A splicing variant of the RON transcript induces constitutive tyrosine kinase activity and an invasive phenotype.

The Ron tyrosine kinase receptor shares with the members of its subfamily (Met and Sea) a unique functional feature: the control of cell dissociation, motility, and invasion of extracellular matrices (scattering). The mature Ron protein is a heterodimer of disulfide-linked alpha and beta chains, originated by proteolytic cleavage of a single-chain precursor of 185 kDa. In a human gastric cancer cell line (KATO-III), we found abnormal accumulation of an uncleaved single-chain protein (delta-Ron) of 165 kDa; this molecule is encoded by a transcript differing from the full-length RON mRNA by an in-frame deletion of 49 amino acids in the beta-chain extracellular domain. The deleted transcript originates by an alternatively spliced cassette exon of 147 bp, flanked by two short introns. The delta-Ron tyrosine kinase is constitutively activated by disulfide-linked intracellular oligomerization because it contains an uneven number of cysteine residues. Oligomerization and constitutive tyrosine phosphorylation of the full-size Ron was obtained by site-directed mutagenesis of a single cysteine residue in the region encoded by the cassette exon, mimicking that occurring in the delta-Ron isoform. Inhibition of thiol-mediated intermolecular disulfide bonding prevented delta-Ron oligomerization. The intracellular activation of Ron is followed by acquisition of invasive properties in vitro. These data (i) provide a novel molecular mechanism for posttranscriptional activation of a tyrosine kinase receptor protein and (ii) suggest a role for the Ron receptor in progression toward malignancy.

Constitutive activation of the RON gene promotes invasive growth but not transformation.

MET, RON, and SEA are members of a gene family encoding tyrosine kinase receptors with distinctive properties. Besides mediating growth, they control cell dissociation, motility ("scattering"), and formation of branching tubules. While there are transforming counterparts of MET and SEA, no oncogenic forms of RON have yet been identified. A chimeric Tpr-Ron, mimicking the oncogenic form of Met (Tpr-Met) was generated to investigate its transforming potential. For comparison, a chimeric Tpr-Sea was also constructed. Fusion with Tpr induced constitutive activation of the Ron and Sea kinases. While Tpr-Sea was more efficient than Tpr-Met in transformation, Tpr-Ron did not transform NIH 3T3 cells. The differences in the transforming abilities of Tpr-Met and Tpr-Ron were linked to the functional features of the respective tyrosine kinases using the approach of swapping subdomains. Kinetic analysis showed that the catalytic efficiency of Tpr-Ron is five times lower than that of Tpr-Met. Moreover, constitutive activation of Ron resulted in activation of the MAP kinase signaling cascade approximately three times lower than that attained by Tpr-Met. However, constitutive activation of Ron did induce a mitogenic-invasive response, causing cell dissociation, motility, and invasion of extracellular matrices. Tpr-Ron also induced formation of long, unbranched tubules in tridimensional collagen gels. These data show that RON has the potential to elicit a motile-invasive rather than a transformed phenotype.

Dynamics of secretion.

In this short review, kinetic aspects of exocytosis are discussed. A special emphasis is put on recent data that highlight dynamic differences between neurotransmission and other forms of secretion.

Kinetics of neuronal and endocrine secretion.

Calcium (Ca2+) regulated secretion/exocytosis is a key mechanism for cell-cell communication. Neurotransmission and hormone release are the most studied and the best characterized of all secretion systems so far. Here, some dynamic aspects of secretory vesicle trafficking will be briefly reviewed with special emphasis on the differences between synaptic vesicle and dense-core vesicle turnover.

RON is a heterodimeric tyrosine kinase receptor activated by the HGF homologue MSP.

RON, a cDNA homologous to the hepatocyte growth factor (HGF) receptor gene (MET), encodes a putative tyrosine kinase. Here we show that the RON gene is expressed in several epithelial tissues as well as in granulocytes and monocytes. The major RON transcript is translated into a glycosylated single chain precursor, cleaved into a 185 kDa heterodimer (p185RON) of 35 (alpha) and 150 kDa (beta) disulfide-linked chains, before exposure at the cell surface. The Ron beta-chain displays intrinsic tyrosine kinase activity in vitro, after immunoprecipitation by specific antibodies. In vivo, tyrosine phosphorylation of p185RON is induced by stimulation with macrophage stimulating protein (MSP), a protease-like factor containing four 'kringle' domains, homologous to HGF. In epithelial cells, MSP-induced tyrosine phosphorylation of p185RON is followed by DNA synthesis. p185RON is not activated by HGF, nor is the HGF receptor activated by MSP in biochemical and biological assays. p185RON is also activated by a pure recombinant protein containing only the N-terminal two kringles of MSP. These data show that p185RON is a tyrosine kinase activated by MSP and that it is member of a family of growth factor receptors with distinct specificities for structurally related ligands.