Conjugated linoleic acid is an activator and ligand for peroxisome proliferator-activated receptor-gamma (PPARγ)
Introduction
The fatty acid, conjugated linoleic acid (18:2), exerts numerous health benefits in experimental animals including action as an anti-cancer agent, anti-atherosclerotic agent, and anti-obesity agent [1]. In addition, we have shown that dietary CLA delays the onset of diabetes in a model of type 2 diabetes using the Zucker diabetic fatty rat [2]. Feeding a diet with CLA (1.5% by weight) to ZDF rats between eight and ten weeks of age normalized fasting glucose, insulin and triglycerides in a manner similar to a diet containing the thiazolidinedione (TZD), troglitazone (Rezulin™). In addition, like the diet with TZD, dietary CLA induced the level of adipocyte lipid binding protein (aP2) mRNA in adipose tissue.
The molecular mechanisms of action of CLA are not clearly understood. We have shown that several isomers of CLA are high affinity ligands and activators for the nuclear hormone receptor, peroxisome proliferator-activated receptor-alpha (PPARα) [3]. In addition, we and others have shown that CLA is readily metabolized via Δ6 desaturase metabolism [4], [5]. TZDs are thought to exert their actions via activation of PPARγ to modulate the transcription PPAR-responsive genes such as aP2 in adipose tissue. Therefore, the primary objective of this study was to determine the extent that several isomers of CLA bind to and activate PPARγ. Because the present study found that CLA isomers activate PPARγ but are weak ligands for the gamma isoform of PPAR (relative to PPARα), we sought to determine the extent that blocking Δ6 desaturase metabolism could modulate the activation of PPARγ by CLA. Blocking metabolism of c9t11-CLA using the Δ6 desaturase inhibitor, SC-26,196, significantly reduced activation of PPARγ by CLA. These data suggest that part of the ability of CLA to activate PPARγ may be due to downstream Δ6 desaturase metabolites of CLA.
Section snippets
Materials
The individual isomers of CLA (c9t11-CLA, c9c11-CLA, t9t11-CLA, t10c12-CLA, and n-hexyl-2-furyl-octanoic acid; all > 98% purity) were purchased from Matreya Inc. (Pleasant Gap, PA). The thiazolidinedione, troglitazone, was provided by Parke-Davis (Rezulin™, Ann Arbor, MI). Δ6 desaturase inhibitor, SC-26,196, was a gift from M. Obukowicz (Monsanto Co., St. Louis, MO). Other chemicals and reagents were the highest grade commercially available.
PPARγ binding assay
The affinity of several CLA isomers for binding to
Binding affinity of CLA isomers to PPARγ
It has been reported that the drug rosiglitazone is a high affinity ligand for the human PPARγ ligand binding domain (EC50 = 40 nM) [7]. In the present study we determined the ability of CLA isomers to inhibit specific binding of [3H]-rosiglitazone to PPARγ by competitive SPA (Fig. 1). Individual CLA isomers were able to bind human PPARγ (Table 1) with Ki values ranging from 4.7 μM to 7.1 μM.
Activation of PPARγ by CLA
The ability of CLA to activate PPARγ was determined in CV-1 cells co-transfected with a
Discussion
Previous studies conducted in our laboratory demonstrated that dietary CLA normalized impaired glucose in a manner similar to the TZD, troglitazone [2]. In addition, rats fed diets with either CLA or TZD accumulated aP2 mRNA in epididymal fat tissues. We have previously demonstrated that several isomers of CLA are high affinity ligands and potent activators of PPARα [3]. However, because agonists for PPARα are poor inducers of adipogenesis, the ability of CLA to induce aP2 mRNA in epididymal
Acknowledgements
This work was supported by ACS RPG-98-038-01-CNE. We thank Dr. Jonathan Tugwood (AstraZeneca Central Toxicology Laboratory, United Kingdom) for providing pcDNA3-mPPARγ and psV-GL2-PPRE-luciferase and Dr. T. Consler (GlaxoSmithKline, Research Triangle Park, NC) for the PPARγ ligand-binding domain. We also are grateful for the generous donation of the Δ6 desaturase inhibitor, 26,196 from Dr. Mark Obukowicz (Searle/Monsanto Pharmaceuticals, St. Louis, MO).
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