MET meet adaptors: functional and structural implications in downstream signalling mediated by the Met receptor

3-phosphoinositide-dependent protein kinase 1 (PDK1) mediates crosstalk between Src and Akt pathways in MET receptor signaling

The high-affinity tyrosine kinase receptor MET plays a pivotal role in several facets of cell regulation. Although its mitogenic effect is well documented, some aspects of connection patterns between signaling pathways involved in cell cycle progression remain to be deciphered. We have used a tractable heterologous expression system, the Xenopus oocyte, to detect connections between distinct MET signaling cascades involved in G2/M progression. Our results reveal that Src acts as an adapter via its SH2 domain to recruit 3-phosphoinositide-dependent protein kinase 1 (PDK1) to the MET signaling complex leading to Akt phosphorylation. These data define an original crosstalk between Src and Akt signaling pathways that contributes to MET-induced entry into the M phase, and deserves further investigation in pathologies harboring deregulation of this receptor.

HGF/MET Signaling in Malignant Brain Tumors

Hepatocyte growth factor (HGF) ligand and its receptor tyrosine kinase (RTK) mesenchymal-epithelial transition factor (MET) are important regulators of cellular processes such as proliferation, motility, angiogenesis, and tissue regeneration. In healthy adult somatic cells, this ligand and receptor pair is expressed at low levels and has little activity except when tissue injuries arise. In cancer cells, HGF/MET are often overexpressed, and this overexpression is found to correlate with tumorigenesis, metastasis, and poorer overall prognosis. This review focuses on the signaling of these molecules in the context of malignant brain tumors. RTK signaling pathways are among the most common and universally dysregulated pathways in gliomas. We focus on the role of HGF/MET in the following primary malignant brain tumors: astrocytomas, glioblastomas, oligodendrogliomas, ependymomas, and embryonal central nervous system tumors (including medulloblastomas and others). Brain metastasis, as well as current advances in targeted therapies, are also discussed.

HGF-MET Signaling Shifts M1 Macrophages Toward an M2-Like Phenotype Through PI3K-Mediated Induction of Arginase-1 Expression

Backgrounds and Aims: Hepatocyte Growth Factor (HGF)-MET signaling is known to promote biological functions such as cell survival, cell motility, and cell proliferation. However, it is unknown if HGF-MET alters the macrophage phenotype. In this study, we aimed to study the effects of HGF-MET signaling on the M1 macrophage phenotype.

Methods and Materials: Bone marrow-derived macrophages (BMDMs) isolated from mice were either polarized to an M1 phenotype by IFN-γ and LPS treatment or to an M2 phenotype by IL-4 treatment. Changes in M1 or M2 markers induced by HGF-MET signaling were evaluated. Mechanisms responsible for alternations in the macrophage phenotype and intracellular metabolism were analyzed.

Results: c-Met was expressed especially in M1 macrophages polarized by treatment with IFN-γ and LPS. In M1 macrophages, HGF-MET signaling induced the expression of Arg-1 mRNA and secretion of IL-10 and TGF-β1 and downregulated the mRNA expression of iNOS, TNF-α, and IL-6. In addition, activation of the PI3K pathway and inactivation of NFκB were also observed in M1 macrophages treated with HGF. The increased Arg-1 expression and IL-10 secretion were abrogated by PI3K inhibition, whereas, no changes were observed in TNF-α and IL-6 expression. The inactivation of NFκB was found to be independent of the PI3K pathway. HGF-MET signaling shifted the M1 macrophages to an M2-like phenotype, mainly through PI3K-mediated induction of Arg-1 expression. Finally, HGF-MET signaling also shifted the M1 macrophage intracellular metabolism toward an M2 phenotype, especially with respect to fatty acid metabolism.

Conclusion: Our results suggested that HGF treatment not only promotes regeneration in epithelial cells, but also leads to tissue repair by altering M1 macrophages to an M2-like phenotype.

Therapeutic Efficacy of ABN401, a Highly Potent and Selective MET Inhibitor, Based on Diagnostic Biomarker Test in MET-Addicted Cancer

The receptor tyrosine kinase c-MET regulates processes essential for tissue remodeling and mammalian development. The dysregulation of c-MET signaling plays a role in tumorigenesis. The aberrant activation of c-MET, such as that caused by gene amplification or mutations, is associated with many cancers. c-MET is therefore an attractive therapeutic target, and inhibitors are being tested in clinical trials. However, inappropriate patient selection criteria, such as low amplification or expression level cut-off values, have led to the failure of clinical trials. To include patients who respond to MET inhibitors, the selection criteria must include MET oncogenic addiction. Here, the efficacy of ABN401, a MET inhibitor, was investigated using histopathologic and genetic analyses in MET-addicted cancer cell lines and xenograft models. ABN401 was highly selective for 571 kinases, and it inhibited c-MET activity and its downstream signaling pathway. We performed pharmacokinetic profiling of ABN401 and defined the dose and treatment duration of ABN401 required to inhibit c-MET phosphorylation in xenograft models. The results show that the efficacy of ABN401 is associated with MET status and they highlight the importance of determining the cut-off values. The results suggest that clinical trials need to establish the characteristics of each sample and their correlations with the efficacy of MET inhibitors.

Activation of the Aryl Hydrocarbon Receptor Leads to Resistance to EGFR TKIs in Non-Small Cell Lung Cancer by Activating Src-mediated Bypass Signaling

Purpose: The aryl hydrocarbon receptor (AhR) has been generally recognized as a ligand-activated transcriptional factor that responds to xenobiotic chemicals. Recent studies have suggested that the expression of AhR varies widely across different cancer types and cancer cell lines, but its significance in cancer treatment has yet to be clarified.Experimental Design: AhR expression in non-small cell lung cancer (NSCLC) was determined by Western blotting and IHC staining. In vitro and in vivo functional experiments were performed to determine the effect of AhR on sensitivity to targeted therapeutics. A panel of biochemical assays was used to elucidate the underlying mechanisms.Results: A high AhR protein level indicated an unfavorable prognosis for lung adenocarcinoma. Inhibition of AhR signaling sensitized EGFR tyrosine kinase inhibitors (TKIs) in NSCLC cells that express high level of endogenous AhR protein. Notably, activation of AhR by pharmacologic and molecular approaches rendered EGFR-mutant cells resistant to TKIs by restoring PI3K/Akt and MEK/Erk signaling through activation of Src. In addition, we found that AhR acts as a protein adaptor to mediate Jak2-Src interaction, which does not require the canonical transcriptional activity of AhR.Conclusions: Our results reveal a transcription-independent function of AhR and indicate that AhR may act as a protein adaptor that recruits kinases bypassing EGFR and drives resistance to TKIs. Accordingly, targeting Src would be a strategy to overcome resistance to EGFR TKIs in AhR-activated NSCLC. Clin Cancer Res; 24(5); 1227-39. ©2017 AACR.

Anti-vascular therapies in ovarian cancer: moving beyond anti-VEGF approaches

Resistance to chemotherapy is among the most important issues in the management of ovarian cancer. Unlike cancer cells, which are heterogeneous as a result of remarkable genetic instability, stromal cells are considered relatively homogeneous. Thus, targeting the tumor microenvironment is an attractive approach for cancer therapy. Arguably, anti-vascular endothelial growth factor (anti-VEGF) therapies hold great promise, but their efficacy has been modest, likely owing to redundant and complementary angiogenic pathways. Components of platelet-derived growth factor (PDGF), fibroblast growth factor (FGF), epidermal growth factor (EGF), and other pathways may compensate for VEGF blockade and allow angiogenesis to occur despite anti-VEGF treatment. In addition, hypoxia induced by anti-angiogenesis therapy modifies signaling pathways in tumor and stromal cells, which induces resistance to therapy. Because of tumor cell heterogeneity and angiogenic pathway redundancy, combining cytotoxic and targeted therapies or combining therapies targeting different pathways can potentially overcome resistance. Although targeted therapy is showing promise, much more work is needed to maximize its impact, including the discovery of new targets and identification of individuals most likely to benefit from such therapies.

The miR-199a/Brm/EGR1 axis is a determinant of anchorage-independent growth in epithelial tumor cell lines

In epithelial cells, miRNA-199a-5p/-3p and Brm, a catalytic subunit of the SWI/SNF complex were previously shown to form a double-negative feedback loop through EGR1, by which human cancer cell lines tend to fall into either of the steady states, types 1 [miR-199a(−)/Brm(+)/EGR1(−)] and 2 [miR-199a(+)/Brm (−)/EGR1(+)]. We show here, that type 2 cells, unlike type 1, failed to form colonies in soft agar, and that CD44, MET, CAV1 and CAV2 (miR-199a targets), all of which function as plasma membrane sensors and can co-localize in caveolae, are expressed specifically in type 1 cells. Single knockdown of any of them suppressed anchorage-independent growth of type 1 cells, indicating that the miR-199a/Brm/EGR1 axis is a determinant of anchorage-independent growth. Importantly, two coherent feedforward loops are integrated into this axis, supporting the robustness of type 1-specific gene expression and exemplifying how the miRNA-target gene relationship can be stably sustained in a variety of epithelial tumors.

Hepatocyte growth factor: A regulator of inflammation and autoimmunity

Hepatocyte growth factor (HGF) is a pleiotropic cytokine that has been extensively studied over several decades, but was only recently recognized as a key player in mediating protection of many types of inflammatory and autoimmune diseases. HGF was reported to prevent and attenuate disease progression by influencing multiple pathophysiological processes involved in inflammatory and immune response, including cell migration, maturation, cytokine production, antigen presentation, and T cell effector function. In this review, we discuss the actions and mechanisms of HGF in inflammation and immunity and the therapeutic potential of this factor for the treatment of inflammatory and autoimmune diseases.

Enhanced prediction of Src homology 2 (SH2) domain binding potentials using a fluorescence polarization-derived c-Met, c-Kit, ErbB, and androgen receptor interactome

Many human diseases are associated with aberrant regulation of phosphoprotein signaling networks. Src homology 2 (SH2) domains represent the major class of protein domains in metazoans that interact with proteins phosphorylated on the amino acid residue tyrosine. Although current SH2 domain prediction algorithms perform well at predicting the sequences of phosphorylated peptides that are likely to result in the highest possible interaction affinity in the context of random peptide library screens, these algorithms do poorly at predicting the interaction potential of SH2 domains with physiologically derived protein sequences. We employed a high throughput interaction assay system to empirically determine the affinity between 93 human SH2 domains and phosphopeptides abstracted from several receptor tyrosine kinases and signaling proteins. The resulting interaction experiments revealed over 1000 novel peptide-protein interactions and provided a glimpse into the common and specific interaction potentials of c-Met, c-Kit, GAB1, and the human androgen receptor. We used these data to build a permutation-based logistic regression classifier that performed considerably better than existing algorithms for predicting the interaction potential of several SH2 domains.

Role of met axis in head and neck cancer

Head and neck cancer is the sixth most common type of cancer worldwide. Despite advances in aggressive multidisciplinary treatments, the 5-year survival rate for this dreadful disease is only 50%, mostly due to high rate of recurrence and early involvement of regional lymph nodes and subsequent metastasis. Understanding the molecular mechanisms responsible for invasion and metastasis is one of the most pressing goals in the field of head and neck cancer. Met, also known as hepatocyte growth factor receptor (HGFR), is a member of the receptor protein tyrosine kinase (RPTK) family. There is compelling evidence that Met axis is dysregulated and plays important roles in tumorigenesis, progression, metastasis, angiogenesis, and drug resistance in head and neck cancer. We describe in this review current understanding of Met axis in head and neck cancer biology and development of therapeutic inhibitors targeting Met axis.

Functional role and analysis of cysteine residues of the salt tolerance protein Sod2

Sod2 is the major salt tolerance plasma membrane protein of Schizosaccharomyces pombe. It functions to remove excess intracellular sodium (or lithium) in exchange for protons. We investigated the role of cysteine residues and created a cysteine-free Sod2 protein. Each cysteine residue of the ten present was individually mutated to serine and the different proteins expressed and characterized in S. pombe. Western blotting revealed that all the individual mutant proteins were expressed. We examined the ability of the mutant proteins to confer salt tolerance to S. pombe with the endogenous Sod2 protein deleted. Only proteins with C26S and C374S mutations were partially reduced in their ability to confer salt tolerance. Additionally, they showed a change in conformation in comparison to the wild-type protein, indicated by differential sensitivity to trypsin. Deletion of all the cysteine residues of Sod2 resulted in a functional protein that was expressed in S. pombe at levels similar to the wild type and also conferred salt tolerance. The conformation of the cysteine-free Sod2 protein was not altered relative to the wild-type protein. We examined the accessibility of amino acids of the cysteineless protein present on putative extracellular loop 2. A cysteine placed at position Ala119 was accessible to externally applied [2-(trimethylammonium)ethyl] methane thiosulfonate bromide. The results demonstrate that cysteines in the Sod2 protein can be changed to serine residues resulting in an expressed, functional protein. The utility of the cysteine-free Sod2 protein for determination of topology and amino acid accessibility is demonstrated.

Suppressor of Cytokine Signaling (SOCS) 5 utilises distinct domains for regulation of JAK1 and interaction with the adaptor protein Shc-1

Suppressor of Cytokine Signaling (SOCS)5 is thought to act as a tumour suppressor through negative regulation of JAK/STAT and epidermal growth factor (EGF) signaling. However, the mechanism/s by which SOCS5 acts on these two distinct pathways is unclear. We show for the first time that SOCS5 can interact directly with JAK via a unique, conserved region in its N-terminus, which we have termed the JAK interaction region (JIR). Co-expression of SOCS5 was able to specifically reduce JAK1 and JAK2 (but not JAK3 or TYK2) autophosphorylation and this function required both the conserved JIR and additional sequences within the long SOCS5 N-terminal region. We further demonstrate that SOCS5 can directly inhibit JAK1 kinase activity, although its mechanism of action appears distinct from that of SOCS1 and SOCS3. In addition, we identify phosphoTyr317 in Shc-1 as a high-affinity substrate for the SOCS5-SH2 domain and suggest that SOCS5 may negatively regulate EGF and growth factor-driven Shc-1 signaling by binding to this site. These findings suggest that different domains in SOCS5 contribute to two distinct mechanisms for regulation of cytokine and growth factor signaling.

Structural and functional analysis of transmembrane segment IV of the salt tolerance protein Sod2

Sod2 is the plasma membrane Na(+)/H(+) exchanger of the fission yeast Schizosaccharomyces pombe. It provides salt tolerance by removing excess intracellular sodium (or lithium) in exchange for protons. We examined the role of amino acid residues of transmembrane segment IV (TM IV) ((126)FPQINFLGSLLIAGCITSTDPVLSALI(152)) in activity by using alanine scanning mutagenesis and examining salt tolerance in sod2-deficient S. pombe. Two amino acids were critical for function. Mutations T144A and V147A resulted in defective proteins that did not confer salt tolerance when reintroduced into S. pombe. Sod2 protein with other alanine mutations in TM IV had little or no effect. T144D and T144K mutant proteins were inactive; however, a T144S protein was functional and provided lithium, but not sodium, tolerance and transport. Analysis of sensitivity to trypsin indicated that the mutations caused a conformational change in the Sod2 protein. We expressed and purified TM IV (amino acids 125-154). NMR analysis yielded a model with two helical regions (amino acids 128-142 and 147-154) separated by an unwound region (amino acids 143-146). Molecular modeling of the entire Sod2 protein suggested that TM IV has a structure similar to that deduced by NMR analysis and an overall structure similar to that of Escherichia coli NhaA. TM IV of Sod2 has similarities to TM V of the Zygosaccharomyces rouxii Na(+)/H(+) exchanger and TM VI of isoform 1 of mammalian Na(+)/H(+) exchanger. TM IV of Sod2 is critical to transport and may be involved in cation binding or conformational changes of the protein.

Role of GRB2-associated binder 1 in epidermal growth factor receptor-induced signaling in head and neck squamous cell carcinoma

The epidermal growth factor receptor (EGFR) plays an important role in the pathogenesis of head and neck squamous cell carcinoma (HNSCC). Despite the high expression of EGFR in HNSCC, EGFR inhibitors have only limited success as monotherapy. The Grb2-associated binder (GAB) family of adaptor proteins acts as docking/scaffolding molecules downstream of tyrosine kinase receptors. We hypothesized that GAB1 may amplify EGFR-induced signaling in HNSCCs and therefore could play a role in the reduced sensitivity of HNSCC to EGFR inhibitors. We used representative human HNSCC cell lines overexpressing wild type EGFR, and expressing GAB1 but not GAB2. We demonstrated that baseline Akt and MAPK signaling were reduced in HNSCC cells in which GAB1 expression was reduced. Furthermore, the maximal EGF-induced activation of the Akt and MAPK pathway was reduced and delayed, and the duration of the EGF-induced activation of these pathways was reduced in cells with GAB1 knock-down. In agreement with this, HNSCC cells in which GAB1 levels were reduced showed an increased sensitivity to the EGFR inhibitor gefitinib. Our work demonstrates that GAB1 plays an important role as part of the mechanism of by which EGFR induces induced activation of the MAPK and AKT pathway. Our results identify GAB1 as an amplifier of the EGFR-initiated signaling, which may also interfere with EGFR degradation. These findings support the emerging notion that reducing GAB1 function may sensitize HNSCC to EGFR inhibitors, hence representing a new therapeutic target HNSCC treatment in combination with EGFR targeting agents.

Structural insights into Met receptor activation

The receptor tyrosine kinase Met plays a pivotal role in vertebrate development and tissue regeneration, its deregulation contributes to cancer. Met is also targeted during the infection by the facultative intracellular bacterium Listeria monocytogenes. The mechanistic basis for Met activation by its natural ligand hepatocyte growth factor/scatter factor (HGF/SF) is only beginning to be understood at a structural level. Crystal structures of Met in complex with L. monocytogenes InlB suggest that Met dimerization by this bacterial invasion protein is mediated by a dimer contact of the ligand. Here, I review the structural basis of Met activation by InlB and highlight parallels and differences to the physiological Met ligand HGF/SF and its splice variant NK1.

Alkali metal cation transport and homeostasis in yeasts

The maintenance of appropriate intracellular concentrations of alkali metal cations, principally K(+) and Na(+), is of utmost importance for living cells, since they determine cell volume, intracellular pH, and potential across the plasma membrane, among other important cellular parameters. Yeasts have developed a number of strategies to adapt to large variations in the concentrations of these cations in the environment, basically by controlling transport processes. Plasma membrane high-affinity K(+) transporters allow intracellular accumulation of this cation even when it is scarce in the environment. Exposure to high concentrations of Na(+) can be tolerated due to the existence of an Na(+), K(+)-ATPase and an Na(+), K(+)/H(+)-antiporter, which contribute to the potassium balance as well. Cations can also be sequestered through various antiporters into intracellular organelles, such as the vacuole. Although some uncertainties still persist, the nature of the major structural components responsible for alkali metal cation fluxes across yeast membranes has been defined within the last 20 years. In contrast, the regulatory components and their interactions are, in many cases, still unclear. Conserved signaling pathways (e.g., calcineurin and HOG) are known to participate in the regulation of influx and efflux processes at the plasma membrane level, even though the molecular details are obscure. Similarly, very little is known about the regulation of organellar transport and homeostasis of alkali metal cations. The aim of this review is to provide a comprehensive and up-to-date vision of the mechanisms responsible for alkali metal cation transport and their regulation in the model yeast Saccharomyces cerevisiae and to establish, when possible, comparisons with other yeasts and higher plants.

Regulated expansion of human pancreatic beta-cells

Although pancreatic beta-cell transplantation may serve as a potential cure for diabetes mellitus (DM), limited donor tissue availability poses a major challenge. Thus, there is a great demand to find new sources for pancreatic beta-cells. Here, we present a lentiviral vector-based approach to achieve beta-cell proliferation through the beta-cell-specific activation of the hepatocyte growth factor (HGF)/cmet signaling pathway. The methodology is based on the beta-cell-specific expression of a ligand-inducible, chimeric receptor (F36Vcmet), under transcriptional control of the promoter from the human insulin gene, and its ability to induce HGF/cmet signaling in the presence of a synthetic ligand (AP20187). High transduction efficiency of human pancreatic islets was achieved utilizing this approach with chimeric receptor expression confined to the beta-cell population. In addition, specific proliferation of human pancreatic beta-cells was induced utilizing this approach. Selective, regulated beta-cell expansion may help to provide greater availability of cells for transplantation in patients with DM.

Effects of Src inhibitors on cell growth and epidermal growth factor receptor and MET signaling in gefitinib-resistant non-small cell lung cancer cells with acquired MET amplification

The efficacy of epidermal growth factor receptor (EGFR)-tyrosine kinase inhibitors such as gefitinib and erlotinib in non-small cell lung cancer (NSCLC) is often limited by the emergence of drug resistance conferred either by a secondary T790M mutation of EGFR or by acquired amplification of the MET gene. We now show that the extent of activation of the tyrosine kinase Src is markedly increased in gefitinib-resistant NSCLC (HCC827 GR) cells with MET amplification compared with that in the gefitinib-sensitive parental (HCC827) cells. In contrast, the extent of Src activation did not differ between gefitinib-resistant NSCLC (PC9/ZD) cells harboring the T790M mutation of EGFR and the corresponding gefitinib-sensitive parental (PC9) cells. This activation of Src in HCC827 GR cells was largely abolished by the MET-TKI PHA-665752 but was only partially inhibited by gefitinib, suggesting that Src activation is more dependent on MET signaling than on EGFR signaling in gefitinib-resistant NSCLC cells with MET amplification. Src inhibitors blocked Akt and Erk signaling pathways, resulting in both suppression of cell growth and induction of apoptosis, in HCC827 GR cells as effectively as did the combination of gefitinib and PHA-665752. Furthermore, Src inhibitor dasatinib inhibited tumor growth in HCC827 GR xenografts to a significantly greater extent than did treatment with gefitinib alone. These results provide a rationale for clinical targeting of Src in gefitinib-resistant NSCLC with MET amplification.

c-Met activation in medulloblastoma induces tissue factor expression and activity: effects on cell migration

Met, the receptor for hepatocyte growth factor (HGF), is a receptor tyrosine kinase that has recently emerged as an important contributor to human neoplasia. In physiological and pathological conditions, Met triggers various cellular functions related to cell proliferation, cell migration and the inhibition of apoptosis, and also regulates a genetic program leading to coagulation. Since medulloblastomas (MBs) express high levels of tissue factor (TF), the main initiator of blood coagulation, we therefore examined the link between Met and TF expression in these pediatric tumors. We observed that stimulation of the MB cell line DAOY with HGF led to a marked increase of TF expression and procoagulant activity, in agreement with analysis of clinical MB tumor specimens, in which tumors expressing high levels of Met also showed high levels of TF. The HGF-dependent increase in TF expression and activity required Src family kinases and led to the translocation of TF to actin-rich structures at the cell periphery, suggesting a role of the protein in cell migration. Accordingly, addition of physiological concentrations of the TF activator factor VIIa (FVII) to HGF-stimulated DAOY cells promoted a marked increase in the migratory potential of these cells. Overall, these results suggest that HGF-induced activation of the Met receptor results in TF expression by MB cells and that this event probably contribute to tumor proliferation by enabling the formation of a provisional fibrin matrix. In addition, TF-mediated non-hemostatic functions, such as migration toward FVIIa, may also play a central role in MB aggressiveness.

Phosphorylation of MUC1 by Met modulates interaction with p53 and MMP1 expression

MUC1, a transmembrane mucin, is a key modulator of several signaling pathways that affect oncogenesis, motility, and cell morphology. The interaction of MUC1 cytoplasmic tail (MUC1CT) with signal transducers and its nuclear translocation and subsequent biological responses are believed to be regulated by phosphorylation status, but the precise mechanisms by which this occurs remain poorly defined. We detected a novel association between the Met receptor tyrosine kinase and the MUC1CT. Met catalyzed phosphorylation of tyrosine at YHPM in the MUC1CT. Stimulation of S2-013.MUC1F pancreatic cancer cells with hepatocyte growth factor facilitated nuclear localization of MUC1CT, as determined by real time confocal imaging analysis. MUC1 overexpression also facilitated faster turnover of Met. Phosphorylation of MUC1CT by Met enhanced its interaction with p53, which led to suppression of AP1 transcription factor activity through interactions at the MMP1 promoter, ultimately leading to reduced transcription of MMP1. This correlated with a decrease in hepatocyte growth factor-induced invasiveness when MUC1 was overexpressed. The results demonstrate that MUC1 modulates Met-mediated oncogenic signaling in cancer.

Intranasal HGF administration ameliorates the physiologic and morphologic changes in lung emphysema

Hepatocyte growth factor (HGF) has multiple biological effects on stem cells, epithelial proliferation, and wound healing. In this study, we investigated a possible therapeutic benefit of intranasal HGF on elastase-induced emphysema, and assessed the role of stem/progenitor cells in this process. HGF was given twice a week for 1-4 weeks after the establishment of emphysema in mice. HGF inhalation significantly ameliorated the enlargement of airspaces and alveolar wall destruction. Also, elevated static lung compliance returned to control levels within 2 weeks of HGF treatment. The expressions of stem-cell markers, c-kit, stem-cell antigen 1 (Sca-1), and CD34 were also significantly influenced by HGF. Most of the c-kit(+) cells were bone marrow derived, while most Sca-1(+) were lung endogenous cells. CD34(+) cells were from both sources, and a portion of the endogenous CD34(+) cells was also Sca-1(+). Further, HGF increased the expression levels of proliferating cell nuclear antigen (PCNA) and cytokeratin-19. Also, their immunohistochemical staining patterns were colocalized, indicative of epithelial multiplication. The results of the study show that intranasal treatment with HGF reverses both the physiological and morphometric changes of lung emphysema, possibly through stem-cell mobilization and alveolar regeneration, providing a nonsurgical treatment and suggesting the possibility of achieving a similar effect in humans.

Crosstalk between the alpha2beta1 integrin and c-met/HGF-R regulates innate immunity

Data from several investigators suggest that the alpha2beta1 integrin, a receptor for collagens, laminins, decorin, E-cadherin, matrix metalloproteinase-1, endorepellin, and several viruses, is required for innate immunity and regulation of autoimmune/allergic disorders. We demonstrated that the innate immune response to Listeria monocytogenes required alpha2beta1 integrin expression by peritoneal mast cells (PMCs). Ligation of the alpha2beta1 integrin by C1q contained in immune complexes comprised of Listeria and antibody was required for PMC activation in vitro and in vivo. However, ligation of the alpha2beta1 integrin alone was insufficient to activate cytokine secretion, suggesting that one or more additional signals emanating from a coreceptor were required for PMC activation. Here, we demonstrate that C1q, but neither other complement proteins nor FcRgamma, is required for early innate immune response to Listeria. The binding of Listeria's Internalin B (InlB) to hepatocyte growth factor receptor (HGF-R)/c-met provides the costimulatory function required for PMC activation. Either HGF or Listeria InlB bound to c-met and either C1q or type I collagen bound to alpha2beta1 integrin stimulates PMC activation. These findings suggest that crosstalk between c-met and the alpha2beta1 integrin may contribute to mast-cell activation in autoimmune and inflammatory disorders.

JAK-STAT pathway in carcinogenesis: Is it relevant to cholangiocarcinoma progression

The features of JAK-STAT signaling in liver cells are discussed in the current review. The role of this signaling cascade in carcinogenesis is accentuated. The possible involvement of this pathway and alteration of its elements are compared for normal cholangiocytes, cholangiocarcinoma predisposition and development. Prolactin and interleukin-6 are described in detail as the best studied examples. In addition, the non-classical nuclear translocation of cytokine receptors is discussed in terms of its possible implication to cholangiocarcinoma development.

Hepatocyte growth factor and c-Met promote dendritic maturation during hippocampal neuron differentiation via the Akt pathway

During central nervous system development, growth factors and their associated receptor protein tyrosine kinases regulate many neuronal functions such as neurite extension and dendrite maturation. Hepatocyte growth factor (HGF) and its receptor, c-Met, can promote formation of neurites and enhance elaboration of dendrites in mature neurons, but their effects on the early stages of dendrite maturation in hippocampal neurons and the signaling pathways by which they promote dendrite formation have not been studied. Exogenous HGF treatment effectively enhanced the phosphorylation and activation of c-Met in cultured hippocampal neurons at 4 days in vitro. HGF treatment increased the number of dendrites and promoted dendrite elongation in these neurons. Consistent with these results, HGF activated Akt, which phosphorylates glycogen synthase kinase-3beta (GSK-3beta) to inactivate it, and reduced phosphorylation of microtubule-associated protein 2 (MAP2), which can promote microtubule polymerization and dendrite elongation when dephosphorylated. Conversely, pharmacological inhibition of c-Met with its specific inhibitor, PHA-665752, or genetic knock-down of c-Met with short hairpin RNAs (shRNAs) suppressed HGF-induced phosphorylation of Akt and GSK-3beta, increased phosphorylation of MAP2, and reduced dendrite number and length in cultured hippocampal neurons. Moreover, suppressing c-Met with PHA-665752 or by shRNA decreased MAP2 expression. Inhibiting Akt activity with the phosphoinositide-3-kinase inhibitor LY294002 or Akt inhibitor X suppressed HGF-induced phosphorylation of GSK-3beta, increased MAP2 phosphorylation, and blocked the ability of HGF to enhance dendritic length. These observations indicate that HGF and c-Met can regulate the early stages of dendrite maturation via activation of the Akt/GSK-3beta pathway.

Lysophosphatidic acid modulates c-Met redistribution and hepatocyte growth factor/c-Met signaling in human bronchial epithelial cells through PKC delta and E-cadherin

Previously we demonstrated that ligation of lysophosphatidic acid (LPA) to G protein-coupled LPA receptors induces transactivation of receptor tyrosine kinases (RTKs), such as platelet-derived growth factor receptor beta (PDGF-Rbeta) and epidermal growth factor receptor (EGF-R), in primary cultures of human bronchial epithelial cells (HBEpCs). Here we examined the role of LPA on c-Met redistribution and modulation of hepatocyte growth factor (HGF)/c-Met pathways in HBEpCs. Treatment of HBEpCs with LPA-induced c-Met serine phosphorylation and redistribution to plasma membrane, while treatment with HGF-induced c-Met internalization. Pretreatment with LPA reversed HGF-induced c-Met internalization. Overexpression of dominant negative (Dn)-PKC delta or pretreatment with Rottlerin or Pertussis toxin (PTx) attenuated LPA-induced c-Met serine phosphorylation and redistribution. Co-immnuoprecipitation and immunocytochemistry showed that E-cadherin interacted with c-Met in HBEpCs. LPA treatment induced E-cadherin and c-Met complex redistribution to plasma membranes. Overexpression of Dn-PKC delta attenuated LPA-induced E-cadherin redistribution and E-cadherin siRNA attenuated LPA-induced c-Met redistribution to plasma membrane. Furthermore, pretreatment of LPA attenuated HGF-induced c-Met tyrosine phosphorylation and downstream signaling, such as Akt kinase phosphorylation and cell motility. These results demonstrate that LPA regulates c-Met function through PKC delta and E-cadherin in HBEpCs, suggesting an alternate function of the cross-talk between G-protein-coupled receptors (GPCRs) and RTKs in HBEpCs.

Up-regulation of MET Expression by α-Melanocyte-stimulating Hormone and MITF Allows Hepatocyte Growth Factor to Protect Melanocytes and Melanoma Cells from Apoptosis

The MET proto-oncogene encodes for the hepatocyte growth factor (HGF) receptor, a plasma membrane tyrosine kinase that is involved in melanocyte growth and melanoma development. In mouse melanoma cells, Met expression is increased by alphaMSH via the activation of the cAMP pathway. However, the mechanism by which cAMP regulates MET and the biological consequences of this increase were not known. In the present report, we show that alphaMSH regulates MET expression in both human melanocytes and mouse melanoma cells through a transcriptional mechanism that requires MITF. Furthermore, the adenovirus driven expression of MITF is sufficient to increase MET in melanoma cells. Functional analysis of the MET promoter allows us to identify an E-box motif conserved in both human and mouse promoter that mediates the effect of MITF. Interestingly, up-regulation of MET expression by cAMP leads to an exacerbated HGF signaling and allows HGF to protect melanocytes and melanoma cells from apoptosis. Thus, physiological stimuli or pathological events that would induce MITF expression may lead to increased MET expression thereby favoring melanoma survival. These observations strengthen the roles of MITF and MET in melanoma development.

Tyrosine phosphorylation of p145met mediated by EGFR and Src is required for serum-independent survival of human bladder carcinoma cells

Here we address the molecular mechanism of serum-independent survival and growth of human bladder carcinoma cell line 5637. Serum starvation promoted tyrosine phosphorylation of a 145-kDa protein and activation of the tyrosine kinase Src and the receptor for epidermal growth factor (EGFR) over a slow time course (>8 hours). The phosphorylated 145-kDa protein was identified as the beta-subunit of c-Met/hepatocyte growth factor (HGF) receptor, p145(met), in which tyrosine residues 1003, 1234, and 1235 were phosphorylated. Inhibitors of Src (PP2, SU6656) or EGFR (AG99), but not p145(met) (K252a), effectively blocked tyrosine phosphorylation of p145(met) and promoted cell death accompanied by activation of caspase-like proteases. Conditioned medium from the serum-starved 5637 cells or purified EGF readily promoted the activation of Src and EGFR, and tyrosine phosphorylation of p145(met) in normally grown 5637 cells, suggesting that autocrine signaling of EGFR ligands is responsible for signal transduction events in serum-starved cells. Consistent with this idea, a monoclonal antibody against EGFR that would interfere with the ligand binding to EGFR blocked tyrosine phosphorylation events and promoted the caspase activation and cell death in serum-free conditions. Such apoptotic cell death was also induced by pretreatment of cells with a high concentration of HGF that downregulated endogenous p145(met). Nevertheless, Cu2+ ions, competitive inhibitors for HGF-binding to p145(met), did not show any effect on cellular functions in serum-free conditions. These results suggest that the serum-independent growth of 5637 cells involves the transmembrane signaling cascade via EGFR ligand(s) (but not HGF), EGFR, Src and p145(met).

c-jun amino-terminal kinase and mitogen activated protein kinase 1/2 mediate hepatocyte growth factor-induced migration of brain endothelial cells

Hepatocyte growth factor (HGF) influences several components of the angiogenic response, including endothelial cell migration. While recent studies indicate a crucial role of HGF in brain angiogenesis, the signaling pathways that regulate brain endothelial cell migration by HGF remain uncharacterized. Herein, we report that HGF stimulated human brain microvascular endothelial cell (HBMEC) migration in a dose- and time-dependent manner. Challenge of HBMECs with HGF activated the c-jun amino-terminal kinase (JNK), increased phosphorylation of the proline-rich tyrosine kinase 2 (Pyk-2) at Tyr(402) and activated c-Src. Inhibition of JNK by SP600125 or expression of a dominant negative JNK1 construct abrogated the migratory response of HBMECs to HGF. Treatment of HBMECs with the Src inhibitor PP2 markedly decreased HGF-stimulated JNK activation and migration to HGF. Moreover, expression of a mutant Pyk-2 construct prevented HGF-induced Pyk-2 phosphorylation at Tyr(402) and stimulation of HBMEC migration. Next, we examined activation of the extracellular signal regulated kinase (ERK) pathway. Stimulation of HBMECs by HGF led to rapid activation of ERK1/2, phosphorylation of Raf-1 at Ser(338) and Tyr(340/341) and MEK1/2 at Ser(222). Moreover, inhibition of ERK activation by UO126 and PD98059 markedly decreased HGF-stimulated HBMEC migration. HGF also activated AKT, while inhibition of AKT by LY294002 induced a modest decrease of HGF-induced HBMEC migration. These results highlight a model whereby JNK and ERK play a critical role in regulation of brain endothelial cell migration by HGF.