Neuromedins are a family of peptides best known for their contractile activity on smooth muscle preparations. The biological mechanism of action of neuromedin U remains unknown, despite the fact that the peptide was first isolated in 1985. Here we show that neuromedin U potently activates the orphan G proteincoupled receptor FM3, with subnanomolar potency, when FM3 is transiently expressed in human HEK-293 cells. Neuromedins B, C, K, and N are all inactive at this receptor. Quantitative reverse transcriptase-polymerase chain reaction analysis of neuromedin U expression in a range of human tissues showed that the peptide is highly expressed in the intestine, pituitary, and bone marrow, with lower levels of expression seen in stomach, adipose tissue, lymphocytes, spleen, and the cortex. Similar analysis of FM3 expression showed that the receptor is widely expressed in human tissue with highest levels seen in adipose tissue, intestine, spleen, and lymphocytes, suggesting that neuromedin U may have a wide range of presently undetermined physiological effects. The discovery that neuromedin U is an endogenous agonist for FM3 will significantly aid the study of the full physiological role of this peptide. G protein-coupled receptors (GPCRs)1 represent one of the largest gene superfamilies identified to date, with more than 1000 members cloned from a wide range of species. The current explosion in the availability of human genomic sequence data is allowing many more members of this family to be identified in man. Most if not all of these newly identified GPCRs fall into the category of orphan receptors, for which the endogenous ligand(s) remain to be identified. Typically these orphan receptors show only low levels of similarity (less than 35% identity) with known GPCRs, too low to classify them with any confidence into a specific receptor subfamily, although one can often predict the likely class of ligand for these receptors, e.g. peptide, nucleotide, lipid, etc., by using phylogenetic analysis.Recently, naturally occurring ligands have been identified for a number of these orphan GPCRs using a "reverse-pharmacological" approach (1), that is using the recombinant orphan receptor as a specific sensor component of a bioassay. Tissue extracts have often been the source of these natural ligands (2, 3), although more recently the ligands for several orphans have been identified as a result of screening large libraries of known or putative GPCR ligands (4 -6). Here, we describe how this latter approach has been used to identify neuromedin U (NmU) as a naturally occurring ligand for the orphan receptor FM3.Neuromedin U was first isolated over 15 years ago from extracts of porcine spinal cord, using a uterine smooth muscle contraction bioassay to monitor purification (7). Two molecular forms were isolated; neuromedin U-8 (NmU-8) and neuromedin U-25 (NmU-25). NmU-like immunoreactivity has since been detected in neurones in the mammalian brain and gastrointestinal tracts of various species (8 -10) and in the thyroid and endocri...

Derangements in skeletal muscle fatty acid (FA) metabolism associated with insulin resistance in obesity appear to involve decreased FA oxidation and increased accumulation of lipids such as ceramides and diacylglycerol (DAG). We investigated potential lipid-related mechanisms of metformin (Met) and/or exercise for blunting the progression of hyperglycemia/hyperinsulinemia and skeletal muscle insulin resistance in female Zucker diabetic fatty rats (ZDF), a high-fat (HF) diet-induced model of diabetes. Lean and ZDF rats consumed control or HF diet (48 kcal %fat) alone or with Met (500 mg/kg), with treadmill exercise, or with both exercise and Met interventions for 8 wk. HF-fed ZDF rats developed hyperglycemia (mean: 24.4 +/- 2.1 mM), impairments in muscle insulin-stimulated glucose transport, increases in the FA transporter FAT/CD36, and increases in total ceramide and DAG content. The development of hyperglycemia was significantly attenuated with all interventions, as was skeletal muscle FAT/CD36 abundance and ceramide and DAG content. Interestingly, improvements in insulin-stimulated glucose transport and increased GLUT4 transporter expression in isolated muscle were seen only in conditions that included exercise training. Reduced FA oxidation and increased triacylglycerol synthesis in isolated muscle were observed with all ZDF rats compared with lean rats (P < 0.01) and were unaltered by therapeutic intervention. However, exercise did induce modest increases in peroxisome proliferator-activated receptor-gamma coactivator-1alpha, citrate synthase, and beta-hydroxyacyl-CoA dehydrogenase activity. Thus reduction of skeletal muscle FAT/CD36 and content of ceramide and DAG may be important mechanisms by which exercise training blunts the progression of diet-induced insulin resistance in skeletal muscle.

Mullen KL, Smith AC, Junkin KA, Dyck DJ. Globular adiponectin resistance develops independently of impaired insulin-stimulated glucose transport in soleus muscle from high-fat-fed rats. Am J Physiol Endocrinol Metab 293: E83-E90, 2007. First published March 13, 2007; doi:10.1152/ajpendo.00545.2006 diets induce insulin resistance and alter lipid metabolism, although controversy exists regarding the impact of saturated vs. polyunsaturated fats. Adiponectin (Ad) stimulates fatty acid (FA) oxidation and improves insulin sensitivity in humans and rodents, due in part to the activation of AMP-activated protein kinase (AMPK) and subsequent deactivation of acetyl coenzyme A carboxylase (ACC). In genetically obese, diabetic mice, this acute stimulatory effect on AMPK in muscle is lost. The ability of a HF diet to induce skeletal muscle Ad resistance has not been examined. The purpose of this study was to determine whether Ad's effects on FA oxidation and AMPK/ACC would be reduced following different HF diets, and if this coincided with the development of impaired maximal insulin-stimulated glucose transport. Rats were fed a control (10% kcal fat, CON), high unsaturated fat (60% kcal safflower oil, SAFF), or high saturated fat diet (60% kcal lard, LARD) for 4 wk. Following the dietary intervention, glucose transport, lipid metabolism, and AMPK/ACC phosphorylation were measured in the presence and absence of globular Ad (gAd, 2.5 g/ml) in isolated soleus muscle. LARD rats showed reduced rates of maximal insulin-stimulated glucose transport compared with CON and SAFF (ϩ68 vs. ϩ172 and ϩ184%, P Յ 0.001). gAd increased pACC (ϩ25%, P Յ 0.01) and FA oxidation (ϩ28%, P Յ 0.05) in CON rats, but not in either HF group. Thus 4 wk of HF feeding results in the loss of gAd stimulatory effect on ACC phosphorylation and muscle FA oxidation, and this can occur independently of impaired maximal insulin-stimulated glucose transport.

. Metformin counters the insulininduced suppression of fatty acid oxidation and stimulation of triacylglycerol storage in rodent skeletal muscle. Am J Physiol Endocrinol Metab 291: E182-E189, 2006. First published February 14, 2006 doi:10.1152/ajpendo.00272.2005.-The present study examined the acute effects of metformin on fatty acid (FA) metabolism in oxidative soleus (SOL) and glycolytic epitrochlearis (EPT) rodent muscle. SOL and EPT were incubated for either 30 or 180 min in the absence or presence of 2 mM metformin and with or without insulin (10 mU/ml). Metformin did not alter basal FA metabolism but countered the effects of insulin on FA oxidation and incorporation into triacylglyerol (TAG). Specifically, metformin prevented the insulin-induced suppression of FA oxidation in SOL but did not alter FA incorporation into lipid pools. In contrast, in EPT metformin blunted the incorporation of FA into TAG when insulin was present but did not alter FA oxidation. In SOL, metformin resulted in a 50% increase in AMPactivated protein kinase ␣2 activity and prevented the insulin-induced increase in malonyl-CoA content. In both fiber types, basal and insulin-stimulated glucose oxidation were not significantly altered by metformin. All effects were similar regardless of whether they were measured after 30 or 180 min. Because increased muscle lipid storage and impaired FA oxidation have been associated with insulin resistance in this tissue, the ability of metformin to reverse these abnormalities in muscle FA metabolism may be a part of the mechanism by which metformin improves glucose clearance and insulin sensitivity. The present data also suggest that increased glucose clearance is not due to its enhanced subsequent oxidation. Additional studies are warranted to determine whether chronic metformin treatment has similar effects on muscle FA metabolism. obesity; diabetes; lipid; adenosine 5Ј-monophosphate-activated protein kinase PHYSICAL INACTIVITY AND OBESITY are major risk factors for the development of type 2 diabetes. An appropriate lifestyle change, including dietary modifications and increased physical activity, is recommended for both the prevention and treatment of diabetes. In addition, pharmacological intervention, either alone or in combination with lifestyle changes, is an important means of treatment. There are various pharmacological agents available for the treatment of diabetes including sulfonylureas, thiazolidinediones, and biguanides. A commonly prescribed biguanide is metformin, also more commonly known as Glucophage. It is recognized that one of the primary mechanisms by which metformin improves the management of blood glucose in type 2 diabetics is to reduce hepatic glucose production (17, 31). Additionally, metformin increases peripheral glucose clearance (7,19,21), which also contributes to the control of blood glucose. This is mediated through a metformin-induced increase in the activity of AMP-activated protein kinase (AMPK), as recently demonstrated in diabetic human skeletal muscle (26). ...

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