Enhanced Thermo-Radiosensitization of Tumor Cells through Suppression of the Transcriptional Stress Response by Inhibiting HSF1 Activity or Expression

Hyperthermia is used in combination with radiation therapy to enhance the radiation response of the target tumor. However, heating of cancer cells activates the HSF1 transcription factor in them and stimulates the HSF1-dependent induction of heat shock proteins (HSPs) that can significantly impair the antitumor effects of hyperthermia and radiation exposure. The aim of this study was to examine the possibility of enhancing the radiosensitizing effect of hyperthermia on cancer cells by suppressing the HSF1-mediated HSP induction in them. The object of the study was HeLa cells derived from a malignant tumor of the human cervix. The cells were subjected to heat stress (42–44°C for 20–60 min) before irradiation (2–7 Gy) without or in the presence of HSF1 transcriptional activity inhibitors (quercetin, triptolide, and KRIBB11). In certain cell samples, HSF1 expression was preliminarily knocked down using small interfering RNAs. Cell death and survival was assessed by the levels of apoptosis/necrosis and clonogenic ability. Expression of HSF1 and HSP was analyzed by immunoblotting. It was found that, compared with the radiosensitizing effects of hyperthermia alone, the combined treatment (HSF1 activity inhibition or HSF1 knockdown + heating) significantly increased the thermo-radiosensitization of cancer cells; which was manifested in the intensification of their post-radiation death (apoptosis + necrosis) and in a decrease in clonogenicity. This enhancement of thermo-radiosensitization was observed under the HSP induction blockade. Thus, the combination of hyperthermia with inhibitors of HSF1 activity or expression can effectively sensitize thermoresistant and radioresistant tumors to radiation therapy.