Elisabetta Mueller

Temporal Recruitment of Transcription Factors and SWI/SNF Chromatin-Remodeling Enzymes during Adipogenic Induction of the Peroxisome Proliferator-Activated Receptor γ Nuclear Hormone Receptor

ABSTRACT The peroxisome proliferator-activated receptor gamma (PPARγ) regulates adipogenesis, lipid metabolism, and glucose homeostasis, and roles have emerged for this receptor in the pathogenesis and treatment of diabetes, atherosclerosis, and cancer. We report here that induction of the PPARγ activator and adipogenesis forced by overexpression of adipogenic regulatory proteins is blocked upon expression of dominant-negative BRG1 or hBRM, the ATPase subunits of distinct SWI/SNF chromatin-remodeling enzymes. We demonstrate that histone hyperacetylation and the binding of C/EBP activators, polymerase II (Pol II), and general transcription factors (GTFs) initially occurred at the inducible PPARγ2 promoter in the absence of SWI/SNF function. However, the polymerase and GTFs were subsequently lost from the promoter in cells expressing dominant-negative SWI/SNF, explaining the inhibition of PPARγ2 expression. To corroborate these data, we analyzed interactions at the PPARγ2 promoter in differentiating preadipocytes. Changes in promoter structure, histone hyperacetylation, and binding of C/EBP activators, Pol II, and most GTFs preceded the interaction of SWI/SNF enzymes with the PPARγ2 promoter. However, transcription of the PPARγ2 gene occurred only upon subsequent association of SWI/SNF and TFIIH with the promoter. Thus, induction of the PPARγ nuclear hormone receptor during adipogenesis requires SWI/SNF enzymes to facilitate preinitiation complex function.

Thyroid Hormone Induced Brown Adipose Tissue and Amelioration of Diabetes in a Patient with Extreme Insulin Resistance

CONTEXTnBrown adipose tissue (BAT) found by positron emission/computed tomography (PET-CT) using flouro-deoxyglucose (FDG) is inducible by cold exposure in men. Factors leading to increased BAT are of great interest for its potential role in the treatment of diabetes and obesity.nnnOBJECTIVEnWe tested whether thyroid hormone (TH) levels are related to the volume and activity of BAT in a patient with a mutation in the insulin receptor gene. DESIGN/SETTING/INTERVENTION: Our work was based on the case report of a patient in an observational study at the National Institutes of Health.nnnPATIENTnThe patient discontinued insulin and oral antidiabetics after thyroidectomy and suppressive-dose levothyroxine therapy for thyroid cancer. PET-CT uptake in BAT was confirmed by histology and molecular analysis.nnnOUTCOMESnPET-CT studies were performed, and we measured hemoglobin A1c and resting energy expenditure before and after levothyroxine discontinuation for thyroid cancer testing. Molecular studies of BAT and white adipose samples are presented.nnnRESULTnSupraclavicular and periumbilical sc adipose tissue demonstrated molecular features of BAT including uncoupling protein-1, type 2 deiodinase, and PR domain containing 16 by quantitative PCR. Activity of type 2 deiodinase activity was increased. The discontinuation of levothyroxine resulted in decreased FDG uptake and diminished volume of BAT depots accompanied by worsening of diabetic control.nnnCONCLUSIONSnThis case demonstrates the TH effect on BAT activity and volume in this patient and an association between BAT activity and glucose levels in this patient. Because the contribution of TH on skeletal muscle energy expenditure and fuel metabolism was not assessed, an association between BAT activity and glucose homeostasis can only be suggested.

Celastrol Protects against Obesity and Metabolic Dysfunction through Activation of a HSF1-PGC1α Transcriptional Axis

Altering the balance between energy intake and expenditure is a potential strategy for treating obesity and metabolic syndrome. Nonetheless, despite years of progress in identifying diverse molecular targets, biological-based therapies are limited. Here we demonstrate that heat shock factor 1 (HSF1) regulates energy expenditure through activation of a PGC1α-dependent metabolic program in adipose tissues and muscle. Genetic modulation of HSF1 levels altered white fat remodeling and thermogenesis, and pharmacological activation of HSF1 via celastrol was associated with enhanced energy expenditure, increased mitochondrial function in fat and muscle and protection against obesity, insulin resistance, and hepatic steatosis during high-fat diet regimens. The beneficial metabolic changes elicitedxa0by celastrol were abrogated in HSF1 knockout mice.xa0Overall, our findings identify the temperature sensor HSF1 as a regulator of energy metabolism and demonstrate that augmenting HSF1 via celastrol represents a possible therapeutic strategy to treat obesity and its myriad metabolic consequences.

Disrupted erythropoietin signalling promotes obesity and alters hypothalamus proopiomelanocortin production

While erythropoietin is the cytokine known that regulates erythropoiesis, erythropoietin receptor (EpoR) expression and associated activity beyond hematopoietic tissue remain uncertain. Here we show that mice with EpoR expression restricted to hematopoietic tissues (Tg) develop obesity and insulin resistance. Tg-mice exhibit a decrease in energy expenditure and an increase in white fat mass and adipocyte number. Conversely, erythropoietin treatment of wild-type mice increases energy expenditure and reduces food intake and fat mass accumulation but showed no effect in body weight of Tg-mice. EpoR is expressed at a high level in white adipose tissue and in the proopiomelanocortin neurons of the hypothalamus. While Epo treatment in wild-type mice induces the expression of the polypeptide hormone precursor gene, proopiomelanocortin, mice lacking EpoR show reduced levels of proopiomelanocortin in the hypothalamus. This study provides the first evidence that mice lacking EpoR in nonhematopoietic tissue become obese and insulin resistant with loss of erythropoietin regulation of energy homeostasis.

Transcriptional coactivator PGC-1α promotes peroxisomal remodeling and biogenesis

Mitochondria and peroxisomes execute some analogous, nonredundant functions including fatty acid oxidation and detoxification of reactive oxygen species, and, in response to select metabolic cues, undergo rapid remodeling and division. Although these organelles share some components of their division machinery, it is not known whether a common regulator coordinates their remodeling and biogenesis. Here we show that in response to thermogenic stimuli, peroxisomes in brown fat tissue (BAT) undergo selective remodeling and expand in number and demonstrate that ectopic expression of the transcriptional coactivator PGC-1α recapitulates these effects on the peroxisomal compartment, both in vitro and in vivo. Conversely, β-adrenergic stimulation of PGC-1α−/− cells results in blunted induction of peroxisomal gene expression. Surprisingly, PPARα was not required for the induction of critical biogenesis factors, suggesting that PGC-1α orchestrates peroxisomal remodeling through a PPARα-independent mechanism. Our data suggest that PGC-1α is critical to peroxisomal physiology, establishing a role for this factor as a fundamental orchestrator of cellular adaptation to energy demands.

Serum lipids regulate dendritic cell CD1 expression and function

Dendritic cells (DCs) are highly potent antigen‐presenting cells (APCs) and play a vital role in stimulating naïve T cells. Treatment of human blood monocytes with the cytokines granulocyte–macrophage colony‐stimulating factor (GM‐CSF) and interleukin (IL)‐4 stimulates them to develop into immature dendritic cells (iDCs) in vitro. DCs generated by this pathway have a high capacity to prime and activate resting T cells and prominently express CD1 antigen‐presenting molecules on the cell surface. The presence of human serum during the differentiation of iDCs from monocytes inhibits the expression of CD1a, CD1b and CD1c, but not CD1d. Correspondingly, T cells that are restricted by CD1c showed poor responses to DCs that were generated in the presence of human serum, while the responses of CD1d‐restricted T cells were enhanced. We chemically fractionated human serum to isolate the bioactive factors that modulate surface expression of CD1 proteins during monocyte to DC differentiation. The human serum components that affected CD1 expression partitioned with polar organic soluble fractions. Lysophosphatidic acid and cardiolipin were identified as lipids present in normal human serum that potently modulate CD1 expression. Control of CD1 expression was mediated at the level of gene transcription and correlated with activation of the peroxisome proliferator‐activated receptor (PPAR) nuclear hormone receptors. These findings indicate that the ability of human DCs to present lipid antigens to T cells through expression of CD1 molecules is sensitively regulated by lysophosphatidic acid and cardiolipin in serum, which are ligands that can activate PPAR transcription factors.

A Multifunctional Protein, EWS, Is Essential for Early Brown Fat Lineage Determination

The recent surge in obesity has provided an impetus to better understand the mechanisms of adipogenesis, particularly in brown adipose tissue (BAT) because of its potential utilization for antiobesity therapy. Postnatal brown adipocytes arise from early muscle progenitors, but how brown fat lineage is determined is not completely understood. Here, we show that a multifunctional protein, Ewing Sarcoma (EWS), is essential for determining brown fat lineage during development. BATs from Ews null embryos and newborns are developmentally arrested. Ews mutant brown preadipocytes fail to differentiate due to loss of Bmp7 expression, a critical early brown adipogenic factor. We demonstrate that EWS, along with its binding partner Y-box binding protein 1 (YBX1), activates Bmp7 transcription. Depletion of either Ews or Ybx1 leads to loss of Bmp7 expressionxa0and brown adipogenesis. Remarkably, Ews null BATs and brown preadipocytes ectopically express myogenic genes. These results demonstrate that EWS is essential for early brown fat lineage determination.

Serum- and glucocorticoid-inducible kinase 1 (SGK1) regulates adipocyte differentiation via forkhead box O1.

The serum and glucocorticoid-inducible kinase 1 (SGK1) is an inducible kinase the physiological function of which has been characterized primarily in the kidney. Here we show that SGK1 is expressed in white adipose tissue and that its levels are induced in the conversion of preadipocytes into fat cells. Adipocyte differentiation is significantly diminished via small interfering RNA inhibition of endogenous SGK1 expression, whereas ectopic expression of SGK1 in mesenchymal precursor cells promotes adipogenesis. The SGK1-mediated phenotypic effects on differentiation parallel changes in the mRNA levels for critical regulators and markers of adipogenesis, such as peroxisome proliferator-activated receptor gamma, CCAAT enhancer binding protein alpha, and fatty acid binding protein aP2. We demonstrate that SGK1 affects differentiation by direct phosphorylation of Foxo1, thereby changing its cellular localization from the nucleus to the cytosol. In addition we show that SGK1-/- cells are unable to relocalize Foxo1 to the cytosol in response to dexamethasone. Together these results show that SGK1 influences adipocyte differentiation by regulating Foxo1 phosphorylation and reveal a potentially important function for this kinase in the control of fat mass and function.

Regulation of Adipocyte Differentiation by the Zinc Finger Protein ZNF638

Zinc finger proteins constitute the largest family of transcription regulators in eukaryotes. These factors are involved in diverse processes in many tissues, including development and differentiation. We report here the characterization of the zinc finger protein ZNF638 as a novel regulator of adipogenesis. ZNF638 is induced early during adipocyte differentiation. Ectopic expression of ZNF638 increases adipogenesis in vitro, whereas its knockdown inhibits differentiation and decreases the expression of adipocyte-specific genes. ZNF638 physically interacts and transcriptionally cooperates with CCAAT/enhancer-binding protein (C/EBP) β and C/EBPδ. This interaction leads to the expression of peroxisome proliferator-activated receptor γ, which is the key regulator of adipocyte differentiation. In summary, ZNF638 is a novel and early regulator of adipogenesis that works as a transcription cofactor of C/EBPs.

Understanding the variegation of fat: Novel regulators of adipocyte differentiation and fat tissue biology☆☆

The differentiation of uncommitted cells into specialized adipocytes occurs through a cascade of transcriptional events culminating in the induction and activation of the nuclear receptor PPARγ, the central coordinator of fat cell function. Since the discovery of PPARγ, two decades ago, our views of how this molecule is activated have been significantly refined. Beyond the cell, we also now know that diverse signals and regulators control PPARγ function in a fat-depot specific manner. The goal of this article is to review the latest in our understanding of the early and late transcriptional events that regulate adipocyte development and their potential impact on energy storage and expenditure in different fat depots. This article is part of a Special Issue entitled: Modulation of Adipose Tissue in Health and Disease.