Transcriptional repression of p21((Waf1/Cip1/Sdi1)) gene by c-jun through Sp1 site

Previously, we found that c-jun represses the tumor suppressor p21((Waf1/Ci p1/Sdi1)) (p21) gene expression. In this study, we further investigated the mechanism of the inhibitory effect of c-jun on p21. After analysis of a se ries of deletion and point mutants of p21 promoter, we found that Sp1-3 sit e (-77 and -83) relative to the transcription start site played an importan t role for c-jun-repressing-responsive element in the p21 promoter. Both Sp 1 and Sp3 transcription factors were the key factors for this event. Howeve r, the data from electrophoretic mobility shift assay indicated that c-jun did not change the Sp1 DNA-binding affinity, suggesting that additional fac tors may be involved in the repression of p21 by c-jun. Furthermore, c-jun could inhibit butyrate-inducing p21 gene expression through Sp1, indicating at least one common pathway whereby p21 expression is affected by c-jun an d butyrate in opposing actions. Moreover, the hyperphosphorylated retinobla stoma protein (Rb) increased in c-jun expressing cells, indicating that pho sphorylated Rb may play a role in regulating Sp1 to repress p21 expression. This is the first demonstration of how housekeeping factors and oncogene p roduct counteract the function of tumor suppressor genes to control cell cy cle progression. (C) 2000 Academic Press.