RREB-1, A NOVEL ZINC-FINGER PROTEIN, IS INVOLVED IN THE DIFFERENTIATION RESPONSE TO RAS IN HUMAN MEDULLARY-THYROID CARCINOMAS

An activated ms oncogene induces a program of differentiation in the h uman medullary thyroid cancer cell line TT. This differentiation proce ss is accompanied by a marked increase in the transcription of the hum an calcitonin (CT) gene. We have localized a unique Ras-responsive tra nscriptional element (RRE) in the CT gene promoter. DNase I protection indicates two domains of protein-DNA interaction, and each domain sep arately can confer Ras-mediated transcriptional inducibility. This bip artite RRE was also found to be Raf responsive. By affinity screening, we have cloned a cDNA coding for a zinc finger transcription factor ( RREB-1) that binds to the distal RRE. The consensus binding site for t his factor is CCCCAAACCACCCC. RREB-1 is expressed ubiquitously in huma n tissues outside the adult brain. Overexpression of RREB-1 protein in TT cells confers the ability to mediate increased transactivation of the CT gene promoter-reporter construct during Ras- or Raf-induced dif ferentiation. These data suggest that RREB-1 may play a role in Ras an d Raf signal transduction in medullary thyroid cancer and other cells.