Vascular Adhesion Protein 1 Mediates Gut Microbial Flagellin-Induced Inflammation, Leukocyte Infiltration, and Hepatic Steatosis

Sirpa Jalkanen,Sami Pietilä,Raine Toivonen,Eveliina Munukka,Satu Pekkala,Pentti Huovinen,Anniina Keskitalo,Sanja Vanhatalo,Maija Hollmén,Laura Elo

doi:10.3390/sci3010013

Abstract

Toll-like receptor 5 ligand, flagellin, and vascular adhesion protein 1 (VAP-1) are involved in non-alcoholic fatty liver disease. This study aimed to determine whether VAP-1 mediates flagellin-induced hepatic fat accumulation. The effects of flagellin on adipocyte VAP-1 expression were first studied in vitro. Then, flagellin (100 ng/mouse) or saline was intraperitoneally injected into C57BL/6J (WT) and C57BL/6-Aoc3-/- (VAP-1 KO) mice on a high-fat diet twice a week every 2 weeks for 10 weeks. After that, the effects on inflammation, insulin signaling, and metabolism were studied in liver and adipose tissues. Hepatic fat was quantified histologically and biochemically. Because flagellin challenge increased VAP-1 expression in human adipocytes, we used VAP-1 KO mice to determine whether VAP-1 regulates the inflammatory and metabolic effects of flagellin in vivo. In mice, VAP-1 mediated flagellin-induced inflammation, leukocyte infiltration, and lipolysis in visceral adipose tissue. Consequently, an increased release of glycerol led to hepatic steatosis in WT, but not in KO mice. Flagellin-induced hepatic fibrosis was not mediated by VAP-1. VAP-1 KO mice harbored more inflammation-related microbes than WT mice, while flagellin did not affect the gut microbiota. Our results suggest that by acting on visceral adipose tissue, flagellin increased leukocyte infiltration that induced lipolysis. Further, the released glycerol participated in hepatic fat accumulation. In conclusion, the results describe that gut microbial flagellin through VAP-1 induced hepatic steatosis.

Highlights

Despite the high occurrence of non-alcoholic fatty liver disease (NAFLD), the exact causes of the disease remain largely unknown
Another study reported that hepatic TLR5 prevented the onset of gut-microbiota-induced liver disease [11]. While these results suggest that TLR5 protects from liver diseases, prolonged exposure to circulating FLG has been linked to an increased risk for hepatocellular carcinoma [12], and, at high dosage, FLG was shown to induce liver injury [13]
We have shown that pro-inflammatory vascular adhesion protein 1 (VAP-1) is involved in NAFLD [14]

Summary

Introduction

Despite the high occurrence of non-alcoholic fatty liver disease (NAFLD), the exact causes of the disease remain largely unknown. Recent studies have highlighted the importance of adipose tissue dysfunction and insulin resistance [5], as well as gut-derived signals [6], in the pathogenesis of NAFLD. Our findings indicated that human subjects with high hepatic fat content over-expressed several pathogen-associated molecular-pattern-recognizing toll-like receptor (TLR) signaling. Sci 2021, 3, 13 genes in subcutaneous adipose tissue [7]. Of the ten TLR family members that are present in humans [8], adipose tissue TLR5 associated positively with hepatic fat content and negatively with adipose tissue insulin sensitivity [9]. Our in vitro experiments suggested that by increasing lipolysis in adipocytes, TLR5 ligand flagellin (FLG) induced fat accumulation in hepatocytes [9]. Whether FLG induces hepatic fat accumulation in vivo is currently unknown

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