Hepatocyte growth factor/c-met signaling pathway is required for efficient liver regeneration and repair

The EGF receptor is required for efficient liver regeneration

Mice lacking the EGF receptor (EGFR) die between midgestation and postnatal day 20 with various defects in neural and epithelial organs. Here, we generated mice carrying a floxed EGFR allele to inactivate the EGFR in fetal and adult liver. Perinatal deletion of EGFR in hepatocytes resulted in decreased body weight, whereas deletion in the adult liver did not affect body mass. Although liver function was not affected, after partial hepatectomy mice lacking EGFR in the liver showed increased mortality accompanied by increased levels of serum transaminases indicating liver damage. Liver regeneration was delayed in the mutants because of reduced hepatocyte proliferation. Analysis of cell cycle progression in EGFR-deficient livers indicated a defective G(1)-S phase entry with delayed transcriptional activation and reduced protein expression of cyclin D1 followed by reduced cdk2 and cdk1 expression. Impaired liver regeneration was accompanied by compensatory up-regulation of TNFalpha in the serum and prolonged activation of c-Jun. Moreover, p38alpha and NF-kappaB activation was reduced in regenerating mutant livers, indicating an impaired stress response after hepatectomy. Our studies demonstrate that EGFR is a critical regulator of hepatocyte proliferation in the initial phases of liver regeneration.

Hepatocyte growth factor: a regenerative drug for acute hepatitis and liver cirrhosis

Liver cirrhosis is a major cause of morbidity worldwide and is characterized by the loss of hepatocytes with interstitial fibrosis. In this review, we discuss the potential uses of hepatocyte growth factor for treating hepatic diseases, focusing on the molecular mechanisms whereby hepatocyte growth factor reverses liver cirrhosis. Hepatic myofibroblasts play a central role in the development of liver cirrhosis, while myofibroblasts acquire c-Met. Using a rat model of liver cirrhosis, we recently delineated the direct effect of hepatocyte growth factor toward myofibroblasts: the induction of apoptotic cell death associated with matrix degradation, the inhibition of overproliferation and the suppression of transforming growth factor-beta1 production in myofibroblasts. Hepatocyte growth factor elicits mitogenic, anti-apoptotic and anti-inflammatory functions in hepatocytes, therefore contributing to reversing liver dysfunction. Considering the insufficient production of hepatocyte growth factor is responsible for the manifestation of chronic hepatitis, supplementation with or reinduction of hepatocyte growth factor represents a new strategy for attenuating intractable liver diseases.

Lack of Fas antagonism by Met in human fatty liver disease

Hepatocytes in fatty livers are hypersensitive to apoptosis and undergo escalated apoptotic activity via death receptor-mediated pathways, particularly that of Fas-FasL, causing hepatic injury that can eventually proceed to cirrhosis and end-stage liver disease. Here we report that the hepatocyte growth factor receptor, Met, plays an important part in preventing Fas-mediated apoptosis of hepatocytes by sequestering Fas. We also show that Fas antagonism by Met is abrogated in human fatty liver disease (FLD). Through structure-function studies, we found that a YLGA amino-acid motif located near the extracellular N terminus of the Met alpha-subunit is necessary and sufficient to specifically bind the extracellular portion of Fas and to act as a potent FasL antagonist and inhibitor of Fas trimerization. Using mouse models of FLD, we show that synthetic YLGA peptide tempers hepatocyte apoptosis and liver damage and therefore has therapeutic potential.

The therapeutic potential of hepatocyte growth factor to sensitize ovarian cancer cells to cisplatin and paclitaxel in vivo

PURPOSE

Advanced ovarian cancers are initially responsive to combinatorial chemotherapy with platinum drugs and taxanes but, in most cases, develop drug resistance. We recently showed that, in vitro, hepatocyte growth factor (HGF) enhances death of human ovarian cancer cell lines treated with cisplatin (CDDP) and paclitaxel. The present study addresses whether in vivo HGF makes ovarian carcinoma cells more responsive to these chemotherapeutics.

EXPERIMENTAL DESIGN

Using Lentiviral vectors carrying the HGF transgene, we transduced SK-OV-3 and NIH:OVCAR-3 ovarian carcinoma cell lines to obtain stable autocrine and paracrine HGF receptor activation. In vitro, we assayed growth, motility, invasiveness, and the response to CDDP and paclitaxel of the HGF-secreting bulk unselected cell populations. In vivo, we tested the cytotoxic effects of the drugs versus s.c. tumors formed by the wild-type and HGF-secreting cells in immunocompromised mice. Tumor-bearing mice were treated with CDDP (i.p.) and paclitaxel (i.v.), combined in different schedules and doses.

RESULTS

In vitro, HGF-secreting cells did not show altered proliferation rates and survival but were strongly sensitized to the death triggered by CDDP and paclitaxel, alone or in combination. In vivo, we found a therapeutic window in which autocrine/paracrine HGF made tumors sensitive to low doses of the drugs, which were ineffective on their own.

CONCLUSIONS

These data provide the proof-of-concept that in vivo gene therapy with HGF might be competent in sensitizing ovarian cancer cells to conventional chemotherapy.

Cell cycle effects resulting from inhibition of hepatocyte growth factor and its receptor c-Met in regenerating rat livers by RNA interference

Hepatocyte growth factor (HGF) and its receptor c-Met are involved in liver regeneration. The role of HGF and c-Met in liver regeneration in rat following two-thirds partial hepatectomy (PHx) was investigated using RNA interference to silence HGF and c-Met in separate experiments. A mixture of 2 c-Met-specific short hairpin RNA (ShRNA) sequences, ShM1 and ShM2, and 3 HGF-specific ShRNA, ShH1, ShH3, and ShH4, were complexed with linear polyethylenimine. Rats were injected with the ShRNA/PEI complex 24 hours before and at the time of PHx. A mismatch and a scrambled ShRNA served as negative controls. ShRNA treatment resulted in suppression of c-Met and HGF mRNA and protein compared with that in controls. The regenerative response was assessed by PCNA, mitotic index, and BrdU labeling. Treatment with the ShHGF mixture resulted in moderate suppression of hepatocyte proliferation. Immunohistochemical analysis revealed severe suppression of incorporation of BrdU and complete absence of mitosis in rats treated with ShMet 24 hours after PHx compared with that in controls. Gene array analyses indicated abnormal expression patterns in many cell-cycle- and apoptosis-related genes. The active form of caspase 3 was seen to increase in ShMet-treated rats. The TUNEL assay indicated a slight increase in apoptosis in ShMet-treated rats compared with that in controls.

CONCLUSION

The data indicated that in vivo silencing of c-Met and HGF mRNA by RNA interference in normal rats results in suppression of mRNA and protein, which had a measurable effect on proliferation kinetics associated with liver regeneration.

Tissue-type plasminogen activator deficiency exacerbates cholestatic liver injury in mice

Recent studies demonstrating a role for plasminogen activator inhibitor (PAI)-1 in cholestatic liver disease in mice suggested that tissue-type plasminogen activator (tPA) or urokinase plasminogen activator (uPA) might be important after biliary tract obstruction. We now demonstrate that blocking tPA exacerbates liver injury after bile duct ligation (BDL). tPA deficient mice have increased bile infarcts, increased TUNEL positive cells, increased neutrophil infiltration, reduced hepatocyte proliferation and reduced ductular reaction 72 hours after BDL compared to wild type mice. In addition, the protective and proliferative effects of plasminogen activator inhibitor 1 (PAI-1) deficiency after BDL are dramatically blocked by the tPA inhibitor tPA-STOP. One potential mechanism for these effects is that both tPA deficiency and tPA-STOP reduce hepatocyte growth factor (HGF) activation and c-Met phosphorylation in the liver after BDL. In support of this hypothesis, HGF treatment reverses the effects of tPA deficiency in mice. Furthermore, preferential tPA activation in areas of injury after BDL might occur because fibrin accumulates in bile infarcts and activates tPA.

CONCLUSION

tPA inactivation accelerates liver injury after BDL and reduces HGF activation. These data suggest that strategies to increase HGF activation might be protective in liver diseases with biliary tract obstruction even without increased HGF production.

Advances in signaling in vertebrate regeneration as a prelude to regenerative medicine

While all animals have evolved strategies to respond to injury and disease, their ability to functionally recover from loss of or damage to organs or appendages varies widely damage to skeletal muscle, but, unlike amphibians and fish, they fail to regenerate heart, lens, retina, or appendages. The relatively young field of regenerative medicine strives to develop therapies aimed at improving regenerative processes in humans and is predicated on >40 years of success with bone marrow transplants. Further progress will be accelerated by implementing knowledge about the molecular mechanisms that regulate regenerative processes in model organisms that naturally possess the ability to regenerate organs and/or appendages. In this review we summarize the current knowledge about the signaling pathways that regulate regeneration of amphibian and fish appendages, fish heart, and mammalian liver and skeletal muscle. While the cellular mechanisms and the cell types involved in regeneration of these systems vary widely, it is evident that shared signals are involved in tissue regeneration. Signals provided by the immune system appear to act as triggers of many regenerative processes. Subsequently, pathways that are best known for their importance in regulating embryonic development, in particular fibroblast growth factor (FGF) and Wnt/beta-catenin signaling (as well as others), are required for progenitor cell formation or activation and for cell proliferation and specification leading to tissue regrowth. Experimental activation of these pathways or interference with signals that inhibit regenerative processes can augment or even trigger regeneration in certain contexts.

Epithelial-vascular cross talk mediated by VEGF-A and HGF signaling directs primary septae formation during distal lung morphogenesis

There is increasing evidence that epithelial-vascular interactions are essential for tissue patterning. Here we identified components of the molecular cross talk between respiratory epithelial cells and pulmonary capillaries necessary for the formation of the gas exchange surface of the lung. Selective inactivation of the Vegf-A gene in respiratory epithelium results in an almost complete absence of pulmonary capillaries, demonstrating the dependence of pulmonary capillary development on epithelium-derived Vegf-A. Deficient capillary formation in Vegf-A deficient lungs is associated with a defect in primary septae formation, a morphogenetic process critical for distal lung morphogenesis, coupled with suppression of epithelial cell proliferation and decreased hepatocyte growth factor (Hgf) expression. Lung endothelial cells express Hgf, and selective deletion of the Hgf receptor gene in respiratory epithelium phenocopies the malformation of septae, confirming the requirement for epithelial Hgf signaling in normal septae formation and suggesting that Hgf serves as an endothelium-derived factor that signals to the epithelium. Our findings support a mechanism for primary septae formation dependent on reciprocal interactions between respiratory epithelium and the underlying vasculature, establishing the dependence of pulmonary capillary development on epithelium-derived Vegf-A, and identify Hgf as a putative endothelium-derived factor that mediates the reciprocal signaling from the vasculature to the respiratory epithelium.

Met signals hepatocyte survival by preventing Fas-triggered FLIP degradation in a PI3k-Akt-dependent manner

The FasL-Fas couple is a general death mediator whose activated signals lead to caspase-8 activation and apoptosis in adult hepatocytes. Suppression of caspase-8 activation and cell death is a protective mechanism modulated by the FLICE-Like Inhibitory Protein (FLIP). Although hepatocyte growth factor (HGF) and its receptor Met are known to mediate cell survival in developing livers, the molecular mechanisms involved in this process are poorly understood. We show here that Met activation by HGF impairs Fas-triggered apoptosis of primary embryonic hepatocytes and cell survival correlates with inhibition of caspase-8 and caspase-3 activities. Furthermore, we found that HGF treatment prevents degradation of FLIPL triggered by Fas activation. In contrast to this, Met activation does not modulate FLIPL levels and its stability in untreated cells, thus showing the specificity of this regulatory mechanism for embryonic hepatocyte survival. Knocking down FLIP expression abolishes the ability of Met to inhibit Fas-triggered hepatocyte death, demonstrating the functional requirement of FLIP in HGF anti-apoptotic signals. By combining genetic and pharmacological approaches, we also demonstrate that the PI3K-Akt pathway is required in embryonic hepatocytes to prevent Fas-triggered FLIP degradation and death. Thus, Met acting on PI3K and Akt ensures high levels of FLIPL, and disruption of this pathway contributes to hepatic apoptosis and possibly to Fas-related liver diseases.

Essential role of ubiquitin-specific protease 8 for receptor tyrosine kinase stability and endocytic trafficking in vivo

Posttranslational modification by ubiquitin controls multiple cellular functions and is counteracted by the activities of deubiquitinating enzymes. UBPy (USP8) is a growth-regulated ubiquitin isopeptidase that interacts with the HRS-STAM complex. Using Cre-loxP-mediated gene targeting in mice, we show that lack of UBPy results in embryonic lethality, whereas its conditional inactivation in adults causes fatal liver failure. The defect is accompanied by a strong reduction or absence of several growth factor receptor tyrosine kinases (RTKs), like epidermal growth factor receptor, hepatocyte growth factor receptor (c-met), and ERBB3. UBPy-deficient cells exhibit aberrantly enlarged early endosomes colocalizing with enhanced ubiquitination and have reduced levels of HRS and STAM2. Congruently immortalized cells gradually stop proliferation upon induced deletion of UBPy. These results unveil a central and nonredundant role of UBPy in growth regulation, endosomal sorting, and the control of RTKs in vivo.

Critical role for transcription coactivator peroxisome proliferator-activated receptor (PPAR)-binding protein/TRAP220 in liver regeneration and PPARalpha ligand-induced liver tumor development

Disruption of the gene encoding for the transcription coactivator peroxisome proliferator-activated receptor (PPAR)-binding protein (PBP/TRAP220/DRIP205/Med1) in the mouse results in embryonic lethality. Here, we have reported that targeted disruption of the Pbp/Pparbp gene in hepatocytes (Pbp(DeltaLiv)) impairs liver regeneration with low survival after partial hepatectomy. Analysis of cell cycle progression suggests a defective exit from quiescence, reduced BrdUrd incorporation, and diminished entry into G(2)/M phase in Pbp(DeltaLiv) hepatocytes after partial hepatectomy. Pbp(DeltaLiv) hepatocytes failed to respond to hepatocyte growth factor/scatter factor, implying that hepatic PBP deficiency affects c-met signaling. Pbp gene disruption also abolishes primary mitogen-induced liver cell proliferative response. Striking abrogation of CCl(4)-induced hepatocellular proliferation and hepatotoxicity occurred in Pbp(DeltaLiv) mice pretreated with phenobarbital due to lack of expression of xenobiotic metabolizing enzymes necessary for CCl(4) activation. Pbp(DeltaLiv) mice, chronically exposed to Wy-14,643, a PPARalpha ligand, revealed a striking proliferative response and clonal expansion of a few Pbp(fl/fl) hepatocytes that escaped Cre-mediated gene deletion in Pbp(DeltaLiv) livers, but no proliferative expansion of PBP null hepatocytes was observed. In these Pbp(DeltaLiv) mice, none of the Wy-14,643-induced hepatic adenomas and hepatocellular carcinomas was derived from PBP(DeltaLiv) hepatocytes; all liver tumors developing in Pbp(DeltaLiv) mice maintained non-recombinant Pbp alleles and retained PBP expression. These studies provide direct evidence in support of a critical role of PBP/TRAP220 in liver regeneration, induction of hepatotoxicity, and hepatocarcinogenesis.

Entry of the bacterial pathogen Listeria monocytogenes into mammalian cells

The bacterial pathogen Listeria monocytogenes causes food-borne illnesses leading to meningitis or abortion. Listeria provokes its internalization ('entry') into mammalian cells that are normally non-phagocytic, such as intestinal epithelial cells and hepatocytes. Entry provides access to a nutrient-rich cytosol and allows translocation across anatomical barriers. Here I discuss the two major internalization pathways used by Listeria. These pathways are initiated by binding of the bacterial surface proteins InlA or InlB to their respective host receptors, E-cadherin or Met. InlA mediates traversal of the intestinal barrier, whereas InlB promotes infection of the liver. At the cellular level, both InlA- and InlB-dependent entry require host signalling that promotes cytoskeletal rearrangements and pathogen engulfment. However, many of the specific signalling proteins in the two entry routes differ. InlA-mediated uptake uses components of adherens junctions that are coupled to F-actin and myosin, whereas InlB-dependent entry involves cytosolic adaptors that bridge Met to regulators of F-actin, including phosphoinositide 3-kinase and activators of the Arp2/3 complex. Unexpectedly, entry directed by InlB also involves endocytic components. Future work on InlA and InlB will lead to a better understanding of virulence, and may also provide novel insights into the normal biological functions of E-cadherin and Met.

Activation of mitogen-activated protein kinase by hepatocyte growth factor is stimulated by both alpha1- and beta2-adrenergic agonists in primary cultures of adult rat hepatocytes

We investigated the effects of alpha(1)- and beta(2)-adrenergic agonists on hepatocyte growth factor (HGF)-stimulated mitogen-activated protein kinase (MAPK) isoforms in primary cultures of adult rat hepatocytes. Hepatocytes were isolated and cultured with HGF (5 ng/ml) and/or alpha- and beta-adrenergic agonists. Phosphorylated MAPK isoforms (p42 and p44 MAPK) were detected by Western blotting analysis using anti-phospho-MAPK antibody. The results show that HGF increased phosphorylation of p42 MAPK by 2.2-fold within 3 min. The HGF-induced MAPK activation was abolished by AG1478 treatment (10(-7) M). The MEK (MAPK kinase) inhibitor PD98059 (10(-6) M) completely inhibited the HGF-dependent increase in MAPK activity. Phenylephrine (10(-6) M) and metaproterenol (10(-6) M) alone had no effect in the absence of HGF, but significantly increased p42 MAPK induction by HGF. Moreover, the cell-permeable cAMP analog, 8-bromo cAMP (10(-7) M), and phorbol 12-myristate 13 acetate (10(-7) M) potentiated HGF-induced MAPK phosphorylation. The effects of these analogs were antagonized by the protein kinase A (PKA) inhibitor H-89 (10(-7) M) and the protein kinase C (PKC) inhibitor sphingosine (10(-6) M), respectively. These results suggest that direct or indirect activation of both PKA and PKC represent a positive regulatory mechanism for stimulating MAPK induction by HGF.

c-Met is essential for wound healing in the skin

Wound healing of the skin is a crucial regenerative process in adult mammals. We examined wound healing in conditional mutant mice, in which the c-Met gene that encodes the receptor of hepatocyte growth factor/scatter factor was mutated in the epidermis by cre recombinase. c-Met-deficient keratinocytes were unable to contribute to the reepithelialization of skin wounds. In conditional c-Met mutant mice, wound closure was slightly attenuated, but occurred exclusively by a few (5%) keratinocytes that had escaped recombination. This demonstrates that the wound process selected and amplified residual cells that express a functional c-Met receptor. We also cultured primary keratinocytes from the skin of conditional c-Met mutant mice and examined them in scratch wound assays. Again, closure of scratch wounds occurred by the few remaining c-Met-positive cells. Our data show that c-Met signaling not only controls cell growth and migration during embryogenesis but is also essential for the generation of the hyperproliferative epithelium in skin wounds, and thus for a fundamental regenerative process in the adult.

From Tpr-Met to Met, tumorigenesis and tubes

The receptor for hepatocyte growth factor (HGF)/scatter factor (SF), Met, controls a program of invasive epithelial growth through the coordination of cell proliferation and survival, cell migration and epithelial morphogenesis. This process is important during embryogenesis and for organ regeneration in the adult. However, when deregulated the HGF/SF-Met signaling axis contributes to tumorigenesis and metastasis. Studies on the oncogenic activation of the Met receptor have shed light on the molecular mechanisms underlying the oncogenic activation of receptor tyrosine kinase (RTKs). More than a decade ago, work on the Met related oncogene, Tpr-Met, revealed the mechanism for activation of RTK-derived oncogenes generated following chromosomal translocation. More recently, studies on the mechanisms of downregulation of the Met RTK highlight a role for loss of downregulation in RTK oncogenic activation.

Liver regeneration: mechanisms, mysteries and more

BACKGROUND

Liver regeneration remains a fascinating topic, still partly clouded to many as to the exact cellular and molecular mechanisms that bring about this phenomenon. It is an area, therefore, of active research today. This review looks at the recent published reports that have led to a greater understanding of this process.

METHODS

A database search was carried out on Medline search using the terms liver regeneration with no linguistic limitations from 1966 to 2006.

RESULTS

There are two randomized controlled trials on the topic and most data and information have come from experimental studies in animals.

CONCLUSION

Liver regeneration is a complex, tightly controlled process involving many inflammatory cells growth factors and hormones. More information about it is awaited in studies on humans.

Met acts on Mdm2 via mTOR to signal cell survival during development

Coordination of cell death and survival is crucial during embryogenesis and adulthood, and alteration of this balance can result in degeneration or cancer. Growth factor receptors such as Met can activate phosphatidyl-inositol-3' kinase (PI3K), a major intracellular mediator of growth and survival. PI3K can then antagonize p53-triggered cell death, but the underlying mechanisms are not fully understood. We used genetic and pharmacological approaches to uncover Met-triggered signaling pathways that regulate hepatocyte survival during embryogenesis. Here, we show that PI3K acts via mTOR (Frap1) to regulate p53 activity both in vitro and in vivo. mTOR inhibits p53 by promoting the translation of Mdm2, a negative regulator of p53. We also demonstrate that the PI3K effector Akt is required for Met-triggered Mdm2 upregulation, in addition to being necessary for the nuclear translocation of Mdm2. Inhibition of either mTOR or Mdm2 is sufficient to block cell survival induced by Hgf-Met in vitro. Moreover, in vivo inhibition of mTOR downregulates Mdm2 protein levels and induces p53-dependent apoptosis. Our studies identify a novel mechanism for Met-triggered cell survival during embryogenesis, involving translational regulation of Mdm2 by mTOR. Moreover, they reinforce mTOR as a potential drug target in cancer.

Antinecrotic and antiapoptotic effects of hepatocyte growth factor on cholestatic hepatitis in a mouse model of bile-obstructive diseases

Cholestasis, an impairment of bile outflux, frequently occurs in liver diseases. In this process, an overaccumulation of bile acids causes hepatocyte necrosis and apoptosis, leading to advanced hepatitis. Hepatocyte growth factor (HGF) is mitogenic toward hepatocytes, but it is still unclear whether HGF has physiological and therapeutic functions during the progression of cholestasis. Using anti-HGF IgG or recombinant HGF in mice that had undergone bile duct ligation (BDL), we investigated the involvement of HGF in cholestasis-induced hepatitis. After the BDL surgery, HGF and c-Met mRNA levels transiently increased in livers during the progression of cholestatic hepatitis. When c-Met tyrosine phosphorylation was blocked in the livers of BDL-treated mice by anti-HGF IgG, hepatic dysfunction became evident, associated with the acceleration of hepatocyte necrosis and apoptosis. Inversely, administration of recombinant HGF into the mice led to the prevention of cholestasis-induced inflammation: HGF suppressed the hepatic expression of intracellular adhesion molecule-1 and neutrophil infiltration in BDL-treated mice. As a result, parenchymal necrosis was suppressed in the HGF-injected BDL mice. In addition, HGF supplement therapy reduced the number of apoptotic hepatocytes in cholestatic mice, associated with the early induction of Bcl-xL. The administration of HGF enhanced hepatic repair, via accelerating G1/S progression in hepatocytes. Our study showed that 1) upregulation of HGF production is required for protective mechanisms against cholestatic hepatitis and 2) enhancement of the intrinsic defense system by adding HGF may be a reasonable strategy to attenuate hepatic inflammation, necrosis, and apoptosis under bile-congestive conditions.

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.

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

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

Liver growth in the embryo and during liver regeneration in zebrafish requires the cell cycle regulator, uhrf1

In contrast to the deregulated hepatocellular division that is a feature of many hepatic diseases and malignancies, physiologic liver growth during embryonic development and after partial hepatectomy (PH) in adults is characterized by tightly controlled cell proliferation. We used forward genetic screening in zebrafish to test the hypothesis that a similar genetic program governs physiologic liver growth during hepatogenesis and regeneration after PH. We identified the uhrf1 gene, a cell cycle regulator and transcriptional activator of top2a expression, as required for hepatic outgrowth and embryonic survival. By developing a methodology to perform PH on adult zebrafish, we found that liver regeneration inuhrf1+/- adult animals is impaired.uhrf1 transcript levels dramatically increase after PH in both mice, and zebrafish and top2a is not up-regulated in uhrf1+/- livers after PH. This indicates that uhrf1 is required for physiologic liver growth in both embryos and adults and illustrates that zebrafish livers regenerate.

The MET receptor tyrosine kinase in invasion and metastasis

Various cytokines and soluble growth factors upon interaction with their membrane receptors are responsible for inducing cellular proliferation, differentiation, movement, and protection from anoikis (a planned suicide activated by normal cells in absence of attachment to neighboring cells or extracellular matrix (EMC)). Among those soluble factors a major position is exerted by hepatocyte growth factor (HGF) together with its receptor MET and macrophage-stimulating protein (MSP) in cooperation with its receptor RON.

Gene Expression Profiles in Living Donors Immediately After Partial Hepatectomy—The Initial Response of Liver Regeneration

BACKGROUND/PURPOSE

Gene expression profiles of liver regeneration are well explored in rat models. However, there are limited relative data in humans. This study aimed to show that mRNA expression profiles change immediately after right hepatectomy in living-related donors and correlate with mechanisms of liver regeneration reported in the literature.

METHODS

Prospective study was conducted from March 2003 to August 2004. Living-related donors who donated right lobe of liver were included. Liver biopsies were performed at the beginning and, 5 hours later, at the end of liver resection. RNAs were isolated to synthesize cRNA. Oligo DNA microarray experiments were conducted and paired signal intensity ratios (Cy3/Cy5) were normalized with rank-invariant global Lowess regression analysis by taking base two logarithms. Genes whose average residuals more than 2.5-fold increased or less than -2.5-fold decreased were selected to get the most pronounced expression changes during this period.

RESULTS

Five of 34 donors were included with qualified samples. The expression patterns of paired DNA microarray experiments were similar in five donors. A total of 28 upregulated and 14 downregulated genes were collected. Acute-phase proteins (serum amyloid A, complement-reactive protein, heme oxygenase-1) were upregulated. Genes related to growth signal transduction (G-protein coupled receptor-30) were downregulated.

CONCLUSION

Gene expression profiles immediately after partial hepatectomy were reported first in humans with the techniques of oligo DNA microarray, which were compatible with the initial gene expression patterns of liver regeneration in rats.

Loss of Embryonic MET Signaling Alters Profiles of Hippocampal Interneurons

Hippocampal interneurons arise in the ventral forebrain and migrate dorsally in response to cues, including hepatocyte growth factor/scatter factor which signals via its receptor MET. Examination of the hippocampus in adult mice in which MET had been inactivated in the embryonic proliferative zones showed an increase in parvalbumin-expressing cells in the dentate gyrus, but a loss of these cells in the CA3 region. An overall loss of calretinin-expressing cells was seen throughout the hippocampus. A similar CA3 deficit of parvalbumin and calretinin cells was observed when MET was eliminated only in postmitotic cells. These data suggest that MET is required for the proper hippocampal development, and embryonic perturbations lead to long-term anatomical defects with possible learning and memory dysfunction.

Reg2 inactivation increases sensitivity to Fas hepatotoxicity and delays liver regeneration post-hepatectomy in mice

Reg2/RegIIIbeta is the murine homologue of the human secreted HIP/PAP C-type lectin. HIP/PAP transgenic mice were protected against acetaminophen-induced acute liver failure and were stimulated to regenerate post-hepatectomy. To assess the role of Reg2, we used Reg2-/- mice in a model of fulminant hepatitis induced by Fas and in the post-hepatectomy regeneration. Within 4 hours of J0-2 treatment (0.5 microg/g), only 50% of the Reg2-/- mice were alive but with an increased sensitivity to Fas-induced oxidative stress and a decreased level of Bcl-xL. In contrast, HIP/PAP transgenic mice were resistant to Fas, with HIP/PAP serving as a sulfhydryl buffer to slow down decreases in glutathione and Bcl-xL. In Reg2-/- mice, liver regeneration was markedly impaired, with 29% mortality and delay of the S-phase and the activation of ERK1/2 and AKT. Activation of STAT3 began on time at 3 hours but persisted strongly up to 72 hours despite significant accumulation of SOCS3. Thus, Reg2 deficiency induced exaggerated IL-6/STAT-3 activation and mito-inhibition. Because the Reg2 gene was activated between 6 and 24 hours after hepatectomy in wild-type mice, Reg2 could mediate the TNF-alpha/IL-6 priming signaling by exerting a negative feed-back on STAT3/IL-6 activation to allow the hepatocytes to progress through the cell cycle. In conclusion, Reg2 deficiency enhanced liver sensitivity to Fas-induced oxidative stress and delayed liver regeneration with persistent TNF-alpha/IL6/STAT3 signaling. In contrast, overexpression of human HIP/PAP promoted liver resistance to Fas and accelerated liver regeneration with early activation/deactivation of STAT3. Reg2/HIP/PAP is therefore a critical mitogenic and antiapoptotic factor for the liver.

Hepatic precursors derived from murine embryonic stem cells contribute to regeneration of injured liver

We established an efficient system for differentiation, expansion and isolation of hepatic progenitor cells from mouse embryonic stem (ES) cells and evaluated their capacity to repopulate injured liver. Using mouse ES cells transfected with the green fluorescent protein (GFP) reporter gene regulated by albumin (ALB) enhancer/promoter, we found that a serum-free chemically defined medium supports formation of embryoid bodies (EBs) and differentiation of hepatic lineage cells in the absence of exogenous growth factors or feeder cell layers. The first GFP+ cells expressing ALB were detected in close proximity to "beating" myocytes after 7 days of EB cultures. GFP+ cells increased in number, acquired hepatocyte-like morphology and hepatocyte-specific markers (i.e., ALB, AAT, TO, and G6P), and by 28 days represented more than 30% of cells isolated from EB outgrowths. The FACS-purified GFP+ cells developed into functional hepatocytes without evidence of cell fusion and participated in the repairing of diseased liver when transplanted into MUP-uPA/SCID mice. The ES cell-derived hepatocytes were responsive to normal growth regulation and proliferated at the same rate as the host hepatocytes after an additional growth stimulus from CCl(4)-induced liver injury. The transplanted GFP+ cells also differentiated into biliary epithelial cells. In conclusion, a highly enriched population of committed hepatocyte precursors can be generated from ES cells in vitro for effective cell replacement therapy.

Serum levels of hepatocyte growth factor/scatter factor in patients with liver metastases from breast cancer

OBJECTIVE

Recent studies have shown that the pleiotropic cytokine hepatocyte growth factor/scatter factor (HGF/SF) and its receptor c-Met play major roles in the malignant progression of numerous tumors. For patients with breast cancer liver metastases, increased serum levels of HGF/SF have been reported. We studied the relationship between the clinical course of the disease and the serum levels of HGF/SF in such patients.

METHODS

We examined 51 patients treated for breast cancer liver metastases. Serum concentrations of HGF/SF were measured before each therapy cycle and compared to the corresponding tumor marker levels.

RESULTS

Mean serum levels of HGF/SF in patients with liver metastases were increased above the reported reference levels of primary breast cancer patients. Serum levels of HGF/SF were correlated with tumor marker levels in a logarithmic relation (r = 0.47, p < 0.001). In some cases serum concentrations of HGF/SF changed similarly to the course of the corresponding tumor markers.

CONCLUSIONS

Serum levels of HGF/SF are associated with the clinical course of metastatic breast cancer patients with liver metastases. Further studies are required to clarify the potential value of the HGF/SF serum concentration as a tumor marker. HGF/SF and its receptor c-Met should be further evaluated as therapeutic targets.

Effect of Akt inhibition on scatter factor-regulated gene expression in DU-145 human prostate cancer cells

The cytokine scatter factor (SF) (hepatocyte growth factor) transduces various biologic actions, including cell motility, invasion, angiogenesis and apoptosis inhibition. The latter is relevant to understanding the role of SF in promoting tumor cell survival in different contexts, for example, detachment from basement membrane, growth in metastatic sites and responses to chemo- and radiotherapy. Previously, we showed that SF protects cells against apoptosis owing to DNA damage, by a mechanism involving phosphoinositol-3-kinase/c-Akt signaling. Here, we used DNA microarray assays to identify c-Akt-regulated genes that might contribute to cell protection. DU-145 human prostate cancer cells were transfected+/-a dominant-negative mutant Akt, treated+/-SF and analysed for gene expression using Affymetrix arrays. These studies identified SF-regulated genes for which induction was c-Akt-dependent vs -independent. Selected microarray findings were confirmed by semiquantitative and quantitative reverse transcription-polymerase chain reaction. We tested the contribution of four SF-inducible/c-Akt-dependent genes (AMPD3, EPHB2, MX1 and WNT4) to protection against adriamycin (a DNA topoisomerase IIalpha inhibitor) using RNA interference. Knockdown of each gene except EPHB2 caused a small but significant reduction in the SF cell protection. The lack of effect of EPHB2 knockdown may be due to the fact that DU-145 cells contain a single-mutant EPHB2 allele. A combination of three small interfering RNAs blocked most of the protection by SF in both DU-145 and T47D cells. These findings identify novel c-Akt-regulated genes, some of which contribute to SF-mediated cytoprotection.

A genetic variant that disrupts MET transcription is associated with autism

There is strong evidence for a genetic predisposition to autism and an intense interest in discovering heritable risk factors that disrupt gene function. Based on neurobiological findings and location within a chromosome 7q31 autism candidate gene region, we analyzed the gene encoding the pleiotropic MET receptor tyrosine kinase in a family based study of autism including 1,231 cases. MET signaling participates in neocortical and cerebellar growth and maturation, immune function, and gastrointestinal repair, consistent with reported medical complications in some children with autism. Here, we show genetic association (P = 0.0005) of a common C allele in the promoter region of the MET gene in 204 autism families. The allelic association at this MET variant was confirmed in a replication sample of 539 autism families (P = 0.001) and in the combined sample (P = 0.000005). Multiplex families, in which more than one child has autism, exhibited the strongest allelic association (P = 0.000007). In case-control analyses, the autism diagnosis relative risk was 2.27 (95% confidence interval: 1.41-3.65; P = 0.0006) for the CC genotype and 1.67 (95% confidence interval: 1.11-2.49; P = 0.012) for the CG genotype compared with the GG genotype. Functional assays showed that the C allele results in a 2-fold decrease in MET promoter activity and altered binding of specific transcription factor complexes. These data implicate reduced MET gene expression in autism susceptibility, providing evidence of a previously undescribed pathophysiological basis for this behaviorally and medically complex disorder.

Increased DNA binding activity of NF-kB, STAT-3, SMAD3 and AP-1 in acutely damaged liver

AIM: To investigate the role of genes and kinetics of specific transcription factors in liver regeneration, and to analyze the gene expression and the activity of some molecules crucially involved in hepatic regeneration.

METHODS: USING gel-shift assay and RT-PCR, transcription factors, such as NF-κB, STAT-3, SMAD3 and AP-1, and gene expression of inducible nitric oxide synthase (iNOS), hepatocyte growth factor (HGF) and c-met were analyzed in an animal model of chemically induced hepatectomy.

RESULTS: Gene expression of HGF and its receptor c-met peaked at 3 h and 24 h after acute CCl₄ intoxi-cation. iNOS expression was only observed from 6 to 48 h. Transcriptional factor NF-κB had an early activation at 30 min after acute liver damage. STAT-3 peaked 3 h post-intoxication, while AP-1 displayed a peak of activation at 48 h. SMAD3 showed a high activity at all analyzed times.

CONCLUSION: TNF-α and IL-6 play a central role in hepatic regeneration. These two molecules are responsible for triggering the cascade of events and switch-on of genes involved in cell proliferation, such as growth factors, kinases and cyclins which are direct participants of cell proliferation.

The Met pathway: master switch and drug target in cancer progression

It has been recognized for more than a century that most tumors tend to become more aggressive in clinical behavior over time, although this time course may be variable. This phenomenon has been termed "cancer progression," a process that appears to develop in a stepwise fashion through qualitatively different stages. Cancer progression relies on the ability of neoplastic cells to abandon their primary site of accretion, trespass tissue boundaries, and penetrate into the vasculature to colonize and repopulate distant sites. Among the various properties associated with cancer progression, the acquisition by neoplastic cells of the capacity to invade locally and to metastasize is of great clinical significance, and is still the fundamental definition of malignancy. This process represents the aberrant counterpart of a physiological morphogenetic program, known as invasive growth, occurring during embryo development and, in some instances, in adulthood for the generation and maintenance of normal organ complexity and architecture. Here we summarize some of the strategies adopted to inhibit cancer cell growth and spreading. We also review the current findings about cancer and metastasis inhibitors. As we suggest possible directions for drug development, we propose the receptor for the hepatocyte growth factor, Met, as an ideal target for tackling cancer progression.

The von Hippel-Lindau tumor suppressor gene product represses oncogenic beta-catenin signaling in renal carcinoma cells

Loss of von Hippel-Lindau (VHL) tumor suppressor gene function occurs in familial and most sporadic clear cell renal cell carcinoma (RCC), resulting in the aberrant expression of genes that control cell proliferation, invasion, and angiogenesis. The molecular mechanisms by which VHL loss leads to tumorigenesis are not yet fully defined. VHL loss has been shown to allow robust RCC cell motility, invasiveness, and morphogenesis in response to hepatocyte growth factor (HGF) stimulation, processes that are known to contribute to tumor invasiveness and metastatic potential. Among the most likely intracellular mediators of these HGF-driven activities is beta-catenin, a structural link between cadherens and the actin cytoskeleton, as well as a gene transactivator. We show that reconstitution of VHL expression in RCC cells repressed HGF-stimulated beta-catenin tyrosyl phosphorylation, adherens junction disruption, cytoplasmic beta-catenin accumulation, and reporter gene transactivation in RCC cells. Ectopic expression of a ubiquitination-resistant beta-catenin mutant specifically restored HGF-stimulated invasion and morphogenesis in VHL-transfected RCC cells. VHL gene silencing in non-RCC renal epithelial cells phenotypically mimicked VHL loss in RCC, and HGF-driven invasiveness was blocked by the expression of a dominant-negative mutant of Tcf. We conclude that, unlike many other cancers, where HGF pathway activation contributes to malignancy through the acquisition of autocrine signaling, receptor overexpression, or mutation, in RCC cells VHL loss enables HGF-driven oncogenic beta-catenin signaling. These findings identify beta-catenin as a potential target in biomarker and drug development for RCC.

c-Jun/AP-1 controls liver regeneration by repressing p53/p21 and p38 MAPK activity

The AP-1 transcription factor c-Jun is a key regulator of hepatocyte proliferation. Mice lacking c-Jun in the liver (c-jun (Deltali*)) display impaired liver regeneration after partial hepatectomy (PH). This phenotype correlates with increased protein levels of the cdk-inhibitor p21 in the liver. We performed PH experiments in several double-knockout mouse models to genetically identify the signaling events regulated by c-Jun. Inactivation of p53 in c-jun (Deltali*) mice abrogated both hepatocyte cell cycle block and increased p21 protein expression. Consistently, liver regeneration was rescued in c-jun (Deltali*) p21 (-/-) double-mutant mice. This indicated that c-Jun controls hepatocyte proliferation by a p53/p21-dependent mechanism. Analyses of p21 mRNA and protein expression in livers of c-jun (Deltali*) mice after PH revealed that the accumulation of p21 protein is due to a post-transcriptional/post-translational mechanism. We have investigated several candidate pathways implicated in the regulation of p21 expression, and observed increased activity of the stress kinase p38 in regenerating livers of c-jun (Deltali*) mice. Importantly, conditional deletion of p38alpha in livers of c-jun (Deltali*) mice fully restored hepatocyte proliferation and attenuated increased p21 protein levels after PH. These data demonstrate that c-Jun/AP-1 regulates liver regeneration through a novel molecular pathway that involves p53, p21, and the stress kinase p38alpha.

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.

Hepatocyte growth factor and the Met system as a mediator of tumor-stromal interactions

Crosstalk between carcinoma cells and host stromal cells such as fibroblasts has a great deal of influence on the invasive and metastatic behavior of cancer cells. The oncogenic action of fibroblasts has been demonstrated through genetic alterations that occur specifically in fibroblasts. Hepatocyte growth factor (HGF), a ligand for the Met receptor tyrosine kinase, plays a definitive role, particularly in the progression to invasive and metastatic cancers, predominantly as a stroma-derived paracrine mediator. Many types of cancer cells secrete molecules that enhance HGF production in fibroblasts, while fibroblast-derived HGF, in turn, is a potent stimulator of the invasion of cancer cells. Fibroblast-specific genetic alterations leading to an overexpression of HGF are associated with the development of epithelial neoplasia and invasive carcinoma. Strategies for targeting the HGF-Met axis are being pursued, in attempts to block the malignant behavior of cancers. In normal tissues, the HGF-Met axis plays diverse roles in organogenesis and in wound healing. The simile that "cancer is a never-healing wound" appears to be pertinent here.

Concerted functions of Gab1 and Shp2 in liver regeneration and hepatoprotection

Liver regeneration is a rapid and concerted response to injury, in which growth factor-generated intracellular signals result in activation of transcription factors, DNA synthesis, and hepatocyte proliferation. However, the link between cytoplasmic signals resulting in proliferative response to liver injury remains to be elucidated. We show here that association of Gab1 adaptor protein and Shp2 tyrosine phosphatase is a critical event at the early phase of liver regeneration. Partial hepatectomy (PH) rapidly and transiently induced assembly of a complex comprising Shp2 and tyrosine-phosphorylated Gab1 in wild-type hepatocytes. Consistently, liver-specific Shp2 knockout (LSKO) and liver-specific Gab1 knockout (LGKO) mice displayed very similar phenotypes of defective liver regeneration triggered by PH, including blunted extracellular signal-regulated kinase 1/2 (Erk1/2) activation, decreased expression of immediate-early genes, and reduced levels of cyclins A, E, and B1, as well as suppression of hepatocyte proliferation. In contrast, the Akt and interleukin-6/Stat3 pathways were up-regulated posthepatectomy in LSKO and LGKO mice, accompanied by improved hepatoprotection. Collectively, this study establishes the physiological significance of the Gab1/Shp2 link in promoting mitogenic signaling through the Erk pathway in mammalian liver regeneration.

Effect of liver regeneration after partial hepatectomy and ischemia-reperfusion on expression of growth factor receptors

AIM: To investigate the effects of experimental partial hepatectomy and normothermic ischemia-reperfusion damage on the time course of the expression of four different growth factor receptors in liver regeneration. This is relevant due to the potential therapeutic use of growth factors in stimulating liver regeneration.

METHODS: For partial hepatectomy (PH) 80% of the liver mass was resected in Sprague Dawley rats. Ischemia and reperfusion (I/R) were induced by occlusion of the portal vein and the hepatic artery for 15 min. The epidermal growth factor receptor, hepatic growth factor receptor, fibroblast growth factor receptor and tumour necrosis factor receptor-1 were analysed by immunohistochemistry up to 72 h after injury. Quantitative RT-PCR was performed at the time point of minimal receptor expression (24 h).

RESULTS: In immunohistochemistry, EGFR, HGFR, FGFR and TNFR1 showed biphasic kinetics after partial hepatectomy with a peak up to 12 h, a nadir after 24 h and another weak increase up to 72 h. During liver regeneration, after ischemia and reperfusion, the receptor expression was lower; the nadir at 24 h after reperfusion was the same. To evaluate whether this nadir was caused by a lack of mRNA transcription, or due to a posttranslational regulation, RT-PCR was performed at 24 h and compared to resting liver. In every probe there was specific mRNA for the receptors. EGFR, FGFR and TNFR1 mRNA expression was equal or lower than in resting liver, HGFR expression after I/R was stronger than in the control.

CONCLUSION: At least partially due to a post-transcrip-tional process, there is a nadir in the expression of the analysed receptors 24 h after liver injury. Therefore, a therapeutic use of growth factors to stimulate liver regeneration 24 h after the damage might be not successful.

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.

Co-ordinate but disproportionate activation of apoptotic, regenerative and inflammatory pathways characterizes the liver response to acute amebic infection

The liver has the remarkable ability to respond to injury with repair and regeneration. The protozoan parasite Entamoeba histolytica is the major cause of liver abscess worldwide. We report a transcriptional analysis of the response of mouse liver to E. histolytica infection, the first study looking at acute liver infection by a non-viral pathogen. Focusing on early time points, we identified 764 genes with altered transcriptional levels in amebic liver abscess. The response to infection is rapid and complex, with concurrent increased expression of genes linked to host defence through IL-1, TLR2, or interferon-induced pathways, liver regeneration via activation of IL-6 pathways, and genes associated with programmed cell death possibly through TNFalpha or Fas pathways. A comparison of amebic liver infection with the liver response to partial hepatectomy or toxins reveals striking similarities between amebic liver abscess and non-infectious injury in key components of the liver regeneration pathways. However, the response in amebic liver abscess is biased towards apoptosis when compared with acute liver injury from hepatectomy, toxins, or other forms of liver infection. E. histolytica infection of the liver simultaneously activates inflammatory, regenerative and apoptotic pathways, but the sum of these early responses is biased towards programmed cell death.

Progenitor cells in liver regeneration: molecular responses controlling their activation and expansion

Although normally quiescent, the adult mammalian liver possesses a great capacity to regenerate after different types of injuries in order to restore the lost liver mass and ensure maintenance of the multiple liver functions. Major players in the regeneration process are mature residual cells, including hepatocytes, cholangiocytes and stromal cells. However, if the regenerative capacity of mature cells is impaired by liver-damaging agents, hepatic progenitor cells are activated and expand into the liver parenchyma. Upon transit amplification, the progenitor cells may generate new hepatocytes and biliary cells to restore liver homeostasis. In recent years, hepatic progenitor cells have been the subject of increasing interest due to their therapeutic potential in numerous liver diseases as alternative or supportive/complementary tools to liver transplantation. While the first investigations on hepatic progenitor cells have focused on their origin and phenotypic characterization, recent attention has focused on the influence of the hepatic microenvironment on their activation and proliferation. This microenvironment comprises the extracellular matrix, epithelial and non-epithelial resident liver cells, and recruited inflammatory cells as well as the variety of growth-modulating molecules produced and/or harboured by these elements. The cellular and molecular responses to different regenerative stimuli seem to depend on the injury inflicted and consequently on the molecular microenvironment created in the liver by a certain insult. This review will focus on molecular responses controlling activation and expansion of the hepatic progenitor cell niche, emphasizing similarities and differences in the microenvironments orchestrating regeneration by recruitment of progenitor cell populations or by replication of mature cells.

Rapid induction and activation of Tec tyrosine kinase in liver regeneration

BACKGROUND

Previous studies have indicated that Tec tyrosine kinase is differentially expressed in the regenerating liver. The purpose of the present study was to further investigate the potential involvement of Tec tyrosine kinase in hepatocyte proliferation and liver regeneration.

METHODS

Tec kinase gene expression after partial (two-thirds) hepatectomy was examined by representational difference analysis. Tissue distribution and potential involvement of Tec kinase in liver regeneration and hepatocyte proliferation were then determined by northern blotting, reverse transcription-polymerase chain reaction (RT-PCR), and western blotting. Full-length rat Tec cDNA was cloned.

RESULTS

Using this cDNA as the probe, northern blotting showed that Tec was specifically expressed in liver and kidney, the highest expression of Tec being detected in embryonic day 15-19 fetal livers. In contrast, the expression level of Tec in adult and neonatal rat livers was significantly decreased. Similar results were obtained from western blotting analyzes. It was thus hypothesized that Tec might be involved in hepatocyte proliferation. To test this hypothesis, Tec expression was examined in regenerating rat livers. An increase in Tec expression and activation of Tec kinase were observed within 1 h after partial hepatectomy. Moreover, it has been shown that hepatocyte growth factor (HGF) dramatically induces Tec expression in primary rat hepatocytes. Additionally, it was observed that Tec gene expression in serum-starved liver tumor cell line HepG2 was substantially decreased. Stimulation with 10% fetal bovine serum and insulin but not epidermal growth factor resulted in dramatic elevation of Tec expression in these cells.

CONCLUSION

Tec is an inducible early response gene that might enhance hepatocyte proliferation and liver regeneration.

Hepatocyte growth factor/scatter factor and MET are involved in arterial repair and atherogenesis

Several studies have shown that in the arterial wall hepatocyte growth factor/scatter factor (HGF/SF) is expressed by smooth muscle cells (SMCs) but acts on endothelial cells, not SMCs. Other studies, however, have indicated that SMCs can respond to HGF/SF. We have reinvestigated expression and activity of HGF/SF and its receptor MET in arterial SMC and endothelial cell cultures and in whole arteries after superficial or deep injury or atherogenesis. High-density cultures of SMCs produced HGF/SF but did not express MET, whereas SMCs, at the leading edge of injured cultures, expressed both ligand and receptor and showed a dramatic motility and growth response to HGF/SF. In line with these results, HGF/SF and MET expression was undetectable in the media of uninjured carotid arteries but was induced after deep arterial injury in areas of SMC migration in the neointima. Strong MET expression was also observed in the SMCs of the atherosclerotic lesions of homozygous apoE(-/-) mice, whereas HGF/SF was expressed by macrophage-derived foam cells. These results demonstrate that MET is induced in migrating and proliferating SMCs and that HGF/SF and MET are key mediators of the SMC response in atherogenesis.

A novel prognostic subtype of human hepatocellular carcinoma derived from hepatic progenitor cells

The variability in the prognosis of individuals with hepatocellular carcinoma (HCC) suggests that HCC may comprise several distinct biological phenotypes. These phenotypes may result from activation of different oncogenic pathways during tumorigenesis and/or from a different cell of origin. Here we address whether the transcriptional characteristics of HCC can provide insight into the cellular origin of the tumor. We integrated gene expression data from rat fetal hepatoblasts and adult hepatocytes with HCC from human and mouse models. Individuals with HCC who shared a gene expression pattern with fetal hepatoblasts had a poor prognosis. The gene expression program that distinguished this subtype from other types of HCC included markers of hepatic oval cells, suggesting that HCC of this subtype may arise from hepatic progenitor cells. Analyses of gene networks showed that activation of AP-1 transcription factors in this newly identified HCC subtype might have key roles in tumor development.

A new description of cellular quiescence

Cellular quiescence, defined as reversible growth/proliferation arrest, is thought to represent a homogenous state induced by diverse anti-mitogenic signals. We used transcriptional profiling to characterize human diploid fibroblasts that exited the cell cycle after exposure to three independent signals—mitogen withdrawal, contact inhibition, and loss of adhesion. We show here that each signal caused regulation of a unique set of genes known to be important for cessation of growth and division. Therefore, contrary to expectation, cells enter different quiescent states that are determined by the initiating signal. However, underlying this diversity we discovered a set of genes whose specific expression in non-dividing cells was signal-independent, and therefore representative of quiescence per se, rather than the signal that induced it. This fibroblast “quiescence program” contained genes that enforced the non-dividing state, and ensured the reversibility of the cell cycle arrest. We further demonstrate that one mechanism by which the reversibility of quiescence is insured is the suppression of terminal differentiation. Expression of the quiescence program was not simply a downstream consequence of exit from the cell cycle, because key parts, including those involved in suppressing differentiation, were not recapitulated during the cell cycle arrest caused by direct inhibition of cyclin-dependent kinases. These studies form a basis for understanding the normal biology of cellular quiescence.

Transcriptional profiling of fibroblasts induced to exit the cell cycle by distinct signals reveals distinctions and commonalities in the pathways to cellular quiescence.

Structural basis of hepatocyte growth factor/scatter factor and MET signalling

The polypeptide growth factor, hepatocyte growth factor/scatter factor (HGF/SF), shares the multidomain structure and proteolytic mechanism of activation of plasminogen and other complex serine proteinases. HGF/SF, however, has no enzymatic activity. Instead, it controls the growth, morphogenesis, or migration of epithelial, endothelial, and muscle progenitor cells through the receptor tyrosine kinase MET. Using small-angle x-ray scattering and cryo-electron microscopy, we show that conversion of pro(single-chain)HGF/SF into the active two-chain form is associated with a major structural transition from a compact, closed conformation to an elongated, open one. We also report the structure of a complex between two-chain HGF/SF and the MET ectodomain (MET928) with 1:1 stoichiometry in which the N-terminal and first kringle domain of HGF/SF contact the face of the seven-blade beta-propeller domain of MET harboring the loops connecting the beta-strands b-c and d-a, whereas the C-terminal serine proteinase homology domain binds the opposite "b" face. Finally, we describe a complex with 2:2 stoichiometry between two-chain HGF/SF and a truncated form of the MET ectodomain (MET567), which is assembled around the dimerization interface seen in the crystal structure of the NK1 fragment of HGF/SF and displays the features of a functional, signaling unit. The study shows how the proteolytic mechanism of activation of the complex proteinases has been adapted to cell signaling in vertebrate organisms, offers a description of monomeric and dimeric ligand-receptor complexes, and provides a foundation to the structural basis of HGF/SF-MET signaling.

Contact Inhibition of Hepatocyte Growth Regulated by Functional Association of the c-Met/Hepatocyte Growth Factor Receptor and LAR Protein-tyrosine Phosphatase

Contact inhibition, the inhibition of cell proliferation by tight cell-cell contact is a fundamental characteristic of normal cells. Using primary cultured hepatocytes, we investigated the mechanisms of contact inhibition that decrease the mitogenic activity of hepatocyte growth factor (HGF), focusing on the regulation of c-Met/HGF-receptor activation. In hepatocytes cultured at a sparse cell density, HGF stimulation induced prolonged c-Met tyrosine phosphorylation for over 5 h and a marked mitogenic response. In contrast, HGF stimulation induced transient c-Met tyrosine phosphorylation in <3 h and failed to induce mitogenic response in hepatocytes cultured at a confluent cell density. Treatment of the confluent cells with HGF plus orthovanadate, a broad spectrum protein-tyrosine phosphatase inhibitor, however, prolonged c-Met tyrosine phosphorylation for over 5 h and permitted the subsequent mitogenic response. The mitogenic response to HGF was associated with the duration of c-Met tyrosine phosphorylation even in the sparse cells. We found that the activity and expression of the protein-tyrosine phosphatase LAR increased following HGF stimulation specifically in confluent hepatocytes and not in sparse hepatocytes. LAR and c-Met were associated, and purified LAR dephosphorylated tyrosine-phosphorylated c-Met in in vitro phosphatase reactions. Furthermore, antisense oligonucleotides specific for LAR mRNA suppressed the expression of LAR, allowed prolonged c-Met tyrosine phosphorylation, and led to acquisition of a mitogenic response in hepatocytes even under the confluent condition. Thus functional association of LAR and c-Met underlies the inhibition of c-Met-mediated mitogenic signaling through the dephosphorylation of c-Met, which specifically occurs under the confluent condition.