Involvement and mechanism of DGAT2 upregulation in the pathogenesis of alcoholic fatty liver disease.
ABSTRACT The mechanisms involved in the development of alcoholic liver disease (ALD) are not well established. We investigated the involvement of acyl-CoA: diacylglycerol acyltransferase 2 (DGAT2) upregulation in mediating hepatic fat accumulation induced by chronic alcohol consumption. Chronic alcohol feeding caused fatty liver and increased hepatic DGAT2 gene and protein expression, concomitant with a significant suppression of hepatic MAPK/ERK kinase/extracellular regulated kinase 1/2 (MEK/ERK1/2) activation. In vitro studies demonstrated that specific inhibitors of the MEK/ERK1/2 pathway increased DGAT2 gene expression and triglyceride (TG) contents in HepG2 cells, whereas epidermal growth factor, a strong ERK1/2 activator, had the opposite effect. Moreover, chronic alcohol feeding decreased hepatic S-adenosylmethionine (SAM): S-adenosylhomocysteine (SAH) ratio, an indicator of disrupted transmethylation reactions. Mechanistic investigations revealed that N-acetyl-S-farnesyl-L-cysteine, a potent inhibitor of isoprenylcysteine carboxyl methyltransferase, suppressed ERK1/2 activation, followed by an enhanced DGAT2 expression and an elevated TG content in HepG2 cells. Lastly, we demonstrated that the beneficial effects of betaine supplementation in ALD were associated with improved SAM/SAH ratio, alleviated ERK1/2 inhibition, and attenuated DGAT2 upregulation. In conclusion, our data suggest that upregulation of DGAT2 plays an important role in the pathogenesis of ALD, and that abnormal methionine metabolism contributes, at least partially, to DGAT2 upregulation via suppression of MEK/ERK1/2 activation.
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ABSTRACT: Alcoholic liver disease is a major cause of illness and death in the United States. In the initial stages of the disease, fat accumulation in hepatocytes leads to the development of fatty liver (steatosis), which is a reversible condition. If alcohol consumption is continued, steatosis may progress to hepatitis and fibrosis, which may lead to liver cirrhosis. Alcoholic fatty liver has long been considered benign; however, increasing evidence supports the idea that it is a pathologic condition. Blunting of the accumulation of fat within the liver during alcohol consumption may block or delay the progression of fatty liver to hepatitis and fibrosis. To achieve this goal, it is important to understand the underlying biochemical and molecular mechanisms by which chronic alcohol consumption leads to fat accumulation in the liver and fatty liver progresses to hepatitis and fibrosis. In addition to alcohol consumption, dietary fatty acids and obesity have been shown to affect the degree of fat accumulation within the liver. Again, it is important to know how these factors modulate the progression of alcoholic liver disease. The National Institute on Alcohol Abuse and Alcoholism and the Office of Dietary Supplements, National Institutes of Health, sponsored a symposium on "Role of Fatty Liver, Dietary Fatty Acid Supplements, and Obesity in the Progression of Alcoholic Liver Disease" in Bethesda, Maryland, USA, October 2003. The following is a summary of the symposium. Alcoholic fatty liver is a pathologic condition that may predispose the liver to further injury (hepatitis and fibrosis) by cytochrome P450 2E1 induction, free radical generation, lipid peroxidation, nuclear factor-kappa B activation, and increased transcription of proinflammatory mediators, including tumor necrosis factor-alpha. Increased acetaldehyde production and lipopolysaccharide-induced Kupffer cell activation may further exacerbate liver injury. Acetaldehyde may promote hepatic fat accumulation by impairing the ability of peroxisome proliferator-activated receptor alpha to bind DNA, and by increasing the synthesis of sterol regulatory binding protein-1. Unsaturated fatty acids (corn oil, fish oil) exacerbate alcoholic liver injury by accentuating oxidative stress, whereas saturated fatty acids are protective. Polyenylphosphatidylcholine may prevent liver injury by down-regulating cytochrome P450 2E1 activity, attenuating oxidative stress, reducing the number of activated hepatic stellate cells, and up-regulating collagenase activity. Nonalcoholic steatohepatitis may develop through several mechanisms, such as oxidative stress, mitochondrial dysfunction and associated impaired fat metabolism, dysregulated cytokine metabolism, insulin resistance, and altered methionine/S-adenosylmethionine/homocysteine metabolism. Obesity (adipose tissue) may contribute to the development of alcoholic liver disease by generating free radicals, increasing tumor necrosis factor-alpha production, inducing insulin resistance, and producing fibrogenic agents, such as angiotensin II, norepinephrine, neuropeptide Y, and leptin. Finally, alcoholic fatty liver transplant failure may be linked to oxidative stress. In vitro treatment of fatty livers with interleukin-6 may render allografts safer for clinical transplantation.Alcohol 09/2004; 34(1):3-8. · 2.26 Impact Factor
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ABSTRACT: The expression of the c-myc gene has previously been shown to be elevated and deregulated in the human hepatoma cell line Hep G2 (B. E. Huber and S. S. Thorgeirsson, Cancer Res., 47: 3414-3420, 1987). We now report that the Hep G2 N-ras gene is activated to a dominant-acting, transforming gene by a missense mutation in codon 61. Hep G2 DNA produced transformed foci when transfected into NIH 3T3 cells. Subsequent to a secondary round of transfection, Southern blot analysis of tumorigenic NIH 3T3 foci demonstrated the presence of human N-ras sequences. Nucleotide sequence analysis of one Hep G2 N-ras allele demonstrated that codons 12, 13, and 59 were normal and that codon 61 had a missense mutation (CAA to CTA). This mutation results in the incorporation of leucine instead of glutamine at residue 61 of the N-ras gene product, p21. N-ras sequences were amplified by the polymerase chain reaction from both Hep G2 genomic DNA and Hep G2 complementary DNA. Analysis of the amplified sequences demonstrated that only one Hep G2 N-ras allele exhibited the codon 61 mutation and that both the mutant and normal alleles were transcribed. Northern blot analysis demonstrated equivalent steady-state levels of N-ras transcripts in Hep G2 cells and normal human liver. The steady-state levels of N-ras and ornithine decarboxylase transcripts were positively correlated suggesting a positive relationship between N-ras expression and the replication rate of Hep G2 cells. c-Ki-ras and c-Ha-ras transcripts were not detected in either Hep G2 cells or normal human liver. Immunoprecipitation experiments using the monoclonal antibody Y13-259 demonstrated the presence of p21 in Hep G2 cells. Expression of a dominant-acting, transforming N-ras gene, in conjunction with the altered regulation of the c-myc gene, documents two important genetic lesions that could be responsible for the transformed phenotype of Hep G2 cells.Cancer Research 04/1990; 50(5):1521-7. · 8.65 Impact Factor
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ABSTRACT: Chronic ethanol consumption can cause sustained hepatocellular injury and inhibit the subsequent regenerative response. These effects of ethanol may be mediated by impaired hepatocyte survival mechanisms. The present study examines the effects of ethanol on survival signaling in the intact liver. Adult Long Evans rats were maintained on ethanol-containing or isocaloric control liquid diets for 8 weeks, after which the livers were harvested to measure mRNA levels, protein expression, and kinase or phosphatase activity related to survival or proapoptosis mechanisms. Chronic ethanol exposure resulted in increased hepatocellular labeling for activated caspase 3 and nuclear DNA damage as demonstrated using the TUNEL assay. These effects of ethanol were associated with reduced levels of tyrosyl phosphorylated (PY) IRS-1 and PI3 kinase, Akt kinase, and Erk MAPK activities and increased levels of phosphatase tensin homologue deleted on chromosome 10 (PTEN) mRNA, protein, and phosphatase activity in liver tissue. In vitro experiments demonstrated that ethanol increases PTEN expression and function in hepatocytes. However, analysis of signaling cascade pertinent to PTEN function revealed increased levels of nuclear p53 and Fas receptor mRNA but without corresponding increases in GSK-3 activity or activated BAD. Although fork-head transcription factor levels were increased in ethanol-exposed livers, virtually all of the fork-head protein detected by Western blot analysis was localized within the cytosolic fraction. In conclusion, chronic ethanol exposure impairs survival mechanisms in the liver because of inhibition of signaling through PI3 kinase and Akt and increased levels of PTEN. However, uncoupling of the signaling cascade downstream of PTEN that mediates apoptosis may account for the relatively modest degrees of ongoing cell loss observed in livers of chronic ethanol-fed rats.Hepatology 10/2003; 38(3):703-14. · 12.00 Impact Factor