Role of poly(ADP-ribose) polymerase in rapid intracellular acidification induced by alkylating DNA damage

Nucleolus activity-dependent recruitment and biomolecular condensation by pH sensing

DEAD-box ATPases are major regulators of biomolecular condensates and orchestrate diverse biochemical processes that are critical for the functioning of cells. How DEAD-box proteins are selectively recruited to their respective biomolecular condensates is unknown. We explored this in the context of the nucleolus and DEAD-box protein DDX21. We find that the pH of the nucleolus is intricately linked to the transcriptional activity of the organelle and facilitates the recruitment and condensation of DDX21. We identify an evolutionarily conserved feature of the C terminus of DDX21 responsible for nucleolar localization. This domain is essential for zebrafish development, and its intrinsically disordered and isoelectric properties are necessary and sufficient for the ability of DDX21 to respond to changes in pH and form condensates. Molecularly, the enzymatic activities of poly(ADP-ribose) polymerases contribute to maintaining the nucleolar pH and, consequently, DDX21 recruitment and nucleolar partitioning. These observations reveal an activity-dependent physicochemical mechanism for the selective recruitment of biochemical activities to biomolecular condensates.

Antitumor Activity of the Cardiac Glycoside αlDiginoside by Modulating Mcl-1 in Human Oral Squamous Cell Carcinoma Cells

We recently isolated a cardiac glycoside (CG), αldiginoside, from an indigenous plant in Taiwan, which exhibits potent tumor-suppressive efficacy in oral squamous cell carcinoma (OSCC) cell lines (SCC2095 and SCC4, IC₅₀ < 0.2 µM; 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assays). Here, we report that αldiginoside caused Sphase arrest and apoptosis, through the inhibition of a series of signaling pathways, including those mediated by cyclin E, phospho-CDC25C (p-CDC25C), and janus kinase/signal transducer and activator of transcription (JAK/STAT)3. αldiginoside induced apoptosis, as indicated by caspase activation and poly (ADP-ribose) polymerase (PARP) cleavage. Equally important, αldiginoside reduced Mcl-1 expression through protein degradation, and overexpression of Mcl-1 partially protected SCC2095 cells from αldiginoside’s cytotoxicity. Taken together, these data suggest the translational potential of αldiginoside to foster new therapeutic strategies for OSCC treatment.

NAD+ consumption by PARP1 in response to DNA damage triggers metabolic shift critical for damaged cell survival

DNA damage signaling is critical for the maintenance of genome integrity and cell fate decision. Poly(ADP-ribose) polymerase 1 (PARP1) is a DNA damage sensor rapidly activated in a damage dose- and complexity-dependent manner playing a critical role in the initial chromatin organization and DNA repair pathway choice at damage sites. However, our understanding of a cell-wide consequence of its activation in damaged cells is still limited. Using the phasor approach to fluorescence lifetime imaging microscopy and fluorescence-based biosensors in combination with laser microirradiation, we found a rapid cell-wide increase of the bound NADH fraction in response to nuclear DNA damage, which is triggered by PARP-dependent NAD+ depletion. This change is linked to the metabolic balance shift to oxidative phosphorylation (oxphos) over glycolysis. Inhibition of oxphos, but not glycolysis, resulted in parthanatos due to rapid PARP-dependent ATP deprivation, indicating that oxphos becomes critical for damaged cell survival. The results reveal the novel prosurvival response to PARP activation through a change in cellular metabolism and demonstrate how unique applications of advanced fluorescence imaging and laser microirradiation-induced DNA damage can be a powerful tool to interrogate damage-induced metabolic changes at high spatiotemporal resolution in a live cell.

P2X7 receptor-mediated PARP1 activity regulates astroglial death in the rat hippocampus following status epilepticus

Poly(ADP-ribose) polymerase-1 (PARP1) plays a regulatory role in apoptosis, necrosis, and other cellular processes after injury. Recently, we revealed that PARP1 regulates the differential neuronal/astroglial responses to pilocarpine-induced status epilepticus (SE) in the distinct brain regions. In addition, P2X7 receptor (P2X7R), an ATP-gated ion channel, activation accelerates astroglial apoptosis, while it attenuates clasmatodendrosis (lysosome-derived autophagic astroglial death). Therefore, we investigated whether P2X7R regulates regional specific astroglial PARP1 expression/activation in response to SE. In the present study, P2X7R activation exacerbates SE-induced astroglial apoptosis, while P2X7R inhibition attenuates it accompanied by increasing PARP1 activity in the molecular layer of the dentate gyrus following SE. In the CA1 region, however, P2X7R inhibition deteriorates SE-induced clasmatodendrosis via PARP1 activation following SE. Taken together, our findings suggest that P2X7R function may affect SE-induced astroglial death by regulating PARP1 activation/expression in regional-specific manner. Therefore, the selective modulation of P2X7R-mediated PARP1 functions may be a considerable strategy for controls in various types of cell deaths.

In vivo chemical exchange saturation transfer imaging allows early detection of a therapeutic response in glioblastoma

Glioblastoma multiforme (GBM), which account for more than 50% of all gliomas, is among the deadliest of all human cancers. Given the dismal prognosis of GBM, it would be advantageous to identify early biomarkers of a response to therapy to avoid continuing ineffective treatments and to initiate other therapeutic strategies. The present in vivo longitudinal study in an orthotopic mouse model demonstrates quantitative assessment of early treatment response during short-term chemotherapy with temozolomide (TMZ) by amide proton transfer (APT) imaging. In a GBM line, only one course of TMZ (3 d exposure and 4 d rest) at a dose of 80 mg/kg resulted in substantial reduction in APT signal compared with untreated control animals, in which the APT signal continued to increase. Although there were no detectable differences in tumor volume, cell density, or apoptosis rate between groups, levels of Ki67 (index of cell proliferation) were substantially reduced in treated tumors. In another TMZ-resistant GBM line, the APT signal and levels of Ki67 increased despite the same course of TMZ treatment. As metabolite changes are known to occur early in the time course of chemotherapy and precede morphologic changes, these results suggest that the APT signal in glioma may be a useful functional biomarker of treatment response or degree of tumor progression. Thus, APT imaging may serve as a sensitive biomarker of early treatment response and could potentially replace invasive biopsies to provide a definitive diagnosis. This would have a major impact on the clinical management of patients with glioma.

Potential applications for biguanides in oncology

Metformin is widely prescribed for the treatment of type II diabetes. Recently, it has been proposed that this compound or related biguanides may have antineoplastic activity. Biguanides may exploit specific metabolic vulnerabilities of transformed cells by acting on them directly, or may act by indirect mechanisms that involve alterations of the host environment. Preclinical data suggest that drug exposure levels are a key determinant of proposed direct actions. With respect to indirect mechanisms, it will be important to determine whether recently demonstrated metformin-induced changes in levels of candidate systemic mediators such as insulin or inflammatory cytokines are of sufficient magnitude to achieve therapeutic benefit. Results of the first generation of clinical trials now in progress are eagerly anticipated. Ongoing investigations may justify a second generation of trials that explore pharmacokinetic optimization, rational drug combinations, synthetic lethality strategies, novel biguanides, and the use of predictive biomarkers.

Resistance to PARP-Inhibitors in Cancer Therapy

The pharmacological inhibitors of poly(ADP-ribose) polymerase (PARP) family of proteins have shown promising results in preclinical studies and clinical trials as a monotherapy or in combination therapy for some cancers. Thus, usage of PARP-inhibitors (PARPi) in cancer therapy is bound to increase with time, but resistance of cancer cells to PARPi is also beginning to be observed. Here we review different known and potential mechanisms by which: (i) PARPi kill cancer cells; and (ii) cancer cells develop resistance to PARPi. Understanding the lethality caused by PARPi and the countermeasures deployed by cancers cells to survive PARPi will help us rationalize the use of this new class of drugs in cancer therapy.

AMPK mediates a pro-survival autophagy downstream of PARP-1 activation in response to DNA alkylating agents

In this study we aim to elucidate the signaling pathway and biological function of autophagy induced by MNNG, a commonly used DNA alkylating agent. We first observed that MNNG is able to induce necrotic cell death and autophagy in Bax-/- Bak-/- double knockout MEFs. We analyzed the critical role of PARP-1 activation and ATP depletion in MNNG-mediated cell death and autophagy via AMPK activation and mTOR suppression. We provide evidence that suppression of AMPK blocks MNNG-induced autophagy and enhances cell death, suggesting the pro-survival function of autophagy in MNNG-treated cells. Taken together, data from this study reveal a novel mechanism in controlling MNNG-mediated autophagy via AMPK activation downstream of PARP-1 activation and ATP depletion.

Oncosis: an important non-apoptotic mode of cell death

It is now increasingly accepted that apoptosis may not be the only form of cell death seen in vitro and in vivo; hence there is a need to study novel forms of cell death. The explosion of cell death research that followed the recognition of apoptosis by Kerr and colleagues in the late 1960s completely obscured the fact that apoptosis is not the only form of cell death. Apoptosis manifests itself by cell shrinkage followed by breakup; another form (oncosis) is almost the opposite: it involves cell swelling and coagulation of the cytoplasm. The name oncosis was chosen over a century ago by von Recklinghausen, a top collaborator of Rudolph Virchow and thereby one of the founders of cellular pathology. Nevertheless, oncosis was forgotten, largely because a satisfactory technique for preparing tissue sections did not exist at the time. Also confusion developed regarding the distinction between oncosis as a mode of cell injury and cell death, and necrosis as a degradation process following cell death. In this review we have described the many characteristics of oncosis from a morphological and biochemical standpoint, and we briefly examine the application of oncosis in disease processes.

Nicotinamide inhibits alkylating agent-induced apoptotic neurodegeneration in the developing rat brain

BACKGROUND

Exposure to the chemotherapeutic alkylating agent thiotepa during brain development leads to neurological complications arising from neurodegeneration and irreversible damage to the developing central nerve system (CNS). Administration of single dose of thiotepa in 7-d postnatal (P7) rat triggers activation of apoptotic cascade and widespread neuronal death. The present study was aimed to elucidate whether nicotinamide may prevent thiotepa-induced neurodegeneration in the developing rat brain.

METHODOLOGY/PRINCIPAL FINDINGS

Neuronal cell death induced by thiotepa was associated with the induction of Bax, release of cytochrome-c from mitochondria into the cytosol, activation of caspase-3 and cleavage of poly (ADP-ribose) polymerase (PARP-1). Post-treatment of developing rats with nicotinamide suppressed thiotepa-induced upregulation of Bax, reduced cytochrome-c release into the cytosol and reduced expression of activated caspase-3 and cleavage of PARP-1. Cresyl violet staining showed numerous dead cells in the cortex hippocampus and thalamus; post-treatment with nicotinamide reduced the number of dead cells in these brain regions. Terminal deoxynucleotidyl transferase (TdT)-mediated dUTP nick end-labeling (TUNEL) and immunohistochemical analysis of caspase-3 show that thiotepa-induced cell death is apoptotic and that it is inhibited by nicotinamide treatment.

CONCLUSION

Nicotinamide (Nic) treatment with thiotepa significantly improved neuronal survival and alleviated neuronal cell death in the developing rat. These data demonstrate that nicotinamide shows promise as a therapeutic and neuroprotective agent for the treatment of neurodegenerative disorders in newborns and infants.

Approaches to detect PARP-1 activation in vivo, in situ, and in vitro

An accurate and sensitive detection of catalytic activation of poly(ADP-ribose) polymerase-1 (PARP-1) is required to be performed in a wide variety of samples because this activity plays a role in various cellular responses to DNA damage ranging from DNA repair to cell death, as well as in housekeeping functions, such as transcription. Since PARP-1 gene is expressed constitutively, its activation cannot be surmised from increased expression of its mRNA or protein, but by demonstrating the consequences of its catalytic -reaction which results in consumption of the substrate nicotinamide adenine dinucleotide (NAD(+)) and formation of three products, namely, polymer of ADP-ribose (pADPr or PAR), nicotinamide, and protons. Here, we describe various approaches commonly used in our laboratory for detection of PARP-1 activation in vivo (cells, tissues, and tumors), in situ, and in vitro via assessment of formation of pADPr, depletion of the substrate NAD, or formation of protons resulting in rapid and reversible intracellular acidification. It is important to note that although some other members of the PARP family can carry out the same catalytic reaction, many of these assays largely reflect PARP-1 activation in a vast majority of the experimental circumstances and more specifically in DNA damage responses. However, if required, PARP-1-specific action should be confirmed by use of PARP-1 knockout or RNAi-mediated knockdown approaches.

Poly ADP-ribose polymerase-1 and health

Niacin (vitamin B(3)) is required to form nicotinamide adenine dinucleotide (NAD) and nicotinamide adenine dinucleotide phosphate (NADP), which are involved in scores of anabolic and catabolic redox reactions throughout metabolism. It is now understood that NAD(+) is also a substrate for several families of ADP-ribosylation reactions, which control processes like DNA repair, replication and transcription, the activity of G-proteins, chromatin structure and intracellular calcium signalling. Poly(ADP-ribose)polymerase-1 (PARP-1) is the most active of the PARP enzymes, and it has been implicated in both prevention and aggravation of disease processes. Inhibition of poly-ADP-ribose formation will tend to cause genomic instability and tumorigenesis in chronic models of DNA damage, but the same inhibition can prevent many acute disease processes, such as stroke, myocardial infarction and septic shock. In models of acute stress, PARP-1 inhibition may protect cellular NAD pools and prevent nuclear factor-kappaB-dependent inflammatory signalling, while long-term protective roles for PARP-1 include DNA repair and regulation of chromatin structure. Promising new PARP-1 inhibitors may display interactions with dietary niacin status and may have long-term deleterious effects on genomic stability, but may be extremely valuable for the treatment of acute inflammatory conditions.

Parp1 activation in mouse embryonic fibroblasts promotes Pol beta-dependent cellular hypersensitivity to alkylation damage

Alkylating agents induce cell death in wild-type (WT) mouse embryonic fibroblasts (MEFs) by multiple mechanisms, including apoptosis, autophagy and necrosis. DNA polymerase beta (Pol beta) knockout (KO) MEFs are hypersensitive to the cytotoxic effect of alkylating agents, as compared to WT MEFs. To test the hypothesis that Parp1 is preferentially activated by methyl methanesulfonate (MMS) exposure of Pol beta KO MEFs, we have examined the relationship between Pol beta expression, Parp1 activation and cell survival following MMS exposure in a series of WT and Pol beta deficient MEF cell lines. Consistent with our hypothesis, we observed elevated Parp1 activation in Pol beta KO MEFs as compared to matched WT MEFs. Both the MMS-induced activation of Parp1 and the MMS-induced cytotoxicity of Pol beta KO MEFs are attenuated by pre-treatment with the Parp1/Parp2 inhibitor PJ34. Further, elevated Parp1 activation is observed following knockdown (KD) of endogenous Pol beta, as compared to WT cells. Pol beta KD MEFs are hypersensitive to MMS and both the MMS-induced hypersensitivity and Parp1 activation is prevented by pre-treatment with PJ34. In addition, the MMS-induced cellular sensitivity of Pol beta KO MEFs is reversed when Parp1 is also deleted (Pol beta/Parp1 double KO MEFs) and we observe no MMS sensitivity differential between Pol beta/Parp1 double KO MEFs and those that express recombinant mouse Pol beta. These studies suggest that Parp1 may function as a sensor of BER to initiate cell death when BER is aborted or fails. Parp1 may therefore function in BER as a tumor suppressor by initiating cell death and preventing the accumulation of cells with chromosomal damage due to a BER defect.

The role of CHOP messenger RNA expression in the link between oxidative stress and apoptosis

Low expression of antioxidant enzymes makes pancreatic beta-cells susceptible to cell damage by oxidative stress. Pancreatic beta-cell loss caused by endoplasmic reticulum stress is associated with the onset of diabetes mellitus. The present studies were undertaken to investigate a possible involvement of proapoptotic gene CHOP in pancreatic beta-cells damage by oxidative stress. The induction of CHOP messenger RNA and apoptosis were investigated in betaHC-9 cells after the oxidative stress by hydrogen peroxide and ribose. Latter was examined after the suppression of CHOP by small interfering RNA. For in vivo study, the pancreatic beta-cells were examined in CHOP-knockout (KO) mice after multiple low-dose streptozotocin (MLDS) administration. In betaHC-9 cells, both hydrogen peroxide and ribose obviously increased apoptotic cells, accompanied with enhanced CHOP messenger RNA expression. However, the number of apoptotic cells by those stimulations was significantly reduced by the addition of small interfering RNA against CHOP. In vivo study also showed that CHOP-KO mice were less susceptible to diabetes after MLDS administration. Although the oxidative stress marker level was similar to that of MLDS-treated wild type, the pancreatic beta-cell area was maintained in CHOP-KO mice. The present studies showed that CHOP should be important in pancreatic beta-cell injury by oxidative stress and indicate that CHOP may play a role in the development of pancreatic beta-cell damage on the onset of diabetes mellitus.

Pyruvate improves recovery after PARP-1-associated energy failure induced by oxidative stress in neonatal rat cerebrocortical slices

Previous neuron and glial cell culture studies of excessive poly (ADP-ribose) polymerase (PARP-1) activation found NAD(+) depletion, glycolytic arrest, and cell death that could be avoided by exogenous tricarboxylic acid cycle (TCA) metabolites, especially pyruvate (pyr). Pyruvate neuroprotection has been attributed to cytosolic NAD(+) replenishment, TCA metabolism, and antioxidant activity. We investigated the first two mechanisms in respiring cerebrocortical slices after a 1-h H(2)O(2) exposure to activate PARP-1. H(2)O(2) was followed by a 4-h recovery with oxy-artificial cerebrospinal fluid superfusion having either: (1) no glucose (glc) or pyruvate; (2) 10 mmol/L glc only; (3) 10 mmol/L pyruvate only; (4) both 10 mmol/L glc and 10 mmol/L pyruvate. Poly-ADP-ribosylation was quantified from Western blots and immunohistochemistry. Perchloric acid extracts were quantified with 14.1 T (31)P nuclear magnetic resonance spectroscopy. Just after H(2)O(2) exposure, ATP and NAD(+) decreased by approximately 50%, PCr decreased by 75%, and the ADP/ATP ratio approximately doubled. ATP and NAD(+) changes, but not PCr changes, were nearly eliminated if PARP inhibitors accompanied the H(2)O(2). Recovery with both pyruvate and glc was better than with glc alone, having higher ATP (0.161 versus 0.075, P<0.01) and PCr levels (0.144 versus 0.078, P<0.01), and higher viable cell counts in TUNEL and Fluoro-Jade B staining. Two-dimensional [(1)H-(13)C] HSQC spectra showed metabolism during recovery of (13)C glc or pyr. Pyruvate metabolism was primarily via pyruvate dehydrogenase, with some via pyruvate carboxylation. Pyruvate superfusion of PARP-injured brain slices helps replenish NAD(+) while providing metabolic fuel. Although this augments recovery, a strong antioxidant role for pyruvate has not been ruled out.

Acid DNases and their interest among apoptotic endonucleases

Apoptosis is characterized by cell shrinkage, nuclear condensation and internucleosomal DNA cleavage. Besides the central role of caspases and other proteases, cell death triggers DNA degradation so that DNases have an active role in apoptotic cell death. The best-characterized apoptotic DNase is CAD, a neutral Mg-dependent endonuclease. Its activity is regulated by its inhibitor, ICAD, which is cleaved by caspases. Other neutral DNases have been shown to cleave nuclear DNA in apoptotic conditions: endonuclease G, GADD. In cells, the cytosolic pH is maintained to 7.2, mostly due to the activity of the Na(+)/H(+) exchanger. In many apoptotic conditions, a decrease of the intracellular pH has been shown. This decrease may activate different acid DNases, mostly when pH decreases below 6.5. Three acidic DNases II are so far known: DNase II alpha, DNase II beta and L-DNase II, a DNase II, derived from the serpin LEI (Leukocyte Elastase Inhibitor). Their activation during cell death is discussed in this review.

Rapid intracellular acidification and cell death by H2O2 and alloxan in pancreatic beta cells

Pancreatic beta-cell death induced by oxidative stress plays an important role in the pathogenesis of diabetes mellitus. We studied the relation between rapid intracellular acidification and cell death of pancreatic beta-cell line NIT-1 cells exposed to H2O2 or alloxan. Intracellular pH was measured by a pH-sensitive dye, and cell damage by double staining with Annexin-V and propidium iodide using flow cytometry. H2O2 and alloxan caused a rapid fall in intracellular pH and suppressed Na+/H+ exchanger activity in the NH4Cl prepulse method. H2O2 induced necrotic cell death, which shifted to apoptotic cell death when initial acidification was prevented by pH clamping to 7.4 using nigericin (unclamped cells vs clamped cells, necrosis 43.8 +/- 5.8% vs 21.1 +/- 10.6%, P < 0.05; apoptosis 8.0 +/- 1.9% vs 44.5 +/- 5.0%, P < 0.01). pH-clamped cells showed enhanced caspase 3 activity and proapoptotic Bax expression. On the other hand, NIT-1 cells were resistant to alloxan toxicity, but treatment with alloxan and nigericin strikingly enhanced the cytotoxicity. Antioxidants partly prevented cell death, although intracellular pH remained similarly acidic. The rapid intracellular acidification was not the cause of cell death but a significant determinant of the mode of death of H2O2 -treated beta cells, whereas no link between cell death and acidification was demonstrated in alloxan toxicity.

Role of poly(ADP-ribose) polymerase-1 activation in the pathogenesis of diabetic complications: endothelial dysfunction, as a common underlying theme

Hyperglycemia-induced overproduction of superoxide by mitochondrial electron-transport chain triggers several pathways of injury involved in the pathogenesis of diabetic complications [protein kinase C (PKC), hexosamine and polyol pathway fluxes, advanced glycation end product (AGE) formation] by inhibiting glyceraldehyde- 3-phosphate dehydrogenase (GAPDH) activity. Increased oxidative and nitrosative stress activates the nuclear enzyme, poly(ADP-ribose) polymerase-1 (PARP). PARP activation, on the one hand, depletes its substrate, NAD+, slowing the rate of glycolysis, electron transport, and ATP formation. On the other hand, it inhibits GAPDH by poly(ADP-ribosy)lation. These processes result in acute endothelial dysfunction in diabetic blood vessels, which importantly contributes to the development of various diabetic complications. Accordingly, hyperglycemia-induced activation of PKC isoforms, hexosaminase pathway flux, and AGE formation is prevented by blocking PARP activity. Furthermore, inhibition of PARP protects against diabetic cardiovascular dysfunction in preclinical models. PARP activation is present in microvasculature of human diabetic subjects. The oxidative/nitrosative stress-PARP pathway leads to diabetes-induced endothelial dysfunction, which may be an important underlying mechanism for the pathogenesis of other diabetic complications (cardiomyopathy, nephropathy, neuropathy, and retinopathy). This review focuses on the role of PARP in diabetic complications and the unique therapeutic potential of PARP inhibition in the prevention or reversal of diabetic complications.

Poly(ADP-ribose) polymerase-mediated cell injury in acute renal failure

Acute Renal Failure (ARF) is the most costly kidney disease in hospitalized patients and remains as a serious problem in clinical medicine. The mortality rate among ARF patients remains around 50% and no pharmaceutical agents are currently available to improve its clinical outcome. Although several successful therapeutic approaches have been developed in animal models of the disease, translation of the results to clinical ARF remains elusive. Understanding the cellular and molecular mechanisms of vascular and tubular dysfunction in ARF is important for developing acceptable therapeutic interventions. Following an ischemic episode, cells of the affected nephron undergo necrotic and/or apoptotic cell death. Necrotic cell death is widely considered to be a futile process that cannot be modulated by pharmacological means as opposed to apoptosis. However, recent reports from various laboratories including ours indicate that inhibition or absence of poly(ADP)-ribose polymerase (PARP), one of the molecules involved in cell death, provides remarkable protection in disease models such as stroke, myocardial infarction and renal ischemia which are characterized predominantly by necrotic type of cell death. Overactivation of PARP in conditions such as ischemic renal injury leads to cellular depletion of its substrate NAD+ and consequently ATP. The severely compromised cellular energetic state induces acute cell injury and diminishes renal functions. PARP activation also enhances the expression of proinflammatory agents and adhesion molecules in ischemic kidneys. Pharmacological inhibition and gene ablation of PARP-1 decreased energy depletion, inflammatory response and improved renal functions in the setting renal ischemia/reperfusion injury. The biochemical pathways and the cellular and molecular mechanisms mediated by PARP-1 activation in eliciting the energy depletion and inflammatory responses in ischemic kidney are not fully elucidated. Dissecting the molecular mechanisms by which PARP activation contributes to oxidant-induced cell death will provide new strategies to interfere in those pathways to modulate cell death in renal ischemia. The current review evaluates the experimental evidences in animal and cell culture models implicating PARP as a pathophysiological modulator of acute renal failure with particular emphasis on ischemic renal injury.

The pathogenesis of diabetic complications: the role of DNA injury and poly(ADP-ribose) polymerase activation in peroxynitrite-mediated cytotoxicity

Recent work has demonstrated that hyperglycemia-induced overproduction of superoxide by the mitochondrial electron-transport chain triggers several pathways of injury [(protein kinase C (PKC), hexosamine and polyol pathway fluxes, advanced glycation end product formation (AGE)] involved in the pathogenesis of diabetic complications by inhibiting glyceraldehyde-3-phosphate dehydrogenase (GAPDH) activity. Increased oxidative and nitrosative stress activates the nuclear enzyme, poly(ADP-ribose) polymerase-1 (PARP). PARP activation, on one hand, depletes its substrate, NAD+, slowing the rate of glycolysis, electron transport and ATP formation. On the other hand, PARP activation results in inhibition of GAPDH by poly-ADP-ribosylation. These processes result in acute endothelial dysfunction in diabetic blood vessels, which importantly contributes to the development of various diabetic complications. Accordingly, hyperglycemia-induced activation of PKC and AGE formation are prevented by inhibition of PARP activity. Furthermore, inhibition of PARP protects against diabetic cardiovascular dysfunction in rodent models of cardiomyopathy, nephropathy, neuropathy, and retinopathy. PARP activation is also present in microvasculature of human diabetic subjects. The present review focuses on the role of PARP in diabetic complications and emphasizes the therapeutic potential of PARP inhibition in the prevention or reversal of diabetic complications.

Mechanism of early biphasic activation of poly(ADP-ribose) polymerase-1 in response to ultraviolet B radiation

The damage to DNA caused by ultraviolet B radiation (280-320 nm) contributes significantly to development of sunlight-induced skin cancers. The susceptibility of mice to ultraviolet B-induced skin carcinogenesis is increased by an inhibitor of the DNA damage-activated nuclear enzyme poly(ADP-ribose) polymerase-1 (PARP), hence PARP activation is likely to be associated with cellular responses that suppress carcinogenesis. To understand the role of activated PARP in these cellular functions, we need to first clearly identify the cause of PARP activation in ultraviolet B-irradiated cells. Ultraviolet B, like ultraviolet C, causes direct DNA damage of cyclobutane pyrimidine dimer and 6, 4-photoproduct types, which are subjected to the nucleotide excision repair. Moreover, ultraviolet B also causes oxidative DNA damage, which is subjected to base excision repair. To identify which of these two types of DNA damage activates PARP, we examined mechanism of early PARP activation in mouse fibroblasts exposed to ultraviolet B and C radiations. The ultraviolet B-irradiated cells rapidly activated PARP in two distinct phases, initially within the first 5 minutes and later between 60-120 minutes, whereas ultraviolet C-irradiated cells showed only the immediate PARP activation. Using antioxidants, local irradiation, chromatin immunoprecipitation and in vitro PARP assays, we identified that ultraviolet radiation-induced direct DNA damage, such as thymine dimers, cause the initial PARP activation, whereas ultraviolet B-induced oxidative damage cause the second PARP activation. Our results suggest that cells can selectively activate PARP for participation in different cellular responses associated with different DNA lesions.

Poly(ADP-ribose) polymerase-1 gene ablation protects mice from ischemic renal injury

Increased generation of reactive oxygen species (ROS) and the subsequent DNA damage and excessive activation of poly(ADP-ribose) polymerase-1 (PARP-1) have been implicated in the pathogenesis of ischemic injury. We previously demonstrated that pharmacological inhibition of PARP protects against ischemic renal injury (IRI) in rats (Martin DR, Lewington AJ, Hammerman MR, and Padanilam BJ. Am J Physiol Regul Integr Comp Physiol 279: R1834-R1840, 2000). To further define the role of PARP-1 in IRI, we tested whether genetic ablation of PARP-1 attenuates tissue injury after renal ischemia. Twenty-four hours after reperfusion following 37 min of bilateral renal pedicle occlusion, the effects of the injury on renal functions in PARP-/- and PARP+/+ mice were assessed by determining glomerular filtration rate (GFR) and the plasma levels of creatinine. The levels of plasma creatinine were decreased and GFR was augmented in PARP-/- mice. Morphological evaluation of the kidney tissues showed that the extent of damage due to the injury in PARP-/- mice was less compared with their wild-type counterparts. The levels of ROS and DNA damage were comparable in the injured kidneys of PARP+/+ and PARP-/- mice. PARP activity was induced in ischemic kidneys of PARP+/+ mice at 6-24 h postinjury. At 6, 12, and 24 h after injury, ATP levels in the PARP+/+ mice kidney declined to 28, 26, and 43%, respectively, whereas it was preserved close to normal levels in PARP-/- mice. The inflammatory cascade was attenuated in PARP-/- mice as evidenced by decreased neutrophil infiltration and attenuated expression of inflammatory molecules such as TNF-alpha, IL-1beta, and intercellular adhesion molecule-1. At 12 h postinjury, no apoptotic cell death was observed in PARP-/- mice kidneys. However, by 24 h postinjury, a comparable number of cells underwent apoptosis in both PARP-/- and PARP+/+ mice kidneys. Thus activation of PARP post-IRI contributes to cell death most likely by ATP depletion and augmentation of the inflammatory cascade in the mouse model. PARP ablation preserved ATP levels, renal functions, and attenuated inflammatory response in the setting of IRI in the mouse model. PARP inhibition may have clinical efficacy in preventing the progression of acute renal failure complications.

Alterations of intracellular pH homeostasis in apoptosis: origins and roles

Intracellular pH (pHi) has an important role in the maintenance of normal cell function, and hence this parameter has to be tightly controlled within a narrow range, largely through the activity of transporters located at the plasma membrane. These transporters can be modulated by endogenous or exogenous molecules as well as, in some pathological situations, leading to pHi changes that have been implicated in both cell proliferation and cell death. Whereas intracellular alkalinization seems to be a common feature of proliferative processes, the precise role of pHi in apoptosis is still unclear. The present review gathers the most recent advances along with previous data on both the origin and the role of pHi alterations in apoptosis and highlights the major concerns that merit further research in the future. Special attention is given to the possible role played by pHi-regulating transporters.

Pathophysiological aspects of cellular pyridine nucleotide metabolism: focus on the vascular endothelium. Review

In recent years, pyridine nucleotides NAD(H) and NADP(H) have been established as an important molecules in physiological and pathophysiological signaling and cell injury pathways. Protein modification is catalyzed by ADP-ribosyl transferases that attach the ADP-ribose moiety of NAD+ to specific aminoacid residues of the acceptor proteins, with significant changes in the function of these acceptors. Mono(ADP-ribosyl)ation reactions have been implicated to play a role both in physiological responses and in cellular responses to bacterial toxins. Cyclic ADP-ribose formation also utilizes NAD+ and primarily serves as physiological, signal transduction mechanisms regulating intracellular calcium homeostasis. In pathophysiological conditions associated with oxidative stress (such as various forms of inflammation and reperfusion injury), activation of the nuclear enzyme poly(ADP-ribose) polymerase (PARP) occurs, with subsequent, substantial fall in cellular NAD+ and ATP levels, which can determine the viability and function of the affected cells. In addition, NADPH oxidases can significantly affect the balance and fate of NAD+ and NADP in oxidatively stressed cells and can facilitate the generation of various positive feedback cycles of injury. Under severe oxidant conditions, direct oxidative damage to NAD+ has also been reported. The current review focuses on PARP and on NADPH oxidases, as pathophysiologically relevant factors in creating disturbances in the cellular pyridine nucleotide balance. A separate section describes how these mechanisms apply to the pathogenesis of endothelial cell injury in selected cardiovascular pathophysiological conditions.

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.

3-Aminobenzamide reduces brain infarction and neutrophil infiltration after transient focal cerebral ischemia in mice

Poly(ADP-ribose) polymerase (PARP) was shown to be detrimental in cerebral ischemia but the mechanisms whereby PARP is deleterious have yet to be determined. They may include a role in neutrophil infiltration known to aggravate ischemic damage. In this context, we investigated the effect of 3-aminobenzamide (3-AB), a PARP inhibitor, on brain damage and neutrophil infiltration after transient focal cerebral ischemia in mice. Ischemia was induced in male Swiss mice, anaesthetized with chloral hydrate (400 mg/kg, i.p.), by a 15-min-occlusion of the left middle cerebral artery using an intraluminal suture. Treatments with 3-AB were first administered intraperitoneally 15 min before reperfusion and endpoints measured at 24 h. Among the range of dosages studied (20-320 mg/kg), 40 mg/kg gave the maximal neuroprotection with a 30% decrease in the infarct volume and tended to improve the neurological score evaluated by a grip test. The same dosage was, however, devoid of effect when injection was delayed 2 or 6 h after reperfusion. Myeloperoxidase (MPO) activity used as an index of neutrophil infiltration showed that infiltration peaked 48 h after reperfusion in our model. At this time point, 3-AB (40 mg/kg given 15 min before reperfusion) markedly reduced the neutrophil infiltration, as evidenced by a 72%-decrease in MPO activity, and was still neuroprotective. Our results confirm that 3-AB reduces brain damage. Moreover, for the first time, a quantitative study shows that 3-AB decreases neutrophil infiltration elicited by cerebral ischemia.

Mammalian DNA base excision repair proteins: their interactions and role in repair of oxidative DNA damage

The DNA base excision repair (BER) is a ubiquitous mechanism for removing damage from the genome induced by spontaneous chemical reaction, reactive oxygen species (ROS) and also DNA damage induced by a variety of environmental genotoxicants. DNA repair is essential for maintaining genomic integrity. As we learn more about BER, a more complex mechanism emerges which supersedes the classical, simple pathway requiring only four enzymatic reactions. The key to understand the complete BER process is to elucidate how multiple proteins interact with one another in a coordinated process under specific physiological conditions.

Poly(ADP-ribose) polymerase in bovine retained and not retained placenta

Poly(ADP-ribose) polymerase (PARP) synthesizes poly(ADP-ribose) in response to DNA strand breaks using NAD+ as a substrate. It leads to consequences for metabolism not only on a cellular level, but also on a tissue level, among others: NAD+ and ATP depletion. Retention of foetal membranes (RF) in cows is supposed to be connected with the imbalance between production and neutralization of reactive oxygen species, leading to oxidative damage to DNA, lipids and proteins. The aim of this preliminary study was to detect the presence of PARP in bovine placenta and to describe the enzyme with respect to type of placental tissue, time and mode of delivery. Placentomes, collected after spontaneous delivery or caesarian section, were divided into maternal and foetal parts of placenta, homogenized, and subjected to electrophoresis. Cows were divided into six groups as follows: (A) caesarian section before term with RF, (B) caesarian section before term without RF, (C) spontaneous delivery at term with RF, (D) spontaneous delivery at term without RF, (E) caesarian section at term with RF, (F) caesarian section at term without RF. PARP was detected by Western blotting using commercially available bovine anti PARP antibody. Bands referred to as bovine PARP standard were present in all examined tissues as well as the products of its cleavage. However, the patterns of bands were different with respect to type of tissue, time, and mode of delivery. Further experiments on detailed relationship between PARP activity and the process of releasing and retaining of bovine placenta are necessary.

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

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

Mitochondrial membrane potential regulates matrix configuration and cytochrome c release during apoptosis

During apoptosis, the mitochondrial membrane potential (MMP) decreases, but it is not known how this relates to the apoptotic process. It was recently suggested that cytochrome c is compartmentalized in closed cristal regions and therefore, matrix remodeling is required to attain complete cytochrome c release from the mitochondria. In this work we show that, at the onset of apoptosis, changes in MMP control matrix remodeling prior to cytochrome c release. Early after growth factor withdrawal the MMP declines and the matrix condenses. Both phenomena are reversed by adding oxidizable substrates. In mitochondria isolated from healthy cells, matrix condensation can be induced by either denying oxidizable substrates or by protonophores that dissipate the membrane potential. Matrix remodeling to the condensed state results in cristal unfolding and exposes cytochrome c to the intermembrane space facilitating its release from the mitochondria during apoptosis. In contrast, when a transmembrane potential is generated due to either electron transport or a pH gradient formed by acidifying the medium, mitochondria maintain an orthodox configuration in which most cytochrome c is sequestered in the cristae and is resistant to release by agents that disrupt the mitochondrial outer membrane.