Expression of the Cancer Stem Cell Marker ALDH1A1 in Primary Breast Cancer: A Mechanism for Chemotherapy Resistance.

Background: Aldehyde dehydrogenase 1 (ALDH1) has been identified as a marker of breast cancer stem cells, and ALDH1 expression in primary breast tumors has been associated with resistance to chemotherapy and poor prognosis. ALDH1 is an enzyme that can detoxify reactive aldehyde compounds; however the mechanism of ALDH1 chemotherapy resistance and its promotion of the breast cancer stem cell phenotype is unclear. Materials and Methods: Illumina cDNA array was performed on primary breast tumors that were snap-frozen, and RNA was isolated from dense, viable tumor regions. Samples were normalized by rank-invariant method and gene expression was analyzed. Immunohistochemistry was performed for ALDH1A1 on initial and post-neoadjuvant tumor samples from patients who have undergone six cycles of chemotherapy. Clinical data including age, tumor pathology, lymph node status, and response to neoadjuvant therapy were available for analysis. Human breast cancer cell lines (MCF-7 and MDA-MB-468) were treated with increasing doses of retinoic acid (RA) with and without the retinoic acid receptor alpha antagonist, Ro 41-5253. Effects on cell proliferation and signal transduction pathways were assessed. Results: The expression of ALDH1A1 and/or ALDH1A3 isoform mRNA was found to be present in primary human breast tumors. The majority of these cases were noted as triple negative or Her-2 positive tumors. Evaluation of RA receptor expression revealed repressed RAR beta levels in all tumors, whereas RAR alpha expression was retained. Observations of patient tumor samples (pre and post neoadjuvant chemotherapy treatment) demonstrated high ALDH1A1 expression in tumor and stromal cell compartments. The expression was predominant in partial and non-responders to the neoadjuvant therapy. Interestingly, the tumor from a partial responder was converted to extremely high expresser of ALDH1A1 post-neoadjuvant therapy indicating a positive survival selection. Since ALDH1A1 can convert retinaldehyde to retinoic acid, breast cancer cell lines were treated with all-trans RA and responded with a significant decrease in cell proliferation. This effect was partially attenuated by pre-treatment with the RAR alpha antagonist, Ro 41-5253. Importantly, RA treatment of MCF-7 cells repressed MAPK signaling, which was also attenuated by pre-treatment with Ro 41-5253. Discussion: The observations from this study suggest that the expression of the breast cancer stem cell marker, ALDH1A1 on tumor cells or in tumor-associated stromal cells can provide a mechanism for survival of cancer during chemotherapy treatment. The conversion of therapeutic oxidant compounds to retinoic acid by ALDH1 may lead to increased RAR alpha signaling and suppression of proliferation. The suppression of proliferation correlates well with chemotherapy resistance in these neoadjuvant cases. This data demonstrates a possible mechanism for ALDH1 promotion of tumor cell survival under chemotherapy and suggests that treatment with RAR alpha antagonists may promote tumor cell susceptibility to current treatment modalities and improve the therapeutic window for primary, metastatic and chemo-refractory breast cancer. Citation Information: Cancer Res 2009;69(24 Suppl):Abstract nr 1135.