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

Abstract

cose-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 dominantnegative forms. Promoter truncation analysis revealed a major loss of promoter activity when the sequence between Ϫ184 to Ϫ145 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. transcriptional regulation; gut; K cell; glucagon-like peptide-1; glucose-dependent insulinotropic polypeptide GLUCOSE-DEPENDENT INSULINOTROPIC POLYPEPTIDE (GIP, also known as gastric inhibitory polypeptide) and glucagon-like peptide-1 (GLP-1) are gastrointestinal hormones with pleiotropic

Phenotype of entero‐endocrine L cells becomes restricted during development

“…It has been proposed that Pdx-1 and Pax6 are required for GIP expression (Offield et al, 1996;Jepeal et al, 2005;Fujita et al, 2008) while Pax6 has been reported to activate the expression of the glucagon gene (Hill et al, 1999). At e-17, ileum of embryos contain GIPþPdx-1þ ( Fig.…”

“…GIP and GLP-1 are termed incretins because they stimulate insulin secretion from pancreatic beta-cells following nutrient ingestion (Baggio and Drucker, 2007). While the primary site of endogenous GIP release is the endocrine K cell of the duodenum and proximal jejunum, several reports indicate the presence of LK cells in adult mouse ileum (Mortensen et al, 2003;Fujita et al, 2008;Grigoryan et al, 2012).…”

“…The differentiation of secretin and CCK lineages in Ngn3-positive precursors is activated by BETA2/NeuroD (Naya et al, 1997) while Nkx2.2 is required for the appearance of the CCK, GIP, somatostatin, GLP-1, and gastrin cells (Desai et al, 2008). The TF Pax6 activates the glucagon gene expression (St-Onge et al, 1997;Larsson et al, 1998;Hill et al, 1999;Katz et al, 2009), while TF's Arx and Pdx1 are required for L and K cell differentiation, respectively (Offield et al, 1996;Fujita et al, 2008;Beucher et al, 2012;Du et al, 2012). Although these factors are important for enteroendocrine lineage development, it is unclear when intestinal progenitor cells become committed to the different cell lineages.…”

Isolated ileal interposition in enteroendocrine L cells differentiation

“…This suggests that the presence of Pdx1 determines whether gastrointestinal L-cells will co-express GIP. 163 Most Nkx6.1-expressing cells also express serotonin, and some express gastrin. Nkx6.1 is not expressed in Pdx1-deficient mice, indicating that Pdx1 acts upstream of Nkx6.1 in enteroendocrine differentiation.…”

A common variant upstream of the PAX6 gene influences islet function in man

“…In mice, PAX6 is needed for the differentiation of gastric inhibitory peptide (GIP) and glucagon-like peptide (GLP)-1-and GLP-2-producing cells in the intestine [8]. Furthermore, PAX6 regulates the production of GIP and proglucagon [9] as well as the main enzymes involved in GIP, proinsulin and proglucagon processing: PC2 and PC1/3 [2,[10][11][12][13]. PAX6 could thus affect incretin levels either by changing the expression of the propeptide genes or the levels of processing enzymes.…”