Beta-cell Dysfunction In Diabetes Research Articles

HNF1A Mutations and Beta Cell Dysfunction in Diabetes.

Understanding the genetic factors of diabetes is essential for addressing the global increase in type 2 diabetes. HNF1A mutations cause a monogenic form of diabetes called maturity-onset diabetes of the young (MODY), and HNF1A single-nucleotide polymorphisms are associated with the development of type 2 diabetes. Numerous studies have been conducted, mainly using genetically modified mice, to explore the molecular basis for the development of diabetes caused by HNF1A mutations, and to reveal the roles of HNF1A in multiple organs, including insulin secretion from pancreatic beta cells, lipid metabolism and protein synthesis in the liver, and urinary glucose reabsorption in the kidneys. Recent studies using human stem cells that mimic MODY have provided new insights into beta cell dysfunction. In this article, we discuss the involvement of HNF1A in beta cell dysfunction by reviewing previous studies using genetically modified mice and recent findings in human stem cell-derived beta cells.

Beta cell dysfunction in diabetes: the islet microenvironment as an unusual suspect.

Cells in different tissues, including endocrine cells in the pancreas, live in complex microenvironments that are rich in cellular and acellular components. Intricate interactions with their microenvironment dictate most cellular properties, such as their function, structure and size, and maintain tissue homeostasis. Pancreatic islets are populated by endocrine, vascular and immune cells that are immersed in the extracellular matrix. While the intrinsic properties of beta cells have been vastly investigated, our understanding of their interactions with their surroundings has only recently begun to unveil. Here, we review current research on the interplay between the islet cellular and acellular components, and the role these components play in beta cell physiology and pathophysiology. Although beta cell failure is a key pathomechanism in diabetes, its causes are far from being fully elucidated. We, thus, propose deleterious alterations of the islet niche as potential underlying mechanisms contributing to beta cell failure. In sum, this review emphasises that the function of the pancreatic islet depends on all of its components. Graphical abstract.

2178-P: Beta-Cell Gene Expression Pattern in Food Restricted db/db Mice and Moderate Dedifferentiation Status in ob/ob Islets

In clinical, the 'fatigue' of beta cells is a famous phenomenon from way back. Nowadays, researchers describe how glucolipotoxicity, endoplasmic reticulum (ER) stress and oxidative stress impair beta cell function. In 2012, Talchai et al. revealed the novel machinery of beta cell dysfunction in diabetes - dedifferentiation. It's a gradual process along with the progressive feature of diabetes, and in several reports, it's proven to be preventable by existing diabetic treatment. Previously, we showed that each diabetic treatment had a different effect on beta cell dedifferentiation. For instance, food restriction had more manifold good effects to restore beta cell gene expression than SGLT inhibitor. In the current study, we aimed to unveil the molecular mechanism of beta cell dedifferentiation. First, we investigated the microarray analysis of islet RNAs in food-restricted db/db mice. Compared to the control db/db, the islets of food-restricted group had higher expressions of genes related to matured beta cell markers and ER maintenance. On the other hand, Gene expressions related to cell stress, cell growth and cell division were lowered by calorie restriction. To understand more about the relationship between beta cell dedifferentiation and ER stress, we utilized the other obese mice model, ob/ob, which is known to have more moderate hyperglycemia than db/db. The other study group demonstrated that ER and oxidative stress markers were maintained in ob/ob islets than that in db/db (Chan, Diabetes 2013). We found that beta cell dedifferentiation markers, such as Aldh1a3 and Sox9, were also lowered in ob/ob. But it's still unknown, whether the dedifferentiated beta cells are feeble against the ER and oxidative stress, or, these stress triggers the beta cell dedifferentiation. Disclosure X. Lei: None. E. Ishida: None. E. Yamada: None. S. Okada: None. M. Yamada: None.

Hypertriglyceridemia per se Is Not an Independent Risk Factor for Beta-Cell Dysfunction

Hypertriglyceridemia results from increased production and decreased clearance of triglyceride-rich very low-density lipoproteins, a pathological condition that accounts for heightened risk of ischemic vascular diseases in obesity and type 2 diabetes. Despite its intimate association with insulin resistance, whether hypertriglyceridemia constitutes an independent risk for beta-cell dysfunction in diabetes is unknown. Answering this fundamental question is stymied by the fact that hypertriglyceridemia is intertwined with hyperglycemia and insulin resistance in obese and diabetic subjects. To circumvent this limitation, we took advantage of apolipoprotein C3 (ApoC3)-transgenic mice, a model with genetic predisposition to hypertriglyceridemia without alterations in glucose metabolism. We showed that ApoC3-transgenic mice, as opposed to age/sex matched wild type littermates, develop hypertriglyceridemia with concomitant elevations in plasma cholesterol and non-esterified fatty acid levels. Anti-insulin and anti-glucagon dual immunohistochemistry in combination with morphometric analysis revealed that ApoC3-transgenic and wild type littermates had similar beta-cell and alpha-cell masses as well as islet size and architecture. These effects correlated with similar amplitudes of glucose-stimulated insulin secretion and similar degrees of postprandial glucose excursion in ApoC3-transgenic vs. wild type littermates. ApoC3-transgenic mice did not visualize lipid infiltration into islets, correlating with the lack of ectopic triglyceride and cholesterol depositions in the pancreata. ApoC3-transgenic mice, despite persistent hypertriglyceridemia, maintained euglycemia under both fed and fasting conditions without manifestation of insulin resistance and fasting hyperinsulinemia. We concluded that hypertriglyceridemia per se is not an independent risk factor for beta-cell dysfunction. Disclosure J. Yamauchi: None. T. Zhang: None. S. Lee: None. H. Dong: None.

Effect of Hypertriglyceridemia on Beta Cell Mass and Function in ApoC3 Transgenic Mice

Hypertriglyceridemia results from increased production and decreased clearance of triglyceride-rich very low-density lipoproteins, a pathological condition that accounts for heightened risk of ischemic vascular diseases in obesity and type 2 diabetes. Despite its intimate association with insulin resistance, whether hypertriglyceridemia constitutes an independent risk for beta cell dysfunction in diabetes is unknown. Answering this fundamental question is stymied by the fact that hypertriglyceridemia is intertwined with hyperglycemia and insulin resistance in obese and diabetic subjects. To circumvent this limitation, we took advantage of apolipoprotein C3 (ApoC3)-transgenic mice, a model with genetic predisposition to hypertriglyceridemia. We showed that ApoC3-transgenic mice, as opposed to age/sex-matched wild-type littermates, develop hypertriglyceridemia with concomitant elevations in plasma cholesterol and non-esterified fatty acid levels. Anti-insulin and anti-glucagon dual immunohistochemistry in combination with morphometric analysis revealed that ApoC3-transgenic and wild-type littermates had similar beta cell and alpha cell masses as well as islet size and architecture. These effects correlated with similar amplitudes of glucose-stimulated insulin secretion and similar degrees of postprandial glucose excursion in ApoC3-transgenic versus wild-type littermates. Oil Red O histology did not visualize lipid infiltration into islets, correlating with the lack of ectopic triglyceride and cholesterol depositions in the pancreata of ApoC3-transgenic versus wild-type littermates. ApoC3-transgenic mice, despite persistent hypertriglyceridemia, maintained euglycemia under both fed and fasting conditions without manifestation of insulin resistance and fasting hyperinsulinemia. Thus, hypertriglyceridemia per se is not an independent risk factor for beta cell dysfunction in ApoC3 transgenic mice.

MicroRNA-185 targets SOCS3 to inhibit beta-cell dysfunction in diabetes.

Diabetes is the most common and complex metabolic disorder, and one of the most important health threats now. MicroRNAs (miRNAs) are a group of small non-coding RNAs that have been suggested to play a vital role in a variety of physiological processes, including glucose homeostasis. In this study, we investigated the role of miR-185 in diabetes. MiR-185 was significantly downregulated in diabetic patients and mice, and the low level was correlated to blood glucose concentration. Overexpression of miR-185 enhanced insulin secretion of pancreatic β-cells, promoted cell proliferation and protected cells from apoptosis. Further experiments using in silico prediction, luciferase reporter assay and western blot assay demonstrated that miR-185 directly targeted SOCS3 by binding to its 3’-UTR. On the contrary to miR-185’s protective effects, SOCS3 significantly suppressed functions of β-cell and inactivated Stat3 pathway. When treating cells with miR-185 mimics in combination with SOCS3 overexpression plasmid, the inhibitory effects of SOCS3 were reversed. While combined treatment of miR-185 mimics and SOCS3 siRNA induced synergistically promotive effects compared to either miR-185 mimics or SOCS3 siRNA treatment alone. Moreover, we observed that miR-185 level was inversely correlated with SOCS3 expression in diabetes patients. In conclusion, this study revealed a functional and mechanistic link between miR-185 and SOCS3 in the pathogenesis of diabetes. MiR-185 plays an important role in the regulation of insulin secretion and β-cell growth in diabetes. Restoration of miR-185 expression may serve a potentially promising and efficient therapeutic approach for diabetes.

Regulation of Insulin Synthesis and Secretion and Pancreatic Beta-Cell Dysfunction in Diabetes

Pancreatic β-cell dysfunction plays an important role in the pathogenesis of both type 1 and type 2 diabetes. Insulin, which is produced in β-cells, is a critical regulator of metabolism. Insulin is synthesized as preproinsulin and processed to proinsulin. Proinsulin is then converted to insulin and C-peptide and stored in secretary granules awaiting release on demand. Insulin synthesis is regulated at both the transcriptional and translational level. The cis-acting sequences within the 5' flanking region and trans-activators including paired box gene 6 (PAX6), pancreatic and duodenal homeobox- 1(PDX-1), MafA, and β-2/Neurogenic differentiation 1 (NeuroD1) regulate insulin transcription, while the stability of preproinsulin mRNA and its untranslated regions control protein translation. Insulin secretion involves a sequence of events in β-cells that lead to fusion of secretory granules with the plasma membrane. Insulin is secreted primarily in response to glucose, while other nutrients such as free fatty acids and amino acids can augment glucose-induced insulin secretion. In addition, various hormones, such as melatonin, estrogen, leptin, growth hormone, and glucagon like peptide-1 also regulate insulin secretion. Thus, the β-cell is a metabolic hub in the body, connecting nutrient metabolism and the endocrine system. Although an increase in intracellular [Ca2+] is the primary insulin secretary signal, cAMP signaling- dependent mechanisms are also critical in the regulation of insulin secretion. This article reviews current knowledge on how β-cells synthesize and secrete insulin. In addition, this review presents evidence that genetic and environmental factors can lead to hyperglycemia, dyslipidemia, inflammation, and autoimmunity, resulting in β-cell dysfunction, thereby triggering the pathogenesis of diabetes.

Regulation of Insulin Synthesis and Secretion and Pancreatic Beta-Cell Dysfunction in Diabetes

Pancreatic β-cell dysfunction plays an important role in the pathogenesis of both type 1 and type 2 diabetes. Insulin, which is produced in -cells, is a critical regulator of metabolism. Insulin is synthesized as preproinsulin and processed to proinsulin. Proinsulin is then converted to insulin and C-peptide and stored in secretary granules awaiting release on demand. Insulin synthesis is regulated at both the transcriptional and translational level. The cis-acting sequences within the 5’ flanking region and trans-activators including paired box gene 6 (PAX6), pancreatic and duodenal homeobox- 1(PDX-1), MafA, and -2/Neurogenic differentiation 1 (NeuroD1) regulate insulin transcription, while the stability of preproinsulin mRNA and its untranslated regions control protein translation. Insulin secretion involves a sequence of events in β-cells that lead to fusion of secretory granules with the plasma membrane. Insulin is secreted primarily in response to glucose, while other nutrients such as free fatty acids and amino acids can augment glucose-induced insulin secretion. In addition, various hormones, such as melatonin, estrogen, leptin, growth hormone, and glucagon like peptide-1 also regulate insulin secretion. Thus, the β-cell is a metabolic hub in the body, connecting nutrient metabolism and the endocrine system. Although an increase in intracellular [Ca2+] is the primary insulin secretary signal, cAMP signaling- dependent mechanisms are also critical in the regulation of insulin secretion. This article reviews current knowledge on how β-cells synthesize and secrete insulin. In addition, this review presents evidence that genetic and environmental factors can lead to hyperglycemia, dyslipidemia, inflammation, and autoimmunity, resulting in β-cell dysfunction, thereby triggering the pathogenesis of diabetes.

Regulation of Insulin Synthesis and Secretion and Pancreatic Beta-Cell Dysfunction in Diabetes

Pancreatic β-cell dysfunction plays an important role in the pathogenesis of both type 1 and type 2 diabetes. Insulin, which is produced in -cells, is a critical regulator of metabolism. Insulin is synthesized as preproinsulin and processed to proinsulin. Proinsulin is then converted to insulin and C-peptide and stored in secretary granules awaiting release on demand. Insulin synthesis is regulated at both the transcriptional and translational level. The cis-acting sequences within the 5’ flanking region and trans-activators including paired box gene 6 (PAX6), pancreatic and duodenal homeobox- 1(PDX-1), MafA, and -2/Neurogenic differentiation 1 (NeuroD1) regulate insulin transcription, while the stability of preproinsulin mRNA and its untranslated regions control protein translation. Insulin secretion involves a sequence of events in β-cells that lead to fusion of secretory granules with the plasma membrane. Insulin is secreted primarily in response to glucose, while other nutrients such as free fatty acids and amino acids can augment glucose-induced insulin secretion. In addition, various hormones, such as melatonin, estrogen, leptin, growth hormone, and glucagon like peptide-1 also regulate insulin secretion. Thus, the β-cell is a metabolic hub in the body, connecting nutrient metabolism and the endocrine system. Although an increase in intracellular [Ca2+] is the primary insulin secretary signal, cAMP signaling- dependent mechanisms are also critical in the regulation of insulin secretion. This article reviews current knowledge on how β-cells synthesize and secrete insulin. In addition, this review presents evidence that genetic and environmental factors can lead to hyperglycemia, dyslipidemia, inflammation, and autoimmunity, resulting in β-cell dysfunction, thereby triggering the pathogenesis of diabetes.

Gossypin, a flavonol glucoside protects pancreatic beta-cells from glucotoxicity in streptozotocin-induced experimental diabetes in rats

Chronic oxidative stress caused by glucotoxicity plays a crucial role in the onset and progression of pancreatic beta-cell dysfunction in diabetes mellitus. Several studies have reported that flavonoids protect tissues from oxidative damage by its antioxidative potential. The present study was aimed to investigate the antioxidant activity of gossypin by DPPH and ABTS radical scavenging assays and its role on glucotoxicity mediated pancreatic beta-cell dysfunction in streptozotocin-induced experimental diabetes in rats. Gossypin effectively scavenges both 2,2-Diphenyl-1-picrylhydrazyl (DPPH) and ABTS radicals generated in the reaction mixtures, which implies the antioxidant property of gossypin. Oral administration of gossypin to diabetic rats significantly improved the antioxidants and reduced the levels of lipid peroxides, nitric oxide, IL-1β and nuclear NF-κB p65 unit in pancreatic tissues. The improved insulin content in pancreas and plasma in the experimental diabetic rats treated with gossypin as well as histological observations demonstrate the possible role of gossypin in the protection of pancreatic beta-cells. In addition, blood glucose, HbA1c, glutathione, lipid peroxides, IL-1β levels in plasma and nitric oxide level were significantly altered upon treatment with gossypin. It is concluded that the pancreatic islet beta-cell protective role of gossypin against glucotoxicity mediated oxidative damage might be attributed by its antioxidative potential.

An intracellular role for ABCG1-mediated cholesterol transport in the regulated secretory pathway of mouse pancreatic β cells

Cholesterol is a critical component of cell membranes, and cellular cholesterol levels and distribution are tightly regulated in mammals. Recent evidence has revealed a critical role for pancreatic beta cell-specific cholesterol homeostasis in insulin secretion as well as in beta cell dysfunction in diabetes and the metabolic response to thiazolidinediones (TZDs), which are antidiabetic drugs. The ATP-binding cassette transporter G1 (ABCG1) has been shown to play a role in cholesterol efflux, but its role in beta cells is currently unknown. In other cell types, ABCG1 expression is downregulated in diabetes and upregulated by TZDs. Here we have demonstrated an intracellular role for ABCG1 in beta cells. Loss of ABCG1 expression impaired insulin secretion both in vivo and in vitro, but it had no effect on cellular cholesterol content or efflux. Subcellular localization studies showed the bulk of ABCG1 protein to be present in insulin granules. Loss of ABCG1 led to altered granule morphology and reduced granule cholesterol levels. Administration of exogenous cholesterol restored granule morphology and cholesterol content and rescued insulin secretion in ABCG1-deficient islets. These findings suggest that ABCG1 acts primarily to regulate subcellular cholesterol distribution in mouse beta cells. Furthermore, islet ABCG1 expression was reduced in diabetic mice and restored by TZDs, implicating a role for regulation of islet ABCG1 expression in diabetes pathogenesis and treatment.

Rap1 Promotes Multiple Pancreatic Islet Cell Functions and Signals through Mammalian Target of Rapamycin Complex 1 to Enhance Proliferation

Recent studies have implicated Epac2, a guanine-nucleotide exchange factor for the Rap subfamily of monomeric G proteins, as an important regulator of insulin secretion from pancreatic beta-cells. Although the Epac proteins were originally identified as cAMP-responsive activators of Rap1 GTPases, the role of Rap1 in beta-cell biology has not yet been defined. In this study, we examined the direct effects of Rap1 signaling on beta-cell biology. Using the Ins-1 rat insulinoma line, we demonstrate that activated Rap1A, but not related monomeric G proteins, promotes ribosomal protein S6 phosphorylation. Using isolated rat islets, we show that this signaling event is rapamycin-sensitive, indicating that it is mediated by the mammalian target of rapamycin complex 1-p70 S6 kinase pathway, a known growth regulatory pathway. This newly defined beta-cell signaling pathway acts downstream of cAMP, in parallel with the stimulation of cAMP-dependent protein kinase, to drive ribosomal protein S6 phosphorylation. Activated Rap1A promotes glucose-stimulated insulin secretion, islet cell hypertrophy, and islet cell proliferation, the latter exclusively through mammalian target of rapamycin complex 1, suggesting that Rap1 is an important regulator of beta-cell function. This newly defined signaling pathway may yield unique targets for the treatment of beta-cell dysfunction in diabetes.

Effects of DHMEQ, a Novel Nuclear Factor-.KAPPA.B Inhibitor, on Beta Cell Dysfunction in INS-1 Cells

Recent studies suggest that nuclear factor-kappaB (NF-kappaB) activation has an important role in leading to beta cell dysfunction in both type 1 and type 2 diabetes. In this study we tested this hypothesis by investigating the effects of dehydroxymethylepoxyquinomicin (DHMEQ), a novel NF-kappaB inhibitor, on tumor necrosis factor-alpha (TNF-alpha)-induced beta cell dysfunction. INS-1 cells were incubated with TNF- alpha and with or without DHMEQ for 24 hours. Glucose-stimulated insulin secretion, cell viability, mRNA expression and NF-kappaB activation were investigated. DHMEQ suppressed TNF-alpha-induced NF-kappaB activation and partially ameliorated glucose-stimulated insulin secretion in a dose-dependent manner. DHMEQ also partially ameliorated decreased cell viability and insulin mRNA level induced by TNF-alpha. DHMEQ suppressed NF-kappaB activation and ameliorated beta cell dysfunction induced by TNF- alpha. Inhibition of activated NF-kappaB in beta cells may be important to ameliorate beta cell dysfunction in diabetes.

Preventing diabetes in patients with hypertension: one more reason to block the renin–angiotensin system

Patients with essential hypertension are at increased risk of type 2 (non-insulin-dependent) diabetes. Recent large studies have been unable to delineate any superiority in one class of antihypertensive drug over another, independent of their effects in reducing blood pressure; however, in the longer term, antihypertensive agents that are able to reduce the risk of diabetes may have a theoretical advantage. To this end, the findings of several recent clinical trials have suggested that blockade of the renin-angiotensin system (RAS) may protect against the development of de-novo diabetes in 'at risk' patients. This beneficial effect appears to outweigh both the adverse metabolic effects of agents used in the control arm of these studies and the control of blood pressure achieved. Furthermore, recent evidence suggests that the RAS may have a direct role in the pathogenesis of diabetes. Angiotensin-mediated increases in oxidative stress, inflammation, and free fatty acids concentrations potentially contribute to beta-cell dysfunction in diabetes. In addition, activation of the RAS appears to potentiate the action of other pathogenic pathways, including glucotoxicity, lipotoxicity, and advanced glycation. In experimental models of type 2 diabetes, blockade of the RAS with angiotensin-converting enzyme inhibitors or angiotensin receptor antagonists also results in the improvement of islet structure and function. At least three large controlled trials are currently under way to study the utility of blockade of the RAS in the development of diabetes, including studies of combination therapy. It is hoped that these studies will demonstrate the true potential of blockade of the RAS for the prevention of diabetes.

Beyond the beta cell in diabetes

A new study shows that mutations in the gene encoding carboxyl ester lipase (CEL) cause beta cell dysfunction and early-onset diabetes. As this gene is expressed in the exocrine, and not the endocrine, pancreas, its identification challenges many of our preconceived ideas about the causes of beta cell dysfunction in diabetes.

Beta-cell mitochondria in the regulation of insulin secretion: a new culprit in type II diabetes.

Insulin is stored in secretory granules in the beta-cell and is secreted by exocytosis. This process is precisely controlled to achieve blood glucose homeostasis. Many forms of diabetes mellitus display impaired glucose-induced insulin secretion. This has been shown to be the primary cause of the disease in the various forms of maturity-onset diabetes of the young (MODY) and has also been implicated in adult-onset Type II (non-insulin-dependent) diabetes mellitus. Glucose generates ATP and other metabolic coupling factors in the beta-cell mitochondria. By plasma membrane depolarisation ATP promotes Ca2+ influx, which raises cytosolic Ca2+ and triggers insulin exocytosis. Through hyperpolarisation of the mitochondrial membrane glucose also increases the Ca2+ concentration in the mitochondrial matrix activating Ca(2+)-sensitive dehydrogenases in the tricarboxylic acid cycle. The resulting generation of glutamate participates in Ca(2+)-stimulated exocytosis. Mitochondrial DNA (mtDNA) encodes some of the polypeptides of the respiratory chain enzyme complexes. Mutations in mtDNA lead to maternally inherited diabetes mellitus characterised by impaired insulin secretion. The impact of altered mtDNA on insulin secretion has been shown in mtDNA-deficient beta-cell lines which have lost glucose-stimulated insulin secretion but retain a Ca(2+)-induced insulin secretion. A cellular model of MODY3 expressing dominant-negative hepatocyte nuclear factor-1 alpha (HNF-1 alpha) also displayed deletion of glucose-induced but not Ca(2+)-induced insulin secretion. Reduced mitochondrial metabolism explains this secretory pattern. Thus, genetically manipulated beta-cell lines are essential tools in the investigation of the molecular basis of beta-cell dysfunction in diabetes and should explain the role of other transcription factors in the disease.

Beneficial effects of antioxidants in diabetes: possible protection of pancreatic beta-cells against glucose toxicity.

Oxidative stress is produced under diabetic conditions and possibly causes various forms of tissue damage in patients with diabetes. The aim of this study was to examine the involvement of oxidative stress in the progression of pancreatic beta-cell dysfunction in type 2 diabetes and to evaluate the potential usefulness of antioxidants in the treatment of type 2 diabetes. We used diabetic C57BL/KsJ-db/db mice, in whom antioxidant treatment (N-acetyl-L-cysteine [NAC], vitamins C plus E, or both) was started at 6 weeks of age; its effects were evaluated at 10 and 16 weeks of age. According to an intraperitoneal glucose tolerance test, the treatment with NAC retained glucose-stimulated insulin secretion and moderately decreased blood glucose levels. Vitamins C and E were not effective when used alone but slightly effective when used in combination with NAC. No effect on insulin secretion was observed when the same set of antioxidants was given to nondiabetic control mice. Histologic analyses of the pancreases revealed that the beta-cell mass was significantly larger in the diabetic mice treated with the antioxidants than in the untreated mice. As a possible cause, the antioxidant treatment suppressed apoptosis in beta-cells without changing the rate of beta-cell proliferation, supporting the hypothesis that in chronic hyperglycemia, apoptosis induced by oxidative stress causes reduction of beta-cell mass. The antioxidant treatment also preserved the amounts of insulin content and insulin mRNA, making the extent of insulin degranulation less evident. Furthermore, expression of pancreatic and duodenal homeobox factor-1 (PDX-1), a beta-cell-specific transcription factor, was more clearly visible in the nuclei of islet cells after the antioxidant treatment. In conclusion, our observations indicate that antioxidant treatment can exert beneficial effects in diabetes, with preservation of in vivo beta-cell function. This finding suggests a potential usefulness of antioxidants for treating diabetes and provides further support for the implication of oxidative stress in beta-cell dysfunction in diabetes.

Transcription factor abnormalities as a cause of beta cell dysfunction in diabetes: a hypothesis.

Well-characterized defects in insulin secretion, most notably a loss of glucose-induced insulin secretion, are found in virtually all forms of NIDDM, as well as in early IDDM. Similar abnormalities have been found in all animal models of diabetes in which they have been studied. A novel hypothesis is being proposed to explain the mechanisms responsible for these alterations. Many abnormalities in the various steps of glucose-induced insulin secretion have been identified in rodent models of diabetes, but none by itself seems sufficient to explain the defects. These include a loss of GLUT2, glycogen accumulation, glucose recycling, abnormal glucokinase or hexokinase, altered mitochondrial glycerol phosphate dehydrogenase (mGPDH) activity, abnormal ion channel function and beta cell degranulation. We propose that optimal secretory function is dependent upon the unique differentiation of beta cells that is maintained by a set of transcription factors and that this control is disrupted by the diabetic state. Therefore, we propose that key transcription factors are affected even when beta cells are stressed by insulin resistance in very earliest stages of diabetes and that the abnormality becomes more severe as full-blown diabetes develops, which leads to loss of beta cell differentiation and a resultant derangement of insulin secretion.

GLUT1 is adequate for glucose uptake in GLUT2-deficient insulin-releasing beta-cells.

GLUT2 may play an important role in pancreatic beta-cell glucose metabolism. A decrease in glucose uptake due to underexpression of GLUT2 has been considered as the cause of beta-cell dysfunction in diabetes with different pathogenesis. However, this view has been challenged by recent studies, in which the underexpression of GLUT2 was not accompanied by a decrease in glucose uptake. Our present aim is to evaluate the presumed importance of GLUT2 in maintaining the efficiency of beta-cell glucose uptake. We studied the kinetic characteristics of 3-O-methylglucose uptake in two beta-cell lines. One of these is the beta TC3 cell line which expresses GLUT1 and the other is the beta HC9 cell line which expresses both GLUT1 and GLUT2. Under equilibrium exchange conditions, 3-O-methylglucose transport in these two cell lines showed similar values of K(m) and V(max). The apparent IC50 of cytochalasin B for inhibiting 3-O-methylglucose transport in beta HC9 cells was nine times as high as in beta TC3 cells, indicating that GLUT1 is the critically important glucose transporter in the beta TC3 cell line and GLUT2 in the beta HC9 cell line. In both cell lines, the rates of glucose uptake were at least three times as fast as that of glucose phosphorylation. Our results suggest that GLUT1 is able to compensate for GLUT2 loss as it occurs in beta TC3 and maintains a commensurately high capacity of glucose uptake to sustain glucose metabolism in pancreatic beta-cells.

Abnormal Patterns of Insulin Secretion in Non-Insulin-Dependent Diabetes Mellitus

To determine whether non-insulin-dependent diabetes is associated with specific alterations in the pattern of insulin secretion, we studied 16 patients with untreated diabetes and 14 matched controls. The rates of insulin secretion were calculated from measurements of peripheral C-peptide in blood samples taken at 15- to 20-minute intervals during a 24-hour period in which the subjects ate three mixed meals. Incremental responses of insulin secretion to meals were significantly lower in the diabetic patients (P less than 0.005), and the increases and decreases in insulin secretion after meals were more sluggish. These disruptions in secretory response were more marked after dinner than after breakfast, and a clear secretory response to dinner often could not be identified. Both the control and diabetic subjects secreted insulin in a series of discrete pulses. In the controls, a total of seven to eight pulses were identified in the period from 9 a.m. to 11 p.m., including the three post-meal periods (an average frequency of one pulse per 105 to 120 minutes), and two to four pulses were identified in the remaining 10 hours. The number of pulses in the patients and controls did not differ significantly. However, in the patients, the pulses after meals had a smaller amplitude (P less than 0.03) and were less frequently concomitant with a glucose pulse (54.7 +/- 4.9 vs. 82.2 +/- 5.0, P less than 0.001). Pulses also appeared less regularly in the patients. During glucose clamping to produce hyperglycemia (glucose level, 16.7 mmol per liter [300 mg per deciliter]), the diabetic subjects secreted, on the average, 70 percent less insulin than matched controls (P less than 0.001). These data suggest that profound alterations in the amount and temporal organization of stimulated insulin secretion may be important in the pathophysiology of beta-cell dysfunction in diabetes.