HEPATOCYTE NUCLEAR FACTOR 3-BETA IS INVOLVED IN PANCREATIC BETA-CELL-SPECIFIC TRANSCRIPTION OF THE PDX-1 GENE

The mammalian homeobox gene pdx-1 is expressed in pluripotent precurso r cells in the dorsal and ventral pancreatic bud and duodenal endoderm , which will produce the pancreas and the rostral duodenum. In the adu lt, pdx-1 is expressed principally within insulin-secreting pancreatic islet beta cells and cells of the duodenal epithelium. Our objective in this study was to localize sequences within the mouse pdx-1 gene me diating selective expression within the islet, Studies of transgenic m ice in which a genomic fragment of the mouse pdx-1 gene from kb -4.5 t o +8.2 was used to drive a beta-galactosidase reporter showed that the control sequences sufficient for appropriate developmental and adult specific expression were contained within this region, Three nuclease- hypersensitive sites, located between bp -2560 and -1880 (site 1), bp -1330 and -800 (site 2), and bp -260 and +180 (site 3), were identifie d within the 5'-flanking region of the endogenous pdx-1 gene. Pancreat ic beta-cell-specific expression was shown to be controlled by sequenc es within site 1 from an analysis of the expression pattern of various pdx-1-herpes simplex virus thymidine kinase promoter expression const ructs in transfected beta-cell and non-beta-cell lines, Furthermore, w e also established that this region was important in vivo by demonstra ting that expression from a site 1-driven beta-galactosidase reporter construct was directed to islet beta-cells in transgenic mice. The act ivity of the site 1-driven constructs was reduced substantially in bet a-cell lines by mutating a hepatocyte nuclear factor 3 (HNF3)-like sit e located between nucleotides -2007 and -1996. Gel shift analysis indi cated that HNF3 beta present in islet beta cells binds to this element . Immunohistochemical studies revealed that HNF3 beta was present with in the nuclei of almost all islet beta cells and subsets of pancreatic acinar cells. Together, these results suggest that HNF3 beta, a key r egulator of endodermal cell lineage development, plays an essential ro le in the cell-type-specific transcription of the pdx-1 gene in the pa ncreas.