Oxidative stress induces nucleo-cytoplasmic translocation of pancreatic transcription factor PDX-1 through activation of c-Jun NH(2)-terminal kinase

Oxidative stress and redox modulation potential in type 1 diabetes

Redox reactions are imperative to preserving cellular metabolism yet must be strictly regulated. Imbalances between reactive oxygen species (ROS) and antioxidants can initiate oxidative stress, which without proper resolve, can manifest into disease. In type 1 diabetes (T1D), T-cell-mediated autoimmune destruction of pancreatic β-cells is secondary to the primary invasion of macrophages and dendritic cells (DCs) into the islets. Macrophages/DCs, however, are activated by intercellular ROS from resident pancreatic phagocytes and intracellular ROS formed after receptor-ligand interactions via redox-dependent transcription factors such as NF-κB. Activated macrophages/DCs ferry β-cell antigens specifically to pancreatic lymph nodes, where they trigger reactive T cells through synapse formation and secretion of proinflammatory cytokines and more ROS. ROS generation, therefore, is pivotal in formulating both innate and adaptive immune responses accountable for islet cell autoimmunity. The importance of ROS/oxidative stress as well as potential for redox modulation in the context of T1D will be discussed.

Neutralizing interleukin-1beta (IL-1beta) induces beta-cell survival by maintaining PDX1 protein nuclear localization

The transcription factor PDX1 plays a critical role during β-cell development and in glucose-induced insulin gene transcription in adult β-cells. Acute glucose exposure leads to translocalization of PDX1 to the nucleoplasm, whereas under conditions of oxidative stress, PDX1 shuttles from the nucleus to the cytosol. Here we show that cytosolic PDX1 expression correlated with β-cell failure in diabetes. In isolated islets from patients with type 2 diabetes and from diabetic mice, we found opposite regulation of insulin and PDX1 mRNA; insulin was decreased in diabetes, but PDX1 was increased. This suggests that elevated PDX1 mRNA levels may be insufficient to regulate insulin. In diabetic islets, PDX1 protein was localized in the cytosol, whereas in non-diabetic controls, PDX1 was in the nucleus. In contrast, overexpression of either IL-1 receptor antagonist or shuttling-deficient PDX1 restored β-cell survival and function and PDX1 nuclear localization. Our results show that nuclear localization of PDX1 is essential for a functional β-cell and provides a novel mechanism of the protective effect of IL-1 receptor antagonist on β-cell survival and function.

Two tales of antioxidant enzymes on β cells and diabetes

Pancreatic islets contain low activities of catalase, selenium-dependent glutathione peroxidase 1 (GPX1), and Cu,Zn-superoxide dismutase 1 (SOD1). Thus, enhancing expression of these enzymes in islets has been unquestionably favored. However, such an attempt has produced variable metabolic outcomes. While β cell-specific overexpression of Sod1 enhanced mouse resistance to streptozotocin-induced diabetes, the same manipulation of catalase aggravated onset of type 1 diabetes in nonobese diabetic mice. Global overexpression of Gpx1 in mice induced type 2 diabetes-like phenotypes. Although knockouts of Gpx1 and Sod1 each alone or together decreased pancreatic β cell mass and plasma insulin concentrations, these knockouts improved body insulin sensitivity to different extents. Pancreatic duodenal homeobox 1, forkhead box A2, and uncoupling protein 2 are three key regulators of β cell mass, insulin synthesis, and glucose-stimulated insulin secretion. Phenotypes resulted from altering GPX1 and/or SOD1 were partly mediated through these factors, along with protein kinase B and c-jun terminal kinase. A shifted reactive oxygen species inhibition of protein tyrosine phosphatases in insulin signaling might be attributed to altered insulin sensitivity. Overall, metabolic roles of antioxidant enzymes in β cells and diabetes depend on body oxidative status and target functions. Revealing regulatory mechanisms for this type of dual role will help prevent potential pro-diabetic risk of antioxidant over-supplementation to humans.

Decreased Expression and Induced Nucleocytoplasmic Translocation of Pancreatic and Duodenal Homeobox 1 in INS-1 Cells Exposed to High Glucose and Palmitate

BACKGROUND

Type 2 diabetes mellitus (T2DM) is often accompanied by increased levels of circulating fatty acid. Elevations in fatty acids and glucose for prolonged periods of time have been suggested to cause progressive dysfunction or apoptosis of pancreatic beta cells in T2DM. However, the precise mechanism of this adverse effect is not well understood.

METHODS

INS-1 rat-derived insulin-secreting cells were exposed to 30 mM glucose and 0.25 mM palmitate for 48 hours.

RESULTS

The production of reactive oxygen species increased significantly. Pancreatic and duodenal homeobox 1 (Pdx1) expression was down-regulated, as assessed by reverse transcription-polymerase chain reaction and Western blot analyses. The promoter activities of insulin and Pdx1 were also diminished. Of note, there was nucleocytoplasmic translocation of Pdx1, which was partially prevented by treatment with an antioxidant, N-acetyl-L-cysteine.

CONCLUSION

Our data suggest that prolonged exposure of beta cells to elevated levels of glucose and palmitate negatively affects Pdx1 expression via oxidative stress.

Knockouts of SOD1 and GPX1 exert different impacts on murine islet function and pancreatic integrity

Metabolic subtlety and clinical relevance of different forms of reactive oxygen species in diabetes remain unclear. Using single knockout of Cu,Zn-superoxide dismutase (SOD1(-/-)) or Se-glutathione peroxidase-1 (GPX1(-/-)) and their double-knockout (DKO) mouse models, we determined if elevating endogenously-derived superoxide and hydroperoxide exerted distinct impacts and mechanisms on body glucose homeostasis. Whereas the three knockout groups displayed decreased plasma insulin concentrations and islet β-cells mass, only SOD1(-/-) showed decreased body weight, increased blood glucose, and blocked glucose-stimulated insulin secretion. Null of SOD1 and GPX1 elevated respective islet superoxide and hydroperoxide production, and upregulated p53 phosphorylation. Knockout of SOD1 downregulated the foxhead box A2/pancreatic and duodenal homeobox 1 pathway in a superoxide-dependent fashion at epigenetic, mRNA, and protein levels in islets, but improved insulin signaling in liver and muscle. The SOD1(-/-) mice showed more apparent pancreatitis than the GPX1(-/-) mice that were more susceptible to the cerulein-induced amylase increase. Knockout of SOD1 impaired islet function, pancreas integrity, and body glucose homeostasis more than that of GPX1. Simultaneous ablation of both enzymes did not result in additive or aggravated metabolic outcomes.

Cyclin-Dependent Kinase 5/p35/p39: A Novel and Imminent Therapeutic Target for Diabetes Mellitus

Present therapies to minify hyperglycaemia and insulin resistance mainly target ATP-sensitive K(+) channels (K(ATP)) of pancreatic cells and PPAR-γ to enhance the insulin secretion and potential for GLUT expression, respectively. These current approaches are frequently associated with the various side effects such as hypoglycaemia and cardiovascular adverse events. CDK5 is a serine/threonine protein kinase, which forms active complexes with p35 or p39 found principally in neurons and in pancreatic β cells. Pieces of evidence from recent studies recommend the vital role of CDK5 in physiological functions in nonneuronal cells such as glucose-stimulated insulin secretion in pancreatic cells. Inhibition of CDK5 averts the decrease of insulin gene expression through the inhibition of nuclear translocation of PDX-1 which is a transcription factor for the insulin gene. The present pieces of evidence designate that CDK5 might be a potential drug target for the regulation of glucose-stimulated insulin secretion in the treatment of diabetes mellitus.

Calcium/calmodulin-dependent protein kinase IV involvement in the pathophysiology of glucotoxicity in rat pancreatic β-cells

Glucotoxicity is a critical component of the pathophysiology of type 2 diabetes mellitus; however, the molecular mechanisms of glucotoxicity are still not fully understood. We have attempted to determine the protein kinases involved in glucotoxicity in pancreatic β-cells by the use of a new technique. Using Multi-PK antibodies, which are capable of detecting a wide variety of protein kinases, we analyzed the protein kinase that correlated with insulin synthesis in INS-1 cells under glucotoxic conditions. When expression patterns of protein kinases in INS-1 cells were analyzed by Western blotting with Multi-PK antibodies, a kinase of 63 kd was significantly reduced concomitant with the decrease of insulin secretion under glucotoxic conditions. To identify the 63-kd kinase, we used a unique 2-dimensional gel electrophoretic technique and MicroRotofor (Bio-Rad Laboratories, Tokyo, Japan) electrophoresis. From the molecular size of a native kinase/cyanogen bromide fragment and pI value, the 63-kd protein kinase was deduced to be CaMKIV. This was confirmed by Western blotting analysis using anti-CaMKIV antibodies. The decreased CaMKIV levels under glucotoxic conditions recovered to original levels after changing the medium to a normal glucose concentration. Recombinant CaMKIV was degraded in a Ca²+-dependent manner by incubation with cell lysates from INS-1 cells under glucotoxic conditions, and degradation was protected by calpain inhibitor. Furthermore, CaMKIV was reduced in the pancreatic islets of diabetic Otsuka Long-Evans Tokushima fatty rats, whereas that of nondiabetic Long-Evans Tokushima Otsuka rats was not. This study suggests that the abnormal regulation of CaMKIV is a component of β-cell dysfunction caused by high glucose.

NADPH oxidase 2-derived reactive oxygen species mediate FFAs-induced dysfunction and apoptosis of β-cells via JNK, p38 MAPK and p53 pathways

Dysfunction of β-cell is one of major characteristics in the pathogenesis of type 2 diabetes. The combination of obesity and type 2 diabetes, characterized as ‘diabesity’, is associated with elevated plasma free fatty acids (FFAs). Oxidative stress has been implicated in the pathogenesis of FFA-induced β-cell dysfunction. However, molecular mechanisms linking between reactive oxygen species (ROS) and FFA-induced β-cell dysfunction and apoptosis are less clear. In the present study, we test the hypothesis that NOX2-derived ROS may play a critical role in dysfunction and apoptosis of β-cells induced by FFA. Our results show that palmitate and oleate (0.5 mmol/L, 48 h) induced JNK activation and AKT inhibition which resulted in decreased phosphorylation of FOXO1 following nuclear localization and the nucleocytoplasmic translocation of PDX-1, leading to the reducing of insulin and ultimately dysfunction of pancreatic NIT-1 cells. We also found that palmitate and oleate stimulated apoptosis of NIT-1 cells through p38MAPK, p53 and NF-κB pathway. More interestingly, our data suggest that suppression of NOX2 may restore FFA-induced dysfunction and apoptosis of NIT-1 cells. Our findings provide a new insight of the NOX2 as a potential new therapeutic target for preservation of β-cell mass and function.

Dietary oxidised frying oil causes oxidative damage of pancreatic islets and impairment of insulin secretion, effects associated with vitamin E deficiency

We previously reported that, in rodents, a diet with a high oxidised frying oil (OFO) content leads to glucose intolerance associated with a reduction in insulin secretion. The present study aimed at investigating the impairment of pancreatic islets caused by dietary OFO. C57BL/6J mice were divided into three groups to receive a low-fat basal diet containing 5 g/100 g of fresh soyabean oil (LF group) or a high-fat diet containing 20 g/100 g of either fresh soyabean oil (HF group) or OFO (HO group). After 8 weeks, mice in the HO group showed glucose intolerance and hypoinsulinaemia, and their islets showed impaired glucose-stimulated insulin secretion (P < 0·05; HO group v. LF and HF groups). Significantly higher oxidative stress and a lower mitochondrial membrane potential were observed in the islets in the HO group compared with the LF and HF groups. Immunoblots showed that the reduction in insulin levels in HO islets was associated with activation of the c-Jun NH2-terminal kinase and a reduction in levels of pancreatic and duodenal homeobox factor-1. In a second study, when dietary OFO-induced tissue vitamin E depletion was prevented by large-dose vitamin E supplementation (500 IU(1·06 mmol all-rac-α-tocopherol acetate)/kg diet; HO+E group), the OFO-mediated reduction in islet size and impairment of glucose tolerance and insulin secretion were significantly attenuated (P < 0·05; HO group v. HO+E group). We conclude that a high level of dietary OFO ingestion impairs glucose metabolism by causing oxidative damage and compromising insulin secretion in pancreatic islets, and that these effects can be prevented by vitamin E supplementation.

Suppression of NADPH oxidase 2 substantially restores glucose-induced dysfunction of pancreatic NIT-1 cells

Defects in insulin secretion by pancreatic cells and/or decreased sensitivity of target tissues to insulin action are the key features of type 2 diabetes. It has been shown that excessive generation of reactive oxygen species (ROS) is linked to glucose-induced β-cell dysfunction. However, cellular mechanisms involved in ROS generation in β-cells and the link between ROS and glucose-induced β-cell dysfunction are poorly understood. Here, we demonstrate a key role of NADPH oxidase 2 (NOX2)-derived ROS in the deterioration of β-cell function induced by a high concentration of glucose. Sprague-Dawley rats were fed a high-fat diet for 24 weeks to induce diabetes. Diabetic rats showed increased glucose levels and elevated ROS generation in blood, but decreased insulin content in pancreatic β-cells. In vitro, increased ROS levels in pancreatic NIT-1 cells exposed to high concentrations of glucose (33.3 mmol·L(-1)) were associated with elevated expression of NOX2. Importantly, decreased glucose-induced insulin expression and secretion in NIT-1 cells could be rescued via siRNA-mediated NOX2 reduction. Furthermore, high glucose concentrations led to apoptosis of β-cells by activation of p38MAPK and p53, and dysfunction of β-cells through phosphatase and tensih homolog (PTEN)-dependent Jun N-terminal kinase (JNK) activation and protein kinase B (AKT/PKB) inhibition, which induced the translocation of forkhead box O1 and pancreatic duodenal homeobox-1, followed by reduced insulin expression and secretion. In conclusion, NOX2-derived ROS could play a critical role in high glucose-induced β-cell dysfunction through PTEN-dependent JNK activation and AKT inhibition.

Identification of epidermal Pdx1 expression discloses different roles of Notch1 and Notch2 in murine Kras(G12D)-induced skin carcinogenesis in vivo

BACKGROUND

The Ras and Notch signaling pathways are frequently activated during development to control many diverse cellular processes and are often dysregulated during tumorigenesis. To study the role of Notch and oncogenic Kras signaling in a progenitor cell population, Pdx1-Cre mice were utilized to generate conditional oncogenic Kras(G12D) mice with ablation of Notch1 and/or Notch2.

METHODOLOGY/PRINCIPAL FINDINGS

Surprisingly, mice with activated Kras(G12D) and Notch1 but not Notch2 ablation developed skin papillomas progressing to squamous cell carcinoma providing evidence for Pdx1 expression in the skin. Immunostaining and lineage tracing experiments indicate that PDX1 is present predominantly in the suprabasal layers of the epidermis and rarely in the basal layer. Further analysis of keratinocytes in vitro revealed differentiation-dependent expression of PDX1 in terminally differentiated keratinocytes. PDX1 expression was also increased during wound healing. Further analysis revealed that loss of Notch1 but not Notch2 is critical for skin tumor development. Reasons for this include distinct Notch expression with Notch1 in all layers and Notch2 in the suprabasal layer as well as distinctive p21 and β-catenin signaling inhibition capabilities.

CONCLUSIONS/SIGNIFICANCE

Our results provide strong evidence for epidermal expression of Pdx1 as of yet not identified function. In addition, this finding may be relevant for research using Pdx1-Cre transgenic strains. Additionally, our study confirms distinctive expression and functions of Notch1 and Notch2 in the skin supporting the importance of careful dissection of the contribution of individual Notch receptors.

Regulation of MafA expression in pancreatic beta-cells in db/db mice with diabetes

OBJECTIVE

Islet beta-cells loose their ability to synthesize insulin under diabetic conditions, which is at least partially due to the decreased activity of insulin transcription factors such as MafA. Although an in vitro study showed that reactive oxygen species (ROS) decrease MafA expression, the underlying mechanism still remains unclear. In this study, we examined the effects of c-Jun, which is known to be upregulated by ROS, on the expression of MafA under diabetic conditions.

RESEARCH DESIGN AND METHODS

To examine the protein levels of MafA and c-Jun, we performed histological analysis and Western blotting using diabetic db/db mice. In addition, to evaluate the possible effects of c-Jun on MafA expression, we performed adenoviral overexpression of c-Jun in the MIN6 beta-cell line and freshly isolated islets. RESULTS MafA expression was markedly decreased in the islets of db/db mice, while in contrast c-Jun expression was increased. Costaining of these factors in the islets of db/db mice clearly showed that MafA and insulin levels are decreased in c-Jun-positive cells. Consistent with these results, overexpression of c-Jun significantly decreased MafA expression, accompanied by suppression of insulin expression. Importantly, MafA overexpression restored the insulin promoter activity and protein levels that were suppressed by c-Jun. These results indicate that the decreased insulin biosynthesis induced by c-Jun is principally mediated by the suppression of MafA activity.

CONCLUSIONS

It is likely that the augmented expression of c-Jun in diabetic islets decreases MafA expression and thereby reduces insulin biosynthesis, which is often observed in type 2 diabetes.

JNK1 and IKKbeta: molecular links between obesity and metabolic dysfunction

Inflammation is thought to underlie the pathogenesis of many chronic diseases. It is now established that obesity results in a state of chronic low-grade inflammation thought to contribute to several metabolic disorders, including insulin resistance and pancreatic islet dysfunction. The protein kinases JNK1 and IKKbeta have been found to serve as critical molecular links between obesity, metabolic inflammation, and disorders of glucose homeostasis. The precise mechanisms of these linkages are still being investigated. However, as we discuss here, JNK1 and IKKbeta are activated by almost all forms of metabolic stress that have been implicated in insulin resistance or islet dysfunction. Furthermore, both JNK1 and IKKbeta are critically involved in the promotion of diet-induced obesity, metabolic inflammation, insulin resistance, and beta-cell dysfunction. Understanding the molecular mechanisms by which JNK1 and IKKbeta mediate obesity-induced metabolic stress is likely to be of importance for the development of new treatments for a variety of obesity-associated diseases.

Role of reactive oxygen species in the progression of type 2 diabetes and atherosclerosis

Type 2 diabetes is the most prevalent and serious metabolic disease all over the world, and its hallmarks are pancreatic beta-cell dysfunction and insulin resistance. Under diabetic conditions, chronic hyperglycemia and subsequent augmentation of reactive oxygen species (ROS) deteriorate beta-cell function and increase insulin resistance which leads to the aggravation of type 2 diabetes. In addition, chronic hyperglycemia and ROS are also involved in the development of atherosclerosis which is often observed under diabetic conditions. Taken together, it is likely that ROS play an important role in the development of type 2 diabetes and atherosclerosis.

Multiple chromatin-bound protein kinases assemble factors that regulate insulin gene transcription

During the onset of diabetes, pancreatic beta cells become unable to produce sufficient insulin to maintain blood glucose within the normal range. Proinflammatory cytokines have been implicated in impaired beta cell function. To understand more about the molecular events that reduce insulin gene transcription, we examined the effects of hyperglycemia alone and together with the proinflammatory cytokine interleukin-1beta (IL-1beta) on signal transduction pathways that regulate insulin gene transcription. Exposure to IL-1beta in fasting glucose activated multiple protein kinases that associate with the insulin gene promoter and transiently increased insulin gene transcription in beta cells. In contrast, cells exposed to hyperglycemic conditions were sensitized to the inhibitory actions of IL-1beta. Under these conditions, IL-1beta caused the association of the same protein kinases, but a different combination of transcription factors with the insulin gene promoter and began to reduce transcription within 2 h; stimulatory factors were lost, RNA polymerase II was lost, and inhibitory factors were bound to the promoter in a kinase-dependent manner.

Pancreatic ductal morphogenesis and the Pdx1 homeodomain transcription factor

Embryonic development of the pancreas is marked by an early phase of dramatic morphogenesis, in which pluripotent progenitor cells of the developing pancreatic epithelium give rise to the full array of mature exocrine and endocrine cell types. The genetic determinants of acinar and islet cell lineages are somewhat well defined; however, the molecular mechanisms directing ductal formation and differentiation remain to be elucidated. The complex ductal architecture of the pancreas is established by a reiterative program of progenitor cell expansion and migration known as branching morphogenesis, or tubulogenesis, which proceeds in mouse development concomitantly with peak Pdx1 transcription factor expression. We therefore evaluated Pdx1 expression with respect to lineage-specific markers in embryonic sections of the pancreas spanning this critical period of duct formation and discovered an unexpected population of nonislet Pdx1-positive cells displaying physical traits of branching. We then established a 3D cell culture model of branching morphogenesis using primary pancreatic duct cells and identified a transient surge of Pdx1 expression exclusive to branching cells. From these observations we propose that Pdx1 might be involved temporally in a program of gene expression sufficient to facilitate the biochemical and morphological changes necessary for branching morphogenesis.

JNK3 is abundant in insulin-secreting cells and protects against cytokine-induced apoptosis

AIMS/HYPOTHESIS

In insulin-secreting cells, activation of the c-Jun NH(2)-terminal kinase (JNK) pathway triggers apoptosis. Whereas JNK1 and JNK2 are ubiquitously produced, JNK3 has been described exclusively in neurons. This report aims to characterise the expression and role in apoptosis of the three JNK isoforms in insulin-secreting cells exposed to cytokines.

METHODS

Sections of human and mouse pancreases were used for immunohistochemistry studies with isoform-specific anti-JNK antibodies. Human, pig, mouse and rat pancreatic islets were isolated by enzymatic digestion and RNA or protein extracts were prepared. RNA and protein levels were determined by quantitative RT-PCR and western blotting respectively, using JNK-isoform-specific primers and isoform-specific antibodies; activities of the three JNK isoforms were determined by kinase assays following quantitative immunoprecipitation/depletion of JNK3. JNK silencing was performed with small interfering RNAs and apoptotic rates were determined in INS-1E cells by scoring cells displaying pycnotic nuclei.

RESULTS

JNK3 and JNK2 mRNAs are the predominant isoforms expressed in human pancreatic islets. JNK3 is nuclear while JNK2 is also cytoplasmic. In INS-1E cells, JNK3 knockdown increases c-Jun levels and caspase-3 cleavage and sensitises cells to cytokine-induced apoptosis; in contrast, JNK1 or JNK2 knockdown is protective.

CONCLUSIONS/INTERPRETATION

In insulin-secreting cells, JNK3 plays an active role in preserving pancreatic beta cell mass from cytokine attacks. The specific localisation of JNK3 in the nucleus, its recruitment by cytokines, and its effects on key transcription factors such as c-Jun, indicate that JNK3 is certainly an important player in the transcriptional control of genes expressed in insulin-secreting cells.

Role of MafA in pancreatic beta-cells

Pancreatic beta-cell-specific insulin gene expression is regulated by a variety of pancreatic transcription factors and the conserved A3, C1 and E1 elements in the insulin gene enhancer region are very important for activation of insulin gene. Indeed, PDX-1 binding to the A3 element and NeuroD binding to the E1 element are crucial for insulin gene transcription. Recently, C1 element-binding transcription factor was identified as MafA, which is a basic-leucine zipper transcription factor and functions as a potent transactivator for the insulin gene. Under diabetic conditions, chronic hyperglycemia gradually deteriorates pancreatic beta-cell function, which is accompanied by decreased expression and/or DNA binding activities of MafA and PDX-1. Furthermore, MafA overexpression, together with PDX-1 and NeuroD, markedly induces insulin biosynthesis in various non-beta-cells and thereby is a useful tool to efficiently induce insulin-producing surrogate beta-cells. These results suggest that MafA plays a crucial role in pancreatic beta-cells and could be a novel therapeutic target for diabetes.

Glucose-responsive gene expression system for gene therapy

Regulation of gene expression by glucose is an important mechanism for mammals in adapting to their nutritional environment. Glucose, the primary fuel for most cells, modulates gene expression that is crucial in the cellular adaptation to glycemic variation. Transcription of the genes for insulin and glycolytic and lipogenic enzymes is stimulated by glucose in pancreatic beta-cells and liver. Recent findings further support the key role of the carbohydrate-responsive element binding protein in the regulation of glycolytic and lipogenic genes by glucose and dietary carbohydrates. Herein, we review the transcriptional regulation of glucose-responsive genes, and recent advances in the gene therapy using glucose-responsive gene expression for diabetes.

The role of FOXO in the regulation of metabolism

Forkhead box O (FOXO) transcription factors play an important role in modulating metabolic functions. FOXO is regulated by several modifications, but one of the most critical is phosphorylation and nuclear exclusion by Akt. Given the impact of insulin signaling on Akt-mediated phosphorylation of FOXO and the relatively high expression of Foxo1 in insulin-responsive tissues, this transcription factor is highly poised to regulate energy metabolism. When nutrient and insulin levels are low, Foxo1 promotes expression of gluconeogenic enzymes. Conversely, in the fed state, insulin levels rise and stimulate uptake of glucose primarily into skeletal muscle and other organs, including adipose tissue. Under certain pathophysiologic conditions, including insulin resistance, negative signaling to Foxo1 is compromised. Further clarification of the role of Foxo1 in insulin-responsive tissues will strengthen our understanding and allow us to better combat insulin resistance and diabetes mellitus.

Role of copper ion in the pathogenesis of type 2 diabetes

Reactive oxygen species (ROS) are induced under diabetic conditions and are likely associated with the development of type 2 diabetes. It is also known that ROS production is facilitated in the presence of copper ion through the Fenton reaction. The aim of this study was to examine the involvement of copper ion in the pathogenesis of type 2 diabetes and to evaluate the potential usefulness of a copper chelating agent for the treatment of type 2 diabetes. First, both serum copper ion and ROS levels in diabetic C57BL/KsJ-db/db mice were significantly higher compared to those in nondiabetic mice. Second, we treated diabetic db/db mice with a copper chelating agent tetrathiomolybdate and examined the effects on the development of type 2 diabetes. As the results, both serum copper ion and ROS levels were significantly decreased by the treatment, which were equivalent to those in non-diabetic mice. Consequently, the treatment with a copper chelating agent reduced insulin resistance and ameliorated glucose intolerance in diabetic db/db mice. In addition, serum triglyceride levels were also decreased by the treatment. In conclusion, our present results suggest that copper ion is involved in the development of type 2 diabetes and thereby a potential therapeutic target for diabetes.

The expression pattern of PDX-1, SHH, Patched and Gli-1 is associated with pathological and clinical features in human pancreatic cancer

BACKGROUND AND AIMS

Pancreatic cancer cells have been shown to possess stem-cell-like properties, especially by reactivating embryonic transcription factors involved in tissue differentiation. We therefore investigated if and to what extent developmental genes of the human pancreas are expressed in pancreatic ductal adenocarcinomas and precursor lesions, pancreatic intraepithelial neoplasia (PanIN), and if this correlates or predicts response to treatment and overall survival.

MATERIAL AND METHODS

Invasive ductal adenocarcinomas of the pancreas [UICC pT3pN0 (n = 13) vs. pT3pN1 (n = 25)] and tumors after neoadjuvant chemotherapy [5-fluorouracil (FU)/folic-acid and gemcitabine; UICC ypN0 (n = 7) vs. ypN1 (n = 6)] resected between 1997 and 2003 were characterized histochemically and immunohistochemically [pancreas duodenum homeobox 1 (PDX-1), Sonic hedgehog protein (SHH), Patched (Ptc) and Gli-1]. Gene distribution was compared with morphological patterns of the pancreatic carcinoma and PanIN as well as with peritumorous reactions of normal pancreas.

RESULTS

The overall expression of PDX-1, SHH, Ptc and Gli-1 was low, but showed a distinctive and topographic linkage inside pancreatic carcinomas as well as inside PanINs. Additionally, a topographic and significant association of these markers with nodal status (PDX-1, Ptc, Gli-1), tumor size (PDX-1, Gli-1) and R status (PDX-1) was found. After stratification with the strongest outcome predictor, grading, survival analysis revealed that Ptc expression in grade 2 and PDX-1 expression in grade 3 carcinomas are independent survival factors.

CONCLUSIONS

Markers of pancreas development are reexpressed in invasive ductal adenocarcinomas and their expression is essentially associated with general clinical and pathological features such as survival or nodal status.

Beta-cell failure as a complication of diabetes

Type 2 diabetes mellitus is a complex disease characterized by beta-cell failure in the setting of insulin resistance. In early stages of the disease, pancreatic beta-cells adapt to insulin resistance by increasing mass and function. As nutrient excess persists, hyperglycemia and elevated free fatty acids negatively impact beta-cell function. This happens by numerous mechanisms, including the generation of reactive oxygen species, alterations in metabolic pathways, increases in intracellular calcium and the activation of endoplasmic reticulum stress. These processes adversely affect beta-cells by impairing insulin secretion, decreasing insulin gene expression and ultimately causing apoptosis. In this review, we will first discuss the regulation of beta-cell mass during normal conditions. Then, we will discuss the mechanisms of beta-cell failure, including glucotoxicity, lipotoxicity and endoplasmic reticulum stress. Further research into mechanisms will reveal the key modulators of beta-cell failure and thus identify possible novel therapeutic targets. Type 2 diabetes mellitus is a multifactorial disease that has greatly risen in prevalence in part due to the obesity and inactivity that characterize the modern Western lifestyle. Pancreatic beta-cells possess the potential to greatly expand their function and mass in both physiologic and pathologic states of nutrient excess and increased insulin demand. beta-cell response to nutrient excess occurs by several mechanisms, including hypertrophy and proliferation of existing beta-cells, increased insulin production and secretion, and formation of new beta-cells from progenitor cells [1, 2]. Failure of pancreatic beta-cells to adequately expand in settings of increased insulin demand results in hyperglycemia and diabetes. In this review, we will first discuss the factors involved in beta-cell growth and then discuss the mechanisms by which beta-cell expansion fails and leads to beta-cell failure and diabetes (Fig. 1).

Multiple kinases regulate mafA expression in the pancreatic beta cell line MIN6

MafA is a basic leucine zipper transcription factor expressed within the beta cells of the pancreas and is required to maintain normal glucose homeostasis as it is involved in various aspects of beta cell biology. MafA protein levels are known to increase in response to high glucose through mechanisms that have yet to be fully characterized. We investigated whether discrete intracellular signaling events control mafA expression. We found that the general kinase inhibitor staurosporine induces mafA expression without altering the stability of the protein. Inhibition of the MAP-kinase JNK mimics the effects of staurosporine on the expression of mafA. Calmodulin kinase and calcium signaling are also important in stimulating mafA expression by high glucose. However, staurosporine, JNK, and calmodulin kinase have different effects on the induction of insulin expression. These data reveal that MafA levels are tightly controlled by the coordinated action of multiple kinase pathways.

Glucose regulation of insulin gene expression in pancreatic β-cells

Production and secretion of insulin from the β-cells of the pancreas is very crucial in maintaining normoglycaemia. This is achieved by tight regulation of insulin synthesis and exocytosis from the β-cells in response to changes in blood glucose levels. The synthesis of insulin is regulated by blood glucose levels at the transcriptional and post-transcriptional levels. Although many transcription factors have been implicated in the regulation of insulin gene transcription, three β-cell-specific transcriptional regulators, Pdx-1 (pancreatic and duodenal homeobox-1), NeuroD1 (neurogenic differentiation 1) and MafA (V-maf musculoaponeurotic fibrosarcoma oncogene homologue A), have been demonstrated to play a crucial role in glucose induction of insulin gene transcription and pancreatic β-cell function. These three transcription factors activate insulin gene expression in a co-ordinated and synergistic manner in response to increasing glucose levels. It has been shown that changes in glucose concentrations modulate the function of these β-cell transcription factors at multiple levels. These include changes in expression levels, subcellular localization, DNA-binding activity, transactivation capability and interaction with other proteins. Furthermore, all three transcription factors are able to induce insulin gene expression when expressed in non-β-cells, including liver and intestinal cells. The present review summarizes the recent findings on how glucose modulates the function of the β-cell transcription factors Pdx-1, NeuroD1 and MafA, and thereby tightly regulates insulin synthesis in accordance with blood glucose levels.

Involvement of chronic stresses in rat islet and INS-1 cell glucotoxicity induced by intermittent high glucose

In order to investigate the toxic effect of intermittent high glucose (IHG) and sustained high glucose (SHG) on rat pancreatic beta cell functions and the potential involved mechanisms, isolated rat islets and INS-1 beta cells were exposed to SHG (25 mmol/l) or IHG (11.1 and 25 mmol/l glucose alternating every 12 h) for 72 h. The results showed that IHG induced a more significant impairment of insulin release response in rat islets and INS-1 cell than SHG. Simultaneously, the intracellular levels of endoplasmic reticulum and oxidative stress were more markedly increased in islets and INS-1 cells exposed to IHG. However, there was no significant difference between reducing cell viability, insulin content and gene expression induced by SHG and IHG. Taken together, this study suggested the more serious toxic effect on rat pancreatic beta cell function induced by IHG treatment may be due to excessive activation of cellular stress.

Molecular mechanisms for hyperinsulinaemia induced by overproduction of selenium-dependent glutathione peroxidase-1 in mice

AIMS/HYPOTHESIS

We previously observed hyperglycaemia, hyperinsulinaemia, insulin resistance and obesity in Gpx1-overexpressing mice (OE). Here we determined whether these phenotypes were eliminated by diet restriction, subsequently testing whether hyperinsulinaemia was a primary effect of Gpx1 overexpression and caused by dysregulation of pancreatic duodenal homeobox 1 (PDX1) and uncoupling protein-2 (UCP2) in islets.

METHODS

First, 24 male OE and wild-type (WT) mice (2 months old) were given 3 g (diet-restricted) or 5 g (full-fed) feed per day for 4 months to compare their glucose metabolism. Thereafter, several mechanistic experiments were conducted with pancreas and islets of the two genotypes (2 or 6 months old) to assay for beta cell mass, reactive oxygen species (ROS) levels, mitochondrial membrane potential (Deltapsi(m)) and expression profiles of regulatory proteins. A functional assay of islets was also performed.

RESULTS

Diet restriction eliminated obesity but not hyperinsulinaemia in OE mice. These mice had greater pancreatic beta cell mass (more than twofold) and pancreatic insulin content (40%) than the WT, along with an enhanced Deltapsi(m) and glucose-stimulated insulin secretion in islets. With diminished ROS production, the OE islets displayed hyperacetylation of H3 and H4 histone in the Pdx1 promoter, elevated PDX1 and decreased UCP2.

CONCLUSIONS/INTERPRETATION

Overproduction of the major antioxidant enzyme, glutathione peroxidase 1, caused seemingly beneficial changes in pancreatic PDX1 and UCP2, but eventually led to chronic hyperinsulinaemia by dysregulating islet insulin production and secretion.

Nutrigenomics, beta-cell function and type 2 diabetes

INTRODUCTION

The present investigation was designed to investigate the accuracy and precision of lactate measurement obtained with contemporary biosensors (Chiron Diagnostics, Nova Biomedical) and standard enzymatic photometric procedures (Sigma Diagnostics, Abbott Laboratories, Analyticon).

MATERIALS AND METHODS

Measurements were performed in vitro before and after the stepwise addition of 1 molar sodium lactate solution to samples of fresh frozen plasma to systematically achieve lactate concentrations of up to 20 mmol/l.

RESULTS

Precision of the methods investigated varied between 1% and 7%, accuracy ranged between 2% and -33% with the variability being lowest in the Sigma photometric procedure (6%) and more than 13% in both biosensor methods.

CONCLUSION

Biosensors for lactate measurement provide adequate accuracy in mean with the limitation of highly variable results. A true lactate value of 6 mmol/l was found to be presented between 4.4 and 7.6 mmol/l or even with higher difference. Biosensors and standard enzymatic photometric procedures are only limited comparable because the differences between paired determinations presented to be several mmol. The advantage of biosensors is the complete lack of preanalytical sample preparation which appeared to be the major limitation of standard photometry methods.

Mutated ATP synthase induces oxidative stress and impaired insulin secretion in beta-cells of female BHE/cdb rats

BACKGROUND

Adenosine triphosphate (ATP) is a critical determinant of beta-cell insulin secretion in response to glucose. BHE/cdb rats have a mutation in ATP synthase that limits ATP production, yet develop mild diabetes only with ageing. We investigated the cellular basis for reduced insulin secretion and compensatory mechanisms that mitigate the effects of the ATP synthase mutation.

METHODS

In vitro beta-cell function in isolated islets and expression of key regulatory genes was compared with in vivo oral glucose tolerance and insulin sensitivity in BHE/cdb and control rats.

RESULTS

BHE/cdb rat islets had reduced responsiveness to glucose stimulation and ATP content was 35% lower than in control islets. Oral glucose tolerance was impaired at both 21 and 43 weeks of age because of a reduction in glucose-stimulated insulin secretion (GSIS). An increase in inducible nitric oxide synthase (INOS, 3-fold) and manganese superoxide dismutase (MnSOD, 1.6-fold), detection of nitrotyrosine, beta-cell apoptosis, and nucleocytoplasmic translocation of pancreas duodenum homeobox-1 (PDX-1) in beta-cells indicated increased oxygen radical formation. However, BHE/cdb rats partially compensated for low glucose responsiveness by increasing the number of small islets and beta-cell hypertrophy. There was also an increase in the proportion of mature insulin relative to proinsulin (PI) detected within beta-cell granules. Increased activation of AMP-dependent kinase (AMPK)-regulated pathways was consistent with increased oxidative stress and with induction of apoptosis and reduction of preproinsulin gene transcription.

CONCLUSIONS

The findings are consistent with impaired but partially compensated mechanisms of insulin secretion early in life, but progressive non-compensated impairments due to oxidative stress occurs by age 43 weeks.

PDX-1 and MafA play a crucial role in pancreatic beta-cell differentiation and maintenance of mature beta-cell function

Pancreatic and duodenal homeobox factor-1 (PDX-1) plays a crucial role in pancreas development, beta-cell differentiation, and maintenance of mature beta-cell function. PDX-1 expression is maintained in pancreatic precursor cells during pancreas development but becomes restricted to beta-cells in mature pancreas. In mature beta-cells, PDX-1 transactivates the insulin and other genes involved in glucose sensing and metabolism such as GLUT2 and glucokinase. MafA is a recently isolated beta-cell-specific transcription factor which functions as a potent activator of insulin gene transcription. Furthermore, these transcription factors play an important role in induction of insulin-producing cells in various non-beta-cells and thus could be therapeutic targets for diabetes. On the other hand, under diabetic conditions, expression and/or activities of PDX-1 and MafA in beta-cells are reduced, which leads to suppression of insulin biosynthesis and secretion. It is likely that alteration of such transcription factors explains, at least in part, the molecular mechanism for beta-cell glucose toxicity found in diabetes.

The unfolded protein response: a pathway that links insulin demand with beta-cell failure and diabetes

The endoplasmic reticulum (ER) is the entry site into the secretory pathway for newly synthesized proteins destined for the cell surface or released into the extracellular milieu. The study of protein folding and trafficking within the ER is an extremely active area of research that has provided novel insights into many disease processes. Cells have evolved mechanisms to modulate the capacity and quality of the ER protein-folding machinery to prevent the accumulation of unfolded or misfolded proteins. These signaling pathways are collectively termed the unfolded protein response (UPR). The UPR sensors signal a transcriptional response to expand the ER folding capacity, increase degradation of malfolded proteins, and limit the rate of mRNA translation to reduce the client protein load. Recent genetic and biochemical evidence in both humans and mice supports a requirement for the UPR to preserve ER homeostasis and prevent the beta-cell failure that may be fundamental in the etiology of diabetes. Chronic or overwhelming ER stress stimuli associated with metabolic syndrome can disrupt protein folding in the ER, reduce insulin secretion, invoke oxidative stress, and activate cell death pathways. Therapeutic interventions to prevent polypeptide-misfolding, oxidative damage, and/or UPR-induced cell death have the potential to improve beta-cell function and/or survival in the treatment of diabetes.

The role of FoxO in the regulation of metabolism

Forkhead proteins, and FoxO1 in particular, play a significant role in regulating whole body energy metabolism. Glucose homeostasis is achieved by adjusting endogenous glucose production as well as glucose uptake by peripheral tissues in response to insulin. In the fasted state, the liver is primarily responsible for maintaining glucose levels, with FoxO1 playing a key role in promoting the expression of gluconeogenic enzymes. Following feeding, pancreatic beta cells secrete insulin, which promotes the uptake of glucose by peripheral tissues including skeletal muscle and adipose tissue, and can in part suppress gluconeogenic enzyme expression in the liver. In addition to directly regulating metabolism, FoxO1 also plays a role in the formation of both adipose tissue and skeletal muscle, two major organs that are critical for maintaining energy homeostasis. The importance of FoxO1 in energy homeostasis is particularly striking under conditions of metabolic dysfunction or insulin resistance. In obese or diabetic states, FoxO1-dependent gene expression promotes some of the deleterious characteristics associated with these conditions, including hyperglycemia and glucose intolerance. In addition, the increase in pancreatic beta cell mass that normally occurs in response to a rise in insulin demand is blunted by nuclear FoxO1 expression. However, under these same pathophysiological conditions, FoxO1 expression may help drive the expression of genes involved in combating oxidative stress, thereby preserving cellular function. FoxO1 may also be involved in promoting the switch from carbohydrate to fatty acid as the major energy source during starvation.

Trp53 deletion stimulates the formation of metastatic pancreatic tumors

The presence of distant metastases is a common finding on diagnosis of pancreatic cancer; however, the mechanisms underlying the dissemination of this tumor type remain poorly understood. Loss of the p53 tumor suppressor protein has been associated with tumor progression and metastasis in several tumor types including pancreatic ductal adenocarcinoma. Here, we describe the generation of a progressive and metastatic pancreatic cancer mouse model after the somatic and sporadic delivery of avian retroviruses encoding the mouse polyoma virus middle T antigen to elastase-tv-a transgenic mice with a pancreas-specific deletion of the Trp53 tumor suppressor locus. In this model, the tumors metastasize most frequently to the liver, consistent with human pancreatic carcinomas. Analysis of metastatic lesions demonstrated that concomitant loss of the Ink4a/Arf locus was not required for metastasis; however, pancreas-specific deletion of a single Ink4a/Arf allele cooperated with Trp53 deletion in a haploinsufficient manner to accelerate tumor development. Thus, our findings illustrate the potential role of p53 loss of function in pancreatic tumor progression, demonstrate the feasibility of modeling pancreatic cancer metastasis after somatic and sporadic oncogene activation, and indicate that our model may provide a useful experimental system for investigation of the molecular mechanisms underlying pancreatic cancer progression and metastasis.

Pleiotropic Roles of PDX-1 in the Pancreas

It is well known that pancreatic and duodenal homeobox factor-1 (PDX-1) plays a pleiotropic role in the pancreas. In the developing pancreas, PDX-1 is involved in both pancreas formation and beta-cell differentiation. In mature beta-cells, PDX-1 transactivates insulin and other beta-cell-related genes such as GLUT2 and glucokinase. Furthermore, PDX-1 plays an important role in the induction of insulin-producing cells in various non-beta-cells and is thereby a possible therapeutic target for diabetes. On the other hand, under diabetic conditions, expression and/or activity of PDX-1 in beta-cells is reduced, which leads to suppression of insulin biosynthesis and secretion. It is likely that PDX-1 inactivation explains, at least in part, the molecular mechanism for beta-cell glucose toxicity found in diabetes.

Endoplasmic reticulum stress and oxidative stress: a vicious cycle or a double-edged sword?

The endoplasmic reticulum (ER) is a well-orchestrated protein-folding machine composed of protein chaperones, proteins that catalyze protein folding, and sensors that detect the presence of misfolded or unfolded proteins. A sensitive surveillance mechanism exists to prevent misfolded proteins from transiting the secretory pathway and ensures that persistently misfolded proteins are directed toward a degradative pathway. The unfolded protein response (UPR) is an intracellular signaling pathway that coordinates ER protein-folding demand with protein-folding capacity and is essential to adapt to homeostatic alterations that cause protein misfolding. These include changes in intraluminal calcium, altered glycosylation, nutrient deprivation, pathogen infection, expression of folding-defective proteins, and changes in redox status. The ER provides a unique oxidizing folding-environment that favors the formation of the disulfide bonds. Accumulating evidence suggests that protein folding and generation of reactive oxygen species (ROS) as a byproduct of protein oxidation in the ER are closely linked events. It has also become apparent that activation of the UPR on exposure to oxidative stress is an adaptive mechanism to preserve cell function and survival. Persistent oxidative stress and protein misfolding initiate apoptotic cascades and are now known to play predominant roles in the pathogenesis of multiple human diseases including diabetes, atherosclerosis, and neurodegenerative diseases.

Reduction of oxidative stress by a new low-molecular-weight antioxidant improves metabolic alterations in a nonobese mouse diabetes model

OBJECTIVES

We have previously established a nonobese diabetes mouse model characterized by moderate hyperglycemic levels, like those usually occurring in human type 2 diabetes. As oxidative stress is considered a major mechanism of progressive beta-cell damage in diabetes, we tested in this model the protective effects of a new low-molecular-weight antioxidant, namely, bis(1-hydroxy-2,2,6,6-tetramethyl-4-piperidinyl)decandioate dihydrochloride (IAC).

METHODS

Diabetes was induced in C57Bl/6J mice by streptozotocin (STZ) and nicotinamide (NA) administration. Two weeks later, STZ-NA mice were treated for 5 weeks with different doses of IAC (15 or 30 mg/kg per day intraperitoneally) and monitored for glycemia, insulinemia, glucose tolerance, and pancreatic insulin content.

RESULTS

Streptozotocin-NA mice showed moderate hyperglycemia, hypoinsulinemia, glucose intolerance, growth impairment, and markedly reduced pancreatic insulin content (22% of controls). IAC-treated STZ-NA mice showed clear-cut reduction of hyperglycemia and attenuation of glucose intolerance, associated to higher residual pancreatic insulin content with respect to untreated diabetic animals. Plasma nitrotyrosine levels (an index of oxidative stress), enhanced 3-fold in diabetic mice, were significantly reduced by IAC treatment. Significant correlations were found between plasma nitrotyrosine values and either blood glucose levels or pancreatic insulin content.

CONCLUSIONS

In the STZ-NA diabetic mouse model, the new antioxidant, IAC, improves diabetic metabolic alterations, likely by counteracting beta-cell dysfunction and loss associated with oxidative stress.

Diabetes associated cell stress and dysfunction: role of mitochondrial and non-mitochondrial ROS production and activity

It is now widely accepted, given the current weight of experimental evidence, that reactive oxygen species (ROS) contribute to cell and tissue dysfunction and damage caused by glucolipotoxicity in diabetes. The source of ROS in the insulin secreting pancreatic beta-cells and in the cells which are targets for insulin action has been considered to be the mitochondrial electron transport chain. While this source is undoubtably important, we provide additional information and evidence for NADPH oxidase-dependent generation of ROS both in pancreatic beta-cells and in insulin sensitive cells. While mitochondrial ROS generation may be important for regulation of mitochondrial uncoupling protein (UCP) activity and thus disruption of cellular energy metabolism, the NADPH oxidase associated ROS may alter parameters of signal transduction, insulin secretion, insulin action and cell proliferation or cell death. Thus NADPH oxidase may be a useful target for intervention strategies based on reversing the negative impact of glucolipotoxicity in diabetes.

Activation of c-Jun NH2-terminal kinase during islet isolation

Pancreatic islet transplantation has been remarkably improved by the Edmonton protocol; however, it is not easy to achieve insulin independence after islet transplantation from one donor pancreas. The islet isolation procedure itself destroys cellular and noncellular components of the pancreas that probably play a role in supporting islet survival. Further islet transplantation exposes cells to a variety of stressful stimuli such as proinflammatory cytokines. The reduction in islet mass immediately after isolation and transplantation implicates beta cell death by apoptosis and the prerecruitment of intracellular death signalling pathways. The c-Jun NH2-terminal kinases (JNKs) are classic stress-activated protein kinases and many cellular stresses have been shown to stimulate JNK activation. JNK in the pancreas is activated during brain death, pancreas procurement, and organ preservation, and its activity is progressively increased during the isolation procedure. Moreover, JNK activity in the transplanted liver after islet transplantation increases markedly within 24 hrs. Use of the JNK inhibitor in pancreas preservation, islet culture, and/or islet transplantation prevents islet apoptosis and improves islet graft function. These findings suggest that the control of JNK activation is important for pancreatic islet transplantation.

NeuroD and reaggregation induce beta-cell specific gene expression in cultured hepatocytes

BACKGROUND

Our goal was to convert adult mouse hepatocytes to pancreatic beta-cells.

METHODS AND RESULTS

To facilitate conversion, cultured primary hepatocytes were dedifferentiated by the removal of dexamethasone (Dex) from the culture media. Removal of Dex caused detachment of hepatocytes from the culture dish, but the addition of betacellulin prevented this from happening. With the combination of lack of Dex and addition of betacellulin, albumin mRNA levels decreased. Cultured hepatocytes had a faint expression of insulin 2 mRNA, Nkx 6.1 and Pax 6 mRNA. Dedifferentiated hepatocytes were transduced with adenoviruses expressing NeuroD1, Ngn 3, or Pax 4. NeuroD1 transduction increased the insulin 2 mRNA but caused detachment of cells. However, when hepatocytes were allowed to reaggregate for 4 and 6 days in hydrophobic plates after transduction with NeuroD1, further increases of insulin 2 mRNA were found along with induction of PDX-1, IAPP, NeuroD1, Ngn3, Pax 4, Isl-1, PC1, PC2 and islet glucokinase. Additionally, glucagon, pancreatic polypeptide and somatostatin expression were induced, but neither elastase 1 nor insulin 1 mRNA could be detected. Ngn 3 and Pax 4 had effects similar to NeuroD1, but did not increase insulin 2 mRNA as much as NeuroD1.

CONCLUSION

We conclude that the combination of NeuroD1 and reaggregation promotes cultured dedifferentiated hepatocytes to differentiate towards a pancreatic beta-cell phenotype.

Involvement of oxidative stress in the pathogenesis of diabetes

Pancreatic beta-cell failure is the common characteristic of type 1 and type 2 diabetes. Type 1 diabetes is induced by pancreatic beta-cell destruction, which is mediated by an autoimmune mechanism and consequent inflammatory process. Various inflammatory cytokines and oxidative stress produced by islet-infiltrating immune cells have been proposed to play an important role in mediating the destruction of beta cells. The JNK pathway is also activated by such cytokines and oxidative stress and is involved in beta-cell destruction. Type 2 diabetes is the most prevalent and serious metabolic disease affecting people all over the world. Pancreatic beta-cell dysfunction and insulin resistance are the hallmark of type 2 diabetes. Once hyperglycemia becomes apparent, beta-cell function gradually deteriorates, and insulin resistance is aggravated. This process is called "glucose toxicity." Under such conditions, oxidative stress is provoked, and the JNK pathway is activated, which is likely involved in pancreatic beta-cell dysfunction and insulin resistance. In addition, oxidative stress and activation of the JNK pathway are involved in the progression of atherosclerosis, which is often observed under diabetic conditions. Taken together, it is likely that oxidative stress and subsequent activation of the JNK pathway are involved in the pathogenesis of type 1 and type 2 diabetes.

Mechanisms of action of glucagon-like peptide 1 in the pancreas

Glucagon-like peptide 1 (GLP-1) is a hormone that is encoded in the proglucagon gene. It is mainly produced in enteroendocrine L cells of the gut and is secreted into the blood stream when food containing fat, protein hydrolysate, and/or glucose enters the duodenum. Its particular effects on insulin and glucagon secretion have generated a flurry of research activity over the past 20 years culminating in a naturally occurring GLP-1 receptor (GLP-1R) agonist, exendin 4 (Ex-4), now being used to treat type 2 diabetes mellitus (T2DM). GLP-1 engages a specific guanine nucleotide-binding protein (G-protein) coupled receptor (GPCR) that is present in tissues other than the pancreas (brain, kidney, lung, heart, and major blood vessels). The most widely studied cell activated by GLP-1 is the insulin-secreting beta cell where its defining action is augmentation of glucose-induced insulin secretion. Upon GLP-1R activation, adenylyl cyclase (AC) is activated and cAMP is generated, leading, in turn, to cAMP-dependent activation of second messenger pathways, such as the protein kinase A (PKA) and Epac pathways. As well as short-term effects of enhancing glucose-induced insulin secretion, continuous GLP-1R activation also increases insulin synthesis, beta cell proliferation, and neogenesis. Although these latter effects cannot be currently monitored in humans, there are substantial improvements in glucose tolerance and increases in both first phase and plateau phase insulin secretory responses in T2DM patients treated with Ex-4. This review will focus on the effects resulting from GLP-1R activation in the pancreas.

Activation of c-Jun NH2-terminal kinase (JNK) pathway during islet transplantation and prevention of islet graft loss by intraportal injection of JNK inhibitor

AIMS/HYPOTHESIS

Although application of the Edmonton protocol has markedly improved the outcome for pancreatic islet transplantation, the insulin independence rate after islet transplantation from one donor pancreas has remained low. During the isolation process and subsequent clinical transplantation, islets are subjected to severe adverse conditions that impair survival and ultimately contribute to graft failure. The aim of this study was to map the c-Jun NH2-terminal kinase (JNK) pathway that mediates islet loss during islet transplantation and to clarify whether intraportal injection with JNK inhibitor during islet transplantation can prevent islet graft loss.

METHODS

We measured JNK activity in the liver, fat and muscle of diabetic mice and in the liver immediately after islet transplantation. We examined the effect of intraportal injection of JNK inhibitory peptide at islet transplantation.

RESULTS

JNK activity became progressively higher at least until 24 h after transplantation. The cell-permeable peptide of JNK inhibitor was delivered not only in the liver but also in other insulin target organs, preventing JNK activation in the liver at least until 24 h after transplantation and reducing JNK activity in these insulin target organs. Moreover, the peptide inhibitor prevented islet graft loss immediately after transplantation and improved islet transplant outcome.

CONCLUSIONS/INTERPRETATION

These findings suggest that control of the JNK pathway is extremely important in islet transplantation and that intraportal injection of JNK inhibitor during islet transplantation (addition of JNK inhibitor to transplant media) could prevent the impairment of islet cells, leading to improved outcome for pancreatic islet transplantation.

The JNK pathway as a therapeutic target for diabetes

The hallmark of Type 2 diabetes is insulin resistance and pancreatic beta-cell dysfunction. Under diabetic conditions, the c-jun N-terminal kinase (JNK) pathway is activated in various tissues, which is involved in both insulin resistance and beta-cell dysfunction. Activation of the JNK pathway interferes with insulin action and reduces insulin biosynthesis, and suppression of the JNK pathway in diabetic mice improves insulin resistance and beta-cell function, leading to amelioration of glucose tolerance. Taken together, the JNK pathway is likely to play a central role in the progression of insulin resistance and beta-cell dysfunction and, thus, could be a potential therapeutic target for diabetes.

Protein transduction technology offers a novel therapeutic approach for diabetes

Diabetes remains a major burden, with more than 200 million people affected worldwide, representing 6% of the population. New technology, known as protein transduction technology, has been recently developed. A variety of peptides, known as protein transduction domains or cell-penetrating peptides, have been characterized for their ability to translocate into live cells. There are numerous examples of biologically active full-length proteins and peptides that have been delivered to cells and tissues both in vitro and in vivo, suggesting new avenues for the treatment of several diseases. Some studies have shown that this technology is useful for the treatment of diabetes. In islet isolation and transplantation, cell-permeable peptides deliver anti-apoptotic molecules to protect islets. Another peptide provides immunosuppression for fully mismatched islet allografts in mice. These findings suggest that peptide drugs could lead to outcome improvement for pancreatic islet transplantation. In mice with type 2 diabetes, a cell-penetrating peptide markedly improves insulin resistance and ameliorates glucose tolerance. Moreover, the technology facilitates the differentiation of stem cells into insulin-producing cells. Protein transduction technology has opened up several possibilities for the development of new peptide/protein drugs for the treatment of diabetes.

PDX-1 and MafA in β-cell differentiation and dysfunction

Pancreatic and duodenal homeobox factor-1 (PDX-1) plays crucial roles in pancreas development and β-cell differentiation, and in maintaining mature β-cell function. MafA is a recently isolated β-cell-specific transcription factor that functions as a potent activator of insulin gene transcription. Also, these pancreatic transcription factors play a crucial role in inducing surrogate β-cells from non-β-cells and, thus, could be therapeutic targets for diabetes. Conversely, expression and/or activities of PDX-1 and MafA in β-cells are reduced under diabetic conditions, which leads to suppression of insulin biosynthesis and secretion. It is likely that alteration of such transcription factors explains, at least in part, the molecular mechanism for β-cell glucose toxicity.

Inhibition of cyclin-dependent kinase 5 activity protects pancreatic beta cells from glucotoxicity

Type 2 diabetes (T2D) and Alzheimer disease are degenerative diseases that may share common pathophysiologic mechanisms. Neuronal dysfunction in Alzheimer patients has been linked to overactivity of the cyclin-dependent kinase 5 (CDK5) and its activator p35. Both of these proteins are expressed in the insulin-producing beta cells of the pancreas. Further, glucose enhances p35 gene expression, promoting the formation of active p35/CDK5 complexes that regulate the expression of the insulin gene. In T2D, chronic elevations of glucose, glucotoxicity, impair beta cell function. We therefore postulated that CDK5 and p35 may be responsible for this beta cell impairment and that inhibition of CDK5 might have a beneficial effect. To test this hypothesis, the pancreatic cell line INS-1 was selected as a known in vitro model of glucotoxicity, and roscovitine (10 mum) was used as a CDK5 inhibitor. Chronic exposure of INS-1 cells to high glucose (20-30 mm) reduced both insulin mRNA levels and the activity of an insulin promoter reporter gene. Inhibition of CDK5 prevented this decrease of insulin gene expression. We used DNA binding (gel shift) assays and Western immunoblots to demonstrate that cellular levels of the transcription factor PDX-1, normally decreased by glucotoxicity, were preserved with CDK5 inhibition, as was the binding of PDX-1 to the insulin promoter. Analyses of nuclear and cytoplasmic PDX-1 protein levels revealed that CDK5 inhibition restores nuclear PDX-1, without affecting its cytoplasmic concentration, suggesting that CDK5 regulates the nuclear/cytoplasm partitioning of PDX-1. Using a Myc-tagged PDX-1 construct, we showed that the translocation of PDX-1 from the nucleus to the cytoplasm during glucotoxic conditions was prevented when CDK5 was inhibited. These studies indicate that CDK5 plays a role in the loss of beta cell function under glucotoxic conditions and that CDK5 inhibitors could have therapeutic value for T2D.