RB regulates transcription of the p21/WAF1/CIP1 gene

We have previously shown that RB plays an important role in the maintenance of the epithelial phenotype, p21 is also involved in several terminal diff erentiation systems including keratinocytes. We report here that p21 is an RB target gene in epithelial cells, but not in fibroblasts where RB is unab le to transactivate p21 transcriptional expression. In epithelial cells, wh en RB family factors were inactivated by SV40 T antigen (LT), p21 expressio n was strongly repressed, whereas its expression was not affected when the cells were transformed by a mutated LT leaving RB active but inactivating p 53. Moreover, retransformation by RB of LT transformed epithelial cells tot ally restored p21 expression. By cotransfection experiments and using delet ions and point mutations of the p21 promoter, we show that the minimal regi on required for the RB-mediated transcriptional activation maps to a GC-ric h region located between -83 and -74. This region is shown to interact spec ifically with the transcription factor Sp1 and Sp3. Thus for the first time , we show a positive transcriptional relationship between RB and p21 in epi thelial cells. Since p21 keeps RB in a hypophosphorylated state important f or its transcriptional activity during differentiation, our results imply a n auto-loop of regulation between RB and p21 that may be essential for the maintenance of the differentiation state. We propose that this transcriptio nal relationship might be necessary of their roles in cell cycle arrest and in several differentiation pathways.