Abstract
Radioresistance is thought to be caused by intrinsic characteristics of cancer stem cells (CSCs) due to their unique DNA damage response (DDR), efficient DNA repair and resistance to apoptosis. Repopulation of surviving CSCs is associated with recurrence of tumors after radiotherapy (RT). Therefore, eradication of CSCs is important to improve the efficacy of RT.
We have established long-term fractionated radiation (FR) cells from HepG2 and A172 by exposure of X-rays every 12 hours for 82 days (82FR-31NR cells). 82FR-31NR cells exhibited CSC like properties, such as overexpression of CSC marker CD133 and the ABC transporter (MDR1 and BCRP1). Furthermore, 82FR-31NR cells have the tumotigenic capacity when the lower number of cells was implanted into nude mice. Thus, we successfully isolated CSCs from HepG2 and A172 by repeated FR for a long-time. In 82FR-31NR cells, the AKT pathway was activated after 5Gy-irradiation but not in parental cells. Inhibition of the AKT pathway by either an AKT inhibitor, API-2 or a Cdk4 inhibitor could be suppressed radioresistance of 82FR-31NR cells. Similarly, knockdown of cyclin D1 or Cdk4 by its siRNA was sufficient to suppress radioresistance of 82FR-31NR cells. These results demonstrated that the AKT/cyclin D1/Cdk4 pathway is important for radioresistance of CSCs.
In conclusion, we would present the newly discovered DDR of CSCs, and propose a strategy for eradication of CSCs using fractionated RT combined with the AKT inhibitor.
