[Regulation of p21(WAF1/CiP1) expression during cellular differentiation]

In the present study, the molecular mechanisms regulating p 21(WAF1/CiP1) expression by basic helix-loop-helix (bHLH) factors during cellular differentiation were investigated. The cyclin dependent kinase inhibitor p 21 plays crucial roles during differentiation of osteoblasts and myoblasts. In the osteoblastic cell line MG 63, expression of the p 21 gene has been shown to be upregulated by E2A factors, members of the bHLH factor family. In addition, E2A-dependent activation of p21 promoter can be inhibited by another bHLH factor, TWIST. Using reporter assays with mutant p 21 promoters, a novel element was identified in the p21 promoter, which is essential for E2A-dependent activation and TWIST-mediated inhibition. Interestingly, in the myoblastic cell line C2C12, this sequence was not involved in E2A-dependent activation of p21 expression. Gel mobility shift assays showed a specific complex of the novel p21 promoter element with nuclear factor(s) of MG 63 cells. Complex formation was inhibited by the addition of anti-TWIST antibody. In contrast no complexes could be identified with C2C12 cells. These results raise the possibility that interactions of the bHLH factors with the novel p21 promoter element are cell type specific. This suggests a novel mechanism regulating p 21 expression.