Aldehyde dehydrogenase activity in serous ovarian carcinoma.

Abstract

e15577 Background: Only specific subpopulations of tumor cells, so called cancer stem cells (CSC) may initiate and maintain tumors. The phenotype and molecular properties of ovarian CSC remain elusive. Aldehyde dehydrogenase (ALDH) activity characterizes (cancer) stem cells in different tissues and has been associated with ovarian CSC (Silva et al, 2011; Kryczek et al, 2012). Contradictory results have been reported on ALDH1 expression and prognosis in ovarian carcinoma. In this study, we explore the role of ALDH in serous ovarian carcinoma (SOC). Methods: Aldefluor-staining was used to assess ALDH activity in different ovarian carcinoma cell-lines and patient samples. ALDH+ and ALDH- cells isolated by FACS and ALDH1 versus control siRNA treated cells were analyzed in sphere forming, proliferation, BrdU and cell cycle assays. In vivo tumorigenicity assays including serial re-transplantations were performed in NOD/SCID/IL2Rγnull mice. ALDH1 and Ki67 expression were assessed immunohistochemically on a tissue microarray of 152 SOC samples. Results: ALDH+ cells formed more tumor spheres than ALDH- cells from the OVCAR-3 cell line and primary SOC and larger spheres (> 5.000 µm²) developed solely from ALDH+ cells. However, in vivo both cell fractions gave rise to tumors. Tumors contained both ALDH+ and ALDH- cells irrespective of the starting population. Notably, ALDH+ cells generated tumors more rapidly and induced larger tumors, suggesting a higher proliferative capacity. Immunohistochemical analysis of a larger cohort of SOC patients confirmed association of ALDH1 expression with the proliferation marker Ki67 (p=0.007). Surprisingly, co-stainings revealed that ALDH1 positive cells were mostly Ki67 negative and cell cycle synchronisation experiments using different agents showed ALDH induction in G0-enriched OVCAR-3 cells. However, inhibition of ALDH by treatment with three distinct siRNAs against ALDH1 did not alter cell cycle distribution. Conclusions: Our data suggest that ALDH is a correlative marker indicating, but not actively sustaining a quiescent stem-cell like state in SOC. Upon exit from G0, ALDH+ cells lose ALDH expression and induce a proliferative response.