Endothelial Integrin Research Articles

Constitutive Association of Tie1 and Tie2 with Endothelial Integrins is Functionally Modulated by Angiopoietin-1 and Fibronectin.

Functional cross-talk between Tie2 and Integrin signaling pathways is essential to coordinate endothelial cell adhesion and migration in response to the extracellular matrix, yet the mechanisms behind this phenomenon are unclear. Here, we examine the possibility that receptor cross-talk is driven through uncharacterized Tie-integrin interactions on the endothelial surface. Using a live cell FRET-based proximity assay, we monitor Tie-integrin receptor recognition and demonstrate that both Tie1 and Tie2 readily associate with integrins α5ß1 and αVß3 through their respective ectodomains. Although not required, Tie2-integrin association is significantly enhanced in the presence of the extracellular component and integrin ligand fibronectin. In vitro binding assays with purified components reveal that Tie-integrin recognition is direct, and further demonstrate that the receptor binding domain of the Tie2 ligand Ang-1, but not the receptor binding domain of Ang-2, can independently associate with α5ß1 or αVß3. Finally, we reveal that cooperative Tie/integrin interactions selectively stimulate ERK/MAPK signaling in the presence of both Ang-1 and fibronectin, suggesting a molecular mechanism to sensitize Tie2 to extracellular matrix. We provide a mechanistic model highlighting the role of receptor localization and association in regulating distinct signaling cascades and in turn, the angiogenic switch.

Mice deficient in endothelial α5 integrin are profoundly resistant to experimental ischemic stroke

Stroke is a disease in dire need of better therapies. We have previously shown that a fragment of the extracellular matrix proteoglycan, perlecan, has beneficial effects following cerebral ischemia via the α5β1 integrin receptor. We now report that endothelial cell selective α5 integrin deficient mice (α5 KO) are profoundly resistant to ischemic infarct after transient middle cerebral artery occlusion. Specifically, α5 KOs had little to no infarct 2–3 days post-stroke, whereas controls had an increase in mean infarct volume over the same time period as expected. Functional outcome is also improved in the α5 KOs compared with controls. Importantly, no differences in cerebrovascular anatomy or collateral blood flow were noted that could account for this difference in ischemic injury. Rather, we demonstrate that α5 KOs have increased blood-brain barrier integrity (increased expression of claudin-5, and absent brain parenchymal IgG extravasation) after stroke compared with controls, which could explain their resistance to ischemic injury. Additionally, inhibition of α5 integrin in vitro leads to decreased permeability of brain endothelial cells following oxygen-glucose deprivation. Together, these findings indicate endothelial cell α5 integrin plays an important role in stroke outcome and blood-brain barrier integrity, suggesting that α5 integrin could be a novel therapeutic target for stroke.

Physiological cerebrovascular remodeling in response to chronic mild hypoxia: A role for activated protein C

Activated protein C (APC) is a serine protease that promotes favorable changes in vascular barrier integrity and post-ischemic angiogenic remodeling in animal models of ischemic stroke, and its efficacy is currently being investigated in clinical ischemic stroke trials. Interestingly, application of sub-clinical chronic mild hypoxia (CMH) (8% O2) also promotes angiogenic remodeling and increased tight junction protein expression, suggestive of enhanced blood-brain barrier (BBB) integrity, though the role of APC in mediating the influence of CMH has not been investigated. To examine this potential link, we studied CMH-induced cerebrovascular remodeling after treating mice with two different reagents: (i) a function-blocking antibody that neutralizes APC activity, and (ii) exogenous recombinant murine APC. While CMH promoted endothelial proliferation, increased vascular density, and upregulated the angiogenic endothelial integrins α5β1 and αvβ3, these events were almost completely abolished by functional blockade of APC. Consistent with these findings, addition of exogenous recombinant APC enhanced CMH-induced endothelial proliferation, expansion of total vascular area and further enhanced the CMH-induced right-shift in vessel size distribution. Taken together, our findings support a key role for APC in mediating physiological remodeling of cerebral blood vessels in response to CMH.

Prostate-specific membrane antigen (PSMA)-mediated laminin proteolysis generates a pro-angiogenic peptide

Prostate-specific membrane antigen (PSMA) is a membrane-bound glutamate carboxypeptidase expressed in a number of tissues. PSMA participates in various biological functions depending on the substrate available in the particular tissue; in the brain, PSMA cleaves the abundant neuropeptide N-acetyl-aspartyl-glutamate to regulate release of key neurotransmitters, while intestinal PSMA cleaves polyglutamated peptides to supply dietary folate. PSMA expression is also progressively upregulated in prostate cancer where it correlates with tumor progression as well as in tumor vasculature, where it regulates angiogenesis. The previous research determined that PSMA cleavage of small peptides generated via matrix metalloprotease-mediated proteolysis of the extracellular matrix protein laminin potently activated endothelial cells, integrin signaling and angiogenesis, although the specific peptide substrates were not identified. Herein, using enzymatic analyses and LC/MS, we unequivocally demonstrate that several laminin-derived peptides containing carboxy-terminal glutamate moieties (LQE, IEE, LNE) are bona fide substrates for PSMA. Subsequently, the peptide products were tested for their effects on angiogenesis in various models. We report that LQ, the dipeptide product of PSMA cleavage of LQE, efficiently activates endothelial cells in vitro and enhances angiogenesis in vivo. Importantly, LQE is not cleaved by an inactive PSMA enzyme containing an active site mutation (E424S). Endothelial cell activation by LQ was dependent on integrin beta-1-induced activation of focal adhesion kinase. These results characterize a novel PSMA substrate, provide a functional rationale for the upregulation of PSMA in cancer cells and tumor vasculature and suggest that inhibition of PSMA could lead to the development of new angiogenic therapies.

Quantification of Endothelial αvβ3 Expression with High-Frequency Ultrasound and Targeted Microbubbles: In Vitro and In Vivo Studies

Angiogenesis is a critical feature of plaque development in atherosclerosis and might play a key role in both the initiation and later rupture of plaques. The precursory molecular or cellular pro-angiogenic events that initiate plaque growth and that ultimately contribute to plaque instability, however, cannot be detected directly with any current diagnostic modality. This study was designed to investigate the feasibility of ultrasound molecular imaging of endothelial αvβ3 expression in vitro and in vivo using αvβ3-targeted ultrasound contrast agents (UCAs). In the in vitro study, αvβ3 expression was confirmed by immunofluorescence in a murine endothelial cell line and detected using the targeted UCA and ultrasound imaging at 18-MHz transmit frequency. In the in vivo study, expression of endothelial αvβ3 integrin in murine carotid artery vessels and microvessels of the salivary gland was quantified using targeted UCA and high-frequency ultrasound in seven animals. Our results indicated that endothelial αvβ3 expression was significantly higher in the carotid arterial wall containing atherosclerotic lesions than in arterial segments without any lesions. We also found that the salivary gland can be used as an internal positive control for successful binding of targeted UCA to αvβ3 integrin. In conclusion, αvβ3-targeted UCA allows non-invasive assessment of the expression levels of αvβ3 on the vascular endothelium and may provide potential insights into early atherosclerotic plaque detection and treatment monitoring.

Abstract 66: Endothelial α5β1 Integrin is Acutely Upregulated and Destabilizes the Blood-brain Barrier After Stroke

Repeated opening of the blood-brain barrier (BBB) after ischemic stroke significantly contributes to brain injury. As the β1 integrin receptor family has been linked to changes in BBB permeability via changes to tight junction (TJ) protein expression and function, we hypothesized that the pro-angiogenic fibronectin receptor α5β1 integrin is acutely upregulated in cerebrovasculature after ischemic stroke and contributes to BBB breakdown via affects on the TJ protein claudin-5. First, α5β1 brain immunohistochemistry after transient middle cerebral artery occlusion (MCAo) in mice demonstrated an increase in core infarct and peri-infarct vascular expression of α5β1 as early as 24 hours after stroke that became even more prominent by post stroke day 2. Next, in vitro studies demonstrated an increase in barrier permeability where stroke was simulated using oxygen-glucose deprivation or TNF-α treatment, as measured by trans-endothelial cell electrical resistance and FITC-dextran permeability, that correlated with an increase in α5β1 expression and changes in claudin-5 expression and localization away from the cell-surface (i.e. away from TJ complexes). Importantly, this increased permeability could be completely prevented by addition of the α5β1 inhibitor ATN-161. Finally, MCAo performed in endothelial cell selective α5β1 knockout mice demonstrated significantly increased post-stroke BBB stability, stable claudin-5 expression, and little to no ischemic injury. Collectively, our results demonstrate that endothelial cell α5β1 integrin expression is increased acutely after stroke, may contribute to BBB breakdown and subsequent expansion of brain injury, and therefore could represent a novel stroke therapeutic target worthy of further investigation.

Buyanghuanwu decoction promotes angiogenesis after cerebral ischemia/reperfusion injury: mechanisms of brain tissue repair.

Buyanghuanwu decoction has been shown to protect against cerebral ischemia/reperfusion injury, but the underlying mechanisms remain unclear. In this study, rats were intragastrically given Buyanghuanwu decoction, 15 mL/kg, for 3 days. A rat model of cerebral ischemia/reperfusion injury was established by middle cerebral artery occlusion. In rats administered Buyanghuanwu decoction, infarct volume was reduced, serum vascular endothelial growth factor and integrin αvβ3 levels were increased, and brain tissue vascular endothelial growth factor and CD34 expression levels were increased compared with untreated animals. These effects of Buyanghuanwu decoction were partially suppressed by an angiogenesis inhibitor (administered through the lateral ventricle for 7 consecutive days). These data suggest that Buyanghuanwu decoction promotes angiogenesis, improves cerebral circulation, and enhances brain tissue repair after cerebral ischemia/reperfusion injury.

β4 integrin is not essential for localization of hemidesmosome proteins plectin and CD151 in cerebral vessels.

Objective:In the central nervous system (CNS), β4 integrin is predominantly expressed by endothelial cells lining arterioles. As β4 integrin plays an essential role in epithelial tissues, organizing structural proteins into specialized adhesive structures called hemidesmosomes (HD), the aim of this study was to determine whether it plays a similar role in CNS endothelium.Methods:Dual-immunofluorescence was used to examine the relationship between β4 integrin expression and co-expression of the HD proteins plectin and CD151 in frozen sections of mouse brain, both under normoxic (control) conditions and following chronic mild hypoxia. The requirement of β4 integrin for the localization of HD proteins was examined in transgenic mice lacking β4 integrin expression specifically in endothelial cells (β4-EC-KO mice).Results:Immunofluorescence revealed that in the normal adult CNS, plectin and CD151 strongly co-localized with β4 integrin in arterioles. However, in the chronic mild hypoxia model, in which extensive cerebrovascular remodeling is observed, plectin and CD151 were strongly upregulated on all cerebral vessels, but surprisingly, in capillaries, this occurred in a β4 integrin-independent manner. Unexpectedly, absence of endothelial β4 integrin (in β4-EC-KO mice) had no impact on the expression level or distribution pattern of plectin and CD151 within stable or remodeling cerebral vessels.Conclusions:These results demonstrate that the HD proteins plectin and CD151 are closely associated with β4 integrin on arterioles in normal brain, and are strongly upregulated on remodeling blood vessels. However, unlike its described role in the epidermis, β4 integrin is not essential for localization or regulation of expression of plectin and CD151 in cerebral vessels.

Platelet-Localized FXI Promotes a Glycoprotein Ib Alpha Dependent Feedback Loop in Arterial Hypertension and Vascular Inflammation

Backgroud: Vascular inflammation is a hallmark of atherosclerosis and arterial and venous thromboembolic disease. Arterial hypertension is a highly significant risk factor for death and cardiovascular disease. High levels of angiotensin II (ATII) cause arterial hypertension by a complex inflammatory pathway requiring leukocyte recruitment and reactive oxygen species production within the vessel wall. Interactions of platelets, leukocytes and the vessel wall play pivotal roles in activating coagulation and precipitating thrombosis. Platelets provide a pro-coagulant surface for amplified thrombin generation in hemostasis and thrombosis, and play important roles in vascular inflammation by preserving vascular integrity. They also promote leukocyte recruitment in wire injury and angiogenesis models through glycoprotein Ib alpha (GPIbalpha) interacting with the integrin alphaMbeta2 (CD11b/CD18 or Mac-1) on leukocytes and regulate monocyte and neutrophil activation. How platelets, coagulation factors, leukocytes and the vessel wall cooperate to promote vascular inflammation in arterial hypertension is unclear.Objective: The aim of this work was to explore the roles of TF, FXI, FXII, thrombin and platelet GPIbalpha on inflammatory monocyte-driven vascular dysfunction and arterial hypertension in ATII infused mice and rats as well as in 5/6 nephrectomized (5/6Nx) rats.Methods: FXII-/-, FXI-/-, and hIL-4R/Ibalpha mice and 5/6NX rats were used for this study. Mice and rats where treated with ATII (1 mg×kg-1 ×d-1 for 7 days) using osmotic minipumps. Blood pressure was recorded using tail cuff measurement and telemetry carotid implants. To assess vasodilator properties, isolated aortic segments were mounted to force transducers in organ chambers to test their response to acetylcholine (ACh) and glyceryl trinitrate (GTN). ROS production was quantified within the aorta using dihydroethidium-derived fluorescence. Thrombin generation in platelet rich plasma (PRP) was measured using calibrated automated thrombography.Results: ATII induces an upregulation of tissue factor, thrombin-dependent endothelial cell VCAM-1 expression and integrin alpha4- and platelet-dependent leukocyte adhesion to arterial conductance vessels. Depletion of platelets using an anti-GPIbalpha antibody as well as injection of anti-Mac-1 directed against the CD11b/CD18 integrin on leukocytes similarly prevented leukocyte rolling and adhesion. Reduced vascular recruitment of leukocytes was paralleled by diminished vascular ROS production assessed by the superoxide-sensitive dye dihydroethidium. ATII-induced vascular dysfunction unexpectedly involved the activation of FXI but not FXII. Moreover, inhibition of FXI synthesis attenuates blood pressure increase in response to ATII. The platelet FXI receptor GPIbalpha supports the upregulation of thrombin feedback activation in ATII-treated mice. Blockade of TF during ATII administration attenuated both endothelial dysfunction (acetylcholine concentration-relaxation curves) and smooth muscle dysfunction as demonstrated by improved vascular relaxation in response to the endothelium independent vasodilator glyceryl trinitrate and reduced ROS within the vessel wall. Five/6Nx rats had endothelial dysfunction that was prevented by FXI Antisense oligonucleotide. Thrombin-induced thrombin generation was increased in PRP of 5/6 Nx rats and markedly diminished by FXI Antisense oligonucleotide, mimicking the results from ATII-infused mice.Conclusion: Our results reveal a critical role of platelet GPIbalpha to promote localized thrombin amplification and a FXI-thrombin feedback loop in ATII-induced vascular inflammation. Importantly, pharmacologic inhibition of FXI synthesis is sufficient to prevent thrombin propagation on platelets, to reduce vessel wall leukocyte infiltration, and to diminish ATII-induced endothelial dysfunction and arterial hypertension in mice and rats.Targeting FXI could be a novel therapeutic possibility to interrupt this heterotypic cellular coagulation-inflammatory circuit. DisclosuresMonia:Isis Pharmaceuticals: Employment.

MEK1/2 inhibitors reverse acute vascular occlusion in mouse models of sickle cell disease.

In sickle cell disease (SCD), treatment of recurrent vasoocclusive episodes, leading to pain crises and organ damage, is still a therapeutic challenge. Vasoocclusion is caused primarily by adherence of homozygous for hemoglobin S (SS) red blood cells (SSRBCs) and leukocytes to the endothelium. We tested the therapeutic benefits of MEK1/2 inhibitors in reversing vasoocclusion in nude and humanized SCD mouse models of acute vasoocclusive episodes using intravital microscopy. Administration of 0.2, 0.3, 1, or 2 mg/kg MEK1/2 inhibitor to TNF-α-pretreated nude mice before human SSRBC infusion inhibited SSRBC adhesion in inflamed vessels, prevented the progression of vasoocclusion, and reduced SSRBC organ sequestration. By use of a more clinically relevant protocol, 0.3 or 1 mg/kg MEK1/2 inhibitor given to TNF-α-pretreated nude mice after human SSRBC infusion and onset of vasoocclusion reversed SSRBC adhesion and vasoocclusion and restored blood flow. In SCD mice, 0.025, 0.05, or 0.1 mg/kg MEK1/2 inhibitor also reversed leukocyte and erythrocyte adhesion after the inflammatory trigger of vasoocclusion and improved microcirculatory blood flow. Cell adhesion was reversed by shedding of endothelial E-selectin, P-selectin, and αvβ3 integrin, and leukocyte CD44 and β2 integrin. Thus, MEK1/2 inhibitors, by targeting the adhesive function of SSRBCs and leukocytes, could represent a valuable therapeutic intervention for acute sickle cell vasoocclusive crises.

Abstract 18216: Essential Role of Platelet Glycoprotein Ibα Dependent Thrombin-FXI Feedback Loop in Arterial Hypertension, Vascular Dysfunction and Inflammation

Interactions of platelets, leukocytes and the vessel wall play pivotal roles in activating coagulation and precipitating thrombosis. High levels of angiotensin II (ATII) cause arterial hypertension by a complex inflammatory pathway requiring leukocyte recruitment and reactive oxygen species production within the vessel wall coupled to vascular dysfunction. Here we report an upregulation of tissue factor, thrombin-dependent endothelial cell VCAM-1 expression and integrin α4- and platelet-dependent leukocyte adhesion to arterial conductance vessels by ATII in vivo (1mg/kg/d for 7d). ATII-induced vascular dysfunction and Ccl2, VCAM-1 and Ly6C mRNA expression were ameliorated by thrombin inhibition, platelet-depletion as well as in hIL-4R/Ibα mice missing the extracellular ligand binding domains of GPIbα. Blockade of TF during ATII administration also attenuated vascular dysfunction and reduced vascular oxidative stress. Platelet rich plasma (PRP) of ATII-treated mice showed an increased thrombin evoked endogenous thrombin potential (ETP), whereas PRP from mice with pharmacological inhibition of FXI production or of hIL-4R/Ibα mice failed to amplify ETP following chronic ATII exposure. These data show that a FXI-dependent thrombin generation feedback loop requires GPIbα on platelets and suggest that TF-initiated coagulation promotes additional thrombin formation on platelets to cause vascular inflammation. The therapeutic potential of blocking FXI synthesis via antisense oligonucleotides was generalized by experiments with 5/6 nephrectomized (5/6 Nx) rats. 5/6 Nx rats showed after 4 wk of surgery an endothelial dysfunction, increased blood pressure, vascular inflammation and oxidative stress, as well as a elevated ETP compared to sham whereas pharmacological inhibition of FXI synthesis attenuated these effects. Our results reveal a critical role of platelet GPIbα to promote localized thrombin amplification and supporting a FXI-thrombin feedback loop in ATII-induced vascular inflammation. By using a FXI targeted approach, we propose a novel therapeutic possibility to interrupt this heterotypic cellular coagulation-inflammatory circuit.

Interactions of DPP-4 and integrin β1 influences endothelial-to-mesenchymal transition

Integrin β1 and dipeptidyl peptidase (DPP)-4 play roles in endothelial cell biology. Vascular endothelial growth factor (VEGF)-A inhibits endothelial-to-mesenchymal transition (EndMT) through VEGF-R2, but through VEGF-R1 promotes EndMT by reducing the bioavailability of VEGF-A. Here we tested whether DPP-4-integrin β1 interactions have a role in EndMT in the renal fibrosis of diabetic nephropathy. In streptozotocin-induced fibrotic kidneys in diabetic CD-1 mice, levels of endothelial DPP-4, integrin β1, and phospho-integrin β1 were all higher and associated with plasma cystatin C elevation. The DPP-4 inhibitor linagliptin ameliorated kidney fibrosis, reduced plasma cystatin C levels, and suppressed endothelial levels of DPP-4, integrin β1, and phospho-integrin β1. In cultured endothelial cells, DPP-4 and integrin β1 physically interacted. Suppression of DPP-4 by siRNA was associated with suppression of integrin β1 and vice versa. Knockdown of either integrin β1 or DPP-4 resulted in the silencing of TGF-β2-induced TGF-β receptor heterodimer formation, smad3 phosphorylation, and EndMT. DPP-4 negatively regulated endothelial viability signaling by VEGF-R2 suppression and VEGF-R1 induction in endothelial cells. Thus, DPP-4 and integrin β1 interactions regulate key endothelial cell signal transduction in both physiological and pathological conditions including EndMT. Hence, inhibiting DPP-4 may be a therapeutic target for treating kidney fibrosis in diabetes.

Integrins mediate mechanical compression-induced endothelium-dependent vasodilation through endothelial nitric oxide pathway.

Cardiac and skeletal muscle contraction lead to compression of intramuscular arterioles, which, in turn, leads to their vasodilation (a process that may enhance blood flow during muscle activity). Although endothelium-derived nitric oxide (NO) has been implicated in compression-induced vasodilation, the mechanism whereby arterial compression elicits NO production is unclear. We cannulated isolated swine (n = 39) myocardial (n = 69) and skeletal muscle (n = 60) arteriole segments and exposed them to cyclic transmural pressure generated by either intraluminal or extraluminal pressure pulses to simulate compression in contracting muscle. We found that the vasodilation elicited by internal or external pressure pulses was equivalent; moreover, vasodilation in response to pressure depended on changes in arteriole diameter. Agonist-induced endothelium-dependent and -independent vasodilation was used to verify endothelial and vascular smooth muscle cell viability. Vasodilation in response to cyclic changes in transmural pressure was smaller than that elicited by pharmacological activation of the NO signaling pathway. It was attenuated by inhibition of NO synthase and by mechanical removal of the endothelium. Stemming from previous observations that endothelial integrin is implicated in vasodilation in response to shear stress, we found that function-blocking integrin α5β1 or αvβ3 antibodies attenuated cyclic compression-induced vasodilation and NOx (NO(-)2 and NO(-)3) production, as did an RGD peptide that competitively inhibits ligand binding to some integrins. We therefore conclude that integrin plays a role in cyclic compression-induced endothelial NO production and thereby in the vasodilation of small arteries during cyclic transmural pressure loading.

Characterization of Neuwiedin, a new disintegrin from Bothrops neuwiedi venom gland with distinct cysteine pattern.

Disintegrins are cysteine-rich toxins containing the RGD motif exposed in a loop that binds integrins such as αIIbβ3, α5β1 and αvβ3. The flexibility of the RGD loop, controlled by the profile of the cysteine pairs and the residues flanking the RGD sequence, are key structural features for the functional activity of these molecules. Recently, our group reported a transcript in the venom gland of Bothrops neuwiedi corresponding to a new P-II SVMP precursor, BnMPIIx, in which the RGD-binding loop includes many substituted residues and unique cysteine residues at the C-terminal. In this paper, we obtained the recombinant disintegrin domain of BnMPIIx, Neuwiedin, which inhibited ADP-induced platelet aggregation, endothelial cell adhesion to fibrinogen and tube formation in Matrigel with no particular selectivity to αIIbβ3 or endothelial cell integrins. This value was also comparable to the inhibition observed with other recombinant disintegrins with conserved cysteine positions and residues in RGD loop. In this regard, Neuwiedin is an important component to understand the functional relevance of the diversity generated by accelerated evolution of venom toxins as well as to find out eventual new disintegrin-dependent targets that may be approached with disintegrins.

Arf6 regulates tumour angiogenesis and growth through HGF-induced endothelial β1 integrin recycling.

Anti-angiogenic drugs targeting vascular endothelial cell growth factor receptor have provided modest clinical benefit, in part, owing to the actions of additional angiogenic factors that stimulate tumour neoangiogenesis in parallel. To overcome this redundancy, approaches targeting these other signalling pathways are required. Here we show, using endothelial cell-targeted mice, that the small GTPase Arf6 is required for hepatocyte growth factor (HGF)-induced tumour neoangiogenesis and growth. Arf6 deletion from endothelial cells abolishes HGF-stimulated β1 integrin recycling. Pharmacological inhibition of the Arf6 guanine nucleotide exchange factor (GEF) Grp1 efficiently suppresses tumour vascularization and growth. Grp1 as well as other Arf6 GEFs, such as GEP100, EFA6B and EFA6D, regulates HGF-stimulated β1 integrin recycling. These findings provide insight into the mechanism of HGF-induced tumour angiogenesis and offer the possibility that targeting the HGF-activated Arf6 signalling pathway may synergize with existing anti-angiogenic drugs to improve clinical outcomes.

αvβ3 Integrins Mediate Flow-Induced NF-κB Activation, Proinflammatory Gene Expression, and Early Atherogenic Inflammation

Endothelial cell interactions with transitional matrix proteins, such as fibronectin, occur early during atherogenesis and regulate shear stress-induced endothelial cell activation. Multiple endothelial cell integrins bind transitional matrix proteins, including α5β1, αvβ3, and αvβ5. However, the role these integrins play in mediating shear stress-induced endothelial cell activation remains unclear. Therefore, we sought to elucidate which integrin heterodimers mediate shear stress-induced endothelial cell activation and early atherogenesis. We now show that inhibiting αvβ3 integrins (S247, siRNA), but not α5β1 or αvβ5, blunts shear stress-induced proinflammatory signaling (NF-κB, p21-activated kinase) and gene expression (ICAM1, VCAM1). Importantly, inhibiting αvβ3 did not affect cytokine-induced proinflammatory responses or inhibit all shear stress-induced signaling, because Akt, endothelial nitric oxide synthase, and extracellular regulated kinase activation remained intact. Furthermore, inhibiting αv integrins (S247), but not α5 (ATN-161), in atherosclerosis-prone apolipoprotein E knockout mice significantly reduced vascular remodeling after acute induction of disturbed flow. S247 treatment similarly reduced early diet-induced atherosclerotic plaque formation associated with both diminished inflammation (expression of vascular cell adhesion molecule 1, plaque macrophage content) and reduced smooth muscle incorporation. Inducible, endothelial cell-specific αv integrin deletion similarly blunted inflammation in models of disturbed flow and diet-induced atherogenesis but did not affect smooth muscle incorporation. Our studies identify αvβ3 as the primary integrin heterodimer mediating shear stress-induced proinflammatory responses and as a key contributor to early atherogenic inflammation.

Coagulation factor XIII: a multifunctional transglutaminase with clinical potential in a range of conditions.

Coagulation factor XIII (FXIII), a plasma transglutaminase, is best known as the final enzyme in the coagulation cascade, where it is responsible for cross-linking of fibrin. However, a growing body of evidence has demonstrated that FXIII targets a wide range of additional substrates that have important roles in health and disease. These include antifibrinolytic proteins, with cross-linking of α2-antiplasmin to fibrin, and potentially fibrinogen, being the principal mechanism(s) whereby plasmin-mediated clot degradation is minimised. FXIII also acts on endothelial cell VEGFR-2 and αvβ3 integrin, which ultimately leads to downregulation of the antiangiogenic protein thrombospondin-1, promoting angiogenesis and neovascularisation. Under infectious disease conditions, FXIII cross-links bacterial surface proteins to fibrinogen, resulting in immobilisation and killing, while during wound healing, FXIII induces cross-linking of the provisional matrix. The latter process has been shown to influence the interaction of leukocytes with the provisional extracellular matrix and promote wound healing. Through these actions, there are good rationales for evaluating the therapeutic potential of FXIII in diseases in which tissue repair is dysregulated or perturbed, including systemic sclerosis (scleroderma), invasive bacterial infections, and tissue repair, for instance healing of venous leg ulcers or myocardial injuries. Adequate levels of FXIII are also required in patients undergoing surgery to prevent or treat perioperative bleeding, and its augmentation in patients with/at risk for perioperative bleeding may also have potential clinical benefit. While there are preclinical and/or clinical data to support the use of FXIII in a range of settings, further clinical evaluation in these underexplored applications is warranted.

P53-Induced inflammation exacerbates cardiac dysfunction during pressure overload

The rates of death and disability caused by severe heart failure are still unacceptably high. There is evidence that the sterile inflammatory response has a critical role in the progression of cardiac remodeling in the failing heart. The p53 signaling pathway has been implicated in heart failure, but the pathological link between p53 and inflammation in the failing heart is largely unknown. Here we demonstrate a critical role of p53-induced inflammation in heart failure. Expression of p53 was increased in cardiac endothelial cells and bone marrow cells in response to pressure overload, leading to up-regulation of intercellular adhesion molecule-1 (ICAM1) expression by endothelial cells and integrin expression by bone marrow cells. Deletion of p53 from endothelial cells or bone marrow cells significantly reduced ICAM1 or integrin expression, respectively, as well as decreasing cardiac inflammation and ameliorating systolic dysfunction during pressure overload. Conversely, overexpression of p53 in bone marrow cells led to an increase of integrin expression and cardiac inflammation that reduced systolic function. Norepinephrine markedly increased p53 expression in endothelial cells and macrophages. Reducing β2-adrenergic receptor expression in endothelial cells or bone marrow cells attenuated cardiac inflammation and improved systolic dysfunction during pressure overload. These results suggest that activation of the sympathetic nervous system promotes cardiac inflammation by up-regulating ICAM1 and integrin expression via p53 signaling to exacerbate cardiac dysfunction. Inhibition of p53-induced inflammation may be a novel therapeutic strategy for heart failure.

Gαs proteins activate p72Syk and p60-c-Src tyrosine kinases to mediate sickle red blood cell adhesion to endothelium via LW-αvβ3 and CD44–CD44 interactions

G protein-coupled receptors (GPCRs) have been suggested as new drug targets to treat a variety of diseases. In sickle cell disease (SCD), the LW erythrocyte adhesion receptor can be activated by stimulation of β2 adrenergic receptors (β2ARs), to mediate sickle erythrocyte (SSRBC) adhesion to endothelium. However, the involvement of tyrosine protein kinases in β2AR signaling to activate SSRBC adhesion to endothelium has not been thoroughly elucidated. Either direct activation with Cholera toxin of Gαs protein, which acts downstream of β2ARs, or inhibition with Pertussis toxin of Gαi, mediating suppression of adenylyl cyclase, increased SSRBC adhesion to endothelium over baseline adhesion. This effect involved the non-receptor tyrosine kinases, p72Syk and p60-c-Src, which were more abundant in SSRBCs than in normal erythrocytes. In contrast, Pertussis toxin and Cholera toxin failed to increase adhesion of normal erythrocytes. SSRBC Gαi inhibition also increased phosphorylation of p72Syk and p60-c-Src. Further, we investigated the relevance of activation of p72Syk and p60-c-Src, and identified LW (ICAM-4, CD242) and CD44 as the erythroid adhesion molecules both physically interacting with activated p60-c-Src. As a result, SSRBC LW underwent increased tyrosine phosphorylation, leading to SSRBC LW and CD44 binding to endothelial αvβ3 integrin and CD44, respectively. These data provide in vitro mechanistic evidence that p60-c-Src, which could act downstream of Gαs/p72Syk, associates with LW and CD44 on SSRBCs leading to their interactions with endothelial αvβ3 and CD44, respectively. Thus, increased activation of these signaling mechanisms in SSRBCs could initiate or exacerbate vascular occlusion, the hallmark of SCD.

Abstract 573: Endothelial α5 Integrins Regulate OxLDL-Induced Fibronectin Deposition, Proinflammatory Gene Expression, and Early Atherosclerosis

Alterations in extracellular matrix quantity and composition contribute to atherosclerosis, with remodeling of the subendothelial basement membrane to a fibronectin-rich matrix preceding lesion development. Published data from our lab and others demonstrate that endothelial cell interactions with fibronectin prime inflammatory responses to a variety of atherogenic stimuli. However, the mechanisms regulating early atherogenic fibronectin accumulation remain unknown. Work from our group previously demonstrated that oxidized LDL (oxLDL) promotes endothelial proinflammatory gene expression by activating the integrin α5β1, a classic mediator of fibronectin fibrillogenesis. We now show that treating endothelial cells with oxLDL induces fibronectin deposition and inhibiting α5β1 (blocking antibodies, α5 knockout cells) completely inhibits oxLDL-induced fibronectin deposition. While endothelial fibronectin expression remains unchanged, oxLDL robustly stimulates the deposition of endothelial cell-derived fibronectin associated with a significant reduction in intracellular fibronectin. Interestingly, loss of endothelial cell-derived fibronectin, but not plasma fibronectin, prevents integrin α5 localization to focal adhesions, reduces fibronectin fibril length, and inhibits oxLDL-induced VCAM-1 expression. In addition, inducible endothelial-specific deletion of α5 integrins significantly blunts atherosclerotic plaque formation in ApoE knockout mice, suggesting an important role for this integrin in early endothelial activation. Taken together, our data demonstrate that oxLDL stimulates α5 integrin-dependent subendothelial matrix remodeling and endothelial proinflammatory gene expression through the deposition of fibronectin.