ALDH1-Bright Epithelial Ovarian Cancer Cells Are Associated with CD44 Expression, Drug Resistance, and Poor Clinical Outcome

The role of aldehyde dehydrogenase 1 (ALDH1) as an ovarian cancer stem cell marker and its clinical significance have rarely been explored. We used an Aldefluor assay to isolate ALDH1-bright (ALDH1^br) cells from epithelial ovarian cancer cell lines and characterized the properties of the stem cells. ALDH1^br cells were enriched in ES-2 (1.3%), TOV-21G (1.0%), and CP70 (1.2%) cells. Both ALDH1^br and ALDH1^low cells repopulated stem cell heterogeneity, formed spheroids, and grew into tumors in immunocompromised mice, although these processes were more efficient in ALDH1^br cells. In the ES-2 and CP70 cells, ALDH1^br cells conferred more chemoresistance, and were more enriched in CD44 (by 1.74-fold and 5.18-fold, respectively) than in CD133 (by 1.39-fold and 1.17-fold, respectively), compared with ALDH1^low cells. Immunohistochemical staining for ALDH1 on a tissue microarray containing 84 epithelial ovarian cancer samples revealed that patients with higher ALDH1 expression (>50%) had poor overall survival, compared with those with lower ALDH1 (P = 0.004) and yielded an odds ratio of death of 2.43 (95% CI = 1.12 to 5.28) by multivariate analysis. The results did not support ALDH1 alone as an ovarian cancer stem cell marker, but demonstrated that ALDH1 is associated with CD44 expression, chemoresistance, and poor clinical outcome. The use of a combination of ALDH1 with other stem cell markers may help define ovarian cancer stem cells more stringently.