Insights into the Role of Phosphatidylethanolamine N-methyltransferase and the PC:PE Ratio in Liver Function

• Author / Creator

  Wan, Sereana

• PEMT is quantitatively important in the liver and is responsible for approximately 30% of hepatic PC synthesis, with the remaining 70% synthesized by the CDP-choline pathway. HFD-fed Pemt-/- mice and LPcyt1a-/- mice both develop NAFLD associated with reduced VLDL secretion and a reduced hepatic PC:PE molar ratio. Interestingly, the severity of NAFLD was greater in HFD-fed Pemt-/- mice, which was associated with a greater decrease in the hepatic PC:PE molar ratio, compared to LPcyt1a-/-. Pemt-/- mice are also protected from DIO and IR but LPcyt1a-/- mice are not suggesting a role for PEMT, beyond PC deficiency, in mediating these effects. However, dietary supplementation of choline into the HFD ameliorates the protection against DIO and IR and improves NAFLD. Aside from the role of PC in VLDL secretion, a large proportion of PC is also secreted into bile. This thesis endeavors to delineate the effects of hepatic PEMT deficiency and PC:PE imbalance in biliary secretion, resistance to DIO and IR, and NAFLD development during over nutrition. Interestingly, portions of the ER located close to the canaliculus are enriched in PEMT. Phospholipid balance and asymmetrical distribution by ATPase Phospholipid Transporting 8B1 (ATP8B1) on the canalicular membrane is required for membrane integrity and biliary processes. We hypothesized that PEMT is an important supplier of PC to the canaliculus and that PEMT activity is critical for the maintenance of canalicular membrane integrity and bile formation upon HFD feeding, when there is an increase in overall hepatic PC demand. Pemt+/+ and Pemt-/- mice were fed a chow diet, an HFD, or a choline-supplemented HFD (CSHFD). Pemt-/- mice developed cholestasis, i.e., elevated plasma bile acid (BA) concentrations and decreased biliary secretion rates of BAs and PC, during HFD feeding. Maximal BA secretory rate was reduced more than 70% in HFD-fed Pemt-/- mice. Hepatic ABCB11/bile salt export protein (BSEP), responsible for BA secretion, was decreased in Pemt-/- mice and appeared to be retained intracellularly. Canalicular membranes of HFD-fed Pemt-/- mice contained fewer invaginations and displayed a smaller surface area than Pemt+/+ mice. Choline supplementation prevented and reversed the development of HFD-induced cholestasis. Thus, we propose that hepatic PC availability is critical for bile formation. Dietary choline supplementation might be a potential noninvasive therapy for specific subset of patients with cholestasis. We have previously suggested that PC insufficiency and the severely reduced hepatic PC:PE molar ratio, which reduces membrane integrity, contributes to the development of cholestasis in HFD-fed Pemt-/- mice. Since the majority of hepatic PC is synthesized through the CDP:choline pathway, we developed an inducible LPcyt1a-/- (iLPcyt1a-/-) mouse to investigate the effects of PC insufficiency on biliary secretion. We report that the biliary secretion of bile acids (BA) is not impaired, and that the biliary secretion of phospholipids is surprisingly increased in iLPcyt1a-/- mice. This suggests that CTα is not required for biliary secretion under HFD conditions and may play a role in regulating biliary phospholipid secretion. Small amounts of PEMT protein have been detected in extra-hepatic tissues, such as white adipose, testis, and kidneys. We investigated if the lack of hepatic PEMT is responsible for the metabolic phenotype, or if the low expression of PEMT in other tissues contributes to the protection against diet induced obesity and insulin resistance in Pemt-/- mice. Furthermore, we investigated if decreasing PEMT expression with antisense oligonucleotides (ASO) would result in metabolic benefits in both lean and obese mice, without negatively impacting liver health. We both restored hepatic PEMT in Pemt-/- mice, via adeno associated virus delivery, and decreased hepatic PEMT with ASOs targeted against PEMT. Weight gain, insulin sensitivity, and indices of liver function were determined.We report that the protection against diet induced weight gain and insulin resistance, as well as the development of NAFLD is dependent on hepatic PEMT activity. NAFLD, associated with a significant decrease in the hepatic PC:PE ratio, was exacerbated by PEMT deficiency in obese mice, suggesting that phospholipid insufficiency promotes NAFLD progression during obesity/over-nutrition. Hepatic PEMT is critical for maintaining phospholipid balance, which is crucial for a healthy liver.

• Subjects / Keywords

  • Cholestasis
  • phospholipids
  • PEMT
  • HFD
  • NASH
  • NAFLD

• Graduation date

  Spring 2019

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/r3-hr7n-9h97

• License

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