The controls of microvascular survival

Evaluation of a Miniaturized Biologically Vascularized Scaffold in vitro and in vivo

In tissue engineering, the generation and functional maintenance of dense voluminous tissues is mainly restricted due to insufficient nutrient supply. Larger three-dimensional constructs, which exceed the nutrient diffusion limit become necrotic and/or apoptotic in long-term culture if not provided with an appropriate vascularization. Here, we established protocols for the generation of a pre-vascularized biological scaffold with intact arterio-venous capillary loops from rat intestine, which is decellularized under preservation of the feeding and draining vascular tree. Vessel integrity was proven by marker expression, media/blood reflow and endothelial LDL uptake. In vitro maintenance persisted up to 7 weeks in a bioreactor system allowing a stepwise reconstruction of fully vascularized human tissues and successful in vivo implantation for up to 4 weeks, although with time-dependent decrease of cell viability. The vascularization of the construct lead to a 1.5× increase in cellular drug release compared to a conventional static culture in vitro. For the first time, we performed proof-of-concept studies demonstrating that 3D tissues can be maintained within a miniaturized vascularized scaffold in vitro and successfully implanted after re-anastomosis to the intrinsic blood circulation in vivo. We hypothesize that this technology could serve as a powerful platform technology in tissue engineering and regenerative medicine.

Genistein modulates the estrogen receptor and suppresses angiogenesis and inflammation in the murine model of peritoneal endometriosis

The purpose of this study was to investigate the effect of genistein administration on the modulation of the estrogen receptor, inhibition of inflammation and angiogenesis in the murine model of peritoneal endometriosis. A total of thirty-six mice (Mus musculus) were divided into six groups (n = 6), including the control group, endometriosis group, endometriosis group treated with various doses of genistein (0.78; 1.04; 1.3 mg/day), and endometriosis group treated with leuprolide acetate (0.00975 mg/day every 5 days for 15 days). Analysis of estrogen receptor-α, estrogen receptor-β, TNF-α, IL-6, VEGF, and HIF-1α were performed immunohistochemically. Expression of estrogen receptor-α, estrogen receptor-β, TNF-α, IL-6, VEGF and HIF-1α increased significantly compared with the control group (p < 0.05). All doses of genistein decreased the expression of estrogen receptor-α, increased estrogen receptor-β, lowered VEGF and HIF-1α significantly compared with endometriosis group (p > 0.05). Genistein also decreased the expression of TNF-α and IL-6 (1.04 and 1.3 mg/day) compared with the endometriosis group, reaching level comparable to that of the control group (p > 0.05). It was concluded that genistein is able to modulate estrogen receptor-α and estrogen receptor-β and inhibit the development of inflammation and angiogenesis in the murine model of peritoneal endometriosis. Thus, genistein can be a candidate in the treatment of endometriosis.

Deficiency in matrix metalloproteinase-2 results in long-term vascular instability and regression in the injured mouse spinal cord

Angiogenesis plays a critical role in wound healing after spinal cord injury. Therefore, understanding the events that regulate angiogenesis has considerable relevance from a therapeutic standpoint. We evaluated the contribution of matrix metalloproteinase (MMP)-2 to angiogenesis and vascular stability in spinal cord injured MMP-2 knockout and wildtype (WT) littermates. While MMP-2 deficiency resulted in reduced endothelial cell division within the lesioned epicenter, there were no genotypic differences in vascularity (vascular density, vascular area, and endothelial cell number) over the first two weeks post-injury. However, by 21days post-injury MMP-2 deficiency resulted in a sharp decline in vascularity, indicative of vascular regression. Complementary in vitro studies of brain capillary endothelial cells confirmed MMP-2 dependent proliferation and tube formation. As deficiency in MMP-2 led to prolonged MMP-9 expression in the injured spinal cord, we examined both short-term and long-term exposure to MMP-9 in vitro. While MMP-9 supported endothelial tube formation and proliferation, prolonged exposure resulted in loss of tubes, findings consistent with vascular regression. Vascular instability is frequently associated with pericyte dissociation and precedes vascular regression. Quantification of PDGFrβ+ pericyte coverage of mature vessels within the glial scar (the reactive gliosis zone), a known source of MMP-9, revealed reduced coverage in MMP-2 deficient animals. These findings suggest that acting in the absence of MMP-2, MMP-9 transiently supports angiogenesis during the early phase of wound healing while its prolonged expression leads to vascular instability and regression. These findings should be considered while developing therapeutic interventions that block MMPs.

Role of vascular networks in extending glucose sensor function: Impact of angiogenesis and lymphangiogenesis on continuous glucose monitoring in vivo

The concept of increased blood vessel (BV) density proximal to glucose sensors implanted in the interstitial tissue increases the accuracy and lifespan of sensors is accepted, despite limited existing experimental data. Interestingly, there is no previous data or even conjecture in the literature on the role of lymphatic vessels (LV) alone, or in combination with BV, in enhancing continuous glucose monitoring (CGM) in vivo. To investigate the impact of inducing vascular networks (BV and LV) at sites of glucose sensor implantation, we utilized adenovirus based local gene therapy of vascular endothelial cell growth factor-A (VEGF-A) to induce vessels at sensor implantation sites. The results of these studies demonstrated that (1) VEGF-A based local gene therapy increases vascular networks (blood vessels and lymphatic vessels) at sites of glucose sensor implantation; and (2) this local increase of vascular networks enhances glucose sensor function in vivo from 7 days to greater than 28 days postsensor implantation. This data provides "proof of concept" for the effective usage of local angiogenic factor (AF) gene therapy in mammalian models in an effort to extend CGM in vivo. It also supports the practice of a variety of viral and nonviral vectors as well as gene products (e.g. anti-inflammatory and anti-fibrosis genes) to engineer "implant friendly tissues" for the usage with implantable glucose sensors as well as other implantable devices.

Apoptosis and angiogenesis: an evolving mechanism for fibrosis

Fibrosis, seen in the liver, lung, heart, kidney, and skin, is a significant global disease burden. Currently, therapeutic treatment is limited, and the number of cases continues to grow. Apoptosis has been identified as a potential initiator and propagator of fibrosis. This review specifically examines the correlation between the presence of apoptotic cells and their effect on fibroblast phenotype and collagen metabolism in several different experimental models of fibrosis. Fibrosis in these models is generally preceded by robust angiogenesis and vascular regression, suggesting that the vascular apoptotic burden may be important to fibrotic outcomes. This review considers the emerging evidence that angiogenesis or vascular regression contributes to fibrosis and identifies initial vascular outgrowth or vascular apoptotic cell presence as possible regulators of fibrosis. A further understanding of the cellular mechanisms of fibrosis may suggest novel methods for the reduction of the fibrotic response and promotion of regeneration.

Pericytes promote selective vessel regression to regulate vascular patterning

Blood vessel networks form in a 2-step process of sprouting angiogenesis followed by selective branch regression and stabilization of remaining vessels. Pericytes are known to function in stabilizing blood vessels, but their role in vascular sprouting and selective vessel regression is poorly understood. The endosialin (CD248) receptor is expressed by pericytes associated with newly forming but not stable quiescent vessels. In the present study, we used the Endosialin(-/-) mouse as a means to uncover novel roles for pericytes during the process of vascular network formation. We demonstrate in a postnatal retina model that Endosialin(-/-) mice have normal vascular sprouting but are defective in selective vessel regression, leading to increased vessel density. Examination of the Endosialin(-/-) mouse tumor vasculature revealed an equivalent phenotype, indicating that pericytes perform a hitherto unidentified function to promote vessel destabilization and regression in vivo in both physiologic and pathologic angiogenesis. Mechanistically, Endosialin(-/-) mice have no defect in pericyte recruitment. Rather, endosialin binding to an endothelial associated, but not a pericyte associated, basement membrane component induces endothelial cell apoptosis and detachment. The results of the present study advance our understanding of pericyte biology and pericyte/endothelial cell cooperation during vascular patterning and have implications for the design of both pro- and antiangiogenic therapies.

Loss of epidermal MMP-14 expression interferes with angiogenesis but not with re-epithelialization

Synthesis and activation of matrix metalloproteinases during wound healing are important for remodeling the extracellular matrix and modulating various cellular functions. The membrane-type 1 matrix metalloproteinase (MMP-14) has been shown to play a key role during these processes. To analyze the function of epidermal-derived MMP-14 during skin repair we generated mice lacking MMP-14 expression in the epidermis (MMP-14(ep-/-)). These mice displayed overall normal skin morphology and epidermal differentiation patterns. Wound repair in MMP-14(ep-/-) followed the same kinetics as in wild type mice (MMP-14(ep+/+)), and infiltration of neutrophils, leukocytes, and macrophages into the wound site was comparable. Microscopic analysis showed no altered re-epithelialization in the absence of epidermal MMP-14. Furthermore, epidermal differentiation at the end of the repair process and scar formation was normal. However, at day 14 post wounding, sustained angiogenesis was observed in MMP-14(ep-/-) mice in contrast to control mice. Interestingly, decreased levels of endostatin were detected in wound lysates of MMP-14(ep-/-) mice as well as in cultured keratinocytes. Taken together, these data indicate that MMP-14 expression in keratinocytes is dispensable for skin homeostasis and repair, but plays a crucial role in the epidermal-dermal crosstalk leading to modulation of vessel density.

Circulating endothelial progenitor cells are up-regulated in a mouse model of endometriosis

Endometriosis is a debilitating disease characterized by the growth of ectopic endometrial tissue. It is widely accepted that angiogenesis plays an integral part in the establishment and growth of endometriotic lesions. Recent data from a variety of angiogenesis-dependent diseases suggest a critical role of bone marrow–derived endothelial progenitor cells (EPCs) in neovascularization. In this study we examined the blood levels of EPCs and mature circulating endothelial cells in a mouse model of surgically induced endometriosis. Fluorescence-activated cell sorting analysis revealed elevated levels of EPCs in the blood of mice with endometriosis compared with control subject that underwent a sham operation. EPC concentrations positively correlated with the amount of endometriotic tissue and peaked 1 to 4 days after induction of disease. In a green fluorescent protein bone marrow transplant experiment we found green fluorescent protein–positive endothelial cells incorporated into endometriotic lesions but not eutopic endometrium, as revealed by flow cytometry and immunohistochemistry. Finally, treatment of endometriosis-bearing mice with the angiogenesis inhibitor Lodamin, an oral nontoxic formulation of TNP-470, significantly decreased EPC levels while suppressing lesion growth. Taken together, our data indicate an important role for bone marrow–derived endothelial cells in the pathogenesis of endometriosis and support the potential clinical use of anti-angiogenic therapy as a novel treatment modality for this disease.

Activation of endothelial ras signaling bypasses senescence and causes abnormal vascular morphogenesis

Angiogenesis is crucial for embryogenesis, reproduction, and wound healing and is a critical determinant of tumor growth and metastasis. The multifunctional signal transducer Ras is a proto-oncogene and frequently becomes mutated in a variety of human cancers, including angiosarcomas. Regulation of Ras is important for endothelial cell function and angiogenesis. Hyperactivation of Ras is linked with oncogene-induced senescence in many cell types. Given links between vascular malformations and angiosarcoma with activated Ras signaling, we sought to determine the consequence of sustained Ras activation on endothelial cell function. We find that sustained Ras activation in primary endothelial cells leads to prolonged activation of progrowth signaling, accompanied by a senescence bypass, enhanced proliferation, autonomous growth, and increased survival. Moreover, Ras severely compromises the ability of these cells to organize into vascular structures, instead promoting formation of planar endothelial sheets. This abnormal phenotype is regulated by phosphoinositide 3-kinase signaling, highlighting the therapeutic potential of agents targeting this axis in dealing with vascular morphogenic disorders and vascular normalization of tumors.

Role of vitamin D receptor in the antiproliferative effects of calcitriol in tumor-derived endothelial cells and tumor angiogenesis in vivo

Calcitriol (1,25-dihydroxycholecalciferol), the major active form of vitamin D, is antiproliferative in tumor cells and tumor-derived endothelial cells (TDEC). These actions of calcitriol are mediated at least in part by vitamin D receptor (VDR), which is expressed in many tissues including endothelial cells. To investigate the role of VDR in calcitriol effects on tumor vasculature, we established TRAMP-2 tumors subcutaneously into either VDR wild-type (WT) or knockout (KO) mice. Within 30 days post-inoculation, tumors in KO mice were larger than those in WT (P < 0.001). TDEC from WT expressed VDR and were able to transactivate a reporter gene whereas TDEC from KO mice were not. Treatment with calcitriol resulted in growth inhibition in TDEC expressing VDR. However, TDEC from KO mice were relatively resistant, suggesting that calcitriol-mediated growth inhibition on TDEC is VDR-dependent. Further analysis of the TRAMP-C2 tumor sections revealed that the vessels in KO mice were enlarged and had less pericyte coverage compared with WT (P < 0.001). Contrast-enhanced magnetic resonance imaging showed an increase in vascular volume of TRAMP tumors grown in VDR KO mice compared with WT mice (P < 0.001) and FITC-dextran permeability assay suggested a higher extent of vascular leakage in tumors from KO mice. Using ELISA and Western blot analysis, there was an increase of hypoxia-inducible factor-1alpha, vascular endothelial growth factor, angiopoietin 1, and platelet-derived growth factor-BB levels observed in tumors from KO mice. These results indicate that calcitriol-mediated antiproliferative effects on TDEC are VDR-dependent and loss of VDR can lead to abnormal tumor angiogenesis.

Estrogen stimulates the human endometrium to express a factor(s) that promotes vascular smooth muscle cell migration as an early step in microvessel remodeling

Vascular smooth muscle cell (VSMC) migration is a pivotal early step in blood vessel remodeling; however, very little is known about the regulation of this process in the human endometrium during the menstrual cycle. In this study, explants of human endometrium were incubated with estradiol and/or progesterone and the conditioned medium (CM) applied to cultures of VSMC to test the hypothesis that estrogen and progesterone stimulate endometrial cells to secrete a factor(s) that promotes VSMC migration. Endometrial explants were composed of highly organized glands and stroma. VSMC migration (cells migrated in 21 h/mm(2) fibronectin-coated semipermeable membrane) in the presence of CM from human endometrial explants obtained in the proliferative phase of the menstrual cycle and incubated for 24 h with estradiol was approximately threefold greater (P < 0.001) than with medium alone and greater (P < 0.05) than with CM from explants treated with estradiol plus progesterone or progesterone. It is concluded, therefore, that estrogen stimulates endometrial secretion of a factor(s) that promotes VSMC migration as an early step in vessel remodeling within the endometrium.

Engineering angiogenesis following spinal cord injury: a coculture of neural progenitor and endothelial cells in a degradable polymer implant leads to an increase in vessel density and formation of the blood-spinal cord barrier

Angiogenesis precedes recovery following spinal cord injury and its extent correlates with neural regeneration, suggesting that angiogenesis may play a role in repair. An important precondition for studying the role of angiogenesis is the ability to induce it in a controlled manner. Previously, we showed that a coculture of endothelial cells (ECs) and neural progenitor cells (NPCs) promoted the formation of stable tubes in vitro and stable, functional vascular networks in vivo in a subcutaneous model. We sought to test whether a similar coculture would lead to the formation of stable functional vessels in the spinal cord following injury. We created microvascular networks in a biodegradable two-component implant system and tested the ability of the coculture or controls (lesion control, implant alone, implant + ECs or implant + NPCs) to promote angiogenesis in a rat hemisection model of spinal cord injury. The coculture implant led to a fourfold increase in functional vessels compared with the lesion control, implant alone or implant + NPCs groups and a twofold increase in functional vessels over the implant + ECs group. Furthermore, half of the vessels in the coculture implant exhibited positive staining for the endothelial barrier antigen, a marker for the formation of the blood-spinal cord barrier. No other groups have shown positive staining for the blood-spinal cord barrier in the injury epicenter. This work provides a novel method to induce angiogenesis following spinal cord injury and a foundation for studying its role in repair.

New role for the protein tyrosine phosphatase DEP-1 in Akt activation and endothelial cell survival

Functional inactivation of the protein tyrosine phosphatase DEP-1 leads to increased endothelial cell proliferation and failure of vessels to remodel and branch. DEP-1 has also been proposed to contribute to the contact inhibition of endothelial cell growth via dephosphorylation of vascular endothelial growth factor receptor 2 (VEGFR2), a mediator of vascular development. However, how DEP-1 regulates VEGF-dependent signaling and biological responses remains ill-defined. We show here that DEP-1 targets tyrosine residues in the VEGFR2 kinase activation loop. Consequently, depletion of DEP-1 results in the increased phosphorylation of all major VEGFR2 autophosphorylation sites, but surprisingly, not in the overall stimulation of VEGF-dependent signaling. The increased phosphorylation of Src on Y529 under these conditions results in impaired Src and Akt activation. This inhibition is similarly observed upon expression of catalytically inactive DEP-1, and coexpression of an active Src-Y529F mutant rescues Akt activation. Reduced Src activity correlates with decreased phosphorylation of Gab1, an adapter protein involved in VEGF-dependent Akt activation. Hypophosphorylated Gab1 is unable to fully associate with phosphatidylinositol 3-kinase, VEGFR2, and VE-cadherin complexes, leading to suboptimal Akt activation and increased cell death. Overall, our results reveal that despite its negative role on global VEGFR2 phosphorylation, DEP-1 is a positive regulator of VEGF-mediated Src and Akt activation and endothelial cell survival.

Capillary sprout endothelial cells exhibit a CD36 low phenotype: regulation by shear stress and vascular endothelial growth factor-induced mechanism for attenuating anti-proliferative thrombospondin-1 signaling

Endothelial cells acquire distinctive molecular signatures in their transformation to an angiogenic phenotype that are indicative of changes in cell behavior and function. Using a rat mesentery model of inflammation-induced angiogenesis and a panel of known endothelial markers (CD31, VE-cadherin, BS-I lectin), we identified a capillary sprout-specific endothelial phenotype that is characterized by the marked down-regulation of CD36, a receptor for the anti-angiogenic molecule thrombospondin-1 (TSP-1). TSP-1/CD36 interactions were shown to regulate angiogenesis in this model as application of TSP-1 inhibited angiogenesis and blockade of both TSP-1 and CD36 accelerated angiogenesis. Vascular endothelial growth factor, which was up-regulated in the in vivo model, elicited a dose- and time-dependent down-regulation of CD36 (ie, to a CD36 low phenotype) in cultured human umbilical vein endothelial cells. Human umbilical vein endothelial cells that had been conditioned to a CD36 low phenotype with VEGF were found to be refractory to anti-proliferative TSP-1 signaling via a CD36-dependent mechanism. The loss of exposure to wall shear stress, which occurs in vivo when previously quiescent cells begin to sprout, also generated a CD36 low phenotype. Ultimately, our results identified the regulation of endothelial cell CD36 expression as a novel mechanism through which VEGF stimulates and sustains capillary sprouting in the presence of TSP-1. Additionally, CD36 was shown to function as a potential molecular linkage through which wall shear stress may regulate both microvessel sprouting and quiescence.

Human esophageal cancer endothelial cells increase tumor growth by incorporating with mouse endothelium

Current in vivo investigations of tumor angiogenesis mainly rely on the results obtained from engrafted models in mice. In the present study, we attempt to assess the potential of human tumor endothelium to form neovasculature in different engrafted tumor models. The tumor endothelial cells were isolated from human esophageal squamous cell carcinoma, and then identified by anti-VEGFR1/2 immunoreactions and tube formation assay. Esophageal and lung cancer cells were subcutaneously inoculated into nude mice with human esophageal cancer endothelial cells (HECECs), respectively. The human umbilical vein endothelial cells (HUVECs) were also co-inoculated into mice with esophageal cancer cells as a control. The engrafted tumor growth was significantly promoted by co-inoculation of HECECs in comparison with injection of esophageal tumor cells alone. Immunohistochemistry of anti-CD31 and anti-huCD31 was performed to detect the micro-vessels in the engrafted tumors which revealed that the HECECs formed humanized micro-vessels and significantly increased the micro-vessel density in engrafted tumors comparing with the tumors without HECECs. However, HUVEC cells could not enhance the esophageal tumor growth and the growth of lung tumors could not be increased by HECECs, either. Few humanized blood vessels were found in these two groups of xenografts. These results suggest that the specific interaction between HECECs and esophageal tumor cells contributes to the neovasculature construction and esophageal tumor growth in xenografts.

PEDF induces apoptosis in human endothelial cells by activating p38 MAP kinase dependent cleavage of multiple caspases

We examined how pigment epithelium derived factor (PEDF), an effective endogenous antiangiogenic protein, decreases survival of primary cultures of human umbilical vein endothelial cells (HUVECs) in a low serum environment supplemented with the endothelial cell growth factor (VEGF). We provide evidence that induction of apoptosis by PEDF is associated with activation of p38 followed by cleavage of caspases 3, 8, and 9 by treatment with PEDF, and PEDF's actions are caspase dependent. A key mediator in the executioner effects of PEDF is p38 since the inhibition of p38 activity blocked apoptosis and prevented cleavage of caspases 3, 8, and 9. Although PEDF-induced phosphorylation of JNK1, the inhibition of JNK1 had no effect on apoptosis, even though it prevented phosphorylation of JNK1 by PEDF. Based on these findings, we propose that the antiangiogenic action of PEDF is dependent on activation of p38 MAPkinase which regulates cleavage of multiple caspases cascades.

Pathogenesis of persistent lymphatic vessel hyperplasia in chronic airway inflammation

Edema occurs in asthma and other inflammatory diseases when the rate of plasma leakage from blood vessels exceeds the drainage through lymphatic vessels and other routes. It is unclear to what extent lymphatic vessels grow to compensate for increased leakage during inflammation and what drives the lymphangiogenesis that does occur. We addressed these issues in mouse models of (a) chronic respiratory tract infection with Mycoplasma pulmonis and (b) adenoviral transduction of airway epithelium with VEGF family growth factors. Blood vessel remodeling and lymphangiogenesis were both robust in infected airways. Inhibition of VEGFR-3 signaling completely prevented the growth of lymphatic vessels but not blood vessels. Lack of lymphatic growth exaggerated mucosal edema and reduced the hypertrophy of draining lymph nodes. Airway dendritic cells, macrophages, neutrophils, and epithelial cells expressed the VEGFR-3 ligands VEGF-C or VEGF-D. Adenoviral delivery of either VEGF-C or VEGF-D evoked lymphangiogenesis without angiogenesis, whereas adenoviral VEGF had the opposite effect. After antibiotic treatment of the infection, inflammation and remodeling of blood vessels quickly subsided, but lymphatic vessels persisted. Together, these findings suggest that when lymphangiogenesis is impaired, airway inflammation may lead to bronchial lymphedema and exaggerated airflow obstruction. Correction of defective lymphangiogenesis may benefit the treatment of asthma and other inflammatory airway diseases.

Fluid shear stress induces endothelial KLF2 gene expression through a defined promoter region

Fluid shear stress is crucial for maintenance of a properly functioning endothelium. In this study we demonstrate that the KLF2 transcription factor is greatly induced by pulsatile shear stress in murine microvascular endothelial cells. The promoter elements responsible for the induction were studied by transfection with luciferase-reporter plasmids including the 5' flanking region of the murine KLF2 gene. Deletion analysis reveals that the responses are regulated by a region from -157 to -95 bp from the start site of transcription. Furthermore, shear stress induces specific nuclear binding within this region. These results define a novel shear stress response region that is highly conserved between mouse and human homologs.

Measuring tumour vascular response to antivascular and antiangiogenic drugs

The tumour vasculature is an attractive target for therapy because of its accessibility to blood-borne anticancer agents and the reliance of most tumour cells on an intact vascular supply for their survival. For convenience, therapeutic targeting of the tumour vasculature can be divided into antiangiogenic approaches, which target the process of new blood vessel development and antivascular approaches, which target the established tumour vasculature. Many agents are now in clinical trial for the treatment of cancer by these methods. The main aim of this article is to describe the vascular effects of some of these agents and identify suitable end-points for measuring efficacy in early clinical trials. For drugs which are active below their maximum tolerated dose (MTD), measurement of vascular end-points is required to determine the most effective dosing/scheduling protocols. In addition, many of the current and developing antiangiogenic agents have additional mechanisms of action unrelated to angiogenesis per se, requiring measurement of vascular end-points to understand their mechanisms of action. Measurement of tumour microvascular density (MVD) from tumour biopsies is a common method for assessing the efficacy of antiangiogenic drugs. The limitations of this method and alternative end-points, which take into account vascular function, are discussed. Pre-clinical data regarding tumour response to the antivascular agent combretastatin A-4 3-0-phosphate (CA-4-P) are discussed in the context of guiding clinical trial planning. Finally, the accessibility of vascular end-points for clinical imaging is addressed.

Regulated angiogenesis and vascular regression in mice overexpressing vascular endothelial growth factor in airways

Angiogenesis and vascular remodeling occurs in many inflammatory diseases, including asthma. In this study, we determined the time course and reversibility of the angiogenesis and vascular remodeling produced by vascular endothelial growth factor (VEGF) in a tet-on inducible transgenic system driven by the CC10 promoter in airway epithelium. One day after switching on VEGF expression, endothelial sprouts arose from venules, grew toward the epithelium, and were abundant by 3 to 5 days. Vessel density reached twice baseline by 7 days. Many new vessels were significantly larger than normal, were fenestrated, and penetrated the epithelium. Despite their mature appearance at 7 days suggested by their pericyte coat and basement membrane, the new vessels started to regress within 3 days when VEGF was switched off, showing stasis and luminal occlusion, influx of inflammatory cells, and retraction and apoptosis of endothelial cells and pericytes. Vessel density returned to normal within 28 days after VEGF withdrawal. Our study showed the dynamic nature of airway angiogenesis and regression. Blood vessels can respond to VEGF by sprouting angiogenesis within a few days, but regress more slowly after VEGF withdrawal, and leave a historical record of their previous extent in the form of empty basement membrane sleeves.

Short-term human prostate primary xenografts: an in vivo model of human prostate cancer vasculature and angiogenesis

Transgenic spontaneously occurring and transplantable xenograft models of adenocarcinoma of the prostate (CaP) are established tools for the study of CaP progression and metastasis. However, no animal model of CaP has been characterized that recapitulates the response of the human prostate vascular compartment to the evolving tumor microenvironment during CaP progression. We report that primary xenografts of human CaP and of noninvolved areas of the human prostate peripheral zone transplanted to athymic nude mice provide a unique model of human angiogenesis occurring in an intact human prostate tissue microenvironment. Angiogenesis in human kidney primary xenografts established from human renal cell carcinoma and noninvolved kidney tissue, a highly vascular organ and cancer, was compared with angiogenesis in xenografts from the relatively less vascularized prostate. Immunohistochemical identification of the human versus mouse host origin of the endothelial cells and of human endothelial cell proliferation in the human prostate and human kidney xenografts demonstrated that: (a) the majority of the vessels in primary xenografts of benign and malignant tissue of both organs were lined with human endothelial cells through the 30-day study period; (b) the mean vessel density was increased in both the CaP and benign prostate xenografts relative to the initial tissue, whereas there was no significant difference in mean vessel density in the renal cell carcinoma and benign kidney xenografts compared with the initial tissue; and (c) the number of vessels with proliferating endothelial cells in primary xenografts of CaP and benign prostate increased compared with their respective initial tissue specimens, whereas the number of vessels with proliferating endothelial cells decreased in the benign kidney xenografts. Short-term primary human prostate xenografts, therefore, represent a valuable in vivo model for the study of human angiogenesis within a human tissue microenvironment and for comparison of angiogenesis in CaP versus benign prostate.

Morphogenesis of embryonic CNS vessels

This chapter focuses on the morphology of blood vessel formation in and around the early central nervous system (CNS, i.e., brain and spinal cord) of avian embryos. We discuss cell lineages, proliferation and interactions of endothelial cells, pericytes and smooth muscle cells, and macrophages. Due to space limitations, we can not review the molecular control of CNS angiogenesis, but refer the reader to other chapters in this book and to recent publications on the assembly of the vasculature (1,2).

Potential role of platelets in endothelial damage observed during treatment with cisplatin, gemcitabine, and the angiogenesis inhibitor SU5416

PURPOSE

An increased incidence of thromboembolic events was observed during treatment with cisplatin-gemcitabine plus SU5416 (CG+SU5416), a tyrosine kinase inhibitor targeting the vascular endothelial growth factor (VEGF) receptor-1 and -2. Nine thromboembolic events occurred in eight of 19 patients. We performed an analysis of parameters of the coagulation cascade and vessel wall activation.

MATERIALS AND METHODS

Markers for thrombin generation and endothelial cell activation were measured in three patients treated with CG+SU5416, two of whom developed a thromboembolic event. The results were compared with measurements in six patients treated with CG alone, and in 17 patients treated with SU5416 alone.

RESULTS

During cycles 1 and 2 of treatment with CG+SU5416, a significant cycle-dependent activation of both the coagulation cascade and endothelial cells occurred, whereas platelet counts decreased. Change in platelet number had a significant negative predictive effect on soluble (s)-E-selectin levels. Significant activation of the coagulation cascade only was observed in the patients treated with CG alone, whereas in patients treated with SU5416 alone, significant endothelial cell activation was observed.

CONCLUSION

We hypothesize that endothelial cells deprived of VEGF after exposure to SU5416 became activated and more susceptible to damage during treatment with CG+SU5416, which was aggravated by a transient decrease in platelets, which are, among other things, carriers of VEGF. These results suggests that VEGF, in addition to being a permeability, proliferation, and migration factor, also is a maintenance and protection factor for endothelial cells, and that platelets may have a role in maintaining vascular integrity.

Fluid shear stress-induced transcriptional activation of the vascular endothelial growth factor receptor-2 gene requires Sp1-dependent DNA binding

Hemodynamic forces play a fundamental role in the regulation of endothelial cell survival. As signaling via the vascular endothelial growth factor (VEGF) receptor-2 pathway has been previously demonstrated to impact endothelial cell survival, we hypothesized that laminar shear stress may facilitate survival in part by inducing VEGF receptor-2 expression. This study shows a time- and dose-dependent upregulation of endothelial VEGF receptor-2 expression by fluid shear stress in microvascular and large-vessel derived endothelial cells. A functional analysis of the 5'-regulatory region of the VEGF receptor-2 promoter localized the shear stress-response element to a sequence between bp -60 and -37 that encompasses two adjacent consensus Sp1 transcription factor binding sites. Constitutive and shear stress-inducible Sp1-dependent complexes are bound to this element, indicating that fluid shear stress-induced transcriptional activation of the VEGF receptor-2 gene requires Sp1-dependent DNA binding. Together, these results suggest that biomechanical stimulation may lead to endothelial cell survival by upregulating VEGF receptor-2 expression.

Id1 regulates angiogenesis through transcriptional repression of thrombospondin-1

Id proteins are helix-loop-helix transcription factors that regulate tumor angiogenesis. In order to identify downstream effectors of Id1 involved in the regulation of angiogenesis, we performed PCR-select subtractive hybridization on wild-type and Id1 knockout mouse embryo fibroblasts (MEFs). Here we demonstrate that thrombospondin-1 (TSP-1), a potent inhibitor of angiogenesis, is a target of transcriptional repression by Id1. We also show that Id1-null MEFs secrete an inhibitor of endothelial cell migration, which is completely inactivated by depletion of TSP-1. Furthermore, in vivo studies revealed decreased neovascularization in matrigel assays in Id1-null mice compared to their wild-type littermates. This decrease was completely reversed by a TSP-1 neutralizing antibody. We conclude that TSP-1 is a major target for Id1 effects on angiogenesis.

Antiproliferative, antiangiogenic and proapoptotic activity of h-R3: A humanized anti-EGFR antibody

The epidermal growth factor receptor (EGFR) proto-oncogene is frequently overexpressed in tumors of epithelial origin. This event is thought to be causative for tumor development and progression and henceforth associated with poor prognosis. The recent considerable interest in developing EGFR-targeting agents resulted in derivation of the monoclonal, humanized, neutralizing antibody h-R3, which binds to the extracellular domain of EGFR with high affinity and strongly inhibits EGFR-dependent cellular transformation. Thus, treatment of A431 squamous cell carcinoma cells with h-R3 in either 2-dimensional or 3-dimensional culture resulted in appreciable antimitotic effects through induction of the G1 arrest. Although h-R3 does not appear to have a direct proapoptotic activity in this setting, it inhibits production of the vascular endothelial growth factor (VEGF) by A431 cells both in vitro and in vivo. In the latter case, h-R3 treatment (0.25-1 mg/mouse; every other day per 2 weeks) not only significantly reduced VEGF mRNA expression of A431 tumors growing subcutaneously in SCID mice but also resulted in reduction of the overall microvascular density (MVD), disappearance of dilated "mother vessels," as well as in suppression of tumor growth followed by regression of established tumors. This apparent antiangiogenic activity of h-R3 was associated with reduction in Ki67-positive tumor cell fraction and (unlike in vitro) also with an elevated apoptotic index, the latter indicative of a cytotoxic mode of action in vivo. Taken together, h-R3 is a promising new antagonist of the EGFR oncogene, the anticancer properties of which are associated with combined and potent antiproliferative, antiangiogenic and proapoptotic activity.

Bartonella-associated endothelial proliferation depends on inhibition of apoptosis

Bartonella is a Gram-negative pathogen that is unique among bacteria in being able to induce angioproliferative lesions. Cultured human endothelial cells have provided an in vitro system in which to study the basis of angioproliferation. Previous studies have attributed the organism's ability to induce angioproliferative lesions to direct mitotic stimulation of endothelial cells by these bacteria. Here we show that Bartonella inhibits apoptosis of endothelial cells in vitro, and that its ability to stimulate proliferation of endothelial cells depends to a large extent on its antiapoptotic activity. Bartonella suppresses both early and late events in apoptosis, namely caspase activation and DNA fragmentation, respectively. Its ability to inhibit death of endothelial cells after serum starvation can be recapitulated by media conditioned by bacteria, indicating that direct cell contact is not necessary. Among tested strains, the activity is produced only by Bartonella species that are significant human pathogens and are associated with angioproliferative lesions. We suggest that endothelial cells normally respond to infection by undergoing apoptosis and that Bartonella evolved the antiapoptotic activity to enhance survival of the host cells and therefore itself. We propose that Bartonella's antiapoptotic mechanism accounts at least in part for its ability to induce vascular proliferation in vivo.

Inducer-stimulated Fas targets activated endothelium for destruction by anti-angiogenic thrombospondin-1 and pigment epithelium-derived factor

Natural inhibitors of angiogenesis are able to block pathological neovascularization without harming the preexisting vasculature. Here we show that two such inhibitors, thrombospondin-1 and pigment epithelium-derived factor, derive specificity for remodeling vessels from their dependence on Fas/Fas ligand (FasL)-mediated apoptosis to block angiogenesis. Both inhibitors upregulated FasL on endothelial cells. Expression of the essential partner of FasL, Fas/CD95 receptor, was low on quiescent endothelial cells and vessels but greatly enhanced by inducers of angiogenesis, thereby specifically sensitizing the stimulated cells to apoptosis by inhibitor-generated FasL. The anti-angiogenic activity of thrombospondin-1 and pigment epithelium-derived factor both in vitro and in vivo was dependent on this dual induction of Fas and FasL and the resulting apoptosis. This example of cooperation between pro- and anti-angiogenic factors in the inhibition of angiogenesis provides one explanation for the ability of inhibitors to select remodeling capillaries for destruction.

New vascular tissue rapidly replaces neural parenchyma and vessels destroyed by a contusion injury to the rat spinal cord

Blood vessels identified by laminin staining were studied in uninjured spinal cord and at 2, 4, 7, and 14 days following a moderate contusion (weight drop) injury. At 2 days after injury most blood vessels had been destroyed in the lesion epicenter; neurons and astrocytes were also absent, and few ED1+ cells were seen infiltrating the lesion center. By 4 days, laminin associated with vessel staining was increased and ED1+ cells appeared to be more numerous in the lesion. By 7 days after injury, the new vessels formed a continuous cordon oriented longitudinally through the lesion center. ED1+ cells were abundant at this time point and were found in the same area as the newly formed vessels. Astrocyte migration from the margins of the lesion into the new cordon was apparent. By 14 days, a decrease in the number of vessels in the lesion center was observed; in contrast, astrocytes were more prominent in those areas. In addition to providing a blood supply to the lesion site, protecting the demise of the newly formed vascular bridge might provide an early scaffold to hasten axonal regeneration across the injury site.