Hyperthermia Sensitizes Breast Cancer Stem Cells to Radiation Therapy.

Abstract

Abstract Background In cancer, metastatic disease and recurrence are hypothesized to result, at least in part, from the residual cancer stem cells (CSCs), also referred to as tumor-initiating cells(TICs), which evade initial treatment. In several genetically engineered mouse models that resemble human breast cancer, the tumor subpopulation CD29+/CD24+/lin- has been shown to be more tumorigenic as compared to other subpopulations by in vitro and in vivo assays. We and others have demonstrated that the tumorigenic CD29+/CD24+/lin- subpopulation repairs radiation-induced DNA strand breaks more efficiently than the other subpopulations. Therefore, we hypothesized that hyperthermia, a documented means of inhibiting DNA repair, may radiosensitize CSCs to radiation therapy(RT) by delaying and inhibiting the repair of DNA strand breaks. Methods To test this hypothesis, we assayed for tumorigenic CD29+/CD24+/lin- cells in p53null mouse mammary tumor model with and without radiation, and in combination with hyperthermia (42°C) administered as optically-activated gold nanoparticles. We then determined if there was a decrease in functional CSCs by FACS analysis and limiting dilution transplantation assays. Results These studies confirmed that CD29+/CD24+/lin- cells were more radiation resistant and that their relative proportion was increased after RT, as compared to the other cell populations. However, the addition of hyperthermia increased the sensitivity of the CD29+/CD24+/lin- subpopulation to radiation. Radiation treatment alone in vivo resulted in a decrease in tumor size, but the percentage of tumorigenic cells increased as compared to untreated controls. In contrast, following post-treatment with 20 minutes of local hyperthermia, using intravenously administered optically activated gold nanoparticles, a larger regression in tumor size was observed, without a concomitant increase in the percentage of CD29+/CD24+/lin- tumorigenic cells. Using the functional limiting dilution transplantation assay to assay for CSCs, we showed that 48 hours after treatment cells derived from tumors treated with RT exhibited an increased frequency of TICs as compared to untreated controls, and tumors treated with radiation and hyperthermia. In addition, clonogenic survival assays of this tumorigenic subpopulation had consistent results with the in vivo data. Conclusion These results suggest that localized hyperthermia serves as a simple strategy to selectively sensitize the radioresistant subpopulation of CSCs to enhance their response to radiation therapy. The mechanisms responsible for these effects of hyperthermia are currently under investigation. Supported by grant NCI R01 CA112305 and CA16303. Citation Information: Cancer Res 2009;69(24 Suppl):Abstract nr 506.