PPARα Ligand-Binding Domain Research Articles

Structural Basis for Anti-non-alcoholic Fatty Liver Disease and Diabetic Dyslipidemia Drug Saroglitazar as a PPAR α/γ Dual Agonist.

Peroxisome proliferator-activated receptors (PPARs) are nuclear receptor-type transcription factors that consist of three subtypes (α, γ, and β/δ) with distinct functions and PPAR dual/pan agonists are expected to be the next generation of drugs for metabolic diseases. Saroglitazar is the first clinically approved PPARα/γ dual agonist for treatment of diabetic dyslipidemia and is currently in clinical trials to treat non-alcoholic fatty liver disease (NAFLD); however, the structural information of its interaction with PPARα/γ remains unknown. We recently revealed the high-resolution co-crystal structure of saroglitazar and the PPARα-ligand binding domain (LBD) through X-ray crystallography, and in this study, we report the structure of saroglitazar and the PPARγ-LBD. Saroglitazar was located at the center of "Y"-shaped PPARγ-ligand-binding pocket (LBP), just as it was in the respective region of PPARα-LBP. Its carboxylic acid was attached to four amino acids (Ser289/His323/His449/Thr473), which contributes to the stabilization of Activating Function-2 helix 12, and its phenylpyrrole moiety was rotated 121.8 degrees in PPARγ-LBD from that in PPARα-LBD to interact with Phe264. PPARδ-LBD has the consensus four amino acids (Thr253/His287/His413/Tyr437) towards the carboxylic acids of its ligands, but it seems to lack sufficient space to accept saroglitazar because of the steric hindrance between the Trp228 or Arg248 residue of PPARδ-LBD and its methylthiophenyl moiety. Accordingly, in a coactivator recruitment assay, saroglitazar activated PPARα-LBD and PPARγ-LBD but not PPARδ-LBD, whereas glycine substitution of either Trp228, Arg248, or both of PPARδ-LBD conferred saroglitazar concentration-dependent activation. Our findings may be valuable in the molecular design of PPARα/γ dual or PPARα/γ/δ pan agonists.

Crystal Structures of the Human Peroxisome Proliferator-Activated Receptor (PPAR)α Ligand-Binding Domain in Complexes with a Series of Phenylpropanoic Acid Derivatives Generated by a Ligand-Exchange Soaking Method.

Peroxisome proliferator-activated receptor (PPAR)α, a member of the nuclear receptor family, is a transcription factor that regulates the expression of genes related to lipid metabolism in a ligand-dependent manner, and has attracted attention as a target for hypolipidemic drugs. We have been developing phenylpropaonic acid derivatives as PPARα-targeted drug candidates for the treatment of metabolic diseases. Recently, we have developed the "ligand-exchange soaking method," which crystallizes the recombinant PPARα ligand-binding domain (LBD) as a complex with intrinsic fatty acids derived from an expression host Escherichia (E.) coli and thereafter replaces them with other higher-affinity ligands by soaking. Here we applied this method for preparation of cocrystals of PPARα LBD with its ligands that have not been obtained with the conventional cocrystallization method. We revealed the high-resolution structures of the cocrystals of PPARα LBD and the three synthetic phenylpropaonic acid derivatives: TIPP-703, APHM19, and YN4pai, the latter two of which are the first observations. The overall structures of cocrystals obtained from the two methods are identical and illustrate the close interaction between these ligands and the surrounding amino acid residues of PPARα LBD. This ligand-exchange soaking method could be applicable to high throughput preparations of co-crystals with another subtype PPARδ LBD for high resolution X-ray crystallography, because it also crystallizes in complex with intrinsic fatty acid(s) while not in the apo-form.

Identification of Binding Regions of Bilirubin in the Ligand-Binding Pocket of the Peroxisome Proliferator-Activated Receptor-A (PPARalpha).

Recent work has shown that bilirubin has a hormonal function by binding to the peroxisome proliferator-activated receptor-α (PPARα), a nuclear receptor that drives the transcription of genes to control adiposity. Our previous in silico work predicted three potential amino acids that bilirubin may interact with by hydrogen bonding in the PPARα ligand-binding domain (LBD), which could be responsible for the ligand-induced function. To further reveal the amino acids that bilirubin interacts with in the PPARα LBD, we harnessed bilirubin’s known fluorescent properties when bound to proteins such as albumin. Our work here revealed that bilirubin interacts with threonine 283 (T283) and alanine 333 (A333) for ligand binding. Mutational analysis of T283 and A333 showed significantly reduced bilirubin binding, reductions of 11.4% and 17.0%, respectively. Fenofibrate competitive binding studies for the PPARα LBD showed that bilirubin and fenofibrate possibly interact with different amino acid residues. Furthermore, bilirubin showed no interaction with PPARγ. This is the first study to reveal the amino acids responsible for bilirubin binding in the ligand-binding pocket of PPARα. Our work offers new insight into the mechanistic actions of a well-known molecule, bilirubin, and new fronts into its mechanisms.

PPARα Ligand-Binding Domain Structures with Endogenous Fatty Acids and Fibrates.

SummaryMost triacylglycerol-lowering fibrates have been developed in the 1960s–1980s before their molecular target, peroxisome proliferator-activated receptor alpha (PPARα), was identified. Twenty-one ligand-bound PPARα structures have been deposited in the Protein Data Bank since 2001; however, binding modes of fibrates and physiological ligands remain unknown. Here we show thirty-four X-ray crystallographic structures of the PPARα ligand-binding domain, which are composed of a “Center” and four “Arm” regions, in complexes with five endogenous fatty acids, six fibrates in clinical use, and six synthetic PPARα agonists. High-resolution structural analyses, in combination with coactivator recruitment and thermostability assays, demonstrate that stearic and palmitic acids are presumably physiological ligands; coordination to Arm III is important for high PPARα potency/selectivity of pemafibrate and GW7647; and agonistic activities of four fibrates are enhanced by the partial agonist GW9662. These results renew our understanding of PPARα ligand recognition and contribute to the molecular design of next-generation PPAR-targeted drugs.

Discovery of Novel Peroxisome Proliferator-Activated Receptor α (PPARα) Agonists by Virtual Screening and Biological Evaluation.

Nonalcoholic steatohepatitis (NASH) is one of the important causes of cirrhosis and hepatocellular carcinoma worldwide. PPARα is highly expressed in the liver and plays a critical role in hepatic lipid metabolism. Our analysis of the gene expression profiles in the liver of humanized mice treated with a PPARα agonist and NASH patients suggested that PPARα might be a potential target for NASH therapy. This promoted us to find novel PPARα agonists. The results of virtual screening and biological evaluation identified compound A-4 as a selective PPARα agonist. It significantly regulated the target genes of PPARα involved in fatty acid metabolism and inflammation, exhibiting cellular anti-inflammatory activity. The key residues involved in the binding between PPARα ligand-binding domain (LBD) and compound A-4 were revealed by molecular dynamics (MD) simulation and further experimentally validated by the mutation study. Together, compound A-4 was well characterized as a novel lead compound for developing potent and selective PPARα agonists.

Elucidation of Molecular Mechanism of a Selective PPARα Modulator, Pemafibrate, through Combinational Approaches of X-ray Crystallography, Thermodynamic Analysis, and First-Principle Calculations.

The selective PPARα modulator (SPPARMα) is expected to medicate dyslipidemia with minimizing adverse effects. Recently, pemafibrate was screened from the ligand library as an SPPARMα bearing strong potency. Several clinical pieces of evidence have proved the usefulness of pemafibrate as a medication; however, how pemafibrate works as a SPPARMα at the molecular level is not fully known. In this study, we investigate the molecular mechanism behind its novel SPPARMα character through a combination of approaches of X-ray crystallography, isothermal titration calorimetry (ITC), and fragment molecular orbital (FMO) analysis. ITC measurements have indicated that pemafibrate binds more strongly to PPARα than to PPARγ. The crystal structure of PPARα-ligand binding domain (LBD)/pemafibrate/steroid receptor coactivator-1 peptide (SRC1) determined at 3.2 Å resolution indicates that pemafibrate binds to the ligand binding pocket (LBP) of PPARα in a Y-shaped form. The structure also reveals that the conformation of the phenoxyalkyl group in pemafibrate is flexible in the absence of SRC1 coactivator peptide bound to PPARα; this gives a freedom for the phenoxyalkyl group to adopt structural changes induced by the binding of coactivators. FMO calculations have indicated that the accumulation of hydrophobic interactions provided by the residues at the LBP improve the interaction between pemafibrate and PPARα compared with the interaction between fenofibrate and PPARα.

Glycogen Synthase Kinase-3α Promotes Fatty Acid Uptake and Lipotoxic Cardiomyopathy

Obesity induces lipotoxic cardiomyopathy, a condition in which lipid accumulation in cardiomyocytes causes cardiac dysfunction. Here, we show that glycogen synthase kinase-3α (GSK-3α) mediates lipid accumulation in the heart. Fatty acids (FAs) upregulate GSK-3α, which phosphorylates PPARα at Ser280 in the ligand-binding domain (LBD). This modification ligand independently enhances transcription of a subset of PPARα targets, selectively stimulating FA uptake and storage, but not oxidation, thereby promoting lipid accumulation. Constitutively active GSK-3α, but not GSK-3β, was sufficient to drive PPARα signaling, while cardiac-specific knockdown of GSK-3α, but not GSK-3β, or replacement ofPPARα Ser280 with Ala conferred resistance to lipotoxicity in the heart. Fibrates, PPARα ligands, inhibited phosphorylation of PPARα at Ser280 by inhibiting the interaction of GSK-3α with the LBD of PPARα, thereby reversing lipotoxic cardiomyopathy. These results suggest that GSK-3α promotes lipid anabolism through PPARα-Ser280 phosphorylation, which underlies the development of lipotoxic cardiomyopathy in the context of obesity.

Antcins, triterpenoids from Antrodia cinnamomea, as new agonists for peroxisome proliferator-activated receptor α

Peroxisome proliferator-activated receptor α (PPARα) is a nuclear hormone receptor that transcriptionally regulates lipid metabolism and inflammation; therefore, PPARα agonists are promising agents to treat dyslipidemia and metabolic disorders. PPARα full agonists, such as fibrates, are effective anti-hypertriglyceride agents, but their use is limited by adverse side effects. Hence, the aim of this study was to identify small molecules that can activate PPARα while minimizing the adverse effects. Antrodia cinnamomea, a rare medical mushroom, has been used widely in Asian countries for the treatment of various diseases, including liver diseases. Antcin B, H and K (antcins) and ergostatrien-3β-ol (EK100) are bioactive compounds isolated from A. cinnamomea with anti-inflammatory actions. Antcins, ergostane-type triterpenoids, contain the polar head with carboxylate group and the sterol-based body. Here, we showed at the first time that sterol-based compounds, antcins, but not EK100, activate PPARα in a cell-based transactivation study. The in silico docking studies presented several significant molecular interactions of antcins, including Tyr314, and His440 in the ligand-binding domain of PPARα, and these interactions are required for helix 12 (H12) stabilization. We propose that PPARα activation activity of antcins is related to their binding mode which requires conventional H12 stabilization, and that antcins can be developed as safe selective PPARα modulators.

Aspirin binds to PPARα to stimulate hippocampal plasticity and protect memory

Despite its long history, until now, no receptor has been identified for aspirin, one of the most widely used medicines worldwide. Here we report that peroxisome proliferator-activated receptor alpha (PPARα), a nuclear hormone receptor involved in fatty acid metabolism, serves as a receptor of aspirin. Detailed proteomic analyses including cheminformatics, thermal shift assays, and TR-FRET revealed that aspirin, but not other structural homologs, acts as a PPARα ligand through direct binding at the Tyr314 residue of the PPARα ligand-binding domain. On binding to PPARα, aspirin stimulated hippocampal plasticity via transcriptional activation of cAMP response element-binding protein (CREB). Finally, hippocampus-dependent behavioral analyses, calcium influx assays in hippocampal slices and quantification of dendritic spines demonstrated that low-dose aspirin treatment improved hippocampal plasticity and memory in FAD5X mice, but not in FAD5X/Ppara-null mice. These findings highlight a property of aspirin: stimulating hippocampal plasticity via direct interaction with PPARα.

Repurposing statins for Alzheimer’s disease

By inducing the abundance of neurotrophins, statins enhance memory in Alzheimer’s disease.

The 4'-hydroxyl group of resveratrol is functionally important for direct activation of PPARα.

Long-term moderate consumption of red wine is associated with a reduced risk of developing lifestyle-related diseases such as cardiovascular disease and cancer. Therefore, resveratrol, a constituent of grapes and various other plants, has attracted substantial interest. This study focused on one molecular target of resveratrol, the peroxisome proliferator activated receptor α (PPARα). Our previous study in mice showed that resveratrol-mediated protection of the brain against stroke requires activation of PPARα; however, the molecular mechanisms involved in this process remain unknown. Here, we evaluated the chemical basis of the resveratrol-mediated activation of PPARα by performing a docking mode simulation and examining the structure-activity relationships of various polyphenols. The results of experiments using the crystal structure of the PPARα ligand-binding domain and an analysis of the activation of PPARα by a resveratrol analog 4-phenylazophenol (4-PAP) in vivo indicate that the 4′-hydroxyl group of resveratrol is critical for the direct activation of PPARα. Activation of PPARα by 5 μM resveratrol was enhanced by rolipram, an inhibitor of phosphodiesterase (PDE) and forskolin, an activator of adenylate cyclase. We also found that resveratrol has a higher PDE inhibitory activity (IC50 = 19 μM) than resveratrol analogs trans-4-hydroxystilbene and 4-PAP (IC50 = 27-28 μM), both of which has only 4′-hydroxyl group, indicating that this 4′-hydroxyl group of resveratrol is not sufficient for the inhibition of PDE. This result is consistent with that 10 μM resveratrol has a higher agonistic activity of PPARα than these analogs, suggesting that there is a feedforward activation loop of PPARα by resveratrol, which may be involved in the long-term effects of resveratrol in vivo.

The dipeptide H-Trp-Arg-OH (WR) Is a PPARα agonist and reduces hepatic lipid accumulation in lipid-loaded H4IIE cells.

Dipeptides absorbed by the intestinal epithelium are delivered to circulation, but their metabolic roles are not yet clearly understood. We investigated the biological activities of a dietary dipeptide, H-Trp-Arg-OH (WR), on the regulation of peroxisome proliferator-activated receptor (PPAR) α activity. Reporter gene assays revealed that WR dose-dependently induced PPARα transactivation. Surface plasmon resonance experiments demonstrated that WR interacts directly with the PPARα ligand binding domain, and time-resolved fluorescence energy transfer analyses revealed recruitment of a co-activator peptide, fluorescein-PGC1α, to PPARα, confirming the direct binding of WR to PPARα and occurrence of conformational changes. WR induced cellular fatty acid uptake and the expression of PPARα response genes in fatty acid oxidation, thus reducing intracellular triglyceride accumulation in lipid-loaded hepatocytes. In conclusion, the dietary dipeptide WR activates PPARα and reduces hepatic lipid accumulation in lipid-loaded hepatocytes.

Trans-Caryophyllene is a natural agonistic ligand for peroxisome proliferator-activated receptor-α

Intake of dietary aroma compounds may regulate cellular lipid metabolism. We demonstrated that trans-caryophyllene, a flavor compound in plant foods and teas, activates peroxisome proliferator-activated receptor (PPAR)-α through direct interaction with the ligand-binding domain of PPAR-α. The agonistic activity of trans-caryophyllene was investigated by the luciferase reporter assay, surface plasmon resonance, and time-resolved fluorescence resonance energy transfer assay. Following the stimulation of cells with trans-caryophyllene, intracellular triglyceride concentrations were significantly reduced by 17%, and hepatic fatty acid uptake was significantly increased by 31%. The rate of fatty acid oxidation was also significantly increased. The expressions of PPAR-α and its target genes and proteins in fatty acid uptake and oxidation were significantly up-regulated as well. In HepG2 cells transfected with small interfering RNA of PPAR-α, the effects of trans-caryophyllene on PPAR-α responsive gene expressions, intracellular triglyceride, fatty acid uptake and oxidation were disappeared. These results indicate that the aroma compound, trans-caryophyllene, is PPAR-α agonist thus regulates cellular lipid metabolism in PPAR-α dependent manners.

The dipeptide H-Trp-Glu-OH (WE) shows agonistic activity to peroxisome proliferator-activated protein-α and reduces hepatic lipid accumulation in lipid-loaded H4IIE cells

Dipeptides digested from dietary proteins can be directly absorbed by the intestine and delivered to the circulatory system. However, the dipeptides’ metabolic roles and biological activities are largely unknown. Lipid-loaded HII4E cells stimulated with H-Trp-Glu-OH (WE) exhibited reduced lipid accumulation, of which the effect was abolished by peroxisome proliferator-activated receptor (PPAR) α gene knock down. A luciferase assay showed that the WE dipeptide induced PPARα transactivation in a dose-dependent manner. Surface plasmon resonance and time-resolved fluorescence resonance energy transfer analyses demonstrated that WE interacts directly with the PPARα ligand binding domain (KD, 120μM; EC50, 83μM). Cells stimulated with WE induced PPARα and its responsive genes and increased cellular fatty acid uptake. In conclusion, WE reduces hepatic lipid accumulation in lipid-loaded hepatocytes via the activation of PPARα by a direct interaction.

Computational and Biological Evaluation of N-octadecyl-N′-propylsulfamide, a Selective PPARα Agonist Structurally Related to N-acylethanolamines

To further understand the pharmacological properties of N-oleoylethanolamine (OEA), a naturally occurring lipid that activates peroxisome proliferator-activated receptor alpha (PPARα), we designed sulfamoyl analogs based on its structure. Among the compounds tested, N-octadecyl-N′-propylsulfamide (CC7) was selected for functional comparison with OEA. The performed studies include the following computational and biological approaches: 1) molecular docking analyses; 2) molecular biology studies with PPARα; 3) pharmacological studies on feeding behavior and visceral analgesia. For the docking studies, we compared OEA and CC7 data with crystallization data obtained with the reference PPARα agonist GW409544. OEA and CC7 interacted with the ligand-binding domain of PPARα in a similar manner to GW409544. Both compounds produced similar transcriptional activation by in vitro assays, including the GST pull-down assay and reporter gene analysis. In addition, CC7 and OEA induced the mRNA expression of CPT1a in HpeG2 cells through PPARα and the induction was avoided with PPARα-specific siRNA. In vivo studies in rats showed that OEA and CC7 had anorectic and antiobesity activity and induced both lipopenia and decreases in hepatic fat content. However, different effects were observed when measuring visceral pain; OEA produced visceral analgesia whereas CC7 showed no effects. These results suggest that OEA activity on the PPARα receptor (e.g., lipid metabolism and feeding behavior) may be dissociated from other actions at alternative targets (e.g., pain) because other non cannabimimetic ligands that interact with PPARα, such as CC7, do not reproduce the full spectrum of the pharmacological activity of OEA. These results provide new opportunities for the development of specific PPARα-activating drugs focused on sulfamide derivatives with a long alkyl chain for the treatment of metabolic dysfunction.

Catalposide is a natural agonistic ligand of peroxisome proliferator-activated receptor-α

Peroxisome proliferator-activated receptor-alpha (PPARα) is a nuclear receptor that regulates the expression of genes related to cellular lipid uptake and oxidation. Thus, PPARα agonists may be important in the treatment of hypertriglyceridemia and hepatic steatosis. In this study, we demonstrated that catalposide is a novel natural PPARα agonist, identified from reporter gene assay-based activity screening with approximately 900 natural plant and seaweed extracts. Results of time-resolved fluorescence resonance energy transfer analyses suggested that the compound interacted directly with the ligand-binding domain of PPARα. Cultured hepatocytes stimulated with catalposide exhibited significantly reduced cellular triglyceride concentrations, by 21%, while cellular uptake of fatty acids was increased, by 70% (P<0.05). Quantitative PCR analysis revealed that the increase in cellular fatty acid uptake was due to upregulation of fatty acid transporter protein-4 (+19% vs. the control) in cells stimulated with catalposide. Additionally, expression of genes related to fatty acid oxidation and high-density lipoprotein metabolism were upregulated, while that of genes related to fatty acid synthesis were suppressed. In conclusion, catalposide is hypolipidemic by activation of PPARα via a ligand-mediated mechanism that modulates the expression of in lipid metabolism genes in hepatocytes.

Structural Basis for Iloprost as a Dual Peroxisome Proliferator-activated Receptor α/δ Agonist

Iloprost is a prostacyclin analog that has been used to treat many vascular conditions. Peroxisome proliferator-activated receptors (PPARs) are ligand-regulated transcription factors with various important biological effects such as metabolic and cardiovascular physiology. Here, we report the crystal structures of the PPARα ligand-binding domain and PPARδ ligand-binding domain bound to iloprost, thus providing unambiguous evidence for the direct interaction between iloprost and PPARs and a structural basis for the recognition of PPARα/δ by this prostacyclin analog. In addition to conserved contacts for all PPARα ligands, iloprost also initiates several specific interactions with PPARs using its unique structural groups. Structural and functional studies of receptor-ligand interactions reveal strong functional correlations of the iloprost-PPARα/δ interactions as well as the molecular basis of PPAR subtype selectivity toward iloprost ligand. As such, the structural mechanism may provide a more rational template for designing novel compounds targeting PPARs with more favorable pharmacologic impact based on existing iloprost drugs.

Discovery of an Oxybenzylglycine Based Peroxisome Proliferator Activated Receptor α Selective Agonist 2-((3-((2-(4-Chlorophenyl)-5-methyloxazol-4-yl)methoxy)benzyl)(methoxycarbonyl)amino)acetic Acid (BMS-687453)

An 1,3-oxybenzylglycine based compound 2 (BMS-687453) was discovered to be a potent and selective peroxisome proliferator activated receptor (PPAR) alpha agonist, with an EC(50) of 10 nM for human PPARalpha and approximately 410-fold selectivity vs human PPARgamma in PPAR-GAL4 transactivation assays. Similar potencies and selectivity were also observed in the full length receptor co-transfection assays. Compound 2 has negligible cross-reactivity against a panel of human nuclear hormone receptors including PPARdelta. Compound 2 demonstrated an excellent pharmacological and safety profile in preclinical studies and thus was chosen as a development candidate for the treatment of atherosclerosis and dyslipidemia. The X-ray cocrystal structures of the early lead compound 12 and compound 2 in complex with PPARalpha ligand binding domain (LBD) were determined. The role of the crystal structure of compound 12 with PPARalpha in the development of the SAR that ultimately resulted in the discovery of compound 2 is discussed.

The Peroxisome Proliferator-Activated Receptor-γ Agonist Pioglitazone Represses Inflammation in a Peroxisome Proliferator-Activated Receptor-α–Dependent Manner In Vitro and In Vivo in Mice

The Peroxisome Proliferator-Activated Receptor-γ Agonist Pioglitazone Represses Inflammation in a Peroxisome Proliferator-Activated Receptor-α–Dependent Manner In Vitro and In Vivo in Mice Gabriela Orasanu, Ouliana Ziouzenkova, Pallavi R. Devchand, Vedika Nehra, Osama Hamdy, Edward S. Horton, Jorge Plutzky The peroxisome proliferator-activated receptor (PPAR)-γ-activating thiazolidinediones (TZDs) pioglitazone and rosiglitazone are insulin sensitizers that limit inflammation and decrease atherosclerosis. Recent data, however, have raised questions regarding distinct TZD effects. In a randomized, placebo-controlled clinical study among patients with recently diagnosed type 2 diabetes, pioglitazone exerted effects consistent with PPARα activation. In studying pioglitazone effects on PPARα responses, including in PPARα-deficient experimental settings in vitro and in vivo, we found that pioglitazone exerted known PPARα effects in vitro but not in the absence of PPARα, including regulation of endothelial vascular cell adhesion molecule-1 and hepatic IκBα expression. Pioglitazone modulated these same targets in vivo but not in PPARα-deficient mice. Our aim was to investigate if the peroxisome proliferator-activated receptor (PPAR)-γ agonist pioglitazone modulates inflammation through PPARα mechanisms. The thiazolidinediones (TZDs) pioglitazone and rosiglitazone are insulin-sensitizing PPARγ agonists used to treat type 2 diabetes (T2DM). Despite evidence for TZDs limiting inflammation and atherosclerosis, questions exist regarding differential responses to TZDs. In a double-blinded, placebo-controlled 16-week trial among recently diagnosed T2DM subjects (n = 34), pioglitazone-treated subjects manifested lower triglycerides and lacked the increase in soluble vascular cell adhesion molecules (sVCAM)-1 evident in the placebo group. Previously we reported PPARα but not PPARγ agonists could repress VCAM-1 expression. Since both triglyceride-lowering and VCAM-1 repression characterize PPARα activation, we studied pioglitazone's effects via PPARα. Pioglitazone effects on known PPARα responses—ligand binding domain activation and PPARα target gene expression—were tested in vitro and in vivo, including in wild-type and PPARα-deficient cells and mice, and compared with the effects of other PPARγ (rosiglitazone) and PPARα (WY14643) agonists. Pioglitazone repressed endothelial TNFα-induced VCAM-1 messenger ribonucleic acid expression and promoter activity, and induced hepatic IκBα in a manner dependent on both pioglitazone exposure and PPARα expression. Pioglitazone also activated the PPARα ligand binding domain and induced PPARα target gene expression, with in vitro effects that were most pronounced in endothelial cells. In vivo, pioglitazone administration modulated sVCAM-1 levels and IκBα expression in wild-type but not PPARα-deficient mice. Pioglitazone regulates inflammatory target genes in hepatic (IκBα) and endothelial (VCAM-1) settings in a PPARα-dependent manner. These data offer novel mechanisms that may underlie distinct TZD responses.

Liver-Specific Peroxisome Proliferator–Activated Receptor α Target Gene Regulation by the Angiotensin Type 1 Receptor Blocker Telmisartan

The angiotensin type 1 receptor blocker (ARB) and peroxisome proliferator-activated receptor (PPAR) gamma modulator telmisartan has been recently demonstrated to reduce plasma triglycerides in nondiabetic and diabetic hypertensive patients. The present study investigates the molecular mechanisms of telmisartans hypolipidemic actions, in particular its effect on the PPARalpha pathway. RESEARCH DESIGN AND METHODS; Regulation of PPARalpha target genes by telmisartan was studied by real-time PCR and Western immunoblotting in vitro and in vivo in liver/skeletal muscle of mice with diet-induced obesity. Activation of the PPARalpha ligand binding domain (LBD) was investigated using transactivation assays. Telmisartan significantly induced the PPARalpha target genes carnitine palmitoyl transferase 1A (CPT1A) in human HepG2 cells and acyl-CoA synthetase long-chain family member 1 (ACSL1) in murine AML12 cells in the micromolar range. Telmisartan-induced CPT1A stimulation was markedly reduced after small interfering RNA-mediated knockdown of PPARalpha. Telmisartan consistently activated the PPARalpha-LBD as a partial PPARalpha agonist. Despite high in vitro concentrations required for PPARalpha activation, telmisartan (3 mg x kg(-1) x day(-1)) potently increased ACSL1 and CPT1A expression in liver from diet-induced obese mice associated with a marked decrease of hepatic and serum triglycerides. Muscular CPT1B expression was not affected. Tissue specificity of telmisartan-induced PPARalpha target gene induction may be the result of previously reported high hepatic concentrations of telmisartan. The present study identifies the ARB/PPARgamma modulator telmisartan as a partial PPARalpha agonist. As a result of its particular pharmacokinetic profile, PPARalpha activation by telmisartan seems to be restricted to the liver. Hepatic PPARalpha activation may provide an explanation for telmisartan's antidyslipidemic actions observed in recent clinical trials.