Antidiabetic actions of a non-agonist PPARγ ligand blocking Cdk5-mediated phosphorylation

Transcriptional Regulation of Adipogenesis

Adipocytes are the defining cell type of adipose tissue. Once considered a passive participant in energy storage, adipose tissue is now recognized as a dynamic organ that contributes to several important physiological processes, such as lipid metabolism, systemic energy homeostasis, and whole-body insulin sensitivity. Therefore, understanding the mechanisms involved in its development and function is of great importance. Adipocyte differentiation is a highly orchestrated process which can vary between different fat depots as well as between the sexes. While hormones, miRNAs, cytoskeletal proteins, and many other effectors can modulate adipocyte development, the best understood regulators of adipogenesis are the transcription factors that inhibit or promote this process. Ectopic expression and knockdown approaches in cultured cells have been widely used to understand the contribution of transcription factors to adipocyte development, providing a basis for more sophisticated in vivo strategies to examine adipogenesis. To date, over two dozen transcription factors have been shown to play important roles in adipocyte development. These transcription factors belong to several families with many different DNA-binding domains. While peroxisome proliferator-activated receptor gamma (PPARγ) is undoubtedly the most important transcriptional modulator of adipocyte development in all types of adipose tissue, members of the CCAAT/enhancer-binding protein, Krüppel-like transcription factor, signal transducer and activator of transcription, GATA, early B cell factor, and interferon-regulatory factor families also regulate adipogenesis. The importance of PPARγ activity is underscored by several covalent modifications that modulate its activity and its ability to modulate adipocyte development. This review will primarily focus on the transcriptional control of adipogenesis in white fat cells and on the mechanisms involved in this fine-tuned developmental process. © 2017 American Physiological Society. Compr Physiol 7:635-674, 2017.

Adipose tissue NAD+ biology in obesity and insulin resistance: From mechanism to therapy

Nicotinamide adenine dinucleotide (NAD⁺ ) biosynthetic pathway, mediated by nicotinamide phosphoribosyltransferase (NAMPT), a key NAD⁺ biosynthetic enzyme, plays a pivotal role in controlling many biological processes, such as metabolism, circadian rhythm, inflammation, and aging. Over the past decade, NAMPT-mediated NAD⁺ biosynthesis, together with its key downstream mediator, namely the NAD⁺ -dependent protein deacetylase SIRT1, has been demonstrated to regulate glucose and lipid metabolism in a tissue-dependent manner. These discoveries have provided novel mechanistic and therapeutic insights into obesity and its metabolic complications, such as insulin resistance, an important risk factor for developing type 2 diabetes and cardiovascular disease. This review will focus on the importance of adipose tissue NAMPT-mediated NAD⁺ biosynthesis and SIRT1 in the pathophysiology of obesity and insulin resistance. We will also critically explore translational and clinical aspects of adipose tissue NAD⁺ biology.

MicroRNA-27a promotes podocyte injury via PPARγ-mediated β-catenin activation in diabetic nephropathy

Podocyte injury has a pivotal role in the pathogenesis of diabetic nephropathy (DN). MicroRNA-27a (miR-27a), peroxisome proliferator-activated receptor γ (PPARγ) and β-catenin pathways have been involved in the pathogenesis of DN. Herein, we asked whether miR-27a mediates podocyte injury through PPARγ/β-catenin signaling in DN. The functional relevance of miR-27a, PPARγ and β-catenin were investigated in cultured podocytes and glomeruli of diabetic rats and patients using in vitro and in vivo approaches. Podocyte injury was assessed by migration, invasion and apoptosis assay. Biological parameters were analyzed using enzyme-linked immunosorbent assay. We found that high glucose stimulated miR-27a expression, which, by negatively targeting PPARγ, activated β-catenin signaling as evidenced by upregulation of β-catenin target genes, snail1 and α-smooth muscle actin (α-SMA) and downregulation of podocyte-specific markers podocin and synaptopodin. These changes caused podocyte injury as demonstrated by increased podocyte mesenchymal transition, disrupted podocyte architectural integrity and increased podocyte apoptosis. Furthermore, we provide evidence that miR-27a contributed to unfavorable renal function and increased podocyte injury in diabetic rats. Notably, miR-27a exhibited clinical and biological relevance as it was linked to elevated serum creatinine, proteinuria and reduced creatinine clearance rate. In addition, miR-27a upregulation and activation of PPARγ/β-catenin signaling were verified in renal biopsy samples from DN patients. We propose a novel role of the miR-27a/PPARγ/β-catenin axis in fostering the progression toward more deteriorated podocyte injury in DN. Targeting miR-27a could be a potential therapeutic approach for DN.

Protein Footprinting Comes of Age: Mass Spectrometry for Biophysical Structure Assessment

Protein footprinting mediated by mass spectrometry has evolved over the last 30 years from proof of concept to commonplace biophysics tool, with unique capabilities for assessing structure and dynamics of purified proteins in physiological states in solution. This review outlines the history and current capabilities of two major methods of protein footprinting: reversible hydrogen-deuterium exchange (HDX) and hydroxyl radical footprinting (HRF), an irreversible covalent labeling approach. Technological advances in both approaches now permit high-resolution assessments of protein structure including secondary and tertiary structure stability mediated by backbone interactions (measured via HDX) and solvent accessibility of side chains (measured via HRF). Applications across many academic fields and in biotechnology drug development are illustrated including: detection of protein interfaces, identification of ligand/drug binding sites, and monitoring dynamics of protein conformational changes along with future prospects for advancement of protein footprinting in structural biology and biophysics research.

Posttranslational Modifications of Lipid-Activated Nuclear Receptors: Focus on Metabolism

Posttranslational modifications (PTMs) occur to nearly all proteins, are catalyzed by specific enzymes, and are subjected to tight regulation. They have been shown to be a powerful means by which the function of proteins can be modified, resulting in diverse effects. Technological advances such as the increased sensitivity of mass spectrometry-based techniques and availability of mutant animal models have enhanced our understanding of the complexities of their regulation and the effect they have on protein function. However, the role that PTMs have in a pathological context still remains unknown for the most part. PTMs enable the modulation of nuclear receptor function in a rapid and reversible manner in response to varied stimuli, thereby dramatically altering their activity in some cases. This review focuses on acetylation, phosphorylation, SUMOylation, and O-GlcNAcylation, which are the 4 most studied PTMs affecting lipid-regulated nuclear receptor biology, as well as on the implications of such modifications on metabolic pathways under homeostatic and pathological situations. Moreover, we review recent studies on the modulation of PTMs as therapeutic targets for metabolic diseases.

Identification of a novel selective PPARγ ligand with a unique binding mode and improved therapeutic profile in vitro

Thiazolidinediones (TZD) function as potent anti-diabetic drugs through their direct action on the nuclear receptor peroxisome proliferator-activated receptor γ (PPARγ), but their therapeutic benefits are compromised by severe side effects. To address this concern, here we developed a potent “hit” compound, VSP-51, which is a novel selective PPARγ-modulating ligand with improved therapeutic profiles in vitro compared to the multi-billion dollar TZD drug rosiglitazone (Rosi). Unlike Rosi, VSP-51 is a partial agonist of PPARγ with improved insulin sensitivity due to its ability to bind PPARγ with high affinity without stimulating adipocyte differentiation and the expression of adipogenesis-related genes. We have determined the crystal structure of the PPARγ ligand-binding domain (LBD) in complex with VSP-51, which revealed a unique mode of binding for VSP-51 and provides the molecular basis for the discrimination between VSP-51 from TZDs and other ligands such as telmisartan, SR1663 and SR1664. Taken together, our findings demonstrate that: a) VSP-51 can serve as a promising candidate for anti-diabetic drug discovery; and b) provide a rational basis for the development of future pharmacological agents targeting PPARγ with advantages over current TZD drugs.

PPARγ and Its Role in Cardiovascular Diseases

Peroxisome proliferator-activated receptor Gamma (PPARγ), a ligand-activated transcription factor, has a role in various cellular functions as well as glucose homeostasis, lipid metabolism, and prevention of oxidative stress. The activators of PPARγ are already widely used in the treatment of diabetes mellitus. The cardioprotective effect of PPARγ activation has been studied extensively over the years making them potential therapeutic targets in diseases associated with cardiovascular disorders. However, they are also associated with adverse cardiovascular events such as congestive heart failure and myocardial infarction. This review aims to discuss the role of PPARγ in the various cardiovascular diseases and summarize the current knowledge on PPARγ agonists from multiple clinical trials. Finally, we also review the new PPARγ agonists under development as potential therapeutics with reduced or no adverse effects.

Dihydromyricetin delays the onset of hyperglycemia and ameliorates insulin resistance without excessive weight gain in Zucker diabetic fatty rats

Many flavonoids are reported to be partial agonists of PPARγ and exert antidiabetic effects with fewer side effects compared with full agonists. Here, we assessed the effects of flavonoid dihydromyricetin (DHM) on glucose homeostasis in male Zucker diabetic fatty rats. Animals were treated with DHM (50-200  mg kg^-1) or rosiglitazone (4  mg kg^-1) once a day for 8 weeks. We found that DHM reduced fasting blood glucose and delayed the onset of hyperglycemia by 4 weeks. Furthermore, DHM preserved pancreatic β-cell mass, elevated adiponectin and improved lipid profile more vigorously than rosiglitazone. Notably, DHM decreased body weight gain and fat accumulation in both liver and adipose tissue, while rosiglitazone caused a significant increase of body weight and fat accumulation. DHM inhibited phosphorylation of PPARγ at serine 273 more efficiently than rosiglitazone. These results suggest that DHM exerts antidiabetic effects without causing excessive body weight gain via inhibition of PPARγ phosphorylation.

Investigations on Binding Pattern of Kinase Inhibitors with PPARγ: Molecular Docking, Molecular Dynamic Simulations, and Free Energy Calculation Studies

Peroxisome proliferator-activated receptor gamma (PPARγ) is a potential target for the treatment of several disorders. In view of several FDA approved kinase inhibitors, in the current study, we have investigated the interaction of selected kinase inhibitors with PPARγ using computational modeling, docking, and molecular dynamics simulations (MDS). The docked conformations and MDS studies suggest that the selected KIs interact with PPARγ in the ligand binding domain (LBD) with high positive predictive values. Hence, we have for the first time shown the plausible binding of KIs in the PPARγ ligand binding site. The results obtained from these in silico investigations warrant further evaluation of kinase inhibitors as PPARγ ligands in vitro and in vivo.

Peroxisome Proliferator-Activated Receptor Modulation during Metabolic Diseases and Cancers: Master and Minions

The prevalence of obesity and metabolic diseases (such as type 2 diabetes mellitus, dyslipidaemia, and cardiovascular diseases) has increased in the last decade, in both industrialized and developing countries. This also coincided with our observation of a similar increase in the prevalence of cancers. The aetiology of these diseases is very complex and involves genetic, nutritional, and environmental factors. Much evidence indicates the central role undertaken by peroxisome proliferator-activated receptors (PPARs) in the development of these disorders. Due to the fact that their ligands could become crucial in future target-therapies, PPARs have therefore become the focal point of much research. Based on this evidence, this narrative review was written with the purpose of outlining the effects of PPARs, their actions, and their prospective uses in metabolic diseases and cancers.

CHARMM Force Field Parameterization of Peroxisome Proliferator-Activated Receptor γ Ligands

The peroxisome proliferator-activated receptor γ (PPARγ) ligands are important therapeutic drugs for the treatment of type 2 diabetes, obesity and cardiovascular diseases. In particular, partial agonists and non-agonists are interesting targets to reduce glucose levels, presenting few side effects in comparison to full agonists. In this work, we present a set of CHARMM-based parameters of a molecular mechanics force field for two PPARγ ligands, GQ16 and SR1664. GQ16 belongs to the thiazolidinedione class of drugs and it is a PPARγ partial agonist that has been shown to promote the “browning” of white adipose tissue. SR1664 is the precursor of the PPARγ non-agonist class of ligands that activates PPARγ in a non-classical manner. Here, we use quantum chemical calculations consistent with the CHARMM protocol to obtain bonded and non-bonded parameters, including partial atomic charges and effective torsion potentials for both molecules. The newly parameterized models were evaluated by examining the behavior of GQ16 and SR1664 free in water and bound to the ligand binding pocket of PPARγ using molecular dynamics simulations. The potential parameters derived here are readily transferable to a variety of pharmaceutical compounds and similar PPARγ ligands.

Browning of White Adipose Tissue with Roscovitine Induces a Distinct Population of UCP1+ Adipocytes

Brown-like adipocytes exist in several adipose depots including white (WAT) as well as brown (BAT). Activation of these UCP1⁺ cells is a potential therapeutic strategy to combat obesity. Studies have shown that posttranslational modifications of PPARγ regulate select adipocyte programs. Deacetylation of K268 and K293 in the ligand-binding domain of PPARγ by Sirt1 induces browning of WAT. Phosphorylation of S273 of PPARγ by CDK5 or ERK stimulates a diabetogenic program of gene expression in WAT. Here, we report that roscovitine, a CDK inhibitor, prevents S273 phosphorylation and promotes formation of UCP1⁺ (brite) adipocytes in WAT. It also enhances energy expenditure as well as prevents diet-induced obesity and insulin resistance. Analysis of fluorescence-activated cell-sorted UCP1⁺ adipocytes shows that the mRNA signature of brite adipocytes is distinct from beige adipocytes, which arise through catecholamine signaling. These results suggest that brown-like adipocytes in WAT may arise from multiple origins.

Importance of 5/6-aryl substitution on the pharmacological profile of 4'-((2-propyl-1H-benzo[d]imidazol-1-yl)methyl)-[1,1'-biphenyl]-2-carboxylic acid derived PPARγ agonists

In this structure-activity relationship study, the influence of aryl substituents at position 5 or 6 on the pharmacological profile of the partial PPARγ agonist 4'-((2-propyl-1H-benzo[d]imidazol-1-yl)methyl)-[1,1'-biphenyl]-2-carboxylic acid was investigated. This lead was previously identified as the essential part of telmisartan to induce PPARγ activation. Para-OCH₃-phenyl substitution strongly increased potency and efficacy independent of the position. Both compounds represent full agonists because of strong hydrophobic contacts with the amino acid Phe363 in the ligand binding domain. Partial agonists with higher potency than telmisartan or the lead were obtained with OH or Cl substituents at the phenyl ring. Molecular modeling suggested additional hydrogen or halogen bonds with Phe360 located at helix 7. It is assumed that these interactions fix helix 7, thereby promoting a partial agonist conformation of the receptor. The theoretical considerations correlate very well with the results from the luciferase transactivation assay using hPPARγ-LBD as well as those from a time-resolved fluorescent resonance energy transfer (TR-FRET) assay in which the coactivator (TRAP220, PGC-1α) recruitment and corepressor (NCoR1) release pattern was investigated.

Epigenomic and transcriptional control of insulin resistance

Insulin resistance is one of the defining features of type 2 diabetes and the metabolic syndrome and accompanies many other clinical conditions, ranging from obesity to lipodystrophy to glucocorticoid excess. Extraordinary efforts have gone into defining the mechanisms that underlie insulin resistance, with most attention focused on altered signalling as well as mitochondrial and endoplasmic reticulum stress. Here, nuclear mechanisms of insulin resistance, including transcriptional and epigenomic effects, will be discussed. Three levels of control involving transcription factors, transcriptional cofactors, and chromatin-modifying enzymes will be considered. Well-studied examples of the first include PPAR-γ in adipose tissue and the glucocorticoid receptor and FoxO1 in a variety of insulin-sensitive tissues. These proteins work in concert with cofactors such as PGC-1α and CRTC2, and chromatin-modifying enzymes including DNA methyltransferases and histone acetyltransferases, to regulate key genes that promote insulin-stimulated glucose uptake, gluconeogenesis or other pathways that affect systemic insulin action. Furthermore, genetic variation associated with increased risk of type 2 diabetes is often related to altered transcription factor binding, either by affecting the transcription factor itself, or more commonly by changing the binding affinity of a noncoding regulatory region. Finally, several avenues for therapeutic exploitation in the battle against metabolic disease will be discussed, including small-molecule inhibitors and activators of these factors and their related pathways.

The role of expression imbalance between adipose synthesis and storage mediated by PPAR-γ/FSP27 in the formation of insulin resistance in catch up growth

Background

Catch up growth (CUG) motivated by under-nutrition can lead to insulin resistance (IR) and visceral fat over-accumulation. However, the precise mechanisms on IR induced by adipose tissue changes during CUG remain unresolved.

Methods

Experimental rats were divided into three groups: normal chow group, catch up growth group and resveratrol administrated group. The whole experiment was carried out in four stages: 4, 6, 8 and 12 weeks.

Peroxisome-proliferator activated receptor gamma (PPAR-γ) and fat-specific protein 27 (FSP27) expression level in epididymal adipose tissues (EAT) and subcutaneous adipose tissues (SAT) were detected along with other IR indicators.

Results

Calorie restriction (CR) significantly increased PPAR-γ expression in EAT while decreased FSP27 expression. During re-feeding, both of the expression of PPAR-γ and FSP27 increased, even FSP27 returned to normal level when CUG for 4 weeks. Although PPAR-γ expression declined slightly at 8 weeks, it was still much stronger than normal chow groups. However, no changes were seen in SAT. Relative insufficiency of FSP27 expression in EAT results in a decrease in lipid storage capacity, causing a series of path physiological changes that led to the formation of IR. Resveratrol inhibited the expression of PPAR-γ and promoted FSP27 expression, thus fundamentally improving IR.

Conclusions

The imbalance between adipose synthesis and storage mediated by PPAR-γ / FSP27 in the EAT plays a pivotal role in the formation of IR during CUG. Resveratrol can correct fat formation and storage imbalance status by up-regulating FSP27 and down-regulating PPAR-γ expression level, ameliorating insulin sensitivity.

FKBP51 Null Mice Are Resistant to Diet-Induced Obesity and the PPARγ Agonist Rosiglitazone

FK506-binding protein-51 (FKBP51) is a molecular cochaperone recently shown to be a positive regulator of peroxisome proliferator-activated receptor (PPAR)γ, the master regulator of adipocyte differentiation and function. In cellular models of adipogenesis, loss of FKBP51 not only reduced PPARγ activity but also reduced lipid accumulation, suggesting that FKBP51 knock-out (KO) mice might have insufficient development of adipose tissue and lipid storage ability. This model was tested by examining wild-type (WT) and FKBP51-KO mice under regular and high-fat diet conditions. Under both diets, FKBP51-KO mice were resistant to weight gain, hepatic steatosis, and had greatly reduced white adipose tissue (WAT) but higher amounts of brown adipose tissue. Under high-fat diet, KO mice were highly resistant to adiposity and exhibited reduced plasma lipids and elevated glucose and insulin tolerance. Profiling of perigonadal and sc WAT revealed elevated expression of brown adipose tissue lineage genes in KO mice that correlated increased energy expenditure and a shift of substrate oxidation to carbohydrates, as measured by indirect calorimetry. To directly test PPARγ involvement, WT and KO mice were fed rosiglitazone agonist. In WT mice, rosiglitazone induced whole-body weight gain, increased WAT mass, a shift of substrate oxidation to lipids, and elevated expression of PPARγ-regulated lipogenic genes in WAT. In contrast, KO mice had reduced rosiglitazone responses for these parameters. Our results identify FKBP51 as an important regulator of PPARγ in WAT and as a potential new target in the treatment of obesity and diabetes.

Small Molecule-Induced Complement Factor D (Adipsin) Promotes Lipid Accumulation and Adipocyte Differentiation

Adipocytes are differentiated by various transcriptional cascades integrated on the master regulator, Pparγ. To discover new genes involved in adipocyte differentiation, preadipocytes were treated with three newly identified pro-adipogenic small molecules and GW7845 (a Pparγ agonist) for 24 hours and transcriptional profiling was analyzed. Four genes, Peroxisome proliferator-activated receptor γ (Pparγ), human complement factor D homolog (Cfd), Chemokine (C-C motif) ligand 9 (Ccl9), and GIPC PDZ Domain Containing Family Member 2 (Gipc2) were induced by at least two different small molecules but not by GW7845. Cfd and Ccl9 expressions were specific to adipocytes and they were altered in obese mice. Small hairpin RNA (shRNA) mediated knockdown of Cfd in preadipocytes inhibited lipid accumulation and expression of adipocyte markers during adipocyte differentiation. Overexpression of Cfd promoted adipocyte differentiation, increased C3a production, and led to induction of C3a receptor (C3aR) target gene expression. Similarly, treatments with C3a or C3aR agonist (C4494) also promoted adipogenesis. C3aR knockdown suppressed adipogenesis and impaired the pro-adipogenic effects of Cfd, further suggesting the necessity for C3aR signaling in Cfd-mediated pro-adipogenic axis. Together, these data show the action of Cfd in adipogenesis and underscore the application of small molecules to identify genes in adipocytes.

Identification of dual ligands targeting angiotensin II type 1 receptor and peroxisome proliferator-activated receptor-γ by core hopping of telmisartan

It has been reported previously that some angiotensin II receptor blockers not only antagonize angiotensin II type 1 receptor (AT₁R), but also exert stimulation in peroxisome proliferator-activated receptor γ (PPARγ) partial activation, among which telmisartan displays the best. Telmisartan has been tested as a bifunctional ligand with antihypertensive and hypoglycemic activity. Aiming at more potent leads with selective AT₁R antagonism and PPARγ partial agonism, the three parts of telmisartan including the distal benzimidazole ring, the biphenyl moiety, and the carboxylic acid group experienced modification by core hopping method in our study. The central benzimidazole ring, however, remained intact considering its great affinity toward AT₁R and PPARγ. We utilized computational techniques for the sake of details on the binding interactions and conformational stability. Standard precision docking analysis and absorption, distribution, metabolism, excretion, and toxicity prediction received 10 molecules with higher Glide scores, similar interactions, and improved pharmacokinetic profiles compared to telmisartan. Comp#91 with highest scores for AT₁R (-11.92 kcal/mol) and PPARγ (-13.88 kcal/mol) exhibited excellent binding modes and pharmacokinetic parameters. Molecular dynamics trajectories on best docking pose of comp#91 confirmed the docking results and verified the conformational stability with both receptors throughout the course of 20-ns simulations. Thus, comp#91 could be identified as a promising lead in the development of dual AT₁R antagonist and PPARγ partial agonist against hypertension and type 2 diabetes.

Does Thiazolidinedione therapy exacerbate fluid retention in congestive heart failure?

The ever-growing global burden of congestive heart failure (CHF) and type 2 diabetes mellitus (T2DM) as well as their co-existence necessitate that anti-diabetic pharmacotherapy will modulate the cardiovascular risk inherent to T2DM while complying with the accompanying restrictions imposed by CHF. The thiazolidinedione (TZD) family of peroxisome proliferator-activated receptor γ (PPARγ) agonists initially provided a promising therapeutic option in T2DM owing to anti-diabetic efficacy combined with pleiotropic beneficial cardiovascular effects. However, the utility of TZDs in T2DM has declined in the past decade, largely due to concomitant adverse effects of fluid retention and edema formation attributed to salt-retaining effects of PPARγ activation on the nephron. Presumably, the latter effects are potentially deleterious in the context of pre-existing fluid retention in CHF. However, despite a considerable body of evidence on mechanisms responsible for TZD-induced fluid retention suggesting that this class of drugs is rightfully prohibited from use in CHF patients, there is a paucity of experimental and clinical studies that investigate the effects of TZDs on salt and water homeostasis in the CHF setting. In an attempt to elucidate whether TZDs actually exacerbate the pre-existing fluid retention in CHF, our review summarizes the pathophysiology of fluid retention in CHF. Moreover, we thoroughly review the available data on TZD-induced fluid retention and proposed mechanisms in animals and patients. Finally, we will present recent studies challenging the common notion that TZDs worsen renal salt and water retention in CHF.

NAMPT-Mediated NAD(+) Biosynthesis in Adipocytes Regulates Adipose Tissue Function and Multi-organ Insulin Sensitivity in Mice

Obesity is associated with adipose tissue dysfunction and multi-organ insulin resistance. However, the mechanisms of such obesity-associated systemic metabolic complications are not clear. Here, we characterized mice with adipocyte-specific deletion of nicotinamide phosphoribosyltransferase (NAMPT), a rate-limiting NAD(+) biosynthetic enzyme known to decrease in adipose tissue of obese and aged rodents and people. We found that adipocyte-specific Nampt knockout mice had severe insulin resistance in adipose tissue, liver, and skeletal muscle and adipose tissue dysfunction, manifested by increased plasma free fatty acid concentrations and decreased plasma concentrations of a major insulin-sensitizing adipokine, adiponectin. Loss of Nampt increased phosphorylation of CDK5 and PPARγ (serine-273) and decreased gene expression of obesity-linked phosphorylated PPARγ targets in adipose tissue. These deleterious alterations were normalized by administering rosiglitazone or a key NAD(+) intermediate, nicotinamide mononucleotide (NMN). Collectively, our results provide important mechanistic and therapeutic insights into obesity-associated systemic metabolic derangements, particularly multi-organ insulin resistance.

PPARG Post-translational Modifications Regulate Bone Formation and Bone Resorption

The peroxisome proliferator-activated receptor gamma (PPARγ) regulates osteoblast and osteoclast differentiation, and is the molecular target of thiazolidinediones (TZDs), insulin sensitizers that enhance glucose utilization and adipocyte differentiation. However, clinical use of TZDs has been limited by side effects including a higher risk of fractures and bone loss. Here we demonstrate that the same post-translational modifications at S112 and S273, which influence PPARγ pro-adipocytic and insulin sensitizing activities, also determine PPARγ osteoblastic (pS112) and osteoclastic (pS273) activities. Treatment of either hyperglycemic or normoglycemic animals with SR10171, an inverse agonist that blocks pS273 but not pS112, increased trabecular and cortical bone while normalizing metabolic parameters. Additionally, SR10171 treatment modulated osteocyte, osteoblast, and osteoclast activities, and decreased marrow adiposity. These data demonstrate that regulation of bone mass and energy metabolism shares similar mechanisms suggesting that one pharmacologic agent could be developed to treat both diabetes and metabolic bone disease.

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PPARγ S273 regulates osteoclast differentiation and insulin sensitivity

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PPARγ S112 regulates osteoblast and adipocyte differentiation

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PPARγ and PPARα regulate osteocyte activities of bone formation and turnover

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SR10171, a PPARγ inverse agonist and PPARα weak agonist, is anabolic for bone

Diabetes is a condition with compromised energy balance and is associated with bone fractures. Some treatment options for diabetes sensitize the patient to insulin via targeting the transcription factor PPARγ. PPARγ is also key regulator of bone formation and bone resorption. Anti-diabetic drugs TZDs target PPARγ protein and this leads to bone loss and increase in fractures in postmenopausal women. Bone mass and energy metabolism share similar regulating pathways, and here we demonstrate a new class of insulin sensitizers that is a selective modulator of PPARγ activity; resulting in a pharmacologic agent that can be beneficial for both diabetes and metabolic bone disease.

Pleiotropic Actions of Peroxisome Proliferator-Activated Receptors (PPARs) in Dysregulated Metabolic Homeostasis, Inflammation and Cancer: Current Evidence and Future Perspectives

BACKGROUND

Peroxisome proliferator-activated receptors (PPARs) have demonstrated a lot of important effects in the regulation of glucose and lipid metabolism and in the correct functioning of adipose tissue. Recently, many studies have evaluated a possible effect of PPARs on tumor cells. The purpose of this review is to describe the effects of PPARs, their action and their future prospective;

METHODS

Narrative review aimed to synthesize cutting-edge evidence retrieved from searches of computerized databases;

RESULTS

PPARs play a key role in metabolic diseases, which include several cardiovascular diseases, insulin resistance, type 2 diabetes, metabolic syndrome, impaired immunity and the increasing risk of cancer; in particular, PPARα and PPARβ/δ mainly enable energy combustion, while PPARγ contributes to energy storage by enhancing adipogenesis;

CONCLUSION

PPAR agonists could represent interesting types of molecules that can treat not only metabolic diseases, but also inflammation and cancer. Additional research is needed for the identification of high-affinity, high-specificity agonists for the treatment of obesity, type 2 diabetes (T2DM) and other metabolic diseases. Further studies are needed also to elucidate the role of PPARs in cancer.

PPARγ signaling and emerging opportunities for improved therapeutics

Peroxisome proliferator-activated receptor gamma (PPARγ) is a ligand-activated nuclear receptor that regulates glucose and lipid metabolism, endothelial function and inflammation. Rosiglitazone (RGZ) and other thiazolidinedione (TZD) synthetic ligands of PPARγ are insulin sensitizers that have been used for the treatment of type 2 diabetes. However, undesirable side effects including weight gain, fluid retention, bone loss, congestive heart failure, and a possible increased risk of myocardial infarction and bladder cancer, have limited the use of TZDs. Therefore, there is a need to better understand PPARγ signaling and to develop safer and more effective PPARγ-directed therapeutics. In addition to PPARγ itself, many PPARγ ligands including TZDs bind to and activate G protein-coupled receptor 40 (GPR40), also known as free fatty acid receptor 1. GPR40 signaling activates stress kinase pathways that ultimately regulate downstream PPARγ responses. Recent studies in human endothelial cells have demonstrated that RGZ activation of GPR40 is essential to the optimal propagation of PPARγ genomic signaling. RGZ/GPR40/p38 MAPK signaling induces and activates PPARγ co-activator-1α, and recruits E1A binding protein p300 to the promoters of target genes, markedly enhancing PPARγ-dependent transcription. Therefore in endothelium, GPR40 and PPARγ function as an integrated signaling pathway. However, GPR40 can also activate ERK1/2, a proinflammatory kinase that directly phosphorylates and inactivates PPARγ. Thus the role of GPR40 in PPARγ signaling may have important implications for drug development. Ligands that strongly activate PPARγ, but do not bind to or activate GPR40 may be safer than currently approved PPARγ agonists. Alternatively, biased GPR40 agonists might be sought that activate both p38 MAPK and PPARγ, but not ERK1/2, avoiding its harmful effects on PPARγ signaling, insulin resistance and inflammation. Such next generation drugs might be useful in treating not only type 2 diabetes, but also diverse chronic and acute forms of vascular inflammation such as atherosclerosis and septic shock.

Bavachinin, as a novel natural pan-PPAR agonist, exhibits unique synergistic effects with synthetic PPAR-γ and PPAR-α agonists on carbohydrate and lipid metabolism in db/db and diet-induced obese mice

AIMS/HYPOTHESIS

Pan-peroxisome proliferator-activated receptor (PPAR) agonists have long been sought as therapeutics against the metabolic syndrome, but current PPAR agonists show limited efficacy and adverse effects. Natural herbs provide a structurally untapped resource to prevent and treat complicated metabolic syndrome.

METHODS

Natural PPAR agonists were screened using reporter gene, competitive binding and 3T3-L1 pre-adipocyte differentiation assays in vitro. The effects on metabolic phenotypes were verified in db/db and diet-induced obese mice. In addition, potentially synergistic actions of bavachinin (BVC, a novel natural pan-PPAR agonist from the fruit of the traditional Chinese glucose-lowering herb malaytea scurfpea) and synthetic PPAR agonists were studied through nuclear magnetic resonance, molecular docking, reporter gene assays and mouse studies.

RESULTS

BVC exhibited glucose-lowering properties without inducing weight gain and hepatotoxicity. Importantly, BVC synergised with thiazolidinediones, which are synthetic PPAR-γ agonists, and fibrates, which are PPAR-α agonists, to induce PPAR transcriptional activity, as well as to lower glucose and triacylglycerol levels in db/db mice. We further found that BVC occupies a novel alternative binding site in addition to the canonical site of synthetic agonists of PPAR, and that the synthetic PPAR-γ agonist rosiglitazone can block BVC binding to this canonical site but not to the alternative site.

CONCLUSIONS/INTERPRETATION

This is the first report of a synergistic glucose- and lipid-lowering effect of BVC and synthetic agonists induced by unique binding with PPAR-γ or -α. This combination may improve the efficacy and decrease the toxicity of marketed drugs for use as adjunctive therapy to treat the metabolic syndrome.

F-box only protein 9 is an E3 ubiquitin ligase of PPARγ

Peroxisome proliferator-activated receptor gamma (PPARγ) is a critical regulator of carbohydrate and lipid metabolism, adipocyte differentiation and inflammatory response. Post-translational modification of PPARγ and its degradation involve several pathways, including the ubiquitin–proteasome system. Here, we identified F-box only protein 9 (FBXO9) as an E3 ubiquitin ligase of PPARγ. We screened interacting partners of PPARγ using immunoprecipitation and mass spectrometric analysis and identified FBXO9 as an E3 ubiquitin ligase of PPARγ. FBXO9 directly interacted with PPARγ through the activation function-1 domain and ligand-binding domain. FBXO9 decreased the protein stability of PPARγ through induction of ubiquitination. We found that the F-box motif of FBXO9 was required for its ubiquitination function. The activity of PPARγ was significantly decreased by FBXO9 overexpression. Furthermore, FBXO9 overexpression in 3T3-L1 adipocytes resulted in decreased levels of endogenous PPARγ and suppression of adipogenesis. These results suggest that FBXO9 is an important enzyme that regulates the stability and activity of PPARγ through ubiquitination.

Tissue Tregs

The immune system is responsible for defending an organism against the myriad of microbial invaders it constantly confronts. It has become increasingly clear that the immune system has a second major function: the maintenance of organismal homeostasis. Foxp3(+)CD4(+) regulatory T cells (Tregs) are important contributors to both of these critical activities, defense being the primary purview of Tregs circulating through lymphoid organs, and homeostasis ensured mainly by their counterparts residing in parenchymal tissues. This review focuses on so-called tissue Tregs. We first survey existing information on the phenotype, function, sustaining factors, and human equivalents of the three best-characterized tissue-Treg populations-those operating in visceral adipose tissue, skeletal muscle, and the colonic lamina propria. We then attempt to distill general principles from this body of work-as concerns the provenance, local adaptation, molecular sustenance, and targets of action of tissue Tregs, in particular.

Mechanistic elucidation guided by covalent inhibitors for the development of anti-diabetic PPARγ ligands

Peroxisome proliferator-activated receptor gamma (PPARγ) is a ligand-regulated transcription factor that plays crucial roles in adipogenesis, lipid metabolism, and glucose homeostasis. Several PPARγ ligands possess anti-diabetic activity and they commonly inhibit the phosphorylation of PPARγ at serine 273 (Ser273). The recently reported PPARγ ligand SR1664, which selectively blocks the phosphorylation of PPARγ without classical agonism, has potent anti-diabetic activity, indicating that the inhibition of Ser273 phosphorylation is sufficient to provoke anti-diabetic effects. In this study, we revealed the X-ray structure of PPARγ co-crystallized with SR1664 bound to the alternate binding site of PPARγ and confirmed that the alternate site binding of SR1664 blocks the phosphorylation of Ser273. Furthermore, using covalent inhibitors as chemical tools, we demonstrated that the inhibition of phosphorylation is attributed to the occupation of a specific site which is a hydrophobic region between helix 3 and β3-β4 at the binding pocket of PPARγ. In high-fat diet-induced obese mice, we confirmed the anti-diabetic activity of our covalent inhibitor SB1453 that was designed to bind at the specific site in PPARγ for blocking the phosphorylation of Ser273. Lastly, the target selectivity of SB1453 was demonstrated by fluorescence-based visualization of target proteins complexed with the covalent probe 11 containing a bioorthogonal functional group.

A high-content screening platform with fluorescent chemical probes for the discovery of first-in-class therapeutics

Phenotypic screening has emerged as a promising approach to discover novel first-in-class therapeutic agents. Rapid advances in phenotypic screening systems facilitate a high-throughput unbiased evaluation of compound libraries. However, limited sets of phenotypic changes are utilized in high-content screening, which require extensive genetic engineering. Therefore, it is critical to develop new chemical probes that can reflect phenotypic changes in any type of cells, especially primary cells, tissues, and organisms. Herein, we introduce our continuous efforts in the development of fluorescent bioprobes and their application to phenotypic screening. In addition, we emphasize the importance of the phenotype-based approach in conjunction with target identification at an early stage of research to accelerate the discovery of therapeutics with new modes of action.

Pioglitazone Enhances the Beneficial Effects of Glucocorticoids in Experimental Nephrotic Syndrome

Glucocorticoids are the primary therapy for nephrotic syndrome (NS), but have serious side effects and are ineffective in ~20-50% of patients. Thiazolidinediones have recently been suggested to be renoprotective, and to modulate podocyte glucocorticoid-mediated nuclear receptor signaling. We hypothesized that thiazolidinediones could enhance glucocorticoid efficacy in NS. We found that puromycin aminonucleoside-induced proteinuria in rats was significantly reduced by both high-dose glucocorticoids (79%) and pioglitazone (61%), but not low-dose glucocorticoids (25%). Remarkably, pioglitazone + low-dose glucocorticoids also reduced proteinuria (63%) comparably to high-dose glucocorticoids, whereas pioglitazone + high-dose glucocorticoids reduced proteinuria to almost control levels (97%). Molecular analysis revealed that both glucocorticoids and pioglitazone enhanced glomerular synaptopodin and nephrin expression, and reduced COX-2 expression, after injury. Furthermore, the glomerular phosphorylation of glucocorticoid receptor and Akt, but not PPARγ, correlated with treatment-induced reductions in proteinuria. Notably, clinical translation of these findings to a child with refractory NS by the addition of pioglitazone to the treatment correlated with marked reductions in both proteinuria (80%) and overall immunosuppression (64%). These findings together suggest that repurposing pioglitazone could potentially enhance the proteinuria-reducing effects of glucocorticoids during NS treatment.

GQ-16, a TZD-Derived Partial PPARγ Agonist, Induces the Expression of Thermogenesis-Related Genes in Brown Fat and Visceral White Fat and Decreases Visceral Adiposity in Obese and Hyperglycemic Mice

Background

Beige adipocytes comprise a unique thermogenic cell type in the white adipose tissue (WAT) of rodents and humans, and play a critical role in energy homeostasis. In this scenario, recruitment of beige cells has been an important focus of interest for the development of novel therapeutic strategies to treat obesity. PPARγ activation by full agonists (thiazolidinediones, TZDs) drives the appearance of beige cells, a process so-called browning of WAT. However, this does not translate into increased energy expenditure, and TZDs are associated with weight gain. Partial PPARγ agonists, on the other hand, do not induce weight gain, but have not been shown to drive WAT browning. The present study was designed to investigate the effects of GQ-16 on BAT and on browning of WAT in obese mice.

Methods

Male Swiss mice with obesity and hyperglycemia induced by high fat diet were treated with vehicle, rosiglitazone (4 mg/kg/d) or the TZD-derived partial PPARγ agonist GQ-16 (40 mg/kg/d) for 14 days. Fasting blood glucose, aspartate aminotransferase, alanine aminotransferase and lipid profile were measured. WAT and brown adipose tissue (BAT) depots were excised for determination of adiposity, relative expression of Ucp-1, Cidea, Prdm16, Cd40 and Tmem26 by RT-qPCR, histological analysis, and UCP-1 protein expression analysis by immunohistochemistry. Liver samples were also removed for histological analysis and determination of hepatic triglyceride content.

Results

GQ-16 treatment reduced high fat diet-induced weight gain in mice despite increasing energy intake. This was accompanied by reduced epididymal fat mass, reduced liver triglyceride content, morphological signs of increased BAT activity, increased expression of thermogenesis-related genes in interscapular BAT and epididymal WAT, and increased UCP-1 protein expression in interscapular BAT and in epididymal and inguinal WAT.

Conclusion

This study suggests for the first time that a partial PPARγ agonist may increase BAT activity and induce the expression of thermogenesis-related genes in visceral WAT.

General Significance

These findings suggest that PPARγ activity might be modulated by partial agonists to induce WAT browning and treat obesity.

Ligands for the Nuclear Peroxisome Proliferator-Activated Receptor Gamma

Nuclear receptors are ligand-activated transcription factors, which represent a primary class of drug targets. The nuclear receptor peroxisome proliferator-activated receptor gamma (PPARγ) is a key player in various biological processes. PPARγ is widely known as the target protein of the thiazolidinediones for treating type 2 diabetes. Moreover, PPARγ ligands can induce anti-inflammatory and potentially additional beneficial effects. Recent mechanistic insights of PPARγ modulation give hope the next generation of efficient PPARγ-based drugs with fewer side effects can be developed. Furthermore, chemical approaches that make use of synergistic action of combinatorial ligands are promising alternatives for providing tailored medicine. Lessons learned from fine-tuning the action of PPARγ can provide avenues for efficient molecular intervention via many other nuclear receptors to combat common diseases.

PPARγ Antagonist Gleevec Improves Insulin Sensitivity and Promotes the Browning of White Adipose Tissue

Blocking phosphorylation of peroxisome proliferator-activated receptor (PPAR)γ at Ser(273) is one of the key mechanisms for antidiabetes drugs to target PPARγ. Using high-throughput phosphorylation screening, we here describe that Gleevec blocks cyclin-dependent kinase 5-mediated PPARγ phosphorylation devoid of classical agonism as a PPARγ antagonist ligand. In high fat-fed mice, Gleevec improved insulin sensitivity without causing severe side effects associated with other PPARγ-targeting drugs. Furthermore, Gleevec reduces lipogenic and gluconeogenic gene expression in liver and ameliorates inflammation in adipose tissues. Interestingly, Gleevec increases browning of white adipose tissue and energy expenditure. Taken together, the results indicate that Gleevec exhibits greater beneficial effects on both glucose/lipid metabolism and energy homeostasis by blocking PPARγ phosphorylation. These data illustrate that Gleevec could be a novel therapeutic agent for use in insulin resistance and type 2 diabetes.

PPARγ Regulation in Hypertension and Metabolic Syndrome

Dysregulation of peroxisome proliferator-activated receptor gamma (PPARγ) activity leads to significant alterations in cardiovascular and metabolic regulation. This is most keenly observed by the metabolic syndrome-like phenotypes exhibited by patients carrying mutations in PPARγ. We will summarize recent findings regarding mechanisms of PPARγ regulation in the cardiovascular and nervous systems focusing largely on PPARγ in the smooth muscle, endothelium, and brain. Canonically, PPARγ exerts its effects by regulating the expression of target genes in these cells, and we will discuss mechanisms by which PPARγ targets in the vasculature regulate cardiovascular function. We will also discuss emerging evidence that PPARγ in the brain is a mediator of appetite and obesity. Finally, we will briefly review how novel PPARγ activators control posttranslational modifications of PPARγ and their prospects to offer new therapeutic options for treatment of metabolic diseases without the adverse side effects of thiazolidinediones which strongly activate transcriptional activity of PPARγ.

Nuclear Mechanisms of Insulin Resistance

Insulin resistance is a sine qua non of type 2 diabetes and is associated with many other clinical conditions. Decades of research into mechanisms underlying insulin resistance have mostly focused on problems in insulin signal transduction and other mitochondrial and cytosolic pathways. By contrast, relatively little attention has been focused on transcriptional and epigenetic contributors to insulin resistance, despite strong evidence that such nuclear mechanisms play a major role in the etiopathogenesis of this condition. In this review, we summarize the evidence for nuclear mechanisms of insulin resistance, focusing on three transcription factors with a major impact on insulin action in liver, muscle, and fat.

SR2067 Reveals a Unique Kinetic and Structural Signature for PPARγ Partial Agonism

Synthetic full agonists of PPARγ have been prescribed for the treatment of diabetes due to their ability to regulate glucose homeostasis and insulin sensitization. While the use of full agonists of PPARγ has been hampered due to severe side effects, partial agonists have shown promise due to their decreased incidence of such side effects in preclinical models. No kinetic information has been forthcoming in regard to the mechanism of full versus partial agonism of PPARγ to date. Here, we describe the discovery of a partial agonist, SR2067. A co-crystal structure obtained at 2.2 Å resolution demonstrates that interactions with the β-sheet are driven exclusively via hydrophobic interactions mediated through a naphthalene group, an observation that is unique from other partial agonists. Surface plasmon resonance revealed that SR2067 binds to the receptor with higher affinity (KD = 513 nM) as compared to that of full agonist rosiglitazone, yet it has a much slower off rate compared to that of rosiglitazone.

Skeletal integration of energy homeostasis: Translational implications

New evidence has recently emerged defining a close relationship between fat and bone metabolism. Adipose tissue is one of the largest organs in the body but its functions vary by location and origin. Adipocytes can act in an autocrine manner to regulate energy balance by sequestering triglycerides and then, depending on demand, releasing fatty acids through lipolysis for energy utilization, and in some cases through uncoupling protein 1 for generating heat. Adipose tissue can also act in an endocrine or paracrine manner by releasing adipokines that modulate the function of other organs. Bone is one of those target tissues, although recent evidence has emerged that the skeleton reciprocates by releasing its own factors that modulate adipose tissue and beta cells in the pancreas. Therefore, it is not surprising that these energy-modulating tissues are controlled by a central regulatory mechanism, primarily the sympathetic nervous system. Disruption in this complex regulatory circuit and its downstream tissues is manifested in a wide range of metabolic disorders, for which the most prevalent is type 2 diabetes mellitus. The aim of this review is to summarize our knowledge of common determinants in the bone and adipose function and the translational implications of recent work in this emerging field.

Effects of diabetes drugs on the skeleton

Type 2 diabetes is associated with increased fracture risk and the mechanisms underlying the detrimental effects of diabetes on skeletal health are only partially understood. Antidiabetic drugs are indispensable for glycemic control in most type 2 diabetics, however, they may, at least in part, modulate fracture risk in exposed patients. Preclinical and clinical data clearly demonstrate an unfavorable effect of thiazolidinediones on the skeleton with impaired osteoblast function and activated osteoclastogenesis. The negative effect of thiazolidinediones on osteoblastogenesis includes decreased activity of osteoblast-specific transcription factors (e.g. Runx2, Dlx5, osterix) and decreased activity of osteoblast-specific signaling pathways (e.g. Wnt, TGF-β/BMP, IGF-1). In contrast, metformin has a positive effect on osteoblast differentiation due to increased activity of Runx2 via the AMPK/USF-1/SHP regulatory cascade resulting in a neutral or potentially protective effect on bone. Recently marketed antidiabetic drugs include incretin-based therapies (GLP-1 receptor agonists, DPP-4 inhibitors) and sodium-glucose co-transporter 2 (SGLT2)-inhibitors. Preclinical studies indicate that incretins (GIP, GLP-1, and GLP-2) play an important role in the regulation of bone turnover. Clinical safety data are limited, however, meta-analyses of trials investigating the glycemic-lowering effect of both, GLP-1 receptor agonists and DPP4-inhibitors, suggest a neutral effect of incretin-based therapies on fracture risk. For SGLT2-inhibitors recent data indicate that due to their mode of action they may alter calcium and phosphate homeostasis (secondary hyperparathyroidism induced by increased phosphate reabsorption) and thereby potentially affect bone mass and fracture risk. Clinical studies are needed to elucidate the effect of SGLT2-inhibitors on bone metabolism. Meanwhile SGLT2-inhibitors should be used with caution in patients with high fracture risk, which is specifically true for the use of thiazolidinediones.

Synthesis of 5-trifluoromethyl-2-sulfonylpyridine PPARβ/δ antagonists: Effects on the affinity and selectivity towards PPARβ/δ

The covalent modification of peroxisome-proliferator activated receptor β/δ (PPARβ/δ) is part of the mode of action of 5-trifluoromethyl-2-sulfonylpyridine PPARβ/δ antagonists such as GSK3787 and CC618. Herein, the synthesis and in vitro biological evaluation of a range of structural analogues of the two antagonists are reported. The new ligands demonstrate that an improvement in the selectivity of 5-trifluoromethyl-2-sulfonylpyridine antagonists towards PPARβ/δ is achievable at the expense of their immediate affinity for PPARβ/δ. However, their putatively covalent and irreversible mode of action may ensure their efficacy over time, as observed in time-resolved fluorescence resonance energy transfer (TR-FRET)-based ligand displacement assays.

Adipose tissue and bone: role of PPARγ in adipogenesis and osteogenesis

Peroxisome proliferator-activated receptor-γ (PPARγ) is a critical factor for the reciprocal regulation of adipogenesis and osteogenesis. Because of their insulin-sensitizing effect, PPARγ agonists, the thiazolidinediones (TZDs), have been widely used for the treatment of type 2 diabetes mellitus; however, the use of TZDs has also been revealed to cause bone loss and bone fractures. The nodal point of regulation of skeletal metabolism by PPARγ activation may reside in its role in cell fate determination of mesenchymal stem cells toward adipogenesis at the expense of osteogenesis. In addition, accumulating evidence demonstrates that PPARγ possesses a circadian expression profile and plays an important role in the skeletal and adipose metabolism regulated by the circadian clock network. Recently, we have shown that nocturnin, a circadian-regulated gene, enhances PPARγ activity, resulting in the suppression of osteogenesis and enhancement of adipogenesis, thus providing additional evidence of the link between circadian networks and PPARγ. In this review, we will focus on the emerging concept of PPARγ as a regulator for skeletal metabolism and summarize recent findings about one of the mechanisms on how PPARγ is connected to the circadian-regulatory system, which involves nocturnin.

New ethanol extraction improves the anti-obesity effects of black tea

Black tea has been reported to have anti-obesity effects in both rodents and humans. Gallic acid, an active component of black tea, decomposes quickly into pyrogallol in high-temperature solutions. This study introduced a new, aqueous ethanol extraction of black tea, which resulted in extracts with higher concentrations of gallic acid than conventional black tea extracts prepared by hot-water extraction or hot-ethanol extraction. We confirmed that, compared with the hot-water extract of black tea, the cold-ethanol extract of black tea (CE-BTE) had greater effects on reducing body weight and body fat, improving fatty liver, regulating blood glucose, and reducing blood cholesterol in the high-fat diet-induced obese mouse model. Nonetheless, although CE-BTE significantly reduced fat content, it did not reduce peroxisome proliferator-activated receptor (PPARγ) protein in epididymal fat tissue of HFD mice. We also showed that CE-BTE did not inhibit the function of PPARγ protein to drive adipogenesis of mouse 3T3-L1 preadipocytes. Considering that PPARγ is a master transcription factor not only for adipocyte differentiation, but also for adipose tissue function, such as glucose and lipid metabolism and insulin sensitivity, these results suggest that CE-BTE reduced fat mass and body weight without dampening fat cell homeostasis and insulin sensitivity.

The Glucose Sensor ChREBP Links De Novo Lipogenesis to PPARγ Activity and Adipocyte Differentiation

Reduced de novo lipogenesis in adipose tissue, often observed in obese individuals, is thought to contribute to insulin resistance. Besides trapping excess glucose and providing for triglycerides and energy storage, endogenously synthesized lipids can function as potent signaling molecules. Indeed, several specific lipids and their molecular targets that mediate insulin sensitivity have been recently identified. Here, we report that carbohydrate-response element-binding protein (ChREBP), a transcriptional inducer of glucose use and de novo lipogenesis, controls the activity of the adipogenic master regulator peroxisome proliferator-activated receptor (PPAR)γ. Expression of constitutive-active ChREBP in precursor cells activated endogenous PPARγ and promoted adipocyte differentiation. Intriguingly, ChREBP-constitutive-active ChREBP expression induced PPARγ activity in a fatty acid synthase-dependent manner and by trans-activating the PPARγ ligand-binding domain. Reducing endogenous ChREBP activity by either small interfering RNA-mediated depletion, exposure to low-glucose concentrations, or expressing a dominant-negative ChREBP impaired differentiation. In adipocytes, ChREBP regulated the expression of PPARγ target genes, in particular those involved in thermogenesis, similar to synthetic PPARγ ligands. In summary, our data suggest that ChREBP controls the generation of endogenous fatty acid species that activate PPARγ. Thus, increasing ChREBP activity in adipose tissue by therapeutic interventions may promote insulin sensitivity through PPARγ.

Mechanisms of peroxisome proliferator activated receptor γ regulation by non-steroidal anti-inflammatory drugs

Non-steroidal anti-inflammatory drugs (NSAIDs) display anti-inflammatory, antipyretic and analgesic properties by inhibiting cyclooxygenases and blocking prostaglandin production. Previous studies, however, suggested that some NSAIDs also modulate peroxisome proliferator activated receptors (PPARs), raising the possibility that such off target effects contribute to the spectrum of clinically relevant NSAID actions. In this study, we set out to understand how peroxisome proliferator activated receptor-γ (PPARγ/PPARG) interacts with NSAIDs using X-ray crystallography and to relate ligand binding modes to effects on receptor activity. We find that several NSAIDs (sulindac sulfide, diclofenac, indomethacin and ibuprofen) bind PPARγ and modulate PPARγ activity at pharmacologically relevant concentrations. Diclofenac acts as a partial agonist and binds to the PPARγ ligand binding pocket (LBP) in typical partial agonist mode, near the β-sheets and helix 3. By contrast, two copies of indomethacin and sulindac sulfide bind the LBP and, in aggregate, these ligands engage in LBP contacts that resemble agonists. Accordingly, both compounds, and ibuprofen, act as strong partial agonists. Assessment of NSAID activities in PPARγ-dependent 3T3-L1 cells reveals that NSAIDs display adipogenic activities and exclusively regulate PPARγ-dependent target genes in a manner that is consistent with their observed binding modes. Further, PPARγ knockdown eliminates indomethacin activities at selected endogenous genes, confirming receptor-dependence of observed effects. We propose that it is important to consider how individual NSAIDs interact with PPARγ to understand their activities, and that it will be interesting to determine whether high dose NSAID therapies result in PPAR activation.

Identification of Bexarotene as a PPARγ Antagonist with HDX

The retinoid x receptors (RXRs) are the pharmacological target of Bexarotene, an antineoplastic agent indicated for the treatment of cutaneous T cell lymphoma (CTCL). The RXRs form heterodimers with several nuclear receptors (NRs), including peroxisome proliferator-activated receptor gamma (PPARγ), to regulate target gene expression through cooperative recruitment of transcriptional machinery. Here we have applied hydrogen/deuterium exchange (HDX) mass spectrometry to characterize the effects of Bexarotene on the conformational plasticity of the intact RXRα:PPARγ heterodimer. Interestingly, addition of Bexarotene to PPARγ in the absence of RXRα induced protection from solvent exchange, suggesting direct receptor binding. This observation was confirmed using a competitive binding assay. Furthermore, Bexarotene functioned as a PPARγ antagonist able to alter rosiglitazone induced transactivation in a cell based promoter:reporter transactivation assay. Together these results highlight the complex polypharmacology of lipophilic NR targeted small molecules and the utility of HDX for identifying and characterizing these interactions.

Design, Synthesis, and Biological Evaluation of Indole Biphenylcarboxylic Acids as PPARγ Antagonists

The thiazolidinediones (TZD) typified by rosiglitazone are the only approved therapeutics targeting PPARγ for the treatment of type-2 diabetes (T2DM). Unfortunately, despite robust insulin sensitizing properties, they are accompanied by a number of severe side effects including congestive heart failure, edema, weight gain, and osteoporosis. We recently identified PPARγ antagonists that bind reversibly with high affinity but do not induce transactivation of the receptor, yet they act as insulin sensitizers in mouse models of diabetes (SR1664).1 This Letter details our synthetic exploration around this novel series of PPARγ antagonists based on an N-biphenylmethylindole scaffold. Structure-activity relationship studies led to the identification of compound 46 as a high affinity PPARγ antagonist that exhibits antidiabetic properties following oral administration in diet-induced obese mice.