Peroxisome proliferators and peroxisome proliferator activated receptors (PPARs) as regulators of lipid metabolism

To gain insight into the function of peroxisome proliferator-activated receptor (PPAR) isoforms in rodents, we disrupted the ligand-binding domain of the alpha isoform of mouse PPAR (mPPAR alpha) by homologous recombination. Mice homozygous for the mutation lack expression of mPPAR alpha protein and yet are viable and fertile and exhibit no detectable gross phenotypic defects. Remarkably, these animals do not display the peroxisome proliferator pleiotropic response when challenged with the classical peroxisome proliferators, clofibrate and Wy-14,643. Following exposure to these chemicals, hepatomegaly, peroxisome proliferation, and transcriptional-activation of target genes were not observed. These results clearly demonstrate that mPPAR alpha is the major isoform required for mediating the pleiotropic response resulting from the actions of peroxisome proliferators. mPPAR alpha-deficient animals should prove useful to further investigate the role of this receptor in hepatocarcinogenesis, fatty acid metabolism, and cell cycle regulation.

Nuclear hormone receptors comprise a superfamily of ligand-modulated transcription factors that mediate the transcriptional activities of steroids, retinoids, and thyroid hormones. A growing number of related proteins have been identified that possess the structural features of hormone receptors, but that lack known ligands. Known as orphan receptors, these proteins represent targets for novel signaling molecules. We have isolated a mammalian orphan receptor that forms a heterodimeric complex with the retinoid X receptor. A screen of candidate ligands identified farnesol and related metabolites as effective activators of this complex. Farnesol metabolites are generated intracellularly and are required for the synthesis of cholesterol, bile acids, steroids, retinoids, and farnesylated proteins. Intermediary metabolites have been recognized as transcriptional regulators in bacteria and yeast. Our results now suggest that metabolite-controlled intracellular signaling systems are utilized by higher organisms.

Peroxisome proliferator-activated receptors (PPARs) are nuclear hormone receptors that regulate gene transcription in response to peroxisome proliferators and fatty acids. PPARs also play an important role in the regulation of adipocyte differentiation. It is unclear, however, what naturally occurring compounds activate each of the PPAR subtypes. To address this issue, a screening assay was established using heterologous fusions of the bacterial tetracycline repressor to several members of the peroxisome proliferator-activated receptor (PPAR) family. This assay was employed to compare the activation of PPAR family members by known PPAR activators including peroxisome proliferators and fatty acids. Interestingly, the activation of PPARs by fatty acids was partially inhibited by the cyclooxygenase inhibitor indomethacin, which prevents prostaglandin synthesis. Indeed, prostaglandins PGA1 and 2, PGD1 and 2, and PGJ2-activated PPARs, while a number of other prostaglandins had no effect. We also screened a variety of hydroxyeicosatetraenoic acids (HETEs) for the ability to activate PPARs. 8(S)-HETE, but not other (S)-HETEs, was a strong activator of PPAR alpha. Remarkably, PPAR activation by 8(S)-HETE was stereoselective. In addition, 8(S)-HETE was able to induce differentiation of 3T3-L1 preadipocytes. These results indicate that PPARs are differentially activated by naturally occurring eicosanoids and related molecules.