CDP-choline pathway

PE can be generated by several mechanisms. The CDP-ethanolamine route may be important under physiological conditions in vivo when an abundant supply of ethanolamine is present. The CDP-ethanolamine pathway is very similar to the CDP-choline pathway described above (Tijburg et al. 1989 Bladergroen and Van Golde, 1997). The pathway comprises (i) an ethanolamine kinase (EK) which converts ethanolamine to phosphoethanolamine, (ii) an CTP phosphoethanolamine cytidyltransferase... 

Recently, it has been shown that cell-permeable cerantides dramatically inhibited the synthesis of the two major membrane phospholipids, PC and PE (Bladergroen et al, 1999b Allan, 2000). The inhibition of phospholipid synthesis was rapid, within 2 h, and resulted in massive apoptosis after 16-24 h. The mechanism by which short-chain cerantides exert their effect on phospholipid synthesis is possibly cell type dependent. In baby-hamster kidney (BHK) fibroblasts rc synthesis was reduced at the level of CT, the putative rate-determining enzyme in the CDP-choline pathway (Allan, 2000). This conclusion was based solely on radio-label studies in combination with an earlier published observation (Wieder et al, 1995) showing that C2-SM (the SM generated from C2-ceramide by SM synthase, which was actively synthesized in the BHK-cells) inhibited CT activity in vitro. On the other hand, data obtained from studies with rat-2 fibroblasts clearly showed that short-chain cerantides regulate the synthesis of PC and PE mainly at the final step of the CDP-pathways. This conclusion was based on the following observations (a) incorporation of [ H]-choline into PC and... 

The most convincing example that inhibition of PC synthesis can induce apoptosis comes from a cell line with a temperature-sensitive defect in one of the enzymes in the CDP-choline pathway (Cui et al, 1996). The MT58 cell line is a Chinese hamster ovary (CHO) derived cell line with a mutation in CTP phosphocholine cytidylyltransferase (CT), which renders the... 

Enzymes in the CDP-choline pathway are targets for other pro-apoptotic compounds besides ceramides... 

The pathway for the synthesis of dipalmitoyl-phos-phatidylcholine is illustrated in figure 19.5. The starting species of phosphatidylcholine is made by the CDP-choline pathway (see fig. 19.4). The fatty acid at the sn-2 position, which is usually unsaturated, is hydrolyzed by phospholi-pase A2, and the lysophosphatidylcholine is reacylated with palmitoyl-CoA. This modification permits alteration of the properties of the phospholipid without resynthesis of the entire molecule, a strategy called remodeling. Deacylation-reacylation of phosphatidylcholine occurs in other tissues and provides an important route for alteration of the fatty acid substituents at both the sn-1 and sn-2 positions. For example, fatty acids at the sn-2 position can be replaced by arachidonic acid, which is stored there until needed for eicosanoid biosynthesis, as we discuss later in this chapter. 

Boggs K. P., Rock C. O., and Jackowski S. (1995). Lysophosphatidylcholine and l-O-octadecyl-2-O- methyl-rac-glycero-3-phosphocholine inhibit the CDP-choline pathway of phosphatidylcholine synthesis at the CTP phosphocholine cytidylyltransferase step. J. Biol. Chem. 270 7757-7764. 

Houweling, M., Cui, Z., and Vance, D.E., 1995, Expression of phosphatidylethanolamine N-methyltransferase-2 cannot compensate for an impaired CDP-choline pathway in mutant Chinese hamster ovary cells.. /. Biol. Chem. 270 16277-16282 Ishidate, K.. 1997, Chohne/ethanolamine kinase from mammalian tissues. Biochim. Biophys. Acta 1348 70-78... 

Cleves, A.E., McGee, T Whitters, E. A., Champion, K.M., Aitken, J.R., Dowhan, W., Goebl, M., and Bankaitis, VA., 1991b, Mutations in the CDP-choline pathway for phospholipid biosynthesis bypass the requirement for an essential phospholipid transfer protein. Cell 64 789-800. 

The CDP-choline pathway to PC (Figure 6.1) occurs in a number of tissues, but the PE methylation pathway (Figure 6.2) occurs mainly in the liver and accounts... 

FIGURE 6.1 Phosphatidylcholine (PC) synthesis by the CDP-choline pathway. First, choline is converted to phosphocholine then the CMP moiety of CTP is transferred to phosphocholine to form CDP-choline. Cleavage of the pyrophosphate (PPj) produced by phosphatases markedly reduces its concentration in the cell and ensures that the reaction is driven in the forward direction. The phosphocholine moiety of CDP-choline then is transferred to diacylglycerol, generating PC. The total number of phosphate-phosphate bonds (phosphodiester bonds) broken in this pathway is three. This number determines the energy cost of PC synthesis. All cells containing a nucleus can make PC via the CDP-choline pathway. 

The CDP-choline pathway is the major pathway for the synthesis of PC in the lung, and cholinephosphotransferase (CPT) is a terminal enzyme in this pathway. Regulation of PC metabolism is one of the vital aspects of the cell cycle with implications in the control of cell proliferation as well as in apoptosis (Cui et al., 1996 Baburina and Jackowski, 1998). 

PC is the most abundant phospholipid in mammalian cells, and it is synthesized via the CDP-choline pathway (Kent, 1995). CPT is the terminal enzyme of this pathway and plays a direct role in the final production of PC in the lung. This pathway is important for both cell proliferation and cell death (Ghosh et al., 2002 Ryan et al., 2003), and selective inhibition of this pathway has been shown to induce cellular apoptosis (Miquel et al., 1998). Any modulation in the expression and activity of this enzyme is expected to result in abnormal functioning of the cells. 

Other laboratories have also shown that enzymes of the CDP-choline pathway for the production of PC in the cells, including CPT, show reduced activity when cells are incubated with cell-permeable C2/C6 ceramides (Bladergroen et al., 1999 Ramos et al., 2002), and it has been predicted that this inhibition may be due to the competitive inhibition by ceramides owing to the similarity in the stmcture to one of the substrates for CPT, DAG. 

Fig. 3. Regulation of PC biosynthesis via the CDP-choline pathway by modulation of the binding of CTP phosphocholine cytidylyltransferase (CT) to membranes. Three different modes of regulation of CT activity are indicated. The abbreviations are CK, choline kinase CPT, CDP-choline 1,2-diacylglycerol cholinephos-photransferase PEMT, phosphatidylethanolamine Af-methyltransferase PC, phosphatidylcholine PE, phos-phatidylethanolamine DG, diacylglycerol.

All nucleated cells contain PC and utilize the CDP-choline pathway. Thus, it was not obvious why the alternative pathway for PC synthesis, the PE methylation pathway (Fig. 4), survived during evolution. Nor was it obvious why PE methyltransferase (PEMT) activity is essentially a liver-specific enzyme whereas 2% or less of the hepatic PEMT activity is found in other tissues. In addition to producing PC, PEMT generates three... 

CTP has also been implicated as a regulator of PC synthesis in animals and yeast. For example, over-expression of CTP synthetase in yeast stimulates the biosynthesis of PC via the CDP-choline pathway (G.M. Carman, 1995). 

The reactions of the CDP-ethanolamine pathway (Figs. 1 and 7) parallel those of the CDP-choline pathway for PC synthesis (Section 3). Ethanolamine is an obligatory precursor of PE synthesis by this route and is derived from the diet with smaller amounts... 

Phosphatidylcholine is the major phospholipid on the surface monolayer of all lipoproteins, including VLDLs. In the liver, phosphatidylcholine is synthesized by two biosynthetic pathways the CDP-choline pathway and the phosphatidylethanolamine A -methyltransferase pathway (Chapter 8). Choline is an essential biosynthetic precursor of phosphatidylcholine via the CDP-choline pathway. When cells or animals are deprived of choline, plasma levels of TG and apo B are markedly reduced and TG accumulates in the liver, resulting in fatty liver. These observations led to the widely held view that the fatty liver caused by choline deficiency is due to inhibition of PC synthesis, which in turn would decrease VLDL secretion. This hypothesis was tested in primary rat hepatocytes cultured in medium lacking choline. Upon removal of choline/methionine from culture medium, the TG content of hepatocytes was increased 6-fold, and the secretion of TG and apo B in VLDL was markedly reduced. The interpretation of these experiments was that hepatic VLDL secretion requires the synthesis of phosphatidylcholine from either the CDP-choline or methylation pathways which require choline or methionine, respectively, as precursors (D.E. Vance, 1988). However, since choline deprivation was induced in a background of methionine insufficiency, it was not clear whether the lack of choline per se, and inhibition of the choline pathway for phosphatidylcholine synthesis, decreased VLDL secretion. More recent experiments have shown, surprisingly, that deficiency of choline in primary mouse hepatocytes does not reduce, but increases, phosphatidylcholine synthesis via the CDP-choline pathway, and does not decrease VLDL secretion (J.E. Vance, 2004). Thus, a deficiency of dietary choline reduces plasma TG and apo B levels by a mechanism that does not involve reduction of phosphatidylcholine synthesis. 

To determine the role of the CDP-choline pathway of phosphatidylcholine synthesis for VLDL secretion, knockout mice were generated in which the gene encoding CTP phos-phocholine cytidylyltransferase-a (Pcytla), a key enzyme in phosphatidylcholine synthesis via the CDP-choline pathway (Chapter 8), was disrupted only in the liver. TG accumulated in livers of these mice and the secretion of TG and apo B was decreased (D.E. Vance, 2004). Thus, elimination of the CDP-choline pathway in the liver inhibits VLDL secretion. In addition, disruption of the gene encoding the liver-specific enzyme, phosphatidylethanolamine V-methyltransferase, in mice fed a high-fat/high-cholesterol diet, also markedly reduces the secretion of TG and apo B in VLDLs (D.E. Vance, 2003). Thus, the hepatic synthesis of phosphatidylcholine via both the CDP-choline and methylation pathways appears to be required for normal VLDL secretion. 

The soluble protein in animals can be aggregated under various conditions and its activity raised. It is probably the rate-limiting step in the CDP-choline pathway in animals (Vance and Choy,... 

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