For more than a century oxygen has been known to be one of the most powerful radiosensitizers. However, despite decades of preclinical and clinical research aimed at overcoming tumor hypoxia, little clinical progress has been made so far. Ionizing radiation damages DNA through generation of free radicals. In the presence of oxygen these lesions are chemically modified, and thus harder to repair while hypoxia protects cells from radiation (Oxygen enhancement ratio (OER)). Breast cancer stem cells (BSCSs) are protected from radiation by high levels of free radical scavengers even in the presence of oxygen. This led us to hypothesize that BCSCs exhibit an OER of 1. Using four established breast cancer cell lines (MCF-7, T47D, MDA-MB-231, SUM159PT) and primary breast cancer samples, we determined the number of BCSCs using cancer stem cell markers (ALDH1, low proteasome activity), compared radiation clonogenic survival and mammosphere formation under normoxic and hypoxic conditions, and correlated these results to the expression levels of key members of the free radical scavenging systems. The number of BCSCs increased with increased aggressiveness of the cancer. This correlated with increased radioresistance (SF8Gy), and decreasing OERs. When cultured as mammospheres, breast cancer cell lines and primary samples were highly radioresistant and not further protected by hypoxia (OER~1). We conclude that because BCSCs are protected from radiation through high expression levels of free radical scavengers, hypoxia does not lead to additional radioprotection of BCSCs.
References
[1]
Schwarz G (1906) Ueber Desensibilisierung gegen Roentgen- und Radiumstrahlen. Muenchner Medizinische Wochenschrift 56: 1217–1218.
[2]
Konerding MA, Steinberg F, Budach V, Steffens C (1989) Scanning electron microscopic studies on the vascular system of xenotransplanted tumours. Strahlentherapie und Onkologie: Organ der Deutschen Rontgengesellschaft 165: 503–504.
[3]
Dewhirst MW (2009) Relationships between cycling hypoxia, HIF-1, angiogenesis and oxidative stress. Radiation research 172: 653–665.
[4]
van Putten LM (1968) Tumour reoxygenation during fractionated radiotherapy; studies with a transplantable mouse osteosarcoma. European Journal of Cancer 4: 172–182.
[5]
Chen FH, Chiang CS, Wang CC, Tsai CS, Jung SM, et al. (2009) Radiotherapy decreases vascular density and causes hypoxia with macrophage aggregation in TRAMP-C1 prostate tumors. Clinical Cancer Research 15: 1721–1729.
[6]
Al-Hajj M, Wicha MS, Benito-Hernandez A, Morrison SJ, Clarke MF (2003) Prospective identification of tumorigenic breast cancer cells. Proc Natl Acad Sci U S A 100: 3983–3988.
[7]
Hemmati HD, Nakano I, Lazareff JA, Masterman-Smith M, Geschwind DH, et al. (2003) Cancerous stem cells can arise from pediatric brain tumors. Proc Natl Acad Sci U S A 100: 15178–15183.
[8]
Ginestier C, Hur MH, Charafe-Jauffret E, Monville F, Dutcher J, et al. (2007) ALDH1 Is a Marker of Normal and Malignant Human Mammary Stem Cells and a Predictor of Poor Clinical Outcome. Cell Stem Cell 1: 555–567.
[9]
Vlashi E, Lagadec C, Vergnes L, Matsutani T, Masui K, et al. (2011) Metabolic state of glioma stem cells and nontumorigenic cells. Proceedings of the National Academy of Sciences of the United States of America 108: 16062–16067.
[10]
Reya T, Morrison SJ, Clarke MF, Weissman IL (2001) Stem cells, cancer, and cancer stem cells. Nature 414: 105–111.
[11]
Phillips TM, McBride WH, Pajonk F (2006) The response of CD24(-/low)/CD44+ breast cancer-initiating cells to radiation. J Natl Cancer Inst 98: 1777–1785.
[12]
Woodward WA, Chen MS, Behbod F, Alfaro MP, Buchholz TA, et al. (2007) WNT/beta-catenin mediates radiation resistance of mouse mammary progenitor cells. Proc Natl Acad Sci U S A 104: 618–623.
[13]
Vlashi E, Kim K, Dealla Donna L, Lagadec C, McDonald T, et al. (2009) In-vivo imaging, tracking, and targeting of cancer stem cells. J Natl Cancer Inst 101: 350–359.
[14]
Lagadec C, Vlashi E, Della Donna L, Meng Y, Dekmezian C, et al. (2010) Survival and self-renewing capacity of breast cancer initiating cells during fractionated radiation treatment. Breast Cancer Res 12: R13.
[15]
Elfadl D, Hodgkinson VC, Long ED, Scaife L, Drew PJ, et al. (2011) A pilot study to investigate the role of the 26S proteasome in radiotherapy resistance and loco-regional recurrence following breast conserving therapy for early breast cancer. Breast 20: 334–337.
[16]
Fillmore CM, Kuperwasser C (2008) Human breast cancer cell lines contain stem-like cells that self-renew, give rise to phenotypically diverse progeny and survive chemotherapy. Breast Cancer Res 10: R25.
[17]
Ricardo S, Vieira AF, Gerhard R, Leitao D, Pinto R, et al. (2011) Breast cancer stem cell markers CD44, CD24 and ALDH1: expression distribution within intrinsic molecular subtype. Journal of clinical pathology 64: 937–46.
[18]
Dontu G, Abdallah WM, Foley JM, Jackson KW, Clarke MF, et al. (2003) In vitro propagation and transcriptional profiling of human mammary stem/progenitor cells. Genes Dev 17: 1253–1270.
[19]
Ponti D, Costa A, Zaffaroni N, Pratesi G, Petrangolini G, et al. (2005) Isolation and in vitro propagation of tumorigenic breast cancer cells with stem/progenitor cell properties. Cancer Res 65: 5506–5511.
[20]
Brizel DM, Sibley GS, Prosnitz LR, Scher RL, Dewhirst MW (1997) Tumor hypoxia adversely affects the prognosis of carcinoma of the head and neck. Int J Radiat Oncol Biol Phys 38: 285–289.
[21]
Fyles A, Milosevic M, Hedley D, Pintilie M, Levin W, et al. (2002) Tumor hypoxia has independent predictor impact only in patients with node-negative cervix cancer. J Clin Oncol 20: 680–687.
[22]
Vaupel P, Briest S, Hockel M (2002) Hypoxia in breast cancer: pathogenesis, characterization and biological/therapeutic implications. Wien Med Wochenschr 152: 334–342.
[23]
Teicher BA, Lazo JS, Sartorelli AC (1981) Classification of antineoplastic agents by their selective toxicities toward oxygenated and hypoxic tumor cells. Cancer research 41: 73–81.
[24]
Overgaard J (2011) Hypoxic modification of radiotherapy in squamous cell carcinoma of the head and neck - A systematic review and meta-analysis. Radiother Oncol 100: 22–32.
[25]
Frommhold H, Guttenberger R, Henke M (1998) The impact of blood hemoglobin content on the outcome of radiotherapy. The Freiburg experience. Strahlenther Onkol 174: Suppl 431–34.
[26]
Hoff CM, Lassen P, Eriksen JG, Hansen HS, Specht L, et al. (2011) Does transfusion improve the outcome for HNSCC patients treated with radiotherapy? - Results from the randomized DAHANCA 5 and 7 trials. Acta oncologica 50: 1006–14.
[27]
Henke M, Laszig R, Rübe C, Sch？fer U, Haase KD, et al. (2003) Erythropoietin to treat head and neck cancer patients with anaemia undergoing radiotherapy: randomised,double-blind, placebo-controlled trial. The Lancet 262: 1255–1260.
[28]
Epperly MW, Gretton JE, Sikora CA, Jefferson M, Bernarding M, et al. (2003) Mitochondrial localization of superoxide dismutase is required for decreasing radiation-induced cellular damage. Radiation research 160: 568–578.
[29]
Semenza GL (2011) Oxygen sensing, homeostasis, and disease. The New England journal of medicine 365: 537–547.
[30]
Heddleston JM, Li Z, McLendon RE, Hjelmeland AB, Rich JN (2009) The hypoxic microenvironment maintains glioblastoma stem cells and promotes reprogramming towards a cancer stem cell phenotype. Cell Cycle 8: 3274–3284.
[31]
Lagadec C, Vlashi E, Della Donna L, Dekmezian C, Pajonk F (2012) Radiation-induced Reprograming of Breast Cancer Cells. Stem Cells. In press.
