RFPL4A increases the G₁ population and decreases sensitivity to chemotherapy in human colorectal cancer cells

Cell cycle-arrested cancer cells are resistant to conventional chemotherapy that acts on the mitotic phases of the cell cycle, although the molecular mechanisms involved in halting cell cycle progression remain unclear. Here, we demonstrated that RFPL4A, an uncharacterized ubiquitin ligase, induced G₁ retention and thus conferred decreased sensitivity to chemotherapy in the human colorectal cancer cell line, HCT116. Long term time lapse observations in HCT116 cells bearing a "fluorescence ubiquitin-based cell cycle indicator" identified a characteristic population that is viable but remains in the G₁ phase for an extended period of time (up to 56 h). Microarray analyses showed that expression of RFPL4A was significantly up-regulated in these G₁-arrested cells, not only in HCT116 cells but also in other cancer cell lines, and overexpression of RFPL4A increased the G₁ population and decreased sensitivity to chemotherapy. However, knockdown of RFPL4A expression caused the cells to resume mitosis and induced their susceptibility to anti-cancer drugs in vitro and in vivo. These results indicate that RFPL4A is a novel factor that increases the G₁ population and decreases sensitivity to chemotherapy and thus may be a promising therapeutic target for refractory tumor conditions.