Scatter-factor and semaphorin receptors: cell signalling for invasive growth

Oncogenes as Novel Targets for Cancer Therapy (Part I)

In the past 10 years, progress made in cancer biology, genetics, and biotechnology has led to a major transition in cancer drug design and development. There has been a change from an emphasis on non-specific, cytotoxic agents to specific, molecular-based therapeutics. Mechanism-based therapy is designed to act on cellular and molecular targets that are causally involved in the formation, growth, and progression of human cancers. These agents, which may have greater selectivity for cancer versus normal cells, and which may produce better anti-tumor efficacy and lower host toxicity, can be small molecules, natural or engineered peptides, proteins, antibodies, or synthetic nucleic acids (e.g. antisense oligonucleotides, ribozymes, and siRNAs). Novel targets are identified and validated by state-of-the-art approaches, including high-throughput screening, combinatorial chemistry, and gene expression arrays, which increase the speed and efficiency of drug discovery and development. Examples of oncogene-based, molecular therapeutics that show promising clinical activity include trastuzumab (Herceptin), imatinib (Gleevec), and gefitinib (Iressa). However, the full potential of oncogenes as novel targets for cancer therapy has not been realized and many challenges remain, from the validation of novel targets, to the design of specific agents, to the evaluation of these agents in both preclinical and clinical settings. In maximizing the benefits of molecular therapeutics in monotherapy or combination therapy of cancer, it is necessary to have an understanding of the underlying molecular abnormalities and mechanisms involved. This is the first part of a four-part review in which we discuss progress made in the last decade as it relates to the discovery of novel oncogenes and signal transduction pathways, in the context of their potential as targets for cancer therapy. This part delineates the latest discoveries about the potential use of growth factors and protein tyrosine kinases as targets for therapy. Later parts focus on intermediate signaling pathways, transcription factors, and proteins involved in cell cycle, DNA damage, and apoptotic pathways.

Stromal fibroblasts in cancer initiation and progression

It is widely accepted that the development of carcinoma--the most common form of human cancer--is due to the accumulation of somatic mutations in epithelial cells. The behaviour of carcinomas is also influenced by the tumour microenvironment, which includes extracellular matrix, blood vasculature, inflammatory cells and fibroblasts. Recent studies reveal that fibroblasts have a more profound influence on the development and progression of carcinomas than was previously appreciated. These new findings have important therapeutic implications.

An uncleavable form of pro-scatter factor suppresses tumor growth and dissemination in mice

Scatter factor (SF), also known as hepatocyte growth factor, is ubiquitously present in the extracellular matrix of tissues in the form of an inactive precursor (pro-SF). In order to acquire biological activity, pro-SF must be cleaved by specific proteases present on the cell surface. The mature form of SF controls invasive cues in both physiological and pathological processes through activation of its receptor, the Met tyrosine kinase. By substituting a single amino acid in the proteolytic site, we engineered an unprocessable form of pro-SF (uncleavable SF). Using lentivirus vector technology, we achieved local or systemic delivery of uncleavable SF in mice. We provide evidence that (a) uncleavable SF inhibits both protease-mediated pro-SF conversion and active SF-induced Met activation; (b) local expression of uncleavable SF in tumors suppresses tumor growth, impairs tumor angiogenesis, and prevents metastatic dissemination; and (c) systemic expression of uncleavable SF dramatically inhibits the growth of transplanted tumors and abolishes the formation of spontaneous metastases without perturbing vital physiological functions. These data show that proteolytic activation of pro-SF is a limiting step in tumor progression, thus suggesting a new strategy for the treatment or prevention of the malignant conversion of neoplastic lesions.

Netrin-1 and its receptors in tumorigenesis

Netrin-1 and its receptors DCC (deleted in colorectal cancer) and the UNC5 orthologues (human UNC5A-D and rodent UNC5H1-4) define a new mechanism for both the positive (induction) and negative (suppression) regulation of apoptosis. Accumulating evidence implies that for human cancers, this positive signalling pathway is frequently inactivated. Surprisingly, binding of netrin-1 to its receptors inhibits tumour suppressor p53-dependent apoptosis, and p53 is directly involved in transcriptional regulation of netrin-1 and its receptors. So, the netrin-1 receptor pathways probably play an important part in tumorigenesis.

To move or not to move? Semaphorin signalling in cell migration

Semaphorins were discovered 11 years ago as molecular cues for axon guidance that are conserved from invertebrates to humans. More than 20 semaphorin genes have been identified in mammals and their protein products are now known to be involved in a range of processes from the guidance of cell migration to the regulation of the immune response, angiogenesis and cancer. Plexins, either alone or in association with neuropilins, constitute high-affinity semaphorin receptors. However, other transmembrane molecules have been implicated in semaphorin receptor complexes, and interactions between plexins and a range of intracellular effectors have been reported. These data indicate that semaphorins might be able to elicit responses through more than one signalling pathway. Interestingly, according to recent findings, the semaphorin-dependent control of cell migration crucially involves integrin-based adhesive structures through which polarized cell-membrane protrusion is coupled to cytoskeletal dynamics. This review focuses on the mechanisms whereby semaphorins are thought to regulate cell migration.

c-Met as a target for human cancer and characterization of inhibitors for therapeutic intervention

Receptor tyrosine kinase (RTK) targeted agents such as trastuzumab, imatinib, bevacizumab, and gefitinib inhibitors have illustrated the utility of targeting this protein class for treatment of selected cancers. A unique member of the RTK family, c-Met, also represents an intriguing target for cancer therapy that is yet to be explored in a clinical setting. The proto-oncogene, c-Met, encodes the high-affinity receptor for hepatocyte growth factor (HGF) or scatter factor (SF). c-Met and HGF are each required for normal mammalian development and have been shown to be particularly important in cell migration, morphogenic differentiation, and organization of three-dimensional tubular structures (e.g. renal tubular cells, gland formation, etc.) as well as cell growth and angiogenesis. Both c-Met and HGF have been shown to be deregulated in and to correlate with poor prognosis in a number of major human cancers. New data describing the constitutive phosphorylation of c-Met in a number of human tumors is presented here along with a variety of mechanisms by which c-Met can become activated, including mutation and gene amplification. In support of the clinical data implicating c-Met activation in the pathogenesis of human cancers, introduction of c-Met and HGF (or mutant c-Met) into cells conferred the properties of motility, invasiveness, and tumorgenicity to the transformed cells. Conversely, the inhibition of c-Met with a variety of receptor antagonists inhibited the motility, invasiveness, and tumorgenicity of human tumor cell lines. Consistent with this observation, small-molecule inhibitors of c-Met were developed that antagonized c-Met/HGF-dependent phenotypes and tumor growth in mouse models. This review will address the potential for development of c-Met inhibitors for treatment of human cancers with particular emphasis on recent findings with small-molecule inhibitors.

Epidermal growth factor receptor, c-MET, beta-catenin, and p53 expression as prognostic indicators in stage II colon cancer: a tissue microarray study

PURPOSE

Through the use of molecular markers, it may be possible to identify aggressive tumor phenotypes and tailor therapies to treat them. This approach would be particularly useful for stage II colon cancer. The purpose of this study was to define the prognostic value of epidermal growth factor receptor (EGFR), c-MET, beta-catenin, and p53 protein expression in TNM stage II colon cancer using tissue microarray technology.

EXPERIMENTAL DESIGN

In this study, we retrospectively analyzed, resected, and otherwise untreated paraffin-embedded specimens from 134 consecutive patients with Tumor-Node-Metastasis stage II colonic carcinomas for EGFR, c-MET, beta-catenin, and p53 protein expression by immunohistochemistry.

RESULTS

Thirty-five percent, 77, and 65% of tumors exhibited strong (+2 and +3 immunopositivity) expression of EGFR, c-MET, and beta-catenin, respectively. Fifty-four percent exhibited nuclear staining for p53 in >10% of the tumor cells. Univariate analysis revealed that increased nuclear p53 expression (P = 0.001), strong membranous EGFR expression (P = 0.04), and lymphovascular invasion (P = 0.01) were significantly related to disease recurrence and that p53 (P = 0.02) and EGFR (P = 0.05) expression were associated with decreased survival. Increased nuclear p53 expression also correlated with the presence of distal metastasis (P = 0.027). No significant association was seen between c-MET expression and prognosis, whereas a strong trend was detected between loss of beta-catenin (P = 0.065) expression and poor outcome. Multivariate analysis indicated that p53 (P = 0.04), EGFR (P = 0.05), and lymphovascular invasion (P = 0.03) were independent predictors of recurrence and that p53 (P = 0.02) and EGFR (P = 0.01) expression were both associated with poor survival.

CONCLUSIONS

This retrospective tumor microarray study, restricted to Tumor-Node-Metastasis stage II colon cancer patients who did not undergo adjuvant therapy, supports the use of EGFR and p53 as biological markers, which may assist in predicting disease recurrence and survival.

Early onset hereditary papillary renal carcinoma: germline missense mutations in the tyrosine kinase domain of the met proto-oncogene

PURPOSE

Hereditary papillary renal carcinoma (HPRC) is characterized by a predisposition to multiple, bilateral papillary type 1 renal tumors caused by inherited activating missense mutations in the tyrosine kinase domain of the MET proto-oncogene. In the current study we evaluated the clinical phenotype and germline MET mutation of 3 new HPRC families. We describe the early onset clinical features of HPRC.

MATERIALS AND METHODS

We identified new HPRC families of Italian (family 177), Spanish (family 223) and Cuban (family 268) descent. We evaluated their clinical features, performed MET mutation analysis by denaturing high performance liquid chromatography and DNA sequencing, and estimated age dependent penetrance and survival using Kaplan-Meier analysis. We characterized renal tumors by histology and fluorescence in situ hybridization.

RESULTS

Identical germline MET c.3522G --> A mutations (V1110I) were identified in families 177 and 268 but no evidence of a founder effect was found. Affected members of family 223 carried a germline c.3906G --> C.3522G --> A MET mutation (V1238I). Age dependent penetrance but not survival was significantly earlier for the c.3522G -->A mutation than for the c.3906G --> A mutation in these HPRC families. Trisomy of chromosome 7 and papillary renal carcinoma type 1 histology were detected in papillary renal tumors.

CONCLUSIONS

HPRC can occur in an early onset form. The median age for renal tumor development in these 3 HPRC families was 46 to 63 years. HPRC associated papillary renal tumors may be aggressive and metastasize, leading to mortality. Median survival age was 60 to 70 years. Families with identical germline mutations in MET do not always share a common ancestor. HPRC is characterized by germline mutations in MET and papillary type 1 renal tumor histology.

Exploitation of host cell cytoskeleton and signalling during Listeria monocytogenes entry into mammalian cells

Deciphering how Listeria monocytogenes exploits the host cell machinery to invade mammalian cells during infection isa key issue for the understanding how this food-borne pathogen causes a pleiotropic disease ranging from gastro-enteritis to meningitis and abortions. Using multidisciplinary approaches, essentially combining bacterial genetics and cell biology, we have identified two bacterial proteins critical for entry into target cells, InlA and InlB. Their cellular ligands have been also identified: InlA interacts with the adhesion molecule E-cadherin, while InlB interacts with the receptor for the globular head of the complement factor Clq (gClq-R), with the hepatocyte growth factor receptor (c-Met) and with glycosaminoglycans(including heparan sulphate). The dynamic interaction between these cellular receptors and the actin cytoskeleton is currently under investigation. Several intracellular molecules have been recognized as key effectors for Listeria entry into target cells,including catenins (implicated in the connection of E-cadherin to actin) and the actin depolymerising factor/cofilin (involved in the rearrangement of the cytoskeleton in the InlB-dependent internalisation pathway). At the organism level, species specificity has been discovered concerning the interaction between InlA and E-cadherin, leading to the generation of transgenic mice expressing the human E-cadherin, in which the critical role of InlA in the crossing of the intestinal barrier has been clearly determined. Listeria appears as an instrumental model for addressing critical questions concerning both the complex process of bacterial pathogenesis and also fundamental molecular processes, such as phagocytosis.

Hepatocyte growth factor/c-Met signaling promotes the progression of experimental human neuroblastomas

Neuroblastoma is the most frequent solid childhood malignancy. Despite aggressive therapy, mortality is high due to rapid tumor progression to advanced stages. The molecules and mechanisms underlying poor prognosis are not well understood. Here, we report that cultured human neuroblastoma cells express the hepatocyte growth factor (HGF) and its receptor c-Met. Binding of HGF to c-Met triggers receptor autophosphorylation, indicating functional relevance of this interaction. HGF activates several downstream effectors of c-Met such as the mitogen-activated protein kinases extracellular signal-regulated kinase 1/extracellular signal-regulated kinase 2 and phospholipase C-gamma, whereas signal transducer and activator of transcription 3 is constitutively activated in neuroblastoma cells expressing c-Met. In addition, HGF is able to stimulate expression and proteolytic activity of matrix metalloproteinase-2 and tissue-type plasminogen activator in neuroblastoma cells, thereby promoting degradation of extracellular matrix components. We show that HGF stimulates invasion of neuroblastoma cells in vitro and in vivo, and it promotes the formation of angiogenic neuroblastomas in vivo. These processes can be blocked by specific inhibitors of the mitogen-activated protein kinase cascade, by inhibitors of phospholipase C-gamma, and also by the expression of a dominant negative signal transducer and activator of transcription 3 mutant. Our data provide the first evidence that the HGF/c-Met pathway is essential for invasiveness and malignant progression of human neuroblastomas. They further suggest that specific inhibitors of this pathway may be suitable as therapeutic agents to improve clinical outcome of neuroblastomas.

Multiple myeloma cells catalyze hepatocyte growth factor (HGF) activation by secreting the serine protease HGF-activator

Multiple myeloma (MM) is a common hematologic neoplasm consisting of malignant plasma cells, which expand in the bone marrow. A potential key signal in the evolution of MM is hepatocyte growth factor (HGF), which acts as a potent paracrine and/or autocrine growth factor and survival factor for MM cells. Proteolytic conversion of HGF into its active form is a critical limiting step in HGF/MET signaling. Here, we show that malignant MM plasma cells convert HGF into its active form and secrete HGF-activator (HGFA), a serine protease specific for HGF activation. By using serine protease inhibitors and neutralizing antibodies, we demonstrate that HGFA produced by the MM cells is responsible for their ability to catalyze HGF activation. We, therefore, suggest that autocatalyzation of HGF conversion by MM cells is an important step in HGF/MET-induced myeloma growth and survival, which may have implications for the management of this incurable form of cancer. (Blood. 2004;104:2172-2175)

Targeting Plasmodium host cells: survival within hepatocytes

Upon entering their host, Plasmodium sporozoites travel directly to the liver. Once there, they migrate through several hepatocytes before they infect a final one. During migration, sporozoites breach the plasma membrane of traversed hepatocytes, but to infect they must form a parasitophorous vacuole, in which the intra-hepatic form of the parasite grows and multiplies. During this period there is a remarkable parasite multiplication, but little is known about the requirements and strategies that are developed to be successful. Hepatocyte growth factor and its receptor on hepatocytes might enhance early Plasmodium development within these cells. We anticipate that this might be the basis for further studies on host-cell requirements for Plasmodium development.

Truncated RON tyrosine kinase drives tumor cell progression and abrogates cell-cell adhesion through E-cadherin transcriptional repression

RON is a tyrosine kinase receptor that triggers scattering of normal cells and invasive growth of cancer cells on ligand binding. We identified a short RON mRNA, which is expressed in human lung, ovary, tissues of the gastrointestinal tract, and also in several human cancers, including ovarian carcinomas and cell lines from pancreatic carcinomas and leukemias. This transcript encodes a truncated protein (short-form RON; sf-RON), lacking most of the RON receptor extracellular domain but retaining the whole transmembrane and intracellular domains. Sf-RON shows strong intrinsic tyrosine kinase activity and is constitutively phosphorylated. Epithelial cells transduced with sf-RON display an aggressive phenotype; they shift to a nonepithelial morphology, are unable to form aggregates, grow faster in monolayer cultures, show anchorage-independent growth, and become motile. We show that in these cells, E-cadherin expression is lost through a dominant transcriptional repression pathway likely mediated by the transcriptional factor SLUG. Altogether, these data show that expression of a naturally occurring, constitutively active truncated RON kinase results in loss of epithelial phenotype and aggressive behavior and, thus, it might contribute to tumor progression.

Class IV semaphorins promote angiogenesis by stimulating Rho-initiated pathways through plexin-B

The semaphorins are a large family of secreted and cell surface proteins that provide attractive and repulsive cues for axon guidance during neuronal development. Semaphorins share a conserved NH(2)-terminal Sema domain with their receptors, the plexins, which mediate neuronal cell adhesion, axon guidance, and maintenance of established neuronal pathways in the adult. Both semaphorins and plexins share structural homology with the extracellular domain of c-Met, a member of the scatter factor family of receptors. However, the highly conserved cytoplasmic region of plexins has no homology with the c-Met tyrosine kinase or with any other known protein. Using a recently developed antibody and RNA analysis, we found that high levels of plexin-B1 are expressed in endothelial cells. Whereas c-Met, with which plexin-B1 can interact, is known to be a potent promoter of angiogenesis, the effects of semaphorin-mediated plexin activation in endothelial cells are still poorly understood. Here, we examined the role of plexin-B1 activation in angiogenesis using a purified, secreted form of its ligand, Semaphorin 4D (Sema4D). Sema4D potently induced chemotaxis and tubulogenesis in endothelial cells and enhanced blood vessel formation in an in vivo mouse model. Interestingly, responses to Sema4D did not require c-Met activation. Instead, the use of chimeric plexin-B1 receptors, Rho inhibitors, and lentiviral gene delivery of interfering molecules revealed that these proangiogenic effects are dependent on a COOH-terminal PDZ-binding motif of plexin-B1, which binds two guanine nucleotide exchange factors for the small GTPase Rho, PDZ-RhoGEF and LARG, and are mediated by the activation of Rho-initiated pathways.

Progressive changes in Met-dependent signaling in a human ovarian surface epithelial model of malignant transformation

We used an experimental in vitro model of human ovarian surface epithelium (OSE), the tissue of origin of >90% of ovarian cancers, to more precisely define the contribution of hepatocyte growth factor (HGF) to various OSE phenotypes at different stages of neoplastic progression. Neoplastic transformation of OSE in cultures was achieved by multiple genetic manipulations, resulting in the nontumorigenic line IOSE-29, the tumorigenic IOSE-Ov29, and the tumor-derived, more highly malignant IOSE-Ov29/T4. We demonstrate here that, compared to IOSE-29, IOSE-Ov29 and IOSE-Ov29/T4 exhibited higher levels of the HGF receptor Met and an increasing duration of ERK1/2 activation with malignant progression, in conjunction with other neoplastic properties. HGF activated Met signaling in all lines but elicited different responses: HGF induced cell dispersion (scattering) and collagen gel invasion in IOSE-Ov29 and IOSE-Ov29/T4 but did not alter the growth pattern of IOSE-29. Inhibition with PD98059 and LY294002 independently prevented HGF-induced invasive growth. Furthermore, our results show that HGF-induced invasion can be mediated through a rapamycin-sensitive p70 S6K cascade, which demonstrates that p70S6K can regulate cell motility in addition to its well-established role in protein synthesis. Taken together, our data correlate specific responses to HGF-mediated signaling with specific signaling pathways and with progressive neoplastic changes.

The Semaphorin 4D receptor Plexin-B1 is a GTPase activating protein for R-Ras

Plexins are cell surface receptors for semaphorin molecules, and their interaction governs cell adhesion and migration in a variety of tissues. We report that the Semaphorin 4D (Sema4D) receptor Plexin-B1 directly stimulates the intrinsic guanosine triphosphatase (GTPase) activity of R-Ras, a member of the Ras superfamily of small GTP-binding proteins that has been implicated in promoting cell adhesion and neurite outgrowth. This activity required the interaction of Plexin-B1 with Rnd1, a small GTP-binding protein of the Rho family. Down-regulation of R-Ras activity by the Plexin-B1-Rnd1 complex was essential for the Sema4D-induced growth cone collapse in hippocampal neurons. Thus, Plexin-B1 mediates Sema4D-induced repulsive axon guidance signaling by acting as a GTPase activating protein for R-Ras.

Insights into function of PSI domains from structure of the Met receptor PSI domain

PSI domains are cysteine-rich modules found in extracellular fragments of hundreds of signaling proteins, including plexins, semaphorins, integrins, and attractins. Here, we report the solution structure of the PSI domain from the human Met receptor, a receptor tyrosine kinase critical for proliferation, motility, and differentiation. The structure represents a cysteine knot with short regions of secondary structure including a three-stranded antiparallel beta-sheet and two alpha-helices. All eight cysteines are involved in disulfide bonds with the pattern consistent with that for the PSI domain from Sema4D. Comparison with the Sema4D structure identifies a structurally conserved core comprising the N-terminal half of the PSI domain. Interestingly, this part links adjacent SEMA and immunoglobulin domains in the Sema4D structure, suggesting that the PSI domain serves as a wedge between propeller and immunoglobulin domains and is responsible for the correct positioning of the ligand-binding site of the receptor.

SU5416 is a potent inhibitor of hepatocyte growth factor receptor (c-Met) and blocks HGF-induced invasiveness of human HepG2 hepatoma cells

BACKGROUND/AIMS

SU5416 is a potent inhibitor of receptor tyrosine kinases, including those of the vascular endothelial growth factor receptor, stem cell factor receptor, and platelet-derived growth factor receptor. Because of the overwhelming evidence favoring the role of aberrant hepatocyte growth factor (HGF)/Met signaling in the pathogenesis of various human cancers, various inhibitor strategies have been employed to therapeutically target this receptor.

METHODS

Cell proliferation was determined by incorporation of [(3)H] thymidine. Invasiveness was assayed in Boyden Chambers with 8 microm Matrigel coated filters. Phosphorylation of ERK1/2, Akt by HGF stimulation was detected by Western blotting.

RESULTS

We found that SU5416 inhibited motility scattering and the invasive activity of a hepatocellular carcinoma cell line HepG2 in vitro and growth in primary cultured hepatocytes induced by HGF. Consequently, tyrosine autophosphorylation of the c-met induced by HGF was inhibited in these cells by SU5416 in a dose-dependent manner. Furthermore, ERK1/2 and Akt phosphorylation, the signaling events down-stream of c-met activation were reduced. Moreover, SU5416 caused reversion in NIH3T3 fibroblasts transformed by the oncogenic form of the receptor, Tpr-Met.

CONCLUSIONS

Inhibition of various solid tumors growth and metastasis by SU5416 may be partially attributed to blocking activation of the hepatocyte growth factor receptor.

Interplay between scatter factor receptors and B plexins controls invasive growth

Met and Ron tyrosine kinases are members of the Scatter Factor Receptor family. Met is the receptor for hepatocyte growth factor while Ron is that for macrophage stimulating protein. On ligand stimulation, activation of these receptors induces 'invasive growth', a complex biological response involved in tissue morphogenesis and, when deregulated, in tumor progression and metastasis. Scatter Factor Receptors share structural homology with Plexins, transmembrane receptors for Semaphorins, a family of ligands originally identified as axon guidance molecules. A physical and functional association between Met and Plexin B1, the prototype of class B Plexin subfamily, has been previously demonstrated. Here, we show that both Met and Ron receptors can interact with each of the three members of class B Plexins, even in the absence of their ligands and that Plexin B1 ligand, Sema 4D, can induce activation of Met and Ron receptors, promoting an invasive response. Furthermore, in some human neoplastic cell lines Plexin B1 is overexpressed, constitutively tyrosine phosphorylated, and associated with Scatter Factor Receptors. These data extend the crosstalk previously described between Met and Plexin B1 to the entire families of Scatter Factor Receptors and class B Plexins and show that interaction with multiple upstream activators can finely tune the invasive growth process both in physiological conditions and in tumor growth and metastatization.

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

Hepatocyte growth factor/scatter factor c-met signaling pathway is of central importance during development as well as in tumorigenesis. Because homozygous null mice for either hgf/sf or c-met die in utero, we used Cre/loxP-mediated gene targeting to investigate the function of c-met specifically in the adult liver. Loss of c-met appeared not to be detrimental to hepatocyte function under physiological conditions. Nonetheless, the adaptive responses of the liver to injury were dramatically affected. Mice lacking c-met gene in hepatocytes were hypersensitive to Fas-induced apoptosis. When injected with a low dose of anti-Fas antibody, the majority of these mice died from massive apoptosis and hemorrhagic necrosis, whereas all wild-type mice survived with signs of minor injury. After a challenge with a single necrogenic dose of CCl4, c-met conditional knockout mice exhibited impaired recovery from centrolobular lesions rather than a deficit in hepatocyte proliferation. The delayed healing was associated with a persistent inflammatory reaction, over-production of osteopontin, early and prominent dystrophic calcification, and impaired hepatocyte scattering/migration into diseased areas. These studies provide direct genetic evidence in support of the critical role of c-met in efficient liver regeneration and suggest that disruption of c-met affects primarily hepatocyte survival and tissue remodeling.

Met decoys: will cancer take the bait?

Inappropriate Met receptor tyrosine kinase signaling can produce proliferative, invasive, angiogenic, and antiapoptotic activities that contribute to malignant growth. Met can be activated by paracrine or autocrine mechanisms in a ligand-dependent fashion, or be constitutively activated by mutation and by other ligand-independent mechanisms. Because Met is inappropriately expressed in almost all types of human cancer, the HGF/SF-Met signaling pathway should be an exceptional target for cancer intervention strategies and therapies. In this issue of Cancer Cell, two reports show that the extracellular domain of Met is an important target for developing anticancer therapies.

Targeting the tumor and its microenvironment by a dual-function decoy Met receptor

Met, the receptor for hepatocyte growth factor (HGF), is activated in human cancer by both ligand-dependent and -independent mechanisms. We engineered a soluble Met receptor (decoy Met) that interferes with both HGF binding to Met and Met homodimerization. By lentiviral vector technology, we achieved local or systemic delivery of decoy Met in mice. We provide evidence that in vivo expression of decoy Met (1) inhibits tumor cell proliferation and survival in a variety of human xenografts, (2) impairs tumor angiogenesis by preventing host vessel arborization, (3) suppresses or prevents the formation of spontaneous metastases, and (4) synergizes with radiotherapy in inducing tumor regression, without (5) affecting housekeeping physiological functions in the adult animal.

Structural and functional basis of the serine protease-like hepatocyte growth factor beta-chain in Met binding and signaling

Hepatocyte growth factor (HGF), a plasminogen-related growth factor, is the ligand for Met, a receptor tyrosine kinase implicated in development, tissue regeneration, and invasive tumor growth. HGF acquires signaling activity only upon proteolytic cleavage of single-chain HGF into its alpha/beta heterodimer, similar to zymogen activation of structurally related serine proteases. Although both chains are required for activation, only the alpha-chain binds Met with high affinity. Recently, we reported that the protease-like HGF beta-chain binds to Met with low affinity (Stamos, J., Lazarus, R. A., Yao, X., Kirchhofer, D., and Wiesmann, C. (2004) EMBO J. 23, 2325-2335). Here we demonstrate that the zymogen-like form of HGF beta also binds Met, albeit with 14-fold lower affinity than the protease-like form, suggesting optimal interactions result from conformational changes upon cleavage of the single-chain form. Extensive mutagenesis of the HGF beta region corresponding to the active site and activation domain of serine proteases showed that 17 of the 38 purified two-chain HGF mutants resulted in impaired cell migration or Met phosphorylation but no loss in Met binding. However, reduced biological activities were well correlated with reduced Met binding of corresponding mutants of HGF beta itself in assays eliminating dominant alpha-chain binding contributions. Moreover, the crystal structure of HGF beta determined at 2.53 A resolution provides a structural context for the mutagenesis data. The functional Met binding site is centered on the "active site region" including "triad" residues Gln(534) [c57], Asp(578) [c102], and Tyr(673) [c195] and neighboring "activation domain" residues Val(692), Pro(693), Gly(694), Arg(695), and Gly(696) [c214-c219]. Together they define a region that bears remarkable resemblance to substrate processing regions of serine proteases. Models of HGF-dependent Met receptor activation are discussed.

Divergent functions of murine Pax3 and Pax7 in limb muscle development

Pax genes encode evolutionarily conserved transcription factors that play critical roles in development. Pax3 and Pax7 constitute one of the four Pax subfamilies. Despite partially overlapping expression domains, mouse mutations for Pax3 and Pax7 have very different consequences. To investigate the mechanism of these contrasting phenotypes, we replaced Pax3 by Pax7 by using gene targeting in the mouse. Pax7 can substitute for Pax3 function in dorsal neural tube, neural crest cell, and somite development, but not in the formation of muscles involving long-range migration of muscle progenitor cells. In limbs in which Pax3 is replaced by Pax7, the severity of the muscle phenotype increases as the number of Pax7 replacement alleles is reduced, with the forelimb more affected than the hindlimb. We show that this hypomorphic activity of Pax7 is due to defects in delamination, migration, and proliferation of muscle precursor cells with inefficient activation of c-met in the hypaxial domain of the somite. Despite this, overall muscle patterning is retained. We conclude that functions already prefigured by the single Pax3/7 gene present before vertebrate radiation are fulfilled by Pax7 as well as Pax3, whereas the role of Pax3 in appendicular muscle formation has diverged, reflecting the more recent origin of this mode of myogenesis.

Crystal structure of the HGF beta-chain in complex with the Sema domain of the Met receptor

The Met tyrosine kinase receptor and its ligand, hepatocyte growth factor (HGF), play important roles in normal development and in tumor growth and metastasis. HGF-dependent signaling requires proteolysis from an inactive single-chain precursor into an active alpha/beta-heterodimer. We show that the serine protease-like HGF beta-chain alone binds Met, and report its crystal structure in complex with the Sema and PSI domain of the Met receptor. The Met Sema domain folds into a seven-bladed beta-propeller, where the bottom face of blades 2 and 3 binds to the HGF beta-chain 'active site region'. Mutation of HGF residues in the area that constitutes the active site region in related serine proteases significantly impairs HGF beta binding to Met. Key binding loops in this interface undergo conformational rearrangements upon maturation and explain the necessity of proteolytic cleavage for proper HGF signaling. A crystallographic dimer interface between two HGF beta-chains brings two HGF beta:Met complexes together, suggesting a possible mechanism of Met receptor dimerization and activation by HGF.

Expression of neuropilin-1 in high-grade dysplasia, invasive cancer, and metastases of the human gastrointestinal tract

Neuropilin-1 (NRP-1) functions as an axonal guidance molecule in the developing nervous system, and recent work has identified NRP-1 up-regulation in several cancers, including neuroblastomas and breast carcinoma. We examined for the first time NRP-1 expression in a large variety of gastrointestinal carcinomas and precursor lesions to determine whether NRP-1 up-regulation correlated with invasive growth in this system. Protein expression and localization of NRP-1 were studied by immunolabeling and semiquantification in >300 dysplastic, invasive, and metastatic lesions of the gastrointestinal tract, and confirmation of NRP-1 protein expression was performed by Western blot analysis on pancreatic cancer cell lines. NRP-1 expression was limited within normal tissues of the gastrointestinal tract, with prominent labeling present only in endothelial cells, pancreatic islet cells, and the most apical colonic epithelium. Invasive cancer of the esophagus, gallbladder, ampulla of Vater, pancreas (endocrine and exocrine), and colon, however, all demonstrated striking NRP-1 expression. NRP-1 was also identified in precursor lesions of gastrointestinal adenocarcinomas, such as Barrett esophagus and colorectal adenomas. Within the spectrum of precursor lesions, a progressive increase in both intensity and area of expression was evident during histologic progression from low-grade to high-grade dysplasia. Notably, the most intense up-regulation of NRP-1 was apparent at or around the point of invasion, with focal expression of NRP-1 at levels equivalent to the invasive cancer (2- to 3-fold increase). Prominent labeling for NRP-1 was apparent in primary invasive cancers, liver metastases, and a subset of lymph node metastases, with a 2- to 3-fold increase of NRP-1 over dysplastic lesions. We conclude that increased expression of NRP-1 occurs in gastrointestinal adenocarcinomas and in a subset of high-grade precursor lesions. This up-regulation appears to parallel invasive behavior and may therefore be used as a potential marker for cancer aggressiveness in this system.

Reactive oxygen species mediate Met receptor transactivation by G protein-coupled receptors and the epidermal growth factor receptor in human carcinoma cells

Cross-communication between the Met receptor tyrosine kinase and the epidermal growth factor receptor (EGFR) has been proposed to involve direct association of both receptors and EGFR kinase-dependent phosphorylation. Here, we demonstrate that in human hepatocellular and pancreatic carcinoma cells the Met receptor becomes tyrosine phosphorylated not only upon EGF stimulation but also in response to G protein-coupled receptor (GPCR) agonists. Whereas specific inhibition of the EGFR kinase activity blocked EGF- but not GPCR agonist-induced Met receptor transactivation, it was abrogated in the presence of a reducing agent or treatment of cells with a NADPH oxidase inhibitor. Both GPCR ligands and EGF are further shown to increase the level of reactive oxygen species within the cell. Interestingly, stimulation of the Met receptor by either GPCR agonists, EGF or its cognate ligand HGF, resulted in release of Met-associated beta-catenin and in its Met-dependent translocation into the nucleus, as analyzed by small interfering RNA-mediated knockdown of the Met receptor. Our results provide a new molecular explanation for cell surface receptor cross-talk involving the Met receptor and thereby link the wide diversity of GPCRs and the EGFR to the oncogenic potential of Met signaling in human carcinoma cells.

Exploitation of host cell cytoskeleton and signalling during Listeria monocytogenes entry into mammalian cells

Deciphering how Listeria monocytogenes exploits the host cell machinery to invade mammalian cells during infection is a key issue for the understanding how this food-borne pathogen causes a pleiotropic disease ranging from gastro-enteritis to meningitis and abortions. Using multidisciplinary approaches, essentially combining bacterial genetics and cell biology, we have identified two bacterial proteins critical for entry into target cells, InlA and InlB. Their cellular ligands have been also identified: InlA interacts with the adhesion molecule E-cadherin, while InlB interacts with the receptor for the globular head of the complement factor C1q (gC1q-R), with the hepatocyte growth factor receptor (c-Met) and with glycosaminoglycans (including heparan sulphate). The dynamic interaction between these cellular receptors and the actin cytoskeleton is currently under investigation. Several intracellular molecules have been recognized as key effectors for Listeria entry into target cells, including catenins (implicated in the connection of E-cadherin to actin) and the actin depolymerising factor/cofilin (involved in the rearrangement of the cytoskeleton in the InlB-dependent internalisation pathway). At the organism level, species specificity has been discovered concerning the interaction between InlA and E-cadherin, leading to the generation of transgenic mice expressing the human E-cadherin, in which the critical role of InlA in the crossing of the intestinal barrier has been clearly determined. Listeria appears as an instrumental model for addressing critical questions concerning both the complex process of bacterial pathogenesis and also fundamental molecular processes, such as phagocytosis.

Hepatocyte growth factor/scatter factor in development, inflammation and carcinogenesis: its expression and role in oral tissues

Hepatocyte growth factor (HGF) was discovered as a potent mitogen for adult hepatocytes from the plasma of patients with fulminant hepatic failure. It is now known to be a broad-spectrum, multi-functional mitogen, motogen and morphogen. The activities of HGF are mediated through the signalling pathway of its receptor, c-Met. During tooth development, HGF is expressed in the dental papilla and c-Met is expressed in the inner enamel epithelium. The expression of HGF and c-Met indicates that HGF is involved in morphogenesis of the tooth by mediating epithelial-mesenchymal interactions. In the mature tooth, HGF expression by fibroblasts is enhanced in pulpitis and mediated through the induction of prostaglandin (PG) E(2); it is induced not only by inflammatory cytokines, but also by components of oral bacteria. Consequently, concentrations of HGF in gingival crevicular fluid (GCF) increase in periodontitis. The mitogenic and other biological activities, such as angiogenesis, of HGF contribute towards wound healing. Both HGF and c-Met are expressed in the developing tongue, and the signalling pathway of the latter is shown to be essential for myogenesis. Dysregulation of c-Met signalling is observed in carcinogenesis, but HGF also has cytotoxic activity to certain tumour cells. The reason for the discrepancy between these observations is not clear at present.

Tenascin-C and SF/HGF produced by myofibroblasts in vitro provide convergent pro-invasive signals to human colon cancer cells through RhoA and Rac

Myofibroblasts are present at the invasion front in colon cancer. In an attempt to understand their putative proinvasive activity, we have developed an in vitro model. Myofibroblasts isolated from colon cancer tissue or obtained through transdifferentiation of colon fibroblasts by transforming growth factor (TGF)-beta stimulate invasion of colon cancer cells into collagen type I and Matrigel. We identified two convergent proinvasive agents secreted by myofibroblasts: namely scatter factor/hepatocyte growth factor (SF/HGF) and the TGF-beta-upregulated extracellular matrix glycoprotein tenascin-C (TNC), each of which is necessary though not sufficient for invasion. Myofibroblast-stimulated invasion into collagen type I is characterized by a change from a round, nonmigratory morphotype with high RhoA and low Rac activity to an elongated, migratory morphotype with low RhoA and high Rac activity. RhoA inactivation is determined by the epidermal growth factor (EGF)-like repeats of TNC through EGF-receptor signaling that confers a permissive and priming signal for the proinvasive activity of SF/HGF that activates Rac via c-Met. We confirmed the validity of this mechanism by using pharmacological modulators and dominant negative or constitutive active mutants that interfere with RhoA-Rho kinase and Rac signaling. Our in vitro results point to a new putative proinvasive signal for colon cancer cells provided by myofibroblasts in the tumor stroma.

Identification of a gene expression signature associated with recurrent disease in squamous cell carcinoma of the head and neck

Molecular studies of squamous cell carcinoma of the head and neck (HNSCC) have demonstrated multiple genetic abnormalities such as activation of various oncogenes (Ras, Myc, epidermal growth factor receptor, and cyclin D1), tumor suppressor gene inactivation (TP53 and p16), and loss of heterozygosity at numerous chromosomal locations. Despite these observations, accurate and reliable biomarkers that predict patients at highest risk for local recurrence have yet to be defined. In an effort to identify gene expression signatures that may serve as biomarkers, we studied 41 squamous cell carcinoma tumors (25 primary and 16 locally recurrent) from various anatomical sites and 13 normal oral mucosal biopsy samples from healthy volunteers with microarray analysis using Affymetrix U133A GeneChip arrays. Differentially expressed genes were identified by calculating generalized t tests (P < 0.001) and applying a series of filtering criteria to yield a highly discriminant list of 2890 genes. Hierarchical clustering and image generation using standard software were used to visualize gene expression signatures. Several gene expression signatures were readily identifiable in the HNSCC tumors, including signatures associated with proliferation, extracellular matrix production, cytokine/chemokine expression, and immune response. Of particular interest was the association of a gene expression signature enriched for genes involved in tumor invasion and metastasis with patients experiencing locally recurrent disease. Notably, these tumors also demonstrated a marked absence of an immune response signature suggesting that modulation of tumor-specific immune responses may play a role in local treatment failure. These data provide evidence for a new gene expression-based biomarker of local treatment failure in HNSCC.

Hepatocyte growth factor sensitizes human ovarian carcinoma cell lines to paclitaxel and cisplatin

The hepatocyte growth factor (HGF) receptor, encoded by the MET oncogene, is expressed in approximately 70% of human ovarian carcinomas and overexpressed in 30% of cases. Because HGF is known to protect cells from apoptosis, we investigated whether receptor expression modifies ovarian cancer cell response to chemotherapy. The apoptotic effect of the front-line chemotherapeutic drugs paclitaxel and cisplatin on cells treated with HGF was studied. In ovarian cancer cell lines, pretreatment with HGF surprisingly enhances the apoptotic response to low doses of paclitaxel and cisplatin. HGF empowers specifically the intrinsic apoptotic pathway, whereas it protects cells from extrinsic Fas-induced apoptosis. Chemotherapy sensitization is specific for HGF because another growth factor (e.g., epidermal growth factor) increases ovarian cancer cell survival. In nonovarian cancer cell models, as expected, HGF provides protection from drug-induced apoptosis. These data show that HGF sensitizes ovarian carcinoma cells to low-dose chemotherapeutic agents. This suggests that HGF may be used to improve response to chemotherapy in a set of human ovarian carcinomas molecularly classified based on the MET oncogene expression.

Tumour-derived TGF-beta1 modulates myofibroblast differentiation and promotes HGF/SF-dependent invasion of squamous carcinoma cells

The development of an altered stromal microenvironment is a common feature of many tumours including squamous cell carcinoma (SCC), and there is increasing evidence that these changes in the stroma, which include increased expression of proteases and cytokines, may actually promote tumour progression. A common finding is that stromal fibroblasts become ‘activated’ myofibroblasts, expressing smooth muscle actin and secreting cytokines, proteases and matrix proteins. We show that myofibroblasts are commonly found in the stroma of oral SCC and are often concentrated at the invasive margin of the tumour. Using oral SCC cells and primary oral fibroblasts, we demonstrate that tumour cells directly induce a myofibroblastic phenotype, and that this transdifferentiation is dependent on SCC-derived TGF-β1. In turn, myofibroblasts secrete significantly higher levels of hepatocyte growth factor/scatter factor compared with fibroblast controls, and this cytokine promotes SCC invasion through Matrigel, a mixture of basement membrane proteins. This is the first time that this double paracrine mechanism has been demonstrated between squamous carcinoma cells and fibroblasts, and emphasises that cancer invasion can be promoted indirectly by the release of tumour-induced host factors from stroma.

Rho Proteins and Cancer

The Rho family of GTPases has been intensively studied for their roles in signal transduction processes leading to cytoskeletal-dependent responses, including cell migration and phagocytosis. In addition, they are important regulators of cell cycle progression and affect the expression of a number of genes, including those for matrix-degrading proteases implicated in cancer invasion. So far, the expression of some Rho family members has been found to be increased in some human cancers, and some cancer-associated mutations in Rho family regulators have been characterized. This makes Rho protein signalling pathways attractive targets for cancer therapy. However, there is little evidence so far from animal studies to define if and how Rho proteins contribute to cancer cell proliferation, survival, invasion and metastasis.

GW domains of the Listeria monocytogenes invasion protein InlB are required for potentiation of Met activation

The Listeria monocytogenes protein InlB promotes intracellular invasion by activating the receptor tyrosine kinase Met. Earlier studies have indicated that the LRR fragment of InlB is sufficient for Met activation, but we show that this is not the case unless the LRR fragment is artificially dimerized through a disulphide bond. In contrast, activation of Met proceeds through monomers of intact InlB and, at physiologically relevant concentrations, requires coordinated action in cis of both InlB N-terminal LRR region and C-terminal GW domains. The GW domains are shown to be crucial for potentiating Met activation and inducing intracellular invasion, with these effects depending on association between GW domains and glycosaminoglycans. Glycosaminoglycans do not alter the monomeric state of InlB, and are likely to enhance Met activation through a receptor-mediated mode, as opposed to the ligand-mediated mode observed for the LRR fragment. Surprisingly, we find that gC1q-R, a host protein implicated in InlB-mediated invasion, specifically antagonizes rather than enhances InlB signalling, and that interaction between InlB and gC1q-R is unnecessary for bacterial invasion. Lastly, we demonstrate that HGF, the endogenous ligand of Met, substitutes for InlB in promoting intracellular invasion, suggesting that no special properties are required of InlB in invasion besides its hormone-like mimicry of HGF.

A New Look at an Old Problem: The Survival and Organ-Specific Growth of Metastases

Despite improvements in cancer detection and therapy, metastatic disease is largely incurable. Recent research indicates that tumor cells disseminate widely early in the process of pathogenesis, and that the survival and proliferation of these cells--and thus the development of metastases--depend on interactions between the disseminated cells and their particular microenvironment. Proliferative signals and the inhibition of proapoptotic responses are both critically involved in the development of clinically significant metastases. Identification of the underlying signaling cascades may provide additional targets for antimetastatic therapy.

Search for unknown tumor-antagonizing genes

Following the ingenious prediction of Alfred Knudson in 1971, the first tumor suppressor gene, RB1, has been isolated. Its product, the RB1 protein, was found to play a major role in the control of the cell cycle. The loss of heterozygosity (LOH) technique, introduced by Cavenee and colleagues, was an important milestone toward the confirmation of Knudson's hypothesis and the identification of the gene. Subsequently, the LOH technique has provided important clues that have led to the discovery of other tumor suppressor genes. Most of them play important roles in the regulation of the cell cycle and/or of apoptosis. Circumstantial evidence suggests that still other and perhaps many unknown genes may participate in the protection of the organism against malignant growth. The numerous genome losses in tumors, detected by LOH, comparative genomic hybridization, and by cytogenetic techniques, support this possibility. The early work of one of us (G.K.), together with Henry Harris and Francis Wiener, had shown that the malignant phenotype can be suppressed by hybridizing malignant with low- or non-tumorigenic cells. However, analysis of this phenomenon failed to assign the inhibition of tumorigenicity to any particular gene. We have pursued the search for new tumor-antagonizing genes with two unconventional approaches, focusing on human chromosomal subband 3p21.3, a region frequently targeted by cytogenetically detectable deletions. We have detected four clusters of candidate tumor suppressor genes at 3p21.3 by a combination of deletion mapping and the "elimination test." These findings raise the question whether the number and variety of genes that may contribute to the defense against uncontrolled proliferation may have been underestimated.

Locus of fragility in robust breast cancer system

Functional heterogeneous redundancy of breast cancer makes this tumor to be robust. Signaling mechanisms which control cancer responses are crucial for controlling robustness. Identification of locus of fragility in cancer represents basic mechanism to target robustness. The goal of this prospect is to present locus of fragility in breast cancer robust system, and how disruption of this locus induces failure of robustness. My recent research show, that locus of fragility in breast cancer cells is suppression of nitric oxide (NO). When it was targeted, dynamics of cancer to generate robustness failed that it blocked cancer cell proliferation dependent on the NO/Rb pathway, blocked cell migration and angiogenesis dependent on the VEGF/PI3K/AKT/NO/ICAM-1 pathway, and induced breast cancer cell apoptosis through the NO/ROCK/FOXO3a signaling pathway. This tiny and trivial perturbation in breast cancer cells such as suppression of NO represents locus of fragility (weakness) and new approach for breast cancer chemotherapy.

Tyrosine kinase signal specificity: lessons from the HGF receptor

There are two major lines of thinking concerning the mechanisms responsible for specificity in receptor tyrosine kinase signalling. On one hand, receptors might provide instructive signals that dictate cell-fate decisions and, on the other, they might generate permissive signals unleashing responses that are inherently defined in the protein repertoire of target cells. Recent data indicate that the signalling activity of the Met receptor for hepatocyte growth factor is affected by association with cell-specific surface molecules, namely the alpha6beta4 integrin, Plexin B1 and CD44. This suggests that integration of cell-restricted expression of receptor partners that modulate kinase outputs with the intrinsic signalling features of receptors is required for specification of biological responses.

Cross‐talk between vascular endothelial growth factor and semaphorin‐3A pathway in the regulation of normal and malignant mesothelial cell proliferation

Vascular endothelial growth factor (VEGF) and semaphorin-3A (Sema-3A) play important roles in the transduction of promitotic and antimitotic signals, respectively. Here, we report that these conflicting signals are integrated via negative feedback between VEGF and Sema-3A pathways in several primary normal, but not malignant, mesothelial cells. Unlike malignant mesothelial (MM) cells, in which VEGF induces cell proliferation, normal mesothelial (NM) cell growth was repressed by VEGF. Although both cell-types expressed an overlapping set of VEGF tyrosine-kinase receptors, only in NM cells VEGF exposure entails a p38 mitogen-activated protein kinase (MAPK)-dependent increased of Sema-3A production. Inhibition of p38 MAPK (by SB202190 and SB203580) or a dominant-negative mutant of Sema-3A receptor plexin-A1 reversed the inhibitory effects of VEGF in NM cells, increasing cyclin D1 synthesis and cell growth. Conversely, sustained activation of p38 MAPK by the p38 MAPK-activating kinases MKK3 and MKK6 or transfection with Sema-3A inhibited VEGF-induced cyclin D1 up-regulation and MM cell proliferation. Therefore, these results delineate a new role of Sema-3A in VEGF function mediated by p38 MAPK and suggest that the abrogation of regulated Sema-3A expression is responsible for VEGF-driven growth of tumor cells.

The transcriptional activator PAX3-FKHR rescues the defects of Pax3 mutant mice but induces a myogenic gain-of-function phenotype with ligand-independent activation of Met signaling in vivo

Pax3 is a key transcription factor implicated in development and human disease. To dissect the role of Pax3 in myogenesis and establish whether it is a repressor or activator, we generated loss- and gain-of-function alleles by targeting an nLacZ reporter and a sequence encoding the oncogenic fusion protein PAX3-FKHR into the Pax3 locus. Rescue of the Pax3 mutant phenotypes by PAX3-FKHR suggests that Pax3 acts as a transcriptional activator during embryogenesis. This is confirmed by a Pax reporter mouse. However, mice expressing PAX3-FKHR display developmental defects, including ectopic delamination and inappropriate migration of muscle precursor cells. These events result from overexpression of c-met, leading to constitutive activation of Met signaling, despite the absence of the ligand SF/HGF. Haploinsufficiency of c-met rescues this phenotype, confirming the direct genetic link with Pax3. The gain-of-function phenotype is also characterized by overactivation of MyoD. The consequences of PAX3-FKHR myogenic activity in the limbs and cervical and thoracic regions point to differential regulation of muscle growth and patterning. This gain-of-function allele provides a new approach to the molecular and cellular analysis of the role of Pax3 and of its target genes in vivo.

Elevated hyaluronan production induces mesenchymal and transformed properties in epithelial cells

During carcinoma progression, tumor cells often undergo changes similar (but not identical) to epithelialmesenchymal transitions in embryonic development. In this study, we demonstrate that experimental stimulation of hyaluronan synthesis in normal epithelial cells is sufficient to induce mesenchymal and transformed characteristics. Using recombinant adenoviral expression of hyaluronan synthase-2, we show that increased hyaluronan production promotes anchorage-independent growth and invasiveness, induces gelatinase production, and stimulates phosphoinositide 3-kinase/Akt pathway activity in phenotypically normal Madin-Darby canine kidney and MCF-10A human mammary epithelial cells. Cells infected with hyaluronan synthase-2 adenovirus also acquired mesenchymal characteristics, including up-regulation of vimentin, dispersion of cytokeratin, and loss of organized adhesion proteins at intercellular boundaries. Furthermore, we show that the transforming effects of two well described agents, hepatocyte growth factor (HGF) and beta-catenin, are dependent on hyaluronan-cell interactions. Perturbation of endogenous hyaluronan polymer interactions by treatment with hyaluronan oligomers is shown here to reverse the transforming effects of HGF and beta-catenin in Madin-Darby canine kidney and MCF-10A human mammary epithelial cells. Also, HGF and beta-catenin induced assembly of hyaluronan-dependent pericellular matrices similar to those surrounding mesenchymal cells. Thus, increased expression of hyaluronan is sufficient to induce epithelial-mesenchymal transition and acquisition of transformed properties in phenotypically normal epithelial cells.

Axis of evil: molecular mechanisms of cancer metastasis

Although the genetic basis of tumorigenesis may vary greatly between different cancer types, the cellular and molecular steps required for metastasis are similar for all cancer cells. Not surprisingly, the molecular mechanisms that propel invasive growth and metastasis are also found in embryonic development, and to a less perpetual extent, in adult tissue repair processes. It is increasingly apparent that the stromal microenvironment, in which neoplastic cells develop, profoundly influences many steps of cancer progression, including the ability of tumor cells to metastasize. In carcinomas, the influences of the microenvironment are mediated, in large part, by bidirectional interactions (adhesion, survival, proteolysis, migration, immune escape mechanisms lymph-/angiogenesis, and homing on target organs) between epithelial tumor cells and neighboring stromal cells, such as fibroblasts as well as endothelial and immune cells. In this review, we summarize recent advances in understanding the molecular mechanisms that govern this frequently lethal metastatic progression along an axis from primary tumor to regional lymph nodes to distant organ sites. Affected proteins include growth factor signaling molecules, chemokines, cell-cell adhesion molecules (cadherins, integrins) as well as extracellular proteases (matrix metalloproteinases). We then discuss promising new therapeutic approaches targeting the microenvironment. We note, however, that there is still too little knowledge of how the many events are coordinated and integrated by the cancer cell, with conspiratorial help by the stromal component of the host. Before drug development can proceed with a legitimate chance of success, significant gaps in basic knowledge need to be filled.

Crystal structure of the tyrosine kinase domain of the hepatocyte growth factor receptor c-Met and its complex with the microbial alkaloid K-252a

The protooncogene c-met codes for the hepatocyte growth factor receptor tyrosine kinase. Binding of its ligand, hepatocyte growth factor/scatter factor, stimulates receptor autophosphorylation, which leads to pleiotropic downstream signaling events in epithelial cells, including cell growth, motility, and invasion. These events are mediated by interaction of cytoplasmic effectors, generally through Src homology 2 (SH2) domains, with two phosphotyrosine-containing sequence motifs in the unique C-terminal tail of c-Met (supersite). There is a strong link between aberrant c-Met activity and oncogenesis, which makes this kinase an important cancer drug target. The furanosylated indolocarbazole K-252a belongs to a family of microbial alkaloids that also includes staurosporine. It was recently shown to be a potent inhibitor of c-Met. Here we report the crystal structures of an unphosphorylated c-Met kinase domain harboring a human cancer mutation and its complex with K-252a at 1.8-A resolution. The structure follows the well established architecture of protein kinases. It adopts a unique, inhibitory conformation of the activation loop, a catalytically noncompetent orientation of helix alphaC, and reveals the complete C-terminal docking site. The first SH2-binding motif (1349YVHV) adopts an extended conformation, whereas the second motif (1356YVNV), a binding site for Grb2-SH2, folds as a type II Beta-turn. The intermediate portion of the supersite (1353NATY) assumes a type I Beta-turn conformation as in an Shc-phosphotyrosine binding domain peptide complex. K-252a is bound in the adenosine pocket with an analogous binding mode to those observed in previously reported structures of protein kinases in complex with staurosporine.

Hepatocyte growth factor and its receptor are required for malaria infection

Plasmodium, the causative agent of malaria, must first infect hepatocytes to initiate a mammalian infection. Sporozoites migrate through several hepatocytes, by breaching their plasma membranes, before infection is finally established in one of them. Here we show that wounding of hepatocytes by sporozoite migration induces the secretion of hepatocyte growth factor (HGF), which renders hepatocytes susceptible to infection. Infection depends on activation of the HGF receptor, MET, by secreted HGF. The malaria parasite exploits MET not as a primary binding site, but as a mediator of signals that make the host cell susceptible to infection. HGF/MET signaling induces rearrangements of the host-cell actin cytoskeleton that are required for the early development of the parasites within hepatocytes. Our findings identify HGF and MET as potential targets for new approaches to malaria prevention.

Hepatocyte growth factor/scatter factor mediates angiogenesis through positive VEGF and negative thrombospondin 1 regulation

Hepatocyte growth factor/scatter factor (HGF/SF), acting through the Met receptor, plays an important role in most human solid tumors, and inappropriate expression of this ligand-receptor pair is often associated with poor prognosis. The molecular basis for the malignant potential of the HGF/SF-Met signal in cancer cells has mostly been attributed to its mitogenic and invasive properties. However, HGF/SF also induces angiogenesis, but the signaling mechanism has not been fully explained, nor has this activity been directly associated with HGF/SF-Met-mediated tumorigenesis. It is known that HGF/SF induces in vitro expression of vascular endothelial growth factor (VEGF), a key agonist of tumor angiogenesis; by contrast, thrombospondin 1 (TSP-1) is a negative regulator of angiogenesis. Here, we show that, in the very same tumor cells, in addition to inducing VEGF expression, HGF/SF dramatically down-regulates TSP-1 expression. We show that TSP-1 shut-off plays an important, extrinsic role in HGF/SF-mediated tumor development, because ectopic expression of TSP-1 markedly inhibits tumor formation through the suppression of angiogenesis. Interestingly, although VEGF-induced expression is sensitive to inhibitors of several pathways, including mitogen-activated protein kinase, phosphoinositide 3-kinase, and signal transducer and activator of transcription 3, TSP-1 shut-off by HGF/SF is prevented solely by inhibiting mitogen-activated protein kinase activation. These studies identify HGF/SF as a key switch for turning on angiogenesis. They suggest that TSP-1 is a useful antagonist to tumor angiogenesis and that it may have therapeutic value when used in conjunction with inhibitors of VEGF.