Genetic dissection of tumor angiogenesis: are PlGF and VEGFR-1 novel anti-cancer targets?

The VEGFs/VEGFRs system in Alzheimer's and Parkinson's diseases: Pathophysiological roles and therapeutic implications

The vascular endothelial growth factors (VEGFs) and their cognate receptors (VEGFRs), besides their well-known involvement in physiological angiogenesis/lymphangiogenesis and in diseases associated to pathological vessel formation, play multifaceted functions in the central nervous system (CNS). In addition to shaping brain development, by controlling cerebral vasculogenesis and regulating neurogenesis as well as astrocyte differentiation, the VEGFs/VEGFRs axis exerts essential functions in the adult brain both in physiological and pathological contexts. In this article, after describing the physiological VEGFs/VEGFRs functions in the CNS, we focus on the VEGFs/VEGFRs involvement in neurodegenerative diseases by reviewing the current literature on the rather complex VEGFs/VEGFRs contribution to the pathogenic mechanisms of Alzheimer's (AD) and Parkinson's (PD) diseases. Thereafter, based on the outcome of VEGFs/VEGFRs targeting in animal models of AD and PD, we discuss the factual relevance of pharmacological VEGFs/VEGFRs modulation as a novel and potential disease-modifying approach for these neurodegenerative pathologies. Specific VEGFRs targeting, aimed at selective VEGFR-1 inhibition, while preserving VEGFR-2 signal transduction, appears as a promising strategy to hit the molecular mechanisms underlying AD pathology. Moreover, therapeutic VEGFs-based approaches can be proposed for PD treatment, with the aim of fine-tuning their brain levels to amplify neurotrophic/neuroprotective effects while limiting an excessive impact on vascular permeability.

The VEGF/VEGFR Axis Revisited: Implications for Cancer Therapy

The vascular endothelial growth factor (VEGF)/vascular endothelial growth factor receptor (VEGFR) axis is indispensable in the process of angiogenesis and has been implicated as a key driver of tumor vascularization. Consequently, several strategies that target VEGF and its cognate receptors, VEGFR-1 and VEGFR-2, have been designed to treat cancer. While therapies targeting full-length VEGF have resulted in an improvement in both overall survival and progression-free survival in various cancers, these benefits have been modest. In addition, the inhibition of VEGFRs is associated with undesirable off-target effects. Moreover, VEGF splice variants that modulate sprouting and non-sprouting angiogenesis have been identified in recent years. Cues within the tumor microenvironment determine the expression patterns of these variants. Noteworthy is that the mechanisms of action of these variants challenge the established norm of VEGF signaling. Furthermore, the aberrant expression of some of these variants has been observed in several cancers. Herein, developments in the understanding of the VEGF/VEGFR axis and the splice products of these molecules, as well as the environmental cues that regulate these variants are reviewed. Furthermore, strategies that incorporate the targeting of VEGF variants to enhance the effectiveness of antiangiogenic therapies in the clinical setting are discussed.

Generating the Engineered Form of a Nanobody Against Placenta Growth Factor with High Antiangiogenesis Potential

Antibody engineering is a dynamic field in antibody industry. Over 30% of novel monoclonal antibodies (mAbs) in R&D and clinical trials are engineered forms. Affinity enhancement contributes to the development of new binders that are not only effective in low dose and cost but also improve some drawbacks of antibody production. After previous successful work on in silico affinity maturation of nanobody against placenta growth factor and finding the best engineered nanobodies (Mut2:S31D and Mut4:R45E), according to bioinformatic parameters and molecular dynamics (MD) simulation results, in this study we focused on experimental confirmation of affinity enhancement of a mutant form of nanobody. So, we cloned and expressed two of four mutant forms in pHEN6c vector. Affinity binding was assayed by enzyme-linked immunosorbent assay on purified mutants, with results showing that 10-time enhancement in affinity compared with the native form associated with MD simulation results. We checked the effectiveness of these mutant nanobodies in angiogenesis inhibition by human umbilical vein endothelial cell proliferation and 3D capillary tube formation. EC50 of mut2, mut4, and native in proliferation assay was 110, 140, and 190 ng/mL, respectively, and that in 3D capillary tube formation was 80, 83, and 100 ng/mL. The results of functional studies revealed strong effectiveness of Mut2 followed by Mut4 compared with the native form. Our study confirmed that in silico approach could facilitate development of novel versions of mAb with better characteristics, which could save cost and time.

The Prognostic Value of the Combination of Low VEGFR-1 and High VEGFR-2 Expression in Endothelial Cells of Colorectal Cancer

Research on tumor angiogenesis has mainly focused on the vascular endothelial growth factor (VEGF) family and on methods to block its actions. However, reports on VEGF receptor (VEGFR) expression in tumor-associated endothelial cells (ECs) are limited. Thus, we evaluated VEGF, VEGFR-1 and VEGFR-2 expression in ECs of colorectal cancer (CRC) using immunohistochemistry. VEGF, VEGFR-1 and -2 expression in ECs was quantitatively evaluated by digital image analysis in a retrospective series of 204 tumor tissue samples and related to clinical variables. The data show that the VEGF, VEGFR-1 and VEGFR-2 expression in ECs is heterogeneous. Multivariate analysis including a set of clinicopathological variables reveals that high EC VEGFR-1 expression is an independent prognostic factor for overall survival (OS). The combination of low VEGFR-1 and high VEGFR-2 expression in ECs outperforms models integrating VEGFR-1 and VEGFR-2 as separate markers. Indeed, this VEGFR-1_VEGFR-2 combination is an independent negative prognostic factor for OS (p = 0.012) and metastasis-free survival (p = 0.007). In conclusion, this work illustrates the importance of studying the distribution of VEGF members in ECs of CRC. Interestingly, our preliminary data suggest that high VEGFR-1 and low VEGFR-2 expression in ECs appear to be involved in the progression of CRC, suggesting that targeting EC VEGFR-1 could offer novel opportunities for CRC treatment. However, a prospective validation study is needed.

A novel kinase mutation in VEGFR-1 predisposes its αC-helix/activation loop towards allosteric activation: Atomic insights from protein simulation

Vascular endothelial growth factor receptor 1 (VEGFR-1) has been implicated in diverse pathologies, including cancers. Although VEGFR-1 is considered as functionally impaired kinase, its decoy characteristics make it an important regulator of VEGFR-mediated signaling, particularly in tumor angiogenesis. VEGFR-1 conveys signaling via its tyrosine kinase (TK) domain whose activation is regulated by phosphorylation of specific tyrosine residues. Thus dysregulation of VEGFR-1 signaling, as reported in most of the cancers, might be a consequence of altered phosphorylation that could be attributed to genotypic variations in its TK domain. Considering the importance of TK domain of VEGFR-1, we carried out its mutational screening in 84 clinically validated and histopathologically confirmed colorectal cancer patients. By means of direct DNA sequencing and SNP analyses, eight novel variations, including one synonymous, two deletion, one missense and four intronic variations, were reported in the TK domain of VEGFR-1. rs730882263:C>G variation specifically reported in colon cancer, representing a single-atomic change (Sulfur to Oxygen) in the predicted (p.Cys1110Ser) protein, was observed as potentially deleterious variation as assessed by multiple single-nucleotide polymorphism prediction servers. Molecular dynamics simulations of VEGFR-1 Wt and (p.Cys1110Ser) variant models revealed major conformational changes in variant protein presumptuously generating an open conformation thereby exposing the activation domain and consequently increasing the probability of phosphorylation events: a condition frequently reported in cancers.

Placenta Growth Factor in Eyes with Neovascular Glaucoma Is Decreased after Intravitreal Ranibizumab Injection

Purpose

To evaluate changes in the concentrations of placental growth factor (PlGF) and vascular endothelial growth factor-A (VEGF-A) in aqueous humor of patients with neovascular glaucoma (NVG) before and after an intravitreal injection of ranibizumab (IVR) and to determine the underlying correlation between the levels.

Methods

The prospective interventional comparative study involved 20 eyes of 20 patients with surgery-required advanced NVG and 20 control subjects from January 2013 to November 2013. The NVG eyes received the IVR treatment before glaucoma surgery. Aqueous humor was collected at the time of the IVR injection (pre- IVR) and at the time of antiglaucomatous surgery (post-IVR). Aqueous humor was also collected at the time of cataract surgery in normal control. Aqueous humor and plasma VEGF-A and PlGF levels were measured with an enzyme-linked immunosorbent assay methods, respectively.

Results

The mean aqueous humor PlGF and VEGF-A concentrations in the pre-IVR eyes were significantly higher than in those of the control subjects (p<0.001), whereas the plasma levels showed no significant difference. There was a statistically significant correlation between the aqueous humor PlGF and the VEGF-A concentration (r = 0.612, p = 0.003). The mean aqueous humor PlGF in the post-IVR eyes dramatically decreased from 1078.36 ± 755.83 to 177.64 ± 151.73 pg/mL (p<0.001). The VEGF-A level showed a similar trend from 3697.64 ± 2104.47 pg/mL to 183.54 ± 130.35 pg/mL (p<0.001).

Conclusions

Aqueous humor concentrations of VEGF-A and PlGF were significantly elevated in the eyes with NVG, and there was a positive correlation between the levels. After an IVR treatment, VEGF-A and PlGF were significantly decreased in NVG eyes.

Inhibition of stromal PlGF suppresses the growth of prostate cancer xenografts

The growth and vascularization of prostate cancer is dependent on interactions between cancer cells and supporting stromal cells. The primary stromal cell type found in prostate tumors is the carcinoma-associated fibroblast, which produces placental growth factor (PlGF). PlGF is a member of the vascular endothelial growth factor (VEGF) family of angiogenic molecules and PlGF mRNA levels increase after androgen deprivation therapy in prostate cancer. In this study, we show that PlGF has a direct dose-dependent proliferative effect on human PC-3 prostate cancer cells in vitro and fibroblast-derived PlGF increases PC-3 proliferation in co-culture. In xenograft tumor models, intratumoral administration of murine PlGF siRNA reduced stromal-derived PlGF expression, reduced tumor burden and decreased the number of Ki-67 positive proliferating cells associated with reduced vascular density. These data show that targeting stromal PlGF expression may represent a therapeutic target for the treatment of prostate cancer.

VEGFR1 and VEGFR2 involvement in extracellular galectin-1- and galectin-3-induced angiogenesis

AIM

Accumulating evidence suggests that extracellular galectin-1 and galectin-3 promote angiogenesis. Increased expression of galectin-1 and/or galectin-3 has been reported to be associated with tumour progression. Thus, it is critical to identify their influence on angiogenesis.

METHODS

We examined the individual and combined effects of galectin-1 and galectin-3 on endothelial cell (EC) growth and tube formation using two EC lines, EA.hy926 and HUVEC. The activation of vascular endothelial growth factor receptors (VEGFR1 and VEGFR2) was determined by ELISA and Western blots. We evaluated the VEGFR1 and VEGFR2 levels in endosomes by proximity ligation assay.

RESULTS

We observed different responses to exogenous galectins depending on the EC line. An enhanced effect on EA.hy926 cell growth and tube formation was observed when both galectins were added together. Focusing on this enhanced effect, we observed that together galectins induced the phosphorylation of both VEGFR1 and VEGFR2, whereas galectin-1 and -3 alone induced VEGFR2 phosphorylation only. In the same way, the addition of a blocking VEGFR1 antibody completely abolished the increase in tube formation induced by the combined addition of both galectins. In contrast, the addition of a blocking VEGFR2 antibody only partially inhibited this effect. Finally, the addition of both galectins induced a decrease in the VEGFR1 and VEGFR2 endocytic pools, with a significantly enhanced effect on the VEGFR1 endocytic pool. These results suggest that the combined action of galectin-1 and galectin-3 has an enhanced effect on angiogenesis via VEGFR1 activation, which could be related to a decrease in receptor endocytosis.

Expression of placenta growth factor in colorectal carcinomas

Purpose

Placenta growth factor (PlGF) is a member of the vascular endothelial growth factor (VEGF) family. PlGF is implicated in several pathologic processes, including the growth and spread of cancer and tumor angiogenesis. The aim of this study was to evaluate the expression and the clinical implications of PlGF in colorectal cancer.

Methods

In order to ascertain the clinical significance of PlGF expression in colorectal cancer, the researcher analyzed the expression pattern of PlGF by using an immunohistochemical method and attempted to establish if a relationship existed between PlGF expression and microvessel density (MVD), and subsequently between PlGF expression and the predicted prognosis. A total of 83 patients with colorectal cancer were included for immunohistochemical staining. Clinicopathological characteristics were defined according to the tumor-node-metastasis (TNM) criteria of the Union for International Cancer Control. Clinicopathologic factors, such as age, sex, histological types of tumors, tumor cell grade, TNM stage, lymphovascular invasion, and lymph-node metastasis, were reviewed.

Results

In this study, the PlGF protein expression level was significantly correlated with MVD, patient survival, and clinicopathological factors such as lymph-node metastasis, TNM staging, lymphatic invasion and vascular invasion.

Conclusion

PlGF may be an important angiogenic factor in human colorectal cancer, and in this study, PlGF expression level was significantly correlated with positive lymph-node metastases, tumor stage, and patient survival. These findings suggest that PlGF expression correlates with disease progression and may be used as a prognostic marker for colorectal cancer.

Phase I trial of OTS11101, an anti-angiogenic vaccine targeting vascular endothelial growth factor receptor 1 in solid tumor

OTS11101 is a novel peptide vaccine that acts as an angiogenesis inhibitor by inducing cytotoxic T lymphocyte (CTL) cells that specifically target vascular endothelial cells expressing vascular endothelial growth factor (VEGF) receptor 1. We conducted a phase I study to evaluate the safety, tolerability, maximum tolerated dose, and pharmacodynamic biomarker status of this vaccine. Nine patients with advanced solid tumors received 1.0, 2.0, or 3.0 mg of OTS11101 subcutaneously, once a week in a 28-day cycle. Three patients experienced grade 1 injection site reactions, which were the most frequent adverse events. Grade 2 proteinuria and hypertension each occurred in one patient. As other toxicities were generally mild, the maximum tolerated dose was not reached. Furthermore, we explored the induction of specific activated CTLs, and biomarkers related to angiogenesis. A pharmacodynamics study revealed that induction of specific CTLs was observed for a dose of 2.0 and 3.0 mg. The serum concentrations of soluble VEGF receptor 1 and 2 after vaccination increased significantly compared with baseline. A microarray was performed to give a comprehensive analysis of gene expression, suggesting that OTS11101 vaccination resulted in T cell activation in a clinical setting. In conclusion, OTS11101 was well tolerated in patients up to 3.0 mg once weekly and our biomarker analysis suggested that this anti-angiogenesis vaccine is biologically active.

The Roles of Angiogenesis in Malignant Melanoma: Trends in Basic Science Research over the Last 100 Years

Blood vessels arose during evolution carrying oxygen and nutrients to distant organs via complex networks of blood vessels penetrating organs and tissues. Mammalian cells require oxygen and nutrients for survival, of which oxygen has a diffusion limit of 100 to 200 μm between cell and blood vessel. For growth beyond this margin, cells must recruit new blood vessels, first by vasculogenesis, where embryonic vessels form from endothelial precursors, then angiogenesis which is the sprouting of interstitial tissue columns into the lumen of preexisting blood vessels. Angiogenesis occurs in many inflammatory diseases and in many malignant disease states, including over 90% of solid tumours. Malignant melanoma (MM) is the most lethal skin cancer, highly angiogenic, highly metastatic, and refractory to all treatments. Raised serum levels of vascular endothelial growth factor (VEGF) strongly correlate MM disease progression and poor prognosis. Melanoma cells secrete several proangiogenic cytokines including VEGF-A, fibroblast growth factor (FGF-2), platelet growth factor (PGF-1), interleukin-8 (IL-8), and transforming growth factor (TGF-1) that modulate the angiogenic switch, changing expression levels during tumour transition from radial to invasive vertical and then metastatic growth. We highlight modern and historical lines of research and development that are driving this exciting area of research currently.

Hypoxia acts through multiple signaling pathways to induce metallothionein transactivation by the metal-responsive transcription factor-1 (MTF-1)

Metal-responsive transcription factor-1 (MTF-1) is essential for the induction of genes encoding metallothionein by metals and hypoxia. Here, we studied the mechanism controlling the activation of MTF-1 by hypoxia. Hypoxia activation of Mt gene transcription is dependent on the presence of metal regulatory elements (MREs) in the promoter of Mt genes. We showed that MREa and MREd are the main elements controlling mouse Mt-1 gene induction by hypoxia. Transfection experiments in Mtf-1-null cells showed that MTF-1 is essential for induction by hypoxia. Chromatin immunoprecipitation analysis showed that MTF-1 DNA-binding activity was strongly enhanced in the presence of zinc but not by hypoxia. Notably, hypoxia inducible factor- (HIF) 1α was recruited to the Mt-1 promoter in response to hypoxia but not to zinc. MTF-1 activation was inhibited by PKC, JNK, and PI3K inhibitors and by the electron transport chain inhibitors rotenone and myxothiazol, but not by the antioxidant N-acetylcysteine. We showed that prolyl-hydroxylase inhibitors can activate MTF-1, but this activation requires the presence of HIF-1α. Finally, HIF-dependent transcription is enhanced in the presence of MTF-1 and induction of an MRE promoter is stimulated by HIF-1α, thus indicating cooperation between these 2 factors. However, coimmunoprecipitation experiments did not suggest direct interaction between MTF-1 and HIF-1α.

Blood vessel maturation, vascular phenotype and angiogenic potential in malignant melanoma: one step forward for overcoming anti-angiogenic drug resistance?

Angiogenesis is a pivotal process for growth, invasion and spread of the majority of solid tumors including melanoma. Anti-angiogenic agents have not been systematically tested in patients with advanced melanoma. Clinical efficacy of angiogenesis inhibitors targeting endothelial cells has not been as affirmative as initially hoped and improved clinical outcomes have been observed in combination with chemotherapy or additional drugs for many types of human cancer. However, angiogenesis is not only dependent on endothelial cell invasion and proliferation, it also requires pericyte coverage of vascular sprouts for stabilization and maturation of vascular walls. Recent data suggest that pericytes might be able to confer resistance to anti-vascular endothelial growth factor (VEGF) therapy. This review will focus on the significance of the vascular phenotype but also on the impact of pericyte-mediated vessel maturation for the susceptibility to anti-angiogenic therapy, including malignant melanoma, which we identified as crucial factor regarding therapeutic efficacy.

Negative regulation of VEGF signaling in human coronary artery endothelial cells by G protein-coupled receptor kinase 5

G protein-coupled receptor kinase 5 (GRK5) is present in endothelial cells (ECs) and has the potential to regulate EC function through seven transmembrane-spanning receptor (7TMR) signaling. Recently, it has been appreciated that GRKs can affect receptor tyrosine kinases (RTKs). VEGF, an RTK, is one of the most potent mediators for EC function and angiogenesis; therefore, we determined the role GRK5 plays in VEGF signaling in human coronary artery ECs (HCAECs). GRK5 levels were increased by VEGF treatment in HCAECs. Adenoviral overexpression of GRK5 inhibited migration and proliferation of HCAECs in response to VEGF. GRK5 overexpression in HCAECs significantly suppressed both acute and late activation of Akt and extracellular signal-related kinase (ERKs) as well as the phosphorylation of GSK-3beta, an endogenous substrate of Akt. Coimmunoprecipitations revealed that GRK5 is physically associated with Akt. This study shows for the first time that GRK5 negatively regulates VEGF signaling in HCAECs and suggests that targeted intervention of GRK5 in ECs might be a novel therapeutic strategy to prevent and treat disorders involving altered EC function.

Angiogenesis and progression in human melanoma

In tumor growth, angiogenesis, the process of new-formation of blood vessels from pre-existing ones, is uncontrolled and unlimited in time. The vascular phase is characterized by the new-formation of vascular channels that enhances tumor cell proliferation, local invasion and hematogenous metastasis. Human malignant melanoma is a highly metastatic tumor with poor prognosis, and high resistance to treatment. Parallel with progression, melanoma acquires a rich vascular network, whereas an increasing number of tumor cells express the laminin receptor, which enables their adhesion to the vascular wall, favouring tumor cell extravasation and metastases. Melanoma neovascularization has been correlated with poor prognosis, overall survival, ulceration and increased rate of relapse. Secretion of various angiogenic cytokines, i.e. VEGF-A, FGF-2, PGF-1 and -2, IL-8, and TGF-1 by melanoma cells promote the angiogenic switch and has been correlated to transition from the radial to the vertical growth phase, and to the metastatic phase. Moreover, melanoma cells overexpress alphavbeta3, alphavbeta5, alpha2beta1 and alpha5beta1 integrins and release, together with stromal cells, higher amount of metalloproteases that increasing their invasive potential and angiogenesis. Basing on these observations, different molecular targets of antiangiogenic molecules has be recognized and various antiangiogenic agents are currently in preclinical and clinical trials for melanoma.

From combinatorial peptide selection to drug prototype (I): targeting the vascular endothelial growth factor receptor pathway

Inhibition of blood vessel formation is a viable therapeutic approach in angiogenesis-dependent diseases. We previously used a combinatorial screening on vascular endothelial growth factor (VEGF)-activated endothelial cells to select the sequence CPQPRPLC and showed that the motif Arg-Pro-Leu targets VEGF receptor-1 and neuropilin-1. Here, we evaluated and validated (D)(LPR), a derivative molecule with strong antiangiogenesis attributes. This prototype drug markedly inhibits neovascularization in three mouse models: Matrigel-based assay, functional human/murine blood vessel formation, and retinopathy of prematurity. In addition to its systemic activity, (D)(LPR) also inhibits retinal angiogenesis when administered in an eye-drop formulation. Finally, in preliminary studies, we have showed targeted drug activity in an experimental tumor-bearing mouse model. These results show that drugs targeting extracellular domains of VEGF receptors are active, affect signal transduction, and have potential for clinical application. On a larger context, this study illustrates the power of ligand-directed selection plus retro-inversion for rapid drug discovery and development.

Elevated levels of placental growth factor represent an adaptive host response in sepsis

Recently, we demonstrated that circulating levels of vascular endothelial growth factor (VEGF) and placental growth factor (PlGF) are increased in sepsis (Yano, K., P.C. Liaw, J.M. Mullington, S.C. Shih, H. Okada, N. Bodyak, P.M. Kang, L. Toltl, B. Belikoff, J. Buras, et al. 2006. J. Exp. Med. 203:1447–1458). Moreover, enhanced VEGF/Flk-1 signaling was shown to contribute to sepsis morbidity and mortality. We tested the hypothesis that PlGF also contributes to sepsis outcome. In mouse models of endotoxemia and cecal ligation puncture, the genetic absence of PlGF or the systemic administration of neutralizing anti-PlGF antibodies resulted in higher mortality compared with wild-type or immunoglobulin G–injected controls, respectively. The increased mortality associated with genetic deficiency of PlGF was reversed by adenovirus (Ad)-mediated overexpression of PlGF. In the endotoxemia model, PlGF deficiency was associated with elevated circulating levels of VEGF, induction of VEGF expression in the liver, impaired cardiac function, and organ-specific accentuation of barrier dysfunction and inflammation. Mortality of endotoxemic PlGF-deficient mice was increased by Ad-mediated overexpression of VEGF and was blocked by expression of soluble Flt-1. Collectively, these data suggest that up-regulation of PlGF in sepsis is an adaptive host response that exerts its benefit, at least in part, by attenuating VEGF signaling.

Angiogenesis in melanoma

The process of angiogenesis is crucial for progression and metastasis of the majority of solid tumors including melanomas. The current review summarizes existing knowledge of the mechanisms of angiogenesis in melanoma, as well as current anti-angiogenic therapeutic strategies and their targets. We focus primarily on the role of key growth factors that are secreted by melanoma cells and known to trigger angiogenic responses, and their receptors as expressed on both endothelial and melanoma cells. Many of these growth factors function in synergy with receptors for extracellular matrix, integrins, and matrix metalloproteinases (MMPs). All of these systems of molecules are activated during major stages of angiogenesis such as endothelial migration, proliferation, and reorganization of surrounding extracellular matrix. The blockade of these molecules and their downstream pathways leads to inhibition of melanoma vascularization. Thus, these classes of molecules are essential for melanoma angiogenesis and, therefore, might serve as promising targets for therapeutic intervention. Many recently developed compounds targeting key pathways in angiogenesis are in their final stages of clinical trials.

Human tumor xenografts recurring after radiotherapy are more sensitive to anti-vascular endothelial growth factor receptor-2 treatment than treatment-naive tumors

The effects of antiangiogenic therapy on tumors relapsing after irradiation are not known. To this end, we irradiated human tumors growing s.c. in nude mice with a single dose of 20 or 30 Gy. Compared with primary (treatment-naive) xenografts, the growth rate of recurrent tumors was 1.6-fold slower, which is consistent with the known "tumor bed effect." For similar size tumors, recurrences had fewer functional vessels, a reduced vessel coverage by perivascular cells, and were more necrotic. Placenta growth factor concentration was significantly lower in relapses, whereas vascular endothelial growth factor (VEGF) levels were similar between primary and recurrent tumors. On the other hand, fibrillar collagen deposition was significantly increased in recurrent tumors. This radiation-induced fibrosis was partially responsible for the slower growth of recurrences; the i.t. injection of collagenase increased the growth rate of tumor relapses without affecting primary tumor growth. The mouse-specific VEGF receptor 2-blocking antibody DC101 induced a 2.2-fold longer growth delay in recurrent tumors compared with treatment-naive tumors. DC101 significantly decreased the interstitial fluid pressure and did not change the functional vessel density and perivascular cell coverage in both tumor variants. Interestingly, DC101 induced a rapid (2 days after treatment initiation) and significant decrease in tumor cell proliferation in recurrent but not in primary tumors. Thus, our results show that the stromal compartment and the response to antiangionenic therapy of primary and in-field recurrent tumors are significantly different. Our findings suggest that antiangiogenic agents could be effective in the treatment of patients with relapses after radiotherapy.

KRN951, a highly potent inhibitor of vascular endothelial growth factor receptor tyrosine kinases, has antitumor activities and affects functional vascular properties

Vascular endothelial growth factor (VEGF) plays a key role in tumor angiogenesis by stimulating the proangiogenic signaling of endothelial cells via activation of VEGF receptor (VEGFR) tyrosine kinases. Therefore, VEGFRs are an attractive therapeutic target for cancer treatment. In the present study, we show that a quinoline-urea derivative, KRN951, is a novel tyrosine kinase inhibitor for VEGFRs with antitumor angiogenesis and antigrowth activities. KRN951 potently inhibited VEGF-induced VEGFR-2 phosphorylation in endothelial cells at in vitro subnanomolar IC50 values (IC50 = 0.16 nmol/L). It also inhibited ligand-induced phosphorylation of platelet-derived growth factor receptor-beta (PDGFR-beta) and c-Kit (IC50 = 1.72 and 1.63 nmol/L, respectively). KRN951 blocked VEGF-dependent, but not VEGF-independent, activation of mitogen-activated protein kinases and proliferation of endothelial cells. In addition, it inhibited VEGF-mediated migration of human umbilical vein endothelial cells. Following p.o. administration to athymic rats, KRN951 decreased the microvessel density within tumor xenografts and attenuated VEGFR-2 phosphorylation levels in tumor endothelium. It also displayed antitumor activity against a wide variety of human tumor xenografts, including lung, breast, colon, ovarian, pancreas, and prostate cancer. Furthermore, dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) analysis revealed that a significant reduction in tumor vascular hyperpermeability was closely associated with the antitumor activity of KRN951. These findings suggest that KRN951 is a highly potent, p.o. active antiangiogenesis and antitumor agent and that DCE-MRI would be useful in detecting early responses to KRN951 in a clinical setting. KRN951 is currently in phase I clinical development for the treatment of patients with advanced cancer.

Inhibition of tumor growth with antiangiogenic cancer vaccine using epitope peptides derived from human vascular endothelial growth factor receptor 1

PURPOSE

Antiangiogenic therapy is now considered to be one of promising approaches to treat various types of cancer. In this study, we examined the possibility of developing antiangiogenic cancer vaccine targeting vascular endothelial growth factor receptor 1 (VEGFR1) overexpressed on endothelial cells of newly formed vessels in the tumor.

EXPERIMENTAL DESIGN

Epitope-candidate peptides were predicted from the amino acid sequence of VEGFR1 based on their theoretical binding affinities to the corresponding HLAs. The A2/Kb transgenic mice, which express the alpha1 and alpha2 domains of human HLA-A*0201, were immunized with the epitope candidates to examine their effects. We also examined whether these peptides could induce human CTLs specific to the target cells in vitro.

RESULTS

The CTL responses in A2/Kb transgenic mice were induced with vaccination using identified epitope peptides restricted to HLA-A*0201. Peptide-specific CTL clones were also induced in vitro with these identified epitope peptides from peripheral blood mononuclear cells donated by healthy volunteers with HLA-A*0201. We established CTL clones in vitro from human peripheral blood mononuclear cells with HLA-A*2402 as well. These CTL clones were shown to have potent cytotoxicities in a HLA class I-restricted manner not only against peptide-pulsed target cells but also against target cells endogenously expressing VEGFR1. Furthermore, immunization of A2/Kb transgenic mice with identified epitope peptides restricted to HLA-A*0201 was associated with significant suppression of tumor-induced angiogenesis and tumor growth without showing apparent adverse effects.

CONCLUSIONS

These results strongly suggest that VEGFR1 is a promising target for antiangiogenic cancer vaccine and warrants further clinical development of this strategy.

A SNP in the flt-1 promoter integrates the VEGF system into the p53 transcriptional network

The VEGF system is essential for angiogenesis. VEGF overexpression frequently correlates with increased microvascularity and metastasis and decreased spontaneous apoptosis. Although a precise mechanism has not been established, studies suggest that VEGF expression is negatively regulated by p53, a master regulator and tumor suppressor. There are no reports of additional components of the VEGF signal transduction pathway being part of the p53 transcriptional network. A target of VEGF, the VEGF receptor 1/flt-1, can regulate growth and migration of endothelial cells and modulate angiogenesis. VEGF appears to be up-regulated in various cancers in which flt-1 may have a role in tumor progression and metastasis. We identified a C-to-T SNP upstream of the transcriptional start site in approximately 6% of the people examined. The SNP is located within a putative p53 response element. Only the promoter with the T SNP (FLT1-T) was responsive to p53 when examined with reporter assays or by endogenous gene expression analysis in cell lines with different SNP status. In response to doxorubicin-induced DNA damage, there was clear allele discrimination based on p53 binding at the FLT1-T but not FLT1-C promoters as well as p53-dependent induction of flt-1 mRNA, which required the presence of FLT1-T. Our results establish that p53 can differentially stimulate transcription at a polymorphic variant of the flt-1 promoter and directly places the VEGF system in the p53 stress-response network via flt-1 in a significant fraction of the human population. We suggest that the p53-VEGF-flt-1 interaction is relevant to risks in angiogenesis-associated diseases, including cancer.

Signaling Mechanisms Regulating Endothelial Permeability

The microvascular endothelial cell monolayer localized at the critical interface between the blood and vessel wall has the vital functions of regulating tissue fluid balance and supplying the essential nutrients needed for the survival of the organism. The endothelial cell is an exquisite “sensor” that responds to diverse signals generated in the blood, subendothelium, and interacting cells. The endothelial cell is able to dynamically regulate its paracellular and transcellular pathways for transport of plasma proteins, solutes, and liquid. The semipermeable characteristic of the endothelium (which distinguishes it from the epithelium) is crucial for establishing the transendothelial protein gradient (the colloid osmotic gradient) required for tissue fluid homeostasis. Interendothelial junctions comprise a complex array of proteins in series with the extracellular matrix constituents and serve to limit the transport of albumin and other plasma proteins by the paracellular pathway. This pathway is highly regulated by the activation of specific extrinsic and intrinsic signaling pathways. Recent evidence has also highlighted the importance of the heretofore enigmatic transcellular pathway in mediating albumin transport via transcytosis. Caveolae, the vesicular carriers filled with receptor-bound and unbound free solutes, have been shown to shuttle between the vascular and extravascular spaces depositing their contents outside the cell. This review summarizes and analyzes the recent data from genetic, physiological, cellular, and morphological studies that have addressed the signaling mechanisms involved in the regulation of both the paracellular and transcellular transport pathways.

Elevated Flk1 (vascular endothelial growth factor receptor 2) signaling mediates enhanced angiogenesis in beta3-integrin-deficient mice

Tumor growth, tumor angiogenesis, and vascular endothelial growth factor (VEGF)-specific angiogenesis are all enhanced in beta(3)-integrin-null mice. Furthermore, endothelial cells isolated from beta(3)-null mice show elevated levels of Flk1 (VEGF receptor 2) expression, suggesting that beta(3)-integrin can control the amplitude of VEGF responses by controlling Flk1 levels or activity. We now show that Flk1 signaling is required for the enhanced tumor growth and angiogenesis seen in beta(3)-null mice. Moreover, beta(3)-null endothelial cells exhibit enhanced migration and proliferation in response to VEGF in vitro, and this phenotype requires Flk1 signaling. Upon VEGF stimulation, beta(3)-null endothelial cells exhibit higher levels of phosphorylated Flk1 and extracellular-related kinases 1 and 2 than wild-type endothelial cells. Furthermore, signaling via ERK1/2 is required to mediate the elevated responses to VEGF observed in beta(3)-null endothelial cells and aortic rings in vitro. These data confirm that VEGF signaling via Flk1 is enhanced in beta(3)-integrin-deficient mice and suggests that this increase may mediate the enhanced angiogenesis and tumor growth observed in these mice in vivo.