Coactivators are linker proteins that recognize the activated PPAR receptor as well as the transcription machinery and thus are also required to turn on gene transcription (4-6). In some cases, negative regulation also occurs through the binding of corepressors (7). Thus gene transcription is regulated by a number of proteins and ligands in a very sensitive manner that might differ depending on the tissue.
PPARa, first cloned in the early 1990's, plays an important role in lipid metabolism. It acts as a dietary fat sensor by upregulating lipid metabolism (predominantly beta-oxidation) in the presence of fatty acids which are presumed to be the natural ligands for PPARa (8, 9). Although the PPARa DNA binding domains (DBDs) are identical across species, the ligand binding domains (LBDs) are not (human and murine receptors are 91% homologous), presumably reflecting evolutionary adaptation to different dietary ligands. This is particularly important when assessing the behavior of PPARa ligands in different animal models and how this behavior might extrapolate to humans. The fibrate drugs (e.g. clofibrate 1, fenofibrate 2 and bezafibrate 3) which are hypolipidemic agents, work through their agonist activity on PPARa. In rats, fibrates cause the formation of peroxisomes which in turn oxidatively metabolize fatty acids.
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