Wnt signaling and adipocyte lineage commitment

Interaction between IGF binding protein-3 and TGFβ in the regulation of adipocyte differentiation

The development of white adipose tissue involves both the hypertrophy of existing adipocytes and the proliferation and differentiation of preadipocytes. Adipogenic differentiation is inhibited by TGFβ signaling through Smad2/3, and IGF binding protein-3 (IGFBP-3) is also known to activate Smad2/3 signaling in some cell types. We previously reported that exogenous or overexpressed IGFBP-3 inhibits adipogenesis in 3T3-L1 cells, but the role of endogenous IGFBP-3 in this process, and its possible interaction with TGFβ, is not known. During 10-d adipogenic differentiation initiated by insulin, dexamethasone, and 3-isobutyl-1-methylxanthine, 3T3-L1 cells expressed increasing levels of IGFBP-3 and TGFβ1, secreting over 1000 pg/ml of both proteins. Exogenous recombinant human IGFBP-3 paralleled TGFβ1 in stimulating Smad2 phosphorylation in 3T3-L1 preadipocytes, but no additive effect was observed for the two agents. In contrast, knockdown of endogenous IGFBP-3 by small interfering RNA (siRNA) significantly impaired Smad2 activation by 0.25 ng/ml TGFβ1. Transient expression of human IGFBP-3 significantly inhibited the induction of adipogenic markers adiponectin and resistin, and the appearance of lipid droplets, but down-regulation of endogenous IGFBP-3 by siRNA had little effect on the expression of either marker during the 10-d differentiation, compared with nonsilencing control siRNA. However, down-regulation of endogenous IGFBP-3 using two different siRNA significantly reversed the inhibitory effect of TGFβ1 on both adiponectin and resistin induction. We conclude that IGFBP-3 activates inhibitory Smad signaling in 3T3-L1 cells and that endogenous IGFBP-3 modulates their adipogenic differentiation by regulating cell sensitivity towards the inhibitory effect of TGFβ.

ZNF281 knockdown induced osteogenic differentiation of human multipotent stem cells in vivo and in vitro

ZNF281 is one of the core transcription factors in embryonic stem cells (ESCs) and has activation and repression roles in the transcription of ESC genes. A known target molecule of Zfp281 (the mouse homologue of ZNF281) is Nanog. However, NANOG is not expressed in most human multipotent stem cells (hMSCs). Here, we investigated the roles of ZNF281 with a gain- and loss-of-function study. The knockdown of ZNF281 in vivo and in vitro resulted in spontaneous osteochondrogenic differentiation and reduced the proliferation of hMSCs, as determined by cell morphology and molecular markers. When ZNF281-knockdown hMSCs were subcutaneously implanted into mice along with β-tricalcium phosphate (β-TCP), many cells were converted into osteoblasts within 4 weeks. In contrast, the overexpression of ZNF281 in hMSCs resulted in accelerated proliferation. The expression pattern of ZNF281 correlated well with the expression of β-CATENIN during differentiation and in the gain/loss-of-function study in hMSCs. The binding of ZNF281 to the promoter region of β-CATENIN was observed using a chromatin immunoprecipitation (ChIP) assay. In conclusion, we propose that ZNF281 plays an important role in the maintenance and osteogenic differentiation of stem cells via the transcriptional regulation of genes including β-CATENIN.

The clock gene, brain and muscle Arnt-like 1, regulates adipogenesis via Wnt signaling pathway

Circadian clocks in adipose tissue are known to regulate adipocyte biology. Although circadian dysregulation is associated with development of obesity, the underlying mechanism has not been established. Here we report that disruption of the clock gene, brain and muscle Arnt-like 1 (Bmal1), in mice led to increased adipogenesis, adipocyte hypertrophy, and obesity, compared to wild-type (WT) mice. This is due to its cell-autonomous effect, as Bmal1 deficiency in embryonic fibroblasts, as well as stable shRNA knockdown (KD) in 3T3-L1 preadipocyte and C3H10T1/2 mesenchymal stem cells, promoted adipogenic differentiation. We demonstrate that attenuation of Bmal1 function resulted in down-regulation of genes in the canonical Wnt pathway, known to suppress adipogenesis. Promoters of these genes (Wnt10a, β-catenin, Dishevelled2, TCF3) displayed Bmal1 occupancy, indicating direct circadian regulation by Bmal1. As a result, Wnt signaling activity was attenuated by Bmal1 KD and augmented by its overexpression. Furthermore, stabilizing β-catenin through Wnt ligand or GSK-3β inhibition achieved partial restoration of blunted Wnt activity and suppression of increased adipogenesis induced by Bmal1 KD. Taken together, our study demonstrates that Bmal1 is a critical negative regulator of adipocyte development through transcriptional control of components of the canonical Wnt signaling cascade, and provides a mechanistic link between circadian disruption and obesity.

The WNT inhibitor Dickkopf 1 and bone morphogenetic protein 4 rescue adipogenesis in hypertrophic obesity in humans

Overweight characterized by inappropriate expansion of adipose cells (hypertrophic obesity) is associated with the metabolic syndrome and is caused by an inability to recruit and differentiate new precursor cells. We examined the role of bone morphogenetic protein 4 (BMP4) and WNT activation in the regulation of human adipose cell differentiation. Cluster of differentiation (CD)14(+)/45(+) and CD31(+) cells were first removed before the remaining stromal vascular cells of human subcutaneous biopsy specimens were differentiated with/without different WNT inhibitors and/or BMP4. Inhibition of WNT and induction of Dickkopf 1 (DKK1) were markers of precursor cells undergoing excellent differentiation. The addition of DKK1 inhibited WNT activation and promoted adipogenesis in cells with a low degree of differentiation. The positive effect of DKK1, inhibiting cellular WNT activation by binding to the Kremen/LDL receptor-related protein receptors, was not seen with inhibitors of secreted WNT ligands. BMP4 increased differentiation, and BMP4 in the presence of DKK1 produced an additive effect. There was an apparent cross-talk between differentiation and commitment because BMP4 expression increased in differentiating adipocytes, and the addition of the BMP4 inhibitor, Noggin, reduced precursor cell differentiation. Thus, differentiated human adipose cells can promote adipogenesis via endogenous BMP4 activation, and the impaired adipogenesis in hypertrophic obesity is mainly due to an inability to suppress canonical WNT and to induce DKK1.

Adipogenesis: from stem cell to adipocyte

Excessive caloric intake without a rise in energy expenditure promotes adipocyte hyperplasia and adiposity. The rise in adipocyte number is triggered by signaling factors that induce conversion of mesenchymal stem cells (MSCs) to preadipocytes that differentiate into adipocytes. MSCs, which are recruited from the vascular stroma of adipose tissue, provide an unlimited supply of adipocyte precursors. Members of the BMP and Wnt families are key mediators of stem cell commitment to produce preadipocytes. Following commitment, exposure of growth-arrested preadipocytes to differentiation inducers [insulin-like growth factor 1 (IGF1), glucocorticoid, and cyclic AMP (cAMP)] triggers DNA replication and reentry into the cell cycle (mitotic clonal expansion). Mitotic clonal expansion involves a transcription factor cascade, followed by the expression of adipocyte genes. Critical to these events are phosphorylations of the transcription factor CCATT enhancer-binding protein β (C/EBPβ) by MAP kinase and GSK3β to produce a conformational change that gives rise to DNA-binding activity. "Activated" C/EBPβ then triggers transcription of peroxisome proliferator-activated receptor-γ (PPARγ) and C/EBPα, which in turn coordinately activate genes whose expression produces the adipocyte phenotype.

Low density lipoprotein (LDL) receptor-related protein 6 (LRP6) regulates body fat and glucose homeostasis by modulating nutrient sensing pathways and mitochondrial energy expenditure

Body fat, insulin resistance, and type 2 diabetes are often linked together, but the molecular mechanisms that unify their association are poorly understood. Wnt signaling regulates adipogenesis, and its altered activity has been implicated in the pathogenesis of type 2 diabetes and metabolic syndrome. LRP6(+/-) mice on a high fat diet were protected against diet-induced obesity and hepatic and adipose tissue insulin resistance compared with their wild-type (WT) littermates. Brown adipose tissue insulin sensitivity and reduced adiposity of LRP6(+/-) mice were accounted for by diminished Wnt-dependent mTORC1 activity and enhanced expression of brown adipose tissue PGC1-α and UCP1. LRP6(+/-) mice also exhibited reduced endogenous hepatic glucose output, which was due to diminished FoxO1-dependent expression of the key gluconeogenic enzyme glucose-6-phosphatase (G6pase). In addition, in vivo and in vitro studies showed that loss of LRP6 allele is associated with increased leptin receptor expression, which is a likely cause of hepatic insulin sensitivity in LRP6(+/-) mice. Our study identifies LRP6 as a nutrient-sensitive regulator of body weight and glucose metabolism and as a potential target for pharmacological interventions in obesity and diabetes.

MicroRNA identity and abundance in developing swine adipose tissue as determined by Solexa sequencing

MicroRNAs (miRNAs) are small ∼22-nt regulatory RNAs that regulate the stability and translation of cognate mRNAs. MiRNAs participate in the regulation of adipogenesis, and identification of the full repertoire of miRNAs expressed in adipose tissue is likely to significantly increase our understanding of adipose tissue growth and development. Here, we adopted a deep sequencing approach to determine the identity and abundance of miRNAs in developing swine adipose tissue. Via this approach, we identified the sequences and relative expression levels of 227 conserved miRNAs (of which 59 were novel) and 66 potential porcine miRNAs. The expression levels displayed a large range, as reflected by the number of sequence reads, which varied from several counts for rare miRNAs to several million reads for the most abundant miRNAs. The abundant miRNAs principally belonged to 32 miRNA gene families, including miR-143, miR-103, let-7, and miR-148. Of the conserved miRNAs, 93 miRNAs were up-regulated and 33 miRNAs were down-regulated in the adult pig adipose tissue. Moreover, we observed sequence variants and seed edits of the miRNAs. KEGG pathway analysis and GO term enrichment suggested that highly expressed miRNAs are involved in adipose tissue development, signal transduction, cell-cell and cell-extracellular matrix communication, neural development and function, and lipid metabolism including carboxylic acid, oxoacid, fatty acid, steroid, glycerolipid, alcohol and phospholipid metabolism. Our results expand the number of known porcine miRNAs and provide a thorough account of the miRNA transcriptome in porcine adipose tissue.

Role of WNT-5a in the determination of human mesenchymal stem cells into preadipocytes

Increasing adipocyte size as well as numbers is important in the development of obesity and type 2 diabetes, with adipocytes being generated from mesenchymal precursor cells. This process includes the determination of mesenchymal stem cells (MSC) into preadipocytes (PA) and the differentiation of PA into mature fat cells. Although the process of differentiation has been highly investigated, the determination in humans is poorly understood. In this study, we compared human MSC and human committed PA on a cellular and molecular level to gain further insights into the regulatory mechanisms in the determination process. Both cell types showed similar morphology and expression patterns of common mesenchymal and hematopoietic surface markers. However, although MSC were able to differentiate into adipocytes and osteocytes, PA were only able to undergo adipogenesis, indicating that PA lost their multipotency during determination. WNT-5a expression showed significantly higher levels in MSC compared with PA suggesting that WNT-5a down-regulation might be important in the determination process. Indeed, incubation of human MSC in medium containing neutralizing WNT-5a antibodies abolished their ability to undergo osteogenesis, although adipogenesis was still possible. An opposite effect was achieved using recombinant WNT-5a protein. On a molecular level, WNT-5a was found to promote c-Jun N-terminal kinase-dependent intracellular signaling in MSC. Activation of this noncanonical pathway resulted in the induction of osteopontin expression further indicating pro-osteogenic effects of WNT-5a. Our data suggest that WNT-5a is necessary to maintain osteogenic potential of MSC and that inhibition of WNT-5a signaling therefore plays a role in their determination into PA in humans.

Molecular switching of osteoblastogenesis versus adipogenesis: implications for targeted therapies

Osteoblasts and adipocytes differentiate from a common precursor, the pluripotent mesenchymal stem cell (MSC) found in bone marrow (BMSC) and adipose tissue (AD-MSC). Numerous transcription factors and multiple extracellular and intracellular signals regulating adipogenesis and osteoblastogenesis have been identified and analyzed. Significantly, inducers of differentiation towards one lineage may inhibit cell differentiation into an alternative lineage. For example, the canonical Wnt/beta-catenin pathway induces osteoblastogenesis and inhibits adipogenesis, whereas the peroxisome proliferator activated receptor-gamma (PPAR-gamma) is a prime inducer of adipogenesis and, as shown in recent studies, inhibits osteoblastogenesis. We have identified two signaling pathways that switch the cell fate decision from adipocytes to osteoblasts by suppressing the transactivation function of PPAR-gamma. In the first pathway, the TNF-alpha- or IL-1-induced TAK1/TAB1/NIK signaling cascade attenuates PPAR-gamma-mediated adipogenesis by inhibiting the binding of PPAR-gamma to the DNA response element. The second is the noncanonical Wnt pathway through the CaMKII-TAK1/TAB2-NLK (nemo-like kinase) signaling cascade. Specifically, Wnt-5a-induced phosphorylation of NLK triggers formation of a complex with the histone methyltransferase SETDB1 (SET domain, bifurcated 1) that represses PPAR-gamma transactivation through histone H3-K9 methylation at the target genes. Thus, two signaling cascades promote osteoblastic differentiation from MSC through two distinct modes of PPAR-gamma transrepression.