Vitamin K₂ induces non-apoptotic cell death along with autophagosome formation in breast cancer cell lines

Background: Vitamin K₂ (VK2) has been reported to induce apoptosis in many types of cancer cells including leukemia. However, there are no precise reports regarding the breast cancer cells. From the stand point of clinical implications of VK2 including chemoprevention, we investigated the effects of VK2 on breast cancer cell lines. Methods: Breast cancer cell lines were cultured with VK2, and the cytotoxicity and cell death phenotype were examined. The HL-60 leukemia cells were used as a control for VK2-induced apoptosis. Results: VK2 exhibited the cytotoxic effect, especially in triple negative breast cancer cell lines, namely, MDA-MB-231 and MDA-MB-468. However, in contrast to HL-60 cells, typical features of the cells undergoing apoptosis, such as chromatin condensation, nuclear fragments, and cleavage of caspase-3 were not detected. Transmission electron microscopy exhibited an increased number of autophagosomes/autolysosomes with plasma membrane integrity. An autophagy inhibitor, 3-methyladenine, apparently attenuated VK2-induced cytotoxicity, which indicated the involvement of autophagy-dependent cell death. Interestingly, both VK2-induced non-apoptotic cell death in MDA-MB-231 cells and VK2-induced apoptosis in HL-60 cells were suppressed in the presence of reactive oxygen species (ROS) scavengers. Therefore, ROS production by VK2 seems to be located up-stream in the molecular machinery for both the types of cell death execution. Conclusion: The VK2 induced non-apoptotic cell death along with autophagy, in triple negative breast cancer cell lines. Cell death phenotype induced by VK2 appears to differ among the type of cancers. This suggests the possibility of using VK2 for the breast cancer therapy.