Identification and Characterization of a Major Liver Lysophosphatidylcholine Acyltransferase

Abstract

Phosphatidylcholine (PC) is synthesized through the Kennedy pathway, but more than 50% of PC is remodeled through the Lands cycle, i.e. the deacylation and reacylation of PC to attain the final and proper fatty acids within PC. The reacylation step is catalyzed by lysophosphatidylcholine acyltransferase (LPCAT), and we report here the identification of a novel LPCAT, which we named LPCAT3. LPCAT3 belongs to the membrane-bound O-acyltransferase (MBOAT) family and encodes a protein of 487 amino acids with a calculated molecular mass of 56 kDa. Membranes from HEK293 cells overexpressing LPCAT3 showed significantly increased LPCAT activity as assessed by thin layer chromatography analysis with substrate preference toward unsaturated fatty acids. LPCAT3 is localized within the endoplasmic reticulum and is primarily expressed in metabolic tissues including liver, adipose, and pancreas. In a human hepatoma Huh7 cells, RNA interference-mediated knockdown of LPCAT3 resulted in virtually complete loss of membrane LPCAT activity, suggesting that LPCAT3 is primarily responsible for hepatic LPCAT activity. Furthermore, peroxisome proliferator-activated receptor ␣ agonists dosedependently regulated LPCAT3 in liver in a peroxisome proliferator-activated receptor ␣-dependent fashion, implicating a role of LPCAT3 in lipid homeostasis. Our studies identify a long-sought enzyme that plays a critical role in PC remodeling in metabolic tissues and provide an invaluable tool for future investigations on how PC remodeling may potentially impact glucose and lipid homeostasis.

Signaling Role for Lysophosphatidylcholine Acyltransferase 3 in Receptor-Regulated Arachidonic Acid Reacylation Reactions in Human Monocytes

“…In mouse, LPCAT2 (53) and LPCAT3 (49) have been suggested to show a preference for AA. In human cells, it is LPCAT3 the form that appears to exhibit preference for AA, although also by linoleic acid (50,52). Shindou et al (53) have recently reported the increase of lyso-platelet-activating factor acetyltransferase activity in RAW364.7 cells transfected with the mouse lyso-platelet-activating factor acetyl transferase/LPCAT2 gene and stimulated with TLR agonists.…”

“…Four isoforms of LPCAT exist in mammalian cells, termed LPCAT1, LPCAT2, LPCAT3, and LPCAT4 (45-51), but only two of them, LPCAT2 and LPCAT3, have been documented to participate in AA reacylation reactions (49)(50)(51)(52)(53). Human peripheral blood monocytes express both LPCAT2 and LPCAT3 (data not shown).…”

Small Intestine but Not Liver Lysophosphatidylcholine Acyltransferase 3 (Lpcat3) Deficiency Has a Dominant Effect on Plasma Lipid Metabolism

“…With the cloning of Lpcat family members in recent years, it has become clear that these enzymes are responsible for PC remodeling and the generation of polyunsaturated PCs in various tissues. Lpcat3 is the major Lpcat isoform in the small intestine and liver (6). It has been shown that ablation of Lpcat3 in the small intestine at early stages of life is neonatally lethal and diminishes the incorporation of arachidonic acid into PCs within the cell membrane, leading to the impaired triglyceride mobilization and lipoprotein production (9, 10).…”

Lipogenesis in Huh7 cells is promoted by increasing the fructose: Glucose molar ratio

“…These experiments employ an established, immortal and metabolically active human hepatocellular carcinoma cell line, Huh7, used extensively in studies of hepatocyte metabolism [13][14][15] . Since facilitated uptake of glucose and fructose by the transmembrane GLUT2 transporter is demonstrated in Huh7 cells [16] , these cells provide an excellent model for studies of carbohydrate-induced lipogenesis. Accordingly, the studies herein were carried out to determine whether hepatocyte lipogenesis, in an in vitro cell culture model, is modulated by adjusting culture media monosaccharide content and concentration.…”