Academic literature on the topic 'Insulin secretory defects'

Pancreatic β-cell-restricted knockout of the insulin receptor results in hyperglycemia due to impaired insulin secretion, suggesting that this cell is an important target of insulin action. The present studies were undertaken in β-cell insulin receptor knockout (βIRKO) mice to define the mechanisms underlying the defect in insulin secretion. On the basis of responses to intraperitoneal glucose, ∼7-mo-old βIRKO mice were either diabetic (25%) or normally glucose tolerant (75%). Total insulin content was profoundly reduced in pancreata of mutant mice compared with controls. Both groups also exhibited reduced β-cell mass and islet number. However, insulin mRNA and protein were similar in islets of diabetic and normoglycemic βIRKO mice compared with controls. Insulin secretion in response to insulin secretagogues from the isolated perfused pancreas was markedly reduced in the diabetic βIRKOs and to a lesser degree in the nondiabetic βIRKO group. Pancreatic islets of nondiabetic βIRKO animals also exhibited defects in glyceraldehyde- and KCl-stimulated insulin release that were milder than in the diabetic animals. Gene expression analysis of islets revealed a modest reduction of GLUT2 and glucokinase gene expression in both the nondiabetic and diabetic mutants. Taken together, these data indicate that loss of functional receptors for insulin in β-cells leads primarily to profound defects in postnatal β-cell growth. In addition, altered glucose sensing may also contribute to defective insulin secretion in mutant animals that develop diabetes.

Insulin, a vital hormone for glucose homeostasis is produced by pancreatic beta-cells and when secreted, stimulates the uptake and storage of glucose from the blood. In the pancreas, insulin is stored in vesicles termed insulin secretory granules (ISGs). In Type 2 diabetes (T2D), defects in insulin action results in peripheral insulin resistance and beta-cell compensation, ultimately leading to dysfunctional ISG production and secretion. ISGs are functionally dynamic and many proteins present either on the membrane or in the lumen of the ISG may modulate and affect different stages of ISG trafficking and secretion. Previously, studies have identified few ISG proteins and more recently, proteomics analyses of purified ISGs have uncovered potential novel ISG proteins. This review summarizes the proteins identified in the current ISG proteomes from rat insulinoma INS-1 and INS-1E cell lines. Here, we also discuss techniques of ISG isolation and purification, its challenges and potential future directions.

Studies were performed in subjects with no known family history of diabetes, normoglycemic subjects who have first-degree relatives with non-insulin-dependent diabetes mellitus (NIDDM), and subjects with nondiagnostic oral glucose tolerance tests (NDX) or impaired glucose tolerance (IGT). Insulin sensitivity index (SI) was similar in all four groups. However, a number of defects in insulin secretion were seen in the NDX and IGT groups, including reduced first-phase insulin secretory responses in intravenous glucose in relation to the degree of insulin resistance, and reduced normalized spectral power of insulin secretion during oscillatory glucose infusion. The latter finding demonstrates a decreased ability of the beta-cell to detect and respond to the successive increases and decreases in glucose and therefore to be entrained by the exogenous glucose infusion. The ability of a low-dose glucose infusion to prime the insulin secretory response to a subsequent glucose stimulus was normal in subjects with IGT but reduced or absent in subjects with overt NIDDM. These studies demonstrate that a number of alterations in beta-cell function are detectable in nondiabetic first-degree relatives of subjects with NIDDM with mild elevations in the 2-h postchallenge glucose level, and these abnormalities antedate the onset of overt hyperglycemia and clinical diabetes.

Increasing prevalence of obesity combined with longevity will produce an epidemic of Type 2 (non-insulin-dependent) diabetes in the next 20 years. This disease is associated with defects in insulin secretion, specifically abnormalities of insulin secretory kinetics and pancreatic β-cell glucose responsiveness. Mechanisms underlying β-cell dysfunction include glucose toxicity, lipotoxicity and β-cell hyperactivity. Defects at various sites in β-cell signal transduction pathways contribute, but no single lesion can account for the common form of Type 2 diabetes. Recent studies highlight diverse β-cell actions of GLP-1 (glucagon-like peptide-1) and GIP (glucose-dependent insulinotropic polypeptide). These intestinal hormones target the β-cell to stimulate glucose-dependent insulin secretion through activation of protein kinase A and associated pathways. Both increase gene expression and proinsulin biosynthesis, protect against apoptosis and stimulate replication/neogenesis of β-cells. Incretin hormones therefore represent an exciting future multi-action solution to correct β-cell defect in Type 2 diabetes.

The rapid insulin secretory pulses that occur in the perfused rat pancreas can be entrained by an oscillatory glucose concentration in pancreata from nondiabetic rats but not from X diabetic Zucker diabetic fatty (ZDF) rats. To investigate whether this defect is present in prediabetic ZDF rats and whether treatment with either pioglitazone or acarbose can prevent or reverse this defect, 39 ZDF and 5 lean ZDF control rats were studied. The ZDF rats were divided into six groups depending on age, form of therapy used, and the time at which pioglitazone was started in relation to the onset of diabetes. The pancreas was isolated and perfused using a sine wave-shaped glucose concentration (mean 7 mM, period 10 min, amplitude 10%). The results, assessed by spectral analysis, revealed that in prediabetic animals and in controls, entrainment of pulsatile insulin secretion was normal. Initiation of pioglitazone therapy in ZDF rats at the time of weaning or before diabetes onset prevented hyperglycemia. However, entrainment was only partially retained. Thus, in these two groups, the spectral power at 10 min was greater than in untreated animals but lower than in prediabetic and control animals. Treatment with acarbose before or with pioglitazone after diabetes onset improved but did not normalize glucose levels, and it did not improve entrainment. The results demonstrate the presence of insulin secretory defects in 13-wk-old ZDF rats in which hyperglycemia was prevented.

Objective. Advanced glycation end products (AGEs) are important in the pathophysiology of type 2 diabetes mellitus (T2DM). They directly cause insulin secretory defects in animal and cell culture models and may promote insulin resistance in nondiabetic subjects. We have developed a highly sensitive liquid chromatography-tandem mass spectrometry method for measuring AGEs in human serum. Here, we use this method to investigate the relationship between AGEs and insulin secretion and resistance in patients with T2DM. Methods. Our study involved 15 participants with T2DM not on medication and 20 nondiabetic healthy participants. We measured the AGE carboxyethyllysine (CEL), carboxymethyllysine (CML), and methyl-glyoxal-hydro-imidazolone (MG-H1). Plasma glucose and insulin were measured in these participants during a meal tolerance test, and the glucose disposal rate was measured during a euglycemic-hyperinsulinemic clamp. Results. CML and CEL levels were significantly higher in T2DM than non-DM participants. CML showed a significant negative correlation with insulin secretion, HOMA-%B, and a significant positive correlation with the insulin sensitivity index in T2DM participants. There was no correlation between any of the AGEs measured and glucose disposal rate. Conclusions. These results suggest that AGE might play a role in the development or prediction of insulin secretory defects in type 2 diabetes.

Thyrotropin releasing hormone (TRH; pGlu-His-ProNH2) is expressed also in pancreatic β cells where it is colocalized in secretory granules with insulin. High perinatal changes of the TRH gene expression and TRH concentrations in rat pancreatic islets coincide with the perinatal maturation of the adequate insulin secretory responsiveness to glucose and other nutrient secretagogues. TRH secretion from pancreatic islets is stimulated by glucose and inhibited by insulin. Disruption of the TRH gene in knockout mice results in hyperglycemia accompanied by impaired insulin secretory response to glucose. Progress in understanding TRH - insulin relations may be substantial for improving knowledge of pathophysiological mechanisms included in changes of insulin secretion with possible clinical impact. Block of the last step of biosynthesis of α-amidated peptides, including TRH by disulfiram (DS) treatment of adult male rats subcutaneously with 200 mg/kg for five days in our experiments resulted in barely detectable levels of peptidyl-glycine α-amidating monooxygenase (PAM) in their pancreatic islets. TRH in physiological concentration (1 nM) does not affect basal insulin secretion from intact rat pancreatic islets. In contrast, basal insulin secretion from islets of DS-treated rats is four times higher compared to controls and could not be further stimulated by high-glucose. The addition of 1 nM TRH into medium decreased immediately basal insulin secretion in DS (TRH lacking) islets to control level and normalized also their response to glucose. Interestingly, absence of the secretory response to glucose in islets from TRH depleted rats was connected with their increase of insulin content during stimulation. Glucose stimulation together with 1 nM TRH normalized also insulin content in DS islets. Apparently, high insulin content in islets from TRH depleted animals is a result of block of regulatory secretion pathway redirected to constitutional secretion which was corrected by the addition of TRH. Type 2 diabetes mellitus is a disease characterized by various range from predominant insulin resistance with relative insulin deficiency to a predominant secretory defect with insulin resistance. These symptoms suggest a possible role of TRH dysregulation. In conclusion, presence of TRH in β cells ensures appropriate low basal (constitutive) insulin secretion. Release of TRH induced by glucose and possibly by other secretagogues has autocrine effect resulting in directing insulin secretion to regulatory pathway reacting to stimulation. If some defects of insulin secretion could be treated by TRH, various ways of applications (also oral and nasal) could be utilized. Moreover, positive side effects shown in animal experiments may accompany the treatment: TRH has the potential to prevent apoptosis and promotes insulin-producing cell proliferation and has also aging-reversing properties.

Inorganic phosphate (P i) plays an important role in cell signaling and energy metabolism. In insulin-releasing cells, P i transport into mitochondria is essential for the generation of ATP, a signaling factor in metabolism-secretion coupling. Elevated P i concentrations, however, can have toxic effects in various cell types. The underlying molecular mechanisms are poorly understood. Here, we have investigated the effect of P i on secretory function and apoptosis in INS-1E clonal β-cells and rat pancreatic islets. Elevated extracellular P i (1∼5 mM) increased the mitochondrial membrane potential (ΔΨm), superoxide generation, caspase activation, and cell death. Depolarization of the ΔΨm abolished P i-induced superoxide generation. Butylmalonate, a nonselective blocker of mitochondrial phosphate transporters, prevented ΔΨm hyperpolarization, superoxide generation, and cytotoxicity caused by P i. High P i also promoted the opening of the mitochondrial permeability transition (PT) pore, leading to apoptosis, which was also prevented by butylmalonate. The mitochondrial antioxidants mitoTEMPO or MnTBAP prevented P i-triggered PT pore opening and cytotoxicity. Elevated extracellular P i diminished ATP synthesis, cytosolic Ca2+ oscillations, and insulin content and secretion in INS-1E cells as well as in dispersed islet cells. These parameters were restored following preincubation with mitochondrial antioxidants. This treatment also prevented high-P i-induced phosphorylation of ER stress proteins. We propose that elevated extracellular P i causes mitochondrial oxidative stress linked to mitochondrial hyperpolarization. Such stress results in reduced insulin content and defective insulin secretion and cytotoxicity. Our data explain the decreased insulin content and secretion observed under hyperphosphatemic states.

Type 2 diabetes accounts for 85-90% of all people with diabetes and is currently estimated to affect more than 180 million people worldwide, a figure estimated to double by the year 2030. Thus understanding the basic biology of glucose homeostasis and how it is altered during disease progression is crucial to the development of safe and effective treatment regimes. The link between high dietary fat and the development of type Il diabetes is well established. Chronic treatment of pancreatic islets with the lipid palmitate induces defects in glucose stimulated insulin secretion (GSIS) akin to those seen in the development of type Il diabetes. Previous studies from our group have identified the lipid-activated kinase protein kinase C epsilon (PKCε) as a potential mediator of some of these effects. Deletion of PKCε in mice results in complete protection from high-fat diet induced glucose intolerance. This protection is associated with enhanced circulating insulin suggesting that PKCε may be involved in the regulation of insulin release from the pancreatic β-Cell. The data presented here suggests that PKCs plays an important role in the regulation of insulin secretion under both physiological and pathophysiological conditions. We demonstrate that PKCε can be activated by chronic lipid treatment and acute cholinergic stimulation. Under these conditions insulin secretion is enhanced by PKCε deletion or inhibition suggesting that PKCε is a negative regulator of insulin secretion. Mechanistically the PKCs mediated inhibition of insulin release by acute or chronic PKCε activation appears to be distinct. The effect of PKCε induced by palmitate pre-treatment appears to be distal to calcium influx. The pool of pre-docked vesicles is enhanced in palmitate pre-treated β-cells lacking PKCε suggesting that PKCε may be involved in the regulation of vesicle dynamics. In contrast, calcium dynamics induced by cholinergic stimulation are altered by PKCε deletion, suggesting an effect on either the calcium channels themselves or on the upstream signalling. Given the ability of PKCε to inhibit insulin secretion, inhibition of PKCε in the β-cells of people suffering from insulin resistance and (or) type II diabetes represents a novel target for the treatment of type II diabetes.

The distinctions between what has previously been termed cell therapy and gene therapy have become blurred. Cell therapy traditionally implied the in vitro expansion of cells that could subsequently be engrafted into patients to elicit a therapeutic effect, while gene therapy was a term applied to the genetic manipulation of tissues or cells in vivo or ex vivo. With the amazing advances that have been achieved using transcription factors to reprogramme cells, this distinction, at least for regenerative medicine applications, no longer exists. In this chapter, following the statement of the unmet clinical need, we review potential sources of new β‎ cells and approaches to β‎ cell replacement therapy; discuss how recent advances in safety and efficacy of gene transfer technology can augment cellular therapeutic approaches, and summarize pure gene therapy approaches dependent on expression of genes encoding insulin and other glucose-lowering hormones in the recipient’s own cells. Since both type 1 and type 2 diabetes are associated with a decline in β‎ cell mass, cell and gene therapy targeted at the β‎ cell and insulin replacement have potential applications for both forms of the disease. In type 1 diabetes, uninterrupted compliance with insulin injection therapy is necessary to prevent potentially fatal ketoacidosis. The landmark Diabetes Control and Complications Trial and Epidemiology of Diabetes Interventions and Complications follow-up study have confirmed that chronic hyperglycaemic microvascular and macrovascular complications can be prevented by tight glycaemic control, but this was at the expense of a threefold increase in severe hypoglycaemia—one of the greatest fears of those living with daily insulin injections. Overall, the health implications and economic costs of type 1 diabetes are massive, and increasing annually. There is, therefore, an unquestionable clinical need for new therapeutic options. While transplantation of whole pancreas together with its blood supply can entirely normalize blood glucose levels, the major surgery required is associated with 5% mortality in the first year, even in the most experienced centres. Isolation and transplantation of purified insulin-secreting islets of Langerhans from a donor pancreas requires only minimally invasive cannulation of the portal vein transhepatically under X-ray guidance. This offers the promise of more widespread implementation restoring excellent control, preventing both long-term complications and severe hypoglycaemia. Capacity will, however, be severely limited by the scarcity of deceased donor organs: currently sufficient for fewer than 1% of those who might benefit from this form of treatment. This has provided impetus to efforts to produce a replenishable supply of glucose-responsive insulin-secreting cells that could be used in transplantation. One potential source might involve the in vitro differentiation of stem cells derived from embryonic and adult tissue. Type 2 diabetes is marked by both a resistance of target tissue to the effects of insulin and impaired function of the β‎ cell. The major β‎-cell defects relate to an impaired secretory response to glucose, altered kinetics of secretion including pulsatility, accumulation of islet amyloid polypeptide, an increase in glucagon-secreting α‎ cells, and a decline in β‎-cell mass. Current therapy for type 2 diabetes involves a combination of drugs directed at improvements in both insulin sensitivity and β‎-cell function, together with management of associated cardiovascular risk factors. Conventional treatment modalities have not been able to prevent the inexorable progressive loss of β‎-cell function necessitating insulin replacement in the majority over time, but this is often insufficient to sustainably achieve target glucose levels outwith intensive clinical trials. It is envisaged that novel cell therapy approaches will enable restoration of β‎-cell mass.