B4 Integrin Is a Transforming Molecule that Unleashes Met Tyrosine Kinase Tumorigenesis

Cell multiplication in the absence of integrin-derived adhesive signals (anchorage-independent growth) is the phenotypic hallmark of neoplastic transformation. Therefore, the frequently observed up-regulation of some integrins in tumors has been interpreted as an epiphenomenon and not as a causative factor of oncogenic conversion. B4 integrin stimulates proliferation and survival of epithelial cells and is overexpressed in human carcinomas, often in concomitance with up-regulation of the Met tyrosine kinase receptor for hepatocyte growth factor. Met is not endowed with transforming ability but can exploit the B4 cytoplasmic tail as a substrate/adaptor for amplification of mitogenic and antiapoptotic responses, independently of cell adhesion. Here, we show that overexpression of B4 is sufficient to transform rodent fibroblasts, enhances anchorage-independent growth of breast carcinoma cells, and induces tumorigenesis in nude mice; conversely, RNA interference–mediated depletion abrogates the transformed phenotype of neoplastic cells. These autonomous oncogenic properties are dramatically exacerbated upon Met coexpression, suggesting that the integrin can instigate the latent tumorigenic potential of the kinase. A B4 nonadhesive variant still cooperates with Met for cellular transformation, confirming the adhesion-independent function of B4 in magnification of Met biological effects. Conversely, a B4 signaling-incompetent mutant that cannot be efficiently tyrosine phosphorylated by Met and displays reduced ability to activate phosphatidylinositol 3-kinase– dependent and Ras-dependent pathways aborts transformation. Our findings define B4 as a signaling accomplice (a ‘‘servo-oncogene’’) of tyrosine kinase proto-oncogenes in primary carcinogenesis, evoke an unorthodox function for a prototypic adhesion molecule in the positive regulation of anchorage-independent growth, and suggest the use of B4 as a target for anticancer therapy. (Cancer Res 2005; 65(23): 10674-9)