Abstract Body: Background: Genome- and epigenome-wide association studies have associated variants and methylation status of carnitine palmitoyltransferase 1a (CPT1a) to reductions in very low-density lipoprotein (VLDL) cholesterol and triglyceride levels. The primary goal of this project is to determine the mechanism by which CPT1a alters hepatic and lipoprotein metabolism.

Methods: Eight-week-old Cpt1a floxed mice expressing the human APOB100 transgene (Cpt1a^fl/fl/B100^Tg) were administered control adenoassociated virus (AAV) or AAV encoding Cre-recombinase under control of a liver specific promoter (TBG-Cre). Control and LKO mice were placed on low-fat control or western-type diet (WTD; 42% kcal fat, 0.2% cholesterol) for 16 weeks. Livers were collected and used for histological and lipid analysis, while gene and protein expression were measured by RNA sequencing (bulk, single cell) and immunoblotting, respectively. Lipoprotein composition in plasma was determined by size exclusion chromatography and nuclear magnetic resonance (NMR). Liquid and gas chromatography-mass spectrometry were employed to measure bile acid species and fecal neutral sterols, respectively.

Results: Mice with liver-specific deletion of Cpt1a (LKO) displayed lower circulating APOB levels consistent with reduced triglyceride rich lipoproteins and LDL particle number. No changes in bile flow rate, cholesterol secretion rate, bile acid secretion rate, total bile acids, or fecal cholesterol levels were observed in control or LKO mice. Despite a reduction in steady-state plasma lipids, VLDL-triglyceride secretion rates were enhanced in LKO mice, suggesting accelerated clearance of apoB-containing lipoproteins. In addition, western diet feeding elevated hepatic triglycerides in LKO mice across both sexes, while cholesterol (free and esterified) increased by ~2.5-fold specifically in females. Greater accumulation of free cholesterol coincided with an increase in inflammation and fibrosis of the liver in female LKO mice.

Conclusions: Liver-specific Cpt1a deletion reduces plasma LDL-cholesterol and triglycerides, despite having accelerated VLDL-secretion. Increases in hepatic free cholesterol levels were observed only in female LKO mice, which associates with a pro-inflammatory gene signature and fibrosis of the liver. These studies provide mechanistic insight into why genetic variants in Cpt1a associate with reductions in circulating apoB-containing lipoproteins in humans.