Uncoupling Ligand-dependent and -independent Mechanisms for Mitogen-activated Protein Kinase Activation by the Murine Ron Receptor Tyrosine Kinase

RON Signaling Is a Key Mediator of Tumor Progression in Many Human Cancers

With an increasing body of literature covering RON receptor tyrosine kinase function in different types of human cancers, it is becoming clear that RON has prominent roles in both cancer cells and in the tumor-associated microenvironment. RON not only activates several oncogenic signaling pathways in cancer cells, leading to more aggressive behavior, but also promotes an immunosuppressive, alternatively activated phenotype in macrophages and limits the antitumor immune response. These two unique functions of this oncogene, the strong correlation between RON expression and poor outcomes in cancer, and the high tolerability of a new RON inhibitor make it an exciting therapeutic target, the blocking of which offers an advantage toward improving the survival of cancer patients. Here, we discuss recent findings on the role of RON signaling in cancer progression and its potential in cancer therapy.

Spontaneous symbiotic reprogramming of plant roots triggered by receptor-like kinases

Symbiosis Receptor-like Kinase (SYMRK) is indispensable for the development of phosphate-acquiring arbuscular mycorrhiza (AM) as well as nitrogen-fixing root nodule symbiosis, but the mechanisms that discriminate between the two distinct symbiotic developmental fates have been enigmatic. In this study, we show that upon ectopic expression, the receptor-like kinase genes Nod Factor Receptor 1 (NFR1), NFR5, and SYMRK initiate spontaneous nodule organogenesis and nodulation-related gene expression in the absence of rhizobia. Furthermore, overexpressed NFR1 or NFR5 associated with endogenous SYMRK in roots of the legume Lotus japonicus. Epistasis tests revealed that the dominant active SYMRK allele initiates signalling independently of either the NFR1 or NFR5 gene and upstream of a set of genes required for the generation or decoding of calcium-spiking in both symbioses. Only SYMRK but not NFR overexpression triggered the expression of AM-related genes, indicating that the receptors play a key role in the decision between AM- or root nodule symbiosis-development.

Autoinhibition of the Ron receptor tyrosine kinase by the juxtamembrane domain

Background

The Ron receptor tyrosine kinase (RTK) has been implicated in the progression of a number of carcinomas, thus understanding the regulatory mechanisms governing its activity is of potential therapeutic significance. A critical role for the juxtamembrane domain in regulating RTK activity is emerging, however the mechanism by which this regulation occurs varies considerably from receptor to receptor.

Results

Unlike other RTKs described to date, tyrosines in the juxtamembrane domain of Ron are inconsequential for receptor activation. Rather, we have identified an acidic region in the juxtamembrane domain of Ron that plays a central role in promoting receptor autoinhibition. Furthermore, our studies demonstrate that phosphorylation of Y¹¹⁹⁸ in the kinase domain promotes Ron activation, likely by relieving the inhibitory constraints imposed by the juxtamembrane domain.

Conclusions

Taken together, our experimental data and molecular modeling provide a better understanding of the mechanisms governing Ron activation, which will lay the groundwork for the development of novel therapeutic approaches for targeting Ron in human malignancies.

RON receptor tyrosine kinase, a negative regulator of inflammation, is decreased during simian immunodeficiency virus-associated central nervous system disease

Expressed on tissue-resident macrophages, the receptor tyrosine kinase, recepteur d'orgine nantais (RON), functions to maintain inflammation homeostasis by activating genes that promote wound repair and resolve inflammation while repressing genes that perpetuate tissue damage and cell death. Chronic HIV-1 infection is associated with dysregulated inflammation, and we hypothesize that diminished RON expression contributes to the development of end organ diseases such as HIV-1-associated CNS disease. To explore RON function in vivo, we used CNS tissue from a well-characterized SIV macaque model and examined the temporal regulation of RON in the brain during the course of infection. Following prolonged SIV infection, RON expression was inversely correlated with the development of CNS disease; RON was maintained in animals that did not develop CNS lesions and was reduced in SIV-infected macaques that demonstrated moderate to severe inflammatory lesions. Arginase-1 expression was reduced in the brain during late infection, whereas expression of the inflammatory genes, IL-12p40 and TNF-α, was elevated. To validate a role for RON in regulating HIV-1 in primary cells, we used human tissue-resident macrophages isolated from tonsil as a tractable cell model. RON signaling in tissue-resident macrophages, both ligand dependent and independent, limited HIV-1 replication. Furthermore, prolonged HIV-1 infection in vitro resulted in downregulation of RON. We propose a model in which, following chronic HIV-1 infection in the brain, RON expression is decreased, genes that quell inflammation are repressed, and inflammatory mediators are induced to promote tissue inflammation.

Host genetic background impacts modulation of the TLR4 pathway by RON in tissue-associated macrophages

Toll-like receptors (TLRs) enable metazoans to mount effective innate immune responses to microbial and viral pathogens, as well as to endogenous host-derived ligands. It is understood that genetic background of the host can influence TLR responsiveness, altering susceptibility to pathogen infection, autoimmunity and cancer. Macrophage stimulatory protein (MSP), which activates the receptor tyrosine kinase recepteur d'origine nantais (RON), promotes key macrophage functions such as motility and phagocytic activity. MSP also acts via RON to modulate signaling by TLR4, which recognizes a range of pathogen or endogenous host-derived molecules. Here, we show that RON exerts divergent control over TLR4 activity in macrophages from different mouse genetic backgrounds. RON potently modulated the TLR4 response in macrophages from M2-prone FVB mice, as compared with M1-skewed C57Bl6 mice. Moreover, global expression analysis revealed that RON suppresses the TLR4-dependent type-I interferon gene signature only in FVB macrophages. This leads to attenuated production of the potent inflammatory mediator, tumor necrosis factor-α. Eliminating RON kinase activity markedly decreased carcinogen-mediated tumorigenesis in M2/Th2-biased FVB mice. We propose that host genetic background influences RON function, thereby contributing to the variability in TLR4 responsiveness in rodents and, potentially, in humans. These findings provide novel insight into the complex interplay between genetic context and immune function.

Crystal structure of the Sema-PSI extracellular domain of human RON receptor tyrosine kinase

Human RON (Recepteur d’Origine Nantais) receptor tyrosine kinase is a cell surface receptor for Macrophage Stimulating Protein (MSP). RON mediates signal transduction pathways that regulate cell adhesion, invasion, motility and apoptosis processes. Elevated levels of RON and its alternatively spliced variants are implicated in the progression and metastasis of tumor cells. The binding of MSP α/β heterodimer to the extracellular region of RON receptor induces receptor dimerization and activation by autophosphorylation of the intracellular kinase domains. The ectodomain of RON, containing the ligand recognition and dimerization domains, is composed of a semaphorin (Sema), Plexins-Semaphorins-Integrins domain (PSI), and four Immunoglobulins-Plexins-Transcription factor (IPT) domains. High affinity association between MSP and RON is mediated by the interaction between MSP β-chain and RON Sema, although RON activation requires intact RON and MSP proteins. Here, we report the structure of RON Sema-PSI domains at 1.85 Å resolution. RON Sema domain adopts a seven-bladed β-propeller fold, followed by disulfide bond rich, cysteine-knot PSI motif. Comparison with the homologous Met receptor tyrosine kinase reveals that RON Sema-PSI contains distinguishing secondary structural features. These define the receptors’ exclusive selectivity towards their respective ligands, RON for MSP and Met for HGF. The RON Sema-PSI crystal packing generates a homodimer with interface formed by the Sema domain. Mapping of the dimer interface using the RON homology to Met, MSP homology to Hepatocyte Growth Factor (HGF), and the structure of the Met/HGF complex shows the dimer interface overlapping with the putative MSPβ binding site. The crystallographically determined RON Sema-PSI homodimer may represent the dimer assembly that occurs during ligand-independent receptor activation and/or the inhibition of the constitutive activity of RONΔ160 splice variant by the soluble RON splice variant, RONΔ85.

Regulation of macrophage arginase expression and tumor growth by the Ron receptor tyrosine kinase

M1 activation of macrophages promotes inflammation and immunity to intracellular pathogens, whereas M2 macrophage activation promotes resolution of inflammation, wound healing, and tumor growth. These divergent phenotypes are characterized, in part, by the expression of inducible NO synthase and arginase I (Arg1) in M1 versus M2 activated macrophages, respectively. In this study, we demonstrate that the Ron receptor tyrosine kinase tips the balance of macrophage activation by attenuating the M1 phenotype while promoting expression of Arg1 through a Stat6-independent mechanism. Induction of the Arg1 promoter by Ron is mediated by an AP-1 site located 433 bp upstream of the transcription start site. Treatment of primary macrophages with macrophage stimulating protein, the ligand for Ron, induces potent MAPK activation, upregulates Fos, and enhances binding of Fos to the AP-1 site in the Arg1 promoter. In vivo, Arg1 expression in tumor-associated macrophages (TAMs) from Ron(-/-) mice was significantly reduced compared with that in TAMs from control animals. Furthermore, we show that Ron is expressed specifically by Tie2-expressing macrophages, a TAM subset that exhibits a markedly skewed M2 and protumoral phenotype. Decreased Arg1 in TAMs from Ron(-/-) mice was associated with reduced syngeneic tumor growth in these animals. These findings indicate that Ron induces Arg1 expression in macrophages through a previously uncharacterized AP-1 site in the Arg1 promoter and that Ron could be therapeutically targeted in the tumor microenvironment to inhibit tumor growth by targeting expression of Arg1.

Inhibition of TLR4-induced IκB kinase activity by the RON receptor tyrosine kinase and its ligand, macrophage-stimulating protein

The RON receptor tyrosine kinase regulates the balance between classical (M1) and alternative (M2) macrophage activation. In primary macrophages, the ligand for Ron, macrophage-stimulating protein (MSP), inhibits the expression of inducible NO synthase, a marker of classically activated macrophages, whereas promoting the expression of arginase I, a marker of alternative activation. Ron(-/-) mice express increased levels of IL-12, a product of classically activated macrophages, after endotoxin administration, resulting in increased serum IFN-γ levels and enhanced susceptibility to septic shock. In this study, we demonstrate that MSP inhibits LPS-induced IL-12p40 expression, and this inhibition is dependent on the docking site tyrosines in Ron. To further define this inhibition, we examined the effect of Ron on signaling pathways downstream of Ron. We found that MSP does not inhibit the MyD88-independent activation of IFN regulatory factor 3 and production of IFN-β in response to LPS, nor does it inhibit MyD88-dependent TGF-β-activated kinase phosphorylation or MAPK activation in primary macrophages. However, the induction of IκB kinase activity, IκB degradation, and DNA binding of NF-κB after LPS stimulation is delayed in the presence of MSP. In addition, Ron inhibits serine phosphorylation of p65 and NF-κB transcriptional activity induced by LPS stimulation of TLR4. Finally, MSP inhibits the NF-κB-dependent upregulation of the nuclear IκB family member, IκBζ, a positive regulator of secondary response genes including IL-12p40. LPS also induces expression of Ron and an N-terminally truncated form of Ron, Sf-Ron, in primary macrophages, suggesting that the upregulation of Ron by LPS could provide classical feedback regulation of TLR signaling.

Tyrosines 868, 966, and 972 in the kinase domain of JAK2 are autophosphorylated and required for maximal JAK2 kinase activity

Janus kinase 2 (JAK2) is activated by a majority of cytokine family receptors including receptors for GH, leptin, and erythropoietin. To identify novel JAK2-regulatory and/or -binding sites, we set out to identify autophosphorylation sites in the kinase domain of JAK2. Two-dimensional phosphopeptide mapping of in vitro autophosphorylated JAK2 identified tyrosines 868, 966, and 972 as sites of autophosphorylation. Phosphorylated tyrosines 868 and 972 were also identified by mass spectrometry analysis of JAK2 activated by an erythropoietin-bound chimeric erythropoietin receptor/leptin receptor. Phosphospecific antibodies suggest that the phosphorylation of all three tyrosines increases in response to GH. Compared with wild-type JAK2, which is constitutively active when overexpressed, JAK2 lacking tyrosine 868, 966, or 972 has substantially reduced activity. Coexpression with GH receptor and protein tyrosine phosphatase1B allowed us to investigate GH-dependent activation of these mutated JAK2s in human embryonic kidney 293T cells. All three mutated JAK2s are activated by GH, although to a lesser extent than wild-type JAK2. The three mutated JAK2s also mediate GH activation of signal transducer and activator of transcription 3 (Stat3), signal transducer and activator of transcription 5b (Stat5b) and ERK1, but at reduced levels. Coexpression with Src-homology 2B1beta (SH2B1beta), like coexpression with GH-bound GH receptor, partially restores the activity of all three JAK2 mutants. Based on these results and the crystal structure of the JAK2 kinase domain, we hypothesize that small changes in the conformation of the regions of JAK2 surrounding tyrosines 868, 966, and 972 due to e.g. phosphorylation, binding to a ligand-bound cytokine receptor, and/or binding to Src-homology 2B1, may be essential for JAK2 to assume a maximally active conformation.

The RON receptor tyrosine kinase promotes MSP-independent cell spreading and survival in breast epithelial cells

The recepteur d’origne nantais (RON) is a receptor tyrosine kinase (RTK) in the scatter factor family, which includes the c-Met receptor. RON exhibits increased expression in a significant number of human breast cancer tissues as well as in many established breast cancer cell lines. Recent studies have indicated that in addition to ligand-dependent signaling events, RON also promotes signals in the absence of its only known ligand, the macrophage stimulating protein, when expressed in epithelial cells. In the current study, we found that when expressed in MCF-10A breast epithelial cells, RON exhibits both MSP-dependent and MSP-independent signaling, which lead to distinct biological outcomes. In the absence of MSP, RON signaling promotes cell survival, increased cell spreading and enhanced migration in response to other growth factors. However, both RON-mediated proliferation and migration require the addition of MSP in MCF-10A cells. Both MSP-dependent and MSP-independent signaling by RON is mediated in part by Src-family kinases. These data suggest that RON has two alternative modes of signaling that can contribute to oncogenic behavior in normal breast epithelial cells.

Receptor tyrosine kinase Tie-1 overexpression in endothelial cells upregulates adhesion molecules

Tie-1 is an endothelial specific cell surface protein whose biology remains poorly understood. Using an overexpression system in vitro, we examined whether Tie-1 activity in endothelial cells in vitro would elicit a proinflammatory response. We found that when overexpressed in endothelial cells in vitro, Tie-1 is tyrosine-phosphorylated. We also showed that Tie-1 upregulates VCAM-1, E-selectin, and ICAM-1, partly through a p38-dependent mechanism. Interestingly, upregulation of VCAM-1 and E-selectin by Tie-1 is significantly higher in human aortic endothelial cells than in human umbilical vein endothelial cells. Additionally, attachment of cells of monocytic lineage to endothelial cells is also enhanced by Tie-1 expression. Collectively, our data show that Tie-1 has a proinflammatory property and may play a role in the endothelial inflammatory diseases such as atherosclerosis.

The receptor tyrosine kinase RON represses HIV-1 transcription by targeting RNA polymerase II processivity

Efficient HIV-1 transcription requires the induction of cellular transcription factors, such as NF-kappaB, and the viral factor Tat, which through the recruitment of P-TEFb enhances processive transcription. However, whether cellular signals repress HIV-1 transcription to establish proviral latency has not been well studied. Previously, it has been shown that the receptor tyrosine kinase RON inhibits HIV transcription. To gain insights into the biochemical mechanisms by which RON inhibits transcription we examined the binding of transcription factors to the HIV provirus long terminal repeat using chromatin immunoprecipitation. RON expression decreased basal levels of NF-kappaB and RNA polymerase II (Pol II) binding to the HIV provirus long terminal repeat but did not prevent the induction of these complexes following treatment with cytokines. However, RON did decrease efficient transcription elongation because reduced RNA Pol II was associated with HIV-1 genomic sequences downstream of the transcriptional start site. There was a correlation between RON expression and increased binding of factors that negatively regulate transcription elongation, NELF, Spt5, and Pcf11. Furthermore, the ability of RON to inhibit HIV-1 transcription was sensitive to a histone deacetylase inhibitor and was associated with nucleosome remodeling. These results indicate that RON represses HIV transcription at multiple transcriptional check points including initiation, elongation and chromatin organization and are the first studies to show that cellular signaling pathways target Pol II pausing to repress gene expression.

Human RON receptor tyrosine kinase induces complete epithelial-to-mesenchymal transition but causes cellular senescence

The RON receptor tyrosine kinase is a member of the MET proto-oncogene family and is important for cell proliferation, differentiation, and cancer development. Here, we created a series of Madin-Darby canine kidney (MDCK) epithelial cell clones that express different levels of RON, and have investigated their biological properties. While low levels of RON correlated with little morphological change in MDCK cells, high levels of RON expression constitutively led to morphological scattering or complete and stabilized epithelial-to-mesenchymal transition (EMT). Unexpectedly, MDCK clones expressing higher levels of RON exhibited retarded proliferation and senescence, despite increased motility and invasiveness. RON was constitutively tyrosine-phosphorylated in MDCK cells expressing high levels of RON and undergoing EMT, and the MAPK signaling pathway was activated. This study reveals for the first time that RON alone is sufficient to induce complete and stabilized EMT in MDCK cells, and overexpression of RON does not cause cell transformation but rather induces cell cycle arrest and senescence, leading to impaired cell proliferation.

Molecular regulation of receptor tyrosine kinases in hematopoietic malignancies

Dysregulation of receptor tyrosine kinase (RTK) activity has been implicated in the progression of a variety of human leukemias. Most notably, mutations and chromosomal translocations affecting regulation of tyrosine kinase activity in the Kit receptor, the Flt3 receptor, and the PDGFbeta/FGF1 receptors have been demonstrated in mast cell leukemia, acute myeloid leukemia (AML), and chronic myelogenous leukemias (CML), respectively. In addition, critical but non-overlapping roles for the Ron and Kit receptor tyrosine kinases in the progression of animal models of erythroleukemia have been demonstrated [Persons, D., Paulson, R., Loyd, M., Herley, M., Bodner, S., Bernstein, A., Correll, P. and Ney, P., 1999. Fv2 encodes a truncated form of the Stk receptor tyrosine kinase. Nat. Gen. 23, 159-165.; Subramanian, A., Teal, H.E., Correll, P.H. and Paulson, R.F., 2005. Resistance to friend virus-induced erythroleukemia in W/Wv mice is caused by a spleen-specific defect which results in a severe reduction in target cells and a lack of Sf-Stk expression. J. Virol. 79 (23), 14586-14594.]. The various classes of RTKs implicated in the progression of leukemia have been recently reviewed [Reilly, J., 2003. Receptor tyrosine kinases in normal and malignant haematopoiesis. Blood Rev. 17 (4), 241-248.]. Here, we will discuss the mechanism by which alterations in these receptors result in transformation of hematopoietic cells, in the context of what is known about the molecular regulation of RTK activity, with a focus on our recent studies of the Ron receptor tyrosine kinase.

Association of RON tyrosine kinase with the Jaagsiekte sheep retrovirus envelope glycoprotein

The envelope (Env) of Jaagsiekte sheep retrovirus (JSRV) functions as an oncoprotein. One of the mechanisms of JSRV-induced cell transformation that has been proposed for epithelial cells involves JSRV Env binding Hyaluronidase 2 (the JSRV receptor), thereby inducing its degradation and allowing the release and activation of RON tyrosine kinase which is normally suppressed by HYAL-2. In this study, we report that HYAL-2 and RON are not critical for the JSRV Env-induced transformation of the rat epithelial cell line IEC-18, while the cytoplasmic tail of the JSRV Env is critical to transform this cell line. We have also determined that RON can associate with the JSRV Env under normal and stringent conditions. In addition, the cytoplasmic tail of the JSRV and the enJS5F16 (non oncogenic JSRV-related endogenous retrovirus) Env proteins appears to have a major influence on the activation status of RON. Thus, it appears that the interaction of the JSRV Env with RON is more complex than previously thought and requires further investigation.