Expression and endocytosis of VEGF and its receptors in human colonic vascular endothelial cells

Management of adverse events associated with cabozantinib plus nivolumab in renal cell carcinoma: A review

Tyrosine kinase inhibitors have been successfully developed in combination with immune checkpoint inhibitors to treat advanced renal cell carcinoma (RCC), further advancing treatment. While safety profiles are generally manageable with combination regimens, overlapping adverse events (AEs) and immune-related AEs can make treatment more complex. The CheckMate 9ER study evaluated the tyrosine kinase inhibitor cabozantinib in combination with the anti-programmed cell death protein-1 antibody nivolumab in patients with previously untreated advanced RCC. Cabozantinib + nivolumab demonstrated superiority over sunitinib for progression-free survival, overall survival, and objective response rate. These outcomes supported the approval of cabozantinib + nivolumab as a first-line therapy for advanced RCC. The safety profile was manageable with prophylaxis, supportive care, dose holds and reductions for cabozantinib, and dose holds and immunosuppressive therapy for nivolumab. This review discusses the safety results of CheckMate 9ER and provides guidance on managing some of the more clinically relevant AEs with a focus on overlapping AEs, including diarrhea, elevated amylase/lipase, hepatotoxicity, dermatologic reactions, fatigue, endocrine disorders, and nephrotoxicity. We discuss AE management strategies (prophylaxis, supportive care, dose modification, and immunosuppressive therapy), and provide recommendations for identifying the causative agent of overlapping AEs and for consulting specialists about organ-specific immune-related AEs. Optimizing AE management can maintain tolerability and should be a priority with cabozantinib + nivolumab treatment.

A Cellular Potts Model for Analyzing Cell Migration across Constraining Pillar Arrays

Cell migration in highly constrained environments is fundamental in a wide variety of physiological and pathological phenomena. In particular, it has been experimentally shown that the migratory capacity of most cell lines depends on their ability to transmigrate through narrow constrictions, which in turn relies on their deformation capacity. In this respect, the nucleus, which occupies a large fraction of the cell volume and is substantially stiffer than the surrounding cytoplasm, imposes a major obstacle. This aspect has also been investigated with the use of microfluidic devices formed by dozens of arrays of aligned polymeric pillars that limit the available space for cell movement. Such experimental systems, in particular, in the designs developed by the groups of Denais and of Davidson, were here reproduced with a tailored version of the Cellular Potts model, a grid-based stochastic approach where cell dynamics are established by a Metropolis algorithm for energy minimization. The proposed model allowed quantitatively analyzing selected cell migratory determinants (e.g., the cell and nuclear speed and deformation, and forces acting at the nuclear membrane) in the case of different experimental setups. Most of the numerical results show a remarkable agreement with the corresponding empirical data.

Cabozantinib for the Treatment of Advanced Hepatocellular Carcinoma: Current Data and Future Perspectives

Cabozantinib (Cabometyx®) is a potent multikinase inhibitor targeting the vascular endothelial growth factor (VEGF) receptor 2, the mesenchymal-epithelial transition factor (MET) receptor, and the "anexelekto" (AXL) receptor tyrosine kinase. It is approved for the treatment of advanced hepatocellular carcinoma (HCC) after failure of sorafenib in Europe (since November 2018) and in the USA (since January 2019). The approval of cabozantinib was based on results of the randomized, placebo-controlled, phase 3 CELESTIAL trial in patients with unresectable HCC, who received one or two prior lines of treatment including sorafenib. At the second planned interim analysis, the trial was stopped, because the primary end point overall survival was clearly in favor for cabozantinib. Additionally, median progression-free survival was superior to placebo. The most common ≥ grade 3 relevant adverse events in patients with HCC treated with cabozantinib were palmar-plantar erythrodysesthesia, hypertension, fatigue, and diarrhea. In this review, current data on cabozantinib for the treatment of patients with advanced HCC, with a focus on the management of common adverse events and ongoing clinical trials, are discussed.

Receptor-Mediated Endocytosis of VEGF-A in Rat Liver Sinusoidal Endothelial Cells

Background and Aims

Vascular endothelial growth factor (VEGF) receptors (VEGFR1 and VEGFR2) bind VEGF-A with high affinity. This study sought to determine the relative contributions of these two receptors to receptor-mediated endocytosis of VEGF-A and to clarify their endocytic itineraries in rat liver sinusoidal endothelial cells (LSECs).

Methods

Isolated LSECs and radiolabeled VEGF-A were used to examine surface binding and receptor-mediated endocytosis. Quantitative real time RT-PCR (Q-RT-PCR) and Western blotting were applied to demonstrate receptor expression.

Results

Q-RT-PCR analysis showed that VEGFR1 and VEGFR2 mRNA were expressed in LSECs. Ligand saturation analysis at 4°C indicated two different classes of [¹²⁵I]-VEGFA binding sites on LSECs with apparent dissociation constants of 8 and 210 pM. At 37°C, LSECs efficiently took up and degraded [¹²⁵I]-VEGF-A for at least 2 hours. Uptake of [¹²⁵I]-VEGF-A by LSECs was blocked by dynasore that inhibits dynamin-dependent internalization, whereas inhibition of cysteine proteases by leupeptin inhibited degradation without affecting the uptake of [¹²⁵I]-VEGF-A, suggesting that it is degraded following transport to lysosomes. Incubation of LSECs in the continued presence of a saturating concentration of unlabeled VEGF-A at 37°C was associated with a loss of as much as 75% of the total VEGFR2 within 30 min as shown by Western blot analysis, whereas there was no appreciable decrease in protein levels for VEGFR1 after 120 min incubation, suggesting that VEGF-A stimulation downregulates VEGFR2, but not VEGFR1, in LSECs. This possibility was supported by the observation that a hexapeptide that specifically blocks VEGF-A binding to VEGFR1 caused a marked reduction in the uptake of [¹²⁵I]-VEGF-A, whereas a control peptide had no effect. Finally, live cell imaging studies using a fluorescently labeled anti-VEGFR2 antibody showed that VEGFR2 was transported via early and late endosomes to reach endolysosomes where degradation of the VEGFR2 takes place.

Conclusion

Our studies suggest that, subsequent to VEGF-A binding and internalization, the unoccupied VEGFR1 may recycle to the cell surface allowing its reutilization, whereas the majority of the internalized VEGFR2 is targeted for degradation.

Management of adverse events associated with tyrosine kinase inhibitors: Improving outcomes for patients with hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is the most common form of primary liver cancer. Sorafenib, regorafenib, lenvatinib and cabozantinib are tyrosine kinase inhibitors (TKIs) that target, in part, vascular endothelial growth factor receptors, and are approved in various regions of the world for the treatment of advanced HCC. All these agents are associated with a range of adverse events (AEs) that can have a substantial impact on patients' health-related quality of life. Fatigue, diarrhoea, hand-foot skin reaction, nausea, vomiting, decreased appetite, hypertension and weight loss are among the most common AEs experienced with these four TKIs. In this review, we discuss strategies for the management of these AEs in patients with advanced HCC, with the aim of maximizing treatment benefits and minimizing the need for TKI treatment discontinuation. We also consider potential TKI-drug interactions and discuss the use of TKIs in patients with liver dysfunction or who have experienced tumour recurrence after liver transplantation. Use of appropriate AE management strategies and avoidance of contraindicated drugs should help patients with advanced HCC to achieve optimal outcomes with TKIs.

Modelling of endothelial cell migration and angiogenesis in microfluidic cell culture systems

Tumour-induced angiogenesis is a complex biological process that involves growth of new blood vessels within the tumour microenvironment and is an important target for cancer therapies. Significant efforts have been undertaken to develop theoretical models as well as in vitro experimental models to study angiogenesis in a highly controllable and accessible manner. Various mathematical models have been developed to study angiogenesis, but these have mostly been applied to in vivo assays. Recently, microfluidic cell culture systems have emerged as useful and powerful tools for studying cell migration and angiogenesis processes, but thus far, mathematical angiogenesis models have not yet been applied to microfluidic geometries. Integrating mathematical and computational modelling with microfluidic-based assays has potential to enable greater control over experimental parameters, provide new insights into fundamental angiogenesis processes and assist in accelerating design and optimization of operating conditions. Here, we describe the development and application of a combined mathematical and computational modelling approach tailored specifically for microfluidic cell culture systems. The objective was to allow optimization of the engineering design of microfluidic systems, where the model may be used to test the impact of various geometric parameters on cell migration and angiogenesis processes, and assist in identifying optimal device dimensions to achieve desired readouts. We employed two separate continuum mathematical models that treated cell density, vessel length density and vascular endothelial growth factor (VEGF) concentration as continuous average variables, and we implemented these models numerically using finite difference discretization and a Method of Lines approach. We examined the average response of cells to VEGF gradients inside our microfluidic device, including the time-dependent changes in cell density and vessel density, and how they were affected by changes in device geometries including the migration port width and length. Our study demonstrated that mathematical modelling can be integrated with microfluidics to offer new perspectives on emerging problems in biomicrofluidics and cancer biology.

The importance of geometry in the corneal micropocket angiogenesis assay

The corneal micropocket angiogenesis assay is an experimental protocol for studying vessel network formation, or neovascularization, in vivo. The assay is attractive due to the ease with which the developing vessel network can be observed in the same animal over time. Measurements from the assay have been used in combination with mathematical modeling to gain insights into the mechanisms of angiogenesis. While previous modeling studies have adopted planar domains to represent the assay, the hemispherical shape of the cornea and asymmetric positioning of the angiogenic source can be seen to affect vascular patterning in experimental images. As such, we aim to better understand: i) how the geometry of the assay influences vessel network formation and ii) how to relate observations from planar domains to those in the hemispherical cornea. To do so, we develop a three-dimensional, off-lattice mathematical model of neovascularization in the cornea, using a spatially resolved representation of the assay for the first time. Relative to the detailed model, we predict that the adoption of planar geometries has a noticeable impact on vascular patterning, leading to increased vessel 'merging', or anastomosis, in particular when circular geometries are adopted. Significant differences in the dynamics of diffusible aniogenesis simulators are also predicted between different domains. In terms of comparing predictions across domains, the 'distance of the vascular front to the limbus' metric is found to have low sensitivity to domain choice, while metrics such as densities of tip cells and vessels and 'vascularized fraction' are sensitive to domain choice. Given the widespread adoption and attractive simplicity of planar tissue domains, both in silico and in vitro, the differences identified in the present study should prove useful in relating the results of previous and future theoretical studies of neovascularization to in vivo observations in the cornea.

Management of Adverse Events Associated with Cabozantinib Therapy in Renal Cell Carcinoma

Cabozantinib was recently approved for the treatment of advanced renal cell carcinoma (RCC) after treatment with vascular endothelial growth factor (VEGF)-targeted therapy. Cabozantinib is a multikinase inhibitor targeting VEGF receptor (VEGFR) 2, mesenchymal-epithelial transition receptor, and "anexelekto" receptor tyrosine kinase. A 60-mg daily dose led to improved overall survival and progression-free survival (PFS) versus everolimus in advanced RCC patients as a second- or later-line treatment in the METEOR trial. Improved PFS with cabozantinib versus sunitinib has also been demonstrated in the first-line setting in CABOSUN. However, cabozantinib, like other VEGFR inhibitors, is associated with toxicity that may affect the patient's quality of life. The most frequent adverse events (AEs) are diarrhea, fatigue, hypertension, hand-foot syndrome, weight loss, nausea, and stomatitis. This article summarizes the safety profile of cabozantinib in RCC patients and offers guidance for the management of these AEs. We discuss the underlying mechanisms of these AEs and, based on our experiences with cabozantinib and other multikinase inhibitors, we present approaches to manage toxicity. Prophylactic and therapeutic solutions are available to help with the management of toxicity associated with cabozantinib, and adequate interventions can ensure optimum adherence and maximize patient outcomes.

IMPLICATIONS FOR PRACTICE

Cabozantinib leads to improved survival outcomes in renal cell carcinoma patients compared with everolimus. However, management of the adverse event profile is crucial to achieve optimum adherence and outcomes with the use of cabozantinib. This review aims to provide appropriate guidance that will minimize the impact of adverse events and help to maximize the utility of this agent in patients with advanced renal cell carcinoma.

Vascular phenotype identification and anti-angiogenic treatment recommendation: A pseudo-multiscale mathematical model of angiogenesis

The development of anti-angiogenic drugs for cancer therapy has yielded some promising candidates, but novel approaches for interventions to angiogenesis have led to disappointing results. In addition, there is a shortage of biomarkers that are predictive of response to anti-angiogenic treatments. Consequently, the complex biochemical and physiological basis for tumour angiogenesis remains incompletely understood. We have adopted a mathematical approach to address these issues, formulating a spatially averaged multiscale model that couples the dynamics of VEGF, Ang1, Ang2 and PDGF, with those of mature and immature endothelial cells and pericyte cells. The model reproduces qualitative experimental results regarding pericyte coverage of vessels after treatment by anti-Ang2, anti-VEGF and combination anti-VEGF/anti-Ang2 antibodies. We used the steady state behaviours of the model to characterise angiogenic and non-angiogenic vascular phenotypes, and used mechanistic perturbations representing hypothetical anti-angiogenic treatments to generate testable hypotheses regarding transitions to non-angiogenic phenotypes that depend on the pre-treatment vascular phenotype. Additionally, we predicted a synergistic effect between anti-VEGF and anti-Ang2 treatments when applied to an immature pre-treatment vascular phenotype, but not when applied to a normalised angiogenic pre-treatment phenotype. Based on these findings, we conclude that changes in vascular phenotype are predicted to be useful as an experimental biomarker of response to treatment. Further, our analysis illustrates the potential value of non-spatial mathematical models for generating tractable predictions regarding the action of anti-angiogenic therapies.

Placenta growth factor and vascular endothelial growth factor a have differential, cell-type specific patterns of expression in vascular cells

OBJECTIVE

PLGF, a VEGF-A related protein, mediates collateral enlargement via monocytes but plays little role in capillary proliferation. In contrast, VEGF-A mediates both collateral enlargement and capillary proliferation. PLGF has been less thoroughly studied than VEGF-A, and questions remain regarding its regulation and function. Therefore, our goal was to characterize the expression of PLGF by vascular cells. We hypothesized that vascular SMC would express more PLGF than EC, since VEGF-A is primarily expressed by non-EC.

METHODS

We compared PLGF and VEGF-A across eight EC and SMC lines, then knocked down PLGF and evaluated cell function. We also assessed the effect of hypoxia on PLGF expression and promoter activity.

RESULTS

PLGF was most highly expressed in EC, whereas VEGF-A was most highly expressed in SMC. PLGF knockdown did not affect EC number, migration, or tube formation, but reduced monocyte migration toward EC. Monocyte migration was rescued by exogenous PLGF. Hypoxia increased PLGF protein without activating PLGF gene transcription.

CONCLUSIONS

PLGF and VEGF-A have distinct patterns of expression in vascular cells. EC derived PLGF may function primarily in communication between EC and circulating cells. Hypoxia increases EC PLGF expression posttranscriptionally.

A hybrid model of the role of VEGF binding in endothelial cell migration and capillary formation

Vascular endothelial growth factor (VEGF) is the most studied family of soluble, secreted mediators of endothelial cell migration, survival, and proliferation. VEGF exerts its function by binding to specific tyrosine kinase receptors on the cell surface and transducing the effect through downstream signaling. In order to study the influence of VEGF binding on endothelial cell motion, we develop a hybrid model of VEGF-induced angiogenesis, based on the theory of reinforced random walks. The model includes the chemotactic response of endothelial cells to angiogenic factors bound to cell-surface receptors, rather than approximating this as a function of extracellular chemical concentrations. This allows us to capture biologically observed phenomena such as activation and polarization of endothelial cells in response to VEGF gradients across their lengths, as opposed to extracellular gradients throughout the tissue. We also propose a novel and more biologically reasonable functional form for the chemotactic sensitivity of endothelial cells, which is also governed by activated cell-surface receptors. This model is able to predict the threshold level of VEGF required to activate a cell to move in a directed fashion as well as an optimal VEGF concentration for motion. Model validation is achieved by comparison of simulation results directly with experimental data.

Bioactive scaffolds for engineering vascularized cardiac tissues

Functional vascularization is a key requirement for the development and function of most tissues, and most critically cardiac muscle. Rapid and irreversible loss of cardiomyocytes during cardiac infarction directly results from the lack of blood supply. Contractile cardiac grafts, engineered using cardiovascular cells in conjunction with biomaterial scaffolds, are an actively studied method for cardiac repair. In this article, we focus on biomaterial scaffolds designed to mediate the development and maturation of vascular networks, by immobilized growth factors. The interactive effects of multiple vasculogenic factors are discussed in the context of cardiac tissue engineering.

Ricinus communis agglutinin I leads to rapid down-regulation of VEGFR-2 and endothelial cell apoptosis in tumor blood vessels

Ricinus communis agglutinin I (RCA I), a galactose-binding lectin from castor beans, binds to endothelial cells at sites of plasma leakage, but little is known about the amount and functional consequences of binding to tumor endothelial cells. We addressed this issue by examining the effects of RCA I on blood vessels of spontaneous pancreatic islet-cell tumors in RIP-Tag2 transgenic mice. After intravenous injection, RCA I bound strongly to tumor vessels but not to normal blood vessels. At 6 minutes, RCA I fluorescence of tumor vessels was largely diffuse, but over the next hour, brightly fluorescent dots appeared as the lectin was internalized by endothelial cells. RCA I injection led to a dose- and time-dependent decrease in vascular endothelial growth factor receptor-2 (VEGFR-2) immunoreactivity in tumor endothelial cells, with 95% loss over 6 hours. By comparison, VEGFR-3, CD31, and CD105 had decreases in the range of 21% to 33%. Loss of VEGFR-2 was followed by increased activated caspase-3 in tumor vessels. Prior inhibition of VEGF signaling by AG-028262 decreased RCA I binding and internalization into tumor vessels. These findings indicate RCA I preferentially binds to and is internalized by tumor endothelial cells, which leads to VEGFR-2 down-regulation, endothelial cell apoptosis, and tumor vessel regression. Together, the results illustrate the selective impact of RCA I on VEGF signaling in tumor blood vessels.

Endothelial cell activation in a VEGF-A gradient: relevance to cell fate decisions

Distribution of vascular endothelial cell growth factor A (VEGF-A) as a gradient determines microvascular endothelial cell (EC) fate during organogenesis. While much is understood about mechanisms of differential distribution, less is known about how EC perceive and interpret a graded VEGF-A signal to generate positional target gene activation. Using microvascular EC, we analyzed the effect of time and graded VEGF-A input on VEGFR2 autophosphorylation, signal kinase activation and induction of immediate-early genes. The threshold and time to peak activation of VEGFR2 were dependent on signal strength over a 50-fold range in concentration with 3-fold concentration differences readily distinguished. Longer duration of exposure did not compensate for low concentration of VEGF-A, suggesting intensity and duration of signal were not interpreted equivalently. With the same conditions, graded and time-sensitive information was transduced through the PLCgamma/p44/p42MAPK signal pathway but not the parallel AKT pathway. Analysis of MAPK-induced angiogenic immediate-early genes determined that EGR-1, EGR-3, and NR4A1 were dependent on graded input while NR4A2 and DSCR1 were independent with 'switch-like' induction. These data demonstrate rapid, linear integration of VEGF-A levels but independent interpretation of duration of signal and identify potential nodes for segregation of gradient-dependent and -independent responses. These results describe how microvascular EC fate decisions can be determined by comparatively moderate changes in VEGF signal strength, resulting in combinatorial changes in the repertoire of immediate-early genes for transcription effectors.

A cell-based model of endothelial cell migration, proliferation and maturation during corneal angiogenesis

The motivation of this work stems from two critical experimental observations associated with corneal angiogenesis: (1) angiogenesis will not succeed without endothelial cell proliferation, and (2) proliferation mainly occurs at the leading edge of developing sprouts (Sholley et al., Lab. Invest. 51:624-634, 1984). To discover the underlying mechanisms of these phenomena, we develop a cell-based mathematical model that integrates a mechanical model of elongation with a biochemical model of cell phenotype variation regulated by angiopoietins within a developing sprout. This model allows for a detailed study of the relative roles of endothelial cell migration, proliferation, and maturation. The model is validated by quantitatively comparing its predictions with data derived from corneal angiogenesis experiments. We conclude that cell elasticity and cell-to-cell adhesion allow only limited sprout extension in the absence of proliferation, and the maturation process combined with bioavailability of VEGF can explain the localization of proliferation to the leading edge. We also use this model to investigate the effects of X-ray irradiation, Ang-2 inhibition, and extracellular matrix anisotropy on sprout morphology and extension.

Minimal effects of VEGF and anti-VEGF drugs on the permeability or selectivity of RPE tight junctions

PURPOSE

Bevacizumab and ranibizumab are currently used to treat age-related macular degeneration by neutralizing vascular endothelial growth factor (VEGF). In this study, the potential side effects on the outer blood-retinal barrier were examined.

METHODS

Human fetal RPE (hfRPE) cells were used because they are highly differentiated in culture. The claudin composition of RPE tight junctions was determined by RT-PCR, immunoblot analysis, and immunofluorescence. ELISA assays monitored the secretion and trafficking of VEGF and a fluid-phase marker, methylpolyethylene glycol (mPEG). Tight junction functions were assessed by the conductance of K(+) and Na(+) (derived from the transepithelial electrical resistance, TER) and the flux of NaCl and mPEG.

RESULTS

Claudin-3, claudin-10, and claudin-19 were detected in RPE tight junctions. VEGF was secreted in equal amounts across the apical and basolateral membranes, but the apical membrane was more active in endocytosing and degrading VEGF. Exogenous VEGF and mPEG crossed the RPE monolayer by transcytosis, predominantly in the apical-to-basal direction. RPE tight junctions were selective for K(+), but did not discriminate between Na(+) and Cl(-). VEGF, bevacizumab, and ranibizumab had minimal effects on TER, permeation of mPEG, and selectivity for K(+), Na(+), and Cl(-). They had minimal effects on the expression and distribution of the claudins.

CONCLUSIONS

RPE has mechanisms for maintaining low concentrations of VEGF in the subretinal space that include endocytosis and degradation and fluid-phase transcytosis in the apical-to-basal direction. RPE tight junctions are selective for K(+) over Na(+) and Cl(-). Permeability and selectivity of the junctions are not affected by VEGF, bevacizumab, or ranibizumab.

Tipping the balance: robustness of tip cell selection, migration and fusion in angiogenesis

Vascular abnormalities contribute to many diseases such as cancer and diabetic retinopathy. In angiogenesis new blood vessels, headed by a migrating tip cell, sprout from pre-existing vessels in response to signals, e.g., vascular endothelial growth factor (VEGF). Tip cells meet and fuse (anastomosis) to form blood-flow supporting loops. Tip cell selection is achieved by Dll4-Notch mediated lateral inhibition resulting, under normal conditions, in an interleaved arrangement of tip and non-migrating stalk cells. Previously, we showed that the increased VEGF levels found in many diseases can cause the delayed negative feedback of lateral inhibition to produce abnormal oscillations of tip/stalk cell fates. Here we describe the development and implementation of a novel physics-based hierarchical agent model, tightly coupled to in vivo data, to explore the system dynamics as perpetual lateral inhibition combines with tip cell migration and fusion. We explore the tipping point between normal and abnormal sprouting as VEGF increases. A novel filopodia-adhesion driven migration mechanism is presented and validated against in vivo data. Due to the unique feature of ongoing lateral inhibition, 'stabilised' tip/stalk cell patterns show sensitivity to the formation of new cell-cell junctions during fusion: we predict cell fates can reverse. The fusing tip cells become inhibited and neighbouring stalk cells flip fate, recursively providing new tip cells. Junction size emerges as a key factor in establishing a stable tip/stalk pattern. Cell-cell junctions elongate as tip cells migrate, which is shown to provide positive feedback to lateral inhibition, causing it to be more susceptible to pathological oscillations. Importantly, down-regulation of the migratory pathway alone is shown to be sufficient to rescue the sprouting system from oscillation and restore stability. Thus we suggest the use of migration inhibitors as therapeutic agents for vascular normalisation in cancer.

Topography of extracellular matrix mediates vascular morphogenesis and migration speeds in angiogenesis

The extracellular matrix plays a critical role in orchestrating the events necessary for wound healing, muscle repair, morphogenesis, new blood vessel growth, and cancer invasion. In this study, we investigate the influence of extracellular matrix topography on the coordination of multi-cellular interactions in the context of angiogenesis. To do this, we validate our spatio-temporal mathematical model of angiogenesis against empirical data, and within this framework, we vary the density of the matrix fibers to simulate different tissue environments and to explore the possibility of manipulating the extracellular matrix to achieve pro- and anti-angiogenic effects. The model predicts specific ranges of matrix fiber densities that maximize sprout extension speed, induce branching, or interrupt normal angiogenesis, which are independently confirmed by experiment. We then explore matrix fiber alignment as a key factor contributing to peak sprout velocities and in mediating cell shape and orientation. We also quantify the effects of proteolytic matrix degradation by the tip cell on sprout velocity and demonstrate that degradation promotes sprout growth at high matrix densities, but has an inhibitory effect at lower densities. Our results are discussed in the context of ECM targeted pro- and anti-angiogenic therapies that can be tested empirically.

A compartment model of VEGF distribution in humans in the presence of soluble VEGF receptor-1 acting as a ligand trap

Vascular endothelial growth factor (VEGF), through its activation of cell surface receptor tyrosine kinases including VEGFR1 and VEGFR2, is a vital regulator of stimulatory and inhibitory processes that keep angiogenesis--new capillary growth from existing microvasculature--at a dynamic balance in normal physiology. Soluble VEGF receptor-1 (sVEGFR1)--a naturally-occurring truncated version of VEGFR1 lacking the transmembrane and intracellular signaling domains--has been postulated to exert inhibitory effects on angiogenic signaling via two mechanisms: direct sequestration of angiogenic ligands such as VEGF; or dominant-negative heterodimerization with surface VEGFRs. In pre-clinical studies, sVEGFR1 gene and protein therapy have demonstrated efficacy in inhibiting tumor angiogenesis; while in clinical studies, sVEGFR1 has shown utility as a diagnostic or prognostic marker in a widening array of angiogenesis-dependent diseases. Here we developed a novel computational multi-tissue model for recapitulating the dynamic systemic distributions of VEGF and sVEGFR1. Model features included: physiologically-based multi-scale compartmentalization of the human body; inter-compartmental macromolecular biotransport processes (vascular permeability, lymphatic drainage); and molecularly-detailed binding interactions between the ligand isoforms VEGF(121) and VEGF(165), signaling receptors VEGFR1 and VEGFR2, non-signaling co-receptor neuropilin-1 (NRP1), as well as sVEGFR1. The model was parameterized to represent a healthy human subject, whereupon we investigated the effects of sVEGFR1 on the distribution and activation of VEGF ligands and receptors. We assessed the healthy baseline stability of circulating VEGF and sVEGFR1 levels in plasma, as well as their reliability in indicating tissue-level angiogenic signaling potential. Unexpectedly, simulated results showed that sVEGFR1 - acting as a diffusible VEGF sink alone, i.e., without sVEGFR1-VEGFR heterodimerization--did not significantly lower interstitial VEGF, nor inhibit signaling potential in tissues. Additionally, the sensitivity of plasma VEGF and sVEGFR1 to physiological fluctuations in transport rates may partially account for the heterogeneity in clinical measurements of these circulating angiogenic markers, potentially hindering their diagnostic reliability for diseases.

Modification of the primary tumor microenvironment by transforming growth factor alpha-epidermal growth factor receptor signaling promotes metastasis in an orthotopic colon cancer model

The transforming growth factor alpha (TGFalpha)/epidermal growth factor receptor (EGFR) signaling pathway appears to play a critical role in colon cancer progression, but the cellular and molecular mechanisms that contribute to metastasis remain unknown. KM12C colon cancer cell clones expressing high (C9) or negligible (C10) levels of TGFalpha were implanted into the cecal walls of nude mice. C9 tumors formed autocrine and paracrine EGFR networks, whereas C10 tumors were unable to signal through EGFR. The tumor microenvironment of C9, but not C10, contained cells enriched in vascular endothelial growth factor (VEGF) A, interleukin-8, and matrix metalloproteinases-2 and -9 and had a high vascular surface area. C9 tumors recruited a macrophage population that co-expressed F4/80 and lymphatic vessel endothelial hyaluronic acid receptor and produced VEGFC. The mean lymphatic density of C9 tumors was threefold higher than that of C10 tumors. C9, but not C10, tumor cells metastasized to regional lymph nodes in all mice and to the liver in 5 of 10 mice. Forced expression of TGFalpha in C10 tumor cells led to the generation of autocrine and paracrine EGFR signaling, macrophage recruitment, enhanced blood and lymphatic vascular surface areas, and increased lymphatic metastasis. Collectively, these data show that activation of TGFalpha-EGFR signaling in colon cancer cells creates a microenvironment that is conducive for metastasis, providing a rationale for efforts to inhibit EGFR signaling in TGFalpha-positive colon cancers.

5-Aza-2'-deoxycytidine increases the expression of anti-angiogenic vascular endothelial growth factor 189b variant in human lung microvascular endothelial cells

Vascular endothelial growth factor (VEGF) is involved in angiogenesis, growth, and tumour cell metastasis. VEGF is expressed as alternative splice variants, which exhibit angiogenic and anti-angiogenic properties. We determined the effect of 5-Aza-2'-deoxycytidine (5-dAzaC) DNA methyltransferase (DNMTs) inhibitor on angiogenic and anti-angiogenic VEGF variants expression in immortalized human lung microvascular endothelial cells (HLMEC). Employing reverse transcription, real-time quantitative PCR (RQ-PCR), and Western blot analysis, we determined that 5-dAzaC decreased VEGF(121a) and VEGF(165a) angiogenic, and VEGF(121b) and VEGF(165b) anti-angiogenic variants expression in HLMEC. However, this DNMTs inhibitor significantly increases expression of VEGF(189b) anti-angiogenic variant transcript and protein in HLMEC. Our results suggest that the DNMTs activity may have an influence on the expression of angiogenic and anti-angiogenic VEGF variants in human lung microvascular endothelial cells.

Dimerization of VEGF receptors and implications for signal transduction: a computational study

Vascular endothelial growth factor (VEGF) is a potent cytokine involved in the induction of neovascularization. Secreted as a cysteine-linked dimer, it has two binding sites at opposite poles through which it may bind VEGF receptors (VEGFRs), receptor tyrosine kinases found on the surface of endothelial and other cells. The binding of a VEGF molecule to two VEGFR molecules induces transphosphorylation of the intracellular domains of the receptors, leading to signal transduction. The dominant mechanism of receptor dimerization is not clear: the receptors may be present in an inactive pre-dimerized form, VEGF binding first to one of the receptors, the second receptor then ideally located for dimerization; or VEGF may bind receptor monomers on the cell surface, which then diffuse and bind to available unligated receptor monomers to complete the activation. Both processes take place and one or other may dominate on different cell types. We demonstrate the impact of dimerization mechanism on the binding of VEGF to the cell surface and on the formation of active signaling receptor complexes. We describe two methods to determine which process dominates, based on binding and phosphorylation assays. The presence of two VEGF receptor populations, VEGFR1 and VEGFR2, can result in receptor heterodimer formation. Our simulations predict that heterodimers will comprise 10-50% of the active, signaling VEGF receptor complexes, and that heterodimers will form at the expense of homodimers of VEGFR1 when VEGFR2 populations are larger. These results have significant implications for VEGF signal transduction and interpretation of experimental studies. These results may be applicable to other ligand-receptor pairs, in particular PDGF.

A cell-based model exhibiting branching and anastomosis during tumor-induced angiogenesis

This work describes the first cell-based model of tumor-induced angiogenesis. At the extracellular level, the model describes diffusion, uptake, and decay of tumor-secreted pro-angiogenic factor. At the cellular level, the model uses the cellular Potts model based on system-energy reduction to describe endothelial cell migration, growth, division, cellular adhesion, and the evolving structure of the stroma. Numerical simulations show: 1), different tumor-secreted pro-angiogenic factor gradient profiles dramatically affect capillary sprout morphology; 2), average sprout extension speeds depend on the proximity of the proliferating region to the sprout tip, and the coordination of cellular functions; and 3), inhomogeneities in the extravascular tissue lead to sprout branching and anastomosis, phenomena that emerge without any prescribed rules. This model provides a quantitative framework to test hypotheses on the biochemical and biomechanical mechanisms that control tumor-induced angiogenesis.

Ca2+-mediated ascorbate release from coronary artery endothelial cells

1.--The addition of Ca(2+) ionophore A23187 or ATP to freshly isolated or cultured pig coronary artery endothelial cells (PCEC) potentiated the release of ascorbate (Asc). Cultured PCEC were used to characterize the Ca(2+)-mediated release. An increase in Ca(2+)-mediated Asc release was observed from PCEC preincubated with Asc, Asc-2-phosphate or dehydroascorbic acid (DHAA). 2.--The effects of various ATP analogs and inhibition by suramin were consistent with the ATP-induced release being mediated by P2Y2-like receptors. 3.--ATP-stimulated Asc release was Ca(2+)-mediated because (a) ATP analogs that increased Asc release also elevated cytosolic [Ca(2+)], (b) Ca(2+) ionophore A23187 and cyclopiazonic acid stimulated the Asc release, (c) removing extracellular Ca(2+) and chelating intracellular Ca(2+)inhibited the ATP-induced release, and (d) inositol-selective phospholipase C inhibitor U73122 also inhibited this release. 4.--Accumulation of Asc by PCEC was examined at Asc concentrations of 10 microM (Na(+)-Asc symporter not saturated) and 5 mM (Na(+)-Asc symporter saturated). At 10 microM Asc, A23187 and ATP caused an inhibition of Asc accumulation but at 5 mM Asc, both the agents caused a stimulation. Substituting gluconate for chloride did not affect the basal Asc uptake but it abolished the effects of A23187. 5.--PCEC but not pig coronary artery smooth muscle cells show a Ca(2+)- mediated Asc release pathway that may be activated by agents such as ATP.

Activation of Tie2 by angiopoietin-1 and angiopoietin-2 results in their release and receptor internalization

The receptor tyrosine kinase Tie2 is highly expressed in endothelial cells and is crucial for angiogenesis and vascular maintenance. The ligands for Tie2 are the angiopoietins, of which angiopoietin-1 and angiopoietin-2 have been the most studied. Angiopoietin-1 has been characterized as the primary activating ligand for Tie2 whereas the role of angiopoietin-2 remains controversial; activating Tie2 in some studies and inhibiting Tie2 in others. Our studies were aimed at understanding the regulation of Tie2 in endothelial cells by angiopoietin-1 and angiopoietin-2 and revealed that both ligands activated Tie2 in a concentration-dependent manner. Angiopoietin-2 was considerably weaker at activating Tie2 compared with angiopoietin-1 suggesting that angiopoietin-2 may be a partial agonist. Activation of Tie2 by these ligands resulted in differential turnover of the receptor where binding of angiopoietin-1, and to a lesser extent angiopoietin-2, induced rapid internalization and degradation of Tie2. Furthermore, our binding studies demonstrate that both ligands are differentially released from the endothelial cell surface after receptor activation and accumulate in the surrounding medium. Altogether, these data begin our understanding of the regulation of Tie2 and the activity of the angiopoietins after engaging the endothelial cell surface.

Role of repulsive factors in vascularization dynamics

Capillary networks are essential in vertebrates to supply tissues with nutrients. Experiments of in vitro capillary formation show that endothelial cells randomly spread on a gel matrix autonomously organize to form vascular networks with a characteristic length independent of the initial cell density. A mathematical model based on free cell migration and on cell cross-talk mediated by soluble chemical factors has been recently proposed and explains the main dynamical and geometrical properties of the networks. We extend this model introducing the action of repulsive factors and we show that their activity results in a larger degree of reorganization of cellular matter and in more robust control over the size of the growing vascular network.

Vesicular Trafficking of Tyrosine Kinase Receptors and Associated Proteins in the Regulation of Signaling and Vascular Function

Receptor tyrosine kinases (RTKs) play a pivotal role in the development and function of the cardiovascular system. Ligand-activated RTKs promote numerous downstream signal transduction pathways that lead to vascular permeability, as well as proliferation, migration, and differentiation of vascular endothelia and smooth muscle cells. Ligand binding also promotes internalization of the activated receptors either to downregulate the signaling via degradation of the ligand/receptor complex or to signal from endosomes. However, the outcomes of receptor internalization via clathrin-dependent or caveolar pathways and trafficking mechanisms are incompletely clarified in vascular systems. Activity modulation through endocytosis and vesicular trafficking significantly impacts downstream targets of RTKs such as endothelial nitric oxide synthase (eNOS) and VE-cadherin. RTKs and their associated targets are also transported to the nucleus, where they may directly impact nuclear signaling. Although the nuclear transport pathways are just beginning to be unraveled, it appears that endocytosis and vesicular trafficking are involved. In this review, we discuss the mechanisms by which activated RTKs and the downstream targets eNOS and VE-cadherin may be internalized and transported to various intracellular compartments. How localization and interacting proteins impact protein function and influence signaling is an important theme, as is the potential for modulating signaling through therapeutic targeting of activated receptors and components of the endocytic machinery.

Immunohistochemical study about the Flt-1/VEGFR1 expression in the gastrointestinal tract of mouse, rat, dog, swine and monkey

Fms-like tyrosine kinase 1 (Flt-1) performs a subordinate effector role in mesenchymal angiogenesis and potentially serves an equally important functional role as a self-contained receptor in epithelial cells. In both endothelial cells and epithelial cells, Flt-1/vascular endothelial growth factor receptor 1 (VEGFR1) downstream signalling is involved in regulating cellular processes such as cytoskeletal changes and cellular survival protection. Cellular renewal of the gastrointestinal mucosa is based on these processes and might involve Flt-1/VEGFR1 pathway activities; the molecular mechanisms regulating these cellular dynamics remain unclear. This study was performed to investigate the presence and distribution of Flt-1/VEGFR1 in epithelial cells of the gastrointestinal tract by immunohistochemistry (IHC). Gastrointestinal tissues were taken from eight anatomical sites from mouse, rat, dog, swine and monkey. Present results revealed a cytosolic Flt-1/VEGFR1 staining pattern in mucosal epithelial cells for all investigated species. Non-epithelial structures also displayed a distinct Flt-1/VEGFR1 positivity and included vascular smooth muscle walls, enteric smooth muscle layers, the enteric nervous system and capillary endothelial cells. Diverse intensities of the Flt-1/VEGFR1 binding reaction within each species were observed in the intestinal mucosa with a strong immunoreaction in enterocytes and with a low protein expression in the ileum in most species. Crypt cells in the large intestine were mostly negative for Flt-1/VEGFR1. A peculiar and mainly intranuclear antibody binding reaction was found in Brunner's gland epithelial cells of mouse and rat whereas Brunner's glands of dog, swine and monkey remained completely negative. These results indicate a potential involvement of Flt-1/VEGFR1 in normal restitution of gastrointestinal structures in the species studied. Additionally, intranuclear Flt-1/VEGFR1 antibody binding in Brunner's glands of rodents may suggest a nuclear translocation of the transmembrane VEGFR1 which has not previously been described.

Transplantation of human umbilical vein endothelial cells improves left ventricular function in a rat model of myocardial infarction

INTRODUCTION

Although cell-based therapy after myocardial infarction (MI) may be beneficial in improving cardiac dysfunction, the underlying mechanisms remain to be clarified. Since human umbilical vein endothelial cells (HUVEC) harbor the potential for transdifferentiation, we studied the functional effects of HUVEC transplantation in a rat model of MI.

METHODS AND RESULTS

HUVEC labeled with BrdU or medium alone were injected into the infarcted area and its margin 4 weeks after ligation of the left coronary artery in cyclosporine-treated rats. BrdU(+) signals could be detected in the area of MI at two weeks and two months after injection only in hearts transplanted with HUVEC. While no signs of transdifferentiation into cardiomyocytes were evident, staining for the rat macrophage marker ED-1, adjacent to or colocalized with BrdU(+) signals, revealed an in.ltration with macrophages and implied the phagocytosis of injected HUVEC. In the vicinity of BrdU(+) signals, the density of CD31(+) microvessels was significantly increased in HUVEC-transplanted as compared to medium-treated hearts after two months. HUVEC transplantation led to improved contractility as assessed by echocardiography and to higher coronary flow rates as well as to improved response to volume strain and catecholamine stress in Langendorff perfused hearts.

CONCLUSIONS

After MI, transplanted HUVEC persist in the host myocardium and trigger an infiltration with macrophages. The ensuing increase in neovascularization and improvement in global left ventricular function may be attributable to the associated inflammatory response.

Analysis of compartmental models of ligand-induced endocytosis

Kinetic models have played a pivotal role in the study of ligand-induced endocytosis. However, an analysis that suggests a systematic way to validate such models is lacking. The current work analyses the base model of ligand-induced endocytosis for three widely used experimental protocols. In protocol I cells initially devoid of ligand are incubated in ligand solution, whereas protocols II and III are desorption experiments in which an initial pool of surface or internalized ligand-receptor complexes, respectively, are released into an elution medium that is initially devoid of ligand. A short-time analysis of protocol I using successive substitutions yielded a corrected pre-factor for the In/Sur plot introduced by Wiley and Cunningham (Cell 25 (1981) 433). In contrast, neglecting the variation in receptor numbers yielded an approximation of protocol I that is valid for long times (e.g. tens of minutes). Similarly, the low cell-concentration limits of protocols II and III are derived by neglecting the concentration of free ligand. The simplicity of these approximations provides a simple and reliable method for estimating the parameters governing ligand kinetics, while their definitive nature implies that they can be used to verify the validity of the base model. This analysis also provides insight on the fast endocytosis and recycling limit of protocol III.

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Model of competitive binding of vascular endothelial growth factor and placental growth factor to VEGF receptors on endothelial cells

Placental growth factor (PlGF) competes with vascular endothelial growth factor (VEGF) for binding to VEGF receptor (VEGFR)-1 but does not bind VEGFR2. Experiments show that PlGF can augment the response to VEGF in pathological angiogenesis and in models of endothelial cell survival, migration, and proliferation. This synergy has been hypothesized to be due to a combination of the following: signaling by PlGF through VEGFR1 and displacement of VEGF from VEGFR1 to VEGFR2 by PlGF, causing increased signaling through VEGFR2. In this study, the relative contribution of PlGF-induced VEGF displacement to the synergy is quantified using a mathematical model of ligand-receptor binding to examine the effect on ligand-receptor complex formation of VEGF and PlGF acting together. Parameters specific to the VEGF-PlGF system are used based on existing data. The model is used to simulate in silico a specific in vitro experiment in which VEGF-PlGF synergy is observed. We show that, whereas a significant change in the formation of endothelial surface growth factor-VEGFR1 complexes is predicted in the presence of PlGF, the increase in the number of VEGFR2-containing signaling complexes is less significant; these results were shown to be robust to significant variation in the kinetic parameters of the model. Synergistic effects observed in that experiment thus appear unlikely to be due to VEGF displacement but to a shift from VEGF-VEGFR1 to PlGF-VEGFR1 complexes and an increase in total VEGFR1 complexes. These results suggest that VEGFR1 signaling can be functional in adult-derived endothelial cells.