ABSTRACT
Monotherapies have proven largely ineffective for the treatment of glioblastomas, suggesting that increased patient benefit may be achieved by combining therapies. Two protumorigenic pathways known to be active in glioblastoma include RAS/RAF/mitogen-activated protein kinase and phosphatidylinositol 3-kinase/AKT/target of rapamycin (TOR). We investigated the efficacy of a combination of novel low molecular weight inhibitors LBT613 and RAD001 (everolimus), which were designed to target RAF and TOR, respectively. LBT613 decreased phosphorylation of extracellular signal-regulated kinase 1 and 2, downstream effectors of RAF, in a human glioma cell line. RAD001 resulted in decreased phosphorylation of the TOR effector S6. To determine if targeting RAF and TOR activities could result in decreased protumorigenic glioma cellular behaviors, we evaluated the abilities of LBT613 and RAD001 to affect the proliferation, migration, and invasion of human glioma cells. Treatment with either LBT613 or RAD001 alone significantly decreased the proliferation of multiple human glioma cell lines. Furthermore, LBT613 and RAD001 in combination synergized to decrease glioma cell proliferation in association with G(1) cell cycle arrest. Glioma invasion is a critical contributor to tumor malignancy. The combination of LBT613 and RAD001 inhibited the invasion of human glioma cells through Matrigel to a greater degree than treatment with either drug alone. These data suggest that the combination of LBT613 and RAD001 reduces glioma cell proliferation and invasion and support examination of the combination of RAF and TOR inhibitors for the treatment of human glioblastoma patients.
Subject(s)
Cell Proliferation/drug effects , Glioma/pathology , Immunosuppressive Agents/therapeutic use , Isoquinolines/therapeutic use , Phosphoinositide-3 Kinase Inhibitors , Sirolimus/analogs & derivatives , raf Kinases/antagonists & inhibitors , Annexin A5/metabolism , Blotting, Western , Cell Line, Tumor/drug effects , Cell Line, Tumor/metabolism , Cell Line, Tumor/pathology , Cell Movement/drug effects , Collagen/metabolism , DNA Replication/drug effects , Drug Combinations , Drug Therapy, Combination , Everolimus , Flow Cytometry , Glioma/drug therapy , Humans , Laminin/metabolism , Mitogen-Activated Protein Kinases/metabolism , Phosphorylation/drug effects , Protein Kinases/chemistry , Protein Kinases/metabolism , Proteoglycans/metabolism , Signal Transduction/drug effects , Sirolimus/therapeutic use , TOR Serine-Threonine Kinases , raf Kinases/metabolismABSTRACT
In normal epithelial tissues, the multifunctional cytokine transforming growth factor-beta (TGF-beta) acts as a tumor suppressor through growth inhibition and induction of differentiation whereas in advanced cancers, TGF-beta promotes tumor progression through induction of tumor invasion, neoangiogenesis, and immunosuppression. The molecular mechanisms through which TGF-beta shifts from a tumor suppressor to a tumor enhancer are poorly understood. We now show a role for the tumor suppressor phosphatase and tensin homologue deleted on chromosome 10 (PTEN) in repressing the protumorigenic effects of TGF-beta. The TGF-beta effector SMAD3 inducibly interacts with PTEN on TGF-beta treatment under endogenous conditions. RNA interference (RNAi) suppression of PTEN expression enhances SMAD3 transcriptional activity and TGF-beta-mediated induction of SMAD3 target genes whereas reconstitution of PTEN in a null cancer cell line represses the expression of TGF-beta-regulated target genes. Targeting PTEN expression through RNAi in a PTEN wild-type cell line increases TGF-beta-mediated invasion but does not affect TGF-beta-mediated growth inhibition. Reconstitution of PTEN expression in a PTEN-null cell line blocks TGF-beta-induced invasion but does not modulate TGF-beta-mediated growth regulation. These effects are distinct from Akt and Forkhead family members that also interact with SMAD3 to regulate apoptosis or proliferation, respectively. Pharmacologic inhibitors targeting TGF-beta receptors and phosphatidylinositol 3-kinase signaling downstream from PTEN cooperate to block TGF-beta-mediated invasion. Thus, the loss of PTEN expression in human cancers may contribute to a role for TGF-beta as a tumor enhancer with specific effects on cellular motility and invasion.
Subject(s)
Microfilament Proteins/metabolism , PTEN Phosphohydrolase/metabolism , Smad3 Protein/genetics , Smad3 Protein/metabolism , Transcription, Genetic , Transforming Growth Factor beta/metabolism , Cell Movement , Cell Nucleus/metabolism , Enzyme Inhibitors/pharmacology , Humans , Microfilament Proteins/genetics , Neoplasm Invasiveness , PTEN Phosphohydrolase/antagonists & inhibitors , PTEN Phosphohydrolase/genetics , Phosphatidylinositol 3-Kinases/metabolism , Phosphoinositide-3 Kinase Inhibitors , Promoter Regions, Genetic/genetics , Proto-Oncogene Proteins c-akt/antagonists & inhibitors , Proto-Oncogene Proteins c-akt/metabolism , RNA, Small Interfering/pharmacology , Signal Transduction , Smad2 Protein/metabolism , Tensins , Tumor Cells, CulturedABSTRACT
Malignant gliomas are highly lethal tumors that display striking genetic heterogeneity. Novel therapies that inhibit a single molecular target may slow tumor progression, but tumors are likely not dependent on a signal transduction pathway. Rather, malignant gliomas exhibit sustained mitogenesis and cell growth mediated in part through the effects of receptor tyrosine kinases and the mammalian target of rapamycin (mTOR). AEE788 is a novel orally active tyrosine kinase inhibitor that decreases the kinase activity associated with the epidermal growth factor receptor and, at higher concentrations, the vascular endothelial growth factor receptor 2 (kinase domain region). RAD001 (everolimus) is an orally available mTOR inhibitor structurally related to rapamycin. We hypothesized that combined inhibition of upstream epidermal growth factor receptor and kinase domain region receptors with AEE788 and inhibition of the downstream mTOR pathway with RAD001 would result in increased efficacy against gliomas compared with single-agent therapy. In vitro experiments showed that the combination of AEE788 and RAD001 resulted in increased rates of cell cycle arrest and apoptosis and reduced proliferation more than either agent alone. Combined AEE788 and RAD001 given orally to athymic mice bearing established human malignant glioma tumor xenografts resulted in greater tumor growth inhibition and greater increases in median survival than monotherapy. These studies suggest that simultaneous inhibition of growth factor receptor and mTOR pathways offer increased benefit in glioma therapy.
Subject(s)
Cell Division/drug effects , Glioma/pathology , Protein Kinases/metabolism , Purines/therapeutic use , Animals , Cell Line, Tumor , DNA Replication/drug effects , ErbB Receptors/antagonists & inhibitors , Glioma/drug therapy , Humans , Mice , Mice, Nude , Receptors, Vascular Endothelial Growth Factor/antagonists & inhibitors , TOR Serine-Threonine Kinases , Transplantation, HeterologousABSTRACT
Transforming growth factor-beta (TGF-beta) is a multifunctional cytokine that promotes malignant glioma invasion, angiogenesis, and immunosuppression. Antisense oligonucleotide suppression of TGF-beta(2) ligand expression has shown promise in preclinical and clinical studies but at least two ligands mediate the effects of TGF-beta in gliomas. Therefore, we examined the effects of SB-431542, a novel, small molecule inhibitor of the type I TGF-beta receptor, on a panel of human malignant glioma cell lines. SB-431542 blocked the phosphorylation and nuclear translocation of the SMADs, intracellular mediators of TGF-beta signaling, with decreased TGF-beta-mediated transcription. Furthermore, SB-431542 inhibited the expression of two critical effectors of TGF-beta-vascular endothelial growth factor and plasminogen activator inhibitor-1. SB-431542 treatment of glioma cultures inhibited proliferation, TGF-beta-mediated morphologic changes, and cellular motility. Together, our results suggest that small molecule inhibitors of TGF-beta receptors may offer a novel therapy for malignant gliomas by reducing cell proliferation, angiogenesis, and motility.
