Upregulation of UCP2 by Adiponectin: The Involvement of Mitochondrial Superoxide and hnRNP K Mingyan Zhou 1 , Aimin Xu1,2, Paul K. H. Tam3, Karen S. L. Lam2, Bosheng Huang 1 , Yan Liang1, In-Kyu Lee4, Donghai Wu 5 , Yu Wang 1 * 1 Department of Pharmacology and Pharmacy, University of Hong Kong, Hong Kong, China, 2 Department of Medicine, University of Hong Kong, Hong Kong, China, 3 Department of Surgery, University of Hong Kong, Hong Kong, China, 4 School of Medicine, Kyungpook National University, Daegu, Korea, 5 Key Laboratory of Regenerative Biology, Guangzhou Institute of Biomedicine and Health, Chinese Academy of Sciences, Guangzhou, China Abstract Background: The adipocyte-derived hormone adiponectin elicits protective functions against fatty liver diseases and hepatic injuries at least in part by stimulating the expression of a mitochondrial inner membrane transporter, uncoupling protein 2 (UCP2). The present study was designed to investigate the cellular and molecular mechanisms underlying adiponectin-induced UCP2 expression. Methodology/Principal Findnigs: Mice were treated with adiponectin and/or different drug inhibitors. Parenchymal (PCs) and nonparenchymal (NPCs) cells were fractionated from the liver tissues for mitochondria isolation, Western blotting and quantitative PCR analysis. Mitochondrial superoxide production was monitored by MitoSOX staining and flow cytometry analysis. Compared to control mice, the expression of UCP2 was significantly lower in NPCs, but not PCs of adiponectin knockout mice (AKO). Both chronic and acute treatment with adiponectin selectively increased the mRNA and protein abundance of UCP2 in NPCs, especially in the enriched endothelial cell fractions. The transcription inhibitor actinomycin D could not block adiponectin-induced UCP2 expression, whereas the protein synthesis inhibitor cycloheximide inhibited the elevation of UCP2 protein but not its mRNA levels. Mitochondrial content of heterogeneous nuclear ribonucleoprotein K (hnRNP K), a nucleic acid binding protein involved in regulating mRNA transportation and stabilization, was significantly enhanced by adiponectin, which also evoked a transient elevation of mitochondrial superoxide. Rotenone, an inhibitor of mitochondrial respiratory complex I, abolished adiponectin-induced superoxide production, hnRNP K recruitment and UCP2 expression. Conclusions/Significance: Mitochondrial superoxide production stimulated by adiponectin serves as a trigger to initiate the translocation of hnRNP K, which in turn promotes UCP2 expressions in liver. Citation: Zhou M, Xu A, Tam PKH, Lam KSL, Huang B, et al. (2012) Upregulation of UCP2 by Adiponectin: The Involvement of Mitochondrial Superoxide and hnRNP K. PLoS ONE 7(2): e32349. doi:10.1371/journal.pone.0032349 Editor: Sookja Chung, The University of Hong Kong, Hong Kong Received July 18, 2011; Accepted January 26, 2012; Published February 16, 2012 Copyright: 2012 Zhou et al. 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 is supported by Research Grants Council of Hong Kong (Project no. HKU777908M), the National Basic Research Program of China (2010CB945500). Adiponectin knockout mice were kindly provided by Dr. Lawrence Chan at Baylor College of Medicine, who generated these mice with the support of the US National Institutes of Health grant HL-51586. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. * E-mail: yuwanghk@hku.hk Introduction Non-alcoholic fatty liver disease (NAFLD) is one of the metabolic syndrome components closely associated with obesity, a worldwide pandemic [1]. The presence of steatosis in liver poses significant risks for the development of Type 2 Diabetes, cardiovascular diseases, viral hepatitis, drug-induced hepatotoxic- ity and alcoholic steatohepatitis [2,3,4]. In western countries, NAFLD is the most frequent hepatic lesion with an estimated prevalence of 10–25% [5]. About 20% to 30% of individuals with NAFLD progress into non-alcoholic steatohepatitis (NASH), cirrhosis and hepatocellular carcinoma [6,7]. Adiponectin is an adipocyte-derived hormone possessing a wide range of beneficial functions against obesity-associated medical complications [8,9,10]. The hepatoprotective activities of adipo- nectin have been demonstrated by evidence derived from clinical, genetic and pharmacological studies [11,12,13,14,15,16,17,18]. Epidemiological investigations suggest that low adiponectin level is an independent risk factor for NAFLD and liver dysfunctions in different ethnic groups [11,12,15,17,18,19,20]. In mice, adipo- nectin deficiency leads to exacerbated liver injuries induced by chemicals, endotoxins, alcohol consumption and obesity [21,22,23,24], whereas administration of this protein protects against fatty liver diseases, as well as various other forms of hepatic injuries [17,25,26,27,28]. In adiponectin knockout (AKO) mice, there is a pre-existing condition of hepatic steatosis and mitochondria dysfunction, characterized by abnormal ultrastruc- tures and defective mitochondrial respiratory chain (MRC) activity [24]. Adiponectin treatment restores mitochondrial functions, depletes lipid accumulation, and up-regulates the mRNA and protein expression of uncoupling protein 2 (UCP2) in liver tissues of AKO mice. UCP2 is a mitochondrial ion carrier encoded by PLoS ONE | www.plosone.org 1 February 2012 | Volume 7 | Issue 2 | e32349