title: Inositol hexakisphosphate kinases promote autophagy.
abstract: 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.
authors.affiliation: Department of Neurology, Tokai University School of Medicine, 143 Shimo-Kasuya, Isehara, Kanagawa 259-1193, Japan.
authors.forename: Eiichiro
authors.initials: E
authors.lastname: Nagata
authors.forename: Adolfo
authors.initials: A
authors.lastname: Saiardi
authors.forename: Hideo
authors.initials: H
authors.lastname: Tsukamoto
authors.forename: Tadayuki
authors.initials: T
authors.lastname: Satoh
authors.forename: Yoshiko
authors.initials: Y
authors.lastname: Itoh
authors.forename: Johbu
authors.initials: J
authors.lastname: Itoh
authors.forename: Mamoru
authors.initials: M
authors.lastname: Shibata
authors.forename: Shunya
authors.initials: S
authors.lastname: Takizawa
authors.forename: Shigeharu
authors.initials: S
authors.lastname: Takagi
mesh_terms: D001343:Autophagy / Q000502:physiology*; D016923:Cell Death / Q000502:physiology; D002470:Cell Survival; D057809:HEK293 Cells; D006367:HeLa Cells; D006801:Humans; D007150:Immunohistochemistry; D007295:Inositol Phosphates / Q000378:metabolism; D010766:Phosphorylation; D017856:Phosphotransferases (Phosphate Group Acceptor) / Q000235:genetics / Q000378:metabolism*; D014162:Transfection
chemical_list: D007295:Inositol Phosphates; D017856:Phosphotransferases (Phosphate Group Acceptor); C099438:inositol hexakisphosphate kinase