The protein kinase MEK1/2 mediate vascular endothelial growth factor- and histamine-induced hyperpermeability in porcine coronary venules

Circulating small extracellular vesicles mediate vascular hyperpermeability in diabetes

AIMS/HYPOTHESIS

A hallmark chronic complication of type 2 diabetes mellitus is vascular hyperpermeability, which encompasses dysfunction of the cerebrovascular endothelium and the subsequent development of associated cognitive impairment. The present study tested the hypothesis that during type 2 diabetes circulating small extracellular vesicles (sEVs) exhibit phenotypic changes that facilitate pathogenic disruption of the vascular barrier.

METHODS

sEVs isolated from the plasma of a mouse model of type 2 diabetes and from diabetic human individuals were characterised for their ability to disrupt the endothelial cell (EC) barrier. The contents of sEVs and their effect on recipient ECs were assessed by proteomics and identified pathways were functionally interrogated with small molecule inhibitors.

RESULTS

Using intravital imaging, we found that diabetic mice (Leprdb/db) displayed hyperpermeability of the cerebrovasculature. Enhanced vascular leakiness was recapitulated following i.v. injection of sEVs from diabetic mice into non-diabetic recipient mice. Characterisation of circulating sEV populations from the plasma of diabetic mice and humans demonstrated increased quantity and size of sEVs compared with those isolated from non-diabetic counterparts. Functional experiments revealed that sEVs from diabetic mice or humans induced the rapid and sustained disruption of the EC barrier through enhanced paracellular and transcellular leak but did not induce inflammation. Subsequent sEV proteome and recipient EC phospho-proteome analysis suggested that extracellular vesicles (sEVs) from diabetic mice and humans modulate the MAPK/MAPK kinase (MEK) and Rho-associated protein kinase (ROCK) pathways, cell-cell junctions and actin dynamics. This was confirmed experimentally. Treatment of sEVs with proteinase K or pre-treatment of recipient cells with MEK or ROCK inhibitors reduced the hyperpermeability-inducing effects of circulating sEVs in the diabetic state.

CONCLUSIONS/INTERPRETATION

Diabetes is associated with marked increases in the concentration and size of circulating sEVs. The modulation of sEV-associated proteins under diabetic conditions can induce vascular leak through activation of the MEK/ROCK pathway. These data identify a new paradigm by which diabetes can induce hyperpermeability and dysfunction of the cerebrovasculature and may implicate sEVs in the pathogenesis of cognitive decline during type 2 diabetes.

Decoding Tumor Angiogenesis for Therapeutic Advancements: Mechanistic Insights

Tumor angiogenesis, the formation of new blood vessels within the tumor microenvironment, is considered a hallmark of cancer progression and represents a crucial target for therapeutic intervention. The tumor microenvironment is characterized by a complex interplay between proangiogenic and antiangiogenic factors, regulating the vascularization necessary for tumor growth and metastasis. The study of angiogenesis involves a spectrum of techniques, spanning from biomarker assessment to advanced imaging modalities. This comprehensive review aims to provide insights into the molecular intricacies, regulatory dynamics, and clinical implications of tumor angiogenesis. By delving into these aspects, we gain a deeper understanding of the processes driving vascularization in tumors, paving the way for the development of novel and effective antiangiogenic therapies in the fight against cancer.

NRP1 interacts with endoglin and VEGFR2 to modulate VEGF signaling and endothelial cell sprouting

Endothelial cells express neuropilin 1 (NRP1), endoglin (ENG) and vascular endothelial growth factor receptor 2 (VEGFR2), which regulate VEGF-A-mediated vascular development and angiogenesis. However, the link between complex formation among these receptors with VEGF-A-induced signaling and biology is yet unclear. Here, we quantify surface receptor interactions by IgG-mediated immobilization of one receptor, and fluorescence recovery after photobleaching (FRAP) measurements of the mobility of another coexpressed receptor. We observe stable ENG/NRP1, ENG/VEGFR2, and NRP1/VEGFR2 complexes, which are enhanced by VEGF-A. ENG augments NRP1/VEGFR2 interactions, suggesting formation of tripartite complexes bridged by ENG. Effects on signaling are measured in murine embryonic endothelial cells expressing (MEEC+/+) or lacking (MEEC-/-) ENG, along with NRP1 and/or ENG overexpression or knockdown. We find that optimal VEGF-A-mediated phosphorylation of VEGFR2 and Erk1/2 requires ENG and NRP1. ENG or NRP1 increase VEGF-A-induced sprouting, becoming optimal in cells expressing all three receptors, and both processes are inhibited by a MEK1/2 inhibitor. We propose a model where the maximal potency of VEGF-A involves a tripartite complex where ENG bridges VEGFR2 and NRP1, providing an attractive therapeutic target for modulation of VEGF-A signaling and biological responses.

The Role and Therapeutic Implications of Inflammation in the Pathogenesis of Brain Arteriovenous Malformations

Brain arteriovenous malformations (bAVMs) are focal vascular lesions composed of abnormal vascular channels without an intervening capillary network. As a result, high-pressure arterial blood shunts directly into the venous outflow system. These high-flow, low-resistance shunts are composed of dilated, tortuous, and fragile vessels, which are prone to rupture. BAVMs are a leading cause of hemorrhagic stroke in children and young adults. Current treatments for bAVMs are limited to surgery, embolization, and radiosurgery, although even these options are not viable for ~20% of AVM patients due to excessive risk. Critically, inflammation has been suggested to contribute to lesion progression. Here we summarize the current literature discussing the role of the immune system in bAVM pathogenesis and lesion progression, as well as the potential for targeting inflammation to prevent bAVM rupture and intracranial hemorrhage. We conclude by proposing that a dysfunctional endothelium, which harbors the somatic mutations that have been shown to give rise to sporadic bAVMs, may drive disease development and progression by altering the immune status of the brain.

Edema and lymphatic clearance: molecular mechanisms and ongoing challenges

Resolution of edema remains a significant clinical challenge. Conditions such as traumatic shock, sepsis, or diabetes often involve microvascular hyperpermeability, which leads to tissue and organ dysfunction. Lymphatic insufficiency due to genetic causes, surgical removal of lymph nodes, or infections, leads to varying degrees of tissue swelling that impair mobility and immune defenses. Treatment options are limited to management of edema as there are no specific therapeutics that have demonstrated significant success for ameliorating microvascular leakage or impaired lymphatic function. This review examines current knowledge about the physiological, cellular, and molecular mechanisms that control microvascular permeability and lymphatic clearance, the respective processes for interstitial fluid formation and removal. Clinical conditions featuring edema, along with potential future directions are discussed.

Cellular mechanisms underlying the impairment of macrophage efferocytosis

The phagocytosis and clearance of dying cells by macrophages, a process termed efferocytosis, is essential for both maintaining homeostasis and promoting tissue repair after infection or sterile injury. If not removed in a timely manner, uncleared cells can undergo secondary necrosis, and necrotic cells lose membrane integrity, release toxic intracellular components, and potentially induce inflammation or autoimmune diseases. Efferocytosis also initiates the repair process by producing a wide range of pro-reparative factors. Accumulating evidence has revealed that macrophage efferocytosis defects are involved in the development and progression of a variety of inflammatory and autoimmune diseases. The underlying mechanisms of efferocytosis impairment are complex, disease-dependent, and incompletely understood. In this review, we will first summarize the current knowledge about the normal signaling and metabolic processes of macrophage efferocytosis and its importance in maintaining tissue homeostasis and repair. We then will focus on analyzing the molecular and cellular mechanisms underlying efferocytotic abnormality (impairment) in disease or injury conditions. Next, we will discuss the potential molecular targets for enhanced efferocytosis in animal models of disease. To provide a balanced view, we will also discuss some deleterious effects of efferocytosis.

Growth Hormone-Releasing Hormone in Endothelial Inflammation

The discovery of hypothalamic hormones propelled exciting advances in pharmacotherapy and improved life quality worldwide. Growth hormone-releasing hormone (GHRH) is a crucial element in homeostasis maintenance, and regulates the release of growth hormone from the anterior pituitary gland. Accumulating evidence suggests that this neuropeptide can also promote malignancies, as well as inflammation. Our review is focused on the role of that 44 - amino acid peptide (GHRH) and its antagonists in inflammation and vascular function, summarizing recent findings in the corresponding field. Preclinical studies demonstrate the protective role of GHRH antagonists against endothelial barrier dysfunction, suggesting that the development of those peptides may lead to new therapies against pathologies related to vascular remodeling (eg, sepsis, acute respiratory distress syndrome). Targeted therapies for those diseases do not exist.

Lactate induces vascular permeability via disruption of VE-cadherin in endothelial cells during sepsis

Circulating lactate levels are a critical biomarker for sepsis and are positively correlated with sepsis-associated mortality. We investigated whether lactate plays a biological role in causing endothelial barrier dysfunction in sepsis. We showed that lactate causes vascular permeability and worsens organ dysfunction in CLP sepsis. Mechanistically, lactate induces ERK-dependent activation of calpain1/2 for VE-cadherin proteolytic cleavage, leading to the enhanced endocytosis of VE-cadherin in endothelial cells. In addition, we found that ERK2 interacts with VE-cadherin and stabilizes VE-cadherin complex in resting endothelial cells. Lactate-induced ERK2 phosphorylation promotes ERK2 disassociation from VE-cadherin. In vivo suppression of lactate production or genetic depletion of lactate receptor GPR81 mitigates vascular permeability and multiple organ injury and improves survival outcome in polymicrobial sepsis. Our study reveals that metabolic cross-talk between glycolysis-derived lactate and the endothelium plays a critical role in the pathophysiology of sepsis.

A narrative review of changes in microvascular permeability after burn

Objective

We aimed to review and discuss some of the latest research results related to post-burn pathophysiological changes and provide some clues for future study.

Background

Burns are one of the most common and serious traumas and consist of a series of pathophysiological changes of thermal injury. Accompanied by thermal damage to skin and soft tissues, inflammatory mediators are released in large quantities. Changes in histamine, bradykinin, and cytokines such as vascular endothelial growth factor (VEGF), metabolic factors such as adenosine triphosphate (ATP), and activated neutrophils all affect the body’s vascular permeability.

Methods

We searched articles with subject words “microvascular permeability”, “burn” “endothelium”, and “endothelial barrier” in PubMed in English published from the beginning of database to Dec, 2020.

Conclusions

The essence of burn shock is the rapid and extensive fluid transfer in burn and non-burn tissue. After severe burns, the local and systemic vascular permeability increase, causing intravascular fluid extravasation, leading to a progressive decrease in effective circulation volume, an increase in systemic vascular resistance, a decrease in cardiac output, peripheral tissue edema, multiple organ failure, and even death. There are many cells, tissues, mediators and structures involved in the pathophysiological process of the damage to vascular permeability. Ulinastatin is a promising agent for this problem.

GHRH antagonists support lung endothelial barrier function

Growth Hormone-Releasing Hormone (GHRH) regulates the release of growth hormone from the anterior pituitary gland. GHRH also acts as a growth and inflammatory factor in a variety of experimental models in oncology. In the current study, we used bovine pulmonary arterial cells in order to investigate the effects of GHRH and its antagonistic and agonistic analogs in key intracellular pathways that regulate endothelial permeability. GHRH antagonists suppressed the activation of MLC2, ERK1/2, JAK2/STAT3 pathway and increased the intracellular P53 and pAMPK levels. In contrast, both GHRH and GHRH agonist MR409 exerted the opposite effects. Furthermore, GHRH antagonists supported the integrity of endothelial barrier, while GHRH and GHRH agonists had the contrary effects, as reflected in measurements of transendothelial resistance. Our observations support the evidence for the anti - inflammatory role of GHRH antagonists in the vasculature. Moreover, our results suggest that GHRH antagonists should be considered as promising therapeutic agents for treating severe respiratory abnormalities, such as the lethal Acute Respiratory Distress Syndrome (ARDS).

Myogenic tone contributes to the regulation of permeability in mesenteric microvessels

The microvascular endothelium plays a key role in regulating solute permeability in the gut, but the contribution of vascular smooth muscle to barrier function is unknown. We sought to determine the role of vascular smooth muscle and its myogenic tone in the vascular barrier to solutes in mesenteric microvessels. We determined vascular permeability to 4.4 kDa and 70 kDa dextrans in isolated mouse mesenteric arteries at increasing pressure increments. The myogenic response was simultaneously monitored using video edge-detection of vessel diameter and wall thickness. We expressed permeability as the apparent permeability coefficient, or the solute flux per second normalized to surface area and concentration gradient. We compared the effects of myogenic tone, L-type calcium channel blockade, calcium elimination, and endothelial removal on the permeability of each dextran. We found arteries resisted changes in 4.4 kDa and 70 kDa dextran permeability coefficients at intravascular pressures associated with myogenic tone. Manipulations that reduced or eliminated myogenic tone (L-type calcium channel blockade or calcium elimination) caused vasodilation and increased permeability coefficients. Thus, the maintenance of a reactive mesenteric vascular smooth muscle layer and its myogenic tone prevents increases in vascular permeability that would otherwise occur with increasing pressure. Conditions that impact vascular tone, such as trauma, stroke, or major surgery could diminish the gut-vascular barrier against dissemination of the microbiome.

Endothelial Protrusions in Junctional Integrity and Barrier Function

Endothelial cells of the microcirculation form a semi-permeable diffusion barrier between the blood and tissues. This permeability of the endothelium, particularly in the capillaries and postcapillary venules, is a normal physiological function needed for blood-tissue exchange in the microcirculation. During inflammation, microvascular permeability increases dramatically and can lead to tissue edema, which in turn can lead to dysfunction of tissues and organs. The molecular mechanisms that control the barrier function of endothelial cells have been under investigation for several decades and remain an important topic due to the potential for discovery of novel therapeutic strategies to reduce edema. This review highlights current knowledge of the cellular and molecular mechanisms that lead to endothelial hyperpermeability during inflammatory conditions associated with injury and disease. This includes a discussion of recent findings demonstrating temporal protrusions by endothelial cells that may contribute to intercellular junction integrity between endothelial cells and affect the diffusion distance for solutes via the paracellular pathway.

Efficient Promotion of Autophagy and Angiogenesis Using Mesenchymal Stem Cell Therapy Enhanced by the Low-Energy Shock Waves in the Treatment of Erectile Dysfunction

Background

Mesenchymal stem cell therapy (MSCT) and defocused low-energy shock wave therapy (ESWT) has been shown to ameliorate erectile dysfunction (ED). However, the interactions and effects of action between MSCT and ESWT remain poorly understood. In this study, we investigated the mechanisms of combination therapy with MSCT and ESWT in a rat model of diabetic ED.

Materials and Methods

Eight-week-old male Sprague-Dawley rats were randomly divided into 2 parts. Diabetic rats induced by streptozotocin (65 mg/kg) were randomly divided into 4 groups: (1) DM control group, (2) DM + ESWT group, (3) DM + MSCT group, and (4) DM + ESWT + MSCT group. The sham group was a normal control group (without streptozotocin). MSCT and (or) ESWT were, respectively, administered to each group according to the proposal for 8 weeks. Immediately after recording of intracavernous pressure (ICP), the penis was then harvested for histologic analysis, ELISA, and Western blotting.

Results

The ratio of ICP/MAP was significantly higher in the DM + ESWT + MSCT group than in ESWT or MSCT treated group (P < 0.05). Also, the treatment stimulated angiogenesis and vasodilatation in the corpus cavernosum (P < 0.05). ESWT increased the quantity of MSCs in the corpus cavernosum and also induced MSCs to express more VEGF in vitro and vivo (P < 0.05) which activated the PI3K/AKT/mTOR and NO/cGMP signaling pathways in the corpus cavernosum. The combination approach stimulated autophagy and decreased apoptosis in the corpus cavernosum. NGF and BDNF expressions were higher in the DM + ESWT + MSCT group than in the DM control group (P < 0.01). Furthermore, the treatment promoted the MSC recruitment by inducing penile tissues to express more PECAM and SDF-1.

Conclusions

Combination of LI-ESWT and MSCT can get a better result than a single treatment by expressing more VEGF which can take part in autophagy by triggering the PI3K/AKT/mTOR signaling pathway. This cooperative therapy would provide a new research direction in ED treatment for the future.

Polarised VEGFA Signalling at Vascular Blood–Neural Barriers

At blood–neural barriers, endothelial VEGFA signalling is highly polarised, with entirely different responses being triggered by luminal or abluminal stimulation. These recent findings were made in a field which is still in its mechanistic infancy. For a long time, endothelial polarity has intuitively been presumed, and likened to that of epithelial cells, but rarely demonstrated. In the cerebral and the retinal microvasculature, the uneven distribution of VEGF receptors 1 and 2, with the former predominant on the luminal and the latter on the abluminal face of the endothelium, leads to a completely polarised signalling response to VEGFA. Luminal VEGFA activates VEGFR1 homodimers and AKT, leading to a cytoprotective response, whilst abluminal VEGFA induces vascular leakage via VEGFR2 homodimers and p38. Whilst these findings do not provide a complete picture of VEGFA signalling in the microvasculature—there are still unclear roles for heterodimeric receptor complexes as well as co-receptors—they provide essential insight into the adaptation of vascular systems to environmental cues that are naturally different, depending on whether they are present on the blood or tissue side. Importantly, sided responses are not only restricted to VEGFA, but exist for other important vasoactive agents.

Effect of aminaphtone on in vitro vascular permeability and capillary-like maintenance

Objectives Aminaphtone, a naphtohydrochinone used in the treatment of capillary disorders, may affect oedema in chronic venous insufficiency. Aim of study is to investigate the effect of aminaphtone on vascular endothelial permeability in vitro and its effects on three-dimensional capillary-like structures formed by human umbilical vein endothelial cells. Method Human umbilical vein endothelial cells were treated with 50 ng/ml VEGF for 2 h and aminaphtone for 6 h. Permeability assay, VE-cadherin expression and Matrigel assay were performed. Results VEGF-induced permeability was significantly decreased by aminaphtone in a range concentration of 1-20 µg/ml. Aminaphtone restored VE-cadherin expression. Finally, 6 h pre-treatment with aminaphtone significantly preserved capillary-like structures formed by human umbilical vein endothelial cells on Matrigel up to 48 h compared to untreated cells. Conclusions Aminaphtone significantly protects endothelium permeability and stabilises endothelial cells organised in capillary-like structures, modulating VE-cadherin expression. These data might explain the clinical benefit of aminaphtone on chronic venous insufficiency.

Involvement of the H1 Histamine Receptor, p38 MAP Kinase, Myosin Light Chains Kinase, and Rho/ROCK in Histamine-Induced Endothelial Barrier Dysfunction

OBJECTIVE

The mechanisms by which histamine increases microvascular permeability remain poorly understood. We tested the hypothesis that H1 receptor activation disrupts the endothelial barrier and investigated potential downstream signals.

METHODS

We used confluent EC monolayers, assessing TER as an index of barrier function. HUVEC, HCMEC, and HDMEC were compared. Receptor expression was investigated using Western blotting, IF confocal microscopy and RT-PCR. Receptor function and downstream signaling pathways were tested using pharmacologic antagonists and inhibitors, respectively.

RESULTS

We identified H1-H4 receptors on all three EC types. H1 antagonists did not affect basal TER but prevented the histamine-induced decrease in TER. Blockade of H2 or H3 attenuated the histamine response only in HDMEC, while inhibition of H4 attenuated the response only in HUVEC. Combined inhibition of both PKC and PI3K caused exaggerated histamine-induced barrier dysfunction in HDMEC, whereas inhibition of p38 MAP kinase attenuated the histamine response in all three EC types. Inhibition of RhoA, ROCK, or MLCK also prevented the histamine-induced decrease in TER in HDMEC.

CONCLUSION

The data suggest that multiple signaling pathways contribute to histamine-induced endothelial barrier dysfunction via the H1 receptor.

Histamine activates p38 MAP kinase and alters local lamellipodia dynamics, reducing endothelial barrier integrity and eliciting central movement of actin fibers

The role of the actin cytoskeleton in endothelial barrier function has been debated for nearly four decades. Our previous investigation revealed spontaneous local lamellipodia in confluent endothelial monolayers that appear to increase overlap at intercellular junctions. We tested the hypothesis that the barrier-disrupting agent histamine would reduce local lamellipodia protrusions and investigated the potential involvement of p38 mitogen-activated protein (MAP) kinase activation and actin stress fiber formation. Confluent monolayers of human umbilical vein endothelial cells (HUVEC) expressing green fluorescent protein-actin were studied using time-lapse fluorescence microscopy. The protrusion and withdrawal characteristics of local lamellipodia were assessed before and after addition of histamine. Changes in barrier function were determined using electrical cell-substrate impedance sensing. Histamine initially decreased barrier function, lamellipodia protrusion frequency, and lamellipodia protrusion distance. A longer time for lamellipodia withdrawal and reduced withdrawal distance and velocity accompanied barrier recovery. After barrier recovery, a significant number of cortical fibers migrated centrally, eventually resembling actin stress fibers. The p38 MAP kinase inhibitor SB203580 attenuated the histamine-induced decreases in barrier function and lamellipodia protrusion frequency. SB203580 also inhibited the histamine-induced decreases in withdrawal distance and velocity, and the subsequent actin fiber migration. These data suggest that histamine can reduce local lamellipodia protrusion activity through activation of p38 MAP kinase. The findings also suggest that local lamellipodia have a role in maintaining endothelial barrier integrity. Furthermore, we provide evidence that actin stress fiber formation may be a reaction to, rather than a cause of, reduced endothelial barrier integrity.

Stromal interaction molecule 1 (STIM1) and Orai1 mediate histamine-evoked calcium entry and nuclear factor of activated T-cells (NFAT) signaling in human umbilical vein endothelial cells

Histamine is an important immunomodulator involved in allergic reactions and inflammatory responses. In endothelial cells, histamine induces Ca(2+) mobilization by releasing Ca(2+) from the endoplasmic reticulum and eliciting Ca(2+) entry across the plasma membrane. Herein, we show that histamine-evoked Ca(2+) entry in human umbilical vein endothelial cells (HUVECs) is sensitive to blockers of Ca(2+) release-activated Ca(2+) (CRAC) channels. RNA interference against STIM1 or Orai1, the activating subunit and the pore-forming subunit of CRAC channels, respectively, abolishes this histamine-evoked Ca(2+) entry. Furthermore, overexpression of dominant-negative CRAC channel subunits inhibits while co-expression of both STIM1 and Orai1 enhances histamine-induced Ca(2+) influx. Interestingly, gene silencing of STIM1 or Orai1 also interrupts the activation of calcineurin/nuclear factor of activated T-cells (NFAT) pathway and the production of interleukin 8 triggered by histamine in HUVECs. Collectively, these results suggest a central role of STIM1 and Orai1 in mediating Ca(2+) mobilization linked to inflammatory signaling of endothelial cells upon histamine stimulation.

Myosin light chain kinase signaling in endothelial barrier dysfunction

Microvascular barrier dysfunction is a serious problem that occurs in many inflammatory conditions, including sepsis, trauma, ischemia-reperfusion injury, cardiovascular disease, and diabetes. Barrier dysfunction permits extravasation of serum components into the surrounding tissue, leading to edema formation and organ failure. The basis for microvascular barrier dysfunction is hyperpermeability at endothelial cell-cell junctions. Endothelial hyperpermeability is increased by actomyosin contractile activity in response to phosphorylation of myosin light chain by myosin light chain kinase (MLCK). MLCK-dependent endothelial hyperpermeability occurs in response to inflammatory mediators (e.g., activated neutrophils, thrombin, histamine, tumor necrosis factor alpha, etc.), through multiple cell signaling pathways and signaling molecules (e.g., Ca(++) , protein kinase C, Src kinase, nitric oxide synthase, etc.). Other signaling molecules protect against MLCK-dependent hyperpermeability (e.g., sphingosine-1-phosphate or cAMP). In addition, individual MLCK isoforms play specific roles in endothelial barrier dysfunction, suggesting that isoform-specific inhibitors could be useful for treating inflammatory disorders and preventing multiple organ failure. Because endothelial barrier dysfunction depends upon signaling through MLCK in many instances, MLCK-dependent signaling comprises multiple potential therapeutic targets for preventing edema formation and multiple organ failure. The following review is a discussion of MLCK-dependent mechanisms and cell signaling events that mediate endothelial hyperpermeability.

Tying the knot: the cystine signature and molecular-recognition processes of the vascular endothelial growth factor family of angiogenic cytokines

The cystine-knot motif, made up of three intertwined disulfide bridges, is a unique feature of several toxins, cyclotides and growth factors, and occurs in a variety of species, including fungi, insects, molluscs and mammals. Growth factor molecules containing the cystine-knot motif serve as ligands for a diverse range of receptors and play an important role in extracellular signalling. This superfamily of polypeptides comprises several homodimeric and heterodimeric molecules that are central characters in both health and disease. Amongst these molecules are a group of proteins that belong to the vascular endothelial growth factor (VEGF) subfamily. The members of this family are known angiogenic factors that regulate processes leading to blood vessel formation in physiological and pathological conditions. The focus of the present review is on the structural characteristics of proteins that belong to the VEGF family and on signal-transduction pathways that become initiated via the VEGF receptors.

Nitric oxide and adverse events of vascular endothelial growth factor inhibitors

BACKGROUND

Angiogenesis inhibitors targeting the vascular endothelial growth factor (VEGF) signalling pathways have demonstrated therapeutic efficacy in a wide variety of malignancies. With their increased use, adverse events, some common and certain rare but characteristic complications, are being recognized. The exact mechanisms for the development of at least some of these side effects are not entirely clear.

SCOPE

Review of the current literature with respect to mechanisms for the development of side effects to VEGF blocking agents was reviewed.

FINDINGS

Nitric oxide is a key molecule in the downstream signalling pathway for VEGF. Blockade of nitric oxide by the VEGF antagonists results in several class-specific adverse events.

CONCLUSION

Nitric oxide can be considered as an important factor in the development of most of the common and rare adverse events related to VEGF antagonists.

Tissue inhibitor of metalloproteinase-2 regulates matrix metalloproteinase-2-mediated endothelial barrier dysfunction and breast cancer cell transmigration through lung microvascular endothelial cells

Matrix metalloproteinases (MMP) have been implicated in multiple stages of cancer metastasis. Tissue inhibitor of metalloproteinase-2 (TIMP-2) plays an important role in regulating MMP-2 activity. By forming a ternary complex with pro-MMP-2 and its activator MMP-14 on the cell surface, TIMP-2 can either initiate or restrain the cleavage and subsequent activation of MMP-2. Our recent work has shown that breast cancer cell adhesion to vascular endothelial cells activates endothelial MMP-2, promoting tumor cell transendothelial migration (TEM(E)). However, the mechanism of MMP-2 regulation during TEM(E) remains unclear. In the current study, we present evidence that MMP-14 is expressed in both invasive breast cancer cells (MDA-MB-231 and MDA-MB-436) and lung microvascular endothelial cells (HBMVEC-L), whereas TIMP-2 is exclusively expressed and released from the cancer cells. The tumor cell-derived TIMP-2 was further identified as a major determinant of endothelial MMP-2 activity during tumor cell transmigration in the presence of MMP-14. This response was associated with endothelial barrier dysfunction because coculture of MDA-MB-231 or MDA-MB-436 with HBMVEC-L caused a significant decrease in transendothelial electrical resistance concomitantly with endothelial cell-cell junction disruption and tumor cell transmigration. Knockdown of TIMP-2 or inhibition of TIMP-2/MMP-14 attenuated MMP-2-dependent transendothelial electrical resistance response and TEM(E). These findings suggest a novel interactive role of breast cancer cells and vascular endothelial cells in regulating the TIMP-2/MMP-14/MMP-2 pathway during tumor metastasis.

Myosin light chain kinase in microvascular endothelial barrier function

Microvascular barrier dysfunction is implicated in the initiation and progression of inflammation, posttraumatic complications, sepsis, ischaemia-reperfusion injury, atherosclerosis, and diabetes. Under physiological conditions, a precise equilibrium between endothelial cell-cell adhesion and actin-myosin-based centripetal tension tightly controls the semi-permeability of microvascular barriers. Myosin light chain kinase (MLCK) plays an important role in maintaining the equilibrium by phosphorylating myosin light chain (MLC), thereby inducing actomyosin contractility and weakening endothelial cell-cell adhesion. MLCK is activated by numerous physiological factors and inflammatory or angiogenic mediators, causing vascular hyperpermeability. In this review, we discuss experimental evidence supporting the crucial role of MLCK in the hyperpermeability response to key cell signalling events during inflammation. At the cellular level, in vitro studies of cultured endothelial monolayers treated with MLCK inhibitors or transfected with specific inhibiting peptides have demonstrated that induction of endothelial MLCK activity is necessary for hyperpermeability. Ex vivo studies of live microvessels, enabled by development of the isolated, perfused venule method, support the importance of MLCK in endothelial permeability regulation in an environment that more closely resembles in vivo tissues. Finally, the role of MLCK in vascular hyperpermeability has been confirmed with in vivo studies of animal disease models and the use of transgenic MLCK210 knockout mice. These approaches provide a more complete view of the role of MLCK in vascular barrier dysfunction.

Vascular endothelial growth factors and vascular permeability

Vascular endothelial growth factors (VEGFs) are key regulators of permeability. The principal evidence behind how they increase vascular permeability in vivo and in vitro and the consequences of that increase are addressed here. Detailed analysis of the published literature has shown that in vivo and in vitro VEGF-mediated permeability differs in its time course, but has common involvement of many specific signalling pathways, in particular VEGF receptor-2 activation, calcium influx through transient receptor potential channels, activation of phospholipase C gamma and downstream activation of nitric oxide synthase. Pathways downstream of endothelial nitric oxide synthase appear to involve the guanylyl cyclase-mediated activation of the Rho–Rac pathway and subsequent involvement of junctional signalling proteins such as vascular endothelial cadherin and the tight junctional proteins zona occludens and occludin linked to the actin cytoskeleton. The signalling appears to be co-ordinated through spatial organization of the cascade into a signalplex, and arguments for why this may be important are considered. Many proteins have been identified to be involved in the regulation of vascular permeability by VEGF, but still the mechanisms through which these are thought to interact to control permeability are dependent on the experimental system, and a synthesis of existing data reveals that in intact vessels the co-ordination of the pathways is still not understood.

ADAM15 regulates endothelial permeability and neutrophil migration via Src/ERK1/2 signalling

AIMS

Endothelial barrier dysfunction is a key event in the pathogenesis of vascular diseases associated with inflammation. ADAM (a disintegrin and metalloprotease) 15 has been shown to contribute to the development of vascular inflammation. However, its role in regulating endothelial barrier function is unknown. The aim of this study was to examine the effect of ADAM15 on endothelial permeability and its underlying mechanisms.

METHODS AND RESULTS

By measuring albumin transendothelial flux and transendothelial electric resistance in cultured human umbilical vein endothelial cell monolayers, we found that depletion of ADAM15 expression via siRNA decreased endothelial permeability and attenuated thrombin-induced barrier dysfunction. In contrast, endothelial cells overexpressing either wild-type or catalytically dead mutant ADAM15 displayed a higher basal permeability and augmented hyperpermeability in response to thrombin. In addition, ADAM15 knockdown inhibited whereas ADAM15 overexpression promoted neutrophil transendothelial migration. Further molecular assays revealed that ADAM15 did not cleave vascular endothelial-cadherin or cause its degradation. However, overexpression of ADAM15 promoted extracellular signal-regulated kinase (ERK)1/2 phosphorylation in both non-stimulated and thrombin-stimulated endothelial cells in a protease activity-independent manner. Pharmacological inhibition of Src kinase or ERK activation reversed ADAM15-induced hyperpermeability and neutrophil transmigration.

CONCLUSION

The data provide evidence for a novel function of ADAM15 in regulating endothelial barrier properties. The mechanisms of ADAM15-induced hyperpermeability involve Src/ERK1/2 signalling independent of junction molecule shedding.

Endothelial contractile cytoskeleton and microvascular permeability

Microvascular barrier dysfunction represents a significant problem in clinical conditions associated with trauma, burn, sepsis, acute respiratory distress syndrome, ischemia-reperfusion injury, and diabetic retinopathy. An important cellular mechanism underlying microvascular leakage is the generation of contractile force from the endothelial cytoskeleton, which counteracts cell-cell and cell-matrix adhesions leading to paracellular hyperpermeability. In this review, we present recent experimental evidence supporting the critical role of MLCK-activated, RhoA/ROCK-regulated contractile cytoskeleton in endothelial permeability response to inflammatory and thrombotic stimuli arising from thermal injury, activated neutrophils, vascular endothelial growth factor, and fibrinogen degradation products. Further understanding the molecular basis of microvascular barrier structure and function would contribute to the development of novel therapeutic targets for treating circulatory disorders and vascular injury.

Molecular mechanisms of endothelial hyperpermeability: implications in inflammation

Endothelial hyperpermeability is a significant problem in vascular inflammation associated with trauma, ischaemia-reperfusion injury, sepsis, adult respiratory distress syndrome, diabetes, thrombosis and cancer. An important mechanism underlying this process is increased paracellular leakage of plasma fluid and protein. Inflammatory stimuli such as histamine, thrombin, vascular endothelial growth factor and activated neutrophils can cause dissociation of cell-cell junctions between endothelial cells as well as cytoskeleton contraction, leading to a widened intercellular space that facilitates transendothelial flux. Such structural changes initiate with agonist-receptor binding, followed by activation of intracellular signalling molecules including calcium, protein kinase C, tyrosine kinases, myosin light chain kinase, and small Rho-GTPases; these kinases and GTPases then phosphorylate or alter the conformation of different subcellular components that control cell-cell adhesion, resulting in paracellular hypermeability. Targeting key signalling molecules that mediate endothelial-junction-cytoskeleton dissociation demonstrates a therapeutic potential to improve vascular barrier function during inflammatory injury.

Hypoxia modulation of caveolin-1 and vascular endothelial growth factor in ovine fetal membranes

During normal pregnancy, amniotic fluid is absorbed from the amniotic compartment into fetal blood through the intramembranous blood vessels in the fetal membranes. It has been hypothesized that this transport process is mediated by transcytosis of caveolae-like vesicles. Because fetal hypoxia increases intramembranous absorption, the authors explore the effects of hypoxia on the gene expression of caveolin-1, a structural protein of caveolae, in ovine fetal membranes and cultured amnion cells. Near-term ovine fetuses were rendered hypoxic for 4 days. Caveolin-1 mRNA and protein levels were significantly reduced in the amnion and chorion but not in the placenta. In cultured ovine amnion cells incubated in 2% oxygen for 24 hours, hypoxia did not significantly alter caveolin-1 mRNA or protein expression. Vascular endothelial growth factor mRNA levels were increased in response to hypoxia in the fetal membranes as well as in cultured amnion cells. The results indicate that hypoxia does not augment but instead down-regulates or has no effect on caveolin-1 gene expression in the amnion and chorion, suggesting that caveolin-1 may play a role as a negative regulator of amnion transport function under hypoxic conditions.

Counter regulatory effects of PKCbetaII and PKCdelta on coronary endothelial permeability

OBJECTIVE

The aim of this study was to examine the endothelial distribution and activity of selected PKC isoforms in coronary vessels with respect to their functional impact on endothelial permeability under the experimental conditions relevant to diabetes.

METHODS AND RESULTS

En face immunohistochemistry demonstrated a significant increase of PKC(betaII) and decrease of PKCdelta expression in coronary arterial endothelium of Zucker diabetic rats. To test whether changes in PKC expression alter endothelial barrier properties, we measured the transcellular electric resistance in human coronary microvascular endothelial monolayers and found that either PKC(betaII) overexpression or PKCdelta inhibition disrupted the cell-cell adhesive barrier. Three-dimensional fluorescence microscopy revealed that hyperpermeability was caused by altered PKC activity in association with distinct translocation of PKC(betaII) to the cell-cell junction and PKCdelta localization to the cytosol. Further analyses in fractionated endothelial lysates confirmed the differential redistribution of these isozymes. Additionally, FRET analysis of PKC subcellular dynamics demonstrated a high PKC(betaII) activity at the cell surface and junction, whereas PKCdelta activity is concentrated in intracellular membrane organelles.

CONCLUSIONS

Taken together, these data suggest that PKC(betaII) and PKCdelta counter-regulate coronary endothelial barrier properties by targeting distinctive subcellular sites. Imbalanced PKC(betaII)/PKCdelta expression and activity may contribute to endothelial hyperpermeability and coronary dysfunction in diabetes.

VE-cadherin and beta-catenin binding dynamics during histamine-induced endothelial hyperpermeability

Beta-catenin plays an important role in the regulation of vascular endothelial cell-cell adhesions and barrier function by linking the VE-cadherin junction complex to the cytoskeleton. The purpose of this study was to evaluate the effect of beta-catenin and VE-cadherin interactions on endothelial permeability during inflammatory stimulation by histamine. We first assessed the ability of a beta-catenin binding polypeptide known as inhibitor of beta-catenin and T cell factor (ICAT) to compete beta-catenin binding to VE-cadherin in vitro. We then overexpressed recombinant FLAG-ICAT in human umbilical vein endothelial cells (HUVECs) to study its impact on endothelial barrier function controlled by cell-cell adhesions. The binding of beta-catenin to VE-cadherin was quantified before and after stimulation with histamine along with measurements of transendothelial electrical resistance (TER) and apparent permeability to albumin (P(a)) under the same conditions. The results showed that ICAT bound to beta-catenin and competitively inhibited binding of the VE-cadherin cytoplasmic domain to beta-catenin in a concentration-dependent manner. Overexpression of FLAG-ICAT in endothelial cell monolayers did not affect their basal permeability properties, as indicated by unaltered TER and P(a); however, the magnitude and duration of histamine-induced decreases in TER were significantly augmented. Likewise, the increase in P(a) in the presence of histamine was exacerbated. Overexpression of FLAG-ICAT also significantly decreased the level of beta-catenin-associated VE-cadherin following histamine stimulation. Taken together, these data suggest that inflammatory agents like histamine cause a transient and reversible disruption of binding between beta-catenin and VE-cadherin, during which endothelial permeability is elevated.

Bradykinin- and substance P-induced edema formation in the hamster cheek pouch is tyrosine kinase dependent

The purpose of this study was to determine whether protein tyrosine kinase, a ubiquitous family of intracellular signaling enzymes that regulates endothelial cell function, modulates bradykinin- and substance P-induced increase in macromolecular efflux from the intact hamster cheek pouch microcirculation. Using intravital microscopy, I found that suffusion of bradykinin or substance P (each, 0.5 and 1.0 microM) onto the cheek pouch elicited significant, concentration-dependent leaky site formation and increase in clearance of fluorescein isothiocyanate-dextran (FITC-dextran; molecular mass, 70 kDa; P < 0.05). These responses were significantly attenuated by suffusion of genistein (1.0 microM) or tyrphostin 25 (10 microM), two structurally unrelated, nonspecific protein tyrosine kinase inhibitors (P < 0.05). Conceivably, the kinase(s) involved in this process could be agonist specific because genistein was more effective than tyrphostin 25 in attenuating bradykinin-induced responses while the opposite was observed with substance P. Both inhibitors had no significant effects on adenosine (0.5 M)-induced responses (P > 0.5). Collectively, these data suggest that the protein tyrosine kinase metabolic pathway modulates, in part, the edemagenic effects of bradykinin and substance P in the intact hamster cheek pouch microcirculation in a specific fashion.

Induction of vascular permeability: beta PIX and GIT1 scaffold the activation of extracellular signal-regulated kinase by PAK

Increased permeability of blood vessels is an important component of inflammation, but in some circumstances it contributes to tissue injury and organ failure. Previous work showed that p21-activated kinase (PAK) is a critical regulator of endothelial cell-cell junctions through effects on myosin light chain phosphorylation and cell contractility. We now show that blocking PAK function inhibits fluid leak in a mouse model of acute lung injury. In cultured endothelial cells, induction of myosin light chain phosphorylation by PAK is mediated by mitogen-activated protein kinase kinase and extracellular signal-regulated kinase (Erk). Erk in lipopolysaccharide (LPS)-treated mouse lung is activated in a PAK-dependent manner in several cell types, most prominently vascular endothelium. Activation of Erk requires the integrity of the complex between PAK, PIX, and GIT1. Several means of disrupting this complex inhibit stimulation of vascular permeability in vitro. A cell-permeant peptide that blocks binding of PAK to PIX inhibits LPS-induced fluid leak in the mouse lung injury model. We conclude that the PAK-PIX-GIT1 complex is critical for Erk-dependent myosin phosphorylation and vascular permeability.

Rho and ROCK signaling in VEGF-induced microvascular endothelial hyperpermeability

OBJECTIVES

Vascular endothelial growth factor (VEGF) plays an important role in the regulation of microvascular permeability under various physiological and pathological conditions. The authors tested the hypothesis that the small GTPase Rho and its downstream effector ROCK (Rho-associated coiled-coil-containing protein kinase) mediate VEGF-induced increases in venular permeability. They also investigated myosin light chain (MLC) phosphorylation and actin polymerization, two well-characterized targets of the Rho-ROCK pathway that are implicated in the regulation of endothelial barrier function.

METHODS

The apparent permeability coefficient of albumin (P(a)) was measured in intact isolated porcine coronary venules and in cultured coronary venular endothelial cell (CVEC) monolayers. RhoA activation was determined using a Rhotekin-agarose pull down assay. MLC phosphorylation was evaluated by immunoblotting with phospho-specific antibodies, and endothelial cellular F-actin was viewed using fluorescence microscopy.

RESULTS

VEGF increased P(a) in both isolated coronary venules and CVEC monolayers. The hyperpermeability response occurred in a similar time course to that of Rho activation, MLC phosphorylation, and actin stress fiber formation. Selective blockage of ROCK with Y27632 dose-dependently inhibited VEGF-induced venular hyperpermeability. Moreover, inhibition of either Rho with exoenzyme C3 or ROCK with Y-27632 attenuated VEGF-induced increases in permeability, MLC phosphorylation, and actin-stress fiber formation in CVEC monolayers.

CONCLUSIONS

Collectively, these findings suggest that the Rho-ROCK signal pathway contributes to VEGF-induced hyperpermeability. Myosin light-chain phosphorylation and actin stress fiber formation occur concomitantly with the increase in permeability upon VEGF stimulation.

Signaling pathways mediating VEGF165-induced calcium transients and membrane depolarization in human endothelial cells

Cytosolic calcium and membrane potential were monitored simultaneously in quiescent human umbilical vein endothelial cells (HUVEC) exposed to vascular endothelial growth factor (VEGF)165 using the fluorescent indicators indo-1 AM and DiSBAC2(3), respectively. Application of VEGF165 to cells elicits a rapid rise in cytosolic calcium followed by a slower decline toward control values. Peak calcium is associated with a slight membrane hyperpolarization; however, as calcium falls toward control, a strong depolarization develops and is sustained throughout a 10-min period of VEGF165 stimulation. Both the VEGF165-mediated rise in cytosolic calcium and membrane depolarization are eliminated by inhibitors of VEGFR-2, tyrosine kinase, src kinase and inositol-1,4,5 triphosphate-operated calcium channels. Calcium entry, which is initially facilitated by transient hyperpolarization, is restricted by a substantial, sustained depolarization that developed during the downstroke of the calcium spike. Inhibition of plasmalemmal calcium channels diminished the magnitude and duration of the calcium spike, suggesting that extracellular calcium influx, secondary to stores release, is a significant component of the calcium transient. Inhibition of chloride channels substantially reduced membrane depolarization. In addition, the depolarization is modulated by PI3 kinase in a ras-independent manner. In summary, intracellular calcium and membrane potential are influenced by several key signaling cascades of VEGFR-2 activation in HUVEC.

Signaling Mechanisms Regulating Endothelial Permeability

The microvascular endothelial cell monolayer localized at the critical interface between the blood and vessel wall has the vital functions of regulating tissue fluid balance and supplying the essential nutrients needed for the survival of the organism. The endothelial cell is an exquisite “sensor” that responds to diverse signals generated in the blood, subendothelium, and interacting cells. The endothelial cell is able to dynamically regulate its paracellular and transcellular pathways for transport of plasma proteins, solutes, and liquid. The semipermeable characteristic of the endothelium (which distinguishes it from the epithelium) is crucial for establishing the transendothelial protein gradient (the colloid osmotic gradient) required for tissue fluid homeostasis. Interendothelial junctions comprise a complex array of proteins in series with the extracellular matrix constituents and serve to limit the transport of albumin and other plasma proteins by the paracellular pathway. This pathway is highly regulated by the activation of specific extrinsic and intrinsic signaling pathways. Recent evidence has also highlighted the importance of the heretofore enigmatic transcellular pathway in mediating albumin transport via transcytosis. Caveolae, the vesicular carriers filled with receptor-bound and unbound free solutes, have been shown to shuttle between the vascular and extravascular spaces depositing their contents outside the cell. This review summarizes and analyzes the recent data from genetic, physiological, cellular, and morphological studies that have addressed the signaling mechanisms involved in the regulation of both the paracellular and transcellular transport pathways.

Macromolecule permeability of in situ and excised rodent skeletal muscle arterioles and venules

In microvessels, acute inflammation is typified by an increase in leukocyte-endothelial cell interactions, culminating in leukocyte transmigration into the tissue, and increased permeability to water and solutes, resulting in tissue edema. The goal of this study was to establish a method to quantify solute permeability (P(s)) changes in microvessels in intact predominantly blood-perfused networks in which leukocyte transmigratory behavior could be precisely described using established paradigms. We used intravital confocal microscopy to measure solute (BSA) flux across microvessel walls, hence P(s). A quantitative fluorescence approach (Huxley VH, Curry FE, and Adamson RH. Am J Physiol Heart Circ Physiol 252: H188-H197, 1987) was adapted to the imaged confocal tissue slice in which the fluorescent source volume and source surface area of the microvessel were restricted to the region of vessel that was contained within the imaged confocal tissue section. P(s) measurements were made in intact cremaster muscle microvasculature of anesthetized mice and compared with measurements of P(s) made in isolated rat skeletal muscle microvessels. Mouse arteriolar P(s) was 9.9 +/- 1.1 x 10(-7) cm/s (n = 16), which was not different from 8.4 +/- 1.3 x 10(-7) cm/s (n = 6) in rat arterioles. Values in venules were significantly (P < 0.05) higher: 44.4 +/- 7.9 x 10(-7) cm/s (n = 14) in mice and 25.0 +/- 3.7 x 10(-7) cm/s in rats. Convective coupling was estimated to contribute <10% to the measured P(s) in both microvessel types and both animal models. We conclude that this approach provides an appropriate quantification of P(s) in the intact microvasculature and that arteriolar P(s), while lower than in venules, is nevertheless consistent with arterioles being a significant source of interstitial protein.

Vagal control of the heart: central serotonergic (5-HT) mechanisms

Cardiac vagal preganglionic neurones (CVPNs) are located within the dorsal vagal nucleus (DVN) and the nucleus ambiguus (nA). In mammals, CVPNs within the nA have small myelinated axons and mediate major chronotropic effects, those in the DVN have non-myelinated axons and mediate smaller chronotropic, dromotropic and inotropic effects. Numerous studies demonstrate important influences of serotonin (5-HT) at multiple sites controlling autonomic outflows including the nucleus tractus solitarius (NTS) where cardiorespiratory afferent fibres terminate, and the CVPNs and rostral ventrolateral medulla (RVLM), the location of sympathetic premotor neurones. We have demonstrated roles for some of the numerous 5-HT receptor subtypes (5-HT1, 5-HT2, 5-HT3, 5-HT4 and 5-HT7) in brainstem regions involved in cardiac control. Intracisternal application of selective ligands was used to study the effect of 5-HT receptors on heart rate and its reflex control. Further electrophysiological studies were also carried out to delineate their location and the mechanisms of action of these ligands. Blocking 5-HT1A receptors attenuated bradycardias evoked by stimulating baroreceptor and cardiopulmonary afferents but not arterial chemoreceptors, whereas antagonizing 5-HT7 receptors markedly attenuated all these reflex bradycardias. Within the DVN, nA and NTS, activation of 5-HT1A receptors could excite or inhibit neurones. In the NTS 5-HT2 receptors also had variable effects; 5-HT2B receptors excite and 5-HT2C receptors inhibit. Antagonism of 5-HT3 receptors attenuated upper airway and cardiopulmonary reflex bradycardias; this is compatible with data showing that 5-HT3 receptors excite DVN and NTS neurones by a glutamate-dependent mechanism. The origin of the glutamate (neuronal or glial) remains unresolved but glia are a possibility as baroreceptor-sensitive NTS neurones receive few direct 5-HT-containing synaptic contacts. Thus, 5-HT plays a critical role in the control of vagal outflow to the heart; however, why so many different receptors are involved, and their relative functional roles, remains unresolved.