Identification and characterization of bipotent tumor stem cells in renal carcinomas

INTRODUCTION AND AIMS: Emerging evidence showed that the capacity of a tumor to grow and propagate resides in a small population of tumor cells, termed cancer stem cells or tumor-initiating cells. The identification of the tumor initiating stem cell population has a great impact in the understanding of tumor biology as well as in tumor therapy. However, no study addressed so far the possible presence of tumor stem cells in renal carcinomas. Recently, stem cells with mesenchymal characteristics have been identified within the adult kidney. As tumor stem cells may originate from mutated stem cells of the tissue, we here investigated whether a mesenchymal stem cell population was present within renal carcinomas, whether this population exhibited tumor initiating properties and stem cell properties. METHODS: Cells were sorted from different renal tumor specimens by magnetic cell sorting using the MACS system with anti-CD105 Ab coupled to magnetic beads, characterized by flow cytometry, RT-PCR and immunofluorescence and expanded in a medium described for multipotent adult progenitor cells. To evaluate epithelial and endothelial differentiation, clones were incubated in differentiating media containing serum or VEGF, respectively. To evaluate the tumorigenic potential, CD105+ and CD105 - cells (1x106-1x102 cells) were collected and implanted subcutaneously into SCID mice. Tumors were analyzed by immunofluorescence and immunohistochemistry. RESULTS: We identified a clonogenic CD105+tumor initiating cell population with the characteristic previously described for cancer stem cells. In particular, the CD105+ cells expressed nestin, Nanog and Oct4 stem cell markers, lacked differenziative epithelial markers, grew in non adhesive spheroids and could generate in vivo serially transplantable tumors. These tumors, despite being derived from clones expressing mesenchymal markers, were epithelial carcinomas resembling the tumor of origin (undifferentiated and clear cell carcinomas) and were characterized by the maintenance of a CD105+ tumorigenic population and by the presence of a non-tumorigenic differentiated CD105- population. In addition, CD105+tumor stem cells differentiated in vitro into endothelial cells and in vivo generated human vessels within the implanted tumors, demonstrating a vasculogenic property. CONCLUSIONS: The identified bipotent CD105+tumor stem cells in renal carcinomas could be responsible both for the process of tumor growth and for its vascularization. CD105+tumor stem cells could be therefore a new target for therapies of renal carcinomas.