title: Combined regulation of mTORC1 and lysosomal acidification by GSK-3 suppresses autophagy and contributes to cancer cell growth.
abstract: There is controversy over the role of glycogen synthase kinase-3 (GSK-3) in cancer progression. Recent work has implicated GSK-3 in the regulation of mammalian target of rapamycin (mTOR), a known player in malignant transformation. Autophagy, a self-degradation pathway, is inhibited by mTOR and is tightly associated with cell survival and tumor growth. Here we show that GSK-3 suppresses autophagy via mTOR complex-1 (mTORC1) and lysosomal regulation. We show that overexpression of GSK-3 isoforms (GSK-3α and GSK-3β) activated mTORC1 and suppressed autophagy in MCF-7 human breast cancer cells as indicated by reduced beclin-1 levels and upregulation of sequestosome 1 (p62/SQSTM1). Further, overexpression of GSK-3 increased the number of autophagosomes and inhibited autophagic flux. This activity was directly related to reduced lysosomal acidification triggered by GSK-3 (in which GSK-3β has a stronger impact). We found that lysosomal acidification is reduced in MCF-7 cells that also exhibit increased levels of autophagosomes and p62/SQSTM1 and increased activity of mTORC1. Subsequently, treating cells with GSK-3 inhibitors restored lysosomal acidification, enhanced autophagic flux and inhibited mTORC1. Furthermore, GSK-3 inhibitors inhibited cell proliferation. We provide evidence that GSK3-mediated mTORC1 activity and GSK-3-mediated lysosomal acidification occur via distinct pathways, yet both mTORC1 and lysosomes control cell growth. Finally, we show that GSK-3-reduced lysosomal acidification inhibits endocytic clearance as demonstrated by reduced endocytic degradation of the epidermal growth factor receptor. Taken together, our study places GSK-3 as a key regulator coordinating cellular homeostasis. GSK-3 inhibitors may be useful in targeting mTORC1 and lysosomal acidification for cancer therapy.
authors.affiliation: Department of Human Molecular Genetics and Biochemistry, Sackler School of Medicine, Tel Aviv University, Tel Aviv, Israel.
authors.forename: I
authors.initials: I
authors.lastname: Azoulay-Alfaguter
authors.affiliation: Department of Human Molecular Genetics and Biochemistry, Sackler School of Medicine, Tel Aviv University, Tel Aviv, Israel.
authors.forename: R
authors.initials: R
authors.lastname: Elya
authors.affiliation: Department of Human Molecular Genetics and Biochemistry, Sackler School of Medicine, Tel Aviv University, Tel Aviv, Israel.
authors.forename: L
authors.initials: L
authors.lastname: Avrahami
authors.affiliation: Department of Human Molecular Genetics and Biochemistry, Sackler School of Medicine, Tel Aviv University, Tel Aviv, Israel.
authors.forename: A
authors.initials: A
authors.lastname: Katz
authors.affiliation: Department of Human Molecular Genetics and Biochemistry, Sackler School of Medicine, Tel Aviv University, Tel Aviv, Israel.
authors.forename: H
authors.initials: H
authors.lastname: Eldar-Finkelman
mesh_terms: D001343:Autophagy; D049109:Cell Proliferation; D004705:Endocytosis; D038362:Glycogen Synthase Kinase 3 / Q000502:physiology*; D006801:Humans; D006863:Hydrogen-Ion Concentration; D008247:Lysosomes / Q000201:enzymology*; D061986:MCF-7 Cells; D000076222:Mechanistic Target of Rapamycin Complex 1; D046912:Multiprotein Complexes / Q000502:physiology*; D010588:Phagosomes / Q000378:metabolism; D058570:TOR Serine-Threonine Kinases / Q000502:physiology*
chemical_list: D046912:Multiprotein Complexes; D000076222:Mechanistic Target of Rapamycin Complex 1; D058570:TOR Serine-Threonine Kinases; D038362:Glycogen Synthase Kinase 3