Met receptor tyrosine kinase: enhanced signaling through adapter proteins

Progestin-regulated luminal cell and myoepithelial cell-specific responses in mammary organoid culture

Normal mammary gland development requires the coordinated proliferation and morphogenesis of both mammary luminal epithelial cells (LECs) and myoepithelial cells (MECs). Cell proliferation in cultured mammary organoids containing both LECs and MECs is not increased by progestin (R5020) or 17beta-estradiol (E2) alone or R5020+E2 but is increased by E2-regulated, mammary stroma-derived Hepatocyte growth factor (HGF) and further increased by HGF+R5020. We investigated the effects of HGF and/or R5020 on morphology and LEC- and MEC-specific in vitro proliferation in organoids. HGF-induced tubulogenesis was initiated and carried out by LECs starting with cellular extensions, followed by the formation of chains and cords, and culminating in tubule formation. MECs did not appear to have an active role in this process. Whereas HGF by itself caused maximal proliferation of LECs, HGF+R5020 produced a synergistic and specific increase in MEC proliferation. Because only LECs expressed progesterone receptors (PRs), we investigated the role of receptor activator of nuclear factor-kappaB ligand (RANKL), a progestin-induced paracrine factor, in mediating increased MEC proliferation. Quantitative RT-PCR showed that RANKL mRNA was induced by R5020 or HGF+R5020 and RANKL protein colocalized with PRs in LECs. The increased proliferation of MECs in response to HGF+R5020 could be blocked by neutralizing antibody to RANKL and reproduced by treatment with HGF plus exogenous RANKL in place of R5020. Neither R5020, nor exogenously administered RANKL increased proliferation of LECs. These results led us to conclude that RANKL, induced by progestin in PR-positive cells, is secreted and interacts with HGF to specifically increase proliferation of PR-negative MECs.

The Met tyrosine kinase receptor in development and cancer

Met is a tyrosine kinase receptor, encoded by an oncogene, whose crucial role has been elucidated during the last two decades. The complex biological program triggered by Met has been dissected and its biological relevance in both physiology and pathology has been proven. Met supports a morphogenetic program, known as invasive growth, taking place both during embryogenesis and adulthood. In tumors Met is often aberrantly activated, giving rise to the pathological counterpart of the invasive growth program: cancer progression towards metastasis. Several approaches have been recently developed to interfere with the tumorigenic and metastatic processes triggered by Met.

Discovery of a novel and potent series of thieno[3,2-b]pyridine-based inhibitors of c-Met and VEGFR2 tyrosine kinases

A series of thieno[3,2-b]pyridine-based inhibitors of c-Met and VEGFR2 tyrosine kinases is described. The compounds demonstrated potency with IC(50) values in the low nanomolar range in vitro while the lead compound also showed in vivo activity against various human tumor xenograft models in mice. Further exploration of this class of compounds is underway.

Deletion of the Met tyrosine kinase in liver progenitor oval cells increases sensitivity to apoptosis in vitro

The hepatocyte growth factor (HGF)/Met signaling system is essential for liver development, homeostasis, and function. In this study, we took advantage of a liver-specific, Met-conditional knockout mouse generated in our laboratory to address the molecular mechanisms of HGF/Met signaling in adult liver progenitor cell (oval cell) biology. For this purpose, we isolated oval cells from 3,5-diethoxycarbonyl-1,4-dihydro-collidine-treated Met(flx/flx) mice and established oval cell-derived cell lines that carried either functional (Met(flx/flx)) or a nonfunctional (Met(-/-)) met gene using virus-mediated Cre-loxP recombination. Oval cells lacking Met tyrosine kinase activity displayed neither Met phosphorylation nor activation of downstream targets and were refractory to HGF stimulation. Although Met(-/-) and Met(flx/flx) cells proliferated at similar rates under 10% serum, Met-deficient cells demonstrated decreased cell viability and were more prone to apoptosis when challenged with either serum starvation or the pro-apoptotic cytokine transforming growth factor-beta. Treatment with HGF reduced transforming growth factor-beta-mediated cell death in Met(flx/flx) but not Met(-/-) cells. Importantly, Met(flx/flx) and Met(-/-) cells both constitutively expressed hgf, and conditioned medium from serum-starved oval cells exhibited anti-apoptotic activity in Met(flx/flx) cells. Furthermore, serum-starved Met(flx/flx) cells showed persistent activation of the Met tyrosine kinase, suggesting HGF/Met autocrine regulation. In conclusion, these data reveal a critical, functional role for Met in oval cell survival through an autocrine mechanism.

Biochemical basis for the functional switch that regulates hepatocyte growth factor receptor tyrosine kinase activation

Ligand-induced dimerization of receptor tyrosine kinases (RTKs) modulates a system of linked biochemical reactions, sharply switching the RTK from a quiescent state to an active state that becomes phosphorylated and triggers intracellular signaling pathways. To improve our understanding of this molecular switch, we developed a quantitative model for hepatocyte growth factor receptor (c-MET) activation using parameters derived in large part from c-MET kinetic and thermodynamic experiments. Our model accurately produces the qualitative and quantitative dynamic features of c-MET phosphorylation observed in cells following ligand binding, including a rapid transient buildup of phosphorylated c-MET at high ligand concentrations. In addition, our model predicts a slow buildup of phosphorylated c-MET under conditions of reduced phosphatase activity and no extracellular agonist. Significantly, this predicted response is observed in cells treated with phosphatase inhibitors, further validating our model. Parameter sensitivity studies clearly show that synergistic oligomerization-dependent changes in c-MET kinetic, thermodynamic, and dephosphorylation properties result in the selective activation of the dimeric receptor, confirming that this model can be used to accurately evaluate the relative importance of linked biochemical reactions important for c-MET activation. Our model suggests that the functional differences observed between c-MET monomers and dimers may have incrementally evolved to optimize cell surface signaling responses.

Hepatocyte growth factor suppresses proinflammatory NFkappaB activation through GSK3beta inactivation in renal tubular epithelial cells

Activation of NFkappaB is a fundamental cellular event central to all inflammatory diseases. Hepatocyte growth factor (HGF) ameliorates both acute and chronic inflammation in a multitude of organ systems through modulating NFkappaB activity; nevertheless, the exact molecular mechanism remains uncertain. Here we report that HGF through inactivation of GSK3beta suppresses NFkappaB p65 phosphorylation specifically at position Ser-468. The Ser-468 of RelA/p65 situates in a GSK3beta consensus motif and could be directly phosphorylated by GSK3beta both in vivo and in vitro, signifying Ser-468 of RelA/p65 as a putative substrate for GSK3beta. In addition, the C terminus of RelA/p65 harbors a highly conserved domain homologue of the consensus docking sequence for GSK3beta. Moreover, this domain was required for efficient phosphorylation of Ser-468 and was indispensable for the physical interaction between RelA/p65 and GSK3beta. HGF substantially intercepted this interaction by inactivating GSK3beta. Functionally, phosphorylation of Ser-468 of RelA/p65 was required for the induced expression of a particular subset of proinflammatory NFkappaB-dependent genes. Diminished phosphorylation at Ser-468 by HGF resulted in a gene-specific inhibition of these genes' expression. The action of HGF on proinflammatory NFkappaB activation was consistently mimicked by a selective GSK3beta inhibitor or GSK3beta knockdown by RNA interference but largely abrogated in cells expressing the mutant uninhibitable GSK3beta. Collectively, our findings suggest that HGF has a potent suppressive effect on NFkappaB activation, which is mediated by GSK3beta, an important signaling transducer controlling RelA/p65 phosphorylation specificity and directing the transcription of selective proinflammatory cytokines implicated in inflammatory kidney disease.

Host-pathogen systems biology: logical modelling of hepatocyte growth factor and Helicobacter pylori induced c-Met signal transduction

Background

The hepatocyte growth factor (HGF) stimulates mitogenesis, motogenesis, and morphogenesis in a wide range of tissues, including epithelial cells, on binding to the receptor tyrosine kinase c-Met. Abnormal c-Met signalling contributes to tumour genesis, in particular to the development of invasive and metastatic phenotypes. The human microbial pathogen Helicobacter pylori can induce chronic gastritis, peptic ulceration and more rarely, gastric adenocarcinoma. The H. pylori effector protein cytotoxin associated gene A (CagA), which is translocated via a type IV secretion system (T4SS) into epithelial cells, intracellularly modulates the c-Met receptor and promotes cellular processes leading to cell scattering, which could contribute to the invasiveness of tumour cells. Using a logical modelling framework, the presented work aims at analysing the c-Met signal transduction network and how it is interfered by H. pylori infection, which might be of importance for tumour development.

Results

A logical model of HGF and H. pylori induced c-Met signal transduction is presented in this work. The formalism of logical interaction hypergraphs (LIH) was used to construct the network model. The molecular interactions included in the model were all assembled manually based on a careful meta-analysis of published experimental results. Our model reveals the differences and commonalities of the response of the network upon HGF and H. pylori induced c-Met signalling. As another important result, using the formalism of minimal intervention sets, phospholipase Cγ1 (PLCγ1) was identified as knockout target for repressing the activation of the extracellular signal regulated kinase 1/2 (ERK1/2), a signalling molecule directly linked to cell scattering in H. pylori infected cells. The model predicted only an effect on ERK1/2 for the H. pylori stimulus, but not for HGF treatment. This result could be confirmed experimentally in MDCK cells using a specific pharmacological inhibitor against PLCγ1. The in silico predictions for the knockout of two other network components were also verified experimentally.

Conclusion

This work represents one of the first approaches in the direction of host-pathogen systems biology aiming at deciphering signalling changes brought about by pathogenic bacteria. The suitability of our network model is demonstrated by an in silico prediction of a relevant target against pathogen infection.

Down-regulation of collagen and connective tissue growth factor expression with hepatocyte growth factor in lung fibroblasts from white scleroderma patients via two signaling pathways

OBJECTIVE

To study the mechanisms by which hepatocyte growth factor (HGF) down-regulates collagen and connective tissue growth factor (CTGF) in scleroderma (systemic sclerosis [SSc]) lung fibroblasts.

METHODS

CTGF, type I collagen, and IkappaBalpha expression, together with MAPK phosphorylation, were studied by immunoblotting of lung fibroblasts derived from white SSc patients. Matrix metalloproteinase 1 (MMP-1) expression in cell culture medium samples was measured by enzyme-linked immunosorbent assay, MMP-1 activity was studied using an MMP-1 assay, and NF-kappaB DNA binding activity was determined using a transcription factor assay.

RESULTS

In lung fibroblasts from white SSc patients, HGF activated MAPK (ERK-1/2) signaling pathways and MMP-1, while it inhibited NF-kappaB and significantly down-regulated CTGF and collagen in a time- and dose-dependent manner. Small interfering RNA (siRNA)-mediated depletion of Grb2 expression disrupted c-Met receptor downstream signaling, which resulted in diminished HGF-induced ERK-1/2 phosphorylation and the recovery of HGF-inhibited expression of MMP-1, NF-kappaB, collagen, and CTGF. The MAPK inhibitor, U0126, blocked MMP-1 activity and restored HGF-inhibited collagen and CTGF accumulation. Inhibition of MMP activity by MMP inhibitor GM1489 and inhibition of MMP-1 expression by siRNA did not prevent HGF-induced ERK-1/2 phosphorylation and NF-kappaB activity, but significantly restored HGF-inhibited collagen and CTGF accumulation. NF-kappaB inhibitor BAY 11-7082 did not interfere with MAPK phosphorylation or MMP-1 expression and activation, but significantly inhibited NF-kappaB DNA binding activity and acted synergistically with HGF to completely diminish the expression of CTGF.

CONCLUSION

In lung fibroblasts from white SSc patients, HGF down-regulates the accumulation of CTGF via MAPK/MMP-1 and NF-kappaB signaling pathways, whereas collagen down-regulation is mediated mainly by a MAPK/MMP-1-dependent pathway.

Loss of hepatocyte growth factor/c-Met signaling pathway accelerates early stages of N-nitrosodiethylamine induced hepatocarcinogenesis

Hepatocyte growth factor (HGF) has been reported to have both positive and negative effects on carcinogenesis. Here, we show that the loss of c-Met signaling in hepatocytes enhanced rather than suppressed the early stages of chemical hepatocarcinogenesis. c-Met conditional knockout mice (c-metfl/fl, AlbCre+/-; MetLivKO) treated with N-nitrosodiethylamine developed significantly more and bigger tumors and with a shorter latency compared with control (w/w, AlbCre+/-; Cre-Ctrl) mice. Accelerated tumor development was associated with increased rate of cell proliferation and prolonged activation of epidermal growth factor receptor (EGFR) signaling. MetLivKO livers treated with N-nitrosodiethylamine also displayed elevated lipid peroxidation, decreased ratio of reduced glutathione to oxidized glutathione, and up-regulation of superoxide dismutase 1 and heat shock protein 70, all consistent with increased oxidative stress. Likewise, gene expression profiling done at 3 and 5 months after N-nitrosodiethylamine treatment revealed up-regulation of genes associated with cell proliferation and stress responses in c-Met mutant livers. The negative effects of c-Met deficiency were reversed by chronic p.o. administration of antioxidant N-acetyl-L-cysteine. N-acetyl-L-cysteine blocked the EGFR activation and reduced the N-nitrosodiethylamine-initiated hepatocarcinogenesis to the levels of Cre-Ctrl mice. These results argue that intact HGF/c-Met signaling is essential for maintaining normal redox homeostasis in the liver and has tumor suppressor effect(s) during the early stages of N-nitrosodiethylamine-induced hepatocarcinogenesis.

c-Met must translocate to the nucleus to initiate calcium signals

Hepatocyte growth factor (HGF) is important for cell proliferation, differentiation, and related activities. HGF acts through its receptor c-Met, which activates downstream signaling pathways. HGF binds to c-Met at the plasma membrane, where it is generally believed that c-Met signaling is initiated. Here we report that c-Met rapidly translocates to the nucleus upon stimulation with HGF. Ca(2+) signals that are induced by HGF result from phosphatidylinositol 4,5-bisphosphate hydrolysis and inositol 1,4,5-trisphosphate formation within the nucleus rather than within the cytoplasm. Translocation of c-Met to the nucleus depends upon the adaptor protein Gab1 and importin beta1, and formation of Ca(2+) signals in turn depends upon this translocation. HGF may exert its particular effects on cells because it bypasses signaling pathways in the cytoplasm to directly activate signaling pathways in the nucleus.

Grb2-associated binder-1 plays a central role in the hepatocyte growth factor enhancement of hepatoma growth inhibition by K vitamin analog compound 5

Compound 5 (Cpd 5), a K vitamin analog, has been shown to inhibit Hep3B human hepatoma cell growth in cultures and rat hepatoma growth in vivo through prolonged epidermal growth factor receptor (EGFR)-extracellular response kinase (ERK) phosphorylation, and hepatocyte growth factor (HGF) synergizes with Cpd 5 to enhance the inhibition of Hep3B cell and rat hepatoma growth. To explore the mechanisms mediating the HGF/Cpd 5 synergy, we examined the possible involvement of the Grb2-associated binder-1 (Gab1) docking protein because it interacts with both EGFR and HGF receptor c-Met pathways. We found that HGF enhanced Cpd 5-induced c-Met phosphorylation at Tyr-1349, a binding site for Gab1, resulting in increased c-Met binding to Gab1, and induced strong and prolonged Gab1 tyrosine phosphorylation. Prolonged Gab1 phosphorylation by HGF/Cpd 5 in turn enhanced the ability of Gab1 to bind to protein tyrosine phosphatase SHP2 and enhanced the activation of its downstream mitogen-activated protein kinase pathway. In contrast, this same HGF/Cpd 5 treatment inhibited Gab1 binding to phosphatidylinositol 3-kinase (PI3K), leading to the inactivation of the PI3K-Akt pathway. The inhibition of Akt phosphorylation by HGF/Cpd 5 further activated the Raf-MEK-ERK signaling cascade via an Akt-Raf1 interaction, leading to strong and prolonged ERK phosphorylation. The transfection of Hep3B cells with mutated Gab1 (Gab1 Y627F), which had lost its ability to bind SHP2, antagonized HGF/Cpd 5-induced ERK phosphorylation, whereas the transfection of Hep3B cells with mutated Gab1 3YF, which lost its ability to bind PI3K, further enhanced HGF/Cpd 5-induced ERK phosphorylation and cell growth inhibition.

CONCLUSION

Gab1 plays a central role in regulating HGF/Cpd 5 synergy in their actions on Hep3B cell growth inhibition.

Administration of antisense DNA for hepatocyte growth factor causes an depressive and anxiogenic response in rats

Hepatocyte growth factor (HGF) is induced in neurons during ischemia and is neuroprotective against post-ischemic delayed neuronal death in the hippocampus. HGF might play an important role in the maturation and functioning of these neurons in the hippocampus. Our aim was to determine what effect HGF antisense has on depression and anxiety in rats. HGF antisense was infused at a constant rate into cerebral lateral ventricles and its effect on anxiety in rats was monitored. In forced swimming test, rats that received antisense DNA increased the length of time that they were immobile in the water. In the elevated plus maze test, the black and white box test and conditioned fear test, HGF antisense administration caused all indicators of anxiety to increase. Number of HGF-positive cells in C1 of hippocampus was significantly decreased in the HGF antisense-infused group compared to the vehicle- and scrambled oligonucleotide-treated group. No significant effect on general locomotor activity was seen. These results indicate that inhibition of HGF induces an increase in depression and anxiety-related behaviors suggesting a depressive and anxiogenic-like effect.

Mimp, a mitochondrial carrier homologue, inhibits Met-HGF/SF-induced scattering and tumorigenicity by altering Met-HGF/SF signaling pathways

We have recently shown that Mimp, a mitochondrial carrier protein homologue, is induced by Met-hepatocyte growth factor/scatter factor (HGF/SF) signaling and decreases the mitochondrial membrane potential in DA3 mammary adenocarcinoma cells. We show here that induction of Mimp leads to growth arrest in response to HGF/SF by arresting cells at the S phase of the cell cycle. Induction of Mimp or its transient expression does not lead to apoptosis. Mimp also attenuates HGF/SF-induced cellular scattering in vitro and tumor growth in vivo. The exogenous induction of Mimp at levels similar to its endogenous induction by HGF/SF increases the level of the Met protein and its phosphorylation by HGF/SF but reduces the levels of Shc and prevents the HGF/SF-induced tyrosine phosphorylation of Grb2 and Shc. In contrast, the level of phosphatidylinositol 3-kinase (PI3K) increases following Mimp induction and the level of phosphorylated PI3K in response to HGF/SF is unaffected by the exogenous induction of Mimp. Moreover, exogenous Mimp prevents the HGF/SF-induced transcription of the serum response element-luciferase reporter gene. Our results show that Mimp expression reduces Met-HGF/SF-induced proliferation and scattering by attenuating and altering the downstream signaling of Met. These data show a new link between a tyrosine kinase growth factor receptor and a mitochondrial carrier homologue that regulates cellular growth, motility, and tumorigenicity.

Effect of a c-Met-specific, ATP-competitive small-molecule inhibitor SU11274 on human ovarian carcinoma cell growth, motility, and invasion

Increased expression of the receptor tyrosine kinase c-Met has been shown to correlate with enhanced cell proliferation, motility, and invasion. The objectives of this study were to characterize total and activated c-Met expression in both normal and malignant human ovarian epithelial cells and to determine the effects of inhibiting the activation of c-Met on ovarian epithelial cell growth, motility, and invasion. Total c-Met was overexpressed in 82 (68%) of 119 ovarian carcinomas, as shown by immunohistochemistry. Quantitative reverse transcription-polymerase chain reaction and Western blot analyses revealed that ovarian carcinoma cell lines had higher levels of c-Met messenger RNA, total protein, and activated protein expression compared to normal ovarian epithelial cell cultures. Using a specific adenosine triphosphate-competitive small-molecule inhibitor, SU11274, activated c-Met was decreased in normal and ovarian carcinoma cell lines. 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assays showed that cell growth inhibition directly correlated to the level of activated c-Met detected in each cell line (r =-0.87, P = 0.012). Using modified Boyden chamber assays, ovarian carcinoma cells treated with SU11274 demonstrated significantly decreased cell motility and invasion compared to untreated cells (P = 0.003 and P < 0.001, respectively). These data indicate that c-Met is overexpressed in the majority of malignant ovarian epithelial cells both in vivo and in vitro and that decreasing activated c-Met in vitro can significantly decrease ovarian carcinoma cell growth, motility, and invasion. Developing therapies that specifically inhibit the activation of c-Met may represent a novel therapeutic modality for patients with ovarian carcinomas expressing high levels of c-Met.

Met, the hepatocyte growth factor receptor, localizes to the nucleus in cells at low density

Some breast cancer cases in our previous immunohistochemical studies show Met expression in the nucleus. Given nuclear localization of other receptor tyrosine kinases, we proceeded to investigate Met. Nuclear Met is seen in numerous cell lines and in germinal regions of many tissues using four unique antibodies. Cell fractionation reveals a 60-kDa band recognized by COOH-terminal Met antibodies that is present independent of hepatocyte growth factor treatment. Green fluorescent protein (GFP) fusion proteins of the cytoplasmic domain of Met transfected into HEK293 cells are found in the nucleus whereas the full-length Met-GFP fusion is membranous. Further deletions of the Met-GFP fusions identify a region of the juxtamembrane domain required for nuclear translocation. In a CaCo2 cell line model for epithelial maturation, we find that Met is initially nuclear, and then becomes membranous, after confluence. This work suggests processing of the Met receptor, analogous to ErbB4, resulting in the release of the cytoplasmic domain and its translocation to the nucleus in cells at low density.

Improving islet transplantation by gene delivery of hepatocyte growth factor (HGF) and its downstream target, protein kinase B (PKB)/Akt

Clinical studies have demonstrated that islet transplantation may be a useful procedure to replace beta cell function in patients with Type 1 diabetes. Islet transplantation faces many challenges, including complications associated with the procedure itself, the toxicity of immunosuppression regimens, and to the loss of islet function and insulin-independence with time. Despite the current successes, and residual challenges, these studies have pointed out an enormous scarcity of islet tissue that precludes the use of islet transplantation in a clinical setting on a wider scale. To address this problem, many research groups are trying to identify different islet growth factors and intracellular molecules capable of improving islet graft survival and function, therefore reducing the number of islets needed for successful transplantation. Among these growth factors, hepatocyte growth factor (HGF), a factor known to improve transplantation of a variety of organs/cells, has shown promising results in increasing islet graft survival and reducing the number of islets needed for successful transplantation in four different rodent models of islet transplantation. Protein kinase B (PKB)/Akt, a pro-survival intracellular signaling molecule is known to be activated in the beta cell by several different growth factors, including HGF. PKB/Akt has also shown promising results for improving human islet graft survival and function in a minimal islet mass model of islet transplantation in diabetic SCID mice. Increasing our knowledge on how HGF, PKB/Akt and other emerging molecules work for improving islet transplantation may provide substrate for future therapeutic approaches aimed at increasing the number of patients in which beta cell function can be successfully replaced.

Regulation of c-Met signaling by the tetraspanin KAI-1/CD82 affects cancer cell migration

It has been proposed that the metastasis suppressor CD82/KAI-1, which is a member of the tetraspanin superfamily, regulates biological activity by associating with cell surface receptors or proteins. We show a novel association between CD82 and the hepatocyte growth factor (HGF) receptor c-Met. Although ectopic expression of CD82 in nonsmall cell lung carcinoma cells did not affect the tyrosine phosphorylation of c-Met, these cells showed significant suppression of HGF-induced lamellipodial protrusion and cell migration. CD82 selectively attenuated c-Met signaling via the Ras-Cdc42/Rac and the phosphatidylinositol 3-kinase/Cdc42/Rac pathways. In contrast, another c-Met signaling pathway that involves phosphatidylinositol 3-kinase/Akt and phosphatidylinositol 3-kinase/mitogen activated protein kinase was not affected by CD82. Signaling adapter proteins for c-Met, such as Grb2 and p85, exhibited reduced association with c-Met in cells that ectopically expressed CD82. These results indicate that the CD82-c-Met complex inhibits HGF-induced cancer cell migration by the inactivation of small GTP-binding proteins of the Rho family via c-Met adapter proteins.

Hepatocyte growth factor receptor c-MET is associated with FAS and when activated enhances drug-induced apoptosis in pediatric B acute lymphoblastic leukemia with TEL-AML1 translocation

Expression of c-MET, the HGF (hepatocyte growth factor) tyrosine kinase receptor, was investigated in pediatric B-acute lymphoblastic leukemia (ALL) patients. c-MET was found to be expressed in normal B cells and in B-ALL patients with the t(12;21) TEL-AML1 translocation, but it is not expressed in the most part of B-ALL without the t(12;21). We also found that c-MET, related to proliferation and protection from apoptosis, is associated with the pro-apoptotic protein FAS in TEL-AML1 B-ALL cells and in normal B lymphocytes. The possible role of this protein complex in drug-induced apoptosis was thus investigated in REH TEL-AML1 B-ALL cell line. REH cells prestimulated with HGF and treated with doxorubicin had shown a higher apoptotic rate than non-HGF-prestimulated ones (p = 0.03). REH cells stimulated with IL-3 and treated with doxorubicin did not undergo apoptosis more than nonstimulated cells, demonstrating that increased proliferation in itself is not directly related to the higher apoptotic sensitivity observed with HGF stimulation. These results indicate that c-MET activation enhances specifically FAS-mediated apoptosis in TEL-AML1 ALL cells and, considering that the c-MET/FAS complex is present only in normal B lymphocytes and in TEL-AML1 leukemias, this implies that it may have an important contribution in cellular homeostasis and in high sensitivity of TEL-AML1 ALL to chemotherapeutic regimens.

Blocking Wnt/LRP5 signaling by a soluble receptor modulates the epithelial to mesenchymal transition and suppresses met and metalloproteinases in osteosarcoma Saos-2 cells

We previously reported the Wnt receptor low-density lipoprotein receptor-related protein 5 (LRP5) was frequently expressed in osteosarcoma (OS) tissue and correlated with metastasis and a lower disease-free survival. Subsequent in vitro analysis revealed that dominant-negative, soluble LRP5 (sLRP5) can reduce in vitro cellular invasion. In the current study, we examined the molecular mechanisms of blocking canonical Wnt signaling by sLRP5 in Saos-2 osteosarcoma cells. Transfection of sLRP5 caused a marked up-regulation of E-cadherin in this cell line. This increase in E-cadherin, seen primarily at the cell-cell contact borders, was associated with down-regulation of Slug and Twist, transcriptional repressors which mediate cancer invasion and metastasis. In contrast, N-cadherin, a mesenchymal marker, was reduced by sLRP5. In addition, blocking Wnt signaling by sLRP5 modulated other epithelial and mesenchymal markers (keratin 8 and 18, fibronectin), suggesting a reversal of epithelial-mesenchymal transition (EMT) seen during cancer progression. SLRP5 also reduced the expression of matrix metalloproteinase (MMP) 2 and 14, consistent with a decrease in invasive capacity. SLRP5 transfection decreased both Met expression and hepatocyte growth factor (HGF)-induced cell motility. Taken together, these results support a role for Wnt/LRP5 signaling in invasiveness of a subset of OS cells.

Signaling pathways involved in cyclooxygenase-2 induction by hepatocyte growth factor in non small-cell lung cancer

Many studies have suggested a role for the hepatocyte growth factor (HGF)/c-Met pathway in tumorigenesis. Some actions of HGF are believed to be mediated by cyclooxygenase-2 (COX-2), resulting in the production of prostaglandin E2 (PGE(2)). We examined four c-Met-positive non-small-cell lung cancer (NSCLC) cell lines for effects of HGF on COX-2. HGF increased COX-2 protein expression 3-fold over basal levels. Induction of COX-2 occurred through both the extracellular signal-regulated kinase 1/2 and p38 pathways. HGF treatment caused activation of the activator protein-1, CCAAT/enhancer-binding protein, and cAMP response element-binding protein transcription factors, and COX-2 induction was blocked by actinomycin D. The half-life of COX-2 mRNA was also increased by HGF. HGF stimulation resulted in a 4-fold increase in PGE(2) secretion, and treatment of NSCLC cells with exogenous PGE(2) significantly increased cell proliferation. The addition of PGE(2) to NSCLC cells also led to rapid phosphorylation of c-Met in the absence of HGF, which was blocked by epidermal growth factor receptor (EGFR) inhibition. EGFR ligands were released in response to PGE(2). This suggests that secretion of PGE(2) induced by HGF/c-Met pathway activation can further activate the c-Met pathway via EGFR in a reinforcing loop that is independent of HGF. HGF and PGE(2) each significantly stimulated invasion in NSCLC cells. Cells transiently transfected with c-Met antisense plasmid showed a significant decrease in HGF- or PGE(2)-induced invasion. PGE(2)-induced invasion was EGFR-dependent, confirming a link between PGE(2), EGFR, and c-Met. Targeting of both the HGF/c-Met and PGE(2) pathways with a neutralizing antibody to HGF and celecoxib resulted in enhanced anti-invasion effects in response to HGF.

Specific Grb2-mediated interactions regulate clathrin-dependent endocytosis of the cMet-tyrosine kinase

Lysosomal degradation of the receptor-tyrosine kinase cMet requires receptor ubiquitination by the E3 ubiquitin ligase Cbl followed by clathrin-dependent internalization. A role for Cbl as an adaptor for cMet internalization has been previously reported. However, the requirement for Cbl ubiquitin ligase activity in this process and its mode of recruitment to cMet has yet to be determined. Cbl can directly bind cMet at phosphotyrosine 1003 or indirectly via Grb2 to phosphotyrosine 1356 in the multisubstrate binding domain of cMet. The direct binding of Cbl with cMet is critical for receptor degradation and not receptor internalization. Here we show a strict requirement for Grb2 and the ubiquitin ligase activity of Cbl for cMet endocytosis. Receptor internalization was impaired by small interfering RNA depletion of Grb2, overexpression of dominant negative Grb2 mutants, and point mutations in the cMet multisubstrate docking site that inhibits the direct association of Grb2 with cMet. The requirement for Grb2 was specific and did not involve the multiadaptor Gab1. cMet internalization was impaired in cells expressing an ubiquitin ligase-deficient Cbl mutant or conjugation-deficient ubiquitin but was unaffected in cells expressing a Cbl mutant that is unable to bind cMet directly. Expression of a Cbl-Grb2 chimera rescued impaired cMet endocytosis in cells depleted of endogenous Grb2. These results indicate that the ubiquitin ligase activity of Cbl is critical for clathrin-dependent cMet internalization and suggest a role for Grb2 as an intermediary linking Cbl ubiquitin ligase activity to this process.

c-Met and hepatocyte growth factor: Potential as novel targets in cancer therapy

Receptor tyrosine kinases have come to fruition as therapeutic targets in a variety of malignancies. In this group of targets, the c-Met receptor tyrosine kinase plays an important role in increased cell growth, reduced apoptosis, altered cytoskeletal function, increased metastasis, and other biologic changes. The ligand for c-Met is hepatocyte growth factor (HGF), also known as scatter factor. Met is overexpressed and mutated in a variety of malignancies, among which germline mutations are of particular interest. Most mutations of Met have been found in the juxtamembrane, the tyrosine kinase, and the semaphorin domain. Met gain-of-function mutations lead to deregulated or prolonged tyrosine kinase activity, which is instrumental to its transforming activity. This review summarizes the biologic functions regulated by Met and its structural requirements as well as related developments in targeted therapy. Treatment approaches, including antagonism of HGF binding to Met, targeting of RNA and the Met protein, and inhibition of the tyrosine kinase domain of Met, are highlighted. Targeting of the HGF/Met pathway, alone or in combination with standard therapies, is likely to improve current therapies in Met-dependent malignancies.

HIC gene, a candidate suppressor gene within a minimal region of loss at 7q31.1 in myeloid neoplasms

We studied monosomy and deletions of chromosome 7 in 170 patients with myeloid disorders and we identified a minimal region of loss in 7q31.1 spanning between the D7S2554 and D7S2460 markers. The closest gene to our most deleted microsatellite, D7S2554, is the human I-mfa domain containing (HIC) gene, alias MyoD family inhibitor domain containing (MDFIC). We investigated the involvement of HIC in myeloid neoplasms by screening for mutations the coding regions and the intron-exon boundaries of this gene in 15 patients who presented chromosome 7 deletions in the region of HIC. No mutations were found in the coding region of this gene.

Identification of the genes for kidney cancer: opportunity for disease-specific targeted therapeutics

Recent advances in understanding the kidney cancer gene pathways has provided the foundation for the development of targeted therapeutic approaches for patients with this disease. Kidney cancer is not a single disease; it includes a number of different types of renal cancers, each with different histologic features, a different clinical course, a different response to therapy, and different genes causing the defects. Most of what is known about the genetic basis of kidney cancer has been learned from study of the inherited forms of kidney cancer: von Hippel Lindau (VHL gene), hereditary papillary renal carcinoma (c-Met gene), Birt Hogg Dubé (BHD gene), and hereditary leiomyomatosis renal cell cancer (fumarate hydratase gene). These Mendelian single-gene syndromes provide a unique opportunity to evaluate the effectiveness of agents that target the VHL, c-Met, BHD, and fumarate hydratase pathways.

Role of carbonic anhydrases in the progression of renal cell carcinoma subtypes: proposal of a unified hypothesis

Renal cell carcinoma (RCC) has the highest rate of occurrence within the US when compared with other countries. Recent advances in the basic research and molecular diagnostics of this malignancy have revealed that RCC is not a single disease, but it is a mixture of several types of malignancies with unique molecular mechanisms and pathological attributes. RCC is now divided into clear cell carcinoma (80% of all kidney cancers), papillary type 1 and papillary type 2 neoplasms (10-15% of all RCC patients) and RCC with chromophobic and oncocytic features, called the Birt-Hogg-Dube (BHD) subtype, in roughly 5% of all patients. Apart from these, neoplasms such as the tuberous sclerosis (TSC) syndrome may occur with a mixed pathological features with a renal presentation. In this review, molecular evidence, both direct and indirect, published so far on all these RCC subtypes have been analyzed to find out whether there is any common thread that could run through these disparate malignancies that happen to occur in a single organ, i.e., the kidney. We believe that the role played by the expression and certain non-traditional activities of the cabonic anhydrase (CA) family members, along with the differing levels of hypoxia induced within these tumors may be the most common denominators. Evidence is presented focusing on how the CA family members could participate in the genesis and progression of each and every one of these RCC subtypes and how their function could be influenced by hypoxia, activities of receptor type protein tyrosine kinases and certain other pre-disposing factors. These rationalizations point towards a unified hypothesis that may help explain the occurrence of all these RCC subtypes in a molecular manner. We hope that these analyses would a) stimulate further studies aimed toward a better understanding of the role played by carbonic anhydrases in RCC subtypes and b) would pave way to a better and rationally designed therapies to interfere with their function to benefit patients with RCC and possibly other cancers.

Hepatocyte growth factor induces glucose uptake in 3T3-L1 adipocytes through A Gab1/phosphatidylinositol 3-kinase/Glut4 pathway

Adipose tissue is a source of hepatocyte growth factor (HGF), and circulating HGF levels have been associated with elevated body mass index in human. However, the effects of HGF on adipocyte functions have not yet been investigated. We show here that in 3T3-L1 adipocytes HGF stimulates the phosphatidylinositol (PI) 3-kinase-dependent protein kinase B (PKB) activity, AS160 phosphorylation, Glut4 translocation, and consequently, glucose uptake. The initial steps involved in HGF- and insulin-induced glucose uptake are different. HGF enhanced the tyrosine phosphorylation of Gab1, leading to the recruitment of the p85-regulated subunit of PI 3-kinase, whereas p85 was exclusively recruited by IRS1 in response to insulin. In adipocytes rendered insulin-resistant by a long-lasting tumor necrosis factor alpha treatment, the protein level of Gab1 was strongly decreased, and HGF-stimulated PKB activation and glucose uptake were also altered. Moreover, treatment of 3T3-L1 adipocytes with thiazolidinedione, an anti-diabetic drug, enhanced the expression of both HGF and its receptor. These data provide the first evidence that in vitro HGF promotes glucose uptake through a Gab1/PI 3-kinase/PKB/AS160 pathway which was altered in tumor necrosis factor alpha-treated adipocytes.

The C-terminal SH3 domain of CRKL as a dynamic dimerization module transiently exposing a nuclear export signal

CRKL plays essential roles in cell signaling. It consists of an N-terminal SH2 domain followed by two SH3 domains. SH2 and SH3N bind to signaling proteins, but the function of the SH3C domain has remained largely enigmatic. We show here that the SH3C of CRKL forms homodimers in protein crystals and in solution. Evidence for dimer formation of full-length CRKL is also presented. In the SH3C dimer, a nuclear export signal (NES) is mostly buried under the domain surface. The same is true for a monomeric SH3C obtained under different crystallization conditions. Interestingly, partial SH3 unfolding, such as occurs upon dimer/monomer transition, produces a fully-accessible NES through translocation of a single beta strand. Our results document the existence of an SH3 domain dimer formed through exchange of the first SH3 domain beta strand and suggest that partial unfolding of the SH3C is important for the relay of information in vivo.

TFE3 fusions activate MET signaling by transcriptional up-regulation, defining another class of tumors as candidates for therapeutic MET inhibition

Specific chromosomal translocations encoding chimeric transcription factors are considered to play crucial oncogenic roles in a variety of human cancers but the fusion proteins themselves seldom represent suitable therapeutic targets. Oncogenic TFE3 fusion proteins define a subset of pediatric renal adenocarcinomas and one fusion (ASPL-TFE3) is also characteristic of alveolar soft part sarcoma (ASPS). By expression profiling, we identified the MET receptor tyrosine kinase gene as significantly overexpressed in ASPS relative to four other types of primitive sarcomas. We therefore examined MET as a direct transcriptional target of ASPL-TFE3. ASPL-TFE3 binds to the MET promoter and strongly activates it. Likewise, PSF-TFE3 and NONO-TFE3 also bind this promoter. Induction of MET by ASPL-TFE3 results in strong MET autophosphorylation and activation of downstream signaling in the presence of hepatocyte growth factor (HGF). In cancer cell lines containing endogenous TFE3 fusion proteins, inhibiting MET by RNA interference or by the inhibitor PHA665752 abolishes HGF-dependent MET activation, causing decreased cell growth and loss of HGF-dependent phenotypes. MET is thus a potential therapeutic target in these cancers. Aberrant transcriptional up-regulation of MET by oncogenic TFE3 fusion proteins represents another mechanism by which certain cancers become dependent on MET signaling. The identification of kinase signaling pathways transcriptionally up-regulated by oncogenic fusion proteins may reveal more accessible therapeutic targets in this class of human cancers.

c-Met ectodomain shedding rate correlates with malignant potential

PURPOSE

Many proteins are proteolytically released from the cell surface by a process known as ectodomain shedding. Shedding occurs under normal physiologic conditions and can be increased in certain pathologies. Among the many receptors for which ectodomain shedding has been shown is c-Met, the hepatocyte growth factor (HGF) receptor tyrosine kinase. HGF stimulates mitogenesis, motogenesis, and morphogenesis in a variety of cellular targets during development, homeostasis, and tissue regeneration. Inappropriate HGF signaling resulting in unregulated cell proliferation, motility, and invasion occurs in several human malignancies. This can occur through paracrine signaling, autocrine loop formation, receptor mutation, gene amplification, or gene rearrangement, accompanied frequently with overexpression of ligand and/or receptor proteins. We hypothesized that c-Met overexpression in cancer might result in increased ectodomain shedding, and that its measure could be a useful biomarker of tumor progression.

EXPERIMENTAL DESIGN

We developed a sensitive electrochemiluminescent immunoassay to quantitate c-Met protein in cell lysates, culture supernatants, and biological samples.

RESULTS

A survey of cultured cell models of oncogenic transformation revealed significant direct correlations (P < 0.001, t test or ANOVA) between malignant potential and the rate of c-Met ectodomain shedding that was independent of steady-state receptor expression level. Moreover, weekly plasma and urine samples from mice harboring s.c. human tumor xenografts (n = 4 per group) displayed soluble human c-Met levels that were measurable before tumors became palpable and that correlated directly with tumor volume (R2 > 0.92, linear regression).

CONCLUSIONS

For a variety of human cancers, c-Met ectodomain shedding may provide a reliable and practical indicator of malignant potential and overall tumor burden.

RNA interference: implications for cancer treatment

RNA interference (RNAi) has emerged as one of the most important discoveries of the last years in the field of molecular biology. Following clarification of this highly conserved endogenous gene silencing mechanism, RNAi has largely been exploited as a powerful tool to uncover the function of specific genes and to understand the effects of selective gene silencing in mammalian cells both in vitro and in vivo. RNAi can be induced by direct introduction of chemically synthesized siRNAs into the cell or by the use of plasmid and viral vectors encoding for siRNA allowing a more stable RNA knockdown. Potential application of this technique both as a research tool and for therapeutic purposes has led to an extensive effort to overcome some critical constraints which may limit its successful application in vivo, including off-target and non-specific effects, as well as the relatively poor stability of siRNA. This review provides a brief overview of the RNAi mechanism and of its application in preclinical animal models of cancer.

Beta4 integrin activates a Shp2-Src signaling pathway that sustains HGF-induced anchorage-independent growth

Despite being a cell–matrix adhesion molecule, β4 integrin can prompt the multiplication of neoplastic cells dislodged from their substrates (anchorage-independent growth). However, the molecular events underlying this atypical behavior remain partly unexplored. We found that activation of the Met receptor for hepatocyte growth factor results in the tyrosine phosphorylation of β4, which is instrumental for integrin-mediated recruitment of the tyrosine phosphatase Shp2. Shp2 binding to β4 enhances the activation of Src, which, in turn, phosphorylates the multiadaptor Gab1 predominantly on consensus sites for Grb2 association, leading to privileged stimulation of the Ras–extracellular signal-regulated kinase (ERK) cascade. This signaling axis can be inhibited by small interfering RNA–mediated β4 depletion, by a β4 mutant unable to bind Shp2, and by pharmacological and genetic inhibition of Shp2 or Src. Preservation of the β4 docking sites for Shp2 as well as the integrity of Shp2, Src, or ERK activity are required for the β4-mediated induction of anchorage-independent growth. These results unravel a novel pathway whereby β4 directs tyrosine kinase–based signals toward adhesion-unrelated outcomes.

Inhibition of c-Met and prevention of spontaneous metastatic spreading by the 2-indolinone RPI-1

Hepatocyte growth factor (HGF) and its tyrosine kinase receptor Met play a pivotal role in the tumor metastatic phenotype and represent attractive therapeutic targets. We investigated the biochemical and biological effects of the tyrosine kinase inhibitor RPI-1 on the human lung cancer cell lines H460 and N592, which express constitutively active Met. RPI-1-treated cells showed down-regulation of Met activation and expression, inhibition of HGF/Met-dependent downstream signaling involving AKT, signal transducers and activators of transcription 3 and paxillin, as well as a reduced expression of the proangiogenic factors vascular endothelial growth factor and basic fibroblast growth factor. Cell growth in soft agar of H460 cells was strongly reduced in the presence of the drug. Furthermore, RPI-1 inhibited both spontaneous and HGF-induced motility/invasiveness of both H460 and human endothelial cells. Targeting of Met signaling by alternative methods (Met small interfering RNA and anti-phosphorylated Met antibody intracellular transfer) produced comparable biochemical and biological effects. Using the spontaneously metastasizing lung carcinoma xenograft H460, daily oral treatment with well-tolerated doses of RPI-1 produced a significant reduction of spontaneous lung metastases (-75%; P < 0.001, compared with control mice). In addition, a significant inhibition of angiogenesis in primary s.c. tumors of treated mice was observed, possibly contributing to limit the development of metastases. The results provide preclinical evidence in support of Met targeting pharmacologic approach as a new option for the control of tumor metastatic dissemination.

Targeting the c-Met signaling pathway in cancer

On binding to the cell surface receptor tyrosine kinase (TK) known as c-Met, hepatocyte growth factor (HGF) stimulates mitogenesis, motogenesis, and morphogenesis in a wide range of cellular targets including, epithelial and endothelial cells, hematopoietic cells, neurons, melanocytes, and hepatocytes. These pleiotropic actions are fundamentally important during development, homeostasis, and tissue regeneration. HGF signaling also contributes to oncogenesis and tumor progression in several human cancers and promotes aggressive cellular invasiveness that is strongly linked to tumor metastasis. Our present understanding of c-Met oncogenic signaling supports at least three avenues of pathway selective anticancer drug development: antagonism of ligand/receptor interaction, inhibition of TK catalytic activity, and blockade of intracellular receptor/effector interactions. Potent and selective preclinical drug candidates have been developed using all three strategies, and human clinical trials in two of the three areas are now under way.

Molecular mechanism of adaphostin-mediated G1 arrest in prostate cancer (PC-3) cells: signaling events mediated by hepatocyte growth factor receptor, c-Met, and p38 MAPK pathways

Adaphostin (NSC680410), a small molecule congener of tyrphostin AG957, has been demonstrated previously to have significant anti-proliferative effects in several leukemia models. However, this effect of adaphostin in adherent cells/solid tumor models has not been examined. In this study, we investigated the anti-proliferative effects of adaphostin in the human prostate cancer cell line PC-3. Specifically, we explored the potential molecular mechanism(s) by which adaphostin elicits its anti-proliferative effect(s). We demonstrate that adaphostin inhibits the proliferation of PC-3 cells by inducing a G(1) phase cell cycle arrest. This adaphostin-induced G(1) arrest was associated with an increase in the expression of p21 and p27 and a decrease in the expression of G(1)-specific cyclins (cyclin A, D1, and D3) and cyclin-dependent kinases 4 and 6. Consequently, a dramatic decrease in the phosphorylation of retinoblastoma protein was also observed. Additionally, we found that adaphostin treatment induced a decrease in the phosphorylation of nucleophosmin, a major nuclear phosphoprotein, and that this decreased phosphorylation was a result of the p21- and p27-mediated inactivation of cyclin E-cyclin-dependent kinase 2 complex kinase activity. Furthermore, we have determined that the adaphostin-mediated cell cycle arrest of PC-3 cells is dependent upon activation of the p38 MAPK. We also demonstrate that the hepatocyte growth factor receptor-c-Met is involved in the adaphostin-mediated signaling events that regulate p38 MAPK. Taken together, these results identify for the first time a signaling cascade of adaphostin-mediated G(1) phase-specific cell cycle arrest in PC-3 cells. These findings suggest that the tyrphostin member has a broader spectrum of activity than originally predicted.

Adaptor molecule Crk is required for sustained phosphorylation of Grb2-associated binder 1 and hepatocyte growth factor-induced cell motility of human synovial sarcoma cell lines

Activation of the c-Met receptor tyrosine kinase through its ligand, hepatocyte growth factor (HGF), promotes mitogenic, motogenic, and morphogenic cellular responses. Aberrant HGF/c-Met signaling has been strongly implicated in tumor cell invasion and metastasis. Both HGF and its receptor c-Met have been shown to be overexpressed in human synovial sarcoma, which often metastasizes to the lung; however, little is known about HGF-mediated biological effects in this sarcoma. Here, we provide evidence that Crk adaptor protein is required for the sustained phosphorylation of c-Met-docking protein Grb2-associated binder 1 (Gab1) in response to HGF, leading to the enhanced cell motility of human synovial sarcoma cell lines SYO-1, HS-SY-II, and Fuji. HGF stimulation induced the sustained phosphorylation on Y307 of Gab1 where Crk was recruited. Crk knockdown by RNA interference disturbed this HGF-induced tyrosine phosphorylation of Gab1. By mutational analysis, we identified that Src homology 2 domain of Crk is indispensable for the induction of the phosphorylation on multiple Tyr-X-X-Pro motifs containing Y307 in Gab1. HGF remarkably stimulated cell motility and scattering of synovial sarcoma cell lines, consistent with the prominent activation of Rac1, extreme filopodia formation, and membrane ruffling. Importantly, the elimination of Crk in these cells induced the disorganization of actin cytoskeleton and complete abolishment of HGF-mediated Rac1 activation and cell motility. Time-lapse microscopic analysis revealed the significant attenuation in scattering of Crk knockdown cells following HGF treatment. Furthermore, the depletion of Crk remarkably inhibited the tumor formation and its invasive growth in vivo. These results suggest that the sustained phosphorylation of Gab1 through Crk in response to HGF contributes to the prominent activation of Rac1 leading to enhanced cell motility, scattering, and cell invasion, which may support the crucial role of Crk in the aggressiveness of human synovial sarcoma.

Hepatocyte growth factor induces cell scattering through MAPK/Egr-1-mediated upregulation of Snail

Hepatocyte growth factor/scatter factor (HGF) exerts several functions in physiological and pathological processes, among them the induction of epithelial cell scattering and motility. Its pivotal role in angiogenesis, tumor progression, and metastasis is evident; however, the underlying molecular mechanisms are still poorly understood. Here, we demonstrate that HGF induces scattering of epithelial cells by upregulating the expression of Snail, a transcriptional repressor involved in epithelial-mesenchymal transition (EMT). Snail is required for HGF-induced cell scattering, since shRNA-mediated ablation of Snail expression prevents this process. HGF-induced upregulation of Snail transcription involves activation of the mitogen-activated protein kinase (MAPK) pathway and requires the activity of early growth response factor-1 (Egr-1). Upon induction by Egr-1, Snail represses the expression of E-cadherin and claudin-3 genes. It also binds to the Egr-1 promoter and represses Egr-1 transcription, thereby establishing a negative regulatory feedback loop. These findings indicate that Snail upregulation by HGF is mediated via the MAPK/Egr-1 signaling pathway and that both Snail and Egr-1 play a critical role in HGF-induced cell scattering, migration, and invasion.

Met-regulated expression signature defines a subset of human hepatocellular carcinomas with poor prognosis and aggressive phenotype

Identification of specific gene expression signatures characteristic of oncogenic pathways is an important step toward molecular classification of human malignancies. Aberrant activation of the Met signaling pathway is frequently associated with tumor progression and metastasis. In this study, we defined the Met-dependent gene expression signature using global gene expression profiling of WT and Met-deficient primary mouse hepatocytes. Newly identified transcriptional targets of the Met pathway included genes involved in the regulation of oxidative stress responses as well as cell motility, cytoskeletal organization, and angiogenesis. To assess the importance of a Met-regulated gene expression signature, a comparative functional genomic approach was applied to 242 human hepatocellular carcinomas (HCCs) and 7 metastatic liver lesions. Cluster analysis revealed that a subset of human HCCs and all liver metastases shared the Met-induced expression signature. Furthermore, the presence of the Met signature showed significant correlation with increased vascular invasion rate and microvessel density as well as with decreased mean survival time of HCC patients. We conclude that the genetically defined gene expression signatures in combination with comparative functional genomics constitute an attractive paradigm for defining both the function of oncogenic pathways and the clinically relevant subgroups of human cancers.

Oncogenic Met receptor induces ectopic structures in Xenopus embryos

When aberrantly expressed or activated, the Met receptor tyrosine kinase is involved in tumor invasiveness and metastasis. In this study, we have used the Xenopus embryonic system to define the role of various Met proximal-binding partners and downstream signaling pathways in regulating an induced morphogenetic event. We show that expression of an oncogenic derivative of the Met receptor (Tpr-Met) induces ectopic morphogenetic structures during Xenopus embryogenesis. Using variant forms of Tpr-Met that are engineered to recruit a specific signaling molecule of choice, we demonstrate that the sole recruitment of either the Grb2 or the Shc adaptor protein is sufficient to induce ectopic structures and anterior reduction, while the recruitment of PI-3Kinase (PI-3K) is necessary but not sufficient for this effect. In contrast, the recruitment of PLCgamma can initiate the induction, but fails to maintain or elongate supernumerary structures. Finally, evidence indicates that the Ras/Raf/MAPK pathway is necessary, but not sufficient to induce these structures. This study also emphasizes the importance of examining signaling molecules in the regulatory context that is provided by receptor/effector interactions when assessing a role in cell growth and differentiation.

Inhibitory effect of luteolin on hepatocyte growth factor/scatter factor-induced HepG2 cell invasion involving both MAPK/ERKs and PI3K–Akt pathways

Hepatocyte growth factor (HGF), also known as scatter factor (SF), and its receptor, the c-Met tyrosine kinase, play roles in cancer invasion and metastasis in a wide variety of tumor cells. Clinical observations suggest that HGF can promote metastasis of hepatoma cells while stimulating tumor invasiveness. We use HGF as an invasive inducer of human hepatoma HepG2 cells to investigate the effect of flavonoids on anti-invasion. In our preliminary study, we investigated the effect of flavonoids including luteolin, quercetin, baicalein, genistein, taxifolin and catechin on HGF-mediated migration and invasion of HepG2 cells. We found that luteolin presented the most potent potential on anti-migration and anti-invasion by Boyden chamber assay. Furthermore, luteolin inhibited HGF-induced cell scattering and cytoskeleton change such as filopodia and lamellipodia was determined by both phase-contrast and fluorescence microscopy studies. In addition, Western blotting and immunoprecipitation were performed to confirm luteolin suppressed the phosphorylation of c-Met, the membrane receptor of HGF, as well as ERK1/2 and Akt, but not JNK1/2, which is activated by HGF. Our investigation demonstrated that luteolin similar to PD98059, which acts as a specific inhibitor of MEK, an up stream kinase regulating ERK1/2, and wortmannin, a PI3K inhibitor, inhibited the invasiveness induced by HGF. In conclusion, the luteolin inhibited HGF-induced HepG2 cell invasion involving both MAPK/ERKs and PI3K-Akt pathways.

Molecular biology of malignant mesothelioma: a review

Malignant mesothelioma (MM) is an uncommon tumor with high mortality and morbidity rates. It arises from mesothelial cells that line the pleural, pericardial, peritoneal, and testicular cavities. This is a disease with an indolent course because tumors arise 20 to 40 years after exposure to an inciting agent. Extensive research has shown that mesothelial cells are transformed into MM cells through various chromosomal and cellular pathway defects. These changes alter the normal cells' ability to survive, proliferate, and metastasize. This article discusses the alterations that occur in transforming normal mesothelial cells into MM. It also details some of the signal transduction pathways that seem to be important in MM with the potential for novel targeted therapeutics.

Molecular targeting of growth factor receptor-bound 2 (Grb2) as an anti-cancer strategy

Growth factor receptor-bound 2 (Grb2) is a ubiquitously expressed adapter protein that provides a critical link between cell surface growth factor receptors and the Ras signaling pathway. As such, it has been implicated in the oncogenesis of several important human malignancies. In addition to this function, research over the last decade has revealed other fundamental roles for Grb2 in cell motility and angiogenesis--processes that also contribute to tumor growth, invasiveness and metastasis. This functional profile makes Grb2 a high priority target for anti-cancer drug development. Knowledge of Grb2 protein structure, its component Src homology domains and their respective structure-function relationships has facilitated the rapid development of sophisticated drug candidates that can penetrate cells, bind Grb2 with high affinity and potently antagonize Grb2 signaling. These novel compounds offer considerable promise in our growing arsenal of rationally designed anti-cancer therapeutics.

Combined signaling through ERK, PI3K/AKT, and RAC1/p38 is required for met-triggered cortical neuron migration

Cell migration is a complex biological process playing a key role in physiological and pathological conditions. During central nervous system development, positioning and function of cortical neurons is tightly regulated by cell migration. Recently, signaling events involving the urokinase-type plasminogen activator receptor, which is a key regulator for the activation of hepatocyte growth factor (HGF), have been implicated in modulating cortical neuron migration. However, the intracellular pathways controlling neuronal migration triggered by the HGF receptor Met have not been elucidated. By combining pharmacological and genetic approaches, we show here that the Ras/ERK pathway and phosphatidylinositol 3-kinase (PI3K) are both required for cortical neuron migration. By dissecting the downstream signals necessary for this event, we found that Rac1/p38 and Akt are required, whereas the c-Jun N-terminal kinase (JNK) and mTOR/p70(s6k) pathways are dispensable. This study demonstrates that concomitant activation of the Ras/ERK, PI3K/Akt, and Rac1/p38 pathways is required to achieve full capacity of cortical neurons to migrate upon HGF stimulation.

Crystal Structures of a High-affinity Macrocyclic Peptide Mimetic in Complex with the Grb2 SH2 Domain

The high-affinity binding of the growth factor receptor-bound protein 2 (Grb2) SH2 domain to tyrosine-phosphorylated cytosolic domains of receptor tyrosine kinases (RTKs) is an attractive target for therapeutic intervention in many types of cancer. We report here two crystal forms of a complex between the Grb2 SH2 domain and a potent non-phosphorus-containing macrocyclic peptide mimetic that exhibits significant anti-proliferative effects against erbB-2-dependent breast cancers. This agent represents a "second generation" inhibitor with greatly improved binding affinity and bio-availability compared to its open-chain counterpart. The structures were determined at 2.0A and 1.8A with one and two domain-swapped dimers per asymmetric unit, respectively. The mode of binding and specific interactions between the protein and the inhibitor provide insight into the high potency of this class of macrocylic compounds and may aid in further optimization as part of the iterative rational drug design process.

Agonist Met antibodies define the signalling threshold required for a full mitogenic and invasive program of Kaposi's Sarcoma cells

We previously showed that the Kaposi Sarcoma line KS-IMM express a functional Met tyrosine kinase receptor, which, upon HGF stimulation, activates motogenic, proliferative, and invasive responses. In this study, we investigated the signalling pathways activated by HGF, as well as by Met monoclonal antibodies (Mabs), acting as full or partial agonists. The full agonist Mab mimics HGF in all biological and biochemical aspects. It elicits the whole spectrum of responses, while the partial agonist Mab induces only wound healing. These differences correlated with a more prolonged and sustained tyrosine phosphorylation of the receptor and MAPK evoked by HGF and by the full agonist Mab, relative to the partial agonist Mab. Since Gab1, JNK and PI 3-kinase are activated with same intensity and kinetics by HGF and by the two agonist antibodies, it is concluded that level and duration of MAPK activation by Met receptor are crucial for the induction of a full HGF-dependent mitogenic and invasive program in KS cells.

Altered exon usage in the juxtamembrane domain of mouse and human RON regulates receptor activity and signaling specificity

Alternative splicing of signaling proteins can contribute to the complexity of signaling networks. We find that expression of mouse RON, but not human RON, results in constitutive receptor autophosphorylation, ligand-independent activation of the mitogen-activated protein kinase pathway, and association of the receptor with c-Src. Using chimeric receptors, we mapped the region for this difference in signaling capacity of mouse and human RON to the juxtamembrane domain. Expression of these receptors in primary erythroid progenitor cells also demonstrated a functional difference in the ability of mouse and human RON to support erythropoietin-independent colony formation that mapped to the juxtamembrane domain. Splicing of the mouse RON receptor tyrosine kinase transcript results in the constitutive deletion of an exon used by all other known RON orthologs that encodes part of the juxtamembrane domain of the receptor. Mutational analysis indicated that the two tyrosines present in this region in human RON, one of which has been previously shown to be a c-Cbl binding site, are not responsible for this difference. However, deletion of this region in the context of human RON enhanced receptor phosphorylation, activation of mitogen-activated protein kinase, and association of c-Src at levels comparable with those observed with mouse RON. These data provide direct evidence that the divergence of exon usage among different species can generate a protein with novel activity and subsequently add to the complexity of cellular signaling regulation.

The Listeria protein internalin B mimics hepatocyte growth factor-induced receptor trafficking

Increased hepatocyte growth factor receptor (HGFR) signaling correlates closely with neoplastic invasion and metastatic potential of many human cancers. Hepatocyte growth factor receptor signaling is initiated by binding the physiological ligand HGF or the internalin B (InlB) protein of Listeria monocytogenes. Subsequent degradation of endocytosed HGFR terminates receptor signaling. Previously reported discrepancies in InlB and HGF-induced HGFR signaling could reflect differences in receptor internalization and degradation in response to these distinct ligands. We report that soluble InlB and HGF are mechanistically equivalent in triggering clathrin-dependent endocytosis and lysosomal degradation of HGFR. After internalization, InlB and HGF colocalize with Rab5, EEA1 and the transferrin receptor in classical early endosomes. Hepatocyte growth factor receptor internalization was prevented by overexpression of dominant negative mutants of dynamin 1 and epidermal growth factor phosphorylation substrate 15, but not caveolin 1, the GTPase Arf6 or the cholesterol-chelating drug Nystatin. Thus, HGFR internalization is principally clathrin-mediated and is not regulated by clathrin- independent pathways. Phosphatidylinositol 3-kinase signaling and HGF-regulated tyrosine kinase substrate were not required for ligand-triggered internalization of HGFR but were essential for subsequent lysosomal degradation. Thus, soluble InlB and HGF induce HGFR endocytosis and degradation by indistinguishable mechanisms, suggesting that InlB may be exploited to regulate pathogenic HGFR signaling.

Hepatocyte Growth Factor Induces Redistribution of p21CIP1 and p27KIP1 through ERK-dependent p16INK4a Up-regulation, Leading to Cell Cycle Arrest at G1 in HepG2 Hepatoma Cells

Hepatocyte growth factor (HGF) has an anti-proliferative effect on many types of tumor cell lines and tumors in vivo. We found previously that inhibition of HGF-induced proliferation in HepG2 hepatoma cells is caused by cell cycle arrest at G1 through a high intensity ERK signal, which represses Cdk2 activity. To examine further the mechanisms of G1 arrest by HGF, we analyzed the Cdk inhibitor p16(INK4a), which has an anti-proliferative function through cell cycle arrest at G1. We found that HGF treatment drastically increased endogenous p16 levels. Knockdown of p16 with small interfering RNA reversed the arrest, indicating that the induction of p16 is required for G1 arrest by HGF. Analysis of the promoter of the human p16 gene identified the proximal Ets-binding site as a responsive element for HGF, and this responded to the high intensity ERK signal. HGF treatment of the cells led to a redistribution of p21(CIP1) and p27(KIP1) from Cdk4 to Cdk2. The redistribution was blocked by the knockdown of p16 with small interfering RNA, which restored the Cdk2 activity repressed by HGF, demonstrating the requirement of p16 induction for the redistribution and eventual repression of Cdk2 activity. Our results reveal a signaling pathway for G1 arrest induced by HGF.

Growth factors and beta cell replication

Recent studies have demonstrated that human islet allograft transplantation can be a successful therapeutic option in the treatment of patients with Type I diabetes. However, this impressive recent advance is accompanied by a very important constraint. There is a critical paucity of pancreatic islets or pancreatic beta cells for islet transplantation to become a large-scale therapeutic option in patients with diabetes. This has prompted many laboratories around the world to invigorate their efforts in finding ways for increasing the availability of beta cells or beta cell surrogates that potentially could be transplanted into patients with diabetes. The number of studies analyzing the mechanisms that govern beta cell proliferation and growth in physiological and pathological conditions has increased exponentially during the last decade. These studies exploring the role of growth factors, intracellular signaling molecules and cell cycle regulators constitute the substrate for future strategies aimed at expanding human beta cells in vitro and/or in vivo after transplantation. In this review, we describe the current knowledge on the effects of several beta cell growth factors that have been shown to increase beta cell proliferation and expand beta cell mass in vitro and/or in vivo and that they could be potentially deployed in an effort to increase the number of patients transplanted with islets. Furthermore, we also analyze in this review recent studies deciphering the relevance of these specific islet growth factors as physiological and pathophysiological regulators of beta cell proliferation and islet growth.

Hepatocyte growth factor effects on mesenchymal stem cells: proliferation, migration, and differentiation

Hepatocyte growth factor (HGF), a pleiotropic cytokine of mesenchymal origin promoting migration, proliferation, and survival in a wide spectrum of cells, can also modulate different biological responses in stem cells, but the mechanisms involved are not completely understood so far. In this context, we show that short-term exposure of mesenchymal stem cells (MSCs) to HGF can induce the activation of its cognate Met receptor and the downstream effectors ERK1/2, p38MAPK, and PI3K/Akt, while long-term exposure to HGF resulted in cytoskeletal rearrangement, cell migration, and marked inhibition of proliferation through the arrest in the G1-S checkpoint. When added to MSCs, the K252A tyrosine kinase inhibitor prevented HGF-induced responses. HGF's effect on MSC proliferation was reversed by p38 inhibitor SB203580, while the effects on cell migration were abrogated by PI3K inhibitor Wortmannin, suggesting that HGF acts through different pathways to determine its complex effects on MSCs. Prolonged treatment with HGF induced the expression of cardiac-specific markers (GATA-4, MEF2C, TEF1, desmin, alpha-MHC, beta-MHC, and nestin) with the concomitant loss of the stem cell markers nucleostemin, c-kit, and CD105.