Mitotic arrest induced by XK469, a novel antitumor agent, is correlated with the inhibition of cyclin B1 ubiquitination.

XK469 (NSC 697887) is a novel antitumor agent with broad activity against a variety of tumors. Previous studies suggest that XK469 is a topoisomerase II beta poison with functional activity similar to that of 4'-(9-acridinylamino) methanesulfon-m-anisidide (m-AMSA). The goal of our study was to investigate its mechanism of action further using a human HCT-116 (H116) colon tumor cell model. Concentration-survival curves with continuous exposure indicated that XK469 had low cytotoxic activity against H116 cells. Cell cycle analysis revealed that XK469 is a phase-specific cell cycle blocker that is associated with increased levels of cyclin B1, cyclin A and p53 but not CDK1 (cdc2) or cyclin E. In contrast, treatment of H116 cells with m-AMSA caused a total degradation of both cyclin A and B1 but enhanced expression of cyclin E and p53. Accumulation of cyclin B1 in XK469-treated cells was correlated with the inhibition of cyclin B1 ubiquitination, a metabolic process mandatory for proteasome-mediated protein turnover. However, no inhibition of cyclin B1 ubiquitination was detected in cells treated with m-AMSA or colchicine, a known mitotic inhibitor. Furthermore, unlike m-AMSA, XK469 did not induce caspase activation or apoptotic cell death in H116 cells. Our results suggest that XK469 is a phase-specific cell cycle inhibitor with a unique mechanism of action that is correlated with the inhibition of cyclin B1 ubiquitination and its accumulation at early M phase.