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

Necroptosis, necrostatins and tissue injury

Cell death is an integral part of the life of an organism being necessary for the maintenance of organs and tissues. If, however, cell death is allowed to proceed unrestricted, tissue damage and degenerative disease may ensue. Until recently, three morphologically distinct types of cell death were recognized, apoptosis (type I), autophagy (type II) and necrosis (type III). Apoptosis is a highly regulated, genetically determined mechanism designed to dismantle cells systematically (e.g. cells that are no longer functionally viable), via protease (caspase) action, and maintain homeostasis. Autophagy is responsible for the degradation of cytoplasmic material, e.g. proteins and organelles, through autophagosome formation and subsequent proteolytic degradation by lysosomes, and is normally considered in the context of survival although it is sometimes associated with cell death. Necrosis was formerly considered to be an accidental, unregulated form of cell death resulting from excessive stress, although it has been suggested that this is an over-simplistic view as necrosis may under certain circumstances involve the mobilization of specific transduction mechanisms. Indeed, recently, an alternative death pathway, termed necroptosis, was delineated and proposed as a form of ‘programmed necrosis’. Identified with the aid of specific inhibitors called necrostatins, necroptosis shares characteristics with both necrosis and apoptosis. Necroptosis involves Fas/tumour necrosis factor-α death domain receptor activation and inhibition of receptor-interacting protein I kinase, and it has been suggested that it may contribute to the development of neurological and myocardial diseases. Significantly, necrostatin-like drugs have been mooted as possible future therapeutic agents for the treatment of degenerative conditions.

Vascular endothelial growth factor confers endothelial resistance to apoptosis through poly(ADP-ribose) polymerase

BACKGROUND

Vascular endothelial growth factor (VEGF) inhibits the endothelial apoptosis that is induced by caspases during vascular remodeling; however, the underlying mechanisms have not been completely elucidated.

OBJECTIVES

We hypothesized that VEGF upregulates poly(ADP-ribose) polymerase-1 (PARP) as a caspase mediator, and sought to investigate the link between apoptosis inhibition by VEGF and PARP regulation in the human vasculature.

METHODS

Human endothelial cells (primary cells, macrovascular/microvascular lines) were incubated with 100 pg mL(-1) to 1 μg mL(-1) VEGF-A(165) in the absence or presence of PARP small interfering RNA (siRNA). Apoptosis induced by integrin inhibition was measured by flow cytometry, trypan blue exclusion, and nuclear morphology. PARP expression and activity were determined by real-time RT-PCR, Western blot, and ribosylation assay. VEGF receptors and signal transduction were analyzed by inhibitor experiments, enzyme assays, western blot, and immunofluorescence microscopy. Immunohistochemistry was applied to a vascular culture model and to 24 atherosclerotic and 10 normal human arteries.

RESULTS

VEGF-A(165) induced resistance to apoptosis caused by caspase activation in endothelial cells in a time-dependent manner. VEGF, but not fibroblast growth factor-2 or transforming growth factor-β, time-dependently and dose-dependently induced PARP expression and activity, involving VEGF receptor-2 colocalized with neuropilin-1 as well as the signal transduction molecules c-Jun N-terminal kinase and Akt. The antiapoptotic effect of VEGF was abrogated by PARP siRNA. The relationship between local VEGF influence and endothelial PARP expression was confirmed in human arteries and the vascular culture model.

CONCLUSIONS

 VEGF exerts its antiapoptotic effect on the endothelium through the regulation of PARP expression. PARP has been attributed an ambiguous role in apoptosis; here, we show that PARP promotes vascular endothelial integrity in VEGF-associated processes.

hPuf-A/KIAA0020 modulates PARP-1 cleavage upon genotoxic stress

Human hPuf-A/KIAA0020 was first identified as a new minor histocompatibility antigen in 2001. Its zebrafish orthologue contains six Pumilio-homology RNA-binding domains and has been shown to participate in the development of eyes and primordial germ cells, but the cellular function of hPuf-A remains unclear. In this report, we showed that hPuf-A predominantly localized in the nucleoli with minor punctate signals in the nucleoplasm. The nucleolar localization of hPuf-A would redistribute to the nucleoplasm after the treatment of RNA polymerase inhibitors (actinomycin D and 5,6-dichlorobenzimidazole riboside) and topoisomerase inhibitors [camptothecin (CPT) and etoposide]. Interestingly, knockdown of hPuf-A sensitized cells to CPT and UV treatment and cells constitutively overexpressing hPuf-A became more resistant to genotoxic exposure. Affinity gel pull-down coupled with mass spectrometric analysis identified PARP-1 as one of the hPuf-A interacting proteins. hPuf-A specifically interacts with the catalytic domain of PARP-1 and inhibits poly(ADP-ribosyl)ation of PARP-1 in vitro. Depletion of hPuf-A increased the cleaved PARP-1 and overexpression of hPuf-A lessened PARP-1 cleavage when cells were exposed to CPT and UV light. Collectively, hPuf-A may regulate cellular response to genotoxic stress by inhibiting PARP-1 activity and thus preventing PARP-1 degradation by caspase-3.

PARP-1 inhibition-induced activation of PI-3-kinase-Akt pathway promotes resistance to taxol

PARP inhibitors combined with DNA-damage inducing cytostatic agents can lead to effective tumor therapy. However, inhibition of poly(ADP-ribose) polymerase (PARP-1; EC 2.4.2.30) induces the activation of PI-3-kinase-Akt pathway, which can counteract the effectiveness of this therapy. To understand the role of Akt activation in the combined use of cytostatic agent and PARP inhibition, we used taxol (paclitaxel) as an antineoplastic agent, which targets microtubules and up-regulates mitochondrial ROS production, together with (i) pharmacological inhibition (PJ-34), (ii) siRNA knock-down and (iii) transdominant expression of the DNA binding domain of PARP-1. In all cases, PARP-1 inhibition leads to suppressed poly-ADP-ribosylation of nuclear proteins, prevention of NAD(+) depletion and significant resistance against taxol induced caspase-3 activation and apoptotic cell death. Paclitaxel induced a moderate increase in Akt activation, which was significantly augmented by PARP inhibition, suggesting that PARP inhibition-induced Akt activation could be responsible for the cytostatic resistance. When activation of the PI-3-kinase-Akt pathway was prevented by LY-294002 or Akt Inhibitor IV, the cytoprotective effect of PARP inhibition was significantly diminished showing that the activation of PI-3-kinase-Akt cascade had significantly contributed to the cytostatic resistance. Our study demonstrates that drug-induced drug resistance can be responsible for the reduced efficacy of antitumor treatment. Although inhibition of PARP-1 can promote cell death in tumor cells by the inhibition of DNA repair, PARP-inhibition promoted activation of the PI-3-kinase-Akt pathway can counteract this facilitating effect, and can cause cytostatic resistance. We suggest augmenting PARP inhibition by the inhibition of the PI-3-kinase-Akt pathway for antitumor therapy.

Identification of a molecular signaling network that regulates a cellular necrotic cell death pathway

Stimulation of death receptors by agonists such as FasL and TNFalpha activates apoptotic cell death in apoptotic-competent conditions or a type of necrotic cell death dependent on RIP1 kinase, termed necroptosis, in apoptotic-deficient conditions. In a genome-wide siRNA screen for regulators of necroptosis, we identify a set of 432 genes that regulate necroptosis, a subset of 32 genes that act downstream and/or as regulators of RIP1 kinase, 32 genes required for death-receptor-mediated apoptosis, and 7 genes involved in both necroptosis and apoptosis. We show that the expression of subsets of the 432 genes is enriched in the immune and nervous systems, and cellular sensitivity to necroptosis is regulated by an extensive signaling network mediating innate immunity. Interestingly, Bmf, a BH3-only Bcl-2 family member, is required for death-receptor-induced necroptosis. Our study defines a cellular signaling network that regulates necroptosis and the molecular bifurcation that controls apoptosis and necroptosis.

Depletion of poly(ADP-ribose) polymerase-1 reduces host cell reactivation of a UV-damaged adenovirus-encoded reporter gene in human dermal fibroblasts

In response to ultraviolet radiation (UV), mammalian cells rapidly activate a nuclear enzyme poly(ADP-ribose) polymerase-1 (PARP), and we recently showed that one of the causes for PARP-activation is UV-induced direct DNA photolesions which are repaired by nucleotide excision repair process (NER). To determine whether PARP can play a role in NER, we stably depleted PARP in NER-proficient human skin fibroblasts GM637 by DNA vector-based RNAi. In these cells, we examined host cell reactivation (HCR) of UVB or UVC-irradiated recombinant adenovirus AdCA35lacZ, encoding a beta-galactosidase (beta-gal) reporter gene. The depletion of PARP decreased the HCR of UVB- or UVC-damaged reporter gene to a similar extent, indicating the role of PARP in NER. Moreover, PARP-depletion reduced the HCR capacity of the NER-competent GM637 cells to a level closer to that in the XP-C and CS-B cell lines, which are deficient in the lesion recognition steps of the global genome repair (GGR) and transcription-coupled repair (TCR) sub-pathways of NER, respectively. In order to identify the potential role of PARP in these two sub-pathways of NER from that of its known role in base excision repair (BER) of UVB-induced oxidant damage, we depleted PARP from XP-C and CS-B cells and examined HCR of the reporter gene damaged by UVB, UVC or photoactivated methylene blue, the latter causing predominantly 8-oxo-2'-deoxyguanosine damage that is repaired by BER. Interestingly, a decreased HCR due to PARP-depletion was observed in both the NER-deficient cell lines in response to virus damaged by these three agents, albeit with different kinetics from 12 to 44h after infection. The role of PARP in NER was highlighted by a decreased clonogenic survival of UV-irradiated NER-competent GM637 cells depleted of PARP. Our results, while confirming the role of PARP in base excision repair, suggest a novel role of PARP in both the GGR and TCR sub-pathways of NER.

The roles of translation initiation regulation in ultraviolet light-induced apoptosis

Ultraviolet light (UV) inhibits translation initiation through activation of kinases that phosphorylate the alpha-subunit of eukaryotic initiation factor 2 (eIF2alpha). Two eIF2alpha kinases, PERK and GCN2, are known to phosphorylate the Serine-51 of eIF2alpha in response to UV-irradiation. In this report, we present evidence that phosphorylation of eIF2alpha plays a role in UV-induced apoptosis. Our data show that wild-type mouse embryo fibroblasts (MEF(s/s)) are less sensitive to UV-induced apoptosis than MEF(A/A) cells in which the phosphorylation site, Ser51, of eIF2alpha is replaced with a non-phosphorylatable Ala (Ser51Ala). PARP expression in MEF(A/A) cells is reduced without being cleaved after UV-irradiation. In contrast, PARP is cleaved without a significant decrease in parental PARP in MEF(S/S) cells after UV-irradiation. Our data also show that MEF(GCN2-/-) cells, in which GCN2 is knocked out, are more sensitive to UV-irradiation, agreeing with the observation from MEF(A/A) cells. However, MEF(PERK-/-) cells, in which PERK is knocked out, are less sensitive to UV-irradiation. In addition, MCF-7-PERKDeltaC cells, which are stably transfected with a kinase domain deleted mutant of PERK (PERKDeltaC), are more resistant to UV-induced apoptosis than parental MCF-7 cells. Overexpression of wild-type PERK sensitizes MCF-7 cells to UV-induced apoptosis without directly inducing cell death. These results suggest that the level of eIF2alpha phosphorylation impacts PARP expression upon UV-irradiation. The eIF2alpha kinases may mediate UV-induced apoptosis via an eIF2alpha dependent or independent signaling pathway.

Pivotal role of Akt activation in mitochondrial protection and cell survival by poly(ADP-ribose)polymerase-1 inhibition in oxidative stress

According to the classical view, the cytoprotective effect of inhibitors of poly(ADP-ribose)polymerase (PARP) in oxidative stress was based on the prevention of NAD+ and ATP depletion, thus the attenuation of necrosis. Our previous data on PARP inhibitors in an inflammatory model suggested that PARP-catalyzed ADP-ribosylations may affect signaling pathways, which can play a significant role in cell survival. To clarify the molecular mechanism of cytoprotection, PARP activity was inhibited pharmacologically by suppressing PARP-1 expression by a small interfering RNA (siRNA) technique or by transdominantly expressing the N-terminal DNA-binding domain of PARP-1 (PARP-DBD) in cultured cells. Cell survival, activation of the phosphatidylinositol 3-kinase (PI3-kinase)/Akt system, and the preservation of mitochondrial membrane potential were studied in hydrogen peroxide-treated WRL-68 cells. Our data showed that suppression of the single-stranded DNA break-induced PARP-1 activation by pharmacological inhibitor, siRNA, or by the transdominant expression of PARP-DBD protected cells from oxidative stress and induced the phosphorylation and activation of Akt. Furthermore, prevention of Akt activation by inhibiting PI3-kinase counteracted the cytoprotective effect of PARP inhibition. Microscopy data showed that PARP inhibition-induced Akt activation was responsible for protection of mitochondria in oxidative stress because PI3-kinase inhibitors diminished the protective effect of PARP inhibition. Similarly, Src kinase inhibitors, which decrease Akt phosphorylation, also counteracted the protection of mitochondrial membrane potential supporting the pivotal role of Akt in cytoprotection. These data together with the finding that PARP inhibition in the absence of oxidative stress induced the phosphorylation and activation of Akt indicate that PARP inhibition-induced Akt activation is dominantly responsible for the cytoprotection in oxidative stress.

Induction of apoptosis by benzamide and its inhibition by aurin tricarboxylic acid (ATA) in Chinese hamster V79 cells

To investigate the effect of benzamide and nicotinamide, well known inhibitors of poly(ADP-ribose) polymerase, in Chinese hamster V79 cells at the physiological condition of cell growth, we have tested the ability of the inhibitors to induce apoptosis. Apoptosis was detected by nuclear fragmentation, nucleosomal ladder formation, cytochrome-c release from the mitochondria and caspase-3 activation. Benzamide treatment alone increased nuclear fragmentation in dose (2.5-10 mM) and time (4-48 h)-dependent manner. Such treatment also increased nucleosomal ladders. However, 5 mM benzamide pre-treatment inhibited the nucleosomal ladders induced by gamma-irradiation indicating the role of poly(ADP-ribose) polymerase was different in irradiated cells and in un-irradiated cells. Release of cytochrome-c from the mitochondria and caspase-3 activity were also increased by such treatment. Treatment with 200 microM of aurin tricarboxylic acid (ATA), an inhibitor of DNases, inhibited the nucleosomal ladders induced by benzamide or gamma-irradiation without changing the cytochrome-c release or caspase-3 activation. This result showed that ATA inhibited the nucleosomal ladders possibly by inhibiting DNase(s) involved in apoptosis.

Cleavage of 14-3-3 protein by caspase-3 facilitates bad interaction with Bcl-x(L) during apoptosis

The 14-3-3 epsilon protein was identified as one of the caspase-3 substrates by the modified yeast two-hybrid system. The cellular 14-3-3 epsilon protein was also cleaved in response to the treatment of apoptosis inducers in cultured mammalian cells. Asp238 of the 14-3-3 epsilon protein was determined as the site of cleavage by caspase-3. The affinity of the cleaved 14-3-3 mutant protein (D238) to Bad, a death-promoting Bcl-2 family protein, was lower than that of wild type or the uncleavable mutant 14-3-3 epsilon protein (D238A). However, Bad associated with the cellular Bcl-x(L) more effectively in human 293T cells co-expressing Bad with the truncated form of the 14-3-3 epsilon protein (D238) than in control cells co-expressing Bad with wild type or the uncleavable mutant 14-3-3 epsilon protein (D238A). The present study suggests that the cleavage of 14-3-3 protein during apoptosis promotes cell death by releasing the associated Bad from the 14-3-3 protein and facilitates Bad translocation to the mitochondria and its interaction with Bcl-x(L).

Necrosis: a specific form of programmed cell death?

For a long time necrosis was considered as an alternative to programmed cell death, apoptosis. Indeed, necrosis has distinct morphological features and it is accompanied by rapid permeabilization of plasma membrane. However, recent data indicate that, in contrast to necrosis caused by very extreme conditions, there are many examples when this form of cell death may be a normal physiological and regulated (programmed) event. Various stimuli (e.g., cytokines, ischemia, heat, irradiation, pathogens) can cause both apoptosis and necrosis in the same cell population. Furthermore, signaling pathways, such as death receptors, kinase cascades, and mitochondria, participate in both processes, and by modulating these pathways, it is possible to switch between apoptosis and necrosis. Moreover, antiapoptotic mechanisms (e.g., Bcl-2/Bcl-x proteins, heat shock proteins) are equally effective in protection against apoptosis and necrosis. Therefore, necrosis, along with apoptosis, appears to be a specific form of execution phase of programmed cell death, and there are several examples of necrosis during embryogenesis, a normal tissue renewal, and immune response. However, the consequences of necrotic and apoptotic cell death for a whole organism are quite different. In the case of necrosis, cytosolic constituents that spill into extracellular space through damaged plasma membrane may provoke inflammatory response; during apoptosis these products are safely isolated by membranes and then are consumed by macrophages. The inflammatory response caused by necrosis, however, may have obvious adaptive significance (i.e., emergence of a strong immune response) under some pathological conditions (such as cancer and infection). On the other hand, disturbance of a fine balance between necrosis and apoptosis may be a key element in development of some diseases.

Maintenance of ATP favours apoptosis over necrosis triggered by benzamide riboside

A new synthetic drug, benzamide riboside (BR) exhibited strong oncolytic activity against leukemic cells in the 5-10 microM range. Higher BR-concentrations (20 microM) predominantly induced necrosis which correlated with DNA strand breaks and subsequent depletion of ATP- and dATP levels. Replenishment of the ATP pool by addition of adenosine prevented necrosis and favoured apoptosis. This effect was not a pecularity of BR-treatment, but was reproduced with high concentrations of all trans-retinoic acid (120 microM) and cyanide (20 mM). Glucose was also capable to suppress necrosis and to favour apoptosis of HL-60 cells, which had been treated with necrotic doses of BR and cyanide. Apoptosis eliminates unwanted cells without affecting the microenvironment, whereas necrosis causes severe inflammation of surrounding tissues due to spillage of cell fluids into the peri-cellular space. Thus, the monitoring and maintenance of cellular energy pools during therapeutic drug treatment may help to minimize nonspecific side effects and to improve attempted drug effects.

Gain-of-function of poly(ADP-ribose) polymerase-1 upon cleavage by apoptotic proteases: implications for apoptosis

Poly(ADP-ribosyl)ation is an important mechanism for the maintenance of genomic integrity in response to DNA damage. The enzyme responsible for poly(ADP-ribose) synthesis, poly(ADP-ribose) polymerase 1 (PARP-1), has been implicated in two distinct modes of cell death induced by DNA damage, namely apoptosis and necrosis. During the execution phase of apoptosis, PARP-1 is specifically proteolyzed by caspases to produce an N-terminal DNA-binding domain (DBD) and a C-terminal catalytic fragment. The functional consequence of this proteolytic event is not known. However, it has recently been shown that overactivation of full-length PARP-1 can result in energy depletion and necrosis in dying cells. Here, we investigate the molecular basis for the differential involvement of PARP-1 in these two types of cellular demise. We show that the C-terminal apoptotic fragment of PARP-1 loses its DNA-dependent catalytic activity upon cleavage with caspase 3. However, the N-terminal apoptotic fragment, retains a strong DNA-binding activity and totally inhibits the catalytic activity of uncleaved PARP-1. This dominant-negative behavior was confirmed and extended in cellular extracts where DNA repair was completely inhibited by nanomolar concentrations of the N-terminal fragment. Furthermore, overexpression of the apoptotic DBD in mouse fibroblast inhibits endogenous PARP-1 activity very efficiently in vivo, thereby confirming our biochemical observations. Taken together, these experiments indicate that the apoptotic DBD of PARP-1 acts cooperatively with the proteolytic inactivation of the enzyme to trans-inhibit NAD hydrolysis and to maintain the energy levels of the cell. These results are consistent with a model in which cleavage of PARP-1 promotes apoptosis by preventing DNA repair-induced survival and by blocking energy depletion-induced necrosis.

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

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

Functional competition between poly(ADP-ribose) polymerase and its 24-kDa apoptotic fragment in DNA repair and transcription

Poly(ADP-ribose) polymerase is a 113-kDa nuclear enzyme that binds to both damaged DNA and to RNA associated with actively transcribed regions of chromatin. Binding of poly(ADP-ribose) polymerase to DNA lesions activates it, catalyzing the covalent addition of multiple ADP-ribose polymers to the enzyme (automodification). During apoptosis, poly(ADP-ribose) polymerase is cleaved by caspase-3, resulting in the formation of an N-terminal 24-kDa fragment, containing the DNA binding domain, and a C-terminal 89-kDa catalytic fragment. The functional relevance of this cleavage is not well understood. We therefore prepared a recombinant 24-kDa poly(ADP-ribose) polymerase fragment and investigated the role of this fragment in DNA repair and transcription. The 24-kDa fragment retained its binding affinity for both DNA breaks and RNA. In an in vitro cell-free DNA repair assay, this fragment inhibited rejoining of DNA breaks and suppressed ADP-ribose polymer formation by competing with poly(ADP-ribose) polymerase in binding to DNA breaks. With regard to transcription, it has recently been demonstrated that binding of poly(ADP-ribose) polymerase to transcribed RNA reduces the rate of transcript elongation and that automodification of poly(ADP-ribose) polymerase bound to DNA breaks results in up-regulation of transcription. We tested the 24-kDa fragment for its ability to suppress transcript elongation, and we found that it competed against the up-regulation of transcription mediated by full-length poly(ADP-ribose) polymerase. The ability of the 24-kDa fragment to inhibit DNA repair, ADP-ribose polymer formation, and damage-dependent up-regulation of transcription may contribute to the apoptotic shift from cell survival to cell death mode.