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Addition of Peroxisome Proliferator-activated Receptor to Guinea Pig Hepatocytes Confers Increased Responsiveness to Peroxisome Proliferators

AstraZeneca Central Toxicology Laboratory, Alderley Park, Macclesfield SK10 4TJ, United Kingdom

The fibrate drugs, such as nafenopin and fenofibrate, show efficacy in hyperlipidemias but cause peroxisome proliferation and liver tumors in rats and mice via nongenotoxic mechanisms. However, humans and guinea pigs appear refractory to these adverse effects. The peroxisome proliferator (PP)-activated receptor (PPAR) mediates the effects of PPs by heterodimerizing with the retinoid X receptor (RXR) to bind to DNA at PP response elements (PPREs) upstream of PP-regulated genes, such as acyl-CoA oxidase. Hepatic expression of PPAR in guinea pigs and humans is low, suggesting that species differences in response to PPs may be due at least in part to quantity of PPAR. To test this hypothesis, we introduced mouse PPAR and its heterodimerization partner, RXR, into guinea pig hepatocytes by transient transfection and determined responsiveness to the PP nafenopin by cyanide-insensitive palmitoyl-CoA oxidation (CIPCO). Expression of the mRNA for mouse PPAR in transfected guinea pig hepatocytes was verified using species-specific PCR. In guinea pig hepatocytes transfected with control plasmids and treated with 50 µM nafenopin in the absence or presence of the RXR ligand, 9-cis-retinoic acid (5 µM) gave only a 1.7 ± 1.5- or 3.3 ± 1.5-fold induction in CIPCO, respectively. However, addition of ligands to hepatocytes co-transfected with both mPPAR and RXR gave a strong induction of CIPCO (14.8 ± 8.6-fold). Mouse, human, and guinea pig PPAR showed equivalent function in the CIPCO assays. Thus, quantity of PPAR plays a significant role in the lack of response to PPs in guinea pigs. In humans, however, lack of PPAR may be only one factor dictating lack of response because recent data show that the human acyl-CoA oxidase gene lacks a functional PP response element.

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