Characterization of Phosphatidylcholine Metabolism in Mouse Hepatocytes after Hepatectomy and in Primary Human Hepatocytes

Abstract

Phosphatidylcholine (PC) is the major component of mammalian membranes and the induction of PC biosynthesis has been shown to be an essential step in cell proliferation in various cell lines. In the liver, PC biosynthesis is regulated by two pathways: the CDP-choline and the phosphatidylethanolamine N-methyltransferase (PEMT) pathways. The major enzymes that regulate these two pathways are CTP:phosphocholine cytidylyltransferase (CT)α and PEMT, respectively. In our first study, we elucidated the role of PC biosynthesis during proliferation by monitoring liver regeneration after 70% partial hepatectomy (PH) in mice lacking hepatic CTα (LCTα-/- mice). In the liver, CDP-choline is the major pathway of PC biosynthesis. To our surprise, liver re-growth, DNA synthesis, and PC mass after surgery were not impaired in LCTα-/- mice despite reduced total PC synthesis. Furthermore, PC synthesis in control mice was not induced after PH. Thus, we concluded that CTα is not essential for hepatocyte proliferation in vivo and that basal hepatic PC biosynthesis is sufficient to sustain regeneration after PH. In the second study, we assessed hepatic PC to phosphatidylethanolamine ratio (PC/PE) as a predictor of non-alcoholic fatty liver disease (NAFLD) and post-operative complications after major hepatectomy. LCTα-/- mice, PEMT-deficient mice (Pemt-/- mice), and their respective controls were fed a high fat diet to induce various degrees of NAFLD, before PH was performed. We found significant correlation of decreased PC/PE (pre-surgery) with the progression of NAFLD and decreased survival rate after PH. Additionally, dietary choline supplementation increased hepatic PC/PE in Pemt-/- mice with NAFLD, decreased inflammation, and increased survival rate after partial hepatectomy. Thus, choline supplementation may serve as a potential therapy to prevent the progression of NAFLD and to improve post-operative outcome after liver surgery. Finally, we characterized some fundamental aspects of lipid and lipoprotein metabolism in primary human hepatocytes. In particular, we evaluated the synthesis of phospholipids in relation to lipoprotein metabolism. Overall, this work has contributed to the understanding of important processes required for liver regeneration as well as discovering the similarities and differences in lipid and lipoprotein metabolism in primary human hepatocytes compared to hepatoma cell lines and rodent primary hepatocytes.