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Vol. 13, Issue 1, 336-347, January 2002
Cold Spring Harbor Laboratory, Cold Spring Harbor, NY 11724
Transformation by oncogenic Ras profoundly alters actin
cytoskeleton organization. We investigated Ras-dependent signaling pathways involved in cytoskeleton disruption by transfecting normal rat
kidney (NRK) cells with different Ras mutants. RasV12S35, a mutant
known to activate specifically the Raf/MAPK pathway, led to stress
fiber and focal contact disruption, whereas the adherens junctions
remained intact. Next, we found that pharmacological inhibition of MEK
was sufficient to restore the cytoskeletal defects of ras-transformed
NRK cells, including assembly of stress fibers and focal contacts, but
it did not induce reorganization of the cell-cell junctions.
Investigating the mechanism underlying this phenotypic reversion, we
found that the sustained MAPK signaling resulting from
Ras-transformation down-regulated the expression of ROCKI and
Rho-kinase, two-Rho effectors required for stress fiber formation, at
the post-transcriptional level. On MEK inhibition, ROCKI/Rho-kinase
expression and cofilin phosphorylation were increased, demonstrating
that the Rho-kinase/LIM-kinase/cofilin pathway was functionally
restored. Finally, using dominant negative or constitutively active
mutants, we demonstrated that expression of ROCKI/Rho-kinase was both
necessary and sufficient to promote cytoskeleton reorganization in
NRK/ras cells. These findings further establish the Ras/MAPK pathway as
the critical pathway involved in cytoskeleton disruption during
Ras-transformation, and they suggest a new mechanism, involving alteration in ROCKI/Rho-kinase expression, by which oncogenic Ras can
specifically target the actin-based cytoskeleton and achieve morphological transformation of the cells.
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