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It is generally assumed that white adipocytes arise from resident adipose tissue mesenchymal progenitor cells. We challenge this paradigm by defining a hematopoietic origin for both the de novo development of a subset of white adipocytes in adults and a previously uncharacterized adipose tissue resident mesenchymal progenitor population. Lineage and cytogenetic analysis revealed that bone marrow progenitor (BMP)-derived adipocytes and adipocyte progenitors arise from hematopoietic cells via the myeloid lineage in the absence of cell fusion. Global gene expression analysis indicated that the BMP-derived fat cells are bona fide adipocytes but differ from conventional white or brown adipocytes in decreased expression of genes involved in mitochondrial biogenesis and lipid oxidation, and increased inflammatory gene expression. The BMP-derived adipocytes accumulate with age, occur in higher numbers in visceral than in subcutaneous fat, and in female versus male mice. BMP-derived adipocytes may, therefore, account in part for adipose depot heterogeneity and detrimental changes in adipose metabolism and inflammation with aging and adiposity. bone marrow | hematopoietic | stem cell doi/ 10.1073/pnas.1003512107

Fibroblast growth factor 21 (FGF21) has been identified as a potent metabolic regulator. Administration of recombinant FGF21 protein to rodents and rhesus monkeys with diet-induced or genetic obesity and diabetes exerts strong antihyperglycemic and triglyceride-lowering effects and reduction of body weight. Despite the importance of FGF21 in the regulation of glucose, lipid, and energy homeostasis, the mechanisms by which FGF21 functions as a metabolic regulator remain largely unknown. Here we demonstrate that FGF21 regulates energy homeostasis in adipocytes through activation of AMP-activated protein kinase (AMPK) and sirtuin 1 (SIRT1), resulting in enhanced mitochondrial oxidative function. AMPK phosphorylation levels were increased by FGF21 treatment in adipocytes as well as in white adipose tissue from ob/ob mice. FGF21 treatment increased cellular [NAD.sup.+] levels, leading to activation of SIRT1 and deacetylation of its downstream targets, peroxisome proliferator-activated receptor-[gamma] coactivator-1[alpha] (PGC-1[alpha]) and histone 3. Activation of AMPK and SIRT1 by FGF21 in adipocytes enhanced mitochondrial oxidative capacity as demonstrated by increases in oxygen consumption, citrate synthase activity, and induction of key metabolic genes. The effects of FGF21 on mitochondrial function require serine/ threonine kinase 11 (STK11/LKB1), which activates AMPK. Inhibition of AMPK, SIRT1, and PGC-1[alpha] activities attenuated the effects of FGF21 on oxygen consumption and gene expression, indicating that FGF21 regulates mitochondrial activity and enhances oxidative capacity through an AMPK--SIRT1-PGC1[alpha]-dependent mechanism in adipocytes. adipocytes | diabetes | obesity | mitochondrial function doi/10.1073/pnas.1006962107

Storage of energy as triglyceride in large adipose-specific lipid droplets is a fundamental need in all mammals. Efficient sequestration of fat in adipocytes also prevents fatty acid overload in skeletal muscle and liver, which can impair insulin signaling. Here we report that the Cide domain-containing protein Cidea, previously thought to be a mitochondrial protein, colocalizes around lipid droplets with perilipin, a regulator of lipolysis. Cidea-GFP greatly enhances lipid droplet size when ectopically expressed in preadipocytes or COS cells. These results explain previous findings showing that depletion of Cidea with RNAi markedly elevates lipolysis in human adipocytes. Like perilipin, Cidea and the related lipid droplet protein Cidec/FSP27 are controlled by peroxisome proliferator-activated receptor [gamma] (PPAR[gamma]). Treatment of lean or obese mice with the PPAR[gamma] agonist rosiglitazone markedly up-regulates Cidea expression in white adipose tissue (WAT), increasing lipid deposition. Strikingly, in both omental and s.c. WAT from BMI-matched obese humans, expression of Cidea, Cidec/FSP27, and perilipin correlates positively with insulin sensitivity (HOMA-IR index). Thus, Cidea and other lipid droplet proteins define a novel, highly regulated pathway of triglyceride deposition in human WAT. The data support a model whereby failure of this pathway results in ectopic lipid accumulation, insulin resistance, and its associated comorbidities in humans. adipocyte | Cide | diabetes | fat droplet | fat metabolism

The ability to store fat in the form of cytoplasmic triglyceride droplets is conserved from Saccharomyces cerevisiae to humans. Although much is known regarding the composition and catabolism of lipid droplets, the molecular components necessary for the biogenesis of lipid droplets have remained obscure. Here we report the characterization of a conserved gene family important for lipid droplet formation named fat-inducing transcript (FIT). FIT1 and FIT2 are endoplasmic reticulum resident membrane proteins that induce lipid droplet accumulation in cell culture and when expressed in mouse liver, shRNA silencing of FIT2 in 3T3-LI adipocytes prevents accumulation of lipid droplets, and depletion of FIT2 in zebrafish blocks diet-induced accumulation of lipid droplets in the intestine and liver, highlighting an important role for FIT2 in lipid droplet formation in vivo. Together these studies identify and characterize a conserved gene family that is important in the fundamental process of storing fat. adipocytes | diabetes | FIT | obesity | triglyceride

The E2F and pocket protein families are known to play an important role in the regulation of both cellular proliferation and terminal differentiation. In this study, we have used compound E2F and pocket protein mutant mouse embryonic fibroblasts to dissect the role of these proteins in adipogenesis. This analysis shows that loss of E2F4 allows cells to undergo spontaneous differentiation. The ability of E2F4 to prevent adipogenesis seems to be quite distinct from the known properties of E2F. First, it can be separated from any change in either E2F-responsive gene expression or cell cycle regulation. Second, it is a specific property of E2F4, and not other E2Fs, and it occurs independently of E2F4's ability to interact with pocket proteins. In addition, E2F4 loss does not override the differentiation defect resulting from pRB loss even though it completely suppresses the proliferation defect of [Rb.sup.-/-] mouse embryonic fibroblasts. This finding definitively separates the known, positive role of pRB in adipogenesis from its cell cycle function and shows that this pocket protein is required to act downstream of E2F4 in the differentiation process.

Lack of leptin (ob) protein causes obesity in mice. The leptin gene product is important for normal regulation of appetite and metabolic rate and is produced exclusively by adipocytes. Leptin mRNA was induced during the adipose conversion of 3T3-L1 cells, which are useful for studying adipocyte differentiation and function under controlled conditions. We studied leptin regulation by antidiabetic thiazolidinedione compounds, which are ligands for the adipocyte-specific nuclear receptor peroxisome proliferator-activated receptor [Gamma] ([PPAR.sub.[Gamma]]) that regulates the transcription of other adipocyte-specific genes. Remarkably, leptin gene expression was dramatically repressed within a few hours after thiazolidinedione treatment. The [ED.sub.50] for inhibition of leptin expression by the thiazolidinedione BRL49653 was between 5 and 50 nM, similar to its [K.sub.d] for binding to [PPAR.sub.[Gamma]]. The relatively weak, nonthiazolidinedione PPAR activator WY 14,643 also inhibited leptin expression, but was [approximately equal to]1000 times less potent than BRL49653. These results indicate that antidiabetic thiazolidinediones down-regulate leptin gene expression with potencies that correlate with their abilities to bind and activate [PPAR.sub.[Gamma]].

SV40 early region genes were placed under the control of the regulatory region from the aP2 gene and created transgenic mice to create a model system for studying adipocyte transformation. Brown fat tumors or hibernomas developed in transgenic mice as early as one day after birth which expressed brown fat-specific uncoupling protein gene as well as the aP2 gene. These results suggest that regulation of the UCP thermogenic gene can be studied in an established cell line.

Adipocyte complement-related protein (30 kDa) (Acrp30), a secreted protein of unknown function, is exclusively expressed in differentiated adipocytes; its mRNA is decreased in obese humans and mice. Here we describe novel pharmacological properties of the protease-generated globular head domain of Acrp30 (gAcrp30). Acute treatment of mice with gAcrp30 significantly decreased the elevated levels of plasma free fatty acids caused either by administration of a high fat test meal or by i.v. injection of Intralipid. This effect of gAcrp30 was caused, at least in part, by an acute increase in fatty acid oxidation by muscle. As a result, daily administration of a very low dose of gAcrp30 to mice consuming a high-fat/sucrose diet caused profound and sustainable weight reduction without affecting food intake. Thus, gAcrp30 is a novel pharmacological compound that controls energy homeostasis and exerts its effect primarily at the peripheral level.

Retinoid X receptor [Alpha] (RXR[Alpha]) is involved in multiple signaling pathways, as a heterodimeric partner of several nuclear receptors. To investigate its function in energy homeostasis, we have selectively ablated the RXR[Alpha] gene in adipocytes of 4-week-old transgenic mice by using the tamoxifen-inducible Cre-[ER.sup.T2] recombination system. Mice lacking RXR[Alpha] in adipocytes were resistant to dietary and chemically induced obesity and impaired in fasting-induced lipolysis. Our results also indicate that RXR[Alpha] is involved in adipocyte differentiation. Thus, our data demonstrate the feasibility of adipocyte-selective temporally controlled gene engineering and reveal a central role of RXR[Alpha] in adipogenesis, probably as a heterodimeric partner for peroxisome proliferator-activated receptor [Gamma]. peroxisome proliferator-activated receptor [Gamma] | conditional mutagenesis | obesity | fasting | adipocyte fatty acid binding protein

CCAAT enhancer-binding protein-[Alpha] (C/EBP[Alpha]) functions as a pleiotropic transcriptional activator of adipocyte genes during adipogenesis. Nuclear factor C/EBP undifferentiated protein (CUP), an isoform of activator protein-2[Alpha] (AP-2[Alpha]), binds to repressive elements in the C/EBP[Alpha] gene promoter, silencing the gene until late in the differentiation program. The CUP regulatory element overlaps a Sp (GT-box) element in the promoter to which Sp3 (or Sp1) can bind. Binding by Sp3 or Sp1 and CUP/AP2-[Alpha] is mutually exclusive. Sp3 is a strong transcriptional activator of the C/EBP[Alpha] gene promoter in 3T3-L1 preadipocytes and Schneider cells, this activation being repressed by CUP/AP-2[Alpha]. Sp3 is expressed throughout differentiation, whereas CUP/AP-2[Alpha], which is expressed only by preadipocytes, is down-regulated during differentiation coincident with transcription of the C/EBP[Alpha] gene. Thus, CUP/AP-2[Alpha] delays access of Sp3 to the Sp regulatory element, preventing premature expression of C/EBP[Alpha] and thereby interference by C/EBP[Alpha] (which is antimitotic) with mitotic clonal expansion, an essential early event in the differentiation program. 3T3-L1 adipocyte | CEBP[Alpha] undifferentiated protein | AP-2 | Sp1 | Sp3