OBJECTIVEFibroblast growth factor 21 (FGF21) is a key mediator of fatty acid oxidation and lipid metabolism. Pharmacological doses of FGF21 improve glucose tolerance, lower serum free fatty acids, and lead to weight loss in obese mice. Surprisingly, however, FGF21 levels are elevated in obese ob/ob and db/db mice and correlate positively with BMI in humans. However, the expected beneficial effects of endogenous FGF21 to increase glucose tolerance and reduce circulating triglycerides are absent in obesity.RESEARCH DESIGN AND METHODSTo test the hypothesis that obesity is a state of FGF21 resistance, we evaluated the response of obese mice to exogenous FGF21 administration. In doing this, we assessed the impact of diet-induced obesity on FGF21 signaling and resultant transcriptional events in the liver and white adipose tissue. We also analyzed the physiologic impact of FGF21 resistance by assessing serum parameters that are acutely regulated by FGF21.RESULTSWhen obese mice are treated with FGF21, they display both a significantly attenuated signaling response as assessed by extracellular mitogen-activated protein kinase 1 and 2 (ERK1/2) phosphorylation as well as an impaired induction of FGF21 target genes, including cFos and EGR1. These effects were seen in both liver and fat. Similarly, changes in serum parameters such as the decline in glucose and free fatty acids are attenuated in FGF21-treated DIO mice.CONCLUSIONSThese data demonstrate that DIO mice have increased endogenous levels of FGF21 and respond poorly to exogenous FGF21. We therefore propose that obesity is an FGF21-resistant state.

Background & Aims Fibroblast growth factor 21 (FGF21) is a hepatic protein that plays a critical role in metabolism, stimulating fatty acid oxidation in liver and glucose uptake in fat. Systemic administration to obese rodents and diabetic monkeys leads to improved glucose homeostasis and weight loss. In rodents, FGF21 increases with fasting and consumption of a ketogenic diet (KD). In humans, FGF21 correlates with body mass index, but studies evaluating other parameters show inconsistent results. We examined FGF21 serum levels in lean and obese individuals and in response to dietary manipulation. We also evaluated FGF21 serum levels and liver mRNA expression in nonalcoholic fatty liver disease (NAFLD) and nonalcoholic steatohepatitis (NASH). Methods Serum FGF21 was measured after an overnight fast in individuals with BMI ranging from normal to obese. Volunteers fasted for 16 or 72 hours and then ate a standard meal. Another group consumed KD for 12 days. Serum FGF21 and hepatic mRNA expression were measured in obese individuals with NAFLD or NASH. Results There was a positive correlation between BMI and FGF21. There was no change in FGF21 in response to a short fast or KD. A non-statistically significant fall in FGF21 levels was seen after a 72 hour fast. Hepatic FGF21 mRNA expression was significantly elevated in NAFLD, which correlated with a substantial increase in serum FGF21. In NASH, serum FGF21 but not liver mRNA was increased. Conclusion FGF21 correlates with BMI and may be a novel biomarker for NAFLD but is not nutritionally regulated in humans.

Adipose tissue plays a central role in the control of energy homeostasis through the storage and turnover of triglycerides and through the secretion of factors that affect satiety and fuel utilization. Agents that enhance insulin sensitivity, such as rosiglitazone, appear to exert their therapeutic effect through adipose tissue, but the precise mechanisms of their actions are unclear. Rosiglitazone changes the morphological features and protein profiles of mitochondria in 3T3-L1 adipocytes. To examine the relevance of these effects in vivo, we studied white adipocytes from ob/ob mice during the development of obesity and after treatment with rosiglitazone. The levels of approximately 50% of gene transcripts encoding mitochondrial proteins were decreased with the onset of obesity. About half of those genes were upregulated after treatment with rosiglitazone, and this was accompanied by an increase in mitochondrial mass and changes in mitochondrial structure. Functionally, adipocytes from rosiglitazone-treated mice displayed markedly enhanced oxygen consumption and significantly increased palmitate oxidation. These data reveal mitochondrial remodeling and increased energy expenditure in white fat in response to rosiglitazone treatment in vivo and suggest that enhanced lipid utilization in this tissue may affect whole-body energy homeostasis and insulin sensitivity. IntroductionThe ability of brown adipose tissue to affect whole-body metabolism upon thermogenic stress has been known for many years (1, 2). More recently, the important contribution of white adipose tissue (WAT) to the control of energy homeostasis has been recognized with the study of tissue-specific knockout models and the discovery of adipocyte-specific secreted factors that have powerful effects on fuel metabolism (3, 4). These observations have raised many as-yetunanswered questions about the mechanisms by which adipocytes maintain their energy homeostasis as well as sense and respond to the metabolic requirements of the organism.Brown adipocytes contain a large complement of mitochondria that serve as a source of heat generated as a consequence of flow through the electron transport chain under uncoupled conditions elicited by uncoupling protein 1 (UCP-1). Recently we reported that adipogenesis of the 3T3-L1 cell line, representative of white adipocytes, is also accompanied by a stimulation of mitochondrial biogenesis (5). The need for a large mitochondrial mass in white fat can be linked to key functions of the adipocyte that require mitochondrial function. For example, adipocytes must generate glycerol 3-phosphate at a rate sufficient to sustain triglyceride synthesis, and for this the glyceroneogenic pathway and

Peroxisome proliferator-activated receptor ␥ (PPAR␥) is the master regulator of adipogenesis as well as the target of thiazolidinedione (TZD) antidiabetic drugs. Many PPAR␥ target genes are induced during adipogenesis, but others, such as glycerol kinase (GyK), are expressed at low levels in adipocytes and dramatically up-regulated by TZDs. Here, we have explored the mechanism whereby an exogenous PPAR␥ ligand is selectively required for adipocyte gene expression. The GyK gene contains a functional PPAR␥-response element to which endogenous PPAR␥ is recruited in adipocytes. However, unlike the classic PPAR␥-target gene aP2, which is constitutively associated with coactivators, the GyK gene is targeted by nuclear receptor corepressors in adipocytes. TZDs trigger the dismissal of corepressor histone deacetylase (HDAC) complexes and the recruitment of coactivators to the GyK gene. TZDs also induce PPAR␥-Coactivator 1␣ (PGC-1␣), whose recruitment to the GyK gene is sufficient to release the corepressors. Thus, selective modulation of adipocyte PPAR␥ target genes by TZDs involves the dissociation of corepressors by direct and indirect mechanisms.

Background & Aims Nonalcoholic fatty liver disease (NAFLD) is a common consequence of human and rodent obesity. Disruptions in lipid metabolism lead to accumulation of triglycerides and fatty acids, which can promote inflammation and fibrosis and lead to nonalcoholic steatohepatitis (NASH). Circulating levels of fibroblast growth factor (FGF)21 increase in patients with NAFLD or NASH, so we assessed the role of FGF21 in the progression of murine fatty liver disease, independent of obesity, caused by methionine and choline deficiency. Methods C57BL/6 wild-type and FGF21-knockout (FGF21-KO) mice were placed on methionine- and choline-deficient (MCD), high-fat, or control diets for 8–16 weeks. Mice were weighed; serum and liver tissues were collected and analyzed for histology, levels of malondialdehyde and liver enzymes, gene expression, and lipid content. Results The MCD diet increased hepatic levels of FGF21 mRNA more than 50-fold and serum levels 16-fold, compared with the control diet. FGF21-KO mice had more severe steatosis, fibrosis, inflammation, and peroxidative damage than wild-type C57BL/6 mice. FGF21-KO mice had reduced hepatic fatty acid activation and β oxidation, resulting in increased levels of free fatty acid. FGF21-KO mice given continuous subcutaneous infusions of FGF21 for 4 weeks while on MCD diets had reduced steatosis and peroxidative damage, compared with mice not receiving FGF21. The expression of genes that regulate inflammation and fibrosis were reduced in FGF21-KO mice given FGF21, similar to those of wild-type mice. Conclusions FGF21 regulates fatty acid activation and oxidation in livers of mice. In the absence of FGF21, accumulation of inactivated fatty acids results in lipotoxic damage and increased steatosis.

Adipose tissue plays a central role in the control of energy homeostasis through the storage and turnover of triglycerides and through the secretion of factors that affect satiety and fuel utilization. Agents that enhance insulin sensitivity, such as rosiglitazone, appear to exert their therapeutic effect through adipose tissue, but the precise mechanisms of their actions are unclear. Rosiglitazone changes the morphological features and protein profiles of mitochondria in 3T3-L1 adipocytes. To examine the relevance of these effects in vivo, we studied white adipocytes from ob/ob mice during the development of obesity and after treatment with rosiglitazone. The levels of approximately 50% of gene transcripts encoding mitochondrial proteins were decreased with the onset of obesity. About half of those genes were upregulated after treatment with rosiglitazone, and this was accompanied by an increase in mitochondrial mass and changes in mitochondrial structure. Functionally, adipocytes from rosiglitazone-treated mice displayed markedly enhanced oxygen consumption and significantly increased palmitate oxidation. These data reveal mitochondrial remodeling and increased energy expenditure in white fat in response to rosiglitazone treatment in vivo and suggest that enhanced lipid utilization in this tissue may affect whole-body energy homeostasis and insulin sensitivity. IntroductionThe ability of brown adipose tissue to affect whole-body metabolism upon thermogenic stress has been known for many years (1, 2). More recently, the important contribution of white adipose tissue (WAT) to the control of energy homeostasis has been recognized with the study of tissue-specific knockout models and the discovery of adipocyte-specific secreted factors that have powerful effects on fuel metabolism (3, 4). These observations have raised many as-yetunanswered questions about the mechanisms by which adipocytes maintain their energy homeostasis as well as sense and respond to the metabolic requirements of the organism.Brown adipocytes contain a large complement of mitochondria that serve as a source of heat generated as a consequence of flow through the electron transport chain under uncoupled conditions elicited by uncoupling protein 1 (UCP-1). Recently we reported that adipogenesis of the 3T3-L1 cell line, representative of white adipocytes, is also accompanied by a stimulation of mitochondrial biogenesis (5). The need for a large mitochondrial mass in white fat can be linked to key functions of the adipocyte that require mitochondrial function. For example, adipocytes must generate glycerol 3-phosphate at a rate sufficient to sustain triglyceride synthesis, and for this the glyceroneogenic pathway and

In addition to its role in energy storage, adipose tissue also accumulates cholesterol. Concentrations of cholesterol and triglycerides are strongly correlated in the adipocyte, but little is known about mechanisms regulating cholesterol metabolism in fat cells. Here we report that antidiabetic thiazolidinediones (TZDs) and other ligands for the nuclear receptor PPARγ dramatically upregulate oxidized LDL receptor 1 (OLR1) in adipocytes by facilitating the exchange of coactivators for corepressors on the OLR1 gene in cultured mouse adipocytes. TZDs markedly stimulate the uptake of oxidized LDL (oxLDL) into adipocytes, and this requires OLR1. Increased OLR1 expression, resulting either from TZD treatment or adenoviral gene delivery, significantly augments adipocyte cholesterol content and enhances fatty acid uptake. OLR1 expression in white adipose tissue is increased in obesity and is further induced by PPARγ ligand treatment in vivo. Serum oxLDL levels are decreased in both lean and obese diabetic animals treated with TZDs. These data identify OLR1 as a novel PPARγ target gene in adipocytes. While the physiological role of adipose tissue in cholesterol and oxLDL metabolism remains to be established, the induction of OLR1 is a potential means by which PPARγ ligands regulate lipid metabolism and insulin sensitivity in adipocytes. IntroductionThe adipocyte is the major site of fatty acid storage in the body and plays a critical role in maintaining normal glucose and lipid homeostasis. In a healthy person, excess fat is stored as triglycerides in the adipose tissue, and fatty acids are released into the bloodstream only in response to an increased energy requirement, for example, during fasting. If the capacity of the adipocyte to store lipids is exceeded, it can no longer regulate the release of FFAs into the circulation, which ultimately leads to the abnormal accumulation of lipid in nonadipose depots. A buildup of triglycerides in the liver, pancreatic islets, and the muscle is thought to lead to metabolic dysregulation of these tissues (1); in particular, increased plasma FFA levels and elevated intramyocellular lipids are highly correlated with insulin resistance (2, 3).Obesity can be viewed as a state of long-term lipid disequilibrium that is marked by massive adipocyte hypertrophy and is a major risk factor for developing insulin resistance and type 2 diabetes. When compared with small fat cells from lean controls, enlarged adipocytes isolated from obese animals or humans demonstrate a decreased ability to store triglycerides (4), increased insulin resistance (5), and increased secretion of leptin and TNF-α (6). Interestingly, adipose tissue from ob/ob mice also exhibits an increase in cholesterol biosynthesis (7), and hypertrophied adipocytes from 2 obese rodent models showed elevated mRNA levels of SREBP-2, 3-hydroxy-3-methylglutaryl-CoA (HMG CoA) reductase, and the LDL receptor (8, 9), which suggests that these cells are relatively cholesterol deficient. Adipocytes normally contain a significant amount of ...

A number of RNA-binding proteins are associated with mRNAs in both the nucleus and the cytoplasm. One of these, Npl3p, is a heterogeneous nuclear ribonucleoprotein-like protein with some similarity to SR proteins and is essential for growth in the yeast S. cerevisiae. Temperature-sensitive alleles have defects in the export of mRNA out of the nucleus (1). In this report, we define a genetic relationship between NPL3 and the nonessential genes encoding the subunits of the cap-binding complex (CBP80 and CBP20). Deletion of CBP80 or CBP20 in combination with certain temperature-sensitive npl3 mutant alleles fail to grow and thus display a synthetic lethal relationship. Further evidence of an interaction between Npl3p and the cap-binding complex was revealed by co-immunoprecipitation experiments; Cbp80p and Cbp20p specifically co-precipitate with Npl3p. However, the interaction of Npl3p with Cbp80p depends on both the presence of Cbp20p and RNA. In addition, we show that Cbp80p is capable of shuttling between the nucleus and the cytoplasm in a manner dependent on the ongoing synthesis of RNA. Taken together, these data support a model whereby mRNAs are co-transcriptionally packaged by proteins including Npl3p and capbinding complex for export out of the nucleus.While in the nucleus, mRNA precursors, referred to as pre-mRNAs or heterogeneous nuclear RNAs undergo a series of processing events before entering the cytoplasm. These maturation events include co-transcriptional capping at the 5Ј-end, splicing, and cleavage and polyadenylation at the 3Ј-end. The proper execution of these steps affects the export of mRNA (reviewed in Refs. 2-4). Thus, the process of mRNA export commences long before the RNA actually reaches the nuclear membrane.