Abstract
The combined effects of hyperthermia or radiation and a new class of furan-fused tetracyclic synthesized compounds DFs on apoptosis in human myelomonocytic lymphoma U937 cells were investigated. Among the tested compounds, the combination treatment of DF3 (10 microM) and hyperthermia showed the greatest potency to induce DNA fragmentation. Enhancement of hyperthermia-induced apoptosis by DF3 in a dose-dependent manner was observed. When the cells were treated with DF3 at a nontoxic concentration of 20 microM, and then exposed to hyperthermia, significant enhancement of heat-induced apoptosis was determined by DNA fragmentation, nuclear morphological change and phosphatidylserine externalization. The Bid cleavage was detected, though no significant changes in Bax and Bcl-2 expression were observed after the combined treatment. The release of cytochrome c from mitochondria to cytosol, which was induced by hyperthermia, was enhanced by DF3. Mitochondrial transmembrane potential was decreased and the activation of caspase-3 and caspase-8 was enhanced in the cells treated with the combination. Flow cytometry using hydroethidine revealed rapid and sustained increase of intracellular superoxide due to DF3, and that using DCFH-DA showed subsequent and transient increase in the formation of intracellular peroxide, which was further increased when hyperthermia was combined. These results indicate the intracellular peroxide generated by DF3 enhances heat-induced apoptosis via the mitochondria-mediated caspase-dependent pathway.
