Hepatic Inactivation of Carnitine Palmitoyltransferase 1a Lowers ApoB-Containing Lipoproteins in Mice

Abstract

Background: Genome- and epigenome-wide association studies have associated variants and methylation status of CPT1a (carnitine palmitoyltransferase 1a) to reductions in VLDL (very low-density lipoprotein) cholesterol and triglyceride levels. The objective of this study was to determine the mechanisms by which CPT1a-dependent mitochondrial fatty acid oxidation influences hepatic and lipoprotein metabolism.

Methods: Eight-week-old male and female Cpt1a-floxed mice (Cpt1a^fl/fl) and Cpt1a-floxed mice expressing the human apo B₁₀₀ transgene (Cpt1a^fl/fl/B100^Tg) were administered control adeno-associated virus or adeno-associated virus encoding Cre-recombinase under control of a liver-specific promoter (TBG-Cre [thyroxin-binding globulin]). Control and liver-specific knockout mice were placed on a low-fat control or western-type diet (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 bulk RNA-sequencing and immunoblotting, respectively. Lipoprotein composition in plasma was determined by size exclusion chromatography and nuclear magnetic resonance. Rates of VLDL-triglyceride secretion were quantified after lipase inhibition with poloxamer 407. Liquid and gas chromatography-mass spectrometry were used to measure bile acid species and fecal neutral sterols, respectively.

Results: We report significant associations between the presence of CPT1a SNPs (single nucleotide polymorphisms) and reductions in plasma cholesterol, as well as positive associations between hepatic Cpt1a expression and plasma cholesterol levels across inbred mouse strains. Mechanistic studies show that both wild-type and human apo B₁₀₀ (apoB)-transgenic mice with liver-specific deletion of Cpt1a (liver-specific knockout) display lower circulating apoB levels consistent with reduced LDL (low-density lipoprotein)-cholesterol and LDL particle number. Despite a reduction in steady-state plasma lipids, VLDL-triglyceride and VLDL cholesterol secretion rates are increased, suggesting accelerated clearance of apoB-LPs (apoB-containing lipoproteins) in liver-specific knockout mice. Mechanistic approaches show greater PPARα (peroxisome proliferator-activated receptor α) signaling which favors enhanced lipoprotein lipase-mediated metabolism of apoB-LPs, including increases in apo AIV and apo CII and reductions in apo CIII and Angptl3 (angiopoietin-like 3).

Conclusions: These studies provide mechanistic insight linking genetic variants and methylation status of CPT1a to reductions in circulating apoB-LPs in humans.

Keywords: cardiovascular diseases; fatty acids; lipolysis; lipoproteins; mitochondria.