title: Localization-independent regulation of homocysteine secretion by phosphatidylethanolamine N-methyltransferase.
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.
authors.affiliation: Department of Biochemistry and Canadian Institutes of Health Research Group on Molecular & Cell Biology of Lipids, University of Alberta, Edmonton, Alberta T6G 2S2, Canada.
authors.forename: David J
authors.initials: DJ
authors.lastname: Shields
authors.forename: Susanne
authors.initials: S
authors.lastname: Lingrell
authors.forename: Luis B
authors.initials: LB
authors.lastname: Agellon
authors.forename: John T
authors.initials: JT
authors.lastname: Brosnan
authors.forename: Dennis E
authors.initials: DE
authors.lastname: Vance
mesh_terms: D000818:Animals; D002460:Cell Line; D017124:Conserved Sequence; D004721:Endoplasmic Reticulum / Q000378:metabolism; D006056:Golgi Apparatus / Q000378:metabolism; D006706:Homeostasis; D006710:Homocysteine / Q000378:metabolism*; D006801:Humans; D008780:Methyltransferases / Q000378:metabolism / Q000502:physiology*; D016297:Mutagenesis, Site-Directed; D050918:Phosphatidylethanolamine N-Methyltransferase; D021382:Protein Sorting Signals; D051381:Rats; D014162:Transfection
chemical_list: D021382:Protein Sorting Signals; D006710:Homocysteine; D008780:Methyltransferases; C494489:PEMT protein, human; D050918:Phosphatidylethanolamine N-Methyltransferase