Cardiac glycosides are clinically used for cardiac arrhythmias. In this study, we investigated the mechanism responsible for anti-cancer and radiosensitizing effects of lanatoside C in colorectal cancer cells. Lanatoside C-treated cells showed classic patterns of autophagy, which may have been caused by lanatoside C-induced mitochondrial aggregation or degeneration. This mitochondrial dysfunction was due to disruption of K+ homeostasis, possibly through inhibition of Na+/K+-ATPase activity. In addition, lanatoside C sensitized HCT116 cells (but not HT-29 cells) to radiation in vitro. γ-H2AX, a representative marker of DNA damage, were sustained longer after combination of irradiation with lanatoside C, suggesting lanatoside C impaired DNA damage repair processes. Recruitment of 53BP1 to damaged DNA, a critical initiation step for DNA damage repair signaling, was significantly suppressed in lanatoside C-treated HCT116 cells. This may have been due to defects in the RNF8- and RNF168-dependent degradation of KDM4A/JMJD2A that increases 53BP1 recruitment to DNA damage sites. Although lanatoside C alone reduced tumor growth in the mouse xenograft tumor model, combination of lanatoside C and radiation inhibited tumor growth more than single treatments. Thus, lanatoside C could be a potential molecule for anti-cancer drugs and radiosensitizing agents.
Keywords: DNA damage repair; autophagy; lanatoside C; mitochondria; radiosensitivity.