Phosphorylation-dependent nucleocytoplasmic shuttling of pancreatic duodenal homeobox-1

The hippo kinases MST1/2 in cardiovascular and metabolic diseases: A promising therapeutic target option for pharmacotherapy

Cardiovascular diseases (CVDs) and metabolic disorders are major components of noncommunicable diseases, causing an enormous health and economic burden worldwide. There are common risk factors and developmental mechanisms among them, indicating the far-reaching significance in exploring the corresponding therapeutic targets. MST1/2 kinases are well-established proapoptotic effectors that also bidirectionally regulate autophagic activity. Recent studies have demonstrated that MST1/2 influence the outcome of cardiovascular and metabolic diseases by regulating immune inflammation. In addition, drug development against them is in full swing. In this review, we mainly describe the roles and mechanisms of MST1/2 in apoptosis and autophagy in cardiovascular and metabolic events as well as emphasis on the existing evidence for their involvement in immune inflammation. Moreover, we summarize the latest progress of pharmacotherapy targeting MST1/2 and propose a new mode of drug combination therapy, which may be beneficial to seek more effective strategies to prevent and treat CVDs and metabolic disorders.

Arsenic in Drinking Water and Diabetes

Arsenic is ubiquitous in soil and water environments and is consistently at the top of the Agency for Toxic Substances Disease Registry (ATSDR) substance priority list. It has been shown to induce toxicity even at low levels of exposure. One of the major routes of exposure to arsenic is through drinking water. This review presents current information related to the distribution of arsenic in the environment, the resultant impacts on human health, especially related to diabetes, which is one of the most prevalent chronic diseases, regulation of arsenic in drinking water, and approaches for treatment of arsenic in drinking water for both public utilities and private wells. Taken together, this information points out the existing challenges to understanding both the complex health impacts of arsenic and to implementing the treatment strategies needed to effectively reduce arsenic exposure at different scales.

TRPM7 kinase is required for insulin production and compensatory islet responses during obesity

Most overweight individuals do not develop diabetes due to compensatory islet responses to restore glucose homeostasis. Therefore, regulatory pathways that promote β cell compensation are potential targets for treatment of diabetes. The transient receptor potential cation channel subfamily M member 7 protein (TRPM7), harboring a cation channel and a serine/threonine kinase, has been implicated in controlling cell growth and proliferation. Here, we report that selective deletion of Trpm7 in β cells disrupted insulin secretion and led to progressive glucose intolerance. We indicate that the diminished insulinotropic response in β cell-specific Trpm7-knockout mice was caused by decreased insulin production because of impaired enzymatic activity of this protein. Accordingly, high-fat-fed mice with a genetic loss of TRPM7 kinase activity displayed a marked glucose intolerance accompanied by hyperglycemia. These detrimental glucoregulatory effects were engendered by reduced compensatory β cell responses because of mitigated protein kinase B (AKT)/ERK signaling. Collectively, our data identify TRPM7 kinase as a potentially novel regulator of insulin synthesis, β cell dynamics, and glucose homeostasis under obesogenic diet.

PDX1 is the cornerstone of pancreatic β-cell functions and identity

Diabetes has been a worldwide healthcare problem for many years. Current methods of treating diabetes are still largely directed at symptoms, aiming to control the manifestations of the pathology. This creates an overall need to find alternative measures that can impact on the causes of the disease, reverse diabetes, or make it more manageable. Understanding the role of key players in the pathogenesis of diabetes and the related β-cell functions is of great importance in combating diabetes. PDX1 is a master regulator in pancreas organogenesis, the maturation and identity preservation of β-cells, and of their role in normal insulin function. Mutations in the PDX1 gene are correlated with many pancreatic dysfunctions, including pancreatic agenesis (homozygous mutation) and MODY4 (heterozygous mutation), while in other types of diabetes, PDX1 expression is reduced. Therefore, alternative approaches to treat diabetes largely depend on knowledge of PDX1 regulation, its interaction with other transcription factors, and its role in obtaining β-cells through differentiation and transdifferentiation protocols. In this article, we review the basic functions of PDX1 and its regulation by genetic and epigenetic factors. Lastly, we summarize different variations of the differentiation protocols used to obtain β-cells from alternative cell sources, using PDX1 alone or in combination with various transcription factors and modified culture conditions. This review shows the unique position of PDX1 as a potential target in the genetic and cellular treatment of diabetes.

PDX-1: A Promising Therapeutic Target to Reverse Diabetes

The pancreatic duodenum homeobox-1 (PDX-1) is a transcription factor encoded by a Hox-like homeodomain gene that plays a crucial role in pancreatic development, β-cell differentiation, and the maintenance of mature β-cell functions. Research on the relationship between PDX-1 and diabetes has gained much attention because of the increasing prevalence of diabetes melitus (DM). Recent studies have shown that the overexpression of PDX-1 regulates pancreatic development and promotes β-cell differentiation and insulin secretion. It also plays a vital role in cell remodeling, gene editing, and drug development. Conversely, the absence of PDX-1 increases susceptibility to DM. Therefore, in this review, we summarized the role of PDX-1 in pancreatic development and the pathogenesis of DM. A better understanding of PDX-1 will deepen our knowledge of the pathophysiology of DM and provide a scientific basis for exploring PDX-1 as a potential target for treating diabetes.

Secreted PDZD2 exerts an insulinotropic effect on INS-1E cells by a PKA-dependent mechanism

Secreted PDZD2 (sPDZD2) is a signaling molecule generated upon proteolytic processing of the multi-PDZ-containing protein PDZD2. Previous analysis of gene-trap mice deficient in the synthesis of full-length PDZD2, but not the secreted form, revealed a role of PDZD2 in the regulation of glucose-stimulated insulin secretion. Here, using the pancreatic INS-1E β cells as in vitro model, we showed that depletion of PDZD2/sPDZD2 by RNA interference suppressed the expression of β-cell genes Ins1, Glut2 and MafA whereas treatment with recombinant sPDZD2 rescued the suppressive effect. Similar to GLP-1, sPDZD2 stimulated intracellular cAMP levels, activated β-cell gene expression in a PKA-dependent manner and induced the phosphorylation and nuclear localization of PDX1. Depletion of PDX1 inhibited the sPDZD2 insulinotropic effect, which could also be demonstrated in mouse islets. In summary, our findings are consistent with sPDZD2 serving a signaling function in regulating β-cell gene expression.

Specific deletion of CDC42 in pancreatic β cells attenuates glucose-induced insulin expression and secretion in mice

Insulin is a key hormone for maintaining glucose homeostasis in organisms. In general, deficiency of insulin synthesis and secretion results in type I diabetes, whereas insulin resistance leads to type 2 diabetes. Cell division cycle 42 (CDC42), a member of Rho GTPases family, has been shown as an essential regulator in the second phase of glucose-induced insulin secretion in pancreatic islets β cells in vitro. However, the effect of CDC42 on insulin expression has not been explored. Here we reported that the glucose-induced insulin expression and secretion were significantly inhibited in mice lacking CDC42 gene in pancreatic β cells (Rip-CDC42cKO) in vivo and in vitro. Deletion of CDC42 gene in pancreatic β cells did not affect survival or reproduction in mice. However, the Rip-CDC42cKO mice showed the systemic glucose intolerance and the decrease of glucose-induced insulin secretion without apparent alterations of peripheral tissues insulin sensitivity and the morphology of islets. Furthermore, we demonstrated that deletion of CDC42 gene in pancreatic β cells significantly attenuated the insulin expression through inhibiting the ERK1/2-NeuroD1 signaling pathway. Taken together, our study presents novel evidence that CDC42 is an important modulator in glucose-induced insulin expression as well as insulin secretion in pancreatic β cells.

Assessment of arsenic-induced modifications in the DNA methylation of insulin-related genes in rat pancreatic islets

Extended exposure to inorganic arsenic through contaminated drinking water has been linked with increased incidence of diabetes mellitus. The most common exposure occurs through the consumption of contaminated drinking water mainly through geogenic sources of inorganic arsenic. Epigenetic modifications are important mechanisms through which environmental pollutants could exert their toxic effects. Bisulfite sequencing polymerase chain reaction method followed by Sanger sequencing was performed for DNA methylation analysis. Our results showed that sodium arsenite treatment significantly reduced insulin secretion in pancreatic islets. It was revealed that the methylation of glucose transporter 2 (Glut2) gene was changed at two cytosine-phosphate-guanine (CpG) sites (-1743, -1734) in the promoter region of the sodium arsenite-treated group comparing to the control. No changes were observed in the methylation status of peroxisome proliferator-activated receptor-gamma (PPARγ), pancreatic and duodenal homeobox 1 (Pdx1) and insulin 2 (Ins2) CpG sites in the targeted regions. Measuring the gene expression level showed increase in Glut2 expression, while the expression of insulin (INS) and Pdx1 were significantly affected by sodium arsenite treatment. This study revealed that exposure to sodium arsenite changed the DNA methylation pattern of Glut2, a key transporter of glucose entry into the pancreatic beta cells (β-cells). Our data suggested possible epigenetic-mediated toxicity mechanism for arsenite-induced β-cells dysfunction. Further studies are needed to dissect the precise epigenetic modulatory activity of sodium arsenite that affect the biogenesis of insulin.

Small Molecule-Induced Pancreatic β-Like Cell Development: Mechanistic Approaches and Available Strategies

Diabetes is a metabolic disease which affects not only glucose metabolism but also lipid and protein metabolism. It encompasses two major types: type 1 and 2 diabetes. Despite the different etiologies of type 1 and 2 diabetes mellitus (T1DM and T2DM, respectively), the defining features of the two forms are insulin deficiency and resistance, respectively. Stem cell therapy is an efficient method for the treatment of diabetes, which can be achieved by differentiating pancreatic β-like cells. The consistent generation of glucose-responsive insulin releasing cells remains challenging. In this review article, we present basic concepts of pancreatic organogenesis, which intermittently provides a basis for engineering differentiation procedures, mainly based on the use of small molecules. Small molecules are more auspicious than any other growth factors, as they have unique, valuable properties like cell-permeability, as well as a nonimmunogenic nature; furthermore, they offer immense benefits in terms of generating efficient functional beta-like cells. We also summarize advances in the generation of stem cell-derived pancreatic cell lineages, especially endocrine β-like cells or islet organoids. The successful induction of stem cells depends on the quantity and quality of available stem cells and the efficient use of small molecules.

Controlled clustering enhances PDX1 and NKX6.1 expression in pancreatic endoderm cells derived from pluripotent stem cells

Pluripotent stem cell (PSC)-derived insulin-producing cells are a promising cell source for diabetes cellular therapy. However, the efficiency of the multi-step process required to differentiate PSCs towards pancreatic beta cells is variable between cell lines, batches and even within cultures. In adherent pancreatic differentiation protocols, we observed spontaneous local clustering of cells expressing elevated nuclear expression of pancreatic endocrine transcription factors, PDX1 and NKX6.1. Since aggregation has previously been shown to promote downstream differentiation, this local clustering may contribute to the variability in differentiation efficiencies observed within and between cultures. We therefore hypothesized that controlling and directing the spontaneous clustering process would lead to more efficient and consistent induction of pancreatic endocrine fate. Micropatterning cells in adherent microwells prompted clustering, local cell density increases, and increased nuclear accumulation of PDX1 and NKX6.1. Improved differentiation profiles were associated with distinct filamentous actin architectures, suggesting a previously overlooked role for cell-driven morphogenetic changes in supporting pancreatic differentiation. This work demonstrates that confined differentiation in cell-adhesive micropatterns may provide a facile, scalable, and more reproducible manufacturing route to drive morphogenesis and produce well-differentiated pancreatic cell clusters.

Long-Term Liraglutide Administration Induces Pancreas Neogenesis in Adult T2DM Mice

In vivo beta-cell neogenesis may be one way to treat diabetes. We aimed to investigate the effect of glucagon-like peptide-1 (GLP-1) on beta-cell neogenesis in type 2 diabetes mellitus (T2DM). Male C57BL/6J mice, 6 wk old, were randomly divided into three groups: Control, T2DM, and T2DM + Lira. T2DM was induced using high-fat diet and intraperitoneal injection of streptozotocin (40 mg/kg/d for 3 d). At 8 wk after streptozotocin injection, T2DM + Lira group was injected intraperitoneally with GLP-1 analog liraglutide (0.8 mg/kg/d) for 4 wk. Apparently for the first time, we report the appearance of a primitive bud connected to pancreas in all adult mice from each group. The primitive bud was characterized by scattered single monohormonal cells expressing insulin, GLP-1, somatostatin, or pancreatic polypeptide, and four-hormonal cells, but no acinar cells and ductal epithelial cells. Monohormonal cells in it were small, newborn, immature cells that rapidly proliferated and expressed cell markers indicative of immaturity. In parallel, Ngn3+ endocrine progenitors and Nestin+ cells existed in the primitive bud. Liraglutide facilitated neogenesis and rapid growth of acinar cells, pancreatic ducts, and blood vessels in the primitive bud. Meanwhile, scattered hormonal cells aggregated into cell clusters and grew into larger islets; polyhormonal cells differentiated into monohormonal cells. Extensive growth of exocrine and endocrine glands resulted in the neogenesis of immature pancreatic lobes in adult mice of T2DM + Lira group. Contrary to predominant acinar cells in mature pancreatic lobes, there were still a substantial number of mesenchymal cells around acinar cells in immature pancreatic lobes, which resulted in the loose appearance. Our results suggest that adult mice preserve the capacity of pancreatic neogenesis from the primitive bud, which liraglutide facilitates in adult T2DM mice. To our knowledge, this is the first time such a phenomenon has been reported.

Effects of boschnaloside from Boschniakia rossica on dysglycemia and islet dysfunction in severely diabetic mice through modulating the action of glucagon-like peptide-1

BACKGROUND

Boschniakia rossica is a well-known traditional Chinese medicine for tonifying kidney and improving impotence. Boschnaloside is the major iridoid glycoside in this herb but therapeutic benefits for diabetes remained to be evaluated.

HYPOTHESIS/PURPOSE

The current investigation aims to study the antidiabetic effect and the underlying pharmacological mechanisms.

STUDY DESIGN AND METHODS

Receptor binding, cAMP production, Ins secretion, glucagon-like peptide 1 (GLP-1) secretion, and dipeptidyl peptidase-4 activity assays were performed. Therapeutic benefits of orally administrated boschnaloside (150 and 300 mg/kg/day) were evaluated using severely 12-week old female diabetic db/db mice (Hemoglobin A1c >10%).

RESULTS

Oral treatment of boschnaloside for 4 weeks improved diabetic symptoms including fasting blood sugar, hemoglobin A1c, glucose intolerance, and Homeostatic Model Assessment of Ins Resistance, accompanied by circulating GLP-1_active and adiponectin levels. In addition, bochnaloside treatment improved islet/β cell function associated with an alteration of the pancreatic and duodenal homeobox 1 level. It was shown that boschnaloside interacted with the extracellular domain of GLP-1 receptor and enhanced glucose stimulated Ins secretion. Boschnaloside also augmented the insulinotropic effect of GLP-1. Finally, the presence of boschnaloside caused a reduction of dipeptidyl peptidase-4 activity while enhanced GLP-1 secretion from STC-1 cells.

CONCLUSION

It appears that bochnaloside at oral dosage greater than 150 mg/kg/day exerts antidiabetic effects in vivo through modulating the action of GLP-1.

DPP4 inhibitor induces beta cell regeneration and DDR-1 protein expression as an endocrine progenitor cell marker in neonatal STZ-diabetic rats

BACKGROUND

We aim to investigate the effects of dipeptidyl-peptidase-4 inhibitor (Vildagliptin-VG) on DDR-1 as a marker for endocrine progenitor cells, β-cell regeneration, and apoptosis in neonatal streptozotocin (n2-STZ) diabetics.

METHODS

Neonatal rats were divided into two main groups as short- and long-term treatment, each consisted of four groups; (1) Control, (2) n2-STZ diabetic (single dose of 100 mg/kg STZ at 2nd day of birth), (3) n2-STZ + VG (60 mg/kg/day VG orally; for 8 and 28 days), (4) VG (60 mg/kg/day orally; for 8 and 28 days). Blood glucose levels and body weights were measured, and the tissue sections were immunostained using insulin, glucagon, somatostatin, PCNA, Pdx-1 and DDR-1 antibodies. The TUNEL method was used for apoptosis.

RESULTS

The number of β cells in islets of the n2-STZ + VG group increased compared to the n2-STZ group; insulin (+) cells were observed individually or as small clusters in exocrine tissue, between pancreatic duct epithelial cells, and around the ducts. The number of Pdx-1 and DDR-1 positive cells in islet and extra-islet pancreas tissue was elevated as a result of VG application compared to the STZ diabetic group; the number of double positive cells for DDR-1 and insulin increased in n2-STZ + VG rats.

CONCLUSION

We showed that vildagliptin promotes β cell neogenesis and regeneration, stimulates DDR-1 expression as an endocrine cell progenitor marker, suppresses apoptosis, induces islet cell proliferation and rearranges islet morphology in the n2-STZ diabetes model.

Defining a Novel Role for the Pdx1 Transcription Factor in Islet β-Cell Maturation and Proliferation During Weaning

The transcription factor encoded by the Pdx1 gene is a critical transcriptional regulator, as it has fundamental actions in the formation of all pancreatic cell types, islet β-cell development, and adult islet β-cell function. Transgenic- and cell line-based experiments have identified 5'-flanking conserved sequences that control pancreatic and β-cell type-specific transcription, which are found within areas I (bp -2694 to -2561), II (bp -2139 to -1958), III (bp -1879 to -1799), and IV (bp -6200 to -5670). Because of the presence in area IV of binding sites for transcription factors associated with pancreas development and islet cell function, we analyzed how an endogenous deletion mutant affected Pdx1 expression embryonically and postnatally. The most striking result was observed in male Pdx1ΔIV mutant mice after 3 weeks of birth (i.e., the onset of weaning), with only a small effect on pancreas organogenesis and no deficiencies in their female counterparts. Compromised Pdx1 mRNA and protein levels in weaned male mutant β-cells were tightly linked with hyperglycemia, decreased β-cell proliferation, reduced β-cell area, and altered expression of Pdx1-bound genes that are important in β-cell replication, endoplasmic reticulum function, and mitochondrial activity. We discuss the impact of these novel findings to Pdx1 gene regulation and islet β-cell maturation postnatally.

Self-Transducible Bimodal PDX1-FOXP3 Protein Lifts Insulin Secretion and Curbs Autoimmunity, Boosting Tregs in Type 1 Diabetic Mice

Type 1 diabetes (T1D) is characterized by massive destruction of insulin-producing β cells by autoreactive T lymphocytes, arising via defective immune tolerance. Therefore, effective anti-T1D therapeutics should combine autoimmunity-preventing and insulin production-restoring properties. We constructed a cell-permeable PDX1-FOXP3-TAT fusion protein (FP) composed of two transcription factors: forkhead box P3 (FOXP3), the master regulator of differentiation and functioning of self-tolerance-promoting Tregs, and pancreatic duodenal homeobox-1 (PDX1), the crucial factor supporting β cell development and maintenance. The FP was tested in vitro and in a non-obese diabetic mouse T1D model. In vitro, FP converted naive CD4⁺ T cells into a functional "Treg-like" subset, which suppressed cytokine secretion, downregulated antigen-specific responses, and curbed viability of diabetogenic effector cells. In hepatic stem-like cells, FP potentiated endocrine transdifferentiation, inducing expression of Insulin2 and other β lineage-specific genes. In vivo, FP administration to chronically diabetic mice triggered (1) a significant elevation of insulin and C-peptide levels, (2) the formation of insulin-containing cell clusters in livers, and (3) a systemic anti-inflammatory shift (higher Foxp3⁺CD4⁺CD25⁺ T cell frequencies, elevated rates of IL-10-producing cells, and reduced rates of IFN-γ-secreting cells). Overall, in accordance with its design, PDX1-FOXP3-TAT FP delivered both Treg-stabilizing anti-autoimmune and de novo insulin-producing effects, proving its anti-T1D therapeutic potential.

MST1: a promising therapeutic target to restore functional beta cell mass in diabetes

The loss of insulin-producing beta cells by apoptosis is a hallmark of all forms of diabetes mellitus. Strategies to prevent beta cell apoptosis and dysfunction are urgently needed to restore the insulin-producing cells and to prevent severe diabetes progression. We recently identified the serine/threonine kinase known as mammalian sterile 20-like kinase 1 (MST1) as a critical regulator of apoptotic beta cell death and dysfunction. MST1 activates several apoptotic signalling pathways, which further stimulate its own cleavage, leading to a vicious cycle of cell death. This led us to hypothesise that MST1 signalling is central to the initiation of beta cell death in diabetes. We found that MST1 is strongly activated in a diabetic beta cell and induces not only its death but also directly impairs insulin secretion through promoting proteasomal degradation of key beta cell transcription factor, pancreatic and duodenal homeobox 1 (PDX1), which is critical for insulin production.Pre-clinical studies in various animal models of diabetes have reported that MST1 deficiency remarkably restores normoglycaemia and beta cell function and prevents the development of diabetes. Importantly, MST1 deficiency can revert fully diabetic beta cells to a non-diabetic state. MST1 may serve as a target for the development of novel therapies for diabetes that trigger the cause of the disease, namely, the destruction of the beta cells. The major current focus of our investigation is to identify and test the efficacy of potent inhibitors of this death signalling pathway to protect beta cells against the effects of autoimmune attack in type 1 diabetes and to preserve beta cell mass and function in type 2 diabetes. This review summarises a presentation given at the 'Can we make a better beta cell?' symposium at the 2015 annual meeting of the EASD. It is accompanied by two other reviews on topics from this symposium (by Heiko Lickert and colleagues, DOI: 10.1007/s00125-016-3949-9 , and by Harry Heimberg and colleagues, DOI: 10.1007/s00125-016-3879-6 ) and a commentary by the Session Chair, Shanta Persaud (DOI: 10.1007/s00125-016-3870-2 ).

Preadipocyte factor 1 induces pancreatic ductal cell differentiation into insulin-producing cells

The preadipocyte factor 1 (Pref-1) is involved in the proliferation and differentiation of various precursor cells. However, the intracellular signaling pathways that control these processes and the role of Pref-1 in the pancreas remain poorly understood. Here, we showed that Pref-1 induces insulin synthesis and secretion via two independent pathways. The overexpression of Pref-1 activated MAPK signaling, which induced nucleocytoplasmic translocation of FOXO1 and PDX1 and led to the differentiation of human pancreatic ductal cells into β-like cells and an increase in insulin synthesis. Concurrently, Pref-1 activated Akt signaling and facilitated insulin secretion. A proteomics analysis identified the Rab43 GTPase-activating protein as a downstream target of Akt. A serial activation of both proteins induced various granular protein syntheses which led to enhanced glucose-stimulated insulin secretion. In a pancreatectomised diabetic animal model, exogenous Pref-1 improved glucose homeostasis by accelerating pancreatic ductal and β-cell regeneration after injury. These data establish a novel role for Pref-1, opening the possibility of applying this molecule to the treatment of diabetes.

PRMT1 promotes glucose toxicity-induced β cell dysfunction by regulating the nucleo-cytoplasmic trafficking of PDX-1 in a FOXO1-dependent manner in INS-1 cells

Protein N-arginine methyltransferase-1 (PRMT1), the major asymmetric arginine methyltransferase, plays important roles in various cellular processes. Previous reports have demonstrated that levels and activities of PRMT1 can vary in animals with type 2 diabetes mellitus. The aim of this study was to assess the expression and mechanism of action of PRMT1 during glucose toxicity-induced β cell dysfunction. Liposome-mediated gene transfection was used to transfect INS-1 cells with siPRMT1, which inhibits PRMT1 expression, and pALTER-FOXO1, which overexpresses forkhead box protein O1 (FOXO1). The cells were then cultured in media containing 5.6 or 25 mmol/L glucose with or without the small molecule PRMT1 inhibitor AMI-1 for 48 h. The protein levels of PRMT1, the arginine methylated protein α-metR, FOXO1, Phospho-FOXO1, pancreas duodenum homeobox-1 (PDX-1), and the intracellular localization of PDX-1 and FOXO1 were then measured by western blotting. FOXO1 methylation was detected by immunoprecipitated with anti-PRMT1 antibody and were immunoblotted with α-metR. The levels of insulin mRNA were measured by real-time fluorescence quantitative PCR. Glucose-stimulated insulin secretion (GSIS) and intracellular insulin content were measured using radioimmunoassays. Intracellular Ca(2+) ([Ca(2+)]i) was detected using Fura-2 AM. Intracellular cAMP levels were measured using ELISA. Chronic exposure to high glucose impaired insulin secretion, decreased insulin mRNA levels and insulin content, increased intracellular [Ca(2+)]i and cAMP levels, and abolishes their responses to glucose. Inhibiting PRMT1 expression improved insulin secretion, increased mRNA levels and insulin content by regulating the intracellular translocation of PDX-1 and FOXO1, decreasing the methylation of FOXO1, and reducing intracellular [Ca(2+)]i and cAMP concentrations. Transient overexpression of constitutively active FOXO1 in nuclear reversed the AMI-1-induced improvement of β cell function without changing arginine methylation. It is concluded therefore that PRMT1 regulates GSIS in INS-1 cells, through enhanced methylation-induced nuclear localization of FOXO1, which subsequently suppresses the nuclear localization of PDX-1. Our results suggest a novel mechanism that might contribute to the deficient insulin secretion observed under conditions of chronically hyperglycemia.

Resveratrol ameliorates the maturation process of β-cell-like cells obtained from an optimized differentiation protocol of human embryonic stem cells

Human embryonic stem cells (hESCs) retain the extraordinary capacity to differentiate into different cell types of an adult organism, including pancreatic β-cells. For this particular lineage, although a lot of effort has been made in the last ten years to achieve an efficient and reproducible differentiation protocol, it was not until recently that this aim was roughly accomplished. Besides, several studies evidenced the impact of resveratrol (RSV) on insulin secretion, even though the mechanism by which this polyphenol potentiates glucose-stimulated insulin secretion (GSIS) is still not clear. The aim of this study was to optimize an efficient differentiation protocol that mimics in vivo pancreatic organogenesis and to investigate whether RSV may improve the final maturation step to obtain functional insulin-secreting cells. Our results indicate that treatment of hESCs (HS-181) with activin-A induced definitive endoderm differentiation as detected by the expression of SOX17 and FOXA2. Addition of retinoic acid (RA), Noggin and Cyclopamine promoted pancreatic differentiation as indicated by the expression of the early pancreatic progenitor markers ISL1, NGN3 and PDX1. Moreover, during maturation in suspension culture, differentiating cells assembled in islet-like clusters, which expressed specific endocrine markers such as PDX1, SST, GCG and INS. Similar results were confirmed with the human induced Pluripotent Stem Cell (hiPSC) line MSUH-001. Finally, differentiation protocols incorporating RSV treatment yielded numerous insulin-positive cells, induced significantly higher PDX1 expression and were able to transiently normalize glycaemia when transplanted in streptozotocin (STZ) induced diabetic mice thus promoting its survival. In conclusion, our strategy allows the efficient differentiation of hESCs into pancreatic endoderm capable of generating β-cell-like cells and demonstrates that RSV improves the maturation process.

Regulation of postnatal pancreatic Pdx1 and downstream target genes after gestational exposure to protein restriction in rats

The study carried out in our laboratory demonstrated that protein restriction (low protein, LP) during fetal and neonatal life alters pancreatic development and impairs glucose tolerance later in life. In this study, we examined the role of the transcription factorPdx1, a master regulator of β-cell differentiation and function along with its downstream target genes insulin,Glut2and glucokinase (GK). The role(s) of these genes and protein products on the pancreata of male offspring from mothers exposed to LP diets were assessed during gestation, weaning, and adult life. Pregnant rats were allocated to two dietary treatments: control (C) 20% protein diet or LP, 8% protein diet. At birth, offspring were divided into four groups: C received control diet all life, LP1 received LP diet all life, LP2 changed the LP diet to C at weaning, and LP3 switched to C after being exposed to LP during gestation only. Body weights (bw) were significantly (P<0.001) decreased in all LP groups at birth. At weaning, only the LP3 offspring had their body weight restored to control levels.Pdx1or any of thePdx1-target genes were similar in all diets at day 21. However, at d130Pdx1mRNA expression and protein abundance were significantly decreased (P<0.05) in all LP groups. In addition, insulin mRNA and protein were decreased in LP1 and LP3 groups compared with C,Glut2mRNA and GLUT2 protein levels were decreased in LP3 and GK did not change between groups. Intraperitoneal glucose tolerance test revealed impaired glucose tolerance in LP3 males, concomitant with decreased β-cell mass, islet area, and PDX1 nuclear protein localization. Collectively, this study suggests that restoring proteins in the diet after birth in LP offspring dramatically impairs glucose homeostasis in early adulthood, by alteringPdx1expression and downstream-target genes increasing the risk to develop type 2 diabetes.

Differences in insulin biosynthesis pathway between small and large islets do not correspond to insulin secretion

In a variety of mammalian species, small islets secrete more insulin per volume than large islets. This difference may be due to diffusional limitations of large islets, or inherent differences in the insulin production pathways. The purpose of this study was to identify possible differences in the early phase of glucose-stimulated insulin biosynthesis between large and small islets. Isolated small and large rat islets were challenged with 30 minutes of high glucose. The expression of insulin gene transcription factors (MafA, NeuroD/ Beta2, and PDX-1), preproinsulin mRNA, proinsulin and insulin were compared between large and small islets. Under basal (low glucose) conditions, MafA and NeuroD had higher mRNA levels and greater protein amounts in large islets compared to small when normalized to GAPDH levels. 30 minutes of high glucose stimulation failed to alter the mRNA or subsequent protein levels of either gene. However, 30 minutes of high glucose suppressed activated PDX-1 protein levels in both small and large islets. High glucose stimulation did not statistically alter the preproinsulin mRNA (insulin 1 and insulin 2) levels. At the translational level, high glucose increased the proinsulin levels, and large islets showed a higher proinsulin content per cell than small islets. Insulin content per cell was not significantly different between small and large islets under basal or high glucose levels. The results fail to explain the higher level of insulin secretion noted in small versus large islets and may suggest that possible differences lie downstream in the secretory pathway rather than insulin biosynthesis.

Pancreatic and duodenal homeobox-1 nuclear localization is regulated by glucose in dispersed rat islets but not in insulin-secreting cell lines

The transcription factor Pancreatic and Duodenal Homeobox-1 (PDX-1) plays a major role in the development and function of pancreatic β-cells and its mutation results in diabetes. In adult β-cells, glucose stimulates transcription of the insulin gene in part by regulating PDX-1 expression, stability and activity. Glucose is also thought to modulate PDX-1 nuclear translocation but in vitro studies examining nucleo-cytoplasmic shuttling of endogenous or ectopically expressed PDX-1 in insulin-secreting cell lines have led to conflicting results. Here we show that endogenous PDX-1 undergoes translocation from the cytoplasm to the nucleus in response to glucose in dispersed rat islets but not in insulin-secreting MIN6, HIT-T15, or INS832/13 cells. Interestingly, however, we found that a PDX-1-GFP fusion protein can shuttle from the cytoplasm to the nucleus in response to glucose stimulation in HIT-T15 cells. Our results suggest that the regulation of endogenous PDX-1 sub-cellular localization by glucose is observed in primary islets and that care should be taken when interpreting data from insulin-secreting cell lines.

In vitro differentiation into insulin-producing β-cells of stem cells isolated from human amniotic fluid and dental pulp

AIM

To investigate the ability of human amniotic fluid stem cells and human dental pulp stem cells to differentiate into insulin-producing cells.

METHODS

Human amniotic fluid stem cells and human dental pulp stem cells were induced to differentiate into pancreatic β-cells by a multistep protocol. Islet-like structures were assessed in differentiated human amniotic fluid stem cells and human dental pulp stem cells after 21 days of culture by dithizone staining. Pancreatic and duodenal homebox-1, insulin and Glut-2 expression were detected by immunofluorescence and confocal microscopy. Insulin secreted from differentiated cells was tested with SELDI-TOF MS and by enzyme-linked immunosorbent assay.

RESULTS

Human amniotic fluid stem cells and human dental pulp stem cells, after 7 days of differentiation started to form islet-like structures that became evident after 14 days of induction. SELDI-TOF MS analysis, revealed the presence of insulin in the media of differentiated cells at day 14, further confirmed by enzyme-linked immunosorbent assay after 7, 14 and 21 days. Both stem cell types expressed, after differentiation, pancreatic and duodenal homebox-1, insulin and Glut-2 and were positively stained by dithizone. Either the cytosol to nucleus translocation of pancreatic and duodenal homebox-1, either the expression of insulin, are regulated by glucose concentration changes. Day 21 islet-like structures derived from both human amniotic fluid stem cells and human dental pulp stem cell release insulin in a glucose-dependent manner.

CONCLUSION

The present study demonstrates the ability of human amniotic fluid stem cells and human dental pulp stem cell to differentiate into insulin-producing cells, offering a non-pancreatic, low-invasive source of cells for islet regeneration.

Per-Arnt-Sim kinase regulates pancreatic duodenal homeobox-1 protein stability via phosphorylation of glycogen synthase kinase 3β in pancreatic β-cells

In pancreatic β-cells, glucose induces the binding of the transcription factor pancreatic duodenal homeobox-1 (PDX-1) to the insulin gene promoter to activate insulin gene transcription. At low glucose levels, glycogen synthase kinase 3β (GSK3β) is known to phosphorylate PDX-1 on C-terminal serine residues, which triggers PDX-1 proteasomal degradation. We previously showed that the serine/threonine Per-Arnt-Sim domain-containing kinase (PASK) regulates insulin gene transcription via PDX-1. However, the mechanisms underlying this regulation are unknown. In this study, we aimed to identify the role of PASK in the regulation of PDX-1 phosphorylation, protein expression, and stability in insulin-secreting cells and isolated rodent islets of Langerhans. We observed that glucose induces a decrease in overall PDX-1 serine phosphorylation and that overexpression of WT PASK mimics this effect. In vitro, PASK directly phosphorylates GSK3β on its inactivating phosphorylation site Ser(9). Overexpression of a kinase-dead (KD), dominant negative version of PASK blocks glucose-induced Ser(9) phosphorylation of GSK3β. Accordingly, GSK3β Ser(9) phosphorylation is reduced in islets from pask-null mice. Overexpression of WT PASK or KD GSK3β protects PDX-1 from degradation and results in increased PDX-1 protein abundance. Conversely, overexpression of KD PASK blocks glucose-induction of PDX-1 protein. We conclude that PASK phosphorylates and inactivates GSK3β, thereby preventing PDX-1 serine phosphorylation and alleviating GSK3β-mediated PDX-1 protein degradation in pancreatic β-cells.

Inactivation of specific β cell transcription factors in type 2 diabetes

Type 2 diabetes (T2DM) commonly arises from islet β cell failure and insulin resistance. Here, we examined the sensitivity of key islet-enriched transcription factors to oxidative stress, a condition associated with β cell dysfunction in both type 1 diabetes (T1DM) and T2DM. Hydrogen peroxide treatment of β cell lines induced cytoplasmic translocation of MAFA and NKX6.1. In parallel, the ability of nuclear PDX1 to bind endogenous target gene promoters was also dramatically reduced, whereas the activity of other key β cell transcriptional regulators was unaffected. MAFA levels were reduced, followed by a reduction in NKX6.1 upon development of hyperglycemia in db/db mice, a T2DM model. Transgenic expression of the glutathione peroxidase-1 antioxidant enzyme (GPX1) in db/db islet β cells restored nuclear MAFA, nuclear NKX6.1, and β cell function in vivo. Notably, the selective decrease in MAFA, NKX6.1, and PDX1 expression was found in human T2DM islets. MAFB, a MAFA-related transcription factor expressed in human β cells, was also severely compromised. We propose that MAFA, MAFB, NKX6.1, and PDX1 activity provides a gauge of islet β cell function, with loss of MAFA (and/or MAFB) representing an early indicator of β cell inactivity and the subsequent deficit of more impactful NKX6.1 (and/or PDX1) resulting in overt dysfunction associated with T2DM.

Update on the protective molecular pathways improving pancreatic beta-cell dysfunction

The primary function of pancreatic beta-cells is to produce and release insulin in response to increment in extracellular glucose concentrations, thus maintaining glucose homeostasis. Deficient beta-cell function can have profound metabolic consequences, leading to the development of hyperglycemia and, ultimately, diabetes mellitus. Therefore, strategies targeting the maintenance of the normal function and protecting pancreatic beta-cells from injury or death might be crucial in the treatment of diabetes. This narrative review will update evidence from the recently identified molecular regulators preserving beta-cell mass and function recovery in order to suggest potential therapeutic targets against diabetes. This review will also highlight the relevance for novel molecular pathways potentially improving beta-cell dysfunction.

mTORC1 and mTORC2 regulate insulin secretion through Akt in INS-1 cells

Regulated associated protein of mTOR (Raptor) and rapamycin-insensitive companion of mTOR (rictor) are two proteins that delineate two different mTOR complexes, mTORC1 and mTORC2 respectively. Recent studies demonstrated the role of rictor in the development and function of β-cells. mTORC1 has long been known to impact β-cell function and development. However, most of the studies evaluating its role used either drug treatment (i.e. rapamycin) or modification of expression of proteins known to modulate its activity, and the direct role of raptor in insulin secretion is unclear. In this study, using siRNA, we investigated the role of raptor and rictor in insulin secretion and production in INS-1 cells and the possible cross talk between their respective complexes, mTORC1 and mTORC2. Reduced expression of raptor is associated with increased glucose-stimulated insulin secretion and intracellular insulin content. Downregulation of rictor expression leads to impaired insulin secretion without affecting insulin content and is able to correct the increased insulin secretion mediated by raptor siRNA. Using dominant-negative or constitutively active forms of Akt, we demonstrate that the effect of both raptor and rictor is mediated through alteration of Akt signaling. Our finding shed new light on the mechanism of control of insulin secretion and production by the mTOR, and they provide evidence for antagonistic effect of raptor and rictor on insulin secretion in response to glucose by modulating the activity of Akt, whereas only raptor is able to control insulin biosynthesis.

Angiotensin II type 2 receptor is critical for the development of human fetal pancreatic progenitor cells into islet-like cell clusters and their potential for transplantation

Local renin-angiotensin systems (RASs) regulate the differentiation of tissue progenitors. However, it is not known whether such systems can regulate the development of pancreatic progenitor cells (PPCs). To address this issue, we characterized the expression profile of major RAS components in human fetal PPC preparations and examined their effects on the differentiation of PPCs into functional islet-like cell clusters (ICCs). We found that expression of RAS components was highly regulated throughout PPC differentiation and that locally generated angiotensin II (Ang II) maintained PPC growth and differentiation via Ang II type 1 and type 2 (AT(1) and AT(2)) receptors. In addition, we observed colocalization of AT(2) receptors with critical β-cell phenotype markers in PPCs/ICCs, as well as AT(2) receptor upregulation during differentiation, suggesting that these receptors may regulate β-cell development. In fact, we found that AT(2) , but not AT(1) , receptor was a key mediator of Ang II-induced upregulation of transcription factors important in β-cell development. Furthermore, lentivirus-mediated knockdown of AT(2) receptor suppressed the expression of these transcription factors in ICCs. Transplantation of AT(2) receptor-depleted ICCs into immune-privileged diabetic mice failed to ameliorate hyperglycemia, implying that AT(2) receptors are indispensable during ICC maturation in vivo. These data strongly indicate that a local RAS is involved in governing the functional maturation of pancreatic progenitors toward the endocrine lineage.

Association between type 2 diabetes and chronic arsenic exposure in drinking water: a cross sectional study in Bangladesh

BACKGROUND

Chronic exposure to high level of inorganic arsenic in drinking water has been associated with Type 2 Diabetes (T2D). Most research has been ecological in nature and has focused on high levels of arsenic exposure with few studies directly measuring arsenic levels in drinking water as an index of arsenic exposure. The effect of low to moderate levels of arsenic exposure on diabetes risk is largely unknown thus our study is adding further knowledge over previous works.

METHODS

This cross sectional study was conducted in 1004 consenting women and men from 1682 eligible participants yielding a participation rate of 60%. These participants are aged >30 years and were living in Bangladesh and had continuously consumed arsenic-contaminated drinking water for at least 6 months. T2D cases were diagnosed using glucometer following the new diagnostic criteria (Fasting Blood Glucose > 126 mg/dl) from the WHO guideline (WHO 2006), or a self-reported physician diagnosis of type 2 diabetes. Association between T2D and chronic arsenic exposure was estimated by multiple logistic regression with adjustment for age, sex, education, Body Mass Index (BMI) and family history of T2D.

RESULTS

A total of 1004 individuals participated in the study. The prevalence of T2D was 9% (95% CI 7-11%). After adjustment for diabetes risk factors, an increased risk of type 2 diabetes was observed for arsenic exposure over 50 μg/L with those in the highest category having almost double the risk of type 2 diabetes (OR=1.9 ; 95% CI 1.1-3.5). For most levels of arsenic exposure, the risk estimates are higher with longer exposure; a dose-response pattern was also observed.

CONCLUSIONS

These findings suggest an association between chronic arsenic exposure through drinking water and T2D. Risks are generally higher with longer duration of arsenic exposure. The risk of T2D is highest among those who were exposed to the highest concentration of arsenic for more than 10 years.

Pdx1 is post-translationally modified in vivo and serine 61 is the principal site of phosphorylation

Maintaining sufficient levels of Pdx1 activity is a prerequisite for proper regulation of blood glucose homeostasis and beta cell function. Mice that are haploinsufficient for Pdx1 display impaired glucose tolerance and lack the ability to increase beta cell mass in response to decreased insulin signaling. Several studies have shown that post-translational modifications are regulating Pdx1 activity through intracellular localization and binding to co-factors. Understanding the signaling cues converging on Pdx1 and modulating its activity is therefore an attractive approach in diabetes treatment. We employed a novel technique called Nanofluidic Proteomic Immunoassay to characterize the post-translational profile of Pdx1. Following isoelectric focusing in nano-capillaries, this technology relies on a pan specific antibody for detection and it therefore allows the relative abundance of differently charged protein species to be examined simultaneously. In all eukaryotic cells tested we find that the Pdx1 protein separates into four distinct peaks whereas Pdx1 protein from bacteria only produces one peak. Of the four peaks in eukaryotic cells we correlate one of them to a phosphorylation Using alanine scanning and mass spectrometry we map this phosphorylation to serine 61 in both Min6 cells and in exogenous Pdx1 over-expressed in HEK293 cells. A single phosphorylation is also present in cultured islets but it remains unaffected by changes in glucose levels. It is present during embryogenesis but is not required for pancreas development.

Deregulation of CREB signaling pathway induced by chronic hyperglycemia downregulates NeuroD transcription

CREB mediates the transcriptional effects of glucose and incretin hormones in insulin-target cells and insulin-producing β-cells. Although the inhibition of CREB activity is known to decrease the β-cell mass, it is still unknown what factors inversely alter the CREB signaling pathway in β-cells. Here, we show that β-cell dysfunctions occurring in chronic hyperglycemia are not caused by simple inhibition of CREB activity but rather by the persistent activation of CREB due to decreases in protein phophatase PP2A. When freshly isolated rat pancreatic islets were chronically exposed to 25 mM (high) glucose, the PP2A activity was reduced with a concomitant increase in active pCREB. Brief challenges with 15 mM glucose or 30 µM forskolin after 2 hour fasting further increased the level of pCREB and consequently induced the persistent expression of ICER. The excessively produced ICER was sufficient to repress the transcription of NeuroD, insulin, and SUR1 genes. In contrast, when islets were grown in 5 mM (low) glucose, CREB was transiently activated in response to glucose or forskolin stimuli. Thus, ICER expression was transient and insufficient to repress those target genes. Importantly, overexpression of PP2A reversed the adverse effects of chronic hyperglycemia and successfully restored the transient activation of CREB and ICER. Conversely, depletion of PP2A with siRNA was sufficient to disrupt the negative feedback regulation of CREB and induce hyperglycemic phenotypes even under low glucose conditions. Our findings suggest that the failure of the negative feedback regulation of CREB is the primary cause for β-cell dysfunctions under conditions of pathogenic hyperglycemia, and PP2A can be a novel target for future therapies aiming to protect β-cells mass in the late transitional phase of non-insulin dependent type 2 diabetes (NIDDM).

Islet-enriched gene expression and glucose-induced insulin secretion in human and mouse islets

AIMS/HYPOTHESIS

Our understanding of the transcription factors that control the development and function of rodent islet beta cells is advancing rapidly, yet less is known of the role they play in similar processes in human islets.

METHODS

To characterise the abundance and regulation of key proteins involved in glucose-regulated insulin secretion in human islets, we examined the expression of MAFA, MAFB, GLUT2 (also known as SLC2A2), βGK (also known as GCK) and PDX1 in isolated, highly purified human islets with an intact insulin secretory pattern. We also assessed these features in islets from two different mouse strains (C57BL/6J and FVB).

RESULTS

Compared with mouse islets, human islets secreted more insulin at baseline glucose (5.6 mmol/l), but less upon stimulation with high glucose (16.7 mmol/l) or high glucose plus 3-isobutyl-1-methyl-xanthine. Human islets had relatively more MAFB than PDX1 mRNA, while mouse islets had relatively more Pdx1 than Mafb mRNA. However, v-maf musculoaponeurotic fibrosarcoma oncogene homologue (MAF) B protein was found in human islet alpha and beta cells. This is unusual as this regulator is only produced in islet alpha cells in adult mice. The expression of insulin, MAFA, βGK and PDX1 was not glucose-regulated in human islets with an intact insulin secretory pattern.

CONCLUSIONS/INTERPRETATION

Our results suggest that human islets have a distinctive distribution and function of key regulators of the glucose-stimulated insulin secretion pathway, emphasising the urgent need to understand the processes that regulate human islet beta cell function.

Incidental CD8 T cell reactivity against caspase-cleaved apoptotic self-antigens from ubiquitously expressed proteins in islets from prediabetic human leucocyte antigen-A2 transgenic non-obese diabetic mice

Apoptosis is known as a major mechanism which contributes to beta cell decay in type 1 diabetes. Commitment to this pathway generally involves caspase-mediated protein cleavage and was found to induce cross-presentation of a specific antigen repertoire under certain inflammatory conditions. We aimed to assess the significance of the CD8 T cell population reactive against such caspase-cleaved apoptotic self-antigens in pancreatic islets of prediabetic human leucocyte antigen (HLA)-A2 transgenic non-obese diabetic chimeric monochain transgene construct (NOD.HHD) mice. We have reproduced a unique peptide library consisting of human CD8 T cell-derived apoptosis-specific antigens, all of which belong to structural proteins expressed ubiquitously in human islets. Pancreatic islets from prediabetic NOD.HHD mice, harbouring humanized major histocompatibilty complex (MHC) class I, were isolated and handpicked at various ages, and islet-infiltrating CD8 T cells were expanded in vitro and used as responders in an interferon (IFN)-γ enzyme-linked immunospot (ELISPOT) assay. Human T2 cells were used as antigen-presenting cells (APC) to avoid endogenous antigen presentation. Analogous to the interindividual variability found with peptides from known islet autoantigens such as islet-specific glucose-6-phosphatase catalytic subunit related protein (IGRP) and insulin, some mice showed variable, low-degree CD8 T cell reactivity against caspase-cleaved self-antigens. Because reactivity was predominantly minor and often undetectable, we conclude that beta cell apoptosis does not routinely provoke the development of dominant cytotoxic T lymphocyte (CTL) reactive against caspase-cleaved self-antigens in the NOD.HHD model.

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.

Rictor/mTORC2 is essential for maintaining a balance between beta-cell proliferation and cell size

OBJECTIVE

We examined the role of Rictor/mammalian target of rapamycin complex 2 (mTORC2), a key component of the phosphotidylinositol-3-kinase (PI3K)/mTORC2/AKT signaling pathway, in regulating both β-cell mass and function.

RESEARCH DESIGN AND METHODS

Mice with β-cell-specific deletions of Rictor or Pten were studied to determine the effects of deleting either or both genes on β-cell mass and glucose homeostasis.

RESULTS

Rictor null mice exhibited mild hyperglycemia and glucose intolerance caused by a reduction in β-cell mass, β-cell proliferation, pancreatic insulin content, and glucose-stimulated insulin secretion. Islets from these mice exhibited decreased AKT-S473 phosphorylation and increased abundance of FoxO1 and p27 proteins. Conversely, Pten null (βPtenKO) mice exhibited an increase in β-cell mass caused by increased cellular proliferation and size. Although β-cell mass was normal in mice lacking both Rictor and Pten (βDKO), their β-cells were larger than those in the βPtenKO mice. Even though the β-cell proliferation rate in the βDKO mice was lower than in the βPtenKO mice, there was a 12-fold increase the phosphorylation of AKT-T308.

CONCLUSIONS

PI3K/AKT signaling through mTORC2/pAKT-S473 plays a key role in maintaining normal β-cell mass. The phosphorylation of AKT-S473, by negatively regulating that of AKT-T308, is essential for maintaining a balance between β-cell proliferation and cell size in response to proliferative stimuli.

Karyopherins in nuclear transport of homeodomain proteins during development

Homeodomain proteins are crucial transcription factors for cell differentiation, cell proliferation and organ development. Interestingly, their homeodomain signature structure is important for both their DNA-binding and their nucleocytoplasmic trafficking. The accurate nucleocytoplasmic distribution of these proteins is essential for their functions. We summarize information on (a) the roles of karyopherins for import and export of homeoproteins, (b) the regulation of their nuclear transport during development, and (c) the corresponding complexity of homeoprotein nucleocytoplasmic transport signals. This article is part of a Special Issue entitled: Regulation of Signaling and Cellular Fate through Modulation of Nuclear Protein Import.

Islet beta-cell-specific MafA transcription requires the 5'-flanking conserved region 3 control domain

MafA is a key transcriptional activator of islet beta cells, and its exclusive expression within beta cells of the developing and adult pancreas is distinct among pancreatic regulators. Region 3 (base pairs -8118 to -7750 relative to the transcription start site), one of six conserved 5' cis domains of the MafA promoter, is capable of directing beta-cell-line-selective expression. Transgenic reporters of region 3 alone (R3), sequences spanning regions 1 to 6 (R1-6; base pairs -10428 to +230), and R1-6 lacking R3 (R1-6(DeltaR3)) were generated. Only the R1-6 transgene was active in MafA(+) insulin(+) cells during development and in adult cells. R1-6 also mediated glucose-induced MafA expression. Conversely, pancreatic expression was not observed with the R3 or R1-6(DeltaR3) line, although much of the nonpancreatic expression pattern was shared between the R1-6 and R1-6(DeltaR3) lines. Further support for the importance of R3 was also shown, as the islet regulators Nkx6.1 and Pax6, but not NeuroD1, activated MafA in gel shift, chromatin immunoprecipitation (ChIP), and transfection assays and in vivo mouse knockout models. Lastly, ChIP demonstrated that Pax6 and Pdx-1 also bound to R1 and R6, potentially functioning in pancreatic and nonpancreatic expression. These data highlight the nature of the cis- and trans-acting factors controlling the beta-cell-specific expression of MafA.

Cyclic AMP signaling in pancreatic islets

Cyclic 3'5'AMP (cAMP) is an important physiological amplifier of glucose-induced insulin secretion by the pancreatic islet beta-cell, where it is formed by the activity of adenylyl cyclases, which are stimulated by glucose, through elevation in intracellular calcium concentrations, and by the incretin hormones (GLP-1 and GIP). cAMP is rapidly degraded in the pancreatic islet beta-cell by various cyclic nucleotide phosphodiesterase (PDE) enzymes. Many steps involved in glucose-induced insulin secretion are modulated by cAMP, which is also important in regulating pancreatic islet beta-cell differentiation, growth and survival. This chapter discusses the formation, destruction and actions of cAMP in the islets with particular emphasis on the beta-cell.

Liver X receptor agonists augment human islet function through activation of anaplerotic pathways and glycerolipid/free fatty acid cycling

Recent studies in rodent models suggest that liver X receptors (LXRs) may play an important role in the maintenance of glucose homeostasis and islet function. To date, however, no studies have comprehensively examined the role of LXRs in human islet biology. Human islets were isolated from non-diabetic donors and incubated in the presence or absence of two synthetic LXR agonists, TO-901317 and GW3965, under conditions of low and high glucose. LXR agonist treatment enhanced both basal and stimulated insulin secretion, which corresponded to an increase in the expression of genes involved in anaplerosis and reverse cholesterol transport. Furthermore, enzyme activity of pyruvate carboxylase, a key regulator of pyruvate cycling and anaplerotic flux, was also increased. Whereas LXR agonist treatment up-regulated known downstream targets involved in lipogenesis, we observed no increase in the accumulation of intra-islet triglyceride at the dose of agonist used in our study. Moreover, LXR activation increased expression of the genes encoding hormone-sensitive lipase and adipose triglyceride lipase, two enzymes involved in lipolysis and glycerolipid/free fatty acid cycling. Chronically, insulin gene expression was increased after treatment with TO-901317, and this was accompanied by increased Pdx-1 nuclear protein levels and enhanced Pdx-1 binding to the insulin promoter. In conclusion, our data suggest that LXR agonists have a direct effect on the islet to augment insulin secretion and expression, actions that should be considered either as therapeutic or unintended side effects, as these agents are developed for clinical use.

Forkhead box O1/pancreatic and duodenal homeobox 1 intracellular translocation is regulated by c-Jun N-terminal kinase and involved in prostaglandin E2-induced pancreatic beta-cell dysfunction

Prostaglandin E(2) (PGE(2)) is a well-known mediator of beta-cell dysfunction in both type 1 and type 2 diabetes. We recently reported that down-regulation of the Akt pathway activity is implicated in PGE(2)-induced pancreatic beta-cell dysfunction. The aim of this study was to further dissect the signaling pathway of this process in pancreatic beta-cell line HIT-T15 cells and primary mouse islets. We found that PGE(2) time-dependently increased the c-Jun N-terminal kinase (JNK) pathway activity. JNK inhibition by the JNK-specific inhibitor SP600125 reversed PGE(2)-inhibited glucose-stimulated insulin secretion (GSIS). PGE(2) induced dephosphorylation of Akt and FOXO1, leading to nuclear localization and transactivation of FOXO1. Activation of FOXO1 induced nuclear exclusion but had no obvious effect on the whole-cell protein level of pancreatic and duodenal homeobox 1 (PDX1). However, these effects were all attenuated by JNK inhibition. Furthermore, adenovirus-mediated overexpression of dominant-negative (DN)-FOXO1 abolished whereas constitutively active (CA)-FOXO1 mimicked the effects of PGE(2) on GSIS in isolated mouse islets. In addition, we demonstrated that DN-JNK1 but not DN-JNK2 or CA-Akt abolished the PGE(2)-induced AP-1 luciferase reporter activity, whereas DN-JNK1 and CA-Akt but not DN-JNK2 reversed the effect of PGE(2) on FOXO1 transcriptional activity, and overexpression of DN-JNK1 rescued PGE(2)-impaired GSIS in mouse islets. Our results revealed that activation of the JNK is involved in PGE(2)-induced beta-cell dysfunction. PGE(2)-mediated JNK1 activation, through dephosphorylation of Akt and FOXO1, leads to nuclear accumulation of FOXO1 and nucleocytoplasmic shuttling of PDX1, finally resulting in defective GSIS in pancreatic beta-cells.

Transcription factors as therapeutic targets for diabetes

BACKGROUND

Islet cell implantation and pancreas transplantation have been used as treatments for diabetes but are limited by the shortage of donors and the requirement for lifelong immunosuppression. As an alternative, the generation of surrogate insulin-producing cells has been an area of interest for many researchers. Understanding how pancreatic beta-cells are generated during pancreas development will provide information that can be applied to generating surrogate beta-cells.

OBJECTIVE

To outline the current knowledge of pancreas development and differentiation, with a focus on the regulatory network of pancreas-enriched transcription factors and their targets.

METHODS

A review of relevant literature.

CONCLUSIONS

Pancreatic and duodenal homeobox 1 (Pdx1), Neurogenin 3 (Ngn3), and musculoaponeurotic fibrosarcoma oncogene homolog A (MafA) have been shown to play essential roles in pancreas development and beta-cell differentiation, and gain-of-function approaches indicate the potency of these factors for inducing differentiation of non-beta-cells into insulin-producing cells, which could lead to a novel therapy to cure 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.

Overexpression of the Pdx-1 homeodomain transcription factor impairs glucose metabolism in cultured rat hepatocytes

The Pdx-1 transcription factor plays crucial functions both during pancreas development and in the adult β cells. Previous studies have indicated that ectopic Pdx-1 expression in liver or intestinal primary and immortalized cells is sufficient to promote activation of insulin gene expression. This work is focused on the molecular and physiological consequences of Pdx-1 overexpression in liver cells. We present evidence that Pdx-1 affects the level of expression of one of the four mammalian hexokinase isozymes. These are glucose phosphorylating enzymes involved in essential cellular functions such as glucose sensing, metabolic energy production and apoptosis. Specifically, our data show that over-expression of Pdx-1 in cultured hepatocytes is able to repress the expression of hexokinase 2 (Hxk 2) and the phenomenon is mediated via binding of Pdx-1 to a specific sequence on the Hxk 2 gene promoter. As a consequence, liver cells over-expressing Pdx-1 present interesting alterations concerning glucose metabolism.

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.

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.

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.