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Molecular and Cellular Biology, January 2000, p. 158-172, Vol. 20, No. 1
0270-7306/0/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.

Phosphatidylinositol 3-Kinase, Cdc42, and Rac1 Act Downstream of Ras in Integrin-Dependent Neurite Outgrowth in N1E-115 Neuroblastoma Cells

Shula Sarner,1 Robert Kozma,1,2,* Sohail Ahmed,1,2 and Louis Lim1,2

Department of Neurochemistry, Institute of Neurology, London WC1N 1PJ, United Kingdom,1 and Glaxo-IMCB Group, Institute of Molecular and Cell Biology, Singapore 0511, Singapore2

Received 9 December 1998/Returned for modification 25 January 1999/Accepted 29 September 1999

Ras and Rho family GTPases have been ascribed important roles in signalling pathways determining cellular morphology and growth. Here we investigated the roles of the GTPases Ras, Cdc42, Rac1, and Rho and that of phosphatidylinositol 3-kinase (PI 3-kinase) in the pathway leading from serum starvation to neurite outgrowth in N1E-115 neuroblastoma cells. Serum-starved cells grown on a laminin matrix exhibited integrin-dependent neurite outgrowth. Expression of dominant negative mutants of Ras, PI 3-kinase, Cdc42, or Rac1 all blocked this neurite outgrowth, while constitutively activated mutants of Ras, PI 3-kinase, or Cdc42 were each sufficient to promote outgrowth even in the presence of serum. A RasH40C;G12V double mutant which binds preferentially to PI 3-kinase also promoted neurite formation. Activated RasG12V-induced outgrowth required PI 3-kinase activity, but activated PI 3-kinase-induced outgrowth did not require Ras activity. Although activated Rac1 by itself did not induce neurites, neurite outgrowth induced by activated Cdc42G12V was Rac1 dependent. Cdc42G12V-induced neurites appeared to lose their normal polarization, almost doubling the average number of neurites produced by a single cell. Outgrowth induced by activated Ras or PI 3-kinase required both Cdc42 and Rac1 activity, but Cdc42G12V-induced outgrowth did not need Ras or PI 3-kinase activity. Active RhoG14V reduced outgrowth promoted by RasG12V. Finally, expression of dominant negative Jun N-terminal kinase or extracellular signal-regulated kinase did not inhibit outgrowth, suggesting these pathways are not essential for this process. Our results suggest a hierarchy of signalling where Ras signals through PI 3-kinase to Cdc42 and Rac1 activation (and Rho inactivation), culminating in neurite outgrowth. Thus, in the absence of serum factors, Ras may initiate cell cycle arrest and terminal differentiation in N1E-115 neuroblastoma cells.

* Corresponding author. Mailing address: Department of Neurochemistry, Institute of Neurology, UCL, 1 Wakefield St., London WC1N 1PJ, United Kingdom. Phone: 0171-278 1552. Fax: 0171-278 7045. E-mail: r.kozma{at}ion.ucl.ac.uk.

Molecular and Cellular Biology, January 2000, p. 158-172, Vol. 20, No. 1
0270-7306/0/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.


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