Identification of cis- and trans-active factors regulating human islet amyloid polypeptide gene expression in pancreatic beta-cells

PDX-1: A Promising Therapeutic Target to Reverse Diabetes

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

Transcription factor Ets-1 links glucotoxicity to pancreatic beta cell dysfunction through inhibiting PDX-1 expression in rodent models

AIMS/HYPOTHESIS

'Glucotoxicity' is a term used to convey the negative effect of hyperglycaemia on beta cell function; however, the underlying molecular mechanisms that impair insulin secretion and gene expression are poorly defined. Our objective was to define the role of transcription factor v-ets avian erythroblastosis virus E26 oncogene homologue 1 (Ets-1) in beta cell glucotoxicity.

METHODS

Primary islets and Min6 cells were exposed to high glucose and Ets-1 expression was measured. Recombinant adenovirus and transgenic mice were used to upregulate Ets-1 expression in beta cells in vitro and in vivo, and insulin secretion was assessed. The binding activity of H3/H4 histone on the Ets-1 promoter, and that of forkhead box (FOX)A2, FOXO1 and Ets-1 on the Pdx-1 promoter was measured by chromatin immunoprecipitation and quantitative real-time PCR assay.

RESULTS

High glucose induced upregulation of Ets-1 expression and hyperacetylation of histone H3 and H4 at the Ets-1 gene promoter in beta cells. Ets-1 overexpression dramatically suppressed insulin secretion and biosynthesis both in vivo and in vitro. Besides, Ets-1 overexpression increased the activity of FOXO1 but decreased that of FOXA2 binding to the pancreatic and duodenal homeobox 1 (PDX-1) homology region 2 (PH2), resulting in inhibition of Pdx-1 promoter activity and downregulation of PDX-1 expression and activity. In addition, high glucose promoted the interaction of Ets-1 and FOXO1, and the activity of Ets-1 binding to the Pdx-1 promoter. Importantly, PDX-1 overexpression reversed the defect in pancreatic beta cells induced by Ets-1 excess, while knockdown of Ets-1 prevented hyperglycaemia-induced dysfunction of pancreatic beta cells.

CONCLUSIONS/INTERPRETATION

Our observations suggest that Ets-1 links glucotoxicity to pancreatic beta cell dysfunction through inhibiting PDX-1 expression in type 2 diabetes.

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.

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.

Review of the neuroanatomic landscape implicated in glucose sensing and regulation of nutrient signaling: immunophenotypic localization of diabetes gene Tcf7l2 in the developing murine brain

Genetic variants in the transcription factor 7-like 2(Tcf7l2) gene have been found to confer a significant risk of type 2 diabetes and attenuated insulin secretion. Based on its genomic wide association Tcf7l2 is considered the single most important predictor of diabetes to date. Previous studies of Tcf7l2 mRNA localization in the adult brain suggest a putative role of Tcf7l2 in the CNS regulation of energy homeostasis. The present study further characterizes the immunophenotypic distribution of peptide expression in the brains of Tcf7l2 progeny during developmental time periods between E12.5 and P1. Tcf7l2(-/-) is lethal beyond P1. Results show that while negligible TCF7L2 expression is found in the developing brains of Tcf7l2(-/-)mice, TCF7L2 protein is relatively widespread and robustly expressed in the brain by E18.5 and exhibits specific expression within neuronal populations and regions of the brain in Tcf7l2(+/-) and Tcf7l2(+/+) progeny. Strong immunophenotypic labeling was found in the diencephalic structure of the thalamus that suggests a role of Tcf7l2 in the development and maintenance of thalamic activity. Strongly expressed TCF7L2 was localized in select hypothalamic and preoptic nuclei indicative of Tcf7l2 function within neurons controlling energy balance. Definitive neuronal staining for TCF7L2 within nuclei of the brain stem and circumventricular organs extends TCF7L2 localization within autonomic neurons and its potential integration with autonomic function. In addition robust TCF7L2 expression was found in the tectal and tegmental structures of the superior and inferior colliculi as well as transient expression in neuroepithelium of the cerebral and hippocampal cortices of E16 and E18.5. Patterns of TCF7L2 peptide localization when compared to the adult protein synthetic chemical/anatomical landscape of glucose sensing exhibit a good correlational fit between its expression and regions, nuclei, and pathways regulating energy homeostasis via integration and response to peripheral endocrine, metabolic and neuronal signaling. TCF was also found co-localized with peptides that regulate energy homeostasis including AgRP, POMC and NPY. TCF7l2, some variants of which have been shown to impair GLP-1-induced insulin secretion, was also found co-localize with GLP-1 in adult TCF wild type progeny. Impaired Tcf7l2-mediated neural regulation may contribute to the risk and/or underlying pathophysiology of type 2 diabetes that has found high expression in genomic studies of Tcf7l2 variants.

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.

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.

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.

TNF-α acutely upregulates amylin expression in murine pancreatic beta cells

AIMS/HYPOTHESIS

Amylin, a secretory protein mainly produced by pancreatic beta cells, is elevated in the circulation of patients with diseases related to acute and chronic inflammation, including acute pancreatitis, pancreas graft rejection, obesity and insulin resistance. TNF-α is involved in these disorders. We investigated the effect of TNF-α on amylin levels and the underlying mechanisms, using murine pancreatic beta cell line MIN6 and pancreatic islets.

METHODS

Amylin, proinsulin and prohormone convertase 1/3, 2 (Pc1/3, Pc2 [also known as Pcsk1/3 and Pcsk2, respectively]) mRNA levels, and amylin promoter and nuclear factor κB (NF-κB) activation were examined by real-time PCR and luciferase reporter assay, respectively. Amylin protein level and mitogen-activated protein kinase phosphorylation were detected by western blot. Activator protein 1 (AP1) activation was examined by electrophoretic mobility shift assay (EMSA).

RESULTS

TNF-α acutely induced amylin expression at the transcriptional level and increased proamylin and the intermediate form of amylin in MIN6 cells and islets. However, it had no effect on proinsulin, Pc1/3 and Pc2 expression. Studies with (1) MIN6 cells treated with inhibitors of MEK1/2, c-Jun-N-terminal kinase (JNK) or protein kinase Cζ (PKC(ζ)), (2) MIN6 cells expressing a c-Jun-dominant negative construct and (3) islets from Fos knockout mice demonstrated that TNF-α induced amylin expression through the PKC(ζ)-extracellular signal-regulated kinase (ERK)/JNK pathways. EMSA showed that (PKC(ζ)), JNK and ERK1/2 were involved in TNF-α-induced AP1 activation, suggesting that TNF-α induces murine amylin expression through the (PKC(ζ)) - ERK1/2 - AP and PKC(ζ) - JNK - AP1 pathways. Further studies showed that TNF-α also induced murine amylin expression through the phosphatidylinositol 3 kinase-NF-κB signalling pathway and enhanced human amylin promoter activation through NF-κB and AP1.

CONCLUSIONS/INTERPRETATION

TNF-α acutely induces amylin gene expression in beta cells through multiple signalling pathways, possibly contributing to amylin elevation in acute inflammation-related pancreatic disorders.

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.

The homeodomain-interacting protein kinase 2 regulates insulin promoter factor-1/pancreatic duodenal homeobox-1 transcriptional activity

The homeodomain transcription factor insulin promoter factor (IPF)-1/pancreatic duodenal homeobox (PDX)-1 plays a crucial role in both pancreas development and maintenance of beta-cell function. Targeted disruption of the Ipf1/Pdx1 gene in beta-cells of mice leads to overt diabetes and reduced Ipf1/Pdx1 gene expression results in decreased insulin expression and secretion. In humans, mutations in the IPF1 gene have been linked to diabetes. Hence, the identification of molecular mechanisms regulating the transcriptional activity of this key transcription factor is of great interest. Herein we analyzed homeodomain-interacting protein kinase (Hipk) 2 expression in the embryonic and adult pancreas by in situ hybridization and RT-PCR. Moreover, we functionally characterized the role of HIPK2 in regulating IPF1/PDX1 transcriptional activity by performing transient transfection experiments and RNA interference. We show that Hipk2 is expressed in the developing pancreatic epithelium from embryonic d 12-15 but that the expression becomes preferentially confined to pancreatic endocrine cells at later developmental stages. Moreover, we show that HIPK2 positively influences IPF1/PDX1 transcriptional activity and that the kinase activity of HIPK2 is required for this effect. We also demonstrate that HIPK2 directly phosphorylates the C-terminal portion of IPF1/PDX1. Taken together, our data provide evidence for a new mechanism by which IPF1/PDX1 transcriptional activity, and thus possibly pancreas development and/or beta-cell function, is regulated.

Pancreatic transcription factors and their role in the birth, life and survival of the pancreatic beta cell

In recent years major progress has been made in understanding the role of transcription factors in the development of the endocrine pancreas in the mouse. Here we describe how a number of these transcription factors play a role in maintaining the differentiated phenotype of the beta cell, and in the mechanisms that allow the beta cell to adapt to changing metabolic demands that occur throughout life. Amongst these factors, Pdx1 plays a critical role in defining the region of the primitive gut that will form the pancreas, Ngn3 expression drives cells towards an endocrine lineage, and a number of additional proteins including Pdx1, in a second wave of expression, Pax4, NeuroD1/beta2, and MafA act as beta cell differentiation factors. In the mature beta cell Pdx1, MafA, beta2, and Nkx2.2 play important roles in regulating expression of insulin and to some extent other genes responsible for maintaining beta cell function. We emphasise here that data from gene expression studies in rodents seldom map on to the known structure of the corresponding human promoters. In the adult the beta cell is particularly susceptible to autoimmune-mediated attack and to the toxic metabolic milieu associated with over-eating, and utilises a number of these transcription factors in its defence. Pdx1 has anti-apoptotic and proliferative activities that help facilitate the maintenance of beta cell mass, while Ngn3 may be involved in the recruitment of progenitor cells, and Pax4 (and possibly HNF1alpha and Hnf4alpha) in the proliferation of beta cells in the adult pancreas. Other transcription factors with a more widespread pattern of expression that play a role in beta cell survival or proliferation include Foxo1, CREB family members, NFAT, FoxM1, Snail and Asc-2.

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.

Islet cell differentiation in liver by combinatorial expression of transcription factors neurogenin-3, BETA2, and RIPE3b1

Transcription factors, such as PDX-1, that normally mediate pancreatic development are capable of inducing hepatic progenitor cells to differentiate into cells with pancreatic islet characteristics. We hypothesized that simultaneous expression of multiple transcription factors involved in islet development might enhance the differentiation of hepatic progenitor cells. Bi- or tri-cistronic constructs were generated in hybrid adenovirus/adeno-associated virus (Ad/AAV) vectors containing neurogenin 3 (NGN3), BETA2 (NeuroD), and RIPE3b1 (MafA), each of which plays a role in islet cell differentiation. These vectors efficiently express multiple transcription factors and stimulate insulin promoter activity in a combinatorial manner. When these multi-cistronic constructs were administered in vivo, they induce hepatic expression of islet-specific markers, including PDX-1, insulin, glucagon, somatostatin, and islet-amyloid peptide. Administration of the Ad/AAV hybrid vectors to streptozotocin-induced diabetic mice reversed hyperglycemia, consistent the differentiation of functional hepatic insulin-secreting cells. These results indicate that Ad/AAV hybrid vectors can be used to administer combinations of factors that induce islet cell differentiation in hepatic progenitor cells.

Pancreas and beta-cell development: from the actual to the possible

The development of insulin-producing pancreatic beta (beta)-cells represents the culmination of a complex developmental program. Cells of the posterior foregut assume a pancreatic identity, cells within the expanding pancreatic primordia adopt an endocrine fate, and a subset of these precursors becomes competent to generate beta-cells. Postnatally, beta-cells are primarily maintained by self-duplication rather than new differentiation. Although major gaps in our knowledge still persist, experiments across several organisms have shed increasing light on the steps of beta-cell specification and differentiation. Increasing our understanding of the extrinsic, as well as intrinsic, mechanisms that control these processes should facilitate efforts to regenerate this important cell type in humans.

Glucose-dependent expansion of pancreatic beta-cells by the protein p8 in vitro and in vivo

p8 protein expression is known to be upregulated in the exocrine pancreas during acute pancreatitis. Own previous work revealed glucose-dependent p8 expression also in endocrine pancreatic beta-cells. Here we demonstrate that glucose-induced INS-1 beta-cell expansion is preceded by p8 protein expression. Moreover, isopropylthiogalactoside (IPTG)-induced p8 overexpression in INS-1 beta-cells (p8-INS-1) enhances cell proliferation and expansion in the presence of glucose only. Although beta-cell-related gene expression (PDX-1, proinsulin I, GLUT2, glucokinase, amylin) and function (insulin content and secretion) are slightly reduced during p8 overexpression, removal of IPTG reverses beta-cell function within 24 h to normal levels. In addition, insulin secretion of p8-INS-1 beta-cells in response to 0-25 mM glucose is not altered by preceding p8-induced beta-cell expansion. Adenovirally transduced p8 overexpression in primary human pancreatic islets increases proliferation, expansion, and cumulative insulin secretion in vitro. Transplantation of mock-transduced control islets under the kidney capsule of immunosuppressed streptozotocin-diabetic mice reduces blood glucose and increases human C-peptide serum concentrations to stable levels after 3 days. In contrast, transplantation of equal numbers of p8-transduced islets results in a continuous decrease of blood glucose and increase of human C-peptide beyond 3 days, indicating p8-induced expansion of transplanted human beta-cells in vivo. This is underlined by a doubling of insulin content in kidneys containing p8-transduced islet grafts explanted on day 9. These results establish p8 as a novel molecular mediator of glucose-induced pancreatic beta-cell expansion in vitro and in vivo and support the notion of existing beta-cell replication in the adult organism.

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.

Phosphorylation marks IPF1/PDX1 protein for degradation by glycogen synthase kinase 3-dependent mechanisms

The transcription factor IPF1/PDX1 plays a crucial role in both pancreas development and maintenance of beta-cell function. Targeted disruption of this transcription factor in beta-cells leads to diabetes, whereas reduced expression levels affect insulin expression and secretion. Therefore, it is essential to determine molecular mechanisms underlying the regulation of this key transcription factor on mRNA levels and, most importantly, on protein levels. Here we show that a minor portion of IPF1/PDX1 is phosphorylated on serine 61 and/or serine 66 in pancreatic beta-cells. This phosphorylated form of IPF1/PDX1 preferentially accumulates following proteasome inhibition, an effect that is prevented by inhibition of glycogen synthase kinase 3 (GSK3) activity. Oxidative stress, which is associated with the diabetic state, (i) increases IPF1/PDX1 Ser61 and/or Ser66 phosphorylation and (ii) increases the degradation rate and decreases the half-life of IPF-1/PDX-1 protein. In addition, we provide evidence that GSK3 activity participates in oxidative stress-induced effects on beta-cells. Thus, this current study uncovers a new mechanism that might contribute to diminished levels of IPF1/PDX1 protein and beta-cell dysfunction during the progression of diabetes.

Heterogeneity in predisposition of hepatic cells to be induced into pancreatic endocrine cells by PDX-1

AIM: The role of Pancreatic and Duodenal Homeobox-1 (PDX-1) as a major regulator of pancreatic development determines the function and phenotype of β cell. In this study, potential plasticity of liver cells into pancreatic endocrine cells induced by PDX-1 was evaluated.

METHODS: Human hepatoma cell line HepG₂ was stably transfected with mammalian expression plasmid pcDNA3-PDX encoding human PDX-1 gene. Ectopic expression of PDX-1 and insulin were detected by RT-PCR, Western blot and/or immunostaining. PDX-1⁺ HepG₂ cells were transplanted under renal capsule of STZ-induced diabetic nude mice (n = 16) to examine the inducing effect in vivo.

RESULTS: Exogenous PDX-1 transgene was proved to express effectively in HepG₂ cell at both mRNA and protein levels. The expression of endogenous insulin and some β cell-specific differentiation markers and transcription factors were not induced in PDX-1⁺ HepG₂ cells. When transplanted under renal capsule of STZ-induced diabetic nude mice, PDX-1⁺ HepG₂ cells did not generate insulin-producing cells. These data indicated that stable transfected PDX-1 could not convert hepatoma cell line HepG₂ to pancreatic cells in vitro or in vivo. Mature hepatocytes might need much more complicated or rigorous conditions to be shifted to insulin-producing cells.

CONCLUSION: The expression of exogenous PDX-1 is not sufficient to induce relatively mature hepatocytes differentiating into insulin-producing cells.

Reduced PDX-1 expression impairs islet response to insulin resistance and worsens glucose homeostasis

In type 2 diabetes mellitus, insulin resistance and an inadequate pancreatic beta-cell response to the demands of insulin resistance lead to impaired insulin secretion and hyperglycemia. Pancreatic duodenal homeodomain-1 (PDX-1), a transcription factor required for normal pancreatic development, also plays a key role in normal insulin secretion by islets. To investigate the role of PDX-1 in islet compensation for insulin resistance, we examined glucose disposal, insulin secretion, and islet cell mass in mice of four different genotypes: wild-type mice, mice with one PDX-1 allele inactivated (PDX-1+/-, resulting in impaired insulin secretion), mice with one GLUT4 allele inactivated (GLUT4+/-, resulting in insulin resistance), and mice heterozygous for both PDX-1 and GLUT4 (GLUT4+/-;PDX-1+/-). The combination of PDX-1 and GLUT4 heterozygosity markedly prolonged glucose clearance. GLUT4+/-;PDX-1+/- mice developed beta-cell hyperplasia but failed to increase their beta-cell insulin content. These results indicate that PDX-1 heterozygosity (approximately 60% of normal protein levels) abrogates the beta-cell's compensatory response to insulin resistance, impairs glucose homeostasis, and may contribute to the pathogenesis of type 2 diabetes.

Islet amyloid polypeptide gene promoter polymorphisms are not associated with Type 2 diabetes or with the severity of islet amyloidosis

The over-expression of the islet amyloid polypeptide (IAPP) gene could be a causal factor for islet amyloidosis and beta-cell destruction in Type 2 diabetes (T2DM). An IAPP gene promoter polymorphism, IAPP-132G to A, has been associated with T2DM in Spain. To investigate this polymorphism in other cohorts and in relation to therapy, DNA from 425 T2DM and 279 unrelated, non-diabetic UK subjects (ND) and 102 T2DM and 80 ND Finnish subjects was examined. The relationship of amyloid severity (percent amyloid/islet) to prevalence (number of islets affected) and the association of IAPP-132G/A with amyloid was determined in post-mortem pancreas from 38 T2DM subjects. The -132G/A was not associated with T2DM in the UK cohorts (4.5% T2DM; 3.2% ND) or associated with requirement for insulin therapy by 6 years. The mutation was and undetected in the Finnish samples but a new variant, -166T/C, was identified in 2 Finnish T2DM subjects. -132G/A was found in 2/38 diabetic, amyloid-containing and 3/19 ND, amyloid-free subjects. The islet amyloid severity was linearly correlated with the prevalence in T2DM. The IAPP-132G/A promoter polymorphism is not associated with T2DM, a requirement for insulin therapy or with the degree of islet amyloidosis in cohorts from the UK or Finland.

Transcriptional regulation of the IAPP gene in pancreatic beta-cells

Islet amyloid polypeptide (IAPP or amylin) is co-secreted with insulin from the pancreatic beta-cells. Transcription of the IAPP gene is controlled by a complex promoter region, spanning from -2798 to +450 relative to the transcriptional start site. In the present study, we have used reporter gene analysis and semi-quantitative RT-PCR to establish that insulin, glucagon, glucagon-like peptide-1 (GLP-1) and the GLP-1 derivatives GLP(7-36)Amide and Exendin-4 all stimulate IAPP promoter activity, as well as endogenous IAPP mRNA levels in isolated islets of Langerhans. In contrast, somatostatin had no effect, and whilst the inflammatory cytokines TNF-alpha, IL-1alpha and IL-1beta had no effect on promoter activity, they all decreased IAPP mRNA levels in isolated islets. Finally, utilising a series of deletion reporter gene constructs of the human IAPP gene promoter, we used overexpression studies to establish that HNF-3beta (FoxA2) negatively regulates the IAPP promoter, whilst the MODY3 transcription factor HNF-1alpha positively regulates promoter activity.

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.

Mutation at position -132 in the islet amyloid polypeptide ( IAPP) gene promoter enhances basal transcriptional activity through a new CRE-like binding site

AIMS/HYPOTHESIS

Mutations in the islet amyloid polypeptide ( IAPP) gene may play a potential role in the abnormal regulation or expression of the peptide. The aim of this study was to determine the functional role of the -132 G/A mutation reported in the promoter region of the IAPP gene in a population of Spanish Type 2 diabetic patients.

METHODS

We investigated the transcriptional activity using MIN6 cells and luciferase reporter plasmids in several culture conditions. Key regulatory elements of the IAPP promoter region were also analysed by electrophoretic mobility shift assays (EMSA).

RESULTS

The mutant construct doubled IAPP transcriptional activity ( p<0.001). Both constructs showed severely reduced promoter activity (four-fold decrease) in the presence of verapamil and diazoxide. In contrast, IAPP promoter activity was doubled after incubation with forskolin or dexamethasone, regardless of the glucose concentrations in the culture media. EMSA revealed that the -132 G/A mutation increased the binding affinity through two DNA-protein complexes. In addition, a cAMP-responsive element binding protein (CREB) was identified by super-shift EMSA.

CONCLUSIONS/INTERPRETATION

Our studies show that the wild-type and the mutant constructs are regulated in a similar pattern under all conditions, strongly indicating that the -132 G/A mutation increases basal but not inducible transcription. These results may be explained by new binding to the mutant region through CREB and other transcription factors not yet identified.

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.

Transcription Factors in Islet Development and Physiology: Role of PDX-1 in Beta-Cell Function

Differentiation of early foregut endoderm into pancreatic endocrine and exocrine cells depends on a cascade of gene activation events controlled by various transcription factors. The first molecular marker identified that specifies the early pancreatic epithelium is the homeodomain-containing transcription factor PDX-1. Its absence in mice and humans during development leads to agenesis of the pancreas. Later, it becomes restricted primarily to beta cells where it regulates the expression of beta cell-specific genes, and, most importantly, mediates the glucose effect on insulin gene transcription. Although exposure of beta cells to high glucose concentrations for relatively short periods stimulates insulin gene expression, chronic exposure has adverse effects on many beta-cell functions, including insulin gene transcription. These events appear to correlate with pdx-1 gene expression and its ability to bind the insulin gene. We consider that loss of PDX-1 function or altered pdx-1 gene expression due to mutations or functional impairment of transcription factors controlling its expression can lead to 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.

Identification of a novel PDX-1 binding site in the human insulin gene enhancer

Islet beta cell type-specific transcription of the insulin gene is regulated by a number of cis-acting elements found within the proximal 5'-flanking region. The control sequences conserved between mammalian insulin genes are acted upon by transcription factors, like PDX-1 and BETA-2, that are also involved in islet beta cell function and formation. In the current study, we investigated the contribution to human insulin expression of the GG2 motif found between nucleotides -145 and -140 relative to the transcription start site. Site-specific mutants were generated within GG2 that displayed a parallel increase (i.e. -144 base pair) or decrease (i.e. -141 base pair) in insulin enhancer-driven reporter and gel shift binding activity in beta cells consistent with human GG2 being under positive regulatory control. In contrast, the corresponding site in the rodent insulin gene, which only differs from the human at nucleotides -144 and -141, is negatively regulated by the Nkx2.2 transcription factor (Cissell, M. A., Zhao, L., Sussel, L., Henderson, E., and Stein, R. (2003) J. Biol. Chem. 278, 751-756). Human GG2 activator binding activity was present in nuclear extracts prepared from human islets and enriched in those from rodent beta cell lines. The human GG2 activator binding factor(s) was shown to be approximately 38-40 kDa and distinct from other size-matched islet-enriched transcription factors, including Nkx2.2, Pax-4, Cdx2/3, and Isl-1. Combined DNA chromatographic purification and mass spectrometry analysis revealed that the GG2 activator was PDX-1. These results demonstrate that the GG2 element, despite its divergence from the core homeodomain consensus binding motif, is a site for PDX-1 activation in the human insulin gene.

PDX-1 and the pancreas

Many transcription factors are critical for ensuring proper embryonic development of the endocrine pancreas and normal islet function. The transcription factor pancreatic duodenal homeobox 1 (PDX-1) is uniformly expressed in early pancreatic buds of embryos as well as the beta and delta cells of the islets of Langerhans. PDX-1 has also been found in dispersed endocrine cells of the duodenum in adults and plays a key role in pancreas formation. It has been reported that null mutation of PDX-1 in mice results in a failure of the pancreatic bud to expand; thus, the mice die 2-3 days after birth from hyperglycemia and dehydration. Heterozygous PDX-1 mice developed a pancreas but were diabetic. It has been shown that PDX-1 is required for maintaining the pancreatic islet functions by activating gene transcriptions including insulin, somatostatin (SST), islet amyloid polypeptide, glucose transporter type 2, and glucokinase. PDX-1 serves a dual role in pancreatic development. It initially contributes to pancreatic formation during embryogenesis and subsequently regulates the pancreatic islet cell physiology in mature islet cells. Understanding the underlying molecular mechanisms of pancreas formation, especially the function of PDX-1, may contribute to the enhanced treatment and prevention of debilitating diseases such as diabetes, insulinomas, and pancreatic carcinomas.

Role of intrinsic DNA binding specificity in defining target genes of the mammalian transcription factor PDX1

PDX1 is a homeodomain transcription factor essential for pancreatic development and mature beta cell function. Homeodomain proteins typically recognize short TAAT DNA motifs in vitro: this binding displays paradoxically low specificity and affinity, given the extremely high specificity of action of these proteins in vivo. To better understand how PDX1 selects target genes in vivo, we have examined the interaction of PDX1 with natural and artificial binding sites. Comparison of PDX1 binding sites in several target promoters revealed an evolutionarily conserved pattern of nucleotides flanking the TAAT core. Using competitive in vitro DNA binding assays, we defined three groups of binding sites displaying high, intermediate and low affinity. Transfection experiments revealed a striking correlation between the ability of each sequence to activate transcription in cultured beta cells, and its ability to bind PDX1 in vitro. Site selection from a pool of oligonucleotides (sequence NNNTAATNNN) revealed a non-random preference for particular nucleotides at the flanking locations, resembling natural PDX1 binding sites. Taken together, the data indicate that the intrinsic DNA binding specificity of PDX1, in particular the bases adjacent to TAAT, plays an important role in determining the spectrum of target genes.

Hyperinsulinism in Infancy: From Basic Science to Clinical Disease

Dunne, Mark J., Karen E. Cosgrove, Ruth M. Shepherd, Albert Aynsley-Green, and Keith J. Lindley. Hyperinsulinism in Infancy: From Basic Science to Clinical Disease. Physiol Rev 84: 239–275, 2004; 10.1152/physrev.00022.2003.—Ion channelopathies have now been described in many well-characterized cell types including neurons, myocytes, epithelial cells, and endocrine cells. However, in only a few cases has the relationship between altered ion channel function, cell biology, and clinical disease been defined. Hyperinsulinism in infancy (HI) is a rare, potentially lethal condition of the newborn and early childhood. The causes of HI are varied and numerous, but in almost all cases they share a common target protein, the ATP-sensitive K+channel. From gene defects in ion channel subunits to defects in β-cell metabolism and anaplerosis, this review describes the relationship between pathogenesis and clinical medicine. Until recently, HI was generally considered an orphan disease, but as parallel defects in ion channels, enzymes, and metabolic pathways also give rise to diabetes and impaired insulin release, the HI paradigm has wider implications for more common disorders of the endocrine pancreas and the molecular physiology of ion transport.

Conserved transcriptional regulatory domains of the pdx-1 gene

The pancreas and duodenum homeobox protein 1 (PDX-1) homeodomain-containing transcription factor affects both pancreatic endocrine cell development and adult islet beta-cell function. Cell-type-specific expression is controlled by sequences 5' flanking the pdx-1 gene transcription start site. One principal control region is located roughly between -2800 and -1600 bp and spans three conserved, distinct, and functionally important subdomains, termed areas I, II, and III. In this study, we found that an upstream control region in the rat pdx-1 gene located between -6200 and -5670 bp is also present in the mouse, chicken, and human genes. This region is capable of independently directing pancreatic beta-cell-selective reporter gene expression and potentiating area I/II-driven activity. This newly recognized conserved subdomain has been termed area IV. The islet-enriched forkhead box A2 (FoxA2), NK2 homeobox 2.2 (Nkx2.2), and pancreas and duodenum homeobox protein 1 (PDX-1) transcription factors have been shown to activate area IV-driven reporter gene expression as well as bind to this region of the endogenous gene in beta-cells. Analysis of the histone H3 and H4 acetylation level also indicated that areas I-IV are within transcriptionally active chromatin in beta-cells. Our data suggests that pdx-1 transcription is also regulated by factors acting upon conserved area IV sequences.

Ectopically expressed PDX-1 in liver initiates endocrine and exocrine pancreas differentiation but causes dysmorphogenesis

To date, the potency of pancreatic and duodenal homeobox gene 1 (PDX-1) in inducing differentiation into insulin-producing cells has been demonstrated in some cells and tissues. In order to carry out efficient screening of somatic tissues and cells that can transdifferentiate into beta-cell-like cells in response to PDX-1, we generated CAG-CAT-PDX1 transgenic mice carrying a transgene cassette composed of the chicken beta-actin gene (CAG) promoter and a floxed stuffer DNA sequence (CAT) linked to PDX-1 cDNA. When the mice were crossed with Alb-Cre mice, which express the Cre recombinase driven by the rat albumin gene promoter, PDX-1 was expressed in more than 50% of hepatocytes and cholangiocytes. The PDX-1 (+) livers expressed a variety of endocrine hormone genes such as insulin, glucagon, somatostatin, and pancreatic polypeptide. In addition, they expressed exocrine genes such as elastase-1 and chymotrypsinogen 1B. However, the mice exhibited marked jaundice due to conjugated hyperbilirubinemia, and the liver tissue displayed abnormal lobe structures and multiple cystic lesions. Thus, the in vivo ectopic expression of PDX-1 in albumin-producing cells was able to initiate but not complete the differentiation of liver cells into pancreatic cells. The conditional PDX-1 transgenic mouse system developed in this study appeared to be useful for efficient screening of PDX-1 responsive somatic tissues and cells.

Amylin gene expression mediated by cAMP/PKA and transcription factors HNF-1 and NFY

The regulation of amylin gene expression is not clearly understood. In this study, we used the pancreatic beta cell line, INS-1, to evaluate the regulation of amylin gene expression. When INS-1 cells were cultured in media containing IBMX, the ratio of amylin secretion to insulin secretion increased by 200%. This coincided with an increase in amylin mRNA content. Moreover, there was a three to four-fold increase in amylin promoter activity which was inhibited by the specific protein kinase A (PKA) inhibitor, H89. Electrophoretic mobility shift assays (EMSA) demonstrated that IBMX induced protein-DNA binding to the FLAT and CAAT elements of the amylin promoter. Competitive EMSA experiments revealed that these proteins are likely to be HNF-1 and NFY, respectively. IBMX-induced amylin promoter activity was inhibited by mutations in the FLAT and CAAT elements. These results indicate that amylin is positively regulated by cAMP and PKA through the transcription factors HNF-1 and NFY.

DNA sequence motifs conserved in endocrine promoters are essential forPax4 expression

The paired box transcription factor Pax4 is required for maturation of insulin-producing beta- and somatostatin secreting delta-cells in the murine pancreas. It starts to be expressed in pancreatic precursors and later is restricted to beta- and delta-cells to finally be switched off after birth. A 0.9-kb genomic DNA fragment has been shown to mediate the Pax4 expression pattern. Transcription factors Pdx1 and NeuroD bind to this fragment at A2- and E1-sequence motifs. In this study, we downscale the size of this fragment to 409 bp. Another genomic fragment of 254 bp is still able to mediate the specificity, but not the strength of Pax4 expression. Deletion of the A2 and E1 elements results in loss or weakening of reporter gene expression. Because A2 and E1 elements are conserved in numerous pancreatic promoters, they might play a general role in regulating endocrine gene expression.

Members of the large Maf transcription family regulate insulin gene transcription in islet beta cells

The C1/RIPE3b1 (-118/-107 bp) binding factor regulates pancreatic-beta-cell-specific and glucose-regulated transcription of the insulin gene. In the present study, the C1/RIPE3b1 activator from mouse beta TC-3 cell nuclear extracts was purified by DNA affinity chromatography and two-dimensional gel electrophoresis. C1/RIPE3b1 binding activity was found in the roughly 46-kDa fraction at pH 7.0 and pH 4.5, and each contained N- and C-terminal peptides to mouse MafA as determined by peptide mass mapping and tandem spectrometry. MafA was detected in the C1/RIPE3b1 binding complex by using MafA peptide-specific antisera. In addition, MafA was shown to bind within the enhancer region (-340/-91 bp) of the endogenous insulin gene in beta TC-3 cells in the chromatin immunoprecipitation assay. These results strongly suggested that MafA was the beta-cell-enriched component of the RIPE3b1 activator. However, reverse transcription-PCR analysis demonstrated that mouse islets express not only MafA but also other members of the large Maf family, specifically c-Maf and MafB. Furthermore, immunohistochemical studies revealed that at least MafA and MafB were present within the nuclei of islet beta cells and not within pancreas acinar cells. Because MafA, MafB, and c-Maf were each capable of specifically binding to and activating insulin C1 element-mediated expression, our results suggest that all of these factors play a role in islet beta-cell function.

Amylin gene promoter mutations predispose to Type 2 diabetes in New Zealand Maori

AIMS/HYPOTHESIS

Amylin gene mutations are known to predispose Chinese and Japanese subjects, but not Caucasian subjects, to Type 2 diabetes. New Zealand Maori, who have a high prevalence of Type 2 diabetes, have genetic origins in South East Asia. Amylin gene mutations could therefore predispose New Zealand Maori to Type 2 diabetes.

METHODS

The amylin gene was screened for mutations in the proximal promoter region, exons 1 and 2, intron 1, and coding region of exon 3 by polymerase chain reaction amplification and direct sequencing of 131 Type 2 diabetic Maori patients and 258 non-diabetic Maori control subjects.

RESULTS

We identified three new amylin gene mutations: two mutations in the promoter region (-215T>G and -132G>A) and a missense mutation in exon 3 (Q10R). The -215T>G mutation was observed in 5.4% of Type 2 Maori diabetic patients and predisposed the carrier to diabetes with a relative risk of 7.23. The -215T>G mutation was inherited with a previously described amylin promoter polymorphism (-230A>C) in 3% of the Maori with Type 2 diabetes, which suggests linkage disequilibrium exists between these two mutations. The -230A>C polymorphism on its own, however, was not associated with Type 2 diabetes in Maori subjects. The -132G>A and Q10R mutations were both observed in 0.76% of Type 2 diabetic patients and were absent in non-diabetic subjects.

CONCLUSION/INTERPRETATION

The amylin gene mutations identified in this study are associated with Type 2 diabetes in 7% of Maori. Amylin is likely to be an important susceptibility gene for Type 2 diabetes in Maori people.

Sumoylation of Pdx1 is associated with its nuclear localization and insulin gene activation

Pancreatic duodenal homeobox-1 (Pdx1) is a transcription factor, and its phosphorylation is thought to be essential for activation of insulin gene expression. This phosphorylation is related to a concomitant shift in molecular mass from 31 to 46 kDa. However, we found that Pdx1 was modified by SUMO-1 (small ubiquitin-related modifier 1) in beta-TC-6 and COS-7 cells, which were transfected with Pdx1 cDNA. This modification contributed to the increase in molecular mass of Pdx1 from 31 to 46 kDa. Additionally, sumoylated Pdx1 localized in the nucleus. The reduction of SUMO-1 protein by use of RNA interference (SUMO-iRNAs) resulted in a significant decrease in Pdx1 protein in the nucleus. A 34-kDa form of Pdx1 was detected by the cells exposed to SUMO-iRNAs in the presence of lactacystin, a proteasome inhibitor. Furthermore, the reduced nuclear sumoylated Pdx1 content was associated with significant lower transcriptional activity of the insulin gene. These findings indicate that SUMO-1 modification is associated with both the localization and stability of Pdx1 as well as its effect on insulin gene activation.

BETA2 activates transcription from the upstream glucokinase gene promoter in islet beta-cells and gut endocrine cells

Glucokinase (GK) gene transcription initiates in the islet (beta-cell), gut, and brain from promoter sequences residing approximately 35 kbp upstream from those used in liver. Expression of betaGK is controlled in beta-cells by cell-enriched (i.e. pancreatic duodenal homeobox 1 [PDX-1]) and ubiquitously (i.e., Pal) distributed factors that bind to and activate from conserved sequence motifs within the upstream promoter region (termed betaGK). Here, we show that a conserved E-box element also contributes to control in the islet and gut. betaGK promoter-driven reporter gene activity was diminished by mutating the specific sequences involved in E-box-mediated basic helix-loop-helix factor activator binding in islet beta-cells and enteroendocrine cells. Gel shift assays demonstrated that the betaGK and insulin gene E-box elements formed the same cell-enriched (BETA2:E47) and generally distributed (upstream stimulatory factor [USF]) protein-DNA complexes. betaGK E-box-driven activity was stimulated in cotransfection assays performed in baby hamster kidney (BHK) cells with BETA2 and E47, but not USF. Chromatin immunoprecipitation assays performed with BETA2 antisera showed that BETA2 occupies the upstream promoter region of the endogenous betaGK gene in beta-cells. We propose that BETA2 (also termed NeuroD1) regulates betaGK promoter activity.

Transcription factor occupancy of the insulin gene in vivo. Evidence for direct regulation by Nkx2.2

Consensus-binding sites for many transcription factors are relatively non-selective and found at high frequency within the genome. This raises the possibility that factors that are capable of binding to a cis-acting element in vitro and regulating transcription from a transiently transfected plasmid, which would not have higher order chromatin structure, may not occupy this site within the endogenous gene. Closed chromatin structure and competition from another DNA-binding protein with similar nucleotide specificity are two possible mechanisms by which a transcription factor may be excluded from a potential binding site in vivo. Multiple transcription factors, including Pdx-1, BETA-2, and Pax6, have been implicated in expression of the insulin gene in pancreatic beta cells. In this study, the chromatin immunoprecipitation assay has been used to show that these factors do, in fact, bind to insulin control region sequences in intact beta cells. In addition, another key islet-enriched transcription factor, Nkx2.2, was found to occupy this region using the chromatin immunoprecipitation assay. In vitro DNA-binding and transient transfection assays defined how Nkx2.2 affected insulin gene expression. Pdx-1 was also shown to bind within a region of the endogenous islet amyloid polypeptide, pax-4, and glucokinase genes that were associated with control in vitro. Because Pdx-1 does not regulate gene transcription in isolation, these sequences were examined for occupancy by the other insulin transcriptional regulators. BETA-2, Pax6, and Nkx2.2 were also found to bind to amyloid polypeptide, glucokinase, and pax-4 control sequences in vivo. These studies reveal the broad application of the Pdx-1, BETA-2, Pax6, and Nkx2.2 transcription factors in regulating expression of genes selectively expressed in islet beta cells.

The role of transcription factors in maturity-onset diabetes of the young

The study of maturity-onset diabetes of the young (MODY), an autosomal dominant form of early-onset diabetes mellitus characterised by defective insulin secretion has been extremely successful in two ways. Firstly it has enabled definitive diagnosis for patients. This allows more accurate prediction of disease and treatment requirements. Secondly it has facilitated an increased understanding of the genes and pathways that are crucial for normal beta-cell function. Five of the six MODY genes, TCF1 (encoding HNF-1alpha), TCF2 (encoding HNF-1beta) HNF4A, insulin promoter factor (IPF)1, and NEUROD1, are transcription factors that operate in a complex network of gene regulation. Several genes have been shown to be regulated by the MODY transcription factors in a beta-cell specific manner. This includes the co-regulation of HNF-1alpha and HNF-4alpha by each other. The exact mechanism of how mutations in these transcription factors result in diabetes in humans remains unknown. However, current opinion favours pleiotropic adverse effects on many genes; extensive in vitro and in vivo studies of these genes has highlighted their importance in both glucose sensing-insulin secretion coupling and maintaining the fully differentiated beta-cell phenotype.