Highlights d Alcohol consumption increases xCT expression in HEPs d xCT-derived glutamate release increases expression of mGluR5 in HSCs d mGluR5 stimulates 2-AG production in HSC to influence lipogenesis of HEPs via CB 1 R d Inhibition of xCT and mGluR5 blocks alcoholic steatosis in liver

The expression of chemokine receptor CX3CR1 is related to migration and signaling in cells of the monocyte-macrophage lineage. The precise roles of CX3CR1 in the liver have been investigated but not clearly elucidated. Here, we investigated the roles of CX3CR1 in hepatic macrophages and liver injury. Hepatic and splenic CX3CR1lowF4/80low monocytes and CX3CR1lowCD16− monocytes were differentiated into CX3CR1highF4/80high or CX3CR1highCD16+ macrophages by co-culture with endothelial cells. Moreover, CX3CL1 deficiency in human umbilical vein endothelial cells (HUVECs) attenuated the expression of interleukin-1β (IL-1β) and tumor necrosis factor-α (TNF-α), whereas recombinant CX3CL1 treatment reversed this expression in co-cultured monocytes. Upon treatment with clodronate liposome, hepatic F4/80high macrophages were successfully depleted at day 2 and recovered similarly in CX3CR1+/GFP and CX3CR1GFP/GFP mice at week 4, suggesting a CX3CR1-independent replacement. However, F4/80high macrophages of CX3CR1+/GFP showed a stronger pro-inflammatory phenotype than CX3CR1GFP/GFP mice. In clodronate-treated chimeric CX3CR1+/GFP and CX3CR1GFP/GFP mice, CX3CR1+F4/80high macrophages showed higher expression of IL-1β and TNF-α than CX3CR1−F4/80high macrophages. In alcoholic liver injury, despite the similar frequency of hepatic F4/80high macrophages, CX3CR1GFP/GFP mice showed reduced liver injury, hepatic fat accumulation, and inflammatory responses than CX3CR1+/GFP mice. Thus, CX3CR1 could be a novel therapeutic target for pro-inflammatory macrophage-mediated liver injury.

Background and Aims The important roles of glutamate and metabotropic glutamate receptor 5 (mGluR5) in HSCs have recently been reported in various liver diseases; however, the mechanism linking the glutamine/glutamate metabolism and mGluR5 in liver fibrosis remains unclear. Here, we report that mGluR5 activation in natural killer (NK) cells attenuates liver fibrosis through increased cytotoxicity and interferon‐γ (IFN‐γ) production in both mice and humans. Approach and Results Following 2‐week injection of carbon tetrachloride (CCl4) or 5‐week methionine‐deficient and choline‐deficient diet, liver fibrosis was more aggravated in mGluR5 knockout mice with significantly decreased frequency of NK cells compared with wild‐type mice. Consistently, NK cell–specific mGluR5 knockout mice had aggravated CCl4‐induced liver fibrosis with decreased production of IFN‐γ. Conversely, in vitro activation of mGluR5 in NK cells significantly increased the expression of anti‐fibrosis‐related genes including Ifng, Prf1 (perforin), and Klrk1 (killer cell lectin like receptor K1) and the production of IFN‐γ through the mitogen‐activated extracellular signal‐regulated kinase/extracellular signal‐related kinase pathway, contributing to the increased cytotoxicity against activated HSCs. However, we found that the uptake of glutamate was increased in activated HSCs, resulting in shortage of extracellular glutamate and reduced stimulation of mGluR5 in NK cells. Consequently, this could enable HSCs to evade NK cell cytotoxicity in advanced liver fibrosis. In vivo, pharmacologic activation of mGluR5 accelerated CCl4‐induced liver fibrosis regression by restoring NK cell cytotoxicity. In humans, mGluR5 activation enhanced the cytotoxicity of NK cells isolated from healthy donors, but not from patients with cirrhosis with significantly reduced mGluR5 expression in NK cells. Conclusions mGluR5 plays important roles in attenuating liver fibrosis by augmenting NK cell cytotoxicity, which could be used as a potential therapeutic target for liver fibrosis.

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