Phosphatidylcholines: Beyond the Membrane
Klatt, Kevin C.
Phosphatidylcholines (PC) are the most abundant phospholipids in mammalian cells, where they serve well-characterized structural roles, maintaining membrane integrity, facilitating lipoprotein assembly and serving as a surfactant in the lung. Over the past decade, seminal papers have been published that describe novel roles for specific PC species derived from both the cytidine diphosphate-choline (CDP-choline) and phosphatidylethanolamine N-methyltransferase (PEMT) pathways of PC synthesis; these roles include both nuclear receptor agonism and generating critical physiological pools of the omega 3 fatty acid, docosahexaenoic acid (DHA). In addition to these novel functions, complex interactions between PC and its substrate, choline, with growth and energetic status have emerged, following the observation that the mammalian target of rapamycin complex 1 (mTORC1) is a major regulator of PC synthesis through the CDP-choline pathway. Presented in this dissertation are the tests of our primary hypotheses informed directly from these emerging areas of phosphatidylcholine biology. Chapter 2 of this dissertation examines the impact of diet on the production of dilauroylphosphatidylcholine (DLPC), a phosphatidylcholine species enriched in lauric acid that was recently identified as a ligand for the nuclear receptor, liver receptor homolog-1 (LRH-1). DLPC binding to LRH-1 results in modulation of LRH-1-dependent hepatic gene expression and improvements in glucose and lipid handling. While DLPC has been convincingly shown to bind and activate LRH-1 when provided exogenously to cultured cells and mice, endogenous production of DLPC has not been observed, questioning its significance in the regulation of mammalian physiology. We hypothesized that the absence of DLPC in mammalian tissues results from substrate insufficiency (i.e. low lauric acid supply) and that provision of lauric acid, either in the culture media or the mammalian diet, will result in endogenous DLPC production, and be associated with functional LRH-1 activation. Cell culture models, animal feeding experiments, and a single-blind, randomized, controlled crossover acute feeding study in human participants were used to test this hypothesis. As hypothesized, provision of lauric acid in the cell culture media, animal diet, and human diet resulted in the acute and chronic production of DLPC. In cultured cells, this was associated with upregulation of LRH-1 dependent transcripts, an effect that was blunted by co-treatment with a LRH-1 antagonist. In animals, feeding of high fat diets containing lauric acid, utilizing purified lauric acid or coconut oil-based diets, resulted in substantial improvements in glucose handling as indicated by an oral glucose tolerance test. Chapter 3 of this dissertation examines the association of dietary choline intake and reproductive stage with plasma lysophosphatidylcholine (LPC)-DHA among women participants of a 10 week controlled feeding study. LPC-DHA has been recently highlighted as a physiological pool of DHA for maintaining the supply of this critical polyunsaturated fatty acid to extrahepatic organs during growth and development. We hypothesized that reproductive life-stage, dietary choline intake (22% provided as deuterium-labeled choline) and genetic variants in one-carbon metabolism would impact plasma unlabeled and labeled LPC-DHA in response to controlled feeding. To address these hypotheses, we measured unlabeled and deuterium-labeled plasma LPC-DHA in samples from a previously conducted controlled feeding study performed in non-pregnant, pregnant, and lactating women randomized to either 480 or 930mg of choline per day. Consistent with our hypotheses, we observed a significant reduction in unlabeled plasma LPC-DHA in pregnant and lactating women relative to non-pregnant women at week 10; only non-pregnant women exhibited significant increases in LPC-DHA from baseline to week 10 while consuming the study diets. Choline intake and one-carbon metabolism variants were not associated with unlabeled plasma LPC-DHA. However, both choline intake and reproductive life-stage altered the enrichment of PEMT- and CDP-choline-derived LPCs. In Chapter 4 of this dissertation, we explore roles for choline and phospholipid metabolism in the context of a novel animal model of adolescent severe acute malnutrition (aSAM). This complex disorder is characterized by negative energy balance, impaired growth, and multiple micronutrient deficiencies; severe forms of aSAM, namely kwashiorkor, are further complicated by an idiopathic fatty liver. The sensitivity of this fatty liver to dietary choline remains unknown. To characterize the potential for choline supplementation to serve as a metabolic therapy in aSAM, we developed a maize vegetable diet (MVD), comprised of foods typically consumed by children who will go on to develop kwashiorkor. The MVD was fed to weanling mice with or without choline supplementation, and compared to typical chow diets, to determine impacts on growth, body composition, and potential to alleviate the hallmark characteristic of kwashiorkor, fatty liver. We further explored the metabolic fate of dietary choline, hypothesizing that choline would be oxidized to betaine to support PC synthesis through the PEMT pathway in this context of lower mTORC1 activity and reduced CDP-choline pathway activity. Mice consuming our novel maize vegetable diet exhibited impaired growth relative to chow fed mice, and developed hepatic steatosis, consistent with human kwashiorkor. The addition of choline to the maize vegetable diet resulted in amelioration of the hepatic steatosis, and a greater hepatic concentration of betaine; choline partitioning to betaine is consistent with a role for the PEMT pathway in supporting PC synthesis in the context of malnutrition.
lauric acid; lysophosphatidylcholine; phosphatidylcholine; Nutrition; Molecular biology; Medicine; coconut oil; dilauroylphosphatidylcholine; docosahexaenoic acid
Caudill, Marie A.
Roberson, Mark Stephen; Brenna, James Thomas; Thalacker-Mercer, Anna E.
Ph. D., Nutrition
Doctor of Philosophy
dissertation or thesis