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 promoter. When tested in reporter gene assays, this sequence had no transcriptional activity per se, but was able to repress the activity of the heterologous SV40 promoter. Conversely, when fused to the homologous NIS gene promoter and thus comprising 3,800 bp 5'-flanking region, the transcription of the proximal NIS promoter was stimulated in the human cell lines FTC-133 (from thyroid) and HeLa, but inhibited in the rat thyroid cell line FRTL-5. This 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 exhibited transcriptional activity in non-thyroid cells. Thyroid-stimulating hormone (TSH) had a moderately stimulating effect on the full length NIS reporter gene construct in FRTL-5 cells. This stimulation is presumably mediated by a putative cAMP responsive element found in the first half of the cloned sequence.