PPAR delta : its role in cholesterol metabolism

Abstract

Peroxisome proliferator activated receptors (PPARs) are ligand-activated transcription factors that regulate several biological processes, many of which are crucial for energy homeostasis. Three different isotypes have been identified; PPARalpha, PPARdelta and PPARgamma that are encoded by different genes and show different expression patterns and functions. PPARalpha controls the expression of genes involved in the catabolism wheras PPARgamma control genes involved in the storage of fatty acids. The role of PPARdelta is not yet fully elucidated although the work presented in this thesis provides evidence that PPARdelta is implicated in cholesterol metabolism in humans.

To better understand the role of PPARdelta in human physiology the structure and expression pattern of the gene was determined. The PPARdelta gene spans approximately 85 kilobases (kb) of DNA and consists of nine exons. Exons 1-3, the 5'-part of exon 4 and the 3'part of exon 9 are untranslated. The TATA box-less gene is transcribed from a unique start site located 380 base pairs (bp) upstream of the ATG initiation codon. PPARdelta is ubiquitously expressed and its mRNA size is 3.8 kb. Radiation hybrid mapping located the PPARdelta gene at chromosomal region 6p21.2.

In order to explore the role of PPARdelta in human cholesterol metabolism we searched for functional polymorphisms in the 5'-region of the gene to use as a tool in association studies. Of four polymorphisms found only the +294T/C variant in exon 4, located 86 bp upstream of the translation start site, showed an association with a metabolic trait in two independent cohorts of healthy middle-aged men. Homozygotes for the rare C-allele had higher plasma LDL-cholesterol concentration than homozygotes for the common T-allele. Transient transfection studies showed that the C-allele conferred higher transcriptional activity than the common Tallele.

Whether the +294C allele is associated with LDL-cholesterol elevation independently of the background lipoprotein phenotype and whether it results in increased risk of coronary heart disease (CHD) was investigated in the West of Scotland Coronary Prevention Study (WOSCOPS). WOSCOPS is a prospective double-blind placebo controlled study designed to assess the value of pravastatin treatment in the prevention of CHD events in moderately hypercholesterolaemic men. Individuals carrying the rare PPARdelta +294C allele had significantly lower plasma HDL-cholesterol concentrations and showed a tendency towards higher risk of CHD than subjects homozygous for the common T-allele.

To identify genes involved in cholesterol metabolism that are regulated by PPARdelta, a potent selective agonist was synthesized. Hepatic cells treated with the agonist showed decreased expression of the LDL receptor gene as measured by microarray and TaqMan analyses. EMSAs identified two potential PPAR response elements in the LDL receptor promoter, suggesting that the effect of PPARdelta on LDL receptor gene expression may be direct. Thus, one might speculate that the increase in plasma LDL-cholesterol concentration observed in the association studies could partly be explained by a reduction in LDL receptor expression.

Taken together, these studies provide evidence that PPARdelta is implicated in cholesterol metabolism and suggest that the receptor plays an atherogenic role in healthy humans.