Abstract
BACKGROUND: Recently, tumor initiating cells (TICs), which possess self-renewal and other stem cell properties, are regarded as the cause of tumor initiation, recurrence and metastasis. The isolation and identification of TICs could help to develop novel therapeutic strategies. METHODS: In this study, we isolated spheroid cells from human renal cell carcinoma (RCC) cell line SN12C in stem cell-conditioned medium. The stemness characteristics of spheroid cells, including tumorigenicity, self-renewal, proliferation and aldehyde dehydrogenase (ALDH) activity were evaluated; the expression levels of stemness genes and related proteins were assessed. Furthermore, study examined the differentiation of TICs into endothelial cells and the relationship between TICs and EMT. RESULTS: Our data demonstrated that spheroid cells cultured in defined serum-free medium possessed TIC properties, such as high tumorigenic capacity, upregulation of TIC-related genes and proteins, persistent self-renewal and extensive proliferation. Furthermore, spheroid cells were more aggressive in growth, invasion, scratch recovery, clonogenic survival and high aldehyde dehydrogenase (ALDH) activity. Interestingly, a marked increase in tumor vascularity compared to adherent tumors in vivo, and spheroid cells can differentiate into functional endothelial-like cells in vitro suggesting a role of tumor initiating cells in tumor angiogenesis. The spheroid cells also demonstrated down-regulated E-cadherin and up-regulated Vimentin expression, which is the typical phenotype of EMT. CONCLUSIONS: These results suggest that spheroid cells with tumor initiating cells-like characteristics contributed to tumor generation, progression, high tumorigenicity, pro-angiogenic capability and relationship with EMT. Further experiments using more refined selection criteria such as a combination of two or multiple markers would be useful to specifically identify and purify TICs.