Macrophage stimulating protein (MSP) binds to its receptor via the MSP beta chain

Macrophage stimulating protein is a novel transcriptional target of estrogen related receptor gamma in alcohol-intoxicated mice

Macrophage stimulating protein (MSP) is a multifunctional serum protein produced in the liver, belonging to the plasminogen-related kringle protein family. It exerts diverse biological functions by activating a transmembrane receptor protein-tyrosine kinase known as RON in humans and SKT in mice. MSP plays a pivotal role in innate immunity and is involved in various activities such as cell survival, migration, and phagocytosis. Elucidating the regulatory mechanisms governing MSP gene expression is of great importance. In this study, we comprehensively elucidate the molecular mechanism underlying hepatic MSP gene expression in response to alcoholism. Exposure to ethanol specifically upregulated the expression of ERRγ and MSP in the liver, while not in other organs. Liver-specific knockout of the cannabinoid receptor type 1 (CB1R), an upstream regulator of ERRγ, inhibited the alcohol-induced upregulation of MSP expression. Overexpression of ERRγ alone was sufficient to enhance MSP expression in hepatic cell lines and in mice. Conversely, knockdown of ERRγ in cell lines or liver-specific knockout of ERRγ in mice reversed ethanol-induced MSP gene expression. Promoter studies revealed the direct binding of ERRγ to the MSP gene promoter at the ERR response element (ERRE), resulting in the positive regulation of MSP gene expression in response to alcohol. This finding was further supported by ERRE-mutated MSP-luciferase reporter assays. Notably, treatment with GSK5182, an ERRγ-specific inverse agonist, significantly suppressed alcohol-induced hepatic MSP expression. Collectively, we exposed a novel mechanistic understanding of how alcohol-induced ERRγ controls the transcriptional regulation of MSP gene expression in the liver.

An Introduction and Overview of RON Receptor Tyrosine Kinase Signaling

RON is a receptor tyrosine kinase (RTK) of the MET receptor family that is canonically involved in mediating growth and inflammatory signaling. RON is expressed at low levels in a variety of tissues, but its overexpression and activation have been associated with malignancies in multiple tissue types and worse patient outcomes. RON and its ligand HGFL demonstrate cross-talk with other growth receptors and, consequentially, positions RON at the intersection of numerous tumorigenic signaling programs. For this reason, RON is an attractive therapeutic target in cancer research. A better understanding of homeostatic and oncogenic RON activity serves to enhance clinical insights in treating RON-expressing cancers.

Serine pseudoproteases in physiology and disease

Serine proteases (SPs) constitute a very important family of enzymes, both physiologically and pathologically. The effects produced by these proteins have been explained by their proteolytic activity. However, the discovery of pharmacologically active SP molecules that show no enzymatic activity, as the so-called pseudo SPs or SP homologs (SPHs), has exposed a profoundly neglected possibility of nonenzymatic functions of these SP molecules. In this review, the most thoroughly described SPHs are presented. The main physiological domains in which SPHs operate appear to be in reproduction, embryonic development, immune response, host defense, and hemostasis. Hitherto unexplained actions of SPs should therefore be considered also as the result of the ligand-like attributes of SPs. The gain of a novel function by an SPH is a consequence of specific amino acid replacements that have resulted in a novel interaction interface or a 'catalytic trap'. Unraveling the SP/SPH interactome will provide a description of previously unknown physiological functions of SPs/SPHs, aiding the creation of innovative medical approaches.

RON in hepatobiliary and pancreatic cancers: Pathogenesis and potential therapeutic targets

The receptor protein tyrosine kinase RON belongs to the c-MET proto-oncogene family. Research has shown that RON has a role in cancer pathogenesis, which places RON on the frontline of the development of novel cancer therapeutic strategies. Hepatobiliary and pancreatic (HBP) cancers have a poor prognosis, being reported as having higher rates of cancer-related death. Therefore, to combat these malignant diseases, the mechanism underlying the aberrant expression and signaling of RON in HBP cancer pathogenesis, and the development of RON as a drug target for therapeutic intervention should be investigated. Abnormal RON expression and signaling have been identified in HBP cancers, and also act as tumorigenic determinants for HBP cancer malignant behaviors. In addition, RON is emerging as an important mediator of the clinical prognosis of HBP cancers. Thus, not only is RON significant in HBP cancers, but also RON-targeted therapeutics could be developed to treat these cancers, for example, therapeutic monoclonal antibodies and small-molecule inhibitors. Among them, antibody-drug conjugates have become increasingly popular in current research and their potential as novel anti-cancer biotherapeutics will be determined in future clinical trials.

RON signalling promotes therapeutic resistance in ESR1 mutant breast cancer

Background

Oestrogen Receptor 1 (ESR1) mutations are frequently acquired in oestrogen receptor (ER)-positive metastatic breast cancer (MBC) patients who were treated with aromatase inhibitors (AI) in the metastatic setting. Acquired ESR1 mutations are associated with poor prognosis and there is a lack of effective therapies that selectively target these cancers.

Methods

We performed a proteomic kinome analysis in ESR1 Y537S mutant cells to identify hyperactivated kinases in ESR1 mutant cells. We validated Recepteur d’Origine Nantais (RON) and PI3K hyperactivity through phospho-immunoblot analysis, organoid growth assays, and in an in vivo patient-derived xenograft (PDX) metastatic model.

Results

We demonstrated that RON was hyperactivated in ESR1 mutant models, and in acquired palbociclib-resistant (PalbR) models. RON and insulin-like growth factor 1 receptor (IGF-1R) interacted as shown through pharmacological and genetic inhibition and were regulated by the mutant ER as demonstrated by reduced phospho-protein expression with endocrine therapies (ET). We show that ET in combination with a RON inhibitor (RONi) decreased ex vivo organoid growth of ESR1 mutant models, and as a monotherapy in PalbR models, demonstrating its therapeutic efficacy. Significantly, ET in combination with the RONi reduced metastasis of an ESR1 Y537S mutant PDX model.

Conclusions

Our results demonstrate that RON/PI3K pathway inhibition may be an effective treatment strategy in ESR1 mutant and PalbR MBC patients. Clinically our data predict that ET resistance mechanisms can also contribute to CDK4/6 inhibitor resistance.

MSP-RON Pathway: Potential Regulator of Inflammation and Innate Immunity

Macrophage-stimulating protein (MSP), a soluble protein mainly synthesized by the liver, is the only known ligand for recepteur d'origine nantais (RON), which is a member of the MET proto-oncogene family. Recent studies show that the MSP-RON signaling pathway not only was important in tumor behavior but also participates in the occurrence or development of many immune system diseases. Activation of RON in macrophages results in the inhibition of nitric oxide synthesis as well as lipopolysaccharide (LPS)-induced inflammatory response. MSP-RON is also associated with chronic inflammatory responses, especially chronic liver inflammation, and might serve as a novel regulator of inflammation, which may affect the metabolism in the body. Another study provided evidence of the relationship between MSP-RON and autoimmune diseases, suggesting a potential role for MSP-RON in the development of drugs for autoimmune diseases. Moreover, MSP-RON plays an important role in maintaining the stability of the tissue microenvironment and contributes to immune escape in the tumor immune microenvironment. Here, we summarize the role of MSP-RON in immunity, based on recent findings, and lay the foundation for further research.

Hepatocyte Growth Factor and Macrophage-stimulating Protein "Hinge" Analogs to Treat Pancreatic Cancer

Pancreatic cancer (PC) ranks twelfth in frequency of diagnosis but is the fourth leading cause of cancer related deaths with a 5 year survival rate of less than 7 percent. This poor prognosis occurs because the early stages of PC are often asymptomatic. Over-expression of several growth factors, most notably vascular endothelial growth factor (VEGF), has been implicated in PC resulting in dysfunctional signal transduction pathways and the facilitation of tumor growth, invasion and metastasis. Hepatocyte growth factor (HGF) acts via the Met receptor and has also received research attention with ongoing efforts to develop treatments to block the Met receptor and its signal transduction pathways. Macrophage-stimulating protein (MSP), and its receptor Ron, is also recognized as important in the etiology of PC but is less well studied. Although the angiotensin II (AngII)/AT1 receptor system is best known for mediating blood pressure and body water/electrolyte balance, it also facilitates tumor vascularization and growth by stimulating the expression of VEGF. A metabolite of AngII, angiotensin IV (AngIV) has sequence homology with the "hinge regions" of HGF and MSP, key structures in the growth factor dimerization processes necessary for Met and Ron receptor activation. We have developed AngIV-based analogs designed to block dimerization of HGF and MSP and thus receptor activation. Norleual has shown promise as tested utilizing PC cell cultures. Results indicate that cell migration, invasion, and pro-survival functions were suppressed by this analog and tumor growth was significantly inhibited in an orthotopic PC mouse model.

RON kinase inhibition reduces renal endothelial injury in sickle cell disease mice

Sickle cell disease patients are at increased risk of developing a chronic kidney disease. Endothelial dysfunction and inflammation associated with hemolysis lead to vasculopathy and contribute to the development of renal disease. Here we used a Townes sickle cell disease mouse model to examine renal endothelial injury. Renal disease in Townes mice was associated with glomerular hypertrophy, capillary dilation and congestion, and significant endothelial injury. We also detected substantial renal macrophage infiltration, and accumulation of macrophage stimulating protein 1 in glomerular capillary. Treatment of human cultured macrophages with hemin or red blood cell lysates significantly increased expression of macrophage membrane-associated protease that might cleave and activate circulating macrophage stimulating protein 1 precursor. Macrophage stimulating protein 1 binds to and activates RON kinase, a cell surface receptor tyrosine kinase. In cultured human renal glomerular endothelial cells, macrophage stimulating protein 1 induced RON downstream signaling, resulting in increased phosphorylation of ERK and AKT kinases, expression of Von Willebrand factor, increased cell motility, and re-organization of F-actin. Specificity of macrophage stimulating protein 1 function was confirmed by treatment with RON kinase inhibitor BMS-777607 that significantly reduced downstream signaling. Moreover, treatment of sickle cell mice with BMS-777607 significantly reduced glomerular hypertrophy, capillary dilation and congestion, and endothelial injury. Taken together, our findings demonstrated that RON kinase is involved in the induction of renal endothelial injury in sickle cell mice. Inhibition of RON kinase activation may provide a novel approach for prevention of the development of renal disease in sickle cell disease.

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.

HIV‑1 downregulates the expression and phosphorylation of receptor tyrosine kinase by targeting the NF‑κB pathway

Macrophages are major targets of human immunodeficiency virus (HIV) and can act as long-term reservoirs of the virus. Chronic HIV-1 infection is associated with dysregulated inflammation. Recepteur d'origine nantais (RON) is expressed in tissue resident macrophages and functions to maintain inflammatory homeostasis. The present study aimed to compare the expression of RON on HIV-positive and -negative participants, and to investigate the mechanism by which HIV-1 influences the expression and function of RON in the JLTRG T cell line. The levels of RON and the RON ligand, macrophage-stimulating protein (MSP), in the peripheral blood of HIV-1-positive patients that were receiving (n=22) or not receiving highly active anti-retroviral therapy (HAART) (n=82) and 37 healthy control participants were determined by enzyme-linked immunosorbent assay. Expression of RON and MSP in the JLTRG T cell line was assessed by western blotting and the subcellular location was analyzed by fluorescence microscopy. JLTRG cells were co-cultured with a cell line that stably expresses HIV, H9/HTLV-IIIB, and alterations in the levels of RON and nuclear factor-κB (NF-κB) in JLTRG cells were assessed by western blotting. The expression of RON and MSP were significantly different in the serum of HIV-1- positive patients that were receiving HAART compared with those not receiving HAART (P<0.05) and healthy control patients (P<0.01). RON was detected in JLTRG cells, and was shown to be downregulated by HIV-1 infection. HIV-1 infection of JLTRG cells also reduced NF-κB phosphorylation. Thus, HIV-1 was shown to downregulate the expression and phosphorylation of RON by targeting the NF-κB pathway.

Structural basis for the binding specificity of human Recepteur d'Origine Nantais (RON) receptor tyrosine kinase to macrophage-stimulating protein

Recepteur d'origine nantais (RON) receptor tyrosine kinase and its ligand, serum macrophage-stimulating protein (MSP), play important roles in inflammation, cell growth, migration, and epithelial to mesenchymal transition during tumor development. The binding of mature MSPαβ (disulfide-linked α- and β-chains) to RON ectodomain modulates receptor dimerization, followed by autophosphorylation of tyrosines in the cytoplasmic receptor kinase domains. Receptor recognition is mediated by binding of MSP β-chain (MSPβ) to the RON Sema. Here we report the structure of RON Sema-PSI-IPT1 (SPI1) domains in complex with MSPβ at 3.0 Å resolution. The MSPβ serine protease-like β-barrel uses the degenerate serine protease active site to recognize blades 2, 3, and 4 of the β-propeller fold of RON Sema. Despite the sequence homology between RON and MET receptor tyrosine kinase and between MSP and hepatocyte growth factor, it is well established that there is no cross-reactivity between the two receptor-ligand systems. Comparison of the structure of RON SPI1 in complex with MSPβ and that of MET receptor tyrosine kinase Sema-PSI in complex with hepatocyte growth factor β-chain reveals the receptor-ligand selectivity determinants. Analytical ultracentrifugation studies of the SPI1-MSPβ interaction confirm the formation of a 1:1 complex. SPI1 and MSPαβ also associate primarily as a 1:1 complex with a binding affinity similar to that of SPI1-MSPβ. In addition, the SPI1-MSPαβ ultracentrifuge studies reveal a low abundance 2:2 complex with ∼ 10-fold lower binding affinity compared with the 1:1 species. These results support the hypothesis that the α-chain of MSPαβ mediates RON dimerization.

Functional consequences of the macrophage stimulating protein 689C inflammatory bowel disease risk allele

Background

Macrophage stimulating protein (MSP) is a serum growth factor that binds to and activates the receptor tyrosine kinase, Recepteur d'Origine Nantais (RON). A non-synonymous coding variant in MSP (689C) has been associated with genetic susceptibility to both Crohn's disease and ulcerative colitis, two major types of inflammatory bowel disease (IBD) characterized by chronic inflammation of the digestive tract. We investigated the consequences of this polymorphism for MSP-RON pathway activity and IBD pathogenesis.

Methods

RON expression patterns were examined on mouse and human cells and tissues under normal and disease conditions to identify cell types regulated by MSP-RON. Recombinant MSP variants were tested for their ability to bind and stimulate RON and undergo proteolytic activation. MSP concentrations were quantified in the serum of individuals carrying the MSP 689R and 689C alleles.

Results

In intestinal tissue, RON was primarily expressed by epithelial cells under normal and disease conditions. The 689C polymorphism had no impact on the ability of MSP to bind to or signal through RON. In a cohort of normal individuals and IBD patients, carriers of the 689C polymorphism had lower concentrations of MSP in their serum.

Conclusions

By reducing the quantities of circulating MSP, the 689C polymorphism, or a variant in linkage disequilibrium with this polymorphism, may impact RON ligand availability and thus receptor activity. Given the known functions of RON in regulating wound healing and our analysis of RON expression patterns in human intestinal tissue, these data suggest that decreased RON activity may impact the efficiency of epithelial repair and thus underlie the increased IBD susceptibility associated with the MSP 689C allele.

Virtual screening and pharmacophore design for a novel theoretical inhibitor of macrophage stimulating factor as a metastatic agent

INTRODUCTION

Metastasis is a crucial aspect of cancer. Macrophage stimulating protein (MSP) is a single chain protein and can be cleaved by serum proteases. MSP has several roles in metastasis. In this in silico study, MSP as a metastatic agent was considered as a drug target.

METHODS

Crystallographic structure of MSP was retrieved from protein data bank. To find a chemical inhibitor of MSP, a library of KEGG compounds was screened and 1000 shape complemented ligands were retrieved with FindSite algorithm. Molegro Virtual Docker (MVD) software was used for docking simulation of shape complemented ligands against MSP. Moldock score was used as scoring function for virtual screening and potential inhibitors with more negative binding energy were obtained. PLANS scoring function was used for revaluation of virtual screening data.

RESULTS

The top found chemical had binding affinity of -183.55 based on MolDock score and equal to -66.733 PLANTs score to MSP structure.

CONCLUSION

Based on pharmacophore model of potential inhibitor, this study suggests that the chemical which was found in this research and its derivate can be used for subsequent laboratory studies.

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

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.

MSP-RON signalling in cancer: pathogenesis and therapeutic potential

Since the discovery of MSP (macrophage-stimulating protein; also known as MST1 and hepatocyte growth factor-like (HGFL)) as the ligand for the receptor tyrosine kinase RON (also known as MST1R) in the early 1990s, the roles of this signalling axis in cancer pathogenesis has been extensively studied in various model systems. Both in vitro and in vivo evidence has revealed that MSP-RON signalling is important for the invasive growth of different types of cancers. Currently, small-molecule inhibitors and antibodies blocking RON signalling are under investigation. Substantial responses have been achieved in human tumour xenograft models, laying the foundation for clinical validation. In this Review, we discuss recent advances that demonstrate the importance of MSP-RON signalling in cancer and its potential as a therapeutic target.

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.

Protein characterization of a candidate mechanism SNP for Crohn's disease: the macrophage stimulating protein R689C substitution

High throughput genome wide associations studies (GWAS) are now identifying a large number of genome loci related to risk of common human disease. Each such locus presents a challenge in identifying the relevant underlying mechanism. Here we report the experimental characterization of a proposed causal single nucleotide polymorphism (SNP) in a locus related to risk of Crohn's disease and ulcerative colitis. The SNP lies in the MST1 gene encoding Macrophage Stimulating Protein (MSP), and results in an R689C amino acid substitution within the β-chain of MSP (MSPβ). MSP binding to the RON receptor tyrosine kinase activates signaling pathways involved in the inflammatory response. We have purified wild-type and mutant MSPβ proteins and compared biochemical and biophysical properties that might impact the MSP/RON signaling pathway. Surface plasmon resonance (SPR) binding studies showed that MSPβ R689C affinity to RON is approximately 10-fold lower than that of the wild-type MSPβ and differential scanning fluorimetry (DSF) showed that the thermal stability of the mutant MSPβ was slightly lower than that of wild-type MSPβ, by 1.6 K. The substitution was found not to impair the specific Arg483-Val484 peptide bond cleavage by matriptase-1, required for MSP activation, and mass spectrometry of tryptic fragments of the mutated protein showed that the free thiol introduced by the R689C mutation did not form an aberrant disulfide bond. Together, the studies indicate that the missense SNP impairs MSP function by reducing its affinity to RON and perhaps through a secondary effect on in vivo concentration arising from reduced thermodynamic stability, resulting in down-regulation of the MSP/RON signaling pathway.

Deletion or insertion in the first immunoglobulin-plexin-transcription (IPT) domain differentially regulates expression and tumorigenic activities of RON receptor Tyrosine Kinase

Background

Activation of the RON receptor tyrosine kinase, a member of the c-MET family, regulates tumorigenic phenotypes. The RON extracellular domains are critical in regulating these activities. The objective of this study was to determine the role of the first IPT domain in regulating RON-mediated tumorigenic activities and the underlying mechanisms.

Results

Two RON variants, RON160 and RON^E5/6in with deletion and insertion in the first IPT domain, respectively, were molecularly cloned. RON160 was a splicing variant generated by deletion of 109 amino acids encoded by exons 5 and 6. In contrast, RON^E5/6in was derived from a transcript with an insertion of 20 amino acids between exons 5 and 6. Both RON160 and RON^E5/6in were proteolytically matured into two-chain receptor and expressed on the cell surface. RON160 was constitutively active with tyrosine phosphorylation. However, activation of RON^E5/6in required ligand stimulation. Deletion resulted in the resistance of RON160 to proteolytic digestion by cell associated trypsin-like enzymes. RON160 also resisted anti-RON antibody-induced receptor internalization. These features contributed to sustained intracellular signaling cascades. On the other hand, RON^E5/6in was highly susceptible to protease digestion, which led to formation of a truncated variant known as RONp110. RON^E5/6in also underwent rapid internalization upon anti-RON antibody treatment, which led to signaling attenuation. Although ligand-induced activation of RON^E5/6in partially caused epithelial to mesenchymal transition (EMT), it was RON160 that showed cell-transforming activities in cell focus formation and anchorage-independent growth. RON160-mediated EMT is also associated with increased motile/invasive activity.

Conclusions

Alterations in the first IPT domain in extracellular region differentially regulate RON mediated tumorigenic activities. Deletion of the first IPT results in formation of oncogenic variant RON160. Enhanced degradation and internalization with attenuated signaling cascades could be the mechanisms underlying non-tumorigenic features of RON^E5/6in.

Structural and mechanistic insight into how antibodies inhibit serine proteases

Antibodies display great versatility in protein interactions and have become important therapeutic agents for a variety of human diseases. Their ability to discriminate between highly conserved sequences could be of great use for therapeutic approaches that target proteases, for which structural features are conserved among family members. Recent crystal structures of antibody-protease complexes provide exciting insight into the variety of ways antibodies can interfere with the catalytic machinery of serine proteases. The studies revealed the molecular details of two fundamental mechanisms by which antibodies inhibit catalysis of trypsin-like serine proteases, exemplified by hepatocyte growth factor activator and MT-SP1 (matriptase). Enzyme kinetics defines both mechanisms as competitive inhibition systems, yet, on the molecular level, they involve distinct structural elements of the active-site region. In the steric hindrance mechanism, the antibody binds to protruding surface loops and inserts one or two CDR (complementarity-determining region) loops into the enzyme's substrate-binding cleft, which results in obstruction of substrate access. In the allosteric inhibition mechanism the antibody binds outside the active site at the periphery of the substrate-binding cleft and, mediated through a conformational change of a surface loop, imposes structural changes at important substrate interaction sites resulting in impaired catalysis. At the centre of this allosteric mechanism is the 99-loop, which is sandwiched between the substrate and the antibody-binding sites and serves as a mobile conduit between these sites. These findings provide comprehensive structural and functional insight into the molecular versatility of antibodies for interfering with the catalytic machinery of proteases.

Variants at the 3p21 locus influence susceptibility and phenotype both in adults and early-onset patients with inflammatory bowel disease

BACKGROUND

To date, a number of high-profile studies have yielded over 50 inflammatory bowel disease (IBD) disease genes/loci. The polymorphisms rs9858542 (BSN) and rs3197999 (MST1), on 3p21 locus, have been found associated with susceptibility to IBD. We aimed to replicate these associations in adult and early-onset cohorts of IBD Italian patients, by analyzing also potential gene-gene interactions with variants in NOD2/CARD15, IL23R, ATG16L1, and IRGM genes, and investigating genotype-phenotype correlation.

METHODS

In all, 1808 patients with IBD, 855 with Crohn's disease (CD) and 953 with ulcerative colitis (UC), including 539 patients with their initial diagnosis <19 years of age, and 651 controls were analyzed for SNPs rs9858542 and rs3197999.

RESULTS

BSN and MST1 were significantly associated with either CD (P(rs9858542) 2.5 x 10(-7); P(rs3197999) 3.9 x 10(-7)), and UC (P(rs9858542) = 3.1 x 10(-4); P(rs3197999) = 8 x 10(-4)). Prevalence of these variants was significantly increased in both adult and early-onset IBD patients. After stepwise logistic regression, the 2 variants were associated in adult UC with distal colitis (P(rs9858542) = 0.013, odds ratio [OR] = 2.04, 95% confidence interval [CI] = 1.16-3.59; P(rs3197999) = 0.018, OR 1.9, 95% CI 1.2-3.3), while the rs3197999 variant was inversely associated with occurrence of extraintestinal manifestations in adult CD(P = 0.017, OR 0.6, 95% CI 0.4-0.9).

CONCLUSIONS

We confirmed the association of BSN and MST1 with IBD susceptibility, either in the adult or the early-onset cohorts. These variants appeared to influence either the distal location of the disease in the UC cohort and extraintestinal manifestations in CD patients.

Transactivation of RON receptor tyrosine kinase by interaction with PDGF receptor beta during steady-state growth of human mesangial cells

Although it is well known that platelet-derived growth factor (PDGF) causes mesangial cell proliferation (presumably contributing to progression of glomerular disease), targeted inhibition of the PDGF receptor system has shown only limited efficacy against glomerular diseases. To examine whether this discrepancy is due to the involvement of other pathways, we used phosphorylated receptor tyrosine kinase arrays and found that RON (recepteur d'origine nantais) was phosphorylated while the PDGF receptor was dephosphorylated (thus inactive) in human mesangial cells (HMCs) at the time of cell cycle entry. Further, RON remained active during steady-state growth. Activation of RON was independent of its canonical ligand, macrophage-stimulating protein, but was mediated by transactivation from the PDGF-engaged PDGF receptor. Following stimulation with PDGF we found that the two receptors physically interacted. Knockdown of RON by siRNA increased the number of apoptotic cells without affecting the rate of DNA synthesis, suggesting that RON has anti-apoptotic functions. Immunohistochemical analysis found phosphorylated RON in glomerular lesions of patients with IgA nephropathy but not those with minimal change nephrotic syndrome, a disease not associated with mesangial proliferation. These results suggest that RON is involved in mesangial cell proliferation under both physiological and pathological conditions, and may be a relevant target for therapeutic intervention.

Metron factor-1 prevents liver injury without promoting tumor growth and metastasis

Hepatocyte growth factor (HGF) is the most powerful hepatotrophic factor identified so far. However, the ability of HGF to promote tumor cell "scattering" and invasion raises some concern about its therapeutic safety. We compared the therapeutic efficacy of HGF with that of Metron Factor-1 (MF-1), an engineered cytokine derived from HGF and the HGF-like factor macrophage stimulating protein (MSP), in mouse models of acute and chronic liver injury. At the same time, we tested the ability of HGF and MF-1 to promote tumor growth, angiogenesis, and invasion in several mouse models of cancer. We show that (1) MF-1 and HGF stimulate hepatocyte proliferation in vitro; (2) MF-1 and HGF protect primary hepatocytes against Fas-induced and drug-induced apoptosis; (3) HGF but not MF-1 induces scattering and matrigel invasion of carcinoma cell lines in vitro; (4) HGF but not MF-1 promotes migration and extracellular matrix invasion of endothelial cells in vitro; (5) MF-1 and HGF prevent CCl(4)-induced acute liver injury as measured by alanine aminotransferase (ALT) levels, histology, terminal deoxynucleotidyl transferase-mediated nick-end labeling (TUNEL) analysis, and phospho-histone-3 immunostaining; (6) MF-1 and HGF attenuate liver fibrosis caused by chronic CCl(4) intoxication and promote regeneration as measured by Sirius red staining, alpha-smooth muscle actin immunostaining, and Ki-67 analysis; (7) HGF but not MF-1 promotes tumor growth, angiogenesis, and metastasis in a variety of xenograft models; (8) HGF but not MF-1 promotes intrahepatic dissemination of hepatocarcinoma cells injected orthotopically.

CONCLUSION

These data suggest that MF-1 is as effective as HGF at preventing liver injury and at promoting hepatocyte regeneration, but therapeutically safer than HGF because it lacks proangiogenic and prometastatic activity.

Coordinate expression and functional profiling identify an extracellular proteolytic signaling pathway

A multidisciplinary method combining transcriptional data, specificity profiling, and biological characterization of an enzyme may be used to predict novel substrates. By integrating protease substrate profiling with microarray gene coexpression data from nearly 2,000 human normal and cancerous tissue samples, three fundamental components of a protease-activated signaling pathway were identified. We find that MT-SP1 mediates extracellular signaling by regulating the local activation of the prometastatic growth factor MSP-1. We demonstrate MT-SP1 expression in peritoneal macrophages, and biochemical methods confirm the ability of MT-SP1 to cleave and activate pro-MSP-1 in vitro and in a cellular context. MT-SP1 induced the ability of MSP-1 to inhibit nitric oxide production in bone marrow macrophages. Addition of HAI-1 or an MT-SP1-specific antibody inhibitor blocked the proteolytic activation of MSP-1 at the cell surface of peritoneal macrophages. Taken together, our work indicates that MT-SP1 is sufficient for MSP-1 activation and that MT-SP1, MSP-1, and the previously shown MSP-1 tyrosine kinase receptor RON are required for peritoneal macrophage activation. This work shows that this triad of growth factor, growth factor activator protease, and growth factor receptor is a protease-activated signaling pathway. Individually, MT-SP1 and RON overexpression have been implicated in cancer progression and metastasis. Transcriptional coexpression of these genes suggests that this signaling pathway may be involved in several human cancers.

Apical oxidative hyaluronan degradation stimulates airway ciliary beating via RHAMM and RON

Hyaluronan (HA) is synthesized in high-molecular-weight form at the apical pole of airway epithelial cells, covering the luminal surface. When human airway epithelial cells grown and redifferentiated at the air-liquid interface (ALI) were exposed to xanthine/xanthine oxidase (X/XO), ciliary beat frequency (CBF) increased. This effect was blocked by superoxide dismutase (SOD) and catalase. Inhibition of hyaluronan synthesis inhibited the CBF response to X/XO, while addition of exogenous HA amplified it. A functionally blocking antibody to the receptor for hyaluronic acid-mediated motility (RHAMM) reduced the CBF response to X/XO. Since RHAMM has no transmembrane domain and thus cannot signal on its own, the association of RHAMM with recepteur d'origine nantais (RON), a member of the hepatocyte growth factor receptor family, was explored. Immunohistochemistry of human airway epithelium showed co-localization of RHAMM and RON at the apex of ciliated cells. Physical association of RHAMM and RON was confirmed with co-immunoprecipitations. Macrophage-stimulating protein (MSP), an agonist of RON, stimulated CBF. Genistein, a nonspecific tyrosine kinase inhibitor, and MSP beta chain (beta-MSP), a specific RON inhibitor, blocked the X/XO-induced CBF increase. HA present in the apical secretions of human airway epithelial cells was shown to degrade upon exposure to X/XO, a process inhibited by SOD. Low-molecular-weight HA fragments stimulated CBF, an effect blocked by anti-RHAMM antibody and genistein. These data suggest that high molecular form HA is broken down by reactive oxygen species to form low-molecular-weight fragments that signal via RHAMM and RON to stimulate CBF.

Oncogenesis of RON receptor tyrosine kinase: a molecular target for malignant epithelial cancers

Recepteur d'origine nantais (RON) belongs to a subfamily of receptor tyrosine kinases (RTK) with unique expression patterns and biological activities. RON is activated by a serum-derived growth factor macrophage stimulating protein (MSP). The RON gene transcription is essential for embryonic development and critical in regulating certain physiological processes. Recent studies have indicated that altered RON expression contributes significantly to cancer progression and malignancy. In primary tumors, such as colon and breast cancers, overexpression of RON exists in large numbers and is often accompanied by the generation of different splicing variants. These RON variants direct a unique program that controls cell transformation, growth, migration, and invasion, indicating that altered RON expression has the ability to regulate motile/invasive phenotypes. These activities were also seen in transgenic mice, in which targeted expression of RON in lung epithelial cells resulted in numerous tumors with pathological features of human bronchioloalveolar carcinoma. Thus, abnormal RON activation is a pathogenic factor that transduces oncogenic signals leading to uncontrolled cell growth and subsequent malignant transformation. Considering these facts, RON and its variants can be considered as potential targets for therapeutic intervention. Experiments using small interfering RNA and neutralizing monoclonal antibodies demonstrated that suppressing RON expression and activation decreases cancer cell proliferation, increases apoptotic death, prevents tumor formation in nude mice, and reduces malignant phenotypes. Thus, blocking RON expression and activation has clinical significance in reversing malignant phenotypes and controlling tumor growth.

Macrophage-stimulating protein differently affects human alveolar macrophages from smoker and non-smoker patients: evaluation of respiratory burst, cytokine release and NF-kappaB pathway

Macrophage activation is a key feature of inflammatory reactions occurring during bacterial infections, immune responses and tissue injury. We previously demonstrated that human macrophages of different origin express the tyrosine kinase receptor recepteur d'origine nantaise, the human receptor for MSP (RON) and produce superoxide anion (O(2)(-)) when challenged with macrophage-stimulating protein (MSP), the endogenous ligand for RON. This study was aimed to evaluate the role of MSP in alveolar macrophages (AM) isolated from healthy volunteers and patients with interstitial lung diseases (sarcoidosis, idiopathic pulmonary fibrosis), either smokers or non-smokers, by evaluating the respiratory burst, cytokine release and nuclear factor-kappa B (NF-kappaB) activation. MSP effects were compared with those induced by known AM stimuli, for example, phorbol myristate acetate, N-formyl-methionyl-leucyl-phenylalanine, lipopolysaccharide.MSP evokes O(2)(-) production, cytokine release and NF-kappaB activation in a concentration-dependent manner. By evaluating the respiratory burst, we demonstrate a significantly increased O(2)(-) production in AM from healthy smokers or smokers with pulmonary fibrosis, as compared to non-smokers, thus suggesting MSP as an enhancer of cigarette smoke toxicity. Besides inducing interleukin-1 beta (IL-1beta) and interleukin-10 (IL-10) production, MSP triggers an enhanced tumor necrosis factor-alpha release, especially in healthy and pulmonary fibrosis smokers. On the contrary, MSP-induced IL-10 release is higher in AM from healthy non-smokers. MSP activates the transcription factor NF-kappaB; this effect is more potent in healthy and fibrosis smokers (2.5-fold increase in p50 subunit translocation). This effect is receptor-mediated, as it is prevented by a monoclonal anti-human MSP antibody. The higher effectiveness of MSP in AM from healthy smokers and patients with pulmonary fibrosis is suggestive of its role in these clinical conditions.

Crystal structure of the beta-chain of human hepatocyte growth factor-like/macrophage stimulating protein

Hepatocyte growth factor like/macrophage stimulating protein (HGFl/MSP) and hepatocyte growth factor/scatter factor (HGF/SF) define a distinct family of vertebrate-specific growth factors structurally related to the blood proteinase precursor plasminogen and with important roles in development and cancer. Although the two proteins share a similar domain structure and mechanism of activation, there are differences between HGFl/MSP and HGF/SF in terms of the contribution of individual domains to receptor binding. Here we present a crystal structure of the 30 kDa beta-chain of human HGFl/MSP, a serine proteinase homology domain containing the high-affinity binding site for the RON receptor. The structure describes at 1.85 Angstrom resolution the region of the domain corresponding to the receptor binding site recently defined in the HGF/SF beta-chain, namely the central cleft harboring the three residues corresponding to the catalytic ones of active proteinases (numbers in brackets define the sequence position according to the standard chymotrypsinogen numbering system) [Gln522 (c57), Gln568 (c102) and Tyr661 (c195)] and an adjacent loop flanking the S1 specificity pocket and containing residues Asn682 (c217) and Arg683 (c218) previously shown to be essential for binding of HGFl/MSP to the RON receptor. The study confirms the concept that the serine proteinase homology domains of HGFl/MSP and HGF/SF bind their receptors in an 'enzyme-substrate' mode, reflecting the common evolutionary origin of the plasminogen-related growth factors and the proteinases of the clotting and fibrinolytic pathways. However, analysis of the intermolecular interactions in the crystal lattice of beta-chain HGFl/MSP fails to show the same contacts seen in the HGF/SF structures and does not support a conserved mode of dimerization of the serine proteinase homology domains of HGFl/MSP and HGF/SF responsible for receptor activation.

Macrophage-Stimulating Protein Promotes Hair Growth Ex Vivo and Induces Anagen from Telogen Stage Hair Follicles In Vivo

Hepatocyte growth factor (HGF) is a promoter of hair follicle growth. We examined another HGF family member, macrophage-stimulating protein (MSP), for its hair follicle-modulating properties. Western blotting revealed presence of mature MSP in cultured human dermal papilla (DP) cells and bulbar dermal sheath (DS) cells, but not non-bulbar DS cells. Immunohistology demonstrated expression of MSP receptor RON in the outer and inner root sheaths, hair matrix cells, DP, and bulbar DS whereas non-follicular epithelium and some cells of the sweat glands exhibited low-level receptor expression. Human hair follicles exposed in vitro for 8 d to 0.1, 1, 10, and 100 ng per mL MSP all yielded a mean net increase in hair follicle length in excess of the mean baseline growth observed in controls. MSP was incubated with agarose beads and injected subcutaneously into mice all 70 d old when a uniform telogen state in dorsal skin was apparent. All eight mice receiving 1 microg MSP, and four of eight receiving 100 ng MSP showed induction of anagen hair growth at the site of bead implantation by 16 d whereas eight mice implanted with saline incubated beads had no hair growth. The data identify MSP as a modulator of hair growth.

Structural and functional basis of the serine protease-like hepatocyte growth factor beta-chain in Met binding and signaling

Hepatocyte growth factor (HGF), a plasminogen-related growth factor, is the ligand for Met, a receptor tyrosine kinase implicated in development, tissue regeneration, and invasive tumor growth. HGF acquires signaling activity only upon proteolytic cleavage of single-chain HGF into its alpha/beta heterodimer, similar to zymogen activation of structurally related serine proteases. Although both chains are required for activation, only the alpha-chain binds Met with high affinity. Recently, we reported that the protease-like HGF beta-chain binds to Met with low affinity (Stamos, J., Lazarus, R. A., Yao, X., Kirchhofer, D., and Wiesmann, C. (2004) EMBO J. 23, 2325-2335). Here we demonstrate that the zymogen-like form of HGF beta also binds Met, albeit with 14-fold lower affinity than the protease-like form, suggesting optimal interactions result from conformational changes upon cleavage of the single-chain form. Extensive mutagenesis of the HGF beta region corresponding to the active site and activation domain of serine proteases showed that 17 of the 38 purified two-chain HGF mutants resulted in impaired cell migration or Met phosphorylation but no loss in Met binding. However, reduced biological activities were well correlated with reduced Met binding of corresponding mutants of HGF beta itself in assays eliminating dominant alpha-chain binding contributions. Moreover, the crystal structure of HGF beta determined at 2.53 A resolution provides a structural context for the mutagenesis data. The functional Met binding site is centered on the "active site region" including "triad" residues Gln(534) [c57], Asp(578) [c102], and Tyr(673) [c195] and neighboring "activation domain" residues Val(692), Pro(693), Gly(694), Arg(695), and Gly(696) [c214-c219]. Together they define a region that bears remarkable resemblance to substrate processing regions of serine proteases. Models of HGF-dependent Met receptor activation are discussed.

Characterization of a single-chain intrabody directed against the human receptor tyrosine kinase Ron

A large human nonimmune phage antibody library was screened by affinity chromatography to select single-chain antibodies directed against the human receptor tyrosine kinase (RTK) Ron. As antigen, we used a GST fusion protein (GST-IRP(-)) containing the whole intracellular portion of Ron except for the carboxyl-terminal arginine-proline-rich motif. One selected phage was highly specific for Ron when tested in an enzyme-linked immunosorbent assay (ELISA). We report here the immunological characterization of this anti-Ron single-chain antibody (sc7) and show that it recognizes both denatured and native forms of the receptor. The epitope bound by sc7 maps within the first 50 amino acid residues of the juxtamembrane domain of Ron. This monoclonal fragment does not cross-react with other receptor tyrosine kinases including the closely related human proto-oncogene Met. We demonstrate that the isolated antibody fragment interacts in vivo with the intracellular domain of Ron in mammalian cells.

The soluble sema domain of the RON receptor inhibits macrophage-stimulating protein-induced receptor activation

RON is a receptor tyrosine kinase of the MET family that is involved in cell proliferation, cell survival, and cell motility in both normal and disease states. Macrophage-stimulating protein (MSP) is the RON ligand whose binding to RON causes receptor activation. RON is a trans-membrane heterodimer comprised of one alpha- and one beta-chain originating from a single-chain precursor and held together by several disulfide bonds. The intracellular part of RON contains the kinase domain and regulatory elements. The extracellular region is characterized by the presence of a sema domain (a stretch of approximately 500 amino acids with several highly conserved cysteine residues), a PSI (plexin, semaphorins, integrins) domain, and four immunoglobulin-like folds. Here we show that a soluble, secreted molecule representing the sema domain of RON (referred to as ron-sema) has a dominant negative effect on the ligand-induced receptor activation and is capable of inhibiting RON-dependent signaling pathways and cellular responses. Results suggest that the sema domain of RON participates in ligand binding by the full-length receptor. The ability of ron-sema to suppress growth of MSP-responsive cells in culture, including cancer cells, points to a potential therapeutic use of this molecule, and forced expression of it could potentially be used as a gene therapy tool for treating MSP-dependent types of cancer.

Oncogenic and invasive potentials of human macrophage-stimulating protein receptor, the RON receptor tyrosine kinase

The product of the RON (recepteur d'origine nantais) gene belongs to the MET proto-oncogene family, a distinct subfamily of receptor tyrosine kinases. The ligand of RON was identified as macrophage-stimulating protein (MSP), a member of the plasminogen-related growth factor family. RON is mainly expressed in cells of epithelial origin and is required for embryonic development. In vitro RON activation results in epithelial cell dissociation, migration and matrix invasion, suggesting that RON might be involved in the pathogenesis of certain epithelial cancers in vivo. Indeed, recent studies have shown that RON expression is significantly altered in several primary human cancers, including those of the breast and colon. Truncation of the RON protein has also been found in primary tumors from the gastrointestinal tract. These alterations lead to constitutive activation of RON that causes cell transformation in vitro, induces neoplasm formation in athymic nude mice, and promotes tumor metastasis into the lung. Studies employing transgenic models further demonstrated that over-expression of RON in lung epithelial cells results in multiple tumor formation with features of large cell undifferentiated carcinoma. The oncogenic activities of RON are mediated by RON-transduced signals that promote unbalanced cell growth and transformation leading to tumor development. Thus, abnormal accumulation and activation of RON could play a critical role in vivo in the progression of certain malignant human epithelial cancers.

Cis-acting elements in the hepatocyte growth factor-like protein gene regulate kidney and liver-specific expression in mice

Previous studies from our laboratory demonstrated that the hepatocyte-specific transcriptional activity of the hepatocyte growth factor-like protein/macrophage stimulating protein (HGFL) promoter is modulated in HepG2 cells by the first 135 base pairs (bp) upstream of the HGFL transcriptional start site. Gel mobility shift and transactivation assays demonstrated that hepatocyte nuclear factor-4 (HNF-4) binds to this region and is responsible, in part, for the liver-specific expression of this gene in HepG2 cells. In an attempt to understand the in vivo mechanism regulating the expression of HGFL, a series of transgenic mice were generated that contained four different regions upstream of the HGFL promoter attached to the coding sequences for chloramphenicol acetyltransferase (CAT). Interestingly, upstream promoter sequences, containing as little as 104 bp upstream of the translational start site, were able to drive reporter expression and protein production specifically in kidney and liver tissue. Strikingly, when the first exon and intron of the HGFL gene was inserted downstream of the 135 bp promoter element, only liver-specific expression was observed. These studies indicate that short sequences upstream of HGFL can drive efficient expression in kidney and liver tissue, and that sequences in the first intron of the HGFL gene contain regulatory elements that direct kidney-specific transcriptional repression in vivo and aid in the proper recapitulation of HGFL expression in mice.

Macrophage-stimulating protein and RON receptor tyrosine kinase: potential regulators of macrophage inflammatory activities

Macrophage-stimulating protein (MSP) is a serum protein belonging to the plasminogen-related growth factor family. The specific receptor for MSP is the RON (recepteur d'origine nantais) receptor tyrosine kinase - a member of the MET proto-oncogene family. Activation of RON by MSP exerts dual functions on macrophages. The stimulatory activities include the induction of macrophage spreading, migration and phagocytosis. However, MSP also inhibits lipopolysaccharide (LPS)-induced production of inflammatory mediators, including inducible nitric oxide and prostaglandins. These suppressive effects are mediated by RON-transduced signals that block LPS-induced enzymatic cascades that activate nuclear factor kappa-B (NFkappaB) pathways. Recent in vivo studies demonstrated that inactivation of the RON gene results in increased inflammatory responses and susceptibility to LPS-induced septic death in mice, suggesting that RON expression is required for attenuating the extent of inflammatory responses in vivo. Thus, MSP and RON are potential regulators that control macrophage activities during bacterial infection in vivo.

Activation of the RON receptor tyrosine kinase by macrophage-stimulating protein inhibits inducible cyclooxygenase-2 expression in murine macrophages

The RON receptor tyrosine kinase is activated by macrophage-stimulating protein, which regulates macrophage migration, phagocytosis, and nitric oxide production. We report here the inhibitory effect of RON on lipopolysaccharide (LPS)-induced cyclooxygenase (Cox)-2 expression in mouse macrophages. In RON-expressing macrophages treated with macrophage stimulating protein, LPS-induced prostaglandin E(2) (PGE(2)) production was significantly reduced. The inhibition was accompanied by reduction of Cox-2 protein and mRNA expression. Transcriptional studies indicated that RON activation inhibits LPS-induced luciferase activity driven by the Cox-2 gene promoter. To determine whether RON activation affects LPS-induced NF-kappa B pathway, which is important for Cox-2 expression. Western blot analyses were performed showing that RON activation inhibits LPS-induced I kappa B alpha degradation. The decreased I kappa B alpha degradation was due to reduced I kappa B alpha phosphorylation at Ser-32 as determined by I kappa B alpha (Ser-32) phosphor-antibody. Moreover, we found that LPS-induced IKK beta activity, an enzyme responsible for phosphorylation of I kappa B alpha, was inhibited upon RON activation. Interestingly, these inhibitory effects were not regulated by RON-mediated phosphatidylinositol-3 kinase. These results suggest that RON activation inhibits LPS-induced macrophage Cox-2 expression. The inhibitory effect is mediated by impairing LPS-activated cascade enzymes that activate NF-kappa B. The inhibition of Cox-2 expression might represent a novel mechanism for the inhibitory functions of RON in vivo against LPS-induced inflammation and septic shock.

Scatter factors and invasive growth

Scatter factors are unequivocal signals governing a genetic program that includes cell detachment, repulsion, protection from apoptosis, invasiveness of extracellular matrices and proliferation. This pleiomorphic response is defined as 'invasive growth'. Under physiological conditions, it leads to morphogenic cell movements through the matrix, and--primarily--to ordered building of epithelial tubules. Dysfunctions in invasive growth cause enhanced proliferation, uncontrolled migration into surrounding tissues, and failure to differentiate, events that foster tumour growth and invasiveness. Scatter factors act through tyrosine kinase receptors that belong to the Met oncogene family. Here we discuss how alterations of these receptors or of their signal transduction pathways are responsible for cancer onset and progression towards metastasis.

Cross-talk between the proto-oncogenes Met and Ron

Scatter Factors control a complex genetic program known as 'invasive growth'. HGF (Scatter factor 1) and MSP (Scatter Factor 2) bind to tyrosine kinase receptors encoded by the proto-oncogenes MET and RON. Using the appropriate 'kinase inactive' mutant receptors, we show that ligand-induced activation of Met results in transphosphorylation of Ron, and vice versa. Transphosphorylation is direct, as it occurs in Met or Ron receptors lacking the docking sites for signal transducers. Phosphate groups are transferred to the tyrosine phosphorylation sites responsible both for kinase up-regulation (Met: Y1234/Y1235 and Ron: Y1238/Y1239) and for generation of signal transducer docking sites (Met: Y1349/Y1356 and Ron Y1353/Y1360). The transphosphorylation specifically takes place for the receptor subfamily, as it is not observed between Met or Ron and ErbB1, ErbB2 or TrkA. Cross-linking experiments show that non-covalent Met-Ron complexes are present on the cell surface, before ligand-induced dimerization. Co-expression of a kinase inactive Ron receptor with naturally-occurring oncogenic Met mutants suppresses the transforming phenotype, suggesting a dominant negative role for the inefficient kinase partner. These data show that, while specific for their ligands, scatter factor receptors cross-talk and cooperate in intracellular signaling.

Plasminogen-related growth factor and semaphorin receptors: a gene superfamily controlling invasive growth

Plasminogen-related growth factors (PRGFs), also known as "scatter factors," trigger a unique biological program leading to "invasive growth." This is a result of the integration of apparently independent biological responses including cell proliferation, cell survival, cell motility, invasion of extracellular matrices, and induction of cell polarity. Under physiological conditions, the coordinated execution of the underlying genetic programs leads to the formation of tubular structures by epithelial organs (the so-called branching morphogenesis). PRGF receptors are tyrosine kinases, encoded by a family of oncogenes: MET and RON. They feature unique signal transduction properties as their cytoplasmic tails contain a two-tyrosine multifunctional docking site that binds multiple SH2-containing intracellular signal transducers. Invasive growth results from the concomitant activation of Ras (growth), phosphatidylinositol 3-kinase ("scattering"), and signal transducer and activator of transcription (cell polarity and morphogenesis). We recently identified a new human gene family, encoding large transmembrane proteins, sex/plexins, sharing homologies with Met. These molecules are receptors for semaphorins, involved in axon guidance and cell-cell repulsion, a process reminiscent of scattering and invasive growth. Deregulated activation of PRGF or semaphorin ligands or receptors, by mutation or overexpression, confers to cancer cells invasive and metastatic properties.

Interaction of macrophage-stimulating protein with its receptor. Residues critical for beta chain binding and evidence for independent alpha chain binding

Macrophage-stimulating protein (MSP) and hepatocyte growth factor/scatter factor (HGF/SF) are plasminogen-related growth and motility factors that interact with cell-surface protein tyrosine kinase receptors. Each one is a heterodimeric protein comprising a disulfide-linked alpha chain and a serine protease-like beta chain. Despite structural similarities between MSP and HGF, the primary receptor binding site is located on the alpha chain of HGF/SF but on the beta chain of MSP. To obtain insight into the structural basis for MSP beta chain binding, beta chain structure was modeled from coordinates of an existing model of the HGF beta chain. The model revealed that the region corresponding to the S1 specificity pocket in trypsin is filled by the Asn(682)/Glu(648) interacting pair, leaving a shallow cavity for possible beta chain interaction with the receptor. Mutants in this region were created, and their binding characteristics were determined. A double mutation of Asn(682)/Glu(648) caused diminished binding of the beta chain to the MSP receptor, and a single mutation of neighboring Arg(683) completely abolished binding. Thus, this region of the molecule is critical for binding. We also found that at equimolar concentrations of free alpha and beta chains, alpha chain binding to receptor was detectable, at levels considerably lower than beta chain binding. The EC(50) values determined by quantitative enzyme-linked immunosorbent assay are 0.25 and 16.9 nM for beta and alpha chain, respectively. The data suggest that MSP has two independent binding sites with high and low affinities located in beta and alpha chain, respectively, and that the two sites together mediate receptor dimerization and subsequent activation.

Chicken macrophage stimulating protein is a ligand of the receptor protein-tyrosine kinase Sea

Affinity chromatography, employing the extracellular domain of the Sea receptor, was used to enrich Sea-binding proteins from chicken serum. One isolated protein bound both a Sea-immunoglobulin fusion protein and an antisera raised against murine macrophage stimulating protein. Amino-terminal sequencing of the dual-reactive protein yielded sequences which were identical to the predicted alpha and beta subunits of chicken macrophage stimulating protein. The partially purified chicken macrophage stimulating protein caused autophosphorylation of the Sea receptor. Previous work showed that recombinant expression of fully activatible human or mouse macrophage stimulating protein required a specific Cys to Ala substitution (Wahl, R. C., Costigan, V. J., Batac, J. P., Chen, K., Cam, L., Courchesne, P. L., Patterson, S. D. Zhang, K., and Pacifici, R. E. (1997) J. Biol. Chem. 272, 1-4). Therefore, we expressed both the wild type and the specific Cys to Ala form of chicken macrophage stimulating protein as recombinant proteins. After proteolytic activation, only conditioned media from COS cells transfected with the C665A chicken macrophage stimulating protein, but not from wild type chicken macrophage-stimulating protein, or control vector, was detected by the Sea-immunoglobulin fusion protein in Western blotting experiments. Conditioned media containing the C665A chicken macrophage-stimulating protein readily caused Sea phosphorylation, while conditioned media containing the wild type chicken macrophage-stimulating protein was only effective at inducing receptor phosphorylation at high concentrations. In addition to receptor phosphorylation, the C665A chicken macrophage-stimulating protein induced phosphorylation of Shc, Erk1, and Erk 2. We conclude that macrophage-stimulating protein is a ligand of the Sea receptor protein-tyrosine kinase.

Macrophage stimulating protein-induced epithelial cell adhesion is mediated by a PI3-K-dependent, but FAK-independent mechanism

Macrophage stimulating protein (MSP) is a growth and motility factor that mediates its activity via the RON/STK receptor tyrosine kinase. MSP promotes integrin-dependent epithelial cell migration, which suggests that MSP may regulate integrin receptor functions. Integrins are cell surface receptors for extracellular matrix. Epithelial cell adhesion and motility are mediated by integrins. We studied the enhancement by MSP of cell adhesion and the molecular mechanisms mediating this effect. MSP decreased the time required for adhesion of 293 and RE7 epithelial cells to substrates coated with collagen or fibronectin. Prevention of adhesion by an RGD-containing peptide showed that the cell-substrate interaction was mediated by integrins. Wortmannin, an inhibitor of phosphatidylinositol 3-kinase (PI3-K), blocked MSP-dependent adhesion, which shows that PI3-K is in the MSP-induced adhesion pathway. MSP also affected focal adhesion kinase (FAK) which is important for some types of cell adhesion and motility. Although MSP caused PI3-K-independent tyrosine phosphorylation and activation of FAK, experiments with dominant-negative FAK constructs showed that FAK does not mediate the effects of MSP on cell adhesion or motility. Thus PI3-K, but not FAK, mediates MSP-induced integrin-dependent adhesion of epithelial cells. Also, we found ligand-independent association between RON and beta1 integrin, which is additional evidence for a relationship between these two receptor systems.

Characterization of free alpha- and beta-chains of recombinant macrophage-stimulating protein

Human serum macrophage-stimulating protein (MSP) induces motile activity of murine resident peritoneal macrophages and is a growth and motility factor for epithelial cells. It belongs to the plasminogen-related family of kringle proteins, and is secreted as a single-chain, 78-kDa, biologically inactive pro-MSP. Proteolytic cleavage of pro-MSP at a single site yields active MSP, a disulfide-linked alphabeta-chain heterodimer. However cleavage of recombinant pro-MSP yielded not only the disulfide-linked heterodimer, but also free alpha- and beta-chains, indicating that some of the recombinant molecules lacked an alphabeta-chain disulfide. We purified the free chains for characterization. The beta-chain of MSP has three extra cysteines, Cys527, Cys562, and Cys672, which are not found in the plasminogen beta-chain. Disulfide bond analysis showed a Cys527-Cys562, but also a Cys588-Cys672. Coopting Cys588 by Cys672 prevented the expected formation of a disulfide between alpha-chain Cys468 and beta-chain Cys588. Concomitant studies determined structures of oligosaccharides at the three Asn-linked glycosylation sites of MSP. The oligosaccharides at the three Asn loci are heterogeneous; 11 different sugars were identified, all being sialylated fucosyl biantennary structures. We also located the pro-MSP signal peptide cleavage site at Gly18-Gln19 and the scissile bond for formation of mature MSP at Arg483-Val484.

Expression of the hepatocyte growth factor-like protein gene in human hepatocellular carcinoma and interleukin-6-induced increased expression in hepatoma cells

Human hepatocellular carcinoma is one of the most frequent malignant tumors. It may occur following exposure to various agents, including viruses and chemical carcinogens; however, the underlying mechanisms of the hepatocarcinogenesis are not known. The present study is the result of our search for genes which may be abundantly expressed in human primary liver carcinoma. One of these genes was found to encode the human hepatocyte growth factor-like protein (HGFLP), also known as macrophage-stimulating protein. HGFLP is structurally homologous to hepatocyte growth factor, a potent growth factor for liver. HGFLP mRNA was also found to be overexpressed in a hepatoblastoma sample and in a sample of subacute fulminant hepatic necrosis. In a study on the effects of cytokines on the expression of HGFLP, we found that IL-6 increased expression of HGFLP mRNA in Hep G2 cells, but IL-1alpha, IL-1beta and TNF-alpha had no effect. An increase in HGFLP could be the result of inflammation and/or tissue injury and its overexpression may prove to be useful as an indicator of hepatoma.

Activation of the RON Receptor Tyrosine Kinase Inhibits Inducible Nitric Oxide Synthase (iNOS) Expression by Murine Peritoneal Exudate Macrophages: Phosphatidylinositol-3 Kinase Is Required for RON-Mediated Inhibition of iNOS Expression

RON (recepteur d’origine nantais) is a receptor tyrosine kinase expressed in murine peritoneal resident macrophages and activated by macrophage-stimulating protein (MSP). The objectives of this investigation were to study the RON expression in exudate macrophages and the mechanisms by which RON inhibits inducible nitric oxide synthase (iNOS) expression induced by LPS and IFN-γ. We found that mouse peritoneal resident and Con A-elicited macrophages collected on day 3 or day 5 express RON. Acute exudate macrophages collected on day 1 did not express RON. Activation of RON inhibited LPS- and IFN-γ-induced macrophage nitric oxide production and iNOS mRNA accumulation. Similar inhibition was observed also in Raw264.7 macrophage cell lines transfected with human RON cDNA. In these cells, MSP induced RON phosphorylation concomitant with reduced iNOS mRNA expression and protein synthesis. Further, we show that activated RON inhibited the iNOS gene transcription activity as assessed by chloramphenicol acetyltransferase activity in Raw264.7 cells expressing RON. Wortmannin, a specific inhibitor of phosphatidylinositol-3 (PI-3) kinase, prevented the inhibitory effect of RON on the iNOS gene promoter activity and on the nitric oxide production induced by LPS and IFN-γ. These effects were confirmed further by introducing a dominant-inhibitory PI-3 kinase p85 subunit in RON-expressing Rwa264.7 cells. Taken together, our results suggest that RON is expressed in peritoneal macrophages at later stages of inflammation. Activation of RON by MSP in mature exudate macrophages inhibits LPS- and IFN-γ-induced iNOS synthesis. PI-3 kinase is an important effector molecule required for RON-mediated inhibition of iNOS expression in macrophages.

Interactions between scatter factors and their receptors: hints for therapeutic applications

The scatter factors, which include hepatocyte growth factor and macrophage stimulating protein, stand out from other cytokines because of their uncommon biological properties. In addition to promoting cell growth and protection from apoptosis, they are involved in the control of cell dissociation, migration into extracellular matrices, and a unique process of differentiation called 'branching morphogenesis'. Through the concerted regulation of these complex phenomena, scatter factors promote development, regeneration, and reconstruction of normal organ architecture. In transformed epithelia, scatter factors can mediate tumor invasive growth, a harmful feature of neoplastic progression in which cancer cells invade surrounding tissues, penetrate across the vascular walls, and eventually disseminate throughout the body, giving rise to systemic metastases. A much-debated issue in basic biology, which has strong implications for experimental medicine, is how to dissociate the favorable effects of growth factors from their adverse ones. Accordingly, to find agonists or antagonists with potential therapeutic applications is a crucial undertaking for current research. Domain-mapping analyses of growth factor molecules can help to isolate specific structural requirements for the induction of selective biological effects. Based on the observation that certain growth factors must undergo posttranslational modifications to exert a full response, it is possible to interfere with their activation mechanisms to modulate their functions. Finally, the identification of cell type-specific coreceptors able to potentiate their activity allows drawing of a functional body map, where some organs or tissues may be more responsive than others to growth factors. This review is focused on how, and to what extent, scatter factors can behave 'well' or 'badly' according to their molecular structure, the way they are activated, and the way they interact with cell surface receptors and coreceptors.

Cooperative interaction between alpha- and beta-chains of hepatocyte growth factor on c-Met receptor confers ligand-induced receptor tyrosine phosphorylation and multiple biological responses

Hepatocyte growth factor (HGF) is a heterodimeric molecule composed of the alpha-chain containing the N-terminal hairpin domain, four kringle domains, and the serine protease-like beta-chain. We prepared HGF/NK4 and HGF/beta from the entire HGF after single-cut digestion with elastase. HGF/NK4 contains the N-terminal hairpin and four kringle domains, while HGF/beta is composed of the C-terminal 16 amino acids of the alpha-chain and the entire beta-chain, linked by a disulfide bridge. HGF/NK4 competitively inhibited the binding of 125I-HGF to the receptor, and affinity cross-linking analysis indicated that HGF/NK4 alone can bind to the c-Met receptor. In contrast, HGF/beta alone did not competitively inhibit the binding of 125I-HGF to the receptor and did not bind to the c-Met/HGF receptor. Scatchard analysis and affinity cross-linking experiments indicated that HGF/beta specifically binds to c-Met in the presence of HGF/NK4 but not HGF/NK2. Neither HGF/NK4 nor HGF/beta alone induced mitogenic, motogenic (cell scattering), and morphogenic (induction of branching tubulogenesis) responses; however, HGF/beta did induce these biological responses in the presence of HGF/NK4. Consistent with these results, although neither HGF/NK4 alone nor HGF/beta alone induced tyrosine phosphorylation of the c-Met/HGF receptor, HGF/beta induced tyrosine phosphorylation of the receptor when c-Met/HGF receptor was occupied by HGF/NK4. These results indicate that HGF/beta binds to the c-Met/HGF receptor that is occupied by HGF/NK4 and induces receptor tyrosine phosphorylation and the subsequent biological activities of HGF. We propose that there exists a unique cooperative interaction between alpha- and beta-chains, this interaction leading to beta-chain-dependent receptor tyrosine phosphorylation and subsequent biological responses.

Mode of receptor binding and activation by plasminogen-related growth factors

Hepatocyte growth factor/scatter factor (HGF/SF) and macrophage stimulating protein (MSP) are plasminogen-related kringle proteins that lost serine protease domain enzymatic activity and became ligands for cell surface tyrosine kinase receptors. They are activated by cleavage to disulfide-linked alphabeta chains. Surprisingly, despite structural similarities, the high affinity receptor binding regions of the two proteins are different: alpha chain for HGF, and beta chain for MSP. We propose that after cleavage exposes a beta chain binding site (high affinity for MSP, low affinity for HGF), monomeric ligand induces receptor dimerization and activation via alpha and beta chain binding sites of different affinity.