Nucleophosmin mediates mammalian target of rapamycin-dependent actin cytoskeleton dynamics and proliferation in neurofibromin-deficient astrocytes.
Neurofibromatosis type 1 (NF1) is a common autosomal dominant tumor predisposition syndrome in which affected individuals develop astrocytic brain tumors (gliomas). To determine how the NF1 gene product (neurofibromin) regulates astrocyte growth and motility relevant to glioma formation, we have used Nf1-deficient primary murine astrocytes. Nf1(-/-) astrocytes exhibit increased protein translation and cell proliferation, which are mediated by Ras-dependent hyperactivation of the mammalian target of rapamycin (mTOR) protein, a serine/threonine protein kinase that regulates ribosomal biogenesis, protein translation, actin cytoskeleton dynamics, and cell proliferation. In this study, we show that Nf1-deficient astrocytes have fewer actin stress fibers and exhibit increased cell motility compared with wild-type astrocytes, which are rescued by pharmacologic and genetic mTOR inhibition. We further show that mTOR-dependent regulation of actin stress fiber formation, motility, and proliferation requires rapamycin-sensitive activation of the Rac1 GTPase but not elongation factor 4E-binding protein 1/S6 kinase. Nf1(-/-) astrocytes also exhibit increased protein translation and ribosomal biogenesis through increased expression of the nucleophosmin (NPM) nuclear-cytoplasmic shuttling protein. We found that NPM expression in Nf1(-/-) astrocytes was blocked by rapamycin in vitro and in vivo and that expression of a dominant-negative NPM mutant protein in Nf1(-/-) astrocytes rescued actin stress fiber formation and restored cell motility and proliferation to wild-type levels. Together, these data show that neurofibromin regulates actin cytoskeleton dynamics and cell proliferation through a mTOR/Rac1-dependent signaling pathway and identify NPM as a critical mTOR effector mediating these biological properties in Nf1-deficient astrocytes.
Sandsmark
Danielle K
DK
Department of Neurology, Washington University School of Medicine, St. Louis, Missouri 63110, USA.
Zhang
Huabiao
H
Hegedus
Balazs
B
Pelletier
Corey L
CL
Weber
Jason D
JD
Gutmann
David H
DH
D000199:Actins / Q000096:biosynthesis / Q000378:metabolism*; D048868:Adaptor Proteins, Signal Transducing; D000818:Animals; D001253:Astrocytes / Q000166:cytology* / Q000378:metabolism*; D002352:Carrier Proteins / Q000378:metabolism; D018797:Cell Cycle Proteins; D048708:Cell Growth Processes / Q000502:physiology; D002465:Cell Movement / Q000502:physiology; D002478:Cells, Cultured; D003599:Cytoskeleton / Q000378:metabolism / Q000502:physiology*; D004789:Enzyme Activation; D039642:Eukaryotic Initiation Factors; D051379:Mice; D009479:Neuropeptides / Q000378:metabolism; D009687:Nuclear Proteins / Q000096:biosynthesis / Q000172:deficiency / Q000378:metabolism / Q000502:physiology*; D000090243:Nucleophosmin; D010750:Phosphoproteins / Q000378:metabolism; D051571:Protein Kinase C-alpha / Q000378:metabolism; D011494:Protein Kinases / Q000378:metabolism; D019893:Ribosomal Protein S6 Kinases / Q000378:metabolism; D058570:TOR Serine-Threonine Kinases; D020744:rac GTP-Binding Proteins / Q000378:metabolism; D020830:rac1 GTP-Binding Protein
D000199:Actins; D048868:Adaptor Proteins, Signal Transducing; D002352:Carrier Proteins; D018797:Cell Cycle Proteins; C499527:Eif4ebp1 protein, mouse; D039642:Eukaryotic Initiation Factors; D009479:Neuropeptides; D009687:Nuclear Proteins; D010750:Phosphoproteins; C484602:Rac1 protein, mouse; D000090243:Nucleophosmin; D011494:Protein Kinases; C546843:mTOR protein, mouse; D019893:Ribosomal Protein S6 Kinases; D058570:TOR Serine-Threonine Kinases; D051571:Protein Kinase C-alpha; D020744:rac GTP-Binding Proteins; D020830:rac1 GTP-Binding Protein