Alterations in a defined extracellular region of the RON receptor tyrosine kinase promote RON-mediated motile and invasive phenotypes in epithelial cells

Cell migration followed by matrix invasion is a critical step required for epithelial cell differentiation, growth and survival. This study determined the RON receptor in regulation of motile-invasive phenotypes of epithelial cells. Two RON variants, RON165.e11p and RON165, with alterations in a defined extracellular domain were used as the model. RON165 is a splicing variant generated by an mRNA transcript with an in-frame deletion of 49 amino acids encoded by exon 11. In contrast, RON165.e11p was produced by a partial deletion of exon 11 with the elimination of the first 40 amino acids. Thus, RON165.e11p differs from RON165 with nine amino acids retained in the fourth immunoglobulin-plexin-transcription (IPT) domain. Biochemically, both RON165 and RON165.e11p exist as a single-chain protein, residing in the cytoplasm, and failed to mature into the two-chain receptor. Both RON165 and RON165.e11p spontaneously formed oligomers in vivo leading to constitutive phosphorylation and the activation of downstream signaling proteins. Although lacking cell-transforming activities, RON165 and RON165.e11p mediated the epithelial to mesenchymal transition with spindle-like cell morphologies, diminished E-cadherin expression, and increased N-cadherin and vimentin expression. These changes facilitated epithelial cell migration and invasion as modeled in Martin-Darby canine kidney (MDCK) cells. Moreover, expression of RON165 or RON165.e11p in breast epithelial MCF-7 cells diminished epithelial cell phenotypes and increased motile and invasive activities. Thus, alterations in the defined extracellular region result in two unique RON variants with similar biological properties. The ability of RON165 or RON165.e11p to promote motile-invasive phenotypes may represent a mechanism by which RON regulates epithelial cell phenotypes and biological activities.