An integrated view of beta-cell dysfunction in type-II diabetes

Hypoglycemia in Oral Glucose Tolerance Test during Pregnancy and Risk for Type 2 Diabetes-A Five-Year Cohort Study

Objective: To evaluate the risk of progression to type 2 diabetes (T2D) following reactive hypoglycemia in 100 g oral glucose tolerance test (oGTT). Methods: A retrospective analysis of parturients with up to 5-year follow-up postpartum. Data were extracted from the computerized laboratory system of Meuhedet, an Israeli HMO and cross-linked with the Israeli National Registry of Diabetes. Included were parturients with no prior diabetesand available oGTT values during pregnancy. Reactive hypoglycemia was defined as glucose levels lower than 60 mg/dL in at least one of 3 post-glucose load values in oGTT. The cohort was divided into 3 groups: normal glucose status, reactive hypoglycemia, and GDM. Maternal characteristics, laboratory data, and progression to T2D over 5 years were compared. Univariate and survival analyses assessed the adjusted hazard ratio for T2D, stratified by obesity Results: Among 14,122 parturients, 16.8% had reactive hypoglycemia, 71% had normal glucose status, and 12.2% had GDM. Adjusted for age, obesity, and hypertension, Parturients with reactive hypoglycemia had similar T2D risk compared to normal glucose status and a lower risk compared to GDM patients, regardless of obesity status. Conclusions: Reactive hypoglycemia during oGTT does not increase the risk of progressing to T2D.

Sleep-Disordered Breathing and Free Fatty Acid Metabolism

BACKGROUND

Sleep-disordered breathing (SDB) is independently associated with insulin resistance, glucose intolerance, and type 2 diabetes mellitus. However, data on whether SDB alters the metabolism of free fatty acids (FFAs) are lacking.

RESEARCH QUESTION

The primary objective of the current study was to characterize alterations in FFA metabolism across the spectrum of SDB severity.

STUDY DESIGN AND METHODS

The study sample included 118 participants with and without SDB who underwent full-montage polysomnography, the frequently sampled IV glucose tolerance test (FSIGTT), and body composition measurements including determination of percent body fat. Parameters of lipolysis suppression, time to FFA nadir, and FFA rebound after an IV glucose challenge were derived using a mathematical model. Multivariable regression analyses were used to characterize the independent associations between SDB severity and parameters of FFA metabolism.

RESULTS

SDB severity, as assessed by the apnea-hypopnea index, was associated with adipocyte insulin resistance, a decrease in the glucose- and insulin-mediated suppression of lipolysis, a longer duration to reach a nadir in FFA levels during the FSIGTT, and a sluggish rebound in FFA levels after suppression. Severity of SDB-related hypoxemia was independently associated with adipocyte insulin resistance and the time to reach the FFA nadir during the FSIGTT. Finally, a higher percentage of stage N3 sleep was positively associated with greater suppression of lipolysis and a faster rebound in the FFA levels during the FSIGTT.

INTERPRETATION

Independent of adiposity, SDB is associated with impairments in FFA metabolism, which may contribute to the development of glucose intolerance and type 2 diabetes in SDB.

Effect of Ishophloroglucin A, A Component of Ishige okamurae, on Glucose Homeostasis in the Pancreas and Muscle of High Fat Diet-Fed Mice

Ishophloroglucin A (IPA), a component of Ishige okamurae (IO), was previously evaluated to standardize the antidiabetic potency of IO. However, the potential of IPA as a functional food for diabetes prevention has not yet been evaluated. Here, we investigated if 1.35 mg/kg IPA, which is the equivalent content of IPA in 75 mg/kg IO, improved glucose homeostasis in high-fat diet (HFD)-induced diabetes after 12 weeks of treatment. IPA significantly ameliorated glucose intolerance, reducing fasting glucose levels as well as 2 h glucose levels in HFD mice. In addition, IPA exerted a protective effect on the pancreatic function in HFD mice via pancreatic β-cells and C-peptide. The level of glucose transporter 4 (GLUT4) in the muscles of HFD mice was stimulated by IPA intake. Our results suggested that IPA, which is a component of IO, can improve glucose homeostasis via GLUT4 in the muscles of HFD mice. IO may be used as a functional food for the prevention of diabetes.

Postprandial Reactive Hypoglycemia

Reactive hypoglycemia (RH) is the condition of postprandially hypoglycemia occurring 2-5 hours after food intake. RH is clinically seen in three different forms as follows: idiopathic RH (at 180 min), alimentary (within 120 min), and late RH (at 240–300 min). When the first-phase insulin response decreases, firstly, blood glucose starts to rise after the meal. This leads to late but excessive secretion of the second-phase insulin secretion. Thus, late reactive hypoglycemia occurs. Elevated insulin levels also cause down-regulation of the insulin post-receptor on the muscle and fat cells, thus decreasing insulin sensitivity. The cause of the increase in insulin sensitivity in IRH at 3 h is not completely clear. However, there is a decrease in insulin sensitivity in late reactive hypoglycaemia at 4 or 5 hours. Thus, patients with hypoglycemia at 4 or 5 h who have a family history of diabetes and obesity may be more susceptible to diabetes than patients with hypoglycemia at 3 h. We believe that some cases with normal glucose tolerance in OGTT should be considered as prediabetes at <55 or 60 mg/dl after 4-5 hours after OGTT. Metformin and AGI therapy may be recommended if there is late RH with IFG. Also Metformin, AGİ, TZD, DPP-IVInhibitors, GLP1RA therapy may be recommended if there is late RH with IGT. As a result, postprandial RH (<55 or 60 mg/dl), especially after 4 hours may predict diabetes. Therefore, people with RH along with weight gain and with diabetes history in the family will benefit from a lifestyle modification as well as the appropriate antidiabetic approach in the prevention of diabetes.

Forkhead box O1 promotes INS‑1 cell apoptosis by reducing the expression of CD24

Type 2 diabetes seriously affects human health and burdens public health systems. Pancreatic β-cell apoptosis contributes to a reduction in β-cell mass, which is responsible for the occurrence of type 2 diabetes. However, the mechanism that underlies this effect remains unclear. In the present study, the role of forkhead box O1 (Foxo1) was investigated (which is a key regulatory factor in β-cell function) in the apoptotic behavior of β-cells and a potential underlying mechanism was determined. It was demonstrated that Foxo1 overexpression significantly reduced the proliferation of INS-1 cells and increased the apoptosis of INS-1 cells, in contrast to foxm1, foxp, foxa1, foxc and foxb1 overexpression. The present study aimed to investigate potential underlying mechanisms using bioinformatics, including Gene Set Enrichment Analysis, and biological experiments, including flow cytometry, cell counting kit-8, immunofluorescence, western blotting, reverse transcription-quantitative polymerase chain reaction analysis and lentiviral transfection. Further experiments conclusively showed that cluster of differentiation (CD)24 expression was significantly decreased when INS-1 cells were treated with Foxo1. Animal experiments showed high CD24 expression in the pancreatic islets of diabetic Goto-Kakizaki rats. Moreover, Gene Set Enrichment Analysis showed that CD24 expression was associated with the adaptive immune response of β-cells. Finally, no significant differences in the proliferation and apoptosis of CD24 overexpressing INS-1 cells were observed after Foxo1 treatment. These results suggested that Foxo1 overexpression in β-cells was able to increase apoptosis by inhibiting CD24 expression. This study may provide an approach for the treatment and prevention of type 2 diabetes.

O-Linked β-N-acetylglucosamine (O-GlcNAc) Acts as a Glucose Sensor to Epigenetically Regulate the Insulin Gene in Pancreatic Beta Cells

The post-translational protein modification O-linked β-N-acetylglucosamine (O-GlcNAc) is a proposed nutrient sensor that has been shown to regulate multiple biological pathways. This dynamic and inducible enzymatic modification to intracellular proteins utilizes the end product of the nutrient sensing hexosamine biosynthetic pathway, UDP-GlcNAc, as its substrate donor. Type II diabetic patients have elevated O-GlcNAc-modified proteins within pancreatic beta cells due to chronic hyperglycemia-induced glucose overload, but a molecular role for O-GlcNAc within beta cells remains unclear. Using directed pharmacological approaches in the mouse insulinoma-6 (Min6) cell line, we demonstrate that elevating nuclear O-GlcNAc increases intracellular insulin levels and preserves glucose-stimulated insulin secretion during chronic hyperglycemia. The molecular mechanism for these observed changes appears to be, at least in part, due to elevated O-GlcNAc-dependent increases in Ins1 and Ins2 mRNA levels via elevations in histone H3 transcriptional activation marks. Furthermore, RNA deep sequencing reveals that this mechanism of altered gene transcription is restricted and that the majority of genes regulated by elevated O-GlcNAc levels are similarly regulated by a shift from euglycemic to hyperglycemic conditions. These findings implicate the O-GlcNAc modification as a potential mechanism for hyperglycemic-regulated gene expression in the beta cell.

Distribution of nociceptin in pancreatic islet cells of normal and diabetic rats

OBJECTIVES

Nociceptin has been reported to play an important role in the regulation of pancreatic exocrine secretion. Most of the studies performed on nociceptin are mainly physiological rather than morphological in nature. The present study investigated the pattern of distribution of nociceptin in the endocrine pancreas of normal and diabetic rats.

METHODS

Immunohistochemistry, immunofluorescence, Western blot, and double-labeled immunoelectron microscopy were used in this study. Diabetes was induced using streptozotocin (60 mg/kg body weight).

RESULTS

Nociceptin-immunoreactive cells were observed in the central and peripheral regions of the islets of both normal and diabetic rat pancreas. The number of nociceptin-positive cells was significantly (P < 0.05) lower in the islet of diabetic rats compared with the control. Immunofluorescence study showed that nociceptin colocalizes with insulin in pancreatic β-cells. The degree of colocalization of nociceptin with insulin was severely deranged after the onset of diabetes. Moreover, immunogold particles conjugated with either nociceptin or insulin were observed on the granules of pancreatic β-cell. The number of nociceptin-labeled colloidal gold particles was significantly lower after the onset of diabetes.

CONCLUSIONS

Nociceptin is present in pancreatic islets cells and colocalizes with insulin. Nociceptin may have a physiological role in the metabolism of insulin.

Impact of sex on insulin secretion in cystic fibrosis

CONTEXT

Cystic fibrosis-related diabetes is primarily due to a defect in insulin secretion. Women with cystic fibrosis (CF) are at higher risk of developing CF-related diabetes.

OBJECTIVE

The objective of the study was to examine sex differences in insulin and glucose homeostasis. We hypothesized that in CF, women would display lower insulin secretion than men.

DESIGN

This was a study based on an ongoing observational CF cohort with a mean follow-up of 19.9 ± 5.2 months.

SETTING

The study was conducted at the CF clinic of the Centre Hospitalier de l'Université de Montréal (Québec, Canada).

PATIENTS

From 230 adults with CF (123 men, 107 women) of similar age and functional pulmonary status, 104 retested after the follow-up. Age-matched healthy individuals (25 men, 19 women) were included in the study.

INTERVENTIONS

Participants underwent a 2-hour oral glucose tolerance test with 30-minute interval sample measurements.

MAIN OUTCOME MEASURE

Plasma insulin and glucose levels were measured.

RESULTS

Women with CF had higher overall insulin secretion as compared with men with CF (P ≤ .05) but similar to healthy women (P = .606). Men with CF had lower overall insulin secretion than healthy men (P = .020) and higher insulin sensitivity (P = .009) than women with CF. PATIENTS with CF displayed higher overall glucose excursions than healthy patients. Sex-related differences were still observed in the CF cohort after follow-up.

CONCLUSIONS

Surprisingly, in CF, adult women presented higher insulin secretion than adult men at a comparable level with what is observed in healthy individuals. Potential implications of this sex dimorphism in CF remain to be established.

Connexin 36 mediates blood cell flow in mouse pancreatic islets

The insulin-secreting β-cells are contained within islets of Langerhans, which are highly vascularized. Blood cell flow rates through islets are glucose-dependent, even though there are no changes in blood cell flow within in the surrounding exocrine pancreas. This suggests a specific mechanism of glucose-regulated blood flow in the islet. Pancreatic islets respond to elevated glucose with synchronous pulses of electrical activity and insulin secretion across all β-cells in the islet. Connexin 36 (Cx36) gap junctions between islet β-cells mediate this synchronization, which is lost in Cx36 knockout mice (Cx36(-/-)). This leads to glucose intolerance in these mice, despite normal plasma insulin levels and insulin sensitivity. Thus, we sought to investigate whether the glucose-dependent changes in intraislet blood cell flow are also dependent on coordinated pulsatile electrical activity. We visualized and quantified blood cell flow using high-speed in vivo fluorescence imaging of labeled red blood cells and plasma. With the use of a live animal glucose clamp, blood cell flow was measured during either hypoglycemia (∼50 mg/dl) or hyperglycemia (∼300 mg/dl). In contrast to the large glucose-dependent islet blood velocity changes observed in wild-type mice, only minimal differences are observed in both Cx36(+/-) and Cx36(-/-) mice. This observation supports a novel model where intraislet blood cell flow is regulated by the coordinated electrical activity in the islet β-cells. Because Cx36 expression and function is reduced in type 2 diabetes, the resulting defect in intraislet blood cell flow regulation may also play a significant role in diabetic pathology.

Ablation of ghrelin receptor in leptin-deficient ob/ob mice has paradoxical effects on glucose homeostasis when compared with ablation of ghrelin in ob/ob mice

The orexigenic hormone ghrelin is important in diabetes because it has an inhibitory effect on insulin secretion. Ghrelin ablation in leptin-deficient ob/ob (Ghrelin(-/-):ob/ob) mice increases insulin secretion and improves hyperglycemia. The physiologically relevant ghrelin receptor is the growth hormone secretagogue receptor (GHS-R), and GHS-R antagonists are thought to be an effective strategy for treating diabetes. However, since some of ghrelin's effects are independent of GHS-R, we have utilized genetic approaches to determine whether ghrelin's effect on insulin secretion is mediated through GHS-R and whether GHS-R antagonism indeed inhibits insulin secretion. We investigated the effects of GHS-R on glucose homeostasis in Ghsr-ablated ob/ob mice (Ghsr(-/-):ob/ob). Ghsr ablation did not rescue the hyperphagia, obesity, or insulin resistance of ob/ob mice. Surprisingly, Ghsr ablation worsened the hyperglycemia, decreased insulin, and impaired glucose tolerance. Consistently, Ghsr ablation in ob/ob mice upregulated negative β-cell regulators (such as UCP-2, SREBP-1c, ChREBP, and MIF-1) and downregulated positive β-cell regulators (such as HIF-1α, FGF-21, and PDX-1) in whole pancreas; this suggests that Ghsr ablation impairs pancreatic β-cell function in leptin deficiency. Of note, Ghsr ablation in ob/ob mice did not affect the islet size; the average islet size of Ghsr(-/-):ob/ob mice is similar to that of ob/ob mice. In summary, because Ghsr ablation in leptin deficiency impairs insulin secretion and worsens hyperglycemia, this suggests that GHS-R antagonists may actually aggravate diabetes under certain conditions. The paradoxical effects of ghrelin ablation and Ghsr ablation in ob/ob mice highlight the complexity of the ghrelin-signaling pathway.

Connexin-36 gap junctions regulate in vivo first- and second-phase insulin secretion dynamics and glucose tolerance in the conscious mouse

Insulin is secreted from the islets of Langerhans in coordinated pulses. These pulses are thought to lead to plasma insulin oscillations, which are putatively more effective in lowering blood glucose than continuous levels of insulin. Gap-junction coupling of β-cells by connexin-36 coordinates intracellular free calcium oscillations and pulsatile insulin release in isolated islets, however a role in vivo has not been shown. We test whether loss of gap-junction coupling disrupts plasma insulin oscillations and whether this impacts glucose tolerance. We characterized the connexin-36 knockout (Cx36(-/-)) mouse phenotype and performed hyperglycemic clamps with rapid sampling of insulin in Cx36(-/-) and control mice. Our results show that Cx36(-/-) mice are glucose intolerant, despite normal plasma insulin levels and insulin sensitivity. However, Cx36(-/-) mice exhibit reduced insulin pulse amplitudes and a reduction in first-phase insulin secretion. These changes are similarly found in isolated Cx36(-/-) islets. We conclude that Cx36 gap junctions regulate the in vivo dynamics of insulin secretion, which in turn is important for glucose homeostasis. Coordinated pulsatility of individual islets enhances the first-phase elevation and second-phase pulses of insulin. Because these dynamics are disrupted in the early stages of type 2 diabetes, dysregulation of gap-junction coupling could be an important factor in the development of this disease.

Fermentation effects of oligosaccharides of Radix Ophiopogonis on alloxan-induced diabetes in mice

In this study, oligosaccharides extracted from Ophiopogon japonicus vinegar (OOV) by alcoholic and acetic acid fermentation with water extracts from Radix Ophiopogon and oligosaccharides extracted from Radix Ophiopogonis (OOJ) were investigated. Characterization of the extracts indicated that OOV are proteoglycans, whereas OOJ are not. Moreover, compared with OOJ, monosaccharide compositions of OOV only include fructose and galactose and not glucose. MALDI-TOF-mass spectrometric results showed that the molecular weight of OOV was smaller after fermentation. Changes in the characteristics of OOV would inevitably lead to changes in its hypoglycemic properties. The OOV inhibition activity against α-glucosidase was stronger than that of OOJ. The inhibition activity became stronger with higher dosages of OOV. The hypoglycemic effect of OOV on alloxan-induced diabetic mice was stronger than that of OOJ. More important, the ability of OOV to reduce damage on islets in diabetic mice was stronger than that of OOJ. Overall, alcoholic and acetic acid fermentation improved the hypoglycemic activity of OOJ.

Effect of ghrelin on glucose-insulin homeostasis: therapeutic implications

Ghrelin is a 28-amino-acid peptide that displays a strong growth hormone- (GH-) releasing activity through the activation of the growth hormone secretagogue receptor (GHSR). The first studies about role of ghrelin were focused on its orexigenic ability, but despite indisputable pharmacological data, the evidence for a physiological role for ghrelin in the control of appetite is much less clear. Mice with targeted deletion of either ghrelin or the GHSR exhibit an essentially normal metabolic phenotype when fed a regular chow diet, suggesting that ghrelin may have a redundant role in the regulation of food intake. RNAs for ghrelin as well as GHSR are expressed in the pancreas of rats and humans and several studies propose that ghrelin could have an important function in glucose homeostasis and insulin release, independent of GH secretion. Low plasma ghrelin levels are associated with elevated fasting insulin levels and insulin resistance, suggesting both physiological and pathophysiological roles for ghrelin. For this reason, at least theoretically, ghrelin and/or its signalling manipulation could be useful for the treatment or prevention of diseases of glucose homeostasis such as type 2 diabetes.

Early metabolic markers of islet allograft dysfunction

BACKGROUND

Islet transplantation can restore normoglycemia to patients with unstable type 1 diabetes mellitus, but long-term insulin independence is usually not sustained. Identification of predictor(s) of islet allograft dysfunction (IGD) might allow for early intervention(s) to preserve functional islet mass.

METHODS

Fourteen islet transplantation recipients with long-term history of type 1 diabetes mellitus underwent metabolic testing by mixed meal tolerance test, intravenous glucose tolerance test, and arginine stimulation test every 3 months postislet transplant completion. Metabolic responses were compared between subjects who maintained insulin independence at 18 months (group 1; n=5) and those who restarted insulin within 18 months (group 2; n=9). Data were analyzed before development of islet graft dysfunction and while insulin independent.

RESULTS

The 90-min glucose, time-to-peak C-peptide, and area under the curve for glucose were consistently higher in group 2 and increased as a function of time. At 12 months, acute insulin release to glucose in group 2 was markedly reduced as compared with baseline (5.62+/-1.21 microIU/mL, n=4 vs. 16.14+/-3.69 microIU/mL, n=8), whereas it remained stable in group 1 (22.36+/-4.98 microIU/mL, n=5 vs. 27.70+/-2.83 microIU/mL, n=5). Acute insulin release to glucose, acute C-peptide release to glucose (ACpRg), and mixed meal stimulation index were significantly decreased and time-to-peak C-peptide, 90-min glucose, and area under the curve for glucose were significantly increased when measured at time points preceding intervals where IGD occurred compared with intervals where there was no IGD.

CONCLUSIONS

The intravenous glucose tolerance test and mixed meal tolerance test may be useful in the prediction of IGD and should be essential components of the metabolic testing of islet transplant recipients.

Postprandial dysmetabolism: the missing link between diabetes and cardiovascular events?

OBJECTIVE

To investigate the association of postprandial dysmetabolism, ie, hyperglycemia, and hyperlipidemia with myocardial disease in diabetic, glucose-intolerant, and glucose-tolerant patients.

METHODS

We performed a MEDLINE search of the English-language literature published between January 1979 and April 2007 for studies regarding postprandial dysmetabolism and heart disease.

RESULTS

Postprandial dysmetabolism is associated with increased inflammation, endothelial dysfunction, decreased fibrinolysis, plaque instability, and cardiac events.

CONCLUSION

There is a direct and proportional association between postprandial dysmetabolism and both coronary artery disease and cardiac events.

Type 2 diabetes: Gaining insight into the disease process using proteomics

The incidence of diabetes mellitus is growing rapidly, with an increasing disease related morbidity and mortality. This is caused by macro- and microvascular complications, as a consequence of the often late diagnosis of type 2 diabetes (T2D), but especially by the difficulties to control glucose homeostasis due to the progressive nature of the disease. T2D is moreover a dual disease, with components of beta-cell failure and components of insulin resistance in peripheral organs, such as liver, fat, and muscle. Understanding the pathogenesis of the disease by gaining insight into the molecular pathways involved in both phenomena is one of the major assets of proteomic approaches. Moreover, proteomics and peptidomics may provide us with robust biomarkers for beta-cell failure, insulin resistance in pheripheral organs, but also for the development of diabetic complications. This review focuses on the knowledge gained by use of proteomic and peptidomic techniques in the study of the pathophysiology of T2D and in the attempts to discover new therapeutic targets.

3T3-L1 adipocytes induce dysfunction of MIN6 insulin-secreting cells via multiple pathways mediated by secretory factors in a co-culture system

Pancreatic beta-cell dysfunction is an important pathological change in type 2 diabetes, which is tightly related to obesity. However, the direct role of adipose tissue in beta-cell dysfunction has not been well understood. In this study, we examined the effects of 3T3-L1 adipocytes on MIN6 insulin-secreting cells in a co-culture system. MIN6 cells used here kept most of beta-cell functions but less sensitive to glucose stimulation. Tolbutamide, the KATP channel blocker, was therefore used to stimulate insulin secretion in this report. MIN6 cells co-cultured with 3T3-L1 adipocytes had significantly reduced intracellular calcium concentration ([Ca2+]i) and lost the ability to secrete insulin in response to tolbutamide, compared to the control cells. 3T3-L1 adipocytes significantly decreased the expression of insulin, glucokinase and Kir6.2 genes but increased the expression of uncoupling protein-2 (UCP-2) in MIN6 cells after one week of co-culture, as measured by semi-quantitative RT-PCR. 3T3-L1 adipocyte-conditioned medium also significantly decreased insulin secretion and the expression of insulin, glucokinase and Kir6.2 genes in MIN6 cells. The conditioned medium also reduced tyrosine kinase activity in MIN6 cells. The inhibitor of protein tyrosine kinase, genistein, decreased the expression of glucokinase and Kir6.2 in MIN6 cells, while two free fatty acids, oleic acid and linoleic acids, were found to increase UCP-2 expression. The present study demonstrates that 3T3-L1 adipocytes directly impair insulin secretion and the expression of important genes in MIN6 cells. The effects of T3-L1 adipocytes on MIN6 cells are ascribed to secreted bioactive factors and may be mediated via multiple pathways, which include the upregulation of UCP-2 expression via free fatty acids, and downregulation of glucokinase and Kir6.2 expression via decreasing protein tyrosine kinase activity.

Central and peripheral roles of ghrelin on glucose homeostasis

Ghrelin, an acylated 28-amino-acid peptide, is an endogenous ligand of the growth hormone secretagogue type 1a (GHS-R1a). Ghrelin is best known for its hypothalamic actions on growth hormone-releasing hormone neurons and neuropeptide Y/agouti-related peptide neurons; however, ghrelin affects multiple organ systems and the complexity of its functions is only now being realized. Although ghrelin is mainly produced in the stomach, it is also produced in low levels by the hypothalamus and by most peripheral tissues. GHS-R1a is expressed predominantly in the anterior pituitary gland, at lower levels in the brain including hypothalamic neurons that regulate feeding behavior and glucose sensing, and at even lower levels in the pancreas. A reciprocal relationship exists between ghrelin and insulin, suggesting that ghrelin regulates glucose homeostasis. Ablation of ghrelin in mice increases glucose-induced insulin secretion, and improves peripheral insulin sensitivity. This review focuses on the newly emerging role of ghrelin in glucose homeostasis and exploration of whether ghrelin is a potential therapeutic target for diabetes.

Glucose induces MafA expression in pancreatic beta cell lines via the hexosamine biosynthetic pathway

MafA is a basic leucine zipper transcription factor that regulates gene expression in both the neuroretina and pancreas. Within the pancreas, MafA is exclusively expressed in the beta cells and is involved in insulin gene transcription, insulin secretion, and beta cell survival. The expression of the mafA gene within beta cells is known to increase in response to high glucose levels by an unknown mechanism. In this study, we demonstrate that pyruvate, which is produced by glycolysis from glucose, is not sufficient to induce mafA gene expression compared with high glucose. This suggests that the signal for MafA induction is independent of ATP levels and that a metabolic event occurring upstream of pyruvate production leads to the induction of MafA. Furthermore, insulin secretion mediated by high glucose is not important for MafA expression. However, the addition of glucosamine to beta cell lines stimulates MafA expression in the absence of high glucose, and inhibition of the hexosamine biosynthetic pathway in the presence of high glucose abolishes MafA induction. Moreover, we demonstrate that the expression of UDP-N-acetylglucosaminyl transferase, the enzyme mediating O-linked glycosylation of cytosolic and nuclear proteins, is essential for glucose-dependent MafA expression. Consistent with this observation, inhibition of N-acetylglucosaminidase, the enzyme involved in the removal of the O-GlcNAc modification from proteins, with O-(2-acetamido-2-deoxy-d-glucopyranosylidene)amino-N-phenylcarbamate stimulates MafA expression under low glucose conditions. The presented data suggest that MafA expression mediated by high glucose requires flux through the hexosamine biosynthetic pathway and the O-linked glycosylation of an unknown protein(s) by UDP-N-acetylglucosaminyl transferase.

Ablation of ghrelin improves the diabetic but not obese phenotype of ob/ob mice

Ghrelin and leptin are suggested to regulate energy homeostasis as mutual antagonists on hypothalamic neurons that regulate feeding behavior. We employed reverse genetics to investigate the interplay between ghrelin and leptin. Leptin-deficient mice (ob/ob) are hyperphagic, obese, and hyperglycemic. Unexpectedly, ablation of ghrelin in ob/ob mice fails to rescue the obese hyperphagic phenotype, indicating that the ob/ob phenotype is not a consequence of ghrelin unopposed by leptin. Remarkably, deletion of ghrelin augments insulin secretion in response to glucose challenge and increases peripheral insulin sensitivity; indeed, the hyperglycemia exhibited by ob/ob mice is markedly reduced when ob/ob mice are bred onto the ghrelin(-/-) background. We further demonstrate that ablation of ghrelin reduces expression of Ucp2 mRNA in the pancreas, which contributes toward enhanced glucose-induced insulin secretion. Hence, chronically, ghrelin controls glucose homeostasis by regulating pancreatic Ucp2 expression and insulin sensitivity.

Genetic and genomic studies of the BTBR ob/ob mouse model of type 2 diabetes

The BTBR mouse strain harbors alleles promoting insulin resistance. When made genetically obese (ob/ob), these mice develop severe type 2 diabetes (fasting glucose >400 mg/dL). By contrast, C57BL/6 ob/ob mice are able to compensate for the obesity-induced insulin resistance by increasing pancreatic insulin secretion and thus maintain only slightly elevated plasma glucose levels (<250 mg/dL). Islet insulin secretory responses to glucose are undiminished in the remaining islets of BTBR ob/ob mice. A genome-wide linkage analysis identified 3 major loci influencing plasma glucose and/or insulin levels in an F2ob/ob sample derived from the 2 strains. A locus on chromosome 2 affects insulin sensitivity and is independent of obesity. Loci on chromosomes 16 and 19 affect fasting glucose and insulin levels and likely affect beta-cell mass or function. Analysis of mRNA expression patterns revealed a reduction in lipogenic gene expression in adipose tissue associated with obesity. Conversely, hepatic lipogenic gene expression increases in obese mice, but to a much greater extent in the diabetes-resistant C57BL/6 strain. We propose that hepatic lipogenic capacity affects susceptibility to obesity-induced diabetes.

Reactive hypoglycemia in lean young women with PCOS and correlations with insulin sensitivity and with beta cell function

Reactive hypoglycemia (RH), which is a postprandial hypoglycemic state, occurs within 2-5 h after food intake. It is classified as idiopathic, alimentary, or diabetic reactive hypoglycemia. We studied the incidence of reactive hypoglycemia and looked for any correlations between it and the presence of insulin sensitivity and/or beta cell function in young lean polycystic ovary syndrome (PCOS) patients. This study was designed as a cross-sectional study in 64 lean young women with PCOS (BMI < or = 25 kg/m2). Various indices of insulin sensitivity and beta cell function derived from the oral glucose tolerance test (OGTT) results were used. We found the rate of RH to be 50% in lean young women with PCOS. DHEA-S and PRL levels were found to be lower in subjects with RH (P < 0.05 and P > 0.05, respectively). Beta cell function indices such as the insulinogenic index (at 120 min), CIR (at 120 min) and HOMA beta cell index were found to be insignificantly higher in the RH group than the nonreactive hypoglycemia (NRH) group. The 4 h glucose level, but not the 3 h glucose level, was significantly correlated with insulin resistance indices, such as fasting insulin level, HOMA-IR, Quicky index, and FIRI in the RH group. Significantly decreased DHEA-S levels were an interesting finding. In conclusion, there is an urgent need to investigate RH in lean young women with PCOS. Our results indicate that more definite insulin resistance occurs in subjects with RH in the fourth hour of the OGTT than those with RH in the third hour. In addition, RH in the fourth hour together with a low DHEA-S level may be predictive of future diabetes in young women with PCOS even when they are not obese.

Glucagon-like peptide-1 prevents beta cell glucolipotoxicity

AIMS/HYPOTHESIS

We have provided evidence that glucagon-like peptide-1, a potential therapeutic agent in the treatment of diabetes, activates phosphatidylinositol-3 kinase/protein kinase B signalling in the pancreatic beta cell. Since this pathway promotes cell survival in a variety of systems, we tested whether glucagon-like peptide-1 protects beta cells against cell death induced by elevated glucose and/or non-esterified fatty acids.

METHODS

Human islets and INS832/13 cells were cultured at glucose concentrations of 5 or 25 mmol/l in the presence or absence of palmitate. Apoptosis was evaluated by monitoring DNA fragmentation and chromatin condensation. Wild-type and protein kinase B mutants were overexpressed in INS832/13 cells using adenoviruses. Nuclear factor-kappa B DNA binding was assayed by electrophoretic mobility shift assay.

RESULTS

In human pancreatic beta cells and INS832/13 cells, glucagon-like peptide-1 prevented beta cell apoptosis induced by elevated concentrations of (i) glucose (glucotoxicity), (ii) palmitate (lipotoxicity) and (iii) both glucose and palmitate (glucolipotoxicity). Overexpression of a dominant-negative protein kinase B suppressed the anti-apoptotic action of glucagon-like peptide-1 in INS832/13 cells, whereas a constitutively active protein kinase B prevented beta cell apoptosis induced by elevated glucose and palmitate. Glucagon-like peptide-1 enhanced nuclear factor-kappa B DNA binding activity and stimulated the expression of inhibitor of apoptosis protein-2 and Bcl-2, two anti-apoptotic genes under the control of nuclear factor-kappa B. Inhibition of nuclear factor-kappa B by BAY 11-7082 abolished the prevention of glucolipotoxicity by glucagon-like peptide-1.

CONCLUSIONS/INTERPRETATION

The results demonstrate a potent protective effect of glucagon-like peptide-1 on beta cell gluco-, lipo- and glucolipotoxicity. This effect is mediated via protein kinase B activation and possibly its downstream target nuclear factor-kappa B.

Chronic effects of fatty acids on pancreatic beta-cell function: new insights from functional genomics

Type 2 diabetes can be viewed as a failure of the pancreatic beta-cell to compensate for peripheral insulin resistance with enhanced insulin secretion. This failure is explained by both a relative loss of beta-cell mass as well as secretory defects that include enhanced basal secretion and a selective loss of sensitivity to glucose. These features are reproduced by chronic exposure of beta-cells to fatty acids (FAs), suggesting that hyperlipidemia might contribute to decompensation. Using MIN6 cells pretreated for 48 h with oleate or palmitate, we have previously defined alterations in global gene expression by transcript profiling and described additional secretory changes to those already established (Busch A-K, Cordery D, Denyer G, Biden TJ: Diabetes 51:977-987, 2002). In contrast to a modest decoupling of glucose-stimulated insulin secretion, FA pretreatment markedly enhanced the secretory response to an acute subsequent challenge with FAs. We propose that this apparent switch in sensitivity from glucose to FAs would be an appropriate response to hyperlipidemia in vivo and thus plays a positive role in beta-cell compensation for insulin resistance. Altered expression of dozens of genes could contribute to this switch, and allelic variations in any of these genes could (to varying degrees) impair beta-cell compensation and thus contribute to conditions ranging from impaired glucose tolerance to frank diabetes.

Saturated fatty acids synergize with elevated glucose to cause pancreatic beta-cell death

We have proposed the "glucolipotoxicity" hypothesis in which elevated free fatty acids (FFAs) together with hyperglycemia are synergistic in causing islet beta-cell damage because high glucose inhibits fat oxidation and consequently lipid detoxification. The effects of 1-2 d culture of both rat INS 832/13 cells and human islet beta-cells were investigated in medium containing glucose (5, 11, 20 mM) in the presence or absence of various FFAs. A marked synergistic effect of elevated concentrations of glucose and saturated FFA (palmitate and stearate) on inducing beta-cell death by apoptosis was found in both INS 832/13 and human islet beta-cells. In comparison, linoleate (polyunsaturated) synergized only modestly with high glucose, whereas oleate (monounsaturated) was not toxic. Treating cells with the acyl-coenzyme A synthase inhibitor triacsin C, or the AMP kinase activators metformin and 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside that redirect lipid partitioning to oxidation, curtailed glucolipotoxicity. In contrast, the fat oxidation inhibitor etomoxir, like glucose, markedly enhanced palmitate-induced cell death. The data indicate that FFAs must be metabolized to long chain fatty acyl-CoA to exert toxicity, the effect of which can be reduced by activating fatty acid oxidation. The results support the glucolipotoxicity hypothesis of beta-cell failure proposing that elevated FFAs are particularly toxic in the context of hyperglycemia.

Deteriorating beta-cell function in type 2 diabetes: a long-term model

BACKGROUND

Type 2 diabetes is characterized by insulin resistance and the progressive loss of islet beta-cell function. Although the former is already established at diagnosis and changes little thereafter, beta-cell function continues to decline, leading to secondary failure of anti-hyperglycaemic therapies.

AIM

To develop a quantitative model of the process of beta-cell function decay over time, using trial data.

DESIGN

Re-analysis of published data.

METHODS

The results of the Belfast Diet Study were re-analysed. Assuming patients are diagnosed at different stages in the disease process, time displacement of data was used to obtain a bi-partite spline model describing loss of insulin secretion over a 6-year period.

RESULTS

The model was developed combining two phases, in which a long slow gradual loss of beta-cell function leads to a crisis in metabolic regulation, precipitating a much more rapid decay phase. This paradigm was consistent with a previous non-linear model of beta-cell mass regulation.

DISCUSSION

This model may have important implications for targeting appropriate therapy to patients in each phase: delaying or avoiding full clinical type 2 diabetes in the first phase; and preventing the development of diabetic complications in the second phase.

Malonyl-CoA signaling, lipid partitioning, and glucolipotoxicity: role in beta-cell adaptation and failure in the etiology of diabetes

Beta-cells possess inherent mechanisms to adapt to overnutrition and the prevailing concentrations of glucose, fatty acids, and other fuels to maintain glucose homeostasis. However, this is balanced by potentially harmful actions of the same nutrients. Both glucose and fatty acids may cause good/adaptive or evil/toxic actions on the beta-cell, depending on their concentrations and the time during which they are elevated. Chronic high glucose dramatically influences beta-cell lipid metabolism via substrate availability, changes in the activity and expression of enzymes of glucose and lipid metabolism, and modifications in the expression level of key transcription factors. We discuss here the emerging view that beta-cell "glucotoxicity" is in part indirectly caused by "lipotoxicity," and that beta-cell abnormalities will become particularly apparent when both glucose and circulating fatty acids are high. We support the concept that elevated glucose and fatty acids synergize in causing toxicity in islets and other organs, a process that may be instrumental in the pleiotropic defects associated with the metabolic syndrome and type 1 and type 2 diabetes. The mechanisms by which hyperglycemia and hyperlipidemia alter insulin secretion are discussed and a model of beta-cell "glucolipotoxicity" that implicates alterations in beta-cell malonyl-CoA concentrations; peroxisome proliferator-activated receptor-alpha and -gamma and sterol regulatory element binding protein-1c expression; and lipid partitioning is proposed.

Optimizing insulin secretagogue therapy in patients with type 2 diabetes: a randomized double-blind study with repaglinide

OBJECTIVE

Repaglinide, a novel antidiabetic agent that has a rapid onset and short duration of action, was developed for mealtime dosing. The purpose of this pharmacodynamic study was to validate a prandial regimen of repaglinide by comparing meal-related dosing with a regimen in which the same total daily dose was divided into only two doses at morning and evening meals.

RESEARCH DESIGN AND METHODS

The study was a double-blind, randomized, parallel-group trial in 19 antidiabetic agent-naive subjects with type 2 diabetes (mean age 58 years, known duration of diabetes 3.5 years, HbA(1c) 7.3%, and BMI 32 kg/m(2)). Patients were randomly assigned to receive repaglinide either before each of the three main meals or before breakfast and before the evening meal. Patients in both groups received the same total daily dose of repaglinide. Twenty-four hour profiles of blood glucose, plasma insulin, and plasma C-peptide concentrations were measured at baseline and after 4 weeks of treatment.

RESULTS

Repaglinide increased postprandial insulin levels and markedly reduced postprandial glucose levels relative to baseline in both groups. Significant reductions were also recorded in fasting blood glucose and HbA(1c) levels. The repaglinide regimen, in which a dose was taken before each main meal, was more effective in improving glycemic control (including postprandial glucose and HbA(1c) levels) than the same total dose of repaglinide divided into morning and evening mealtime doses.

CONCLUSIONS

These data support the strategy of mealtime dosing with repaglinide. The improvements in glycemic control observed in these patients are encouraging. In addition to classic parameters of glycemic control, improvements in postprandial glucose excursions may prove to be important because postprandial hyperglycemia has been suggested to be an independent risk factor for cardiovascular disease in diabetes.

The role of hepatic nuclear factor 1 alpha and PDX-1 in transcriptional regulation of the pdx-1 gene

The PDX-1 homeodomain transcription factor regulates pancreatic development and adult islet beta cell function. Expression of the pdx-1 gene is almost exclusively localized to beta cells within the adult endocrine pancreas. Islet beta cell-selective transcription is controlled by evolutionarily conserved subdomain sequences (termed Areas I (-2839 to -2520 base pairs (bp)), II (-2252 to -2023 bp), and III (-1939 to -1664 bp)) found within the 5'-flanking region of the pdx-1 gene. Areas I and II are independently capable of directing beta cell-selective reporter gene activity in transfection assays, with Area I-mediated stimulation dependent upon binding of hepatic nuclear factor 3 beta (HNF3 beta), a key regulator of islet beta cell function. To identify other transactivators of Area I, highly conserved sequence segments within this subdomain were mutagenized, and their effect on activation was determined. Several of the sensitive sites were found by transcription factor data base analysis to potentially bind endodermally expressed transcription factors, including HNF1 alpha (-2758 to -2746 bp, Segment 2), HNF4 (-2742 to -2730 bp, Segment 4; -2683 to -2671 bp, Segment 7-8), and HNF6 (-2727 to -2715 bp, Segment 5). HNF1 alpha, but not HNF4 and HNF6, binds specifically to Area I sequences in vitro. HNF1 alpha was also shown to specifically activate Area I-driven transcription through Segment 2. In addition, PDX-1 itself was found to stimulate Area I activation. The chromatin immunoprecipitation assay performed with PDX-1 antisera also demonstrated that this factor bound to Area I within the endogenous pdx-1 gene in beta cells. Our results indicate that regulatory factors binding to Area I conserved sequences contribute to the selective transcription pattern of the pdx-1 gene and that control is mediated by endodermal regulators like HNF1 alpha, HNF3 beta, and PDX-1.

Uncoupling protein-2 negatively regulates insulin secretion and is a major link between obesity, beta cell dysfunction, and type 2 diabetes

beta cells sense glucose through its metabolism and the resulting increase in ATP, which subsequently stimulates insulin secretion. Uncoupling protein-2 (UCP2) mediates mitochondrial proton leak, decreasing ATP production. In the present study, we assessed UCP2's role in regulating insulin secretion. UCP2-deficient mice had higher islet ATP levels and increased glucose-stimulated insulin secretion, establishing that UCP2 negatively regulates insulin secretion. Of pathophysiologic significance, UCP2 was markedly upregulated in islets of ob/ob mice, a model of obesity-induced diabetes. Importantly, ob/ob mice lacking UCP2 had restored first-phase insulin secretion, increased serum insulin levels, and greatly decreased levels of glycemia. These results establish UCP2 as a key component of beta cell glucose sensing, and as a critical link between obesity, beta cell dysfunction, and type 2 diabetes.

Characterization of the mouse islet-specific glucose-6-phosphatase catalytic subunit-related protein gene promoter by in situ footprinting: correlation with fusion gene expression in the islet-derived betaTC-3 and hamster insulinoma tumor cell lines

Glucose-6-phosphatase (G6Pase) is a multicomponent system located in the endoplasmic reticulum comprising a catalytic subunit and transporters for glucose-6-phosphate, inorganic phosphate, and glucose. We have recently cloned a novel gene that encodes an islet-specific G6Pase catalytic subunit-related protein (IGRP) (Ebert et al., Diabetes 48:543-551, 1999). To begin to investigate the molecular basis for the islet-specific expression of the IGRP gene, a series of truncated IGRP-chloramphenicol acetyltransferase (CAT) fusion genes were transiently transfected into the islet-derived mouse betaTC-3 and hamster insulinoma tumor cell lines. In both cell lines, basal fusion gene expression decreased upon progressive deletion of the IGRP promoter sequence between -306 and -66, indicating that multiple promoter regions are required for maximal IGRP-CAT expression. The ligation-mediated polymerase chain reaction footprinting technique was then used to compare trans-acting factor binding to the IGRP promoter in situ in betaTC-3 cells, which express the endogenous IGRP gene, and adrenocortical Y1 cells, which do not. Multiple trans-acting factor binding sites were selectively identified in betaTC-3 cells that correlate with regions of the IGRP promoter identified as being required for basal IGRP-CAT fusion gene expression. The data suggest that hepatocyte nuclear factor 3 may be important for basal IGRP gene expression, as it is for glucagon, GLUT2, and Pdx-1 gene expression. In addition, binding sites for several trans-acting factors not previously associated with islet gene expression, as well as binding sites for potentially novel proteins, were identified.

The calcium/calmodulin-dependent phosphodiesterase PDE1C down-regulates glucose-induced insulin secretion

To understand the role cAMP phosphodiesterases (PDEs) play in the regulation of insulin secretion, we analyzed cyclic nucleotide PDEs of a pancreatic beta-cell line and used family and isozyme-specific PDE inhibitors to identify the PDEs that counteract glucose-stimulated insulin secretion. We demonstrate the presence of soluble PDE1C, PDE4A and 4D, a cGMP-specific PDE, and of particulate PDE3, activities in betaTC3 insulinoma cells. Selective inhibition of PDE1C, but not of PDE4, augmented glucose-stimulated insulin secretion in a dose-dependent fashion thus demonstrating that PDE1C is the major PDE counteracting glucose-dependent insulin secretion from betaTC3 cells. In pancreatic islets, inhibition of both PDE1C and PDE3 augmented glucose-dependent insulin secretion. The PDE1C of betaTC3 cells is a novel isozyme possessing a K(m) of 0.47 microM for cAMP and 0.25 microM for cGMP. The PDE1C isozyme of betaTC3 cells is sensitive to 8-methoxymethyl isobutylmethylxanthine and zaprinast (IC(50) = 7.5 and 4.5 microM, respectively) and resistant to vinpocetine (IC(50) > 100 microM). Increased responsiveness of PDE1C activity to calcium/calmodulin is evident upon exposure of cells to glucose. Enhanced cAMP degradation by PDE1C, due to increases in its responsiveness to calcium/calmodulin and in intracellular calcium, constitutes a glucose-dependent feedback mechanism for the control of insulin secretion.

Changes in islet capillary angioarchitecture coincide with impaired B-cell function but not with insulin resistance in male Otsuka-Long-Evans-Tokushima fatty rats: dimorphism of the diabetic phenotype at an advanced age

The Otsuka-Long-Evans-Tokushima fatty (OLETF) rat is a genetic model of spontaneous development of non-insulin-dependent diabetes mellitus (NIDDM) established as an inbred strain after 20 generations of selective breeding. Although they are thought to be genetically homogeneous, they show a dimorphism regarding the diabetic phenotype at an advanced age, with one remaining obese and modestly diabetic while the other becomes lean and overtly diabetic. To clarify the causes for this divergence, we examined the physical, biochemical, and histopathological features in rats at 50 weeks of age, including an analysis of islet angioarchitecture. Sixty-one of 85 male OLETF rats lost weight, while the remainder remained obese. Mean nonfasting plasma glucose in the lean group was 21.8+/-4.6 mmol/L, significantly higher versus the obese group (10.5+/-1.4 mmol/L) and the age-matched control Long-Evans-Tokushima-Otsuka (LETO) group (7.1+/-0.6 mmol/L). Morphological studies of the pancreas from the lean group showed enlarged multilobulated fibrotic islets with a paucity of B cells, whereas islets from the obese group appeared slightly enlarged and showed a relative abundance of B cells. The fine capillaries that form a network in the islets were extremely sparse in the lean group, resulting in a defective glomerular-like configuration, whereas those from the obese group were dense, forming a nearly typical glomerular-like configuration. Increased plasma insulin responses to oral and intravenous (i.v.) glucose and i.v. glucagon loads were nearly absent in the lean group, while they were evident in the obese group, although to a lesser extent compared with the LETO group. Mean insulin secretory output from the perfused pancreas in response to 11.1 mmol/L glucose in the lean group (3.5+/-2.2 pmol/20 min) was significantly lower versus the obese group (8.8+/-6.5 pmol/20 min) and LETO group (22.0+/-10.8 pmol/20 min). Similarly, pancreatic insulin content was significantly lower in the lean group (9.3+/-6.1 microg) versus the others (26.1+/-17.3 microg for obese and 41.1+/-24.8 microg for LETO). In vivo insulin-stimulated glucose uptake measured by a euglycemic clamp technique was significantly higher in the lean group compared with the obese group. These results demonstrate that the dimorphism regarding the diabetic phenotype in male OLETF rats at 50 weeks of age was due to differences in the number of islet B cells, which could be the result of a variation in the capacity for B-cell proliferation among male OLETF rats.

Tissue-specific knockout of the insulin receptor in pancreatic beta cells creates an insulin secretory defect similar to that in type 2 diabetes

Dysfunction of the pancreatic beta cell is an important defect in the pathogenesis of type 2 diabetes, although its exact relationship to the insulin resistance is unclear. To determine whether insulin signaling has a functional role in the beta cell we have used the Cre-loxP system to specifically inactivate the insulin receptor gene in the beta cells. The resultant mice exhibit a selective loss of insulin secretion in response to glucose and a progressive impairment of glucose tolerance. These data indicate an important functional role for the insulin receptor in glucose sensing by the pancreatic beta cell and suggest that defects in insulin signaling at the level of the beta cell may contribute to the observed alterations in insulin secretion in type 2 diabetes.

Glucose stimulation of pancreatic beta-cell lines induces expression and secretion of dynorphin

To investigate adaptive responses of pancreatic beta-cells to hyperglycemia, genes induced by glucose stimulation were identified by subtraction cloning. Among 53 clones representing differentially expressed genes, 20 encoded the endogenous opioid precursor, prodynorphin. The amino acid sequence of murine prodynorphin is identical to the rat protein in sequences comprising the opioid peptides and 86% identical in the remainder of the molecule. Stimulation of MIN6 cells increased prodynorphin RNA levels to more than 20-fold in proportion to physiological glucose concentrations. Similar induction levels were observed in murine betaTC3 and rat Rinm5F beta-cell lines. Prodynorphin RNA expression increased within 1 h of glucose stimulation, achieved maximal levels by 4 h, and remained elevated for at least 24 h. By using RIA, MIN6 cells were shown to contain and secrete increased amounts of dynorphin-A following glucose stimulation. Treatment of MIN6 cells with KCl, forskolin, or isobutyl-methyl-xanthine strongly induced prodynorphin RNA expression, suggesting that induction may be related to secretion-coupled signaling pathways. The induction of prodynorphin in several beta-cell lines is consistent with previous demonstrations of beta-cell synthesis of other endogenous opioids, including beta-endorphin, and suggests that opioids may have a potentially significant role in regulating beta-cell secretion.

Implications of blood glucose, insulin resistance and beta-cell function in impaired glucose tolerance

Insulin secretion is stimulated by ingestion of food. The combination of hyperinsulinaemia plus hyperglycaemia effectively promotes glucose uptake by the liver and by peripheral tissues, such as muscle and fat cells, and suppresses hepatic glucose output. These simultaneous processes maintain normal glucose homeostasis in a co-ordinated fashion. Type 2 diabetes mellitus is associated with impaired insulin in target tissues due to insulin resistance and/or insulin deficiency. At first, increased insulin secretion overcomes insulin resistance, but ultimately this fails, leading progressively to increased blood glucose levels. Individuals pass through a phase of impaired glucose tolerance (IGT) and increased fasting plasma glucose levels (IFG) before developing overt type 2 diabetes. Therefore, IGT/IFG is a dysglycaemic state that is intermediate between normal glucose tolerance and diabetes. In this article, we discuss the relative importance of hyperglycaemia, insulin resistance and beta-cell function in the development of glucose intolerance, taking the new diagnostic criteria into consideration. New recommendations from the Expert Committee on the Diagnosis and Classification of Diabetes Mellitus are discussed where appropriate.

Differential expression of the insulin gene transcriptional repressor CCAAT/enhancer-binding protein beta and transactivator islet duodenum homeobox-1 in rat pancreatic beta cells during the development of diabetes mellitus

Impairment of insulin secretion due to prolonged hyperglycemia is believed to contribute to the manifestation of diabetes mellitus, often referred to as glucose toxicity of pancreatic beta cells. In addition, impaired beta cell function has been associated with elevated islet triglyceride content (lipotoxicity). Impaired functions of the transactivating factors islet duodenum homeobox-1 (IDX-1) and RIPE3b-binding proteins have been implicated in the pathological downregulation of insulin gene transcription by high glucose levels in pancreatic beta cell lines in vitro, and, similarly, the exposure of pancreatic islets to fatty acids decreases IDX-1 expression. Previously, we identified the basic leucine zipper transcription factor CCAAT/enhancer-binding protein beta (C/ EBPbeta) as an inhibitor of insulin gene transcription in pancreatic beta cells and showed that the expression of C/EBPbeta is upregulated in insulinoma-derived beta cell lines by sustained high glucose concentrations. Here we describe the regulation of the expression of IDX-1, C/EBPbeta, and insulin at the mRNA and protein levels in pancreatic islets in animal models of diabetes mellitus. Concomitant with a downregulation of IDX-1 and insulin expression, C/EBPbeta is upregulated in association with the manifestation of hyperglycemia during the development of diabetes in the Zucker diabetic fatty (fa/fa) rat and in the 90% pancreatectomy rat model of diabetes. This regulation is demonstrated to influence both the amount of cellular protein and the level of steady state messenger RNA. Our findings indicate that the differential dysregulation of both IDX-1 and C/EBPbeta, in response to sustained hyperglycemia or hyperlipidemia, may be involved in the impairment of insulin gene expression during the manifestation of diabetes mellitus.