The prevalence of GAD antibodies in Korean women with gestational diabetes mellitus and their clinical characteristics during and after pregnancy

BACKGROUND

We conducted this research in order to evaluate the prevalence of glutamic acid decarboxylase antibodies (GADA) in Korean women with gestational diabetes mellitus (GDM), to identify the clinical characteristics of these women, and to gauge the prevalence of diabetes among them after childbirth.

METHODS

We studied 887 Korean women with GDM who were screened for GADA, and assessed their antepartum clinical characteristics and the outcomes of their pregnancies. At 6 weeks' postpartum, 75 g oral glucose tolerance tests were performed to determine the diabetic status of GDM women with GADA.

RESULTS

The prevalence of GADA in Korean women with GDM was 1.7%. Plasma glucose levels at 0- and 3-h during oral glucose tolerance tests were significantly different between GADA-positive and GADA-negative women with GDM. There were no significant differences between them in terms of age, body mass index, family history of diabetes, fasting insulin, and lipid profiles. However, GADA-positive women with GDM required insulin treatment during pregnancy more frequently than GADA-negative patients. The development of diabetes at early postpartum was significantly higher in GADA-positive women with GDM than those who were GADA negative.

CONCLUSIONS

The prevalence of GADA in Korean women with GDM was low. However, GADA-positive women with GDM are more susceptible to subsequently developing type 1 diabetes, even in the early postpartum period. Long-term follow up studies and intervention to prevent type 1 diabetes among GADA-positive GDM women are needed.

Retinoids synergize with insulin to induce hepatic Gck expression

Hepatic GK (glucokinase) plays a key role in maintaining glucose homoeostasis. Many stimuli regulate GK activity by controlling its gene transcription. We hypothesized that endogenous lipophilic molecules modulate hepatic Gck expression. Lipophilic molecules were extracted from rat livers, saponified and re-constituted as an LE (lipophilic extract). LE synergized with insulin to induce primary hepatocyte, but not beta-cell, Gck expression in an SREBP-1c (sterol-regulatory-element-binding protein-1c)-independent manner. The dramatic induction of Gck mRNA resulted in a significant increase in GK activity. Subsequently, the active molecules were identified as retinol and retinal by MS after the purification of the active LE fractions. Retinoids synergized with insulin to induce Gck expression by the activation of both RAR [RA (retinoic acid) receptor] and RXR (retinoid X receptor). Inhibition of RAR activation completely abolished the effect of retinal. The hepatic GK specific activity and Gck mRNA levels of Zucker lean rats fed with a VAD [VA (vitamin A)-deficient] diet were significantly lower than those of rats fed with VAS (VA-sufficient) diet. Additionally, the hepatic Gck mRNA expression of Sprague-Dawley rats fed with a VAD diet was lower than that of rats fed with VA-marginal, -adequate or -supplemented diets. The reduced expression of Gck mRNA was increased after an intraperitoneal dose of RA in VAD rats. Furthermore, an intravenous injection of RA rapidly raised hepatic Gck expression in rats fed with a VAS control diet. Understanding the underlying mechanism that mediates the synergy may be helpful for developing a treatment strategy for patients with diabetes.

Therapy with GAD in diabetes

The enzyme glutamic acid decarboxylase (GAD) is of great importance for the neurotransmission in the central nervous system, and therefore of interest for treatment of pain and neurological disease. However, it is also released in pancreas although its role is not quite clear. GAD is a major auto-antigen in the process leading to type 1 diabetes with both a clear cell-mediated immune response to GAD and auto-antibodies to GAD (GADA), which can be used as a predictor of diabetes. Administration of the isoform GAD65 can prevent autoimmune destruction of pancreatic beta cells in non-obese diabetic (NOD) mice and the subsequent need for exogenous insulin replacement. In Phase I and II studies an alum-formulated vaccine (Diamyd) has shown to be safe, and in a dose-finding study in Latent Autoimmune Diabetes in Adults (LADA) patients 20-microg was given subcutaneously one month apart indicating preservation of residual insulin secretion. A double-blind randomized Phase II trial in 70 patients (10-18 years old) with recent-onset type 1 diabetes showed significant preservation of residual insulin secretion and a GAD-specific immune response, both humoral and cell-mediated, but no treatment-related adverse events. With this promising background further studies are on their way, both intervention in newly diagnosed type 1 diabetic patients, and trials to prevent the disease.

Survey of the human pancreatic beta-cell G1/S proteome reveals a potential therapeutic role for cdk-6 and cyclin D1 in enhancing human beta-cell replication and function in vivo

OBJECTIVES

To comprehensively inventory the proteins that control the G1/S cell cycle checkpoint in the human islet and compare them with those in the murine islet, to determine whether these might therapeutically enhance human beta-cell replication, to determine whether human beta-cell replication can be demonstrated in an in vivo model, and to enhance human beta-cell function in vivo.

RESEARCH DESIGN AND METHODS

Thirty-four G1/S regulatory proteins were examined in human islets. Effects of adenoviruses expressing cdk-6, cdk-4, and cyclin D1 on proliferation in human beta-cells were studied in both in vitro and in vivo models.

RESULTS

Multiple differences between murine and human islets occur, most strikingly the presence of cdk-6 in human beta-cells versus its low abundance in the murine islet. Cdk-6 and cyclin D1 in vitro led to marked activation of retinoblastoma protein phosphorylation and cell cycle progression with no induction of cell death. Human islets transduced with cdk-6 and cyclin D1 were transplanted into diabetic NOD-SCID mice and markedly outperformed native human islets in vivo, maintaining glucose control for the entire 6 weeks of the study.

CONCLUSIONS

The human G1/S proteome is described for the first time. Human islets are unlike their rodent counterparts in that they contain easily measurable cdk-6. Cdk-6 overexpression, alone or in combination with cyclin D1, strikingly stimulates human beta-cell replication, both in vitro as well as in vivo, without inducing cell death or loss of function. Using this model, human beta-cell replication can be induced and studied in vivo.

The role of immunomodulation therapy in autoimmune diabetes

Type 1 diabetes (T1DM) is characterized by loss of virtually all endogenous insulin secretion. If residual insulin secretion is preserved, this will lead to improved metabolic balance, less acute and late complications, improved quality of life, and, in case of pronounced improvement of residual insulin secretion, complete remission and even cure of the disease. Immune suppression or immune modulation have been demonstrated as a proof of principle to stop/decrease the destructive process and thereby preserve beta-cell function. Several methods to save residual beta-cell function have been tried for more than three decades with little or no evidence of efficacy. Positive effects have been seen mainly in adult patients but have been minimal or absent in children with diabetes. Furthermore, the safety of these immune interventions and/or their benefit to risk relationships have not been found to justify clinical use. More specific immune modulation with anti-CD3 monoclonal antibodies has resulted in more encouraging postponement of C-peptide decline, but with frequent and serious adverse effects. Still more promising are the autoantigen therapies, of which glutamic acid decarboxylase (GAD) vaccination has shown significant preservation of residual insulin secretion in 10-18-year-old type 1 diabetes patients with recent onset. Efficacy was most impressive in the subgroup of patients with diabetes of short duration (<3 months). The treatment was simple, well tolerated, and showed no treatment-related adverse events. If these results can be confirmed, there is a realistic hope that GAD vaccination, perhaps in combination with vaccinations with other autoantigens and/or other therapies, will result in remission for some patients. The prospects of cure and prevention of T1DM will become less remote.

HIV-1 protease inhibitor induced oxidative stress suppresses glucose stimulated insulin release: protection with thymoquinone

The highly active anti-retroviral therapy (HAART) regimen has considerably reduced the mortality rate in HIV-1 positive patients. However, long-term exposure to HAART is associated with a metabolic syndrome manifesting cardiovascular dysfunction, lipodystrophy, and insulin resistance syndrome (IRS). The inclusion of HIV-1 protease inhibitors (PIs) in HAART has been linked to the induction of IRS. Although several molecular mechanisms of PI-induced effects on insulin action have been postulated, the deleterious effects of PIs on insulin production by pancreatic beta-cells have not been fully investigated and therapeutic strategies to ameliorate insulin dysregulation at this level have not been targeted. The present study showed that exposure to several different PIs, nelfinavir (5-10 microM), saquinavir (5-10 microM) and atazanavir (8-20 microM), decreases glucose stimulated insulin secretion from rat pancreatic beta-cells (INS-1). Nelfinavir significantly increased reactive oxygen species (ROS) generation and suppressed cytosolic, but not mitochondrial superoxide dismutase (SOD) levels. Nelfinvair also decreased both glutathione and ATP and increased UCP2 levels in these cells. Simultaneous treatment with thymoquinone (TQ) (2.5 microM), an active ingredient of black seed oil, significantly inhibited the effect of nelfinavir on augmented ROS production and suppressed SOD levels. Both TQ and black seed oil exposure increased glucose stimulated insulin secretion and ameliorated the suppressive effect of nelfinavir. The present findings imply a direct role of ROS in PI induced deleterious effects on pancreatic beta-cells. Our findings also suggest that TQ may be used as a potential therapeutic agent to normalize the dysregulated insulin production observed in HAART treated patients.

A beta cell-specific knockout of hormone-sensitive lipase in mice results in hyperglycaemia and disruption of exocytosis

AIMS/HYPOTHESIS

The enzyme hormone-sensitive lipase (HSL) is produced and is active in pancreatic beta cells. Because lipids are known to play a crucial role in normal control of insulin release and in the deterioration of beta cell function, as observed in type 2 diabetes, actions of HSL in beta cells may be critical. This notion has been addressed in different lines of HSL knockout mice with contradictory results.

METHODS

To resolve this, we created a transgenic mouse lacking HSL specifically in beta cells, and characterised this model with regard to glucose metabolism and insulin secretion, using both in vivo and in vitro methods.

RESULTS

We found that fasting basal plasma glucose levels were significantly elevated in mice lacking HSL in beta cells. An IVGTT at 12 weeks revealed a blunting of the initial insulin response to glucose with delayed elimination of the sugar. Additionally, arginine-stimulated insulin secretion was markedly diminished in vivo. Investigation of the exocytotic response in single HSL-deficient beta cells showed an impaired response to depolarisation of the plasma membrane. Beta cell mass and islet insulin content were increased, suggesting a compensatory mechanism, by which beta cells lacking HSL strive to maintain normoglycaemia.

CONCLUSIONS/INTERPRETATION

Based on these results, we suggest that HSL, which is located in close proximity of the secretory granules, may serve as provider of a lipid-derived signal essential for normal insulin secretion.

Synergistic induction of heme oxygenase-1 by the components of the antioxidant supplement Protandim

Protandim is an antioxidant supplement that consists of five ingredients, namely, ashwagandha, bacopa extract, green tea extract, silymarin, and curcumin, each with known therapeutic properties. Protandim was formulated with the objective of combining multiple phytochemicals at low nontoxic doses to gain synergy among them. A recent clinical study demonstrated the in vivo antioxidant effects of Protandim (S.K. Nelson et al., 2006, Free Radic. Biol. Med. 40, 341-347). The objective of the present study was to determine if the components of Protandim induce heme oxygenase-1 (HO-1) in a synergistic manner in cultured MIN6 cells, a mouse beta-cell line, and in SK-N-MC cells, a human neuroblastoma cell line. When the components of Protandim were tested alone at low doses, curcumin showed minimal induction, whereas the others were unable to induce the HO-1 promoter, assayed by transient transfection. All components together, however, produced a strongly synergistic induction of around three- to ninefold in a dose-dependent manner, greatly exceeding the sum of the parts. Similar findings were obtained for the expression of HO-1 at the mRNA and protein levels. Protandim-mediated HO-1 induction involved the presence of ARE sites in the HO-1 promoter and nuclear translocalization of the transcription factor Nrf2, which binds to ARE sites. The involvement of multiple signaling pathways, including PI3-kinase/Akt, p38MAPK, and PKCdelta, in HO-1 induction seems to be the probable mechanism of synergy between the components of Protandim. There were significant increases in the levels of total glutathione in Protandim-treated cells. These findings suggest that the use of a combination of phytochemicals may be an efficient method for the induction of antioxidant enzymes.

Islet adaptive changes to fructose-induced insulin resistance: beta-cell mass, glucokinase, glucose metabolism, and insulin secretion

Beta-cell mass, hexokinase/glucokinase (HK/GK) activity, glucose metabolism and insulin secretion were studied in the islets of rats with fructose-induced insulin resistance (IR). Normal male Wistar rats were fed a standard commercial diet and water without (control, C) or with 10% fructose-rich diet (FRD) for 3 weeks. Blood glucose (strips), triglyceride (commercial kit), and insulin (RIA) levels were measured at the time of death. Glucose-induced insulin release, glucose metabolism ((14)CO(2) and (3)H(2)O production from D-[U-(14)C]- and D-[5-(3)H]-glucose) and HK/GK activity (G-6-P production), transcription (RT-PCR), protein expression (Western blot), and cellular compartmentalization were measured in isolated islets (collagenase digestion). FRD rats presented normoglycemia but impaired glucose tolerance, hypertriglyceridemia, hyperinsulinemia, and increased HOMA-IR index. In these rats, beta-cell mass decreased significantly by 33%, with a 44% increase in the percentage of apoptotic cells. Glucose-induced insulin release and islet glucose metabolism were higher in FRD rats. While GK activity (total and cytosolic fraction) and protein expression were significantly higher in FRD islets, HK showed no change in any of these parameters. Our results demonstrate that the changes induced by dietary-induced IR upon beta-cell function and mass are strongly conditional on the nutrient model used. In our model (intact animals with impaired glucose tolerance), GK activity increases through mechanisms previously shown only in vitro or under highly hyperglycemic conditions. Such an increase plays a pivotal role in the adaptive increased release of insulin in response to IR, even in the presence of marked beta-cell mass reduction.

The AMP-regulated kinase family: enigmatic targets for diabetes therapy

AMP-activated protein kinase (AMPK) is a widely conserved Ser/Thr-specific protein kinase, homologous to Saccharomyces cerevisiae Snf1, and involved in nutrient sensing in lower organisms. In 2003, we reviewed the role of this enzyme in glucose homeostasis in mammals [Rutter, G.A., daSilvaXavier, G., Leclerc, I., 2003. Roles of 5'-AMP-activated protein kinase (AMPK) in mammalian glucose homoeostasis. Biochem. J. 375 (Pt 1), 1-16]. In the subsequent 5 years, dramatic strides have taken place in our understanding of the role of AMPK in the control of whole body metabolic homeostasis, the regulation of the enzyme by upstream kinases, and its molecular structure. These new studies and earlier work arguably propel AMPK, and perhaps related family members into a "super league" of potential therapeutic targets for maladies including diabetes, cancer, heart disease, and obesity. Here, we survey some of these recent advances, focussing on the role of this and related enzymes in the control of pancreatic beta-cell function and glucose homeostasis.

Regulation of beta cell replication

Beta cell mass, at any given time, is governed by cell differentiation, neogenesis, increased or decreased cell size (cell hypertrophy or atrophy), cell death (apoptosis), and beta cell proliferation. Nutrients, hormones and growth factors coupled with their signalling intermediates have been suggested to play a role in beta cell mass regulation. In addition, genetic mouse model studies have indicated that cyclins and cyclin-dependent kinases that determine cell cycle progression are involved in beta cell replication, and more recently, menin in association with cyclin-dependent kinase inhibitors has been demonstrated to be important in beta cell growth. In this review, we consider and highlight some aspects of cell cycle regulation in relation to beta cell replication. The role of cell cycle regulation in beta cell replication is mostly from studies in rodent models, but whether these findings can be extended to human beta cells remains to be shown.

Emerging roles for the ubiquitin-proteasome system and autophagy in pancreatic beta-cells

Protein degradation in eukaryotic cells is mediated primarily by the ubiquitin-proteasome system and autophagy. Turnover of protein aggregates and other cytoplasmic components, including organelles, is another function attributed to autophagy. The ubiquitin-proteasome system and autophagy are essential for normal cell function but under certain pathological conditions can be overwhelmed, which can lead to adverse effects in cells. In this review we will focus primarily on the insulin-producing pancreatic beta-cell. Pancreatic beta-cells respond to glucose levels by both producing and secreting insulin. The inability of beta-cells to secrete sufficient insulin is a major contributory factor in the development of type 2 diabetes. The aim of this review is to examine some of the crucial roles of the ubiquitin-proteasome system and autophagy in normal pancreatic beta-cell function and how these pathways may become dysfunctional under pathological conditions associated with metabolic syndromes.

The role of pyruvate carboxylase in insulin secretion and proliferation in rat pancreatic beta cells

AIMS/HYPOTHESIS

Pyruvate carboxylase (PC) or pyruvate dehydrogenase (PDH) is required to transfer carbons from pyruvate into the Krebs cycle. PC activity is preserved in the islets of obese animals, but it is reduced in the islets of animal models of type 2 diabetes, suggesting that PC is important in beta cell adaptation to insulin resistance and that PC reduction may lead to beta cell failure.

METHODS

To confirm the significance of PC, we first lowered activity using Pc (now known as Pcx) small interfering RNA (siRNA) in INS-1 cells and in dispersed rat islet cells. Second, we overexpressed PC in INS-1 cells, and third, we inhibited PDH by overexpressing the gene encoding pyruvate dehydrogenase kinase 4 (Pdk4) in INS-1 cells.

RESULTS

Treatment of INS-1 cells or dispersed rat islet cells with Pc siRNA resulted in a significant reduction in insulin secretion in both cell types and reduced proliferation in INS-1 cells. This treatment also reduced the content of oxaloacetate, malate and ATP, as well as the NADPH:NADP(+) ratio and activity of the pyruvate-malate shuttle. Overexpression of PC in INS-1 cells led to an elevation of insulin secretion and cell proliferation, whereas inhibition of PDH activity by overexpressing Pdk4 in INS-1 cells did not reduce insulin secretion.

CONCLUSIONS/INTERPRETATION

Our findings indicate that the PC pathway in beta cells might play a key role in pyruvate metabolism, insulin secretion and cell proliferation.

Beneficial effect of Flos magnoliae extract on multiple low dose streptozotocin-induced type 1 diabetes development and cytokine-induced beta-cell damage

In the present study, Flos magnoliae extract (FME) was evaluated to determine if it could protect pancreatic beta-cells against multiple low dose streptozotocin (MLDS) and interleukin-1beta and interferon-gamma. Injection of mice with MLDS resulted in hyperglycemia and hypoinsulinemia, which was confirmed by immunohistochemical staining. However, the induction of diabetes by MLDS was completely prevented when mice were pretreated with FME. FME also effectively protected beta-cells against cytokine toxicity, which was demonstrated by an increase in the viability of rat insulinoma RINm5F cells and by preserved insulin secreting responses to glucose in isolated rat islets. Moreover, cytokine-induced nitric oxide production and iNOS mRNA and protein expression were significantly reduced in RINm5F cells and islets that were preincubated with FME. The molecular mechanism by which FME inhibits iNOS gene expression in in vitro and in vivo appears to involve inhibition of NF-kappaB activation. Taken together, these results reveal the possible therapeutic value of FME for the prevention of type 1 diabetes progression.

A novel model of insulin-dependent diabetes with renal and retinal lesions by transgenic expression of CaMKIIalpha (Thr286Asp) in pancreatic beta-cells

BACKGROUND

The activation of Ca2+/calmodulin-dependent protein kinase II (CaMKII) in pancreatic beta-cells has been thought to play a central role in Ca2+-mediated insulin secretion. However, the physiological and pathological significance of CaMKII activation in pancreatic beta-cells has never been investigated in vivo.

METHODS

We generated transgenic (TG) mice overexpressing the constitutively active-type CaMKIIalpha (Thr286Asp) in beta-cells. The mice were extensively examined histologically and biochemically. Time-course changes of blood glucose, haemoglobin A1C and insulin were also determined.

RESULTS

Western blot and immunohistochemical analyses showed overexpression of CaMKIIalpha proteins in pancreatic beta-cells of TG mice. All TG mice developed severe hypoinsulinaemic diabetes by P28. In vivo BrdU labelling analysis revealed that cell proliferation in TG islets is severely impaired. Immunohistochemical examination revealed accumulations of NF-kappaB in nuclei of TG beta-cells at P21, which are associated with DNA laddering, a hallmark of apoptosis. At P28, pancreatic and serum insulin levels were both significantly (p < 0.05) lower in TG mice (0.037 +/- 0.005 ng/microg and 0.50 +/- 0.01 ng/mL) than in wild-type mice (0.997 +/- 0.093 ng/microg and 2.50 +/- 0.22 ng/mL). TG mice at P140 showed enlargement of kidney, mesangial expansion and glomerulosclerosis, which are associated with urinary albumin excretion. TG mice at P140-P168 developed severe retinal lesions such as disrupted ganglion cells and showed a flat pattern in electroretinography.

CONCLUSIONS

The TG mice established herein will be valuable as a novel model of severe insulin-dependent diabetes accompanied by an early progression of diabetic micro-vascular complications.

Dominance of cytokine- over FasL-induced impairment of the mitochondrial transmembrane potential (Deltapsim) in the pancreatic beta-cell line NIT-1

Mitochondria of pancreatic beta-cells are potential targets of intrinsic and extrinsic apoptotic pathways in the autoimmune pathogenesis of type 1 diabetes. We aimed to investigate whether cytokine- and FasLigand (FasL)-induced apoptosis is associated with impaired mitochondrial transmembrane potential (Deltapsim) in the pancreatic beta-cell line NIT-1. NIT-1 cells were exposed to the interleukin-1beta/interferon-gamma (IL-1beta/IFN-gamma) cytokine combination to induce apoptosis in vitro. Low concentrations of cytokines resulted in Deltapsim impairment, and increasing concentrations had only a minor additional effect. Treatment with the inducible nitric oxide synthase (iNOS) inhibitor Nw-nitro-L-arginine methyl ester hydrochloride (L-NAME) prevented cytokine-mediated Deltapsim impairment, implying that cytokines affect Deltapsim via nitric oxide. The broad-spectrum caspase inhibitor Z-VAD(Ome)-FMK (ZVAD) revealed dichotomic actions. In the presence of ZVAD, cytokine-induced nitrite generation was increased but cell death and Deltapsim impairment were reduced. Deltapsim impairment was also reduced by inhibitors of caspases 1, 6 and 8. Induction of Fas by IL-1beta/IFN-gamma coupled with activation by Super-FasL augmented cytokine-induced cell death. We observed a clear dominance of cytokine- over FasL-induced effects on Deltapsim. Our findings show that IL-1beta/IFN-gamma cytokines have a strong effect to impair Deltaym and prime beta-cells for apoptosis via the intrinsic pathway mediated by iNOS and caspases. Furthermore, at least in NIT-1 cells, the extrinsic FasL/Fas pathway has only a minor additive effect on cytokine-induced Deltapsim impairment.

AMP-activated protein kinase agonist dose dependently improves function and reduces apoptosis in glucotoxic beta-cells without changing triglyceride levels

Prolonged hyperglycaemia leads to impaired glucose-stimulated insulin secretion (GSIS) and apoptosis in insulin-producing beta-cells. The detrimental effects have been connected with glucose-induced lipid accumulation in the beta-cell. AMP-activated protein kinase (AMPK) agonist, 5-aminoimidazole-4-carboxamide ribonucleoside (AICAR), promotes utilization of nutrient stores for energy production. It was tested how impaired GSIS and elevated apoptosis observed in insulinoma (INS)-1E cells after prolonged culture at 27 mM glucose were affected by the inclusion of 0.3 or 1 mM AICAR during culture. Glucose-induced impairment of insulin release was reverted by the inclusion of 0.3 but not 1 mM AICAR, which did not affect insulin content. The glucose-induced rise in triglyceride (TG) content observed in the cells cultured at 27 mM glucose was not altered by the inclusion of either 0.3 or 1 mM AICAR. Inclusion of 1 but not 0.3 mM AICAR during culture induced phosphorylation of AMPK and its downstream target acyl-CoA carboxylase. Phosphorylation was paralleled by reduced number of apoptotic cells and lowered expression of pro-apoptotic C/EBP homologous protein (CHOP). In conclusion, AICAR dose dependently improves beta-cell function and reduces apoptosis in beta-cells exposed to prolonged hyperglycaemia without changing TG levels.

Beta-cell alpha-ketoglutarate hydroxylases may acutely participate in insulin secretion

The presence of Fe(II) alpha-ketoglutarate hydroxylases in rat and human pancreatic islets and INS-1 832/13 cells was demonstrated with the reverse transcriptase polymerase chain reaction (PHD1, 2, and 3; lysyl hydroxylases 1, 2, and 3; and phytanoyl-coenzyme A hydroxylase were seen) and/or immunoblotting (high levels of proline hydroxylase P4Halpha1, PHD2, and PHD4 and low levels of PHD2 and PHD3 in human islets, and high levels of PHD2 in rat islets and INS-1 cells were seen). Prolyl hydroxylase enzyme activity in INS-1 832/13 cells was purified with polyproline affinity chromatography. Inhibitors of alpha-ketoglutarate hydroxylases lowered glucose-induced and leucine-plus-glutamine-induced insulin release in rat pancreatic islets, suggesting that there may be acute unknown effects of alpha-ketoglutarate hydroxylases in insulin secretion. It is possible that an increase in mitochondrially generated alpha-ketoglutarate derived from insulin secretagogue carbon and translocated to the cytosol may be part of the signal for insulin secretion.

Fructose-1,6-bisphosphatase overexpression in pancreatic beta-cells results in reduced insulin secretion: a new mechanism for fat-induced impairment of beta-cell function

OBJECTIVE

Fructose-1,6-bisphosphatase (FBPase) is a gluconeogenic enzyme that is upregulated in islets or pancreatic beta-cell lines exposed to high fat. However, whether specific beta-cell upregulation of FBPase can impair insulin secretory function is not known. The objective of this study therefore is to determine whether a specific increase in islet beta-cell FBPase can result in reduced glucose-mediated insulin secretion.

RESEARCH DESIGN AND METHODS

To test this hypothesis, we have generated three transgenic mouse lines overexpressing the human FBPase (huFBPase) gene specifically in pancreatic islet beta-cells. In addition, to investigate the biochemical mechanism by which elevated FBPase affects insulin secretion, we made two pancreatic beta-cell lines (MIN6) stably overexpressing huFBPase.

RESULTS

FBPase transgenic mice showed reduced insulin secretion in response to an intravenous glucose bolus. Compared with the untransfected parental MIN6, FBPase-overexpressing cells showed a decreased cell proliferation rate and significantly depressed glucose-induced insulin secretion. These defects were associated with a decrease in the rate of glucose utilization, resulting in reduced cellular ATP levels.

CONCLUSIONS

Taken together, these results suggest that upregulation of FBPase in pancreatic islet beta-cells, as occurs in states of lipid oversupply and type 2 diabetes, contributes to insulin secretory dysfunction.

Anti-proliferative effect of pro-inflammatory cytokines in cultured beta cells is associated with extracellular signal-regulated kinase 1/2 pathway inhibition: protective role of glucagon-like peptide -1

Pancreatic beta-cell homeostasis is a balance between programmed cell death (apoptosis) and regeneration. Although autoimmune diabetes mellitus type 1 (DM1) is the most-studied cause of beta-cell mass loss by pro-inflammatory cytokine-induced apoptosis, influences of a pro-inflammatory environment on beta-cell regenerative response have been poorly studied. In this study, we assess the anti-proliferative effect of pro-inflammatory cytokines and glucose concentration on rat pancreatic beta cells and the potential protective role of glucagon-like peptide (GLP-1). Apoptotic and proliferating islet cells were stained using the DeadEnd Fluorimetric TUNEL System and 5-bromo-2'-deoxyuridine label respectively, in the presence-absence of varying concentrations of glucose, pro-inflammatory cytokines, and GLP-1. The potential signaling pathways involved were evaluated by western blot. Considerable anti-proliferative effects of pro-inflammatory cytokines interleukin (IL)-1beta, interferon (IFN)-gamma, and tumour necrosis factor-alpha (TNF-alpha) were observed. The effects were synergistic and independent of glucose concentration, and appeared to be mediated by the inhibition of extracellular signal-regulated kinase 1/2 (ERK1/2) activation, the signaling pathway involved in beta-cell replication. GLP-1 completely reversed the cytokine-induced inhibition of ERK phosphorylation and increased beta-cell proliferation threefold in cytokine-treated cultures. While pro-inflammatory cytokines reduced islet cell ERK1/2 activation and beta-cell proliferation in pancreatic islet culture, GLP-1 was capable of reversing this effect. These data suggest a possible pharmacological application of GLP-1 in the treatment of early stage DM1, to prevent the loss of pancreatic beta cells as well as to delay the development of overt diabetes.

Cytochrome P-450(17alpha) in beta-cells of rat pancreas and its local steroidogenesis

We have found cytochrome P-450(17alpha) in the islets of Langerhans of rat pancreas. Its existence coincided with that of insulin and demarcated those of glucagon and somatostatin, demonstrating the localization in beta-cells. The enzyme has not only 17alpha-hydroxylase activity but also lyase one, which is a prerequisite for androgen biosynthesis. The pancreatic microsomes converted progesterone mainly to androstenedione with a minor production of 17alpha-hydroxyprogesterone. Due to a low activity of the built-in lyase, cytochrome P-450(17alpha) requires a sufficient electron-transfer from P-450 reductase or presence of an activator to promote the C-C bond cleavage. In beta-cells, P-450 reductase was abundant and could efficiently transfer electrons to P-450(17alpha). Actually, inhibition with anti-P-450 reductase or limitation of NADPH preferentially reduced the lyase activity. Androstenedione was accumulated when its further metabolism was suppressed. We also found localization of cytochrome P-450scc and 3beta-hydroxysteroid dehydrogenase in beta-cells. These results indicate that the immediate substrate for androgen formation, progesterone, is intracellularly produced and is converted mainly to androstenedione with support by an efficient electron supply from P-450 reductase. The product was supposed to be further metabolized to the reduced derivatives such as testosterone, 5alpha-androstanedione, and dihydrotestosterone, which would act as local steroids in the islets of Langerhans.

The intrauterine metabolic environment modulates the gene expression pattern in fetal rat islets: prevention by maternal taurine supplementation

AIMS/HYPOTHESIS

Events during fetal life may in critical time windows programme tissue development leading to organ dysfunction with potentially harmful consequences in adulthood such as diabetes. In rats, the beta cell mass of progeny from dams fed with a low-protein (LP) diet during gestation is decreased at birth and metabolic perturbation lasts through adulthood even though a normal diet is given after birth or after weaning. Maternal and fetal plasma taurine levels are suboptimal. Maternal taurine supplementation prevents these induced abnormalities. In this study, we aimed to reveal changes in gene expression in fetal islets affected by the LP diet and how taurine may prevent these changes.

METHODS

Pregnant Wistar rats were fed an LP diet (8% [wt/wt] protein) supplemented or not with taurine in the drinking water or a control diet (20% [wt/wt] protein). At 21.5 days of gestation, fetal pancreases were removed, digested and cultured for 7 days. Neoformed islets were collected and transcriptome analysis was performed.

RESULTS

Maternal LP diet significantly changed the expression of more than 10% of the genes. Tricarboxylic acid cycle and ATP production were highly targeted, but so too were cell proliferation and defence. Maternal taurine supplementation normalised the expression of all altered genes.

CONCLUSIONS/INTERPRETATION

Development of the beta cells and particularly their respiration is modulated by the intrauterine environment, which may epigenetically modify expression of the genome and programme the beta cell towards a pre-diabetic phenotype. This mis-programming by maternal LP diet was prevented by early taurine intervention.

Rab GTPase-activating protein AS160 is a major downstream effector of protein kinase B/Akt signaling in pancreatic beta-cells

OBJECTIVE

Protein kinase B/Akt plays a central role in beta-cells, but little is known regarding downstream Akt substrates in these cells. Recently, Rab GTPase-activating protein AS160, a substrate of Akt, was shown to be involved in insulin modulation of GLUT4 trafficking in skeletal muscle and adipose tissue. The aim of this study was to investigate the expression and potential role of AS160 in beta-cells.

RESEARCH DESIGN AND METHODS

AS160 mRNA expression was measured in mouse and human islets and fluorescence-activated cell sorted beta-cells and compared in islets from control subjects versus individuals with type 2 diabetes. For knockdown experiments, transformed mouse insulin-secreting MIN6B1 cells were transfected with pSUPER-GFP plasmid encoding a small hairpin RNA against insulin receptor substrate (IRS)-2, AS160, or a negative control. Primary mouse islet cells were transfected with AS160 small interfering RNA.

RESULTS

AS160 was expressed in human and mouse pancreatic beta-cells and phosphorylated after glucose stimulation. AS160 mRNA expression was downregulated in pancreatic islets from individuals with type 2 diabetes. In MIN6B1 cells, glucose induced phosphorylation of Akt and AS160, and this was mediated by insulin receptor/IRS-2/phosphatidylinositol 3-kinase independently of changes in cytosolic Ca(2+). Knockdown of AS160 resulted in increased basal insulin secretion, whereas glucose-stimulated insulin release was abolished. Furthermore, beta-cells with decreased AS160 showed increased apoptosis and loss of glucose-induced proliferation.

CONCLUSIONS

This study shows for the first time that AS160, previously recognized as a key player in insulin signaling in skeletal muscle and adipose tissue, is also a major effector of protein kinase B/Akt signaling in the beta-cell.

Exendin-4 protects beta-cells from interleukin-1 beta-induced apoptosis by interfering with the c-Jun NH2-terminal kinase pathway

OBJECTIVE

The pro-inflammatory cytokine interleukin-1 beta (IL-1 beta) generates pancreatic beta-cells apoptosis mainly through activation of the c-Jun NH(2)-terminal kinase (JNK) pathway. This study was designed to investigate whether the long-acting agonist of the hormone glucagon-like peptide 1 (GLP-1) receptor exendin-4 (ex-4), which mediates protective effects against cytokine-induced beta-cell apoptosis, could interfere with the JNK pathway.

RESEARCH DESIGN AND METHODS

Isolated human, rat, and mouse islets and the rat insulin-secreting INS-1E cells were incubated with ex-4 in the presence or absence of IL-1 beta. JNK activity was assessed by solid-phase JNK kinase assay and quantification of c-Jun expression. Cell apoptosis was determined by scoring cells displaying pycnotic nuclei.

RESULTS

Ex-4 inhibited induction of the JNK pathway elicited by IL-1 beta. This effect was mimicked with the use of cAMP-raising agents isobutylmethylxanthine and forskolin and required activation of the protein kinase A. Inhibition of the JNK pathway by ex-4 or IBMX and forskolin was concomitant with a rise in the levels of islet-brain 1 (IB1), a potent blocker of the stress-induced JNK pathway. In fact, ex-4 as well as IBMX and forskolin induced expression of IB1 at the promoter level through cAMP response element binding transcription factor 1. Suppression of IB1 levels with the use of RNA interference strategy impaired the protective effects of ex-4 against apoptosis induced by IL-1 beta.

CONCLUSIONS

The data establish the requirement of IB1 in the protective action of ex-4 against apoptosis elicited by IL-1 beta and highlight the GLP-1 mimetics as new potent inhibitors of the JNK signaling induced by cytokines.

Anti-Apoptotic Effects of Arachidonic Acid and Prostaglandin E2 in Pancreatic β-Cells

BACKGROUND/AIMS

The polyunsaturated fatty acid arachidonic acid (AA) has been implicated in beta-cell defence mechanisms and prostaglandin (PG) products of cyclooxygenase (COX) 2 action confer resistance to alloxan-induced apoptosis in insulin-secreting RIN cells. We have now investigated the anti-apoptotic effects of AA and its metabolite, PGE(2), in the MIN6 mouse insulin-secreting beta-cell line and mouse islets.

METHODS

Apoptosis was determined in MIN6 beta-cell and mouse islet extracts by measurement of capase-3 activity, and COX2 mRNA levels were quantified by real-time RT-PCR.

RESULTS

Exposure of MIN6 cells to AA (3.1-12.5 microM) caused concentration-dependent reductions in apoptosis, and similar results were obtained when endogenous AA levels were elevated in cytosolic phospholipase A(2)-overexpressing MIN6 cells. 25mM glucose caused both a significant up-regulation of MIN6 cell COX2 mRNA levels and a decrease in apoptosis. Inhibition of MIN6 cell COX2 activity with a selective inhibitor, NS-398 (10-100 microM), increased apoptosis and exogenous PGE(2) (0.2-5 microM) reduced NS-398-induced apoptosis in a concentration-dependent manner. The protective effects of AA and PGE(2) were also observed in primary mouse islets.

CONCLUSION

These data show that AA and its COX2-generated metabolite, PGE(2), can protect beta-cells from apoptosis.

Mice with beta cell overexpression of glycogen synthase kinase-3beta have reduced beta cell mass and proliferation

AIMS/HYPOTHESIS

Glycogen synthase kinase-3 (GSK3) has been implicated in the pathophysiology of several prevalent diseases, including diabetes. However, despite recent progress in our understanding of the role of GSK3 in the regulation of glucose metabolism in peripheral tissues, the involvement of GSK3 in islet beta cell growth and function in vivo is unknown. We therefore sought to determine whether over-activation of GSK3beta would lead to alterations in islet beta cell mass and/or function.

METHODS

Transgenic mice overexpressing a constitutively active form of human GSK3beta (S9A) under the control of the rat insulin promoter (RIP-GSK3betaCA) were created. Studies using mouse insulinoma cells (MIN6) were conducted to investigate the regulation of GSK3beta activity and its impact on pancreas/duodenum homeobox protein-1 (PDX-1) levels.

RESULTS

We demonstrated that phosphorylation of GSK3beta was decreased, indicating increased GSK3beta activity in two animal models of diabetes, Lepr(-/- ) mice and Ins2 (Akita/+) mice. In MIN6 cells, the activity of GSK3beta was regulated by glucose, in a fashion largely dependent on phosphatidylinositol 3-kinase. RIP-GSK3betaCA transgenic mice showed impaired glucose tolerance after 5 months of age. Histological studies revealed that transgenic mice had decreased beta cell mass and decreased beta cell proliferation, with a 50% decrease (p<0.05) in the level of PDX-1.

CONCLUSIONS/INTERPRETATION

We showed direct evidence that GSK3beta activity is associated with beta cell failure in diabetic mouse models and that its overactivation resulted in decreased pancreatic beta cell proliferation and mass. GSK3 modulates PDX-1 stability in both cultured insulinoma cells and islets in vivo. These results may ultimately facilitate the development of potential therapeutic interventions targeting type 2 diabetes and/or islet transplantation.

Diminished phosphodiesterase-8B potentiates biphasic insulin response to glucose

cAMP activates multiple signal pathways, crucial for the pancreatic beta-cells function and survival and is a major potentiator of insulin release. A family of phosphodiesterases (PDEs) terminate the cAMP signals. We examined the expression of PDEs in rat beta-cells and their role in the regulation of insulin response. Using RT-PCR and Western blot analyses, we identified PDE3A, PDE3B, PDE4B, PDE4D, and PDE8B in rat islets and in INS-1E cells and several possible splice variants of these PDEs. Specific depletion of PDE3A with small interfering (si) RNA (siPDE3A) led to a small (67%) increase in the insulin response to glucose in INS-1E cells but not rat islets. siPDE3A had no effect on the glucagon-like peptide-1 (10 nmol/liter) potentiated insulin response in rat islets. Depletion in PDE8B levels in rat islets using similar technology (siPDE8B) increased insulin response to glucose by 70%, the potentiation being of similar magnitude during the first and second phase insulin release. The siPDE8B-potentiated insulin response was further increased by 23% when glucagon-like peptide-1 was included during the glucose stimulus. In conclusion, PDE8B is expressed in a small number of tissues unrelated to glucose or fat metabolism. We propose that PDE8B, an 3-isobutyl-1-methylxanthine-insensitive cAMP-specific phosphodiesterase, could prove a novel target for enhanced insulin response, affecting a specific pool of cAMP involved in the control of insulin granule trafficking and exocytosis. Finally, we discuss evidence for functional compartmentation of cAMP in pancreatic beta-cells.

The protein kinases ERK1/2 and their roles in pancreatic beta cells

Extracellular signal-regulated protein kinases 1 and 2 (ERK1/2) activities are modulated in a manner that reflects the secretory demand on beta cells to integrate long- and short-term nutrient sensing information. Our studies have focused on the mechanisms of ERK1/2 activation in beta cells and on the actions of ERK1/2 that regulate beta cell function. Insulin and growth factors regulate ERK1/2 in beta cells in a largely calcium-independent manner. Nutrients and anticipatory hormones, in contrast, activate ERK1/2 in a calcium-dependent manner in these cells. We are exploring the key intermediates in these distinct activation pathways and find that calcineurin is essential for the nutrient pathway but is not essential for the growth factor pathway. Using reporter assays, heterologous reconstitution, electrophoretic mobility shift assays, Northern analysis, Q-PCR and chromatin immunoprecipitation, we have examined several genes that are regulated by ERK1/2, primarily the insulin gene and the apoptotic factor C/EBP-homologous protein (CHOP)-10 (GADD153/DDIT-3), a bZIP protein. ERK1/2-sensitive transcriptional regulators common to these two genes are C/EBP-beta and MafA. The insulin promoter is both positively and negatively regulated by glucose and other nutrients. Exposure to glucose for minutes to hours causes an increase in the rate of insulin gene transcription. In contrast, exposure to elevated glucose for 48 h or more results in inhibition of the insulin gene promoter. Both of these processes depend on ERK1/2 activity. Expression of CHOP is induced by stresses including nutrient deprivation and endoplasmic reticulum stress. CHOP gene expression, especially that regulated by nutrients, is also ERK1/2-dependent in beta cells, These studies support the hypothesis that the genes regulated by ERK1/2 and the mechanisms employed are key to maintaining normal beta cell function.

Essential role of Skp2-mediated p27 degradation in growth and adaptive expansion of pancreatic beta cells

Diabetes results from an inadequate mass of functional beta cells, due to either beta cell loss caused by immune assault or the lack of compensation to overcome insulin resistance. Elucidating the mechanisms that regulate beta cell mass has important ramifications for fostering beta cell regeneration and the treatment of diabetes. We report here that Skp2, a substrate recognition component of Skp1-Cul1-F-box (SCF) ubiquitin ligase, played an essential and specific role in regulating the cellular abundance of p27 and was a critical determinant of beta cell proliferation. In Skp2(-/-) mice, accumulation of p27 resulted in enlarged polyploid beta cells as a result of endoreduplication replacing proliferation. Despite beta cell hypertrophy, Skp2(-/-) mice exhibited diminished beta cell mass, hypoinsulinemia, and glucose intolerance. Increased insulin resistance resulting from diet-induced obesity caused Skp2(-/-) mice to become overtly diabetic, because beta cell growth in the absence of cell division was insufficient to compensate for increased metabolic demand. These results indicate that the Skp2-mediated degradation pathway regulating the cellular degradation of p27 is essential for establishing beta cell mass and to respond to increased metabolic demand associated with insulin resistance.

Protein kinase C-zeta activation markedly enhances beta-cell proliferation: an essential role in growth factor mediated beta-cell mitogenesis

OBJECTIVE

Diabetes results from a deficiency of functional beta-cells. Previous studies have identified hepatocyte growth factor (HGF) and parathyroid hormone-related protein (PTHrP) as two potent beta-cell mitogens. The objective of this study is to determine 1) whether HGF and PTHrP have additive/synergistic effects on beta-cell growth and proliferation; 2) the signaling pathways through which these growth factors mediate beta-cell mitogenesis; and 3) whether activation of this/these signaling pathway(s) enhances human beta-cell replication.

RESEARCH DESIGN AND METHODS

We generated and phenotypically analyzed doubly transgenic mice overexpressing PTHrP and HGF in the beta-cell. INS-1 and primary mouse and human islet cells were used to identify mitogenic signaling pathways activated by HGF and/or PTHrP.

RESULTS

Combined overexpression of HGF and PTHrP in the beta-cell of doubly transgenic mice did not result in additive/synergistic effects on beta-cell growth and proliferation, suggesting potential cross-talk between signaling pathways activated by both growth factors. Examination of these signaling pathways in INS-1 cells revealed atypical protein kinase C (PKC) as a novel intracellular target activated by both HGF and PTHrP in beta-cells. Knockdown of PKC zeta, but not PKC iota/lambda, expression using specific small-interfering RNAs blocked growth factor-induced INS-1 cell proliferation. Furthermore, adenovirus-mediated delivery of kinase-dead PKC zeta completely inhibited beta-cell proliferation in primary islet cells overexpressing PTHrP and/or HGF. Finally, adenovirus-mediated delivery of constitutively active PKC zeta in mouse and human primary islet cells significantly enhanced beta-cell proliferation.

CONCLUSIONS

PKC zeta is essential for PTHrP- and HGF-induced beta-cell proliferation. PKC zeta activation could be useful in therapeutic strategies for expanding beta-cell mass in vitro and in vivo.

Radix asari extract protects pancreatic beta cells against cytokine-induced toxicity: implication of the NF-kappaB-iNOS signaling cascade

In this study, we assessed the preventive effects of Radix asari extract (RAE) against cytokine-induced beta-cell destruction. Cytokines secreted by immune cells that have infiltrated pancreatic islets are crucial mediators of beta-cell destruction in insulin-dependent diabetes mellitus. Treatment of RINm5F (RIN) cells with interleukin (IL)-1beta and interferon (IFN)-gamma resulted in a reduction of cell viability and proliferation. However, treatment of RIN cells with RAE protected the IL-1beta and IFN-gamma- mediated viability and proliferation reduction in a concentration-dependent manner. Incubation with RAE also resulted in significant suppression of IL-1beta and IFN-gamma-induced nitric oxide (NO) production, and this reduction was correlated with reduced levels of mRNA and protein associated with the inducible form of NO synthase (iNOS). The molecular mechanism by which RAE inhibited iNOS gene expression appeared to involve the inhibition of NF-kappaB activation as a result of RAE's suppression of IL-1beta and IFN-gamma-induced IkappaBalpha degradation. The protective effects of RAE were verified via the observation of reduced NO generation and iNOS expression, as well as the observation of normal insulin-secretion responses to glucose in IL-1beta and IFN-gamma-treated rat islets. These results suggest that RAE protects beta cells from cytokine toxicity by suppression of NF-kappaB activation.

Regulation of beta-cell mass and function by the Akt/protein kinase B signalling pathway

The insulin receptor substrate-2/phosphoinositide 3-kinase (PI3K) pathway plays a critical role in the regulation of beta-cell mass and function, demonstrated both in vitro and in vivo. The serine threonine kinase Akt is one of the promising downstream molecules of this pathway that has been identified as a potential target to regulate function and induce proliferation and survival of beta cells. Here we summarize some of the molecular mechanisms, downstream signalling pathways and critical components involved in the regulation of beta-cell mass and function by Akt.

Resistin Is a Key Mediator of Glucose-dependent Insulinotropic Polypeptide (GIP) Stimulation of Lipoprotein Lipase (LPL) Activity in Adipocytes

Studies on the physiological roles of the incretin hormone, glucose-dependent insulinotropic polypeptide (GIP) have largely focused on its insulinotropic action and ability to regulate beta-cell mass. In previous studies on the stimulatory effect of GIP on adipocyte lipoprotein lipase (LPL), a pathway was identified involving increased phosphorylation of protein kinase B (PKB) and reduced phosphorylation of LKB1 and AMP-activated protein kinase (AMPK). The slow time of onset of the responses suggested that GIP may have induced release of an intermediary molecule, and the current studies focused on the possible contribution of the adipokine resistin. In differentiated 3T3-L1 adipocytes, GIP, in the presence of insulin, increased resistin secretion through a pathway involving p38 mitogen-activated protein kinase (p38 MAPK) and the stress-activated protein kinase/Jun amino-terminal kinase (SAPK/JNK). The other major incretin hormone, glucagon-like peptide-1 (GLP-1), exhibited no significant effects. Chronic elevation of circulating GIP levels in the Vancouver Diabetic Fatty (VDF) Zucker rat resulted in increases in circulating resistin levels and activation of p38 MAPK or SAPK/JNK in epididymal fat tissue, suggesting the existence of identical pathways in vivo as well as in vitro. Administration of resistin to 3T3-L1 adipocytes mimicked the effects of GIP on the PKB/LKB1/AMPK/LPL pathway: increasing phosphorylation of PKB, reducing levels of phosphorylated LKB1 and AMPK, and increasing LPL activity. Knockdown of resistin using RNA interference attenuated the effect of GIP on the PKB/LKB1/AMPK/LPL pathway in 3T3-L1 adipocytes, supporting a role for resistin as a mediator.

Brain fatty acid synthase activates PPARalpha to maintain energy homeostasis

Central nervous system control of energy balance affects susceptibility to obesity and diabetes, but how fatty acids, malonyl-CoA, and other metabolites act at this site to alter metabolism is poorly understood. Pharmacological inhibition of fatty acid synthase (FAS), rate limiting for de novo lipogenesis, decreases appetite independently of leptin but also promotes weight loss through activities unrelated to FAS inhibition. Here we report that the conditional genetic inactivation of FAS in pancreatic beta cells and hypothalamus produced lean, hypophagic mice with increased physical activity and impaired hypothalamic PPARalpha signaling. Administration of a PPARalpha agonist into the hypothalamus increased PPARalpha target genes and normalized food intake. Inactivation of beta cell FAS enzyme activity had no effect on islet function in culture or in vivo. These results suggest a critical role for brain FAS in the regulation of not only feeding, but also physical activity, effects that appear to be mediated through the provision of ligands generated by FAS to PPARalpha. Thus, 2 diametrically opposed proteins, FAS (induced by feeding) and PPARalpha (induced by starvation), unexpectedly form an integrative sensory module in the central nervous system to orchestrate energy balance.

Release of glutamate decarboxylase-65 into the circulation by injured pancreatic islet beta-cells

The enzyme glutamate decarboxylase-65 (GAD65) is a major autoantigen in autoimmune diabetes. The mechanism whereby autoreactivity to GAD65, an intracellular protein, is triggered is unknown, and it is possible that immunoreactive GAD65 is released by injured pancreatic islet beta-cells. There is a great need for methods by which to detect and monitor ongoing islet injury. If GAD65 were released and, furthermore, were able to reach the circulation, it could function as a marker of beta-cell injury. Here, a novel GAD65 plasma immunoassay is used to test the hypotheses that beta-cell injury induces GAD65 discharge in vivo and that discharged GAD65 reaches the bloodstream. Plasma GAD65 levels were determined in rats treated with alloxan, and with diabetogenic and low, subdiabetogenic doses of streptozotocin. beta-Cell injury resulted in GAD65 release into the circulation in a dose-dependent manner, and low-dose streptozotocin resulted in a more gradual increase in plasma GAD65 levels than did diabetogenic doses. Plasma GAD65 levels were reduced in rats that had undergone partial pancreatectomy and remained undetectable in mice. Together, these data demonstrate that GAD65 can be released into the circulation by injured beta-cells. Autoantigen shedding may contribute to the pathogenesis of islet autoimmunity in the multiple low-dose streptozocin model and perhaps, more generally, in other forms of autoimmune diabetes. These results demonstrate that, as is true with other tissues, islet injury, at least in some circumstances, can be monitored by use of discharged, circulating proteins. GAD65 is the first such confirmed protein marker of islet injury.

Insulin VNTR I/III genotype is associated with autoantibodies against glutamic acid decarboxylase in newly diagnosed type 1 diabetes

BACKGROUND

In type 1 diabetes (T1D), the influence of age at diagnosis and of the IDDM1 and IDDM2 genetic susceptibility loci on the profile of beta-cell autoantibodies has been demonstrated. We studied these associations in a group of 92 patients (children, adolescents and adults, aged 2-62 years) with newly diagnosed T1D.

METHODS

The prevalence of the HLA-DQB1*02 and *0302 alleles and of the classes of variable number of tandem repeats (VNTR) of the insulin gene (INS), and of beta-cell autoantibodies (GADA, IA-2A, ICA and IAA) was determined. Statistical analysis was performed using linear and logistic regression models.

RESULTS

The presence of IAA, IA-2A and ICA, but not of GADA, was negatively associated with age at diagnosis. Younger patients were more likely to have multiple autoantibodies. There was a tendency of a higher prevalence of IAA in patients with the HLA-DQB1*02/0302 genotype or with the DQB1*0302 allele compared to patients lacking these markers. As a novel observation, the INS VNTR I/III genotype was significantly associated with the presence of GADA (OR = 4.79; p = 0.018).

CONCLUSION

The association between the INS VNTR I/III genotype and GADA may suggest that in patients with T1D lacking the INS VNTR I/I genotype, the effect of other susceptibility factors prevails, which promotes the development of autoimmunity to beta-cell antigens other than insulin.

Endocrine regulation of energy metabolism by the skeleton

The regulation of bone remodeling by an adipocyte-derived hormone implies that bone may exert a feedback control of energy homeostasis. To test this hypothesis we looked for genes expressed in osteoblasts, encoding signaling molecules and affecting energy metabolism. We show here that mice lacking the protein tyrosine phosphatase OST-PTP are hypoglycemic and are protected from obesity and glucose intolerance because of an increase in beta-cell proliferation, insulin secretion, and insulin sensitivity. In contrast, mice lacking the osteoblast-secreted molecule osteocalcin display decreased beta-cell proliferation, glucose intolerance, and insulin resistance. Removing one Osteocalcin allele from OST-PTP-deficient mice corrects their metabolic phenotype. Ex vivo, osteocalcin can stimulate CyclinD1 and Insulin expression in beta-cells and Adiponectin, an insulin-sensitizing adipokine, in adipocytes; in vivo osteocalcin can improve glucose tolerance. By revealing that the skeleton exerts an endocrine regulation of sugar homeostasis this study expands the biological importance of this organ and our understanding of energy metabolism.

[The effect of metformin on insulin receptor protein tyrosine kinase activity of HIT-T15 cell exposed to high concentration glucose and free fatty acid]

OBJECTIVE

To study the effect of metformin on insulin receptor (IRc) protein tyrosine kinase (PTK) activity of HIT-T15 cell exposed to high glucose and free fatty acid (FFA) concentration, and to explore the mechanism of metformin (MF) improving beta cell insulin resistance.

METHODS

HIT-T15 cells were incubated for 48 h in a medium containing 5.5-16.7 mmol/L glucose and 0.5 mmol/L palmitic acid respectively. The cells were re-incubated for another 24 h with or without 2.5 microg/mL MF. The PTK activities of IRc were measured by radioactive enzyme assay.

RESULTS

The enzymatic activities of IRc PTK were significantly decreased to HIT-T15 cells having the exposure to high glucose or high FFA concentration, when compared to control [(52.5 +/- 18.6) or (54.6 +/- 14.0) vs. (119.4 +/- 29.1) pmol/(min x microg), P < 0.01 respectively. The enzymatic activities of IRc PTK in HIT-T15 cells reincubated with 2.5 microg/mL MF for an additional 24 h were significantly increased vs MF free group [(113.0 +/- 29.8) vs. (52.5 +/- 18.6) pmol/(min x microg), x 98.6 +/- 26.1) vs. (54.6 +/- 14.0) pmol/(min x microg), P < 0.01 respectively], and were no significant difference in comparison with control group (P > 0.05).

CONCLUSION

The enzymatic activities of IRc PTK are significantly decreased in HIT-T15 cells chronically exposed to elevated glucose or free fatty acids levels. Metformin can restore approximately normal enzymatic activities of PTK of HIT-T15 cells, of which the PTK activities have been impaired by chronic exposure to high glucose or free fatty acids levels.

Distinct in vivo roles of caspase-8 in beta-cells in physiological and diabetes models

Inadequate pancreatic beta-cell mass resulting from excessive beta-cell apoptosis is a key defect in type 1 and type 2 diabetes. Caspases are the major molecules involved in apoptosis; however, in vivo roles of specific caspases in diabetes are unclear. The purpose of this study is to examine the role of Caspase (Casp)8 in beta-cells in vivo. Using the Cre-loxP system, mice lacking Casp8 in beta-cells (RIPcre(+)Casp8(fl/fl) mice) were generated to address the role of Casp8 in beta-cells in physiological and diabetes models. We show that islets isolated from RIPcre(+)Casp8(fl/fl) mice were protected from Fas ligand (FasL)-and ceramide-induced cell death. Furthermore, RIPcre(+)Casp8(fl/fl) mice were protected from in vivo models of type 1 and type 2 diabetes. In addition to being the central mediator of apoptosis in diabetes models, we show that Casp8 is critical for maintenance of beta-cell mass under physiological conditions. With aging, RIPcre(+)Casp8(fl/fl) mice gradually develop hyperglycemia and a concomitant decline in beta-cell mass. Their islets display decreased expression of molecules involved in insulin/IGF-I signaling and show decreased pancreatic duodenal homeobox-1 and cAMP response element binding protein expression. At the level of individual islets, we observed increased insulin secretory capacity associated with increased expression of exocytotic proteins. Our results show distinct context-specific roles of Casp8 in physiological and disease states; Casp8 is essential for beta-cell apoptosis in type 1 and type 2 diabetes models and in regulating beta-cell mass and insulin secretion under physiological conditions.

The Rational Design of β Cell Cytoprotective Gene Transfer Strategies: Targeting Deleterious iNOS Expression

Islet transplantation represents a promising therapeutic strategy for the treatment of type 1 diabetes mellitus (T1DM) [Hakim and Papalois (Ann Ital Chir 75:1-7, 2004); Jaeckel et al. (Internist (Berl) 45:1268-1280, 2004); Sutherland et al. (Transplant Proc 36:1697-1699, 2004)]. The insulin-secreting pancreatic beta cells of the islet allograft are, however, subject to recurrent immune-mediated damage. Principal among the molecular culprits involved in this destructive process is the proinflammatory cytokine IL-1beta. IL-1beta-induced beta cell destruction may be mediated by the generation of NO and/or ROS, although the relative importance of NO and ROS in this process remains unclear. This study broadly encompassed three arms of investigation: the first of these was geared toward the establishment of a robust in vitro cell system for the study of IL-1beta-induced pathophysiology; the second arm aimed to provide a comparative analysis of the gene transfer profiles of the three most commonly used gene transfer vehicles, namely plasmid vectors, adenoviral vectors, and lentiviral vectors, in the aforementioned cell system; the final arm aimed to screen an array of potentially cytoprotective gene transfer strategies incorporating the optimal gene transfer vectors. Briefly, we established an in vitro beta cell system that accurately reflected primary beta cell cytokine-induced pathophysiology. That is, IL-1beta exposure (100 U/ml) induced a time-dependent decrease in rat insulinoma (RIN) cell viability, which coincided with an induction in iNOS expression and nitrite accumulation. Gene transfer studies using plasmid, adenoviral, or lentiviral vectors underscored the superiority of viral vector-based gene transfer strategies for the manipulation of this beta cell line. Using these vectors, we provide evidence that NF-kappaB-based iNOS inhibition confers significant protection against IL-1beta-induced damage whereas antioxidant overexpression fails to provide protection. Conferred cytoprotection was associated with a suppression of iNOS expression and nitrite accumulation. From a therapeutic standpoint, gene transfer strategies employing efficient viral vectors to target iNOS activation may harbour therapeutic potential in preserving beta cell survival against proinflammatory cytokine exposure.

Differential regulation of CHOP-10/GADD153 gene expression by MAPK signaling in pancreatic beta-cells

CHOP-10 (GADD153/DDIT-3) is a bZIP protein involved in differentiation and apoptosis. Its expression is induced in response to stresses such as nutrient deprivation, perturbation of the endoplasmic reticulum, redox imbalance, and UV exposure. Here we show that CHOP expression is induced in cultured pancreatic beta-cells maintained in a basal glucose concentration of 5.5 mM and repressed by stimulatory glucose (>or=11 mM). Both induction and repression of CHOP are dependent on the MAPKs ERK1 and ERK2. Two regulatory composite sites containing overlapping MafA response elements (MARE) and CAAT enhancer binding (CEB) elements regulate transcription in an ERK1/2-dependent manner. One site (MARE-CEB), from -320 to -300 bp in the promoter, represses transcription. The other site (CEB-MARE), from +2,628 to +2,641 bp in the first intron of the CHOP gene, activates it. MafA can influence transcription of both sites. The MARE-CEB is repressed by MafA, whereas the CEB-MARE site, which is homologous to the A2C1 component of the glucose-sensitive RIPE3b region of the insulin gene promoter, is activated by MafA. These results indicate that ERK1/2 have dual roles in regulating CHOP gene expression via both promoter and intronic regions, depending on environmental and metabolic stresses imposed on pancreatic beta-cells.

Coptidis rhizoma extract protects against cytokine-induced death of pancreatic beta-cells through suppression of NF-kappaB activation

We demonstrated previously that Coptidis rhizoma extract (CRE) prevented S-nitroso-N-acetylpenicillamine-induced apoptotic cell death via the inhibition of mitochondrial membrane potential disruption and cytochrome c release in RINm5F (RIN) rat insulinoma cells. In this study, the preventive effects of CRE against cytokine-induced beta-cell death was assessed. Cytokines generated by immune cells infiltrating pancreatic islets are crucial mediators of beta-cell destruction in insulin-dependent diabetes mellitus. The treatment of RIN cells with IL-1beta and IFN-gamma resulted in a reduction of cell viability. CRE completely protected IL-1beta and IFN-gamma-mediated cell death in a concentration-dependent manner. Incubation with CRE induced a significant suppression of IL-1beta and IFN-gamma-induced nitric oxide (NO) production, a finding which correlated well with reduced levels of the iNOS mRNA and protein. The molecular mechanism by which CRE inhibited iNOS gene expression appeared to involve the inhibition of NF-kappaB activation. The IL-1beta and IFN-gamma-stimulated RIN cells showed increases in NF-kappaB binding activity and p65 subunit levels in nucleus, and IkappaB alpha degradation in cytosol compared to unstimulated cells. Furthermore, the protective effects of CRE were verified via the observation of reduced NO generation and iNOS expression, and normal insulin-secretion responses to glucose in IL-1beta and IFN-gamma-treated islets.

Glutaminase activity is confined to the mantle of the islets of Langerhans

Glutamatergic signalling plays an important role in the coordination of hormone secretion from the endocrine pancreas. Thus, glutamate production must be a process exquisitely regulated to ensure a proper transmitter function. Recently we have reported that the endocrine pancreas co-expresses two isoforms of the protein glutaminase (GA), denoted as kidney-type (KGA) and liver-type (LGA). However, how GA activity, and therefore glutamate production, is regulated in the islets represents a critical issue that remains unresolved. Since the purification of these enzymes from rat islets is a daunting task, in order to characterize each isoform we have taken advantage of the spatial segregation of these isoenzymes in pancreas. To assist us with this goal, we have developed a new procedure that enables us to assay GA activity in situ. The assay is highly specific for GA as indicated by its dependence on glutamine and orthophosphate. Surprisingly, LGA, which is abundantly expressed by beta-cells, did not show detectable activity under the assay conditions. All the GA activity detected in pancreatic islets was attributed to KGA and was confined to the mantle of the islets. Double labelling analyses strongly suggested that alpha-cells should be regarded as the site of glutamate production in the endocrine pancreas.

Depletion of the catalytic subunit of protein phosphatase-2A (PP2Ac) markedly attenuates glucose-stimulated insulin secretion in pancreatic beta-cells

Among various phosphatases, the protein phosphatase 2A (PP2A) is relatively well studied in the islet. Previously, we have demonstrated that the catalytic subunit of PP2A (PP2Ac) undergoes okadaic acid (OKA)-sensitive, reversible carboxylmethylation (CML), which appears to be requisite for glucose-stimulated insulin secretion (GSIS). Using the siRNA approach, we examined, herein, the contributory roles of PP2Ac in GSIS from insulin-secreting pancreatic beta-(INS-1 832/13) cells. Immunologically, PP2Ac was detectable in all the subcellular fractions studied in rank order of: cytosol > microsomes > secretory granules = nucleus > mitochondria. Transfection of PP2Ac-specific, but not scrambled-siRNA, markedly attenuated PP2A activity and GSIS in these cells. Together, our findings provide a direct evidence for a positive modulatory role for PP2Ac in signaling steps leading to GSIS.

Human high-density lipoprotein particles prevent activation of the JNK pathway induced by human oxidised low-density lipoprotein particles in pancreatic beta cells

AIMS/HYPOTHESIS

We explored the potential adverse effects of pro-atherogenic oxidised LDL-cholesterol particles on beta cell function.

MATERIALS AND METHODS

Isolated human and rat islets and different insulin-secreting cell lines were incubated with human oxidised LDL with or without HDL particles. The insulin level was monitored by ELISA, real-time PCR and a rat insulin promoter construct linked to luciferase gene reporter. Cell apoptosis was determined by scoring cells displaying pycnotic nuclei.

RESULTS

Prolonged incubation with human oxidised LDL particles led to a reduction in preproinsulin expression levels, whereas the insulin level was preserved in the presence of native LDL-cholesterol. The loss of insulin production occurred at the transcriptional levels and was associated with an increase in activator protein-1 transcriptional activity. The rise in activator protein-1 activity resulted from activation of c-Jun N-terminal kinases (JNK, now known as mitogen-activated protein kinase 8 [MAPK8]) due to a subsequent decrease in islet-brain 1 (IB1; now known as MAPK8 interacting protein 1) levels. Consistent with the pro-apoptotic role of the JNK pathway, oxidised LDL also induced a twofold increase in the rate of beta cell apoptosis. Treatment of the cells with JNK inhibitor peptides or HDL countered the effects mediated by oxidised LDL.

CONCLUSIONS/INTERPRETATION

These data provide strong evidence that oxidised LDL particles exert deleterious effects in the progression of beta cell failure in diabetes and that these effects can be countered by HDL particles.

Cytokine-induced osteoprotegerin expression protects pancreatic beta cells through p38 mitogen-activated protein kinase signalling against cell death

AIMS/HYPOTHESIS

Pro-inflammatory cytokines play a crucial role in immune-mediated beta cell destruction, an essential mechanism in the pathogenesis of type 1 diabetes mellitus. Microarray analysis recently identified osteoprotegerin (OPG; now known as tumour necrosis factor receptor superfamily, member 11b [TNFRSF11B]) as a cytokine-induced gene in beta cells. The aim of the present study was to characterise the functional role and signalling pathways of OPG that are involved in cytokine-induced beta cell death.

MATERIALS AND METHODS

As cellular models, the rat beta cell line INS-1E and human primary pancreatic islets were employed. The effects of IL-1beta and TNF-alpha on OPG expression were characterised by northern blot and immunoassay. The effect of OPG on beta cell survival was assessed by the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay. Signalling pathways were evaluated by western blot analysis using antibodies against p38 mitogen-activated protein kinases (MAPK), c-Jun N-terminal kinase and extracellular signal-regulated kinase 1/2.

RESULTS

The INS-1E cell line and primary pancreatic islets expressed OPG mRNA and secreted OPG protein, both of which were enhanced by IL-1beta and TNF-alpha. Exposure to IL-1beta resulted in sustained phosphorylation of p38 MAPK in INS-1E cells and subsequent cell death. Administration of exogenous OPG prevented both IL-1beta-induced beta cell death and sustained p38 MAPK phosphorylation.

CONCLUSIONS/INTERPRETATION

Our data indicate that cytokine-induced production of OPG may protect beta cells from further damage. This protective effect is, at least in part, mediated through inhibition of p38 MAPK phosphorylation. Thus OPG is an autocrine or paracrine survival factor for beta cells.