Tumor Endothelial Cells Express Epidermal Growth Factor Receptor (EGFR) but not ErbB3 and Are Responsive to EGF and to EGFR Kinase Inhibitors

Epidermal growth factor (EGF) receptor family members are expressed by tumor cells and contribute to tumor progression. The expression and activity of EGF receptors in endothelial cells are less well characterized. Analysis of tumor-derived endothelial cells showed that they express EGFR, ErbB2, and ErbB4, whereas their normal counterparts express ErbB2, ErbB3, and ErbB4. The gain in expression of EGFR and the loss of ErbB3 expression in tumor vasculature was also observed in vivo. As a consequence of their expressing EGFR, tumor endothelial cells responded to EGF and other EGF family members by activating both EGFR and ErbB2, by activating the downstream mitogen-activated protein kinase pathway, and by enhanced proliferation. On the other hand, normal endothelial cells did not respond to EGF but instead were responsive to neuregulin (NRG), a ligand for ErbB3 and ErbB4. NRG activated ErbB3 in normal endothelial cells and inhibited growth of these cells. In contrast, tumor endothelial cells, which do not express ErbB3, were not growth inhibited by NRG. Furthermore, due to their expression of EGFR, tumor endothelial cells, unlike normal endothelial cells, are direct targets for EGFR kinase inhibitors. These low-molecular-weight compounds block EGF-induced EGFR activation and proliferation of tumor endothelial cells. These results suggest that a gain of EGF-induced endothelial cell proliferation, and loss of NRG-induced growth inhibition in tumor endothelial cells constitutes a switch that promotes tumor angiogenesis. In addition, these results suggest that EGFR kinase inhibitors may be effective for antiangiogenesis therapy by specifically targeting the tumor, but not the normal, vasculature.

Neuron Restrictive Silencer Factor NRSF/REST Is a Transcriptional Repressor of Neuropilin-1 and Diminishes the Ability of Semaphorin 3A to Inhibit Keratinocyte Migration

Neuropilin-1 (NRP1) is expressed by endothelial cells and neurons and serves as a receptor for both vascular endothelial growth factor (VEGF), an angiogenesis factor, and semaphorin 3A (Sema3A), a mediator of axonal guidance. We show here that NRP1 is also expressed in keratinocytes in vitro and in vivo. However, nothing has been reported about the regulation or function of keratinocyte NRP1. Using NRP1 promoter constructs in HaCaT cells, a keratinocyte cell line, we could demonstrate that a neuron restrictive silencer element (NRSE) was implicated in transcriptional repression of the NRP1 gene. Electrophoretic mobility shift assays demonstrated that the neuron restrictive silencer factor (NRSF) binds to NRSE. Overexpression of NRSF in HaCaT cells decreased NRP1 RNA and protein, whereas a dominant negative NRSF increased NRP1. Furthermore, the histone deacetylase inhibitor trichostatin A, an inhibitor of NRSF silencing activity, also increased NRP1 levels. NRP2 expression was not affected. Epidermal growth factor (EGF) and heparin-binding EGF-like growth factor (HB-EGF) strongly up-regulated NRP1 expression, concomitant with down-regulation of NRSF. Other keratinocyte mitogens such as keratinocyte growth factor (KGF) had no effect. To address function, HaCaT cells were exposed to two NRP1 ligands, VEGF165 and Sema3A. Neither had an effect on proliferation, whereas Sema3A, but not VEGF165, inhibited cell migration. Down-regulation of NRP1 by NRSF overexpression reduced Sema3A activity. It was concluded that NRSF is a transcription factor that silences NRP1 expression and thereby diminishes the Sema3A mediated inhibition of HaCaT keratinocyte migration.

Neuropilins in neoplasms: expression, regulation, and function

Neuropilins (NRP) are membranous receptors capable of binding two disparate ligands, class 3 semaphorins (SEMA) and vascular endothelial growth factors (VEGF), and regulating two diverse systems, neuronal guidance and angiogenesis. The neuropilin genes, NRP1 and NRP2, share similar protein structure, but differ in their expression patterns, regulation, and ligand-binding specificities. NRPs vary in their expression patterns; for example, endothelial cells express both NRP1 and NRP2, lymphatic endothelial cells predominantly express NRP2, and epidermal cells predominantly express NRP1. NRP expression can be differentially regulated by transcription factors, e.g. prox-1 induces NRP2 while suppressing NRP1, or by growth factors, e.g. epidermal growth factor (EGF) induces NRP1 but not NRP2. Nearly all tumor cells express NRP1, NRP2, or both. Carcinomas express NRP1, whereas neuronal tumors and melanomas predominantly express NRP2. SEMAs play a role in neoplasms as angiogenesis inhibitors. For example, SEMA3F, which binds specifically to NRP2, inhibits tumor angiogenesis and metastasis. Metastatic tumor cells lose SEMA3F expression during progression. Therefore, SEMA3F may have therapeutic potential. This article focuses on the role of NRPs and SEMAs in tumor progression and angiogenesis.

Soluble neuropilin targeted to the skin inhibits vascular permeability

Neuropilin 1 (NRP1) is a co-receptor for vascular endothelial growth factor (VEGF(165)), an inducer of vascular permeability and angiogenesis. Numerous physiological factors enhance VEGF expression and function but only a few have been shown to be negative regulators. Previously, we have shown that the naturally occurring soluble form of NRP1 (sNRP1) inhibits binding of VEGF(165) to endothelial cells in vitro and impairs tumor growth in vivo. To investigate the role of sNRP1 in the regulation of vascular development and function, sNRP1 expression was targeted to the skin, where it is not normally expressed, using a keratin 14 (K14) promoter expression construct. K14-sNRP1 transgenic mice displayed normal skin architecture with a subtle abnormal vascular phenotype. While the overall number of skin blood vessels remained unchanged, the lumen size of smooth muscle-associated dermal vessels was reduced. K14-sNRP1 mice had reduced vascular permeability in response to VEGF(165), but also to VEGF(121) and platelet activating factor, suggesting that the lack of permeability was not solely due to the sequestration of VEGF. sNRP1 also reversed the increase in inflammation and edema induced by transgenic VEGF overexpression in cutaneous delayed-type hypersensitivity reactions. In summary, sNRP1 appears to primarily regulate vessel permeability while its effect on physiological angiogenesis is less evident in this model.

Endothelial progenitor cell sprouting in spheroid cultures is resistant to inhibition by osteoblasts: A model for bone replacement grafts

Survival of tissue transplants generated in vitro is strongly limited by the slow process of graft vascularization in vivo. A method to enhance graft vascularization is to establish a primitive vascular plexus within the graft prior to transplantation. Endothelial cells (EC) cultured as multicellular spheroids within a collagen matrix form sprouts resembling angiogenesis in vitro. However, osteoblasts integrated into the graft suppress EC sprouting. This inhibition depends on direct cell-cell-interactions and is characteristic of mature ECs isolated from preexisting vessels. In contrast, sprouting of human blood endothelial progenitor cells is not inhibited by osteoblasts, making these cells suitable for tissue engineering of pre-vascularized bone grafts.

Overexpression of neuropilin-1 promotes constitutive MAPK signalling and chemoresistance in pancreatic cancer cells

Neuropilin-1 (NRP-1) is a novel co-receptor for vascular endothelial growth factor (VEGF). Neuropilin-1 is expressed in pancreatic cancer, but not in nonmalignant pancreatic tissue. We hypothesised that NRP-1 expression by pancreatic cancer cells contributes to the malignant phenotype. To determine the role of NRP-1 in pancreatic cancer, NRP-1 was stably transfected into the human pancreatic cancer cell line FG. Signal transduction was assessed by Western blot analysis. Susceptibility to anoikis (detachment induced apoptosis) was evaluated by colony formation after growth in suspension. Chemosensitivity to gemcitabine or 5-fluorouracil (5-FU) was assessed by MTT assay in pancreatic cancer cells following NRP-1 overexpression or siRNA-induced downregulation of NRP-1. Differential expression of apoptosis-related genes was determined by gene array and further evaluated by Western blot analysis. Neuropilin-1 overexpression increased constitutive mitogen activated protein kinase (MAPK) signalling, possibly via an autocrine loop. Neuropilin-1 overexpression in FG cells enhanced anoikis resistance and increased survival of cells by >30% after exposure to clinically relevant levels of gemcitabine and 5-FU. In contrast, downregulation of NRP-1 expression in Panc-1 cells markedly increased chemosensitivity, inducing >50% more cell death at clinically relevant concentrations of gemcitabine. Neuropilin-1 overexpression also increased expression of the antiapoptotic regulator, MCL-1. Neuropilin-1 overexpression in pancreatic cancer cell lines is associated with (a) increased constitutive MAPK signalling, (b) inhibition of anoikis, and (c) chemoresistance. Targeting NRP-1 in pancreatic cancer cells may downregulate survival signalling pathways and increase sensitivity to chemotherapy.

Neuropilin-1a is involved in trunk motor axon outgrowth in embryonic zebrafish

Neuropilin-1, a receptor for axon-repellent semaphorins and vascular endothelial growth factor (VEGF), functions both in angiogenesis and axon growth. Here, we show strong expression of neuropilin-1a in primary motor neurons in the trunk of embryonic zebrafish. Reducing the expression of neuropilin-1a using antisense morpholino oligonucleotides induced aberrant branching of motor nerves, additional exit points of motor nerves from the spinal cord, and migration of neurons out of the spinal cord along the motor axon pathway in a dose-dependent manner. These phenotypes could be partially rescued by co-injecting neuropilin-1a mRNA. Other axons in the spinal cord and head appeared unaffected by the morpholino treatment. In addition, neuropilin-1a morpholino treatment disturbed normal formation of blood vessels in the trunk of 24 hours postfertilization embryos, as shown by microangiography. Morpholinos to VEGF also disturbed formation of blood vessels but did not affect motor axons, indicating that correct formation of blood vessels is not needed for the growth of primary motor axons. Morpholinos to the semaphorin 3A homologs semaphorin 3A1 and semaphorin 3A2 also had no effect on motor axon growth. However, combined injections of neuropilin-1a morpholino, at a concentration that did not elicit axonal aberrations when injected alone, with VEGF, semaphorin 3A1, or semaphorin 3A2 morpholinos synergistically increased the proportion of embryos showing aberrant motor axon growth. Thus, neuropilin-1a in primary motor neurons may integrate signals from several ligands and is needed for proper segmental growth of primary motor nerves in zebrafish.

A role for axon guidance receptors and ligands in blood vessel development and tumor angiogenesis

Nerves and blood vessels resemble each other in their ability to form branching networks. They are in close proximity suggesting possible molecular interactions. The patterning of nerves and blood vessels are not random but are regulated by attractive and repulsive cues. Four major neuronal guidance factors that are sensed by growth cones have been identified, Semaphorin, Ephrin, Slit and Netrin, and their cognate receptors, neuropilin, Eph, roundabouts (Robo) and uncoordinated-5 (UNC5). Unexpectedly, these ligand/receptor pairs also regulate developmental and tumor angiogenesis. Together, there is strong evidence that development of the nervous and vascular systems are regulated by common cues.

A new perspective on tumor endothelial cells: unexpected chromosome and centrosome abnormalities

It has been shown that endothelial cells in solid tumors are cytogenetically abnormal. These cells are aneuploid with multiple chromosomes and multiple centrosomes. Unlike normal endothelial cells which remain diploid in long-term culture, the aneuploidy of tumor endothelial cells is exacerbated in culture suggesting that these cells are inherently unstable. It is speculated that this instability might compromise the effectiveness of antiangiogenesis therapy.

Tumor-associated endothelial cells with cytogenetic abnormalities

Tumor angiogenesis is necessary for solid tumor progression and metastasis. Tumor blood vessels have been shown to differ from normal counterparts, for example, by changes in morphology. An important concept in tumor angiogenesis is that tumor endothelial cells are assumed to be genetically normal, although these endothelial cells are structurally and functionally abnormal. However, we hypothesized that given the phenotypic differences between tumor and normal blood vessels, there may be genotypic alterations as well. Mouse endothelial cells were isolated from two different human tumor xenografts, melanoma and liposarcoma, and from two normal endothelial cell counterparts, skin and adipose. Tumor-associated endothelial cells expressed typical endothelial cell markers, such as CD31. They had relatively large, heterogeneous nuclei. Unexpectedly, tumor endothelial cells were cytogenetically abnormal. Fluorescence in situ hybridization (FISH) analysis showed that freshly isolated uncultured tumor endothelial cells were aneuploid and had abnormal multiple centrosomes. The degree of aneuploidy was exacerbated by passage in culture. Multicolor FISH indicated that the structural chromosomal aberrations in tumor endothelial cells were heterogeneous, indicating that the cytogenetic alterations were not clonal. There was no evidence of human tumor-derived chromosomal material in the mouse tumor endothelial cells. In marked contrast, freshly isolated normal skin and adipose endothelial cells were diploid, had normal centrosomes, and remained cytogenetically stable in culture even up to 20 passages. FISH analysis of tumor sections also showed endothelial cell aneuploidy. We conclude that tumor endothelial cells can acquire cytogenetic abnormalities while in the tumor microenvironment.

Semaphorin 3F, a chemorepulsant for endothelial cells, induces a poorly vascularized, encapsulated, nonmetastatic tumor phenotype

Melanoma is the most lethal skin cancer. Most deaths from melanoma result from metastases. Semaphorins have been shown to inhibit neuronal and endothelial cell migration, but the effects of semaphorins on tumor metastasis have not been documented. We found that semaphorin 3F (SEMA3F) was markedly downregulated in highly metastatic human cell lines in vitro and in vivo, which suggested that it may be a metastasis inhibitor. Metastatic human melanoma cells were transfected with SEMA3F and implanted into mice; the resultant tumors did not metastasize. Rather, the primary tumors resembled benign nevi characterized by large areas of apoptosis, diminished vascularity, inhibition of hyperplasia in overlying epidermal cells, and encapsulated tumor borders delineated by thick layers of fibroblasts and collagen matrix. This phenotype is in stark contrast to highly invasive, vascular mock-transfected tumors. In vitro, tumor cells expressing SEMA3F had a diminished capacity to adhere and migrate on fibronectin. Consistent with semaphorin-mediated chemorepulsion of neurons, tumor cells expressing SEMA3F were chemorepulsive for vascular and lymphatic endothelial cells expressing neuropilin-2 (NRP2), a novel mechanism for a tumor angiogenesis inhibitor. The repulsive activity was abrogated by NRP2 RNA interference. Together these results indicate that SEMA3F is a potent metastasis inhibitor that targets both tumor and stromal cells and raise the possibility of SEMA3F having therapeutic potential.

VEGF is required for the maintenance of dorsal root ganglia blood vessels but not neurons during development

Vascular endothelial growth factor (VEGF) is a potent regulator of vascular function through its control of multiple endothelial cell functions. In addition to its key role in vascularization, VEGF has recently been shown to have neurotrophic activity during hypoxic stress. In the central and peripheral motor nervous system, VEGF treatment increased neuronal vascularization and perfusion, as well as having direct trophic effects on neurons and Schwann cells. However, the role of VEGF in the sensory nervous system remains unclear. To characterize the differential effects of VEGF on endothelial cells and neurons in sensory ganglia, we used explanted mouse dorsal root ganglia (DRG), a culture system containing neurons and endothelial cells in close apposition. We show that VEGF is expressed by neurons and satellite cells, but not by endothelial cells or pericytes. On the other hand, the tyrosine kinase VEGF receptor VEGFR-2 was robustly expressed by endothelial cells throughout the extensive DRG capillary network, but not found at either the transcript or protein level in sensory neurons or other nonendothelial cells of the DRG. Both soluble receptor sequestration of VEGF and small molecule kinase inhibition of VEGFR-2 signaling rapidly disrupted the connectivity, branching, and structural integrity of the capillary network of embryonic DRG; this effect was no longer evident postnatally. However, VEGF inhibition showed no detectable effect on neuronal health at any stage analyzed. These data suggest that endogenous VEGF is a strict requirement for vascular, but not neuronal, maintenance in developing sensory ganglia.

Neuropilin-1 in human colon cancer: expression, regulation, and role in induction of angiogenesis

Neuropilin-1 (NRP-1), a recently identified co-receptor for vascular endothelial growth factor, is expressed by several nongastrointestinal tumor types and enhances prostate cancer angiogenesis and growth in preclinical models. We investigated the expression and regulation of NRP-1 and the effect of NRP-1 overexpression on angiogenesis and growth of human colon adenocarcinoma by immunohistochemistry and in situ hybridization. NRP-1 was expressed in 20 of 20 human colon adenocarcinoma specimens but not in the adjacent nonmalignant colonic mucosa. By reverse transcriptase-polymerase chain reaction analysis, NRP-1 mRNA was expressed in seven of seven colon adenocarcinoma cell lines. Subcutaneous xenografts of stably transfected KM12SM/LM2 human colon cancer cells overexpressing NRP-1 led to increased tumor growth and angiogenesis in nude mice. In in vitro assays, conditioned medium from NRP-1-transfected cell lines led to an increase in endothelial cell migration, but did not affect endothelial cell growth. Epidermal growth factor (EGF) led to induction of NRP-1 in human colon adenocarcinoma cells and selective blockade of the epidermal growth factor receptor (EGFR) decreased constitutive and EGF-induced NRP-1 expression. Blockade of the Erk 1/2 and P38 mitogen-activated protein kinase signaling pathways also led to a decrease in constitutive and EGF-induced NRP-1 expression. These findings demonstrate the ubiquitous expression of NRP-1 in human colon cancer and suggest that NRP-1 may contribute to colon cancer angiogenesis and growth. This study also suggests that EGF and mitogen-activated protein kinase signaling pathways play an important role in NRP-1 regulation in colon cancer cells.

Expression and mapping of duplicate neuropilin-1 and neuropilin-2 genes in developing zebrafish

Previously, we described the isolation and characterization of the first zebrafish neuropilin gene, which we now call nrp1a, and found its protein to be a mediator of vascular endothelial growth factor (VEGF)-dependent angiogenesis [Proc. Natl Acad. Sci. USA 99 (2002) 10470]. Subsequently, we have isolated three other full-length neuropilin genes (nrp1b, nrp2a, and nrp2b) and find that they map to independent zebrafish linkage groups. The nrp1s and nrp2s had differential spatio-temporal gene expression profiles with nrp1a being most prominent in the gut, brain, retina, hypochord, motorneurons, fin bud and mandibular cartilage, nrp1b in the brain, dorsal aorta, melanophores, ventral fin, and heart, nrp2a in the brain, retina, heart, and caudal vessels, and nrp2b in the brain, retina, gut, fin bud, melanophores, heart, and caudal vessels. In addition, we have identified an alternatively-spliced transcript of the nrp1b gene (denoted as nrp1b(s)) which is predicted to encode a soluble form of Nrp1b, containing only the a, b, and c extracellular domains. Transcript expression of nrp1b(s) was different from full-length nrp1b transcript, with prominence in the brain, developing mouth, heart, and fin bud. The NRP1s were tested for VEGF-binding ability. Both 125 kDa Nrp1a and 145 kDa Nrp1b bound 125I-labelled VEGFA165. In summary, two nrp1 and two nrp2 genes, with expression patterns similar to higher vertebrates, have been isolated from zebrafish.

Vascular Endothelial Growth Factor and Its Receptors in Embryonic Zebrafish Blood Vessel Development

There is intense interest in how blood vessel development is regulated. A number of vascular growth factors and their receptors have been described. The vascular endothelial growth factor (VEGF) and its receptors are major contributors to normal mammalian vascular development. These receptors include VEGFR-1, VEGFR-2, VEGFR-3, neuropilin-1 (NRP1), and NRP2. The function of these genes have been determined to some degree in mouse gene targeting studies. These knockouts are embryonically lethal, and early death can be attributed in part to lack of normal blood and lymphatic vessel development. More recently, it has been demonstrated that zebrafish are an excellent model for studying the genes and proteins that regulate embryonic vascular development. Zebrafish have a number of advantages compared to mice, including rapid embryonic development and the ability to examine and manipulate embryos outside of the animal. In this review, we describe some of the earlier mouse VEGF/receptor functional studies and emphasize the development of the zebrafish vasculature. We describe the zebrafish vasculature, zebrafish VEGF and VEGF receptors, advantages of the zebrafish model, resources, and methods of determining growth factor and receptor function.

Activation of syndecan-1 ectodomain shedding by Staphylococcus aureus alpha-toxin and beta-toxin

Exploitation of host components by microbes to promote their survival in the hostile host environment has been a recurring theme in recent years. Available data indicate that bacterial pathogens activate ectodomain shedding of host cell surface molecules to enhance their virulence. We reported previously that several major bacterial pathogens activate ectodomain shedding of syndecan-1, the major heparan sulfate proteoglycan of epithelial cells. Here we define the molecular basis of how Staphylococcus aureus activates syndecan-1 shedding. We screened mutant S. aureus strains devoid of various toxin and protease genes and found that only strains lacking both alpha-toxin and beta-toxin genes do not stimulate shedding. Mutations in the agr global regulatory locus, which positively regulates expression of alpha- and beta-toxins and other exoproteins, also abrogated the capacity to stimulate syndecan-1 shedding. Furthermore, purified S. aureus alpha- and beta-toxins, but not enterotoxin A and toxic shock syndrome toxin-1, rapidly potentiated shedding in a concentration-dependent manner. These results establish that S. aureus activates syndecan-1 ectodomain shedding via its two virulence factors, alpha- and beta-toxins. Toxin-activated shedding was also selectively inhibited by antagonists of the host cell shedding mechanism, indicating that alpha- and beta-toxins shed syndecan-1 ectodomains through stimulation of the host cell's shedding machinery. Interestingly, beta-toxin, but not alpha-toxin, also enhanced ectodomain shedding of syndecan-4 and heparin-binding epidermal growth factor. Because shedding of these ectodomains has been implicated in promoting bacterial pathogenesis, activation of ectodomain shedding by alpha-toxin and beta-toxin may be a previously unknown virulence mechanism of S. aureus.

Inhibition of zebrafish epidermal growth factor receptor activity results in cardiovascular defects

The physiological role of any of the epidermal growth factor (EGF) receptor tyrosine kinases has yet to be determined in zebrafish. We isolated a zebrafish homologue of EGFR (egfr) that shows a 63% amino acid overall identity to human EGFR but with 90% amino acid identity in the kinase domain. Whole mount in situ hybridization showed ubiquitous distribution of egfr transcripts during gastrulation, somitogenesis and later stages. When expressed in Chinese hamster ovary cells, zebrafish Egfr was a functional receptor that responded to EGF by receptor tyrosine phosphorylation and activation of MAP kinase. The function of zebrafish Egfr in vivo was determined by inhibiting its activity using EGFR kinase inhibitors and antisense morpholinos (MO), which inhibited Egfr kinase activity and translation of egfr messenger RNA into protein, respectively. The zebrafish is a particularly excellent model for studying cardiovascular development because zebrafish are transparent allowing direct visualization of the heart and circulation in the blood vessels. Inhibition of zebrafish Egfr activity in vivo impeded blood flow via the outflow tract into the aorta and impeded circulation in the axial and intersegmental vessels by 80 h post-fertilization. Analysis of the heart showed that the heart chambers and pericardial sacs were dilated and the outflow tracts were narrowed. Together these results suggested that zebrafish Egfr has a cardiovascular function in the developing zebrafish that is required for normal circulation.

The role of neuropilin in vascular and tumor biology

Neuropilin-1 (NRP1) and NRP2 are related transmembrane receptors that function as mediators of neuronal guidance and angiogenesis. NRPs bind members of the class 3 semaphorin family, regulators of neuronal guidance, and of the vascular endothelial growth factor (VEGF) family of angiogenesis factors. There is substantial evidence that NRPs serve as mediators of developmental and tumor angiogenesis. NRPs are expressed in endothelial cells (EC) and bind VEGF165. NRP1 is a co-receptor for VEGF receptor-2 (VEGFR2) that enhances the binding of VEGF165 to VEGFR2 and VEGF165-mediated chemotaxis. NRP1 expression is regulated in EC by tumor necrosis factor-alpha, the transcription factors dHAND and Ets-1, and vascular injury. During avian blood vessel development NRP1 is expressed only in arteries whereas NRP2 is expressed in veins. Transgenic mouse models demonstrate that NRP1 plays a critical role in embryonic vascular development. Overexpression of NRP1 results in the formation of excess capillaries and hemorrhaging. NRP1 knockouts have defects in yolk sac, embryo and neuronal vascularization, and in development of large vessels in the heart. Tumor cells express NRPs and bind VEGF165. NRP1 upregulation is positively correlated with the progression of various tumors. Overexpression of NRPI in rat tumor cells results in enlarged tumors and substantially enhanced tumor angiogenesis. On the other hand, soluble NRP1 (sNRP1) is an antagonist of tumor angiogenesis. Semaphorin 3A binds to EC and tumor cells. It also inhibits EC motility and capillary sprouting in vitro. VEGF165 and Sema3A are competitive inhibitors for NRP1 mediated functions in EC and neurons. These results suggest that NRP1 is a novel regulator of the vascular system.

Heparin-binding EGF-like growth factor and ErbB signaling is essential for heart function

The heparin-binding epidermal growth factor (EGF)-like growth factor (HB-EGF) is a member of the EGF family of growth factors that binds to and activates the EGF receptor (EGFR) and the related receptor tyrosine kinase, ErbB4. HB-EGF-null mice (HB(del/del)) were generated to examine the role of HB-EGF in vivo. More than half of the HB(del/del) mice died in the first postnatal week. The survivors developed severe heart failure with grossly enlarged ventricular chambers. Echocardiographic examination showed that the ventricular chambers were dilated and that cardiac function was diminished. Moreover, HB(del/del) mice developed grossly enlarged cardiac valves. The cardiac valve and the ventricular chamber phenotypes resembled those displayed by mice lacking EGFR, a receptor for HB-EGF, and by mice conditionally lacking ErbB2, respectively. HB-EGF-ErbB interactions in the heart were examined in vivo by administering HB-EGF to WT mice. HB-EGF induced tyrosine phosphorylation of ErbB2 and ErbB4, and to a lesser degree, of EGFR in cardiac myocytes. In addition, constitutive tyrosine phosphorylation of both ErbB2 and ErbB4 was significantly reduced in HB(del/del) hearts. It was concluded that HB-EGF activation of receptor tyrosine kinases is essential for normal heart function.

Characterization of the neuropilin-1 promoter; gene expression is mediated by the transcription factor Sp1

Neuropilin-1 (NRP1) is a receptor for the vascular endothelial growth factor (VEGF) family of angiogenesis factors and for the semaphorin family of secreted neuronal guidance polypeptides. Very little is known, however, about how NRP1 gene expression is regulated. In this study, it was demonstrated that the tumor promoter, TPA (12-O-tetradecanoylphorbol-13-acetate) significantly up-regulated NRP1 mRNA levels by increasing its gene transcription rate in a manner dependent on de novo protein synthesis. To determine which elements regulate functional NRP1 expression, the promoter regions of human and mouse NRP1 genes were cloned and characterized. Promoter-reporter gene transfection experiments using deletion and point mutations demonstrated that two Sp1 elements are major contributors to both the constitutive and TPA-induced activity of the NRP1 promoter. Gel shift analysis showed a specific binding of the Sp1 transcription factor to those elements. Further mutational analysis revealed that an AP-1, and a CCAAT box also contributed to NRP1 constitutive and TPA-induced promoter activity. It was concluded that NRP1 expression is regulated by the cooperation of several regulatory elements including AP-1, Sp1, and a CCAAT box.

Neurovascular congruence results from a shared patterning mechanism that utilizes Semaphorin3A and Neuropilin-1

Peripheral nerves and blood vessels have similar patterns in quail forelimb development. Usually, nerves extend adjacent to existing blood vessels, but in a few cases, vessels follow nerves. Nerves have been proposed to follow vascular smooth muscle, endothelium, or their basal laminae. Focusing on the major axial blood vessels and nerves, we found that when nerves grow into forelimbs at E3.5-E5, vascular smooth muscle was not detectable by smooth muscle actin immunoreactivity. Additionally, transmission electron microscopy at E5.5 confirmed that early blood vessels lacked smooth muscle and showed that the endothelial cell layer lacks a basal lamina, and we did not observe physical contact between peripheral nerves and these endothelial cells. To test more generally whether lack of nerves affected blood vessel patterns, forelimb-level neural tube ablations were performed at E2 to produce aneural limbs; these had completely normal vascular patterns up to at least E10. To test more generally whether vascular perturbation affected nerve patterns, VEGF(165), VEGF(121), Ang-1, and soluble Flt-1/Fc proteins singly and in combination were focally introduced via beads implanted into E4.5 forelimbs. These produced significant alterations to the vascular patterns, which included the formation of neo-vessels and the creation of ectopic avascular spaces at E6, but in both under- and overvascularized forelimbs, the peripheral nerve pattern was normal. The spatial distribution of semaphorin3A protein immunoreactivity was consistent with a negative regulation of neural and/or vascular patterning. Semaphorin3A bead implantations into E4.5 forelimbs caused failure of nerves and blood vessels to form and to deviate away from the bead. Conversely, semaphorin3A antibody bead implantation was associated with a local increase in capillary formation. Furthermore, neural tube electroporation at E2 with a construct for the soluble form of neuropilin-1 caused vascular malformations and hemorrhage as well as altered nerve trajectories and peripheral nerve defasciculation at E5-E6. These results suggest that neurovascular congruency does not arise from interdependence between peripheral nerves and blood vessels, but supports the hypothesis that it arises by a shared patterning mechanism that utilizes semaphorin3A.

The metalloendopeptidase nardilysin (NRDc) is potently inhibited by heparin-binding epidermal growth factor-like growth factor (HB-EGF)

Nardilysin (N-arginine dibasic convertase, or NRDc) is a cytosolic and cell-surface metalloendopeptidase that, in vitro, cleaves substrates upstream of Arg or Lys in basic pairs. NRDc differs from most of the other members of the M16 family of metalloendopeptidases by a 90 amino acid acidic domain (DAC) inserted close to its active site. At the cell surface, NRDc binds heparin-binding epidermal growth factor-like growth factor (HB-EGF) and enhances HB-EGF-induced cell migration. An active-site mutant of NRDc fulfills this function as well as wild-type NRDc, indicating that the enzyme activity is not required for this process. We now demonstrate that NRDc starts at Met(49). Furthermore, we show that HB-EGF not only binds to NRDc but also potently inhibits its enzymic activity. NRDc-HB-EGF interaction involves the 21 amino acid heparin-binding domain (P21) of the growth factor, the DAC of NRDc and most probably its active site. Only disulphide-bonded P21 dimers are inhibitory. We also show that Ca(2+), via the DAC, regulates both NRDc activity and HB-EGF binding. We conclude that the DAC is thus a key regulatory element for the two distinct functions that NRDc fulfills, i.e. as an HB-EGF modulator and a peptidase.