Characterization of peroxisome proliferator-activated receptor alpha (PPARa) regulated acyl-CoA thioesterases involved in lipid metabolism

Sammanfattning: The peroxisome proliferator-activated receptor alpha (PPARalpha) is a key nuclear receptor in the control of lipid metabolism. Target genes for PPARalpha include many enzymes involved in the oxidation of fatty acids in mitochondria and peroxisomes. Our work involves characterization of a group of acyl-CoA thioesterase enzymes, that catalyze the hydrolysis of acyl-CoAs to the free fatty acid and coenzyme A (CoASH). These enzymes are targets for PPARalpha action and are strongly induced by hypolipidemic drugs, which act as ligands for PPARalpha. We have now cloned and characterized a family of acyl-CoA thioesterase genes in mouse and human, which show a high degree of sequence similarity. Four genes were identified in mouse, namely cytosolic acyl-CoA thioesterase (mCTE-I), mitochondrial acyl-CoA thioesterase (mMTE-I), and two peroxisomal acyl-CoA thioesterases (mPTE-la and -Ib), located in a cluster on mouse chromosome 12. The human genes, hCTE-la and -Ib, hMTE-I and hPTE-I were localized in a cluster on human chromosome 14. These acyl-CoA thioesterases showed sequence similarity only to the bile acid-CoA:amino acid N- acyltransferase involved in conjugation of bile acids to glycine or taurine. Regulation of the mouse enzymes showed that each individual enzyme was upregulated at mRNA level by treatment with peroxisome proliferators, and fasting, showing that these enzymes are inducible by both pharmacological treatments, and changes in physiological conditions. We have also identified a further acyl-CoA thioesterase, peroxisomal acyl-CoA thioesterase 2, PTE-2, as a novel PPARalpha target gene, and have shown that this enzyme acts as a 'general' acyl-CoA thioesterase in peroxisomal lipid metabolism. In searching for other previously uncharacterized PPARalpha target genes, we identified the sterol 12alpha- hydroxylase as a novel PPARalpha target. The sterol 12alpha-hydroxylase is involved in bile acid synthesis, and results in the production of cholic acid. The induction of this enzyme by fibrates, which act as ligands for the PPARalpha, suggests a link between cholesterol and lipid metabolism mediated via the PPARalpha.