Spheres derived from the human SK-RC-42 renal cell carcinoma cell line are enriched in cancer stem cells

Various types of malignant tumor have been found to contain a subpopulation of cancer stem cells (CSCs). In this study, we sought to enrich CSCs from the renal cell carcinoma (RCC) cell line SK-RC-42 using the sphere culture system and characterize their immunophenotype. We demonstrated that a subpopulation of SK-RC-42 cells were capable of growing as tumor spheres in serum-free medium supplemented with EGF and bFGF. The sphere-forming cells (SFCs) had many properties similar to CSCs: ability of self-renewing in vitro and in vivo, higher mRNA expression levels of several 'stemness' genes, stronger tumorigenicity and resistance to chemotherapeutic agents and irradiation compared with the monolayer adherent cells (MACs). The SFCs expressed high levels of MHC class I but low levels of MHC class II, CD80 and CD86. In contrast with MACs, the SFCs had lower expression levels of FasL and Fas, Her2 and hTERT and activating natural killer receptors. Finally, SK-RC-42 SFCs and MACs both expressed significant and comparable levels of the transcription factor forkhead box protein 3 (FoxP3) and membrane complement regulatory proteins (mCRPs). Taken together, these findings indicate that CSCs can be enriched from RCC by culturing the tumor cells as spheres. The immunophenotype of the SFCs demonstrated in this study suggests that CSCs might play an important role in the evasion of tumor growth from immune surveillance.