C-MYB TRANS-ACTIVATES THE HUMAN DNA TOPOISOMERASE II-ALPHA GENE PROMOTER

DNA topoisomerase II alpha (topo II alpha) is an essential proliferati on-dependent nuclear enzyme which has been exploited as an anti-tumor drug target. Since the proliferative status of human leukemia cells is associated with expression of the c-myb proto oncogene, c-Myb was inv estigated as a trans-activator of the topo II alpha gene, Using topo I I alpha promoter-luciferase reporter plasmids, c-myb expression caused trans-activation of the topo II alpha promoter a maximum of similar t o 4.5-fold over basal levels in HL-60 human promyelocytic leukemia cel ls. Trans-activation was submaximal with higher levels of c-myb expres sion plasmid but a Myb protein lacking its negative regulatory domain resulted in similar to 19-fold trans-activation. Mutagenesis and 5'-de letion studies revealed that Myb trans-activation was mediated via a M yb-binding site at positions -16 to -11 and that this region governed the bulk of basal topo II alpha promoter activity in human leukemia ce lls, Trans activation of topo II alpha by c-Myb was lymphoid- or myelo id-dependent, However, B-Myb, a more widely-expressed Myb family membe r, caused topo II alpha trans-activation in both HL-60 cells and HeLa epithelial cervical carcinoma cells. These data provide evidence for a new Myb-responsive gene which is directly linked to and required for cellular proliferation.