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BACKGROUND: Naturally occurring variation in Membrane-bound O-acyltransferase domain-containing 7 (MBOAT7), encoding for an enzyme involved in phosphatidylinositol acyl-chain remodelling, has been associated with fatty liver and hepatic disorders. Here, we examined the relationship between hepatic Mboat7 down-regulation and fat accumulation. METHODS: Hepatic MBOAT7 expression was surveyed in 119 obese individuals and in experimental models. MBOAT7 was acutely silenced by antisense oligonucleotides in C57Bl/6 mice, and by CRISPR/Cas9 in HepG2 hepatocytes. FINDINGS: In obese individuals, hepatic MBOAT7 mRNA decreased from normal liver to steatohepatitis, independently of diabetes, inflammation and MBOAT7 genotype. Hepatic MBOAT7 levels were reduced in murine models of fatty liver, and by hyper-insulinemia. In wild-type mice, Mboat7 was down-regulated by refeeding and insulin, concomitantly with insulin signalling activation. Acute hepatic Mboat7 silencing promoted hepatic steatosis in vivo and enhanced expression of fatty acid transporter Fatp1. MBOAT7 deletion in hepatocytes reduced the incorporation of arachidonic acid into phosphatidylinositol, consistently with decreased enzymatic activity, determining the accumulation of saturated triglycerides, enhanced lipogenesis and FATP1 expression, while FATP1 deletion rescued the phenotype. INTERPRETATION: MBOAT7 down-regulation by hyper-insulinemia contributes to hepatic fat accumulation, impairing phosphatidylinositol remodelling and up-regulating FATP1. FUNDING: LV was supported by MyFirst Grant AIRC n.16888, Ricerca Finalizzata Ministero della Salute RF-2016–02,364,358, Ricerca corrente Fondazione IRCCS Ca' Granda Ospedale Maggiore Policlinico; LV and AG received funding from the European Union Programme Horizon 2020 (No. 777,377) for the project LITMUS-"Liver Investigation: Testing Marker Utility in Steatohepatitis". MM was supported by Fondazione Italiana per lo Studio del Fegato (AISF) 'Mario Coppo' fellowship.

Funding Information: This study was supported by Academy of Finland Grant 309263 (H.Y.-J.) and Grant 138006 (J.P.), EU H2020 project ?Elucidating Pathways of Steatohepatitis? EPoS Grant 634413 (H.Y.-J.), H2020-JTI-IMI2 EU project 777377-2 Liver Investigation: Testing Marker Utility in Steatohepatitis (LITMUS) (H.Y.-J.), Government Funding (H.Y.-J.), Novo Nordisk Foundation (H.Y.-J., P.K.L., M.A.-K.), Ralph Gr?sbeck Scholarship of the Minerva Foundation (P.K.L.), Novo Nordisk Foundation (P.K.L.), Juho Vainio Foundation (J.P.), Finnish Medical Foundation (V.M.), British Heart Foundation Senior Research Fellowship in Basic Science (FS/15/56/31645) (L.H.) and Kuopio University Hospital Project grant (J.P., EVO/VTR grants 2005-2019). Funding Information: This study was supported by Academy of Finland Grant 309263 (H.Y.-J.) and Grant 138006 (J.P.), EU H2020 project 'Elucidating Pathways of Steatohepatitis' EPoS Grant 634413 (H.Y.-J.), H2020-JTI-IMI2 EU project 777377-2 Liver Investigation: Testing Marker Utility in Steatohepatitis (LITMUS) (H.Y.-J.), Government Funding (H.Y.-J.), Novo Nordisk Foundation (H.Y.-J., P.K.L., M.A.-K.), Ralph Gräsbeck Scholarship of the Minerva Foundation (P.K.L.), Novo Nordisk Foundation (P.K.L.), Juho Vainio Foundation (J.P.), Finnish Medical Foundation (V.M.), British Heart Foundation Senior Research Fellowship in Basic Science (FS/15/56/31645) (L.H.) and Kuopio University Hospital Project grant (J.P., EVO/VTR grants 2005-2019). Publisher Copyright: © 2021 The Author(s) ; Background & Aims: There is substantial inter-individual variability in the risk of non-alcoholic fatty liver disease (NAFLD). Part of which is explained by insulin resistance (IR) ('MetComp') and part by common modifiers of genetic risk ('GenComp'). We examined how IR on the one hand and genetic risk on the other contribute to the pathogenesis of NAFLD. Methods: We studied 846 individuals: 492 were obese patients with liver histology and 354 were individuals who underwent intrahepatic triglyceride measurement by proton magnetic resonance spectroscopy. A genetic risk score was calculated using the number of risk alleles in PNPLA3, TM6SF2, MBOAT7, HSD17B13 and MARC1. Substrate concentrations were assessed by serum NMR metabolomics. In subsets of participants, non-esterified fatty acids (NEFAs) and their flux were assessed by D5-glycerol and hyperinsulinemic-euglycemic clamp (n = 41), and hepatic de novo lipogenesis (DNL) was measured by D2O (n = 61). Results: We found that substrate surplus (increased concentrations of 28 serum metabolites including glucose, glycolytic intermediates, and amino acids; increased NEFAs and their flux; increased DNL) characterized the 'MetComp'. In contrast, the 'GenComp' was not accompanied by any substrate excess but was characterized by an increased hepatic mitochondrial redox state, as determined by serum β-hydroxybutyrate/acetoacetate ratio, and inhibition of hepatic pathways dependent on tricarboxylic acid cycle activity, such as DNL. Serum β-hydroxybutyrate/acetoacetate ratio correlated strongly with all histological features of NAFLD. IR and hepatic mitochondrial redox state conferred additive increases in histological features of NAFLD. Conclusions: These data show that the mechanisms underlying 'Metabolic' and 'Genetic' components of NAFLD are fundamentally different. These findings may have implications with respect to the diagnosis and treatment of NAFLD. Lay summary: The pathogenesis of non-alcoholic fatty liver disease can be explained in part by a metabolic component, including obesity, and in part by a genetic component. Herein, we demonstrate that the mechanisms underlying these components are fundamentally different: the metabolic component is characterized by hepatic oversupply of substrates, such as sugars, lipids and amino acids. In contrast, the genetic component is characterized by impaired hepatic mitochondrial function, making the liver less able to metabolize these substrates. ; Peer reviewed