Hypoxia perturbs endothelium by re-organizing cellular actin architecture: Nitric oxide offers limited protection.

Exposure to hypoxia causes structural changes in the endothelial cell (EC) monolayer that alter its permeability. There was a report earlier of impairment of nitric oxide (NO) production in endothelium. The intervention of NO in the altered cellular arrangements of actin cytoskeleton in endothelium for rectification of paracellular gaps in endothelium under hypoxia was observed. The present study demonstrates hypoxia inducing paracellular gaps in hypoxia-exposed blood capillaries in chick embryo extravascular model. Phalloidin staining confirmed significant polymerization of actin and unique cellular localization of the F-actin bands under hypoxia treatments. Addition of spermine NONOate (SPNO), a NO donor, or reoxygenation to endothelial monolayer attenuated hypoxia-mediated effects on endothelial permeability with partial recovery of endothelial integrity through actin remodeling. The present study indicates link of hypoxia-induced actin-associated cytoskeletal rearrangements and paracellular gaps in the endothelium with a low NO availability in the hypoxia milieu. The author concludes that NO confers protection against hypoxia-mediated cytoskeletal remodeling and endothelial leakiness.

Rho-kinase as a therapeutic target in vascular diseases: striking nitric oxide signaling.

Rho GTPases are a globular, monomeric group of small signaling G-protein molecules. Rho-associated protein kinase/Rho-kinase (ROCK) is a downstream effector protein of the Rho GTPase. Rho-kinases are the potential therapeutic targets in the treatment of cardiovascular diseases. Here, we have primarily discussed the intriguing roles of ROCK in cardiovascular health in relation to nitric oxide signaling. Further, we highlighted the biphasic effects of Y-27632, a ROCK inhibitor under shear stress, which acts as an agonist of nitric oxide production in endothelial cells. The biphasic effects of this inhibitor raised the question of safety of the drug usage in treating cardiovascular diseases.

A comparative study of NONOate based NO donors: spermine NONOate is the best suited NO donor for angiogenesis.

Nitric oxide (NO) is a known modulator of angiogenesis. The NONOate subfamily of NO donors has long been used in experimental and clinical studies to promote angiogenesis. However, no studies have been conducted yet to compare the angiogenesis potential of these NO donors in respect to their pattern of NO release. We hypothesize that having different pattern of NO release, each of the NO donors in NONOate subfamily can promote key stages of angiogenesis in differential manner. To verify our hypothesis, NO donors with half life ranging from seconds to several hours and having very different pattern of NO release were selected to evaluate their efficacy in modulating angiogenesis. Endothelial tube formation using EAhy926 cells was maximally increased by Spermine NONOate (SP) treatment. SP treatment maximally induced both ex vivo and in vivo angiogenesis using egg yolk and cotton plug angiogenesis models respectively. Experiment using chick embryo partial ischemia model revealed SP as the best suited NO donor to recover ischemia driven hampered angiogenesis. The present study elaborated that differential release pattern of NO by different NO donors can modulate angiogenesis differentially and also suggested that SP have a unique pattern of NO release that best fits for angiogenesis.

Endothelial dysfunction and diabetes: effects on angiogenesis, vascular remodeling, and wound healing.

Diabetes mellitus (DM) is a chronic metabolic disorder characterized by inappropriate hyperglycemia due to lack of or resistance to insulin. Patients with DM are frequently afflicted with ischemic vascular disease or wound healing defect. It is well known that type 2 DM causes amplification of the atherosclerotic process, endothelial cell dysfunction, glycosylation of extracellular matrix proteins, and vascular denervation. These complications ultimately lead to impairment of neovascularization and diabetic wound healing. Therapeutic angiogenesis remains an attractive treatment modality for chronic ischemic disorders including PAD and/or diabetic wound healing. Many experimental studies have identified better approaches for diabetic cardiovascular complications, however, successful clinical translation has been limited possibly due to the narrow therapeutic targets of these agents or the lack of rigorous evaluation of pathology and therapeutic mechanisms in experimental models of disease. This paper discusses the current body of evidence identifying endothelial dysfunction and impaired angiogenesis during diabetes.

Simulated microgravity promotes nitric oxide-supported angiogenesis via the iNOS-cGMP-PKG pathway in macrovascular endothelial cells.

Angiogenesis is a physiological process involving the growth of blood vessel in response to specific stimuli. The present study shows that limited microgravity treatments induce angiogenesis by activating macrovascular endothelial cells. Inhibition of nitric oxide production using pharmacological inhibitors and inducible nitric oxide synthase (iNOS) small interfering ribo nucleic acid (siRNA) abrogated microgravity induced nitric oxide production in macrovascular cells. The study further delineates that iNOS acts as a molecular switch for the heterogeneous effects of microgravity on macrovascular, endocardial and microvascular endothelial cells. Further dissection of nitric oxide downstream signaling confirms that simulated microgravity induces angiogenesis via the cyclic guanosine monophosphate (cGMP)-PKG dependent pathway.

Inhibition of dynamin-2 confers endothelial barrier dysfunctions by attenuating nitric oxide production.

Hypoxia induces barrier dysfunctions in endothelial cells. Nitric oxide is an autacoid signalling molecule that confers protection against hypoxia-mediated barrier dysfunctions. Dyn-2 (dynamin-2), a large GTPase and a positive modulator of eNOS (endothelial nitric oxide synthase), plays an important role in maintaining vascular homeostasis. The present study aims to elucidate the role of dyn-2 in hypoxia-mediated leakiness of the endothelial monolayer in relation to redox milieu. Inhibition of dyn-2 by transfecting the cells with K44A, a dominant negative construct of dyn-2, elevated leakiness of the endothelial monolayer under hypoxia. Sodium nitroprusside (nitric oxide donor) and uric acid (peroxynitrite quencher) were used to evaluate the role of nitric oxide and peroxynitrite in maintaining endothelial barrier functions under hypoxia. Administration of nitric oxide and uric acid recovered hypoxia-mediated leakiness of K44A-overexpressed endothelial monolayer. Our study confirms that inhibition of dyn-2 induces leakiness in the endothelial monolayer by increasing the load of peroxynitrite under hypoxia.

eNOS phosphorylation in health and disease.

Endothelium plays a fundamental role in maintaining the vascular tone by releasing various biochemical factors that modulate the contractile and relaxatory behavior of the underlying vascular smooth muscle, regulation of inflammation, immunomodulation, platelet aggregation, and thrombosis. Endothelium regulates these cellular processes by activating endothelial nitric oxide synthase (eNOS) responsible for nitric oxide (NO) production. eNOS is constitutively expressed in ECs in response to humoral, mechanical or pharmacological stimulus. eNOS activity is regulated mainly by protein-protein interactions and multisite phosphorylations. The phosphorylation state of specific serine, threonine and tyrosine residues of the enzyme plays a pivotal role in regulation of eNOS activity. Perturbations of eNOS phosphorylation have been reported in a number of diseases thereby emphasizing the importance of regulation of eNOS activity. This review summarizes the mechanism of eNOS regulation through multi-site phosphorylation in different pathologies. Attempts have been made to highlight phosphorylation of eNOS at various residues, regulation of the enzyme activity via posttranslational modifications and its implications on health and disease.

Shear stress promotes nitric oxide production in endothelial cells by sub-cellular delocalization of eNOS: A basis for shear stress mediated angiogenesis.

This study aims to investigate the role of shear stress in cellular remodeling and angiogenesis with relation to nitric oxide (NO). We observed a 2-fold increase in endothelial cell (EC) migration in relation to actin re-arrangements under 15 dyne/cm(2) shear stress. Blocking NO production inhibited the migration and ring formation of ECs by 6-fold and 5-fold, respectively under shear stress. eNOS-siRNA knockdown technique also ascertained a 3-fold reduction in shear stress mediated ring formation. In ovo artery ligation model with a half and complete flow block for 30 min showed a reduction of angiogenesis by 50% and 70%, respectively. External stimulation with NO donor showed a 2-fold recovery in angiogenesis under both half and complete flow block conditions. NO intensity clustering studies by using Diaminofluorescein diacetate (DAF-2DA) probed endothelial monolayer depicted pattern-changes in NO distribution and cluster formation of ECs under shear stress. Immunofluorescence and live cell studies revealed an altered sub-cellular localization pattern of eNOS and phospho-eNOS under shear stress. In conclusion, shear-induced angiogenesis is mediated by nitric oxide dependent EC migration.

Simulated microgravity perturbs actin polymerization to promote nitric oxide-associated migration in human immortalized Eahy926 cells.

Microgravity causes endothelium dysfunctions and vascular endothelium remodeling in astronauts returning from space flight. Cardiovascular deconditioning occurs as a consequence of an adaptive response to microgravity partially due to the effects exerted at cellular level. Directional migration of endothelial cell which are central in maintaining the structural integrity of vascular walls is regulated by chemotactic, haptotactic, and mechanotactic stimuli which are essential for vasculogenesis. We explored the migration property of transformed endothelial cells (EC) exposed to 2-h microgravity, simulated using a three-dimensional clinostat constructed based on blueprint published by the Fokker Space, Netherlands. Migration of EC was measured using the scrap wound healing in the presence or absence of actin polymerization inhibitor-cytochalasin D (CD) in Eahy926 cell lines. Simulated microgravity increased cellular migration by 25% while CD-blocked microgravity induced cellular migration. The key migratory structures of cells, filopodia and lamellipodia, formed by EC were more in simulated microgravity compared to gravity. Parallel experiments with phalloidin and diaminorhodamine-4M (DAR-4M) showed that simulated microgravity caused actin rearrangements that lead to 25% increase in nitric oxide production. Further nitric oxide measurements showed a higher nitric oxide production which was not abrogated by phosphoinositol 3 kinase inhibitor (Wortmanin). Bradykinin, an inducer of nitric oxide, prompted two folds higher nitric oxide production along with simulated microgravity in a synergistic manner. We suggest that limited exposure to simulated microgravity increases Eahy926 cell migration by modulating actin and releasing nitric oxide.

Secreted frizzled-related protein 4: an angiogenesis inhibitor.

Wnt signaling is involved in developmental processes, cell proliferation, and cell migration. Secreted frizzled-related protein 4 (sFRP4) has been demonstrated to be a Wnt antagonist; however, its effects on endothelial cell migration and angiogenesis have not yet been reported. Using various in vitro assays, we show that sFRP4 inhibits endothelial cell migration and the development of sprouts and pseudopodia as well as disrupts the stability of endothelial rings in addition to inhibiting proliferation. sFRP4 interfered with endothelial cell functions by antagonizing the canonical Wnt/beta-catenin signaling pathway and the Wnt/planar cell polarity pathway. Furthermore, sFRP4 blocked the effect of vascular endothelial growth factor on endothelial cells. sFRP4 also selectively induced apoptotic events in endothelial cells by increasing cellular levels of reactive oxygen species. In vivo assays demonstrated a reduction in vascularity after sFRP4 treatment. Most importantly, sFRP4 restricted tumor growth in mice by interfering with endothelial cell function. The data demonstrate sFRP4 to be a potent angiogenesis inhibitor that warrants further investigation as a therapeutic agent in the control of angiogenesis-associated pathology.

Thalidomide attenuates nitric oxide-driven angiogenesis by interacting with soluble guanylyl cyclase.

BACKGROUND AND PURPOSE: Nitric oxide (NO) promotes angiogenesis by activating endothelial cells. Thalidomide arrests angiogenesis by interacting with the NO pathway, but its putative targets are not known. Here, we have attempted to identify these targets. EXPERIMENTAL APPROACH: Cell-based angiogenesis assays (wound healing of monolayers and tube formation in ECV304, EAhy926 and bovine arterial endothelial cells), along with ex vivo and in vivo angiogenesis assays, were used to explore interactions between thalidomide and NO. We also carried out in silico homology modelling and docking studies to elucidate possible molecular interactions of thalidomide and soluble guanylyl cyclase (sGC). KEY RESULTS: Thalidomide inhibited pro-angiogenic functions in endothelial cell cultures, whereas 8-bromo-cGMP, sildenafil (a phosphodiesterase inhibitor) or a NO donor [sodium nitroprusside (SNP)] increased these functions. The inhibitory effects of thalidomide were reversed by adding 8-bromo-cGMP or sildenafil, but not by SNP. Immunoassays showed a concentration-dependent decrease of cGMP in endothelial cells with thalidomide, without affecting the expression level of sGC protein. These results suggested that thalidomide inhibited the activity of sGC. Molecular modelling and docking experiments revealed that thalidomide could interact with the catalytic domain of sGC, which would explain the inhibitory effects of thalidomide on NO-dependent angiogenesis. CONCLUSION AND IMPLICATIONS: Our results showed that thalidomide interacted with sGC, suppressing cGMP levels in endothelial cells, thus exerting its anti-angiogenic effects. These results could lead to the formulation of thalidomide-based drugs to curb angiogenesis by targeting sGC.

Cadmium attenuates bradykinin-driven nitric oxide production by interplaying with the localization pattern of endothelial nitric oxide synthase.

Cadmium, a ubiquitous heavy metal, interferes with endothelial functions and angiogenesis. Bradykinin is a Ca-mobilizing soluble peptide that acts via nitric oxide to promote vasodilation and capillary permeability. The objective of the present study was to explore the Cd implications in bradykinin-dependent endothelial functions. An egg yolk angiogenesis model was employed to evaluate the effect of Cd on bradykinin-induced angiogenesis. The results demonstrate that 100 nmol/L Cd attenuated bradykinin-dependent angiogenesis. The results of the in vitro wound healing and tube formation assays by using EAhy 926, a transformed endothelial cell line, suggest that Cd blocked bradykinin-mediated endothelial migration and tube formation by 38% and 67%, respectively, while nitric oxide supplementation could reverse the effect of Cd on bradykinin-induced endothelial migration by 94%. The detection of nitric oxide by using a DAF-2DA fluorescent probe, Griess assay, and ultrasensitive electrode suggests that Cd blocked bradykinin-induced nitric oxide production. Fluorescence imaging of eNOS-GFP transfected endothelial cells, immunofluorescence, and Western blot studies of Cd and bradykinin-treated cells show that Cd interfered with the localization pattern of eNOS, which possibly attenuates nitric oxide production in part. Additionally, Ca imaging of Cd- and bradykinin-treated cells suggests that Cd blocked bradykinin-dependent Ca influx into the cells, thus partially blocking Ca-dependent nitric oxide production in endothelial cells. The results of this study conclude that Cd blunted the effect of bradykinin by interfering with the Ca-associated NOS activity specifically by impeding subcellular trafficking of eNOS.

Cadmium reduces nitric oxide production by impairing phosphorylation of endothelial nitric oxide synthase.

Cadmium (Cd) perturbs vascular health and interferes with endothelial function. However, the effects of exposing endothelial cells to low doses of Cd on the production of nitric oxide (NO) are largely unknown. The objective of the present study was to evaluate these effects by using low levels of CdCl2 concentrations, ranging from 10 to 1000 nmol/L. Cd perturbations in endothelial function were studied by employing wound-healing and MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) assays. The results suggest that a CdCl2 concentration of 100 nmol/L maximally attenuated NO production, cellular migration, and energy metabolism in endothelial cells. An egg yolk angiogenesis model was employed to study the effect of Cd exposure on angiogenesis. The results demonstrate that NO supplementation restored Cd-attenuated angiogenesis. Immunofluorescence, Western blot, and immuno-detection studies showed that low levels of Cd inhibit NO production in endothelial cells by blocking eNOS phosphorylation, which is possibly linked to processes involving endothelial function and dysfunction, including angiogenesis.

Nitric oxide/cGMP protects endothelial cells from hypoxia-mediated leakiness.

Leakiness of the endothelial bed is attributed to the over-perfusion of the pulmonary bed, which leads to high altitude pulmonary edema (HAPE). Inhalation of nitric oxide has been successfully employed to treat HAPE patients. We hypothesize that nitric oxide intervenes in the permeability of the pulmonary macrovascular endothelial bed to rectify the leaky bed under hypoxia. Our present work explores the underlying mechanism of 'hypoxia-mediated' endothelial malfunction by using human umbilical cord-derived immortalized endothelial cells, ECV-304, and bovine pulmonary artery primary endothelial cells. The leakiness of the endothelial monolayer was increased by two-fold under hypoxia in comparison to cells under normoxia, while optical tweezers-based tethering assays reported a higher membrane tension of endothelial cells under hypoxia. Phalloidin staining demonstrated depolymerization of F-actin stress fibers and highly polarized F-actin patterns in endothelial cells under hypoxia. Nitric oxide, 8-Br-cGMP and sildenafil citrate (phosphodiesterase type 5 inhibitor) led to recovery from hypoxia-induced leakiness of the endothelial monolayers. Results of the present study also suggest that 'hypoxia-induced' cytoskeletal rearrangements and membrane leakiness are associated with the low nitric oxide availability under hypoxia. We conclude that nitric oxide-based recovery of hypoxia-induced leakiness of endothelial cells is a cyclic guanosine monophosphate (cGMP)-dependent phenomenon.

Activated pericyte attenuates endothelial functions: nitric oxide-cGMP rescues activated pericyte-associated endothelial dysfunctions.

Hepatic stellate cells are liver-specific pericytes and exist in close proximity with endothelial cells. The activation of liver pericytes is intrinsic to liver pathogenesis, and leads to endothelial dysfunction, including the low bioavailability of nitric oxide (NO). However, the role of nitric oxide in pericyte-endothelium cross-talk has not yet been elucidated. This work examines the cellular mechanism of action of NO in pericyte-mediated endothelial dysfunction. We used in vitro coculture and conditioned medium systems to study the effects of activated liver pericytes on endothelial function, and an egg yolk vascular bed model was used to study the effects of activated pericytes on angiogenesis. This study also demonstrates that activated pericytes attenuate the migration, proliferation, permeability, and NO production of endothelial cells. Our results demonstrate that activated pericytes restrict angiogenesis in egg yolk vascular bed models, and NO supplementation recovers 70% of the inhibition. Our results also demonstrate that supplementation with NO, sildenafil citrate (phosphodiesterase inhibitor), and 8-bromo-cGMP (cGMP analog) partially recovers activated-pericyte-mediated endothelium dysfunction. We conclude that NO-cGMP alleviates activated-pericyte-associated endothelial dysfunction, including angiogenesis, in a cGMP-dependent manner.

Cadmium induced endothelial dysfunction: consequence of defective migratory pattern of endothelial cells in association with poor nitric oxide availability under cadmium challenge.

Recent advances in cadmium toxicity research suggest an association between cadmium and vascular diseases. However, the mechanisms of cadmium implications in vascular diseases are not yet explained. The objective of our present study is to explore the mechanism of cadmium induced endothelial dysfunction. Doses of 0, 1 and 5microM cadmium chloride were used to test the effects of cadmium on nitric oxide induced tube formation, cellular migration and subcellular actin polymerization in ECV-304 endothelial cells. An egg-yolk vascular bed model was used to study the effects of cadmium on angiogenesis. Results of the present study show that 5microM cadmium chloride effectively inhibited angiogenesis, cellular migration and tube formation. Phalloidin staining, which represents actin polymerization of endothelial cells, reveals that cadmium induces an altered F-actin pattern, which could be the prime cause for cadmium mediated inhibition of cellular migration and angiogenesis. Cadmium was also found to inhibit nitric oxide production in endothelial cells in a calcium free medium, which further hints that cadmium might impair endothelial functions by inhibiting endothelial nitric oxide synthase.

Thalidomide attenuates nitric oxide mediated angiogenesis by blocking migration of endothelial cells.

BACKGROUND: Thalidomide is an immunomodulatory agent, which arrests angiogenesis. The mechanism of anti-angiogenic activity of thalidomide is not fully understood. As nitric oxide is involved in angiogenesis, we speculate a cross-talk between thalidomide and nitric oxide signaling pathway to define angiogenesis. The aim of present study is to understand the mechanistic aspects of thalidomide-mediated attenuation of angiogenesis induced by nitric oxide at the cellular level. METHODS: To study the cellular mechanism of thalidomide-mediated blocking of angiogenesis triggered by nitric oxide, we used two endothelial cell based models: 1) wound healing and 2) tube formation using ECV 304, an endothelial cell line. These cell-based models reflect pro-angiogenic events in vivo. We also studied the effects of thalidomide on nitric oxide mediated egg yolk angiogenesis. Thalidomide could block the formation of blood vessels both in absence and presence of nitric oxide. Thalidomide effects on migration of, and actin polymerization in, ECV 304 cells were studied at the single cell level using live cell imaging techniques and probes to detect nitric oxide. RESULTS: Results demonstrate that thalidomide blocks nitric oxide-mediated angiogenesis in egg yolk model and also reduces the number of tubes formed in endothelial cell monolayers. We also observed that thalidomide arrests wound healing in presence and absence of nitric oxide in a dose-dependent fashion. Additionally, thalidomide promotes actin polymerization and antagonizes the formation of membrane extensions triggered by nitric oxide in endothelial cells. Experiments targeting single tube structure with thalidomide, followed by nitric oxide treatment, show that the tube structures are insensitive to thalidomide and nitric oxide. These observations suggest that thalidomide interferes with nitric oxide-induced migration of endothelial cells at the initial phase of angiogenesis before cells co-ordinate themselves to form organized tubes in endothelial cells and thereby inhibits angiogenesis. CONCLUSION: Thalidomide exerts inhibitory effects on nitric oxide-mediated angiogenesis by altering sub-cellular actin polymerization pattern, which leads to inhibition of endothelial cell migration.