Understanding the Physiological Role of ETNPPL in Hepatic Lipid Metabolism
Creators: Elmihi, Kholoud A.
Publisher: name: University of Alberta Library
Year: 2024

<p>Phospholipids (PLs) are an essential structural component of the cell membrane and play a role in regulating lipid metabolism. Previous work in our laboratory investigated the functions of phosphatidylcholine (PC) in maintaining liver health and regulating very low-density lipoprotein (VLDL) secretion. Phosphatidylethanolamine (PE) is the second most abundant PL in mammalian membranes. Disturbance of PE metabolism has been linked to the development of fatty liver and increased VLDL secretion. In this thesis, we focused on an enzyme related to PE metabolism, ethanolamine phosphate phospholyase (ETNPPL). ETNPPL irreversibly degradesphospho-ethanolamine (p-ETN) to acetaldehyde, ammonia, and inorganic phosphate. p-ETN is an intermediate in the Kennedy pathway of PE biosynthesis. We generated whole-body knockout (KO) mice lacking Etnppl expression. We hypothesized that genetic deletion of Etnppl wouldinduce accumulation of p-ETN and PE, impacting hepatic lipid metabolism. We fed mice with three different diets: a chow diet, a high-fat diet (HFD) containing 45% kcal fat and a western-type diet (WTD) containing 42% kcal fat and 0.2% cholesterol. When fed a chow diet, plasma triglycerides (TG), PC, cholesterol ester (CE), and free cholesterol (FC) were elevated in Etnppl-/- mice compared to Etnppl+/+ mice. Furthermore, there was an elevation in plasma level of apolipoprotein B100 in KO mice. However, there was no difference in the secretion of VLDL particles from the liver. We did observe no difference in hepatic TG, CE, FC, PC, or PE levels between groups. Even though, there was a downregulation in the expression of genes related to cholesterol metabolism in Etnppl-/- mice. The changes in genes were not translated into the protein level. The liver was able to maintain hepatic PE level constantthrough a homeostatic mechanism although there is an absence of ETNPPL to eliminate p-ETN, the substrate for PE biosynthesis. In the second feeding trial, we aimed to induce steatohepatitis through feeding the mice with a WTD for 24 weeks. There was no difference in weight gain and in hepatic pathological score between both groups. Like the chow diet, KO mice had elevated levels of plasma TG, cholesterol and apolipoprotein B100, without any changes in hepatic neutral lipids. Interestingly, hepatic PE levels were increased in KO mice fed a WTD, suggesting that ETNPPL may impact PE synthesis under certain dietary conditions.In the third feeding trial, mice were fed with a HFD for 10 weeks to induce obesity and fatty liver. Etnppl-/- mice gained less weight compared to Etnppl+/+ mice with less fat tissue. In Etnppl-/- mice, there was an increase in plasma TG level and apolipoprotein B100, but no change in VLDL secretion. Additionally, there was a decrease in plasma CE and FC in KO mice compared to wild type (WT) mice. Unexpectedly, hepatic CE and FC levels were reduced in KO mice when compared to WT mice with attenuated hepatic lipid accumulation and an increase in the concentration of some bile acids in the collected bile. At the same time, no change in hepatic TG, PC, or PE was noticed between both genotypes. In contrast to chow diet fed mice, there was an upregulation in genes related to cholesterol metabolism in Etnppl-/- mice fed a HFD that may be a compensatory mechanism for decreased hepatic cholesterol level. Consequently, we fed mice with HFD- containing cholesterol. No change in hepatic lipid accumulation was observed between KO and WT mice fed HFD-containing cholesterol. This HFD differs in the fatty acid profile and sucrose content compared to WTD.To conclude, we have provided novel evidence that ETNPPL plays a role in regulating plasma lipoprotein metabolism and hepatic cholesterol levels. Diet composition impacts thephenotype in KO mice. Further investigation is warranted to understand the mechanisms.</p>