The Ras-ERK pathway modulates cytoskeleton organization, cell motility and lung metastasis signature genes in MDA-MB-231 LM2

MDA-MB-231 LM2 (herein referred to as LM2) is a derivative of MDA-MB-231 cells that was selected for its ability to metastasize to lung tissue in vivo. We investigated cellular properties of LM2 including actin cytoskeleton organization, motility and signaling pathways that drive the expression of genes associated with the lung metastasis signature. Parental cells exhibit well-developed stress fibers, whereas LM2 had poorly organized stress fibers. LM2 exhibited higher levels of K-Ras protein and corresponding higher levels of phosphorylated ERK compared with parental cells. The Ras-ERK pathway was responsible for the disruption of stress fibers because inhibition of MEK with UO126 or small interfering RNA (siRNA) against K-Ras or ERK1/2 resulted in restoration of stress fibers and focal adhesions. We observed that the K-Ras-ERK pathway is important for the expression of genes associated with the lung metastasis signature. Paradoxically, inhibition of the Ras-ERK pathway did not result in inhibition of cell motility but was accompanied by activation of the phosphatidylinositol 3-kinase (PI3K) pathway. Inhibition of both ERK and PI3K pathways was required to inhibit motility of LM2 cells. These results suggest that both ERK and PI3K pathways drive motile functions of metastatic LM2 cells and genes associated with the lung metastasis signature.