Attainment of Brown Adipocyte Features in White Adipocytes of Hormone-Sensitive Lipase Null Mice Kristoffer Stro¨ m1*, Ola Hansson1¤, Ste´ phanie Lucas1, Pernilla Nevsten2, Ce´ line Fernandez1, Cecilia Klint1, Sofia Move´ rare-Skrtic3, Frank Sundler1, Claes Ohlsson 3 , Cecilia Holm1 1 Department of Experimental Medical Science, Lund University, Lund, Sweden, 2 National Center for High Resolution Electron Microscopy, Lund University, Lund, Sweden, 3 Research Center for Endocrinology and Metabolism, The Sahlgrenska Academy at Gothenburg University, Gothenburg, Sweden Abstract Background: Hormone-sensitive lipase (HSL) is expressed predominantly in adipose tissue, where it plays an important role in catecholamine-stimulated hydrolysis of stored tri- and diglycerides, thus mobilizing fatty acids. HSL exhibits broad substrate specificity and besides acylglycerides it hydrolyzes cholesteryl esters, retinyl esters and lipoidal esters. Despite its role in fatty acid mobilization, HSL mice have been shown to be resistant to diet-induced obesity. Methodology/Principal Findings: Following a high-fat diet (HFD) regimen, energy expenditure, measured using indirect calorimetry, was increased in HSL mice. White adipose tissue of HSL mice was characterized by reduced mass and reduced protein expression of PPARc, a key transcription factor in adipogenesis, and stearoyl-CoA desaturase 1, the expression of which is known to be positively correlated to the differentiation state of the adipocyte. The protein expression of uncoupling protein-1 (UCP-1), the highly specific marker of brown adipocytes, was increased 7-fold in white adipose tissue of HSL mice compared to wildtype littermates. Transmission electron microscopy revealed an increase in the size of mitochondria of white adipocytes of HSL mice. The mRNA expression of pRb and RIP140 was decreased in isolated white adipocytes, while the expression of UCP-1 and CPT1 was increased in HSL mice compared to wildtype littermates. Basal oxygen consumption was increased almost 3-fold in white adipose tissue of HSL mice and was accompanied by increased uncoupling activity. Conclusions: These data suggest that HSL is involved in the determination of white versus brown adipocytes during adipocyte differentiation The exact mechanism(s) underlying this novel role of HSL remains to be elucidated, but it seems clear that HSL is required to sustain normal expression levels of pRb and RIP140, which both promote differentiation into the white, rather than the brown, adipocyte lineage. Citation: Stro¨ m K, Hansson O, Lucas S, Nevsten P, Fernandez C, et al. (2008) Attainment of Brown Adipocyte Features in White Adipocytes of Hormone-Sensitive Lipase Null Mice. PLoS ONE 3(3): e1793. doi:10.1371/journal.pone.0001793 Editor: Adrian Vella, Mayo Clinic College of Medicine, United States of America Received November 21, 2007; Accepted February 11, 2008; Published March 12, 2008 Copyright: 2008 Str o¨m 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: Financial support was provided by the Swedish Research Council (project no 11284 to C.H. and project no 621-2203-300 to Reine Wallenberg at HREM), a Center of Excellence Grant from the Juvenile Diabetes Foundation, USA, and Knut and Alice Wallenberg Foundation, Sweden, the Swedish Diabetes Association, and the following foundations: Novo Nordisk, A. Pa˚hlsson, Salubrin/Druvan, and Torsten and Ragnar So¨ derberg. O.H. was supported by the Cell Factory for Functional Genomics, a program funded by the Swedish Foundation for Strategic Research, S.L was supported by the Foundation of Tage Blu¨ cher for Medical Research and C.F. was supported by the Swedish Research School of Genomics and Bioinformatics. The funding agencies did not take any part in the design and conduct of the study, in the collection, analysis, and interpretation of the data, or in the preparation, review, or approval of the manuscript. Competing Interests: The authors have declared that no competing interests exist. * E-mail: kristoffer.strom@med.lu.se ¤ Current address: Department of Clinical Sciences, University Hospital MAS, Lund University, Malmo¨ , Sweden Introduction Obesity is a growing health problem globally, predisposing individuals to disorders such as type 2 diabetes, cardiovascular disease and certain cancers. Understanding the molecular events underlying adipogenesis and determination of the differentiation of white versus brown adipocytes is thus of both basic and clinical importance. Brown adipocytes are characterized by expression of UCP1 and high energy expenditure potential. Thus, attainment of brown adipocyte features in white adipocytes has evolved as a legitimate strategy to combat obesity. Considerable efforts over the last decades have led to the identification of a large plethora of transcription factors involved in differentiation and determination of adipocytes. The CCAAT/enhancer binding protein C/EBPa [1] and PPARc [2,3] are master regulators of adipogenesis. With regard to determination of the differentiation into the white versus the brown adipocyte lineage, key factors include transcriptional intermediary factor 2 (TIF2) [4] and the retinoblastoma protein (pRB) [5], promoting white adipocyte differentiation, and the PPARc co-activator a (PGC-1a) [6,7] and steroid receptor coactivator 1 (SRC-1) [4], promoting development into a brown adipocyte-like phenotype. Receptor interacting protein 140 (RIP140) antagonizes PGC coactivators and thus the appearance of a brown adipocyte phenotype [8,9]. Very recently it was shown that brown, but not white, preadipocytes express myogenic markers, suggesting that brown and white adipocytes originate from distinct cell lineages [10]. HSL is a key enzyme in fatty acid mobilization in white adipose tissue (WAT) [11]. It has a wide tissue distribution and is, besides PLoS ONE | www.plosone.org 1 2008 | Volume 3 | Issue 3 | e1793