Molecular and Cellular Pharmacology Activation of PPARα and PPARγ reduces triacylglycerol synthesis in rat hepatoma cells by reduction of nuclear SREBP-1 Bettina König ⁎, Alexander Koch, Julia Spielmann, Christian Hilgenfeld, Frank Hirche, Gabriele I. Stangl, Klaus Eder Institute of Agricultural and Nutritional Sciences, Martin-Luther-University of Halle-Wittenberg, Emil-Abderhalden-Straße 26, D-06108 Halle (Saale), Germany abstract article info Article history: Received 17 June 2008 Received in revised form 26 November 2008 Accepted 9 January 2009 Available online 21 January 2009 Keywords: PPARα PPARγ SREBP-1c Insig Triacylglycerol synthesis Fao cells Fibrates and thiazolidinediones, agonists of PPARα and PPARγ, respectively, reduce triglyceride concentra- tions in rat liver and plasma. Fatty acid and triacylglycerol synthesis in mammals is regulated by sterol regulatory element-binding protein (SREBP)-1c. Recently, it was shown that insulin-induced gene (Insig)-1, the key regulator of SREBP activity, is up-regulated by both activation of PPARα and PPARγ. In order to elucidate whether inhibition of SREBP-1 activation may contribute to the triacylglycerol lowering effect of PPARα and PPARγ agonists, we incubated rat hepatoma Fao cells with WY 14,643 and troglitazone, strong and selective agonists of PPARα and PPARγ, respectively. Activation of both, PPARα and PPARγ led to increased concentrations of Insig-1 and Insig-2a, with the most prominent effect on Insig-2a after troglitazone incubation. As a result, the amount of nuclear SREBP-1 was reduced in Fao cells by both WY 14,643 and troglitazone treatment. The reduction of nuclear SREBP-1 was associated with decreased mRNA concentrations of its target genes fatty acid synthase and glycerol-3-phosphate acyltransferase, implicated in fatty acid and triacylglycerol synthesis. This was finally reflected in reduced rates of newly synthesized triacylglycerols from de novo-derived fatty acids and decreased intracellular and secreted triacylglycerol concentrations in Fao cells treated with WY 14,643 and troglitazone, respectively. Thus, these data suggest that the triacylglycerol reducing effect of fibrates and thiazolidinediones is partially caused by inhibition of SREBP-1 activation via up-regulation of Insig. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Fibrates are a class of hypolipidemic drugs that are effective in lowering elevated plasma triacylglycerol and cholesterol levels (Frick et al., 1987; Zimetbaum et al., 1991). Thiazolidinediones are used in the treatment of insulin-resistant states (reviewed in Quinn et al., 2008). Their mode of action is based on their structural similarity to free fatty acids which enables them to bind to peroxisome proliferator-activated receptors (PPAR). PPAR are transcription factors belonging to the superfamily of nuclear receptors and regulate nutrient metabolism and energy homeostasis (reviewed in Desvergne and Wahli, 1999; Chinetti et al., 2000; Duval et al., 2002). There are three PPAR isotypes, PPARα, PPARβ/δ and PPARγ (Schoonjans et al., 1997). PPARα is highly expressed in tissues with high fatty acid oxidation (Mandard et al., 2004); PPARγ is predominantly expressed in adipose tissue but also to a lesser extent in other tissues like skeletal muscle and liver (Fajas et al., 1997). Both, fibrates and thiazolidinediones, decrease plasma triacylglycerol concentrations in rats (Naderali et al., 2004; Lee et al., 2004; Festuccia et al., 2006; Laplante et al., 2007; König et al., 2007). Upon binding to PPARα, fibrates decrease circulating triacylglycerol levels by transcriptional modulation of genes involved in lipolysis, cellular uptake and β-oxidation of fatty acids and decreased synthesis of fatty acids and triacylglycerols (reviewed in Schoonjans et al., 1996; Staels et al., 1998). Mechanisms underlying the reduced fatty acid and triacylglycerol synthesis are, however, not completely understood. Induction of the β-oxidation pathway results in lower availability of fatty acids for triacylglycerol synthesis. Furthermore, PPARα activation leads to reduced expression of fatty acid synthase (FAS) in the liver of rats (Eder et al., 2003). The triacylglycerol lowering effect of thiazolidinediones results at least in part from an increase in adipose lipoprotein lipase activity and triacylglycerol-derived fatty acid uptake and retention in adipose tissue (Laplante et al., 2007). In mammals, lipogenesis is regulated by the transcription factor family designated sterol regulatory element-binding proteins (SREBP; Brown and Goldstein, 1999). Three isoforms of SREBP are known in mammals, SREBP-1a, SREBP-1c and SREBP-2. SREBP-1c, the predomi- nant isoform in adult liver, preferentially activates genes required for fatty acid synthesis and their incorporation into triacylglycerols and phospholipids (Horton et al., 2002). Retention of the SCAP/SREBP complex in the endoplasmic reticulum is mediated by sterol-dependent European Journal of Pharmacology 605 (2009) 23–30 ⁎ Corresponding author. Fax: +49 345 5527124. E-mail address: bettina.koenig@landw.uni-halle.de (B. König). 0014-2999/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.ejphar.2009.01.009 Contents lists available at ScienceDirect European Journal of Pharmacology journal homepage: www.elsevier.com/locate/ejphar