E2F-1 COOPERATES WITH TOPOISOMERASE-II INHIBITION AND DNA-DAMAGE TO SELECTIVELY AUGMENT P53-INDEPENDENT APOPTOSIS

Mutations in the retinoblastoma (pRb) tumor suppressor pathway includi ng its cyclin-cdk regulatory kinases, or cdk inhibitors, are a hallmar k of most cancers and allow unrestrained E2F-1 transcription factor ac tivity, which leads to unregulated G(1)-to-S-phase cell cycle progress ion. Moderate levels of E2F-1 overexpression are tolerated in interleu kin 3 (IL3)-dependent 32D.3 myeloid progenitor cells, yet this induces apoptosis when these cells are deprived of IL-3. However, when E2F ac tivity is augmented by coexpression of its heterodimeric partner, DP-1 , the effects of survival factors are abrogated. To determine whether enforced E2F-1 expression selectively sensitizes cells to cytotoxic ag ents, we examined the effects of chemotherapeutic agents and radiation used in cancer therapy. E2F-1 overexpression in the myeloid cells pre ferentially sensitized cells to apoptosis when they were treated with the topoisomerase II inhibitor etoposide. Although E2F-1 alone induces moderate levels of p53 and treatment with drugs markedly increased p5 3, the deleterious effects of etoposide in E2F-1-overexpressing cells were independent of p53 accumulation. Coexpression of Bcl-2 and E2F-1 in 32D.3 cells protected them from etoposide-mediated apoptosis. Howev er, Bcl-2 also prevented apoptosis of these cells upon exposure to 5-f luorouracil and doxorubicin, which were also cytotoxic for control cel ls. Pretreating E2F-1-expressing cells with ICRF-193, a second topoiso merase II inhibitor that does not damage DNA, protected the cells from etoposide-induced apoptosis. However, ICRF-193 cooperated with DNA-da maging agents to induce apoptosis. Therefore, topoisomerase II inhibit ion and DNA damage can cooperate to selectively induce p53-independent apoptosis in cells that have unregulated E2F-1 activity resulting fro m mutations in the pRb pathway.