title: Feedback activation of neurofibromin terminates growth factor-induced Ras activation.
abstract: BACKGROUND
  Growth factors induce a characteristically short-lived Ras activation in cells emerging from quiescence. Extensive work has shown that transient as opposed to sustained Ras activation is critical for the induction of mitogenic programs. Mitogen-induced accumulation of active Ras-GTP results from increased nucleotide exchange driven by the nucleotide exchange factor Sos. In contrast, the mechanism accounting for signal termination and prompt restoration of basal Ras-GTP levels is unclear, but has been inferred to involve feedback inhibition of Sos. Remarkably, how GTP-hydrolase activating proteins (GAPs) participate in controlling the rise and fall of Ras-GTP levels is unknown.
  
  
  RESULTS
  Monitoring nucleotide exchange of Ras in permeabilized cells we find, unexpectedly, that the decline of growth factor-induced Ras-GTP levels proceeds in the presence of unabated high nucleotide exchange, pointing to GAP activation as a major mechanism of signal termination. Experiments with non-hydrolysable GTP analogues and mathematical modeling confirmed and rationalized the presence of high GAP activity as Ras-GTP levels decline in a background of high nucleotide exchange. Using pharmacological and genetic approaches we document a raised activity of the neurofibromatosis type I tumor suppressor Ras-GAP neurofibromin and an involvement of Rsk1 and Rsk2 in the down-regulation of Ras-GTP levels.
  
  
  CONCLUSIONS
  Our findings show that, in addition to feedback inhibition of Sos, feedback stimulation of the RasGAP neurofibromin enforces termination of the Ras signal in the context of growth-factor signaling. These findings ascribe a precise role to neurofibromin in growth factor-dependent control of Ras activity and illustrate how, by engaging Ras-GAP activity, mitogen-challenged cells play safe to ensure a timely termination of the Ras signal irrespectively of the reigning rate of nucleotide exchange.
authors: Institute of Molecular Cell Biology, Center for Molecular Biomedicine, University Hospital, Hans-Knöll-Str.2, 07745, Jena, Germany. anne.hennig@med.uni-jena.de.
authors: Anne
authors: A
authors: Hennig
authors: Institute of Molecular Cell Biology, Center for Molecular Biomedicine, University Hospital, Hans-Knöll-Str.2, 07745, Jena, Germany. robby.markwart@gmail.com.
authors: Robby
authors: R
authors: Markwart
authors: Institute of Molecular Cell Biology, Center for Molecular Biomedicine, University Hospital, Hans-Knöll-Str.2, 07745, Jena, Germany. katharina@wolff-braunschweig.de.
authors: Katharina
authors: K
authors: Wolff
authors: Institute of Molecular Cell Biology, Center for Molecular Biomedicine, University Hospital, Hans-Knöll-Str.2, 07745, Jena, Germany. katja.schubert@hki-jena.de.
authors: Katja
authors: K
authors: Schubert
authors: Leibniz Institute for Age Research - Fritz Lipmann Institute, 07745, Jena, Germany. cuiyan@fli-leibniz.de.
authors: Yan
authors: Y
authors: Cui
authors: Division of Cellular and Molecular Physiology, Institute of Translational Medicine, University of Liverpool, Liverpool, L69 3BX, UK. iprior@liverpool.ac.uk.
authors: Ian A
authors: IA
authors: Prior
authors: Systems Biology Doctoral Training Centre, University of Warwick, Coventry, CV4 7AL, UK. M.A.Esparza-Franco@warwick.ac.uk.
authors: Manuel A
authors: MA
authors: Esparza-Franco
authors: Department of Pharmacology, University of Cambridge, Cambridge, CB2 1PD, UK. grl30@cam.ac.uk.
authors: Graham
authors: G
authors: Ladds
authors: Institute of Molecular Cell Biology, Center for Molecular Biomedicine, University Hospital, Hans-Knöll-Str.2, 07745, Jena, Germany. ignacio.rubio@med.uni-jena.de.|Center for Sepsis Control and Care, University Hospital, 07747, Jena, Germany. ignacio.rubio@med.uni-jena.de.
authors: Ignacio
authors: I
authors: Rubio
mesh_terms: D000818:Animals; D002460:Cell Line; D004789:Enzyme Activation; D004815:Epidermal Growth Factor / Q000378:metabolism*; D006160:Guanosine Triphosphate / Q000378:metabolism; D057809:HEK293 Cells; D006367:HeLa Cells; D006801:Humans; D020935:MAP Kinase Signaling System; D051379:Mice; D025542:Neurofibromin 1 / Q000378:metabolism*; D010766:Phosphorylation; D038744:Ribosomal Protein S6 Kinases, 90-kDa / Q000378:metabolism; D015398:Signal Transduction; D020839:Son of Sevenless Protein, Drosophila / Q000378:metabolism; D018631:ras Proteins / Q000378:metabolism*
chemical_list: D025542:Neurofibromin 1; D020839:Son of Sevenless Protein, Drosophila; D004815:Epidermal Growth Factor; D006160:Guanosine Triphosphate; C069329:RPS6KA1 protein, human; D038744:Ribosomal Protein S6 Kinases, 90-kDa; D018631:ras Proteins