Mutant Met-mediated transformation is ligand-dependent and can be inhibited by HGF antagonists

Mutations in the genes encoding for Met, Ret and Kit receptor tyrosine kinases invariably result in increased kinase activity and in the acquisition of transforming potential. However, the requirement of receptor ligands for the transformation process is still unclear. We have investigated the role of hepatocyte growth factor (HGF), the high-affinity ligand for Met, in mutant Met-mediated cell transformation. We provide evidence that the transforming potential displayed by mutant forms of Met found in human cancer is not only sensitive but entirely dependent on the presence of HGF, by showing that mutant Met transforms NIH3T3 fibroblasts, which produce endogenous HGF, but is not able to transform epithelial cells, unless exogenous HGF is supplied. Accordingly, mutant Met-induced transformation of NIH3T3 cells can be inhibited by HGF antagonists and increased by HGF stimulation. We also show that an engineered Met receptor which contains an oncogenic mutation but is impaired in its ability to bind HGF completely loses its transforming activity, which can be rescued by causing receptor dimerization using a monoclonal antibody. These results indicate that point mutations resulting in Met kinase activation are necessary but not sufficient to cause cell transformation, the latter being dependent on ligand-induced receptor dimerization. They also suggest that mutant Met-driven tumour growth depends on the availability and tissue distribution of active HGF, and provide proof-of-concept for the treatment of mutant-Met related pathologies by HGF-antagonizing drugs.

Agonistic monoclonal antibodies against the Met receptor dissect the biological responses to HGF

Hepatocyte growth factor, also known as scatter factor, is a pleiotropic cytokine, which stimulates cell motility, invasion, proliferation, survival and morphogenesis, and induces the expression of specific genes by activating its receptor tyrosine kinase. In this work we have isolated, characterized and used as agonists two monoclonal antibodies (mAbs) directed against the extracellular domain of HGF receptor to investigate the requirements for receptor activation and for the different biological responses. The two mAbs display similar affinities, react with epitopes different from the hepatocyte growth factor binding site, and behave as either full or partial agonists. The full agonist mAb (DO-24) triggers all the biological effects elicited by hepatocyte growth factor, namely motility, proliferation, cell survival, invasion, tubulogenesis and angiogenesis. The partial agonist mAb (DN-30) induces only motility. Only the full agonist mAb is able to induce and sustain the expression of urokinase-type plasminogen activator receptor for prolonged periods of time, while both mAbs up-regulate the constitutive expression of urokinase-type plasminogen activator. Both mAbs activate receptor phosphorylation, which, being strictly dependent on mAb bivalence, requires receptor dimerization. Since simple receptor dimerization is not sufficient to trigger full biological responses, we propose that the region on the ss chain of the receptor recognized by the full agonist mAb is crucial for optimal receptor activation.

Overexpression and activation of hepatocyte growth factor/scatter factor in human non-small-cell lung carcinomas

Hepatocyte growth factor/scatter factor (HGF/SF) stimulates the invasive growth of epithelial cells via the c-MET oncogene-encoded receptor. In normal lung, both the receptor and the ligand are detected, and the latter is known to be a mitogenic and a motogenic factor for both cultured bronchial epithelial cells and non-small-cell carcinoma lines. Here, ligand and receptor expression was examined in 42 samples of primary human non-small-cell lung carcinoma of different histotype. Each carcinoma sample was compared with adjacent normal lung tissue. The Met/HGF receptor was found to be 2 to 10-fold increased in 25% of carcinoma samples (P = 0.0113). The ligand, HGF/SF, was found to be 10 to 100-fold overexpressed in carcinoma samples (P < 0.0001). Notably, while HGF/SF was occasionally detectable and found exclusively as a single-chain inactive precursor in normal tissues, it was constantly in the biologically-active heterodimeric form in carcinomas. Immunohistochemical staining showed homogeneous expression of both the receptor and the ligand in carcinoma samples, whereas staining was barely detectable in their normal counterparts. These data show that HGF/SF is overexpressed and consistently activated in non-small-cell lung carcinomas and may contribute to the invasive growth of lung cancer.