Inhibition of adipogenesis through MAP kinase-mediated phosphorylation of PPARgamma

Dexamethasone and rosiglitazone are sufficient and necessary for producing functional adipocytes from mesenchymal stem cells

The final product of adipogenesis is a functional adipocyte. This mature cell acquires the necessary machinery for lipid metabolism, loses its proliferation potential, increases its insulin sensitivity, and secretes adipokines. Multipotent mesechymal stromal cells have been recognized as a source of adipocytes both in vivo and in vitro. The in vitro adipogenic differentiation of human MSC (hMSC) has been induced up to now by using a complex stimulus which includes dexamethasone, 3-isobutyl-1-methylxanthine, indomethacin, and insulin (a classical cocktail) and evaluated according to morphological changes. The present work was aimed at demonstrating that the simultaneous activation of dexamethasone's canonical signaling pathways, through the glucocorticoid receptor and CCAAT-enhancer-binding proteins (C/EBPs) and rosiglitazone through peroxisome proliferator-activated receptor gamma (PPAR-gamma) is sufficient yet necessary for inducing hMSC adipogenic differentiation. It was also ascertained that hMSC exposed just to dexamethasone and rosiglitazone (D&R) differentiated into cells which accumulated neutral lipid droplets, expressed C/EBP-alpha, PPAR-gamma, aP2, lipoprotein lipase, acyl-CoA synthetase, phosphoenolpyruvate carboxykinase, adiponectin, and leptin genes but did not proliferate. Glucose uptake was dose dependent on insulin stimulus and high levels of adipokines were secreted (i.e. displaying not only the morphology but also expressing mature adipocytes' specific genes and functional characteristics). This work has demonstrated that (i) the activating C/EBPs and PPAR-gamma signaling pathways were sufficient to induce adipogenic differentiation from hMSC, (ii) D&R producing functional adipocytes from hMSC, (iii) D&R induce adipogenic differentiation from mammalian MSC (including those which are refractory to classical adipogenic differentiation stimuli). D&R would thus seem to be a useful tool for MSC characterization, studying adipogenesis pathways and producing functional adipocytes.

EPA-enriched phospholipids ameliorate cancer-associated cachexia mainly via inhibiting lipolysis

EPA-PL rescues the cancer-associated cachexia via inhibiting lipolysis.

MiR-378a-3p enhances adipogenesis by targeting mitogen-activated protein kinase 1

Previous studies showed that miR-378a plays important roles in adipogenesis and obesity; however, the precise mechanisms of action remain unknown. Here, we found that miR-378a-3p expression is up-regulated in adipose tissues of high fat diet-induced obese mice, as well as during the differentiation of 3T3-L1 preadipocytes. Mir-378a-3p induced adipogenesis by targeting mitogen-activated protein kinase 1 (MAPK1). Overexpression of miR-378a-3p or silencing MAPK1 reduced MAPK1 expression and enhanced adipogenesis, whereas blockage of endogenous miR-378a-3p had the opposite effect, suggesting that miR-378a-3p promotes the adipogenesis of 3T3-L1 cells by targeting MAPK1.

Thy1 (CD90) controls adipogenesis by regulating activity of the Src family kinase, Fyn

Worldwide obesity rates are at epidemic levels, and new insight into the regulation of obesity and adipogenesis are required. Thy1 (CD90), a cell surface protein with an enigmatic function, is expressed on subsets of fibroblasts and stem cells. We used a diet-induced obesity model to show that Thy1-null mice gain weight at a faster rate and gain 30% more weight than control C57BL/6 mice. During adipogenesis, Thy1 expression is lost in mouse 3T3-L1 cells. Overexpression of Thy1 blocked adipocyte formation and reduced mRNA and protein expression of an adipocyte marker, fatty acid-binding protein 4, by 5-fold. Although preadipocyte fibroblasts expressed Thy1 mRNA and protein, adipocytes from mouse and human fat tissue had almost undetectable Thy1 levels. Thy1 decreases the activity of the adipogenic transcription factor PPARγ by more than 60% as shown by PPARγ-dependent reporter assays. Using both genetic and pharmacologic approaches, we show Thy1 expression dampens PPARγ by inhibiting the activity of the Src-family kinase, Fyn. Thus, these studies reveal Thy1 blocks adipogenesis and PPARγ by inhibiting Fyn and support the idea that Thy1 is a novel therapeutic target in obesity.

ATF3 inhibits PPARγ-stimulated transactivation in adipocyte cells

Previously, we reported that activating transcription factor 3 (ATF3) downregulates peroxisome proliferator activated receptor (PPARγ) gene expression and inhibits adipocyte differentiation in 3T3-L1 cells. Here, we investigated another role of ATF3 on the regulation of PPARγ activity. ATF3 inhibited PPARγ-stimulated transactivation of PPARγ responsive element (PPRE)-containing reporter or GAL4/PPARγ chimeric reporter. Thus, ATF3 effectively repressed rosiglitazone-stimulated expression of adipocyte fatty acid binding protein (aP2), PPARγ target gene, in 3T3-L1 cells. Coimmunoprecipitation and GST pulldown assay demonstrated that ATF3 interacted with PPARγ. Accordingly, ATF3 prevented PPARγ from binding to PPRE on the aP2 promoter. Furthermore, ATF3 suppressed p300-mediated transcriptional coactivation of PPRE-containing reporter. Chromatin immunoprecipitation assay showed that overexpression of ATF3 blocked both binding of PPARγ and recruitment of p300 to PPRE on aP2 promoter induced by rosiglitazone treatment in 3T3-L1 cells. Taken together, these results suggest that ATF3 interacts with PPARγ and represses PPARγ-mediated transactivation through suppression of p300-stimulated coactivation in 3T3-L1 cells, which may play a role in inhibition of adipocyte differentiation.

An ERK/Cdk5 axis controls the diabetogenic actions of PPARγ

Obesity-linked insulin resistance is a major precursor to the development of type 2 diabetes. Previous work has shown that phosphorylation of PPARγ at serine 273 by Cdk5 stimulates diabetogenic gene expression in adipose tissues¹. Inhibition of this modification is a key therapeutic mechanism for anti-diabetic PPARγ ligand drugs, such as the thiazolidinediones and PPARγ partial/non-agonists². To better understand the importance of this obesity-linked PPARγ phosphorylation, we created mice that ablated Cdk5 specifically in adipose tissues. Surprisingly, these mice have both a paradoxical increase in PPARγ phosphorylation at S273 and worsened insulin resistance. Unbiased proteomic studies show that ERK kinases are activated in these KO animals. We show here that ERK directly phosphorylates S273 of PPARγ in a robust manner and that Cdk5 suppresses ERKs through direct action on a novel site in MEK, the ERK kinase. Importantly, pharmacological MEK and ERK inhibition markedly improves insulin resistance in both obese wild type and ob/ob mice, and also completely reverses the deleterious effects of the Cdk5 ablation. These data show that an ERK/Cdk5 axis controls PPARγ function and suggest that MEK/ERK inhibitors may hold promise for the treatment of type 2 diabetes.

c-Abl tyrosine kinase promotes adipocyte differentiation by targeting PPAR-gamma 2

Adipocyte differentiation, or adipogenesis, is a complex and highly regulated process. A recent proteomic analysis has predicted that the nonreceptor tyrosine kinase Abelson murine leukemia viral oncogene (c-Abl) is a putative key regulator of adipogenesis, but the underlying mechanism remained obscure. We found that c-Abl was activated during the early phase of mouse 3T3-L1 preadipocyte differentiation. Moreover, c-Abl activity was essential and its inhibition blocked differentiation to mature adipocytes. c-Abl directly controlled the expression and activity of the master adipogenic regulator peroxisome proliferator-activator receptor gamma 2 (PPARγ2). PPARγ2 physically associated with c-Abl and underwent phosphorylation on two tyrosine residues within its regulatory activation function 1 (AF1) domain. We demonstrated that this process positively regulates PPARγ2 stability and adipogenesis. Remarkably, c-Abl binding to PPARγ2 required the Pro12 residue that has a phenotypically well-studied common human genetic proline 12 alanine substitution (Pro12Ala) polymorphism. Our findings establish a critical role for c-Abl in adipocyte differentiation and explain the behavior of the known Pro12Ala polymorphism.

Minireview: Challenges and opportunities in development of PPAR agonists

The clinical impact of the fibrate and thiazolidinedione drugs on dyslipidemia and diabetes is driven mainly through activation of two transcription factors, peroxisome proliferator-activated receptors (PPAR)-α and PPAR-γ. However, substantial differences exist in the therapeutic and side-effect profiles of specific drugs. This has been attributed primarily to the complexity of drug-target complexes that involve many coregulatory proteins in the context of specific target gene promoters. Recent data have revealed that some PPAR ligands interact with other non-PPAR targets. Here we review concepts used to develop new agents that preferentially modulate transcriptional complex assembly, target more than one PPAR receptor simultaneously, or act as partial agonists. We highlight newly described on-target mechanisms of PPAR regulation including phosphorylation and nongenomic regulation. We briefly describe the recently discovered non-PPAR protein targets of thiazolidinediones, mitoNEET, and mTOT. Finally, we summarize the contributions of on- and off-target actions to select therapeutic and side effects of PPAR ligands including insulin sensitivity, cardiovascular actions, inflammation, and carcinogenicity.

2,4,5-trimethoxybenzaldehyde, a bitter principle in plants, suppresses adipogenesis through the regulation of ERK1

Because of the prevalence of obesity, there is particular interest in finding potential therapeutic targets. In a previous study, we demonstrated that 2,4,5-trimethoxybenzaldehyde (2,4,5-TMBA), a bitter principle in plants and a natural cyclooxygenase II (COX-2) inhibitor, suppressed the differentiation of preadipocyts into adipocytes at the concentration of 0.5 mM. In this current study, we aimed to investigate the stage during adipogenesis that is critically affected by 2,4,5-TMBA and the effects of 2,4,5-TMBA on the time-course expression of signaling molecules MAP kinase kinase (MAPKK, represented by MEK) and extracellular signal-regulated kinase (ERK), transcription factors CCAAT/enhancer binding protein (C/EBP)α, β, and δ and peroxisome proliferator-activated receptor (PPAR)γ, lipogenic enzymes acetyl-CoA carboxylase (ACC) and fatty acid synthase (FAS), and lipid droplet-coating protein perilipin A. When preadipocytes were co-cultured with 2,4,5-TMBA (0.5 mM) specifically at post-induction days 0-2, 2-4, 4-6, or 6-8 only, relative lipid accumulation was decreased by 67.93, 34.65, 49.56, and 34.32%, respectively. A time-course study showed that treatment of 2,4,5-TMBA suppressed the phosphorylation of ERK1 at the initial stage of adipogenesis but upregulated the phosphorylation at the late stage, which is opposite to the conditions required for the differentiation process. The overall expression of C/EBPα, β, and δ, PPARγ2, ACC, FAS, and perilipin A in preadipocytes was downregulated by the treatment of 2,4,5-TMBA. Taken together, our findings suggest that 2,4,5-TMBA suppresses adipogenesis through the regulation of ERK1 phosphorylation. Although results from in vitro studies cannot be directly extrapolated into clinical effects, our study will help to elucidate the anti-adipogenic potential of 2,4,5-TMBA.

Thiazolidinediones and the promise of insulin sensitization in type 2 diabetes

Type 2 diabetes is caused by insulin resistance coupled with an inability to produce enough insulin to control blood glucose, and thiazolidinediones (TZDs) are the only current antidiabetic agents that function primarily by increasing insulin sensitivity. However, despite clear benefits in glycemic control, this class of drugs has recently fallen into disuse due to concerns over side effects and adverse events. Here we review the clinical data and attempt to balance the benefits and risks of TZD therapy. We also examine potential mechanisms of action for the beneficial and harmful effects of TZDs, mainly via agonism of the nuclear receptor PPARγ. Based on critical appraisal of both preclinical and clinical studies, we discuss the prospect of harnessing the insulin sensitizing effects of PPARγ for more effective, safe, and potentially personalized treatments of type 2 diabetes.

Inhibition of HDAC3 promotes ligand-independent PPARγ activation by protein acetylation

Peroxisome proliferator-activated receptor gamma (PPARγ) is a nuclear receptor whose activation is dependent on a ligand. PPARγ activation by exogenous ligands, such as thiazolidinediones (TZDs), is a strategy in the treatment of type 2 diabetes mellitus for the improvement of insulin sensitivity. In addition to a ligand, PPARγ function is also regulated by posttranslational modifications, such as phosphorylation, sumoylation, and ubiquitination. Herein, we report that the PPARγ protein is modified by acetylation, which induces the PPARγ function in the absence of an external ligand. We observed that histone deacetylase 3 (HDAC3) interacted with PPARγ to deacetylate the protein. In immunoprecipitation assays, the HDAC3 protein was associated with the PPARγ protein. Inhibition of HDAC3 using RNAi-mediated knockdown or HDAC3 inhibitor increased acetylation of the PPARγ protein. Furthermore, inhibition of HDAC3 enhanced the expression of PPARγ target genes such as adiponectin and aP2. The expression was associated with an increase in glucose uptake and insulin signaling in adipocytes. HDAC3 inhibition enhanced lipid accumulation during differentiation of adipocytes. PPARγ acetylation was also induced by pioglitazone and acetylation was required for PPARγ activation. In the absence of TZDs, the acetylation from HDAC3 inhibition was sufficient to induce the transcriptional activity of PPARγ. Treating diet-induced obesity mice with HDAC3 inhibitor or pioglitazone for 2 weeks significantly improved high-fat-diet-induced insulin resistance. Our results indicate that acetylation of PPARγ is a ligand-independent mechanism of PPARγ activation. HDAC3 inhibitor is a potential PPARγ activator for the improvement of insulin sensitivity.

Phloretin promotes adipocyte differentiation in vitro and improves glucose homeostasis in vivo

Adipocyte dysfunction is associated with many metabolic diseases such as obesity, insulin resistance and diabetes. Previous studies found that phloretin promotes 3T3-L1 cells differentiation, but the underlying mechanisms for phloretin's effects on adipogenesis remain unclear. In this study, we demonstrated that phloretin enhanced the lipid accumulation in porcine primary adipocytes in a time-dependent manner. Furthermore, phloretin increased the utilization of glucose and nonesterified fatty acid, while it decreased the lactate output. Microarray analysis revealed that genes associated with peroxisome proliferator-activated receptor-γ (PPARγ), mitogen-activated protein kinase and insulin signaling pathways were altered in response to phloretin. We further confirmed that phloretin enhanced expression of PPARγ, CAAT enhancer binding protein-α (C/EBPα) and adipose-related genes, such as fatty acids translocase and fatty acid synthase. In addition, phloretin activated the Akt (Thr308) and extracellular signal-regulated kinase, and therefore, inactivated Akt targets protein. Wortmannin effectively blocked the effect of phloretin on Akt activity and the protein levels of PPARγ, C/EBPα and fatty acid binding protein-4 (FABP4/aP2). Oral administration of 5 or 10 mg/kg phloretin to C57BL BKS-DB mice significantly decreased the serum glucose level and improved glucose tolerance. In conclusion, phloretin promotes the adipogenesis of porcine primary preadipocytes through Akt-associated signaling pathway. These findings suggested that phloretin might be able to increase insulin sensitivity and alleviate the metabolic diseases.

Acetylation-defective mutant of Pparγ is associated with decreased lipid synthesis in breast cancer cells

In our prior publications we characterized a conserved acetylation motif (K(R)xxKK) of evolutionarily related nuclear receptors. Recent reports showed that peroxisome proliferator activated receptor gamma (PPARγ) deacetylation by SIRT1 is involved in delaying cellular senescence and maintaining the brown remodeling of white adipose tissue. However, it still remains unknown whether lysyl residues 154 and 155 (K154/155) of the conserved acetylation motif (RIHKK) in Pparγ1 are acetylated. Herein, we demonstrate that Pparγ1 is acetylated and regulated by both endogenous TSA-sensitive and NAD-dependent deacetylases. Acetylation of lysine 154 was identified by mass spectrometry (MS) while deacetylation of lysine 155 by SIRT1 was confirmed by in vitro deacetylation assay. An in vivo labeling assay revealed K154/K155 as bona fide acetylation sites. The conserved acetylation sites of Pparγ1 and the catalytic domain of SIRT1 are both required for the interaction between Pparγ1 and SIRT1. Sirt1 and Pparγ1 converge to govern lipid metabolism in vivo. Acetylation-defective mutants of Pparγ1 were associated with reduced lipid synthesis in ErbB2 overexpressing breast cancer cells. Together, these results suggest that the conserved lysyl residues K154/K155 of Pparγ1 are acetylated and play an important role in lipid synthesis in ErbB2-positive breast cancer cells.

Oxidative stress-induced alterations in PPAR-γ and associated mitochondrial destabilization contribute to kidney cell apoptosis

The mechanism(s) underlying renoprotection by peroxisome proliferator-activated receptor (PPAR)-γ agonists in diabetic and nondiabetic kidney disease are not well understood. Mitochondrial dysfunction and oxidative stress contribute to kidney disease. PPAR-γ upregulates proteins required for mitochondrial biogenesis. Our aim was to determine whether PPAR-γ has a role in protecting the kidney proximal tubular epithelium (PTE) against mitochondrial destabilisation and oxidative stress. HK-2 PTE cells were subjected to oxidative stress (0.2-1.0 mM H₂O₂) for 2 and 18 h and compared with untreated cells for apoptosis, mitosis (morphology/biomarkers), cell viability (MTT), superoxide (dihydroethidium), mitochondrial function (MitoTracker red and JC-1), ATP (luminescence), and mitochondrial ultrastructure. PPAR-γ, phospho-PPAR-γ, PPAR-γ coactivator (PGC)-1α, Parkin (Park2), p62, and light chain (LC)3β were investigated using Western blots. PPAR-γ was modulated using the agonists rosiglitazone, pioglitazone, and troglitazone. Mitochondrial destabilization increased with H₂O₂concentration, ATP decreased (2 and 18 h; P < 0.05), Mitotracker red and JC-1 fluorescence indicated loss of mitochondrial membrane potential, and superoxide increased (18 h, P < 0.05). Electron microscopy indicated sparse mitochondria, with disrupted cristae. Mitophagy was evident at 2 h (Park2 and LC3β increased; p62 decreased). Impaired mitophagy was indicated by p62 accumulation at 18 h (P < 0.05). PPAR-γ expression decreased, phospho-PPAR-γ increased, and PGC-1α decreased (2 h), indicating aberrant PPAR-γ activation and reduced mitochondrial biogenesis. Cell viability decreased (2 and 18 h, P < 0.05). PPAR-γ agonists promoted further apoptosis. In summary, oxidative stress promoted mitochondrial destabilisation in kidney PTE, in association with increased PPAR-γ phosphorylation. PPAR-γ agonists failed to protect PTE. Despite positive effects in other tissues, PPAR-γ activation appears to be detrimental to kidney PTE health when oxidative stress induces damage.

Ginkgo biloba extract inhibits oxidized low-density lipoprotein (oxLDL)-induced matrix metalloproteinase activation by the modulation of the lectin-like oxLDL receptor 1-regulated signaling pathway in human umbilical vein endothelial cells

BACKGROUND

The overexpression of matrix metalloproteinases (MMPs) induced by oxidized low-density lipoprotein (oxLDL) has been found in atherosclerotic lesions. Previous reports have identified that oxLDL, via the upregulation of lectin-like ox-LDL receptor 1 (LOX-1), modulates the expression of MMPs in endothelial cells. Ginkgo biloba extract (GbE), from Ginkgo biloba leaves, has often been considered as a therapeutic compound for cardiovascular and neurologic diseases. However, further investigation is needed to ascertain the probable molecular mechanisms underlying the antiatherogenic effects of GbE. The aim of this study was to investigate the effects of GbE on oxLDL-activated MMPs of human endothelial cells and to test the involvement of LOX-1 and protein kinase C (PKC)-α, extracellular signal-regulated kinase (ERK), and peroxisome proliferator-activated receptor-γ (PPAR-γ).

METHODS

Human umbilical vein endothelial cells were stimulated with oxLDL, with or without GbE treatment. LOX-1 signaling and MMPs expression were tested by Western blotting or activity assay. Further, protein expression levels of PKC-α, ERK, nuclear factor-κB, and PPAR-γ were investigated by Western blotting.

RESULTS

GbE inhibited the oxLDL-caused upregulation of MMP-1, MMP-2, and MMP-3. Pretreating with GbE reduced oxLDL-activated LOX-1 expression. Furthermore, pharmacologic inhibitors of free radicals, Ca(++), PKC, and GbE, inhibited the oxLDL-induced ERK and nuclear factor-κB activation. Lastly, GbE ameliorated the oxLDL-inhibited PPAR-γ function.

CONCLUSIONS

Data obtained in this study indicate that GbE actives its protective effects by regulating the LOX-1-mediated PKC-α/ERK/PPAR-γ/MMP pathway, resulting in the suppression of reactive oxygen species formation and, ultimately, the reduction of MMPs expression in endothelial cells treated with oxLDL.

PPARγ and PGC-1α as therapeutic targets in Parkinson's

The peroxisome proliferator-activated receptor gamma (PPARγ) is a ligand-activated transcriptional factor that belongs to the nuclear hormone receptor superfamily. PPARγ was initially identified through its role in the regulation of glucose and lipid metabolism and cell differentiation. It also influences the expression or activity of a number of genes in a variety of signalling networks. These include regulation of redox balance, fatty acid oxidation, immune responses and mitochondrial function. Recent studies suggest that the PPARγ agonists may serve as good candidates for the treatment of several neurodegenerative disorders including Parkinson’s disease (PD), Alzheimer’s disease, Huntington’s disease and amyotrophic lateral sclerosis, even though multiple etiological factors contribute to the development of these disorders. Recent reports have also signposted a role for PPARγ coactivator-1α (PGC-1α) in several neurodegenerative disorders including PD. In this review, we explore the current knowledge of mechanisms underlying the beneficial effects of PPARγ agonists and PGC-1α in models of PD.

Identification of Posttranslational Modifications in Peroxisome Proliferator-Activated Receptor γ Using Mass Spectrometry

Posttranslational modification (PTM) of proteins is critical for various cellular processes. However, there are few studies examining PTMs in specific proteins using unbiased approaches. Here we report the attempt to identify the PTMs in peroxisome proliferator-activated receptor γ (PPARγ) proteins using our previously established PTM analysis system. In this study, we identified several PTMs in exogenously expressed PPARγ2 proteins from 293T cells as well as endogenous PPARγ1 proteins from a Caco-2 colon cancer cell line. The identified PTMs include phosphorylation of serine 112 and serine 81 in PPARγ2 and PPARγ1, respectively, both of which are well-known mitogen-activated protein kinase- (MAP kinase-) mediated PTMs in PPARγ proteins, thus confirming our experimental approach. Furthermore, previously unknown PTMs were also identified, demonstrating that this method can be applied to find previously unidentified PTMs in PPARγ proteins and other proteins including nuclear receptors.

Nuclear MEK1 sequesters PPARγ and bisects MEK1/ERK signaling: a non-canonical pathway of retinoic acid inhibition of adipocyte differentiation

Uncontrolled adipogenesis and adipocyte proliferation have been connected to human comorbidities. Retinoic acid (RA) is known to inhibit adipocyte differentiation, however the underlying mechanisms have not been adequately understood. This study reports that RA acting as a ligand to RA receptors (RARs and RXRs) is not a sine qua non to the inhibition of adipogenesis. Our intriguing observation of a negative correlation between increased retinoylation and adipogenesis led us to explore retinoylated proteins in adipocytes. Exportin (CRM1) was found to be retinoylated, which in turn can affect the spatio-temporal regulation of the important signaling molecule mitogen-activated protein kinase kinase 1 (MEK1), likely by disrupting its export from the nucleus. Nuclear enrichment of MEK1 physically sequesters peroxisome proliferator-activated receptor gamma (PPARγ), the master regulator of adipogenesis, from its target genes and thus inhibits adipogenesis while also disrupting the MEK1-extracellular-signal regulated kinase (ERK) signaling cascade. This study is first to report the inhibition of adipocyte differentiation by retinoylation.

Expression of hepatic fat-specific protein 27 depends on the specific etiology of fatty liver

Fat-specific protein 27 gene (FSP27), isolated by screening for genes specifically expressed in fully differentiated mouse adipocytes, belongs to the cell death-inducing DNA fragmentation factor, alpha subunit-like effector family. FSP27 is induced in not only adipose tissue but also the liver of ob/ob mice, and it promotes the development of fatty liver. The FSP27 gene is expressed in a fatty liver-specific manner and is not detected in the normal mouse liver. FSP27 expression is directly regulated by the induction of the hepatic peroxisome proliferator-activated receptor γ (PPARγ) in ob/ob fatty liver. In the present study, expression of hepatic FSP27 mRNA was determined in non-genetic fatty liver models. The FSP27 gene was markedly induced in the high-fat- or methionine- and choline-deficient (MCD) diet-induced fatty liver, but it was not elevated in alcohol-induced fatty liver. Interestingly, the induction of FSP27 mRNA due to the MCD diet was independent of PPARγ levels and completely absent in the liver from PPARγ-null mice. These results suggest that FSP27 mRNA expression in the liver depends on the etiology of fatty liver.

FKBP51 reciprocally regulates GRα and PPARγ activation via the Akt-p38 pathway

FK506-binding protein 51 (FKBP51) is a negative regulator of glucocorticoid receptor-α (GRα), although the mechanism is unknown. We show here that FKBP51 is also a chaperone to peroxisome proliferator-activated receptor-γ (PPARγ), which is essential for activity, and uncover the mechanism underlying this differential regulation. In COS-7 cells, FKBP51 overexpression reduced GRα activity at a glucocorticoid response element-luciferase reporter, while increasing PPARγ activity at a peroxisome proliferator response element reporter. Conversely, FKBP51-deficient (knockout) (51KO) mouse embryonic fibroblasts (MEFs) showed elevated GRα but reduced PPARγ activities compared with those in wild-type MEFs. Phosphorylation is known to exert a similar pattern of reciprocal modulation of GRα and PPARγ. Knockdown of FKBP51 in 3T3-L1 preadipocytes increased phosphorylation of PPARγ at serine 112, a phospho-residue that inhibits activity. In 51KO cells, elevated phosphorylation of GRα at serines 220 and 234, phospho-residues that promote activity, was observed. Because FKBP51 is an essential chaperone to the Akt-specific phosphatase PH domain leucine-rich repeat protein phosphatase, Akt signaling was investigated. Elevated Akt activation and increased activation of p38 kinase, a downstream target of Akt that phosphorylates GRα and PPARγ, were seen in 51KO MEFs, causing activation and inhibition, respectively. Inactivation of p38 with PD169316 reversed the effects of FKBP51 deficiency on GRα and PPARγ activities and reduced PPARγ phosphorylation. Last, loss of FKBP51 caused a shift of PPARγ from cytoplasm to nucleus, as previously shown for GRα. A model is proposed in which FKBP51 loss reciprocally regulates GRα and PPARγ via 2 complementary mechanisms: activation of Akt-p38-mediated phosphorylation and redistribution of the receptors to the nucleus for direct targeting by p38.

FKBP51 controls cellular adipogenesis through p38 kinase-mediated phosphorylation of GRα and PPARγ

Glucocorticoid receptor-α (GRα) and peroxisome proliferator-activated receptor-γ (PPARγ) are critical regulators of adipogenic responses. We have shown that FK506-binding protein 51 (FKBP51) represses the Akt-p38 kinase pathway to reciprocally inhibit GRα but stimulate PPARγ by targeting serine 112 (PPARγ) and serines 220 and 234 (GRα). Here, this mechanism is shown to be essential for GRα and PPARγ control of cellular adipogenesis. In 3T3-L1 cells, FKBP51 was a prominent marker of the differentiated state and knockdown of FKBP51 showed reduced lipid accumulation and expression of adipogenic genes. Compared with wild-type (WT), FKBP51 knockout (51KO) mouse embryonic fibroblasts (MEFs) showed dramatic resistance to differentiation, with almost no lipid accumulation and greatly reduced adipogenic gene expression. These features were rescued by reexpression of FKBP51 in 51KO cells. 51KO MEFs exhibited reduced fatty acid synthase activity, increased sensitivity to GRα-induced lipolysis, and reduced PPARγ activity at adipogenic genes (adiponectin, CD36, and perilipin) but elevated GRα transrepression at these same genes. A p38 kinase inhibitor increased lipid content in WT cells and also restored lipid levels in 51KO cells, showing that elevated p38 kinase activity is a major contributor to adipogenic resistance in the 51KO cells. In 51KO cells, the S112A mutant of PPARγ and the triple S212A/S220A/S234A mutant of GRα both increased lipid accumulation, identifying these residues as targets of the FKBP51/p38 axis. Our combined investigations have uncovered FKBP51 as a key regulator of adipogenesis via the Akt-p38 pathway and as a potential target in the treatment of obesity and related disorders.

The chalcones cardamonin and flavokawain B inhibit the differentiation of preadipocytes to adipocytes by activating ERK

AIM

We searched for polyphenols capable of inhibiting the lipid accumulation in 3T3-L1 cells, and investigated the mechanisms of two effective chalcones cardamonin and flavokawain B on differentiation of preadipocytes.

METHOD AND RESULTS

We treated 3T3-L1 cells with a panel of 46 polyphenols and measured intracellular lipid accumulation by Sudan II staining. Four of them, including cardamonin and flavokawain B, inhibited lipid accumulation. In the further study, cardamonin and flavokawain B inhibited lipid accumulation by downregulating the expression of CCAAT/enhancer binding protein (C/EBP)-β, C/EBPα, and peroxisome proliferator-activated receptor-γ (PPARγ) at both mRNA and protein levels. Cardamonin and flavokawain B also increased phosphorylation of extracellular signal-regulated kinase (ERK) in the early phase of adipocyte differentiation. PD98059, an ERK inhibitor, restored C/EBPβ, PPARγ expression and intracellular lipid accumulation in adipocytes. Moreover, cardamonin and flavokawain B also modulated the secretion of C-reactive protein, dipeptidyl peptidase IV, interleukin-6, tumor necrosis factor-α and fibroblast growth factor-21 in mature adipocytes.

CONCLUSIONS

These results indicate that ERK activation and consequent downregulation of adipocyte-specific transcription factors are involved in the inhibitory effects of the chalcones cardamonin and flavokawain B on adipocyte differentiation. Moreover, cardamonin and flavokawain B are able to modulate secretion of adipokines in mature adipocytes.

Partial agonist, telmisartan, maintains PPARγ serine 112 phosphorylation, and does not affect osteoblast differentiation and bone mass

Peroxisome proliferator activated receptor gamma (PPARγ) controls both glucose metabolism and an allocation of marrow mesenchymal stem cells (MSCs) toward osteoblast and adipocyte lineages. Its activity is determined by interaction with a ligand which directs posttranscriptional modifications of PPARγ protein including dephosphorylation of Ser112 and Ser273, which results in acquiring of pro-adipocytic and insulin-sensitizing activities, respectively. PPARγ full agonist TZD rosiglitazone (ROSI) decreases phosphorylation of both Ser112 and Ser273 and its prolonged use causes bone loss in part due to diversion of MSCs differentiation from osteoblastic toward adipocytic lineage. Telmisartan (TEL), an anti-hypertensive drug from the class of angiotensin receptor blockers, also acts as a partial PPARγ agonist with insulin-sensitizing and a weak pro-adipocytic activity. TEL decreased ^S273pPPARγ and did not affect ^S112pPPARγ levels in a model of marrow MSC differentiation, U-33/γ2 cells. In contrast to ROSI, TEL did not affect osteoblast phenotype and actively blocked ROSI-induced anti-osteoblastic activity and dephosphorylation of ^S112pPPARγ. The effect of TEL on bone was tested side-by-side with ROSI. In contrast to ROSI, TEL administration did not affect bone mass and bone biomechanical properties measured by micro-indentation method and did not induce fat accumulation in bone, and it partially protected from ROSI-induced bone loss. In addition, TEL induced “browning” of epididymal white adipose tissue marked by increased expression of UCP1, FoxC2, Wnt10b and IGFBP2 and increased overall energy expenditure. These studies point to the complexity of mechanisms by which PPARγ acquires anti-osteoblastic and pro-adipocytic activities and suggest an importance of Ser112 phosphorylation status as being a part of the mechanism regulating this process. These studies showed that TEL acts as a full PPARγ agonist for insulin-sensitizing activity and as a partial agonist/partial antagonist for pro-adipocytic and anti-osteoblastic activities. They also suggest a relationship between PPARγ fat “browning” activity and a lack of anti-osteoblastic activity.

Low intensity pulsed ultrasound (LIPUS) influences the multilineage differentiation of mesenchymal stem and progenitor cell lines through ROCK-Cot/Tpl2-MEK-ERK signaling pathway

Mesenchymal stem cells (MSCs) are pluripotent cells that can differentiate into multilineage cell types, including adipocytes and osteoblasts. Mechanical stimulus is one of the crucial factors in regulating MSC differentiation. However, it remains unknown how mechanical stimulus affects the balance between adipogenesis and osteogenesis. Low intensity pulsed ultrasound (LIPUS) therapy is a clinical application of mechanical stimulus and facilitates bone fracture healing. Here, we applied LIPUS to adipogenic progenitor cell and MSC lines to analyze how multilineage cell differentiation was affected. We found that LIPUS suppressed adipogenic differentiation of both cell types, represented by impaired lipid droplet appearance and decreased gene expression of peroxisome proliferator-activated receptor γ2 (Pparg2) and fatty acid-binding protein 4 (Fabp4). LIPUS also down-regulated the phosphorylation level of peroxisome proliferator-activated receptor γ2 protein, inhibiting its transcriptional activity. In contrast, LIPUS promoted osteogenic differentiation of the MSC line, characterized by increased cell calcification as well as inductions of runt-related transcription factor 2 (Runx2) and Osteocalcin mRNAs. LIPUS induced phosphorylation of cancer Osaka thyroid oncogene/tumor progression locus 2 (Cot/Tpl2) kinase, which was essential for the phosphorylation of mitogen-activated kinase kinase 1 (MEK1) and p44/p42 extracellular signal-regulated kinases (ERKs). Notably, effects of LIPUS on both adipogenesis and osteogenesis were prevented by a Cot/Tpl2-specific inhibitor. Furthermore, effects of LIPUS on MSC differentiation as well as Cot/Tpl2 phosphorylation were attenuated by the inhibition of Rho-associated kinase. Taken together, these results indicate that mechanical stimulus with LIPUS suppresses adipogenesis and promotes osteogenesis of MSCs through Rho-associated kinase-Cot/Tpl2-MEK-ERK signaling pathway.

Evaluation of protein phosphorylation during adipogenesis

Adipocyte differentiation is a complex process that involves the sequential expression of various adipocyte-specific genes controlled by signaling pathways and transcription factors for which phosphorylation plays a crucial regulatory role. CCAAT/enhancer-binding proteins and peroxisome proliferator-activated receptors are the most important transcriptional regulators in adipogenesis, and the functions of these proteins are regulated by various phosphorylation events. Because cultured 3T3-L1 preadipocytes are commonly used as a model for adipocyte differentiation, we used these cells for a proteomic analysis to identify kinases, phosphatases, and phosphosites that participate in adipogenesis. In addition to the phosphoproteomic analysis, we provide a detailed description of Western blotting, an in vitro phosphorylation assay, enzyme-linked immunosorbent assay, and phosphorylation site mutagenesis to fully characterize the phosphorylation of proteins and verify their roles in adipogenesis.

CDK5-induced p-PPARγ(Ser 112) downregulates GFAP via PPREs in developing rat brain: effect of metal mixture and troglitazone in astrocytes

The peroxisome proliferator-activated receptor gamma (PPARγ), a group of ligand-activated transcriptional factors, is expressed in glial fibrillary acidic protein (GFAP)-immunoreactive astrocytes. Here, we investigated the role of PPARγ in regulating GFAP using a mixture of As, Cd and Pb (metal mixture, MM) that induces apoptosis and aberrant morphology in rat brain astrocytes. We observed a phospho PPARγ (serine 112 (S112)) (p-PPARγ (S112))-mediated downregulation of GFAP in the MM-exposed astrocytes. We validated this using pure PPARγ agonist, troglitazone (TZ). As reported with MM, TZ induced astrocyte damage owing to reduced GFAP. In silico analysis in the non-coding region of GFAP gene revealed two PPARγ response elements (PPREs); inverted repeat 10 and direct repeat 1 sequences. Gel shift and chromatin immunoprecipitation assays demonstrated enhancement in binding of p-PPARγ (S112) to the sequences, and luciferase reporter assay revealed strong repression of GFAP via PPREs, in response to both MM and TZ. This indicated that suppression in GFAP indeed occurs through direct regulation of these elements by p-PPARγ (S112). Signaling studies proved that MM, as well as TZ, activated the cyclin-dependent kinase 5 (CDK5) and enhanced its interaction with PPARγ resulting into increased p-PPARγ (S112). The p-CDK5 levels were dependent on proximal activation of extracellular signal-regulated protein kinase 1/2 and downstream Jun N-terminal kinase. Taken together, these results are the first to delineate downregulation of GFAP through genomic and non-genomic signaling of PPARγ. It also brings forth a resemblance of TZ with MM in terms of astrocyte disarray in developing brain.

MicroRNA-143 regulates adipogenesis by modulating the MAP2K5-ERK5 signaling

A better understanding of the molecular mechanisms that regulate adipose tissue-derived stromal cell (ADSC) differentiation could provide new insight into some adipose-tissue-related disease. The differentiation of ADSCs into adipocytes is a complex physiological process that includes clonal expansion, growth arrest, and terminal differentiation. Here the role of microRNA-143 (miR-143) during ADSC adipogenic differentiation was systematically investigated. We found that miR-143 expression was transiently decreased after adipogenic induction while increased from day 3 and peaked on day 7 after induction. We show for the first time that the role of miR-143 is not consistent in the differentiation process. The regulatory role depends on the differentiation stage that miR-143 acts on. When miR-143 is overexpressed during the clonal expansion stage, the adipogenic differentiation of ADSCs is inhibited, whereas the overexpression of miR-143 during the growth arrest stage or terminal differentiation stage promotes differentiation. Further we firstly demonstrate that miR-143 plays the modulational role by directly repressing MAP2K5, a key member of the MAPKK family in the MAPK signaling pathway. These findings suggest that miR-143 plays an important role in adipose tissue formation, with special implications for some metabolic disease in which the amount and/or function of adipose tissue is altered.

A comprehensive microRNA expression profile of the backfat tissue from castrated and intact full-sib pair male pigs

Background

It is widely known that castration has a significant effect on the accumulation of adipose tissue. microRNAs (miRNAs) are known to be involved in fat deposition and to be regulated by the androgen-induced androgen receptor (AR). However, there is little understanding of the relationship between miRNAs and fat deposition after castration. In this study, the high-throughput SOLiD sequencing approach was used to identify and characterize miRNA expression in backfat from intact and castrated full-sib male 23-week-old pigs. The patterns of adipogenesis and fat deposition were compared between castrated and intact male pigs.

Results

A total of 366 unique miRNA genes were identified, comprising 174 known pre-miRNAs and 192 novel pre-miRNAs. One hundred and sixty-seven pre-miRNAs were common to both castrated (F3) and intact (F4) male pig small RNA libraries. The novel pre-miRNAs encoded 153 miRNAs/miRNA*s and 141 miRNAs/miRNA*s in the F3 and F4 libraries, respectively. One hundred and seventy-seven miRNAs, including 45 up- and 132 down-regulated, had more than 2-fold differential expression between the castrated and intact male pigs (p-value < 0.001). Thirty-five miRNAs were further selected, based on the expression abundance and differentiation between the two libraries, to predict their targets in KEGG pathways. KEGG pathway analyses suggested that miRNAs differentially expressed between the castrated and intact male pigs are involved in proliferation, apoptosis, differentiation, migration, adipose tissue development and other important biological processes. The expression patterns of eight arbitrarily selected miRNAs were validated by stem-loop reverse-transcription quantitative polymerase chain reaction. These data confirmed the expression tendency observed with SOLiD sequencing. miRNA isomiRs and mirtrons were also investigated in this study. Mirtrons are a recently described category of miRNA relying on splicing rather than processing by the microprocessor complex to generate the RNAi pathway. The functions of miRNAs important for regulating fat deposition were also investigated in this study.

Conclusions

This study expands the number of fat-deposition-related miRNAs in pig. The results also indicate that castration can significantly affect the expression patterns of fat-related miRNAs. The differentially expressed miRNAs may play important roles in fat deposition after castration.

PPARγ1 phosphorylation enhances proliferation and drug resistance in human fibrosarcoma cells

Post-translational regulation plays a critical role in the control of cell growth and proliferation. The phosphorylation of peroxisome proliferator-activated receptor γ (PPARγ) is the most important post-translational modification. The function of PPARγ phosphorylation has been studied extensively in the past. However, the relationship between phosphorylated PPARγ1 and tumors remains unclear. Here we investigated the role of PPARγ1 phosphorylation in human fibrosarcoma HT1080 cell line. Using the nonphosphorylation (Ser84 to alanine, S84A) and phosphorylation (Ser84 to aspartic acid, S84D) mutant of PPARγ1, the results suggested that phosphorylation attenuated PPARγ1 transcriptional activity. Meanwhile, we demonstrated that phosphorylated PPARγ1 promoted HT1080 cell proliferation and this effect was dependent on the regulation of cell cycle arrest. The mRNA levels of cyclin-dependent kinase inhibitor (CKI) p21(Waf1/Cip1) and p27(Kip1) descended in PPARγ1(S84D) stable HT1080 cell, whereas the expression of p18(INK4C) was not changed. Moreover, compared to the PPARγ1(S84A), PPARγ1(S84D) up-regulated the expression levels of cyclin D1 and cyclin A. Finally, PPARγ1 phosphorylation reduced sensitivity to agonist rosiglitazone and increased resistance to anticancer drug 5-fluorouracil (5-FU) in HT1080 cell. Our findings establish PPARγ1 phosphorylation as a critical event in human fibrosarcoma growth. These findings raise the possibility that chemical compounds that prevent the phosphorylation of PPARγ1 could act as anticancer drugs.

The alliance of mesenchymal stem cells, bone, and diabetes

Bone fragility has emerged as a new complication of diabetes. Several mechanisms in diabetes may influence bone homeostasis by impairing the action between osteoblasts, osteoclasts, and osteocytes and/or changing the structural properties of the bone tissue. Some of these mechanisms can potentially alter the fate of mesenchymal stem cells, the initial precursor of the osteoblast. In this review, we describe the main factors that impair bone health in diabetic patients and their clinical impact.

Lanthanides inhibit adipogenesis with promotion of cell proliferation in 3T3-L1 preadipocytes

Lanthanides are widely used in various fields for industrial, agricultural and medical purposes. They have also been used in Chinese agriculture either as fertilizers in plant production or as performance-enhancers in animal nutrition for many years. In view of their possible application for growth enhancing purposes and new medical applications, detailed information on how lanthanides influence physiological processes in biological systems is indispensable. In the present work, the effects of lanthanides (LaCl3, CeCl3 and GdCl3) on cell proliferation and adipogenesis in 3T3-L1 preadipocytes were evaluated. The results demonstrate that lanthanides inhibit adipogenesis in 3T3-L1 preadipocytes, evidenced by decreased triglyceride content and expression of C/EBPα and PPARγ. Simultaneously, the results show that lanthanides can promote cell proliferation during the different stages of differentiation. Firstly, prior to the addition of differentiation inducers (MDI), all the three types of lanthanides resulted in a significant increase of cell growth. Secondly, during the mitotic clonal expansion (MCE) process, GdCl3, as a representative compound, is able to promote cell-cycle entry into the S phase and levels of cell cycle-regulating proteins. Third, at the late stage of the terminal differentiation, on day 8, in the presence of GdCl3, cells exhibited higher levels of G1/S regulatory proteins and proliferating cell nuclear antigen (PCNA). In addition, GdCl3 caused stronger sustained ERK activation during the differentiation process of 3T3-L1 cells. The present study demonstrates that lanthanides may modulate lipid metabolism by inhibition of adipocyte differentiation. The sustained activation of the ERK pathway might be responsible for their inhibition of differentiation and a possible link between their pro-proliferative ability and inhibition of the differentiation process is indicated.

Down-regulated peroxisome proliferator-activated receptor γ (PPARγ) in lung epithelial cells promotes a PPARγ agonist-reversible proinflammatory phenotype in chronic obstructive pulmonary disease (COPD)

Chronic obstructive pulmonary disease (COPD) is a progressive inflammatory condition and a leading cause of death, with no available cure. We assessed the actions in pulmonary epithelial cells of peroxisome proliferator-activated receptor γ (PPARγ), a nuclear hormone receptor with anti-inflammatory effects, whose role in COPD is largely unknown. We found that PPARγ was down-regulated in lung tissue and epithelial cells of COPD patients, via both reduced expression and phosphorylation-mediated inhibition, whereas pro-inflammatory nuclear factor-κB (NF-κB) activity was increased. Cigarette smoking is the main risk factor for COPD, and exposing airway epithelial cells to cigarette smoke extract (CSE) likewise down-regulated PPARγ and activated NF-κB. CSE also down-regulated and post-translationally inhibited the glucocorticoid receptor (GR-α) and histone deacetylase 2 (HDAC2), a corepressor important for glucocorticoid action and whose down-regulation is thought to cause glucocorticoid insensitivity in COPD. Treating epithelial cells with synthetic (rosiglitazone) or endogenous (10-nitro-oleic acid) PPARγ agonists strongly up-regulated PPARγ expression and activity, suppressed CSE-induced production and secretion of inflammatory cytokines, and reversed its activation of NF-κB by inhibiting the IκB kinase pathway and by promoting direct inhibitory binding of PPARγ to NF-κB. In contrast, PPARγ knockdown via siRNA augmented CSE-induced chemokine release and decreases in HDAC activity, suggesting a potential anti-inflammatory role of endogenous PPARγ. The results imply that down-regulation of pulmonary epithelial PPARγ by cigarette smoke promotes inflammatory pathways and diminishes glucocorticoid responsiveness, thereby contributing to COPD pathogenesis, and further suggest that PPARγ agonists may be useful for COPD treatment.

Ursodeoxycholic acid but not tauroursodeoxycholic acid inhibits proliferation and differentiation of human subcutaneous adipocytes

Stress of endoplasmic reticulum (ERS) is one of the molecular triggers of adipocyte dysfunction and chronic low inflammation accompanying obesity. ERS can be alleviated by chemical chaperones from the family of bile acids (BAs). Thus, two BAs currently used to treat cholestasis, ursodeoxycholic and tauroursodeoxycholic acid (UDCA and TUDCA), could potentially lessen adverse metabolic effects of obesity. Nevertheless, BAs effects on human adipose cells are mostly unknown. They could regulate gene expression through pathways different from their chaperone function, namely through activation of farnesoid X receptor (FXR) and TGR5, G-coupled receptor. Therefore, this study aimed to analyze effects of UDCA and TUDCA on human preadipocytes and differentiated adipocytes derived from paired samples of two distinct subcutaneous adipose tissue depots, abdominal and gluteal. While TUDCA did not alter proliferation of cells from either depot, UDCA exerted strong anti-proliferative effect. In differentiated adipocytes, acute exposition to neither TUDCA nor UDCA was able to reduce effect of ERS stressor tunicamycin. However, exposure of cells to UDCA during whole differentiation process decreased expression of ERS markers. At the same time however, UDCA profoundly inhibited adipogenic conversion of cells. UDCA abolished expression of PPARγ and lipogenic enzymes already in the early phases of adipogenesis. This anti-adipogenic effect of UDCA was not dependent on FXR or TGR5 activation, but could be related to ability of UDCA to sustain the activation of ERK1/2 previously linked with PPARγ inactivation. Finally, neither BAs did lower expression of chemokines inducible by TLR4 pathway, when UDCA enhanced their expression in gluteal adipocytes. Therefore while TUDCA has neutral effect on human preadipocytes and adipocytes, the therapeutic use of UDCA different from treating cholestatic diseases should be considered with caution because UDCA alters functions of human adipose cells.

Influence of dietary fat on intestinal microbes, inflammation, barrier function and metabolic outcomes

Recent studies using germ-free, gnotobiotic microbial transplantation/conventionalization or antibiotic treatment in rodent models have highlighted the critical role of intestinal microbes on gut health and metabolic functions of the host. Genetic and environmental factors influence the abundance and type of mutualistic vs. pathogenic bacteria, each of which has preferred substrates for growth and unique products of fermentation. Whereas some fermentation products or metabolites promote gut function and health, others impair gut function, leading to compromised nutrient digestion and barrier function that adversely impact the host. Such products may also influence food intake, energy harvest and expenditure, and insulin action, thereby influencing adiposity and related metabolic outcomes. Diet composition influences gut microbiota and subsequent fermentation products that impact the host, as demonstrated by prebiotic studies using oligosaccharides or other types of indigestible fiber. Recent studies also show that dietary lipids affect specific populations of gut microbes and their metabolic end products. This review will focus on studies examining the influence of dietary fat amount and type on the gut microbiome, intestinal health and positive and negative metabolic consequences. The protective role of omega-3-rich fatty acids on intestinal inflammation will also be examined.

Brd2 inhibits adipogenesis via the ERK1/2 signaling pathway in 3T3-L1 adipocytes

Bromodomain-containing protein 2 (Brd2) is a nuclear serine/threonine kinase involved in transcriptional regulation. In 3T3-L1 adipocytes, Brd2 normally co-represses PPARγ (peroxisome proliferator-activated receptor gamma) and inhibits adipogenesis. Here, we show that Brd2 over-expression in preadipocytes inhibits their differentiation into adipocytes, while Brd2 knockdown promotes adipogenic differentiation in vitro and forces cells to undergo adipogenesis independent of the MDI (methyisobutylxanthane, dexamethasone and insulin) induction. In this study, the two key transcription factors for adipogenesis, PPARγ and C/EBPα (CCAAT/enhancer binding protein-α) were persistently expressed during the differentiation of preadipocytes to mature adipocytes in Brd2 knockdown 3T3-L1 cells, but their expression was inhibited in cells in which Brd2 was overexpressed. To investigate the role of Brd2 in signal transduction we examined the expression of several signaling molecules involved in the regulation of gene expression and cell differentiation by immunoblotting assay. Down-regulation of Brd2 expression in 3T3-L1 cells led to a decrease in extracellular signal-regulated kinase1/2 (ERK1/2) activity and, conversely, the up-regulation of Brd2 leads to increase in ERK1/2 phosphorylation. Nevertheless, changes in Brd2 expression do not affect the activities of JNK and p38 MAPK. In addition, the phosphorylation of Rafs is not affected by changes in Brd2 expression in 3T3-L1 cells. MEK inhibitor UO126 partly restores differentiation of 3T3-L1 cells that overexpress Brd2. In conclusion, these results indicate that Brd2 regulates ERK1/2 activity independently of Raf signaling in 3T3-L1 adipocytes.

The anti-adipogenic effect of PGRN on porcine preadipocytes involves ERK1,2 mediated PPARγ phosphorylation

Recent researches indicate that PGRN is closely related to diabetes and is regarded as a novel adipokine associated with obesity development, affecting adipocyte biology. In the present study, we investigated the effects and mechanisms of PGRN on porcine preadipocytes differentiation. Porcine preadipocytes were induced to differentiation with the addition of lentivirius-expressed PGRN shRNA at the early or late stage of induction period, and in the presence or absence of recombinant PGRN protein. The effects of PGRN on adipogenic genes expression and ERK activation were investigated. At the early stage of induction, knockdown of PGRN promoted differentiation, evidenced by enhanced lipid accumulation, upregulation of adipocyte markers, as well as master adipogenic transcription factors, PPARγ and C/EBPα. While, decreasing PGRN expression at the late stage of induction (day 3) had no effect on differentiation. These results suggested that PGRN functions in the early adipogenic events. Conversely, porcine preadipocytes differentiation was impaired by MDI and recombinant PGRN protein induction, the expressions of adipocyte markers were decreased. Further studies revealed that PGRN can specifically facilitate ERK1,2 activation, and this activation can be abolished by U0126. Moreover, PPARγ phosphorylation at serine 112 site was increased by PGRN treatment, which could reduce the transcriptional activity of PPARγ. We conclude that PGRN inhibits adipogenesis in porcine preadipocytes partially through ERK activation mediated PPARγ phosphorylation.

Herpesvirus-associated ubiquitin-specific protease (HAUSP) modulates peroxisome proliferator-activated receptor γ (PPARγ) stability through its deubiquitinating activity

The peroxisome proliferator-activated receptor γ (PPARγ) is a central regulator of adipogenesis and modulates glucose and lipid metabolism. In this study, herpesvirus-associated ubiquitin-specific protease (HAUSP) was isolated as a binding partner of PPARγ. Both endogenous and exogenous PPARγ associated with HAUSP in co-immunoprecipitation analysis. HAUSP, but not the catalytically inactive HAUSP C223S mutant, increased the stability of both endogenous and exogenous PPARγ through its deubiquitinating activity. Site-directed mutagenesis experiments showed that the Lys(462) residue of PPARγ is critical for ubiquitination. HBX 41,108, a specific inhibitor of HAUSP, abolished the increase in PPARγ stability induced by HAUSP. In addition, knockdown of endogenous HAUSP using siRNA decreased PPARγ protein levels. HAUSP enhanced the transcriptional activity of both exogenous and endogenous PPARγ in luciferase activity assays. Quantitative RT-PCR analysis showed that HAUSP increased the transcript levels of PPARγ target genes in HepG2 cells, resulting in the enhanced uptake of glucose and fatty acids, and vice versa, upon siRNA knockdown of HAUSP. In vivo analysis using adenoviruses confirmed that HAUSP, but not the HAUSP C223S mutant, decreased blood glucose and triglyceride levels, which are associated with the increased expression of endogenous PPARγ and lipid accumulation in the liver. Our results demonstrate that the stability and activity of PPARγ are modulated by the deubiquitinating activity of HAUSP, which may be a target for the development of anti-diabetic drugs.