Mechanistic Study On The Role Of TM6SF2 In Lipid Metabolism And Non-alcoholic Fatty Liver Disease

Non-alcoholic fatty liver disease(NAFLD)is a metabolic disorder characterized by excess lipid accumulation in the liver without significant consumption of alcohol.It covers a wide degree of liver damage,including simple fat deposition(steatosis),inflammation(non-alcoholic steatohepatitis),and irreversible scarring(fibrosis and cirrhosis).NAFLD is highly prevalent affecting 20% to 30% adults around the world.Although obesity,metabolic syndrome and insulin resistance are important risk factors of NAFLD,results from,familial,twin and epidemiological studies show that genetic variations may also predispose NAFLD susceptibility.Genome-wide association studies have identified several genetic polymorphisms associated with NAFLD,including non-synonymous single nucleotide polymorphisms in PNPLA3 and transmembrane 6 superfamily member 2(TM6SF2).TM6SF2 encodes a multi-transmembrane protein with 351 amino acids,and TM6SF2(E167K)missense variant strongly associates with NAFLD in humans.In the terms of subcellular localization,TM6SF2 is localized in the endoplasmic reticulum(ER),the ER-Golgi intermediate compartment,and Golgi of liver cells and enterocytes.The E167 K mutation destabilizes TM6SF2,resulting in hepatic lipid accumulation and low serum lipid levels.However,the molecular mechanism of TM6SF2 in lipid metabolism remains unclear.By using tandem affinity purification in combination with mass spectrometry,we found that apolipoprotein(APOB),ER lipid raft protein(ERLIN)1 and 2 were TM6SF2-interacting proteins.ERLINs and TM6SF2 mutually stabilized each other.TM6SF2 bound and stabilized APOB via two luminal loops.ERLINs did not interact with APOB directly but still increased APOB stability by stabilizing TM6SF2.This APOB stabilization was hampered by the low protein levels of TM6SF2(E167K)mutant.In mice,knockout of Tm6sf2 and knockdown of Tm6sf2 or Erlins decreased hepatic APOB protein level,causing lipid accumulation in the liver and reducing lipid levels in the serum.We conclude that defective APOB stabilization,as a result regardless of ERLINs or TM6SF2 deficiency or E167 K mutation,is a key factor contributing to NAFLD.