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      Regulation of TLR2 and NLRP3 in Primary Murine Renal Tubular Epithelial Cells

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          Abstract

          Pattern recognition receptors (PRRs) are now recognized to be key triggers of injury in a variety of renal diseases. Several families of these receptors are present in the kidney, and recent data suggest that they are differentially expressed and regulated in the kidney. This study evaluated the interaction between two distinct PRRs that are expressed in the kidney, i.e. TLR2 (Toll-like receptor 2) and the NLRP3 inflammasome. The regulation and activation of these receptors in primary renal tubular epithelial (RTE) cells from murine kidneys were evaluated. RTE cells were extracted from WT and NLRP3-mutant mice and treated ex vivo with ligands specific for TLR2 or NLRP3. We found that TLR2 upregulated NLRP3 as well as its substrate IL-1β, and that signaling through the NLRP3 inflammasome induced RTE cell necrosis. The results of this study suggest a previously unknown interaction between TLR2 and NLRP3 in primary RTE cells and highlight the importance of the cross talk that occurs in kidney-related PRRs. Understanding how PRRs are regulated is important for the design of rationale therapeutic strategies to modulate these receptors in renal disease.

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          Most cited references11

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          Necrotic cells trigger a sterile inflammatory response through the Nlrp3 inflammasome.

          Dying cells are capable of activating the innate immune system and inducing a sterile inflammatory response. Here, we show that necrotic cells are sensed by the Nlrp3 inflammasome resulting in the subsequent release of the proinflammatory cytokine IL-1beta. Necrotic cells produced by pressure disruption, hypoxic injury, or complement-mediated damage were capable of activating the Nlrp3 inflammasome. Nlrp3 inflammasome activation was triggered in part through ATP produced by mitochondria released from damaged cells. Neutrophilic influx into the peritoneum in response to necrotic cells in vivo was also markedly diminished in the absence of Nlrp3. Nlrp3-deficiency moreover protected animals against mortality, renal dysfunction, and neutrophil influx in an in vivo renal ischemic acute tubular necrosis model. These findings suggest that the inhibition of Nlrp3 inflammasome activity can diminish the acute inflammation and damage associated with tissue injury.
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            IRAK-1 bypasses priming and directly links TLRs to rapid NLRP3 inflammasome activation.

            Pathogenic infections and tissue injuries trigger the assembly of inflammasomes, cytosolic protein complexes that activate caspase-1, leading to cleavage of pro-IL-1β and pro-IL-18 and to pyroptosis, a proinflammatory cell death program. Although microbial recognition by Toll-like receptors (TLRs) is known to induce the synthesis of the major caspase-1 substrate pro-IL-1β, the role of TLRs has been considered limited to up-regulation of the inflammasome components. During infection with a virulent microbe, TLRs and nucleotide-binding oligomerization domain-like receptors (NLRs) are likely activated simultaneously. To examine the requirements and outcomes of combined activation, we stimulated TLRs and a specific NLR, nucleotide binding and oligomerization, leucine-rich repeat, pyrin domain-containing 3 (NLRP3), simultaneously and discovered that such activation triggers rapid caspase-1 cleavage, leading to secretion of presynthesized inflammatory molecules and pyroptosis. This acute caspase-1 activation is independent of new protein synthesis and depends on the TLR-signaling molecule IL-1 receptor-associated kinase (IRAK-1) and its kinase activity. Importantly, Listeria monocytogenes induces NLRP3-dependent rapid caspase-1 activation and pyroptosis, both of which are compromised in IRAK-1-deficient macrophages. Our results reveal that simultaneous sensing of microbial ligands and virulence factors by TLRs and NLRP3, respectively, leads to a rapid TLR- and IRAK-1-dependent assembly of the NLRP3 inflammasome complex, and that such activation is important for release of alarmins, pyroptosis, and early IFN-γ production by memory CD8 T cells, all of which could be critical for early host defense.
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              Inflammasome-mediated secretion of IL-1β in human monocytes through TLR2 activation; modulation by dietary fatty acids.

              Many studies have shown that TLR4- and TLR2-deficient mice are protected from high-fat diet-induced inflammation and insulin resistance, suggesting that saturated fatty acids derived from the high-fat diet activate TLR-mediated proinflammatory signaling pathways and induce insulin resistance. However, evidence that palmitic acid, the major dietary saturated fatty acid, can directly activate TLR has not been demonstrated. In this article, we present multiple lines of evidence showing that palmitic acid directly activates TLR2, a major TLR expressed on human monocytes, by inducing heterodimerization with TLR1 in an NADPH oxidase-dependent manner. Dimerization of TLR2 with TLR1 was inhibited by the n-3 fatty acid docosahexaenoic acid. Activation of TLR2 by palmitic acid leads to expression of pro-IL-1β that is cleaved by caspase-1, which is constitutively present in monocytes, to release mature IL-1β. Our results reveal mechanistic insight about how palmitic acid activates TLR2, upregulates NALP3 expression, and induces inflammasome-mediated IL-1β production in human monocytes, which can trigger enhanced inflammation in peripheral tissues, and suggest that these processes are dynamically modulated by the types of dietary fat we consume.
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                Author and article information

                Journal
                NEC
                Nephron Clin Pract
                10.1159/issn.1660-2110
                Nephron Clinical Practice
                S. Karger AG
                1660-2110
                2014
                September 2014
                24 September 2014
                : 127
                : 1-4
                : 119-123
                Affiliations
                Division of Nephrology-Hypertension, Department of Medicine, University of California San Diego, San Diego, Calif., USA
                Author notes
                *Dr. Dianne McKay, Division of Nephrology-Hypertension, Department of Medicine, University of California San Diego, 9500 Gilman Drive, San Diego, CA 92093 (USA), E-Mail d1mckay@ucsd.edu
                Article
                363208 PMC4896108 Nephron Clin Pract 2014;127:119-123
                10.1159/000363208
                PMC4896108
                25343834
                8e29aa2f-fac9-43f0-a61b-6153dde86fba
                © 2014 S. Karger AG, Basel

                Copyright: All rights reserved. No part of this publication may be translated into other languages, reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying, recording, microcopying, or by any information storage and retrieval system, without permission in writing from the publisher. Drug Dosage: The authors and the publisher have exerted every effort to ensure that drug selection and dosage set forth in this text are in accord with current recommendations and practice at the time of publication. However, in view of ongoing research, changes in government regulations, and the constant flow of information relating to drug therapy and drug reactions, the reader is urged to check the package insert for each drug for any changes in indications and dosage and for added warnings and precautions. This is particularly important when the recommended agent is a new and/or infrequently employed drug. Disclaimer: The statements, opinions and data contained in this publication are solely those of the individual authors and contributors and not of the publishers and the editor(s). The appearance of advertisements or/and product references in the publication is not a warranty, endorsement, or approval of the products or services advertised or of their effectiveness, quality or safety. The publisher and the editor(s) disclaim responsibility for any injury to persons or property resulting from any ideas, methods, instructions or products referred to in the content or advertisements.

                History
                Page count
                Figures: 1, Pages: 5
                Categories
                Original Paper

                Cardiovascular Medicine,Nephrology
                Pattern recognition receptors,Renal tubular epithelial cells,Regulation

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