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Originally published as MBC in Press, 10.1091/mbc.E07-09-0895 on November 28, 2007

Vol. 19, Issue 2, 498-508, February 2008

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B-RAF Regulation of Rnd3 Participates in Actin Cytoskeletal and Focal Adhesion Organization

R. Matthew Klein, Laurie S. Spofford, Ethan V. Abel, Arisa Ortiz, and Andrew E. Aplin

Center for Cell Biology and Cancer Research, Albany Medical College, Albany, NY 12208

Submitted September 13, 2007; Revised November 5, 2007; Accepted November 19, 2007
Monitoring Editor: Jean Schwarzbauer

The actin cytoskeleton controls multiple cellular functions, including cell morphology, movement, and growth. Accumulating evidence indicates that oncogenic activation of the mitogen-activated protein kinase kinase/extracellular signal-regulated kinase 1/2 (MEK/ERK1/2) pathway is accompanied by actin cytoskeletal reorganization. However, the signaling events contributing to actin cytoskeleton remodeling mediated by aberrant ERK1/2 activation are largely unknown. Mutant B-RAF is found in a variety of cancers, including melanoma, and it enhances activation of the MEK/ERK1/2 pathway. We show that targeted knockdown of B-RAF with small interfering RNA or pharmacological inhibition of MEK increased actin stress fiber formation and stabilized focal adhesion dynamics in human melanoma cells. These effects were due to stimulation of the Rho/Rho kinase (ROCK)/LIM kinase-2 signaling pathway, cumulating in the inactivation of the actin depolymerizing/severing protein cofilin. The expression of Rnd3, a Rho antagonist, was attenuated after B-RAF knockdown or MEK inhibition, but it was enhanced in melanocytes expressing active B-RAF. Constitutive expression of Rnd3 suppressed the actin cytoskeletal and focal adhesion effects mediated by B-RAF knockdown. Depletion of Rnd3 elevated cofilin phosphorylation and stress fiber formation and reduced cell invasion. Together, our results identify Rnd3 as a regulator of cross talk between the RAF/MEK/ERK and Rho/ROCK signaling pathways, and a key contributor to oncogene-mediated reorganization of the actin cytoskeleton and focal adhesions.

This was published online ahead of print in MBC in Press (http://www.molbiolcell.org/cgi/doi/10.1091/mbc.E07-09-0895) on November 28, 2007.

Address correspondence to: A. E. Aplin (aplina{at}mail.amc.edu)

Abbreviations used: NHEM, normal human epidermal melanocyte; VGP, vertical growth phase.






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