Evidence for a peroxisome proliferator-activated receptor (PPAR)-mediated mechanism for conjugated linoleic acid (CLA)
We have previously shown that a mixture of conjugated derivatives of linoleic acid (CLA) induces peroxisome proliferator-responsive enzymes and modulates hepatic lipid metabolism in vivo. The present studies demonstrate that CLA is a high affinity ligand and activator of peroxisome proliferator-activated receptor (PPAR) with high specificity for the PPARα subtype. Moreover, CLA induces accumulation of PPAR-responsive mRNAs in a rat hepatoma cell line. Using scintillation proximity assays, CLA isomers were shown to be ligands for human PPARα with a rank order of potency of (9Z, 11E) > (10E, 12Z) > (9E, 11E) > furan-CLA (Ki values from 140nM to 400nM). Levels of acyl-CoA oxidase (ACO), liver fatty acid binding protein (L-FABP) and cytochrome P450IVA1 (CYP4A1) mRNA were induced by CLA in FaO hepatoma cells in culture. Even though linoleate and CLA were incorporated into lipids of hepatoma cells to the same extent, linoleate had no effect on CYP4A1 or L-FABP mRNA. In agreement with its binding potency, of the CLAs tested, (9Z, 11E) was the most efficacious PPARα activator in the mouse PPARα-GAL4(UAS)5-CAT reporter system. Also (9Z, 11E)-CLA is one of the most avid fatty acids yet described as a PPARα ligand. Because CLA acts as peroxisome proliferator, we compared the effects of dietary CLA (0.0, 0.5, 1.0 and 1.5% by weight) with a prototypical peroxisome proliferator (0.01% Wy-14,643) in female and male Sprague-Dawley (SD) rats. Dietary CLA had little effect on body, liver weight, and hepatic peroxisome proliferation, compared to male rats fed Wy-14,643 diets. Lipid content in livers from rats fed 1.5% CLA and Wy-14,643 diets was significantly increased when compared to rats fed control diets regardless of gender. Hepatic acyl-CoA oxidase (ACO) mRNA was slightly increased in male rats fed 1.5% CLA compared to rats fed control diets while Wy-14,643 supported ∼30-fold ACO mRNA accumulation. A similar response was observed for L-FABP mRNA. The effect of dietary treatments of CLA 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. We also studied the capacity of different geometric isomers to bind and activate human PPARγ. Using scintillation proximity assays, CLA isomers were shown to be ligands for human PPARγ with Ki values ranging from 5.5 μM to 7.1 μM. CLA isomers had lower affinity for PPARγ when compared to the affinities for the PPARα subtype under the same experimental conditions. We found differences in activation among isomers in transiently co-transfected CV-1 cells with a PPRE-luciferase reporter gene and the mouse PPARγ expression vectors. The (9Z, 11E) isomer was the most efficacious activator of mouse PPARγ of the CLAs tested. Taken together these data demonstrate that CLA is a ligand and activator of PPARα and to a lesser extent of PPARα, suggesting that effects on lipid metabolism may be attributed to transcriptional event associated with these nuclear receptors. Furthermore, we demonstrated that while CLA was able to activate PPARs it is not a peroxisome proliferator in Sprague-Dawley rats. Therefore, CLA may have a beneficial impact for those metabolic disorders related to impaired lipid metabolism, such as non-insulin dependent diabetes mellitus, atherosclerosis and obesity.
Belury, Purdue University.
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