title: Synergistic augmentation of rapamycin-induced autophagy in malignant glioma cells by phosphatidylinositol 3-kinase/protein kinase B inhibitors.
abstract: The mammalian target of rapamycin (mTOR) is a downstream effector of the phosphatidylinositol 3-kinase (PI3K)/protein kinase B (Akt) signaling pathway and a central modulator of cell proliferation in malignant gliomas. Therefore, the targeting of mTOR signaling is considered a promising therapy for malignant gliomas. However, the mechanisms underlying the cytotoxic effects of a selective mTOR inhibitor, rapamycin, on malignant glioma cells are poorly understood. The purpose of this study was thus to elucidate how rapamycin exerts its cytotoxic effects on malignant glioma cells. We showed that rapamycin induced autophagy but not apoptosis in rapamycin-sensitive malignant glioma U87-MG and T98G cells by inhibiting the function of mTOR. In contrast, in rapamycin-resistant U373-MG cells, the inhibitory effect of rapamycin was minor, although the phosphorylation of p70S6 kinase, a molecule downstream of mTOR, was remarkably inhibited. Interestingly, a PI3K inhibitor, LY294002, and an Akt inhibitor, UCN-01 (7-hydroxystaurosporine), both synergistically sensitized U87-MG and T98G cells as well as U373-MG cells to rapamycin by stimulating the induction of autophagy. Enforced expression of active Akt in tumor cells suppressed the combined effects of LY294002 or UCN-01, whereas dominant-negative Akt expression was sufficient to increase the sensitivity of tumor cells to rapamycin. These results indicate that rapamycin exerts its antitumor effect on malignant glioma cells by inducing autophagy and suggest that in malignant glioma cells a disruption of the PI3K/Akt signaling pathway could greatly enhance the effectiveness of mTOR inhibitors.
authors.affiliation: Department of Neurosurgery, University of Texas M.D. Anderson Cancer Center, 1515 Holcombe Boulevard, Houston, TX 77030, USA.
authors.forename: Hayato
authors.initials: H
authors.lastname: Takeuchi
authors.forename: Yasuko
authors.initials: Y
authors.lastname: Kondo
authors.forename: Keishi
authors.initials: K
authors.lastname: Fujiwara
authors.forename: Takao
authors.initials: T
authors.lastname: Kanzawa
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Aoki
authors.forename: Gordon B
authors.initials: GB
authors.lastname: Mills
authors.forename: Seiji
authors.initials: S
authors.lastname: Kondo
mesh_terms: D000903:Antibiotics, Antineoplastic / Q000008:administration & dosage / Q000494:pharmacology*; D000971:Antineoplastic Combined Chemotherapy Protocols / Q000494:pharmacology*; D017209:Apoptosis / Q000187:drug effects; D001343:Autophagy / Q000187:drug effects; D045744:Cell Line, Tumor; D002867:Chromones / Q000008:administration & dosage; D004357:Drug Synergism; D004791:Enzyme Inhibitors / Q000008:administration & dosage / Q000494:pharmacology*; D005910:Glioma / Q000188:drug therapy* / Q000201:enzymology / Q000473:pathology; D006801:Humans; D009025:Morpholines / Q000008:administration & dosage; D000081082:Phosphoinositide-3 Kinase Inhibitors; D047428:Protein Kinase Inhibitors / Q000494:pharmacology; D011494:Protein Kinases / Q000378:metabolism; D017346:Protein Serine-Threonine Kinases / Q000037:antagonists & inhibitors*; D011518:Proto-Oncogene Proteins / Q000037:antagonists & inhibitors*; D051057:Proto-Oncogene Proteins c-akt; D020123:Sirolimus / Q000008:administration & dosage / Q000494:pharmacology*; D019311:Staurosporine / Q000008:administration & dosage / Q000031:analogs & derivatives*; D058570:TOR Serine-Threonine Kinases
chemical_list: D000903:Antibiotics, Antineoplastic; D002867:Chromones; D004791:Enzyme Inhibitors; D009025:Morpholines; D000081082:Phosphoinositide-3 Kinase Inhibitors; D047428:Protein Kinase Inhibitors; D011518:Proto-Oncogene Proteins; C085911:2-(4-morpholinyl)-8-phenyl-4H-1-benzopyran-4-one; C054852:7-hydroxystaurosporine; D011494:Protein Kinases; C546842:MTOR protein, human; C494918:AKT1 protein, human; D017346:Protein Serine-Threonine Kinases; D051057:Proto-Oncogene Proteins c-akt; D058570:TOR Serine-Threonine Kinases; D019311:Staurosporine; D020123:Sirolimus