Cardiac Fibroblasts Contribute to Myocardial Dysfunction in Mice with Sepsis: The Role of NLRP3 Inflammasome Activation

Myocardial contractile dysfunction in sepsis is associated with the increased morbidity and mortality. Although the underlying mechanisms of the cardiac depression have not been fully elucidated, an exaggerated inflammatory response is believed to be responsible. Nucleotide-binding oligomerization domain-like receptor containing pyrin domain 3 (NLRP3) inflammasome is an intracellular platform that is involved in the maturation and release of interleukin (IL)-1β. The aim of the present study is to evaluate whether sepsis activates NLRP3 inflammasome/caspase-1/IL-1β pathway in cardiac fibroblasts (CFs) and whether this cytokine can subsequently impact the function of cardiomyocytes (cardiac fibroblast-myocyte cross-talk). We show that treatment of CFs with lipopolysaccharide (LPS) induces upregulation of NLRP3, activation of caspase-1, as well as the maturation (activation) and release of IL-1β. In addition, the genetic (small interfering ribonucleic acid [siRNA]) and pharmacological (glyburide) inhibition of the NLRP3 inflammasome in CFs can block this signaling pathway. Furthermore, the inhibition of the NLRP3 inflammasome in cardiac fibroblasts ameliorated the ability of LPS-chalenged CFs to impact cardiomyocyte function as assessed by intracellular cyclic adenosine monophosphate (cAMP) responses in cardiomyocytes. Salient features of this the NLP3 inflammasome/ caspase-1 pathway were confirmed in in vivo models of endotoxemia/sepsis. We found that inhibition of the NLRP3 inflammasome attenuated myocardial dysfunction in mice with LPS and increased the survival rate in mice with feces-induced peritonitis. Our results indicate that the activation of the NLRP3 inflammasome in cardiac fibroblasts is pivotal in the induction of myocardial dysfunction in sepsis.     

Vascular endothelial cells senescence is associated with NOD-like receptor family pyrin domain-containing 3 (NLRP3) inflammasome activation via reactive oxygen species (ROS)/thioredoxin-interacting protein (TXNIP) pathway

Endothelial dysfunction caused by endothelial cells senescence and chronic inflammation is tightly linked to the development of cardiovascular diseases. NLRP3 (NOD-like receptor family pyrin domain-containing3) inflammasome plays a central role in inflammatory response that is associated with diverse inflammatory diseases. This study explores the effects and possible mechanisms of NLRP3 inflammasome in endothelial cells senescence. Results show an increment of pro-inflammatory cytokine interleukin (IL) −1β secretion and caspase-1 activation during the senescence of endothelial cells induced by bleomycin. Moreover, secreted IL-1β promoted endothelial cells senescence through up-regulation of p53/p21 protein expression. NLRP3 inflammasome was found to mediate IL-1β secretion through the production of ROS (reactive oxygen species) during the senescence of endothelial cells. Furthermore, the association of TXNIP (thioredoxin-interacting protein) with NLRP3 induced by ROS promoted NLRP3 inflammasome activation in senescent endothelial cells. In addition, the expressions of NLRP3 inflammasome related genes, ASC (apoptosis associated speck-like protein containing a CARD), TXNIP, cleaved caspase-1 and IL-1β, were also increased in vitro and in vivo studies. These findings indicate that endothelial senescence could be mediated through ROS and NLRP3 inflammasome signaling pathways, suggesting a potential target for the prevention of endothelial senescence-related cardiovascular diseases.     

Differential recognition of HIV-stimulated IL-1β and IL-18 secretion through NLR and NAIP signalling in monocyte-derived macrophages

Macrophages are important drivers of pathogenesis and progression to AIDS in HIV infection. The virus in the later phases of the infection is often predominantly macrophage-tropic and this tropism contributes to a chronic inflammatory and immune activation state that is observed in HIV patients. Pattern recognition receptors of the innate immune system are the key molecules that recognise HIV and mount the inflammatory responses in macrophages. The innate immune response against HIV-1 is potent and elicits caspase-1-dependent pro-inflammatory cytokine production of IL-1β and IL-18. Although, NLRP3 has been reported as an inflammasome sensor dictating this response little is known about the pattern recognition receptors that trigger the “priming” signal for inflammasome activation, the NLRs involved or the HIV components that trigger the response. Using a combination of siRNA knockdowns in monocyte derived macrophages (MDMs) of different TLRs and NLRs as well as chemical inhibition, it was demonstrated that HIV Vpu could trigger inflammasome activation via TLR4/NLRP3 leading to IL-1β/IL-18 secretion. The priming signal is triggered via TLR4, whereas the activation signal is triggered by direct effects on Kv1.3 channels, causing K⁺ efflux. In contrast, HIV gp41 could trigger IL-18 production via NAIP/NLRC4, independently of priming, as a one-step inflammasome activation. NAIP binds directly to the cytoplasmic tail of HIV envelope protein gp41 and represents the first non-bacterial ligand for the NAIP/NLRC4 inflammasome. These divergent pathways represent novel targets to resolve specific inflammatory pathologies associated with HIV-1 infection in macrophages.     

Myxoma virus lacking the pyrin-like protein M013 is sensed in human myeloid cells by both NLRP3 and multiple Toll-like receptors, which independently activate the inflammasome and NF-κB innate response pathways

The myxoma virus (MYXV)-encoded pyrin domain-containing protein M013 coregulates inflammatory responses mediated by both the inflammasome and the NF-κB pathways. Infection of human THP-1 monocytic cells with a MYXV construct deleted for the M013 gene (vMyxM013-KO), but not the parental MYXV, activates both the inflammasome and NF-κB pathways and induces a spectrum of proinflammatory cytokines and chemokines, like interleukin-1β (IL-1β), tumor necrosis factor (TNF), IL-6, and monocyte chemoattractant protein 1. Here, we report that vMyxM013-KO virus-mediated activation of inflammasomes and secretion of IL-1β are dependent on the adaptor protein ASC, caspase-1, and NLRP3 receptor. However, vMyxM013-KO virus-mediated activation of NF-κB signaling, which induces TNF secretion, was independent of ASC, caspase-1, and either the NLRP3 or AIM2 inflammasome receptors. We also report that early synthesis of pro-IL-1β in response to vMyxM013-KO infection is dependent upon the components of the inflammasome complex. Activation of the NLRP3 inflammasome and secretion of IL-1β was also dependent on the release of cathepsin B and production of reactive oxygen species (ROS). By using small interfering RNA screening, we further demonstrated that, among the RIG-I-like receptors (RLRs) and Toll-like receptors (TLRs), only TLR2, TLR6, TLR7, and TLR9 contribute to the NF-κB-dependent secretion of TNF and the inflammasome-dependent secretion of IL-1β in response to vMyxM013-KO virus infection. Additionally, we demonstrate that early triggering of the mitogen-activated protein kinase pathway by vMyxM013-KO virus infection of THP-1 cells plays a critical common upstream role in the coordinate induction of both NF-κB and inflammasome pathways. We conclude that an additional cellular sensor(s)/receptor(s) in addition to the known RLRs/TLRs plays a role in the M013 knockout virus-induced activation of NF-κB pathway signaling, but the activation of inflammasomes entirely depends on sensing by the NLRP3 receptor in response to vMyxM013-KO infection of human myeloid cells.     

Mitochondrial NLRP3 Protein Induces Reactive Oxygen Species to Promote Smad Protein Signaling and Fibrosis Independent from the Inflammasome

Nucleotide-binding domain and leucine-rich repeat containing PYD-3 (NLRP3) is a pattern recognition receptor that is implicated in the pathogenesis of inflammation and chronic diseases. Although much is known regarding the NLRP3 inflammasome that regulates proinflammatory cytokine production in innate immune cells, the role of NLRP3 in non-professional immune cells is unclear. Here we report that NLRP3 is expressed in cardiac fibroblasts and increased during TGFβ stimulation. NLRP3-deficient cardiac fibroblasts displayed impaired differentiation and R-Smad activation in response to TGFβ. Only the central nucleotide binding domain of NLRP3 was required to augment R-Smad signaling because the N-terminal Pyrin or C-terminal leucine-rich repeat domains were dispensable. Interestingly, NLRP3 regulation of myofibroblast differentiation proceeded independently from the inflammasome, IL-1β/IL-18, or caspase 1. Instead, mitochondrially localized NLRP3 potentiated reactive oxygen species to augment R-Smad activation. In vivo, NLRP3-deficient mice were protected against angiotensin II-induced cardiac fibrosis with preserved cardiac architecture and reduced collagen 1. Together, these results support a distinct role for NLRP3 in non-professional immune cells independent from the inflammasome to regulate differential aspects of wound healing and chronic disease.     

The SGK1 inhibitor EMD638683, prevents Angiotensin II–induced cardiac inflammation and fibrosis by blocking NLRP3 inflammasome activation

Inflammation has emerged as a critical biological process contributing to hypertensive cardiac remodeling. Effective pharmacological treatments targeting the cardiac inflammatory response, however, are still lacking. Prior studies suggested that the serum- and glucocorticoid-inducible kinase (SGK1) plays a key role in inflammation and cardiac remodeling. Recently, a highly selective SGK1 inhibitor, EMD638683, was developed, though whether EMD638683 can prevent hypertension-induced cardiac fibrosis and the mechanisms by which this inhibitor may alter the disease process remain unknown. Using a murine Angiotension II (Ang II) infusion-induced hypertension model we found that EMD638683 treatment inhibited cardiac fibrosis and remodeling, with significant abatement of cardiac inflammation. EMD638683 was shown to suppress Ang II infusion-induced interleukin (IL)-1β release, and substantially reduce nucleotide-binding oligomerization domain-like receptor with pyrin domain 3 (NLRP3) expression and caspase-1 activation in cardiac tissues. In vitro experiments revealed that EMD638683 ameliorated Ang II-stimulated IL-1β secretion in macrophages by blocking NLRP3 inflammasome activation. By reducing IL-1β production in macrophages, the transformation of fibroblasts to myofibroblasts was inhibited. The effects of EMD638683 on cardiac fibrosis were abolished by supplementation with exogenous IL-1β. Administration of the NLRP3 inflammasome inhibitor MCC950 indicated that EMD638683 attenuated Ang II-induced cardiac inflammation and fibrosis by inhibiting the NLRP3 inflammasome/IL-1β secretion axis. These findings indicate that the SGK1 inhibitor EMD638683 can negatively regulate NLRP3 inflammasome activation, and may represent a promising approach to the treatment of hypertensive cardiac damage.     

Suppression of ribosomal function triggers innate immune signaling through activation of the NLRP3 inflammasome

Some inflammatory stimuli trigger activation of the NLRP3 inflammasome by inducing efflux of cellular potassium. Loss of cellular potassium is known to potently suppress protein synthesis, leading us to test whether the inhibition of protein synthesis itself serves as an activating signal for the NLRP3 inflammasome. Murine bone marrow-derived macrophages, either primed by LPS or unprimed, were exposed to a panel of inhibitors of ribosomal function: ricin, cycloheximide, puromycin, pactamycin, and anisomycin. Macrophages were also exposed to nigericin, ATP, monosodium urate (MSU), and poly I:C. Synthesis of pro-IL-? and release of IL-1? from cells in response to these agents was detected by immunoblotting and ELISA. Release of intracellular potassium was measured by mass spectrometry. Inhibition of translation by each of the tested translation inhibitors led to processing of IL-1?, which was released from cells. Processing and release of IL-1? was reduced or absent from cells deficient in NLRP3, ASC, or caspase-1, demonstrating the role of the NLRP3 inflammasome. Despite the inability of these inhibitors to trigger efflux of intracellular potassium, the addition of high extracellular potassium suppressed activation of the NLRP3 inflammasome. MSU and double-stranded RNA, which are known to activate the NLRP3 inflammasome, also substantially inhibited protein translation, supporting a close association between inhibition of translation and inflammasome activation. These data demonstrate that translational inhibition itself constitutes a heretofore-unrecognized mechanism underlying IL-1? dependent inflammatory signaling and that other physical, chemical, or pathogen-associated agents that impair translation may lead to IL-1?-dependent inflammation through activation of the NLRP3 inflammasome. For agents that inhibit translation through decreased cellular potassium, the application of high extracellular potassium restores protein translation and suppresses activation of the NLRP inflammasome. For agents that inhibit translation through mechanisms that do not involve loss of potassium, high extracellular potassium suppresses IL-1? processing through a mechanism that remains undefined.     

The NLRP3 inflammasome: activation and regulation

The NOD-, LRR- and pyrin domain-containing protein 3 (NLRP3) inflammasome is a cytoplasmic supramolecular complex that is activated in response to cellular perturbations triggered by infection and sterile injury. Assembly of the NLRP3 inflammasome leads to activation of caspase-1, which induces the maturation and release of interleukin-1β (IL-1β) and IL-18, as well as cleavage of gasdermin D (GSDMD), which promotes a lytic form of cell death. Production of IL-1β via NLRP3 can contribute to the pathogenesis of inflammatory disease, whereas aberrant IL-1β secretion through inherited NLRP3 mutations causes autoinflammatory disorders. In this review, we discuss recent developments in the structure of the NLRP3 inflammasome, and the cellular processes and signaling events controlling its assembly and activation.     

Tripartite-motif protein 30 negatively regulates NLRP3 inflammasome activation by modulating reactive oxygen species production

The NLR family, pyrin domain-containing 3 (NLRP3) inflammasome is critical for caspase-1 activation and the proteolytic processing of pro-IL-1β. However, the mechanism that regulates NLRP3 inflammasome activation remains unclear. In this paper, we demonstrate that tripartite-motif protein 30 (TRIM30) negatively regulates NLRP3 inflammasome activation. After stimulation with ATP, an agonist of the NLRP3 inflammasome, knockdown of TRIM30 enhanced caspase-1 activation and increased production of IL-1β in both J774 cells and bone marrow-derived macrophages. Similarly with ATP, knockdown of TRIM30 increased caspase-1 activation and IL-1β production triggered by other NLRP3 inflammasome agonists, including nigericin, monosodium urate, and silica. Production of reactive oxygen species was increased in TRIM30 knockdown cells, and its increase was required for enhanced NLRP3 inflammasome activation, because antioxidant treatment blocked excess IL-1β production. Conversely, overexpression of TRIM30 attenuated reactive oxygen species production and NLRP3 inflammasome activation. Finally, in a crystal-induced NLRP3 inflammasome-dependent peritonitis model, monosodium urate-induced neutrophil flux and IL-1β production was reduced significantly in TRIM30 transgenic mice as compared with that in their nontransgenic littermates. Taken together, our results indicate that TRIM30 is a negative regulator of NLRP3 inflammasome activation and provide insights into the role of TRIM30 in maintaining inflammatory responses.     

Acinetobacter baumannii outer membrane protein 34 elicits NLRP3 inflammasome activation via mitochondria-derived reactive oxygen species in RAW264.7 macrophages

Acinetobacter baumannii (A. baumannii) is a Gram-negative bacterium, which acts as an opportunistic pathogen and causes hospital-acquired pneumonia and bacteremia by infecting the alveoli of epithelial cells and macrophages. Evidence reveals that A. baumannii outer membrane protein 34 (Omp34) elicits cellular immune responses and inflammation. The innate immunity NOD-like receptor 3 (NLRP3) inflammasome exerts critical function against pneumonia caused by A. baumannii infection, however, the role of Omp34 in the activation of the NLRP3 inflammasome and its corresponding regulatory mechanism are not clearly elucidated. The present study aimed to investigate whether Omp34 elicited NLRP3 inflammasome activation through the mitochondria-derived reactive oxygen species (ROS). Our results showed that Omp34 triggered cell pyroptosis by up-regulating the expression of NLRP3 inflammasome-associated proteins and IL-1β release in a time- and dose-dependent manner. Omp34 induced the expression of caspase-1-p10 and IL-1β, which was significantly attenuated by NLRP3 gene silencing in RAW264.7 mouse macrophage cells. Additionally, Omp34 stimulated RAW264.7 mitochondria to generate ROS, while the ROS scavenger Mito-TEMPO inhibited the Omp34-triggered expression of NLRP3 inflammasome-associated proteins and IL-1β synthesis. The above findings indicate that mitochondria-derived ROS play an important role in the process of NLRP3 inflammasome activation. In summary, our study demonstrates that the A. baumannii pathogen pattern recognition receptor Omp34 activates NLRP3 inflammasome via mitochondria-derived ROS in RAW264.7 cells. Accordingly, down-regulating the mitochondria-derived ROS prevents the severe infection consequences caused by A. baumannii-induced NLRP3 inflammasome hyper-activation.     

Purple sweet potato color attenuated NLRP3 inflammasome by inducing autophagy to delay endothelial senescence

Autophagy is a vital negative factor regulating cellular senescence. Purple sweet potato color (PSPC), one type of flavonoid, has been demonstrated to suppress endothelial senescence and restore endothelial function in diabetic mice by inhibiting the nucleotide-binding oligomerization domain, leucine rich repeat and pyrin domain containing protein 3 (NLRP3) inflammasome. However, the roles of autophagy in the inflammatory response during endothelial senescence are unknown. Here, we found that PSPC augmented autophagy to restrict high-glucose-induced premature endothelial senescence. In addition, PSPC administration impaired endothelium aging in diabetic mice by increasing autophagy. Inhibition of autophagy accelerated endothelial senescence, while enhancement of autophagy delayed senescence. Moreover, deactivation of the NLRP3 inflammasome triggered by PSPC was autophagy-dependent. Autophagy receptor microtubule-associated protein 1 light chain 3 and p62 interacted with the inflammasome component NLRP3, suggesting that autophagosomes target the NLRP3 inflammasome and deliver it to the lysosome for degradation. Altogether, PSPC amplified cellular autophagy, subsequently attenuated NLRP3 inflammasome activity and finally delayed endothelial senescence to ameliorate cardiovascular complication. These results suggest a potential therapeutic target in senescence-related cardiovascular diseases.     

Toll-Like Receptor 9 Mediated Responses in Cardiac Fibroblasts

Altered cardiac Toll-like receptor 9 (TLR9) signaling is important in several experimental cardiovascular disorders. These studies have predominantly focused on cardiac myocytes or the heart as a whole. Cardiac fibroblasts have recently been attributed increasing significance in mediating inflammatory signaling. However, putative TLR9-signaling through cardiac fibroblasts remains non-investigated. Thus, our aim was to explore TLR9-signaling in cardiac fibroblasts and investigate the consequence of such receptor activity on classical cardiac fibroblast cellular functions. Cultivated murine cardiac fibroblasts were stimulated with different TLR9 agonists (CpG A, B and C) and assayed for the secretion of inflammatory cytokines (tumor necrosis factor α [TNFα], CXCL2 and interferon α/β). Expression of functional cardiac fibroblast TLR9 was proven as stimulation with CpG B and –C caused significant CXCL2 and TNFα-release. These responses were TLR9-specific as complete inhibition of receptor-stimulated responses was achieved by co-treatment with a TLR9-antagonist (ODN 2088) or chloroquine diphosphate. TLR9-stimulated responses were also found more potent in cardiac fibroblasts when compared with classical innate immune cells. Stimulation of cardiac fibroblasts TLR9 was also found to attenuate migration and proliferation, but did not influence myofibroblast differentiation in vitro. Finally, results from in vivo TLR9-stimulation with subsequent fractionation of specific cardiac cell-types (cardiac myocytes, CD45+ cells, CD31+ cells and cardiac fibroblast-enriched cell-fractions) corroborated our in vitro data and provided evidence of differentiated cell-specific cardiac responses. Thus, we conclude that cardiac fibroblast may constitute a significant TLR9 responder cell within the myocardium and, further, that such receptor activity may impact important cardiac fibroblast cellular functions.     

Resolvin E1 attenuates doxorubicin-induced cardiac fibroblast senescence: A key role for IL-1β

Cardiac fibroblasts (CFs) undergo senescence in reaction to different stressors, leading to a poor prognosis of cardiac disease. Doxorubicin (Doxo) is an antineoplastic drug with strong cardiotoxic effects, which induces IL-1β secretion and thus, triggers a potent pro-inflammatory response. Doxo induces CFs senescence; however, the mechanisms are not fully understood. Different pharmacological strategies have been used to eliminate senescent cells by inducing their apoptosis or modifying their secretome. However, Resolvin E1 (RvE1), a lipid derivative resolutive mediator with potent anti-inflammatory effects has not been used before to prevent CFs senescence.CFs were isolated from adult male C57BL/6J mice and subsequently stimulated with Doxo, in the presence or absence of RvE1. Senescence-associated β-galactosidase activity (SA-β-gal), γ-H2A.X, p53, p21, and senescence-associated secretory phenotype (SASP) were evaluated. The involvement of the NLRP3 inflammasome/interleukin-1 receptor (IL-1R) signaling pathway on CFs senescence was studied using an NLRP3 inhibitor (MCC950) and an endogenous IL-1R antagonist (IR1A).Doxo is able to trigger CFs senescence, as evidenced by an increase of γ-H2A.X, p53, p21, and SA-β-gal, and changes in the SASP profile. These Doxo effects were prevented by RvE1. Doxo triggers IL-1β secretion, which was dependent on NLRP3 activation. Doxo-induced CFs senescence was partially blocked by MCC950 and IR1A. In addition, IL-1β also triggered CFs senescence, as evidenced by the increase of γ-H2A.X, p53, p21, SA-β-gal activity, and SASP. All these effects were also prevented by RvE1 treatment.ConclusionThese data show the anti-senescent role of RvE1 in Doxo-induced CFs senescence, which could be mediated by reducing IL-1β secretion.