The tyrosine kinase encoded by the MET proto-oncogene is activated by autophosphorylation

Conserved regulatory motifs in the juxtamembrane domain and kinase N-lobe revealed through deep mutational scanning of the MET receptor tyrosine kinase domain

MET is a receptor tyrosine kinase (RTK) responsible for initiating signaling pathways involved in development and wound repair. MET activation relies on ligand binding to the extracellular receptor, which prompts dimerization, intracellular phosphorylation, and recruitment of associated signaling proteins. Mutations, which are predominantly observed clinically in the intracellular juxtamembrane and kinase domains, can disrupt typical MET regulatory mechanisms. Understanding how juxtamembrane variants, such as exon 14 skipping (METΔEx14), and rare kinase domain mutations can increase signaling, often leading to cancer, remains a challenge. Here, we perform a parallel deep mutational scan (DMS) of the MET intracellular kinase domain in two fusion protein backgrounds: wild type and METΔEx14. Our comparative approach has revealed a critical hydrophobic interaction between a juxtamembrane segment and the kinase αC-helix, pointing to potential differences in regulatory mechanisms between MET and other RTKs. Additionally, we have uncovered a β5 motif that acts as a structural pivot for the kinase domain in MET and other TAM family of kinases. We also describe a number of previously unknown activating mutations, aiding the effort to annotate driver, passenger, and drug resistance mutations in the MET kinase domain.

Reduced expression of phosphorylated ataxia-telangiectasia mutated gene is related to poor prognosis and gemcitabine chemoresistance in pancreatic cancer

BACKGROUND

Loss of expression of the gene ataxia-telangiectasia mutated (ATM), occurring in patients with multiple primary malignancies, including pancreatic cancer, is associated with poor prognosis. In this study, we investigated the detailed molecular mechanism through which ATM expression affects the prognosis of patients with pancreatic cancer.

METHODS

The levels of expression of ATM and phosphorylated ATM in patients with pancreatic cancer who had undergone surgical resection were analyzed using immunohistochemistry staining. RNA sequencing was performed on ATM-knockdown pancreatic-cancer cells to elucidate the mechanism underlying the invlovement of ATM in pancreatic cancer.

RESULTS

Immunohistochemical analysis showed that 15.3% and 27.8% of clinical samples had low levels of ATM and phosphorylated ATM, respectively. Low expression of phosphorylated ATM substantially reduced overall and disease-free survival in patients with pancreatic cancer. In the pancreatic cancer cell lines with ATM low expression, resistance to gemcitabine was demonstrated. The RNA sequence demonstrated that ATM knockdown induced the expression of MET and NTN1. In ATM knockdown cells, it was also revealed that the protein expression levels of HIF-1α and antiapoptotic BCL-2/BAD were upregulated.

CONCLUSIONS

These findings demonstrate that loss of ATM expression increases tumor development, suppresses apoptosis, and reduces gemcitabine sensitivity. Additionally, loss of phosphorylated ATM is associated with a poor prognosis in patients with pancreatic cancer. Thus, phosphorylated ATM could be a possible target for pancreatic cancer treatment as well as a molecular marker to track patient prognosis.

Reduced HGF/MET Signaling May Contribute to the Synaptic Pathology in an Alzheimer's Disease Mouse Model

Alzheimer's disease (AD) is a neurodegenerative disorder strongly associates with aging. While amyloid plagues and neurofibrillary tangles are pathological hallmarks of AD, recent evidence suggests synaptic dysfunction and physical loss may be the key mechanisms that determine the clinical syndrome and dementia onset. Currently, no effective therapy prevents neuropathological changes and cognitive decline. Neurotrophic factors and their receptors represent novel therapeutic targets to treat AD and dementia. Recent clinical literature revealed that MET receptor tyrosine kinase protein is reduced in AD patient's brain. Activation of MET by its ligand hepatocyte growth factor (HGF) initiates pleiotropic signaling in the developing brain that promotes neurogenesis, survival, synaptogenesis, and plasticity. We hypothesize that if reduced MET signaling plays a role in AD pathogenesis, this might be reflected in the AD mouse models and as such provides opportunities for mechanistic studies on the role of HGF/MET in AD. Examining the 5XFAD mouse model revealed that MET protein exhibits age-dependent progressive reduction prior to overt neuronal pathology, which cannot be explained by indiscriminate loss of total synaptic proteins. In addition, genetic ablation of MET protein in cortical excitatory neurons exacerbates amyloid-related neuropathology in 5XFAD mice. We further found that HGF enhances prefrontal layer 5 neuron synaptic plasticity measured by long-term potentiation (LTP). However, the degree of LTP enhancement is significantly reduced in 5XFAD mice brain slices. Taken together, our study revealed that early reduction of HGF/MET signaling may contribute to the synaptic pathology observed in AD.

Control of cortical synapse development and plasticity by MET receptor tyrosine kinase, a genetic risk factor for autism

The key developmental milestone events of the human brain, such as neurogenesis, synapse formation, maturation, and plasticity, are determined by a myriad of molecular signaling events, including those mediated by a number of receptor tyrosine kinases (RTKs) and their cognate ligands. Aberrant or mistimed brain development and plasticity can lead to maladaptive changes, such as dysregulated synaptic connectivity and breakdown of circuit functions necessary for cognition and adaptive behaviors, which are hypothesized pathophysiologies of many neurodevelopmental and neuropsychiatric disorders. Here we review recent literature that supports autism spectrum disorder as a likely result of aberrant synapse development due to mistimed maturation and plasticity. We focus on MET RTK, a prominent genetic risk factor for autism, and discuss how a pleiotropic molecular signaling system engaged by MET exemplifies a genetic program that controls cortical circuit development and plasticity by modulating the anatomical and functional connectivity of cortical circuits, thus conferring genetic risk for neurodevelopmental disorders.

Biological Pathways Leading From ANGPTL8 to Diabetes Mellitus-A Co-expression Network Based Analysis

Angiopoietin like protein 8 (ANGPTL8) is a newly identified hormone with unique nature due to its ability to regulate both glucose and lipid metabolic pathways. It is characterized as an important molecular player of insulin induced nutrient storage and utilization pathway during fasting to re-feeding metabolic transition. Several studies have contributed to increase our knowledge regarding its function and mechanism of action. Moreover, its altered expression levels have been observed in Insulin Resistance, Diabetes Mellitus (Types I & II) and Non Alcohlic Fatty Liver Disease emphasizing its assessment as a drug target. However, there is still a great deal of information that remains to be investigated including its associated biological processes, partner proteins in these processes, its regulators and its association with metabolic pathogenesis. In the current study, the analysis of a transcriptomic data set was performed for functional assessment of ANGPTL8 in liver. Weighted Gene Co-expression Network Analysis coupled with pathway analysis tools was performed to identify genes that are significantly co-expressed with ANGPTL8 in liver and investigate their presence in biological pathways. Gene ontology term enrichment analysis was performed to select the gene ontology classes that over-represent the hepatic ANGPTL8-co-expressed genes. Moreover, the presence of diabetes linked SNPs within the genes set co-expressed with ANGPTL8 was investigated. The co-expressed genes of ANGPTL8 identified in this study (n = 460) provides narrowed down list of molecular targets which are either co-regulated with it and/or might be regulation partners at different levels of interaction. These results are coherent with previously demonstrated roles and regulators of ANGPTL8. Specifically, thirteen co-expressed genes (MAPK8, CYP3A4, PIK3R2, PIK3R4,PRKAB2, G6PC, MAP3K11, FLOT1, PIK3C2G, SHC1, SLC16A2, and RAPGEF1) are also present in the literature curated pathway of ANGPTL8 (WP3915¹). Moreover, the gene-SNP analysis of highly associated biological processes with ANGPTL8 revealed significant genetic signals associated to Diabetes Mellitus and similar phenotypic traits. It provides meaningful insights on the influencing genes involved and co-expressed in these pathways. Findings of this study have implications in functional characterization of ANGPTL8 with emphasis on the identified genes and pathways and their possible involvement in the pathogenesis of Diabetes Mellitus and Insulin Resistance.

Disruption of MET Receptor Tyrosine Kinase, an Autism Risk Factor, Impairs Developmental Synaptic Plasticity in the Hippocampus

As more genes conferring risks to neurodevelopmental disorders are identified, translating these genetic risk factors into biological mechanisms that impact the trajectory of the developing brain is a critical next step. Here, we report that disrupted signaling mediated MET receptor tyrosine kinase (RTK), an established risk factor for autism spectrum disorders, in the developing hippocampus glutamatergic circuit leads to profound deficits in neural development, synaptic transmission, and plasticity. In cultured hippocampus slices prepared from neonatal mice, pharmacological inhibition of MET kinase activity suppresses dendritic arborization and disrupts normal dendritic spine development. In addition, single-neuron knockdown (RNAi) or overexpression of Met in the developing hippocampal CA1 neurons leads to alterations, opposite in nature, in basal synaptic transmission and long-term plasticity. In forebrain-specific Met conditional knockout mice (Met^fx/fx ;emx1^cre ), an enhanced long-term potentiation (LTP) and long-term depression (LTD) were observed at early developmental stages (P12-14) at the Schaffer collateral to CA1 synapses compared with wild-type littermates. In contrast, LTP and LTD were markedly reduced at young adult stage (P56-70) during which wild-type mice show robust LTP and LTD. The altered trajectory of synaptic plasticity revealed by this study indicate that temporally regulated MET signaling as an intrinsic, cell autonomous, and pleiotropic mechanism not only critical for neuronal growth and functional maturation, but also for the timing of synaptic plasticity during forebrain glutamatergic circuits development.

Isoquercitrin, ingredients in Tetrastigma hemsleyanum Diels et Gilg, inhibits hepatocyte growth factor/scatter factor-induced tumor cell migration and invasion

Aberrant activation of hepatocyte growth factor/scatter factor (HGF/SF) and its receptor, Met, is involved in the development and progression of many human cancers. In the screening assay of extracts from the root tuber of Tetrastigma hemsleyanum Diels et Gilg, isoquercitrin inhibited HGF/SF-Met signaling as indicated by its inhibitory activity on HGF/SF-induced cell scattering. Further analysis revealed that isoquercitrin specifically inhibited HGF/SF-induced tyrosine phosphorylation of Met. We also found that isoquercitrin decreased HGF-induced migration and invasion by parental or HGF/SF-transfected bladder carcinoma cell line NBT-II cells. Furthermore, isoquercitrin inhibited HGF/SF-induced epithelial mesenchymal transition in vitro and the invasion/metastasis of HGF/SF-transfected NBT-II cells in vivo. Our data suggest the possible use of isoquercitrin in human cancers associated with dysregulated HGF/SF-Met signaling.

Activated HGF-c-Met Axis in Head and Neck Cancer

Head and neck squamous cell carcinoma (HNSCC) is a highly morbid disease. Recent developments including Food and Drug Administration (FDA) approved molecular targeted agent’s pembrolizumab and cetuximab show promise but did not improve the five-year survival which is currently less than 40%. The hepatocyte growth factor receptor; also known as mesenchymal–epithelial transition factor (c-Met) and its ligand hepatocyte growth factor (HGF) are overexpressed in head and neck squamous cell carcinoma (HNSCC); and regulates tumor progression and response to therapy. The c-Met pathway has been shown to regulate many cellular processes such as cell proliferation, invasion, and angiogenesis. The c-Met pathway is involved in cross-talk, activation, and perpetuation of other signaling pathways, curbing the cogency of a blockade molecule on a single pathway. The receptor and its ligand act on several downstream effectors including phospholipase C gamma (PLCγ), cellular Src kinase (c-Src), phosphotidylinsitol-3-OH kinase (PI3K) alpha serine/threonine-protein kinase (Akt), mitogen activate protein kinase (MAPK), and wingless-related integration site (Wnt) pathways. They are also known to cross-talk with other receptors; namely epidermal growth factor receptor (EGFR) and vascular endothelial growth factor receptor (VEGFR) and specifically contribute to treatment resistance. Clinical trials targeting the c-Met axis in HNSCC have been undertaken because of significant preclinical work demonstrating a relationship between HGF/c-Met signaling and cancer cell survival. Here we focus on HGF/c-Met impact on cellular signaling in HNSCC to potentiate tumor growth and disrupt therapeutic efficacy. Herein we summarize the current understanding of HGF/c-Met signaling and its effects on HNSCC. The intertwining of c-Met signaling with other signaling pathways provides opportunities for more robust and specific therapies, leading to better clinical outcomes.

HGF potentiates extracellular matrix-driven migration of human myoblasts: involvement of matrix metalloproteinases and MAPK/ERK pathway

Background

The hepatocyte growth factor (HGF) is required for the activation of muscle progenitor cells called satellite cells (SC), plays a role in the migration of proliferating SC (myoblasts), and is present as a soluble factor during muscle regeneration, along with extracellular matrix (ECM) molecules. In this study, we aimed at determining whether HGF is able to interact with ECM proteins, particularly laminin 111 and fibronectin, and to modulate human myoblast migration.

Methods

We evaluated the expression of the HGF-receptor c-Met, laminin, and fibronectin receptors by immunoblotting, flow cytometry, or immunofluorescence and used Transwell assays to analyze myoblast migration on laminin 111 and fibronectin in the absence or presence of HGF. Zymography was used to check whether HGF could modulate the production of matrix metalloproteinases by human myoblasts, and the activation of MAPK/ERK pathways was evaluated by immunoblotting.

Results

We demonstrated that human myoblasts express c-Met, together with laminin and fibronectin receptors. We observed that human laminin 111 and fibronectin have a chemotactic effect on myoblast migration, and this was synergistically increased when low doses of HGF were added. We detected an increase in MMP-2 activity in myoblasts treated with HGF. Conversely, MMP-2 inhibition decreased the HGF-associated stimulation of cell migration triggered by laminin or fibronectin. HGF treatment also induced in human myoblasts activation of MAPK/ERK pathways, whose specific inhibition decreased the HGF-associated stimulus of cell migration triggered by laminin 111 or fibronectin.

Conclusions

We demonstrate that HGF induces ERK phosphorylation and MMP production, thus stimulating human myoblast migration on ECM molecules. Conceptually, these data state that the mechanisms involved in the migration of human myoblasts comprise both soluble and insoluble moieties. This should be taken into account to optimize the design of therapeutic cell transplantation strategies by improving the migration of donor cells within the host tissue, a main issue regarding this approach.

Electronic supplementary material

The online version of this article (10.1186/s13395-017-0138-6) contains supplementary material, which is available to authorized users.

The autism-associated MET receptor tyrosine kinase engages early neuronal growth mechanism and controls glutamatergic circuits development in the forebrain

The human MET gene imparts a replicated risk for autism spectrum disorder (ASD), and is implicated in the structural and functional integrity of brain. MET encodes a receptor tyrosine kinase, MET, which has a pleiotropic role in embryogenesis and modifies a large number of neurodevelopmental events. Very little is known, however, on how MET signaling engages distinct cellular events to collectively affect brain development in ASD-relevant disease domains. Here, we show that MET protein expression is dynamically regulated and compartmentalized in developing neurons. MET is heavily expressed in neuronal growth cones at early developmental stages and its activation engages small GTPase Cdc42 to promote neuronal growth, dendritic arborization and spine formation. Genetic ablation of MET signaling in mouse dorsal pallium leads to altered neuronal morphology indicative of early functional maturation. In contrast, prolonged activation of MET represses the formation and functional maturation of glutamatergic synapses. Moreover, manipulating MET signaling levels in vivo in the developing prefrontal projection neurons disrupts the local circuit connectivity made onto these neurons. Therefore, normal time-delimited MET signaling is critical in regulating the timing of neuronal growth, glutamatergic synapse maturation and cortical circuit function. Dysregulated MET signaling may lead to pathological changes in forebrain maturation and connectivity, and thus contribute to the emergence of neurological symptoms associated with ASD.

Regulation of the MET oncogene: molecular mechanisms

The MET oncogene is a predictive biomarker and an attractive therapeutic target for various cancers. Its expression is regulated at multiple layers via various mechanisms. It is subject to epigenetic modifications, i.e. DNA methylation and histone acetylation. Hypomethylation and acetylation of the MET gene have been associated with its high expression in some cancers. Multiple transcription factors including Sp1 and Ets-1 govern its transcription. After its transcription, METmRNA is spliced into multiple species in the nucleus before being transported to the cytoplasm where its translation is modulated by at least 30 microRNAs and translation initiation factors, e.g. eIF4E and eIF4B. METmRNA produces a single chain pro-Met protein of 170 kDa which is cleaved into α and β chains. These two chains are bound together through disulfide bonds to form a heterodimer which undergoes either N-linked or O-linked glycosylation in the Golgi apparatus before it is properly localized in the membrane. Upon interactions with its ligand, i.e. hepatocyte growth factor (HGF), the activity of Met kinase is boosted through various phosphorylation mechanisms and the Met signal is relayed to downstream pathways. The phosphorylated Met is then internalized for subsequent degradation or recycle via proteasome, lysosome or endosome pathways. Moreover, the Met expression is subject to autoregulation and activation by other EGFRs and G-protein coupled receptors. Since deregulation of the MET gene leads to cancer and other pathological conditions, a better understanding of the MET regulation is critical for Met-targeted therapeutics.

cMET in triple-negative breast cancer: is it a therapeutic target for this subset of breast cancer patients?

INTRODUCTION

The identification and validation of a targeted therapy for triple-negative breast cancer (TNBC) is currently one of the most urgent needs in breast cancer therapeutics. The cMET oncogene encodes a membrane-bound tyrosine kinase implicated in the formation and/or progression of several cancer types, including TNBC. Currently, inhibitors targeting cMET are undergoing clinical trials for a variety of cancers, including TNBC. These include anti-cMET and anti-hepatocyte growth factor (HGF) monoclonal antibodies and tyrosine kinase inhibitors.

AREAS COVERED

This article reviews the structure and mode of action of cMET, the role of cMET in cancer formation/development, with particular emphasis on its role in basal/TNBC and its potential as a therapeutic target for this subtype of breast cancer.

EXPERT OPINION

Due to cancer heterogeneity, it is unlikely that all TNBC patients will be responsive to anti-cMET drugs. Therefore, if cMET is to be used as a target for treatment, it will be important to identify predictive biomarkers to select, upfront, those patients likely to benefit. Potential predictive biomarkers for anti-cMET treatments in basal/TNBC include cMET, phospho-cMET, downstream signaling proteins or HGF. These putative predictive biomarkers should be evaluated in a large panel of basal/TNBC cell lines before incorporation into clinical trials involving anti-cMET drugs.

Targeting MET kinase with the small-molecule inhibitor amuvatinib induces cytotoxicity in primary myeloma cells and cell lines

BACKGROUND

MET is a receptor tyrosine kinase that is activated by the ligand HGF and this pathway promotes cell survival, migration, and motility. In accordance with its oncogenic role, MET is constitutively active, mutated, or over-expressed in many cancers. Corollary to its impact, inhibition of MET kinase activity causes reduction of the downstream signaling and demise of cells. In myeloma, a B-cell plasma malignancy, MET is neither mutated nor over-expressed, however, HGF is increased in plasma or serum obtained from myeloma patients and this was associated with poor prognosis. The small-molecule, amuvatinib, inhibits MET receptor tyrosine kinase. Based on this background, we hypothesized that targeting the HGF/MET signaling pathway is a rational approach to myeloma therapy and that myeloma cells would be sensitive to amuvatinib.

METHODS

Expression of MET and HGF mRNAs in normal versus malignant plasma cells was compared during disease progression. Cell death and growth as well as MET signaling pathway were assessed in amuvatinib treated primary myeloma cells and cell lines.

RESULTS

There was a progressive increase in the transcript levels of HGF (but not MET) from normal plasma cells to refractory malignant plasma cells. Amuvatinib readily inhibited MET phosphorylation in primary CD138+ cells from myeloma patients and in concordance, increased cell death. A 48-hr amuvatinib treatment in high HGF-expressing myeloma cell line, U266, resulted in growth inhibition. Levels of cytotoxicity were time-dependent; at 24, 48, and 72 h, amuvatinib (25 μM) resulted in 28%, 40%, and 55% cell death. Consistent with these data, there was an amuvatinib-mediated decrease in MET phosphorylation in the cell line. Amuvatinib at concentrations of 5, 10, or 25 μM readily inhibited HGF-dependent MET, AKT, ERK and GSK-3-beta phosphorylation. MET-mediated effects were not observed in myeloma cell line that has low MET and/or HGF expression.

CONCLUSIONS

These data suggest that at the cellular level MET/HGF pathway inclines with myeloma disease progression. Amuvatinib, a small molecule MET kinase inhibitor, is effective in inducing growth inhibition and cell death in myeloma cell lines as well as primary malignant plasma cells. These cytostatic and cytotoxic effects were associated with an impact on MET/HGF pathway.

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.

Neuronal connectivity as a convergent target of gene × environment interactions that confer risk for Autism Spectrum Disorders

Evidence implicates environmental factors in the pathogenesis of Autism Spectrum Disorders (ASD). However, the identity of specific environmental chemicals that influence ASD risk, severity or treatment outcome remains elusive. The impact of any given environmental exposure likely varies across a population according to individual genetic substrates, and this increases the difficulty of identifying clear associations between exposure and ASD diagnoses. Heritable genetic vulnerabilities may amplify adverse effects triggered by environmental exposures if genetic and environmental factors converge to dysregulate the same signaling systems at critical times of development. Thus, one strategy for identifying environmental risk factors for ASD is to screen for environmental factors that modulate the same signaling pathways as ASD susceptibility genes. Recent advances in defining the molecular and cellular pathology of ASD point to altered patterns of neuronal connectivity in the developing brain as the neurobiological basis of these disorders. Studies of syndromic ASD and rare highly penetrant mutations or CNVs in ASD suggest that ASD risk genes converge on several major signaling pathways linked to altered neuronal connectivity in the developing brain. This review briefly summarizes the evidence implicating dysfunctional signaling via Ca(2+)-dependent mechanisms, extracellular signal-regulated kinases (ERK)/phosphatidylinositol-3-kinases (PI3K) and neuroligin-neurexin-SHANK as convergent molecular mechanisms in ASD, and then discusses examples of environmental chemicals for which there is emerging evidence of their potential to interfere with normal neuronal connectivity via perturbation of these signaling pathways.

MET receptor tyrosine kinase as an autism genetic risk factor

In this chapter, we will briefly discuss recent literature on the role of MET receptor tyrosine kinase (RTK) in brain development and how perturbation of MET signaling may alter normal neurodevelopmental outcomes. Recent human genetic studies have established MET as a risk factor for autism, and the molecular and cellular underpinnings of this genetic risk are only beginning to emerge from obscurity. Unlike many autism risk genes that encode synaptic proteins, the spatial and temporal expression pattern of MET RTK indicates this signaling system is ideally situated to regulate neuronal growth, functional maturation, and establishment of functional brain circuits, particularly in those brain structures involved in higher levels of cognition, social skills, and executive functions.

c-Met: A potential therapeutic target for hepatocellular carcinoma

The approval of receptor tyrosine kinase (RTK) targeted agent sorafenib as the first effective drug for the systemic treatment of advanced hepatocellular carcinoma (HCC) represents a milestone in the treatment of this disease. A better understanding of HCC pathogenesis will lead to development of novel targeted treatments. As a typical member of the RTK family, c-Met represents an intriguing target for cancer therapy. The c-Met signaling pathway has been shown to be deregulated and to correlate with poor prognosis in a number of major human cancers. This review discusses the possibility of c-Met as a target in HCC treatment from the following respects: i) c-Met expression and activation profile in HCC, ii) relationship between c-Met and clinicopathologic state and prognosis of HCC, iii) role of c-Met signaling activity in HCC genesis and progression, and iv) strategy of c-Met pathway targeting therapy in HCC treatment.

Epigallocatechin-3-gallate inhibits paracrine and autocrine hepatocyte growth factor/scatter factor-induced tumor cell migration and invasion

Aberrant activation of hepatocyte growth factor/scatter factor (HGF/SF) and its receptor, Met, is involved in the development and progression of many human cancers. In the cell-based screening assay, (-)epigallocatechin-3-gallate (EGCG) inhibited HGF/SF-Met signaling as indicated by its inhibitory activity on HGF/SF-induced cell scattering and uPA activation (IC50=15.8 microgram/ml). Further analysis revealed that EGCG at low doses specifically inhibited HGF/SF-induced tyrosine phosphorylation of Met but not epidermal growth factor (EGF)-induced phosphorylation of EGF receptor (EGFR). On the other hand, high-dose EGCG decreased both Met and EGFR proteins. We also found that EGCG did not act on the intracellular portion of Met receptor tyrosine kinase, i.e., it inhibited InlB-dependent activation of Met but not NGF-induced activation of Trk-Met hybrid receptor. This inhibition decreased HGF-induced migration and invasion by parental or HGF/SF-transfected B16F10 melanoma cells in vitro in either a paracrine or autocrine manner. Furthermore, EGCG inhibited the invasion/metastasis of HGF/SF-transfected B16F10 melanoma cells in mice. Our data suggest the possible use of EGCG in human cancers associated with dysregulated paracrine or autocrine HGF/SF-Met signaling.

Induction of MET by ionizing radiation and its role in radioresistance and invasive growth of cancer

BACKGROUND

Ionizing radiation (IR) is effectively used in cancer therapy. However, in subsets of patients, a few radioresistant cancer cells survive and cause disease relapse with metastatic progression. The MET oncogene encodes the hepatocyte growth factor (HGF) receptor and is known to drive "invasive growth", a regenerative and prosurvival program unduly activated in metastasis.

METHODS

Human tumor cell lines (MDA-MB-231, MDA-MB-435S, U251) were subjected to therapeutic doses of IR. MET mRNA, and protein expression and signal transduction were compared in treated and untreated cells, and the involvement of the DNA-damage sensor ataxia telangiectasia mutated (ATM) and the transcription factor nuclear factor kappa B (NF-κB) in activating MET transcription were analyzed by immunoblotting, chromatin immunoprecipitation, and use of NF-κB silencing RNA (siRNA). Cell invasiveness was measured in wound healing and transwell assays, and cell survival was measured in viability and clonogenic assays. MET was inhibited by siRNA or small-molecule kinase inhibitors (PHA665752 or JNJ-38877605). Combinations of MET-targeted therapy and radiotherapy were assessed in MDA-MB-231 and U251 xenografts (n = 5-6 mice per group). All P values were from two-sided tests.

RESULTS

After irradiation, MET expression in cell lines was increased up to fivefold via activation of ATM and NF-κB. MET overexpression increased ligand-independent MET phosphorylation and signal transduction, and rendered cells more sensitive to HGF. Irradiated cells became more invasive via a MET-dependent mechanism that was further enhanced in the presence of HGF. MET silencing by siRNA or inhibition of its kinase activity by treatment with PHA665752 or JNJ-38877605 counteracted radiation-induced invasiveness, promoted apoptosis, and prevented cells from resuming proliferation after irradiation in vitro. Treatment with MET inhibitors enhanced the efficacy of IR to stop the growth of or to induce the regression of xenografts (eg, at day 13, U251 xenografts, mean volume increase relative to mean tumor volume at day 0: vehicle = 438%, 5 Gy IR = 151%, 5 Gy IR + JNJ-38877605 = 76%; difference, IR vs JNJ-38877604 + IR = 75%, 95% CI = 59% to 91%, P = .01).

CONCLUSION

IR induces overexpression and activity of the MET oncogene through the ATM-NF-κB signaling pathway; MET, in turn, promotes cell invasion and protects cells from apoptosis, thus supporting radioresistance. Drugs targeting MET increase tumor cell radiosensitivity and prevent radiation-induced invasiveness.

HGF-independent potentiation of EGFR action by c-Met

The c-Met receptor is a potential therapeutic target for non-small cell lung cancer (NSCLC). Signaling interactions between c-Met and the mutant Epidermal Growth Factor Receptor (EGFR) have been studied extensively, but signaling intermediates and biological consequences of lateral signaling to c-Met in EGFR wild-type tumors is minimally understood. Our observations indicate that delayed c-Met activation in NSCLC cell lines is initiated by wild-type EGFR, the receptor most often found in NSCLC tumors. EGFR ligands induce accumulation of activated c-Met which begins at 8 h continues for 48 h. This effect is accompanied by an increase in c-Met expression and phosphorylation of critical c-Met tyrosine residues without activation of MAPK or Akt. Gene transcription is required for delayed c-Met activation; however, phosphorylation of c-Met by EGFR occurs without production of HGF or another secreted factor, supporting a ligand-independent mechanism. Lateral signaling is blocked by two selective c-Met tyrosine kinase inhibitors (TKIs), PF2341066 and SU11274, or with gefitinib, an EGFR TKI, suggesting kinase activity of both receptors is required for this effect. Prolonged c-Src phosphorylation is observed, and c-Src pathway is essential for EGFR to c-Met communication. Pre-treatment with pan-SFK inhibitors, PP2 and dasatinib, abolishes delayed c-Met phosphorylation. A c-Src dominant-negative construct reduces EGF-induced c-Met phosphorylation compared to control, further, confirming a c-Src requirement. Inhibition of c-Met with PF2341066 and siRNA decreases EGF-induced phenotypes of invasion by ~86% and motility by ~81%, suggesting that a novel form of c-Met activation is utilized by EGFR to maximize these biological effects. Combined targeting of c-Met and EGFR leads to increased xenograft anti-tumor activity, demonstrating that inhibition of downstream and lateral signaling from the EGFR-c-Src-c-Met axis might be effective in treatment of NSCLC.

Targeting the MET oncogene in cancer and metastases

IMPORTANCE OF THE FIELD

'Invasive growth' is a genetic program involved in embryonic development and adult organ regeneration and usurped by cancer cells. Although its control is complex, tumor- and context-specific and regulated by several cytokines and growth factors, the role played by the MET oncogene is well documented. In human cancers the contribution of MET to invasive growth is mainly through overexpression, driven by unfavorable microenvironmental conditions. MET activation confers a selective advantage to neoplastic cells in tumor progression and drug resistance. A subset of tumors feature alterations of the MET gene and a consequent MET-addicted phenotype.

AREAS COVERED IN THIS REVIEW

The molecular basis and rationale of MET inhibition in cancer and metastases are discussed. A number of molecules designed to block MET signaling are under development and several Phase II trials are ongoing.

WHAT THE READER WILL GAIN

Knowledge of the state of the art of anti-MET targeted approaches and the molecular basis and strategies to select patients eligible for treatment with MET inhibitors.

TAKE HOME MESSAGE

Due to its versatile functions MET is a promising candidate for cancer therapy. Understanding molecular mechanisms of sensitization and resistance to MET inhibitors is a priority to guide tailored therapies and select patients that are most likely to achieve a clinical benefit.

The PI3K-Akt mediates oncogenic Met-induced centrosome amplification and chromosome instability

The oncogenic ability of aberrant hepatocyte growth factor receptor (Met) signaling is thought to mainly rely on its mitogenic and anti-apoptotic effects. Recently, however, cumulating evidences suggest that genomic instability may be a crucial factor in tumorigenesis. Here, we address whether oncogenic Met receptor is linked to the centrosome abnormality and genomic instability. We showed that expression of the constitutive active Met (CA-Met) induced supernumerary centrosomes probably due to deregulated centrosome duplication, which was accompanied with multipolar spindle formation and aneuploidy. Interestingly, LY294002, a phosphoinositide 3-kinase (PI3K) inhibitor, significantly suppressed the appearance of supernumerary centrosomes. Moreover, knockdown of Akt with small interfering RNAs and overexpression of phosphatase and tensin homolog or dominant-negative Akt abrogated supernumerary centrosome formation, evidencing the involvement of PI3K signaling. We further showed that expression of CA-Met significantly increased aneuploidy in p53(-/-) HCT116 cells, but not in p53(+/+) HCT116 cells, indicating that the ability of CA-Met to induce chromosomal instability (CIN) phenotype is related with p53 status. Together, our data demonstrate that aberrant hepatocyte growth factor/Met signaling induces centrosome amplification and CIN via the PI3K-Akt pathway, providing an example that oncogenic growth factor signals prevalent in a wide variety of cancers have cross talks to centrosome abnormality and CIN.

Ectodomain shedding of E-cadherin and c-Met is induced by Helicobacter pylori infection

Helicobacter pylori, a microaerophilic gram-negative bacterium, colonizes the human stomach. About 50% of the world's population is infected, and this infection is considered as the major risk factor for the development of gastric adenocarcinomas in 1% of infected subjects. Carcinogenesis is characterized by the process of epithelial-to-mesenchymal transition (EMT), in the course of which fully differentiated epithelial cells turn into depolarized and migratory cells. Concomitant disruption of adherence junctions (AJ) is facilitated by growth factors like hepatocyte growth factor 1 (HGF-1), but has been also shown to depend on ectodomain shedding of E-cadherin. The aim of this study was to investigate the impact of infection with H. pylori of NCI-N87 gastric epithelial cells on the shedding of E-cadherin and HGF-receptor c-Met. Our results show that infection with H. pylori provokes shedding of the surface proteins c-Met and E-cadherin. Evidence is provided that ADAM10 contributes to the shedding of c-Met and E-cadherin.

MET pathway as a therapeutic target

Dysregulation of mesenchymal-epithelial transition factor receptor tyrosine kinase pathway leads to cell proliferation, protection from apoptosis, angiogenesis, invasion, and metastasis. It can be dysregulated through overexpression, constitutive activation, gene amplification, ligand-dependent activation or mutation. New drugs targeting various mesenchymal-epithelial transition factor pathways are being investigated with promising results.

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.

The ERK-RSK1 activation by growth factors at G2 phase delays cell cycle progression and reduces mitotic aberrations

Growth factors accelerate G0 to S progression in the cell cycle, however, the roles of growth factors in other cell cycle phases are largely unknown. Here, we show that treatment of HeLa cells with hepatocyte growth factor (HGF) at G2 phase induced the G2/M transition delay as evidenced by FACS analysis as well as by mitotic index and time-lapse analyses. Growth factors such as epidermal growth factor (EGF) and fibroblast growth factor (FGF) also induced G2/M transition delay like HGF. HGF treatment at G2 phase causes a delayed activation of cyclin B1-associated kinase and a diminished nuclear translocation of cyclin B1. Either U0126, a MAPK kinase (MEK) inhibitor, or kinase-dead mutant of ribosomal S6 kinase (RSK) abolished the delay. Additionally, knockdown of RSK1, but not RSK2, with siRNA abrogated the delay, indicating that the extracellular-regulated protein kinase (ERK)-RSK1 mediates the HGF-induced delay. We further found that the delay in G2/M transition of cells expressing oncogenic HGF receptor, M1268T, was abolished by RSK1 knockdown. Intriguingly, we observed that HGF induced chromosomal segregation defects, and depletion of RSK1, but not RSK2, aggravated these chromosomal aberrations. Taken together, the ERK-RSK1 activation by growth factors delays G2/M transition and this might be required to maintain genomic integrity during growth factor stimulation.

Molecular cancer therapy: can our expectation be MET?

Altered regulation of tyrosine kinase receptors (RTKs) is frequent in solid tumours and it is often associated with the acquisition of an aggressive phenotype. Thus, therapies targeting these receptors have been proposed as molecular approaches to treat human cancers. The MET proto-oncogene, encoding the tyrosine kinase receptor for hepatocyte growth factor (HGF), controls genetic programmes leading to cell growth, invasion and protection from apoptosis. Germ-line mutations of MET in patients affected by hereditary papillary renal carcinomas (HPRC) have provided strong genetic evidences for its role in human malignancies; moreover, constitutive activation of this receptor, as a consequence of different mechanisms such as over-expression, autocrine stimulation or point mutations, is frequent in sporadic cancers. Several strategies to block the activation of MET are under development, such as the use of tyrosine kinase inhibitors or monoclonal antibodies and some of these compounds have already been used in clinical trials. In this review, we will discuss the molecular mechanisms underlying MET involvement in tumourigenesis and present pre-clinical and clinical data obtained with compounds aimed at targeting MET in the frame of cancer therapy.

Grb2-associated binder-1 plays a central role in the hepatocyte growth factor enhancement of hepatoma growth inhibition by K vitamin analog compound 5

Compound 5 (Cpd 5), a K vitamin analog, has been shown to inhibit Hep3B human hepatoma cell growth in cultures and rat hepatoma growth in vivo through prolonged epidermal growth factor receptor (EGFR)-extracellular response kinase (ERK) phosphorylation, and hepatocyte growth factor (HGF) synergizes with Cpd 5 to enhance the inhibition of Hep3B cell and rat hepatoma growth. To explore the mechanisms mediating the HGF/Cpd 5 synergy, we examined the possible involvement of the Grb2-associated binder-1 (Gab1) docking protein because it interacts with both EGFR and HGF receptor c-Met pathways. We found that HGF enhanced Cpd 5-induced c-Met phosphorylation at Tyr-1349, a binding site for Gab1, resulting in increased c-Met binding to Gab1, and induced strong and prolonged Gab1 tyrosine phosphorylation. Prolonged Gab1 phosphorylation by HGF/Cpd 5 in turn enhanced the ability of Gab1 to bind to protein tyrosine phosphatase SHP2 and enhanced the activation of its downstream mitogen-activated protein kinase pathway. In contrast, this same HGF/Cpd 5 treatment inhibited Gab1 binding to phosphatidylinositol 3-kinase (PI3K), leading to the inactivation of the PI3K-Akt pathway. The inhibition of Akt phosphorylation by HGF/Cpd 5 further activated the Raf-MEK-ERK signaling cascade via an Akt-Raf1 interaction, leading to strong and prolonged ERK phosphorylation. The transfection of Hep3B cells with mutated Gab1 (Gab1 Y627F), which had lost its ability to bind SHP2, antagonized HGF/Cpd 5-induced ERK phosphorylation, whereas the transfection of Hep3B cells with mutated Gab1 3YF, which lost its ability to bind PI3K, further enhanced HGF/Cpd 5-induced ERK phosphorylation and cell growth inhibition.

CONCLUSION

Gab1 plays a central role in regulating HGF/Cpd 5 synergy in their actions on Hep3B cell growth inhibition.

Inactivation of von Hippel-Lindau gene induces constitutive phosphorylation of MET protein in clear cell renal carcinoma

It is well known that inactivation of von Hippel-Lindau (VHL) gene predisposes for human clear cell renal carcinoma (CCRC). However, details about critical roles of VHL inactivation during tumorigenesis are still unknown. MET protein is a tyrosine kinase receptor for hepatocyte growth factor/scatter factor (HGF/SF), which regulates cell growth, cell morphology, and cell motility. We showed that MET protein overexpressed in CCRC cells was phosphorylated without HGF/SF. This constitutive phosphorylation of MET protein in CCRC cells was inhibited by the rescue of exogenous wild-type VHL gene without a decrease in expression level of MET protein. Interestingly, wild-type VHL gene suppressed the phosphorylation of MET protein only under high cell density conditions. Additionally, MET protein activated by the inactivation of VHL gene modified cell adherence, including N-cadherin and beta-catenin. When activation of MET protein in CCRC cells was inhibited by the MET inhibitor K252a, the growth of CCRC cells in vitro and the tumorigenesis induced by CCRC cells in nude mice were suppressed. From these results, we concluded that inactivation of VHL gene induced constitutive phosphorylation of MET protein and modified intercellular adherence structure to trigger the cell growth released from contact inhibition, finally resulting in tumorigenesis. This is one of the mechanisms of CCRC oncogenesis, and MET protein has potential as a molecular target for novel CCRC therapies.

Cancer therapy: can the challenge be MET?

The deregulation of tyrosine kinase receptors (RTKs) is frequent in human tumors and is often associated with the acquisition of an aggressive phenotype. The Met oncogene, encoding the RTK for hepatocyte growth factor (HGF), controls genetic programs leading to cell growth, invasion and protection from apoptosis. The deregulated activation of Met is crucial not only for the acquisition of tumorigenic properties but also to achieve an invasive phenotype. The involvement of MET in human tumors has been definitively established and can be achieved through several mechanisms, including MET interaction with unrelated membrane receptors, such as integrins, plexins, CD44, FAS and other RTKs. Interfering with Met activation is thus a new and challenging approach to hamper tumorigenic and metastatic processes.

Met receptor dynamics and signalling

The receptor for hepatocyte growth factor (HGF), Met, controls a programme of invasive growth that combines proliferation with various moto- and morphogenetic processes. This process is important for development and organ regeneration, but dysregulation in transformed tissues can contribute to cancer progression and metastasis. Acute stimulation of tissue culture cells with HGF leads to Met downregulation via degradation through an endocytic mechanism that also requires proteasome activity. Perturbation of Met trafficking on the endocytic pathway, either at the level of the internalisation step or during sorting at the early endosome, leads to altered signalling outputs. Ubiquitination of Met through the E3-ligase Cbl is required for receptor downregulation, and a mutant receptor defective in Cbl binding is able to transform cells. We discuss the hypothesis that some naturally occurring Met mutants implicated in cancer may transform cells owing to defects in their trafficking along the endosomal degradation pathway.

Des-gamma-carboxy prothrombin is a potential autologous growth factor for hepatocellular carcinoma

Des-gamma-carboxyl prothrombin (DCP) is a well recognized tumor marker for hepatocellular carcinoma (HCC). In the present study, we demonstrate that DCP has a mitogenic effect on HCC cell lines. Purified DCP stimulated DNA synthesis of Hep3B and SK-Hep-1 cells in a dose-dependent manner. DCP was found to bind with cell surface receptor Met causing Met autophosphorylation and also to activate STAT3 signaling pathway through Janus kinase 1. Luciferase gene reporter analysis showed that DCP induced STAT3-related transcription. Small interfering RNAs against both STAT3 and Met abrogated DCP-induced cell proliferation. DCP did not affect the mitogen-activated protein kinase pathway, Myc signaling pathway, or phosphoinositide 3-kinase/Akt pathway. Based on these results, we believe that DCP acts as an autologous mitogen for HCC cell lines. The Met-Janus kinase 1-STAT3 signaling pathway may be a major signaling pathway for DCP-induced cell proliferation.

Hepatocyte growth factor enhances protein phosphatase Cdc25A inhibitor compound 5-induced hepatoma cell growth inhibition via Akt-mediated MAPK pathway

We have previously shown that Compound 5 (Cpd 5), an inhibitor of protein phosphatase Cdc25A, inhibits Hep3B human hepatoma cell growth. We now show that hepatocyte growth factor (HGF), a hepatocyte growth stimulant, can strongly enhance Cpd 5-induced growth inhibition in Hep3B cells, and this enhancement in cell growth inhibition is correlated with a much stronger ERK phosphorylation when compared to cells treated with Cpd 5 or HGF separately. We found that HGF/Cpd 5-induced ERK phosphorylation and cell growth inhibition were mediated by Akt (protein kinase B) pathway, since combination HGF/Cpd 5 treatment of Hep3B cells inhibited Akt phosphorylation at Ser-473 and its kinase activity, which led to the suppression of Raf-1 phosphorylation at Ser-259. The suppression of Raf-1 Ser-259 phosphorylation caused the induction of Raf-1 kinase activity, as well as hyper-ERK phosphorylation. Transient transfection of Hep3B cells with dominant negative Akt c-DNA further enhanced both Cpd 5- and HGF/Cpd 5-induced ERK phosphorylation, while over-expression of wild-type Akt c-DNA diminished their effects. In contrast, HGF antagonized the growth inhibitory actions of Cpd 5 on normal rat hepatocytes, thus showing a selective effect on tumor cells compared to normal cells. Our data suggest that Akt kinase negatively regulates MAPK activity at the Akt-Raf level. Suppression of Akt activity by either combination HGF/Cpd 5 treatment or by dominant negative Akt c-DNA transfection antagonizes the Akt inhibitory effect on Raf-1, resulting in an enhancement of Cpd 5-induced MAPK activation and cell growth inhibition.

Novel interaction partners of the TPR/MET tyrosine kinase

A large variety of biological processes is mediated by stimulation of the receptor tyrosine kinase MET. Screening a mouse embryo cDNA library, we were able to identify several novel, putative intracellular TPR/MET-substrates: SNAPIN, DCOHM, VAV-1, Sorting nexin 2, Death associated protein kinase 3, SMC-1, Centromeric protein C, and hTID-1. Interactions as identified by yeast two-hybrid analysis were validated in vitro and in vivo by mammalian two-hybrid studies, a far-western assay and coimmunoprecipitation. Participation in apoptosis-regulating mechanisms through interaction with DAPK-3 and cell cycle control via binding to nuclear proteins such as CENPC and SMC-1 are possible new aspects of intracellular MET signaling.

c-Met as a target for human cancer and characterization of inhibitors for therapeutic intervention

Receptor tyrosine kinase (RTK) targeted agents such as trastuzumab, imatinib, bevacizumab, and gefitinib inhibitors have illustrated the utility of targeting this protein class for treatment of selected cancers. A unique member of the RTK family, c-Met, also represents an intriguing target for cancer therapy that is yet to be explored in a clinical setting. The proto-oncogene, c-Met, encodes the high-affinity receptor for hepatocyte growth factor (HGF) or scatter factor (SF). c-Met and HGF are each required for normal mammalian development and have been shown to be particularly important in cell migration, morphogenic differentiation, and organization of three-dimensional tubular structures (e.g. renal tubular cells, gland formation, etc.) as well as cell growth and angiogenesis. Both c-Met and HGF have been shown to be deregulated in and to correlate with poor prognosis in a number of major human cancers. New data describing the constitutive phosphorylation of c-Met in a number of human tumors is presented here along with a variety of mechanisms by which c-Met can become activated, including mutation and gene amplification. In support of the clinical data implicating c-Met activation in the pathogenesis of human cancers, introduction of c-Met and HGF (or mutant c-Met) into cells conferred the properties of motility, invasiveness, and tumorgenicity to the transformed cells. Conversely, the inhibition of c-Met with a variety of receptor antagonists inhibited the motility, invasiveness, and tumorgenicity of human tumor cell lines. Consistent with this observation, small-molecule inhibitors of c-Met were developed that antagonized c-Met/HGF-dependent phenotypes and tumor growth in mouse models. This review will address the potential for development of c-Met inhibitors for treatment of human cancers with particular emphasis on recent findings with small-molecule inhibitors.

Activation of c-Met (hepatocyte growth factor receptor) in human gastric cancer tissue

c-Met is a high-affinity receptor for hepatocyte growth factor (HGF) and plays a crucial role in embryonic development, as well as in the process of tissue repair. Overexpression and amplification of c-Met are often observed in various cancer tissues, especially in gastric carcinoma. It has, however, been unclear whether the overexpression leads to activation of the c-Met receptor. To address this point, we prepared an antibody (anti-phospho-Met) which specifically recognizes c-Met that is phosphorylated at Y1235, a major phosphorylation site of c-Met. Normal as well as cancerous gastric tissue was positive for anti-total-Met staining, whereas only cancerous tissue was strongly positive for anti-phospho-Met staining; cells near the basal layer were moderately positive, and the proliferative zone in normal tissue was only weakly positive. Among cancerous tissues from seven patients examined in the present study, those from six patients were strongly positive for phospho-Met staining. These results indicate that c-Met is actually activated in gastric carcinoma tissue, and may trigger proliferation/anti-apoptotic signals.

Hepatocyte growth factor stimulates cell motility in cultures of the striatal progenitor cells ST14A

Hepatocyte growth factor/scatter factor (HGF/SF) is a growth factor with pleiotropic effects on different cell types. It acts as a mitogen and motility factor for many epithelial cells. HGF/SF and its receptor Met are present in the developing and adult mammalian brain and control neuritogenesis of sympathetic and sensory neurons. We report that the striatal progenitor ST14A cells express the Met receptor, which is activated after binding with HGF/SF. The interaction between Met and HGF/SF triggers a signaling cascade that leads to increased levels of c-Jun, c-Fos, and Egr-1 proteins, in agreement with data reported on the signaling events evoked by HGF in other cellular types. We also studied the effects of the exposure of ST14A cells to HGF/SF. By time-lapse photography, we observed that a 24-hr treatment with 50 ng/ml HGF/SF induced modification in cell morphology, with a decrease in cell-cell interactions and increase of cell motility. In contrast, no effect on cell proliferation was observed. To investigate which intracellular pathway is primarily involved we used PD98059 and LY294002, two specific inhibitors of mitogen-activated protein kinase/extracellular signal-regulated kinase (MAP-kinase/ERK-kinase) and phosphoinositide 3-OH kinase (PI3-K), respectively. Cell motility in HGF/SF treated cultures was inhibited by LY294002 but not by PD98059, suggesting that PI3-K plays a key role in mediating the HGF/SF-induced dissociation of ST14A cells. Previous evidence of HGF stimulation of motility in nervous system has been obtained on postmitotic neurons, which have already acquired their specificity. Data reported here of a motogenic response of ST14A cell line, which displays properties of neuronal progenitors, seem of interest because they suggest that HGF could play a role in very early steps of neurogenesis.

Src phosphorylates Grb2-associated binder 1 upon hepatocyte growth factor stimulation

Grb2-associated binder 1 (Gab1) is known to play an important role in hepatocyte growth factor (HGF) signaling, which rapidly becomes tyrosine-phosphorylated upon HGF stimulation. In this study, we found that the tyrosine phosphorylation of Gab1 in the cells derived from Src/Yes/Fyn null mouse embryos was approximately 40% lower than that in their wild type counterparts upon HGF stimulation. Increased expression of wild-type Src enhanced HGF-induced phosphorylation of Gab1, and, in contrast, expression of the Src kinase-deficient mutant or treatment of the specific Src inhibitor PP1 suppressed it. Expression of a constitutively active Src mutant (Y527F) or oncogenic v-Src led to a prominent increase in Gab1 phosphorylation independent of HGF stimulation. Moreover, Src interacted with Gab1 via both its Src homology 2 and 3 domains and was capable of phosphorylating purified Gab1 in vitro. Finally, the increased phosphorylation of Gab1 by Src selectively potentiated HGF-induced activation of ERK and AKT. Taken together, our results establish a new role for Src in HGF-induced Gab1 phosphorylation.

Oligomerization-induced modulation of TPR-MET tyrosine kinase activity

Phosphorylation, although necessary, may not be sufficient to fully activate many receptor tyrosine kinases (RTKs). Oligomerization-induced conformational changes may be necessary to modulate the kinetic properties of RTKs and render them fully functional. To investigate this regulatory mechanism, recombinant TPR-MET, a functionally active oncoprotein derivative of the RTK c-MET, has been expressed and purified for quantitative enzymatic analysis. This naturally occurring oncoprotein contains the cytoplasmic domain of c-MET fused to a coiled coil motif from the nuclear pore complex (TPR). cytoMET, the monomeric analog of TPR-MET, has also been expressed and purified for comparative enzymatic analysis. ATP and peptide substrates have been kinetically characterized for both TPR-MET and cytoMET. Significantly, phosphorylated TPR-MET has smaller Km values for ATP (Km,ATP) and peptide substrates (Km,peptide) and a larger kcat relative to phosphorylated cytoMET. This provides the first direct evidence that receptor oligomerization and not simply activation loop phosphorylation modulates RTK enzymatic activity. The ATP dissociation constants (Kd,ATP) for the two enzymes also displayed significant differences. In contrast, the KI values for the ATP competitive inhibitor staurosporin are similar for the two phosphorylated enzymes. These results suggest that much of the oligomerization-induced kinetic changes occur with respect to peptide substrate binding or catalytic efficiency. The possibility that oligomerization-induced conformational changes occur within the cytoplasmic domain of receptor tyrosine kinases has significant implications for structure-based design of RTK inhibitors and the development of a detailed mechanistic model of RTK activation.

Helicobacter pylori CagA protein targets the c-Met receptor and enhances the motogenic response

Infection with the human microbial pathogen Helicobacter pylori is assumed to lead to invasive gastric cancer. We find that H. pylori activates the hepatocyte growth factor/scatter factor receptor c-Met, which is involved in invasive growth of tumor cells. The H. pylori effector protein CagA intracellularly targets the c-Met receptor and promotes cellular processes leading to a forceful motogenic response. CagA could represent a bacterial adaptor protein that associates with phospholipase Cgamma but not Grb2-associated binder 1 or growth factor receptor-bound protein 2. The H. pylori-induced motogenic response is suppressed and blocked by the inhibition of PLCgamma and of MAPK, respectively. Thus, upon translocation, CagA modulates cellular functions by deregulating c-Met receptor signaling. The activation of the motogenic response in H. pylori-infected epithelial cells suggests that CagA could be involved in tumor progression.

Hypoxia promotes invasive growth by transcriptional activation of the met protooncogene

Hypoxia unleashes the invasive and metastatic potential of tumor cells by largely unknown mechanisms. The Met tyrosine kinase, a high affinity receptor for hepatocyte growth factor (HGF), plays a crucial role in controlling invasive growth and is often overexpressed in cancer. Here we show that: (1) hypoxia activates transcription of the met protooncogene, resulting in higher levels of Met; (2) hypoxic areas of tumors overexpress Met; (3) hypoxia amplifies HGF signaling; (4) hypoxia synergizes with HGF in inducing invasion; (5) the proinvasive effects of hypoxia are mimicked by Met overexpression; and (6) inhibition of Met expression prevents hypoxia-induced invasive growth. These data show that hypoxia promotes tumor invasion by sensitizing cells to HGF stimulation, providing a molecular basis to explain Met overexpression in cancer.

Synergistic effect of focal adhesion kinase overexpression and hepatocyte growth factor stimulation on cell transformation

Although an elevated level of focal adhesion kinase (FAK) has been observed in a variety of invasive human tumors, forced expression of FAK alone in cultured cells does not cause them to exhibit transformed phenotypes. Therefore, the role of FAK in oncogenic transformation remains unclear. In this study, we have demonstrated that FAK overexpression in Madin-Darby canine kidney epithelial cells rendered them susceptible to transformation by hepatocyte growth factor (HGF). Using various FAK mutants, we found that the simultaneous bindings of Src and p130(cas) were required for FAK to potentiate cell transformation. Expression of FAK-related nonkinase, kinase-deficient Src, or the Src homology 3 domain of p130(cas), which respectively serve as dominant negative versions of FAK, Src, and p130(cas), apparently reversed the transformed phenotypes of FAK-overexpressed cells upon HGF stimulation. Moreover, FAK overexpression was able to enhance HGF-elicited signals, leading to sustained activation of ERK, JNK, and AKT, which could be prevented by the expression of the Src homology 3 domain of p130(cas). Taken together, our results indicate that the synergistic effect of FAK overexpression and HGF stimulation leads to cell transformation and implicate a critical role of p130(cas) in this process.