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

Molecular mechanisms of cell death by parthanatos: More questions than answers

Regulated cell death by a non-apoptotic pathway known as parthanatos is increasingly recognised as a central player in pathological processes, including ischaemic tissue damage and neurodegenerative diseases. Parthanatos is activated under conditions that induce high levels of DNA damage, leading to hyperactivation of the DNA damage sensor PARP1. While this strict dependence on PARP1 activation is a defining feature of parthanatos that distinguishes it from other forms of cell death, the molecular events downstream of PARP1 activation remain poorly understood. In this mini-review, we highlight a number of important questions that remain to be answered about this enigmatic form of cell death.

Poly (ADP-ribose) polymerase 1 and neurodegenerative diseases: Past, present, and future

Poly (ADP-ribose) polymerase 1 (PARP1) is a first responder that recognizes DNA damage and facilitates its repair. Neurodegenerative diseases, characterized by progressive neuron loss driven by various risk factors, including DNA damage, have increasingly shed light on the pivotal involvement of PARP1. During the early phases of neurodegenerative diseases, PARP1 experiences controlled activation to swiftly address mild DNA damage, thereby contributing to maintain brain homeostasis. However, in late stages, exacerbated PARP1 activation precipitated by severe DNA damage exacerbates the disease condition. Consequently, inhibition of PARP1 overactivation emerges as a promising therapeutic approach for neurodegenerative diseases. In this review, we comprehensively synthesize and explore the multifaceted role of PARP1 in neurodegenerative diseases, with a particular emphasis on its over-activation in the aggregation of misfolded proteins, dysfunction of the autophagy-lysosome pathway, mitochondrial dysfunction, neuroinflammation, and blood-brain barrier (BBB) injury. Additionally, we encapsulate the therapeutic applications and limitations intrinsic of PARP1 inhibitors, mainly including limited specificity, intricate pathway dynamics, constrained clinical translation, and the heterogeneity of patient cohorts. We also explore and discuss the potential synergistic implementation of these inhibitors alongside other agents targeting DNA damage cascades within neurodegenerative diseases. Simultaneously, we propose several recommendations for the utilization of PARP1 inhibitors within the realm of neurodegenerative disorders, encompassing factors like the disease-specific roles of PARP1, combinatorial therapeutic strategies, and personalized medical interventions. Lastly, the encompassing review presents a forward-looking perspective along with strategic recommendations that could guide future research endeavors in this field.

Cell death induction and protection by activation of ubiquitously expressed anion/cation channels. Part 3: the roles and properties of TRPM2 and TRPM7

Cell volume regulation (CVR) is a prerequisite for animal cells to survive and fulfill their functions. CVR dysfunction is essentially involved in the induction of cell death. In fact, sustained normotonic cell swelling and shrinkage are associated with necrosis and apoptosis, and thus called the necrotic volume increase (NVI) and the apoptotic volume decrease (AVD), respectively. Since a number of ubiquitously expressed ion channels are involved in the CVR processes, these volume-regulatory ion channels are also implicated in the NVI and AVD events. In Part 1 and Part 2 of this series of review articles, we described the roles of swelling-activated anion channels called VSOR or VRAC and acid-activated anion channels called ASOR or PAC in CVR and cell death processes. Here, Part 3 focuses on therein roles of Ca2+-permeable non-selective TRPM2 and TRPM7 cation channels activated by stress. First, we summarize their phenotypic properties and molecular structure. Second, we describe their roles in CVR. Since cell death induction is tightly coupled to dysfunction of CVR, third, we focus on their participation in the induction of or protection against cell death under oxidative, acidotoxic, excitotoxic, and ischemic conditions. In this regard, we pay attention to the sensitivity of TRPM2 and TRPM7 to a variety of stress as well as to their capability to physicall and functionally interact with other volume-related channels and membrane enzymes. Also, we summarize a large number of reports hitherto published in which TRPM2 and TRPM7 channels are shown to be involved in cell death associated with a variety of diseases or disorders, in some cases as double-edged swords. Lastly, we attempt to describe how TRPM2 and TRPM7 are organized in the ionic mechanisms leading to cell death induction and protection.

TRP Channels: The Neglected Culprits in Breast Cancer Chemotherapy Resistance?

Breast cancer is a major health concern worldwide, and resistance to therapies remains a significant challenge in treating this disease. In breast cancer, Transient Receptor Potential (TRP) channels are well studied and constitute key players in nearly all carcinogenesis hallmarks. Recently, they have also emerged as important actors in resistance to therapy by modulating the response to various pharmaceutical agents. Targeting TRP channels may represent a promising approach to overcome resistance to therapies in breast cancer patients.

PARG suppresses tumorigenesis and downregulates genes controlling angiogenesis, inflammatory response, and immune cell recruitment

Chemokines are highly expressed in tumor microenvironment and play a critical role in all aspects of tumorigenesis, including the recruitment of tumor-promoting immune cells, activation of cancer-associated fibroblasts, angiogenesis, metastasis, and growth. Poly (ADP-ribose) polymerase (PARP) is a multi-target transcription regulator with high levels of poly(ADP-ribose) (pADPr) being reported in a variety of cancers. Furthermore, poly (ADP-ribose) glycohydrolase (PARG), an enzyme that degrades pADPr, has been reported to be downregulated in tumor tissues with abnormally high levels of pADPr. In conjunction to this, we have recently reported that the reduction of pADPr, by either pharmacological inhibition of PARP or PARG's overexpression, disrupts renal carcinoma cell malignancy in vitro. Here, we use 3 T3 mouse embryonic fibroblasts, a universal model for malignant transformation, to follow the effect of PARG upregulation on cells' tumorigenicity in vivo. We found that the overexpression of PARG in mouse allografts produces significantly smaller tumors with a delay in tumor onset. As downregulation of PARG has also been implicated in promoting the activation of pro-inflammatory genes, we also followed the gene expression profile of PARG-overexpressing 3 T3 cells using RNA-seq approach and observed that chemokine transcripts are significantly reduced in those cells. Our data suggest that the upregulation of PARG may be potentially useful for the tumor growth inhibition in cancer treatment and as anti-inflammatory intervention.

Selenium and Resveratrol Attenuated Diabetes Mellitus-Mediated Oxidative Retinopathy and Apoptosis via the Modulation of TRPM2 Activity in Mice

Diabetes mellitus induces optic nerve injury via the excessive generation of mitochondria reactive free oxygen radical (mitROS). TRPM2 channel is activated by mitROS, although it is inhibited by selenium (Se) and resveratrol (RSV). The activation of TRPM2 induces apoptosis and oxidative injury in the optic nerve. The inhibition of TRPM2 may decrease the optic nerve injury action of diabetes mellitus after the treatments of Se and RSV. Present study aimed to investigate the protective actions of Se and RSV on the excessive Ca²⁺ influx and mitROS generation-mediated optic nerve oxidative injury via the modulation of TRPM2. Fifty-six C57BL/6j male mice were divided into seven groups as control, Se, RSV, streptozotocin (STZ), STZ + Se, STZ + RSV, and STZ + Se + RSV. The STZ-mediated stimulation of TRPM2 increased the cytosolic Ca²⁺, lipid peroxidation, mitROS, cytosolic ROS, apoptosis, caspase-3, caspase-8, and caspase-9 concentrations in the mice, although their concentrations were decreased in the optic nerve by the treatments of Se and RSV. The STZ-induced decrease of optic nerve viability, glutathione, glutathione peroxidase, vitamin A, and vitamin E concentrations was also upregulated by the treatments of Se and RSV. The STZ-induced increase of TRPM2, PARP-1, caspase-3, and caspase-9 protein band expressions was diminished by the treatments of Se and RSV. In conclusion, STZ induced the optic nerve oxidative injury and apoptosis via the upregulation of TRPM2 stimulation, although the treatments of Se and RSV decreased the injury and apoptosis via the downregulation of TRPM2 activity.

TRPM2 in ischemic stroke: Structure, molecular mechanisms, and drug intervention

Ischemic stroke has a high lethality rate worldwide, and novel treatments are limited. Calcium overload is considered to be one of the mechanisms of cerebral ischemia. Transient receptor potential melastatin 2 (TRPM2) is a reactive oxygen species (ROS)-sensitive calcium channel. Cerebral ischemia-induced TRPM2 activation triggers abnormal intracellular Ca2+ accumulation and cell death, which in turn causes irreversible brain damage. Thus, TRPM2 has emerged as a new therapeutic target for ischemic stroke. This review provides data on the expression, structure, and function of TRPM2 and illustrates its cellular and molecular mechanisms in ischemic stroke. Natural and synthetic TRPM2 inhibitors (both specific and nonspecific) are also summarized. The three-dimensional protein structure of TRPM2 has been identified, and we speculate that molecular simulation techniques will be essential for developing new drugs that block TRPM2 channels. These insights about TRPM2 may be the key to find potent therapeutic approaches for the treatment of ischemic stroke.

Structural and functional basis of the selectivity filter as a gate in human TRPM2 channel

Transient receptor potential melastatin 2 (TRPM2), a Ca²⁺-permeable cation channel, is gated by intracellular adenosine diphosphate ribose (ADPR), Ca²⁺, warm temperature, and oxidative stress. It is critically involved in physiological and pathological processes ranging from inflammation to stroke to neurodegeneration. At present, the channel's gating and ion permeation mechanisms, such as the location and identity of the selectivity filter, remain ambiguous. Here, we report the cryo-electron microscopy (cryo-EM) structure of human TRPM2 in nanodisc in the ligand-free state. Cryo-EM map-guided computational modeling and patch-clamp recording further identify a quadruple-residue motif as the ion selectivity filter, which adopts a restrictive conformation in the closed state and acts as a gate, profoundly contrasting with its widely open conformation in the Nematostella vectensis TRPM2. Our study reveals the gating of human TRPM2 by the filter and demonstrates the feasibility of using cryo-EM in conjunction with computational modeling and functional studies to garner structural information for intrinsically dynamic but functionally important domains.

Emerging role of PARP-1 and PARthanatos in ischemic stroke

Cell death is a key feature of neurological diseases, including stroke and neurodegenerative disorders. Studies in a variety of ischemic/hypoxic mouse models demonstrate that poly(ADP-ribose) polymerase 1 (PARP-1)-dependent cell death, also named PARthanatos, plays a pivotal role in ischemic neuronal cell death and disease progress. PARthanatos has its unique triggers, processors, and executors that convey a highly orchestrated and programmed signaling cascade. In addition to its role in gene transcription, DNA damage repair, and energy homeostasis through PARylation of its various targets, PARP-1 activation in neuron and glia attributes to brain damage following ischemia/reperfusion. Pharmacological inhibition or genetic deletion of PARP-1 reduces infarct volume, eliminates inflammation, and improves recovery of neurological functions in stroke. Here, we reviewed the role of PARP-1 and PARthanatos in stroke and their therapeutic potential.

The parp-1 and bax genes as potential targets for treatment of the heart functioning impairments induced by type 1 diabetes mellitus

Objectives. The present study was designed to assess whether apoptosis-related genes as parp-1 and bax could be targets for treatment of diabetes mellitus and whether vitamin D may exert beneficial effects. Methods. Vitamin D₃ treatment for 4 weeks, starting after 4 weeks of the diabetes duration. The expression of parp-1 and bax genes was estimated on mRNA levels using real time quantitative polymerase chain reaction. Results. After 8 weeks, diabetic rats had weight loss, while blood glucose was increased about 4.9-fold compared to control group. Vitamin D₃ administration to diabetic animals had no effect on these parameters. It was found that total serum alkaline phosphatase activity was significantly elevated in diabetic rats as compared to control animals and was restored by vitamin D₃. Diabetes was accompanied by reduction of nicotinamidadenindinucleotide, a substrate of poly-ADP-ribosylation, level by 31.7% as compared to control rats, which was not reversed in response to vitamin D₃ treatment. In diabetic hearts, the mRNA expression level of parp-1 gene was 2.8-fold higher compared to control rats and partially decreased by vitamin D3 treatment. Less significant alterations were observed in diabetic hearts for the mRNA expression level of bax gene that was 2.0-fold higher compared to control animals and vitamin D₃ normalized it. These results indicate that cardiomyocytes have a tendency to apoptosis. Conclusions. The findings suggest that investigated genes can be targets at the transcriptional level for vitamin D action that may be contributed to the improving metabolic/signaling pathways induced by diabetes mellitus.

Revisiting Traumatic Brain Injury: From Molecular Mechanisms to Therapeutic Interventions

Studying the complex molecular mechanisms involved in traumatic brain injury (TBI) is crucial for developing new therapies for TBI. Current treatments for TBI are primarily focused on patient stabilization and symptom mitigation. However, the field lacks defined therapies to prevent cell death, oxidative stress, and inflammatory cascades which lead to chronic pathology. Little can be done to treat the mechanical damage that occurs during the primary insult of a TBI; however, secondary injury mechanisms, such as inflammation, blood-brain barrier (BBB) breakdown, edema formation, excitotoxicity, oxidative stress, and cell death, can be targeted by therapeutic interventions. Elucidating the many mechanisms underlying secondary injury and studying targets of neuroprotective therapeutic agents is critical for developing new treatments. Therefore, we present a review on the molecular events following TBI from inflammation to programmed cell death and discuss current research and the latest therapeutic strategies to help understand TBI-mediated secondary injury.

Poly(ADP-ribose) metabolism in human parasitic protozoa

Poly(ADP-ribosyl)ation reactions constitute a post-translational protein modification synthesized in higher eukaryotes by a family of poly(ADP-ribose)polymerases (PARP) and catabolized mainly by poly(ADP-ribose) glycohydrolase (PARG). The best understood role of PARP is the maintenance of genomic integrity via the promotion of DNA repair that leads to cell survival when low levels of genotoxic stress occur. The participation of PARP in unleashing cell death at higher levels of damage has also been broadly studied. The biology of poly(ADP-ribosyl)ation in protozoan parasites, however, still remains a mystery. This review will examine the presence of the key enzyme involved in ADP-ribose polymer (PAR) metabolism in protozoan parasites associated with human diseases. Theoretical and experimental data obtained up to date have revealed the presence of PAR metabolism only in the trypanosomatids Trypanosoma cruzi and T. brucei, the apicomplexan Toxoplasma gondii and Entamoeba histolytica. T. cruzi and T. brucei, as opposed to humans and other organisms, have only one PARP and one PARG with subcellular localizations that are distinct from the ones described for their mammalian counterparts. The topics discussed in this review describe the first studies on PAR metabolism in trypanosomatids, specially the role of PAR on DNA damage response, cell cycle progression and cell death after genotoxic stimuli. The results described show differences in some aspects of PAR metabolism in trypanosomatids in comparison to other eukaryotes. New questions about the function of this metabolic pathway in the parasites under study are open and we hope it encourages the research community to explore this signaling pathway as a new possible target of clinical relevance in these and other disease-causing parasites.

Targeting poly(ADP-ribose) glycohydrolase to draw apoptosis codes in cancer

Poly(ADP-ribosyl)ation is a unique post-translational modification of proteins. The metabolism of poly(ADP-ribose) (PAR) is tightly regulated mainly by poly(ADP-ribose) polymerases (PARP) and poly(ADP-ribose) glycohydrolase (PARG). Accumulating evidence has suggested the biological functions of PAR metabolism in control of many cellular processes, such as cell proliferation, differentiation and death by remodeling chromatin structure and regulation of DNA transaction, including DNA repair, replication, recombination and transcription. However, the physiological roles of the catabolism of PAR catalyzed by PARG remain less understood than those of PAR synthesis by PARP. Noteworthy biochemical studies have revealed the importance of PAR catabolic pathway generating nuclear ATP via the coordinated actions of PARG and ADP-ribose pyrophosphorylase (ADPRPPL) for the driving of DNA repair and the maintenance of DNA replication apparatus while repairing DNA damage. Furthermore, genetic studies have shown the value of PARG as a therapeutic molecular target for PAR-mediated diseases, such as cancer, inflammation and many pathological conditions. In this review, we present the current knowledge of de-poly(ADP-ribosyl)ation catalyzed by PARG focusing on its role in DNA repair, replication and apoptosis. Furthermore, the induction of apoptosis code of DNA replication catastrophe by synthetic lethality of PARG inhibition and the recent progresses regarding the development of small molecule PARG inhibitors and their therapeutic potentials in cancer chemotherapy are highlighted in this review.

PARG regulates the proliferation and differentiation of DCs and T cells via PARP/NF‑κB in tumour metastases of colon carcinoma

The present study investigated the effect of poly(ADP‑ribose) glycohydrolase (PARG) on the immune response in tumour metastases of colon carcinoma. CT26 cells were transfected with lentivirus PARG‑short hairpin RNA (shRNA). A liver metastasis model of colon carcinoma was successfully established by splenic subcapsular inoculation of the various groups of CT26 cells into BALB/c mice. Next, changes in the liver metastases of colon carcinoma nodules and alterations in the survival times were observed in tumour‑bearing mice. The numbers of B220+DEC205+ dendritic cells (B220+DEC205+DC) and CD11c+CD11b+ dendritic cells (CD11c+CD11b+DC) in the spleen and liver were measured by the double‑label immunofluorescence assay. The distribution pattern of CD4+T cells and CD8+T cells in the spleen and liver was investigated by immunofluorescence staining. The expression levels of PARG, PARP and nuclear factor‑κB (NF‑κB) proteins in spleen transplant tumours and liver metastases of colon carcinoma were detected by western blotting. An ELISA was used to detect the levels of IL‑10 and TGF‑β in the serum of tumour‑bearing mice and from the supernatant of tumour cells. The numbers and grading of metastatic liver nodules in the PARG‑silenced group were clearly lower than those in the control group. The survival time of the PARG‑silenced group mice was longer than that in the control group. In the PARG‑silenced group, the levels of B220+DEC205+DC in the spleen and liver were lower and the numbers of CD11c+CD11b+DC in the spleen and liver were more than those in the control group. The ratio of CD4+/CD8+ in the spleen and liver in the PARG‑silenced group was increased compared with that in the control group (P<0.05). The levels of PARG, PARP and NF‑κB in spleen transplant tumours and liver metastases of colon carcinoma were lower in the PARG‑silenced group than in the control group. In addition, the levels of IL‑10 and TGF‑β in the serum of tumour‑bearing mice and supernatants of tumour cells were both reduced in the PARG‑silenced group compared with those in the control group. The present research suggests that the liver metastases of colon carcinoma could be restrained by silencing PARG. Likely, the silencing of PARG could suppress the expression of PARP and NF‑κB and subsequently suppress the secretion of IL‑10 and TGF‑α, finally affecting the proliferation and differentiation of DC and T cells.

Adenine nucleotides as paracrine mediators and intracellular second messengers in immunity and inflammation

Adenine nucleotides (AdNs) play important roles in immunity and inflammation. Extracellular AdNs, such as adenosine triphosphate (ATP) or nicotinamide adenine dinucleotide (NAD) and their metabolites, act as paracrine messengers by fine-tuning both pro- and anti-inflammatory processes. Moreover, intracellular AdNs derived from ATP or NAD play important roles in many cells of the immune system, including T lymphocytes, macrophages, neutrophils and others. These intracellular AdNs are signaling molecules that transduce incoming signals into meaningful cellular responses, e.g. activation of immune responses against pathogens.

Multiple Roles for Mono- and Poly(ADP-Ribose) in Regulating Stress Responses

Although stress-induced synthesis of mono(ADP-ribose) (mADPr) and poly(ADP-ribose) (pADPr) conjugates by pADPr polymerase (PARP) enzymes has been studied extensively, the removal and degradation of pADPr, as well as the fate of ADPr metabolites, have received less attention. The observations that stress-induced pADPr undergoes rapid turnover, and that deficiencies in ADPr degradation phenocopy loss of pADPr synthesis, suggest that ADPr degradation is fundamentally important to the cellular stress response. Recent work has identified several distinct families of pADPr hydrolases that can degrade pADPr to release pADPr or mADPr into the cytoplasm. Further, many stress-response proteins contain ADPr-binding domains that can interact with these metabolites. We discuss how pADPr metabolites generated during pADPr degradation can function as signaling intermediates in processes such as inflammation, apoptosis, and DNA damage responses. These studies highlight that the full cycle of ADPr metabolism, including both synthesis and degradation, is necessary for responses to genotoxic stress.

TRPM2: a candidate therapeutic target for treating neurological diseases

Transient receptor potential melastatin 2 (TRPM2) is a calcium (Ca²⁺)-permeable non-selective cation channel belonging to the TRP ion channel family. Oxidative stress-induced TRPM2 activation provokes aberrant intracellular Ca²⁺ accumulation and cell death in a variety of cell types, including neurons. Aberrant TRPM2 function has been implicated in several neurological disorders including ischemia/stroke, Alzheimer's disease, neuropathic pain, Parkinson's disease and bipolar disorder. In addition to research identifying a role for TRPM2 in disease, progress has been made in the identification of physiological functions of TRPM2 in the brain, including recent evidence that TRPM2 is necessary for the induction of N-methyl-D-aspartate (NMDA) receptor-dependent long-term depression, an important form of synaptic plasticity at glutamate synapses. Here, we summarize recent evidence on the role of TRPM2 in the central nervous system (CNS) in health and disease and discuss the potential therapeutic implications of targeting TRPM2. Collectively, these studies suggest that TRPM2 represents a prospective novel therapeutic target for neurological disorders.

Alkylation induced cerebellar degeneration dependent on Aag and Parp1 does not occur via previously established cell death mechanisms

Alkylating agents are ubiquitous in our internal and external environments, causing DNA damage that contributes to mutations and cell death that can result in aging, tissue degeneration and cancer. Repair of methylated DNA bases occurs primarily through the base excision repair (BER) pathway, a multi-enzyme pathway initiated by the alkyladenine DNA glycosylase (Aag, also known as Mpg). Previous work demonstrated that mice treated with the alkylating agent methyl methanesulfonate (MMS) undergo cerebellar degeneration in an Aag-dependent manner, whereby increased BER initiation by Aag causes increased tissue damage that is dependent on activation of poly (ADP-ribose) polymerase 1 (Parp1). Here, we dissect the molecular mechanism of cerebellar granule neuron (CGN) sensitivity to MMS using primary ex vivo neuronal cultures. We first established a high-throughput fluorescent imaging method to assess primary neuron sensitivity to treatment with DNA damaging agents. Next, we verified that the alkylation sensitivity of CGNs is an intrinsic phenotype that accurately recapitulates the in vivo dependency of alkylation-induced CGN cell death on Aag and Parp1 activity. Finally, we show that MMS-induced CGN toxicity is independent of all the cellular events that have previously been associated with Parp-mediated toxicity, including mitochondrial depolarization, AIF translocation, calcium fluxes, and NAD+ consumption. We therefore believe that further investigation is needed to adequately describe all varieties of Parp-mediated cell death.

The role of poly ADP-ribosylation in the first wave of DNA damage response

Poly ADP-ribose polymerases (PARPs) catalyze massive protein poly ADP-ribosylation (PARylation) within seconds after the induction of DNA single- or double-strand breaks. PARylation occurs at or near the sites of DNA damage and promotes the recruitment of DNA repair factors via their poly ADP-ribose (PAR) binding domains. Several novel PAR-binding domains have been recently identified. Here, we summarize these and other recent findings suggesting that PARylation may be the critical event that mediates the first wave of the DNA damage response. We also discuss the potential for functional crosstalk with other DNA damage-induced post-translational modifications.

From the Cover: ROS-Induced Store-Operated Ca2+ Entry Coupled to PARP-1 Hyperactivation Is Independent of PARG Activity in Necrotic Cell Death

2,3,5-tris(Glutathion-S-yl)hydroquinone, a potent nephrotoxic and nephrocarcinogenic metabolite of benzene and hydroquinone, generates reactive oxygen species (ROS) causing DNA strand breaks and the subsequent activation of DNA repair enzymes, including poly(ADP-ribose) polymerase (PARP)-1. Under robust oxidative DNA damage, PARP-1 is hyperactivated, resulting in the depletion of NAD+ and ATP with accompanying elevations in intracellular calcium concentrations (iCa2+), and ultimately necrotic cell death. The role of Ca2+ during PARP-dependent necrotic cell death remains unclear. We therefore sought to determine the relationship between Ca2+ and PARP-1 during ROS-induced necrotic cell death in human renal proximal tubule epithelial cells (HK-2). Our experiments suggest that store-operated Ca2+ channel (SOC) entry contributes to the coupling of PARP-1 activation to increases in iCa2+ during ROS-induced cell death. Poly(ADP-ribose)glycohydrolase (PARG), which catalyzes the degradation of PARs to yield free ADP-ribose (ADPR), is known to activate Ca2+ channels such as TRPM2. However, siRNA knockdown of PARG did not restore cell viability, indicating that free ADPR is not responsible for SOC activation in HK-2 cells. The data indicate that PARP-1 and iCa2+ are coupled through activation of SOC mediated Ca2+ entry in an apparently ADPR-independent fashion; alternative PAR-mediated signaling likely contributes to PARP-dependent necrotic cell death, perhaps via PAR-mediated signaling proteins that regulate iCa2+ homeostasis.

Protective Functions of PJ34, a Poly(ADP-ribose) Polymerase Inhibitor, Are Related to Down-Regulation of Calpain and Nuclear Factor-κB in a Mouse Model of Traumatic Brain Injury

OBJECTIVES

Poly(ADP-ribose) polymerase (PARP), calpain, and nuclear factor-κB (NF-κB) are reported to participate in inflammatory reactions in pathologic conditions and are involved in traumatic brain injury. The objective of this study was to investigate whether PARP participates in inflammation related to calpain and NF-κB in a mouse model of controlled cortical impact (CCI).

METHODS

PJ34 (10 mg/kg), a selective PARP inhibitor, was administered intraperitoneally 5 minutes and 8 hours after experimental CCI. We then performed a histopathologic analysis, and we measured calpain activity and protein levels in all animals. The cytosolic, mitochondria, and nuclear fractions were prepared and used to determine the levels of PARP, calpastatin, NF-κB p65, inhibitory-κB-α, tumor necrosis factor-α, interleukin-1β, intracellular adhesion molecule-1, inducible nitric oxide synthase, and cyclooxygenase-2. We then measured blood-brain barrier disruption using electron microscopy at 6 and 24 hours after CCI.

RESULTS

Treatment with PJ34 markedly reduced the extent of both cerebral contusion and edema, improved neurologic scores, and attenuated blood-brain barrier damage resulting from CCI. Our data showed that the cytosolic and nuclear fractions of calpain and NF-κB were up-regulated in the injured cortex and that these changes were reversed by PJ34. Moreover, PJ34 significantly enhanced the calpastatin and inhibitory-κB levels and decreased the levels of inflammatory mediators.

CONCLUSIONS

PARP inhibition by PJ34 suppresses the overactivation of calpain and the production of inflammatory factors that are caused by NF-κB activation and attenuates neuronal cell death in a mouse model of CCI.

PARPing for balance in the homeostasis of poly(ADP-ribosyl)ation

Despite more than 50 years of research, the vast majority of the biology of poly(ADP-ribosyl)ation (PARylation) still remains a gross mystery. Originally described to be a part of the DNA repair machinery, poly(ADP-ribose) (PAR) is synthesized immediately by poly(ADP-ribose) polymerases (PARPs, also known as ARTDs) upon DNA damage and then rapidly removed by degrading enzymes. PAR provides a delicate and spatiotemporal interaction scaffold for numerous target proteins. Thus, the multifaceted PARylation system, consisting of PAR itself and its synthesizers and erasers, plays diverse roles in the DNA damage response (DDR), in DNA repair, transcription, replication, chromatin remodelling, metabolism and cell death. In this review, we summarize the current understanding of the biology of PARylation, focusing on the functionality and the activities of the PARPs' founding member PARP1/ARTD1, which is modulated by a variety of posttranslational modifications. We also discuss the homeostasis of PAR - a process which is maintained by the balance of PAR synthesizers and erasers. We aim to sensitize the scientific community to the complexity of PAR homeostasis. Finally, we provide some perspective on how future research could try to disentangle the biology of PARylation - perhaps the most sophisticated, but still intricate posttranslational modification described to date.

Protective effects of PARP inhibitor, PJ34, is related to down-regulation of calpain and NF-κB in a mouse model of TBI

BACKGROUND

Poly(ADP-ribose) polymerase (PARP), calpain and nuclear factor-κB (NF-κB) are reported to participate in inflammatory reactions in pathological conditions and are involved in traumatic brain injury. The objective of this study was to investigate whether PARP participated in inflammation related to calpain and NF-κB in a mouse model of controlled cortical impact (CCI).

MATERIALS AND METHODS

PJ34 (10 mg kg-1), a selective PARP inhibitor, was administered intraperitoneally 5 minutes and 8 hours after experimental CCI. A neurobehavioural evaluation and a histopathological analysis were then performed and the contusion volume, calpain activity and protein levels were measured in all animals.

RESULTS

Treatment with PJ34 markedly reduced neurological deficits, decreased contusion volume and attenuated necrotic and apoptotic neuronal cell death 24 hours after CCI. The data showed that the cytosolic and nuclear fractions of calpain and NF-κB were up-regulated in the injured cortex and that these changes were reversed by PJ34. Moreover, PJ34 significantly enhanced the calpastatin and IκB levels and decreased the levels of inflammatory mediators.

CONCLUSIONS

PARP inhibition by PJ34 suppresses the over-activation of calpain and the production of inflammatory factors that are caused by NF-κB activation and it improves neurological functioning, decreases the contusion volume and attenuates neuronal cell death in a mouse model of CCI.

TRPM2 channels in alveolar epithelial cells mediate bleomycin-induced lung inflammation

Lung inflammation is a major adverse effect of therapy with the antitumor drug bleomycin (BLM). Transient receptor potential melastatin 2 (TRPM2) is a Ca(2+)-permeable channel that is activated by oxidative stress through the production of ADP-ribose. We herein investigated whether TRPM2 channels contributed to BLM-induced lung inflammation. The intratracheal instillation of BLM into wild-type (WT) mice increased the number of polymorphonuclear leukocytes (PMNs) and inflammatory cytokine levels in the lung. Increases in inflammatory markers in WT mice were markedly reduced in trpm2 knockout (KO) mice, which demonstrated that the activation of TRPM2 channels was involved in BLM-induced lung inflammation. The expression of TRPM2 mRNA was observed in alveolar macrophages, alveolar epithelial cells, and lung fibroblasts. Actually, TRPM2 protein was expressed in lung tissues. Of these, TRPM2 channels in epithelial cells were activated by the addition of H2O2 following a BLM pretreatment, resulting in the secretion of macrophage inflammatory protein-2 (MIP-2). The H2O2-induced activation of TRPM2 by the BLM pretreatment was blocked by the poly(ADP-ribose) polymerase (PARP) inhibitors PJ34 and 3-aminobenzamide. The accumulation of poly(ADP-ribose) in the nucleus, a marker for ADP-ribose production, was strongly induced by H2O2 following the BLM pretreatment. Furthermore, administration of PRAP inhibitors into WT mice markedly reduced recruitment of inflammatory cells and MIP-2 secretion induced by BLM instillation. These results suggest that the induction of MIP-2 secretion through the activation of TRPM2 channels in alveolar epithelial cells is an important mechanism in BLM-induced lung inflammation, and the TRPM2 activation is likely to be mediated by ADP-ribose production via PARP pathway. TRPM2 channels may be new therapeutic target for BLM-induced lung inflammation.

Complex role of nicotinamide adenine dinucleotide in the regulation of programmed cell death pathways

Over the past few years, a growing body of experimental observations has led to the identification of novel and alternative programs of regulated cell death. Recently, autophagic cell death and controlled forms of necrosis have emerged as major alternatives to apoptosis, the best characterized form of regulated cell demise. These recently identified, caspase-independent, forms of cell death appear to play a role in the response to several forms of stress, and their importance in different pathological conditions such as ischemia, infection and inflammation has been recognized. The functional link between cell metabolism and survival has also been the matter of recent studies. Nicotinamide adenine dinucleotide (NAD(+)) has gained particular interest due to its role in cell energetics, and as a substrate for several families of enzymes, comprising poly ADP-ribose polymerases (PARPs) and sirtuins, involved in numerous biological functions including cell survival and death. The recently uncovered diversity of cell death programs has led us to reevaluate the role of this important metabolite as a universal pro-survival factor, and to discuss the potential benefits and limitations of pharmacological approaches targeting NAD(+) metabolism.

Enhanced cytotoxicity in triple-negative and estrogen receptor‑positive breast adenocarcinoma cells due to inhibition of the transient receptor potential melastatin-2 channel

We previously demonstrated a unique protective role for the transient receptor potential, melastatin-2 (TRPM2) cation channel in breast cancer cells. In the present study, we investigated the chemotherapeutic effects elicited by inhibiting this protective role in metastatic breast adenocarcinoma cells. TRPM2 inhibition led to dose-dependent increases in MDA-MB-231 breast adenocarcinoma cell death after treatment with doxorubicin or the DNA-methylating agent, N-methyl-N'-nitro-N-nitrosoguanidine. Similar results were observed after RNAi silencing of TRPM2 in these cells after doxorubicin treatment. However, TRPM2 RNAi silencing also led to increased MCF-7 breast adenocarcinoma cell death after tamoxifen treatment, yet not in non-cancerous human mammary epithelial cells. These results thus revealed that TRPM2 inhibition selectively increased cytotoxicity in a triple-negative and an estrogen receptor-positive breast cancer cell line, with minimal deleterious effects in non-cancerous breast cells. Analysis of DNA damage revealed enhanced DNA damage levels in MCF-7 cells treated with doxorubicin due to TRPM2 inhibition. Analysis of cell death demonstrated that inhibition of apoptosis, caspase-independent cell death or autophagy failed to significantly reduce cell death induced by TRPM2 inhibition and chemotherapy. These results indicate that TRPM2 inhibition activates alternative pathways of cell death in breast cancer cells. Taken together, our results provide significant evidence that TRPM2 inhibition is a potential strategy to induce triple-negative and estrogen receptor-positive breast adenocarcinoma cell death via alternative cell death pathways. This is expected to provide a basis for inhibiting TRPM2 for the improved treatment of breast cancer, which potentially includes treating breast tumors that are resistant to chemotherapy due to their evasion of apoptosis.

The rise and fall of poly(ADP-ribose): An enzymatic perspective

Human cells respond to DNA damage with an acute and transient burst in production of poly(ADP-ribose), a posttranslational modification that expedites damage repair and plays a pivotal role in cell fate decisions. Poly(ADP-ribose) polymerases (PARPs) and glycohydrolase (PARG) are the key set of enzymes that orchestrate the rise and fall in cellular levels of poly(ADP-ribose). In this perspective, we focus on recent structural and mechanistic insights into the enzymes involved in poly(ADP-ribose) production and turnover, and we highlight important questions that remain to be answered.

The role of excitotoxic programmed necrosis in acute brain injury

Excitotoxicity involves the excessive release of glutamate from presynaptic nerve terminals and from reversal of astrocytic glutamate uptake, when there is excessive neuronal depolarization. N-methyl-d-aspartate (NMDA) receptors, a subtype of glutamate receptor, are activated in postsynaptic neurons, opening their receptor-operated cation channels to allow Ca^2 + influx. The Ca^2 + influx activates two enzymes, calpain I and neuronal nitric oxide synthase (nNOS). Calpain I activation produces mitochondrial release of cytochrome c (cyt c), truncated apoptosis-inducing factor (tAIF) and endonuclease G (endoG), the lysosomal release of cathepsins B and D and DNase II, and inactivation of the plasma membrane Na⁺–Ca^2 + exchanger, which add to the buildup of intracellular Ca^2 +. tAIF is involved in large-scale DNA cleavage and cyt c may be involved in chromatin condensation; endoG produces internucleosomal DNA cleavage. The nuclear actions of the other proteins have not been determined. nNOS forms nitric oxide (NO), which reacts with superoxide (O₂⁻) to form peroxynitrite (ONOO⁻). These free radicals damage cellular membranes, intracellular proteins and DNA. DNA damage activates poly(ADP-ribose) polymerase-1 (PARP-1), which produces poly(ADP-ribose) (PAR) polymers that exit nuclei and translocate to mitochondrial membranes, also releasing AIF. Poly(ADP-ribose) glycohydrolase hydrolyzes PAR polymers into ADP-ribose molecules, which translocate to plasma membranes, activating melastatin-like transient receptor potential 2 (TRPM-2) channels, which open, allowing Ca^2 + influx into neurons. NADPH oxidase (NOX1) transfers electrons across cellular membranes, producing O₂⁻. The result of these processes is neuronal necrosis, which is a programmed cell death that is the basis of all acute neuronal injury in the adult brain.

Inhibition of the transient receptor potential melastatin-2 channel causes increased DNA damage and decreased proliferation in breast adenocarcinoma cells

Transient receptor potential, melastatin-2 (TRPM2) is a plasma membrane cation channel with important roles in sensory functions and promoting cell death. However, we demonstrated here that TRPM2 was present in the nuclei of MCF-7 and MDA-MB-231 human breast adenocarcinoma cells, and its pharmacologic inhibition or RNAi silencing caused decreased cell proliferation. Neither an effect on proliferation nor a localization of TRPM2 in the nucleus was observed in noncancerous HMEC and MCF-10A human mammary epithelial cells. Investigation of possible effects of TRPM2 function in the nucleus demonstrated that pharmacologic inhibition or RNAi silencing of TRPM2 in MCF-7 and MDA-MB-231 human breast adenocarcinoma cells caused up to 4-fold increases in DNA damage levels, as compared to noncancerous breast cells after equivalent treatments. These results indicate that TRPM2 has a novel nuclear function in human breast adenocarcinoma cells that facilitates the integrity of genomic DNA, a finding that is distinct from its previously reported role as a plasma membrane cation channel in noncancerous cells. In summary, we report here a novel effect promoted by TRPM2, where it functions to minimize DNA damage and thus may have a role in the protection of genomic DNA in breast cancer cells. Our study therefore provides compelling evidence that TRPM2 has a unique role in breast adenocarcinoma cells. Accordingly, these studies suggest that TRPM2 is a potential therapeutic target, where its pharmacologic inhibition may provide an innovative strategy to selectively increase DNA damage levels in breast cancer cells.

The key role of transient receptor potential melastatin-2 channels in amyloid-β-induced neurovascular dysfunction

Alzheimer's dementia is a devastating and incurable disease afflicting over 35 million people worldwide. Amyloid-β (Aβ), a key pathogenic factor in this disease, has potent cerebrovascular effects that contribute to brain dysfunction underlying dementia by limiting the delivery of oxygen and glucose to the working brain. However, the downstream pathways responsible for the vascular alterations remain unclear. Here we report that the cerebrovascular dysfunction induced by Aβ is mediated by DNA damage caused by vascular oxidative-nitrosative stress in cerebral endothelial cells, which, in turn, activates the DNA repair enzyme poly(ADP)-ribose polymerase. The resulting increase in ADP ribose opens transient receptor potential melastatin-2 (TRPM2) channels in endothelial cells leading to intracellular Ca(2+) overload and endothelial dysfunction. The findings provide evidence for a previously unrecognized mechanism by which Aβ impairs neurovascular regulation and suggest that TRPM2 channels are a potential therapeutic target to counteract cerebrovascular dysfunction in Alzheimer's dementia and related pathologies.

Redox regulation of transient receptor potential channels

SIGNIFICANCE

Environmental and endogenous reactive species such as reactive oxygen species (ROS), reactive nitrogen species (RNS), and other electrophiles are not only known to exert toxic effects on organisms, but are also emerging as molecules that mediate cell signaling responses. However, the mechanisms underlying this cellular redox signaling by reactive species remains largely uncharacterized.

RECENT ADVANCES

Ca2+-permeable cation channels encoded by the transient receptor potential (trp) gene superfamily are characterized by a wide variety of activation triggers that act from outside and inside the cell. Recent studies have revealed that multiple TRP channels sense reactive species and induce diverse physiological and pathological responses, such as cell death, chemokine production, and pain transduction. TRP channels sense reactive species either indirectly through second messengers or directly via oxidative modification of cysteine residues. In this review, we describe the activation mechanisms and biological roles of redox-sensitive TRP channels, including TRPM2, TRPM7, TRPC5, TRPV1, and TRPA1.

CRITICAL ISSUES

The sensitivity of TRP channels to reactive species in vitro has been well characterized using molecular and pharmacological approaches. However, the precise activation mechanism(s) and in vivo function(s) of ROS/RNS-sensitive TRP channels remain elusive.

FUTURE DIRECTIONS

Redox sensitivity of TRP channels has been shown to mediate previously unexplained biological phenomena and is involved in various pathologies. Understanding the physiological significance and activation mechanisms of TRP channel regulation by reactive species may lead to TRP channels becoming viable pharmacological targets, and modulators of these channels may offer therapeutic options for previously untreatable diseases.

Water-filtered infrared a irradiation in combination with visible light inhibits acute chlamydial infection

New therapeutic strategies are needed to overcome drawbacks in treatment of infections with intracellular bacteria. Chlamydiaceae are Gram-negative bacteria implicated in acute and chronic diseases such as abortion in animals and trachoma in humans. Water-filtered infrared A (wIRA) is short wavelength infrared radiation with a spectrum ranging from 780 to 1400 nm. In clinical settings, wIRA alone and in combination with visible light (VIS) has proven its efficacy in acute and chronic wound healing processes. This is the first study to demonstrate that wIRA irradiation combined with VIS (wIRA/VIS) diminishes recovery of infectious elementary bodies (EBs) of both intra- and extracellular Chlamydia (C.) in two different cell lines (Vero, HeLa) regardless of the chlamydial strain (C. pecorum, C. trachomatis serovar E) as shown by indirect immunofluorescence and titration by subpassage. Moreover, a single exposure to wIRA/VIS at 40 hours post infection (hpi) led to a significant reduction of C. pecorum inclusion frequency in Vero cells and C. trachomatis in HeLa cells, respectively. A triple dose of irradiation (24, 36, 40 hpi) during the course of C. trachomatis infection further reduced chlamydial inclusion frequency in HeLa cells without inducing the chlamydial persistence/stress response, as ascertained by electron microscopy. Irradiation of host cells (HeLa, Vero) neither affected cell viability nor induced any molecular markers of cytotoxicity as investigated by Alamar blue assay and Western blot analysis. Chlamydial infection, irradiation, and the combination of both showed a similar release pattern of a subset of pro-inflammatory cytokines (MIF/GIF, Serpin E1, RANTES, IL-6, IL-8) and chemokines (IL-16, IP-10, ENA-78, MIG, MIP-1α/β) from host cells. Initial investigation into the mechanism indicated possible thermal effects on Chlamydia due to irradiation. In summary, we demonstrate a non-chemical reduction of chlamydial infection using the combination of water-filtered infrared A and visible light.

PARP-1 hyperactivation and reciprocal elevations in intracellular Ca2+ during ROS-induced nonapoptotic cell death

The generation of reactive oxygen species (ROS) has been implicated in the pathogenesis of renal ischemia/reperfusion injury, and many other pathological conditions. DNA strand breaks caused by ROS lead to the activation of poly(ADP-ribose)polymerase-1 (PARP-1), the excessive activation of which can result in cell death. We have utilized a model in which 2,3,5-tris(glutathion-S-yl)hydroquinone (TGHQ), a nephrotoxic and nephrocarcinogenic metabolite of hydroquinone, causes ROS-dependent cell death in human renal proximal tubule epithelial cells (HK-2), to further elucidate the role of PARP-1 in ROS-dependent cell death. TGHQ-induced ROS generation, DNA strand breaks, hyperactivation of PARP-1, rapid depletion of nicotinamide adenine dinucleotide (NAD), elevations in intracellular Ca(2+) concentrations, and subsequent nonapoptotic cell death in both a PARP- and Ca(2+)-dependent manner. Thus, inhibition of PARP-1 with PJ34 completely blocked TGHQ-mediated accumulation of poly(ADP-ribose) polymers and NAD consumption, and delayed HK-2 cell death. In contrast, chelation of intracellular Ca(2+) with BAPTA completely abrogated TGHQ-induced cell death. Ca(2+) chelation also attenuated PARP-1 hyperactivation. Conversely, inhibition of PARP-1 modulated TGHQ-mediated changes in Ca(2+) homeostasis. Interestingly, PARP-1 hyperactivation was not accompanied by the translocation of apoptosis-inducing factor (AIF) from mitochondria to the nucleus, a process usually associated with PARP-dependent cell death. Thus, pathways coupling PARP-1 hyperactivation to cell death are likely to be context-dependent, and therapeutic strategies designed to target PARP-1 need to recognize such variability. Our studies provide new insights into PARP-1-mediated nonapoptotic cell death, during which PARP-1 hyperactivation and elevations in intracellular Ca(2+) are reciprocally coupled to amplify ROS-induced nonapoptotic cell death.

Sex stratified neuronal cultures to study ischemic cell death pathways

Sex differences in neuronal susceptibility to ischemic injury and neurodegenerative disease have long been observed, but the signaling mechanisms responsible for those differences remain unclear. Primary disassociated embryonic neuronal culture provides a simplified experimental model with which to investigate the neuronal cell signaling involved in cell death as a result of ischemia or disease; however, most neuronal cultures used in research today are mixed sex. Researchers can and do test the effects of sex steroid treatment in mixed sex neuronal cultures in models of neuronal injury and disease, but accumulating evidence suggests that the female brain responds to androgens, estrogens, and progesterone differently than the male brain. Furthermore, neonate male and female rodents respond differently to ischemic injury, with males experiencing greater injury following cerebral ischemia than females. Thus, mixed sex neuronal cultures might obscure and confound the experimental results; important information might be missed. For this reason, the Herson Lab at the University of Colorado School of Medicine routinely prepares sex-stratified primary disassociated embryonic neuronal cultures from both hippocampus and cortex. Embryos are sexed before harvesting of brain tissue and male and female tissue are disassociated separately, plated separately, and maintained separately. Using this method, the Herson Lab has demonstrated a male-specific role for the ion channel TRPM2 in ischemic cell death. In this manuscript, we share and discuss our protocol for sexing embryonic mice and preparing sex-stratified hippocampal primary disassociated neuron cultures. This method can be adapted to prepare sex-stratified cortical cultures and the method for embryo sexing can be used in conjunction with other protocols for any study in which sex is thought to be an important determinant of outcome.

p53 regulates a non-apoptotic death induced by ROS

DNA damage induced by reactive oxygen species and several chemotherapeutic agents promotes both p53 and poly (ADP-ribose) polymerase (PARP) activation. p53 activation is well known to regulate apoptotic cell death, whereas robust activation of PARP-1 has been shown to promote a necrotic cell death associated with energetic collapse. Here we identify a novel role for p53 in modulating PARP enzymatic activity to regulate necrotic cell death. In mouse embryonic fibroblasts, human colorectal and human breast cancer cell lines, loss of p53 function promotes resistance to necrotic, PARP-mediated cell death. We therefore demonstrate that p53 can regulate both necrotic and apoptotic cell death, mutations or deletions in this tumor-suppressor protein may be selected by cancer cells to provide not only their resistance to apoptosis but also to necrosis, and explain resistance to chemotherapy and radiation even when it kills via non-apoptotic mechanisms.

Variations in the mRNA expression of poly(ADP-ribose) polymerases, poly(ADP-ribose) glycohydrolase and ADP-ribosylhydrolase 3 in breast tumors and impact on clinical outcome

In order to assess the variation in expression of poly(ADP-ribose) polymerase (PARP) family members and the hydrolases that degrade the poly(ADP-ribose) polymers they generate and possible associations with classical pathological parameters, including long-term outcome, the mRNA levels of PARP1, PARP2, PARP3, poly(ADP-ribose) glycohydrolase (PARG) and ADP-ribosylhydrolase 3 (ARH3) were examined using quantitative reverse transcription polymerase chain reaction in 443 unilateral invasive breast cancers and linked to hormonal status, tumor proliferation and clinical outcome. PARP1 mRNA levels were the highest among these five genes in both normal and tumor tissues, with a 2.45-fold higher median level in tumors compared to normal tissues. Tumors (34.1%) showed PARP1 overexpression (>3 fold relative to normal breast tissues) compared to underexpression (<0.33 fold) in only 0.5%. This overexpression was seen in all breast tumor subgroups, with the highest fraction (51%) seen in the HR-positive/ERBB2-positive subgroup and was not highly associated with any other classical predictive factors. No correlation was seen between PARP1 mRNA and PARP-1 protein levels in a subset of 31 tumors. PARP3 was underexpressed in 10.4% of tumors, more frequently in the HR-negative tumors (25.4%) than the HR-positive tumors (5.9%). This PARP3 underexpression was mutually exclusive with a PARP1 overexpression. PARP2 levels were unchanged between normal and tumor tissues and few tumors showed overexpression of PARG (3.8%) or ARH3 (3.4%). Within the subgroup of triple negative tumors, PARG mRNA levels below the median were associated with a higher risk of developing metastases (p = 0.039) raising the possibility this might be marker of clinical outcome.

Loss of TRPM2 function protects against irradiation-induced salivary gland dysfunction

Xerostomia as a result of salivary gland damage is a permanent and debilitating side effect of radiotherapy for head and neck cancers. Effective treatments for protecting, or restoring, salivary gland function are not available. Here we report that irradiation treatment leads to activation of the calcium-permeable channel, transient potential melastatin-like 2 (TRPM2), via stimulation of poly-ADP-ribose polymerase. Importantly, irradiation induced an irreversible loss of salivary gland fluid secretion in TRPM2+/+ mice while a transient loss was seen in TRPM2-/- mice with >60% recovery by 30 days after irradiation. Treatment of TRPM2+/+ mice with the free radical scavenger Tempol or the PARP1 inhibitor 3-aminobenzamide attenuated irradiation-induced activation of TRPM2 and induced significant recovery of salivary fluid secretion. Furthermore, TPL (4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl) induced complete recovery of function in irradiated TRPM2-/- mice. These novel data demonstrate that TRPM2 is activated by irradiation, via PARP1 activation, and contributes to irreversible loss of salivary gland function.

The TRPM2 ion channel, an oxidative stress and metabolic sensor regulating innate immunity and inflammation

TRPM2 (transient receptor potential melastatin 2) is the unique fusion of a Ca(2+)-permeable pore with an enzymatic domain that binds the NAD(+)-metabolite ADP-ribose (ADPR), resulting in channel opening. ADPR formation is a metabolic corollary of cellular stress, but can also be elicited enzymatically through NAD glycohydrolases like CD38. TRPM2 thus functions as a metabolic and oxidative stress sensor and translates this information into ion fluxes that can affect Ca(2+) signaling and the membrane potential. TRPM2 is strongly represented in immune cells of the phagocytic lineage, themselves professional generators of oxidants. The recent characterization of TRPM2-deficient mouse models has revealed the involvement of this channel in various aspects of immunity. Monocytes lacking TRPM2 show reduced production of the CXCL2 chemokine, resulting in diminished neutrophilic influx to the colon in chemically induced colitis, and thus protection against tissue ulceration in TRPM2(-/-) mice. However, the insufficient production of proinflammatory cytokines leads to high morbidity and lethality of the TRPM2(-/-) mice following infection with the bacterial pathogen Listeria monocytogenes. In the context of endotoxin-induced pulmonary inflammation, TRPM2's absence was found to promote inflammation and ROS production. TRPM2 acts thereby as a negative feedback loop by interfering through membrane depolarization with ROS generation by NADPH oxidases. In dendritic cells, TRPM2 is a lysosomal Ca(2+)-release channel that promotes chemokine responsiveness and cell migration, which is reminiscent of CD38-mediated functions. The discovery of TRPM2 has unveiled an unsuspected signaling pathway and established ADPR as a novel second messenger. Understanding TRPM2's complex involvement in inflammation is crucial to evaluating the potential of manipulating TRPM2 activity and ADPR metabolism for therapeutic intervention.

Host cell poly(ADP-ribose) glycohydrolase is crucial for Trypanosoma cruzi infection cycle

Trypanosoma cruzi, etiological agent of Chagas’ disease, has a complex life cycle which involves the invasion of mammalian host cells, differentiation and intracellular replication. Here we report the first insights into the biological role of a poly(ADP-ribose) glycohydrolase in a trypanosomatid (TcPARG). In silico analysis of the TcPARG gene pointed out the conservation of key residues involved in the catalytic process and, by Western blot, we demonstrated that it is expressed in a life stage-dependant manner. Indirect immunofluorescence assays and electron microscopy using an anti-TcPARG antibody showed that this enzyme is localized in the nucleus independently of the presence of DNA damage or cell cycle stage. The addition of poly(ADP-ribose) glycohydrolase inhibitors ADP-HPD (adenosine diphosphate (hydroxymethyl) pyrrolidinediol) or DEA (6,9-diamino-2-ethoxyacridine lactate monohydrate) to the culture media, both at a 1 µM concentration, reduced in vitro epimastigote growth by 35% and 37% respectively, when compared to control cultures. We also showed that ADP-HPD 1 µM can lead to an alteration in the progression of the cell cycle in hydroxyurea synchronized cultures of T. cruzi epimastigotes. Outstandingly, here we demonstrate that the lack of poly(ADP-ribose) glycohydrolase activity in Vero and A549 host cells, achieved by chemical inhibition or iRNA, produces the reduction of the percentage of infected cells as well as the number of amastigotes per cell and trypomastigotes released, leading to a nearly complete abrogation of the infection process. We conclude that both, T. cruzi and the host, poly(ADP-ribose) glycohydrolase activities are important players in the life cycle of Trypanosoma cruzi, emerging as a promising therapeutic target for the treatment of Chagas’ disease.

Sexually dimorphic response of TRPM2 inhibition following cardiac arrest-induced global cerebral ischemia in mice

Transient global cerebral ischemia due to cardiac arrest followed by resuscitation (CA/CPR) causes significant neurological damage in vulnerable neuron populations within the brain, such as hippocampal CA1 neurons. In recent years, we have implicated the transient receptor potential M2 (TRPM2) channel as a mediator of ischemic injury to neurons. We previously demonstrated that genetic and pharmacological strategies that reduce TRPM2 function preferentially protect male neurons in vitro and reduce infarct volume following experimental stroke. Due to the narrow therapeutic window for intervention following ischemic stroke, it is important to assess the role of TRPM2 in other models of cerebral ischemia. Therefore, this study utilized a modified mouse model of CA/CPR to mimic more accurately the clinical condition by maintaining body and head temperatures near the physiological range throughout. Here, we report that inhibition of TRPM2 activity with clotrimazole reduces hippocampal CA1 neuronal injury when administered 30 min after resuscitation from cardiac arrest. Consistent with our previous observations, neuroprotection was observed in male mice and no effect on injury was observed in the female. These findings provide further evidence for TRPM2 as a target for protection against cerebral ischemia in the male brain.

Poly(ADP-ribose) signaling in cell death

Poly(ADP-ribosyl)ation (PARylation) is a reversible protein modification carried out by the concerted actions of poly(ADP-ribose) polymerase (PARP) enzymes and poly(ADP-ribose) (PAR) decomposing enzymes such as PAR glycohydrolase (PARG) and ADP-ribosyl hydrolase 3 (ARH3). Reversible PARylation is a pleiotropic regulator of various cellular functions but uncontrolled PARP activation may also lead to cell death. The cellular demise pathway mediated by PARylation in oxidatively stressed cells has been described almost thirty years ago. However, the underlying molecular mechanisms have only begun to emerge relatively recently. PARylation has been implicated in necroptosis, autophagic cell death but its role in extrinsic and intrinsic apoptosis appears to be less predominant and depends largely on the cellular model used. Currently, three major pathways have been made responsible for PARP-mediated necroptotic cell death: (1) compromised cellular energetics mainly due to depletion of NAD, the substrate of PARPs; (2) PAR mediated translocation of apoptosis inducing factor (AIF) from mitochondria to nucleus (parthanatos) and (3) a mostly elusive crosstalk between PARylation and cell death/survival kinases and phosphatases. Here we review how these PARP-mediated necroptotic pathways are intertwined, how PARylation may contribute to extrinsic and intrinsic apoptosis and discuss recent developments on the role of PARylation in autophagy and autophagic cell death.

Cytosolic Ca2+ shifts as early markers of cytotoxicity

The determination of the cytotoxic potential of new and so far unknown compounds as well as their metabolites is fundamental in risk assessment. A variety of strategic endpoints have been defined to describe toxin-cell interactions, leading to prediction of cell fate. They involve measurement of metabolic endpoints, bio-energetic parameters or morphological cell modifications. Here, we evaluated alterations of the free cytosolic Ca²⁺ homeostasis using the Fluo-4 dye and compared results with the metabolic cell viability assay Alamar Blue. We investigated a panel of toxins (As₂O₃, gossypol, H₂O₂, staurosporine, and titanium(IV)-salane complexes) in four different mammalian cell lines covering three different species (human, mouse, and African green monkey). All tested compounds induced an increase in free cytosolic Ca²⁺ within the first 5 s after toxin application. Cytosolic Ca²⁺ shifts occurred independently of the chemical structure in all tested cell systems and were persistent up to 3 h. The linear increase of free cytosolic Ca²⁺ within the first 5 s of drug treatment correlates with the EC₂₅ and EC₇₅ values obtained in Alamar Blue assays one day after toxin exposure. Moreover, a rise of cytosolic Ca²⁺ was detectable independent of induced cell death mode as assessed by caspase and poly(ADP-ribose) polymerase (PARP) activity in HeLa versus MCF-7 cells at very low concentrations. In conclusion, a cytotoxicity assay based on Ca²⁺ shifts has a low limit of detection (LOD), is less time consuming (at least 24 times faster) compared to the cell viability assay Alamar Blue and is suitable for high-troughput-screening (HTS).

Poly(ADP-ribose): PARadigms and PARadoxes

Poly(ADP-ribosyl)ation (PARylation) is a posttranslational protein modification (PTM) catalyzed by members of the poly(ADP-ribose) polymerase (PARP) enzyme family. PARPs use NAD(+) as substrate and upon cleaving off nicotinamide they transfer the ADP-ribosyl moiety covalently to suitable acceptor proteins and elongate the chain by adding further ADP-ribose units to create a branched polymer, termed poly(ADP-ribose) (PAR), which is rapidly degraded by poly(ADP-ribose) glycohydrolase (PARG) and ADP-ribosylhydrolase 3 (ARH3). In recent years several key discoveries changed the way we look at the biological roles and mode of operation of PARylation. These paradigm shifts include but are not limited to (1) a single PARP enzyme expanding to a PARP family; (2) DNA-break dependent activation extended to several other DNA dependent and independent PARP-activation mechanisms; (3) one molecular mechanism (covalent PARylation of target proteins) underlying the biological effect of PARPs is now complemented by several other mechanisms such as protein-protein interactions, PAR signaling, modulation of NAD(+) pools and (4) one principal biological role in DNA damage sensing expanded to numerous, diverse biological functions identifying PARP-1 as a real moonlighting protein. Here we review the most important paradigm shifts in PARylation research and also highlight some of the many controversial issues (or paradoxes) of the field such as (1) the mostly synergistic and not antagonistic biological effects of PARP-1 and PARG; (2) mitochondrial PARylation and PAR decomposition, (3) the cross-talk between PARylation and signaling pathways (protein kinases, phosphatases, calcium) and the (4) divergent roles of PARP/PARylation in longevity and in age-related diseases.

Roles of poly(ADP-ribose) glycohydrolase in DNA damage and apoptosis

Poly(ADP-ribose) glycohydrolase (PARG) is the primary enzyme that catalyzes the hydrolysis of poly(ADP-ribose) (PAR), an essential biopolymer that is synthesized by poly(ADP-ribose) polymerases (PARPs) in the cell. By regulating the hydrolytic arm of poly(ADP-ribosyl)ation, PARG participates in a number of biological processes, including the repair of DNA damage, chromatin dynamics, transcriptional regulation, and cell death. Collectively, the research investigating the roles of PARG in the cell has identified the importance of PARG and its value as a therapeutic target. However, the biological role of PARG remains less understood than the role of PAR synthesis by the PARPs. Further complicating the study of PARG is the existence of multiple PARG isoforms in the cell, the lack of optimal PARG inhibitors, and the lack of viable PARG-null animals. This review will present our current knowledge of PARG, with a focus on its roles in DNA-damage repair and cell death.

Alpha-tocopheryl succinate inhibits autophagic survival of prostate cancer cells induced by vitamin K3 and ascorbate to trigger cell death

Background

The redox-silent vitamin E analog α-tocopheryl succinate (α-TOS) was found to synergistically cooperate with vitamin K3 (VK3) plus ascorbic acid (AA) in the induction of cancer cell-selective apoptosis via a caspase-independent pathway. Here we investigated the molecular mechanism(s) underlying cell death induced in prostate cancer cells by α-TOS, VK3 and AA, and the potential use of targeted drug combination in the treatment of prostate cancer.

Methodology/Principal Findings

The generation of ROS, cellular response to oxidative stress, and autophagy were investigated in PC3 prostate cancer cells by using drugs at sub-toxic doses. We evaluated whether PARP1-mediated apoptosis-inducing factor (AIF) release plays a role in apoptosis induced by the combination of the agents. Next, the effect of the combination of α-TOS, VK3 and AA on tumor growth was examined in nude mice. VK3 plus AA induced early ROS formation associated with induction of autophagy in response to oxidative stress, which was reduced by α-TOS, preventing the formation of autophagosomes. α-TOS induced mitochondrial destabilization leading to the release of AIF. Translocation of AIF from mitochondria to the nucleus, a result of the combinatorial treatment, was mediated by PARP1 activation. The inhibition of AIF as well as of PARP1 efficiently attenuated apoptosis triggered by the drug combination. Using a mouse model of prostate cancer, the combination of α-TOS, VK3 and AA was more efficient in tumor suppression than when the drugs were given separately, without deleterious side effects.

Conclusions/Significance

α-TOS, a mitochondria-targeting apoptotic agent, switches at sub-apoptotic doses from autophagy-dependent survival of cancer cells to their demise by promoting the induction of apoptosis. Given the grim prognosis for cancer patients, this finding is of potential clinical relevance.

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

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

Immunomodulatory Effects of Danshen (Salvia miltiorrhiza) in BALB/c Mice

Danshen, the root and rhizome of Salvia miltiorrhiza Bge, a Traditional Chinese Medicine, especially for cardiovascular and cerebrovascular diseases, has unique immunomodulatory effects. Danshen is capable of anti-inflammation and antiallergy, which are immunosuppressive activities, whereas it is also able to promote immunity against cancer, viruses, and bacteria. Most previous reports were performed with use of a purified compound or compounds of Danshen. Since there are more than twenty active compounds in Danshen, it is very difficult to predict that one compound will act the same way when it is combined with other compounds. In order to overcome this limitation, we used the crude form of Danshen to study its immunomodulatory effects in a mouse model. The mice were fed daily diet supplements of Danshen for three months and then tested for their immunity, including leukocyte subsets in peripheral blood, humoral and cell-mediated immune responses, and host defenses against a Listeria monocytogenes (LM) infection. Different doses of Danshen caused different immunomodulatory effects. Danshen at 0.5% decreased serum IgE production in BALB/c mice; 1% Danshen promoted cell-mediated immunity; Danshen at 0.5 and 1% inhibited the production of oxygen free radicals in liver and spleen and NO production in liver; 2% Danshen enhanced the host resistance against LM with increased numbers of peripheral monocytes and natural killer (NK) cells and decreased production of IL-1 β and NO.

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

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