ABSTRACT
The human glucagon-like peptide-1 (GLP-1) receptor mediates the insulinotropic effects of the incretin hormone GLP-1. It is expressed in a cell- and tissue-specific manner. Recently, we cloned the 5'-region of the GLP-1 receptor gene and found that tissue and cell specificity is lost by 5'-deletion to -574. In this region proximal to the main transcription start point three putative binding sites for Sp1 were localized. Now, in vitro binding of Sp1 was shown by deoxyribonuclease footprint analysis with DNA fragments using either recombinant Sp1 or nuclear extracts from HIT cells. To elucidate the roles of the three Sp1-binding sites, we mutated each of the sites individually as well as in different combinations. The activity of each construct was analyzed in comparison to the wild-type promoter. Mutation of two adjacent Sp1-binding sites showed a clear reduction of activity. Contrasting results were obtained after mutation of the third, more distal Sp1-binding site. Here, a clear increase (approximately 150%) revealed a silencing effect of this cis-regulatory element, possibly resembling a Sp3-binding site. Electrophoretic mobility shift analysis revealed binding of Sp1 and Sp3, which was demonstrated by supershifts using specific antibodies. Cotransfection with Sp1 and Sp3 expression vectors in insect cells lacking endogenous Sp factors clearly demonstrated the involvement of Sp1 and Sp3. Therefore, the basal activity of the GLP-1 receptor gene is mediated by two proximal Sp1-binding sites, whereas a more distal site acts as a repressor.
