Composite action of three GC/GT boxes in the proximal promoter region is important for gastrin gene transcription.

The proximal region of the human gastrin gene promoter contains three GC/GT boxes at positions -140 to -134 bp, -108 to -102 bp and -67 to -61 bp. In this study we have examined the significance of the three elements, and their role in Sp1 and Sp3 mediated gastrin transcription. In AGS cells, mutation of each of the boxes caused a moderate decrease in promoter activity from 33 to 63%, whereas double or triple mutations reduced activity to 3-12%. In Drosophila cells Sp1 activated the promoter, mainly through the distal GC box. Similarly, co-transfection of heterologous promoter constructs revealed that only the distal GC box increased activation by Sp1. The effect of Sp3 was cell-line dependent, since Sp3 inhibited the gastrin promoter activity in AGS cells and caused a synergistic activation of the Sp1 stimulated gastrin promoter in Drosophila cells. Both effects were dependent on the C-terminal DNA binding domain of Sp3. The results indicates that the combined effect of the GC/GT boxes and the ratio between Sp1 and Sp3 are important for gastrin gene expression.