To Örebro University

Background and aims: Phosphatidylcholines (PCs) are a major hepatic reservoir of polyunsaturated fatty acids (PUFAs). In murine models of metabolic dysfunction-associated steatotic liver disease (MASLD), liver injury is induced by PUFA-PC depletion. The lipid droplet enzyme HSD17B13 metabolizes PUFA derivatives, and its loss-of-function variant rs72613567:TA protects against steatohepatitis (MASH) and fibrosis, although the mechanism underlying this protection remains unknown. We investigated whether hepatic PUFA-PC metabolism is influenced by MASLD or the protective HSD17B13 variant.

Method: Obese patients with a liver biopsy (n = 135) underwent genotyping for HSD17B13 rs72613567:TA and analysis of the hepatic lipidome by UPLC-MS. A state-of-the-art deep learning image analysis method (Aiforia Technologies) quantified steatosis via the hepatic parenchymal fat fraction. Using a recruit-by-genotype approach, we studied homozygous rs72613567:TA carriers (n = 13) and non-carriers (n = 13) to determine whether the variant affects incorporation of 13C-labeled PUFAs linoleic acid (LA) and alphalinolenic acid (ALA) into triglycerides (TGs) and phospholipids (PLs) secreted by the liver in very-low-density lipoprotein (VLDL). We tested whether targeting HSD17B13 with a small molecule inhibitor, INI-822, affects PC metabolism using two models: in vitro, a primary human liver-on-a-chip system challenged with high-fat media for 20 days; and in vivo, Zucker obese rats fed either an atherogenic diet or a choline-deficient, L-amino acid-defined high-fat diet for 21 days.

Results: Human MASLD livers were characterized by a marked depletion of hepatic PUFA-PCs containing 5–8 double bonds. Steatosis had a particularly accentuated effect to lower PUFA-PCs in individuals without the HSD17B13 rs72613567:TA variant. However, this effect was abolished in variant carriers due to markedly increased concentrations of hepatic PUFA-PCs compared to non-carriers. Homozygous carriers had significantly decreased incorporation of [U-13 C]LA into VLDL-TG (P < 0.001) and of [U-13 C] LA and[U-13 C]ALA into VLDL-PL (P = 0.01 and 0.05), consistent with retention of these PUFAs in the liver. In vitro, inhibition of HSD17B13 by INI-822 in the human liver-on-a-chip system lowered fibrotic markers while stabilizing choline utilization and increasing PC concentrations. In vivo, inhibition of HSD17B13 in Zucker obese rats fed MASH-inducing diets reduced liver enzymes and dose-dependently increased hepatic PC concentrations.

Conclusion: In humans, HSD17B13 rs72613567:TA prevents MASLD-induced hepatic PUFA-PC depletion by retaining PUFAs within the liver. Pharmacological inhibition of HSD17B13 recapitulates the human phenotype in vitro and in vivo. These findings suggest tha thepatic PC enrichment is central to the protective effects associated with HSD17B13 loss-of-function.