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Molecular Medicine

, Volume 17, Issue 5–6, pp 486–494 | Cite as

GNMT Expression Increases Hepatic Folate Contents and Folate-Dependent Methionine Synthase-Mediated Homocysteine Remethylation

  • Yi-Cheng Wang
  • Yi-Ming Chen
  • Yan-Jun Lin
  • Shih-Ping Liu
  • En-Pei Isabel Chiang
Research Article

Abstract

Glycine N-methyltransferase (GNMT) is a major hepatic enzyme that converts S-adenosylmethionine to S-adenosylhomocys-teine while generating sarcosine from glycine, hence it can regulate mediating methyl group availability in mammalian cells. GNMT is also a major hepatic folate binding protein that binds to, and, subsequently, may be inhibited by 5-methyltetrafolate. GNMT is commonly diminished in human hepatoma; yet its role in cellular folate metabolism, in tumorigenesis and antifolate therapies, is not understood completely. In the present study, we investigated the impacts of GNMT expression on cell growth, folate status, methylfolate-dependent reactions and antifolate cytotoxicity. GNMT-diminished hepatoma cell lines transfected with GNMT were cultured under folate abundance or restriction. Folate-dependent homocysteine remethylation fluxes were investigated using stable isotopic tracers and gas chromatography/mass spectrometry. Folate status was compared between wild-type (WT), GNMT transgenic (GNMTtg) and GNMT knockout (GNMTko) mice. In the cell model, GNMT expression increased folate concentration, induced folate-dependent homocysteine remethylation, and reduced antifolate methotrexate cytotoxicity. In the mouse models, GNMTtg had increased hepatic folate significantly, whereas GNMTko had reduced folate. Liver folate levels correlated well with GNMT expressions (r = 0.53, P = 0.002); and methionine synthase expression was reduced significantly in GNMTko, demonstrating impaired methylfolate-dependent metabolism by GNMT deletion. In conclusion, we demonstrated novel findings that restoring GNMT assists methylfolate-dependent reactions and ameliorates the consequences of folate depletion. GNMT expression in vivo improves folate retention and bioavailability in the liver. Studies on how GNMT expression impacts the distribution of different folate cofactors and the regulation of specific folate dependent reactions are underway.

Notes

Acknowledgments

This project was supported in part by National Science Council (NSC98-2320-B005-004MY3, EP Chiang) and by the Department of Health in Taiwan (DOH 97-TD-D-113-97011, EP Chiang).

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Copyright information

© The Feinstein Institute for Medical Research 2011

Authors and Affiliations

  • Yi-Cheng Wang
    • 1
  • Yi-Ming Chen
    • 2
  • Yan-Jun Lin
    • 1
  • Shih-Ping Liu
    • 2
  • En-Pei Isabel Chiang
    • 1
  1. 1.Department of Food Science and BiotechnologyNational Chung Hsing UniversityTaichungTaiwan
  2. 2.Institute of Microbiology and ImmunologyNational Yang-Ming UniversityTaipeiTaiwan

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