title: Disruption of parallel and converging signaling pathways contributes to the synergistic antitumor effects of simultaneous mTOR and EGFR inhibition in GBM cells.
abstract: Elevated epidermal growth factor receptor (EGFR) and mammalian target of rapamycin (mTOR) signaling are known to contribute to the malignant properties of glioblastoma multiforme (GBM), which include uncontrolled cell proliferation and evasion of apoptosis. Small molecule inhibitors that target these protein kinases have been evaluated in multiple clinical trials for cancer patients, including those with GBM. Here we have examined the cellular and molecular effects of a combined kinase inhibition of mTOR (rapamycin) and EGFR (EKI-785) in U87 and U251 GBM cells. Simultaneous treatment with rapamycin and EKI-785 results in synergistic antiproliferative as well as proapoptotic effects. At a molecular level, rapamycin alone significantly decreases S6 phosphorylation, whereas EKI-785 alone promotes substantially reduced signal transducer and activator of transcription (STAT3) phosphorylation. Treatment with rapamycin alone also increases Akt phosphorylation on Ser-473, but this effect is blocked by a simultaneous administration of EKI-785. Individually, EKI-785 diminishes while rapamycin promotes the binding of the translation inhibitor eukaryotic initiation factor 4E binding protein (4EBP1) to the eukaryotic translation initiation factor 4E (eIF4E). In spite of these opposing effects, the highest level of 4EBP1-eIF4E binding occurs with the combination of the two inhibitors. These results indicate that the inhibition of EGFR and mTOR has distinct as well as common signaling consequences and provides a molecular rationale for the synergistic antitumor effects of EKI-785 and rapamycin administration.
authors.affiliation: Department of Oncology, Mayo Clinic, Rochester, MN 55905, USA.
authors.forename: Ravi D
authors.initials: RD
authors.lastname: Rao
authors.forename: Ann C
authors.initials: AC
authors.lastname: Mladek
authors.forename: Jeffrey D
authors.initials: JD
authors.lastname: Lamont
authors.forename: Jennie M
authors.initials: JM
authors.lastname: Goble
authors.forename: Charles
authors.initials: C
authors.lastname: Erlichman
authors.forename: C David
authors.initials: CD
authors.lastname: James
authors.forename: Jann N
authors.initials: JN
authors.lastname: Sarkaria
mesh_terms: D048868:Adaptor Proteins, Signal Transducing; D000970:Antineoplastic Agents / Q000494:pharmacology*; D017209:Apoptosis; D002352:Carrier Proteins / Q000378:metabolism; D018797:Cell Cycle Proteins; D045744:Cell Line, Tumor; D004305:Dose-Response Relationship, Drug; D066246:ErbB Receptors / Q000378:metabolism*; D039561:Eukaryotic Initiation Factor-4E / Q000378:metabolism; D015972:Gene Expression Regulation, Neoplastic; D005909:Glioblastoma / Q000188:drug therapy*; D006801:Humans; D015151:Immunoblotting; D047468:Immunoprecipitation; D008954:Models, Biological; D010750:Phosphoproteins / Q000378:metabolism; D010766:Phosphorylation; D011494:Protein Kinases / Q000378:metabolism*; D011799:Quinazolines / Q000494:pharmacology; D019893:Ribosomal Protein S6 Kinases / Q000378:metabolism; D015398:Signal Transduction; D020123:Sirolimus / Q000494:pharmacology; D058570:TOR Serine-Threonine Kinases; D013936:Thymidine / Q000737:chemistry
chemical_list: D048868:Adaptor Proteins, Signal Transducing; D000970:Antineoplastic Agents; D002352:Carrier Proteins; D018797:Cell Cycle Proteins; C087000:EIF4EBP1 protein, human; D039561:Eukaryotic Initiation Factor-4E; D010750:Phosphoproteins; D011799:Quinazolines; C118029:CL 387785; D011494:Protein Kinases; C546842:MTOR protein, human; D066246:ErbB Receptors; D019893:Ribosomal Protein S6 Kinases; D058570:TOR Serine-Threonine Kinases; D013936:Thymidine; D020123:Sirolimus