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

p38 differentially regulates ERK, p21, and mitogenic signalling in two pancreatic carcinoma cell lines

Whereas the p38 MAP kinase has largely been associated with anti-proliferative functions, several observations have indicated that it may also have positive effects on proliferation. In hepatocytes, we have found that p38 has opposing effects on DNA synthesis when activated by EGF and HGF. Here we have studied the function of p38 in EGF- and HGF-induced DNA synthesis in the two pancreatic carcinoma cell lines AsPC-1 and Panc-1. In Panc-1 cells, the MEK inhibitor PD98059 reduced EGF- and HGF-induced DNA synthesis, while the p38 inhibitor SB203580 strongly increased the basal DNA synthesis and reduced expression of the cyclin-dependent kinase inhibitor (CDKI) p21. In contrast, in AsPC-1 cells, EGF- and HGF-induced DNA synthesis was not significantly reduced by PD98059 but was inhibited by SB203580. Treatment with SB203580 amplified the sustained ERK phosphorylation induced by these growth factors and caused a marked upregulation of the expression of p21, which could be blocked by PD98059. These results suggest that while DNA synthesis in Panc-1 cells is enhanced by ERK and strongly suppressed by p38, in AsPC-1 cells, p38 exerts a pro-mitogenic effect through MEK/ERK-dependent downregulation of p21. Thus, p38 may have suppressive or stimulatory effects on proliferation depending on the cell type, due to differential cross-talk between the p38 and MEK/ERK pathways.

Response of Cultured Fetal and Adult Rat Hepatocytes to Growth Factors and Cyclosporine

Hepatocyte transplantation is a promising alternative to orthotopic liver transplantation in experimental animal models with genetic disorders of liver metabolism and liver failure. Fetal hepatocytes have several characteristics that make them potentially suitable as donor cells. In contrast to adult hepatocytes, fetal hepatocytes are thought to be highly proliferative, which may facilitate engraftment, expansion of transplanted cell population, and gene transfer requiring active DNA synthesis. The present study was undertaken to evaluate the proliferative capacity of fetal and adult rat hepatocytes under standardized culture conditions. Fetal (20 days of gestation) and adult hepatocytes were cultured in serum-free media at low densities and treated with growth factors. Proliferation was assessed by [3H]-thymidine incorporation and cell cycle analysis by flow cytometry. In nonstimulated cells, DNA synthesis at 4 h was about × 100 higher and after 10 days in culture ×20 higher in fetal compared to adult hepatocytes. When epidermal growth factor (EGF) was added, maximal DNA synthesis in fetal hepatocytes was seen at 48 h, whereas in adult hepatocytes at 72 h. For adult hepatocytes, the average increase compared to untreated cells was × 13.8 with EGF, ×18.5 with transforming growth factor alpha (TGF-α), and ×7.6 with hepatocyte growth factor (HGF). For fetal hepatocytes, the increase was twofold with either EGF, TGF-α or HGF. EGF-, TGF-α- and HGF-dependent DNA synthesis was inhibited by transfroming growth factor beta-1 (TGF-β1) in both fetal and adult hepatocyte cultures; this antiproliferative effect was significantly stronger in adult hepatocyte cultures. With cyclosporine, EGF-, TGF-α- and HGF-dependent DNA synthesis in fetal hepatocyte cultures decreased by 36–46%, whereas in adult hepatocytes by 19–27%. These results show that in contrast to adult hepatocytes, fetal hepatocytes have high spontaneous proliferative activity independently of growth factors and are relatively resistant to the inhibitory effect of TGF-β1. It was also found that cyclosporine suppresses proliferation of cultured fetal hepatocytes.

HGF-induced DNA synthesis in hepatocytes is suppressed by p38

Previous studies in rat hepatocytes have shown that the MEK/ERK, PI3K/Akt and p38 pathways are all involved in the activation of DNA synthesis by EGF and that sustained activation of MEK/ERK is required. Here, we show that although HGF stimulated DNA synthesis and activated signaling in the same manner as EGF, the contribution of the signaling pathways to the induction of DNA synthesis differed. While HGF-induced DNA synthesis was dependent on MEK/ERK, with no significant contribution from PI3K/Akt, p38 suppressed HGF-induced DNA synthesis. The p38 inhibitor SB203580 increased HGF-induced DNA synthesis and enhanced the phosphorylation of ERK. In contrast, SB203580 decreased EGF-induced ERK phosphorylation. This suggests that p38 has distinct effects on DNA synthesis induced by EGF and HGF. Due to differential regulation of signaling through the MEK/ERK pathway, p38 acts as an enhancer of EGF-induced DNA synthesis and as a suppressor of HGF-induced DNA synthesis.

Gab1 amplifies signaling in response to low-intensity stimulation by HGF

The receptor tyrosine kinases EGFR and Met induce phosphorylation of the docking protein Gab1, and there is evidence that Gab1 may have a role in the signaling from these receptors. Studying hepatocytes, we previously found that although Gab1 mechanistically interacted in different ways with EGFR and Met, it was involved in mitogenic signaling induced by both EGF and HGF. It has been reported that in EGFR, Gab1 is required particularly at a low dose of EGF. Whether this also applies to HGF/Met signaling has not been investigated. We have studied the role of Gab1 in activation of the Akt and ERK pathways at low- and high-intensity stimulation with EGF and HGF in cultured hepatocytes. In cells where Gab1 was depleted by a specific Gab1-directed siRNA, the EGF-induced phosphorylation of ERK was lowered and HGF-induced phosphorylation of both ERK and Akt was substantially reduced. These effects were more marked at low-dose HGF stimulation. The inhibitory consequence of Gab1 depletion was particularly pronounced for HGF-induced Akt phosphorylation. The results suggest that Gab1 is an important signal amplifier for low-intensity stimulation by HGF.

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.

The involvement of the docking protein Gab1 in mitogenic signalling induced by EGF and HGF in rat hepatocytes

Grb2-associated binder (Gab) family proteins are docking molecules that can interact with receptor tyrosine kinases (RTKs) and cytokine receptors and bind several downstream signalling proteins. Studies in several cell types have shown that Gab1 may have a role in signalling mediated by the two RTKs epidermal growth factor (EGF) receptor (EGFR) and Met, the receptor for hepatocyte growth factor (HGF), but the involvement of Gab1 in EGFR and Met signalling has not been directly compared in the same cell. We have studied mechanisms of activation and role in mitogenic signalling of Gab1 in response to EGF and HGF in cultured rat hepatocytes. Gab1, but not Gab2, was expressed in the hepatocytes and was phosphorylated upon stimulation with EGF or HGF. Depletion of Gab1, using siRNA, decreased the ERK and Akt activation, cyclin D1 expression, and DNA synthesis in response to both EGF and HGF. Studies of mechanisms of recruitment to the receptors showed that HGF induced co-precipitation of Gab1 and Met while EGF induced binding of Gab1 to Grb2 but not to EGFR. Gab1 activation in response to both EGF and HGF was dependent on PI3K. While EGF activated Gab1 and Shc equally, within the same concentration range, HGF very potently and almost exclusively activated Gab1, having only a minimal effect on Shc. Collectively, our results strongly suggest that although Gab1 interacts differently with EGFR and Met, it is involved in mitogenic signalling mediated by both these growth factor receptors in hepatocytes.

Gene transfection of multicellular spheroid of hepatocytes on an artificial substrate

The handling of hepatocytes, a major cell population in the liver, is an important technique in both liver tissue engineering and hepatology. However, these cells are so fragile that it has been impossible to harvest hepatocytes with high viability from tissue culture dishes after a period of culture in vitro. In this study, we employed an artificial substrate for transfection of multilayer hepatocytes and harvested these cells with high viability after transfection. Hepatocytes cultured on an amphiphilic artificial substrate form multilayer aggregates (spheroids) in the presence of growth factors during gene transfection with cation liposomes. Compared to cells cultured on a collagen-coated plate, these spheroids are easily harvested with high viability by pipetting in EDTA solution. In addition, these spheroids rapidly spread on collagen after transfer from the artificial substrate, demonstrating that hepatocytes in the center of the spheroids were viable. Epidermal growth factor (EGF) increased the transfection efficiency into hepatocytes while hepatocyte growth factor (HGF) alone did not increase the efficiency. However, HGF synergestically increased the effect of EGF on transfection. Interestingly, this transfection required the process of spheroid formation because the gene was not transfected once the spheroid formation completed or under conditions where hepatocytes did not form spheroids. This method using spheroidal hepatocytes for in vitro transfection is promising for the development of ex vivo gene therapy.

Motility behavior of hepatocytes on extracellular matrix substrata during aggregation

Aggregation of hepatocytes in culture is an important phenomenon to control in tissue engineering applications. Aggregation generally enhances maintenance of differentiated functions but inhibits cell growth. At present there exists insufficient information for rational design of substrata that control aggregation. Indeed, the cellular mechanism(s) underlying the aggregation process is poorly understood, although cell motility is generally considered to be an essential phenomenon. In this article we provide the first study investigating the relationship between hepatocyte aggregation and motility behavior on various extracellular matrix substrata, including Matrigel, laminin, and fibronectin. We find that the extent of aggregation depends on the concentration of the extracellular matrix proteins, as well as on the type. Furthermore, we find that the extent of aggregation appears to be independent of classical single-cell locomotion. In fact, under conditions giving rise to substantial aggregation, the fraction of cells exhibiting classical locomotion is essentially negligible. Instead, aggregation appears to involve intracellular contacts accomplished via a different form of cell motility: active cell membrane extensions followed by adhesive cell-cell interactions. An implication of these findings is that aggregation may be largely governed by relative strengths of cell-cell versus cell-substratum interactions. These observations could be helpful for improved design of cell transplantation devices and cell culture substrata. (c) 1996 by John Wiley & Sons, Inc.

Autocrine-controlled formation and function of tissue-like aggregates by primary hepatocytes in micropatterned hydrogel arrays

The liver carries out a variety of essential functions regulated in part by autocrine signaling, including hepatocyte-produced growth factors and extracellular matrix (ECM). The local concentrations of autocrine factors are governed by a balance between receptor-mediated binding at the cell surface and diffusion into the local matrix and are thus expected to be influenced by the dimensionality of the cell culture environment. To investigate the role of growth factor and ECM-modulated autocrine signaling in maintaining appropriate primary hepatocyte survival, metabolic functions, and polarity, we created three-dimensional cultures of defined geometry using micropatterned semisynthetic polyethylene glycol-fibrinogen hydrogels to provide a mechanically compliant, nonadhesive material platform that could be modified by cell-secreted factors. We found that in the absence of exogenous peptide growth factors or ECM, hepatocytes retain the epidermal growth factor (EGF) receptor ligands (EGF and transforming growth factor-α) and the proto-oncogenic mesenchymal epithelial transition factor (c-MET) ligand hepatocyte growth factor (HGF), along with fibronectin. Further, hepatocytes cultured in this three-dimensional microenvironment maintained high levels of liver-specific functions over the 10-day culture period. Function-blocking inhibitors of α5β1 or EGF receptor dramatically reduced cell viability and function, suggesting that signaling by both these receptors is needed for in vitro survival and function of hepatocytes in the absence of other exogenous signals.

Epithelial-to-mesenchymal transition of murine liver tumor cells promotes invasion

Epithelial-to-mesenchymal transition (EMT) is predicted to play a critical role in metastatic disease in hepatocellular carcinoma. In this study, we used a novel murine model of EMT to elucidate a mechanism of tumor progression and metastasis. A total of 2 x 10(6) liver cells isolated from Pten(loxp/loxp)/Alb-Cre(+) mice, expanded from a single CD133(+)CD45(-) cell clone, passage 0 (P0), were sequentially transplanted to obtain two passages of tumor cells, P1 and P2. Cells were analyzed for gene expression using microarray and real-time polymerase chain reaction. Functional analysis included cell proliferation, migration, and invasion in vitro and orthotopic tumor metastasis assays in vivo. Although P0, P1, and P2 each formed tumors consistent with mixed liver epithelium, within the P2 cells, two distinct cell types were clearly visible: cells with epithelial morphology similar to P0 cells and cells with fibroblastoid morphology. These P2 mesenchymal cells demonstrated increased locomotion on wound healing; increased cell invasion on Matrigel basement membrane; increased EMT-associated gene expression of Snail1, Zeb1, and Zeb2; and down-regulated E-cadherin. P2 mesenchymal cells demonstrated significantly faster tumor growth in vivo compared with P2 epithelial counterparts, with invasion of intestine, pancreas, spleen, and lymph nodes. Furthermore, P2 mesenchymal cells secreted high levels of hepatocyte growth factor (HGF), which we propose acts in a paracrine fashion to drive epithelial cells to undergo EMT. In addition, a second murine liver cancer stem cell line with methionine adenosyltransferase 1a deficiency acquired EMT after sequential transplantations, indicating that EMT was not restricted to Pten-deleted tumors.

CONCLUSION

EMT is associated with a high rate of liver tumor proliferation, invasion, and metastasis in vivo, which is driven by HGF secreted from mesenchymal tumor cells in a feed-forward mechanism.

Effect of a hepatocyte growth factor/heparin-immobilized collagen system on albumin synthesis and spheroid formation by hepatocytes

A hepatocyte growth factor (HGF)/heparin-immobilized collagen system was used as a synthetic extracellular matrix for hepatocyte culture. The albumin synthesis, nucleus numbers and morphology of the hepatocytes were determined separately to evaluate the hepatocyte number and hepatocyte-specific function under this system. The benefits of the HGF/heparin-immobilized collagen system for hepatocyte culture were confirmed by three types of culture methods in vitro, namely 2D film cultures, 2D gel cultures and 3D gel cultures. In 2D collagen film cultures, hepatocytes exhibited the highest albumin synthesis (1.42 microg/well/day) in HGF/heparin-immobilized collagen films at 7 days of culture. Heparin inhibited hepatocyte adhesion while HGF promoted hepatocyte migration, and spheroid formation was easily detected in HGF/heparin-immobilized collagen films. In 2D collagen gel cultures, albumin synthesis of around 15 microg/well/day was detected and maintained for more than 18 days on HGF/heparin-immobilized collagen gels. Similar findings were obtained in 3D HGF/heparin-immobilized collagen gel cultures, which exhibited albumin synthesis of up to 30 microg/well/day. The albumin synthesis by hepatocytes was two-fold higher in 3D gel cultures compared with 2D gel cultures, and was maintained for over 2 weeks compared with 2D film cultures using the HGF/heparin-immobilized collagen system. Taken together, the HGF/heparin-immobilized collagen system was effective for albumin synthesis by hepatocytes in both 2D film cultures and 3D gel cultures, and therefore shows good potential for tissue engineering use.

PIK3IP1, a negative regulator of PI3K, suppresses the development of hepatocellular carcinoma

Phosphatidylinositol-3-kinase (PI3K) is a well-known regulator of cell division, motility, and survival in most cell types. Recently, we characterized a novel protein that we call PI3K Interacting Protein 1 (PIK3IP1), which binds to the p110 catalytic subunit of PI3K and reduces its activity in vitro. Little is known about the role of PIK3IP1 in normal and neoplastic growth in vivo. Proper liver function and development depend on intact PI3K signal transduction; when dysregulated, the PI3K pathway is linked to the development of liver cancer. To begin to dissect the contribution of PIK3IP1 to hepatic PI3K signaling in vivo and to liver tumorigenesis in particular, we formulated the following hypothesis: because PIK3IP1 down-regulates PI3K signaling and uncontrolled PI3K signaling is associated with liver cancer, then PIK3IP1-mediated down-regulation of the PI3K pathway should inhibit hepatocellular carcinoma (HCC) development. To test this idea, we generated transgenic mice overexpressing PIK3IP1 in hepatocytes in a mouse strain prone to develop HCC. Isolated PIK3IP1 transgenic mouse hepatocytes showed blunted PI3K signaling, DNA synthetic activity, motility, and survival compared with controls. In vivo, spontaneous liver tumorigenesis was significantly dampened in the transgenic animals. This was accompanied by decreased hepatic PI3K activity and reduced hepatocyte proliferation in the transgenics compared with controls. We also observed that human HCC expressed less PIK3IP1 protein than adjacent matched liver tissue. Our data show that PIK3IP1 is an important regulator of PI3K in vivo, and its dysregulation can contribute to liver carcinogenesis.

An MLCK-dependent window in late G1 controls S phase entry of proliferating rodent hepatocytes via ERK-p70S6K pathway

We show that MLCK (myosin light chain kinase) plays a key role in cell cycle progression of hepatocytes: either chemical inhibitor ML7 or RNA interference led to blockade of cyclin D1 expression and DNA replication, providing evidence that MLCK regulated S phase entry. Conversely, inhibition of RhoK by specific inhibitor Y27632 or RhoK dominant-negative vector did not influence progression in late G1 and S phase entry. Inhibition of either MLCK or RhoK did not block ERK1/2 phosphorylation, whereas MLCK regulated ERK2-dependent p70S6K activation. In addition, DNA synthesis was reduced in hepatocytes treated with p70S6K siRNA, demonstrating the key role played by the kinase in S phase entry. Interestingly, after the G1/S transition, DNA replication in S phase was no longer dependent on MLCK activity. We strengthened this result by ex vivo experiments and evidenced an MLCK-dependent window in late G1 phase of regenerating liver after two-thirds partial hepatectomy. In conclusion, our results underline an MLCK-dependent restriction point in G1/S transition, occurring downstream of ERK2 through the regulation of p70S6K activation, and highlighting a new signaling pathway critical for hepatocyte proliferation.

Transcription factors control invasion: AP-1 the first among equals

Metastasis, the aggressive spread of a malignant tumor to distant organs, is a major cause of death in cancer patients. Despite this critical role in cancer outcomes, the molecular mechanisms that control this process are just beginning to be understood. Metastasis is largely dependent upon the ability of tumor cells to invade the barrier formed by the basement membrane and to migrate through neighboring tissues. This review will summarize the evidence that tumor cell invasion is the result of oncogene-mediated signal transduction pathways that control the expression of a specific set of genes that together mediate tumor cell invasion. We focus on the role of the transcription factor AP-1 to both induce the expression of genes that function as invasion effectors and repress other genes that function as invasion suppressors. This identifies AP-1 as a critical regulator of a complex program of gene expression that defines the invasive phenotype.

ALR and Liver Regeneration

Liver possesses the capacity to restore its tissue mass and attain optimal volume in response to physical, infectious and toxic injury. The extraordinary ability of liver to regenerate is the effect of cross-talk between growth factors, cytokines, matrix components and many other factors. In this review we present recent findings and existing information about mechanisms that regulate liver growth, paying attention to augmenter of liver regeneration.

Controlled and reversible induction of differentiation and activation of adult human hepatocytes by a biphasic culture technique

AIM: Clinical application of human hepatocytes (HC) is hampered by the progressive loss of growth and differentiation in vitro. The object of the study was to evaluate the effect of a biphasic culture technique on expression and activation of growth factor receptors and differentiation of human adult HC.

METHODS: Isolated HC were sequentially cultured in a hormone enriched differentiation medium (DM) containing nicotinamide, insulin, transferrin, selenium, and dexame-thasone or activation medium (AM) containing hepatocyte growth factor (HGF), epidermal growth factor (EGF), and granulocyte-macrophage colony-stimulating factor (GM-CSF). Expression, distribution and activation of the HC receptors (MET and EGFR) and the pattern of characteristic cytokeratin (CK) filaments were measured by fluorometry, confocal microscopy and Western blotting.

RESULTS: In the biphasic culture system, HC underwent repeated cycles of activation (characterized by expression and activation of growth factor receptors) and re-differentiation (illustrated by distribution of typical filaments CK-18 but low or absent expression of CK-19). In AM increased expression of MET and EGFR was associated with receptor translocation into the cytoplasm and induction of atypical CK-19. In DM low expression of MET and EGFR was localized on the cell membrane and CK-19 was reduced. Receptor phosphorylation required embedding of HC in collagen type I gel.

CONCLUSION: Control and reversible modulation of growth factor receptor activation of mature human HC can be accomplished in vitro, when defined signals from the extracellular matrix and sequential growth stimuli are provided. The biphasic technique helps overcome de-differentiation, which occurs during continuous stimulation by means of growth factors.

p53 May positively regulate hepatocyte proliferation in rats

p53, known as a tumor suppressor gene, is a transcription factor that regulates various cellular functions. Recently, several growth factor gene promoters, including that of transforming growth factor alpha (TGF-alpha), were shown to be direct targets of p53-mediated transcription. Hepatic p53 mRNA is up-regulated during liver regeneration in rats. The aim of this study is to examine the role of p53 in hepatocyte proliferation. p53 protein levels were examined in rat hepatocytes cultured in the medium containing hepatocyte growth factor (HGF). p53 levels began to increase after 6 hours of incubation, reached a maximum at 18 hours, and decreased thereafter. DNA synthesis increased at 12 hours and peaked at 30 hours. When hepatocytes were incubated with p53 antisense oligonucleotide in addition to HGF, increases of p53 and TGF-alpha levels were suppressed, and DNA synthesis was reduced. The increases of TGF-alpha levels and DNA synthesis were also suppressed by a chemical inhibitor of p53, pifithrin-alpha. In rats after two-thirds partial hepatectomy, hepatic p53 increased and reached maximal levels around 16 hours when hepatic HGF levels have been shown to reach a maximum followed by an increase in hepatic TGF-alpha levels or hepatocyte proliferation. In contrast, sham-operated rats showed minor elevations of hepatic p53 levels. In conclusion, p53 production is stimulated by HGF and may contribute to the proliferation of rat hepatocytes. Considering previous findings indicating the importance of endogenous TGF-alpha for the proliferation of hepatocytes stimulated by HGF, TGF-alpha might play a role in HGF-p53 mediated hepatocyte proliferation.

Engineering hepatocyte functional fate through growth factor dynamics: the role of cell morphologic priming

We have reported previously that cellular stimulation induced by variable mechanochemical properties of the extracellular microenvironment can significantly alter liver-specific function in cultured hepatocytes (Semler et al., Biotech Bioeng 69:359-369, 2000). Cell activation via time-invariant presentation of biochemical growth factors was found to either enhance or repress cellular differentiation of cultured hepatocytes depending on the mechanical properties of the underlying substrate. In this work, we investigated the effects of dynamic growth factor stimulation on the cell growth and differentiation behavior of hepatocytes cultured on either compliant or rigid substrates. Specifically, hepatotrophic growth factors (epidermal and hepatocyte) were either temporally added or withdrawn from hepatocyte cultures on Matrigel that was crosslinked to yield differential degrees of mechanical compliance. We determined that the functional responsiveness of hepatocytes to fluctuations in GF stimulation is substrate specific but only in conditions in which the initial mechanochemical environment induced significant cell morphogenesis. Our studies indicate that in conditions under which hepatocytes adopted a "rounded" phenotype, they exhibited increased levels of differentiated function upon soluble stimulation and markedly decreased function upon the depletion of GF stimulation. In contrast, hepatocytes that assumed a "spread" phenotype exhibited slightly increased function upon the depletion of GF stimulation. By examining the functional responsiveness of hepatocytes of differential morphology to varied fluctuations in GF activation, insights into the ability of cell shape to "prime" hepatocyte behavior in dynamic microenvironments were elucidated. We report on the possibility of uncoupling and, thus, selectively manipulating, the concerted contributions of GF-induced cellular activation and substrate- and GF-induced cell morphogenesis toward induction of cell function.

Epidermal growth factor induces Egr-1 promoter activity in hepatocytes in vitro and in vivo

Early growth response-1 (Egr-1) is a transcription factor that couples short-term changes in the extracellular milieu to long-term changes in gene expression. Under in vitro conditions, the Egr-1 gene has been shown to respond to many extracellular signals. In most cases, these findings have not been extended to the in vivo setting. The goal of the present study was to explore the role of epidermal growth factor (EGF) in mediating Egr-1 expression in hepatocytes under both in vitro and in vivo conditions. In HepG2 cells, Egr-1 protein and mRNA were upregulated in the presence of EGF. In stable transfections of HepG2 cells, a 1,200-bp Egr-1 promoter contained information for EGF response via a protein kinase C-independent, mitogen-activated protein kinase-dependent signaling pathway. A promoter region containing the two most proximal serum response elements was sufficient to transduce the EGF signal. In transgenic mice that carry the Egr-1 promoter coupled to the LacZ reporter gene, systemic delivery of EGF by intraperitoneal injection resulted in an induction of the endogenous Egr-1 gene and the Egr-1-lacZ transgene in hepatocytes. Together, these results suggest that the 1,200-bp promoter contains information for EGF response in hepatocytes both in vitro and in intact animals.

Hepatocyte growth factor/scatter factor stimulates migration of rat mammary fibroblasts through both mitogen-activated protein kinase and phosphatidylinositol 3-kinase/Akt pathways

Hepatocyte growth factor/scatter factor (HGF/SF) is considered to be a mesenchymal-derived factor that acts via a dual system receptor, consisting of the MET receptor and proteoglycans present on adjacent epithelial cells. Surprisingly, HGS/SF stimulated the migration of rat mammary (Rama) 27 fibroblasts, although it failed to stimulate their proliferation. HGF/SF stimulated a transient activation of mitogen-activated protein kinases p44 and p42 (p42/44(MAPK)), with a maximum level of dual phosphorylation of p42/44(MAPK) occurring 10-15 min after the addition of the growth factor, which was followed by a rapid decrease to near basal levels after 20 min. Interestingly, a second phase of p42/44(MAPK) dual phosphorylation was observed at later times (3 h to 10 h). PD098059, a specific inhibitor of MEK-1, prevented the dual phosphorylation of p42/44(MAPK) and also the phosphorylation of p90(RSK) (ribosomal subunit S6 kinase), which mirrored the kinetics of p42/44(MAPK) phosphorylation. Moreover, PD098059 prevented the HGF/SF-induced migration of Rama 27 cells. HGF/SF also induced an early increase in the phosphorylation of protein kinase B/Akt. Akt phosphorylation was elevated 15 min after the addition of HGF/SF and then declined to basal levels by 30 min. Wortmannin, an inhibitor of phosphatidylinositol 3-kinase (PtdIns3K), prevented the increase in Akt phosphorylation and abolished HGF/SF-induced migration of fibroblasts. PD098059 also inhibited the stimulation of Akt phosphorylation by HGF/SF and wortmannin similarly inhibited the stimulation of p42/44(MAPK) dual phosphorylation. These results suggest that HGF/SF-induced motility depends on both the transient dual phosphorylation of p42/44(MAPK) and the activation of PtdIns3K in Rama 27 fibroblasts and that these pathways are mutually dependent.

The mitogenic activity of hepatocyte growth factor on rat hepatocytes is dependent upon endogenous transforming growth factor-alpha

Both transforming growth factor-alpha (TGF-alpha) and hepatocyte growth factor (HGF) induce DNA synthesis in hepatocytes in vitro and in vivo. Hepatic and circulating levels of HGF have been reported to increase before an increase in TGF-alpha levels in several rat models of liver regeneration. In addition, serum TGF-alpha levels increase after an increase in serum HGF levels in patients with either partial hepatectomy or acute hepatitis. In this study, we investigate the significance of TGF-alpha in hepatocyte proliferation. TGF-alpha contents and DNA synthesis in cultured rat hepatocytes increased in response to HGF addition to the culture medium in a dose-related manner. These increases were suppressed by the addition of anti-sense TGF-alpha mRNA oligonucleotide. Furthermore, the addition of anti-TGF-alpha rabbit IgG suppressed the increase in DNA synthesis. When the anti-TGF-alpha antibody was administered to rats after partial hepatectomy, the number of mitotic hepatocytes was reduced in comparison to rats treated with normal rabbit IgG. These results were observed even though hepatic HGF levels were increased equally in rats given either anti-TGF-alpha antibody or normal rabbit IgG. Our results suggest that HGF stimulates TGF-alpha production in rat hepatocytes, and that the mitogenic activity of HGF depends on endogenous TGF-alpha activity.

Mechanochemical manipulation of hepatocyte aggregation can selectively induce or repress liver-specific function

Controlled activation of hepatocyte aggregation is critical to three-dimensional (3D) multicellular morphogenesis during native regeneration of liver as well as tissue reconstruction therapies. In this work, we quantify the stimulatory effects of two model hepatotrophic activators, epidermal growth factor (EGF) and hepatocyte growth factor (HGF), on the aggregation kinetics and liver-specific function of hepatocytes cultured on organotypic substrates with differing mechanical resistivity. Substrate-specific morphogenesis of cultured hepatocytes is induced on a tissue basement membrane extract, Matrigel, formulated at two distinct levels of mechanical compliance (storage modulus G', at oscillatory shear rate 1 rad/s, was 34 Pa for basal Matrigel and 118 Pa for crosslinked Matrigel). Overall, we report that growth factor stimulation selectively promotes the kinetics of aggregation in the form of two-dimensional corded aggregates on basal Matrigel and three-dimensional spheroidal aggregates on crosslinked Matrigel. Our analysis also indicates that costimulation with EGF and HGF (20 ng/mL each) cooperatively maximizes the kinetics of aggregation in a substrate-specific manner. In addition, we show that the role of growth factor stimulation on hepatocyte function is sensitively governed by the mechanical compliance of the substrate. In particular, on matrices with high compliance, costimulatory aggregation is shown to elicit a marked increase in albumin secretion rate, whereas on matrices with low compliance aggregation results in effective functional repression to basal, unstimulated levels. Thus, our studies highlight a novel interplay of physicochemical parameters of the culture microenvironment, leading to selective enhancement or repression of differentiated functions of hepatocytes, in concert with the activation of cellular morphogenesis.

Tri-iodothyronine and a deleted form of hepatocyte growth factor act synergistically to enhance liver proliferation and enable in vivo retroviral gene transfer via the peripheral venous system

Retroviral vectors integrate into the target cell genome in a stable manner and therefore offer the potential for permanent correction of the genetic diseases that affect the liver. These vectors, however, usually require cell division to occur in order to allow provirus entry into the nucleus. We have explored clinically acceptable methods to improve the efficiency of retroviral gene transfer to the liver, which avoid the need for liver damage. Tri-iodothyronine (T3) and recombinant hepatocyte growth factor have previously been used to induce hepatocyte proliferation in rat livers and allow in vivo retroviral gene transfer. We investigated the combined effects of these growth factors, with their differing mechanisms of action, on hepatocyte proliferation in vivo and assessed their effectiveness in priming cells for retroviral gene transfer. During the phase of hepatocyte proliferation retrovirus was administered via either the portal or tail vein. Acting synergistically, T3 and a truncated form of recombinant hepatocyte growth factor (dHGF) induced 30% of hepatocytes in normal rat liver to enter DNA synthesis at 24 h. This increased proliferation enabled the liver to be transduced in vivo by retroviral vectors via either the portal or peripheral venous system, achieving transduction efficiencies of 6.9 +/- 1.6% and 4.3 +/- 0.4% respectively. Thus, the liver can be simply and conveniently transduced in vivo with integrating vectors, introduced via the peripheral venous system during a wave of growth factor-induced proliferation, pointing the way to clinically applicable gene transfer techniques.

Hepatocyte growth factor promotes migration of human hepatocellular carcinoma via phosphatidylinositol 3-kinase

Hepatocyte growth factor (HGF) is known to be a potent mitogen and motogen for epithelial cells. Hepatocellular carcinoma (HCC) often metastasizes, and the c-Met/HGF receptor is highly expressed by HCC cells. The aim of this study was to investigate the signaling pathways associated with the motogenic effect of HGF on HCC cells via c-Met. HCC cell lines (Hep3B, HepG2, PLC, and Huh-7) and HCC cells harvested from patients were used for the Boyden chamber assay of chemotactic activity as well as for immunoprecipitation and immunoblotting studies. HGF stimulated the motility of Hep3B, HepG2, and Huh-7 cells in a dose-dependent manner in association with tyrosine phosphorylation of c-Met and activation of phosphatidylinositol 3-kinase (PI3-K). A tyrosine kinase inhibitor (genistein) and a PI3-K inhibitor (wortmannin) prevented the migration of HCC cells. However, migration was not prevented by calphostin C, an inhibitor of protein kinase C (PKC), which is a downstream target of phospholipase Cgamma (PLCgamma). HGF also stimulated the migration of HCC cells obtained from three patients, while wortmannin prevented the migration of these cells. These results indicate that HGF stimulates the migration of HCC cells through the tyrosine phosphorylation of c-Met via activation of PI3-K.

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.

Epidermal growth factor and insulin-induced deoxyribonucleic acid synthesis in primary rat hepatocytes is phosphatidylinositol 3-kinase dependent and dissociated from protooncogene induction

The mitogenic response to insulin and epidermal growth factor (EGF) was studied in subconfluent and confluent cultures of primary rat hepatocytes. In subconfluent cultures, wortmannin, LY294002, and rapamycin reversed insulin- and EGF-induced [3H]thymidine incorporation into DNA. The mitogen-activated protein kinase (MAPK) kinase 1 (MEK1) inhibitor PD98059 was without significant effect on either insulin- or EGF-induced [3H]thymidine incorporation. Insulin treatment did not alter levels of messenger RNAs (mRNAs) for c-fos, c-jun, and c-myc. EGF induced an increase in c-myc, but not c-fos or c-jun, mRNA levels in subconfluent hepatocyte cultures. This increase in c-myc mRNA was abolished by PD98059. In confluent cells that could not be induced to synthesize DNA, EGF treatment also promoted an increase in c-myc mRNA to levels seen in subconfluent cultures. This increase was also abrogated by PD98059. These data indicate that in primary rat hepatocyte cultures, 1) the phosphoinositol 3-kinase pathway, perhaps through p70s6k activation, regulates DNA synthesis in response to insulin and EGF; 2) the MAPKpathway is not involved in insulin- and EGF-induced DNA synthesis; and 3) p44/42 MAPKs are involved the induction of c-myc mRNA levels, although this induction is not required for DNA synthesis. These studies define two distinct signal transduction pathways that independently mediate growth-related responses in a physiologically relevant, normal cell system.

HGF triggers activation of the COX-2 gene in rat gastric epithelial cells: action mediated through the ERK2 signaling pathway

Although it is established that growth factors and prostaglandins function in the maintenance of gastric mucosal integrity and in the healing of gastric mucosal injury and ulceration, the regulatory relationship between growth factors and prostaglandins in the gastric mucosa is not well characterized. Therefore, we investigated whether hepatocyte growth factor (HGF) affects expression of COX-2 (the inducible form of the prostaglandin synthesizing enzyme, cyclooxygenase) in gastric epithelial cells and whether this action is mediated through the MAP (ERK) kinase signaling pathway. In RGM1 cells (an epithelial cell line derived from normal rat gastric mucosa), HGF caused an increase in COX-2 mRNA and protein by 236% and 175%, respectively (both P<0.05). This induction of COX-2 expression was abolished by pretreatment with the MAPK kinase (MEK) inhibitor PD98059. HGF also triggered a 13-fold increase in c-Met/HGF receptor phosphorylation (P<0.005) and increased ERK2 activity by 684% (P<0.01). Pretreatment with PD98059 abolished the HGF-induced increase in ERK2 activity, but not c-Met/HGF receptor phosphorylation. The specific inhibitor of p38 MAP kinase, SB203580, had no effect on HGF-induced COX-2 expression. Thus, HGF triggers activation of the COX-2 gene in gastric epithelial cells through phosphorylation of c-Met/HGF receptor and activation of the ERK2 signaling pathway.-Jones, M. K., Sasaki, E., Halter, F., Pai, R., Nakamura, T., Arakawa, T., Kuroki, T., Tarnawski, A. S. HGF triggers activation of the COX-2 gene in rat gastric epithelial cells: action mediated through the ERK2 signaling pathway.

Lipopolysaccharide potentiates the effect of hepatocyte growth factor on hepatocyte replication in rats by augmenting AP-1 activity

The liver regenerates by replication of differentiated hepatocytes after damage or removal of part of the liver. Although several growth factors and signaling pathways are activated during regeneration, it is unclear as to which of these are essential for hepatocyte replication. We show here that low- (1 mg/kg) and high- (10 mg/kg) dose hepatocyte growth factor (HGF) induced replication of 2.1% and 11.1% of hepatocytes in rats, respectively. Lipopolysaccharide (LPS), an inducer of the acute phase response, augmented hepatocyte replication in response to low- and high-dose HGF by 4- and 2-fold, respectively. HGF alone induced moderate levels of c-Jun-N-terminal kinase (JNK) and p44/p42 mitogen-activated protein kinase (MAPK), resulting in moderate levels of AP-1-DNA binding activity. The combination of LPS + HGF increased JNK and AP-1-DNA binding activity more than levels seen with LPS or HGF alone. The activation of Stat3 that was observed after administration of LPS + HGF, but not HGF alone, could contribute to increased transcription of AP-1 components. Because phosphorylation of the c-Jun component of AP-1 by JNK increases its ability to activate transcription, the AP-1 in hepatocytes from animals treated with LPS + HGF may be more active than in rats treated with LPS or HGF alone. LPS may contribute to hepatocyte replication by potentiating the effect of HGF on the activation of both AP-1-DNA binding and transcriptional activity.

STAT 1alpha/1beta, STAT 3 and STAT 5: expression and association with c-MET and EGF-receptor in long-term cultures of human hepatocytes

Long-term cultures of human hepatocytes were maintained serum-free in a chemically defined medium in the presence of HGF and EGF for up to 30 days. STAT 1alpha/1beta, STAT 3, and STAT 5 were present and tyrosine phosphorylated throughout the culture period in the cytosol as well as the nucleus. We show by co-immunoprecipitation that a portion of the cellular pools of STAT 1alpha/1beta and STAT 5 is physically associated with c-MET and EGF-receptor. Co-immunoprecipitation of STAT 3 with STAT 5 did occur in the cytosol but not in the nucleus, suggesting dimerization of the two STAT family members. The observed differences of protein amounts and tyrosine phosphorylation between cytosol and nucleus, the association of STAT proteins with EGF-receptor and c-MET and with each other may all be involved in regulating the activity of the STAT transcription factors. It is intriguing to speculate that STAT 5 may have a modulating role in the regulation of STAT 3 activity.

Cationic colloidal silica membrane perturbation as a means of examining changes at the sinusoidal surface during liver regeneration

By employing the cationic colloidal silica membrane density perturbation technique, we examined growth factor receptor and extracellular matrix (ECM) changes at the sinusoidal surface during rat liver regeneration 72 hours after 70% partial hepatectomy (PHx). At this time after PHx, hepatocyte division has mostly subsided, while sinusoidal endothelial cell (SEC) proliferation is initiating, resulting in avascular hepatocyte islands. Because of the discontinuous nature of the surface of liver SEC, ECM proteins underlying the SEC, as well as SEC luminal membrane proteins, are available to absorption to the charged silica beads when the liver is perfused with the colloid. Subsequent liver homogenization and density centrifugation yield two separate fractions, enriched in SECs as well as hepatocyte basolateral membrane-specific proteins up to 50-fold over whole liver lysates. This technique facilitates examination of changes in protein composition that influence or occur as a result of SEC mitogenesis and migration during regeneration of the liver. When ECM and receptor proteins from SEC-enriched fractions were examined by Western immunoblotting, urokinase plasminogen activator receptor, fibronectin, and plasmin increased at the SEC surface 72 hours after PHx. Epidermal growth factor receptor, plasminogen, SPARC (secreted protein, acidic and rich in cysteine, also called osteonectin or BM40), and collagen IV decreased, and fibrinogen subunits and c-Met expression remained constant 72 hours after PHx when compared to control liver. These results display the usefulness of the cationic colloidal silica membrane isolation protocol. They also show considerable modulation of surface components that may regulate angiogenic processes at the end stage of liver regeneration during the reformation of sinusoids.

The mitogen-activated protein kinase kinase/extracellular signal-regulated kinase cascade activation is a key signalling pathway involved in the regulation of G(1) phase progression in proliferating hepatocytes

In this study, activation of the mitogen-activated protein kinase kinase (MEK)/extracellular signal-regulated kinase (ERK) signalling pathway was analyzed in proliferating rat hepatocytes both in vivo after partial hepatectomy and in vitro following epidermal growth factor (EGF)-pyruvate stimulation. First, a biphasic MEK/ERK activation was evidenced in G(1) phase of hepatocytes from regenerating liver but not from sham-operated control animals. One occurred in early G(1) (30 min to 4 h), and the other occurred in mid-late G(1), peaking at around 10.5 h. Interestingly, the mid-late G(1) activation peak was located just before cyclin D1 induction in both in vivo and in vitro models. Second, the biological role of the MEK/ERK cascade activation in hepatocyte progression through the G(1)/S transition was assessed by adding a MEK inhibitor (PD 98059) to EGF-pyruvate-stimulated hepatocytes in primary culture. In the presence of MEK inhibitor, cyclin D1 mRNA accumulation was inhibited, DNA replication was totally abolished, and the MEK1 isoform was preferentially targeted by this inhibition. This effect was dose dependent and completely reversed by removing the MEK inhibitor. Furthermore, transient transfection of hepatocytes with activated MEK1 construct resulted in increased cyclin D1 mRNA accumulation. Third, a correlation between the mid-late G(1) MEK/ERK activation in hepatocytes in vivo after partial hepatectomy and the mitogen-independent proliferation capacity of these cells in vitro was established. Among hepatocytes isolated either 5, 7, 9, 12 or 15 h after partial hepatectomy, only those isolated from 12- and 15-h regenerating livers were able to replicate DNA without additional growth stimulation in vitro. In addition, PD 98059 intravenous administration in vivo, before MEK activation, was able to inhibit DNA replication in hepatocytes from regenerating livers. Taken together, these results show that (i) early induction of the MEK/ERK cascade is restricted to hepatocytes from hepatectomized animals, allowing an early distinction of primed hepatocytes from those returning to quiescence, and (ii) mid-late G(1) MEK/ERK activation is mainly associated with cyclin D1 accumulation which leads to mitogen-independent progression of hepatocytes to S phase. These results allow us to point to a growth factor dependency in mid-late G(1) phase of proliferating hepatocytes in vivo as observed in vitro in proliferating hepatocytes and argue for a crucial role of the MEK/ERK cascade signalling pathway.

Role of diacylglycerol (DAG) in hormonal induction of S phase in hepatocytes: the DAG-dependent protein kinase C pathway is not activated by epidermal growth factor (EGF), but is involved in mediating the enhancement of responsiveness to EGF by vasopressin, angiotensin II, and norepinephrine

The role of diacylglycerol (DAG) in hormonal induction of S phase was investigated in primary cultures of rat hepatocytes. In this model, several agonists that bind to G protein-coupled receptors act as comitogens when added to the cells soon after plating (i.e., in Go/early Gl phase), while the cells are most responsive to the mitogenic effect of epidermal growth factor (EGF) at 24-48 h of culturing (i.e., mid/late Gl). It was found that the cellular concentration of DAG rose markedly and progressively during the first 24 h of culturing. Exposure of the hepatocytes at 3 h to alpha1-adrenergic stimulation (norepinephrine with timolol), vasopressin, or angiotensin II further increased this rise, producing a sustained increase in the DAG level. Norepinephrine, which was the most efficient comitogen, produced the most prolonged DAG elevation. In contrast, no significant increase of DAG was found in response to EGF, neither at 3 nor at 24 h, using concentrations that markedly stimulated the ERK subgroup of the mitogen-activated protein kinases (MAPK) and DNA synthesis. Addition of Bacillus cereus phosphatidylcholine-specific phospholipase C (PC-PLC) strongly elevated DAG, while Streptomyces phospholipase D (PLD) increased phosphatidic acid (PA) but not DAG. B. cereus PC-PLC and the protein kinase C (PKC) activator tetradecanoyl phorbol-acetate (TPA), like norepinephrine, vasopressin, and angiotensin II, stimulated MAPK and enhanced the stimulatory effect of EGF on DNA synthesis. The PKC inhibitor GF109203X did not diminish the effect of EGF on MAPK or DNA synthesis, but strongly inhibited the effects of norepinephrine, vasopressin, angiotensin II, TPA and B. cereus PC-PLC on MAPK and almost abolished the enhancement by these agents of EGF-stimulated DNA synthesis. These results suggest that although generation of DAG is not a direct downstream response mediating the effects of the EGF receptor in hepatocytes, a sustained elevation of DAG with activation of PKC markedly increases the responsiveness to EGF. Mechanisms involving DAG and PKC seem to play a role in the comitogenic effects of various agents that bind to G protein-coupled receptors and activate the cells early in Gl, such as norepinephrine, angiotensin II, and vasopressin.

Skp2 induction and phosphorylation is associated with the late G1 phase of proliferating rat hepatocytes

The changes in phosphoproteins purified with the affinity peptide p9CKShs1 were analyzed from extracts of regenerating rat livers in order to define some G1 and G1/S regulations characteristic of mature hepatocytes stimulated to proliferate. We observed a 47 kDa phosphoprotein that occurred first at the end of G1 before peaking in the S phase. P47 was also found to be phosphorylated in late G1 in primary hepatocyte cultures stimulated with mitogens. P47 was still phosphorylated in extracts depleted of Cdc2, but to a lesser extent after Cdk2 depletion. This phosphoprotein was identified as Skp2. (i) P47 shared the same electrophoretic mobility than Skp2, a cell cycle protein essential for S phase entry in human fibroblasts; (ii) Skp2, like P47, started to be expressed and was highly phosphorylated during the G1/S transition of hepatocytes stimulated to proliferate in vivo and in vitro; (iii) P47 was specifically immunoprecipitated by an antibody directed against Skp2. In addition, cyclin A/Cdk2 complexes from regenerating liver clearly interacted with Skp2. This is the first demonstration that Skp2 is induced and phosphorylated in the late G1 and S phase of hepatocytes in vivo in regenerating liver as well as in vitro in mitogen-stimulated hepatocytes.

Expression of the human hepatocyte growth factor cDNA in primary cultures of rat hepatocytes

Hepatocyte growth factor (HGF) and epidermal growth factor (EGF) are primary mitogens for hepatocytes in culture. hepatocytes express the HGF-receptor MET but not HGF itself. To investigate the influence of autocrine HGF expression on the proliferative potential of hepatocytes, primary cultures were submitted to retrovirus-mediated transduction of the human hgf (huHGF) cDNA. Expression of the transduced cDNA revealed a minimum 2-fold increase in HGF-mRNA, whereas expression of the Escherichia coli beta-galactosidase gene remained even. Estimation of huHGF copy numbers showed there was a minimum 4-fold increase, suggesting an increase in the population of transduced cells. Immunoprecipitation of excreted huHGF and growth bioassays proofed that HGF was present and functional. HGF is excreted into the medium and therefore, by diffusion, available to transduced and non-transduced cells. The increase in huHGF-transduced cells suggests that the autocrine pathway as opposed to the paracrine pathway, which are both present at the same time, confers a growth advantage to these cells.

HGF-mediated apoptosis via p53/bax-independent pathway activating JNK1

Current studies have indicated both positive and negative roles for the hepatocyte growth factor (HGF)/c-met receptor signaling system in tumor development. Recently, we have shown that HGF has the capacity to induce both growth inhibition and programmed cell death in aflatoxin-transformed (AFLB8) rat liver epithelial cells. Using the same cell line, we have now investigated a potential mechanism for HGF-induced apoptosis. Immunoblot analysis of bcl-2 gene family member (bax, bcl-2, bclX-s/l) expression showed no correlation with HGF treatment, suggesting that HGF-mediated apoptosis is bax independent. Following HGF treatment retinoblastoma protein (pRB) was present in the hypophosphorylated state. HGF treatment increased cyclin A, cyclin G1 and nuclear transcriptional factor (NFkappaB) protein expression. However, electrophoretic mobility shift analysis showed that NFkappaB activity decreased with HGF treatment. Under these apoptotic conditions, c-Jun N-terminal kinase (JNK1) and extracellular signal-regulated kinase (ERK2) were activated with lower level activation of ERK2, while no involvement of phosphatidylinositol-3 kinase was observed. Epidermal growth factor (EGF) was not protective, and actually induced cells to undergo apoptosis to a level similar to that of HGF alone or EGF/HGF in combination. These results suggest the possibility of cross-talk between HGF/c-met and EGF/EGFR signaling pathways, and the involvement of JNK1 induction in HGF-mediated apoptotic cell death.

Establishment of a functional HGF/C-MET autocrine loop in spontaneous transformants of WB-F344 rat liver stem-like cells

A model of spontaneous malignant transformation was used to evaluate the molecular changes that take place in WB-F344 rat liver epithelial cells during neoplastic transformation and tumorigenesis. A comparison of wild-type low-passage WB-F344 cells to spontaneously transformed tumor cell lines revealed that the majority of the tumor cell lines have an increased capacity for autonomous proliferation and motility when maintained in serum-free media. In the current study, we show that c-met is expressed at some level in wild-type WB-F344 cells and in all of the spontaneously transformed tumor cell lines, and that 9/16 of the tumor cell lines have acquired hepatocyte growth factor (HGF) expression. In vitro growth of HGF-expressing tumor cell lines is inhibited as much as 68% by the addition of neutralizing antibodies to HGF or antisense HGF oligonucleotides, indicating that the production of HGF by the tumor cells is partially responsible for driving autonomous proliferation in a subset of tumor cell lines. Furthermore, conditioned media collected from HGF-expressing tumor cell lines stimulates DNA synthesis in wild-type WB-F344 cells, and this effect can be abrogated by pre-incubation of the conditioned media with neutralizing antibodies to HGF. Because HGF is a motility-promoting growth factor, all cell lines were evaluated to determine if expression of HGF stimulated motogenesis. All tumor cell lines (regardless of HGF expression) were highly motile in comparison with wild-type WB-F344 cells, with a 3.5-fold to 20-fold greater number of motile cells. The high basal rate of motility characteristic of the tumor cell lines is not a result of the production of HGF, because it is also a property of the cell lines that do not express HGF messenger RNA. Furthermore, tumor cell motility is not inhibited by antisense oligonucleotides or neutralizing antibodies. Establishment of an autocrine HGF/c-met loop in a subset of spontaneously transformed WB-F344 cell lines may influence development and/or expression of the tumorigenic phenotype by driving cellular proliferation.

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

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

Hepatocyte growth factor enables enhanced integrin-cytoskeleton linkage by affecting integrin expression in subconfluent epithelial cells

Hepatocyte growth factor (HGF) exerts mitogenic and motogenic effects in different cell types. In the epithelial cell line mHepR1 we found that HGF induced pronounced alterations in cell morphology and promoted cell adhesion and spreading. To analyze the mechanisms how HGF affects these integrin mediated functions we studied the physical linkage of integrins with the cytoskeleton. First we found that HGF increased the expression of different integrin subunits in subconfluent cells and influenced the distribution of integrins on the cell surface. To address the physical association of integrins with the cytoskeleton we analyzed Triton X-100-extracted cell fractions using flow cytometry. Here we show that cultivation of the cells with HGF for 24 h prior to integrin cross-linking significantly enhanced the cytoskeletal anchorage of integrins. To further find out whether HGF directly induces an integrin-cytoskeleton link without subsequent cross-linking we added HGF to suspended cells but failed to detect cytoskeletally immobilized integrins in the detergent-insoluble cell fraction which could be related to the absence of a calcium response induced by HGF. Overall, the results indicate that HGF promotes the physical linkage of integrins to the cytoskeleton which requires additional stimulation of integrins.

Transforming growth factor-beta (TGF-beta) and EGF promote cord-like structures that indicate terminal differentiation of fetal hepatocytes in primary culture

When fetal hepatocytes were cultured in the presence of transforming growth factor-beta (TGF-beta 1) and epidermal growth factor (EGF), some morphological changes were observed. Under these conditions, cells migrated, from typical clusters that hepatocytes adopt in culture, to form elongated, cord-like structures similar to the hepatic acinus organization. Immunocytochemical analysis of these cells revealed high levels of albumin and cytokeratin 18, phenotypic markers of parenchymal hepatocytes. Although some of the cells in the cord-like structures presented a cortical ring distribution of F-actin filaments, the cord also presented thick peripheral bundles and cells of the tips showed thin stress fibers oriented to the cell edges, typical of a migratory phenotype. In addition to these morphological effects, flow cytometric analysis of the cells revealed a larger size, granularity and intracellular lipid content (as a parameter related to liver metabolic function), in TGF-beta + EGF-treated hepatocytes. Western blot analysis of the albumin levels revealed that both expression and secretion of albumin were increased in EGF + TGF-beta-treated cells. Finally, all these changes were coincident with an enhancement in the DNA-binding activity for hepatocyte nuclear factors (HNF1, HNF3, and HNF4), as revealed in gel-shift experiments with nuclear extracts. We conclude that a cooperative action between TGF-beta and EGF might modulate terminal maturation of fetal hepatocytes.

Response to transforming growth factor alpha (TGFalpha) and epidermal growth factor (EGF) in hepatocytes: lower EGF receptor affinity of TGFalpha is associated with more sustained activation of p42/p44 mitogen-activated protein kinase and greater efficacy in stimulation of DNA synthesis

The epidermal growth factor (EGF) receptor mediates the effects of both EGF and transforming growth factor alpha (TGFalpha). Recent data suggested that EGF acts as a partial agonist/antagonist in hepatocytes, TGFalpha exerting a larger maximal stimulation of DNA synthesis than EGF. To further study the mechanisms involved in mediating the different effects of EGF and TGFalpha, we have examined receptor binding of the two growth factors and their action on the p42/p44 mitogen-activated protein (MAP) kinase activity in hepatocytes. Single-ligand concentration curves and competition experiments showed that the binding affinity to a common population of surface binding sites was about 20-fold lower for TGFalpha than for EGF. MAP kinase activity responded to EGF and TGFalpha with different kinetics. While the two agents produced almost identical acute (5 min) stimulation (peak about fivefold), TGFalpha produced a more sustained MAP kinase activity than EGF. The difference between EGF and TGFalpha was still detectable 24 h after growth factor addition. The results show that in hepatocytes a lower receptor affinity of TGFalpha, as compared to EGF, is associated with a more sustained activation of the MAP kinase and a greater efficacy in the stimulation of DNA synthesis. This suggests that differential interaction of these two agents with the EGF receptor results in differences in the downstream events elicited at a given level of receptor occupancy. The data also are compatible with a role of a prolonged MAP kinase activity in the mitogenic effects of EGF and TGFalpha.

Differential modulation of hepatocyte growth factor-stimulated motility by transforming growth factor beta1 on rat liver epithelial cells in vitro

We have previously shown that transforming growth factor-beta1 (TGF-beta1) enhances the epidermal growth factor- (EGF) and transforming growth factor-alpha (TGF-alpha)-stimulated motility of rat hepatocytes in an extracellular matrix (ECM)-dependent fashion (Stolz and Michalopoulos, 1997, J. Cell. Physiol., 170:57-68). We have extended this study to examine the effects of TGF-beta1 on hepatocyte growth factor (HGF) and EGF-stimulated motility of rat nonparenchymal liver epithelial cells (RLECs) in vitro and determined that chemotaxis, scattering, and monolayer wound healing by EGF was synergistically enhanced by TGF-beta1 on all ECMs examined. However, HGF-based motility, unlike EGF-stimulated motility, was modulated in an assay-dependent manner by TGF-beta1. HGF-stimulated chemotaxis was dramatically decreased by addition of TGF-beta1, but wound healing was synergistically enhanced by TGF-beta1 on all ECMs examined. HGF-based scattering was not consistently affected by TGF-beta1 on any ECM tested except on laminin, where scattering was often reduced by the concomitant addition of TGF-beta1. TGF-beta1 enhanced the motility associated with monolayer wound healing by HGF or EGF independent of DNA synthesis, because tritiated thymidine uptake was consistently reduced by 60% in the presence of TGF-beta1. The data indicate that HGF and EGF motility do not follow redundant signal-transduction pathways and that specific growth factor motility-related events, as measured by wound healing, scattering, and chemotaxis, are modulated independently by ECM and TGF-beta1.

Growth inhibition and induction of apoptosis by HGF in transformed rat liver epithelial cells

Recently we demonstrated in a transgenic mouse model that hepatocyte growth factor (HGF) inhibits c-myc dependent hepatocarcinogenesis. The inhibitory effects of HGF in carcinogenesis were further characterized using a series of rat liver epithelial (RLE) cell lines which were transformed in vitro with either aflatoxin or oncogenes, or spontaneously. HGF caused a cytostatic effect and enhanced cell motility in spontaneously and aflatoxin-transformed cells. In normal RLE cells HGF was slightly stimulatory and did not induce scattering. The HGF receptor was tyrosine phosphorylated in all cell lines, indicating that it is functionally active and capable of signaling events. In the aflatoxin transformed cells HGF also induced apoptosis, associated with constitutive c-myc expression and 1 Kb bax-alpha transcripts. These findings indicate that transformed RLE cell lines may provide a useful model to further examine the mechanism(s) by which HGF and its receptor modulate neoplastic development.

Synergistic enhancement of EGF, but not HGF, stimulated hepatocyte motility by TGF-beta 1 in vitro

The ability of TGF-beta 1 (transforming growth factor-beta 1) to suppress growth factor induced proliferation of many cell types in vitro is well documented; however, TGF-beta 1 increases within a similar time frame as the hepatocyte mitogens HGF (hepatocyte growth factor), EGF (epidermal growth factor), and TGF-alpha (transforming growth factor-alpha) prior to hepatocyte proliferation during liver regeneration. This has raised the issue that TGF-beta 1 may have effects on hepatocytes additional to mito-inhibition and that these effects may be relevant to the regenerative process. To this end, we examined the effect of TGF-beta 1 on both the mitogenesis and the motility of growth factor stimulated primary rat hepatocytes and the hepatoblastoma cell line HepG2 in vitro. TGF-beta 1 significantly enhanced the chemotactic motility of EGF or TGF-alpha, and not HGF, stimulated hepatocytes on a collagen I substratum. TGF-beta 1 was not chemotactic when added alone and decreased the DNA synthesis of all hepatocyte cultures to near control levels. HepG2 cells were chemotactic toward HGF, EGF, and TGF-beta 1 alone and displayed an additive chemotactic response when TGF-beta 1 was added to either HGF or EGF. Additionally, HepG2 cells were refractory to the growth stimulatory effects of HGF or EGF and the growth inhibitory effects of TGF-beta 1. Hepatocytes plated onto other collagen-containing substrates (collagen IV, Matrigel, or ECL, an entactin-collagen IV-laminin matrix), but not on fibronectin or laminin alone, also displayed enhanced EGF stimulated motility by TGF-beta 1. The data indicate that an additional, novel role for TGF-beta 1 during liver tissue remodeling following PHx may include the synergistic enhancement EGF stimulated hepatocyte motility responses, and this enhancement is observed only on collagen-containing extracellular matrices.

Detection of alphafetoprotein-expressing cells in the blood of patients with hepatoma and hepatitis

The presence of tumour cells in the blood circulation may predict disease recurrence and metastasis. We have evaluated the specificity and sensitivity of detecting hepatoma cells in blood using nested polymerase chain reaction with primers specific for the alphafetoprotein (AFP) gene. The nested polymerase chain reaction amplified a 270-base pair AFP DNA fragment from cDNA of Hep 3B hepatoma cells. In a reconstitution experiment, AFP mRNA was detected from peripheral mononuclear cells isolated from 10 ml of blood containing as few as ten Hep 3B cells. Peripheral mononuclear cells from the blood of 20 hepatoma patients were analysed, and 19 patients showed positive AFP mRNA expression. Seven of 13 samples from hepatitis patients also showed positive AFP mRNA expression. All five paired samples of peripheral blood or umbilical cord blood from pregnant mothers and their babies, respectively, showed positive AFP expression. None of 22 control samples was positive. The presence of AFP mRNA in the blood of hepatitis or hepatoma patients suggests the presence of circulating hepatoma cells or hepatocytes in the circulation. The high incidence of AFP mRNA in the blood of hepatoma patients supports the notion of early haematogenous spreading of the disease.

Thapsigargin induces phosphorylation of the 27-kDa heat shock protein in human keratinocytes

In the human keratinocyte line HaCaT, the nonphosphorylated 27-kDa heat shock protein (HSP27) isoform A (pI 6.5) is constitutively expressed. Application of thapsigargin, which inhibits Ca2+-ATPase in the endoplasmic reticulum, results in the rapid formation of the phosphorylated HSP27 isoform B (pI 6.0) and reduction of HSP27 A without affecting the synthesis of HSP27. The thapsigargin-dependent HSP27 isoform change is similar to that induced by 43 degrees C heat shock, but different from that induced by arsenite, where the biphosphorylated isoform HSP27 C (pI 5.7) is observed. The receptor agonist bradykinin, which increases intracellular Ca2+ (Ca(i)) level, shows no effect on the distribution of HSP27 isoforms. The responses of HSP27 isoforms to thapsigargin are independent of Ca(i) concentration in HaCaT cells. These observations suggest that the thapsigargin-induced change in HSP27 isoforms is dependent on the depletion of internal Ca2+ stores rather than on the increase in Ca(i) concentration. The thapsigargin-induced change in HSP27 isoforms is reduced by the tyrosine kinase inhibitor, genistein, but not the protein kinase C inhibitor, H-7. We propose that the modulation of HSP27 phosphorylation status by Ca(i) homeostasis may be mechanistically linked to control of keratinocyte growth and differentiation and responses of keratinocytes to extracellular stresses.