Cell-specific expression of glucose-dependent-insulinotropic polypeptide is regulated by the transcription factor PDX-1

Glucose-dependent insulinotropic polypeptide (GIP)

BACKGROUND

Glucose-dependent insulinotropic polypeptide (GIP) was the first incretin identified and plays an essential role in the maintenance of glucose tolerance in healthy humans. Until recently GIP had not been developed as a therapeutic and thus has been overshadowed by the other incretin, glucagon-like peptide 1 (GLP-1), which is the basis for several successful drugs to treat diabetes and obesity. However, there has been a rekindling of interest in GIP biology in recent years, in great part due to pharmacology demonstrating that both GIPR agonism and antagonism may be beneficial in treating obesity and diabetes. This apparent paradox has reinvigorated the field, led to new lines of investigation, and deeper understanding of GIP.

SCOPE OF REVIEW

In this review, we provide a detailed overview on the multifaceted nature of GIP biology and discuss the therapeutic implications of GIPR signal modification on various diseases.

MAJOR CONCLUSIONS

Following its classification as an incretin hormone, GIP has emerged as a pleiotropic hormone with a variety of metabolic effects outside the endocrine pancreas. The numerous beneficial effects of GIPR signal modification render the peptide an interesting candidate for the development of pharmacotherapies to treat obesity, diabetes, drug-induced nausea and both bone and neurodegenerative disorders.

RFX6 regulates human intestinal patterning and function upstream of PDX1

The gastrointestinal (GI) tract is complex and consists of multiple organs with unique functions. Rare gene variants can cause congenital malformations of the human GI tract, although the molecular basis of these has been poorly studied. We identified a patient with compound-heterozygous variants in RFX6 presenting with duodenal malrotation and atresia, implicating RFX6 in development of the proximal intestine. To identify how mutations in RFX6 impact intestinal patterning and function, we derived induced pluripotent stem cells from this patient to generate human intestinal organoids (HIOs). We identified that the duodenal HIOs and human tissues had mixed regional identity, with gastric and ileal features. CRISPR-mediated correction of RFX6 restored duodenal identity. We then used gain- and loss-of-function and transcriptomic approaches in HIOs and Xenopus embryos to identify that PDX1 is a downstream transcriptional target of RFX6 required for duodenal development. However, RFX6 had additional PDX1-independent transcriptional targets involving multiple components of signaling pathways that are required for establishing early regional identity in the GI tract. In summary, we have identified RFX6 as a key regulator in intestinal patterning that acts by regulating transcriptional and signaling pathways.

Dietary Vitamin A Affects the Function of Incretin-Producing Enteroendocrine Cells in Male Mice Fed a High-Fat Diet

BACKGROUND

Retinol-binding protein 2 (RBP2) is an intracellular carrier for vitamin A in the absorptive enterocytes. Mice lacking RBP2 (Rbp2-/-) display an unexpected phenotype of obesity, glucose intolerance, and elevated glucose-dependent insulinotropic polypeptide (GIP) levels. GIP and glucagon-like peptide 1 (GLP-1) are incretin hormones secreted by enteroendocrine cells (EECs). We recently demonstrated the presence of RBP2 and other retinoid-related proteins in EECs.

OBJECTIVES

Given RBP2's role in intracellular retinoid trafficking, we aimed to evaluate whether dietary vitamin A affects incretin-secreting cell function and gene expression.

METHODS

Male Rbp2-/- mice and sex- and age-matched controls (n = 6-9) were fed a high-fat diet (HFD) for 18 wk containing normal (VAN, 4000 IU/kg of diet) or low (VAL, 25% of normal) vitamin A concentrations. Body weight was recorded biweekly. Plasma GIP and GLP-1 levels were obtained fasting and 30 min after an oral fat gavage at week 16. Glucose tolerance tests were also performed. Mice were killed at week 18, and blood and tissue samples were obtained.

RESULTS

Rbp2-/- mice displayed greater weight gain on the VAN compared with the VAL diet from week 7 of the intervention (P ≤ 0.01). Stimulated GIP levels were elevated in Rbp2-/- mice compared with their controls fed the VAN diet (P = 0.02), whereas their GIP response was lower when fed the VAL diet (P = 0.03). Although no differences in GLP-1 levels were observed in the VAN diet group, a lower GLP-1 response was seen in Rbp2-/- mice fed the VAL diet (P = 0.02). Changes in incretin gene expression and that of other genes associated with EEC lineage and function were consistent with these observations. Circulating and hepatic retinoid levels revealed no systemic vitamin A deficiency across dietary groups.

CONCLUSIONS

Our data support a role for RBP2 and dietary vitamin A in incretin secretion and gene expression in mice fed a HFD.

First Japanese Family With PDX1-MODY (MODY4): A Novel PDX1 Frameshift Mutation, Clinical Characteristics, and Implications

CONTEXT

The PDX1 gene encodes pancreatic and duodenal homeobox, a critical transcription factor for pancreatic β-cell differentiation and maintenance of mature β-cells. Heterozygous loss-of-function mutations cause PDX1-MODY (MODY4).

CASE DESCRIPTION

Our patient is an 18-year-old lean man who developed diabetes at 16 years of age. Given his early-onset age and leanness, we performed genetic testing. Targeted next-generation sequencing and subsequent Sanger sequencing detected a novel heterozygous frameshift mutation (NM_00209.4:c.218delT. NP_000200.1: p.Leu73Profs*50) in the PDX1 transactivation domain that resulted in loss-of-function and was validated by an in vitro functional study. The proband and his 56-year-old father, who had the same mutation, both showed markedly reduced insulin and gastric inhibitory polypeptide (GIP) secretion compared with the dizygotic twin sister, who was negative for the mutation and had normal glucose tolerance. The proband responded well to sitagliptin, suggesting its utility as a treatment option. Notably, the proband and his father showed intriguing phenotypic differences: the proband had been lean for his entire life but developed early-onset diabetes requiring an antihyperglycemic agent. In contrast, his father was overweight, developed diabetes much later in life, and did not require medication, suggesting the oligogenic nature of PDX1-MODY. A review of all reported cases of PDX1-MODY also showed heterogeneous phenotypes regarding onset age, obesity, and treatment, even in the presence of the same mutation.

CONCLUSIONS

We identified the first Japanese family with PDX1-MODY. The similarities and differences found among the cases highlight the wide phenotypic spectrum of PDX1-MODY.

Transcriptional factor Pdx1 is involved in age-related GIP hypersecretion in mice

Fat accumulation with aging is a serious problem; glucose-dependent insulinotropic polypeptide/gastric inhibitory polypeptide (GIP) is an incretin that plays an important role in fat accumulation. GIP receptor knockout mice show reduced fat mass and improved insulin sensitivity associated with aging. Therefore, GIP is involved in fat accumulation and insulin resistance with aging. However, age-related changes of GIP secretion remain unclear. The present study aimed to elucidate age-related changes of GIP secretion and enteroendocrine K cells using GIP reporter [GIP-green fluorescent protein (GFP) knock-in heterozygous (GIP^gfp/+)] mice. Aged 1-yr-old GIP^gfp/+ mice exhibited a phenotype of fat accumulation, insulin resistance, and GIP hypersecretion compared with young (3-4 mo old) GIP^gfp/+ mice. In aged mice, K-cell number in the small intestine and the mRNA expression levels of GIP and transcriptional factor pancreatic and duodenal homeobox-1 (Pdx1) in K cells were increased. K-cell number, GIP mRNA expression and content in small intestine, and GIP secretion were decreased after posteriori suppression of Pdx1 using intestine-specific gene transfer. Thus, Pdx1 positively regulates GIP mRNA and K-cell number in small intestine. Increased Pdx1 expression might be involved in GIP hypersecretion with aging. NEW & NOTEWORTHY Age-related changes of glucose-dependent insulinotropic polypeptide/gastric inhibitory polypeptide (GIP) secretion and K cells were investigated. We found that K-cell number and GIP and pancreatic and duodenal homeobox-1 (Pdx1) expression in K cells were increased in aged mice, which showed greater GIP secretion compared with young mice. In addition, we have succeeded in posteriori suppression of Pdx1 in small intestine using the method of intestine-specific gene transfer, and showed that K-cell number, GIP expression, and GIP secretion were decreased in the Pdx1-knockdown intestine.

Patterning the gastrointestinal epithelium to confer regional-specific functions

The gastrointestinal (GI) tract, in simplest terms, can be described as an epithelial-lined muscular tube extending along the cephalocaudal axis from the oral cavity to the anus. Although the general architecture of the GI tract organs is conserved from end to end, the presence of different epithelial tissue structures and unique epithelial cell types within each organ enables each to perform the distinct digestive functions required for efficient nutrient assimilation. Spatiotemporal regulation of signaling pathways and downstream transcription factors controls GI epithelial morphogenesis during development to confer essential regional-specific epithelial structures and functions. Here, we discuss the fundamental functions of each GI tract organ and summarize the diversity of epithelial structures present along the cephalocaudal axis of the GI tract. Next, we discuss findings, primarily from genetic mouse models, that have defined the roles of key transcription factors during epithelial morphogenesis, including p63, SOX2, SOX15, GATA4, GATA6, HNF4A, and HNF4G. Additionally, we examine how the Hedgehog, WNT, and BMP signaling pathways contribute to defining unique epithelial features along the cephalocaudal axis of the GI tract. Lastly, we examine the molecular mechanisms controlling regionalized cytodifferentiation of organ-specific epithelial cell types within the GI tract, concentrating on the stomach and small intestine. The delineation of GI epithelial patterning mechanisms in mice has provided fundamental knowledge to guide the development and refinement of three-dimensional GI organotypic culture models such as those derived from directed differentiation of human pluripotent stem cells and those derived directly from human tissue samples. Continued examination of these pathways will undoubtedly provide vital insights into the mechanisms of GI development and disease and may afford new avenues for innovative tissue engineering and personalized medicine approaches to treating GI diseases.

Mechanisms of fat-induced gastric inhibitory polypeptide/glucose-dependent insulinotropic polypeptide secretion from K cells

Gastric inhibitory polypeptide/glucose‐dependent insulinotropic polypeptide (GIP) is one of the incretins, which are gastrointestinal hormones released in response to nutrient ingestion and potentiate glucose‐stimulated insulin secretion. Single fat ingestion stimulates GIP secretion from enteroendocrine K cells; chronic high‐fat diet (HFD) loading enhances GIP secretion and induces obesity in mice in a GIP‐dependent manner. However, the mechanisms of GIP secretion from K cells in response to fat ingestion and GIP hypersecretion in HFD‐induced obesity are not well understood. We generated GIP‐green fluorescent protein knock‐in (GIP^gfp/+) mice, in which K cells are labeled by enhanced GIP‐green fluorescent protein. Microarray analysis of isolated K cells from GIP^gfp/+ mice showed that both fatty acid‐binding protein 5 and G protein‐coupled receptor 120 are highly expressed in K cells. Single oral administration of fat resulted in significant reduction of GIP secretion in both fatty acid‐binding protein 5‐ and G protein‐coupled receptor 120‐deficient mice, showing that fatty acid‐binding protein 5 and G protein‐coupled receptor 120 are involved in acute fat‐induced GIP secretion. Furthermore, the transcriptional factor, regulatory factor X6 (Rfx6), is highly expressed in K cells. In vitro experiments using the mouse enteroendocrine cell line, STC‐1, showed that GIP messenger ribonucleic acid levels are upregulated by Rfx6. Expression levels of Rfx6 messenger ribonucleic acid as well as that of GIP messenger ribonucleic acid were augmented in the K cells of HFD‐induced obese mice, in which GIP content in the small intestine is increased compared with that in lean mice fed a control diet. These results suggest that Rfx6 is involved in hypersecretion of GIP in HFD‐induced obese conditions by increasing GIP gene expression.

Enteroendocrine Cells: Chemosensors in the Intestinal Epithelium

The enteroendocrine system orchestrates how the body responds to the ingestion of foods, employing a diversity of hormones to fine-tune a wide range of physiological responses both within and outside the gut. Recent interest in gut hormones has surged with the realization that they modulate glucose tolerance and food intake through a variety of mechanisms, and such hormones are therefore excellent therapeutic candidates for the treatment of diabetes and obesity. Characterizing the roles and functions of different enteroendocrine cells is an essential step in understanding the physiology, pathophysiology, and therapeutics of the gut-brain-pancreas axis.

Reprogramming of enteroendocrine K cells to pancreatic β-cells through the combined expression of Nkx6.1 and Neurogenin3, and reaggregation in suspension culture

Recent studies have demonstrated that adult cells such as pancreatic exocrine cells can be converted to pancreatic β-cells in a process called cell reprogramming. Enteroendocrine cells and β-cells share similar pathways of differentiation during embryonic development. Notably, enteroendocrine K cells express many of the key proteins found in β-cells. Thus, K cells could be reprogrammed to β-cells under certain conditions. However, there is no clear evidence on whether these cells convert to β-cells. K cells were selected from STC-1 cells, an enteroendocrine cell line expressing multiple hormones. K cells were found to express many genes of transcription factors crucial for islet development and differentiation except for Nkx6.1 and Neurogenin3. A K cell clone stably expressing Nkx6.1 (Nkx6.1(+)-K cells) was established. Induction of Neurogenin3 expression in Nkx6.1(+)-K cells, by either treatment with a γ-secretase inhibitor or infection with a recombinant adenovirus expressing Neurogenin3, led to a significant increase in Insulin1 mRNA expression. After infection with the adenovirus expressing Neurogenin3 and reaggregation in suspension culture, about 50% of Nkx6.1(+)-K cells expressed insulin as determined by immunostaining. The intracellular insulin content was increased markedly. Electron microscopy revealed the presence of insulin granules. However, glucose-stimulated insulin secretion was defective, and there was no glucose lowering effect after transplantation of these cells in diabetic mice. In conclusion, we demonstrated that K cells could be reprogrammed partially to β-cells through the combined expression of Nkx6.1 and Neurogenin3, and reaggregation in suspension culture.

Transcriptional regulatory factor X6 (Rfx6) increases gastric inhibitory polypeptide (GIP) expression in enteroendocrine K-cells and is involved in GIP hypersecretion in high fat diet-induced obesity

Gastric inhibitory polypeptide (GIP) is an incretin released from enteroendocrine K-cells in response to nutrient ingestion. GIP potentiates glucose-stimulated insulin secretion and induces energy accumulation into adipose tissue, resulting in obesity. Plasma GIP levels are reported to be increased in the obese state. However, the molecular mechanisms of GIP secretion and high fat diet (HFD)-induced GIP hypersecretion remain unclear, primarily due to difficulties in separating K-cells from other intestinal epithelial cells in vivo. In this study, GIP-GFP knock-in mice that enable us to visualize K-cells by enhanced GFP were established. Microarray analysis of isolated K-cells from these mice revealed that transcriptional regulatory factor X6 (Rfx6) is expressed exclusively in K-cells. In vitro experiments using the mouse intestinal cell line STC-1 showed that knockdown of Rfx6 decreased mRNA expression, cellular content, and secretion of GIP. Rfx6 bound to the region in the gip promoter that regulates gip promoter activity, and overexpression of Rfx6 increased GIP mRNA expression. HFD induced obesity and GIP hypersecretion in GIP-GFP heterozygous mice in vivo. Immunohistochemical and flow cytometry analysis showed no significant difference in K-cell number between control fat diet-fed (CFD) and HFD-fed mice. However, GIP content in the upper small intestine and GIP mRNA expression in K-cells were significantly increased in HFD-fed mice compared with those in CFD-fed mice. Furthermore, expression levels of Rfx6 mRNA were increased in K-cells of HFD-fed mice. These results suggest that Rfx6 increases GIP expression and content in K-cells and is involved in GIP hypersecretion in HFD-induced obesity.

Phenotype of entero-endocrine L cells becomes restricted during development

BACKGROUND

Glucagon-like peptide (GLP)-1 and glucose-dependent insulinotropic polypeptide (GIP) are hormones secreted by L and K cells, respectively, and by LK cells. To characterize L and K cells during development, we examined ileum from embryonic (e)- 12 to e-17.

RESULTS

GLP-1 cells were first seen at e-15 and their number increased at e-17. At e-17, most GLP-1 cells co-expressed GIP. The transcription factors Pax6 and Pdx-1 are required for GIP expression, while Pax6 activates the expression of GLP-1. At e-17, the mucosa has GIP+ Pax6+, GIP+ Pdx-1+, GLP-1+ Pax6+, and GLP-1+ Pdx-1+ cells. Unlike ileal L cells of postnatal and adult mice, a subset of ileal L cells of e-17 embryos co-expressed GLP-1 and glucagon (Glu). Glu-positive cells contain proprotein-convertase 2 (PC2) and PC3/1, the enzymes responsible for Glu and GLP-1 synthesis, respectively.

CONCLUSIONS

Our findings indicate that most GLP-1+ cells of ileum of e-17 embryos co-express GIP and, therefore, are LK cells. In addition, a subset of GLP-1+ cells of embryos but not of neonates co-express glucagon, indicating that the expression of Glu in GLP-1+ cells disappears after birth.

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.

Regulation of mouse intestinal L cell progenitors proliferation by the glucagon family of peptides

Glucagon like peptide-1 (GLP-1) and GLP-2 are hormones secreted by intestinal L cells that stimulate glucose-dependent insulin secretion and regulate intestinal growth, respectively. Mice with deletion of the glucagon receptor (Gcgr) have high levels of circulating GLP-1 and GLP-2. We sought to determine whether the increased level of the glucagon-like peptides is due to L cell hyperplasia. We found, first, that high levels of the glucagon-like peptides increase L cell number but does not affect the number of other intestinal epithelial cell types. Second, a large proportion of ileal L cells of Gcgr(-/-) mice coexpressed glucose-dependent insulinotropic peptide (GIP). Cells coexpressing GIP and GLP-1 are termed LK cells. Third, the augmentation in L cell number was due to a higher rate of proliferation of L cell progenitors rather than to the entrance of mature L cells into the cell cycle. Fourth, a high concentration of the glucagon-like peptides in the circulation augmented the mRNA levels of transcription factors expressed by late but not early enteroendocrine progenitors. Fifth, the administration of exendin 9-39, a GLP-1 receptor antagonist, resulted in a decrease in the rate of L cell precursor proliferation. Finally, we determined that L cells do not express the GLP-1 receptor, suggesting that the effect of GLP-1 is mediated by paracrine and/or neuronal signals. Our results suggest that GLP-1 plays an important role in the regulation of L cell number.

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.

Pancreas organogenesis: from bud to plexus to gland

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

K-cells and glucose-dependent insulinotropic polypeptide in health and disease

In the 1970s, glucose-dependent insulinotropic polypeptide (GIP, formerly gastric inhibitory polypeptide), a 42-amino acid peptide hormone, was discovered through a search for enterogastrones and subsequently identified as an incretin, or an insulinotropic hormone secreted in response to intraluminal nutrients. Independent of the discovery of GIP, the K-cell was identified in small intestine by characteristic ultrastructural features. Subsequently, it was realized that K-cells are the predominant source of circulating GIP. The density of K-cells may increase under conditions including high-fat diet and obesity, and generally correlates with plasma GIP levels. In addition to GIP, K-cells secrete xenin, a peptide with as of yet poorly understood physiological functions, and GIP is often colocalized with the other incretin hormone glucagon-like peptide-1 (GLP-1). Differential posttranslational processing of proGIP produces 30 and 42 amino acid versions of GIP. Its secretion is elicited by intraluminal nutrients, especially carbohydrate and fat, through the action of SGLT1, GPR40, GPR120, and GPR119. There is also evidence of regulation of GIP secretion via neural pathways and somatostatin. Intracellular signaling mechanisms of GIP secretion are still elusive but include activation of adenylyl cyclase, protein kinase A (PKA), and protein kinase C (PKC). GIP has extrapancreatic actions on adipogenesis, neural progenitor cell proliferation, and bone metabolism. However, the clinical or physiological relevance of these extrapancreatic actions remain to be defined in humans. The application of GIP as a glucose-lowering drug is limited due to reduced efficacy in humans with type 2 diabetes and its potential obesogenic effects demonstrated by rodent studies. There is some evidence to suggest that a reduction in GIP production or action may be a strategy to reduce obesity. The meal-dependent nature of GIP release makes K-cells a potential target for genetically engineered production of satiety factors or glucose-lowering agents, for example, insulin. Transgenic mice engineered to produce insulin from intestinal K-cells are resistant to diabetes induced by a beta-cell toxin. Collectively, K-cells and GIP play important roles in health and disease, and both may be targets for novel therapies.

High-fat feeding stimulates endocrine, glucose-dependent insulinotropic polypeptide (GIP)-expressing cell hyperplasia in the duodenum of Wistar rats

AIMS/HYPOTHESIS

Incretins are hormones released by enteroendocrine cells in response to meals, depending upon absorption of nutrients. The present study aimed to elucidate the mechanisms through which a high-fat diet (HFD) induces insulin resistance and insulin hypersecretion by focusing on the effects on enteroendocrine cells, especially those secreting glucose-dependent insulinotropic polypeptide (GIP).

METHODS

Forty male Wistar rats, 4 months old, were randomised into two groups; one group received a chow diet and the other one received a purified tripalmitin-based HFD ad libitum. An OGTT was performed every 10 days and histological and immunofluorescence evaluations of the duodenum were obtained at 60 days from the beginning of the diets. Plasma glucose, insulin, GIP and glucagon-like peptide-1 (GLP-1) levels were measured. Immunofluorescence analysis of duodenal sections for pancreatic duodenal homeobox-1 (PDX-1), KI67, GLP-1, GIP and insulin were performed.

RESULTS

Compared with chow diet, HFD induced a progressive significant increase of the glucose, insulin and GIP responses to OGTT, whereas GLP-1 circulating levels were reduced over time. After 60 days of HFD, cellular agglomerates of KI67 and PDX-1 positive cells, negative for insulin and GLP-1 but positive for GIP staining, were found inside the duodenal mucosa, and apoptosis was significantly increased.

CONCLUSIONS/INTERPRETATION

With the limitation that we could not establish a causal relationship between events, our study shows that HFD stimulates duodenal proliferation of endocrine cells differentiating towards K cells and oversecreting GIP. The progressive increment of GIP levels might represent the stimulus for insulin hypersecretion and insulin resistance.

Evolutionary conservation of glucose-dependent insulinotropic polypeptide (GIP) gene regulation and the enteroinsular axis

Glucose-dependent insulinotropic polypeptide (GIP), an important component of the enteroinsular axis, is a potent stimulator of insulin secretion, functioning to maintain nutrient efficiency. Although well-characterized in mammals, little is known regarding GIP transcriptional regulation in Danio rerio (Dr). We previously demonstrated that DrGIP is expressed in the intestine and the pancreas, and we therefore cloned the Dr promoter to compare GIP transcriptional regulation in Dr and mammals. Although no significant homology was indentified between the highly conserved mammalian promoter and the DrGIP promoter, 1072-bp of the DrGIP promoter conferred tissue-specific expression in mammalian cell lines. Deletional analysis of the DrGIP promoter identified two regions that, when deleted, reduced transcription by 75% and 95%, respectively. Mutational analysis of the upstream region suggested involvement of an Nkx binding site, although we were unable to identify the factor binding to this site. The cis element in the downstream region was found to be a GATA binding site. Lastly, overexpression and shRNA experiments identified PAX4 as a potential repressor of DrGIP expression. These findings provide evidence that despite the identification of species-specific transcriptional regulators and differences in GIP expression patterns between D. rerio and mammals, a moderate degree of regulatory conservation appears to exist.

The Incretins and Pancreatic beta-Cells: Use of Glucagon-Like Peptide-1 and Glucose-Dependent Insulinotropic Polypeptide to Cure Type 2 Diabetes Mellitus

Type 2 diabetes mellitus (T2DM) is increasing in prevalence worldwide. The complications associated with T2DM result in increased mortality and financial cost for those affected. T2DM has long been known to be associated with insulin resistance in peripheral tissues and a relative degree of insulin deficiency. However, the concept that insulin secretion and insulin sensitivity are not linked through a hyperbolic relationship in T2DM has continuously been demonstrated in many clinical trials. Thus, in order to prevent and treat T2DM, it is necessary to identify the substance(s) that will improve the function and survival of the pancreatic beta-cells in both normal and pathologic conditions, so that production and secretion of insulin can be enhanced. Incretin hormones, such as glucagon-like peptide (GLP)-1 and glucose-dependent insulinotropic polypeptide (GIP), have been shown to lower the postprandial and fasting glucose and the glycated hemoglobin levels, suppress the elevated glucagon level, and stimulate glucose-dependent insulin synthesis and secretion. In this report, we will review the biological actions and mechanisms associated with the actions of incretin hormones, GLP-1 and GIP, on beta-cell health and compare the differences between GLP-1 and GIP.

An endoderm-specific transcriptional enhancer from the mouse Gata4 gene requires GATA and homeodomain protein-binding sites for function in vivo

Several transcription factors function in the specification and differentiation of the endoderm, including the zinc finger transcription factor GATA4. Despite its essential role in endoderm development, the transcriptional control of the Gata4 gene in the developing endoderm and its derivatives remains incompletely understood. Here, we identify a distal enhancer from the Gata4 gene, which directs expression exclusively to the visceral and definitive endoderm of transgenic mouse embryos. The activity of this enhancer is initially broad within the definitive endoderm but later restricts to developing endoderm-derived tissues, including pancreas, glandular stomach, and duodenum. The activity of this enhancer in vivo is dependent on evolutionarily-conserved HOX- and GATA-binding sites, which are bound by PDX-1 and GATA4, respectively. These studies establish Gata4 as a direct transcriptional target of homeodomain and GATA transcription factors in the endoderm and support a model in which GATA4 functions in the transcriptional network for pancreas formation.

Cdx2 regulates patterning of the intestinal epithelium

Cdx1, Cdx2 and Cdx4 encode homeodomain transcription factors that are involved in vertebral anterior-posterior (AP) patterning. Cdx1 and Cdx2 are also expressed in the intestinal epithelium during development, suggesting a role in this tissue. Intestinal defects have not been reported in Cdx1 null mutants, while Cdx2 null mutants die at embryonic day 3.5 (E3.5), thus precluding assessment of the null phenotype at later stages. To circumvent this latter shortcoming, we have used a conditional Cre-lox strategy to inactivate Cdx2 in the intestinal epithelium. Using this approach, we found that ablation of Cdx2 at E13.5 led to a transformation of the small intestine to a pyloric stomach-like identity, although the molecular nature of the underlying mesenchyme remained unchanged. Further analysis of Cdx1-Cdx2 double mutants suggests that Cdx1 does not play a critical role in the development of the small intestine, at least after E13.5.

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.

WNT/beta-catenin increases the production of incretins by entero-endocrine cells

AIMS/HYPOTHESIS

Glucose-dependent insulinotropic peptide (GIP) plays a pivotal role in the regulation of glucose homeostasis. Rates of diet-induced obesity, insulin resistance and type 2 diabetes are decreased when GIP signalling is disturbed in mice, suggesting that GIP plays a role in the onset of type 2 diabetes. WNT signalling is linked to type 2 diabetes and induces synthesis of the other incretin, glucagon-like peptide 1 (GLP-1). GLP-1 analogues improve treatment of type 2 diabetes patients in whom GLP-1 signalling is intact and have captured clinical attention. GIP levels are altered at the onset of type 2 diabetes and later on, while GIP signalling is impaired. Thus, GIP is not a candidate for treatment but might be an important target from a prevention perspective. Hypothesising that hypersecretion of GIP links altered WNT signalling to the onset of type 2 diabetes, we sought to determine whether WNT signalling induces GIP production by entero-endocrine cells.

METHODS

RT-PCR and chromatin immunoprecipitation (ChIP) were used to study Gip gene induction. Gip promoter elements mediating WNT/lithium induction were identified (electrophoretic mobility shift assay, co-transfection of deletion mutants, ChIP).

RESULTS

Lithium or WNT/beta-catenin signalling enhanced GIP production by entero-endocrine cells through a conserved site in the proximal Gip promoter. Lithium favours lymphoid enhancer factor-1/beta-catenin binding to Gip promoter and diminishes ChIP through T cell factor-4 and histone deacetylase 1.

CONCLUSIONS/INTERPRETATION

Lithium and WNT are incretin inducers in general. This work provides a novel link between WNT signalling, obesity and diabetes.

The role of incretins in glucose homeostasis and diabetes treatment

Incretins are gut hormones that are secreted from enteroendocrine cells into the blood within minutes after eating. One of their many physiological roles is to regulate the amount of insulin that is secreted after eating. In this manner, as well as others to be described in this review, their final common raison d'être is to aid in disposal of the products of digestion. There are two incretins, known as glucose-dependent insulinotropic peptide (GIP) and glucagon-like peptide-1 (GLP-1), that share many common actions in the pancreas but have distinct actions outside of the pancreas. Both incretins are rapidly deactivated by an enzyme called dipeptidyl peptidase 4 (DPP4). A lack of secretion of incretins or an increase in their clearance are not pathogenic factors in diabetes. However, in type 2 diabetes (T2DM), GIP no longer modulates glucose-dependent insulin secretion, even at supraphysiological (pharmacological) plasma levels, and therefore GIP incompetence is detrimental to beta-cell function, especially after eating. GLP-1, on the other hand, is still insulinotropic in T2DM, and this has led to the development of compounds that activate the GLP-1 receptor with a view to improving insulin secretion. Since 2005, two new classes of drugs based on incretin action have been approved for lowering blood glucose levels in T2DM: an incretin mimetic (exenatide, which is a potent long-acting agonist of the GLP-1 receptor) and an incretin enhancer (sitagliptin, which is a DPP4 inhibitor). Exenatide is injected subcutaneously twice daily and its use leads to lower blood glucose and higher insulin levels, especially in the fed state. There is glucose-dependency to its insulin secretory capacity, making it unlikely to cause low blood sugars (hypoglycemia). DPP4 inhibitors are orally active and they increase endogenous blood levels of active incretins, thus leading to prolonged incretin action. The elevated levels of GLP-1 are thought to be the mechanism underlying their blood glucose-lowering effects.

Developmental biology of the pancreas: a comprehensive review

Pancreatic development represents a fascinating process in which two morphologically distinct tissue types must derive from one simple epithelium. These two tissue types, exocrine (including acinar cells, centro-acinar cells, and ducts) and endocrine cells serve disparate functions, and have entirely different morphology. In addition, the endocrine tissue must become disconnected from the epithelial lining during its development. The pancreatic development field has exploded in recent years, and numerous published reviews have dealt specifically with only recent findings, or specifically with certain aspects of pancreatic development. Here I wish to present a more comprehensive review of all aspects of pancreatic development, though still there is not a room for discussion of stem cell differentiation to pancreas, nor for discussion of post-natal regeneration phenomena, two important fields closely related to pancreatic development.

Pax6 and Pdx1 are required for production of glucose-dependent insulinotropic polypeptide in proglucagon-expressing L cells

Glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) are incretin hormones that play important roles in maintaining glucose homeostasis and are being actively pursued as novel therapeutic agents for diabetes. GIP is produced by dispersed enteroendocrine cells and interestingly at times is coexpressed with GLP-1. We sought to determine the factors that selectively define GIP- vs. GLP-1-expressing cells. We performed comparative immunostaining of Pax6 and Pdx1 in GIP- and GLP-1-secreting cells. We investigated whether Pax6 and Pdx1 activate the human GIP promoter in control IEC-6 cells and GIP-expressing STC-1 cells. EMSA was performed to assess the binding of these transcription factors to the GIP promoter. Pax6 and Pdx1 consistently colocalized in GIP-immunoreactive cells. Cells that coexpress GIP and GLP-1 were Pax6 and Pdx1 positive, whereas cells expressing only GLP-1 were Pax6 positive but did not express Pdx1. GIP promoter activity was enhanced in IEC-6 cells by exogenous Pax6 or Pdx1 and diminished in STC-1 cells by inhibition of endogenous Pax6 or Pdx1 by dominant-negative forms. Promoter truncation analysis revealed a major loss of promoter activity when the sequence between -184 to -145 bp was deleted. EMSA studies indicated that Pax6 and Pdx1 bind to this proximal sequence of the human GIP promoter. Our findings indicate that concomitant expression of Pax6 and Pdx1 is important for GIP expression. Our results also suggest that the presence of Pdx1 defines whether GLP-1-expressing gastrointestinal L cells also coexpress GIP.

GATA-4 upregulates glucose-dependent insulinotropic polypeptide expression in cells of pancreatic and intestinal lineage

A thorough examination of glucose-dependent insulinotropic polypeptide (GIP) expression has been hampered by difficulty in isolating widely dispersed, GIP-producing enteroendocrine K-cells. To elucidate the molecular mechanisms governing the regulation of GIP expression, 14 intestinal and pancreatic cell lines were assessed for their suitability for studies examining GIP expression. Both STC-1 cells and the pancreatic cell line betaTC-3 were found to express GIP mRNA and secrete biologically active GIP. However, levels of GIP mRNA and bioactive peptide and the activity of transfected GIP reporter constructs were significantly lower in betaTC-3 than STC-1 cells. When betaTC-3 cells were analyzed for transcription factors known to be important for GIP expression, PDX-1 and ISL-1, but not GATA-4, were detected. Double staining for GIP-1 and GATA-4 in mouse duodenum demonstrated GATA-4 expression in intestinal K-cells. Exogenous expression of GATA-4 in betaTC-3 cells led to marked increases in both GIP transcription and secretion. Lastly suppression of GATA-4 via RNA interference, in GTC-1 cells, a subpopulation of STC-1 cells with high endogenous GIP expression resulted in a marked an attenuation of GIP promoter activity. Our data support the hypothesis that GATA-4 may function to augment or enhance GIP expression rather than act as an initiator of GIP transcription.

Genetically engineered K cells provide sufficient insulin to correct hyperglycemia in a nude murine model

A gene therapy-based treatment of type 1 diabetes mellitus requires the development of a surrogate beta cell that can synthesize and secrete functionally active insulin in response to physiologically relevant changes in ambient glucose levels. In this study, the murine enteroendocrine cell line STC-1 was genetically modified by stable transfection. Two clone cells were selected (STC-1-2 and STC-1-14) that secreted the highest levels of insulin among the 22 clones expressing insulin from 0 to 157.2 microIU/ml/10(6) cells/d. After glucose concentration in the culture medium was increased from 1 mM to 10 mM, secreted insulin rose from 40.3+/-0.8 to 56.3+/-3.2 microIU/ml (STC-1-2), and from 10.8+/-0.8 to 23.6+/-2.3 microIU/ml (STC-1-14). After STC-1-14 cells were implanted into diabetic nude mice, their blood glucose levels were reduced to normal. Body weight loss was also ameliorated. Our data suggested that genetically engineered K cells secrete active insulin in a glucose-regulated manner, and in vivo study showed that hyperglycemia could be reversed by implantation of the cells, suggesting that the use of genetically engineered K cells to express human insulin might provide a glucose-regulated approach to treat diabetic hyperglycemia.

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

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

Biology of incretins: GLP-1 and GIP

This review focuses on the mechanisms regulating the synthesis, secretion, biological actions, and therapeutic relevance of the incretin peptides glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1). The published literature was reviewed, with emphasis on recent advances in our understanding of the biology of GIP and GLP-1. GIP and GLP-1 are both secreted within minutes of nutrient ingestion and facilitate the rapid disposal of ingested nutrients. Both peptides share common actions on islet beta-cells acting through structurally distinct yet related receptors. Incretin-receptor activation leads to glucose-dependent insulin secretion, induction of beta-cell proliferation, and enhanced resistance to apoptosis. GIP also promotes energy storage via direct actions on adipose tissue, and enhances bone formation via stimulation of osteoblast proliferation and inhibition of apoptosis. In contrast, GLP-1 exerts glucoregulatory actions via slowing of gastric emptying and glucose-dependent inhibition of glucagon secretion. GLP-1 also promotes satiety and sustained GLP-1-receptor activation is associated with weight loss in both preclinical and clinical studies. The rapid degradation of both GIP and GLP-1 by the enzyme dipeptidyl peptidase-4 has led to the development of degradation-resistant GLP-1-receptor agonists and dipeptidyl peptidase-4 inhibitors for the treatment of type 2 diabetes. These agents decrease hemoglobin A1c (HbA1c) safely without weight gain in subjects with type 2 diabetes. GLP-1 and GIP integrate nutrient-derived signals to control food intake, energy absorption, and assimilation. Recently approved therapeutic agents based on potentiation of incretin action provide new physiologically based approaches for the treatment of type 2 diabetes.

Aire-dependent alterations in medullary thymic epithelium indicate a role for Aire in thymic epithelial differentiation

The prevalent view of thymic epithelial differentiation and Aire activity holds that Aire functions in terminally differentiated medullary thymic epithelial cells (MTECs) to derepress the expression of structural tissue-restricted Ags, including pancreatic endocrine hormones. An alternative view of these processes has proposed that Aire functions to regulate the differentiation of immature thymic epithelial cells, thereby affecting tissue-restricted Ag expression and negative selection. In this study, we demonstrate that Aire impacts several aspects of murine MTECs and provide support for this second model. Expression of transcription factors associated with developmental plasticity of progenitor cells, Nanog, Oct4, and Sox2, by MTECs was Aire dependent. Similarly, the transcription factors that regulate pancreatic development and the expression of pancreatic hormones are also expressed by wild-type MTECs in an Aire-dependent manner. The altered transcriptional profiles in Aire-deficient MTECs were accompanied by changes in the organization and composition of the medullary epithelial compartment, including a reduction in the medullary compartment defined by keratin (K) 14 expression, altered patterns of K5 and K8 expression, and more prominent epithelial cysts. These findings implicate Aire in the regulation of MTEC differentiation and the organization of the medullary thymic compartment and are compatible with a role for Aire in thymic epithelium differentiation.

Glucose-dependent insulinotropic polypeptide and its role in obesity

PURPOSE OF REVIEW

As we strive to improve our understanding of the factors that contribute to the pathogenesis of obesity, it is certainly logical to speculate that a nutrition-dependent component emanating from the gastrointestinal tract, which has the capacity to regulate insulin expression, could represent an etiologic factor in this serious health issue. One such regulatory peptide that possesses these properties is glucose-dependent insulinotropic polypeptide. Release of this peptide is stimulated by the ingestion of glucose and fat from the upper portion of the small intestine, which in turn enhances the release of insulin from the pancreas.

RECENT FINDINGS

In addition to its indirect effects in inducing obesity through the stimulation of insulin, a potent efficiency hormone, glucose-dependent insulinotropic polypeptide, like insulin, also directly modulates a variety of factors that are important in adipocyte homeostasis. Such factors include lipoprotein lipase, Akt, and GLUT-4, all of which promote the storage of fat.

SUMMARY

In this review, various mechanisms by which glucose-dependent insulinotropic polypeptide may serve as a link that indirectly and directly contributes to the development of obesity will be discussed. Understanding the peptide's role will potentially help provide novel ways to prevent and treat obesity.

Complementation rescue of Pdx1 null phenotype demonstrates distinct roles of proximal and distal cis-regulatory sequences in pancreatic and duodenal expression

The unique, well-demarcated expression domain of Pdx1 within the posterior foregut suggests that investigating its transcriptional regulation will provide insight into mechanisms that regionally pattern the endoderm. Previous phylogenetic comparison identified conserved noncoding regions that stimulate transcriptional activity selectively in cultured pancreatic beta cells. Characterization of these regulatory elements is helping to dissect the transcription factor networks that operate within beta cells, which is important for understanding the etiology of beta cell dysfunction and diabetes, as well as for developing methods to produce beta cells in vitro for cell-based therapies. We recently reported that deletion of three proximally located conserved areas (Area I-II-III) from the endogenous Pdx1 locus resulted in severely reduced expression of Pdx1 in the pancreas, and a milder decrease in other foregut tissues. Here, we report transgene-based complementation experiments on Pdx1 null mice, which reveal that the proximal promoter/enhancer region, including Area I-II-III, rescues the pancreatic defects caused by Pdx1 deficiency, but only weakly promotes expression of Pdx1 in the postnatal stomach and duodenum. These results reveal a role for distal cis-regulatory elements in achieving the correct level of extra-pancreatic Pdx1 expression, which is necessary for the production of duodenal GIP cells and stomach gastrin cells.

Bitter stimuli induce Ca2+ signaling and CCK release in enteroendocrine STC-1 cells: role of L-type voltage-sensitive Ca2+ channels

We previously demonstrated the expression of bitter taste receptors of the type 2 family (T2R) and the alpha-subunits of the G protein gustducin (Galpha(gust)) in the rodent gastrointestinal (GI) tract and in GI endocrine cells. In this study, we characterized mechanisms of Ca(2+) fluxes induced by two distinct T2R ligands: denatonium benzoate (DB) and phenylthiocarbamide (PTC), in mouse enteroendocrine cell line STC-1. Both DB and PTC induced a marked increase in intracellular [Ca(2+)] ([Ca(2+)](i)) in a dose- and time-dependent manner. Chelating extracellular Ca(2+) with EGTA blocked the increase in [Ca(2+)](i) induced by either DB or PTC but, in contrast, did not prevent the effect induced by bombesin. Thapsigargin blocked the transient increase in [Ca(2+)](i) induced by bombesin, but did not attenuate the [Ca(2+)](i) increase elicited by DB or PTC. These results indicate that Ca(2+) influx mediates the increase in [Ca(2+)](i) induced by DB and PTC in STC-1 cells. Preincubation with the L-type voltage-sensitive Ca(2+) channel (L-type VSCC) blockers nitrendipine or diltiazem for 30 min inhibited the increase in [Ca(2+)](i) elicited by DB or PTC. Furthermore, exposure to the L-type VSCCs opener BAY K 8644 potentiated the increase in [Ca(2+)](i) induced by DB and PTC. Stimulation with DB also induced a marked increase in the release of cholecystokinin from STC-1 cells, an effect also abrogated by prior exposure to EGTA or L-type VSCC blockers. Collectively, our results demonstrate that bitter tastants increase [Ca(2+)](i) and cholecystokinin release through Ca(2+) influx mediated by the opening of L-type VSCCs in enteroendocrine STC-1 cells.

Features of Medullary Thymic Epithelium Implicate Postnatal Development in Maintaining Epithelial Heterogeneity and Tissue-Restricted Antigen Expression

Although putative thymic epithelial progenitor cells have been identified, the developmental potential of these cells, the extent of medullary thymic epithelium (mTEC) heterogeneity, and the mechanisms that mediate the expression of a wide range of peripheral tissue-restricted Ags (TRAs) by mTECs remain poorly defined. Here we have defined several basic properties of the mTEC population that refine our understanding of these cells and impose important constraints for any model of mTEC differentiation and function. We report here that mTECs from adult mice are mitotically active, implying continual turnover, differentiation, and replacement of mTEC populations in the adult thymus. The mTEC population in adult thymus expresses transcription factors implicated in the maintenance of multipotential progenitor cell populations, suggesting that epithelial progenitors in the adult thymus may not be restricted to a thymic fate. mTECs also express multiple transcription factors required for the specification of multiple epithelial lineages in peripheral tissues. Thus, expression of some TRAs by mTECs may represent coordinated gene expression that reflects alternate programs of epithelial differentiation among mTECs. Analysis of TRA expression in individual and small pools of sorted mTECs show that mTECs are highly heterogeneous; each individual mTEC expresses a limited spectrum of TRAs, and the frequency of mTECs that express any individual TRA is quite low (>0.4-2%). Collectively, these findings suggest that the differentiation of mTECs can involve some of the developmental programs used by other epithelial lineages and that expression of some TRAs by mTECs may reflect this activity.

Targeted deletion of a cis-regulatory region reveals differential gene dosage requirements for Pdx1 in foregut organ differentiation and pancreas formation

Pdx1 (IPF-1 in humans, which is altered in MODY-4) is essential for pancreas development and mature beta-cell function. Pdx1 is expressed dynamically within the developing foregut, but how its expression characteristics are linked to the various steps of organ specification, differentiation, and function is unknown. Deletion of a conserved enhancer region (Area I-II-III) from Pdx1 produced a hypomorphic allele (Pdx1(DeltaI-II-III)) with altered timing and level of expression, which was studied in combination with wild-type and protein-null alleles. Lineage labeling in homozygous Area I-II-III deletion mutants (Pdx1(DeltaI-II-III/DeltaI-II-III)) revealed lack of ventral pancreatic bud specification and early-onset hypoplasia in the dorsal bud. Acinar tissue formed in the hypoplastic dorsal bud, but endocrine maturation was greatly impaired. While Pdx1(-/-) (protein-null) mice have nonpancreatic abnormalities (e.g., distorted pylorus, absent Brunner's glands), these structures formed normally in Pdx1(DeltaI-II-III/DeltaI-II-III) and Pdx1(DeltaI-II-III/-) mice. Surprisingly, heterozygous (Pdx1(+/DeltaI-II-III)) mice had abnormal islets and a more severe prediabetic condition than Pdx1(+/-) mice. These findings provide in vivo evidence of the differential requirements for the level of Pdx1 gene activity in the specification and differentiation of the various organs of the posterior foregut, as well as in pancreas and gut endocrine cell differentiation.

Role of glucagon-like peptide-1 in the pathogenesis and treatment of diabetes mellitus

Glucagon-like peptide-1 (GLP-1) is an incretin hormone secreted from enteroendocrine L cells in response to ingested nutrients. The first recognized and most important action of GLP-1 is the potentiation of glucose-stimulated insulin secretion in beta-cells, mediated by activation of its seven transmembrane domain G-protein-coupled receptor. In addition to its insulinotropic actions, GLP-1 exerts islet-trophic effects by stimulating replication and differentiation and by decreasing apoptosis of beta-cells. The GLP-1 receptor is expressed in a variety of other tissues important for carbohydrate metabolism, including pancreatic alpha-cells, hypothalamus and brainstem, and proximal intestinal tract. GLP-1 also appears to exert important actions in liver, muscle and fat. Thus, GLP-1 suppresses glucagon secretion, promotes satiety, delays gastric emptying and stimulates peripheral glucose uptake. The impaired GLP-1 secretion observed in type 2 diabetes suggests that GLP-1 plays a role in the pathogenesis of this disorder. Thus, because of its multiple actions, GLP-1 is an attractive therapeutic target for the treatment of type 2 diabetes, and major interest has resulted in the development of a variety of GLP-1 receptor agonists for this purpose. Ongoing clinical trials have shown promising results and the first analogs of GLP-1 are expected to be available in the near future.