Vitamin K2 (menaquinone-2: VK2) is now known to be a potent inducer for apoptosis in leukemia cells in vitro. HL-60bcl-2 cells, which are derived from a stable transfectant clone of human bcl-2 gene into HL-60 leukemia cell line, show 5-fold greater expression of Bcl-2 protein compared with that in HL-60neo cells, a control clone transfected with vector alone. Although HL-60neo cells are induced apoptosis in response to VK2, HL-60bcl-2 cells are resistant against apoptosis induction but still show cell growth inhibition along with an increase of cytoplasmic vacuoles during exposure to VK2. Electron microscopy revealed autophagosomes and autolysosomes formation in HL-60bcl-2 cells after exposure to VK2. An increase of acid vesicular organelles (AVO) detected by acridine orange staining for lysosomes as well as conversion of LC3B-I into LC3B-II by immunonoblotting and an increased punctuated pattern of cytoplasmic LC3B by fluorescent immunostaining all supported enhanced autophagy induction in response to VK2 in HL-60bcl-2 cells. However, during shorter exposure to VK2, autophagosome formation was rather prominent in HL-60neo cells although nuclear chromatin condensations and nuclear fragments were also observed at the same time. These findings indicated the mixed morphologic features of apoptosis and autophagy. Inhibition of autophagy by either addition of 3-methyladenine, siRNA for Atg7, or Tet-off Atg5 system all resulted in attenuation of VK2-incuded cell death, indicating autophagy-mediated cell death in response to VK2. These data demonstrate that autophagy and apoptosis can be simultaneously induced by VK2. However, autophagy becomes prominent when the cells were protected from rapid apoptotic death by higher expression level of Bcl-2.
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