Fenoxycarb promotes adipogenesis in 3T3‐L1 preadipocytes

Lipophilic insect hormones and their analogs affect mammalian physiology by regulating the expression of metabolic genes. Therefore, we determined the effect of fenoxycarb, a juvenile hormone analog, on lipid metabolism in adipocytes. Here, we demonstrated that fenoxycarb dose‐dependently promoted lipid accumulation in 3T3‐L1 adipocytes during adipocyte differentiation and that its lipogenic effect was comparable to that of rosiglitazone, a well‐known ligand for peroxisome proliferator‐activated receptor gamma (PPARγ). Furthermore, fenoxycarb stimulated PPARγ activity without affecting other nuclear receptors, such as liver X receptor (LXR), farnesoid X‐activated receptor (FXR) and Nur77. In addition, fenoxycarb treatment increased the expression of PPARγ and fatty acid transporter protein 1 (FATP1) in 3T3‐L1 adipocytes, suggesting that fenoxycarb may facilitate adipocyte differentiation by enhancing PPARγ signaling, the master regulator of adipogenesis. Together, our results suggest that fenoxycarb promoted lipid accumulation in adipocytes, in part, by stimulating PPARγ.     

Trigonelline attenuates the adipocyte differentiation and lipid accumulation in 3T3-L1 cells

Trigonelline is a natural alkaloid mainly found in Trigonella Foenum Graecum (fenugreek) Fabaceae and other edible plants with a variety of medicinal applications. Therefore, we investigated the molecular mechanism of trigonelline (TG) on the inhibition of adipocyte differentiation and lipid accumulation in 3T3-L1 cells. Trigonelline suppressed lipid droplet accumulation in a concentration (75 and 100 μM) dependent manner. Treatment of adipocyte with of TG down regulates the peroxisome proliferator-activated receptor (PPARγ) and CCAAT element binding protein (C/EBP-α) mRNA expression, which leads to further down regulation of other gene such as adiponectin, adipogenin, leptin, resistin and adipocyte fatty acid binding protein (aP2) as compared with respective control cells on 5th and 10th day of differentiation. Further, addition of triognelline along with troglitazone to the adipocyte attenuated the troglitazone effects on PPARγ mediated differentiation and lipid accumulation in 3T3-L1 cells. Trigonelline might compete against troglitazone for its binding to the PPARγ. In addition, adipocyte treated with trigonelline and isoproterenol separately. Isoproterenol, a lipolytic agent which inhibits the fatty acid synthase and GLUT-4 transporter expression via cAMP mediated pathway, we found that similar magnitude response of fatty acid synthase and GLUT-4 transporter expression in trigonelline treated adipocyte. These results suggest that the trigonelline inhibits the adipogenesis by its influences on the expression PPARγ, which leads to subsequent down regulation of PPAR-γ mediated pathway during adipogenesis. Our findings provide key approach to the mechanism underlying the anti-adipogenic activity of trigonelline.     

Medium chain fatty acids are selective peroxisome proliferator activated receptor (PPAR) γ activators and pan-PPAR partial agonists

Thiazolidinediones (TZDs) act through peroxisome proliferator activated receptor (PPAR) γ to increase insulin sensitivity in type 2 diabetes (T2DM), but deleterious effects of these ligands mean that selective modulators with improved clinical profiles are needed. We obtained a crystal structure of PPARγ ligand binding domain (LBD) and found that the ligand binding pocket (LBP) is occupied by bacterial medium chain fatty acids (MCFAs). We verified that MCFAs (C8-C10) bind the PPARγ LBD in vitro and showed that they are low-potency partial agonists that display assay-specific actions relative to TZDs; they act as very weak partial agonists in transfections with PPARγ LBD, stronger partial agonists with full length PPARγ and exhibit full blockade of PPARγ phosphorylation by cyclin-dependent kinase 5 (cdk5), linked to reversal of adipose tissue insulin resistance. MCFAs that bind PPARγ also antagonize TZD-dependent adipogenesis in vitro. X-ray structure B-factor analysis and molecular dynamics (MD) simulations suggest that MCFAs weakly stabilize C-terminal activation helix (H) 12 relative to TZDs and this effect is highly dependent on chain length. By contrast, MCFAs preferentially stabilize the H2-H3/β-sheet region and the helix (H) 11-H12 loop relative to TZDs and we propose that MCFA assay-specific actions are linked to their unique binding mode and suggest that it may be possible to identify selective PPARγ modulators with useful clinical profiles among natural products.     

N-Acetylfarnesylcysteine is a novel class of peroxisome proliferator-activated receptor γ ligand with partial and full agonist activity in vitro and in vivo

The thiazolidedione (TZD) class of drugs is clinically approved for the treatment of type 2 diabetes. The therapeutic actions of TZDs are mediated via activation of peroxisome proliferator-activated receptor γ (PPARγ). Despite their widespread use, concern exists regarding the safety of currently used TZDs. This has prompted the development of selective PPARγ modulators (SPPARMs), compounds that promote glucose homeostasis but with reduced side effects due to partial PPARγ agonism. However, this also results in partial agonism with respect to PPARγ target genes promoting glucose homeostasis. Using a gene expression-based screening approach we identified N-acetylfarnesylcysteine (AFC) as both a full and partial agonist depending on the PPARγ target gene (differential SPPARM). AFC activated PPARγ as effectively as rosiglitazone with regard to Adrp, Angptl4, and AdipoQ, but was a partial agonist of aP2, a PPARγ target gene associated with increased adiposity. Induction of adipogenesis by AFC was also attenuated compared with rosiglitazone. Reporter, ligand binding assays, and dynamic modeling demonstrate that AFC binds and activates PPARγ in a unique manner compared with other PPARγ ligands. Importantly, treatment of mice with AFC improved glucose tolerance similar to rosiglitazone, but AFC did not promote weight gain to the same extent. Finally, AFC had effects on adipose tissue remodeling similar to those of rosiglitazone and had enhanced antiinflammatory effects. In conclusion, we describe a new approach for the identification of differential SPPARMs and have identified AFC as a novel class of PPARγ ligand with both full and partial agonist activity in vitro and in vivo.     

Minireview: PPARγ as the target of obesogens

The peroxisome proliferator-activated receptor gamma (PPARγ) is a key regulator of adipogenesis and is medically important for its connections to obesity and the treatment of type II diabetes. Activation of this receptor by certain natural or xenobiotic compounds has been shown to stimulate adipogenesis in vitro and in vivo. Obesogens are chemicals that ultimately increase obesity through a variety of potential mechanisms, including activation of PPARγ. The first obesogen for which a definitive mechanism of action has been elucidated is the PPARγ and RXR activator tributyltin; however, not all chemicals that activate PPARγ are adipogenic or correlated with obesity in humans. There are multiple mechanisms through which obesogens can target PPARγ that may not involve direct activation of the receptor. Ligand-independent mechanisms could act through obesogen-mediated post-translational modification of PPARγ which cause receptor de-repression or activation. PPARγ is active in multipotent stem cells committing to the adipocyte fate during fat cell development. By modifying chromatin structure early in development, obesogens have the opportunity to influence the promoter activity of PPARγ, or the ability of PPARγ to bind to its target genes, ultimately biasing the progenitor pool towards the fat lineage. Obesogens that act by directly or indirectly activating PPARγ, by increasing the levels of PPARγ protein, or enhancing its recruitment to promoters of key genes in the adipogenic pathway may ultimately play an important role in adipogenesis and obesity.     

Ginsenoside Rg3 stereoisomers differentially inhibit vascular smooth muscle cell proliferation and migration in diabetic atherosclerosis

Ginsenoside 20(R/S)‐Rg3, as a natural peroxisome proliferator‐activated receptor gamma (PPARγ) ligand, has been reported to exhibit differential biological effects. It is of great interest to understand the stereochemical selectivity of 20(R/S)‐Rg3 and explore whether differential PPARγ activation by Rg3 stereoisomers, if it exists, could lead to differential physiological outcome and therapeutic effects in diabetic atherosclerosis. Here, we investigated the binding modes of 20(R/S)‐Rg3 stereoisomers in the PPARγ ligand‐binding domain (PPARγ‐LBD) using molecular modelling and their effects on smooth muscle cell proliferation and migration induced by advanced glycation end products (AGEs). The results revealed that 20(S)‐Rg3 exhibited stronger antiproliferative and antimigratory effects due to stronger PPARγ activation. To validate the in vitro results, we used a mice model with diabetic atherosclerosis and obtained that 20(S)‐Rg3 markedly reduced the plaque size secondary to reducing the proliferation and migration of VSMCs, while the plaques were more stable due to improvements in other plaque compositions. The results shed light on the structural difference between Rg3 stereoisomers that can lead to significant differential physiological outcome, and the (S)‐isomer seems to be the more potent isomer to be developed as a promising drug for diabetic atherosclerosis.     

Flavanone exhibits PPARγ ligand activity and enhances differentiation of 3T3-L1 adipocytes

Flavanones are class of polyphenolic compounds, some of which are found in foods and provide health benefits. In this study, we show that flavanone significantly enhances differentiation of 3T3-L1 preadipocytes. During adipogenesis, flavanone enhanced expression of genes and accumulation of proteins that are involved in adipocyte function. Some reports have indicated that flavanone inhibits proliferation of mammalian cells, and down-regulates expression of growth-related proteins. Such proteins include phosphorylated ERK1/2, cyclins, and Cdks that are important for an early event in adipogenesis, mitotic clonal expansion (MCE). We demonstrated that flavanone did not inhibit MCE or expression of MCE-related proteins, except for a modest inhibition of cyclin D1 expression. Using luciferase reporter assays, we found that flavanone acted as a peroxisome proliferator-activated receptor γ (PPARγ) ligand in a dose-dependent manner. Together, our results suggest that flavanone enhances adipogenesis, at least in part, through its PPARγ ligand activity.