Physiological Regulation of Phospholipid Methylation Alters Plasma Homocysteine in Mice

René L Jacobs,Lori M Stead,Cecilia Devlin,Ira Tabas,Margaret E Brosnan,John T Brosnan,Dennis E Vance

doi:10.1074/jbc.m501971200

Abstract

Biological methylation reactions and homocysteine (Hcy) metabolism are intimately linked. In previous work, we have shown that phosphatidylethanolamine N-methyltransferase, an enzyme that methylates phosphatidylethanolamine to form phosphatidylcholine, plays a significant role in the regulation of plasma Hcy levels through an effect on methylation demand (Noga, A. A., Stead, L. M., Zhao, Y., Brosnan, M. E., Brosnan, J. T., and Vance, D. E. (2003) J. Biol. Chem. 278, 5952-5955). We have further investigated methylation demand and Hcy metabolism in liver-specific CTP:phosphocholine cytidylyltransferase-alpha (CTalpha) knockout mice, since flux through the phosphatidylethanolamine N-methyltransferase pathway is increased 2-fold to meet hepatic demand for phosphatidylcholine. Our data show that plasma Hcy is elevated by 20-40% in mice lacking hepatic CTalpha. CTalpha-deficient hepatocytes secrete 40% more Hcy into the medium than do control hepatocytes. Liver activity of betaine:homocysteine methyltransferase and methionine adenosyltransferase are elevated in the knockout mice as a mechanism for maintaining normal hepatic S-adenosylmethionine and S-adenosylhomocysteine levels. These data suggest that phospholipid methylation in the liver is a major consumer of AdoMet and a significant source of plasma Hcy.

Highlights

Biological methylation reactions and homocysteine (Hcy) metabolism are intimately linked
We have further investigated methylation demand and Hcy metabolism in liver-specific CTP:phosphocholine cytidylyltransferase-␣ (CT␣) knockout mice, since flux through the phosphatidylethanolamine N-methyltransferase pathway is increased 2-fold to meet hepatic demand for phosphatidylcholine
A deficiency of hepatic CT␣ did not change the amount of either metabolite, suggesting that the induction of betaine:Hcy methyltransferase (BHMT) and MAT was successful in maintaining a normal hepatic ratio of AdoMet to AdoHcy in the knockout mice. These results indicate that AdoMet is not limiting for Hcy formation and that the increased secretion of Hcy from CT␣deficient hepatocytes is attributable to an increased flux through the phosphatidylethanolamine N-methyltransferase (PEMT) pathway

Summary

Introduction

Biological methylation reactions and homocysteine (Hcy) metabolism are intimately linked. Liver activity of betaine:homocysteine methyltransferase and methionine adenosyltransferase are elevated in the knockout mice as a mechanism for maintaining normal hepatic S-adenosylmethionine and S-adenosylhomocysteine levels. These data suggest that phospholipid methylation in the liver is a major consumer of AdoMet and a significant source of plasma Hcy. Elevations in plasma homocysteine (Hcy), a nonprotein sulfur-containing amino acid, is an independent risk factor for cardiovascular [1, 2] and atherosclerotic diseases [3]. § Recipient of postdoctoral fellowships from the Canadian Institutes of Health Research and the Alberta Heritage Foundation for Medical Research. Altered Hcy metabolism has been observed in Alzheimer’s disease [4] and in the elderly with cognitive impairment [5]

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Conclusion