Proteolytic cleavage and activation of pro-macrophage-stimulating protein and upregulation of its receptor in tissue injury

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.

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.

Prostate tumor RON receptor signaling mediates macrophage recruitment to drive androgen deprivation therapy resistance through Gas6-mediated Axl and RON signaling

BACKGROUND

Androgen deprivation therapy (ADT), or chemical castration, is the first-line therapy for prostate cancer; however, resistance leaves few treatment options. Prostatic tumor-associated macrophages (TAMs) have been shown to promote prostate cancer growth and are abundant in castration-resistant prostate cancer (CRPC), suggesting a role in promoting CRPC. We recently showed a tumor cell-intrinsic mechanism by which RON promotes CRPC. Given previous reports that RON alters prostate cancer cell chemokine production and RON-overexpressing tumors alter macrophage function, we hypothesized that a macrophage-dependent mechanism regulated by tumor cell intrinsic RON also promotes CRPC.

METHODS

Using RON-modulated genetically engineered mouse models (GEMMs) and GEMM-derived cell lines and co-cultures with bone marrow-derived macrophages, we show functional and molecular characteristics of signaling pathways in supporting CRPC. Further, we used an unbiased phosphokinase array to identify pathway interactions regulated by RON. Finally, using human prostate cancer cell lines and prostate cancer patient data sets, we show the relevance of our findings to human prostate cancer.

RESULTS

Studies herein show that macrophages recruited into the prostate tumor microenvironment (TME) serve as a source for Gas6 secretion which serves to further enhance RON and Axl receptor activation in prostate tumor cells thereby driving CRPC. Further, we show targeting RON and macrophages in a murine model promotes CRPC sensitization to ADT.

CONCLUSIONS

We discovered a novel role for the RON receptor in prostate cancer cells in promoting CRPC through the recruitment of macrophages into the prostate TME. Macrophage-targeting agents in combination with RON/Axl inhibition are likely to provide clinical benefits for patients with CRPC.

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.

Genetic Investigation of Inverse Psoriasis

Inverse psoriasis is considered to be a rare variant of plaque-type psoriasis and is associated with significantly impaired quality of life. Clinical manifestations and treatment options are somewhat different for each subtype. Identifying genetic variants that contribute to the susceptibility of different types of psoriasis might improve understanding of the etiology of the disease. Since we have no current knowledge about the genetic background of inverse psoriasis, whole exome sequencing was used to comprehensively assess genetic variations in five patients with exclusively inverse lesions. We detected six potentially pathogenic rare (MAF < 0.01) sequence variants that occurred in all investigated patients. The corresponding mutated genes were FN1, FBLN1, MYH7B, MST1R, RHOD, and SCN10A. Several mutations identified in this study are known to cause disease, but roles in psoriasis or other papulosquamous diseases have not previously been reported. Interestingly, potentially causative variants of established psoriasis-susceptibility genes were not identified. These outcomes are in agreement with our hypothesis that the inverse subtype is a different entity from plaque-type psoriasis.

RON Expression Mediates Lipopolysaccharide-Mediated Dendritic Cell Maturation via March-I

The macrophage stimulating protein (MSP)-Recepteur d'origine nantais (RON) signaling pathway regulates macrophage function. Here, we verified RON receptor expression in bone marrow-derived dendritic cells (BMDCs) by real time-PCR, Western blot, and flow cytometry. Flow cytometry was used to detect the changes in MHC II and CD86 expression following the inhibition of RON in BMDCs and splenic dendritic cells (DCs). Immunoprecipitation and Western blot were used to detect the level of MHC II and CD86 ubiquitination. An enzyme-linked immunosorbent assay was used to detect cytokine release, and a mixed lymphocyte reaction was performed to evaluate DC maturity. The results show that the inhibition of RON leads to an increase in March-1 transcription, which intensifies the ubiquitination of MHC II and CD86 and ultimately leads to a decreased level of these two molecules. The mixed lymphocyte reaction provided evidence that RON inhibition decreased the ability of DCs to promote the proliferation of T cells. The MSP-RON signaling pathway may play an important role in lipopolysaccharide (LPS)-stimulated DC maturation through March-I and may protect DC differentiation following LPS stimulation.

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.

A Novel S100A8/A9 Induced Fingerprint of Mesenchymal Stem Cells associated with Enhanced Wound Healing

We here investigated whether the unique capacity of mesenchymal stem cells (MSCs) to re-establish tissue homeostasis depends on their potential to sense danger associated molecular pattern (DAMP) and to mount an adaptive response in the interest of tissue repair. Unexpectedly, after injection of MSCs which had been pretreated with the calcium-binding DAMP protein S100A8/A9 into murine full-thickness wounds, we observed a significant acceleration of healing even exceeding that of non-treated MSCs. This correlates with a fundamental reprogramming of the transcriptome in S100A8/A9 treated MSCs as deduced from RNA-seq analysis and its validation. A network of genes involved in proteolysis, macrophage phagocytosis, and inflammation control profoundly contribute to the clean-up of the wound site. In parallel, miR582-5p and genes boosting energy and encoding specific extracellular matrix proteins are reminiscent of scar-reduced tissue repair. This unprecedented finding holds substantial promise to refine current MSC-based therapies for difficult-to-treat wounds and fibrotic conditions.

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.

α-Ketobenzothiazole Serine Protease Inhibitors of Aberrant HGF/c-MET and MSP/RON Kinase Pathway Signaling in Cancer

Upregulation of the HGF and MSP growth-factor processing serine endopeptidases HGFA, matriptase and hepsin is correlated with increased metastasis in multiple tumor types driven by c-MET or RON kinase signaling. We rationally designed P1' α-ketobenzothiazole mechanism-based inhibitors of these proteases. Structure-activity studies are presented, which resulted in the identification of potent inhibitors with differential selectivity. The tetrapeptide inhibitors span the P1-P1' substrate cleavage site via a P1' amide linker off the benzothiazole, occupying the S3' pocket. Optimized inhibitors display sub-nanomolar enzyme inhibition against one, two, or all three of HGFA, matriptase, and hepsin. Several compounds also have good selectivity against the related trypsin-like proteases, thrombin and Factor Xa. Finally, we show that inhibitors block the fibroblast (HGF)-mediated migration of invasive DU145 prostate cancer cells. In addition to prostate cancer, breast, colon, lung, pancreas, gliomas, and multiple myeloma tumors all depend on HGF and MSP for tumor survival and progression. Therefore, these unique inhibitors have potential as new therapeutics for a diverse set of tumor types.

The curious ability of polyethylene glycol fusion technologies to restore lost behaviors after nerve severance

Traumatic injuries to PNS and CNS axons are not uncommon. Restoration of lost behaviors following severance of mammalian peripheral nerve axons (PNAs) relies on regeneration by slow outgrowths and is typically poor or nonexistent when after ablation or injuries close to the soma. Behavioral recovery after severing spinal tract axons (STAs) is poor because STAs do not naturally regenerate. Current techniques to enhance PNA and/or STA regeneration have had limited success and do not prevent the onset of Wallerian degeneration of severed distal segments. This Review describes the use of a recently developed polyethylene glycol (PEG) fusion technology combining concepts from biochemical engineering, cell biology, and clinical microsurgery. Within minutes after microsuturing carefully trimmed cut ends and applying a well-specified sequence of solutions, PEG-fused axons exhibit morphological continuity (assessed by intra-axonal dye diffusion) and electrophysiological continuity (assessed by conduction of action potentials) across the lesion site. Wallerian degeneration of PEG-fused PNAs is greatly reduced as measured by counts of sensory and/or motor axons and maintenance of axonal diameters and neuromuscular synapses. After PEG-fusion repair, cut-severed, crush-severed, or ablated PNAs or crush-severed STAs rapidly (within days to weeks), more completely, and permanently restore PNA- or STA-mediated behaviors compared with nontreated or conventionally treated animals. PEG-fusion success is enhanced or decreased by applying antioxidants or oxidants, trimming cut ends or stretching axons, and exposure to Ca(2+) -free or Ca(2+) -containing solutions, respectively. PEG-fusion technology employs surgical techniques and chemicals already used by clinicians and has the potential to produce a paradigm shift in the treatment of traumatic injuries to PNAs and STAs.

Expression of Hepatocyte Growth Factor-Like Protein in Human Wound Tissue and Its Biological Functionality in Human Keratinocytes

human keratinocyte model, may indicate a role for HGFl in active wound healing.

The effects of macrophage-stimulating protein on the migration, proliferation, and collagen synthesis of skin fibroblasts in vitro and in vivo

Macrophage-stimulating protein (MSP), an important cytokine with multiple functions, is highly expressed in adipose-derived stem cells-conditioned medium (ASC-CM). ASCs can effectively promote wound healing through paracrine mechanism, suggesting that MSP may play a critical role in wound healing. Through binding to its receptor, RON (Receptuerd'OrigineNantaise, also called macrophage stimulation 1 receptor; MST1R), it can activate epithelial cells and work as an inflammatory mediator. In this study, we found RON was also expressed on dermal fibroblasts and investigated the effects of MSP on proliferation, migration, and collagen synthesis of fibroblasts. With the treatment of different concentrations of MSP (0, 1, 10, 20, 50, and 100 ng/mL) on fibroblasts, proliferation, migration, and collagen synthesis were analyzed by Cell Counting Kit-8 (CCK-8), transwell and real-time polymerase chain reaction. Under the treatment of MSP, the migration, Collagen I, III synthesis, and matrix metalloproteinase-1 (MMP-1) mRNA expression of fibroblasts were upregulated significantly, although there was no effect on fibroblasts proliferation, and the optimal concentration of MSP for migration and collagen synthesis was 10 ng/mL. In the in vivo study, 10 ng/mL MSP was applied to full-thickness skin wound with bacterial cellulose membranes, and this treatment could accelerate the wound healing rate and increased the collagen synthesis of wound sites. This study suggested that MSP appears to promote the migration of fibroblasts, enhances collagen synthesis and remodeling, and effectively improves wound healing.

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.

Hepatocyte growth factor-like protein is a positive regulator of early mammary gland ductal morphogenesis

The Ron receptor tyrosine kinase regulates multiple cellular processes and is important during mammary gland development and tumor progression. Hepatocyte growth factor-like protein [HGFL] is the only known ligand for the Ron receptor and recent studies have identified major roles for HGFL during breast cancer metastasis. Understanding the functional importance HGFL during mammary gland development will provide significant insights onto its contribution during tumor development and metastasis. In this study, we assessed the role of HGFL during postnatal mammary gland development using mice that were either proficient [HGFL +/+] or deficient [HGFL-/-] for HGFL. Postnatal ductal morphology and stromal cell associations were analyzed at multiple time points through puberty until adulthood. HGFL deficiency resulted in several mammary gland developmental defects including smaller terminal end buds [TEBs], significantly fewer TEBs, and delayed ductal outgrowth during early puberty. Additionally, HGFL deficient animals exhibited significantly altered TEB epithelial cell turnover with decreased proliferation and increased apoptosis coupled with decreased TEB diameter. Macrophage recruitment to the TEBs was also significantly decreased in the HGFL-/- mice compared to controls. Moreover, the levels of STAT3 mRNA as well as the phosphorylation status of this protein were lower in the HGFL-/- mammary glands compared to controls. Taken together, our data provide the first evidence for HGFL as a positive regulator of mammary gland ductal morphogenesis by controlling overall epithelial cell turnover, macrophage recruitment, and STAT3 activation in the developing mammary gland. With a function in early mammary gland development, HGFL represents a potential target for the development of novel breast cancer therapies.

Ron receptor signaling is protective against DSS-induced colitis in mice

Inflammatory bowel diseases (IBD) are chronic inflammatory disorders of the intestine that result in painful and debilitating complications. Currently no cure exists for IBD, and treatments are primarily aimed at reducing inflammation to alleviate symptoms. Genome-wide linkage studies have identified the Ron receptor tyrosine kinase (TK) and its ligand, hepatocyte growth factor-like protein (HGFL), as genes highly associated with IBD. However, only scant information exists on the role of Ron or HGFL in IBD. Based on the linkage of Ron to IBD, we directly examined the biological role of Ron in colitis. Wild-type mice and mice lacking the TK signaling domain of Ron (TK-/- mice) were utilized in a well-characterized model of chronic colitis induced by cyclic exposure to dextran sulfate sodium. In this model, TK-/- mice were more susceptible to injury as judged by increased mortality compared with control mice and developed more severe colitis. Loss of Ron led to significantly reduced body weights and more aggressive clinical and histopathologies. Ron loss also resulted in a dramatic reduction in colonic epithelial cell proliferation and increased proinflammatory cytokine production, which was associated with alterations in important signaling pathways known to regulate IBD. Examination of human gene expression data further supports the contention that loss of Ron signaling is associated with IBD. In total, our studies point to important functional roles for Ron in IBD by regulating healing of the colonic epithelium and by controlling cytokine secretion.

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.

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

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

Inhibition of ron kinase blocks conversion of micrometastases to overt metastases by boosting antitumor immunity

Many "nonmetastatic" cancers have spawned undetectable metastases before diagnosis. Eventual outgrowth of these microscopic lesions causes metastatic relapse and death, yet the events that dictate when and how micrometastases convert to overt metastases are largely unknown. We report that macrophage-stimulating protein and its receptor, Ron, are key mediators in conversion of micrometastases to bona fide metastatic lesions through immune suppression. Genetic deletion of Ron tyrosine kinase activity specifically in the host profoundly blocked metastasis. Our data show that loss of Ron function promotes an effective antitumor CD8(+) T-cell response, which specifically inhibits outgrowth of seeded metastatic colonies. Treatment of mice with a Ron-selective kinase inhibitor prevented outgrowth of lung metastasis, even when administered after micrometastatic colonies had already been established. Our findings indicate that Ron inhibitors may hold potential to specifically prevent outgrowth of micrometastases in patients with cancer in the adjuvant setting.

Myeloid-specific expression of Ron receptor kinase promotes prostate tumor growth

Ron receptor kinase (MST1R) is important in promoting epithelial tumorigenesis, but the potential contributions of its specific expression in stromal cells have not been examined. Herein, we show that the Ron receptor is expressed in mouse and human stromal cells of the prostate tumor microenvironment. To test the significance of stromal Ron expression, prostate cancer cells were orthotopically implanted into the prostates of either wild-type or Ron tyrosine kinase deficient (TK(-/-); Mst1r(-/-)) hosts. In TK(-/-) hosts, prostate cancer cell growth was significantly reduced as compared with tumor growth in TK(+/+) hosts. Prostate tumors in TK(-/-) hosts exhibited an increase in tumor cell apoptosis, macrophage infiltration and altered cytokine expression. Reciprocal bone marrow transplantation studies and myeloid cell-specific ablation of Ron showed that loss of Ron in myeloid cells is sufficient to inhibit prostate cancer cell growth. Interestingly, depletion of CD8(+) T cells, but not CD4(+) T cells, was able to restore prostate tumor growth in hosts devoid of myeloid-specific Ron expression. These studies show a critical role for the Ron receptor in the tumor microenvironment, whereby Ron loss in tumor-associated macrophages inhibits prostate cancer cell growth, at least in part, by derepressing the activity of CD8(+) T cells.

Macrophage stimulating protein promotes liver metastases of small cell lung cancer cells by affecting the organ microenvironment

The organ microenvironment significantly affects the processes of cancer metastasis. Elucidating the molecular mechanisms of interaction between tumor cells and the organ microenvironment is crucial for the development of effective therapeutic strategies to eradicate cancer metastases. Macrophage stimulating protein (MSP), an activator of macrophages, regulates a pleiotropic array of effects, including proliferation, cellular motility, invasiveness, angiogenesis, and resistance to anoikis. However, the role of MSP in cancer metastasis is still largely unknown. In this study, the action of MSP on the production of metastases was determined in a multiple-organ metastasis model. The murine MSP gene was transfected into two human SCLC cell lines, SBC-5 and H1048, to establish transfectants secreting biologically active MSP. MSP gene transduction did not affect cell proliferation and motility in vitro. Intravenously inoculated MSP transfectants produced significantly larger numbers of liver metastases than parental cells or vector control clones, while there were no significant differences in bone or lung metastases among them. Immunohistochemical analyses of liver metastases revealed that tumor-associated microvessel density and tumor-infiltrating macrophages were significantly increased in lesions produced by MSP transfectants. MSP could stimulate the migration of murine macrophages and endothelial cells in vitro. Consequently, MSP may be one of the major determinants that affects the properties of tumor stroma and that produces a permissive microenvironment to promote cancer metastasis.

Macrophage-stimulating protein polymorphism rs3197999 is associated with a gain of function: implications for inflammatory bowel disease

Crohn's disease and ulcerative colitis, the two main types of inflammatory bowel disease (IBD), were reported to be associated with a variety of genetic polymorphisms. A subset of these polymorphisms was identified in both diseases and only three of them were found in primary sclerosing cholangitis (PSC). rs3197999 (Arg689Cys) located in the MST1 gene is one of the most convincingly replicated IBD/PSC-associated polymorphisms but its functional consequences have not been investigated, yet. We expressed both MST1 gene variants (Arg(689) (MSP(wt)) and Cys(689) (MSP(mut)) in a eukaryotic cell system and compared their stimulatory effects on macrophage-like THP-1 cells. Except for the rate of apoptosis that remained unchanged, MSP(mut) significantly increased the stimulatory effect of MSP (macrophage-stimulating protein) on chemotaxis and proliferation by THP-1 cells, indicating a gain of function associated with the Arg689Cys exchange. A broad set of evidence reported previously suggests that pro-inflammatory changes in macrophage function have a major role in the initiation of the inflammatory process in IBD and PSC. Therefore, the gain of function observed with rs3197999 in MST1 might provide a cellular mechanism for the consistent association of this polymorphism with an increased risk for IBD and PSC.

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.

Role of STK in mouse liver macrophage and endothelial cell responsiveness during acute endotoxemia

Acute endotoxemia is associated with excessive production of proinflammatory mediators by hepatic macrophages and endothelial cells, which have been implicated in liver injury and sepsis. In these studies, we analyzed the role of MSP and its receptor STK in regulating the activity of these cells. Acute endotoxemia, induced by administration of LPS (3 mg/kg) to mice, resulted in increased expression of STK mRNA and protein in liver macrophages and endothelial cells, an effect that was dependent on TLR-4. This was correlated with decreased MSP and increased pro-MSP in serum. In Kupffer cells, but not endothelial cells, MSP suppressed LPS-induced NOS-2 expression, with no effect on COX-2. LPS treatment of mice caused a rapid (within 3 h) increase in the proinflammatory proteins NOS-2, IL-1beta, and TNF-alpha, as well as TREM-1 and TREM-3 and the anti-inflammatory cytokine IL-10 in liver macrophages and endothelial cells. Whereas LPS-induced expression of proinflammatory proteins was unchanged in STK-/- mice, IL-10 expression was reduced significantly. Enzymes mediating eicosanoid biosynthesis including COX-2 and mPGES-1 also increased in macrophages and endothelial cells after LPS administration. In STK-/- mice treated with LPS, mPGES-1 expression increased, although COX-2 expression was reduced. LPS-induced up-regulation of SOD was also reduced in STK-/- mice in liver macrophages and endothelial cells. These data suggest that MSP/STK signaling plays a role in up-regulating macrophage and endothelial cell anti-inflammatory activity during hepatic inflammatory responses. This may be important in protecting the liver from tissue injury.

The RON receptor tyrosine kinase mediates oncogenic phenotypes in pancreatic cancer cells and is increasingly expressed during pancreatic cancer progression

Pancreatic cancer is an aggressive disease characterized by rapid growth and early metastasis. The recepteur d'origine nantais (RON) receptor tyrosine kinase is overexpressed and/or constitutively active in several epithelial cancers, but its role in pancreatic cancer is unknown. In this study, we have characterized RON expression in both murine and human pancreatic cancer. Immunoblotting indicates that RON is expressed in pancreatic intraepithelial neoplasia (PanIN), primary, and metastatic cell lines both in the human and mouse. Immunostaining revealed that 93% of high-grade PanIN, 79% of primary, and 83% of metastatic lesions from human pancreatic tissue samples expressed RON, with minimal expression in normal ducts and low-grade PanIN (6% and 18%, respectively). Moreover, we show a dose-dependent effect of hepatocyte growth factor-like protein (HGFL), the RON-specific ligand, on pancreatic cancer cell migration and invasion, which was reversed by RON inhibition. Although stimulation with HGFL had no effect on proliferation, concurrent RON receptor blockade and gemcitabine treatment increased apoptosis of RON-expressing pancreatic cancer cells versus gemcitabine treatment alone. Finally, HGFL stimulation of pancreatic cancer cells resulted in increased expression of phospho-mitogen-activated protein kinase and phospho-Akt. Taken together, these findings suggest that RON receptor signaling may contribute to pancreatic carcinogenesis, and that further investigation is warranted to assess the potential of RON-directed therapies in this deadly disease.

Current insights into the formation and breakdown of hemidesmosomes

Hemidesmosomes are multiprotein adhesion complexes that promote epithelial stromal attachment in stratified and complex epithelia. Modulation of their function is of crucial importance in a variety of biological processes, such as differentiation and migration of keratinocytes during wound healing and carcinoma invasion, in which cells become detached from the substrate and acquire a motile phenotype. Although much is known about the signaling potential of the alpha6beta4 integrin in carcinoma cells, the events that coordinate the disassembly of hemidesmosomes during differentiation and wound healing remain unclear. The binding of alpha6beta4 to plectin has a central role in hemidesmosome assembly and it is becoming clear that disrupting this interaction is a crucial event in hemidesmosome disassembly. In addition, further insight into the functional interplay between alpha3beta1 and alpha6beta4 has contributed to our understanding of hemidesmosome disassembly and cell migration.

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 tyrosine kinase receptor regulates papilloma growth and malignant conversion in a murine model of skin carcinogenesis

Recent studies demonstrate that the receptor tyrosine kinase (TK) Ron is tumorigenic when overexpressed and plays a role in regulating skin homeostasis. We hypothesized that Ron signaling promotes skin carcinogenesis. To test this hypothesis, mice deficient in the TK domain of Ron (TK(-/-) mice) were crossed with v-Ha-ras (Tg.AC) transgenic mice; the resulting TK(-/-) Tg.AC(+/-) mice, and their controls, were utilized in a model of chemically induced Ras-mediated skin carcinogenesis. The mice were treated with 2.5 microg of 12-O-tetradecanoylphorbol-13-acetate applied weekly to the shaved back of 36 control (TK(+/+) Tg.AC(+/-)) and 35 experimental (TK(-/-) Tg.AC(+/-)) mice. In an analysis of the resulting papillomas, a reduction in cellular proliferation and papilloma volume was found in the TK(-/-) Tg.AC(+/-) mice compared to controls. Further, Ron protein expression was upregulated during papilloma formation. Ablation of Ron signaling resulted in partial defects in MAPK and Akt signaling that may account for the decreased papilloma growth in the TK(-/-) Tg.AC(+/-) mice. The papilloma-bearing mice were monitored for the occurrence of malignant skin tumors and other malignant tumor types for a period of 48 weeks. Loss of Ron receptor signaling significantly reduced the percent of papillomas that underwent malignant conversion as well as the number of mice developing other malignant tumor types. In conclusion, these studies demonstrate that Ron signaling augments papilloma growth and malignant conversion in vivo.

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 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.

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.

The MSP receptor regulates alpha6beta4 and alpha3beta1 integrins via 14-3-3 proteins in keratinocyte migration

Growth factors, integrins, and the extracellular matrix (ECM) are known to play key roles in epidermal wound healing, although the interplay between these proteins is not fully understood. We show that growth factor macrophage stimulating protein (MSP)- and its receptor Ron-mediated PI3K activation in keratinocytes induces phosphorylation of both Ron and alpha6beta4 integrin at specific 14-3-3 binding sites. Consequently, a Ron/alpha6beta4 complex formed via 14-3-3 binding displaces alpha6beta4 from its location at hemidesmosomes (structures supporting cell adhesion) and relocalizes it to lamellipodia. Concomitant activation of alpha3beta1 and keratinocyte spreading/migration on laminin-5 occurs. Further, MSP-dependent beta4 tyrosine phosphorylation evokes p38 and NF-kappaB signaling required for keratinocyte wound closure. Based on these results, we propose a mechanism based on MSP-Ron-dependent phosphorylation and 14-3-3 association, whereby the function of alpha6beta4 switches from a mechanical adhesive device into a signaling component, and might be critically involved in human epidermal wound healing.

Regulation of wound healing by growth factors and cytokines

Cutaneous wound healing is a complex process involving blood clotting, inflammation, new tissue formation, and finally tissue remodeling. It is well described at the histological level, but the genes that regulate skin repair have only partially been identified. Many experimental and clinical studies have demonstrated varied, but in most cases beneficial, effects of exogenous growth factors on the healing process. However, the roles played by endogenous growth factors have remained largely unclear. Initial approaches at addressing this question focused on the expression analysis of various growth factors, cytokines, and their receptors in different wound models, with first functional data being obtained by applying neutralizing antibodies to wounds. During the past few years, the availability of genetically modified mice has allowed elucidation of the function of various genes in the healing process, and these studies have shed light onto the role of growth factors, cytokines, and their downstream effectors in wound repair. This review summarizes the results of expression studies that have been performed in rodents, pigs, and humans to localize growth factors and their receptors in skin wounds. Most importantly, we also report on genetic studies addressing the functions of endogenous growth factors in the wound repair process.

The Ron proto-oncogene product is a phenotypic marker of renal oncocytoma

The proto-oncogene product Ron is the receptor for macrophage stimulating protein, a scatter factor that stimulates cell proliferation, prevents apoptosis, and induces an invasive cell phenotype. We investigated the expression of Ron, Ki-67 (proliferation index), p53, and bcl-2 (proapoptotic and antiapoptotic proteins, respectively) in 50 renal tumors (19 clear cell carcinomas, 18 oncocytomas, 7 papillary cell carcinomas, 5 chromophobe cell carcinomas, and 1 carcinoma with sarcomatoid areas). In addition, we studied Ron in normal kidney and in the renal carcinoma cell line Caki-1. Immunostaining and Western blot showed Ron in normal kidney and in all oncocytomas but never in renal cell carcinomas or in Caki-1. In addition, Western blot showed that Ron was expressed in phosphorylated, i.e., active, form. Bcl-2 was strongly expressed in oncocytomas, whereas Ki-67 and p53 were much less expressed in oncocytomas than in carcinomas. These results indicate in Ron a marker that differentiates oncocytoma from the other renal epithelial tumors. We therefore think that Ron may prove to be a new tool for a sound and precise diagnosis of oncocytoma, a benign tumor that cannot always be distinguished from carcinomas at histologic examination. The overexpression of bcl-2, but not p53 in oncocytoma, suggests that the MSP/Ron system sustains the growth of oncocytoma by opposing apoptosis.

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.

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.

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.

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.

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.

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.

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.

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.