| Radiotherapy is a primary treatment modality for prostate cancer but its efficacy is limited by the recurrence of resistant tumors. Cancer stem cells (CSCs), also known as tumor initiating cells, are a small subset of tumor cells that are responsible for transplantability of tumors, but its role in tumor recurrence after treatment remains unproven. To investigate the biology of CSCs in tumor recurrence after radiotherapy, tumor cells were irradiated in vitro and then injected s.c. into mice to model tumor recurrence from radioresistant tumor cells. It was found that DU145 cells, after irradiation at 800 cGy, were still able to form palpable tumors but the growth of tumors was significantly compromised. To investigate whether CSCs survived from radiation and gave rise to radioresistant tumors, we evaluated the presence of CD133 antigen, a putative marker for stem cells, in the recurring tumors by immunohistochemistry. Most cells were negative for CD133 antigen in tumors derived from sham-irradiated cells. In the small recurrent tumors derived from irradiated cells, most cells were positive for CD133 antigens, but when the tumors grew bigger, the CD133 positivity was reduced. To further study the role of CSCs in radioresistance, we enriched and expanded CSCs from DU145 or LNCaP cells as prostaspheres. The cells in prostaspheres retained a high percentage of CD133(+) cells and were able to differentiate in serum-rich media and form tumors when transplanted into mice. When compared to isogenic parental DU145 cells, the CSCs isolated from DU145 presented a higher resistance toward radiation at 200 and 800 cGy as indicated by colony formation assay. CSCs isolated from LNCaP cells presented a much higher resistance toward radiation at 800 and 2,000 cGy than parental LNCaP cells. Taken together, our data suggest that prostate CSCs are inherently more resistant to radiotherapy than non-CSCs and surviving CSCs may lead to recurrence of tumors after radiotherapy. |
