Inhibition of aldehyde dehydrogenase (ALDH) activity reduces chemotherapy and radiation resistance of stem-like [ALDH.sup.hi][CD44.sup.+] human breast cancer cells

The majority of breast cancer deaths are because of ineffective treatment of metastatic disease. We previously identified a subpopulation of cells in human breast cancer cell lines that demonstrate high activity of aldehyde dehydrogenase (ALDH) and high expression of CD44. These [ALDH.sup.hi][CD44.sup.+] cells displayed enhanced metastatic behavior in vitro and in vivo relative to [ALDH.sup.low][CD44.sup.-] cells. The goal of this study was to test the hypothesis that [ALDH.sup.hi][CD44.sup.+] breast cancer cells are more resistant to standard cancer therapy, and that inhibiting ALDH activity through all-trans retinoic acid (ATRA) or the specific ALDH inhibitor diethylaminobenzaldehyde (DEAB) sensitizes these cells to treatment. [ALDH.sup.hi][CD44.sup.+] and [ALDH.sup.low][CD44.sup.-] populations were isolated from MDA-MB-231 and MDA-MB-468 cells lines and exposed to chemotherapy (doxorubicin/paclitaxel) or radiotherapy ± ATRA or DEAB. Cell populations were assessed for differences in survival, colony formation, and protein expression related to therapy resistance and differentiation. Significantly more [ALDH.sup.hi][CD44.sup.+] cells survived chemotherapy/radiotherapy relative to [ALDH.sup.low][CD44.sup.-] cells (P < 0.001). Glutathione-Stransferase pi, p-glycoprotein, and/or CHK1 were overexpressed in [ALDH.sup.hi][CD44.sup.+] populations compared with [ALDH.sup.low][CD44.sup.-] populations (P < 0.05). Pre-treatment of cell populations with DEAB or ATRA had no effect on [ALDH.sup.low][CD44.sup.-] cells, but resulted in significant initial sensitization of [ALDH.sup.hi][CD44.sup.+] cells to chemotherapy/ radiotherapy. However, only DEAB had a long-term effect, resulting in reduced colony formation (P < 0.01). ATRA also significantly increased expression of CK8/18/19 in MDA-MB-468 [ALDH.sup.hi][CD44.sup.+] cells compared with control (P < 0.05). Our novel findings indicate that [ALDH.sup.hi][CD44.sup.+] breast cancer cells contribute to both chemotherapy and radiation resistance and suggest a much broader role for ALDH in treatment response than previously reported. Keywords Breast cancer * Stem-like cells * Aldehyde dehydrogenase * Therapy resistance * All-trans retinoic acid