against nitric oxide, oxygen radicals and streptozotocin toxicity in vitro

When cultures of pancreatic islet cells are exposed to the nitric oxide donor sodium nitroprusside, to enzymatically generated reactive oxygen intermediates or to streptozotocin cell lysis occurs after 4-12 h. We report here that a heat shock at 43 degrees C for 90 min reduces cell lysis from nitric oxide (0.45 mM sodium nitroprusside) by 70%, from reactive oxygen intermediates (12 mU xanthine oxidase and 0.05 mM hypoxanthine) by 80% and from streptozotocin (1.5 mM) by 90%. Heat shock induced resistance was observed immediately after termination of the 90 min culture at 43 degrees C and correlated with enhanced expression of hsp70. The occurrence of DNA strand breaks, a major early consequence of nitric oxide, reactive oxygen intermediates, or streptozotocin action, was not suppressed by heat shock treatment. However, the depletion of NAD+, the major cause of radical induced islet cell death, was suppressed after heat shock (P < 0.01). We conclude that pancreatic islet cells can rapidly activate defence mechanisms against nitric oxide, reactive oxygen intermediates and streptozotocin by culture at 43 degrees C. Islet cell survival is due to the prevention of lethal NAD+ depletion during DNA repair, probably by slowing down poly(ADP-ribose)polymerase activation.

Modulation of poly(ADP-ribose) polymerase during neutrophilic and monocytic differentiation of promyelocytic (NB4) and myelocytic (HL-60) leukaemia cells

Poly(ADP-ribose) polymerase (PARP) is a nuclear enzyme which has been shown to play a role in the differentiation of haematopoietic cells. We report here that neutrophils are the first nucleated mammalian cell type demonstrated to be devoid of immunoreactive PARP. Both NB4 acute promyelocytic leukaemia and HL-60 (acute myelocytic leukaemia) cells were differentiated into non-malignant neutrophils with all-trans-retinoic acid (ATRA). Western blot analysis demonstrated that ATRA had no effect on PARP expression in HL-60 cells. However, PARP was completely down-regulated in NB4 cells within 36 h of treatment initiation. This decrease in PARP polypeptide coincided with growth arrest and preceded the appearance of neutrophilic differentiation features. NB4 cells require a combination of 1,25-dihydroxyvitamin D3 (1,25-D3) and phorbol 12-myristate 13-acetate (PMA) to differentiate completely into monocyte/macrophages, whereas HL-60 cells can be made to differentiate by combined or single agents. PARP expression was up-regulated 90-fold when NB4 cells were treated with PMA and 1,25-D3 together, and this increase accompanied expression of the monocyte/macrophage phenotype. Only modest changes in PARP expression were observed when each agent was used alone in NB4 cells or when HL-60 cells were differentiated along the monocyte/macrophage pathway. In addition, PARP activity was modulated in a pattern similar to protein levels when NB4 cells were induced to differentiate along the neutrophilic and monocyte/macrophage pathways. This suggests that the activity of PARP may be controlled through regulation of protein levels during NB4 cell differentiation. We conclude that PARP levels are dramatically modulated during monocyte/macrophage and neutrophilic differentiation. On the basis of the tremendous changes in PARP polypeptide and total activity during myeloid differentiation, we propose that modulation of PARP gene expression is required for cellular maturation in both lineages.

Cell culture of tumors alters endogenous poly(ADPR)polymerase expression and activity

Poly(ADP-ribose)polymerase, a chromatin-bound enzyme, actively participates in processes such as cell proliferation, differentiation, and DNA repair and replication. This enzyme is also implicated in cell transformation, and its inhibition has been proposed to potentiate anti-cancer drug activity. Since cells prepared from tumor biopsies and established tumor cell lines are commonly used to evaluate the efficiency of anticancer therapies, we have compared poly(ADP-ribose)polymerase activity in animal tumor cells growing in vivo and in cell culture. Three tumor types were tested: a mastocytoma (P815), a lymphoma (RDM4), and a glioma (C6). Our results show that cell culture alters poly(ADP-ribose)polymerase levels and activity. Endogenous poly(ADP-ribose) activity was several fold higher in exponentially growing cells than in cells freshly recovered from solid or ascitic tumors. Moreover, polymerase activity increased with culture time, reaching a maximum when cells became confluent. Measurements of poly(ADP-ribose)polymerase gene expression and protein amount indicate that lower enzyme activity in tumors grown in vivo are sustained by decreases in poly(ADP-ribose)polymerase mRNA and protein amount. In contrast, the increase in endogenous poly(ADP-ribose)polymerase activity observed in cultured cells was due to enzyme activation and not to de novo protein synthesis. Such differences must be considered when assessing the applicability of cell-culture results to in vivo situations.

Poly(ADP-ribose) polymerase activity in mononuclear leukocytes of 13 mammalian species correlates with species-specific life span

Poly(ADP-ribosyl)ation is a eukaryotic posttranslational modification of proteins that is strongly induced by the presence of DNA strand breaks and plays a role in DNA repair and the recovery of cells from DNA damage. We compared poly(ADP-ribose) polymerase (PARP; EC 2.4.2.30) activities in Percoll gradient-purified, permeabilized mononuclear leukocytes from mammalian species of different maximal life span. Saturating concentrations of a double-stranded octameric oligonucleotide were applied to provide a direct and maximal stimulation of PARP. Our results on 132 individuals from 13 different species yield a strong positive correlation between PARP activity and life span (r = 0.84; P << 0.001), with human cells displaying approximately 5 times the activity of rat cells. Intraspecies comparisons with both rat and human cells from donors of all age groups revealed some decline of PARP activity with advancing age, but it was only weakly correlated. No significant polymer degradation was detectable under our assay conditions, ruling out any interference by poly(ADP-ribose) glycohydrolase activity. By Western blot analysis of mononuclear leukocytes from 11 species, using a crossreactive antiserum directed against the extremely well-conserved NAD-binding domain, no correlation between the amount of PARP protein and the species' life spans was found, suggesting a greater specific enzyme activity in longer-lived species. We propose that a higher poly(ADP-ribosyl)ation capacity in cells from long-lived species might contribute to the efficient maintenance of genome integrity and stability over their longer life span.