Peroxisome proliferator-activated receptor alpha induces rat sterol carrier protein x promoter activity through two peroxisome proliferator-response elements

Mechanisms Mediating the Regulation of Peroxisomal Fatty Acid Beta-Oxidation by PPARα

In mammalian cells, two cellular organelles, mitochondria and peroxisomes, share the ability to degrade fatty acid chains. Although each organelle harbors its own fatty acid β-oxidation pathway, a distinct mitochondrial system feeds the oxidative phosphorylation pathway for ATP synthesis. At the same time, the peroxisomal β-oxidation pathway participates in cellular thermogenesis. A scientific milestone in 1965 helped discover the hepatomegaly effect in rat liver by clofibrate, subsequently identified as a peroxisome proliferator in rodents and an activator of the peroxisomal fatty acid β-oxidation pathway. These peroxisome proliferators were later identified as activating ligands of Peroxisome Proliferator-Activated Receptor α (PPARα), cloned in 1990. The ligand-activated heterodimer PPARα/RXRα recognizes a DNA sequence, called PPRE (Peroxisome Proliferator Response Element), corresponding to two half-consensus hexanucleotide motifs, AGGTCA, separated by one nucleotide. Accordingly, the assembled complex containing PPRE/PPARα/RXRα/ligands/Coregulators controls the expression of the genes involved in liver peroxisomal fatty acid β-oxidation. This review mobilizes a considerable number of findings that discuss miscellaneous axes, covering the detailed expression pattern of PPARα in species and tissues, the lessons from several PPARα KO mouse models and the modulation of PPARα function by dietary micronutrients.

Liver lipid metabolism is altered by increased circulating estrogen to androgen ratio in male mouse

Estrogens are suggested to lower the risk of developing metabolic syndrome in both sexes. In this study, we investigated how the increased circulating estrogen-to-androgen ratio (E/A) alters liver lipid metabolism in males. The cytochrome P450 aromatase (P450arom) is an enzyme converting androgens to estrogens. Male mice overexpressing human aromatase enzyme (AROM+ mice), and thus have high circulating E/A, were used as a model in this study. Proteomics and gene expression analyses indicated an increase in the peroxisomal β-oxidation in the liver of AROM+ mice as compared with their wild type littermates. Correspondingly, metabolomic analysis revealed a decrease in the amount of phosphatidylcholines with long-chain fatty acids in the plasma. With interest we noted that the expression of Cyp4a12a enzyme, which specifically metabolizes arachidonic acid (AA) to 20-hydroxy AA, was dramatically decreased in the AROM+ liver. As a consequence, increased amounts of phospholipids having AA as a fatty acid tail were detected in the plasma of the AROM+ mice. Overall, these observations demonstrate that high circulating E/A in males is linked to indicators of higher peroxisomal β-oxidation and lower AA metabolism in the liver. Furthermore, the plasma phospholipid profile reflects the changes in the liver lipid metabolism.

PPAR-alpha cloning, expression, and characterization

Peroxisome proliferator-activated receptor α (PPARα) is a member of the nuclear/steroid receptor gene superfamily that also comprises β, δ, and γ isoforms. PPARα is a ligand-activated transcription factor that plays an important role in the regulation of many genes involved in key metabolic processes. Today, PPARα has been cloned from mammalian, marsupial, and a number of marine species and its expression has been found to be relatively tissue- and species-specific. Here, we describe the methods for cloning of PPARα genes by RT-PCR and RACE approaches and related protocols for studying the expression of cloned PPARα cDNAs in mammalian cell systems.

The Peroxisomal 3-keto-acyl-CoA thiolase B Gene Expression Is under the Dual Control of PPARα and HNF4α in the Liver

PPARα and HNF4α are nuclear receptors that control gene transcription by direct binding to specific nucleotide sequences. Using transgenic mice deficient for either PPARα or HNF4α, we show that the expression of the peroxisomal 3-keto-acyl-CoA thiolase B (Thb) is under the dependence of these two transcription factors. Transactivation and gel shift experiments identified a novel PPAR response element within intron 3 of the Thb gene, by which PPARα but not HNF4α transactivates. Intriguingly, we found that HNF4α enhanced PPARα/RXRα transactivation from TB PPRE3 in a DNA-binding independent manner. Coimmunoprecipitation assays supported the hypothesis that HNF4α was physically interacting with RXRα. RT-PCR performed with RNA from liver-specific HNF4α-null mice confirmed the involvement of HNF4α in the PPARα-regulated induction of Thb by Wy14,643. Overall, we conclude that HNF4α enhances the PPARα-mediated activation of Thb gene expression in part through interaction with the obligate PPARα partner, RXRα.

Role of Jhdm2a in regulating metabolic gene expression and obesity resistance

Recent studies indicate that the methylation state of histones can be dynamically regulated by histone methyltransferases and demethylases. The H3K9-specific demethylase Jhdm2a (also known as Jmjd1a and Kdm3a) has an important role in nuclear hormone receptor-mediated gene activation and male germ cell development. Through disruption of the Jhdm2a gene in mice, here we demonstrate that Jhdm2a is critically important in regulating the expression of metabolic genes. The loss of Jhdm2a function results in obesity and hyperlipidemia in mice. We provide evidence that the loss of Jhdm2a function disrupts beta-adrenergic-stimulated glycerol release and oxygen consumption in brown fat, and decreases fat oxidation and glycerol release in skeletal muscles. We show that Jhdm2a expression is induced by beta-adrenergic stimulation, and that Jhdm2a directly regulates peroxisome proliferator-activated receptor alpha (Ppara) and Ucp1 expression. Furthermore, we demonstrate that beta-adrenergic activation-induced binding of Jhdm2a to the PPAR responsive element (PPRE) of the Ucp1 gene not only decreases levels of H3K9me2 (dimethylation of lysine 9 of histone H3) at the PPRE, but also facilitates the recruitment of Ppargamma and Rxralpha and their co-activators Pgc1alpha (also known as Ppargc1a), CBP/p300 (Crebbp) and Src1 (Ncoa1) to the PPRE. Our studies thus demonstrate an essential role for Jhdm2a in regulating metabolic gene expression and normal weight control in mice.

Tumor-induced upregulation of Twist, Snail, and Slug represses the activity of the human VE-cadherin promoter

Endothelial integrity is dependent on intracellular adherens junctions formed by complexes of vascular endothelial (VE)-cadherin and catenins. We have previously demonstrated that exposing endothelial cells (EC) to breast cancer cell-conditioned media (CM) for 24h results in a reduction in VE-cadherin protein and mRNA levels. Herein, we examined the mechanism(s) involved in the downregulation of VE-cadherin by CM. Human dermal microvascular EC exposed to CM showed a downregulation in VE-cadherin promoter activity and upregulation of Twist, Slug, and Snail expression. Reporter gene analysis demonstrated a direct repression of the VE-cadherin promoter by Slug, Snail, and Twist expression plasmids. At least two E-box motifs appear to be involved in this regulatory process as shown by electrophoretic mobility shift assays. These results suggest that factors released by breast cancer cells are able to upregulate Twist, Slug, and Snail expression in EC, which in turn downregulate the activity of the VE-cadherin promoter.

Lauric acid dependent enhancement in hepatic SCPx protein requires an insulin deficient environment

Sterol carrier protein X (SCPx) is a peroxisomal protein with both lipid transfer and thiolase activity. Treating with the fatty acid, lauric acid, induced SCPx mRNA levels in rat liver and in rat hepatoma H4IIE cells but enhanced protein levels of SCPx and the thiolase produced as a post-translational modification of SCPx were only seen in H4IIE cells. Further investigation revealed that the presence of insulin can mask lauric acid effects on the SCPx gene especially at the protein level. These data are in agreement with the findings that diabetes, a medical condition characterized by high levels of fatty acids in an insulin deficient environment, enhances the hepatic expression of SCPx.

Activation of the SCPx promoter in mouse adrenocortical Y1 cells

Sterol carrier protein X (SCPx) is a peroxisomal protein with both lipid transfer and thiolase activity. Treatment of mouse adrenal Y1 cells with cAMP for 24h caused a significant induction of SCPx mRNA levels. Reporter gene studies demonstrated that treatment with cAMP and SF-1 was able to activate the SCPx promoter. Sequence analysis revealed the presence of three putative steroidogenic factor-1 (SF-1) binding motifs (designated SFB1, SFB2, and SFB3) and one CRE. Only SFB1 and SFB3 were able to bind recombinant SF-1 protein in electrophoretic mobility shift assays. The CRE was able to form a DNA/protein complex in the presence of Y1 nuclear extracts. Mutational analysis studies demonstrated that SFB3 is required for full activation of the SCPx promoter by cAMP treatment. Regulation of the SCPx gene by SF-1 and cAMP is similar to the regulatory mechanisms observed for other steroidogenic genes.

Activation of the hepatic LDL receptor promoter by thyroid hormone

The question of whether mature sterol regulatory element binding protein-2 (SREBP-2) mediates transcriptional activation of the hepatic low density lipoprotein (LDL) receptor by thyroid hormone was investigated. Western blotting analysis and electrophoretic mobility shift assays demonstrated that mature nuclear SREBP-2 protein could be detected in liver nuclear extracts prepared from normal animals but not in extracts prepared from rats rendered hypothyroid either by hypophysectomy (Hx) or thyroidectomy (Tx). Treatment of Hx rats with T3 restored LDL receptor mRNA levels in about 1 h and caused a 6-fold increase 2.5 h after T3 administration. However, no detectable mature SREBP-2 was seen in this time period despite a substantial reduction in serum cholesterol levels caused by the T3 treatment. Deletion of the SRE region from the LDL receptor promoter did not decrease the T3 response. Thus, the possibility that T3 may be mediating LDL receptor induction directly via a thyroid response element (TRE) was investigated. Reporter gene analysis and electrophoretic mobility shift assays demonstrated that the rat LDL receptor promoter contains two functional TREs (US-TRE and 2H-TRE). Either one of these elements could support T3 induction. However, the stronger of these elements is US-TRE at-612 which binds TRbeta1 more tightly and when mutated results in a diminished T3 response. These results indicate that the rapid induction of the hepatic LDL receptor by thyroid hormone is likely due to direct interaction with TREs rather than indirectly by a mechanism involving SREBP-2.

Effect of SCP-x gene ablation on branched-chain fatty acid metabolism

Despite the importance of peroxisomal oxidation in branched-chain lipid (phytol, cholesterol) detoxification, little is known regarding the factors regulating the peroxisomal uptake, targeting, and metabolism of these lipids. Although in vitro data suggest that sterol carrier protein (SCP)-x plays an important role in branched-chain lipid oxidation, the full physiological significance of this peroxisomal enzyme is not completely clear. To begin to resolve this issue, SCP-x-null mice were generated by gene ablation of SCP-x from the SCP-x/SCP-2 gene and fed a phytol-enriched diet to characterize the effects of lipid overload in a system with minimal 2/3-oxoacyl-CoA thiolytic activity. It was shown that SCP-x gene ablation 1) did not result in reduced expression of SCP-2 (previously thought to be derived in considerable part by posttranslational cleavage of SCP-x); 2) increased expression levels of key enzymes involved in alpha- and beta-oxidation; and 3) altered lipid distributions, leading to decreased hepatic fatty acid and triglyceride levels. In response to dietary phytol, lack of SCP-x resulted in 1) accumulation of phytol metabolites despite substantial upregulation of hepatic peroxisomal and mitochondrial enzymes; 2) reduced body weight gain and fat tissue mass; and 3) hepatic enlargement, increased mottling, and necrosis. In summary, the present work with SCP-x gene-ablated mice demonstrates, for the first time, a direct physiological relationship between lack of SCP-x and decreased ability to metabolize branched-chain lipids.

Activation of the rat scavenger receptor class B type I gene by PPARalpha

Peroxisomal proliferator activated receptor alpha (PPARalpha) is activated by fibrate drugs which are known to protect against atherosclerosis. The present study examines the effects of PPARalpha on SR-BI expression. For this study, a rat SR-BI promoter-luciferase reporter gene construct was co-transfected into different cell lines with expression vectors that encode for PPARalpha+/-retinoic X receptor alpha (RXRalpha). PPARalpha/RXR increased the activity of the SR-BI promoter, an effect that was enhanced by clofibrate. Sequence analysis of the rat SR-BI promoter revealed the presence of a putative peroxisomal proliferator response element (PPRE) at bp -1,622. Electrophoretic mobility shift assays demonstrated that PPARalpha and RXRalpha are able to bind to the SR-BI PPRE motif. In addition, mutational analysis studies confirmed that this PPRE motif is responsible for the PPARalpha/RXRalpha-dependent activation of the rat SR-BI promoter in the cell lines examined.

Characterization of the rat LDL receptor 5′-flanking region

A 1.5-kb genomic DNA fragment corresponding to the 5'-flanking region of the rat LDL receptor gene was cloned and putative regulatory regions were identified. A major transcription start site was identified at -154 bp relative to the ATG translation initiation codon, within a region containing two thyroid hormone response element half-site motifs (2H-TRE). Binding of thyroid hormone receptors alpha and beta1 to this element was demonstrated. Mutations within this 2H-TRE region abolished basal transcription levels of the rat LDL receptor gene. Reporter gene studies indicated that the promoter region between -300 and -200 bp, which contains one sterol response element (SRE) and two specificity protein-1 sites (Sp1) sites, is crucial for basal transcription of the rat LDL receptor gene. The functionality of the SRE motif was confirmed using electrophoretic mobility shift assays and reporter gene studies.

Overexpression of Peroxisome Proliferator-activated Receptor-α (PPARα)-regulated Genes in Liver in the Absence of Peroxisome Proliferation in Mice Deficient in both l- and d-Forms of Enoyl-CoA Hydratase/Dehydrogenase Enzymes of Peroxisomal β-Oxidation System

Peroxisomal beta-oxidation system consists of peroxisome proliferator-activated receptor alpha (PPARalpha)-inducible pathway capable of catalyzing straight-chain acyl-CoAs and a second noninducible pathway catalyzing the oxidation of 2-methyl-branched fatty acyl-CoAs. Disruption of the inducible beta-oxidation pathway in mice at the level of fatty acyl-CoA oxidase (AOX), the first and rate-limiting enzyme, results in spontaneous peroxisome proliferation and sustained activation of PPARalpha, leading to the development of liver tumors, whereas disruptions at the level of the second enzyme of this classical pathway or of the noninducible system had no such discernible effects. We now show that mice with complete inactivation of peroxisomal beta-oxidation at the level of the second enzyme, enoyl-CoA hydratase/L-3-hydroxyacyl-CoA dehydrogenase (L-PBE) of the inducible pathway and D-3-hydroxyacyl-CoA dehydratase/D-3-hydroxyacyl-CoA dehydrogenase (D-PBE) of the noninducible pathway (L-PBE-/-D-PBE-/-), exhibit severe growth retardation and postnatal mortality with none surviving beyond weaning. L-PBE-/-D-PBE-/- mice that survived exceptionally beyond the age of 3 weeks exhibited overexpression of PPARalpha-regulated genes in liver, despite the absence of morphological evidence of hepatic peroxisome proliferation. These studies establish that peroxisome proliferation in rodent liver is highly correlatable with the induction mostly of the L- and D-PBE genes. We conclude that disruption of peroxisomal fatty acid beta-oxidation at the level of second enzyme in mice leads to the induction of many of the PPARalpha target genes independently of peroxisome proliferation in hepatocytes, raising the possibility that intermediate metabolites of very long-chain fatty acids and peroxisomal beta-oxidation act as ligands for PPARalpha.

Regulation of sterol carrier protein gene expression by the forkhead transcription factor FOXO3a

The SCP gene encodes two proteins, sterol carrier protein X (SCPx) and SCP2, that are independently regulated by separate promoters. SCPx has been shown to be the thiolase involved in the breakdown of branched-chain fatty acids and in the biosynthesis of bile acids. The in vivo function of SCP2 however remains to be established. The transcriptional regulation of SCPx and SCP2 is unclear, but their promoter regions contain several putative regulatory domains. We show here that both SCPx and SCP2 are upregulated by the daf-16-like Forkhead transcription factor FOXO3a (also known as FKHRL1) on the level of promoter activity. It was recently described that Forkheads regulate protection against (oxidative) stress in both Caenorhabditis elegans and mammalian cells. We looked into a role for SCP2 in the cellular defense against oxidative damage and found that a fluorescent fatty acid analog bound to SCP2 is protected against H2O2/Cu2+-induced oxidative damage. We propose a model for the way in which SCP2 could protect fatty acids from peroxidation.