Role of the Sp family on glucagon receptor of transcription factors gene expression

The glucagon receptor mediates the actions of glucagon on carbohydrate meta bolism by the liver and on insulin release by the pancreatic beta -cell, wh ich are key processes in the control of glucose homeostasis. The 5 ' -regio n of the mouse glucagon receptor gene has been recently cloned and two func tional promoters were characterized. In the present study, we show that mos t of the glucagon receptor mRNA was transcribed from the distal promoter, i n the mouse liver. In the distal promoter region, a GC-rich sequence with f ive putative binding sites for the Sp family of transcription factors was l ocalized. To elucidate the role of these Sp1-binding sites in the mouse MIN 6 beta -cell line, the expression of reporter gene constructs containing de letion or point mutation of each site was carried out. Selective mutation o f the second Sp1-binding site decreased the activity of the distal promoter . Electrophoretic mobility shift assay with a DNA fragment spanning the thr ee first Sp1 sites confirmed that the second site bound specifically MIN6 n uclear proteins, and supershift using specific Sp antibodies demonstrated t hat it interacted with Sp3 but not Sp1 transcription factors. These data il lustrate that the basal expression of the mouse glucagon receptor gene, dri ven by the distal promoter, requires an Sp1-binding site that binds Sp3 pro teins. (C) 2001 Academic Press.