No abstract

Scatter Factor (SF) is a fibroblast‐secreted protein which promotes motility and matrix invasion of epithelial cells. Hepatocyte Growth Factor (HGF) is a powerful mitogen for hepatocytes and other epithelial tissues. SF and HGF, purified according to their respective biological activities, were interchangeable and equally effective in assays for cell growth, motility and invasion. Both bound with identical affinities to the same sites in target cells. The receptor for SF and HGF was identified as the product of the MET oncogene by: (i) ligand binding and coprecipitation in immunocomplexes; (ii) chemical crosslinking to the Met beta subunit; (iii) transfer of binding activity in insect cells by a baculovirus carrying the MET cDNA; (iv) ligand‐induced tyrosine phosphorylation of the Met beta subunit. SF and HGF cDNA clones from human fibroblasts, placenta and liver had virtually identical sequences. We conclude that the same molecule (SF/HGF) acts as a growth or motility factor through a single receptor in different target cells.

Growth factor receptors with protein tyrosine kinase activity are central to the control of proliferation of both normal and malignant cells. Using anti-phosphotyrosine antibodies, we have previously identified a transmembrane glycoprotein with abnormally high protein tyrosine kinase activity in a human gastric tumour cell line (GTL-16). Electrophoresis under non-reducing conditions revealed that this kinase (relative molecular mass 145,000 (145 K)) is disulphide-linked to a 50K chain in an alpha beta-complex of 190K (p190). From its novel two-chain structure, we deduced that p190 was the prototype of a new class of tyrosine kinase receptors. We now show that p190 is indistinguishable from the protein encoded by the c-met proto-oncogene and that the alpha beta-subunit structure is conserved in other human cell lines. We also show that the high level of p190 found in the GTL-16 cell line is accompanied by amplification and overexpression of c-met. This provides the first example of a functional alteration of c-met in a human tumour cell line.

Many microRNAs (miRNAs), posttranscriptional regulators of numerous cellular processes and developmental events, are downregulated in tumors. However, their role in tumorigenesis remains largely unknown. In this work, we examined the role of the muscle-specific miRNAs miR-1 and miR-206 in human rhabdomyosarcoma (RMS), a soft tissue sarcoma thought to arise from skeletal muscle progenitors. We have shown that miR-1 was barely detectable in primary RMS of both the embryonal and alveolar subtypes and that both miR-1 and miR-206 failed to be induced in RMS cell lines upon serum deprivation. Moreover, reexpression of miR-206 in RMS cells promoted myogenic differentiation and blocked tumor growth in xenografted mice by switching the global mRNA expression profile to one that resembled mature muscle. Finally, we showed that the product of the MET proto-oncogene, the Met tyrosine-kinase receptor, which is overexpressed in RMS and has been implicated in RMS pathogenesis, was downregulated in murine satellite cells by miR-206 at the onset of normal myogenesis. Thus, failure of posttranscriptional modulation may underlie Met overexpression in RMS and other types of cancer. We propose that tissue-specific miRNAs such as miR-1 and miR-206, given their ability to modulate hundreds of transcripts and to act as nontoxic differentiating agents, may override the genomic heterogeneity of solid tumors and ultimately hold greater therapeutic potential than single gene-directed drugs.

Skeletal muscle in vertebrates is derived from somites, epithelial structures of the paraxial mesoderm, yet many unrelated reports describe the occasional appearance of myogenic cells from tissues of nonsomite origin, suggesting either transdifferentiation or the persistence of a multipotent progenitor. Here, we show that clonable skeletal myogenic cells are present in the embryonic dorsal aorta of mouse embryos. This finding is based on a detailed clonal analysis of different tissue anlagen at various developmental stages. In vitro, these myogenic cells show the same morphology as satellite cells derived from adult skeletal muscle, and express a number of myogenic and endothelial markers. Surprisingly, the latter are also expressed by adult satellite cells. Furthermore, it is possible to clone myogenic cells from limbs of mutant c-Met−/− embryos, which lack appendicular muscles, but have a normal vascular system. Upon transplantation, aorta-derived myogenic cells participate in postnatal muscle growth and regeneration, and fuse with resident satellite cells.The potential of the vascular system to generate skeletal muscle cells may explain observations of nonsomite skeletal myogenesis and raises the possibility that a subset of satellite cells may derive from the vascular system.