Ablation of ghrelin improves the diabetic but not obese phenotype of ob/ob mice

Ghrelin and leptin are suggested to regulate energy homeostasis as mutual antagonists on hypothalamic neurons that regulate feeding behavior. We employed reverse genetics to investigate the interplay between ghrelin and leptin. Leptin-deficient mice (ob/ob) are hyperphagic, obese, and hyperglycemic. Unexpectedly, ablation of ghrelin in ob/ob mice fails to rescue the obese hyperphagic phenotype, indicating that the ob/ob phenotype is not a consequence of ghrelin unopposed by leptin. Remarkably, deletion of ghrelin augments insulin secretion in response to glucose challenge and increases peripheral insulin sensitivity; indeed, the hyperglycemia exhibited by ob/ob mice is markedly reduced when ob/ob mice are bred onto the ghrelin(-/-) background. We further demonstrate that ablation of ghrelin reduces expression of Ucp2 mRNA in the pancreas, which contributes toward enhanced glucose-induced insulin secretion. Hence, chronically, ghrelin controls glucose homeostasis by regulating pancreatic Ucp2 expression and insulin sensitivity.     

Leptin induces the hepatic high density lipoprotein receptor scavenger receptor B type I (SR-BI) but not cholesterol 7alpha-hydroxylase (Cyp7a1) in leptin-deficient (ob/ob) mice

Cholesterol elimination from the body involves reverse cholesterol transport from peripheral tissues in which the elimination of high density lipoprotein (HDL) and low density lipoprotein (LDL) cholesterol by the liver and subsequent biliary excretion as free cholesterol and bile acids are important. In situations of peripheral fat and cholesterol accumulation, such as obesity, these pathways may be overloaded, contributing to increased cholesterol deposition. Leptin has an important role in obesity, suppressing food intake and increasing energy expenditure. This hormone, which is absent in genetically obese ob/ob mice, is also thought to be involved in the coordination of lipid excretion pathways, although available data are somewhat inconsistent. We therefore studied the expression of the hepatic HDL receptor, scavenger receptor class B type I (SR-BI), and the LDL receptor as well as the rate-limiting enzyme in bile acid synthesis, cholesterol 7alpha-hydroxylase (Cyp7a1), in leptin-deficient ob/ob mice and their wild-type controls. In ob/ob mice, protein levels of both LDL receptor and SR-BI were reduced, whereas LDL receptor mRNA levels were increased and those of SR-BI were reduced, regardless of challenge with a 2% cholesterol diet. In ob/ob mice, the enzymatic activity and mRNA for Cyp7a1 were reduced, and the increase in response to dietary cholesterol was blunted. Upon short-term (2 days) treatment with leptin, a dose-dependent increase was seen in the SR-BI protein and mRNA, whereas the Cyp7a1 protein and mRNA were reduced. Our findings indicate that leptin is an important regulator of hepatic SR-BI expression and, thus, HDL cholesterol levels, whereas it does not stimulate Cyp7a1 and bile acid synthesis.     

Ablation of ghrelin receptor in leptin-deficient ob/ob mice has paradoxical effects on glucose homeostasis when compared with ablation of ghrelin in ob/ob mice

The orexigenic hormone ghrelin is important in diabetes because it has an inhibitory effect on insulin secretion. Ghrelin ablation in leptin-deficient ob/ob (Ghrelin(-/-):ob/ob) mice increases insulin secretion and improves hyperglycemia. The physiologically relevant ghrelin receptor is the growth hormone secretagogue receptor (GHS-R), and GHS-R antagonists are thought to be an effective strategy for treating diabetes. However, since some of ghrelin's effects are independent of GHS-R, we have utilized genetic approaches to determine whether ghrelin's effect on insulin secretion is mediated through GHS-R and whether GHS-R antagonism indeed inhibits insulin secretion. We investigated the effects of GHS-R on glucose homeostasis in Ghsr-ablated ob/ob mice (Ghsr(-/-):ob/ob). Ghsr ablation did not rescue the hyperphagia, obesity, or insulin resistance of ob/ob mice. Surprisingly, Ghsr ablation worsened the hyperglycemia, decreased insulin, and impaired glucose tolerance. Consistently, Ghsr ablation in ob/ob mice upregulated negative β-cell regulators (such as UCP-2, SREBP-1c, ChREBP, and MIF-1) and downregulated positive β-cell regulators (such as HIF-1α, FGF-21, and PDX-1) in whole pancreas; this suggests that Ghsr ablation impairs pancreatic β-cell function in leptin deficiency. Of note, Ghsr ablation in ob/ob mice did not affect the islet size; the average islet size of Ghsr(-/-):ob/ob mice is similar to that of ob/ob mice. In summary, because Ghsr ablation in leptin deficiency impairs insulin secretion and worsens hyperglycemia, this suggests that GHS-R antagonists may actually aggravate diabetes under certain conditions. The paradoxical effects of ghrelin ablation and Ghsr ablation in ob/ob mice highlight the complexity of the ghrelin-signaling pathway.     

Ablation of Ghrelin O-Acyltransferase Does Not Improve Glucose Intolerance or Body Adiposity in Mice on a Leptin-Deficient ob/ob Background

Type 2 Diabetes is a global health burden and based on current estimates will become an even larger problem in the future. Developing new strategies to prevent and treat diabetes is a scientific challenge of high priority. The stomach hormone ghrelin has been associated with playing a role in the regulation of glucose homeostasis. However, its precise mechanism and impact on whole glucose metabolism remains to be elucidated. This study aims to clarify the role of the two ghrelin isoforms acyl- and desacyl ghrelin in regulating glucose homeostasis. Therefore ghrelin activating enzyme Ghrelin-O-acyltransferase (GOAT) was ablated in leptin-deficient ob/ob mice to study whether specific acyl ghrelin deficiency or desacyl ghrelin abundance modifies glucose tolerance on a massively obese background. As targeted deletion of acyl ghrelin does not improve glucose homeostasis in our GOAT-ob/ob mouse model we conclude that neither acyl ghrelin nor the increased ratio of desacyl/acyl ghrelin is crucial for controlling glucose homeostasis in the here presented model of massive obesity induced by leptin deficiency.     

Role of adrenal glands in the development of abnormal glucose and insulin homeostasis in genetically obese (ob/ob) mice

This study evaluates the role of adrenal hormones in the development of hyperinsulinaemia and impaired glucose homeostasis in genetically obese hyperglycaemic C57BL/6J ob/ob mice. Lean (+/?) and obese mice were bilaterally adrenalectomised or sham operated at 5 weeks of age, and glucose tolerance was examined after 7 and 14 days. Adrenalectomy temporarily reduced food intake and body weight gain in lean mice, and improved glucose tolerance without a significant change in plasma insulin concentrations at both intervals studied. In obese mice adrenalectomy permanently reduced body weight gain and food intake to values comparable with lean mice. Glucose tolerance was improved in adrenalectomised obese mice at both intervals studied, resulting in plasma glucose concentrations similar to adrenalectomised lean mice. Plasma insulin concentrations during the tolerance tests were reduced in adrenalectomised obese mice, but remained higher than in lean mice. Adrenalectomy did not improve the poor insulin response to parenteral glucose in obese mice. The results indicate that adrenal hormones play an important role in the development of glucose intolerance and contribute to the hyperinsulinaemia in obese (ob/ob) mice, in part by promoting hyperphagia.     

Neuropeptide Y promotes beta-cell replication via extracellular signal-regulated kinase activation

AIMS/HYPOTHESIS: Previous studies have shown that neuropeptide Y (NPY) gene expression and release are increased in hyperphagic ob/ob mice and diabetic rats. Therefore, we hypothesized that orexigenic agent, NPY, has the effect on the obesity and diabetes. To elucidate the relationship, we have studied the regulatory role of NPY on islet cells. METHODS: Isolated islets were incubated with NPY or NPY Y1 receptor specific antagonist, BIBP3226. Proliferation, apoptosis, and Y1 receptor expression were identified by immunohistochemistry. We studied that ERK1/2 mediates the NPY pathway with PD98059 (MAP kinase inhibitor), wortmannin (phosphatidylinositol 3-kinase inhibitor), and BIM-1 (protein kinase C inhibitor). After NPY-treated islets were exposed to high glucose, insulin levels were detected. RESULTS: beta-Cell replication was enhanced in a dose-dependent manner, but without any changes on the other cells in islet. NPY Y1 receptors were expressed on islet and NPY induced phosphorylation of ERK1/2 rapidly and transiently. PD98059 (MAPK kinase inhibitor) and BIM-1 (protein kinase C inhibitor) inhibited activation of ERK1/2 by NPY, but wortmannin (phosphatidylinositol 3-kinase inhibitor) did not. Exposure of NPY-treated islets to high glucose showed the decreasing trend of insulin secretion. CONCLUSION/INTERPRETATION: Our data suggest that NPY promotes beta-cell replication via extracellular signal-regulated kinase activation and inhibits glucose-stimulated insulin secretion.     

Effects of zinc supplementation on the plasma glucose level and insulin activity in genetically obese (ob/ob) mice

The effects of zinc supplementation (20 mM ZnCl₂ from the drinking water for eight weeks) on plasma glucose and insulin levels, as well as its in vitro effect on lipogenesis and lipolysis in adipocytes were studied in genetically obese (ob/ob) mice and their lean controls (+/?). Zinc supplementation reduced the fasting plasma glucose levels in both obese and lean mice by 21 and 25%, respectively (p < 0.05). Fasting plasma insulin levels were significantly decreased by 42% in obese mice after zinc treatment. In obese mice, zinc supplementation also attenuated the glycemic response by 34% after the glucose load. The insulin-like effect of zinc on lipogenesis in adipocytes was significantly increased by 80% in lean mice. However, the increment of 74% on lipogenesis in obese mice was observed only when the zinc plus insulin treatment was given. This study reveals that zinc supplementation alleviated the hyperglycemia of ob/ob mice, which may be related to its effect on the enhancement of insulin activity.     

Leptin treatment markedly increased plasma adiponectin but barely decreased plasma resistin of ob/ob mice

Adiponectin (ApN) and leptin are two adipocytokines that control fuel homeostasis, body weight, and insulin sensitivity. Their interplay is still poorly studied. These hormones are either undetectable or decreased in obese, diabetic ob/ob mice. We examined the effects of leptin treatment on ApN gene expression, protein production, secretion, and circulating levels of ob/ob mice. We also briefly tackled the influence of this treatment on resistin, another adipocytokine involved in obesity-related insulin resistance. Leptin-treated (T) obese mice (continuous sc infusion for 6 days) were compared with untreated lean (L), untreated obese (O), and untreated pair-fed obese (PF) mice. Blood was collected throughout the study. At day 3 or day 6, fat pads were either directly analyzed (mRNA, ApN content) or cultured for up to 24 h (ApN secretion). The direct effect of leptin was also studied in 3T3-F442A adipocytes. Compared with L mice, ApN content of visceral or subcutaneous fat and ApN secretion by adipose explants were blunted in obese mice. Accordingly, plasma ApN levels of O mice were decreased by 50%. Leptin treatment of ob/ob mice increased ApN mRNAs, ApN content, and secretion from the visceral depot by 50-80%. Leptin also directly stimulated ApN mRNAs and secretion from 3T3-F442A adipocytes. After 6 days of treatment, plasma ApN of ob/ob mice increased 2.5-fold, a rise that did not occur in PF mice. Plasma resistin of T mice was barely decreased. Leptin treatment, but not mere calorie restriction, corrects plasma ApN in obese mice by restoring adipose tissue ApN concentrations and secretion, at least in part, via a direct stimulation of ApN gene expression. Such a treatment only minimally affects circulating resistin. ApN restoration could, in concert with leptin, contribute to the metabolic effects classically observed during leptin administration.     

Neuropeptide Y-mediated inhibition of proopiomelanocortin neurons in the arcuate nucleus shows enhanced desensitization in ob/ob mice

NPY and alphaMSH are expressed in distinct neurons in the arcuate nucleus of the hypothalamus, where alphaMSH decreases and NPY increases food intake and body weight. Here we use patch-clamp electrophysiology from GFP-labeled POMC and NPY neurons to demonstrate that NPY strongly hyperpolarized POMC neurons through the Y1R-mediated activation of GIRK channels, while the alphaMSH analog, MTII, had no effect on activity of NPY neurons. While initially NPY had similar effects on POMC neurons derived from ob/ob mice, further studies revealed a significant increase in desensitization of the NPY-induced currents in POMC neurons from ob/ob mice. This increase in desensitization was specific to NPY, as GABA(B) and microOR agonists showed unaltered desensitization in POMC neurons from ob/ob mice. These data reveal an intricate and asymmetric interplay between NPY and POMC neurons in the hypothalamus and have important implications for the delineation of the neural circuits that regulate feeding behavior.     

Macrophage-specific transgenic expression of cholesteryl ester hydrolase attenuates hepatic lipid accumulation and also improves glucose tolerance in ob/ob mice

Cellular cholesterol homeostasis is increasingly being recognized as an important determinant of the inflammatory status of macrophages, and a decrease in cellular cholesterol levels polarizes macrophages toward an anti-inflammatory or M2 phenotype. Cholesteryl ester hydrolase (CEH) catalyzes the hydrolysis of stored intracellular cholesteryl esters (CE) and thereby enhances free cholesterol efflux and reduces cellular CE content. We have reported earlier reduced atherosclerosis as well as lesion necrosis and improved insulin sensitivity (due to decreased adipose tissue inflammation) in macrophage-specific CEH transgenic (CEHTg) mice in the LDLR(-/-) background. In the present study, we examined the effects of reduced intracellular accumulation of CE in CEHTg macrophages in an established diabetic mouse model, namely the leptin-deficient ob/ob mouse. Macrophage-specific transgenic expression of CEH improved glucose tolerance in ob/ob-CEHTg mice significantly compared with ob/ob nontransgenic littermates, but with no apparent change in macrophage infiltration into the adipose tissue. However, there was a significant decrease in hepatic lipid accumulation in ob/ob-CEHTg mice. Consistently, decreased [(14)C]acetate incorporation into total lipids and triglycerides was noted in precision-cut liver slices from ob/ob-CEHTg mice. In the primary hepatocyte-macrophage coculture system, macrophages from CEHTg mice significantly reduced the incorporation of [(14)C]acetate into triglycerides in hepatocytes, indicating a direct effect of macrophages on hepatocyte triglyceride biosynthesis. Kupffer cells isolated from ob/ob-CEHTg mice were polarized toward an anti-inflammatory M2 (Ly6C(lo)) phenotype. Taken together, these studies demonstrate that transgenic overexpression of CEH in macrophages polarizes hepatic macrophages (Kupffer cells) to an anti-inflammatory M2 phenotype that attenuates hepatic lipid synthesis and accumulation.     

Globular adiponectin protected ob/ob mice from diabetes and ApoE-deficient mice from atherosclerosis

The adipocyte-derived hormone adiponectin has been shown to play important roles in the regulation of energy homeostasis and insulin sensitivity. In this study, we analyzed globular domain adiponectin (gAd) transgenic (Tg) mice crossed with leptin-deficient ob/ob or apoE-deficient mice. Interestingly, despite an unexpected similar body weight, gAd Tg ob/ob mice showed amelioration of insulin resistance and beta-cell degranulation as well as diabetes, indicating that globular adiponectin and leptin appeared to have both distinct and overlapping functions. Amelioration of diabetes and insulin resistance was associated with increased expression of molecules involved in fatty acid oxidation such as acyl-CoA oxidase, and molecules involved in energy dissipation such as uncoupling proteins 2 and 3 and increased fatty acid oxidation in skeletal muscle of gAd Tg ob/ob mice. Moreover, despite similar plasma glucose and lipid levels on an apoE-deficient background, gAd Tg apoE-deficient mice showed amelioration of atherosclerosis, which was associated with decreased expression of class A scavenger receptor and tumor necrosis factor alpha. This is the first demonstration that globular adiponectin can protect against atherosclerosis in vivo. In conclusion, replenishment of globular adiponectin may provide a novel treatment modality for both type 2 diabetes and atherosclerosis.     

Orexins/hypocretins in the ob/ob mouse: hypothalamic gene expression, peptide content and metabolic effects

Orexins (forms A and B) belong to a new family of peptides that, as neuropeptide Y (NPY), stimulate food intake when centrally injected. The ob/ob mouse is a well-characterized model of hyperphagia and obesity associated with strong metabolic disturbances and a central dysregulation of peptides involved in the control of feeding. In the present report, we investigated the hypocretin (Hcrt)/orexin (OX) peptide pathway in lean and ob/ob mice. Prepro-Hcrt/OX mRNA expression, measured by in situ hybridization was restricted to the lateral hypothalamus area. It was significantly decreased in ob/ob mice (-18%; p<0.01). When estimated by real time RT-PCR in the whole hypothalamus, this decrease amounted to 65% (p<0.001). Hcrt-1/OX-A peptide concentrations, measured by RIA in microdissected hypothalamic nuclei were high in the lateral hypothalamus (LH) and lower in the arcuate (ARC) and paraventricular nuclei (PVN). In ob/ob mice, OX-A levels were significantly lower than in lean mice in the LH (-34%; p<0.02) and in the PVN (-72%; p<0.005). Acute intracerebroventricular injection of Hcrt-1/OX-A (1-10 nmol) stimulated feeding in lean, but not in ob/ob mice, whereas Hcrt-2/OX-B (1-10 nmol) had the opposite effect. Acute third ventricle (i3vt) injections of Hcrt/OX peptides in ob/ob mice transiently increased their metabolic rate and stimulated lipid substrate utilization. These findings provide direct evidence that Hcrt/OX peptides are down-regulated in the hypothalamus of ob/ob mice, contrary to the NPY system. The present data argues that Hcrt/OX peptides are not primarily responsible for the metabolic syndrome of the ob/ob mice. The diminution in the OX tone might participate in a counterregulatory system necessary to limit the adverse effects of NPY on food intake and body weight.     

Changes in hepatic gene expression upon oral administration of taurine-conjugated ursodeoxycholic acid in ob/ob mice

Nonalcoholic fatty liver disease (NAFLD) is highly prevalent and associated with considerable morbidities. Unfortunately, there is no currently available drug established to treat NAFLD. It was recently reported that intraperitoneal administration of taurine-conjugated ursodeoxycholic acid (TUDCA) improved hepatic steatosis in ob/ob mice. We hereby examined the effect of oral TUDCA treatment on hepatic steatosis and associated changes in hepatic gene expression in ob/ob mice. We administered TUDCA to ob/ob mice at a dose of 500 mg/kg twice a day by gastric gavage for 3 weeks. Body weight, glucose homeostasis, endoplasmic reticulum (ER) stress, and hepatic gene expression were examined in comparison with control ob/ob mice and normal littermate C57BL/6J mice. Compared to the control ob/ob mice, TUDCA treated ob/ob mice revealed markedly reduced liver fat stained by oil red O (44.2±5.8% vs. 21.1±10.4%, P<0.05), whereas there was no difference in body weight, oral glucose tolerance, insulin sensitivity, and ER stress. Microarray analysis of hepatic gene expression demonstrated that oral TUDCA treatment mainly decreased the expression of genes involved in de novo lipogenesis among the components of lipid homeostasis. At pathway levels, oral TUDCA altered the genes regulating amino acid, carbohydrate, and drug metabolism in addition to lipid metabolism. In summary, oral TUDCA treatment decreased hepatic steatosis in ob/ob mice by cooperative regulation of multiple metabolic pathways, particularly by reducing the expression of genes known to regulate de novo lipogenesis.     

Dietary copper supplementation increases the catecholamine levels in genetically obese (ob/ob) mice

The interactive relationship between Cu deficiency and depressed synthesis of certain neurotransmitters has been recognized. To investigate the effects of dietary Cu supplementation on the catecholamine levels in genetically obese mice, male obese (ob/ob) mice and their lean (+/?) counterparts were administered either a control diet (4.0 mg/kg) or a Cu-supplemented diet (50 mg/kg) for 4 wk. The ob/ob mice that were fed a control diet showed lower liver and higher plasma levels of Cu. Depressed levels of plasma and brain catecholamines were also found in ob/ob mice that were fed the control diet. The ob/ob mice that received a Cu-supplemented diet showed significant increases in the levels of catecholamine in the plasma and brain. This study showed that catecholamine levels in ob/ob mice can be increased by dietary Cu supplementation. However, the interaction between Cu and sympathetic nervous activity in obesity was not elucidated in this study.     

Energy Metabolic Profile of Mice after Chronic Activation of Central NPY Y1, Y2, or Y5 Receptors**

Objective: Neuropeptide Y (NPY), a 36‐amino acid peptide with orexigenic properties, is expressed abundantly in the central nervous system and binds to several NPY receptor subtypes. This study examines the roles of the NPY Y1, Y2, and Y5 receptor(s) in energy homeostasis. Research Methods and Procedures: We administered intracerebroventricular NPY (3 μg/d) or selective peptide agonists for the Y1, Y2, and Y5 receptor subtypes to C57Bl/6 mice for 6 days by mini‐osmotic pumps to assess the role of each receptor subtype in NPY‐induced obesity. Energy expenditure (EE) and respiratory quotient (RQ) were studied using indirect calorimetry. Adiposity was measured by DXA scanning and fat pad dissection. Insulin sensitivity was tested by whole‐blood glucose measurement after an insulin challenge. Results: Central administration of the selective Y1 agonist, Y5 agonist, or NPY for 6 days in mice significantly increased body weight, adiposity, and RQ, with significant hyperphagia in the Y5 agonist‐ and NPY‐treated groups but not in the Y1 agonist‐treated group. The NPY, Y1, or Y5 agonist‐treated mice had little change in total EE during ad libitum and pair‐feeding conditions. Conversely, selective activation of the Y2 receptor reduced feeding and resulted in a significant, but transient, weight loss. Discussion: Central activation of both Y1 and Y5 receptors increases RQ and adiposity, whereas only Y5 receptor activation reduces energy expended per energy ingested. Selective activation of Y2 autoreceptors leads to hypophagia and transient weight loss, with little effect on total EE. Our study indicates that all three NPY receptor subtypes may play a role in regulating energy homeostasis in mice.     

Hepatic knockdown of mitochondrial GPAT1 in ob/ob mice improves metabolic profile

Glycerol-3-phosphate acyltransferase (GPAT) controls the first step of triglyceride (TAG) synthesis. Three distinct GPAT activities have been identified, two localized in mitochondria and one in microsomes. Mitochondrial GPAT1 (mtGPAT1) is abundantly expressed in the liver and constitutes approximately 50% of total GPAT activities in this organ. Hepatic mtGPAT1 activity is elevated in obese rodents. Mice deficient in mtGPAT1 have an improved lipid profile. To investigate if beneficial effects can result from reduced hepatic expression of mtGPAT1 in adult obese mice, adenoviral vector-based short hairpin RNA interference (shRNA) technology was used to knockdown mtGPAT1 expression in livers of ob/ob mice. Reduced expression of mtGPAT1 mRNA in liver of ob/ob mice resulted in dramatic and dose dependent reduction in mtGPAT1 activity. Reduced hepatic TAG, diacylglycerol, and free fatty acid, as well as reduced plasma cholesterol and glucose, were also observed. Fatty acid composition analysis revealed decrease of C16:0 in major lipid species. Our results demonstrate that acute reduction of mtGPAT1 in liver of ob/ob mice reduces TAG synthesis, which points to a role for mtGPAT1 in the correction of obesity and related disorders.     

The antihyperglycemic effect of estrone sulfate in genetically obese-diabetic (ob/ob) mice is associated with reduced hepatic glucose-6-phosphatase.

Excessive glucose production by the liver contributes significantly to diabetic hyperglycemia. The enzyme system glucose-6-phosphatase plays a key role in regulating hepatic glucose production and therefore its inhibition is a potential therapeutic target for the correction of hyperglycemia. It has previously been shown that sulfated steroids, such as estrone sulfate and dehydroepiandrosterone sulfate, inhibit the glucose-6-phosphatase system in vitro, principally through inhibition of endoplasmic reticulum glucose-6-phosphate transport. We report here that in the obese/diabetic ob/ob mouse model, orally administered estrone sulfate reduces the abnormally elevated hepatic glucose-6-phosphatase enzyme activity and enzyme protein levels that are characteristic in the ob/ob mouse, and that this reduction is associated with normalization of blood glucose levels. Other sulfated and non-sulfated steroids also reduced, to a lesser extent, glucose-6-phosphatase enzyme activity - with the exception of dehydroepiandrosterone sulfate, which had no apparent effect on this system in ob/ob mice. Estrone sulfate is therefore an effective antihyperglycemic agent in ob/ob mice, and the glucose-6-phosphatase system can be successfully targeted for the therapeutic management of hyperglycemia in this animal model of non-insulin-dependent diabetes mellitus.     

Reduced Histone H3K9 Acetylation of Clock Genes and Abnormal Glucose Metabolism in ob/ob Mice

Recent chronobiological studies found significant correlation between lack of clock function and metabolic abnormalities. We previously showed that clock gene expressions were dampened in the peripheral tissues of obese and diabetic ob/ob mice. However, the molecular mechanism of the disturbance remained to be determined. In this study, we demonstrated for the first time that acetylation levels of histone H3 lysine 9 (H3K9) at the promoter regions of clock genes, such as Dbp, Per2, and Bmal1, in the adipose tissue of ob/ob mice were significantly reduced compared with those of its control C57BL/6J mice. Treatment with histone deacetylase (HDAC) inhibitors increased Dbp, but not Per2 or Bmal1, mRNA expression in adipose tissue, and it decreased blood glucose in these animals. In addition, 2-deoxyglucose uptake activity was significantly suppressed by silencing Dbp expression in cultured adipocytes. These results suggest that reduced H3K9 acetylation and subsequent decreased mRNA expression of the Dbp gene in adipose tissue are involved in the mechanism of development of abnormal glucose metabolism in ob/ob mice. (Author correspondence: akiofuji@jichi.ac.jp ).     

Glucoregulatory effects of bombesin in lean and genetically obese hyperglycaemic (ob/ob) mice

1. Plasma glucose and insulin responses to bombesin were examined in 12-15-week-old 12 hr fasted lean and genetically obese hyperglycaemic (ob/ob) mice. 2. Bombesin (1 mg/kg ip) produced a prompt but transient increase of plasma insulin in lean mice (maximum increase of 50% at 5 min), and a more slowly generated but protracted insulin response in ob/ob mice (maximum increase of 80% at 30 min). Plasma glucose concentrations of both groups of mice were increased by bombesin (maximum increases of 40 and 48% respectively in lean and ob/ob mice at 15 min). 3. When administered with glucose (2 g/kg ip), bombesin (1 mg/kg ip) rapidly increased insulin concentrations of lean and ob/ob mice (maximum increases of 39 and 63% respectively at 5 min). Bombesin did not significantly alter the rise of plasma glucose after exogenous glucose administration to these mice. 4. The results indicate that bombesin exerts an insulin-releasing effect in lean and ob/ob mice. The greater insulin-releasing effect in ob/ob mice renders bombesin a possible component of the overactive entero-insular axis in the ob/ob mutant, especially if it acts within the islets as a neurotransmitter or paracrine agent.