Met receptor dynamics and signalling

Negative regulation of receptor tyrosine kinases by ubiquitination: Key roles of the Cbl family of E3 ubiquitin ligases

Receptor tyrosine kinases (RTKs) serve as transmembrane receptors that participate in a broad spectrum of cellular processes including cellular growth, motility, differentiation, proliferation, and metabolism. Hence, elucidating the regulatory mechanisms of RTKs involved in an assortment of diseases such as cancers attracts increasing interest from researchers. Members of the Cbl family ubiquitin ligases (c-Cbl, Cbl-b and Cbl-c in mammals) have emerged as negative regulators of activated RTKs. Upon activation of RTKs by growth factors, Cbl binds to RTKs via its tyrosine kinase binding (TKB) domain and targets them for ubiquitination, thus facilitating their degradation and negative regulation of RTK signaling. RTKs such as epidermal growth factor receptor (EGFR), platelet-derived growth factor receptor (PDGF), fibroblast growth factor receptor (FGFR) and hepatocyte growth factor receptor (HGFR) undergo ubiquitination upon interaction with Cbl family members. In this review, we summarize the current knowledge related to the negative regulation of RTKs by Cbl family proteins.

Establishment of an indirect ELISA for detection of the novel antifibrotic peptide M10

OBJECTIVE

M10 is a ten amino acid peptide generated from the intracellular cytoplasmic tail of the hepatocyte growth factor (HGF) receptor c-Met following cleavage by caspase-3. Recently we reported that M10 interacts with Smad2 and demonstrates antifibrotic properties in vitro and in vivo and can be advanced into a novel antifibrotic remedy. The current study was undertaken to develop an immunoassay to measure M10 concentration in biological specimens.

EXPERIMENTAL DESIGN

An Indirect Enzyme-Linked Immunosorbent Assay (ELISA) for detection of M10 in biological fluids was developed using pharmaceutical grade synthetic M10 as a calibrator and commercially available anti-c-Met C12 antibody.

RESULTS

M10 ELISA specifically detected in plasma M10, but not a scrambled peptide, following a single intraperitoneal administration of M10 (1mg/kg) to mice. The detection limit was 9.6 ng/ml, and the measuring limit was between 15 ng/ml and 200 ng/ml. The recovery limits of M10 were between 80% and 120%; intra-assay coefficient of variation was between 5.3% and 6.3%; inter-assay coefficient of variation was between 5.0% and 8.0% over the buffer concentration tested in the range from 15 ng /ml to 250 ng /ml. The peak of M10 concentration following a single intraperitoneal injection (1mg/kg) was achieved within 6 hours and declined to minimal levels by 48 hours. The experimentally obtained half-life for M10 was comparable to the theoretically predicted half-life for M10.

CONCLUSIONS

We have established a highly sensitive ELISA to detect the antifibrotic peptide M10 in plasma samples, which should prove to be a novel tool to study the pharmacokinetics and efficacy of M10 in the treatment of fibroproliferative disorders.

The role of endocytic Rab GTPases in regulation of growth factor signaling and the migration and invasion of tumor cells

Metastasis is characterized pathologically by uncontrolled cell invasion, proliferation, migration and angiogenesis. It is a multistep process that encompasses the modulation of membrane permeability and invasion, cell spreading, cell migration and proliferation of the extracellular matrix, increase in cell adhesion molecules and interaction, decrease in cell attachment and induced survival signals and propagation of nutrient supplies (blood vessels). In cancer, a solid tumor cannot expand and spread without a series of synchronized events. Changes in cell adhesion receptor molecules (e.g., integrins, cadherin-catenins) and protease expressions have been linked to tumor invasion and metastasis. It has also been determined that ligand-growth factor receptor interactions have been associated with cancer development and metastasis via the endocytic pathway. Specifically, growth factors, which include IGF-1 and IGF-2 therapy, have been associated with most if not all of the features of metastasis. In this review, we will revisit some of the key findings on perhaps one of the most important hallmarks of cancer metastasis: cell migration and cell invasion and the role of the endocytic pathway in mediating this phenomenon.

Advances in research of signaling pathways in cholangiocarcinoma

Epidemiological data indicate that the incidence and mortality of cholangiocarcinoma (CC) show an increasing trend worldwide over the past several years. Many pathophysiologic aspects of this neoplasia are still unknown and need to be fully discovered. However, progress has been recently made in understanding molecular mechanisms involved in the transformation and growth of malignant cholangiocytes. It is found that cholangiocarcinogenesis is a multistep cellular process evolving from a normal condition of the epithelial biliary cells and ending with malignant transformation through a chronic inflammation status. The bad prognosis related to CC justifies why a better identification of the molecular mechanisms involved in the growth and progression of this cancer is required for the development of effective preventive measures and valid treatment regimens. Signaling pathways can regulate substance and energy metabolism in organisms and are closely related to biological growth and development. This paper mainly introduces signaling pathways which occur in cholangiocarcinoma and their roles in cholangiocarcinoma cells.

Protein-tyrosine phosphatase 1B modulates early endosome fusion and trafficking of Met and epidermal growth factor receptors

The endoplasmic reticulum-localized non-receptor protein-tyrosine phosphatase 1B (PTP1B) is associated with oncogenic, metabolic, and cytokine-related signaling and functionally targets multiple receptor tyrosine kinases (RTKs) for dephosphorylation. Loss of PTP1B activity leads to enhanced ligand-dependent biological activity of the Met RTK among others. Here, we demonstrate that knockdown of PTP1B or expression of a PTP1B trapping aspartic acid-to-alanine substitution (D/A) mutant delayed ligand-induced degradation of the Met and EGF RTKs. Loss of PTP1B function abrogated trafficking of Met and EGF receptor to Rab5- and phosphatidylinositol 3-phosphate (Pl3P)-positive early endosomes and subsequent trafficking through the degradative pathway. Under these conditions, internalization of the Met and EGF receptors was unaltered, suggesting a block at the level of early endosome formation. We show that the N-ethylmaleimide-sensitive factor (NSF), an essential component of the vesicle fusion machinery, was hyperphosphorylated in PTP1B knockdown or PTP1B D/A-expressing cells and was a target for PTP1B. NSF knockdown phenocopied PTP1B knockdown, demonstrating a mechanism through which PTP1B regulates endocytic trafficking. Finally, we show that PTP1B dephosphorylated NSF and that this interaction was required for physiological RTK trafficking and appropriate attenuation of downstream signaling.

Comparative analyses of differentially induced T-cell receptor-mediated phosphorylation pathways in T lymphoma cells

Activation of T lymphoma cells expressing Syk, but not ZAP-70 tyrosine kinase, has been shown to negatively regulate cell activation and activation-induced cell death (AICD), perhaps due to differential induction of tyrosine phosphorylation modified proteins. To better understand the role of these proteins and their associated molecules/pathways, we studied a previously described model of T lymphoma cells expressing either a kinase-activated chimeric Syk or ZAP-70 genetically linked to T-cell receptor (TCR) ζ chain (Z/Syk or Z/ZAP cells, respectively). To help identify molecules and pathways linked to cell activation or AICD, a comparative semi-quantitative proteomics-based approach was utilized to analyze tyrosine-phosphorylated protein immunoprecipitates from two-minute short-term activated Z/Syk or Z/ZAP cells. Using the resulting bioinformatics data-sets, we identified several differentially immunoprecipitated proteins that could be validated biochemically. More tyrosine-phosphorylated and phosphotyrosine-associated proteins were found in Z/Syk than in Z/ZAP cells. Proteins involved in different unique functional pathways were induced in these cells and showed altered intermolecular interactions in varied pathways. Remarkably, 41% of differentially identified proteins in Z/Syk cells belonged to cell cycle or vesicle/trafficking pathways. In contrast, 21% of such proteins in Z/ZAP cells belonged to metabolism pathways. Therefore, molecular pathways involved in post-translational modifications linked to distinct cellular/physiological functions are differentially activated, which may contribute to varied activation and AICD responses of these cells. In summary, we identified proteins belonging to novel differentially activated pathways involved in TCR-mediated signaling, which may be targets for regulating activation and AICD of T lymphoma cells and for potential cancer therapy.

Involvement of PI3K and ERK1/2 pathways in hepatocyte growth factor-induced cholangiocarcinoma cell invasion

AIM: To investigate the role of hepatocyte growth factor (HGF) in cholangiocarcinoma (CCA) cell invasiveness and the mechanisms underlying such cellular responses.

METHODS: Effects of HGF on cell invasion and motility were investigated in two human CCA cell lines, HuCCA-1 and KKU-M213, using Transwell in vitro assay. Levels of proteins of interest and their phosphorylated forms were determined by Western blotting. Localization of E-cadherin was analyzed by immunofluorescence staining and visualized under confocal microscope. Activities of matrix degrading enzymes were determined by zymography.

RESULTS: Both CCA cell lines expressed higher Met levels than the H69 immortalized cholangiocyte cell line. HGF induced invasion and motility of the cell lines and altered E-cadherin from membrane to cytoplasm localization, but did not affect the levels of secreted matrix metalloproteinase (MMP)-2, MMP-9 and urokinase plasminogen activator, key matrix degrading enzymes involved in cell invasion. Concomitantly, HGF stimulated Akt and extracellular signal-regulated kinase (ERK)1/2 phosphorylation but with slightly different kinetic profiles in the two cell lines. Inhibition of the phosphoinositide 3-kinase (PI3K)/Akt pathway by the PI3K inhibitor, LY294002, markedly suppressed HGF-stimulated invasion of both CCA cell lines, and inhibition of the ERK pathway by U0126 suppressed HGF-induced invasion of the KKU-M213 cell line but had a moderate effect on HuCCA-1 cells.

CONCLUSION: These data indicate that HGF promotes CCA cell invasiveness through dys-localization of E-cadherin and induction of cell motility by distinct signaling pathways depending on cell line type.

PKC and the control of localized signal dynamics

Networks of signal transducers determine the conversion of environmental cues into cellular actions. Among the main players in these networks are protein kinases, which can acutely and reversibly modify protein functions to influence cellular events. One group of kinases, the protein kinase C (PKC) family, have been increasingly implicated in the organization of signal propagation, particularly in the spatial distribution of signals. Examples of where and how various PKC isoforms direct this tier of signal organization are becoming more evident.

PHA665752, a small-molecule inhibitor of c-Met, inhibits hepatocyte growth factor-stimulated migration and proliferation of c-Met-positive neuroblastoma cells

BACKGROUND

c-Met is a tyrosine kinase receptor for hepatocyte growth factor/scatter factor (HGF/SF), and both c-Met and its ligand are expressed in a variety of tissues. C-Met/HGF/SF signaling is essential for normal embryogenesis, organogenesis, and tissue regeneration. Abnormal c-Met/HGF/SF signaling has been demonstrated in different tumors and linked to aggressive and metastatic tumor phenotypes. In vitro and in vivo studies have demonstrated inhibition of c-Met/HGF/SF signaling by the small-molecule inhibitor PHA665752. This study investigated c-Met and HGF expression in two neuroblastoma (NBL) cell lines and tumor tissue from patients with NBL, as well as the effects of PHA665752 on growth and motility of NBL cell lines. The effect of the tumor suppressor protein PTEN on migration and proliferation of tumor cells treated with PHA665752 was also evaluated.

METHODS

Expression of c-Met and HGF in NBL cell lines SH-EP and SH-SY5Y and primary tumor tissue was assessed by immunohistochemistry and quantitative RT-PCR. The effect of PHA665752 on c-Met/HGF signaling involved in NBL cell proliferation and migration was evaluated in c-Met-positive cells and c-Met-transfected cells. The transwell chemotaxis assay and the MTT assay were used to measure migration and proliferation/cell-survival of tumor cells, respectively. The PPAR-gamma agonist rosiglitazone was used to assess the effect of PTEN on PHA665752-induced inhibition of NBL cell proliferation/cell-survival and migration

RESULTS

High c-Met expression was detected in SH-EP cells and primary tumors from patients with advanced-stage disease. C-Met/HGF signaling induced both migration and proliferation of SH-EP cells. Migration and proliferation/cell-survival were inhibited by PHA665752 in a dose-dependent manner. We also found that induced overexpression of PTEN following treatment with rosiglitazone significantly enhanced the inhibitory effect of PHA665752 on NBL-cell migration and proliferation.

CONCLUSION

c-Met is highly expressed in most tumors from patients with advanced-stage, metastatic NBL. Furthermore, using the NBL cell line SH-EP as a model, PHA665752 was shown to inhibit cMet/HGF/SF signaling in vitro, suggesting c-Met inhibitors may have efficacy for blocking local progression and/or metastatic spread of c-Met-positive NBL in vivo. These are novel findings for this disease and suggest that further studies of agents targeting the c-Met/HGF axis in NBL are warranted.

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.

Integration of transforming growth factor beta and RAS signaling silences a RAB5 guanine nucleotide exchange factor and enhances growth factor-directed cell migration

Transforming growth factor beta (TGF-beta) receptor (TbetaR) signaling contributes to normal development as well as tumorigenesis. Here we report that RIN1, a RAB5 guanine nucleotide exchange factor (GEF) and down regulator of receptor tyrosine kinases (RTKs), promotes TbetaR signaling through enhanced endocytosis. TbetaR activation induces SNAI1 (Snail), a transcription repressor that reduces RIN1 expression, providing a negative feedback mechanism to control TbetaR trafficking and downstream signaling. Persistent RAS signaling disrupts this equilibrium by stabilizing SNAI1 protein, resulting in strong silencing of RIN1 and stabilization of RTKs. TGF-beta-induced RIN1 silencing in breast cancer cells prolonged sensitivity to hepatocyte growth factor, a ligand for the MET-type RTK, and enhanced growth factor-directed cell motility. We conclude that in some tumor cells TbetaR and RAS signals are integrated through the silencing of RIN1, leading to a reduction in RAB5-mediated endocytosis. These findings shed new light on the basis for distinct interpretations of TGF-beta signaling by normal versus transformed cells.

Integration of transforming growth factor beta and RAS signaling silences a RAB5 guanine nucleotide exchange factor and enhances growth factor-directed cell migration

Transforming growth factor beta (TGF-beta) receptor (TbetaR) signaling contributes to normal development as well as tumorigenesis. Here we report that RIN1, a RAB5 guanine nucleotide exchange factor (GEF) and down regulator of receptor tyrosine kinases (RTKs), promotes TbetaR signaling through enhanced endocytosis. TbetaR activation induces SNAI1 (Snail), a transcription repressor that reduces RIN1 expression, providing a negative feedback mechanism to control TbetaR trafficking and downstream signaling. Persistent RAS signaling disrupts this equilibrium by stabilizing SNAI1 protein, resulting in strong silencing of RIN1 and stabilization of RTKs. TGF-beta-induced RIN1 silencing in breast cancer cells prolonged sensitivity to hepatocyte growth factor, a ligand for the MET-type RTK, and enhanced growth factor-directed cell motility. We conclude that in some tumor cells TbetaR and RAS signals are integrated through the silencing of RIN1, leading to a reduction in RAB5-mediated endocytosis. These findings shed new light on the basis for distinct interpretations of TGF-beta signaling by normal versus transformed cells.

Impairment of the antifibrotic effect of hepatocyte growth factor in lung fibroblasts from African Americans: possible role in systemic sclerosis

OBJECTIVE

To compare the composition of cytokines in African American and Caucasian patients with systemic sclerosis (SSc; scleroderma) and in healthy individuals, particularly the expression and function of hepatocyte growth factor (HGF).

METHODS

Bronchoalveolar lavage (BAL) fluid samples were analyzed using cytokine array techniques. HGF in plasma and cell culture medium samples was measured by enzyme-linked immunosorbent assay. Connective tissue growth factor (CTGF), type I collagen expression, and c-Met receptor phosphorylation were studied by immunoblotting.

RESULTS

Overall greater expression of cytokines in BAL fluid from African American patients as compared with Caucasian patients was observed. Significant increases in HGF concentrations were detected in BAL fluid, plasma, and fibroblast culture medium from Caucasian SSc patients. In contrast, African American SSc patients did not demonstrate an increase in HGF. Recombinant HGF readily abolished CTGF expression and collagen accumulation in lung fibroblasts isolated from Caucasian SSc patients. Pretreatment of lung fibroblasts with neutralizing anti-c-Met antibody abolished the effects of HGF on CTGF expression and collagen accumulation, suggesting that the antifibrotic activity of HGF is mediated via c-Met receptor tyrosine kinase. Whereas recombinant HGF rapidly induced c-Met receptor phosphorylation in lung fibroblasts from Caucasian patients, c-Met receptor phosphorylation was significantly reduced in lung fibroblasts from African American subjects. Moreover, recombinant HGF failed to prevent CTGF expression and collagen accumulation in lung fibroblasts derived from African American subjects.

CONCLUSION

Ethnic differences exist in terms of antifibrotic HGF expression in lung fibroblasts derived from Caucasian and African American subjects. Reduced levels of HGF as well as a deficiency in c-Met receptor function appear to be present in African American patients with SSc. These findings may explain in part the greater disease severity and worse prognosis observed in African Americans with SSc.

Gene expression profiling of long-term changes in rat liver following burn injury

The inflammatory response initiated upon burn injury is also associated with extensive metabolic adjustments. While there is a significant body of literature on the characterization of these changes at the metabolite level, little is known on the mechanisms of induction, especially with respect to the role of gene expression. We have comprehensively analyzed changes in gene expression in rat livers during the first 7 d after 20% total body surface area burn injury using Affymetrix microarrays. A total of 740 genes were significantly altered in expression at 1, 2, 4, and 7 d after burn injury compared to sham-burn controls. Functional classification based on gene ontology terms indicated that metabolism, transport, signaling, and defense/inflammation response accounted for more than 70% of the significantly altered genes. Fisher least-significant difference post-hoc testing of the 740 differentially expressed genes indicated that over 60% of the genes demonstrated significant changes in expression either on d 1 or on d 7 postburn. The gene expression trends were corroborated by biochemical measurements of triglycerides and fatty acids 24 h postburn but not at later time points. This suggests that fatty acids are used, at least in part, in the liver as energy substrates for the first 4 d after injury. Our data also suggest that long-term regulation of energy substrate utilization in the liver following burn injury is primarily at the posttranscriptional level. Last, relevance networks of significantly expressed genes indicate the involvement of key small molecules in the hepatic response to 20% total body surface area burn injury.

A clash to conquer: the malaria parasite liver infection

All mammalian malaria parasite species have an initial tissue stage in liver cells. The liver stage produces new parasite forms that can enter and live inside red blood cells. Accordingly, the first place of residence provides parasites with a radically different cellular and molecular environment from their subsequent red blood cell home. Liver stages have remained refractory to reveal their secrets, yet the last few years have seen several advances in elucidating their biology. This review looks at the more recent findings concerning the liver stage-host hepatocyte association, some of which may become powerful weapons in the prevention of malaria infection. We also outline areas of liver stage research and technological development that provide promising foci to accelerate a better understanding of this most elusive of the parasites many life cycle stages.

LRIG1 is a novel negative regulator of the Met receptor and opposes Met and Her2 synergy

The Met receptor tyrosine kinase regulates a complex array of cellular behaviors collectively known as "invasive growth." While essential for normal development and wound repair, this program is frequently co-opted by tumors to promote their own growth, motility, and invasion. Met is overexpressed in a variety of human tumors, and this aberrant expression correlates with poor patient prognosis. Previous studies indicate that Met receptor levels are governed in part by cbl-mediated ubiquitination and degradation, and uncoupling of Met from cbl-mediated ubiquitination promotes its transforming activity. Here we describe a novel mechanism for Met degradation. We find that the Met receptor interacts with the transmembrane protein LRIG1 independent of hepatocyte growth factor (HGF) stimulation and that LRIG1 destabilizes the Met receptor in a cbl-independent manner. Overexpression of LRIG1 destabilizes endogenous Met receptor in breast cancer cells and impairs their ability to respond to HGF. LRIG1 knockdown increases Met receptor half-life, indicating that it plays an essential role in Met degradation. Finally, LRIG1 opposes Met synergy with the ErbB2/Her2 receptor tyrosine kinase in driving cellular invasion. We conclude that LRIG1 is a novel suppressor of Met function, serving to regulate cellular receptor levels by promoting Met degradation in a ligand- and cbl-independent manner.

Inhibition of c-Met as a therapeutic strategy for esophageal adenocarcinoma

The hepatocyte growth factor (HGF) receptor c-Met is a tyrosine kinase receptor with established oncogenic properties. We have previously shown that c-Met is usually overexpressed in esophageal adenocarcinoma (EA), yet the implications of c-Met inhibition in EA remain unknown. Three c-Met-overexpressing EA cell lines (Seg-1, Bic-1, and Flo-1) were used to examine the effects of a c-Met-specific small molecule inhibitor (PHA665752) on cell viability, apoptosis, motility, invasion, and downstream signaling pathways. PHA665752 demonstrated dose-dependent inhibition of constitutive and/or HGF-induced phosphorylation of c-Met, which correlated with reduced cell viability and inhibition of extracellular regulated kinase 1/2 phosphorylation in all three EA cell lines. In contrast, PHA665752 induced apoptosis and reduced motility and invasion in only one EA cell line, Flo-1. Interestingly, Flo-1 was the only cell line in which phosphatidylinositol 3-kinase (PI3K)/Akt was induced following HGF stimulation. The PI3K inhibitor LY294002 produced effects equivalent to those of PHA665752 in these cells. We conclude that inhibition of c-Met may be a useful therapeutic strategy for EA. Factors other than receptor overexpression, such as c-Met-dependent PI3K/Akt signaling, may be predictive of an individual tumor's response to c-Met inhibition.