Macrophage-stimulating protein inhibits induction of nitric oxide production by endotoxin- or cytokine-stimulated mouse macrophages

A detailed analysis of F-MuLV- and SFFV-infected cells in Friend virus-infected mice reveals the contribution of both F-MuLV- and SFFV-infected cells to the interleukin-10 host response

BACKGROUND

Friend virus (FV) is a complex of the Friend murine leukemia virus (F-MuLV) and the replication-defective, pathogenic spleen focus forming virus (SFFV). In the past, we used a fluorescently labeled F-MuLV to analyze FV target cells. To build on these findings, we have now created a double-labeled FV that contains a Katushka-labeled F-MuLV and an mTagBFP-labeled SFFV, which we have used to study the infection by the two individual viruses in the FV infection of highly susceptible BALB/c mice.

RESULTS

Our data show that the target cells of SFFV largely mirror those of F-MuLV, with the highest virus loads in erythroblasts, B cells and myeloid cells. The early phase of infection was dominated by cells infected by either SFFV or F-MuLV, whereas double-infected cells became dominant later in the course of infection with increasing viral loads. In the late phase of infection, the frequency of double-infected cells was similarly high as the frequencies of SFFV or F-MuLV single-infected cells, and single- and double-infected cells outnumbered the uninfected cells in the most highly infected cell populations such as erythroblasts. FV and retroviruses in general have been shown to induce interleukin 10 (IL-10) as a means of suppressing immune responses. Interestingly, we found in infected IL-10-eGFP reporter mice that SFFV-infected cells contributed to the IL-10-producing cell pool much more significantly than F-MuLV-infected cells, suggesting that the truncated SFFV envelope protein gp55 might play a role in IL-10 induction. Even though BALB/c mice mount notoriously weak immune responses against FV, infection of mice with an ablation of IL-10 expression in T cells showed transiently lower viral loads and stronger T cell activation, suggesting that IL-10 induction by FV and by SFFV in particular may contribute to a suppressed immune response in BALB/c mice.

CONCLUSION

Our data provide detailed information about both F-MuLV- and SFFV-infected cells during the course of FV infection in highly susceptible mice and imply that the pathogenic SFFV contributes to immune suppression.

Macrophage-mediated RON signaling supports breast cancer growth and progression through modulation of IL-35

Tumor associated macrophages (TAMs) play a major role in regulating mammary tumor growth and in directing the responses of tumor infiltrating leukocytes in the microenvironment. However, macrophage-specific mechanisms regulating the interactions of macrophages with tumor cells and other leukocytes that support tumor progression have not been extensively studied. In this study, we show that the activation of the RON receptor tyrosine kinase signaling pathway specifically in macrophages supports breast cancer growth and metastasis. Using clinically relevant murine models of breast cancer, we demonstrate that loss of macrophage RON expression results in decreases in mammary tumor cell proliferation, survival, cancer stem cell self-renewal, and metastasis. Macrophage RON signaling modulates these phenotypes via direct effects on the tumor proper and indirectly by regulating leukocyte recruitment including macrophages, T-cells, and B-cells in the mammary tumor microenvironment. We further show that macrophage RON expression regulates the macrophage secretome including IL-35 and other immunosuppressive factors. Overall, our studies implicate activation of RON signaling in macrophages as a key player in supporting a thriving mammary pro-tumor microenvironment through novel mechanisms including the augmentation of tumor cell properties through IL-35.

Molecular Mechanisms Modulating the Phenotype of Macrophages and Microglia

Macrophages and microglia play crucial roles during central nervous system development, homeostasis and acute events such as infection or injury. The diverse functions of tissue macrophages and microglia are mirrored by equally diverse phenotypes. A model of inflammatory/M1 versus a resolution phase/M2 macrophages has been widely used. However, the complexity of macrophage function can only be achieved by the existence of varied, plastic and tridimensional macrophage phenotypes. Understanding how tissue macrophages integrate environmental signals via molecular programs to define pathogen/injury inflammatory responses provides an opportunity to better understand the multilayered nature of macrophages, as well as target and modulate cellular programs to control excessive inflammation. This is particularly important in MS and other neuroinflammatory diseases, where chronic inflammatory macrophage and microglial responses may contribute to pathology. Here, we perform a comprehensive review of our current understanding of how molecular pathways modulate tissue macrophage phenotype, covering both classic pathways and the emerging role of microRNAs, receptor-tyrosine kinases and metabolism in macrophage phenotype. In addition, we discuss pathway parallels in microglia, novel markers helpful in the identification of peripheral macrophages versus microglia and markers linked to their phenotype.

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.

Plasma macrophage-stimulating protein and hepatocyte growth factor levels are associated with prostate cancer progression

Hepatocyte growth factor (HGF) is a well-known multifunctional growth factor, and evidence has accumulated indicating that the HGF/MET (HGF receptor) signaling axis is involved in the progression of cancer. Macrophage-stimulating protein (MSP) is also known as a growth factor which activates not only macrophages but also cancer cells and osteoclasts through the activation of the specific Receptor d'origine nantais (RON). Pro-HGF and pro-MSP lack biological activity and, therefore, require proteolytic activation for conversion to an active two-chain form by HGF activator (HGFA). Although, there are several studies on HGF/MET signaling with castration-resistant prostate cancer (CRPC) and bone metastasis, reports on plasma protein are rare. In addition, the MSP/RON signaling axis in PC is not well understood. Here, we analyzed associations between PC progression and plasma HGF and MSP levels. We tested plasma samples from 58 patients with PC: 36 with castration-resistant (CR) PC and 22 with pretreatment for PC as control. We used enzyme-linked immunosorbent assay (ELISA) kit to determine plasma levels of HGF, MSP and HGFA, and examined correlations with clinicopathological characteristics such as Gleason grade and bone metastasis. PCR was used to evaluate HGF and MSP-related molecules in PC cell lines. Plasma levels of HGF, MSP and HGFA in the CRPC group were higher than in the control group (HGF: P < 0.001; MSP: P = 0.008; HGFA: P < 0.001). HGF and MSP levels were significantly correlated (P = 0.003). In the CRPC group, plasma HGF and MSP levels and Gleason score were not correlated; however, high plasma MSP level correlated with bone metastasis. (P = 0.016). In cell lines, PC3 expressed significantly more HGF, MET and RON than did LNCaP (P < 0.001), and both cell lines expressed MSP. Plasma concentrations of HGF, MSP and HGFA are significantly elevated in patients with CRPC. Also, as plasma MSP levels are significantly associated with bone metastasis in CRPC patients, MSP may be a candidate for serum marker of bone metastasis. Our results show the importance of the HGF/MET and MSP/RON signaling systems in CRPC.

Deficiency in macrophage-stimulating protein results in spontaneous intestinal inflammation and increased susceptibility toward epithelial damage in zebrafish

Several genome-wide association studies have identified the genes encoding for macrophage-stimulating protein (MSP) and its receptor RON (Recepteur d'Origine Nantais) as possible susceptibility factors in inflammatory bowel disease. While it has been shown that the MSP-RON signaling pathway is involved in tissue injury responses, current mouse models for MSP and RON deficiency have not clearly demonstrated a role of MSP-RON signaling in the context of intestinal inflammation. In this study, we report that the recently identified zebrafish Msp mutant (msp(t34230)) develops spontaneous intestinal inflammation over time. From 14 to 28 weeks postfertilization Msp-deficient zebrafish show intestinal eosinophilia, increased intestinal expression of inflammatory marker mmp9, and activation of intestinal goblet cells. Moreover, these Msp mutant zebrafish are more susceptible toward ethanol-induced epithelial damage, which resulted in increased infiltration and proliferation of immune cells within the lamina propria and prolonged intestinal proinflammatory cytokine responses in some mutant fish. In light of the recent development of many tools to visualize, monitor, and genetically modify zebrafish, these Msp-deficient zebrafish will enable in-depth in vivo analysis of epithelial and macrophage-specific MSP-RON signaling in the context of intestinal inflammation.

Palmitoylethanolamide regulates development of intestinal radiation injury in a mast cell-dependent manner

BACKGROUND

Mast cells and neuroimmune interactions regulate the severity of intestinal radiation mucositis, a dose-limiting toxicity during radiation therapy of abdominal malignancies.

AIM

Because endocannabinoids (eCB) regulate intestinal inflammation, we investigated the effect of the cannabimimetic, palmitoylethanolamide (PEA), in a mast competent (+/+) and mast cell-deficient (Ws/Ws) rat model.

METHODS

Rats underwent localized, fractionated intestinal irradiation, and received daily injections with vehicle or PEA from 1 day before until 2 weeks after radiation. Intestinal injury was assessed noninvasively by luminol bioluminescence, and, at 2 weeks, by histology, morphometry, and immunohistochemical analysis, gene expression analysis, and pathway analysis.

RESULTS

Compared with +/+ rats, Ws/Ws rats sustained more intestinal structural injury (p = 0.01), mucosal damage (p = 0.02), neutrophil infiltration (p = 0.0003), and collagen deposition (p = 0.004). PEA reduced structural radiation injury (p = 0.02), intestinal wall thickness (p = 0.03), collagen deposition (p = 0.03), and intestinal inflammation (p = 0.02) in Ws/Ws rats, but not in +/+ rats. PEA inhibited mast cell-derived cellular immune response and anti-inflammatory IL-6 and IL-10 signaling and activated the prothrombin pathway in +/+ rats. In contrast, while PEA suppressed nonmast cell-derived immune responses, it increased anti-inflammatory IL-10 and IL-6 signaling and decreased activation of the prothrombin pathway in Ws/Ws rats.

CONCLUSIONS

These data demonstrate that the absence of mast cells exacerbate radiation enteropathy by mechanisms that likely involve the coagulation system, anti-inflammatory cytokine signaling, and the innate immune system; and that these mechanisms are regulated by PEA in a mast cell-dependent manner. The eCB system should be explored as target for mitigating intestinal radiation injury.

MSP: an emerging player in metabolic syndrome

MSP (Macrophage Stimulating Protein; also known as Hepatocyte Growth Factor-like protein (HGFL) and MST1) is a secreted protein and the ligand for transmembrane receptor tyrosine kinase Recepteur d'Origine Nantais (RON; also known as MST1R). Since its discovery, MSP has been demonstrated to play a key role in regulating inflammation in the peripheral tissues of multiple disease models. Recent evidences also point toward a beneficial role of MSP in the regulation of hepatic lipid and glucose metabolism, thereby implicating MSP as a crucial regulator in maintaining metabolic homeostasis while simultaneously suppressing inflammatory processes. In this review, we discuss the recent advances that demonstrate the significance of MSP in metabolic syndrome and build a strong case supporting its therapeutic potential.

Intron retention in the alternatively spliced region of RON results from weak 3' splice site recognition

The RON gene encodes a tyrosine kinase receptor for macrophage-stimulating protein. A constitutively active isoform that arises by skipping of exon 11 is expressed in carcinomas and contributes to an invasive phenotype. However, a high proportion of the mRNA expressed from the endogenous gene, or from transfected minigenes, appears to retain introns 10 and 11. It is not known whether this represents specific repression or the presence of weak splicing signals. We have used chimeric pre-mRNAs spliced in vitro to investigate the reason for intron retention. A systematic test showed that, surprisingly, the exon sequences known to modulate exon 11 skipping were not limiting, but the 3’ splice site regions adjacent to exons 11 and 12 were too weak to support splicing when inserted into a globin intron. UV-crosslinking experiments showed binding of hnRNP F/H just 5’ of these regions, but the hnRNP F/H target sequences did not mediate inhibition. Instead, the failure of splicing is linked to weak binding of U2AF65, and spliceosome assembly stalls prior to formation of any of the ATP-dependent complexes. We discuss mechanisms by which U2AF65 binding is facilitated in vivo.

Microsatellite (GT)n polymorphism at 3'UTR of SLC11A1 influences the expression of brucella LPS induced MCP1 mRNA in buffalo peripheral blood mononuclear cells

A (GT)n microsatellite polymorphism at 3'UTR of SLC11A1(solute carrier family 11A1) is associated with the natural resistance to bovine brucellosis. A pleiotropic effect of SLC11A1 on other candidate genes influencing the host resistance including monocyte chemotactic/chemoattractant protein 1 (MCP1) is also hypothesized. In the present study, we report the cloning and characterization of the complete coding sequence of bubaline (bu) MCP1 and its tissue distribution at the transcript level. The buMCP1 exhibited as high as 99% and >80% of sequence identities with the bovine and other domestic animal species homologues. The buMCP1 mRNA was abundant across the different tissues: most abundant in liver and mammary gland, moderate in ovary, skeletal muscle and testis, and least in uterus. Further, quantitative real-time PCR (RTqPCR) analysis revealed that PBMCs carrying so called resistant GT13 allele produced more MCP1 mRNA endogenously as well as when induced with brucella LPS suggesting the pleiotropic roles of SLC11A1 in conferring resistance against the intracellular pathogens particularly against brucellosis. However, the underlying molecular mechanisms by which 3'UTR SLC11A1 concomitantly increases the production of chemokines like MCP1 are yet to be investigated.

Ron receptor tyrosine kinase signaling as a therapeutic target

INTRODUCTION

Since its discovery nearly 20 years ago, the Ron receptor tyrosine kinase has been extensively studied. These studies have elucidated many of the major signaling pathways activated by Ron. In the context of the inflammation and cancer, studies have shown that Ron plays differential roles; Ron activation limits the inflammatory response, whereas in cancer, Ron activation is associated with increased metastases and poor prognosis.

AREAS COVERED

This review discusses the current literature with regard to Ron signaling and consequences of its activation in cancer as well as its role in cancer therapy. Further, we discuss the mechanisms by which Ron influences the inflammatory response and its role in chronic inflammatory diseases. Finally, we discuss Ron's connection between chronic inflammation and progression to cancer.

EXPERT OPINION

The complex nature of Ron's signaling paradigm necessitates additional studies to understand the pathways by which Ron is functioning and how these differ in inflammation and cancer. This will be vital to understanding the impact that Ron signaling has in disease states. Additional studies of targeted therapies, either alone or in conjunction with current therapies are needed to determine if inhibition of Ron signaling will provide long-term benefits to cancer patients.

Proteolytic activation of pro-macrophage-stimulating protein by hepsin

Macrophage-stimulating protein (MSP) is a plasminogen-related growth factor and ligand for the receptor tyrosine kinase RON. The MSP/RON system promotes wound healing and invasive tumor growth and suppresses proinflammatory immune response. MSP binding to RON requires proteolytic conversion of the inactive single-chain form (pro-MSP) into the disulfide-linked α/β heterodimer. The pro-MSP cleavage sequence (Ser-Lys-Leu-Arg(483)↓Val(484)) closely matches the substrate recognition sequences of hepsin, a type II transmembrane serine protease, that is overexpressed in several cancers. Here, we show that recombinant hepsin cleaves pro-MSP at the consensus site Arg(483)-Val(484) with superior efficiency compared with the known activators MT-SP1 and hepatocyte growth factor activator (HGFA). At least 50% of pro-MSP was processed within 1 hour at a hepsin concentration of 2.4 nmol/L and at a molar enzyme to substrate ratio of 1:500. An uncleavable single-chain variant of MSP weakly bound to a RON-Fc fusion protein, whereas hepsin-cleaved MSP bound with a K(D) of 10.3 nmol/L, suggesting that the high-affinity binding site in MSP β-chain was properly formed. LNCaP prostate cancer cells overexpressing hepsin on the cell surface efficiently activated pro-MSP, which was blocked by a specific anti-hepsin antibody. Incubation of pro-MSP with hepsin led to robust RON-mediated phosphorylation of mitogen-activated protein kinase, ribosomal S6 protein, and Akt in human A2780 ovarian carcinoma cells stably expressing RON protein. In macrophages, pro-MSP with hepsin induced chemotaxis and attenuated lipopolysaccharide-dependent production of nitric oxide. These findings suggest that the MSP/RON signaling pathway may be regulated by hepsin in tissue homeostasis and in disease pathologies, such as in cancer and immune disorders.

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.

Monocyte chemoattractant protein-1 (MCP-1), not MCP-3, is the primary chemokine required for monocyte recruitment in mouse peritonitis induced with thioglycollate or zymosan A

MCP-1/CCL2 plays a critical role in monocyte recruitment into sites of immune responses and cancer. However, the role of other MCPs remains unclear. In this study, we generated a novel MCP-1-deficient (designated as MCP-1(Delta/Delta)) mouse model by deleting a 2.3-kb DNA fragment from the mouse genome using the Cre/loxP system. MCP-1 was not produced by LPS-activated MCP-1(Delta/Delta) macrophages; however, the production of MCP-3, coded by the immediate downstream gene, was significantly increased. In contrast, macrophages from another mouse line with a neo-gene cassette in intron 2 produced a significantly lower level of MCP-1 and MCP-3. Decreased MCP-3 production was also detected in previously generated MCP-1-deficient mice in which a neo-gene cassette was inserted in exon 2 (designated as MCP-1 knockout (KO)). Altered MCP-1 and/or MCP-3 production was also observed in vivo in each mouse model in response to i.p. injection of thioglycolate or zymosan. The up- and down-regulation of MCP-3 production in MCP-1(Delta/Delta) and MCP-1 KO mice, respectively, provided us with a unique opportunity to evaluate the role for MCP-3. Despite the increased MCP-3 production in MCP-1(Delta/Delta) mice, thioglycolate- or zymosan-induced monocyte/macrophage accumulation was still reduced by approximately 50% compared with wild-type mice, similar to the reduction detected in MCP-1 KO mice. Thus, up-regulated MCP-3 production did not compensate for the loss of MCP-1, and MCP-3 appears to be a less effective mediator of monocyte recruitment than MCP-1. Our results also indicate the presence of other mediators regulating the recruitment of monocytes in these models.

Bach1 deficiency ameliorates hepatic injury in a mouse model

Bach1 is a basic region-leucine zipper (bZip) protein that forms heterodimers with the small Maf proteins and functions as a repressor of gene expression. One of the target genes of Bach1 is Hmox-1 that encodes heme oxygenase-1 (HO-1). HO-1 degrades heme into carbon monoxide (CO), biliverdin, and iron. HO-1 is strongly induced by various stresses as well as its substrate heme, and protects cells and tissues against insults through diverse cytoprotective functions of the reaction products CO and biliverdin. Bach1-deficiency in mice leads to higher expression of Hmox-1 in various tissues. Here we investigated the effects of Bach1-deficiency in mice on tissue injuries: hepatic injury induced by D-galactosamine (GalN) and lipopolysaccharide (LPS), and mouse paw edema induced by carrageenin, polysaccharide derived from various seaweeds. Bach1-deficiency suppressed induction of plasma alanine aminotransferase (ALT) and aspartate aminotransferase (AST) activities in response to the GalN/LPS-treatment. However, production of tumor necrosis factor alpha (TNF-alpha) and nitric oxide (NO), both being cytotoxic mediators in LPS-induced hepatic injury, in Bach1-deficient mice and their peritoneal macrophages was similar to wild type controls. In contrast, Bach1-deficiency did not affect extent of mouse paw edema induced by carrageenin, which enhances vascular permeability by activating kinin release. These results indicate that Bach1 plays an inhibitory role in the cytoprotection of LPS-induced liver injury but not in the kinin-mediated inflammatory edema. The inhibitory role for Bach1 may stem from its activity to repress gene expression including HO-1.

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.

HIV-1 Tat mediates degradation of RON receptor tyrosine kinase, a regulator of inflammation

HIV encodes several proteins, including Tat, that have been demonstrated to modulate the expression of receptors critical for innate immunity, including MHC class I, mannose receptor, and beta(2)-microglobulin. We demonstrate that Tat targets the receptor tyrosine kinase recepteur d'origine nantais (RON), which negatively regulates inflammation and HIV transcription, for proteosome degradation. Tat decreases cell surface RON expression in HIV-infected monocytic cells, and Tat-mediated degradation of RON protein is blocked by inhibitors of proteosome activity. Tat specifically induced down-regulation of RON and not other cell surface receptors, such as the transferrin receptor, the receptor tyrosine kinase TrkA, or monocytic markers CD14 and ICAM-1. The Tat trans activation domain is required for RON degradation, and this down-regulation is dependent on the integrity of the kinase domain of RON receptor. We propose that Tat mediates degradation of RON through a ubiquitin-proteosome pathway, and suggest that by targeting signals that modulate inflammation, Tat creates a microenvironment that is optimal for HIV replication and progression of AIDS-associated diseases.

Met-related receptor tyrosine kinase Ron in tumor growth and metastasis

The Ron receptor is a member of the Met family of cell surface receptor tyrosine kinases and is primarily expressed on epithelial cells and macrophages. The biological response of Ron is mediated by binding of its ligand, hepatocyte growth factor-like protein/macrophage stimulating-protein (HGFL). HGFL is primarily synthesized and secreted from hepatocytes as an inactive precursor and is activated at the cell surface. Binding of HGFL to Ron activates Ron and leads to the induction of a variety of intracellular signaling cascades that leads to cellular growth, motility and invasion. Recent studies have documented Ron overexpression in a variety of human cancers including breast, colon, liver, pancreas, and bladder. Moreover, clinical studies have also shown that Ron overexpression is associated with both worse patient outcomes as well as metastasis. Forced overexpression of Ron in transgenic mice leads to tumorigenesis in both the lung and the mammary gland and is associated with metastatic dissemination. While Ron overexpression appears to be a hallmark of many human cancers, the mechanisms by which Ron induces tumorigenesis and metastasis are still unclear. Several strategies are currently being undertaken to inhibit Ron as a potential therapeutic target; current strategies include the use of Ron blocking proteins, small interfering RNA (siRNA), monoclonal antibodies, and small molecule inhibitors. In total, these data suggest that Ron is a critical factor in tumorigenesis and that inhibition of this protein, alone or in combination with current therapies, may prove beneficial in the treatment of cancer patients.

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.

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.

Short-form Ron receptor is required for normal IFN-gamma production in concanavalin A-induced acute liver injury

Abrogation of Ron receptor tyrosine kinase function results in defects in macrophage activation and dysregulated acute inflammatory responses in vivo. Several naturally occurring constitutively active alternative forms of Ron have been identified, including from primary human tumors and tumor cell lines. One of these alternative forms, short-form (SF) Ron, is generated from an alternative start site in intron 10 of the Ron gene that eliminates most of the extracellular portion of the receptor and is overexpressed in several human cancers. To test the physiological significance of SF-Ron in vivo, mice were generated that solely express the full-length form of Ron (FL-Ron). Our results show that elimination of the capacity to express SF-Ron in vivo leads to augmented production of IFN-gamma from splenocytes following stimulation ex vivo with either concanavalin A or anti-CD3/T cell receptor monoclonal antibody. Moreover, in a concanavalin A-induced murine model of acute liver injury, FL-Ron mice have increased production of serum INF-gamma and serum alanine aminotransferase levels and worsened liver histology and overall survival compared with wild-type control mice. Taken together, these results suggest for the first time that SF-Ron impacts the progression of inflammatory immune responses in vivo and further support a role for the Ron receptor and its various forms in liver pathophysiology.

Production of reactive oxygen species and expression of inducible nitric oxide synthase in rat isolated Kupffer cells stimulated by Leptospira interrogans and Borrelia burgdorferi

AIM: To evaluate the production of reactive oxygen species (ROS) and the expression of inducible nitric oxide synthase (iNOS) in rat isolated Kupffer cells (KCs) stimulated by Leptospira interrogans and Borrelia burgdorferi.

METHODS: Rat Kupffer cells were separated by perfusion of the liver with 0.05% collagenase, and purified by Percoll gradients. Purified Kupffer cells were tested in vitro with alive L.interogans and B. burgdorferi preparations. The production of ROS was determined by chemiluminescence, whereas iNOS protein expression was evaluated by Western blot assay using anti-iNOS antibodies.

RESULTS: B. burgdorferi and to a less extent L. interrogans induced ROS production with a peak 35 min after infection. The chemiluminescence signal progressively diminished and was undetectable by 180 min of incubation. Leptospirae and borreliae induced an increased iNOS expression in Kupffer cells that peaked at 6 hours and was still evident 22 h after infection.

CONCLUSION: Both genera of spirochetes induced ROS and iNOS production in rat Kupffer cells. Since the cause of liver damage both in leptospiral as well as in borrelial infections are still unknown, we suggest that leptospira and borrelia damage of the liver can be initially mediated by oxygen radicals, and is then maintained at least in part by nitric oxide.

RON-regulated innate immunity is protective in an animal model of multiple sclerosis

The tyrosine kinase receptor RON and its ligand, macrophage stimulating protein (MSP), exert inhibitory effects on systemic innate immunity, but their CNS expression and impact on human neuroinflammatory diseases are unknown were RON and MSP present in human brain perivascular macrophages and microglia, but RON mRNA and protein abundance in the CNS were diminished in both MS patients and the MS animal model, experimental autoimmune encephalomyelitis (EAE). Treatment of differentiated human monocytoid cells with MSP resulted in significant reduction of interleukin (IL)-1beta, tumor necrosis factor (TNF)-alpha and MMP-9 mRNA levels, whereas minimal effects were observed in human astrocytes. After induction of EAE, RON knockout and heterozygote animals exhibited significantly increased CNS proinflammatory gene (TNF-alpha, MMP-12) expression compared with wild-type littermate controls, although IL-4 levels were suppressed in both RON-deficient groups. Neurological disease in RON-deficient animals showed a more rapid onset with overall worsened severity, together with exacerbated demyelination, axonal injury, and neuroinflammation after EAE induction. The proto-oncogene, c-Cbl, which modulates ubiquitylation of RON, was increased in glia in both MS brains and EAE spinal cords. Thus, the MSP-RON pathway represents a novel regulatory mechanism within the CNS by which innate immunity and its pathogenic effects could be targeted for future therapeutic interventions.

RON Receptor Tyrosine Kinase, a Negative Regulator of Inflammation, Inhibits HIV-1 Transcription in Monocytes/Macrophages and Is Decreased in Brain Tissue from Patients with AIDS

Activation of macrophages and microglia cells after HIV-1 infection and their production of inflammatory mediators contribute to HIV-associated CNS diseases. The mechanisms that initiate and maintain inflammation after HIV-1 infection in the brain have not been well studied. Furthermore, it is not understood why in HIV-associated CNS disease, macrophages and microglia are biased toward inflammation rather than production of mediators that control inflammation. We have focused on the receptor tyrosine kinase RON, a critical negative regulator of macrophage function and inflammation, to determine whether this receptor regulates HIV-1 expression. Overexpressing RON in monocytes/macrophages demonstrates that RON inhibits HIV-1 proviral transcription in part by decreasing the binding activity of NF-kappaB to the HIV-1 long terminal repeat. Because macrophages and microglia cells are a critical reservoir for HIV-1 in the CNS, we examined brain tissues for RON expression and detected RON in astrocytes, cortical neurons, and monocytoid cells. RON was detected in all control patients who were HIV seronegative (n = 7), whereas six of nine brain samples obtained from AIDS patients exhibited reduced RON protein. These data suggest that RON initiates signaling pathways that negatively regulate HIV-1 transcription in monocytes/macrophages and that HIV-1 suppresses RON function by decreasing protein levels in the brain to assure efficient replication. Furthermore, HIV-1 infection would compromise the ability of RON to protect against inflammation and consequent CNS damage.

Macrophage-stimulating protein, the ligand for the stem cell-derived tyrosine kinase/RON receptor tyrosine kinase, inhibits IL-12 production by primary peritoneal macrophages stimulated with IFN-gamma and lipopolysaccharide

IL-12, produced by APCs during the initial stages of an immune response, plays a pivotal role in the induction of IFN-gamma by NK and gammadeltaT cells and in driving the differentiation of Th1 cells, thus providing a critical link between innate and acquired immunity. Due to the unique position occupied by IL-12 in the regulation of immunity, many mechanisms have evolved to modulate IL-12 production. We have shown previously that macrophage-stimulating protein (MSP), the ligand for the stem cell-derived tyrosine kinase/recepteur d'origine nantais (RON) receptor, inhibits NO production by macrophages in response to IFN-gamma and enhances the expression of arginase. Mice lacking RON exhibit increased inflammation in a delayed-type hypersensitivity reaction and increased susceptibility to endotoxic shock. In this study we demonstrate that pretreatment of macrophages with MSP before IFN-gamma and LPS results in the complete inhibition of IL-12 production due to suppression of p40 expression. This response is mediated by the RON receptor, and splenocytes from RON(-/-) animals produce increased levels of IFN-gamma. MSP pretreatment of macrophages resulted in decreased tyrosine phosphorylation of Stat-1 and decreased expression of IFN consensus sequence binding protein in response to inflammatory cytokines. In addition to IL-12, the expression of IL-15 and IL-18, cytokines that are also dependent on IFN consensus sequence binding protein activation, is inhibited by pretreatment with MSP before IFN-gamma and LPS. We also show that the ability of MSP to inhibit IL-12 production is independent of IL-10. Taken together, these results suggest that MSP may actively suppress cell-mediated immune responses through its ability to down-regulate IL-12 production and thus inhibit classical activation of macrophages.

Cross-talk between the receptor tyrosine kinases Ron and epidermal growth factor receptor

Heterogeneous receptor-receptor interactions may play a role in intracellular signaling. Accordingly, the interaction of two dissimilar tyrosine kinase receptors, Ron and epidermal growth factor receptor (EGFR) was investigated. The functional interaction of Ron and EGFR in cell scatter and oncogenic transformation was investigated in vivo. Transfection of a dominant negative form of EGFR into human embryonic kidney cells stably expressing Ron (293-Ron) dramatically reduced the scatter response induced by the Ron ligand hepatocyte growth factor-like protein/macrophage stimulating protein (HGFL). The scatter response of the 293-Ron cells was also attenuated by treatment of the cells with the specific EGFR inhibitor AG 1478. Co-transfection of Ron and dominant-negative EGFR, or co-transfection of EGFR and a dominant-negative form of Ron reduced focus formation in NIH/3T3 cells. Western analysis of NIH/3T3 cells overexpressing murine Ron and expressing endogenous levels of EGFR was used to demonstrate that Ron and EGFR co-immunoprecipitate. Stimulation of the cells in vitro with the Ron ligand HGFL or with the EGFR ligand epidermal growth factor (EGF) appeared to induce phosphorylation of both receptors. Co-immunoprecipitation and phosphorylation of phosphatidyl inositol 3-kinase (PI3-K) was also observed. This novel finding of a functional and biochemical interaction between Ron and EGFR suggests that heterologous tyrosine kinase receptor interactions may play a role in cellular processes such as scatter and transformation.

The receptor tyrosine kinase Ron is expressed in the mouse ovary and regulates inducible nitric oxide synthase levels and ovulation

OBJECTIVE

To determine the reproductive effects in mice of the deletion of the tyrosine kinase domain of the Ron receptor.

DESIGN

Controlled animal studies.

SETTING

Academic research environment.

ANIMAL(S)

Immature mice with deletion of the tyrosine kinase domain of the Ron receptor (TK-/-) at 22-30 days of age and adult black Swiss female mice at 5-6 weeks of age.

INTERVENTION(S)

Hormonal stimulation of immature female TK-/- animals to induce ovulation.

MAIN OUTCOME MEASURE(S)

Ovulation rates measured by counting the total number of cumulus oocyte complexes in the ampullar region of the murine oviduct after hormonal stimulation. Western blot analysis measured murine ovarian protein levels of endothelial and inducible nitric oxide synthase (iNOS). Immunohistochemical analysis localized iNOS in the developing murine ovarian follicle.

RESULT

Immature TK-/- mice (22-30 days) ovulate significantly fewer cumulus oocyte complexes. Western blot analyses demonstrated increased levels of iNOS before and after ovulation compared with controls. Conversely, endothelial nitric oxide synthase levels were similar and remained constant during corresponding time periods. Immunohistochemical analyses demonstrated a significant increase in iNOS staining throughout the ovary in TK-/- mice with a significant amount of iNOS in granulosa cells surrounding the oocyte when compared with controls.

CONCLUSION(S)

The increased level of nitric oxide in the TK-/- mice is likely due to an elevated level of iNOS, which may contribute to a decrease in the size of the ovaries and ovulation rates of immature TK-/- animals.

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.

Activation of CR3-mediated phagocytosis by MSP requires the RON receptor, tyrosine kinase activity, phosphatidylinositol 3-kinase, and protein kinase C zeta

Macrophage-stimulating protein (MSP) promotes the phagocytosis of C3bi-coated erythrocytes by resident peritoneal macrophages, although the mechanism by which this occurs is largely unknown. We show that MSP-induced complement-mediated phagocytosis requires the RON receptor tyrosine kinase and the alphaMbeta2 integrin, as evidenced by the inability of RON-/- and alphaM-/- peritoneal macrophages to augment phagocytosis of complement-coated sheep erythrocytes in response to MSP. MSP stimulation of macrophages results in tyrosine phosphorylation and AKT activation, and inhibitor studies demonstrate a phagocytic requirement for tyrosine kinase and phosphatidylinositol 3-kinase (PI-3K) activity as well as activity of the atypical protein kinase C (PKC) isoform zeta, which localizes to MSP-induced phagosomes containing complement-coated beads. Additionally, MSP augments the ability of peritoneal macrophages to bind to intercellular adhesion molecule-1 (ICAM-1) via the alphaMbeta2 integrin. MSP-induced ICAM-1 adhesion is also dependent on tyrosine kinase activity, PI-3K, and PKC zeta, indicating that these signaling requirements are upstream of complement receptor 3 activation.

Receptor tyrosine kinase Ron is expressed in mouse reproductive tissues during embryo implantation and is important in trophoblast cell function

Ron is a receptor tyrosine kinase that is activated by the binding of hepatocyte growth factor-like (HGFL) protein. Mutations in the catalytic domain of this receptor result in an aggressively invasive phenotype. Conversely, deletion of the entire receptor results in an embryonic lethality by Embryonic Day 7.5. The specific cellular localization and mechanisms of action of Ron and HGFL during embryo implantation are not known. Therefore, this report characterizes the temporal and spatial distribution of this receptor during mouse embryo implantation and placentation. Reverse transcription-polymerase chain reaction analysis demonstrated the presence of Ron transcripts in the uterus, placenta, testis, and epididymis, whereas HGFL transcripts were found in the cervix, placenta, epididymis, and testis. In situ hybridization and immunohistochemical analyses demonstrated that Ron was present in the cells of the ectoplacental cone and trophoblast giant cell regions surrounding the implanting embryo. Ron expression was also observed in SM9-1, SM9-2, and SM-10 murine trophoblast cell lines. To determine the effects of Ron activation on trophoblast function, Matrigel invasion and cell survival assays were performed using the SM9-1 and SM-10 trophoblast cell lines. The HGFL stimulation of these cells increased invasion and enhanced cell survival. These observations suggest that activation of the Ron receptor by HGFL binding may aid in implantation by way of trophoblast function and viability.

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.

Deletion of the Ron receptor tyrosine kinase domain in mice provides protection from endotoxin-induced acute liver failure

The targeted deletion of the cytoplasmic domain of the Ron receptor tyrosine kinase (TK) in mice leads to exaggerated responses to injury in several murine models of inflammation as well as increased lethality in response to endotoxin (lipopolysaccharide [LPS]). Using a well-characterized model of LPS-induced acute liver failure (ALF) in galactosamine (GalN)-sensitized mice, we show that Ron TK(-/-) mice display marked protection compared with control Ron TK(+/+) mice. Whereas control mice have profound elevation of serum aminotransferase levels (a marker of hepatocyte injury) and hemorrhagic necrosis of the liver, in dramatic contrast, Ron TK(-/-) mice have mild elevation of aminotransferase levels and relatively normal liver histology. These findings are associated with a reduction in the number of liver cells undergoing apoptosis in Ron TK(-/-) mice. Paradoxically, treatment of Ron TK(-/-) mice with LPS/GalN leads to markedly elevated (3.5-fold) serum levels of tumor necrosis factor (TNF) alpha, a key inflammatory mediator in this liver injury model, as well as reduced amounts of interleukin (IL) 10 (a suppressor of TNF-alpha production) and interferon (IFN)-gamma (a TNF-alpha sensitizer). These results show that ablation of the TK activity of the Ron receptor leads to protection from the development of hepatocellular apoptosis in response to treatment with LPS/GalN, even in the presence of excessive levels of serum TNF-alpha. In conclusion, our studies show that the Ron receptor TK plays a critical role in modulating the response of the liver to endotoxin.

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.

Macrophage-stimulating protein is produced by tubular cells and activates mesangial cells

Until now, hepatocytes have been the only known cell source of macrophage-stimulating protein (MSP), and tissue macrophages have been the cells on which the biologic effects of MSP have been proved. To extend the understanding of the biologic meaning of MSP, it was investigated whether MSP operates in the kidney. MSP protein was evaluated by Western blot in supernatant of cultured human tubular cells (HK2) and human mesangial cells (HMC). MSP mRNA was investigated in HK2 by reverse transcription-polymerase chain reaction (RT-PCR). The expression of the MSP receptor, RON, was evaluated in HMC and HK2 by Western blot. RON mRNA was investigated in HMC by RT-PCR. The expression of MSP and RON in normal human renal tissue was studied by immunohistochemistry. HMC were stimulated with recombinant MSP (rMSP) and HK2 supernatant to study cell growth, migration, and the capacity to invade an artificial collagen matrix and synthesize interleukin-6 (IL-6). HK2 produced MSP and expressed RON in a form that was phosphorylated by rMSP. HMC expressed RON but did not produce MSP. MSP in HK2 supernatant and rMSP induced in HMC phosphorylation of RON, growth, migration, invasion, and IL-6 synthesis. In normal human kidney, tubules expressed MSP and RON. These results indicate a novel field of operation for MSP and suggest a pathogenic role of the MSP/RON system in renal disease. In fact, MSP released by tubular cells may recruit monocytes/macrophages in inflammatory tubulointerstitial disorders. In addition, MSP either circulating or as paracrine product may sustain glomerular mesangioproliferative disease.

Activation of the stem cell-derived tyrosine kinase/RON receptor tyrosine kinase by macrophage-stimulating protein results in the induction of arginase activity in murine peritoneal macrophages

Regulation of macrophage activities in response to inflammatory stimuli must be finely tuned to promote an effective immune response while, at the same time, preventing damage to the host. Our lab and others have previously shown that macrophage-stimulating protein (MSP), through activation of its receptor RON, negatively regulates NO production in response to IFN-gamma and LPS by inhibiting the expression of inducible NO synthase (iNOS). Furthermore, activated macrophages from mice harboring targeted mutations in RON produce increased levels of NO both in vitro and in vivo, rendering them more susceptible to LPS-induced endotoxic shock. In this study, we demonstrate that stimulation of murine peritoneal macrophages with MSP results in the RON-dependent up-regulation of arginase, an enzyme associated with alternative activation that competes with iNOS for the substrate L-arginine, the products of which are involved in cell proliferation and matrix synthesis. Expression of other genes associated with alternative activation, including scavenger receptor A and IL-1R antagonist, is also up-regulated in MSP-stimulated murine macrophages. Stimulation of cells with IFN-gamma and LPS blocks the ability of MSP to induce arginase activity. However, pretreatment of cells with MSP results in the up-regulation of arginase and inhibits their ability to produce NO in response to IFN-gamma and LPS, even in the presence of excess substrate, suggesting that the inhibition of NO by MSP occurs primarily through its ability to regulate iNOS expression.

STK receptor tyrosine kinase regulates susceptibility to infection with Listeria monocytogenes

We have previously identified the STK receptor tyrosine kinase as a key regulator of macrophage activation and cell-mediated immune responses. Here we demonstrate that, although MSP activation of STK inhibits NO production by macrophages in response to heat-killed Listeria monocytogenes, STK-deficient mice exhibit increased susceptibility to infection with Listeria.

The role of the receptor tyrosine kinase Ron in nickel-induced acute lung injury

Acute lung injury (ALI), a severe respiratory syndrome, develops in response to numerous insults and responds poorly to therapeutic intervention. Recently, cDNA microarray analyses were performed that indicated several pathogenic responses during nickel-induced ALI, including marked macrophage activation. Macrophage activation is mediated, in part, via the receptor tyrosine kinase Ron. To address the role of Ron in ALI, the response of mice deficient in the cytoplasmic domain of Ron (Ron tk-/-) were assessed in response to nickel exposure. Ron tk-/- mice succumb to nickel-induced ALI earlier, express larger, early increases in interleukin-6, monocyte chemoattractant protein-1, and macrophage inflammatory protein-2, display greater serum nitrite levels, and exhibit earlier onset of pulmonary pathology and augmented pulmonary tyrosine nitrosylation. Increases in cytokine expression and cellular nitration can lead to tissue damage and are consistent with the differences between genotypes in the early onset of pathology and mortality in Ron tk-/- mice. These analyses indicate a role for the tyrosine kinase receptor Ron in ALI.

Macrophage stimulating protein (MSP) evokes superoxide anion production by human macrophages of different origin

1. Macrophage Stimulating Protein (MSP), a serum factor related to Hepatocyte Growth Factor, was originally discovered to stimulate chemotaxis of murine resident peritoneal macrophages. MSP is the ligand for Ron, a member of the Met subfamily of tyrosine kinase receptors. The effects of MSP on human macrophages and the role played in human pathophysiology have long been elusive. 2. We show here that human recombinant MSP (hrMSP) evokes a dose-dependent superoxide anion production in human alveolar and peritoneal macrophages as well as in monocyte-derived macrophages, but not in circulating human monocytes. Consistently, the mature Ron protein is expressed by the MSP responsive cells but not by the unresponsive monocytes. The respiratory burst evoked by hrMSP is quantitatively higher than the one induced by N-formylmethionyl-leucyl-phenylalanine and similar to phorbol myristate acetate-evoked one. 3. To investigate the mechanisms involved in NADPH oxidase activation, leading to superoxide anion production, different signal transduction inhibitors were used. By using the non selective tyrosine kinase inhibitor genistein, the selective c-Src inhibitor PP1, the tyrosine phosphatase inhibitor sodium orthovanadate, the phosphatidylinositol 3-kinase inhibitor wortmannin, the p38 inhibitor SB203580, the MEK inhibitor PD098059, we demonstrate that hrMSP-evoked superoxide production is mediated by tyrosine kinase activity, requires the activation of Src but not of PI 3-kinase. We also show that MAP kinase and p38 signalling pathways are involved. 4. These results clearly indicate that hrMSP induces the respiratory burst in human macrophages but not in monocytes, suggesting for the MSP/Ron complex a role of activator as well as of possible marker for human mature macrophages.

Point mutations and overexpression of Ron induce transformation, tumor formation, and metastasis

The receptor tyrosine kinase Ron is a member of the receptor family that includes the proto-oncogene Met and the avian oncogene Sea. The interaction of Ron with its ligand, known as hepatocyte growth factor-like protein (HGFL) or macrophage stimulating protein (MSP), induces crucial cellular responses including invasive growth, proliferation, cell scattering, and branching morphogenesis. Based on the homology and functional similarities between Met and Ron it was hypothesized that Ron may be important in tumor formation and metastasis. To test this hypothesis, wild-type mouse Ron and three mutant forms of Ron containing mutations similar to those found in the Met gene in human hereditary papillary renal carcinoma (HPRC), were expressed in NIH3T3 cells. A transformed phenotype was produced in cell lines expressing either wild-type Ron or the mutated Ron proteins. Further, these cell lines displayed oncogenic potential by exhibiting increased proliferation and constitutive phosphorylation of Ron. These cell lines were also tested for the ability to form solid tumors. Cells expressing wild-type Ron and the three proteins with single amino acid substitutions were highly tumorigenic in vivo. In a model of experimental metastasis, two of the cell lines with altered Ron protein formed highly aggressive tumors in the lungs. These results suggest that Ron may be an aggressive oncogene when either overexpressed or when activated by mutation.

Macrophage stimulating protein is a novel neurotrophic factor

Macrophage stimulating protein (MSP), also known as hepatocyte growth factor-like, is a soluble cytokine that belongs to the family of the plasminogen-related growth factors (PRGFs). PRGFs are alpha/beta heterodimers that bind to transmembrane tyrosine kinase receptors. MSP was originally isolated as a chemotactic factor for peritoneal macrophages. Through binding to its receptor, encoded by the RON gene, it stimulates dissociation of epithelia and works as an inflammatory mediator by repressing the production of nitric oxide (NO). Here, we identify a novel role for MSP in the central nervous system. As a paradigm to analyze this function we chose the hypoglossal system of adult mice. We demonstrate in vivo that either administration of exogenous MSP or transplantation of MSP-producing cells at the proximal stump of the resected nerve is sufficient to prevent motoneuron atrophy upon axotomy. We also show that the MSP gene is expressed in the tongue, the target of the hypoglossal nerve, and that MSP induces biosynthesis of Ron receptor in the motoneuron somata. Finally, we show that MSP suppresses NO production in the injured hypoglossal nuclei. Together, these data suggest that MSP is a novel neurotrophic factor for cranial motoneurons and, by regulating the production of NO, may have a role in brain plasticity and regeneration.

Ron-mediated cytoplasmic signaling is dispensable for viability but is required to limit inflammatory responses

Ron receptor activation induces numerous cellular responses in vitro, including proliferation, dissociation, and migration. Ron is thought to be involved in blood cell development in vivo, as well as in many aspects of the immune response including macrophage activation, antigen presentation, and nitric oxide regulation. In previous studies to determine the function of Ron in vivo, mice were generated with a targeted deletion of the extracellular and transmembrane regions of this gene. Mice homologous for this deletion appear to die early during embryonic development. To ascertain the in vivo function of Ron in more detail, we have generated mice with a germline ablation of the tyrosine kinase domain. Strikingly, our studies indicate that this domain of Ron, and therefore Ron cytoplasmic signaling, is not essential for embryonic development. While mice deficient in this domain are overtly normal, mice lacking Ron signaling have an altered ability to regulate nitric oxide levels and, in addition, have enhanced tissue damage following acute and cell-mediated inflammatory responses.

alpha 1-Antichymotrypsin is the human plasma inhibitor of macrophage ectoenzymes that cleave pro-macrophage stimulating protein

Macrophage stimulating protein (MSP) is secreted as 78-kDa single chain pro-MSP, which is converted to biologically active, disulfide-linked alphabeta chain MSP by cleavage at Arg(483)-Val(484). Murine resident peritoneal macrophages have two cell surface proteolytic activities that cleave pro-MSP. One is a pro-MSP convertase, which cleaves pro-MSP to active MSP; the other degrades pro-MSP. The degrading protease is inhibited by soybean trypsin inhibitor or by low concentrations of blood plasma, which allows the convertase to cleave pro-MSP to MSP. Using pro-MSP cleavage as the assay, we purified the inhibitor from human plasma. The bulk of the plasma protein was removed by salting out and by isoelectric precipitation of albumin. Highly purified inhibitor was then obtained in three steps: dye-ligand binding and elution, ion exchange chromatography, and high performance liquid chromatography gel filtration. After SDS-polyacrylamide gel electrophoresis and transfer to a polyvinylidene membrane, N-terminal sequencing of the product identified it as alpha(1)-antichymotrypsin. The mean concentration of alpha(1)-antichymotrypsin in human plasma is 7 micrometer. At this concentration, alpha(1)-antichymotrypsin inhibits both macrophage enzymes. A concentration of 0.4 micrometer, which is in the expected concentration range in extracellular fluid, preferentially inhibits the degrading enzyme, which allows for cleavage to active MSP by the pro-MSP convertase.

Regulation of the RON receptor tyrosine kinase expression in macrophages: blocking the RON gene transcription by endotoxin-induced nitric oxide

Previous studies have shown that activation of the RON receptor tyrosine kinase inhibits inducible NO production in murine peritoneal macrophages. The purpose of this study is to determine whether inflammatory mediators such as LPS, IFN-gamma, and TNF-alpha regulate RON expression. Western blot analysis showed that RON expression is reduced in peritoneal macrophages collected from mice injected with a low dose of LPS. The inhibition was seen as early as 8 h after LPS challenge. Experiments in vitro also demonstrated that the levels of the RON mRNA and protein are diminished in cultured peritoneal macrophages following LPS stimulation. TNF-alpha plus IFN-gamma abrogated macrophage RON expression, although individual cytokines had no significant effect. Because LPS and TNF-alpha plus IFN-gamma induce NO production, we reasoned that NO might be involved in the RON inhibition. Two NO donors, S-nitroglutathione (GSNO) and (+/-)-S-nitroso-N-acetylpenicillamine (SNAP), directly inhibited macrophage RON expression when added to the cell cultures. Blocking NO production by NO inhibitors like TGF-beta prevented the LPS-mediated inhibitory effect. In Raw264.7 cells transiently transfected with a report vector, GSNO or SNAP inhibited the luciferase activities driven by the RON gene promoter. Moreover, GSNO or SNAP inhibited the macrophage-stimulating protein-induced RON phosphorylation and macrophage migration. We concluded from these data that RON expression in macrophages is regulated during inflammation. LPS and TNF-alpha plus IFN-gamma are capable of down-regulating RON expression through induction of NO production. The inhibitory effect of NO is mediated by suppression of the RON gene promoter activities.

Negative Regulation of Macrophage Activation in Response to IFN-γ and Lipopolysaccharide by the STK/RON Receptor Tyrosine Kinase

IFN-γ primes macrophages for antimicrobial activity, increased killing of intracellular pathogens, and Ag processing and presentation to lymphocytes by cooperating with a second signal (provided by LPS or endogenous TNF-α) to promote increased proinflammatory cytokine production, NO production, and MHC class II expression. Macrophage-stimulating protein (MSP) suppresses NO production by activated peritoneal macrophages in vitro. Furthermore, targeted deletion of the receptor for MSP, stem cell-derived tyrosine kinase receptor (STK/RON), resulted in increased production of NO by activated macrophages both in vitro and in vivo. Here we demonstrate that expression of STK in RAW264.7 cells resulted in suppression of NO production following IFN-γ+/− LPS stimulation in the presence of MSP, reflecting a decrease in the levels of inducible NO synthase (iNOS) mRNA and protein, which was confirmed by decreased trans-activation of an iNOS reporter. The iNOS expression is regulated by the coordinate activity of the inducible transcription factors STAT-1, IFN response factor-1, and NF-κB. The presence of the STK receptor did not significantly alter the expression of the IFN-γ receptor, STAT1 phosphorylation, or the up-regulation of IFN response factor-1 expression following IFN-γ stimulation. However, nuclear translocation of NF-κB following stimulation of RAW cells with IFN-γ and LPS was reduced in the presence of the MSP/STK signaling pathway. These results suggest that the negative regulation of macrophage responses by MSP/STK occurs at least in part via inhibition of costimulatory signals, resulting in NF-κB activation, that cooperate with IFN-γ to promote activation.

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.