Tim-4 Inhibits NLRP3 Inflammasome via the LKB1/AMPKα Pathway in Macrophages

Abstract

Nonalcoholic fatty liver disease (NAFLD), characterized by excessive inflammation and lipid deposition, is one of the most common metabolic liver diseases. The expression of NLRP3 inflammasome in macrophages is significantly increased in NAFLD, and its activation aggravates NAFLD greatly. Tim-4, as the phosphatidylserine (PS) receptor, is expressed highly in macrophages, and macrophage Tim-4 inhibits inflammation under various conditions of immune activation. However, the precise role of Tim-4 in NLRP3 inflammasome regulation and NAFLD pathogenesis remains completely unknown. Using NAFLD mice models, we confirmed that the expression of Tim-4 was increased in liver tissues by Western blot, real-time PCR, immunohistochemistry, and immunofluorescence, especially higher expression in liver macrophages, and Tim-4 knockout mice displayed more severe liver inflammation and hepatic steatosis than controls in NAFLD mice model. In vitro, we found that Tim-4 could inhibit NLRP3 inflammasome activation, and the inhibition was dependent on PS binding domain in the IgV domain. Mechanistically, Tim-4 induced the degradation of NLRP3 inflammasome components through activating AMPKa-mediated autophagy. Specifically, Tim-4 promoted AMPKa phosphorylation by interacting with LKB1 and AMPKa. In addition, PS binding motif was responsible for Tim-4-mediated AMPKa and LKB1 interaction. In conclusion, NAFLD microenvironments upregulate Tim-4 expression in macrophages, and elevated Tim-4, in turn, suppresses NLRP3 inflammasome activation by activating LKB1/AMPKa-mediated autophagy, thereby ameliorating the release of IL-1b and IL-18. Collectively, this study unveils the novel function of Tim-4 in suppressing NLRP3 inflammasome, which would shed new lights on intervention of NAFLD or inflammatory liver diseases by targeting Tim-4.

Tim-4 in Health and Disease: Friend or Foe?

“…Furthermore, Tim-4 expression was greatly enhanced by IL-6 or TGF-β, which were highly abundant in tumor microenvironment (19,22,23). Our studies also showed that microenvironment of non-alcohol fatty liver disease increased Tim-4 expression in liver tissues greatly, especially in macrophages (24). These data suggest that Tim-4 is inducible under specific disease conditions.…”

“…Furthermore, we revealed that Tim-4 promoted AMPKα phosphorylation depending on LKB1. Tim-4/LKB1/AMPKα interaction triggered autophagy and inhibited NLRP3 inflammasome activation, which might be responsible for NAFLD progress (24). However, we could not exclude whether Tim-4 was involved in lipid metabolism, a possibility that warrants further investigation.…”

Targeting phosphatidylserine for Cancer therapy: prospects and challenges

“…Moreover, TIM-1 is a binding site for Ebola virus on T lymphocytes and blocking TIM1-PS interactions reduces viral binding, T-cell activation, and cytokine production 106 . TIM-4 is highly expressed on dendritic cells and macrophages, and TIM-4 on macrophages participates in inflammatory responses 107 . TIM-3 is not expressed on naïve T cells, but it is expressed on fully differentiated Th1 cells 108 .…”

Crosstalk Between Liver Macrophages and Surrounding Cells in Nonalcoholic Steatohepatitis

“…KCs were thought to be the most important hepatic efferocytes with the expression of several different PtdSer receptors, such as T cell immunoglobulin, mucin domain-containing molecule 3 (Tim3), Tim4, macrophage c-mer tyrosine kinase (MerTK), stabilin-1, and stabilin-2 (53). Strikingly, both Tim3 and Tim4 were overexpressed in all detected liver macrophage subsets in methionine-and choline-deficient diet (MCD)-induced NASH mice (55,56). Their absence led to increased production of reactive oxygen species (ROS), IL-1β, and IL-18 in macrophages, concomitant with the aggravation of steatosis and liver fibrosis (55,56).…”

“…Strikingly, both Tim3 and Tim4 were overexpressed in all detected liver macrophage subsets in methionine-and choline-deficient diet (MCD)-induced NASH mice (55,56). Their absence led to increased production of reactive oxygen species (ROS), IL-1β, and IL-18 in macrophages, concomitant with the aggravation of steatosis and liver fibrosis (55,56). Further studies are urgently needed to explore that how those PtdSer receptors participate in efferocytosis mechanisms of macrophages during NASH development.…”