Notch and VEGF pathways play distinct but complementary roles in tumor angiogenesis
- Sonia L Hernandez†1,
- Debarshi Banerjee†1,
- Alejandro Garcia2,
- Thaned Kangsamaksin3,
- Wei-Yi Cheng4,
- Dimitris Anastassiou4,
- Yasuhiro Funahashi3,
- Angela Kadenhe-Chiweshe2,
- Carrie J Shawber3,
- Jan K Kitajewski3, 5,
- Jessica J Kandel†2 and
- Darrell J Yamashiro†1, 2, 5Email author
© Hernandez et al.; licensee BioMed Central Ltd. 2013
Received: 3 May 2013
Accepted: 20 September 2013
Published: 25 September 2013
Anti-angiogenesis is a validated strategy to treat cancer, with efficacy in controlling both primary tumor growth and metastasis. The role of the Notch family of proteins in tumor angiogenesis is still emerging, but recent data suggest that Notch signaling may function in the physiologic response to loss of VEGF signaling, and thus participate in tumor adaptation to VEGF inhibitors.
We asked whether combining Notch and VEGF blockade would enhance suppression of tumor angiogenesis and growth, using the NGP neuroblastoma model. NGP tumors were engineered to express a Notch1 decoy construct, which restricts Notch signaling, and then treated with either the anti-VEGF antibody bevacizumab or vehicle.
Combining Notch and VEGF blockade led to blood vessel regression, increasing endothelial cell apoptosis and disrupting pericyte coverage of endothelial cells. Combined Notch and VEGF blockade did not affect tumor weight, but did additively reduce tumor viability.
Our results indicate that Notch and VEGF pathways play distinct but complementary roles in tumor angiogenesis, and show that concurrent blockade disrupts primary tumor vasculature and viability further than inhibition of either pathway alone.
KeywordsNeuroblastoma Vascular endothelial growth factor Notch Angiogenesis Bevacizumab
The anti-vascular endothelial growth factor (VEGF) antibody bevacizumab (BV) is clinically validated to treat human cancers in adults, and is currently being evaluated as a therapy for pediatric tumors, including neuroblastoma . However, it appears that essentially all patients, even those who respond to BV initially, will ultimately progress despite ongoing treatment. Thus, a similar limitation is anticipated in children treated with this agent. The mechanisms of tumor progression during BV treatment are not well understood.
We have previously shown that VEGF inhibition disrupts the blood vessels of the neuroblastoma NGP xenograft model, but resistance occurs [2–4]. VEGF blockade can increase cooption of existing vascular structures by encroaching tumors , a mechanism which may partly compensate for defective VEGF signaling. An additional mechanism for tumor progression during VEGF blockade could be the activation of an alternative angiogenic pathway. In support of this concept, we have shown that the Notch pathway is up-regulated in neuroblastoma when the VEGF receptor 2 (VEGFR2) is blocked with the anti-VEGFR2 antibody DC101 .
To examine the role of Notch in tumor angiogenesis, we have used the Notch1 decoy (N1D), a soluble construct derived from the extracellular domain of the Notch1 receptor that blocks Notch activation by sequestering Notch ligands . Notch blockade using the N1D disrupts tumor vasculature and decreases tumor viability . Based on our previous studies, we thus hypothesized that blocking the Notch and VEGF pathways simultaneously would disrupt the vasculature and inhibit tumor growth more effectively than either agent alone. To test this hypothesis, we evaluated the effects of expressing N1D in combination with bevacizumab (BV) in the NGP neuroblastoma xenograft model. Here, we report that combining N1D with BV had a profound effect on the tumor viability and the vasculature of NGP xenografts. N1D and BV additively increased tumor endothelial cell apoptosis, leading to blood vessel regression, and cell death.
Transfections and cell culture
NGP-LacZ and NGP-N1D cells were maintained as described . pLKO.1 empty vector (control) and pLKO.1 Notch1 shRNA (Notch1 KD) plasmids (Sigma Aldrich) were stably transfected into NGP cells, selected and maintained in 1 mg/ml puromycin (Sigma Aldrich).
NGP xenograft model
1 × 106 cells were injected intrarenally into 4–6 week old NCR female nude mice (Taconic, Germantown, NY) as previously described . All animal experiments were approved by the Columbia University Institutional Animal Care and Use Committee. At sacrifice, organs were harvested and weighed as previously described .
Bevacizumab and placebo administration
NGP-LacZ and NGP-Notch1 decoy (N1D) tumor-bearing mice were randomized one week after implantation. At this time, 28 mice in each group were treated with 100 μg mouse albumin (Sigma Aldrich) (placebo), and another 28 with 250 μg bevacizumab (10 mg/kg BV, Genentech) by intraperitoneal injection biweekly until time of sacrifice. Both the N1D and BV are fused to a human Fc fragment. Immunohistochemistry (Additional file 1: Figure S1A) and SDS-PAGE (Additional file 1: Figure S1B) to human Fc was performed on tumors, and confirmed expression of N1D in the tumors at time of sacrifice. Since BV is also fused to human Fc, it could also be detected in the NGP-LacZ + BV and NGP-N1D + BV tumors by SDS-PAGE (Additional file 1: Figure S1B, lower band).
Primary antibodies: αSMA (Lab vision), cleaved Notch1 (Cell Signaling), PECAM-1 (Angioproteomie), Notch1 (Upstate), collagen IV (Cosmobio), biotinylated hFc (Jackson Labs). Alexafluor probes (Invitrogen) were used for fluorescent stains.
Hypoxyprobe and quantification
Thirty minutes prior to sacrifice, mice received an intraperitoneal injection of 0.1 ml of pimonidazole (Hypoxyprobe®, Chemicon), and paraffin sections were used to stain for pimonidazole with MAB-1 (Chemicon), following the manufacturer’s instructions. Four tumors per treatment group were quantified, with an average of 10 images taken from each tumor section at 20×. The images were then analyzed using Adobe Photoshop using an arbitrary threshold applied to all images.
Lectin perfusion and quantification
An intracardiac injection of 100 μl of Fluorescein labeled Lycopersicon esculentum (Vector Labs) was delivered 2 minutes prior to 1% PFA and PBS perfusion, followed by tumor collection. Tumors were fixed in 4% PFA and 50 μm thick sections were analyzed using confocal microscopy.
TUNEL assay quantification
TUNEL staining and quantification was performed as described . An average of 20 images from each tumor section, and 5 tumors per group were analyzed.
Collagen IV, PECAM-1 and EC apoptosis quantifications
Fresh-frozen, serial sections were used for Collagen IV, PECAM-1 and EC apoptosis quantifications. Viable tumor areas immunostained for collagen IV or PECAM-1 were photographed at 10× (Collagen IV) or 20× (PECAM-1). An average of 15 (Collagen IV) or 30 (PECAM-1) images per tumor, and three tumors per group were analyzed. For EC quantification, TUNEL, PECAM-1, stained sections were mounted with DAPI (Vector) and images obtained at 40× magnification.
5 μm sections of paraffin-embedded livers or kidneys were stained with hematoxylin and eosin (H&E). Tumor sections were scanned using an Epson 4870 Scanner. Glomeruli surrounded by at least one layer of tumor cells were counted as coopted vasculature. One section per tumor, and 28–30 tumors per group were analyzed.
3 × 106 NGP-LacZ or NGP-N1D cells were plated on a 10 cm dish, and cultured in 6 ml of serum-free EBM-2 (Invitrogen) for 48 hrs. Supernatant from three independent collections was measured, using a human VEGF ELISA kit (R&D) following the manufacturer’s instructions.
HUVEC cell death and BrdU incorporation assays
8,000 HUVEC per well (48 well plate) were seeded on a fibrinogen gel (Sigma), in basal EBM-2 media (Lonza). Two hrs later, media was replaced by conditioned media (CM) from NGP-LacZ or NGP-N1D cells (3×106 cells per 10 cm2 plate, cultured for 24 hrs in EBM-2 basal media. We added 50 ng/ml BV or 20 ng/ml rhVEGF165 (R&D) to the CM. For BrdU assays, 2 μl BrdU (Millipore) was added per ml of CM. Cell death assays (Roche) or BrdU incorporation assays (Millipore) were measured 23 hrs later, following the manufacturer’s instructions.
Immunoprecipitation and Western Blots
Tumors were lysed in a ratio of 100 ml of lysis buffer to 10 mg of tumor. 100 μl of tumor lysate was immunoprecipitated with A/G agarose beads (Santa Cruz Biotechnology) and blotted with anti-human Fc (Pierce).
Reverse transcription and quantitative PCR
RNA was extracted from cells following the manufacturer’s instructions (Qiagen). Tissue RNA extraction was performed with Ambion ToTALLY RNA kit (Ambion) following the manufacturer’s specifications. RNA from tissue or cells was reverse transcribed following the manufacturer’s instructions (Invitrogen, DNAseI and Superscript). Quantitative PCR was performed with the Taqman system (Applied Biosystems) following the manufacturer’s specifications, using primers designed not to amplify genomic DNA. GeNorm was used to select the normalization values out of 6 housekeeping genes.
Microarrays and probes preparation
HG-U133A 2.0 (human genome) Gene Chips (Affymetrix, Santa Clara, CA) were used to investigate gene expression in xenograft tumors. The cRNA probes were synthesized as recommended by Affymetrix, purified using RNeasy and fragmented according to the Affymetrix protocol, and 15 μg of biotinylated cRNA were hybridized to the microarrays. The samples were scanned with Affymetrix Gene Chip Scanner 3000. The data set, corresponding to 24 tumors (6 arrays per group), has been deposited in NCBI’s Gene Expression Omnibus (GEO) and is available through accession number GSE (pending). Data were RMA normalized using the Bioconductor open source software.
Statistical analysis was performed using the Prism 5 software (GraphPad Software, Inc. La Jolla, CA). One-way analysis of variance was performed with post-hoc analysis by Tukey’s Multiple Comparison Test. Graphs represent mean and standard deviation in all cases.
Combined blockade of notch and VEGF increases necrosis and apoptosis in primary tumors
Although there was no difference in tumor weight as compared with BV treatment alone, the NGP-N1D + BV tumors were noticeably more necrotic (Figure 1B), which was confirmed by quantification of necrosis in H&E sections (Figure 1C). Trichrome staining confirmed that these differences were due to necrosis and not fibrosis (data not shown). When assessed by TUNEL, the NGP-N1D + BV tumors exhibited significantly more apoptosis then NGP-LacZ controls (14.4 fold), BV alone (3.6 fold), or N1D alone (2.5 fold) (Figure 1D). Blockade of Notch or VEGF did not have a direct effect on tumor cell proliferation, as there was no difference in phospho-histone H3 positive cells in areas of viable tumor (Figure 1E). These data indicate that combined Notch and VEGF blockade additively decreases tumor viability.
Combined blockade of notch and VEGF markedly decreases perfusion
The decrease in lectin-positive area for NGP-N1D + BV relative to NGP-BV tumors suggested that there could be a decrease in the number of coopted glomeruli. We quantified coopted vessels by counting the number of glomeruli surrounded by tumor cells in H&E sections (Figure 2D, arrows). The number of coopted glomeruli in NGP-LacZ + BV tumors was increased 2.3 fold compared to NGP-LacZ tumors (Figure 2E). Notch blockade significantly attenuated this BV-induced increase in cooption, with NGP-N1D + BV tumors having significantly fewer coopted glomeruli as compared to the NGP-LacZ + BV group (115 vs 178, respectively, Figure 2E). These findings suggest that Notch blockade attenuates BV-induced cooption in NGP xenografts. Furthermore, immunostaining for cleaved Notch1, reflecting processing of the Notch receptor after ligand binding, showed increased activated Notch1 in coopted glomeruli as a result of BV treatment in NGP tumors (Additional file 2: Figure S2). These data implicate Notch signaling in BV-induced cooption of host vasculature.
Blockade of VEGF results in tumor hypoxia
We also quantified the expression of two hypoxia responsive genes, VEGF and PlGF, in the tumors by real-time PCR. VEGF expression was significantly increased 3.1 fold (P < 0.01) in NGP-LacZ + BV and by 2.7 fold (P < 0.01) in NGP-N1D + BV tumors compared to NGP-LacZ controls and to NGP-N1D (Figure 3C). PlGF expression behaved in a similar pattern: relative to NGP-LacZ controls, NGP-N1D tumors had no change, while NGP-LacZ + BV tumors had 4.7 fold more (p < 0.001), and NGP-N1D + BV tumors had 3.79 fold more PlGF than controls (p < 0.001).
Hypoxia associated gene sets* are enriched in BV treated tumors compared with LacZ tumors
ENRICHED GENE SETS FOR N1D+BV TUMORS
ENRICHED GENE SETS FOR LacZ+BV TUMORS
POSITIVELY ENRICHED GENE SETS
POSITIVELY ENRICHED GENE SETS
NEGATIVELY ENRICHED GENE SETS
NEGATIVELY ENRICHED GENE SETS
ENRICHED GENE SETS FOR N1D TUMORS
POSITIVELY ENRICHED GENE SETS
NEGATIVELY ENRICHED GENE SETS
Combined notch and VEGF blockade disrupts tumor vasculature
Notch and VEGF blockade additively decrease survival of tumor endothelial cells
Previous studies have suggested that both Notch and VEGF can act as survival signals for ECs. We therefore hypothesized that combined blockade would result in increased EC apoptosis. Quantification of apoptotic ECs was done using TUNEL (Figure 5C, red), PECAM-1 (Figure 5C, green), and nuclear DAPI immunostaining (Figure 5C, blue). A cell was considered an apoptotic EC when its nucleus, stained by DAPI, overlapped with TUNEL stain (red), and was surrounded by PECAM-1 (green) (Figure 5C, arrowheads). The number of apoptotic ECs in each section was then normalized to the PECAM positive area in each tumor. This quantification showed a significant increase in apoptotic EC in NGP-N1D + BV tumors (Figure 5D, P < 0.01). This increase in apoptotic ECs suggested that Notch and VEGF are both survival signals in tumor ECs, which act independently.
In order to test the effects of VEGF and Notch on EC survival in vitro, we used human umbilical vein endothelial cells (HUVEC). Conditioned media (CM) from NGP-N1D cells was collected as a source of N1D, with CM from NGP-LacZ as a control. CM from NGP-LacZ and NGP-N1D contained similar amounts of VEGF (1.26 ± 0.79 vs. 1.28 ± 1.4 ng/ml, respectively, P = n.s.). We therefore added 50 ng/ml of BV to block the VEGF that was present in the CM. We incubated the HUVECs in CM + BV or CM + VEGF (20 ng/ml of rhVEGF165) for 23 hrs and quantified apoptosis using a cell death ELISA. HUVECs incubated with N1D CM + BV had 1.5 to 2 fold higher apoptosis levels than HUVECs incubated with control CM + BV (Figure 5E). HUVEC apoptosis was significantly reduced when VEGF was added to the CM containing N1D (Figure 5E).
Inhibition of Notch signaling with γ-secretase inhibitors or anti-Dll4 antibodies have found that there is increased EC proliferation in vitro, while Notch1 activation leads to endothelial cell cycle arrest . Using conditioned medium from NGP-LacZ or NGP-N1D cells, with either 50 ng/ml BV or 20 ng/ml rhVEGF165, we found that N1D caused a small increase (9%) in HUVEC proliferation in vitro, and which was not affected by addition of VEGF (10%, Figure 5F). Together, these data suggest that both endothelial proliferation and apoptosis increase simultaneously in NGP-N1D, as compared to NGP-LacZ tumors. ECs in NGP-N1D tumors treated with BV, proliferate more than controls, but also display more EC apoptosis than either N1D or BV treatment alone.
Notch1 decoy and BV alter pericyte coverage of blood vessels
Confocal imaging for endothelial marker PECAM-1 and αSMA revealed that, while NGP-LacZ control tumor EC had neighboring pericytes (arrows), NGP-N1D tumors developed segments of ECs that lacked pericyte coverage (Figure 6C, right panel, arrowheads). NGP-LacZ + BV tumors contained ECs still covered by pericytes, although the pericyte layer was thinned compared to controls (Figure 6C, lower left panel). Similar to the NGP-N1D tumor vessels, NGP-N1D + BV vessels displayed segmental defects in pericyte coverage (Figure 6C, lower right, white arrows).
Notch1 decoy effects on vasculature are direct
A growing body of literature demonstrates crosstalk between Notch and VEGF pathways in angiogenesis in vitro[10–13], as well as in the mouse retina model [5, 8, 14, 15]. The interaction between Notch and VEGF in tumor angiogenesis, however, is still in the process of definition. Our data suggests that Notch and VEGF have independent but coordinated functions in tumor angiogenesis, leading to additive effects when both pathways are simultaneously inhibited. VEGF blockade limits initial vessel sprouting and branching, and then later promotes vascular cooption  and the recruitment of pericytes that can stabilize the vasculature in the absence of VEGF [16, 17]. Our results demonstrate that Notch blockade inhibits the later processes of cooption and EC/pericyte interaction.
Our data indicate that while VEGF blockade induces vascular cooption, this does not rescue tumors from hypoxia. Furthermore, while Notch blockade decreases cooption, this does not increase the VEGF-blockade induced hypoxia. This suggests that vascular cooption is relatively inefficient as a mechanism of providing oxygen to tumors; other potential contributions of co-opted vessels to tumor homeostasis remain unclear.
Our current results demonstrate that while Notch blockade can disrupt EC/pericyte interaction, it is only in the absence of VEGF that there is marked loss of vasculature, perfusion, and subsequent induction of tumor necrosis. In the context of loss of VEGF, Notch blockade causes EC apoptosis both in vitro and in vivo. This suggests that VEGF can rescue ECs when Notch is blocked.
Other groups have shown that antibodies against Dll4 and Notch1 increase non-functional EC vasculature in subcutaneous tumor models [8, 14, 15]. Endothelial Jagged1, however, has been shown to decrease sprouting in the mouse retinal model . Therefore, the ability of the N1D to block both signaling by both Dll4 and Jagged1  may explain the observed decrease in EC in NGP-N1D tumors. Our studies also suggest that the effect of Notch blockade on tumor angiogenesis is context-dependent. Thus, this effect may vary with different tumor models, and perhaps with tumor types.
Our data lead us to propose that the Notch and VEGF pathways play distinct but complementary roles in tumor angiogenesis, and that Notch is required for VEGF-mediated vascular remodeling. Here we show that concurrent blockade disrupts primary tumor vasculature and viability further than inhibition of either pathway alone, and that manipulation of each pathway is reflected in distinct vascular defects. Further investigation of the interaction between VEGF and Notch signaling in vasculature may allow refinement of a combined approach to targeting tumor angiogenesis.
These studies were supported by the National Institutes of Health (1R01CA124644, to DJY), (3R01CA124644-03S1, to AKC), the Pediatric Cancer Foundation (to DJY and JJK), the Sorkin Fund (to JJK), and the Children’s Neuroblastoma Cancer Foundation (to SLH and DJY).
- Glade Bender JL, Adamson PC, Reid JM, Xu L, Baruchel S, Shaked Y, Kerbel RS, Cooney-Qualter EM, Stempak D, Chen HX, et al: Phase I trial and pharmacokinetic study of bevacizumab in pediatric patients with refractory solid tumors: a Children’s Oncology Group Study. J Clin Oncol. 2008, 26: 399-405. 10.1200/JCO.2007.11.9230.View ArticlePubMedGoogle Scholar
- Kim ES, Serur A, Huang J, Manley CA, McCrudden KW, Frischer JS, Soffer SZ, Ring L, New T, Zabski S, et al: Potent VEGF blockade causes regression of coopted vessels in a model of neuroblastoma. Proc Natl Acad Sci USA. 2002, 99: 11399-11404. 10.1073/pnas.172398399.PubMed CentralView ArticlePubMedGoogle Scholar
- Zaghloul N, Hernandez SL, Bae JO, Huang J, Fisher JC, Lee A, Kadenhe-Chiweshe A, Kandel JJ, Yamashiro DJ: Vascular endothelial growth factor blockade rapidly elicits alternative proangiogenic pathways in neuroblastoma. Int J Oncol. 2009, 34: 401-407.PubMed CentralPubMedGoogle Scholar
- Kim KJ, Li B, Winer J, Armanini M, Gillett N, Phillips HS, Ferrara N: Inhibition of vascular endothelial growth factor-induced angiogenesis suppresses tumour growth in vivo. Nature. 1993, 362: 841-844. 10.1038/362841a0.View ArticlePubMedGoogle Scholar
- Funahashi Y, Hernandez SL, Das I, Ahn A, Huang J, Vorontchikhina M, Sharma A, Kanamaru E, Borisenko V, Desilva DM, et al: A notch1 ectodomain construct inhibits endothelial notch signaling, tumor growth, and angiogenesis. Cancer Res. 2008, 68: 4727-4735. 10.1158/0008-5472.CAN-07-6499.PubMed CentralView ArticlePubMedGoogle Scholar
- Subramanian A, Tamayo P, Mootha VK, Mukherjee S, Ebert BL, Gillette MA, Paulovich A, Pomeroy SL, Golub TR, Lander ES, Mesirov JP: Gene set enrichment analysis: a knowledge-based approach for interpreting genome-wide expression profiles. Proc Natl Acad Sci USA. 2005, 102: 15545-15550. 10.1073/pnas.0506580102.PubMed CentralView ArticlePubMedGoogle Scholar
- Baluk P, Morikawa S, Haskell A, Mancuso M, McDonald DM: Abnormalities of basement membrane on blood vessels and endothelial sprouts in tumors. Am J Pathol. 2003, 163: 1801-1815. 10.1016/S0002-9440(10)63540-7.PubMed CentralView ArticlePubMedGoogle Scholar
- Ridgway J, Zhang G, Wu Y, Stawicki S, Liang WC, Chanthery Y, Kowalski J, Watts RJ, Callahan C, Kasman I, et al: Inhibition of Dll4 signalling inhibits tumour growth by deregulating angiogenesis. Nature. 2006, 444: 1083-1087. 10.1038/nature05313.View ArticlePubMedGoogle Scholar
- Noseda M, Chang L, McLean G, Grim JE, Clurman BE, Smith LL, Karsan A: Notch activation induces endothelial cell cycle arrest and participates in contact inhibition: role of p21Cip1 repression. Mol Cell Biol. 2004, 24: 8813-8822. 10.1128/MCB.24.20.8813-8822.2004.PubMed CentralView ArticlePubMedGoogle Scholar
- Liu ZJ, Shirakawa T, Li Y, Soma A, Oka M, Dotto GP, Fairman RM, Velazquez OC, Herlyn M: Regulation of Notch1 and Dll4 by vascular endothelial growth factor in arterial endothelial cells: implications for modulating arteriogenesis and angiogenesis. Mol Cell Biol. 2003, 23: 14-25. 10.1128/MCB.23.1.14-25.2003.PubMed CentralView ArticlePubMedGoogle Scholar
- Funahashi Y, Shawber CJ, Vorontchikhina M, Sharma A, Outtz HH, Kitajewski J: Notch regulates the angiogenic response via induction of VEGFR-1. J Angiogenes Res. 2010, 2: 3-10.1186/2040-2384-2-3.PubMed CentralView ArticlePubMedGoogle Scholar
- Shawber CJ, Funahashi Y, Francisco E, Vorontchikhina M, Kitamura Y, Stowell SA, Borisenko V, Feirt N, Podgrabinska S, Shiraishi K, et al: Notch alters VEGF responsiveness in human and murine endothelial cells by direct regulation of VEGFR-3 expression. J Clin Invest. 2007, 117: 3369-3382. 10.1172/JCI24311.PubMed CentralView ArticlePubMedGoogle Scholar
- Taylor KL, Henderson AM, Hughes CC: Notch activation during endothelial cell network formation in vitro targets the basic HLH transcription factor HESR-1 and downregulates VEGFR-2/KDR expression. Microvasc Res. 2002, 64: 372-383. 10.1006/mvre.2002.2443.View ArticlePubMedGoogle Scholar
- Noguera-Troise I, Daly C, Papadopoulos NJ, Coetzee S, Boland P, Gale NW, Lin HC, Yancopoulos GD, Thurston G: Blockade of Dll4 inhibits tumour growth by promoting non-productive angiogenesis. Nature. 2006, 444: 1032-1037. 10.1038/nature05355.View ArticlePubMedGoogle Scholar
- Wu Y, Cain-Hom C, Choy L, Hagenbeek TJ, de Leon GP, Chen Y, Finkle D, Venook R, Wu X, Ridgway J, et al: Therapeutic antibody targeting of individual Notch receptors. Nature. 2010, 464: 1052-1057. 10.1038/nature08878.View ArticlePubMedGoogle Scholar
- Soffer SZ, Moore JT, Kim E, Huang J, Yokoi A, Manley C, O’Toole K, Stolar C, Middlesworth W, Yamashiro DJ, Kandel JJ: Combination antiangiogenic therapy: increased efficacy in a murine model of Wilms tumor. Journal of pediatric surgery. 2001, 36: 1177-1181. 10.1053/jpsu.2001.25747.View ArticlePubMedGoogle Scholar
- Bergers G, Hanahan D: Modes of resistance to anti-angiogenic therapy. Nature reviews Cancer. 2008, 8: 592-603. 10.1038/nrc2442.PubMed CentralView ArticlePubMedGoogle Scholar
- Benedito R, Roca C, Sorensen I, Adams S, Gossler A, Fruttiger M, Adams RH: The notch ligands Dll4 and Jagged1 have opposing effects on angiogenesis. Cell. 2009, 137: 1124-1135. 10.1016/j.cell.2009.03.025.View ArticlePubMedGoogle Scholar
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.