Pax-2 activates the proglucagon gene promoter but is not essential for proglucagon gene expression or development of proglucagon-producing cell lineages in the murine pancreas or intestine

Divergent roles for the gut intraepithelial lymphocyte GLP-1R in control of metabolism, microbiota, and T cell-induced inflammation

Gut intraepithelial lymphocytes (IELs) are thought to calibrate glucagon-like peptide 1 (GLP-1) bioavailability, thereby regulating systemic glucose and lipid metabolism. Here, we show that the gut IEL GLP-1 receptor (GLP-1R) is not required for enteroendocrine L cell GLP-1 secretion and glucose homeostasis nor for the metabolic benefits of GLP-1R agonists (GLP-1RAs). Instead, the gut IEL GLP-1R is essential for the full effects of GLP-1RAs on gut microbiota. Moreover, independent of glucose control or weight loss, the anti-inflammatory actions of GLP-1RAs require the gut IEL GLP-1R to selectively restrain local and systemic T cell-induced, but not lipopolysaccharide-induced, inflammation. Such effects are mediated by the suppression of gut IEL effector functions linked to the dampening of proximal T cell receptor signaling in a protein-kinase-A-dependent manner. These data reposition key roles of the L cell-gut IEL GLP-1R axis, revealing mechanisms linking GLP-1R activation in gut IELs to modulation of microbiota composition and control of intestinal and systemic inflammation.

PAX proteins and their role in pancreas

Gene regulatory factors that govern the expression of heritable information come in an array of flavors, chiefly with transcription factors, the proteins which bind to regions of specific genes and modulate gene transcription, subsequently altering cellular function. PAX transcription factors are sequence-specific DNA-binding proteins exerting its regulatory activity in many tissues. Notably, three members of the PAX family namely PAX2, PAX4 and PAX6 have emerged as crucial players at multiple steps of pancreatic development and differentiation and also play a pivotal role in the regulation of pancreatic islet hormones synthesis and secretion. Providing a comprehensive outline of these transcription factors and their primordial and divergent roles in the pancreas is far-reaching in contemporary diabetes research. Accordingly, this review furnishes an outline of the role of pancreatic specific PAX regulators in the development of the pancreas and its associated disorders.

Targeting pancreatic expressed PAX genes for the treatment of diabetes mellitus and pancreatic neuroendocrine tumors

INTRODUCTION

Four members of the PAX family, PAX2, PAX4, PAX6 and PAX8 are known to be expressed in the pancreas. Accumulated evidences indicate that several pancreatic expressed PAX genes play a significant role in pancreatic development/functionality and alterations in these genes are involved in the pathogenesis of pancreatic diseases. Areas covered: In this review, we summarize the ongoing research related to pancreatic PAX genes in diabetes mellitus and pancreatic neuroendocrine tumors. We dissect the current knowledge at different levels; from mechanistic studies in cell lines performed to understand the molecular processes controlled by pancreatic PAX genes, to in vivo studies using rodent models that over-express or lack specific PAX genes. Finally, we describe human studies associating variants on pancreatic-expressed PAX genes with pancreatic diseases. Expert opinion: Based on the current literature, we propose that future interventions to treat pancreatic neuroendocrine tumors and diabetes mellitus could be developed via the modulation of PAX4 and/or PAX6 regulated pathways.

Sleeve gastrectomy activates the GLP-1 pathway in pancreatic β cells and promotes GLP-1-expressing cells differentiation in the intestinal tract

Db/db mouse was used to study the underlying mechanisms by which Sleeve gastrectomy (SG) improves β-cell function. We investigated β-cell function, plasma active GLP-1 levels, the GLP-1R pathway in β cells and L cell differentiation. After SG, β-cell function was significantly increased, and the GLP-1R-PKCζ-PDX-1 pathway was active in β cells. Plasma active GLP-1 levels, as well as the number of L cells in the jejunum, were significantly increased after SG. The expression of early transcription factors (TF), including Ngn3, FoxA1 and Nkx2.2, was not compromised by chronic hyperglycemia. In contrast, the expression of the downstream TF PAX6 was affected, and this down-regulation could be reversed by SG. So, SG can maintain L cell differentiation, increase plasma active GLP-1 level, sustain the activation of the GLP-1R pathway and improve β cell function in Db/db mice. Our results show that SG can overall improve the function of the entero-insular axis.

Tumor suppressor menin represses paired box gene 2 expression via Wilms tumor suppressor protein-polycomb group complex

Tumor suppressor menin, the product of the MEN1 gene, plays a key role in controlling histone 3 lysine 4 trimethylation (H3K4me3) and gene transcription, which can regulate proliferation, apoptosis, and differentiation. However, little is known as to whether menin controls gene expression and cell proliferation and survival via regulating Polycomb group (PcG) protein complex/H3K27me3. Here we show that menin specifically represses transcription factor Paired box gene 2 (Pax2) through PcG-mediated H3K27me3 and Wilms tumor suppressor protein (WT1), a zinc finger domain-containing DNA-binding protein. Menin does not directly bind to the Pax2 locus, instead, it up-regulates WT1 expression. WT1 recruits PcG complex to the Pax2 promoter and represses expression of Pax2 through PcG-dependent H3K27me3. Moreover, WT1 also interacts with DNA methyltransferase 1 (DNMT1), and recruits DNMT1 to the Pax2 promoter, resulting in hypermethylation of CpG in the Pax2 promoter. Together, these studies have uncovered a novel epigenetic mechanism whereby menin regulates H3K27me3 and promoter DNA methylation via WT1 and suggest that WT1 protein plays an important, yet previously unappreciated role in regulating the function of the menin/PcG axis, H3K27 methylation, and DNA methylation, resulting in repression of gene transcription.

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.

Neuronal apoptosis inhibitory protein is expressed in developing kidney and is regulated by PAX2

During fetal kidney development, the extent of ureteric bud (UB) branching will determine final nephron endowment for life. Nephron number varies widely among normal humans and those who are born at the low end of the nephron number spectrum may be at risk for essential hypertension in adulthood. Little is known about how nephron number is set. However, we previously showed that the transcription factor, Pax2, suppresses apoptosis in UB cells during kidney development and optimizes branching morphogenesis. Here, we report that PAX2 directly binds to a specific recognition motif in the human neuronal apoptosis inhibitory protein (NAIP) gene promoter. NAIP is an endogenous inhibitor of apoptosis, inactivating caspase-3 and caspase-7 in neuronal tissues. PAX2 activates NAIP gene transcription (7-fold) in vitro and NAIP transcript level is increased fourfold in HEK293 cells stably transfected with PAX2. We show that Naip is expressed in embryonic day 15 (E15) fetal kidney tissue (RT-PCR) and NAIP protein is demonstrated by immunohistochemistry in E15 mouse kidney collecting ducts and P1 proximal tubules. Naip mRNA is significantly reduced (50%) in heterozygous Pax2 mutant mice. Finally, we show that an antisense Naip1 cDNA transfected into murine collecting duct cells doubles caspase-3/7 activity induced by Baxalpha. These observations suggest that the powerful effects of PAX2 on renal branching morphogenesis and final nephron number may be mediated by activation of Naip which then suppresses apoptosis in UB cells.

Integrating global proteomic and genomic expression profiles generated from islet alpha cells: opportunities and challenges to deriving reliable biological inferences

Systematic profiling of expressed gene products represents a promising research strategy for elucidating the molecular phenotypes of islet cells. To this end, we have combined complementary genomic and proteomic methods to better assess the molecular composition of murine pancreatic islet glucagon-producing alphaTC-1 cells as a model system, with the expectation of bypassing limitations inherent to either technology alone. Gene expression was measured with an Affymetrix MG_U74Av2 oligonucleotide array, while protein expression was examined by performing high-resolution gel-free shotgun MS/MS on a nuclear-enriched cell extract. Both analyses were carried out in triplicate to control for experimental variability. Using a stringent detection p value cutoff of 0.04, 48% of all potential mRNA transcripts were predicted to be expressed (probes classified as present in at least two of three replicates), while 1,651 proteins were identified with high-confidence using rigorous database searching. Although 762 of 888 cross-referenced cognate mRNA-protein pairs were jointly detected by both platforms, a sizeable number (126) of gene products was detected exclusively by MS alone. Conversely, marginal protein identifications often had convincing microarray support. Based on these findings, we present an operational framework for both interpreting and integrating dual genomic and proteomic datasets so as to obtain a more reliable perspective into islet alpha cell function.

The HeLa Cell Glucagon-Like Peptide-2 Receptor Is Coupled to Regulation of Apoptosis and ERK1/2 Activation through Divergent Signaling Pathways

Glucagon-like peptide-2 (GLP-2) regulates proliferative and cytoprotective pathways in the intestine; however GLP-2 receptor (GLP-2R) signal transduction remains poorly understood, and cell lines that express the endogenous GLP-2R have not yet been isolated. We have now identified several expressed sequence tags from human cervical carcinoma cDNA libraries that correspond to GLP-2R nucleotide sequences. GLP-2R mRNA transcripts were detected by RT-PCR in two human cervical carcinoma cell lines, including HeLa cells. GLP-2 increased cAMP accumulation and activated ERK1/2 in HeLa cells transiently expressing the cloned human HeLa cell GLP-2R cDNA. However, the GLP-2R-induced activation of ERK1/2 was not mediated through Gαs, adenylyl cyclase, or transactivation of the epidermal growth factor receptor, but was pertussis toxin sensitive, inhibited by dominant negative Ras, and dependent on βγ-subunits. GLP-2 also induced a significant increase in bromodeoxyuridine incorporation that was blocked by dominant negative Ras. Furthermore, GLP-2 inhibited HeLa cell apoptosis induced by LY294002 in a protein kinase A-dependent, but ERK-independent, manner. These findings demonstrate that the HeLa cell GLP-2R differentially signals through both Gαs/cAMP- and Gi/Go-dependent pathways, illustrating for the first time that the GLP-2R is capable of coupling to multiple heterotrimeric G proteins defining distinct GLP-2R-dependent biological actions.

beta-Cell Pdx1 expression is essential for the glucoregulatory, proliferative, and cytoprotective actions of glucagon-like peptide-1

Glucagon-like peptide-1 (GLP-1) regulates energy intake, gastrointestinal motility, and nutrient disposal. The relative importance of the islet beta-cell for GLP-1 actions remains unclear. We determined the role of the islet beta-cell and the pancreatic duodenal homeobox-1 (Pdx1) transcription factor for GLP-1 receptor (GLP-1R)-dependent actions through analysis of mice with beta-cell-specific inactivation of the Pdx1 gene (beta-cell(Pdx1-/-) mice). The GLP-1R agonist exendin-4 (Ex-4) reduced glycemic excursion following intraperitoneal (i.p.) glucose challenge in control littermates (beta-cell(Pdx1+/+) mice) but not in beta-cell(Pdx1-/-) mice. Similarly, Ex-4 failed to increase levels of plasma insulin, pancreatic insulin content, and pancreatic insulin mRNA transcripts in beta-cell(Pdx1-/-) mice. Furthermore, Ex-4 significantly increased beta-cell proliferation and reduced beta-cell apoptosis in beta-cell(Pdx1+/+) mice but not in beta-cell(Pdx1-/-) mice. Moreover, Ex-4 increased the levels of insulin and amylin mRNA transcripts and augmented glucose-stimulated insulin secretion in islets from beta-cell(Pdx1+/+) mice but not in beta-cell(Pdx1-/-) islets. Surprisingly, Ex-4 failed to reduce levels of plasma glucagon in beta-cell(Pdx1-/-) mice. These findings demonstrate that Pdx1 expression is essential for integrating GLP-1R-dependent signals regulating alpha-cell glucagon secretion and for the growth, differentiated function, and survival of islet beta-cells.

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

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

Cell and molecular biology of the small intestine: new insights into differentiation, growth and repair

PURPOSE OF REVIEW

This paper will discuss recent research that has provided new insights into the molecular and cell biology of the small intestine.

RECENT FINDINGS

Differentiation of the epithelial cell lineages, including the enterocytes, enteroendocrine, Goblet and Paneth cells, from the stem cells is better understood. Important interactions have been demonstrated between these cells, luminal bacteria, and underlying mesenchymal tissue. Intestine-specific gene expression is regulated by transcription factors that are becoming well characterized, including CDX1, CDX2 and HNF1. The actions of growth factors such as GLP-2 and EGF are now known to be complex, demonstrating multiple effects in this tissue at a number of levels.

SUMMARY

Progress in the cellular and molecular biology of the small intestine is producing many intriguing new findings.

Aberrant regulation of human intestinal proglucagon gene expression in the NCI-H716 cell line

Despite interest in understanding glucagon-like peptide-1 (GLP-1) production, the factors important for GLP-1 biosynthesis remain poorly understood. We examined control of human proglucagon gene expression in NCI-H716 cells, a cell line that secretes GLP-1 in a regulated manner. Insulin, phorbol myristate acetate, or forskolin, known regulators of rodent proglucagon gene expression, had no effect, whereas sodium butyrate decreased levels of NCI-H716 proglucagon mRNA transcripts. The inhibitory effect of sodium butyrate was mimicked by trichostatin A but was not detected with sodium acetate or isobutyrate. The actions of butyrate were not diminished by the ERK1/2 inhibitor PD98059, p38 inhibitor SB203580, or soluble guanylate cyclase inhibitor LY83583 or following treatment of cells with KT5823, a selective inhibitor of cGMP-dependent protein kinase. NCI-H716 cells expressed multiple proglucagon gene transcription factors including isl-1, pax-6, pax-2, cdx-2/3, pax-4, hepatocyte nuclear factor (HNF)-3 alpha, HNF-3beta, HNF-3 gamma, and Nkx2.2. Nevertheless, the butyrate-dependent inhibition of proglucagon gene expression was not associated with coordinate changes in transcription factor expression and both the human and rat transfected proglucagon promoters were transcriptionally inactive in NCI-H716 cells. Hence, NCI-H716 cells may not be a physiologically optimal model for studies of human enteroendocrine proglucagon gene transcription.