Inositol hexakisphosphate kinases promote autophagy

We and other authors have previously reported that increasing cellular diphosphoinositol pentakisphosphate (InsP(7)) levels increases cell sensitivity to cell death. In the present study, we elucidated the relationship between inositol hexakisphosphate kinases (InsP(6)Ks), which form InsP(7), and autophagy using InsP(6)Ks overexpression and disruption systems. A large number of autophagosomes were induced in cells transfected with InsP(6)Ks, as revealed by the conversion of LC3-I to LC3-II, which was examined using immunoblotting, immunocytochemistry, and immuno-electron microscopy for LC3; consequently, the rate of cell death was higher among these cells than among cells transfected with a control vector, as shown using propidium iodide staining. However, the reduction of InsP(6)Ks levels using RNAi suppressed the formation of autophagosomes. Moreover, the number of autophagosomes and the rate of cell death were significantly higher among cells transfected with InsP(6)Ks subjected to staurosporine-induced stress than among cells transfected with InsP(6)Ks subjected to normal conditions. The cell death induced by InsP(6)Ks was not completely suppressed by z-VAD-fmk, a pan-caspase inhibitor. The phosphorylation of mammalian target of rapamycin (mTOR) was also depressed in cells overexpressing InsP(6)Ks, suggesting that the mTOR pathway regulates autophagosomes generated by InsP(6)Ks. These findings imply that InsP(6)Ks promote autophagy and induce caspase-independent cell death. This phenomenon opens a new pathway of autophagy via InsP(6)Ks.