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      Ginsenoside Rg3 Reduces Lipid Accumulation with AMP-Activated Protein Kinase (AMPK) Activation in HepG2 Cells

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          Abstract

          Cardiovascular disease (CVD) is one of the main causes of mortality worldwide, and dyslipidemia is a major risk factor for CVD. Ginseng has been widely used in the clinic to treat CVD. Ginsenoside Rg3, one of the major active components of ginseng, has been reported to exhibit antiobesity, antidiabetic, and cardioprotective effects. However, the effect of ginsenoside Rg3 on hepatic lipid metabolism remains unclear. Therefore, we investigated whether ginsenoside Rg3 would regulate hepatic lipid metabolism with AMP-activated protein kinase (AMPK) activation in HepG2 cells. Ginsenoside Rg3 significantly reduced hepatic cholesterol and triglyceride levels. Furthermore, ginsenoside Rg3 inhibited expression of sterol regulatory element binding protein-2 (SREBP-2) and 3-hydroxy-3-methyl glutaryl coenzyme A reductase (HMGCR). Ginsenoside Rg3 increased activity of AMPK, a major regulator of energy metabolism. These results suggest that ginsenoside Rg3 reduces hepatic lipid accumulation with inhibition of SREBP-2 and HMGCR expression and stimulation of AMPK activity in HepG2 cells. Therefore, ginsenoside Rg3 may be beneficial as a food ingredient to lower the risk of CVD by regulating dyslipidemia.

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          Most cited references 20

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          Activation of cholesterol synthesis in preference to fatty acid synthesis in liver and adipose tissue of transgenic mice overproducing sterol regulatory element-binding protein-2.

          We produced transgenic mice that express a dominant-positive truncated form of sterol regulatory element-binding protein-2 (SREBP-2) in liver and adipose tissue. The encoded protein lacks the membrane-binding and COOH-terminal regulatory domains, and it is therefore not susceptible to negative regulation by cholesterol. Livers from the transgenic mice showed increases in mRNAs encoding multiple enzymes of cholesterol biosynthesis, the LDL receptor, and fatty acid biosynthesis. The elevations in mRNA for 3-hydroxy-3-methylglutaryl coenzyme A (HMG CoA) synthase and HMG CoA reductase were especially marked (13-fold and 75-fold, respectively). As a result, the transgenic livers showed a 28-fold increase in the rate of cholesterol synthesis and a lesser fourfold increase in fatty acid synthesis, as measured by intraperitoneal injection of [3H]water. These results contrast with previously reported effects of dominant-positive SREBP-1a, which activated fatty acid synthesis more than cholesterol synthesis. In adipose tissue of the SREBP-2 transgenics, the mRNAs for cholesterol biosynthetic enzymes were elevated, but the mRNAs for fatty acid biosynthetic enzymes were not. We conclude that SREBP-2 is a relatively selective activator of cholesterol synthesis, as opposed to fatty acid synthesis, in liver and adipose tissue of mice.
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            AMP-activated protein kinase is required for the lipid-lowering effect of metformin in insulin-resistant human HepG2 cells.

            The antidiabetic drug metformin stimulates AMP-activated protein kinase (AMPK) activity in the liver and in skeletal muscle. To better understand the role of AMPK in the regulation of hepatic lipids, we studied the effect of metformin on AMPK and its downstream effector, acetyl-CoA carboxylase (ACC), as well as on lipid content in cultured human hepatoma HepG2 cells. Metformin increased Thr-172 phosphorylation of the alpha subunit of AMPK in a dose- and time-dependent manner. In parallel, phosphorylation of ACC at Ser-79 was increased, which was consistent with decreasing ACC activity. Intracellular triacylglycerol and cholesterol contents were also decreased. These effects of metformin were mimicked or completely abrogated by adenoviral-mediated expression of a constitutively active AMPKalpha or a kinase-inactive AMPKalpha, respectively. An insulin-resistant state was induced by exposing cells to 30 mm glucose as indicated by decreased phosphorylation of Akt and its downstream effector, glycogen synthase kinase 3alpha/beta. Under these conditions, the phosphorylation of AMPK and ACC was also decreased, and the level of hepatocellular triacylglycerols increased. The inhibition of AMPK and the accumulation of lipids caused by high glucose concentrations were prevented either by metformin or by expressing the constitutively active AMPKalpha. The kinase-inactive AMPKalpha increased lipid content and blocked the ability of metformin to decrease lipid accumulation caused by high glucose concentrations. Taken together, these results indicate that AMPKalpha negatively regulates ACC activity and hepatic lipid content. Inhibition of AMPK may contribute to lipid accumulation induced by high concentrations of glucose associated with insulin resistance. Metformin lowers hepatic lipid content by activating AMPK, thereby mediating beneficial effects in hyperglycemia and insulin resistance.
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              SREBP cleavage-activating protein (SCAP) is required for increased lipid synthesis in liver induced by cholesterol deprivation and insulin elevation.

              In liver, the synthesis of cholesterol and fatty acids increases in response to cholesterol deprivation and insulin elevation, respectively. This regulatory mechanism underlies the adaptation to cholesterol synthesis inhibitors (statins) and high calorie diets (insulin). In nonhepatic cells, lipid synthesis is controlled by sterol regulatory element-binding proteins (SREBPs), membrane-bound transcription factors whose active domains are released proteolytically to enter the nucleus and activate genes involved in the synthesis and uptake of cholesterol and fatty acids. SCAP (SREBP cleavage-activating protein) is a sterol-regulated escort protein that transports SREBPs from their site of synthesis in the endoplasmic reticulum to their site of cleavage in the Golgi. Here, we produced a conditional deficiency of SCAP in mouse liver by genomic recombination mediated by inducible Cre recombinase. SCAP-deficient mice showed an 80% reduction in basal rates of cholesterol and fatty acid synthesis in liver, owing to decreases in mRNAs encoding multiple biosynthetic enzymes. Moreover, these mRNAs failed to increase normally in response to cholesterol deprivation produced by a cholesterol synthesis inhibitor and to insulin elevation produced by a fasting-refeeding protocol. These data provide in vivo evidence that SCAP and the SREBPs are required for hepatic lipid synthesis under basal and adaptive conditions.
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                Author and article information

                Journal
                Int J Mol Sci
                Int J Mol Sci
                ijms
                International Journal of Molecular Sciences
                Molecular Diversity Preservation International (MDPI)
                1422-0067
                2012
                11 May 2012
                : 13
                : 5
                : 5729-5739
                Affiliations
                [1 ]Department of Nutritional Science and Food Management, Ewha Womans University, Seoul 120-750, Korea; E-Mails: hayeeun@ 123456empas.com (S.L.); troph@ 123456hanmail.net (M.-S.L.)
                [2 ]Food Bio-Nano Research Group, Korea Food Research Institute, Seongnam, Gyeonggi 463-746, Korea; E-Mail: ctkim@ 123456kfri.re.kr
                [3 ]Department of Food and Nutrition, College of Health Sciences, Korea University, Seoul 136-703, Korea; E-Mail: k610in@ 123456korea.ac.kr
                Author notes
                [* ]Author to whom correspondence should be addressed; E-Mail: yhmoon@ 123456ewha.ac.kr ; Tel.:+82-2-3277-3101; Fax: +82-2-3277-4425.
                [†]

                These authors contributed equally to the work.

                Article
                ijms-13-05729
                10.3390/ijms13055729
                3382760
                22754327
                © 2012 by the authors; licensee Molecular Diversity Preservation International, Basel, Switzerland.

                This article is an open-access article distributed under the terms and conditions of the Creative Commons Attribution license ( http://creativecommons.org/licenses/by/3.0/).

                Categories
                Article

                Molecular biology

                ginsenoside rg3, cholesterol, srebp-2, cardiovascular disease (cvd), ampk, triglyceride, hmgcr

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