Binding Peroxisome Proliferator-activated Receptor Research Articles

In Silico Discovery of Stapled Peptide Inhibitor Targeting the Nur77-PPARγ Interaction and Its Anti-Breast-Cancer Efficacy.

The binding of peroxisome proliferator-activated receptor γ (PPARγ) to the orphan nuclear receptor Nur77 facilitates the ubiquitination and degradation of Nur77, and leads to aberrant fatty acid uptake for breast cancer progression. Because of its crucial role in clinical prognosis, the interaction between Nur77 and PPARγ is an attractive target for anti-breast-cancer therapy. However, developing an inhibitor of the Nur77-PPARγ interaction poses a technical challenge due to the absence of the crystal structure of PPARγ and its corresponding interactive model with Nur77. Here, ST-CY14, a stapled peptide,is identified as a potent modulator of Nur77 with a KD value of 3.247×10-8M by in silico analysis, rational design, and structural modification. ST-CY14 effectively increases Nur77 protein levels by blocking the Nur77-PPARγ interaction, thereby inhibiting lipid metabolism in breast tumor cells. Notably, ST-CY14 significantly suppresses breast cancer growth and bone metastasis in mice. The findings demonstrate the feasibility of exploiting directly Nur77-PPARγ interaction in breast cancer, and generate what to the best knowledge is the first direct inhibitor of the Nur77-PPARγ interaction available for impeding fatty acid uptake and therapeutic development.

Abstract 1247: PPARsing epigenetic memory in intestinal stem cells: High-fat diet and oncogenic susceptibility

Abstract Diet has a profound impact on health and susceptibility to cancer. Adaptation to a Western diet is strongly associated with increased colorectal cancer (CRC) and obesity prevalence. This study demonstrates the epigenetic changes in intestinal stem cells (ISCs) induced by a pro-obesity high-fat Western diet (HFD) and the heightened risk of oncogenic transformation. A HFD induces a phenotype characterized by increased ISC frequency, proliferation, regenerative capacity, and ability to initiate adenomas. We hypothesized that a HFD generates lasting epigenetic alterations in ISCs thereby enhancing oncogenic susceptibility. We evaluated the chromatin landscape of intestinal stem cells (ISCs) and the reversibility of the epigenetic changes. Mice carrying a fluorescent reporter of the ISC marker Lgr5, were subjected to a control diet, HFD, or HFD with a return to control. We isolated ISCs by flow cytometry and used an assay for transposase-accessible chromatin sequencing (ATAC-seq) to identify differentially accessible genomic regions correlated with the ISC phenotype. We found that a HFD induces distinct changes in the chromatin landscape that persist after removal from the HFD. These accessible regions are associated with genes involved in lipid metabolism and are enriched with PPAR (Peroxisome Proliferator-Activated Receptor) binding motifs. We demonstrate that these lipid metabolism regulators, PPAR-delta and PPAR-alpha, are necessary for the majority of significant accessible regions. We further show that HFD-induced accessibility significantly correlates with early chromatin alterations seen by loss of tumor suppressor APC. Our data supports a mechanistic pathway for diet-induced epigenetic reprogramming due to PPAR activity and suggests that the HFD imposes a preneoplastic stem cell chromatin state that stokes the risk of oncogenic change. Citation Format: Dominic R. Saiz, Thomas Hartley-McDerrmott, Yesenia Barrera-Millan, Karla Fabiola Castro-Ochoa, Abhigyan Shukla, Matthew Torel, Miyeko D. Mana. PPARsing epigenetic memory in intestinal stem cells: High-fat diet and oncogenic susceptibility [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 1247.

3,4,5-Trimethoxycinnamate thymol ester inhibits melanogenesis in normal human melanocytes and 3D human epidermal equivalents via the PGC-1α-independent PPARγ partial agonism

Background3,4,5-Trimethoxycinnamate thymol ester (TCTE), an anti-melanogenic cosmetic agent prescribed currently, promotes adiponectin synthesis during adipogenesis in human bone marrow mesenchymal stem cells (hBM-MSCs). Adiponectin inhibits melanin biosynthesis and its biosynthesis is directly regulated by peroxisome proliferator-activated receptor (PPAR) γ. In this regard, TCTE may potentially affect PPARγ activity. However, contradicting effects of PPARγ agonists with different chemical structures on human melanogenesis have been reported. ObjectiveA molecular target of TCTE was investigated to elucidate the association of both adiponectin and PPARγ with anti-melanogenic activity. MethodsThe adiponectin secretion-promoting activity of TCTE was tested in an adipogenesis model of hBM-MSCs. A molecular target of TCTE for adiponectin secretion was evaluated via time-resolved fluorescence resonance energy transfer-based receptor binding and transactivation of PPARs. ResultsTCTE significantly promoted adiponectin secretion (EC50, 27.9 μM) during adipogenesis in hBM-MSCs and directly bound to PPARγ (Ki, 13.2 μM). The TCTE-bound PPARγ increased the recruitment of SRC-1, SRC-3, and TRAP220/DRIP-1 coactivator peptides without affecting PGC-1α coactivation. In the docking analysis, the optimal ligand binding mode of TCTE exhibited typical ligand-receptor interactions of PPARγ partial agonists. The PPARγ partial agonism of TCTE was established experimentally and the anti-melanogenic activity of TCTE was decreased by treatment with a PPARγ antagonist in cultured normal human melanocytes and a 3D model of human epidermis. ConclusionThe anti-melanogenic activity of TCTE was associated with a PGC-1α-independent PPARγ partial agonism.

The Journey of Thiazolidinediones as Modulators of PPARs for the Management of Diabetes: A Current Perspective.

Peroxisome Proliferator-Activated Receptors (PPARs) also known as glitazone receptors are a family of receptors that regulate the expression of genes and have an essential role in carbohydrate, lipid and protein metabolism apart from other functions. PPARs come in 3 sub-types: PPAR-α, PPAR-β/δ and PPAR-γ - with PPAR-γ having 2 isoforms - γ1 and γ2. Upon activation, the PPARs regulate the transcription of various genes involved in lipid and glucose metabolism, adipocyte differentiation, increasing insulin sensitivity, prevention of oxidative stress and to a certain extent, modulation of immune responses via macrophages that have been implicated in the pathogenesis of insulin resistance. Hence, PPARs are an attractive molecular target for designing new anti-diabetic drugs. This has led to a boost in the research efforts directed towards designing of PPAR ligands - particularly ones that can selectively and specifically activate one or more of the PPAR subtypes. Though, PPAR- γ full agonists such as Thiazolidinediones (TZDs) are well established agents for dyslipidemia and type 2 diabetes mellitus (T2D), the side effect profile associated with TZDs has potentiated an imminent need to come up with newer agents that act through this pathway. Several newer derivatives having TZD scaffold have been designed using structure based drug designing technique and computational tools and tested for their PPAR binding affinity and efficacy in combating T2D and some have shown promising activities. This review would focus on the role of PPARs in the management of T2D; recently reported TZD derivatives which acted as agonists of PPAR- γ and its subtypes and are potentially useful in the new drug discovery for the disease.

Novel PPAR pan agonist, ZBH ameliorates hyperlipidemia and insulin resistance in high fat diet induced hyperlipidemic hamster.

Effective and safe pharmacological interventions for hyperlipidemia remains badly needed. By incorporating the key pharmacophore of fibrates into the natural scaffold of resveratrol, a novel structural compound ZBH was constructed. In present study, we found ZBH reserved approximately one third of the sirtuin 1 (SIRT1) activation produced by resveratrol at in-vitro enzyme activity assay, directly bound to and activated all three peroxisome proliferator-activated receptor (PPAR) subtypes respectively in PPAR binding and transactivation assays. Moreover, ZBH (EC50, 1.75 µM) activate PPARα 21 fold more efficiently than the well-known PPAR pan agonist bezafibrate (EC50, 37.37 µM) in the cellular transactivation assays. In the high fat diet induced hyperlipidemic hamsters, 5-week treatment with ZBH significantly lowered serum triglyceride, total cholesterol, LDL-C, FFA, hyperinsulinemia, and improved insulin sensitivity more potently than bezafibrate. Meanwhile, serum transaminases, creatine phosphokinase and CREA levels were found not altered by ZBH intervention. Mechanism study indicated ZBH promoted the expression of PPARα target genes and SIRT1 mRNA. Hepatic lipogenesis was markedly decreased via down-regulation of lipogenic genes, and fatty acid uptake and oxidation was simultaneously increased in the liver and skeletal muscle via up-regulation of lipolysis genes. Glucose uptake and utilization was also significantly promoted in skeletal muscle. These results suggested that ZBH significantly lowered hyperlipidemia and ameliorated insulin resistance more efficiently than bezafibrate in the hyperlipidemic hamsters primarily by activating of PPARα, and SIRT1 promotion and activation. ZBH thus presents a potential new agent to combat hyperlipidemia.

Potent Anti-Diabetic Effects of MHY908, a Newly Synthesized PPAR α/γ Dual Agonist in db/db Mice

Peroxisome proliferator-activated receptor (PPAR) α/γ dual agonists have been developed to alleviate metabolic disorders and have the potential to be used as therapeutic agents for the treatment of type 2 diabetes. In this study, we investigated the effects of a newly synthesized PPAR α/γ dual agonist, 2-[4-(5-chlorobenzo [d] thiazol-2-yl) phenoxy]-2-methylpropanoic acid (MHY908) on type 2 diabetes in vitro and in vivo. To obtain initial evidence that MHY908 acts as a PPAR α/γ dual agonist, ChIP and reporter gene assays were conducted in AC2F rat liver cells, and to investigate the anti-diabetic effects and molecular mechanisms, eight-week-old, male db/db mice were allowed to eat ad libitum, placed on calorie restriction, or administered MHY908 (1 mg or 3 mg/kg/day) mixed in food for 4 weeks. Age-matched male db/m lean mice served as non-diabetic controls. It was found that MHY908 enhanced the binding and transcriptional activity of PPAR α and γ in AC2F cells, and it reduced serum glucose, triglyceride, and insulin levels, however increased adiponectin levels without body weight gain. In addition, MHY908 significantly improved hepatic steatosis by enhancing CPT-1 levels. Remarkably, MHY908 reduced endoplasmic reticulum (ER) stress and c-Jun N-terminal kinase (JNK) activation in the livers of db/db mice, and subsequently reduced insulin resistance. The study shows MHY908 has beneficial effects on type 2 diabetes by simultaneously activating PPAR α/γ and improving ER stress, and suggests that MHY908 could have a potent anti-diabetic effect as a PPAR α/γ dual agonist, and potential for the treatment of type 2 diabetes.

β-Carotene regulates expression of β-carotene 15,15′-monoxygenase in human alveolar epithelial cells

β-Carotene 15,15′-monooxygenase (CMO1, BCMO1) converts β-carotene to retinaldehyde (retinal) and is a key enzyme in vitamin A metabolism. CMO1 activity is robust in the intestine and liver, where cmo1 gene transcription may be subject to negative feedback by accumulation of its metabolic products. Evidence from CMO1 null animals also indicates that non-gastrointestinal CMO1 may be required for tissue-specific conversion of β-carotene into vitamin A. The aim of this study was to investigate the effects of the enzymatic substrate, β-carotene, on regulation of CMO1 in a cell model of human alveolar pneumocytes. We demonstrate that CMO1 is expressed in human alveolar epithelial (A549) cells and converts β-carotene into retinal and biologically active retinoic acids (RA). Exposure to β-carotene suppresses CMO1 expression at both mRNA and protein levels. β-Carotene, but not all-trans RA, decreases CMO1 promoter activity in a time- and dosage-dependent manner. This β-carotene-mediated inhibition of CMO1 expression results from decreased binding of peroxisome proliferator-activated receptor γ (PPARγ) and retinoid X receptor α (RXRα) in the CMO1 promoter. β-Carotene treatment also antagonizes PPARγ activity in HEK293 cells that stably express CMO1 wild-type, but not in cells that express the CMO1 mutant or vector alone. These findings have implications for local vitamin A synthesis in the lung, especially during systemic vitamin A insufficiency and may also help to explain, in part, the mechanism underlying the increased lung cancer risk upon β-carotene supplementation in smokers.

Fatty acids regulate perilipin5 in muscle by activating PPARδ

The surface of lipid droplets (LDs) in various cell types is coated with perilipin proteins encoded by the Plin genes. Perilipins regulate LD metabolism by selectively recruiting lipases and other proteins to LDs. We have studied the expression of perilipins in mouse muscle. The glycolytic fiber-enriched gastrocnemius muscle expresses predominantly Plin2-4. The oxidative fiber-enriched soleus muscle expresses Plin2-5. Expression of Plin2 and Plin4-5 is elevated in gastrocnemius and soleus muscles from mice fed a high-fat diet. This effect is preserved in peroxisome proliferator-activated receptor (PPAR)α-deficient mice. Mouse muscle derived C2C12 cells differentiated into glycolytic fibers increase transcription of these Plins when exposed to various long chain fatty acids (FAs). To understand how FAs regulate Plin genes, we used specific activators and antagonists against PPARs, Plin promoter reporter assays, chromatin immunoprecipitation, siRNA, and animal models. Our analyses demonstrate that FAs require PPARδ to induce transcription of Plin4 and Plin5. We further identify a functional PPAR binding site in the Plin5 gene and establish Plin5 as a novel direct PPARδ target in muscle. Our study reveals that muscle cells respond to elevated FAs by increasing transcription of several perilipin LD-coating proteins. This induction renders the muscle better equipped to sequester incoming FAs into cytosolic LDs.

Molecular properties of fatty acids related to PPAR binding and metabolic diseases

Peroxisome proliferator-activated receptor (PPAR) is a class of nuclear receptors responsible for the transcription of genes that regulate the metabolism of carbohydrates and lipids. The main endogenous ligands of PPARs are fatty acids, leukotrienes and derivatives. The PPAR family has three isoforms: α, γ, and δ. To understand the main interactions responsible for the PPAR selectivity of a subset of ligands, we performed molecular modeling studies on a set of 16 fatty acids. From the results obtained, we found that stereochemical, hydrophobic, and polar properties are correlated with PPAR selectivity and can be used to design new ligands with improved biological selectivity.

UDP-Glucuronosyltransferase Expression in Mouse Liver Is Increased in Obesity- and Fasting-Induced Steatosis

UDP-glucuronosyltransferases (Ugt) catalyze phase II conjugation reactions with glucuronic acid, which enhances chemical polarity and the elimination from the body. Few studies have addressed whether Ugt expression and activity are affected by liver disease, such as steatosis. The purpose of this study was to determine whether steatosis induced by obesity or fasting could affect liver Ugt mRNA expression and activity. Male C57BL/6J and Lep(ob/ob) (ob/ob) mice were fed ad libitum or food was withheld for 24 h. In steatotic livers of ob/ob mice, Ugt1a1, -1a6, -1a9, -2a3, -3a1, and -3a2 mRNA expression increased. Fasting, which also induced steatosis, increased hepatic Ugt1a1, -1a6, -1a7, -1a9, -2b1, -2b5, -2a3, -3a1, and -3a2 mRNA expression in mouse liver. Likewise, acetaminophen glucuronidation increased by 47% in hepatic microsomes from ob/ob mice compared with that in C57BL/6J mice, but not after fasting. In both steatosis models, Ugt induction was accompanied by increased aryl hydrocarbon receptor, constitutive androstane receptor (CAR), peroxisome proliferator-activated receptor (PPAR)-α, pregnane X receptor, nuclear factor (erythroid-derived 2)-like 2 (Nrf2), and peroxisome proliferator-activated receptor-γ coactivator-1α mRNA expression. In addition, fasting increased CAR, PPAR, and Nrf2 binding activity. The work points to hepatic triglyceride concentrations corresponding with nuclear receptor and Ugt expression. The findings indicate that steatosis significantly alters hepatic Ugt expression and activity, which could have a significant impact on determining circulating hormone levels, drug efficacy, and environmental chemical clearance.

Structural Insight Into the Crucial Role of Ligand Chirality in the Activation of PPARs by Crystallographic Methods

Peroxisome Proliferator-Activated Receptors (PPARs) are ligand-activated transcription factors that govern lipid and glucose homeostasis playing a central role in cardiovascular disease, obesity, and diabetes. These receptors show a high degree of stereoselectivity towards several classes of drugs. This review provides an overview of most papers reporting the influence of stereochemistry on PPAR activation. Some cases in which chirality is a crucial point in determining the PPAR binding mode are reviewed and discussed with the aim to show how enantiomeric recognition originates at the molecular level. The structural characterization by crystallographic methods of complexes formed by PPARs with their ligands turns out to be an essential tool to explain receptor stereoselectivity.

Rice protein isolate improves lipid and glucose homeostasis in rats fed high fat/high cholesterol diets

In order to understand the molecular mechanisms underlying effects of feeding rice protein on lipid and glucose homeostasis, weanling rats were fed AIN-93G diets made with casein or rice protein isolate (RPI) for 14 d. Peroxisome proliferator-activated receptor (PPAR)alpha genes and proteins involved in fatty acid degradation were upregulated by feeding RPI (P < 0.05), accompanied by increased promoter binding and nuclear expression of PPARalpha and its heterodimerization partner retinoid X receptor (P < 0.05). Effects of RPI feeding on hepatic PPARgamma signaling were significant but less robust. Feeding RPI also increased hepatic genes involved in cholesterol metabolism and transport. However, feeding RPI had no effect on binding of liver X-receptor (LXR)alpha to the cytochrome P450 (CYP)7A1 promoter. The effect of RPI feeding on PPARalpha signaling appeared to be direct and was reversed when RPI diets were switched to casein. In another experiment, male Sprague-Dawley rats were fed casein diets from postnatal day (PND) 24 to PND64 or were fed high fat 'Western' diets containing 0.5% cholesterol made with either casein or RPI. Increased liver triglyceride content, hypercholesterolemia and insulin resistance in the 'Western' diet-fed rats were partially prevented by feeding RPI (P < 0.05). mRNA and protein expression of hepatic enzymes involved in fatty acid synthesis were suppressed by feeding 'Western diets' containing RPI (P < 0.05), despite a lack of effects on nuclear concentrations of sterol regulatory element binding protein-1c. These data suggest that attenuation of metabolic syndrome observed in RPI-fed rats after consumption of diets high in fat and cholesterol occur as a result of improved lipid and glucose homeostasis partly as a result of activation of PPARs.

The MicroRNAs and Stroke: No Need to be Coded to be Counted

The MicroRNAs and Stroke: No Need to be Coded to be Counted

Molecular characterization and functional analysis of porcine macrophage migration inhibitory factor (MIF) gene

Originally identified as a cytokine that inhibits the migration of macrophages from capillaries, MIF has now emerged to be a multifunctional factor involved in immune response, glucose and lipid metabolism. We report here the molecular characterization of porcine MIF gene, which resides on chromosome 14q21.3 and encodes 114 amino acids. Porcine MIF mRNA was found to be highly expressed in stomach, to a lesser extent in spleen, and at the lowest levels in skeletal muscle and heart. To gain further insight into the functional regulation of MIF, we cloned the porcine MIF promoter and verified its functionality using luciferase reporter assay. In NIH 3T3 cells, the endogenous MIF mRNA level was up-regulated by rosiglitazone treatment in a dose-dependent manner, which was accompanied by a concurrent increase in peroxisome proliferator-activated receptor (PPAR)gamma2 transcription. Over-expression of porcine PPARgamma2 in NIH 3T3 cells could also up-regulate the transcription of MIF in the absence of rosiglitazone treatment. These observations are consistent with the bioinformatic analysis that reveals two potential PPAR binding sites within the MIF promoter. Additionally, the over-expression of resistin, a factor considered to be a link between inflammation and metabolic signals, could remarkably down-regulate the transcription of MIF in NIH 3T3 cells. Taken together, we proposed that the transcription of MIF is induced by activation of PPARgamma2 and inhibited by excessive resistin.

Scutellariae radix suppresses hepatitis B virus production in human hepatoma cells

The traditional Chinese medicine Xiao-Chai-Hu-Tang (HD-7) has been widely used to treat liver diseases in China and Japan. HD-7 consists of seven different ingredients, but which one provides the therapeutic benefits is still unknown. Here, we identified the "Minister herb" Scutellariae radix (HD-1S), but not the "King herb" Bupleuri radix (HD-1B) in HD-7, as the active component that suppresses HBV gene expression and virus production in human hepatoma cells. We have found that an aqueous extract of HD-1S not only suppressed wild-type virus but also lamivudine-resistant HBV mutant in human hepatoma cells. We show that HD-1S selectively suppresses HBV core promoter activity. Electrophoretic mobility shift assay analysis has revealed that HD-1S treatment decreases the DNA-binding activity of nuclear extract of HepA2 cells to a specific cis-element of the HBV core promoter, which includes the peroxisome proliferator-activated receptor binding site and HNF4. Furthermore, ectopic expression of PGC-1 abolished the suppression of HD-1S on HBV core promoter activity suggesting that HD-1S may act through modulating hepatic transcriptional machinery to suppress HBV viral gene expression and virus production.

Time Course of Changes in the Expression of Insulin Sensitivity-Related Genes after an Acute Load of Virgin Olive Oil

Our aim was to examine whether an acute fat load could induce changes in the expression of insulin sensitivity-related genes in human peripheral blood mononuclear cells. Selection of candidate genes was based on previous studies with sustained virgin olive oil (VOO) consumption and biological plausibility in relation to insulin sensitivity. Eleven healthy volunteers ingested raw VOO (50 mL). Blood samples were collected at 0, 1 and 6 h. Plasma glucose, insulin and hydroxytyrosol increased at 1 h and decreased at 6 h. Lipid oxidative damage increased at 6 h (p < 0.05). Gene expression changes were characterized based on quantification of the samples relative to a reference sample [i.e., relative quantification (RQ) method]. A 1 h downregulation was observed in O-linked-N-acetylglucosamine transferase (OGT, RQ: 0.62 +/- 0.32) and arachidonate-5-lipoxygenase-activating protein (ALOX5AP, RQ: 0.64 +/- 0.31) genes (p < 0.005). OGT was upregulated at 6 h (RQ: 1.88 +/- 0.28, p < 0.05). CD36 (thrombospondin receptor) was upregulated at 1 h (RQ: 1.6 +/- 0.8, p < 0.05) returning to the basal values at 6 h. Lipoic acid synthetase (LIAS), peroxisome proliferator-activated receptor binding protein (PPARBP), a disintegrin and metallopeptidase domain 17 (ADAM17), and adrenergic beta-2-receptor (ADRB2) genes were upregulated at 6 h (range for the mean RQ: 1.33-1.56) following an increasing linear trend (p < 0.05) from baseline to 6 h. ALOX5AP and OGT genes inversely correlated with insulin and glucose levels at 1 h. ADAM17 and ADRB2 inversely correlated with oxLDL at 6 h (p < 0.05). Taken together, these observations may inform the future clinical nutrigenomics study designs and indicate that a single dose of VOO can elicit quantifiable and rapid changes in gene expression in targets that are mechanistically relevant for insulin sensitivity and the metabolic syndrome.

Comparative analysis of fatty acid-binding protein 4 promoters: Conservation of peroxisome proliferator-activated receptor binding sites1

The objective of this research was to further characterize the promoter regions of the bovine and porcine fatty acid-binding protein 4 (FABP4) genes relative to those of other mammals. The DNA sequences of FABP4 promoters for the human, mouse, cow, pig, and dog were obtained from the genomic database of the National Center for Biotechnology Information and also from the sequencing of bovine and porcine genomic DNA clones obtained by 5' PCR racing of genomic DNA. Sequence alignments of these FABP4 promoters using the basic local alignment search tool of the National Center for Biotechnology Information revealed 3 highly conserved promoter regions across the species. Two computational bioinformatics databases and the literature identified the conserved transcription factor-binding sites for C/EBP, adapter primer-1, and boxes of CAAT and TATA in the first conserved proximal promoter region, a direct repeat 1-type PPAR responsive element in the second distal conserved region, and another PPAR responsive element in the third distal conserved promoter region of FABP4 in all 5 mammals. Five new short interspersed repetitive elements (SINE) in the bovine FABP4 promoter and 2 new SINE in the porcine were found, but these SINE did not disrupt the 3 conserved regions, indicating that important regulatory elements are maintained regardless of evolutionary pressure. In conclusion, the conserved cis-acting elements, especially the 2 key adipocyte transcription factors C/EBP and PPAR, may contribute greatly to adipogenic regulation and adipose tissue-specific expression of FABP4 in these mammals. This helps to further characterize and decipher important cis-acting elements that are important for adipocyte development in adipose and muscle tissue.

The Expression of GPIHBP1, an Endothelial Cell Binding Site for Lipoprotein Lipase and Chylomicrons, Is Induced by Peroxisome Proliferator-Activated Receptor-γ

Glycosylphosphatidylinositol-anchored high-density lipoprotein-binding protein 1 (GPIHBP1), a protein in the lymphocyte antigen 6 (Ly-6) family, plays a key role in the lipolytic processing of triglyceride-rich lipoproteins. GPIHBP1 binds lipoprotein lipase and chylomicrons and is expressed along the luminal surface of microvascular endothelial cells. Lipolysis is known to be regulated by metabolic factors and is controlled at multiple levels, including the number of LPL binding sites on capillaries. Here, we tested the possibility that GPIHBP1 expression could be regulated by dietary perturbations and by peroxisome proliferator-activated receptors (PPARs). Gpihbp1 transcript levels in the heart and in brown and white adipose tissue increased with fasting and returned toward baseline after refeeding. A PPARgamma agonist increased Gpihbp1 expression in adipose tissue, heart, and skeletal muscle, whereas PPARalpha and PPARdelta agonists had no effect. Gpihbp1 was expressed in endothelial cells of embryoid bodies generated from mouse embryonic stem cells, and Gpihbp1 expression in embryoid bodies was up-regulated by a PPARgamma agonist. Sequences upstream from exon 1 of Gpihbp1 contain a strong PPAR binding site, and that site exhibited activity in a luciferase reporter assay. Gpihbp1 transcript levels in brown and white adipose tissue were lower in endothelial cell PPARgamma knockout mice than in littermate control mice, suggesting that PPARgamma regulates Gpihbp1 expression in vivo. We conclude that GPIHBP1 is regulated by dietary factors and by PPARgamma.

PPARα regulates the hepatotoxic biomarker alanine aminotransferase (ALT1) gene expression in human hepatocytes

In this work, we investigated a potential mechanism behind the observation of increased aminotransferase levels in a phase I clinical trial using a lipid-lowering drug, the peroxisome proliferator-activated receptor (PPAR) α agonist, AZD4619. In healthy volunteers treated with AZD4619, serum alanine aminotransferase (ALT) and aspartate aminotransferase (AST) activities were elevated without an increase in other markers for liver injury. These increases in serum aminotransferases have previously been reported in some patients receiving another PPARα agonist, fenofibrate. In subsequent in vitro studies, we observed increased expression of ALT1 protein and mRNA in human hepatocytes after treatment with fenofibric acid. The PPAR effect on ALT1 expression was shown to act through a direct transcriptional mechanism involving at least one PPAR response element (PPRE) in the proximal ALT1 promoter, while no effect of fenofibrate and AZD4619 was observed on the ALT2 promoter. Binding of PPARs to the PPRE located at − 574 bp from the transcriptional start site was confirmed on both synthetic oligonucleotides and DNA in hepatocytes. These data show that intracellular ALT expression is regulated by PPAR agonists and that this mechanism might contribute to increased ALT activity in serum.

Screening for PPAR Responsive Regulatory Modules in Cancer

Peroxisome proliferator-activated receptors (PPARs) have via their large set of target genes a critical impact on numerous diseases including cancer. Cancer development involves numerous regulatory cascades that drive the progression of the malignancy of the cells. On a genomic level, these pathways converge on regulatory modules, some of which contain colocalizing PPAR binding sites (PPREs). We developed an in silico screening method that incorporates experiment- and informatics-derived evidence for a more reliable prediction of PPREs and PPAR target genes. This method is based on DNA-binding data of PPAR subtypes to a panel of DR1-type PPREs and tracking the enrichment of binding sites from multiple species. The ability of PPARγ to induce cellular differentiation and the existence of FDA-approved PPARγ agonists encourage the exploration of possibilities to activate or inactivate PPRE containing modules to arrest cancer progression. Recent advances in genomic techniques combined with computational analysis of binding modules are discussed in the review with the example of our recent screen for PPREs on human chromosome 19.