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  • Abstract: Adipose x
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Rui Wang, Jie Hong, Ruixin Liu, Maopei Chen, Min Xu, Wiqiong Gu, Yifei Zhang, Qinyun Ma, Feng Wang, Juan Shi, Jiqiu Wang, Weiqing Wang and Guang Ning

WNT/β-catenin signalling is involved in regulating adipogenesis, and its dysregulation occurs in obesity. Secreted frizzled-related protein 5 (SFRP5) is a WNT protein inhibitor; however, its role in adipogenesis and obesity is controversial. In this study, we observed that SFRP5 mRNA levels were increased in the fat tissues of obese humans and mice. Sfrp5 expression was gradually induced during differentiation of white and brown adipocytes and was highly increased in mature adipocytes rather than preadipocytes. However, the effects of the exogenous overexpression of Sfrp5 indicated that Sfrp5 may not directly regulate adipogenesis in vitro under the conditions studied. Moreover, SFRP5 did not inhibit the canonical WNT/β-catenin signalling pathway in preadipocytes. Subsequently, we measured the levels of circulating SFRP5 in obese patients and non-obese subjects using ELISA and did not find any significant difference. Collectively, these findings indicate that Sfrp5 represents a candidate for a mature adipocyte marker gene. Our data provide new evidence concerning the role of SFRP5 in adipogenesis of white and brown adipocytes and obesity.

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Jason F Ohlstein, Amy L Strong, John A McLachlan, Jeffrey M Gimble, Matthew E Burow and Bruce A Bunnell

Exposure of humans to the endocrine disrupter bisphenol A (BPA) has been associated with increased weight and obesity. However, the mechanism(s) by which BPA increases adipose tissue in humans remains to be determined. The goal of this study was to determine the effects of BPA on adipogenesis of cultured human adipose stromal/stem cells (ASCs), precursors to mature adipocytes. ASCs from three donors were cultured for either 14 or 21 days in adipogenic differentiation media containing increasing concentrations of BPA (100 pM–10 μM). The extent of adipogenic differentiation in the ASCs was assessed by staining with Oil Red O to visualize adipogenic differentiation and then quantified by extraction and optical density measurement of the retained dye. BPA significantly enhanced adipogenesis at a concentration of 1 μM after 21 days of culture. Additionally, we found that BPA increased transcription of the estrogen receptor (ER (ESR1)) and that treatment with the ER antagonist ICI 182 780, blocked the effects of BPA, indicating that BPA may act via an ER-mediated pathway. The results of molecular analyses indicated that the expression of the adipogenesis-associated genes dual leucine zipper-bearing kinase (DLK (MAP3K12)), IGF1, CCAAT/enhancer-binding protein alpha (C/EBP α (CEBPA)), peroxisome proliferator-activated receptor gamma (PPAR γ (PPARG)), and lipoprotein lipase (LPL) was temporally accelerated and increased by BPA. In summary, these results indicate that BPA significantly enhances adipogenesis in ASCs through an ER-mediated pathway at physiologically relevant concentrations.

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Stephen Fitter, Kate Vandyke, Stan Gronthos and Andrew C W Zannettino

Improved glucose and lipid metabolism is a unique side effect of imatinib therapy in some chronic myeloid leukaemia (CML) patients. We recently reported that plasma levels of adiponectin, an important regulator of insulin sensitivity, are elevated following imatinib therapy in CML patients, which could account for these improved metabolic outcomes. Adiponectin is secreted exclusively from adipocytes, suggesting that imatinib modulates adiponectin levels directly, by transcriptional upregulation of adiponectin in pre-existing adipocytes, and/or indirectly, by stimulating adipogenesis. In this report, we have demonstrated that imatinib promotes adipogenic differentiation of human mesenchymal stromal cells (MSCs), which in turn secrete high-molecular-weight adiponectin. Conversely, imatinib does not stimulate adiponectin secretion from mature adipocytes. We hypothesise that inhibition of PDGFRα (PDGFRA) and PDGFRβ (PDGFRB) is the mechanism by which imatinib promotes adipogenesis. Supporting this, functional blocking antibodies to PDGFR promote adipogenesis and adiponectin secretion in MSC cultures. We have shown that imatinib is a potent inhibitor of PDGF-induced PI3 kinase activation and, using a PI3 kinase p110α-specific inhibitor (PIK-75), we have demonstrated that suppression of this pathway recapitulates the effects of imatinib on MSC differentiation. Furthermore, using mitogens that activate the PI3 kinase pathway, or MSCs expressing constitutively activated Akt, we have shown that activation of the PI3 kinase pathway negates the pro-adipogenic effects of imatinib. Taken together, our results suggest that imatinib increases plasma adiponectin levels by promoting adipogenesis through the suppression of PI3 kinase signalling downstream of PDGFR.

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Matthias Laudes

The development of obesity is characterised not only by increased storage of lipids in existing fat cells but also by the generation of new adipocytes from progenitor cells. This process, called adipogenesis, can be divided into two related steps. First, during determination, multipotent mesenchymal stem cells commit to preadipocytes. These cells exhibit similar morphology compared with stem cells; however, they are committed to the adipogenic lineage and are not longer able to transform into osteoblasts, myocytes or chondrocytes. Secondly, during differentiation, preadipocytes become mature fat cells. As in other developmental processes, adipogenesis is tightly regulated at a molecular level by several transcription factors. Within the last decade, it has also become clear how the activity of these transcription factors is coordinated by extracellular signals. In this respect, secreted WNT signalling molecules are particularly important. Several members of the WNT family have been shown to inhibit early steps of adipogenesis. Conversely, endogenous inhibitors of WNT signalling were found to promote generation of adipocytes, indicating a fundamental role of these bioactive peptides in adipogenesis. From a pathophysiological point of view, it is of interest that polymorphisms in genes of the WNT signalling system have been associated with the development of obesity and type 2 diabetes in humans. Moreover, recent findings indicate that certain WNT molecules are involved in the so-called low-grade inflammation of adipose tissue, which is crucial in the development of obesity-associated insulin resistance. These important findings in nutritional and metabolic medicine will be summarised in the present review.

Open access

Rihua Zhang, Dongming Su, Weidong Zhu, Qiong Huang, Menglan Liu, Yi Xue, Yuanyuan Zhang, Dong li, Allan Zhao and Yun Liu

The aim of this study is to determine the effects of E2 on metabolic syndrome and the molecular mechanisms involving S100A16. Ovariectomized (OVX) rat models and mouse embryonic fibroblasts cell models were used. E2 loss in OVX rats induced body weight gain and central abdominal fat accumulation, which were ameliorated by E2 treatment under chow and high-fat diet (HFD) conditions. E2 decreased the expression of the adipocyte marker genes PPAR γ, aP2, C/EBP α, and S100A16. E2 inhibited adipogenesis. Overexpression of S100A16 reversed the E2-induced adipogenesis effect. A luciferase assay showed that E2 inhibited the expression of S100A16. E2 treatment decreased body weight gain and central abdominal fat accumulation under both chow and HFD conditions. Also, E2 suppressed adipogenesis by inhibiting S100A16 expression.

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Chen Chen, Yongdong Peng, Yinglin Peng, Jian Peng and Siwen Jiang

MicroRNAs are endogenous, conserved, and non-coding small RNAs that function as post-transcriptional regulators of fat development and adipogenesis. Adipogenic marker genes, such as CCAAT/enhancer binding protein α (Cebpa), peroxisome proliferator-activated receptor γ (Pparg), adipocyte fatty acid binding protein (Ap2), and fatty acid synthase (Fas), are regarded as the essential transcriptional regulators of preadipocyte differentiation and lipid storage in mature adipocytes. Canonical Wnt/β-catenin signaling is recognized as a negative molecular switch during adipogenesis. In the present work we found that miR-135a-5p is markedly downregulated during the process of 3T3-L1 preadipocyte differentiation. Overexpression of miR-135a-5p impairs the expressions of adipogenic marker genes as well as lipid droplet accumulation and triglyceride content, indicating the importance of miR-135a-5p for adipogenic differentiation and adipogenesis. Further studies show that miR-135a-5p directly targets adenomatous polyposis coli (Apc), contributes to the translocation of β-catenin from cytoplasm to nucleus, and then activates the expressions of cyclin D1 (Ccnd1) and Cmyc, indicating the induction of canonical Wnt/β-catenin signaling. In addition, inhibition of APC with siRNA exhibits the same effects as overexpression of miR-135a-5p. Our findings demonstrate that miR-135a-5p suppresses 3T3-L1 preadipocyte differentiation and adipogenesis through the activation of canonical Wnt/β-catenin signaling by directly targeting Apc. Taken together, these results offer profound insights into the adipogenesis mechanism and the development of adipose tissue.

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D Pasquali, GM Pierantoni, A Fusco, S Staibano, V Colantuoni, A De Bellis, A Bellastella and AA Sinisi

Expansion of adipose tissue in the orbits is a key feature of Graves' ophthalmopathy. Recent evidence shows that orbital fibroblasts are committed to differentiate into adipocytes under appropriate stimuli. Rosiglitazone, an agonist of the nuclear hormone receptor, peroxisome proliferator-activated receptor gamma (PPARgamma) is able to induce both differentiation of orbital fibroblasts into mature adipocytes and expression of the TSH receptor (TSHr) gene. Several studies have suggested an important role of the high mobility group AT-hook 2 (HMGA2) gene in adipocytic cell growth and development. To investigate further the association between adipogenesis-related genes and orbital fibroblasts, we treated fibroblasts from Graves' ophthalmopathy (FGOs) and from normal orbital tissues with fenofibrate, a specific agonist for PPARalpha. We then evaluated the expression of the PPARalpha, PPARgamma2, HMGA2, leptin and TSHr genes before and after 24 h of fenofibrate treatment, using semiquantitative and real-time PCR. For up to 96 h after exposure to fenofibrate, FGOs differentiated into adipocytes. PPARalpha and PPARgamma2 were expressed more in FGOs than in normal cultures, whereas TSHr mRNA was detected only in FGOs. Expression of HMGA2 mRNA and protein was significantly increased in FGOs from 6 to 24 h after fenofibrate, confirming its role in the early phase of adipocyte differentiation. Treatment with fenofibrate for 24 h significantly increased the expression of leptin and TSHr genes. Moreover, TSH treatment significantly increased the accumulation of cAMP, demonstrating that FGOs express functional TSHr. The high level of expression of PPARalpha other than PPARgamma2 transcripts and the stimulating effect of fenofibrate on adipogenesis and on HMGA2, leptin and TSHr genes also indicate that the PPARalpha pathway plays an important part in the adipocyte differentiation of FGOs. These findings suggest that novel drugs to antagonize PPARalpha, other than the PPARgamma signalling system, may also need to be considered in the treatment or prevention of Graves' ophthalmopathy.

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Masanobu Kawai, Noriyuki Namba, Sotaro Mushiake, Yuri Etani, Riko Nishimura, Makoto Makishima and Keiichi Ozono

Growth hormone-deficient (GHD) patients show a decreased number of adipocytes, which is normalized by GH replacement, indicating an adipogenic effect of GH. However, the precise mechanisms underlying this effect remain to be clarified. In this study, we investigated the adipogenic effect of GH. GH stimulated MDI (3-isobutyl-1-methylxanthine, dexamethasone, and insulin)-induced adipogenesis of 3T3-L1 cells with early induction of peroxisome proliferator-activated receptors (PPAR)γ2 expression. This adipogenic effect of GH was suppressed by overexpression of Stat5A mutant (Stat5A-Y694F), a transcriptional suppressor for the GH–Stat5A/5B signaling pathway, with the reduction of PPARγ2 expression. Next, we investigated the relationship between Stat5A/5B and CCAAT/enhancer binding protein (C/EBP)β/δ orPPARγ in 3T3-L1 cells. Stat5A/5B stimulated C/EBPβ- and C/EBPδ-induced adipogenesis with enhancement of PPARγ2 expression. In addition, Stat5A/5B enhanced the transcriptional activity of C/EBPβ/δ in the PPARγ gene promoter. Furthermore, Stat5A/5B stimulated PPARγ-induced adipogenesis and enhanced the transcriptional activity of PPARγ. These results suggest that the GH–Stat5A/5B signaling pathway stimulates adipogenesis in cooperation with C/EBPβ/δ and PPARγ. To completely understand the effect of GH, cDNA microarray analysis was performed to screen genes affected by GH during MDI-induced adipogenesis. Among 4277 genes, 18 and 19 genes were up- and down-regulated respectively. cDNA microarray analysis also indicated the up-regulation of PPARγ and the modulation of expression of genes coding for growth factors or growth factor receptors, suggesting that GH stimulates adipogenesis in association with the modulation of cell growth. Thus, the GH–Stat5A/B signaling pathway stimulates adipogenesis through two distinct steps. In addition, cDNA microarray data provide us the further insights underlying the adipogenic effect of GH.

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Alok Mishra, Xu-guang Zhu, Kai Ge and Sheue-Yann Cheng

To understand the roles of thyroid hormone receptors (TRs) in adipogenesis, we adopted a loss-of-function approach. We generated 3T3-L1 cells stably expressing either TRα1 mutant (TRα1PV) or TRβ1 mutant (TRβ1PV). TRα1PV and TRβ1PV are dominant negative mutations with a frameshift in the C-terminal amino acids. In control cells, the thyroid hormone, tri-iodothyronine (T3), induced a 2.5-fold increase in adipogenesis in 3T3-L1 cells, as demonstrated by increased lipid droplets. This increase was mediated by T3-induced expression of the peroxisome proliferator-activated receptor γ (PPARγ) and CCAAT/enhancer-binding protein α (C/EBPα), which are master regulators of adipogenesis at both the mRNA and protein levels. In 3T3-L1 cells stably expressing TRα1PV (L1-α1PV cells) or TRβ1PV (L1-β1PV cells), adipogenesis was reduced 94 or 54% respectively, indicative of differential inhibitory activity of mutant TR isoforms. Concordantly, the expression of PPARγ and C/EBPα at the mRNA and protein levels was more repressed in L1-α1PV cells than in L1-β1PV cells. In addition, the expression of PPARγ downstream target genes involved in fatty acid synthesis – the lipoprotein lipase (Lpl) and aP2 involved in adipogenesis – was more inhibited by TRα1PV than by TRβ1PV. Chromatin immunoprecipitation assays showed that TRα1PV was more avidly recruited than TRβ1PV to the promoter to preferentially block the expression of the C/ebpα gene. Taken together, these data indicate that impaired adipogenesis by mutant TR is isoform dependent. The finding that induction of adipogenesis is differentially regulated by TR isoforms suggests that TR isoform-specific ligands could be designed for therapeutic intervention for lipid abnormalities.

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Maria Cernea, Wei Tang, Haiyan Guan and Kaiping Yang

Adipose tissue expansion, resulting from adipocyte hyperplasia and/or hypertrophy, is a hallmark of obesity. Adipocytes are derived from mesenchymal stem cells (MSCs) through adipogenesis, a process involving three key steps: proliferation, commitment and differentiation. Although studies have elaborated on the mechanisms regulating adipocyte commitment and differentiation, the factors that control MSC proliferation remain largely unknown. Previously, we demonstrated that bone morphogenetic protein 3 (Bmp3), the expression of which was upregulated in our rat model of hyperplasic visceral adiposity, potently stimulated MSC proliferation. In the present study, we investigate the molecular target of Bmp3. We conducted DNA microarray analysis on MSCs treated with and without Bmp3 and identified WNT1-inducible signaling pathway protein 1 (Wisp1) as a differentially expressed gene, whose expression was upregulated 3.7-fold by Bmp3. Wisp1 is a proliferative agent in various non-adipose cell types and is implicated in adipogenesis. Therefore, we tested the hypothesis that Wisp1 mediates Bmp3 stimulation of MSC proliferation. We showed that Bmp3 increased the expression of Wisp1 as early as 3 h following Bmp3 treatment in MSCs. Importantly, the upregulated Wisp1 expression preceded Bmp3-induced MSC proliferation, as determined by [3H]-thymidine incorporation. Furthermore, treatment of MSCs with recombinant Wisp1 led to a concentration-dependent increase in [3H]-thymidine incorporation with a maximal increase of 300%. In addition, siRNA-mediated knockdown of Wisp1 expression attenuated Bmp3-induced MSC proliferation. Taken together, our present findings reveal Wisp1 as a novel target of Bmp3 and suggest that the Bmp3/Wisp1 signaling pathway play a key role in MSC proliferation, and consequently adipogenesis.