REGULATION OF THE HUMAN TOPOISOMERASE II-ALPHA GENE PROMOTER IN CONFLUENCE-ARRESTED CELLS

Expression of DNA topoisomerase II alpha mRNA and protein reflects the proliferative state of mammalian cell lines and tissues with high lev els in actively cycling cells but marked down-regulation during serum deprivation or cell density-induced growth arrest, Using stably integr ated gene fusions comprising the human topoisomerase II alpha promoter with a growth hormone reporter gene, we have localized elements requi red for the differential activity of the topoisomerase II alpha promot er in proliferating and confluence-arrested cells, Deletion analysis l ocalized the region of the promoter that responded to changes in the c ellular growth state to between -101 and -144 base pairs, Mutation ana lysis identified an inverted CCAAT box (ICE) located at -108 to -104 a s necessary for promoter down-regulation in confluence-arrested cells, while several other potential cis-acting elements, including four add itional ICBs, were shown not to be required, The critical ICE was reco gnized in vitro by the CCAAT box binding factor, NF-Y, with levels of binding activity higher in extracts from proliferating cells than from confluence-arrested cells, We conclude that the differential regulati on of topoisomerase II alpha gene expression in cycling and confluence -arrested cells is mediated, at least in part, through proliferation-s pecific binding of factors to an ICE element in the gene promoter.