Clinical trials of bone marrow stem/progenitor cell therapy after myocardial infarction (MI) have shown promising results, but the mechanism of benefit is unclear. We examined the nature of endogenous myocardial repair that is dependent on the function of the c-kit receptor, which is expressed on bone marrow stem/progenitor cells and on recently identified cardiac stem cells. MI increased the number of c-kit+ cells in the heart. These cells were traced back to a bone marrow origin, using genetic tagging in bone marrow chimeric mice. The recruited c-kit+ cells established a proangiogenic milieu in the infarct border zone by increasing VEGF and by reversing the cardiac ratio of angiopoietin-1 to angiopoietin-2. These oscillations potentiated endothelial mitogenesis and were associated with the establishment of an extensive myofibroblast-rich repair tissue. Mutations in the c-kit receptor interfered with the mobilization of the cells to the heart, prevented angiogenesis, diminished myofibroblast-rich repair tissue formation, and led to precipitous cardiac failure and death. Replacement of the mutant bone marrow with wild-type cells rescued the cardiomyopathic phenotype. We conclude that, consistent with their documented role in tumorigenesis, bone marrow c-kit+ cells act as key regulators of the angiogenic switch in infarcted myocardium, thereby driving efficient cardiac repair.

OBJECTIVE

Vascular endothelial growth factor (VEGF) is a potent molecule that mediates tumor angiogenesis primarily through VEGF receptor 2 (VEGFR2). Bevacizumab, a recombinant humanized monoclonal antibody to VEGF, was administered to previously untreated patients to evaluate parameters of angiogenesis.

METHODS

Twenty-one patients with inflammatory and locally advanced breast cancer were treated with bevacizumab for cycle 1 (15 mg/kg on day 1) followed by six cycles of bevacizumab with doxorubicin (50 mg/m(2)) and docetaxel (75 mg/m(2)) every 3 weeks. After locoregional therapy, patients received eight cycles of bevacizumab alone, and hormonal therapy when indicated. Tumor biopsies and dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) were obtained at baseline, and after cycles 1, 4, and 7.

RESULTS

A median decrease of 66.7% in phosphorylated VEGFR2 (Y951) in tumor cells (P = .004) and median increase of 128.9% in tumor apoptosis (P = .0008) were seen after bevacizumab alone. These changes persisted with the addition of chemotherapy. There were no significant changes in microvessel density or VEGF-A expression. On DCE-MRI, parameters reflecting reduced angiogenesis, a median decrease of 34.4% in the inflow transfer rate constant (P = .003), 15.0% in the backflow extravascular- extracellular rate constant (P = .0007) and 14.3% in extravascular-extracellular volume fraction (P = .002) were seen after bevacizumab alone.

CONCLUSIONS

Bevacizumab has inhibitory effects on VEGF receptor activation and vascular permeability, and induces apoptosis in tumor cells.

OBJECTIVE

Vascular endothelial growth factor (VEGF) plays an important role in the biology of ovarian cancer (OC). Inhibitors of VEGF suppress tumor growth in OC models. Metronomic chemotherapy, defined as frequent administration of low doses of cytotoxic chemotherapy, suppresses tumor growth, possibly by inhibiting angiogenesis. A phase II trial was conducted to evaluate the antitumor activity and adverse effects of bevacizumab and metronomic oral cyclophosphamide in women with recurrent OC.

METHODS

Patients with measurable disease and prior treatment with a platinum-containing regimen were eligible. Up to two different regimens for recurrent disease were allowed. Treatment consisted of bevacizumab 10 mg/kg intravenously every 2 weeks and oral cyclophosphamide 50 mg/d. The primary end point was progression-free survival at 6 months. Plasma levels of VEGF, E-selectin, and thrombospondin-1 were obtained serially.

RESULTS

Seventy patients were enrolled. The probability of being alive and progression free at 6 months was 56% (+/- 6% SE). A partial response was achieved in 17 patients (24%). Median time to progression and survival were 7.2 and 16.9 months, respectively. The most common serious toxicities were hypertension, fatigue, and pain. Bevacizumab-related toxicities included four episodes of gastrointestinal perforation or fistula, two episodes each of CNS ischemia and pulmonary hypertension, and one episode each of gastrointestinal bleeding and wound healing complication. There were three treatment-related deaths. Levels of VEGF, E-selectin, and thrombospondin-1 were not associated with clinical outcome.

CONCLUSIONS

The combination of bevacizumab and metronomic cyclophosphamide is active in recurrent OC. Further study of this combination is warranted.

Genetic studies in mice and humans have shown that the transforming growth factor-beta (TGF-beta) type-I receptor activin receptor-like kinase 1 (ALK1) and its co-receptor endoglin play an important role in vascular development and angiogenesis. Here, we demonstrate that ALK1 is a signalling receptor for bone morphogenetic protein-9 (BMP-9) in endothelial cells (ECs). BMP-9 bound with high affinity to ALK1 and endoglin, and weakly to the type-I receptor ALK2 and to the BMP type-II receptor (BMPR-II) and activin type-II receptor (ActR-II) in transfected COS cells. Binding of BMP-9 to ALK2 was greatly facilitated when BMPR-II or ActR-II were co-expressed. Whereas BMP-9 predominantly bound to ALK1 and BMPR-II in ECs, it bound to ALK2 and BMPR-II in myoblasts. In addition, we observed binding of BMP-9 to ALK1 and endoglin in glioblastoma cells. BMP-9 activated Smad1 and/or Smad5, and induced ID1 protein and endoglin mRNA expression in ECs. Furthermore, BMP-9 was found to inhibit basic fibroblast growth factor (bFGF)-stimulated proliferation and migration of bovine aortic ECs (BAECs) and to block vascular endothelial growth factor (VEGF)-induced angiogenesis. Taken together, these results suggest that BMP-9 is a physiological ALK1 ligand that plays an important role in the regulation of angiogenesis.

Akt, or protein kinase B, is a multifunctional serine-threonine protein kinase implicated in a diverse range of cellular functions including cell metabolism, survival, migration, and gene expression. However, the in vivo roles and effectors of individual Akt isoforms in signaling are not explicitly clear. Here we show that the genetic loss of Akt1, but not Akt2, in mice results in defective ischemia and VEGF-induced angiogenesis as well as severe peripheral vascular disease. Akt1 knockout (Akt1-/-) mice also have reduced endothelial progenitor cell (EPC) mobilization in response to ischemia, and reintroduction of WT EPCs, but not EPCs isolated from Akt1-/- mice, into WT mice improves limb blood flow after ischemia. Mechanistically, the loss of Akt1 reduces the basal phosphorylation of several Akt substrates, the migration of fibroblasts and ECs, and NO release. Reconstitution of Akt1-/- ECs with Akt1 rescues the defects in substrate phosphorylation, cell migration, and NO release. Thus, the Akt1 isoform exerts an essential role in blood flow control, cellular migration, and NO synthesis during postnatal angiogenesis.

The first angiogenesis inhibitors for cancer have now been approved by the F.D.A. in the U.S. and in 28 other countries, including China. The majority of these are monotherapies that block VEGF. However, mutant tumor cells may over time produce redundant angiogenic factors. Therefore, for long-term use in cancer, combinations of angiogenesis inhibitors or broad spectrum angiogenesis inhibitors will be needed. The two most broad spectrum and least toxic angiogenesis inhibitors are Caplostatin and endostatin. Endostatin inhibits 65 different tumor types and modifies 12% of the human genome to downregulate pathological angiogenesis without side-effects. The recent discovery that small increases in circulating endostatin can suppress tumor growth and that orally available small molecules can increase endostatin in the plasma suggests the possible development of a new pharmaceutical field.

Screening of phage libraries expressing random peptides for binding to prostate cancer cells primarily yielded peptides that had a C-terminal arginine (or rarely lysine) residue, usually in a consensus context R/KXXR/K. Phage expressing these sequences and synthetic nanoparticles coated with them bound to and were internalized into cells. The C-terminal arginine (or lysine) was essential to the activity; adding another amino acid, or even blocking the free carboxyl group of this arginine residue by amidation, eliminated the binding and internalizing activity. An internal R/KXXR/K can be exposed and switched on by a cleavage by a protease. The strict requirement for C-terminal exposure of the motif prompted us to term the phenomenon the C-end rule (CendR). Affinity chromatography showed that the CendR peptides bind to neuropilin-1 (NRP-1) on the target cells. NRP-1 is a cell-surface receptor that plays an essential role in angiogenesis, regulation of vascular permeability, and the development of the nervous system. VEGF-A165 and other ligands of NRP-1 possess a C-terminal CendR sequence that interacts with the b1 domain of NRP-1 and causes cellular internalization and vascular leakage. Our CendR peptides have similar effects, particularly when made multivalent through coupling to a particle. We also noted a unique and important activity of these peptides: penetration and transportation through tissues. The peptides were able to take payloads up to the nanoparticle size scale deep into extravascular tissue. Our observations have implications in drug delivery and penetration of tissue barriers and tumors.

OBJECTIVE

Validated biomarkers of prognosis and response to drug have not been identified for patients with hepatocellular carcinoma (HCC). One of the objectives of the phase III, randomized, controlled Sorafenib HCC Assessment Randomized Protocol (SHARP) trial was to explore the ability of plasma biomarkers to predict prognosis and therapeutic efficacy.

METHODS

In SHARP, 602 patients with advanced HCC were randomized to receive either oral sorafenib 400 mg twice a day per os or matching placebo daily on a continuous basis. Ten plasma biomarkers implicated in the pathogenesis of HCC were measured in 491 patients at baseline and in 305 after 12 weeks of treatment. The candidate biomarkers were analyzed to identify correlates of prognosis or predictors of response to sorafenib.

RESULTS

In both the entire patient population and the placebo cohort, baseline angiopoietin 2 (Ang2) and VEGF concentrations independently predicted survival. Clinical variables such as macroscopic vascular invasion, Eastern Cooperative Oncology Group (ECOG) performance status, and baseline α-fetoprotein and alkaline phosphatase concentrations also independently predicted survival in these groups. In the sorafenib cohort, trends toward enhanced survival benefit from sorafenib were observed in patients with high s-c-KIT or low hepatocyte growth factor concentration at baseline (P of interaction = 0.081 and 0.073, respectively).

CONCLUSIONS

The angiogenesis biomarkers Ang2 and VEGF were independent predictors of survival in patients with advanced HCC. In contrast, none of the biomarkers tested significantly predicted response to sorafenib.

Amyotrophic lateral sclerosis (ALS) causes adult-onset, progressive motor neuron degeneration in the brain and spinal cord, resulting in paralysis and death three to five years after onset in most patients. ALS is still incurable, in part because its complex aetiology remains insufficiently understood. Recent reports have indicated that reduced levels of vascular endothelial growth factor (VEGF), which is essential in angiogenesis and has also been implicated in neuroprotection, predispose mice and humans to ALS. However, the therapeutic potential of VEGF for the treatment of ALS has not previously been assessed. Here we report that a single injection of a VEGF-expressing lentiviral vector into various muscles delayed onset and slowed progression of ALS in mice engineered to overexpress the gene coding for the mutated G93A form of the superoxide dismutase-1 (SOD1(G93A)) (refs 7-10), even when treatment was only initiated at the onset of paralysis. VEGF treatment increased the life expectancy of ALS mice by 30 per cent without causing toxic side effects, thereby achieving one of the most effective therapies reported in the field so far.

Reactive oxygen species (ROS) are associated with multiple cellular functions such as cell proliferation, differentiation, and apoptosis. However, the direct roles of endogenous ROS production still remain to be elucidated. In this study, we found that high levels of ROS were spontaneously produced by ovarian and prostate cancer cells. This elevated ROS production was inhibited by NADPH oxidase inhibitor diphenylene iodonium (DPI) and mitochondria electron chain inhibitor rotenone in the cells. To further analyze the source of ROS production, we found that ovarian cancer cells have much higher expression of NOX4 NADPH oxidase, and that specific inhibition of NADPH oxidase subunit p47(phox) diminished ROS production. To analyze the functional relevance of ROS production, we showed that ROS regulated hypoxia-inducible factor 1 (HIF-1) and vascular endothelial growth factor (VEGF) expression in ovarian cancer cells. Elevated levels of endogenous ROS were required for inducing angiogenesis and tumor growth. NOX4 knockdown in ovarian cancer cells decreased the levels of VEGF and HIF-1 alpha and tumor angiogenesis. This study suggests a new mechanism of higher ROS production in ovarian cancer cells and provides strong evidence that endogenous ROS play an important role for cancer cells to induce angiogenesis and tumor growth. This information may be useful to understand the new mechanism of cancer cells in inducing tumorigenesis and to develop new therapeutic strategy by targeting ROS signaling in human cancer in the future.

Vascular endothelial growth factor (VEGF) is a hypoxia-inducible angiogenic peptide with recently identified neurotrophic effects. Because some neurotrophic factors can protect neurons from hypoxic or ischemic injury, we investigated the possibility that VEGF has similar neuroprotective properties. In HN33, an immortalized hippocampal neuronal cell line, VEGF reduced cell death associated with an in vitro model of cerebral ischemia: at a maximally effective concentration of 50 ng/ml, VEGF approximately doubled the number of cells surviving after 24 h of hypoxia and glucose deprivation. To investigate the mechanism of neuroprotection by VEGF, the expression of known target receptors for VEGF was measured by Western blotting, which showed that HN33 cells expressed VEGFR-2 receptors and neuropilin-1, but not VEGFR-1 receptors. The neuropilin-1 ligand placenta growth factor-2 failed to reproduce the protective effect of VEGF, pointing to VEGFR-2 as the site of VEGF's neuroprotective action. Two phosphatidylinositol 3'-kinase inhibitors, wortmannin and LY294002, reversed the neuroprotective effect of VEGF, implicating the phosphatidylinositol 3'-kinase/Akt signal transduction system in VEGF-mediated neuroprotection. VEGF also protected primary cultures of rat cerebral cortical neurons from hypoxia and glucose deprivation. We conclude that in addition to its known role as an angiogenic factor, VEGF may exert a direct neuroprotective effect in hypoxic-ischemic injury.

The angiopoietins and members of the vascular endothelial growth factor (VEGF) family are the only growth factors thought to be largely specific for vascular endothelial cells. Targeted gene inactivation studies in mice have shown that VEGF is necessary for the early stages of vascular development and that angiopoietin-1 is required for the later stages of vascular remodeling. Here it is shown that transgenic overexpression of angiopoietin-1 in the skin of mice produces larger, more numerous, and more highly branched vessels. These results raise the possibility that angiopoietins can be used, alone or in combination with VEGF, to promote therapeutic angiogenesis.

Potent sequence selective gene inhibition by siRNA 'targeted' therapeutics promises the ultimate level of specificity, but siRNA therapeutics is hindered by poor intracellular uptake, limited blood stability and non-specific immune stimulation. To address these problems, ligand-targeted, sterically stabilized nanoparticles have been adapted for siRNA. Self-assembling nanoparticles with siRNA were constructed with polyethyleneimine (PEI) that is PEGylated with an Arg-Gly-Asp (RGD) peptide ligand attached at the distal end of the polyethylene glycol (PEG), as a means to target tumor neovasculature expressing integrins and used to deliver siRNA inhibiting vascular endothelial growth factor receptor-2 (VEGF R2) expression and thereby tumor angiogenesis. Cell delivery and activity of PEGylated PEI was found to be siRNA sequence specific and depend on the presence of peptide ligand and could be competed by free peptide. Intravenous administration into tumor-bearing mice gave selective tumor uptake, siRNA sequence-specific inhibition of protein expression within the tumor and inhibition of both tumor angiogenesis and growth rate. The results suggest achievement of two levels of targeting: tumor tissue selective delivery via the nanoparticle ligand and gene pathway selectivity via the siRNA oligonucleotide. This opens the door for better targeted therapeutics with both tissue and gene selectivity, also to improve targeted therapies with less than ideal therapeutic targets.

The concept of treating solid tumors by inhibiting tumor angiogenesis was first articulated almost 30 years ago. For the next 10 years it attracted little scientific interest. This situation changed, relatively slowly, over the succeeding decade with the discovery of the first pro-angiogenic molecules such as basic fibroblast growth factor and vascular endothelial growth factor (VEGF), and the development of methods of successfully growing vascular endothelial cells in culture as well as in vivo assays of angiogenesis. However, the 1990s have witnessed a striking change in both attitude and interest in tumor angiogenesis and anti-angiogenic drug development, to the point where a remarkably diverse group of over 24 such drugs is currently undergoing evaluation in phase I, II or III clinical trials. In this review I will discuss the many reasons for this. These features, together with other recent discoveries have created intense interest in initiating and expanding anti-angiogenic drug discovery programs in both academia and industry, and the testing of such newly developed drugs, either alone, or in various combinations with conventional cytotoxic therapeutics. However, significant problems remain in the clinical application of angiogenesis inhibitors such as the need for surrogate markers to monitor the effects of such drugs when they do not cause tumor regressions, and the design of clinical trials. Also of concern is that the expected need to use anti-angiogenic drugs chronically will lead to delayed toxic side effects in humans, which do not appear in rodents, especially in short-term studies.

Many solid tumors produce vascular endothelial growth factor C (VEGF-C), and its receptor, VEGFR-3, is expressed in tumor blood vessels. To study the role of VEGF-C in tumorigenesis, we implanted MCF-7 human breast carcinoma cells overexpressing recombinant VEGF-C orthotopically into severe combined immunodeficient mice. VEGF-C increased tumor growth, but unlike VEGF, it had little effect on tumor angiogenesis. Instead, VEGF-C strongly promoted the growth of tumor-associated lymphatic vessels, which in the tumor periphery were commonly infiltrated with the tumor cells. These effects of VEGF-C were inhibited by a soluble VEGFR-3 fusion protein. Our data suggest that VEGF-C facilitates tumor metastasis via the lymphatic vessels and that tumor spread can be inhibited by blocking the interaction between VEGF-C and its receptor.

The white adipose tissue, especially of humans, is now recognized as the central player in the mild inflammatory state that is characteristic of obesity. The question is how the increased accumulation of lipid seen in obesity causes an inflammatory state and how this is linked to the hypertension and type 2 diabetes that accompanies obesity. Once it was thought that adipose tissue was primarily a reservoir for excess calories that were stored in the adipocytes as triacylglycerols. In times of caloric deprivation these stored lipids were mobilized as free fatty acids and the insulin resistance of obesity was attributed to free fatty acids. It is now clear that in humans the expansion of adipose tissue seen in obesity results in more blood vessels, more connective tissue fibroblasts, and especially more macrophages. There is an enhanced secretion of some interleukins and inflammatory cytokines in adipose tissue of the obese as well as increased circulating levels of many cytokines. The central theme of this chapter is that human adipose tissue is a potent source of inflammatory interleukins plus other cytokines and that the majority of this release is due to the nonfat cells in the adipose tissue except for leptin and adiponectin that are primarily secreted by adipocytes. Human adipocytes secrete at least as much plasminogen activator inhibitor-1 (PAI-1), MCP-1, interleukin-8 (IL-8), and IL-6 in vitro as they do leptin but the nonfat cells of adipose tissue secrete even more of these proteins. The secretion of leptin, on the other hand, by the nonfat cells is negligible. The amount of serum amyloid A proteins 1 & 2 (SAA 1 & 2), haptoglobin, nerve growth factor (NGF), macrophage migration inhibitory factor (MIF), and PAI-1 secreted by the adipocytes derived from a gram of adipose tissue is 144%, 75%, 72%, 37%, and 23%, respectively, of that by the nonfat cells derived from the same amount of human adipose tissue. However, the release of IL-8, MCP-1, vascular endothelial growth factor (VEGF), TGF-beta1, IL-6, PGE(2), TNF-alpha, cathepsin S, hepatocyte growth factor (HGF), IL-1beta, IL-10, resistin, C-reactive protein (CRP), and interleukin-1 receptor antagonist (IL-1Ra) by adipocytes is less than 12% of that by the nonfat cells present in human adipose tissue. Obesity markedly elevates the total release of TNF-alpha, IL-6, and IL-8 by adipose tissue but only that of TNF-alpha is enhanced in adipocytes. However, on a quantitative basis the vast majority of the TNF-alpha comes from the nonfat cells. Visceral adipose tissue also releases more VEGF, resistin, IL-6, PAI-1, TGF-beta1, IL-8, and IL-10 per gram of tissue than does abdominal subcutaneous adipose tissue. In conclusion, there is an increasing recognition that adipose tissue is an endocrine organ that secretes leptin and adiponectin along with a host of other paracrine and endocrine factors in addition to free fatty acids.

High Gleason grade prostate carcinomas are aggressive, poorly differentiated tumors that exhibit diminished estrogen receptor beta (ERbeta) expression. We report that a key function of ERbeta and its specific ligand 5alpha-androstane-3beta,17beta-diol (3beta-adiol) is to maintain an epithelial phenotype and repress mesenchymal characteristics in prostate carcinoma. Stimuli (TGF-beta and hypoxia) that induce an epithelial-mesenchymal transition (EMT) diminish ERbeta expression, and loss of ERbeta is sufficient to promote an EMT. The mechanism involves ERbeta-mediated destabilization of HIF-1alpha and transcriptional repression of VEGF-A. The VEGF-A receptor neuropilin-1 drives the EMT by promoting Snail1 nuclear localization. Importantly, this mechanism is manifested in high Gleason grade cancers, which exhibit significantly more HIF-1alpha and VEGF expression, and Snail1 nuclear localization compared to low Gleason grade cancers.

Our analyses in several different tumor settings challenge the prevailing view that malignancies and metastases generally initiate as avascular masses that only belatedly induce vascular support. Instead, we find that malignant cells rapidly co-opt existing host vessels to form an initially well-vascularized tumor mass. Paradoxically, the co-opted vasculature does not undergo angiogenesis to support the growing tumor, but instead regresses (perhaps as part of a normal host defense mechanism) via a process that involves disruption of endothelial cell/smooth muscle cell interactions and endothelial cell apoptosis. This vessel regression in turn results in necrosis within the central part of the tumor. However, robust angiogenesis is initiated at the tumor margin, rescuing the surviving tumor and supporting further growth. The expression patterns of Angiopoietin-2 (the natural antagonist for the angiogenic Tie2 receptor) and vascular endothelial growth factor (VEGF) strongly implicate these factors in the above processes. Angiopoietin-2 is highly induced in co-opted vessels, prior to VEGF induction in the adjacent tumor cells, providing perhaps the earliest marker of tumor vasculature and apparently marking the co-opted vessels for regression. Subsequently, VEGF upregulation coincident with Angiopoietin-2 expression at the tumor periphery is associated with robust angiogenesis. Thus, in tumors, Angiopoietin-2 and VEGF seem to reprise the roles they play during vascular remodeling in normal tissues, acting to regulate the previously underappreciated balance between vascular regression and growth.

Angiogenesis depends on the cooperation of growth factors and cell adhesion events. Although alphav integrins have been shown to play critical roles in angiogenesis, recent studies in alphav-null mice suggest that other adhesion receptors and their ligands also regulate this process. Evidence is now provided that the integrin alpha5beta1 and its ligand fibronectin are coordinately up-regulated on blood vessels in human tumor biopsies and play critical roles in angiogenesis, resulting in tumor growth in vivo. Angiogenesis induced by multiple growth factors in chick embryos was blocked by monoclonal antibodies to the cell-binding domain of fibronectin. Furthermore, application of fibronectin or a proteolytic fragment of fibronectin containing the central cell-binding domain to the chick chorioallantoic membrane enhanced angiogenesis in an integrin alpha5beta1-dependent manner. Importantly, antibody, peptide, and novel nonpeptide antagonists of integrin alpha5beta1 blocked angiogenesis induced by several growth factors but had little effect on angiogenesis induced by vascular endothelial growth factor (VEGF) in both chick embryo and murine models. In fact, these alpha5beta1 antagonists inhibited tumor angiogenesis, thereby causing regression of human tumors in animal models. Thus, fibronectin and integrin alpha5beta1, like integrin alphavbeta3, contribute to an angiogenesis pathway that is distinct from VEGF-mediated angiogenesis, yet important for the growth of tumors.

We describe a new mechanism regulating the tumor endothelial barrier and T cell infiltration into tumors. We detected selective expression of the death mediator Fas ligand (FasL, also called CD95L) in the vasculature of human and mouse solid tumors but not in normal vasculature. In these tumors, FasL expression was associated with scarce CD8(+) infiltration and a predominance of FoxP3(+) T regulatory (Treg) cells. Tumor-derived vascular endothelial growth factor A (VEGF-A), interleukin 10 (IL-10) and prostaglandin E2 (PGE2) cooperatively induced FasL expression in endothelial cells, which acquired the ability to kill effector CD8(+) T cells but not Treg cells because of higher levels of c-FLIP expression in Treg cells. In mice, genetic or pharmacologic suppression of FasL produced a substantial increase in the influx of tumor-rejecting CD8(+) over FoxP3(+) T cells. Pharmacologic inhibition of VEGF and PGE2 produced a marked increase in the influx of tumor-rejecting CD8(+) over FoxP3(+) T cells that was dependent on attenuation of FasL expression and led to CD8-dependent tumor growth suppression. Thus, tumor paracrine mechanisms establish a tumor endothelial death barrier, which has a critical role in establishing immune tolerance and determining the fate of tumors.

Angiogenesis is critical during tumour initiation and malignant progression. Different strategies aimed at blocking vascular endothelial growth factor (VEGF) and its receptors have been developed to inhibit angiogenesis in cancer patients. It has become increasingly clear that in addition to its effect on angiogenesis, other mechanisms including a direct effect of VEGF on tumour cells may account for the efficiency of VEGF-blockade therapies. Cancer stem cells (CSCs) have been described in various cancers including squamous tumours of the skin. Here we use a mouse model of skin tumours to investigate the impact of the vascular niche and VEGF signalling on controlling the stemness (the ability to self renew and differentiate) of squamous skin tumours during the early stages of tumour progression. We show that CSCs of skin papillomas are localized in a perivascular niche, in the immediate vicinity of endothelial cells. Furthermore, blocking VEGFR2 caused tumour regression not only by decreasing the microvascular density, but also by reducing CSC pool size and impairing CSC renewal properties. Conditional deletion of Vegfa in tumour epithelial cells caused tumours to regress, whereas VEGF overexpression by tumour epithelial cells accelerated tumour growth. In addition to its well-known effect on angiogenesis, VEGF affected skin tumour growth by promoting cancer stemness and symmetric CSC division, leading to CSC expansion. Moreover, deletion of neuropilin-1 (Nrp1), a VEGF co-receptor expressed in cutaneous CSCs, blocked VEGF's ability to promote cancer stemness and renewal. Our results identify a dual role for tumour-cell-derived VEGF in promoting cancer stemness: by stimulating angiogenesis in a paracrine manner, VEGF creates a perivascular niche for CSCs, and by directly affecting CSCs through Nrp1 in an autocrine loop, VEGF stimulates cancer stemness and renewal. Finally, deletion of Nrp1 in normal epidermis prevents skin tumour initiation. These results may have important implications for the prevention and treatment of skin cancers.

Here we describe a triple transgenic mouse system, which combines the tissue specificity of any Cre-transgenic line with the inducibility of the reverse tetracycline transactivator (rtTA)/tetracycline-responsive element (tet-O)-driven transgenes. To ensure reliable rtTA expression in a broad range of cell types, we have targeted the rtTA transgene into the ROSA26 locus. The rtTA expression, however, is conditional to a Cre recombinase-mediated excision of a STOP region from the ROSA26 locus. We demonstrate the utility of this technology through the inducible expression of the vascular endothelial growth factor (VEGF-A) during embryonic development and postnatally in adult mice. Our results of adult induction recapitulate several different hepatic and immune cell pathological phenotypes associated with increased systemic VEGF-A protein levels. This system will be useful for studying genes in which temporal control of expression is necessary for the discovery of the full spectrum of functions. The presented approach abrogates the need to generate tissue-specific rtTA transgenes for tissues where well-characterized Cre lines already exist.

BACKGROUND

The effect of aging on angiogenesis in ischemic vascular disease has not been studied. Accordingly, we investigated the hypothesis that angiogenesis is impaired as a function of age.

RESULTS

Forty days after the resection of 1 femoral artery, collateral vessel development was significantly impaired in old (aged 4 to 5 years; n=7) versus young (aged 6 to 8 months; n=6) New Zealand White (NZW) rabbits on the basis of reduced hindlimb perfusion (ischemic: normal blood pressure ratio=0.58+/-0.05 versus 0.77+/-0.06; P<0.005), reduced number of angiographically visible vessels (angiographic score=0.48+/-0.05 versus 0.70+/-0.05; P<0.01), and lower capillary density in the ischemic limb (130.3+/-5.8/mm2 versus 171.4+/-9.5/mm2; P<0.001). Angiogenesis was also impaired in old (aged 2 years) versus young (aged 12 weeks) mice as shown by reduced hindlimb perfusion (measured by laser Doppler imaging) and lower capillary density (353.0+/-14.3/mm2 versus 713.3+/-63.4/mm2; P<0.01). Impaired angiogenesis in old animals was the result of impaired endothelial function (lower basal NO release and decreased vasodilation in response to acetylcholine) and a lower expression of vascular endothelial growth factor (VEGF) in ischemic tissues (by Northern blot, Western blot, and immunohistochemistry). When recombinant VEGF protein was administered to young and old rabbits, both groups exhibited a significant and similar increase in blood pressure ratio, angiographic score, and capillary density.

CONCLUSIONS

Angiogenesis responsible for collateral development in limb ischemia is impaired with aging; responsible mechanisms include age-related endothelial dysfunction and reduced VEGF expression. Advanced age, however, does not preclude augmentation of collateral vessel development in response to exogenous angiogenic cytokines.

Systemic administration of the nitric oxide (NO) synthase inhibitor Nomega-nitro--arginine methyl ester (L-NAME) to rabbits bearing a corneal implant blocked vascular endothelial growth factor (VEGF), but not basic fibroblast growth factor (bFGF)-induced angiogenesis. L-NAME completely blocked angiogenesis induced by VEGF-transfected MCF-7 breast carcinoma cells and the cells remained dormant in the cornea. Postcapillary endothelial cell migration and growth induced by VEGF were blocked by both the NO synthase inhibitor Nomega-mono-methyl--arginine and by the guanylate cyclase inhibitor LY 83583. We conclude that NO is a downstream imperative of VEGF-, but not bFGF-induced angiogenesis, and propose that the NO synthase/guanylate cyclase pathway is a potential target for controlling tumor angiogenesis in response to VEGF. Our studies support recent evidence that VEGF and bFGF induce angiogenesis by different mechanistic pathways using the alphavbeta5 and alphavbeta3 integrins, respectively.