Abstract
Autophagy is a conserved catabolic process that maintains homeostasis by regulating the energy balance of the cell. Cancer cells use autophagy to remove damaged organelles and aggregated proteins, and to recycle nutrients in high demand to support tumor growth. Radiation therapy (RT) and temozolomide (TMZ), are front-line treatments for glioblastoma (GBM), the common and most malignant brain tumors in humans. However, RT and TMZ are known activating the autophagic response in tumor cells, which protects GBM cells from therapy-induced cell death. Thus, improved understanding of mechanisms regulating autophagy could reveal targets for selective and specific inhibition, which would enhance the anti-tumor activity of RT and TMZ while reducing toxic effects of treatment. In this study, we determined the roles of MST4, a less known protein serine/threonine kinase in its cellular functions in regulation of GBM tumorigenicity and therapy responses through activating autophagic activities. By using proteomic, biochemical and genetic approaches, we identified ATG4B as a novel substrate of MST4. ATG4B is a key regulator that facilitates autophagic process through reversible modification of ATG8/LC3. MST4 phosphorylates ATG4B at serine residue 383, which stimulates ATG4B enzymatic activity towards LC3, increasing autophagic flux. Inhibition of MST4 or ATG4B activities suppresses autophagic activities and tumorigenicity of patient-derived glioma stem cells (GSCs) in vitro and in the brain of mice. Furthermore, RT induces MST4 expression, ATG4B phosphorylation and autophagic activity. Inhibiting ATG4B by using a novel inhibitor NSC185058 in combination with RT in treating mice with intracranial GBM tumor xenografts markedly slows tumor growth and provides significant survival benefit to animal subjects. This study not only describes a novel regulatory mechanism by which the MST4-ATG4B axis accelerates autophagic process, regulates GBM tumorigenicity, and responses to RT, but also explores imminent clinical utility of combination of ATG4B inhibition with RT to suppress orthotopic GBM tumor xenografts.