Cloning and characterization of repressory and stimulatory DNA sequences upstream the Na/I-symporter gene promoter

To investigate the existence of potential enhancer or silencer elements in the 5'-flanking region of the human Na+/I-symporter (NIS) gene, we cloned 2 512 bp of genomic DNA further upstream of the previously defined proximal p romoter. When tested in reporter gene assays, this sequence had no transcri ptional activity per se, but was able to repress the activity of the hetero logous SV40 promoter. Conversely, when fused to the homologous NIS gene pro moter and thus comprising 3800 bp 5'-flanking region, the transcription of the proximal NIS promoter was stimulated in the human cell lines FTC-133 (f rom thyroid) and HeLa, but inhibited in the rat thyroid cell line FRTL-5. T his might be due to differences between the upstream regions of the rat and human NIS gene. Comparative analysis with standard promoters (SV40) led to the conclusion that the 5'-flanking region of the human NIS gene also exhi bited transcriptional activity in non-thyroid cells. Thyroid-stimulating ho rmone (TSH) had a moderately stimulating effect on the full length NIS repo rter gene construct in FRTL-5 cells. This stimulation is presumably mediate d by a putative cAMP responsive element found in the first half of the clon ed sequence.