Combined regulation of mTORC1 and lysosomal acidification by GSK-3 suppresses autophagy and contributes to cancer cell growth

Oncogene. 2015 Aug 27;34(35):4613-23. doi: 10.1038/onc.2014.390. Epub 2014 Dec 15.

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.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Autophagy*
  • Cell Proliferation*
  • Endocytosis
  • Glycogen Synthase Kinase 3 / physiology*
  • Humans
  • Hydrogen-Ion Concentration
  • Lysosomes / enzymology*
  • MCF-7 Cells
  • Mechanistic Target of Rapamycin Complex 1
  • Multiprotein Complexes / physiology*
  • Phagosomes / metabolism
  • TOR Serine-Threonine Kinases / physiology*

Substances

  • Multiprotein Complexes
  • TOR Serine-Threonine Kinases
  • Mechanistic Target of Rapamycin Complex 1
  • Glycogen Synthase Kinase 3