Radiation-induced glucocorticoid receptor promotes CD44+ prostate cancer stem cell growth through activation of SGK1-Wnt/β-catenin signaling.

We observed cancer stem cell (CSC) population increase in radioresistant LNCaP (LNCaPR18) and C4-2 (C4-2R26) prostate cancer (PCa) cells compared with respective parental cells. Since the CD44 level increase was most significant in radioresistant PCa cells compared with parental cells among CSC markers tested, we isolated the CD44+ population from LNCaP/LNCaPR18 and C4-2/C4-2R26 cell sets via the immunomagnetic separation method and used them as CSC sources. We detected lower AR level, but higher glucocorticoid receptor (GR) level in CD44+ CSCs than CD44- non-CSCs. Higher GR level in CD44+ CSCs than CD44- cells was also detected when cells were isolated from mouse tumor tissues of LNCaPR18 cell and C4-2R26 cell-derived human xenografts and grown in culture. We then found blocking the GR signaling by adding the anti-GR agent mifepristone into the cell culture inhibited the CD44+ CSC growth while the addition of the anti-AR agent enzalutamide enhanced the CSC growth. In xenograft mouse studies in which tumors were developed from the injection of CD44+ CSCs of LNCaPR18 or C4-2R26 cell lines, retarded tumor growth in mifepristone-injected mice was observed compared with vehicle-treated mice. We next discovered the GR regulation of Wnt/β-catenin signaling pathway. We further found that the serum/glucocorticoid regulated kinase 1 (SGK1) is the GR downstream molecule that mediates Wnt/β-catenin signaling activation. Therefore, inhibition of either SGK1 or Wnt/β-catenin signaling impaired the in vitro CD44+ CSC growth. From these results, we suggest that blocking GR signaling or its downstream SGK1-Wnt/β-catenin signaling axis may suppress the radiation-induced CSC increase in PCa. KEY MESSAGES: Higher CSC population exists in radioresistant PCa cells than parental cells. Higher GR levels (and lower AR level) in CD44+ CSCs than CD44- non-CSCs. Use of anti-GR agent blocked the growth of CD44+ CSCs in in vitro/in vivo tests. GR downstream SGK1-Wnt/β-catenin signaling axis mediates the CSC increase. Targeting this signaling axis may enhance the radiotherapy efficacy in treating PCa.