Dexamethasone suppresses phospholipase C activation and insulin secretion from isolated rat islets

Impact of therapeutic doses of prednisolone and other glucocorticoids on insulin secretion from human islets

INTRODUCTION

Glucocorticoid-induced diabetes (GCID) is a prevalent health issue, generally attributed to insulin resistance. High doses of dexamethasone (DEX) are known to inhibit glucose-stimulated insulin secretion (GSIS), but the effects of lower doses, commonly used in chronic therapy, and equipotent doses of other glucocorticoids (GCs) such as hydrocortisone (HC) and prednisone (PRED) remain underexplored. This study aimed to investigate these effects in vitro, and explore variations between patients.

MATERIALS AND METHODS

Dynamic perifusion assays were conducted on human islets to evaluate the impact of different GCs on GSIS. The islets were treated for 24h with 250nM PRED and other GCs at equipotent anti-inflammatory doses (HC: 1μM; DEX: 38nM).

RESULTS

In 11 human islet donor preparations, 250nM PRED, corresponding to a clinical oral dose of 5mg/day, significantly inhibited the first and second phase of GSIS: area under the curve (AUC) decreased by 32.3% (P<0.001), first phase by 41.5% (P<0.001), and second phase by 38.4% (P<0.001). Despite interindividual differences in GSIS response to PRED, no significant differences were observed according to body mass index, gender or age. Comparing the effects of GCs at equipotent anti-inflammatory doses, DEX had a more pronounced inhibitory effect on GSIS than HC or PRED.

CONCLUSIONS

In vitro, low-dose PRED treatment significantly impacted GSIS. DEX had a more unfavorable impact on GSIS than HC or PRED, indicating that metabolic effects do not align with anti-inflammatory potency.

The protective effect of imatinib against pancreatic β-cell apoptosis induced by dexamethasone via increased GSTP1 expression and reduced oxidative stress

Glucocorticoids (GCs) are known to stimulate pancreatic beta (β)-cell apoptosis via several mechanisms, including oxidative stress. Our previous study suggested an increase in dexamethasone-induced pancreatic β-cell apoptosis via a reduction of glutathione S-transferase P1 (GSTP1), which is an antioxidant enzyme. Imatinib, which is a tyrosine kinase inhibitor, also exerts antioxidant effect. This study aims to test our hypothesis that imatinib would prevent pancreatic β-cell apoptosis induced by dexamethasone via increased GSTP1 expression and reduced oxidative stress. Our results revealed that dexamethasone significantly increased apoptosis in INS-1 cells when compared to the control, and that imatinib significantly decreased INS-1 cell apoptosis induced by dexamethasone. Moreover, dexamethasone significantly increased superoxide production in INS-1 cells when compared to the control; however, imatinib, when combined with dexamethasone, significantly reduced superoxide production in INS-1 cells. Dexamethasone significantly decreased GSTP1, p-ERK1/2, and BCL2 protein expression, but significantly increased p-JNK, p-p38, and BAX protein expression in INS-1 cells-all compared to control. Importantly, imatinib significantly ameliorated the effect of dexamethasone on the expression of GSTP1, p-ERK1/2, p-JNK, p-p38 MAPK, BAX, and BCL2. Furthermore-6-(7-nitro-2,1,3-benzoxadiazol-4-ylthio) hexanol (NBDHEX), which is a GSTP1 inhibitor, neutralized the protective effect of imatinib against pancreatic β-cell apoptosis induced by dexamethasone. In conclusion, imatinib decreases pancreatic β-cell apoptosis induced by dexamethasone via increased GSTP1 expression and reduced oxidative stress.

Glucocorticoid receptor-NECAB1 axis can negatively regulate insulin secretion in pancreatic β-cells

The mechanisms of impaired glucose-induced insulin secretion from the pancreatic β-cells in obesity have not yet been completely elucidated. Here, we aimed to assess the effects of adipocyte-derived factors on the functioning of pancreatic β-cells. We prepared a conditioned medium using 3T3-L1 cell culture supernatant collected at day eight (D8CM) and then exposed the rat pancreatic β-cell line, INS-1D. We found that D8CM suppressed insulin secretion in INS-1D cells due to reduced intracellular calcium levels. This was mediated by the induction of a negative regulator of insulin secretion-NECAB1. LC-MS/MS analysis results revealed that D8CM possessed steroid hormones (cortisol, corticosterone, and cortisone). INS-1D cell exposure to cortisol or corticosterone increased Necab1 mRNA expression and significantly reduced insulin secretion. The increased expression of Necab1 and reduced insulin secretion effects from exposure to these hormones were completely abolished by inhibition of the glucocorticoid receptor (GR). NECAB1 expression was also increased in the pancreatic islets of db/db mice. We demonstrated that the upregulation of NECAB1 was dependent on GR activation, and that binding of the GR to the upstream regions of Necab1 was essential for this effect. NECAB1 may play a novel role in the adipoinsular axis and could be potentially involved in the pathophysiology of obesity-related diabetes mellitus.

Transient Dexamethasone Loading Induces Prolonged Hyperglycemia in Male Mice With Histone Acetylation in Dpp-4 Promoter

Glucocorticoid causes hyperglycemia, which is common in patients with or without diabetes. Prolonged hyperglycemia can be experienced even after the discontinuation of glucocorticoid use. In the present study, we examined the time course of blood glucose level in hospital patients who received transient glucocorticoid treatment. In addition, the mechanism of prolonged hyperglycemia was investigated by using dexamethasone (Dexa)-treated mice and cultured cells. The blood glucose level in glucose tolerance tests, level of insulin and glucagon-like peptide 1 (GLP-1), and the activity of dipeptidyl peptidase 4 (DPP-4) were examined during and after Dexa loading in mice, with histone acetylation level of the promoter region. Mice showed prolonged hyperglycemia during and after transient Dexa loading accompanied by persistently lower blood GLP-1 level and higher activity of DPP-4. The expression level of Dpp-4 was increased in the mononuclear cells and the promoter region of Dpp-4 was hyperacetylated during and after the transient Dexa treatment. In vitro experiments also indicated development of histone hyperacetylation in the Dpp-4 promoter region during and after Dexa treatment. The upregulation of Dpp-4 in cultured cells was significantly inhibited by a histone acetyltransferase inhibitor. Moreover, the histone hyperacetylation induced by Dexa was reversible by treatment with a sirtuin histone deacetylase activator, nicotinamide mononucleotide. We identified persistent reduction in blood GLP-1 level with hyperglycemia during and after Dexa treatment in mice, associated with histone hyperacetylation of promoter region of Dpp-4. The results unveil a novel mechanism of glucocorticoid-induced hyperglycemia, and suggest therapeutic intervention through epigenetic modification of Dpp-4.

Localized drug delivery graphene bioscaffolds for cotransplantation of islets and mesenchymal stem cells

In the present work, we developed, characterized, and tested an implantable graphene bioscaffold which elutes dexamethasone (Dex) that can accommodate islets and adipose tissue–derived mesenchymal stem cells (AD-MSCs). In vitro studies demonstrated that islets in graphene–0.5 w/v% Dex bioscaffolds had a substantial higher viability and function compared to islets in graphene-alone bioscaffolds or islets cultured alone (P < 0.05). In vivo studies, in which bioscaffolds were transplanted into the epididymal fat pad of diabetic mice, demonstrated that, when islet:AD-MSC units were seeded into graphene–0.5 w/v% Dex bioscaffolds, this resulted in complete restoration of glycemic control immediately after transplantation with these islets also showing a faster response to glucose challenges (P < 0.05). Hence, this combination approach of using a graphene bioscaffold that can be functionalized for local delivery of Dex into the surrounding microenvironment, together with AD-MSC therapy, can significantly improve the survival and function of transplanted islets.

Potent Anti-Inflammatory, Arylpyrazole-Based Glucocorticoid Receptor Agonists That Do Not Impair Insulin Secretion

Glucocorticoids (GCs) are widely used in medicine for their role in the treatment of autoimmune-mediated conditions, certain cancers, and organ transplantation. The transcriptional activities GCs elicit include transrepression, postulated to be responsible for the anti-inflammatory activity, and transactivation, proposed to underlie the undesirable side effects associated with long-term use. A GC analogue that could elicit only transrepression and beneficial transactivation properties would be of great medicinal value and is highly sought after. In this study, a series of 1-(4-substituted phenyl)pyrazole-based GC analogues were synthesized, biologically screened, and evaluated for SARs leading to the desired activity. Activity observed in compounds bearing an electron deficient arylpyrazole moiety showed promise toward a dissociated steroid, displaying transrepression while having limited transactivation activity. In addition, compounds 11aa and 11ab were found to have anti-inflammatory efficacy comparable to that of dexamethasone at 10 nM, with minimal transactivation activity and no reduction of insulin secretion in cultured rat 832/13 beta cells.

New-Onset Diabetes after Kidney Transplantation

New-onset diabetes mellitus after transplantation (NODAT) is a frequent complication in kidney allograft recipients. It may be caused by modifiable and non-modifiable factors. The non-modifiable factors are the same that may lead to the development of type 2 diabetes in the general population, whilst the modifiable factors include peri-operative stress, hepatitis C or cytomegalovirus infection, vitamin D deficiency, hypomagnesemia, and immunosuppressive medications such as glucocorticoids, calcineurin inhibitors (tacrolimus more than cyclosporine), and mTOR inhibitors. The most worrying complication of NODAT are major adverse cardiovascular events which represent a leading cause of morbidity and mortality in transplanted patients. However, NODAT may also result in progressive diabetic kidney disease and is frequently associated with microvascular complications, eventually determining blindness or amputation. Preventive measures for NODAT include a careful assessment of glucose tolerance before transplantation, loss of over-weight, lifestyle modification, reduced caloric intake, and physical exercise. Concomitant measures include aggressive control of systemic blood pressure and lipids levels to reduce the risk of cardiovascular events. Hypomagnesemia and low levels of vitamin D should be corrected. Immunosuppressive strategies limiting the use of diabetogenic drugs are encouraged. Many hypoglycemic drugs are available and may be used in combination with metformin in difficult cases. In patients requiring insulin treatment, the dose and type of insulin should be decided on an individual basis as insulin requirements depend on the patient's diet, amount of exercise, and renal function.

Tissue Lipidomic Alterations Induced by Prolonged Dexamethasone Treatment

Dexamethasone is a synthetic glucocorticoid medication vastly used to treat abnormal immune responses and inflammation. Although the medication is well-established in the medical community, the prolonged treatment with high dosages of dexamethasone may lead to severe adverse effects through mechanisms that are not yet well-known. Lipids are a large class of hydrophobic molecules involved in energy storage, signaling, modulation of gene expression, and membranes. Hence, untargeted lipidomics may help unravel the biochemical alterations following prolonged treatment with high dosages of dexamethasone. We performed comprehensive lipidomic analyses of brain, heart, kidney, liver, and muscle samples obtained from rats that were treated with intramuscular injections of dexamethasone for 14 weeks compared to healthy controls. The employed methodology and statistical analysis showed that phosphatidic acids, glycerophospholipids, plasmalogens, and fatty acids are deeply affected by prolonged use of the medication. Brain tissue was only mildly affected, but skeletal muscle showed a strong accumulation of lipids that may be correlated with alterations in the energy metabolism, myopathy, and oxidative processes. This work provides new insights into the mechanisms of action and adverse effects for one of the most commonly prescribed class of drugs in the world.

Suppression of Fibrotic Reactions of Chitosan-Alginate Microcapsules Containing Porcine Islets by Dexamethasone Surface Coating

BACKGROUND

The microencapsulation is an ideal solution to overcome immune rejection without immunosuppressive treatment. Poor biocompatibility and small molecular antigens secreted from encapsulated islets induce fibrosis infiltration. Therefore, the aims of this study were to improve the biocompatibility of microcapsules by dexamethasone coating and to verify its effect after xenogeneic transplantation in a streptozotocin-induced diabetes mice.

METHODS

Dexamethasone 21-phosphate (Dexa) was dissolved in 1% chitosan and was cross-linked with the alginate microcapsule surface. Insulin secretion and viability assays were performed 14 days after microencapsulation. Dexa-containing chitosan-coated alginate (Dexa-chitosan) or alginate microencapsulated porcine islets were transplanted into diabetic mice. The fibrosis infiltration score was calculated from the harvested microcapsules. The harvested microcapsules were stained with trichrome and for insulin and macrophages.

RESULTS

No significant differences in glucose-stimulated insulin secretion and islet viability were noted among naked, alginate, and Dexa-chitosan microencapsulated islets. After transplantation of microencapsulated porcine islets, nonfasting blood glucose were normalized in both the Dexa-chitosan and alginate groups until 231 days. The average glucose after transplantation were lower in the Dexa-chitosan group than the alginate group. Pericapsular fibrosis and inflammatory cell infiltration of microcapsules were significantly reduced in Dexa-chitosan compared with alginate microcapsules. Dithizone and insulin were positive in Dexa-chitosan capsules. Although fibrosis and macrophage infiltration was noted on the surface, some alginate microcapsules were stained with insulin.

CONCLUSION

Dexa coating on microcapsules significantly suppressed the fibrotic reaction on the capsule surface after transplantation of xenogenic islets containing microcapsules without any harmful effects on the function and survival of the islets.

Prunetin Protects Against Dexamethasone-Induced Pancreatic Β-Cell Apoptosis via Modulation of p53 Signaling Pathway

Long-term administration of dexamethasone results in insulin resistance and pancreatic β-cell apoptosis. Prunetin (an O-methylated isoflavone, a type of flavonoid) is demonstrated to protect diabetes, but the molecular mechanism of this protection is still unclear. This study thus aims to investigate how prunetin protects against dexamethasone-induced pancreatic β-cell apoptosis. Rat insulinoma (INS-1) cells were cultured in medium with or without dexamethasone in the presence or absence of prunetin or pifithrin-α, a p53 inhibitor. Cell apoptosis was measured by Annexin V/propidium iodide staining. Dexamethasone significantly induced INS-1 apoptosis but dexamethasone plus prunetin significantly reduced INS-1 apoptosis. Dexamethasone-treated INS-1 upregulated p53 protein expression; the induction of p53 was also reduced in the presence of RU486, a glucocorticoid receptor (GR) inhibitor. This suggested that dexamethasone induced P53 via GR. Dexamethasone-treated INS-1 significantly increased p53, Bax, and Rb protein expressions, whereas treatments of dexamethasone plus prunetin or pifithrin-α significantly decreased these protein expressions. In addition, dexamethasone significantly decreased B-cell lymphoma 2 (Bcl2), while dexamethasone plus prunetin or pifithrin-α significantly increased Bcl2. Dexamethasone significantly increased caspase-3 activity while co-treatment of dexamethasone plus prunetin or pifithrin-α significantly decreased caspase-3 activity to the control level. Taken together, our results revealed that prunetin protected against dexamethasone-induced pancreatic β-cells apoptosis via modulation of the p53 signaling pathway.

Polymeric Scaffolds for Pancreatic Tissue Engineering: A Review

In recent years, there has been an alarming increase in the incidence of diabetes, with one in every eleven individuals worldwide suffering from this debilitating disease. As the available treatment options fail to reduce disease progression, novel avenues such as the bioartificial pancreas are being given serious consideration. In the past decade, the research focus has shifted towards the field of tissue engineering, which helps to design biological substitutes for repair and replacement of non-functional or damaged organs. Scaffolds constitute an integral part of tissue engineering; they have been shown to mimic the native extracellular matrix, thereby supporting cell viability and proliferation. This review offers a novel compilation of the recent advances in polymeric scaffolds, which are used for pancreatic tissue engineering. Furthermore, in this article, the design strategies for bioartificial pancreatic constructs and their future applications in cell-based therapy are discussed.

Glucocorticoids, genes and brain function

The identification of key genes in transcriptomic data constitutes a huge challenge. Our review of microarray reports revealed 88 genes whose transcription is consistently regulated by glucocorticoids (GCs), such as cortisol, corticosterone and dexamethasone, in the brain. Replicable transcriptomic data were combined with biochemical and physiological data to create an integrated view of the effects induced by GCs. The most frequently reported genes were Errfi1 and Ddit4. Their up-regulation was associated with the altered transcription of genes regulating growth factor and mTORC1 signaling (Gab1, Tsc22d3, Dusp1, Ndrg2, Ppp5c and Sesn1) and progression of the cell cycle (Ccnd1, Cdkn1a and Cables1). The GC-induced reprogramming of cell function involves changes in the mRNA level of genes responsible for the regulation of transcription (Klf9, Bcl6, Klf15, Tle3, Cxxc5, Litaf, Tle4, Jun, Sox4, Sox2, Sox9, Irf1, Sall2, Nfkbia and Id1) and the selective degradation of mRNA (Tob2). Other genes are involved in the regulation of metabolism (Gpd1, Aldoc and Pdk4), actin cytoskeleton (Myh2, Nedd9, Mical2, Rhou, Arl4d, Osbpl3, Arhgef3, Sdc4, Rdx, Wipf3, Chst1 and Hepacam), autophagy (Eva1a and Plekhf1), vesicular transport (Rhob, Ehd3, Vps37b and Scamp2), gap junctions (Gjb6), immune response (Tiparp, Mertk, Lyve1 and Il6r), signaling mediated by thyroid hormones (Thra and Sult1a1), calcium (Calm2), adrenaline/noradrenaline (Adcy9 and Adra1d), neuropeptide Y (Npy1r) and histamine (Hdc). GCs also affected genes involved in the synthesis of polyamines (Azin1) and taurine (Cdo1). The actions of GCs are restrained by feedback mechanisms depending on the transcription of Sgk1, Fkbp5 and Nr3c1. A side effect induced by GCs is increased production of reactive oxygen species. Available data show that the brain's response to GCs is part of an emergency mode characterized by inactivation of non-core activities, restrained inflammation, restriction of investments (growth), improved efficiency of energy production and the removal of unnecessary or malfunctioning cellular components to conserve energy and maintain nutrient supply during the stress response.

Glucose Metabolism Abnormalities in Cushing Syndrome: From Molecular Basis to Clinical Management

An impaired glucose metabolism, which often leads to the onset of diabetes mellitus (DM), is a common complication of chronic exposure to exogenous and endogenous glucocorticoid (GC) excess and plays an important part in contributing to morbidity and mortality in patients with Cushing syndrome (CS). This article reviews the pathogenesis, epidemiology, diagnosis, and management of changes in glucose metabolism associated with hypercortisolism, addressing both the pathophysiological aspects and the clinical and therapeutic implications. Chronic hypercortisolism may have pleiotropic effects on all major peripheral tissues governing glucose homeostasis. Adding further complexity, both genomic and nongenomic mechanisms are directly induced by GCs in a context-specific and cell-/organ-dependent manner. In this paper, the discussion focuses on established and potential pathologic molecular mechanisms that are induced by chronically excessive circulating levels of GCs and affect glucose homeostasis in various tissues. The management of patients with CS and DM includes treating their hyperglycemia and correcting their GC excess. The effects on glycemic control of various medical therapies for CS are reviewed in this paper. The association between DM and subclinical CS and the role of screening for CS in diabetic patients are also discussed.

Impact of Glucocorticoid Excess on Glucose Tolerance: Clinical and Preclinical Evidence

Glucocorticoids (GCs) are steroid hormones that exert important physiological actions on metabolism. Given that GCs also exert potent immunosuppressive and anti-inflammatory actions, synthetic GCs such as prednisolone and dexamethasone were developed for the treatment of autoimmune- and inflammatory-related diseases. The synthetic GCs are undoubtedly efficient in terms of their therapeutic effects, but are accompanied by significant adverse effects on metabolism, specifically glucose metabolism. Glucose intolerance and reductions in insulin sensitivity are among the major concerns related to GC metabolic side effects, which may ultimately progress to type 2 diabetes mellitus. A number of pre-clinical and clinical studies have aimed to understand the repercussions of GCs on glucose metabolism and the possible mechanisms of GC action. This review intends to summarize the main alterations that occur in liver, skeletal muscle, adipose tissue, and pancreatic islets in the context of GC-induced glucose intolerance. For this, both experimental (animals) and clinical studies were selected and, whenever possible, the main cellular mechanisms involved in such GC-side effects were discussed.

Glucocorticoids Inhibit Basal and Hormone-Induced Serotonin Synthesis in Pancreatic Beta Cells

Diabetes is a major complication of chronic Glucocorticoids (GCs) treatment. GCs induce insulin resistance and also inhibit insulin secretion from pancreatic beta cells. Yet, a full understanding of this negative regulation remains to be deciphered. In the present study, we investigated whether GCs could inhibit serotonin synthesis in beta cell since this neurotransmitter has been shown to be involved in the regulation of insulin secretion. To this aim, serotonin synthesis was evaluated in vitro after treatment with GCs of either islets from CD1 mice or MIN6 cells, a beta-cell line. We also explored the effect of GCs on the stimulation of serotonin synthesis by several hormones such as prolactin and GLP 1. We finally studied this regulation in islet in two in vivo models: mice treated with GCs and with liraglutide, a GLP1 analog, and mice deleted for the glucocorticoid receptor in the pancreas. We showed in isolated islets and MIN6 cells that GCs decreased expression and activity of the two key enzymes of serotonin synthesis, Tryptophan Hydroxylase 1 (Tph1) and 2 (Tph2), leading to reduced serotonin contents. GCs also blocked the induction of serotonin synthesis by prolactin or by a previously unknown serotonin activator, the GLP-1 analog exendin-4. In vivo, activation of the Glucagon-like-Peptide-1 receptor with liraglutide during 4 weeks increased islet serotonin contents and GCs treatment prevented this increase. Finally, islets from mice deleted for the GR in the pancreas displayed an increased expression of Tph1 and Tph2 and a strong increased serotonin content per islet. In conclusion, our results demonstrate an original inhibition of serotonin synthesis by GCs, both in basal condition and after stimulation by prolactin or activators of the GLP-1 receptor. This regulation may contribute to the deleterious effects of GCs on beta cells.

Identification of novel genes for glucose metabolism based upon expression pattern in human islets and effect on insulin secretion and glycemia

Normal glucose homeostasis is characterized by appropriate insulin secretion and low HbA1c. Gene expression signatures associated with these two phenotypes could be essential for islet function and pathophysiology of type 2 diabetes (T2D). Herein, we employed a novel approach to identify candidate genes involved in T2D by correlating islet microarray gene expression data (78 donors) with insulin secretion and HbA1c level. The expression of 649 genes (P < 0.05) was correlated with insulin secretion and HbA1c. Of them, five genes (GLR1A, PPP1R1A, PLCDXD3, FAM105A and ENO2) correlated positively with insulin secretion/negatively with HbA1c and one gene (GNG5) correlated negatively with insulin secretion/positively with HbA1c were followed up. The five positively correlated genes have lower expression levels in diabetic islets, whereas GNG5 expression is higher. Exposure of human islets to high glucose for 24 h resulted in up-regulation of GNG5 and PPP1R1A expression, whereas the expression of ENO2 and GLRA1 was down-regulated. No effect was seen on the expression of FAM105A and PLCXD3. siRNA silencing in INS-1 832/13 cells showed reduction in insulin secretion for PPP1R1A, PLXCD3, ENO2, FAM105A and GNG5 but not GLRA1. Although no SNP in these gene loci passed the genome-wide significance for association with T2D in DIAGRAM+ database, four SNPs influenced gene expression in cis in human islets. In conclusion, we identified and confirmed PPP1R1A, FAM105A, ENO2, PLCDX3 and GNG5 as potential regulators of islet function. We provide a list of candidate genes as a resource for exploring their role in the pathogenesis of T2D.

Glucocorticoid treatment and endocrine pancreas function: implications for glucose homeostasis, insulin resistance and diabetes

Glucocorticoids (GCs) are broadly prescribed for numerous pathological conditions because of their anti-inflammatory, antiallergic and immunosuppressive effects, among other actions. Nevertheless, GCs can produce undesired diabetogenic side effects through interactions with the regulation of glucose homeostasis. Under conditions of excess and/or long-term treatment, GCs can induce peripheral insulin resistance (IR) by impairing insulin signalling, which results in reduced glucose disposal and augmented endogenous glucose production. In addition, GCs can promote abdominal obesity, elevate plasma fatty acids and triglycerides, and suppress osteocalcin synthesis in bone tissue. In response to GC-induced peripheral IR and in an attempt to maintain normoglycaemia, pancreatic β-cells undergo several morphofunctional adaptations that result in hyperinsulinaemia. Failure of β-cells to compensate for this situation favours glucose homeostasis disruption, which can result in hyperglycaemia, particularly in susceptible individuals. GC treatment does not only alter pancreatic β-cell function but also affect them by their actions that can lead to hyperglucagonaemia, further contributing to glucose homeostasis imbalance and hyperglycaemia. In addition, the release of other islet hormones, such as somatostatin, amylin and ghrelin, is also affected by GC administration. These undesired GC actions merit further consideration for the design of improved GC therapies without diabetogenic effects. In summary, in this review, we consider the implication of GC treatment on peripheral IR, islet function and glucose homeostasis.

Diabetes mellitus in the transplanted kidney

Diabetes mellitus (DM) is the most common cause of chronic kidney disease and end stage renal disease. New onset diabetes mellitus after transplant (NODAT) has been described in approximately 30% of non-diabetic kidney-transplant recipients many years post transplantation. DM in patients with kidney transplantation constitutes a major comorbidity, and has significant impact on the patients and allografts' outcome. In addition to the major comorbidity and mortality that result from cardiovascular and other DM complications, long standing DM after kidney-transplant has significant pathological injury to the allograft, which results in lowering the allografts and the patients' survivals. In spite of the cumulative body of data on diabetic nephropathy (DN) in the native kidney, there has been very limited data on the DN in the transplanted kidney. In this review, we will shed the light on the risk factors that lead to the development of NODAT. We will also describe the impact of DM on the transplanted kidney, and the outcome of kidney-transplant recipients with NODAT. Additionally, we will present the most acceptable data on management of NODAT.

Exogenous glucocorticoids and a high-fat diet cause severe hyperglycemia and hyperinsulinemia and limit islet glucose responsiveness in young male Sprague-Dawley rats

Corticosterone (CORT) and other glucocorticoids cause peripheral insulin resistance and compensatory increases in β-cell mass. A prolonged high-fat diet (HFD) induces insulin resistance and impairs β-cell insulin secretion. This study examined islet adaptive capacity in rats treated with CORT and a HFD. Male Sprague-Dawley rats (age ∼6 weeks) were given exogenous CORT (400 mg/rat) or wax (placebo) implants and placed on a HFD (60% calories from fat) or standard diet (SD) for 2 weeks (N = 10 per group). CORT-HFD rats developed fasting hyperglycemia (>11 mM) and hyperinsulinemia (∼5-fold higher than controls) and were 15-fold more insulin resistant than placebo-SD rats by the end of ∼2 weeks (Homeostatic Model Assessment for Insulin Resistance [HOMA-IR] levels, 15.08 ± 1.64 vs 1.0 ± 0.12, P < .05). Pancreatic β-cell function, as measured by HOMA-β, was lower in the CORT-HFD group as compared to the CORT-SD group (1.64 ± 0.22 vs 3.72 ± 0.64, P < .001) as well as acute insulin response (0.25 ± 0.22 vs 1.68 ± 0.41, P < .05). Moreover, β- and α-cell mass were 2.6- and 1.6-fold higher, respectively, in CORT-HFD animals compared to controls (both P < .05). CORT treatment increased p-protein kinase C-α content in SD but not HFD-fed rats, suggesting that a HFD may lower insulin secretory capacity via impaired glucose sensing. Isolated islets from CORT-HFD animals secreted more insulin in both low and high glucose conditions; however, total insulin content was relatively depleted after glucose challenge. Thus, CORT and HFD, synergistically not independently, act to promote severe insulin resistance, which overwhelms islet adaptive capacity, thereby resulting in overt hyperglycemia.

Thyroid hormone promotes postnatal rat pancreatic β-cell development and glucose-responsive insulin secretion through MAFA

Neonatal β cells do not secrete glucose-responsive insulin and are considered immature. We previously showed the transcription factor MAFA is key for the functional maturation of β cells, but the physiological regulators of this process are unknown. Here we show that postnatal rat β cells express thyroid hormone (TH) receptor isoforms and deiodinases in an age-dependent pattern as glucose responsiveness develops. In vivo neonatal triiodothyronine supplementation and TH inhibition, respectively, accelerated and delayed metabolic development. In vitro exposure of immature islets to triiodothyronine enhanced the expression of Mafa, the secretion of glucose-responsive insulin, and the proportion of responsive cells, all of which are effects that were abolished in the presence of dominant-negative Mafa. Using chromatin immunoprecipitation and electrophoretic mobility shift assay, we show that TH has a direct receptor-ligand interaction with the Mafa promoter and, using a luciferase reporter, that this interaction was functional. Thus, TH can be considered a physiological regulator of functional maturation of β cells via its induction of Mafa.

Cellular and physiological mechanisms of new-onset diabetes mellitus after solid organ transplantation

New-onset diabetes after transplantation is recognized as one of the metabolic consequences which may increase the risk of morbidity and mortality after solid organ transplantation. The pathophysiology of new-onset diabetes after transplantation has not been clearly defined and may resemble that of Type 2 diabetes, characterized by predominantly insulin resistance or defective insulin secretion, or both. This review aims to summarize the current state of knowledge regarding the prevalence, consequences, pathogenesis, and management of new-onset diabetes after transplantation, with a major focus on the possible mechanisms involved in the pathogenesis of the disorder. The aetiology of new-onset diabetes after transplantation is multifactorial, with diabetogenic immunosuppressive drugs playing a major role. Multiple cellular and physiologic mechanisms are involved in the process. Selection of an appropriate maintenance immunosuppressive regimen should involve balancing the risk of patient and graft survival vs. the potential for new-onset diabetes after transplantation.

Involvement of thioredoxin-interacting protein (TXNIP) in glucocorticoid-mediated beta cell death

AIM/HYPOTHESIS

Glucocorticoid hormones (GCs) are widely used to treat a variety of inflammatory and immune diseases. However, their long-term administration is associated with adverse metabolic effects, including glucose intolerance and diabetes. Our objective was to elucidate the mechanisms by which GCs affect beta cell survival with a specific emphasis on the role of the thioredoxin-interacting protein (TXNIP) in beta cell apoptosis.

METHODS

Human and mouse islets, together with MIN6 beta cells, were exposed to dexamethasone (Dex) and apoptosis was assessed by measuring the percentage of sub-G1 cells, the appearance of cleaved caspase-3 or by using a TUNEL assay. Dex-upregulated expression of Txnip mRNA was analysed by real-time PCR, and GC-modulated production and modification of proteins were determined by western blotting.

RESULTS

We provide evidence that TXNIP, a negative regulator of the antioxidant thioredoxin (TRX), is strongly induced in beta cells by GCs and that its induction is dependent on p38 mitogen-activated protein kinase (MAPK) activation. TXNIP downregulation by RNA interference, overexpression of the radical scavenger TRX1 or elevation of intracellular cAMP levels attenuated the Dex-mediated apoptosis. Dex-induced Txnip expression and beta cell apoptosis are mediated by the glucocorticoid receptor (GR), as the GR antagonist RU486 fully abolishes these effects.

CONCLUSIONS/INTERPRETATION

Altogether, our data suggest TXNIP as a novel mediator of GC-induced apoptosis in beta cells and further contribute to our understanding of beta cell death pathways.

Nuclear PLCs affect insulin secretion by targeting PPARγ in pancreatic β cells

Type 2 diabetes is a heterogeneous disorder caused by concomitant impairment of insulin secretion by pancreatic β cells and of insulin action in peripheral target tissues. Studies with inhibitors and agonists established a role for PLC in the regulation of insulin secretion but did not distinguish between effects due to nuclear or cytoplasmic PLC signaling pathways that act in a distinct fashion. We report that in MIN6 β cells, PLCβ1 localized in both nucleus and cytoplasm, PLCδ4 in the nucleus, and PLCγ1 in the cytoplasm. By silencing each isoform, we observed that they all affected glucose-induced insulin release both at basal and high glucose concentrations. To elucidate the molecular basis of PLC regulation, we focused on peroxisome proliferator-activated receptor-γ (PPARγ), a nuclear receptor transcription factor that regulates genes critical to β-cell maintenance and functions. Silencing of PLCβ1 and PLCδ4 resulted in a decrease in the PPARγ mRNA level. By means of a PPARγ-promoter-luciferase assay, the decrease could be attributed to a PLC action on the PPARγ-promoter region. The effect was specifically observed on silencing of the nuclear and not the cytoplasmic PLC. These findings highlight a novel pathway by which nuclear PLCs affect insulin secretion and identify PPARγ as a novel molecular target of nuclear PLCs.

Detailed protocol for evaluation of dynamic perifusion of human islets to assess β-cell function

The definitive measure of β-cell quality in an islet is the measurement of β-cell function, i.e., the ability of the islets to release insulin in a controlled manner in response to minute changes in ambient glucose levels. Continuous flow or dynamic perifusion of the solution containing glucose and secretagogues through the islets is the most accurate assessment of regulated insulin release in vitro. Here, we describe in detail a low cost, mini-perifusion system that can be adapted to any laboratory to assess islet function by examining dynamic insulin release in response to elevated glucose concentrations and addition of secretagogues. Human islets with purity > 80% and viability > 90% were perifused with low glucose (1 mM) and subsequently challenged with high glucose (16.8 mM ± KCl, 25 mM). A prototypical biphasic response to elevated glucose concentrations was observed with an average 8-fold (above basal) increase in insulin concentration at peak values. Similarly, perifusion with carbachol or exendin-4 (Byetta) with glucose (6 mM) resulted in 1.32- and 1.35-fold increase in insulin secretion above basal. Islets could be maintained in the perifusion apparatus and continued to respond to glucose for up to 3 h. At minimal financial cost and technical expertise, this apparatus can be set-up in any biological laboratory to evaluate regulated hormone release from many cell types in less than 6 h. This will allow other laboratories to measure insulin responses to their drug or modifier of interest in vitro, in a manner that better approximates islet function in vivo.

Physiologic implications of phosphoinositides and phospholipase C in the regulation of insulin secretion

The secretion of insulin from the pancreatic beta-cell must be commensurate to satisfy the insulin requirements of the organism. This cell has a great flexibility to meet these requirements which are increased not only by the ingestion of nutrients (increase of plasma glucose) but also by the sensitivity of target tissues to insulin as well. The insulin secretion is a complex biochemical event regulated by a host of potential second messenger molecules acting alone or in concert. These events include the cation calcium, which gains access to the beta-cell via the opening of voltage-regulated channels, cAMP and phosphoinositide-derived second messenger molecules, generated as a consequence of phospholipase C (PLC) activation. In this review, we focused on phosphoinositides, PLC/Phosphokinase C (PKC) and phosphatidylinositol 3-kinase (PI3K) cascade in the regulation of insulin secretion. We also described our studies on the mechanism of the beta-cell desensitization using perifused islets. It is suggested that a failure of the signaling events contribute to the pathogenesis of diabetes in which the beta-cell can no longer secrete the required amounts of insulin. It has been observed that chronic exposure to high glucose desensitizes the beta-cells to subsequent stimulation. We suggested that the failure of PLC activation can be attributed in the impairment of insulin secretion by chronic sustained glucose exposure. It may contribute to the vicious circle of impaired insulin secretion leading up to diabetes.

Novel insights into glucocorticoid-mediated diabetogenic effects: towards expansion of therapeutic options?

At pharmacological concentrations, glucocorticoids (GCs) display potent anti-inflammatory effects, and are therefore frequently prescribed by physicians to treat a wide variety of diseases. Despite excellent efficacy, GC therapy is hampered by their notorious metabolic side effect profile. Chronic exposure to increased levels of circulating GCs is associated with central adiposity, dyslipidaemia, skeletal muscle wasting, insulin resistance, glucose intolerance and overt diabetes. Remarkably, many of these side-effects of GC treatment resemble the various components of the metabolic syndrome (MetS), in which indeed subtle disturbances in the hypothalamic-pituitary-adrenal (HPA) axis and/or increased tissue sensitivity to GCs have been reported. Recent developments have led to renewed interest in the mechanisms of GC's diabetogenic effects. First, 'selective dissociating glucocorticoid receptor (GR) ligands', which aim to segregate GC's anti-inflammatory and metabolic actions, are currently being developed. Second, at present, selective 11beta-hydroxysteroid dehydrogenase type 1 (11beta-HSD1) inhibitors, which may reduce local GC concentrations by inhibiting cortisone to cortisol conversion, are evaluated in clinical trials as a novel treatment modality for the MetS. In this review, we provide an update of the current knowledge on the mechanisms that underlie GC-induced dysmetabolic effects. In particular, recent progress in research into the role of GCs in the pathogenesis of insulin resistance and beta-cell dysfunction will be discussed.

Oscillatory control of insulin secretion

Pancreatic beta-cells possess an inherent ability to generate oscillatory signals that trigger insulin release. Coordination of the secretory activity among beta-cells results in pulsatile insulin secretion from the pancreas, which is considered important for the action of the hormone in the target tissues. This review focuses on the mechanisms underlying oscillatory control of insulin secretion at the level of the individual beta-cell. Recent studies have demonstrated that oscillations of the cytoplasmic Ca(2+) concentration are synchronized with oscillations in beta-cell metabolism, intracellular cAMP concentration, phospholipase C activity and plasma membrane phosphoinositide lipid concentrations. There are complex interdependencies between the different messengers and signalling pathways that contribute to amplitude regulation and shaping of the insulin secretory response to nutrient stimuli and neurohormonal modulators. Several of these pathways may be important pharmacological targets for improving pulsatile insulin secretion in type 2 diabetes.

Glucocorticoid receptor interaction with TrkB promotes BDNF-triggered PLC-gamma signaling for glutamate release via a glutamate transporter

An increase in glucocorticoid levels and down-regulation of BDNF (brain-derived neurotrophic factor) are supposed to be involved in the pathophysiology of depressive disorders. However, possible crosstalk between glucocorticoid- and BDNF-mediated neuronal functions in the CNS has not been elucidated. Here, we examined whether chronic glucocorticoid exposure influences BDNF-triggered intracellular signaling for glutamate release via a glutamate transporter. We found that chronic exposure to dexamethasone (DEX, a synthetic glucocorticoid) suppressed BDNF-induced glutamate release via weakening the activation of the PLC-gamma (phospholipase C-gamma)/Ca(2+) system in cultured cortical neurons. We demonstrated that the GR (glucocorticoid receptor) interacts with receptor tyrosine kinase for BDNF (TrkB). Following DEX treatment, TrkB-GR interaction was reduced due to the decline in GR expression. Corticosterone, a natural glucocorticoid, also reduced TrkB-GR interaction, BDNF-stimulated PLC-gamma, and BDNF-triggered glutamate release. Interestingly, BDNF-dependent binding of PLC-gamma to TrkB was diminished by DEX. SiRNA transfection to induce a decrease in endogenous GR mimicked the inhibitory action of DEX. Conversely, DEX-inhibited BDNF-activated PLC-gamma signaling for glutamate release was recovered by GR overexpression. We propose that TrkB-GR interaction plays a critical role in the BDNF-stimulated PLC-gamma pathway, which is required for glutamate release, and the decrease in TrkB-GR interaction caused by chronic exposure to glucocorticoids results in the suppression of BDNF-mediated neurotransmitter release via a glutamate transporter.

Biphasic insulin secretion from freshly isolated or cultured, perifused rodent islets: comparative studies with rats and mice

In the present report, we compared the insulin secretory responses of freshly isolated, perifused rat and mouse islets to glucose. Prestimulatory glucose levels were changed to assess their influence on the subsequent secretory responses. Additional studies included experiments with the incretin factor glucagon-like peptide-1 (GLP-1), the cholinergic agonist carbachol, and the alpha2 agonist epinephrine. Our findings demonstrate that under conditions where glucose (8.5-11.1 mmol/L) evokes a dramatic biphasic insulin secretory response from perifused rat islets, mouse islets exhibit little response. Increasing the prestimulatory glucose level to 8.5 mmol/L dramatically distorts subsequently measured glucose-induced insulin secretion from rat islets but allows the evocation of a modest but clear biphasic response from mouse islets in response to 30 mmol/L, but not 11.1 or 16.7 mmol/L, glucose. In the presence of a minimally effective glucose level (10 mmol/L), mouse islets remain exquisitely sensitive to the combined stimulatory effects of GLP-1 (2.5 nmol/L) plus carbachol (0.5 micromol/L) and to the inhibitory influence of epinephrine (10 nmol/L). Short-term culture of rat islets in CMRL 1066 containing 5.6 mmol/L glucose resulted in a significant decrease in the secretory response to 11.1 mmol/L glucose, whereas the same manipulation improved mouse islet responses. It is concluded that the process of collagenase isolating islets does not alter mouse islet sensitivity in any adverse way and that increasing the prestimulatory glucose level can indeed alter the pattern of insulin secretion in either a positive or negative manner depending upon the species being investigated. Prior short-term culture of rodent islets differentially affects secretion from these 2 species.