VEGF-dependent plasticity of fenestrated capillaries in the normal adult microvasculature

Vascular endothelial growth factor mediates the estrogen-induced breakdown of tight junctions between and increase in proliferation of microvessel endothelial cells in the baboon endometrium

To assess whether there is a link between estrogen, vascular endothelial growth factor (VEGF), and early aspects of uterine angiogenesis, an acute temporal study was conducted in which ovariectomized baboons were pretreated with VEGF Trap, which sequesters endogenous VEGF, and administered estradiol at time 0 h. Serum estradiol levels approximated 500 pg/ml 4-6 h after estradiol administration. VEGF mRNA levels in endometrial glandular epithelial and stromal cells were increased to values 6 h after estradiol that were 3.74 +/- 0.99-fold (mean +/- se) and 5.70 +/- 1.60-fold greater (P < 0.05), respectively, than at 0 h. Microvessel interendothelial cell tight junctions, which control paracellular permeability, were present in the endometrium at time 0 h, but not evident 6 h after estradiol administration. Thus, microvessel paracellular cleft width increased (P < 0.01, ANOVA) from 5.03 +/- 0.22 nm at 0 h to 7.27 +/- 0.48 nm 6 h after estrogen. In contrast, tight junctions remained intact, and paracellular cleft widths were unaltered in estradiol/VEGF Trap and vehicle-treated animals. Endometrial microvessel endothelial cell mitosis, i.e. percent Ki67+/Ki67- immunolabeled endothelial cells, increased (P < 0.05) from 2.9 +/- 0.3% at 0 h to 21.4 +/- 7.0% 6 h after estrogen treatment but was unchanged in estradiol/VEGF Trap and vehicle-treated animals. In summary, the estrogen-induced disruption of endometrial microvessel endothelial tight junctions and increase in endothelial cell proliferation were prevented by VEGF Trap. Therefore, we propose that VEGF mediates the estrogen-induced increase in microvessel permeability and endothelial cell proliferation as early steps in angiogenesis in the primate endometrium.

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.

The FGF system has a key role in regulating vascular integrity

The integrity of the endothelial monolayer is essential to blood vessel homeostasis and active regulation of endothelial permeability. The FGF system plays important roles in a wide variety of physiologic and pathologic conditions; however, its role in the adult vasculature has not been defined. To assess the role of the FGF system in the adult endothelial monolayer, we disrupted FGF signaling in bovine aortic endothelial cells and human saphenous vein endothelial cells in vitro and in adult mouse and rat endothelial cells in vivo using soluble FGF traps or a dominant inhibitor of all FGF receptors. The inhibition of FGF signaling using these approaches resulted in dissociation of the VE-cadherin/p120-catenin complex and disassembly of adherens and tight junctions, which progressed to loss of endothelial cells, severe impairment of the endothelial barrier function, and finally, disintegration of the vasculature. Thus, FGF signaling plays a key role in the maintenance of vascular integrity.

Transgenic induction of vascular endothelial growth factor-C is strongly angiogenic in mouse embryos but leads to persistent lymphatic hyperplasia in adult tissues

Vascular endothelial growth factor-C (VEGF-C) is the quintessential lymphangiogenic growth factor that is required for the development of the lymphatic system and is capable of stimulating lymphangiogenesis in adults by activating its receptor, VEGFR-3. Although VEGF-C is a major candidate molecule for the development of prolymphangiogenic therapy for defective lymphatic vessels in lymphedema, the stability of lymph vessels generated by exogenous VEGF-C administration is not currently known. We studied VEGF-C-stimulated lymphangiogenesis in inducible transgenic mouse models in which growth factor expression can be spatially and temporally controlled without side effects, such as inflammation. VEGF-C induction in adult mouse skin for 1 to 2 weeks caused robust lymphatic hyperplasia that persisted for at least 6 months. VEGF-C induced lymphangiogenesis in numerous tissues and organs when expressed in the vascular endothelium in either neonates or adult mice. Very few or no effects were observed in either blood vessels or collecting lymph vessels. Additionally, VEGF-C stimulated lymphangiogenesis in embryos after the onset of lymphatic vessel development. Strikingly, a strong angiogenic effect was observed after VEGF-C induction in vascular endothelium at any point before embryonic day 16.5. Our results indicate that blood vessels can undergo VEGF-C-induced angiogenesis even after down-regulation of VEGFR-3 in embryos; however, transient VEGF-C expression in adults can induce long-lasting lymphatic hyperplasia with no obvious side effects on the blood vasculature.

Nonclinical safety evaluation of sunitinib: a potent inhibitor of VEGF, PDGF, KIT, FLT3, and RET receptors

Sunitinib malate (SUTENT) is a multitargeted receptor tyrosine kinase (RTK) inhibitor that is approved multinationally for the treatment of imatinib-resistant/-intolerant gastrointestinal stromal tumor and advanced renal cell carcinoma. This paper characterizes the organ toxicity of sunitinib in Sprague-Dawley rats and cynomolgus monkeys, and the reversibility of any treatment-induced effects. Rats and monkeys received sunitinib (0-15 and 0-20 mg/kg/day, respectively) orally on a consecutive daily dosing schedule for thirteen weeks or on an intermittent daily dosing schedule for up to nine months. Clinical observations and laboratory parameters were recorded. Necropsy was conducted following treatment/recovery periods, and histologic examinations were performed. In rats, sunitinib was generally tolerated at 0.3 and 1.5 mg/kg/day, and findings were reversible. In monkeys, the level at which there were no observed adverse effects was 1.5 mg/kg/day, and findings were similarly reversible (except for uterine/ovarian weight changes and skin pallor). Data suggest that inhibition of multiple RTK pathways may induce pharmacologic effects on organ systems in nonclinical species. Key pharmacologic effects of sunitinib included reversible inhibition of neovascularization into the epiphyseal growth plate, and impaired corpora lutea formation and uterine development during estrus. Similar observations have been noted with this class of RTK signaling inhibitors and are consistent with pharmacologic perturbations of physiologic/angiogenic processes associated with the intended molecular targets.

Endogenous VEGF is required for visual function: evidence for a survival role on müller cells and photoreceptors

Background

Vascular endothelial growth factor (VEGF) is well known for its role in normal and pathologic neovascularization. However, a growing body of evidence indicates that VEGF also acts on non-vascular cells, both developmentally as well as in the adult. In light of the widespread use of systemic and intraocular anti-VEGF therapies for the treatment of angiogenesis associated with tumor growth and wet macular degeneration, systematic investigation of the role of VEGF in the adult retina is critical.

Methods and Findings

Using immunohistochemistry and Lac-Z reporter mouse lines, we report that VEGF is produced by various cells in the adult mouse retina and that VEGFR2, the primary signaling receptor, is also widely expressed, with strong expression by Müller cells and photoreceptors. Systemic neutralization of VEGF was accomplished in mice by adenoviral expression of sFlt1. After 14 days of VEGF neutralization, there was no effect on the inner and outer retina vasculature, but a significant increase in apoptosis of cells in the inner and outer nuclear layers. By four weeks, the increase in neural cell death was associated with reduced thickness of the inner and outer nuclear layers and a decline in retinal function as measured by electroretinograms. siRNA-based suppression of VEGF expression in a Müller cell line in vitro supports the existence of an autocrine role for VEGF in Müller cell survival. Similarly, the addition of exogenous VEGF to freshly isolated photoreceptor cells and outer-nuclear-layer explants demonstrated VEGF to be highly neuroprotective.

Conclusions

These results indicate an important role for endogenous VEGF in the maintenance and function of adult retina neuronal cells and indicate that anti-VEGF therapies should be administered with caution.

Blood glucose levels in patients with metastatic renal cell carcinoma treated with sunitinib

Sunitinib, a multitargeted tyrosine-kinase inhibitor, extends survival of patients with metastatic renal cell carcinoma (mRCC) and gastrointestinal stromal tumours. Between October 2005 and March 2007, we retrospectively reviewed blood glucose level variations associated with sunitinib therapy in patients treated for mRCC. Nineteen of the patients had type II diabetes. All 19 patients had a decrease in blood glucose level (mean 1.77 mmol l(-1)) after 4 weeks of treatment. This was followed by re-elevation in the 2-week rest period. After two cycles of sunitinib administration, two patients had stopped blood glucose-lowering drugs whereas five other patients had normalised their blood glucose level. On the basis of pre-clinical data, we hypothesise that several mechanisms could be involved in this process, such as capillary regression of pancreatic islets, IGF-1 modulation through HIF1-alpha or NF-kappaB activation. In addition, a decrease of glucose uptake in the context of concomitant gastrointestinal toxicity cannot be excluded. Glycaemic control should be carefully evaluated in diabetic patients treated with sunitinib, and routine monitoring is warranted.

Thyrotoxicosis during sunitinib treatment for renal cell carcinoma

CONTEXT

Sunitinib malate is an oral tyrosine kinase inhibitor used in the treatment of renal cell carcinoma (RCC) and gastrointestinal stromal tumours. Hypothyroidism has been observed in patients treated with sunitinib, but the mechanism whereby sunitinib induces hypothyroidism is unknown.

OBJECTIVE

To describe a series of six patients who developed thyrotoxicosis while on sunitinib for metastatic RCC.

SETTING

The study was conducted at Austin Health, a tertiary teaching hospital in Melbourne, Australia.

RESULTS

Two patients developed severe thyrotoxicosis within 10 weeks after commencing sunitinib. In contrast, in the four patients who presented with later onset (16-30 weeks) thyrotoxicosis, the thyrotoxicosis was relatively mild, self-limiting and rapidly progressed to hypothyroidism. These patients experienced recurrent episodes of thyrotoxicosis in temporal relation to their cyclical sunitinib treatment. One patient had cytological evidence of lymphocytic thyroiditis.

CONCLUSIONS

These findings suggest that sunitinib-induced hypothyroidism may be a consequence of preceding thyroiditis with associated transient thyrotoxicosis. As predictive factors are currently unknown, we suggest regular monitoring of thyroid function in all patients commenced on sunitinib. Clinicians treating patients with sunitinib or other similar kinase inhibitors should to be alerted to thyroid dysfunction as a potential toxicity of these agents.

Vascular endothelial growth factor-C, a potential paracrine regulator of glomerular permeability, increases glomerular endothelial cell monolayer integrity and intracellular calcium

We have previously reported expression of vascular endothelial growth factor (VEGF)-A and -C in glomerular podocytes and actions of VEGF-A on glomerular endothelial cells (GEnC) that express VEGF receptor-2 (VEGFR-2). Here we define VEGFR-3 expression in GEnC and investigate the effects of the ligand VEGF-C. Renal cortex and cultured GEnC were examined by microscopy, and both cell and glomerular lysates were assessed by Western blotting. VEGF-C effects on trans-endothelial electrical resistance and albumin flux across GEnC monolayers were measured. The effects of VEGF-C156S, a VEGFR-3-specific agonist, and VEGF-A were also studied. VEGF-C effects on intracellular calcium ([Ca2+]i) were measured using a fluorescence technique, receptor phosphorylation was examined by immunoprecipitation assays, and phosphorylation of myosin light chain-2 and VE-cadherin was assessed by blotting with phospho-specific antibodies. GEnC expressed VEGFR-3 in tissue sections and culture, and VEGF-C increased trans-endothelial electrical resistance in a dose-dependent manner with a maximal effect at 120 minutes of 6.8 Omega whereas VEGF-C156S had no effect. VEGF-C reduced labeled albumin flux by 32.8%. VEGF-C and VEGF-A increased [Ca2+]i by 15% and 39%, respectively. VEGF-C phosphorylated VEGFR-2 but not VEGFR-3, myosin light chain-2, or VE-cadherin. VEGF-C increased GEnC monolayer integrity and increased [Ca2+]i, which may be related to VEGF-C-S particular receptor binding and phosphorylation induction characteristics. These observations suggest that podocytes direct GEnC behavior through both VEGF-C and VEGF-A.

Role of cell and matrix-bound VEGF isoforms in lens development

PURPOSE

To determine the role of vascular endothelial growth factor (VEGF) in embryonic eye development and lens differentiation.

METHODS

Expression of components of the VEGF signaling pathway during lens development and in adults was characterized by beta-galactosidase staining of VEGF-LacZ mice, immunohistochemistry, and real-time (q) PCR. Embryonic eyes from wild-type mice and VEGF120/120 mice were analyzed by light microscopy and immunohistochemistry. VEGF function during lens development was analyzed using eye explants treated with VEGF-neutralizing antibody. Direct function of VEGF was demonstrated on the human lens epithelial cell line, HLE-B3.

RESULTS

Embryonic lens epithelium and posterior lens fibers expressed VEGF and VEGFR2. qPCR revealed VEGF164 as the major isoform in embryonic lens. Transgenic mice expressing only VEGF120 (VEGF120/120 mice) showed major defects in eye development, including microphthalmia, failed lens differentiation, and hyperplastic hyaloid vessels. The lens displayed abnormal cell patterning and differentiation associated with altered c-Maf, Prox1, and p57 expression pattern in the anterior epithelium. The number of proliferating epithelial cells was drastically reduced in VEGF120/120 lenses. Altered MIP26 cellular localization and reduced E-cadherin expression in the lens epithelium were observed. VEGF-neutralization led to reduced fiber elongation of eye explants. Exogenous VEGF increased survival and proliferation of HLE-B3 cell in a dose-dependent manner.

CONCLUSIONS

Abnormalities in ocular development in VEGF120/120 mice suggest a role for VEGF not only in the formation of ocular vascular beds but also in the differentiation of the lens itself.

The clinical implications of sunitinib-induced hypothyroidism: a prospective evaluation

Sunitinib is approved for the treatment of metastatic renal cell carcinoma (RCC) and imatinib-resistant or -intolerant gastrointestinal stromal tumours (GIST). Several studies have identified unexpected rates of thyroid dysfunction with sunitinib treatment. We performed a prospective observational study with the aim of more accurately defining the incidence and severity of hypothyroidism in RCC or GIST patients receiving sunitinib. Thyroid function was assessed at baseline and on days 1 and 28 of each treatment cycle. Thyroid antibodies were assessed at baseline and during follow-up if abnormal thyroid function tests were recorded. Sixteen patients (27%) developed sub- or clinical hypothyroidism and required hormone replacement and 20 patients (34%) showed at least one elevated thyroid-stimulating hormone not requiring therapeutic intervention. Twenty patients (34%) did not develop any biochemical thyroid abnormality. Thus, sunitinib can induce (sub-) clinical hypothyroidism, warranting close monitoring of thyroid function. We propose a new algorithm for managing this side effect in clinical practise.

Prolonged blockade of VEGF family members does not cause identifiable damage to retinal neurons or vessels

Several ocular diseases complicated by neovascularization are being treated by repeated intraocular injections of vascular endothelial growth factor (VEGF) antagonists. While substantial benefits have been documented, there is concern that unrecognized damage may be occurring, because blockade of VEGF may damage the fenestrated vessels of the choroicapillaris and deprive retinal neurons of input from a survival factor. One report has suggested that even temporary blockade of all isoforms of VEGF-A results in significant loss of retinal ganglion cells. In this study, we utilized double transgenic mice with doxycycline-inducible expression of soluble VEGF receptor 1 coupled to an Fc fragment (sVEGFR1Fc), a potent antagonist of several VEGF family members, including VEGF-A, to test the effects of VEGF blockade in the retina. Expression of sVEGFR1Fc completely blocked VEGF-induced retinal vascular permeability and significantly suppressed the development of choroidal neovascularization at rupture sites in Bruch's membrane, but did not cause regression of established choroidal neovascularization. Mice with constant expression of sVEGFR1Fc in the retina for 7 months had normal electroretinograms and normal retinal and choroidal ultrastructure including normal fenestrations in the choroicapillaris. They also showed no significant difference from control mice in the number of ganglion cell axons in optic nerve cross sections and the retinal level of mRNA for 3 ganglion cell-specific genes. These data indicate that constant blockade of VEGF for up to 7 months has no identifiable deleterious effects on the retina or choroid and support the use of VEGF antagonists in the treatment of retinal diseases.

Pegaptanib sodium for neovascular age-related macular degeneration: third-year safety results of the VEGF Inhibition Study in Ocular Neovascularisation (VISION) trial

Aims:

To evaluate the safety of up to 3 years of pegaptanib sodium therapy in the treatment of neovascular age-related macular degeneration (NV-AMD).

Methods:

Two concurrent, prospective, multicentre, double-masked studies randomised subjects with all angiographic lesion compositions of NV-AMD to receive intravitreous pegaptanib sodium (0.3, 1 and 3 mg) or sham injections every 6 weeks for 54 weeks. Those initially assigned to pegaptanib were rerandomised to continue or discontinue therapy for 48 more weeks; sham-treated subjects continued sham, discontinued or received pegaptanib. At 102 weeks, subjects receiving pegaptanib 0.3 mg or 1 mg in years 1 or 2 continued; those receiving pegaptanib 3 mg or who did not receive treatment in years 1 and 2 were rerandomised to 0.3 mg or 1 mg for year 3.

Results:

As in years 1 and 2, pegaptanib was well tolerated in year 3. Adverse events were mainly ocular in nature, mild, transient and injection-related. Serious adverse events were rare. No evidence of systemic safety signals attributed to vascular endothelial growth factor inhibition arose in year 3. There were no findings in relation to vital signs or electrocardiogram results suggesting a relationship to pegaptanib treatment.

Conclusion:

The 3-year safety profile of pegaptanib sodium was favourable in patients with NV-AMD.

VEGF blockade inhibits angiogenesis and reepithelialization of endometrium

Despite extensive literature on vascular endothelial growth factor (VEGF) expression and regulation by steroid hormones, the lack of clear understanding of the mechanisms of angiogenesis in the endometrium is a major limitation for use of antiangiogenic therapy targeting endometrial vessels. In the current work, we used the rhesus macaque as a primate model and the decidualized mouse uterus as a murine model to examine angiogenesis during endometrial breakdown and regeneration. We found that blockade of VEGF action with VEGF Trap, a potent VEGF blocker, completely inhibited neovascularization during endometrial regeneration in both models but had no marked effect on preexisting or newly formed vessels, suggesting that VEGF is essential for neoangiogenesis but not survival of mature vessels in this vascular bed. Blockade of VEGF also blocked reepithelialization in both the postmenstrual endometrium and the mouse uterus after decidual breakdown, evidence that VEGF has pleiotropic effects in the endometrium. In vitro studies with a scratch wound assay showed that the migration of luminal epithelial cells during repair involved signaling through VEGF receptor 2-neuropilin 1 (VEGFR2-NP1) receptors on endometrial stromal cells. The leading front of tissue growth during endometrial repair was strongly hypoxic, and this hypoxia was the local stimulus for VEGF expression and angiogenesis in this tissue. In summary, we provide novel experimental data indicating that VEGF is essential for endometrial neoangiogenesis during postmenstrual/postpartum repair.

Markers for microscopic imaging of lymphangiogenesis and angiogenesis

Imaging of lymphangiogenesis and angiogenesis requires robust and unambiguous markers of lymphatic and blood vessels. Although much progress has been made in recent years in identifying molecules specifically expressed on lymphatic and blood vessels, no perfect marker has been found that works reliably in all species, tissues, vascular beds, and in all physiological and pathologic conditions. The heterogeneity of expression of markers in both blood and lymphatic vessels reflects underlying differences in the phenotype of endothelial cells. Use of only one marker can lead to misleading interpretations, but these pitfalls can usually be avoided by use of multiple markers and three-dimensional whole-mount preparations. LYVE-1, VEGFR-3, Prox1, and podoplanin are among the most useful markers for microscopic imaging of lymphatic vessels, but, depending on histologic location, each marker can be expressed by other cell types, including vascular endothelial cells. Other markers, including CD31, junctional proteins, and receptors, such as VEGF-2, are shared by lymphatic and blood vessels.

Improved vascular engraftment and graft function after inhibition of the angiostatic factor thrombospondin-1 in mouse pancreatic islets

OBJECTIVE

Insufficient development of a new intra-islet capillary network after transplantation may be one contributing factor to the failure of islet grafts in clinical transplantation. The present study tested the hypothesis that the angiostatic factor thrombospondin-1 (TSP-1), which is normally present in islets, restricts intra-islet vascular expansion posttransplantation.

RESEARCH DESIGN AND METHODS

Pancreatic islets of TSP-1-deficient (TSP-1(-/-)) mice or wild-type islets transfected with siRNA for TSP-1 were transplanted beneath the renal capsule of syngeneic or immunocompromised recipient mice.

RESULTS

Both genetically TSP-1(-/-) islets and TSP-1 siRNA-transfected islet cells demonstrated an increased vascular density when compared with control islets 1 month after transplantation. This was also reflected in a markedly increased blood perfusion and oxygenation of the grafts. The functional importance of the improved vascular engraftment was analyzed by comparing glucose-stimulated insulin release from islet cells transfected with either TSP-1 siRNA or scramble siRNA before implantation. These experiments showed that the increased revascularization of grafts composed of TSP-1 siRNA-transfected islet cells correlated to increments in both their first and second phase of glucose-stimulated insulin secretion.

CONCLUSIONS

Our findings demonstrate that inhibition of TSP-1 in islets intended for transplantation may be a feasible strategy to improve islet graft revascularization and function.

Bevacizumab for the treatment of advanced non-small-cell lung cancer

Until recently, advances in non-small-cell lung cancer (NSCLC) care have been limited; new chemotherapy regimens have not significantly impacted patient survival. With our improved understanding of tumor biology, novel biological therapies targeting key tumorigenic processes targeting factors essential for tumor growth, such as angiogenesis, have been developed that improve patient outcomes beyond those achieved with chemotherapy alone. One of these, bevacizumab (Avastin), specifically targets VEGF, which is key to the malignant growth and progression of solid tumors. Bevacizumab-based therapy until progression significantly delays disease progression, has a well-characterized and acceptable safety profile in bevacizumab-eligible patients and was the first treatment to improve the overall survival of patients with advanced NSCLC beyond 1 year, a significant breakthrough in advanced NSCLC care. Furthermore, bevacizumab-based therapy significantly delays disease progression and has a well-characterized and acceptable safety profile. Based on these data, bevacizumab has received approval for the first-line treatment of NSCLC in the USA and Europe. A number of ongoing trials will potentially expand the eligible patient population for bevacizumab and further define its role in NSCLC treatment.

Axitinib is an active treatment for all histologic subtypes of advanced thyroid cancer: results from a phase II study

PURPOSE

Patients with advanced, incurable thyroid cancer not amenable to surgery or radioactive iodine ((131)I) therapy have few satisfactory therapeutic options. This multi-institutional study assessed the activity and safety of axitinib, an oral, potent, and selective inhibitor of vascular endothelial growth factor receptors (VEGFR) 1, 2, and 3 in patients with advanced thyroid cancer.

PATIENTS AND METHODS

Patients with thyroid cancer of any histology that was resistant or not appropriate for (131)I were enrolled onto a single-arm phase II trial to receive axitinib orally (starting dose, 5 mg twice daily). Objective response rate (ORR) by Response Evaluation Criteria in Solid Tumors was the primary end point. Secondary end points included duration of response, progression-free survival (PFS), overall survival, safety, and modulation of soluble (s) VEGFR.

RESULTS

Sixty patients were enrolled. Partial responses were observed in 18 patients, yielding an ORR of 30% (95% CI, 18.9 to 43.2). Stable disease lasting > or = 16 weeks was reported in another 23 patients (38%).

OBJECTIVE

responses were noted in all histologic subtypes. Median PFS was 18.1 months (95% CI, 12.1 to not estimable). Axitinib was generally well tolerated, with the most common grade > or = 3 treatment-related adverse event being hypertension (n = 7; 12%). Eight patients (13%) discontinued treatment because of adverse events. Axitinib selectively decreased sVEGFR-2 and sVEGFR-3 plasma concentrations versus sKIT, demonstrating its targeting of VEGFR.

CONCLUSION

Axitinib is a selective inhibitor of VEGFR with compelling antitumor activity in all histologic subtypes of advanced thyroid cancer.

Effect of sunitinib on growth and function of FRTL-5 thyroid cells

BACKGROUND

Sunitinib, a multitargeted vascular endothelial growth factor and receptor tyrosine kinase inhibitor, causes hypothyroidism in patients who take it for treatment of cancer. Although the pathophysiologic mechanism of the hypothyroidism is unclear, it has been claimed that it is due to inhibition of iodide uptake.

METHODS

To evaluate the pathologic mechanism of induction of the hypothyroidism, we studied the effect of sunitinib on FRTL-5 rat thyroid cells. We measured the effect of sunitinib on cell growth, (125)I-iodide uptake and efflux, TSH receptor (TSH-R), and sodium-iodide symporter (NIS) message.

RESULTS

At 48 hours, sunitinib caused a dose-related inhibition of growth with LC(50) of 14.6 muM, but there was no apparent inhibition of growth at 24 hours at concentrations of 0.1-25 microM. Preincubation with sunitinib did not impair the response to TSH, indicating that it did not affect the TSH-R. Incubation with sunitinib for 24 hours caused a dose-related increase of (125)I-iodide uptake and did not reduce iodide efflux or NIS mRNA expression.

CONCLUSION

The data indicate that sunitinib is unlikely to cause hypothyroidism by inhibition of iodide uptake.

Modulation of the antitumor activity of metronomic cyclophosphamide by the angiogenesis inhibitor axitinib

The promising but still limited efficacy of angiogenesis inhibitors as monotherapies for cancer treatment indicates a need to integrate these agents into existing therapeutic regimens. Presently, we investigate the antitumor activity of the small-molecule angiogenesis inhibitor axitinib (AG-013736) and its potential for combination with metronomic cyclophosphamide. Axitinib significantly inhibited angiogenesis in rat 9L tumors grown s.c. in scid mice but only moderately delayed tumor growth. Combination of axitinib with metronomic cyclophosphamide fully blocked 9L tumor growth on initiation of drug treatment. In contrast, metronomic cyclophosphamide alone required multiple treatment cycles to halt tumor growth. However, in contrast to the substantial tumor regression that is ultimately induced by metronomic cyclophosphamide, the axitinib/cyclophosphamide combination was tumor growth static. Axitinib did not inhibit hepatic activation of cyclophosphamide or export of its activated metabolite, 4-hydroxy-cyclophosphamide (4-OH-CPA), to extrahepatic tissues; rather, axitinib selectively decreased 9L tumor uptake of 4-OH-CPA by 30% to 40%. The reduced tumor penetration of 4-OH-CPA was associated with a decrease in cyclophosphamide-induced tumor cell apoptosis and a block in the induction of the endogenous angiogenesis inhibitor thrombospondin-1 in tumor-associated host cells, which may contribute to the absence of tumor regression with the axitinib/cyclophosphamide combination. Finally, axitinib transiently increased 9L tumor cell apoptosis, indicating that its effects are not limited to the endothelial cell population. These findings highlight the multiple effects that may characterize antiangiogenic agent/metronomic chemotherapy combinations and suggest that careful optimization of drug scheduling and dosages will be required to maximize antitumor responses.

Hantaviruses direct endothelial cell permeability by sensitizing cells to the vascular permeability factor VEGF, while angiopoietin 1 and sphingosine 1-phosphate inhibit hantavirus-directed permeability

Hantaviruses infect human endothelial cells and cause two vascular permeability-based diseases: hemorrhagic fever with renal syndrome and hantavirus pulmonary syndrome. Hantavirus infection alone does not permeabilize endothelial cell monolayers. However, pathogenic hantaviruses inhibit the function of alphav beta3 integrins on endothelial cells, and hemorrhagic disease and vascular permeability deficits are consequences of dysfunctional beta3 integrins that normally regulate permeabilizing vascular endothelial growth factor (VEGF) responses. Here we show that pathogenic Hantaan, Andes, and New York-1 hantaviruses dramatically enhance the permeability of endothelial cells in response to VEGF, while the nonpathogenic hantaviruses Prospect Hill and Tula have no effect on endothelial cell permeability. Pathogenic hantaviruses directed endothelial cell permeability 2 to 3 days postinfection, coincident with pathogenic hantavirus inhibition of alphav beta3 integrin functions, and hantavirus-directed permeability was inhibited by antibodies to VEGF receptor 2 (VEGFR2). These studies demonstrate that pathogenic hantaviruses, similar to alphav beta3 integrin-deficient cells, specifically enhance VEGF-directed permeabilizing responses. Using the hantavirus permeability assay we further demonstrate that the endothelial-cell-specific growth factor angiopoietin 1 (Ang-1) and the platelet-derived lipid mediator sphingosine 1-phosphate (S1P) inhibit hantavirus directed endothelial cell permeability at physiologic concentrations. These results demonstrate the utility of a hantavirus permeability assay and rationalize the testing of Ang-1, S1P, and antibodies to VEGFR2 as potential hantavirus therapeutics. The central importance of beta3 integrins and VEGF responses in vascular leak and hemorrhagic disease further suggest that altering beta3 or VEGF responses may be a common feature of additional viral hemorrhagic diseases. As a result, our findings provide a potential mechanism for vascular leakage after infection by pathogenic hantaviruses and the means to inhibit hantavirus-directed endothelial cell permeability that may be applicable to additional vascular leak syndromes.

VEGF in biological control

Vascular endothelial growth factor A (VEGF-A) belongs to a family of heparin binding growth factors that include VEGF-B, VEGF-C, VEGF-D, and placental-like growth factor (PLGF). First discovered for its ability to regulate vascular endothelial cell permeability, VEGF is a well-known angiogenic factor that is important for vascular development and maintenance in all mammalian organs. The development of molecular tools and pharmacological agents to selectively inhibit VEGF function and block angiogenesis and/or vascular permeability has led to great promise in the treatment of various cancers, macular degeneration, and wound healing. However, VEGF is also important in animals for the regulation of angiogenesis, stem cell and monocyte/macrophage recruitment, maintenance of kidney and lung barrier functions and neuroprotection. In addition to its role in regulating endothelial cell proliferation, migration, and cell survival, VEGF receptors are also located on many non-endothelial cells and act through autrocrine pathways to regulate cell survival and function. The following review will discuss the role of VEGF in physiological angiogenesis as well as its role in non-angiogenic processes that take place in adult organs.

Optimizing the dose and schedule of anti-vascular endothelial growth factor antibodies in non-small-cell lung cancer

Lung cancer is the world's leading cause of cancer death. Most patients with non-small-cell lung cancer (NSCLC) present with advanced disease. Median survival is approximately 8-10 months for patients who receive standard platinum-based doublet therapy. In 2006 the FDA approved the anti-vascular endothelial growth factor (VEGF) antibody bevacizumab for patients with advanced, non-squamous, NSCLC based on the Eastern Cooperative Oncology Group E4599 trial. This trial demonstrated a 2-month improvement in overall survival when bevacizumab was added to carboplatin/paclitaxel. European investigators presented further data supporting improvement in progression-free survival with the use of bevacizumab and a cisplatin doublet in the Avastin in Lung Cancer (AVAiL) trial. Bevacizumab at doses of 7.5 mg/kg and 15 mg/kg are both effective and safe for patients with advanced NSCLC. Fatal hemorrhage has been reported for patients receiving the antiangiogenesis antibody. According to a retrospective study, the only significant clinical and radiographic variable associated with increased risk of pulmonary hemorrhage is the presence of cavitation. Common side effects include hypertension, proteinuria and minor mucosal bleeding. Bevacizumab monotherapy given every 21 days can be safely continued for patients without evidence of progression and for whom side effects of therapy are acceptable. Many questions remain, such as the role of the anti-VEGF antibody in early-stage disease, the safety of bevacizumab in patients with squamous histology NSCLC, and the benefit of combination therapy in elderly patients.

Oligosaccharides as anti-angiogenic agents

BACKGROUND

Several studies of drugs that inhibit tumour angiogenesis have shown improvements in the survival of cancer patients, thus validating angiogenesis as a clinically relevant target. Both intracellular and extracellular approaches have shown promising results in clinical situations.

OBJECTIVES

To compare and contrast oligosaccharide therapies and other anti-angiogenic compounds for their benefits and toxicity.

METHODS

Analysis of the relevant literature including presentations at recent conferences.

RESULTS

Receptor tyrosine kinase inhibitors are orally available but have a broad spectrum of activity which is associated with toxicity. Antibodies are associated with different toxicities, however, they are administered parenterally. Oligosaccharides that act as competitive inhibitors of heparan sulfate (HS) are in the early and late phases of clinical development. The advantage of oligosaccharides should be that they can be designed to target several angiogenic molecules, that they are relatively safe and that they can be administered subcutaneously at home. The key questions concerning their development focus on whether compounds with sufficient affinity and relative specificity can be generated, whether they are active at doses that do not perturb the coagulation cascade to a clinically dangerous level, whether the synthetic routes are scalable and, whether the current Phase III trials will yield positive results.

CONCLUSIONS

Saccharides represent a novel and exciting therapeutic approach that targets a spectrum of angiogenic molecules that cannot be inhibited through established drug development programmes.

Regulation of retinal blood flow in health and disease

Optimal retinal neuronal cell function requires an appropriate, tightly regulated environment, provided by cellular barriers, which separate functional compartments, maintain their homeostasis, and control metabolic substrate transport. Correctly regulated hemodynamics and delivery of oxygen and metabolic substrates, as well as intact blood-retinal barriers are necessary requirements for the maintenance of retinal structure and function. Retinal blood flow is autoregulated by the interaction of myogenic and metabolic mechanisms through the release of vasoactive substances by the vascular endothelium and retinal tissue surrounding the arteriolar wall. Autoregulation is achieved by adaptation of the vascular tone of the resistance vessels (arterioles, capillaries) to changes in the perfusion pressure or metabolic needs of the tissue. This adaptation occurs through the interaction of multiple mechanisms affecting the arteriolar smooth muscle cells and capillary pericytes. Mechanical stretch and increases in arteriolar transmural pressure induce the endothelial cells to release contracting factors affecting the tone of arteriolar smooth muscle cells and pericytes. Close interaction between nitric oxide (NO), lactate, arachidonic acid metabolites, released by the neuronal and glial cells during neural activity and energy-generating reactions of the retina strive to optimize blood flow according to the metabolic needs of the tissue. NO, which plays a central role in neurovascular coupling, may exert its effect, by modulating glial cell function involved in such vasomotor responses. During the evolution of ischemic microangiopathies, impairment of structure and function of the retinal neural tissue and endothelium affect the interaction of these metabolic pathways, leading to a disturbed blood flow regulation. The resulting ischemia, tissue hypoxia and alterations in the blood barrier trigger the formation of macular edema and neovascularization. Hypoxia-related VEGF expression correlates with the formation of neovessels. The relief from hypoxia results in arteriolar constriction, decreases the hydrostatic pressure in the capillaries and venules, and relieves endothelial stretching. The reestablished oxygenation of the inner retina downregulates VEGF expression and thus inhibits neovascularization and macular edema. Correct control of the multiple pathways, such as retinal blood flow, tissue oxygenation and metabolic substrate support, aiming at restoring retinal cell metabolic interactions, may be effective in preventing damage occurring during the evolution of ischemic microangiopathies.

VEGF and TGF-beta are required for the maintenance of the choroid plexus and ependyma

Although the role of vascular endothelial growth factor (VEGF) in developmental and pathological angiogenesis is well established, its function in the adult is less clear. Similarly, although transforming growth factor (TGF) β is involved in angiogenesis, presumably by mediating capillary (endothelial cell [EC]) stability, its involvement in quiescent vasculature is virtually uninvestigated. Given the neurological findings in patients treated with VEGF-neutralizing therapy (bevacizumab) and in patients with severe preeclampsia, which is mediated by soluble VEGF receptor 1/soluble Fms-like tyrosine kinase receptor 1 and soluble endoglin, a TGF-β signaling inhibitor, we investigated the roles of VEGF and TGF-β in choroid plexus (CP) integrity and function in adult mice. Receptors for VEGF and TGF-β were detected in adult CP, as well as on ependymal cells. Inhibition of VEGF led to decreased CP vascular perfusion, which was associated with fibrin deposition. Simultaneous blockade of VEGF and TGF-β resulted in the loss of fenestrae on CP vasculature and thickening of the otherwise attenuated capillary endothelium, as well as the disappearance of ependymal cell microvilli and the development of periventricular edema. These results provide compelling evidence that both VEGF and TGF-β are involved in the regulation of EC stability, ependymal cell function, and periventricular permeability.

Biological therapy and other novel therapies in early-stage disease: are they appropriate?

For nearly two decades, adjuvant chemotherapy has been the standard of care in patients with early-stage colon cancer at high risk of recurrence. Until now, treatment has been based on the use of cytotoxic drugs that have well-demonstrated efficacy in advanced colorectal cancer. Most recently, targeted biological agents [i.e., antibodies against the epidermal growth factor receptor and vascular endothelial growth factor] have become essential components of the palliative medical treatment of colorectal cancer. Proof of efficacy of these agents in advanced disease has led to the initiation of several trials testing epidermal growth factor receptor and vascular endothelial growth factor antibodies in the adjuvant setting. Although definitive results of ongoing adjuvant studies will not be available for 2 to 3 years, some oncologists might already inappropriately consider the use of these targeted agents as a component of adjuvant therapy in selected patients. Whether the results obtained in advanced colorectal cancer can be readily translated into a projected efficacy in early-stage colon cancer, however, is unclear. In addition, the long-term safety of biological agents in potentially surgically cured patients has not yet been established. This review discusses the potential caveats and concerns associated with the uncritical use of targeted agents as adjuvant therapy before their safety and efficacy in this setting has been indisputably established in definitive phase III trials.

Coordinated vascular endothelial growth factor expression and signaling during skeletal myogenic differentiation

Angiogenesis is largely controlled by hypoxia-driven transcriptional up-regulation and secretion of vascular endothelial growth factor (VEGF) and its binding to the endothelial cell tyrosine receptor kinases, VEGFR1 and VEGFR2. Recent expression analysis suggests that VEGF is expressed in a cell-specific manner in normoxic adult tissue; however, the transcriptional regulation and role of VEGF in these tissues remains fundamentally unknown. In this report we demonstrate that VEGF is coordinately up-regulated during terminal skeletal muscle differentiation. We reveal that this regulation is mediated in part by MyoD homo- and hetero-dimeric transcriptional mechanisms. Serial deletions of the VEGF promoter elucidated a region containing three tandem CANNTG consensus MyoD sites serving as essential sites of direct interaction for MyoD-mediated up-regulation of VEGF transcription. VEGF-null embryonic stem (ES) cells exhibited reduced myogenic differentiation compared with wild-type ES cells, suggesting that VEGF may serve a role in skeletal muscle differentiation. We demonstrate that VEGFR1 and VEGFR2 are expressed at low levels in myogenic precursor cells and are robustly activated upon VEGF stimulation and that their expression is coordinately regulated during skeletal muscle differentiation. VEGF stimulation of differentiating C2C12 cells promoted myotube hypertrophy and increased myogenic differentiation, whereas addition of sFlt1, a VEGF inhibitor, resulted in myotube hypotrophy and inhibited myogenic differentiation. We further provide evidence indicating VEGF-mediated myogenic marker expression, mitogenic activity, migration, and prosurvival functions may contribute to increased myogenesis. These data suggest a novel mechanism whereby VEGF is coordinately regulated as part of the myogenic differentiation program and serves an autocrine function regulating skeletal myogenesis.

VEGF Trap complex formation measures production rates of VEGF, providing a biomarker for predicting efficacious angiogenic blockade

VEGF is the best characterized mediator of tumor angiogenesis. Anti-VEGF agents have recently demonstrated impressive efficacy in human cancer trials, but the optimal dosing of such agents must still be determined empirically, because biomarkers to guide dosing have yet to be established. The widely accepted (but unverified) assumption that VEGF production is quite low in normal adults led to the notion that increased systemic VEGF levels might quantitatively reflect tumor mass and angiogenic activity. We describe an approach to determine host and tumor production of VEGF, using a high-affinity and long-lived VEGF antagonist now in clinical trials, the VEGF Trap. Unlike antibody complexes that are usually rapidly cleared, the VEGF Trap forms inert complexes with tissue- and tumor-derived VEGF that remain stably in the systemic circulation, where they are readily assayable, providing unprecedented capability to accurately measure VEGF production. We report that VEGF production is surprisingly high in non-tumor-bearing rodents and humans, challenging the notion that systemic VEGF levels can serve as a sensitive surrogate for tumor load; tumor VEGF contribution becomes significant only with very large tumor loads. These findings have the important corollary that anti-VEGF therapies must be sufficiently dosed to avoid diversion by host-derived VEGF. We further show that our assay can indicate when VEGF is optimally blocked; such biomarkers to guide dosing do not exist for other anti-VEGF agents. Based on this assay, VEGF Trap doses currently being assessed in clinical trials are in the efficacious range.

Endothelial stomatal and fenestral diaphragms in normal vessels and angiogenesis

Vascular endothelium lines the entire cardiovascular system where performs a series of vital functions including the control of microvascular permeability, coagulation inflammation, vascular tone as well as the formation of new vessels via vasculogenesis and angiogenesis in normal and disease states. Normal endothelium consists of heterogeneous populations of cells differentiated according to the vascular bed and segment of the vascular tree where they occur. One of the cardinal features is the expression of specific subcellular structures such as plas-malemmal vesicles or caveolae, transendothelial channels, vesiculo-vacuolar organelles, endothelial pockets and fenestrae, whose presence define several endothelial morphological types. A less explored observation is the differential expression of such structures in diverse settings of angiogenesis. This review will focus on the latest developments on the components, structure and function of these specific endothelial structures in normal endothelium as well as in diverse settings of angiogenesis.

The VEGF family, the inside story

The role of paracrine signaling by vascular endothelial growth factor (VEGF) in the formation and maintenance of blood vessels has been studied extensively. In this issue, Lee et al. (2007) report unexpected results showing that endogenous VEGF produced by endothelial cells is also crucial for vascular homeostasis.

Autocrine VEGF signaling is required for vascular homeostasis

Vascular endothelial growth factor (VEGF) is essential for developmental and pathological angiogenesis. Here we show that in the absence of any pathological insult, autocrine VEGF is required for the homeostasis of blood vessels in the adult. Genetic deletion of vegf specifically in the endothelial lineage leads to progressive endothelial degeneration and sudden death in 55% of mutant mice by 25 weeks of age. The phenotype is manifested without detectable changes in the total levels of VEGF mRNA or protein, indicating that paracrine VEGF could not compensate for the absence of endothelial VEGF. Furthermore, wild-type, but not VEGF null, endothelial cells showed phosphorylation of VEGFR2 in the absence of exogenous VEGF. Activation of the receptor in wild-type cells was suppressed by small molecule antagonists but not by extracellular blockade of VEGF. These results reveal a cell-autonomous VEGF signaling pathway that holds significance for vascular homeostasis but is dispensable for the angiogenic cascade.

Antiangiogenic therapy for normalization of atherosclerotic plaque vasculature: a potential strategy for plaque stabilization

Angiogenesis within human atherosclerotic plaques has an important role in plaque progression as immature blood vessels leak red blood cells and inflammatory mediators into the plaque center. Accumulation of free cholesterol from red blood cell membranes potentially increases the size of the necrotic core and triggers a chain of events that promote plaque destabilization. Antiangiogenic agents have been shown to prune some tumor vessels and 'normalize' the structure and function of the remaining vasculature, thereby improving the access of chemotherapeutic agents to tumors. We propose that antiangiogenic therapy can similarly stabilize vulnerable 'rupture-prone' plaques by pruning and normalizing immature intraplaque vessels, preventing further intraplaque hemorrhage. This normalization would limit necrotic core enlargement, further luminal narrowing and the degree of inflammation. Such normalization has been realized using vascular endothelial growth factor antagonists for the treatment of cancer and age-related macular degeneration. The development of this novel approach to prevent plaque progression might add to the armamentarium of preventive measures for acute myocardial infarction, stroke and sudden cardiac death.

Acute Pancreatitis Associated with Sorafenib

Since its FDA approval in December 2005, sorafenib (Nexavar) has been in use for the treatment of metastatic renal cell carcinoma. With this increased use have come reports of adverse effects of sorafenib. To the best of the authors' knowledge, they are the first to describe an 80-year-old Asian male with a history of metastatic renal cell carcinoma who developed acute pancreatitis confirmed by computed tomography (CT) one month after taking sorafenib 400 mg orally twice a day. Sorafenib was eventually discontinued, and the pancreatitis resolved. The molecular biologic mechanism causing this side effect is discussed. Patients should be informed of this rare but potentially serious adverse effect before initiation of sorafenib therapy. Early recognition of this complication and complete discontinuation of sorafenib are recommended.

Angiogenesis modulates adipogenesis and obesity

Substantial evidence shows that neoplastic and nonneoplastic tissue growth is dependent on angiogenesis. Neovascularization and adipogenesis are temporally and spatially coupled processes during prenatal life and they continue to reciprocally interact via paracrine signaling systems throughout adult life. Activated adipocytes produce multiple angiogenic factors including leptin, angiopoietins, HGF, GM-CSF, VEGF, FGF-2, and TGF-beta, which either alone or collectively stimulate neovascularization during fat mass expansion. Thus antiangiogenic agents provide a novel therapeutic option for prevention and treatment of human obesity and its related disorders.