Ras stimulates DNA topoisomerase II alpha through MEK: A link between oncogenic signaling and a therapeutic target

Topoisomerase II alpha (topo II alpha) is a major target of antitumor treat ments. In an effort to determine why this protein might be a better target in tumor cells than in normal cells, we attempted to determine if the alter ed proliferative signaling in a tumor cell might effect the levels of expre ssion of the topo II alpha gene. In support of this idea, it was found that topo II alpha was elevated following microinjection of oncogenic Ras prote in. Oncogenic was was further shown to stimulate the topo II alpha promoter . Stimulation by pas was independent of the normal cell cycle regulation of this promoter. Transactivation of topo II alpha by ras required both the M EK/ERK pathway, and the stress-associated protein kinase (SAPK) signaling p athway. As a direct confirmation that both ERK and SAPK were involved in to po II alpha regulation, a constitutively active MEKK that stimulates these two kinases simultaneously was shown to strongly induce topo II alpha promo ter activity. Activation of either pathway alone, on the other hand, only s lightly stimulated the topo II alpha promoter. Deletion analyses showed tha t elements near both the 5' and 3' ends of the promoter were responsible fo r the ras stimulation. Site-directed mutagenesis further demonstrated that an Ets-like binding site near the 5' end (-480 to -475) was one of the resp onsive elements. Taken together, these studies demonstrate the direct role of Ras signaling in stimulation of topo II alpha expression, and thereby es tablish a link between the action of a common tumor mutation and the target of multiple anti-tumor reagents.