Abstract 3047: Hypoxia leads to deregulation of PI3K/AKT/mTOR signaling in prostate cancer stem cells

Abstract

Abstract Tumor initiating subpopulations of cancer cells, also known as cancer progenitor or cancer stem cells (CSCs) were recently identified and characterized in prostate cancer. Emerging evidence suggests that hypoxia-inducible factor 1 alpha (HIF1A) contributes to CSC maintenance and influences the PI3K/AKT/mTOR axis, a pathway which is frequently altered in prostate cancer. In our study, we assessed the effects of hypoxia on PI3K/AKT/mTOR signaling in a subset of basal prostate cancer stem cells in a murine model of prostate cancer. First, we successfully isolated Lin-/Sca-1+/CD49f+ basal stem cell subpopulations from primary prostate tumors of TRAMP (transgenic adenocarcinoma of mouse prostate) mice. We observed a consistent decrease in S6 phosphorylation within the CSC subpopulation, in agreement with an mTOR pathway inhibition. However, AKT phosphorylation in the CSC subpopulation was simultaneously elevated, thus prompting further investigations. To analyze the molecular pathways leading to this deregulation, we used a cell line model and isolated Sca-1+/CD49f+ CSC-like subpopulations from the murine prostate cancer cell line TRAMP-C1 and CD44+/CD49f+ CSC-like subpopulations from the androgen independent human prostate cancer cell line DU145. We used sphere formation assays and expression of Notch 1 and Oct 3/4 to confirm the stem and progenitor cell properties of the sorted subpopulations. We could confirm the decrease in mTOR/S6 signaling and identify the upregulation of AKT through IRS-1 mediated feedback loop in prostate CSC-like cells under hypoxic conditions. This AKT/mTOR deregulation was promptly reversed in normoxia, suggestive of the involvement of hypoxia inducible factors. We found elevated HIF1A protein levels as well as increased target gene expression in CSC subpopulations in vitro and in vivo. Further, lentiviral knockdown of HIF1A restored AKT/mTOR activity in CSC-like cells. We could show that CSC-like prostate cancer cells with elevated expression of basal stem cell markers, such as CD44, Sca-1 or CD49f, show a hypoxic deactivation of feedback inhibition on AKT signaling through IRS-1. In light of these results, we propose that deregulation of the PI3K/AKT/mTOR pathway via HIF1A may be critical for cancer stem cell quiescence and maintenance, slowing down cancer stem cell metabolism and growth via mTOR and promoting their survival via AKT signaling. Citation Format: Maximilian Marhold, Erwin Tomasich, Zuzana Pernicová, Radek Fedr, Karel Soucek, Andreas Spittler, Michael Krainer, Peter Horak. Hypoxia leads to deregulation of PI3K/AKT/mTOR signaling in prostate cancer stem cells. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 3047. doi:10.1158/1538-7445.AM2014-3047