Increased expression of death receptors 4 and 5 synergizes the apoptosis response to combined treatment with etoposide and TRAIL

Chemotherapeutic genotoxins induce apoptosis in epithelial-cell-derived can cer cells. The death receptor ligand TRAIL also induces apoptosis in epithe lial-cell-derived cancer cells but generally fails to induce apoptosis in n ontransformed cells. We show here that the-treatment of four different epit helial cell lines with the topoisomerase II inhibitor etoposide in combinat ion with TRAIL (tumor necrosis factor [TNF]-related apoptosis-inducing liga nd) induces a synergistic apoptotic response. The mechanism of the synergis tic effect results from the etoposide-mediated increase in the expression o f the death receptors 4 (DR4) and 5 (DR5). Inhibition of NF-kappa B activat ion by expression of kinase-inactive MEK kinase 1(MEKK1) or dominant-negati ve I kappa B (Delta I kappa B) blocked the increase in DR4 and DR5 expressi on following etoposide treatment. Addition of a soluble decoy DR4 fusion pr otein (DR4:Fc) to cell cultures reduced the amount of etoposide-induced apo ptosis in a dose-dependent manner. The addition of a soluble TNF decoy rece ptor (TNFR:Fc) was without effect, demonstrating the specificity of DR4 bin ding ligands in the etoposide-induced apoptosis response. Thus, genotoxin t reatment in combination with TRAIL is an effective inducer of epithelial-ce ll derived tumor cell apoptosis relative to either treatment alone.