Leptin administration normalizes insulin secretion from islets of Lep(ob)/Lep(ob) mice by food intake-dependent and -independent mechanisms

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.

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.

Resistin induces rat insulinoma cell RINm5F apoptosis

Beta-cell apoptosis induced by adipokines may result in beta-cell dysfunction in type 2 diabetes. Resistin, an adipokine-linked obesity with type 2 diabetes, impairs glucose-stimulated insulin secretion (GSIS) in beta-cells. Presently, the effects of resistin on rat insulinoma cells RINm5F were examined. Treatment of RINm5F with resistin induced cell damage. Tissue Inhibitor of Metalloproteinase-1 (TIMP-1) protected resistin-mediated cytotoxicity in RINm5F. Incubation with resistin up-regulated caspase-3 activity and induced the formation of a DNA ladder. TIMP-1 attenuated these effects. The molecular mechanism of TIMP-1 inhibition of resistin-mediated cytotoxicity appeared to involve Akt phosphorylation and activation of IkB-alpha phosphorylation. Resistin treatment suppressed Akt phosphorylation and activated IkB-alpha phosphorylation, which could be attenuated by TIMP-1. We conclude that resistin can induce beta-cell apoptosis and that resistin-related beta-cell apoptosis can be prevented by TIMP-1.

Contribution of adipocyte-derived factors to beta-cell dysfunction in diabetes

In addition to serving as an energy reservoir, the adipocyte has been characterized as an endocrine cell, secreting many bioactive factors which influence energy homeostasis. Being overweight, with excessive adipose tissue, is considered to be part of the pathogenesis of type 2 diabetes. Insulin resistance and beta-cell dysfunction are two major pathophysiological changes seen in type 2 diabetes. In addition to inducing insulin resistance in insulin-responsive tissues, adipocyte-derived factors play an important role in the pathogenesis of beta-cell dysfunction. Leptin, free fatty acids, adiponectin, tumor necrosis factor-alpha and interleukin-6 are all produced and secreted by adipocytes, and may directly influence aspects of beta-cell function, including insulin synthesis and secretion, insulin cell survival and apoptosis. During the progression from normal weight to obesity and on to overt diabetes, the adipocyte-derived factors contribute to the occurrence and development of beta-cell dysfunction and type 2 diabetes.

Leptin acts peripherally to limit meal-induced increases in plasma insulin concentrations in mice: a brief communication

Leptin inhibits food intake and lowers plasma insulin concentrations. This study was designed to determine whether leptin acts independent of food-intake regulation to affect meal-induced increases in plasma insulin concentrations. Leptin-deficient, Lep(ob)/Lep(ob) mice were administered 1 microg leptin intracerebroventricularly (ICV) or intraperitoneally. Food intake and plasma insulin concentrations of mice administered leptin ICV before a meal were lower, as expected, than were intakes and plasma insulin concentrations of mice administered vehicle ICV. However when food intake was controlled, meal-induced increases in plasma insulin were unaffected by ICV administration of leptin. Intraperitoneal administration of 1 microg leptin before a meal lowered meal-induced increases in plasma insulin concentrations without influencing the size of the meal. We conclude that plasma leptin concentrations can affect meal-induced insulin secretion independent of the central nervous system actions of leptin associated with food-intake regulation.

Cytolipotoxicity-induced involution of the female reproductive tract following expression of obese (ob/ob) and diabetes (db/db) genotype mutations: progressive, hyperlipidemic transformation into adipocytic tissues

Both diabetes (db/db) and obese (ob/ob) single gene mutations induce a progressive, hyperglycemic-hyperinsulinemic endometabolic environment which promotes hypercytolipidemic, utero-ovarian involution in C57BL/KsJ mice. The progressive expression of the induced diabetes-obesity syndrome (DOS) results in female reproductive sterility and eventual organoatrophy. In order to define the intra-cytoplasmic alterations induced by the progressive cytolipidemia on cellular vitality, utero-ovarian tissue samples were collected from both control (+/?) and littermate-matched ob/ob or db/db C57BL/KsJ mice at either 4 weeks (initial-onset DOS phase), 8 weeks (progressive, overt DOS phase), or 16 weeks (chronic-DOS phase) of age for cytolipid distribution analysis. All db/db and ob/ob mutant groups exhibited phenotypic obesity and systemic hyperglycemia-hyperinsulinemia relative to age-matched littermate +/? groups. In all db/db and ob/ob age groups, a progressive hypercytolipidemia was noted relative to +/? groups. When analyzed for lipid channeling, a progressive perinuclear mapping pattern of cytolipid distribution was noted. The primary locus of initial db/db and ob/ob cytolipid deposition was localized to the baso-polar regions in endometrial epithelia samples, or to the interstitium-thecal layer border of ovarian follicular compartments, during the initial-onset DOS phase. Progressively, intra-cytoplasmic lipid mobilization promoted a consistent perinuclear channeling of lipid vacuoles, ultimately isolating nuclear loci from the peripherally displaced cytoplasmic organelles within uterine epithelial layers. In db/db and ob/ob ovarian tissue samples, a progressive, gradient-related lipid infiltration of interstitial, thecal and, ultimately, granulosa cell layers promoted an enhanced rate of follicular-lipidemic atresia relative to +/? groups. In each tissue layer, the cytolipidemia promoted a dramatic perinuclear lipid-isolation barrier from intra-cytoplasmic organelle domains. With age-related exacerbation of the DOS syndrome, cytoplasmic nuclear-organelle displacement and lipoisolation resulted in cellular atresia, promoting the eventual utero-ovarian organoatrophy which characterized the chronic-DOS phase in db/db and ob/ob C57BL/KsJ mutants. These results indicate that the cytoinvolution associated with reproductive tract atrophy in these genetically mutant, diabetic-obese models is promoted by the disruption of the normal cytoarchitecture of utero-ovarian tissue layers induced by the progressive lipid sequestration, accumulation and ultimate isolation-induced disruption of intra-cellular organelle compartmentalization.

Leptin effects on pancreatic beta-cell gene expression and function

The hormone leptin is secreted from white adipocytes, and serum levels of leptin correlate with adipose tissue mass. Leptin was first described to act on the satiety center in the hypothalamus through specific receptors (leptin receptor [ObR]) to restrict food intake and enhance energy expenditure. Important peripheral actions of leptin involve inhibition of insulin biosynthesis and secretion in pancreatic beta-cells. In turn, insulin stimulates leptin secretion from adipose tissue, establishing a hormonal regulatory feedback loop-the so-called "adipo-insular axis." Multiple signal transduction pathways are involved in leptin signaling in pancreatic beta-cells. We have identified the proinsulin gene and protein phosphatase 1 gene as leptin repressed genes and the gene for the suppressor of cytokine signaling 3 protein as a leptin-induced gene in pancreatic beta-cells. The molecular effects of leptin culminate to restrict insulin secretion and biosynthesis to adapt glucose homeostasis to the amount of body fat. In most overweight individuals, however, physiological regulation of body weight by leptin seems to be disturbed, representing "leptin resistance." This leptin resistance at the level of the pancreatic beta-cell may contribute to dysregulation of the adipo-insular axis and promote the development of hyperinsulinemia and manifest type 2 diabetes in overweight patients.