MET signaling regulates glioblastoma stem cells

Glioblastomas multiforme (GBM) contain highly tumorigenic, self-renewing populations of stem/initiating cells [glioblastoma stem cells (GSC)] that contribute to tumor propagation and treatment resistance. However, our knowledge of the specific signaling pathways that regulate GSCs is limited. The MET tyrosine kinase is known to stimulate the survival, proliferation, and invasion of various cancers including GBM. Here, we identified a distinct fraction of cells expressing a high level of MET in human primary GBM specimens that were preferentially localized in perivascular regions of human GBM biopsy tissues and were found to be highly clonogenic, tumorigenic, and resistant to radiation. Inhibition of MET signaling in GSCs disrupted tumor growth and invasiveness both in vitro and in vivo, suggesting that MET activation is required for GSCs. Together, our findings indicate that MET activation in GBM is a functional requisite for the cancer stem cell phenotype and a promising therapeutic target.     

The MET axis as a therapeutic target

The MET receptor tyrosine kinase and its ligand hepatocyte growth factor (HGF) have been implicated in transformation of a variety of malignancies. Chronic or dysregulated activation of the MET/HGF pathway may lead to increased cell growth, invasion, angiogenesis, and metastasis, reduced apoptosis, altered cytoskeletal functions and other biological changes. It has been suggested that ligand activated MET stimulation can be sufficient for a transforming phenotype. In addition, amplification and activation mutations (germline and/or somatic) within the tyrosine kinase domain, juxtamembrane domain, or semaphorin domain have been identified for MET. MET gain-of-function mutations lead to either deregulated or prolonged tyrosine kinase activity, which are instrumental to its transforming activity. A number of therapeutic strategies targeting ligand-dependent activation or the kinase domain have been employed to inhibit MET. The different structural requirements for activation of signaling events and biological functions regulated by MET will be summarized. Therapeutic targets and current pre-clinical and clinical approaches will be described. Targeting the HGF/MET pathway, alone or in combination with standard therapies, is likely to improve present therapies in MET-dependent malignancies.     

MET as a target for treatment of chest tumors

The receptor tyrosine kinase MET has been studied of a large variety of human cancers, including lung and mesothelioma. The MET receptor and its ligand HGF (hepatocyte growth factor) play important roles in cell growth, survival and migration, and dysregulation of the HGF-MET pathway leads to oncogenic changes including tumor proliferation, angiogenesis and metastasis. In small cell lung cancer (SCLC), non-small cell lung cancer (NSCLC), and malignant pleural mesothelioma (MPM), MET is dysregulated via overexpression, constitutive activation, gene amplification, ligand-dependent activation, mutation or epigenetic mechanisms. New drugs targeted against MET and HGF are currently being investigated in vitro and in vivo, with promising results. These drugs function at a variety of steps within the HGF-MET pathway, including MET expression at the RNA or protein level, the ligand-receptor interaction, and tyrosine kinase function. This paper will review the structure, function, mechanisms of tumorigenesis, and potential for therapeutic inhibition of the MET receptor in lung cancer and mesothelioma.     

Multiple mutations and bypass mechanisms can contribute to development of acquired resistance to MET inhibitors

Therapies targeting receptor tyrosine kinases have shown efficacy in molecularly defined subsets of cancers. Unfortunately, cancers invariably develop resistance, and overcoming or preventing resistance will ultimately be key to unleashing their full therapeutic potential. In this study, we examined how cancers become resistant to MET inhibitors, a class of drugs currently under clinical development. We utilized the highly sensitive gastric carcinoma cell line, SNU638, and two related MET inhibitors PHA-665752 and PF-2341066. To our surprise, we observed at least two mechanisms of resistance that arose simultaneously. Both resulted in maintenance of downstream PI3K (phosphoinositide 3-kinase)-AKT and MEK (MAP/ERK kinase)-ERK signaling in the presence of inhibitor. One mechanism, observed by modeling resistance both in vitro and in vivo, involved the acquisition of a mutation in the MET activation loop (Y1230). Structural analysis indicates that this mutation destabilizes the autoinhibitory conformation of MET and abrogates an important aromatic stacking interaction with the inhibitor. The other cause of resistance was activation of the epidermal growth factor receptor (EGFR) pathway due to increased expression of transforming growth factor α. Activation of EGFR bypassed the need for MET signaling to activate downstream signaling in these cells. This resistance could be overcome by combined EGFR and MET inhibition. Thus, therapeutic strategies that combine MET inhibitors capable of inhibiting Y1230 mutant MET in combination with anti-EGFR-based therapies may enhance clinical benefit for patients with MET-addicted cancers. Importantly, these results also underscore the notion that a single cancer can simultaneously develop resistance induced by several mechanisms and highlight the daunting challenges associated with preventing or overcoming resistance.     

Ron kinase transphosphorylation sustains MET oncogene addiction

Receptors for the scatter factors HGF and MSP that are encoded by the MET and RON oncogenes are key players in invasive growth. Receptor cross-talk between Met and Ron occurs. Amplification of the MET oncogene results in kinase activation, deregulated expression of an invasive growth phenotype, and addiction to MET oncogene signaling (i.e., dependency on sustained Met signaling for survival and proliferation). Here we show that cancer cells addicted to MET also display constitutive activation of the Ron kinase. In human cancer cell lines coexpressing the 2 oncogenes, Ron is specifically transphosphorylated by activated Met. In contrast, Ron phosphorylation is not triggered in cells harboring constitutively active kinase receptors other than Met, including Egfr or Her2. Furthermore, Ron phosphorylation is suppressed by Met-specific kinase inhibitors (PHA-665752 or JNJ-38877605). Last, Ron phosphorylation is quenched by reducing cell surface expression of Met proteins by antibody-induced shedding. In MET-addicted cancer cells, short hairpin RNA-mediated silencing of RON expression resulted in decreased proliferation and clonogenic activity in vitro and tumorigenicity in vivo. Our findings establish that oncogene addiction to MET involves Ron transactivation, pointing to Ron kinase as a target for combinatorial cancer therapy.     

The Role of MET Receptor Tyrosine Kinase in Non‐Small Cell Lung Cancer and Clinical Development of Targeted Anti‐MET Agents

A better understanding of the pathophysiology and evolution of non‐small cell lung cancer (NSCLC) has identified a number of molecular targets and spurred development of novel targeted therapeutic agents. The MET receptor tyrosine kinase and its ligand hepatocyte growth factor (HGF) are implicated in tumor cell proliferation, migration, invasion, and angiogenesis in a broad spectrum of human cancers, including NSCLC. Amplification of MET has been reported in approximately 5%–22% of lung tumors with acquired resistance to small‐molecule inhibitors of the epidermal growth factor receptor (EGFR). Resistance to EGFR inhibitors is likely mediated through downstream activation of the phosphoinositide 3‐kinase /AKT pathway. Simultaneous treatment of resistant tumors with a MET inhibitor plus an EGFR inhibitor can abrogate activation of downstream effectors of cell growth, proliferation, and survival, thereby overcoming acquired resistance to EGFR inhibitors. Development and preclinical testing of multiple agents targeting the HGF–MET pathway, including monoclonal antibodies targeting HGF or the MET receptor and small‐molecule inhibitors of the MET tyrosine kinase, have confirmed the crucial role of this pathway in NSCLC. Several agents are now in phase III clinical development for the treatment of NSCLC. This review summarizes the role of MET in the pathophysiology of NSCLC and in acquired resistance to EGFR inhibitors and provides an update on progress in the clinical development of inhibitors of MET for treatment of NSCLC.     

Seminars in clinical pharmacology: an introduction to MET inhibitors for the medical oncologist

MET is a tyrosine kinase receptor for hepatocyte growth factor (HGF), primarily expressed on epithelial cells; the activation of MET induces several biological responses relevant for the development and growth of many human cancers. Several human malignancies present altered expression of MET and this is usually associated with poor prognosis and aggressive phenotype. The majority of MET inhibitors in clinical development target directly the receptor through the use of monoclonal antibodies (MAbs) or through small molecule inhibitors of MET kinase activity; small molecule inhibitors are very potent but less specific than MAbs. MET inhibitors are of great clinical interest because of the extensive crosstalk of the HGF/MET axis with many other signaling pathways, including growth factor-dependent pathways (like PI3K/AKT/mTOR,RAS/RAF/ERK) and vascular endothelial growth factor (VEGF) axis. In preclinical studies, the treatment with MET inhibitors could prevent or reverse resistance to inhibitors of growth factor-dependent signaling; this hypothesis is currently tested in phase III trials with anti-epidermal growth factor receptor (EGFR) inhibitors in non-small-cell lung cancer (NSCLC). Based on preclinical and preliminary clinical results, a rational strategy for the clinical development of MET antagonists should include a selection of the tumors with MET overexpression, the identification of prognostic/predictive biomarkers, the evaluation of combinations with anti-VEGF compounds.     

Impact of MET inhibition on small‐cell lung cancer cells showing aberrant activation of the hepatocyte growth factor/MET pathway

Small‐cell lung cancer (SCLC) accounts for approximately 15% of all lung cancers, and is characterized as extremely aggressive, often displaying rapid tumor growth and multiple organ metastases. In addition, the clinical outcome of SCLC patients is poor due to early relapse and acquired resistance to standard chemotherapy treatments. Hence, novel therapeutic strategies for the treatment of SCLC are urgently required. Accordingly, several molecular targeted therapies were evaluated in SCLC; however, they failed to improve the clinical outcome. The receptor tyrosine kinase MET is a receptor for hepatocyte growth factor (HGF), and aberrant activation of HGF/MET signaling is known as one of the crucial mechanisms enabling cancer progression and invasion. Here, we found that the HGF/MET signaling was aberrantly activated in chemoresistant or chemorelapsed SCLC cell lines (SBC‐5, DMS273, and DMS273‐G3H) by the secretion of HGF and/or MET copy number gain. A cell‐based in vitro assay revealed that HGF/MET inhibition, induced either by MET inhibitors (crizotinib and golvatinib), or by siRNA‐mediated knockdown of HGF or MET, constrained growth of chemoresistant SCLC cells through the inhibition of ERK and AKT signals. Furthermore, treatment with either crizotinib or golvatinib suppressed the systemic metastasis of SBC‐5 cell tumors in natural killer cell‐depleted SCID mice, predominantly through cell cycle arrest. These findings reveal the therapeutic potential of targeting the HGF/MET pathway for inhibition, to constrain tumor progression of SCLC cells showing aberrant activation of HGF/MET signaling. We suggest that it would be clinically valuable to further investigate HGF/MET‐mediated signaling in SCLC cells.     

Clinical significance of MET in gastric cancer

Chemotherapy has become the global standard treatment for patients with metastatic or unresectable gastric cancer (GC), although outcomes remain unfavorable. Many molecular-targeted therapies inhibiting signaling pathways of various tyrosine kinase receptors have been developed, and monoclonal antibodies targeting human epidermal growth factor receptor 2 or vascular endothelial growth factor receptor 2 have become standard therapy for GC. Hepatocyte growth factor and its receptor, c-MET (MET), play key roles in tumor growth through activated signaling pathways from receptor in GC cells. Genomic amplification of MET leads to the aberrant activation found in GC tumors and is related to survival in patients with GC. This review discusses the clinical significance of MET in GC and examines MET as a potential therapeutic target in patients with GC. Preclinical studies in animal models have shown that MET antibodies or small-molecule MET inhibitors suppress tumor-cell proliferation and tumor progression in MET-amplified GC cells. These drugs are now being evaluated in clinical trials as treatments for metastatic or unresectable GC.     

Targeting MET by Tyrosine Kinase Inhibitor Suppresses Growth and Invasion of Nasopharyngeal Carcinoma Cell Lines

Nasopharyngeal carcinoma (NPC) represents a common cancer in endemic areas with high invasive and metastatic potential. It is now known that the HGF-MET signaling pathway plays an important role in mediating the invasive growth of many different types of cancer, including head and neck squamous cell carcinoma. HGF has been shown to stimulate NPC cell growth and invasion in cell line model. The current study aims at demonstrating the effect of MET inhibition by small molecule tyrosine kinase inhibitor PHA665752 on the growth and invasive potential of NPC cell lines. NPC cell lines were used for immunohistochemistry for the MET protein, as well as western blot analysis on MET together with its downstream cascade signaling proteins after treatment with PHA665752. The effect on cell growth, migration and invasion after PHA665752 treatment was also studied. MET inhibition by PHA665752 resulted in highly significant inhibition on NPC cell growth, migration and invasion in vitro. Down-regulation of phospho-MET, phospho-Akt, phospho-MAPK, phospho-STAT3, cyclin D1, β-catenin and PCNA was detected in NPC cells after PHA665752 treatment. MET inhibition with tyrosine kinase inhibitor resulted in suppression of NPC cell growth and invasive potential via down-regulation of a variety of signaling onco-proteins. MET is an important therapeutic target for NPC that warrants further studies and clinical trials.     

c-MET通路和抑制剂在非小细胞肺癌中的研究进展

c-MET被认为是继表皮生长因子受体(epidermal growth factor receptor,EGFR)基因突变和间变性淋巴瘤激酶(anaplastic lymphoma kinase,ALK)基因融合之后,非小细胞肺癌(non-small cell lung cancer,NSCLC)又一个重要的驱动基因.MET的激活包括突变、扩增和蛋白质过表达,是NSCLC潜在的治疗靶点,并提示与预后相关.临床证据表明,MET既可以作为肺癌的原发致癌驱动基因,也是EGFR靶向治疗获得性耐药的原因之一.本文主要对c-MET通路在NSCLC中的活性形式及治疗的研究进展进行综述.     

MET signaling: novel targeted inhibition and its clinical development in lung cancer

MET is a versatile receptor tyrosine kinase within the human kinome which is activated by its specific natural ligand hepatocyte growth factor (HGF). MET signaling plays an important physiologic role in embryogenesis and early development, whereas its deregulation from an otherwise quiescent signaling state in mature adult tissues can lead to upregulated cell proliferation, survival, scattering, motility and migration, angiogenesis, invasion, and metastasis in tumorigenesis and tumor progression. Studies have shown that MET pathway is activated in many solid and hematological malignancies, including lung cancer, and can be altered through ligand or receptor overexpression, genomic amplification, MET mutations, and alternative splicing. The MET signaling pathway is known to be an important novel target for therapeutic intervention in human cancer. A number of novel therapeutic agents that target the MET/HGF pathway have been tested in early-phase clinical studies with promising results. Phase 3 studies of MET targeting agents have just been initiated. We will review the MET signaling pathway and biology in lung cancer and the recent clinical development and advances of MET/HGF targeting agents with emphasis on discussion of issues and strategies needed to optimize the personalized therapy and further clinical development.     

Hepatocyte growth factor/MET in cancer progression and biomarker discovery

Signaling driven by hepatocyte growth factor (HGF) and MET receptor facilitates conspicuous biological responses such as epithelial cell migration, 3‐D morphogenesis, and survival. The dynamic migration and promotion of cell survival induced by MET activation are bases for invasion–metastasis and resistance, respectively, against targeted drugs in cancers. Recent studies indicated that MET in tumor‐derived exosomes facilitates metastatic niche formation and metastasis in malignant melanoma. In lung cancer, gene amplification‐induced MET activation and ligand‐dependent MET activation in an autocrine/paracrine manner are causes for resistance to epidermal growth factor receptor tyrosine kinase inhibitors and anaplastic lymphoma kinase inhibitors. Hepatocyte growth factor secreted in the tumor microenvironment contributes to the innate and acquired resistance to RAF inhibitors. Changes in serum/plasma HGF, soluble MET (sMET), and phospho‐MET have been confirmed to be associated with disease progression, metastasis, therapy response, and survival. Higher serum/plasma HGF levels are associated with therapy resistance and/or metastasis, while lower HGF levels are associated with progression‐free survival and overall survival after treatment with targeted drugs in lung cancer, gastric cancer, colon cancer, and malignant melanoma. Urinary sMET levels in patients with bladder cancer are higher than those in patients without bladder cancer and associated with disease progression. Some of the multi‐kinase inhibitors that target MET have received regulatory approval, whereas none of the selective HGF‐MET inhibitors have shown efficacy in phase III clinical trials. Validation of the HGF‐MET pathway as a critical driver in cancer development/progression and utilization of appropriate biomarkers are key to development and approval of HGF‐MET inhibitors for clinical use.     

The emerging role of MET/HGF inhibitors in oncology

The N-methyl-N′-nitroso-guanidine human osteosarcoma transforming gene (MET) receptor tyrosine kinase and its ligand hepatocyte growth factor (HGF) control cellular signaling cascades that direct cell growth, proliferation, survival, and motility. Aberrant MET/HGF activation has been observed in many tumor types, can occur by multiple mechanisms, and promotes cellular proliferation and metastasis via growth factor receptors and other oncogenic receptor pathways. Thus, MET/HGF inhibition has emerged as targeted anticancer therapies. Preclinically, neoplastic and metastatic phenotypes of several tumor cells, including non-small cell lung cancer, hepatocellular carcinoma, and gastric cancer, were abrogated by MET inhibition. Ongoing clinical development with tivantinib, cabozantinib, onartuzumab, crizotinib, rilotumumab, and ficlatuzumab has shown encouraging results. These trials have established a key role for MET in a variety of tumor types. Evidence is emerging for identification of aberrant MET activity biomarkers and selection of patient subpopulations that may benefit from targeted MET and HGF inhibitor treatment.