POSITIVE REGULATION OF THE PEROXISOMAL BETA-OXIDATION PATHWAY BY FATTY-ACIDS THROUGH ACTIVATION OF PEROXISOME PROLIFERATOR-ACTIVATED RECEPTORS (PPAR)

Peroxisome proliferators regulate the transcription of genes by activating ligand-dependent transcription factors, which, due to their structure and function, can be assigned to the superfamily of nuclear hormone receptors. Three such peroxisome proliferator-activated receptors (PPARalpha, beta, and gamma) have been cloned in Xenopus laevis. Their mRNAs are expressed differentially; xPPARalpha and beta but not xPPARgamma are expressed in oocytes and embryos. In the adult, expression of xPPARalpha and beta appears to be ubiquitous, and xPPARgamma is mainly observed in adipose tissue and kidney. Immunocytochemical analysis revealed that PPARs are nuclear proteins, and that their cytoplasmic-nuclear translocation is independent of exogenous activators. A target gene of PPARs is the gene encoding acyl-CoA oxidase (ACO), which catalyzes the rate-limiting step in the peroxisomal beta-oxidation of fatty acids. A peroxisome proliferator response element (PPRE), to which PPARs bind, has been identified within the promoter of the ACO gene. Besides the known xenobiotic activators of PPARs, such as hypolipidemic drugs, natural activators have been identified. Polyunsaturated fatty acids at physiological concentrations are efficient activators of PPARs, and 5,8,11,14-eicosatetraynoic acid (ETYA), which is the alkyne homolog of arachidonic acid, is the most potent activator of xPPARalpha described to date. Taken together, our data suggest that PPARs have an important role in lipid metabolism.