Normothermic Microwave Irradiation Induces Death of HL-60 Cells through Heat-Independent Apoptosis

Microwaves have been used in various cancer therapies to generate heat and increase tumor cell temperature; however, their use is limited by their side-effects in normal cells and the acquisition of heat resistance. We previously developed a microwave irradiation method that kills cultured cancer cells, including a human promyelomonocytic leukemia (HL-60) cell line, by maintaining a cellular temperature of 37 °C during treatment. In the present study, we investigated the mechanisms underlying HL-60 cell death during this treatment. The microwave-irradiated HL-60 cells appear to undergo caspase-independent apoptosis, whereby DNA fragmentation was induced by mitochondrial dysfunction-related expression of apoptosis-inducing factor (AIF). Caspase-dependent apoptosis was also interrupted by the loss of apoptotic protease-activating factor 1 (Apaf-1) and caspase 9. Moreover, these cells did not exhibit a heat-stress response, as shown by the lack of heat shock protein 70 (HSP70) upregulation. Alternatively, in HL-60 cells heated at 42.5 °C, HSP70 expression was upregulated and a pathway resembling death receptor-induced apoptosis was activated while mitochondrial function was maintained. Collectively, these results suggest that the cell death pathway activated by our 37 °C microwave irradiation method differs from that induced during other heating methods and support the use of normothermic microwave irradiation in clinical cancer treatments.