The paired-domain transcription factor Pax8 binds to the upstream enhancerof the rat sodium/iodide symporter gene and participates in both thyroid-specific and cyclic-AMP-dependermt transcription

The gene encoding the Na/I symporter (NIS) is expressed at high levels only in thyroid follicular cells, where its expression is regulated by the thyr oid-stimulating hormone via the second messenger, cyclic AMP (cAMP). In thi s study, we demonstrate the presence of an enhancer that is located between nucleotides -2264 and -2495 in the 5'-flanking region of the NIS gene and that recapitulates the most relevant aspects of NIS regulation. When fused to either its own or a heterologous promoter, the NIS upstream enhancer, wh ich we call NUE, stimulates transcription in a thyroid-specific and cAMP-de pendent manner. The activity of NUE depends on the four most relevant sites , identified by mutational analysis. The thyroid-specific transcription fac tor Pax8 binds at two of these sites. Mutations that interfere with Pax8 bi nding also decrease transcriptional activity of the NUE. Furthermore, expre ssion of Pax8 in nonthyroid cells results in transcriptional activation of NUE, strongly suggesting that the paired-domain protein Pax8 plays an impor tant role in NUE activity. The NUE responds to cAMP in both protein kinase A-dependent and -independent manners, indicating that this enhancer could r epresent a novel type of cAMP responsive element. Such a cAMP response requ ires Pax8 but also depends on the integrity of a cAMP responsive element (C RE)-like sequence, thus suggesting a functional interaction between Pax8 an d factors binding at the CRE-like site.