PANCREATIC-SPECIFIC EXPRESSION OF THE GLUCOSE-TRANSPORTER TYPE-2 GENE- IDENTIFICATION OF CIS-ELEMENTS AND ISLET-SPECIFIC TRANS-ACTING FACTORS

A defect in glucose sensing of the pancreatic beta-cells has been obse rved in several animal models of type If diabetes and has been correla ted with a reduced gene expression of the glucose transporter type 2 ( Glut2). In a transgenic mouse model, expression of Glut2 antisense RNA in pancreatic beta-cells has recently been shown to be associated wit h an impaired glucose-induced insulin secretion and the development of diabetes. To identify factors that may be involved in the specific de crease of Glut2 in the beta-cells of the diabetic animal, an attempt w as made to localize the cis-elements and trans-acting factors involved in the control of Glut:! expression in the endocrine pancreas. It was demonstrated by transient transfection studies that only 338 base pai rs (bp) of the murine Glut2 proximal promoter are needed for reporter gene expression in pancreatic islet-derived cell lines, whereas no act ivity was detected in nonpancreatic cells. Three cis-elements, GTI, GT II, and GTIII, have been identified by DNAse I footprinting and gel re tardation experiments within these 338 bp. GTI and GTIII bind distinct but ubiquitously expressed trans-acting factors. On the other hand, n uclear proteins specifically expressed in pancreatic cell lines intera ct with GTII, and their relative abundance correlates with endogenous Glut2 expression. These GTII-binding factors correspond to nuclear pro teins of 180 and 90 kilodaltons as defined by Southwestern analysis. T he 180-kilodalton factor is present in pancreatic beta-cell lines but not in an alpha-cell line. Mutation of the GTI or GTIII cis-elements d ecreases transcriptional activity directed by the 338-bp promoter, whe reas mutation of GTII increases gene transcription. Thus negative and positive regulatory sequences are identified within the proximal 338 b p of the GLUT2 promoter and may participate in the islet-specific expr ession of the gene by binding beta-cell specific trans-acting factors.