p145, a protein with associated tyrosine kinase activity in a human gastric carcinoma cell line

Ovotransferrin Supplementation Improves the Iron Absorption: An In Vitro Gastro-Intestinal Model

Transferrins constitute the most important iron regulation system in vertebrates and some invertebrates. Soluble transferrins, such as bovine lactoferrin and hen egg white ovotransferrin, are glycoproteins with a very similar structure with lobes that complex with iron. In this in vitro study, a comparison of bovine lactoferrin and ovotransferrin was undertaken to confirm the comparability of biological effects. An in vitro gastric barrier model using gastric epithelial cells GTL-16 and an in vitro intestinal barrier model using CaCo-2 cells was employed to evaluate iron absorption and barrier integrity. An analysis of the molecular pathways involving DMT-1 (divalent metal transporter 1), ferritin and ferroportin was also carried out. These in vitro data demonstrate the activity of both 15% saturated and 100% saturated ovotransferrin on the iron regulation system. Compared with the commercial bovine lactoferrin, both 15% saturated and 100% saturated ovotransferrin were found to act in a more physiological manner. Based on these data, it is possible to hypothesise that ovotransferrin may be an excellent candidate for iron supplementation in humans; in particular, 15% saturated ovotransferrin is the overall best performing product. In vivo studies should be performed to confirm this in vitro data.

Iron Absorption from Three Commercially Available Supplements in Gastrointestinal Cell Lines

This study compares the absorption characteristics of two iron-based dietary supplements and their biocompatibility to bisglycinate iron, a common chelated iron form. The Caco-2 cell line—a model of human intestinal absorption—and GTL-16 cell line—a model of gastric epithelial cells—were used to perform the experiments; in the first experiments, the kinetics of absorption have been evaluated analyzing the divalent metal transporter 1 (DMT1) expression. Three different iron combinations containing 50 µM iron (named Fisioeme^®, Sideral^® and bisglycinate) were used for different stimulation times (1–24 h). After this, the effects of the three iron formulations were assessed in both a short and a long time, in order to understand the extrusion mechanisms. The effects of the three different formulations were also analyzed at the end of stimulation period immediately after iron removal, and after some time in order to clarify whether the mechanisms were irreversibly activated. Findings obtained in this study demonstrate that Fisioeme^® was able to maintain a significant beneficial effect on cell viability compared to control, to Sideral^®, and to iron bisglycinate. This observation indicates that Fisioeme^® formulation is the most suitable for gastric and intestinal epithelial cells.

Role of vitamin D3 combined to alginates in preventing acid and oxidative injury in cultured gastric epithelial cells

Background

Gastric diseases are a worldwide problem in modern society, as reported in the USA, in the range of 0.5–2 episodes/year/person and an incidence of 5–100 episodes/1000/week according to seasons and age. There is convincing evidence that oxidative stress is involved in the pathogenesis of acute gastric injury. Acid secreted from gastric parietal cells determines mucosal injuries which in turn cause inflammation and oxidative stress. Consequent inflammation produces free radicals by mitochondria thus causing lipid peroxidation, oxidative and acidic stress, which can lead to cell apoptosis. Vitamin D_3, the active form of vitamin D, may counteract intracellular cell death and improve epithelial regeneration.

Methods

This study was planned to assess whether vitamin D₃ is a protective factor against acid injury and oxidative stress in gastric epithelial cells. Primary epithelial cells and GTL-16 cells have been used to test the effects of Grisù® alone or in combination with vitamin D₃ during oxidative stress or high acid exposition measuring cell viability, ROS production, cellular adhesion time along with apoptotic, autophagic and survival pathways. The combined effect of Grisù® and vitamin D₃ was found more effective in counteracting the negative consequences of oxidative stress and acidity conditions than some other gastroprotective agents, such as Maalox® or Gaviscon®.

Results

In case of oxidative stress or acidity condition the stimulation with Grisù® alone caused an improvement of cell viability and a reduction of ROS production on epithelial gastric cells. In addition, the adhesion time of the cells was improved. All these effects were increased by the presence of vitamin D₃. Similar data were also observed in primary gastric epithelial cells confirming the results obtained in GTL-16 cells.

Conclusions

These results suggest that Grisù® in combination with vitamin D₃ may exert a gastroprotective effect to maintain or restore the integrity of gastric epithelium through an antioxidant pathway, inhibiting apoptosis and activating survival kinases. Moreover, the combination of Grisù® and vitamin D₃ improves cell viability and decreases ROS production compared to other gastroprotective agents combined with vitamin D₃. All these data were validated using primary cells isolated from gastric tissue.

Electronic supplementary material

The online version of this article (doi:10.1186/s12876-016-0543-z) contains supplementary material, which is available to authorized users.

Dual Constant Domain-Fab: A novel strategy to improve half-life and potency of a Met therapeutic antibody

The kinase receptor encoded by the Met oncogene is a sensible target for cancer therapy. The chimeric monovalent Fab fragment of the DN30 monoclonal antibody (MvDN30) has an odd mechanism of action, based on cell surface removal of Met via activation of specific plasma membrane proteases. However, the short half-life of the Fab, due to its low molecular weight, is a severe limitation for the deployment in therapy. This issue was addressed by increasing the Fab molecular weight above the glomerular filtration threshold through the duplication of the constant domains, in tandem (DCD-1) or reciprocally swapped (DCD-2). The two newly engineered molecules showed biochemical properties comparable to the original MvDN30 in vitro, acting as full Met antagonists, impairing Met phosphorylation and activation of downstream signaling pathways. As a consequence, Met-mediated biological responses were inhibited, including anchorage-dependent and -independent cell growth. In vivo DCD-1 and DCD-2 showed a pharmacokinetic profile significantly improved over the original MvDN30, doubling the circulating half-life and reducing the clearance. In pre-clinical models of cancer, generated by injection of tumor cells or implant of patient-derived samples, systemic administration of the engineered molecules inhibited the growth of Met-addicted tumors.

Identification of a novel MET mutation in high-grade glioma resulting in an auto-active intracellular protein

MET has gained interest as a therapeutic target for a number of malignancies because of its involvement in tumorigenesis, invasion and metastasis. At present, a number of inhibitors, both antibodies against MET or its ligand hepatocyte growth factor, and small molecule MET tyrosine kinase inhibitors are in clinical trials. We here describe a novel variant of MET that is expressed in 6 % of high-grade gliomas. Characterization of this mutation in a glioma cell line revealed that it consists of an intronic deletion, resulting in a splice event connecting an intact splice donor site in exon 6 with the next splice acceptor site being that of exon 9. The encoded protein lacks parts of the extracellular IPT domains 1 and 2, encoded by exons 7 and 8, resulting in a novel pseudo-IPT and is named MET^Δ7−8. MET^Δ7−8 is located predominantly in the cytosol and is constitutively active. The auto-activating nature of MET^Δ7−8, in combination with a lack of transmembrane localization, renders MET^Δ7−8 not targetable using antibodies, although the protein is efficiently deactivated by MET-specific tyrosine kinase inhibitors. Testing of MET-expressing tumors for the presence of this variant may be important for treatment decision making.

Inhibition of ligand-independent constitutive activation of the Met oncogenic receptor by the engineered chemically-modified antibody DN30

An awesome number of experimental and clinical evidences indicate that constitutive activation of the Met oncogenic receptor plays a critical role in the progression of cancer toward metastasis and/or resistance to targeted therapies. While mutations are rare, the common mechanism of Met activation is overexpression, either by gene amplification ('addiction') or transcriptional activation ('expedience'). In the first instance ligand-independent kinase activation plays a major role in sustaining the transformed phenotype. Anti-Met antibodies directed against the receptor binding site behave essentially as ligand (Hepatocyte Growth Factor, HGF) antagonists and are ineffective to counteract ligand-independent activation. The monovalent chimeric MvDN30 antibody fragment, PEGylated to extend its half-life, binds the fourth IPT domain and induces 'shedding' of the Met extracellular domain, dramatically reducing both the number of receptors on the surface and their phosphorylation. Downstream signaling is thus inhibited, both in the absence or in the presence of the ligand. In vitro, MvDN30 is a strong inhibitor not only of ligand-dependent invasive growth, sustained by both paracrine and autocrine HGF, but notably, also of ligand-independent growth of 'Met-addicted' cells. In immunocompromised mice, lacking expression of Hepatocyte Growth Factor cross-reacting with the human receptor - thus providing, by definition, a model of 'ligand-independent' Met activation - PEGylated MvDN30 impairs growth of Met 'addicted' human gastric carcinoma cells. In a Met-amplified patient-derived colo-rectal tumor (xenopatient) MvDN30-PEG overcomes the resistance to EGFR targeted therapy (Cetuximab). The PEGylated MvDN30 is thus a strong candidate for targeting tumors sustained by ligand-independent Met oncogenic activation.

cMET in NSCLC: Can We Cut off the Head of the Hydra? From the Pathway to the Resistance

In the last decade, the tyrosine kinase receptor cMET, together with its ligand hepatocyte growth factor (HGF), has become a target in non-small cell lung cancer (NSCLC). Signalization via cMET stimulates several oncological processes amongst which are cell motility, invasion and metastasis. It also confers resistance against several currently used targeted therapies, e.g., epidermal growth factor receptor (EGFR) inhibitors. In this review, we will discuss the basic structure of cMET and the most important signaling pathways. We will also look into aberrations in the signaling and the effects thereof in cancer growth, with the focus on NSCLC. Finally, we will discuss the role of cMET as resistance mechanism.

Increase of MET gene copy number confers resistance to a monovalent MET antibody and establishes drug dependence

The relevant role in cancer played by the tyrosine kinase receptor encoded by the MET oncogene led to the development of specific inhibitors, some of which are now in advanced phases of clinical trials. Previous experience has shown that the main limit to the efficacy of most targeted treatments is the advent of resistance. Mechanisms underlying resistance to MET-specific small tyrosine kinase inhibitors (TKIs) have been already described, while nothing is known about resistance to MET monoclonal antibodies, nor about bypassing resistance to chemical TKIs by antibodies or vice-versa. EBC1 lung cancer cells are MET-addicted as a consequence of gene amplification and thus sensitive to MET inhibitors, including the monovalent form of a MET monoclonal antibody (MV-DN30). We generated cells resistant to this antibody and found that resistance was due to a further increase of gene copy number and a dramatic overexpression of the MET receptor. Such an excess of expression saturated the 'shedding' activity of MV-DN30, and prevented both the efficient down-regulation of the MET receptor from the surface and the inhibition of the ensuing constitutive activation. Notably, antibody-resistant cells remained MET-'addicted' and were still sensitive to MET TKIs. Moreover, antibody-resistant cells became 'drug-dependent', since the removal of MV-DN30 led them to death due to excess of signal. In the mirror experiment, cells made resistant to MET-specific TKIs were still sensitive to treatment with the antibody MV-DN30. These findings suggest that a discontinuous, combined treatment by antibodies and chemical kinase inhibitors may increase the clinical response and bypass resistance to anti-MET targeted therapies.

An 'in-cell trial' to assess the efficacy of a monovalent anti-MET antibody as monotherapy and in association with standard cytotoxics

In clinical practice, targeted therapies are usually administered together with chemotherapeutics. However, little is known whether conventional cytotoxic agents enhance the efficacy of targeted compounds, and whether a possible synergy would be dictated by drug-sensitizing genetic alterations. To explore these issues, we leveraged the design of clinical studies in humans to conduct a multi-arm trial in an 'in-cell' format. Using the MET oncogene as a model target and a panel of genetically characterized cell lines as a reference population, we found that two different chemotherapeutic regimens - cisplatin and 5-fluorouracil - exerted widespread cytotoxic activity that was not further enhanced by MET inhibition with a monovalent anti-MET antibody. From a complementary perspective, targeted MET inhibition was successful in a selected complement of cells harboring MET genomic lesions. In this latter setting, addition of chemotherapy did not provide a therapeutic advantage. Mechanistically, chemotherapeutics did not influence the basal activity of MET in cells with normal MET genomic status nor did they contribute to neutralize MET signals in cells with MET amplification. These data suggest that tumors displaying MET aberrations achieve plateau responses by MET monotherapy and do not receive further benefit by addition of cytotoxic treatments.

Soluble c-Met protein as a susceptible biomarker for gastric cancer risk: A nested case-control study within the Korean Multicenter Cancer Cohort

This study was conducted to evaluate the relevance of the soluble form of c-Met protein, a truncated form of the c-Met membrane receptor involved in the CagA pathway, as a potential biomarker for gastric cancer. Among 290 gastric cancer case-control sets selected from the Korean Multicenter Cancer Cohort, the plasma concentrations of soluble c-Met protein were measured with enzyme-linked immunosorbent assays. Using analysis of variance and covariance models with age, sex, smoking, Helicobacter pylori infection, and CagA seropositivity, the mean concentrations of soluble c-Met protein between cases and controls were compared. To evaluate the association between gastric cancer and a c-Met protein level, odds ratios and 95% confidence intervals were estimated using conditional logistic regression models. Interactions between CagA-related genes and the soluble c-Met protein concentration were also investigated. The overall median plasma concentration of soluble c-Met among cases was significantly lower than those of controls (1.390 vs. 1.610 ng/mL, p < 0.0001). Closer to the onset of gastric cancer, the soluble c-Met protein level decreased linearly in a time-dependent manner (p for trend = 0.0002). The combined effects between the CagA-related genes and the soluble c-Met protein concentration significantly intensified risks for gastric cancer. Restricted analyses including cases that had been diagnosed within 1 year after entering the cohort had a fair degree of ability (area under the receiver operating characteristic curve of 0.73-0.77) to discriminate gastric cancer cases from normal controls. Our findings demonstrate the potential of the soluble form of c-Met protein as a novel biomarker for gastric cancer. The beneficial effects of a high soluble c-Met concentration in human plasma are strongly supported.

A preclinical algorithm of soluble surrogate biomarkers that correlate with therapeutic inhibition of the MET oncogene in gastric tumors

The MET oncogene is amplified in a fraction of human gastric carcinoma cell lines, with consequent overexpression and constitutive activation of the corresponding protein product, the Met tyrosine kinase receptor. This genetically driven hyperactivation of Met is necessary for cancer cell growth and survival, so that Met pharmacological blockade results in cell-cycle arrest or apoptosis (oncogene addiction). MET gene amplification also occurs in vivo in a number of human gastric carcinomas, and clinical trials are now ongoing to assess the therapeutic efficacy of Met inhibitors in this type of malignancy. The aim of our study was to identify a preclinical algorithm of soluble surrogate biomarkers indicative of response to Met inhibition in gastric tumors, as a potential tool to integrate imaging criteria during patient follow-up. We started from a survey of candidate molecules based on antibody proteomics and gene expression profiling; after ELISA validation and analytical quantification, four biomarkers were identified that appeared to be strongly and consistently modulated by Met inhibition in a panel of Met-addicted gastric carcinoma cell lines, but not in Met-independent cell lines. Pharmacologic blockade of Met using specific small-molecule inhibitors led to reduced secretion of IL-8, GROα and the soluble form of uPAR and to increased production of IL-6 both in vitro (in culture supernatants) and in vivo (in the plasma of xenografted mice). If confirmed in patients, this information might prove useful to monitor clinical response to Met-targeted therapies in MET-amplified gastric carcinomas.

Hepatocyte growth factor/c-met signaling in regulating urokinase plasminogen activator in human stomach cancer: A potential therapeutic target for human stomach cancer

BACKGROUND

Up-regulation of the hepatocyte growth factor (HGF), its transmembrane tyrosine kinase receptor (c-Met), and urokinase type plasminogen activator (uPA), is associated with the development and metastasis of various types of cancers. However, the mechanisms by which HGF/c-Met signaling mediates cancer progression and metastasis are unclear.

METHODS

We investigated the roles of HGF/c-Met in tumor progression and metastasis in NUGC-3 and MKN-28 stomach cancer cell lines.

RESULTS

Treatment with HGF increased c-Met phosphorylation in a dose-dependent manner, as well as increasing cell proliferation. HGF treatment also increased the protein level and the activity of uPA in NUGC-3 and MKN-28 cells. A monoclonal antibody against human uPA receptor (uPAR), mAb 3936, inhibited HGF-mediated tumor cell invasion in a dose-dependent manner. Down-regulation of uPA using uPA-shRNA induced a decrease in in vitro cell invasion in NUGC-3 cells.

CONCLUSIONS

These results suggest that NUGC-3 and MKN-28 cells express functional c-Met, which may provide a therapeutic target for interfering with metastases of cancer cells by inhibiting uPA and uPAR-mediated proteolysis.

Effects of COX-2 inhibitor on growth of human gastric cancer cells and its relation to hepatocyte growth factor

It is known that hepatocyte growth factor binding to its receptor regulates gastric cancer progression and metastasis. HGF was found to up-regulate the expression of cyclooxygenase-2 gene and increases prostaglandin (PG) synthesis in gastric mucosa cells. Overexpression of COX-2 and increased PG secretion have also been found to be involved in the regulation of growth and metastasis of gastric cancer. Results from this study showed that c-Met and COX-2 are expressed in 28 cases (93.3%) and 16 cases (53.3%) of 30 human gastric cancer tissues, respectively. Expressions of c-Met positively correlated with that of COX-2 (r=0.41; P=0.024). Using in vivo and in vitro models to further examine the interaction between c-MET and COX-2, we found that HGF stimulated the growth of SC-M1 cells in a dose-dependent manner. COX-2-specific inhibitor-NS398 inhibited the growth of human gastric cancer SC-M1 cells as well as HGF stimulated the growth of SC-M1 cells in a dose-dependent manner. HGF treatment of SC-M1 cells increased the secretion of PGE2 and this stimulation was blocked by NS398. In vivo SC-M1 tumor model showed that HGF stimulated the tumor growth and NS398 retarded the tumor growth. These results suggest that COX-2-specific inhibitors may play some role on the therapy of gastric cancer patients with high serum HGF level and overexpression of c-Met in tumor.

Hepatocyte growth factor exerts a proliferative effect on oval cells through the PI3K/AKT signaling pathway

Hepatocyte growth factor (HGF) is a potent mitogen for a variety of cells including hepatocytes. While rat oval cells are supposed to be one of hepatic stem cells, biological effects of HGF on oval cells and their relevant signal transduction pathways remain to be determined. We sought to investigate them on OC/CDE22 rat oval cells, which are established from the liver of rats fed a choline-deficient/DL-ethionine-supplemented diet. The oval cells were cultured on fibronectin-coated dishes and stimulated with recombinant HGF, transforming growth factor-alpha (TGF-alpha), and thrombopoietin (TPO) under the serum-free medium condition. HGF treatment enhanced [3H]thymidine incorporation into oval cells in a dose-dependent manner. On the contrary, treatment with TGF-alpha or TPO had no significant effects on [3H]thymidine incorporation into the oval cells. c-Met protein was phosphorylated at the tyrosine residues after the HGF treatment. AKT, extracellular signal-regulated kinase 1/2 (ERK1/2), and p70(s6k) were simultaneously activated after the HGF stimulation, peaking at 30min after the treatment. The activation of AKT, p70(s6k), and ERK1/2 induced by HGF was abolished by pre-treatment with LY294002, a phosphoinositide 3-OH kinase (PI3K) inhibitor, and U0126, a mitogen-activated protein kinase/ERK kinase (MEK) inhibitor, respectively. When the cells were pre-treated with LY294002 prior to the HGF stimulation, the proliferative action of HGF was completely abrogated, implying that the PI3K/AKT signaling pathway is responsible for the biological effect of HGF. These in vitro data indicate that HGF exerts a proliferative action on hepatic oval cells via activation of the PI3K/AKT signaling pathway.

Mutations in the met oncogene unveil a "dual switch" mechanism controlling tyrosine kinase activity

The met oncogene, encoding the high affinity hepatocyte growth factor receptor, is the only known gene inherited in human cancer that is invariably associated with somatic duplication of the mutant locus. Intriguingly, mutated Met requires ligand stimulation in order to unleash its transforming potential. Furthermore, individuals bearing a germ line met mutation develop cancer only late in life and with incomplete penetrance. To date, there is no molecular explanation for this unique behavior, which is unusual for a dominant oncogene. Here we investigate the molecular mechanisms underlying met oncogenic conversion by generating antibodies specific for the differently phosphorylated forms of the Met protein. Using these antibodies, we show that activation of wild-type Met is achieved through sequential phosphorylation of Tyr1235 and Tyr1234 in the activation loop and that mutagenesis of either tyrosine dramatically impairs kinase function. Surprisingly, oncogenic Met mutants never become phosphorylated on Tyr1234 despite their high enzymatic activity, and mutagenesis of Tyr1234 does not affect their biochemical or biological function. By analyzing the enzymatic properties of the mutant proteins in different conditions, we demonstrate that oncogenic mutations do not elicit constitutive kinase activation but simply overcome the requirement for the second phosphorylation step, thus reducing the threshold for activation. In the presence of activating signals, these mutations result therefore in a dynamic imbalance toward the active conformation of the kinase. This explains why mutant met provides an oncogenic predisposition but needs a second activating "hit," provided by sustained ligand stimulation or receptor overexpression, to achieve a fully transformed phenotype.

Correlation of P-cadherin and beta-catenin expression and phosphorylation with carcinogenesis in rat tongue cancer induced with 4-nitroquinoline 1-oxide

Using biochemical and immunohistochemical techniques, we have investigated P-cadherin, beta-catenin, c-src and c-met protein expression, and phosphorylation of beta-catenin in a rat model of tongue cancer induced with 4-nitroquinoline 1-oxide. Six-week-old male Sprague-Dawley rats were given either normal drinking water (controls) or 50 ppm 4NQO solution as drinking water for 16 and 20 weeks. This treatment produced dysplasia and well-differentiated squamous cell cancer in rat tongues after 16 and 20 weeks, respectively. In controls, P-cadherin and beta-catenin were expressed only in cell membranes of tongue suprabasal epithelial cells, whereas strong reaction to P-cadherin antibody was observed during carcinogenesis, especially in nests of cancer cells. However, dysplastic and cancer cells expressed beta-catenin not only in cell membranes but also in the nuclear and cytoplasmic compartments. During carcinogenesis, immunohistochemical reaction to phosphotyrosine increased gradually. Reaction to the c-src product was strongest at the dysplastic stage and, to the c-met product, at the cancer stage. In addition, western blotting analysis showed a marked increase in the expression of beta-catenin and phosphotyrosine in dysplastic and cancer cells compared with the controls. Using immunoprecipitation and western blotting techniques, we found that phosphorylated beta-catenin gradually increased during carcinogenesis. These experiments demonstrate that cell-cell adhesion in epithelial cells was reduced by phosphorylation of beta-catenin and that beta-catenin overexpression in nuclear and cytoplasmic compartments during carcinogenesis and the production of the c-met product that is associated with the phosphorylation of beta-catenin in tongue cancer.

Vascular origin of a soluble truncated form of the hepatocyte growth factor receptor (c-met)

Hepatocyte growth factor (scatter factor) is an angiogenic growth factor that binds to its cellular transmembrane receptor, c-met. Both HGF and c-met are expressed by vascular smooth muscle and endothelial cells, where HGF may exert autocrine and paracrine effects. We have found that human aortic smooth muscle cells (HASMCs) and human umbilical vein endothelial cells (HUVECs) release a soluble, truncated form of c-met. Receptor shedding was induced by treatment of the cells with phorbol 12-myristate 13-acetate (PMA) and by the ligand, HGF. Shedding was inhibited by cycloheximide, a metalloproteinase inhibitor, and protein kinase C inhibitors. The soluble form of c-met was able to bind HGF, although with reduced affinity (K(d) approximately 10 nmol/L) compared with the membrane bound receptor. Conditioned medium containing soluble c-met inhibited the induction of Akt phosphorylation by HGF in HUVECs. The soluble truncated form of c-met was detectable in the plasma of 5 healthy volunteers. The shedding of c-met may represent a novel mechanism for regulating the mitogenic, motogenic, and morphogenic effects of hepatocyte growth factor.

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.

Plasminogen-related growth factor and semaphorin receptors: a gene superfamily controlling invasive growth

Plasminogen-related growth factors (PRGFs), also known as "scatter factors," trigger a unique biological program leading to "invasive growth." This is a result of the integration of apparently independent biological responses including cell proliferation, cell survival, cell motility, invasion of extracellular matrices, and induction of cell polarity. Under physiological conditions, the coordinated execution of the underlying genetic programs leads to the formation of tubular structures by epithelial organs (the so-called branching morphogenesis). PRGF receptors are tyrosine kinases, encoded by a family of oncogenes: MET and RON. They feature unique signal transduction properties as their cytoplasmic tails contain a two-tyrosine multifunctional docking site that binds multiple SH2-containing intracellular signal transducers. Invasive growth results from the concomitant activation of Ras (growth), phosphatidylinositol 3-kinase ("scattering"), and signal transducer and activator of transcription (cell polarity and morphogenesis). We recently identified a new human gene family, encoding large transmembrane proteins, sex/plexins, sharing homologies with Met. These molecules are receptors for semaphorins, involved in axon guidance and cell-cell repulsion, a process reminiscent of scattering and invasive growth. Deregulated activation of PRGF or semaphorin ligands or receptors, by mutation or overexpression, confers to cancer cells invasive and metastatic properties.

The prognostic significance of amplification and overexpression of c-met and c-erb B-2 in human gastric carcinomas

BACKGROUND

The c-met and the c-erb B-2 protooncogenes belong to a family of tyrosine kinase growth factor receptors. Abnormalities of these oncogenes and protein products have been reported in several cancers. The authors investigated the correlation between clinical factors and amplification or overexpression of the c-met and/or c-erb B-2 gene in Japanese patients with gastric carcinoma patients, with a focus on prognostic significance.

METHODS

Amplification and overexpression of c-met and c-erb B-2 were investigated retrospectively in 128 gastric carcinoma patients by using immunohistochemistry and Southern blot hybridization. Survival analysis was performed with the Kaplan-Meier test, and the log rank test was used for statistical analysis.

RESULTS

Overexpression of c-met and c-erb B-2 was observed in 46.1% and 16.4% of gastric carcinoma cases, respectively. Gene amplification of c-met and c-erb B-2 was detected in 10.2% and 11.7% of gastric carcinoma cases, respectively. Amplification and overexpression of c-met were correlated significantly with depth of tumor invasion and lymph node metastasis, whereas amplification and overexpression of c-erb B-2 were correlated significantly with histologic type. The survival rate of patients with amplification and/or overexpression of c-met or c-erb B-2 was significantly poorer than that of patients with no amplification or overexpression. Multivariate analysis revealed that c-met overexpression and lymph node metastasis were independent prognostic factors.

CONCLUSIONS

These data suggest that overexpression and/or gene amplification of c-met and c-erb B-2 may be prognostic factors in gastric carcinoma.

Interactions between scatter factors and their receptors: hints for therapeutic applications

The scatter factors, which include hepatocyte growth factor and macrophage stimulating protein, stand out from other cytokines because of their uncommon biological properties. In addition to promoting cell growth and protection from apoptosis, they are involved in the control of cell dissociation, migration into extracellular matrices, and a unique process of differentiation called 'branching morphogenesis'. Through the concerted regulation of these complex phenomena, scatter factors promote development, regeneration, and reconstruction of normal organ architecture. In transformed epithelia, scatter factors can mediate tumor invasive growth, a harmful feature of neoplastic progression in which cancer cells invade surrounding tissues, penetrate across the vascular walls, and eventually disseminate throughout the body, giving rise to systemic metastases. A much-debated issue in basic biology, which has strong implications for experimental medicine, is how to dissociate the favorable effects of growth factors from their adverse ones. Accordingly, to find agonists or antagonists with potential therapeutic applications is a crucial undertaking for current research. Domain-mapping analyses of growth factor molecules can help to isolate specific structural requirements for the induction of selective biological effects. Based on the observation that certain growth factors must undergo posttranslational modifications to exert a full response, it is possible to interfere with their activation mechanisms to modulate their functions. Finally, the identification of cell type-specific coreceptors able to potentiate their activity allows drawing of a functional body map, where some organs or tissues may be more responsive than others to growth factors. This review is focused on how, and to what extent, scatter factors can behave 'well' or 'badly' according to their molecular structure, the way they are activated, and the way they interact with cell surface receptors and coreceptors.

Regulation of spermidine/spermine N1-acetyltransferase expression by cytokines and polyamines in human hepatocarcinoma cells (HepG2)

Spermidine/spermine N1-acetyltransferase (cSAT), a key enzyme in polyamine degradation, is induced by various hepatotoxins and liver tumor promoters. In this paper we demonstrate that physiological factors, such as cytokines, control cSAT expression in HepG2 human hepatocarcinoma cells. Hepatocyte growth factor (HGF) induced the cSAT mRNA precursor (3.5 kb) at 4 h. The mature form of mRNA (1.3 kb) increased 6-8-fold between 8 and 10 h, and remained elevated until 18 h. An increase in cSAT activity (2-fold) and high levels of N1-acetylspermidine were observed concomitantly. Interleukin-1 beta (IL-1 beta) enhanced cSAT expression (both mRNA and enzyme activity) similar to HGF, while tumor necrosis factor-alpha (TNF-alpha) was less effective. This system also provides a useful means for examining the involvement of negative and positive changes of polyamines in the induction of cSAT and c-jun, a gene that participates in the control of cSAT expression. alpha-Difluoromethylornithine (DFMO) pretreatment, by lowering putrescine and spermidine in HGF- or IL-1 beta-treated cells, prevented the induction of cSAT. This effect was reversed by exogenous putrescine or spermidine. IL-1 beta induced c-jun mRNA more than HGF. DFMO prevented almost completely the enhancement of c-jun mRNA expression by IL-1 beta, and this effect was reversed by exogenous putrescine or spermidine. Therefore, we suggest that cSAT and c-jun expression is specifically regulated by polyamine-mediated mechanisms in IL-1 beta treated HepG2 cells. Since cSAT is inducibile by cytokines that control tumor metabolism and growth as well as tumor-host interaction, we hypothesize an involvement of cSAT in hepatoma growth.

Control of invasive growth by hepatocyte growth factor (HGF) and related scatter factors

Hepatocyte growth factor (HGF) is the prototype of a family of structurally related soluble molecules, named scatter factors (SFs). These control a complex genetic programme leading to cell-dissociation, migration in the extracellular matrix, growth, acquisition of polarity and tubule formation. This programme is pivotal during the embryonic development of epithelial and some mesodermal-derived tissues. In the adult HGF sustains cell survival and regeneration. A structurally related molecule, originally identified as macrophage stimulating protein (MSP), triggers the same complex genetic programme in epithelial and neural cells. The receptors for HGF and MSP are the tyrosine kinases encoded by the homologous genes MET and RON. As a distinctive feature, these receptors act via a two-phosphotyrosine docking site, capable of concomitant activation of multiple intracellular transducers and signalling pathways. In a number of malignant tumours, MET and RON constitutively sustain the genetic programme of scattering, leading to invasive growth and metastatic phenotype. Four MET-related receptors have been recently identified (the SEX protein family). These molecules are predominantly expressed during development and are likely to mediate repelling cues between cells of different type.

The hepatocyte growth factor receptor kinase-mediated phosphorylation of lipocortin-1 transduces the proliferating signal of the hepatocyte growth factor

Hepatocyte growth factor (HGF), which is identical to scatter factor (SF) through coupling to its receptor the product of c-met oncogene, was found to induce proliferation of A549 lung carcinoma cell line, accompanied by release of prostaglandin E2 (PGE2). This activity was sensitive to 0.1-100 microM indomethacin and to 5-50 nM of verapamil. Lipocortin-1, a dexamethasone-inducible inhibitor of phospholipase A2, was shown to be phosphorylated on tyrosine 10 min upon addition of HGF and to translocate to the membrane fraction for up to 6 h upon ligand stimulation. Lipocortin-1 was found to associate in vivo with the HGF receptor species, and this association was independent of the phosphorylation state of the beta-subunit of the HGF receptor (p145betaMET. Immobilized HGF receptor kinase species associated and phosphorylated in vitro lipocortin-1, thus providing evidence that lipocortin-1 is directly phosphorylated by the p145betaMET. Incubation of A549 cells with antisense 21-mer lipocortin-1 oligonucleotides reduced the synthesis and the HGF-stimulated phosphorylation of lipocortin-1 as well as the HGF-stimulated cell proliferation. In processes where the HGF receptor tyrosine kinase is activated, phosphorylation of lipocortin-1 may function as a "signal amplifier" promoting the release of intercellular messengers (PGE2) with pluripotent roles in cell proliferation, chemotaxis, and vascular remodeling.

Hepatocyte growth factor induces activation of Nck and phospholipase C-gamma in lung carcinoma cells

Hepatocyte growth factor (HGF), a mesenchyme derived growth factor, promotes cell growth, cell motility, and morphogenesis in a variety of epithelial cells. The diverse responses are transduced across the cell membrane by the met/HGF receptor, a product of c-met protooncogene. The met/HGF receptor recruits a variety of second messenger molecules which relay the diverse intracellular responses of HGF. In this study, we show that HGF autophosphorylates and activates met/HGF receptor. The activated met/HGF receptor then physically associates with and activates phospholipase C-gamma (PLC-gamma). Furthermore, upon ligand stimulation, tyrosine-autophosphorylated met/HGF receptor also activates Nck oncogene product. Taken together, our results suggest that the receptor activation leads to formation of a complex in which PLC-gamma and Nck oncogene product co-exist with the activated met/HGF receptor, and that the Nck oncogene product is an important component of HGF signaling in Calu-1 and A549 cells.

Hepatocyte growth factor and Met receptor expression in human pancreatic carcinogenesis

We have used immunohistochemistry to evaluate the clinicopathological significance of hepatocyte growth factor (HGF) and Met/HGF receptor expression in ductal lesions of 46 human pancreata. Normal duct epithelium shows no significant immunoreactivity for either HGF or Met. Strong immunostaining for HGF was respectively demonstrated in hyperplastic and severely dysplastic epithelia in 35.5 and 40% of cases with such duct lesions, whereas 37% of ductal adenocarcinoma showed diffuse HGF immunostaining. Positive Met immunostaining was demonstrated in 58, 80, and 78%, respectively, of specimens demonstrating the above duct lesions. Patients with resectable ductal adenocarcinoma demonstrating diffuse Met immunostaining have a significantly longer survival than those with tumors showing negative/focal staining (19.7 versus 8.1 months at P = 0.026). In contrast, HGF immunoreactivity did not significantly correlate with the survival of the patients. The results suggest that HGF and Met expression may play significant bifunctional roles during human pancreatic ductal carcinogenesis and progression. Whereas an upregulation of Met receptor expression and HGF-Met interaction may have an important pathogenetic role during the early stages of neoplastic promotion, a lack or subsequent loss of Met expression in invasive adenocarcinoma appears to result in a more aggressive clinical behavior.

Suramin modulates cellular levels of hepatocyte growth factor receptor by inducing shedding of a soluble form

Several growth factor receptors undergo shedding from the cell surface as a result of limited proteolysis via mechanisms that are at present poorly understood. By Western blotting of the conditioned media and cell lysates of several cell lines expressing the hepatocyte growth factor receptor, we found that suramin, a pharmacological agent that inhibits the activity of many growth factors, was able to induce shedding of this receptor. Increased levels of soluble hepatocyte growth factor receptor were observed in the conditioned media of GTL-16, a cell line over-expressing the receptor, as early as ten minutes after initial exposure to the agent, and incubation of this line with 300 microM suramin caused a 50% reduction in cell-associated levels of receptor after 6 hours. Although protein kinase C activation by treatment of cells with phorbol esters has previously been found to stimulate shedding of the hepatocyte growth factor receptor, this hitherto undescribed activity of suramin was not affected by protein kinase C inhibitors. Since shedding represents a possible means of down-modulation of receptor activity, suramin may inhibit the hepatocyte growth factor ligand/receptor system, not only by abrogation of hepatocyte growth factor binding to intact receptor, but also by induction of receptor shedding.

Selective tumorigenesis in non-parenchymal liver epithelial cell lines by hepatocyte growth factor transfection

Hepatocyte growth factor (HGF) has been recently suggested to contribute to tumorigenesis by an autocrine mechanism in fibroblast cells overexpressing its receptor, the MET/HGFR protein. Since epithelial cells represent the primary physiologic target of HGF, we investigated whether inappropriate expression of HGF by epithelial cells which normally express MET/HGFR may also contribute to tumorigenesis. We have transfected a full length rat HGF gene into three mouse epithelial cell lines, one derived from breast (MM55) and two (BNL CL.2 and NMuLi) representing liver non-parenchymal epithelial cells (NPEC). We confirmed the presence of the transfected gene by Southern blot analysis, the production of HGF protein by immunofluorescence, and the preservation of HGF biologic activity by bio-assay. In comparison to untransfected cells, all three HGF-transfected cell lines displayed high level MET/HGFR autophosphorylation and increased ability to proliferate in media containing low serum. The two HGF-transfected liver NPEC lines, but not the HGF-transfected mammary cell line, displayed loss of cell contact growth-inhibition and acquired a markedly increased ability to form colonies in soft agar. Furthermore, the NPEC HGF-transfected cell lines formed much larger tumors in nude mice than the untransfected counterparts, with extensive invasion and sporadic lung metastases. These results demonstrate that overexpression of HGF in at least some epithelial cells contribute to tumorigenesis, and furthermore suggest a possible role for the HGF-MET/HGFR system in the progression of certain epithelial tumors.

Role of hepatocyte growth factor in breast cancer: a novel mitogenic factor secreted by adipocytes

Stromal cells can dramatically affect the growth and metastatic capability of breast carcinoma cells. Growth factors, considered to be important mediators of this process, act as either mitogenic or mito-inhibitory regulators. We have developed an in vitro coculture system to examine the influence of adipocytes, a dominant mammary stromal cell type, on the growth of a murine mammary carcinoma, SP1. Previously, we have reported that conditioned medium (CM) from 3T3-L1 adipocytes can promote in vitro growth of SP1 cells. We now show that the major mitogenic signal derived from 3T3-L1 adipocyte CM is mediated by hepatocyte growth factor (HGF). Neutralizing antibody against HGF at 15 micrograms/ml completely abrogated mitogenic activity of 3T3-L1 CM. Furthermore, heparin, an inhibitor of biological activity of HGF, inhibited the mitogenic activity of 3T3-L1 CM. Western blot analysis also confirmed the presence of HGF in 3T3-L1 CM. Although basic fibroblast growth factor (bFGF) and insulin-like growth factor I (IGF-I) were mitogenic for SP1 cells, neutralizing antibodies against IGF-I, bFGF, platelet-derived growth factor (PDGF), and epidermal growth factor (EGF) did not inhibit the mitogenic activity of 3T3-L1 CM. Immunoprecipitation and immunoblotting of HGF receptor/c-met showed that c-met is expressed at high level in SP1 cells, and is phosphorylated following HGF ligation. Together, our present data demonstrate that 3T3-L1 adipocytes secrete HGF, which stimulates SP1 cell growth by a paracrine mechanism. Furthermore, the mitogenic effect of 3T3-L1 CM requires HGF receptor ligation and activation of tyrosine kinase signaling cascades in SP1 cells. These results highlight the importance of stromal-tumor cell interactions and suggest that HGF secreted by adipocytes may be a key regulator of mammary tumor growth.

Tyrosine protein kinase inhibition and cancer

The various aspects of the research on tyrosine protein kinase inhibition and its connections with cancer are presented. The emphasis was made on the theoretical low toxic side effects of specific tyrosine protein kinase inhibitors. Particularly, the strategy of finding peptidic substrate-derived inhibitors or modulators is discussed, with an almost complete compendium of the tyrosine protein kinase peptidic substrates published so far. A series of data has been gathered that may serve as a basis for the discovery of selective and specific tyrosine protein kinase inhibitors by screening on molecular and cellular models. The potential of SH2 domain-interfering agents are also presented as a promising route to new anticancer compounds.

Identification of functional domains in the hepatocyte growth factor and its receptor by molecular engineering

Hepatocyte growth factor/scatter factor (HGF/SF) is a heparin-binding polypeptide which shares structural domains with enzymes of the blood clotting cascade. HGF/SF is secreted by cells of mesodermal origin and has powerful mitogenic, motogenic and morphogenic activity on epithelial and endothelial cells. HGF/SF is produced as a biologically inactive single-chain precursor (pro-HGF/SF) most of which is sequestered on the cell surface or bound to the extracellular matrix. Maturation into the active alpha beta heterodimer results from proteolytic cleavage by a urokinase-type protease, which acts as a pro-HGF/SF convertase. The primary determinant for receptor binding appears to be located within the alpha-chain. The interaction of the alpha-chain with the receptor is sufficient for the activation of the signal cascade involved in the motility response. However, the complete HGF/SF protein seems to be required to elicit a mitogenic response. HGF/SF binds with high affinity to a transmembrane receptor, p190MET, encoded by the MET proto-oncogene. p190MET is the prototype of a distinct subfamily of heterodimeric tyrosine kinases, including the putative receptors Ron and Sea. The mature form of p190MET is a heterodimer of two disulfide-linked subunits (alpha and beta). The alpha-subunit is extracellular and heavily glycosylated. The beta-subunit consists of an extracellular portion involved in ligand binding, a membrane spanning segment, and a cytoplasmic tyrosine kinase domain. Both subunits derive from glycosylation and proteolytic cleavage of a common precursor of 170 kDa. In polarized epithelial cells the HGF/SF receptor is selectively exposed in the basolateral plasmalemma, where it is associated with detergent-insoluble components. Two Met isoforms, carrying an intact ligand binding domain but lacking the kinase domain due to truncation of the beta-subunit, arise from alternative post-transcriptional processing of the mature form. One truncated form is soluble and released from the cells. HGF/SF binding triggers tyrosine autophosphorylation of the receptor beta-subunit. Autophosphorylation on the major phosphorylation site Y1235 upregulates the kinase activity of the receptor, increasing the Vmax of the phosphotransfer reaction. Negative regulation of the kinase activity occurs through phosphorylation of a unique serine residue (S985) located in the juxtamembrane domain of the receptor. This phosphorylation is triggered by two distinct pathways involving either protein kinase C activation or increase in intracellular Ca2+ concentration. Upon ligand binding, the HGF/SF receptor recruits and activates several cytoplasmic effectors, including phosphatidylinositol 3-kinase (PI 3-K), phospholipase C-gamma (PLC-gamma), pp60c-Src, a tyrosine phosphatase, and a Ras-guanine nucleotide exchanger.(ABSTRACT TRUNCATED AT 400 WORDS)

c-met mRNA overexpression in human hepatocellular carcinoma

This study was aimed at assessing the presence of c-met overexpression in human hepatocellular carcinoma and at determining whether this feature is associated with a definite clinical or pathological characteristic. Expression of c-met was determined by Northern-blot hybridization of a specific probe (human met proto-oncogene) in 18 tumoral and nontumoral liver samples obtained in 18 cirrhotic patients with hepatocellular carcinoma submitted to surgical treatment. Eight of the 18 hepatocellular carcinomas exhibited c-met overexpression, with an increase ranging between 2-fold and 10-fold when compared by densitometry with the surrounding liver. By contrast, in the remaining 10 cases c-met expression was almost identical to that of the surrounding nontumoral liver tissue. Overexpression of c-met was not related to either the age, sex, etiology or functional status of the underlying liver disease, or to the size of the tumor, to its differentiation degree or to the presence of pseudocapsule invasion and existence of additional neoplastic nodules. These data indicate that almost half of the human hepatocellular carcinomas exhibit c-met overexpression. Nevertheless, the biological relevance of this characteristic is not known.

The hepatocyte growth factor and its receptor

Hepatocyte growth factor/scatter factor (HGF/SF) is secreted by cells of mesodermal origin and shows powerful mitogenic, motogenic and morphogenic activities on epithelial and endothelial cells. It is a heparin-binding polypeptide with an alpha/beta heterodimeric structure, showing structural homologies with enzymes of the blood clotting cascade. HGF binds with high affinity to the receptor encoded by the MET protooncogene (p190MET). The MET receptor is a heterodimer of two disulfide-linked subunits (alpha and beta); the alpha subunit is extracellular, while the beta is transmembrane and endowed with tyrosine kinase activity. The HGF-triggered signalling is mediated by different cytoplasmic effectors, including phosphatidylinositol 3-kinase, phospholipase C-gamma, and Src-related tyrosine kinases. p190MET is expressed in several normal epithelial tissues (e.g., liver, gastrointestinal tract, kidney) and is often overexpressed in neoplastic cells. p190MET expression has been reported also in central nervous system microglia, a monocyte-derived cell population. We recently found that p190MET is expressed in selected peripheral blood cell populations, such as macrophages. The amount of both mRNA and protein is barely detectable, while it is dramatically increased upon activation. These findings suggest that HGF may play a role in hemopoietic cell signaling, during activation and differentiation of blood cell lineages.

Aberrant elevation of tyrosine-specific phosphorylation in human gastric cancer cells

Phosphotyrosine-containing proteins in various human cancer cell lines were studied by immunoblotting with anti-phosphotyrosine antibody. Of 29 cell lines derived from oral epidermoid cancer, esophageal cancer, gastric cancer, colon cancer, pancreatic cancer, hepatocellular carcinoma and malignant melanoma, 3 of the 6 gastric cancer cells showed aberrant elevation of tyrosine-specific phosphorylation. On the other hand, both esophageal cancer cells and colon cancer cells, which were reported to have amplified epidermal growth factor receptor and activated p60v-src kinase, respectively, showed no apparent elevation of tyrosine-specific phosphorylation, and their profiles of phosphorylation were similar to that of normal human fibroblasts. Two gastric cancer cells, NUGC-4 and MKN-45, showed similar profiles of phosphorylation but their responses to growth factors differed from each other. Tyrosine phosphorylation in NUGC-4 was strongly activated by treatment with epidermal growth factor and quickly reduced by the acid treatment which is effective in removing growth factors from cellular surface receptors. On the contrary, phosphorylation in MKN-45 did not respond to either growth factor or acid treatment. These results suggest that NUGC-4 and MKN-45 have tyrosine kinases which are activated by different mechanisms but share similar substrates.

The receptor encoded by the human c-MET oncogene is expressed in hepatocytes, epithelial cells and solid tumors

The human c-MET oncogene encodes a transmembrane tyrosine kinase (p190c-met) with structural and functional features of a growth-factor receptor. Monoclonal antibodies (MAbs) have been used to investigate the distribution of the c-Met protein in human normal and neoplastic tissues. By immunofluorescence microscopy homogeneous expression was detected in normal hepatocytes as well as in epithelial cells lining the stomach, the small and the large intestine. Positive staining was also found in epithelial cells of the endometrium and ovary, and in basal keratinocytes of esophagus and skin. By Northern blot analysis, high levels of c-met messenger RNA were detected in specimens of liver, gastro-intestinal tract and kidney. c-met-specific mRNA was also found in thyroid, pancreas and placenta, in which organs c-Met protein was barely detectable by immunofluorescence. The antibodies revealed expression of c-MET protein in hepatomas (11/14), carcinomas of colon and rectum (19/21), stomach (11/22), kidney (16/19), ovary (9/17) and skin (7/17). Carcinomas of the lung (13/20), thyroid (11/13) and pancreas (5/7) were also positive. In these last cases (lung, thyroid and pancreas) tumor cells were homogeneously stained by the antibodies, whereas in their normal counterparts staining was barely detectable. These data suggest that the receptor encoded by c-MET plays a physiological role in epithelial cell growth and that its expression is altered in human carcinomas.

Protein tyrosine kinases in human breast cancer: kinetic properties and evidence for the presence of two forms of native enzyme

The protein tyrosine kinase (PTK) of human breast tumors classified as positive (TM+) or negative (TM-) according to their estrogen and progestin receptor levels was partially characterized with regard to its distribution, kinetic parameters, molecular size, and ability to phosphorylate endogenous mammary proteins. For both types of tumors, PTK activity depended upon the presence of Mn++ (2-5 mM) and/or Mg++ (10-20 mM). The activities, total (per g of tissue) and specific (per mg of protein), were similar for both types of tumors, and an average of 60% of activity was located in cytosolic fractions. The autoradiographic detection of alkali-resistant phosphoproteins after SDS-PAGE showed very similar patterns between corresponding fractions from both types of tumors. Upon gel filtration, two peaks of activity of apparent Mr 245 kDa (peak I) and 47 kDa (peak II) were observed. Peak II was found in both cytosols and extracts from particulate fractions, while peak I was present only in the latter fraction for both TM+ and TM- tumors. The apparent Km's for ATP ranged from 4.1 to 6.6 microM, and from 11 to 34 micrograms/ml for the synthetic substrate poly [Glu80, Tyr20], at an optimal pH of 6.5-7.5. When endogenous alkali-resistant phosphorylation of peaks I and II was determined by autoradiography after SDS-PAGE, two major mammary proteins of Mr 60 and 45 kDa were phosphorylated by peak II and three, Mr 145, 74, and 62 kDa, by peak I.(ABSTRACT TRUNCATED AT 250 WORDS)

Tyrosine kinase receptor indistinguishable from the c-met protein

Growth factor receptors with protein tyrosine kinase activity are central to the control of proliferation of both normal and malignant cells. Using anti-phosphotyrosine antibodies, we have previously identified a transmembrane glycoprotein with abnormally high protein tyrosine kinase activity in a human gastric tumour cell line (GTL-16). Electrophoresis under non-reducing conditions revealed that this kinase (relative molecular mass 145,000 (145 K)) is disulphide-linked to a 50K chain in an alpha beta-complex of 190K (p190). From its novel two-chain structure, we deduced that p190 was the prototype of a new class of tyrosine kinase receptors. We now show that p190 is indistinguishable from the protein encoded by the c-met proto-oncogene and that the alpha beta-subunit structure is conserved in other human cell lines. We also show that the high level of p190 found in the GTL-16 cell line is accompanied by amplification and overexpression of c-met. This provides the first example of a functional alteration of c-met in a human tumour cell line.

Evidence for autocrine activation of a tyrosine kinase in a human gastric carcinoma cell line

Phosphotyrosine (P-Tyr) antibodies have been used to identify the phosphorylated forms of growth factor receptors and oncogene-coded tyrosine kinases. Western blot analysis of a gastric carcinoma cell line with P-Tyr antibodies revealed a tyrosine-phosphorylated protein of Mr 145,000 (P145). In addition, in vitro phosphorylation with (gamma-32P)ATP or P-Tyr immunoprecipitates of the same cells resulted in labelling of this protein on tyrosine. P145 appears to be a transmembrane glycoprotein, with features suggestive of a growth factor receptor. However, the in vivo or in vitro addition of known growth factors did not affect P145 tyrosine phosphorylation. We now report that P145 is rapidly dephosphorylated in vivo when cells are exposed to low pH, a condition known to dissociate ligands from their receptors. The addition of serum-free medium, conditioned by the gastric carcinoma cells, fully restores the tyrosine phosphorylation lost with acid treatment. These data suggest that the activity responsible for P145 phosphorylation on tyrosine, whether intrinsic to P145 itself or due to an associated kinase, is stimulated by a factor secreted by the tumor cells themselves.