Structural mechanism underlying ligand binding and activation of PPARγ

Insights into the chirality-dependent recognition of Danshensu Bingpian Zhi stereoisomers with PPARγ

Peroxisome proliferator-activated receptor γ (PPARγ), a nuclear receptor involved in metabolic processes, inflammation, and energy balance, represents a promising therapeutic target for cardiovascular diseases. Danshensu Bingpian Zhi (DBZ), a chiral compound derived from traditional Chinese medicine, exhibits potential as a PPARγ agonist. Using an ensemble-based docking approach, molecular dynamics (MD) simulations, and the molecular mechanics generalized born surface area (MM/GBSA) methods, we explored the binding modes and energetics of DBZ stereoisomers with the PPARγ ligand-binding domain (LBD). The results indicated that the right-handed stereoisomer (DBZR) binds like a full agonist, while the left-handed stereoisomer (DBZS) binds as a partial agonist with stronger binding energies (ΔGbind), indicating a robust interaction with PPARγ. Both the stereoisomers stabilize the β-sheet region of PPARγ-LBD, potentially protecting Ser245 from phosphorylation by Cdk5, a process implicated in atherosclerosis. Principal component analysis (PCA) and dynamic cross-correlation matrices (DCCM) revealed the complex structural dynamics within the Ω loop, β-sheet, and AF-2 region of PPARγ-LBD upon ligand binding, which may contribute to the unique binding mode and efficacy of DBZS. These findings provide insights into the molecular recognition of PPARγ-LBD by DBZ stereoisomers and their impact on the conformational dynamics of PPARγ, highlighting the therapeutic potential of DBZ and the significance of chirality in drug design.

Structure-based screening, optimization and biological evaluation of novel chrysin-based derivatives as selective PPARγ modulators for the treatment of T2DM and hepatic steatosis

In consideration of several serious side effects induced by the classical AF-2 involved "lock" mechanism, recently disclosed PPARγ-Ser273 phosphorylation mode of action has become an alternative and mainstream mechanism for currently PPARγ-based drug discovery and development with an improved therapeutic index. In this study, by virtue of structure-based virtual high throughput screening (SB-VHTS), structurally chemical optimization by targeting the inhibition of the PPARγ-Ser273 phosphorylation as well as in vitro biological evaluation, which led to the final identification of a chrysin-based potential hit (YGT-31) as a novel selective PPARγ modulator with potent binding affinity and partial agonism. Further in vivo evaluation demonstrated that YGT-31 possessed potent glucose-lowering and relieved hepatic steatosis effects without involving the TZD-associated side effects. Mechanistically, YGT-31 presented such desired therapeutic index, mainly because it effectively inhibited the CDK5-mediated PPARγ-Ser273 phosphorylation, selectively elevated the level of insulin sensitivity-related Glut4 and adiponectin but decreased the expression of insulin-resistance-associated genes PTP1B and SOCS3 as well as inflammation-linked genes IL-6, IL-1β and TNFα. Finally, the molecular docking study was also conducted to uncover an interesting hydrogen-bonding network of YGT-31 with PPARγ-Ser273 phosphorylation-related key residues Ser342 and Glu343, which not only gave a clear verification for our targeting modification but also provided a proof of concept for the abovementioned molecular mechanism.

Unanticipated mechanisms of covalent inhibitor and synthetic ligand cobinding to PPARγ

Peroxisome proliferator-activated receptor gamma (PPARγ) is a nuclear receptor transcription factor that regulates gene expression programs in response to ligand binding. Endogenous lipids and synthetic ligands, including covalent antagonist inhibitors such as GW9662 and T0070907, are thought to compete for the orthosteric pocket in the ligand-binding domain (LBD). However, we previously showed that synthetic PPARγ ligands can cooperatively cobind with and reposition a bound endogenous orthosteric ligand to an alternate site, synergistically regulating PPARγ structure and function (Shang et al., 2018). Here, we reveal the structural mechanism of cobinding between a synthetic covalent antagonist inhibitor with other synthetic ligands. Biochemical and NMR data show that covalent antagonist inhibitors weaken-but do not prevent-the binding of other synthetic ligands via an allosteric mechanism rather than direct ligand clashing. The covalent ligands shift the LBD ensemble toward a transcriptionally repressive conformation, which structurally clashes with and reduces the orthosteric binding affinity of non-covalent synthetic ligands. Crystal structures reveal different non-covalent synthetic ligand-specific cobinding mechanisms ranging from alternate site binding to unexpectedly adopting an orthosteric binding mode by altering the covalent ligand binding pose. Our findings not only highlight the significant flexibility of the PPARγ orthosteric pocket and its ability to accommodate multiple ligands simultaneously, but also demonstrate that GW9662 and T0070907 should not be used as reliable chemical tools to inhibit the binding of other ligands to PPARγ.

Cladosporols and PPARγ: Same Gun, Same Bullet, More Targets

Several natural compounds have been found to act as PPARγ agonists, thus regulating numerous biological processes, including the metabolism of carbohydrates and lipids, cell proliferation and differentiation, angiogenesis, and inflammation. Recently, Cladosporols, secondary metabolites purified from the fungus Cladosporium tenuissimum, have been demonstrated to display an efficient ability to control cell proliferation in human colorectal and prostate cancer cells through a PPARγ-mediated modulation of gene expression. In addition, Cladosporols exhibited a strong anti-adipogenetic activity in 3T3-L1 murine preadipocytes, preventing their in vitro differentiation into mature adipocytes. These data interestingly point out that the interaction between Cladosporols and PPARγ, in the milieu of different cells or tissues, might generate a wide range of beneficial effects for the entire organism affected by diabetes, obesity, inflammation, and cancer. This review explores the molecular mechanisms by which the Cladosporol/PPARγ complex may simultaneously interfere with a dysregulated lipid metabolism and cancer promotion and progression, highlighting the potential therapeutic benefits of Cladosporols for human health.

Ligand efficacy shifts a nuclear receptor conformational ensemble between transcriptionally active and repressive states

Nuclear receptors (NRs) are thought to dynamically alternate between transcriptionally active and repressive conformations, which are stabilized upon ligand binding. Most NR ligand series exhibit limited bias, primarily consisting of transcriptionally active agonists or neutral antagonists, but not repressive inverse agonists-a limitation that restricts understanding of the functional NR conformational ensemble. Here, we report a NR ligand series for peroxisome proliferator-activated receptor gamma (PPARγ) that spans a pharmacological spectrum from repression (inverse agonism) to activation (agonism) where subtle structural modifications switch compound activity. While crystal structures provide snapshots of the fully repressive state, NMR spectroscopy and conformation-activity relationship analysis reveals that compounds within the series shift the PPARγ conformational ensemble between transcriptionally active and repressive conformations that are populated in the apo/ligand-free ensemble. Our findings reveal a molecular framework for minimal chemical modifications that enhance PPARγ inverse agonism and elucidate their influence on the dynamic PPARγ conformational ensemble.

Raloxifene-driven benzothiophene derivatives: Discovery, structural refinement, and biological evaluation as potent PPARγ modulators based on drug repurposing

By virtue of the drug repurposing strategy, the anti-osteoporosis drug raloxifene was identified as a novel PPARγ ligand through structure-based virtual high throughput screening (SB-VHTS) of FDA-approved drugs and TR-FRET competitive binding assay. Subsequent structural refinement of raloxifene led to the synthesis of a benzothiophene derivative, YGL-12. This compound exhibited potent PPARγ modulation with partial agonism, uniquely promoting adiponectin expression and inhibiting PPARγ Ser273 phosphorylation by CDK5 without inducing the expression of adipongenesis associated genes, including PPARγ, aP2, CD36, FASN and C/EBPα. This specific activity profile resulted in effective hypoglycemic properties, avoiding major TZD-related adverse effects like weight gain and hepatomegaly, which were demonstrated in db/db mice. Molecular docking studies showed that YGL-12 established additional hydrogen bonds with Ile281 and enhanced hydrogen-bond interaction with Ser289 as well as PPARγ Ser273 phosphorylation-related residues Ser342 and Glu343. These findings suggested YGL-12 as a promising T2DM therapeutic candidate, thereby providing a molecular framework for the development of novel PPARγ modulators with an enhanced therapeutic index.

PPARγ Modulators in Lung Cancer: Molecular Mechanisms, Clinical Prospects, and Challenges

Lung cancer is one of the most lethal malignancies worldwide. Peroxisome proliferator-activated receptor gamma (PPARγ, NR1C3) is a ligand-activated transcriptional factor that governs the expression of genes involved in glucolipid metabolism, energy homeostasis, cell differentiation, and inflammation. Multiple studies have demonstrated that PPARγ activation exerts anti-tumor effects in lung cancer through regulation of lipid metabolism, induction of apoptosis, and cell cycle arrest, as well as inhibition of invasion and migration. Interestingly, PPARγ activation may have pro-tumor effects on cells of the tumor microenvironment, especially myeloid cells. Recent clinical data has substantiated the potential of PPARγ agonists as therapeutic agents for lung cancer. Additionally, PPARγ agonists also show synergistic effects with traditional chemotherapy and radiotherapy. However, the clinical application of PPARγ agonists remains limited due to the presence of adverse side effects. Thus, further research and clinical trials are necessary to comprehensively explore the actions of PPARγ in both tumor and stromal cells and to evaluate the in vivo toxicity. This review aims to consolidate the molecular mechanism of PPARγ modulators and to discuss their clinical prospects and challenges in tackling lung cancer.

Mechanisms of Nelumbinis folium targeting PPARγ for weight management: A molecular docking and molecular dynamics simulations study

The lotus leaf, Nelumbinis folium (NF), has frequently appeared in obesity clinical trials as an intervention to promote weight loss and improve metabolic profiles. However, the molecular mechanisms by which it interacts with important obesity targets and pathways, such as the peroxisome proliferator-activated receptor gamma (PPARγ) within the PPAR signalling pathway, were not well understood. This study aims to screen for candidate compounds from NF with desirable pharmacokinetic properties and examine their binding feasibility at the PPARγ ligand-binding domain (LBD). Ligand- and structure-based screening of NF compounds were performed, and a consensus approach has been applied to identify druggable candidates. By examining the pharmacokinetic profiles, a large proportion of NF compounds exhibited favourable drug-likeness and oral bioavailability properties. Furthermore, the binding affinity scores and poses provided new insights on the distinctive binding behaviours of NF compounds at the LBD of PPARγ in its inactive form. Several NF compounds could bind strongly to PPARγ at sub-pockets where partial agonists and antagonists were found to bind and may induce conformational changes that influence co-repressor binding, trans-repression, and gene expression inhibition. Subsequent molecular dynamics simulations of a candidate compound (NF129 narcissin) bound to PPARγ revealed conformational stability, residue fluctuation, and binding behaviours comparable to that of the known inhibitor, SR1664. Therefore, it can be proposed that narcissin exhibits characteristics of a PPARγ antagonist. Further experimental validation to support the development of NF129 as a future anti-obesity agent is warranted.

Melatonin and Its Metabolites Can Serve as Agonists on the Aryl Hydrocarbon Receptor and Peroxisome Proliferator-Activated Receptor Gamma

Melatonin is widely present in Nature. It has pleiotropic activities, in part mediated by interactions with high-affinity G-protein-coupled melatonin type 1 and 2 (MT1 and MT2) receptors or under extreme conditions, e.g., ischemia/reperfusion. In pharmacological concentrations, it is given to counteract the massive damage caused by MT1- and MT2-independent mechanisms. The aryl hydrocarbon receptor (AhR) is a perfect candidate for mediating the latter effects because melatonin has structural similarity to its natural ligands, including tryptophan metabolites and indolic compounds. Using a cell-based Human AhR Reporter Assay System, we demonstrated that melatonin and its indolic and kynuric metabolites act as agonists on the AhR with EC50's between 10-4 and 10-6 M. This was further validated via the stimulation of the transcriptional activation of the CYP1A1 promoter. Furthermore, melatonin and its metabolites stimulated AhR translocation from the cytoplasm to the nucleus in human keratinocytes, as demonstrated by ImageStream II cytometry and Western blot (WB) analyses of cytoplasmic and nuclear fractions of human keratinocytes. These functional analyses are supported by in silico analyses. We also investigated the peroxisome proliferator-activated receptor (PPAR)γ as a potential target for melatonin and metabolites bioregulation. The binding studies using a TR-TFRET kit to assay the interaction of the ligand with the ligand-binding domain (LBD) of the PPARγ showed agonistic activities of melatonin, 6-hydroxymelatonin and N-acetyl-N-formyl-5-methoxykynuramine with EC50's in the 10-4 M range showing significantly lower affinities that those of rosiglitazone, e.g., a 10-8 M range. These interactions were substantiated by stimulation of the luciferase activity of the construct containing PPARE by melatonin and its metabolites at 10-4 M. As confirmed by the functional assays, binding mode predictions using a homology model of the AhR and a crystal structure of the PPARγ suggest that melatonin and its metabolites, including 6-hydroxymelatonin, 5-methoxytryptamine and N-acetyl-N-formyl-5-methoxykynuramine, are excellent candidates to act on the AhR and PPARγ with docking scores comparable to their corresponding natural ligands. Melatonin and its metabolites were modeled into the same ligand-binding pockets (LBDs) as their natural ligands. Thus, functional assays supported by molecular modeling have shown that melatonin and its indolic and kynuric metabolites can act as agonists on the AhR and they can interact with the PPARγ at high concentrations. This provides a mechanistic explanation for previously reported cytoprotective actions of melatonin and its metabolites that require high local concentrations of the ligands to reduce cellular damage under elevated oxidative stress conditions. It also identifies these compounds as therapeutic agents to be used at pharmacological doses in the prevention or therapy of skin diseases.

Peroxisome Proliferator-Activated Receptor α in Lipoprotein Metabolism and Atherosclerotic Cardiovascular Disease

Peroxisome proliferator-activated receptors (PPARs) are a group of ligand-binding transcription factors with pivotal action in regulating pleiotropic signaling pathways of energetic metabolism, immune responses and cell proliferation and differentiation. A significant body of evidence indicates that the PPARα receptor is an important modulator of plasma lipid and lipoprotein metabolism, with pluripotent effects influencing the lipid and apolipoprotein cargo of both atherogenic and antiatherogenic lipoproteins and their functionality. Clinical evidence supports an important role of PPARα agonists (fibric acid derivatives) in the treatment of hypertriglyceridemia and/or low high-density lipoprotein (HDL) cholesterol levels, although the effects of clinical trials are contradictory and point to a reduction in the risk of nonfatal and fatal myocardial infarction events. In this manuscript, we provide an up-to-date critical review of the existing relevant literature.

Design, Synthesis, and Biological Evaluation of Novel Quercetin Derivatives as PPAR-γ Partial Agonists by Modulating Epithelial-Mesenchymal Transition in Lung Cancer Metastasis

Epithelial-to-mesenchymal transition (EMT) is responsible for driving metastasis of multiple cancer types including lung cancer. Peroxisome proliferator-activated receptor (PPAR)-γ, a ligand-activated transcription factor, controls expression of variety of genes involved in EMT. Although several synthetic compounds act as potent full agonists for PPAR-γ, their long term application is restricted due to serious adverse effects. Therefore, partial agonists involving reduced and balanced PPAR-γ activity are more effective and valued. A previous study discerned the efficacy of quercetin and its derivatives to attain favorable stabilization with PPAR-γ. Here this work is extended by synthesizing five novel quercetin derivatives (QDs) namely thiosemicarbazone (QUETSC)) and hydrazones (quercetin isonicotinic acid hydrazone (QUEINH), quercetin nicotinic acid hydrazone (QUENH), quercetin 2-furoic hydrazone (QUE2FH), and quercetin salicyl hydrazone (QUESH)) and their effects are analyzed in modulating EMT in lung cancer cell lines via PPAR-γ partial activation. QDs-treated A549 cells diminish cell proliferation strongly at nanomolar concentration compared to NCI-H460 cells. Of the five screened derivatives, QUETSC, QUE2FH, and QUESH exhibit the property of partial activation as compared to the overexpressive level of rosiglitazone. Consistently, these QDs also suppress EMT process by markedly downregulating the levels of mesenchymal markers (Snail, Slug, and zinc finger E-box binding homeobox 1) and concomitant upregulation of epithelial marker (E-cadherin).

Targeting the Alternative Vitamin E Metabolite Binding Site Enables Noncanonical PPARγ Modulation

The lipid-sensing transcription factor PPARγ is the target of antidiabetic thiazolidinediones (TZD). At two sites within its ligand binding domain, it also binds oxidized vitamin E metabolites and the vitamin E mimetic garcinoic acid. While the canonical interaction within the TZD binding site mediates classical PPARγ activation, the effects of the second binding on PPARγ activity remain elusive. Here, we identified an agonist mimicking dual binding of vitamin E metabolites and developed a selective ligand of the second site, unveiling potential noncanonical regulation of PPARγ activities. We found that this alternative binding event can simultaneously occur with orthosteric ligands and it exerted different effects on PPARγ-cofactor interactions compared to both orthosteric PPARγ agonists and antagonists, indicating the diverse roles of the two binding sites. Alternative site binding lacked the pro-adipogenic effect of TZD and mediated no classical PPAR signaling in differential gene expression analysis but markedly diminished FOXO signaling, suggesting potential therapeutic applications.

Selective PPAR-Delta/PPAR-Gamma Activation Improves Cognition in a Model of Alzheimer's Disease

Background: The continuously increasing association of Alzheimer's disease (AD) with increased mortality rates indicates an unmet medical need and the critical need for establishing novel molecular targets for therapeutic potential. Agonists for peroxisomal proliferator activating receptors (PPAR) are known to regulate energy in the body and have shown positive effects against Alzheimer's disease. There are three members of this class (delta, gamma, and alpha), with PPAR-gamma being the most studied, as these pharmaceutical agonists offer promise for AD because they reduce amyloid beta and tau pathologies, display anti-inflammatory properties, and improve cognition. However, they display poor brain bioavailability and are associated with several adverse side effects on human health, thus limiting their clinical application. Methods: We have developed a novel series of PPAR-delta and PPAR-gamma agonists in silico with AU9 as our lead compound that displays selective amino acid interactions focused upon avoiding the Tyr-473 epitope in the PPAR-gamma AF2 ligand binding domain. Results: This design helps to avoid the unwanted side effects of current PPAR-gamma agonists and improve behavioral deficits and synaptic plasticity while reducing amyloid-beta levels and inflammation in 3xTgAD animals. Conclusions: Our innovative in silico design of PPAR-delta/gamma agonists may offer new perspectives for this class of agonists for AD.

A New Antidiabetic Agent Showing Short- and Long-Term Effects Due to Peroxisome Proliferator-Activated Receptor Alpha/Gamma Dual Agonism and Mitochondrial Pyruvate Carrier Inhibition

A new series of analogues or derivatives of the previously reported PPARα/γ dual agonist LT175 allowed the identification of ligand 10, which was able to potently activate both PPARα and -γ subtypes as full and partial agonists, respectively. Docking studies were performed to provide a molecular explanation for this different behavior on the two different targets. In vivo experiments showed that this compound induced a significant reduction in blood glucose and lipid levels in an STZ-induced diabetic mouse model displaying no toxic effects on bone, kidney, and liver. By examining in depth the antihyperglycemic activity of 10, we found out that it produced a slight but significant inhibition of the mitochondrial pyruvate carrier, acting also through insulin-independent mechanisms. This is the first example of a PPARα/γ dual agonist reported to show this inhibitory effect representing, therefore, the potential lead of a new class of drugs for treatment of dyslipidemic type 2 diabetes.

Structural Biology Inspired Development of a Series of Human Peroxisome Proliferator-Activated Receptor Gamma (PPARγ) Ligands: From Agonist to Antagonist

Recent progress in the structural and molecular pharmacological understanding of the nuclear receptor, peroxisome proliferator-activated receptor gamma (hPPARγ)-a transcription factor with pleiotropic effects on biological responses-has enabled the investigation of various graded hPPARγ ligands (full agonist, partial agonist, and antagonist). Such ligands are useful tools to investigate the functions of hPPARγ in detail and are also candidate drugs for the treatment of hPPARγ-mediated diseases, such as metabolic syndrome and cancer. This review summarizes our medicinal chemistry research on the design, synthesis, and pharmacological evaluation of a covalent-binding and non-covalent-binding hPPARγ antagonist, both of which have been created based on our working hypothesis of the helix 12 (H12) holding induction/inhibition concept. X-ray crystallographic analyses of our representative antagonists complexed with an hPPARγ ligand binding domain (LBD) indicated the unique binding modes of hPPARγ LBD, which are quite different from the binding modes observed for hPPARγ agonists and partial agonists.

Structures of human TR4LBD-JAZF1 and TR4DBD-DNA complexes reveal the molecular basis of transcriptional regulation

Testicular nuclear receptor 4 (TR4) modulates the transcriptional activation of genes and plays important roles in many diseases. The regulation of TR4 on target genes involves direct interactions with DNA molecules via the DNA-binding domain (DBD) and recruitment of coregulators by the ligand-binding domain (LBD). However, their regulatory mechanisms are unclear. Here, we report high-resolution crystal structures of TR4DBD, TR4DBD-DNA complexes and the TR4LBD-JAZF1 complex. For DNA recognition, multiple factors come into play, and a specific mutual selectivity between TR4 and target genes is found. The coactivators SRC-1 and CREBBP can bind at the interface of TR4 originally occupied by the TR4 activation function region 2 (AF-2); however, JAZF1 suppresses the binding through a novel mechanism. JAZF1 binds to an unidentified surface of TR4 and stabilizes an α13 helix never reported in the nuclear receptor family. Moreover, the cancer-associated mutations affect the interactions and the transcriptional activation of TR4 in vitro and in vivo, respectively. Overall, our results highlight the crucial role of DNA recognition and a novel mechanism of how JAZF1 reinforces the autorepressed conformation and influences the transcriptional activation of TR4, laying out important structural bases for drug design for a variety of diseases, including diabetes and cancers.

Molecular Modeling of Allosteric Site of Isoform-Specific Inhibition of the Peroxisome Proliferator-Activated Receptor PPARγ

The peroxisome proliferator-activated receptor gamma (PPARγ) is a nuclear receptor and controls a number of gene expressions. The ligand binding domain (LBD) of PPARγ is large and involves two binding sites: orthosteric and allosteric binding sites. Increased evidence has shown that PPARγ is an oncogene and thus the PPARγ antagonists have potential as anticancer agents. In this paper, we use Glide Dock approach to determine which binding site, orthosteric or allosteric, would be a preferred pocket for PPARγ antagonist binding, though antidiabetic drugs such as thiazolidinediones (TZDs) bind to the orthosteric site. The Glide Dock results show that the binding of PPARγ antagonists at the allosteric site yielded results that were much closer to the experimental data than at the orthosteric site. The PPARγ antagonists seem to selectively bind to residues Lys265, Ser342 and Arg288 at the allosteric binding site, whereas PPARγ agonists would selectively bind to residues Leu228, Phe363, and His449, though Phe282 and Lys367 may also play a role for agonist binding at the orthosteric binding pocket. This finding will provide new perspectives in the design and optimization of selective and potent PPARγ antagonists or agonists.

Determination of Molecule Category of Ligands Targeting the Ligand-Binding Pocket of Nuclear Receptors with Structural Elucidation and Machine Learning

The mechanism of transcriptional activation/repression of the nuclear receptors (NRs) involves two main conformations of the NR protein, namely, the active (agonistic) and inactive (antagonistic) conformations. Binding of agonists or antagonists to the ligand-binding pocket (LBP) of NRs can regulate the downstream signaling pathways with different physiological effects. However, it is still hard to determine the molecular type of a LBP-bound ligand because both the agonists and antagonists bind to the same position of the protein. Therefore, it is necessary to develop precise and efficient methods to facilitate the discrimination of agonists and antagonists targeting the LBP of NRs. Here, combining structural and energetic analyses with machine-learning (ML) algorithms, we constructed a series of structure-based ML models to determine the molecular category of the LBP-bound ligands. We show that the proposed models work robustly and with high accuracy (ACC > 0.9) for determining the category of molecules derived from docking-based and crystallized poses. Furthermore, the models are also capable of determining the molecular category of ligands with dual opposite functions on different NRs (i.e., working as an agonist in one NR target, whereas functioning as an antagonist in another) with reasonable accuracy. The proposed method is expected to facilitate the determination of the molecular properties of ligands targeting the LBP of NRs with structural interpretation.

Di-(2-ethylhexyl) phthalate substitutes accelerate human adipogenesis through PPARγ activation and cause oxidative stress and impaired metabolic homeostasis in mature adipocytes

The obesity pandemic is presumed to be accelerated by endocrine disruptors such as phthalate-plasticizers, which interfere with adipose tissue function. With the restriction of the plasticizer di-(2-ethylhexyl)-phthalate (DEHP), the search for safe substitutes gained importance. Focusing on the master regulator of adipogenesis and adipose tissue functionality, the peroxisome proliferator-activated receptor gamma (PPARγ), we evaluated 20 alternative plasticizers as well as their metabolites for binding to and activation of PPARγ and assessed effects on adipocyte lipid accumulation. Among several compounds that showed interaction with PPARγ, the metabolites MINCH, MHINP, and OH-MPHP of the plasticizers DINCH, DINP, and DPHP exerted the highest adipogenic potential in human adipocytes. These metabolites and their parent plasticizers were further analyzed in human preadipocytes and mature adipocytes using cellular assays and global proteomics. In preadipocytes, the plasticizer metabolites significantly increased lipid accumulation, enhanced leptin and adipsin secretion, and upregulated adipogenesis-associated markers and pathways, in a similar pattern to the PPARγ agonist rosiglitazone. Proteomics of mature adipocytes revealed that both, the plasticizers and their metabolites, induced oxidative stress, disturbed lipid storage, impaired metabolic homeostasis, and led to proinflammatory and insulin resistance promoting adipokine secretion. In conclusion, the plasticizer metabolites enhanced preadipocyte differentiation, at least partly mediated by PPARγ activation and, together with their parent plasticizers, affected the functionality of mature adipocytes similar to reported effects of a high-fat diet. This highlights the need to further investigate the currently used plasticizer alternatives for potential associations with obesity and the metabolic syndrome.

Alternatives for the worse: Molecular insights into adverse effects of bisphenol a and substitutes during human adipocyte differentiation

Bisphenol A (BPA), which is used in a variety of consumer-related plastic products, was reported to cause adverse effects, including disruption of adipocyte differentiation, interference with obesity mechanisms, and impairment of insulin- and glucose homeostasis. Substitute compounds are increasingly emerging but are not sufficiently investigated.We aimed to investigate the mode of action of BPA and four of its substitutes during the differentiation of human preadipocytes to adipocytes and their molecular interaction with peroxisome proliferator-activated receptor γ (PPARγ), a pivotal regulator of adipogenesis.Binding and effective biological activation of PPARγ were investigated by surface plasmon resonance and reporter gene assay, respectively. Human preadipocytes were continuously exposed to BPA, BPS, BPB, BPF, BPAF, and the PPARγ-antagonist GW9662. After 12 days of differentiation, lipid production was quantified via Oil Red O staining, and global protein profiles were assessed using LC-MS/MS-based proteomics. All tested bisphenols bound to human PPARγ with similar efficacy as the natural ligand 15d-PGJ2in vitroand provoked an antagonistic effect on PPARγ in the reporter gene assay at non-cytotoxic concentrations. During the differentiation of human preadipocytes, all bisphenols decreased lipid production. Global proteomics displayed a down-regulation of adipogenesis and metabolic pathways, similar to GW9662. Interestingly, pro-inflammatory pathways were up-regulated, MCP1 release was increased, and adiponectin decreased. pAKT/AKT ratios revealed significantly reduced insulin sensitivity by BPA, BPB, and BPS upon insulin stimulation.Thus, our results show that not only BPA but also its substitutes disrupt crucial metabolic functions and insulin signaling in adipocytes under low, environmentally relevant concentrations. This effect, mediated through inhibition of PPARγ, may promote hypertrophy of adipose tissue and increase the risk of developing metabolic syndrome, including insulin resistance.

Two steps, one ligand: How PPARγ binds small-molecule agonists

In this issue of Structure,Shang and Kojetin (2021) present insights into the binding mechanism of artificial agonists to the PPARγ nuclear receptor. These data support a two-step model with induced fit and conformational selection aspects. This mechanism may exist in related receptors, providing new opportunities for drug development.