Sp1 and Sp3 activate p21 (WAF1/CIP1) gene transcription in the Caco-2 colon adenocarcinoma cell line

The CDK inhibitor p21(WAF1/CIP1) is a negative regulator of the cell cycle, and its expression is induced during terminal differentiation in vitro and in vivo. Expression of p21 is controlled at the transcriptional level by b oth p53-dependent and -independent mechanisms. Our previous studies establi shed that p21 is expressed in the Caco-2 adenocarcinoma cell line, and its expression is induced by a p53-independent mechanism during differentiation of these cells, Here we have found that transcription of p21 in Caco-2 cel ls is controlled primarily by the transcription factors Sp1 and Sp3 through two Spl binding sites, Sp1-1 and Sp1-2 located between -119 and -114 bp an d between -109 and -104 bp of the p21 promoter, respectively, Spl and Sp3 b inding to the p21 promoter increased during Caco-2 cell differentiation, wh ile the absolute level of Spl did not change and the absolute level of Sp3 increased approximately twofold. Transfection experiments in the SL2 Drosop hila cell line that lacks endogenous Sp3 activity demonstrated that Spl tra nsactivates the p21 promoter primarily through the Sp1-2 site, while Sp3 ac ts through the Spl-l site, In these cells Sp3 is a stronger transactivator of the p21 promoter than Spl, Our data suggest that induction of p21 transc ription during Caco-2 differentiation is modulated by Sp1/Sp3 interactions with the p21 promoter.