Ovarian Cancer Stem Cells Are Enriched in Side Population and Aldehyde Dehydrogenase Bright Overlapping Population

Abstract

Cancer stem-like cells (CSCs)/cancer-initiaiting cells (CICs) are defined as a small population of cancer cells that have self-renewal capacity, differentiation potential and high tumor-initiating ability. CSCs/CICs of ovarian cancer have been isolated by side population (SP) analysis, ALDEFLUOR assay and using cell surface markers. However, these approaches are not definitive markers for CSCs/CICs, and it is necessary to refine recent methods for identifying more highly purified CSCs/CICs. In this study, we analyzed SP cells and aldehyde dehydrogenese bright (ALDHBr) cells from ovarian cancer cells. Both SP cells and ALDHBr cells exhibited higher tumor-initiating ability and higher expression level of a stem cell marker, sex determining region Y-box 2 (SOX2), than those of main population (MP) cells and ALDHLow cells, respectively. We analyzed an SP and ALDHBr overlapping population (SP/ALDHBr), and the SP/ALDHBr population exhibited higher tumor-initiating ability than that of SP cells or ALDHBr cells, enabling initiation of tumor with as few as 102 cells. Furthermore, SP/ADLHBr population showed higher sphere-forming ability, cisplatin resistance, adipocyte differentiation ability and expression of SOX2 than those of SP/ALDHLow, MP/ALDHBr and MP/ALDHLow cells. Gene knockdown of SOX2 suppressed the tumor-initiation of ovarian cancer cells. An SP/ALDHBr population was detected in several gynecological cancer cells with ratios of 0.1% for HEC—1 endometrioid adenocarcinoma cells to 1% for MCAS ovary mucinous adenocarcinoma cells. Taken together, use of the SP and ALDHBr overlapping population is a promising approach to isolate highly purified CSCs/CICs and SOX2 might be a novel functional marker for ovarian CSCs/CICs.

Epigenetic Targeting of Ovarian Cancer Stem Cells

“…First, we assert that ALDH1A1 expression and activity characterizes OC cells with stem cell properties, in agreement with reports from other groups (6,30,38). Aldefluor positivity detectable by FACS identifies the enzymatic activity of ALDH1, a member of the ALDH family that metabolizes reactive aldehydes (39).…”

Suppression of ABHD2, identified through a functional genomics screen, causes anoikis resistance, chemoresistance and poor prognosis in ovarian cancer

“…It is also known that resistance to anoikis is a characteristics of cancer stem cells (CSC), and CSC have high drug efflux potential, thus inherent platinum resistance [6]. In our study, suppression of ABHD2 increased the SP fraction of OVCA420 cells, a characteristic feature of CSC [33, 34, 35] (Supplementary Figure S5). The increase in colony formation in soft agar may indeed reflect an increase in stemness characteristics, but to demonstrate this would require a great deal of further work that is beyond the scope of the present manuscript.…”

Establishment and Characterization of an In Vitro Model of Ovarian Cancer Stem-like Cells with an Enhanced Proliferative Capacity

“…Although high levels of ALDH activity are associated with CSC-related features in ovarian cancer cell lines (20), there are no reports demonstrating the proliferation of ALDH-positive CSCs in in vitro culture, suggesting that there are some unknown technical difficulties for their maintenance under in vitro conditions.…”