The role of hepatic nuclear factor 1 alpha and PDX-1 in transcriptional regulation of the pdx-1 gene

From glucose sensing to exocytosis: takes from maturity onset diabetes of the young

Monogenic diabetes gave us simplified models of complex molecular processes occurring within β-cells, which allowed to explore the roles of numerous proteins from single protein perspective. Constellation of characteristic phenotypic features and wide application of genetic sequencing techniques to clinical practice, made the major form of monogenic diabetes - the Maturity Onset Diabetes of the Young to be distinguishable from type 1, type 2 as well as neonatal diabetes mellitus and understanding underlying molecular events for each type of MODY contributed to the advancements of antidiabetic therapy and stem cell research tremendously. The functional analysis of MODY-causing proteins in diabetes development, not only provided better care for patients suffering from diabetes, but also enriched our comprehension regarding the universal cellular processes including transcriptional and translational regulation, behavior of ion channels and transporters, cargo trafficking, exocytosis. In this review, we will overview structure and function of MODY-causing proteins, alterations in a particular protein arising from the deleterious mutations to the corresponding gene and their consequences, and translation of this knowledge into new treatment strategies.

Novel Loss-of-Function Variant in HNF1a Induces β-Cell Dysfunction through Endoplasmic Reticulum Stress

Heterozygous variants in the hepatocyte nuclear factor 1a (HNF1a) cause MODY3 (maturity-onset diabetes of the young, type 3). In this study, we found a case of novel HNF1a p.Gln125* (HNF1a-Q125ter) variant clinically. However, the molecular mechanism linking the new HNF1a variant to impaired islet β-cell function remains unclear. Firstly, a similar HNF1a-Q125ter variant in zebrafish (hnf1a^+/−) was generated by CRISPR/Cas9. We further crossed hnf1a^+/− with several zebrafish reporter lines to investigate pancreatic β-cell function. Next, we introduced HNF1a-Q125ter and HNF1a shRNA plasmids into the Ins-1 cell line and elucidated the molecular mechanism. hnf1a^+/− zebrafish significantly decreased the β-cell number, insulin expression, and secretion. Moreover, β cells in hnf1a^+/− dilated ER lumen and increased the levels of ER stress markers. Similar ER-stress phenomena were observed in an HNF1a-Q125ter-transfected Ins-1 cell. Follow-up investigations demonstrated that HNF1a-Q125ter induced ER stress through activating the PERK/eIF2a/ATF4 signaling pathway. Our study found a novel loss-of-function HNF1a-Q125ter variant which induced β-cell dysfunction by activating ER stress via the PERK/eIF2a/ATF4 signaling pathway.

Nurses' leadership in psychiatric care-A qualitative interview study of nurses' experience of leadership in an adult psychiatric inpatient care setting

WHAT IS KNOWN ON THE SUBJECT?: Previous studies on leadership in psychiatric care have focussed on a diversity of staff and on different healthcare settings. Nurses in both Sweden and internationally, working with patients newly diagnosed with psychosis and addiction, describe an overwhelming workload. Existing research points out that experience and leadership training are the most important factors to exert a good nursing leadership. In Sweden, requirements for leadership exists already from the first day of a nurse's career. The relationship and communication between nurses in psychiatric care and members of the staff is decisive for whether the leadership will work. WHAT THE PAPER ADDS TO EXISTING KNOWLEDGE: The study adds knowledge regarding the challenges leading the nursing care for patients diagnosed with psychosis and addiction. Ambivalence in the leadership role is prominent among nurses in psychiatric care and feelings of responsibility and meaningfulness are mixed with feelings of powerlessness and uncertainty. Swedish psychiatric nurses lack mandate to lead psychiatric nursing care. This may increase the feelings of uncertainty in their leadership role. WHAT ARE THE IMPLICATIONS FOR PRACTICE?: A mandate to lead as well as a leadership guidance in communication and teambuilding will enhance the leadership, especially among newly graduated nurses. Heightened awareness within the healthcare organization about nurse's experience of difficulties in leading the psychiatric nursing care of the most severe psychiatric illnesses could increase the right prerequisites for leadership. ABSTRACT: Introduction Research shows that psychiatric nursing care puts additional demands on the nurse as a leader due to the psychological complexity of care. Experience and leadership training are most important to exert leadership. In Sweden, demands for leadership exist already at the beginning of a nursing career, and in psychiatry, it may lead to an overwhelming workload. Aim/Question The aim of the present study is to highlight nurses' experiences of leading the psychiatric nursing care in an adult psychiatric context. Method A qualitative interview study of eleven registered nurses within psychiatric inpatient care. Content analysis was used for analysis. Results Leading with combined feelings of both meaningfulness and uncertainty were the theme arising from the result. Discussion Findings from Swedish and international studies stress special demands on leadership in psychiatric care. The result shows that nurses perceived an ambivalence of their leadership in terms of both meaningfulness and uncertainty. Implications for Practice An official mandate to lead as well as leadership guidance in communication and teambuilding will enhance leadership, especially among newly graduated nurses. Heightened awareness within healthcare organizations about difficulties in leading psychiatric nursing care could increase the possibility to create right prerequisites for leadership.

HNF1A Mutations and Beta Cell Dysfunction in Diabetes

Understanding the genetic factors of diabetes is essential for addressing the global increase in type 2 diabetes. HNF1A mutations cause a monogenic form of diabetes called maturity-onset diabetes of the young (MODY), and HNF1A single-nucleotide polymorphisms are associated with the development of type 2 diabetes. Numerous studies have been conducted, mainly using genetically modified mice, to explore the molecular basis for the development of diabetes caused by HNF1A mutations, and to reveal the roles of HNF1A in multiple organs, including insulin secretion from pancreatic beta cells, lipid metabolism and protein synthesis in the liver, and urinary glucose reabsorption in the kidneys. Recent studies using human stem cells that mimic MODY have provided new insights into beta cell dysfunction. In this article, we discuss the involvement of HNF1A in beta cell dysfunction by reviewing previous studies using genetically modified mice and recent findings in human stem cell-derived beta cells.

GATA6 mutations in hiPSCs inform mechanisms for maldevelopment of the heart, pancreas, and diaphragm

Damaging GATA6 variants cause cardiac outflow tract defects, sometimes with pancreatic and diaphragmic malformations. To define molecular mechanisms for these diverse developmental defects, we studied transcriptional and epigenetic responses to GATA6 loss of function (LoF) and missense variants during cardiomyocyte differentiation of isogenic human induced pluripotent stem cells. We show that GATA6 is a pioneer factor in cardiac development, regulating SMYD1 that activates HAND2, and KDR that with HAND2 orchestrates outflow tract formation. LoF variants perturbed cardiac genes and also endoderm lineage genes that direct PDX1 expression and pancreatic development. Remarkably, an exon 4 GATA6 missense variant, highly associated with extra-cardiac malformations, caused ectopic pioneer activities, profoundly diminishing GATA4, FOXA1/2, and PDX1 expression and increasing normal retinoic acid signaling that promotes diaphragm development. These aberrant epigenetic and transcriptional signatures illuminate the molecular mechanisms for cardiovascular malformations, pancreas and diaphragm dysgenesis that arise in patients with distinct GATA6 variants.

Point mutations in the PDX1 transactivation domain impair human β-cell development and function

OBJECTIVE

Hundreds of missense mutations in the coding region of PDX1 exist; however, if these mutations predispose to diabetes mellitus is unknown.

METHODS

In this study, we screened a large cohort of subjects with increased risk for diabetes and identified two subjects with impaired glucose tolerance carrying common, heterozygous, missense mutations in the PDX1 coding region leading to single amino acid exchanges (P33T, C18R) in its transactivation domain. We generated iPSCs from patients with heterozygous PDX1P33T/+, PDX1C18R/+ mutations and engineered isogenic cell lines carrying homozygous PDX1P33T/P33T, PDX1C18R/C18R mutations and a heterozygous PDX1 loss-of-function mutation (PDX1+/-).

RESULTS

Using an in vitro β-cell differentiation protocol, we demonstrated that both, heterozygous PDX1P33T/+, PDX1C18R/+ and homozygous PDX1P33T/P33T, PDX1C18R/C18R mutations impair β-cell differentiation and function. Furthermore, PDX1+/- and PDX1P33T/P33T mutations reduced differentiation efficiency of pancreatic progenitors (PPs), due to downregulation of PDX1-bound genes, including transcription factors MNX1 and PDX1 as well as insulin resistance gene CES1. Additionally, both PDX1P33T/+ and PDX1P33T/P33T mutations in PPs reduced the expression of PDX1-bound genes including the long-noncoding RNA, MEG3 and the imprinted gene NNAT, both involved in insulin synthesis and secretion.

CONCLUSIONS

Our results reveal mechanistic details of how common coding mutations in PDX1 impair human pancreatic endocrine lineage formation and β-cell function and contribute to the predisposition for diabetes.

Novel codons in rat Pdx-1 complementary DNA

Objectives

Pancreatic and duodenal homeobox-1 (Pdx-1) is a homeodomain-containing transcription factor essential for pancreatic development, beta-cell differentiation and the maintenance of mature beta cell function. To transfect the expression vectors of Pdx-1 in the mammalian cells, the complementary DNA (cDNA) of Pdx-1 was conducted.

Results

Novel codons and amino acids sequences were detected in rat Pdx-1 cDNA. Comparing the previous reports regarding rat Pdx-1 cDNA, 3 novel codons (ACA141CCA, AAG720CCG, GTT742GCT) were detected. The amino acids sequences based on the detected cDNA sequences confirmed those, which were already available in public databases. The present study described novel codons in rat Pdx-1 cDNA. The results may be useful for an effective research against pancreatic development, regeneration or carcinogenesis regarding Pdx-1 expressions.

HMGA1 is a novel transcriptional regulator of the FoxO1 gene

PURPOSE

The forkhead transcription factor (FoxO1) is a master transcriptional regulator of fundamental cellular processes ranging from cell proliferation and differentiation to inflammation and metabolism. However, despite its relevance, the mechanism(s) underlying FoxO1 gene regulation are largely unknown. We have previously shown that the chromatin factor high-mobility group A1 (HMGA1) plays a key role in the transcriptional regulation of glucose-responsive genes, including some that are involved in FoxO1-mediated glucose metabolism. Here we investigated the impact of HMGA1 on FoxO1 gene expression.

METHODS

FoxO1 protein and gene expression studies were performed by Western blot analysis combined with qRT-PCR of material from human cultured cells and EBV-transformed lymphoblasts, and from primary cultured hepatocytes from wild-type and Hmga1 ^-/- mice. Reporter gene assays and chromatin immunoprecipitation for binding of HMGA1 to the endogenous FoxoO1 locus were performed in cells overexpressing HMGA1 and in cells pretreated with siRNA targeting HMGA1.

RESULTS

HMGA1 increased FoxO1 mRNA and protein expression in vitro, in cultured HepG2 and HEK-293 cells by binding FoxO1 gene promoter, thereby activating FoxO1 gene transcription. Forced expression of HMGA1 in primary cultured hepatocytes from Hmga1 ^-/- mice and in EBV-transformed lymphoblasts from subjects with reduced expression of endogenous HMGA1 increased FoxO1 mRNA and protein levels.

CONCLUSION

These findings may contribute to the understanding of FoxO1 gene regulation and its role in metabolism.

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.

Transcriptional regulation of pancreas development and β-cell function [Review]

A small number of cells in the adult pancreas are endocrine cells. They are arranged in clusters called islets of Langerhans. The islets make insulin, glucagon, and other endocrine hormones, and release them into the blood circulation. These hormones help control the level of blood glucose. Therefore, a dysfunction of endocrine cells in the pancreas results in impaired glucose homeostasis, or diabetes mellitus. The pancreas is an organ that originates from the evaginations of pancreatic progenitor cells in the epithelium of the foregut endoderm. Pancreas organogenesis and maturation of the islets of Langerhans occurs via a coordinated and complex interplay of transcriptional networks and signaling molecules, which guide a stepwise and repetitive process of the propagation of progenitor cells and their maturation, eventually resulting in a fully functional organ. Increasing our understanding of the extrinsic, as well as intrinsic mechanisms that control these processes should facilitate the efforts to generate surrogate β cells from ES or iPS cells, or to reactivate the function of important cell types within pancreatic islets that are lost in diabetes.

Foxa2 and Pdx1 cooperatively regulate postnatal maturation of pancreatic β-cells

OBJECTIVE

The transcription factors (TF) Foxa2 and Pdx1 are key regulators of beta-cell (β-cell) development and function. Mutations of these TFs or their respective cis-regulatory consensus binding sites have been linked to maturity diabetes of the young (MODY), pancreas agenesis, or diabetes susceptibility in human. Although Foxa2 has been shown to directly regulate Pdx1 expression during mouse embryonic development, the impact of this gene regulatory interaction on postnatal β-cell maturation remains obscure.

METHODS

In order to easily monitor the expression domains of Foxa2 and Pdx1 and analyze their functional interconnection, we generated a novel double knock-in homozygous (FVFPBF^DHom) fluorescent reporter mouse model by crossing the previously described Foxa2-Venus fusion (FVF) with the newly generated Pdx1-BFP (blue fluorescent protein) fusion (PBF) mice.

RESULTS

Although adult PBF homozygous animals exhibited a reduction in expression levels of Pdx1, they are normoglycemic. On the contrary, despite normal pancreas and endocrine development, the FVFPBF^DHom reporter male animals developed hyperglycemia at weaning age and displayed a reduction in Pdx1 levels in islets, which coincided with alterations in β-cell number and islet architecture. The failure to establish mature β-cells resulted in loss of β-cell identity and trans-differentiation towards other endocrine cell fates. Further analysis suggested that Foxa2 and Pdx1 genetically and functionally cooperate to regulate maturation of adult β-cells.

CONCLUSIONS

Our data show that the maturation of pancreatic β-cells requires the cooperative function of Foxa2 and Pdx1. Understanding the postnatal gene regulatory network of β-cell maturation will help to decipher pathomechanisms of diabetes and identify triggers to regenerate dedifferentiated β-cell mass.

The mammal-specific Pdx1 Area II enhancer has multiple essential functions in early endocrine cell specification and postnatal β-cell maturation

The transcription factor Pdx1 is required for multiple aspects of pancreatic organogenesis. It remains unclear to what extent Pdx1 expression and function depend upon trans-activation through 5' conserved cis-regulatory regions and, in particular, whether the mammal-specific Area II (-2139 to -1958 bp) affects minor or major aspects of organogenesis. We show that Area II is a primary effector of endocrine-selective transcription in epithelial multipotent cells, nascent endocrine progenitors, and differentiating and mature β cells in vivo Pdx1^ΔAREAII/- mice exhibit a massive reduction in endocrine progenitor cells and progeny hormone-producing cells, indicating that Area II activity is fundamental to mounting an effective endocrine lineage-specification program within the multipotent cell population. Creating an Area II-deleted state within already specified Neurog3-expressing endocrine progenitor cells increased the proportion of glucagon⁺ α relative to insulin⁺ β cells, associated with the transcriptional and epigenetic derepression of the α-cell-determining Arx gene in endocrine progenitors. There were also glucagon and insulin co-expressing cells, and β cells that were incapable of maturation. Creating the Pdx1^ΔAREAII state after cells entered an insulin-expressing stage led to immature and dysfunctional islet β cells carrying abnormal chromatin marking in vital β-cell-associated genes. Therefore, trans-regulatory integration through Area II mediates a surprisingly extensive range of progenitor and β-cell-specific Pdx1 functions.

Transcription factor regulation of pancreatic organogenesis, differentiation and maturation

Lineage tracing studies have revealed that transcription factors play a cardinal role in pancreatic development, differentiation and function. Three transitions define pancreatic organogenesis, differentiation and maturation. In the primary transition, when pancreatic organogenesis is initiated, there is active proliferation of pancreatic progenitor cells. During the secondary transition, defined by differentiation, there is growth, branching, differentiation and pancreatic cell lineage allocation. The tertiary transition is characterized by differentiated pancreatic cells that undergo further remodeling, including apoptosis, replication and neogenesis thereby establishing a mature organ. Transcription factors function at multiple levels and may regulate one another and auto-regulate. The interaction between extrinsic signals from non-pancreatic tissues and intrinsic transcription factors form a complex gene regulatory network ultimately culminating in the different cell lineages and tissue types in the developing pancreas. Mutations in these transcription factors clinically manifest as subtypes of diabetes mellitus. Current treatment for diabetes is not curative and thus, developmental biologists and stem cell researchers are utilizing knowledge of normal pancreatic development to explore novel therapeutic alternatives. This review summarizes current knowledge of transcription factors involved in pancreatic development and β-cell differentiation in rodents.

Dynamic recruitment of functionally distinct Swi/Snf chromatin remodeling complexes modulates Pdx1 activity in islet β cells

Pdx1 is a transcription factor of fundamental importance to pancreas formation and adult islet β cell function. However, little is known about the positive- and negative-acting coregulators recruited to mediate transcriptional control. Here, we isolated numerous Pdx1-interacting factors possessing a wide range of cellular functions linked with this protein, including, but not limited to, coregulators associated with transcriptional activation and repression, DNA damage response, and DNA replication. Because chromatin remodeling activities are essential to developmental lineage decisions and adult cell function, our analysis focused on investigating the influence of the Swi/Snf chromatin remodeler on Pdx1 action. The two mutually exclusive and indispensable Swi/Snf core ATPase subunits, Brg1 and Brm, distinctly affected target gene expression in β cells. Furthermore, physiological and pathophysiological conditions dynamically regulated Pdx1 binding to these Swi/Snf complexes in vivo. We discuss how context-dependent recruitment of coregulatory complexes by Pdx1 could impact pancreas cell development and adult islet β cell activity.

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.

Dominant and context-specific control of endodermal organ allocation by Ptf1a

The timing and gene regulatory logic of organ-fate commitment from within the posterior foregut of the mammalian endoderm is largely unexplored. Transient misexpression of a presumed pancreatic-commitment transcription factor, Ptf1a, in embryonic mouse endoderm (Ptf1a(EDD)) dramatically expanded the pancreatic gene regulatory network within the foregut. Ptf1a(EDD) temporarily suppressed Sox2 broadly over the anterior endoderm. Pancreas-proximal organ territories underwent full tissue conversion. Early-stage Ptf1a(EDD) rapidly expanded the endogenous endodermal Pdx1-positive domain and recruited other pancreas-fate-instructive genes, thereby spatially enlarging the potential for pancreatic multipotency. Early Ptf1a(EDD) converted essentially the entire glandular stomach, rostral duodenum and extrahepatic biliary system to pancreas, with formation of many endocrine cell clusters of the type found in normal islets of Langerhans. Sliding the Ptf1a(EDD) expression window through embryogenesis revealed differential temporal competencies for stomach-pancreas respecification. The response to later-stage Ptf1a(EDD) changed radically towards unipotent, acinar-restricted conversion. We provide strong evidence, beyond previous Ptf1a inactivation or misexpression experiments in frog embryos, for spatiotemporally context-dependent activity of Ptf1a as a potent gain-of-function trigger of pro-pancreatic commitment.

C-Abl inhibitor imatinib enhances insulin production by β cells: c-Abl negatively regulates insulin production via interfering with the expression of NKx2.2 and GLUT-2

Chronic myelogenous leukemia patients treated with tyrosine kinase inhibitor, Imatinib, were shown to have increased serum levels of C-peptide. Imatinib specifically inhibits the tyrosine kinase, c-Abl. However, the mechanism of how Imatinib treatment can lead to increased insulin level is unclear. Specifically, there is little investigation into whether Imatinib directly affects β cells to promote insulin production. In this study, we showed that Imatinib significantly induced insulin expression in both glucose-stimulated and resting β cells. In line with this finding, c-Abl knockdown by siRNA and overexpression of c-Abl markedly enhanced and inhibited insulin expression in β cells, respectively. Unexpectedly, high concentrations of glucose significantly induced c-Abl expression, suggesting c-Abl may play a role in balancing insulin production during glucose stimulation. Further studies demonstrated that c-Abl inhibition did not affect the major insulin gene transcription factor, pancreatic and duodenal homeobox-1 (PDX-1) expression. Of interest, inhibition of c-Abl enhanced NKx2.2 and overexpression of c-Abl in β cells markedly down-regulated NKx2.2, which is a positive regulator for insulin gene expression. Additionally, we found that c-Abl inhibition significantly enhanced the expression of glucose transporter GLUT2 on β cells. Our study demonstrates a previously unrecognized mechanism that controls insulin expression through c-Abl-regulated NKx2.2 and GLUT2. Therapeutic targeting β cell c-Abl could be employed in the treatment of diabetes or β cell tumor, insulinoma.

Silymarin induces expression of pancreatic Nkx6.1 transcription factor and β-cells neogenesis in a pancreatectomy model

A physio-pathological feature of diabetes mellitus is a significant reduction of β-pancreatic cells. The growth, differentiation and function maintenance of these cells is directed by transcription factors. Nkx6.1 is a key transcription factor for the differentiation, neogenesis and maintenance of β-pancreatic cells. We reported that silymarin restores normal morphology and endocrine function of damaged pancreatic tissue after alloxan-induced diabetes mellitus in rats. The aim of this study was to analyze the effect of silymarin on Nkx6.1 transcription factor expression and its consequence in β cells neogenesis. Sixty male Wistar rats were partially pancreatectomized and divided into twelve groups. Six groups were treated with silymarin (200 mg/Kg p.o) for periods of 3, 7, 14, 21, 42 and 63 days. Additionally, an unpancreatectomized control group was used. Nkx6.1 and insulin gene expression were assessed by RT-PCR assay in total pancreatic RNA. β-Cell neogenesis was determined by immunoperoxidase assay. Silymarin treated group showed an increase of Nkx6.1 and insulin genic expression. In this group, there was an increment of β-cell neogenesis in comparison to pancreatectomized untreated group. Silymarin treatment produced a rise in serum insulin and serum glucose normalization. These results suggest that silymarin may improve the reduction of β pancreatic cells observed in diabetes mellitus.

Nkx6.1 controls a gene regulatory network required for establishing and maintaining pancreatic Beta cell identity

All pancreatic endocrine cell types arise from a common endocrine precursor cell population, yet the molecular mechanisms that establish and maintain the unique gene expression programs of each endocrine cell lineage have remained largely elusive. Such knowledge would improve our ability to correctly program or reprogram cells to adopt specific endocrine fates. Here, we show that the transcription factor Nkx6.1 is both necessary and sufficient to specify insulin-producing beta cells. Heritable expression of Nkx6.1 in endocrine precursors of mice is sufficient to respecify non-beta endocrine precursors towards the beta cell lineage, while endocrine precursor- or beta cell-specific inactivation of Nkx6.1 converts beta cells to alternative endocrine lineages. Remaining insulin⁺ cells in conditional Nkx6.1 mutants fail to express the beta cell transcription factors Pdx1 and MafA and ectopically express genes found in non-beta endocrine cells. By showing that Nkx6.1 binds to and represses the alpha cell determinant Arx, we identify Arx as a direct target of Nkx6.1. Moreover, we demonstrate that Nkx6.1 and the Arx activator Isl1 regulate Arx transcription antagonistically, thus establishing competition between Isl1 and Nkx6.1 as a critical mechanism for determining alpha versus beta cell identity. Our findings establish Nkx6.1 as a beta cell programming factor and demonstrate that repression of alternative lineage programs is a fundamental principle by which beta cells are specified and maintained. Given the lack of Nkx6.1 expression and aberrant activation of non-beta endocrine hormones in human embryonic stem cell (hESC)–derived insulin⁺ cells, our study has significant implications for developing cell replacement therapies.

Diabetes is a disease caused by the loss or dysfunction of insulin-producing beta cells in the pancreas. Recent studies suggest that modification of the beta cells' differentiation state is among the earliest events marking the progressive failure of beta cells in diabetes. Currently, very little is known about the factors that instruct cells to adopt beta cell characteristics and maintain the differentiated state of beta cells. We have discovered that a single transcription factor can instruct precursor cells of other endocrine cell types to change their identity and differentiate into beta cells. Conversely, inactivation of the transcription factor in endocrine precursors prevents their differentiation into beta cells and results in excess production of other endocrine cell types. When the factor is specifically inactivated in beta cells, beta cells lose their identity and adopt characteristics of other endocrine cell types, similar to what is seen in animal models of diabetes. Thus, we have identified a single factor that is both sufficient to program beta cells and necessary for maintaining their differentiated state. This factor could be an important target for diabetes therapy and could help reprogram other cell types into beta cells.

GATA4 and GATA6 control mouse pancreas organogenesis

Recently, heterozygous mutations in GATA6 have been found in neonatal diabetic patients with failed pancreatic organogenesis. To investigate the roles of GATA4 and GATA6 in mouse pancreas organogenesis, we conditionally inactivated these genes within the pancreas. Single inactivation of either gene did not have a major impact on pancreas formation, indicating functional redundancy. However, double Gata4/Gata6 mutant mice failed to develop pancreata, died shortly after birth, and displayed hyperglycemia. Morphological defects in Gata4/Gata6 mutant pancreata were apparent during embryonic development, and the epithelium failed to expand as a result of defects in cell proliferation and differentiation. The number of multipotent pancreatic progenitors, including PDX1+ cells, was reduced in the Gata4/Gata6 mutant pancreatic epithelium. Remarkably, deletion of only 1 Gata6 allele on a Gata4 conditional knockout background severely reduced pancreatic mass. In contrast, a single WT allele of Gata4 in Gata6 conditional knockout mice was sufficient for normal pancreatic development, indicating differential contributions of GATA factors to pancreas formation. Our results place GATA factors at the top of the transcriptional network hierarchy controlling pancreas organogenesis.

HMGA1 is a novel downstream nuclear target of the insulin receptor signaling pathway

High-mobility group AT-hook 1 (HMGA1) protein is an important nuclear factor that activates gene transcription by binding to AT-rich sequences in the promoter region of DNA. We previously demonstrated that HMGA1 is a key regulator of the insulin receptor (INSR) gene and individuals with defects in HMGA1 have decreased INSR expression and increased susceptibility to type 2 diabetes mellitus. In addition, there is evidence that intracellular regulatory molecules that are employed by the INSR signaling system are involved in post-translational modifications of HMGA1, including protein phosphorylation. It is known that phosphorylation of HMGA1 reduces DNA-binding affinity and transcriptional activation. In the present study, we investigated whether activation of the INSR by insulin affected HMGA1 protein phosphorylation and its regulation of gene transcription. Collectively, our findings indicate that HMGA1 is a novel downstream target of the INSR signaling pathway, thus representing a new critical nuclear mediator of insulin action and function.

Pancreas organogenesis: from bud to plexus to gland

Pancreas oganogenesis comprises a coordinated and highly complex interplay of signaling events and transcriptional networks that guide a step-wise process of organ development from early bud specification all the way to the final mature organ state. Extensive research on pancreas development over the last few years, largely driven by a translational potential for pancreatic diseases (diabetes, pancreatic cancer, and so on), is markedly advancing our knowledge of these processes. It is a tenable goal that we will one day have a clear, complete picture of the transcriptional and signaling codes that control the entire organogenetic process, allowing us to apply this knowledge in a therapeutic context, by generating replacement cells in vitro, or perhaps one day to the whole organ in vivo. This review summarizes findings in the past 5 years that we feel are amongst the most significant in contributing to the deeper understanding of pancreas development. Rather than try to cover all aspects comprehensively, we have chosen to highlight interesting new concepts, and to discuss provocatively some of the more controversial findings or proposals. At the end of the review, we include a perspective section on how the whole pancreas differentiation process might be able to be unwound in a regulated fashion, or redirected, and suggest linkages to the possible reprogramming of other pancreatic cell-types in vivo, and to the optimization of the forward-directed-differentiation of human embryonic stem cells (hESC), or induced pluripotential cells (iPSC), towards mature β-cells.

Suppression of the pancreatic duodenal homeodomain transcription factor-1 (Pdx-1) promoter by sterol regulatory element-binding protein-1c (SREBP-1c)

Overexpression of sterol regulatory element-binding protein-1c (SREBP-1c) in β cells causes impaired insulin secretion and β cell dysfunction associated with diminished pancreatic duodenal homeodomain transcription factor-1 (PDX-1) expression in vitro and in vivo. To identify the molecular mechanism responsible for this effect, the mouse Pdx-1 gene promoter (2.7 kb) was analyzed in β cell and non-β cell lines. Despite no apparent sterol regulatory element-binding protein-binding sites, the Pdx-1 promoter was suppressed by SREBP-1c in β cells in a dose-dependent manner. PDX-1 activated its own promoter. The E-box (-104/-99 bp) in the proximal region, occupied by ubiquitously expressed upstream stimulatory factors (USFs), was crucial for the PDX-1-positive autoregulatory loop through direct PDX-1·USF binding. This positive feedback activation was a prerequisite for SREBP-1c suppression of the promoter in non-β cells. SREBP-1c and PDX-1 directly interact through basic helix-loop-helix and homeobox domains, respectively. This robust SREBP-1c·PDX-1 complex interferes with PDX-1·USF formation and inhibits the recruitment of PDX-1 coactivators. SREBP-1c also inhibits PDX-1 binding to the previously described PDX-1-binding site (-2721/-2646 bp) in the distal enhancer region of the Pdx-1 promoter. Endogenous up-regulation of SREBP-1c in INS-1 cells through the activation of liver X receptor and retinoid X receptor by 9-cis-retinoic acid and 22-hydroxycholesterol inhibited PDX-1 mRNA and protein expression. Conversely, SREBP-1c RNAi restored Pdx-1 mRNA and protein levels. Through these multiple mechanisms, SREBP-1c, when induced in a lipotoxic state, repressed PDX-1 expression contributing to the inhibition of insulin expression and β cell dysfunction.

Disruption of a novel Kruppel-like transcription factor p300-regulated pathway for insulin biosynthesis revealed by studies of the c.-331 INS mutation found in neonatal diabetes mellitus

Krüppel-like transcription factors (KLFs) have elicited significant attention because of their regulation of essential biochemical pathways and, more recently, because of their fundamental role in the mechanisms of human diseases. Neonatal diabetes mellitus is a monogenic disorder with primary alterations in insulin secretion. We here describe a key biochemical mechanism that underlies neonatal diabetes mellitus insulin biosynthesis impairment, namely a homozygous mutation within the insulin gene (INS) promoter, c.-331C>G, which affects a novel KLF-binding site. The combination of careful expression profiling, electromobility shift assays, reporter experiments, and chromatin immunoprecipitation demonstrates that, among 16 different KLF proteins tested, KLF11 is the most reliable activator of this site. Congruently, the c.-331C>G INS mutation fails to bind KLF11, thus inhibiting activation by this transcription factor. Klf11^−/− mice recapitulate the disruption in insulin production and blood levels observed in patients. Thus, these data demonstrate an important role for KLF11 in the regulation of INS transcription via the novel c.-331 KLF site. Lastly, our screening data raised the possibility that other members of the KLF family may also regulate this promoter under distinct, yet unidentified, cellular contexts. Collectively, this study underscores a key role for KLF proteins in biochemical mechanisms of human diseases, in particular, early infancy onset diabetes mellitus.

Defining the cancer master switch

BACKGROUND

Recent research has focused on signaling cascades and their interactions yielding considerable insight into which genetic pathways are targeted and how they tend to be altered in tumors. Therapeutic interventions now can be designed based on the knowledge of pathways vital to tumor growth and survival. These critical targets for intervention, master switches for cancer, are termed so because the tumor attempts to "flip the switch" in a way that promotes its survival, whereas molecular therapy aims to "switch off" signals important for tumor-related processes.

METHODS

Literature review.

CONCLUSIONS

Defining useful targets for therapy depends on identifying pathways that are crucial for tumor growth, survival, and metastasis. Because not all signaling cascades are created equal, selecting master switches or targets for intervention needs to be done in a systematic fashion. This discussion proposes a set of criteria to define what it means to be a cancer master switch and provides examples to illustrate their application.

The pancreatic islet β-cell-enriched transcription factor Pdx-1 regulates Slc30a8 gene transcription through an intronic enhancer

The SLC30A8 gene encodes the zinc transporter ZnT-8, which provides zinc for insulin-hexamer formation. Genome-wide association studies have shown that a polymorphic variant in SLC30A8 is associated with altered susceptibility to Type 2 diabetes and we recently reported that glucose-stimulated insulin secretion is decreased in islets isolated from Slc30a8-knockout mice. The present study examines the molecular basis for the islet-specific expression of Slc30a8. VISTA analyses identified two conserved regions in Slc30a8 introns 2 and 3, designated enhancers A and B respectively. Transfection experiments demonstrated that enhancer B confers elevated fusion gene expression in both βTC-3 cells and αTC-6 cells. In contrast, enhancer A confers elevated fusion gene expression selectively in βTC-3 and not αTC-6 cells. These data suggest that enhancer A is an islet β-cell-specific enhancer and that the mechanisms controlling Slc30a8 expression in α- and β-cells are overlapping, but distinct. Gel retardation and ChIP (chromatin immunoprecipitation) assays revealed that the islet-enriched transcription factor Pdx-1 binds enhancer A in vitro and in situ respectively. Mutation of two Pdx-1-binding sites in enhancer A markedly reduces fusion gene expression suggesting that this factor contributes to Slc30a8 expression in β-cells, a conclusion consistent with developmental studies showing that restriction of Pdx-1 to pancreatic islet β-cells correlates with the induction of Slc30a8 gene expression and ZnT-8 protein expression in vivo.

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.

Locus co-occupancy, nucleosome positioning, and H3K4me1 regulate the functionality of FOXA2-, HNF4A-, and PDX1-bound loci in islets and liver

The liver and pancreas share a common origin and coexpress several transcription factors. To gain insight into the transcriptional networks regulating the function of these tissues, we globally identify binding sites for FOXA2 in adult mouse islets and liver, PDX1 in islets, and HNF4A in liver. Because most eukaryotic transcription factors bind thousands of loci, many of which are thought to be inactive, methods that can discriminate functionally active binding events are essential for the interpretation of genome-wide transcription factor binding data. To develop such a method, we also generated genome-wide H3K4me1 and H3K4me3 localization data in these tissues. By analyzing our binding and histone methylation data in combination with comprehensive gene expression data, we show that H3K4me1 enrichment profiles discriminate transcription factor occupied loci into three classes: those that are functionally active, those that are poised for activation, and those that reflect pioneer-like transcription factor activity. Furthermore, we demonstrate that the regulated presence of H3K4me1-marked nucleosomes at transcription factor occupied promoters and enhancers controls their activity, implicating both tissue-specific transcription factor binding and nucleosome remodeling complex recruitment in determining tissue-specific gene expression. Finally, we apply these approaches to generate novel insights into how FOXA2, PDX1, and HNF4A cooperate to drive islet- and liver-specific gene expression.

Novel monoclonal antibodies against Pdx1 reveal feedback regulation of Pdx1 protein levels

The aim of this study was to characterize two monoclonal antibodies (F6A11 and F109-D12) generated against Pdx1 (pancreatic and duodenal homeobox-1), a homeodomain transcription factor, which is critical for pancreas formation as well as for normal pancreatic beta cell function. For production of monoclonal antibodies, we immunized Robertsonian POSF (RBF)mice with a GST-Pdx1 fusion protein containing a 68-amino acid C-terminal fragment of rat Pdx1. These monoclonal antibodies detect Pdx1 by western blotting and allow immunohistochemical detection of Pdx1 in both mouse and rat tissue. F6A11 and F109-D12 produce IHC staining patterns indistinguishable from that obtained with highly specific polyclonal Pdx1 antisera raised in rabbits and goats, when applied to embryonic or adult mouse pancreatic tissue. In contrast to previously generated polyclonal anti-Pdx1 antisera, we also demonstrate that F6A11 works for intracellular fluorescence activated cell sorting (FACS) staining of Pdx1. By using F6A11, we characterize the induction of Pdx1 in the Doxycycline (DOX) inducible insulinoma cell line INSrαβ-Pdx1 and follow the reduction of Pdx1 after removing Dox. Finally, we show that induction of exogenous Pdx1 leads to a reduction in endogenous Pdx1 levels, which suggests that a negative feedback loop is involved in maintaining correct levels of Pdx1 in the cell.

MODY7 gene, KLF11, is a novel p300-dependent regulator of Pdx-1 (MODY4) transcription in pancreatic islet beta cells

Pdx-1 (pancreatic-duodenal homeobox-1), a MODY4 homeodomain transcription factor, serves as a master regulator in the pancreas because of its importance during organogenesis and in adult islet insulin-producing beta cell activity. Here, we show that KLF11, an SP/Krüppel-like (SP/KLF) transcription factor, mutated in French maturity onset diabetes of the young patients (MODY7), regulates Pdx-1 transcription in beta cells through two evolutionarily conserved GC-rich motifs in conserved Area II, a control region essential to islet beta cell-enriched expression. These regulatory elements, termed GC1 (human base pair -2061/-2055) and GC2 (-2036/-2027), are also nearly identical to the consensus KLF11 binding sequence defined here by random oligonucleotide binding analysis. KLF11 specifically associates with Area II in chromatin immunoprecipitation assays, while preventing binding to GC1- and/or GC2-compromised Pdx1-driven reporter activity in beta cell lines. Mechanistically, we find that KLF11 interacts with the coactivator p300 via its zinc finger domain in vivo to mediate Pdx-1 activation. Together, our data identified a hierarchical regulatory cascade for these two MODY genes, suggesting that gene regulation in MODY is more complex than anticipated previously. Furthermore, because KLF11 like most MODY-associated transcription factors uses p300, these data further support a role for this coactivator as a critical chromatin link in forms of type 2 diabetes.

The diabetes gene Pdx1 regulates the transcriptional network of pancreatic endocrine progenitor cells in mice

Heterozygous mutations in the gene encoding the pancreatic homeodomain transcription factor pancreatic duodenal homeobox 1 (PDX1) are associated with maturity onset diabetes of the young, type 4 (MODY4) and type 2 diabetes. Pdx1 governs the early embryonic development of the pancreas and the later differentiation of the insulin-producing islet beta cells of the endocrine compartment. We derived a Pdx1 hypomorphic allele that reveals a role for Pdx1 in the specification of endocrine progenitors. Mice homozygous for this allele displayed a selective reduction in endocrine lineages associated with decreased numbers of endocrine progenitors and a marked reduction in levels of mRNA encoding the proendocrine transcription factor neurogenin 3 (Ngn3). During development, Pdx1 occupies an evolutionarily conserved enhancer region of Ngn3 and interacts with the transcription factor one cut homeobox 1 (Hnf6) to activate this enhancer. Furthermore, mRNA levels of all 4 members of the transcription factor network that regulates Ngn3 expression, SRY-box containing gene 9 (Sox9), Hnf6, Hnf1b, and forkhead box A2 (Foxa2), were decreased in homozygous mice. Pdx1 also occupied regulatory sequences in Foxa2 and Hnf1b. Thus, Pdx1 contributes to specification of endocrine progenitors both by regulating expression of Ngn3 directly and by participating in a cross-regulatory transcription factor network during early pancreas development. These results provide insights that may be applicable to beta cell replacement strategies involving the guided differentiation of ES cells or other progenitor cell types into the beta cell lineage, and they suggest a molecular mechanism whereby human PDX1 mutations cause diabetes.

Dynamic regulation of Pdx1 enhancers by Foxa1 and Foxa2 is essential for pancreas development

The onset of pancreas development in the foregut endoderm is marked by activation of the homeobox gene Pdx1 (IPF1). Pdx1 is essential for the expansion of the pancreatic primordium and the development of endocrine islets. The control of Pdx1 expression has been only partially elucidated. We demonstrate here that the winged-helix transcription factors Foxa1 and Foxa2 co-occupy multiple regulatory domains in the Pdx1 gene. Compound conditional ablation of both Foxa1 and Foxa2 in the pancreatic primordium results in complete loss of Pdx1 expression and severe pancreatic hypoplasia. Mutant mice exhibit hyperglycemia with severely disrupted acinar and islet development, and die shortly after birth. Assessment of developmental markers in the mutant pancreas revealed a failure in the expansion of the pancreatic anlage, a blockage of exocrine and endocrine cell differentiation, and an arrest at the primitive duct stage. Comparing their relative developmental activity, we find that Foxa2 is the major regulator in promoting pancreas development and cell differentiation. Using chromatin immunoprecipitations (ChIP) and ChIP sequencing (ChIPSeq) of fetal pancreas and islet chromatin, we demonstrate that Foxa1 and Foxa2 predominantly occupy a distal enhancer at -6.4 kb relative to the transcriptional start site in the Pdx1 gene. In addition, occupancy of the well-characterized proximal Pdx1 enhancer by Foxa1 and Foxa2 is developmental stage-dependent. Thus, the regulation of Pdx1 expression by Foxa1 and Foxa2 is a key early event controlling the expansion and differentiation of the pancreatic primordia.

Dynamic regulation of Pdx1 enhancers by Foxa1 and Foxa2 is essential for pancreas development

The onset of pancreas development in the foregut endoderm is marked by activation of the homeobox gene Pdx1 (IPF1). Pdx1 is essential for the expansion of the pancreatic primordium and the development of endocrine islets. The control of Pdx1 expression has been only partially elucidated. We demonstrate here that the winged-helix transcription factors Foxa1 and Foxa2 co-occupy multiple regulatory domains in the Pdx1 gene. Compound conditional ablation of both Foxa1 and Foxa2 in the pancreatic primordium results in complete loss of Pdx1 expression and severe pancreatic hypoplasia. Mutant mice exhibit hyperglycemia with severely disrupted acinar and islet development, and die shortly after birth. Assessment of developmental markers in the mutant pancreas revealed a failure in the expansion of the pancreatic anlage, a blockage of exocrine and endocrine cell differentiation, and an arrest at the primitive duct stage. Comparing their relative developmental activity, we find that Foxa2 is the major regulator in promoting pancreas development and cell differentiation. Using chromatin immunoprecipitations (ChIP) and ChIP sequencing (ChIPSeq) of fetal pancreas and islet chromatin, we demonstrate that Foxa1 and Foxa2 predominantly occupy a distal enhancer at -6.4 kb relative to the transcriptional start site in the Pdx1 gene. In addition, occupancy of the well-characterized proximal Pdx1 enhancer by Foxa1 and Foxa2 is developmental stage-dependent. Thus, the regulation of Pdx1 expression by Foxa1 and Foxa2 is a key early event controlling the expansion and differentiation of the pancreatic primordia.

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.

DREAM mediates cAMP-dependent, Ca2+-induced stimulation of GFAP gene expression and regulates cortical astrogliogenesis

In the developing mouse brain, once the generation of neurons is mostly completed during the prenatal period, precisely coordinated signals act on competent neural precursors to direct their differentiation into astrocytes, which occurs mostly after birth. Among these signals, those provided by neurotrophic cytokines and bone morphogenetic proteins appear to have a key role in triggering the neurogenic to gliogenic switch and in regulating astrocyte numbers. In addition, we have reported previously that the neurotrophic peptide pituitary adenylate cyclase-activating polypeptide (PACAP) is able to promote astrocyte differentiation of cortical precursors via activation of a cAMP-dependent pathway. Signals acting on progenitor cells of the developing cortex to generate astrocytes activate glial fibrillary acidic protein (GFAP) gene expression, but the transcriptional mechanisms that regulate this activation are unclear. Here, we identify the previously known transcriptional repressor downstream regulatory element antagonist modulator (DREAM) as an activator of GFAP gene expression. We found that DREAM occupies specific sites on the GFAP promoter before and after differentiation is initiated by exposure of cortical progenitor cells to PACAP. PACAP raises intracellular calcium concentration via a mechanism that requires cAMP, and DREAM-mediated transactivation of the GFAP gene requires the integrity of calcium-binding domains. Cortical progenitor cells from dream(-/-) mice fail to express GFAP in response to PACAP. Moreover, the neonatal cortex of dream(-/-) mice exhibits a reduced number of astrocytes and increased number of neurons. These results identify the PACAP-cAMP-Ca(2+)-DREAM cascade as a new pathway to activate GFAP gene expression during astrocyte differentiation.

Embryonic stem cells to beta-cells by understanding pancreas development

Insulin injections treat but do not cure Type 1 diabetes (T1DM). The success of islet transplantation suggests cell replacement therapies may offer a curative strategy. However, cadaver islets are of insufficient number for this to become a widespread treatment. To address this deficiency, the production of beta-cells from pluripotent stem cells offers an ambitious far-sighted opportunity. Recent progress in generating insulin-producing cells from embryonic stem cells has shown promise, highlighting the potential of trying to mimic normal developmental pathways. Here, we provide an overview of the current methodology that has been used to differentiate stem cells toward a beta-cell fate. Parallels are drawn with what is known about normal development, especially regarding the human pancreas.

MafA and MafB regulate Pdx1 transcription through the Area II control region in pancreatic beta cells

Pancreatic-duodenal homeobox factor-1 (Pdx1) is highly enriched in islet β cells and integral to proper cell development and adult function. Of the four conserved 5′-flanking sequence blocks that contribute to transcription in vivo, Area II (mouse base pairs -2153/-1923) represents the only mammalian specific control domain. Here we demonstrate that regulation of β-cell-enriched Pdx1 expression by the MafA and MafB transcription factors is exclusively through Area II. Thus, these factors were found to specifically activate through Area II in cell line transfection-based assays, and MafA, which is uniquely expressed in adult islet β cells was only bound to this region in quantitative chromatin immunoprecipitation studies. MafA and MafB are produced in β cells during development and were both bound to Area II at embryonic day 18.5. Expression of a transgene driven by Pdx1 Areas I and II was also severely compromised during insulin⁺ cell formation in MafB^-/- mice, consistent with the importance of this large Maf in β-cell production and Pdx1 expression. These findings illustrate the significance of large Maf proteins to Pdx1 expression in β cells, and in particular MafB during pancreatic development.

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.

Reversal of streptozotocin-induced diabetes in mice by cellular transduction with recombinant pancreatic transcription factor pancreatic duodenal homeobox-1: a novel protein transduction domain-based therapy

OBJECTIVE

The key pancreatic transcription factor pancreatic duodenal homeobox-1 (Pdx1), known to control development and maintenance of pancreatic beta-cells, possesses a protein transduction domain (PTD) that facilitates its entry into cells. We therefore sought to evaluate the capacity of in vivo-administered recombinant Pdx1 (rPdx1) to ameliorate hyperglycemia in mice with streptozotocin-induced diabetes.

RESEARCH DESIGN AND METHODS

Cell entry and transcriptional regulatory properties of rPdx1 protein and its PTD-deletion mutant rPdx1Delta protein, as well as a PTD-green fluorescent protein, were evaluated in vitro. After intraperitoneal rPdx1 injection into mice with streptozotocin-induced diabetes, we assessed its action on blood glucose levels, insulin content, intraperitoneal glucose tolerance test (IPGTT), Pdx1 distribution, pancreatic gene expression, islet cell proliferation, and organ histology.

RESULTS

Restoration of euglycemia in Pdx1-treated diabetic mice was evident by improved IPGTT and glucose-stimulated insulin release. Insulin, glucagon, and Ki67 immunostaining revealed increased islet cell number and proliferation in pancreata of rPdx1-treated mice. Real-time PCR of pancreas and liver demonstrated upregulation of INS and PDX1 genes and other genes relevant to pancreas regeneration. While the time course of beta-cell gene expression and serum/tissue insulin levels indicated that both liver- and pancreas-derived insulin contributed to restoration of normoglycemia, near-total pancreatectomy resulted in hyperglycemia, suggesting that beta-cell regeneration played the primary role in rPdx1-induced glucose homeostasis.

CONCLUSIONS

rPdx1 treatment of mice with streptozotocin-induced diabetes promotes beta-cell regeneration and liver cell reprogramming, leading to restoration of normoglycemia. This novel PTD-based protein therapy offers a promising way to treat patients with diabetes while avoiding potential side effects associated with the use of viral vectors.

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.

ParaHox genes in pancreatic cell cultures: effects on the insulin promoter regulation

The gene encoding PDX1 (pancreatic duodenum homeobox 1), the main transcription factor regulating the glucose-dependent transactivation of the insulin promoter in pancreatic β-cells, clusters with two closely related homeobox genes (Gsh1 and Cdx2/3), all of them belonging to the ParaHox gene family. The ParaHox gene evolutionary history in the vertebrate lineage involved duplications of the cluster and subsequent loss of some members, so that eventually, the human and murine genomes contain only 6 ParaHox genes. The crucial role of PDX1 in pancreas development, beta-cell formation and insulin transcription regulation has long been established. There is some data on CDX2/3 function in α-cells, but remarkably, nothing is known on the role of the other ParaHox genes, which are also expressed in the endocrine pancreas. Homeobox transcription factors that belong to the same family show high conservation of the homeodomain and share similar target sites and oligomeric partners, and thus may act redundantly, synergistically or antagonistically on the same promoters. Therefore, we explored the effects of the Parahox proteins (GSH1, GSH2, CDX1, CDX2/3 and CDX4) on the regulation of the insulin promoter in transfected α- and β- cultured cell lines at different glucose concentrations and compared them to those of PDX1. Noticeably, several ParaHox transcription factors are able to transactivate or inhibit the insulin promoter, depending on the cell type and glucose concentration, thus suggesting their possible participation in the regulation of similar target genes, such as insulin, either by silencing or activating them, in the absence of PDX1.

Role of PDX-1 and MafA as a potential therapeutic target for diabetes

Pancreatic and duodenal homeobox factor-1 (PDX-1) plays a crucial role in pancreas development, beta-cell differentiation, and maintaining mature beta-cell function. During pancreas development, PDX-1 expression is maintained in precursor cells, and later it becomes restricted to beta-cells. In mature beta-cells, PDX-1 regulates gene expression of various beta-cell-related factors including insulin. Also, PDX-1 has potency to induce insulin-producing cells from non-beta-cells in various tissues, and PDX-1-VP16 fusion protein more efficiently induces insulin-producing cells, especially in the presence of NeuroD or Ngn3. MafA is a recently isolated beta-cell-specific transcription factor which functions as a potent activator of insulin gene transcription. During pancreas development, MafA expression is first detected at the beginning of the principal phase of insulin-producing cell production. Furthermore, MafA markedly enhances insulin gene promoter activity and ameliorates glucose tolerance in diabetic mice, especially in the presence of PDX-1 and NeuroD. Taken together, PDX-1 and MafA play a crucial role in inducing surrogate beta-cells and could be a therapeutic target for diabetes.

Establishment of a non-invasive mouse reporter model for monitoring in vivo pdx-1 promoter activity

It is well known that pancreatic and duodenal homeobox gene-1 (PDX-1) plays a crucial role in beta-cell differentiation, and maintaining mature beta-cell function. Thus, it is important to understand how pdx-1 gene is regulated under various pathophysiological conditions in vivo. In this study, to non-invasively and quantitatively monitor pdx-1 promoter activity in vivo, we constructed a pdx-1 promoter-SEAP-IRES-GFP reporter plasmid. In this construct, the -4.6kb pdx-1 promoter region sufficient for driving beta-cell-selective PDX-1 expression was inserted to the upstream of the secreted alkaline phosphatase (SEAP) reporter gene. It is noted here that the pdx-1 promoter-mediated SEAP activity can be distinguished from endogenous alkaline phosphatase activity. First, we transfected the construct in mouse beta-cell line MIN6 and human hepatocellular carcinoma cell line HepG2. SEAP activity was readily detected in the media of MIN6 cells, but not in HepG2 cells. These results indicate that this construct specifically reports beta-cell-specific pdx-1 promoter activity in a cell culture system. Based on these in vitro findings, we next generated transgenic mice using the same construct. SEAP activity was readily detected in serum of the transgenic mice, but not in their littermate mice. Furthermore, SEAP activity was detected in protein extract from the transgenic pancreas and slightly from the transgenic duodenum, but not from the liver, and brain. These results indicate that serum SEAP activity likely represents in vivo pdx-1 promoter activity. This transgenic mouse model would be useful to non-invasively monitor in vivo pdx-1 promoter activity and to screen new molecules which regulate PDX-1 expression.

A feat of metabolic proportions: Pdx1 orchestrates islet development and function in the maintenance of glucose homeostasis

Emerging evidence over the past decade indicates a central role for transcription factors in the embryonic development of pancreatic islets and the consequent maintenance of normal glucose homeostasis. Pancreatic and duodenal homeobox 1 (Pdx1) is the best studied and perhaps most important of these factors. Whereas deletion or inactivating mutations of the Pdx1 gene causes whole pancreas agenesis in both mice and humans, even haploinsufficiency of the gene or alterations in its expression in mature islet cells causes substantial impairments in glucose tolerance and the development of a late-onset form of diabetes known as maturity onset diabetes of the young. The study of Pdx1 has revealed crucial phenotypic interrelationships of the varied cell types within the pancreas, particularly as these impinge upon cellular differentiation in the embryo and neogenesis and regeneration in the adult. In this review, we describe the actions of Pdx1 in the developing and mature pancreas and attempt to unify these actions with its known roles in modulating transcriptional complex formation and chromatin structure at the molecular genetic level.

Characterization of pancreatic transcription factor Pdx-1 binding sites using promoter microarray and serial analysis of chromatin occupancy

The homeobox transcription factor Pdx-1 is necessary for pancreas organogenesis and beta cell function, however, most Pdx-1-regulated genes are unknown. To further the understanding of Pdx-1 in beta cell biology, we have characterized its genomic targets in NIT-1 cells, a mouse insulinoma cell line. To identify novel targets, we developed a microarray that includes traditional promoters as well as non-coding conserved elements, micro-RNAs, and elements identified through an unbiased approach termed serial analysis of chromatin occupancy. In total, 583 new Pdx-1 target genes were identified, many of which contribute to energy sensing and insulin release in pancreatic beta cells. By analyzing 31 of the protein-coding Pdx-1 target genes, we show that 29 are expressed in beta cells and, of these, 68% are down- or up-regulated in cells expressing a dominant negative mutant of Pdx-1. We additionally show that many Pdx-1 targets also interact with NeuroD1/BETA2, including the micro-RNA miR-375, a known regulator of insulin secretion.

Ptf1a and RBP-J cooperate in activating Pdx1 gene expression through binding to Area III

Pancreatic and duodenal homeobox factor 1 (Pdx1) has been demonstrated to play a crucial role in pancreas development and in maintenance of mature beta-cell function. However, it remains to be elucidated how Pdx1 gene expression is regulated in non-beta cells during pancreas development. Pdx1 and Ptf1a are expressed in pancreatic progenitor cells, which give rise to all three types of pancreatic tissue. In addition, Ptf1a has been shown to bind the mammalian Suppressor of Hairless (RBP-J) within the PTF1 complex. Furthermore, loss-of-function approaches have revealed that all three factors are essential for early pancreas development. We therefore hypothesized that Ptf1a and RBP-J regulate expression of the Pdx1 gene in pancreatic precursors. Reporter gene analyses showed that Ptf1a transactivated Pdx1 promoter in pancreatic Panc-1 cells, which was enhanced by RBP-J. Deletion/mutation analyses of the Pdx1 promoter and electrophoretic gel-mobility shift assays identified the Ptf1a binding site in the well-conserved regulatory sequence domain, termed Area III, which was also confirmed by the chromatin immunoprecipitation assay. Furthermore, adenovirus-mediated overexpression of Ptf1a, together with RBP-J, markedly increased Pdx1 protein levels in pancreatic AR42J-B13 cells. Our data suggest a novel transcriptional network, where Ptf1a and RBP-J cooperatively regulate Pdx1 gene expression through binding to Area III.

MafA regulates expression of genes important to islet beta-cell function

Insulin transcription factor MafA is unique in being exclusively expressed at the secondary and principal phase of insulin-expressing cell production during pancreas organogenesis and is the only transcriptional activator present exclusively in islet beta-cells. Here we show that ectopic expression of MafA is sufficient to induce a small amount of endogenous insulin expression in a variety of non-beta-cell lines. Insulin mRNA and protein expression was induced to a much higher level when MafA was provided with two other key insulin activators, pancreatic and duodenal homeobox (PDX-1) and BETA2. Potentiation by PDX-1 and BETA2 was entirely dependent upon MafA, and MafA binding to the insulin enhancer region was increased by PDX-1 and BETA2. Treatment with activin A and hepatocyte growth factor induced even larger amounts of insulin in AR42J pancreatic acinar cells, compared with other non-beta endodermal cells. The combination of PDX-1, BETA2, and MafA also induced the expression of other important regulators of islet beta-cell activity. These results support a critical role of MafA in islet beta-cell function.

Regulation of pancreas duodenum homeobox-1 expression by early growth response-1

The homeodomain transcription factor pancreas duodenum homeobox-1 (PDX-1) is a key regulator of pancreatic beta-cell development, function, and survival. Deficits in PDX-1 expression result in insulin deficiency and hyperglycemia. We previously found that the glucose-responsive transcription factor early growth response-1 (Egr-1) activates the insulin promoter in part by increasing expression levels of PDX-1. We now report that Egr-1 binds and activates multiple regulatory sites within the pdx-1 promoter. We identified consensus Egr-1 recognition sequences within proximal and distal regions of the mouse pdx-1 promoter and demonstrated specific binding of Egr-1 by chromatin immunoprecipitation and electrophoretic mobility shift assays. Overexpression of Egr-1 increased transcriptional activation of the -4500 proximal pdx-1 promoter and of the highly conserved regulatory Areas I, II, and III. Mutagenesis of a specific Egr-1 binding site within Area III substantially decreased Egr-1-mediated activation. Egr-1 increased the transcriptional activation of Areas I and II, despite the absence of Egr-1 recognition sequences within this promoter segment, suggesting that Egr-1 also can regulate the pdx-1 promoter indirectly. Egr-1 increased, and a dominant-negative Egr-1 mutant repressed, the transcriptional activation of distal pdx-1 promoter sequences. Mutagenesis of a specific Egr-1 binding site within regulatory Area IV reduced basal and Egr-1-mediated transcriptional activation. Our data indicate that Egr-1 regulates expression of PDX-1 in pancreatic beta-cells by both direct and indirect activation of the pdx-1 promoter. We propose that Egr-1 expression levels may act as a sensor in pancreatic beta-cells to translate extracellular signals into changes in PDX-1 expression levels and pancreatic beta-cell function.

Alternate mRNA processing of the hepatocyte nuclear factor genes and its role in monogenic diabetes

Variation in mRNA processing has the capacity to exert fine control over gene expression in most cell types. The hepatic nuclear factor genes, like approximately 74% of the genome, produce multiple transcripts. Hepatic nuclear factor isoforms exhibit both spatial and temporal variation in expression. In this review, the known isoforms of the hepatocyte nuclear factor-1α, hepatocyte nuclear factor-1β and hepatocyte nuclear factor-4α genes are described and their properties are compared. Finally, data are discussed regarding the influence of hepatocyte nuclear factor-1α alternate mRNA processing on the clinical phenotype of maturity-onset diabetes of the young.