Mechanisms of protection by the betaine-homocysteine methyltransferase/betaine system in HepG2 cells and primary mouse hepatocytes

Authors

  • Cheng Ji,

    Corresponding author
    1. From the University of Southern California Research Center for Liver Disease, Department of Medicine, Keck School of Medicine, University of Southern California, Los Angeles, California
    • Gastroenterology/Liver Division, Keck School of Medicine, University of Southern California, HMR-101, 2011 Zonal Avenue, Los Angeles, CA 90033===

    Search for more papers by this author
    • fax: 323-442-5425

  • Masao Shinohara,

    1. From the University of Southern California Research Center for Liver Disease, Department of Medicine, Keck School of Medicine, University of Southern California, Los Angeles, California
    Search for more papers by this author
  • John Kuhlenkamp,

    1. From the University of Southern California Research Center for Liver Disease, Department of Medicine, Keck School of Medicine, University of Southern California, Los Angeles, California
    Search for more papers by this author
  • Christine Chan,

    1. From the University of Southern California Research Center for Liver Disease, Department of Medicine, Keck School of Medicine, University of Southern California, Los Angeles, California
    Search for more papers by this author
  • Neil Kaplowitz

    1. From the University of Southern California Research Center for Liver Disease, Department of Medicine, Keck School of Medicine, University of Southern California, Los Angeles, California
    Search for more papers by this author

  • Potential conflict of interest: Nothing to report.

Abstract

Betaine-homocysteine methyltransferase (BHMT) regulates homocysteine levels in the liver. We previously reported that the alteration of BHMT is associated with alcoholic liver steatosis and injury. In this study, we tested whether BHMT protects hepatocytes from homocysteine-induced injury and lipid accumulation. Both BHMT transfectants of HepG2 cells and primary mouse hepatocytes with suppressed BHMT were generated. Comparisons were made between the cell models with respect to their response to homocysteine treatments. Homocysteine metabolism was impaired in HepG2 cells, and the expression of BHMT in HepG2 cells ameliorated the impairment and stabilized the levels of intracellular homocysteine after the addition of exogenous homocysteine. BHMT expression inhibited homocysteine-induced glucose-regulated protein 78 (GRP78) and C/EBP-homologous protein (CHOP) and homocysteine-induced cell death. A betaine treatment protected primary mouse hepatocytes from a homocysteine-induced increase in GRP78 and cell death but not a tunicamycin-induced increase. Homocysteine induced greater CHOP expression (2.7-fold) in BHMT small interfering RNA (siRNA)–transfected cells than in a control (1.9-fold). Homocysteine-induced cell death was increased by 40% in the siRNA-treated cells in comparison with the control. Apolipoprotein B (apoB) expression was higher and triglycerides and cholesterol were lower in HepG2 expressing BHMT. In primary mouse hepatocytes, homocysteine induced the accumulation of triglycerides and cholesterol, which was reduced in the presence of betaine. Betaine partially reduced homocysteine-induced sterol regulatory element binding protein 1 expression in HepG2 cells and increased S-adenosylmethionine in primary mouse hepatocytes. Conclusion: The BHMT/betaine system directly protects hepatocytes from homocysteine-induced injury but not tunicamycin-induced injury, including an endoplasmic reticulum stress response, lipid accumulation, and cell death. This system also exhibits a more generalized effect on liver lipids by inducing ApoB expression and increasing S-adenosylmethionine/S-adenosylhomocysteine. (HEPATOLOGY 2007.)

Ancillary