Differential radiosensitizing effect of valproic acid in differentiation versus self-renewal promoting culture conditions.
OBJECTIVE
It has been shown that valproic acid (VA) enhances the proliferation and self-renewal of normal hematopoietic stem cells and that breast cancer stem/progenitor cells can be resistant to radiation. From these data, we hypothesized that VA would fail to radiosensitize breast cancer stem/progenitor cells grown to three-dimensional (3D) mammospheres.


METHODS
We used the MCF7 breast cancer cell line grown under stem cell-promoting culture conditions (3D mammosphere) and standard nonstem cell monolayer culture conditions (two-dimensional) to examine the effect of pretreatment with VA on radiation sensitivity in clonogenic survival assays and on the expression of embryonic stem cell transcription factors.


RESULTS
3D-cultured MCF-7 cells expressed higher levels of Oct4, Nanog, and Sox2. The 3D passage enriched self-renewal and increased radioresistance in the 3D mammosphere formation assays. VA radiosensitized adherent cells but radioprotected 3D cells in single-fraction clonogenic assays. Moreover, fractionated radiation sensitized VA-treated adherent MCF7 cells but did not have a significant effect on VA-treated single cells grown to mammospheres.


CONCLUSIONS
We have concluded that VA might preferentially radiosensitize differentiated cells compared with those expressing stem cell surrogates and that stem cell-promoting culture is a useful tool for in vitro evaluation of novel cancer therapeutic agents and radiosensitizers.
Division of Radiation Oncology, University of Texas MD Anderson Cancer Center, Houston, TX 77030, USA.
Bisrat G
BG
Debeb
Wei
W
Xu
Henry
H
Mok
Li
L
Li
Fredika
F
Robertson
Naoto T
NT
Ueno
Jim
J
Reuben
Anthony
A
Lucci
Massimo
M
Cristofanilli
Wendy A
WA
Woodward
D018929:Cell Culture Techniques / Q000379:methods; D002454:Cell Differentiation / Q000187:drug effects; D045744:Cell Line, Tumor; D049109:Cell Proliferation; D018398:Homeodomain Proteins / Q000378:metabolism; D006801:Humans; D000071317:Nanog Homeobox Protein; D014411:Neoplastic Stem Cells / Q000166:cytology / Q000187:drug effects / Q000378:metabolism / Q000528:radiation effects*; D050814:Octamer Transcription Factor-3 / Q000378:metabolism; D011836:Radiation Tolerance / Q000187:drug effects* / Q000502:physiology; D011837:Radiation-Protective Agents / Q000494:pharmacology; D011838:Radiation-Sensitizing Agents / Q000494:pharmacology*; D055748:SOXB1 Transcription Factors / Q000378:metabolism; D018874:Spheroids, Cellular / Q000187:drug effects / Q000378:metabolism / Q000473:pathology / Q000528:radiation effects; D014410:Tumor Stem Cell Assay / Q000379:methods; D014635:Valproic Acid / Q000494:pharmacology*
D018398:Homeodomain Proteins; C486780:NANOG protein, human; D000071317:Nanog Homeobox Protein; D050814:Octamer Transcription Factor-3; D011837:Radiation-Protective Agents; D011838:Radiation-Sensitizing Agents; C527782:SOX2 protein, human; D055748:SOXB1 Transcription Factors; D014635:Valproic Acid