Microarray analyses during adipogenesis: understanding the effects of Wnt signaling on adipogenesis and the roles of liver X receptor alpha in adipocyte metabolism

Expression and regulation of transcript for the novel transmembrane protein Tmem182 in the adipocyte and muscle lineage

Background

White adipose tissue is not only an energy storage organ; it also functions as an endocrine organ. The coordination and integration of numerous gene expression events is required to establish and maintain the adipocyte phenotype.

Findings

We previously observed a 45-fold upregulation for a transcript encoding a novel predicted transmembrane protein, Tmem182, upon brown preadipocyte to adipocyte conversion. Here we use real-time PCR analysis to further characterize Tmem182 transcript expression in the adipocyte lineage. Analysis across a panel of 10 murine tissues revealed highest Tmem182 transcript expression in white adipose tissues (WAT), with 10-fold to 20-fold higher levels than in brown adipose tissue (BAT). Tmem182 transcript expression is ~3-fold upregulated in BAT of genetically obese (ob/ob) mice vs. wild type C57BL/6. Analysis of three in vitro models of white adipogenesis indicates markedly enriched expression of Tmem182 transcript in adipocytes vs. preadipocytes. Compared to 3T3-L1 preadipocytes, a 157-fold higher level of Tmem182 transcript is detected at 3 day post-induction of adipogenesis and an ~2500-fold higher level in mature 3T3-L1 adipocytes. TNFα treatment of 3T3-L1 adipocytes resulted in a ~90% decrease in Tmem182 transcript level. As skeletal muscle and heart were also found to express Tmem182 transcript, we assessed expression in C2C12 myogenesis and observed a ~770-fold upregulation upon conversion of myoblasts to myocytes.

Conclusion

WAT is the most prominent site of Tmem182 transcript expression and levels of transcript for Tmem182 are altered in adipose tissues of ob/ob mice and upon exposure of 3T3-L1 adipocytes to the proinflammatory cytokine TNFα. The dramatic upregulation of Tmem182 transcript during in vitro adipogenesis and myogenesis suggests Tmem182 may function in intracellular pathways important in these two cell types.

Oleanolic acid derivative NPLC441 potently stimulates glucose transport in 3T3-L1 adipocytes via a multi-target mechanism

The natural product oleanolic acid (OA) has been discovered to exhibit varied pharmacological functions including anti-inflammation, anti-tumor and anti-diabetes, while appropriate synthetic oleanolic acid derivatives seem to possess more potent activities. Here we identified a new oleanolic acid derivative, 3-beta-(2-carboxybenzoyloxy)-oleanolic acid (NPLC441), which functioned as a competitive PTP1B inhibitor and enhanced insulin-stimulated phosphorylation of IR and AKT in HepG2 cells. As an RXRalpha antagonist, it could selectively activate LXRalpha:RXRalpha heterodimer and increase the promoter activities of ABCA1 and ABCG1 genes in transient transfection assays. Quantitative RT-PCR and Western blot analyses suggested that NPLC441 could up-regulate GLUT4 expression in 3T3-L1 adipocytes, and such effect was further proved to be dependent on LXRalpha:RXRalpha activation. Moreover, 2-deoxyglucose uptake technology-based characterization demonstrated that this compound could stimulate glucose uptake in 3T3-L1 adipocytes. Finally, NPLC441 was observed to be able to suppress 11beta-HSD(1) expression in HepG2 cells, following the discovery that activation of LXRalpha:RXRalpha could repress the expression of 11beta-HSD(1). Compared with NPLC441, OA showed no effects on the transactivation of either LXRalpha:RXRalpha heterodimer or RXRalpha-LBD. Our work is thus expected to provide a new insight into the anti-diabetic application for oleanolic acid derivatives via multi-target mechanism, and NPLC441 could be used as a potential lead compound for further research.

Methods that resolve different contributions of clonal expansion to adipogenesis in 3T3-L1 and C3H10T1/2 cells

The mouse embryo fibroblast cell lines 3T3-L1 and C3H10T1/2 differentiate to adipocytes that exhibit similar insulin regulation of lipogenesis. These cell lines, however, differ appreciably in the processes that produce the major regulator PPAR gamma. Each line is stimulated by a mixture of insulin, dexamethasone, and methylisobutylxanthine (IDM). In the first 24h, IDM activates each cell type to produce similar regulatory changes and cell contraction. However, the increase in PPARy is delayed by 24h in typical 3T3-L1 cells compared with C3H10T1/2 cells. This delay is caused by the need for one or two rounds of cell division (clonal expansion) for PPAR gamma synthesis in 3T3-L1 cells. This expansion also occurs in C3H10T1/2 cells, but is not needed for PPAR gamma synthesis and differentiation. Other 3T3-L1 sublines have been described that follow the C3H10T1/2 pattern of differentiation. Culture conditions and inhibitors are described here that remove clonal expansion in C3H10T1/2 cells. With these constraints the cells retain full commitment to differentiation. This distinction is significant because many agents suppress differentiation in 3T3-L1 cells through inhibition of clonal expansion. Other effects on differentiation may be seen in C3H10T1/2 cells that are obscured in 3T3-L1 cells due to this inhibition of proliferation. Human preadipocytes do not need clonal expansion for adipogenesis, thus paralleling C3H10T1/2 cells.

Disruption of cell-matrix interactions by heparin enhances mesenchymal progenitor adipocyte differentiation

Differentiation of marrow-derived mesenchymal progenitors to either the osteoblast or adipocyte lineage is reciprocally regulated. Factors that promote osteoblastogenesis inhibit adipogenesis, while adipogenic factors are inhibitory to osteoblast differentiation. Heparin, a soluble glycosaminoglycan, inhibits bone formation in vivo and osteoblast cell differentiation and function in vitro, and has been shown to promote adipocyte differentiation. To elucidate the role that heparin plays in the adipogenic induction of murine mesenchymal progenitors, we studied immortalized marrow stromal cells (IM-MSC), the MSC cell line, ST2, and 3T3L1 pre-adipocytes. Heparin alone was not sufficient to induce adipogenesis, but enhanced the induction under a variety of adipogenic cocktails. This effect was both dose- and time-dependent. Heparin showed a positive effect at concentrations > 0.1 microg/ml when applied before day 3 during the induction course. Heparin's effect on adipogenesis was independent of cell proliferation, cell density, and extracellular lipid. This effect is likely related to the unique structure of heparin because another polyanionic glycosaminoglycan, dextran sulfate, did not promote adipogenic differentiation. Heparin treatment altered morphology and adhesion characteristics of progenitor cells, resulting in cell rounding and aggregation. As well, heparin counteracted the known inhibitory effect of fibronectin on adipogenesis and decreased basal focal adhesion kinase and paxillin phosphorylation. We conclude that heparin-mediated disruption of cell-matrix adhesion enhances adipogenic potential.

Wdnm1-like, a new adipokine with a role in MMP-2 activation

White adipose tissue functions in energy storage and as an endocrine organ. DNA microarray analysis led us to identify Wdnm1-like, a distant member of the whey acidic protein/four-disulfide core (WAP/4-DSC) family, as a differentiation-dependent gene in white and brown adipogenesis. Wdnm1-like is a novel 6.8-kDa protein, and Western blot analysis reveals secretion into culture media. Wdnm1-like transcript is selectively expressed in adipose tissue and liver and is enriched approximately 500-fold in white adipose depots vs. brown. Cellular fractionation of WAT demonstrates Wdnm1-like transcript expression is restricted to the adipocyte population. Studies in 3T3-L1 preadipocytes, an in vitro model of white adipogenesis, indicate Wdnm1-like transcript increases within 6 h of adipogenic induction with an approximately 17,000-fold increase by day 7. Dramatic upregulation of Wdnm1-like also accompanies white adipogenesis of ScAP-23 preadipocytes and primary preadipocytes. TNF-alpha treatment of 3T3-L1 adipocytes increased Wdnm1-like transcript level 2.4-fold and was attenuated by pretreatment with the p38 MAP kinase inhibitor SB203580. A number of WAP/4-DSC family proteins function as protease inhibitors. This, taken with the role of extracellular remodeling in adipogenesis, led us to address effects of Wdnm1-like on matrix metalloproteinase (MMP) activity. Gelatin zymography of HT1080 fibrosarcoma cells transfected with a Wdnm1-like expression construct revealed markedly increased levels of active MMP-2. Our findings identify a new member of the adipocyte "secretome" that functions to enhance MMP-2 activity. We postulate that Wdnm1-like may play roles in remodeling of the extracellular milieu in adipogenesis, as well as in tumor microenvironments where adipocytes are key stromal components.

Transcriptional regulation of ketone body-utilizing enzyme, acetoacetyl-CoA synthetase, by C/EBPalpha during adipocyte differentiation

Acetoacetyl-CoA synthetase (AACS), an essential enzyme for the synthesis of fatty acid and cholesterol from ketone bodies, was found to be highly expressed in mouse adipose tissue, and GC box and C/EBPs motif were crucial for AACS promoter activity in 3T3-L1 adipocytes. Moreover, we found that AACS promoter activity was controlled mainly by C/EBPalpha during adipogenesis.

Phosphorylation of CCAAT/enhancer-binding protein alpha regulates GLUT4 expression and glucose transport in adipocytes

The transcription factor CCAAT/enhancer-binding protein alpha (C/EBPalpha) is required during adipogenesis for development of insulin-stimulated glucose uptake. Modes for regulating this function of C/EBPalpha have yet to be determined. Phosphorylation of C/EBPalpha on Ser-21 has been implicated in the regulation of granulopoiesis and hepatic gene expression. To explore the role of Ser-21 phosphorylation on C/EBPalpha function during adipogenesis, we developed constructs in which Ser-21 was mutated to alanine (S21A) to model dephosphorylation. In two cell culture models deficient in endogenous C/EBPalpha, enforced expression of S21A-C/EBPalpha resulted in normal lipid accumulation and expression of many adipogenic markers. However, S21A-C/EBPalpha had impaired ability to activate the Glut4 promoter specifically, and S21A-C/EBPalpha expression resulted in diminished GLUT4 and adiponectin expression, as well as reduced insulin-stimulated glucose uptake. No defects in insulin signaling or GLUT4 vesicle trafficking were identified with S21A-C/EBPalpha expression, and when exogenous GLUT4 expression was enforced to normalize expression in S21A-C/EBPalpha cells, insulin-responsive glucose transport was reconstituted, suggesting that the primary defect was a deficit in GLUT4 levels. Mice in which endogenous C/EBPalpha was replaced with S21A-C/EBPalpha displayed reduced GLUT4 and adiponectin protein expression in epididymal adipose tissue and increased blood glucose compared with wild-type littermates. These results suggest that phosphorylation of C/EBPalpha on Ser-21 may regulate adipocyte gene expression and whole body glucose homeostasis.

Fat-specific protein 27 regulates storage of triacylglycerol

FSP27 (fat-specific protein 27) is a member of the cell death-inducing DNA fragmentation factor-alpha-like effector (CIDE) family. Although Cidea and Cideb were initially characterized as activators of apoptosis, recent studies have demonstrated important metabolic roles for these proteins. In this study, we investigated the function of another member of this family, FSP27 (Cidec), in apoptosis and adipocyte metabolism. Although overexpression of FSP27 is sufficient to increase apoptosis of 293T and 3T3-L1 cells, more physiological levels of expression stimulate spontaneous lipid accumulation in several cell types without induction of adipocyte genes. Increased triacylglycerol is likely due to decreased beta-oxidation of nonesterified fatty acids. Altered flux of fatty acids into triacylglycerol may be a direct effect of FSP27 function, which is localized to lipid droplets in 293T cells and 3T3-L1 adipocytes. Stable knockdown of FSP27 during adipogenesis of 3T3-L1 cells substantially decreases lipid droplet size, increases mitochondrial and lipid droplet number, and modestly increases glucose uptake and lipolysis. Expression of FSP27 in subcutaneous adipose tissue of a human diabetes cohort decreases with total fat mass but is not associated with measures of insulin resistance (e.g. homeostasis model assessment). Together, these data indicate that FSP27 binds to lipid droplets and regulates their enlargement.

Fat paradox of steatohepatitis

Alcoholic and non-alcoholic steatohepatitis (ASH and NASH) constitute two major types of chronic liver disease with worldwide prevalence and are histologically indistinguishable with shared pathogenetic mechanisms. More importantly, they have synergistic interactions for liver pathology. Comparative studies on ASH and NASH have been hampered by the use of different animal models with confounding variables, particularly those with extreme genetic, toxic, and malnutrition etiologies. The mouse intragastric model circumvents these problems and reproduces the natural course and etiological background of ASH and NASH. Further, our recent work reproduces a profound synergism between the two in the model. Intracellular accumulation of neural lipids is a hallmark biochemical feature of ASH and NASH. Although impaired lipid oxidation and export may contribute to this pathological change, enhanced lipogenic regulation is frequently encountered, as characterized by induction of lipogenic or adipogenic transcription factors (peroxisome proliferator-activated receptor [PPAR gamma], liver X receptor alpha[LXR alpha], sterol-regulatory element-binding protein-1c [SREBP-1c]). In contrast, we have recently defined transdifferentiation of hepatic stellate cells (HSC), a pivotal event in liver fibrogenesis, as an 'antilipogenic' or 'anti-adipogenic' phenomenon. Thus, there is an apparent paradox between hepatocytes and HSC in steatohepatitis in terms of the outcome of lipogenic regulation. Our recent work suggests that defective insulin signaling in activated HSC may be responsible for this paradox. Further, activated Wnt signaling is implicated in 'anti-adipogenic' stellate cell transdifferentiation in liver fibrogenesis.

Correlation of obesity and osteoporosis: effect of fat mass on the determination of osteoporosis

It was previously believed that obesity and osteoporosis were two unrelated diseases, but recent studies have shown that both diseases share several common genetic and environmental factors. Body fat mass, a component of body weight, is one of the most important indices of obesity, and a substantial body of evidence indicates that fat mass may have beneficial effects on bone. Contrasting studies, however, suggest that excessive fat mass may not protect against osteoporosis or osteoporotic fracture. Differences in experimental design, sample structure, and even the selection of covariates may account for some of these inconsistent or contradictory results. Despite the lack of a clear consensus regarding the impact of effects of fat on bone, a number of mechanistic explanations have been proposed to support the observed epidemiologic and physiologic associations between fat and bone. The common precursor stem cell that leads to the differentiation of both adipocytes and osteoblasts, as well the secretion of adipocyte-derived hormones that affect bone development, may partially explain these associations. Based on our current state of knowledge, it is unclear whether fat has beneficial effects on bone. We anticipate that this will be an active and fruitful focus of research in the coming years.

TNF-alpha and adipocyte biology

Dyslipidemia and insulin resistance are commonly associated with catabolic or lipodystrophic conditions (such as cancer and sepsis) and with pathological states of nutritional overload (such as obesity-related type 2 diabetes). Two common features of these metabolic disorders are adipose tissue dysfunction and elevated levels of tumour necrosis factor-alpha (TNF-alpha). Herein, we review the multiple actions of this pro-inflammatory adipokine on adipose tissue biology. These include inhibition of carbohydrate metabolism, lipogenesis, adipogenesis and thermogenesis and stimulation of lipolysis. TNF-alpha can also impact the endocrine functions of adipose tissue. Taken together, TNF-alpha contributes to metabolic dysregulation by impairing both adipose tissue function and its ability to store excess fuel. The molecular mechanisms that underlie these actions are discussed.

Wnt/beta-catenin signaling in adipogenesis and metabolism

Adipocyte differentiation consists of a complex series of events in which scores of cellular and extracellular factors interact to transform a fibroblast-like preadipocyte into a mature, lipid-filled adipocyte. Many of the pathways influencing this process have been identified using well-characterized preadipocyte culture systems and have subsequently been confirmed in animal models. Research conducted over the past decade has established the Wnt/beta-catenin signaling pathway as an important regulator of adipocyte differentiation. While initial reports implicated activators of Wnt/beta-catenin signaling as potent inhibitors of adipogenesis, recent investigations of mesenchymal cell fate, obesity, and type 2 diabetes highlight significant additional roles for Wnt signaling in metabolism and adipocyte biology.

PPARgamma regulates the function of human dendritic cells primarily by altering lipid metabolism

Activation of the lipid-regulated nuclear receptor peroxisome proliferator-activated receptor gamma (PPARgamma) modifies the immunophenotype of monocyte-derived dendritic cells (DCs). However it has not been analyzed in a systematic manner how lipid metabolism and immune regulation are connected at the transcriptional level via this receptor. Here we present the genome-wide expression analyses of PPARgamma-instructed human DCs. Receptor activation was achieved by exogenous, synthetic as well as endogenous, natural means. More than 1000 transcripts are regulated during DC development by activation of PPARgamma; half of the changes are positive effects. These changes appear to enhance and modulate the robust gene expression alterations associated with monocyte to DC transition. Strikingly, only genes related to lipid metabolism are overrepresented among early induced genes. As a net consequence, lipid accumulation appears to be diminished in these cells. In contrast, genes related to immune response are regulated after 24 hours, implying the existence of indirect mechanisms of modulation. Receptor dependence was established by using DCs of patients harboring a dominant-negative mutation of PPARgamma. Our data show that PPARgamma acts as a mostly positive transcriptional regulator in human developing DCs, acting primarily through controlling genes involved in lipid metabolism and via this, indirectly modifying the immune phenotype.

Characterization of ScAP-23, a new cell line from murine subcutaneous adipose tissue, identifies genes for the molecular definition of preadipocytes

The 3T3-L1 model of in vitro adipogenesis has provided key insights into the molecular nature of this process. However, given that 3T3-L1 are of an embryonic origin, it is not clear to what extent they represent adipogenesis as it occurs in white adipose tissue (WAT). With the goal of better defining preadipocytes and adipogenesis in WAT, we have generated a new cell culture model from adipocyte precursors present in C57BL/6 mouse subcutaneous WAT. ScAP-23 preadipocytes show fibroblastic morphology, and on treatment with dexamethasone, 3-methylisobutylxanthine, insulin, and indomethacin, convert to nearly 100% adipocyte morphology. ScAP-23 adipocytes contain abundant lipid droplets and express transcripts for PPARγ, C/EBP family, and SREBP-1c transcription factors, SCD1, aFABP, ATGL, GLUT4, FAS, LDL, and GPDH, and are insulin responsive. Differential screening of 1,176 genes using nylon DNA arrays identified 10 transcripts enriched in ScAP-23 adipocytes vs. preadipocytes and 26 transcripts enriched in ScAP-23 preadipocytes vs. adipocytes. Semiquantitative or real-time PCR analyses identified a common cohort of 14 transcripts markedly downregulated in both ScAP-23 and 3T3-L1 adipogenesis. These included catenin-β1, chemokine ligand-2, serine or cysteine peptidase inhibitor f1, aurora kinase B, thrombospondin2, and solute carrier-7a5. Five of these transcripts (Ccl2, Serpinf1, Aurkb, Thbs2, and Slc7a5) demonstrated at least a twofold increase in WAT from obese ( ob/ob) mice compared with that of wild-type mice. This suggests that comparative gene expression studies of ScAP-23 and 3T3-L1 adipogenesis may be particularly fruitful in identifying preadipocyte-expressed genes that play a role in adipose tissue physiology and/or pathophysiology.

Liver X receptor antagonist reduces lipid formation and increases glucose metabolism in myotubes from lean, obese and type 2 diabetic individuals

AIMS/HYPOTHESIS

Liver X receptors (LXRs) play important roles in lipid and carbohydrate metabolism. The purpose of the present study was to evaluate effects of the endogenous LXR agonist 22-R-hydroxycholesterol (22-R-HC) and its stereoisomer 22-S-hydroxycholesterol (22-S-HC), in comparison with the synthetic agonist T0901317 on lipid and glucose metabolism in human skeletal muscle cells (myotubes).

METHODS

Myotubes established from lean and obese control volunteers and from obese type 2 diabetic volunteers were treated with LXR ligands for 4 days. Lipid and glucose metabolisms were studied with labelled precursors, and gene expression was analysed using real-time PCR.

RESULTS

Treatment with T0901317 increased lipogenesis (de novo lipid synthesis) and lipid accumulation in myotubes, this increase being more pronounced in myotubes from type 2 diabetic volunteers than from lean volunteers. Furthermore, 22-S-HC efficiently counteracted the T0901317-induced enhancement of lipid formation. Moreover, synthesis of diacylglycerol, cholesteryl ester and free cholesterol from acetate was reduced below baseline by 22-S-HC, whereas glucose uptake and oxidation were increased. Both 22-S-HC and 22-R-HC, in contrast to T0901317, decreased the expression of genes involved in cholesterol synthesis, whereas only 22-R-HC, like T0901317, increased the expression of the gene encoding the reverse cholesterol transporter ATP-binding cassette subfamily A1 (ABCA1).

CONCLUSIONS/INTERPRETATION

T0901317-induced lipogenesis and lipid formation was more pronounced in myotubes from type 2 diabetic patients than from lean individuals. 22-S-HC counteracted these effects and reduced de novo lipogenesis below baseline, while glucose uptake and oxidation were increased.

Ectopic expression of Wnt10b decreases adiposity and improves glucose homeostasis in obese rats

The Wnt family of secreted glycoproteins had previously been shown to regulate diverse processes during early development. Wnt signaling also plays a key role in the homeostasis of adult tissues maintaining stem cell pluripotency and determining differentiating cell fate. The age-related decrease in Wnt signaling may contribute to increased muscle adiposity and diminished bone strength. In the current study, we investigated the long-term metabolic consequences of the upregulated Wnt/beta-catenin signaling in skeletal muscles of adult diet-induced obese (DIO) rats. To this end, we generated a recombinant adeno-associated virus (rAAV) vector encoding murine Wnt10b cDNA. The long-term expression of rAAV1-Wnt10b was tested after intramuscular injection in the female DIO rat. Animals fed high-fat diet and treated with rAAV1-Wnt10b showed a sustained reduction in body weight compared with controls, and expression of Wnt10b was accompanied by a reduction in hyperinsulinemia and triglyceride plasma levels as well as improved glucose homeostasis. Nuclear magnetic resonance methods revealed that ectopic expression of Wnt10b resulted in a decrease in both global and muscular fat deposits in DIO rats. The long-range effect of locally expressed Wnt10b was also manifested through the increased bone mineral density. The detailed analysis of molecular markers revealed fibroblast growth factor-4 and vascular endothelial growth factor as possible mediators of the systemic effect of Wnt10b transgene expression. Our data demonstrate that altering Wnt/beta-catenin signaling in the skeletal muscle of an adult animal invokes moderate responses with favorable metabolic profile, bringing the notion of alternative therapeutic modality in the treatment of obesity, diabetes, and osteoporosis.

Wnt/Lrp/beta-catenin signaling suppresses adipogenesis by inhibiting mutual activation of PPARgamma and C/EBPalpha

Wnt/beta-catenin signaling has been implicated in repressing adipogenesis. Several lines of evidence show that the possible mechanism is blockade of PPARgamma induction. However, the precise mechanisms remain to be elucidated. In this study, we demonstrated that Wnt3a conditioned medium suppresses C/EBPbeta/delta-induced adipogenesis of 3T3-L1 cells by inhibiting PPARgamma induction. In addition, the mutual activation of PPARgamma and C/EBPalpha was also repressed in the presence of Wnt3a. To further investigate the role of the canonical Wnt pathway in adipogenesis, we used mouse embryonic fibroblasts (MEFs) isolated from Lrp6-deficient embryos. Contrary to wild-type MEFs, Lrp6-deficient MEFs showed spontaneous adipogenesis and escaped the suppressive effect of exogenous Wnt3a. These findings suggest a critical role of Wnt/Lrp6/beta-catenin signaling in adipogenesis and cell fate decision of mesenchymal stem cells.

Cytosolic aspartate aminotransferase, a new partner in adipocyte glyceroneogenesis and an atypical target of thiazolidinedione

We show that cytosolic aspartate aminotransferase (cAspAT) is involved in adipocyte glyceroneogenesis, a regulated pathway that controls fatty acid homeostasis by promoting glycerol 3-phosphate formation for fatty acid re-esterification during fasting. cAspAT activity, as well as the incorporation of [(14)C]aspartate into the neutral lipid fraction of 3T3-F442A adipocytes was stimulated by the thiazolidinedione (TZD) rosiglitazone. Conversely, the ratio of fatty acid to glycerol released into the medium decreased. Regulation of cAspAT gene expression was specific to differentiated adipocytes and did not require any peroxisome proliferator-activated receptor gamma (PPARgamma)/retinoid X receptor-alpha direct binding. Nevertheless, PPARgamma is indirectly necessary for both cAspAT basal expression and TZD responsiveness because they are, respectively, diminished and abolished by ectopic overexpression of a dominant negative PPARgamma. The cAspAT TZD-responsive site was restricted to a single AGGACA hexanucleotide located at -381 to -376 bp whose mutation impaired the specific RORalpha binding. RORalpha ectopic expression activated the cAspAT gene transcription in absence of rosiglitazone, and its protein amount in nuclear extracts is 1.8-fold increased by rosiglitazone treatment of adipocytes. Finally, the amounts of RORalpha and cAspAT mRNAs were similarly increased by TZD treatment of human adipose tissue explants, confirming coordinated regulation. Our data identify cAspAT as a new member of glyceroneogenesis, transcriptionally regulated by TZD via the control of RORalpha expression by PPARgamma in adipocytes.

Differentiation-dependent expression of Adhfe1 in adipogenesis

We have determined that adipocytes are a major site of expression of the transcript for the novel alcohol dehydrogenase (ADH), Adhfe1. Adhfe1 is unique in that the sequence of its encoded protein places it among the iron-activated ADHs. Western blot analysis reveals Adhfe1 encodes a 50 kDa protein and immunocytochemical staining indicates mitochondrial localization. Adhfe1 transcript exhibits differentiation-dependent expression during in vitro brown and white adipogenesis. Unlike many adipocyte-enriched genes, however, Adhfe1 transcript expression in adipocytes is refractory to TNFalpha-mediated downregulation. However, use of pharmacological inhibitors reveals PI 3-kinase-mediated signals maintain the basal level of Adhfe1 transcript in 3T3-L1 adipocytes. Tissue profiling studies show Adhfe1 transcript is restricted to white and brown adipose tissues, liver, and kidney. In comparison to C57BL/6 mice, Adhfe1 transcript is downregulated 40% in white adipose tissue of ob/ob obese mice. Further characterization of Adhfe1 should yield new insights into adipocyte function and energy metabolism.

SREBP-1-independent regulation of lipogenic gene expression in adipocytes

Sterol regulatory element-binding protein (SREBP)-1c is now well established as a key transcription factor for the regulation of lipogenic enzyme genes such as FAS in hepatocytes. Meanwhile, the mechanisms of lipogenic gene regulation in adipocytes remain unclear. Here, we demonstrate that those in adipocytes are independent of SREBP-1c. In adipocytes, unlike in hepatocytes, the stimulation of SREBP-1c expression by liver X receptor agonist does not accompany lipogenic gene upregulation, although nuclear SREBP-1c protein is concomitantly increased, indicating that the activation process of SREBP-1c by the cleavage system is intact in adipocytes. Supportively, transcriptional activity of the mature form of SREBP-1c for the FAS promoter was negligible when measured by reporter analysis. As an underlying mechanism, accessibility of SREBP-1c to the functional elements was involved, because chromatin immunoprecipitation assays revealed that SREBP-1c does not bind to the functional SRE/E-box site on the FAS promoter in adipocytes. Moreover, genetic disruption of SREBP-1 did not cause any changes in lipogenic gene expression in adipose tissue. In summary, in adipocytes, unlike in hepatocytes, increments in nuclear SREBP-1c are not accompanied by transactivation of lipogenic genes; thus, SREBP-1c is not committed to the regulation of lipogenesis.

A novel pathway to enhance adipocyte differentiation of 3T3-L1 cells by up-regulation of lipocalin-type prostaglandin D synthase mediated by liver X receptor-activated sterol regulatory element-binding protein-1c

Lipocalin-type prostaglandin (PG) D synthase (L-PGDS) is expressed in adipocytes and is proposed to be involved in the regulation of glucose tolerance and atherosclerosis in type 2 diabetes, because L-PGDS gene knock-out mice show abnormalities in these functions. However, the role of L-PGDS and the regulation mechanism governing its gene expression in adipocytes remain unclear. Here, we applied small interference RNA of L-PGDS to mouse 3T3-L1 cells and found that it suppressed differentiation of these cells into adipocytes. Reporter analysis of the mouse L-PGDS promoter demonstrated that a responsive element for liver receptor homolog-1 (LRH-1) at -233 plays a critical role in preadipocytic 3T3-L1 cells. Moreover, we identified two sterol regulatory elements (SREs) at -194 to be cis-elements for activation of L-PGDS gene expression in adipocytic 3T3-L1 cells. L-PGDS mRNA was induced in response to synthetic liver X receptor agonist, T0901317, through activation of the expression of SRE-binding protein-1c (SREBP-1c) in the adipocytic 3T3-L1 cells. The results of electrophoretic mobility shift assay and chromatin immunoprecipitation assay revealed that LRH-1 and SREBP-1c bound to their respective binding elements in the promoter of L-PGDS gene. Small interference RNA-mediated suppression of LRH-1 or SREBP-1c decreased L-PGDS gene expression in preadipocytic or adipocytic 3T3-L1 cells, respectively. These results indicate that L-PGDS gene expression is activated by LRH-1 in preadipocytes and by SREBP-1c in adipocytes. Liver X receptor-mediated up-regulation of L-PGDS through activation of SREBP-1c is a novel path-way to enhance adipocyte differentiation.

Chibby promotes adipocyte differentiation through inhibition of beta-catenin signaling

The canonical Wnt/beta-catenin signaling pathway plays diverse roles in embryonic development and disease. Activation of this pathway, likely by Wnt-10b, has been shown to inhibit adipogenesis in cultured 3T3-L1 preadipocytes and in mice. Here, we report that the beta-catenin antagonist Chibby (Cby) is required for adipocyte differentiation. Cby is expressed in adipose tissue in mice, and Cby protein levels increase during adipogenic differentiation of 3T3-L1 cells. Ectopic expression of Cby induces spontaneous differentiation of these cells into mature adipocytes to an extent similar to that of dominant-negative Tcf-4. In contrast, depletion of Cby by RNA interference potently blocks adipogenesis of 3T3-L1 and mouse embryonic stem cells. In support of this, embryonic fibroblasts obtained from Cby-deficient embryos display attenuated differentiation to the adipogenic lineage. Mechanistically, Cby promotes adipocyte differentiation, in part by inhibiting beta-catenin, since gain or loss of function of Cby influences beta-catenin signaling in 3T3-L1 cells. Our results therefore establish Cby as a novel proadipogenic factor required for adipocyte differentiation.

Wnt10b inhibits obesity in ob/ob and agouti mice

The Wnt family of secreted signaling molecules has profound effects on diverse developmental processes, including the fate of mesenchymal progenitors. While activation of Wnt signaling blocks adipogenesis, inhibition of endogenous Wnt/beta-catenin signaling by Wnt10b promotes spontaneous preadipocyte differentiation. Transgenic mice with expression of Wnt10b from the FABP4 promoter (FABP4-Wnt10b) have less adipose tissue when maintained on a normal chow diet and are resistant to diet-induced obesity. Here we demonstrate that FABP4-Wnt10b mice largely avert weight gain and metabolic abnormalities associated with genetic obesity. FABP4-Wnt10b mice do not gain significant body weight on the ob/ob background, and at 8 weeks of age, they have an approximately 70% reduction in visceral and subcutaneous adipose tissues compared with ob/ob mice. Similarly, on the lethal yellow agouti (A(y)) background, FABP4-Wnt10b mice have 50-70% less adipose tissue weight and circulating leptin at 5 months of age. Wnt10b-Ay mice are more glucose tolerant and insulin sensitive than A(y) controls, perhaps due to reduced expression and circulation of resistin. Reduced expression of inflammatory cytokines may also contribute to improved glucose homeostasis.

Liver X receptor gene polymorphisms and adipose tissue expression levels in obesity

OBJECTIVE

LXRA and LXRB genes regulate adiposity, energy dissipation, as well as glucose and lipid homeostasis in mice. We investigated the LXR genes in human obesity.

METHODS

LXRA and LXRB mRNAs were quantified in abdominal subcutaneous adipose tissue of obese and nonobese women. The LXRA and LXRB genes were screened for polymorphisms and common single nucleotide polymorphisms genotyped in obese and nonobese women.

RESULTS

Relative LXRA mRNA expression levels were higher in obese women (P=0.03). One LXRA single nucleotide polymorphism, rs2279238, and one common haplotype, CAAGCC, as well as two LXRB single nucleotide polymorphisms, LB44732G>A and rs2695121, were associated with obesity phenotypes (nominal P values of 0.0075, 0.0014, 0.008 and 0.02, respectively). Furthermore, there was evidence of interaction between LXRA and LXRB alleles in determining body mass index.

CONCLUSION

Our results support a role for LXRA in human adipose tissue. The nominal associations of LXRA and LXRB alleles with obesity are interesting and should be further investigated in independent data sets.

Adipocyte differentiation from the inside out

Improved knowledge of all aspects of adipose biology will be required to counter the burgeoning epidemic of obesity. Interest in adipogenesis has increased markedly over the past few years with emphasis on the intersection between extracellular signals and the transcriptional cascade that regulates adipocyte differentiation. Many different events contribute to the commitment of a mesenchymal stem cell to the adipocyte lineage including the coordination of a complex network of transcription factors, cofactors and signalling intermediates from numerous pathways.

Regulation of secreted protein acidic and rich in cysteine during adipose conversion and adipose tissue hyperplasia

OBJECTIVE

To explore the regulation of secreted protein acidic and rich in cysteine (SPARC) expression and its role in adipose tissue.

RESEARCH METHODS AND PROCEDURES

We studied the regulation of SPARC expression in transgenic mice expressing the human beta3 and alpha2 adrenergic receptors on a murine beta3 adrenergic receptor null background that became obese under a high-fat diet mainly as a result of adipose tissue hyperplasia. Furthermore, we analyzed its expression in human adipose tissue and its regulation during adipocyte differentiation.

RESULTS

SPARC protein in adipose tissue was increased in obese transgenic mice compared with control mice, indicating that SPARC expression was associated with adipose tissue hyperplasia. Both SPARC mRNA and protein were detected in human adipose tissue. Comparing adipocytes and vascular stroma, we found that SPARC expression was mainly associated with the adipocyte fraction. Consistent with this, SPARC transcript increased during differentiation of human primary preadipocytes. 3T3-L1 preadipocytes showed an increase in SPARC expression in differentiated cells but with biphasic expression during the process. After induction in committed cells, SPARC mRNA and protein levels declined as differentiation began and returned to elevated levels in fully differentiated adipocytes.

DISCUSSION

SPARC expression correlated with adipose tissue hyperplasia and adipogenesis. Therefore, SPARC seems to play a role in adipose tissue physiology as it is involved in growth and differentiation.

Secreted proteins and genes in fetal and neonatal pig adipose tissue and stromal-vascular cells

Although microarray and proteomic studies have indicated the expression of unique and unexpected genes and their products in human and rodent adipose tissue, similar studies of meat animal adipose tissue have not been reported. Thus, total RNA was isolated from stromal-vascular (S-V) cell cultures (n = 4; 2 arrays; 2 cultures/array) from 90-d (79% of gestation) fetuses and adipose tissue from 105-d (92% of gestation) fetuses (n = 2) and neonatal (5-d-old) pigs (n = 2). Duplicate adipose tissue microarrays (n = 4) represented RNA samples from a pig and a fetus. Dye-labeled cDNA probes were hybridized to custom microarrays (70-mer oligonucleotides) representing more than 600 pig genes involved in growth and reproduction. Microarray studies showed significant expression of 40 genes encoding for known adipose tissue secreted proteins in fetal S-V cell cultures and adipose tissue. Expression of 10 genes encoding secreted proteins not known to be expressed by adipose tissue was also observed in neonatal adipose tissue and fetal S-V cell cultures. Additionally, the agouti gene was detected by reverse transcription-PCR in pig S-V cultures and adipose tissue. Proteomic analysis of adipose tissue and fetal and young pig S-V cell culture-conditioned media identified multiple secreted proteins including heparin-like epidermal growth factor-like growth factor and several apolipoproteins. Another adipose tissue secreted protein, plasminogen activator inhibitor-1, was identified by ELISA in S-V cell culture media. A group of 20 adipose tissue secreted proteins were detected or identified using the gene microarray and the proteomic and protein assay approaches including apolipoprotein-A1, apolipoprotein-E, relaxin, brain-derived neurotrophic factor, and IGF binding protein-5. These studies demonstrate, for the first time, the expression of several major secreted proteins in pig adipose tissue that may influence local and central metabolism and growth.

Regulatory circuits controlling white versus brown adipocyte differentiation

Adipose tissue is a major endocrine organ that exerts a profound influence on whole-body homoeostasis. Two types of adipose tissue exist in mammals: WAT (white adipose tissue) and BAT (brown adipose tissue). WAT stores energy and is the largest energy reserve in mammals, whereas BAT, expressing UCP1 (uncoupling protein 1), can dissipate energy through adaptive thermogenesis. In rodents, ample evidence supports BAT as an organ counteracting obesity, whereas less is known about the presence and significance of BAT in humans. Despite the different functions of white and brown adipocytes, knowledge of factors differentially influencing the formation of white and brown fat cells is sparse. Here we summarize recent progress in the molecular understanding of white versus brown adipocyte differentiation, including novel insights into transcriptional and signal transduction pathways. Since expression of UCP1 is the hallmark of BAT and a key factor determining energy expenditure, we also review conditions associated with enhanced energy expenditure and UCP1 expression in WAT that may provide information on processes involved in brown adipocyte differentiation.

The Wnt antagonist Dickkopf-1 and its receptors are coordinately regulated during early human adipogenesis

Secretion of Wnts by adipose cells has an important role in the control of murine adipogenesis. We present the first evidence that a Wnt antagonist, Dickkopf 1 (Dkk1), is secreted by human preadipocytes and promotes adipogenesis. DKK1 mRNA increases six hours after onset of human adipogenesis and this is followed by an increase in Dkk1 protein. With further differentiation, the mRNA and protein levels progressively decline such that they are undetectable in mature adipocytes. The transient induction in DKK1 correlates with downregulation of cytoplasmic and nuclear beta-catenin levels, this being a surrogate marker of canonical Wnt signalling, and Wnt/beta-catenin transcriptional activity. In addition, constitutive expression of Dkk1 in 3T3-L1 preadipocytes promotes their differentiation, further supporting the functional significance of increased Dkk1 levels during human adipogenesis. Concomitant downregulation of the Dkk1 receptors LRP5 and LRP6 is likely to potentiate the ability of Dkk1 to inhibit Wnt signalling and promote differentiation. Notably, Dkk1 is not expressed in primary murine preadipocytes or cell lines. The involvement of Dkk1 in human but not murine adipogenesis indicates that inter-species differences exist in the molecular control of this process. Given the public health importance of disorders of adipose mass, further knowledge of the pathways involved specifically in human adipocyte differentiation might ultimately be of clinical relevance.

Transcriptional regulation of metabolism

Our understanding of metabolism is undergoing a dramatic shift. Indeed, the efforts made towards elucidating the mechanisms controlling the major regulatory pathways are now being rewarded. At the molecular level, the crucial role of transcription factors is particularly well-illustrated by the link between alterations of their functions and the occurrence of major metabolic diseases. In addition, the possibility of manipulating the ligand-dependent activity of some of these transcription factors makes them attractive as therapeutic targets. The aim of this review is to summarize recent knowledge on the transcriptional control of metabolic homeostasis. We first review data on the transcriptional regulation of the intermediary metabolism, i.e., glucose, amino acid, lipid, and cholesterol metabolism. Then, we analyze how transcription factors integrate signals from various pathways to ensure homeostasis. One example of this coordination is the daily adaptation to the circadian fasting and feeding rhythm. This section also discusses the dysregulations causing the metabolic syndrome, which reveals the intricate nature of glucose and lipid metabolism and the role of the transcription factor PPARgamma in orchestrating this association. Finally, we discuss the molecular mechanisms underlying metabolic regulations, which provide new opportunities for treating complex metabolic disorders.

CREB activation induced by mitochondrial dysfunction triggers triglyceride accumulation in 3T3-L1 preadipocytes

Several mitochondrial pathologies are characterized by lipid redistribution and microvesicular cell phenotypes resulting from triglyceride accumulation in lipid-metabolizing tissues. However, the molecular mechanisms underlying abnormal fat distribution induced by mitochondrial dysfunction remain poorly understood. In this study, we show that inhibition of respiratory complex III by antimycin A as well as inhibition of mitochondrial protein synthesis trigger the accumulation of triglyceride vesicles in 3T3-L1 fibroblasts. We also show that treatment with antimycin A triggers CREB activation in these cells. To better delineate how mitochondrial dysfunction induces triglyceride accumulation in preadipocytes, we developed a low-density DNA microarray containing 89 probes, which allows gene expression analysis for major effectors and/or markers of adipogenesis. We thus determined gene expression profiles in 3T3-L1 cells incubated with antimycin A and compared the patterns obtained with differentially expressed genes during the course of in vitro adipogenesis induced by a standard pro-adipogenic cocktail. After an 8-day treatment, a set of 39 genes was found to be differentially expressed in cells treated with antimycin A, among them CCAAT/enhancer-binding protein alpha (C/EBPalpha), C/EBP homologous protein-10 (CHOP-10), mitochondrial glycerol-3-phosphate dehydrogenase (GPDmit), and stearoyl-CoA desaturase 1 (SCD1). We also demonstrate that overexpression of two dominant negative mutants of the cAMP-response element-binding protein CREB (K-CREB and M1-CREB) and siRNA transfection, which disrupt the factor activity and expression, respectively, inhibit antimycin-A-induced triglyceride accumulation. Furthermore, CREB knockdown with siRNA also downregulates the expression of several genes that contain cAMP-response element (CRE) sites in their promoter, among them one that is potentially involved in synthesis of triglycerides such as SCD1. These results highlight a new role for CREB in the control of triglyceride metabolism during the adaptative response of preadipocytes to mitochondrial dysfunction.

Wnt Signaling: A Key Regulator of Bone Mass

The identification of a link between bone mass in humans and gain- [high bone mass (HBM) trait] or loss-of-function [osteoporosis pseudoglioma (OPPG) syndrome] mutations in the Wnt coreceptor lipoprotein receptor-related protein (LRP)5 or in the Wnt antagonist sclerostin (sclerosteosis, Van Buchem syndrome) has called the attention of academic and industry scientists and clinicians to the importance of this signaling pathway in skeletal biology and disease. Multiple genetic and pharmacological manipulations of Wnt signaling in mice have since then confirmed the central role of this pathway in both the establishment of peak bone mass and its maintenance throughout life. Wnt signaling appears to be located downstream of bone morphogenetic proteins (BMPs), itself induced by Hedgehog (Hh) signaling, suggesting that it is the successive recruitment of these three intracellular signaling cascades that allow the full expression of the genetic patterns that characterize the osteoblast, the cell responsible for the formation of bone.

Playing with bone and fat

The relationship between bone and fat formation within the bone marrow microenvironment is complex and remains an area of active investigation. Classical in vitro and in vivo studies strongly support an inverse relationship between the commitment of bone marrow-derived mesenchymal stem cells or stromal cells to the adipocyte and osteoblast lineage pathways. In this review, we focus on the recent literature exploring the mechanisms underlying these differentiation events and discuss their implications relevant to osteoporosis and regenerative medicine.

Molecular processes during fat cell development revealed by gene expression profiling and functional annotation

In-depth bioinformatics analyses of expressed sequence tags found to be differentially expressed during differentiation of 3T3-L1 pre-adipocyte cells were combined with de novo functional annotation and mapping onto known pathways to generate a molecular atlas of fat-cell development.

Background

Large-scale transcription profiling of cell models and model organisms can identify novel molecular components involved in fat cell development. Detailed characterization of the sequences of identified gene products has not been done and global mechanisms have not been investigated. We evaluated the extent to which molecular processes can be revealed by expression profiling and functional annotation of genes that are differentially expressed during fat cell development.

Results

Mouse microarrays with more than 27,000 elements were developed, and transcriptional profiles of 3T3-L1 cells (pre-adipocyte cells) were monitored during differentiation. In total, 780 differentially expressed expressed sequence tags (ESTs) were subjected to in-depth bioinformatics analyses. The analysis of 3'-untranslated region sequences from 395 ESTs showed that 71% of the differentially expressed genes could be regulated by microRNAs. A molecular atlas of fat cell development was then constructed by de novo functional annotation on a sequence segment/domain-wise basis of 659 protein sequences, and subsequent mapping onto known pathways, possible cellular roles, and subcellular localizations. Key enzymes in 27 out of 36 investigated metabolic pathways were regulated at the transcriptional level, typically at the rate-limiting steps in these pathways. Also, coexpressed genes rarely shared consensus transcription-factor binding sites, and were typically not clustered in adjacent chromosomal regions, but were instead widely dispersed throughout the genome.

Conclusions

Large-scale transcription profiling in conjunction with sophisticated bioinformatics analyses can provide not only a list of novel players in a particular setting but also a global view on biological processes and molecular networks.

C/EBPalpha activates the transcription of triacylglycerol hydrolase in 3T3-L1 adipocytes

TGH (triacylglycerol hydrolase) catalyses the lipolysis of intracellular stored triacylglycerol. To explore the mechanisms that regulate TGH expression in adipose tissue, we studied the expression of TGH during the differentiation of 3T3-L1 adipocytes. TGH mRNA and protein levels increased dramatically in 3T3-L1 adipocytes compared with pre-adipocytes. Electrophoretic mobility shift assays demonstrated enhanced binding of nuclear proteins of adipocytes to the distal murine TGH promoter region (-542/-371 bp), yielding one adipocyte-specific migrating complex. Competitive and supershift assays demonstrated that the distal TGH promoter fragment bound C/EBPalpha (CCAAT/enhancer-binding protein alpha). Transient transfections of different mutant TGH promoter-luciferase constructs into 3T3-L1 adipocytes and competitive electromobility shift assays showed that the C/EBP-binding elements at positions -470/-459 bp and -404/-390 bp are important for transcriptional activation. Co-transfection with C/EBPalpha cDNA and TGH promoter constructs in 3T3-L1 pre-adipocytes demonstrated that C/EBPalpha increased TGH promoter activity. Ectopic expression of C/EBPalpha in NIH 3T3 cells activated TGH mRNA expression without causing differentiation into adipocytes. These experiments directly link increased TGH expression in adipocytes to transcriptional regulation by C/EBPalpha. This is the first evidence that C/EBPalpha participates directly in the regulation of an enzyme associated with lipolysis.

CCR2 modulates inflammatory and metabolic effects of high-fat feeding

The C-C motif chemokine receptor-2 (CCR2) regulates monocyte and macrophage recruitment and is necessary for macrophage-dependent inflammatory responses and the development of atherosclerosis. Although adipose tissue expression and circulating concentrations of CCL2 (also known as MCP1), a high-affinity ligand for CCR2, are elevated in obesity, the role of CCR2 in metabolic disorders, including insulin resistance, hepatic steatosis, and inflammation associated with obesity, has not been studied. To determine what role CCR2 plays in the development of metabolic phenotypes, we studied the effects of Ccr2 genotype on the development of obesity and its associated phenotypes. Genetic deficiency in Ccr2 reduced food intake and attenuated the development of obesity in mice fed a high-fat diet. In obese mice matched for adiposity, Ccr2 deficiency reduced macrophage content and the inflammatory profile of adipose tissue, increased adiponectin expression, ameliorated hepatic steatosis, and improved systemic glucose homeostasis and insulin sensitivity. In mice with established obesity, short-term treatment with a pharmacological antagonist of CCR2 lowered macrophage content of adipose tissue and improved insulin sensitivity without significantly altering body mass or improving hepatic steatosis. These data suggest that CCR2 influences the development of obesity and associated adipose tissue inflammation and systemic insulin resistance and plays a role in the maintenance of adipose tissue macrophages and insulin resistance once obesity and its metabolic consequences are established.

Interaction of nuclear receptors with the Wnt/beta-catenin/Tcf signaling axis: Wnt you like to know?

The cross-regulation of Wnt/beta-catenin/Tcf ligands, kinases, and transcription factors with members of the nuclear receptor (NR) family has emerged as a clinically and developmentally important area of endocrine cell biology. Interactions between these signaling pathways result in a diverse array of cellular effects including altered cellular adhesion, tissue morphogenesis, and oncogenesis. Analyses of NR interactions with canonical Wnt signaling reveal two broad themes: Wnt/beta-catenin modulation of NRs (theme I), and ligand-dependent NR inhibition of the Wnt/beta-catenin/Tcf cascade (theme II). Beta-catenin, a promiscuous Wnt signaling member, has been studied intensively in relation to the androgen receptor (AR). Beta-catenin acts as a coactivator of AR transcription and is also involved in co-trafficking, increasing cell proliferation, and prostate pathogenesis. T cell factor, a transcriptional mediator of beta-catenin and AR, engages in a dynamic reciprocity of nuclear beta-catenin, p300/CREB binding protein, and transcriptional initiation factor 2/GC receptor-interaction protein, thereby facilitating hormone-dependent coactivation and transrepression. Beta-catenin responds in an equally dynamic manner with other NRs, including the retinoic acid (RA) receptor (RAR), vitamin D receptor (VDR), glucocorticoid receptor (GR), progesterone receptor, thyroid receptor (TR), estrogen receptor (ER), and peroxisome proliferator-activated receptor (PPAR). The NR ligands, vitamin D(3), trans/cis RA, glucocorticoids, and thiazolidines, induce dramatic changes in the physiology of cells harboring high Wnt/beta-catenin/Tcf activity. Wnt signaling regulates, directly or indirectly, developmental processes such as ductal branching and adipogenesis, two processes dependent on NR function. Beta-catenin has been intensively studied in colorectal cancer; however, it is now evident that beta-catenin may be important in cancers of the breast, prostate, and thyroid. This review will focus on the cross-regulation of AR and Wnt/beta-catenin/Tcf but will also consider the dynamic manner in which RAR/RXR, GR, TR, VDR, ER, and PPAR modulate canonical Wnt signaling. Although many commonalities exist by which NRs interact with the Wnt/beta-catenin signaling pathway, striking cell line and tissue-specific differences require deciphering and application to endocrine pathology.

Gene expression analysis suggests that EBF-1 and PPARγ2 induce adipogenesis of NIH-3T3 cells with similar efficiency and kinetics

Differentiation of multipotent mesenchymal stem cells into lipid-accumulating adipocytes is a physiological process induced by transcription factors in combination with hormonal stimulation. We have used Affymetrix microarrays to compare the adipogenic differentiation pathways of NIH-3T3 fibroblasts induced to undergo in vitro differentiation by ectopic expression of early B cell factor (EBF)-1 or peroxisome proliferator-activated receptor (PPAR)γ2. These experiments revealed that commitment to the adipogenic pathway in the NIH-3T3 cells was not reflected in gene expression until 4 days after induction of differentiation. Furthermore, gene expression patterns at the earlier time points after stimulation indicated that EBF-1 and PPARγ2 induced different sets of genes, while the similarities increased upon differentiation, and that several genes linked to adipocyte differentiation were also transiently induced in the vector-transduced cells. These data suggest that the initial activation of genes associated with adipocyte development is independent of commitment to the adipogenic pathway and that EBF-1 and PPARγ2 induce adipocyte differentiation with comparable kinetics and efficiency.

Microarray analysis of early adipogenesis in C3H10T1/2 cells: cooperative inhibitory effects of growth factors and 2,3,7,8-tetrachlorodibenzo-p-dioxin

C3H10T1/2 mouse embryo fibroblasts differentiate into adipocytes when stimulated by a standard hormonal mixture (IDMB). 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD), via the aryl hydrocarbon receptor (AhR), inhibits induction of the key adipogenic gene peroxisome proliferator-activated receptor gamma (PPARgamma) and subsequent adipogenesis. This TCDD-mediated inhibition requires activation of the extracellular signal-regulated kinase (ERK) pathway, which can be accomplished by serum, epidermal growth factor (EGF), or fibroblast growth factor (FGF). In the absence of serum or growth factors, IDMB induced adipogenesis without mitosis. Microarray analysis identified 200 genes that exhibited expression changes of at least twofold after 24 h of IDMB treatment. This time precedes most PPARgamma stimulation but follows the period of TCDD/ERK cooperation and periods of increased cell contraction and DNA synthesis. Functionally related gene clusters include genes associated with cell structure, triglyceride and cholesterol metabolism, oxidative regulation, and secreted proteins. In the absence of growth factors TCDD inhibited 30% of these IDMB responses without inhibiting the process of differentiation. A combination of EGF and TCDD that blocks differentiation cooperatively blocked a further 44 IDMB-responsive genes, most of which have functional links to differentiation, including PPARgamma. Cell cycle regulators that are stimulated by EGF were substantially inhibited by IDMB but these responses were unaffected by TCDD. By contrast, TCDD and EGF cooperatively reversed IDMB-induced changes in cell adhesion complexes immediately prior to increases in PPARgamma1 expression. Changes in adhesion-linked signaling may play a key role in TCDD affects on differentiation.

Gene expression profiling of orbital adipose tissue from patients with Graves' ophthalmopathy: a potential role for secreted frizzled-related protein-1 in orbital adipogenesis

CONTEXT

The signs and symptoms of Graves' ophthalmopathy (GO) result from inflammation and increased volume of the orbital adipose tissues and extraocular muscles.

OBJECTIVE

Our objective was to identify differentially regulated genes that may be involved in stimulating the orbital adipose tissue expansion seen in GO.

DESIGN

Gene expression profiling was used to compare genes expressed in orbital adipose tissues from GO patients and normal individuals.

SETTING

The study took place at a private practice tertiary referral center.

PATIENTS

Orbital adipose tissues were collected at transantral orbital decompression surgery from 20 euthyroid patients undergoing this procedure for severe GO and at early autopsy from eight normal individuals having no evidence of thyroid or ocular disease.

RESULTS

Of the 12,686 genes analyzed, 25 known genes were increased in expression (>4-fold) in GO orbital tissues, whereas 11 genes were decreased (>4-fold). Up-regulated genes, confirmed by quantitative RT-PCR, included secreted frizzled-related protein-1 (sFRP-1; 18.5-fold) and several adipocyte-related genes, including peroxisome proliferator activated receptor-gamma (44.1-fold) and adiponectin (25-fold). Treatment in vitro of GO orbital preadipocytes with recombinant sFRP-1 (100 nm) significantly increased adiponectin (2.0-fold; P < 0.05), leptin (7-fold; P < 0.002), and TSH receptor mRNA (13-fold; P < 0.003) levels and enhanced Oil red-O staining in the cultures.

CONCLUSIONS

These results support the concept that orbital adipogenesis is enhanced in GO and suggest that elevated levels of sFRP-1 in the GO orbit may be involved in stimulating this pathogenic process.

The CB1 receptor antagonist rimonabant reverses the diet-induced obesity phenotype through the regulation of lipolysis and energy balance

We investigated the molecular events involved in the long-lasting reduction of adipose mass by the selective CB1 antagonist, SR141716. Its effects were assessed at the transcriptional level both in white (WAT) and brown (BAT) adipose tissues in a diet-induced obesity model in mice. Our data clearly indicated that SR141716 reversed the phenotype of obese adipocytes at both macroscopic and genomic levels. First, oral treatment with SR141716 at 10 mg/kg/d for 40 days induced a robust reduction of obesity, as shown by the 50% decrease in adipose mass together with a major restoration of white adipocyte morphology similar to lean animals. Second, we found that the major alterations in gene expression levels induced by obesity in WAT and BAT were mostly reversed in SR141716-treated obese mice. Importantly, the transcriptional patterns of treated obese mice were similar to those obtained in the CB1 receptor knockout mice fed a high-fat regimen and which are resistant to obesity, supporting a CB1 receptor-mediated process. Functional analysis of these modulations indicated that the reduction of adipose mass by the molecule resulted from an enhanced lipolysis through the induction of enzymes of the beta-oxidation and TCA cycle, increased energy expenditure, mainly through futile cycling (calcium and substrate), and a tight regulation of glucose homeostasis. These changes accompanied a significant cellular remodeling and contributed to a reduction of the obesity-related inflammatory status. In addition to a transient reduction of food consumption, increases of both fatty acid oxidation and energy expenditure induced by the molecule summate leading to a sustained weight loss. Altogether, these data strongly indicate that the endocannabinoid system has a major role in the regulation of energy metabolism.

LXR is crucial in lipid metabolism

Liver X receptors (LXRalpha and LXRbeta) are members of the nuclear receptor superfamily and are activated by oxysterols and intermediates in the cholesterol synthetic pathway. The pivotal role of LXRs in the metabolic conversion of cholesterol to bile acids is well established. Analysis of gene expression in LXRalpha and LXRbeta deficient mice have confirmed that LXR regulates a number of target genes involved in both cholesterol and fatty acid metabolism in liver, macrophages and intestine. The observation that LXRalpha is responsive to fatty acids and is expressed in metabolic tissues suggests that it also plays a general role in lipid metabolism. Adipose tissue is the main storage site for fat in the body and plays a crucial role in overall lipid handling. Both LXRalpha and LXRbeta are expressed and activated by endogenous and synthetic ligands, which lead to lipid accumulation into adipocytes. This indicates an important regulatory role of LXR in several metabolic signaling pathways in the adipose tissue, such as glucose uptake and de novo fatty acid synthesis. Here, we review recent studies that provide new insights into the mechanisms by which LXRs act to influence fatty acid synthesis in liver and adipose tissue.

Wnt5b partially inhibits canonical Wnt/beta-catenin signaling pathway and promotes adipogenesis in 3T3-L1 preadipocytes

To elucidate the functional roles of Wnt5b in adipogenesis, we characterized gene expression profiles in Wnt5b overexpressing 3T3-L1 cells using microarray analysis. Of the approximately 20,000 genes screened, we found that 85 genes were up-regulated and 211 genes were down-regulated in 3T3-L1 cells overexpressing Wnt5b. Among the genes regulated by Wnt5b, the expressions of insulin like growth factor-1 (IGF-1), vascular endothelial growth factor-C (VEGF-C), and WNT1 inducible signaling pathway protein 1 (WISP-1), which were known to be up-regulated by Wnt1/beta-catenin signaling, were decreased in the Wnt5b overexpressing cells. This result was subsequently confirmed by real-time quantitative RT-PCR (IGF-1; 0.74+/-0.08 and 0.56+/-0.08, WISP-1; 0.71+/-0.03 and 0.56+/-0.08, and VEGF-C; 0.67+/-0.01 and 0.80+/-0.07, mean+/-SEM, compared with the control at zero and two days after induction of differentiation, respectively). We also found that Wnt5b overexpression in 3T3-L1 preadipocytes was able to partially prevent the inhibitory effect of Wnt3a on adipogenesis. Furthermore, the overexpression of Wnt5b was able to inhibit Wnt3a-induced activation of the canonical Wnt/beta-catenin pathway as evidenced by the reduced translocation of beta-catenin into the nucleus. These findings indicate that Wnt5b may promote adipogenesis in 3T3-L1 cells, at least in part, by antagonizing the canonical Wnt/beta-catenin pathway.

LXRβ Is Required for Adipocyte Growth, Glucose Homeostasis, and β Cell Function

Liver X receptors (LXR) alpha and beta are nuclear oxysterol receptors with established roles in cholesterol, lipid, and carbohydrate metabolism. Although LXRs have been extensively studied in liver and macrophages, the importance for development and metabolism of other tissues and cell types is not as well characterized. We demonstrate here that although LXRalpha and LXRbeta are not required for adipocyte development per se, LXRbeta is required for the increase in adipocyte size that normally occurs with aging and diet-induced obesity. Similar food intake and oxygen consumption in LXRbeta-/- mice suggests that reduced storage of lipid in adipose tissue is not due to altered energy balance. Despite reduced amounts of adipose tissue, LXRbeta-/- mice on a chow diet have insulin sensitivity and levels of adipocyte hormones similar to wild type mice. However, these mice are glucose-intolerant due to impaired glucose-induced insulin secretion. Lipid droplets in pancreatic islets may result from accumulation of cholesterol esters as analysis of islet gene expression reveals that LXRbeta is required for expression of the cholesterol transporters, ABCA1 and ABCG1. Our data establish novel roles for LXRbeta in adipocyte growth, glucose homeostasis, and beta cell function.

Role of Gas-6 in Adipogenesis and Nutritionally Induced Adipose Tissue Development in Mice

Objective— A potential role of growth arrest-specific gene 6 (Gas-6) in energy storage in adipose tissue was investigated in murine models of obesity. Gas-6 is a ligand for the Axl, C-Mer, and Sky family of tyrosine kinase receptors.

Methods and Results— Whereas Gas-6, C-Mer, and Sky were expressed in mature murine adipocytes, the expression of Axl was restricted to the stromal-vascular fraction, which includes pre-adipocytes. During the in vitro conversion of adipogenic 3T3-F442A cells into mature adipocytes, the expression of Gas-6 increased in undifferentiated confluent pre-adipocytes during a transient phase of growth arrest. On treatment of these cells with an adipogenic medium, Gas-6 expression decreased sharply, coinciding with expression of early adipocytes markers. This modulation was not observed in the nonadipogenic 3T3-C2 cells. The Gas-6 mRNA level was transiently downregulated during nutritionally induced expansion of adipose tissues in vivo. When kept on a standard diet, no significant difference in either total body weight or weight of gonadal or subcutaneous fat pads was observed between Gas-6 deficient and wild-type mice. On exposure to a high-fat diet, however, Gas-6- deficient mice had significantly less fat mass than their wild-type counterparts.

Conclusions— Gas-6 enhances the accumulation of adipose tissue in diet-induced obese mice.

Gene array analysis of Wnt-regulated genes in C3H10T1/2 cells

Wnt/beta-catenin signaling is involved in a large variety of modeling and remodeling processes including cell polarity, cell differentiation, and cell migration. Recently, a role of the Wnt pathway in bone biology has been demonstrated. However, the precise mechanism by which Wnt proteins regulate bone formation still remains to be elucidated. We have previously shown that the Wnt pathway mediates induction of alkaline phosphatase, an osteoblast differentiation marker, in the pluripotent mesenchymal cells C3H10T1/2. In the present study, we performed a genome-wide expression analysis using Affymetrix oligonucleotide chips to determine the Wnt3a-induced gene expression profile in C3H10T1/2 cells. The expression profiles of 447 Wnt3a-regulated genes, classified into distinct functional families, are presented here. Our data reveal that Wnt3a regulates several genes that are involved in osteoblast and adipocyte differentiation. Importantly, Wnt3a induces the expression of osteoprotegerin by a beta-catenin dependent mechanism indicating that the Wnt pathway may also affect osteoclastogenesis. Through the analysis of our expression profiling data, we have established a TaqMan panel as a tool to rapidly compare the expression profiles of a specific set of genes induced by distinct stimuli acting in the Wnt/beta-catenin pathway. Using the TaqMan panel, we have compared the gene expression profiles induced by Wnt1, Wnt2, and Wnt3a in C3H10T1/2 cells, and also by two different GSK-3beta inhibitors: LiCl and SB216773. Our data show that Wnt1 and Wnt3a act in a similar manner, distinct from Wnt2. Finally, we found that LiCl and SB216773 displayed different profiles in the TaqMan panel evidencing their distinct inhibitory action toward GSK-3beta. Overall, data presented herein will aid further understanding of the involvement of the Wnt signaling pathway in its regulation of osteoblast and adipocyte differentiation and function and, in addition, will enhance current knowledge of the Wnt signaling pathway itself.

Skeletal muscle lipid accumulation in type 2 diabetes may involve the liver X receptor pathway

Liver X receptors (LXRs) are important regulators of cholesterol and lipid metabolism and are also involved in glucose metabolism. However, the functional role of LXRs in human skeletal muscle is at present unknown. This study demonstrates that chronic ligand activation of LXRs by a synthetic LXR agonist increases the uptake, distribution into complex cellular lipids, and oxidation of palmitate as well as the uptake and oxidation of glucose in cultured human skeletal muscle cells. Furthermore, the effect of the LXR agonist was additive to acute effects of insulin on palmitate uptake and metabolism. Consistently, activation of LXRs induced the expression of relevant genes: fatty acid translocase (CD36/FAT), glucose transporters (GLUT1 and -4), sterol regulatory element-binding protein-1c, peroxisome proliferator-activated receptor-gamma, carnitine palmitoyltransferase-1, and uncoupling protein 2 and 3. Interestingly, in response to activation of LXRs, myotubes from patients with type 2 diabetes showed an elevated uptake and incorporation of palmitate into complex lipids but an absence of palmitate oxidation to CO(2). These results provide evidence for a functional role of LXRs in both lipid and glucose metabolism and energy uncoupling in human myotubes. Furthermore, these data suggest that increased intramyocellular lipid content in type 2 diabetic patients may involve an altered response to activation of components in the LXR pathway.