Met-Related Receptor Tyrosine Kinase Ron in Tumor Growth and Metastasis

Purnima K. Wagh*,†,§; Belinda E. Peace*,§; Susan E. Waltz*,†,‡    * Department of Surgery, University of Cincinnati College of Medicine, Cincinnati, OH 45267-0558
† Graduate Program in Cell and Cancer Biology, University of Cincinnati College of Medicine, Cincinnati, OH 45267-0558
‡ Department of Research, Shriner’s Hospital for Children, Cincinnati, OH
§ Contributed equally to this work

I Ron Structure and Function

Cell surface growth factor receptors play a vital role in translating signals from the extracellular environment into an intracellular biologic response. One such receptor is the Ron receptor tyrosine kinase. Ron, also referred to as macrophage stimulating 1-receptor (MST1R), is a receptor tyrosine kinase (RTK) of the hepatocyte growth factor (HGF)/Met receptor family. Ron was first identified as a novel protein tyrosine kinase by screening a library prepared from a mixture of human tumors. The full-length Ron cDNA was then identified using a human foreskin keratinocyte library (Ronsin et al., 1993). The Ron ortholog in the mouse was first cloned from hemapoietic stems cells and is also referred to as stem cell derived tyrosine kinase (STK) (Iwama et al., 1994). Met and Ron are the only two members of this RTK family, in contrast to other receptor tyrosine kinase families with multiple members (Manning et al., 2002). Ron was classified based upon its homology to Met and also by its homology to the Sea receptor found in chicken. The c-Sea receptor is the cellular homolog of the avian oncoprotein v-sea, and is structurally similar to Ron and Met (Huff et al., 1993; Huff et al., 1996). Sea is activated by chicken macrophage stimulating-protein (MSP) (Wahl et al., 1999). To date, homologs of Ron and its ligand have been identified by sequence analysis in many mammalian species including Rattus norvegicus (rat), Canis lupus (dog), Bos taurus (cow), Equus caballus (horse), and Macaca mulatta (rhesus monkey) (BLAST sequence analysis, 2007). Homologs of Ron have also been found in nonmammalian species, including Fugu rubripes (puffer fish) and Strongylocentrotus purpuratus (sea urchin) (Cottage et al., 1999; Lapraz et al., 2006).

The Ron and Met receptors are structurally very similar. Both Ron and Met receptors contain an extracellular ligand binding domain, a single pass hydrophobic membrane spanning domain, and an intracellular region containing a tyrosine kinase domain. Ron is synthesized as a 185 kDa precursor glycosylated protein and is further processed by furin-like proteases before being delivered as a mature receptor to the cell surface (Gaudino et al., 1994). On the cell surface, Ron exists as a heterodimeric receptor, consisting of a 35 kDa alpha chain and 150 kDa beta chain. The alpha chain is entirely extracellular whereas the beta chain contains the extracellular, transmembrane, and intracellular regions of the receptor (Gaudino et al., 1994). The 50-amino acid tyrosine kinase domain of Ron shares 80% identity to the Met tyrosine kinase domain and overall the receptors exhibit 34% identity (BLAST sequence analysis, 2007) (Fig. 1). Human and murine Ron cDNAs share about 74% identity overall, with about 88% identity in the intracellular domains (Iwama et al., 1994). The human Ron transcript consists of 20 exons while murine Ron codes for 19 exons. Altered splicing of the murine Ron gene creates a deletion of a small juxtamembrane region that is present in the human Ron gene (Wei et al., 2005). An analysis of the mouse Ron gene promoter region showed the presence of a number of putative transcription factor binding sites important in tumor progression, including binding sites for NF-kβ, Ets-1, and the estrogen receptor (Waltz et al., 1998).

II Ron Ligand Structure and Function

The ligand for Ron is hepatocyte growth factor-like (HGFL) protein and is also known as MSP. HGFL was originally cloned from a human genomic library by screening for the characteristic kringle domains present in prothrombin and several other proteins in the blood coagulation system (Han et al., 1991). The protein sequence of the isolated gene was predicted to contain four kringle domains followed by a serine protease-like domain. On the basis of domain structure, this protein was predicted to be similar to HGF, the ligand for the Met receptor. By sequence comparison, however, HGF and HGFL are only about 45% identical (BLAST sequence analysis, 2007) (Fig. 1). This newly identified protein was localized to human chromosome 3p21, a region that often displays loss of heterozygosity in cancerous tissue. The mouse gene and cDNA for HGFL were then isolated from mouse liver (Degen et al., 1991). The mouse homolog of HGFL was predicted to display the same domain structure as human HGFL and to be about 80% identical. The expression pattern of HGFL was determined by Northern analysis of tissues in the pregnant rat. The liver represented the primary site of expression for HGFL, with low levels detected in lung, adrenal gland, and placenta. Another group similarly cloned a cDNA for MSP from a library prepared from HepG2 cells, a human hepatocarcinoma cell line (Yoshimura et al., 1993). The probe for this clone was derived from the peptide sequence of MSP that had previously been isolated from human serum. The predicted amino acid sequence of MSP also included four kringle domains and was subsequently found, like HGFL, to be most similar to HGF. HGFL and MSP were soon determined to be identical (Shimamoto et al., 1993). Two independent groups later determined HGFL to be the ligand for the Ron receptor. Further, in spite of sequence similarities, no cross activation is seen between HGFL and Met, or HGF and Ron (Gaudino et al., 1994; Wang et al., 1994b).

Despite the structural similarity of HGF and...