Metastasis is the predominant cause of death in breast cancer patients. Several lines of evidence have shown that microRNAs (miRs) can have an important role in cancer metastasis. Using isogenic pairs of low and high metastatic lines derived from a human breast cancer line, we have identified miR-149 to be a suppressor of breast cancer cell invasion and metastasis. We also identified GIT1 (G-protein-coupled receptor kinase-interacting protein 1) as a direct target of miR-149. Knockdown of GIT1 reduced migration/invasion and metastasis of highly invasive cells. Re-expression of GIT1 significantly rescued miR-149-mediated inhibition of cell migration/invasion and metastasis. Expression of miR-149 impaired fibronectin-induced focal adhesion formation and reduced phosphorylation of focal adhesion kinase and paxillin, which could be restored by re-expression of GIT1. Inhibition of GIT1 led to enhanced protein degradation of paxillin and α5β1 integrin via proteasome and lysosome pathways, respectively. Moreover, we found that GIT1 depletion in metastatic breast cancer cells greatly reduced α5β1-integrin-mediated cell adhesion to fibronectin and collagen. Low level of miR-149 and high level of GIT1 was significantly associated with advanced stages of breast cancer, as well as with lymph node metastasis. We conclude that miR-149 suppresses breast cancer cell migration/invasion and metastasis by targeting GIT1, suggesting potential applications of the miR-149-GIT1 pathway in clinical diagnosis and therapeutics.

Transforming growth factor (TGF)-b1 has a biphasic effect on rat intestinal epithelial (RIE) cells. By itself, TGF-b1 functions as a tumor suppressor by inhibiting the growth, migration and invasion of RIE cells. We show in this study that in conjunction with epidermal growth factor (EGF), TGF-b1 helped to augment migration, invasion and anchorage-independent growth (AIG) compared to that by EGF alone. EGF plus TGF-b1 induced a dramatic morphological change characteristic of epithelial-mesenchymal transition (EMT). The mechanism for this enhanced effect of TGF-b1 and EGF on oncogenic properties was explored by analysis of EGF-and TGF-b1-mediated signaling pathways and complementary DNA arrays. TGF-b1 augmented EGF-mediated signaling of mitogen-activated protein kinase (MAPK) and AKT by enhancing and prolonging the activation of the former and prolonging the activation of the latter. Inhibition of MAPK, but not phosphoinositide-3 kinase (PI3K), abolished TGF-b1 plus EGF-induced EMT and downregulation of E-cadherin at mRNA and protein levels. By contrast, cell migration and invasion were sensitive to inhibition of either MAPK or PI3 kinase. TGF-b1 plus EGF-induced AIG was significantly more resistant to inhibition of PI3K and MAPK compared to that induced by EGF alone. EGF and TGF-b1 synergistically induced the expression of a series of proteases including matrix metalloproteinase (MMP) 1 (collagenase), MMP3, MMP9, MMP10, MMP14 and cathepsin. Among them, the expression of MMP1, MMP3, MMP9 and MMP10 was MAPK dependent. Inhibition of the MMPs or cathepsin significantly blocked EGF plus TGF-b1-induced invasion, but had no effect on colony formation.Phospholipase C (PLC) and Cox2 induced by EGF plus TGF-b1 also played a significant role in invasion, whereas PLC was also important for colony formation. Our study reveals specific signaling functions and induction of genes differentially required for enhanced effect of EGF-and TGF-b1-induced oncogenic properties, and helps to explain the tumor-promoting effect of TGF-b1 in human cancer with elevated expression or activation of TGF-b1 and receptor protein tyrosine kinases.

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