Treatment of agarose-agarose RENCA macrobeads with docetaxel selects for OCT4(+) cells with tumor-initiating capability

The cancer stem cell (CSC) theory depicts such cells as having the capacity to produce both identical CSCs (symmetrical division) and tumor-amplifying daughter cells (asymmetric division). CSCs are thought to reside in niches similar to those of normal stem cells as described for neural, intestinal, and epidermal tissue, are resistant to chemotherapy, and are responsible for tumor recurrence. We recently described the niche-like nature of mouse renal adenocarcinoma (RENCA) cells following encapsulation in agarose macrobeads. In this paper we tested the hypothesis that encapsulated RENCA colonies function as an in vitro model of a CSC niche and that the majority of cells would undergo chemotherapy-induced death, followed by tumor recurrence. After exposure to docetaxel (5 µg/ml), 50% of cells were lost one week post-treatment while only one or two cells remained in each colony by 6 weeks. Surviving cells expressed OCT4 and reformed tumors at 16 weeks post-treatment. Docetaxel-resistant cells also grew as monolayers in cell culture (16-17 weeks post-exposure) or as primary tumors following transplantation to Balb/c mice (6 of 10 mice) or NOD.CB17-Prkdc(scid)/J mice (9 of 9 mice; 10 weeks post-transplantation or 28 weeks post-exposure). These data support the hypothesis that a rare subpopulation of OCT4(+) cells are resistant to docetaxel and these cells are sufficient for tumor recurrence. The reported methodology can be used to obtain purified populations of tumor-initiating cells, to screen for anti-tumor-initiating cell agents, and to investigate the in vitro correlate of a CSC niche, especially as it relates to chemo-resistance and tumor recurrence.