The role of phosphatidylcholine biosynthesis in the secretion of lipoproteins from hepatocytes

Abstract

An investigation of the role of phospholipids in lipoprotein assembly and secretion is important since phospholipids, particularly phosphatidylcholine, are prominent components of all plasma lipoproteins. The fatty acid composition of phosphatidylcholine is virtually identical in human very low (VLDL), low, and high density lipoproteins, which supports the idea that phosphatidylcholine exchanges freely among plasma lipoproteins. However, the fatty acid composition of phosphatidylcholine from cultured rat hepatocytes is different from that in the secreted lipoproteins. In addition, the composition of molecular species of phosphatidylcholine is quite different in the rat liver, plasma, and red cells.Phosphatidylcholine is made in liver by two alternate pathways, by the CDP-choline pathway and by the methylation of phosphatidylethanolamine. Regulation of phosphatidylcholine biosynthesis by the CDP-choline pathway in rat liver is well established. In most instances, the rate of phosphatidylcholine synthesis is governed by the activity of CTP:phosphocholine cytidylyltransferase, which is present in the cytosol and also associated with microsomes. The cytosolic enzyme is inactive but can be reversibly translocated to the microsomes, where it is active. Translocation of this enzyme to the microsomes can be achieved either by a dephosphorylation reaction or by the presence of fatty acids in the cytosol.Once synthesized, how is phosphatidylcholine assembled into lipoprotein particles? The sequence of assembly of phospholipids into VLDL has been investigated in several studies. In a pulse–chase experiment, there was an initial labelling (within 15 min of the pulse) of phospholipids in secreted VLDL, which probably reflected the rapid movement of the phospholipids from their site of synthesis (the endoplasmic reticulum) to the Golgi. There appears to be a rapid exchange of phospholipid between the Golgi membranes and contents. There was also a delayed labelling (after 30 min) of the phospholipids and triacylglycerols (from [3H]glycerol) and the apoproteins (from [3H]leucine) in the secreted VLDL. This lag was attributed to the time taken for the nascent VLDL particles to move from the lumen of the endoplasmic reticulum to the Golgi and into the medium.Is phosphatidylcholine biosynthesis required for lipoprotein secretion? This question was investigated in rats maintained on a choline-deficient diet for 10 days. The total amount of plasma phosphatidylcholine decreased by approximately 40% and the rats developed fatty livers. The mechanisms for these effects of choline deficiency have not been fully explained, although one might anticipate that a deficiency of choline would inhibit the synthesis of phosphatidylcholine via the CDP-choline pathway.In addition, the possibility was considered that the alternative pathway for phosphatidylcholine biosynthesis (the methylation of phosphatidylethanolamine) might be necessary for lipoprotein secretion. Inhibition of the methylation pathway for phosphatidylcholine synthesis by greater than 90% in cultured rat hepatocytes did not inhibit the secretion of phosphatidylcholine, phosphatidylethanolamine, or apoproteins of the lipoproteins in the culture medium.Since phosphatidylcholine, triacylglycerol, and cholesterol are required for lipoprotein assembly and secretion, there might be some coordination among the synthesis and secretion of phosphatidylcholine, triacylglycerol, and cholesterol. Such coordinate regulation has been observed. For example, fatty acids stimulate the synthesis and secretion of both triacylglycerol and phosphatidylcholine by cultured rat hepatocytes. In addition, glucagon and AMP inhibit fatty acid synthesis, phosphatidylcholine biosynthesis, and the secretion of both phosphatidylcholine and triacylglycerol. Surprisingly, insulin appears to inhibit lipoprotein secretion even though it promotes fatty acid and cholesterol biosynthesis. In other studies in which there was an increased supply of cholesterol and cholesterol esters in the diet of rats, the percent of cholesterol esters in the core of the secreted VLDL particles was increased. Moreover, cholesterol feeding increased the plasma concentrations of both phosphatidylcholine and cholesterol. Presumably, there was a coordination of the synthesis of phosphatidylcholine with the level of cholesterol in the cells and (or) plasma.We conclude that phosphatidylcholine biosynthesis is a critical component for the synthesis and secretion of lipoproteins from liver.