Peroxisome proliferator-activated receptors (PPARs) are nuclear hormone receptors that mediate the effects of fatty acids and their derivatives at the transcriptional level. These receptors stimulate transcription after activation by their cognate ligand and binding to the promoter of target genes. In this review, we discuss how fatty acids affect PPAR functions in the cell. We first describe the structural features of the ligand binding domains of PPARs, as defined by crystallographic analyses. We then present the ligand-binding characteristics of each of the three PPARs (alpha, beta/delta, gamma) and relate ligand activation to various cellular processes: (i) fatty acid catabolism and modulation of the inflammatory response for PPARalpha, (ii) embryo implantation, cell proliferation and apoptosis for PPARbeta, and (iii) adipocytic differentiation, monocytic differentiation and cell cycle withdrawal for PPARgamma. Finally, we present possible cross-talk between the PPAR pathway and different endocrine routes within the cell, including the thyroid hormone and retinoid pathways.

PPARs are nuclear hormone receptors which, like the retinoid, thyroid hormone, vitamin D, and steroid hormone receptors, are ligand-activated transcription factors mediating the hormonal control of gene expression. Two lines of evidence indicate that PPARs have an important function in fatty acid metabolism. First, PPARs are activated by hypolipidemic drugs and physiological concentrations of fatty acids, and second, PPARs control the peroxisomal beta-oxidation pathway of fatty acids through transcriptional induction of the gene encoding the acyl-CoA oxidase (ACO), which is the rate-limiting enzyme of the pathway. Furthermore, the PPAR signaling pathway appears to converge with the 9-cis retinoic acid receptor (RXR) signaling pathway in the regulation of the ACO gene because heterodimerization between PPAR and RXR is essential for in vitro binding to the PPRE and because the strongest stimulation of this gene is observed when both receptors are exposed simultaneously to their activators. Thus, it appears that PPARs are involved in the 9-cis retinoic acid signaling pathway and that they play a pivotal role in the hormonal control of lipid metabolism.

Ubiquinone (UQ) is a lipid co-factor that is involved in numerous enzymatic processes and is present in most cellular membranes. In particular, UQ is a crucial electron carrier in the mitochondrial respiratory chain. Recently, it was shown that clk-1 mutants of the nematode worm Caenorhabditis elegans do not synthesize UQ 9 but instead accumulate demethoxyubiquinone (DMQ 9 ), a biosynthetic precursor of UQ 9 (the subscript refers to the length of the isoprenoid side chain). DMQ 9 is capable of carrying out the function of UQ 9 in the respiratory chain, as demonstrated by the functional competence of mitochondria isolated from clk-1 mutants, and the ability of DMQ 9 to act as a co-factor for respiratory enzymes in vitro. However, despite the presence of functional mitochondria, clk-1 mutant worms fail to complete development when feeding on bacteria that do not produce UQ 8 . Here we show that clk-1 mutants cannot grow on bacteria producing only DMQ 8 and that worm coq-3 mutants, which produce neither UQ 9 nor DMQ 9 , arrest development even on bacteria producing UQ 8 . These results indicate that UQ is required for nematode development at mitochondrial and non-mitochondrial sites and that DMQ cannot functionally replace UQ at those non-mitochondrial sites. Ubiquinone (UQ)1 is a prenylated benzoquinone that is an essential co-factor in the mitochondrial respiratory chain, where its function is best characterized. UQ is also found in many other locations in the cell, such as the lysosome and Golgi membranes, as well as in nuclear and plasma membranes (1). The exact role of UQ at these extramitochondrial sites is being actively explored (e.g. Refs. 2 and 3).The gene clk-1 of the nematode Caenorhabditis elegans affects many physiological rates, including embryonic and postembryonic development, rhythmic behaviors, reproduction, and life span (4). clk-1 encodes a 187-amino acid protein that is localized in mitochondria (5) and that is homologous to the yeast protein Coq7p, which has been shown to be required for UQ biosynthesis (6). clk-1 has also been shown to be necessary for UQ biosynthesis in worms (7,8) and in the mouse (9). Indeed, UQ 9 is entirely absent from mitochondria purified from worm and mouse clk-1 mutants (8, 9) (the subscript refers to the length of the isoprenoid side chain). Instead, these mitochondria accumulate demethoxyubiquinone (DMQ 9 ), which is an intermediate in the synthesis of UQ 9 (8, 9). Consistently, recent evidence suggests that clk-1 encodes a DMQ hydroxylase (10), which converts DMQ to ubiquinol. In Escherichia coli, DMQ 8 is able to sustain respiration in isolated membranes although at a lower rate than Q 8 (11). Similarly, DMQ 9 is capable of sustaining electron transport in eukaryotic mitochondria, as the function of purified mitochondria (5), and mitochondrial enzymes (8), from clk-1 worm mutants appears to be almost intact compared with the wild type. In addition, synthetic DMQ 2 has been shown to function in vitro as a co-factor for electron transport from worm complex I and, ...

Peroxisome proliferator-activated receptor ␣ (PPAR␣) is a nuclear receptor for various fatty acids, eicosanoids, and hypolipidemic drugs. In the presence of ligand, this transcription factor increases expression of target genes that are primarily associated with lipid homeostasis. We have previously reported PPAR␣ as a nuclear receptor of the inflammatory mediator leukotriene B 4 (LTB 4 ) and demonstrated an anti-inflammatory function for PPAR␣ in vivo (Devchand, P. R., Keller, H., Peters, J. M., Vazquez, M., Gonzalez, F. J., and Wahli, W. (1996) Nature 384, 39 -43). LTB 4 also has a cell surface receptor (BLTR) that mediates proinflammatory events, such as chemotaxis and chemokinesis (Yokomizo, T., Izumi, T., Chang, K., Takuwa, Y., and Shimizu, T. (1997) Nature 387, 620 -624). In this study, we report on chemical probes that differentially modulate activity of these two LTB 4 receptors. The compounds selected were originally characterized as synthetic BLTR effectors, both agonists and antagonists. Here, we evaluate the compounds as effectors of the three PPAR isotypes (␣, ␤, and ␥) by transient transfection assays and also determine whether the compounds are ligands for these nuclear receptors by coactivator-dependent receptor ligand interaction assay, a semifunctional in vitro assay. Because the compounds are PPAR␣ selective, we further analyze their potency in a biological assay for the PPAR␣-mediated activity of lipid accumulation. These chemical probes will prove invaluable in dissecting processes that involve nuclear and cell surface LTB 4 receptors and also aid in drug discovery programs.Hormones and nutrient-derived molecules, such as retinoids and fatty acid derivatives, are important signals in many biological processes. Dysregulation or disruption of their signaling pathways can manifest in various ways, with defects that range in severity, rate of onset, and organ systems affected. For instance, a prolonged disturbance of lipid homeostasis is often associated with many late-onset inflammatory conditions, obesity, diabetes, and cardiovascular disease. In order to efficiently treat and prevent these prominent metabolic problems, a better understanding of the mechanisms involved in lipid regulation is required.Intracellular targets for lipid mediators have been postulated for many years (1). However, it is only recently that we have seen the emergence of reports describing nuclear receptors for fatty acids and their derivatives (see Refs. 2 and 3 and references therein). Particular attention has focused on a group of ligand-activated transcription factors called peroxisome proliferator-activated receptors (PPARs).1 The three PPAR isotypes (␣, ␤/␦, and ␥) form a distinct subclass of the nuclear hormone receptor superfamily (4). The functional complex is a heterodimer of PPAR and the retinoid X acid receptor (RXR) that binds to a consensus sequence in the promoter of target genes and can up-regulate transcription in the presence of a PPAR ligand. Although the PPAR target genes identified so far, are gener...

Orphan receptors of the FTZ-F1-related group of nuclear receptors (xFFlr) were identified in Xenopus laevis by isolation of cDNAs from a neurula stage library. Two cDNAs were found, which encode full-length, highly related receptor proteins, xFFlrA and B, whose closest relative known so far is the murine LRH-1 orphan receptor. xFFIrA protein expressed by a recombinant vaccinia virus system specifically binds to f1Z-F1 response elements (FRE; PyCAAGGPyCPu). In cotransfection studies, xFF1rA constitutively activates transcription, in a manner dependent on the number of FREs. The amounts of at least four mRNAs encoding full-length receptors greatly increase between gastrula and early tailbud stages and decrease at later stages. At early tailbud stages, xFTZ-F1-related antigens are found in all nuclei of the embryo.The nuclear hormone receptor superfamily includes receptors for steroid and thyroid hormones, vitamin D and retinoic acid, which regulate gene expression in the presence of their respective ligands by binding to cis-acting DNA sequences called hormone response elements (HRE) (53). These nuclear receptors, which control different developmental and physiological processes, share several structural and functional features. They are composed of six domains, A to F, of which the most highly conserved Zn-finger-containing C domain is responsible for specific DNA binding (25). In addition to ligand binding, the E domain appears to be involved in dimerization and transcriptional activation or repression (12,53).Recently, a number of receptors have been identified for which a ligand has not yet been found or may not exist. However, these orphan receptors are structurally related to other nuclear receptors and, thus, are included in the nuclear receptor superfamily (27,41). This superfamily can be divided into subfamilies on the basis of either characteristic amino acid sequence motifs of its members (53), their evolutionary relationships (27), or functional criteria, such as the ability to bind to distinct HREs (27,42,46