Growth factor-dependent activation of alphavbeta3 integrin in normal epithelial cells: implications for tumor invasion.

Integrin activation is a multifaceted phenomenon leading to increased affinity and avidity for matrix ligands. To investigate whether cytokines produced during stromal infiltration of carcinoma cells activate nonfunctional epithelial integrins, a cellular system of human thyroid clones derived from normal glands (HTU-5) and papillary carcinomas (HTU-34) was employed. In HTU-5 cells, alphavbeta3 integrin was diffused all over the membrane, disconnected from the cytoskeleton, and unable to mediate adhesion. Conversely, in HTU-34 cells, alphavbeta3 was clustered at focal contacts (FCs) and mediated firm attachment and spreading. alphavbeta3 recruitment at FCs and ligand-binding activity, essentially identical to those of HTU-34, occurred in HTU-5 cells upon treatment with hepatocyte growth factor/scatter factor (HGF/SF). The HTU-34 clone secreted HGF/SF and its receptor was constitutively tyrosine phosphorylated suggesting an autocrine loop responsible for alphavbeta3 activated state. Antibody-mediated inhibition of HGF/SF function in HTU-34 cells disrupted alphavbeta3 enrichment at FCs and impaired adhesion. Accordingly, activation of alphavbeta3 in normal cells was produced by HTU-34 conditioned medium on the basis of its content of HGF/SF. These results provide the first example of a growth factor-driven integrin activation mechanism in normal epithelial cells and uncover the importance of cytokine-based autocrine loops for the physiological control of integrin activation.