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.