title: Loss of Pten, a tumor suppressor, causes the strong inhibition of autophagy without affecting LC3 lipidation.
abstract: (1)Pten (phosphatase and tensin homolog deleted on chromosome ten), a tumor suppressor, is a phosphatase with a variety of substrate specificities. Its function as a negative regulator of the class I phosphatidyl-inositol 3-kinase/Akt pathway antagonizes insulin-dependent cell signaling. The targeted deletion of Pten in mouse liver leads to insulin hypersensitivity and the upregulation of the phosphatidyl-inositol 3-kinase/Akt signaling pathway. In this study, we investigated the effects of Pten deficiency on autophagy, a major cellular degradative system responsible for the turnover of cell constituents. The autophagic degradation of [(14)C-leucine-labeled proteins of hepatocytes isolated from Pten-deficient livers was strongly inhibited, compared with that of control hepatocytes. However, no significant difference was found in the levels of the Atg12-Atg5 conjugate and LC3-II, the lipidated form of LC3, an intrinsic autophagosomal membrane marker, between control and Pten-deficient livers. Electron microscopic analyses showed that numerous autophagic vacuoles (autophagosomes plus autolysosomes) were present in the livers of control mice that had been starved for 48 hours, whereas they were markedly reduced in Pten-deficient livers under the same conditions. In vivo administration of leupeptin to control livers caused the inhibition of autophagic proteolysis, resulting in the accumulation of autolysosomes. These autolysosomes could be separated as a denser autolysosomal fraction from other cell membranes by Percoll density gradient centrifugation. In leupeptin-administered mutant livers, however, the accumulation of denser autolysosomes was reduced substantially. Collectively, we conclude that enhanced insulin signaling in Pten deficiency suppresses autophagy at the formation and maturation steps of autophagosomes, without inhibiting ATG conjugation reactions.
authors.affiliation: The Department of Biochemistry, Juntendo University School of Medicine, Hongo, Tokyo, Japan.
authors.forename: Takashi
authors.initials: T
authors.lastname: Ueno
authors.forename: Wataru
authors.initials: W
authors.lastname: Sato
authors.forename: Yasuo
authors.initials: Y
authors.lastname: Horie
authors.forename: Masaaki
authors.initials: M
authors.lastname: Komatsu
authors.forename: Isei
authors.initials: I
authors.lastname: Tanida
authors.forename: Mitsutaka
authors.initials: M
authors.lastname: Yoshida
authors.forename: Shigetoshi
authors.initials: S
authors.lastname: Ohshima
authors.forename: Tak Wah
authors.initials: TW
authors.lastname: Mak
authors.forename: Sumio
authors.initials: S
authors.lastname: Watanabe
authors.forename: Eiki
authors.initials: E
authors.lastname: Kominami
mesh_terms: D000818:Animals; D051017:Apoptosis Regulatory Proteins / Q000037:antagonists & inhibitors / Q000502:physiology; D001343:Autophagy / Q000235:genetics* / Q000502:physiology; D002478:Cells, Cultured; D016147:Genes, Tumor Suppressor / Q000502:physiology*; D022781:Hepatocytes / Q000378:metabolism* / Q000473:pathology; D007328:Insulin / Q000502:physiology; D050356:Lipid Metabolism / Q000502:physiology*; D051379:Mice; D008810:Mice, Inbred C57BL; D008822:Mice, Transgenic; D008869:Microtubule-Associated Proteins / Q000378:metabolism*; D051059:PTEN Phosphohydrolase / Q000037:antagonists & inhibitors / Q000172:deficiency* / Q000235:genetics; D010588:Phagosomes / Q000378:metabolism; D015398:Signal Transduction / Q000235:genetics; D013217:Starvation / Q000378:metabolism
chemical_list: D051017:Apoptosis Regulatory Proteins; D007328:Insulin; C480860:Map1lc3b protein, mouse; D008869:Microtubule-Associated Proteins; D051059:PTEN Phosphohydrolase; C494929:PTEN protein, human