Structure-based design, synthesis, and nonalcoholic steatohepatitis (NASH)-preventive effect of phenylpropanoic acid peroxisome proliferator-activated receptor (PPAR) α-selective agonists

Diaryl Ether: A Privileged Scaffold for Drug and Agrochemical Discovery

Diaryl ether (DE) is a functional scaffold existing widely both in natural products (NPs) and synthetic organic compounds. Statistically, DE is the second most popular and enduring scaffold within the numerous medicinal chemistry and agrochemical reports. Given its unique physicochemical properties and potential biological activities, DE nucleus is recognized as a fundamental element of medicinal and agrochemical agents aimed at different biological targets. Its drug-like derivatives have been extensively synthesized with interesting biological features including anticancer, anti-inflammatory, antiviral, antibacterial, antimalarial, herbicidal, fungicidal, insecticidal, and so on. In this review, we highlight the medicinal and agrochemical versatility of the DE motif according to the published information in the past decade and comprehensively give a summary of the target recognition, structure-activity relationship (SAR), and mechanism of action of its analogues. It is expected that this profile may provide valuable guidance for the discovery of new active ingredients both in drug and pesticide research.

Structural Basis for PPARα Activation by 1H-pyrazolo-[3,4-b]pyridine Derivatives

Small-molecule agonism of peroxisome proliferator-activated receptor α (PPARα), a ligand-activated transcriptional factor involved in regulating fatty acid metabolism, is an important approach for treating dyslipidemia. Here, we determined the structures of the ligand-binding domain (LBD) of PPARα in complex with 1H-pyrazolo[3,4-b]pyridine-4-carboxylic acid derivatives, which were recently identified as PPARα-selective activators with markedly different structures from those of the well-known PPARα agonists fibrates. The crystal structures of the complexes showed that they form a canonical hydrogen-bond network involving helix 12 in the LBD, which is thought to be essential for PPARα activation, as also observed for fibrates. However, the phenyl side chain of the compounds occupies a small cavity between Ile272 and Ile354, which is rarely accessed by fibrates. This unique feature may be essential for subtype selectivity and combine with the well-characterized binding mode of fibrates to improve activity. These findings demonstrate the advantage of using 1H-pyrazolo-[3,4-b]pyridine as a skeleton of PPARα agonists and provide insight into the design of molecules for treating dyslipidemia.

Structural development of 1H-pyrazolo-[3,4-b]pyridine-4-carboxylic acid derivatives as human peroxisome proliferator-activated receptor alpha (PPARα)-selective agonists

We previously reported that 1H-pyrazolo-[3,4-b]pyridine-4-carboxylic acid derivative 6 is an agonist of human peroxisome proliferator-activated receptor alpha (hPPARα). Here, we prepared a series of 1H-pyrazolo-[3,4-b]pyridine-4-carboxylic acid derivatives in order to examine the structure-activity relationships (SAR). SAR studies clearly indicated that the steric bulkiness of the substituent on 1H-pyrazolo-[3,4-b]pyridine ring, the position of the distal hydrophobic tail part, and the distance between the distal hydrophobic tail part and the acidic head part are critical for hPPARα agonistic activity. These SAR results are somewhat different from those reported for fibrate-class hPPARα agonists. A representative compound (10f) was as effective as fenofibrate in reducing the elevated plasma triglyceride levels in a high-fructose-fed rat model.

An update about the crucial role of stereochemistry on the effects of Peroxisome Proliferator-Activated Receptor ligands

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 covers the most relevant findings that have been made in the last decade and takes into consideration only those compounds in which stereochemistry led to unexpected results or peculiar interactions with the receptors. These cases are reviewed and discussed with the aim to show how enantiomeric recognition originates at the molecular level. The structural characterization by crystallographic methods and docking experiments of complexes formed by PPARs with their ligands turns out to be an essential tool to explain receptor stereoselectivity.

Discovery of peroxisome proliferator-activated receptor α (PPARα) activators with a ligand-screening system using a human PPARα-expressing cell line

Peroxisome proliferator-activated receptor α (PPARα) is a ligand-activated transcription factor that belongs to the superfamily of nuclear hormone receptors. PPARα is mainly expressed in the liver, where it activates fatty acid oxidation and lipoprotein metabolism and improves plasma lipid profiles. Therefore, PPARα activators are often used to treat patients with dyslipidemia. To discover additional PPARα activators as potential compounds for use in hypolipidemic drugs, here we established human hepatoblastoma cell lines with luciferase reporter expression from the promoters containing peroxisome proliferator-responsive elements (PPREs) and tetracycline-regulated expression of full-length human PPARα to quantify the effects of chemical ligands on PPARα activity. Using the established cell-based PPARα-activator screening system to screen a library of >12,000 chemical compounds, we identified several hit compounds with basic chemical skeletons different from those of known PPARα agonists. One of the hit compounds, a 1H-pyrazolo[3,4-b]pyridine-4-carboxylic acid derivative we termed compound 3, selectively up-regulated PPARα transcriptional activity, leading to PPARα target gene expression both in vitro and in vivo Of note, the half-maximal effective concentrations of the hit compounds were lower than that of the known PPARα ligand fenofibrate. Finally, fenofibrate or compound 3 treatment of high fructose-fed rats having elevated plasma triglyceride levels for 14 days indicated that compound 3 reduces plasma triglyceride levels with similar efficiency as fenofibrate. These observations raise the possibility that 1H-pyrazolo[3,4-b]pyridine-4-carboxylic acid derivatives might be effective drug candidates for selective targeting of PPARα to manage dyslipidemia.

Characterization of long non-coding RNA transcriptome in high-energy diet induced nonalcoholic steatohepatitis minipigs

Today, obesity and nonalcoholic steatohepatitis are a worldwide epidemic, although how these syndromes are regulated with respect to lncRNAs remains largely unknown. Our previous studies have revealed important pathological features and molecular characteristics of nonalcoholic steatohepatitis in the minipig model, and in this study, we analyze the features of lncRNAs and their potential target genes. Minipig samples only from liver were analyzed using next-generation deep sequencing. In total, we obtained 585 million raw reads approximately 70.4 Gb of high quality data. After a strict five-step filtering process, 1,179 lncRNAs were identified, including 89 differentially expressed lncRNAs (P < 0.05) in the experiment group relative to the control group. The cis and trans analysis identified target genes that were enriched for specific GO terms (P < 0.01), including immune processes, chemokine activity, cytokine activity, and G-protein coupled receptor binding, which are closely related to nonalcoholic steatohepatitis. The predicted protein-coding targets of the differentially expressed lncRNAs were further analyzed, such as PPAR, FADS2, DGAT2, ACAA2, CYP2E1, ADH4, and Fos. This study reveals a wealth of candidate lncRNAs involved in NASH and their regulated pathways, which should facilitate further research into the molecular mechanisms of this disorder.

Molecular dynamics study-guided identification of cyclic amine structures as novel hydrophobic tail components of hPPARγ agonists

We previously reported that a α-benzylphenylpropanoic acid-type hPPARγ-selective agonist with a piperidine ring as the hydrophobic tail part (3) exhibited sub-micromolar-order hPPARγ agonistic activity. In order to enhance the activity, we planned to carry out structural development based on information obtained from the X-ray crystal structure of hPPARγ ligand binding domain (LBD) complexed with 3. However, the shape and/or nature of the binding pocket surrounding the piperidine ring of 3 could not be precisely delineated because the structure of the omega loop of the LBD was poorly defined. Therefore, we constructed and inserted a plausible omega loop by means of molecular dynamics simulation. We then used the reconstructed LBD structure to design new mono-, bi- and tricyclic amine-bearing compounds that might be expected to show greater binding affinity for the LBD. Here, we describe synthesis and evaluation of α-benzylphenylpropanoic acid derivatives 8. As expected, most of the newly synthesized compounds exhibited more potent hPPARγ agonistic activity and greater hPPARγ binding affinity than 3. Some of these compounds also showed comparable aqueous solubility to 3.

Bidirectional fluorescence properties of pyrene-based peroxisome proliferator-activated receptor (PPAR) α/δ dual agonist

Based on X-ray crystallographic analysis of a peroxisome proliferator-activated receptor (PPAR) α/δ dual agonist complexed with human PPARs ligand binding domain (LBD), we previously reported the design and synthesis of a pyrene-based fluorescent PPARα/δ co-agonist 2. Here, we found that the fluorescence intensity of 2 increased upon binding to hPPARα-LBD, in a manner dependent upon the concentration of the LBD. But, surprisingly, the fluorescence intensity of 2 decreased concentration-dependently upon binding to hPPRδ-LBD. Site-directed mutagenesis of the two hPPAR subtypes clearly indicated that Trp264 of hPPARδ-LBD, located between H2' helix and H3 helix (omega loop), is critical for the concentration-dependent decrease in fluorescence intensity, which is suggested to be due to fluorescence resonance energy transfer (FRET) from the pyrene moiety of bound 2 to the nearby side-chain indole moiety of Trp264 in the hPPARδ-LBD.

Roles of PPARs in NAFLD: potential therapeutic targets

Non-alcoholic fatty liver disease (NAFLD) is a liver pathology with increasing prevalence due to the obesity epidemic. Hence, NAFLD represents a rising threat to public health. Currently, no effective treatments are available to treat NAFLD and its complications such as cirrhosis and liver cancer. Peroxisome proliferator-activated receptors (PPARs) are ligand-activated nuclear receptors which regulate lipid and glucose metabolism as well as inflammation. Here we review recent findings on the pathophysiological role of PPARs in the different stages of NAFLD, from steatosis development to steatohepatitis and fibrosis, as well as the preclinical and clinical evidence for potential therapeutical use of PPAR agonists in the treatment of NAFLD. PPARs play a role in modulating hepatic triglyceride accumulation, a hallmark of the development of NAFLD. Moreover, PPARs may also influence the evolution of reversible steatosis toward irreversible, more advanced lesions. Presently, large controlled trials of long duration are needed to assess the long-term clinical benefits of PPAR agonists in humans. This article is part of a Special Issue entitled Triglyceride Metabolism and Disease.