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      Keap1-Knockdown Decreases Fasting-Induced Fatty Liver via Altered Lipid Metabolism and Decreased Fatty Acid Mobilization from Adipose Tissue

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          Abstract

          Aims

          The purpose of this study was to determine whether Nrf2 activation, via Keap1-knockdown (Keap1-KD), regulates lipid metabolism and mobilization induced by food deprivation (e.g. fasting).

          Methods and Results

          Male C57BL/6 (WT) and Keap1-KD mice were either fed ad libitum or food deprived for 24 hours. After fasting, WT mice exhibited a marked increase in hepatic lipid accumulation, but Keap1-KD mice had an attenuated increase of lipid accumulation, along with reduced expression of lipogenic genes (acetyl-coA carboxylase, stearoyl-CoA desaturase-1, and fatty acid synthase) and reduced expression of genes related to fatty acid transport, such as fatty acid translocase/CD36 (CD36) and Fatty acid transport protein (FATP) 2, which may attribute to the reduced induction of Peroxisome proliferator-activated receptor (Ppar) α signaling in the liver. Additionally, enhanced Nrf2 activity by Keap1-KD increased AMP-activated protein kinase (AMPK) phosphorylation in liver. In white adipose tissue, enhanced Nrf2 activity did not change the lipolysis rate by fasting, but reduced expression of fatty acid transporters — CD36 and FATP1, via a PPARα-dependent mechanism, which impaired fatty acid transport from white adipose tissue to periphery circulation system, and resulted in increased white adipose tissue fatty acid content. Moreover, enhanced Nrf2 activity increased glucose tolerance and Akt phosphorylation levels upon insulin administration, suggesting Nrf2 signaling pathway plays a key role in regulating insulin signaling and enhanced insulin sensitivity in skeletal muscle.

          Conclusion

          Enhanced Nrf2 activity via Keap1-KD decreased fasting-induced steatosis, pointing to an important function of Nrf2 on lipid metabolism under the condition of nutrient deprivation.

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          Most cited references 42

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          Peroxisome proliferator-activated receptor alpha mediates the adaptive response to fasting.

          Prolonged deprivation of food induces dramatic changes in mammalian metabolism, including the release of large amounts of fatty acids from the adipose tissue, followed by their oxidation in the liver. The nuclear receptor known as peroxisome proliferator-activated receptor alpha (PPARalpha) was found to play a role in regulating mitochondrial and peroxisomal fatty acid oxidation, suggesting that PPARalpha may be involved in the transcriptional response to fasting. To investigate this possibility, PPARalpha-null mice were subjected to a high fat diet or to fasting, and their responses were compared with those of wild-type mice. PPARalpha-null mice chronically fed a high fat diet showed a massive accumulation of lipid in their livers. A similar phenotype was noted in PPARalpha-null mice fasted for 24 hours, who also displayed severe hypoglycemia, hypoketonemia, hypothermia, and elevated plasma free fatty acid levels, indicating a dramatic inhibition of fatty acid uptake and oxidation. It is shown that to accommodate the increased requirement for hepatic fatty acid oxidation, PPARalpha mRNA is induced during fasting in wild-type mice. The data indicate that PPARalpha plays a pivotal role in the management of energy stores during fasting. By modulating gene expression, PPARalpha stimulates hepatic fatty acid oxidation to supply substrates that can be metabolized by other tissues.
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            Hepatocyte-specific deletion of SIRT1 alters fatty acid metabolism and results in hepatic steatosis and inflammation.

            Hepatic metabolic derangements are key components in the development of fatty liver, insulin resistance, and atherosclerosis. SIRT1, a NAD+-dependent protein deacetylase, is an important regulator of energy homeostasis in response to nutrient availability. Here we demonstrate that hepatic SIRT1 regulates lipid homeostasis by positively regulating peroxisome proliferators-activated receptor alpha (PPARalpha), a nuclear receptor that mediates the adaptive response to fasting and starvation. Hepatocyte-specific deletion of SIRT1 impairs PPARalpha signaling and decreases fatty acid beta-oxidation, whereas overexpression of SIRT1 induces the expression of PPARalpha targets. SIRT1 interacts with PPARalpha and is required to activate PPARalpha coactivator PGC-1alpha. When challenged with a high-fat diet, liver-specific SIRT1 knockout mice develop hepatic steatosis, hepatic inflammation, and endoplasmic reticulum stress. Taken together, our data indicate that SIRT1 plays a vital role in the regulation of hepatic lipid homeostasis and that pharmacological activation of SIRT1 may be important for the prevention of obesity-associated metabolic diseases.
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              PGC-1alpha: a key regulator of energy metabolism.

              Peroxisome proliferator-activated receptor-gamma coactivator (PGC)-1alpha is a member of a family of transcription coactivators that plays a central role in the regulation of cellular energy metabolism. It is strongly induced by cold exposure, linking this environmental stimulus to adaptive thermogenesis. PGC-1alpha stimulates mitochondrial biogenesis and promotes the remodeling of muscle tissue to a fiber-type composition that is metabolically more oxidative and less glycolytic in nature, and it participates in the regulation of both carbohydrate and lipid metabolism. It is highly likely that PGC-1alpha is intimately involved in disorders such as obesity, diabetes, and cardiomyopathy. In particular, its regulatory function in lipid metabolism makes it an inviting target for pharmacological intervention in the treatment of obesity and Type 2 diabetes.
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                Author and article information

                Contributors
                Role: Editor
                Journal
                PLoS One
                PLoS ONE
                plos
                plosone
                PLoS ONE
                Public Library of Science (San Francisco, USA )
                1932-6203
                2013
                4 November 2013
                : 8
                : 11
                Affiliations
                [1 ]Department of Biomedical and Pharmaceutical Sciences, University of Rhode Island, Kingston, Rhode Island, United States of America
                [2 ]Institute of Biochemistry and Molecular Biology, College of Life and Health Sciences, Northeastern University, Shenyang, P. R. China
                Nihon University School of Medicine, Japan
                Author notes

                Competing Interests: The authors have declared that no competing interests exist.

                Conceived and designed the experiments: JX ALS. Performed the experiments: JX ACD JM ALS. Analyzed the data: JX. Wrote the manuscript: JX ALS.

                Article
                PONE-D-13-21057
                10.1371/journal.pone.0079841
                3817107

                This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

                Funding
                This work was supported by National Institute of Health [5R01ES016042-04; 3R01ES016042-2S2; 5K22ES013782-03], and also supported, in part, by Rhode Island IDeA Network of Biomedical Research Excellence grants from the National Center for Research Resources (5P20RR016457-11) and the National Institute for General Medical Science (8P20GM103430-11), components of the National Institutes of Health (NIH). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
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