LPIAT1/MBOAT7 depletion increases triglyceride synthesis fueled by high phosphatidylinositol turnover

Abstract

ObjectiveNon-alcoholic fatty liver disease (NAFLD) is a common prelude to cirrhosis and hepatocellular carcinoma. The genetic rs641738 C>T variant in the lysophosphatidylinositol acyltransferase 1 (LPIAT1)/membrane bound O-acyltransferase domain-containing 7, which incorporates arachidonic acid into phosphatidylinositol (PI), is associated with the entire spectrum of NAFLD. In this study, we investigated the mechanism underlying this association in mice and cultured human hepatocytes.DesignWe generated the hepatocyte-specific Lpiat1 knockout mice to investigate the function of Lpiat1 in vivo. We also depleted LPIAT1 in cultured human hepatic cells using CRISPR-Cas9 systems or siRNA. The effect of LPIAT1-depletion on liver fibrosis was examined in mice fed high fat diet and in liver spheroids. Lipid species were measured using liquid chromatography-electrospray ionisation mass spectrometry. Lipid metabolism was analysed using radiolabeled glycerol or fatty acids.ResultsThe hepatocyte-specific Lpiat1 knockout mice developed hepatic steatosis spontaneously, and hepatic fibrosis on high fat diet feeding. Depletion of LPIAT1 in cultured hepatic cells and in spheroids caused triglyceride accumulation and collagen deposition. The increase in hepatocyte fat content was due to a higher triglyceride synthesis fueled by a non-canonical pathway. Indeed, reduction in the PI acyl chain remodelling caused a high PI turnover, by stimulating at the same time PI synthesis and breakdown. The degradation of PI was mediated by a phospholipase C, which produces diacylglycerol, a precursor of triglyceride.ConclusionWe found a novel pathway fueling triglyceride synthesis in hepatocytes, by a direct metabolic flow of PI into triglycerides. Our findings provide an insight into the pathogenesis and therapeutics of NAFLD.

Roles of polyunsaturated fatty acids, from mediators to membranes

“…Genetic variants of MBOAT7 in humans are reported to affect nonalcoholic fatty liver disease, but inconsistencies are also suggested ( 107 ). In mice, liver-specific MBOAT7 knockdown or knockout promotes fatty liver ( 108 , 109 ). Triglyceride synthesis is promoted under MBOAT7 insufficiency, in part due to the higher turnover of PI (both synthesis and degradation are promoted) and the higher production of diacylglycerol thereof (see “PI cycle” in Fig.…”

New Perspectives on Genetic Prediction for Pediatric Metabolic Associated Fatty Liver Disease

“…The membrane-bound O-acyltransferase domain-containing protein 7 (MBOAT7) is a 6 transmembrane domain (100). Hepatocyte specific inactivation of this gene caused an increase in hepatic fat content due to a non-canonical triglyceride synthesis pathway related to a high turnover of phosphatidyl inositol (101). Down-regulation of MBOAT7 predisposes subjects to MAFLD (102).…”

Genetic Susceptibility to Chronic Liver Disease in Individuals from Pakistan

“…With the present study, we also show for the first time that the minor and risk allele for the MBOAT7 rs641738 and for GCKR rs1260326 variants are enriched in Pakistani individuals with chronic liver disease and each of them induces a 35% increased risk for chronic liver disease. The MBOAT7 variant, originally reported as intergenic variant between MBOAT7 and Transmembrane Channel Like 4 (TMC4), is located in the 3'UTR of the MBOAT7 gene and results in its decreased expression level, which consequently induces liver disease by altering phosphatidylinositol remodeling [8] and by triggering a novel non-canonical hepatic triglyceride synthesis pathway fuelled by phospholipid turnover [24]. Our findings are in line with results previously reported in other ethnicities describing the association between the MBOAT7 variant and chronic liver disease and suggest that the rs641738 is the causal genetic variant influencing expression level of MBOAT7 and increasing the risk to liver disease [8,9].…”

“…The proteins encoded by these genes are all involved in the hepatic lipid metabolism. More specifically: PNPLA3 is a lipase localized on the hepatic lipid droplets surface [18] with a triglyceride hydrolase activity (in hepatocytes) [19,20] and a retinyl-palmitate lipase activity (in hepatic stellate cells) [21]; MBOAT7 is a transmembrane acyltransferase protein localized on the endoplasmic reticulum, mitochondria-associated membranes and lipid droplets [8,22], involved in the hepatic phospholipids remodeling [23] and in a non-canonical hepatic triglyceride synthesis pathway fuelled by the phosphatidylinositol turnover [24]; GCKR is involved in the hepatic glucose metabolism [25] and responsible of inverse modulation of fasting plasma glucose and triglyceride levels [26,27]; TM6SF2 is involved in the APOB lipidation and modulates the hepatic lipid droplets secretion of triglycerides through very-low-density lipoproteins (VLDL) [13,[28][29][30]; HSD17B13 is expressed on the hepatic lipid droplet membrane [14], is involved in the hepatic lipogenesis and modulates lipid droplets number and size [31]; PPP1R3B is primarily involved in the hepatic glycogen synthesis and, as results of this, secondarily modulates hepatic triglycerides synthesis through insulin-dependent de novo lipogenesis [16]. The fact that the main genetic determinants of liver disease encode for proteins involved in the hepatic lipid metabolism supports the role of dysregulated lipid metabolism in the pathogenesis of both viral and non-viral liver disease.…”