Phospholipase D and Choline Metabolism

Abstract

Phospholipases D (PLDs) catalyze hydrolysis of the diester bond of phospholipids to generate phosphatidic acid and the free lipid headgroup. In mammals, PLD enzymes comprise the intracellular enzymes PLD1 and PLD2 and possibly the proteins encoded by related genes, as well as a class of cell surface and secreted enzymes with structural homology to ectonucleotide phosphatases/phosphodiesterases as typified by autotaxin (ENPP2) that have lysoPLD activities. Genetic and pharmacological loss-of-function approaches implicate these enzymes in intra- and intercellular signaling mediated by the lipid products phosphatidic acid, lysophosphatidic acid, and their metabolites, while the possibility that the water-soluble product of their reactions is biologically relevant has received far less attention. PLD1 and PLD2 are highly selective for phosphatidylcholine (PC), whereas autotaxin has broader substrate specificity for lysophospholipids but by virtue of the high abundance of lysophosphatidylcholine (LPC) in extracellular fluids predominantly hydrolyses this substrate. In all cases, the water-soluble product of these PLD activities is choline. Although choline can be formed de novo by methylation of phosphatidylethanolamine, this activity is absent in most tissues, so mammals are effectively auxotrophic for choline. Dietary consumption of choline in both free and esterified forms is substantial. Choline is necessary for synthesis of the neurotransmitter acetylcholine and of the choline-containing phospholipids PC and sphingomyelin (SM) and also plays a recently appreciated important role as a methyl donor in the pathways of "one-carbon (1C)" metabolism. This review discusses emerging evidence that some of the biological functions of these intra- and extracellular PLD enzymes involve generation of choline with a particular focus on the possibility that these choline and PLD dependent processes are dysregulated in cancer.

Keywords: Choline; One carbon metabolism; Phospholipase D.

Figure 1.. Role of Diet and Phospholipase D enzyme in the complex network of choline pathways and Choline-Mediated 1C metabolism.

Phospholipase D (PLD) catalyzes release of choline from either diet-derived or endogenously synthesized phosphatidylcholine (PC). The free choline product of PLD activity, or obtained from diet, is a substrate for the synthesis of acetylcholine, trimethylamine (TMA), and phosphatidylcholine (PC) via Cytidine diphosphate choline (CDP-choline). Formation of phosphocholine catalyzed by choline kinase α (ChoKα) is the initial step of de novo PC synthesis. Choline can also be oxidized to betaine. The oxidation of choline to betaine links PLD enzyme activity to 1C metabolism through de novo synthesis of methionine from homocysteine. The byproduct of this reaction dimethylglycine (DMG) is also a potential methyl donor. The methionine product is utilized for synthesis of S-adenocylmethionine (SAM) catalyzed by methionine adenosyltransferase 2A (MAT2A). SAM is the universal methyl donor for a wide range of substrates including phosphatidylethanolamine (PE), DNA, RNA and proteins including histones, and generating S-adenosylhomocysteine (SAH). SAH is hydrolyzed to homocysteine which couples 1C metabolism to the transsulfuration pathway involving sequential synthesis of cystathionine and cysteine and ultimately forming glutathione, taurine and hydrogen sulfide (H₂S). SAM can also be decarboxylated to decarboxylated S-adenosylmethionine (dc-SAM) which supports synthesis of the polyamines, spermine and spermidine. The byproduct 50 -methylthioadenosine (MTA) is salvaged back to methionine for SAM generation. The highlighted enzymes PLD, ChoKα and MAT2A are presently attractive therapeutic targets for treatment of cancer.