Chronic treatment with compounds activating peroxisome proliferator-activated receptor (PPAR)␥ and -␣ influences body energy stores, but the underlying mechanisms are only partially known. In a chronic-dosing study, equiefficacious antihyperglycemic doses of the PPAR␥ agonist pioglitazone and PPAR␣/␥ dual activator ragaglitazar were administered to obesity-prone male rats. The PPAR␣ agonist fenofibrate had no effect on insulin sensitivity. Pioglitazone transiently increased and fenofibrate transiently decreased food intake, whereas ragaglitazar had no impact on feeding. As a result, body adiposity increased in pioglitazone-treated rats and decreased in fenofibrate-treated rats. PPAR␥ compounds markedly increased feed efficiency, whereas PPAR␣ agonist treatment decreased feed efficiency. In fenofibrate-treated rats, plasma acetoacetate was significantly elevated. Plasma levels of this potentially anorectic ketone body were unaffected in pioglitazoneand ragaglitazar-treated rats. High-fat feeding markedly increased visceral fat pads, and this was prevented by pioglitazone and ragaglitazar treatment. Pioglitazone treatment enlarged subcutaneous adiposity in high-fatfed rats. In conclusion, PPAR␥ activation increases both food intake and feed efficiency, resulting in net accumulation of subcutaneous body fat. The impact of PPAR␥ activation on feeding and feed efficiency appears to be partially independent because the PPAR␣ component of ragaglitazar completely counteracts the orexigenic actions of PPAR␥ activation without marked impact on feed efficiency. Diabetes 52:2249 -2259, 2003 T he nuclear receptors, peroxisome proliferatoractivated receptors (PPARs), constitute a family of three genes, PPAR␣, -␥, and -␤(␦), all of which are involved in control of energy homeostasis (1,2). Unequivocal evidence of endogenous ligands for PPAR␣ and -␥ is lacking, but a number of synthetic PPAR activating ligands exist, of which hypolipidemic fibric acids are typical examples of PPAR␣ activators while hypoglycemic thiazolidinediones are typical examples of PPAR␥ activators.The antidiabetic effects of PPAR␥ agonists are partly mediated via increased insulin sensitivity of adipose tissue and skeletal muscle. From clinical experience, PPAR␥ agonists are associated with weight gain, whereas PPAR␣ agonists appear body weight neutral (3). Part of the body weight increase may be caused by their oedema-inducing class effect, but activators of PPAR␥ also induce adipogenesis (4,5). They act preferentially on subcutaneous adipocytes, which in comparison to intrabdominal adipocytes express higher levels of PPAR␥ (6). The long-term metabolic consequences of the increased fat accumulation accompanying treatment with PPAR␥ agonists are not fully elucidated. Intrabdominal body fat accumulation is one of several hallmarks typifying the metabolic syndrome and, as such, an independent risk factor of type 2 diabetes (7-9). Many clinical trials of oral antidiabetic agents (including PPAR␥ agonists) are conspicuous by their absence of obese test subjec...

The acyl-CoA-binding protein (ACBP) is a 10-kDa intracellular protein that specifically binds acyl-CoA esters with high affinity and is structurally and functionally conserved from yeast to mammals. In vitro studies indicate that ACBP may regulate the availability of acylCoA esters for various metabolic and regulatory purposes. The protein is particularly abundant in cells with a high level of lipogenesis and de novo fatty acid synthesis and is significantly induced during adipocyte differentiation. However, the molecular mechanisms underlying the regulation of ACBP expression in mammalian cells have remained largely unknown. Here we report that ACBP is a novel peroxisome proliferator-activated receptor (PPAR)␥ target gene. The rat ACBP gene is directly activated by PPAR␥/retinoid X receptor ␣ (RXR␣) and PPAR␣/RXR␣, but not by PPAR␦/RXR␣, through a PPAR-response element in intron 1, which is functionally conserved in the human ACBP gene. The intronic PPAR-response element (PPRE) mediates induction by endogenous PPAR␥ in murine adipocytes and confers responsiveness to the PPAR␥-selective ligand BRL49653. Finally, we have used chromatin immunoprecipitation to demonstrate that the intronic PPRE efficiently binds PPAR␥/RXR in its natural chromatin context in adipocytes. Thus, the PPRE in intron 1 of the ACBP gene is a bona fide PPAR␥-response element.

Using restriction fragment differential display (RFDD) technology, we have identified the imprinted gene neuronatin (Nnat) as a hypothalamic target under the influence of leptin. Nnat mRNA expression is decreased in several key appetite regulatory hypothalamic nuclei in rodents with impaired leptin signaling and during fasting conditions. Furthermore, peripheral administration of leptin to ob/ob mice normalizes hypothalamic Nnat expression. Comparative immunohistochemical analysis of human and rat hypothalami demonstrates that NNAT protein is present in anatomically equivalent nuclei, suggesting human physiological relevance of the gene product(s). A putative role of Nnat in human energy homeostasis is further emphasized by a consistent association between single nucleotide polymorphisms (SNPs) in the human Nnat gene and severe childhood and adult obesity.

Stimulation of the G protein coupled receptor GPR120 has been shown to have anti-inflammatory and insulin-sensitizing effects, to promote glucagon like peptide-1 (GLP-1) secretion, and to play a key role in sensing dietary fat and control energy balance. In a search for differentially expressed genes potentially involved in food intake and body-weight regulation we identified GPR120 to be differentially regulated in the intestine of selectively bred diet induced obese (DIO) and diet resistant (DR) rats. Subsequently we investigated the effect of GPR120 receptor stimulation with the long chain fatty acid alpha linolenic acid (ALA) on GLP-1 secretion in rats. Independent of diet (high or low fat), GPR120 expression showed a two-fold increase in the intestine of DIO compared to DR rats. In situ hybridization revealed a broad expression of GPR120 in the gut mucosa in both intestinal epithelial and endocrine cells. Using double in situ hybridization GPR120 mRNA did not appear to be enriched in preproglucagon expressing L-cells. In line with the anatomical data, ALA administration did not increase circulating GLP-1 levels. Our data shows a widespread expression of GPR120 in the gut epithelium and can not confirm a major role for GPR120 in the regulation of GLP-1 secretion. The broad expression of GPR120 in the gut epithelium supports reports indicating a putative role of GPR120 as a sensor of dietary fat.

Pref-1 is a highly glycosylated Delta-like transmembrane protein containing six epidermal growth factorlike repeats in the extracellular domain. Pref-1 is abundantly expressed in preadipocytes, but expression is down-regulated during adipocyte differentiation. Forced expression of Pref-1 in 3T3-L1 cells was reported to inhibit adipocyte differentiation. Here we show that efficient and regulated processing of Pref-1 occurs in 3T3-L1 preadipocytes releasing most of the extracellular domain as a 50-kDa heterogeneous protein, previously isolated and characterized as FA1. Unexpectedly, we found that forced expression of the soluble form, FA1, or full-length Pref-1 did not inhibit adipocyte differentiation of 3T3-L1 cells when differentiation was induced by standard treatment with methylisobutylxanthine, dexamethasone, and high concentrations of insulin. However, forced expression of either form of Pref-1/FA1 in 3T3-L1 or 3T3-F442A cells inhibited adipocyte differentiation when insulin or insulin-like growth factor-1 (IGF-1) was omitted from the differentiation mixture. We demonstrate that the level of the mature form of the IGF-1 receptor is reduced and that IGF-1-dependent activation of p42/p44 mitogen-activated protein kinases (MAPKs) is compromised in preadipocytes with forced expression of Pref-1. This is accompanied by suppression of clonal expansion and terminal differentiation. Accordingly, supplementation with insulin or IGF-1 rescued p42/p44 MAPK activation, clonal expansion, and adipocyte differentiation in a dose-dependent manner.