NK1, a natural splice variant of hepatocyte growth factor/scatter factor, is a partial agonist in vivo

A pilot study on nitration/dysfunction of NK1 segment of myogenic stem cell activator HGF

Protein tyrosine residue (Y) nitration, a post-translational chemical-modification mode, has been associated with changes in protein activity and function; hence the accumulation of specific nitrated proteins in tissues may be used to monitor the onset and progression of pathological disorders. To verify the possible impact of nitration on postnatal muscle growth and regeneration, a pilot study was designed to examine the nitration/dysfunction of hepatocyte growth factor (HGF), a key ligand that is released from the extracellular tethering and activates myogenic stem satellite cells to enter the cell cycle upon muscle stretch and injury. Exposure of recombinant HGF (a hetero-dimer of α- and β-chains) to peroxynitrite induces Y nitration in HGF α-chain under physiological conditions. Physiological significance of this finding was emphasized by Western blotting that showed the NK1 segment of HGF (including a K1 domain critical for signaling-receptor c-met binding) undergoes nitration with a primary target of Y198. Peroxynitrite treatment abolished HGF-agonistic activity of the NK1 segment, as revealed by in vitro c-met binding and bromodeoxyuridine-incorporation assays. Importantly, direct-immunofluorescence microscopy of rat lower hind-limb muscles from two aged-groups (2-month-old “young” and 12-month-old “retired/adult”) provided in vivo evidence for age-related nitration of extracellular HGF (Y198). Overall, findings provide the insight that HGF/NK1 nitration/dysfunction perturbs myogenic stem cell dynamics and homeostasis; hence NK1 nitration may stimulate progression of muscular disorders and diseases including sarcopenia.

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NK1 segment of hepatocyte growth factor (HGF) undergoes tyrosine (Y) nitration.

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Y198 was identified as a primary target for nitration of NK1.

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NK1 nitration may abolish HGF-agonistic activity that activates myogenic stem cells.

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Nitration of extracellular HGF-Y198 was detected in vivo at early aging-phase of rat.

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Findings may provide a possible strategy to combat progressive muscle-atrophy.

The Role of HGF/MET Signaling in Metastatic Uveal Melanoma

Simple Summary

Hepatocyte growth factor (HGF)/mesenchymal-epithelial transition factor (MET) signaling plays an important role in the metastatic formation and therapeutic resistance to uveal melanoma. Here, we review the various functions of MET signaling contributing to metastatic formation, as well as review resistance to treatments in metastatic uveal melanoma.

Abstract

Hepatocyte growth factor (HGF)/mesenchymal-epithelial transition factor (MET) signaling promotes tumorigenesis and tumor progression in various types of cancer, including uveal melanoma (UM). The roles of HGF/MET signaling have been studied in cell survival, proliferation, cell motility, and migration. Furthermore, HGF/MET signaling has emerged as a critical player not only in the tumor itself but also in the tumor microenvironment. Expression of MET is frequently observed in metastatic uveal melanoma and is associated with poor prognosis. It has been reported that HGF/MET signaling pathway activation is the major mechanism of treatment resistance in metastatic UM (MUM). To achieve maximal therapeutic benefit in MUM patients, it is important to understand how MET signaling drives cellular functions in uveal melanoma cells. Here, we review the HGF/MET signaling biology and the role of HGF/MET blockades in uveal melanoma.

The constitutive high-affinity Met-binding site in the kringle domain is dispensable for the signalling activity of hepatocyte growth factor

Activation of a tyrosine kinase receptor Met by hepatocyte growth factor (HGF) requires binding of proteolytically activated, two-chain (tc) HGF, but the biochemical detail of this ligand–receptor interaction specificity remains elusive because biologically inactive single chain (sc) HGF can also bind to Met with high affinity. We found that this proteolysis-independent Met binding can be eliminated by mutagenesis introduced in the kringle domain without losing the ability to bind and activate cellular Met receptor after proteolytic activation, arguing against this site’s involvement in the physiological signalling. This non-signal producing Met–HGF interaction can also be eliminated by addition of a heparin mimetic sucrose octasulphate (SOS). By including SOS in the running buffer, we succeeded in detecting cleavage-dependent tcHGF–Met complex formation by size exclusion chromatography.

CAR T-cell immunotherapy of MET-expressing malignant mesothelioma

Mesothelioma is an incurable cancer for which effective therapies are required. Aberrant MET expression is prevalent in mesothelioma, although targeting using small molecule-based therapeutics has proven disappointing. Chimeric antigen receptors (CARs) couple the HLA-independent binding of a cell surface target to the delivery of a tailored T-cell activating signal. Here, we evaluated the anti-tumor activity of MET re-targeted CAR T-cells against mesothelioma. Using immunohistochemistry, MET was detected in 67% of malignant pleural mesotheliomas, most frequently of epithelioid or biphasic subtype. The presence of MET did not influence patient survival. Candidate MET-specific CARs were engineered in which a CD28+CD3ζ endodomain was fused to one of 3 peptides derived from the N and K1 domains of hepatocyte growth factor (HGF), which represents the minimum MET binding element present in this growth factor. Using an NIH3T3-based artificial antigen-presenting cell system, we found that all 3 candidate CARs demonstrated high specificity for MET. By contrast, these CARs did not mediate T-cell activation upon engagement of other HGF binding partners, namely CD44v6 or heparan sulfate proteoglycans, including Syndecan-1. NK1-targeted CARs demonstrated broadly similar in vitro potency, indicated by destruction of MET-expressing mesothelioma cell lines, accompanied by cytokine release. In vivo anti-tumor activity was demonstrated following intraperitoneal delivery to mice with an established mesothelioma xenograft. Progressive tumor regression occurred without weight loss or other clinical indicators of toxicity. These data confirm the frequent expression of MET in malignant pleural mesothelioma and demonstrate that this can be targeted effectively and safely using a CAR T-cell immunotherapeutic strategy.

Exploring the chemical space of the lysine-binding pocket of the first kringle domain of hepatocyte growth factor/scatter factor (HGF/SF) yields a new class of inhibitors of HGF/SF-MET binding

The growth/motility factor hepatocyte growth factor/scatter factor (HGF/SF) and its receptor, the tyrosine kinase MET, constitute a signalling system essential for embryogenesis and for tissue/organ regeneration in post-natal life. HGF/SF-MET signalling, however, also plays a key role in the onset of metastasis of a large number of human tumours. Both HGF/SF and MET are high molecular weight proteins that bury an extensive interface upon complex formation and thus constitute a challenging target for the development of low molecular weight inhibitors. Here we have used surface plasmon resonance (SPR), nuclear magnetic resonance (NMR) and X-ray crystallography to screen a diverse fragment library of 1338 members as well as a range of piperazine-like compounds. Several small molecules were found to bind in the lysine-binding pocket of the kringle 1 domain of HGF/SF and its truncated splice variant NK1. We have defined the binding mode of these compounds, explored their biological activity and we show that selected fragments inhibit MET downstream signalling. Thus we demonstrate that targeting the lysine-binding pocket of NK1 is an effective strategy to generate MET receptor antagonists and we offer proof of concept that the HGF/SF-MET interface may be successfully targeted with small molecules. These studies have broad implications for the development of HGF/SF-MET therapeutics and cancer treatment.

An engineered dimeric fragment of hepatocyte growth factor is a potent c-MET agonist

Hepatocyte growth factor (HGF), through activation of the c-MET receptor, mediates biological processes critical for tissue regeneration; however, its clinical application is limited by protein instability and poor recombinant expression. We previously engineered an HGF fragment (eNK1) that possesses increased stability and expression yield and developed a c-MET agonist by coupling eNK1 through an introduced cysteine residue. Here, we further characterize this eNK1 dimer and show it elicits significantly greater c-MET activation, cell migration, and proliferation than the eNK1 monomer. The efficacy of the eNK1 dimer was similar to HGF, suggesting its promise as a c-MET agonist.

Hepatocyte Growth Factor Isoforms in Tissue Repair, Cancer, and Fibrotic Remodeling

Hepatocyte growth factor (HGF), also known as scatter factor (SF), is a pleotropic factor required for normal organ development during embryogenesis. In the adult, basal expression of HGF maintains tissue homeostasis and is up-regulated in response to tissue injury. HGF expression is necessary for the proliferation, migration, and survival of epithelial and endothelial cells involved in tissue repair in a variety of organs, including heart, lung, kidney, liver, brain, and skin. The administration of full length HGF, either as a protein or using exogenous expression methodologies, increases tissue repair in animal models of tissue injury and increases angiogenesis. Full length HGF is comprised of an N-terminal hairpin turn, four kringle domains, and a serine protease-like domain. Several naturally occurring alternatively spliced isoforms of HGF were also identified. The NK1 variant contains the N-terminal hairpin and the first kringle domain, and the NK2 variant extends through the second kringle domain. These alternatively spliced forms of HGF activate the same receptor, MET, but they differ from the full length protein in their cellular activities and their biological functions. Here, we review the species-specific expression of the HGF isoforms, their regulation, the signal transduction pathways they activate, and their biological activities.

Prenatal expression of MET receptor tyrosine kinase in the fetal mouse dorsal raphe nuclei and the visceral motor/sensory brainstem

Signaling via MET receptor tyrosine kinase (MET) has been implicated in a number of neurodevelopmental events, including cell migration, dendritic and axonal development and synaptogenesis. Related to its role in the development of forebrain circuitry, we recently identified a functional promoter variant of the MET gene that is associated with autism spectrum disorder (ASD). The association of the MET promoter variant rs1858830 C allele is significantly enriched in families with a child who has ASD and co-occurring gastrointestinal conditions. The expression of MET in the forebrain had been mapped in detail in the developing mouse and rhesus macaque. However, in mammals, its expression in the developing brainstem has not been studied extensively throughout developmental stages. Brainstem and autonomic circuitry are implicated in ASD pathophysiology and in gastrointestinal dysfunction. To advance our understanding of the neurodevelopmental influences of MET signaling in brainstem circuitry development, we employed in situ hybridization and immunohistochemistry to map the expression of Met and its ligand, Hgf, through prenatal development of the mouse midbrain and hindbrain. Our results reveal a highly selective expression pattern of Met in the brainstem, including a subpopulation of neurons in cranial motor nuclei (nVII, nA and nXII), B6 subgroup of the dorsal raphe, Barrington's nucleus, and a small subset of neurons in the nucleus of solitary tract. In contrast to Met, neither full-length nor known splice variants of Hgf were localized in the prenatal brainstem. RT-PCR revealed Hgf expression in target tissues of Met-expressing brainstem neurons, suggesting that MET in these neurons may be activated by HGF from peripheral sources. Together, these data suggest that MET signaling may influence the development of neurons that are involved in central regulation of gastrointestinal function, tongue movement, swallowing, speech, stress and mood.

Structural basis of MET receptor dimerization by the bacterial invasion protein InlB and the HGF/SF splice variant NK1

The structural basis of ligand-induced dimerization of the receptor tyrosine kinase MET by its natural ligand hepatocyte growth factor/scatter factor (HGF/SF) is not well understood. However, interesting insight into the molecular mechanism of MET dimerization has emerged from crystal structures of MET in complex with a bacterial agonist, the invasion protein internalin B (InlB) from pathogenic Listeria monocytogenes. MET activation by InlB promotes uptake of bacteria into host cells. Structural and biophysical data suggest that InlB is monomeric on its own but dimerizes upon binding to the membrane-anchored MET receptor promoting the formation of a signaling active 2:2 complex. The dimerization interface is small and unusually located on the convex side of the curved InlB leucine-rich repeat (LRR) domain. As InlB does not dimerize in solution, the dimerization site could only be identified by studying packing contacts of InlB in various crystal forms and had to be proven by scrutinizing its biological relevance in cellular assays. InlB dimerization is thus an example of a low-affinity contact that appears irrelevant in solution but becomes physiologically significant in the context of 2-dimensional diffusion restricted to the membrane plane. The resulting 2:2 InlB:MET complex has an InlB dimer at its center with one MET molecule bound peripherally to each InlB. This model of ligand-mediated MET dimerization may serve as a blue-print to understand MET activation by NK1, a naturally occurring HGF/SF splice variant and MET agonist. Crystal structures of NK1 repeatedly show a NK1 dimer, in which residues implicated in MET-binding are located on the outside. Thus, MET dimerization by NK1 may also be ligand-mediated with a NK1 dimer at the center of the 2:2 complex with one MET molecule bound peripherally to each NK1. This article is part of a Special Issue entitled: Emerging recognition and activation mechanisms of receptor tyrosine kinases.

Targeting MET in cancer: rationale and progress

Uncontrolled cell survival, growth, angiogenesis and metastasis are essential hallmarks of cancer. Genetic and biochemical data have demonstrated that the growth and motility factor hepatocyte growth factor/scatter factor (HGF/SF) and its receptor, the tyrosine kinase MET, have a causal role in all of these processes, thus providing a strong rationale for targeting these molecules in cancer. Parallel progress in understanding the structure and function of HGF/SF, MET and associated signalling components has led to the successful development of blocking antibodies and a large number of small-molecule MET kinase inhibitors. In this Review, we discuss these advances, as well as results from recent clinical studies that demonstrate that inhibiting MET signalling in several types of solid human tumours has major therapeutic value.

Intracellular signaling cascades triggered by the NK1 fragment of hepatocyte growth factor in human prostate epithelial cell line PNT1A

Hepatocyte Growth Factor (HGF)/c-MET signaling has an emerging role in promoting cell proliferation, survival, migration, wound repair and branching in a variety of cell types. HGF plays a crucial role as a mediator of stromal-epithelial interactions in the normal prostate but the precise biological function of HGF/c-Met interaction in the normal prostate and in prostate cancer is not clear. HGF has two naturally occurring splice variants and NK1, the smallest of these HGF variants, consists of the HGF amino terminus through the first kringle domain. We evaluated the intracellular signaling cascades and the morphological changes triggered by NK1 in human prostate epithelial cell line PNT1A which shows molecular and biochemical properties close to the normal prostate epithelium. We demonstrated that these cells express a functional c-MET, and cell exposure to NK1 induces the phosphorylation of tyrosines 1313/1349/1356 residues of c-MET which provide docking sites for signaling molecules. We observed an increased phosphorylation of ERK1/2, Akt, c-Src, p125FAK, SMAD2/3, and STAT3, down-regulation of the expression of epithelial cell-cell adhesion marker E-cadherin, and enhanced expression levels of mesenchymal markers vimentin, fibronectin, vinculin, α-actinin, and α-smooth muscle actin. This results in cell proliferation, in the appearance of a mesenchymal phenotype, in morphological changes resembling cell scattering and in wound healing. Our findings highlight the function of NK1 in non-tumorigenic human prostatic epithelial cells and provide a picture of the signaling pathways triggered by NK1 in a unique cell line.

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.

In vivo expression of HGF/NK1 and GLP-1 From dsAAV vectors enhances pancreatic ß-cell proliferation and improves pathology in the db/db mouse model of diabetes

OBJECTIVE

The purpose of the current study was to determine whether double-stranded adeno-associated virus (dsAAV)-mediated in vivo expression of β-cell growth factors, glucagon-like peptide-1 (GLP-1) and the NK1 fragment of hepatocyte growth factor (HGF/NK1) in β-cells, improves pathology in the db/db mouse model of type 2 diabetes. RESEARCH DESIGN AND METHODS; The glucoregulatory actions of GLP-1 and full-length HGF are well characterized. Here, we test the ability of HGF/NK1 to induce proliferation of exogenous islets and MIN6 β-cells. In addition, we target both GLP-1 and HGF/NK1 to endogenous β-cells using dsAAV vectors containing the mouse insulin-II promoter. We compare the abilities of these gene products to induce islet proliferation in vitro and in vivo and characterize their abilities to regulate diabetes after AAV-mediated delivery to endogenous islets of db/db mice.

RESULTS

Recombinant HGF/NK1 induces proliferation of isolated islets, and dsAAV-mediated expression of both GLP-1 and HGF/NK1 induces significant β-cell proliferation in vivo. Furthermore, both GLP-1 and HGF/NK1 expressed from dsAAV vectors enhance β-cell mass and insulin secretion in vivo and significantly delay the onset of hyperglycemia in db/db mice.

CONCLUSIONS

A single treatment with dsAAV vectors expressing GLP-1 or HGF/NK1 enhances islet growth and significantly improves pathology in a mouse model of type 2 diabetes. This represents the first example of a successful use of HGF/NK1 for diabetes therapy, providing support for direct AAV-mediated in vivo delivery of β-cell growth factors as a novel therapeutic strategy for the treatment of type 2 diabetes.

Noncoding mutations of HGF are associated with nonsyndromic hearing loss, DFNB39

A gene causing autosomal-recessive, nonsyndromic hearing loss, DFNB39, was previously mapped to an 18 Mb interval on chromosome 7q11.22-q21.12. We mapped an additional 40 consanguineous families segregating nonsyndromic hearing loss to the DFNB39 locus and refined the obligate interval to 1.2 Mb. The coding regions of all genes in this interval were sequenced, and no missense, nonsense, or frameshift mutations were found. We sequenced the noncoding sequences of genes, as well as noncoding genes, and found three mutations clustered in intron 4 and exon 5 in the hepatocyte growth factor gene (HGF). Two intron 4 deletions occur in a highly conserved sequence that is part of the 3' untranslated region of a previously undescribed short isoform of HGF. The third mutation is a silent substitution, and we demonstrate that it affects splicing in vitro. HGF is involved in a wide variety of signaling pathways in many different tissues, yet these putative regulatory mutations cause a surprisingly specific phenotype, which is nonsydromic hearing loss. Two mouse models of Hgf dysregulation, one in which an Hgf transgene is ubiquitously overexpressed and the other a conditional knockout that deletes Hgf from a limited number of tissues, including the cochlea, result in deafness. Overexpression of HGF is associated with progressive degeneration of outer hair cells in the cochlea, whereas cochlear deletion of Hgf is associated with more general dysplasia.

Engineering the NK1 fragment of hepatocyte growth factor/scatter factor as a MET receptor antagonist

The growth and motility factor hepatocyte growth factor/scatter factor (HGF/SF) and its receptor MET, the tyrosine kinase encoded by the c-MET proto-oncogene, exert major roles in cancer invasion and metastasis and are key targets for therapy. NK1 is an alternative spliced variant of HGF/SF that consists of the N-terminal (N) and first kringle (K1) domains and has partial agonistic activity. NK1 crystallizes as a head-to-tail dimer with an extensive inter-protomeric interface resulting from contacts between the two short interdomain linkers and reciprocal contacts between the N and K1 domains. Here we show that a subset of mutants at the NK1 dimer interface, such as the linker mutants Y124A or N127A or the kringle mutant V140A:I142A, bind the MET receptor with affinities comparable to wild-type NK1 but fail to assemble a dimeric, signalling competent NK1-MET complex. These NK1 variants have no detectable agonistic activity on, behave as bona fide receptor antagonists by blocking cell migration and DNA synthesis in target cells and have strong prospects as therapeutics for human cancer.

A mechanistic basis for converting a receptor tyrosine kinase agonist to an antagonist

Hepatocyte growth factor (HGF) activates the Met receptor tyrosine kinase by binding and promoting receptor dimerization. Here we describe a mechanistic basis for designing Met antagonists based on NK1, a natural variant of HGF containing the N-terminal and the first kringle domain. Through detailed biochemical and structural analyses, we demonstrate that both mouse and human NK1 induce Met dimerization via a conserved NK1 dimer interface. Mutations designed to alter the NK1 dimer interface abolish its ability to promote Met dimerization but retain full Met-binding activity. Importantly, these NK1 mutants act as Met antagonists by inhibiting HGF-mediated cell scattering, proliferation, branching, and invasion. The ability to separate the Met-binding activity of NK1 from its Met dimerization activity thus provides a rational basis for designing Met antagonists. This strategy of antagonist design may be applicable for other growth factor receptors by selectively abolishing the receptor activation ability but not the receptor binding of the growth factors.

Insights into the structure/function of hepatocyte growth factor/scatter factor from studies with individual domains

Hepatocyte growth factor/scatter factor (HGF/SF), the ligand for the receptor tyrosine kinase encoded by the c-Met proto-oncogene, is a multidomain protein structurally related to the pro-enzyme plasminogen and with major roles in development, tissue regeneration and cancer. We have expressed the N-terminal (N) domain, the four kringle domains (K1 to K4) and the serine proteinase homology domain (SP) of HGF/SF individually in yeast or mammalian cells and studied their ability to: (i) bind the Met receptor as well as heparan sulphate and dermatan sulphate co-receptors, (ii) activate Met in target cells and, (iii) map their binding sites onto the beta-propeller domain of Met. The N, K1 and SP domains bound Met directly with comparable affinities (K(d)=2.4, 3.3 and 1.4 microM). The same domains also bound heparin with decreasing affinities (N>K1>>SP) but only the N domain bound dermatan sulphate. Three kringle domains (K1, K2 and K4) displayed agonistic activity on target cells. In contrast, the N and SP domains, although capable of Met binding, displayed no or little activity. Further, cross-linking experiments demonstrated that both the N domain and kringles 1-2 bind the beta-chain moiety (amino acid residues 308-514) of the Met beta-propeller. In summary, the K1, K2 and K4 domains of HGF/SF are sufficient for Met activation, whereas the N and SP domains are not, although the latter domains contribute additional binding sites necessary for receptor activation by full length HGF/SF. The results provide new insights into the structure/function of HGF/SF and a basis for engineering the N and K1 domains as receptor antagonists for cancer therapy.

Fully Human Monoclonal Antibodies to Hepatocyte Growth Factor with Therapeutic Potential against Hepatocyte Growth Factor/c-Met–Dependent Human Tumors

c-Met is a well-characterized receptor tyrosine kinase for hepatocyte growth factor (HGF). Compelling evidence from studies in human tumors and both cellular and animal tumor models indicates that signaling through the HGF/c-Met pathway mediates a plethora of normal cellular activities, including proliferation, survival, migration, and invasion, that are at the root of cancer cell dysregulation, tumorigenesis, and tumor metastasis. Inhibiting HGF-mediated signaling may provide a novel therapeutic approach for treating patients with a broad spectrum of human tumors. Toward this goal, we generated and characterized five different fully human monoclonal antibodies that bound to and neutralized human HGF. Antibodies with subnanomolar affinities for HGF blocked binding of human HGF to c-Met and inhibited HGF-mediated c-Met phosphorylation, cell proliferation, survival, and invasion. Using a series of human-mouse chimeric HGF proteins, we showed that the neutralizing antibodies bind to a unique epitope in the β-chain of human HGF. Importantly, these antibodies inhibited HGF-dependent autocrine-driven tumor growth and caused significant regression of established U-87 MG tumor xenografts. Treatment with anti-HGF antibody rapidly inhibited tumor cell proliferation and significantly increased the proportion of apoptotic U-87 MG tumor cells in vivo. These results suggest that an antibody to an epitope in the β-chain of HGF has potential as a novel therapeutic agent for treating patients with HGF-dependent tumors. (Cancer Res 2006; 66(3): 1721-9)

The Interactions of Hepatocyte Growth Factor/Scatter Factor and Its NK1 and NK2 Variants with Glycosaminoglycans Using a Modified Gel Mobility Shift Assay

Full-length hepatocyte growth factor/scatter factor interacts with both heparan and dermatan sulfates and is critically dependent upon them as cofactors for activation of the tyrosine kinase receptor Met. Two C-terminally truncated variants (NK1 and NK2) of this growth factor also occur naturally. Their glycosaminoglycan binding properties are not clear. We have undertaken a comparative study of the heparan/dermatan sulfate binding characteristics of all three proteins. This has entailed the development of a modified gel mobility shift assay, utilizing fluorescence end-tagged oligosaccharides, that is also widely applicable to the analysis of many glycosaminoglycan-protein interactions. Using this we have shown that all three hepatocyte growth factor/scatter factor variants share identical heparan/dermatan sulfate binding properties and that both glycosaminoglycans occupy the same binding site. The minimal size of the oligosaccharide that binds with high affinity in all cases is a tetrasaccharide from heparan sulfate but a hexasaccharide from dermatan sulfate. These findings demonstrate that functional glycosaminoglycan binding is restricted to a binding site situated solely within the small N-terminal domain. The same minimal size fractions are also able to promote hepatocyte growth factor/scatter factor-mediated activation of Met and consequent downstream signaling in the glycosaminoglycan-deficient Chinese hamster ovary pgsA-745 cells. A covalent complex of heparan sulfate tetrasaccharide with monovalent growth factor is also active. The binding and activity of tetrasaccharides put constraints upon the possible interactions and molecular geometry within the ternary signaling complex.

The N-terminal domain of hepatocyte growth factor inhibits the angiogenic behavior of endothelial cells independently from binding to the c-met receptor

Hepatocyte growth factor (HGF) is a pleiotropic factor that plays an important role in complex biological processes such as embryogenesis, tissue regeneration, cancerogenesis, and angiogenesis. HGF promotes cell proliferation, survival, motility, and morphogenesis through binding to its receptor, a transmembrane tyrosine kinase encoded by the MET proto-oncogene (c-met). Structurally speaking, HGF is a polypeptide related to the enzymes of the blood coagulation cascade. Thus, it comprises kringle domains that in some other proteins have been shown to be responsible for the anti-angiogenic activity. To check whether the isolated kringles of HGF were able to inhibit angiogenesis, we produced them as recombinant proteins and compared their biological activity with that of the recombinant HGF N-terminal domain (N). We showed that (i) none of the isolated HGF kringle exhibits an anti-angiogenic activity; (ii) N is a new anti-angiogenic polypeptide; (iii) the inhibitory action of N is not specific toward HGF, because it antagonized the angiogenic activity of other growth factors, such as fibroblast growth factor-2 and vascular endothelial growth factor; and (iv) in contrast with full-length HGF, N does not bind to the c-met receptor in vitro, but fully retains its heparin-binding capacity. Our results suggest that N inhibits angiogenesis not by disrupting the HGF/c-met interaction but rather by interfering with the endothelial glycosaminoglycans, which are the secondary binding sites of HGF.

Plasma hepatocyte growth factor as a marker for small-for-gestational age fetuses

OBJECTIVE

To study the association between hepatocyte growth factor (HGF) levels and pregnancy outcome.

STUDY DESIGN

Hepatocyte growth factor levels were measured in 42 plasma samples between weeks 14 and 21 of gestation using an enzyme-linked immunosorbent assay (ELISA). Results were correlated to pregnancy outcome and Mann-Whitney U-test applied to study the differences.

RESULTS

Hepatocyte growth factor values in pregnancies that develop preeclampsia (n=12) were not significantly different from unaffected pregnancies (n=21, multiples of the median (MoM)=1.38, P=0.47). However, hepatocyte growth factor values were significantly elevated in pregnancies of small-for-gestational age (SGA) fetuses (n=9) compared to uncomplicated pregnancies (MoM=2.66, P<0.001).

CONCLUSION

Measurement of hepatocyte growth factor in peripheral blood between 14 and 21 weeks gestation may offer new possibilities in the early diagnosis and prediction of fetal birth weight but not of preeclampsia.

Immunohistochemical localization of hepatocyte growth factor activator (HGFA) in developing mouse liver tissues. Heterogeneous distribution of HGFA protein

Hepatocyte growth factor activator (HGFA) can activate the single-chain hepatocyte growth factor (HGF) required for embryonic development. We studied the immunohistochemical (IHC) localization of HGFA in adult mouse liver and its developmental changes from embryonic day 12 to postnatal day 30. A heterogeneous distribution of HGFA was observed in adult liver tissues. The hepatocytes around the hepatic veins were preferentially positive for HGFA, whereas those in other areas were negative. Depending on the vascular diameter, the hepatic veins were bordered by a one- to three-cell-thick layer of hepatocytes positive for HGFA, which showed evidence of cell-cell heterogeneity in staining intensity. Immunoelectron microscopy detected ubiquitous distribution of the gold particle reaction product for HGFA in the cytoplasm of these hepatocytes, especially in the rough endoplasmic reticulum. Developmental analysis indicated that there was hardly any staining of HGFA until postnatal day 0 and that noticeable staining was initially detected in the pericentral hepatocytes on postnatal day 3. Subsequently, immunoreactivity increased and the distinct staining pattern had been established by postnatal day 30. These results suggest that HGFA proteins are produced in the hepatocytes surrounding the efferent hepatic veins in the mouse and that development of the unique distributing pattern takes place postnatally.

The HGF/SF antagonist NK4 reverses fibroblast- and HGF-induced prostate tumor growth and angiogenesis in vivo

Our study examined the in vitro and in vivo responses of a newly discovered HGF/SF antagonist, NK4, on HGF/SF-promoted growth of human prostate cancer cells (PC-3). Nude mice were s.c. injected with either PC-3- and/or HGF/SF-producing fibroblasts (MRC5), and tumor size was measured over a 4-week period. rh-HGF/SF and/or NK4 were introduced by osmotic minipumps. An in vitro study found that NK4 significantly suppressed HGF/SF-induced invasion (HGF/SF; p < 0.01 vs. HGF/SF+NK4) and migration (HGF/SF; p < 0.05 vs. HGF/SF+NK4). Similarly, NK4 also suppressed the invasion (MRC5; p < 0.01 vs. MRC5+NK4) and migration (MRC5; p < 0.05 vs. MRC5+NK4) induced by MRC5 cells. NK4 also suppressed HGF/SF- and MRC5-induced tyrosine phosphorylation of the HGF/SF receptor Met as assessed by immunoprecipitation. Using a nude mouse model, prostate tumor volume (mm(3)) was significantly increased in both HGF/SF- (HGF/SF; p < 0.05 vs. control) and MRC5- (MRC5; p < 0.01 vs. control) treated groups compared to the control. In contrast, NK4 alone significantly reduced the growth of prostate tumors (NK4; p < 0.01 vs. control). In addition, NK4 also suppressed both HGF/SF- (HGF/SF; p < 0.01 vs. HGF/SF+NK4) and MRC5- (MRC5; p < 0.05 vs. MRC5+NK4) induced tumor growth in vivo by significantly reducing (p < 0.05) the degree of tumor angiogenesis using a recently discovered family of tumor endothelial markers (TEMs) by Q-RT-PCR analysis. In conclusion, NK4 suppresses both HGF/SF- and MRC5-induced invasion/migration of PC-3 cells in vitro. Furthermore, the HGF/SF antagonist NK4 significantly reduces prostate tumor growth in vivo by inhibiting the degree of tumor angiogenesis as determined by TEM-1 and TEM-8. Finally, our study provides evidence of the therapeutic potential of NK4 in prostate cancer development by antagonising HGF/SF-mediated events.

CCl4-induced acute liver injury in mice is inhibited by hepatocyte growth factor overexpression but stimulated by NK2 overexpression

Hepatocyte growth factor (HGF) inhibits acute liver injury. NK2 acts as an antagonist to HGF in vitro, but its in vivo function has reached no consensus conclusions. We have investigated in vivo effects of HGF and NK2 on CCl4-induced acute liver injury. Elevation of the serum alanine aminotransferase level and extension of centrilobular necrosis were inhibited in HGF transgenic mice but were promoted in NK2 transgenic mice. Hepatocyte proliferation after liver injury was not inhibited in NK2 transgenic mice. Thus, this study indicates that HGF inhibits liver injury, and NK2 antagonizes HGF on liver injury, however, NK2 may not antagonize HGF on hepatocyte proliferation.

A new crystal form of the NK1 splice variant of HGF/SF demonstrates extensive hinge movement and suggests that the NK1 dimer originates by domain swapping

NK1 is a splice variant of the polypeptide growth factor HGF/SF that consists of the N terminal (N) and first kringle (K) domains and retains receptor binding and signalling. While NK1 behaves as a monomer in solution, two independent crystallographic structures have previously shown an identical, tightly packed dimer. Here we report a novel orthorhombic crystal form of NK1 at 2.5 A resolution in which four NK1 protomers are packed in two distinct dimers in the asymmetric unit. Although the basic architecture of the new NK1 dimers is similar to the two described earlier, the new crystal form demonstrates extensive hinge movement between the N and K domain that leads to re-orientation of the receptor-binding sites. The hinge bending is evidence of the paucity of strong interactions between domains within the protomer, in contrast to the extensive interactions between protomers in the dimer. These observations are consistent with domain swapping in the dimer, such that the interdomain interactions of the monomer are replaced by equivalent interprotomer interactions in the dimer and offer a route for protein engineering of NK1 variants which may act as receptor antagonists.

Differential mitogenic effects of single chain hepatocyte growth factor (HGF)/scatter factor and HGF/NK1 following cleavage by factor Xa

Hepatocyte growth factor/scatter factor (HGF/SF) is a multifunctional cytokine that is involved in many normal as well as pathological conditions. HGF/NK1, a splice variant of HGF/SF, has been reported to have either antagonistic or agonistic effects with regard to c-Met signaling depending on the cell type. In these experiments, we have determined that HGF/NK1 is a potent mitogen for rat hepatocytes in culture. Furthermore, we have found that coagulation factor Xa (fXa) is capable of cleaving HGF/NK1 and single chain HGF/SF (scHGF/SF). The products resulting from cleavage of HGF/NK1 or scHGF/SF by fXa appear as single bands under non-reducing conditions. The reaction products from the digestion of HGF/NK1 by fXa were separated under reducing conditions, and the cleavage site, as determined by N-terminal sequencing, was located C-terminal to arginine 134. Previous work established that the heparin-binding domain for HGF/SF is located in the N domain of HGF/SF. Additionally, the dimerization of the HGF/SF receptor (c-Met) by the ligand HGF/NK1 is facilitated by heparin and related sulfonated sugars on the cell surface, whereas heparin is not required for HGF/SF-mediated dimerization. Cleavage of single chain HGF/SF or HGF/NK1 by factor Xa does not alter the affinity of the respective molecules for heparin, but it did variably affect the associated mitogenic activity of these factors. The associated mitogenic activity of HGF/NK1 was reduced by more than 90%, whereas the mitogenic activity of scHGF/SF was unaffected. This suggests mandatory maintenance of a steric interaction of the N domain and the first kringle domain for HGF/NK1 to act as an agonist for rat hepatocyte growth but is not required by full-length HGF/SF.

Requirement of Stat3 signaling for HGF/SF-Met mediated tumorigenesis

Hepatocyte Growth Factor/Scatter Factor (HGF/SF) mediates a wide variety of cellular responses by acting through the Met tyrosine kinase receptor. Inappropriate expression of HGF/SF and/or Met has been found in most types of solid tumors and is often associated with poor prognosis. Importantly, constitutional and sporadic activating mutations in Met have been discovered in human papillary renal carcinomas and other cancers, while autocrine and paracrine signaling of this receptor/ligand pair has been shown to contribute to tumorigenesis and metastasis. Numerous downstream signaling molecules have been implicated in HGF/SF-Met mediated tumorigenesis and metastasis. Stat3 is a downstream signaling molecule activated by HGF/SF-Met signaling, and is reported to contribute to cell transformation induced by a diverse set of oncoproteins. Stat3 is constitutively activated in many primary tumors and tumor cell lines, suggesting that signaling by this molecule may be important for cell transformation. To address whether Stat3 is required for HGF/SF-Met mediated tumorigenesis and metastasis, we introduced a dominant-negative form of Stat3, Stat3beta into the human leiomyosarcoma cell line SK-LMS-1. We found that Stat3beta has no effect on the transformed morphology, proliferation, invasion or branching morphogenesis in vitro. By contrast, expression of Stat3beta affected HGF/SF-Met mediated anchorage-independent colony formation and prevented tumorigenic growth in athymic nu/nu mice. Thus, Met signaling through Stat3 provides an essential function for tumorigenic growth, which is manifested in vitro by loss of anchorage-independent growth.

SF/HGF-c-Met autocrine and paracrine promote metastasis of hepatocellular carcinoma

AIM: To explore the role of SF/HGF-Met autocrine and paracrine in met astasis of hepatocellular carcinoma (HCC).

METHODS: SF/HGF and c-met transcri ption and protein expression in HCC were examined by RT-PCR and Western Blot in 4 HCC cell lines, including HepG2, Hep3B, SMMC7721 and MHCC-1, the last cell line had a higher potential of metastasis. sf/hgf cDNA was transfected by the method of Lipofectin into SMMC7721. SF/HGF and c-met antibody were used to stimulate and block SF/HGF-c-met signal transduction. Cell morphology, mobility, and proliferation were respectively compared by microscopic observation, wound healing assay and cell growth curve.

RESULTS: HCC malignancy appeared to be relative to its met-SF/HGF expression. In MHCC-1, c-met expression was much stronger than that in other cell lines with lower potential of metastasis and only SF/HG F autocrine existed in MHCC-1. After sf/hgf cDNA transfection or conditioned medium of MHCC-1 stimulation, SMMC7721 changed into elongated morphology, and the abilities of proliferation (P < 0.05) and mobility increased. Such bio-activity could be blocked by c-met antibody (P < 0.05).

CONCLUSION: The system of SF/HGF-c- met autocrine and paracrine played an important role in development and metastas is potential of HCC. Inhibition of SF/HGF-c-met signal transduction system may reduce the growth and metastasis of HCC.

Crystal structures of NK1-heparin complexes reveal the basis for NK1 activity and enable engineering of potent agonists of the MET receptor

NK1 is a splice variant of the polypeptide growth factor HGF/SF, which consists of the N-terminal (N) and first kringle (K) domain and requires heparan sulfate or soluble heparin for activity. We describe two X-ray crystal structures of NK1-heparin complexes that define a heparin-binding site in the N domain, in which a major role is played by R73, with further contributions from main chain atoms of T61, K63 and G79 and the side chains of K60, T61, R76, K62 and K58. Mutagenesis experiments demonstrate that heparin binding to this site is essential for dimerization in solution and biological activity of NK1. Heparin also comes into contact with a patch of positively charged residues (K132, R134, K170 and R181) in the K domain. Mutation of these residues yields NK1 variants with increased biological activity. Thus, we uncover a complex role for heparan sulfate in which binding to the primary site in the N domain is essential for biological activity whereas binding to the K domain reduces activity. We exploit the interaction between heparin and the K domain site in order to engineer NK1 as a potent receptor agonist and suggest that dual (positive and negative) control may be a general mechanism of heparan sulfate-dependent regulation of growth factor activity.

Cutting edge: Identification of a hybrid cytokine consisting of IL-7 and the beta-chain of the hepatocyte growth factor/scatter factor

Pre-pro-B cell growth-stimulating factor (PPBSF) is a heterodimer of IL-7 and a 30-kDa cofactor. Unlike monomeric IL-7, PPBSF selectively induces proliferation and differentiation of pre-pro-B cells and up-regulates IL-7Ralpha-chain expression. Here we clone the PPBSF cofactor from bone marrow stromal cells and identify it as a variant beta-chain of hepatocyte growth factor (HGF), a pleiotropic cytokine homologous to plasminogen that regulates cell growth, motility, and morphogenesis. We further demonstrate that, in the presence of low m.w. heparin sulfate-derived oligosaccharides, rHGFbeta combines with rIL-7 to form a biologically active heterodimer having the properties of PPBSF. The results indicate that PPBSF is a novel form of cytokine (hybrid cytokine) consisting of the bioactive components of two unrelated cytokines. Based on its heparin-binding and mitogenic properties, we postulate that the HGFbeta-chain in PPBSF enables IL-7 to participate in cognate interactions at the stromal cell surface and to transduce signals effectively at low levels of IL-7R.

Dissociation of heparan sulfate and receptor binding domains of hepatocyte growth factor reveals that heparan sulfate-c-met interaction facilitates signaling

Hepatocyte growth factor (HGF) is a secreted, heparan sulfate (HS) glycosaminoglycan-binding protein that stimulates mitogenesis, motogenesis, and morphogenesis in a wide array of cellular targets, including hepatocytes and other epithelial cells, melanocytes, endothelial cells, and hematopoietic cells. NK1 is an alternative HGF isoform that consists of the N-terminal (N) and first kringle (K1) domains of full-length HGF and stimulates all major HGF biological activities. Within NK1, the N domain retains the HS binding properties of full-length HGF and mediates HS-stimulated ligand oligomerization but lacks significant mitogenic or motogenic activity. In contrast, K1 does not bind HS, but it stimulates receptor and mitogen-activated protein kinase activation, mitogenesis, and motogenesis, demonstrating that structurally distinct and dissociable domains of HGF are the primary mediators of HS binding and receptor activation. Despite the absence of HS-K1 binding, K1 mitogenic activity in HS-negative cells is strictly dependent on added soluble heparin, whereas K1-stimulated motility is not. We also found that, like the receptors for fibroblast growth factors, the HGF receptor c-Met binds tightly to HS. These data suggest that HS can facilitate HGF signaling through interaction with c-Met that is independent of HGF-HS interaction and that the recruitment of specific intracellular effectors that mediate distinct HGF responses such as mitogenesis and motility is regulated by HS-c-Met interaction at the cell surface.

Hepatocyte growth factor is predominantly expressed by the carcinoma cells in non-small-cell lung cancer

Hepatocyte growth factor (HGF) exerts multifunctional regulatory roles in the growth, morphogenesis, differentiation, and motility of epithelial cells, and putatively plays important roles in tumor angiogenesis and metastasis. Aside from the full-length protein, 2 naturally occurring truncated HGF isoforms (NK1 and NK2) have been identified. Recent evidence suggests that a high level of HGF in surgically resected non-small-cell lung carcinoma (NSCLC) is a negative prognostic marker for NSCLC patients' survival. The origin of HGF in these tumors remains uncertain. We show here by in situ hybridization and immunohistochemistry that HGF messenger RNA (mRNA) and protein were predominantly expressed by the tumor cells in a high percentage of primary NSCLC. Stromal cell expression of HGF was limited to some lymphocytes and endothelial cells. Normal bronchial and bronchiolar epithelial cells also expressed HGF mRNA and immunoreactive protein. The mRNA transcripts and putative proteins of all 3 known HGF isoforms were detected in both normal lung and lung cancer tissues, but the full-length HGF was predominantly expressed. Our findings indicate that both autocrine and paracrine functions of HGF are likely to contribute to the pathobiology of lung cancer in vivo.

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.

Hepatocyte growth factor in renal failure: promise and reality

Can science discover some secrets of Greek mythology? In the case of Prometheus, we can now suppose that his amazing hepatic regeneration was caused by a peptide growth factor called hepatocyte growth factor (HGF). Increasing evidence indicates that HGF acts as a multifunctional cytokine on different cell types. This review addresses the molecular mechanisms that are responsible for the pleiotropic effects of HGF. HGF binds with high affinity to its specific tyrosine kinase receptor c-met, thereby stimulating not only cell proliferation and differentiation, but also cell migration and tumorigenesis. The three fundamental principles of medicine-prevention, diagnosis, and therapy-may be benefited by the rational use of HGF. In renal tubular cells, HGF induces mitogenic and morphogenetic responses. In animal models of toxic or ischemic acute renal failure, HGF acts in a renotropic and nephroprotective manner. HGF expression is rapidly up-regulated in the remnant kidney of nephrectomized rats, inducing compensatory growth. In a mouse model of chronic renal disease, HGF inhibits the progression of tubulointerstitial fibrosis and kidney dysfunction. Increased HGF mRNA transcripts were detected in mesenchymal and tubular epithelial cells of rejecting kidney. In transplanted patients, elevated HGF levels may indicate renal rejection. When HGF is considered as a therapeutic agent in human medicine, for example, to stimulate kidney regeneration after acute injury, strategies need to be developed to stimulate cell regeneration and differentiation without an induction of tumorigenesis.

Disassociation of met-mediated biological responses in vivo: the natural hepatocyte growth factor/scatter factor splice variant NK2 antagonizes growth but facilitates metastasis

Hepatocyte growth factor/scatter factor (HGF/SF) stimulates numerous cellular activities capable of contributing to the metastatic phenotype, including growth, motility, invasiveness, and morphogenetic transformation. When inappropriately expressed in vivo, an HGF/SF transgene induces numerous hyperplastic and neoplastic lesions. NK1 and NK2 are natural splice variants of HGF/SF; all interact with a common receptor, Met. Although both agonistic and antagonistic properties have been ascribed to each isoform in vitro, NK1 retains the full spectrum of HGF/SF-like activities when expressed as a transgene in vivo. Here we report that transgenic mice broadly expressing NK2 exhibit none of the phenotypes characteristic of HGF/SF or NK1 transgenic mice. Instead, when coexpressed in NK2-HGF/SF bitransgenic mice, NK2 antagonizes the pathological consequences of HGF/SF and discourages the subcutaneous growth of transplanted Met-containing melanoma cells. Remarkably, the metastatic efficiency of these same melanoma cells is dramatically enhanced in NK2 transgenic host mice relative to wild-type recipients, rivaling levels achieved in HGF/SF and NK1 transgenic hosts. Considered in conjunction with reports that in vitro NK2 induces scatter, but not other activities, these data strongly suggest that cellular motility is a critical determinant of metastasis. Moreover, our results demonstrate how alternatively structured ligands can be exploited in vivo to functionally dissociate Met-mediated activities and their downstream pathways.

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.

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.

Insights into the structure of hepatocyte growth factor/scatter factor (HGF/SF) and implications for receptor activation

The modular structure of HGF/SF offers a reductionist or 'divide and rule' approach to the analysis of structure and function. Domain deletion experiments have established that the N domain, kringle 1 and kringle 2 are essential for HGF/SF activity and that truncated variants containing the N domain and kringle 1 (NK1) or kringles 1 and 2 (NK2) can exhibit partial agonistic or antagonistic activity depending on target cells. Comparative modelling has been used to predict the 3D structures of the six domains of HGF/SF. More recently, NMR methods have shown that the N domain has a novel fold, the charge distribution of which suggests a heparin binding site. Crystals of NK1 indicate the relationship of this domain to the kringle 1, offering further insights into the mechanism of domain interactions and receptor activation.