Elsevier

Nutrition Research

Volume 22, Issue 7, July 2002, Pages 817-824
Nutrition Research

Conjugated linoleic acid is an activator and ligand for peroxisome proliferator-activated receptor-gamma (PPARγ)

https://doi.org/10.1016/S0271-5317(02)00393-7Get rights and content

Abstract

The goal of the present study was to elucidate the extent that CLA binds to and activates PPARγ. All of the isomers of CLA tested activated PPARγ in a manner similar to the polyunsaturated fatty acid, linoleic acid (18:2n6). When the binding affinity of CLA to PPARγ was determined using a scintillation proximity assay (SPA), isomers of CLA were ligands for PPARγ with micromolar affinity. To determine the extent that metabolism of CLA via Δ6 desaturase could alter activation of PPARγ, we used a Δ6 desaturase inhibitor, SC-26,196, to block Δ6 desaturase metabolism. Blocking Δ6 desaturase significantly reduced activation of PPARγ by c9t11-CLA. These data suggest that the ability of CLA to induce PPAR-responsive genes may be via both direct binding of CLA to the nuclear hormone receptor, PPARγ, as well as active metabolites of CLA via Δ6 desaturase. Further work is needed to determine the ability of Δ6 desaturase metabolites of CLA to bind and activate 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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