NLRP3 inflammasome in endothelial dysfunction

Abstract

Inflammasomes are a class of cytosolic protein complexes. They act as cytosolic innate immune signal receptors to sense pathogens and initiate inflammatory responses under physiological and pathological conditions. The NLR-family pyrin domain-containing protein 3 (NLRP3) inflammasome is the most characteristic multimeric protein complex. Its activation triggers the cleavage of pro-interleukin (IL)-1β and pro-IL-18, which are mediated by caspase-1, and secretes mature forms of these mediators from cells to promote the further inflammatory process and oxidative stress. Simultaneously, cells undergo pro-inflammatory programmed cell death, termed pyroptosis. The danger signals for activating NLRP3 inflammasome are very extensive, especially reactive oxygen species (ROS), which act as an intermediate trigger to activate NLRP3 inflammasome, exacerbating subsequent inflammatory cascades and cell damage. Vascular endothelium at the site of inflammation is actively involved in the regulation of inflammation progression with important implications for cardiovascular homeostasis as a dynamically adaptable interface. Endothelial dysfunction is a hallmark and predictor for cardiovascular ailments or adverse cardiovascular events, such as coronary artery disease, diabetes mellitus, hypertension, and hypercholesterolemia. The loss of proper endothelial function may lead to tissue swelling, chronic inflammation, and the formation of thrombi. As such, elimination of endothelial cell inflammation or activation is of clinical relevance. In this review, we provided a comprehensive perspective on the pivotal role of NLRP3 inflammasome activation in aggravating oxidative stress and endothelial dysfunction and the possible underlying mechanisms. Furthermore, we highlighted the contribution of noncoding RNAs to NLRP3 inflammasome activation-associated endothelial dysfunction, and outlined potential clinical drugs targeting NLRP3 inflammasome involved in endothelial dysfunction. Collectively, this summary provides recent developments and perspectives on how NLRP3 inflammasome interferes with endothelial dysfunction and the potential research value of NLRP3 inflammasome as a potential mediator of endothelial dysfunction.

Fig. 1. Domain structure of representative inflammasome.

Inflammasome is a protein complex formed by the aggregation of inflammasome sensor, adaptor protein ASC, and effector protein caspase-1. PYD pyrin domain, NBD nucleotide-binding domain, LRR leucine-rich-repeat domain, FIIND function-to-find domain, CARD caspase activation and recruitment domain, C–C coiled-coil domain, B B-box domain, BIR baculovirus inhibitor of apoptosis repeat, ASC apoptosis-associated speck-like protein containing a CARD, CASP1 caspase-1.

Fig. 2. The canonical inflammasome activation pathway occurs by sensing diverse pathogen-associated molecular patterns (PAMPs) and damage-associated molecular patterns (DAMPs).

a NLRP3, NLRP6, AIM2, and pyrin, containing a PYD, binds to the PYD of ASC, allowing the ASC to activate caspase-1 by interacting with the CARD of pro-CASP1. Activated caspase-1 triggers the cleavage of pro-interleukin (IL)-1β, pro-IL-18, and gasdermin D (GSDMD), and then releases the N-terminal domain of GSDMD to induce pyroptosis, followed by the release of IL-1β and IL-18. b NLRP1b and NLRC4, containing a CARD, activate caspase-1 by directly binding the CARD of pro-caspase-1 without ASC or binding the paired ASC scaffold. The presence of ASC can enhance the assembly of the sensor protein containing a CARD and the activation of caspase-1. The molecular process after caspase-1 activation is the same as part A.

Fig. 3. Reactive oxygen species (ROS) contribute to the NLRP3 inflammasome activation in endothelial cells.

A wide range of pathogen-associated molecular patterns (PAMPs) or damage-associated molecular patterns (DAMPs) trigger NLRP3 inflammasome activation by inducing potassium (K⁺) efflux, calcium (Ca²⁺) influx, lysosomal leakage, mitochondrial dysfunction, and ROS production. ROS mainly derived from endoplasmic reticulum (ER) stress, damaged mitochondria, and NADPH oxidase. ER stress could activate the NF-κB, TXNIP, and SREBP signaling pathways, Ca²⁺ release, and ROS production. Moreover, athero-prone flow can also mediate SREBP signaling pathways. The release of mtROS and mtDNA in damaged mitochondria might activate NLRP3 inflammasome. Furthermore, mitochondrial antiviral signaling protein (MAVS) is capable of molulating the recruitment and localization of NLRP3. However, NF-E2-related factor 2 (Nrf2) activated under ROS-induced stress conditions can inhibit NLRP3 inflammasome activation. Taken together, ROS is an intermediate factor involved in multiple signaling pathways and can trigger the activation of NLRP3 inflammasome.

Fig. 4. The role of NLRP3 inflammasome activation in endothelial dysfunction.

a The secretion of mature forms of IL-1β, IL-18, and HMGB1 is the result of activation of NLRP3 inflammasome. These mediators possess properties of pro-inflammatory activation. Therefore, NLRP3 inflammasome can induce a potent inflammatory response, oxidative stress, and pro-inflammatory cell death called pyroptosis. b Inflammation and oxidative stress can induce endothelium DNA damage, activate NF-κB signaling pathway, increase P53/P21/P16 transcription, and inhibit autophagy, which may promote the process of endothelial cell senescence. Senescent cells can secrete senescence-associated secretory phenotype (SASP), including pro-inflammatory mediators, which may promote endothelial dysfunction and eventually lead to vascular sclerosis. c IL-1β binding to its cell surface receptor IL-1 receptor 1 (IL-1R1) recruits IL-1 receptor accessory protein (IL-1RacP) to activate intracellular signaling molecules, including myeloid differentiation factor 88 (MyD88), IL-1 receptor-associated kinase 1/4 (IRAK1/4), and TNF receptor-associated factor (TRAF), which then causes NF-κB activation. IL-18 binding to its cell surface receptor IL-18 receptor α chain (IL-18Rα) recruits IL-18 receptor β chain (IL-18Rβ) to activate similar intracellular signaling molecules. HMGB1 can also activate the NF-κB signaling pathway, which is downstream of toll-like receptor (TLR)2/4 activation. The activation of NF-κB signaling pathway increases the secretion of pro-inflammatory mediators such as cytokines and chemokines to mediate the adhesion of leukocyte and promote leukocyte extravasation. Additionally, the binding of HMGB1 to the RAGE receptor leads to the activation of downstream p38 MAP kinase, resulting in phosphorylation of the actin-binding protein Hsp27 and caldesmon, which causes actin stress fibers to form, cytoskeletal remodeling, and endothelial contraction. All of these reactions increase endothelial permeability by altering cell contractility and disrupting intercellular connections.

Fig. 5. The involvement of noncoding RNAs (ncRNAs) in regulating NLRP3 inflammasome-mediated endothelial dysfunction and diseases.

Schematic representation of ncRNAs upregulated or downregulated in multiple endothelial dysfunction-related disease models. These ncRNAs mediate endothelial dysfunction by directly or indirectly activating the NLRP3 inflammasome signaling pathway. The reported diseases mainly include myocardial ischemia/reperfusion (I/R) injury, sepsis-induced brain injury, type 2 diabetes mellitus (T2MD), diabetic retinopathy, and atherosclerosis. Moreover, miR-20b can be downregulated in H₂O₂-treated endothelial cells (ECs) and induces EC senescence by activating NLRP3 inflammasome. In addition, miR-101-3p is downregulated in serum deprivation-induced EC apoptosis, which is partially induced by activating the NLRP3 inflammasome signaling pathway. In this figure, the red upward arrow indicates upward adjustment, and the black downward arrow indicates downward adjustment.

Fig. 6. Drugs targeting NLRP3 inflammasome signaling pathway and their targets in endothelial dysfunction-related diseases.

These drugs, including statins, hypoglycemic agents, and other anti-inflammatory or antioxidant drugs, ameliorate endothelial dysfunction by inhibiting NLRP3 inflammasome signaling pathway. The drugs on the left of the figure have been proved to inhibit NLRP3 inflammasome activation by mediating different targets, while the targets of the drug on the right of the figure still need further study to verify.