Localization-independent Regulation of Homocysteine Secretion by Phosphatidylethanolamine N-Methyltransferase

David J Shields,Susanne Lingrell,Luis B Agellon,John T Brosnan,Dennis E Vance

doi:10.1074/jbc.m504658200

Abstract

Genetic ablation of phosphatidylethanolamine N-methyltransferase (PEMT) in mice causes a 50% reduction in plasma homocysteine (Hcy) levels. Because hyperhomocysteinemia is an independent risk factor for cardiovascular disease, resolution of the molecular basis for this reduction is of significant clinical interest. The PEMT pathway is a metabolically channeled process localized to the endoplasmic reticulum (ER). To assess the importance of PEMT localization for Hcy homeostasis, we identified and ablated the minimal ER targeting motif. Mutagenesis of a conserved, C-terminal lysine residue (197) relocalized the enzyme to the Golgi, demonstrating that Lys-197 is essential for targeting PEMT to the ER. To evaluate the functional significance of PEMT localization, hepatoma cell lines were generated that stably expressed either ER- or Golgi-localized PEMT only. Intriguingly, stable expression of PEMT in either the ER or the Golgi caused increased Hcy secretion. Moreover, PEMT-mediated Hcy secretion correlated with the methyltransferase activity of the enzyme, independently of subcellular localization. Thus, our data suggest that Hcy homeostasis is regulated concomitantly with PEMT activity but independently of PEMT localization.

Highlights

Elevated Hcy1 levels are a risk factor for cardiovascular disease, in particular atherosclerosis and myocardial infarction [1, 2]
Because the phosphatidylethanolamine N-methyltransferase (PEMT) pathway is a metabolically channeled process [22], we investigated whether precise subcellular localization might be essential to the role of PEMT in Hcy homeostasis
Hcy Secretion Increases Concomitantly with PEMT Activity— To further define the molecular determinants of PEMT-dependent Hcy homeostasis, we investigated whether Hcy secretion is modulated coordinately with PEMT activity

Summary

Introduction

Elevated Hcy levels are a risk factor for cardiovascular disease, in particular atherosclerosis and myocardial infarction [1, 2]. Meta-analyses of clinical studies have revealed that hyperhomocysteinemia is positively associated with an increased risk of cardiovascular disease and stroke. § Supported by a studentship from the Alberta Heritage Foundation for Medical Research. ** Canada Research Chair in Molecular and Cell Biology of Lipids and Heritage Medical Scientist of the Alberta Heritage Foundation for Medical Research. Hyperhomocysteinemia is an independent risk factor for Alzheimer disease and dementia [7]. Primary hepatocytes cultured from PEMT null mice secrete ϳ50% less Hcy [8]. Our data reveal a key, yet previously unrecognized, role for PEMT in Hcy homeostasis

Objectives

Results

Conclusion