Regulation of the Fructose Transporter Gene Slc2a5 Expression by Glucose in Cultured Microglial Cells.

Microglia play a role in the regulation of metabolism and pathogenesis of obesity. Microglial activity is altered in response to changes in diet and the body's metabolic state. Solute carrier family 2 member 5 (Slc2a5) that encodes glucose transporter 5 (GLUT5) is a fructose transporter primarily expressed in microglia within the central nervous system. However, little is known about the nutritional regulation of Slc2a5 expression in microglia and its role in the regulation of metabolism. The present study aimed to address the hypothesis that nutrients affect microglial activity by altering the expression of glucose transporter genes. Murine microglial cell line SIM-A9 cells and primary microglia from mouse brain were exposed to different concentrations of glucose and levels of microglial activation markers and glucose transporter genes were measured. High concentration of glucose increased levels of the immediate-early gene product c-Fos, a marker of cell activation, Slc2a5 mRNA, and pro-inflammatory cytokine genes in microglial cells in a time-dependent manner, while fructose failed to cause these changes. Glucose-induced changes in pro-inflammatory gene expression were partially attenuated in SIM-A9 cells treated with the GLUT5 inhibitor. These findings suggest that an increase in local glucose availability leads to the activation of microglia by controlling their carbohydrate sensing mechanism through both GLUT5-dependent and -independent mechanisms.

Stimulation of white adipose tissue lipolysis by xenin, a neurotensin-related peptide.

Xenin is a gastrointestinal hormone that belongs to the neurotensin family. Central administration of xenin to obese mice reduces food intake and body weight gain and causes alterations in the expression of lipid metabolism-related genes and proteins in white adipose tissue (WAT). However, it has not been tested whether or not xenin directly acts on adipose tissue and alters lipid metabolism. The present study was performed to address this possibility by examining the effect of xenin treatment on the levels of glycerol and free fatty acids (FFA) and expression levels of lipolysis marker proteins ex vivo in cultured mouse WAT. Xenin treatment significantly increased concentrations of glycerol and FFA in culture media and increased phosphorylation of hormone sensitive lipase (HSL) in ex vivo cultured WAT. These findings support the hypothesis that xenin directly acts on adipose tissues and stimulates lipolysis. Thus, enhancement of xenin action and its downstream signaling may offer a novel and effective therapy for obese patients by reducing the amount of stored fat in adipose tissue.

Central action of xenin affects the expression of lipid metabolism-related genes and proteins in mouse white adipose tissue.

Xenin is a gastrointestinal hormone that reduces food intake when administered centrally and it has been hypothesized that central action of xenin participates in the regulation of whole-body metabolism. The present study was performed to address this hypothesis by investigating the central effect of xenin on the expression of genes and proteins that are involved in the regulation of lipid metabolism in white adipose tissue (WAT). Male obese ob/ob mice received intracerebroventricular (i.c.v.) injections of xenin (5µg) twice 12h apart. Food intake and body weight change during a 24-h period after the first injection were measured. Epididymal WAT was collected at the end of the 24-h treatment period and levels of lipid metabolism-related genes and proteins were measured. Xenin treatment caused significant reductions in food intake and body weight compared to control vehicle treatment. Levels of fatty acid synthase (FASN) protein were significantly reduced by xenin treatment, while levels of adipose triglyceride lipase (Atgl) and beta-3 adrenergic receptor (Adrb3) mRNA and phosphorylated hormone sensitive lipase (Ser660-pHSL and Ser563-pHSL) were significantly increased by xenin treatment. These findings suggest that central action of xenin causes alterations in lipid metabolism in adipose tissue toward reduced lipogenesis and increased lipolysis, possibly contributing to xenin-induced body weight reduction. Thus, enhancing central action of xenin and its downstream targets may be possible targets for the treatment of obesity by reducing the amount of stored fat in adipose tissue.

Negative regulation of hepatic fat mass and obesity associated (Fto) gene expression by insulin.

AIMS: To investigate the role of glucose and insulin in the regulation of hepatic fat mass and obesity associated (Fto) gene expression and the role of hepatic Fto in the regulation of gluconeogenic gene expression. MAIN METHODS: To determine the effect of hyperglycemia on hepatic Fto expression, levels of Fto mRNA in liver were compared between normoglycemic/normoinsulinemic, hypereglycemic/hyperinsulinemic, and hyperglycemic/hypoinsulinemic mice. To determine the direct effect of insulin on Fto expression, levels of Fto, glucose-6-phosphatase (G6pase), and phosphoenolpyruvate carboxykinase (Pepck) mRNA levels were compared between control and insulin-treated mouse liver tissues cultured ex vivo and immortalized mouse hepatocytes AML12. To determine the role of Fto in the regulation of gluconeogenic gene expression, we examined the effect of enhanced Fto expression on G6pase and Pepck mRNA levels in AML12 cells. KEY FINDINGS: Fto mRNA levels were significantly reduced in hyperglycemic/hyperinsulinemic mice compared to normoglycemic/normoinsulinemic mice, while they were indistinguishable between hyperglycemic/hypoinsulinemic mice and normoglycemic/normoinsulinemic mice. Insulin treatment reduced Fto, G6pase, and Pepck mRNA levels compared to control vehicle treatment in both ex vivo cultured mouse liver tissues and AML12 cells. Enhanced Fto expression significantly increased G6pase and Pepck mRNA level in AML12 cells. SIGNIFICANCE: Our findings support the hypothesis that hepatic Fto participates in the maintenance of glucose homeostasis possibly by mediating the inhibitory effect of glucose and insulin on gluconeogenic gene expression in liver. It is further suggested that impairments in nutritional and hormonal regulation of hepatic Fto expression may lead to impairments in glycemic control in diabetes.

Xenin-induced feeding suppression is not mediated through the activation of central extracellular signal-regulated kinase signaling in mice.

Xenin is a gut hormone that reduces food intake by partly acting through the hypothalamus via neurotensin receptor 1 (Ntsr1). However, specific signaling pathways that mediate xenin-induced feeding suppression are not fully understood. Activation of Ntsr1 leads to the activation of the extracellular signal-regulated kinase (ERK). Hypothalamic ERK participates in the regulation of food intake by mediating the effect of hormonal signals. Therefore, we hypothesized that the anorectic effect of xenin is mediated by hypothalamic ERK signaling. To address this hypothesis, we compared levels of phosphorylation of ERK1/2 (pERK1/2) in the hypothalamus of both control and xenin-treated mice. The effect of xenin on ERK1/2 phosphorylation was also examined in mouse hypothalamic neuronal cell lines with or without Ntsr1. We also examined the effect of the blockade of central ERK signaling on xenin-induced feeding suppression in mice. The intraperitoneal (i.p.) injection of xenin caused a significant increase in the number of pERK1/2-immunoreactive cells in the hypothalamic arcuate nucleus. The majority of pERK1/2-positive cells expressed neuronal nuclei (NeuN), a marker for neurons. Xenin treatment increased pERK1/2 levels in one cell line expressing Ntsr1 but not another line without Ntsr1 expression. Both i.p. and intracerebroventricular (i.c.v.) injections of xenin reduced food intake in mice. The i.c.v. pre-treatment with U0126, a selective inhibitor of ERK1/2 upstream kinases, did not affect xenin-induced reduction in food intake. These findings suggest that although xenin activates ERK signaling in subpopulations of hypothalamic neurons, xenin does not require the activation of hypothalamic ERK signaling pathway to elicit feeding suppression.

Mediation of glucose-induced anorexia by central nervous system interleukin 1 signaling.

Hypothalamic glucose sensing plays a critical role in the regulation of food intake and metabolism. Glucose injection, either centrally or peripherally suppresses food intake. However, the mechanism of glucose-induced feeding suppression is not fully understood. It has been demonstrated that hypothalamic interleukin 1 beta (IL-1ß) mRNA levels are altered by metabolic states and IL-1 signaling participates in the regulation of food intake. Therefore, we hypothesized that hypothalamic IL-1 gene expression is regulated by glucose and glucose-induced feeding suppression is mediated via hypothalamic IL-1 signaling. To address this hypothesis, we examined the effect of glucose on IL-1α and IL-1ß mRNA expression in the hypothalamus. We also examined the effect of intraperitoneal injection of glucose on food intake in wild-type and type I IL-1 receptor (IL-1RI)-deficient mice. Levels of IL-1α and IL-1ß mRNA in the hypothalamus were increased in response to feeding and intraperitoneal injection of glucose, and were positively correlated with blood glucose levels in mice. Exposure of hypothalamic explants to high glucose (10 mM) media increased IL-1α and IL-1ß mRNA levels compared to low glucose (1 mM) media. Intraperitoneal glucose administration reduced food intake in wild-type mice, while the feeding-suppressing effect of glucose was attenuated in IL-1RI-deficient mice. These findings support the role for hypothalamic IL-1 signaling in the mediation of the anorectic effect of glucose.

Central melanocortin receptor agonist reduces hepatic lipogenic gene expression in streptozotocin-induced diabetic mice.

AIMS: The central melanocortin system regulates a variety of metabolic functions including lipid metabolism and hepatic lipogenic gene expression. The objective of the present study was to determine whether central melanocortin regulates hepatic lipogenic gene expression under insulin insufficient condition. MAIN METHODS: We examined the effect of intracerebroventricular (i.c.v.) injection of MTII, a melanocortin agonist, on hepatic gene expression in a mouse model of the insulin-deficient diabetes. Diabetes was induced in male C57BL/6J mice by intraperitoneal injections of streptozotocin (STZ). Diabetic mice received daily i.c.v. injections of MTII (3 nmol) for 11 days. Hepatic expression levels of lipogenic genes and their transcription factors were measured. KEY FINDINGS: MTII treatment significantly reduced hepatic expression levels of genes encoding lipid biosynthetic enzymes, stearoyl-CoA desaturase 1 (SCD1), glycerol-3-phosphate acyltransferase 1 (GPAT1), acyl-CoA:diacylglycerol acyltransferase 1 (DGAT1), and DGAT2 mRNA without significant changes in serum insulin levels, homeostasis model-assessment of insulin resistance (HOMA-IR) and glucose tolerance in STZ-induced diabetic mice. MTII treatment also reduced fatty acid synthase (FAS) and SCD1 protein levels in the liver of diabetic mice. Expression levels of genes encoding transcription factors of these lipogenic genes, sterol regulatory element-binding protein 1c (SREBP-1c) and peroxisome proliferator-activated receptor γ2 (PPARγ2) were also significantly reduced by MTII treatment. SIGNIFICANCE: These data suggest that the insulin-independent mechanism is involved in the regulation of hepatic lipogenic gene expression. Enhanced central melanocortin signaling may be effective in improving abnormal lipid metabolism associated with insulin-deficiency or insulin-insufficiency.

Treatment with a melanocortin agonist improves abnormal lipid metabolism in streptozotocin-induced diabetic mice.

Impairments in leptin-melanocortin signaling are associated with insulin-deficient diabetes and leptin treatment has been shown to be effective in reversing hyperglycemia in animal models of type 1 diabetes. Therefore, we hypothesized that enhanced central melanocortin signaling reverses the metabolic impairments associated with type 1 diabetes. To address this hypothesis, streptozotocin (STZ)-induced diabetic mice were treated with daily intracerebroventricular injection of MTII, a melanocortin agonist, for 11days. STZ-induced hyperglycemia and glucose intolerance were not improved by MTII treatment. MTII treatment did not alter expression levels of genes encoding gluconeogenic enzymes including glucose-6-phosphatase (G6Pase) and phosphoenolpyruvate carboxykinase (PEPCK), in the liver of diabetic mice. Skeletal muscle and white adipose tissue glucose transporter 4 (GLUT4) mRNA levels were not altered by MTII treatment in diabetic mice. In contrast, serum nonesterified fatty acid (NEFA) levels were significantly increased in STZ-induced diabetic mice compared to non-diabetic control mice and MTII treatment significantly reduced serum NEFA levels in diabetic mice. MTII treatment also significantly reduced expression levels of hormone sensitive lipase (HSL) and adipose triglyceride lipase (ATGL) mRNA in white adipose tissue of diabetic mice without a significant change in serum insulin levels. Expression levels of lipoprotein lipase (LPL) and fatty acid translocase (FAT/CD36) mRNA in white adipose tissue and skeletal muscle were not changed by MTII treatment. These data suggest that central melanocortin signaling regulates lipid metabolism and that enhancing central melanocortin signaling is effective in reversing abnormal lipid metabolism, but not carbohydrate metabolism, at least partly by reducing lipolysis in type 1 diabetes.

Relationship between blood glucose levels and hepatic Fto mRNA expression in mice.

Common variants in the fat mass and obesity associated (FTO) gene are associated with obesity and type 2 diabetes. Fto-deficient mice develop hepatic insulin resistance, leading to the hypothesis that hepatic Fto plays a role in the regulation of glucose metabolism and that hepatic Fto expression is regulated by metabolic states. We found that hepatic Fto mRNA levels were increased by fasting in mice. Intraperitoneal glucose injection reduced hepatic Fto mRNA levels without significant changes in body weight in fasted mice. The inverse correlation between Fto mRNA and glucose remained significant after adjusting for body weight. There were positive correlations between hepatic Fto mRNA expression and gluconeogenic gene expression. These data support the hypothesis that hepatic Fto expression changes in response to metabolic states and glucose reduces hepatic Fto mRNA expression independently of body weight. Hepatic Fto may participate in the feedback regulation of glucose metabolism via gluconeogenesis.

Tail suspension increases energy expenditure independently of the melanocortin system in mice.

Space travelers experience anorexia and body weight loss in a microgravity environment, and microgravity-like situations cause changes in hypothalamic activity. Hypothalamic melanocortins play a critical role in the regulation of metabolism. Therefore, we hypothesized that microgravity affects metabolism through alterations in specific hypothalamic signaling pathways, including melanocortin signaling. To address this hypothesis, the microgravity-like situation was produced by an antiorthostatic tail suspension in wild-type and agouti mice, and the effect of tail suspension on energy expenditure and hypothalamic gene expression was examined. Energy expenditure was measured using indirect calorimetry before and during the tail suspension protocol. Hypothalamic tissues were collected for gene expression analysis at the end of the 3 h tail suspension period. Tail suspension significantly increased oxygen consumption, carbon dioxide production, and heat production in wild-type mice. Tail suspension-induced increases in energy expenditure were not attenuated in agouti mice. Although tail suspension did not alter hypothalamic proopiomelanocortin (POMC) and agouti-related protein (AGRP) mRNA levels, it significantly increased hypothalamic interleukin 6 (Il-6) mRNA levels. These data are consistent with the hypothesis that microgravity increases energy expenditure and suggest that these effects are mediated through hypothalamic signaling pathways that are independent of melanocortins, but possibly used by Il-6.