Analysis of the thyroid transcription factor-1(TTF-1) gene expression in thyroid neoplastic tissues.

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Regulation of the thyroid transcription factor-1(TTF-1) gene expression in the thyroid was investigated. We identified a new transcription start site as nucleotide (ut)-1917, -1700 bp upstream of previously described site, and the region encom passing nt -1242 to -14 as the first intron. Northern blot analysis with FRTL-5 cell mRNA revealed that a probe targeted to exon 2 recognized 3.7 and 2.7 kb-transcripts, whereas both probes targeted to the untranslated exon 1 and the second intron deteced only the 3.7 kb-transcript, suggesting that the 3.7 kb-mRNA is transcribed from nt -1917 and that it contains the sacond intron. Chloramphenicol acetyltransferase (CAT) reporter gene assays demonstrated that the 5'-flanking region exhibited promoter activity in FRTL-5 cells but not in rat liver cells, indicating that this region confers the property of thyroid-specific exprression. Two consensus TTF-1 binding motifs were detected in this promoter region, and electrophoretic mobility-shift assays showed that oligonucleotide probes, each containing one of these motifs, formed a complex with the recombinant TTF-1 homeodomain. Moreover, recombinant TTF-1 increased the transcriptional activity in cells lacking TTF-1. These results suggest that transcriptional from the newly identified start site in the TTF-1 gene is positively regulated by TTF-1 in the thyroid.