Objective Non-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.
Design We 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.
Results The 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.
Conclusion We 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.
- fatty liver
- hepatic fibrosis
- lipid metabolism
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YT, YS and AC are joint first authors.
SR and NK are joint senior authors.
Contributors YT, YS and AC designed and performed the experiments, analysed the data, interpreted the results and wrote the manuscript. TK performed the lipid analysis and interpreted the results. YM performed biochemical and histological studies. TK, TY and NK performed oxygen consumption experiments. RMM analysed the data and interpreted the results from lipidomic and transcriptomic data in human liver; GB and LV contributed to transcriptomic data generation, analysis and interpretation in human liver; PL and HA contributed to lipidomic data generation, analysis and interpretation in human liver; JP contributed to lipidomic data interpretation in human liver; HA, SR and NK designed the experiments, interpreted the results and wrote the manuscript.
Funding This work was supported by Grants-in-Aid for Scientific Research (S) (grant numbers 23 227 004 and 17H06164 to HA) and for Scientific Research on Innovative Areas (17H06418 to HA); by AMED-CREST, AMED (JP 18gm0710002 to HA); by the Swedish Research Council (Vetenskapsrådet (VR), 2016–01527 to SR); by the Swedish state under the Agreement between the Swedish government and the county councils (the ALF-agreement) (SU 2018–04276 to SR); by the Novonordisk Foundation Grant for Excellence in Endocrinology (Excellence Project, 9321–430 to SR); by Wallenberg Academy Fellows from the Knut and Alice Wallenberg Foundation (KAW 2017.0203 to SR); by the Novonordisk Project grants in Endocrinology and Metabolism (SR); by Astra Zeneca Agreement for Research (SR); by Grant SSF ITM17-0384, Swedish Foundation for Strategic Research (SR) and by the Novo Nordisk and Sigrid Jusélius Foundations (PL).
Competing interests None declared.
Patient and public involvement Patients and/or the public were not involved in the design, or conduct, or reporting, or dissemination plans of this research.
Patient consent for publication Not required.
Provenance and peer review Not commissioned; externally peer reviewed.
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