Regulation of the human Na(+)-glucose cotransporter gene, SGLT1, by HNF-1 and Sp1
The Na(+)-glucose cotransporter (SGLT1) is expressed primarily by small intestinal epithelial cells and transports the monosaccharides glucose and galactose across the apical membrane. Here we describe the isolation and characterization of 5.3 kb of the 5'-flanking region of the SGLT1 gene by transiently transfecting reporter constructs into a variety of epithelial cell lines. A fragment (nt -235 to +22) of the promoter showed strong activity in the intestinal cell line Caco-2 but was inactive in a nonintestinal epithelial cell line (Chinese hamster ovary). Within this region, three cis-elements, a hepatocyte nuclear factor-1 (HNF-1) and two GC box sites are critical for maintaining the gene's basal level of expression. The two GC boxes bind to several members of the Sp1 family of transcription factors and, in the presence of HNF-1, synergistically upregulate transactivation of the promoter. A novel 16-bp element just downstream of one GC box was also shown to influence the interaction of Sp1 to its binding site. In summary, we report the identification and characterization of the human SGLT1 minimal promoter and the critical role that HNF-1 and Sp1-multigene members have in enhancing the basal level of its transcription in Caco-2 cells.