Siah2 Protein Mediates Early Events in Commitment to an Adipogenic Pathway.

Adipose tissue expansion occurs by increasing the size of existing adipocytes or by increasing the number of adipocytes via adipogenesis. Adipose tissue dysfunction in obesity is associated with adipocyte hypertrophy and impaired adipogenesis. We recently demonstrated that deletion of the ubiquitin ligase Siah2 is associated with enlarged adipocytes in lean or obese mice. In this study, we find that adipogenesis is impaired in 3T3-L1 preadipocytes stably transfected with Siah2 shRNA and that overexpression of Siah2 in non-precursor fibroblasts promotes adipogenesis. In the 3T3-L1 model, loss of Siah2 is associated with sustained ß-catenin expression post-induction, but depletion of ß-catenin only partially restores PPARγ expression and adipocyte formation. Using wild-type and Siah2-/- adipose tissue and adipose stromal vascular cells, we observe that Siah2 influences the expression of several factors that control adipogenesis, including Wnt pathway genes, ß-catenin, Zfp432, and Bmp-4 Consistent with increased ß-catenin levels in shSiah2 preadipocytes, Wnt10b is elevated in Siah2-/- adipose tissue and remains elevated in Siah2-/- primary stromal cells after addition of the induction mixture. However, addition of BMP-4 to Siah2-/- stromal cells reduces Wnt10b expression, reduces Zfp521 protein levels, and increases expression of Zfp423, a transcriptional regulator of peroxisome proliferator-activated receptor γ expression that controls commitment to adipogenesis and is repressed by Zfp521. These results indicate that Siah2 acts upstream of BMP-4 to regulate factors that control the commitment of adipocyte progenitors to an adipogenic pathway. Our findings reveal an essential role for Siah2 in the early events that signal undifferentiated progenitor cells to become mature adipocytes.

An improved method for isolation of RNA from bone.

BACKGROUND: Bone physiology is increasingly appreciated as an important contributor to metabolic disorders such as type 2 diabetes. However, progress in understanding the role of bone in determining metabolic health is hampered by the well-described difficulty of obtaining high quality RNA from bone for gene expression analysis using the currently available approaches. RESULTS: We developed a simple approach to isolate bone RNA that combines pulverizing the bone and the phenol-guanidinium based RNA extraction in a single step while maintaining near-freezing temperatures. This single step method increases the yield of high quality RNA by eight-fold, with RNA integrity numbers ranging from 6.7 to 9.2. CONCLUSIONS: Our streamlined approach substantially increases the yield of high-quality RNA from bone tissue while facilitating safe and efficient processing of multiple samples using readily available platforms. The RNA obtained from this method is suitable for use in gene expression analysis in real-time quantitative PCR, microarray, and next generation sequencing applications.

SIAH2 is Expressed in Adipocyte Precursor Cells and Interacts with EBF1 and ZFP521 to Promote Adipogenesis.

OBJECTIVE: Expression of zinc finger protein 423 (ZFP423), a key proadipogenic transcription factor in adipocyte precursor cells, is regulated by interaction of the proadipogenic early B-cell factor 1 (EBF1) and antiadipogenic ZFP521. The ubiquitin ligase seven-in-absentia homolog 2 (SIAH2) targets ZFP521 for degradation. This study asked whether SIAH2 is expressed in adipocyte precursor cells and whether SIAH2 interacts with ZFP521 and EBF1 to regulate ZFP521 protein levels during adipogenesis. METHODS: SIAH2 expression in precursor cells was assessed in primary cells and tissues from wild-type and SIAH2 null mice fed a control or high-fat diet. Primary cells, 3T3-L1 preadipocytes, and HEK293T cells were used to analyze Siah2, Ebf1, and Zfp521 expression and SIAH2-mediated changes in ZFP521 and EBF1 protein levels. RESULTS: Siah2 is expressed in platelet-derived growth factor receptor α (PDGFRα)+ and stem cell antigen-1 (SCA1)+ adipocyte precursor cells. SIAH2 depletion reduces Ebf1 gene expression and increases EBF1 protein levels in early but not late adipogenesis. In early adipogenesis, SIAH2 forms a protein complex with EBF1 and ZFP521 to enhance SIAH2-mediated ubiquitylation and degradation of ZFP521 while increasing EBF1 protein levels. CONCLUSIONS: Siah2 is expressed in PDGFRα+ adipocyte precursor cells and is linked to precursor cell commitment to adipogenesis by interacting with EBF1 and ZFP521 proteins to target the antiadipogenic ZFP521 for degradation.

Adipogenic human adenovirus Ad-36 induces commitment, differentiation, and lipid accumulation in human adipose-derived stem cells.

Human adenovirus Ad-36 is causatively and correlatively linked with animal and human obesity, respectively. Ad-36 enhances differentiation of rodent preadipocytes, but its effect on adipogenesis in humans is unknown. To indirectly assess the role of Ad-36-induced adipogenesis in human obesity, the effect of the virus on commitment, differentiation, and lipid accumulation was investigated in vitro in primary human adipose-derived stem/stromal cells (hASC). Ad-36 infected hASC in a time- and dose-dependent manner. Even in the presence of osteogenic media, Ad-36-infected hASC showed significantly greater lipid accumulation, suggestive of their commitment to the adipocyte lineage. Even in the absence of adipogenic inducers, Ad-36 significantly increased hASC differentiation, as indicated by a time-dependent expression of genes within the adipogenic cascade-CCAAT/Enhancer binding protein-beta, peroxisome proliferator-activated receptor-gamma, and fatty acid-binding protein-and consequentially increased lipid accumulation in a time- and viral dose-dependent manner. Induction of hASC to the adipocyte state by Ad-36 was further supported by increased expression of lipoprotein lipase and the accumulation of its extracellular fraction. hASC from subjects harboring Ad-36 DNA in their adipose tissue due to natural infection had significantly greater ability to differentiate compared with Ad-36 DNA-negative counterparts, which offers a proof of concept. Thus, Ad-36 has the potential to induce adipogenesis in hASC, which may contribute to adiposity induced by the virus.

Correction to: Siah2 modulates sex-dependent metabolic and inflammatory responses in adipose tissue to a high-fat diet challenge.

Following publication of the original article [1], the authors reported that additional file 1 was incorrect. The corrected additional file 1 is given below.

Siah2 modulates sex-dependent metabolic and inflammatory responses in adipose tissue to a high-fat diet challenge.

BACKGROUND: The obesity-related risk of developing metabolic syndrome is higher in males than in females of reproductive age, likely due to estrogen-mediated reduced adipose tissue inflammation and fibrosis with hypertrophied adipocytes. Depletion of the ubiquitin ligase Siah2 reduced white adipose tissue inflammation and improved glucose metabolism in obese male mice. Siah2 is a transcriptional target of estrogen, but data is lacking about the effect of Siah2 on adipose tissue of females. We therefore evaluated the impact of Siah2 deficiency on white and brown adipose tissue in females of reproductive age. METHODS: Body composition, adipose tissue morphology, brown adipose tissue gene, and protein expression and adipocyte sizing were evaluated in wild-type and Siah2KO female and male mice fed a low-fat or high-fat diet. Glucose and insulin tolerance, fasting glucose, insulin, fatty acids and triglycerides, and gene expression of inflammation markers in perigonadal fat were evaluated in wild-type and Siah2KO female mice. Microarray analysis of brown fat gene expression was carried out in both sexes. Statistical analysis was assessed by unpaired two-tailed t test and repeated measures ANOVA. RESULTS: Siah2 deficiency improves glucose and insulin tolerance in the presence of hypertrophied white adipocytes in high-fat-fed female mice with percent fat comparable to male mice. While previous studies showed Siah2KO reduces the white adipose tissue inflammatory response in male mice, the response in females is biased toward the upregulation of M2-like markers in white adipose tissue. In contrast, loss of Siah2 leads to increased whitening of brown fat in males, but not in females. This corresponded to increased expression of markers of inflammation (F4/80, Ccl2) and thermogenic genes (Pgc1alpha, Dio2, Ucp-1) and proteins (PGC-1α, UCP-1) in females. Contrary to expectations, increased expression of thermogenic markers in females was coupled with a downregulation of ERalpha and ERRgamma protein levels. CONCLUSIONS: The most striking sex-related effect of Siah2 deficiency is reduced whitening of brown fat in high-fat-fed females. Protection from accumulating unilocular adipocytes in the brown fat corresponds to increased expression of thermogenic genes and proteins in female, but not in male mice. These results raise the possibility that Siah2 contributes to the estrogen-related effects on brown fat function in males and females.

Modulation of peroxisome proliferator-activated receptor gamma stability and transcriptional activity in adipocytes by resveratrol.

The peroxisome proliferator-activated receptor (PPAR) gamma is essential for the formation and function of adipocytes. It is also involved in regulating insulin sensitivity and is the functional target of the thiazolidinedione class of insulin-sensitizing drugs. Whereas thiazolidinediones activate PPARgamma and decrease PPARgamma protein levels, genetic models indicate that decreased expression of PPARgamma is also associated with increased insulin sensitivity. In this study, we show that resveratrol modulates PPARgamma protein levels in 3T3-L1 adipocytes via inhibition of PPARgamma gene expression coupled with increased ubiquitin-proteasome-dependent degradation of PPARgamma proteins. Resveratrol-mediated decreases in PPARgamma expression are associated with repression of PPARgamma transcriptional activity when assayed using a panel of PPARgamma target genes in adipocytes. Finally, we demonstrate that resveratrol inhibits insulin-dependent changes in glucose uptake and glycogen levels and decreases insulin receptor substrate 1 and glucose transporter 4 protein levels, indicating that resveratrol represses insulin sensitivity in adipocytes. These results indicate that the resveratrol-mediated effects in adipocytes involve regulation of PPARgamma expression and transcriptional activity along with decreased responsiveness to insulin.

Isolation of human adipose-derived stem cells from biopsies and liposuction specimens.

Adipose tissue has proven to serve as an abundant, accessible, and rich source of adult stem cells with multipotent properties suitable for tissue engineering and regenerative medical applications. Here, we describe a detailed method for the isolation and expansion of adipose-derived stem cells (ASCs). We present a large scale procedure suitable for processing >100 mL volumes of lipoaspirate tissue specimens and a small scale procedure suitable for processing adipose tissue biopsy specimens of < 0.5 g. Although we have focused on the isolation of ASCs from human adipose tissue, the procedure can be applied to adipose tissues from other species with minimal modifications.

Isolation of Murine Adipose-Derived Stromal/Stem Cells for Adipogenic Differentiation or Flow Cytometry-Based Analysis.

Murine models of obesity or reduced adiposity are a valuable resource for understanding the role of adipocyte dysfunction in metabolic disorders. Adipose tissue stromal vascular cells or primary adipocytes derived from murine adipose tissue and grown in culture are essential tools for studying the mechanisms underlying adipocyte development and function. Herein, we describe methods for the isolation, expansion, and long-term storage of murine adipose-derived stromal/stem cells along with protocols for inducing adipogenesis in this cell population or isolating the adipose stromal vascular fraction cells for flow cytometric analysis.

Circadian oscillation of gene expression in murine calvarial bone.

UNLABELLED: The genes encoding the core circadian transcription factors display an oscillating expression profile in murine calvarial bone. More than 26% of the calvarial bone transcriptome exhibits a circadian rhythm, comparable with that observed in brown and white adipose tissues and liver. Thus, circadian mechanisms may directly modulate oxidative phosphorylation and multiple metabolic pathways in bone homeostasis. INTRODUCTION: Although circadian rhythms have been associated historically with central regulatory mechanisms, there is emerging evidence that the circadian transcriptional apparatus exists in peripheral tissues. The aim of this study was to determine the presence and extent of circadian oscillation in the transcriptome of murine calvarial bone. MATERIALS AND METHODS: Cohorts of 8-week-old male AKR/J mice were maintained in a controlled 12-h light:12-h dark cycle on an ad libitum diet for 2 weeks. Groups of three mice were killed every 4 h over a 48-h period. The level of gene expression at successive times-points was determined by quantitative RT-PCR and Affymetrix microarray. Data were analyzed using multiple statistical time series algorithms, including Cosinor, Fisher g-test, and the permutation time test. RESULTS: Both the positive (Bmal1, Npas2) and negative (Cry1, Cry2, Per1, Per2, Per3) elements of the circadian transcriptional apparatus and their immediate downstream targets and mediators (Dbp, Rev-erbalpha, Rev-erbbeta) exhibited oscillatory expression profiles. Consistent with findings in other tissues, the positive and negative elements were in antiphase relative to each other. More than 26% of the genes present on the microarray displayed an oscillatory profile in calvarial bone, comparable with the levels observed in brown and white adipose tissues and liver; however, only a subset of 174 oscillating genes were shared among all four tissues. CONCLUSIONS: Our findings show that the components of the circadian transcriptional apparatus are represented in calvarial bone and display coordinated oscillatory behavior. However, these are not the only genes to display an oscillatory expression profile, which is seen in multiple pathways involving oxidative phosphorylation and lipid, protein, and carbohydrate metabolism.

Characterization of peripheral circadian clocks in adipose tissues.

First described in the suprachiasmatic nucleus, circadian clocks have since been found in several peripheral tissues. Although obesity has been associated with dysregulated circadian expression profiles of leptin, adiponectin, and other fat-derived cytokines, there have been no comprehensive analyses of the circadian clock machinery in adipose depots. In this study, we show robust and coordinated expression of circadian oscillator genes (Npas2, Bmal1, Per1-3, and Cry1-2) and clock-controlled downstream genes (Rev-erb alpha, Rev-erb beta, Dbp, E4bp4, Stra13, and Id2) in murine brown, inguinal, and epididymal (BAT, iWAT, and eWAT) adipose tissues. These results correlated with respective gene expression in liver and the serum markers of circadian function. Through Affymetrix microarray analysis, we identified 650 genes that shared circadian expression profiles in BAT, iWAT, and liver. Furthermore, we have demonstrated that temporally restricted feeding causes a coordinated phase-shift in circadian expression of the major oscillator genes and their downstream targets in adipose tissues. The presence of circadian oscillator genes in fat has significant metabolic implications, and their characterization may have potential therapeutic relevance with respect to the pathogenesis and treatment of diseases such as obesity, type 2 diabetes, and the metabolic syndrome.

Characterization of equine adipose tissue-derived stromal cells: adipogenic and osteogenic capacity and comparison with bone marrow-derived mesenchymal stromal cells.

OBJECTIVE: To characterize equine adipose tissue-derived stromal cell (ASC) frequency and growth characteristics and assess of their adipogenic and osteogenic differentiation potential. STUDY DESIGN: In vitro experimental study. ANIMALS: Horses (n=5; aged, 9 months to 5 years). METHODS: Cell doubling characteristics of ASCs harvested from supragluteal subcutaneous adipose tissue were evaluated over 10 passages. Primary, second (P2), and fourth (P4) passage ASCs were induced under appropriate conditions to undergo adipogenesis and osteogenesis. Limit dilution assays were performed on each passage to determine the frequency of colony-forming units with a fibroblastic (CFU-F) phenotype and the frequency of ASC differentiation into the adipocyte (CFU-Ad) and osteoblast (CFU-Ob) phenotype. RESULTS: ASC isolates exhibited an average cell-doubling time of 2.1+/-0.9 days during the first 10 cell doublings. Approximately 1 in 2.3+/-0.4 of the total stromal vascular fraction nucleated cells were ASCs, based on the CFU-F assays, and 1 in 3.6+/-1.3 expressed alkaline phosphatase, an osteogenic marker. Primary ASCs differentiated in response to adipogenic (1 in 4.9+/-5.4, CFU-Ad) and osteogenic (1 in <2.44, CFU-Ob) inductive conditions and maintained their differentiation potential during subsequent passages (P2 and P4). CONCLUSION: The frequency, in vitro growth rate, and adipogenic and osteogenic differentiation potential of equine ASCs show some differences to those documented for ASCs in other mammalian species. CLINICAL RELEVANCE: Adipose tissue is a potential source of adult stem cells for tissue engineering applications in equine veterinary medicine.

Targeted deletion of melanocortin receptor subtypes 3 and 4, but not CART, alters nutrient partitioning and compromises behavioral and metabolic responses to leptin.

Mouse lines with targeted disruption of the cocaine amphetamine-related transcript (CART), melanocortin receptor 3 (MCR3), or melanocortin receptor 4 (MCR4) were used to assess the role of each component in mediating the anorectic and metabolic effects of leptin, and in regulating the partitioning of nutrient energy between fat and protein deposition. Leptin was administered over a 3 day period using either intraperitoneal or intracerebroventricular routes of injection. The absence of MCR4 blocked leptin's ability to increase UCP1 mRNA in both brown and white adipose tissue, but not its ability to reduce food consumption. In contrast, deletion of MCR3 compromised leptin's ability to reduce food consumption, but not its ability to reduce fat deposition or increase UCP1 expression in adipose tissue. Leptin-dependent effects on food consumption and adipocyte gene expression were unaffected by the absence of CART. Repeated measures of body composition over time indicate that the absence of either MCR3 or MCR4, but not CART, increased lipid deposition and produced comparable degrees of adiposity in both lines. Moreover, modest increases in fat content of the diet (4 to 11%) accentuated fat deposition and produced a rapid and comparable 10-12% increase in % body fat in both genotypes. The results indicate that nutrient partitioning, as well as the anorectic and metabolic responses to leptin, are dependent on integrated but separable inputs from the melanocortin 3 and 4 receptor subtypes.

The ubiquitin ligase Siah2 regulates obesity-induced adipose tissue inflammation.

OBJECTIVE: Chronic, low-grade adipose tissue inflammation associated with adipocyte hypertrophy is an important link in the relationship between obesity and insulin resistance. Although ubiquitin ligases regulate inflammatory processes, the role of these enzymes in metabolically driven adipose tissue inflammation is relatively unexplored. Herein, the effect of the ubiquitin ligase Siah2 on obesity-related adipose tissue inflammation was examined. METHODS: Wild-type and Siah2KO mice were fed a low- or high-fat diet for 16 weeks. Indirect calorimetry, body composition, and glucose and insulin tolerance were assayed along with glucose and insulin levels. Gene and protein expression, immunohistochemistry, adipocyte size distribution, and lipolysis were also analyzed. RESULTS: Enlarged adipocytes in obese Siah2KO mice were not associated with obesity-induced insulin resistance. Proinflammatory gene expression, stress kinase signaling, fibrosis, and crown-like structures were reduced in the Siah2KO adipose tissue, and Siah2KO adipocytes were more responsive to insulin-dependent inhibition of lipolysis. Loss of Siah2 increased expression of PPARγ target genes involved in lipid metabolism and decreased expression of proinflammatory adipokines regulated by PPARγ. CONCLUSIONS: Siah2 links adipocyte hypertrophy with adipocyte dysfunction and recruitment of proinflammatory immune cells to adipose tissue. Selective regulation of PPARγ activity is a Siah2-mediated mechanism contributing to obesity-induced adipose tissue inflammation.

Prolonged proteasome inhibition cyclically upregulates Oct3/4 and Nanog gene expression, but reduces induced pluripotent stem cell colony formation.

There is ample evidence that the ubiquitin-proteasome system is an important regulator of transcription and its activity is necessary for maintaining pluripotency and promoting cellular reprogramming. Moreover, proteasome activity contributes to maintaining the open chromatin structure found in pluripotent stem cells, acting as a transcriptional inhibitor at specific gene loci generally associated with differentiation. The current study was designed to understand further the role of proteasome inhibition in reprogramming and its ability to modulate endogenous expression of pluripotency-related genes and induced pluripotent stem cells (iPSCs) colony formation. Herein, we demonstrate that acute combinatorial treatment with the proteasome inhibitors MG101 or MG132 and the histone deacetylase (HDAC) inhibitor valproic acid (VPA) increases gene expression of the pluripotency marker Oct3/4, and that MG101 alone is as effective as VPA in the induction of Oct3/4 mRNA expression in fibroblasts. Prolonged proteasome inhibition cyclically upregulates gene expression of Oct3/4 and Nanog, but reduces colony formation in the presence of the iPSC induction cocktail. In conclusion, our results demonstrate that the 26S proteasome is an essential modulator in the reprogramming process. Its inhibition enhances expression of pluripotency-related genes; however, efficient colony formation requires proteasome activity. Therefore, discovery of small molecules that increase proteasome activity might lead to more efficient cell reprogramming and generation of pluripotent cells.

An ethanolic extract of Artemisia dracunculus L. regulates gene expression of ubiquitin-proteasome system enzymes in skeletal muscle: potential role in the treatment of sarcopenic obesity.

OBJECTIVE: Obesity is linked to insulin resistance, a primary component of metabolic syndrome and type 2 diabetes. The problem of obesity-related insulin resistance is compounded when age-related skeletal muscle loss, called sarcopenia, occurs with obesity. Skeletal muscle loss results from elevated levels of protein degradation and prevention of obesity-related sarcopenic muscle loss will depend on strategies that target pathways involved in protein degradation. An extract from Artemisia dracunculus, termed PMI 5011, improves insulin signaling and increases skeletal muscle myofiber size in a rodent model of obesity-related insulin resistance. The aim of this study was to examine the effect of PMI 5011 on the ubiquitin-proteasome system, a central regulator of muscle protein degradation. METHODS: Gastrocnemius and vastus lateralis skeletal muscle was obtained from KK-A(y) obese diabetic mice fed a control or 1% (w/w) PMI 5011-supplemented diet. Regulation of genes encoding enzymes of the ubiquitin-proteasome system was determined using real-time quantitative reverse transcriptase polymerase chain reaction. RESULTS: Although MuRF-1 ubiquitin ligase gene expression is consistently down-regulated in skeletal muscle, atrogin-1, Fbxo40, and Traf6 expression is differentially regulated by PMI 5011. Genes encoding other enzymes of the ubiquitin-proteasome system ranging from ubiquitin to ubiquitin-specific proteases are also regulated by PMI 5011. Additionally, expression of the gene encoding the microtubule-associated protein-1 light chain 3 (LC3), a ubiquitin-like protein pivotal to autophagy-mediated protein degradation, is down-regulated by PMI 5011 in the vastus lateralis. CONCLUSION: PMI 5011 alters the gene expression of ubiquitin-proteasome system enzymes that are essential regulators of skeletal muscle mass. This suggests that PMI 5011 has therapeutic potential in the treatment of obesity-linked sarcopenia by regulating ubiquitin-proteasome-mediated protein degradation.

An extract of Artemisia dracunculus L. inhibits ubiquitin-proteasome activity and preserves skeletal muscle mass in a murine model of diabetes.

Impaired insulin signaling is a key feature of type 2 diabetes and is associated with increased ubiquitin-proteasome-dependent protein degradation in skeletal muscle. An extract of Artemisia dracunculus L. (termed PMI5011) improves insulin action by increasing insulin signaling in skeletal muscle. We sought to determine if the effect of PMI5011 on insulin signaling extends to regulation of the ubiquitin-proteasome system. C2C12 myotubes and the KK-A(y) murine model of type 2 diabetes were used to evaluate the effect of PMI5011 on steady-state levels of ubiquitylation, proteasome activity and expression of Atrogin-1 and MuRF-1, muscle-specific ubiquitin ligases that are upregulated with impaired insulin signaling. Our results show that PMI5011 inhibits proteasome activity and steady-state ubiquitylation levels in vitro and in vivo. The effect of PMI5011 is mediated by PI3K/Akt signaling and correlates with decreased expression of Atrogin-1 and MuRF-1. Under in vitro conditions of hormonal or fatty acid-induced insulin resistance, PMI5011 improves insulin signaling and reduces Atrogin-1 and MuRF-1 protein levels. In the KK-A(y) murine model of type 2 diabetes, skeletal muscle ubiquitylation and proteasome activity is inhibited and Atrogin-1 and MuRF-1 expression is decreased by PMI5011. PMI5011-mediated changes in the ubiquitin-proteasome system in vivo correlate with increased phosphorylation of Akt and FoxO3a and increased myofiber size. The changes in Atrogin-1 and MuRF-1 expression, ubiquitin-proteasome activity and myofiber size modulated by PMI5011 in the presence of insulin resistance indicate the botanical extract PMI5011 may have therapeutic potential in the preservation of muscle mass in type 2 diabetes.

The ubiquitin ligase Siah2 regulates PPARγ activity in adipocytes.

Moderate reductions in peroxisome proliferator-activated receptor (PPAR)γ levels control insulin sensitivity as effectively as activation of PPARγ in adipocytes by the thiazolidinediones. That observation suggests that PPARγ activity can be regulated by modulating the amount of PPARγ protein in adipocytes. Activation of PPARγ in adipocytes is linked to changes in PPARγ protein levels via increased degradation of PPARγ proteins by the ubiquitin proteasome system. Identification of the ubiquitin ligase or ligases that recognize ligand bound PPARγ is an essential step in determining the physiological significance of the relationship between activation and ubiquitin-dependent degradation of PPARγ. Using an RNA interference-based screen, we identified five RING (really interesting new gene)-type ubiquitin ligases that alter PPARγ protein levels in adipocytes. Here, we demonstrate that Drosophila seven-in-absentia homolog 2 (Siah2), a mammalian homolog of Drosophila seven-in-absentia, regulates PPARγ ubiquitylation and ligand-dependent activation of PPARγ in adipocytes. We also demonstrate that Siah2 expression is up-regulated during adipogenesis and that PPARγ interacts with Siah2 during adipogenesis. In addition, Siah2 is required for adipogenesis. These data suggest that modulation of PPARγ protein levels by the ubiquitin ligase Siah2 is essential in determining the physiological effects of PPARγ activation in adipocytes.

Isolation of murine adipose-derived stem cells.

Murine models of obesity or reduced adiposity are a valuable resource for understanding the role of adipocyte dysfunction in metabolic disorders. Primary adipocytes grown in culture and derived from murine adipose tissue are essential for studying the mechanisms underlying adipocyte development and function. Herein, we describe methods for the isolation, expansion, and long-term storage of murine adipose-derived stem cells along with a protocol for inducing adipogenesis in this cell population.

PPAR-gamma AF-2 domain functions as a component of a ubiquitin-dependent degradation signal.

The nuclear hormone receptor peroxisome proliferator-activated receptor-gamma (PPAR-gamma) functions as the "master switch" in adipocyte development and is important in regulating glucose metabolism. PPAR-gamma is rapidly degraded in adipocytes by the ubiquitin proteasome pathway under basal and ligand-activated conditions. Proteasome inhibition increases PPAR-gamma activity, indicating disposal of PPAR-gamma by the ubiquitin proteasome system regulates PPAR-gamma activity. However, the signals and factors required for recognition of PPAR-gamma by the ubiquitin proteasome pathway are unknown. To begin understanding how the ubiquitin-proteasome pathway interacts with PPAR-gamma, we designed a series of constructs containing each PPAR-gamma domain expressed as a fusion protein with the GAL4 DNA-binding domain. The ability of each PPAR-gamma domain to alter the stability of the GAL4 DNA-binding domain and to undergo ubiquitylation was assessed via western blot analysis. In addition, luciferase reporter assays were used to assay PPAR-gamma transcriptional activity. Using this approach, we determined that the AF-1 and ligand-binding domains (LBDs) of PPAR-gamma are targeted to the proteasome for degradation. However, only the LBD is conjugated to ubiquitin. The AF-2 helix of the LBD is required for maximum ubiquitylation, but is not essential for ligand-dependent ubiquitin conjugation. Finally, luciferase reporter assays show a fully functional ubiquitin system is required for PPAR-gamma activation. These results indicate that the ubiquitin-proteasome pathway is an integral determinant of PPAR-gamma activity, targeting PPAR-gamma for proteasomal degradation via ubiquitin independent and ubiquitin dependent mechanisms.