Uric acid induces trophoblast IL-1β production via the inflammasome: implications for the pathogenesis of preeclampsia

Citation
Mulla MJ, Myrtolli K, Potter J, Boeras C, Kavathas PB, Sfakianaki AK, Tadesse S, Norwitz ER, Guller S, Abrahams VM. Uric acid induces trophoblast IL‐1β production via the inflammasome: implications for the pathogenesis of preeclampsia. Am J Reprod Immunol 2010; 65: 542–548 Problem  Preeclampsia is associated with hyperuricemia, which correlates with the disease severity. Levels of circulating uric acid increase before the clinical manifestations, suggesting that they may be causally related. Uric acid, or monosodium urate (MSU), activates the Nod‐like receptor, Nalp3, leading to inflammasome activation and IL‐1β processing. Because preeclampsia is associated with placental immune/inflammatory dysregulation, we sought to determine in the trophoblast, the presence of the Nalp3 inflammasome, and the effect of MSU on its activation. Method of study  Isolated first‐ and third‐trimester trophoblasts were assessed for expression of the inflammasome components, Nalp1, Nalp3, and ASC. First‐trimester trophoblast cells were incubated with or without MSU, and after which, IL‐1β secretion and processing and caspase‐1 activation were determined. Results  Trophoblast cells expressed Nalp1, Nalp3, and ASC under basal conditions. Following incubation with MSU, first‐trimester trophoblast IL‐1β secretion was upregulated. This correlated with increased expression levels of active IL‐1β and active caspase‐1. ASC knockdown reduced MSU‐induced IL‐1β secretion. Conclusion  These findings demonstrate that uric acid activates the inflammasome in the trophoblast, leading to IL‐1β production. This may provide a novel mechanism for the induction of inflammation at the maternal–fetal interface leading to placental dysfunction and adverse pregnancy outcome, including preeclampsia.     

Brucella abortus nitric oxide metabolite regulates inflammasome activation and IL‐1β secretion in murine macrophages

NLRP3 inflammasome is a protein complex crucial to caspase‐1 activation and IL‐1β and IL‐18 maturation. This receptor participates in innate immune responses to different pathogens, including the bacteria of genus Brucella. Our group recently demonstrated that Brucella abortus‐induced IL‐1β secretion involves NLRP3 inflammasome and it is partially dependent on mitochondrial ROS production. However, other factors could be involved, such as P2X7‐dependent potassium efflux, membrane destabilization, and cathepsin release. Moreover, there is increasing evidence that nitric oxide acts as a modulator of NLRP3 inflammasome. The aim of this study was to unravel the mechanism of NLRP3 inflammasome activation induced by B. abortus, as well as the involvement of bacterial nitric oxide (NO) as a modulator of this inflammasome pathway. We demonstrated that NO produced by B. abortus can be used by the bacteria to modulate IL‐1β secretion in infected murine macrophages. Additionally, our results suggest that B. abortus‐induced IL‐1β secretion depends on a P2X7‐independent potassium efflux, lysosomal acidification, cathepsin release, mechanisms clearly associated to NLRP3 inflammasome. In summary, our results help to elucidate the molecular mechanisms of NLRP3 activation and regulation during an intracellular bacterial infection.     

Pentraxin 3 recruits complement factor H to protect against oxidative stress‐induced complement and inflammasome overactivation

The discovery that genetic abnormalities in complement factor H (FH) are associated with an increased risk for age‐related macular degeneration (AMD), the most common cause of blindness among the elderly, raised hope of new treatments for this vision‐threatening disease. Nonetheless, over a decade after the identification of this important association, how innate immunity contributes to AMD remains unresolved. Pentraxin 3 (PTX3), an essential component of the innate immunity system that plays a non‐redundant role in controlling inflammation, regulates complement by interacting with complement components. Here, we show that PTX3 is induced by oxidative stress, a known cause of AMD, in the retinal pigmented epithelium (RPE). PTX3 deficiency in vitro and in vivo magnified complement activation induced by oxidative stress, leading to increased C3a, FB, and C3d, but not C5b‐9 complex formation. Increased C3a levels, resulting from PTX3 deficiency, raised the levels of Il1b mRNA and secretion of activated interleukin (IL)‐1β by interacting with C3aR. Importantly, PTX3 deficiency augmented NLRP3 inflammasome activation, resulting in enhanced IL‐1β, but not IL‐18, production by the RPE. Thus, in the presence of PTX3 deficiency, the complement and inflammasome pathways worked in concert to produce IL‐1β in sufficient abundance to, importantly, result in macrophages accumulating in the choroid. These results demonstrate that PTX3 acts as an essential brake for complement and inflammasome activation by regulating the abundance of FH in the RPE, and provide critical insights into the complex interplay between oxidative stress and innate immunity in the early stages of AMD development. Copyright © 2016 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.     

A novel “complement–metabolism–inflammasome axis” as a key regulator of immune cell effector function

The inflammasomes are intracellular multiprotein complexes that induce and regulate the generation of the key pro‐inflammatory cytokines IL‐1β and IL‐18 in response to infectious microbes and cellular stress. The activation of inflammasomes involves several upstream signals including classic pattern or danger recognition systems such as the TLRs. Recently, however, the activation of complement receptors, such as the anaphylatoxin C3a and C5a receptors and the complement regulator CD46, in conjunction with the sensing of cell metabolic changes, for instance increased amino acid influx and glycolysis (via mTORC1), have emerged as additional critical activators of the inflammasome. This review summarizes recent advances in our knowledge about complement‐mediated inflammasome activation, with a specific focus on a novel “complement – metabolism – NLRP3 inflammasome axis.”     

Secreted aspartic proteases of Candida albicans activate the NLRP3 inflammasome

In a recent report, we demonstrated that distinct members of the secreted aspartic protease (Sap) family of Candida albicans are able to induce secretion of proinflammatory cytokines by human monocytes, independently of their proteolytic activity and specific pH optima. In particular, C. albicans Sap2 and Sap6 potently induced IL‐1β, TNF‐α, and IL‐6 production. Here, we demonstrate that Sap2 and Sap6 proteins trigger IL‐1β and IL‐18 production through inflammasome activation. This occurs via NLRP3 and caspase‐1 activation, which cleaves pro‐IL‐1β into secreted bioactive IL‐1β, a cytokine that was induced by Saps in monocytes, in monocyte‐derived macrophages and in dendritic cells. Downregulation of NLRP3 by RNA interference strongly reduced the secretion of bioactive IL‐1β. Inflammasome activation required Sap internalization via a clathrin‐dependent mechanism, intracellular induction of K⁺ efflux, and ROS production. Inflammasome activation of monocytes induced by Sap2 and Sap6 differed from that induced by LPS‐ATP in several aspects. Our data reveal novel immunoregulatory mechanisms of C. albicans and suggest that Saps contribute to the pathogenesis of candidiasis by fostering rather than evading host immunity.     

Caspase‐11: The driving factor for noncanonical inflammasomes

Inflammasomes are large multiprotein platforms that mediate the processing of caspase‐1, which in turn promotes the maturation and release of IL‐1β and IL‐18 in response to microbial and danger signals. While the canonical pathway of inflammasome activation has been known for some time, a novel mechanism of noncanonical inflammasome activation mediated by caspase‐11 was more recently identified. This pathway engages caspase‐11 to trigger both caspase‐1‐dependent and ‐independent production of the inflammatory cytokines IL‐1β, IL‐18, and IL‐1α, as well as to promote pyroptosis, a form of genetically programmed cell death that is associated with the release of such cytokines. In this review, we gather together studies on both the mechanisms and implications of caspase‐11‐mediated noncanonical inflammasome activation, and discuss the emerging importance of this pathway in regulating host defense against intracellular bacterial pathogens.     

NLRP3 inflammasome activation downstream of cytoplasmic LPS recognition by both caspase‐4 and caspase‐5

Humans encode two inflammatory caspases that detect cytoplasmic LPS, caspase‐4 and caspase‐5. When activated, these trigger pyroptotic cell death and caspase‐1‐dependent IL‐1β production; however the mechanism underlying this process is not yet confirmed. We now show that a specific NLRP3 inhibitor, MCC950, prevents caspase‐4/5‐dependent IL‐1β production elicited by transfected LPS. Given that both caspase‐4 and caspase‐5 can detect cytoplasmic LPS, it is possible that these proteins exhibit some degree of redundancy. Therefore, we generated human monocytic cell lines in which caspase‐4 and caspase‐5 were genetically deleted either individually or together. We found that the deletion of caspase‐4 suppressed cell death and IL‐1β production following transfection of LPS into the cytoplasm, or in response to infection with Salmonella typhimurium. Although deletion of caspase‐5 did not confer protection against transfected LPS, cell death and IL‐1β production were reduced after infection with Salmonella. Furthermore, double deletion of caspase‐4 and caspase‐5 had a synergistic effect in the context of Salmonella infection. Our results identify the NLRP3 inflammasome as the specific platform for IL‐1β maturation, downstream of cytoplasmic LPS detection by caspase‐4/5. We also show that both caspase‐4 and caspase‐5 are functionally important for appropriate responses to intracellular Gram‐negative bacteria.     

PMA and crystal‐induced neutrophil extracellular trap formation involves RIPK1‐RIPK3‐MLKL signaling

Neutrophil extracellular trap (NET) formation contributes to gout, autoimmune vasculitis, thrombosis, and atherosclerosis. The outside‐in signaling pathway triggering NET formation is unknown. Here, we show that the receptor‐interacting protein kinase (RIPK)‐1‐stabilizers necrostatin‐1 or necrostatin‐1s and the mixed lineage kinase domain‐like (MLKL)‐inhibitor necrosulfonamide prevent monosodium urate (MSU) crystal‐ or PMA‐induced NET formation in human and mouse neutrophils. These compounds do not affect PMA‐ or urate crystal‐induced production of ROS. Moreover, neutrophils of chronic granulomatous disease patients are shown to lack PMA‐induced MLKL phosphorylation. Genetic deficiency of RIPK3 in mice prevents MSU crystal‐induced NET formation in vitro and in vivo. Thus, neutrophil death and NET formation may involve the signaling pathway defining necroptosis downstream of ROS production. These data imply that RIPK1, RIPK3, and MLKL could represent molecular targets in gout or other crystallopathies.     

Properdin deficiency protects from 5‐fluorouracil‐induced small intestinal mucositis in a complement activation‐independent,interleukin‐10‐dependent mechanism

Intestinal mucositis is a serious complication of chemotherapy that leads to significant morbidity that may require dose or drug adjustments. Specific mitigating strategies for mucositis are unavailable, due partly to an incomplete understanding of the pathogenic mechanisms. We have previously shown an effect of properdin, a positive regulator of complement activation, in models of colitis. Here we use properdin‐deficient (P^KO) mice to interrogate the role of properdin and complement in small intestinal mucositis. Mucositis was induced by five daily injections of 5‐fluorouracil (5‐FU) in wild‐type (WT), P^KO, interleukin (IL)‐10^–/– and properdin/IL‐10^–/– double knock‐out (DKO) mice. At the time of euthanasia their jejunum was collected for histology, immunohistochemistry and cytokine and complement activation measurements. Complement became activated in mice receiving 5‐FU, indicated by increased intestinal levels of C3a and C5a. Compared to WT, P^KO mice experienced significantly less mucositis, despite C3a levels as high as inflamed WT mice and slightly less C5a. Conversely, P^KO mice had higher intestinal levels of IL‐10. IL‐10 expression was mainly by epithelial cells in both uninflamed and inflamed P^KO mice. IL‐10^–/– mice proved to be highly susceptible to mucositis and DKO mice were equally susceptible, demonstrating that a lack of properdin does not protect mice lacking IL‐10. We interpret our findings to indicate that, to a significant extent, the inflammation of mucositis is properdin‐dependent but complement activation‐independent. Additionally, the benefit achieved in the absence of properdin is associated with increased IL‐10 levels, and IL‐10 is important in limiting mucositis.     

Caspase‐4 mediates non‐canonical activation of the NLRP3 inflammasome in human myeloid cells

Inflammasome activation culminates in activation of caspase‐1, which leads to the maturation and subsequent release of cytokines of the interleukin 1 (IL‐1) family and results in a particular form of cell death known as pyroptosis. In addition, in the murine system, a so‐called non‐canonical inflammasome involving caspase‐11 has been described that directly responds to cytosolic LPS. Here, we show that the human monocytic cell line THP1 activates the inflammasome in response to cytosolic LPS in a TLR4‐independent fashion. This response is mediated by caspase‐4 and accompanied by caspase‐1 activation, pyroptosis, and IL‐1β maturation. In addition to caspase‐4, efficient IL‐1β conversion upon intracellular LPS delivery relies on potassium efflux, NLRP3, ASC, and caspase‐1, indicating that although caspase‐4 activation alone is sufficient to induce pyroptosis, this process depends on the NLRP3 inflammasome activation to drive IL‐1β maturation. Altogether, this study provides evidence for the presence of a non‐canonical inflammasome in humans and its dependence on caspase‐4.     

Histones trigger sterile inflammation by activating the NLRP3 inflammasome

Sterile cell death mediated inflammation is linked to several pathological disorders and involves danger recognition of intracellular molecules released by necrotic cells that activate different groups of innate pattern recognition receptors. Toll‐like receptors directly interact with their extrinsic or intrinsic agonists and induce multiple proinflammatory mediators. In contrast, the NLRP3 inflammasome is rather thought to represent a downstream element integrating various indirect stimuli into proteolytic cleavage of interleukin (IL)–1β and IL‐18. Here, we report that histones released from necrotic cells induce IL‐1β secretion in an NLRP3–ASC‐caspase‐1‐dependent manner. Genetic deletion of NLRP3 in mice significantly attenuated histone‐induced IL‐1β production and neutrophil recruitment. Furthermore, necrotic cells induced neutrophil recruitment, which was significantly reduced by histone‐neutralizing antibodies or depleting extracellular histones via enzymatic degradation. These results identify cytosolic uptake of necrotic cell‐derived histones as a triggering mechanism of sterile inflammation, which involves NLRP3 inflammasome activation and IL‐1β secretion via oxidative stress.     

Potassium efflux fires the canon: Potassium efflux as a common trigger for canonical and noncanonical NLRP3 pathways

Murine caspase‐11 and its human orthologues, caspase‐4 and caspase‐5, activate an inflammatory response following cytoplasmic recognition of cell wall constituents from Gram‐negative bacteria, such as LPS. This inflammatory response involves pyroptotic cell death and the concomitant release of IL‐1α, as well as the production of IL‐1β and IL‐18 through the noncanonical NLR family, pyrin domain containing 3 (NLRP3) pathway. This commentary discusses three papers in this issue of the European Journal of Immunology that advance our understanding of the roles of caspase‐11, ‐4, and ‐5 in the noncanonical pathway. By utilizing the new gene editing technique, clustered regularly interspaced short palindromic repeats (CRISPR), as well as sensitive cell imaging techniques, these papers establish that cytoplasmic LPS‐dependent IL‐1β production requires the NLRP3 inflammasome and that its activation is dependent on K⁺ efflux, whereas IL‐1α release and pyroptotic cell death pathways are NLRP3‐independent. These findings expand on previous research implicating K⁺ efflux as the principal trigger for NLRP3 activation and suggest that canonical and noncanonical NLRP3 pathways are not as dissimilar as first thought.     

Interleukin‐17A participates in podocyte injury by inducing IL‐1β secretion through ROS‐NLRP3 inflammasome‐caspase‐1 pathway

Studies show that the Th17/IL ‐17A axis plays an important role in the pathogenesis of kidney diseases. Previously, we also showed that IL ‐17A may play a role in the pathogenesis of primary nephrotic syndrome; however, the underlying mechanism(s) is unclear. The aim of this study was to explore the molecular mechanism of IL ‐17A‐inducing podocyte injury in vitro. In this study, the NLRP 3 inflammasome activation and the morphology of podocytes were detected by Western blot and immunofluorescence. The results showed that podocytes persistently expressed IL ‐17A receptor and that NLRP 3 inflammasome in these cells was activated upon exposure to IL ‐17A. Also, activity of caspase‐1 and secretion of IL ‐1β increased in the presence of IL ‐17A. In addition, IL ‐17A disrupted podocyte morphology by decreasing expression of podocin and increasing expression of desmin. Blockade of intracellular ROS or inhibition of caspase‐1 prevented activation of the NLRP 3 inflammasome, thereby restoring podocyte morphology. Taken together, the results suggest that IL ‐17A induces podocyte injury by activating the NLRP 3 inflammasome and IL ‐1β secretion and contributes to disruption of the kidney's filtration system.