Fibroblasts induce expression of FGF4 in ovarian cancer stem-like cells/cancer-initiating cells and upregulate their tumor initiation capacity.

Cancer stem-like cells (CSCs)/cancer-initiating cells (CICs) are defined as a small population of cells within cancer that contribute to cancer initiation and progression. Cancer-associated fibroblasts (CAFs) are stromal fibroblasts surrounding tumor cells, and they have important roles in tumor growth and tumor progression. It has been suggested that stromal fibroblasts and CSCs/CICs might mutually cooperate to enhance their growth and tumorigenic capacity. In this study, we investigated the effects of fibroblasts on tumor-initiating capacity and stem-like properties of ovarian CSCs/CICs. CSCs/CICs were isolated from the ovarian carcinoma cell line HTBoA as aldehyde dehydrogenase 1 high (ALDH1(high)) population by the ALDEFLUOR assay. Histological examination of tumor tissues derived from ALDH1(high) cells revealed few fibrous stroma, whereas those derived from fibroblast-mixed ALDH1(high) cells showed abundant fibrous stroma formation. In vivo tumor-initiating capacity and in vitro sphere-forming capacity of ALDH1(high) cells were enhanced in the presence of fibroblasts. Gene expression analysis revealed that fibroblast-mixed ALDH1(high) cells had enhanced expression of fibroblast growth factor 4 (FGF4) as well as stemness-associated genes such as SOX2 and POU5F1. Sphere-forming capacity of ALDH1(high) cells was suppressed by small-interfering RNA (siRNA)-mediated knockdown of FGFR2, the receptor for FGF4 which was expressed preferentially in ALDH1(high) cells. Taken together, the results indicate that interaction of fibroblasts with ovarian CSCs/CICs enhanced tumor-initiating capacity and stem-like properties through autocrine and paracrine FGF4-FGFR2 signaling.