Novel mechanism regulating endothelial permeability via T-cadherin-dependent VE-cadherin phosphorylation and clathrin-mediated endocytosis

Decoding the enigmatic role of T-cadherin in tumor angiogenesis

The cadherin family, which includes T-cadherin, plays a significant role in angiogenesis, a critical process involved in tumor growth, metastasis, and recurrence. T-cadherin is extensively expressed in both normal and tumor vascular tissues and has been shown to facilitate the proliferation and migration of vascular cells in some studies. However, T-cadherin also exerts inhibitory effects on angiogenesis in various tumor tissues. The functional role of T-cadherin may vary depending on the tumor type and the interaction between tumor cells and vascular cells, suggesting that it acts as a modulator rather than a primary driver of angiogenesis. Additionally, T-cadherin exhibits distinct characteristics depending on the tumor microenvironment. This review provides an overview of recent research on the role of T-cadherin in tumor angiogenesis and discusses its potential as a diagnostic or therapeutic marker in the field of tumor biology.

New evidence for T-cadherin in COVID-19 pathogenesis, endothelial dysfunction, and lung fibrosis

The COVID-19 pandemic had an unprecedented impact on all aspects of human activity worldwide, frequently resulting in post-acute sequelae and affecting multiple organ systems. The underlying mechanisms driving both acute and post-acute manifestations of COVID-19 are still poorly understood, warranting further investigation for new targets. The study represents the first attempt to explore the role of T-cadherin in COVID-19 pathogenesis as well as its implications in pulmonary fibrosis and endothelial dysfunction. First, we revealed a significant decrease in T-cadherin expression in post-mortem lung samples from COVID-19 patients. This downregulated T-cadherin expression correlated with the elevated levels of VE-cadherin and reduced levels of β-catenin, suggesting a disruption in endothelial cell-cell contact integrity and function. Second, the reciprocal relation of T-cadherin and VE-cadherin expression was further confirmed using cultured human endothelial Ea.hy926 cells. T-cadherin overexpression caused a decrease in VE-cadherin mRNA expression in cultured endothelial cells providing additional evidence in favor of their interplay. Third, employing Cdh13 -/- mice, we unveiled the protective role of T-cadherin deficiency against bleomycin-induced lung fibrosis. Fourth, we demonstrated the mice lacking T-cadherin to have downregulated reactive oxygen species production and Nox2 mRNA expression in an angiotensin II-mediated endothelial dysfunction model. Our findings provide rationale for further studies into T-cadherin-mediated mechanisms in these processes.

Alveolar-capillary endocytosis and trafficking in acute lung injury and acute respiratory distress syndrome

Acute respiratory distress syndrome (ARDS) is associated with high morbidity and mortality but lacks specific therapeutic options. Diverse endocytic processes play a key role in all phases of acute lung injury (ALI), including the initial insult, development of respiratory failure due to alveolar flooding, as a consequence of altered alveolar-capillary barrier function, as well as in the resolution or deleterious remodeling after injury. In particular, clathrin-, caveolae-, endophilin- and glycosylphosphatidyl inositol-anchored protein-mediated endocytosis, as well as, macropinocytosis and phagocytosis have been implicated in the setting of acute lung damage. This manuscript reviews our current understanding of these endocytic pathways and subsequent intracellular trafficking in various phases of ALI, and also aims to identify potential therapeutic targets for patients with ARDS.

Circulatory T-cadherin is a potential biomarker for atherosclerosis

T-cadherin, a special member of cadherin family, expresses with blood circulation involving the heart i.e. CVS. Cadherin is connected with the healthy conditions of an individual and normal functioning of cardio-vascular metabolism. T-cadherin is mainly associated with blood vascular system of human. Previous studies analysed this cadherin been unexpressed within the fat storing tissues i.e. adipose tissue of peri-aortic and peri-coronary, it is present within endothelium as well as in vascularized smooth muscular cells which includes the area nearby coronary vessels and aorta. The area and site of this cadherin is attention-grabbing because it particularly related to atherosclerosis and cardiovascular disease (CVD). T-cadherin - a protein acting as the receptor for low density lipoproteins (LDL). It may act as a special biomarker for atherosclerosis. Previous studies on T-cadherin showed that it has cardio-protective role. Furthermore, research is essential to enumerate the cardio-protective function of T-cadherin. It can be an important therapeutic target in developing new medicine to decrease incident of heart disease and its complications.

Revisiting the multiple roles of T-cadherin in health and disease

As a non-canonical member of cadherin superfamily, T-cadherin was initially described as a molecule involved in homophilic recognition in the nervous and vascular systems. The ensuing decades clearly demonstrated that T-cadherin is a remarkably multifunctional molecule. It was validated as a bona fide receptor for both: LDL exerting adverse atherogenic action and adiponectin mediating many protective metabolic and cardiovascular effects. Motivated by the latest progress and accumulated data unmasking important roles of T-cadherin in blood vessel function and tissue regeneration, here we revisit the original function of T-cadherin as a guidance receptor for the growing axons and blood vessels, consider the recent data on T-cadherin-induced exosomes' biogenesis and their role in myocardial regeneration and revascularization. The review expands upon T-cadherin contribution to mesenchymal stem/stromal cell compartment in adipose tissue. We also dwell upon T-cadherin polymorphisms (SNP) and their possible therapeutic applications. Furthermore, we scrutinize the molecular hub of insulin and adiponectin receptors (AdipoR1 and AdipoR2) conveying signals to their downstream targets in quest for defining a putative place of T-cadherin in this molecular circuitry.

VE-Cadherin and Vesicles Differentially Regulate Lymphatic Vascular Permeability to Solutes of Various Sizes

Lymphatic vascular permeability prevents lymph leakage that is associated with lymphedema, lymphatic malformations, obesity, and inflammation. However, the molecular control of lymphatic permeability remains poorly understood. Recent studies have suggested that adherens junctions and vesicle transport may be involved in regulating lymphatic vessel permeability. To determine the contribution of each transport pathway, we utilized an ex vivo permeability assay to directly measure the solute flux of various molecular weight solutes across a range of pressures in intact murine collecting lymphatic vessels. Pharmacological and biological tools were used to probe the relative contributions of vesicles and junction proteins in the lymphatic vasculature. We show that the permeability of collecting lymphatic vessels is inversely related to the solute molecular weight. Further, our data reveal that vesicles selectively transport BSA, as an inhibitor of vesicle formation significantly decreased the permeability to BSA (∼60% decrease, n = 8, P = 0.02), but not to 3 kDa dextran (n = 7, P = 0.41), α-lactalbumin (n = 5, P = 0.26) or 70 kDa dextran (n = 8, P = 0.13). In contrast, disruption of VE-cadherin binding with a function blocking antibody significantly increased lymphatic vessel permeability to both 3 kDa dextran (5.7-fold increase, n = 5, P < 0.0001) and BSA (5.8-fold increase, n = 5, P < 0.0001). Thus, in the lymphatic vasculature, adherens junctions did not exhibit selectivity for any of the solutes tested here, whereas vesicles specifically transport BSA. Overall, the findings suggest that disease states that disrupt VE-cadherin localization or expression will cause significant leakage of solutes and fluid from the lymphatic vasculature.

Proteolytic enzyme and adiponectin receptors as potential targets for COVID-19 therapy

The coronavirus disease 2019 (COVID-19) pandemic requires not only the creation of vaccines to prevent the spread of the disease, but also the development of novel drugs aimed at reducing viral load, suppressing an excessive immune response and preventing the severe complications such as lung fibrosis and acute respiratory distress syndrome. One of the promising targets for studying the development of pneumonia, systemic inflammation and disseminated intravascular coagulation in COVID-19 is the plasminogen activator system. In patients with a severe disease course, impaired activity or expression of plasminogen activators significantly increases the blood level of D-dimer and fibrinogen, as well as correlates with intravascular coagulation and thrombus formation. The second promising target for studying the pathogenesis of COVID-19 is the adiponectin/T-cadherin system: adiponectin is able to reduce the content of pro-inflammatory cytokines, the increase of which is characteristic of COVID-19, and stimulate the production of an anti-inflammatory cytokine interleukin-10. The review describes the role of plasminogen and T-cadherin activators in their possible participation in the development of pulmonary fibrosis in COVID-19 and hemostasis regulation, as well as cardio- and vasculoprotective function of adiponectin and its receptor, T-cadherin.

Oxygen-Glucose Deprivation/Reoxygenation Induces Human Brain Microvascular Endothelial Cell Hyperpermeability Via VE-Cadherin Internalization: Roles of RhoA/ROCK2

The destruction of the blood-brain barrier (BBB) contributes to a spectrum of neurological diseases such as stroke, and the hyperpermeability of endothelial cells is one of the characters of stroke, which is possibly exacerbated after reperfusion. However, the underlying mechanisms involving hyperpermeability after reperfusion between the endothelial cells remain poorly understood. Therefore, in the present study, the human microvascular endothelial cells (HBMECs) were exposed to oxygen-glucose deprivation/reperfusion (OGD/R) to mimic ischemic stroke condition in vitro with the aim to investigate the potential mechanisms induced by OGD/R. The permeability of cultured HBMECs was measured using FITC-labeled dextran in a Transwell system and transendothelial electrical resistance (TEER), while the RhoA activity was detected by pull-down assay. In addition, the phosphorylation of MYPT1, which reflects the activation of ROCK and the internalization of VE-cadherin, was detected by Western blot. It showed that OGD/R treatment significantly increased the permeability of HBMEC monolayers and facilitated the internalization of VE-cadherin in HBMEC monolayers. Pull-down assay showed that RhoA activation was obviously enhanced after OGD/R treatment, while RhoA and ROCK inhibitor significantly reversed OGD/R-induced HBMEC monolayers hyperpermeability and the internalization of VE-cadherin. Meanwhile, the knockdown assay showed that RhoA small interfering RNA (siRNA) led to similar effects. The inactivation of the downstream effector protein ROCK was also examined. Intriguingly, ROCK2 rather than ROCK1 exerted its adverse effects on HBMEC monolayer integrity, since ROCK2 knockdown markedly reverses the injury of OGD/R in HBMEC monolayers. In conclusion, the present study provides evidence that OGD/R may induce HBMEC monolayer hyperpermeability via RhoA/ROCK2-mediated VE-cadherin internalization, which may provide an impetus for the development of therapeutics targeting BBB damage in ischemic stroke.

Effect of adiponectin level and genetic variation of its receptors on diabetic retinopathy: A case-control study

Adiponectin (APN) and its receptors have been reported to be associated with metabolic phenotypes. To better understand the effects of APN levels and its receptors on diabetic retinopathy (DR), we investigated the association of the plasma APN level and variations in APN-related genes with DR, individually and in combination.Patients with type 2 diabetes (T2D; N = 1604), above 20 years of age from the Taiwanese population participated in the study. Demographic information, blood pressure, and serological markers were recorded at enrollment. Genomic DNA was isolated and genotyped. The plasma APN levels were measured by enzyme-linked immunosorbent assay.T2D patients with DR (N = 632) had diabetes for a longer duration, and had higher HbA1c, and systolic and diastolic blood pressure compared to those without DR (N = 972) (P < .001, for all the parameters). Overall, 10 single nucleotide polymorphisms (SNPs) in ADIPOQ and CDH13 susceptibility loci were associated with DR. Gene risk score (GRS) was calculated based on 10 SNPs for each subject and the cumulative effect of genes was observed. Among the subjects with plasma APN level (N = 518), natural logarithm (LN) of APN (LN [APN]; odds ratio [OR] = 1.63, 95% confidence interval [CI] = 1.19-2.25) and GRS (OR = 1.90, 95% CI = 1.11-3.26 for middle range of GRS, and OR = 2.61, 95% CI = 1.48-4.59 for high range of GRS) were independent risk factors for DR after adjustment for other parameters.In conclusion, the plasma APN level and the genetic variations in adiponectin receptors were associated with DR.

Cytokine-Ion Channel Interactions in Pulmonary Inflammation

The lungs conceptually represent a sponge that is interposed in series in the bodies’ systemic circulation to take up oxygen and eliminate carbon dioxide. As such, it matches the huge surface areas of the alveolar epithelium to the pulmonary blood capillaries. The lung’s constant exposure to the exterior necessitates a competent immune system, as evidenced by the association of clinical immunodeficiencies with pulmonary infections. From the in utero to the postnatal and adult situation, there is an inherent vital need to manage alveolar fluid reabsorption, be it postnatally, or in case of hydrostatic or permeability edema. Whereas a wealth of literature exists on the physiological basis of fluid and solute reabsorption by ion channels and water pores, only sparse knowledge is available so far on pathological situations, such as in microbial infection, acute lung injury or acute respiratory distress syndrome, and in the pulmonary reimplantation response in transplanted lungs. The aim of this review is to discuss alveolar liquid clearance in a selection of lung injury models, thereby especially focusing on cytokines and mediators that modulate ion channels. Inflammation is characterized by complex and probably time-dependent co-signaling, interactions between the involved cell types, as well as by cell demise and barrier dysfunction, which may not uniquely determine a clinical picture. This review, therefore, aims to give integrative thoughts and wants to foster the unraveling of unmet needs in future research.

Desacetylvinblastine Monohydrazide Disrupts Tumor Vessels by Promoting VE-cadherin Internalization

Vinca alkaloids, the well-known tubulin-binding agents, are widely used for the clinical treatment of malignant tumors. However, little attention has been paid to their vascular disrupting effects, and the underlying mechanisms remain largely unknown. This study aims to investigate the vascular disrupting effect and the underlying mechanisms of vinca alkaloids.

Methods: The capillary disruption assay and aortic ring assay were performed to evaluate the in vitro vascular disrupting effect of desacetylvinblastine monohydrazide (DAVLBH), a derivate of vinblastine, and the in vivo vascular disrupting effect was assessed on HepG2 xenograft model using magnetic resonance imaging, hematoxylin and eosin staining and immunohistochemistry. Tubulin polymerization, endothelial cell monolayer permeability, western blotting and immunofluorescence assays were performed to explore the underlying mechanisms of DAVLBH-mediated tumor vascular disruption.

Results: DAVLBH has potent vascular disrupting activity both in vitro and in vivo. DAVLBH disrupts tumor vessels in a different manner than classical tubulin-targeting VDAs; it inhibits microtubule polymerization, promotes the internalization of vascular endothelial cadherin (VE-cadherin) and inhibits the recycling of internalized VE-cadherin to the cell membrane, thus increasing endothelial cell permeability and ultimately resulting in vascular disruption. DAVLBH-mediated promotion of VE-cadherin internalization and inhibition of internalized VE-cadherin recycling back to the cell membrane are partly dependent on inhibition of microtubule polymerization, and Src activation is involved in DAVLBH-induced VE-cadherin internalization.

Conclusions: This study sheds light on the tumor vascular disrupting effect and underlying mechanisms of vinca alkaloids and provides new insight into the molecular mechanism of tubulin-targeting VDAs.

Inhibitory effects of pigment epithelium-derived factor on epithelial-mesenchymal transition, migration and invasion of breast cancer

OBJECTIVE

Pigment epithelium-derived factor (PEDF) is a ubiquitously expressed secreted protein that suppresses tumor growth and metastasis by targeting tumor cells and their microenvironment. However, the exact mechanism of PEDF on breast cancer metastasis including liver and lung metastasis remains unclear. Epithelial-mesenchymal transition (EMT) is a pivotal event in the progression of cancer towards metastasis. In the present report, we investigated whether PEDF inhibits breast cancer metastasis through epithelial-mesenchymal transition and elucidate the association of PEDF expression and EMT in vitro.

METHODS

Our analyses were performed on 102 tissue samples of patients with primary BC and a set of 20 control samples of healthy women, respectively. Lentiviruses were used to stably express PEDF in SkBr3 breast cancer cell line to determine EMT factors changes of invasion ability following PEDF re-expression. PEDF and EMT factors protein levels were measured in SkBr3 breast cancer cell line using western blot analyses.

RESULTS

We show that the important inhibitor of angiogenesis, pigment epithelium-derived factor expression positively correlatedwith lymph node-positive tumor status and tumor size, low expression level of vimentin, and high expression levels of membranous E-cadherin. In addition, we found that PEDF activation suppressed migration and invasion in SKBR3 (luminal) cellsand led to morphologic and molecular changes of epithelial-mesenchymal transition (EMT). Loss of PEDF promotesmesenchymal phenotype, whereas PEDF was shown to effectively promotes epithelial phenotyperesulted ininhibited the growthof endocrine-resistant SkBr3 breast cancer cells invitro. Finally, western blot examination of PEDF/siRNA-expressing tumor showed down-regulation of E-cadherin and up-regulation of vimentin.

CONCLUSIONS

These findings suggest that PEDF is directly linked to the mechanisms that suppress metastasis of breast cancer through regulating epithelial-mesenchymal transition. In the future, contribute to evaluate the efficacy of PEDF targetedtherapy early during the course of the disease, may be beneficial in the treatment of breast cancer patients.

The Role of IL-33 on LPS-Induced Acute Lung Injury in Mice

The objective of the study is to investigate the role and specific molecular mechanism of interleukin-33 (IL-33) acted on acute lung injury (ALI) induced by lipopolysaccharide (LPS). C57BL/6 mice intratracheally instilled LPS to induce ALI model. The mice were randomly divided into three groups: the sham operation group (Sham), ALI group (ALI), and pretreatment with IL-33 of ALI group (IL-33). By observing the survival rate, inflammatory cytokines in bronchoalveolar lavage fluid (BALF), myeloperoxidase (MPO) levels in lung tissue, lung histopathological examination, pulmonary capillary leakage, lung wet/dry (W/D) weight ratio, fibrosis levels in lung tissue, and associated pathways changes among the different groups, comparing to explore the role of IL-33 pretreatment on ALI mice and the possible molecular mechanisms. IL-33 pretreatment overall decreased the survival rate of ALI mice. IL-33 aggravated inflammation reaction showing as increasing the release of proinflammatory cytokines TNF-α and IL-6, increasing MPO levels in lung tissue, and aggravating lung pathology injury. In addition, IL-33 pretreatment further destroyed adherens junctions (AJs) by increasing the phosphorylation of VE-cadherin, resulting in the concomitantly pulmonary capillary barrier damage and pulmonary edema. During this process, mitogen-activated protein kinase (MAPK) pathways further activated. However, IL-33 pretreatment had no significant impact on collagen content of lung tissue. Our results indicated that IL-33 aggravated inflammatory reaction and increased microvascular permeability, but had little effect on pulmonary fibrosis, associated with the further activation of MAPK family proteins in the process. To sum up, IL-33 decreased survival rate and aggravated LPS-induced ALI.

T-cadherin deficiency increases vascular vulnerability in T2DM through impaired NO bioactivity

Background

Endothelial dysfunction plays a critical role in the development of type 2 diabetes (T2DM). T-cadherin (T-cad) has gained recognition as a regulator of endothelial cell (EC) function. The present study examined whether T-cad deficiency increases vascular vulnerability in T2DM.

Methods

Vascular segments were isolated from WT or T-cad knockout mice. Endothelial function, total NO accumulation, and the expression of T-cad related proteins were determined.

Results

Ach and acidified NaNO2 induced similar vasorelaxation in WT groups. T-cad KO mice exhibited normal response to acidified NaNO2, but manifested markedly reduced response to Ach. NO accumulation was also decreased in T-cad KO group. T-cad expression was reduced in WT mice fed 8 weeks of high fat diet (HFD). Furthermore, exacerbated reduction of vasorelaxation was observed in T-cad KO mice fed 8 weeks of HFD.

Conclusions

In the current study, we provide the first in vivo evidence that T-cadherin deficiency causes endothelial dysfunction in T2DM vascular segments, suggesting the involvement of T-cad deficiency in T2DM pathogenesis.

The VE-cadherin cytoplasmic domain undergoes proteolytic processing during endocytosis

VE-cadherin is cleaved by calpain to remove the β-catenin–binding domain upon entry into clathrin-enriched membrane domains. Calpain cleavage of VE-cadherin cytoplasmic tail appears to fate cadherin for degradation rather than recycling and thus alters the cadherin trafficking itinerary after endocytosis.

VE-cadherin trafficking to and from the plasma membrane has emerged as a critical mechanism for regulating cadherin surface levels and adhesion strength. In addition, proteolytic processing of cadherin extracellular and cytoplasmic domains has been reported to regulate cadherin adhesion and signaling. Here we provide evidence that VE-cadherin is cleaved by calpain upon entry into clathrin-enriched domains. This cleavage event occurs between the β-catenin and p120-binding domains within the cadherin cytoplasmic tail. Of interest, VE-cadherin mutants that are resistant to endocytosis are similarly resistant to cleavage. Furthermore, p120-catenin overexpression blocks cadherin internalization and cleavage, coupling entry into the endocytic pathway with proteolytic processing. Of importance, the cleavage of the VE-cadherin tail alters the postendocytic trafficking itinerary of the cadherin, resulting in a higher turnover rate due to decreased recycling and increased degradation. In conclusion, this study identifies a novel proteolytic event that regulates the trafficking of VE-cadherin after endocytosis.

Pharmacoproteomic analysis reveals that metapristone (RU486 metabolite) intervenes E-cadherin and vimentin to realize cancer metastasis chemoprevention

Metapristone is the most predominant biological active metabolite of mifepristone, and being developed as a novel cancer metastasis chemopreventive agent by us. Despite its prominent metastasis chemopreventive effect, the underlying mechanism remains elusive. Our study, for the first time, demonstrated that metapristone had the ability to prevent breast cancer cells from migration, invasion, and interfere with their adhesion to endothelial cells. To explore the underlying mechanism of metapristone, we employed the iTRAQ technique to assess the effect of metapristone on MDA-MB-231 cells. In total, 5,145 proteins were identified, of which, 311 proteins showed significant differences in metapristone-treated cells compared to the control group (P-value < 0.05). Bioinformatic analysis showed many differentially expressed proteins (DEPs) functionally associated with post-translational modification, chaperones, translation, transcription, replication, signal transduction, etc. Importantly, many of the DEPs, such as E-cadherin, vimentin, TGF-β receptor I/II, smad2/3, β-catenin, caveolin, and dystroglycan were associated with TGF-β and Wnt signaling pathways, which were also linked to epithelial-to-mesenchymal transition (EMT) process. Further validation of the epithelial marker "E-caderin" and mesenchymal marker "vimetin" were carried out using immunoblot and immunofluorescence. These results have revealed a novel mechanism that metapristone-mediated metastasis chemoprevention is through intervening the EMT-related signaling pathways.

Integrative proteomics and targeted transcriptomics analyses in cardiac endothelial cells unravel mechanisms of long-term radiation-induced vascular dysfunction

Epidemiological data from radiotherapy patients show the damaging effect of ionizing radiation on heart and vasculature. The endothelium is the main target of radiation damage and contributes essentially to the development of cardiac injury. However, the molecular mechanisms behind the radiation-induced endothelial dysfunction are not fully understood. In the present study, 10-week-old C57Bl/6 mice received local X-ray heart doses of 8 or 16 Gy and were sacrificed after 16 weeks; the controls were sham-irradiated. The cardiac microvascular endothelial cells were isolated from the heart tissue using streptavidin-CD31-coated microbeads. The cells were lysed and proteins were labeled with duplex isotope-coded protein label methodology for quantification. All samples were analyzed by LC-ESI-MS/MS and Proteome Discoverer software. The proteomics data were further studied by bioinformatics tools and validated by targeted transcriptomics, immunoblotting, immunohistochemistry, and serum profiling. Radiation-induced endothelial dysfunction was characterized by impaired energy metabolism and perturbation of the insulin/IGF-PI3K-Akt signaling pathway. The data also strongly suggested premature endothelial senescence, increased oxidative stress, decreased NO availability, and enhanced inflammation as main causes of radiation-induced long-term vascular dysfunction. Detailed data on molecular mechanisms of radiation-induced vascular injury as compiled here are essential in developing radiotherapy strategies that minimize cardiovascular complications.

Exendin-4 promotes endothelial barrier enhancement via PKA- and Epac1-dependent Rac1 activation

Among emerging antidiabetic agents, glucagon-like peptide-1 (GLP-1)-based therapies carry special cardiovascular implications, exerting both direct and indirect effects. The control of vascular permeability is of pivotal importance in vascular pathologies. The objective of the present study was to determine the effect of GLP-1 on endothelial barrier function and assess the underlying mechanism(s). Here we show for the first time that the stable GLP-1 analog exendin-4 attenuated the leakage of subcutaneous blood vessels in mice indexed by dye extravasation caused by injections of thrombin. Moreover, in cultured endothelial cells, exendin-4 significantly prevented the thrombin-induced FITC-dextran permeability of endothelial monolayers via GLP-1 receptor. Immunofluorescence microscopy reveals that exendin-4 abrogates detrimental effects of thrombin on VE-cadherin and the F-actin cytoskeleton, with decreased stress fiber and gap formation. Importantly, exendin-4 reduced thrombin-induced tyrosine phosphorylation of VE-cadherin at Y731 and Y658. Moreover, small GTPase Rac1 was significantly activated as a result of exendin-4 treatment. The efficacy of exendin-4 to counteract the barrier-compromising effect of thrombin was blunted when Rac1 was inactivated by Rac1 inhibitor NSC-23766. Inhibition of PKA activity or small-interfering RNA for exchange protein directly activated by cAMP 1 (Epac1) decreased exendin-4-induced Rac1 activation and barrier enhancement, indicating the participation of both PKA and Epac1 in the barrier-stabilizing effect of exendin-4 elicited on thrombin-impaired barrier function. Thus, our findings have uncovered an unpredicted role for exendin-4 in the coordination of vascular permeability and clarified the molecular underpinnings that contribute to barrier restoration initiated by exendin-4.