Background: Tumor microenvironment is an important factor influencing radiosensitivity of cancer cells. Amongst multiple soluble factors released by tumor-associated stromal cells, chemokine CXCL12 was shown to confer pro-survival effects on cancer cells expressing chemokine receptor 4 (CXCR4). It has been recently shown that stromal microenvironment protects prostate cancer cells from chemotherapy-induced cell death via CXCR4 signaling, which was reversed by AMD3100, a specific CXCR4 inhibitor. In this study, AMD3100 was tested for its radiosensitizing capacity in prostate cancer. Methods: Both short-term (72 h) and long-term survival (17 days) of PC3 prostate cancer cells in the absence or presence of murine stromal cells (MS5) after increasing doses of gamma-irradiation (0-15 Gy) was assessed. To analyze the short-term effect of stromal cells on PC3 viability after radiation, fluorescently labeled PC3 cells were cultured with or without MS5 cells and exposed to gamma-irradiation (0-15 Gy) in the presence or absence of 25 µg/ml AMD3100. DAPI nuclear staining was used to assess the viability of PC3 after 72 hours, Long-term survival of PC3 cells was evaluated by clonogenic assay of PC3 cells seeded alone or on feeder layer of MS5-GFP stromal cells and irradiated with 0-15 Gy. Results: The short-term viability assay showed that PC3 in mono-culture show dose-dependent loss of viability after increasing doses of irradiation, showing only 18±6% (mean ± SD) viability when exposed to 15 Gy as compared to non-irradiated control cells. However, in the presence of stromal cells, viability of PC3 cells exposed to the same radiation dose remained high, reaching levels of 69±2% viability (P=0.004 vs 15 Gy in monoculture). This effect was potently reversed by AMD3100, which lowered viability of cells to 46±3% (P=0.001 vs 15 Gy in co-culture without AMD3100). Importantly, addition of AMD3100 to PC3 cultured in the absence of stromal cells did not change their viability after radiation. The long-term survival measured in clonogenic assay showed that in presence of MS5-GFP murine stromal cell line, 8% PC3-luc cells survived 6 Gy gamma-radiation dose, as compared to only 3.6% of PC3 cells that were cultured without the stromal cells (P=0.03). Conclusion: Murine stromal cells protect PC3 prostate cancer cells against irradiation-induced cell death in an in vitro co-culture system. Treatment with AMD3100 potently inhibits the protective effect of stromal cells on the prostate cancer cells exposed to gamma-irradiation. These data underscore the relevance of stromal microenvironment in tumor response to irradiation and warrant further research on the role of CXCR4 inhibitors as a therapeutic option to potentiate radiotherapy.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 1511. doi:1538-7445.AM2012-1511