Evaluation of the tyrosine kinase domain of the Met proto-oncogene in sporadic ovarian carcinomas*

HGF/c-Met Signalling in the Tumor Microenvironment

Recently, it has become clearer that tumor plasticity increases the chance that cancer cells could acquire new mechanisms to escape immune surveillance, become resistant to conventional drugs, and spread to distant sites.Effectively, tumor plasticity drives adaptive response of cancer cells to hypoxia and nutrient deprivation leading to stimulation of neoangionesis or tumor escape. Therefore, tumor plasticity is believed to be a great contributor in recurrence and metastatic dissemination of cancer cells. Importantly, it could be an Achilles' heel of cancer if we could identify molecular mechanisms dictating this phenotype.The reactivation of stem-like signalling pathways is considered a great determinant of tumor plasticity; in addition, a key role has been also attributed to tumor microenvironment (TME). Indeed, it has been proved that cancer cells interact with different cells in the surrounding extracellular matrix (ECM). Interestingly, well-established communication represents a potential allied in maintenance of a plastic phenotype in cancer cells supporting tumor growth and spread. An important signalling pathway mediating cancer cell-TME crosstalk is represented by the HGF/c-Met signalling.Here, we review the role of the HGF/c-Met signalling in tumor-stroma crosstalk focusing on novel findings underlying its role in tumor plasticity, immune escape, and development of adaptive mechanisms.

MET Inhibitors in Small Cell Lung Cancer: From the Bench to the Bedside

Small cell lung cancer (SCLC) is the most aggressive type of lung cancer. The different systemic treatment approaches attempted in the last 35 years have not improved overall survival in the advanced stage. Targeted therapies assessed in clinical trials have failed to show efficacy against SCLC. Within the potentially interesting targets, the hepatocyte growth factor (HGF)/mesenchymal-epithelial transition (MET) pathway activation is associated with worse survival and chemoresistance in SCLC. Preclinical data suggest that the inhibition of the MET pathway can revert chemoresistance and prevent tumor growth. Recently, immunotherapy has shown modest but relevant activity in SCLC. Interestingly, MET modulation seems to be involved in increasing the efficacy of standard checkpoint inhibitors. Here, we review the preclinical and clinical data of MET inhibition in SCLC, and the role of this pathway in the immune response.

A first-in-human study of AMG 208, an oral MET inhibitor, in adult patients with advanced solid tumors

BACKGROUND

This first-in-human study evaluated AMG 208, a small-molecule MET inhibitor, in patients with advanced solid tumors.

METHODS

Three to nine patients were enrolled into one of seven AMG 208 dose cohorts (25, 50, 100, 150, 200, 300, and 400 mg). Patients received AMG 208 orally on days 1 and days 4-28 once daily. The primary objectives were to evaluate the safety, tolerability, pharmacokinetics, and maximum tolerated dose (MTD) of AMG 208.

RESULTS

Fifty-four patients were enrolled. Six dose-limiting toxicities were observed: grade 3 increased aspartate aminotransferase (200 mg), grade 3 thrombocytopenia (200 mg), grade 4 acute myocardial infarction (300 mg), grade 3 prolonged QT (300 mg), and two cases of grade 3 hypertension (400 mg). The MTD was not reached. The most frequent grade ≥3 treatment-related adverse event was anemia (n = 3) followed by hypertension, prolonged QT, and thrombocytopenia (two patients each). AMG 208 exposure increased linearly with dose; mean plasma half-life estimates were 21.4-68.7 hours. One complete response (prostate cancer) and three partial responses (two in prostate cancer, one in kidney cancer) were observed.

CONCLUSIONS

In this study, AMG 208 had manageable toxicities and showed evidence of antitumor activity, particularly in prostate cancer.

Biology of MET: a double life between normal tissue repair and tumor progression

MNNG HOS transforming gene (MET) is a class IV receptor tyrosine kinase, expressed on the surface of epithelial cells. The interaction with the hepatocyte grow factor (HGF) induces MET dimerization and the activation of multiple intracellular pathways leading to cell proliferation, anti-apoptosis, morphogenic differentiation, motility, invasion, and angiogenesis. Knock out mice have demonstrated that MET is necessary for normal embryogenesis including the formation of striate muscles, liver and trophoblastic structures. The overexpression of MET and HGF are common in solid tumors and contribute to determine their growth. Indeed, MET has been cloned as a transforming gene from a chemically induced human osteosarcoma cell line and therefore is considered a proto-oncogene. Germline MET mutations are characteristic of hereditary papillary kidney cancers and MET amplification is observed in tumors including lung and gastric adenocarcinomas. The inhibition of MET signaling is the target for specific drugs that are raising exciting expectation for medical treatment of cancer.

MET nucleotide variations and amplification in advanced ovarian cancer: characteristics and outcomes with c-Met inhibitors

Purpose

MET alterations including amplifications and nucleotide variations have been associated with resistance to therapy and aggressive clinical behavior.

Experimental Design

The medical records of patients presenting to the University of Texas MD Anderson Cancer Center Phase I Clinic with relapsed or metastatic ovarian cancers and known MET nucleotide variation or amplification status were reviewed retrospectively (n=178). Categorical and continuous clinical and molecular characteristics were compared using Fisher's exact and Wilcoxon rank-sum tests, respectively. Univariate and multivariate survival were assessed via Kaplan-Meier and Cox regression analysis, respectively.

Results

MET amplification occurred in 4 (3.5%) of 113 patients, whereas nonsynonomous nucleotide variations were present in 9 (7.4%) of 122 patients. No patients exhibited concomitant amplification and variation. MET variations were observed only in white women with high-grade ovarian tumors, whereas amplifications were observed in both black and white women with high-grade serous ovarian primary tumors. No patients (n=4) exhibiting a MET alteration achieved an objective response when treated on a c-Met inhibitor phase I trial. In addition, ovarian cancer patients treated with a c-Met inhibitor with multikinase activity trended towards a longer time-to-failure compared with those treated with a c-Met-specific inhibitor (median: 1.5 vs. 4.5 months, p=0.07).

Conclusions

MET alterations occur in a minority of patients with ovarian cancer. c-Met inhibitors with multikinase activity may exhibit less activity in ovarian cancer than c-Met specific drugs. These findings warrant further investigation.

MET: a promising anticancer therapeutic target

The MET pathway is dysregulated in many human cancers and promotes tumour growth, invasion and dissemination. Abnormalities in MET signalling have been reported to correlate with poor clinical outcomes and drug resistance in patients with cancer. Thus, MET has emerged as an attractive target for cancer therapy. Several MET inhibitors have been introduced into the clinic, and are currently in all phases of clinical trials. In general, initial results from these studies indicate only a modest benefit in unselected populations. In this Review, we discuss current challenges in developing MET inhibitors--including identification of predictive biomarkers--as well as the most-efficient ways to combine these drugs with other targeted agents or with classic chemotherapy or radiotherapy.

Accumulative copy number increase of MET drives tumor development and histological progression in a subset of ovarian clear-cell adenocarcinomas

Our previous study demonstrated that, among ovarian carcinomas, amplification of the MET gene and overexpression of MET specifically and commonly occur in clear-cell adenocarcinoma histology. This study was conducted to address how these alterations contribute to development and progression of this highly chemoresistant form of ovarian cancer. We histologically reviewed 21 previously described MET amplification-positive clear-cell adenocarcinoma cases, and selected 11 tumors with synchronous endometriosis and 2 tumors with adjacent clear-cell adenofibroma (CCAF) components. Using double in situ hybridization and immunohistochemistry, copy number alterations of the MET gene and levels of MET protein expression were analyzed in these putative precursor lesions and the corresponding invasive carcinoma components in this selected cohort. All of the non-atypical precursor lesions analyzed (ie, non-atypical endometrioses and the benign CCAFs) were negative for MET gain. However, low-level (≥3 MET copies in ≥10% and ≥4 MET copies in 10-40% of tumor cells) gain of MET was detected in 4 (40%) of the 10 atypical endometrioses and 1 of the 2 borderline CCAFs. Moreover, high-level (≥4 MET copies in ≥40% of tumor cells) gain of MET were detected in five (50%) of the atypical endometrioses. In 4 (31%) of the 13 cases enrolled, intratumoral heterogeneity for MET gain was documented in invasive carcinoma components, wherein all the relatively differentiated carcinoma components showed low-level gain of MET and all the corresponding poorly differentiated carcinomas showed high-level gain. The overall incidence of MET overexpression gradually increased from the precursors of non-atypical form (0%), through those of atypical form (67%) and the relatively differentiated carcinoma components (92%), to the poorly differentiated carcinoma components (100%). These results suggest that accumulative MET gene copy number alterations causing MET overexpression are associated with higher tumor grade and might drive the development and progression of the MET amplification-positive ovarian clear-cell adenocarcinoma.

Gene amplification and protein overexpression of MET are common events in ovarian clear-cell adenocarcinoma: their roles in tumor progression and prognostication of the patient

The aim of this study was to assess protein overexpression and gene copy number alterations of MET in ovarian clear-cell adenocarcinoma, and to assess its potential as a novel therapeutic target. Ninety cases of clear-cell adenocarcinoma were analyzed for MET protein overexpression and copy number alterations of the MET gene by immunohistochemistry and brightfield double in situ hybridization, respectively. In addition, 101 cases of the non-clear-cell type ovarian carcinomas at advanced stages were also evaluated for comparison. MET overexpression was assigned when complete membrane staining with moderate or strong intensity was observed in at least 10% of the tumor cells examined. Double in situ hybridization was determined as positive when the tumor exhibited high-level polysomy (≥4 copies in ≥40% of tumor cells) or MET gene amplification. MET overexpression was detected in 20 of 90 clear-cell adenocarcinomas (22%) and none of 111 non-clear-cell type ovarian carcinomas. Double in situ hybridization was positive in 21 of 89 informative clear-cell adenocarcinomas (24%) and only 3 non-clear-cell type ovarian carcinomas (3%). In the whole population, true amplification of the MET gene was detected only in the clear-cell adenocarcinoma histology (five cases, 6%). In clear-cell adenocarcinomas, double in situ hybridization positivity was highly correlated with the presence of MET overexpression and a poorly differentiated histology of tumors (P=0.0105 and 0.00038, respectively). For the patients with clear-cell adenocarcinomas, MET overexpression, as well as advanced clinical stage and the poorly differentiated histology of tumors, was identified as an independent unfavorable prognostic factor for overall survival. In conclusion, among ovarian carcinomas, the amplification of the MET proto-oncogene is highly selective and commonly occurs in clear-cell adenocarcinoma. MET could serve as a biomarker for the prognostication of patients with clear-cell adenocarcinoma and tumor progression, and has potential as a novel therapeutic target for this carcinoma.

Development of c-MET pathway inhibitors

INTRODUCTION

The aberrantly upregulated c-mesenchymal-epithelia transition factor (c-MET) signaling pathway has been considered to be an attractive target for cancer intervention owing to the important roles it plays in tumor formation, progression, metastasis, angiogenesis and drug resistance. Based on the historical preclinical evidence, a number of c-MET pathway targeted agents are being developed in the clinic, and recent clinical data have begun to provide some insight into which tumor types and patient populations a c-MET pathway inhibitor may be beneficial for.

AREAS COVERED

Through reviewing recent publications in the literature and information disclosed in other public forums, we describe the current understanding of c-MET biology in human malignancies and discuss the latest progress in the development of c-MET pathway inhibitors for cancer treatment.

EXPERT OPINION

The c-MET pathway inhibitors currently being evaluated in the clinic have demonstrated compelling evidence of clinical activity in different cancer types and may provide significant therapeutic opportunities. The challenges, however, are to identify the tumor types and patient populations that benefit most, and find the most effective combinations of therapies while minimizing potential toxicity.

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.

Selection criteria for c-Met-targeted therapies: emerging evidence for biomarkers

Extensive development of targeted therapies emphasize the critical need for biomarkers and major efforts have been engaged to identify screening, prognostic, stratification and therapy-monitoring markers. One of the challenges in translating preclinical studies into effective clinical therapies remains the accurate identification of a responsive subsets of patients. Studies on trastuzumab demonstrated that patient response could be specifically correlated with the amplification of the Her2 gene. However, for the EGF receptor, it has been more difficult to find the right stratification biomarker and recent data demonstrate that genetic alterations for the EGF receptor have to be considered. Taken together, these data underline the need for a deeper understanding of both targeted receptor and human disease to determine pathways that might be investigated during early clinical trials in order to define relevant biomarkers for patient selection. This article, dealing with the c-Met tyrosine kinase receptor, provides an overview of c-Met alterations observed in cancer and proposes approaches for stratification biomarker selection.

Inhibition of tumor cell growth, invasion, and metastasis by EXEL-2880 (XL880, GSK1363089), a novel inhibitor of HGF and VEGF receptor tyrosine kinases

The Met receptor tyrosine kinase and its ligand, hepatocyte growth factor (HGF), are overexpressed and/or activated in a wide variety of human malignancies. Vascular endothelial growth factor (VEGF) receptors are expressed on the surface of vascular endothelial cells and cooperate with Met to induce tumor invasion and vascularization. EXEL-2880 (XL880, GSK1363089) is a small-molecule kinase inhibitor that targets members of the HGF and VEGF receptor tyrosine kinase families, with additional inhibitory activity toward KIT, Flt-3, platelet-derived growth factor receptor beta, and Tie-2. Binding of EXEL-2880 to Met and VEGF receptor 2 (KDR) is characterized by a very slow off-rate, consistent with X-ray crystallographic data showing that the inhibitor is deeply bound in the Met kinase active site cleft. EXEL-2880 inhibits cellular HGF-induced Met phosphorylation and VEGF-induced extracellular signal-regulated kinase phosphorylation and prevents both HGF-induced responses of tumor cells and HGF/VEGF-induced responses of endothelial cells. In addition, EXEL-2880 prevents anchorage-independent proliferation of tumor cells under both normoxic and hypoxic conditions. In vivo, these effects produce significant dose-dependent inhibition of tumor burden in an experimental model of lung metastasis. Collectively, these data indicate that EXEL-2880 may prevent tumor growth through a direct effect on tumor cell proliferation and by inhibition of invasion and angiogenesis mediated by HGF and VEGF receptors.

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.

Specific Grb2-mediated interactions regulate clathrin-dependent endocytosis of the cMet-tyrosine kinase

Lysosomal degradation of the receptor-tyrosine kinase cMet requires receptor ubiquitination by the E3 ubiquitin ligase Cbl followed by clathrin-dependent internalization. A role for Cbl as an adaptor for cMet internalization has been previously reported. However, the requirement for Cbl ubiquitin ligase activity in this process and its mode of recruitment to cMet has yet to be determined. Cbl can directly bind cMet at phosphotyrosine 1003 or indirectly via Grb2 to phosphotyrosine 1356 in the multisubstrate binding domain of cMet. The direct binding of Cbl with cMet is critical for receptor degradation and not receptor internalization. Here we show a strict requirement for Grb2 and the ubiquitin ligase activity of Cbl for cMet endocytosis. Receptor internalization was impaired by small interfering RNA depletion of Grb2, overexpression of dominant negative Grb2 mutants, and point mutations in the cMet multisubstrate docking site that inhibits the direct association of Grb2 with cMet. The requirement for Grb2 was specific and did not involve the multiadaptor Gab1. cMet internalization was impaired in cells expressing an ubiquitin ligase-deficient Cbl mutant or conjugation-deficient ubiquitin but was unaffected in cells expressing a Cbl mutant that is unable to bind cMet directly. Expression of a Cbl-Grb2 chimera rescued impaired cMet endocytosis in cells depleted of endogenous Grb2. These results indicate that the ubiquitin ligase activity of Cbl is critical for clathrin-dependent cMet internalization and suggest a role for Grb2 as an intermediary linking Cbl ubiquitin ligase activity to this process.

The MET receptor tyrosine kinase contributes to invasive tumour growth in rhabdomyosarcomas

The receptor tyrosine kinase MET and its ligand hepatocyte growth factor (HGF), have been implicated in the genesis of the paediatric tumour rhabdomyosarcoma (RMS). Addition of exogenous HGF to RH30 RMS cells enhanced non-chemotactic migration. Stable transfection of dominant negative MET into RH30 cells attenuated Matrigel invasion and in vivo tumour growth. To assess the role of a putative HGF-MET pathway in human RMS, we measured their expression in a panel of 68 human primary tumours. All tumours expressed MET but with a three orders of magnitude variation of expression and 62% of tumours co-expressed HGF. In contrast with other tumour types, neither high-MET expression nor HGF/MET coexpression correlated with metastatic disease. In a microarray screen, we identified CCN1 as being 7.8-fold up regulated following addition of HGF to RH30 cells and in RMS tumours, CCN1 expression correlated with HGF expression. Surprisingly, we identified MET as a consistent feature of embryonal and not alveolar RMS.

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.

MET receptor is overexpressed but not mutated in oral squamous cell carcinomas

Oral squamous cell carcinoma (SCC) is a neoplasm characterized by a high degree of local invasion and an elevated rate of metastasis to cervical lymph nodes. It has been shown that the Hepatocyte Growth Factor/Scatter Factor Receptor Met is constitutively activated in many human tumors of epithelial origin and that it plays a critical role to confer invasive properties to neoplastic cells. Most frequently, Met activation is due to receptor overexpression, but also point mutations in the tyrosine kinase domain can lead to deregulated activation. Here we show that in all the primary tumors examined this receptor is overexpressed. Direct sequencing of Met mRNAs failed to find any activating mutation in its intracellular domain. Moreover, in cell lines derived from squamous cell carcinomas, HGF-induced activation of Met resulted in the acquisition of invasive properties. All together these data suggest that the MET oncogene is involved in progression of squamous cell carcinoma toward an invasive-metastatic behavior.

Met receptor tyrosine kinase: enhanced signaling through adapter proteins

The Met receptor tyrosine kinase is the prototypic member of a small subfamily of growth factor receptors that when activated induce mitogenic, motogenic, and morphogenic cellular responses. The ligand for Met is hepatocyte growth factor/scatter factor (HGF/SF) and while normal HGF/SF-Met signaling is required for embryonic development, abnormal Met signaling has been strongly implicated in tumorigenesis, particularly in the development of invasive and metastatic phenotypes. Following ligand binding and autophosphorylation, Met transmits intercellular signals using a unique multisubstrate docking site present within the C-terminal end of the receptor. The multisubstrate docking site mediates the binding of several adapter proteins such as Grb2, SHC, Crk/CRKL, and the large adapter protein Gab1. These adapter proteins in turn recruit several signal transducing proteins to form an intricate signaling complex. Analysis of how these adapter proteins bind to the Met receptor and what signal transducers they recruit have led to more substantial models of HGF/SF-Met signal transduction and have uncovered new potential pathways that may be involved into Met mediated tumor cell invasion and metastasis.

A novel germ line juxtamembrane Met mutation in human gastric cancer

Activating mutations in the Met receptor tyrosine kinase, both germline and somatic, have been identified in human papillary renal cancer. Here we report a novel germline missense Met mutation, P1009S, in a patient with primary gastric cancer. The dosage of the mutant Met DNA was elevated in the tumor when compared to its matched normal DNA. Therefore, as with hereditary renal papillary cancer, the mutant Met allele may also be selectively duplicated in the tumor. Different from previously reported Met mutations, which occur in the tyrosine kinase domain, this missense mutation is located at the juxtamembrane domain, and is not constitutively activated. However, following treatment with HGF/SF, the P1009S mutant Met protein, expressed in NIH3T3 cells, displays increased and persistent tyrosine phosphorylation compared to the wild-type Met. Importantly, these cells also form colonies in soft agar, and are highly tumorigenic in athymic nude mice. A second nucleotide change in this region of Met, T1010I, was found in a breast cancer biopsy and a large cell lung cancer cell line. Although this previously reported 'polymorphism' did not stimulate NIH3T3 cell growth in soft agar, it was more active than the wild-type Met in the athymic nude mice tumorigenesis assay, suggesting that it may have effects on tumorigenesis. Met has been shown to be highly expressed in human gastric carcinoma cell lines, and our results raise the possibility that activating missense Met mutations could contribute to tumorigenesis of gastric cancer.