Contribution of Extrahepatic Aldehyde Oxidase Activity to Human Clearance.

Aldehyde oxidase (AOX) is a soluble, cytosolic enzyme that metabolizes various N-heterocyclic compounds and organic aldehydes. It has wide tissue distribution with highest levels found in liver, kidney, and lung. Human clearance projections of AOX substrates by in vitro assessments in isolated liver fractions (cytosol, S9) and even hepatocytes have been largely underpredictive of clinical outcomes. Various hypotheses have been suggested as to why this is the case. One explanation is that extrahepatic AOX expression contributes measurably to AOX clearance and is at least partially responsible for the often observed underpredictions. Although AOX expression has been confirmed in several extrahepatic tissues, activities therein and potential contribution to overall human clearance have not been thoroughly studied. In this work, the AOX enzyme activity using the S9 fractions of select extrahepatic human tissues (kidney, lung, vasculature, and intestine) were measured using carbazeran as a probe substrate. Measured activities were scaled to a whole-body clearance using best-available parameters and compared with liver S9 fractions. Here, the combined scaled AOX clearance obtained from the kidney, lung, vasculature, and intestine is very low and amounted to <1% of liver. This work suggests that AOX metabolism from extrahepatic sources plays little role in the underprediction of activity in human. One of the notable outcomes of this work has been the first direct demonstration of AOX activity in human vasculature. SIGNIFICANCE STATEMENT: This work demonstrates aldehyde oxidase (AOX) activity is measurable in a variety of extrahepatic human tissues, including vasculature, yet activities and potential contributions to human clearance are relatively low and insignificant when compared with the liver. Additionally, the modeling of the tissue-specific in vitro kinetic data suggests that AOX may be influenced by the tissue it resides in and thus show different affinity, activity, and modified activity over time.

Possible Mechanism of Human Recombinant Leptin-Induced VEGF A Synthesis via PI3K/Akt/mTOR/S6 Kinase Signaling Pathway while Inducing Angiogenesis: An Analysis Using Chicken Chorioallantoic Membrane Model.

INTRODUCTION: The present study aimed to realize human recombinant leptin 's ability to synthesize VEGF A while inducing neovascularization through PI3K/Akt/mTOR/S6 kinase involved signaling pathway. METHODS: To examine the PI3K/Akt/mTOR/S6 kinase pathway involvement in leptin-induced VEGF A synthesis, the chick chorioallantoic membrane (CAM) was incubated with human recombinant leptin and specific inhibitors of the proposed signaling molecules (rapamycin and wortmannin). We analyzed the role of specified signaling molecules in human recombinant leptin-induced physiological angiogenesis via VEGF A synthesis in detail with the support of various methodologies. RESULTS: Human recombinant leptin's ability to synthesize VEGF A is diminished significantly in the presence of inhibitors. This observation supported the role of PI3K/Akt/mTOR/S6 kinase signaling molecules in human recombinant leptin-mediated VEGF A synthesis while inducing angiogenesis in CAM. CONCLUSION: Synthesis of VEGF A, followed by the growth of new blood vessels, by human recombinant leptin via the activation of the PI3K/Akt/mTOR/S6 kinase signaling pathway reflects mechanistic therapeutic application of human recombinant leptin. The data also signify the role of mTOR and S6 kinase molecules in angiogenesis under a physiological environment.

ACE2/Ang-(1-7)/Mas1 axis and the vascular system: vasoprotection to COVID-19-associated vascular disease.

The two axes of the renin-angiotensin system include the classical ACE/Ang II/AT1 axis and the counter-regulatory ACE2/Ang-(1-7)/Mas1 axis. ACE2 is a multifunctional monocarboxypeptidase responsible for generating Ang-(1-7) from Ang II. ACE2 is important in the vascular system where it is found in arterial and venous endothelial cells and arterial smooth muscle cells in many vascular beds. Among the best characterized functions of ACE2 is its role in regulating vascular tone. ACE2 through its effector peptide Ang-(1-7) and receptor Mas1 induces vasodilation and attenuates Ang II-induced vasoconstriction. In endothelial cells activation of the ACE2/Ang-(1-7)/Mas1 axis increases production of the vasodilator's nitric oxide and prostacyclin's and in vascular smooth muscle cells it inhibits pro-contractile and pro-inflammatory signaling. Endothelial ACE2 is cleaved by proteases, shed into the circulation and measured as soluble ACE2. Plasma ACE2 activity is increased in cardiovascular disease and may have prognostic significance in disease severity. In addition to its enzymatic function, ACE2 is the receptor for severe acute respiratory syndrome (SARS)-coronavirus (CoV) and SARS-Cov-2, which cause SARS and coronavirus disease-19 (COVID-19) respectively. ACE-2 is thus a double-edged sword: it promotes cardiovascular health while also facilitating the devastations caused by coronaviruses. COVID-19 is associated with cardiovascular disease as a risk factor and as a complication. Mechanisms linking COVID-19 and cardiovascular disease are unclear, but vascular ACE2 may be important. This review focuses on the vascular biology and (patho)physiology of ACE2 in cardiovascular health and disease and briefly discusses the role of vascular ACE2 as a potential mediator of vascular injury in COVID-19.

The Effect of Immunosuppressive Drugs on MMPs Activity in The Walls of Blood Vessels - A Systematic Review.

The current study focuses on the role of MMPs in the pathogenesis of the vascular damage and at the same time it offers the review referring to the influence of the immunosuppressive treatment on this interdependence. Contemporary immunosuppressive treatment constitutes of four groups of medications, such as: calcineurin inhibitors including cyclosporine A and tacrolimus; inhibitors of the inosine monophosphate dehydrogenase - the only agent from this group currently used in transplantation is mycophenalate mofetil (MMF); mTOR inhibitors, consisting of everolimus and glucocorticosteroids. Due to the fact that the properties of immunosuppressive drugs still remain unclear and transplant recipients need to use these medicines every day, knowledge of this should be further expanded. The deceases of the patients with the functioning graft who were diagnosed with the cardiovascular system diseases, constitute 50% of all renal transplant recipients. Immunosuppressive treatment leads to many pathological alterations within the organs and tissues and additionally they undoubtedly affect the activity of MMPs in the wall of the vessels.

Pathophysiological Roles of Stress-Activated Protein Kinases in Pulmonary Fibrosis.

Idiopathic pulmonary fibrosis (IPF) is one of the most symptomatic progressive fibrotic lung diseases, in which patients have an extremely poor prognosis. Therefore, understanding the precise molecular mechanisms underlying pulmonary fibrosis is necessary for the development of new therapeutic options. Stress-activated protein kinases (SAPKs), c-Jun N-terminal kinase (JNK), and p38 mitogen-activated protein kinase (p38) are ubiquitously expressed in various types of cells and activated in response to cellular environmental stresses, including inflammatory and apoptotic stimuli. Type II alveolar epithelial cells, fibroblasts, and macrophages are known to participate in the progression of pulmonary fibrosis. SAPKs can control fibrogenesis by regulating the cellular processes and molecular functions in various types of lung cells (including cells of the epithelium, interstitial connective tissue, blood vessels, and hematopoietic and lymphoid tissue), all aspects of which remain to be elucidated. We recently reported that the stepwise elevation of intrinsic p38 signaling in the lungs is correlated with a worsening severity of bleomycin-induced fibrosis, indicating an importance of this pathway in the progression of pulmonary fibrosis. In addition, a transcriptome analysis of RNA-sequencing data from this unique model demonstrated that several lines of mechanisms are involved in the pathogenesis of pulmonary fibrosis, which provides a basis for further studies. Here, we review the accumulating evidence for the spatial and temporal roles of SAPKs in pulmonary fibrosis.

Sex-differences in cholinergic, nicotinic, and ß-adrenergic cutaneous vasodilation: Roles of nitric oxide synthase, cyclooxygenase, and K+ channels.

Previous studies indicate that sex-related differences exist in the regulation of cutaneous vasodilation, however, the mechanisms remain unresolved. We assessed if sex-differences in young adults exist for cholinergic, nicotinic, and ß-adrenergic cutaneous vasodilation with a focus on nitric oxide synthase (NOS), cyclooxygenase (COX), and K+ channel mechanisms. In twelve young men and thirteen young women, four intradermal forearm skin sites were perfused with the following: 1) lactated Ringer's solution (control), 2) 10 mM Nω-nitro-l-arginine, a non-selective NOS inhibitor, 3) 10 mM ketorolac, a non-selective COX inhibitor, or 4) 50 mM BaCl2, a nonspecific K+ channel blocker. At all four sites, cutaneous vasodilation was induced by 1) 10 mM nicotine, a nicotinic receptor agonist, 2) 100 µM isoproterenol, a nonselective ß-adrenergic receptor agonist, and 3) 2 mM and 2000 mM acetylcholine, an acetylcholine receptor agonist. Nicotine and isoproterenol were administered for 3 min, whereas each acetylcholine dose was administered for 25 min. Regardless of treatment site, cutaneous vasodilation in response to nicotine and a high dose of acetylcholine (2000 mM) were lower in women than men. By contrast, isoproterenol induced cutaneous vasodilation was greater in women vs. men. Irrespective of sex, NOS inhibition or K+ channel blockade attenuated isoproterenol-mediated cutaneous vasodilation, whereas K+ channel blockade decreased nicotine-induced cutaneous vasodilation. Taken together, our findings indicate that while the mechanisms underlying cutaneous vasodilation are comparable between young men and women, sex-related differences in the magnitude of cutaneous vasodilation do exist and this response differs as a function of the receptor agonist.

Activity of Gas Transmitters in Vessels of the Anterior Abdominal Wall after Implantation of a Polypropylene Mesh.

The distribution of NO and H2S in the arterial vessels of the anterior abdominal wall after implantation of a polypropylene mesh was studied by immunohistochemical methods at different stages of healing of the surgical wound in mature male Wistar rats. The presence of enzymes of NO and H2S synthesis in the wall of arterial vessels of the soft tissues of the anterior abdominal wall has been established. It has been shown that endothelial NO synthase is localized exclusively in the endothelium of both large and small vessels. Cystathionine γ lyase in small vessels is located only in the endothelial lining, whereas in large arteries and vessels of medium caliber, it is located in the endothelium and in myocytes. Inducible NO synthase appears in the artery wall only in animals with implanted polypropylene mesh by day 5 of the postoperative period, reaching the maximum by day 10. The content and localization of cystathionine γ lyase in the vascular wall of sham-operated and experimental rats did not much differ from the control values.

Temporal and Spatial Post-Transcriptional Regulation of Zebrafish tie1 mRNA by Long Noncoding RNA During Brain Vascular Assembly.

OBJECTIVE: Tie1 (tyrosine kinase containing immunoglobulin and epidermal growth factor homology 1), an endothelial and hematopoietic cell-specific receptor tyrosine kinase, is an important regulator of angiogenesis and critical for maintaining vascular integrity. The post-transcriptional regulation of tie1 mRNA expression is not understood, but it might partly explain Tie1's differential expression pattern in endothelium. Following up on our previous work that identified natural antisense transcripts from the tie1 locus-tie1 antisense (tie1AS), which regulates tie1 mRNA levels in zebrafish-we attempted to identify the mechanism of this regulation. APPROACH AND RESULTS: Through in vitro and in vivo ribonucleoprotein binding studies, we demonstrated that tie1AS long noncoding RNA interacts with an RNA binding protein-embryonic lethal and abnormal vision Drosophila-like 1 (Elavl1)-that regulates tie1 mRNA levels. When we disrupted the interaction between tie1AS and Elavl1 by using constitutively active antisense morpholino oligonucleotides or photoactivatable morpholino oligonucleotides, tie1 mRNA levels increased between 26 and 31 hours post-fertilization, particularly in the head. This increase correlated with dilation of primordial midbrain channels, smaller eyes, and reduced ventricular space. We also observed these phenotypes when we used CRISPR (clustered regularly interspaced short palindromic repeats)-mediated CRISPRi (CRISPR-mediated interference) to knock down tie1AS. Treatment of the morpholino oligonucleotide-injected embryos with a small molecule that decreased tie1 mRNA levels rescued all 3 abnormal phenotypes. CONCLUSIONS: We identified a novel mode of temporal and spatial post-transcriptional regulation of tie1 mRNA. It involves long noncoding RNA, tie1AS, and Elavl1 (an interactor of tie1AS).

Vascular thiol isomerases.

Thiol isomerases are multifunctional enzymes that influence protein structure via their oxidoreductase, isomerase, and chaperone activities. These enzymes localize at high concentrations in the endoplasmic reticulum of all eukaryotic cells where they serve an essential function in folding nascent proteins. However, thiol isomerases can escape endoplasmic retention and be secreted and localized on plasma membranes. Several thiol isomerases including protein disulfide isomerase, ERp57, and ERp5 are secreted by and localize to the membranes of platelets and endothelial cells. These vascular thiol isomerases are released following vessel injury and participate in thrombus formation. Although most of the activities of vascular thiol isomerases that contribute to thrombus formation are yet to be defined at the molecular level, allosteric disulfide bonds that are modified by thiol isomerases have been described in substrates such as αIIbß3, αvß3, GPIbα, tissue factor, and thrombospondin. Vascular thiol isomerases also act as redox sensors. They respond to the local redox environment and influence S-nitrosylation of surface proteins on platelets and endothelial cells. Despite our rudimentary understanding of the mechanisms by which thiol isomerases control vascular function, the clinical utility of targeting them in thrombotic disorders is already being explored in clinical trials.

Resveratrol Mitigates High-Fat Diet-Induced Vascular Dysfunction by Activating the Akt/eNOS/NO and Sirt1/ER Pathway.

We investigated whether resveratrol (RSV) can attenuate obesity and diabetes progression and improve diabetes-induced vascular dysfunction, and we attempted to delineate its underlying mechanisms. Male C57Bl/6 mice were administered a high-fat diet (HFD) for 17 weeks. Mice developed type 2 diabetes with increased body weight, hyperglycemia, hyperinsulinemia, and hyperlipidemia. Oral gavage with RSV significantly reversed the symptoms induced by the HFD. Insulin sensitivity likewise improved after the RSV intervention in these mice. Phenylephrine-induced cremaster arteriolar constriction was impaired, whereas RSV treatment significantly mitigated the vessel responsiveness to phenylephrine. The obese diabetic mice exhibited increased leukocyte rolling, adhesion, and transmigration in the postcapillary venules of the cremaster muscle. By contrast, RSV treatment significantly attenuated HFD-induced extravasation. RSV significantly recovered phosphorylated Akt and eNOS expression in the thoracic aorta. In addition, activated adenosine monophosphate-activated protein kinase in the thoracic aorta was involved in the improvement of epithelial function after RSV intervention. RSV considerably upregulated the plasma NO level in HFD mice. Moreover, RSV-enhanced human umbilical vein endothelial cells healing through Sirt1/ER pathway may be involved in the prevention of leukocyte extravasation. Collectively, RSV attenuates diabetes-induced vascular dysfunction by activating Akt/eNOS/NO and Sirt1/ER pathway. Our mechanistic study provides a potential RSV-based therapeutic strategy against cardiovascular disease.

Heat shock protein 70-1A is a novel angiogenic regulator.

Heat shock protein 70-1A (HSP70-1A) is a stress-inducible protein that provides an essential intracellular molecular chaperone function; however, the mechanism of HSP70-1A in angiogenesis has not been clarified. Herein, HSP70-1A gene silencing implicated this protein in angiogenesis. Additionally, recombinant human HSP70-1A (rhHSP70-1A) was able to stimulate human umbilical vein endothelial cell (HUVEC) migration and tube formation in vitro and microvessel formation in vivo similarly to recombinant human vascular endothelial growth factor (rhVEGF). Furthermore, rhHSP70-1A was tightly bound to the surface of HUVECs and participated in extracellular signal-related kinase (ERK)-dependent angiogenesis. Together, these results implicate HSP70-1A as a novel angiogenic regulator.

Characterisation and vascular expression of nitric oxide synthase 3 in amphibians.

In mammals, nitric oxide (NO) produced by nitric oxide synthase 3 (NOS3) localised in vascular endothelial cells is an important vasodilator but the presence of NOS3 in the endothelium of amphibians has been concluded to be absent, based on physiological studies. In this study, a nos3 cDNA was sequenced from the toad, Rhinella marina. The open reading frame of R. marina nos3 encoded an 1170 amino acid protein that showed 81 % sequence identity to the recently cloned Xenopus tropicalis nos3. Rhinella marina nos3 mRNA was expressed in a range of tissues and in the dorsal aorta and pulmonary, mesenteric, iliac and gastrocnemius arteries. Furthermore, nos3 mRNA was expressed in the aorta of Xenopus laevis and X. tropicalis. Quantitative real-time PCR showed that removal of the endothelium of the lateral aorta of R. marina significantly reduced the expression of nos3 mRNA compared to control aorta with the endothelium intact. However, in situ hybridisation was not able to detect any nos3 mRNA in the dorsal aorta of R. marina. Immunohistochemistry using a homologous R. marina NOS3 antibody showed immunoreactivity (IR) within the basal region of many endothelial cells of the dorsal aorta and iliac artery. NOS3-IR was also observed in the proximal tubules and collecting ducts of the kidney but not within the capillaries of the glomeruli. This is the first study to demonstrate that vascular endothelial cells of an amphibian express NOS3.

Tissue Non-specific Alkaline Phosphatase (TNAP) in Vessels of the Brain.

The microvessels of the brain represent around 3-4 % of the brain compartment but constitute the most important length (400 miles) and surface of exchange (20 m(2)) between the blood and the parenchyma of brain. Under influence of surrounding tissues, the brain microvessel endothelium expresses a specific phenotype that regulates and restricts the entry of compounds and cells from blood to brain, and defined the so-called blood-brain barrier (BBB). Evidences that alkaline phosphatase (AP) is a characteristic feature of the BBB phenotype that allows differentiating capillary endothelial cells from brain to those of the periphery have rapidly emerge. Thenceforth, AP has been rapidly used as a biomarker of the blood-brain barrier phenotype. In fact, brain capillary endothelial cells (BCECs) express exclusively tissue non-specific alkaline phosphatase (TNAP). There are several lines of evidence in favour of an important role for TNAP in brain function. TNAP is thought to be responsible for the control of transport of some compounds across the plasma membrane of the BCECs. Here, we report that levamisole-mediated inhibition of TNAP provokes an increase of the permeability to Lucifer Yellow of the endothelial monolayer. Moreover, we illustrate the disruption of the cytoskeleton organization. Interestingly, all observed effects were reversible 24 h after levamisole removal and correlated with the return of a full activity of the TNAP. This reversible effect remains to be studied in details to evaluate the potentiality of a levamisole treatment to enhance the entry of drugs in the brain parenchyma.

Cellular and molecular mechanisms of HGF/Met in the cardiovascular system.

Met tyrosine kinase receptor, also known as c-Met, is the HGF (hepatocyte growth factor) receptor. The HGF/Met pathway has a prominent role in cardiovascular remodelling after tissue injury. The present review provides a synopsis of the cellular and molecular mechanisms underlying the effects of HGF/Met in the heart and blood vessels. In vivo, HGF/Met function is particularly important for the protection of the heart in response to both acute and chronic insults, including ischaemic injury and doxorubicin-induced cardiotoxicity. Accordingly, conditional deletion of Met in cardiomyocytes results in impaired organ defence against oxidative stress. After ischaemic injury, activation of Met provides strong anti-apoptotic stimuli for cardiomyocytes through PI3K (phosphoinositide 3-kinase)/Akt and MAPK (mitogen-activated protein kinase) cascades. Recently, we found that HGF/Met is also important for autophagy regulation in cardiomyocytes via the mTOR (mammalian target of rapamycin) pathway. HGF/Met induces proliferation and migration of endothelial cells through Rac1 (Ras-related C3 botulinum toxin substrate 1) activation. In fibroblasts, HGF/Met antagonizes the actions of TGFß1 (transforming growth factor ß1) and AngII (angiotensin II), thus preventing fibrosis. Moreover, HGF/Met influences the inflammatory response of macrophages and the immune response of dendritic cells, indicating its protective function against atherosclerotic and autoimmune diseases. The HGF/Met axis also plays an important role in regulating self-renewal and myocardial regeneration through the enhancement of cardiac progenitor cells. HGF/Met has beneficial effects against myocardial infarction and endothelial dysfunction: the cellular and molecular mechanisms underlying repair function in the heart and blood vessels are common and include pro-angiogenic, anti-inflammatory and anti-fibrotic actions. Thus administration of HGF or HGF mimetics may represent a promising therapeutic agent for the treatment of both coronary and peripheral artery disease.

Immunohistochemical Analysis of MMP-2 Expression in the Myocardium During the Postinfarction Period.

Immunohistochemical analysis revealed 2.5-fold increased of expression MMP-2 in myocardium samples during the early period (up to 3 days) of postinfarction reparative regeneration. During this period, MMP-2 was detected mainly in monocytes/macrophages circulating in the blood and migrating to the necrotic zone, while expression in the intermuscular and perivascular connective tissue was lower. At later terms, with development of large focal and diffuse cardiosclerosis, MMP-2 expression significantly decreased (to the initial level) and was detected mainly in the foci of intermuscular and perivascular fibrosis, its area in the sections increased by 1.8 times. Evaluation of MMP-2 expression in the blood vessels showed that the immunohistochemical reaction was the most pronounced in the walls of new sinusoidal vessels and the minimum in the intramural arteries of medium diameter. These results attest to an important role of MMP in connective tissue remodeling (proteolytic degradation) during the early period of postinfarction reparative regeneration. The decrease in MMP-2 expression observed at later terms correlated with myocardial fibrosis progression.

[UROKINASE SYSTEM INVOLVES IN VASCULAR CELLS MIGRATION AND REGULATES THE GROWTH AND BRANCHING OF CAPILLARIES].

Urokinase system representing urokinase-type plasminogen activator (urokinase, uPA) and urokinase re- ceptor (uPAR) plays an important regulatory role in the vascular wall and has the ability to run a proteolytic cascade, degradation of extracellular matrix and activate intracellular signaling in vascular cells. In this work, we have firstly shown a fundamental mechanism of urokinase system-dependent regulation of the trajectory of growth and branching of blood vessels what may be of particular importance in the growth of blood vessels in early embryogenesis and in adults during the repair/regeneration of tissues.

Lipid phosphate phosphatase (LPP3) and vascular development.

Lipid phosphate phosphatases (LPP) are integral membrane proteins with broad substrate specificity that dephosphorylate lipid substrates including phosphatidic acid, lysophosphatidic acid, ceramide 1-phosphate, sphingosine 1-phosphate, and diacylglycerol pyrophosphate. Although the three mammalian enzymes (LPP1-3) demonstrate overlapping catalytic activities and substrate preferences in vitro, the phenotypes of mice with targeted inactivation of the Ppap2 genes encoding the LPP enzymes reveal nonredundant functions. A specific role for LPP3 in vascular development has emerged from studies of mice lacking Ppap2b. A meta-analysis of multiple, large genome-wide association studies identified a single nucleotide polymorphism in PPAP2B as a novel predictor of coronary artery disease. In this review, we will discuss the evidence that links LPP3 to vascular development and disease and evaluate potential molecular mechanisms. This article is part of a Special Issue entitled Advances in Lysophospholipid Research.

Inhibition of AMP-activated protein kinase accentuates lipopolysaccharide-induced lung endothelial barrier dysfunction and lung injury in vivo.

The aim of this study was to determine the role of AMP-activated protein kinase (AMPK) in lipopolysaccharide (LPS)-induced lung endothelial barrier dysfunction and lung injury in vivo. Both cultured human pulmonary artery endothelial cells (HPAECs) and experimental animals [AMPK subunit α-deficient mice and wild-type (WT) control mice (C57BL/6J)] were used. In cultured HPAECs, LPS increased endothelial permeability in parallel with a decrease in AMPK activity. Consistent with this observation, AMPK activation with the potent AMPK activator 5-aminoimidazole-4-carboxamide-1-d-ribofuranoside (AICAR) attenuated LPS-induced endothelial hyperpermeability in vitro. Intratracheal administration of LPS (1 mg/kg) in WT mice reduced AMPK phosphorylation at Thr172 in lung tissue extracts, increased protein content and cell count in bronchial alveolar lavage fluid, and increased Evans Blue dye infiltration into the lung. These same attributes were similarly enhanced in AMPKα-knockout mice, compared with WT mice. Pretreatment with AICAR reduced these lung injury indicators in LPS-treated WT mice. AMPK activation with AICAR attenuated LPS-induced endothelial hyperpermeability by activating the Rac/Cdc42/PAK pathway, with concomitant inhibition of the Rho pathway, and decreased VE-cadherin phosphorylation at Tyr658. We conclude that AMPK activity supports normal endothelial barrier function and that LPS exposure inhibits AMPK, thereby contributing to endothelial barrier dysfunction and lung injury.

Role of epidermal growth factor receptor in vascular structure and function.

PURPOSE OF THE REVIEW: The epidermal growth factor receptor (EGFR) is a receptor tyrosine kinase with a wide implication in tumor biology, wound healing and development. Besides acting as a growth factor receptor activated by ligands such as EGF, the EGFR can also be transactivated and thereby mediate cross-talk with different signaling pathways. The aim of this review is to illustrate the Janus-faced function of the EGFR in the vasculature with its relevance for vascular biology and disease. RECENT FINDINGS: Over recent years, the number of identified signaling partners of the EGFR has steadily increased, as have the biological processes in which the EGFR is thought to be involved. Recently, new models have allowed investigation of EGFR effects in vivo, shedding some light on the overall function of the EGFR in the vasculature. At the same time, EGFR inhibitors and antibodies have become increasingly established in cancer therapy, providing potential therapeutic tools for decreasing EGFR signaling. SUMMARY: The EGFR is a versatile signaling pathway integrator associated with vascular homeostasis and disease. In addition to modulating basal vascular tone and tissue homeostasis, the EGFR also seems to be involved in proinflammatory, proliferative, migratory and remodeling processes, with enhanced deposition of extracellular matrix components, thereby promoting vascular diseases such as hypertension or atherosclerosis.

Reduced mural cell coverage and impaired vessel integrity after angiogenic stimulation in the Alk1-deficient brain.

OBJECTIVE: Vessels in brain arteriovenous malformations are prone to rupture. The underlying pathogenesis is not clear. Hereditary hemorrhagic telangiectasia type 2 patients with activin receptor-like kinase 1 (Alk1) mutation have a higher incidence of brain arteriovenous malformation than the general population. We tested the hypothesis that vascular endothelial growth factor impairs vascular integrity in the Alk1-deficient brain through reduction of mural cell coverage. METHODS AND RESULTS: Adult Alk1(1f/2f) mice (loxP sites flanking exons 4-6) and wild-type mice were injected with 2×10(7) PFU adenovious-cre recombinase and 2×10(9) genome copies of adeno-associated virus-vascular endothelial growth factor to induce focal homozygous Alk1 deletion (in Alk1(1f/2f) mice) and angiogenesis. Brain vessels were analyzed 8 weeks later. Compared with wild-type mice, the Alk1-deficient brain had more fibrin (99±30×10(3) pixels/mm(2) versus 40±13×10(3); P=0.001), iron deposition (508±506 pixels/mm(2) versus 6±49; P=0.04), and Iba1(+) microglia/macrophage infiltration (888±420 Iba1(+) cells/mm(2) versus 240±104 Iba1(+); P=0.001) after vascular endothelial growth factor stimulation. In the angiogenic foci, the Alk1-deficient brain had more α-smooth muscle actin negative vessels (52±9% versus 12±7%, P<0.001), fewer vascular-associated pericytes (503±179/mm(2) versus 931±115, P<0.001), and reduced platelet-derived growth factor receptor-ß expression. CONCLUSIONS: Reduction of mural cell coverage in response to vascular endothelial growth factor stimulation is a potential mechanism for the impairment of vessel wall integrity in hereditary hemorrhagic telangiectasia type 2-associated brain arteriovenous malformation.