ARIH2 Ubiquitinates NLRP3 and Negatively Regulates NLRP3 Inflammasome Activation in Macrophages

NLRP3 Inflammasome as a Common Denominator of Atherosclerosis and Abdominal Aortic Aneurysm

Atherosclerosis and abdominal aortic aneurysm (AAA) are multifactorial diseases characterized by inflammatory cell infiltration, matrix degradation, and thrombosis in the arterial wall. Although there are some differences between atherosclerosis and AAA, inflammation is a prominent common feature of these disorders. The nucleotide-binding oligomerization domain-like receptor family pyrin domain containing 3 (NLRP3) inflammasome is a cytosolic multiprotein complex that activates caspase-1 and regulates the release of proinflammatory cytokines interleukin (IL)-1β and IL-18, as well as the induction of lytic cell death, termed pyroptosis, thereby leading to inflammation. Previous experimental and clinical studies have demonstrated that inflammation in atherosclerosis and AAA is mediated primarily through the NLRP3 inflammasome. Furthermore, recent results of the Canakinumab Anti-inflammatory Thrombosis and Outcome Study (CANTOS) showed that IL-1β inhibition reduces systemic inflammation and prevents atherothrombotic events; this supports the concept that the NLRP3 inflammasome is a promising therapeutic target for cardiovascular diseases, including atherosclerosis and AAA. This review summarizes current knowledge with a focus on the role of the NLRP3 inflammasome in atherosclerosis and AAA, and discusses the prospects of NLRP3 inflammasome-targeted therapy.

An update on the regulatory mechanisms of NLRP3 inflammasome activation

The NOD-, LRR-, and pyrin domain-containing protein 3 (NLRP3) inflammasome is a multiprotein complex involved in the release of mature interleukin-1β and triggering of pyroptosis, which is of paramount importance in a variety of physiological and pathological conditions. Over the past decade, considerable advances have been made in elucidating the molecular mechanisms underlying the priming/licensing (Signal 1) and assembly (Signal 2) involved in NLRP3 inflammasome activation. Recently, a number of studies have indicated that the priming/licensing step is regulated by complicated mechanisms at both the transcriptional and posttranslational levels. In this review, we discuss the current understanding of the mechanistic details of NLRP3 inflammasome activation with a particular emphasis on protein-protein interactions, posttranslational modifications, and spatiotemporal regulation of the NLRP3 inflammasome machinery. We also present a detailed summary of multiple positive and/or negative regulatory pathways providing upstream signals that culminate in NLRP3 inflammasome complex assembly. A better understanding of the molecular mechanisms underlying NLRP3 inflammasome activation will provide opportunities for the development of methods for the prevention and treatment of NLRP3 inflammasome-related diseases.

Roles of Inflammasomes in Epstein-Barr Virus-Associated Nasopharyngeal Cancer

Epstein-Barr virus (EBV) infection is recognised as one of the causative agents in most nasopharyngeal carcinoma (NPC) cases. Expression of EBV viral antigens can induce host's antiviral immune response by activating the inflammasomes to produce pro-inflammatory cytokines, such as interleukin-1β (IL-1β) and IL-18. These cytokines are known to be detrimental to a wide range of virus-infected cells, in which they can activate an inflammatory cell death program, called pyroptosis. However, aberrant inflammasome activation and production of its downstream cytokines lead to chronic inflammation that may contribute to various diseases, including NPC. In this review, we summarise the roles of inflammasomes during viral infection, how EBV evades inflammasome-mediated immune response, and progress into tumourigenesis. The contrasting roles of inflammasomes in cancer, as well as the current therapeutic approaches used in targeting inflammasomes, are also discussed in this review. While the inflammasomes appear to have dual roles in carcinogenesis, there are still many questions that remain unanswered. In particular, the exact molecular mechanism responsible for the regulation of the inflammasomes during carcinogenesis of EBV-associated NPC has not been explored thoroughly. Furthermore, the current practical application of inflammasome inhibitors is limited to specific tumour types, hence, further studies are warranted to discover the potential of targeting the inflammasomes for the treatment of NPC.

Inflammasome activation controlled by the interplay between post-translational modifications: emerging drug target opportunities

Controlling the activation of the NLRP3 inflammasome by post-translational modifications (PTMs) of critical protein subunits has emerged as a key determinant in inflammatory processes as well as in pathophysiology. In this review, we put into context the kinases, ubiquitin processing and other PTM enzymes that modify NLRP3, ASC/PYCARD and caspase-1, leading to inflammasome regulation, activation and signal termination. Potential target therapeutic entry points for a number of inflammatory diseases focussed on PTM enzyme readers, writers and erasers, leading to the regulation of inflammasome function, are discussed. Video Abstract.

HUWE1 mediates inflammasome activation and promotes host defense against bacterial infection

The mechanism by which inflammasome activation is modulated remains unclear. In this study, we identified an AIM2-interacting protein, the E3 ubiquitin ligase HUWE1, which was also found to interact with NLRP3 and NLRC4 through the HIN domain of AIM2 and the NACHT domains of NLRP3 and NLRC4. The BH3 domain of HUWE1 was important for its interaction with NLRP3, AIM2, and NLRC4. Caspase-1 maturation, IL-1β release, and pyroptosis were reduced in Huwe1-deficient bone marrow-derived macrophages (BMDMs) compared with WT BMDMs in response to stimuli to induce NLRP3, NLRC4, and AIM2 inflammasome activation. Furthermore, the activation of NLRP3, NLRC4, and AIM2 inflammasomes in both mouse and human cells was remarkably reduced by treatment with the HUWE1 inhibitor BI8622. HUWE1 mediated the K27-linked polyubiquitination of AIM2, NLRP3, and NLRC4, which led to inflammasome assembly, ASC speck formation, and sustained caspase-1 activation. Huwe1-deficient mice had an increased bacterial burden and decreased caspase-1 activation and IL-1β production upon Salmonella, Francisella, or Acinetobacter baumannii infection. Our study provides insights into the mechanisms of inflammasome activation as well as a potential therapeutic target against bacterial infection.

Regulation of the NLRP3 Inflammasome by Post-Translational Modifications and Small Molecules

Inflammation is a host protection mechanism that eliminates invasive pathogens from the body. However, chronic inflammation, which occurs repeatedly and continuously over a long period, can directly damage tissues and cause various inflammatory and autoimmune diseases. Pattern recognition receptors (PRRs) respond to exogenous infectious agents called pathogen-associated molecular patterns and endogenous danger signals called danger-associated molecular patterns. Among PRRs, recent advancements in studies of the NOD-, LRR- and pyrin domain-containing protein 3 (NLRP3) inflammasome have established its significant contribution to the pathology of various inflammatory diseases, including metabolic disorders, immune diseases, cardiovascular diseases, and cancer. The regulation of NLRP3 activation is now considered to be important for the development of potential therapeutic strategies. To this end, there is a need to elucidate the regulatory mechanism of NLRP3 inflammasome activation by multiple signaling pathways, post-translational modifications, and cellular organelles. In this review, we discuss the intracellular signaling events, post-translational modifications, small molecules, and phytochemicals participating in the regulation of NLRP3 inflammasome activation. Understanding how intracellular events and small molecule inhibitors regulate NLRP3 inflammasome activation will provide crucial information for elucidating the associated host defense mechanism and the development of efficient therapeutic strategies for chronic diseases.

NLRP3 inflammasome as a key driver of vascular disease

NLRP3 (nucleotide-binding oligomerization domain-like receptor family pyrin domain containing 3) is an intracellular innate immune receptor that recognizes a diverse range of stimuli derived from pathogens, damaged or dead cells, and irritants. NLRP3 activation causes the assembly of a large multiprotein complex termed the NLRP3 inflammasome, and leads to the secretion of bioactive interleukin (IL)-1β and IL-18 as well as the induction of inflammatory cell death termed pyroptosis. Accumulating evidence indicates that NLRP3 inflammasome plays a key role in the pathogenesis of sterile inflammatory diseases, including atherosclerosis and other vascular diseases. Indeed, the results of the Canakinumab Anti-inflammatory Thrombosis Outcome Study (CANTOS) trial demonstrated that IL-1β-mediated inflammation plays an important role in atherothrombotic events and suggested that NLRP3 inflammasome is a key driver of atherosclerosis. In this review, we will summarize the current state of knowledge regarding the role of NLRP3 inflammasome in vascular diseases, in particular in atherosclerosis, vascular injury, aortic aneurysm, and Kawasaki disease vasculitis, and discuss NLRP3 inflammasome as a therapeutic target for these disorders.

Ubiquitination in the regulation of inflammatory cell death and cancer

The ubiquitin system is complex, multifaceted, and is crucial for the modulation of a vast number of cellular processes. Ubiquitination is tightly regulated at different levels by a range of enzymes including E1s, E2s, and E3s, and an array of DUBs. The UPS directs protein degradation through the proteasome, and regulates a wide array of cellular processes including transcription and epigenetic factors as well as key oncoproteins. Ubiquitination is key to the dynamic regulation of programmed cell death. Notably, the TNF signaling pathway is controlled by competing ubiquitin conjugation and deubiquitination, which governs both proteasomal degradation and signaling complex formation. In the inflammatory response, ubiquitination is capable of both activating and dampening inflammasome activation through the control of either protein stability, complex formation, or, in some cases, directly affecting receptor activity. In this review, we discuss the enzymes and targets in the ubiquitin system that regulate fundamental cellular processes regulating cell death, and inflammation, as well as disease consequences resulting from their dysregulation. Finally, we highlight several pre-clinical and clinical compounds that regulate ubiquitin system enzymes, with the aim of restoring homeostasis and ameliorating diseases.

Role of NLRP3 Inflammasome Activation in Obesity-Mediated Metabolic Disorders

NLRP3 inflammasome is one of the multimeric protein complexes of the nucleotide-binding domain, leucine-rich repeat (NLR)-containing pyrin and HIN domain family (PYHIN). When activated, NLRP3 inflammasome triggers the release of pro-inflammatory interleukins (IL)-1β and IL-18, an essential step in innate immune response; however, defective checkpoints in inflammasome activation may lead to autoimmune, autoinflammatory, and metabolic disorders. Among the consequences of NLRP3 inflammasome activation is systemic chronic low-grade inflammation, a cardinal feature of obesity and insulin resistance. Understanding the mechanisms involved in the regulation of NLRP3 inflammasome in adipose tissue may help in the development of specific inhibitors for the treatment and prevention of obesity-mediated metabolic diseases. In this narrative review, the current understanding of NLRP3 inflammasome activation and regulation is highlighted, including its putative roles in adipose tissue dysfunction and insulin resistance. Specific inhibitors of NLRP3 inflammasome activation which can potentially be used to treat metabolic disorders are also discussed.

Modulatory mechanisms of NLRP3: Potential roles in inflammasome activation

The NLRP3 inflammasome regulates innate immune and inflammatory responses by promoting pro-inflammatory cytokines such as IL-18 and IL-1β. NLRP3 is one of the main factors restricting the activation of the inflammasome, which is closely related to the abundance and localization of NLRP3. A substantial number of studies have focused on specifically targeting NLRP3 to develop inhibitors against NLRP3 inflammasome. Here, we succinctly review the regulation of NLRP3 expression at DNA/chromosome, transcriptional, post-transcriptional, and translation levels. These are critical for the fine regulation of the NLRP3 inflammasome.

Recent insights into the regulatory networks of NLRP3 inflammasome activation

The NACHT, LRR and PYD domains-containing protein 3 (NLRP3) inflammasome is a fascinating cellular machinery endowed with the capacity for rapid proteolytic processing of the pro-inflammatory cytokine IL-1β and the cell death effector gasdermin D (GSDMD). Although its activity is essential to fight infection and support tissue homeostasis, the inflammasome complex, which consists of the danger sensor NLRP3, the adaptor apoptosis-associated speck-like protein containing a CARD (ASC; also known as PYCARD), caspase-1 and probably other regulatory proteins, also bears considerable potential for detrimental inflammation, as observed in human conditions such as gout, heart attack, stroke and Alzheimer's disease. Thus, multi-layered regulatory networks are required to ensure the fine balance between rapid responsiveness versus erroneous activation (sufficient and temporally restricted versus excessive and chronic activity) of the inflammasome. These involve multiple activation, secretion and cell death pathways, as well as modulation of the subcellular localization of NLRP3, and its structure and activity, owing to post-translational modification by other cellular proteins. Here, we discuss the exciting progress that has recently been made in deciphering the regulation of the NLRP3 inflammasome. Additionally, we highlight open questions and describe areas of research that warrant further exploration to obtain a more comprehensive molecular and cellular understanding of the NLRP3 inflammasome.

UAF1 deubiquitinase complexes facilitate NLRP3 inflammasome activation by promoting NLRP3 expression

NOD-like receptor protein 3 (NLRP3) detects microbial infections or endogenous danger signals and activates the NLRP3 inflammasome, which has important functions in host defense and contributes to the pathogenesis of inflammatory diseases, and thereby needs to be tightly controlled. Deubiquitination of NLRP3 is considered a key step in NLRP3 inflammasome activation. However, the mechanisms by which deubiquitination controls NLRP3 inflammasome activation are unclear. Here, we show that the UAF1/USP1 deubiquitinase complex selectively removes K48-linked polyubiquitination of NLRP3 and suppresses its ubiquitination-mediated degradation, enhancing cellular NLRP3 levels, which are indispensable for subsequent NLRP3 inflammasome assembly and activation. In addition, the UAF1/USP12 and UAF1/USP46 complexes promote NF-κB activation, enhance the transcription of NLRP3 and proinflammatory cytokines (including pro-IL-1β, TNF, and IL-6) by inhibiting ubiquitination-mediated degradation of p65. Consequently, Uaf1 deficiency attenuates NLRP3 inflammasome activation and IL-1β secretion both in vitro and in vivo. Our study reveals that the UAF1 deubiquitinase complexes enhance NLRP3 and pro-IL-1β expression by targeting NLRP3 and p65 and licensing NLRP3 inflammasome activation.

NLRP3 inflammasome activation is regulated by various signaling pathways to ensure inflammation does not go unchecked. Here the authors show how deubiquitination avoids this regulation to activate the NLRP3 inflammasome through the function of UAF1/USP deubiquitinase complexes.

Importance of Deubiquitination in Macrophage-Mediated Viral Response and Inflammation

Ubiquitination and deubiquitination play a fundamental role in the signaling pathways associated with innate and adaptive immune responses. Macrophages are key sentinels for the host defense, triggering antiviral and inflammatory responses against various invading pathogens. Macrophages recognize the genetic material of these pathogens as pathogen-associated molecular patterns (PAMPs) and danger-associated molecular patterns (DAMPs) through the activation of its pattern recognition receptors (PRRs), initiating the cascade of immune signaling, which leads to the production of pro- and anti-inflammatory cytokines that initiates the appropriate immune response. Macrophage-mediated immune response is highly regulated and tightly controlled by the ubiquitin system since its abnormal activation or dysregulation may result in the severe pathogenesis of numerous inflammatory and autoimmune diseases. Deubiquitinating enzymes (DUBs) play a crucial role in reversing the ubiquitination and controlling the magnitude of the immune response. During infection, pathogens manipulate the host defense system by regulating DUBs to obtain nutrients and increase proliferation. Indeed, the regulation of DUBs by small molecule inhibitors has been proposed as an excellent way to control aberrant activation of immune signaling molecules. This review is focused on the complex role of DUBs in macrophage-mediated immune response, exploring the potential use of DUBs as therapeutic targets in autoimmune and inflammatory diseases by virtue of small molecule DUB inhibitors.

Role of Gate-16 and Gabarap in Prevention of Caspase-11-Dependent Excess Inflammation and Lethal Endotoxic Shock

Sepsis is a life-threating multi-organ disease induced by host innate immunity to pathogen-derived endotoxins including lipopolysaccharide (LPS). Direct sensing of LPS by caspase-11 activates inflammasomes and causes lethal sepsis in mice. Inhibition of caspase-11 inflammasomes is important for the prevention of LPS-induced septic shock; however, whether a caspase-11 inflammasome-specific suppressive mechanism exists is unclear. Here we show that deficiency of GABARAP autophagy-related proteins results in over-activation of caspase-11 inflammasomes but not of canonical inflammasomes. Gate-16^−/−Gabarap^−/− macrophages exhibited elevated guanylate binding protein 2 (GBP2)-dependent caspase-11 activation and inflammatory responses. Deficiency of GABARAPs resulted in formation of GBP2-containing aggregates that promote IL-1β production. High mortality after low dose LPS challenge in Gate-16^−/−Gabarap^−/− mice primed with poly(I:C) or polymicrobial sepsis was ameliorated by compound GBP2 deficiency. These results reveal a critical function of Gate-16 and Gabarap to suppress GBP2-dependent caspase-11-induced inflammation and septic shock.

ASC regulates platelet activation and contributes to thrombus formation independent of NLRP3 inflammasome

BACKGROUND

Platelets are critical mediators of vascular homeostasis and thrombosis, and also contribute to the development of inflammation. NLRP3 inflammasome is a cytosolic multi-protein complex that consists of NLRP3, ASC and caspase-1, and regulates IL-1β-mediated inflammation.

METHOD AND RESULTS

Using two mouse models of thrombosis (i.e., occlusion of the middle cerebral artery and inferior vena cava), we found that thrombus formation was significantly enhanced in ASC-deficient (ASC^-/-) mice, compared to that in wild-type (WT) and IL-1β^-/- mice. ASC deficiency had no effects on blood coagulation parameters (i.e., prothrombin time [PT] and activated partial thromboplastin time [APTT]). Platelets from WT mice express ASC, but neither NLRP3 nor caspase-1. ASC deficiency significantly enhanced the expression of P-selectin and GPIIb/IIIa in response to a GPVI agonist (collagen-related peptide [CRP]), but not to thrombin, in platelets. CRP induced ASC speck formation in WT platelets. ASC deficiency also enhanced cytosolic Ca²⁺ elevation and phosphorylation of ERK1/2 and Akt in platelets.

CONCLUSION

Our results demonstrate that ASC negatively regulates GPVI signaling in platelets and enhances thrombus formation, independent of NLRP3 inflammasome and IL-1β, and provide novel insights into the link between inflammation and thrombosis.

Role of interleukin-1 and inflammasomes in oral disease

BACKGROUND

Inflammation promotes immune cell infiltration into tissues and induces production of pro-inflammatory cytokines that mediate innate immune responses. Acute or temporary inflammation results in the required repair of the inflamed tissues. However, chronic inflammation leads to pathogenesis of inflammatory conditions such as periodontal disease. In periodontal tissues, pro-inflammatory cytokines mediate inflammatory responses and accelerate the bone-resorbing activity of osteoclasts, resulting in destruction of alveolar bone. Levels of interleukin-1 (IL-1), a major pro-inflammatory cytokine that strongly promotes osteoclastic activity, are elevated in oral tissues of patients with periodontitis. Therefore, elucidation of the mechanisms underlying IL-1 production will enhance our understanding of the pathogenesis of periodontal disease.

HIGHLIGHT

IL-1 has two isoforms: IL-1α and IL-1β. Both isoforms bind to the same IL-1 receptor and have identical biological activity. Unlike that of IL-1α, the IL-1β precursor is not bioactive. To induce its bioactivity, the IL-1β precursor is cleaved by caspase-1, whose activation is mediated by multiprotein complexes termed inflammasomes. Thus, IL-1β maturation and activity are strictly regulated by inflammasomes. This review highlights the current understanding of the molecular mechanisms underlying IL-1 production and the related inflammasome activity.

CONCLUSION

Inhibition of IL-1 production or the inflammasomes via their regulatory mechanisms may facilitate prevention or treatment of periodontal disease and other inflammatory diseases.

NLRP3 inflammasome priming: A riddle wrapped in a mystery inside an enigma

The NLRP3 (NOD-, LRR-, and pyrin domain-containing protein 3) inflammasome is an immunological sensor that detects a wide range of microbial- and host-derived signals. Inflammasome activation results in the release of the potent pro-inflammatory cytokines IL-1β and IL-18 and triggers a form of inflammatory cell death known as pyroptosis. Excessive NLRP3 activity is associated with the pathogenesis of a wide range of inflammatory diseases, thus NLRP3 activation mechanisms are an area of intensive research. NLRP3 inflammasome activation is a tightly regulated process that requires both priming and activation signals. In particular, recent research has highlighted the highly complex nature of the priming step, which involves transcriptional and posttranslational mechanisms, and numerous protein binding partners. This review will describe the current understanding of NLRP3 priming and will discuss the potential opportunities for targeting this process therapeutically to treat NLRP3-associated diseases.

NLRP3 Inflammasome Activation in Lung Vascular Endothelial Cells Contributes to Intestinal Ischemia/Reperfusion-Induced Acute Lung Injury

Intestinal ischemia/reperfusion (I/R) injury is a life-threatening complication that leads to inflammation and remote organ damage. The NLRP3 inflammasome regulates the caspase-1-dependent release of IL-1β, an early mediator of inflammation after I/R injury. In this study, we investigated the role of the NLRP3 inflammasome in mice with intestinal I/R injury. Deficiency of NLRP3, ASC, caspase-1/11, or IL-1β prolonged survival after intestinal I/R injury, but neither NLRP3 nor caspase-1/11 deficiency affected intestinal inflammation. Intestinal I/R injury caused acute lung injury (ALI) characterized by inflammation, reactive oxygen species generation, and vascular permeability, which was markedly improved by NLRP3 deficiency. Bone marrow chimeric experiments showed that NLRP3 in non-bone marrow-derived cells was the main contributor to development of intestinal I/R-induced ALI. The NLRP3 inflammasome in lung vascular endothelial cells is thought to be important to lung vascular permeability. Using mass spectrometry, we identified intestinal I/R-derived lipid mediators that enhanced NLRP3 inflammasome activation in lung vascular endothelial cells. Finally, we confirmed that serum levels of these lipid mediators were elevated in patients with intestinal ischemia. To our knowledge, these findings provide new insights into the mechanism underlying intestinal I/R-induced ALI and suggest that endothelial NLRP3 inflammasome-driven IL-1β is a novel potential target for treating and preventing this disorder.

Role of the NLRP3 inflammasome in autoimmune diseases

NOD-like receptor family pyrin domain containing 3 (NLRP3) is an intracellular receptor that senses foreign pathogens and endogenous danger signals. It assembles with apoptosis-associated speck-like protein containing a CARD (ASC) and caspase-1 to form a multimeric protein called the NLRP3 inflammasome. Among its various functions, the NLRP3 inflammasome can induce the release of the pro-inflammatory cytokines interleukin (IL)-1β and IL-18 while also promoting gasdermin D (GSDMD)-mediated pyroptosis. Previous studies have established a vital role for the NLRP3 inflammasome in innate and adaptive immune system as well as its contribution to several autoimmune diseases including rheumatoid arthritis (RA), systemic lupus erythematosus (SLE), Sjögren's syndrome (SS), systemic sclerosis (SSc), and ankylosing spondylitis (AS). In this review, we briefly introduce the biological features of the NLRP3 inflammasome and present the mechanisms underlying its activation and regulation. We also summarize recent studies that have reported on the roles of NLRP3 inflammasome in the immune system and several autoimmune diseases, with a focus on therapeutic and clinical applications.

Cellular Models and Assays to Study NLRP3 Inflammasome Biology

The NLRP3 inflammasome is a multi-protein complex that initiates innate immunity responses when exposed to a wide range of stimuli, including pathogen-associated molecular patterns (PAMPs) and danger-associated molecular patterns (DAMPs). Inflammasome activation leads to the release of the pro-inflammatory cytokines interleukin (IL)-1β and IL-18 and to pyroptotic cell death. Over-activation of NLRP3 inflammasome has been associated with several chronic inflammatory diseases. A deep knowledge of NLRP3 inflammasome biology is required to better exploit its potential as therapeutic target and for the development of new selective drugs. To this purpose, in the past few years, several tools have been developed for the biological characterization of the multimeric inflammasome complex, the identification of the upstream signaling cascade leading to inflammasome activation, and the downstream effects triggered by NLRP3 activation. In this review, we will report cellular models and cellular, biochemical, and biophysical assays that are currently available for studying inflammasome biology. A special focus will be on those models/assays that have been used to identify NLRP3 inhibitors and their mechanism of action.

Increasing complexity of NLRP3 inflammasome regulation

Inflammasomes are multiprotein complexes that assemble upon detection of danger signals to activate the inflammatory enzyme caspase-1, trigger secretion of the highly proinflammatory cytokine IL-1β, and induce an inflammatory cell death called pyroptosis. Distinctiveness of the nucleotide-binding oligomerization (NOD), Leucine-rich repeat (LRR)-containing protein (NLRP3) inflammasome resides in the diversity of molecules that induce its activation, indicating a certain intricacy. Furthermore, besides the canonical activation of NLRP3 in response to various stimuli, caspase-11-dependent detection of intracellular LPS activates NLRP3 through a noncanonical pathway. Several aspects of the NLRP3 inflammasome are not characterized or remain unclear. In this review, we summarize the different modes of NLRP3 activation. We describe recent insights into post-translational and cellular regulation that confer further complexity to NLRP3 inflammasomes.

Inflammasome activation and regulation: toward a better understanding of complex mechanisms

Inflammasomes are cytoplasmic multiprotein complexes comprising a sensor protein, inflammatory caspases, and in some but not all cases an adapter protein connecting the two. They can be activated by a repertoire of endogenous and exogenous stimuli, leading to enzymatic activation of canonical caspase-1, noncanonical caspase-11 (or the equivalent caspase-4 and caspase-5 in humans) or caspase-8, resulting in secretion of IL-1β and IL-18, as well as apoptotic and pyroptotic cell death. Appropriate inflammasome activation is vital for the host to cope with foreign pathogens or tissue damage, while aberrant inflammasome activation can cause uncontrolled tissue responses that may contribute to various diseases, including autoinflammatory disorders, cardiometabolic diseases, cancer and neurodegenerative diseases. Therefore, it is imperative to maintain a fine balance between inflammasome activation and inhibition, which requires a fine-tuned regulation of inflammasome assembly and effector function. Recently, a growing body of studies have been focusing on delineating the structural and molecular mechanisms underlying the regulation of inflammasome signaling. In the present review, we summarize the most recent advances and remaining challenges in understanding the ordered inflammasome assembly and activation upon sensing of diverse stimuli, as well as the tight regulations of these processes. Furthermore, we review recent progress and challenges in translating inflammasome research into therapeutic tools, aimed at modifying inflammasome-regulated human diseases.

Does NLRP3 Inflammasome and Aryl Hydrocarbon Receptor Play an Interlinked Role in Bowel Inflammation and Colitis-Associated Colorectal Cancer?

Inflammation is a hallmark in many forms of cancer; with colitis-associated colorectal cancer (CAC) being a progressive intestinal inflammation due to inflammatory bowel disease (IBD). While this is an exemplification of the negatives of inflammation, it is just as crucial to have some degree of the inflammatory process to maintain a healthy immune system. A pivotal component in the maintenance of such intestinal homeostasis is the innate immunity component, inflammasomes. Inflammasomes are large, cytosolic protein complexes formed following stimulation of microbial and stress signals that lead to the expression of pro-inflammatory cytokines. The NOD-, LRR- and pyrin domain-containing protein 3 (NLRP3) inflammasome has been extensively studied in part due to its strong association with colitis and CAC. The aryl hydrocarbon receptor (AhR) has recently been acknowledged for its connection to the immune system aside from its role as an environmental sensor. AhR has been described to play a role in the inhibition of the NLRP3 inflammasome activation pathway. This review will summarise the signalling pathways of both the NLRP3 inflammasome and AhR; as well as new-found links between these two signalling pathways in intestinal immunity and some potential therapeutic agents that have been found to take advantage of this link in the treatment of colitis and CAC.

RBR E3 ubiquitin ligases in tumorigenesis

RING-in-between-RING (RBR) E3 ligases are one class of E3 ligases that is characterized by the unique RING-HECT hybrid mechanism to function with E2s to transfer ubiquitin to target proteins for degradation. Emerging evidence has demonstrated that RBR E3 ligases play essential roles in neurodegenerative diseases, infection, inflammation and cancer. Accumulated evidence has revealed that RBR E3 ligases exert their biological functions in various types of cancers by modulating the degradation of tumor promoters or suppressors. Hence, we summarize the differential functions of RBR E3 ligases in a variety of human cancers. In general, ARIH1, RNF14, RNF31, RNF144B, RNF216, and RBCK1 exhibit primarily oncogenic roles, whereas ARIH2, PARC and PARK2 mainly have tumor suppressive functions. Moreover, the underlying mechanisms by which different RBR E3 ligases are involved in tumorigenesis and progression are also described. We discuss the further investigation is required to comprehensively understand the critical role of RBR E3 ligases in carcinogenesis. We hope our review can stimulate the researchers to deeper explore the mechanism of RBR E3 ligases-mediated carcinogenesis and to develop useful inhibitors of these oncogenic E3 ligases for cancer therapy.

Toward targeting inflammasomes: insights into their regulation and activation

Inflammasomes are multi-component signaling complexes critical to the initiation of pyroptotic cell death in response to invading pathogens and cellular damage. A number of innate immune receptors have been reported to serve as inflammasome sensors. Activation of these sensors leads to the proteolytic activation of caspase-1, a proinflammatory caspase responsible for the cleavage of proinflammatory cytokines interleukin-1β and interleukin-18 and the effector of pyroptotic cell death, gasdermin D. Though crucial to the innate immune response to infection, dysregulation of inflammasome activation can lead to the development of inflammatory diseases, neurodegeneration, and cancer. Therefore, clinical interest in the modulation of inflammasome activation is swiftly growing. As such, it is imperative to develop a mechanistic understanding of the regulation of these complexes. In this review, we divide the regulation of inflammasome activation into three parts. We discuss the transcriptional regulation of inflammasome components and related proteins, the post-translational mechanisms of inflammasome activation, and advances in the understanding of the structural basis of inflammasome activation.

Palmitic acid activates NLRP3 inflammasome and induces placental inflammation during pregnancy in mice

Maternal obesity is one of the major risk factors for pregnancy complications and is associated with low-grade chronic systemic inflammation due to higher levels of pro-inflammatory cytokines such as interleukin (IL)-1β. Pregnant women with obesity have abnormal lipid profiles, characterized by higher levels of free fatty acids, especially palmitic acid (PA). Previously, we reported that PA stimulated IL-1β secretion via activation of NLRP3 inflammasome in human placental cells. These observations led us to hypothesize that higher levels of PA induce NLRP3 inflammasome activation and placental inflammation, resulting in pregnancy complications. However, the effects of PA on NLRP3 inflammasome during pregnancy in vivo remain unclear. Therefore, PA solutions were administered intravenously into pregnant mice on day 12 of gestation. Maternal body weight was significantly decreased and absorption rates were significantly higher in PA-injected mice. The administration of PA significantly increased IL-1β protein and the mRNA expression of NLRP3 inflammasome components (NLRP3, ASC, and caspase-1) within the placenta. In murine placental cell culture, PA significantly stimulated IL-1β secretion, and this secretion was suppressed by a specific NLRP3 inhibitor (MCC950). Simultaneously, the number of macrophages/monocytes and neutrophils, together with the mRNA expression of these chemokines increased significantly in the placentas of PA-treated mice. Treatment with PA induced ASC assembling and IL-1β secretion in macrophages, and this PA-induced IL-1β secretion was significantly suppressed in NLRP3-knockdown macrophages. These results indicate that transient higher levels of PA exposure in pregnant mice activates NLRP3 inflammasome and induces placental inflammation, resulting in the incidence of absorption.

How to Inactivate Human Ubiquitin E3 Ligases by Mutation

E3 ubiquitin ligases are the ultimate enzymes involved in the transfer of ubiquitin to substrate proteins, a process that determines the fate of the modified protein. Numerous diseases are caused by defects in the ubiquitin-proteasome machinery, including when the activity of a given E3 ligase is hampered. Thus, inactivation of E3 ligases and the resulting effects at molecular or cellular level have been the focus of many studies during the last few years. For this purpose, site-specific mutation of key residues involved in either protein interaction, substrate recognition or ubiquitin transfer have been reported to successfully inactivate E3 ligases. Nevertheless, it is not always trivial to predict which mutation(s) will block the catalytic activity of a ligase. Here we review over 250 site-specific inactivating mutations that have been carried out in 120 human E3 ubiquitin ligases. We foresee that the information gathered here will be helpful for the design of future experimental strategies.

Atherothrombosis and the NLRP3 inflammasome - endogenous mechanisms of inhibition

Recently, the CANTOS (Canakinumab Anti-Inflammatory Thrombosis Outcomes Study) showed the successful anti-inflammatory benefit of canakinumab, a monoclonal antibody targeting interleukin-1ß (IL-1ß) toward major cardiovascular events (MACE) in patients with a previous myocardial infarction (MI). The magnitude of reduction in MACE was directly attributed to a reduction witnessed in IL-6 and C-reactive protein (CRP) and highlighted the therapeutic potential of selectively targeting IL-1ß for atherosclerotic disease, a notion previously introduced in animal models. IL-1ß is involved in the downstream activation of the IL-6 receptor, which itself has been previously implicated as a target for atherothrombosis from Mendelian randomization studies. Further support has been garnered with the results of CIRT (Cardiovascular Inflammation Reduction Trial), which showed the inability of low-dose methotrexate to reduce IL-1ß, IL-6, or high-sensitivity CRP (hsCRP) in addition to MACE among patients with prior MI or multivessel coronary artery disease (CAD) but with normal hsCRP levels. Therefore, elucidation of therapeutic targets against the IL-1ß pathway is of immense interest currently in treating atherothrombosis. Upstream and serving as an activator of IL-1ß lies the nucleotide-binding oligomerization domain-like receptor protein 3 (NLRP3) inflammasome that has been well described in animal models to be activated by cholesterol crystals or hypoxia to promote cleavage and secretion of IL-1ß and IL-18 that lead to atherosclerotic deposition in arteries. Given the direct implication of an atherogenic role to the NLRP3 inflammasome in generating these cytokines, NLRP3 inhibitors are of interest with the consideration to move upstream from the initial success of anti-IL-1ß therapy. With further discussion of the existing knowledge on the proinflammatory relationship of the NLRP3 inflammasome with atherosclerosis, this review summarizes and critically evaluates the preclinical and interventional findings of endogenous NLRP3 inflammasome inhibition in attempts to elucidate anti-inflammatory mechanisms, and therapeutic targets against atherothrombosis. Further investigation focusing on the endogenous mechanisms of inhibition of the NLRP3 inflammasome would uncover diagnostic routes from defective means in inflammatory resolution. Specifically, pro-resolving lipid mediators, autophagy, and phosphorylation/dephosphorylation mechanisms are 3 points of worthy investigation from existing evidence.

Control of the inflammasome by the ubiquitin system

Inflammation is the body's response to danger. One of the first immune cell types to encounter danger is the macrophage. Macrophages sense danger signals such as extracellular ATP or bacterial toxins, derived from tissue damage or infection, and initiate the activation of an intracellular molecular complex called the inflammasome. The inflammasome consists of a cytosolic pattern recognition receptor, an adaptor molecule ASC (apoptosis-associated speck-like protein containing a CARD) and the protease caspase-1. Assembly of the complex leads to the cleavage and activation of caspase-1 that triggers processing and release of the cytokines interleukin (IL)-1β and IL-18, and ultimately cell death via the process of pyroptosis. The ability to sense and respond to danger appropriately is critical for maintaining immune homeostasis. Dysregulation of inflammasomes contributes to the progression of chronic diseases prevalent in the ageing population, such as Alzheimer's disease, COPD and metabolic disease; hence, it is critical that activation of the inflammatory response and inflammasome activation are tightly regulated. Post-translational modifications (PTMs) such as ubiquitination have recently emerged as important regulators of inflammasome assembly. However, the mechanisms by which PTMs regulate the inflammasome are still not understood. This review aims to summarize our knowledge to date on how the ubiquitin system controls inflammasome activation and where this area of research is heading.

Role of the NLRP3 Inflammasome in Preeclampsia

Reproduction involves tightly regulated series of events and the immune system is involved in an array of reproductive processes. Disruption of well-controlled immune functions leads to infertility, placental inflammation, and numerous pregnancy complications, including preeclampsia (PE). Inflammasomes are involved in the process of pathogen clearance and sterile inflammation. They are large multi-protein complexes that are located in the cytosol and play key roles in the production of the pivotal inflammatory cytokines, interleukin (IL)-1β and IL-18, and pyroptosis. The nucleotide-binding oligomerization domain, leucine-rich repeat-, and pyrin domain-containing 3 (NLRP3) inflammasome is a key mediator of sterile inflammation induced by various types of damage-associated molecular patterns (DAMPs). Recent evidence indicates that the NLRP3 inflammasome is involved in pregnancy dysfunction, including PE. Many DAMPs (uric acid, palmitic acid, high-mobility group box 1, advanced glycation end products, extracellular vesicles, cell-free DNA, and free fatty acids) are increased and associated with pregnancy complications, especially PE. This review focuses on the role of the NLRP3 inflammasome in the pathophysiology of PE.

The strategies of targeting the NLRP3 inflammasome to treat inflammatory diseases

The NLRP3 inflammasome is a cytoplasmic multiprotein complex, the assembly of which can be initiated in response to various exogenous or endogenous danger signals. Excessive activation of the NLRP3 inflammasome has been implicated in the pathogenesis of a wide variety of human inflammatory diseases, suggesting that the NLRP3 inflammasome is a potential target for the treatment of these diseases. However, clinical drugs targeting the NLRP3 inflammasome are still not available. Recent data have elucidated the different signaling pathways or events that can control NLRP3 inflammasome activation and have provided some potential compounds with anti-NLRP3 inflammasome activity. Here, we summarize the molecular mechanisms and diseases involved in NLRP3 inflammasome activation and discuss the potential strategies targeting different aspects of the NLRP3 inflammasome and its implications for the treatment of inflammatory diseases.

Genome-wide CRISPR screening reveals genetic modifiers of mutant EGFR dependence in human NSCLC

EGFR-mutant NSCLCs frequently respond to EGFR tyrosine kinase inhibitors (TKIs). However, the responses are not durable, and the magnitude of tumor regression is variable, suggesting the existence of genetic modifiers of EGFR dependency. Here, we applied a genome-wide CRISPR-Cas9 screening to identify genetic determinants of EGFR TKI sensitivity and uncovered putative candidates. We show that knockout of RIC8A, essential for G-alpha protein activation, enhanced EGFR TKI-induced cell death. Mechanistically, we demonstrate that RIC8A is a positive regulator of YAP signaling, activation of which rescued the EGFR TKI sensitizing phenotype resulting from RIC8A knockout. We also show that knockout of ARIH2, or other components in the Cullin-5 E3 complex, conferred resistance to EGFR inhibition, in part by promoting nascent protein synthesis through METAP2. Together, these data uncover a spectrum of previously unidentified regulators of EGFR TKI sensitivity in EGFR-mutant human NSCLC, providing insights into the heterogeneity of EGFR TKI treatment responses.

Cancer is caused by cells growing and dividing uncontrollably as a result of mutations in certain genes. Many human lung cancers have a mutation in the gene that makes the protein EGFR. In healthy cells, EGFR allows a cell to respond to chemical signals that encourage healthy growth. In cancer, the altered EGFR is always on, which allows the cell to rapidly grow without any control, resulting in cancer.

One approach to treating these cancers is with drugs that block the activity of mutant EGFR. Although these drugs have been very successful, they do not always succeed in completely treating the cancer. This is because over time the cancer cells can become resistant to the drug and start forming new tumors. One way that this can happen is if random mutations lead to changes in other proteins that make the drug less effective or stop it from accessing the EGFR proteins. However, it is unclear how other proteins in cancer cells affect the response to these EGFR inhibiting drugs.

Now, Zeng et al. have used gene editing to systematically remove every protein from human lung cancer cells grown in the laboratory to see how this affects resistance to EGFR inhibitor treatment. This revealed that a number of different proteins could change how cancer cells responded to the drug. For instance, cells lacking the protein RIC8A were more sensitive to EGFR inhibitors and less likely to develop resistance. This is because loss of RIC8A turns down a key cell survival pathway in cancer cells. Whereas, cancer cells lacking the ARIH2 protein were able to produce more proteins that are needed for cancer cell growth, which resulted in them having increased resistance to EGFR inhibitors.

The proteins identified in this study could be used to develop new drugs that improve the effectiveness of EGFR inhibitors. Understanding how cancer cells respond to EGFR inhibitor treatment could help determine how likely a patient is to develop resistance to these drugs.

Arih2 gene influences immune response and tissue development in chicken

Ariadne homolog 2 (ARIH2), as an E3 ubiquitin ligase, is one of the important factors involved in regulating biological functions, such as inflammation and skeletal muscle degeneration. In the present study, the full-length coding sequence of Arih2 gene was cloned from Hy-Line Brown chicken. The tissue transcriptional profiles of Arih2 gene at different developmental stages were detected using quantitative real-time PCR (qRT-PCR), and the Arih2 functional characteristics in immune response were analyzed. The results showed that the full-length coding sequence of Arih2 gene was 1473 bp, encoding 490 amino acids, and conservative between different species. The Arih2 gene was transcribed in various tissues at different developmental stages, and its transcriptional activities varied significantly between multiple tissues. With the development of chicken, Arih2 gene was basically up-regulated in heart, liver, kidney, skeletal muscle and glandular stomach, but fluctuated significantly in large intestine. In immune response, the transcriptional activities of Arih2 gene exhibited significant changes in the bursa, thymus and blood (P<0.05). The results showed that Arih2 might be a multifunctional gene involved in tissue development and immune response in chicken, and have a potential possible application as diagnostic marker for identifying immune response.

Role of NLRP3 Inflammasome in Cardiac Inflammation and Remodeling after Myocardial Infarction

An accumulating body of evidence indicates that inflammation plays a crucial role in the pathophysiology of myocardial infarction (MI). Nucleotide-binding oligomerization domain-like receptor (NLR) family pyrin domain containing 3 (NLRP3) inflammasome is an intracellular multiprotein complex that regulates caspase-1 activation and the subsequent processing of the potent inflammatory cytokine interleukin (IL)-1β as well as triggering inflammatory cell death pyroptosis. We and other investigators demonstrated that deficiency of the NLRP3 inflammasome components reduces inflammation and improves cardiac dysfunction and remodeling in rodent models of MI. Therefore, the regulation of NLRP3 inflammasome has been regarded as a potential therapeutic target for MI. Furthermore, a recent Canakinumab Antiinflammatory Thrombosis Outcome Study (CANTOS) trial revealed the efficacy of IL-1β inhibition in preventing recurrent cardiovascular events in patients with MI. This review focuses on the role of NLRP3 inflammasome in the process of cardiac inflammation and remodeling after MI, and discusses its potential as a therapeutic target for the prevention and treatment of MI.

The NLRP3 Inflammasome: An Overview of Mechanisms of Activation and Regulation

The NLRP3 inflammasome is a critical component of the innate immune system that mediates caspase-1 activation and the secretion of proinflammatory cytokines IL-1β/IL-18 in response to microbial infection and cellular damage. However, the aberrant activation of the NLRP3 inflammasome has been linked with several inflammatory disorders, which include cryopyrin-associated periodic syndromes, Alzheimer's disease, diabetes, and atherosclerosis. The NLRP3 inflammasome is activated by diverse stimuli, and multiple molecular and cellular events, including ionic flux, mitochondrial dysfunction, and the production of reactive oxygen species, and lysosomal damage have been shown to trigger its activation. How NLRP3 responds to those signaling events and initiates the assembly of the NLRP3 inflammasome is not fully understood. In this review, we summarize our current understanding of the mechanisms of NLRP3 inflammasome activation by multiple signaling events, and its regulation by post-translational modifications and interacting partners of NLRP3.

Ubiquitination-Mediated Inflammasome Activation during Bacterial Infection

Inflammasome activation is essential for host immune responses during pathogenic infection and sterile signals insult, whereas excessive activation is injurious. Thus, inflammasome activation is tightly regulated at multiple layers. Ubiquitination is an important post-translational modification for orchestrating inflammatory immune responses during pathogenic infection, and a major target hijacked by pathogenic bacteria for promoting their survival and proliferation. This review summarizes recent insights into distinct mechanisms of the inflammasome activation and ubiquitination process triggered by bacterial infection. We discuss the complex regulatory of inflammasome activation mediated by ubiquitination machinery during bacterial infection, and provide therapeutic approaches for specifically targeting aberrant inflammasome activation.

CAPS and NLRP3

Cryopyrin-associated periodic syndrome (CAPS) is a rare inherited autoinflammatory disorder characterized by systemic, cutaneous, musculoskeletal, and central nervous system inflammation. Gain-of-function mutations in NLRP3 in CAPS patients lead to activation of the cryopyrin inflammasome, resulting in the inappropriate release of inflammatory cytokines including IL-1β and CAPS-related inflammatory symptoms. Several mechanisms have been identified that are important for the normal regulation of the cryopyrin inflammasome in order to prevent uncontrolled inflammation. Investigators have taken advantage of some of these pathways to develop and apply novel targeted therapies, which have resulted in improved quality of life for patients with this orphan disease.

Recent advances in the mechanisms of NLRP3 inflammasome activation and its inhibitors

The NLRP3 inflammasome is a multimeric protein complex that initiates an inflammatory form of cell death and triggers the release of proinflammatory cytokines IL-1β and IL-18. The NLRP3 inflammasome has been implicated in a wide range of diseases, including Alzheimer’s disease, Prion diseases, type 2 diabetes, and some infectious diseases. It has been found that a variety of stimuli including danger-associated molecular patterns (DAMPs, such as silica and uric acid crystals) and pathogen-associated molecular patterns (PAMPs) can activate NLRP3 inflammasome, but the specific regulatory mechanisms of NLRP3 inflammasome activation remain unclear. Understanding the mechanisms of NLRP3 activation will enable the development of its specific inhibitors to treat NLRP3-related diseases. In this review, we summarize current understanding of the regulatory mechanisms of NLRP3 inflammasome activation as well as inhibitors that specifically and directly target NLRP3.

Cullin1 binds and promotes NLRP3 ubiquitination to repress systematic inflammasome activation

Activation of the NACHT, leucine-rich repeat, and pyrin domains-containing protein 3 (collectively known as NLRP3) inflammasome plays a key role in host immune response, which is the first line of defense against cellular stresses and pathogen infections. However, excessive inflammasome activation damages host cells, and therefore it must be precisely controlled. Here, we discover that Cullin1 (CUL1), a key component of the Skp1-Cullin1-F-box E3 ligase, plays a critical role in controlling the NLRP3 inflammasome. CUL1 represses inflammasome assembly in cultured cells, suppresses NLRP3 function in human monocytic cell line macrophages, and attenuates inflammatory responses in mouse model. Detailed studies demonstrate that CUL1 interacts with NLRP3 and promotes NLRP3 ubiquitination, but not protein degradation, to repress the NLRP3 inflammasome activation. Moreover, upon inflammatory stimuli, including ATP and nigericin treatments, CUL1 disassociates from NLRP3 to release the repression of the NLRP3 inflammasome. Thus, this study reveals a distinct and unique mechanism underlying the control of systematic activation of the NLRP3 inflammasome.-Wan, P., Zhang, Q., Liu, W., Jia, Y., Ai, S., Wang, T., Wang, W., Pan, P., Yang, G., Xiang, Q., Huang, S., Yang, Q., Zhang, W., Liu, F., Tan, Q., Zhang, W., Wu, K., Liu, Y., Wu, J. Cullin1 binds and promotes NLRP3 ubiquitination to repress systematic inflammasome activation.

Updates on autoinflammatory diseases

Autoinflammatory diseases are hyperinflammatory, immune dysregulatory diseases caused by innate immune cells dysregulation that present typically in the perinatal period with systemic and organ-targeted inflammation, but with improved genetic testing and the development of diagnostic criteria, milder and later-onset forms are being detected in adulthood. While the discovery of gain-of-function mutations in innate sensors linked to the production of proinflammatory cytokines provided the bases for anti-cytokine therapies that changed disease and patient outcomes, the field is expanding with the increasing discovery of disease-causing loss-of-function mutations in genes with cellular house-keeping functions that affect cell homeostasis and when dysregulated trigger innate inflammatory pathways. This review focuses on updates on molecular pathways and diseases that cause predominantly IL-1β and Type-I IFN-mediated autoinflammatory diseases.

Exogenous nanoparticles and endogenous crystalline molecules as danger signals for the NLRP3 inflammasomes

Inflammasome mechanisms are involved as some of the pathways of sterile inflammation. Inflammasomes are large multiprotein complexes in the cytosol and are a key system for the production of the pivotal inflammatory cytokines, interleukin (IL)-1β and IL-18, and inflammatory cell death called pyroptosis. Although a number of inflammasomes have been described, the nucleotide-binding oligomerization domain-, leucine-rich repeat-, and pyrin domain-containing 3 (NLRP3) is the most extensively investigated inflammasome. Exogenous pathogen-associated molecular patterns released during infection and endogenous crystalline danger/damage-associated molecular patterns (DAMPs) are well-known activators of NLRP3 inflammasomes. In addition, nanoparticle-associated molecular patterns (NAMPs), which are mediated by synthetic materials, including nanomaterials and nanoparticles, are proposed to be new danger signals of NLRP3 inflammasomes. Importantly, NAMP- and DAMP-triggered inflammation, a defining characteristic in inflammatory diseases, is termed as sterile inflammation because it occurs in the absence of foreign pathogens. This review focuses on the role of inflammasomes in exogenous NAMP- and endogenous crystalline DAMP-mediated sterile inflammation. Moreover, many regulatory mechanisms have been identified to attenuate NLRP3 inflammasomes. Therefore, we also summarize endogenous negative regulators of NLRP3 inflammasome activation, particularly induced by NAMPs or crystalline DAMPs.

Spotlight on the NLRP3 inflammasome pathway

Inflammation is triggered by a repertoire of receptors detecting infections and damages. Some of these receptors directly bind microbial ligands, while others recognize endogenous molecules exposed under stress conditions, including infections. Most of these receptors can be engaged by a relatively limited number of stimuli. Differently, NLRP3 acts as a broad sensor of cell homeostasis rupture and can be activated downstream of a plethora of stimuli. NLRP3 then assembles a multiprotein platform resulting in caspase-1 activation, which controls, by direct cleavage, the maturation of cytosolic pro-cytokines including pro-interleukin-1β. In addition, caspase-1 processes cytosolic gasdermin-D and unleashes its pore-forming N-terminal domain, leading to the release of mature cytosolic cytokines and alarmins, as well as pyroptotic cell lysis. Accumulating evidences of the aggravating role of NLRP3-mediated inflammation in various highly prevalent human conditions, including diabetes, neurodegenerative and cardiovascular diseases, raises a huge clinical interest. Nevertheless, the molecular mechanism governing NLRP3 activation remains insufficiently understood. In line with the detrimental consequences of NLRP3 activation illustrated by the aforementioned pathologies, this process is tightly regulated. In this review, we address the current understanding of the control of NLRP3 activity which can be divided into two coordinated processes referred to as priming and activation. In particular, we detail the emerging role of NLRP3 post-translational modifications critical in inflammasome assembly regulation.

SUMO-mediated regulation of NLRP3 modulates inflammasome activity

The NLRP3 inflammasome responds to infection and tissue damage, and rapidly escalates the intensity of inflammation by activating interleukin (IL)-1β, IL-18 and cell death by pyroptosis. How the NLRP3 inflammasome is negatively regulated is poorly understood. Here we show that NLRP3 inflammasome activation is suppressed by sumoylation. NLRP3 is sumoylated by the SUMO E3-ligase MAPL, and stimulation-dependent NLRP3 desumoylation by the SUMO-specific proteases SENP6 and SENP7 promotes NLRP3 activation. Defective NLRP3 sumoylation, either by NLRP3 mutation of SUMO acceptor lysines or depletion of MAPL, results in enhanced caspase-1 activation and IL-1β release. Conversely, depletion of SENP7 suppresses NLRP3-dependent ASC oligomerisation, caspase-1 activation and IL-1β release. These data indicate that sumoylation of NLRP3 restrains inflammasome activation, and identify SUMO proteases as potential drug targets for the treatment of inflammatory diseases.

Posttranslational Regulation of the NLR Family Pyrin Domain-Containing 3 Inflammasome

The NOD-like receptor family pyrin domain-containing 3 (NLRP3) inflammasome is a multi-protein complex that can be activated by a variety of pathogen-associated molecular patterns or damage-associated molecular patterns. Inappropriate NLRP3 inflammasome activation can induce autoinflammatory, autoimmune, or metabolic disorders. Therefore, NLRP3 is an attractive target against NLRP3 inflammasome activation, and specific targeting of NLRP3 might be an intriguing approach to the development of drugs for the treatment of NLRP3 inflammasome-related diseases. Although many studies with varied mechanistic approaches were reported in inhibition of NLRP3 inflammasome activation, mechanisms related to regulation of posttranslational modification (PTM) of NLRP3, as a focal point has not been thoroughly addressed. Recently, extensive investigations of PTMs of NLRP3 have led to partial understanding of the mechanisms involved in NLRP3 inflammasome activation. In this review, we focused on the role of PTMs regulating NLRP3 inflammasome activation.

NS1 Protein of 2009 Pandemic Influenza A Virus Inhibits Porcine NLRP3 Inflammasome-Mediated Interleukin-1 Beta Production by Suppressing ASC Ubiquitination

The inflammasome represents a molecular platform for innate immune regulation and controls proinflammatory cytokine production. The NLRP3 inflammasome is comprised of NLRP3, ASC, and procaspase-1. When the NLRP3 inflammasome is activated, it causes ASC speck formation and caspase-1 activation, resulting in the maturation of interleukin-1β (IL-1β). The NLRP3 inflammasome is regulated at multiple levels, with one level being posttranslational modification. Interestingly, ubiquitination of ASC has been reported to be indispensable for the activation of the NLRP3 inflammasome. Influenza A virus (IAV) infection induces NLRP3 inflammasome-dependent IL-1β secretion, which contributes to the host antiviral defense. However, IAVs have evolved multiple antagonizing mechanisms, one of which is executed by viral NS1 protein to suppress the NLRP3 inflammasome. In this study, we compared IL-1β production in porcine alveolar macrophages in response to IAV infection and found that the 2009 pandemic H1N1 induced less IL-1β than swine influenza viruses (SIVs). Further study revealed that the NS1 C terminus of pandemic H1N1 but not that of SIV was able to significantly inhibit NLRP3 inflammasome-mediated IL-1β production. This inhibitory function was attributed to impaired ASC speck formation and suppression of ASC ubiquitination. Moreover, we identified two target lysine residues, K110 and K140, which are essential for both porcine ASC ubiquitination and NLRP3 inflammasome-mediated IL-1β production. These results revealed a novel mechanism by which the NS1 protein of the 2009 pandemic H1N1 suppresses NLRP3 inflammasome activation.IMPORTANCE Influenza A virus (IAV) infection activates the NLRP3 inflammasome, resulting in the production of IL-1β, which contributes to the host innate immune response. ASC, an adaptor protein of NLRP3, forms specks that are critical for inflammasome activation. Here, we report that the NS1 C terminus of the 2009 pandemic H1N1 has functions to suppress porcine IL-1β production by inhibiting ASC speck formation and ASC ubiquitination. Furthermore, the ubiquitination sites on porcine ASC were identified. The information gained here may contribute to an in-depth understanding of porcine inflammasome activation and regulation in response to different IAVs, helping to further enhance our knowledge of innate immune responses to influenza virus infection in pigs.