Many homeostatic processes, including appetite and food intake, are controlled by neuroendocrine circuits involving the CNS. The CNS also directly regulates adipocyte metabolism, as we have shown here by examining central action of the orexigenic hormone ghrelin. Chronic central ghrelin infusion resulted in increases in the glucose utilization rate of white and brown adipose tissue without affecting skeletal muscle. In white adipocytes, mRNA expression of various fat storage-promoting enzymes such as lipoprotein lipase, acetyl-CoA carboxylase alpha, fatty acid synthase, and stearoyl-CoA desaturase-1 was markedly increased, while that of the rate-limiting step in fat oxidation, carnitine palmitoyl transferase-1alpha, was decreased. In brown adipocytes, central ghrelin infusion resulted in lowered expression of the thermogenesis-related mitochondrial uncoupling proteins 1 and 3. These ghrelin effects were dose dependent, occurred independently from ghrelin-induced hyperphagia, and seemed to be mediated by the sympathetic nervous system. Additionally, the expression of some fat storage enzymes was decreased in ghrelin-deficient mice, which led us to conclude that central ghrelin is of physiological relevance in the control of cell metabolism in adipose tissue. These results unravel the existence of what we believe to be a new CNS-based neuroendocrine circuit regulating metabolic homeostasis of adipose tissue.

Disruptions of the melanocortin signaling system have been linked to obesity. We investigated a possible role of the central nervous melanocortin system (CNS-Mcr) in the control of adiposity through effects on nutrient partitioning and cellular lipid metabolism independent of nutrient intake. We report that pharmacological inhibition of melanocortin receptors (Mcr) in rats and genetic disruption of Mc4r in mice directly and potently promoted lipid uptake, triglyceride synthesis, and fat accumulation in white adipose tissue (WAT), while increased CNS-Mcr signaling triggered lipid mobilization. These effects were independent of food intake and preceded changes in adiposity. In addition, decreased CNS-Mcr signaling promoted increased insulin sensitivity and glucose uptake in WAT while decreasing glucose utilization in muscle and brown adipose tissue. Such CNS control of peripheral nutrient partitioning depended on sympathetic nervous system function and was enhanced by synergistic effects on liver triglyceride synthesis. Our findings offer an explanation for enhanced adiposity resulting from decreased melanocortin signaling, even in the absence of hyperphagia, and are consistent with feeding-independent changes in substrate utilization as reflected by respiratory quotient, which is increased with chronic Mcr blockade in rodents and in humans with loss-of-function mutations in MC4R. We also reveal molecular underpinnings for direct control of the CNS-Mcr over lipid metabolism. These results suggest ways to design more efficient pharmacological methods for controlling adiposity. IntroductionEnergy homeostasis, the balance of caloric intake and energy expenditure, is regulated by closely interconnected neuroendocrine and autonomic pathways emanating from and controlled by the CNS. Specific neurocircuitry, which is mainly located in hypothalamic and brain stem areas, continuously monitors signals reflecting energy status and initiates appropriate behavioral and metabolic responses to fluctuations in nutrient availability (1-4). Melanocortin neurons expressing genes encoding the endogenous ligands for melanocortin receptors (Mcr) are essential components of the system within the CNS that controls nutrient intake and energy metabolism (5-10). The central nervous melanocortin system (CNS-Mcr) is also the direct central

Activation of peroxisome proliferator-activated receptor γ (PPARγ) by thiazolidinediones (TZDs) improves insulin resistance by increasing insulin-stimulated glucose disposal in skeletal muscle. It remains debatable whether the effect of TZDs on muscle is direct or indirect via adipose tissue. We therefore generated mice with muscle-specific PPARγ knockout (MuPPARγKO) using Cre/loxP recombination. Interestingly, MuPPARγKO mice developed excess adiposity despite reduced dietary intake. Although insulin-stimulated glucose uptake in muscle was not impaired, MuPPARγKO mice had whole-body insulin resistance with a 36% reduction (P < 0.05) in the glucose infusion rate required to maintain euglycemia during hyperinsulinemic clamp, primarily due to dramatic impairment in hepatic insulin action. When placed on a high-fat diet, MuPPARγKO mice developed hyperinsulinemia and impaired glucose homeostasis identical to controls. Simultaneous treatment with TZD ameliorated these high fat-induced defects in MuPPARγKO mice to a degree identical to controls. There was also altered expression of several lipid metabolism genes in the muscle of MuPPARγKO mice. Thus, muscle PPARγ is not required for the antidiabetic effects of TZDs, but has a hitherto unsuspected role for maintenance of normal adiposity, whole-body insulin sensitivity, and hepatic insulin action. The tissue crosstalk mediating these effects is perhaps due to altered lipid metabolism in muscle.

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Reactive oxygen species (ROS) are at the centre of many physiological and pathological processes. NOX1, a ROS-producing NADPH oxidase, is highly expressed in the colon but its function in colonic physiology or pathology is still poorly understood. It has been suggested to play a role in host defence, but also in cell growth and possibly malignant transformation. In this study we characterized NOX1 expression in human colon samples derived from healthy control subjects and patients with colon cancer or inflammatory bowel disease (IBD). NOX1 mRNA expression was assessed by dot-blot hybridization, real-time PCR and in situ hybridization, using samples derived from surgical specimens from patients undergoing colon resection. In normal tissues, NOX1 expression was low in the ileum, intermediate in the right colon, and high in the left colon (p = 0.0056 right vs. left colon). NOX1 mRNA levels were not influenced by factors linked to colon tumourigenesis, such as age or sex. Moreover, there was no statistical difference in NOX1 expression between samples derived from adenomas, well differentiated or poorly differentiated colon adenocarcinomas. At a cellular level, NOX1 was highly expressed in colon epithelial cells, both within the crypts and on the luminal surface. In addition, a population of lymphocytes, particularly in the appendix, showed NOX1 expression. Lymphocytes in lesions of Crohn's disease and ulcerative colitis were also strongly positive for NOX1. In conclusion, NOX1 is an enzyme that is constitutively expressed in colon epithelium and is not associated with tumourigenesis. Its distribution in crypts and on the luminal surface, as well as its left-to-right gradient in the colon, suggests a role in host defence function. In addition to the known epithelial localization, we define lymphocytes as a novel site of NOX1 expression, where it may potentially be involved in the pathogenesis of inflammatory bowel diseases.

Leptin is a 16-kDa hormone secreted by adipocytes and plays an important role in control of feeding behavior and energy expenditure. In obesity, circulating levels of leptin and insulin are high because of the presence of increased body fat mass and insulin resistance. Recent reports have suggested that leptin can act through some of the components of the insulin signaling cascade, such as insulin receptor substrates (IRS-1 and IRS-2), phosphatidylinositol 3-kinase (PI 3-kinase), and mitogen-activated protein kinase, and can modify insulin-induced changes in gene expression in vitro and in vivo. Well differentiated hepatoma cells (Fao) possess both the long and short forms of the leptin receptor and respond to leptin with a stimulation of c-fos gene expression. In Fao cells, leptin alone had no effects on the insulin signaling pathway, but leptin pretreatment transiently enhanced insulininduced tyrosine phosphorylation and PI 3-kinase binding to IRS-1, while producing an inhibition of tyrosine phosphorylation and PI 3-kinase binding to IRS-2. Leptin alone also induced serine phosphorylation of Akt and glycogen synthase kinase 3 but to a lesser extent than insulin, and the combination of these hormones was not additive. These results suggest complex interactions between the leptin and insulin signaling pathways that can potentially lead to differential modification of the metabolic and mitotic effects of insulin exerted through IRS-1 and IRS-2 and the downstream kinases that they activate.T he product of the ob gene is leptin, a 16-kDa peptide hormone produced by adipocytes that acts in the hypothalamus and plays a central role in regulation of feeding behavior and energy homeostasis (1-4). The leptin receptor (OB-R) occurs in several isoforms that differ in the length of their intracellular domains because of alternative splicing of the gene (5, 6). The long form is termed OB-Rb or OB-R L and is expressed abundantly in specific nuclei of the hypothalamus. The short forms, OB-Ra, c, d, and e (collectively referred to OB-R S ), have a wide tissue distribution. The long form of the OB-R belongs to the gp130 family of cytokine receptors that also includes the receptor for IL-6, leukocyte inhibitory factor, and granulocyte colony stimulating factor. These receptors act by activating cytoplasmic tyrosine kinases of the Janus kinase (JAK) family that in return phosphorylate specific transcription factors of the Stat (signal transducer and activator of transcription) family (7-10). On phosphorylation, the Stat proteins dimerize and translocate to the nucleus where they bind to specific nucleotide sequences and induce gene expression. Despite the abundance of the short forms of receptor, little is known about their physiological significance. Cells transfected with the short form of receptor may be capable of activating JAK kinases but fail to phosphorylate Stat proteins or activate gene expression (6,8).In vivo and in vitro evidence supports the hypothesis that leptin and insulin signaling networks may be connected at s...

Activation of peroxisome proliferator-activated receptor γ (PPARγ) by thiazolidinediones (TZDs) improves insulin resistance by increasing insulin-stimulated glucose disposal in skeletal muscle. It remains debatable whether the effect of TZDs on muscle is direct or indirect via adipose tissue. We therefore generated mice with muscle-specific PPARγ knockout (MuPPARγKO) using Cre/loxP recombination. Interestingly, MuPPARγKO mice developed excess adiposity despite reduced dietary intake. Although insulin-stimulated glucose uptake in muscle was not impaired, MuPPARγKO mice had whole-body insulin resistance with a 36% reduction (P < 0.05) in the glucose infusion rate required to maintain euglycemia during hyperinsulinemic clamp, primarily due to dramatic impairment in hepatic insulin action. When placed on a high-fat diet, MuPPARγKO mice developed hyperinsulinemia and impaired glucose homeostasis identical to controls. Simultaneous treatment with TZD ameliorated these high fat-induced defects in MuPPARγKO mice to a degree identical to controls. There was also altered expression of several lipid metabolism genes in the muscle of MuPPARγKO mice. Thus, muscle PPARγ is not required for the antidiabetic effects of TZDs, but has a hitherto unsuspected role for maintenance of normal adiposity, whole-body insulin sensitivity, and hepatic insulin action. The tissue crosstalk mediating these effects is perhaps due to altered lipid metabolism in muscle.

NADPH oxidases synthesize reactive oxygen species that may participate in fibrosis progression. NOX4 and NOX2 are NADPH oxidases expressed in the kidneys, with the former being the major renal isoform, but their contribution to renal disease is not well understood. Here, we used the unilateral urinary obstruction model of chronic renal injury to decipher the role of these enzymes using wild-type, NOX4-, NOX2-, and NOX4/NOX2-deficient mice. Compared with wild-type mice, NOX4-deficient mice exhibited more interstitial fibrosis and tubular apoptosis after obstruction, with lower interstitial capillary density and reduced expression of hypoxia-inducible factor-1a and vascular endothelial growth factor in obstructed kidneys. Furthermore, NOX4-deficient kidneys exhibited increased oxidative stress. With NOX4 deficiency, renal expression of other NOX isoforms was not altered but NRF2 protein expression was reduced under both basal and obstructed conditions. Concomitant deficiency of NOX2 did not modify the phenotype exhibited by NOX4-deficient mice after obstruction. NOX4 silencing in a mouse collecting duct (mCCD cl1 ) cell line increased TGF-b1-induced apoptosis and decreased NRF2 protein along with expression of its target genes. In addition, NOX4 silencing decreased hypoxia-inducible factor-1a and expression of its target genes in response to hypoxia. In summary, these results demonstrate that the absence of NOX4 promotes kidney fibrosis, independent of NOX2, through enhanced tubular cell apoptosis, decreased microvascularization, and enhanced oxidative stress. Thus, NOX4 is crucial for the survival of kidney tubular cells under injurious conditions.