THE THYROID TRANSCRIPTION FACTOR-I GENE IS A CANDIDATE TARGET FOR REGULATION BY HOX PROTEINS

Vertebrate Hox homeobox genes are transcription factors which regulate antero-posterior axial identity in embryogenesis, presumably through activation and/or repression of downstream target genes. Some of these targets were reported to code for molecules involved in cell-cell int eractions, whereas no relationship has yet been demonstrated between H ox genes and other transcription factors involved in determining and/o r maintaining tissue specificity. The thyroid transcription factor-1 ( TTF-1) is a homeodomain-containing protein required for expression of thyroid-specific genes. A 862 bp 5' genomic fragment of the rat TTF-1 gene, conferring thyroid-specific expression to a reporter gene, was s ufficient to mediate transactivation by the human HOXB3 gene in co-tra nsfection assay in both NIH3T3 or HeLa cells. HOXB3 is expressed in ea rly mammalian embryogenesis in the anterior neuroectoderm, branchial a rches and their derivatives, including the area of the thyroid primord ia and thyroid gland, Transcription of the TTF-1 promoter is induced o nly by HOXB3, while its paralogous gene HOXD3 or other Hox genes expre ssed more posteriorly (HOXA4, HOXD4, HOXC5, HOXC6, HOXC8 and Hoxd-8) h ave no effect. Transactivation by HOXB3 is mediated by two binding sit es containing an ATTA core located at -100 and +30 from the transcript ion start site. DNase I footprinting experiments show that the two sit es bind HOXB3 protein synthesized in both Escherichia coli and eukaryo tic cells, as well as nuclear factor(s) present in protein extracts ob tained from mouse embryonic tissues which express group 3 Hox genes an d TTF-1. Some of the DNA-protein complexes formed by the embryonic ext racts are indistinguishable from those generated by HOXB3. These data suggest that HOXB3 might be a transcriptional regulator of the TTF-1 g ene in early embryogenesis, and could therefore participate in the spe cification and development of the thyroid gland.