Host myeloid cells are necessary for creating bioengineered human vascular networks in vivo

The recruitment of myeloid cells has been consistently associated with the formation of new blood vessels during pathological angiogenesis. However, the participation of myeloid cells in bioengineered vascular networks remains unclear. Therefore, we tested whether host myeloid cells play a role in the formation of bioengineered vascular networks that occurs in vivo upon coimplantation of blood-derived endothelial progenitor cells and bone-marrow-derived mesenchymal progenitor cells, suspended as single cells in Matrigel, into immune-deficient mice. We observed an influx of spatially organized host CD11b(+) myeloid cells into the Matrigel implant 1 to 3 days after implantation, which was shown to be cell mediated rather than a nonspecific response. Myeloid cells were significantly reduced once the implants were fully vascularized at days 6 and 7, suggesting an active role during steps that precede formation of functional anastomoses and perfused vessels. Importantly, depletion of circulating myeloid cells resulted in a significant reduction in microvessel density in the implants. In summary, the recruitment of myeloid cells occurs rapidly after coimplantation of endothelial and mesenchymal progenitor cells and is necessary for full vascularization in this model. This is the first demonstration of a role for recruited myeloid cells in the formation of bioengineered vascular networks.

Semaphorin 3E, an exception to the rule

Class 3 semaphorins (Sema3s) regulate axon guidance, angiogenesis, tumor growth, and tumor metastasis. Neuropilins (NRPs; NRP1 and NRP2) are the cell surface receptors for the Sema3s. However, to signal, interaction of Sema3s and NRPs with plexins is obligatory. In this issue of the JCI, Casazza and colleagues report data that challenge the conventional wisdom about the role of Sema3s in tumor metastasis. As a rule, Sema3B and Sema3F, for example, are inhibitors of tumor angiogenesis, progression, and metastasis. However, Casazza et al. found that Sema3E inhibited tumor growth but atypically promoted invasiveness and metastasis. This metastatic potential was dependent on Plexin D1 expression but was independent of NRP expression. Of clinical importance, Sema3E and Plexin D1 were found to be upregulated in human colon cancer, liver metastasis, and melanoma progression.

A mutated soluble neuropilin-2 B domain antagonizes vascular endothelial growth factor bioactivity and inhibits tumor progression

Neuropilins (NRP1 and NRP2) are coreceptors for vascular endothelial growth factor (VEGF) and mediate angiogenesis and tumor progression. VEGF binds to the NRP1 and NRP2 B domains. Previously, it was shown that mutagenesis of the soluble NRP2 B domain (MutB-NRP2) increased affinity to VEGF by 8-fold. Here, we show that MutB-NRP2 inhibited (125)I-VEGF binding to NRP1, NRP2, and VEGFR-2. It antagonized VEGF-induced VEGFR-2/NRP2 complex formation and inhibited VEGF-induced activation of AKT, a mediator of cell survival, without affecting activation of VEGFR-2. In three-dimensional embryoid bodies, a model of VEGF-induced angiogenesis, MutB-NRP2 inhibited VEGF-induced sprouting. When overexpressed in human melanoma cells, MutB-NRP2 inhibited tumor growth compared with control tumors. Avastin (bevacizumab), a monoclonal antibody to VEGF, inhibited VEGF interactions with VEGFR-2, but not with NRPs. The combination of MutB-NRP2 and Avastin resulted in an enhanced inhibition of human melanoma tumor growth compared with MutB-NRP2 treatment only or Avastin treatment only. In conclusion, these results indicate that MutB-NRP2 is a novel antagonist of VEGF bioactivity and tumor progression.

Abstract 1308: Cytogenetic abnormalities of tumor endothelial cells in human malignant tumors

An important concept in tumor angiogenesis has been that tumor blood vessels contain genetically normal and stable endothelial cells (ECs), unlike tumor cells, which typically display genetic instability. Chromosomal aberration in human tumor endothelial cells (hTECs) in carcinoma has not yet been investigated. Here we isolated TECs from 20 human renal cell carcinomas (RCCs) and analyzed their cytogenetic abnormalities. The degree of aneuploidy was analyzed by fluorescence in situ hybridization using chromosome 7 and chromosome 8 DNA probes in isolated hTECs. In human RCCs, 22-58% (median 33%) of uncultured hTECs were aneuploid, whereas normal ECs were diploid. The mechanisms of TEC aneuploidy were studied using mouse TECs (mTECs) isolated from xenografts of human epithelial tumors. To investigate the contribution of progenitor cells to aneuploidy in mTECs, CD133 positive and CD133 negative mTECs were compared for aneuploidy. CD133 positive mTECs showed aneuploidy more frequently than CD133 negative mTECs. This is the first report showing cytogenetic abnormality of hTECs in carcinoma, contrary to traditional belief. It is suggested that cytogenetic alterations in tumor vessels of carcinoma can occur and may play a signicant role in modifying tumor-stromal interactions.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 1308.

Id2 promotes tumor cell migration and invasion through transcriptional repression of semaphorin 3F

Id proteins (Id1 to Id4) are helix-loop-helix transcription factors that promote metastasis. It was found that Semaphorin 3F (SEMA3F), a potent inhibitor of metastasis, was repressed by Id2. High metastatic human tumor cell lines had relatively high amounts of Id2 and low SEMA3F levels compared with their low metastatic counterparts. No correlation between metastatic potential and expression of the other Id family members was observed. Furthermore, ectopic expression of Id2 in low metastatic tumor cells downregulated SEMA3F and, as a consequence, enhanced their ability to migrate and invade, two requisite steps of metastasis in vivo. Id2 overexpression was driven by the c-myc oncoprotein. SEMA3F was a direct target gene of the E47/Id2 pathway. Two E-box sites, which bind E protein transcription factors including E47, were identified in the promoter region of the SEMA3F gene. E47 directly activated SEMA3F promoter activity and expression and promoted SEMA3F biological activities, including filamentous actin depolymerization, inactivation of RhoA, and inhibition of cell migration. Silencing of SEMA3F inhibited the E47-induced SEMA3F expression and biological activities, confirming that these E47-induced effects were SEMA3F dependent. E47 did not induce expression of the other members of the SEMA3 family. Id2, a dominant-negative inhibitor of E proteins, abrogated the E47-induced SEMA3F expression and biological activities. Thus, high metastatic tumor cells overexpress c-myc, leading to upregulation of Id2 expression; the aberrantly elevated amount of Id2 represses SEMA3F expression and, as a consequence, enhances the ability of tumor cells to migrate and invade.

Cytogenetic abnormalities of tumor-associated endothelial cells in human malignant tumors

Tumor blood vessels are thought to contain genetically normal and stable endothelial cells (ECs), unlike tumor cells, which typically display genetic instability. Yet, chromosomal aberration in human tumor-associated ECs (hTECs) in carcinoma has not yet been investigated. Here we isolated TECs from 20 human renal cell carcinomas and analyzed their cytogenetic abnormalities. The degree of aneuploidy was analyzed by fluorescence in situ hybridization using chromosome 7 and chromosome 8 DNA probes in isolated hTECs. In human renal cell carcinomas, 22-58% (median, 33%) of uncultured hTECs were aneuploid, whereas normal ECs were diploid. The mechanisms governing TEC aneuploidy were then studied using mouse TECs (mTECs) isolated from xenografts of human epithelial tumors. To investigate the contribution of progenitor cells to aneuploidy in mTECs, CD133(+) and CD133(-) mTECs were compared for aneuploidy. CD133(+) mTECs showed aneuploidy more frequently than CD133(-) mTECs. This is the first report showing cytogenetic abnormality of hTECs in carcinoma, contrary to traditional belief. Cytogenetic alterations in tumor vessels of carcinoma therefore can occur and may play a significant role in modifying tumor- stromal interactions.

Tumor endothelial cells join the resistance

The field of antiangiogenesis research has been met with some surprises, including the realization that tumor blood vessels are more complex and labile than expected. In this issue of Clinical Cancer Research, Xiong and colleagues show that tumor-specific endothelial cells are less sensitive to cytotoxic and antiangiogenic drugs compared to their normal counterparts.

Tumor endothelial cells have features of adult stem cells

We recently reported that prostate tumor-specific endothelial cells had features of mesenchymal stem cells and could transdifferentiate to form cartilage and bone-like tissues. Plasticity in the tumor vasculature may be related to well-known tumor blood vessel abnormalities and could underlie an intrinsic adaptive mechanism in tumor endothelial cells for circumventing anti-angiogenic strategies.

Calcification of multipotent prostate tumor endothelium

Solid tumors require new blood vessels for growth and metastasis, yet the biology of tumor-specific endothelial cells is poorly understood. We have isolated tumor endothelial cells from mice that spontaneously develop prostate tumors. Clonal populations of tumor endothelial cells expressed hematopoietic and mesenchymal stem cell markers and differentiated to form cartilage- and bone-like tissues. Chondrogenic differentiation was accompanied by an upregulation of cartilage-specific col2a1 and sox9, whereas osteocalcin and the metastasis marker osteopontin were upregulated during osteogenic differentiation. In human and mouse prostate tumors, ectopic vascular calcification was predominately luminal and colocalized with the endothelial marker CD31. Thus, prostate tumor endothelial cells are atypically multipotent and can undergo a mesenchymal-like transition.

Attenuated p53 activation in tumour-associated stromal cells accompanies decreased sensitivity to etoposide and vincristine

Alterations in the tumour suppressor p53 have been reported in tumour-associated stromal cells; however, the consequence of these alterations has not been elucidated. We investigated p53 status and responses to p53-activating drugs using tumour-associated stromal cells from A375 melanoma and PC3 prostate carcinoma xenografts, and a spontaneous prostate tumour model (TRAMP). p53 accumulation after treatment with different p53-activating drugs was diminished in tumour-associated stromal cells compared to normal stromal cells. Tumour-associated stromal cells were also less sensitive to p53-activating drugs - this effect could be reproduced in normal stromal cells by p53 knockdown. Unlike normal stromal cells, tumour stromal cells failed to arrest in G(2) after etoposide treatment, failed to upregulate p53-inducible genes, and failed to undergo apoptosis after treatment with vincristine. The lower levels of p53 in tumour stromal cells accompanied abnormal karyotypes and multiple centrosomes. Impaired p53 function in tumour stroma might be related to genomic instability and could enable stromal cell survival in the destabilising tumour microenvironment.

ABL2/ARG tyrosine kinase mediates SEMA3F-induced RhoA inactivation and cytoskeleton collapse in human glioma cells

Class three semaphorins (SEMAs) were originally shown to be mediators of axon guidance that repelled axons and collapsed growth cones, but it is now evident that SEMA3F, for example, has similar effects on tumor cells and endothelial cells (EC). In both human U87MG glioma cells and human umbilical vein EC, SEMA3F induced rapid cytoskeletal collapse, suppressed cell contractility, decreased phosphorylation of cofilin, and inhibited cell migration in culture. Analysis of the signaling pathways showed that SEMA3F formed a complex with NRP2 (neuropilin-2) and plexin A1. These interactions eventually led to inactivation of the small GTPase, RhoA, which is necessary for stress fiber formation and cytoskeleton integrity. A novel upstream RhoA mediator was shown to be ABL2, also known as ARG, a membrane-anchored nonreceptor tyrosine kinase. Within minutes after the addition of SEMA3F, ABL2 directly bound plexin A1 but not to a plexin A1 mutant lacking the cytoplasmic domain. In addition, ABL2 phosphorylated and thereby activated p190RhoGAP, which inactivated RhoA (GTP to GDP), resulting in cytoskeleton collapse and inhibition of cell migration. On the other hand, cells overexpressing an ABL2 inactive kinase mutant or treated with ABL2 small interfering RNA did not inactivate RhoA. Cells treated with p190RhoGAP small interfering RNA also did not inactivate RhoA. Together, these results suggested that ABL2/ARG is a novel mediator of SEMA3F-induced RhoA inactivation and collapsing activity.

Targeting EGFR activity in blood vessels is sufficient to inhibit tumor growth and is accompanied by an increase in VEGFR-2 dependence in tumor endothelial cells

Epidermal growth factor receptor (EGFR) targeting agents such as kinase inhibitors reduce tumor growth and progression. We have previously reported that EGFR is not only expressed by the tumor cells but by the tumor endothelial cells (EC) as well (Amin, D. N., Hida, K., Bielenberg, D. R., Klagsbrun, M., 2006. Tumor endothelial cells express epidermal growth factor receptor (EGFR) but not ErbB3 and are responsive to EGF and to EGFR kinase inhibitors. Cancer Res. 66, 2173-80). Thus, targeting tumor blood vessel EGFR may be a viable strategy for tumor growth inhibition. We describe here a melanoma xenograft model where the tumor cells express very little or no EGFR but the tumor blood vessels express activated EGFR. The EGFR kinase inhibitor, gefitinib (Iressa), retarded tumor growth with a size decrease of 38% compared to control mice, ostensibly due to targeting of the blood vessels. EC were isolated from tumors of gefitinib-treated mice. These EC were unable to proliferate in response to EGF and displayed relatively weaker activation of MAPK and AKT signaling in response to EGF compared to tumor EC isolated from vehicle-treated mice. In contrast, the tumor EC from gefitinib-treated mice expressed higher levels of VEGFR-2 both at the mRNA and protein level. In addition, these cells were less sensitive to EGFR kinase inhibitors in vitro but more sensitive to a VEGFR-2 kinase inhibitor. These results suggest that in tumor EC from gefitinib-treated mice there is a switch from dependence on EGFR activity to signaling via VEGFR-2. Our data provide a molecular rationale for combination therapies targeting both EGF and VEGF signaling on the tumor vasculature.

Semaphorin-induced cytoskeletal collapse and repulsion of endothelial cells

Class 3 semaphorins (SEMA3) are mediators of neuronal guidance first shown to repel axons and collapse axonal growth cones by depolymerization of cytoskeletal F-actin. Subsequently, it was found that SEMA3 could also mediate angiogenesis. SEMA3F binds to its receptor, neuropilin 2 (NRP2), a transmembrane protein expressed on neurons, EC (EC), and tumor cells. In vitro, SEMA3F collapses the F-actin cytoskeleton, repels EC, and inhibits EC and tumor cell adhesion and migration in a manner similar to what occurs with axons. In a mouse tumor model, SEMA3F is a potent inhibitor of tumor angiogenesis, tumor progression, and metastasis. SEMA3F is encoded in a region of chromosome 3p21.3 that is commonly deleted in small cell lung cancers, suggesting that SEMA3F is a tumor suppressor. SEMA3F may have therapeutic potential. Therefore, this chapter is focused primarily on the detailed methods to purify SEMA3F and to assay its biologic activity, including cytoskeleton collapse and repulsion.

Targeting endothelial and tumor cells with semaphorins

Neuropilins (NRP) are receptors for the class 3 semaphorin (SEMA3) family of axon guidance molecules and the vascular endothelial growth factor (VEGF) family of angiogenesis factors. Although the seminal studies on SEMA3s and NRPs first showed them to be mediators of axon guidance, it has become very apparent that these proteins play an important role in vascular and tumor biology as well. Neuronal guidance and angiogenesis are regulated similarly at the molecular level. For example, SEMA3s not only repel neurons and collapse axon growth cones, but have similar effects on endothelial cells and tumor cells. Preclinical studies indicate that SEMA3F is a potent inhibitor of tumor angiogenesis and metastasis. In addition, neutralizing antibodies to NRP1 enhance the effects of anti-VEGF antibodies in suppressing tumor growth in xenograft models. This article reviews NRP and SEMA3 structural interactions and their role in developmental angiogenesis, tumor angiogenesis and metastasis based on cell culture, zebrafish and murine studies.

Duplicate VegfA genes and orthologues of the KDR receptor tyrosine kinase family mediate vascular development in the zebrafish

Vascular endothelial growth factor A (VEGFA) and the type III receptor tyrosine kinase receptors (RTKs) are both required for the differentiation of endothelial cells (vasculogenesis) and for the sprouting of new capillaries (angiogenesis). We have isolated a duplicated zebrafish VegfA locus, termed VegfAb, and a duplicate RTK locus with homology to KDR/FLK1 (named Kdrb). Morpholino-disrupted VegfAb embryos develop a normal circulatory system until approximately 2 to 3 days after fertilization (dpf), when defects in angiogenesis permit blood to extravasate into many tissues. Unlike the VegfAa(121) and VegfAa(165) isoforms, the VegfAb isoforms VegfAb(171) and VegfAb(210) are not normally secreted when expressed in mammalian tissue culture cells. The Kdrb locus encodes a 1361-amino acid transmembrane receptor with strong homology to mammalian KDR. Combined knockdown of both RTKs leads to defects in vascular development, suggesting that they cooperate in mediating the vascular effects of VegfA in zebrafish development. Both VegfAa and VegfAb can individually bind and promote phosphorylation of both Flk1 (Kdra) and Kdrb proteins in vitro. Taken together, our data support a model in the zebrafish, in which duplicated VegfA and multiple type III RTKs mediate vascular development.

Site-directed Mutagenesis in the B-Neuropilin-2 Domain Selectively Enhances Its Affinity to VEGF165, but Not to Semaphorin 3F

Neuropilins (NRPs) are 130-kDa receptors that bind and respond to the class 3 semaphorin family of axon guidance molecules (SEMAs) and to members of the vascular endothelial growth factor (VEGF) family of angiogenic factors. Two NRPs have been reported so far, NRP1 and NRP2. Unlike NRP1, little is known about NRP2 interactions with its ligands, VEGF165 and SEMA3F. Cell binding studies reveal that VEGF165 and SEMA3F bind NRP2 with similar affinities, 5.2 and 3.9 nM, respectively, and are competitive NRP2 ligands. Immunoprecipitation studies show that the B (b1b2) extracellular domain of NRP2 is sufficient for VEGF165 binding, whereas SEMA3F requires both the A (a1a2) and B domains. To identify residues of B-NRP2 involved in VEGF165 binding, point mutations were introduced by site-directed mutagenesis. VEGF165 is a basic protein. Reduction of the electronegative potential of B-NRP2 by exchanging acidic residues for uncharged alanine (B-NRP2 E284A,E291A) in the 280-290 b1-NRP2 loop resulted in a 2-fold reduction in VEGF165 affinity. Conversely, enhancing the electronegative potential (B-NRP2 R287E,N290D and R287E,N290S) significantly increased VEGF165 affinity for B-NRP2 by 8- and 6.6-fold, respectively. The mutagenesis did not affect SEMA3F/B-NRP2 interactions. These results demonstrate that it is possible to alter VEGF165 affinity for NRP2 without affecting SEMA3F affinity. They also identify NRP2 residues involved in VEGF165 binding and suggest that modifications of B-NRP2 could lead to potentially high affinity selective inhibitors of VEGF165/NRP2 interactions.

Neuropilins: novel targets for anti-angiogenesis therapies

It is now well established that neuropilins (NRP1 and NRP2), first described as mediators of neuronal guidance, are also mediators of angiogenesis and tumor progression. NRPs are receptors for the class-3 semaphorin (SEMA) family of axon guidance molecules and also for the vascular endothelial growth factor (VEGF) family of angiogenic factors. VEGF-NRP interactions promote developmental angiogenesis as shown in mouse knockout and zebrafish knockdown studies. There is also evidence that NRPs mediate tumor progression. For example, overexpression of NRP1 enhances tumor growth whereas NRP1 antagonists, such as soluble NRP1 and anti-NRP1 antibodies, inhibit tumor growth. Furthermore, some class-3 SEMAs acting via NRPs inhibit tumor angiogenesis, progression and metastasis. Clinical data suggest that high NRP levels correlate with poor prognosis and survival in a variety of cancer types. Taken together, these results suggest that NRPs are potentially valuable targets for new anti-cancer therapies. We analyze here the current knowledge on NRPs and their role in angiogenesis and tumor progression and enumerate strategies for targeting these receptors.

AlphaA-crystallin expression prevents gamma-crystallin insolubility and cataract formation in the zebrafish cloche mutant lens

Cataracts, the loss of lens transparency, are the leading cause of human blindness. The zebrafish embryo, with its transparency and relatively large eyes, is an excellent model for studying ocular disease in vivo. We found that the zebrafish cloche mutant, both the cloche(m39) and cloche(S5) alleles, which have defects in hematopoiesis and blood vessel development, also have lens cataracts. Quantitative examination of the living zebrafish lens by confocal microscopy showed significant increases in lens reflectance. Histological analysis revealed retention of lens fiber cell nuclei owing to impeded terminal differentiation. Proteomics identified gamma-crystallin as a protein that was substantially diminished in cloche mutants. Crystallins are the major structural proteins in mouse, human and zebrafish lens. Defects in crystallins have previously been shown in mice and humans to contribute to cataracts. The loss of gamma-crystallin protein in cloche was not due to lowered mRNA levels but rather to gamma-crystallin protein insolubility. AlphaA-crystallin is a chaperone that protects proteins from misfolding and becoming insoluble. The cloche lens is deficient in both alphaA-crystallin mRNA and protein during development from 2-5 dpf. Overexpression of exogenous alphaA-crystallin rescued the cloche lens phenotype, including solubilization of gamma-crystallin, increased lens transparency and induction of lens fiber cell differentiation. Taken together, these results indicate that alphaA-crystallin expression is required for normal lens development and demonstrate that cataract formation can be prevented in vivo. In addition, these results show that proteomics is a valuable tool for detecting protein alterations in zebrafish.