Signaling requirements for oncogenic forms of the Met tyrosine kinase receptor

Role of cMET in the development and progression of colorectal cancer

Mesenchymal-epithelial transition (MET) is a member of a distinct subfamily of heterodimeric receptor tyrosine kinase receptors that specifically binds the hepatocyte growth factor (HGF). Binding to HGF leads to receptor dimerization/multimerization and phosphorylation, resulting in its catalytic activation. MET activation drives the malignant progression of several tumor types, including colorectal cancer (CRC), by promoting signaling cascades that mainly result in alterations of cell motility, survival, and proliferation. MET is aberrantly activated in many human cancers through various mechanisms, including point mutations, gene amplification, transcriptional up-regulation, or ligand autocrine loops. MET promotes cell scattering, invasion, and protection from apoptosis, thereby acting as an adjuvant pro-metastatic gene for many tumor types. In CRC, MET expression confers more aggressiveness and worse clinical prognosis. With all of this rationale, inhibitors that target the HGF/MET axis with different types of response have been developed. HGF and MET are new promising targets to understand the pathogenesis of CRC and for the development of new, targeted therapies.

Current understanding of the molecular mechanisms of kidney cancer: a primer for urologists

Renal cell carcinoma (RCC), the fifth leading malignant condition for men and tenth for women, accounts for 3% of all malignancies in Canada. It is a heterogeneous epithelial malignancy with different subtypes and varied tumour biology. Although most cases of RCC are sporadic, up to 4% of patients have an inherited predisposition for the disease. In this article, we review the current molecular genetics of the different subtypes in hereditary and sporadic RCC. Significant developments in understanding the underlying genetic basis of RCC over the last 2 decades are attributed to intensive research about rare inherited renal cancer syndromes and the identification of the genes responsible for them. Many of these genes are also found in sporadic RCC. Understanding the molecular mechanisms involved in the pathogenesis of RCC has aided the development of molecular-targeted drugs for this disease.

Germline met mutations in mice reveal mutation- and background-associated differences in tumor profiles

Background

The receptor tyrosine kinase Met is involved in the progression and metastasis of numerous human cancers. Although overexpression and autocrine activation of the Met signaling pathway are commonly found in human cancers, mutational activation of Met has been observed in small cell and non-small cell lung cancers, lung adenocarcinomas, renal carcinomas, and mesotheliomas.

Methodology/Principal Findings

To investigate the influence of mutationally activated Met in tumorigenesis, we utilized a novel mouse model. Previously, we observed that various Met mutations developed unique mutation-specific tumor spectra on a C57BL/6 background. Here, we assessed the effect of genetic background on the tumorigenic potential of mutationally activated Met. For this purpose, we created congenic knock-in lines of the Met mutations D1226N, M1248T, and Y1228C on the FVB/N background. Consistent with the mutation-specific tumor spectra, several of the mutations were associated with the same tumor types as observed on C57BL/6 background. However, on the FVB/N background most developed a high incidence of mammary carcinomas with diverse histopathologies.

Conclusions/Significance

This study demonstrates that on two distinct mouse backgrounds, Met is able to initiate tumorigenesis in multiple cell types, including epithelial, hematopoietic, and endothelial. Furthermore, these observations emphasize that even a modest increase in Met activation can initiate tumorigenesis with both the Met mutational spectra and host background having profound influence on the type of tumor generated. Greater insight into the interaction of genetic modifiers and Met signaling will significantly enhance our ability to tailor combination therapies for Met-driven cancers.

Differential inhibition sensitivities of MET mutants to the small molecule inhibitor SU11274

Point mutations emerge as one of the rate-limiting steps in tumor response to small molecule inhibitors of protein kinases. Here we characterized the response of the MET mutated variants, V1110I, V1238I, V1206L and H1112L to the small molecule SU11274. Our results reveal a distinct inhibition pattern of the four mutations with IC(50) values for autophosphorylation inhibition ranging between 0.15 and 1.5muM. Differences were further seen on the ability of SU11274 to inhibit phosphorylation of downstream MET transducers such as AKT, ERK, PLCgamma and STAT3 and a variety of MET-dependent biological endpoints. In all the assays, H1112L was the most sensitive to SU11274, while V1206L was less affected under the used concentration range. The differences in responses to SU11274 are discussed based on a structural model of the MET kinase domain.

Deletion of the ectodomain unleashes the transforming, invasive, and tumorigenic potential of the MET oncogene

The c-MET proto-oncogene, encoding the p190 hepatocyte growth factor tyrosine kinase receptor, can acquire oncogenic potential by multiple mechanisms, such as gene rearrangement, amplification and overexpression, point mutation, and ectopic expression, all resulting in its constitutive activation. Hepatocyte growth factor receptor truncated forms are generated by post-translational cleavage: p140 and p130 lack the kinase domain and are inactive. Their C-terminal remnant fragments are generally undetectable in normal cells, but a membrane-associated truncated form is recognized by anti-C-terminus antibodies in some human tumors, suggesting that a hepatocyte growth factor receptor lacking the ectodomain, but retaining the transmembrane and intracellular domains (Met-EC-), could acquire oncogenic properties. Herein we show that NIH-3T3 cells transduced with MET-EC- expressed a membrane-associated constitutively tyrosine-phosphorylated 60-kDa protein and, similarly to NIH-3T3 cells expressing the cytosolic oncoprotein Tpr-Met, showed activated extracellular regulated kinase 1/2 mitogen-activated protein kinase and Akt downstream transducers. Compared to control NIH-3T3 cells, NIH-3T3-Met-EC- cells grew faster and showed anchorage-independent growth and invasive properties in all aspects similar to cells expressing the transforming TPR-MET. Nude female mice injected subcutaneously with NIH-3T3-Met-EC- cells developed visible tumors, displaying the typical morphology of carcinomas with polygonal cells, in contrast to sarcomas with spindle-shaped cells induced by the injection of NIH-3T3-Tpr-Met cells. It is suggested that the different subcellular localization of the oncoproteins, more than differences in signal transduction, could be responsible for the tumor phenotype. All together, these data show that deletion of the ectodomain activates the hepatocyte growth factor receptor and its downstream signaling pathways, unleashing its transforming, invasive, and tumorigenic potential.

Progesterone receptor A and c-Met mediates spheroids-endometrium attachment

BACKGROUND

Implantation in humans involves cross talk between an active blastocyst and receptive endometrium. The role of the endometrial receptors in this complex embryo-maternal interaction is still unclear. We tested gene and protein expression of endometrial receptors (Progesterone receptor (PR) and c-Met) and the effect of theses receptors in endometrial receptivity.

METHODS

Two endometrial cell lines were used: HEC-1A and RL95-2 considered as being of low and high receptivity, respectively. Western blot and RT-PCR analysis were utilized to study the receptor expression profile.The role of endometrial receptors in endometrial receptivity was studied by attachment and invasion assays of JAR spheroids (made of a trophoblast cell line) on endometrial cells. Different manipulations of inhibition and stimulation of the endometrial receptors were used including: inhibition by specific antibodies against the receptors, or antagonist of the receptors, as well as transfection with antisense for the endometrial receptors, stimulation by specific ligands for the receptors and transfection with the gene for endometrial receptors.

RESULTS

Different protein expression patterns of endometrial receptors were observed between the tested endometrial cell lines. The expression levels of PRA ratio to PRB, and the 50 kDa c-MET isoform were significantly lower in HEC-1A as compared with RL95-2. Attachment rates and growth of JAR spheroids into HEC-1A were significantly lower as compared with RL95-2. Stimulation of PR with progesterone altered attachment rates to HEC-1A. Inhibition of PR with RU-486 mildly increased attachment rate to HEC-1A whereas it slightly decreased attachment rate to RL95-2. c-Met inhibition decreased attachment rates only to HEC-1A cells that expressing high levels of Plexin-B1 (PB1). Immunoprecipitation studies revealed that c-Met and PB1 associate in complexes in the endometrial cell lines.

CONCLUSION

Differential endometrial receptor profiles are expressed during the receptivity period. The attachment and invasion processes are separately regulated. We suggest a biologically functional role for PRA in endometrial receptivity and in the attachment process. c-Met contribution is minor and related with creation of a complex with PB1.

Suppression of human colon tumor growth by adenoviral vector-mediated NK4 expression in an athymic mouse model

AIM: To investigate the suppressive effects of adenoviral vector-mediated expression of NK4, an antagonist of hepatocyte growth factor (HGF), on human colon cancer in an athymic mouse model to explore the possibility of applying NK4 to cancer gene therapy.

METHODS: A human colon tumor model was developed by subcutaneous implantation of tumor tissue formed by LS174T cells grown in athymic mice. Fifteen tumor-bearing mice were randomized into three groups (n = 5 in each group) at d 3 after tumor implantation and mice were injected intratumorally with phosphate-buffered saline (PBS) or with recombinant adenovirus expressing β-galactosidase (Ad-LacZ) or NK4 (rvAdCMV/NK4) at a 6-d interval for total 5 injections in each mouse. Tumor sizes were measured during treatment to draw a tumor growth curve. At d 26 after the first treatment, all animals were sacrificed and the tumors were removed to immunohistochemically examine proliferating cell nuclear antigen (PCNA), microvessel density (represented by CD31), and apoptotic cells. In a separate experiment, 15 additional athymic mice were employed to develop a tumor metastasis model by intraperitoneal injection (ip) of LS174T cells. These mice were randomized into 3 groups (n = 5 in each group) at d 1 after injection and were treated by ip injection of PBS, or Ad-LacZ, or rvAdCMV/NK4 at a 6-d interval for total two injections in each mouse. All animals were sacrificed at d 14 and the numbers and weights of disseminated tumors within the abdominal cavity were measured.

RESULTS: Growth of human colon tumors were significantly suppressed in the athymic mice treated with rvAdCMV/NK4 (2537.4 ± 892.3 mm³) compared to those treated by either PBS (5175.2 ± 1228.6 mm³) or Ad-LacZ (5578.8 ± 1955.7 mm³) (P < 0.05). The tumor growth inhibition rate was as high as 51%. Immunohistochemical staining revealed a similar PCNA labeling index (75.1% ± 11.2% in PBS group vs 72.8% ± 7.6% in Ad-LacZ group vs 69.3% ± 9.4% in rvAdCMV/NK4 group) in all groups, but significantly lower microvessel density (10.7 ± 2.4 in rvAdCMV/NK4 group vs 25.6 ± 3.8 in PBS group or 21.3 ± 3.5 in Ad-LacZ group, P < 0.05), and a markedly higher apoptotic index (7.3% ± 2.4% in rvAdCMV/NK4 group vs 2.6 ± 1.1% in PBS group or 2.1% ± 1.5% in Ad-LacZ group, P < 0.05) in the rvAdCMV/NK4 group compared to the two control groups. In the tumor metastasis model, the number and weight of disseminated tumors of mice treated with rvAdCMV/NK4 were much lower than those of the control groups (tumor number: 6.2 ± 3.3 in rvAdCMV/NK4 group vs 22.9 ± 7.6 in PBS group or 19.8 ± 8.5 in Ad-LacZ group, P < 0.05; tumor weight: 324 ± 176 mg in rvAdCMV/NK4 group vs 962 ± 382 mg in PBS group or 1116 ± 484 mg in Ad-LacZ group, P < 0.05).

CONCLUSION: The recombinant adenovirus, rvAdCMV/NK4, can attenuate the growth of colon cancer in vivo, probably by suppressing angiogenesis and inducing tumor cell apoptosis, but not by direct suppression of tumor cell proliferation. Moreover, rvAdCMV/NK4 may inhibit peritoneal dissemination of colon cancer cells in a murine tumor metastasis model. These findings indicate that NK4 gene transfer may be an effective tool for the treatment of colon cancer.

c-Met overexpression is a prognostic factor in ovarian cancer and an effective target for inhibition of peritoneal dissemination and invasion

The hepatocyte growth factor receptor c-Met is a receptor tyrosine kinase that plays an important role in tumor growth by activating mitogenic signaling pathways. The goal of this study was to evaluate the role of c-Met in the biology of ovarian cancer and to determine its potential as a therapeutic target. c-Met protein expression was detected by immunohistochemistry in 138 advanced-stage ovarian cancers using a tissue microarray annotated with disease-specific patient follow-up. Fifteen of 138 (11%) tissues had c-Met overexpression. Median survival for patients with high c-Met levels was 17 months versus 32 months (P = 0.001) for patients with low c-Met expression. Infection of SKOV-3ip1 cells with an adenovirus expressing a small interfering RNA (siRNA) against c-Met efficiently inhibited c-Met protein and mRNA expression as well as extracellular signal-regulated kinase and phosphatidylinositol 3-kinase signaling. It also inhibited adhesion to different extracellular matrix components, human primary mesothelial cells, and full-thickness human peritoneum and, in vivo, to mouse peritoneum. This was paralleled by a significant reduction in alpha(5) and beta(1) integrin protein and mRNA expression as well as a reduction of urokinase and matrix metalloproteinase (MMP)-2/MMP-9 activity. In SKOV-3ip1 ovarian cancer xenografts, i.p. treatment with the c-Met siRNA significantly reduced tumor burden, ascites formation, protease activity, and the number of peritoneal implants but not tumor size or angiogenesis. These results suggest that c-Met overexpression is a prognostic factor in ovarian cancer and that targeting c-Met in vivo inhibits peritoneal dissemination and invasion through an alpha(5)beta(1) integrin-dependent mechanism. Therefore, c-Met should be explored further as a therapeutic target in ovarian cancer.

Oncogenic Met receptor induces cell-cycle progression in Xenopus oocytes independent of direct Grb2 and Shc binding or Mos synthesis, but requires phosphatidylinositol 3-kinase and Raf signaling

Biological responses of hepatocyte growth factor (HGF) are mediated by the Met receptor tyrosine kinase. Although HGF is a potent mitogen for a variety of cells, the signals required for cell-cycle progression by the Met/HGF receptor are poorly defined. In this study, we have used the Xenopus oocyte system to define the role of various Met proximal-binding partners and downstream signaling pathways in cell-cycle regulation. We show that cell-cycle progression and activation of MAPK and JNK mediated by the oncogenic Met receptor, Tpr-Met, are dependent on its kinase activity and the presence of the twin phosphotyrosine (Y482 & Y489) residues in its C-terminus, but that the recruitment of Grb2 and Shc adaptor proteins is dispensable, implicating other signaling molecules. However, using Met receptor oncoproteins engineered to recruit specific signaling proteins, we demonstrate that recruitment of Grb2 or Shc adaptor proteins is sufficient to induce cell-cycle progression and activation of MAPK and JNK, while the binding of phospholipase-Cgamma or phosphatidylinositol 3-kinase alone fails to elicit these responses. Using various means to block phosphatidylinositol 3-kinase, phospholipase-Cgamma, MEK, JNK, Mos, and Raf1 activity, we show that unlike the fibroblast growth factor receptor, MEK-dependent and independent signaling contribute to Met receptor-mediated cell-cycle progression, but phospholipase-Cgamma or JNK activity and Mos synthesis are not critical. Notably, we demonstrate that Raf1 and phosphatidylinositol 3-kinase signaling are required for cell-cycle progression initiated by the Met receptor, a protein frequently deregulated in human tumors.

Oncogenic Met receptor induces ectopic structures in Xenopus embryos

When aberrantly expressed or activated, the Met receptor tyrosine kinase is involved in tumor invasiveness and metastasis. In this study, we have used the Xenopus embryonic system to define the role of various Met proximal-binding partners and downstream signaling pathways in regulating an induced morphogenetic event. We show that expression of an oncogenic derivative of the Met receptor (Tpr-Met) induces ectopic morphogenetic structures during Xenopus embryogenesis. Using variant forms of Tpr-Met that are engineered to recruit a specific signaling molecule of choice, we demonstrate that the sole recruitment of either the Grb2 or the Shc adaptor protein is sufficient to induce ectopic structures and anterior reduction, while the recruitment of PI-3Kinase (PI-3K) is necessary but not sufficient for this effect. In contrast, the recruitment of PLCgamma can initiate the induction, but fails to maintain or elongate supernumerary structures. Finally, evidence indicates that the Ras/Raf/MAPK pathway is necessary, but not sufficient to induce these structures. This study also emphasizes the importance of examining signaling molecules in the regulatory context that is provided by receptor/effector interactions when assessing a role in cell growth and differentiation.

C-Met overexpression in node-positive breast cancer identifies patients with poor clinical outcome independent of Her2/neu

Receptor tyrosine kinases play an important role in malignant transformation of epithelial cells by activating signal transduction pathways important for proliferation, invasion and metastasis. In a pilot study (n = 40), we evaluated expression of the c-Met and Her2/neu receptor tyrosine kinases and the c-Met ligand hepatocyte growth factor/scatter factor (HGF/SF) in primary breast cancers and their lymph node metastases using both conventional immunohistochemistry and confocal immunofluorescence. Neither c-Met and HGF/SF nor Her2/neu expression correlated with established prognostic factors such as age, lymph node involvement, estrogen receptor (ER), progesterone receptor (PR), tumor size, or grade. Both staining methods confirmed a significant correlation between c-Met overexpression and a high risk of disease progression. Furthermore, among tumors with c-Met overexpression, only 50% also overexpress Her2/neu, thus identifying a subset of patients with aggressive disease in addition to Her2/neu. Median disease-free survival in patients with c-Met overexpressing tumors was 8 months compared to 53 months when c-Met expression was low (p = 0.037; RR = 3.0). This significant impact of c-Met on tumor aggressiveness independent of Her2/neu was also confirmed by multivariate analysis. In conclusion, the role of c-Met expression as a prognostic variable and consequently as an interesting target for novel therapeutic approaches deserves further analysis in a larger cohort of patients.

Hepatocyte growth factor activates several transduction pathways in rat pancreatic acini

The receptor of hepatocyte growth factor (HGF), c-met induces different physiological responses in several cell types. Little is known about the role of HGF in exocrine pancreas. However, abnormal HGF signaling has been strongly implicated in pancreatic tumorigenesis and association of HGF with pancreatitis has been demonstrated. We have studied the presence of c-met and activation of their intracellular pathways associated in rat pancreatic acini in comparison with cholecystokinin (CCK) and epidermal growth factor (EGF). C-met expression in rat exocrine pancreas was identified by immunohistochemistry and immunoprecipitation followed by Western analysis. Tyrosine phosphorylation of c-met is strongly stimulated as well as kinase pathways leading to ERK1/2 cascade. HGF, but not CCK or EGF, selectively caused a consistent increase in the amount of p85 regulatory subunit of PI3-K present in anti-phosphotyrosine immunoprecipitates. Downstream of PI3-K, HGF increased Ser473 phosphorylation of Akt selectively, as CCK or EGF did not affect it. HGF selectively stimulated tyrosine phosphorylation of phosphatase PTP1D. HGF failed to promote the well-known CCK effects in pancreatic acini such as amylase secretion and intracellular calcium mobilization. Although HGF shares activation of ERK1/2 with CCK, we demonstrate that it promotes the selective activation of intracellular pathways not regulated by CCK or EGF. Our results suggest that HGF is an in vivo stimulus of pancreatic acini and provide novel insight into the transduction pathways and effects of c-met/HGF in normal pancreatic acinar cells.

Use of signal specific receptor tyrosine kinase oncoproteins reveals that pathways downstream from Grb2 or Shc are sufficient for cell transformation and metastasis

Many human cancers have been associated with the deregulation of receptor tyrosine kinases (RTK). However, the individual contribution of receptor-associated signaling proteins in cellular transformation and metastasis is poorly understood. To examine the role of RTK activated signal transduction pathways to processes involved in cell transformation, we have exploited the oncogenic derivative of the Met RTK (Tpr-Met). Unlike other RTKs, twin tyrosine residues in the carboxy-terminal tail of the Met oncoprotein and receptor are required for all biological and transforming activities, and a mutant lacking these tyrosines is catalytically active but non transforming. Using this mutant we have inserted oligonucleotide cassettes, each encoding a binding site for a specific signaling protein derived from other RTKs. We have generated variant forms of the Tpr-Met oncoprotein with the ability to bind individually to the p85 subunit of PI3'K, PLCgamma, or to the Grb2 or Shc adaptor proteins. Variants that recruit the Shc or Grb2 adaptor proteins generated foci of morphologically transformed fibroblast cells and induced anchorage-independent growth, scattering of epithelial cells and experimental metastasis. In contrast, variants that bind and activate PI3'K or PLCgamma failed to generate readily detectable foci. Although cell lines expressing the PI3'K variant grew in soft-agar, these cells were non metastatic. Using this unique RTK oncoprotein model, we have established that Grb2 or Shc dependent signaling pathways are sufficient for cell transformation and metastatic spread.

Role of the hepatocyte growth factor receptor, c-Met, in oncogenesis and potential for therapeutic inhibition

Receptor tyrosine kinases have become important therapeutic targets for anti-neoplastic molecularly targeted therapies. c-Met is a receptor tyrosine kinase shown to be over-expressed and mutated in a variety of malignancies. Stimulation of c-Met via its ligand hepatocyte growth factor also known as scatter factor (HGF/SF), leads to a plethora of biological and biochemical effects in the cell. There has been considerable knowledge gained on the role of c-Met-HGF/SF axis in normal and malignant cells. This review summarizes the structure of c-Met and HGF/SF and their family members. Since there are known mutations of c-Met in solid tumors, particularly in papillary renal cell carcinoma, we have summarized the various mutations and over-expression of c-Met known thus far. Stimulation of c-Met can lead to scattering, angiogenesis, proliferation, enhanced cell motility, invasion, and eventual metastasis. The biological functions altered by c-Met are quite unique and described in detail. Along with biological functions, various signal transduction pathways, including the cytoskeleton are altered with the activation of c-Met-HGF/SF loop. We have recently shown the phosphorylation of focal adhesion proteins, such as paxillin and p125FAK in response to c-Met stimulation in lung cancer cells, and this is detailed here. Finally, c-Met when mutated or over-expressed in malignant cells serves as an important therapeutic target and the most recent data in terms of inhibition of c-Met and downstream signal transduction pathways is summarized.

Absence of tpr-met and expression of c-met in human gastric mucosa and carcinoma

The c-met proto-oncogene, encoding the hepatocyte growth factor receptor, can be activated by various mechanisms. These include, among others, gene amplification with concomitant overexpression and the tpr-met oncogenic rearrangement. In the case of gastric cancer, contradictory results on the presence of the tpr-met oncogenic rearrangement have been published. The current study aimed therefore to assess the prevalence of tpr-met expression in Caucasian gastric adenocarcinomas, to evaluate the importance of this oncogene in their carcinogenesis. In addition, the level of c-met expression was determined, to evaluate the role of this alternative mode of activation of the proto-oncogene. A series of Caucasian gastric adenocarcinomas (n=43) and normal gastric mucosal samples (n=14) was analysed for tpr-met and c-met expression. Expression of tpr-met mRNA in the samples was performed by two reverse transcriptase polymerase chain reaction (RT-PCR) assays, with excellent correlation. The specificity of both methods was confirmed by direct sequencing of the PCR products of the MNNG-HOS cell line, which is known to contain the rearrangement. The level of c-met expression was assessed using semi-quantitative RT-PCR assays and immunohistochemistry (IHC). None of the normal gastric mucosal or gastric adenocarcinoma samples expressed tpr-met mRNA, as determined by both RT-PCR assays. Seventy per cent of the adenocarcinomas showed overexpression of c-met, according to elevated c-met mRNA levels, compared with the expression level of normal gastric mucosa. A significant correlation was found between the level of c-met mRNA and protein expression. In conclusion, these results strongly suggest that tpr-met activation does not play a role in Caucasian gastric carcinogenesis, while overexpression of the c-met gene occurs in the majority of Caucasian gastric adenocarcinomas.

Potent blockade of hepatocyte growth factor-stimulated cell motility, matrix invasion and branching morphogenesis by antagonists of Grb2 Src homology 2 domain interactions

Hepatocyte growth factor (HGF) stimulates mitogenesis, motogenesis, and morphogenesis in a wide range of cellular targets during development, homeostasis and tissue regeneration. Inappropriate HGF signaling occurs in several human cancers, and the ability of HGF to initiate a program of protease production, cell dissociation, and motility has been shown to promote cellular invasion and is strongly linked to tumor metastasis. Upon HGF binding, several tyrosines within the intracellular domain of its receptor, c-Met, become phosphorylated and mediate the binding of effector proteins, such as Grb2. Grb2 binding through its SH2 domain is thought to link c-Met with downstream mediators of cell proliferation, shape change, and motility. We analyzed the effects of Grb2 SH2 domain antagonists on HGF signaling and observed potent blockade of cell motility, matrix invasion, and branching morphogenesis, with ED(50) values of 30 nm or less, but only modest inhibition of mitogenesis. These compounds are 1000-10,000-fold more potent anti-motility agents than any previously characterized Grb2 SH2 domain antagonists. Our results suggest that SH2 domain-mediated c-Met-Grb2 interaction contributes primarily to the motogenic and morphogenic responses to HGF, and that these compounds may have therapeutic application as anti-metastatic agents for tumors where the HGF signaling pathway is active.

Structural basis of oncogenic activation caused by point mutations in the kinase domain of the MET proto-oncogene: Modeling studies

Missense mutations in the tyrosine kinase domain of the MET proto-oncogene occur in selected cases of papillary renal carcinoma. In biochemical and biological assays, these mutations produced constitutive activation of the MET kinase and led to tumor formation in nude mice. Some mutations caused transformation of NIH 3T3 cells. To elucidate the mechanism of ligand-independent MET kinase activation by point mutations, we constructed several 3D models of the wild-type and mutated MET catalytic core domains. Analysis of these structures showed that some mutations (e.g., V1110I, Y1248H/D/C, M1268T) directly alter contacts between residues from the activation loop in its inhibitory conformation and those from the main body of the catalytic domain; others (e.g., M1149T, L1213V) increase flexibility at the critical points of the tertiary structure and facilitate subdomain movements. Mutation D1246N plays a role in stabilizing the active form of the enzyme. Mutation M1268T affects the S+1 and S+3 substrate-binding pockets. Models implicate that although these changes do not compromise the affinity toward the C-terminal autophosphorylation site of the MET protein, they allow for binding of the substrate for the c-Abl tyrosine kinase. We provide biochemical data supporting this observation. Mutation L1213V affects the conformation of Tyr1212 in the active form of MET. Several somatic mutations are clustered at the surface of the catalytic domain in close vicinity of the probable location of the MET C-terminal docking site for cytoplasmic effectors.

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.

The tyrosine phosphatase SHP-2 is required for sustained activation of extracellular signal-regulated kinase and epithelial morphogenesis downstream from the met receptor tyrosine kinase

Epithelial morphogenesis is critical during development and wound healing, and alterations in this program contribute to neoplasia. Met, the hepatocyte growth factor (HGF) receptor, promotes a morphogenic program in epithelial cell lines in matrix cultures. Previous studies have identified Gab1, the major phosphorylated protein following Met activation, as important for the morphogenic response. Gab1 is a docking protein that couples the Met receptor with multiple signaling proteins, including phosphatidylinositol-3 kinase, phospholipase Cgamma, the adapter protein Crk, and the tyrosine specific phosphatase SHP-2. HGF induces sustained phosphorylation of Gab1 and sustained activation of extracellular signal-regulated kinase (Erk) in epithelial Madin-Darby canine kidney cells. In contrast, epidermal growth factor fails to promote a morphogenic program and induces transient Gab1 phosphorylation and Erk activation. To elucidate the Gab1-dependent signals required for epithelial morphogenesis, we undertook a structure-function approach and demonstrate that association of Gab1 with the tyrosine phosphatase SHP-2 is required for sustained Erk activation and for epithelial morphogenesis downstream from the Met receptor. Epithelial cells expressing a Gab1 mutant protein unable to recruit SHP-2 elicit a transient activation of Erk in response to HGF. Moreover, SHP-2 catalytic activity is required, since the expression of a catalytically inactive SHP-2 mutant, C/S, abrogates sustained activation of Erk and epithelial morphogenesis by the Met receptor. These data identify SHP-2 as a positive modulator of Erk activity and epithelial morphogenesis downstream from the Met receptor.

Expression of c-Met and heparan-sulfate proteoglycan forms of CD44 in colorectal cancer

In colorectal cancer patients, prognosis is not determined by the primary tumor but by the formation of distant metastases. Molecules that have been implicated in the metastatic process are the proto-oncogene product c-Met and CD44 glycoproteins. Recently, we obtained evidence for functional collaboration between these two molecules: CD44 isoforms decorated with heparan sulfate chains (CD44-HS) can bind the c-Met ligand, the growth and motility factor hepatocyte growth factor/scatter factor (HGF/SF). This interaction strongly promotes signaling through the receptor tyrosine kinase c-Met. In the present study, we explored the expression of CD44-HS, c-Met, and HGF/SF in the normal human colon mucosa, and in colorectal adenomas and carcinomas, as well as their interaction in colorectal cancer cell lines. Compared to the normal colon, CD44v3 isoforms, which contain a site for HS attachment, and c-Met, were both overexpressed on the neoplastic epithelium of colorectal adenomas and on most carcinomas. Likewise, HGF/SF was expressed at increased levels in tumor tissue. On all tested colorectal cancer cell lines CD44v3 and c-Met were co-expressed. As was shown by immunoprecipitation and Western blotting, CD44 on these cells lines was decorated with HS. Interaction with HS moieties on colorectal carcinoma (HT29) cells promoted HGF/SF-induced activation of c-Met and of the Ras-MAP kinase pathway. Interestingly, survival analysis showed that CD44-HS expression predicts unfavorable prognosis in patients with invasive colorectal carcinomas. Taken together, our findings indicate that CD44-HS, c-Met, and HGF/SF are simultaneously overexpressed in colorectal cancer and that HS moieties promote c-Met signaling in colon carcinoma cells. These observations suggest that collaboration between CD44-HS and the c-Met signaling pathway may play an important role in colorectal tumorigenesis.

The proto-oncogene c-Cbl is a positive regulator of Met-induced MAP kinase activation: a role for the adaptor protein Crk

Hepatocyte growth factor triggers a complex biological program leading to invasive cell growth by activating the c-Met receptor tyrosine kinase. Following activation, Met signaling is elicited via its interactions with SH2-containing proteins, or via the phosphorylation of the docking protein Gab1, and the subsequent interaction of Gab1 with additional SH2-containing effector molecules. We have previously shown that the interaction between phosphorylated Gab1 and the adaptor protein Crk mediates activation of the JNK pathway downstream of Met. We report here that c-Cbl, which is a Gab1-like docking protein, also becomes tyrosine-phosphorylated in response to Met activation and serves as a docking molecule for various SH2-containing molecules, including Crk. We further show that Cbl is similarly capable of enhancing Met-induced JNK activation, and several lines of experimentation suggests that it does so by interacting with Crk. We also show that both Cbl and Gab1 enhance Met-induced activation of another MAP kinase cascade, the ERK pathway, in a Crk-independent manner. Taken together, our studies demonstrate a previously unidentified functional role for Cbl in Met signaling and suggest that Met utilizes at least two docking proteins, Gab1 and Cbl, to activate downstream signaling pathways. Oncogene (2000) 19, 4058 - 4065.

The hepatocyte growth factor/Met pathway in development, tumorigenesis, and B-cell differentiation

This article summarizes the structure, signal transduction and physiologic functions of the HGF/Met pathway, as well as its role in tumor growth, invasion, and metastasis. Moreover, it highlights recent studies indicating a role for the HGF/Met pathway in antigen-specific B-cell development and B-cell neoplasia.

Inherited epithelial tumors of the kidney: old and new diseases

This review summarizes information on inherited epithelial tumors of the kidney. Emphasis is placed on identifying clinically distinct inherited forms of renal cancer because each distinct clinical syndrome defines a different renal cancer susceptibility gene. So far, two genes that predispose to epithelial cancers of the kidney have been identified, VHL and the MET proto-oncogene. Available evidence suggests that several renal cancer genes remain to be identified.

Cross-talk between the proto-oncogenes Met and Ron

Scatter Factors control a complex genetic program known as 'invasive growth'. HGF (Scatter factor 1) and MSP (Scatter Factor 2) bind to tyrosine kinase receptors encoded by the proto-oncogenes MET and RON. Using the appropriate 'kinase inactive' mutant receptors, we show that ligand-induced activation of Met results in transphosphorylation of Ron, and vice versa. Transphosphorylation is direct, as it occurs in Met or Ron receptors lacking the docking sites for signal transducers. Phosphate groups are transferred to the tyrosine phosphorylation sites responsible both for kinase up-regulation (Met: Y1234/Y1235 and Ron: Y1238/Y1239) and for generation of signal transducer docking sites (Met: Y1349/Y1356 and Ron Y1353/Y1360). The transphosphorylation specifically takes place for the receptor subfamily, as it is not observed between Met or Ron and ErbB1, ErbB2 or TrkA. Cross-linking experiments show that non-covalent Met-Ron complexes are present on the cell surface, before ligand-induced dimerization. Co-expression of a kinase inactive Ron receptor with naturally-occurring oncogenic Met mutants suppresses the transforming phenotype, suggesting a dominant negative role for the inefficient kinase partner. These data show that, while specific for their ligands, scatter factor receptors cross-talk and cooperate in intracellular signaling.

Direct binding of the signaling adapter protein Grb2 to the activation loop tyrosines on the nerve growth factor receptor tyrosine kinase, TrkA

We demonstrate that the signaling adapter, Grb2, binds directly to the neurotrophin receptor tyrosine kinase, TrkA. Grb2 binding to TrkA is independent of Shc, FRS-2, phospholipase Cgamma-1, rAPS, and SH2B and is observed in in vitro binding assays, yeast two-hybrid assays, and in co-immunoprecipitation assays. Grb2 binding to TrkA is mediated by the central SH2 domain, requires a kinase-active TrkA, and is phosphotyrosine-dependent. By analyzing a series of rat TrkA mutants, we demonstrate that Grb2 binds to the carboxyl-terminal residue, Tyr(794), as well as to the activation loop tyrosines, Tyr(683) and Tyr(684). By using acidic amino acid substitutions of the activation loop tyrosines on TrkA, we can stimulate constitutive kinase activity and TrkA-Shc interactions but, importantly, abolish TrkA/Grb2 binding. Thus, in addition to providing the first evidence of direct Grb2 binding to the neurotrophin receptor, TrkA, these data provide the first direct evidence that the activation loop tyrosines of a receptor tyrosine kinase, in addition to their essential role in kinase activation, also serve a direct role in the recruitment of intracellular signaling molecules.

Tumorigenesis mediated by MET mutant M1268T is inhibited by dominant-negative Src

We recently described germline and somatic mutations in the MET gene associated with papillary renal carcinoma type 1. MET mutation M1268T was located in a codon highly conserved among receptor tyrosine kinases, and homologous to the codon mutated in multiple endocrine neoplasia type 2B, and many cases of sporadic medullary carcinoma of the thyroid gland (Ret M918T). Ret M918T and MET M1268T have previously been shown to be highly active in mouse NIH3T3 transformation assays, and to change the substrate specificity of the kinase. We studied the mechanism of transformation mediated by MET M1268T by analysing a clone, F4, derived from NIH3T3 cells transformed by MET M1268T. In contrast to NIH3T3 cells, F4 cells grew in suspension in tissue culture, and rapidly formed tumors in nude mice. We found that c-Src was constitutively bound to MET proteins in F4 cells, and that Src kinase activity was elevated. Transfection of dominant negative Src constructs into F4 cells eliminated the ability of F4 cells to grow in suspension culture and retarded the growth of F4 cells in vivo. The ability of transfected dominant negative Src constructs to inhibit the growth of F4 cells correlated with the inhibition of phosphorylation of paxillin and focal adhesion kinase. Transfection of dominant negative Src constructs into F4 cells had no effect on Grb2 binding or PLC gamma phosphorylation. The results suggest that c-Src participates in the tumorigenic phenotype induced in NIH3T3 cells by MET M1268T by signaling through focal adhesion kinase and paxillin. Oncogene (2000).

Enhanced transformation by a plasma membrane-associated met oncoprotein: activation of a phosphoinositide 3'-kinase-dependent autocrine loop involving hyaluronic acid and CD44

A Met-hepatocyte growth factor receptor oncoprotein, Tpr-Met, generated by chromosomal rearrangement, fuses a protein dimerization motif with the cytoplasmic domain of the Met receptor, producing a cytosolic, constitutively activated tyrosine kinase. Although both the Met receptor and the Tpr-Met oncoprotein associate with the same substrates, activating mutations of the Met receptor in hereditary papillary renal carcinomas have different signaling requirements for transformation than Tpr-Met. This suggests differential activation of membrane-localized pathways by oncogenic forms of the membrane-bound Met receptor but not by the cytoplasmic Tpr-Met oncoprotein. To establish which pathways might be differentially regulated, we have localized the constitutively activated Tpr-Met oncoprotein to the membrane using the c-src myristoylation signal. Membrane localization enhances cellular transformation, focus formation, and anchorage-independent growth and induces tumors with a distinct myxoid phenotype. This correlates with the induction of hyaluronic acid (HA) and the presence of a distinct form of its receptor, CD44. A pharmacological inhibitor of phosphoinositide 3' kinase (PI3'K), inhibits the production of HA, and conversely, an activated, plasma membrane-targeted form of PI3'K is sufficient to enhance HA production. Furthermore, the multisubstrate adapter protein Gab-1, which couples the Met receptor with PI3'K, enhances Met receptor-dependent HA synthesis in a PI3'K-dependent manner. These results provide a positive link to a role for HA and CD44 in Met receptor-mediated oncogenesis and implicate PI3'K in these events.

Two mutations affecting conserved residues in the Met receptor operate via different mechanisms

We have investigated the mechanism by which two oncogenic mutations (M1268T and D1246H/N; Amino-acids are numbered according to Schmidt et al., 1999) affecting conserved residues in the catalytic domain of the Met receptor, activate its transforming potential. Both mutations were previously found in tumorigenic forms of the Ret and Kit receptors, respectively. The mutated residues are located either in the P+1 loop (M) or within the activation loop (A-loop) (D), which in a number of receptor tyrosine kinases harbors a pair of tandem tyrosines (Y1252-1253 in Met). Ligand-induced dimerization promotes their phosphorylation, and locks the A-loop into an open conformation. When unphosphorylated, the tandem tyrosines inhibit enzymatic activity by blocking the active site. Upon Y-->F mutation of Y1252-1253, neither ligand binding nor Tpr-mediated dimerization can release this block. Here we show that the M1268T mutation partially rescues the kinase activity (and the transforming ability) of the Y1252-1253F Tpr-Met mutant, but is completely dependent on dimerization for its effect. In contrast, the two D1246H/N mutants strictly depend on Y1252-1253 for activity. Surprisingly, however, they constitutively activate the isolated cytoplasmic TK domain of Met (Cyto-Met). These data indicate that the two mutations operate via distinct mechanisms.

Activating mutations in the Met receptor overcome the requirement for autophosphorylation of tyrosines crucial for wild type signaling

Mutations in Met have been identified in human cancer, and we have previously shown that these mutations deregulate the enzymatic activity of this tyrosine kinase receptor, thereby unleashing its oncogenic potential. Signal transduction via wild type Met has been shown to require the autophosphorylation of two tyrosine doublets; Y8,9 which functions to enhance enzymatic activity, and Y14,15 which provides docking sites for signaling molecules, and in the present investigation we examine the importance of these residues for signaling via mutationally activated Met. We find that activating mutations introduced into a membrane-spanning Met receptor circumvent the normally stringent requirement for Y8,9 phosphorylation, and do so in a largely ligand-dependent fashion. Similarly, activating mutations introduced into a constitutively dimerized cytoplasmic form of Met (i.e. Tpr-Met) facilitate its autophosphorylation and oncogenic activity in the absence of Y8,9 phosphorylation. We also find that activating mutations allow a membrane-spanning Met receptor to overcome the requirement for the Y14,15 phosphorylation in a manner which is largely ligand-independent. These findings support a model whereby activating mutations stabilize an active conformation of the Met kinase via a mechanism which can function independently of Y8,9 autophosphorylation and suggest that signaling via wild type Met and mutationally activated Met may proceed through distinct pathways.