I. Introduction II. Molecular Aspects A. PPAR isotypes: identity, genomic organization and chromosomal localization B. DNA binding properties C. PPAR ligand-binding properties D. Alternative pathways for PPAR activation E. PPAR-mediated transactivation properties III. Physiological Aspects A. Differential expression of PPAR mRNAs B. PPAR target genes and functions in fatty acid metabolism C. PPARs and control of inflammatory responses D. PPARs and atherosclerosis E. PPARs and the development of the fetal epidermal permeability barrier F. PPARs, carcinogenesis, and control of the cell cycle IV. Conclusions

Peroxisome proliferator-activated receptors: nuclear control of metabolism.

Nuclear receptors (NR) comprise a family of transcription factors that regulate gene expression in a liganddependent manner. Members of the NR superfamily include receptors for steroid hormones, such as estrogens (ER) and glucocorticoids (GR), receptors for nonsteroidal ligands, such as thyroid hormones (TR) and retinoic acid (RAR), as well as receptors that bind diverse products of lipid metabolism, such as fatty acids and prostaglandins (for review, see Beato et al. 1995; Chambon 1995; Mangelsdorf and Evans 1995). The NR superfamily also includes a large number of so-called orphan receptors for which regulatory ligands have not been identified (Mangelsdorf and Evans 1995). Although many orphan receptors are likely to be regulated by small-molecular-weight ligands, other mechanisms of regulation, such as phosphorylation (Hammer et al. 1999; Tremblay et al. 1999) have also proven to be of importance. Remarkably, the sequence of the Caenorhabditis elegans genome has revealed the presence of >200 members of the NR family, suggesting a critical role of these proteins in environmental adaptation (Sluder et al. 1999). Although mammalian genomes are unlikely to contain such a large complement of these factors, >24 distinct classes of NR have been identified in humans, and these factors exert diverse roles in the regulation of growth, development, and homeostasis. Based on their importance in biology and medicine, as well as the relatively simple mechanism of regulation, NR represent one of the most intensively studied and best-understood classes of transcription factors at the molecular level. Members of the NR family regulate transcription by several mechanisms (Fig. 1). Nuclear receptors can activate or repress target genes by binding directly to DNA response elements as homoor heterodimers or by binding to other classes of DNA-bound transcription factors. A subset of NRs, including TR and RAR, can actively repress target genes in the presence or absence of ligand binding, and many NR have been demonstrated to inhibit transcription in a ligand-dependent manner by antagonizing the transcriptional activities of other classes of transcription factors. These activities have been linked to interactions with general classes of molecules that appear to serve coactivator or corepressor function. In this review, we will discuss recent progress concerning the molecular mechanisms by which NR cofactor interactions serve to activate or repress transcription.

/pdf/the-coregulator-exchange-in-transcriptional-functions-of-15vt39po2d.pdf

The coregulator exchange in transcriptional functions of nuclear receptors

Peroxisome proliferator-activated receptor α (PPARα) plays a key role in the transcriptional control of genes encoding mitochondrial fatty acid β-oxidation (FAO) enzymes. In this study we sought to determine whether the recently identified PPAR gamma coactivator 1 (PGC-1) is capable of coactivating PPARα in the transcriptional control of genes encoding FAO enzymes. Mammalian cell cotransfection experiments demonstrated that PGC-1 enhanced PPARα-mediated transcriptional activation of reporter plasmids containing PPARα target elements. PGC-1 also enhanced the transactivation activity of a PPARα-Gal4 DNA binding domain fusion protein. Retroviral vector-mediated expression studies performed in 3T3-L1 cells demonstrated that PPARα and PGC-1 cooperatively induced the expression of PPARα target genes and increased cellular palmitate oxidation rates. Glutathione S-transferase “pulldown” studies revealed that in contrast to the previously reported ligand-independent interaction with PPARγ, PGC-1 binds PPARα in a ligand-influenced manner. Protein-protein interaction studies and mammalian cell hybrid experiments demonstrated that the PGC-1–PPARα interaction involves an LXXLL domain in PGC-1 and the PPARα AF2 region, consistent with the observed ligand influence. Last, the PGC-1 transactivation domain was mapped to within the NH2-terminal 120 amino acids of the PGC-1 molecule, a region distinct from the PPARα interacting domains. These results identify PGC-1 as a coactivator of PPARα in the transcriptional control of mitochondrial FAO capacity, define separable PPARα interaction and transactivation domains within the PGC-1 molecule, and demonstrate that certain features of the PPARα–PGC-1 interaction are distinct from that of PPARγ–PGC-1.

The coactivator PGC-1 cooperates with peroxisome proliferator-activated receptor alpha in transcriptional control of nuclear genes encoding mitochondrial fatty acid oxidation enzymes.

PPARalpha is a nuclear receptor that regulates liver and skeletal muscle lipid metabolism as well as glucose homeostasis. Acting as a molecular sensor of endogenous fatty acids (FAs) and their derivatives, this ligand-activated transcription factor regulates the expression of genes encoding enzymes and transport proteins controlling lipid homeostasis, thereby stimulating FA oxidation and improving lipoprotein metabolism. PPARalpha also exerts pleiotropic antiinflammatory and antiproliferative effects and prevents the proatherogenic effects of cholesterol accumulation in macrophages by stimulating cholesterol efflux. Cellular and animal models of PPARalpha help explain the clinical actions of fibrates, synthetic PPARalpha agonists used to treat dyslipidemia and reduce cardiovascular disease and its complications in patients with the metabolic syndrome. Although these preclinical studies cannot predict all of the effects of PPARalpha in humans, recent findings have revealed potential adverse effects of PPARalpha action, underlining the need for further study. This Review will focus on the mechanisms of action of PPARalpha in metabolic diseases and their associated vascular pathologies.

https://dm5migu4zj3pb.cloudfront.net/manuscripts/27000/27989/JCI0627989.pdf

Sorting out the roles of PPARα in energy metabolism and vascular homeostasis

▪ Abstract β-Oxidation occurs in both mitochondria and peroxisomes. Mitochondria catalyze the β-oxidation of the bulk of short-, medium-, and long-chain fatty acids derived from diet, and this pathway constitutes the major process by which fatty acids are oxidized to generate energy. Peroxisomes are involved in the β-oxidation chain shortening of long-chain and very-long-chain fatty acyl-coenzyme (CoAs), long-chain dicarboxylyl-CoAs, the CoA esters of eicosanoids, 2-methyl-branched fatty acyl-CoAs, and the CoA esters of the bile acid intermediates di- and trihydroxycoprostanoic acids, and in the process they generate H2O2. Long-chain and very-long-chain fatty acids (VLCFAs) are also metabolized by the cytochrome P450 CYP4A ω-oxidation system to dicarboxylic acids that serve as substrates for peroxisomal β-oxidation. The peroxisomal β-oxidation system consists of (a) a classical peroxisome proliferator–inducible pathway capable of catalyzing straight-chain acyl-CoAs by fatty acyl-CoA oxidase, L-bifunctiona...

PEROXISOMAL β-OXIDATION AND PEROXISOME PROLIFERATOR–ACTIVATED RECEPTOR α: An Adaptive Metabolic System

Peroxisome proliferator-activated receptors (PPARs) are ligand-dependent transcription factors, and it is assumed that the biological effects of these receptors depend on interactions with recently identified coactivators, including steroid receptor coactivator-1 (SRC-1). We assessed the in vivo function of SRC-1 on the PPARalpha-regulated gene expression in liver by generating mice in which the SRC-1 gene was inactivated by gene targeting. The homozygous (SRC-1(-/-)) mice were viable and fertile and exhibited no detectable gross phenotypic defects. When challenged with a PPARalpha ligand, such as ciprofibrate or Wy-14,643, the SRC-1(-/-) mice displayed typical pleiotropic responses, including hepatomegaly, peroxisome proliferation in hepatocytes, and increased mRNA and protein levels of genes that are regulated by PPARalpha. These alterations were indistinguishable from those exhibited by SRC-1(+/+) wild-type mice fed either ciprofibrate- or Wy-14, 643-containing diets. These results indicate that SRC-1 is not essential for PPARalpha-mediated transcriptional activation in vivo and suggest redundancy in nuclear receptor coactivators.

https://www.pnas.org/content/pnas/96/4/1585.full.pdf

Mouse steroid receptor coactivator-1 is not essential for peroxisome proliferator-activated receptor α-regulated gene expression

Under a copper-deficient regimen, pancreatic cells in the adult rat can be found to undergo differentiation into hepatocytes. Pancreatic hepatocytes induced in male and female rats were examined for the expression of the androgen-inducible hepatic protein, alpha 2u-globulin. Alpha 2u-Globulin protein was demonstrable by immunoperoxidase method in all the pancreatic hepatocytes of male rats. Northern blot analysis confirmed the presence of 1.3 kb alpha 2u-globulin mRNA transcript in the pancreas of male rats with hepatocytes. Orchiectomy resulted in marked decrease of alpha 2u-globulin protein and its mRNA. Administration of dihydrotestosterone to castrated rats resulted in increased levels of alpha 2u-globulin mRNA and the amount of alpha 2u-globulin protein in the pancreatic hepatocytes. Unlike normal males, in intact and ovariectomized females alpha 2u-globulin was not detectable in pancreatic hepatocytes. These results indicate that similar to hepatic parenchymal cells pancreatic hepatocytes synthesize alpha 2u-globulin under androgenic regulation. Furthermore, unlike in liver where it is expressed predominantly in perivenular and midlobular hepatocytes, there is no localized difference in the expression of this gene in the transdifferentiated pancreatic hepatocytes.

/pdf/androgen-regulated-expression-of-the-alpha-2u-globulin-gene-3jkhb6ze4j.pdf

Androgen regulated expression of the alpha 2u-globulin gene in pancreatic hepatocytes of rat.

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Mouse steroid receptor coactivator-1 is not essential for peroxisome proliferator-activated receptor α-regulated gene expression

Androgen regulated expression of the alpha 2u-globulin gene in pancreatic hepatocytes of rat.