Macrophage Sphingosine 1-Phosphate Receptor 2 Blockade Attenuates Liver Inflammation and Fibrogenesis Triggered by NLRP3 Inflammasome.

Lei Hou,Xinhao Zhao,Xuan Zhou,Chengbin Dong,Na Chang,Fuquan Liu,Liying Li,Le Yang,Lin Yang

doi:10.3389/fimmu.2020.01149

Abstract

NLR family pyrin domain containing 3 (NLRP3) inflammasome accompanies chronic liver injury and is a critical mediator of inflammation-driven liver fibrosis. Sphingosine 1-phosphate (S1P)/S1P Receptor (S1PR) signaling participates in liver fibrogenesis by affecting bone marrow (BM)-derived monocytes/macrophage (BMM) activation. However, the relationship between S1P/S1PR signaling and NLRP3 inflammasome in BMMs remains unclear. Here, we found significantly elevated gene expression of NLRP3 inflammasome components (NLRP3, pro-interleukin-1β, and pro-interleukin-18) and the activation of NLRP3 inflammasome significantly elevated during murine chronic liver injury induced by a bile duct ligation operation, a methionine-choline–deficient and high-fat diet, or carbon tetrachloride intraperitoneal injection. Moreover, the increased expression of sphingosine kinase 1 (SphK1), the rate-limiting synthetic enzyme of S1P, was positively correlated with NLRP3 inflammasome components in both patients and mouse model livers. Flow cytometry analysis and immunofluorescence staining showed BMMs contributed to the significant proportion of NLRP3+ cells in murine inflammatory livers, but not Kupffer cells, dendritic cells, endothelial cells, T cells, and hepatocytes. Focusing on macrophages, S1P promoted NLRP3 inflammasome priming and activation in a dose-dependent manner. Blockade of S1PR2 by JTE-013 (antagonist of S1PR2) or S1PR2-siRNA inhibited S1P-induced NLRP3 inflammasome priming and inflammatory cytokine (interleukin-1β and interleukin-18) secretion, whereas blockade of S1PR1 or S1PR3 had no such effect. in vivo, a β1,3-d-glucan-encapsulated siRNA particle (GeRP) delivery system is capable of silencing genes in macrophages specifically. Treatment with S1PR2 siRNA-GeRPs markedly reduced NLRP3 inflammasome priming and activation and attenuated liver inflammation and fibrosis. Together, the conclusions indicated that targeting macrophage S1PR2 retarded liver inflammation and fibrogenesis via downregulating NLRP3 inflammasome, which may represent an effective therapeutic strategy for chronic liver injury.

Highlights

The inflammasomes are oligomeric complexes formed by innate immune sensors, including NOD-like receptor (NLR) family members NLRP1b, NLR family pyrin domain containing 3 (NLRP3), and NLRC4, as well as other nonNLR receptors, such as absent in melanoma 2 (AIM2) [1]
To examine the role of NLRP3 inflammasome in liver injury of different etiologies, we first detected the changes of NLRP3 inflammasome components (NLRP3, pro-IL-1β, and pro-interleukin 18 (IL18)) expression in three mouse models of liver injury induced sphingosine kinase 1 (Sphk1)
Our data identified that bone marrow-derived monocyte/macrophage (BMM) contributed to a significant proportion of NLRP3+ cells and might act as an important part in NLRP3 inflammasome priming and activation during bile duct ligation (BDL)-induced liver injury

Summary

Introduction

The inflammasomes are oligomeric complexes formed by innate immune sensors, including NOD-like receptor (NLR) family members NLRP1b, NLRP3, and NLRC4, as well as other nonNLR receptors, such as AIM2 [1]. Recent results have shown that the expression of hepatic NLRP3 inflammasome components increases during liver fibrogenesis [2], and NLRP3 expression level is closely correlated with the severity of liver fibrosis [3]. Aberrant activation of NLRP3 inflammasome results in severe liver inflammation with immune cell infiltration, hepatic stellate cell activation, and collagen deposition [4, 5]. Classical activation of the NLRP3 inflammasome requires two independent steps [6]. The first step has been referred to as priming, in which activators, such as lipopolysaccharide (LPS), induce nuclear factor-κB (NFκB) and mitogen-activated protein kinase (MAPK)-dependent expression of NLRP3 inflammasome components, including NLRP3 and pro-interleukin (IL)-1β [6]. Active caspase-1 p10/20 cleaves its target substrates pro-IL-1β and pro-IL-18 into their released mature forms [6]

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Conclusion