Hepatocyte growth factor induces proliferation and differentiation of multipotent and erythroid hemopoietic progenitors

In vivo transfer of hepatocyte growth factor gene accelerates proliferation of hepatic oval cells in a 2-acetylaminofluorene/partial hepatectomy model in rats

To clarify the effect of hepatocyte growth factor (HGF) on proliferation of hepatic oval cells, we transferred HGF gene into liver of the Solt-Farber rat model. Male Fisher 344 rats were infected with a recombinant adenovirus carrying the cDNA for HGF (pAxCAHGF) from tail vein. HGF mRNA showed its peak at 4 days, and diminished thereafter. The total and proliferating cell nuclear antigen-positive hepatic oval cells were significantly elevated in HGF-transferred rats, in which stem cell factor and c-kit mRNA increased at each time point. Our results suggest that in vivo transfer of the HGF gene into liver accelerates proliferation of hepatic oval cells in the Solt-Farber model in rats.

Hepatocyte growth factor/scatter factor-induced intracellular signalling

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

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

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

Mutant Met-mediated transformation is ligand-dependent and can be inhibited by HGF antagonists

Mutations in the genes encoding for Met, Ret and Kit receptor tyrosine kinases invariably result in increased kinase activity and in the acquisition of transforming potential. However, the requirement of receptor ligands for the transformation process is still unclear. We have investigated the role of hepatocyte growth factor (HGF), the high-affinity ligand for Met, in mutant Met-mediated cell transformation. We provide evidence that the transforming potential displayed by mutant forms of Met found in human cancer is not only sensitive but entirely dependent on the presence of HGF, by showing that mutant Met transforms NIH3T3 fibroblasts, which produce endogenous HGF, but is not able to transform epithelial cells, unless exogenous HGF is supplied. Accordingly, mutant Met-induced transformation of NIH3T3 cells can be inhibited by HGF antagonists and increased by HGF stimulation. We also show that an engineered Met receptor which contains an oncogenic mutation but is impaired in its ability to bind HGF completely loses its transforming activity, which can be rescued by causing receptor dimerization using a monoclonal antibody. These results indicate that point mutations resulting in Met kinase activation are necessary but not sufficient to cause cell transformation, the latter being dependent on ligand-induced receptor dimerization. They also suggest that mutant Met-driven tumour growth depends on the availability and tissue distribution of active HGF, and provide proof-of-concept for the treatment of mutant-Met related pathologies by HGF-antagonizing drugs.

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

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

Characterization of a functional relationship between hepatocyte growth factor and mouse bone marrow-derived mast cells

During the early stage (at 4 weeks) of interleukin-3 (IL-3)-induced development, mouse bone marrow-derived mast cells (BMMC) express alpha 4, alpha 5 and alpha 6 integrins, whereas with further maturation beyond 10 weeks, only alpha 5 integrin remains stably expressed. Hepatocyte growth factor (HGF) modulates the growth and movement of diverse cell types upon binding to its receptor, encoded by the proto-oncogene c-met. We report here the expression of c-met by BMMC throughout the course of their development. In addition, HGF stimulated migration of early week-4 BMMC, but not of the later stage week-10 BMMC, on fibronectin and laminin substrates. The developmental stage-dependent effect of HGF on BMMC was due to specific stimulation of the migratory function of alpha 4 and alpha 6, but not alpha 5 integrins. In addition, HGF had no effect on BMMC growth, either alone or in combination with IL-3. While HGF is stimulatory of the migratory function of BMMC, our results show that BMMC in turn can modulate HGF function. Thus, upon activation via the IgE receptors, BMMC released proteases that abolished HGF activities. Analyses of the degradation products by two-dimensional sodium dodecyl sulfate polyacrylamide gel electrophoresis and Western blot using antisera prepared against recombinant HGF and the kringle 3 domain of HGF revealed specific degradation of HGF alpha but not beta/beta' subunits. Therefore, our results suggest that: 1) the motogenic effect of HGF on BMMC varies according to the stage of their development, 2) HGF stimulation of BMMC migration is due to selective activation of alpha 4 and alpha 6, but not alpha 5 integrin function, and 3) there exists a two-way relationship between BMMC and HGF such that HGF stimulates the beta 1 integrin-mediated migratory function of BMMC, which can, in turn, modulate HGF function by release of serine proteases.

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

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

Heparan sulfate-modified CD44 promotes hepatocyte growth factor/scatter factor-induced signal transduction through the receptor tyrosine kinase c-Met

CD44 has been implicated in tumor progression and metastasis, but the mechanism(s) involved is as yet poorly understood. Recent studies have shown that CD44 isoforms containing the alternatively spliced exon v3 carry heparan sulfate side chains and are able to bind heparin-binding growth factors. In the present study, we have explored the possibility of a physical and functional interaction between CD44 and hepatocyte growth factor/scatter factor (HGF/SF), the ligand of the receptor tyrosine kinase c-Met. The HGF/SF-c-Met pathway mediates cell growth and motility and has been implicated in tumor invasion and metastasis. We demonstrate that a CD44v3 splice variant efficiently binds HGF/SF via its heparan sulfate side chain. To address the functional relevance of this interaction, Namalwa Burkitt's lymphoma cells were stably co-transfected with c-Met and either CD44v3 or the isoform CD44s, which lacks heparan sulfate. We show that, as compared with CD44s, CD44v3 promotes: (i) HGF/SF-induced phosphorylation of c-Met, (ii) phosphorylation of several downstream proteins, and (iii) activation of the MAP kinases ERK1 and -2. By heparitinase treatment and the use of a mutant HGF/SF with greatly decreased affinity for heparan sulfate, we show that the enhancement of c-Met signal transduction induced by CD44v3 was critically dependent on heparan sulfate moieties. Our results identify heparan sulfate-modified CD44 (CD44-HS) as a functional co-receptor for HGF/SF which promotes signaling through the receptor tyrosine kinase c-Met, presumably by concentrating and presenting HGF/SF. As both CD44-HS and c-Met are overexpressed on several types of tumors, we propose that the observed functional collaboration might be instrumental in promoting tumor growth and metastasis.

Fibroblast growth factor-2 induces hepatocyte growth factor/scatter factor expression in osteoblasts

Hepatocyte growth factor/scatter factor (HGF/SF) is a multifunctional growth factor with a major role in tissue morphogenesis and repair. It stimulates the proliferation of cells of the osteoblast and osteoclast lineages. Mitogenic factors playing a role in fracture repair may act by regulating HGF/SF expression or activity in bone-forming cells. We investigated the effect of fibroblast growth factor-2 (FGF-2) on the expression of HGF/SF and its receptor, encoded by c-met, in the MC3T3-E1 osteoblastic cell line. MC3T3-E1 cells expressed low levels of HGF/SF messenger RNA (mRNA), which were markedly increased by FGF-2 in a dose- and time-dependent manner. FGF-2 also induced HGF/SF polypeptide synthesis. The stimulation of HGF/SF mRNA expression by FGF-2 was blocked by cycloheximide, a protein synthesis inhibitor, but not by DNA or prostaglandin synthesis inhibitors. FGF-2 increased the rate of HGF/SF gene transcription by approximately 2-fold, as determined by nuclear run-on assays, and did not modify the decay of HGF/SF mRNA in transcriptionally arrested cells. FGF-2 also caused a dose- and time-dependent stimulation of c-met mRNA. In conclusion, FGF-2 induces HGF/SF expression in osteoblasts and may promote HGF/SF activity by increasing the expression of its receptor. Through these mechanisms, HGF/SF could mediate FGF actions on bone repair.

The role of hepatocyte growth factor and its receptor c-Met in multiple myeloma and other blood malignancies

The cytokine hepatocyte growth factor (HGF) and its receptor c-Met are a ligand-receptor pair with important functions in a communicative interplay between HGF-producing, mesenchymal cells and c-Met-expressing target cells. HGF is cytoprotective and causes regeneration of parenchyma after tissue damage in several organs. The receptor c-Met was first characterized as an oncogene product being responsible for the transformation of an osteosarcoma cell line. HGF or c-Met is overexpressed in several human cancers, including various carcinomas. Some cells of hematopoietic origin also seem to be capable of c-Met expression, but the precise role of HGF in normal hematopoiesis is yet to be determined. In blood malignancies like acute myelogenous leukemia and, notably, multiple myeloma, HGF is overproduced and has implications for the prognosis of the patients. Biological significance of HGF overexpression in multiple myeloma is discussed and is likely to include effects on bone turnover and angiogenesis.

Regulation of adhesion and migration in the germinal center microenvironment

T cell dependent humoral immune responses are initiated by the activation of naive B cells in the T cell areas of the secondary lymphoid tissues. This primary B cell activation leads to migration of germinal center (GC) cell precursors into B cell follicles where they engage follicular dendritic cells (FDC) and T cells, and differentiate into memory B cells or plasma cells. Both B cell homing to the GC and interaction with FDC critically depend on integrin-mediated adhesion. We have recently indentified the c-met-encoded receptor tyrosine kinase and its ligand, the growth and motility factor hepatocyte growth factor/scatter factor (HGF/SF), as a novel paracrine signalling pathway regulating B cell adhesion (van der Voort et al., 1997, J. Exp. Med. 185, 2121-2131). The c-Met protein is expressed on B cells localized in the dark zone of the GC (centroblasts) and is induced by CD40 plus BCR ligation. Stimulation of c-Met with HGF/SF, which is produced at high levels by tonsillar stromal cells and FDC, leads to receptor phosphorylation and to enhanced integrin-mediated adhesion of B cells to both VCAM-1 and fibronectin. Interestingly, these responses to HGF/SF are promoted by heparan-sulfate proteoglycan forms of CD44 (CD44-HS). Like c-Met, CD44-HS is induced on B cells by CD40 ligation. It efficiently binds HGF/SF and strongly promotes signalling through c-Met. We conclude that integrin regulation during antigen specific B cell differentiation involves cross-talk between the HGF/SF-c-Met pathway and CD44-HS.

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

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

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.

Cooperative interaction between alpha- and beta-chains of hepatocyte growth factor on c-Met receptor confers ligand-induced receptor tyrosine phosphorylation and multiple biological responses

Hepatocyte growth factor (HGF) is a heterodimeric molecule composed of the alpha-chain containing the N-terminal hairpin domain, four kringle domains, and the serine protease-like beta-chain. We prepared HGF/NK4 and HGF/beta from the entire HGF after single-cut digestion with elastase. HGF/NK4 contains the N-terminal hairpin and four kringle domains, while HGF/beta is composed of the C-terminal 16 amino acids of the alpha-chain and the entire beta-chain, linked by a disulfide bridge. HGF/NK4 competitively inhibited the binding of 125I-HGF to the receptor, and affinity cross-linking analysis indicated that HGF/NK4 alone can bind to the c-Met receptor. In contrast, HGF/beta alone did not competitively inhibit the binding of 125I-HGF to the receptor and did not bind to the c-Met/HGF receptor. Scatchard analysis and affinity cross-linking experiments indicated that HGF/beta specifically binds to c-Met in the presence of HGF/NK4 but not HGF/NK2. Neither HGF/NK4 nor HGF/beta alone induced mitogenic, motogenic (cell scattering), and morphogenic (induction of branching tubulogenesis) responses; however, HGF/beta did induce these biological responses in the presence of HGF/NK4. Consistent with these results, although neither HGF/NK4 alone nor HGF/beta alone induced tyrosine phosphorylation of the c-Met/HGF receptor, HGF/beta induced tyrosine phosphorylation of the receptor when c-Met/HGF receptor was occupied by HGF/NK4. These results indicate that HGF/beta binds to the c-Met/HGF receptor that is occupied by HGF/NK4 and induces receptor tyrosine phosphorylation and the subsequent biological activities of HGF. We propose that there exists a unique cooperative interaction between alpha- and beta-chains, this interaction leading to beta-chain-dependent receptor tyrosine phosphorylation and subsequent biological responses.

Expression of hepatocyte growth factor and its receptor c-met in human leukemia-lymphoma cell lines

Recent studies have shown that hepatocyte growth factor (HGF) is a regulatory protein for the proliferation and differentiation of hematopoietic progenitors. The proto-oncogene c-met encodes a tyrosine kinase receptor that binds HGF. To obtain information about their possible involvement in the pathogenesis of hematopoietic tumors, we have examined the expression of HGF and c-met in a large panel of leukemia-lymphoma cell lines encompassing all major hematopoietic cell lineages. HGF and c-met mRNAs were detected by reverse transcriptase-polymerase chain reaction (RT-PCR) and Northern blotting. The panel of 92 cell lines analyzed comprised seven B-cell precursor, ten B-cell, six plasma cell, 13 T-cell, four natural killer (NK) cell, 16 myelocytic, 12 monocytic, 13 erythroid-megakaryocytic and 11 Hodgkin-anaplastic large cell lymphoma (ALCL) lines. In total 64 (70%) were RT-PCR-positive for HGF and 43 (47%) for c-met. The highest percentages of expression were found for HGF in the plasma cell (100%), NK (100%) and myeloid (75-92%) cell line categories, whereas c-met was found predominantly in plasma cell (100%) and Hodgkin-ALCL (91%) cell lines. The concomitant expression of HGF and c-met in plasma cell lines (100%) and Hodgkin-ALCL (73%) cell lines should be noted. The high HGF expression in myelocytic-monocytic cell lines (75 and 92%) contrasts with the low c-met expression (18 and 8%) in these cell lineages. In 50 cell lines, mRNA expression of these two genes was also examined at the Northern blot level: 12/50 (24%) and 4/48 (8%) were positive for HGF and c-met mRNA expression, respectively. Of note, three of the four c-met + lines belonged to the category Hodgkin-ALCL; the Hodgkin cell line SUP-HD-1 showed both HGF and c-met mRNA bands suggesting the possibility of an autocrine loop. In conclusion, we detected HGF expression in various types of leukemia-lymphoma cell lines, particularly in plasma cell and myeloid malignancies; c-met expression was found in plasma cell and Hodgkin-ALCL cell lines. Further detailed analysis of the role of this ligand-receptor pair in the pathogenesis of hematopoietic neoplasms is indicated; to this end the HGF + and c-met + cell lines described here represent exquisite model systems.

Role of hepatocyte growth factor and its receptor c-met in multiple myeloma

Multiple myeloma is characterised by the clonal expansion of malignant plasma cells. Recently, we reported that a new cytokine, hepatocyte growth factor (HGF), and its receptor c-met are related to this disease. Here we review the observations that associate HGF with myeloma. Malignant plasma cells produce HGF and express the receptor c-met. Many patients have elevated HGF levels, which is unfavourable both in terms of survival and response to treatment. Possible biological roles of HGF in this disease are discussed, with special focus on bone homeostasis and its binding to heparan sulphate proteoglycans.

Retinal pigment epithelial cells secrete and respond to hepatocyte growth factor

Hepatocyte growth factor (HGF) is normally expressed by mesenchymal cells while its receptor, c-Met, is expressed in epithelial cells. Since HGF is critically involved in epithelial-mesenchyme interactions and the retinal pigment epithelium (RPE) is present at the interface between the retina and choroid, this study was initiated to determine whether the RPE expresses or responds to HGF in vitro. Cultured adult and fetal human RPE expressed mRNA for HGF and c-Met by RT-PCR. ELISA assay demonstrated the secretion of HGF into RPE culture supernatants. Tyrosine phosphorylation of c-Met was constitutively found in 72 hour RPE cultures and could be rapidly induced in serum-starved cells by concentrated RPE supernatants. HGF was mitogenic for cultured RPE (100 ng/ml.) and stimulated their chemotaxis (maximal response at 50 ng/ml). RPE are one of only a very limited number of epithelia that express both HGF and its receptor, suggesting the possibility of an autocrine action for this growth factor.

Wound healing in the liver with particular reference to stem cells

The efficiency of liver regeneration in response to the loss of hepatocytes is widely acknowledged, and this is usually accomplished by the triggering of normally proliferatively quiescent hepatocytes into the cell cycle. However, when regeneration is defective, tortuous ductular structures, initially continuous with the biliary tree, proliferate and migrate into the surrounding hepatocyte parenchyma. In humans, these biliary cells have variously been referred to as ductular structures, neoductules and neocholangioles, and have been observed in many forms of chronic liver disease, including cancer. In experimental animals, similar ductal cells are usually called oval cells, and their association with impaired regeneration has led to the conclusion that they are the progeny of facultative stem cells. Oval cells are of considerable biological interest as they may represent a target population for hepatic carcinogens, and they may also be useful vehicles for ex vivo gene therapy for the correction of inborn errors of metabolism. This review proposes that the liver harbours stem cells that are located in the biliary epithelium, that oval cells are the progeny of these stem cells, and that these cells can undergo massive expansion in their numbers before differentiating into hepatocytes. This is a conditional process that only occurs when the regenerative capacity of hepatocytes is overwhelmed, and thus, unlike the intestinal epithelium, the liver is not behaving as a classical, continually renewing, stem cell-fed lineage. We focus on the biliary network, not merely as a conduit for bile, but also as a cell compartment with the ability to proliferate under appropriate conditions and give rise to fully differentiated hepatocytes and other cell types.

Long-term cultures of human fetal liver cells: a three-dimensional experimental model for monitoring liver tissue development

BACKGROUND/AIMS

The present study describes an embryonic-fetal liver culture system which allows morphogenetic interactions consistent with the development of the hepatocellular function.

METHODS

Intact livers from 8-12-week embryos were soaked in an extracellular matrix at 4 degrees C and gently dissociated without any enzymatic treatment. The resulting spherical hepatic units were cultured in a chemically defined serum-free medium and seeded into an extracellular matrix layer. Adherent three-dimensional tissue specimens were examined at various times by light and electron microscopy to evaluate the maintenance of hepatocyte morphology.

RESULTS

The liver cells were viable for over 4 months; erythropoietic burst colonies were detected for longer than 6 weeks. Parallel detection of bile salt production in the medium by high performance liquid chromatography proved liver tissue functionality. Bile salt composition revealed predominance of taurine-conjugates rather than glycine. Maximum bile salt concentration (approximately 3 months) coincided with structural and ultrastructural observations indicating a marked decline in hematopoiesis, well-defined biliary canaliculi and formation of an organ-like structure.

CONCLUSIONS

This three-dimensional culture system recapitulates fetal liver development with: (i) initial proliferation of both fetal erythropoietic and hepatic cells and (ii) subsequent shut-off of erythropoiesis and a shift to a more advanced stage of hepatocyte function, such as bile salt secretion.

The role of hepatocyte growth factor (HGF) at progressive stages of metanephric development

Several lines of evidence suggest that hepatocyte growth factor (HGF), a soluble protein secreted by mesenchymal cells, may elicit a morphogenic response in the developing metanephros. We investigated the role of HGF at three different stages of murine metanephric development utilizing serum-free organ culture. Cultures were initiated at E-13, E-15, and E-17; treated with exogenous HGF or antibodies to HGF (to block endogenous HGF) for 120 h in vitro; and evaluated for growth and differentiation in comparison to control explants cultured for 120 h in basal medium. HGF treatment of E-13 explants resulted in a reduction of growth and differentiation compared to control explants. Treatment of E-13 explants with antibodies to HGF produced explant growth and differentiation indistinguishable from control explants. In contrast to the results of E-13 cultures, explants initiated at E-15 and E-17 demonstrated an increased growth and differentiation profile when treated with HGF compared to controls. Treatment of E-15 and E-17 explants with antibodies to HGF resulted in a decrease growth and differentiation profile compared to control or HGF-treated explants. These data demonstrate that HGF has differential effects on renal morphogenesis at progressive developmental stages of metanephric development.

Elevated Serum Concentrations of Hepatocyte Growth Factor in Patients With Multiple Myeloma

Serum from 398 myeloma patients at diagnosis and serial samples from 29 patients were analysed for hepatocyte growth factor (HGF). HGF was elevated at diagnosis in 43% of myeloma patients compared with healthy controls (median 1.00 ng/mL and 0.44 ng/mL, respectively;P < .00001). In the group with elevated HGF levels 46% of the patients reached plateau phase, as compared with 60% of the patients with low HGF levels (P = .005), and the median survival time was 21 and 32 months, respectively (P = .002). In a univariate Cox regression analysis, HGF was a significant predictor of mortality (P = .02). In the subgroup of patients with β2-microglobulin levels less than or equal to 6 mg/L, high versus low HGF was a prognostic factor when a multivariate Cox regression analysis was performed. In serial samples HGF was higher at the time of diagnosis and relapse (median 0.57 ng/mL and 0.52 ng/mL, respectively; P = .0018) than at response (median 0.24 ng/mL, P = .008). We conclude that HGF may be a useful follow-up parameter in myeloma patients. Measurement of HGF may identify a group of patients with poor response to melphalan-prednisone treatment and short survival. HGF was a prognostic factor in patients with high levels of β2-microglobulin.

Elevated Serum Concentrations of Hepatocyte Growth Factor in Patients With Multiple Myeloma

Serum from 398 myeloma patients at diagnosis and serial samples from 29 patients were analysed for hepatocyte growth factor (HGF). HGF was elevated at diagnosis in 43% of myeloma patients compared with healthy controls (median 1.00 ng/mL and 0.44 ng/mL, respectively;P < .00001). In the group with elevated HGF levels 46% of the patients reached plateau phase, as compared with 60% of the patients with low HGF levels (P = .005), and the median survival time was 21 and 32 months, respectively (P = .002). In a univariate Cox regression analysis, HGF was a significant predictor of mortality (P = .02). In the subgroup of patients with β2-microglobulin levels less than or equal to 6 mg/L, high versus low HGF was a prognostic factor when a multivariate Cox regression analysis was performed. In serial samples HGF was higher at the time of diagnosis and relapse (median 0.57 ng/mL and 0.52 ng/mL, respectively; P = .0018) than at response (median 0.24 ng/mL, P = .008). We conclude that HGF may be a useful follow-up parameter in myeloma patients. Measurement of HGF may identify a group of patients with poor response to melphalan-prednisone treatment and short survival. HGF was a prognostic factor in patients with high levels of β2-microglobulin.

Neoexpression of the c-met/Hepatocyte Growth Factor-Scatter Factor Receptor Gene in Activated Monocytes

Hepatocyte growth factor-scatter factor (HGF-SF ) mediates mito-, moto-, and morphogenic effects through the MET receptor, a membrane bound tyrosine kinase. HGF-SF/MET signaling is mitogenic for a large number of epithelial and endothelial cells and activates organ regeneration. HGF-SF transcripts have been detected in various myeloid cell lines. Therefore, the potential role of HGF-SF/MET signaling for circulating cells of the immune system, especially under conditions of inflammation, was evaluated. Several B-lymphoid and myeloid cell lines were found to express HGF-SF or c-met transcripts, while activity of both genes was mutually exclusive with the exception of low level coexpression in two B-cell lines. HGF-SF transcripts were present in low quantities in freshly isolated peripheral blood mononuclear cells (PBMNCs). In contrast, c-met expression was not detected in freshly isolated cells from peripheral blood, but was induced in monocytes by activation of monocytic or T-cell function. HGF-SF incubation led to an increased c-fos steady state transcript level in myeloblastic K562 cells and moderately promoted cell viability of freshly isolated preactivated monocytes. c-met expression is thus established in activated monocytes, in particular under conditions resembling inflammation, making these cells accessible to functional effects of HGF-SF.

Neoexpression of the c-met/Hepatocyte Growth Factor-Scatter Factor Receptor Gene in Activated Monocytes

Hepatocyte growth factor-scatter factor (HGF-SF ) mediates mito-, moto-, and morphogenic effects through the MET receptor, a membrane bound tyrosine kinase. HGF-SF/MET signaling is mitogenic for a large number of epithelial and endothelial cells and activates organ regeneration. HGF-SF transcripts have been detected in various myeloid cell lines. Therefore, the potential role of HGF-SF/MET signaling for circulating cells of the immune system, especially under conditions of inflammation, was evaluated. Several B-lymphoid and myeloid cell lines were found to express HGF-SF or c-met transcripts, while activity of both genes was mutually exclusive with the exception of low level coexpression in two B-cell lines. HGF-SF transcripts were present in low quantities in freshly isolated peripheral blood mononuclear cells (PBMNCs). In contrast, c-met expression was not detected in freshly isolated cells from peripheral blood, but was induced in monocytes by activation of monocytic or T-cell function. HGF-SF incubation led to an increased c-fos steady state transcript level in myeloblastic K562 cells and moderately promoted cell viability of freshly isolated preactivated monocytes. c-met expression is thus established in activated monocytes, in particular under conditions resembling inflammation, making these cells accessible to functional effects of HGF-SF.

Developmental regulation of erythropoietin and erythropoiesis

It is well established that erythropoiesis occurs first in the yolk sac, then in the liver, subsequently moving to the bone marrow and, in rodents, the spleen during development. The origin of the erythropoietic precursors and some factors suggested to be important for the changing location of erythropoiesis are discussed in this review. Until recently, the major site of erythropoietin (Epo) production in the fetus was thought to be the liver, but studies have shown now that the Epo gene is expressed strongly in the fetal kidney, even in the temporary mesonephros. The metanephric Epo mRNA is upregulated by anemia, downregulated by glucocorticoids, and contributes substantially to circulating hormone levels in hemorrhaged ovine fetuses. Other sites of Epo and Epo receptor production, likely to have important actions during development, are the placenta and the brain.

The HMC-1 human mast cell line expresses the hepatocyte growth factor receptor c-met

Hepatocyte growth factor (HGF) was originally characterized as a strong inducer of liver regeneration. However, it is now clear that HGF and its receptor, the proto-oncogene c-met, can be expressed in many other tissues, and that HGF can mediate diverse biological activities. We investigated the expression and function of c-met in a human mast cell line (HMC-1). We found that HMC-1 cells express c-met and that c-met expression can be upregulated by treatment of the cells with phorbol 12-myristate 13-acetate (PMA). Although HGF did not detectably influence the proliferation or morphology of HMC-1 cells, HGF inhibited the cells' ability to release tumor necrosis factor-alpha (TNF-alpha) in response to stimulation with PMA and the calcium ionophore, A23187. These results add the inhibition of TNF-alpha production to the other recognized effects of HGF/c-met on cellular function.

Paracrine regulation of germinal center B cell adhesion through the c-met-hepatocyte growth factor/scatter factor pathway

T cell-dependent humoral immune responses are initiated by the activation of naive B cells in the T cell areas of the secondary lymphoid tissues. This primary B cell activation leads to migration of germinal center (GC) cell precursors into B cell follicles where they engage follicular dendritic cells (FDC) and T cells, and differentiate into memory B cells or plasma cells. Both B cell migration and interaction with FDC critically depend on integrin-mediated adhesion. To date, the physiological regulators of this adhesion were unkown. In the present report, we have identified the c-met-encoded receptor tyrosine kinase and its ligand, the growth and motility factor hepatocyte growth factor/scatter factor (HGF/SF), as a novel paracrine signaling pathway regulating B cell adhesion. We observed that c-Met is predominantly expressed on CD38(+)CD77(+) tonsillar B cells localized in the dark zone of the GC (centroblasts). On tonsil B cells, ligation of CD40 by CD40-ligand, induces a transient strong upregulation of expression of the c-Met tyrosine kinase. Stimulation of c-Met with HGF/SF leads to receptor phosphorylation and, in addition, to enhanced integrin-mediated adhesion of B cells to both VCAM-1 and fibronectin. Importantly, the c-Met ligand HGF/SF is produced at high levels by tonsillar stromal cells thus providing signals for the regulation of adhesion and migration within the lymphoid microenvironment.

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

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

Expression of hepatocyte growth factor/scatter factor and its receptor, MET, suggests roles in human embryonic organogenesis

Hepatocyte growth factor/scatter factor (HGF/SF) is secreted by mesenchymal cells and elicits proliferation, motility, differentiation, and morphogenesis of epithelia and other cells. These effects are mediated by binding to MET, a receptor tyrosine kinase. Genetically engineered mice lacking HGF/SF die in utero due to a failure of placental and hepatocyte differentiation, but little information exists regarding the expression of this signaling system in human development. Using reverse transcriptase-polymerase chain reaction, Western blots, and immunohistochemistry, we report that HGF/SF and MET are expressed during critical early periods of human organogenesis from 6 to 13 wk of gestation. Organs that expressed both genes included liver, metanephric kidney, intestine, and lung, each of which develop by inductive interactions between mesenchyme and epithelia. Of all organs studied, the placenta contained the highest levels of HGF/SF protein, and MET was detected in trophoblastic cells of chorionic villi as early as the 5th wk of gestation. Finally, examination of a human multicystic dysplastic kidney demonstrated that malformed, hyperproliferative tubules expressed MET, whereas HGF/SF protein was immunolocalized to the same epithelia and also to the surrounding undifferentiated cells. Hence HGF/SF might be an important growth factor in normal human embryogenesis and may additionally play a role in human organ malformations.

Hepatocyte Growth Factor Is Constitutively Produced by Human Bone Marrow Stromal Cells and Indirectly Promotes Hematopoiesis

Bone marrow (BM) stromal cells are required for normal hematopoiesis. A number of soluble factors secreted by these cells that mediate hematopoiesis have been characterized. However, the mechanism of hematopoiesis cannot be explained solely by these known factors, and the existence of other, still unknown stromal factors has been postulated. We showed that hepatocyte growth factor (HGF ) is one such cytokine produced by human BM stromal cells. BM stromal cells were shown to constitutively produce HGF and also to express the c-MET/HGF receptor. The production of HGF was enhanced by addition of heparin and phorbol ester. Dexamethasone and tumor growth factor-β (TGF-β) inhibited the production of HGF. Interleukin-1α (IL-1α) tumor necrosis factor-α (TNF-α), and N6,2′-o-dibutyryl-adenosine-3′:5′-cyclic monophosphate (dbc-AMP) showed no obvious influence on HGF production. Western blot analysis of HGF derived from BM stromal cells showed two bands at 85 and 28 kD corresponding to native and variant HGF, respectively. Addition of recombinant HGF significantly promoted the formation of burst-forming unit-erythroid (BFU-E) and colony-forming unit-granulocyte erythroid macrophage (CFU-GEM) by BM mononuclear cells in the presence of erythropoietin and granulocyte-macrophage colony-stimulating factor (GM-CSF ), but the formation of CFU-GM was not modified. However, HGF had no effects on colony formation by purified CD34+ cells. Within BM mononuclear cells, c-MET was expressed on a proportion of cells (CD34−, CD33+, CD13+, CD14+, and CD15+), but was not found on CD34+ cells. We conclude that HGF is constitutively produced by BM stromal cells and that it enhances hematopoiesis. In addition, expression of c-MET on the stromal cells suggests the presence of an autocrine mechanism, operating through HGF, among stromal cells.

Hepatocyte Growth Factor Is Constitutively Produced by Human Bone Marrow Stromal Cells and Indirectly Promotes Hematopoiesis

Bone marrow (BM) stromal cells are required for normal hematopoiesis. A number of soluble factors secreted by these cells that mediate hematopoiesis have been characterized. However, the mechanism of hematopoiesis cannot be explained solely by these known factors, and the existence of other, still unknown stromal factors has been postulated. We showed that hepatocyte growth factor (HGF ) is one such cytokine produced by human BM stromal cells. BM stromal cells were shown to constitutively produce HGF and also to express the c-MET/HGF receptor. The production of HGF was enhanced by addition of heparin and phorbol ester. Dexamethasone and tumor growth factor-β (TGF-β) inhibited the production of HGF. Interleukin-1α (IL-1α) tumor necrosis factor-α (TNF-α), and N6,2′-o-dibutyryl-adenosine-3′:5′-cyclic monophosphate (dbc-AMP) showed no obvious influence on HGF production. Western blot analysis of HGF derived from BM stromal cells showed two bands at 85 and 28 kD corresponding to native and variant HGF, respectively. Addition of recombinant HGF significantly promoted the formation of burst-forming unit-erythroid (BFU-E) and colony-forming unit-granulocyte erythroid macrophage (CFU-GEM) by BM mononuclear cells in the presence of erythropoietin and granulocyte-macrophage colony-stimulating factor (GM-CSF ), but the formation of CFU-GM was not modified. However, HGF had no effects on colony formation by purified CD34+ cells. Within BM mononuclear cells, c-MET was expressed on a proportion of cells (CD34−, CD33+, CD13+, CD14+, and CD15+), but was not found on CD34+ cells. We conclude that HGF is constitutively produced by BM stromal cells and that it enhances hematopoiesis. In addition, expression of c-MET on the stromal cells suggests the presence of an autocrine mechanism, operating through HGF, among stromal cells.

Hepatocyte growth factor promotes motor neuron survival and synergizes with ciliary neurotrophic factor

Hepatocyte growth factor (HGF) has been shown to function as a potent mitogen for a variety of cells, transducing its signal through the c-met tyrosine kinase receptor. Ciliary neurotrophic factor (CNTF) is a cytokine that has been shown to promote survival of motor neurons. We show here that c-met mRNA is present in the embryonic rat spinal cord. Peak expression of c-met (at E14) coincides with the period of naturally occurring cell death in motor neurons, suggesting a possible role of HGF in the regulation of this process. Utilizing a neuron-enriched culture system, we established that HGF, like CNTF, stimulates choline acetyltransferase (CAT) activity in motor neurons. When co-administered to motor neuron cultures, saturating concentrations of HGF and CNTF produced a synergistic increase in CAT levels. We show that this synergy reflects enhanced motor neuron survival. Exposure of motor neuron cultures to the cytostatic agent vincristine markedly decreased CAT levels; co-treatment with HGF and CNTF (but not either factor alone) restored CAT activity to control levels. Our findings indicate that HGF is a survival factor for motor neurons, that it acts synergistically with CNTF, and that HGF and CNTF can together be neuroprotective in the face of vincristine toxicity.

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

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

STK/RON receptor tyrosine kinase mediates both apoptotic and growth signals via the multifunctional docking site conserved among the HGF receptor family

STK/RON tyrosine kinase, a member of the hepatocyte growth factor (HGF) receptor family, is a receptor for macrophage-stimulating protein (MSP). To examine the STK/RON signalling pathway, we generated STK/ RON transfectants showing opposite features in growth. STK/RON-expressing Ba/F3 pro-B cells (BaF/STK) exhibited MSP-dependent growth, whereas STK/ RON-expressing mouse erythroleukaemia cells (MEL/ STK) displayed MSP-induced apoptosis. This apoptosis was accompanied by the prolonged activation of c-Jun N-terminal kinase (JNK), which has recently been implicated in the initiation of apoptosis. Co-immunoprecipitation analyses showed that autophosphorylated STK/RON associated with PLC-gamma, P13-kinase, Shc and Grb2 in both transfectants. However, major tyrosine-phosphorylated proteins, p61 and p65, specifically associated with STK/RON in MEL/STK cells. Mutations at two C-terminal tyrosine residues, Y1330 and Y1337, in the counterpart of the multifunctional docking site of the HGF receptor abolished both MSP-induced growth and apoptosis. Analyses of these mutants and in vitro association revealed that signalling proteins including p61 and p65 directly bound to the phosphotyrosines in the multifunctional docking site. These results demonstrate that positive or negative signals toward cell growth are generated through the multifunctional docking site and suggest the involvement of p61 and p65 as well as JNK in apoptosis. Our findings provide the first evidence for apoptosis via a receptor tyrosine kinase.

Role of hepatocyte growth factor in hemopoiesis

Hepatocyte growth factor (HGF) is a polypeptide that stimulates proliferation, motility, and morphogenesis of various cells, particularly epithelial cells. There is considerable evidence that HGF is a regulator in hemopoiesis not only in mice but also in humans. In mice, HGF and c-met (its receptor) mRNA are coexpressed in the fetal liver in the middle and late stages, when hemopoiesis is most active. HGF and c-met mRNA are also expressed in the stromal cells of both fetal liver and bone marrow. Human HGF (2 to 20 ng/ml) enhances colony-forming units in culture (CFU-C) counts and cobblestone colony counts in the long-term cultures of the fetal liver and bone marrow, although HGF has no effect on freshly isolated bone marrow or fetal liver cells in the CFU-C assay. However, when the bone marrow or fetal liver cells are cocultured with HGF in the presence of IL-3, CFU-C counts increase. In humans, it has also been shown that HGF in the presence of erythropoietin induces the formation of erythroid burst-forming unit (BFU-E) colonies from CD34+ cells purified from the bone marrow, peripheral blood, or cord blood. This review discusses the role of HGF as a regulator in hemopoiesis.

Reactive biliary epithelium: the product of a pluripotential stem cell compartment?

Liver parenchymal cells (hepatocytes) have a low rate of turnover, but can nevertheless mount a rapid and efficient regenerative response. However, in some cases of extreme hepatotoxicity hepatocyte proliferation is restricted or even abolished, and instead biliary epithelial cells, commonly referred to as ductular oval cells, migrate into the periportal and midzonal parenchyma. Initially these cells behave as authentic biliary epithelium with expression of the biliary cytokeratin intermediate filaments, but then show hepatocytic traits such as alpha fetoprotein and albumin synthesis. Thereafter these biliary ducts rapidly vanish to be replaced by either small hepatocytes or intestinal-type cells. The proliferation and differentiation of oval cells is probably strongly influenced by paracrine signalling from liver stellate cells. Oval cells appear to be the progeny of facultative pluripotential stem cells which have the lineage potential of uncommitted gastrointestinal stem cells; these stem cells are likely to be located in the cholangioles and small interlobular bile ducts. Oval cells thus constitute an important reserve compartment for hepatocytes when hepatocyte regeneration is compromised.

Synergistic effects of hepatocyte growth factor on human cord blood CD34+ progenitor cells are the result of c-met receptor expression

Hepatocyte growth factor (HGF) is a pleiotropic growth factor which, in addition to its mitogenic potency for primary hepatocytes, also has a role in the regulation of cell motility, cell growth and morphogenesis. In the present study, we show that c-met, the high-affinity receptor for HGF, is expressed on human cord blood (CB) CD34+ progenitor cells and CD34+Thy-1+ Lin-(lin-) cells. We have investigated the capacity of HGF to synergize with other growth factors to induce colony formation by CB CD34+ progenitor cells. CD34+ cells were cultured in semisolid medium containing serum with increasing concentrations of GM-CSF, G-CSF, macrophage colony-stimulating factor (M-CSF), stem cell factor (SCF), interleukin 3 (IL-3) and IL-11 alone or in combination with HGF. HGF acted as a potent synergist and enhanced, up to fourfold, colony formation induced by GM-CSF, G-CSF or M-CSF. HGF in combination with SCF, IL-3 or IL-11 did not induce proliferation of colony forming units-granulocyte macrophage (CFU-GM) above control levels. In serum-deprived cultures, HGF was only detectably synergistic with IL-11, and all other culture combinations showed no proliferation. To determine whether the stimulatory effect of IL-11 and the synergistic effect of HGF in the absence of serum could be attributed to the effect of these two cytokines on stem cells, IL-11-stimulated and unstimulated lin- cells were analyzed for expression of c-met. CD34+Thy-1+Lin- cells were positive for c-met, both in the presence and absence of IL-11 stimulation, and Northern analysis indicated that c-met RNA expression was upregulated in response to IL-11 compared to unstimulated controls. These results provide strong evidence for upregulation of the HGF receptor on primitive hematopoietic cells by IL-11, and for the synergistic role of HGF in colony formation by hematopoietic stem cells.

Hepatocyte growth factor is a coupling factor for osteoclasts and osteoblasts in vitro

Hepatocyte growth factor (HGF), also known as scatter factor, is a powerful motogen, mitogen, and morphogen produced by cells of mesodermal origin, acting on epithelial and endothelial cells. Its receptor is the tyrosine kinase encoded by the c-MET protooncogene. We show that the HGF receptor is expressed by human primary osteoclasts, by osteoclast-like cell lines, and by osteoblasts. In both cell lineages, HGF stimulation triggers the receptor kinase activity and autophosphorylation. In osteoclasts, HGF receptor activation is followed by increase in intracellular Ca2+ concentration and by activation of the pp60c-Src kinase. HGF induces changes in osteoclast shape and stimulates chemotactic migration and DNA replication. Osteoblasts respond to HGF by entering the cell cycle, as indicated by stimulation of DNA synthesis. Interestingly, osteoclasts were found to synthesize and secrete biologically active HGF. These data strongly suggest the possibility of an autocrine regulation of the osteoclast by HGF and a paracrine regulation of the osteoblast by the HGF produced by the osteoclast.

Pluripotential liver stem cells: facultative stem cells located in the biliary tree

The ability of the liver to regenerate after parenchymal damage is usually accomplished by the ephemeral entry of normally proliferatively quiescent (G0) hepatocytes into the cell cycle. However, when hepatocyte regeneration is defective, arborizing ductules which are continuous with the biliary tree, proliferate and migrate into the surrounding parenchyma. In man these biliary cells have variously been referred to as ductular structures, neoductules and neocholangioles, and have been observed in many forms of chronic liver disease, including cancer. In experimental animals similar ductal cells are usually called oval cells, and their association with defective regeneration has led to the belief that these cells represent a progenitor cell population. Oval cells are thought to take over the burden of regenerative growth after substantial hepatocyte loss, suggesting that they are the progeny of facultative stem cells. The liver is not, however, generally considered as a stem cell-fed hierarchy, although this is disputed by others. Despite this, the subject of oval cells has aroused intense interest as these cells may represent a target population for hepatic carcinogens, and they may be useful vehicles for ex vivo gene therapy. This review proposes that the liver does harbour stem cells which are located throughout the biliary epithelium, and that oval cells represent the progeny of these stem cells and function as an amplification compartment for the generation of 'new' hepatocytes. This is a conditional process which only occurs when the regenerative capacity of hepatocytes is overwhelmed and thus, unlike the intestinal epithelium, the liver is not behaving as a classical continually renewing stem cell-fed lineage. We focus on the biliary network, not merely as a conduit for bile, but also as a cell compartment with the potential to proliferate under appropriate conditions and give rise to fully differentiated hepatocytes and other cell types.

Overexpression of the met/HGF receptor in renal cell carcinomas

The c-met oncogene encodes the receptor for hepatocyte growth factor/scatter factor (HGF/SF), a multifunctional cytokine able to mediate morphogenesis as well as mitogenesis, motogenesis and invasiveness of epithelial cells. HGF/SF has been implicated in branching tubulogenesis of the developing kidney and in regeneration after renal injury and nephrectomy. We have examined the expression of the met/HGF receptor in normal human kidney and tissues of the genito-urinary tract, and in 50 kidney neoplasms of different histotypes, using monoclonal antibodies (MAbs) against the met/HGF receptor and immunohistochemistry. In normal kidneys, weak staining restricted to the distal tubules was observed. Transitional cell carcinomas were consistently negative, whereas increased expression at various levels was found in 87% of renal cell carcinomas with different cytological features and histological patterns. Western blot analysis of samples showed that the met/HGF receptor found in the malignant cells exhibits features of the normal receptor. The met/HGF receptor is also overexpressed in a renal cell carcinoma cell line, whose motility is triggered by HGF/SF. Our data suggest that expression of the met/HGF receptor may be involved in the onset and progression of renal cell carcinomas.

Hepatocyte growth factor (HGF)/NK1 is a naturally occurring HGF/scatter factor variant with partial agonist/antagonist activity

Hepatocyte growth factor/scatter factor (HGF/SF) stimulates cell proliferation, motility, and morphogenesis by activation of its receptor, the c-Met tyrosine kinase. HGF/SF is structurally related to plasminogen, including an amino-terminal hairpin loop, four kringle domains, and a serine protease-like region. A truncated HGF/SF isoform, designated HGF/NK2, which extends through the second kringle domain and behaves as a competitive HGF/SF antagonist, was previously shown to be encoded by an alternative HGF/SF transcript. In this study, we describe a second naturally occurring HGF/SF variant, HGF/NK1, consisting of the HGF/SF amino-terminal sequence and first kringle domain. This product is encoded by a 2-kilobase alternative transcript containing intronic sequence that was contiguous with exon K1b. Analysis of baculovirus-expressed HGF/NK1 revealed that this isoform possesses the heparin binding properties of HGF/SF and modest mitogenic and scattering activity relative to HGF/SF. However, at a 40-fold molar excess, HGF/NK1 inhibited HGF/SF-dependent DNA synthesis. HGF/NK1 stimulated tyrosine phosphorylation of Met, and covalent affinity cross-linking demonstrated a direct HGF/NK1-receptor interaction. These findings establish that the HGF/SF gene encodes multiple alternative products, which include not only a mitogenic agonist (HGF/SF) and a pure antagonist (HGF/NK2) but also a molecule with partial agonist/antagonist properties.

Mediators, cytokines, and growth factors in liver-lung interactions

Multiple mediators have been implicated in the interactions between the liver and the lungs in various disease states. The best characterized mediator of liver-lung interaction is alpha 1-antitrypsin. Several cytokines and mediators may be involved in the pathogenesis of the hepatopulmonary syndrome and in the cytokine cascades that are activated in systemic inflammatory states such as acute respiratory distress syndrome. Hepatocyte growth factor or scatter factor is a recently described peptide with a broad range of biologic effects that may mediate lung-liver interactions.

Erythropoietin-independent erythrocyte production: signals through gp130 and c-kit dramatically promote erythropoiesis from human CD34+ cells

Erythropoietin (EPO) is the primary humoral regulator of erythropoiesis and no other factor has previously been reported to support proliferation and terminal maturation of erythroid cells from hemopoietic stem cells. Here we show that stimulation of glycoprotein (gp130) by a combination of recombinant human soluble interleukin 6 receptor (sIL-6R) and IL-6 but not sIL-6R or IL-6 alone can support proliferation, differentiation, and terminal maturation of erythroid cells in the absence of EPO from purified human CD34+ cells in suspension culture containing stem cell factor (SCF). A number of erythroid bursts and mixed erythroid colonies also developed in methylcellulose culture under the same combination. The addition of anti-gp130 monoclonal antibodies but not anti-EPO antibody to the same culture completely abrogated the generation of erythroid cells. These results clearly demonstrate that mature erythroid cells can be emerged from hemopoietic progenitors without EPO in vitro. Together with the previous reports that human sera contain detectable levels of sIL-6R, IL-6, and SCF, current data suggest that gp130 signaling in association with c-kit activation may play a role in human erythropoiesis in vivo.

Relationship of hepatocyte growth factor in human umbilical vein serum to gestational age in normal pregnancies

Hepatocyte growth factor (HGF) is expressed in placental syncitium and fetal organs and acts as a mitogen, motogen, and morphogen in vitro, suggesting a role in fetal growth and development. We aimed to examine the correlates of serum HGF in human cord blood. HGF was measured by ELISA using recombinant human HGF and mouse MAb to recombinant human HGF (Immunology Institute, Tokyo). Umbilical vein blood was collected prospectively at 148 deliveries including 94 normal pregnancies and 54 pregnancies complicated by medical conditions, primarily diabetes mellitus and pregnancy-induced hypertension. Growth parameters, gestation, pregnancy history, and perinatal events were recorded. Sera from 54 adolescents and 32 adult controls were also analyzed. Cord HGF [0.97 (0.66-1.33) ng/mL] [median (25-75 percentile)] was higher than HGF levels in adolescent sera [0.28 (0.21-0.35) ng/mL, p < 0.0001] and adult control sera [0.23 (0.14-0.31) ng/mL, p < 0.0001]. Cord HGF correlated with gestational age (r = 0.42, p = 0.0001) in normal pregnancies, with term babies (n = 69) having higher cord HGF than babies less than 37 wk of gestation (n = 25) [1.11 (0.78-1.45), 0.78 (0.46-1.03) ng/mL, p = 0.0007]. However, there was no relationship between gestation and cord HGF in complicated pregnancies. Cord HGF did not differ at term between appropriate for gestational age babies and small for gestational age babies. There were no independent correlations between cord HGF and birth weight, birth length and placental weight. We provide evidence for the first time that cord HGF levels are high and relate to gestation in normal pregnancies. HGF may have a significant role in fetal development during pregnancy.

Effect of hemorrhage and nephrectomy on erythropoietin gene expression in the ovine fetus

The purposes of this study were to determine whether (1) erythropoietin (Epo) gene expression could be stimulated, by severe hemorrhage, in both kidney and liver, at three stages of gestation (75-80, 106-112, and 140-142 days; term approximately equal to 150 days) in chronically-cannulated ovine fetuses and (2) whether the liver would compensate for the lack of kidneys at 110 days. Blood was removed (20% of estimated blood volume) at each of 0, 1, 19 h, and tissues collected at 24 h. In hemorrhaged fetuses (H) the liver mRNA levels were 2.7-, 5-, and 3-times that of control fetuses (C) at 78, 110 and 141 days, respectively. The kidney H:C ratios, for Epo mRNA, were 5, 6.4 and 43, respectively, at these three stages of gestation. In six fetuses at 108 days, nephrectomized 5-7 days before hemorrhage (HN) Epo mRNA increased 5-fold in the liver, and plasma Epo values were significantly lower (P < 0.05) than in intact (H). Thus hemorrhage stimulates increased Epo gene expression in both liver and kidney from mid-gestation, in the ovine fetus, but the liver does not compensate for the lack of kidneys at 110 days.