Sp1 and Sp3 transactivate the RET proto-oncogene promoter

The RET proto-oncogene plays an important role in the initiation and progre ssion of tumors derived from the neural crest. The cis-regulatory elements responsible for RET basal promoter activity have not been identified. To ch aracterize these elements, a RET promoter DNA fragment (-453 to +227 bp) wa s fused to a luciferase reporter and introduced into TT, a neural crest-der ived cell line. Sequential 5' deletions of the promoter revealed that optim al expression of the RET promoter in TT cells required only 70 bp of sequen ce upstream of the transcription start site, and contains two Spl binding s ites. DNase I footprinting, electrophoretic mobility shift analysis (EMSA), and supershift assays revealed that this region binds both Spl and its rel ated protein, Sp3. Additionally, RET basal promoter activity was abrogated by removal of these Sp1/Sp3 binding sites. The proximal two GC boxes were s ufficient to allow transactivation of the RET promoter in Drosophila SL2 ce lls. Sp3 expression in these cells caused an additional activation of the p romoter. These results demonstrate that the transactivation of the RET prom oter within a neural crest-derived cell line is dependent on Sp1 and Sp3. ( C) 2000 Elsevier Science B.V. All rights reserved.