Induction of insulin and islet amyloid polypeptide production in pancreatic islet glucagonoma cells by insulin promoter factor 1

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.

Pancreatic and duodenal homeobox-1 (PDX1) contributes to β-cell mass expansion and proliferation induced by Akt/PKB pathway

Maintenance of pancreatic β-cell mass and function is fundamental to glucose homeostasis and to prevent diabetes. The PI3 K-Akt-mTORC1 pathway is critical for β-cells mass and function, while PDX1 has been implicated in β-cell development, maturation, and function. Here we tested whether Akt signaling requires PDX1 expression to regulate β-cell mass, proliferation, and glucose homeostasis. In order to address that, we crossed a mouse model overexpressing constitutively active Akt mutant in β-cells (β-caAkt) with mice lacking one allele of PDX1gene (β-caAkt/pdx1^+/-). While the β-caAkt mice exhibit higher plasma insulin levels, greater β-cell mass and improved glucose tolerance compared to control mice, the β-caAkt/pdx1^+/- mice are hyperglycemic and intolerant to glucose. The changes in glucose homeostasis in β-caAkt/pdx1^+/- were associated with a 60% reduction in β-cell mass compared to β-caAkt mice. The impaired β-cell mass in the β-caAkt/pdx1^+/- mice can be explained by a lesser β-cell proliferation measured by the number of Ki67 positive β-cells. We did not observe any differences in apoptosis between β-caAkt/pdx1^+/- and β-caAkt mice. In conclusion, PDX1 contributes to β-cell mass expansion and glucose metabolism induced by activation of Akt signaling.

Metformin Restrains Pancreatic Duodenal Homeobox-1 (PDX-1) Function by Inhibiting ERK Signaling in Pancreatic Ductal Adenocarcinoma

Pancreatic ductal adenocarcinoma (PDAC) is one of the most potent and perilous diseases known, with a median survival rate of 3-5 months due to the combination of only advanced stage diagnosis and ineffective therapeutic options. Metformin (1,1-Dimethylbiguanide hydrochloride), the leading drug used for type 2 diabetes mellitus, emerges as a potential therapy for PDAC and other human cancers. Metformin exerts its anticancer action via a variety of adenosine monophosphate (AMP)-activated protein kinase (AMPK)- dependent and/or AMPK-independent mechanisms. We present data here showing that metformin downregulated pancreatic transcription factor pancreatic duodenal homeobox-1 (PDX-1), suggesting a potential novel mechanism by which metformin exerts its anticancer action. Metformin inhibited PDX-1 expression at both protein and mRNA levels and PDX-1 transactivity as well in PDAC cells. Extracellular signal-regulated kinase (ERK) was identified as a PDX-1-interacting protein by antibody array screening in GFP-PDX-1 stable HEK293 cells. Co-transfection of ERK1 with PDX-1 resulted in an enhanced PDX-1 expression in HEK293 cells in a dose-dependent manner. Immunoprecipitation/Western blotting analysis confirmed the ERK-PDX-1 interaction in PANC-1 cells stimulated by epidermal growth factor (EGF). EGF induced an enhanced PDX-1 expression in PANC-1 cells and this stimulation was inhibited by MEK inhibitor PD0325901. Metformin inhibited EGF-stimulated PDX-1 expression with an accompanied inhibition of ERK kinase activation in PANC- 1 cells. Taken together, our studies show that PDX-1 is a potential novel target for metformin in PDAC cells and that metformin may exert its anticancer action in PDAC by down-regulating PDX-1 via a mechanism involving inhibition of ERK signaling.

THERAPY OF ENDOCRINE DISEASE: Islet transplantation for type 1 diabetes: so close and yet so far away

Over the past decades, tremendous efforts have been made to establish pancreatic islet transplantation as a standard therapy for type 1 diabetes. Recent advances in islet transplantation have resulted in steady improvements in the 5-year insulin independence rates for diabetic patients. Here we review the key challenges encountered in the islet transplantation field which include islet source limitation, sub-optimal engraftment of islets, lack of oxygen and blood supply for transplanted islets, and immune rejection of islets. Additionally, we discuss possible solutions for these challenges.

Using stem cells to produce insulin

INTRODUCTION

Tremendous progress has been made in generating insulin-producing cells from pluripotent stem cells. The best outcome of the refined protocols became apparent in the first clinical trial announced by ViaCyte, based on the implantation of pancreatic progenitors that would further mature into functional insulin-producing cells inside the patient's body.

AREAS COVERED

Several groups, including ours, have contributed to improve strategies to generate insulin-producing cells. Of note, the latest results have gained a substantial amount of interest as a method to create a potentially functional and limitless supply of β-cell to revert diabetes mellitus. This review analyzes the accomplishments that have taken place over the last few decades, summarizes the state-of-art methods for β-cell replacement therapies based on the differentiation of embryonic stem cells into glucose-responsive and insulin-producing cells in a dish and discusses alternative approaches to obtain new sources of insulin-producing cells.

EXPERT OPINION

Undoubtedly, recent events preface the beginning of a new era in diabetes therapy. However, in our opinion, a number of significant hurdles still stand in the way of clinical application. We believe that the combination of the private and public sectors will accelerate the process of obtaining the desired safe and functional β-cell surrogates.

Down-regulation of pancreatic and duodenal homeobox-1 by somatostatin receptor subtype 5: a novel mechanism for inhibition of cellular proliferation and insulin secretion by somatostatin

Somatostatin (SST) is a regulatory peptide and acts as an endogenous inhibitory regulator of the secretory and proliferative responses of target cells. SST’s actions are mediated by a family of seven transmembrane domain G protein-coupled receptors that comprise five distinct subtypes (SSTR1-5). SSTR5 is one of the major SSTRs in the islets of Langerhans. Homeodomain-containing transcription factor pancreatic and duodenal homeobox-1 (PDX-1) is essential for pancreatic development, β cell differentiation, maintenance of normal β cell functions in adults and tumorigenesis. Recent studies show that SSTR5 acts as a negative regulator for PDX-1 expression and that SSTR5 mediates somatostatin’s inhibitory effect on cell proliferation and insulin expression/excretion through down-regulating PDX-1 expression. SSTR5 exerts its inhibitory effect on PDX-1 expression at both the transcriptional level by down-regulating PDX-1 mRNA and the post-translational level by enhancing PDX-1 ubiquitination. Identification of PDX-1 as a transcriptional target for SSTR5 may help in guiding the choice of therapeutic cancer treatments.

Grg3/TLE3 and Grg1/TLE1 induce monohormonal pancreatic β-cells while repressing α-cell functions

In the pancreas, α- and β-cells possess a degree of plasticity. In vitro differentiation of pluripotent cells yields mostly α- and polyhormonal β-like cells, indicating a gap in understanding of how functional monohormonal β-cells are formed and of the endogenous repressive mechanisms used to maintain β-cell identity. We show that the corepressor Grg3 is expressed in almost all β-cells throughout embryogenesis to adulthood. However, Grg3 is expressed in fewer nascent α-cells and is progressively lost from α-cells as endocrine cells mature into adulthood. We show that mouse Grg3(+/-) β-cells have increased α-specific gene expression, and Grg3(+/-) pancreata have more α-cells and more polyhormonal cells, indicating that Grg3 is required for the physiologic maintenance of monohormonal β-cell identity. Ectopic expression of Grg3 in α-cells represses glucagon and Arx, and the addition of Pdx1 induces Glut2 expression and glucose-responsive insulin secretion. Furthermore, we found that Grg1 is the predominant Groucho expressed in human β-cells but acts functionally similarly to Grg3. Overall, we find that Grg3 and Grg1 establish a monohormonal β-cell identity, and Groucho family members may be useful tools or markers for making functional β-cells.

Vertically integrated translational studies of PDX1 as a therapeutic target for pancreatic cancer via a novel bifunctional RNAi platform

RNA interference (RNAi) represents a powerful, new tool for scientific investigation as well as a promising new form of targeted gene therapy, with applications currently in clinical trials. Bifunctional short hairpin RNA (shRNA) are synthetic RNAi molecules, engineered to utilize multiple endogenous RNAi pathways to specifically silence target genes. Pancreatic and duodenal homeobox 1 (PDX1) is a key regulator of pancreatic development, β-cell differentiation, normal β-cell function and pancreatic cancer. Our aim is to review the process of identifying PDX1 as a specific, potential RNAi target in pancreatic cancer, as well as the underlying mechanisms and various forms of RNAi, with subsequent testing and development of PDX1-targeted bifunctional shRNA therapy.

Generating NMR chemical shift assignments of intrinsically disordered proteins using carbon-detected NMR methods

There is an extraordinary need to describe the structures of intrinsically disordered proteins (IDPs) due to their role in various biological processes involved in signaling and transcription. However, general study of IDPs by NMR spectroscopy is limited by the poor (1)H amide chemical shift dispersion typically observed in their spectra. Recently, (13)C direct-detected NMR spectroscopy has been recognized as enabling broad structural study of IDPs. Most notably, multidimensional experiments based on the (15)N,(13)C CON spectrum make complete chemical shift assignment feasible. Here we document a collection of NMR-based tools that efficiently lead to chemical shift assignment of IDPs, motivated by a case study of the C-terminal disordered region from the human pancreatic transcription factor Pdx1. Our strategy builds on the combination of two three-dimensional (3D) experiments, (HN-flip)N(CA)CON and 3D (HN-flip)N(CA)NCO, that enable daisy chain connections to be built along the IDP backbone, facilitated by acquisition of amino acid-specific (15)N,(13)C CON-detected experiments. Assignments are completed through carbon-detected, total correlation spectroscopy (TOCSY)-based side chain chemical shift measurement. Conducting our study required producing valuable modifications to many previously published pulse sequences, motivating us to announce the creation of a database of our pulse programs, which we make freely available through our website.

Thermodynamic and structural determinants of differential Pdx1 binding to elements from the insulin and IAPP promoters

In adult mammals, the production of insulin and other peptide hormones, such as the islet amyloid polypeptide (IAPP), is limited to β-cells due to tissue-specific expression of a set of transcription factors, the best known of which is pancreatic duodenal homeobox protein 1 (Pdx1). Like many homeodomain transcription factors, Pdx1 binds to a core DNA recognition sequence containing the tetranucleotide 5'-TAAT-3'; its consensus recognition element is 5'-CTCTAAT(T/G)AG-3'. Currently, a complete thermodynamic profile of Pdx1 binding to near-consensus and native promoter sequences has not been established, obscuring the mechanism of target site selection by this critical transcription factor. Strikingly, while Pdx1 responsive elements in the human insulin promoter conform to the pentanucleotide 5'-CTAAT-3' sequence, the Pdx1 responsive elements in the human iapp promoter all contain a substitution to 5'-TTAAT-3'. The crystal structure of Pdx1 bound to the consensus nucleotide sequence does not explain how Pdx1 identifies this natural variation, if it does at all. Here we report a combination of isothermal calorimetric titrations, NMR spectroscopy, and extensive multi-microsecond molecular dynamics calculations of Pdx1 that define its interactions with a panel of natural promoter elements and consensus-derived sequences. Our results show a small preference of Pdx1 for a C base 5' relative to the core TAAT promoter element. Molecular mechanics calculations, corroborated by experimental NMR data, lead to a rational explanation for sequence discrimination at this position. Taken together, our results suggest a molecular mechanism for differential Pdx1 affinity to elements from the insulin and iapp promoter sequences.

The role of single-cell analyses in understanding cell lineage commitment

The study of cell lineage commitment is critical for improving our understanding of tissue development and regeneration, and for realizing stem cell-based therapies and engineered tissue replacements. Recently, the discovery of an unanticipated degree of variability in fundamental biological processes, including divergent responses of genetically identical cells to various stimuli, has provided mechanistic insight into cellular decision making and the collective behavior of cell populations. Therefore, the study of lineage commitment with single-cell resolution could provide greater knowledge of cellular differentiation mechanisms and the influence of noise on cellular processes. This will require the adoption of new technologies for single-cell analysis as traditional methods typically measure average values of bulk population behavior. This review discusses the recent developments in methods for analyzing the behavior of individual cells, and how these approaches are leading to a deeper understanding and better control of cellular decision making.

PDX-1 is a therapeutic target for pancreatic cancer, insulinoma and islet neoplasia using a novel RNA interference platform

Pancreatic and duodenal homeobox-1 (PDX-1) is a transcription factor that regulates insulin expression and islet maintenance in the adult pancreas. Our recent studies demonstrate that PDX-1 is an oncogene for pancreatic cancer and is overexpressed in pancreatic cancer. The purpose of this study was to demonstrate that PDX-1 is a therapeutic target for both hormonal symptoms and tumor volume in mouse models of pancreatic cancer, insulinoma and islet neoplasia. Immunohistochemistry of human pancreatic and islet neoplasia specimens revealed marked PDX-1 overexpression, suggesting PDX-1 as a "drugable" target within these diseases. To do so, a novel RNA interference effector platform, bifunctional shRNA(PDX-1), was developed and studied in mouse and human cell lines as well as in mouse models of pancreatic cancer, insulinoma and islet neoplasia. Systemic delivery of bi-shRNA(humanPDX-1) lipoplexes resulted in marked reduction of tumor volume and improved survival in a human pancreatic cancer xenograft mouse model. bi-shRNA(mousePDX-1) lipoplexes prevented death from hyperinsulinemia and hypoglycemia in an insulinoma mouse model. shRNA(mousePDX-1) lipoplexes reversed hyperinsulinemia and hypoglycemia in an immune-competent mouse model of islet neoplasia. PDX-1 was overexpressed in pancreatic neuroendocrine tumors and nesidioblastosis. These data demonstrate that PDX-1 RNAi therapy controls hormonal symptoms and tumor volume in mouse models of pancreatic cancer, insulinoma and islet neoplasia, therefore, PDX-1 is a potential therapeutic target for these pancreatic diseases.

PDX-1 is a therapeutic target for pancreatic cancer, insulinoma and islet neoplasia using a novel RNA interference platform

Pancreatic and duodenal homeobox-1 (PDX-1) is a transcription factor that regulates insulin expression and islet maintenance in the adult pancreas. Our recent studies demonstrate that PDX-1 is an oncogene for pancreatic cancer and is overexpressed in pancreatic cancer. The purpose of this study was to demonstrate that PDX-1 is a therapeutic target for both hormonal symptoms and tumor volume in mouse models of pancreatic cancer, insulinoma and islet neoplasia. Immunohistochemistry of human pancreatic and islet neoplasia specimens revealed marked PDX-1 overexpression, suggesting PDX-1 as a "drugable" target within these diseases. To do so, a novel RNA interference effector platform, bifunctional shRNA(PDX-1), was developed and studied in mouse and human cell lines as well as in mouse models of pancreatic cancer, insulinoma and islet neoplasia. Systemic delivery of bi-shRNA(humanPDX-1) lipoplexes resulted in marked reduction of tumor volume and improved survival in a human pancreatic cancer xenograft mouse model. bi-shRNA(mousePDX-1) lipoplexes prevented death from hyperinsulinemia and hypoglycemia in an insulinoma mouse model. shRNA(mousePDX-1) lipoplexes reversed hyperinsulinemia and hypoglycemia in an immune-competent mouse model of islet neoplasia. PDX-1 was overexpressed in pancreatic neuroendocrine tumors and nesidioblastosis. These data demonstrate that PDX-1 RNAi therapy controls hormonal symptoms and tumor volume in mouse models of pancreatic cancer, insulinoma and islet neoplasia, therefore, PDX-1 is a potential therapeutic target for these pancreatic diseases.

Intestinal Pdx1 mediates nutrient metabolism gene networks and maternal expression is essential for perinatal growth in mice

The homeodomain transcription factor Pdx1 is essential for pancreas formation and functions in pancreatic islets cells to regulate genes involved in maintenance of glucose homeostasis. In order to investigate a role for Pdx1 in intestinal cells, we analyzed the functions and networks associated with genes differentially expressed by Pdx1 overexpression in human Caco-2 cells. In agreement with previous results for intestine isolated from mice with Pdx1 inactivation, functional analysis of genes differentially expressed with Pdx1 overexpression revealed functions significantly associated with nutrient metabolism. Similarly, network analysis examining the interactions among the differentially expressed genes revealed gene networks involved in lipid metabolism. Consistent with defects in maternal nutrient metabolism, mouse pups born to dams with intestine-specific Pdx1 inactivation are underweight and fail to thrive in the neonatal period compared to pups born to control dams. We conclude that Pdx1 mediates lipid metabolism gene networks in intestinal cells and that maternal expression is essential for perinatal growth in mice.

PDX1 regulation of FABP1 and novel target genes in human intestinal epithelial Caco-2 cells

The transcription factor pancreatic and duodenal homeobox 1 (PDX1) plays an essential role in pancreatic development and in maintaining proper islet function via target gene regulation. Few intestinal PDX1 targets, however, have been described. We sought to define novel PDX1-regulated intestinal genes. Caco-2 human intestinal epithelial cells were engineered to overexpress PDX1 and gene expression profiles relative to control cells were assessed. Expression of 80 genes significantly increased while that of 49 genes significantly decreased more than 4-fold following PDX1 overexpression in differentiated Caco-2 cells. Analysis of the differentially regulated genes with known functional annotations revealed genes encoding transcription factors, growth factors, kinases, digestive glycosidases, nutrient transporters, nutrient binding proteins, and structural components. The gene for fatty acid binding protein 1, liver, FABP1, is repressed by PDX1 in Caco-2 cells. PDX1 overexpression in Caco-2 cells also results in repression of promoter activity driven by the 0.6kb FABP1 promoter. PDX1 regulation of promoter activity is consistent with the decrease in FABP1 RNA abundance resulting from PDX1 overexpression and identifies FABP1 as a candidate PDX1 target. PDX1 repression of FABP1, LCT, and SI suggests a role for PDX1 in patterning anterior intestinal development.

Expression profiling identifies novel gene targets and functions for Pdx1 in the duodenum of mature mice

Transcription factor pancreatic and duodenal homeobox 1 (Pdx1) plays an essential role in the pancreas to regulate its development and maintain proper islet function. However, the functions of Pdx1 in mature small intestine are less known. We aimed to investigate the intestinal role of Pdx1 by profiling the expression of genes differentially regulated in response to inactivation of Pdx1 specifically in the intestinal epithelium. Pdx1 was conditionally inactivated in the intestinal epithelium of Pdx1(flox/flox);VilCre mice. Total RNA was isolated from the first 5 cm of the small intestine from mature Pdx1(flox/flox);VilCre and littermate control mice. Microarray analysis identified 86 probe sets representing 68 genes significantly upregulated or downregulated 1.5-fold or greater in Pdx(flox/flox);VilCre mice maintained under standard conditions. Ingenuity Pathway Analysis revealed that functions of the differentially expressed genes are significantly associated with metabolism of nutrients including lipids and iron. Network analysis examining the interactions among the differentially expressed genes further supports the notion that Pdx1 may modulate metabolism of lipids and iron from mature intestinal epithelium. Following forced oil feeding, Pdx1(flox/flox);VilCre mice showed diminished lipid staining in the duodenal epithelium and decreased serum triglyceride levels, indicating reduced lipid absorption compared with control duodenal epithelium. Blood samples from Pdx1(flox/flox);VilCre mice have significantly lower mean values for mean corpuscular volume and mean corpuscular hemoglobin, consistent with iron deficiency. The absence of nonheme iron in the villous epithelium and lamina propria of Pdx1(flox/flox);VilCre duodenum indicates that the duodenal epithelium lacking Pdx1 may have defects in importing iron through enterocytes, resulting in iron deficiency in Pdx1(flox/flox);VilCre mice.

Foetal proglucagon processing in relation to adult appetite control: lessons from a transplantable rat glucagonoma with severe anorexia

We have previously reported severe anorexia abruptly induced in rats 2-3 weeks after they have been transplanted subcutaneously with the glucagonoma MSL-G-AN. Vagotomy did not affect the time of onset and severity of anorexia, and the anorectic state resembles hunger with strongly elevated neuropeptide Y (NPY) mRNA levels in the nucleus arcuatus. We now show that circulating levels of bioactive glucagon-like peptide-1 (GLP-1) (7-36amide) start to increase above control levels exactly at the time of onset of anorexia. At this time-point, bioactive glucagon as well as total glucagon precursors and GLP-1 metabolites are already vastly elevated compared to controls. We further show that intravenous administration of very high concentrations of GLP-1 to hungry schedule-fed rats causes anorexia in a dose-dependent manner, which is blocked by the GLP-1 receptor antagonist exendin (9-39). GLP-1 (7-36amide) has a well-characterized anorectic effect but also causes taste aversion when administered centrally. The anorectic effect is blocked in rats treated neonatally by monosodium glutamate (MSG). We show that MSG treatment does not prevent the MSL-G-AN-induced anorexia, thereby suggesting a different type of anorectic function. We show a very strong component of taste aversion as anorectic rats, when presented to novel or known alternative food items, will resume normal feeding for 1 day, and then redevelop anorexia. We hypothetize that the anorexia in MSL-G-AN tumour-bearing rats correlates with a foetal processing pattern of proglucagon to both glucagon and GLP-1 (7-36amide), and is due to taste aversion. The sudden onset is characterized by a dramatic increase in circulating levels of biologically active GLP-1 (7-36amide), suggesting eventual saturation of proteolytic inactivation of its N-terminus.

The hypofunctional effect of P335L single nucleotide polymorphism on SSTR5 function

BACKGROUND

Somatostatin receptor subtype 5 (SSTR5) mediates the inhibitory effect of somatostatin on insulin expression/secretion and cell proliferation. A number of single nucleotide polymorphisms (SNPs) of SSTR5 have been identified, including P335L, a nonsynonymous SNP located in the protein C-terminal region and encrypted by the codon CCG (proline) or the codon CTG (leucine). In the present study we sought to determine the distribution of the SSTR5 P335L SNP in a cohort of pancreatic cancer patients and whether the P335L SNP affected cellular function of SSTR5 in human pancreatic cancer.

METHODS

The P335L germline genotype of 246 patients with pancreatic cancer (213 Caucasians, 16 Hispanics, and 17 African Americans) and 17 human pancreatic cell lines was determined with the TaqMan SNP Genotyping assay. Human SSTR5 leucine variant (L335) was generated by performing site-directed mutagenesis using SSTR5 proline variant (P335) as a template. Transient transfections were performed in HEK293, Mia PaCa-2, and β-TC-6 cells using Lipofectamine 2000. The expression of SSTR5 L335 was determined with a mouse monoclonal anti-SSTR5 L335 antibody generated in our laboratory. The cell proliferation rate was measured by performing MTS assays. Insulin concentration was measured by performing ELISA assays.

RESULTS

Genotyping of the patients' blood indicated that the frequency of the T allele (CT and TT genotypes) in codon 335 of SSTR5 in Caucasians, Hispanics, and African Americans was 52, 69, and 35%, respectively, which was race-dependent. Statistical analysis indicated that association between the frequency of the T allele and the existence of pancreatic cancer in each race missed significance perhaps due to limited sample size. In 17 tested human pancreatic cancer cell lines, 5 (Capan-2, HPAF-II, Panc03.27, Panc-1, and -3) were homozygous (TT genotype) and 9, including Mia PaCa-2, were heterozygous (CT genotype). Overexpression of SSTR5 L335 in Mia PaCa-2 cells enhanced cell proliferation compared to overexpression of SSTR5 P335. Overexpression of SSTR5 P335 enhanced the inhibitory effect of SSTR5 agonist RPL-1980 on cell proliferation of Mia PaCa-2 cells and glucose-stimulated insulin secretion from mouse insulinoma cells, while overexpression of SSTR5 L335 blocked the inhibitory effect of RPL-1980. Overexpression of SSTR5 L335 enhanced PDX-1 expression in Mia PaCa-2 cells. A specific monoclonal antibody was generated to detect SSTR5 P335L.

CONCLUSION

SSTR5 P335L SNP widely exists in the human population, in patients with pancreatic cancer, and is race-dependent. The SNP is also present in selected human pancreatic cancer cell lines. In contrast to SSTR5 P335, overexpression of the SSTR5 L335 variant resulted in cellular proliferation and PDX-1 overexpression in human pancreatic cancer cells. Its overexpression blocked the inhibitory effect of an SSTR5-specific analog on human pancreatic cancer cell proliferation and on glucose-stimulated insulin secretion from mouse insulinoma cells. These data suggest that SSTR5 P335L is a hypofunctional protein with a potentially harmful effect on function, as well as potential latent effect, and therefore it could affect the clinical response to somatostatin analog therapy for patients with pancreatic cancer.

Milk signalling in the pathogenesis of type 2 diabetes

The presented hypothesis identifies milk consumption as an environmental risk factor of Western diet promoting type 2 diabetes (T2D). Milk, commonly regarded as a valuable nutrient, exerts important endocrine functions as an insulinotropic, anabolic and mitogenic signalling system supporting neonatal growth and development. The presented hypothesis substantiates milk's physiological role as a signalling system for pancreatic β-cell proliferation by milk's ability to increase prolactin-, growth hormone and incretin-signalling. The proposed mechanism of milk-induced postnatal β-cell mass expansion mimics the adaptive prolactin-dependent proliferative changes observed in pregnancy. Milk signalling down-regulates the key transcription factor FoxO1 leading to up-regulation of insulin promoter factor-1 which stimulates β-cell proliferation, insulin secretion as well as coexpression of islet amyloid polypeptide (IAPP). The recent finding that adult rodent β-cells only proliferate by self-duplication is of crucial importance, because permanent milk consumption beyond the weaning period may continuously over-stimulate β-cell replication thereby accelerating the onset of replicative β-cell senescence. The long-term use of milk may thus increase endoplasmic reticulum (ER) stress and toxic IAPP oligomer formation by overloading the ER with cytotoxic IAPPs thereby promoting β-cell apoptosis. Both increased β-cell proliferation and β-cell apoptosis are hallmarks of T2D. This hypothesis gets support from clinical states of hyperprolactinaemia and progeria syndromes with early onset of cell senescence which are both associated with an increased incidence of T2D and share common features of milk signalling. Furthermore, the presented milk hypothesis of T2D is compatible with the concept of high ER stress in T2D and the toxic oligomer hypothesis of T2D and may explain the high association of T2D and Alzheimer disease.

AR42J-B-13 cell: an expandable progenitor to generate an unlimited supply of functional hepatocytes

Hepatocytes are the preparation of choice for Toxicological research in vitro. However, despite the fact that hepatocytes proliferate in vivo during liver regeneration, they are resistant to proliferation in vitro, do not tolerate sub-culture and tend to enter a de-differentiation program that results in a loss of hepatic function. These limitations have resulted in the search for expandable rodent and human cells capable of being directed to differentiate into functional hepatocytes. Research with stem cells suggests that it may be possible to provide the research community with hepatocytes in vitro although to date, significant challenges remain, notably generating a sufficiently pure population of hepatocytes with a quantitative functionality comparable with hepatocytes. This paper reviews work with the AR42J-B-13 (B-13) cell line. The B-13 cell was cloned from the rodent AR42J pancreatic cell line, express genes associated with pancreatic acinar cells and readily proliferates in simple culture media. When exposed to glucocorticoid, 75-85% of the cells trans-differentiate into hepatocyte-like (B-13/H) cells functioning at a level quantitatively similar to freshly isolated rat hepatocytes (with the remaining cells retaining the B-13 phenotype). Trans-differentiation of pancreatic acinar cells also appears to occur in vivo in rats treated with glucocorticoid; in mice with elevated circulating glucocorticoid and in humans treated for long periods with glucocorticoid. The B-13 response to glucocorticoid therefore appears to be related to a real pathophysiological response of a pancreatic cell to glucocorticoid. An understanding of how this process occurs and if it can be generated or engineered in human cells would result in a cell line with the ability to generate an unlimited supply of functional human hepatocytes in a cost effective manner.

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.

Islet cell development

Over the last years, there has been great success in driving stem cells toward insulin-expressing cells. However, the protocols developed to date have some limitations, such as low reliability and low insulin production. The most successful protocols used for generation of insulin-producing cells from stem cells mimic in vitro pancreatic organogenesis by directing the stem cells through stages that resemble several pancreatic developmental stages. Islet cell fate is coordinated by a complex network of inductive signals and regulatory transcription factors that, in a combinatorial way, determine pancreatic organ specification, differentiation, growth, and lineage. Together, these signals and factors direct the progression from multipotent progenitor cells to mature pancreatic cells. Later in development and adult life, several of these factors also contribute to maintain the differentiated phenotype of islet cells. A detailed understanding of the processes that operate in the pancreas during embryogenesis will help us to develop a suitable source of cells for diabetes therapy. In this chapter, we will discuss the main transcription factors involved in pancreas specification and beta-cell formation.

Pdx1 inactivation restricted to the intestinal epithelium in mice alters duodenal gene expression in enterocytes and enteroendocrine cells

Null mutant mice lacking the transcription factor pancreatic and duodenal homeobox 1 (Pdx1) are apancreatic and survive only a few days after birth. The role of Pdx1 in regulating intestinal gene expression has therefore yet to be determined in viable mice with normal pancreatic development. We hypothesized that conditional inactivation of Pdx1 restricted to the intestinal epithelium would alter intestinal gene expression and cell differentiation. Pdx1(flox/flox);VilCre mice with intestine-specific Pdx1 inactivation were generated by crossing a transgenic mouse strain expressing Cre recombinase, driven by a mouse villin 1 gene promoter fragment, with a mutant mouse strain homozygous for loxP site-flanked Pdx1. Pdx1 protein is undetectable in all epithelial cells in the intestinal epithelium of Pdx1(flox/flox);VilCre mice. Goblet cell number and mRNA abundance for mucin 3 and mucin 13 genes in the proximal small intestine are comparable between Pdx1(flox/flox);VilCre and control mice. Similarly, Paneth cell number and expression of Paneth cell-related genes Defa1, Defcr-rs1, and Mmp7 in the proximal small intestine remain statistically unchanged by Pdx1 inactivation. Although the number of enteroendocrine cells expressing chromogranin A/B, gastric inhibitory polypeptide (Gip), or somatostatin (Sst) is unaffected in the Pdx1(flox/flox);VilCre mice, mRNA abundance for Gip and Sst is significantly reduced in the proximal small intestine. Conditional Pdx1 inactivation attenuates intestinal alkaline phosphatase (IAP) activity in the duodenal epithelium, consistent with an average 91% decrease in expression of the mouse enterocyte IAP gene, alkaline phosphatase 3 (a novel Pdx1 target candidate), in the proximal small intestine following Pdx1 inactivation. We conclude that Pdx1 is necessary for patterning appropriate gene expression in enterocytes and enteroendocrine cells of the proximal small intestine.

Glucose-responsive gene expression system for gene therapy

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

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.

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.

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.

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.

PDX-1 expression is associated with islet proliferation in vitro and in vivo

BACKGROUND

Transcription factor pancreatic duodenal homeobox-1 (PDX-1) is critical for beta-cell differentiation and insulin gene expression. In this study, we investigated the role of PDX-1 in ductal-to-islet cell transdifferentiation, islet cell apoptosis, and proliferation in addition to other regulators associated with these processes in two developing beta-cell models.

MATERIALS AND METHODS

CAPAN-1 cells were cultured with the GLP-1 analogue Exendin-4 (Ex-4) to induce transdifferentiation to an insulin-producing phenotype. Expression patterns of PDX-1, somatostatin receptors (SSTR) 1, 2, and 5, p27, and p38 were analyzed. To model pancreatic regeneration in vivo, subtotal pancreatectomies were performed in rats and remnant pancreata were compared to sham laparotomy controls to determine islet size, morphology, apoptosis, and PDX-1 expression.

RESULTS

In Ex-4-treated cells, PDX-1 expression increased 67% above basal levels within 24 h and was followed by a 10-fold decline in expression by the end of the study. Expression of cell-cycle inhibitor p27 was down-regulated by 81% at 24 h, while levels of the pro-apoptotic modulator p38 significantly increased 4-fold. When compared to controls, SSTR1 expression declined, while SSTR2 and SSTR5 expression were significantly up-regulated in treated cells. Immunofluorescence of pancreatic remnants following subtotal pancreatectomy revealed increased PDX-1 staining at 24 h followed by a significant decline at 72 h post-pancreatectomy.

CONCLUSION

GLP-1 analogue Ex-4 resulted in up-regulation of PDX-1 in CAPAN-1 cells and PDX-1 was up-regulated in proliferating islets following subtotal pancreatectomy in rats. The increase was seen in the first 24 h. These findings suggest a possible relationship between PDX-1 and the state of islet proliferation, islet-to-ductal transdifferentiation, apoptosis, and the expression of SSTRs.

Pdx-1 modulates histone H4 acetylation and insulin gene expression in terminally differentiated alpha-TC-1 cells

OBJECTIVES

Islet-associated transcription factors play a critical role in regulating pancreatic endocrine cell differentiation and islet hormone gene expression. Both alpha- and beta-cells differentiate from a common endocrine precursor cell. Therefore, it is important to reveal the differential gene expression profiles between alpha- and beta-cells that can direct their terminal cell fates. alpha-TC-1 and beta-TC-1 are 2 terminally differentiated islet tumor cell lines derived from islets transformed by promoter-specific driven SV40 T antigen overexpression. In this study, we demonstrated that Pdx-1 is a potent transcriptional regulator of endogenous insulin gene expression in alpha-TC-1 cells.

METHODS

Reverse transcriptase-polymerase chain reaction and chromatin immunoprecipitation assays were used to analyze gene expression patterns and chromatin modifications in the insulin promoter.

RESULTS

Differential transcription factor expression profiles of alpha-TC-1 and beta-TC-1 cells revealed that INSM-1 and Pdx-1 transcription factors were expressed exclusively in beta-TC-1 cells. Overexpression of Ad-Pdx-1 in alpha-TC-1 cells induced insulin gene expression. Chromatin immunoprecipitation assays in Ad-Pdx-1 alpha-TC-1 cells demonstrated Pdx-1 occupancy and the hyperacetylation of histone H4 in the insulin promoter region.

CONCLUSIONS

Collectively, these experiments revealed that Pdx-1 is a potent transcriptional regulator that is capable of modulating histone H4 acetylation and activates the insulin gene in a terminally differentiated glucagonoma cell line.

Somatostatin and its receptors in the development of the endocrine pancreas

The development of the endocrine pancreas is regulated by numerous transcription and growth factors. Somatostatin (SST) is present in many tissues and acts as a neurotransmitter and autocrine/paracrine/endocrine regulator in response to ions, nutrients, peptides, and hormones as well as neurotransmitters. In the pancreas, there is evidence that SST acts an inhibitory paracrine regulator of hormone secretion. Somatostatin receptors (SSTRs) are a family of 5 transmembrane G protein-coupled receptors, which are widely expressed in mammals including humans. SSTRs regulate multiple downstream signal transduction pathways that mediate inhibitory effects. These receptors also exhibit age- and tissue-specific expression patterns. Interactions of SST and SSTRs are not only important during normal pancreas development, but have also been implicated in many pancreatic diseases such as diabetes mellitus and pancreatic cancer. In this review article, we use evidence from recently published animal studies to present the critical roles of SST and SSTRs proteins in the development of the endocrine pancreas.

Relative contribution of PDX-1, MafA and E47/beta2 to the regulation of the human insulin promoter

The insulin promoter binds a number of tissue-specific and ubiquitous transcription factors. Of these, the homoeodomain protein PDX-1 (pancreatic duodenal homeobox factor-1), the basic leucine zipper protein MafA and the basic helix-loop-helix heterodimer E47/BETA2 (beta-cell E box transactivator 2; referred to here as beta2) bind to important regulatory sites. Previous studies have shown that PDX-1 can interact synergistically with E47 and beta2 to activate the rat insulin 1 promoter. The aim of the present study was to determine the relative contribution of PDX-1, MafA and E47/beta2 in regulating the human insulin promoter, and whether these factors could interact synergistically in the context of the human promoter. Mutagenesis of the PDX-1, MafA and E47/beta2 binding sites reduced promoter activity by 60, 74 and 94% respectively, in INS-1 beta-cells. In the islet glucagonoma cell line alphaTC1.6, overexpression of PDX-1 and MafA separately increased promoter activity approx. 2.5-3-fold, and in combination approx. 6-fold, indicating that their overall effect was additive. Overexpression of E47 and beta2 had no effect. In HeLa cells, PDX-1 stimulated the basal promoter by approx. 40-fold, whereas MafA, E47 and beta2 each increased activity by less than 2-fold. There was no indication of any synergistic effects on the human insulin promoter. On the other hand, the rat insulin 1 promoter and a mutated version of the human insulin promoter, in which the relevant regulatory elements were separated by the same distances as in the rat insulin 1 promoter, did exhibit synergy. PDX-1 was shown further to activate the endogenous insulin 1 gene in alphaTC1.6 cells, whereas MafA activated the insulin 2 gene. In combination, PDX-1 and MafA activated both insulin genes. Chromatin immunoprecipitation assays confirmed that PDX-1 increased the association of acetylated histones H3 and H4 with the insulin 1 gene and MafA increased the association of acetylated histone H3 with the insulin 2 gene.

Minireview: transcriptional regulation in pancreatic development

Considerable progress has been made in the understanding of the sequential activation of signal transduction pathways and the expression of transcription factors during pancreas development. Much of this understanding has been obtained by analyses of the phenotypes of mice in which the expression of key genes has been disrupted (knockout mice). Knockout of the genes for Pdx1, Hlxb9, Isl1, or Hex results in an arrest of pancreas development at a very early stage (embryonic d 8-9). Disruption of genes encoding components of the Notch signaling pathway, e.g. Hes1 or neurogenin-3, abrogates development of the endocrine pancreas (islets of Langerhans). Disruption of transcription factor genes expressed more downstream in the developmental cascade (Beta2/NeuroD, Pax4, NKx2.2, and Nkx6.1) curtails the formation of insulin-producing beta-cells. An understanding of the importance of transcription factor genes during pancreas development has provided insights into the pathogenesis of diabetes, in which the mass of insulin-producing beta-cells is reduced.

The islet beta cell-enriched MafA activator is a key regulator of insulin gene transcription

The islet-enriched MafA, PDX-1, and BETA2 activators contribute to both beta cell-specific and glucose-responsive insulin gene transcription. To investigate how these factors impart activation, their combined impact upon insulin enhancer-driven expression was first examined in non-beta cell line transfection assays. Individual expression of PDX-1 and BETA2 led to little or no activation, whereas MafA alone did so modestly. MafA together with PDX-1 or BETA2 produced synergistic activation, with even higher insulin promoter activity found when all three proteins were present. Stimulation was attenuated upon compromising either MafA transactivation or DNA-binding activity. MafA interacted with endogenous PDX-1 and BETA2 in coimmunoprecipitation and in vitro GST pull-down assays, suggesting that regulation involved direct binding. Dominant-negative acting and small interfering RNAs of MafA also profoundly reduced insulin promoter activity in beta cell lines. In addition, MafA was induced in parallel with insulin mRNA expression in glucose-stimulated rat islets. Insulin mRNA levels were also elevated in rat islets by adenoviral-mediated expression of MafA. Collectively, these results suggest that MafA plays a key role in coordinating and controlling the level of insulin gene expression in islet beta cells.

Pdx-1 is not sufficient for repression of proglucagon gene transcription in islet or enteroendocrine cells

Pdx-1 plays a key role in the development of the pancreas and the control of islet gene transcription and has also been proposed as a dominant regulator of the alpha- vs. beta-cell phenotype via extinction of proglucagon expression. To ascertain the relationship between Pdx-1 and proglucagon gene expression, we examined the effect of enhanced pdx-1 expression on proglucagon gene expression in murine islet alphaTC-1 and GLUTag enteroendocrine cells. Although adenoviral transduction increased the levels of pdx-1 mRNA transcripts and nuclear Pdx-1 protein, overexpression of pdx-1 did not repress endogenous proglucagon gene expression in alphaTC-1 or GLUTag cells or murine islets. Immunohistochemical analysis of cells transduced with Ad-pdx-1 demonstrated multiple individual islet or enteroendocrine cells exhibiting both nuclear Pdx-1 and cytoplasmic glucagon-like peptide-1 immunopositivity. The failure of pdx-1 to inhibit endogenous proglucagon gene expression was not attributable to defects in Pdx-1 nuclear translocation or DNA binding as demonstrated using Western blotting and EMSA analyses. Furthermore, Ad-pdx-1 transduction did not repress proglucagon promoter activity in alphaTC-1 or GLUTag cells. Taken together, these findings demonstrate that pdx-1 alone is not sufficient for specification of the hormonal phenotype or extinction of proglucagon gene expression in islet or enteroendocrine cells.

NeuroD1 in the endocrine pancreas: Localization and dual function as an activator and repressor

The basic helix-loop-helix transcription factor NeuroD1 regulates cell fate in the nervous system but previously has not been considered to function similarly in the endocrine pancreas due to its reported expression in all islet cell types in the newborn mouse. Because we found that NeuroD1 potently represses somatostatin expression in vitro, its pattern of expression was examined in both strains of mice in which lacZ has been introduced into the NeuroD1 locus by homologous recombination. Analysis of adult transgenic mice revealed that NeuroD1 is predominantly expressed in beta-cells and either absent or expressed below the limit of lacZ detection in mature alpha-, delta-, or PP cells. Consistent with a previous report, NeuroD1 colocalizes with glucagon as well as insulin in immature islets of the newborn mouse. However, no colocalization of NeuroD1with somatostatin was detected in the newborn. In vitro, ectopic expression of NeuroD1 in TRM-6/PDX-1, a human pancreatic delta-cell line, resulted in potent repression of somatostatin concomitant with induction of the beta-cell hormones insulin and islet amyloid polypeptide. Additionally, NeuroD1 induced expression of Nkx2.2, a transcription factor expressed in beta- but not delta-cells. Transfection studies using insulin and somatostatin promoters confirm the ability of NeuroD1 to act as both a transcriptional repressor and activator in the same cell, suggesting a more complex role for NeuroD1 in the establishment and/or maintenance of mature endocrine cells than has been recognized previously.

Effects of dietary fatty acids on insulin sensitivity and secretion

Globalization and global market have contributed to increased consumption of high-fat, energy-dense diets, particularly rich in saturated fatty acids( SFAs). Polyunsaturated fatty acids (PUFAs) regulate fuel partitioning within the cells by inducing their own oxidation through the reduction of lipogenic gene expression and the enhancement of the expression of those genes controlling lipid oxidation and thermogenesis. Moreover, PUFAs prevent insulin resistance by increasing membrane fluidity and GLUT4 transport. In contrast, SFAs are stored in non-adipocyte cells as triglycerides (TG) leading to cellular damage as a sequence of their lipotoxicity. Triglyceride accumulation in skeletal muscle cells (IMTG) derives from increased FA uptake coupled with deficient FA oxidation. High levels of circulating FAs enhance the expression of FA translocase the FA transport proteins within the myocites. The biochemical mechanisms responsible for lower fatty acid oxidation involve reduced carnitine palmitoyl transferase (CPT) activity, as a likely consequence of increased intracellular concentrations of malonyl-CoA; reduced glycogen synthase activity; and impairment of insulin signalling and glucose transport. The depletion of IMTG depots is strictly associated with an improvement of insulin sensitivity, via a reduced acetyl-CoA carboxylase (ACC) mRNA expression and an increased GLUT4 expression and pyruvate dehydrogenase (PDH) activity. In pancreatic islets, TG accumulation causes impairment of insulin secretion. In rat models, beta-cell dysfunction is related to increased triacylglycerol content in islets, increased production of nitric oxide, ceramide synthesis and beta-cell apoptosis. The decreased insulin gene promoter activity and binding of the pancreas-duodenum homeobox-1 (PDX-1) transcription factor to the insulin gene seem to mediate TG effect in islets. In humans, acute and prolonged effects of FAs on glucose-stimulated insulin secretion have been widely investigated as well as the effect of high-fat diets on insulin sensitivity and secretion and on the development of type 2 diabetes.

Transcriptional regulation of the lactase-phlorizin hydrolase promoter by PDX-1

Lactase-phlorizin hydrolase gene expression is spatially restricted along the anterior-posterior gut axis. Lactase gene transcription is maximal in the distal duodenum and jejunum in adult mammals and is barely detectable in the proximal duodenum. By contrast, pancreatic duodenal homeobox-1 (PDX-1) protein is expressed maximally in the proximal duodenum. This study aimed to determine the role of PDX-1 in regulating lactase gene promoter activity in intestinal epithelial cells. Caco-2 cells were cotransfected with lactase promoter-reporter constructs in the presence of a PDX-1 expression vector and assayed for luciferase activity. PDX-1 cotransfection results in repression of lactase promoter activity. Sequence analysis of the lactase promoter revealed a putative PDX-1 DNA binding site in the proximal 100-bp lactase gene promoter. EMSAs demonstrated that PDX-1 can interact with the lactase promoter binding site but not with a site in which the core PDX-1 binding sequence TAAT is mutated. Site-directed mutagenesis of the PDX-1 core binding site in the lactase promoter-reporter construct suggests that PDX-1 can function independently of DNA binding to its consensus binding site. Stable overexpression of PDX-1 results in repression of the endogenous human lactase gene in differentiated Caco-2 cells. Given the contrasting spatial expression pattern, PDX-1 may function to specify the anterior boundary of lactase expression in the small intestine and is thus a candidate regulator of anterior spatial restriction in the gut.

Oligonucleotide microarray analysis reveals PDX1 as an essential regulator of mitochondrial metabolism in rat islets

Mutations in the transcription factor IPF1/PDX1 have been associated with type 2 diabetes. To elucidate beta-cell dysfunction, PDX1 was suppressed by transduction of rat islets with an adenoviral construct encoding a dominant negative form of PDX1. After 2 days, there was a marked inhibition of insulin secretion in response to glucose, leucine, and arginine. Increasing cAMP levels with forskolin and isobutylmethylxanthine restored glucose-stimulated insulin secretion, indicating normal capacity for exocytosis. To identify molecular targets implicated in the altered metabolism secretion coupling, DNA microarray analysis was performed on PDX1-deficient and control islets. Of the 2640 detected transcripts, 70 were up-regulated and 56 were down-regulated. Transcripts were subdivided into 12 clusters; the most prevalent were associated with metabolism. Quantitative reverse transcriptase-PCR confirmed increases in succinate dehydrogenase and ATP synthase mRNAs as well as pyruvate carboxylase and the transcript for the malate shuttle. In parallel there was a 50% reduction in mRNA levels for the mitochondrially encoded nd1 gene, a subunit of the NADH dehydrogenase comprising complex I of the mitochondrial respiratory chain. As a consequence, total cellular ATP concentration was drastically decreased by 75%, and glucose failed to augment cytosolic ATP, explaining the blunted glucose-stimulated insulin secretion. Rotenone, an inhibitor of complex I, mimicked this effect. Surprisingly, TFAM, a nuclear-encoded transcription factor important for sustaining expression of mitochondrial genes, was down-regulated in islets expressing DN79PDX1. In conclusion, loss of PDX1 function alters expression of mitochondrially encoded genes through regulation of TFAM leading to impaired insulin secretion.

Genome-wide scans reveal quantitative trait Loci on 8p and 13q related to insulin action and glucose metabolism: the San Antonio Family Heart Study

Type 2 diabetes is a complex disease that arises from physiological disruptions of the body's sensitivity to insulin and ability to metabolize glucose. Multipoint linkage analyses for insulin sensitivity phenotypes were conducted in 1,280 Mexican Americans from 41 families who participated in the San Antonio Family Heart Study. A significant linkage signal (logarithm of odds [LOD] = 2.98) affecting corrected insulin response to glucose was detected on chromosome 13q between D13787 and D13S252, in the region where the MODY-4 gene has previously been mapped. Another signal on chromosome 13 was observed at D13S285 (LOD = 1.86), where the insulin receptor substrate 2 gene resides. Significant linkage (LOD = 3.09) for insulin response to glucose was found on chromosome 8 between D8S1130 and D8S1106, near the lipoprotein lipase and macrophage scavenger receptor genes. Multipoint analysis of abdominal skinfold with an LOD of 2.68 showed signals in the same region. There was also suggestive evidence for linkage of quantitative insulin sensitivity check index and fasting glucose to a previously reported location at D9S301 (LOD = 2.19). These results indicate that chromosomal locations on 8p and 13q might harbor genes that affect a variety of insulin- and glucose-related phenotypes that contribute to the observed variations in these important risk factors for diabetes in Mexican Americans.

Expression in murine teratocarcinoma f9 cells of transcription factors involved in pancreas development

BACKGROUND

Although it has been established that formation and functional differentiation of the pancreas from embryonic endoderm is associated with activation/inactivation of many genes controlled by specific sets of transcription factors, the role and activation sequence of individual transcription factors has not yet been fully elucidated. This study sought to differentiate a murine teratocarcinoma cell line, F9, to endodermal-like cells and, subsequently; to investigate the effects of regulated expression of transcription factors in pancreas development.

METHODS

Following differentiation using retinoic acid and db cAMP (RAC), resulting F9 cells (F9-RAC) were transfected with cDNAs for PDX-1, ngn3, beta 2/NeuroD (beta 2), and Nkx2.2, singly or in combination. Expression of these transcription factors was investigated using RT-PCR and immunofluorescence techniques. RT-PCR analysis was used to assess the subsequent effects of expression of these factors on endogenous genes related to pancreas development.

RESULTS

Regulated differentiation of F9 cells generated endodermal-like cell types. Following transfection, PDX-1, ngn3, beta 2, and Nkx2.2 were expressed in F9-RAC cells, with their proteins localized mainly in cellular nuclei. Expression of these factors apparently did not affect the endogenous expression of preproinsulin, PDX-1, beta 2, Isl1, Pax4, Pax6, Sonic hedgehog, and Indian hedgehog.

CONCLUSION

This study describes the successful transient expression of transcription factors related to pancreas development, following directed differentiation of F9 cells to endoderm-like cells, and shows that treatment of F9 cells with a combination of RAC causes up-regulation of genes relevant to pancreatic development. The lack of further effect of regulated transcription factor expression on these genes may suggest that parietal endoderm- like cells derived from F9 cells is not the optimal lineage from which to develop beta cells. It may be useful to include F9-derived visceral endoderm in future studies.

Transcriptional networks controlling pancreatic development and beta cell function

Transcription factors provide the genetic instructions that drive pancreatic development and enable mature beta cells to function properly. To understand fully how this is accomplished, it is necessary to unravel the regulatory networks formed by transcription factors acting on their genomic targets. This article discusses recent advances in our understanding of how transcriptional networks control early pancreas organogenesis, embryonic endocrine cell formation and the differentiated function of adult beta cells. We discuss how mutations in several transcription factor genes involved in such networks cause Maturity onset diabetes of the young (MODY). Finally, we propose that pancreatic gene programs might be manipulated to generate beta cells or to enhance the function of existing beta cells, thereby providing a possible treatment of different forms of diabetes.

PDX1, a Cellular Homeoprotein, Binds to and Regulates the Activity of Human Cytomegalovirus Immediate Early Promoter

Cellular homeoproteins have been shown to regulate the transcription of several viruses, including herpes simplex viruses, human papillomaviruses, and mouse mammary tumor viruses. Previous studies investigating the anti-viral mechanisms of several cyclin-dependent kinase inhibitors showed that the homeoproteins, pre B-cell leukemia transcription factor 1 (PBX1) and PBX-regulating protein-1 (PREP1), function as transcriptional activators of Moloney murine leukemia virus. Here, we examined the involvement of cellular homeoproteins in regulating the activity of the human cytomegalovirus immediate early (CMV IE) promoter. We identified a 45-bp element located at position -593 to -549 upstream of the transcription start site of the CMV IE gene, which contains multiple putative homeoprotein binding motifs. Gel shift assays demonstrated the physical association between a homeodomain protein, pancreatic-duodenal homeobox factor-1 (PDX1) and the 45-bp cytomegalovirus (CMV) region. We further determined that PDX1 represses the CMV IE promoter activity in 293 cells. Overexpression of PDX1 resulted in a decrease in transcription of the CMV IE gene. Conversely, blocking PDX1 protein synthesis and mutating the PDX1 binding sites enhanced CMV IE-dependent transcription. Collectively, our results represent the first work demonstrating that a cellular homeoprotein, PDX1, may be a repressor involved in regulation of human CMV gene expression.