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

Crocin Improves Insulin Sensitivity and Ameliorates Adiposity by Regulating AMPK-CDK5-PPARγ Signaling

Crocin is a carotenoid compound which possesses multiple biological activities. Our and other laboratory's previous findings show that crocin alleviates obesity and type 2 diabetes-related complications. We have found that crocin activates AMP-activated protein kinase (AMPK) signaling and inhibition of AMPK suppresses crocin-induced protective effects. However, the causal role of AMPK activation in the biological role of crocin is still not verified. In the present study, we showed that crocin markedly inhibits the changes of glucose metabolic parameters and serum lipid profiles in wild type diabetic mice. In AMPKα KO diabetic mice, those protective effects of crocin against glucose and lipid metabolic dysfunction were abolished. These results demonstrated AMPK activation was responsible for the beneficial effects of crocin on metabolic dysfunction. Moreover, we have shown that the antiobese effect of crocin has been abolished by the deficiency of AMPKα. We also showed that crocin induced a significant decrease of CDK5 protein level in wild type diabetic mice, while this effect was abolished in AMPKα KO diabetic mice. The regulation of downstream targets of CDK5/PPARγ by crocin was abolished by the deficiency of AMPK. In conclusion, our study verified that activation of AMPK is involved in crocin-induced protective effects against glucose and lipid metabolic dysfunction. Activation of AMPK downregulates the protein level of CDK5, followed by the decrease of PPARγ phosphorylation, leading to the inhibition of adipose formation and metabolic dysfunction. Our study provides new insights into the mechanism of protective effects of crocin and interaction of AMPK and CDK5/PPARγ signaling.

Positive Reinforcing Mechanisms between GPR120 and PPARγ Modulate Insulin Sensitivity

G protein-coupled receptor 120 (GPR120) and PPARγ agonists each have insulin sensitizing effects. But whether these two pathways functionally interact and can be leveraged together to markedly improve insulin resistance has not been explored. Here, we show that treatment with the PPARγ agonist rosiglitazone (Rosi) plus the GPR120 agonist Compound A leads to additive effects to improve glucose tolerance and insulin sensitivity, but at lower doses of Rosi, thus avoiding its known side effects. Mechanistically, we show that GPR120 is a PPARγ target gene in adipocytes, while GPR120 augments PPARγ activity by inducing the endogenous ligand 15d-PGJ2 and by blocking ERK-mediated inhibition of PPARγ. Further, we used macrophage- (MKO) or adipocyte-specific GPR120 KO (AKO) mice to show that GRP120 has anti-inflammatory effects via macrophages while working with PPARγ in adipocytes to increase insulin sensitivity. These results raise the prospect of a safer way to increase insulin sensitization in the clinic.

Identifying adipogenic chemicals: Disparate effects in 3T3-L1, OP9 and primary mesenchymal multipotent cell models

3T3-L1 pre-adipocytes are used commonly to identify new adipogens, but this cell line has been shown to produce variable results. Here, potential adipogenic chemicals (identified in the ToxCast dataset using the Toxicological Priority Index) were tested for their ability to induce adipocyte differentiation in 3T3-L1 cells, OP9 cells and primary mouse bone marrow multipotent stromal cells (BM-MSC). Ten of the 36 potential adipogens stimulated lipid accumulation in at least one model (novel: fenthion, quinoxyfen, prallethrin, allethrin, pyrimethanil, tebuconzaole, 2,4,6-tris (tert-butyl)phenol; known: fentin, pioglitazone, 3,3',5,5'-tetrabromobisphenol A). Only prallethrin and pioglitazone enhanced lipid accumulation in all models. OP9 cells were significantly more sensitive to chemicals known to activate PPARγ through RXR than the other models. Coordinate effects on adipocyte and osteoblast differentiation were investigated further in BM-MSCs. Lipid accumulation was correlated with the ability to stimulate expression of the PPARγ target gene, Plin1. Induction of lipid accumulation also was associated with reduction in alkaline phosphatase activity. Allethrin, prallethrin, and quinoxyfen strongly suppressed osteogenic gene expression. BM-MSCs were useful in coordinately investigating pro-adipogenic and anti-osteogenic effects. Overall, the results show that additional models should be used in conjunction with 3T3-L1 cells to identify a broader spectrum of adipogens and their coordinate effects on osteogenesis.

Correlation of serum vitamin D, adipose tissue vitamin D receptor, and peroxisome proliferator-activated receptor γ in women with gestational diabetes mellitus

Background:

Gestational diabetes mellitus (GDM) is a common complication during pregnancy. Obesity and overweight are closely related to metabolic diseases and diabetes. However, the role of adipose tissue in the pathogenesis of GDM remains to be studied. The aim of this study was to investigate the correlation of vitamin D (VD) levels, VD receptor (VDR), and peroxisome proliferator-activated receptor γ (PPARγ) expression with GDM in overweight or obese women.

Methods:

One hundred and forty pregnant women with full-term single-birth cesarean-section were selected as the study subjects and grouped (70 GDM women, including 35 non-overweight/non-obese women [group G1] and 35 women with overweight or obesity [group G2]; 70 pregnant women with normal glucose tolerance, including 35 non-overweight/non-obese women [group N1] and 35 overweight/obese women [group N2]). The levels of serum VD, blood biochemistry, and adiponectin were compared in these women. Subcutaneous adipose tissue was isolated from the abdominal wall incision. VDR and PPARγ messenger RNA (mRNA) transcript levels in these adipose tissues were quantified by real-time polymerase chain reaction. The differences between the levels of PPARγ protein and phosphorylated PPARγ Ser273 were detected by Western blotting.

Results:

The serum VD level of GDM women was lower in comparison to that of women with normal glucose tolerance (G1 vs. N1: 20.62 ± 7.87 ng/mL vs. 25.85 ± 7.29 ng/mL, G2 vs. N2: 17.06 ± 6.74 ng/mL vs. 21.62 ± 7.18 ng/mL, P < 0.05), and the lowest in overweight/obese GDM women. VDR and PPARγ mRNA expression was higher in the adipose tissues of GDM women in comparison to that of women with normal glucose tolerance (VDR mRNA: G1 vs. N1: 210.00 [90.58–311.46] vs. 89.34 [63.74–159.92], G2 vs. N2: 298.67 [170.84–451.25] vs. 198.28 [119.46–261.23], PPARγ mRNA: G1 vs. N1: 100.72 [88.61–123.87] vs. 87.52 [66.37–100.04], G2 vs. N2: 117.33 [100.08–149.00] vs. 89.90 [76.95–109.09], P < 0.05), and their expression was the highest in GDM + overweight/obese women. VDR mRNA levels positively correlated with the pre-pregnancy body mass index (BMI), pre-delivery BMI, fasting blood glucose (FBG), homeostasis model assessment of insulin resistance (HOMA-IR), and PPARγ mRNA while it negatively correlated with the VD and the adiponectin levels (r = 0.395, 0.336, 0.240, 0.190, 0.235, –0.350, –0.294, respectively, P < 0.05). The degree of PPARγ Ser273 phosphorylation increased in obese and GDM pregnant women. PPARγ mRNA levels positively correlated with pre-pregnancy BMI, pre-delivery BMI, FBG, HOMA-IR, serum total cholesterol, triglyceride, free fatty acid, and VDR mRNA, while it negatively correlated with the VD and adiponectin levels (r = 0.276, 0.199, 0.210, 0.230, 0.182, 0.214, 0.270, 0.235, –0.232, –0.199, respectively, P < 0.05).

Conclusions:

Both GDM and overweight/obese women had decreased serum VD levels and up-regulated VDR and PPARγ mRNA expression in adipose tissue, which was further higher in the overweight or obese women with GDM. VD may regulate the formation and differentiation of adipocytes through the VDR and PPARγ pathways and participate in the occurrence of GDM.

Cyclin-Dependent Kinase 5 Inhibitor Butyrolactone I Elicits a Partial Agonist Activity of Peroxisome Proliferator-Activated Receptor γ

Adiponectin is an adipocyte-derived cytokine having an insulin-sensitizing activity. During the phenotypic screening of secondary metabolites derived from the marine fungus Aspergillus terreus, a poly cyclin-dependent kinase (CDK) inhibitor butyrolactone I affecting CDK1 and CDK5 was discovered as a potent adiponectin production-enhancing compound in the adipogenesis model of human bone marrow-derived mesenchymal stem cells (hBM-MSCs). CDK5 inhibitors exhibit insulin-sensitizing activities by suppressing the phosphorylation of peroxisome proliferator-activated receptor γ (PPARγ). However, the adiponectin production-enhancing activities of butyrolactone I have not been correlated with the potency of CDK5 inhibitor activities. In a target identification study, butyrolactone I was found to directly bind to PPARγ. In the crystal structure of the human PPARγ, the ligand-binding domain (LBD) in complex with butyrolactone I interacted with the amino acid residues located in the hydrophobic binding pockets of the PPARγ LBD, which is a typical binding mode of the PPARγ partial agonists. Therefore, the adiponectin production-enhancing effect of butyrolactone I was mediated by its polypharmacological dual modulator activities as both a CDK5 inhibitor and a PPARγ partial agonist.

PPM1A Controls Diabetic Gene Programming through Directly Dephosphorylating PPARγ at Ser273

Peroxisome proliferator-activated receptor γ (PPARγ) is a master regulator of adipose tissue biology. In obesity, phosphorylation of PPARγ at Ser273 (pSer273) by cyclin-dependent kinase 5 (CDK5)/extracellular signal-regulated kinase (ERK) orchestrates diabetic gene reprogramming via dysregulation of specific gene expression. Although many recent studies have focused on the development of non-classical agonist drugs that inhibit the phosphorylation of PPARγ at Ser273, the molecular mechanism of PPARγ dephosphorylation at Ser273 is not well characterized. Here, we report that protein phosphatase Mg²⁺/Mn²⁺-dependent 1A (PPM1A) is a novel PPARγ phosphatase that directly dephosphorylates Ser273 and restores diabetic gene expression which is dysregulated by pSer273. The expression of PPM1A significantly decreases in two models of insulin resistance: diet-induced obese (DIO) mice and db/db mice, in which it negatively correlates with pSer273. Transcriptomic analysis using microarray and genotype-tissue expression (GTEx) data in humans shows positive correlations between PPM1A and most of the genes that are dysregulated by pSer273. These findings suggest that PPM1A dephosphorylates PPARγ at Ser273 and represents a potential target for the treatment of obesity-linked metabolic disorders.

Identification and structural insight of an effective PPARγ modulator with improved therapeutic index for anti-diabetic drug discovery

Peroxisome proliferator-activated receptor γ (PPARγ) is a key regulator of glucose homeostasis and lipid metabolism, and an important target for the development of modern anti-diabetic drugs. However, current PPARγ-targeting anti-diabetic drugs such as classical thiazolidinediones (TZDs) are associated with undesirable side effects. To address this concern, we here describe the structure-based design, synthesis, identification and detailed in vitro and in vivo characterization of a novel, decanoic acid (DA)-based and selective PPARγ modulator (SPPARγM), VSP-77, especially (S)-VSP-77, as the potential "hit" for the development of improved and safer anti-diabetic therapeutics. We have also determined the co-crystal structure of the PPARγ ligand-binding domain (LBD) in complex with two molecules of (S)-VSP-77, which reveal a previously undisclosed allosteric binding mode. Overall, these findings not only demonstrate the therapeutic advantage of (S)-VSP-77 over current TZD drugs and representative partial agonist INT131, but also provide a rational basis for the development of future SPPARγMs as safe and highly efficacious anti-diabetic drugs.

Unwinding Complexities of Diabetic Alzheimer by Potent Novel Molecules

Diabetes mellitus is one of the aggressive disorders in global society. No pharmacotherapy is available for permanent diabetes cure, although management is possible with drugs and physical activities. One of the recent complications noticed in type 2 diabetes mellitus includes diabetes-induced Alzheimer. It has been proposed that the possible diabetes-induced Alzheimer could be of type 3 diabetes. A variety of cross-sectional studies have proved that type 2 diabetes mellitus is one of the factors responsible for the pathophysiology of Alzheimer. New drug molecules developed by pharmaceutical companies with adequate neuroprotective effect have demonstrated their efficacy in treatment of Alzheimer in various preclinical diabetic studies. Patients of type 2 diabetes mellitus may show the benefit with existing drugs but may not cause complete cure. Extensive studies are being carried out to find new drug molecules that show their potential as antidiabetic drug and could treat type 2 diabetes-induced Alzheimer as well. This review provides an overview about the recent advancement in pharmacotherapy of diabetes-induced Alzheimer. The pathomechanistic links between diabetes and Alzheimer as well as neurochemical changes in diabetes-induced Alzheimer are also briefed.

AMPK leads to phosphorylation of the transcription factor Nrf2, tuning transactivation of selected target genes

The transcription factor Nrf2 (nuclear factor (erythroid-derived 2)-like 2) and the kinase AMPK (AMP-activated protein kinase) participate in the cellular adaptive response to redox or energy stress. Despite accumulating evidence for positive cooperativity between both proteins, information about direct post-translational modification of Nrf2 by AMPK in living cells is scarce. Here, MS-based analysis of immunoprecipitated Nrf2 revealed serine 374, 408 and 433 in human Nrf2 to be hyperphosphorylated as a function of activated AMPK. A direct phosphate-transfer by AMPK to those sites was indicated by in vitro kinase assays with recombinant proteins as well as interaction of AMPK and Nrf2 in cells, evident by co-immunoprecipitation. Mutation of serine 374, 408 and 433 to alanine did not markedly affect half-life, nuclear accumulation or induction of reporter gene expression upon Nrf2 activation with sulforaphane. However, some selected endogenous Nrf2 target genes responded with decreased induction when the identified phosphosites were mutated, whereas others remained unaffected. Notably, the genes susceptible to the mutation of the phosphorylation sites in Nrf2 consistently showed reduced induction in AMPKα1 -/-cells. Overall, our data reveal AMPK-triggered phosphorylation of Nrf2 at three serine residues, apparently determining the extent of transactivation of selected target genes.

(-)-O-Methylcubebin from Vitex trifolia Enhanced Adipogenesis in 3T3-L1 Cells via the Inhibition of ERK1/2 and p38MAPK Phosphorylation

In this study, for the purpose of elucidation for antidiabetic components, we isolated and identified compounds that could become lead compounds for the development of antidiabetic agents from the herbal medicine Vitex trifolia, which is used for liver protection in Myanmar. Three kinds of lignan, (-)-O-methylcubebin (MC), (-)-hinokinin, and (-)-cubebin, were isolated from the ethyl acetate extract of the leaves of V. trifolia, using various chromatography. Among the three isolated compounds, MC showed the strongest effects to increase intracellular lipid accumulation in 3T3-L1 cells. From the results of the elucidation of the MC’s effects on the adipogenesis of 3T3-L1 cells, the downsizing of adipocytes and the promotion of the expression of adipogenesis-related proteins, as well as adiponectin, were observed. On the other hand, since the activity of MC was inhibited by antagonists of PPARγ and improved by inhibitors of the classical mitogen-activated protein kinase (MAPK) pathway and p38MAPK pathway, MC was considered to be an agonist of PPARγ, and furthermore promoted adipogenesis via the inhibition of extracellular signal-regulated kinase 1/2 (ERK1/2) and p38MAPK phosphorylation. Although MC showed similar effects to those of rosiglitazone (RO) used as a positive control, RO promoted the migration of GLUT4 from the cytoplasm to the cell membrane, whereas MC did not show such an effect. From the abovementioned results, it was considered that MC could be a lead compound for the development of antidiabetic drugs that does not show weight gain, which is a side effect of RO.

Multitarget and promising role of dihydromyricetin in the treatment of metabolic diseases

Dihydromyricetin (DMY or DHM), also known as ampelopsin, is the main natural flavonol compound extracted from the plant Ampelopsis grossedentata (Hand. -Mazz) W.T. Wang. In recent years, accumulating studies have been conducted to explore the extensive biological functions of DMY, including antitumor, anti-inflammation, organ-protective, and metabolic regulation effects. DMY acts as a potential preventive or therapeutic agent in treating multiple diseases, such as diabetes mellitus, atherosclerosis, nonalcoholic fatty liver disease and osteoporosis. This review article summarizes the preventive and therapeutic potential of DMY in multiple metabolic diseases and the main signaling pathways in which DMY participates to offer a comprehensive understanding and guidance for future studies.

AMPK leads to phosphorylation of the transcription factor Nrf2, tuning transactivation of selected target genes

The transcription factor Nrf2 (nuclear factor (erythroid-derived 2)-like 2) and the kinase AMPK (AMP-activated protein kinase) participate in the cellular adaptive response to redox or energy stress. Despite accumulating evidence for positive cooperativity between both proteins, information about direct post-translational modification of Nrf2 by AMPK in living cells is scarce. Here, MS-based analysis of immunoprecipitated Nrf2 revealed serine 374, 408 and 433 in human Nrf2 to be hyperphosphorylated as a function of activated AMPK. A direct phosphate-transfer by AMPK to those sites was indicated by in vitro kinase assays with recombinant proteins as well as interaction of AMPK and Nrf2 in cells, evident by co-immunoprecipitation. Mutation of serine 374, 408 and 433 to alanine did not markedly affect half-life, nuclear accumulation or induction of reporter gene expression upon Nrf2 activation with sulforaphane. However, some selected endogenous Nrf2 target genes responded with decreased induction when the identified phosphosites were mutated, whereas others remained unaffected. Notably, the genes susceptible to the mutation of the phosphorylation sites in Nrf2 consistently showed reduced induction in AMPKα1 -/-cells. Overall, our data reveal AMPK-triggered phosphorylation of Nrf2 at three serine residues, apparently determining the extent of transactivation of selected target genes.

Structural analysis identifies an escape route from the adverse lipogenic effects of liver X receptor ligands

Liver X receptors (LXRs) are attractive drug targets for cardiovascular disease treatment due to their role in regulating cholesterol homeostasis and immunity. The anti-atherogenic properties of LXRs have prompted development of synthetic ligands, but these cause major adverse effects-such as increased lipogenesis-which are challenging to dissect from their beneficial activities. Here we show that LXR compounds displaying diverse functional responses in animal models induce distinct receptor conformations. Combination of hydrogen/deuterium exchange mass spectrometry and multivariate analysis allowed identification of LXR regions differentially correlating with anti-atherogenic and lipogenic activities of ligands. We show that lipogenic compounds stabilize active states of LXRα and LXRβ while the anti-atherogenic expression of the cholesterol transporter ABCA1 is associated with the ligand-induced stabilization of LXRα helix 3. Our data indicates that avoiding ligand interaction with the activation helix 12 while engaging helix 3 may provide directions for development of ligands with improved therapeutic profiles.

Development of a novel class of peroxisome proliferator-activated receptor (PPAR) gamma ligands as an anticancer agent with a unique binding mode based on a non-thiazolidinedione scaffold

We previously identified dibenzooxepine derivative 1 as a potent PPARγ ligand with a unique binding mode owing to its non-thiazolidinedione scaffold. However, while 1 showed remarkably potent MKN-45 gastric cancer cell aggregation activity, an indicator of cancer differentiation-inducing activity induced by PPARγ activation, we recognized that 1 was metabolically unstable. In the present study, we identified a metabolically soft spot, and successfully discovered 3-fluoro dibenzooxepine derivative 9 with better metabolic stability. Further optimization provided imidazo[1,2-a]pyridine derivative 17, which showed potent MKN-45 gastric cancer cell aggregation activity and excellent PK profiles compared with 9. Compound 17 exerted a growth inhibitory effect on AsPC-1/AG1 pancreatic tumor in mice. Furthermore, the decrease in the hematocrit (an indicator of localized edema, a serious adverse effect of PPARγ ligands) was tolerable even with oral administration at 200 mg/kg in healthy mice.

A novel specific peroxisome proliferator-activated receptor γ (PPARγ) modulator YR4-42 ameliorates hyperglycaemia and dyslipidaemia and hepatic steatosis in diet-induced obese mice

AIMS

To evaluate a novel tetrahydroisoquinoline derivative YR4-42 as a selective peroxisome proliferator-activated receptor γ (PPARγ) modulator (SPPARM) and explore its anti-diabetic effects in vitro and in vivo.

MATERIALS AND METHODS

Using two standard full PPARγ agonists rosiglitazone and pioglitazone as controls, the PPARγ binding affinity and transactivation action of YR4-42 were evaluated using biochemical and cell-based reporter gene assays. The capacity of YR4-42 to recruit coactivators of PPARγ was also assessed. The effects of YR4-42 on adipogenesis and glucose consumption and PPARγ Ser273 phosphorylation were investigated in 3T3-L1 adipocytes. The effects of YR4-42 and pioglitazone, serving as positive control, on glucose and lipids metabolism were investigated in high-fat diet-induced obese (DIO) C57BL/6J mice. The expression of PPARγ target genes involved in glucose and lipid metabolism was also assessed in vitro and in vivo.

RESULTS

In vitro biochemical and cell-based functional assays showed that YR4-42 has much weaker binding affinity, transactivation, and recruitment to PPARγ of the coactivators thyroid hormone receptor-associated protein complex 220 kDa component (TRAP220) and PPARγ coactivator 1-α (PGC1α) compared to full agonists. In 3 T3-L1 adipocytes, YR4-42 significantly improved glucose consumption without a lipogenesis effect, while blocking tumour necrosis factor α-mediated phosphorylation of PPARγ at Ser273, thereby upregulating the expression of the PPARγ Ser273 phosphorylation-dependent genes. Furthermore, in DIO mice, oral administration of YR4-42 ameliorated the hyperglycaemia, with a similar insulin sensitization effect to that of pioglitazone. Importantly, YR4-42 also improved hyperlipidaemia-associated hepatic steatosis without weight gain, which avoids a major side effect of pioglitazone. Thus, YR4-42 appeared to selectively modulate PPARγ responses. This finding was supported by the gene expression analysis, which showed that YR4-42 selectively targets PPARγ-regulated genes mapped to glucose and lipid metabolism in DIO mice.

CONCLUSIONS

We conclude that YR4-42 is a novel anti-diabetic drug candidate with significant advantages compared to standard PPARγ agonists. YR4-42 should be further investigated in preclinical and clinical studies.

Redox Biology of Peroxisome Proliferator-Activated Receptor-γ in Pulmonary Hypertension

Significance: Peroxisome proliferator-activated receptor-gamma (PPARγ) maintains pulmonary vascular health through coordination of antioxidant defense systems, inflammation, and cellular metabolism. Insufficient PPARγ contributes to pulmonary hypertension (PH) pathogenesis, whereas therapeutic restoration of PPARγ activity attenuates PH in preclinical models. Recent Advances: Numerous studies in the past decade have elucidated the complex mechanisms by which PPARγ in the pulmonary vasculature and right ventricle (RV) protects against PH. The scope of PPARγ-interconnected pathways continues to expand and includes induction of antioxidant genes, transrepression of inflammatory signaling, regulation of mitochondrial biogenesis and bioenergetic integrity, control of cell cycle and proliferation, and regulation of vascular tone through interactions with nitric oxide and endogenous vasoactive molecules. Furthermore, PPARγ interacts with an extensive regulatory network of transcription factors and microRNAs leading to broad impact on cell signaling. Critical Issues: Abundant evidence suggests that targeting PPARγ exerts diverse salutary effects in PH and represents a novel and potentially translatable therapeutic strategy. However, progress has been slowed by an incomplete understanding of how specific PPARγ pathways are critically disrupted across PH disease subtypes and lack of optimal pharmacological ligands. Future Directions: Recent studies indicate that ligand-induced post-translational modifications of the PPARγ receptor differentially induce therapeutic benefits versus adverse side effects of PPARγ receptor activation. Strategies to selectively target PPARγ activity in diseased cells of pulmonary circulation and RV, coupled with development of ligands designed to specifically regulate post-translational PPARγ modifications, may unlock the full therapeutic potential of this versatile master transcriptional and metabolic regulator in PH.

Structural Basis for the Regulation of PPARγ Activity by Imatinib

Imatinib is an effective anticancer drug for the treatment of leukemia. Interestingly, when an FDA-approved drug library was tested for agents that block peroxisome proliferator-activated receptor γ (PPARγ) phosphorylation at Ser245 to evaluate possibilities of antidiabetic drug repositioning, imatinib was determined as a PPARγ antagonist ligand. However, it is not well understood how imatinib binds to PPARγ or would improve insulin sensitivity without classical agonism. Here, we report the crystal structure of the PPARγ R288A mutant in complex with imatinib. Imatinib bound to Arm2 and Arm3 regions in the ligand-binding domain (LBD) of PPARγ, of which the Arm3 region is closely related to the inhibition of PPARγ phosphorylation at Ser245. The binding of imatinib in LBD induced a stable conformation of helix H2′ and the Ω loop compared with the ligand-free state. In contrast, imatinib does not interact with Tyr473 on PPARγ helix H12, which is important for the classical agonism associated with side effects. Our study provides new structural insights into the PPARγ regulation by imatinib and may contribute to the development of new antidiabetic drugs targeting PPARγ while minimizing known side effects.

Design, synthesis, and evaluation of potent novel peroxisome proliferator-activated receptor γ indole partial agonists

Peroxisome Proliferator-Activated Receptor γ (PPARγ) is a nuclear receptor important for glucose homeostasis and insulin sensitivity. The anti-diabetic drugs thiazolidinediones improve insulin sensitivity by blocking PPARγ phosphorylation at S273; however, their full agonism on PPARγ also causes significant unwanted side effects. The indole derivative UHC1 displays insulin-sensitizing effect by acting as a partial agonist through the inhibition of PPARγ S273 phosphorylation, but without full agonist-associated side effects; however, its potency leaves much to be desired. Herein we report the design and synthesis of potent indole analogs as partial PPARγ agonists via the structure-activity relationship studies. Our studies revealed that vanillylamine and piperonyl benzylamine at Site 1 are favored to bind PPARγ with either biphenyl or 3-trifluoromethyl benzyl group at Site 2. In particular, compound WO91A with vanillylamine at Site 1 displays highly potent PPARγ binding affinity (IC₅₀ = 16.7 nM), over 30-fold more potent than the parental compound UHC1, yet with less side effect-associated transactivation activity.

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

Peroxisome Proliferator-Activated Receptors (PPARs) are ligand-activated transcription factors that govern lipid and glucose homeostasis playing a central role in cardiovascular disease, obesity, and diabetes. These receptors show a high degree of stereoselectivity towards several classes of drugs. This review covers the most relevant findings that have been made in the last decade and takes into consideration only those compounds in which stereochemistry led to unexpected results or peculiar interactions with the receptors. These cases are reviewed and discussed with the aim to show how enantiomeric recognition originates at the molecular level. The structural characterization by crystallographic methods and docking experiments of complexes formed by PPARs with their ligands turns out to be an essential tool to explain receptor stereoselectivity.

Elusive Conformational Dynamics of PPARγ Inactivation Tied Down by Chemical Cross-Linking

In this issue of Structure, Zheng et al. (2018) have described the dynamics of PPARγ in complex with a non-agonist by exploring its solution-phase conformational landscape through chemical cross-linking in combination with a multitude of different treatment conditions, including their new synthetic anti-diabetic non-agonist, revealing the physical mechanism of PPARγ inactivation.

Structural basis for the inhibitory effects of a novel reversible covalent ligand on PPARγ phosphorylation

Peroxisome proliferator-activated receptor γ (PPARγ) is a major therapeutic target for the treatment of type 2 diabetes. However, the use of PPARγ-targeted drugs, such as rosiglitazone and pioglitazone, is limited owing to serious side effects caused by classical agonism. Using a rational drug discovery approach, we recently developed SB1495, a novel reversible covalent inhibitor of the cyclin-dependent kinase 5 (Cdk5)-mediated phosphorylation of PPARγ at Ser245, a key factor in the insulin-sensitizing effect of PPARγ-targeted drugs. In this study, we report the crystal structures of PPARγ in complex with SB1495 and its enantiomeric analogue SB1494, which rarely exhibits inhibitory activity, to visualize the mechanistic basis for their distinct activities. SB1495 occupies the Arm3 region near the Ω loop of the PPARγ ligand-binding domain, whereas its enantiomeric analogue SB1494 binds to the Arm2 region. In addition, the piperazine moiety of SB1495 directly pushes the helix H2′, resulting in the stabilization of the Ω loop just behind the helix H2′. Our results may contribute to the development of a new generation of antidiabetic drugs that selectively block PPARγ phosphorylation without classical agonism.

Exploring the mechanism of PPARγ phosphorylation mediated by CDK5

Peroxisome proliferator-activated receptor gamma (PPARγ) is a nuclear receptor with a key role in metabolic processes and is target of CDK5 kinase phosphorylation at S245 (S273 in PPARγ isoform 2), thereby inducing insulin resistance. A remarkable effort has been addressed to find PPARγ ligands that inhibit S245 phosphorylation, but the poor understanding in this field challenges the design of such ligands. Here, through computational and biophysical methods, we explored an experimentally validated model of PPARγ-CDK5 complex, and we presented K261, K263 or K265, which are conserved in mammals, as important anchor residues for this interaction. In addition, we observed, from structural data analysis, that PPARγ ligands that inhibit S245 phosphorylation are not in direct contact with these residues; but induce structural modifications in PPARγ:CDK5/p25 interface. In summary, our PPARγ and CDK5/p25 interaction analyses open new possibilities for the rational design of novel inhibitors that impair S245 phosphorylation.

Phloretin and phloridzin improve insulin sensitivity and enhance glucose uptake by subverting PPARγ/Cdk5 interaction in differentiated adipocytes

Activators of peroxisome proliferator-activated receptor-γ (PPARγ agonists) are therapeutically promising candidates against insulin resistance and hyperglycemia. Synthetic PPARγ agonists are known to effectively enhance insulin sensitivity, but these are also associated with adverse side-effects and rising cost of treatment. Therefore, natural PPARγ targeting ligands are desirable alternatives for the management of insulin resistance associated with type 2 diabetes. Phloretin (PT) and Phloridzin (PZ) are predominant apple phenolics, which are recognized for their various pharmacological functions. The present study assessed the potential of PT and PZ in enhancing insulin sensitivity and glucose uptake by inhibiting Cdk5 activation and corresponding PPARγ phosphorylation in differentiated 3T3L1 cells. In silico docking and subsequent validation using 3T3L1 cells revealed that PT and PZ not only block the ser273 site of PPARγ but also inhibit the activation of Cdk5 itself, thereby, indicating their potent PPARγ regulatory attributes. Corroborating this, application of PT and PZ significantly enhanced the accumulation of cellular triglycerides as well as expression of insulin-sensitizing genes in adipocytes ultimately resulting in improved glucose uptake. Taken together, the present study reports that PT and PZ inhibit Cdk5 activation, which could be directly influencing the apparent PPARγ inhibition at ser273, ultimately resulting in improved insulin sensitivity and glucose uptake.

The brominated flame retardant PBDE 99 promotes adipogenesis via regulating mitotic clonal expansion and PPARγ expression

"Obesogens" have been widely accepted as chemicals that promote obesity, and there are many environmental pollutants that were functionally identified as obesogens. PBDE 99 is one of the most abundant PBDE congeners detected in human. However, its obesogenic effects are poorly understood. Here, we explore the in vitro effects of PBDE 99 on adipogenesis, which is a key process in obesogenesis. We observed an increase in adipogenesis when differentiating cells were exposed to PBDE 99. Further, the promoting effects of PBDE 99 on adipogenesis were most efficient during the first 4 days of 3T3-L1 differentiation. Consistent with this, early transcriptional factor CCAAT/enhancer-binding proteins β (C/EBPβ) was upregulated at Days 1 and 2 during differentiation, which is accompanied with the acceleration of mitotic clonal expansion (MCE) and the upregulation of terminal transcriptional factors C/EBPα and PPARγ2 from Day 2 or Day 4. Additionally, bisulfite genomic sequencing analysis revealed that PBDE 99 decreased methylation status of the CpG sites at PPARγ promoter region. Collectively, these findings demonstrate that PBDE 99 may be a potential environmental obesogen by promoting adipogenesis through facilitating MCE progression at early differentiation stage and upregulating key adipogenic factor PPARγ2 expression both in direct transcriptional and epigenetic regulation dependent manner.

Evidence that peroxisome proliferator-activated receptor γ suppresses squamous carcinogenesis through anti-inflammatory signaling and regulation of the immune response

A variety of evidence suggests that peroxisome proliferator-activated receptor (PPAR)γ agonists may represent a potential pharmacologic target in the prevention or treatment of skin cancer. In particular, recent reports suggest that PPARγ activation may exert at least some of its anti-neoplastic effects through the suppression of tumor promoting chronic inflammation as well as by strengthening antitumor immune responses. This activity is thought to occur through a distinct mode of ligand interaction with PPARγ that causes transrepression of transcription factors that are involved in inflammatory and immunomodulatory signaling. However, current thiazolidinedione (TZD)-type PPARγ agonists have significant safety concerns that limit their usefulness as a preventive or therapeutic option. Due to the relatively large ligand binding pocket of PPARγ, a diverse group of ligands can be seen to interact with distinct modes of binding to PPARγ, leading to the phenomenon of partial agonist activity and selective PPARγ modulators (SPPARγM). This has led to the development of ligands that are tailored to deliver desired pharmacologic activity, but lack some of the negative side effects associated with full agonists, such as the currently utilized TZD-type PPARγ agonists. In addition, there is evidence that a number of phytochemicals that are currently being touted as antineoplastic nutraceuticals also possess PPARγ activity that may partially explain their pharmacologic activity. We propose that one or more of these partial agonists, SPPARγMs, or putative phytochemical PPARγ ligands could presumably be used as a starting point to design more efficacious anti-neoplastic PPARγ ligands that lack adverse pharmacological effects.

Unique Polypharmacology Nuclear Receptor Modulator Blocks Inflammatory Signaling Pathways

Obesity and rheumatic disease are mechanistically linked via chronic inflammation. The orphan receptor TREM-1 (triggering receptor expressed on myeloid cells-1) is a potent amplifier of proinflammatory and noninfectious immune responses. Here, we show that the pan modulator SR1903 effectively blocks TREM-1 activation. SR1903 emerged from a chemical series of potent RORγ inverse agonists, although unlike close structural analogues, it has modest agonist activity on LXR and weak repressive activity (inverse agonism) of PPARγ, three receptors that play essential roles in inflammation and metabolism. The anti-inflammatory and antidiabetic efficacy of this unique modulator in collagen-induced arthritis and diet-induced obesity mouse models is demonstrated. Interestingly, in the context of obesity, SR1903 aided in the maintenance of the thymic homeostasis unlike selective RORγ inverse agonists. SR1903 was well-tolerated following chronic administration, and combined, these data suggest that it may represent a viable strategy for treatment of both metabolic and inflammatory disease. More importantly, the ability of SR1903 to block LPS signaling suggests the potential utility of this unique polypharmacological modulator for treatment of innate immune response disorders.

Highly Selective Protein Tyrosine Phosphatase Inhibitor, 2,2',3,3'-Tetrabromo-4,4',5,5'-tetrahydroxydiphenylmethane, Ameliorates Type 2 Diabetes Mellitus in BKS db Mice

Protein tyrosine phosphatase 1B (PTP1B) is a widely confirmed target of the type 2 diabetes mellitus (T2DM) treatment. Herein, we reported a highly specific PTP1B inhibitor 2,2',3,3'-tetrabromo-4,4',5,5'-tetrahydroxydiphenylmethane (compound 1), which showed promising hypoglycemic activity in diabetic BKS db mice. With the IC50 value of 2.4 μM, compound 1 could directly bind to the catalytic pocket of PTP1B through a series of hydrogen bonds. Surface plasmon resonance analysis revealed that the target affinity [KD (equilibrium dissociation constant) value] of compound 1 binding to PTP1B was 2.90 μM. Moreover, compound 1 could activate the insulin signaling pathway in C2C12 skeletal muscle cells. We further evaluated the long-term effects of compound 1 in diabetic BKS db mice. Notably, oral administration of compound 1 significantly reduced the blood glucose levels of diabetic mice with increasing insulin sensitivity. In addition, the dyslipidemia of diabetic mice was also significantly improved by compound 1 gavage. The histological experiments showed that compound 1 treatment significantly ameliorated the disordered hepatic and pancreatic architecture and increased the glycogen content in the liver tissues as well as improved the insulin secretion function of pancreas. Taken together, our results manifested that the natural product compound 1 was a highly specific PTP1B inhibitor, which could activate insulin signaling pathway and ameliorate hyperglycemia and dyslipidemia in diabetic BKS db mice.

Agonism activities of lyso-phosphatidylcholines (LPC) Ligands binding to peroxisome proliferator-activated receptor gamma (PPARγ)

PPARγ is an isoform of peroxisome proliferator-activated receptor (PPAR) belonging to a super family of nuclear receptors and is a primary target of the effective drug to treat the type II diabetes. The experiments found that Lyso-phosphatidylcholines (LPC) could bind to PPARγ, but the binding modes remain unknown. We used the Molecular Docking and Molecular Dynamic (MD) simulations to study the binding of four LPC ligands (LPC16:0, LPC18:0, LPC18:1-1 and LPC18:1-2) to PPARγ. The two-step MD simulations were employed to determine the final binding modes. The 20 ns MD simulations for four final LPC-PPARγ complexes were performed to analyze their structures, the binding key residues, and agonism activities. The results reveal that three LPC ligands (LPC16:0, LPC18:0 and LPC18:1-1) bind to Arm II and III regions of the Ligand Binding Domain (LBD) pocket, whereas they do not interact with Tyr473 of Helix 12 (H12). In contrast, LPC18:1-2 can form the hydrogen bonds with Tyr473 and bind into Arm I and II regions. Comparing with the paradigm systems of the full agonist (Rosiglitazone-PPARγ) and the partial agonist (MRL24-PPARγ), our results indicate that LPC16:0, LPC18:0 and LPC18:1-1 could be the potential partial agonists and LPC18:1-2 could be a full agonist. The in-depth analysis of the residue fluctuations and structure alignment confirm the present prediction of the LPC agonism activities.Communicated by Ramaswamy H. Sarma.

Allosteric small molecule modulators of nuclear receptors

Nuclear Receptors (NRs) are multi-domain proteins, whose natural regulation occurs via ligands for a classical, orthosteric, binding pocket and via intra- and inter-domain allosteric mechanisms. Allosteric modulation of NRs via synthetic small molecules has recently emerged as an interesting entry to address the need for small molecules targeting NRs in pathology, via novel modes of action and with beneficial profiles. In this review the general concept of allosteric modulation in drug discovery is first discussed, serving as a background and inspiration for NRs. Subsequently, the review focuses on examples of small molecules that allosterically modulate NRs, with a strong focus on structural information and the ligand binding domain. Recently discovered nanomolar potent allosteric site NR modulators are catapulting allosteric targeting of NRs to the center of attention. The obtained insights serve as a basis for recommendations for the next steps to take in allosteric small molecular targeting of NRs.

Zafirlukast Is a Dual Modulator of Human Soluble Epoxide Hydrolase and Peroxisome Proliferator-Activated Receptor γ

Cysteinyl leukotriene receptor 1 antagonists (CysLT1RA) are frequently used as add-on medication for the treatment of asthma. Recently, these compounds have shown protective effects in cardiovascular diseases. This prompted us to investigate their influence on soluble epoxide hydrolase (sEH) and peroxisome proliferator activated receptor (PPAR) activities, two targets known to play an important role in CVD and the metabolic syndrome. Montelukast, pranlukast and zafirlukast inhibited human sEH with IC₅₀ values of 1.9, 14.1, and 0.8 μM, respectively. In contrast, only montelukast and zafirlukast activated PPARγ in the reporter gene assay with EC₅₀ values of 1.17 μM (21.9% max. activation) and 2.49 μM (148% max. activation), respectively. PPARα and δ were not affected by any of the compounds. The activation of PPARγ was further investigated in 3T3-L1 adipocytes. Analysis of lipid accumulation, mRNA and protein expression of target genes as well as PPARγ phosphorylation revealed that montelukast was not able to induce adipocyte differentiation. In contrast, zafirlukast triggered moderate lipid accumulation compared to rosiglitazone and upregulated PPARγ target genes. In addition, we found that montelukast and zafirlukast display antagonistic activities concerning recruitment of the PPARγ cofactor CBP upon ligand binding suggesting that both compounds act as PPARγ modulators. In addition, zafirlukast impaired the TNFα triggered phosphorylation of PPARγ2 on serine 273. Thus, zafirlukast is a novel dual sEH/PPARγ modulator representing an excellent starting point for the further development of this compound class.

Sonic hedgehog signaling instigates high-fat diet-induced insulin resistance by targeting PPARγ stability

Obesity is a major risk for patients with chronic metabolic disorders including type 2 diabetes. Sonic hedgehog (Shh) is a morphogen that regulates the pancreas and adipose tissue formation during embryonic development. Peroxisome proliferator-activated receptor γ (PPARγ) is a member of the nuclear receptor superfamily and one of the most important regulators of insulin action. Here, we evaluated the role and mechanism of Shh signaling in obesity-associated insulin resistance and characterized its effect on PPARγ. We showed that Shh expression was up-regulated in subcutaneous fat from obese mice. In differentiated 3T3-L1 and primary cultured adipocytes from rats, recombinant Shh protein and SAG (an agonist of Shh signaling) activated an extracellular signal-regulated kinase (ERK)-dependent noncanonical pathway and induced PPARγ phosphorylation at serine 112, which decreased PPARγ activity. Meanwhile, Shh signaling degraded PPARγ protein via binding of PPARγ to neural precursor cell-expressed developmentally down-regulated protein 4-1 (NEDD4-1). Furthermore, vismodegib, an inhibitor of Shh signaling, attenuated ERK phosphorylation induced by a high fat diet (HFD) and restored PPARγ protein level, thus ameliorating glucose intolerance and insulin resistance in obese mice. Our finding suggests that Shh in subcutaneous fat decreases PPARγ activity and stability via activation of an ERK-dependent noncanonical pathway, resulting in impaired insulin action. Inhibition of Shh may serve as a potential therapeutic approach to treat obesity-related diabetes.

deMix: Decoding Deuterated Distributions from Heterogeneous Protein States via HDX-MS

Characterization of protein structural changes in response to protein modifications, ligand or chemical binding, or protein-protein interactions is essential for understanding protein function and its regulation. Amide hydrogen/deuterium exchange (HDX) coupled with mass spectrometry (MS) is one of the most favorable tools for characterizing the protein dynamics and changes of protein conformation. However, currently the analysis of HDX-MS data is not up to its full power as it still requires manual validation by mass spectrometry experts. Especially, with the advent of high throughput technologies, the data size grows everyday and an automated tool is essential for the analysis. Here, we introduce a fully automated software, referred to as ‘deMix’, for the HDX-MS data analysis. deMix deals directly with the deuterated isotopic distributions, but not considering their centroid masses and is designed to be robust over random noises. In addition, unlike the existing approaches that can only determine a single state from an isotopic distribution, deMix can also detect a bimodal deuterated distribution, arising from EX1 behavior or heterogeneous peptides in conformational isomer proteins. Furthermore, deMix comes with visualization software to facilitate validation and representation of the analysis results.

Structural Basis of Altered Potency and Efficacy Displayed by a Major in Vivo Metabolite of the Antidiabetic PPARγ Drug Pioglitazone

Pioglitazone (Pio) is a Food and Drug Administration-approved drug for type-2 diabetes that binds and activates the nuclear receptor peroxisome proliferator-activated receptor γ (PPARγ), yet it remains unclear how in vivo Pio metabolites affect PPARγ structure and function. Here, we present a structure-function comparison of Pio and its most abundant in vivo metabolite, 1-hydroxypioglitazone (PioOH). PioOH displayed a lower binding affinity and reduced potency in co-regulator recruitment assays. X-ray crystallography and molecular docking analysis of PioOH-bound PPARγ ligand-binding domain revealed an altered hydrogen bonding network, including the formation of water-mediated bonds, which could underlie its altered biochemical phenotype. NMR spectroscopy and hydrogen/deuterium exchange mass spectrometry analysis coupled to activity assays revealed that PioOH better stabilizes the PPARγ activation function-2 (AF-2) co-activator binding surface and better enhances co-activator binding, affording slightly better transcriptional efficacy. These results indicating that Pio hydroxylation affects its potency and efficacy as a PPARγ agonist contributes to our understanding of PPARγ-drug metabolite interactions.

A role of peroxisome proliferator-activated receptor γ in non-alcoholic fatty liver disease

Non-alcoholic fatty liver disease is becoming a major health burden, as prevalence increases and there are no approved treatment options. Thiazolidinediones target the nuclear receptor peroxisome proliferator-activated receptor γ (PPARγ) and have been investigated in several clinical trials for their potential in treating non-alcoholic fatty liver disease (NAFLD) and non-alcoholic steatohepatitis (NASH). PPARγ has specialized roles in distinct tissues and cell types, and although the primary function of PPARγ is in adipose tissue, where the highest expression levels are observed, hepatic expression levels of PPARγ are significantly increased in patients with NAFLD. Thus, NAFLD patients receiving treatment with PPARγ agonists might have a liver response apart from the one in adipose tissue. Owing to the different roles of PPARγ, new treatment strategies include development of compounds harnessing the beneficial effects of PPARγ while restricting PPARγ unwanted effects such as adipogenesis resulting in weight gain. Furthermore, dual or pan agonists targeting two or more of the PPARs have shown promising results in pre-clinical research and some are currently proceeding to clinical trials. This MiniReview explores adipose- and liver-specific actions of PPARγ, and how this knowledge may contribute in the search for new treatment modalities in NAFLD/NASH.

Bronchopulmonary Dysplasia: Crosstalk Between PPARγ, WNT/β-Catenin and TGF-β Pathways; The Potential Therapeutic Role of PPARγ Agonists

Bronchopulmonary dysplasia (BPD) is a serious pulmonary disease which occurs in preterm infants. Mortality remains high due to a lack of effective treatment, despite significant progress in neonatal resuscitation. In BPD, a persistently high level of canonical WNT/β-catenin pathway activity at the canalicular stage disturbs the pulmonary maturation at the saccular and alveolar stages. The excessive thickness of the alveolar wall impairs the normal diffusion of oxygen and carbon dioxide, leading to hypoxia. Transforming growth factor (TGF-β) up-regulates canonical WNT signaling and inhibits the peroxysome proliferator activated receptor gamma (PPARγ). This profile is observed in BPD, especially in animal models. Following a premature birth, hypoxia activates the canonical WNT/TGF-β axis at the expense of PPARγ. This gives rise to the differentiation of fibroblasts into myofibroblasts, which can lead to pulmonary fibrosis that impairs the respiratory function after birth, during childhood and even adulthood. Potential therapeutic treatment could target the inhibition of the canonical WNT/TGF-β pathway and the stimulation of PPARγ activity, in particular by the administration of nebulized PPARγ agonists.

GVS-12 attenuates non-alcoholic steatohepatitis by suppressing inflammatory responses via PPARγ/STAT3 signaling pathways

Non-alcoholic steatohepatitis (NASH), a type of fatty liver disease, is characterized by excessive inflammation and fat accumulation in the liver. Peroxisome proliferator-activated receptor γ (PPARγ) agonist rosiglitazone has great potential in protecting against the development of NASH. However, long-term usage of rosiglitazone probably leads to many adverse reactions. In this research, GVS-12 was designed and synthesized as a PPARγ agonist with high selectivity, evidenced by increasing the activity of the PPARγ reporter gene and promoting the mRNA expression of the PPARγ responsive gene cluster of differentiation 36 (CD36). It was noteworthy that GVS-12 could ameliorate dysfunction and lipid accumulation by down-regulating the mRNA expression of interleukin-1β (IL-1β), interleukin-6 (IL-6) and tumor necrosis factor-α (TNF-α) in the liver of high fat diet (HFD)-induced rats and palmitic acid (PA)-stimulated hepatocellular carcinoma G2 (HepG2) cells. Moreover, PPARγ siRNA (siPPARγ) markedly diminished GVS-12 induced the down-regulation of mRNA expression of IL-1β, IL-6 and TNF-α in PA-stimulated HepG2 cells. Additionally, GVS-12 could reduce the phosphorylation level of STAT3 and up-regulate the protein expression of a suppressor of cytokine signaling 3 (SOCS3), which could be reversed by siPPARγ. In detail, SOCS3 siRNA (siSOCS3) diminished the inhibitory effect of GVS-12 on the mRNA expression of IL-1β, IL-6 and TNF-α. In conclusion, GVS-12 suppressed the development of NASH by down-regulating the mRNA expression of IL-1β, IL-6 and TNF-α via PPARγ/STAT3 signaling pathways.

Non-alcoholic steatohepatitis (NASH), a type of fatty liver disease, is characterized by excessive inflammation and fat accumulation in the liver.

Cooperative cobinding of synthetic and natural ligands to the nuclear receptor PPARγ

Crystal structures of peroxisome proliferator-activated receptor gamma (PPARγ) have revealed overlapping binding modes for synthetic and natural/endogenous ligands, indicating competition for the orthosteric pocket. Here we show that cobinding of a synthetic ligand to the orthosteric pocket can push natural and endogenous PPARγ ligands (fatty acids) out of the orthosteric pocket towards an alternate ligand-binding site near the functionally important omega (Ω)-loop. X-ray crystallography, NMR spectroscopy, all-atom molecular dynamics simulations, and mutagenesis coupled to quantitative biochemical functional and cellular assays reveal that synthetic ligand and fatty acid cobinding can form a 'ligand link' to the Ω-loop and synergistically affect the structure and function of PPARγ. These findings contribute to a growing body of evidence indicating ligand binding to nuclear receptors can be more complex than the classical one-for-one orthosteric exchange of a natural or endogenous ligand with a synthetic ligand.

Growth hormone controls lipolysis by regulation of FSP27 expression

Growth hormone (GH) has long been known to stimulate lipolysis and insulin resistance; however, the molecular mechanisms underlying these effects are unknown. In the present study, we demonstrate that GH acutely induces lipolysis in cultured adipocytes. This effect is secondary to the reduced expression of a negative regulator of lipolysis, fat-specific protein 27 (FSP27; aka Cidec) at both the mRNA and protein levels. These effects are mimicked in vivo as transgenic overexpression of GH leads to a reduction of FSP27 expression. Mechanistically, we show GH modulation of FSP27 expression is mediated through activation of both MEK/ERK- and STAT5-dependent intracellular signaling. These two molecular pathways interact to differentially manipulate peroxisome proliferator-activated receptor gamma activity (PPARγ) on the FSP27 promoter. Furthermore, overexpression of FSP27 is sufficient to fully suppress GH-induced lipolysis and insulin resistance in cultured adipocytes. Taken together, these data decipher a molecular mechanism by which GH acutely regulates lipolysis and insulin resistance in adipocytes.

PPARγ-K107 SUMOylation regulates insulin sensitivity but not adiposity in mice

The nuclear receptor peroxisome proliferator-activated receptor γ (PPARγ) is a master regulator of adipocyte differentiation and is the target for the insulin-sensitizing thiazolidinedione (TZD) drugs used to treat type 2 diabetes. In cell-based in vitro studies, the transcriptional activity of PPARγ is inhibited by covalent attachment of small ubiquitin-related modifier (SUMOylation) at K107 in its N terminus. However, whether this posttranslational modification is relevant in vivo remains unclear. Here, using mice homozygous for a mutation (K107R) that prevents SUMOylation at this position, we demonstrate that PPARγ is SUMOylated at K107 in white adipose tissue. We further show that in the context of diet-induced obesity PPARγ-K107R-mutant mice have enhanced insulin sensitivity without the corresponding increase in adiposity that typically accompanies PPARγ activation by TZDs. Accordingly, the PPARγ-K107R mutation was weaker than TZD treatment in stimulating adipocyte differentiation in vitro. Moreover, we found that both the basal and TZD-dependent transcriptomes of inguinal and epididymal white adipose tissue depots were markedly altered in the K107R-mutant mice. We conclude that PPARγ SUMOylation at K107 is physiologically relevant and may serve as a pharmacologic target for uncoupling PPARγ's beneficial insulin-sensitizing effect from its adverse effect of weight gain.

A structural mechanism for directing corepressor-selective inverse agonism of PPARγ

Small chemical modifications can have significant effects on ligand efficacy and receptor activity, but the underlying structural mechanisms can be difficult to predict from static crystal structures alone. Here we show how a simple phenyl-to-pyridyl substitution between two common covalent orthosteric ligands targeting peroxisome proliferator-activated receptor (PPAR) gamma converts a transcriptionally neutral antagonist (GW9662) into a repressive inverse agonist (T0070907) relative to basal cellular activity. X-ray crystallography, molecular dynamics simulations, and mutagenesis coupled to activity assays reveal a water-mediated hydrogen bond network linking the T0070907 pyridyl group to Arg288 that is essential for corepressor-selective inverse agonism. NMR spectroscopy reveals that PPARγ exchanges between two long-lived conformations when bound to T0070907 but not GW9662, including a conformation that prepopulates a corepressor-bound state, priming PPARγ for high affinity corepressor binding. Our findings demonstrate that ligand engagement of Arg288 may provide routes for developing corepressor-selective repressive PPARγ ligands.

Peroxisome proliferator-activated receptor gamma (PPARγ) is a target for insulin sensitizing drugs. Here the authors combine NMR, X-ray crystallography and MD simulations and report a structural mechanism for eliciting PPARγ inverse agonism, where coactivator binding is inhibited and corepressor binding promoted, which causes PPARγ repression.

Development of Dihydrodibenzooxepine Peroxisome Proliferator-Activated Receptor (PPAR) Gamma Ligands of a Novel Binding Mode as Anticancer Agents: Effective Mimicry of Chiral Structures by Olefinic E/ Z-Isomers

A novel class of PPARγ ligand 1 (EC₅₀ = 197 nM) with a dibenzoazepin scaffold was identified through high-throughput screening campaign. To avoid the synthetically troublesome chiral center of 1, its conformational analysis using the MacroModel was conducted, focusing on conformational flip of the tricyclic ring and the conformational restriction by the methyl group at the chiral center. On the basis of this analysis, scaffold hopping of dibenzoazepine into dibenzo[ b, e]oxepine by replacing the chiral structures with the corresponding olefinic E/ Z isomers was performed. Consequently, dibenzo[ b, e]oxepine scaffold 9 was developed showing extremely potent PPARγ reporter activity (EC₅₀ = 2.4 nM, efficacy = 9.5%) as well as differentiation-inducing activity against a gastric cancer cell line MKN-45 that was more potent than any other well-known PPARγ agonists in vitro (94% at 30 nM). The X-ray crystal structure analysis of 9 complexed with PPARγ showed that it had a unique binding mode to PPARγ ligand-binding domain that differed from that of any other PPARγ agonists identified thus far.

Asiatic acid protests against myocardial ischemia/reperfusion injury via modulation of glycometabolism in rat cardiomyocyte

Background

Asiatic acid is a reported glycogen phosphorylase inhibitor derived from the tropical medicinal plant Centella asiatica and exhibits myocardial protection both in vivo and in vitro. The purpose of this study was to evaluate the effects of asiatic acid on myocardial ischemia/reperfusion (MI/R) injury and investigate the underlying mechanisms associated with the modulation of glycometabolism in cardiomyocyte.

Materials and methods

The rats were subjected to MI/R with or without asiatic acid pretreatment. The cardiac function indexes, the size of myocardial infarction, and plasma lactate dehydrogenase (LDH) and creatine kinase (CK) activities were detected. Cardiomyocyte apoptosis was analyzed by TUNEL assay. The Akt/GSK-3β activation was measured by Western blot. The glycogen content, plasma glucose and lactate concentrations were determined following MI/R. The mRNA and protein levels of PPARγ and GLUT4 were determined by real-time PCR and Western blot, respectively.

Results

Asiatic acid pretreatment significantly improved the cardiac function indexes, attenuated the size of myocardial infarction, reduced LDH and CK activities, and suppressed cardiomyocyte apoptosis after MI/R. Asiatic acid activated Akt/GSK-3β signal pathway in the myocardium following MI/R injury. In addition, asiatic acid effectively suppressed MI/R-induced glycogen breakdown and inhibited the elevation of plasma glucose and lactate concentrations. Asiatic acid treatment increased PPARγ expression at both mRNA and protein levels, and promoted the translocation of GLUT4 to plasma membrane after MI/R insult. However, the effects mediated by asiatic acid on glycometabolism and GLUT4 translocation were reversed by the administration of LY294002, the Akt inhibitor.

Conclusion

These findings demonstrated that asiatic acid exerts beneficial effects on MI/R injury in rats. This protection may be related to the modulation of glycometabolism via Akt-dependent GLUT4 translocation and PPARγ activation in ischemic cardiomyocyte.

Screening of FDA-approved drugs identifies sutent as a modulator of UCP1 expression in brown adipose tissue

BACKGROUND

The pharmacological activation of thermogenesis in brown adipose tissue has long been considered promising strategies to treat obesity. However, identification of safe and effective agents remains a challenge. In this study, we addressed this challenge by developing a cellular system with a fluorescence readout, and applied in a high-throughput manner to screen for FDA-approved drugs that may activate endogenous UCP1 expression in adipocytes.

METHODS

We have generated a Ucp1-2A-GFP reporter mouse, in which GFP intensity serves as a surrogate of the endogenous expression level of UCP1 protein; and immortalized brown adipocytes were derived from this mouse model and applied in drug screening. Candidate drugs were further tested in mouse models either fed with normal chow or high fat diet to induce obesity.

FINDINGS

By using the cellular screening platform, we identified a group of FDA-approved drugs that can upregulate UCP1 expression in brown adipocyte, including previously known UCP1 activators and new candidate drugs. Further studies focusing on a previously unreported drug-sutent, revealed that sutent treatment could increase the energy expenditure and inhibit lipid synthesis in mouse adipose and liver tissues, resulting in improved metabolism and resistance to obesity.

INTERPRETATION

This study offered an easy-to-use cellular screening system for UCP1 activators, and provided a candidate list of FDA-approved drugs that can potentially treat obesity. Further study of these candidates may shed new light on the drug discovery towards obesity. FUND: National Key Research and Development Program and the Strategic Priority Research Program of the Chinese Academy of Sciences, etc. (250 words).

The E3 ubiquitin ligase TRIM25 regulates adipocyte differentiation via proteasome-mediated degradation of PPARγ

Peroxisome proliferator-activated receptor gamma (PPARγ) is a ligand-dependent transcription factor that regulates adipocyte differentiation and glucose homeostasis. The transcriptional activity of PPARγ is regulated not only by ligands but also by post-translational modifications (PTMs). In this study, we demonstrate that a novel E3 ligase of PPARγ, tripartite motif-containing 25 (TRIM25), directly induced the ubiquitination of PPARγ, leading to its proteasome-dependent degradation. During adipocyte differentiation, both TRIM25 mRNA and protein expression significantly decreased and negatively correlated with the expression of PPARγ. The stable expression of TRIM25 reduced PPARγ protein levels and suppressed adipocyte differentiation in 3T3-L1 cells. In contrast, the specific knockdown of TRIM25 increased PPARγ protein levels and stimulated adipocyte differentiation. Furthermore, TRIM25-knockout mouse embryonic fibroblasts (MEFs) exhibited an increased adipocyte differentiation capability compared with wild-type MEFs. Taken together, these data indicate that TRIM25 is a novel E3 ubiquitin ligase of PPARγ and that TRIM25 is a novel target for PPARγ-associated metabolic diseases.