Evidence from experimental and clinical

Evidence from experimental and clinical studies indicates that cancer-initiating cells, or cancer stem ARQ197 (CSC), are resistant to radiotherapy [135]. In this context, CD44 expression has the potential to predict the outcome of radiotherapy assessed by CSC density [135]. Enhanced CSC phenotypes are associated with the activation of the PI3K/Akt/mTOR pathway in radioresistant prostate cancer [136]. Moreover, it has been demonstrated that the expression of Akt1 and Akt2 increased after irradiation in MCF-7 mammosphere CD24(−/low)/CD44(+) expressing cells, but not in the bulk population of MCF-7 CD24(+)/CD44(+) expressing cells [137]. In this study [137], targeting of Akt sensitized MCF-7 mammosphere cells, but not MCF-7 monolayer cells to ionizing radiation. Thus, it seems that Akt isoforms, through an as yet unknown mechanism, are involved in radioresistance in CSCs. In this context, varying effects of Akt isoforms on the expression of the cancer stem cell markers CD133 and CD44 [138], in association with radioresistance in tumors of different origin [139] and [140], have been reported. The role of Akt activity in radioresistance of CSCs has been demonstrated in both mammospheres in vitro and in vivo [141]. Zhang et al. [141] reported that inhibition of the Akt pathway selectively inhibited canonical Wnt signaling as well as repair of DNA damage in cancer initiating cells and sensitized them to ionizing radiation in vitro and in vivo [141]. At least for Akt1, it has been described that it phosphorylates stem cell marker Oct4 at threonine 235 in embryonal carcinoma cells [142]. Phosphorylated Oct4 increases its stability and facilitated Oct4 nuclear localization. In the nucleus, Oct4 interacts with Sox2, which promotes the transcription of the core stemness genes POU5F1 and NANOG [142]. With respect to the described functional role of the PI3K/Akt pathway in CSCs [129] and [143] and the role of cancer stem cells in radiotherapy outcomes [135], inhibition of the Akt pathway might offer an improved response to radiation in CSCs [137], [141] and [144]. Based on the assumption that CSCs are more radioresistant and that they present Akt pathway hyperactivation, our laboratory has shown that a selected radioresistant subpopulations of NSCLC-A549 cells that present the CSC marker ALDH1 can be radiosensitized by the PI3K inhibitor LY294002 [145]. The importance of PI3K/Akt signaling to radioresistance in CSCs is also underlined by data indicating the Akt-dependence of accelerated repair of radiation-induced DNA-DSB [141], especially in brain tumor stem cells [146]. Thus, regulation of tumor cell stemness might be one of the mechanisms by which Akt can promote the survival and tumorigenicity.