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

The Modulation of Phospho-Extracellular Signal-Regulated Kinase and Phospho-Protein Kinase B Signaling Pathways plus Activity of Macrophage-Stimulating Protein Contribute to the Protective Effect of Stachydrine on Acetaminophen-Induced Liver Injury

Stachydrine, a prominent bioactive alkaloid derived from Leonurus heterophyllus, is a significant herb in traditional medicine. It has been noted for its anti-inflammatory and antioxidant characteristics. Consequently, we conducted a study of its hepatoprotective effect and the fundamental mechanisms involved in acetaminophen (APAP)-induced liver injury, utilizing a mouse model. Mice were intraperitoneally administered a hepatotoxic dose of APAP (300 mg/kg). Thirty minutes after APAP administration, mice were treated with different concentrations of stachydrine (0, 2.5, 5, and 10 mg/kg). Animals were sacrificed 16 h after APAP injection for serum and liver tissue assays. APAP overdose significantly elevated the serum alanine transferase levels, hepatic pro-inflammatory cytokines, malondialdehyde activity, phospho-extracellular signal-regulated kinase (ERK), phospho-protein kinase B (AKT), and macrophage-stimulating protein expression. Stachydrine treatment significantly decreased these parameters in mice with APAP-induced liver damage. Our results suggest that stachydrine may be a promising beneficial target in the prevention of APAP-induced liver damage through attenuation of the inflammatory response, inhibition of the ERK and AKT pathways, and expression of macrophage-stimulating proteins.

The Emerging Role of c-Met in Carcinogenesis and Clinical Implications as a Possible Therapeutic Target

Background

c-MET is a receptor tyrosine kinase receptor (RTK) for the hepatocyte growth factor (HGF). The binding of HGF to c-MET regulates several cellular functions: differentiation, proliferation, epithelial cell motility, angiogenesis, and epithelial-mesenchymal transition (EMT). Moreover, it is known to be involved in carcinogenesis. Comprehension of HGF-c-MET signaling pathway might have important clinical consequences allowing to predict prognosis, response to treatment, and survival rates based on its expression and dysregulation. Discussion. c-MET represents a useful molecular target for novel engineered drugs. Several clinical trials are underway for various solid tumors and the development of new specific monoclonal antibodies depends on the recent knowledge about the definite c-MET role in each different malignance. Recent clinical trials based on c-MET molecular targets result in good safety profile and represent a promising therapeutic strategy for solid cancers, in monotherapy or in combination with other target drugs.

Conclusion

The list of cell surface receptors crosslinking with the c-MET signaling is constantly growing, highlighting the importance of this pathway for personalized target therapy. Research on the combination of c-MET inhibitors with other drugs will hopefully lead to discovery of new effective treatment options.

Macrophage Stimulating Protein Enhances Hepatic Inflammation in a NASH Model

Non-alcoholic steatohepatitis (NASH) is a common liver disease characterized by hepatic lipid accumulation (steatosis) and inflammation. Currently, therapeutic options are poor and the long-term burden to society is constantly increasing. Previously, macrophage stimulating protein (MSP)—a serum protein mainly secreted by liver—was shown to inhibit oxidized low-density lipoprotein (OxLDL)/lipopolysaccharides (LPS)-induced inflammation in mouse macrophages. Additionally, MSP could reduce palmitic acid (PA)-induced lipid accumulation and lipogenesis in the HepG2 cell line. Altogether, these data suggest MSP as a suppressor for metabolic inflammation. However, so far the potential of MSP to be used as a treatment for NASH was not investigated. We hypothesized that MSP reduces lipid accumulation and hepatic inflammation. To investigate the effects of MSP in the early stage of NASH, low-density lipoprotein receptor (Ldlr^-/-) mice were fed either a regular chow or a high fat, high cholesterol (HFC) diet for 7 days. Recombinant MSP or saline (control) was administrated to the mice by utilizing subcutaneously-implanted osmotic mini-pumps for the last 4 days. As expected, mice fed an HFC diet showed increased plasma and hepatic lipid accumulation, as well as enhanced hepatic inflammation, compared with chow-fed controls. Upon MSP administration, the rise in cholesterol and triglyceride levels after an HFC diet remained unaltered. Surprisingly, while hepatic macrophage and neutrophil infiltration was similar between the groups, MSP-treated mice showed increased gene expression of pro-inflammatory and pro-apoptotic mediators in the liver, compared with saline-treated controls. Contrary to our expectations, MSP did not ameliorate NASH. Observed changes in inflammatory gene expression suggest that further research is needed to clarify the long-term effects of MSP.

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.

Hepatocyte growth factor-like protein is required for prostate tumor growth in the TRAMP mouse model

The Ron receptor is deregulated in a variety of cancers. Hepatocyte growth factor-like protein (HGFL) is the ligand for Ron and is constitutively secreted from hepatocytes into the circulation. While a few recent reports have emerged analyzing ectopic HGFL overexpression in cancer cells, no studies have examined the effect of host-produced HGFL in tumorigenesis. To examine HGFL function in prostate cancer, the TRAMP mouse model, which is predisposed to develop prostate tumors, was utilized. Prostate tumors from TRAMP mice exhibit elevated levels of HGFL, which correlated with upregulation in human prostate cancer. To directly implicate HGFL in prostate tumorigenesis, TRAMP mice deficient in HGFL (HGFL-/-TRAMP+) were generated. HGFL-/- TRAMP+ mice developed significantly smaller prostate tumors compared to controls. Analysis of HGFL-/- tumors revealed reduced tumor vascularization. No differences in cancer cell proliferation were detected between HGFL-/- TRAMP+ and HGFL+/+ TRAMP+ mice. However, a significant increase in cancer cell death was detected in HGFL-/- TRAMP+ prostates which correlated with decreased pro-survival targets. In vitro analysis demonstrated robust STAT3 activation resulting in Bcl2-dependent survival following treatment of prostate cancer cells with HGFL. These data document a novel function for endogenous HGFL in prostate cancer by imparting a critical survival signal to tumor cells.

Loss of Ron receptor signaling leads to reduced obesity, diabetic phenotypes and hepatic steatosis in response to high-fat diet in mice

The Ron receptor tyrosine kinase is a heterodimeric, membrane-spanning glycoprotein that participates in divergent processes, including proliferation, motility, and modulation of inflammatory responses. We observed male C57BL/6 mice with a global deletion of the Ron tyrosine kinase signaling domain (TK(-/-)) to be leaner compared with control (TK(+/+)) mice under a standard diet. When fed a high-fat diet (HFD), TK(-/-) mice gained 50% less weight and were more insulin sensitive and glucose tolerant than controls. Livers from HFD TK(-/-) mice were considerably less steatotic and weighed significantly less than TK(+/+) livers. Serum cytokine levels of HFD TK(-/-) mice were also significantly altered compared with TK(+/+) mice. Fewer and smaller adipocytes were present in the TK(-/-) mice on both control and HFD and were accompanied by diminished adiponectin and peroxisome proliferator-activated receptor-γ expression. In vitro adipogenesis experiments suggested reduced differentiation in TK(-/-) embryonic fibroblasts (MEFs) that was rescued by Ron reconstitution. Likewise, signal transducer and activator of transcription (STAT)-3 phosphorylation was diminished in TK(-/-) MEFs but was increased after Ron reconstitution. The adipogenic inhibitors, preadipocyte factor 1 and Sox9, were elevated in TK(-/-) MEFs and increased in both groups after STAT3 silencing. In total, these studies document a previously unknown function for the Ron receptor in mediating HFD-induced obesity and metabolic dysregulation.

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.

Ron receptor-dependent gene regulation in a mouse model of endotoxin-induced acute liver failure

BACKGROUND

Prior experimentation has shown that loss of the tyrosine kinase (TK) signaling domain of the Ron receptor leads to marked hepatocyte protection in a model of lipopolysaccharide-induced acute liver failure (ALF) in D-galactosamine (GalN)-sensitized mice. The aim of this study was to identify the role of Ron in the regulation of hepatic gene expression.

METHODS

Microarray analyses were performed on liver RNA isolated sequentially from wild-type (WT) and TK-/- mice during the progression of ALF. Gene array data were validated using Western and immunohistochemistry analyses as well as with ex vivo culture systems.

RESULTS

At baseline, 101 genes were differentially expressed between WT and TK-/- livers, which regulate processes involved in hypoxia, proliferation, apoptosis and metabolism. One hour after ALF induction, WT livers exhibited increased cytokine expression compared to TK-/- livers, and after 4 hours, an induction of suppressor of cytokine signaling (SOCS) genes as well as JAK-STAT pathway activation were prominent in TK-/- livers compared to controls.

CONCLUSION

Our studies suggest a novel hepato-protective mechanism in Ron TK-/- mice wherein increased and sustained SOCS production and JAK-STAT activation in the hepatocyte may inhibit the destructive proinflammatory milieu and promote survival factors which blunt hepatic death and the ensuing development of ALF.

Regulation of alternative macrophage activation in the liver following acetaminophen intoxication by stem cell-derived tyrosine kinase

Stem cell-derived tyrosine kinase (STK) is a transmembrane receptor reported to play a role in macrophage switching from a classically activated/proinflammatory phenotype to an alternatively activated/wound repair phenotype. In the present studies, STK⁻/⁻ mice were used to assess the role of STK in acetaminophen-induced hepatotoxicity as evidence suggests that the pathogenic process involves both of these macrophage subpopulations. In wild type mice, centrilobular hepatic necrosis and increases in serum transaminase levels were observed within 6h of acetaminophen administration (300 mg/kg, i.p.). Loss of STK resulted in a significant increase in sensitivity of mice to the hepatotoxic effects of acetaminophen and increased mortality, effects independent of its metabolism. This was associated with reduced levels of hepatic glutathione, rapid upregulation of inducible nitric oxide synthase, and prolonged induction of heme oxygenase-1, suggesting excessive oxidative stress in STK⁻/⁻ mice. F4/80, a marker of mature macrophages, was highly expressed on subpopulations of Kupffer cells in livers of wild type, but not STK⁻/⁻ mice. Whereas F4/80⁺ macrophages rapidly declined in the livers of wild type mice following acetaminophen intoxication, they increased in STK⁻/⁻ mice. In wild type mice hepatic expression of tumor necrosis factor (TNF)-α, interleukin (IL)-1β, and IL-12, products of classically activated macrophages, increased after acetaminophen administration. Monocyte chemotactic protein-1 (MCP-1) and its receptor, CCR2, as well as IL-10, mediators involved in recruiting and activating anti-inflammatory/wound repair macrophages, also increased in wild type mice after acetaminophen. Loss of STK blunted the effects of acetaminophen on expression of TNFα, IL-1β, IL-12, MCP-1 and CCR2, while expression of IL-10 increased. Hepatic expression of CX3CL1, and its receptor, CX3CR1 also increased in STK⁻/⁻ mice treated with acetaminophen. These data demonstrate that STK plays a role in regulating macrophage recruitment and activation in the liver following acetaminophen administration, and in hepatotoxicity.

The Ron receptor promotes prostate tumor growth in the TRAMP mouse model

The Ron receptor tyrosine kinase is overexpressed in many cancers, including prostate cancer. In order to examine the significance of Ron in prostate cancer in vivo, we utilized a genetically engineered mouse model, referred to as TRAMP mice, that is predisposed to develop prostate tumors. In this model, we demonstrate that prostate tumors from 30-week-old TRAMP mice have increased Ron expression compared to age-matched wild-type prostates. Based on the upregulation of Ron in human prostate cancers and in this murine model of prostate tumorigenesis, we hypothesized that this receptor plays a functional role in the development of prostate tumors. To test this hypothesis, we crossed TRAMP mice with mice that are deficient in Ron signaling (TK−/−). Interestingly, TK−/− TRAMP+ mice show a significant decrease in prostate tumor mass relative to TRAMP mice containing functional Ron. Moreover, TK−/− TRAMP+ prostate tumors exhibited decreased tumor vascularization relative to TK+/+ TRAMP+ prostate tumors, which correlated with reduced levels of the angiogenic molecules VEGF and CXCL2. While Ron loss did not alter tumor cell proliferation, a significant decrease in cell survival was observed. Similarly, murine prostate cancer cell lines containing a Ron deficiency exhibited decreased levels of active NF-kappaB suggesting that Ron may be important in regulating prostate cell survival at least partly through this pathway. In total, our data show for the first time that Ron promotes prostate tumor growth, prostate tumor angiogenesis, and prostate cancer cell survival in vivo.

Ron receptor regulates Kupffer cell-dependent cytokine production and hepatocyte survival following endotoxin exposure in mice

Previous studies demonstrated that targeted deletion of the Ron receptor tyrosine kinase (TK) domain in mice leads to marked hepatocyte protection in a well-characterized model of lipopolysaccharide (LPS)-induced acute liver failure in D-galactosamine (GalN)-sensitized mice. Hepatocyte protection in TK-/- mice was observed despite paradoxically elevated serum levels of tumor necrosis factor alpha (TNF-α). To understand the role of Ron in the liver, purified populations of Kupffer cells and hepatocytes from wildtype (TK+/+) and TK-/- mice were studied. Utilizing quantitative reverse-transcription polymerase chain reaction (RT-PCR), we demonstrated that Ron is expressed in these cell types. Moreover, we also recapitulated the protected hepatocyte phenotype and exaggerated cytokine production observed in the TK-/- mice in vivo through the use of purified cultured cells ex vivo. We show that isolated TK-/- Kupffer cells produce increased levels of TNF-α and select cytokines compared to TK+/+ cells following LPS stimulation. We also show that conditioned media from LPS-treated TK-/- Kupffer cells was more toxic to hepatocytes than control media, suggesting the exaggerated levels of cytokines produced from the TK-/- Kupffer cells are detrimental to wildtype hepatocytes. In addition, we observed that TK-/- hepatocytes were more resistant to cell death compared to TK+/+ hepatocytes, suggesting that Ron functions in both the epithelial and inflammatory cell compartments to regulate acute liver injury. These findings were confirmed in vivo in mice with hepatocyte and macrophage cell-type-specific conditional Ron deletions. Mice with Ron loss selectively in hepatocytes exhibited less liver damage and increased survival compared to mice with Ron loss in macrophages.

CONCLUSION

We dissected cell-type-specific roles for Ron such that this receptor modulates cytokine production from Kupffer cells and inhibits hepatocyte survival in response to injury.

Ron receptor tyrosine kinase negatively regulates TNFalpha production in alveolar macrophages by inhibiting NF-kappaB activity and Adam17 production

The Ron receptor tyrosine kinase (TK) plays a regulatory role in the inflammatory response to acute lung injury induced by intranasal administration of bacterial LPS. Previously, we have shown that mice with a targeted deletion of the TK signaling domain of the Ron receptor exhibited more severe lung injury in response to intranasal LPS administration as evidenced by an increased leakage of albumin in the lungs and a greater thickening of the alveolar septa compared with wild-type mice. In addition, lung injury in the Ron TK-deficient (TK(-/-)) mice was associated with increased activation of the transcription factor, nuclear factor-kappaB (NF-kappaB), and significantly increased intrapulmonary expression of TNFalpha. TNFalpha, a multifunctional proinflammatory cytokine, is a central mediator in several disease states, including rheumatoid arthritis and sepsis. On the basis of the observation that TNFalpha production is increased in the Ron TK-/- mice and that macrophages are a major source of this cytokine, we hypothesized that the alterations observed in the Ron TK(-/-) mice may be due, in part, to Ron signaling, specifically in alveolar macrophages. To test this hypothesis, we used the wild-type and Ron TK(-/-) primary alveolar macrophages and the murine alveolar macrophage cell line, MH-S, to examine the effects of Ron activation on LPS-induced TNFalpha production and NF-kappaB activity. Here, we reported that Ron is expressed on alveolar macrophages and MH-S cells. Activation of Ron by its ligand, hepatocyte growth factor-like protein, decreases TNFalpha production in alveolar macrophages after LPS challenge. Decreased TNFalpha is associated with hepatocyte growth factor-like protein-induced decreases in NF-kappaB activation and increases in the NF-kappaB inhibitory protein, IkappaB. We also provided the first evidence for Ron as a negative regulator of Adam17, the metalloprotease involved in TNFalpha processing. These results indicate that Ron plays a critical role in regulation of alveolar macrophage signaling and validates this receptor as a target in TNFalpha-mediated pulmonary pathologies.

The Ron receptor tyrosine kinase is not required for adenoma formation in Apc(Min/+) mice

The Ron receptor tyrosine kinase is overexpressed in approximately half of all human colon cancers. Increased Ron expression positively correlates with tumor progression, and reduction of Ron levels in human colon adenocarcinoma cells reverses their tumorigenic properties. Nearly all colon tumors demonstrate loss of the adenomatous polyposis coli (APC) tumor suppressor, an early initiating event, subsequently leading to beta-catenin stabilization. To understand the role of Ron in early stage intestinal tumorigenesis, we generated Apc-mutant (Apc(Min/+)) mice with and without Ron signaling. Interestingly, we report here that significantly more Apc(Min/+) Ron-deficient mice developed higher tumor burden than Apc(Min/+) mice with wild-type Ron. Even though baseline levels of intestinal crypt proliferation were increased in the Apc(Min/+) Ron-deficient mice, loss of Ron did not influence tumor size or histological appearance of the Apc(Min/+) adenomas, nor was beta-catenin localization changed compared to Apc(Min/+) mice with Ron. Together, these data suggest that Ron may be important in normal intestinal tissue homeostasis, but that the expression of this receptor is not required for the formation and growth of adenomas in Apc(Min/+) mice.

Urinary trypsin inhibitor protects against liver injury and coagulation pathway dysregulation induced by lipopolysaccharide/D-galactosamine in mice

Urinary trypsin inhibitor (UTI), a serine protease inhibitor, has been widely used for patients with inflammatory disorders including disseminated intravascular coagulation, shock, and pancreatitis in Japan. Our recent studies using UTI-null (-/-) mice have shown that UTI protects against systemic inflammatory responses and acute lung injury. However, the role of UTI in liver injury has not been elucidated. This study determined the contribution of UTI to liver injury and coagulatory disturbance induced by lipopolysaccharide and D-galactosamine (LPS/D-GalN) using UTI (-/-) and wild-type (WT) mice. LPS/D-GalN treatment caused severe liver injury characterized by neutrophilic inflammation, hemorrhagic change, necrosis, and apoptosis, which was more prominent in UTI (-/-) than in WT mice. In both genotypes of mice, LPS/D-GalN challenge caused elevations of aspartate amino-transferase and alanine amino-transferase, prolongation of the prothrombin and activated partial thromboplastin time, and decreases in fibrinogen and platelet counts, as compared with vehicle challenge. These changes, however, were significantly greater in UTI (-/-) than in WT mice. Circulatory levels of tumor necrosis factor (TNF)-alpha (P<0.05) and interferon (IFN)-gamma were also greater in UTI (-/-) than in WT mice after LPS/D-GalN challenge. These results suggest that UTI protects against severe liver injury and subsequent coagulatory disturbance induced by LPS/D-GalN, which was mediated, at least partly, through the suppression of TNF-alpha production along with its antiprotease activity.

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.

Ron receptor tyrosine kinase-dependent hepatic neutrophil recruitment and survival benefit in a murine model of bacterial peritonitis

OBJECTIVE

To determine whether Ron receptor tyrosine kinase signaling affects the in vivo response to bacterial peritonitis.

DESIGN

Experimental study.

SETTING

University laboratory.

SUBJECTS

Male mice 8-11 wks of age (22-28 g).

INTERVENTIONS

A genetic approach comparing wild-type mice to mice with a targeted deletion of the Ron tyrosine kinase signaling domain (TK-/-) was undertaken to determine the influence of Ron receptor in the in vivo response to a well-characterized model of bacterial peritonitis and sepsis induced by cecal ligation and puncture.

MEASUREMENTS AND MAIN RESULTS

Several clinical (i.e., survival curves, blood and tissue bacterial burdens, and neutrophil oxidative burst), morphologic (i.e., liver histology and leukocyte trafficking), and biochemical variables (i.e., serum aminotransferases and select serum cytokine and chemokine levels) important for assessing inflammatory responses to bacterial infection were assessed in mice following cecal ligation and puncture. Ron TK-/- mice had a significant decrease in survival time compared with controls, and this was associated with a significant increase in bacterial colony-forming units found in the blood and several end-organs. Moreover, this increased bacterial load was associated with increased liver necrosis and serum alanine aminotransferase levels. Neutrophils isolated from TK-/- mice exhibited decreased spontaneous oxidative burst capacity ex vivo, and by intravital microscopy, a reduced level of neutrophil migration to and translocation within the liver was observed. Loss of Ron signaling resulted in significantly reduced production of serum monocyte chemoattractant protein-1 and interleukin-6 levels following cecal ligation and puncture, and peritoneal macrophage isolated from TK-/- mice exhibited blunted production of monocyte chemoattractant protein-1, interleukin-6, and macrophage inflammatory protein-2 following stimulation with endotoxin ex vivo.

CONCLUSIONS

Ron signaling negatively regulates the response to polymicrobial infection by regulating the activation and recruitment of inflammatory cells necessary for clearing a systemic bacterial burden. This effect may be regulated in part through the Ron-dependent, macrophage-mediated production of cytokines and chemokines, namely monocyte chemoattractant protein-1, interleukin-6, and macrophage inflammatory protein-2, important for neutrophil mobilization.

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

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

CONCLUSION

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

Ron-receptor tyrosine kinase in tumorigenesis and metastasis

The Ron-receptor tyrosine kinase has been increasingly recognized for its tumorigenic potential in the last decade. Ron-receptor activation leads to the activation of common receptor tyrosine kinase downstream-signaling pathways, and most prominently in tumor models, activation of MAPK, PI3K and beta-catenin. Numerous experimental models of mammalian tumorigenesis have demonstrated that increased Ron-receptor activity correlates with increased tumorigenesis in a variety of organs of epithelial origin. The evidence for Ron as an oncogene in human tumor biology is growing. The Ron receptor is overexpressed and over activated in a large number of human tumors, and overexpression of Ron correlates with a worse clinical outcome for patients in at least two human cancer states, namely breast and bladder cancer. Several experimental approaches have been demonstrated to successfully block Ron activity and function, and given these convincing data, approaches to block Ron-receptor activity in targeted human cancers should prove to be fruitful in the setting of future clinical research trials.

The Ron receptor tyrosine kinase regulates acute lung injury and suppresses nuclear factor kappaB activation

Emerging information implies that the Ron receptor tyrosine kinase may play a role in the inflammatory response. However, the manner in which this receptor contributes to the response is not well understood. In the present studies, we investigated the role of the Ron receptor in the acute lung inflammatory response. Wild-type and mutant mice lacking the tyrosine kinase domain of Ron (Ron TK-/-) were subjected to acute lung injury induced by intranasal administration of bacterial lipopolysaccharide (LPS). Wild-type mice showed increased lung injury after LPS administration, as determined by the leakage of albumin into the lung and by histopathological changes. Ron TK-/- mice had more than twice the amount of albumin leak and much greater thickening of the alveolar septae. Lipopolysaccharide administration caused neutrophil recruitment into the lungs, as measured by myeloperoxidase. However, Ron TK-/- mice had much higher baseline levels of myeloperoxidase, which did not increase further after LPS. Lung injury in wild-type mice occurred with activation of the transcription factor, nuclear factor kappaB (NF-kappaB), and subsequent increases in intrapulmonary generation of tumor necrosis factor alpha. In TK-/- mice, there was far less IkappaB-alpha and IkappaB-beta protein and greater activation of NF-kappaB. This was associated with substantially increased production of tumor necrosis factor alpha and the nitric oxide (NO) by-product, nitrite. The data suggest that the Ron receptor tyrosine kinase plays an important regulatory role in acute inflammatory lung injury by suppressing signals leading to activation of NF-kappaB.

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.

Therapeutic implications of a human neutralizing antibody to the macrophage-stimulating protein receptor tyrosine kinase (RON), a c-MET family member

RON is a member of the c-MET receptor tyrosine kinase family. Like c-MET, RON is expressed by a variety of epithelial-derived tumors and cancer cell lines and it is thought to play a functional role in tumorigenesis. To date, antagonists of RON activity have not been tested in vivo to validate RON as a potential cancer target. In this report, we used an antibody phage display library to generate IMC-41A10, a human immunoglobulin G1 (IgG1) antibody that binds with high affinity (ED50 = 0.15 nmol/L) to RON and effectively blocks interaction with its ligand, macrophage-stimulating protein (MSP; IC50 = 2 nmol/L). We found IMC-41A10 to be a potent inhibitor of receptor and downstream signaling, cell migration, and tumorigenesis. It antagonized MSP-induced phosphorylation of RON, mitogen-activated protein kinase (MAPK), and AKT in several cancer cell lines. In HT-29 colon, NCI-H292 lung, and BXPC-3 pancreatic cancer xenograft tumor models, IMC-41A10 inhibited tumor growth by 50% to 60% as a single agent, and in BXPC-3 xenografts, it led to tumor regressions when combined with Erbitux. Western blot analyses of HT-29 and NCI-H292 xenograft tumors treated with IMC-41A10 revealed a decrease in MAPK phosphorylation compared with control IgG-treated tumors, suggesting that inhibition of MAPK activity may be required for the antitumor activity of IMC-41A10. To our knowledge, this is the first demonstration that a RON antagonist and specifically an inhibitory antibody of RON negatively affects tumorigenesis. Another major contribution of this report is an extensive analysis of RON expression in approximately 100 cancer cell lines and approximately 300 patient tumor samples representing 10 major cancer types. Taken together, our results highlight the potential therapeutic usefulness of RON activity inhibition in human cancers.

Gene expression profiles of Mst1r-deficient mice during nickel-induced acute lung injury

Previous studies have shown that mice deficient in the tyrosine kinase domain (TK-/-) of the receptor Mst1r have an increased susceptibility to nickel (Ni)-induced acute lung injury (ALI). Mst1r TK-/- mice have decreased survival times, alterations in cytokine and nitric oxide regulation, and an earlier onset of pulmonary pathology compared with control mice, suggesting that Mst1r signaling, in part, may regulate the response to ALI. To examine the role of Mst1r in ALI in more detail, we compared the gene expression profiles of murine lung mRNA from control and Mst1r TK-/- mice at baseline and after 24 h of particulate Ni sulfate exposure. Microarray analyses showed a total of 343 transcripts that were significantly changed, either by Ni treatment, or between genotypes. Genes responsible for inflammation, edema, and lymphocyte function were altered in the Mst1r TK-/- mice. Interestingly, the genes for several granzymes were increased in Mst1r TK-/- mice before Ni exposure, compared with controls. In addition, the Mst1r TK-/- lungs showed clusters of cells near the vascular endothelium and airways. Immunohistochemistry indicates these clusters are composed of macrophages, T cells, and neutrophils, and that the clusters display granzyme protein production. These results suggest that Mst1r signaling may be involved in the regulation of macrophage and T-lymphocyte activation in vivo during injury. This assessment of gene expression indicates the importance of genetic factors in contributing to lung injury, and points to strategies for intervention in the progression of inflammatory diseases.

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 signaling augments mammary tumor formation and metastasis in a murine model of breast cancer

The tyrosine kinase receptor Ron has been implicated in several types of cancer, including overexpression in human breast cancer. This is the first report describing the effect of Ron signaling on tumorigenesis and metastasis in a mouse model of breast cancer. Mice with a targeted deletion of the Ron tyrosine kinase signaling domain (TK-/-) were crossed to mice expressing the polyoma virus middle T antigen (pMT) under the control of the mouse mammary tumor virus promoter. Both pMT-expressing wild-type control (pMT+/- TK+/+) and pMT+/- TK-/- mice developed mammary tumors and lung metastases. However, a significant decrease in mammary tumor initiation and growth was found in the pMT+/- TK-/- mice compared with controls. An examination of mammary tumors showed that there was a significant decrease in microvessel density, significantly decreased cellular proliferation, and a significant increase in terminal deoxynucleotidyl transferase-mediated nick end labeling-positive staining in mammary tumor cells from the pMT+/- TK-/- mice compared with the pMT+/- TK+/+ mice. Biochemical analyses on mammary tumor lysates showed that whereas both the pMT-expressing TK+/+ and TK-/- tumors have increased Ron expression compared with normal mammary glands, the pMT-expressing TK-/- tumors have deficits in mitogen-activated protein kinase and AKT activation. These results indicate that Ron signaling synergizes with pMT signaling to induce mammary tumor formation, growth, and metastasis. This effect may be mediated in part through the regulation of angiogenesis and through proliferative and cell survival pathways regulated by mitogen-activated protein kinase and AKT.

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.

Lipopolysaccharides Indirectly Stimulate Apoptosis and Global Induction of Apoptotic Genes in Fibroblasts

Following Gram-negative bacterial infection there is a reduction in matrix-producing cells. The goal of the present study was to examine the apoptotic effects of lipopolysaccharide (LPS) on fibroblastic cells and to investigate the role that the host response plays in this reaction. This was accomplished in vivo by subcutaneous inoculation of LPS in wild type and TNFR1(-/-)R2(-/-) mice. The direct effects of LPS on fibroblast apoptosis was studied in vitro with normal diploid human fibroblasts. The results indicate that LPS in vivo induces apoptosis of fibroblasts. By RNA profiling we demonstrated that LPS stimulates global expression of apoptotic genes and down-regulates anti-apoptotic genes. Fluorometric studies demonstrated that LPS in vivo significantly increased caspase-8 and caspase-3 activity and by use of specific inhibitors, the activation of caspase-3 was shown to be initiated by caspase-8 with no contribution from caspase-9. In vitro studies demonstrated that LPS did not induce apoptosis of fibroblasts, whereas tumor necrosis factor (TNF) did. In addition, the pattern of apoptotic gene expression induced by TNF in vitro was nearly identical to that induced by LPS in vivo, as measured by RNase protection assay. Moreover, pre-treatment of cells with TNF greatly enhanced apoptosis induced by a second stimulation with TNF 24 h later, suggesting that the global induction of pro-apoptotic genes was functionally significant. Thus, LPS acts to modulate the expression of a large number of genes that favor apoptosis of fibroblastic cells that is dependent upon activation of caspase-8 and is largely mediated by TNF.

Nuclear factor-kappa B plays a central role in tumour necrosis factor-mediated liver disease

Deregulation of the apoptotic program is considered an important cause in liver disease. It became clear that the cytokine tumour necrosis factor (TNF) is of specific interest in this context. Therefore, from a clinical point of view, therapeutic control of TNF-receptor signalling pathways is highly desirable. These TNF-initiated signalling pathways result in a direct apoptotic response as well as potent activation of proinflammatory gene expression via activation of the transcription factor nuclear factor-kappa B (NF-kappaB). Since the latter pathway contributes to a series of liver pathologies, inhibition of hepatic NF-kappaB activation was viewed as a potential therapy for liver injury. However, the more recent finding that NF-kappaB activation in hepatocytes is anti-apoptotic shows that NF-kappaB signalling represents a problematic therapeutic target. Here we review the role of TNF and NF-kappaB in liver pathophysiology, and the underlying mechanisms of hepatocyte sensitisation to TNF toxicity in vivo. Based on this knowledge, we suggest some potential strategies for the treatment of TNF-mediated liver disease.

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.

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.