The role of de novo phosphatidylcholine synthesis in the gut-liver axis and intestinal homeostasis
Creators: Carlin, Stephanie D
Publisher: name: University of Alberta Library
Year: 2023

<p>The intestine and liver are in constant communication to maintain lipid processes and to prevent disease states such as non-alcoholic fatty liver disease (NAFLD) and colitis. Phosphatidylcholine (PC) can be synthesized in the liver by the CDP-choline pathway or the phosphatidylethanolamine N-methyltransferase pathway, while the small intestine is only capable of synthesizing PC through the CDP-choline pathway. Cytidine triphosphate:phosphocholine cytidylyltransferase-α (CTα) is the rate limiting enzyme in the CDP-choline pathway.<br>
Hepatic CTα deletion in mice from birth causes reduced circulating very low-density lipoprotein levels and NAFLD when fed a high-fat diet (HFD). Our aim was to determine the impact of an acute deletion of hepatic CTα in adult mice on lipid handling in the liver (fasting) and intestine (postprandial). We found that chow- and HFD-fed acute CTα liver knockout (CTαLKO) mice quickly lose weight, have reduced fasting TG levels, and develop NAFLD. When analyzing lipid metabolism in the postprandial state, we also found that chow and HFD-fed CTαLKO mice have reduced lipid absorption, leading to a reduction in the appearance of plasma TG. This data suggests that that hepatic CTα-derived PC synthesis is important for regulating lipid metabolism in fasting and postprandial states.<br>
When CTα is deleted from intestinal epithelial cells (IECs) of HFD-fed adult mice (CTαIKO mice), they present with weight loss, lipid malabsorption, and high postprandial plasma glucagon-like peptide 1 (GLP-1) levels. We aimed to characterize the changes that occur in the small intestines of CTαIKO mice. We found that impaired de novo PC synthesis in the gut is linked to altered lipid metabolism and induction of endoplasmic reticulum (ER) stress, cell death, and inflammation. Induction of the host defence response in CTαIKO mice was also associated with loss of goblet cells. Additionally, we found that impaired fatty acid uptake occurs in isolated intestinal<br>
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sacs from CTαIKO mice. Antibiotic treatment prevented acute weight loss and normalized jejunum TG concentrations after refeeding but did not alter enhanced postprandial GLP-1 secretion, induction of host defence and ER stress transcripts, or loss of goblet cells in CTαIKO mice. Dietary PC supplementation partially prevented loss of goblet cells but was unable to normalize jejunal TG or plasma GLP-1 concentrations after refeeding in CTαIKO mice. Together these data show that there is a specific requirement from de novo PC synthesis in maintaining small intestinal homeostasis.<br>
Patients with ulcerative colitis have low concentrations of the major membrane lipid, PC, in gastrointestinal mucus. Therefore, we aimed to determine the role that PC plays in colonic barrier function. Inducible loss of CTα in the intestinal epithelium reduced colonic PC concentrations and resulted in spontaneous colitis. Colitis development in CTαIKO mice was traced to an ER stress response caused by an altered phospholipid composition. This ER stress response was linked to the necroptotic death of IECs leading to excessive loss of goblet cells, formation of a thin mucus barrier, elevated intestinal permeability, and infiltration of the epithelium by microbes. These experiments show that maintaining the PC content of IECs protects against colitis development and maintains colonic homeostasis.<br>
Enterohepatic circulation and biliary homeostasis are critical for the digestion, absorption, and processing of lipids. PC has an important role in this cycle as it is the second most abundant component found in bile, after bile acids, and is heavily involved in lipid metabolism. We discovered that CTαIKO mice have reduced postprandial circulating cholecystokinin (CCK) levels leading to enlarged gallbladders. Our aim was to determine whether improving the presence of bile in CTαIKO mice could improve weight loss or lipid malabsorption. When CTαIKO mice were injected with exogenous CCK they did not acutely lose weight yet still experienced lipid<br>
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malabsorption. When CTαIKO mice were fed a diet supplemented with bile acids, there were no improvements in weight loss or lipid malabsorption.<br>
In conclusion, CDP-derived PC is an important regulator of the gut-liver axis and is involved in maintaining cellular, organ and systemic homeostasis. This research demonstrates that maintaining appropriate levels of CDP-derived PC in the liver and intestine is necessary for appropriate fasting and postprandial lipid metabolism. Additionally, CDP-derived PC has an intricate role within the gut-liver axis and altering the pathway of CDP-choline synthesis leads to the development of disease states such as NAFLD and colitis.</p>