Identification of Promoter Elements Responsible for Transcriptional Inhibition of Polo-like Kinase 1 and Topoisomerase IIα Genes by p21WAF1/CIP1/SDI1

Induction of p21(WAF1/CIP1/SDI1), a physiological mediator of cell cycle arrest, inhibits multiple genes involved in cell division. We have investigated the determinants of p21-mediated inhibition of two of these genes, polo-like kinase 1 (PLK1) and topoisomerase IIa (TOPO II alpha). p21 expression from an inducible promoter in human HT1080 cells rapidly decreases cellular levels of PLK1 and TOPO II alpha RNA without decreasing their RNA stability. p21 also inhibits reporter gene expression from the PLK1 and TOPO II alpha promoters in transient and stable transfection assays. Promoter mutagenesis studies show that inhibition of the PLK1 promoter by p21 is mediated in part by tandem sequences CDE (cell cycle-dependent element) and CHR (cell cycle genes homology region). p21 response of the TOPO II alpha promoter was found to be mediated through CDE (but not CHR) and the inverted CCAAT box 1 (ICB1). The extent of PLK1 and TOPO II alpha promoter inhibition and the effects of promoter mutations differ under the conditions of growth arrest produced by p21 induction or by mimosine, a cell cycle inhibitor that increases p21 RNA but not protein expression in HT1080 cells. These results indicate that inhibition of cell division-associated genes by p21 is mediated by different but overlapping mechanisms, which are not a general consequence of cell cycle arrest.