PARP-1 regulates inflammasome activity by poly-ADP-ribosylation of NLRP3 and interaction with TXNIP in primary macrophages

Current insights on mitochondria-associated endoplasmic reticulum membranes (MAMs) and their significance in the pathophysiology of ocular disorders

The intricate interaction network necessary for essential physiological functions underscores the interdependence among eukaryotic cells. Mitochondria-Associated Endoplasmic Reticulum Membranes (MAMs), specialized junctions between mitochondria and the ER, were recently discovered. These junctions participate in various cellular processes, including calcium level regulation, lipid metabolism, mitochondrial integrity maintenance, autophagy, and inflammatory responses via modulating the structure and molecular composition of various cellular components. Therefore, MAMs contribute to the pathophysiology of numerous ocular disorders, including Diabetic Retinopathy (DR), Age-related Macular Degeneration (AMD) and glaucoma. In addition to providing a concise overview of the architectural and functional aspects of MAMs, this review explores the key pathogenetic pathways involving MAMs in the development of several ocular disorders.

Poly (ADP-Ribose) Polymerase-1 (PARP-1) Inhibitors in Diabetic Retinopathy: An Attractive but Elusive Choice for Drug Development

Owing to its promiscuous roles, poly (ADP-ribose) polymerase-1 (PARP-1) is involved in various neurological disorders including several retinal pathologies. Diabetic retinopathy (DR) is the most common microvascular complication of diabetes mellitus affecting the retina. In the present review, we highlight the importance of PARP-1 participation in pathophysiology of DR and discuss promising potential inhibitors for treatment. A high glucose level enhances PARP-1 expression; PARP inhibitors have gained attention due to their potential therapeutic effects in DR. They target different checkpoints (blocking nuclear transcription factor (NF-κB) activation; oxidative stress protection, influence on vascular endothelial growth factor (VEGF) expression, impacting neovascularization). Nowadays, there are several improved clinical PARP-1 inhibitors with different allosteric effects. Combining PARP-1 inhibitors with other compounds is another promising option in DR treatments. Besides pharmacological inhibition, genetic disruption of the PARP-1 gene is another approach in PARP-1-initiated therapies. In terms of future treatments, the limitations of single-target approaches shift the focus onto combined therapies. We emphasize the importance of multi-targeted therapies, which could be effective not only in DR, but also in other ischemic conditions.

The involvement of Sting in exacerbating acute lung injury in sepsis via the PARP-1/NLRP3 signaling pathway

BACKGROUND

Interferon gene stimulator (Sting) is an indispensable adaptor protein that plays a crucial role in acute lung injury (ALI) induced by sepsis, and the PARP-1/NLRP3 signaling pathway may be an integral component of the inflammatory response mediated by Sting. However, the regulatory role of Sting in the PARP-1/NLRP3 pathway in ALI remains insufficiently elucidated.

METHODS

Using lipopolysaccharide (LPS) to induce ALI in C57BL/6 mice and HUVEC cells, an in vivo and in vitro model was established. In vivo, Sting agonists and inhibitors were administered, while in vitro, Sting was knocked down using siRNA. ELISA was employed to quantify the levels of IL-1β, IL-6, and TNF-α. TUNEL staining was conducted to assess cellular apoptosis, while co-immunoprecipitation was utilized to investigate the interaction between Sting and NLRP3. Expression levels of Sting, NLRP3, PARP-1, among others, were assessed via Western blotting and RT-qPCR. Lung HE staining and lung wet/dry ratio were evaluated in the in vivo mouse model. To validate the role of the PARP-1/NLRP3 signaling pathway, PARP-1 inhibitors were employed both in vivo and in vitro.

RESULTS

In vitro experiments revealed that the Sting agonist group exacerbated LPS-induced pulmonary pathological damage, pulmonary edema, inflammatory response (increased levels of IL-6, TNF-α, and IL-1β), and cellular injury, whereas the Sting inhibitor group significantly ameliorated the aforementioned injuries, with further improvement observed in the combination therapy of Sting inhibitor and PARP-1 inhibitor. Western blotting and RT-qPCR results demonstrated significant suppression of ICAM-1, VCAM-1, NLRP3, and PARP-1 expression in the Sting inhibitor group, with this reduction further enhanced in the Sting inhibitor + PARP-1 inhibitor treatment group, exhibiting opposite outcomes to the agonist. Furthermore, in vitro experiments using HUVEC cell lines validated these findings.

CONCLUSIONS

Our study provides new insights into the roles of Sting and the PARP-1/NLRP3 signaling pathway in inflammatory responses, offering novel targets for the development of therapeutic interventions against inflammatory reactions.

Poly(ADP-Ribose) Polymerase-1 Regulates Pyroptosis Independent Function of NLRP3 Inflammasome in Neutrophil Extracellular Trap Formation

Neutrophil extracellular traps (NETs) function to control infectious agents as well as to propagate inflammatory response in a variety of disease conditions. DNA damage associated with chromatin decondensation and NACHT domain-leucine-rich repeat-and pyrin domain-containing protein 3 (NLRP3) inflammasome activation have emerged as crucial events in NET formation, but the link between the two processes is unknown. In this study, we demonstrate that poly(ADP-ribose) polymerase-1 (PARP-1), a key DNA repair enzyme, regulates NET formation triggered by NLRP3 inflammasome activation in neutrophils. Activation of mouse neutrophils with canonical NLRP3 stimulants LPS and nigericin induced NET formation, which was significantly abrogated by pharmacological inhibition of PARP-1. We found that PARP-1 is required for NLRP3 inflammasome assembly by regulating post-transcriptional levels of NLRP3 and ASC dimerization. Importantly, this PARP-1-regulated NLRP3 activation for NET formation was independent of inflammasome-mediated pyroptosis, because caspase-1 and gasdermin D processing as well as IL-1β transcription and secretion remained intact upon PARP-1 inhibition in neutrophils. Accordingly, pharmacological inhibition or genetic ablation of caspase-1 and gasdermin D had no effect on NLRP3-mediated NET formation. Mechanistically, PARP-1 inhibition increased p38 MAPK activity, which was required for downmodulation of NLRP3 and NETs, because concomitant inhibition of p38 MAPK with PARP-1 restored NLRP3 activation and NET formation. Finally, mice undergoing bacterial peritonitis exhibited increased survival upon treatment with PARP-1 inhibitor, which correlated with increased leukocyte influx and improved intracellular bacterial clearance. Our findings reveal a noncanonical pyroptosis-independent role of NLRP3 in NET formation regulated by PARP-1 via p38 MAPK, which can be targeted to control NETosis in inflammatory diseases.

ADP-ribosylation, a multifaceted modification: Functions and mechanisms in aging and aging-related diseases

Aging, a complex biological process, plays key roles the development of multiple disorders referred as aging-related diseases involving cardiovascular diseases, stroke, neurodegenerative diseases, cancers, lipid metabolism-related diseases. ADP-ribosylation is a reversible modification onto proteins and nucleic acids to alter their structures and/or functions. Growing evidence support the importance of ADP-ribosylation and ADP-ribosylation-associated enzymes in aging and age-related diseases. In this review, we summarized ADP-ribosylation-associated proteins including ADP-ribosyl transferases, the ADP-ribosyl hydrolyses and ADP-ribose binding domains. Furthermore, we outlined the latest knowledge about regulation of ADP-ribosylation in the pathogenesis and progression of main aging-related diseases, organism aging and cellular senescence, and we also speculated the underlying mechanisms to better disclose this novel molecular network. Moreover, we discussed current issues and provided an outlook for future research, aiming to revealing the unknown bio-properties of ADP-ribosylation, and establishing a novel therapeutic perspective in aging-related diseases and health aging via targeting ADP-ribosylation.

Poly (ADP-ribose) Polymerase-1 modulations in the genesis of thrombosis

Thrombosis, a coagulation disorder, occurs due to altered levels of coagulation, fibrinolytic and immune factors, which are otherwise known to maintain hemostasis in normal physiological conditions. Here, we review the direct and indirect participation of a multifunctional nuclear enzyme poly (ADP-ribose) polymerase-1 (PARP1) in the expression of key genes and cellular processes involved in thrombotic pathogenesis. PARP1 biological activities range from maintenance of genomic integrity, chromatin remodeling, base excision DNA repair, stress responses to cell death, angiogenesis and cell cycle pathways. However, under homeostatic imbalances, PARP1 activities are linked with the pathogenesis of diseases, including cancer, aging, neurological disorders, and cardiovascular diseases. Disease-associated distressed cells employ a variety of PARP-1 functions such as oxidative damage exacerbations, cellular energetics and apoptosis pathways, regulation of inflammatory mediators, promotion of endothelial dysfunction, and ERK-mediated signaling in pathogenesis. Thrombosis is one such pathogenesis that comprises exacerbation of coagulation cascade due to biochemical alterations in endothelial cells, platelet activation, overexpression of adhesion molecules, cytokines release, and leukocyte adherence. Thus, the activation of endothelial and inflammatory cells in thrombosis implicates a potential role of PARP1 activation in thrombogenesis. This review article explores the direct impact of PARP1 activation in the etiology of thrombosis and discusses PARP1-mediated endothelial dysfunction, inflammation, and epigenetic regulations in the disease manifestation. Understanding PARP1 functions associated with thrombosis may elucidate novel pathogenetic mechanisms and help in better disease management through newer therapeutic interventions targeting PARP1 activity.

Post-translational control of NLRP3 inflammasome signaling

Inflammasomes serve as critical sensors for disruptions to cellular homeostasis, with inflammasome assembly leading to inflammatory caspase activation, gasdermin cleavage, and cytokine release. While the canonical pathways leading to priming, assembly, and pyroptosis are well characterized, recent work has begun to focus on the role of post-translational modifications (PTMs) in regulating inflammasome activity. A diverse array of PTMs, including phosphorylation, ubiquitination, SUMOylation, acetylation, and glycosylation, exert both activating and inhibitory influences on members of the inflammasome cascade through effects on protein-protein interactions, stability, and localization. Dysregulation of inflammasome activation is associated with a number of inflammatory diseases, and evidence is emerging that aberrant modification of inflammasome components contributes to this dysregulation. This review provides insight into PTMs within the NLRP3 inflammasome pathway and their functional consequences on the signaling cascade and highlights outstanding questions that remain regarding the complex web of signals at play.

Targeting epigenetic and post-translational modifications regulating pyroptosis for the treatment of inflammatory diseases

Inflammatory diseases, including infectious diseases, diabetes-related diseases, arthritis-related diseases, neurological diseases, digestive diseases, and tumor, continue to threaten human health and impose a significant financial burden despite advancements in clinical treatment. Pyroptosis, a pro-inflammatory programmed cell death pathway, plays an important role in the regulation of inflammation. Moderate pyroptosis contributes to the activation of native immunity, whereas excessive pyroptosis is associated with the occurrence and progression of inflammation. Pyroptosis is complicated and tightly controlled by various factors. Accumulating evidence has confirmed that epigenetic modifications and post-translational modifications (PTMs) play vital roles in the regulation of pyroptosis. Epigenetic modifications, which include DNA methylation and histone modifications (such as methylation and acetylation), and post-translational modifications (such as ubiquitination, phosphorylation, and acetylation) precisely manipulate gene expression and protein functions at the transcriptional and post-translational levels, respectively. In this review, we summarize the major pathways of pyroptosis and focus on the regulatory roles and mechanisms of epigenetic and post-translational modifications of pyroptotic components. We also illustrate these within pyroptosis-associated inflammatory diseases. In addition, we discuss the effects of novel therapeutic strategies targeting epigenetic and post-translational modifications on pyroptosis, and provide prospective insight into the regulation of pyroptosis for the treatment of inflammatory diseases.

NLRX1 Inhibits LPS-Induced Microglial Death via Inducing p62-Dependent HO-1 Expression, Inhibiting MLKL and Activating PARP-1

The activation of microglia and the production of cytokines are key factors contributing to progressive neurodegeneration. Despite the well-recognized neuronal programmed cell death regulated by microglial activation, the death of microglia themselves is less investigated. Nucleotide-binding oligomerization domain, leucine-rich repeat-containing X1 (NLRX1) functions as a scaffolding protein and is involved in various central nervous system diseases. In this study, we used the SM826 microglial cells to understand the role of NLRX1 in lipopolysaccharide (LPS)-induced cell death. We found LPS-induced cell death is blocked by necrostatin-1 and zVAD. Meanwhile, LPS can activate poly (ADP-ribose) polymerase-1 (PARP-1) to reduce DNA damage and induce heme oxygenase (HO)-1 expression to counteract cell death. NLRX1 silencing and PARP-1 inhibition by olaparib enhance LPS-induced SM826 microglial cell death in an additive manner. Less PARylation and higher DNA damage are observed in NLRX1-silencing cells. Moreover, LPS-induced HO-1 gene and protein expression through the p62-Keap1-Nrf2 axis are attenuated by NLRX1 silencing. In addition, the Nrf2-mediated positive feedback regulation of p62 is accordingly reduced by NLRX1 silencing. Of note, NLRX1 silencing does not affect LPS-induced cellular reactive oxygen species (ROS) production but increases mixed lineage kinase domain-like pseudokinase (MLKL) activation and cell necroptosis. In addition, NLRX1 silencing blocks bafilomycin A1-induced PARP-1 activation. Taken together, for the first time, we demonstrate the role of NLRX1 in protecting microglia from LPS-induced cell death. The underlying protective mechanisms of NLRX1 include upregulating LPS-induced HO-1 expression via Nrf2-dependent p62 expression and downstream Keap1-Nrf2 axis, mediating PARP-1 activation for DNA repair via ROS- and autophagy-independent pathway, and reducing MLKL activation.

A blood-based multi-pathway biomarker assay for early detection and staging of Alzheimer's disease across ethnic groups

INTRODUCTION

Existing blood-based biomarkers for Alzheimer's disease (AD) mainly focus on its pathological features. However, studies on blood-based biomarkers associated with other biological processes for a comprehensive evaluation of AD status are limited.

METHODS

We developed a blood-based, multiplex biomarker assay for AD that measures the levels of 21 proteins involved in multiple biological pathways. We evaluated the assay's performance for classifying AD and indicating AD-related endophenotypes in three independent cohorts from Chinese or European-descent populations.

RESULTS

The 21-protein assay accurately classified AD (area under the receiver operating characteristic curve [AUC] = 0.9407 to 0.9867) and mild cognitive impairment (MCI; AUC = 0.8434 to 0.8945) while also indicating brain amyloid pathology. Moreover, the assay simultaneously evaluated the changes of five biological processes in individuals and revealed the ethnic-specific dysregulations of biological processes upon AD progression.

DISCUSSION

This study demonstrated the utility of a blood-based, multi-pathway biomarker assay for early screening and staging of AD, providing insights for patient stratification and precision medicine.

HIGHLIGHTS

The authors developed a blood-based biomarker assay for Alzheimer's disease. The 21-protein assay classifies AD/MCI and indicates brain amyloid pathology. The 21-protein assay can simultaneously assess activities of five biological processes. Ethnic-specific dysregulations of biological processes in AD were revealed.

Poly-(ADP-ribose) polymerases inhibition by olaparib attenuates activities of the NLRP3 inflammasome and of NF-κB in THP-1 monocytes

Poly-(ADP-ribose) polymerases (PARPs) are a protein family that make ADP-ribose modifications on target genes and proteins. PARP family members contribute to the pathogenesis of chronic inflammatory diseases, including atherosclerosis, in which monocytes/macrophages play important roles. PARP inhibition is protective against atherosclerosis. However, the mechanisms by which PARP inhibition exerts this beneficial effect are not well understood. Here we show that in THP-1 monocytes, inhibition of PARP by olaparib attenuated oxidized low-density lipoprotein (oxLDL)-induced protein expressions of nucleotide-binding oligomerization domain, leucine-rich repeat and pyrin domain-containing-3 (NLRP3) inflammasome components: NLRP3, apoptosis-associated speck-like protein containing a caspase activation and recruitment domain (ASC), and caspase-1. Consistent with this effect, olaparib decreased oxLDL-enhanced interleukin (IL)-1β and IL-18 protein expression. Olaparib also decreased the oxLDL-mediated increase in mitochondrial reactive oxygen species. Similar to the effects of the NLRP3 inhibitor, MCC950, olaparib attenuated oxLDL-induced adhesion of monocytes to cultured human umbilical vein endothelial cells and reduced foam cell formation. Furthermore, olaparib attenuated the oxLDL-mediated activation of nuclear factor (NF)-κB through the oxLDL-mediated increase in IκBα phosphorylation and assembly of NF-κB subunits, demonstrated by co-immunoprecipitation of IκBα with RelA/p50 and RelB/p52 subunits. Moreover, PARP inhibition decreased oxLDL-mediated protein expression of a NF-κB target gene, VCAM1, encoding vascular cell adhesion molecule-1. This finding indicates an important role for NF-κB activity in PARP-mediated activation of the NLRP3 inflammasome. Thus, PARP inhibition by olaparib attenuates NF-κB and NLRP3 inflammasome activities, lessening monocyte cell adhesion and macrophage foam cell formation. These inhibitory effects of olaparib on NLRP3 activity potentially protect against atherosclerosis.

ADP-ribosylation: An emerging direction for disease treatment

ADP-ribosylation (ADPr) is a dynamically reversible post-translational modification (PTM) driven primarily by ADP-ribosyltransferases (ADPRTs or ARTs), which have ADP-ribosyl transfer activity. ADPr modification is involved in signaling pathways, DNA damage repair, metabolism, immunity, and inflammation. In recent years, several studies have revealed that new targets or treatments for tumors, cardiovascular diseases, neuromuscular diseases and infectious diseases can be explored by regulating ADPr. Here, we review the recent research progress on ART-mediated ADP-ribosylation and the latest findings in the diagnosis and treatment of related diseases.

Role of NLRP3 Inflammasome in Stroke Pathobiology: Current Therapeutic Avenues and Future Perspective

Neuroinflammation is a key pathophysiological feature of stroke-associated brain injury. A local innate immune response triggers neuroinflammation following a stroke via activating inflammasomes. The nucleotide-binding oligomerization domain leucine-rich repeat and pyrin domain-containing protein 3 (NLRP3) inflammasome has been heavily implicated in stroke pathobiology. Following a stroke, several stimuli have been suggested to trigger the assembly of the NLRP3 inflammasome. Recent studies have advanced the understanding and revealed several new players regulating NLRP3 inflammasome-mediated neuroinflammation. This article discussed recent advancements in NLRP3 assembly and highlighted stroke-induced mitochondrial dysfunction as a major checkpoint to regulating NLRP3 activation. The NLRP3 inflammasome activation leads to caspase-1-dependent maturation and release of IL-1β, IL-18, and gasdermin D. In addition, genetic or pharmacological inhibition of the NLRP3 inflammasome activation and downstream signaling has been shown to attenuate brain infarction and improve the neurological outcome in experimental models of stroke. Several drug-like small molecules targeting the NLRP3 inflammasome are in different phases of development as novel therapeutics for various inflammatory conditions, including stroke. Understanding how these molecules interfere with NLRP3 inflammasome assembly is paramount for their better optimization and/or development of newer NLRP3 inhibitors. In this review, we summarized the assembly of the NLRP3 inflammasome and discussed the recent advances in understanding the upstream regulators of NLRP3 inflammasome-mediated neuroinflammation following stroke. Additionally, we critically examined the role of the NLRP3 inflammasome-mediated signaling in stroke pathophysiology and the development of therapeutic modalities to target the NLRP3 inflammasome-related signaling for stroke treatment.

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.

SIRT1/PARP-1/NLRP3 cascade as a potential target for niacin neuroprotective effect in lipopolysaccharide-induced depressive-like behavior in mice

Depression is a serious mood disorder characterized by monoamines deficiency, oxidative stress, neuroinflammation, and cell death. Niacin (vitamin B3 or nicotinic acid, NA), a chief mediator of neuronal development and survival in the central nervous system, exerts neuroprotective effects in several experimental models.

AIMS

This study aimed to investigate the effect of NA in lipopolysaccharide (LPS) mouse model of depression exploring its ability to regulate sirtuin1/poly (ADP-ribose) polymerase-1 (PARP-1)/nod-likereceptor protein 3 (NLRP3) signaling.

MAIN METHODS

Mice were injected with LPS (500 µg/kg, i.p) every other day alone or concurrently with oral doses of either NA (40 mg/kg/day) or escitalopram (10 mg/kg/day) for 14 days.

KEY FINDINGS

Administration of NA resulted in significant attenuation of animals' despair reflected by decreased immobility time in forced swimming test. Moreover, NA induced monoamines upsurge in addition to sirtuin1 activation with subsequent down regulation of PARP-1 in the hippocampus. Further, it diminished nuclear factor-κB (NF-κB) levels and inhibited NLRP3 inflammasome with consequent reduction of caspase-1, interleukin-1β and tumor necrosis factor-α levels, thus mitigating LPS-induced neuroinflammation. NA also reduced tumor suppressor protein (p53) while elevating brain-derived neurotrophic factor levels. LPS-induced decline in neuronal survival was reversed by NA administration with an obvious increase in the number of intact cells recorded in the histopathological micrographs.

SIGNIFICANCE

Accordingly, NA is deemed as a prosperous candidate for depression management via targeting SIRT1/PARP-1 pathway.

Research progress on the activation mechanism of NLRP3 inflammasome in septic cardiomyopathy

Sepsis is an uncontrolled host response to infection, resulting in a clinical syndrome involving multiple organ dysfunctions. Cardiac damage is the most common organ damage in sepsis. Uncontrolled inflammatory response is an important mechanism in the pathogenesis of septic cardiomyopathy (SCM). NLRP3 inflammasome promotes inflammatory response by controlling the activation of caspase-1 and the release of pro-inflammatory cytokines interleukin IL-1β and IL-18. The role of NLRP3 inflammasome has received increasing attention, but its activation mechanism and regulation of inflammation in SCM remain to be investigated.

Posttranslational modifications of NLRP3 and their regulatory roles in inflammasome activation

The NACHT, LRR, and PYD domains-containing protein 3 (NLRP3) inflammasome is a multimolecular complex that plays a fundamental role in inflammation. Optimal activation of NLRP3 inflammasome is crucial for host defense against pathogens and the maintenance of immune homeostasis. Aberrant NLRP3 inflammasome activity has been implicated in various inflammatory diseases. Posttranslational modifications (PTMs) of NLRP3, a key inflammasome sensor, play critical roles in directing inflammasome activation and controlling the severity of inflammation and inflammatory diseases, such as arthritis, peritonitis, inflammatory bowel disease, atherosclerosis, and Parkinson's disease. Various NLRP3 PTMs, including phosphorylation, ubiquitination, and SUMOylation, could direct inflammasome activation and control inflammation severity by affecting the protein stability, ATPase activity, subcellular localization, and oligomerization of NLRP3 as well as the association between NLRP3 and other inflammasome components. Here, we provide an overview of the PTMs of NLRP3 and their roles in controlling inflammation and summarize potential anti-inflammatory drugs targeting NLRP3 PTMs.

The 3'UTR region of the DNA repair gene PARP-1 May increase the severity of COVID-19 by altering the binding of antiviral miRNAs

COVID-19 may cause the release of systemic inflammatory cytokines resulting in severe inflammation. PARP-1 has been identified as a nuclear enzyme that is activated by DNA strand breaks. It has been suggested that PARP-1 has a role in the cytokine storm shown as a cause of mortality in COVID-19, and its inhibition may adversely affect the replication of SARS -CoV-2. We aimed to investigate the relationship between PARP-1 gene polymorphisms and the clinical severity of COVID-19. rs8679 TT genotype was found to increase with the COVID-19 disease severity. The 3'UTR polymorphism rs8679 may cause PARP-1 activity as a result of viral replication increase by changing the binding site of antiviral or anti-inflammatory miRNAs. PARP-1 may affect the severity of COVID-19 by cytokine release and maybe a possible treatment target.

IFN-Induced PARPs—Sensors of Foreign Nucleic Acids?

Cells have developed different strategies to cope with viral infections. Key to initiating a defense response against viruses is the ability to distinguish foreign molecules from their own. One central mechanism is the perception of foreign nucleic acids by host proteins which, in turn, initiate an efficient immune response. Nucleic acid sensing pattern recognition receptors have evolved, each targeting specific features to discriminate viral from host RNA. These are complemented by several RNA-binding proteins that assist in sensing of foreign RNAs. There is increasing evidence that the interferon-inducible ADP-ribosyltransferases (ARTs; PARP9—PARP15) contribute to immune defense and attenuation of viruses. However, their activation, subsequent targets, and precise mechanisms of interference with viruses and their propagation are still largely unknown. Best known for its antiviral activities and its role as RNA sensor is PARP13. In addition, PARP9 has been recently described as sensor for viral RNA. Here we will discuss recent findings suggesting that some PARPs function in antiviral innate immunity. We expand on these findings and integrate this information into a concept that outlines how the different PARPs might function as sensors of foreign RNA. We speculate about possible consequences of RNA binding with regard to the catalytic activities of PARPs, substrate specificity and signaling, which together result in antiviral activities.

Efficacy of Clinically Used PARP Inhibitors in a Murine Model of Acute Lung Injury

Poly(ADP-ribose) polymerase 1 (PARP1), as a potential target for the experimental therapy of acute lung injury (ALI), was identified over 20 years ago. However, clinical translation of this concept was not possible due to the lack of clinically useful PARP inhibitors. With the clinical introduction of several novel, ultrapotent PARP inhibitors, the concept of PARP inhibitor repurposing has re-emerged. Here, we evaluated the effect of 5 clinical-stage PARP inhibitors in oxidatively stressed cultured human epithelial cells and monocytes in vitro and demonstrated that all inhibitors (1-30 µM) provide a comparable degree of cytoprotection. Subsequent in vivo studies using a murine model of ALI compared the efficacy of olaparib and rucaparib. Both inhibitors (1-10 mg/kg) provided beneficial effects against lung extravasation and pro-inflammatory mediator production-both in pre- and post-treatment paradigms. The underlying mechanisms include protection against cell dysfunction/necrosis, inhibition of NF-kB and caspase 3 activation, suppression of the NLRP3 inflammasome, and the modulation of pro-inflammatory mediators. Importantly, the efficacy of PARP inhibitors was demonstrated without any potentiation of DNA damage, at least as assessed by the TUNEL method. These results support the concept that clinically approved PARP inhibitors may be repurposable for the experimental therapy of ALI.

A Double-Edged Sword: The Two Faces of PARylation

Poly ADP-ribosylation (PARylation) is a post-translational modification process. Following the discovery of PARP-1, numerous studies have demonstrated the role of PARylation in the DNA damage and repair responses for cellular stress and DNA damage. Originally, studies on PARylation were confined to PARP-1 activation in the DNA repair pathway. However, the interplay between PARylation and DNA repair suggests that PARylation is important for the efficiency and accuracy of DNA repair. PARylation has contradicting roles; however, recent evidence implicates its importance in inflammation, metabolism, and cell death. These differences might be dependent on specific cellular conditions or experimental models used, and suggest that PARylation may play two opposing roles in cellular homeostasis. Understanding the role of PARylation in cellular function is not only important for identifying novel therapeutic approaches; it is also essential for gaining insight into the mechanisms of unexplored diseases. In this review, we discuss recent reports on the role of PARylation in mediating diverse cellular functions and homeostasis, such as DNA repair, inflammation, metabolism, and cell death.

P2X7 Activation Enhances Lipid Accumulation During Adipocytes Differentiation Through Suppressing the Expression of Sirtuin-3, Sirtuin-5, and Browning Genes

P2X7 signaling has been explored in adipose tissue because of its potential to promote ATP-activated inflammatory cascades during obesogenic environments. However, limited literature has investigated the role of the P2X7 receptor in lipid metabolism during adipocyte differentiation. This study sought to explore the regulatory roles of P2X7 in adipocytes. This study utilized the in vitro 3T3-L1 differentiation model. Lipid accumulation, intracellular triglyceride, and extracellular glycerol were determined. The selective P2X7 agonist BzATP and antagonist A438079 were administered to investigate the functions of P2X7. We found that the expression of P2X7 and the lipid accumulation increased during adipocyte differentiation from D0 to D4. When administered at D0/D2, A438079 attenuated, while BzATP enhanced the degree of lipid accumulation during adipocyte differentiation. Neither did BzATP and A438079 administration affect the expression of PPARγ and C/EBPα genes that increased at D4. In addition, both intracellular triglyceride and extracellular glycerol levels at D4 were reduced by A438079 treatment and enhanced by BzATP administration. When administered at stage 2 of adipocyte differentiation, BzATP consistently enhanced lipid accumulation and intracellular triglyceride and extracellular glycerol levels without affecting mRNA and protein levels of PPARγ and C/EBPα that increased at D4. However, treating A438079 or BzATP at D4 did not affect intracellular triglyceride formation and extracellular glycerol release in differentiated adipocytes at D7. Notably, BzATP administration at stage 2 exerted a concentration-dependent inhibition on the enhanced expression of PRDM16, PGC-1α, and UCP-1 at D4. Furthermore, BzATP administration at D0/D2 inhibited the protein and mRNA levels of sirtuin-3/5 at D4. BzATP treatment at stage 2 also suppressed the mRNA levels of sirtuin-3/5 genes upregulated by insulin. In conclusion, this study demonstrated P2X7 enhances lipid accumulation during adipogenesis by suppressing the expression of sirtuin-3/5 and the browning genes.