Role of selective leptin resistance in diet-induced obesity hypertension

Leptin is an adipocyte-derived hormone that plays a key role in the regulation of body weight through its actions on appetite and metabolism. Leptin also increases sympathetic nerve activity (SNA) and blood pressure. We tested the hypothesis that diet-induced obesity is associated with resistance to the metabolic actions of leptin but preservation of its renal SNA and arterial pressure effects, leading to hypertension. Mice were fed a high-fat diet for 10 weeks to induce moderate obesity. The decrease in food intake and body weight induced by intraperitoneal or intracerebroventricular leptin was significantly attenuated in the obese mice. Regional SNA responses to leptin were differentially altered in diet-induced obese mice. Renal SNA response to leptin was preserved, whereas lumbar and brown adipose tissue SNA responses were attenuated in obese mice. Radiotelemetric arterial pressure was approximately 10 mmHg higher in obese mice. Furthermore, the increase in arterial pressure in response to long-term (12 days) leptin treatment was preserved in obese mice. Thus, mice with diet-induced obesity exhibit circulating hyperleptinemia and resistance to the metabolic actions of leptin. However, there is preservation of the renal sympathetic and arterial pressure responses to leptin, which represent a potential mechanism for the adverse cardiovascular consequences of obesity.     

Leptin, obesity and cardiovascular disease

PURPOSE OF REVIEW: Obesity is a risk factor for cardiovascular diseases. Leptin levels are increased in obesity and leptin exhibits cardiovascular actions that may contribute to increased cardiovascular risk. We review the sympathetic, renal and vascular actions of leptin and their relevance to cardiovascular disease. RECENT FINDINGS: Leptin possesses cardio-renal actions potentially contributing to obesity-related hypertension including generalized sympathoactivation. However, given that leptin resistance occurs in obesity, it has been difficult to link hyperleptinemia with hypertension. One possibility is that leptin resistance is confined to the metabolic effects of leptin, with preservation of its sympathoexcitatory actions. Other mechanisms may contribute to the pressor effects of leptin. For instance, angiotensin II induces leptin generation. Leptin also potentiates the pressor effect of insulin. Therefore, interactions between angiotensin II and insulin with leptin could have deleterious cardiovascular effects in obesity. Additionally, leptin appears to stimulate vascular inflammation, oxidative stress and hypertophy. These actions may contribute to the pathogenesis of hypertension, atherosclerosis, and left ventricular hypertrophy. SUMMARY: The potential actions of leptin in the pathophysiology of cardiovascular complications of obesity are diverse, despite evidence of leptin resistance to its metabolic actions. However, most information about cardiovascular actions of leptin derives from in-vitro and animal studies. Future research in humans is widely awaited.     

Ciliary neurotrophic factor restores gallbladder contractility in leptin-resistant obese diabetic mice

BACKGROUND: Obesity and diabetes are major risk factors for cholesterol gallstones, and the majority of obese people are leptin-resistant. Our previous work has shown that both leptin-deficient (Lepob) and leptin-resistant (Lepdb) obese diabetic mice have decreased in vitro gallbladder motility. Leptin administration to leptin-deficient (Lepob) animals restores gallbladder motility and reverses obesity and hyperinsulinemia. However, additional leptin in leptin-resistant obesity would not be expected to improve obesity-related parameters. Recent studies demonstrate that ciliary neurotrophic factor (CNTF) reduces weight and hyperinsulinemia in leptin-resistant obesity. Our hypothesis is that CNFT would cause weight loss, lower blood sugars, and restore gallbladder contractility in leptin-resistant (Lepdb) mice. MATERIALS AND METHODS: 20 C57b/6J and 20 Lepdb 8-week-old female mice were injected daily with either intraperitoneal saline or 0.3 microg/g CNTFAx15 for 17 days. Gallbladders were mounted in muscle baths and stimulated with acetylcholine, neuropeptide Y, and cholecystokinin. Gallbladder volume, serum glucose, insulin, liver weight, liver fat, and gallbladder responses were measured. Data were analyzed by ANOVA. RESULTS: Saline treated obese mice had greater body weight and obesity parameters, but decreased gallbladder contractility to neurotransmitters compared to saline treated lean mice. CNTF administration to obese mice decreased body weight and obesity parameters, and restored gallbladder contractility. CNTF treated lean animals had weight loss and decreased gallbladder contraction to acetylcholine and cholecystokinin compared to saline treated lean animals. CONCLUSIONS: Ciliary neurotrophic factor (CNTF) causes 1) weight loss, 2) improvement of diabetes, and 3) alterations in gallbladder motility that is improved in obese mice but decreased in lean mice. We conclude that CNTF may improve gallbladder contractility in leptin-resistant obesity with diabetes.     

Leptin-resistant obese mice do not form biliary crystals on a high cholesterol diet

INTRODUCTION: Human obesity is associated with leptin resistance and cholesterol gallstone formation. Previously, we demonstrated that leptin-resistant (Lep(db)) obese mice fed a low cholesterol diet have enlarged gallbladders, but a decreased cholesterol saturation index, despite elevated serum cholesterol. Obese humans, however, consume a high cholesterol diet. Therefore, we hypothesized that on a high cholesterol diet, leptin-resistant mice would have cholesterol saturated bile and would form biliary crystals. METHODS: Eight-week old female lean control (n = 70) and leptin-resistant (n = 72) mice were fed a 1% cholesterol diet for 4 weeks. All animals then had cholecystectomies. Bile was collected, grouped into pools to determine cholesterol saturation index (CSI), and examined for cholesterol crystals. Serum cholesterol and leptin were also measured. RESULTS: Gallbladder volumes for Lep(db) mice were enlarged compared with the lean mice (35.8 microl versus 19.1 microl, P < 0.001), but the CSI for the Lep(db) mice was lower than for the lean animals (0.91 versus 1.15, P < 0.03). The obese animals did not form cholesterol crystals, whereas the lean animals averaged 2.2 crystals per high-powered field (hpf) (P < 0.001). Serum cholesterol and leptin were also elevated (P < 0.001) in the obese animals. CONCLUSIONS: These data suggest that Lep(db) obese mice fed a high cholesterol diet have increased gallbladder volume and decreased biliary cholesterol saturation and crystal formation despite elevated serum cholesterol compared with lean control mice. We conclude that the link among obesity, diet, and gallstone formation may not require hypersecretion of biliary cholesterol and may be related to the effects of diabetes, hyperlipidemia, or both on gallbladder motility.     

Gallbladder motility in agouti-yellow and leptin-resistant obese mice

BACKGROUND: Obesity is a polygenic disorder that is associated with gallstone disease. We have previously shown that leptin deficiency in obese mice correlates with decreased gallbladder motility, suggesting that leptin plays a role in the link between gallstone disease and obesity. However, most obese humans are leptin-resistant, and relatively few are leptin-deficient. To confirm that leptin dysfunction is responsible for impaired gallbladder motility in obese mice, we hypothesized that leptin-resistant obese mice (Lep(db)) would have abnormal gallbladder motility while obese mice with intact leptin function (Agouti Yellow, A(y)) would have normal gallbladder motility. MATERIALS AND METHODS: Eighteen lean control (C57BL/6J), 10 A(y) and 12 Lep(db) female mice were fasted overnight, weighed, and livers and gallbladders were harvested. Liver weights and gallbladder volumes were measured. Gallbladder contractile responses (N/cm(2)) to acetylcholine (10(-5)M), neuropeptide Y (10(-8,-7,-6) M) and cholecystokinin (10(-10,-9,-8,-7)M) were determined in muscle bath chambers. Results were analyzed by analysis of various (ANOVA) and with the Mann-Whitney Rank Sum Test. RESULTS: Both Agouti yellow (A(y)) and leptin-resistant (Lep(db)) obese mice had body weights, liver weights and gallbladder volumes that were significantly greater (P < 0.01) than lean control mice. Leptin-resistant obese mice had gallbladder responses to acetylcholine, neuropeptide Y and cholecystokinin that were significantly less (P < 0.01) than both lean control and Agouti yellow obese mice. CONCLUSIONS: These data suggest that (1). leptin-resistant obese mice (Lep(db)) have abnormal gallbladder motility and (2). obese mice with normal leptin metabolism (A(y)) have normal gallbladder response to neurotransmitters. We conclude that leptin represents a link between obesity, gallbladder motility and gallstone formation.     

Central infusion of histamine reduces fat accumulation and upregulates UCP family in leptin-resistant obese mice

Leptin resistance has recently been confirmed not only in animal obese models but in human obesity. Evidence is rapidly emerging that suggests that activation of histamine signaling in the hypothalamus may have substantial anti-obesity and antidiabetic actions, particularly in leptin-resistant states. To address this issue, effects of central, chronic treatment with histamine on food intake, adiposity, and energy expenditure were examined using leptin-resistant obese and diabetic mice. Infusion of histamine (0.05 pmol x g body wt(-1) x day(-1)) into the lateral cerebroventricle (i.c.v.) for 7 successive days reduced food intake and body weight significantly in both diet-induced obesity (DIO) and db/db mice. Histamine treatment reduced body fat weight, ob gene expression, and serum leptin concentration more in the model mice than in pair-fed controls. The suppressive effect on fat deposition was significant in visceral fat but not in subcutaneous fat. Serum concentrations of glucose and/or insulin were reduced, and tests for glucose and insulin tolerance showed improvement of insulin sensitivity in those mice treated with histamine compared with pair-fed controls. On the other hand, gene expression of uncoupling protein (UCP)-1 in brown adipose tissue and UCP-3 expression in white adipose tissue were upregulated more in mice with i.c.v. histamine infusion than in the pair-fed controls. These upregulating effects of histamine were attenuated by targeted disruption of the H1-receptor in DIO and db/db mice. Sustained i.c.v. treatment with histamine thus makes it possible to partially restore the distorted energy intake and expenditure in leptin-resistant mice. Together, i.c.v. treatment with histamine contributes to improvement of energy balance even in leptin-resistant DIO and db/db mice.     

Leptin-resistant obese mice have paradoxically low biliary cholesterol saturation

BACKGROUND: Human obesity is associated with leptin resistance, elevated serum glucose and lipids, hepatic steatosis, and cholesterol gallstone formation. These gallstones are thought to result from hypersecretion of biliary cholesterol as well as biliary stasis. Leptin-resistant Lep(db) obese mice, which are known to have elevated serum leptin, glucose, and lipids, as well as hepatic steatosis, should be an appropriate model for human gallstone formation. Therefore, we tested the hypothesis that leptin-resistant mice would have increased gallbladder volume, biliary cholesterol saturation, and cholesterol crystal formation. METHODS: Sixty lean control mice and 60 Lep(db) obese mice on a low cholesterol chow diet were studied. Gallbladder volumes were measured and bile was pooled to calculate cholesterol saturation index. Serum cholesterol, glucose, and leptin levels were determined from pooled serum. Hepatic fat vacuoles were counted. Bile from a second group of 90 lean control and 59 obese mice was observed microscopically for cholesterol crystal formation. RESULTS: Leptin-resistant obese mice have significantly higher serum cholesterol, glucose, and leptin levels, hepatic fat vacuoles, and gallbladder volume than lean control mice. However, biliary cholesterol saturation index and cholesterol crystal formation were significantly diminished in the obese mice. CONCLUSIONS: These data suggest that leptin-resistant Lep(db) obese mice have (1) increased gallbladder volume, (2) decreased biliary cholesterol saturation despite elevated serum cholesterol and hepatic steatosis, and (3) decreased in vitro cholesterol crystal formation. We conclude that the link between obesity and gallstone formation does not require hypersecretion of biliary cholesterol.     

Leptin potentiates thermogenic sympathetic responses to hypothermia: a receptor-mediated effect

Leptin contributes to the regulation of thermogenesis. In rodents, sympathetic nerve activity efferent to interscapular brown adipose tissue (IBAT-SNA) is involved. On the basis of the hypotheses that 1) leptin acutely potentiates hypothermia-induced increases in IBAT-SNA; 2) this action of leptin is specific to IBAT-SNA, i.e., it does not occur with renal sympathetic nerve activity (R-SNA); and 3) this effect of leptin depends on intact and functional leptin receptors, we measured IBAT-SNA and R-SNA in anesthetized lean and diet-induced obese Sprague-Dawley and in obese Zucker rats, randomly assigned to low-dose leptin or vehicle. Before the start of leptin or vehicle and 5 min, 90 min, and 180 min after, hypothermia (30 degrees C) was induced. Compared with vehicle, leptin did not significantly alter baseline R-SNA or IBAT-SNA. In lean Sprague-Dawley rats, hypothermia-induced increases in IBAT-SNA were significantly augmented by leptin but not by vehicle. In obese Sprague-Dawley rats, leptin did not potentiate hypothermia-induced increases in IBAT-SNA. In Zucker rats, IBAT-SNA did not increase with hypothermia and leptin was not able to induce sympathoactivation with cooling. Changes in R-SNA during hypothermia were not significantly modified by leptin in either group. Thus, low-dose leptin, although not altering baseline SNA, acutely enhances hypothermia-induced sympathetic outflow to IBAT in lean rats. This effect is specific for thermogenic SNA because leptin does not significantly alter the response of R-SNA to hypothermia. The effect depends on intact and functional leptin receptors because it occurs neither in rats with a leptin receptor defect nor in rats with acquired leptin resistance.     

Leptin activation of corticosterone production in hepatocytes may contribute to the reversal of obesity and hyperglycemia in leptin-deficient ob/ob mice

Glucocorticoids have been implicated as pathophysiological mediators of obesity and insulin resistance and are regulated by 11beta-hydroxysteroid dehydrogenase type 1 (11beta-HSD1). This enzyme regenerates active corticosterone from inactive 11-keto forms. To assess the role of 11beta-HSD1-mediated synthesis of active corticosterone in leptin-related obesity and diabetes, we examined the peripheral effect of leptin on 11beta-HSD1 activity and gene expression in vivo and in vitro in hepatocytes from ob/ob mice and in liver of streptozotocin (STZ)-treated ob/ob mice. We observed an inverse relationship between hepatic 11beta-HSD1 expression and body weight in ob/ob mice and lean littermates. Leptin treatment of ob/ob mice markedly increased hepatic 11beta-HSD1 activity and mRNA expression. This induction of 11beta-HSD1 expression corresponded to reduced levels of circulating corticosterone and weight loss in ob/ob mice treated with leptin, indicating that impaired hepatic 11beta-HSD1 expression may contribute to the pathogenesis of obesity in ob/ob mice. In addition, leptin treatment of STZ-treated ob/ob mice caused marked increases in hepatic 11beta-HSD1 levels associated with decreased body weight and a significant reduction in hyperglycemia due to pancreatic beta-cell damage. Addition of leptin to ob/ob mouse primary hepatocytes led to a dose-dependent increase in 11beta-HSD1 mRNA expression. In contrast, leptin did not influence 11beta-HSD1 expression in primary hepatocytes from db/db mice, indicating that leptin regulation of 11beta-HSD1 expression is probably mediated by the functional leptin receptor. Thus, leptin appears to be an important metabolic signal that directly activates intrahepatic corticosterone production. These findings suggest that the liver-specific interaction of leptin with 11beta-HSD1 is involved in the development of obesity and insulin resistance in ob/ob mice.     

Diabetes and hyperlipidemia correlate with gallbladder contractility in leptin-related murine obesity

Obesity is associated with many comorbid conditions including diabetes, hyperlipidemia, and gallstones. However, the interaction among these modalities remains unclear. We recently demonstrated that both leptin-deficient and leptin-resistant obese mice have impaired biliary motility. These obese mice also are diabetic and hyperlipidemic. Therefore, we tested the hypothesis that serum glucose, insulin, cholesterol, and triglyceride levels would correlate with gallbladder contractility. Thirty-four lean control, 10 lean heterozygous leptin-deficient, 18 obese homozygous leptin-deficient, and 12 obese homozygous leptin-resistant mice were fed a nonlithogenic chow diet while nine lean control and nine obese homozygous leptin-deficient mice were fed a high-cholesterol diet for 4 weeks. In vitro gallbladder responses to cholecystokinin (CCK; 10^-8 mol/L), acetylcholine (ACh; 10^-5 mol/L), and neuropeptide Y (NPY; 10^-6 mol/L) were measured. Serum glucose, insulin, cholesterol, and triglyceride levels were measured from pooled serum from an additional 704 animals. Gallbladder responses were greatest for CCK, intermediate for ACh, and least for NPY. Serum glucose, insulin, cholesterol, and triglyceride levels and body weight all correlated similarly, negatively, and significantly (P < 0.001) with gallbladder contractility. Hyperglycemia, insulin-resistance, hyperlipidemia, and body weight in obese mice with leptin dysfunction are associated with poor gallbladder contractility, which in turn may contribute to the association between obesity and gallstone formation. Presented in part at Digestive Disease Week 2003, SSAT Plenary Session and Residents’ Conference, Orlando, Florida, May 17–22, 2003 (oral presentation); and at the Association for Academic Surgery, Poster Session, Boston, Massachusetts, November 7–9, 2002. Supported by grant NIH R-01 DK44279 from the National Institutes of Health.     

Update on adipocyte hormones: regulation of energy balance and carbohydrate/lipid metabolism

Hormones produced by adipose tissue play a critical role in the regulation of energy intake, energy expenditure, and lipid and carbohydrate metabolism. This review will address the biology, actions, and regulation of three adipocyte hormones-leptin, acylation stimulating protein (ASP), and adiponectin-with an emphasis on the most recent literature. The main biological role of leptin appears to be adaptation to reduced energy availability rather than prevention of obesity. In addition to the well-known consequences of absolute leptin deficiency, subjects with heterozygous leptin gene mutations have low circulating leptin levels and increased body adiposity. Leptin treatment dramatically improves metabolic abnormalities (insulin resistance and hyperlipidemia) in patients with relative leptin deficiency due to lipoatrophy. Leptin production is primarily regulated by insulin-induced changes of adipocyte metabolism. Dietary fat and fructose, which do not increase insulin secretion, lead to reduced leptin production, suggesting a mechanism for high-fat/high-sugar diets to increase energy intake and weight gain. ASP increases the efficiency of triacylglycerol synthesis in adipocytes leading to enhanced postprandial lipid clearance. In mice, ASP deficiency results in reduced body fat, obesity resistance, and improved insulin sensitivity. Adiponectin production is stimulated by thiazolidinedione agonists of peroxisome proliferator-activated receptor-gamma and may contribute to increased insulin sensitivity. Adiponectin and leptin cotreatment normalizes insulin action in lipoatrophic insulin-resistant animals. These effects may be mediated by AMP kinase-induced fat oxidation, leading to reduced intramyocellular and liver triglyceride content. The production of all three hormones is influenced by nutritional status. These hormones, the pathways controlling their production, and their receptors are promising targets for managing obesity, hyperlipidemia, and insulin resistance.     

Association of serum leptin with hypoventilation in human obesity

BACKGROUND: Leptin is a protein hormone produced by fat cells of mammals. It acts within the hypothalamus via a specific receptor to reduce appetite and increase energy expenditure. Plasma leptin levels correlate closely with total body fat mass operating via a central feedback mechanism. In human obesity serum leptin levels are up to four times higher than in lean subjects, indicating a failure of the feedback loop and central leptin resistance. In leptin deficient obese mice (ob/ob mice) leptin infusion reverses hypoventilation. It was hypothesised that a relative deficiency in CNS leptin, indicated by high circulating leptin levels, may be implicated in the pathogenesis of obesity hypoventilation syndrome (OHS). METHODS: Fasting morning leptin levels were measured in obese and non-obese patients with and without daytime hypercapnia (n=56). Sleep studies, anthropometric data, spirometric parameters, and awake arterial blood gas tensions were measured in each patient. RESULTS: In the whole group serum leptin levels correlated closely with % body fat (r=0.77). Obese hypercapnic patients (mean (SD) % body fat 43.8 (6.0)%) had higher fasting serum leptin levels than eucapnic patients (mean % body fat 40.8 (6.2)%), with mean (SD) leptin levels of 39.1 (17.9) and 21.4 (11.4) ng/ml, respectively (p<0.005). Serum leptin (odds ratio (OR) 1.12, 95% CI 1.03 to 1.22) was a better predictor than % body fat (OR 0.92, 95% CI 0.76 to 1.1) for the presence of hypercapnia. CONCLUSIONS: Hyperleptinaemia is associated with hypercapnic respiratory failure in obesity. Treatment with leptin or its analogues may have a role in OHS provided central leptin resistance can be overcome.     

Responsiveness to peripherally administered melanocortins in lean and obese mice

To elucidate mechanisms of melanocortin action, we investigated the effects of a melanocortin receptor agonist (melanotetan II [MTII]) in lean C57BL/6J and obese (DIO, ob/ob, UCP1-DTA) mice. MTII administration (100 microg q.i.d. i.p.) for 24 h results in similar weight loss but a more pronounced decrease of food intake in DIO mice. After 4 and 8 days of MTII treatment, however, the reduction in both food intake and body weight is more pronounced in DIO mice than in lean mice. MTII administration for 24 h prevents food deprivation-induced alterations in hypothalamic neuropeptide Y (NPY) and liver adiponectin receptor 1 and adiponectin receptor 2 mRNA expression, but does not alter hypothalamic mRNA expression of melanocortin 4 receptor or adiponectin serum and mRNA expression levels. NPY and agouti gene-related protein (AgRP) mRNA expression after 8 days of MTII is increased to levels comparable to pair-fed mice. In summary, 1) MTII is an effective treatment for obesity and related metabolic defects in leptin-resistant (DIO, UCP1-DTA) and leptin-sensitive (ob/ob) mouse models of obesity; 2) the effects of MTII on food intake and body weight are more pronounced in DIO mice than in lean mice; 3) the tachyphylactic effect after prolonged MTII administration appears to be, at least in part, caused by a compensatory upregulation of NPY and AgRP mRNA levels, whereas decreasing leptin levels may play a very minor role in mediating tachyphylaxis; and 4) alterations in adiponectin receptor mRNA expression after fasting or MTII treatment may contribute to altered insulin sensitivity and needs to be studied further.     

Acute intravenous leptin infusion increases glucose turnover but not skeletal muscle glucose uptake in ob/ob mice.

The mouse ob gene encodes leptin, an adipocyte hormone that regulates body weight and energy expenditure. Leptin has potent metabolic effects on fat and glucose metabolism. A mutation of the ob gene results in mice with severe hereditary obesity and diabetes that can be corrected by treatment with the hormone. In lean mice, leptin acutely increases glucose metabolism in an insulin-independent manner, which could account, at least in part, for some of the antidiabetic effect of the hormone. To investigate further the acute effect of leptin on glucose metabolism in insulin-resistant obese diabetic mice, leptin (40 ng x g(-1) x h(-1)) was administered intravenously for 6 h in C57Bl/6J ob/ob mice. Leptin increased glucose turnover and stimulated glucose uptake in brown adipose tissue (BAT), brain, and heart with no increase in heart rate. A slight increase in all splanchnic tissues was also noticed. Conversely, no increase in skeletal muscle or white adipose tissue (WAT) glucose uptake was observed. Plasma insulin concentration increased moderately but neither glucose, glucagon, thyroid hormones, growth hormone, nor IGF-1 levels were different from phosphate-buffered saline-infused C57Bl/6J ob/ob mice. In addition, leptin stimulated hepatic glucose production, which was associated with increased glucose-6-phosphatase activity. Conversely, PEPCK activity was rather diminished. Interestingly, hepatic insulin receptor substrate (IRS)1-associated phosphatidylinositol 3-kinase activity was slightly elevated, but neither the content of glucose transporter GLUT2 nor the phosphorylation state of the insulin receptor and IRS-1 were changed by acute leptin treatment. Hepatic lipid metabolism was not stimulated during the acute leptin infusion, since the content of triglycerides, glycerol, and citrate was unchanged. These findings suggest that in ob/ob mice, the antidiabetic antiobesity effect of leptin could be the result of a profound alteration of glucose metabolism in liver, BAT, heart, and consequently, glucose turnover. Insulin resistance of skeletal muscle and WAT, while not affected by acute leptin treatment, could also be corrected in the long term and account for some of leptin's antidiabetic effects.     

Inducible nitric oxide synthase has divergent effects on vascular and metabolic function in obesity

Previous studies have suggested an involvement of inducible nitric oxide synthase (iNOS) in obesity, but the relation, if any, between this and mechanisms underlying endothelial dysfunction in obesity is unknown. We studied mice fed an obesogenic high-fat or standard diet for up to 8 weeks. Obesity was associated with elevated blood pressure; resistance to the glucoregulatory actions of insulin; resistance to the vascular actions of insulin, assessed as the reduction in phenylephrine constrictor response of aortic rings after insulin preincubation (lean -21.7 +/- 11.5 vs. obese 18.2 +/- 15.5%; P < 0.05); and evidence of reactive oxygen species (ROS)-dependent vasodilatation in response to acetylcholine in aortic rings (change in maximal relaxation to acetylcholine after exposure to catalase: lean -2.1 +/- 6.0 vs. obese -15.0 +/- 3.8%; P = 0.04). Obese mice had increased expression of iNOS in aorta, with evidence of increased vascular NO production, assessed as the increase in maximal constriction to phenylephrine after iNOS inhibition with 1400W (lean -3.5 +/- 9.1 vs. obese 42.1 +/- 11.2%; P < 0.001). To further address the role of iNOS in obesity-induced vascular and metabolic dysfunction, we studied the effect of a high-fat diet in iNOS knockout mice (iNOS KO). Obese iNOS KO mice were protected against the development of resistance to insulin's glucoregulatory and vascular effects (insulin-dependent reduction in maximal phenylephrine response: obese wild-type 11.2 +/- 15.0 vs. obese iNOS KO -20.0 +/- 7.7%; P = 0.02). However, obese iNOS KO mice remained hypertensive (124.0 +/- 0.7 vs. 114.9 +/- 0.5 mmHg; P < 0.01) and had evidence of increased vascular ROS production. Although these data support iNOS as a target to protect against the adverse effects of obesity on glucoregulation and vascular insulin resistance, iNOS inhibition does not prevent the development of raised blood pressure or oxidative stress.     

Leptin Modulates Allograft Survival by Favoring a Th2 and a Regulatory Immune Profile

Leptin, an adipose‐secreted hormone, links metabolism and immunity. Our aim was to determine whether leptin affects the alloimmune response. We used an allogeneic skin transplant model as a means to analyze the allograft immune response in Lep^ob/ob and wild‐type mice. Leptin deficiency results in an increased frequency of Treg and Th2 cells and a prolonged graft survival. These effects of leptin deficiency indicate the importance of leptin and obesity in modulating the allograft immune responses. Our data suggest a possible explanation for the increased susceptibility of hyperleptinemic obese patients to acute and chronic graft rejection.     

Altered small intestinal absorptive enzyme activities in leptin-deficient obese mice: influence of bowel resection

Background

Residual bowel increases absorption after massive small bowel resection. Leptin affects intestinal adaptation, carbohydrate, peptide, and lipid handling. Sucrase, peptidase, and acyl coenzyme A:monoacylglycerol acyltransferase (MGAT) are involved in carbohydrate, protein, and lipid absorption. We hypothesized that leptin-deficient obese mice would have altered absorptive enzymes compared with controls before and after small bowel resection.

Methods

Sucrase, peptidase (aminopeptidase N [ApN], dipeptidyl peptidase IV [DPPIV]), and MGAT activities were determined from lean control (C57BL/6J, n = 16) and leptin-deficient (Lep^ob, n = 16) mice small bowel before and after 50% resection.

Results

Ileal sucrase activity was greater in obese mice before and after resection. Jejunal ApN and DPPIV activities were lower for obese mice before resection; ileal ApN activity was unaltered after resection for both strains. Resection increased DPPIV activity in both strains. Jejunal MGAT in obese mice decreased postresection. In both strains, ileal MGAT activity decreased after resection, and obese mice had greater activity in remnant ileum.

Conclusions

After small bowel resection, leptin-deficient mice have increased sucrase activity and diminished ileal ApN, DPPIV, and MGAT activity compared with controls. Therefore, we conclude that leptin deficiency alters intestinal enzyme activity in unresected animals and after small bowel resection. Altered handling of carbohydrate, protein, and lipid may contribute to obesity and diabetes in leptin-deficient mice.     

Obesity is associated with a decreased leptin transport across the blood-brain barrier in rats

Leptin exerts important effects on the regulation of food intake and energy expenditure by acting in the brain. Leptin is secreted by adipocytes into the bloodstream and must gain access to specific regions in the brain involved in regulating energy balance. Its action is mediated by interaction with a receptor that is mainly expressed in the hypothalamus but is also present in other cerebral areas. To reach these target areas, leptin most likely needs to cross the blood-brain barrier (BBB). In this study, we compared the permeability of leptin at the BBB in homozygous lean (FA/FA), high-fat diet-induced (HFD) obese rats (FA/FA rats on a highfat diet), and genetically obese fa/fa Zucker rats by quantifying the permeability coefficient surface area (PS) product after correction for the residual plasma volume (Vp) occupied by leptin in the vessel bed of different brain regions. The intravenous bolus injection technique was used in the cannulated brachial vein and artery using leptin radioiodinated with 2 isotopes of iodine (125I and 131I) to separately determine the PS and Vp values. The PS for leptin at the BBB in lean FA/FA rats ranged from 11.0 +/- 1.6 at the cortex to 14.8 +/- 1.4 x 10(-6) ml x g(-1) x ml(-1) at the posterior hypothalamus. The PS for leptin in HFD obese FA/FA and obese fa/fa rats ranged from 3.0- to 4.0-fold lower than in lean FA/FA rats. The Vp values were not significantly different among the 3 groups studied. SDS-PAGE analysis of the radioiodinated leptin after 60 min of uptake revealed intact protein in the 8 different brain regions. Plasma leptin levels were significantly higher in both obese rat groups compared with those in lean FA/FA rats. Leptin levels in cerebrospinal fluid were not significantly different among the 3 groups of rats. These findings strongly suggest that the leptin receptor (OB-R) in the BBB can be easily saturated. Saturation of the BBB OB-R in obese individuals would explain the defect in leptin transport into the brain described in this study.