A novel evolutionarily conserved element is a general transcriptional repressor of p21WAF¹/CIP

The effective induction of p21(WAF1/CIP1/Cdkn1a) (p21) expression in p53-negative cancer cells is an important avenue in cancer management. We investigated the ability of various common chemotherapeutic drugs to induce p21 expression in p53-negative cancer cells and showed that the induction of p21 expression by oxaliplatin is caused by the derepression of a previously unrecognized negative regulatory element with a Sp1/Sp3 palindrome sequence core at -216 to -236 of the p21 proximal promoter. Electrophoretic mobility shift and antibody super-shift assays confirmed the specific binding of Sp1/Sp3, and showed that the oxaliplatin-mediated derepression of p21 transcription was associated with an increased Sp1/Sp3 phosphorylation and binding affinity to the oxaliplatin-responsive element. A search of the ENCODE database for vertebrate-conserved genomic elements identified the Sp1/Sp3 palindrome element as the only vertebrate-conserved element within the 500-bp proximal p21 promoter region, indicating its fundamental importance. In in vivo competition assays, transfected synthetic Sp1/Sp3 palindrome elements derepressed the cotransfected or endogenous p21 promoter in a dosage-dependent manner. This derepression was not seen in oxaliplatin-treated cells, suggesting that the exogenous Sp1/Sp3 palindrome and oxaliplatin had the same downstream signaling target. Taken together, our results revealed, for the first time, this evolutionarily conserved Sp1/Sp3 palindrome element in the proximal p21 promoter that serves as a regulatory repressor to maintain p21 basal level expression.