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


CONCLUSIONS
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.
Ji
Cheng
C
University of Southern California Research Center for Liver Disease, Department of Medicine, Keck School of Medicine, University of Southern California, Los Angeles, California, USA. chengji@usc.edu
Shinohara
Masao
M
Kuhlenkamp
John
J
Chan
Christine
C
Kaplowitz
Neil
N
D000818:Animals; D001055:Apolipoproteins B / Q000378:metabolism; D001622:Betaine / Q000378:metabolism; D050940:Betaine-Homocysteine S-Methyltransferase / Q000378:metabolism*; D006528:Carcinoma, Hepatocellular / Q000378:metabolism; D016923:Cell Death / Q000502:physiology; D045744:Cell Line, Tumor; D004721:Endoplasmic Reticulum / Q000502:physiology*; D000091342:Endoplasmic Reticulum Chaperone BiP; D005234:Fatty Liver / Q000378:metabolism*; D018014:Gene Transfer Techniques; D022781:Hepatocytes / Q000378:metabolism*; D006710:Homocysteine / Q000378:metabolism / Q000502:physiology*; D006801:Humans; D050356:Lipid Metabolism / Q000502:physiology; D008113:Liver Neoplasms / Q000378:metabolism; D051379:Mice; D012435:S-Adenosylhomocysteine / Q000378:metabolism; D012436:S-Adenosylmethionine / Q000378:metabolism; D051780:Sterol Regulatory Element Binding Protein 1 / Q000378:metabolism
D001055:Apolipoproteins B; D000091342:Endoplasmic Reticulum Chaperone BiP; C000717247:HSPA5 protein, human; C000717248:Hspa5 protein, mouse; D051780:Sterol Regulatory Element Binding Protein 1; D006710:Homocysteine; D001622:Betaine; D012436:S-Adenosylmethionine; D012435:S-Adenosylhomocysteine; D050940:Betaine-Homocysteine S-Methyltransferase
J Hepatol. 2006 Aug;45(2):321-33
16797772
J Hepatol. 2006 Nov;45(5):717-24
16879892
Environ Health Perspect. 2006 Oct;114(10):1547-52
17035141
Biochem J. 2007 Jan 1;401(1):87-96
16953798
Cell Mol Life Sci. 2006 Dec;63(23):2792-803
17086380
J Hepatol. 2007 Feb;46(2):314-21
17156888
Am J Clin Nutr. 2007 Jan;85(1):19-25
17209172
Gene. 2000 May 30;250(1-2):31-40
10854776
Biochem Biophys Res Commun. 2000 Jul 14;273(3):1156-60
10891388
Mol Genet Metab. 2000 Nov;71(3):511-9
11073719
J Hepatol. 2000 Dec;33(6):907-14
11131452
J Clin Invest. 2001 May;107(10):1263-73
11375416
Hepatology. 2002 Oct;36(4 Pt 1):829-39
12297830
J Biol Chem. 2003 Feb 28;278(9):7459-68
12397072
Am J Clin Nutr. 2003 May;77(5):1094-5
12716657
Gastroenterology. 2003 May;124(5):1488-99
12730887
Hepatology. 2003 Jun;37(6):1425-34
12774022
Atherosclerosis. 2003 Apr;167(2):205-14
12818402
J Theor Biol. 2004 Jan 7;226(1):33-43
14637052
Hepatology. 2004 Aug;40(2):442-51
15368449
J Biol Chem. 1984 Aug 10;259(15):9508-13
6746658
Am J Clin Nutr. 1992 Jan;55(1):131-8
1728812
Alcohol Clin Exp Res. 1993 Jun;17(3):552-5
8333583
Proc Natl Acad Sci U S A. 1995 Feb 28;92(5):1585-9
7878023
J Biol Chem. 1996 Sep 13;271(37):22831-8
8798461
J Anim Sci. 1998 Feb;76(2):606-10
9498371
Arch Biochem Biophys. 1998 Aug 1;356(1):93-8
9681996
Biochem J. 1999 Aug 1;341 ( Pt 3):639-45
10417327
Am J Med Genet A. 2005 Jun 15;135(3):274-7
15887275
Am J Physiol Gastrointest Liver Physiol. 2005 Jul;289(1):G54-63
15705656
Biochem J. 2006 Apr 15;395(2):363-71
16396637
Cell Metab. 2006 May;3(5):321-31
16679290
J Nutr. 2006 Jun;136(6 Suppl):1741S-1749S
16702349
J Nutr. 2006 Jun;136(6 Suppl):1750S-1754S
16702350