LAUSR

Hyperthermia (HT) is a non-invasive cancer therapy. Treatment temperature between 41 to 44°C has no cytotoxic damage in normal cells, while the temperature does in cancer cells because of the underdeveloped vascular system. HT often used with other cancer therapy such as radiation-therapy and chemotherapy. However mechanisms of synergistic effects among these therapies remains unclear. The 42°C environment is a cellular mild heat stress generating O2‒ from mitochondrial electron transport chain. We have previously reported that the expression of ATP-binding cassette sub-family G member 2 (ABCG2), which is known as breast cancer resistant protein (BCRP), was suppressed by increasing mitochondrial reactive oxygen species (ROS) to induce cancer specific porphyrin accumulations. Since ABCG2 is a transporter of doxorubicin (DOX), we hypothesized that synergistic effect of HT and chemotherapy may be induced by down-regulation of ABCG2 expression via intracellular ROS increase. In this study, we elucidated whether HT with intracellular ROS increase by HT can enhance the cytotoxic effect of DOX for breast cancer cells. The murine breast cancer cell line, 4T1E was incubated at 37 or 42°C for 1h. Intracellular ROS generation was detected by electron spin resonance (ESR). Cytotoxicity of DOX was measured using the Cell Counting Kit 8. ABCG2 expression was analyzed by Western blotting. ESR signal peak with HT treatment became high as compared to without HT treatment, indicating intracellular ROS level was increased by HT treatment. Cell viability and ABCG2 expression were decreased by DOX exposure and by HT treatment. Thus we conclude that the enhancement of HT treatment effect by DOX is considered to be result of down-regulation of BCRP expression by ROS.