Experimental reduction of B-cell mass: implications for the pathogenesis of diabetes

Streptozotocin-Induced Diabetes Mellitus in Neonatal Rats: An Insight into its Applications to Induce Diabetic Complications

BACKGROUND

Diabetic complications are the major contributor in the mortality of diabetic patients despite controlling blood glucose level. In the journey of new drug discovery, animal models have to play a major role. A large number of chemical-induced and genetically modified animal models have been investigated to induce diabetic complications but none of them was found to be mimicking the pathophysiology of the human. Therefore, the search and identification of the appropriate animal model become essential.

OBJECTIVE

In the present review, we have made an attempt to understand the pathophysiology of diabetic complication in the neonatal streptozotocin-diabetic rat model and tried to identify the targets for therapeutic agents. The review will help the researchers to explore the animal model to induce diabetic complications, to identify targets and further to find lead molecules for treatment or prevention of diabetic complications.

METHODS

We have compiled the available research work from 1974 by using prominent databases, organized the available information and analyzed the data to improve the understanding of the pathophysiology of streptozotocin-induced diabetic complications in neonates of rats.

RESULTS

The neonatal streptozotocin-diabetic rat model is frequently used and well-established animal model for type 2 diabetes mellitus. We have found that this model has been used to study the pathogenesis of various micro and macrovascular diabetic complications and also investigated for its effects on the liver, thymus gland, and brain. The underlying pathophysiology for complications had a resemblance to the human.

CONCLUSION

The neonatal streptozotocin-diabetic rat model may demonstrate symptomatic diabetic complications due to persistent hyperglycemia at the age of approximately 18-24 weeks. Critical interpretations of available research work showed that the researcher can explore split dose STZ (90- 100mg/kg b.w) model to induce Type 2 DM in neonates of rats at 2nd or 3rd postnatal day.

Identification of miR-9 as a negative factor of insulin secretion from beta cells

MicroRNA is a novel class of small noncoding RNA that has been implicated in a variety of physiological and pathological processes, including glucose homeostasis and diabetes mellitus. So far, a few studies have reported that miRNAs may be an important regulator in glucose-stimulated insulin secretion (GSIS) pathway. However, the role of miRNAs in this process remains unclear. The levels of miRNAs in mouse islets and MIN6 cells were determined by quantitative RT-PCR. Concentration of insulin was determined by ELISA, and the expression of the target protein was determined with western blot assay. The overexpression and downregulation of miRNAs in MIN6 were conducted using cell transfection methods. And luciferase assay was used to measure the direct interaction between miRNAs and target messenger RNAs 3'UTR. miR-9 was screened out for it was downregulated under the effects of short-term high glucose, while long-term high glucose relatively increased miR-9 expression. The Stxbp1 expression was decreased with the overexpression of miR-9 in MIN6 cells and increased when miR-9 was downregulated. Moreover, it was verified by luciferase assay that miR-9 regulated Stxbp1 gene expression by directly targeting Stxbp1 messenger RNA 3'UTR. This study suggests that the pathway consisting of miR-9 and Stxbp1 plays a key role in β-cell function, thus contributing to the network of miRNA-insulin secretion and offering a new candidate for diabetes therapy.

Insulin Secretion and Glucose Tolerance after Islet Transplantation in Rats with Noninsulin-Dependent Diabetes Induced by Neonatal Streptozotocin

The present study was designed to identify in a model of noninsulin-dependent diabetes induced by neonatal streptozotocin (n0-STZ), the long-term consequences of an islet graft upon 1) glucose handling of the recipient and, 2) glucose response of the residual β cells in the recipient pancreas. We have examined, 4 and 8 wk after islet implantation under the kidney capsule of syngeneic diabetic n0-STZ rats, their tolerance to glucose administered in vivo, together with their insulin release in response to glucose in vivo (oral glucose tolerance test) as well as in vitro (perfused pancreas). The results in the islet-grafted n0-STZ rats, were compared to those obtained in nongrafted nondiabetic rats and nongrafted n0-STZ rats. Our study shows that transplanting a limited number (900) of adult islets under the kidney capsule reverses to normal, many parameters of the noninsulin-dependent diabetic state in the n0-STZ rat model: these include body weight, basal plasma glucose in both the nonfasted and postabsorptive states, and basal plasma insulin in the postabsorptive state. Furthermore, tolerance to oral glucose administration was greatly improved in the transplanted rats and it was correlated with restoration of a manifest glucose-induced insulin secretion in vivo as evaluated (ΔI) during an oral glucose tolerance test. Our data clearly show that the insulin response to glucose from the endogenous pancreas of n0-STZ diabetic rat was not really improved by long-term (8 wk) basal normoglycemia. More precisely, we were able to detect a slight but significant improvement of the early phase of insulin release in vitro in response to glucose; however, the overall insulin response remained 15 times lower than the normal one with no reapparance of the late phase of insulin release. After cessation of glucose stimulation in vivo, off-response of insulin, which is also a landmark of the impaired insulin release by the β cells of n0-STZ rats, was still detectable in the perfused pancreas of the transplanted n0-STZ rats. Finally, because the reactivity to glucose of the endogenous residual β cells was not regained, the insulin released in vivo during the oral glucose test in the graft-bearing n0-STZ rats can be attributed mainly to functioning of the grafted islets population. Copyright © 1997 Elsevier Science Inc.

β-cell mass in people with type 2 diabetes

The early occurrence of β‐cell dysfunction has been broadly recognized as a critical determinant of the development and progression of type 2 diabetes. β‐cell dysfunction might be induced by insufficient β‐cell mass, by a dysfunction of the β‐cells, or both. Whether or not β‐cell dysfunction constitutes a cause of reduced β‐cells or vice‐versa currently remains unclear. The results of some studies have measured the loss of β‐cells in type 2 diabetic patients at between 22 and 63% by planimetric measurements. Because β‐cell hypertrophy has been noted in type 2 diabetic patients, the loss of β‐cell number should prove more profound than what has thus far been reported. Furthermore, β‐cell volumes are reduced even in patients with impaired fasting glucose. Such defects in β‐cell mass are associated with increased apoptosis rather than insufficient replication or neogenesis of β‐cells. With these results, although they still require clarification, the peak β‐cell mass might be determined at quite an early stage of life, and then might decline progressively over time as the result of exposure to harmful environmental influences over one’s lifetime. In this review, we have summarized the relevant studies regarding β‐cell mass in patients with type 2 diabetes, and then presented a review of the various causes of β‐cell loss in adults. (J Diabetes Invest, doi: 10.1111/j.2040‐1124.2010.00072.x, 2010)

Development of an alternative non-obese non-genetic rat model of type 2 diabetes using caffeine and streptozotocin

BACKGROUND

The aim of the present study was to develop an alternative non-obese non-genetic rat model of type 2 diabetes (T2D).

METHODS

Six-week-old male SD rats were randomly divided into six groups, namely: Normal Control (NC), Diabetic Control (DBC), Caffeine 5mg/kg BW+STZ (CAF5), Caffeine 10mg/kg BW+STZ (CAF10), Caffeine 20mg/kg BW+STZ (CAF20) and Caffeine 40mg/kg BW+STZ (CAF40) and were fed a normal rat pellet diet and drinking water ad libitum throughout the experimental period. After a one week acclimatization period, diabetes was induced in the animals in DBC and all CAF groups with an injection (i.p.) of the respective dosages of caffeine (mg/kg BW) 15min before the injection of STZ (65mg/kg BW) when normal saline was injected to the DBC group instead of caffeine. The NC group received normal saline and buffer instead of caffeine and STZ, respectively. One week after the STZ injection, animals with non-fasting blood glucose>300mg/dl were considered as diabetic. Three weeks after the STZ injection, the animals in the CAF5 and CAF10 groups were eliminated from the study due to the severity of diabetes and the experiment was continued with the remainder groups for a 13 weeks period.

RESULTS AND CONCLUSION

The data of food and fluid intake, body weight, blood glucose, glucose tolerance test, HOMA-IR, HOMA-beta, serum insulin, fructosamine, lipid profile and organ specific enzymes, anti-diabetic drug response tests, and pancreatic histopathology suggest that CAF20 group can be a better alternative non-genetic model of non-obese T2D.

Cellular glucose availability and glucagon-like peptide-1

Glucagon-like peptide (GLP)-1 and gastric inhibitory polypeptide (GIP, glucose-dependent insulinotropic polypeptide) are produced in enteroendocrine L-cells and K-cells, respectively. They are known as incretins because they potentiate postprandial insulin secretion. Although unresponsiveness of type 2 diabetes (T2D) patients to GIP has now been reconsidered, GLP-1 mimetics and inhibitors of the GLP-1 degradation enzyme dipeptidyl peptidase (DPP)-4 have now been launched as drugs against T2D. The major roles of GLP-1 in T2D are reduction of appetite, gastric motility, glucagon secretion, enhancement of insulin secretion and β-cell survival. For insulin secretion and peripheral insulin function, GLP-1 and its mimetics sensitise β-cells to glucose; accelerate blood glucose withdrawal, in-cell glucose utilisation and glycogen synthesis in insulin-sensitive tissues; and assist in the function and survival of neurons mainly using glucose as an energy source. Taken together, GLP-1 acts to potentiate glucose availability of various cells or tissues to assist with their essential functions and/or survival. Herein, we review the signalling pathways and clinical relevance of GLP-1 in enhancing cellular glucose availability. On the basis of our recent research results, we also describe a mechanism that regulates GLP-1 for glucokinase activity. Because diabetic tissues including β-cells resist glucose, GLP-1 may be useful for treating T2D.

Experimental model proposed to deduce pharmacological evidence of the beta-cell neogenesis activity of Argyrolobium roseum vis-a-vis an evaluation of its hypoglycemic activity

Background:

Argyrolobium roseum is described in texts for its botanical aspects with no mention of its therapeutic uses. Recently, a solvent fraction of this plant has been reported to possess insulin secretagogue activity.

Objective:

Primary objective of the study was to evaluate the hypoglycemic activity of different fractions of Argyrolobium roseum plant. Secondary objective was to devise a model for detecting its beta-cell neogenesis activity.

Materials and Methods:

Alcoholic fraction (AR) was studied in fasting, glucose-loaded and streptozotocin (STZ)-treated hyperglycemic rats. For deducing beta-cell neogenesis activity, a sub-lethal dose of STZ, causing delayed death in the animals, was administered.

Results:

AR in a dose of 200 mg/kg showed hypoglycemic activity in fasting and STZ-treated rats. With sub-lethal dose of STZ, recovery from hyperglycemia was seen.

Conclusion:

AR of Argyrolobium roseum possesses hypoglycemic and a possible beta-cell neogenesis activity. Further studies are required to confirm its beta-cell neogenesis potential by cytological methods and to establish its role in the treatment of type 2 diabetes.

Cytokine production by islets in health and diabetes: cellular origin, regulation and function

Islets produce a variety of cytokines and chemokines in response to physiologic and pathologic stimulation by nutrients. The cellular source of these inflammatory mediators includes alpha-, beta-, endothelial-, ductal- and recruited immune cells. Islet-derived cytokines promote alpha- and beta-cell adaptation and repair in the short term. Eventually, chronic metabolic stress can induce a deleterious autoinflammatory process in islets leading to insulin secretion failure and type 2 diabetes. Understanding the specific role of islet derived cytokines and chemokines has opened the door to targeted clinical interventions aimed at remodeling islet inflammation from destruction to adaptation. In this article, we review the islet cellular origin of various cytokines and chemokines and describe their regulation and respective roles in physiology and diabetes.

Free fatty acids induce a proinflammatory response in islets via the abundantly expressed interleukin-1 receptor I

Islets of patients with type 2 diabetes mellitus (T2DM) display features of an inflammatory process including elevated levels of the cytokine IL-1beta, various chemokines, and macrophages. IL-1beta is a master regulator of inflammation, and IL-1 receptor type I (IL-1RI) blockage improves glycemia and insulin secretion in humans with T2DM and in high-fat-fed mice pointing to a pivotal role of IL-1RI activity in intra-islet inflammation. Given the association of dyslipidemia and T2DM, we tested whether free fatty acids (FFA) promote the expression of proinflammatory factors in human and mouse islets and investigated a role for the IL-1RI in this response. A comparison of 22 mouse tissues revealed the highest IL-1RI expression levels in islets and MIN6 beta-cells. FFA induced IL-1beta, IL-6, and IL-8 in human islets and IL-1beta and KC in mouse islets. Elevated glucose concentrations enhanced FFA-induced proinflammatory factors in human islets. Blocking the IL-1RI with the IL-1R antagonist (IL-1Ra) strongly inhibited FFA-mediated expression of proinflammatory factors in human and mouse islets. Antibody inhibition of IL-1beta revealed that FFA stimulated IL-1RI activity via the induction of the receptor ligand. FFA-induced IL-1beta and KC expression in mouse islets was completely dependent on the IL-1R/Toll-like receptor (TLR) docking protein Myd88 and partly dependent on TLR2 and -4. Activation of TLR2 in purified human beta-cells and islets stimulated the expression of proinflammatory factors, and IL-1RI activity increased the TLR2 response in human islets. We conclude that FFA and TLR stimulation induce proinflammatory factors in islets and that IL-1RI engagement results in signal amplification.

A rice bran oil diet improves lipid abnormalities and suppress hyperinsulinemic responses in rats with streptozotocin/nicotinamide-induced type 2 diabetes

The aim of this study was to determine the effects of rice bran oil (RBO) on lipid metabolism and insulin resistance in rats with streptozotocin/nicotinamide-induced type 2 diabetes mellitus (T2DM). Rats were divided into two groups: the control group (15% soybean oil, contains 0 g γ-oryzanol and 0 g γ-tocotrienol/150 g oil for 5 weeks) and the RBO group (15% RBO, contains 5.25 g γ-oryzanol and 0.9 g γ-tocotrienol/150 g oil for 5 weeks). Compared with the control group, the RBO group had a lower plasma nonesterified fatty acid concentration, ratio of total to high-density-lipoprotein cholesterol, hepatic cholesterol concentration, and area under the curve for insulin. The RBO group had a higher high-density-lipoprotein cholesterol concentration and greater excretion of fecal neutral sterols and bile acid than did the control group. RBO may improve lipid abnormalities, reduce the atherogenic index, and suppress the hyperinsulinemic response in rats with streptozotocin/nicotinamide-induced T2DM. In addition, RBO can lead to increased fecal neutral sterol and bile acid excretion.

Cell therapy for type 2 diabetes: is it desirable and can we get it?

The functional mass of beta-cells is decreased in type 2 diabetes. Replacing missing beta-cells or triggering their regeneration may thus allow for improved treatment of type 2 diabetes, to the extent that this is combined with therapy for improved insulin sensitivity. Although progress has been made in deriving beta-cell-like cells from stem or precursor cells in vitro, these cannot yet be obtained in sufficient quantities or well enough differentiated to envisage their therapeutic use in beta-cell replacement therapy. Likewise, our very limited understanding of beta-cell regeneration in adult man does not yet allow for development of a valid strategy for kick-starting such a process in individuals with type 2 diabetes, whether by bona fide neogenesis or self-replication of existing beta-cells. Regardless of how beta-cell mass is restored in type 2 diabetes, it will be important to prevent any renewed decrease thereafter. Current understanding suggests that islet inflammation as well as signals from (insulin-resistant/inflamed) adipose tissue and skeletal muscle contribute towards decreased beta-cell mass in type 2 diabetes. It will likely be important to protect newly formed or implanted beta-cells from these negative influences to ensure their long-term survival.

Beta-cell apoptosis in type 2 diabetes: quantitative and functional consequences

Type 2 diabetes, the most common form of diabetes in humans, is characterized by impaired insulin secretion paralleled by a progressive decline in beta-cell function and chronic insulin resistance. Several authors have showed that in type 2 diabetes there is a reduction of islet and/or insulin-containing cell mass or volume. Regulation of the beta-cell mass appears to involve a balance of beta-cell replication and apoptosis but, at the molecular level, pancreatic beta-cell loss by apoptosis appears to play an important role in the development of insulin deficiency and the onset and/or progression of the disease. The mechanisms favoring apoptosis in type 2 diabetic pancreatic islets and new potential therapeutic approaches to prevent beta-cell death and maintain beta-cell mass are discussed.

Evaluation of beta-cell mass and function in the Göttingen minipig

Increased knowledge about beta-cell mass and function is important for our understanding of the pathophysiology of type 2 diabetes (T2DM). The relationship between the two is difficult to study in humans, whereas animal models allow studies of consequences of, for example, reduction of beta-cell mass and induction of obesity and procurement of the pancreas for histological examination. An overview of results obtained in the Göttingen minipig in relation to beta-cell function, and mass is provided here. Effects of a primary reduction of beta-cell mass have indicated that not all of the defects of pulsatile insulin secretion in human T2DM can be explained by reduced beta-cell mass. Furthermore, induction of obesity has shown deterioration of beta-cell function and morphological changes in the pancreas. As in humans, obesity leads to an increased beta-cell volume in the minipig, and based on the increased number of islets, neogenesis of islets is an important factor in expansion of beta-cell mass in this species. Measurement of beta-cell function as an estimate of beta-cell mass is, at present, the only method possible in humans, and this approach has been validated using lean and obese minipigs with a range of beta-cell mass. The effects on beta-cell function and mass of obesity of longer duration and/or more pronounced hyperglycaemia remains to be determined, but the models developed so far represent a valuable tool for such investigations.

beta-cell failure in diabetes and preservation by clinical treatment

There is a progressive deterioration in beta-cell function and mass in type 2 diabetics. It was found that islet function was about 50% of normal at the time of diagnosis, and a reduction in beta-cell mass of about 60% was shown at necropsy. The reduction of beta-cell mass is attributable to accelerated apoptosis. The major factors for progressive loss of beta-cell function and mass are glucotoxicity, lipotoxicity, proinflammatory cytokines, leptin, and islet cell amyloid. Impaired beta-cell function and possibly beta-cell mass appear to be reversible, particularly at early stages of the disease where the limiting threshold for reversibility of decreased beta-cell mass has probably not been passed. Among the interventions to preserve or "rejuvenate" beta-cells, short-term intensive insulin therapy of newly diagnosed type 2 diabetes will improve beta-cell function, usually leading to a temporary remission time. Another intervention is the induction of beta-cell "rest" by selective activation of ATP-sensitive K+ (K(ATP)) channels, using drugs such as diazoxide. A third type of intervention is the use of antiapoptotic drugs, such as the thiazolidinediones (TZDs), and incretin mimetics and enhancers, which have demonstrated significant clinical evidence of effects on human beta-cell function. The TZDs improve insulin secretory capacity, decrease beta-cell apoptosis, and reduce islet cell amyloid with maintenance of neogenesis. The TZDs have indirect effects on beta-cells by being insulin sensitizers. The direct effects are via peroxisome proliferator-activated receptor gamma activation in pancreatic islets, with TZDs consistently improving basal beta-cell function. These beneficial effects are sustained in some individuals with time. There are several trials on prevention of diabetes with TZDs. Incretin hormones, which are released from the gastrointestinal tract in response to nutrient ingestion to enhance glucose-dependent insulin secretion from the pancreas, aid the overall maintenance of glucose homeostasis through slowing of gastric emptying, inhibition of glucagon secretion, and control of body weight. From the two major incretins, glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP), only the first one or its mimetics or enhancers can be used for treatment because the diabetic beta-cell is resistant to GIP action. Because of the rapid inactivation of GLP-1 by dipeptidyl peptidase (DPP)-IV, several incretin analogs were developed: GLP-1 receptor agonists (incretin mimetics) exenatide (synthetic exendin-4) and liraglutide, by conjugation of GLP-1 to circulating albumin. The acute effect of GLP-1 and GLP-1 receptor agonists on beta-cells is stimulation of glucose-dependent insulin release, followed by enhancement of insulin biosynthesis and stimulation of insulin gene transcription. The chronic action is stimulating beta-cell proliferation, induction of islet neogenesis, and inhibition of beta-cell apoptosis, thus promoting expansion of beta-cell mass, as observed in rodent diabetes and in cultured beta-cells. Exenatide and liraglutide enhanced postprandial beta-cell function. The inhibition of the activity of the DPP-IV enzyme enhances endogenous GLP-1 action in vivo, mediated not only by GLP-1 but also by other mediators. In preclinical studies, oral active DPP-IV inhibitors (sitagliptin and vildagliptin) also promoted beta-cell proliferation, neogenesis, and inhibition of apoptosis in rodents. Meal tolerance tests showed improvement in postprandial beta-cell function. Obviously, it is difficult to estimate the protective effects of incretin mimetics and enhancers on beta-cells in humans, and there is no clinical evidence that these drugs really have protective effects on beta-cells.

Mechanisms of impaired fasting glucose and glucose intolerance induced by an approximate 50% pancreatectomy

Impaired fasting glucose (IFG) and impaired glucose tolerance (IGT) often coexist and as such represent a potent risk factor for subsequent development of type 2 diabetes. beta-Cell mass is approximately 50% deficient in IFG and approximately 65% deficient in type 2 diabetes. To establish the effect of a approximately 50% deficit in beta-cell mass on carbohydrate metabolism, we performed a approximately 50% partial pancreatectomy versus sham surgery in 14 dogs. Insulin secretion was quantified from insulin concentrations measured in the portal vein at 1-min sampling intervals under basal conditions, after a 30-g oral glucose, and during a hyperglycemic clamp. Insulin sensitivity was measured by a hyperinsulinemic-euglycemic clamp combined with isotope dilution. Partial pancreatectomy resulted in IFG and IGT. After partial pancreatectomy both basal and glucose-stimulated insulin secretion were decreased through the mechanism of a selective approximately 50 and approximately 80% deficit in insulin pulse mass, respectively (P < 0.05). These defects in insulin secretion were partially offset by decreased hepatic insulin clearance (P < 0.05). Partial pancreatectomy also caused a approximately 40% decrease in insulin-stimulated glucose disposal (P < 0.05), insulin sensitivity after partial pancreatectomy being related to insulin pulse amplitude (r = 0.9, P < 0.01). We conclude that a approximately 50% deficit in beta-cell mass can recapitulate the alterations in glucose-mediated insulin secretion and insulin action in humans with IFG and IGT. These data support a mechanistic role of a deficit in beta-cell mass in the evolution of IFG/IGT and subsequently type 2 diabetes.

Measurements of insulin responses as predictive markers of pancreatic beta-cell mass in normal and beta-cell-reduced lean and obese Göttingen minipigs in vivo

At present, the best available estimators of beta-cell mass in humans are those based on measurement of insulin levels or appearance rates in the circulation. In several animal models, these estimators have been validated against beta-cell mass in lean animals. However, as many diabetic humans are obese, a correlation between in vivo tests and beta-cell mass must be evaluated over a range of body weights to include different levels of insulin sensitivity. For this purpose, obese (n = 10) and lean (n = 25) Göttingen minipigs were studied. Beta-cell mass had been reduced (n = 16 lean, n = 5 obese) with a combination of nicotinamide (67 mg/kg) and streptozotocin (125 mg/kg), acute insulin response (AIR) to intravenous glucose and/or arginine was tested, pulsatile insulin secretion was evaluated by deconvolution (n = 30), and beta-cell mass was determined histologically. AIR to 0.3 (r(2) = 0.4502, P < 0.0001) or 0.6 g/kg glucose (r(2) = 0.6806, P < 0.0001), 67 mg/kg arginine (r(2) = 0.5730, P < 0.001), and maximum insulin concentration (r(2) = 0.7726, P < 0.0001) were all correlated to beta-cell mass when evaluated across study groups, and regression lines were not different between lean and obese groups except for AIR to 0.3 g/kg glucose. Baseline pulse mass was not significantly correlated to beta-cell mass across the study groups (r(2) = 0.1036, NS), whereas entrained pulse mass did show a correlation across groups (r(2) = 0.4049, P < 0.001). This study supports the use of in vivo tests of insulin responses to evaluate beta-cell mass over a range of body weights in the minipig. Extensive stimulation of insulin secretion by a combination of glucose and arginine seems to give the best correlation to beta-cell mass.

Animal models of type 2 diabetes with reduced pancreatic beta-cell mass

Type 2 diabetes is increasingly viewed as a disease of insulin deficiency due not only to intrinsic pancreatic beta-cell dysfunction but also to reduction of beta-cell mass. It is likely that, in diabetes-prone subjects, the regulated beta-cell turnover that adapts cell mass to body's insulin requirements is impaired, presumably on a genetic basis. We still have a limited knowledge of how and when this derangement occurs and what might be the most effective therapeutic strategy to preserve beta-cell mass. The animal models of type 2 diabetes with reduced beta-cell mass described in this review can be extremely helpful (a) to have insight into the mechanisms underlying the defective growth or accelerated loss of beta-cells leading to the beta-cell mass reduction; (b) to investigate in prospective studies the mechanisms of compensatory adaptation and subsequent failure of a reduced beta-cell mass. Furthermore, these models are of invaluable importance to test the effectiveness of potential therapeutic agents that either stimulate beta-cell growth or inhibit beta-cell death.

Reversal of experimental diabetes by multiple bone marrow transplantation

Therapeutic utility of bone marrow transplantation in diabetic patients to overcome deficient beta-cell population is an attractive proposal. However, the status of bone marrow stem cells (BMSCs) under hyperglycemia is not known. In the present study, we investigated the status of BMSCs in experimental-diabetic mice and demonstrated the rescue of experimental diabetes by multiple diabetic bone marrow transplantation. Our flow-cytometry analysis for CD34+, CD45+, flk1+, c-kit+, and CD34+CD45+ revealed that BMSC reserve remains unaffected under sustained hyperglycemia. We found that single injection of diabetic bone marrow cells (approximately 10(6)) resulted in reduction and stabilization of moderate hyperglycemia. However, multiple injections at regular intervals led to restoration of stabilized normoglycemia during a 30 day follow-up. Reversal of diabetes was evidenced by disappearance of hyperglycemia, normal intra-peritoneal glucose tolerance test, and histology and morphometry of pancreas. The present study thus demonstrates that diabetic bone marrow retains its stemness and potential to induce pancreatic regeneration on transplantation.

Reduction of beta cell mass: partial insulin secretory compensation from the residual beta cell population in the nicotinamide-streptozotocin Göttingen minipig after oral glucose in vivo and in the perfused pancreas

AIMS/HYPOTHESIS

A progressive loss of beta cell function and mass are important contributory factors in the development and progression of type 2 diabetes. The aim of this study was to evaluate the effects of a primary reduction in beta cell mass on beta cell function in vivo and in the perfused pancreas and to relate these characteristics to beta cell mass.

METHODS

The beta cell mass of six Göttingen minipigs was reduced chemically (using 67 mg/kg nicotinamide and 125 mg/kg streptozotocin). Six untreated minipigs were kept as control animals. Insulin responses were evaluated in vivo using the mixed meal tolerance test (2 g/kg oral glucose) and in the isolated perfused pancreata from the same animals by stimulation with glucose, glucagon-like peptide-1 or arginine.

RESULTS

Beta cell mass was reduced in the beta-cell-reduced animals compared with the control minipigs (182+/-76 vs 464+/-156 mg, p<0.01). AUC(glucose) was increased in the beta-cell-reduced animals (1383+/-385 vs 853+/-113 mmol.l(-1).min in control minipigs, p<0.01), as was the insulin response to oral glucose per unit of beta cell mass (123+/-84 vs 56+/-24 pmol.l(-1).min.mg(-1), p<0.05). Total in vitro insulin secretion was increased per unit of beta cell mass in nicotinamide + streptozotocin pancreata compared to controls (83.7+/-45.9 vs 34.6+/-14.4 nmol/mg beta cells, p<0.05) with responses to glucose and glucagon-like peptide-1 showing a partial compensation for reduced beta cell mass, whereas no compensation was seen in response to arginine.

CONCLUSIONS/INTERPRETATION

A primary reduction in beta cell mass impairs glucose tolerance and leads to a compensatory increase in insulin secretion from the remaining beta cells after oral glucose in vivo, which is even more apparent in the perfused pancreas. It remains to be determined whether this compensation can be maintained in the long term.

Glucose- and interleukin-1beta-induced beta-cell apoptosis requires Ca2+ influx and extracellular signal-regulated kinase (ERK) 1/2 activation and is prevented by a sulfonylurea receptor 1/inwardly rectifying K+ channel 6.2 (SUR/Kir6.2) selective potassium channel opener in human islets

Increasing evidence indicates that a progressive decrease in the functional beta-cell mass is the hallmark of both type 1 and type 2 diabetes. The underlying causes, beta-cell apoptosis and impaired secretory function, seem to be partly mediated by macrophage production of interleukin (IL)-1beta and/or high-glucose-induced beta-cell production of IL-1beta. Treatment of type 1 and type 2 diabetic patients with the potassium channel opener diazoxide partially restores insulin secretion. Therefore, we studied the effect of diazoxide and of the novel potassium channel opener NN414, selective for the beta-cell potassium channel SUR1/Kir6.2, on glucose- and IL-1beta-induced apoptosis and impaired function in human beta-cells. Exposure of human islets for 4 days to 11.1 and 33.3 mmol/l glucose, 2 ng/ml IL-1beta, or 10 and 100 micromol/l of the sulfonylurea tolbutamide induced beta-cell apoptosis and impaired glucose-stimulated insulin secretion. The deleterious effects of glucose and IL-1beta were blocked by 200 micromol/l diazoxide as well as by 3 and 30 micromol/l NN414. By Western blotting with phosphospecific antibodies, glucose and IL-1beta were shown to activate the extracellular signal-regulated kinase (ERK) 1/2, an effect that was abrogated by 3 micromol/l NN414. Similarly, 1 micromol/l of the mitogen-activated protein kinase/ERK kinase 1/2 inhibitor PD098059 or 1 micromol/l of the l-type Ca(2+) channel blocker nimodipine prevented glucose- and IL-1beta-induced ERK activation, beta-cell apoptosis, and impaired function. Finally, islet release of IL-1beta in response to high glucose could be abrogated by nimodipine, NN414, or PD098059. Thus, in human islets, glucose- and IL-1beta-induced beta-cell secretory dysfunction and apoptosis are Ca(2+) influx and ERK dependent and can be prevented by the beta-cell selective potassium channel opener NN414.

Decreased beta-cell mass in diabetes: significance, mechanisms and therapeutic implications

Increasing evidence indicates that decreased functional beta-cell mass is the hallmark of both Type 1 and Type 2 diabetes. This underlies the absolute or relative insulin insufficiency in both conditions. In this For Debate, we consider the possible mechanisms responsible for beta-cell death and impaired function and their relative contribution to insulin insufficiency in diabetes. Beta-cell apoptosis and impaired proliferation consequent to hyperglycaemia is one pathway that could be operating in all forms of diabetes. Autoimmunity and other routes to beta-cell death are also considered. Recognition of decreased functional beta-cell mass and its overlapping multifactorial aetiology in diabetic states, leads us to propose a unifying classification of diabetes.

Evaluation of the site(s) of glycation in human proinsulin by ion-trap LCQ electrospray ionization mass spectrometry

The glycation of beta cell proteins is known to occur under hyperglycemic states. The site(s) of glycation in human proinsulin was investigated following exposure to a hyperglycemic environment under reducing conditions in vitro. Proinsulin and glycated proinsulin were separated by reversed-phase high-performance liquid chromatography (RP-HPLC) and identified using LCQ ion-trap electrospray ionization mass spectrometry. This revealed a major peak (>70% total) of monoglycated proinsulin (M(r) 9552.2 Da), a second peak (approximately 27%) of nonglycated proinsulin (M(r) 9389.8 Da), and a third minor peptide peak (approximately 3%) corresponding to diglycated proinsulin (M(r) 9717.9 Da). Following reduction of disulphide bridges with dithiothreitol, intact peptides were incubated with endoproteinase Glu-C to release nine daughter fragments for LC-MS analysis. This strategy revealed an N-terminal fragment of monoglycated proinsulin Phe(1)-Glu(13), which contained a single glucitol adduct (M(r) 1642.0 Da). A similar treatment of small amounts of purified diglycated proinsulin revealed a fragment with Phe(1)-Glu(13) linked by a disulphide bridge to Gln(70)-Glu(82) containing two glucitol adducts (M(r) 3292.7 Da). In summary, these studies indicate that the major site of glycation in proinsulin, like insulin, is the amino terminal Phe(1) residue. However, small amounts of diglycated proinsulin occur naturally, involving an additional site of glycation located between Gln(70) and Glu(82).

Loss of beta-cell mass leads to a reduction of pulse mass with normal periodicity, regularity and entrainment of pulsatile insulin secretion in Göttingen minipigs

AIMS/HYPOTHESIS

Type 2 diabetes is associated with impaired insulin action and secretion, including disturbed pulsatile release. Impaired pulsatility has been related to impaired insulin action, thus providing a possible link between release and action of insulin. Furthermore, progressive loss of beta-cell mass has been implicated in the pathogenesis of Type 2 diabetes. The aim of this study was to evaluate a possible link between loss of beta-cell mass and impaired pulsatile insulin secretion with special focus on glucose responsiveness of insulin secretion.

METHODS

The kinetic and dynamic profiles of insulin in Göttingen minipigs are favourable for studies on pulsatility and a model of diabetes with reduced beta-cell mass has recently been established. Pigs were studied before (n=14) and after (n=10) reduction of beta-cell mass by nicotinamide (67 mg/kg) and streptozotocin (125 mg/kg) from 17.7+/-4.7 (normal animals, n=5) to 6.1+/-2.0 mg/kg. Pulsatile insulin secretion was examined during basal (n=8 normal, n=6 beta-cell reduced) and glucose entrained (n=6 normal, n=4 beta-cell reduced) conditions. Insulin concentration time series were analysed by autocorrelation and spectral analyses for periodicities and regularity, and by deconvolution for pulse frequency, mass and amplitude.

RESULTS

Reduction of beta-cell mass and secondary hyperglycaemia resulted in correspondingly (r=0.7421, p=0.0275) reduced pulse mass (42% of normal during basal and 31% during entrained conditions) with normal periodicity (6.6+/-2.2 vs 5.8+/-2.4 min, p=0.50), regularity and entrainability of insulin secretion.

CONCLUSION/INTERPRETATION

Neither beta-cell loss, nor 2 weeks of slight hyperglycaemia, as seen in the beta-cell-reduced minipig, probably accounts for the disturbed insulin pulsatility observed in human Type 2 diabetes.

Measurements of insulin secretory capacity and glucose tolerance to predict pancreatic beta-cell mass in vivo in the nicotinamide/streptozotocin Göttingen minipig, a model of moderate insulin deficiency and diabetes

Knowledge about beta-cell mass and/or function could be of importance for the early diagnosis and treatment of diabetes. However, measurement of beta-cell function as an estimate of beta-cell mass is currently the only method possible in humans. The present study was performed to investigate different functional tests as predictors of beta-cell mass in the Göttingen minipig. beta-cell mass was reduced in the Göttingen minipig with a combination of nicotinamide (100 [n = 6], 67 [n = 25], 20 [n = 2], or 0 mg/kg [n = 4]) and streptozotocin (125 mg/kg). Six normal pigs were included. An oral glucose tolerance test (OGTT) (n = 43) and insulin secretion test (n = 30) were performed and pancreata obtained for stereological determination of beta-cell mass. During OGTT, fasting glucose (r(2) = 0.1744, P < 0.01), area under the curve for glucose (r(2) = 0.2706, P < 0.001), maximum insulin secretion (r(2) = 0.2160, P < 0.01), and maximum C-peptide secretion (r(2) = 0.1992, P < 0.01) correlated with beta-cell mass. During the insulin secretion test, acute insulin response to 0.3 g/kg (r(2) = 0.6155, P < 0.0001) and 0.6 g/kg glucose (r(2) = 0.7321, P < 0.0001) and arginine (67 mg/kg) (r(2) = 0.7732, P < 0.0001) and maximum insulin secretion (r(2) = 0.8192, P < 0.0001) correlated with beta-cell mass. This study supports the use of functional tests to evaluate beta-cell mass in vivo and has established a validated basis for developing a mathematical method for estimation of beta-cell mass in vivo in the Göttingen minipig.

Glucosamine-induced beta-cell dysfunction: a possible involvement of glucokinase or glucose-transporter type 2

INTRODUCTION

The mechanism for beta-cell dysfunction induced by glucosamine has not yet fully been investigated previously.

AIM

To investigate the effects of glucosamine on insulin release or gene expression related to glucose metabolism in rat islets cultured with glucosamine for 24 hours.

METHODOLOGY

After islets were cultured with glucosamine or diazoxide, we measured glucose- or arginine-induced insulin release by using radioimmunoassay (RIA) and gene expressions by semiquantitative polymerase/chain reaction.

RESULTS

Coculture with glucosamine inhibited 27 mmol/L glucose-induced insulin release with no effects on 20 mmol/L arginine-induced insulin release. Coculture with diazoxide did not restore the impaired glucose-induced insulin release. In glucosamine-cultured islets, glucose-transporter type 2 or glucokinase mRNA expression decreased, whereas hexokinase mRNA increased. Phosphofructokinase-A, pyruvate dehydrogenase E1alpha, or pyruvate carboxylase mRNA was not affected by the addition of glucosamine. Pancreatic and duodenal homeobox-1, preproinsulin, or p21 (induced by oxidative stress) mRNA expression did not change, whereas uncoupling protein 2 mRNA, which plays an important role in thermogenesis, decreased in glucosamine-cultured islets.

CONCLUSION

These data imply that glucosamine impairs glucose-induced insulin release probably through the inhibition of glucose metabolism.

Effects of bezafibrate on beta-cell function of rat pancreatic islets

Bezafibrate is an activator of peroxisome proliferator-activated receptors (PPAR) alpha. The present study was performed to investigate the effects of bezafibrate and the PPAR alpha activator, 4-Cholro-6-(2.3-xylidino)-2-pyrimidin-ylthio acetic acid (WY14643), on the beta-cell function of rat pancreatic islets in vitro. In islets cultured with 300 microM bezafibrate or WY14643 for 8 h, a low glucose concentration induced insulin release and increased the levels of mRNA for PPAR alpha, acyl CoA oxidase, carnitine palmitoyl transferase-1, pyruvate dehydrogenase E1 alpha or pyruvate carboxylase. In contrast, after a 48-h culture period, a high glucose concentration induced insulin release and islet insulin content, but decreased the levels of mRNA for glucose transporter-2 (GLUT-2), preproinsulin or pancreatic/duodenal homeobox-1. Diazoxide, the KATP channel opener, restored these responses. We conclude that bezafibrate enhances insulin release through the activation of PPAR alpha gene expression during a short culture period, whereas it may contribute to beta-cell dysfunction through the mechanism of "excessive stimulation" during longer culture periods.

Glucagon-like peptide-1 and exendin-4 stimulate beta-cell neogenesis in streptozotocin-treated newborn rats resulting in persistently improved glucose homeostasis at adult age

In neonatal Wistar rats injected with streptozotocin (STZ) at birth (n0-STZ model), a recognized model of beta-cell regeneration, we investigated the capacity of early treatment with glucagon-like peptide 1 (GLP-1) or exendin-4 to promote beta-cell regeneration and thereby improve islet function in the long term, when animals become adults. To this end, n0-STZ rats were submitted to GLP-1 or exendin-4 from postnatal day 2 to day 6 only, and their beta-cell mass and pancreatic functions were tested on day 7 and at 2 months. On day 7, both treatments increased body weight, decreased basal plasma glucose, decreased insulinemia, and increased pancreatic insulin content in n0-STZ rats. At the same age, the beta-cell mass, measured by immunocytochemistry and morphometry methods, was strongly increased in n0-STZ/GLP-1 and n0-STZ/Ex rats compared with n0-STZ rats, representing 51 and 71%, respectively, of the beta-cell mass in Wistar rats, whereas n0-STZ beta-cell mass represented only 21% of the Wistar control value. Despite such early improved beta-cell mass, which is maintained at adult age, the basal and glucose-stimulated insulin secretion (in vivo after intravenous glucose load or in vitro using perfused pancreas) were not improved in the 2-month-old n0-STZ rats previously treated with GLP-1 or exendin-4 compared with untreated n0-STZ rats. However, both treated groups significantly exhibited a decreased basal plasma glucose level and an increased plasma glucose clearance rate compared with the 2-month-old untreated n0-STZ group at adult age. These findings in the n0-STZ model indicate for the first time that GLP-1 or exendin-4 applied during the neonatal diabetic period exert both short- and long-term beneficial effects on beta-cell mass recovery and glucose homeostasis. However, the increase in beta-cell mass, which is still present in the adult n0-STZ rats previously treated, contrasts with the poor beta-cell responsiveness to glucose. Further studies are needed to understand the dissociation between beta-cell regeneration and the lack of improvement in beta-cell function.

Diabetic rats transplanted with adult porcine islets and immunosuppressed with cyclosporine A, mycophenolate mofetil, and leflunomide remain normoglycemic for up to 100 days

BACKGROUND

Transplantation of adult porcine islets (APIs) offers a possible means of treating diabetes. However, isolating APIs has been notoriously difficult. Furthermore, islet xenograft rejection must be prevented.

MATERIALS AND METHODS

APIs were isolated by a modified automated method. API quality was assessed by static glucose stimulation (SGS), by transplantation to diabetic nude mice and by intraperitoneal glucose tolerance tests (IPGTTs). The morphologic characteristics of API xenograft rejection in rats were studied immunohistochemically. Furthermore, APIs were transplanted to diabetic rats that were either left untreated or immunosuppressed with cyclosporine A (CsA), mycophenolate mofetil (MMF) and leflunomide (LEF). B-glucose and porcine C-peptide levels were monitored and grafts were studied morphologically.

RESULTS

Large numbers of APIs were isolated. At SGS, insulin release increased significantly. All nude mice transplanted with APIs were normoglycemic within 24 hr and remained so for up to 1 year. During IPGTTs, B-glucose levels were rapidly regulated to porcine levels. In untreated rats, API xenografts were destroyed within 6 days by a cellular infiltrate consisting mainly of macrophages. In untreated diabetic rats normoglycemia was sustained for 5.5+/-0.3 days. Rats immunosuppressed with CsA+MMF+LEF remained normoglycemic for 59.6+/-11.3 days. In 3 of 11 rats, normoglycemia was sustained for up to 101 days. Porcine C-peptide was detected in serum. At recurrence of hyperglycemia, many mononuclear cells were found close to the xenografts. However, only occasional cells infiltrated the grafts and many APIs were intact.

CONCLUSIONS

Well-functioning APIs can be isolated in large numbers. API xenografts can be protected from rejection and can maintain an adequate function for up to 100 days, in rats immunosuppressed with CsA+MMF+LEF.

The role of cyclic AMP in the pathogenesis of glucose desensitization in rat pancreatic islets

Adenosine-3',5'-cyclic monophosphate (cyclic AMP) promotes exocytosis of insulin in pancreatic beta cells. This study was performed to investigate the role of cyclic AMP in the pathogenesis of glucose desensitization in rat pancreatic islets. In islets cultured with high glucose for 48 hours, 27 mmol/L glucose-induced insulin release was markedly impaired, while 3.3 mmol/L glucose-or arginine-induced insulin release was enhanced, indicating glucose desensitization. Islet cyclic AMP content was 190% enhanced in high glucose-culture islets for 48 hours. In islets cultured with dibutyryl-cyclic AMP (dbcAMP) or 3-isobutyl methy-xanthine (IBMX), islet insulin content or 27 mmol/L glucose-induced insulin release was deteriorated. In contrast, 3.3 mmol/L glucose- or arginine-induced insulin release was increased, which was similar to glucose-desensitized islets. Wash-out of dbc AMP for the last 24 hours of the 48-hour culture period restored impaired high glucose-induced insulin release in the same manner as wash-out of high glucose. Diazoxide, the KATP channel opener, also restored impaired high glucose-induced insulin release from dbcAMP-cultured islets. The data suggest that enhancement of cyclic AMP in high glucose-culture islets may be one of the pathogenesis of glucose desensitization.

Overproduction of insulin in the chromium-deficient rat

The hypothesis that the insulin secretory hyperresponsiveness observed in rats with diet-induced insulin resistance may be a basic characteristic of dietary chromium (Cr) deficiency was evaluated. Two groups of weanling rats were fed ad libitum a purified diet containing 64% sucrose, 20% casein, 5% corn oil, and the recommended levels of vitamins and minerals without added Cr. Cr-deficient (-Cr) rats were provided with distilled drinking water only, while Cr-supplemented (+Cr) rats received water containing 5 ppm Cr as CrCl3. A third group of rats fed a commercial chow diet served as sucrose controls. Effects of Cr deficiency were assessed by comparing fasting levels of glucose, insulin, and plasma lipids in blood samples collected biweekly from the -Cr and +Cr groups over a 3-month period. Both groups of rats fed the low-Cr sucrose diet developed a transient hyperinsulinemia and hyperlipidemia relative to the chow-fed control rats. There were significant effects of Cr supplementation on plasma triglycerides during the initial 2 weeks of dietary adaptation. Effects of the low-Cr diet were evaluated after the 12-week period by comparing the insulin response area and glucose clearance during a 40-minute intravenous glucose tolerance test (IVGTT). The rates of glucose clearance (KG) in -Cr and +Cr rats were similar (4.2 +/- 1.0 and 4.3 +/- 0.8%/min, respectively) and were comparable to the K(G) in chow-fed rats (4.6 +/- 0.8). In contrast, insulin secretory responses in -Cr rats were exaggerated (area, 14,083 +/- 3,399 microU/mL x min), being twofold greater (P < .05) relative to the +Cr group (6,183 +/- 864). The insulin secretory response area in chow-fed rats (7,081 +/- 408 microU/mL x min) was similar to the value in the +Cr group. These observations provide support for the hypothesis that Cr deficiency can lead to elevated insulin secretory responses to glucose.

Insulin secretion and biosynthesis by the perfused pancreas of spontaneously hypertensive rats

Insulin secretory activity was compared in the spontaneously hypertensive rat (SHR) versus the normotensive Wistar-Kyoto (WKY) rat and Wistar rat. When the isolated pancreas was perfused with 16.7 mmol/L glucose, insulin release was significantly greater in the SHR versus the other groups. On the other hand, there was no difference in arginine (19 mmol/L)-induced insulin secretion among the three groups. To determine insulin biosynthesis during glucose stimulation, the pancreas was perfused with 16.7 mmol/L glucose for 180 minutes. Insulin secretion was greater in SHR versus WKY and Wistar rats, but the net increase in insulin content was not different between the three groups. These results strongly suggest that in vivo hyperinsulinemia in the SHR is associated with increased in vitro insulin secretion in response to glucose. The mechanisms by which enhanced glucose-induced insulin secretion is linked to hypertension in the SHR remain unclear.

Chronic sympathetic innervation of islets in transgenic mice results in differential desensitization of alpha-adrenergic inhibition of insulin secretion

The effects of chronic sympathetic hyperinnervation on pancreatic beta-cell insulin secretion were investigated utilizing the in vitro perfused pancreas from transgenic mice. These mice exhibit islet hyperinnervation of sympathetic neurons resulting from overexpression of nerve growth factor in their beta-cells (1). The goal was to determine whether sympathetic hyperinnervation increased classic alpha-adrenergic inhibition of beta-cell insulin secretion or, in contrast, down-regulated beta-cell sensitivity to adrenergic input resulting in enhanced insulin secretion. Both fasting and fed blood sugars and pancreatic insulin content were normal in the transgenics. Response of the transgenic perfused pancreas to low glucose (7 mM) was primarily first phase and normal whereas high glucose (22 mM) caused enhanced, rather than reduced, insulin secretion of both first and second phases. The alpha-antagonist, phentolamine, caused a six-fold increase in glucose-stimulated insulin secretion from the control pancreas, an effect that was blunted for the transgenic pancreas. A similarly blunted response to phentolamine occurred when this agent was superimposed on a combined glucose-forskolin stimulus. (The positive effect on insulin secretion by phentolamine in normal beta-cell preparations has arguably been ascribed to non-specific ionic effects.) Therefore, as a test of possible changes in the ATP regulated K+ channel or the linked Ca++ channels, glyburide was perfused during glucose stimulation. Insulin secretion in response to glyburide was increased two fold in the control pancreas. However, with the transgenic pancreas, in contrast to the enhanced response to glucose, the effect of glyburide was almost completely inhibited. It is concluded that: 1) chronic adrenergic hyperinnervation results in enhanced glucose-stimulated insulin secretion by desensitization of a major alpha-adrenergic inhibitory site(s); and 2) adrenergic hyperinnervation acts directly or indirectly on ion flux to partially inhibit insulin release, an effect which is not desensitized. Since down-regulation of a single alpha-adrenergic receptor would be expected to desensitize both phenomena the observed differential desensitization indicates that different post receptor events or more than one adrenergic receptor are involved.

Insulin secretion and glucose tolerance after islet transplantation in rats with noninsulin-dependent diabetes induced by neonatal streptozotocin

The present study was designed to identify in a model of noninsulin-dependent diabetes induced by neonatal streptozotocin (n0-STZ), the long-term consequences of an islet graft upon 1) glucose handling of the recipient and, 2) glucose response of the residual beta cells in the recipient pancreas. We have examined, 4 and 8 wk after islet implantation under the kidney capsule of syngeneic diabetic n0-STZ rats, their tolerance to glucose administered in vivo, together with their insulin release in response to glucose in vivo (oral glucose tolerance test) as well as in vitro (perfused pancreas). The results in the islet-grafted n0-STZ rats, were compared to those obtained in nongrafted nondiabetic rats and nongrafted n0-STZ rats. Our study shows that transplanting a limited number (900) of adult islets under the kidney capsule reverses to normal, many parameters of the noninsulin-dependent diabetic state in the n0-STZ rat model: these include body weight, basal plasma glucose in both the nonfasted and postabsorptive states, and basal plasma insulin in the postabsorptive state. Furthermore, tolerance to oral glucose administration was greatly improved in the transplanted rats and it was correlated with restoration of a manifest glucose-induced insulin secretion in vivo as evaluated (delta 1) during an oral glucose tolerance test. Our data clearly show that the insulin response to glucose from the endogenous pancreas of n0-STZ diabetic rat was not really improved by long-term (8 wk) basal normoglycemia. More precisely, we were able to detect a slight but significant improvement of the early phase of insulin release in vitro in response to glucose; however, the overall insulin response remained 15 times lower than the normal one with no reappearance of the late phase of insulin release. After cessation of glucose stimulation in vivo, off-response of insulin, which is also a landmark of the impaired insulin release by the beta cells of n0-STZ rats, was still detectable in the perfused pancreas of the transplanted n0-STZ rats. Finally, because the reactivity to glucose of the endogenous residual beta cells was not regained, the insulin released in vivo during the oral glucose test in the graft-bearing n0-STZ rats can be attributed mainly to functioning of the grafted islets population.

Dissociated insulinotropic sensitivity to glucose and carbachol in high-fat diet-induced insulin resistance in C57BL/6J mice

To study islet function following reduced insulin sensitivity, we examined mice of the C57BL/6J strain, the genotype of which carries an increased propensity to develop insulin resistance when metabolically challenged. The mice received either a high-fat diet (58% fat on an energy basis) or a control diet (11% fat) for 12 weeks. The body weight of mice on the high-fat diet increased significantly more than that of mice on the control diet (25.8 +/- 0.4 v 21.3 +/- 0.2 g, P < .001). Already after 1 week on the high-fat diet, a significant hyperglycemia accompanied by hyperinsulinemia had evolved, indicative of insulin resistance. After 12 weeks, plasma glucose levels for high-fat diet-treated mice were 7.5 +/- 0.1 mmol/L, versus 6.5 +/- 0.1 mmol/L in controls (P < .001); corresponding values for plasma insulin were 248 +/- 17 and 104 +/- 7 pmol/L, respectively (P < .001). Mice given a high-fat diet also had elevated levels of total cholesterol, triglycerides, and free fatty acids (FFAs) compared with controls. After 4, 8, and 12 weeks, glucose (2.8, 8.3, or 16.7 mmol/kg) or the cholinergic agonist carbachol (0.16 or 0.53 micromol/kg) was injected intraperitoneally. The insulinotropic response to glucose was not different between the two groups after 4 or 8 weeks, whereas after 12 weeks, glucose-induced insulin secretion was markedly impaired in high-fat diet-treated mice (P < .001). In contrast, after 8 and 12 weeks on a high-fat diet, carbachol-stimulated insulin secretion was potentiated (P < .01), whereas carbachol-stimulated glucagon secretion was not significantly altered. Furthermore, after 12 weeks on the high-fat diet, insulin secretion from isolated islets was impaired at glucose levels of 8.3, 11.1, and 16.7 mmol/L (P < or = .05). Moreover, islet morphology as examined by immunocytochemistry using insulin antibodies and islet innervation, as revealed by immunostaining of tyrosine hydroxylase (TH), neuropeptide Y (NPY), galanin, vasoactive intestinal polypeptide (VIP), and substance P (SP) were unaffected by the high-fat diet for 12 weeks. However, quantitative in situ hybridization showed a 3.5-fold upregulation of insulin gene expression in response to the high-fat diet (P < .001) despite unaltered B-cell mass and pancreatic insulin content. We conclude that as little as 1 week of treatment with a high-fat diet induces insulin resistance in C57BL/6J mice. This is accompanied later by hyperlipemia, potentiated carbachol-stimulated insulin secretion, and increased insulin gene expression but impaired glucose-stimulated insulin secretion. We suggest that after several weeks' duration, insulin resistance is accompanied by enhanced islet sensitivity to cholinergic activation and exaggerated insulin gene expression, whereas the failing islet sensitivity to glucose represents decompensation.

Decreased glucose-induced cAMP and insulin release in islets of diabetic rats: reversal by IBMX, glucagon, GIP

The first aim of the study was to investigate the possibility that a defect on the islet adenosine 3',5'-cyclic monophosphate (cAMP) production could be involved in the failure of the glucose-induced insulin secretion in the neonatal streptozotocin diabetic rats. Exposure to glucose concentration that induced a rise of the cAMP content in the control islets did not elicit any significant increase in cAMP in diabetic islets. Forskolin, isobutyl methylxanthine (IBMX), glucagon, or pertussis toxin amplified the cAMP accumulation and the insulin release to the same extent in both types of islets. Somatostatin, prostaglandin E2, UK-14304, or galanin inhibited cAMP accumulation and insulin release to the same extent in both types of islets. Our second purpose was to investigate whether the use of activators of adenylate cyclase could restore the beta-cell competence to glucose in diabetic rats. The addition of IBMX, glucagon, or gastric inhibitory polypeptide (GIP) to perifused islets of diabetic rats amplified their insulin response to glucose, and a clear biphasic pattern of the release was regained. In conclusion, although there is no major alteration of the functionality of the adenylate cyclase in the beta-cells of the diabetic rats, we have identified a defective glucose-induced cAMP generation that could be explained by a block in the step(s) linking glucose metabolism and activation of adenylate cyclase.

Beta-cell growth in adolescent and adult rats treated with streptozotocin during the neonatal period

Regeneration of neonatal beta cells after subtotal streptozotocin (STZ)-induced destruction is incomplete but nevertheless leads to rapid remission of hyperglycaemia. To study the proliferative and functional capacity of regenerated beta cells in adolescent and adult rats after early beta-cell damage and to determine the time point after birth which is decisive for regeneration, beta-cell growth and metabolic capacity were analysed in rats treated during the neonatal period with STZ (100 micrograms/g body weight) and studied over 3 to 20 weeks. Using immunocytochemistry combined with morphometry we found that the regenerated beta cells continue to increase in number till week 6 of life, when they reached values of more than 50% of those of controls. After week 6, the regenerated beta cells had enlarged in size but failed to further increase their number, although their proliferative activity, determined by bromodeoxyuridine (BrdU) pulse labelling, was still higher at 6 and 10 weeks than that of normal rats. The inability of regenerated beta cells to further increase their number coincided with a deterioration of their function (week 10, male rats; week 20, female rats). When beta cells were destroyed on day 2 or 5 instead of the day of birth, regeneration of beta cells markedly decreased and the rats were already on the threshold of development of glucose intolerance at 3 weeks of age. We concluded that the partially regenerated beta-cell population in rats treated with STZ on the day of birth ceases to grow 10 to 20 weeks later. This growth arrest may be due to the sustained functional demand to which these beta cells are exposed in order to compensate for their reduced number. Beta-cell capacity for regeneration declines rapidly during the first days of life.

Impairment of glucose-induced insulin secretion in human pancreatic islets transplanted to diabetic nude mice

Hyperglycemia-induced beta-cell dysfunction may be an important component in the pathogenesis of non-insulin-dependent diabetes mellitus. However, most available data in this field were obtained from rodent islets. To investigate the relevance of this hypothesis for human beta-cells in vivo, human pancreatic islets were transplanted under the renal capsule of nude mice. Experimental groups were chosen so that grafted islets were exposed to either hyper- or normoglycemia or combinations of these for 4 or 6 wk. Grafts of normoglycemic recipients responded with an increased insulin release to a glucose stimulus during perfusion, whereas grafts of hyperglycemic recipients failed to respond to glucose. The insulin content of the grafts in the latter groups was only 10% of those observed in controls. Recipients initially hyperglycemic (4 wk), followed by 2 wk of normoglycemia regained a normal graft insulin content, but a decreased insulin response to glucose remained. No ultrastructural signs of beta-cell damage were observed, with the exception of increased glycogen deposits in animals hyperglycemic at the time of killing. It is concluded that prolonged exposure to a diabetic environment induces a long-term secretory defect in human beta-cells, which is not dependent on the size of the islet insulin stores.

Beta-cell proliferation in normal and streptozotocin-treated newborn rats: site, dynamics and capacity

Regeneration of neonatal beta cells after streptozotocin (STZ)-induced destruction may be due to either replication from pre-existing intra-islet beta cells or extra-islet precursor cells. To further investigate this issue, beta-cell growth was analysed in normal and streptozotocin-treated newborn rats (100 micrograms/g body weight) at several time points during the first 20 days of life. Beta cells were identified by insulin immunostaining, non-isotopic in situ hybridization for rat preproinsulin mRNA, and electron microscopy. Their proliferative activity was recorded by bromodeoxyuridine-pulse labelling. Beta-cell size and total volume were determined by computerized morphometry. In normal rats, there was a threefold increase in total beta-cell volume during the first 5 days of life, with no further expansion till day 20. The bromodeoxyuridine labelling index of the intra-islet beta cells was smaller than that of the extra-islet beta cells (2-3% vs 15-20%). Comparison of the cell birth rate, calculated from the beta-cell labelling index, with the observed increase in beta-cell volume suggested that in normal neonatal rats proliferation of the intra-islet beta-cell population could account for only 10% of the observed expansion. Administration of streptozotocin at birth resulted in more than 90% reduction of the total beta-cell volume at day 2 which then increased to 39% of the normal value by day 20. During this period of partial regeneration, which restored normoglycaemia, the labelling index of intra-islet beta cells was higher than in normal rats (9% vs 2%, p < 0.001), whereas no change was seen in the extra-islet beta-cell labelling index.(ABSTRACT TRUNCATED AT 250 WORDS)

The role of islet secretory function in the development of diabetes in the GK Wistar rat

Insulin secretion and glucose metabolism were compared in islets isolated from GK Wistar rats (a non-obese, spontaneous model of non-insulin-dependent diabetes mellitus) and control Wistars aged 8 and 14 weeks. By 8 weeks of age, GK Wistar rats were clearly diabetic as indicated by non-fasting plasma glucose concentrations and impaired glucose tolerance. Islet insulin content was not significantly different to controls at either age. In islets from 14-week-old GK Wistar rats glucose-stimulated insulin release (6-16 mmol/l glucose) was significantly reduced to 25-50% of controls in static incubations (p < 0.001). In perifusion, glucose-stimulated insulin release was reduced by 90% for first phase (p < 0.01) and by 75% for second phase (p < 0.05). The responses to arginine and 2 alpha Ketoisocaproate in islets were similar to those in controls. In contrast, islets isolated from 8-week-old GK Wistar rats exhibited no significant reduction in glucose-stimulated insulin secretion in static incubations. In perifusion, although both first and second phases of glucose-stimulated insulin release were slightly reduced, these were not significantly different to controls. Islets from 8-week-old GK Wistar rats failed however to respond to stimulation by glyceraldehyde. Raising the medium glucose concentration to 16 mmol/l significantly increased rates of glucose utilisation ([3H] H2O production from 5-[3H] glucose) and oxidation ([14C] CO2 production from U-[14C] glucose) in islets isolated from 8-week-old control and GK Wistar rats, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Irreversible loss of normal beta-cell regulation by glucose in neonatally streptozotocin diabetic rats

Animals with NIDDM display abnormal glucose regulation of insulin secretion and biosynthesis. We tested reversibility of abnormal regulation by normoglycaemia using an islet transplantation technique. Inbred non-diabetic and neonatally STZ diabetic rats (n-STZ) were used. Transplantations insufficient to normalize the blood glucose levels (200 islets under kidney capsule) were performed from diabetic to normal (D-N) and from diabetic to diabetic (D-D), as well as from normal to normal (N-N) and from normal to diabetic (N-D) rats. Four weeks after transplantation, graft bearing kidneys were isolated and perfused with Krebs-Henseleit bicarbonate buffer to measure insulin secretion in response to 27.8 mmol/l glucose and 10 mmol/l arginine. Four weeks of normoglycaemia failed to restore glucose-induced insulin secretion from n-STZ islets (glucose induced increment: -1.7 +/- 2.5 fmol/min in D-N, 1.2 +/- 7.1 fmol/min in D-D). In contrast to normal islets, normoglycaemia reduced insulin mRNA contents (60 +/- 24 in D-N, 496 +/- 119 in D-D; O.D.-arbitrary units). However, arginine-induced secretion was markedly enhanced by diabetic environment in both normal and n-STZ islet grafts. These results indicate that selected aspects of glucose recognition are irreversibly damaged by a long-term diabetic state or, alternatively, by a lasting effect of STZ administration.

Enhancement by streptozotocin-induced diabetes of pancreatic prolyl endopeptidase activity in neonatal rats

The effect of streptozotocin (STZ)-induced diabetes on the concentrations of deamidated TRH (TRH-OH), a metabolite of thyrotropin-releasing hormone (TRH) and prolyl endopeptidase (PE) activity were studied in the pancreas of neonatal rats to determine the contribution of beta-cells to PE activity and TRH-OH levels that we have previously found in this tissue. STZ treatment caused a significant reduction of immunoreactive TRH-OH levels on day-3 and -5 compared to untreated control rats. Reverse-phase high performance liquid chromatography of pooled extracts of 3-day-old normal rat pancreas revealed that about 50% of immunoreactive TRH-OH was found in the fractions representing authentic TRH-OH, whereas the remaining 50% eluted earlier. In STZ treated rats, all of the TRH-OH immunoreactive was associated with this early peak, no authentic TRH-OH could be detected. The specific activity of PE, on the other hand, rose 2.5-fold in diabetic 3-day-old pups (4.06 +/- 0.13 compared with 1.59 +/- 0.83 nmol/min/mg protein, P < 0.01, in controls). This increase declined with age (1.6- and 1.3-fold in 5 day- and 7-day-old pups, respectively). STZ treatment did not change pancreatic PE levels in 20-day-old rats control. Normalization of STZ induced hyperglycemia by sodium metavanadate treatment or by replacement of exogenous insulin did not restore pancreatic PE activity. The enhancement of PE activity following STZ treatment was specific for pancreas tissue. Furthermore, beta cytoxin drugs other than STZ that cause permanent diabetes such as alloxan enhanced PE activity to the same extend. Kinetic studies for PE activity show that Vmax is 3-fold higher in 3-day-old STZ-treated than in rat controls. In contrast, values for Km were comparable in rats of both groups (25 to 34 microM). We then tested whether the decrease of Vmax might have been caused by the presence of an PE inhibiting factor in these preparations. Gel-filtration experiments of pancreatic extracts revealed that the total apparent activity of eluted PE in 3-day-old control rats was 2-fold higher than in the original extract. In contrast, the recovery of eluted PE activity was not increased in the case of STZ-treated 3-day-old and untreated 20-day-old rats. These findings demonstrate that TRH-OH is identified in beta-cells and that an inhibiting factor(s) present in beta-cells appear(s) to be responsible for the unexpected enhancement of PE activity observed in STZ-treated neonatal rats.(ABSTRACT TRUNCATED AT 400 WORDS)