A Role for Hepatic Scavenger Receptor Class B, Type I in Decreasing High Density Lipoprotein Levels in Mice That Lack Phosphatidylethanolamine N-Methyltransferase

Julie C Robichaud,Gordon A Francis,Dennis E Vance

doi:10.1074/jbc.m807433200

Abstract

Phosphatidylethanolamine N-methyltransferase (PEMT) is a liver-specific enzyme that converts phosphatidylethanolamine to phosphatidylcholine (PC). Mice that lack PEMT have reduced plasma levels of PC and cholesterol in high density lipoproteins (HDL). We have investigated the mechanism responsible for this reduction with experiments designed to distinguish between a decreased formation of HDL particles by hepatocytes or an increased hepatic uptake of HDL lipids. Therefore, we analyzed lipid efflux to apoA-I and HDL lipid uptake using primary cultured hepatocytes isolated from Pemt(+/+) and Pemt(-/-) mice. Hepatic levels of the ATP-binding cassette transporter A1 are not significantly different between Pemt genotypes. Moreover, hepatocytes isolated from Pemt(-/-) mice released cholesterol and PC into the medium as efficiently as did hepatocytes from Pemt(+/+) mice. Immunoblotting of liver homogenates showed a 1.5-fold increase in the amount of the scavenger receptor, class B, type 1 (SR-BI) in Pemt(-/-) compared with Pemt(+/+) livers. In addition, there was a 1.5-fold increase in the SR-BI-interacting protein PDZK1. Lipid uptake experiments using radiolabeled HDL particles revealed a greater uptake of [(3)H]cholesteryl ethers and [(3)H]PC by hepatocytes derived from Pemt(-/-) compared with Pemt(+/+) mice. Furthermore, we observed an increased association of [(3)H]cholesteryl ethers in livers of Pemt(-/-) compared with Pemt(+/+) mice after tail vein injection of [(3)H]HDL. These results strongly suggest that PEMT is involved in the regulation of plasma HDL levels in mice, mainly via HDL lipid uptake by SR-BI.

Highlights

Found in plasma lipoproteins, bile and lung surfactant [1]
Plasma high density lipoproteins (HDL) Lipids Are Decreased by PE N-methyltransferase (PEMT) Deficiency—A significant reduction of plasma lipid levels was previously observed in chow-fed PemtϪ/Ϫ mice compared with Pemtϩ/ϩ mice [18]
ABCA1-mediated Lipid Efflux Is Not Decreased by the Absence of PEMT—We evaluated the role of PEMT-derived PC in HDL formation

Summary

Introduction

Found in plasma lipoproteins, bile and lung surfactant [1]. All mammalian cells can synthesize PC from choline via the Kennedy (CDP-choline) pathway [2]. PEMT deficiency in male mice challenged with a high fat/high cholesterol diet resulted in a 50% decrease in apoB100, PC and triacylglycerol (TG) secretion from the liver [9] These observations are in agreement with earlier work in which levels of secreted apoB100 and lipids (PC and TG) by hepatocytes from PemtϪ/Ϫ mice were 50 –70% lower than from Pemtϩ/ϩ mice [16]. SR-BI in Livers of Pemt؊/؊ Mice lation of lipid flux in and out of hepatocytes to maintain PC levels constant and ensure optimal liver functions In this respect, a significant reduction of cholesterol and PC in plasma HDL is observed in both genders of PemtϪ/Ϫ mice fed an HF/HC diet. Because hepatocytes secrete apoA-I, express ABCA1 on their cell surface, and can generate HDL by both ABCA1-dependent and -independent pathways [28, 29], we considered the possibility that ABCA1-mediated efflux of PC and cholesterol from PEMT-deficient hepatocytes might be decreased as a mechanism for maintaining PC homeostasis in the liver

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