A NOVEL THYROID TRANSCRIPTION FACTOR IS ESSENTIAL FOR THYROTROPIN-INDUCED UP-REGULATION OF NA+ I- SYMPORTER GENE-EXPRESSION/

The stimulation of iodide (I-) transport by TSH in FRTL-5 thyroid cell s is partly due to an increase in Na+/I- symporter (NIS) gene expressi on. The identification of a TSH-responsive element (TRE) in the NIS pr omoter and its relationship to the action of thyroid transcription fac tor-1 (TTF-l) on the promoter are the subjects of this report. By tran sfecting NIS promoter-luciferase chimeric plasmids into FRTL-5 cells i n the presence or absence of TSH, we identify a TRE between -420 and - 370 bp of the NIS 5'-flanking region. Nuclear extracts from FRTL-5 cel ls cultured in the absence of TSH form two groups of protein-DNA compl exes, A and B, in gel mobility shift assays using an oligonucleotide h aving the sequence from -420 to -385 bp. Only the A complex is increas ed by exposure of FRTL-5 cells to TSH or forskolin. The addition of TS H to FRTL-5 cells can increase the A complex at 3-6 h, reaching a maxi mum at 12 h. FRTL-5, but not nonfunctioning FRT thyroid or Buffalo rat liver (BRL) cell nuclear extracts, form the A complex. The TSH-increa sed nuclear factor in FRTL-5 cells interacting with the NIS TRE is dis tinct from TTF-l, thyroid transcription factor-2, or Pax-8, as evidenc ed by the absence of competition using oligonucleotides specific for t hese factors in gel shift assays. Neither is it the nuclear protein in teracting with cAMP response element. The TRE is in the upstream of a TTF-l-binding site, -245 to -230 bp. Mutation of the TRE causing a los s of TSH responsiveness also decreases TTF-1-induced promoter activity in a transfection experiment. The formation of the A complex between FRTL-5 nuclear extracts and the NIS TRE is redox-regulated. In sum, TS H/cAMP-induced up-regulation of the NIS requires a novel thyroid trans cription factor, which also appears to be involved in TTF-1-mediated t hyroid-specific NIS gene expression.