Survival and self-renewing capacity of breast cancer initiating cells during fractionated radiation treatment.
BACKGROUND
Recent data indicate a hierarchical organization of many solid cancers, including breast cancer, with a small number of cancer initiating cells (CICs) that have the ability to self-renew and exhibit multi-lineage potency. We, and others, have demonstrated that CICs in breast cancer and glioma are relatively resistant to ionizing radiation if compared to their non-tumorigenic counterparts. However, the extent of the remaining self-renewing capacity of CICs after fractions of radiation is currently unknown. We hypothesized that CICs, in contrast to their non-tumorigenic counterparts, not only survive fractions of ionizing radiation but also retain the CIC phenotype as defined by operational means.


METHODS
We used two marker systems to identify breast CICs (CD24-/low/CD44high, or lack of proteasome activity) and performed sphere-forming assays after multiple clinical fractions of radiation. Lineage tracking was performed by membrane staining. Cell cycle distribution and RNA content were assessed by flow cytometry and senescence was assessed via beta-galactosidase staining.


RESULTS
We demonstrated that irradiated CICs survived and retained their self-renewal capacity for at least four generations. We show that fractionated radiation not only spared CICs but also mobilized them from a quiescent/G0 phase of the cell cycle into actively cycling cells, while the surviving non-tumorigenic cells were driven into senescence.


CONCLUSIONS
The breast CIC population retains increased self-renewal capacity over several generations and therefore, we conclude that increases in the number of CICs after sublethal doses of radiation have potential clinical importance. Prevention of this process may lead to improved clinical outcome.
Division of Molecular and Cellular Oncology, Department of Radiation Oncology, David Geffen School of Medicine at University of California, Los Angeles, 10833 Le Conte Avenue, Los Angeles, CA 90095-1714, USA.
Chann
C
Lagadec
Erina
E
Vlashi
Lorenza
L
Della Donna
Yonghong
Y
Meng
Carmen
C
Dekmezian
Kwanghee
K
Kim
Frank
F
Pajonk
D001943:Breast Neoplasms / Q000473:pathology / Q000532:radiotherapy*; D051927:CD24 Antigen / Q000032:analysis; D002453:Cell Cycle / Q000528:radiation effects; D045744:Cell Line, Tumor; D049109:Cell Proliferation / Q000528:radiation effects; D016922:Cellular Senescence / Q000528:radiation effects; D019583:Dose Fractionation, Radiation; D005260:Female; D006801:Humans; D018960:Hyaluronan Receptors / Q000032:analysis; D014411:Neoplastic Stem Cells / Q000473:pathology / Q000528:radiation effects*; D046988:Proteasome Endopeptidase Complex / Q000502:physiology
D051927:CD24 Antigen; C497889:CD24 protein, human; C497483:CD44 protein, human; D018960:Hyaluronan Receptors; D046988:Proteasome Endopeptidase Complex