Adipose tissue is central to the regulation of energy balance. Two functionally different types of fat are present in mammals: white adipose tissue (WAT), the primary site of triglyceride storage, and brown adipose tissue (BAT), which is specialized in energy expenditure and can counteract obesity1. Factors that specify the developmental fate and function of white and brown adipose tissue remain poorly understood2,3. Here, we demonstrate that while some members of the family of bone morphogenetic proteins (BMP) support white adipocyte differentiation, BMP-7 singularly promotes differentiation of brown preadipocytes even in the absence of the normally required hormonal induction cocktail. BMP-7 activates a full program of brown adipogenesis including induction of early regulators of brown fat fate PRDM164 and PGC-1 (PPARγ coactivator-1) α5, increased expression of brown fat defining marker uncoupling protein-1 (UCP-1) and adipogenic transcription factors peroxisome proliferator-activated receptor (PPAR)γ and CCAAT/enhancer-binding proteins (C/EBPs), and mitochondrial biogenesis via a p38 MAP kinase and PGC-1 dependent pathway. Moreover, BMP-7 triggers commitment of mesenchymal progenitor cells to a brown adipocyte lineage, and implantation of these cells into nude mice results in development of adipose tissue containing mostly brown adipocytes. BMP-7 knockout embryos show a marked paucity of brown fat and near complete absence of UCP-1 protein. Adenoviral-mediated expression of BMP-7 in mice results in a significant increase in brown, but not white, fat mass and leads to an increase in energy expenditure and reduced weight gain. These data reveal an important role of BMP-7 in promoting brown adipocyte differentiation and thermogenesis in vivo and in vitro, and provide a potential novel therapeutic approach for the treatment of obesity.

The serine/threonine kinase Akt has been implicated in the control of cell survival and metabolism. Here we report the disruption of the most ubiquitously expressed member of the akt family of genes, akt1, in the mouse.

Adipose tissue expression and circulating concentrations of monocyte chemoattractant protein-1 (MCP-1) correlate positively with adiposity. To ascertain the roles of MCP-1 overexpression in adipose, we generated transgenic mice by utilizing the adipocyte P2 (aP2) promoter (aP2-MCP-1 mice). These mice had higher plasma MCP-1 concentrations and increased macrophage accumulation in adipose tissues, as confirmed by immunochemical, flow cytometric, and gene expression analyses. Tumor necrosis factor-␣ and interleukin-6 mRNA levels in white adipose tissue and plasma nonesterified fatty acid levels were increased in transgenic mice. aP2-MCP-1 mice showed insulin resistance, suggesting that inflammatory changes in adipose tissues may be involved in the development of insulin resistance. Insulin resistance in aP2-MCP-1 mice was confirmed by hyperinsulinemic euglycemic clamp studies showing that transgenic mice had lower rates of glucose disappearance and higher endogenous glucose production than wild-type mice. Consistent with this, insulin-induced phosphorylations of Akt were significantly decreased in both skeletal muscles and livers of aP2-MCP-1 mice. MCP-1 pretreatment of isolated skeletal muscle blunted insulin-stimulated glucose uptake, which was partially restored by treatment with the MEK inhibitor U0126, suggesting that circulating MCP-1 may contribute to insulin resistance in aP2-MCP-1 mice. We concluded that both paracrine and endocrine effects of MCP-1 may contribute to the development of insulin resistance in aP2-MCP-1 mice.

Adiponectin has been shown to stimulate fatty acid oxidation and enhance insulin sensitivity through the activation of AMP-activated protein kinase (AMPK) in the peripheral tissues. The effects of adiponectin in the central nervous system, however, are still poorly understood. Here, we show that adiponectin enhances AMPK activity in the arcuate hypothalamus (ARH) via its receptor AdipoR1 to stimulate food intake; this stimulation of food intake by adiponectin was attenuated by dominant-negative AMPK expression in the ARH. Moreover, adiponectin also decreased energy expenditure. Adiponectin-deficient mice showed decreased AMPK phosphorylation in the ARH, decreased food intake, and increased energy expenditure, exhibiting resistance to high-fat-diet-induced obesity. Serum and cerebrospinal fluid levels of adiponectin and expression of AdipoR1 in the ARH were increased during fasting and decreased after refeeding. We conclude that adiponectin stimulates food intake and decreases energy expenditure during fasting through its effects in the central nervous system.

Adiponectin/Acrp30 is a hormone secreted by adipocytes, which acts as an antidiabetic and antiatherogenic adipokine. We reported previously that AdipoR1 and -R2 serve as receptors for adiponectin and mediate increased fatty acid oxidation and glucose uptake by adiponectin. In the present study, we examined the expression levels and roles of AdipoR1/R2 in several physiological and pathophysiological states such as fasting/refeeding, obesity, and insulin resistance. Here we show that the expression of AdipoR1/R2 in insulin target organs, such as skeletal muscle and liver, is significantly increased in fasted mice and decreased in refed mice. Insulin deficiency induced by streptozotocin increased and insulin replenishment reduced the expression of AdipoR1/R2 in vivo. Thus, the expression of AdipoR1/R2 appears to be inversely correlated with plasma insulin levels in vivo. Interestingly, the incubation of hepatocytes or myocytes with insulin reduced the expression of AdipoR1/R2 via the phosphoinositide 3-kinase/Foxo1-dependent pathway in vitro. Moreover, the expressions of AdipoR1/R2 in ob/ob mice were significantly decreased in skeletal muscle and adipose tissue, which was correlated with decreased adiponectin binding to membrane fractions of skeletal muscle and decreased AMP kinase activation by adiponectin. This adiponectin resistance in turn may play a role in worsening insulin resistance in ob/ob mice. In conclusion, the expression of AdipoR1/R2 appears to be inversely regulated by insulin in physiological and pathophysiological states such as fasting/refeeding, insulin deficiency, and hyperinsulinemia models via the insulin/phosphoinositide 3-kinase/Foxo1 pathway and is correlated with adiponectin sensitivity.Adiponectin/Acrp30 (1-4) is a hormone secreted by adipocytes, which acts as an antidiabetic (5-12) and antiatherogenic (8, 12, 13) adipokine. This insulin-sensitizing effect of adiponectin appears to be mediated by an increase in fatty acid oxidation via activation of the 5Ј-AMP-activated protein kinase (AMPK) 1 (10, 11) and peroxisome proliferator-activated receptor-␣ (5, 6, 12). Very recently, we have reported the cloning of complementary DNAs encoding adiponectin receptors AdipoR1 and -R2 by expression cloning (14). AdipoR1 is abundantly expressed in skeletal muscle, whereas AdipoR2 is predominantly expressed in the liver. AdipoR1 and -R2 are predicted to contain seven transmembrane domains (14) but to be structurally and functionally distinct from G-protein-coupled receptors (15-17). AdipoR1 and -R2 serve as receptors for globular and full-length adiponectin and mediate increased AMPK (10, 11), peroxisome proliferator-activated receptor-␣ ligand activities (12), and fatty acid oxidation and glucose uptake by adiponectin (14).It has not yet been determined whether the expressions of AdipoR1 and -R2 are altered in physiological and pathophysiological states. To address these questions, we first studied the expressions of AdipoR1 and -R2 during fasting and refeeding. We also analyzed the expressions of Ad...

To investigate the role of insulin receptor substrate (IRS)-2 in vivo, we generated IRS-2-deficient mice by gene targeting. Although homozygous IRS-2-deficient mice (IRS-2 -/-mice) had a body weight similar to wildtype mice, they progressively developed type 2 diabetes at 10 weeks. IRS-2 -/-mice showed insulin resistance and a defect in the insulin-stimulated signaling pathway in liver but not in skeletal muscle. Despite insulin resistance, the amount of ␤-cells was reduced to 83% of that in wild-type mice, which was in marked contrast to the 85% increase in the amount of ␤-cells in IRS-1-deficient mice (IRS-1 -/-mice) to compensate for insulin resistance. Thus, IRS-2 plays a crucial role in the regulation of ␤-cell mass. On the other hand, insulin secretion by the same number of cells in response to glucose measured ex vivo was significantly increased in IRS-2 -/-mice compared with wild-type mice but was decreased in IRS-1 -/-mice. These results suggest that IRS-1 and IRS-2 may play different roles in the regulation of ␤-cell mass and the function of individual ␤-cells.

Glucokinase (Gck) functions as a glucose sensor for insulin secretion, and in mice fed standard chow, haploinsufficiency of β cell-specific Gck (Gck +/-) causes impaired insulin secretion to glucose, although the animals have a normal β cell mass. When fed a high-fat (HF) diet, wild-type mice showed marked β cell hyperplasia, whereas Gck +/-mice demonstrated decreased β cell replication and insufficient β cell hyperplasia despite showing a similar degree of insulin resistance. DNA chip analysis revealed decreased insulin receptor substrate 2 (Irs2) expression in HF diet-fed Gck +/-mouse islets compared with wild-type islets. Western blot analyses confirmed upregulated Irs2 expression in the islets of HF diet-fed wild-type mice compared with those fed standard chow and reduced expression in HF diet-fed Gck +/-mice compared with those of HF diet-fed wild-type mice. HF diet-fed Irs2 +/-mice failed to show a sufficient increase in β cell mass, and overexpression of Irs2 in β cells of HF diet-fed Gck +/-mice partially prevented diabetes by increasing β cell mass. These results suggest that Gck and Irs2 are critical requirements for β cell hyperplasia to occur in response to HF diet-induced insulin resistance.

Krüppel-like factor 5 (KLF5) is a zinc-finger transcription factor known to play a pivotal role in the pathogenesis of cardiovascular disease. Here, we show that neonatal heterozygous KLF5 knockout mice exhibit a marked deficiency in white adipose tissue development, suggesting that KLF5 is also required for adipogenesis. In 3T3-L1 preadipocytes, KLF5 expression was induced at an early stage of differentiation, and this was followed by expression of PPARgamma2. Constitutive overexpression of dominant-negative KLF5 inhibited adipocyte differentiation, whereas overexpression of wild-type KLF5 induced differentiation even without hormonal stimulation. Moreover, embryonic fibroblasts obtained from KLF5+/- mice showed much attenuated adipocyte differentiation, confirming the key role played by KLF5 in adipocyte differentiation. KLF5 expression is induced by C/EBPbeta and delta. KLF5, in turn, acts in concert with C/EBPbeta/delta to activate the PPARgamma2 promoter. This study establishes KLF5 as a key component of the transcription factor network controlling adipocyte differentiation.