Multiple biological responses are induced by glycosylation-deficient hepatocyte growth factor

Dimer Interface in Natural Variant NK1 Is Dispensable for HGF-Dependent Met Receptor Activation

NK1, a splicing variant of hepatocyte growth factor (HGF), binds to and activates Met receptor by forming an NK1 dimer and 2:2 complex with Met. Although the structural mechanism underlying Met activation by HGF remains incompletely resolved, it has been proposed that the NK1 dimer structure participates in this activation. We investigated the NK1 dimer interface’s role in Met activation by HGF. Because N127, V140, and K144 are closely involved in the head-to-tail NK1 dimer formation, mutant NK1 proteins with replacement of these residues by alanine were prepared. In Met tyrosine phosphorylation assays, N127-NK1, V140-NK1, and K144-NK1 showed 8.3%, 23.8%, and 52.2% activity, respectively, compared with wild-type NK1. Although wild-type NK1 promoted cell migration and scattering, N127-NK1, V140-NK1, and K144-NK1 hardly or marginally promoted them, indicating loss of activity of these mutant NK1 proteins to activate Met. In contrast, mutant HGFs (N127-HGF, V140-HGF, and K144-HGF) with the same amino acid replacements as in NK1 induced Met tyrosine phosphorylation and biological responses at levels comparable to those of wild-type HGF. These results indicate that the structural basis responsible for NK1-dependent Met dimer formation and activation differs from, or is at least distinguishable from, the structural basis responsible for HGF-dependent Met activation.

Structural and Functional Insight Into the Glycosylation Impact Upon the HGF/c-Met Signaling Pathway

Upon interactions with its specific ligand hepatocyte growth factor (HGF), the c-Met signal is relayed to series of downstream pathways, exerting essential biological roles. Dysregulation of the HGF-c-Met signaling pathway has been implicated in the onset, progression and metastasis of various cancers, making the HGF-c-Met axis a promising therapeutic target. Both c-Met and HGF undergo glycosylation, which appears to be biologically relevant to their function and structural integrity. Different types of glycoconjugates in the local cellular environment can also regulate HGF/c-Met signaling by distinct mechanisms. However, detailed knowledge pertaining to the glycosylation machinery of the HGF-c-Met axis as well as its potential applications in oncology research is yet to be established. This mini review highlights the significance of the HGF-c-Met signaling pathway in physiological and pathological context, and discusses the molecular mechanisms by which affect the glycosylation of the HGF-c-Met axis. Owing to the crucial role played by glycosylation in the regulation of HGF/c-Met activity, better understanding of this less exploited field may contribute to the development of novel therapeutics targeting glycoepitopes.

Distinct Localization of Mature HGF from its Precursor Form in Developing and Repairing the Stomach

Hepatocyte growth factor (HGF) is secreted as an inactive single-chain HGF (scHGF); however, only proteolytically processed two-chain HGF (tcHGF) can activate the MET receptor. We investigated the localization of tcHGF and activated/phosphorylated MET (pMET) using a tcHGF-specific antibody. In day 16.5 mouse embryos, total HGF (scHGF + tcHGF) was mainly localized in smooth muscle cells close to, but separate from, MET-positive epithelial cells in endodermal organs, including the stomach. In the adult stomach, total HGF was localized in smooth muscle cells, and tcHGF was mainly localized in the glandular base region. Immunostaining for pMET and Lgr5-driven green fluorescent protein (GFP) indicated that pMET localization overlapped with Lgr5⁺ gastric stem cells. HGF promoted organoid formation similar to EGF, indicating the potential for HGF to promote the survival and growth of gastric stem cells. pMET and tcHGF localizations changed during regeneration following gastric injury. These results indicate that MET is constantly activated in gastric stem cells and that the localization of pMET differs from the primary localization of precursor HGF but has a close relationship to tcHGF. Our results suggest the importance of the microenvironmental generation of tcHGF in the regulation of development, regeneration, and stem cell behavior.

Stable Ectopic Expression of ST6GALNAC5 Induces Autocrine MET Activation and Anchorage-Independence in MDCK Cells

The epithelial-mesenchymal transition (EMT) is a complex cancer progression that can boost the metastatic potential of transformed cells by inducing migration, loss of cell adhesion, and promoting proliferation under anchorage-independent conditions. A DNA microarray analysis was performed comparing parental anchorage-dependent MDCK cells and anchorage-independent MDCK cells that were engineered to express human siat7e (ST6GALNAC5). The comparison identified several genes involved in the EMT process that were differentially expressed between the anchorage-dependent and the anchorage-independent MDCK cell lines. The hepatocyte growth factor gene (hgf) was found to be over-expressed in the engineered MDCK-siat7e cells at both transcription and protein expression levels. Phosphorylation analysis of the MET receptor tyrosine kinase confirmed the activation of an autocrine loop of the HGF/ MET signaling pathway in the MDCK-siat7e cells. When MET activities were suppressed by using the small-molecular inhibitor drug PF-02341066 (Crizotinib), the anchorage-independent MDCK-siat7e cells reverted to the cellular morphology of the parental anchorage-dependent MDCK cells. These observations indicate that the MET receptor plays a central role in the growth properties of the MDCK cells and its phosphorylation status is likely dependent on sialylation. Further investigation of the downstream signaling targets in the MET network showed that the degree of MDCK cell adhesion correlated with secretion levels of a matrix metalloproteinase, MMP1, suggesting a role of metalloproteinases in the EMT process. These results demonstrate that in addition to its application in biotechnology processes, MDCK-siat7e may serve as a model cell for metastasis studies to decipher the sequence of events leading up to the activation of EMT.

Physiological Signaling and Structure of the HGF Receptor MET

The "hepatocyte growth factor" also known as "scatter factor", is a multifunctional cytokine with the peculiar ability of simultaneously triggering epithelial cell proliferation, movement and survival. The combination of those proprieties results in the induction of an epithelial to mesenchymal transition in target cells, fundamental for embryogenesis but also exploited by tumor cells during metastatization. The hepatocyte growth factor receptor, MET, is a proto-oncogene and a prototypical transmembrane tyrosine kinase receptor. Inhere we discuss the MET molecular structure and the hepatocyte growth factor driven physiological signaling which coordinates epithelial proliferation, motility and morphogenesis.

An investigation into the stability of commercial versus MG63-derived hepatocyte growth factor under flow cultivation conditions

The scale-up of tissue engineering cell culture must ensure that conditions are maintained while also being cost effective. Here we analyse the stability of hepatocyte growth factor (HGF) to investigate whether concentrations change under dynamic conditions, and compare commercial recombinant human HGF as an additive in 'standard medium', to HGF secreted by the osteosarcoma cell line MG63 as a 'preconditioned medium'. After 3 h under flow conditions, HGF in the standard medium degraded to 40% of its original concentration but HGF in the preconditioned medium remained at 100%. The concentration of secreted HGF was 10 times greater than the working concentration of commercially-available HGF. Thus HGF within this medium has increased stability; MG63-derived HGF should therefore be investigated as a cost-effective alternative to current lyophilised powders for use in in vitro models. Furthermore, we recommend that those intending to use HGF (or other growth factors) should consider similar stability testing before embarking on experiments with media flow.

Dissociation of c-Met phosphotyrosine sites in human cells in response to mouse hepatocyte growth factor but not human hepatocyte growth factor: the possible roles of different amino acids in different species

Hepatocyte growth factor (HGF) is essential for embryogenesis, tissue regeneration and tumour malignancy through the activation of its receptor, c-Met. We previously demonstrated that HGF α-chain hairpin-loop, K1 domain and β-chain are required for c-Met signalling. The sequential phosphorylation of tyrosine residues, from c-Met kinase domain to multidocking regions, is required for HGF-signalling transduction. Herein, we provide evidence that the disconcerted activation of c-Met tyrosine regions fails to induce biological functions. When human cells were incubated with 'mouse HGF', kinase domain activation (i.e. phospho-Tyr-1230/34/35) became evident, but the multidocking site (i.e. Tyr-1349) was not phosphorylated, resulting in unsuccessful induction of migration and mitogenesis. The binding ability of mouse HGF α-chain, or of β-chain, to human c-Met was lower than that of human HGF, as evidenced by HGF-chimera assay. Notably, only four amino acid positions in HGF α-chain hairpin-loop and K1 domain and six positions in β-chain differed between human HGF and mouse HGF. The human-specific amino acids (such as Gln-95 in hairpin-loop, Arg-134 in K1 domain and Cys-561 in β-chain) may be important for accurate c-Met assembly and signalling transduction.

Identification of mannose receptor as receptor for hepatocyte growth factor β-chain: novel ligand-receptor pathway for enhancing macrophage phagocytosis

Hepatocyte growth factor (HGF), a heterodimer composed of the α-chain and β-chain, exerts multifunctional actions for tissue repair and homeostasis via its receptor, MET. HGF is cleaved by proteases secreted from inflammatory cells, and NK4 and β-chain remnant (HGF-β) are generated. Here, we provide evidence that HGF-β binds to a new receptor other than MET for promoting a host cell clearance system. By an affinity cross-linking, radiolabeled HGF-β was bound to liver non-parenchymal cells, particularly to Kupffer cells and sinusoidal endothelial cells, but not to parenchymal hepatocytes. The cross-linked complex was immunoprecipitated by anti-HGF antibody, but not anti-MET antibody, implying that HGF-β binds to non-parenchymal cells at a site distinct from MET. Mass spectrometric detection of the ligand receptor complex revealed that the binding site of HGF-β was the mannose receptor (MR). Actually, an ectopic expression of MR in COS-7 cells, which express no endogenous MR or MET, enabled HGF-β to bind these cells at a K(D) of 89 nM, demonstrating that MR is the new receptor for HGF-β. Interaction of HGF-β and MR was diminished by EGTA, and by an enzymatic digestion of HGF-β sugar chains, suggesting that MR may recognize the glycosylation site(s) of HGF-β in a Ca(2+)-dependent fashion. Notably, HGF-β, but not other MR ligands, enhanced the ingestion of latex beads, or of apoptotic neutrophils, by Kupffer cells, possibly via an F-actin-dependent pathway. Thus, the HGF-β·MR complex may provide a new pathway for the enhancement of cell clearance systems, which is associated with resolution of inflammation.

Overproduction of recombinant human hepatocyte growth factor in Chinese hamster ovary cells

Hepatocyte growth factor (HGF) is a potent multi-functional protein that affects morphogenesis, cell migration, organ regeneration, and tumor invasion in various tissues, and has thus been considered to have potential as a therapeutic target in various diseases. In our current study, we established Chinese hamster ovary (CHO) cells overexpressing recombinant human HGF (rhHGF) protein and in a 5 day batch culture process using a 7.5l bioreactor (5l working volume) and serum-free medium these cells could produce over 13 mg/l of rhHGF protein. The recombinant protein was then purified to homogeneity from the culture supernatant using a two-step chromatographic procedure that resulted in about a 35% recovery rate. This purified rhHGF was found to be a mixture of inactive pro-HGF and an active heterodimeric form of this protein with a higher molecular weight than its counterpart expressed from insect cells. This finding suggests that the glycosylation of rhHGF protein in CHO cells differs from that in insect cells. Inactive pro-HGF was found to rapidly convert to the active heterodimeric form of HGF in the presence of FBS (Fetal Bovine Serum), suggesting that this process would occur also when injected into human body. We further demonstrate in cell proliferation and scattering activity assays that our purified rhHGF protein preparation is functionally active with a half-maximal effective concentration of 36.3 ng/ml.

Efficient inefficiency: biochemical "junk" may represent molecular bridesmaids awaiting emergent function as a buffer against environmental fluctuation

The biochemical function of many parts of the genome, transcriptome, proteome, and interactome remain largely unknown. We propose that portions of these fundamental building blocks of life have no current biochemical function per se. Rather, sections of these "omes" may contribute to an inventory of biochemical parts and circuits that participate in the development of emergent functions. Low fidelity deoxyribonucleic acid replication, transcription, translation, and post-translational modification all represent potential mechanisms to produce an inventory of parts. Stochastic processes that influence the conformations of ribonucleic acid molecules and proteins may also contribute to potential biochemical inventory. Some components of the biochemical inventory may enable future adaptations, some may produce disease, and some may remain useless. The function of many of these components await discovery, not by science, but by evolution. While carrying such purposeless biochemical units may appear to dilute fitness by exacting a thermodynamic cost, we argue that net fitness becomes enhanced when considering the value for potential future innovations. One can envision components that intermingle, interact, and act out mock pathways, but in most cases remain molecular bridesmaids. Given sufficiently low thermodynamic cost, such stochastic cycling may persist until a markedly advantageous or cataclysmically disadvantageous trait emerges. Maladaptive screening and utilization of inventory content can lead to disease phenotypes, a process buffered and regulated in part by the heat shock protein and stress response network. Whereas failure of the ubiquitin pathway to recycle misfolded proteins has become increasingly recognized as a source of disease, protein misfolding may itself represent one step in a process that maximizes functional innovation through increasing proteomic diversity. Fractal correlates of these processes occur at the organizational level of cells and organisms. That the abnormal accumulation of units induces local collapse may serve to limit the extension of damage to the greater system at large. The immune and cognitive systems that selectively sample and prune environmental content may serve as additional portals for innovation.