Cellular effects of low-intensity pulsed ultrasound and X-irradiation in combination in two human leukaemia cell lines

Abstract

Previously, we have shown that a combination between X-irradiation and low-intensity pulsed ultrasound (US) could synergistically suppress cell survival post exposure (Buldakov et al., 2014). In this study, the cellular effects underlying the enhanced cell killing are investigated. U937 and Molt-4 cell lines were exposed to 1.0MHz US with 50% duty factor at 0.3W/cm2 and pulsed at 1, 5 and 10Hz immediately after exposure to X-rays at 0, 0.5, 2.5 and 5Gy. The cells were assayed at different time points to depict the major cellular events that culminated in cell death. For instance, membrane damage and cell lysis were estimated immediately following exposure and 24h later. Intracellular reactive oxygen species (ROS) were also determined flow cytometrically after treatment. Moreover, the extent of DNA damage and cell cycle progression were determined at 6 and 24h, respectively. Despite the general trend for synergism, there was a disproportionation of mediating factors depending on the cell type and its specific biological makeup. Immediately, US could induce appreciable necrotic cell death through extensive membrane damage in U937 but induced cell lysis in Molt-4 cells. ROS might have contributed to cell killing in Molt-4 but not in U937 cells. Although both of the physical modalities are significantly DNA-damaging alone, no additional damage was observed in combination. Moreover, override in some arrested cell cycle phases was also observed following combination. Collectively, the interaction between X-rays and US seems to depend mainly on the acoustic environment determined by the setup and this might explain the contradictory data among reports.