Mechanisms that lead to the regulation of NLRP3 inflammasome expression and activation in human dental pulp fibroblasts

Background: The NLRP3 inflammasome plays an important role in the cellular defense against invading pathogens and is reported to be expressed in human dental pulp fibroblasts (HDPFs). However, the role of the NLRP3 inflammasome in HDPFs during pulpal infection and inflammation remains unclear. Objectives: To elucidate the function of the NLRP3 inflammasome and the mechanisms that lead to its expression and activation in HDPFs. Methods: The test model used lipopolysaccharide (LPS) and adenosine triphosphate (ATP) to simulate an inflammatory environment. Lentiviral vectors encoding short hairpin RNAs were used to knock down NLRP3 and caspase-1 in HDPFs. Specific inhibitors were used to determine whether the toll-like receptor 4 (TLR4), myeloid differentiating factor 88 (MyD88), or nuclear factor-kappa B (NF-kappa B) pathways were involved in the regulation of NLRP3 expression. Reactive oxygen species (ROS) production was measured by fluorescent microscopy and flow cytometry using the total ROS/superoxide detection kit. Gene and protein expression were quantified by real-time polymerase chain reaction and Western blot, while cytokine release was measured by an enzyme-linked immunosorbent assay. Results: LPS up-regulated NLRP3 and IL-1 beta expression while ATP induced the activation of caspase-1 and the release of IL-1 beta in LPS-primed HDPFs. The knockdown of NLRP3 or caspase-1 expression significantly inhibited 1L-1 beta secretion. Pretreatment with a TLR4 inhibitor, a MyD88 inhibitory peptide, or an I Kappa B alpha (1 kappa B alpha) phosphorylation inhibitor significantly inhibited LPS-induced NLRP3 and IL-1 beta expression. ATP potently promoted ROS generation in HDPFs; N-acetyl cysteine inhibited ROS production, caspase-1 activation and IL-1 beta secretion induced by ATP. Conclusions: Our results demonstrated that the NLRP3 inflammasome in HDPFs is crucial for IL-1 beta secretion in response to LPS plus ATP. LPS engaged the TLR4/MyD88/NF-kappa B pathway to enhance NLRP3 and pro-IL-1 beta expression in HDPFs. ATP promoted the generation of ROS and activated the NLRP3 inflammasome in a ROS-dependent manner. (C) 2015 The Authors. Published by Elsevier Ltd.