Ethanol increases sensitivity of human myeloid U937 cells to hyperthermia - induced Apoptosis

Abstract

Hyperthermia is a therapeutic approach which has emerged as a potent cancer treatment combined with radiotherapy or chemotherapy and, therefore, the search of chemosensitizers is highly valuable to improve the antitumor properties of hyperthermia. Previous studies suggest that ethanol may increase sensitivity of cells to some chemotherapeutic agents. In the present study, the effect of low concentrations of ethanol (0.25%-1%) in combination with mild hyperthermia (43 ºC for 30 min) was investigated. The human histiocytic lymphoma cell line U937 was selected since it is a widely used model for the study of hyperthermia-induced cell death. The results indicate that ethanol enhances cytotoxicity of hyperthermia, as revealed by the MTT assay and by the trypan blue exclusion procedure. No evidences of cytotoxicity were appreciated in the cells treated only with ethanol. In accordance with these results, the flow cytometric studies showed an increase in the percentage of apoptotic cells (2- to 4-fold) in the group treated with ethanol plus hyperthermia as compared to the cells exposed only to hyperthermia. The effect of ethanol was associated with an increase in the activity of initiator caspases -8 and -9 and also in the executioner caspase-3. Inhibition of caspases activities using the broad-spectrum inhibitor z-VAD-fmk blocked the effect of ethanol. A main role seems to play caspase-8 since its specific inhibitor zIETD-fmk but not the specific caspase-9 inhibitor zLEHD-fmk almost completely abolished apoptosis triggered by hyperthermia or hyperthermia plus ethanol. In conclusion, ethanol may be an effective strategy to augment sensitivity of the cells to hyperthermia-induced apoptosis.