B-Raf Acts via the ROCKII/LIMK/Cofilin Pathway To Maintain Actin Stress Fibers in Fibroblasts

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American Society for Microbiology

RESUMO

Recent data have shown that the BRAF gene is mutated at a high frequency in human malignancies. We have analyzed the migratory characteristics of B-raf−/− mouse embryonic fibroblasts (MEFs) and compared these with the organization of the actin cytoskeleton and the activity of signaling pathways that are known to influence this organization. Disruption of B-raf significantly reduced the levels of phospho-ERK1/2 and, surprisingly, induced an ≈1.5-fold increase in cell migration. Consistent with these findings, the high level of actin stress fibers normally present in MEFs was considerably reduced following disruption of B-raf, and the F-actin content of B-raf−/− cells was less than half that of B-raf+/+ cells. Phosphorylation of the myosin light chain on Thr18/Ser19 residues was not reduced in B-raf−/− cells. Rather, reduced ROCKII expression and attenuated phosphorylation of ADF/cofilin on serine 3 occurred. Normal stress fiber and phosphocofilin levels were restored by the expression of human B-Raf and catalytically active MEK and by the overexpression of LIM kinase (LIMK). These results have important implications for the role of the B-Raf/ERK signaling pathway in regulating cell motility in normal and malignant cells. They suggest that B-Raf is involved in invasiveness by regulating the proper assembly of actin stress fibers and contractility through a ROCKII/LIMK/cofilin signaling pathway.

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