MYC regulation of CHK1 and CHK2 promotes radioresistance in a stem cell-like population of nasopharyngeal carcinoma cells

Abstract

Radiotherapy is the most successful nonsurgical treatment for nasopharyngeal carcinoma (NPC). Despite this, the prognosis remains poor. Although NPCs initially respond well to a full course of radiation, recurrence is frequent. The cancer stem cell (CSC) hypothesis provides a framework for explaining the discrepancy between the response of NPC to therapy and the poor survival rate. In this study, a stem cell-like subpopulation (PKH26+) was identified in NPC cell lines using a label retention technique. PKH26+ cells were enriched for clonogenicity, sphere-formation, side-population cells, and resistance to radiotherapy. Using genomic approaches, we show that the proto-oncogene c-Myc (MYC) regulates radio-tolerance through transcriptional activation of Chk1 (CHEK1) and Chk2 (CHEK2) checkpoint kinases through direct binding to the Chk1 and Chk2 promoters. Overexpression of c-Myc in the PKH26+ subpopulation leads to increased expression of Chk1 and Chk2 and subsequent activation of the DNA damage checkpoint response, resulting in radioresistance. Furthermore, loss of Chk1 and Chk2 expression reverses radioresistance in PKH26+ (c-Myc high expression) cells in vitro and in vivo. This study elucidates the role of the c-Myc-Chk1/Chk2 axis in regulating DNA damage checkpoint responses and stem cell characteristics in the PKH26+ subpopulation. Furthermore, these data reveal a potential therapeutic application in reversal of radioresistance through inhibition of the c-Myc-Chk1/Chk2 pathway.