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

Abstract

Induction of p21WAF1/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 IIa). p21 expression from an inducible promoter in human HT1080 cells rapidly decreases cellular levels of PLK1 and TOPO IIa RNA without decreasing their RNA stability. p21 also inhibits reporter gene expression from the PLK1 and TOPO IIa 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 IIa promoter was found to be mediated through CDE (but not CHR) and the inverted CCAAT box 1 (ICB1). The extent of PLK1 and TOPO IIa 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. Key Words p21WAF1/CIP1/SDI1, Polo-like Kinase, Topoisomerase II, Regulation of Transcription, CDE, CHR, ICB