Molecular Distinction of Phosphatidylcholine Synthesis between the CDP-Choline Pathway and Phosphatidylethanolamine Methylation Pathway

Abstract

In addition to the CDP-choline pathway for phosphatidylcholine (PC) synthesis, the liver has a unique phosphatidylethanolamine (PE) methyltransferase activity for PC synthesis via three methylations of the ethanolamine moiety of PE. Previous studies indicate that the two pathways are functionally different and not interchangeable even though PC is the common product of both pathways. This study was designed to test the hypothesis that these two pathways produce different profiles of PC species. The PC species from these two pathways were labeled with specific stable isotope precursors, D9-choline and D4-ethanolamine, and analyzed by electrospray tandem mass spectrometry. Our studies revealed a profound distinction in PC profiles between the CDP-choline pathway and the PE methylation pathway. PC molecules produced from the CDP-choline pathway were mainly comprised of medium chain, saturated (e.g. 16:0/18:0) species. On the other hand, PC molecules from the PE methylation pathway were much more diverse and were comprised of significantly more long chain, polyunsaturated (e.g. 18:0/20:4) species. PC species from the methylation pathway contained a higher percentage of arachidonate and were more diverse than those from the CDP-choline pathway. This profound distinction of PC profiles may contribute to the different functions of these two pathways in the liver.Phosphatidylcholine (PC) 1 is a major group of phospholipid in all mammalian cells (1). PC is comprised of hydrocarbon chains attached to glycerophosphocholine via acyl, alkyl, or alkenyl linkages. The molecular diversity of PC and other phospholipids is dictated by the combination of different lengths, number of double bonds, and types of linkages of hydrocarbon chains. As a result, a single mammalian cell contains at least a thousand species of phospholipids (2). In most mammalian cells, PC is synthesized mainly via the CDPcholine pathway (3). This pathway uses choline as an initial substrate and is catalyzed by three enzymes: choline kinase, CTP:phosphocholine cytidylyltransferase (CT), and cholinephosphate transferase, with CT as the rate-limiting enzyme (1). Hepatocytes are unique because they also possess a high activity of phosphatidylethanolamine methyltransferase (PEMT) that converts PE to PC via three sequential steps of methylation (4) in addition to a high level of CDP-choline pathway activity. The significance of the PEMT pathway is not completely understood (5).The PEMT pathway seems redundant because its product, PC, is also synthesized by the CDP-choline pathway in hepatocytes. Therefore, the PEMT pathway is traditionally considered a backup pathway for PC synthesis in hepatocytes (3). However, recent studies indicate that these two pathways apparently have opposite effects on proliferative characteristics of the liver and liver-derived cell lines (6 -10). These studies suggest that the higher activity of the CDP-choline pathway favors the faster proliferation of hepatocytes. Conversely, expression of PEMT strongly inhibits the growth of hepatom...

Targeted Deletion of Hepatic CTP:phosphocholine Cytidylyltransferase α in Mice Decreases Plasma High Density and Very Low Density Lipoproteins

“…The Lipid Content of the Liver Is Altered in CT␣ Knockout Mice-The Kennedy (CDP-choline) pathway has been shown to produce ϳ70% of PC in the liver (7,9). It was, therefore, expected that deletion of CT␣ would alter hepatic lipid concentrations.…”

“…Liver cells are unique in that they can also synthesize PC via the sequential methylation of phosphatidylethanolamine catalyzed by phosphatidylethanolamine N-methyltransferase (PEMT) (6). The PEMT pathway accounts for ϳ30% of hepatic PC biosynthesis, whereas the enzymes of the Kennedy pathway produce the remaining 70% (7)(8)(9).…”

Choline kinase inhibition induces the increase in ceramides resulting in a highly specific and selective cytotoxic antitumoral strategy as a potential mechanism of action

“…The inability of PEMT to rescue MT58 cells under restrictive temperature from apoptosis suggests that the PC species generated from the Kennedy pathway and the PEMT pathway are functionally different (Cui and Houweling, 2002). Indeed, the PC species derived from PE methylation contain longer and more unsaturated fatty acid chains than the PC species generated through the Kennedy pathway (DeLong et al, 1999). The only PC capable of rescuing MT58 cells at the nonpermissive temperature from apoptosis is PC generated through the Kennedy pathway (Esko et al, 1982;Ostrander et al, 2001).…”

“…The only PC capable of rescuing MT58 cells at the nonpermissive temperature from apoptosis is PC generated through the Kennedy pathway (Esko et al, 1982;Ostrander et al, 2001). Changes in the nature of PC in membranes may cause alterations in the fluidity, structural integration and association of membrane proteins, vesicular traffic, affinity and activity of lipases (DeLong et al, 1999).…”

Membrane Topography of Human Phosphatidylethanolamine N-Methyltransferase

“…Recent studies investigating the proportion of hepatic PC that is derived from each pathway defined the PEMT-controlled pathway as the source of 30% of hepatic PC with the CDP-choline pathway accounting for 70% (7)(8)(9). Significantly perhaps, data from one group also revealed that the PEMT pathway is a metabolically channeled process suggesting that PEMT-derived PC may be destined for a specific function (9).…”