Constitutive activation of the RON gene, known to code for the tyrosine-kinase receptor for Macrophage Stimulating Protein (also known as Scatter Factor 2), has been shown to induce invasive-metastatic phenotype in vitro. As yet, nothing is known about the expression of this novel member of the MET-oncogene family in spontaneously occurring human cancers. Here we report that Ron is expressed at abnormally high levels in about 50% primary breast carcinomas (35/74 patients). Among these, the expression is increased more than 20-fold in 12 cases and the overexpressed protein is constitutively phosphorylated on tyrosine residues. Notably, Ron is only barely detectable in epithelial cells of the mammary gland, and its expression remains unchanged in benign breast lesions (including adenomas and papillomas). Overexpression was observed in di erent histotypic variants of carcinomas; it is associated with the disease at any stage and correlates with the post-menopausal status. In breast carcinoma cells grown in vitro, activation of the Ron receptor resulted in proliferation, migration and invasion through reconstituted basement membranes. Altogether, these data suggest a role for the RON gene in progression of human breast carcinomas to the invasive-metastatic phenotype.

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Activation of the HGF receptor, encoded by the c-MET protooncogene (Met receptor), triggers motility, matrixinvasion and branching morphogenesis in epithelial cells. It has recently been shown that the Met receptor interacts with Gab-1, an IRS-like adaptor protein, via the docking site (Y 1349 VHVNATY 1356 VNV) known to bind Grb2 and multiple SH2-containing signal transducers. Here we show that Gab1 is the major phosphorylation-substrate of the Met receptor and of its oncogenic variant Tpr-Met. A series of point mutations in the docking site established a direct correlation between the ability to recruit and phosphorylate Gab1 and the transforming potential. Interestingly, the mutations of either Y 1356 or N 1358 abolished the binding of both Grb2 and Gab1 in intact cells. Furthermore, peptides designed to block either the SH2 or the SH3 domains of Grb2 interfered with the receptor-Gab1 interaction. These data indicate that Gab1 coupling to the Met receptor requires binding of Grb2 and correlates with the transforming potential of Tpr-Met.

Met, the high affinity receptor for hepatocyte growth factor, is one of the most frequently activated tyrosine kinases in human cancer and a validated target for cancer therapy. We previously developed a mouse monoclonal antibody directed against the extracellular portion of Met (

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The Slit protein acts through the Roundabout receptor as a paracrine chemorepellent in axon guidance and as an inhibitor in leukocyte chemotaxis, but its role in epithelial cell motility and morphogenesis remains largely unexplored. We report that nontransformed epithelial cells and cancerous cells empower the Slit-2/Robo1 signaling system to limit outward migration in response to motogenic attractants and to remain positionally confined within their primitive location. Short hairpin RNA-mediated depletion of SLIT-2 or ectopic expression of a soluble decoy Robo enhance hepatocyte growth factor (HGF)-induced migration, matrix invasion, and tubulogenesis, concomitantly with the up-regulation of Cdc-42 and the down-modulation of Rac-1 activities. Accordingly, autocrine overexpression or exogenous administration of Slit-2 prevent HGF-triggered motile responses, reduce Cdc-42 activation, and stimulate Rac-1. This antimigratory activity of Slit-2 derives from the inhibition of actin-based protrusive forces and from an increased adhesive strength of cadherinmediated intercellular contacts. These results disclose a novel function for Slit and Robo in the inhibition of growth factor-mediated epithelial cell motility and morphogenesis, invoking a critical role for both molecules as natural antagonists of neoplastic invasive growth.

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