Long-term leptin treatment of ob/ob mice improves glucose-induced insulin secretion

An Overview of Hormone-Sensitive Lipase (HSL)

Hormone-sensitive lipase (HSL) is a pivotal enzyme that mediates triglyceride hydrolysis to provide free fatty acids and glycerol in adipocytes in a hormonally controlled lipolysis process. Elevated plasma-free fatty acids were accompanied by insulin resistance, type-2 diabetes, and obesity. Inhibition of lipolysis through HSL inhibition may provide a mechanism to prevent the accumulation of free fatty acids and to improve the affectability of insulin and blood glucose handling in type II diabetes. The published studies that examine the structure, regulation, and function of HSL and major inhibitors were reviewed in this paper.

Metreleptin treatment of non-HIV lipodystrophy syndromes

Lipodystrophy syndromes (LS) constitute a group of rare diseases of the adipose tissue, characterized by a complete or selective deficiency of the fat mass. These disorders are associated with important insulin resistance, cardiovascular and metabolic comorbidities that impact patient's survival and quality of life. Management is challenging and includes diet, physical activity, and specific pharmacological treatment of LS-associated comorbidities. Because of a common pathophysiology involving decreased concentration of the adipokine leptin, efforts have been made to develop therapeutic strategies with leptin replacement therapy. Metreleptin, a recombinant human leptin analogue, has been proposed in hypoleptinemic patients since the beginning of 2000's. The treatment leads to an improvement in metabolic parameters, more important in generalized than in partial LS forms. In this review, the current knowledge about the development of the drug, its outcomes in the treatment of lipodystrophic patients as well as the peculiarities of its use will be presented.

Leptin contributes to the beneficial effects of insulin treatment in streptozotocin-diabetic male mice

It was long thought that the only hormone capable of reversing the catabolic consequences of diabetes was insulin. However, various studies have demonstrated that the adipocyte-derived hormone leptin can robustly lower blood glucose levels in rodent models of insulin-deficient diabetes. In addition, it has been suggested that some of the metabolic manifestations of insulin-deficient diabetes are due to hypoleptinemia as opposed to hypoinsulinemia. Because insulin therapy increases leptin levels, we sought to investigate the contribution of leptin to the beneficial effects of insulin therapy. To do this, we tested insulin therapy in streptozotocin (STZ) diabetic mice that were either on an ob/ ob background or that were given a leptin antagonist to determine if blocking leptin action would blunt the glucose-lowering effects of insulin therapy. We found that STZ diabetic ob/ ob mice have a diminished blood glucose-lowering effect in response to insulin therapy compared with STZ diabetic controls and exhibited more severe weight loss post-STZ injection. In addition, STZ diabetic mice administered a leptin antagonist through daily injection or plasmid expression respond less robustly to insulin therapy as assessed by both fasting blood glucose levels and blood glucose levels during an oral glucose tolerance test. However, leptin antagonism did not prevent the insulin-induced reduction in β-hydroxybutyrate and triglyceride levels. Therefore, we conclude that elevated leptin levels can contribute to the glucose-lowering effect of insulin therapy in insulin-deficient diabetes.

Leptin stimulates bone formation in ob/ob mice at doses having minimal impact on energy metabolism

Leptin, the protein product of the ob gene, is essential for normal bone growth, maturation and turnover. Peripheral actions of leptin occur at lower serum levels of the hormone than central actions because entry of leptin into the central nervous system (CNS) is limited due to its saturable transport across the blood-brain barrier (BBB). We performed a study in mice to model the impact of leptin production associated with different levels of adiposity on bone formation and compared the response with well-established centrally mediated actions of the hormone on energy metabolism. Leptin was infused (0, 4, 12, 40, 140 or 400 ng/h) for 12 days into 6-week-old female ob/ob mice (n = 8/group) using sc-implanted osmotic pumps. Treatment resulted in a dose-associated increase in serum leptin. Bone formation parameters were increased at EC₅₀ infusion rates of 7-17 ng/h, whereas higher levels (EC₅₀, 40-80 ng/h) were required to similarly influence indices of energy metabolism. We then analyzed gene expression in tibia and hypothalamus at dose rates of 0, 12 and 140 ng/h; the latter dose resulted in serum leptin levels similar to WT mice. Infusion with 12 ng/h leptin increased the expression of genes associated with Jak/Stat signaling and bone formation in tibia with minimal effect on Jak/Stat signaling and neurotransmitters in hypothalamus. The results suggest that leptin acts peripherally to couple bone acquisition to energy availability and that limited transport across the BBB insures that the growth-promoting actions of peripheral leptin are not curtailed by the hormone's CNS-mediated anorexigenic actions.

Leptin Production by Encapsulated Adipocytes Increases Brown Fat, Decreases Resistin, and Improves Glucose Intolerance in Obese Mice

The neuroendocrine effects of leptin on metabolism hold promise to be translated into a complementary therapy to traditional insulin therapy for diabetes and obesity. However, injections of leptin can provoke inflammation. We tested the effects of leptin, produced in the physiological adipocyte location, on metabolism in mouse models of genetic and dietary obesity. We generated 3T3-L1 adipocytes constitutively secreting leptin and encapsulated them in a poly-L-lysine membrane, which protects the cells from immune rejection. Ob/ob mice (OB) were injected with capsules containing no cells (empty, OB^[Emp]), adipocytes (OB^[3T3]), or adipocytes overexpressing leptin (OB^[Lep]) into both visceral fat depots. Leptin was found in the plasma of OB^[Lep], but not OB^[Emp] and OB^[3T3] mice at the end of treatment (72 days). The OB^[Lep] and OB^[3T3] mice have transiently suppressed appetite and weight loss compared to OB^[Emp]. Only OB^[Lep] mice have greater brown fat mass, metabolic rate, and reduced resistin plasma levels compared to OB^[Emp]. Glucose tolerance was markedly better in OB^[Lep]vs. OB^[Emp] and OB^[3T3] mice as well as in wild type mice with high-fat diet-induced obesity and insulin resistance treated with encapsulated leptin-producing adipocytes. Our proof-of-principle study provides evidence of long-term improvement of glucose tolerance with encapsulated adipocytes producing leptin.

Novel stable isotope analyses demonstrate significant rates of glucose cycling in mouse pancreatic islets

A polymorphism located in the G6PC2 gene, which encodes an islet-specific glucose-6-phosphatase catalytic subunit, is the most important common determinant of variations in fasting blood glucose (FBG) levels in humans. Studies of G6pc2 knockout (KO) mice suggest that G6pc2 represents a negative regulator of basal glucose-stimulated insulin secretion (GSIS) that acts by hydrolyzing glucose-6-phosphate (G6P), thereby reducing glycolytic flux. However, this conclusion conflicts with the very low estimates for the rate of glucose cycling in pancreatic islets, as assessed using radioisotopes. We have reassessed the rate of glucose cycling in pancreatic islets using a novel stable isotope method. The data show much higher levels of glucose cycling than previously reported. In 5 mmol/L glucose, islets from C57BL/6J chow-fed mice cycled ∼16% of net glucose uptake. The cycling rate was further increased at 11 mmol/L glucose. Similar cycling rates were observed using islets from high fat-fed mice. Importantly, glucose cycling was abolished in G6pc2 KO mouse islets, confirming that G6pc2 opposes the action of the glucose sensor glucokinase by hydrolyzing G6P. The demonstration of high rates of glucose cycling in pancreatic islets explains why G6pc2 deletion enhances GSIS and why variants in G6PC2 affect FBG in humans.

Leptin deficiency in rats results in hyperinsulinemia and impaired glucose homeostasis

Leptin, an adipocyte-derived hormone, has well-established anorexigenic effects but is also able to regulate glucose homeostasis independent of body weight. Until recently, the ob/ob mouse was the only animal model of global leptin deficiency. Here we report the effects of leptin deficiency on glucose homeostasis in male and female leptin knockout (KO) rats. Leptin KO rats developed obesity by 6 to 7 weeks of age, and lipid mass was increased by more than 2-fold compared with that of wild-type (WT) littermates at 18 weeks of age. Hyperinsulinemia and insulin resistance were evident in both males and females and were sustained with aging. Male KO rats experienced transient mild fasting hyperglycemia between 14 and 25 weeks of age, but thereafter fasting glucose levels were comparable to those of WT littermates up to 36 weeks of age. Fasting glucose levels of female KO rats were similar to those of WT littermates. Male KO rats exhibited a 3-fold increase in the proportion of β-cell area relative to total pancreas at 36 weeks of age. Islets from 12-week-old KO rats secreted more insulin when stimulated than islets from WT littermates. Leptin replacement via miniosmotic pump (100 μg/d) reduced food intake, attenuated weight gain, normalized glucose tolerance, and improved glucose-stimulated insulin secretion and insulin sensitivity. Together, these data demonstrate that the absence of leptin in rats recapitulates some of the phenotype previously observed in ob/ob mice including development of hyperinsulinemia, obesity, and insulin resistance.

Reporter islets in the eye reveal the plasticity of the endocrine pancreas

The islets of Langerhans constitute the endocrine part of the pancreas and are responsible for maintenance of blood glucose homeostasis. They are deeply embedded in the exocrine pancreas, limiting their accessibility for functional studies. Understanding regulation of function and survival and assessing the clinical outcomes of individual treatment strategies for diabetes requires a monitoring system that continuously reports on the endocrine pancreas. We describe the application of a natural body window that successfully reports on the properties of in situ pancreatic islets. As proof of principle, we transplanted "reporter islets" into the anterior chamber of the eye of leptin-deficient mice. These islets displayed obesity-induced growth and vascularization patterns that were reversed by leptin treatment. Hence, reporter islets serve as optically accessible indicators of islet function in the pancreas, and also reflect the efficacy of specific treatment regimens aimed at regulating islet plasticity in vivo.

Localization of lipoprotein lipase and GPIHBP1 in mouse pancreas: effects of diet and leptin deficiency

BACKGROUND

Lipoprotein lipase (LPL) hydrolyzes triglycerides in plasma lipoproteins and enables uptake of lipolysis products for energy production or storage in tissues. Our aim was to study the localization of LPL and its endothelial anchoring protein glycosylphosphatidylinositol-anchored high density lipoprotein-binding protein 1 (GPIHBP1) in mouse pancreas, and effects of diet and leptin deficiency on their expression patterns. For this, immunofluorescence microscopy was used on pancreatic tissue from C57BL/6 mouse embryos (E18), adult mice on normal or high-fat diet, and adult ob/ob-mice treated or not with leptin. The distribution of LPL and GPIHBP1 was compared to insulin, glucagon and CD31. Heparin injections were used to discriminate between intracellular and extracellular LPL.

RESULTS

In the exocrine pancreas LPL was found in capillaries, and was mostly co-localized with GPIHBP1. LPL was releasable by heparin, indicating localization on cell surfaces. Within the islets, most of the LPL was associated with beta cells and could not be released by heparin, indicating that the enzyme remained mostly within cells. Staining for LPL was found also in the glucagon-producing alpha cells, both in embryos (E18) and in adult mice. Only small amounts of LPL were found together with GPIHBP1 within the capillaries of islets. Neither a high fat diet nor fasting/re-feeding markedly altered the distribution pattern of LPL or GPIHBP1 in mouse pancreas. Islets from ob/ob mice appeared completely deficient of LPL in the beta cells, while LPL-staining was normal in alpha cells and in the exocrine pancreas. Leptin treatment of ob/ob mice for 12 days reversed this pattern, so that most of the islets expressed LPL in beta cells.

CONCLUSIONS

We conclude that both LPL and GPIHBP1 are present in mouse pancreas, and that LPL expression in beta cells is dependent on leptin.

Leptin in anorexia and cachexia syndrome

Leptin is a product of the obese (OB) gene secreted by adipocytes in proportion to fat mass. It decreases food intake and increases energy expenditure by affecting the balance between orexigenic and anorexigenic hypothalamic pathways. Low leptin levels are responsible for the compensatory increase in appetite and body weight and decreased energy expenditure (EE) following caloric deprivation. The anorexia-cachexia syndrome is a complication of many chronic conditions including cancer, chronic obstructive pulmonary disease, congestive heart failure, chronic kidney disease, and aging, where the decrease in body weight and food intake is not followed by a compensatory increase in appetite or decreased EE. Crosstalk between leptin and inflammatory signaling known to be activated in these conditions may be responsible for this paradox. This manuscript will review the evidence and potential mechanisms mediating changes in the leptin pathway in the setting of anorexia and cachexia associated with chronic diseases.

Leptin in human physiology and pathophysiology

Leptin, discovered through positional cloning 15 years ago, is an adipocyte-secreted hormone with pleiotropic effects in the physiology and pathophysiology of energy homeostasis, endocrinology, and metabolism. Studies in vitro and in animal models highlight the potential for leptin to regulate a number of physiological functions. Available evidence from human studies indicates that leptin has a mainly permissive role, with leptin administration being effective in states of leptin deficiency, less effective in states of leptin adequacy, and largely ineffective in states of leptin excess. Results from interventional studies in humans demonstrate that leptin administration in subjects with congenital complete leptin deficiency or subjects with partial leptin deficiency (subjects with lipoatrophy, congenital or related to HIV infection, and women with hypothalamic amenorrhea) reverses the energy homeostasis and neuroendocrine and metabolic abnormalities associated with these conditions. More specifically, in women with hypothalamic amenorrhea, leptin helps restore abnormalities in hypothalamic-pituitary-peripheral axes including the gonadal, thyroid, growth hormone, and to a lesser extent adrenal axes. Furthermore, leptin results in resumption of menses in the majority of these subjects and, in the long term, may increase bone mineral content and density, especially at the lumbar spine. In patients with congenital or HIV-related lipoatrophy, leptin treatment is also associated with improvements in insulin sensitivity and lipid profile, concomitant with reduced visceral and ectopic fat deposition. In contrast, leptin's effects are largely absent in the obese hyperleptinemic state, probably due to leptin resistance or tolerance. Hence, another emerging area of research pertains to the discovery and/or usefulness of leptin sensitizers. Results from ongoing studies are expected to further increase our understanding of the role of leptin and the potential clinical applications of leptin or its analogs in human therapeutics.

Swim training prevents hyperglycemia in ZDF rats: mechanisms involved in the partial maintenance of beta-cell function

Exercise improves glucose tolerance in obese rodent models and humans; however, effects with respect to mechanisms of beta-cell compensation remain unexplained. We examined exercise's effects during the progression of hyperglycemia in male Zucker diabetic fatty (ZDF) rats until 19 wk of age. At 6 wk old, rats were assigned to 1) basal--euthanized for baseline values; 2) exercise--swam individually for 1 h/day, 5 days/wk; and 3) controls (n = 8-10/group). Exercise (13 wk) resulted in maintenance of fasted hyperinsulinemia and prevented increases in fed and fasted glucose (P < 0.05) compared with sham-exercised and sedentary controls (P < 0.05). Beta-cell function calculations indicate prolonged beta-cell adaptation in exercised animals alone. During an intraperitoneal glucose tolerance test (IPGTT), exercised rats had lower 2-h glucose (P < 0.05) vs. controls. Area-under-the-curve analyses from baseline for IPGTT glucose and insulin indicate improved glucose tolerance with exercise was associated with increased insulin production and/or secretion. Beta-cell mass increased in exercised vs. basal animals; however, mass expansion was absent at 19 wk in controls (P < 0.05). Hypertrophy and replication contributed to expansion of beta-cell mass; exercised animals had increased beta-cell size and bromodeoxyuridine incorporation rates vs. controls (P < 0.05). The relative area of GLUT2 and protein kinase B was significantly elevated in exercised vs. sedentary controls (P < 0.05). Last, we show formation of ubiquitinated protein aggregates, a response to cellular/oxidative stress, occurred in nonexercised 19 wk-old ZDF rats but not in lean, 6 wk-old basal, or exercised rats. In conclusion, improved beta-cell compensation through increased beta-cell function and mass occurs in exercised but not sedentary ZDF rats and may be in part responsible for improved glucoregulation.

Leptin decreases apoptosis and alters BCL-2 : Bax ratio in clonal rodent pancreatic beta-cells

AIMS/HYPOTHESIS

The adipocyte derived peptide hormone leptin is known to regulate apoptosis and cell viability in several cells and tissues, as well as having several pancreatic islet beta-cell specific effects such as inhibition of glucose-stimulated insulin secretion. This study investigated the effects of leptin upon apoptosis induced by serum depletion and on expression of the apoptotic regulators B-cell leukaemia 2 gene product (BCL-2) and BCL2-associated X protein (Bax) in the glucose-responsive BRIN-BD11 beta-cell line.

METHODS

BRIN-BD11 cells were cultured in RPMI 1640 and subsequently serum depleted +/- leptin (10 and 50 ng/mL) for 24 h. Cell viability and apoptosis were measured using a modified MTS assay and TUNEL/YO-PRO-1 assays, respectively. BCL-2 and Bax expression were measured by real-time PCR and Western blotting.

RESULTS

Leptin caused a reduction in serum-depleted apoptosis, although it failed to have any effect on the overall cell viability, causing a 68% shift from apoptosis to necrosis. Leptin significantly increased the level of BCL-2 mRNA expression (150% compared to serum depletion alone), without altering Bax mRNA expression. At the protein level, leptin increased BCL-2 and decreased Bax, altering the BCL-2 : Bax ratio.

CONCLUSIONS

We conclude that leptin reduces apoptosis in beta-cells at physiological concentrations, possibly via its ability to up-regulate BCL-2 and Bax expression.

Leptin predicts the development of diabetes in Mauritian men, but not women: a population–based study

OBJECTIVE

To determine if levels of the adipocyte-derived hormone, leptin, predict the development of type 2 diabetes.

METHODS

Population-based surveys were undertaken in the multiethnic nation of Mauritius in 1987, 1992 and 1998. Questionnaires, anthropometric measurements, and a 2-h 75-g oral glucose tolerance test were included. A cohort of 2330 participants who were free of diabetes, aged 25-79 years in 1987, and who were followed-up in 1992 and 1998 was studied. Serum leptin was measured in baseline samples. Glucose tolerance was classified according to WHO (World Health Organization) 1999 criteria.

RESULTS

In total, 456 subjects developed diabetes over 11 years with similar incidences in all ethnic groups (P=0.2). Baseline leptin correlated positively with anthropometric measurements, fasting and postload insulin and homeostasis model assessment indices (all P<0.001), and inversely with subsequent weight increase. Participants with incident diabetes had higher serum levels of leptin at baseline than those remaining nondiabetic (P<0.001). After adjustment for confounders, high leptin levels and high leptin/body mass index ratio were independently associated with incident diabetes over 11 years in men (odds ratio for top versus bottom quartile of leptin 2.18; 95% CI: 1.09-4.35), but not in women.

CONCLUSION

We conclude that high leptin levels are associated with the future development of diabetes, and the association is independent of other factors in men, but not in women.

Effects of weight loss in obese subjects with normal fasting plasma glucose or impaired glucose tolerance on insulin release and insulin resistance according to a minimal model analysis

We investigated effects of weight loss from diet and exercise regimen in obese subjects with normal fasting plasma glucose or impaired glucose tolerance (IGT) on insulin release capacity and insulin sensitivity. Eight subjects were recruited among visceral obesity patients (4 men, 4 women; age range, 24 to 57 years; body mass index [BMI], 32.8 to 60.3 kg/m(2)). All were admitted to Chiba University Hospital for 2 weeks, were treated with a tapering 5,023 to 2,930 kJ diet, and were given exercise equivalent to 628 kJ/d. For assessments, we used a combination of C-peptide secretion rate determination and minimal model analysis as previously reported. BMI and visceral fat area (V) significantly decreased (BMI on initiation v after intervention, 43.0 +/- 3.2 v 40.3 +/- 3.1 kg/m(2), P <.05; V, 224 +/- 22 v 188 +/- 22 cm(2); P <.05). Fasting immunoreactive insulin (F-IRI) and leptin concentrations decreased significantly. Capacity for insulin release in response to glucose increased in all subjects (first-phase insulin secretion [CS1], 4.66 +/- 4.05 v 6.81 +/- 4.57 ng/mL/5 min, P <.05), but the insulin sensitivity index (S(i)) did not change significantly. These data suggest that weight reduction early in development of type 2 diabetes can oppose progression of diabetes by improving capacity for insulin release.

Hormone-sensitive lipase: control of intracellular tri-(di-)acylglycerol and cholesteryl ester hydrolysis

Hormone-sensitive lipase (HSL) is an intracellular neutral lipase that is capable of hydrolyzing triacylglycerols, diacylglycerols, monoacylglycerols, and cholesteryl esters, as well as other lipid and water soluble substrates. HSL activity is regulated post-translationally by phosphorylation and also by pretranslational mechanisms. The enzyme is highly expressed in adipose tissue and steroidogenic tissues, with lower amounts expressed in cardiac and skeletal muscle, macrophages, and islets. Studies of the structure of HSL have identified several amino acids and regions of the molecule that are critical for enzymatic activity and regulation of HSL. This has led to important insights into its function, including the interaction of HSL with other intracellular proteins, such as adipocyte lipid binding protein. Accumulating evidence has defined important functions for HSL in normal physiology, affecting adipocyte lipolysis, steroidogenesis, spermatogenesis, and perhaps insulin secretion and insulin action; however, direct links between abnormal expression or genetic variations of HSL and human disorders, such as obesity, insulin resistance, type 2 diabetes, and hyperlipidemia, await further clarification. The published reports examining the regulation, and function of HSL in normal physiology and disease are reviewed in this paper.

The association between leptin and proinsulin is lost with central obesity

OBJECTIVE

Hyperproinsulinaemia and hyperleptinaemia are interrelated features of the insulin resistance syndrome that are linked to the prospective risk of cardiovascular diseases. Whether the association between leptin and proinsulin is different between groups displaying different degrees of risk for cardiovascular diseases is not known. We therefore examined this association in men versus women and in pre- versus postmenopausal women from a population-based sample.

DESIGN AND SUBJECTS

Healthy subjects (n = 158; 85 men and 73 pre- and postmenopausal women) from the Northern Sweden Monitoring of Trends and Determinants in Cardiovascular Disease population were studied with a cross-sectional design.

METHODS

Anthropometric measurements (body mass index and waist circumference) and oral glucose tolerance tests were performed. Enzyme-linked immunosorbent assays were used for the analyses of specific insulin and proinsulin, and radioimmunoassay for leptin. Insulin resistance and beta-cell function were calculated according to the homeostasis assessment model. Partial correlation coefficients adjusted for age and measures of adiposity were calculated and multiple linear regression analyses were performed with leptin as dependent variable.

RESULTS

In nonobese men and premenopausal women and in obese postmenopausal women, leptin was significantly associated with proinsulin after stratification for waist circumference. Furthermore, a multivariate analyses taking age and measures of adiposity into account, showed that high fasting proinsulin was a significant predictor of high leptin in these groups. In contrast, this association was lost with increasing central obesity in men and premenopausal women.

CONCLUSIONS

This study shows that both the degree of adiposity and the hormonal milieu influence the association between circulating leptin and proinsulin in a normal population. Therefore, the insulin resistance syndrome seems to be characterized by lost association between leptin and proinsulin, which may be explained by dysfunction in the adipoinsular axis.

Altered beta-cell distribution of pdx-1 and GLUT-2 after a short-term challenge with a high-fat diet in C57BL/6J mice

Mechanisms involved in the islet adaptation to insulin resistance were examined in mice of the C57BL/6J strain challenged with a high-fat (58%) diet for 8 weeks. Basal hyperglycemia commenced after 1 week, whereas hyperinsulinemia evolved after 8 weeks. Glucose elimination after an intravenous glucose challenge (1 g/kg) was significantly delayed after 1, 4, and 8 weeks on the high-fat diet compared with normal-diet-fed mice. This result was associated with unchanged insulin responses. However, glucose-stimulated insulin secretion from isolated islets was increased in a compensatory fashion at all glucose levels over a wide range (3.3-22 mmol/l) after 8 weeks on the high-fat diet, whereas no compensatory hypersecretion of insulin was evident after 1 or 4 weeks, except at 22 mmol/l glucose. Immunohistochemistry revealed that the islet architecture of insulin and glucagon cells remained intact in islets from mice fed a high-fat diet. However, the nuclear translocation of the homeobox transcription factor, pdx-1, and the plasma membrane translocation of GLUT2 were both impaired in high-fat-fed animals after 1 week. In contrast, the expression of the full-length leptin receptor (ObRb) was not affected by high-fat feeding. The study thus shows that 8 weeks are required for the development of a compensatory hypersecretion of insulin after high-fat feeding in mice, and even then the in vivo insulin secretion is insufficient to normalize impaired glucose tolerance. The early-onset islet dysfunction is accompanied by impaired beta-cell trafficking of two factors, pdx-1 and GLUT-2, which are involved in beta-cell proliferation and glucose recognition. The mechanisms compromising this beta-cell trafficking remain to be established.