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Hyperhomocysteinemia from trimethylation of hepatic phosphatidylethanolamine during cholesterol cholelithogenesis in inbred mice

Authors

  • Ji Zhang,

    1. Department of Medicine, Gastroenterology Division, Brigham and Women's Hospital, Harvard Medical School, Boston, MA
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  • Diane E. Handy,

    1. Department of Medicine, Cardiovascular Division, Brigham and Women's Hospital, Harvard Medical School, Boston, MA
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  • Yufang Wang,

    1. Molecular Pathology Unit, Center for Cancer Research, and Center for Systems Biology, Massachusetts General Hospital, Harvard Medical School, Charlestown, MA
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  • Guylaine Bouchard,

    1. Department of Medicine, Gastroenterology Division, Brigham and Women's Hospital, Harvard Medical School, Boston, MA
    Current affiliation:
    1. Département de Nutrition, Faculté de médicine, Université de Montréal, C.P. 6128, Montréal, Québec, Canada, H3C 3J7; and Hôpital Sainte-Justine, 3175, chemin de la Côte-Ste-Catherine, Montréal, Québec, Canada, H3T 1C5.
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  • Jacob Selhub,

    1. Vitamin Metabolism Laboratory, Jean Mayer USDA Human Nutrition Research Center on Aging at Tufts University, Boston, MA
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  • Joseph Loscalzo,

    1. Department of Medicine, Cardiovascular Division, Brigham and Women's Hospital, Harvard Medical School, Boston, MA
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  • Martin C. Carey

    Corresponding author
    1. Department of Medicine, Gastroenterology Division, Brigham and Women's Hospital, Harvard Medical School, Boston, MA
    • Gastroenterology Division, Brigham and Women's Hospital, 75 Francis St., Boston, MA 02115
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    • fax: (617) 730-5807


  • Potential conflict of interest: Nothing to report.

  • Supported in part by NIH (USPHS) grants: DK36588, HL61795, HL81587, HL70819, HL48743, HL89734; and USDA agreements: 1950-51520-008-00D and 58-1950-9-001.

Abstract

Because hyperhomocysteinemia can occur in cholesterol gallstone disease, we hypothesized that this may result from trimethylation of phosphatidylethanolamine (PE), which partakes in biliary phosphatidylcholine (PC) hypersecretion during cholesterol cholelithogenesis. We fed murine strains C57L/J, C57BL/6J, SWR/J, AKR/J, PE N-methyltransferase (PEMT) knockout (KO), PEMT heterozygous (HET), and wildtype (WT) mice a cholesterol/cholic acid lithogenic diet (LD) for up to 56 days and documented biliary lipid phase transitions and secretion rates. We quantified plasma total homocysteine (tHcy), folate, and vitamin B12 in plasma and liver, as well as biliary tHcy and cysteine secretion rates. Rate-limiting enzyme activities of PC synthesis, PEMT and cytidine triphosphate: phosphocholine cytidylyltransferase (PCT), S-adenosylmethionine (SAM), and S-adenosylhomocysteine (SAH) were measured in liver homogenates. Other potential sources of plasma tHcy, glycine N-methyltransferase (GNMT) and guanidinoacetate N-methyltransferase (GAMT), were assayed by gene expression. Plasma tHcy and PEMT activities became elevated during cholelithogenesis in gallstone-susceptible C57L, C57BL/6, and SWR mice but not in the gallstone-resistant AKR mice. Persisting in C57L mice, which exhibit the greatest Lith gene burden, these increases were accompanied by elevated hepatic SAM/SAH ratios and augmented biliary tHcy secretion rates. Counter-regulation included remethylation of Hcy to methionine concurrent with decreased folate and vitamin B12 levels and Hcy transsulfuration to cysteine. Concomitantly, methylenetetrahydrofolate reductase (Mthfr), betaine-homocysteine methyltransferase (Bhmt), and cystathionine-β-synthase (Cbs) were up-regulated, but Gnmt and Gamt genes were down-regulated. PEMT KO and HET mice displayed biliary lipid secretion rates and high gallstone prevalence rates similar to WT mice without any elevation in plasma tHcy levels. Conclusion: This work implicates up-regulation of PC synthesis by the PEMT pathway as a source of elevated plasma and bile tHcy during cholesterol cholelithogenesis. (HEPATOLOGY 2011;)

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