Conjugated linoleic acid activates peroxisome proliferator-activated receptor α and β subtypes but does not induce hepatic peroxisome proliferation in Sprague–Dawley rats

https://doi.org/10.1016/S0005-2760(98)00121-0Get rights and content

Abstract

Since conjugated linoleic acid (CLA) has structural and physiological characteristics similar to peroxisome proliferators, we hypothesized that CLA would activate peroxisome proliferator-activated receptor (PPAR). We compared the effects of dietary CLA (0.0, 0.5, 1.0 and 1.5% by weight) with a peroxisome proliferator (0.01% Wy-14,643) in female and male Sprague–Dawley (SD) rats. Dietary CLA had little effect on body weight, liver weight, and hepatic peroxisome proliferation, compared to male rats fed Wy-14,643 diet. Lipid content in livers from rats fed 1.5% CLA and Wy-14,643 diets was increased (P<0.01) when compared to rats fed control diets regardless of gender. Hepatic acyl–CoA oxidase (ACO) mRNA levels were increased 3-fold in male rats fed 1.5% CLA diet compared to rats fed control diets while Wy-14,643 supported ∼30-fold ACO mRNA accumulation. A similar response was observed for liver fatty acid-binding protein (L-FABP) mRNA. The effect of dietary treatments on hepatic PPAR-responsive genes in female rats was weaker than in male rats. The (9Z,11E)-CLA isomer activated PPARα in transfected cells to a similar extent as Wy-14,643, whereas the furan-CLA metabolite was comparable to bezafibrate on activating PPARβ. These data suggest that while CLA was able to activate PPARs it is not a peroxisome proliferator in SD rats.

Introduction

CLA is chemoprotective in several organs (mammary, skin, forestomach, colon) during different stages of carcinogenesis [1], [2], [3], [4]. Anti-atherogenic and anti-diabetogenic properties have also been attributed to CLA in experimental rodent models [5], [6]. Several hypotheses have been proposed for CLA’s anticancer effects including, modulation of tissue maturation/differentiation [7], action as an antioxidant [4], [8], and mediation of eicosanoid synthesis [9]. Recently we have found evidence to support that CLA may act through a steroid hormone receptor-mediated mechanism, i.e., by PPAR [10].

Peroxisome proliferators refers to a group of chemicals including the hypolipidemic fibrate class of drugs (i.e., clofibrate and its analogues), plasticizers, pesticides, and other compounds that cause an increase in the number and/or size of liver peroxisomes. Peroxisome proliferators are generally considered non-genotoxic hepatocarcinogens in rodents. It is thought that peroxisome proliferators result in their pleiotropic effects through activation of PPARα [11]. Like other steroid hormone receptors, PPAR is a ligand-activated transcription factor that affects gene expression in a tissue-, sex- and species-specific manner.

CLA has some structural and physiological characteristics similar to peroxisome proliferators. Furthermore, dietary CLA is able to induce accumulation of PPAR-responsive enzymes at the mRNA and protein level in female SENCAR mice [10]. We were able to reproduce these findings in FaO hepatoma cells (M.A. Belury, unpublished data). These observations lead us to conclude that CLA acts similar to peroxisome proliferators, chemicals that are known to exert differential effects on PPAR-associated gene expression and carcinogenesis. Most studies showing CLA is chemoprotective in extrahepatic tumor models have utilized rodents (mice and rats). We have already shown PPAR-associated responses of CLA in mice, therefore it was important to compare CLA’s effects with a prototypical peroxisome proliferator (Wy-14,643) and evaluate these responses in rats.

The present study compared the effect of diets containing various levels of CLA (0.0, 0.5, 1.0, and 1.5% by weight) with a known peroxisome proliferator (0.01% Wy-14,643) in female and male SD rats on (1) body and liver weights, (2) hepatic lipid composition, (3) peroxisome proliferator-responsive gene mRNA steady state levels, and (4) hepatic proliferation of peroxisomes. The extent of CLA in activating PPARα was studied in a rat hepatoma cell line that stably harbors a PPRE-luciferase reporter gene. PPARβ activation by CLA was also determined by two reporter systems: mouse PPARβ-GAL4/(UAS)5-CAT and mouse PPARβ/PPRE-luciferase.

Section snippets

Materials, animals, and diets

CLA was purchased from NuCheck Prep (Elysian, MN) (43% (9Z,11E)- and (9E,11Z)-CLA, 45% (10E,12Z)-CLA, 6% (9Z,11Z)-, (10Z,12Z)-, (9E,11E), (10E,12E)-CLA, 2% linoleate and 4% unidentified compound). Wy-14,643 was purchased from Chemsyn (Lenexa, KS). Pure CLA isomers and fatty acid standards were obtained from Matreya (Pleasant Gap, PA).

Female and male Sprague–Dawley rats (Rattus norvegicus), 20–22 days old, weighing 58–74 g, were from Harlan Sprague–Dawley (Indianapolis, IN). Female rats (n=5 per

Effect of dietary treatments on body and liver weight

A typical physiological response of peroxisome proliferators is a decrease in body weight and an increase in liver weight of male rodents. Dietary CLA did not affect body weight of female or male at the dietary levels studied (data not shown). As expected, only male rats fed diets containing the prototypical peroxisome proliferator Wy-14,643 had lower body weight than male rats fed control diets (255.6 g vs. 332.0 g; P<0.01) after 6 weeks. Wy-14,643 did not have an effect on female body weight.

Discussion

We have previously shown that female SENCAR mice fed CLA-containing diets increased hepatic lipid content and decreased body weight [21]. Furthermore, CLA induced the accumulation of peroxisome-responsive enzymes at mRNA and protein levels in livers from these mice [10]. These findings support the hypothesis that CLA regulates hepatic lipid metabolism through a PPAR-mediated mechanism.

Polyunsaturated fatty acids (PUFA) are known to induce peroxisomal β-oxidation in rat liver [22]. Furthermore,

Acknowledgements

The authors thank Dr. J. Tugwood (Zeneca Central Toxicology Laboratory, UK) for providing pSG5-mPPARβ and psV-GL2-PPRE-luciferase reporter plasmid and Dr. S.A. Kliewer for mPPARβ-GAL4 and (UAS)5-CAT plasmids. D. Van Horn and K. Nickel are thanked for electron microscopy work and peroxisome proliferation analysis. We also thank K.L. Liu, C. Kavanaugh, A. Kempa-Steczko and M. Lu for their excellent technical assistance. This work was supported by AICR Grant no. 96B071. S.Y.M.-C. was supported by

References (45)

  • K.N Lee et al.

    Atherosclerosis

    (1994)
  • K.L Houseknecht et al.

    Biochem. Biophys. Res. Commun.

    (1998)
  • M.A Belury et al.

    Nutr. Biochem.

    (1997)
  • T Flatmark et al.

    Biochim. Biophys. Acta

    (1988)
  • Ch Dreyer et al.

    Biol. Cell

    (1993)
  • C.D Brown et al.

    Toxicol. Appl. Pharmacol.

    (1996)
  • I.H Hall et al.

    J. Pharm. Sci.

    (1985)
  • B.J Blaauboer et al.

    Biochem. Pharmacol.

    (1990)
  • W.T Stott et al.

    Fundam. Appl. Toxicol.

    (1995)
  • L.B Biegel et al.

    Fundam. Appl. Toxicol.

    (1992)
  • K.-L Liu et al.

    Cancer Lett.

    (1998)
  • J.L Sébédio et al.

    Biochim. Biophys. Acta

    (1997)
  • S Banni et al.

    Nutr. Biochem.

    (1996)
  • Y.L Ha et al.

    Carcinogenesis

    (1987)
  • C Ip et al.

    Cancer Res.

    (1994)
  • M.A Belury et al.

    Nutr. Cancer

    (1996)
  • Y.L Ha et al.

    Cancer Res.

    (1990)
  • C Ip et al.

    Nutr. Cancer

    (1995)
  • C Ip et al.

    Carcinogenesis

    (1996)
  • K.-L Liu et al.

    Lipids

    (1997)
  • S.S.-T Lee et al.

    Mol. Cell. Biol.

    (1995)
  • American Institute of Nutrition (1997) J. Nutr. 107,...
  • Cited by (0)

    View full text