NLRP6 suppresses the inflammatory response of human periodontal ligament cells by inhibiting NF‐κB and ERK signal pathways

Abstract

Aim To explore the function and mechanisms of NLRP6 (NOD‐, LRR‐ and pyrin domain‐containing 6) in the inflammatory response of human periodontal ligament cells (HPDLCs). Methodology Tissues associated with apical periodontitis were obtained from three patients who underwent endodontic microsurgery. The expression of NLRP6 in 3 human apical periodontitis tissues and HPDLCs was examined by immunohistochemistry and immunofluorescence, respectively. The expressions of NLRP6, Phospho(p)‐ p65, p65, IκB‐α, p‐ IκB‐α, ERK, p‐ ERK, NLRP3, Pro interleukin (IL)‐1β, Pro caspase‐1 and apoptosis‐associated speck‐like protein containing a CARD (ASC) were examined by western blot. The gene expression and secretion of proinflammatory cytokines were detected using quantitative real‐time polymerase chain reaction and enzyme‐linked immunosorbent assay. Data were analysed statistically with independent sample t‐tests. Results NLRP6 was expressed in inflammatory periapical tissues and HPDLCs. Lipopolysaccharide (LPS) from Escherichia coli induced NLRP6 in HPDLCs (P < 0.05). After silencing NLRP6, E. coli LPS‐induced activation of NF‐κB and ERK signalling was enhanced, which was also accompanied by elevated levels of IL‐6 and tumour necrosis factor‐α (TNF‐α) (P < 0.05). Moreover, knockdown of NLRP6 led to up‐regulation of NLRP3, Pro IL‐1β and Pro caspase‐1 (P < 0.05), whereas down‐regulation of ASC (P < 0.05), which may contribute to unchanged levels of IL‐1β in HPDLCs inflammation. Conclusion NLRP6 was functionally expressed in inflamed periapical tissues and HPDLCs. NLRP6 negatively regulated the production of IL‐6 and TNF‐α in HPDLCs inflammation by inhibiting NF‐κB and ERK signal pathways.

The NLRP6 inflammasome

“…Among others, bacteria of different taxa were shown to regulate NLPR6 inflammasome formation 27‐29 . In line, the microbial ligands lipopolysaccharide (LPS), MDP, iE‐DAP and Pam3CSK4 have been reported to induce NLPR6 activation in human periodontal ligament cells 19 . Additionally, lipoteichoic acid (LTA), a surface‐associated adhesion amphiphile of Gram‐positive bacteria, induces the formation of NLRP6 inflammasome, and interestingly, in this case NLRP6 activates the non‐canonical pathway involving caspase‐11 in vivo 27 .…”

“…Chen et al show that alkali‐burn injury induces NLRP3 and NLRP6 inflammasome formation, whereby NLRP3 is activated by NF‐κB, while NLRP6 dampens NF‐κB activity, demonstrating the opposing roles of these two inflammasomes in this context 30 . In human periodontal ligament cells, NLRP6 inhibits NF‐κB and ERK signalling, thereby dampening the release of IL‐6 and TNF‐α 19 . Additionally, NLRP6 inhibits the activation of NF‐κB signalling pathway and p38 mitogen‐activated protein kinase (MAPK) signalling pathway after allogeneic haematopoietic stem cell transplantation‐induced liver damage in mice.…”

“…The NLRP6 protein is mainly expressed in the lung and liver but is highest at the intestine, 12,18 making the gastrointestinal tract the most widely studied organ for the investigation of the NLRP6 inflammasome in health and disease. Interestingly, in human periodontium and gingiva, NLPR6 seems to also impact on the homeostasis of the oral cavity 19,20 …”

Knockdown of TRIM52 alleviates LPS-induced inflammatory injury in human periodontal ligament cells through the TLR4/NF-κB pathway

“…TLR4, a transmembrane receptor, is responsible for the activation of a wide range of signal pathways involved in the bactericidal response [26]. Accumulating evidence has shown that the NF-κB signaling pathway serves as a crucial role in the progression of periodontitis [27]. Growing evidence indicated that TLR4-mediated NF-κB signaling plays an important role in the progression of periodontitis [28,29].…”

The NLRP6 inflammasome in health and disease

“…In another study of patients undergoing endodontic microsurgery, analysis of tissues associated with apical periodontitis revealed higher expression of NLRP6. 68 In addition, when NLRP6 was silenced in periodontal ligament cells and then stimulated with E. coli-derived LPS, both NF-κB and ERK signaling were found to be augmented as were secretion of IL-6 and TNF-α. This suggests that NLRP6 suppresses the inflammatory response during periodontitis, which is in line with mouse studies where NLRP6 has been reported to negatively regulate NF-κB and ERK pathways.…”