C-Met

c-Met, also called MET and hepatocyte growth factor receptor (HGFR), is a is_associated_with::protein that in humans is encoded by the MET gene (MET proto-oncogene, receptor tyrosine kinase), which earlier in the discovery process had also been called MNNG HOS transforming gene. The protein possesses is_associated_with::tyrosine kinase activity. The primary single chain precursor protein is post-translationally cleaved to produce the alpha and beta subunits, which are disulfide linked to form the mature receptor.

MET is a membrane receptor that is essential for embryonic development and wound healing. is_associated_with::Hepatocyte growth factor (HGF) is the only known ligand of the MET receptor. MET is normally expressed by cells of epithelial origin, while expression of HGF is restricted to cells of mesenchymal origin. Upon HGF stimulation, MET induces several biological responses that collectively give rise to a program known as invasive growth.

Abnormal MET activation in cancer correlates with poor prognosis, where aberrantly active MET triggers tumor growth, formation of new blood vessels (is_associated_with::angiogenesis) that supply the tumor with nutrients, and cancer spread to other organs (is_associated_with::metastasis). MET is deregulated in many types of human malignancies, including cancers of kidney, liver, stomach, breast, and brain. Normally, only is_associated_with::stem cells and is_associated_with::progenitor cells express MET, which allows these cells to grow invasively in order to generate new tissues in an embryo or regenerate damaged tissues in an adult. However, is_associated_with::cancer stem cells are thought to hijack the ability of normal stem cells to express MET, and thus become the cause of cancer persistence and spread to other sites in the body.

Various mutations in the MET gene are associated with papillary renal carcinoma.

Gene
MET proto-oncogene (GeneID: 4233) has a total length of 125,982 bp, and it is located in the 7q31 locus of chromosome 7. MET is transcribed into a 6,641 bp mature mRNA, which is then translated into a 1,390 amino-acid MET protein.

Protein


MET is a is_associated_with::receptor tyrosine kinase (RTK) that is produced as a single-chain precursor. The precursor is proteolytically cleaved at a is_associated_with::furin site to yield a highly glycosylated extracellular α-subunit and a transmembrane β-subunit, which are linked together by a is_associated_with::disulfide bridge.

Extracellular

 * Region of homology to is_associated_with::semaphorins (Sema domain), which includes the full α-chain and the N-terminal part of the β-chain


 * Cysteine-rich MET-related sequence (MRS domain)


 * Glycine-proline-rich repeats (G-P repeats)


 * Four immunoglobulin-like structures (Ig domains), a typical protein-protein interaction region.

Intracellular
A Juxtamembrane segment that contains:


 * a serine residue (Ser 985), which inhibits the receptor kinase activity upon phosphorylation


 * a tyrosine (Tyr 1003), which is responsible for MET is_associated_with::polyubiquitination, is_associated_with::endocytosis, and degradation upon interaction with the is_associated_with::ubiquitin ligase CBL


 * Tyrosine kinase domain, which mediates MET biological activity. Following MET activation, transphosphorylation occurs on Tyr 1234 and Tyr 1235


 * C-terminal region contains two crucial tyrosines (Tyr 1349 and Tyr 1356), which are inserted into the multisubstrate docking site, capable of recruiting downstream adapter proteins with Src homology-2 (SH2) domains. The two tyrosines of the docking site have been reported to be necessary and sufficient for the signal transduction both in vitro.

MET signaling pathway


MET activation by its ligand HGF induces MET kinase catalytic activity, which triggers transphosphorylation of the tyrosines Tyr 1234 and Tyr 1235. These two tyrosines engage various signal transducers, thus initiating a whole spectrum of biological activities driven by MET, collectively known as the invasive growth program. The transducers interact with the intracellular multisubstrate docking site of MET either directly, such as is_associated_with::GRB2, SHC, SRC, and the p85 regulatory subunit of phosphatidylinositol-3 kinase (is_associated_with::PI3K), or indirectly through the scaffolding protein Gab1

Tyr 1349 and Tyr 1356 of the multisubstrate docking site are both involved in the interaction with GAB1, SRC, and SHC, while only Tyr 1356 is involved in the recruitment of GRB2, phospholipase C γ (PLC-γ), p85, and SHP2.

GAB1 is a key coordinator of the cellular responses to MET and binds the MET intracellular region with high is_associated_with::avidity, but low affinity. Upon interaction with MET, GAB1 becomes phosphorylated on several tyrosine residues which, in turn, recruit a number of signalling effectors, including is_associated_with::PI3K, SHP2, and PLC-γ. GAB1 phosphorylation by MET results in a sustained signal that mediates most of the downstream signaling pathways.

Activation of signal transduction
MET engagement activates multiple is_associated_with::signal transduction pathways:


 * The RAS pathway mediates HGF-induced is_associated_with::scattering and proliferation signals, which lead to is_associated_with::branching morphogenesis. Of note, HGF, differently from most is_associated_with::mitogens, induces sustained RAS activation, and thus prolonged MAPK activity.


 * The PI3K pathway is activated in two ways: PI3K can be either downstream of RAS, or it can be recruited directly through the multifunctional docking site. Activation of the PI3K pathway is currently associated with is_associated_with::cell motility through remodeling of adhesion to the extracellular matrix as well as localized recruitment of transducers involved in cytoskeletal reorganization, such as is_associated_with::RAC1 and PAK. PI3K activation also triggers a survival signal due to activation of the is_associated_with::AKT pathway.


 * The STAT pathway, together with the sustained MAPK activation, is necessary for the HGF-induced is_associated_with::branching morphogenesis. MET activates the is_associated_with::STAT3 is_associated_with::transcription factor directly, through an is_associated_with::SH2 domain.


 * The is_associated_with::beta-catenin pathway, a key component of the is_associated_with::Wnt signaling pathway, translocates into the nucleus following MET activation and participates in transcriptional regulation of numerous genes.


 * The Notch pathway, through transcriptional activation of Delta ligand (see is_associated_with::DLL3).



Role in development
MET mediates a complex program known as invasive growth. Activation of MET triggers is_associated_with::mitogenesis, and is_associated_with::morphogenesis.

During is_associated_with::embryonic development, transformation of the flat, two-layer is_associated_with::germinal disc into a three-dimensional body depends on transition of some cells from an epithelial is_associated_with::phenotype to spindle-shaped cells with motile behaviour, a mesenchymal is_associated_with::phenotype. This process is referred to as is_associated_with::epithelial-mesenchymal transition (EMT). Later in embryonic development, MET is crucial for is_associated_with::gastrulation, is_associated_with::angiogenesis, is_associated_with::myoblast migration, is_associated_with::bone remodeling, and is_associated_with::nerve sprouting among others. MET is essential for is_associated_with::embryogenesis, because MET −/− mice die in utero due to severe defects in placental development. Furthermore, MET is required for such critical processes as is_associated_with::liver regeneration and is_associated_with::wound healing during adulthood.

Tissue distribution
MET is normally expressed by is_associated_with::epithelial cells. However, MET is also found on is_associated_with::endothelial cells, is_associated_with::neurons, is_associated_with::hepatocytes, is_associated_with::hematopoietic cells, and is_associated_with::melanocytes. HGF expression is restricted to cells of is_associated_with::mesenchymal origin.

Transcriptional control
MET transcription is activated by HGF and several is_associated_with::growth factors. MET promoter has four putative binding sites for Ets, a family of is_associated_with::transcription factors that control several invasive growth genes. is_associated_with::ETS1 activates MET transcription in vitro. MET transcription is activated by is_associated_with::hypoxia-inducible factor 1 (HIF1), which is activated by low concentration of intracellular oxygen. HIF1 can bind to one of the several hypoxia response elements (HREs) in the MET promoter. Hypoxia also activates transcription factor AP-1, which is involved in MET transcription.

Role in cancer
MET pathway plays an important role in the development of cancer through:
 * activation of key oncogenic pathways (RAS, PI3K, is_associated_with::STAT3, is_associated_with::beta-catenin);
 * is_associated_with::angiogenesis (sprouting of new blood vessels from pre-existing ones to supply a tumor with nutrients);
 * scatter (cells dissociation due to is_associated_with::metalloprotease production), which often leads to is_associated_with::metastasis.

Coordinated down-regulation of both MET and its downstream effector extracellular signal-regulated kinase 2 (ERK2) by miR-199a* may be effective in inhibiting not only cell proliferation but also motility and invasive capabilities of tumor cells.

MET amplification has emerged as a potential biomarker of the clear cell tumor subtype.

The amplification of the is_associated_with::cell surface receptor MET drives resistance to anti-EGFR therapies in is_associated_with::colorectal cancer.

Role in Autism
The SFARIgene database lists MET with an is_associated_with::autism score of 2.0, which indicates that it is a strong candidate for playing a role in cases of autism. The database also identifies at least one study that found a role for MET in cases of is_associated_with::schizophrenia.

PTEN
PTEN (phosphatase and tensin homolog) is a is_associated_with::tumor suppressor gene encoding a protein PTEN, which possesses lipid and protein phosphatase-dependent as well as phosphatase-independent activities. PTEN protein is_associated_with::phosphatase is able to interfere with MET signaling by dephosphorylating either PIP3 generated by is_associated_with::PI3K, or the p52 isoform of SHC. SHC dephosphorylation inhibits recruitment of the is_associated_with::GRB2 adapter to activated MET.

VHL
There is evidence of correlation between inactivation of is_associated_with::VHL is_associated_with::tumor suppressor gene and increased MET signaling in is_associated_with::renal cell carcinoma (RCC).

Cancer therapies targeting HGF/MET


Since tumor invasion and metastasis are the main cause of death in cancer patients, interfering with MET signaling appears to be a promising therapeutic approach. A comprehensive list of HGF and MET targeted experimental therapeutics for oncology now in human clinical trials can be found here.

MET kinase inhibitors
Kinase inhibitors are low molecular weight molecules that prevent ATP binding to MET, thus inhibiting receptor transphosphorylation and recruitment of the downstream effectors. The limitations of kinase inhibitors include the facts that they only inhibit kinase-dependent MET activation, and that none of them is fully specific for MET.


 * is_associated_with::K252a (is_associated_with::Fermentek Biotechnology) is a is_associated_with::staurosporine analogue isolated from Nocardiopsis sp. soil is_associated_with::fungi, and it is a potent inhibitor of all is_associated_with::receptor tyrosine kinases (RTKs). At nanomolar concentrations, K252a inhibits both the is_associated_with::wild type and the mutant (M1268T) MET function.


 * SU11274 (is_associated_with::SUGEN) specifically inhibits MET kinase activity and its subsequent signaling. SU11274 is also an effective inhibitor of the M1268T and H1112Y MET mutants, but not the L1213V and Y1248H mutants. SU11274 has been demonstrated to inhibit HGF-induced motility and invasion of epithelial and carcinoma cells.


 * PHA-665752 (is_associated_with::Pfizer) specifically inhibits MET kinase activity, and it has been demonstrated to represses both HGF-dependent and constitutive MET phosphorylation. Furthermore, some tumors harboring MET amplifications are highly sensitive to treatment with PHA-665752.
 * ARQ197 (ArQule) is a promising selective inhibitor of MET, which entered a phase 2 clinical trial in 2008.
 * is_associated_with::Foretinib (XL880, Exelixis) targets multiple is_associated_with::receptor tyrosine kinases (RTKs) with growth-promoting and angiogenic properties. The primary targets of foretinib are MET, is_associated_with::VEGFR2, and KDR. Foretinib has completed a phase 2 clinical trials with indications for papillary is_associated_with::renal cell carcinoma, is_associated_with::gastric cancer, and is_associated_with::head and neck cancer.
 * SGX523 (SGX Pharmaceuticals) specifically inhibits MET at low nanomolar concentrations.
 * MP470 (SuperGen) is a novel inhibitor of is_associated_with::c-KIT, MET, is_associated_with::PDGFR, Flt3, and AXL. Phase I clinical trial of MP470 had been announced in 2007.

HGF inhibitors
Since HGF is the only known ligand of MET, formation of a HGF:MET complex blocks MET is_associated_with::biological activity. For this purpose, truncated HGF, anti-HGF neutralizing antibodies, and an uncleavable form of HGF have been utilized so far. The major limitation of HGF inhibitors is that they block only HGF-dependent MET activation.


 * NK4 competes with HGF as it binds MET without inducing receptor activation, thus behaving as a full is_associated_with::antagonist. NK4 is a molecule bearing the N-terminal hairpin and the four kringle domains of HGF. Moreover, NK4 is structurally similar to angiostatins, which is why it possesses anti-angiogenic activity.


 * Neutralizing anti-HGF antibodies were initially tested in combination, and it was shown that at least three is_associated_with::antibodies, acting on different HGF is_associated_with::epitopes, are necessary to prevent MET tyrosine kinase activation. More recently, it has been demonstrated that fully human is_associated_with::monoclonal antibodies can individually bind and neutralize human HGF, leading to regression of tumors in mouse models. Two anti-HGF antibodies are currently available: the humanized AV299 (AVEO), and the fully human AMG102 (is_associated_with::Amgen).


 * Uncleavable HGF is an engineered form of pro-HGF carrying a single amino-acid substitution, which prevents the maturation of the molecule. Uncleavable HGF is capable of blocking MET-induced biological responses by binding MET with high affinity and displacing mature HGF. Moreover, uncleavable HGF competes with the wild-type endogenous pro-HGF for the catalytic domain of is_associated_with::proteases that cleave HGF precursors. Local and systemic expression of uncleavable HGF inhibits tumor growth and, more importantly, prevents is_associated_with::metastasis.

Decoy MET
Decoy MET refers to a soluble truncated MET receptor. Decoys are able to inhibit MET activation mediated by both HGF-dependent and independent mechanisms, as decoys prevent both the ligand binding and the MET receptor homodimerization. CGEN241 (Compugen) is a decoy MET that is highly efficient in inhibiting tumor growth and preventing is_associated_with::metastasis in animal models.

Immunotherapy targeting MET
Drugs used for is_associated_with::immunotherapy can act either passively by enhancing the immunologic response to MET-expressing tumor cells, or actively by stimulating is_associated_with::immune cells and altering differentiation/growth of tumor cells.

Passive immunotherapy
Administering is_associated_with::monoclonal antibodies (mAbs) is a form of passive immunotherapy. MAbs facilitate destruction of tumor cells by complement-dependent cytotoxicity (CDC) and cell-mediated cytotoxicity (ADCC). In CDC, mAbs bind to specific is_associated_with::antigen, leading to activation of the is_associated_with::complement cascade, which in turn leads to formation of pores in tumor cells. In ADCC, the Fab domain of a mAb binds to a is_associated_with::tumor antigen, and Fc domain binds to Fc receptors present on effector cells (is_associated_with::phagocytes and is_associated_with::NK cells), thus forming a bridge between an effector and a target cells. This induces the effector cell activation, leading to is_associated_with::phagocytosis of the tumor cell by is_associated_with::neutrophils and is_associated_with::macrophages. Furthermore, is_associated_with::NK cells release is_associated_with::cytotoxic molecules, which lyse tumor cells.


 * DN30 is monoclonal anti-MET antibody that recognizes the extracellular portion of MET. DN30 induces both shedding of the MET is_associated_with::ectodomain as well as cleavage of the intracellular domain, which is successively degraded by is_associated_with::proteasome machinery. As a consequence, on one side MET is inactivated, and on the other side the shed portion of extracellular MET hampers activation of other MET receptors, acting as a decoy. DN30 inhibits tumour growth and prevents is_associated_with::metastasis in animal models.


 * OA-5D5 is one-armed monoclonal anti-MET antibody that was demonstrated to inhibit orthotopic pancreatic and glioblastoma tumor growth and to improve survival in tumor xenograft models. OA-5D5 is produced as a recombinant protein in Escherichia coli. It is composed of murine variable domains for the heavy and light chains with human IgG1 constant domains. The antibody blocks HGF binding to MET in a competitive fashion.

Active immunotherapy
is_associated_with::Active immunotherapy to MET-expressing tumors can be achieved by administering is_associated_with::cytokines, such as is_associated_with::interferons (IFNs) and is_associated_with::interleukins (IL-2), which triggers non-specific stimulation of numerous immune cells. IFNs have been tested as therapies for many types of cancers and have demonstrated therapeutic benefits. IL-2 has been approved by the U.S. Food and Drug Administration (FDA) for the treatment of is_associated_with::renal cell carcinoma and metastatic melanoma, which often have deregulated MET activity.

Interactions
Met has been shown to interact with:


 * CDH1,
 * is_associated_with::Cbl gene,
 * is_associated_with::GLMN,
 * is_associated_with::Grb2,
 * is_associated_with::Hepatocyte growth factor,
 * PTPmu, and
 * is_associated_with::RANBP9