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

Differential requirement of the last C-terminal tail of Met receptor for cell transformation and invasiveness

Biological responses to Hepatocyte Growth Factor are mediated by the tyrosine kinase receptor encoded by the Met oncogene. Under physiological conditions, Met triggers a multi-step genetic program called 'invasive growth' including cell-dissociation, invasion of extracellular matrices and growth. When constitutively activated, Met can induce cell transformation and metastasis. Phosphorylation of two docking tyrosines in the receptor tail is essential for all biological responses. To investigate the role of the C-terminal part of Met, we have generated mutants lacking either the last 26 or 47 amino acids. As expected, mutants lacking the docking sites fail to mediate cell transformation and invasion. Interestingly, while Met Delta26 can mediate invasion, its transforming ability is severely impaired. Moreover, the lack of the last 26 amino acids strongly reduces Met ability to phosphorylate substrates in vitro and in vivo. These data indicate that the last 26 amino acids are required to confer the kinase its full enzymatic activity, which is critical for cell transformation but dispensable for invasive properties. Finally, we also show that up-regulation of Met enzymatic activity by insertion of a point mutation in the kinase domain (M1250T) overcomes the regulatory role played by the last 26 amino acids of the tail. It is concluded that the C-terminal domain of Met is crucial not only for recruitment of transducers but also for regulation of receptor enzymatic activity.

Sustained activation of extracellular signal-regulated kinase stimulated by hepatocyte growth factor leads to integrin alpha 2 expression that is involved in cell scattering

We have previously shown that hepatocyte growth factor (HGF) selectively increases the expression of integrin alpha(2) in Madin-Darby canine kidney (MDCK) cells. In this study, we have further investigated the signal transduction pathways responsible for the event and its role in HGF-induced cell scattering. We found that the level of integrin alpha(2)beta(1) expression induced by HGF correlated with the extent of cell scattering and that a functional blocking antibody against integrin alpha(2) at the concentration of 25 microg/ml partially (40%) inhibited the HGF-induced cell scattering. However, in the presence of the specific phosphatidylinositol 3-kinase inhibitor LY294002 or the selective Src family kinase inhibitor PP1, although cells retained their response to HGF for increasing integrin alpha(2) expression, they failed to scatter, indicating that increased expression of integrin alpha(2) alone is not sufficient for cell scattering. Moreover, epidermal growth factor, which induced a transient (1 h) activation of extracellular signal-regulated kinase (ERK) in MDCK cells, only slightly increased integrin alpha(2) expression and failed to trigger cell scattering. Conversely, HGF induced a sustained (at least 12 h) activation of ERK in the cells. Expression of constitutively active ERK kinase (MEK) in MDCK cells led to increased expression of integrin alpha(2) even in the absence of HGF stimulation. In contrast, expression of ERK phosphatase or dominant negative MEK inhibited HGF-induced integrin alpha(2) expression. Taken together, our results suggest that the increased expression of integrin alpha(2)beta(1) by HGF is at least partially required for cell scattering and that the duration of MEK/ERK activation is likely to be a crucial determinant for cells to activate integrin alpha(2) expression and cell scattering.

Activation of the Met receptor by cell attachment induces and sustains hepatocellular carcinomas in transgenic mice

Overexpression is the most common abnormality of receptor tyrosine kinases (RTKs) in human tumors. It is presumed that overexpression leads to constitutive activation of RTKs, but the mechanism of that activation has been uncertain. Here we show that overexpression of the Met RTK allows activation of the receptor by cell attachment and that this form of activation can be tumorigenic. Transgenic mice that overexpressed Met in hepatocytes developed hepatocellular carcinoma (HCC), one of the human tumors in which Met has been implicated previously. The tumorigenic Met was activated by cell attachment rather than by ligand. Inactivation of the transgene led to regression of even highly advanced tumors, apparently mediated by apoptosis and cessation of cellular proliferation. These results reveal a previously unappreciated mechanism by which the tumorigenic action of RTKs can be mediated, provide evidence that Met may play a role in both the genesis and maintenance of HCC, and suggest that Met may be a beneficial therapeutic target in tumors that overexpress the receptor.

Scatter factors and invasive growth

Scatter factors are unequivocal signals governing a genetic program that includes cell detachment, repulsion, protection from apoptosis, invasiveness of extracellular matrices and proliferation. This pleiomorphic response is defined as 'invasive growth'. Under physiological conditions, it leads to morphogenic cell movements through the matrix, and--primarily--to ordered building of epithelial tubules. Dysfunctions in invasive growth cause enhanced proliferation, uncontrolled migration into surrounding tissues, and failure to differentiate, events that foster tumour growth and invasiveness. Scatter factors act through tyrosine kinase receptors that belong to the Met oncogene family. Here we discuss how alterations of these receptors or of their signal transduction pathways are responsible for cancer onset and progression towards metastasis.

Expression of the c-met proto-oncogene and its ligand, hepatocyte growth factor, in Hodgkin disease

The receptor for hepatocyte growth factor (HGF) is a transmembrane tyrosine kinase that is encoded by the proto-oncogene c-met. Recently, c-MET was detected in Reed-Sternberg (RS) cells from Epstein-Barr virus-positive (EBV(+)) Hodgkin disease (HD). The c-MET, EBER-1, and LMP-1 expression in 45 lymph node biopsies and 12 bone marrow biopsies obtained from patients with HD was analyzed. In addition, HGF levels in serum samples from 80 healthy individuals and 135 HD patients in different phases of disease. In all 45 lymph node and 12 bone marrow samples examined, RS cells expressed c-MET but not HGF(+). These results were independent of the EBV infection. Interestingly, several HGF(+) dendritic-reticulum cells were found scattered around c-MET(+) RS cells. The mean +/- SEM serum HGF levels in HD patients at diagnosis and at the time of relapse were 1403 +/- 91 (95% confidence interval [CI], 1221-1585) and 1497 +/- 242 pg/mL (95% CI, 977-2017), respectively. HGF values were significantly higher than those of healthy individuals (665 +/- 28 pg/mL; 95% CI, 600-721; and P <.001 for both groups of patients) and of HD patients in remission (616 +/- 49 pg/mL; 95% CI, 517-714; and P <.001 for both groups of patients). A significant correlation was found between serum HGF levels and B symptoms at diagnosis (P =.014). In conclusion, this study indicates that HGF and c-MET constitute an additional signaling pathway between RS cells and the reactive cellular background, thereby affecting adhesion, proliferation, and survival of RS cells. Furthermore, the serum concentration of HGF in HD patients may be a useful tool in monitoring the status of disease.

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 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.

The expression of Met/hepatocyte growth factor receptor gene in giant cell tumors of bone and other benign musculoskeletal tumors

Overexpression of the hepatocyte growth factor receptor (Met/HGF receptor), a transmembrane tyrosine kinase encoded by the MET proto-oncogene, is involved in transformation and invasive behavior of human carcinomas and sarcomas. We have previously found that bone sarcomas express high levels of Met/HGF receptor while in some cases the ligand HGF is co-expressed with the receptor, activating an autocrine loop. In this study, we analyzed 40 biopsy samples of a collection of giant cell tumors and other rare benign tumors of bone for expression of the MET proto-oncogene. These included nonossifying fibromas, osteoblastomas, desmoplastic fibromas of bone, chondroblastomas, and giant cell tumors of bone. Snap frozen samples were tested for the MET and HGF gene expression by immuno-histochemistry and Western blotting with anti-MET antibodies and RT-PCR. Over 50% of all cases scored positive for MET expression being constantly positive in recurrent or locally aggressive lesions. Sporadic co-expression of the Met/HGF receptor and ligand is also demonstrated. Met/HGF receptor expression in benign bone neoplasms suggests its early involvement in sarcomagenesis.

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.

Involvement of focal adhesion kinase in hepatocyte growth factor-induced scatter of Madin-Darby canine kidney cells

Focal adhesion kinase (FAK) has been implicated to play a critical role in integrin-mediated control of cell behavior. However, it is unclear whether FAK also participates in the regulation of growth factor-elicited cellular functions. In this study, we have demonstrated that although overexpression of FAK in Madin-Dardy canine kidney cells did not alter their growth property or ability to form tubules within collagen gel upon hepatocyte growth factor (HGF) stimulation, it apparently enhanced HGF-induced cell scattering. This enhancement was largely because of an increase in the third phase (i.e. cell migration) of cell scattering rather than the first two phases (i.e. cell spreading and cell-cell dissociation). Conversely, the expression of FAK-related nonkinase significantly ( approximately 60%) inhibited HGF-induced cell migration. Moreover, we have found that the effect of FAK on promoting HGF-induced cell motility was greatly dependent on cell-matrix interactions. We showed that HGF treatment selectively increased the expression of integrins alpha(2) and, to a lesser extent, alpha(3) in Madin-Dardy canine kidney cells and that a monoclonal antibody against integrin alpha(2) efficiently blocked HGF-enhanced cell migration on collagen. In our efforts to determine the mechanism by which FAK promotes HGF-induced cell migration, we found that FAK mutants deficient in phosphatidylinositol 3-kinase or p130(Cas) binding failed to promote HGF-induced cell migration. Interestingly, cells expressing a FAK mutant defective in Grb2 binding exhibited a rate of migration approximately 50% lower than that of cells expressing wild type FAK in response to HGF stimulation. Taken together, our results suggest a link between HGF-increased integrin expression, FAK activation, and enhanced cell motility and implicate a role for FAK in the facilitation of growth factor-induced cell motility.

Hepatocyte growth factor/scatter factor-induced intracellular signalling

Hepatocyte growth factor (HGF) identical to scatter factor (SF) is a glycoprotein involved in the development of a number of cellular phenotypes, including proliferation, mitogenesis, formation of branching tubules and, in the case of tumour cells, invasion and metastasis. This fascinating cytokine transduces its activities via its receptor encoded by the c-met oncogene, coupled to a number of transducers integrating the HGF/SF signal to the cytosol and the nucleus. The downstream transducers coupled to HGF/MET, most of which participate in overlapping pathways, determine the development of the cell's phenotype, which in most cell types is dual.

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.

A peptide representing the carboxyl-terminal tail of the met receptor inhibits kinase activity and invasive growth

Interaction of the hepatocyte growth factor (HGF) with its receptor, the Met tyrosine kinase, results in invasive growth, a genetic program essential to embryonic development and implicated in tumor metastasis. Met-mediated invasive growth requires autophosphorylation of the receptor on tyrosines located in the kinase activation loop (Tyr(1234)-Tyr(1235)) and in the carboxyl-terminal tail (Tyr(1349)-Tyr(1356)). We report that peptides derived from the Met receptor tail, but not from the activation loop, bind the receptor and inhibit the kinase activity in vitro. Cell delivery of the tail receptor peptide impairs HGF-dependent Met phosphorylation and downstream signaling. In normal and transformed epithelial cells, the tail receptor peptide inhibits HGF-mediated invasive growth, as measured by cell migration, invasiveness, and branched morphogenesis. The Met tail peptide inhibits the closely related Ron receptor but does not significantly affect the epidermal growth factor, platelet-derived growth factor, or vascular endothelial growth factor receptor activities. These experiments show that carboxyl-terminal sequences impair the catalytic properties of the Met receptor, thus suggesting that in the resting state the nonphosphorylated tail acts as an intramolecular modulator. Furthermore, they provide a strategy to selectively target the MET proto-oncogene by using small, cell-permeable, peptide derivatives.

Mechanism of action and in vivo role of platelet-derived growth factor

Platelet-derived growth factor (PDGF) is a major mitogen for connective tissue cells and certain other cell types. It is a dimeric molecule consisting of disulfide-bonded, structurally similar A- and B-polypeptide chains, which combine to homo- and heterodimers. The PDGF isoforms exert their cellular effects by binding to and activating two structurally related protein tyrosine kinase receptors, denoted the alpha-receptor and the beta-receptor. Activation of PDGF receptors leads to stimulation of cell growth, but also to changes in cell shape and motility; PDGF induces reorganization of the actin filament system and stimulates chemotaxis, i.e., a directed cell movement toward a gradient of PDGF. In vivo, PDGF has important roles during the embryonic development as well as during wound healing. Moreover, overactivity of PDGF has been implicated in several pathological conditions. The sis oncogene of simian sarcoma virus (SSV) is related to the B-chain of PDGF, and SSV transformation involves autocrine stimulation by a PDGF-like molecule. Similarly, overproduction of PDGF may be involved in autocrine and paracrine growth stimulation of human tumors. Overactivity of PDGF has, in addition, been implicated in nonmalignant conditions characterized by an increased cell proliferation, such as atherosclerosis and fibrotic conditions. This review discusses structural and functional properties of PDGF and PDGF receptors, the mechanism whereby PDGF exerts its cellular effects, and the role of PDGF in normal and diseased tissues.

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.

HGF/SF and its receptor c-MET play a minor role in the dissemination of human B-lymphoma cells in SCID mice

The MET protooncogene, c-MET, encodes a cell surface tyrosine kinase receptor. The ligand for c-MET is hepatocyte growth factor (HGF), also known as scatter factor (SF), which is known to affect proliferation and motility of primarily epithelial cells. Recently, HGF/SF was also shown to affect haemopoiesis. Studies with epithelial and transfected NIH3T3 cells indicated that the HGF/SF-c-MET interaction promotes invasion in vitro and in vivo. We previously demonstrated that HGF/SF induces adhesion of c-MET-positive B-lymphoma cells to extracellular matrix molecules, and promoted migration and invasion in in vitro assays. Here, the effect of HGF/SF on tumorigenicity of c-MET-positive and c-MET-negative human B-lymphoma cell lines was studied in C.B-17 scid/scid (severe combined immune deficient) mice. Intravenously (i.v.) injected c-MET-positive (BJAB) as well as c-MET-negative (Daudi and Ramos cells) B-lymphoma cells formed tumours in SCID mice. The B-lymphoma cells invaded different organs, such as liver, kidney, lymph nodes, lung, gonads and the central nervous system. We assessed the effect of human HGF/SF on the dissemination of the B-lymphoma cells and found that administration of 5 microg HGF/SF to mice, injected (i.v.) with c-MET-positive lymphoma cells, significantly (P = 0.018) increased the number of metastases in lung, liver and lymph nodes. In addition, HGF/SF did not significantly influence dissemination of c-MET-negative lymphoma cells (P = 0.350 with Daudi cells and P= 0.353 with Ramos cells). Thus the effect of administration of HGF/SF on invasion of lymphoma cells is not an indirect one, e.g. via an effect on endothelial cells. Finally, we investigated the effect of HGF/SF on dissemination of c-MET-transduced Ramos cells. In response to HGF/SF, c-MET-transduced Ramos cells showed an increased migration through Matrigel in Boyden chambers compared to wild-type and control-transduced Ramos cells. The dissemination pattern of c-MET-transduced cells did not differ from control cells in in vivo experiments using SCID mice. Also no effect of HGF/SF administration could be documented, in contrast to the in vitro experiments. From our experiments can be concluded that the HGF/SF-c-MET interaction only plays a minor role in the dissemination of human B-lymphoma cells.

Loss of the exon encoding the juxtamembrane domain is essential for the oncogenic activation of TPR-MET

TPR-MET, a transforming counterpart of the c-MET proto-oncogene detected in experimental and human cancer, results from fusion of the MET kinase domain with a dimerization motif encoded by TPR. In this rearrangement the exons encoding the Met extracellular, transmembrane and juxtamembrane domains are lost. The juxtamembrane domain has been suggested to be a regulatory region endowed with negative feedback control. To understand whether its absence is critical for the generation of the Tpr-Met transforming potential, we produced a chimeric molecule (Tpr-juxtaMet) with a conserved juxtamembrane domain. The presence of the domain (aa 962-1009) strongly inhibited Tpr-Met dependent cell transformation. Cell proliferation, anchorage-independent growth, motility and invasion were also impaired. The enzymatic behavior of Tpr-Met and Tpr-juxtaMet was the same, while Tpr-juxtaMet ability to associate cytoplasmic signal transducers and to elicit downstream signaling was severely impaired. These data indicate that the presence of the juxtamembrane domain counterbalances the Tpr-Met transforming potential and therefore the loss of the exon encoding the juxtamembrane domain is crucial in the generation of the active TPR-MET oncogene.

Overexpression of C-NEU and C-MET during rat liver cholangiocarcinogenesis: A link between biliary intestinal metaplasia and mucin-producing cholangiocarcinoma

Based on limited but compelling immunohistochemical data demonstrating individual overexpression of the tyrosine kinase growth factor receptors, c-erbB-2 and c-met, in significant percentages of human cholangiocarcinoma (ChC), we investigated if combined overexpression of both c-neu, the rat homologue of c-erbB-2, and c-met, the receptor for hepatocyte growth factor/scatter factor (HGF/SF), might represent a characteristic, early event associated with furan-induced cholangiocarcinogenesis in rat liver. Specifically, through the use of immunohistochemistry, in situ hybridization (ISH), and Western and Northern blotting, we found that both c-neu and c-met are prominently overexpressed in intestinal metaplastic lesions in early putative precancerous cholangiofibrotic tissue formed in the livers of rats after 6 weeks of furan treatment when compared with normal and hyperplastic intrahepatic biliary epithelia. We further demonstrated that c-neu and c-met are concordantly overexpressed in neoplastic glandular epithelia in later-developed primary "intestinal-type" of ChC formed in the livers of furan-treated rats, as well as in subsequently derived transplantable mucin-producing tumors. Overexpression of c-neu and c-met correlated with increased proliferating cell nuclear antigen (PCNA)-labeling indices, which were determined to be three to four times higher in intestinal metaplastic glands in precancerous cholangiofibrotic tissue and in neoplastic glands in the primary "intestinal type" of ChC than in hyperplastic bile ductular structures within either cholangiofibrotic or bile duct-ligated (BDL) livers. The c-neu and c-met receptor proteins overexpressed in different in vivo passages of a transplantable ChC each contained immunoreactive phosphotyrosines, indicating an activated state. However, we did not detect evidence of either gene amplification of c-neu or c-met or of a common transmembrane-activating mutation in c-neu expressed in transplantable ChC. Our findings indicate that altered expression of c-neu and c-met occurs relatively early in the process of furan-induced cholangiocarcinogenesis in rat liver and may play a potentially important role in its pathogenesis. They further indicate a common alteration in tyrosine kinase growth factor receptor expression linking early putative precancerous intestinal metaplastic lesions in liver to later-developed mucin-producing biliary cancer.

HGF activates signal transduction from EPO receptor on human cord blood CD34+/CD45+ cells

Hepatocyte growth factor (HGF) is a multifunctional cytokine with early hematopoiesis-stimulatory activity. Here, we focus on its erythropoiesis-stimulatory effect on highly purified human hematopoietic progenitor cells (CD34+/CD45+ cells) derived from the cord blood. In immunoblot analyses, c-met protein (a receptor of HGF) was detected in the CD34+/CD45+ cells, although the expression levels were different among samples. The c-met expression was facilitated by incubation of the cells with stem cell factor (SCF) or interleukin 3 (IL-3), even if the expression level had been low. IL-6, G-CSF, or erythropoietin (EPO) did not show such a stimulatory effect on the c-met expression of the cells. When HGF was added to the CD34+/CD45+ cells in the presence of SCF, the numbers of CD36+/CD11b- cells (very early erythroid lineage cells) and BFU-E increased. EPO-dependent tyrosine phosphorylation of Stat 5 also increased, but the EPO receptor (EPO-R) expression remained unchanged in the CD34+/CD45+ cells treated with SCF + HGF. Our present study suggests that stimulation of the HGF/c-met signal is concomitant with induction of c-met protein by SCF. The subsequent enhancement of signal transduction via the activation of Stat 5 from the EPO-R plays a crucial role in the commitment of hematopoietic stem cells into erythroid lineage cells.

The expression of mRNAs for hepatocyte growth factor/scatter factor, its receptor c-met, and one of its activators tissue-type plasminogen activator show a systematic relationship in the developing and adult cerebral cortex and hippocampus

The temporal and spatial expression in brain of the mRNAs for the pleiotropic cytokine hepatocyte growth factor/scatter factor (HGF/SF) and its receptor c-met were compared to those of a known HGF/SF activator, tissue-type plasminogen activator (tPA). In addition to the previously described expression in the developing and adult olfactory system [D.P. Thewke, N.W. Seeds, Expression of hepatocyte growth factor/scatter factor, its receptor, c-met, and tissue-type plasminogen activator during development of the murine olfactory system, J. Neurosci. 16 (1996) 6933-6944] two other regions of the mouse brain were found where the expression of tPA mRNA appeared to co-localized with HGF/SF and/or c-met mRNA. In the developing hippocampus, tPA mRNA was expressed coincident with HGF/SF and c-met mRNAs in the CA1 field. tPA mRNA was expressed in all areas of the adult hippocampus, while HGF/SF expression was restricted to the CA2 and CA3 fields, and c-met mRNA was seen primarily in the CA1 field. In the developing cerebral cortex, the expression of tPA mRNA was observed in the subplate and inner cortical plate between two layers of c-met expression, whereas HGF/SF mRNA was localized to the proliferative zone lining the lateral ventricle. Layer specific expression of both HGF/SF and c-met mRNA were observed in the adult cortex, where HGF/SF was expressed in layers IV and V and c-met in layers II-III, IV and V. The expression of tPA mRNA in the adult cortex was low and not layer specific, although homogenates of adult cortex did have detectable levels of tPA activity when subjected to zymography. Immunohistochemical analysis using HGF/SF and c-met antibodies on adult brain sections showed a distribution similar to the in situ hybridization results. C-met antibodies appeared to stain large neurons in the cortex and hippocampus. These results are consistent with the hypothesis that HGF/SF plays a role in the development and maintenance of both the cerebral cortex and hippocampus, and that tPA may act as a regulator of HGF/SF activity in these structures.

Hepatocyte growth factor and the kidney: it is not just for the liver

Mesenchymal-epithelial interactions are important for many biological processes in epithelial organs such as the kidney. Hepatocyte growth factor (HGF) is a mesenchymally derived polypeptide cytokine that acts through its tyrosine kinase c-met receptor and is an important mediator of these interactions. This article reviews data showing the in vitro actions of HGF on renal epithelial cells that result in such diverse responses as mitogenesis, motogenesis, and morphogenesis. It also examines the in vivo evidence linking HGF and the c-met receptor to kidney development, regeneration following injury, and renal disease. Elucidating cellular mechanisms underlying the coordinated control of diverse HGF-induced phenotypic changes in renal epithelia in vitro should contribute to a clearer understanding of complex biological processes such as organogenesis, regeneration, and carcinogenesis in epithelial organs such as the kidney.

Tyrosine phosphorylation of focal adhesion kinase stimulated by hepatocyte growth factor leads to mitogen-activated protein kinase activation

Focal adhesion kinase (FAK) is a cytoplasmic tyrosine kinase involved in integrin-mediated signal transduction pathway. In this report, we describe that the treatment of hepatocyte growth factor (HGF) stimulates a significant increase in the tyrosine phosphorylation of FAK in human embryonic kidney 293 cells. This stimulation is independent of cell adhesion or the integrity of the actin cytoskeleton, suggesting potentially different mechanisms by which the HGF receptors and integrins regulate the tyrosine phosphorylation of FAK. Our results also suggest that the activation of Src upon HGF stimulation is likely to be one, if not the only, of the mechanisms responsible for the HGF-induced tyrosine phosphorylation of FAK. Furthermore, we showed that a mutation in the Grb2 binding site Tyr-925 of FAK partially abolishes its increase in HGF-induced phosphorylation. Finally, we demonstrated that HGF stimulates the association of FAK with Grb2 in vitro and in intact cells and provided evidence that FAK might contribute to the activation of mitogen-activated protein kinase through Ras in HGF signaling by functioning as an adapter molecule.

Possible involvement of p21/waf1 in the growth inhibition of HepG2 cells induced by hepatocyte growth factor

Hepatocyte growth factor (HGF) is a potent mitogen for a variety of cell types, but it is also known as an antimitogenic factor for several types of tumor cell lines. The biological processes by which HGF inhibits tumor cell growth remain poorly understood. Here we report a comparative study of HGF-mediated signal transduction events between two opposite responding types of human hepatoblastoma cell lines, HuH6 and HepG2. Following serum starvation, both cell lines were cultured in hepatocyte growth medium (HGM), a chemically defined medium, in the presence or absence of HGF. Under these culture conditions, cell growth in HuH6 was promoted by HGF, while it was inhibited in HepG2. Phosphorylation of p42/mitogen-activated protein (MAP) kinase was observed within 10 min after HGF stimulation in both cell lines. The level of phosphorylated MAP kinase in HuH6 declined to basal levels after 2 hr. However, in HepG2 the phosphorylated form was detectable at 6 hr. p21/waf1 was induced in both cell lines where levels peaked 4-6 hr after HGF stimulation. In HuH6, a marked decrease of p21/waf1 was observed at 8-12 hr, while a high level of p21/waf1 was sustained for at least 24 hr in HepG2. HGF treatment depressed cdk2 activity in a time-dependent manner in HepG2 while the activity increased in HuH6. When serum-starved HepG2 was growth stimulated with serum in the presence or absence of HGF, the cells treated with HGF underwent growth inhibition correlating with a sustained induction of p21/waf1 and a decrease of cdk2 activity. Immunoprecipitation analysis revealed accumulation of cdk2-associated p21/waf1 in the HGF-treated HepG2. Together, the results suggest that sustained induction of p21/waf1 mediates growth inhibition in HepG2 in the presence of HGF.

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.

Signal transduction via platelet-derived growth factor receptors

Platelet-derived growth factor (PDGF) exerts its stimulatory effects on cell growth and motility by binding to two related protein tyrosine kinase receptors. Ligand binding induces receptor dimerization and autophosphorylation, allowing binding and activation of cytoplasmic SH2-domain containing signal transduction molecules. Thereby, a number of different signaling pathways are initiated leading to cell growth, actin reorganization migration and differentiation. Recent observations suggest that extensive cross-talk occurs between different signaling pathways, and that stimulatory signals are modulated by inhibitory signals arising in parallel.

p125Fak focal adhesion kinase is a substrate for the insulin and insulin-like growth factor-I tyrosine kinase receptors

The focal adhesion kinase p125(Fak) is a widely expressed cytosolic tyrosine kinase, which is involved in integrin signaling and in signal transduction of a number of growth factors. In contrast to tyrosine kinase receptors such as the platelet-derived growth factor and the hepatocyte growth factor receptors, which induce p125(Fak) phosphorylation, insulin has been shown to promote its dephosphorylation. In this study, we compared p125(Fak) phosphorylation in insulin-stimulated cells maintained in suspension or in an adhesion state. We found that, in nonattached cells, insulin promotes p125(Fak) phosphorylation, whereas dephosphorylation occurred in attached cells. This was observed in Rat-1 fibroblasts overexpressing the insulin receptor, as well as in Hep G2 hepatocytes and in 3T3-L1 adipocytes expressing more natural levels of insulin receptors. Insulin-induced p125(Fak) phosphorylation correlated with an increase in paxillin phosphorylation, indicating that p125(Fak) kinase activity may be stimulated by insulin. Mixing of purified insulin or insulin-like growth factor-I (IGF-I) receptors with p125(Fak) resulted in an increase in p125(Fak) phosphorylation. Using a kinase-deficient p125(Fak) mutant, we found that this protein is a direct substrate of the insulin and IGF-I receptor tyrosine kinases. This view is supported by two additional findings. (i) A peptide corresponding to p125(Fak) sequence comprising amino acids 568-582, which contains tyrosines 576 and 577 of the kinase domain regulatory loop, is phosphorylated by the insulin receptor; and (ii) p125(Fak) phosphorylation by the insulin receptor is prevented by addition of this peptide. Finally, we observed that p125(Fak) phosphorylation by the receptor results in its activation. Our results show that the nature of the cross-talk between the insulin/IGF-I receptors and p125(Fak) is dependent on the cell architecture, and hence the interaction of the insulin/IGF-I signaling system with the integrin system will vary accordingly.

Differential requirement of Grb2 and PI3-kinase in HGF/SF-induced cell motility and tubulogenesis

Hepatocyte growth factor/scatter factor (HGF/SF) is a multifunctional cytokine that induces mitogenesis, motility, invasion, and morphogenesis of several epithelial and endothelial cell lines in culture. The receptor for HGF/SF has been identified as the Met tyrosine kinase. To investigate the signaling pathways that are involved in these events, we have generated chimeric receptors containing the extracellular domain of the colony stimulating factor-1 (CSF-1) receptor fused to the transmembrane and intracellular domains of the Met receptor (MET). Madin-Darby canine kidney (MDCK) epithelial cells, expressing the CSF-MET chimera dissociate, scatter and form branching tubules in response to CSF-1. However, cells expressing a mutant CSF-MET receptor containing a phenylalanine substitution for tyrosine 1356 (Y1356F) are unable to scatter or form branching tubules following stimulation with CSF-1. Tyrosine 1356 is essential for the recruitment of multiple substrates including Grb2, the p85 subunit of PI3-kinase, and PLC gamma. To investigate the role of these signaling pathways, we have generated a mutant receptor that selectively fails to associate with Grb2, and have treated MDCK cells with potent inhibitors of PLC gamma, PI3-kinase, and p70S6K, a downstream target of PI3-kinase. Our results implicate pathways downstream from PI3-kinase in cell dissociation and scatter, whereas pathways downstream from Grb2 are required for branching tubulogenesis in MDCK cells.

Characterization of the novel focal adhesion kinase RAFTK in hematopoietic cells

Protein tyrosine kinases (PTKs) mediate signals that respond to many pivotal cellular functions. Tyrosine phosphorylation, controlled by the coordinated actions of protein tyrosine phosphatases (PTPs) and PTKs, is a critical control mechanism for various physiological processes, including cell growth, differentiation, metabolism, cell cycle regulation and cytoskeleton function. The focal adhesion kinase (FAK) is a widely expressed non-receptor tyrosine kinase that is implicated in integrin-mediated signaling and plays a role in signal transduction pathways mediating cell adhesion, motility and anchorage-independent growth. Recently, we and others have identified a novel protein tyrosine kinase termed RAFTK, (also known as Pyk2 or Cak-beta), which is related to FAK. This review describes the role of RAFTK in various signaling cascades mainly in reference to hematopoietic cell lineages.

Association of the multisubstrate docking protein Gab1 with the hepatocyte growth factor receptor requires a functional Grb2 binding site involving tyrosine 1356

Hepatocyte growth factor/scatter factor is a multifunctional factor that induces mitogenesis, motility, invasion, and branching tubulogenesis of several epithelial and endothelial cell lines in culture. The receptor for hepatocyte growth factor has been identified as the Met-tyrosine kinase. Upon stimulation with hepatocyte growth factor, the Met beta subunit becomes highly phosphorylated on tyrosine residues, one of which, tyrosine 1356 within the carboxyl terminus, is crucial for dissociation, motility, and branching tubule formation in Madin-Darby canine kidney epithelial cells. Tyrosine 1356 forms a multisubstrate binding site for the Grb2 and Shc adaptor proteins, the p85 subunit of phosphatidylinositol 3'-kinase, phospholipase Cgamma, and a phosphatase, SHP2. To investigate additional signaling molecules that are activated by the Met receptor, we have identified hepatocyte growth factor-induced phosphoproteins in tubular epithelial cells. We have established that proteins of 100-130 kDa are highly phosphorylated following stimulation of epithelial cells and that one of these is the Grb2-associated binding protein Gab1, a possible insulin receptor substrate-1-like signal transducer. We show that Gab1 is the major substrate for the Met kinase in vitro and in vivo. Association of Gab1 with Met requires a functional Grb2 binding site involving tyrosine 1356 and to a lesser extent tyrosine 1349. Met receptor mutants that fail to induce branching tubulogenesis are impaired in their ability to interact with Gab1, suggesting that Gab1 may play a role in these processes.

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.

Transcriptional induction of the agp/ebp (c/ebp beta) gene by hepatocyte growth factor

Hepatocyte growth factor (HGF) is a pleiotropic factor with mitogenic, morphogenic, motogenic, cytotoxic, or growth inhibitory activity. Although the signaling of HGF is mediated through the cell membrane receptor c-Met, the molecular mechanism of downstream signal transduction remains obscure. In this report, we present evidence that shows HGF can stimulate the expression of AGP/EBP (C/EBP beta) and NF-kappaB, which are both key transcription factors responsible for the regulation of many genes under stress conditions or during the acute-phase response. Biochemical and functional analysis indicates that the HGF-responsive element is located in the region -376 to -352 (URE1) of the 5'-upstream regulatory sequence of agp/ebp. Activation of NF-kappaB by HGF was observed to precede the induction of agp/ebp. Further studies indicate that NF-kappaB can cooperate with AGP/EBP or other members of the C/EBP family to activate the agp/ebp gene in both URE1 and URE2-dependent manner. These results suggest that the induction of the agp/ebp gene by HGF is mediated at least in part by its activation of NF-kappaB. The activated NF-kappaB then interacts with AGP/EBP, resulting in the induction of agp/ebp.

Hepatocyte Growth Factor/Scatter Factor Promotes Adhesion of Lymphoma Cells to Extracellular Matrix Molecules Via α4β1 and α5β1 Integrins

Hepatocyte growth factor (HGF )/scatter factor (SF ) is the ligand for a tyrosine kinase cell surface receptor encoded by the MET protooncogene (c-MET). HGF/SF can induce proliferation and motility in epithelial cells and promotes invasion of carcinoma cells and NIH3T3 fibroblasts transfected with both HGF/SF and c-MET genes. Our results show that HGF/SF and c-MET also play a role in adhesion and invasion of human lymphoma cells. c-MET mRNA is expressed in hemopoietic cells, such as hemopoietic progenitor cells (CD34+ cells) in bone marrow (BM) and mobilized peripheral blood, immature B cells in cord blood and BM, and germinal center B-centroblasts. In normal peripheral blood B cells, which are c-MET−, c-MET expression was induced by PMA, ConA, HGF/SF, and Epstein-Barr virus (EBV) infection. Using immunohistochemistry, we detected c-MET on the cell surface of large activated centroblasts in lymph nodes from patients with B-non–Hodgkin's lymphoma and Hodgkin's disease. In the latter group, c-MET expression correlated well with the presence of EBV. Because HGF/SF and c-MET promote metastasis of carcinoma cells, we studied the effects of c-MET stimulation by HGF/SF of B-lymphoma cells on properties relevant for metastasis, ie, adhesion, migration, and invasion. HGF/SF stimulated adhesion of the c-MET+ B-cell lines to the extracellular matrix molecules fibronectin (FN) and collagen (CN) in a dose dependent manner. However, adhesion to laminin was not affected by HGF/SF. Adhesion to FN was mediated by β1-integrins α4β1 (VLA4) and α5β1 (VLA5) since blocking antibodies against β1- (CD29), α4- (CD49d), or α5- (CD49e) integrin subunits, completely reversed the effect of HGF/SF. Furthermore, HGF/SF induced adhesion was abrogated by addition of genistein, which blocks protein tyrosine kinases, including c-MET. Addition of HGF/SF resulted in a sixfold increase in migration of c-MET B-lymphoma cells through Matrigel, compared to medium alone. In rat fibroblast cultures, HGF/SF doubled the number of c-MET+ B-lymphoma cells that invaded the fibroblast monolayer. In these adhesion, migration and invasion assays HGF/SF had no effect on c-MET− cell lines. In conclusion, c-MET is expressed or can be induced on immature, activated, and certain malignant B cells. HGF/SF increased adhesion of c-MET+ B-lymphoma cells to FN and CN, mediated via β1-integrins α4β1 and α5β1 , and furthermore promoted migration and invasion.

C-Met signalling in an HGF/SF-insensitive variant MDCK cell line with constitutive motile/invasive behaviour

The Met protein is a receptor tyrosine kinase for hepatocyte growth factor/scatter factor (HGF/SF), a multifunctional growth factor with mitogenic, motogenic and morphogenic properties. A morphologically altered variant of the MDCK cell line, MDCK-1, spontaneously exhibits a number of features associated with a partial HGF/SF-Met induced phenotype (less adhesive colonies in culture, enhanced invasion and motility, nascent tubule formation), but paradoxically does not respond to HGF/SF treatment. Although the overall cell surface expression and distribution of Met were found to be similar in parental MDCK cells and the MDCK-1 cell line, p145met autophosphorylation (+/ HGF/SF) was significantly reduced in MDCK-1 cells in vitro and in vivo when compared with parental MDCK cells. In contrast, EGF induced cell proliferation and EGF receptor autophosphorylation to similar levels in both cell lines. The basal levels of protein tyrosine phosphorylation were higher in MDCK-1 cells when compared with parental MDCK cells, including that of two prominent proteins with molecular masses of approximately 185 kDa and 220 kDa. Moreover, both p185 and p220 are present and tyrosine phosphorylated in Met immunoprecipitates from MDCK-1 cells (+/-HGF/SF), but not parental MDCK cells. In addition, Met immunocomplexes from MDCK-1 cells exhibited an approximately 3-fold increased tyrosine kinase activity in vitro when compared with MDCK cells, correlating with the higher basal levels of total phosphotyrosine. Treatment of MDCK-1 cells with the tyrosine kinase inhibitor herbimycin A reverted the cell phenotype to a more MDCK-like morphology in culture, with a concomitant reduction in the tyrosine phosphorylation predominantly of p220. Taken together these data suggest that aberrations in Met activity and associated signalling render MDCK-1 cells insensitive to HGF/SF, and may also mediate alterations in MDCK-1 cell behaviour.

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.

Dissociation of c-fos induction and mitogen-activated-protein kinase activation from the hepatocyte-growth-factor-induced motility response in human gastric carcinoma cells

The function of hepatocyte growth factor/scatter factor (HGF/SF) is to increase proliferation as well as to stimulate motility and disperse cell colonies of epithelial cells. In this study, we examined the motogenic and mitogenic responses of two human gastric carcinoma cell types, MKN7 and MKN74. Cell motility of both cell lines was markedly stimulated by HGF/SF. In contrast, HGF/SF stimulated cell growth of MKN74 cells, but did not stimulate growth of MKN7 cells. To address the cause of the difference in response of these cells, which may reflect some differences in signaling pathways downstream from the HGF/SF receptor, c-Met, we investigated the induction of the proto-oncogene c-fos. The level of c-fos mRNA increased and reached a maximum approximately 40 min after HGF/SF stimulation in MKN74 cells, and thereafter its level rapidly decreased. In contrast, the level of c-fos expression was very low irrespective of the stimulation in MKN7 cells. c-Fos protein was transiently induced only in MKN74 cells l h after treatment with HGF/SF, and its levels subsequently decreased. We subsequently examined the activation of mitogen-activated-protein kinase, which is a major mediator in the signaling pathway leading to the stimulation of c-fos transcription, after HGF/SF treatment in both cell lines. Mitogen-activated-protein kinase was markedly activated by this treatment in MKN74 cells, but was only slightly activated in MKN7 cells. These results suggest that although mitogen-activated-protein kinase activation and c-fos induction play an essential role in the signaling pathway leading to cell growth, they are not required for the motility response induced by HGF/SF.

Hepatocyte growth factor-induced scatter of Madin-Darby canine kidney cells requires phosphatidylinositol 3-kinase

Hepatocyte growth factor/scatter factor (HGF/SF) is a multifunctional cytokine that induces mitogenesis, motility, invasion, and morphogenesis of several epithelial and endothelial cell lines in culture. The receptor for HGF/SF has been identified as the Met tyrosine kinase. To investigate the signaling pathways that are involved in these events, we have generated chimeric receptors containing the extracellular domain of the colony-stimulating factor-1 (CSF-1) receptor fused to the transmembrane and intracellular domains of the Met receptor (MET). Madin-Darby canine kidney (MDCK) epithelial cells expressing the CSF-MET chimera dissociate and scatter in response to CSF-1. However, cells expressing a mutant CSF-MET receptor containing a phenylalanine substitution for tyrosine 1356 were unable to scatter or form branching tubules following stimulation with CSF-1. Tyrosine 1356 is essential for the recruitment of multiple substrates including the p85 subunit of PI3-kinase, phospholipase C gamma, and Grb2. In this study, we have investigated the role of PI3-kinase and a downstream target of PI3-kinase, pp70S6K, in the induction of MDCK cell scatter in response to HGF/SF. Our results demonstrate that following stimulation with HGF/SF, activation of PI3-kinase but not pp70S6K is essential for MDCK cell scatter.

Tyrosine kinase inhibitors potentiate the induction of low density lipoprotein receptor gene expression by hepatocyte growth factor

The role of tyrosine kinase, protein kinase C, cyclic nucleotide- and Ca(2+)-calmodulin-dependent protein kinase second messenger pathways in the induction of LDL receptor gene expression by hepatocyte growth factor (HGF) was studied in the human hepatoma cell line Hep-G2. Incubation with media containing HGF increased the level of LDL receptor mRNA by 6.5-fold. Co-incubation with HGF and either of two tyrosine kinase inhibitors genistein (2.0-20.0 micrograms/ml) and herbimycin A (0.5-500.0 ng/ml) increased the level of LDL receptor mRNA above that observed with HGF alone by 40-60%. Incubation with HGF in the presence of the calmodulin antagonist W7 (10-30 microM) also super-induced the level of LDL receptor mRNA by nearly 230%. The protein kinase C and A inhibitors chelerythrine (0.1-10.0 microM) and H8 (0.5-5.0 microM), respectively, had no significant effects on the induction of LDL receptor mRNA by HGF. Taken together, these data suggest that tyrosine kinase, protein kinases C and A, and Ca(2+)-calmodulin dependent protein kinase activities are not essential for activation of LDL receptor gene expression in Hep-G2 cells by HGF.

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)

Comparative effects of hepatocyte growth factor and epidermal growth factor on motility, morphology, mitogenesis, and signal transduction of primary rat hepatocytes

Hepatocyte growth factor (HGF) and epidermal growth factor (EGF) are major hepatocyte mitogens, but HGF, also known as scatter factor (SF), has also been shown as a potent motogen for epithelial and endothelial cells. The mechanisms by which HGF is a stronger motogen compared to other mitogens are not understood. Here we report a comparative study of the effect of the two growth factors on cultured primary rat hepatocytes regarding their differential effects on morphology, mitogenicity, and motility as well as the phosphorylation of cytoskeletal-associated proteins. Using three different motility assays, both HGF and EGF increased the motility of hepatocytes, but HGF consistently elicited a significantly greater motility response than EGF. Additionally, HGF induced a more flattened, highly spread morphology compared to EGF. To examine if HGF and EGF phosphorylated different cytoskeletal elements as signal transduction targets in view of the observed variation in morphology and motility, primary cultures of 32P-loaded rat hepatocytes were stimulated by either HGF or EGF for up to 60 min. Both mitogens rapidly stimulated four isoforms of MAP kinase with similar kinetics and also rapidly facilitated the phosphorylation of cytoskeletal-associated F-actin. Two cytoskeletal-associated proteins, however, were observed to undergo rapid phosphorylation by HGF and not EGF during the time points described. One protein of 28 kDa was observed to become phosphorylated fivefold over controls, while the EGF-stimulated cells showed only a slight increase in the phosphorylation of this protein. Another protein with an apparent mwt of 42 kDa was phosphorylated 20-fold at 1 min and remained phosphorylated over 50-fold over control up to the 60 min time point. This protein was observed to become phosphorylated by EGF only after 10 min, and to a lesser extent (20-fold). Taken together, the data suggest that HGF and EGF stimulate divergent as well as redundant signal transduction pathways in the hepatocyte cytoskeleton, and this may result in unique HGF- or EGF-specific motility, morphology, and mitogenicity in hepatocytes.

A scatter factor-like factor is produced by a metastatic variant of a rat bladder carcinoma cell line

The rat bladder carcinoma epithelial NBT-II cell line undergoes, in vitro, a morphological transition to a fibroblast-like state in the presence of different growth factors. We have selected, in vivo, a metastatic clone, designated M-NBT-II, which has a mesenchymal phenotype and secretes into the culture medium a factor able to dissociate epithelial clusters of NBT-II or MDCK cells. This factor was designated scatter factor-like (SFL) by analogy to the HGF/SF, which has the same dissociating effect in these two cell lines. Here, we show that SFL factor and HGF/SF are different factors: (i) no HGF/SF transcripts could be detected using either specific rat HGF/SF cDNA probes or PCR; (ii) blocking antibodies against rat HGF/SF do not inhibit the SFL activity; and (iii) crude culture medium or partially purified SFL factor-containing fractions do not induce MDCK tubulogenesis, a biological assay that is specific for HGF/SF activity in vitro. We report the partial purification of the SFL factor, based on ion exchange and reverse-phase chromatography. The results indicate that the M-NBT-II metastatic variant secretes a dissociating factor sharing some common biological properties with the HGF/SF, which suggests that the SFL factor is a member of the HGF/SF family and may be involved in tumor progression.