Endocrine Abstracts

The sodium iodide symporter (NIS) mediates the uptake of iodide into thyroid follicular cells. The pituitary tumor transforming gene (PTTG) is a multifunctional oncogene which stimulates expression of fibroblast growth factor-2 (FGF-2) via PTTG binding factor (PBF). PTTG and FGF-2 inhibit iodide uptake in rat thyroid FRTL5 cells and PTTG and PBF repress NIS mRNA expression and iodide uptake in primary human thyroid cultures. To determine whether this regulation is a direct transcriptional event, we co-transfected FRTL5 cells with PTTG and PBF cDNAs along with luciferase reporter constructs containing 1) the human NIS proximal promoter, (2) the human NIS upstream enhancer (hNUE) coupled to the human NIS proximal promoter and (3) hNUE coupled to the SV40 promoter. Both PTTG and PBF repressed promoter activity via the hNUE element (PTTG:45±7% repression; PBF:59±4%; P<0.001; N=9). A PTTG mutant unable to transactivate FGF-2 could not repress hNUE. hNUE contains a PAX8 site, critical to basal NIS activity, within which is a putative USF1 consensus. As USF1 sites exist upstream of genes regulated by PTTG, we hypothesised that PTTG may repress NIS via this site, and created mutants disrupting either the PAX8 or USF1 binding sites. Basal hNUE activity was reduced with both PAX8 (47±3%; P<0.001; N=9) and USF1 (76±3%; P<0.001; N=9) mutations, suggesting that the USF1 binding site is also crucial for hNUE activity. Further, a mutated USF1 binding site caused a loss of repression by PTTG and PBF. The PAX8 mutation also prevented repression of hNUE by PBF but not PTTG. We conclude that the repression of NIS by PTTG and PBF in thyroid cells occurs via its upstream enhancer region, hNUE, at the co-localised PAX8/USF1 binding site. In thyroid cancer, where PTTG and PBF are up-regulated, NIS activity, and hence radioiodine uptake, is likely to be repressed via a promoter-specific mechanism.