Leakiness of blood vessels in tumors may contribute to disease progression and is key to certain forms of cancer therapy, but the structural basis of the leakiness is unclear. We sought to determine whether endothelial gaps or transcellular holes, similar to those found in leaky vessels in inflammation, could explain the leakiness of tumor vessels. Blood vessels in MCa-IV mouse mammary carcinomas, which are known to be unusually leaky (functional pore size 1.2-2 m), were compared to vessels in three less leaky tumors and normal mammary glands

Endothelial cells of tumor vessels have well-documented alterations, but it is less clear whether pericytes on these vessels are abnormal or even absent.Here we report that ␣-smooth muscle actin (␣-SMA) and desmin-immunoreactive pericytes were present on >97% of blood vessels viewed by confocal microscopy in 100-m-thick sections of three different spontaneous or implanted tumors in mice. However, the cells had multiple abnormalities. Unlike pericytes on capillaries in normal pancreatic islets, which had desmin but not ␣-SMA immunoreactivity, pericytes on capillary-size vessels in insulinomas in RIP-Tag2 transgenic mice expressed both desmin and ␣-SMA. Furthermore, pericytes in RIP-Tag2 tumors, as well as those in MCa-IV breast carcinomas and Lewis lung carcinomas, had an abnormally loose association with endothelial cells and extended cytoplasmic processes deep into the tumor tissue. ␣-SMA-positive pericytes also covered 73% of endothelial sprouts in RIP-Tag2 tumors and 92% of sprouts in the other tumors. Indeed, pericyte sleeves were significantly longer than the CD31-immunoreactive endothelial cell sprouts themselves in all three types of tumors. All three tumors also contained ␣-SMA-positive myofibroblasts that resembled pericytes but were not associated with blood vessels. We conclude that pericytes are present on most tumor vessels but have multiple abnormalities, including altered expression of marker proteins. In contrast to some previous studies, the almost ubiquitous presence of pericytes on tumor vessels found in the present study may be attributed to our use of both desmin and ␣-SMA

Angiogenesis inhibitors are receiving increased attention as cancer therapeutics, but little is known of the cellular effects of these inhibitors on tumor vessels. We sought to determine whether two agents, AG013736 and VEGF-Trap, that inhibit vascular endothelial growth factor (VEGF) signaling, merely stop angiogenesis or cause regression of existing tumor vessels. Here, we report that treatment with these inhibitors caused robust and early changes in endothelial cells, pericytes, and basement membrane of vessels in spontaneous islet-cell tumors of RIP-Tag2 transgenic mice and in subcutaneously implanted Lewis lung carcinomas. Strikingly, within 24 hours, endothelial fenestrations in RIP-Tag2 tumors disappeared, vascular sprouting was suppressed, and patency and blood flow ceased in some vessels. By 7 days, vascular density decreased more than 70%, and VEGFR-2 and VEGFR-3 expression was reduced in surviving endothelial cells.

Recirculation of fluid and cells through lymphatic vessels plays a key role in normal tissue homeostasis, inflammatory diseases, and cancer. Despite recent advances in understanding lymphatic function (Alitalo, K., T. Tammela, and T.V. Petrova. 2005. Nature. 438:946–953), the cellular features responsible for entry of fluid and cells into lymphatics are incompletely understood. We report the presence of novel junctions between endothelial cells of initial lymphatics at likely sites of fluid entry. Overlapping flaps at borders of oak leaf–shaped endothelial cells of initial lymphatics lacked junctions at the tip but were anchored on the sides by discontinuous button-like junctions (buttons) that differed from conventional, continuous, zipper-like junctions (zippers) in collecting lymphatics and blood vessels. However, both buttons and zippers were composed of vascular endothelial cadherin (VE-cadherin) and tight junction–associated proteins, including occludin, claudin-5, zonula occludens–1, junctional adhesion molecule–A, and endothelial cell–selective adhesion molecule. In C57BL/6 mice, VE-cadherin was required for maintenance of junctional integrity, but platelet/endothelial cell adhesion molecule–1 was not. Growing tips of lymphatic sprouts had zippers, not buttons, suggesting that buttons are specialized junctions rather than immature ones. Our findings suggest that fluid enters throughout initial lymphatics via openings between buttons, which open and close without disrupting junctional integrity, but most leukocytes enter the proximal half of initial lymphatics.

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.

Exaggerated levels of VEGF (vascular endothelial growth factor) are present in persons with asthma, but the role(s) of VEGF in normal and asthmatic lungs has not been defined. We generated lung-targeted VEGF(165) transgenic mice and evaluated the role of VEGF in T-helper type 2 cell (T(H)2)-mediated inflammation. In these mice, VEGF induced, through IL-13-dependent and -independent pathways, an asthma-like phenotype with inflammation, parenchymal and vascular remodeling, edema, mucus metaplasia, myocyte hyperplasia and airway hyper-responsiveness. VEGF also enhanced respiratory antigen sensitization and T(H)2 inflammation and increased the number of activated DC2 dendritic cells. In antigen-induced inflammation, VEGF was produced by epithelial cells and preferentially by T(H)2 versus T(H)1 cells. In this setting, it had a critical role in T(H)2 inflammation, cytokine production and physiologic dysregulation. Thus, VEGF is a mediator of vascular and extravascular remodeling and inflammation that enhances antigen sensitization and is crucial in adaptive T(H)2 inflammation. VEGF regulation may be therapeutic in asthma and other T(H)2 disorders.

Endothelial adherens junctions maintain vascular integrity. Arteries and veins differ in their permeability but whether organization and strength of their adherens junctions vary has not been demonstrated in vivo. Here we report that vascular endothelial cadherin, an endothelial specific adhesion protein located at adherens junctions, is phosphorylated in Y658 and Y685 in vivo in veins but not in arteries under resting conditions. This difference is due to shear stress-induced junctional Src activation in veins. Phosphorylated vascular endothelial-cadherin is internalized and ubiquitinated in response to permeability-increasing agents such as bradykinin and histamine. Inhibition of Src blocks vascular endothelial cadherin phosphorylation and bradykinin-induced permeability. Point mutation of Y658F and Y685F prevents vascular endothelial cadherin internalization, ubiquitination and an increase in permeability by bradykinin in vitro. Thus, phosphorylation of vascular endothelial cadherin contributes to a dynamic state of adherens junctions, but is not sufficient to increase vascular permeability in the absence of inflammatory agents.

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