Pathogenesis of RON receptor tyrosine kinase in cancer cells: activation mechanism, functional crosstalk, and signaling addiction

Abstract

The RON receptor tyrosine kinase, a member of the MET proto-oncogene family, is a pathogenic factor implicated in tumor malignancy. Specifically, aberrations in RON signaling result in increased cancer cell growth, survival, invasion, angiogenesis, and drug resistance. Biochemical events such as ligand binding, receptor overexpression, generation of structure-defected variants, and point mutations in the kinase domain contribute to RON signaling activation. Recently, functional crosstalk between RON and signaling proteins such as MET and EFGR has emerged as an additional mechanism for RON activation, which is critical for tumorigenic development. The RON signaling crosstalk acts either as a regulatory feedback loop that strengthens or enhances tumorigenic phenotype of cancer cells or serves as a signaling compensatory pathway providing a growth/survival advantage for cancer cells to escape targeted therapy. Moreover, viral oncoproteins derived from Friend leukemia or Epstein-Barr viruses interact with RON to drive viral oncogenesis. In cancer cells, RON signaling is integrated into cellular signaling network essential for cancer cell growth and survival. These activities provide the molecular basis of targeting RON for cancer treatment. In this review, we will discuss recent data that uncover the mechanisms of RON activation in cancer cells, review evidence of RON signaling crosstalk relevant to cancer malignancy, and emphasize the significance of the RON signaling addiction by cancer cells for tumor therapy. Understanding aberrant RON signaling will not only provide insight into the mechanisms of tumor pathogenesis, but also lead to the development of novel strategies for molecularly targeted cancer treatment.

Keywords: Receptor tyrosine kinase (RON); activation mechanism; oncogene addiction; signaling crosstalk; signaling pathway; tumorigenesis.

Fig. 1. Schematic representation of RON and RON variant.

A: General features of MET, RON, and V-SEA. MET is the classical example of this family. Mature RON consists of a 35 kDa α-chain and a 145 kDa β-chain linked by a disulfide bond. The α-chain resides extracellularly and contains a portion of Semaphorin (Sema). The β-chain comprises a large extracellular domain, a short transmembrane (TM) segment, and a cytoplasmic portion harboring a tyrosine kinase (TK) domain and a C-terminal tail. The Sema domain harbors a ligand-binding pocket for the MSP β-chain. Regulatory tyrosine residues Tyr1238 and Tyr1239 in the TK domain and Tyr1353 and Tyr1360 in the C-terminal tail are marked. V-SEA is an oncogenic protein fused by the avian S13 retroviral envelope protein with the chicken SEA sequences. PSI, Plexins-Semaphorins-Integrins; IPT, immunoglobulin-plexin-transcription. B: Different RON variants. RONΔ55 is derived from alternative initiation at Met913. RONΔ165 is formed by deletion of exon 11 coding 49 amino acids. RONΔ160 has a deletion of exons 5 and 6 coding 109 amino acids. RONΔ155 has a combined deletion of exons 5, 6 and 11. RONΔ170 is derived from deletion of exon 19 in the kinase domain. RONΔ110 is formed by N-terminal truncation at Arg631. RONΔ85 is a free variant with C-terminal truncation at Asp634 caused by insertion. RONΔ160e is derived by deletion of exon 2.

Fig. 2. RON activation mechanisms and classical signaling pathways.

Activation of RON is mediated by MSP binding, overexpression, splicing/truncation, and point mutations. Upon activation, the C-terminal docking site recruits cytoplasmic molecules son of sevenless (SOS) and growth factor receptor-bound protein (GRB2) to initiate two classical signaling pathways, Ras-MAPK and PI-3K-AKT. The RAS-MAPK pathway regulates RON-mediated cell growth, survival, and invasiveness. Activated Erk1/2 also stimulates p90 ribosomal S6 kinase (RSK)-2 to regulate gene transcription and cytoskeleton reorganization to cause EMT. The PI-3K-AKT pathway regulates RON-mediated cell shape change, migration and matrix invasion. It also stimulates mTOR signaling to promote HIF-1α activation for gene transcription. AKT also stimulates 14-3-3 phosphorylation, which regulates α6β4 integrin for cell motility. CM, cell membrane; ELK-1, ETS domain-containing protein-1; Erk, extracellular signal-regulated kinase; MITF, microphthalmia-associated transcription factor; mTOR, mammalian target of rapamycin; NM nuclear membrane, SRF, specific response factors.

Fig. 3. Functional crosstalk between RON and signaling protein.

The crosstalk of RON with MET, EGFR, and IGF-1R occurs in various cancer cells and cause increased tumorigenic activity. RON also crosstalks with viral envelope oncoproteins derived from JSRV and FLV to cell transformation and proliferation. At least four signaling pathways are activated upon crosstalking. The β-catenin pathway is stimulated through RON-mediated PI-3K-AKT pathway that activates protein dishevel (DVL) and inactivates glycogen synthase kinase (GSK)-3β leading to cytoplasmic β-catenin accumulation and nuclear translocation. The crosstalk between RON and the NF-κB pathway causes cancer cell growth, angiogenesis, and survival. NF-κB also directly binds the RON promoter, increases RON transcription, and enhances RON-mediated cancer cell migration. In epithelial cells, RON crosstalks with TGF-β signaling to induce EMT for cancer cell invasiveness. Moreover, RONΔ55 binds the FLV envelope protein and interacts with the JAK-Stat3 pathway to induce erythropoietin-independent proliferation of erythroid cells. CM, cell membrane; CXCL, Chemokine (C-X-C motif) ligand; Gab, GRB2-associated-binding protein; IKK, IκB Kinase; IRS-1, insulin receptor substrate-1; JAK, Janus kinase; MMP, matrix metallopeptidase; NM nuclear membrane. SMA, smooth muscle actin; Smad, mothers against decapentaplegic homolog; Stat, signal transducer and activator of transcription; and VEGF, vascular endothelial growth factor.