ABSTRACT
Loss-of-function mutations in the NF1 tumor suppressor result in deregulated Ras signaling and drive tumorigenesis in the familial cancer syndrome neurofibromatosis type I. However, the extent to which NF1 inactivation promotes sporadic tumorigenesis is unknown. Here we report that NF1 is inactivated in sporadic gliomas via two mechanisms: excessive proteasomal degradation and genetic loss. NF1 protein destabilization is triggered by the hyperactivation of protein kinase C (PKC) and confers sensitivity to PKC inhibitors. However, complete genetic loss, which only occurs when p53 is inactivated, mediates sensitivity to mTOR inhibitors. These studies reveal an expanding role for NF1 inactivation in sporadic gliomagenesis and illustrate how different mechanisms of inactivation are utilized in genetically distinct tumors, which consequently impacts therapeutic sensitivity.
Subject(s)
Genes, Tumor Suppressor , Glioblastoma/genetics , Glioma/genetics , Mutation , Neurofibromatosis 1/genetics , Neurofibromin 1/antagonists & inhibitors , Neurofibromin 1/genetics , Neurofibromin 1/metabolism , Proteasome Endopeptidase Complex/genetics , 3T3 Cells , Animals , Down-Regulation , Genes, p53 , Genes, ras , Glioblastoma/enzymology , Glioma/enzymology , Mice , Neurofibromatosis 1/metabolism , Protein Kinase C/antagonists & inhibitors , Protein Kinase C/metabolism , ras Proteins/genetics , ras Proteins/metabolismABSTRACT
Oncogene-induced senescence functions to limit tumor development. However, a complete understanding of the signals that trigger this type of senescence is currently lacking. We found that mutations affecting NF1, Raf, and Ras induce a global negative feedback response that potently suppresses Ras and/or its effectors. Moreover, these signals promote senescence by inhibiting the Ras/PI3K pathway, which can impact the senescence machinery through HDM2 and FOXO. This negative feedback program is regulated in part by RasGEFs, Sprouty proteins, RasGAPs, and MKPs. Moreover, these signals function in vivo in benign human tumors. Thus, the ultimate response to the aberrant activation of the Ras pathway is a multifaceted negative feedback signaling network that terminates the oncogenic signal and participates in the senescence response.
Subject(s)
Cellular Senescence , Genes, ras/physiology , Signal Transduction/physiology , Animals , Cells, Cultured , Feedback , Genes, Neurofibromatosis 1/physiology , Genes, Retinoblastoma/physiology , Genes, p53/physiology , Humans , Mice , Neoplasms/genetics , Neoplasms/pathology , Phosphatidylinositol 3-Kinases/physiology , Stem Cells/pathology , raf Kinases/physiologyABSTRACT
The intermediate filament (IF)-binding protein desmoplakin (DP) is essential for desmosome function and tissue integrity, but its role in junction assembly is poorly understood. Using time-lapse imaging, we show that cell-cell contact triggers three temporally overlapping phases of DP-GFP dynamics: (1) the de novo appearance of punctate fluorescence at new contact zones after as little as 3 min; (2) the coalescence of DP and the armadillo protein plakophilin 2 into discrete cytoplasmic particles after as little as 15 min; and (3) the cytochalasin-sensitive translocation of cytoplasmic particles to maturing borders, with kinetics ranging from 0.002 to 0.04 microm/s. DP mutants that abrogate or enhance association with IFs exhibit delayed incorporation into junctions, altering particle trajectory or increasing particle pause times, respectively. Our data are consistent with the idea that DP assembles into nascent junctions from both diffusible and particulate pools in a temporally overlapping series of events triggered by cell-cell contact and regulated by actin and DP-IF interactions.
