Leptin suppression of insulin secretion and gene expression in human pancreatic islets: implications for the development of adipogenic diabetes mellitus.
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Previously we demonstrated the expression of the long form of the leptin receptor in rodent pancreatic beta-cells and an inhibition of insulin secretion by leptin via activation of ATP-sensitive potassium channels. Here we examine pancreatic islets isolated from pancreata of human donors for their responses to leptin. The presence of leptin receptors on islet beta-cells was demonstrated by double fluorescence confocal microscopy after binding of a fluorescent derivative of human leptin (Cy3-leptin). Leptin (6.25 nM) suppressed insulin secretion of normal islets by 20% at 5.6 mM glucose. Intracellular calcium responses to 16.7 mM glucose were rapidly reduced by leptin. Proinsulin messenger ribonucleic acid expression in islets was inhibited by leptin at 11.1 mM, but not at 5.6 mM glucose. Leptin also reduced proinsulin messenger ribonucleic acid levels that were increased in islets by treatment with 10 nM glucagon-like peptide-1 in the presence of either 5.6 or 11.1 mM glucose. These findings demonstrate direct suppressive effects of leptin on insulin-producing beta-cells in human islets at the levels of both stimulus-secretion coupling and gene expression. The findings also further indicate the existence of an adipoinsular axis in humans in which insulin stimulates leptin production in adipocytes and leptin inhibits the production of insulin in beta-cells. We suggest that dysregulation of the adipoinsular axis in obese individuals due to defective leptin reception by beta-cells may result in chronic hyperinsulinemia and may contribute to the pathogenesis of adipogenic diabetes. (+info)
Relative contribution of insulin and its precursors to fibrinogen and PAI-1 in a large population with different states of glucose tolerance. The Insulin Resistance Atherosclerosis Study (IRAS).
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Hyperinsulinemia is associated with the development of coronary heart disease. However, the underlying mechanisms are still poorly understood. Hypercoagulability and impaired fibrinolysis are possible candidates linking hyperinsulinism with atherosclerotic disease, and it has been suggested that proinsulin rather than insulin is the crucial pathophysiological agent. The aim of this study was to investigate the relationship of insulin and its precursors to markers of coagulation and fibrinolysis in a large triethnic population. A strong and independent relationship between plasminogen activator inhibitor-1 (PAI-1) antigen and insulin and its precursors (proinsulin, 32-33 split proinsulin) was found consistently across varying states of glucose tolerance (PAI-1 versus fasting insulin [proinsulin], r=0.38 [r=0.34] in normal glucose tolerance; r=0.42 [r=0.43] in impaired glucose tolerance; and r=0.38 [r=0.26] in type 2 diabetes; all P<0.001). The relationship remained highly significant even after accounting for insulin sensitivity as measured by a frequently sampled intravenous glucose tolerance test. In a stepwise multiple regression model after adjusting for age, sex, ethnicity, and clinic, both insulin and its precursors were significantly associated with PAI-1 levels. The relationship between fibrinogen and insulin and its precursors was significant in the overall population (r=0.20 for insulin and proinsulin; each P<0.001) but showed a more inconsistent pattern in subgroup analysis and after adjustments for demographic and metabolic variables. Stepwise multiple regression analysis showed that proinsulin (split products) but not fasting insulin significantly contributed to fibrinogen levels after adjustment for age, sex, clinic, and ethnicity. Decreased insulin sensitivity was independently associated with higher PAI-1 and fibrinogen levels. In summary, we were able to demonstrate an independent relationship of 2 crucial factors of hemostasis, fibrinogen and PAI-1, to insulin and its precursors. These findings may have important clinical implications in the risk assessment and prevention of macrovascular disease, not only in patients with overt diabetes but also in nondiabetic subjects who are hyperinsulinemic. (+info)
Intact proinsulin and beta-cell function in lean and obese subjects with and without type 2 diabetes.
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OBJECTIVE: Type 2 diabetes is a heterogeneous disease in which both beta-cell dysfunction and insulin resistance are pathogenetic factors. Disproportionate hyperproinsulinemia (elevated proinsulin/insulin) is another abnormality in type 2 diabetes whose mechanism is unknown. Increased demand due to obesity and/or insulin resistance may result in secretion of immature beta-cell granules with a higher content of intact proinsulin. RESEARCH DESIGN AND METHODS: We investigated the impact of obesity on beta-cell secretion in normal subjects and in type 2 diabetic patients by measuring intact proinsulin, total proinsulin immunoreactivity (PIM), intact insulin, and C-peptide (by radioimmunoassay) by specific enzyme-linked immunosorbent assays in the fasting state and during a 120-min glucagon (1 mg i.v.) stimulation test. Lean (BMI 23.5 +/- 0.3 kg/m2) (LD) and obese (30.1 +/- 0.4 kg/m2) (OD) type 2 diabetic patients matched for fasting glucose (10.2 +/- 0.6 vs. 10.3 +/- 0.4 mmol/l) were compared with age- and BMI-matched lean (22.4 +/- 0.6 kg/m2) (LC) and obese (30.8 +/- 0.9 kg/m2) (OC) normal control subjects. RESULTS: Diabetic patients (LD vs. LC and OD vs. OC) had elevated fasting levels of intact proinsulin 6.6 +/- 1.0 vs. 1.6 +/- 0.3 pmol/l and 7.7 +/- 2.0 vs. 1.2 +/- 0.2 pmol/l; PIM: 19.9 +/- 2.5 vs. 5.4 +/- 1.0 pmol/l and 29.6 +/- 6.1 vs. 6.1 +/- 0.9 pmol/l; and total PIM/intact insulin: 39 +/- 4 vs. 15 +/- 2% and 35 +/- 5 vs. 13 +/- 2%, all P < 0.01. After glucagon stimulation, PIM levels were disproportionately elevated (PIM/intact insulin based on area under the curve analysis) in diabetic patients (LD vs. LC and OD vs. OC): 32.6 +/- 6.7 vs. 9.2 +/- 1.1% and 22.7 +/- 5.2 vs. 9.1 +/- 1.1%, both P < 0.05. Intact insulin and C-peptide net responses were significantly reduced in type 2 diabetic patients, most pronounced in the lean group. The ratio of intact proinsulin to PIM was higher in diabetic patients after stimulation in both LD versus LC: 32 +/- 3 vs. 23 +/- 2%, and OD versus OC: 28 +/- 4 vs. 16 +/- 2%, both P < 0.01. In obese normal subjects, intact proinsulin/PIM was lower both in the fasting state and after glucagon stimulation: OC versus LC: 22 +/- 3 vs. 33 +/- 3% (fasting) and 16 +/- 2 vs. 23 +/- 2% (stimulated), both P < 0.05. CONCLUSIONS: Increased secretory demand from obesity-associated insulin resistance cannot explain elevated intact proinsulin and disproportionate hyperproinsulinemia in type 2 diabetes. This abnormality may be an integrated part of pancreatic beta-cell dysfunction in this disease. (+info)
Adenovirus-mediated overexpression of uncoupling protein-2 in pancreatic islets of Zucker diabetic rats increases oxidative activity and improves beta-cell function.
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The discovery of uncoupling protein (UCP)-2, a ubiquitously expressed protein homologous to UCP-1, has raised the possibility that energy balance of cells might be regulated in tissues other than brown adipocytes. In normal pancreatic islets, UCP-2 is upregulated by leptin and is low in leptin-resistant islets of ZDF rats. To determine whether UCP-2 does, in fact, have uncoupling activity and, if so, whether such activity would favorably influence the abnormalities in leptin-unresponsive UCP-2-underexpressing islets of diabetic ZDF rats, we transferred the UCP-2 gene to the islets of diabetic ZDF rats and lean (+/+) ZDF control rats. Although ATP was reduced by 23% in both groups of islets, the ATP:ADP ratio increased by 42 and 141%, respectively. [3H]palmitate oxidation was increased by 50%, and [3H]glucose oxidation was 42-63% higher. Preproinsulin mRNA was 2.9-fold above control levels, and glucose-stimulated insulin secretion, which was negligible in control ZDF rat islets, was improved in UCP-2-overexpressing islets. The high fat content of the islets was not reduced, however. We conclude that UCP-2 has uncoupling function when overexpressed in leptin-insensitive islets and that its overexpression corrects the underexpression of the insulin gene and ameliorates glucose-stimulated insulin secretion, possibly by increasing the ATP:ADP ratio. (+info)
Fasting proinsulin concentrations predict the development of type 2 diabetes.
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OBJECTIVE: The development of specific assays allows the different molecules in the proinsulin processing pathway to be measured separately. 32,33 Split proinsulin is the predominant form of proinsulin and accounts for the disproportionate hyperproinsulinemia seen in individuals with prevalent type 2 diabetes. This study was established to examine whether the concentration of this molecule predicts diabetes. RESEARCH DESIGN AND METHODS: A population-based longitudinal cohort study was conducted in Ely, Cambridgeshire. At baseline, 1,122 individuals completed a 75-g oral glucose tolerance test (OGTT). At the 4.5-year follow-up study, repeat OGTTs were performed on 937 of the cohort of 1,071 individuals who had been nondiabetic at baseline. RESULTS: A total of 26 people progressed to diabetes as determined by the OGTTs. The risk of progression was strongly related to the fasting glucose concentration (relative risk [RR] comparing top with bottom quartile 17.6 [95% CI 2.4-130.4]) and fasting 32,33 split proinsulin (RR 16.4 [2.2-121.9]), but less strongly to the fasting insulin (RR 4.41 [1.5-12.9]) or intact proinsulin (RR 5.2 [1.5-17.3]). In multivariate analyses, these associations were independent of age, sex, BMI, and baseline glucose tolerance category. Subjects in the top quartile for fasting glucose and total proinsulin with a family history of diabetes were a high-risk subgroup (incidence 65.8 per 1,000 person-years of follow-up [pyfu]); 30% of them progressed to diabetes at follow-up. CONCLUSIONS: Fasting 32,33 split proinsulin independently predicts the development of diabetes. This prediction was better than that observed for either the insulin or intact proinsulin concentrations. The combination of family history, fasting glucose, and total proinsulin identified a subgroup of individuals at high risk of progression who might benefit from targeted interventions. (+info)
Cellular immune responses against proinsulin: no evidence for enhanced reactivity in individuals with IDDM.
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Investigations of humans and nonobese diabetic mice suggest that proinsulin and/or a fragment of the region spanning C-peptide and the B-chain of insulin (i.e., proinsulin peptide) may serve as key autoantigens in IDDM. Therefore, we analyzed cellular immune reactivities against these molecules in people with or at varying risks for the disease to clarify their role in the pathogenesis of IDDM. In vitro peripheral blood mononuclear cell (PBMC) responses against these antigens, a control antigen (tetanus toxoid), and phytohemaglutinin were determined in 60 individuals with newly diagnosed IDDM (< or = 1 day from diagnosis) in 34 islet cell cytoplasmic autoantibody- and/or insulin autoantibody-negative first-degree relatives of the IDDM subjects, and in 28 autoantibody-negative control subjects. Unlike previous reports suggesting diabetes-associated elevations in cellular immunity to other beta-cell antigens (e.g., GAD, IA-2, etc.), we observed equivalent levels of phytohemaglutinin stimulation and cellular proliferation in all groups against these antigens (all P values were not significant). The mean stimulation index +/- SD and frequency of reactivity to proinsulin for healthy control subjects and IDDM patients, respectively, were as follows: 1 microg/ml (1.5 +/- 1.0, 1 out of 17 [6%]; 1.9 +/- 1.4, 4 out of 33 [12%]); 10 microg/ml (1.7 +/- 1.3, 1 out of 17 [6%]; 1.2 +/- 0.6, 0 out of 28 [0%]); and 50 microg/ml (1.2 +/- 0.6, 1 out of 16 [6%]; 1.1 +/- 0.6, 1 out of 27 [4%]). The response in healthy control subjects, autoantibody-negative relatives, and IDDM patients, respectively, against the proinsulin peptide fragment were as follows: 1 microg/ml (0.9 +/- 0.4, 1 out of 12 [8%]; 1.3 +/- 1.1, 4 out of 34 [11%]; 1.1 +/- 0.3, 2 out of 28 [7%]); 10 microg/ml (0.9 +/- 0.6, 1 out of 12 [8%]; 1.2 +/- 0.6, 3 out of 34 [9%] 1.4 +/- 1.7, 2 out of 28 [7%]); and 50 microg/ml (1.0 +/- 0.7, 1 out of 12 [8%]; 1.2 +/- 0.5, 2 out of 34 [6%]; 1.3 +/- 0.5, 2 out of 28 [7%]). Taken together with previous studies reporting relatively infrequent occurrences of autoantibodies to proinsulin, the role of immunity to this molecule in the pathogenesis of IDDM in humans remains unclear. (+info)
Impaired beta-cell functions induced by chronic exposure of cultured human pancreatic islets to high glucose.
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In type 2 diabetes, chronic hyperglycemia has been suggested to be detrimental to beta-cell function, causing reduced glucose-stimulated insulin secretion and disproportionately elevated proinsulin. In the present study, we investigated the effect on several beta-cell functions of prolonged in vitro exposure of human pancreatic islet cultures to high glucose concentrations. Islets exposed to high glucose levels (33 mmol/l) for 4 and 9 days showed dramatic decreases in glucose-induced insulin release and in islet insulin content, with increased proportion of proinsulin-like peptides relative to insulin. The depletion in insulin stores correlated with the reduction in insulin mRNA levels and human insulin promoter transcriptional activity. We also demonstrated that high glucose dramatically lowered the binding activity of pancreatic duodenal homeobox 1 (the glucose-sensitive transcription factor), whereas the transcription factor rat insulin promoter element 3b1 activator was less influenced and insulin enhancer factor 1 remained unaffected. Most of these beta-cell impairments were partially reversible when islets first incubated for 6 days in high glucose were transferred to normal glucose (5.5 mmol/l) concentrations for 3 days. We conclude that cultured human islets are sensitive to the deleterious effect of high glucose concentrations at multiple functional levels, and that such mechanisms may play an important role in the decreased insulin production and secretion of type 2 diabetic patients. (+info)
Insulin secretion: feed-forward control of insulin biosynthesis?
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It has long been accepted wisdom that insulin secreted from islet beta cells has either no effect, or an inhibitory feedback effect, on insulin synthesis and secretion. Recent work suggests, instead, that secreted insulin acts directly on beta cells, via its own receptor, to enhance insulin production in an autocrine feed-forward loop. (+info)