Constitutive Activation of the RON Gene Promotes Invasive Growth but Not Transformation

Abstract

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.In human malignancies a number of tyrosine kinase receptors are constitutively activated by gene rearrangements that fuse their kinase domains with N-terminal unrelated sequences (for a review, see reference 46). The hepatocyte growth factor (HGF) receptor is converted into an oncogene by rearrangement of the MET proto-oncogene with a gene designated TPR (6). Similar rearrangements involving TPR have also been reported for RAF and TRK (19,26). In Tpr-Met the extracellular, transmembrane, and part of the juxtamembrane domains of the HGF receptor are replaced by the N-terminal sequence of Tpr (36). The kinase activity of the resulting hybrid protein is deregulated, since two leucine-zipper motifs present in the Tpr moiety promote its constitutive dimerization (17,41). This conformation mimics receptor activation following ligand binding.A distinctive property of the HGF receptor (Met) is the ability to evoke a complex response including cell growth, "scattering," and tubulogenesis (for a review, see reference 5). Scattering involves cytoskeletal reorganization and loss of intercellular junctions, followed by active cell migration (14,16,55,61). Epithelial tubulogenesis results from polarized cell growth and invasion of collagen matrices (33). These pleiotropic effects are elicited by the activation of several signalling pathways. Met-mediated signal transduction depends on ligand-induced phosphorylation of two critical carboxy-terminal tyrosine residues, which act as docking sites for multiple SH2-containing cytoplasmic effecto...

Cross-talk between the proto-oncogenes Met and Ron

“…Ron is less e cient than Met as a kinase (Santoro et al, 1996), thus, the formation of complexes Met/Ron, resulting in a more e cient Ron transphosphorylation by Met, may lead to a more sustained signal than that induced by the Ron/Ron homodimer. Di erent subsets of transducers may be engaged by the heterodimeric receptor complex, providing a mechanism of ®ne tuning of signal transduction.…”

“…The response to HGF and MSP through the activation of Met and Ron is mediated by the same pathways, mainly by Ras and by PI-3 Kinase (Wang et al, 1996;Bardelli et al, 1999). Met can regulate growth and transformation by activating the MAP kinase cascade through Grb2/Sos/Ras complex whereas the Ron pathway is directed more towards cellular movement and matrix invasion (Santoro et al, 1996;Wang et al, 1997). When Met and Ron receptors are co-expressed, the formation of heterodimers may balance the activation of Ras and PI-3 kinase pathways.…”

Overexpression of the RON gene in human breast carcinoma

“…RON belongs to a tyrosine kinase gene family whose prototype is MET. A number of reports suggest that members of this family are involved in the progression of cancer: (I) germline and somatic missense mutations in the tyrosine kinase domain of MET are involved in papillary renal carcinogenesis (Schmidt et al, 1997); (II) Met is overexpressed in some carcinoma types at advanced stages (Di Renzo et al, 1995); (III) activation of the Met receptor via an experimental autocrine circuit (Rong et al, 1994) or by rearrangement (Giordano et al, 1997) confers metastatic properties; (IV) all the known members of the family stimulate invasion in vitro (Giordano et al, 1993;Medico et al, 1996); (V) although no transforming counterpart of the RON gene has been identi®ed so far, constitutive activation of the Ron kinase elicits a motogenic-invasive phenotype Santoro et al, 1996).…”

Altered expression of the RON receptor tyrosine kinase in primary human colorectal adenocarcinomas: generation of different splicing RON variants and their oncogenic potential

“…Previous studies have shown that the cDNA encoding wtRON or RONDp165 does not have cell-transforming activity when introduced into rodent fibroblasts Santoro et al, 1996). To determine if cloned RON variants have the ability to transform cells in vitro, the focus-forming assay was performed using NIH3T3 cells as the indicator.…”