Flt-1, a receptor for vascular endothelial growth factor, has transforming and morphogenic potentials

Molecular basis of VEGFR1 autoinhibition at the plasma membrane

Ligand-independent activation of VEGFRs is a hallmark of diabetes and several cancers. Like EGFR, VEGFR2 is activated spontaneously at high receptor concentrations. VEGFR1, on the other hand, remains constitutively inactive in the unligated state, making it an exception among VEGFRs. Ligand stimulation transiently phosphorylates VEGFR1 and induces weak kinase activation in endothelial cells. Recent studies, however, suggest that VEGFR1 signaling is indispensable in regulating various physiological or pathological events. The reason why VEGFR1 is regulated differently from other VEGFRs remains unknown. Here, we elucidate a mechanism of juxtamembrane inhibition that shifts the equilibrium of VEGFR1 towards the inactive state, rendering it an inefficient kinase. The juxtamembrane inhibition of VEGFR1 suppresses its basal phosphorylation even at high receptor concentrations and transiently stabilizes tyrosine phosphorylation after ligand stimulation. We conclude that a subtle imbalance in phosphatase activation or removing juxtamembrane inhibition is sufficient to induce ligand-independent activation of VEGFR1 and sustain tyrosine phosphorylation.

The role of mRNA in the development, diagnosis, treatment and prognosis of neural tumors

Neural tumors can generally be divided into central nervous system tumors and peripheral nervous tumors. Because this type of tumor is located in the nerve, even benign tumors are often difficult to remove by surgery. In addition, the majority of neural tumors are malignant, and it is particular the same for the central nervous system tumors. Even treated with the means such as chemotherapy and radiotherapy, they are also difficult to completely cure. In recent years, an increasingly number of studies have focused on the use of mRNA to treat tumors, representing an emerging gene therapy. The use of mRNA can use the expression of some functional proteins for the treatment of genetic disorders or tissue repair, and it can also be applied to immunotherapy through the expression of antigens, antibodies or receptors. Therefore, although these therapies are not fully-fledged enough, they have a broad research prospect. In addition, there are many ways to treat tumors using mRNA vaccines and exosomes carrying mRNA, which have drawn much attention. In this study, we reviewed the current research on the role of mRNA in the development, diagnosis, treatment and prognosis of neural tumors, and examine the future research prospects of mRNA in neural tumors and the opportunities and challenges that will arise in the future application of clinical treatment.

Role of VEGF-A and LRG1 in Abnormal Angiogenesis Associated With Diabetic Nephropathy

Diabetic nephropathy (DN) is an important public health concern of increasing proportions and the leading cause of end-stage renal disease (ESRD) in diabetic patients. It is one of the most common long-term microvascular complications of diabetes mellitus that is characterized by proteinuria and glomerular structural changes. Angiogenesis has long been considered to contribute to the pathogenesis of DN, whereas the molecular mechanisms of which are barely known. Angiogenic factors associated with angiogenesis are the major candidates to explain the microvascular and pathologic finds of DN. Vascular endothelial growth factor A (VEGF-A), leucine-rich α-2-glycoprotein 1, angiopoietins and vasohibin family signal between the podocytes, endothelium, and mesangium have important roles in the maintenance of renal functions. An appropriate amount of VEGF-A is beneficial to maintaining glomerular structure, while excessive VEGF-A can lead to abnormal angiogenesis. LRG1 is a novel pro-angiogenic factors involved in the abnormal angiogenesis and renal fibrosis in DN. The imbalance of Ang1/Ang2 ratio has a role in leading to glomerular disease. Vasohibin-2 is recently shown to be in diabetes-induced glomerular alterations. This review will focus on current understanding of these angiogenic factors in angiogenesis and pathogenesis associated with the development of DN, with the aim of evaluating the potential of anti-angiogenesis therapy in patients with DN.

Vascular endothelial growth factor control mechanisms in skeletal growth and repair

Vascular endothelial growth factor A (VEGF) is a critical regulator of vascular development and postnatal angiogenesis and homeostasis, and it is essential for bone development and repair. Blood vessels serve both as structural templates for bone formation and they provide essential cells, growth factors and minerals needed for synthesis and mineralization, as well as turnover, of the extracellular matrix in bone. Through its regulation of angiogenesis, VEGF contributes to coupling of osteogenesis to angiogenesis, and it directly controls the differentiation and function of osteoblasts and osteoclasts. In this review, we summarize the properties of VEGF and its receptors that are relevant to bone formation and repair; the roles of VEGF during development of endochondral and membranous bones; and the contributions of VEGF to bone healing during different phases of bone repair. Finally, we discuss contributions of altered VEGF function in inherited disorders with bone defects as part of their phenotypes, and we speculate on what will be required before therapeutic strategies based on VEGF modulation can be developed for clinical use to treat patients with bone growth disorders and/or compromised bone repair. Developmental Dynamics 246:227-234, 2017. © 2016 Wiley Periodicals, Inc.

The roles of vascular endothelial growth factor in bone repair and regeneration

Vascular endothelial growth factor-A (VEGF) is one of the most important growth factors for regulation of vascular development and angiogenesis. Since bone is a highly vascularized organ and angiogenesis plays an important role in osteogenesis, VEGF also influences skeletal development and postnatal bone repair. Compromised bone repair and regeneration in many patients can be attributed to impaired blood supply; thus, modulation of VEGF levels in bones represents a potential strategy for treating compromised bone repair and improving bone regeneration. This review (i) summarizes the roles of VEGF at different stages of bone repair, including the phases of inflammation, endochondral ossification, intramembranous ossification during callus formation and bone remodeling; (ii) discusses different mechanisms underlying the effects of VEGF on osteoblast function, including paracrine, autocrine and intracrine signaling during bone repair; (iii) summarizes the role of VEGF in the bone regenerative procedure, distraction osteogenesis; and (iv) reviews evidence for the effects of VEGF in the context of repair and regeneration techniques involving the use of scaffolds, skeletal stem cells and growth factors.

Fibroblast VEGF-receptor 1 expression as molecular target in periodontitis

AIM

Degradation of extracellular matrices is an integral part in periodontitis. For antagonizing this pathophysiological mechanism, we aimed at identifying gene expression profiles in disease progression contributing periodontitis-associated fibroblasts (PAFs) versus normal gingival fibroblasts to determine their molecular repertoire, and exploit it for therapeutic intervention.

MATERIALS AND METHODS

Applying an exploratory analysis using a small number of microarrays in combination with a three dimensional (3D) in vitro culture model that incorporates some aspects of periodontitis, PAFs were initially characterized by gene-expression analyses, followed by targeted gene down-regulation and pharmacological intervention in vitro. Further, immunohistochemistry was applied for phosphorylation analyses in tissue specimens.

RESULTS

PAFs were characterized by 42 genes being commonly up-regulated >1.5-fold, and by five genes that were concordantly down-regulated (<0.7-fold). Expression of vascular endothelial growth factor (VEGF)-receptor 1 (Flt-1) was highly enhanced, and was thus further explored in in vitro culture models of periodontal fibroblasts without accounting for the microbiome. Phosphorylation of the VEGF-receptor 1 was enhanced in PAFs. Receptor inhibition by a specific VEGF-receptor inhibitor or intrinsic down-regulation by RNAi of the VEGF-receptor kinase in 3D gel cultures resulted in significant reduction in collagen degradation associated with increased tissue inhibitor of metalloproteinase expression, suggesting that Flt-1 may contribute to periodontitis.

CONCLUSION

Based on the finding that VEGF-receptor kinase inhibition impaired collagen degradation pathways, Flt-1 may represent a candidate for therapeutic approaches in periodontitis.

A tyrosine-rich amelogenin peptide promotes neovasculogenesis in vitro and ex vivo

The formation of new blood vessels has been shown to be fundamental in the repair of many damaged tissues, and we have recently shown that the adult human periodontal ligament contains multipotent stem/progenitor cells that are capable of undergoing vasculogenic and angiogenic differentiation in vitro and ex vivo. Enamel matrix protein (EMP) is a heterogeneous mixture of mainly amelogenin-derived proteins produced during tooth development and has been reported to be sometimes effective in stimulating these processes, including in clinical regeneration of the periodontal ligament. However, the identity of the specific bioactive component of EMP remains unclear. In the present study we show that, while the high-molecular-weight Fraction A of enamel matrix derivative (a heat-treated form of EMP) is unable to stimulate the vasculogenic differentiation of human periodontal ligament cells (HPC) in vitro, the low-molecular-weight Fraction C significantly up-regulates the expression of the endothelial markers VEGFR2, Tie-1, Tie-2, VE-cadherin and vWF and markedly increases the internalization of low-density lipoprotein. Furthermore, we also demonstrate, for the first time, that the synthetic homolog of the 45-amino acid tyrosine-rich amelogenin peptide (TRAP) present in Fraction C is likely to be responsible for its vasculogenesis-inducing activity. Moreover, the chemically synthesized TRAP peptide is also shown here to be capable of up-regulating the angiogenic differentiation of the HPC, based on its marked stimulation of in vitro cell migration and tubule formation and of blood vessel formation assay in a chick embryo chorioallantoic membrane model ex vivo. This novel peptide, and modified derivatives, might thereby represent a new class of regenerative drug that has the ability to elicit new blood vessel formation and promote wound healing in vivo.

VEGF targets the tumour cell

The function of vascular endothelial growth factor (VEGF) in cancer is not limited to angiogenesis and vascular permeability. VEGF-mediated signalling occurs in tumour cells, and this signalling contributes to key aspects of tumorigenesis, including the function of cancer stem cells and tumour initiation. In addition to VEGF receptor tyrosine kinases, the neuropilins are crucial for mediating the effects of VEGF on tumour cells, primarily because of their ability to regulate the function and the trafficking of growth factor receptors and integrins. This has important implications for our understanding of tumour biology and for the development of more effective therapeutic approaches.

Immunohistochemical study of angiogenesis and angiogenic factors in equine granulosa cell tumours

The first part of our study (Müller et al., 2009) characterized angiogenesis in the equine cycling ovary through histomorphological and immunohistochemical examinations (vascular endothelial growth factors A and B [VEGF A, VEGF B], vascular endothelial growth factor receptors 1 and 2 [VEGF-R1, VEGF-R2], vascular angiopoietins 1 and 2 [Ang1, Ang2], angiopoietin receptor [Tie2], and von Willebrand Factor). Since angiogenesis plays an important role in development and growth of numerous tumours, the second part of our study involved a similar examination of 70 equine granulosa cell tumours (GCTt). The results of the second study were compared with those of the normal equine ovary. Certain similarities in the expression pattern could be detected between normal, cyclical ovaries (Müller et al., 2009) and GCTt. The immunoreactivity of granulosa cells and Leydig-like cells in GCTt resembles granulosa cells and luteinized thecal cells in periovulatory cycling ovaries. The neoplastic cells support circulation, supply and growth of GCTt by contributing to angiogenesis.

Vascular endothelial growth factor receptor-1 in human cancer: concise review and rationale for development of IMC-18F1 (Human antibody targeting vascular endothelial growth factor receptor-1)

The human vascular endothelial growth factor receptor-1 (VEGFR-1, or Flt-1) is widely expressed in normal and pathologic tissue and contributes to the pathogenesis of both neoplastic and inflammatory diseases. In human cancer, VEGFR-1 mediated signaling is responsible for both direct tumor activation and angiogenesis. VEGFR-1 mediated activation of nonmalignant supporting cells, particularly stromal, dendritic, hematopoietic cells, and macrophages, is also likely important for cancer pathogenesis. VEGFR-1 is also hypothesized to enable the development of cancer metastases by means of activation and premetastatic localization in distant organs of bone marrow-derived hematopoietic progenitor cells, which express VEGFR-1. IMC-18F1 is a fully human IgG(1) antibody that binds to VEGFR-1 and has been associated with the inhibition of cancer growth in multiple in vitro and human tumor xenograft models. The preliminary results of phase 1 investigations have also indicated a favorable safety profile for IMC-18F1 at doses that confer antibody concentrations that are associated with relevant antitumor activity in preclinical models.

Logical structures extracted from metastasis experiments

An inductive argument of metastasis with a metaphor of seed and soil was made by Stephen Paget in 1889. It is commonly held that metastasis is dependent on both the organ from which the primary tumors originate, and the organs to which the tumor cells travel. The assumption is based on the statistical observation of a number of autopsy samples. Here I attempt to establish a theory on the mechanisms of metastasis with experimental evidence. I propose that dysregulation of pro-inflammatory Toll-like receptor 4 signaling, stimulated by its endogenous ligands, establishes pre-metastatic soil. Once specific parameters are established, deductive judgments could be possible to predict to which organ a given tumor metastasizes.

FLT1 and its ligands VEGFB and PlGF: drug targets for anti-angiogenic therapy?

Less than 5 years ago, it was still not clear whether anti-angiogenic drugs would prove successful in the clinic. After numerous patients with cancer or age-related macular degeneration have been treated with these drugs, they have now become part of the standard range of therapeutic tools. Despite this milestone, anti-angiogenic therapy still faces a number of clinical hurdles, such as improving efficacy, avoiding escape and resistance, and minimizing toxicity. Hopefully, other agents with complementary mechanisms, such as those that target placental growth factor, will offer novel opportunities for improved treatment.

IGF-2 and FLT-1/VEGF-R1 mRNA levels reveal distinctions and similarities between congenital and common infantile hemangioma

Common infantile hemangioma appears postnatally, grows rapidly, and regresses slowly. Two types of congenital vascular tumors present fully grown at birth and behave differently from infantile hemangioma. These rare congenital tumors have been designated rapidly involuting congenital hemangioma (RICH) and noninvoluting congenital hemangioma (NICH). RICH and NICH are similar in appearance, location, and size, and have some overlapping histologic features with infantile hemangioma. At a molecular level, neither expresses glucose transporter-1, a diagnostic marker of infantile hemangioma. To gain further insight into the molecular differences and similarities between congenital and common hemangioma, we analyzed expression of insulin-like growth factor-2, known to be highly expressed in infantile hemangioma and VEGF-receptors, by quantitative real-time PCR, in three RICH and five NICH specimens. We show that insulin-like growth factor-2 mRNA was expressed in both RICH and NICH, at a level comparable with that detected in common hemangioma over 4 y of age. In contrast, mRNA levels for membrane-associated fms-like tyrosine-kinase receptor, also known as VEGF receptor-1, were uniformly increased in congenital hemangiomas compared with proliferating or involuting phase common hemangioma. These results provide the first evidence of the molecular distinctions and similarities between congenital and postnatal hemangioma.

Vascular endothelial growth factor receptor-2 expression in the pulp of human primary and young permanent teeth

The purpose of this study was to evaluate the expression of vascular endothelial growth factor receptor-2 (VEGFR-2) in endothelial cells within the dental pulp of human primary and young permanent teeth and the spatial distribution of VEGFR-2-positive cells. Nine sound primary teeth and 4 sound young permanent teeth were evaluated by immunohistochemistry with a human anti-VEGFR-2 antibody. Stained tissue sections were analyzed qualitatively under light microscopy. Here we observed that endothelial cells of both primary and permanent teeth showed positive immunostaining for VEGFR-2. Notably, VEGFR-2-positive cells in the primary teeth tended to be found close to the subodontoblastic layer, whereas the spatial distribution of VEGFR-2 immunostaining in the permanent teeth was more uniform. In conclusion, VEGFR-2 was expressed in the microvascular endothelial cells of both primary and young permanent teeth, which suggests that these cells are capable of responding to the morphogenetic and survival signals mediated by VEGF.

Vascular endothelial growth factor and its relationship with the dental pulp

The dental pulp is a loose connective tissue located within rigid dentinal walls. Therefore, when subjected to a stimulus, the pulpal tissue has little expansion capacity. The defense mechanisms of this tissue include the formation of tertiary dentin as well as the production of signaling molecules that help in the repair. The dentin matrix is rich in growth factors (GFs) that, when diluted and diffused into the pulp tissue, aid the healing process of the dentinopulpar complex. The angiogenic GFs participate in this event. Vascular endothelial growth factor (VEGF), a potent mitogen for endothelial cells, promotes endothelial cell survival and angiogenesis. Among its receptors, VEGFR-2 seems to be the most intimately associated with mitogenic activities, cell migration, vascular permeability, and survival of endothelial cells. This literature review addresses the cell-signaling process that occurs in response to a pulp stimulus up to its transduction in the target cell, describing the VEGF, as well as its characteristics and receptors. The reported studies have correlated the expression of VEGF and its potential functions that may have an impact on several dental specialties, thus indicating that further clinical investigations should be conducted in order to translate the results obtained until this moment primarily in laboratory experiments.

Vascular endothelial growth factor (VEGF) in acute lung injury (ALI) and acute respiratory distress syndrome (ARDS): paradox or paradigm?

Acute respiratory distress syndrome (ARDS), the most severe form of acute lung injury (ALI), remains a devastating condition with a high mortality. It is characterised by alveolar injury and increased pulmonary vascular permeability. Vascular endothelial cell growth factor (VEGF) was identified by its properties to increase permeability and act as a cellular growth factor, hence its potential for a key role in the pathogenesis of ALI/ARDS. This review describes the basic biology of VEGF and its receptors as an essential prerequisite to discussing the available and sometimes paradoxical published data, before considering a paradigm for the role of VEGF in the human lung.

Oxidative stress-responsive transcription factor ATF3 potentially mediates diabetic angiopathy

Previous results of our cDNA microarray analysis to look for genes whose expression level correlates well with in vitro tubulogenesis by NP31 endothelial cells revealed the transcription factor ATF3 known to be responsive to stress such as reactive oxygen species (ROS). Anti-ATF3 small interfering RNA gave an inhibitory influence on tube formation by NP31 cells expressing an activated form of the vascular endothelial growth factor receptor 1 (VEGFR-1) kinase. When expression of ATF3 was regulated under the control of tetracycline system in NP31 cells, they acquired the tubulogenic ability upon ATF3 induction. While ATF3 failed to induce expressions of VEGF and VEGFR, it regulated those of CDK2, CDK4, p8, plasminogen activator inhibitor 1, integrin alpha1, subunit and matrix metalloprotease MMP13. In H2O2-stimulated NP31 cells as well as endothelial cells of glomerulus and aorta of Otsuka-Long-Evans-Tokushima-Fatty diabetic model rats, concomitantly enhanced expressions of ATF3, PAI-1, and p8 were observed. Given the proposed hypothesis of the close linkage between diabetic angiopathy and ROS, those data suggest that ROS-associated diabetic complication may involve ATF3-mediated pathological angiogenesis.

A single amino acid substitution in the activation loop defines the decoy characteristic of VEGFR-1/FLT-1

VEGFR-1 is a kinase-defective receptor tyrosine kinase (RTK) and negatively modulates angiogenesis by acting as a decoy receptor. The decoy characteristic of VEGFR-1 is required for normal development and angiogenesis. To date, there is no molecular explanation for this unusual characteristic of VEGFR-1. Here we show that the molecular mechanisms underlying the decoy characteristic of VEGFR-1 is linked to the replacement of a highly conserved amino acid residue in the activation loop. This amino acid is highly conserved among all the type III RTKs and corresponds to aspartic acid, but in VEGFR-1 it is substituted to asparagine. Mutation of asparagine (Asn(1050)) within the activation loop to aspartic acid promoted enhanced ligand-dependent tyrosine autophosphorylation and kinase activation in vivo and in vitro. The mutant VEGFR-1 (Asp(1050)) promoted endothelial cell proliferation but not tubulogenesis. It also displayed an oncogenic phenotype as its expression in fibroblast cells elicited transformation and colony growth. Furthermore, mutation of the invariable aspartic acid to asparagine in VEGFR-2 lowered the autophosphorylation of activation loop tyrosines 1052 and 1057. We propose that the conserved aspartic acid in the activation loop favors the transphosphorylation of the activation loop tyrosines, and its absence renders RTK to a less potent enzyme by disfavoring transphosphorylation of activation loop tyrosines.

Therapeutic targeting of receptor tyrosine kinases in lung cancer

Lung cancer is a difficult illness with a poor overall survival. Even though combination strategies with chemotherapy, radiation therapy and surgery have all been utilised, the overall outcome for this disease continues to be relatively disappointing. In order to make a difference in the treatment of lung cancer, novel therapeutics will have to be developed. Through basic biological studies, a number of receptor tyrosine kinases have been implicated in the pathogenesis and progression of lung cancer. In this review, the authors summarise the mechanisms of several major receptor tyrosine kinases in lung cancer, especially epidermal growth factor receptor, Her2/neu, MET, vascular endothelial growth factor and KIT. The biology associated with these receptors is described, and the various novel therapeutic inhibitory strategies that are ongoing in preclinical and clinical studies for lung cancer are detailed. Through understanding of receptor tyrosine kinases and the utilisation of specific inhibitors, it is hopeful that a dramatic impact will be made on the biology and therapy for lung cancer.

N-acetyl-cysteine enhances growth in BCR-ABL-transformed cells

N-acetyl-cysteine (NAC) has been reported to have anticancer properties such as counteractions against mutagens and prevention of tumor progression by scavenging reactive oxygen species (ROS). However, here we report that NAC can enhance the anchorage-independent growth of cells transformed by activated ABL tyrosine kinases or Ras. This effect was not dependent on loss of focal adhesion kinase activation. NAC rescued cell growth that was suppressed by heat shock protein (Hsp) 90 inhibitors possibly by chemical modification of their quinone moiety. NAC rendered Rat1/BCR-ABL cells resistance to a Ras inhibitor manumycin in soft agar colony formation. In the absence of Hsp90 inhibitors, NAC stimulated the activation of MAP kinase in BCR-ABL-transformed but not in the parental Rat1 cells. We propose that NAC should be used carefully in cancer treatment.

Nox1 regulates apoptosis and potentially stimulates branching morphogenesis in sinusoidal endothelial cells

Tubulogenic transformation of a nontubulogenic endothelial cell line NP31 by a constitutively activated form of the Flt-1 kinase (NP31/kinase) was accompanied by an increased expression of Nox1 by sixfold over NP31. Overexpression of Nox1 in NP31 cells (NP31/Nox1) stimulated branching morphogenesis in Matrigel but surprisingly cords lacked a lumen. The branching morphogenesis by NP31/kinase and NP31/Nox1 cells was blocked either by N-acetyl-l-cysteine (NAC) or Tiron. Vascular endothelial growth factor (VEGF)-dependent sinusoidal endothelial cells (SEC) in primary culture showed fivefold increase in Nox1 expression 4 days after VEGF stimulation. Interestingly, VEGF-resistant apoptosis in SEC at day 7 was inhibited by NAC or by anti-Nox1 siRNA. These results suggest that Nox1 regulates apoptosis in SEC and can potentially stimulate branching morphogenesis in SEC-derived NP 31 cells.

Vascular endothelial growth factor: basic science and clinical progress

Vascular endothelial growth factor (VEGF) is an endothelial cell-specific mitogen in vitro and an angiogenic inducer in a variety of in vivo models. Hypoxia has been shown to be a major inducer of VEGF gene transcription. The tyrosine kinases Flt-1 (VEGFR-1) and Flk-1/KDR (VEGFR-2) are high-affinity VEGF receptors. The role of VEGF in developmental angiogenesis is emphasized by the finding that loss of a single VEGF allele results in defective vascularization and early embryonic lethality. VEGF is critical also for reproductive and bone angiogenesis. Substantial evidence also implicates VEGF as a mediator of pathological angiogenesis. In situ hybridization studies demonstrate expression of VEGF mRNA in the majority of human tumors. Anti-VEGF monoclonal antibodies and other VEGF inhibitors block the growth of several tumor cell lines in nude mice. Clinical trials with various VEGF inhibitors in a variety of malignancies are ongoing. Very recently, an anti-VEGF monoclonal antibody (bevacizumab; Avastin) has been approved by the Food and Drug Administration as a first-line treatment for metastatic colorectal cancer in combination with chemotherapy. Furthermore, VEGF is implicated in intraocular neovascularization associated with diabetic retinopathy and age-related macular degeneration.

Dynamic regulation of gene expression by the Flt-1 kinase and Matrigel in endothelial tubulogenesis

A nontubulogenic endothelial cell line, NP31, can be transformed by the active form of the Flt-1 kinase (BCR-FLTm1) into Tb3 cells, which show a tubulogenic property only when cultured in Matrigel. By utilizing this strict dependence of NP31 on BCR-FLTm1 and Matrigel for experimental angiogenesis, we performed microarray analyses under several conditions and found 97 genes whose dynamically regulated profiles of gene expression are divided into nine groups, in two major clusters. In one major cluster, gene expression is interdependently regulated by BCR-FLTm1 or Matrigel. The second major cluster contains genes whose expression patterns under BCR-FLTm1 influence are reversed by Matrigel. Based on these gene expression patterns in NP31 driven by BCR-FLTm1 and/or Matrigel, we propose a model in which sequential and alternate stimulation by BCR-FLTm1 and Matrigel induces cooperative regulation of subsets of genes. Microarray analyses of Tb3 under 11 different conditions revealed 5 candidate genes whose gene expression regulation is most closely associated with tubulogenesis.

The biology of VEGF and its receptors

Vascular endothelial growth factor (VEGF) is a key regulator of physiological angiogenesis during embryogenesis, skeletal growth and reproductive functions. VEGF has also been implicated in pathological angiogenesis associated with tumors, intraocular neovascular disorders and other conditions. The biological effects of VEGF are mediated by two receptor tyrosine kinases (RTKs), VEGFR-1 and VEGFR-2, which differ considerably in signaling properties. Non-signaling co-receptors also modulate VEGF RTK signaling. Currently, several VEGF inhibitors are undergoing clinical testing in several malignancies. VEGF inhibition is also being tested as a strategy for the prevention of angiogenesis, vascular leakage and visual loss in age-related macular degeneration.

Vasculogenic mimicry and pseudo-comedo formation in breast cancer

Tumors require a blood supply for growth and hematogenous metastases. Until recently, most research in this area has focused on the role of angiogenesis, the recruitment of new vessels into a tumor from preexisting vessels. Previously, in a study of breast cancer (IBC), in which we used established inflammatory breast cancer (IBC) xenografts (WIBC-9) originating from a patient with IBC (Shirakawa et al., Cancer Res 2001:61:445-451), we reported observing vasculogenic mimicry (VM), a condition in which bloodstreams within cancer tissue are not accompanied by a lining of endothelial cells (ECs) (Shirakawa et al., Cancer Res 2002:62:560-566). In the present study, we examined 331 surgically resected breast cancer specimens for evidence of VM, using immunohistochemistry and laser-captured microdissection (LCM) followed by nested reverse transcriptase polymerase chain reaction (RT-PCR). Surprisingly, 7.9% (26 specimens) of the 331 specimens exhibited evidence of VM. Of these 26 VM specimens, 84.6% (22 specimens) exhibited pseudo-comedo formation. RT-PCR analysis of 8 microdissected typical VM specimens revealed expression of Tie-2, Flt-1, thrombin receptor and CD31 in 63, 50, 0 and 0% of specimens, respectively. In contrast, results of RT-PCR analysis of 8 specimens from non-VM tumors were negative for expression of these genes. The 26 VM cases tended to have a higher percentage of hematogenous recurrence (p = 0.059) and a lower percentage of 5-year survival (p = 0.071) than the 305 non-VM cases. However, there were no significant differences in tumor size, lymph node metastasis, estrogen receptors or progesterone receptors between the 2 groups (p > 0.1). Our results suggest that the existence of VM increases the likelihood of hematogenous metastases and is in inverse proportion to prognosis.

Soluble Flt-1 (soluble VEGFR-1), a potent natural antiangiogenic molecule in mammals, is phylogenetically conserved in avians

The flt-1 gene encodes for both the full-length receptor Flt-1 (VEGFR-1) and a soluble form designated sFlt-1. sFlt-1 carries the VEGF-binding domain of Flt-1 as well as a 31-amino-acid stretch derived from an intron and tightly binds VEGF, suppressing its angiogenic activity. The flt-1 gene has so far been identified only in mammals and is highly expressed in placenta as well as in vascular endothelial cells. In placenta, sFlt-1 is abundant in the trophoblast layer during pregnancy, suggesting that it is a negative regulator to excess angiogenesis and vascular permeability at the feto-maternal border in mammals. However, we show here for the first time that the flt-1 gene exists and is highly conserved in chickens. Surprisingly, the chicken flt-1 gene also encodes for sFlt-1 in addition to the full-length receptor. Similar to the mammalian sFlt-1, chicken sFlt-1 carries the VEGF-binding domain and a 31-amino-acid carboxyl region derived from an intron, which was significantly homologous to that in mammals. Chicken sFlt-1 is expressed early in embryogenesis. These findings strongly suggest that the natural antiangiogenic molecule sFlt-1 is widely conserved in vertebrates and regulates the angiogenic process.

Homophilic complex formation of MT1-MMP facilitates proMMP-2 activation on the cell surface and promotes tumor cell invasion

Activation of proMMP-2 by MT1-MMP is considered to be a critical event in cancer cell invasion. In the activation step, TIMP-2 bound to MT1-MMP on the cell surface acts as a receptor for proMMP-2. Subsequently, adjacent TIMP-2-free MT1-MMP activates the proMMP-2 in the ternary complex. In this study, we demonstrate that MT1-MMP forms a homophilic complex through the hemopexin-like (PEX) domain that acts as a mechanism to keep MT1-MMP molecules close together to facilitate proMMP-2 activation. Deletion of the PEX domain in MT1-MMP, or swapping the domain with the one derived from MT4-MMP, abolished the ability to activate proMMP-2 on the cell surface without affecting the proteolytic activities. In addition, expression of the mutant MT1-MMP lacking the catalytic domain (MT1PEX-F) efficiently inhibited complex formation of the full-length enzymes and activation of pro MMP-2. Furthermore, expression of MT1PEX-F inhibited proMMP-2 activation and Matrigel invasion activity of invasive human fibrosarcoma HT1080 cells. These findings elucidate a new function of the PEX domain: regulating MT1-MMP activity on the cell surface, which accelerates cellular invasiveness in the tissue.

Vascular endothelial growth factor receptor-1 modulates vascular endothelial growth factor-mediated angiogenesis via nitric oxide

The known responses of vascular endothelial growth factor (VEGF) are mediated through VEGF receptor-2 (VEGFR-2/KDR) in endothelial cells. However, it is unknown whether VEGFR-1 (Flt-1) is an inert decoy or a signaling receptor for VEGF during physiological or pathological angiogenesis. Here we report that VEGF-stimulated nitric oxide (NO) release is inhibited by blockade of VEGFR-1 and that VEGFR-1 via NO negatively regulates of VEGFR-2-mediated proliferation and promotes formation of capillary networks in human umbilical vein endothelial cells (HUVECs). Inhibition of VEGFR-1 in a murine Matrigel angiogenesis assay induced large aneurysm-like structures. VEGF-induced capillary growth over 14 days was inhibited by anti-VEGFR-2-blocking antibody as determined by reduced tube length between capillary connections (P < 0.0001) in an in vitro angiogenesis assay. In contrast, loss of VEGFR-1 activity with a neutralizing anti-VEGFR-1 antibody resulted in an increase in the accumulation of endothelial cells (P < 0.0001) and a dramatic decrease in the number of capillary connections that were restored by the addition of NO donor. Porcine aortic endothelial (PAE) cells expressing human VEGFR-1 but not VEGFR-2 plated on growth factor-reduced Matrigel rearranged into tube-like structures that were prevented by anti-VEGFR-1 antibody or a cGMP inhibitor. VEGF stimulated NO release from VEGFR-1- but not VEGFR-2-transfected endothelial cells and placenta growth factor-1 stimulated NO release in HUVECs. Blockade of VEGFR-1 increased VEGF-mediated HUVEC proliferation that was inhibited by NO donors, and potentiated by NO synthase inhibitors. These data indicate that VEGFR-1 is a signaling receptor that promotes endothelial cell differentiation into vascular tubes, in part by limiting VEGFR-2-mediated endothelial cell proliferation via NO, which seems to be a molecular switch for endothelial cell differentiation.

The tubulogenic activity associated with an activated form of Flt-1 kinase is dependent on focal adhesion kinase

Focal adhesion kinase (FAK) is known to be located at the intersection between extracellular matrix and growth factor signaling pathways to regulate cell motility. We have shown previously that an activated form (BCR-FLTm1) of Flt-1 kinase, a receptor for vascular endothelial growth factor, had a tubulogenic activity not only in endothelial cells but also in fibroblastic cells. Here we show that tubulogenesis by BCR-FLTm1 depends on FAK and that FAK tyrosine phosphorylation and association with an activated Flt-1 receptor complex is increased after vascular endothelial growth factor stimulation of NIH3T3 cells overexpressing Flt-1.

Semaphorin 3A-vascular endothelial growth factor-165 balance mediates migration and apoptosis of neural progenitor cells by the recruitment of shared receptor

The dynamic and coordinated interaction between cells and their microenvironment controls cell migration, proliferation, and apoptosis, mediated by different cell surface molecules. We have studied the response of a neuroectodermal progenitor cell line, Dev, to a guidance molecule, semaphorin 3A (Sema3A), described previously as a repellent-collapsing signal for axons, and we have shown that Sema3A acts as a repellent guidance cue for migrating progenitor cells and, on prolonged application, induces apoptosis. Both repulsion and induction of cell death are mediated by neuropilin-1, the ligand-binding component of the Sema3A receptor. The vascular endothelial growth factor, VEGF165, antagonizes Sema3A-induced apoptosis and promotes cell survival, migration, and proliferation. Surprisingly, repulsion by Sema3A also depends on expression of VEGFR1, a VEGF165 receptor, expressed in Dev cells. Moreover, we found that these repulsive effects of Sema3A require tyrosine kinase activity, which can be attributed to VEGFR1. These results indicate that the balance between guidance molecules and angiogenic factors can modulate the migration, apoptosis (or survival), and proliferation of neural progenitor cells through shared receptors.

Molecular biology of chronic myeloid leukemia

Multistep carcinogenesis is exemplified by chronic myeloid leukemia with clinical manifestation consisting of a chronic phase and blast crisis. Pathological generation of BCR-ABL (breakpoint cluster region-Abelson) results in growth promotion, differentiation, resistance to apoptosis, and defect in DNA repair in targeted blood cells. Domains in BCR and ABL sequences work in concert to elicit a variety of leukemogenic signals including Ras, STAT5 (signal transducer and activator of transcription-5), Myc, cyclin D1, P13 (phosphatidylinositol 3-kinase), RIN1 (Ras interaction/interference), and activation of actin cytoskeleton. However, the mechanism of differentiation of transformed cells is poorly understood. A mutator phenotype of BCR-ABL could explain the transformation to blast crisis. The aim of this review is to integrate molecular and biological information on BCR, ABL, and BCR-ABL and to focus on how signaling from those molecules mirrors the biological phenotypes of chronic myeloid leukemia.

A repressor sequence in the juxtamembrane domain of Flt-1 (VEGFR-1) constitutively inhibits vascular endothelial growth factor-dependent phosphatidylinositol 3'-kinase activation and endothelial cell migration

Vascular endothelial growth factor (VEGF) has two highly homologous tyrosine kinase receptors: Flt-1 (VEGFR-1) and KDR (VEGFR-2). KDR is strongly phosphorylated on tyrosines and can transmit mitogenic and motogenic signals following VEGF binding, while Flt-1 is markedly less effective in mediating such functions. To dissect the regions that account for the differences between the two receptors, we generated a series of chimeric Flt-1-KDR molecules. We found that the juxtamembrane region of Flt-1 prevents key signaling functions. When the juxtamembrane region of Flt-1 is replaced by that of KDR, Flt-1 becomes competent to mediate endothelial cell migration and phosphatidylinositol 3'-kinase activation in response to VEGF. Further mutational analysis shows that a short divergent sequence is responsible for such repressor function. However, mutant Flt-1 receptors lacking this sequence do not transmit effective proliferative signals, suggesting that this receptor function is regulated separately. These results define a novel functional domain that serves to repress Flt-1 activity in endothelial cells.

An oncogenic form of the Flt-1 kinase has a tubulogenic potential in a sinusoidal endothelial cell line

We have previously reported a constitutively activated form of the Flt-1 kinase (BCR-FLTm) molecularly engineered based on the structural backbone of the activated tyrosine kinase BCR-ABL. Here we show that it can induce not only growth stimulation but also tubulogenic differentiation of non-tubulogenic NP31 (non parenchymal) sinusoidal endothelial cells of rat liver in basement membrane matrix. Tubules formed in vitro were accompanied by fenestration structures and allowed circulation when transplanted into syngeneic animals. This biological response was not observed in other activated forms of kinases constructed in a similar fashion, which include Trk (BCR-TRK), KDR (BCR-KDR), and the parental BCR-ABL. Interestingly, formation of fine tubules was accomplished with lower but not higher expression levels of BCR-FLTm. Compared to NP cells in primary culture NP31 is deficient in expression of alpha1 integrin subunit, which was restored by expression of BCR-FLTm that had tubulogenic ability. Matrix-induced tyrosine phosphorylation of an adaptor protein Shc with recruitment of Grb-2 was observed even when tubulogenesis was nearly completed at G1 stage of the cell cycle in 2-3 weeks. Activation of matrix metalloproteinase 2 (MMP-2) and expression of urokinase type plasminogen activator (uPA) was observed with cellular invasion into matrix at the depth of 200-300 microm. Inhibitors for MAP kinase activator MEK1 and for serine proteases showed deleterious effects on the tubulogenesis. We suppose that matrix ligand-induced integrin signals cooperate with a low level of Flt-1 kinase activity to promote tubulogenic behaviors of endothelial cells in this system.

KDR/Flk-1 is a major regulator of vascular endothelial growth factor-induced tumor development and angiogenesis in murine hepatocellular carcinoma cells

Vascular endothelial growth factor (VEGF), which is one of the most potent angiogenic factors, has been shown to play a pivotal role in tumor angiogenesis, including hepatocellular carcinoma (HCC). The effects of VEGF are mediated mainly through two distinct receptors, flt-1 and KDR/Flk-1. It has been suggested that KDR/Flk-1 plays an important role in tumor development. However, the role of KDR/Flk-1 in HCC has not been examined. We previously reported that VEGF tightly regulated murine HCC development, based on the results of a study using a retroviral tetracycline-regulated (Retro-Tet) gene expression system. This system allows VEGF gene expression to be manipulated in vivo by providing tetracycline in the drinking water. In the present study, we combined the KDR/Flk-1-specific neutralizing monoclonal antibody (KDR/Flk-1mAb) and the Retro-Tet system to elucidate the role of KDR/Flk-1 in VEGF-induced tumor development and angiogenesis in a murine HCC experimental model. In a xenograft study, tumor augmentation induced by VEGF overexpression was almost abolished by means of KDR/Flk-1mAb treatment, with accompanying inhibition of angiogenesis, KDR/Flk-1 autophosphorylation, but not interference of flt-1 activation. This inhibitory effect was achieved even on established tumors and regardless of whether the tumor size was small or large. On the contrary, KDR/Flk-1mAb treatment significantly increased the apoptosis in the tumor. With orthotopic transplantation, KDR/Flk-1mAb also inhibited HCC development in the liver. These results suggest that KDR/Flk-1 is a major regulator of VEGF-mediated HCC development and angiogenesis not only at the initial stage, but also after the tumor has fully developed.

Role of vascular endothelial growth factor in the regulation of angiogenesis

Compelling evidence indicates that vascular endothelial growth factor (VEGF) is a fundamental regulator of normal and abnormal angiogenesis. The loss of a single VEGF allele results in defective vascularization and early embryonic lethality. VEGF plays also a critical role in kidney development, and its inactivation during early postnatal life results in the suppression of glomerular development and kidney failure. Recent evidence indicates that VEGF is also essential for angiogenesis in the female reproductive tract and for morphogenesis of the epiphyseal growth plate and endochondral bone formation. Substantial experimental evidence also implicates VEGF in pathological angiogenesis. Anti-VEGF monoclonal antibodies or other VEGF inhibitors block the growth of several human tumor cell lines in nude mice. Furthermore, the concentrations of VEGF are elevated in the aqueous and vitreous humors of patients with proliferative retinopathies such as the diabetic retinopathy. In addition, VEGF-induced angiogenesis results in a therapeutic benefit in several animal models of myocardial or limb ischemia. Currently, both therapeutic angiogenesis using recombinant VEGF or VEGF gene transfer and inhibition of VEGF-mediated pathological angiogenesis are being pursued clinically.

Expression of vascular endothelial growth factor and its receptors during lung carcinogenesis by N-nitrosobis(2-hydroxypropyl)amine in rats

The expression of vascular endothelial growth factor (VEGF) and its receptors (VEGFRs), VEGFR-1/Flt-1 and VEGFR-2/Flk-1, was investigated by immunohistochemical and northern blot analysis during lung carcinogenesis by N-nitrosobis(2-hydroxypropyl)amine (BHP) in male Wistar rats. After BHP was given in the drinking water for 12 wk, the rats were maintained without further treatment until they were killed at 20-28 wk. Immunohistochemical studies revealed VEGF expression in almost all malignancies, the reaction being strongly positive in most adenocarcinomas (15 of 18; 83.3%) and squamous cell carcinomas (four of five; 80.0%), but less so in a total of 120 adenomas and 136 alveolar hyperplasias. In addition, VEGF mRNA and VEGFR mRNAs were found to be overexpressed in most adenocarcinomas and squamous cell carcinomas as well as in one to three of the five adenomas tested. The results indicated that VEGF and VEGFRs play important roles in the acquisition of malignant potential by preneoplastic lung lesions induced by BHP in rats. Moreover, overexpression of VEGF was related to upregulation of VEGFR-1/Flt-1 and VEGFR-2/Flk-1 expression in malignant and premalignant lung lesions.