Cloning and characterization of rat pancreatic beta-cell/liver type glucose transporter gene: a unique exon/intron organization.

Four overlapping lambda genomic clones encoding rat pancreatic beta-cell/liver type glucose transporter (GLUT2) have been isolated and characterized. The gene is about 35 kb long and contains 14 exons and 13 introns. Contrary to the exon 1 of the human or mouse counterpart, the rat GLUT2 gene has three additional noncoding exons which were identified by 5'-RACE and all four were designated exon 1a, 1b, 1c, and 1d. The intron sequences bordering the splice site junctions generally follow the GT/AG rule except for one intron which begins with GC. The exon sequences determined from genomic DNA sequencing showed some differences when compared to the published rat GLUT2 cDNA. Transcription initiation site was determined by primer extension and located 661 bp upstream of the ATG translation initiation codon. Several potential binding sites for transcription factors such as C/EBP, Sp1, AP1, HNF-5, and UPE were observed and they may be responsible for the regulation of GLUT2 gene expression. The promoter region of rat GLUT2 showed little homology when compared with those of human or mouse. However, striking sequence identity (84%) was found when the adjacent intron regions flanking exon 1c were compared with the -970/-721 region of the mouse GLUT2 promoter. A series of deleted mutant constructs of the putative promoter region linked to the CAT reporter gene showed promoter activity in the primary hepatocyte culture. The region containing -4542/+240 showed the highest CAT activity and further deletion of the region showed gradual decrease in CAT activity.