NK1, a Natural Splice Variant of Hepatocyte Growth Factor/Scatter Factor, Is a Partial Agonist In Vivo

Hepatocyte growth factor/scatter factor (HGF/SF) is a potent mitogen, motogen, and morphogen for epithelial cells expressing its tyrosine kinase receptor, the c-met proto-oncogene product, and is required for normal development in the mouse. Inappropriate stimulation of Met signal transduction induces aberrant morphogenesis and oncogenesis in mice and has been implicated in human cancer. NK1 is a naturally occurring HGF/SF splice variant composed of only the amino terminus and first kringle domain. While the biological activities of NK1 have been controversial, in vitro data suggest that it may have therapeutic value as an HGF/SF antagonist. Here, we directly test this hypothesis in vivo by expressing mouse NK1 in transgenic mice and comparing the consequent effects with those observed for mice carrying an HGF/SF transgene. Despite robust expression, NK1 did not behave as an HGF/SF antagonist in vivo. Instead, NK1-transgenic mice displayed most of the phenotypic characteristics associated with HGF/SF-transgenic mice, including enlarged livers, ectopic skeletal-muscle formation, progressive renal disease, aberrant pigment cell localization, precocious mammary lobuloalveolar development, and the appearance of mammary, hepatocellular, and melanocytic tumors. And like HGF/SF-transgenic livers, NK1 livers had higher levels of tyrosine-phosphorylated complexes associated with Met, suggesting that the mechanistic basis for the effects of NK1 overexpression in vivo was autocrine activation of Met. We conclude that NK1 acts in vivo as a partial agonist. As such, the efficacy of NK1 as a therapeutic HGF/SF antagonist must be seriously questioned.     

Structural insights into Met receptor activation

The receptor tyrosine kinase Met plays a pivotal role in vertebrate development and tissue regeneration, its deregulation contributes to cancer. Met is also targeted during the infection by the facultative intracellular bacterium Listeria monocytogenes. The mechanistic basis for Met activation by its natural ligand hepatocyte growth factor/scatter factor (HGF/SF) is only beginning to be understood at a structural level. Crystal structures of Met in complex with L. monocytogenes InlB suggest that Met dimerization by this bacterial invasion protein is mediated by a dimer contact of the ligand. Here, I review the structural basis of Met activation by InlB and highlight parallels and differences to the physiological Met ligand HGF/SF and its splice variant NK1.     

Enhanced tumorigenicity and invasion-metastasis by hepatocyte growth factor/scatter factor-met signalling in human cells concomitant with induction of the urokinase proteolysis network.

Hepatocyte growth factor/scatter factor (HGF/SF) is a pleiotropic effector of cells expressing the Met tyrosine kinase receptor. Although HGF/SF is synthesized by mesenchymal cells and acts predominantly on epithelial cells, we have recently demonstrated that human sarcoma cell lines often inappropriately express high levels of Met and respond mitogenically to HGF/SF. In the present report we show that HGF/SF-Met signalling in the human leiomyosarcoma cell line SK-LMS-1 enhances its in vivo tumorigenicity, an effect for which the mitogenicity of this signalling pathway is likely to play a role. In addition, we found that HGF/SF-Met signalling dramatically induces the in vitro invasiveness and in vivo metastatic potential of these cells. We have studied the molecular basis by which HGFSF-Met signalling mediates the invasive phenotype. A strong correlation has previously been demonstrated between the activation of the urokinase plasminogen activator (uPA) proteolysis network and the acquisition of the invasive-metastatic phenotype, and we show here that HGF/SF-Met signalling significantly increases the protein levels of both uPA and its cellular receptor in SK-LMS-1 cells. This results in elevated levels of cell-associated uPA and enhanced plasmin-generating ability by these cells. These studies couple HGF/SF-Met signalling to the activation of proteases that mediate dissolution of the extracellular matrix-basement membrane, and important property for cellular invasion-metastasis.     

Degradation of the Met tyrosine kinase receptor by the ubiquitin-proteasome pathway.

The Met tyrosine kinase receptor is a widely expressed molecule which mediates pleiotropic cellular responses following activation by its ligand, hepatocyte growth factor/scatter factor (HGF/SF). In this communication we demonstrate that significant Met degradation is induced by HGF/SF and that this degradation can be blocked by lactacystin, an inhibitor of proteasome activity. We also show that Met is rapidly polyubiquitinated in response to ligand and that polyubiquitinated Met molecules, which are normally unstable, are stabilized by lactacystin. Both HGF/SF-induced degradation and polyubiquitination of Met were shown to be dependent on the receptor possessing intact tyrosine kinase activity. Finally, we found that a normally highly labile 55-kDa fragment of the Met receptor is stabilized by lactacystin and demonstrate that it represents a cell-associated remnant that is generated following the ligand-independent proteolytic cleavage of the Met receptor in its extracellular domain. This truncated Met molecule encompasses the kinase domain of the receptor and is itself tyrosine phosphorylated. We conclude that the ubiquitin-proteasome pathway plays a significant role in the degradation of the Met tyrosine kinase receptor as directed by ligand-dependent and -independent signals. We propose that this proteolytic pathway may be important for averting cellular transformation by desensitizing Met signaling following ligand stimulation and by eliminating potentially oncogenic fragments generated via extracellular cleavage of the Met receptor.     

Hepatocyte growth factor induces delayed STAT3 phosphorylation through interleukin-6 expression

Met receptor tyrosine kinase mediates pleiotropic cellular responses following its activation by hepatocyte growth factor or scatter factor (HGF/SF). STAT3 was reported to be one of direct downstream molecules in HGF/SF-Met signaling. In the present study, however, we observed that Tyr705 of STAT3 was phosphorylated from 2 h or 6 h in NIH3T3 and Chang liver cells, respectively, after HGF/SF treatment. Blocking of the phosphorylation by cycloheximide or actinomycin D and the rapid STAT3 phosphorylation with the conditioned medium from HGF/SF-treated NIH3T3 cells suggested that a newly synthesized secretory protein was responsible for the delayed STAT3 phosphorylation. Among the known mediators to induce STAT3 phosphorylation, interleukin-6 (IL-6) mRNA and protein were induced by HGF/SF, and the released IL-6 was accumulated in the conditioned medium after HGF/SF treatment. Furthermore, the neutralizing IL-6 antibody abolished the STAT3 phosphorylation. Treatment with LY294002, a PI3 kinase inhibitor, but not with other signal inhibitors, resulted in the loss of delayed STAT3 phosphorylation by HGF/SF, showing the involvement of PI3 kinase pathway. Collectively, these results demonstrate that HGF/SF-Met signal cascade stimulates IL-6 production via PI3 kinase pathway, leading to STAT3 phosphorylation as a secondary effect.     

Met signaling mutants as tools for developmental studies

The Met receptor is widely expressed in embryonic and adult epithelial tissues; its ligand (hepatocyte growth factor/scatter factor, HGF/SF) is expressed in the mesenchymal component of various organs. The generation of hgf and met null mice has revealed an essential role for this ligand-receptor pair in the development of the placenta, liver, and limb muscles. However the early lethality of the null mutants has precluded analysis of Met function in late development. To extend the possible observation period, we generated mutant metalleles of different severity. This was done by impairing the ability of the receptor to transduce the HGF/SF signal, via mutation of consensus sequences in the multifunctional docking site present in the C-terminal tail of the receptor. Mice expressing a Met mutant still active as a kinase, but unable to recruit its effectors, died in mid-gestation with the same phenotype as the metknockout, proving the importance of phosphotyrosine-SH2 interactions in vivo. Mice expressing a Met receptor with partial loss of signaling function survived until birth and revealed novel aspects of HGF/SF-Met function during muscle development.     

Hepatocyte growth factor is a pleiotropic factor protecting epithelial cells from apoptosis

Hepatocyte Growth Factor (HGF) is a pleiotropic factor, capable of evoking complex biological responses such as mitogenesis, motogenesis and morphogenesis in a variety of epithelial and endothelial cells. Nonetheless, the meaning of the acronym is consistent with the key role of the factor in liver regeneration, in vivo and in liver development during embryogenesis. The receptor for HGF is the tyrosine kinase encoded by the c-MET proto-oncogene. Upon ligand binding, the receptor kinase is activated by tyrosine autophosphorylation and recruits cytoplasmic transducers involved in HGF-triggered signal transduction. We investigated the role of HGF as a survival factor in protecting cells from apoptosis and we show that HGF is able to counteract staurosporin-induced apoptosis of epithelial cells.     

Making tubes: step by step

Branched hollow tubes form the architectural basis of many mammalian organs. The growth factor HGF/SF and its receptor, the Met receptor tyrosine kinase, stimulate epithelial cells to undergo tubulogenesis in vitro. In this issue of Developmental Cell, O'Brien et al. (2004) look at temporal regulation and the role of two HGF/SF effectors, the ERK 1/2 MAP kinases and matrix metalloproteases, in this process.     

An in vitro tubule assay identifies HGF as a morphogen for the formation of seminiferous tubules in the postnatal mouse testis.

We have been working with a recently immortalized Sertoli cell line, SF7, that appears to produce sleeves, or hollow tubules, when cultivated on a layer of growth factor-reduced Matrigel (GFR-Matrigel) in medium supplemented with serum. We tried to determine which components of GFR-Matrigel and serum provide the environment needed for tubule formation. While laminin and collagen IV were essential for the formation of flat cords, none of the basement membrane constituents, when taken alone or in combination, would support the formation of tubules in minimal culture medium. Moreover, none of the growth factors present in GFR-Matrigel could induce tubulogenesis. Recently, much attention has been focused upon the role of hepatocyte growth factor (HGF) and its receptor c-met in the induction of tubulogenesis by epithelial cells. Therefore, we investigated the expression of HGF/c-met in the mouse testis at different postnatal stages and in the adult and evaluated the contribution of HGF/c-met in the production of Sertoli cell tubules by SF7 and primary Sertoli cells in vitro. Our results confirm that laminin and collagen IV are essential for the formation of testicular cords and reveal that HGF/c-met are necessary for the further remodeling of these cords into tubules.     

Involvement of hepatocyte growth factor/scatter factor and met receptor signaling in hair follicle morphogenesis and cycling.

HGF/SF and its receptor (Met) are principal mediators of mesenchymal-epithelial interactions in several different systems and have recently been implicated in the control of hair follicle (HF) growth. We have studied their expression patterns during HF morphogenesis and cycling in C57BL/6 mice, whereas functional hair growth effects of HGF/SF were assessed in vivo by analysis of transgenic mice and in skin organ culture. In normal mouse skin, follicular expression of HGF/SF and Met was strikingly localized: HGF/SF was found only in the HF mesenchyme (dermal papilla fibroblasts) and Met in the neighboring hair bulb keratinocytes. Both HGF/SF and Met expression peaked during the initial phases of HF morphogenesis, the stage of active hair growth (early and mid anagen), and during the apoptosis-driven HF regression (catagen). Met+ cells in the regressing epithelial strand appeared to be protected from undergoing apoptosis. Compared to wild-type controls, transgenic mice overexpressing HGF/SF under the control of the MT-1 promoter had twice as many developing HF and displayed accelerated HF development on postnatal day 3. They also showed significant catagen retardation on P17. In organ culture and in vivo, HGF/SF i.c. resulted in a significant catagen retardation. These results demonstrate an important role of HGF/SF and Met in murine hair growth control and suggest that Met-mediated signaling might be exploited for therapeutic manipulation of human hair growth disorders.-Lindner, G., Menrad, A., Gherardi, E., Merlino, G., Welker, P., Handjiski, B., Roloff, B., Paus, R. Involvement of hepatocyte growth factor/scatter factor and Met receptor signaling in hair follicle morphogenesis and cycling.     

肝细胞生长因子及受体传递的调节与内吞作用

肝细胞生长因子(HGF)是一种具有多重功能的细胞调控因子。HGF与其受体Met酪氨酸激酶(c-Met)的结合可激发多种生物学反应,从而调节细胞的增殖、分化、形态发生和侵袭运动等。有多种因素参与了HGF/c-Met信号传导的调控,从而防止信号的过度放大,其中Cbl1、Rab、泛素化激酶和HGF/c-Met的内吞等发挥了重要的作用。因此,对HGF/c-Met内吞过程的研究,了解内吞对于HGF/c-Met的信号传导及其调控的影响,探讨HGF/c-Met信号传导通路的调控机理和相互作用模式,可进一步阐明HGF/c-Met信号传导的调控机制,从而验证肝细胞中内吞作用直接调节HGF/c-Met信号通路的作用机制。     

The HGF/SF-induced phosphorylation of paxillin, matrix adhesion, and invasion of prostate cancer cells were suppressed by NK4, an HGF/SF variant

Hepatocyte growth factor/scatter factor (HGF/SF) plays a crucial role in cancer cell migration, matrix adhesion, invasion, and angiogenesis, via the phosphorylation of the c-met tyrosine kinase. This study examined the ability of NK4, a recently discovered HGF/SF variant, to inhibit the influence of HGF/SF on cell-matrix interaction, paxillin phosphorylation, and invasion of prostate cancer cells. HGF/SF was shown to dramatically enhance tumour cell motility, invasion, cell-matrix adhesion, together with an increase in the degree of paxillin phosphorylation and formation of focal adhesion complexes. However, these HGF/SF-induced effects were suppressed by the presence of NK4. NK4 effectively inhibited the degree of HGF/SF-induced paxillin phosphorylation and matrix adhesion. As a consequence, the matrix invasion of these prostate cancer cells was also suppressed by NK4. In conclusion, this study shows that these HGF/SF-enhanced events, which are critical steps in metastasis, can be inhibited through the addition of NK4, thus warranting further in vivo studies on the implication of NK4 as a potential antimetastasis agent in prostate cancer.     

Structure of the human receptor tyrosine kinase met in complex with the Listeria invasion protein InlB

The tyrosine kinase Met, the product of the c-met proto-oncogene and the receptor for hepatocyte growth factor/scatter factor (HGF/SF), mediates signals critical for cell survival and migration. The human pathogen Listeria monocytogenes exploits Met signaling for invasion of host cells via its surface protein InlB. We present the crystal structure of the complex between a large fragment of the human Met ectodomain and the Met-binding domain of InlB. The concave face of the InlB leucine-rich repeat region interacts tightly with the first immunoglobulin-like domain of the Met stalk, a domain which does not bind HGF/SF. A second contact between InlB and the Met Sema domain locks the otherwise flexible receptor in a rigid, signaling competent conformation. Full Met activation requires the additional C-terminal domains of InlB which induce heparin-mediated receptor clustering and potent signaling. Thus, although it elicits a similar cellular response, InlB is not a structural mimic of HGF/SF.     

HGF/SF-met signaling in the control of branching morphogenesis and invasion

Hepatocyte growth factor/Scatter factor (HGF/SF) is a multifunctional growth factor which can induce diverse biological events. In vitro, these include scattering, invasion, proliferation and branching morphogenesis. In vivo, HGF/SF is responsible for many processes during embryonic development and a variety of activities in adults, and many of these normal activities have been implicated in its role in tumorgenesis and metastasis. The c-Met receptor tyrosine kinase is the only known receptor for HGF/SF and mediates all HGF/SF induced biological activities. Upon HGF/SF stimulation, the c-Met receptor is tyrosine-phosphorylated which is followed by the recruitment of a group of signaling molecules and/or adaptor proteins to its cytoplasmic domain and its multiple docking sites. This action leads to the activation of several different signaling cascades that form a complete network of intra and extracellular responses. Different combinations of signaling pathways and signaling molecules and/or differences in magnitude of responses contribute to these diverse series of HGF/SF-Met induced activities and most certainly are influenced by cell type as well as different cellular environments. In this review, we focus on HGF/SF-induced branching morphogenesis and invasion, and bring together recent new findings which provide insight into how HGF/SF, via c-Met induces this response.     

Requirement of regulated activation of Ras for response of MDCK cells to hepatocyte growth factor/scatter factor

Hepatocyte growth factor/scatter factor (HGF/SF) induces cell scattering, migration, and branching tubule formation of MDCK cells. To examine the role of the Ras protein in the HGF/SF-induced responses, we constructed MDCK cell clones expressing either inducible dominant-negative Ras or constitutively activated Ras and analyzed their effects on responses of cells to HGF/SF. Induced expression of dominant-negative Ras prevented cell dissociation required for cell scattering, migration, and cystic formation as well as branching morphology required for branching tubule formation. Constitutively activated Ras induced cell dissociation, but not a scattered fibroblastic morphology even in the presence of HGF/SF. MDCK cells expressing constitutively activated Ras migrated at a level similar to that of wild-type MDCK cells stimulated by HGF/SF. MDCK cells expressing constitutively activated Ras showed disorganized growth in three-dimensional culture and did not form the branching tubule structures. These results indicate that activation of the Ras protein is essential for the cell scattering, migration, and branching tubule formation of MDCK cells induced by HGF/SF, and a properly regulated activation is required for some stages of the HGF/SF-induced responses of MDCK cells.     

The Met receptor tyrosine kinase transduces motility, proliferation, and morphogenic signals of scatter factor/hepatocyte growth factor in epithelial cells

Depending on the target cells and culture conditions, scatter factor/hepatocyte growth factor (SF/HGF) mediates several distinct activities, i.e., cell motility, proliferation, invasiveness, tubular morphogenesis, angiogenesis, or cytotoxicity. A small isoform of SF/HGF encoded by a natural splice variant, which consists of the NH2-terminal hairpin structure and the first two kringle domains but not the protease homology region, induces cell motility but not mitogenesis. Two types of SF/HGF receptors have recently been discovered in epithelial cells, the high affinity c-Met receptor tyrosine kinase, and low affinity/high capacity binding sites, which are probably located on heparan sulfate proteoglycans. In the present study, we have addressed the question whether the various biological activities of SF/HGF are transduced into cells by a single type of receptor. We have here examined MDCK epithelial cells transfected with a hybrid cDNA encoding the ligand binding domain of the nerve growth factor (NGF) receptor and the membrane-spanning and tyrosine kinase domains of the Met receptor. We demonstrate that all biological effects of SF/HGF upon epithelial cells such as the induction of cell motility, proliferation, invasiveness, and tubular morphogenesis can now be triggered by the addition of NGF. Thus, it is likely that all known biological signals of SF/HGF are transduced through the receptor tyrosine kinase encoded by the c-Met protooncogene.     

The shadow of death on the MET tyrosine kinase receptor

The MET tyrosine kinase receptor is a high-affinity receptor for hepatocyte growth factor/scatter factor (HGF/SF). HGF/SF-MET system is necessary for embryonic development, and aberrant MET signalling favours tumorigenesis and metastasis. MET is a prototype of tyrosine kinase receptor, which is able to counteract apoptosis through the initiation of a survival signal involving notably the PI3K-Akt pathway. Paradoxically, the MET receptor is also able to promote apoptosis when activated by HGF/SF or independently of ligand stimulation. The molecular mechanisms underlying this uncommon response have been recently investigated and revealed dual antiapoptotic or proapoptotic property of MET according to the cell type or stress conditions. Although the involvement of MET in the regulation of integrated biological responses mostly took into account its efficient antiapoptotic function, its proapoptotic responses could also be important for regulation of the survival/apoptosis balance and play a role during the development or tumour progression.     

c-Cbl is involved in Met signaling in B cells and mediates hepatocyte growth factor-induced receptor ubiquitination

Hepatocyte growth factor/scatter factor (HGF) and its receptor tyrosine kinase Met are key regulators of epithelial motility and morphogenesis. Recent studies indicate that the HGF/Met pathway also plays a role in B cell differentiation, whereas uncontrolled Met signaling may lead to B cell neoplasia. These observations prompted us to explore HGF/Met signaling in B cells. In this study, we demonstrate that HGF induces strong tyrosine phosphorylation of the proto-oncogene product c-Cbl in B cells and increases Cbl association with the Src family tyrosine kinases Fyn and Lyn, as well as with phosphatidylinositol-3 kinase and CrkL. In addition, we demonstrate that c-Cbl mediates HGF-induced ubiquitination of Met. This requires the juxtamembrane tyrosine Y1001 (Y2) of Met, but not the multifunctional docking site (Y14/15) or any additional C-terminal tyrosine residues (Y13-16). In contrast to wild-type c-Cbl, the transforming mutants v-Cbl and 70Z/3 Cbl, which lack the ubiquitin ligase RING finger domain, suppress Met ubiquitination. Our findings identify c-Cbl as a negative regulator of HGF/Met signaling in B cells, mediating ubiquitination and, consequently, proteosomal degradation of Met, and suggest a role for Cbl in Met-mediated tumorigenesis.     

Hepatocyte growth factor/scatter factor induces a variety of tissue- specific morphogenic programs in epithelial cells

Hepatocyte growth factor/scatter factor (HGF/SF) is the mesenchymal ligand of the epithelial tyrosine kinase receptor c-Met. In vitro, HGF/SF has morphogenic properties, e.g., induces kidney epithelial cells to form branching ducts in collagen gels. Mutation of the HGF/SF gene in mice results in embryonic lethality due to severe liver and placenta defects. Here, we have evaluated the morphogenic activity of HGF/SF with a large variety of epithelial cells grown in three- dimensional collagen matrices. We found that HGF/SF induces SW 1222 colon carcinoma cells to form crypt-like structures. In these organoids, cells exhibit apical/basolateral polarity and build a well- developed brush border towards the lumen. Capan 2 pancreas carcinoma cells, upon addition of HGF/SF, develop large hollow spheroids lined with a tight layer of polarized cells. Collagen inside the cysts is digested and the cells show features of pancreatic ducts. HGF/SF induces EpH4 mammary epithelial cells to form long branches with end- buds that resemble developing mammary ducts. pRNS-1-1 prostate epithelial cells in the presence of HGF/SF develop long ducts with distal branching as found in the prostate. Finally, HGF/SF simulates alveolar differentiation in LX-1 lung carcinoma cells. Expression of transfected HGF/SF cDNA in LX-1 lung carcinoma and EpH4 mammary epithelial cells induce morphogenesis in an autocrine manner. In the cell lines tested, HGF/SF activated the Met receptor by phosphorylation of tyrosine residues. These data show that HGF/SF induces intrinsic, tissue-specific morphogenic activities in a wide variety of epithelial cells. Apparently, HGF/SF triggers respective endogenous programs and is thus an inductive, not an instructive, mesenchymal effector for epithelial morphogenesis.