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Gout is a type of inflammatory arthritis caused by the deposition of monosodium uric acid (MSU) crystals in tissues. The etiology of gout is directly linked to the NLRP3 inflammasome, since MSU crystals are NLRP3 inflammasome activators. Therefore, we decided to search for a small-molecule inhibitor of the NLRP3 inflammasome for the prevention of gout inflammation. We found that loganin suppressed MSU crystals-induced caspase-1 (p20) and interleukin (IL)-1β production and apoptosis-associated speck-like protein containing a caspase recruitment domain (ASC) specks formation in mouse primary macrophages, showing its ability to inhibit the NLRP3 inflammasome. In an air pouch inflammation model, oral administration of loganin to mice prevented MSU crystals-induced production of mature IL-1β and IL-18 in air pouch exudates, resulting in decreased neutrophil recruitment. Furthermore, oral administration of loganin suppressed MSU crystals-induced gout inflammation in a mouse foot gout model, which was accompanied by the inhibition of the NLRP3 inflammasome. Loganin blocked de novo synthesis of mitochondrial DNA in air pouches and foot tissues injected with MSU crystals. Consistently, loganin prevented MSU crystals-induced mitochondrial damage in macrophages, as it increased mitochondrial membrane potential and decreased the amount of mitochondrial reactive oxygen species. These data demonstrate that loganin suppresses NLRP3 inflammasome activation by inhibiting mitochondrial stress. These results suggest a novel pharmacological strategy to prevent gout inflammation by blocking NLRP3 inflammasome activation and mitochondrial dysfunction.

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., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2021 Seok, Kang, Cho, Lee and Lee.)

Inflammasomes are cytosolic pattern recognition receptors that recognize pathogen-associated molecular patterns (PAMPs) and danger-associated molecular patterns (DAMPs) derived from invading pathogens and damaged tissues, respectively. Upon activation, the inflammasome forms a complex containing a receptor protein, an adaptor, and an effector to induce the autocleavage and activation of procaspase-1 ultimately culminating in the maturation and secretion of IL-1β and IL-18 and pyroptosis. Inflammasome activation plays an important role in host immune responses to pathogen infections and tissue repair in response to cellular damage. The NLRP3 inflammasome is a well-characterized pattern recognition receptor and is well known for its critical role in the regulation of immunity and the development and progression of various inflammatory diseases. In this review, we summarize recent efforts to develop therapeutic applications targeting the NLRP3 inflammasome to cure and prevent chronic inflammatory diseases. This review extensively discusses NLRP3 inflammasome-related diseases and current development of small molecule inhibitors providing beneficial information on the design of therapeutic strategies for NLRP3 inflammasome-related diseases. Additionally, small molecule inhibitors are classified depending on direct or indirect targeting mechanism to describe the current status of the development of pharmacological inhibitors.

Objective: The NLRP3 inflammasome is closely linked to the pathophysiology of a wide range of inflammatory diseases. This study was undertaken to identify small molecules that directly bind to NLRP3 in order to develop pharmacologic interventions for NLRP3-related diseases., Methods: A structure-based virtual screening analysis was performed with ~62,800 compounds to select efficient NLRP3 inhibitors. The production of caspase 1-p10 and interleukin-1β (IL-1β) was measured by immunoblotting and enzyme-linked immunosorbent assay to examine NLRP3 inflammasome activation. Two gouty arthritis models and an air pouch inflammation model induced by monosodium urate monohydrate (MSU) crystal injection were used for in vivo experiments. Primary synovial fluid cells from gout patients were used to determine the relevance of NLRP3 inflammasome inhibition in human gout., Results: Beta-carotene (provitamin A) suppressed the NLRP3 inflammasome activation induced by various activators, including MSU crystals, in mouse bone marrow-derived primary macrophages (P < 0.05). Surface plasmon resonance analysis demonstrated the direct binding of β-carotene to the pyrin domain (PYD) of NLRP3 (K D = 3.41 × 10 -6 ). Molecular modeling and mutation assays revealed the interaction mode between β-carotene and the NLRP3 PYD. Inflammatory symptoms induced by MSU crystals were attenuated by oral administration of β-carotene in gouty arthritis mouse models (P < 0.05), correlating with its suppressive effects on the NLRP3 inflammasome in inflamed tissues. Furthermore, β-carotene reduced IL-1β secretion from human synovial fluid cells isolated from gout patients (P < 0.05), showing its inhibitory efficacy in human gout., Conclusion: Our results present β-carotene as a selective and direct inhibitor of NLRP3, and the binding of β-carotene to NLRP3 PYD as a novel pharmacologic strategy to combat NLRP3 inflammasome-driven diseases, including gouty arthritis., (© 2020, American College of Rheumatology.)

Gout is a chronic inflammatory disease evoked by the deposition of monosodium urate (MSU) crystals in joint tissues. The nucleotide-binding oligomerization domain (NOD)-like receptor (NLR) family pyrin domain containing 3 (NLRP3) inflammasome is responsible for the gout inflammatory symptoms induced by MSU crystals. We investigated whether epigallocatechin-3-gallate (EGCG) suppresses the activation of the NLRP3 inflammasome, thereby effectively preventing gouty inflammation. EGCG blocked MSU crystal-induced production of caspase-1(p10) and interleukin-1β in primary mouse macrophages, indicating its suppressive effect on the NLRP3 inflammasome. In an acute gout mouse model, oral administration of EGCG to mice effectively alleviated gout inflammatory symptoms in mouse foot tissue injected with MSU crystals. The in vivo suppressive effects of EGCG correlated well with the suppression of the NLRP3 inflammasome in mouse foot tissue. EGCG inhibited the de novo synthesis of mitochondrial DNA as well as the production of reactive oxygen species in primary mouse macrophages, contributing to the suppression of the NLRP3 inflammasome. These results show that EGCG suppresses the activation of the NLRP3 inflammasome in macrophages via the blockade of mitochondrial DNA synthesis, contributing to the prevention of gouty inflammation. The inhibitory effects of EGCG on the NLRP3 inflammasome make EGCG a promising therapeutic option for NLRP3-dependent diseases such as gout.

Activation of the NACHT, LRR and PYD domains-containing protein 3 (NLRP3) inflammasome by Propionibacterium acnes (P. acnes) is critical for inducing inflammation and aggravating the development of acne lesions. We searched for available small-molecule inhibitors of the NLRP3 inflammasome that could be topically administered for the treatment of acne. We found that licochalcone A, a chalconoid isolated from the root of Glycyrrhiza inflate, was an effective inhibitor for P. acnes-induced NLRP3 inflammasome activation. Licochalcone A blocked P. acnes-induced production of caspase-1(p10) and IL-1β in primary mouse macrophages and human SZ95 sebocytes, indicating the suppression of NLRP3 inflammasome. Licochalcone A suppressed P. acnes-induced ASC speck formation and mitochondrial reactive oxygen species. Topical application of licochalcone A to mouse ear skin attenuated P. acnes-induced skin inflammation as shown by histological assessment, ear thickness measurement, and inflammatory gene expression. Licochalcone A reduced caspase-1 activity and IL-1β production in mouse ear injected with P. acnes. This study demonstrated that licochalcone A is effective in the control of P. acnes-induced skin inflammation as an efficient inhibitor for NLRP3 inflammasome. Our study provides a new paradigm for the development of anti-acne therapy via targeting NLRP3 inflammasome., (© 2018 John Wiley & Sons, Ltd.)

Atopic dermatitis is a chronic inflammatory skin disease, of which incidence is closely related to exposure to environmental pollutants and allergens. Thymic stromal lymphopoietin (TSLP) plays an important role in the early stages of atopic dermatitis development by inducing Th2 immune responses. In addition, TSLP regulates activation of group 2 innate lymphoid cells (ILC2), promoting the pathogenesis of atopic dermatitis. The aim of this study was to investigate whether celastrol alleviated atopic dermatitis symptoms by regulating TSLP expression and ILC2 stimulation. Celastrol suppressed TSLP production in mouse keratinocyte cells by inhibiting NF-ĸB activation. Topical application of celastrol significantly improved atopic dermatitis symptoms induced by house dust mite (HDM) in NC/Nga mice as determined by dermatitis score and histological assessment. Celastrol decreased the levels of TSLP in atopic dermatitis skin lesions of HDM-stimulated NC/Nga mice. Celastrol reduced levels of Th2 cytokines including IL-4, IL-5, and IL-13 in atopic dermatitis skin lesions of NC/Nga mice. Further, celastrol significantly reduced ILC2 population in atopic dermatitis skin lesions of NC/Nga mice. These results indicate that topical application of celastrol improved atopic dermatitis symptoms by lowering TSLP levels and concomitant immune responses. Data demonstrated that reduced TSLP levels and associated lower number of ILC2 cells alleviate atopic dermatitis symptoms induced by house dust mite. [ABSTRACT FROM AUTHOR]

Objective. The aetiology of gout is closely linked to the deposition of monosodium uric acid (MSU) crystals and the consequent activation of the NOD-like receptor family, pyrin domain containing 3 (NLRP3) inflammasome. In this study, we investigated whether oral administration of an NLRP3 inhibitor would be effective to attenuate the symptoms of gout. Methods. The effects of oral administration with sulforaphane (SFN) were examined in two mouse models of acute gout induced by injection of MSU crystals into footpads or air pouch. The production of caspase- 1 (p10) and IL-1β was examined by immunoblotting and ELISA as hallmarks of NLRP3 inflammasome activation. Results. Oral administration of SFN attenuated MSU crystal-induced swelling and neutrophil recruitment in a mouse foot acute gout model, correlating with the suppression of the NLRP3 inflammasome activation in foot tissues. Consistently, oral administration of SFN blocked MSU-crystal-induced activation of the NLRP3 inflammasome in a mouse air pouch gout model. SFN suppressed NLRP3 inflammasome activation induced by MSU crystals, adenosine triphosphate and nigericin but not by poly(dA:dT) in primary mouse macrophages, independent of the reactive oxygen species pathway. SFN inhibited ligand-independent activation of the NLRP3 inflammasome, suggesting that SFN may act directly on the NLRP3 inflammasome complex. Conclusion. Oral administration of SFN effectively alleviated acute gouty inflammation by suppression of the NLRP3 inflammasome. Our results provide a novel strategy in which oral treatment with SFN may be beneficial in preventing acute attacks of gout. [ABSTRACT FROM AUTHOR]

Non-alcoholic steatohepatitis (NASH), a type of non-alcoholic fatty liver disease, is characterized as steatosis and inflammation in the liver. NLRP3 inflammasome activation is associated with NASH pathology. We hypothesized that suppressing the NLRP3 inflammasome could be effective in preventing NASH. We searched substances that could inhibit the activation of the NLRP3 inflammasome and identified sweroside as an NLRP3 inhibitor. We investigated whether sweroside can be applied to prevent the pathological symptoms associated with NASH in a methionine–choline-deficient (MCD) diet-induced NASH mouse model. The activation of the NLRP3 inflammasome was determined by detecting the production of caspase-1 and IL-1β from pro-caspase-1 and pro-IL-1β in primary mouse macrophages and mouse liver. In a NASH model, mice were fed an MCD diet for two weeks with daily intraperitoneal injections of sweroside. Sweroside effectively inhibited NLRP3 inflammasome activation in primary macrophages as shown by a decrease in IL-1β and caspase-1 production. In a MCD diet-induced NASH mouse model, intraperitoneal injection of sweroside significantly reduced serum aspartate transaminase and alanine transaminase levels, hepatic immune cell infiltration, hepatic triglyceride accumulation, and liver fibrosis. The improvement of NASH symptoms by sweroside was accompanied with its inhibitory effects on the hepatic NLRP3 inflammasome as hepatic IL-1β and caspase-1 were decreased. Furthermore, sweroside blocked de novo synthesis of mitochondrial DNA in the liver, contributing to suppression of the NLRP3 inflammasome. These results suggest that targeting the NLRP3 inflammasome with sweroside could be beneficially employed to improve NASH symptoms. [ABSTRACT FROM AUTHOR]

Regnase-1 is an endoribonuclease that regulates the stability of target genes. Here, we investigated whether Regnase-1 plays a regulatory role in the pathophysiology of atopic dermatitis, a chronic inflammatory skin disease. Regnase-1 levels were decreased in skin and serum of atopic dermatitis patients and mice. Regnase-1 +/- mice exhibited more severe atopic dermatitis symptoms than wild-type mice in a house dust mite allergen-induced atopic dermatitis model. Regnase-1 deficiency led to the global changes in gene expression related with innate immune and inflammatory responses, in particular chemokines. The skin Regnase-1 level had an inverse relationship with chemokine expression when we analyzed samples of atopic dermatitis patients and Regnase-1-deficient mice, suggesting that potentiated chemokine production contributes to the augmented inflammation at lesion sites. Subcutaneous administration of recombinant Regnase-1 to mice significantly ameliorated atopic dermatitis-like skin inflammation with reduced chemokine production in a house dust mite-induced atopic dermatitis NC/Nga mouse model. These results indicate that Regnase-1 plays an essential role in maintaining skin immune homeostasis as a regulator of chemokine expression. Modulating Regnase-1 activity may be an efficient therapeutic strategy for treating chronic inflammatory diseases, including atopic dermatitis. [Display omitted] • Regnase-1 expression is decreased in skin and serum of atopic dermatitis patients. • Regnase-1 deficiency is linked to aggravated inflammation in atopic dermatitis. • Regnase-1 deficiency exacerbates chemokine expression in the lesional skin. • Recombinant Regnase-1 ameliorates atopic dermatitis symptoms and chemokine production. • Regnase-1 is an efficient therapeutic target for maintaining skin immune homeostasis. [ABSTRACT FROM AUTHOR]