Inhibition of endothelial nitric oxide synthase activity by proline-rich tyrosine kinase 2 in response to fluid shear stress and insulin

Inhibition of proline-rich tyrosine kinase 2 restores cardioprotection by remote ischaemic preconditioning in type 2 diabetes

BACKGROUND AND PURPOSE

Remote ischaemic preconditioning (rIPC) for cardioprotection is severely impaired in diabetes, and therapeutic options to restore it are lacking. The vascular endothelium plays a key role in rIPC. Given that the activity of endothelial nitric oxide synthase (eNOS) is inhibited by proline-rich tyrosine kinase 2 (Pyk2), we hypothesized that pharmacological Pyk2 inhibition could restore eNOS activity and thus restore remote cardioprotection in diabetes.

EXPERIMENTAL APPROACH

New Zealand obese (NZO) mice that demonstrated key features of diabetes were studied. The consequence of Pyk2 inhibition on endothelial function, rIPC and infarct size after myocardial infarction were evaluated. The impact of plasma from mice and humans with or without diabetes was assessed in isolated buffer perfused murine hearts and aortic rings.

KEY RESULTS

Plasma from nondiabetic mice and humans, both subjected to rIPC, caused remote tissue protection. Similar to diabetic humans, NZO mice demonstrated endothelial dysfunction. NZO mice had reduced circulating nitrite levels, elevated arterial blood pressure and a larger infarct size after ischaemia and reperfusion than BL6 mice. Pyk2 increased the phosphorylation of eNOS at its inhibitory site (Tyr656), limiting its activity in diabetes. The cardioprotective effects of rIPC were abolished in diabetic NZO mice. Pharmacological Pyk2 inhibition restored endothelial function and rescued cardioprotective effects of rIPC.

CONCLUSION AND IMPLICATIONS

Endothelial function and remote tissue protection are impaired in diabetes. Pyk2 is a novel target for treating endothelial dysfunction and restoring cardioprotection through rIPC in diabetes.

Health position paper and redox perspectives - Disease burden by transportation noise

Transportation noise is a ubiquitous urban exposure. In 2018, the World Health Organization concluded that chronic exposure to road traffic noise is a risk factor for ischemic heart disease. In contrast, they concluded that the quality of evidence for a link to other diseases was very low to moderate. Since then, several studies on the impact of noise on various diseases have been published. Also, studies investigating the mechanistic pathways underlying noise-induced health effects are emerging. We review the current evidence regarding effects of noise on health and the related disease-mechanisms. Several high-quality cohort studies consistently found road traffic noise to be associated with a higher risk of ischemic heart disease, heart failure, diabetes, and all-cause mortality. Furthermore, recent studies have indicated that road traffic and railway noise may increase the risk of diseases not commonly investigated in an environmental noise context, including breast cancer, dementia, and tinnitus. The harmful effects of noise are related to activation of a physiological stress response and nighttime sleep disturbance. Oxidative stress and inflammation downstream of stress hormone signaling and dysregulated circadian rhythms are identified as major disease-relevant pathomechanistic drivers. We discuss the role of reactive oxygen species and present results from antioxidant interventions. Lastly, we provide an overview of oxidative stress markers and adverse redox processes reported for noise-exposed animals and humans. This position paper summarizes all available epidemiological, clinical, and preclinical evidence of transportation noise as an important environmental risk factor for public health and discusses its implications on the population level.

Nitric Oxide: Physiological Functions, Delivery, and Biomedical Applications

Nitric oxide (NO) is a gaseous molecule that has a central role in signaling pathways involved in numerous physiological processes (e.g., vasodilation, neurotransmission, inflammation, apoptosis, and tumor growth). Due to its gaseous form, NO has a short half-life, and its physiology role is concentration dependent, often restricting its function to a target site. Providing NO from an external source is beneficial in promoting cellular functions and treatment of different pathological conditions. Hence, the multifaceted role of NO in physiology and pathology has garnered massive interest in developing strategies to deliver exogenous NO for the treatment of various regenerative and biomedical complexities. NO-releasing platforms or donors capable of delivering NO in a controlled and sustained manner to target tissues or organs have advanced in the past few decades. This review article discusses in detail the generation of NO via the enzymatic functions of NO synthase as well as from NO donors and the multiple biological and pathological processes that NO modulates. The methods for incorporating of NO donors into diverse biomaterials including physical, chemical, or supramolecular techniques are summarized. Then, these NO-releasing platforms are highlighted in terms of advancing treatment strategies for various medical problems.

Functional regeneration at the blood-biomaterial interface

The use of cardiovascular implants is commonplace in clinical practice. However, reproducing the key bioactive and adaptive properties of native cardiovascular tissues with an artificial replacement is highly challenging. Exciting new treatment strategies are under development to regenerate (parts of) cardiovascular tissues directly in situ using immunomodulatory biomaterials. Direct exposure to the bloodstream and hemodynamic loads is a particular challenge, given the risk of thrombosis and adverse remodeling that it brings. However, the blood is also a source of (immune) cells and proteins that dominantly contribute to functional tissue regeneration. This review explores the potential of the blood as a source for the complete or partial in situ regeneration of cardiovascular tissues, with a particular focus on the endothelium, being the natural blood-tissue barrier. We pinpoint the current scientific challenges to enable rational engineering and testing of blood-contacting implants to leverage the regenerative potential of the blood.

Modulation of the nitric oxide/cGMP pathway in cardiac contraction and relaxation: Potential role in heart failure treatment

Evidence exists that heart failure (HF) has an overall impact of 1-2 % in the global population being often associated with comorbidities that contribute to increased disease prevalence, hospitalization, and mortality. Recent advances in pharmacological approaches have significantly improved clinical outcomes for patients with vascular injury and HF. Nevertheless, there remains an unmet need to clarify the crucial role of nitric oxide/cyclic guanosine 3',5'-monophosphate (NO/cGMP) signalling in cardiac contraction and relaxation, to better identify the key mechanisms involved in the pathophysiology of myocardial dysfunction both with reduced (HFrEF) as well as preserved ejection fraction (HFpEF). Indeed, NO signalling plays a crucial role in cardiovascular homeostasis and its dysregulation induces a significant increase in oxidative and nitrosative stress, producing anatomical and physiological cardiac alterations that can lead to heart failure. The present review aims to examine the molecular mechanisms involved in the bioavailability of NO and its modulation of downstream pathways. In particular, we focus on the main therapeutic targets and emphasize the recent evidence of preclinical and clinical studies, describing the different emerging therapeutic strategies developed to counteract NO impaired signalling and cardiovascular disease (CVD) development.

Clinical Applications for Gasotransmitters in the Cardiovascular System: Are We There Yet?

Ischemia is the underlying mechanism in a wide variety of acute and persistent pathologies. As such, understanding the fine intracellular events occurring during (and after) the restriction of blood supply is pivotal to improving the outcomes in clinical settings. Among others, gaseous signaling molecules constitutively produced by mammalian cells (gasotransmitters) have been shown to be of potential interest for clinical treatment of ischemia/reperfusion injury. Nitric oxide (NO and its sibling, HNO), hydrogen sulfide (H2S), and carbon monoxide (CO) have long been proven to be cytoprotective in basic science experiments, and they are now awaiting confirmation with clinical trials. The aim of this work is to review the literature and the clinical trials database to address the state of development of potential therapeutic applications for NO, H2S, and CO and the clinical scenarios where they are more promising.

Effects of shear stress on vascular endothelial functions in atherosclerosis and potential therapeutic approaches

Different blood flow patterns in the arteries can alter the adaptive phenotype of vascular endothelial cells (ECs), thereby affecting the functions of ECs and are directly associated with the occurrence of lesions in the early stages of atherosclerosis (AS). Atherosclerotic plaques are commonly found at curved or bifurcated arteries, where the blood flow pattern is dominated by oscillating shear stress (OSS). OSS can induce ECs to transform into pro-inflammatory phenotypes, increase cellular inflammation, oxidative stress response, mitochondrial dysfunction, metabolic abnormalities and endothelial permeability, thereby promoting the progression of AS. On the other hand, the straight artery has a stable laminar shear stress (LSS), which promotes the transformation of ECs into an anti-inflammatory phenotype, improves endothelial cell function, thereby inhibits atherosclerotic progression. ECs have the ability to actively sense, integrate, and convert mechanical stimuli by shear stress into biochemical signals that further induces intracellular changes (such as the opening and closing of ion channels, activation and transcription of signaling pathways). Here we not only outline the relationship between functions of vascular ECs and different forms of fluid shear stress in AS, but also aim to provide new solutions for potential atherosclerotic therapies targeting intracellular mechanical transductions.

Increased cytochrome C threonine 50 phosphorylation in aging heart as a novel defensive signaling against hypoxia/reoxygenation induced apoptosis

Previous studies have shown that aging promotes myocardial apoptosis. However, the detailed mechanisms remain unclear. Our recent studies revealed that aging not only activates apoptosis, but also activates some anti-apoptotic factors. By quantitative phosphoproteomics, here we demonstrated that aging increases cytochrome c (Cytc) phosphorylation at threonine 50 (T50), a post-translational modification with unknown functional impact. With point mutation and lentivirus transfection, cardiomyocytes were divided into four groups: empty vector group, WT (wild type), T50E (as a phosphomimic variant), and T50A (non-phosphorylatable). TUNEL staining and flow cytometry were used to determine the apoptosis ratio in different groups after hypoxic/reoxygenated (H/R) treatment. The results showed that T50-phosphorylated Cytc suppressed myocardial apoptosis induced by H/R. Furthermore, Western Blot and ELISA measurements revealed that Cytc T50 phosphorylation inhibited caspase-9 and caspase-3 activity without altering caspase-8, BCL-2, BCL-XL, and Bax expression. In our study, we demonstrated that aging increases phosphorylation Cytc at T50 and this aging-increasing phosphorylation site can suppress H/R-induced apoptosis.

The Effects of Acidosis on eNOS in the Systemic Vasculature: A Focus on Early Postnatal Ontogenesis

The activity of many vasomotor signaling pathways strongly depends on extracellular/intracellular pH. Nitric oxide (NO) is one of the most important vasodilators produced by the endothelium. In this review, we present evidence that in most vascular beds of mature mammalian organisms metabolic or respiratory acidosis increases functional endothelial NO-synthase (eNOS) activity, despite the observation that direct effects of low pH on eNOS enzymatic activity are inhibitory. This can be explained by the fact that acidosis increases the activity of signaling pathways that positively regulate eNOS activity. The role of NO in the regulation of vascular tone is greater in early postnatal ontogenesis compared to adulthood. Importantly, in early postnatal ontogenesis acidosis also augments functional eNOS activity and its contribution to the regulation of arterial contractility. Therefore, the effect of acidosis on total peripheral resistance in neonates may be stronger than in adults and can be one of the reasons for an undesirable decrease in blood pressure during neonatal asphyxia. The latter, however, should be proven in future studies.

Redox Switches in Noise-Induced Cardiovascular and Neuronal Dysregulation

Environmental exposures represent a significant health hazard, which cumulatively may be responsible for up to 2/3 of all chronic non-communicable disease and associated mortality (Global Burden of Disease Study and The Lancet Commission on Pollution and Health), which has given rise to a new concept of the exposome: the sum of environmental factors in every individual’s experience. Noise is part of the exposome and is increasingly being investigated as a health risk factor impacting neurological, cardiometabolic, endocrine, and immune health. Beyond the well-characterized effects of high-intensity noise on cochlear damage, noise is relatively well-studied in the cardiovascular field, where evidence is emerging from both human and translational experiments that noise from traffic-related sources could represent a risk factor for hypertension, ischemic heart disease, diabetes, and atherosclerosis. In the present review, we comprehensively discuss the current state of knowledge in the field of noise research. We give a brief survey of the literature documenting experiments in noise exposure in both humans and animals with a focus on cardiovascular disease. We also discuss the mechanisms that have been uncovered in recent years that describe how exposure to noise affects physiological homeostasis, leading to aberrant redox signaling resulting in metabolic and immune consequences, both of which have considerable impact on cardiovascular health. Additionally, we discuss the molecular pathways of redox involvement in the stress responses to noise and how they manifest in disruptions of the circadian rhythm, inflammatory signaling, gut microbiome composition, epigenetic landscape and vessel function.

CFTR limits F-actin formation and promotes morphological alignment with flow in human lung microvascular endothelial cells

Micro- and macrovascular endothelial dysfunction in response to shear stress has been observed in cystic fibrosis (CF), and has been associated with inflammation and oxidative stress. We tested the hypothesis that the cystic fibrosis transmembrane conductance regulator (CFTR) regulates endothelial actin cytoskeleton dynamics and cellular alignment in response to flow. Human lung microvascular endothelial cells (HLMVEC) were cultured with either the CFTR inhibitor GlyH-101 (20 µM) or CFTRinh-172 (20 µM), tumor necrosis factor (TNF)-α (10 ng/ml) or a vehicle control (0.1% dimethyl sulfoxide) during 24 and 48 h of exposure to shear stress (11.1 dynes/cm2 ) or under static control conditions. Cellular morphology and filamentous actin (F-actin) were assessed using immunocytochemistry. [Nitrite] and endothelin-1 ([ET-1]) were determined in cell culture supernatant by ozone-based chemiluminescence and ELISA, respectively. Treatment of HLMVECs with both CFTR inhibitors prevented alignment of HLMVEC in the direction of flow after 24 and 48 h of shear stress, compared to vehicle control (both p < 0.05). Treatment with TNF-α significantly increased total F-actin after 24 h versus control (p < 0.05), an effect that was independent of shear stress. GlyH-101 significantly increased F-actin after 24 h of shear stress versus control (p < 0.05), with a significant (p < 0.05) reduction in cortical F-actin under both static and flow conditions. Shear stress decreased [ET-1] after 24 h (p < 0.05) and increased [nitrite] after 48 h (p < 0.05), but neither [nitrite] nor [ET-1] was affected by GlyH-101 (p > 0.05). CFTR appears to limit cytosolic actin polymerization, while maintaining a cortical rim actin distribution that is important for maintaining barrier integrity and promoting alignment with flow, without effects on endothelial nitrite or ET-1 production.

Endothelial Yin Yang 1 Phosphorylation at S118 Induces Atherosclerosis Under Flow

Rationale: Disturbed flow occurring in arterial branches and curvatures induces vascular endothelial cell (EC) dysfunction and atherosclerosis. We postulated that disturbed flow plays important roles in modulating phosphoprotein expression profiles to regulate endothelial functions and atherogenesis. Objective: The goal of this study is to discover novel site-specific phosphorylation alterations induced by disturbed flow in ECs to contribute to atherosclerosis. Methods and Results: Quantitative phosphoproteomics analysis of ECs exposed to disturbed flow with low and oscillatory shear stress (OS, 0.5plusminus4 dynes/cm²) vs. pulsatile flow with high shear stress (PS, 124plusminus dynes/cm²) revealed that OS induces serine (S)118 phosphorylation of Yin Yang 1 (phospho-YY1^S118) in ECs. Elevated phospho-YY1^S118 level in ECs was further confirmed to be present in the disturbed flow regions in experimental animals and human atherosclerotic arteries. This disturbed flow-induced EC phospho-YY1^S118 is mediated by casein kinase 2α (CK2α) through its direct interaction with YY1. Yeast two-hybrid library screening and in situ proximity ligation assays demonstrate that phospho-YY1^S118 directly binds zinc finger with KRAB and SCAN domains 4 (ZKSCAN4) to induce promoter activity and gene expression of human double minute 2 (HDM2), which consequently induces EC proliferation through down-regulations of p53 and p21^CIP1. Administration of apolipoprotein E-deficient (ApoE^-/-) mice with CK2-specific inhibitor tetrabromocinnamic acid or atorvastatin inhibits atherosclerosis formation through down-regulations of EC phospho-YY1^S118 and HDM2. Generation of novel transgenic mice bearing EC-specific overexpression of S118-non-phosphorylatable mutant of YY1 in ApoE^-/- mice confirms the critical role of phospho-YY1^S118 in promoting atherosclerosis through EC HDM2. Conclusions: Our findings provide new insights into the mechanisms by which disturbed flow induces endothelial phospho-YY1^S118 to promote atherosclerosis, thus indicating phospho-YY1^S118 as a potential molecular target for atherosclerosis treatment.

Role of nitric oxide in orthodontic tooth movement (Review)

Nitric oxide (NO) is an ubiquitous signaling molecule that mediates numerous cellular processes associated with cardiovascular, nervous and immune systems. NO also plays an essential role in bone homeostasis regulation. The present review article summarized the effects of NO on bone metabolism during orthodontic tooth movement in order to provide insight into the regulatory role of NO in orthodontic tooth movement. Orthodontic tooth movement is a process in which the periodontal tissue and alveolar bone are reconstructed due to the effect of orthodontic forces. Accumulating evidence has indicated that NO and its downstream signaling molecule, cyclic guanosine monophosphate (cGMP), mediate the mechanical signals during orthodontic-related bone remodeling, and exert complex effects on osteogenesis and osteoclastogenesis. NO has a regulatory effect on the cellular activities and functional states of osteoclasts, osteocytes and periodontal ligament fibroblasts involved in orthodontic tooth movement. Variations of NO synthase (NOS) expression levels and NO production in periodontal tissues or gingival crevicular fluid (GCF) have been found on the tension and compression sides during tooth movement in both orthodontic animal models and patients. Furthermore, NO precursor and NOS inhibitor administration increased and reduced the tooth movement in animal models, respectively. Further research is required in order to further elucidate the underlying mechanisms and the clinical application prospect of NO in orthodontic tooth movement.

VE-PTP inhibition elicits eNOS phosphorylation to blunt endothelial dysfunction and hypertension in diabetes

AIMS

Receptor-type vascular endothelial protein tyrosine phosphatase (VE-PTP) dephosphorylates Tie-2 as well as CD31, VE-cadherin, and vascular endothelial growth factor receptor 2 (VEGFR2). The latter form a signal transduction complex that mediates the endothelial cell response to shear stress, including the activation of the endothelial nitric oxide (NO) synthase (eNOS). As VE-PTP expression is increased in diabetes, we investigated the consequences of VE-PTP inhibition (using AKB-9778) on blood pressure in diabetic patients and the role of VE-PTP in the regulation of eNOS activity and vascular reactivity.

METHODS AND RESULTS

In diabetic patients AKB-9778 significantly lowered systolic and diastolic blood pressure. This could be linked to elevated NO production, as AKB increased NO generation by cultured endothelial cells and elicited the NOS inhibitor-sensitive relaxation of endothelium-intact rings of mouse aorta. At the molecular level, VE-PTP inhibition increased the phosphorylation of eNOS on Tyr81 and Ser1177 (human sequence). The PIEZO1 activator Yoda1, which was used to mimic the response to shear stress, also increased eNOS Tyr81 phosphorylation, an effect that was enhanced by VE-PTP inhibition. Two kinases, i.e. abelson-tyrosine protein kinase (ABL)1 and Src were identified as eNOS Tyr81 kinases as their inhibition and down-regulation significantly reduced the basal and Yoda1-induced tyrosine phosphorylation and activity of eNOS. VE-PTP, on the other hand, formed a complex with eNOS in endothelial cells and directly dephosphorylated eNOS Tyr81 in vitro. Finally, phosphorylation of eNOS on Tyr80 (murine sequence) was found to be reduced in diabetic mice and diabetes-induced endothelial dysfunction (isolated aortic rings) was blunted by VE-PTP inhibition.

CONCLUSIONS

VE-PTP inhibition enhances eNOS activity to improve endothelial function and decrease blood pressure indirectly, through the activation of Tie-2 and the CD31/VE-cadherin/VEGFR2 complex, and directly by dephosphorylating eNOS Tyr81. VE-PTP inhibition, therefore, represents an attractive novel therapeutic option for diabetes-induced endothelial dysfunction and hypertension.

The Calcium Signaling Mechanisms in Arterial Smooth Muscle and Endothelial Cells

The contractile state of resistance arteries and arterioles is a crucial determinant of blood pressure and blood flow. Physiological regulation of arterial contractility requires constant communication between endothelial and smooth muscle cells. Various Ca²⁺ signals and Ca²⁺ -sensitive targets ensure dynamic control of intercellular communications in the vascular wall. The functional effect of a Ca²⁺ signal on arterial contractility depends on the type of Ca²⁺ -sensitive target engaged by that signal. Recent studies using advanced imaging methods have identified the spatiotemporal signatures of individual Ca²⁺ signals that control arterial and arteriolar contractility. Broadly speaking, intracellular Ca²⁺ is increased by ion channels and transporters on the plasma membrane and endoplasmic reticular membrane. Physiological roles for many vascular Ca²⁺ signals have already been confirmed, while further investigation is needed for other Ca²⁺ signals. This article focuses on endothelial and smooth muscle Ca²⁺ signaling mechanisms in resistance arteries and arterioles. We discuss the Ca²⁺ entry pathways at the plasma membrane, Ca²⁺ release signals from the intracellular stores, the functional and physiological relevance of Ca²⁺ signals, and their regulatory mechanisms. Finally, we describe the contribution of abnormal endothelial and smooth muscle Ca²⁺ signals to the pathogenesis of vascular disorders. © 2021 American Physiological Society. Compr Physiol 11:1831-1869, 2021.

The role of vascular endothelial protein tyrosine phosphatase on nitric oxide synthase function in diabetes: from molecular biology to the clinic

Endothelial nitric oxide synthase (eNOS) and receptor-type vascular endothelial protein tyrosine phosphatase (VE-PTP) are one of the majors signaling pathways related to endothelial health in diabetes. Several reports have shown that the inhibition of VE-PTP can lead the nitric oxide production, although repeated studies showed that VE-PTP regulated the eNOS exclusive at Ser1177 in indirect-manner. A recent, exciting paper (Siragusa et al. in Cardiovasc Res, 2020. https://doi.org/10.1093/cvr/cvaa213 ), showing that VE-PTP regulates eNOS in a direct-manner, dephosphorylating eNOS at Tyr81 and indirect at Ser1177 and the effects of a VE-PTP inhibitor, AKB-9778, in the blood pressure from diabetic patients.

Endothelium-dependent remote signaling in ischemia and reperfusion: Alterations in the cardiometabolic continuum

Intact endothelial function plays a fundamental role for the maintenance of cardiovascular (CV) health. The endothelium is also involved in remote signaling pathway-mediated protection against ischemia/reperfusion (I/R) injury. However, the transfer of these protective signals into clinical practice has been hampered by the complex metabolic alterations frequently observed in the cardiometabolic continuum, which affect redox balance and inflammatory pathways. Despite recent advances in determining the distinct roles of hyperglycemia, insulin resistance (InR), hyperinsulinemia, and ultimately diabetes mellitus (DM), which define the cardiometabolic continuum, our understanding of how these conditions modulate endothelial signaling remains challenging. It is widely accepted that endothelial cells (ECs) undergo functional changes within the cardiometabolic continuum. Beyond vascular tone and platelet-endothelium interaction, endothelial dysfunction may have profound negative effects on outcome during I/R. In this review, we summarize the current knowledge of the influence of hyperglycemia, InR, hyperinsulinemia, and DM on endothelial function and redox balance, their influence on remote protective signaling pathways, and their impact on potential therapeutic strategies to optimize protective heterocellular signaling.

Mapping the Endothelial Cell S-Sulfhydrome Highlights the Crucial Role of Integrin Sulfhydration in Vascular Function

BACKGROUND

In vascular endothelial cells, cysteine metabolism by the cystathionine γ lyase (CSE), generates hydrogen sulfide-related sulfane sulfur compounds (H₂S_n), that exert their biological actions via cysteine S-sulfhydration of target proteins. This study set out to map the "S-sulfhydrome" (ie, the spectrum of proteins targeted by H₂S_n) in human endothelial cells.

METHODS

Liquid chromatography with tandem mass spectrometry was used to identify S-sulfhydrated cysteines in endothelial cell proteins and β3 integrin intraprotein disulfide bond rearrangement. Functional studies included endothelial cell adhesion, shear stress-induced cell alignment, blood pressure measurements, and flow-induced vasodilatation in endothelial cell-specific CSE knockout mice and in a small collective of patients with endothelial dysfunction.

RESULTS

Three paired sample sets were compared: (1) native human endothelial cells isolated from plaque-free mesenteric arteries (CSE activity high) and plaque-containing carotid arteries (CSE activity low); (2) cultured human endothelial cells kept under static conditions or exposed to fluid shear stress to decrease CSE expression; and (3) cultured endothelial cells exposed to shear stress to decrease CSE expression and treated with solvent or the slow-releasing H₂S_n donor, SG1002. The endothelial cell "S-sulfhydrome" consisted of 3446 individual cysteine residues in 1591 proteins. The most altered family of proteins were the integrins and focusing on β3 integrin in detail we found that S-sulfhydration affected intraprotein disulfide bond formation and was required for the maintenance of an extended-open conformation of the β leg. β3 integrin S-sulfhydration was required for endothelial cell mechanotransduction in vitro as well as flow-induced dilatation in murine mesenteric arteries. In cultured cells, the loss of S-sulfhydration impaired interactions between β3 integrin and Gα13 (guanine nucleotide-binding protein subunit α 13), resulting in the constitutive activation of RhoA (ras homolog family member A) and impaired flow-induced endothelial cell realignment. In humans with atherosclerosis, endothelial function correlated with low H₂S_n generation, impaired flow-induced dilatation, and failure to detect β3 integrin S-sulfhydration, all of which were rescued after the administration of an H₂S_n supplement.

CONCLUSIONS

Vascular disease is associated with marked changes in the S-sulfhydration of endothelial cell proteins involved in mediating responses to flow. Short-term H₂S_n supplementation improved vascular reactivity in humans highlighting the potential of interfering with this pathway to treat vascular disease.

Functional implications of vascular endothelium in regulation of endothelial nitric oxide synthesis to control blood pressure and cardiac functions

The endothelium is the innermost vascular lining performing significant roles all over the human body while maintaining the blood pressure at physiological levels. Malfunction of endothelium is thus recognized as a biomarker linked with many vascular diseases including but not limited to atherosclerosis, hypertension and thrombosis. Alternatively, prevention of endothelial malfunctioning or regulating the functions of its associated physiological partners like endothelial nitric oxide synthase can prevent the associated vascular disorders which account for the highest death toll worldwide. While many anti-hypertensive drugs are available commercially, a comprehensive description of the key physiological roles of the endothelium and its regulation by endothelial nitric oxide synthase or vice versa is the need of the hour to understand its contribution in vascular homeostasis. This, in turn, will help in designing new therapeutics targeting endothelial nitric oxide synthase or its interacting partners present in the cellular pool. This review describes the central role of vascular endothelium in the regulation of endothelial nitric oxide synthase while outlining the emerging drug targets present in the vasculature with potential to treat vascular disorders including hypertension.

Shear stress regulates cystathionine γ lyase expression to preserve endothelial redox balance and reduce membrane lipid peroxidation

Cystathionine γ lyase (CSE) is the major source of hydrogen sulfide-derived species (H2Sn) in endothelial cells and plays an important role in protecting against atherosclerosis. Here we investigated the molecular mechanisms underlying the regulation of CSE expression in endothelial cells by fluid shear stress/flow. Fluid shear stress decreased CSE expression in human and murine endothelial cells and was negatively correlated with the transcription factor Krüppel-like factor (KLF) 2. CSE was identified as a direct target of the KLF2-regulated microRNA, miR-27b and high expression of CSE in native human plaque-derived endothelial cells, was also inversely correlated with KLF2 and miR-27b levels. One consequence of decreased CSE expression was the loss of Prx6 sulfhydration (on Cys47), which resulted in Prx6 hyperoxidation, decamerization and inhibition, as well as a concomitant increase in endothelial cell reactive oxygen species and lipid membrane peroxidation. H2Sn supplementation in vitro was able to reverse the redox state of Prx6. Statin therapy, which is known to activate KLF2, also decreased CSE expression but increased CSE activity by preventing its phosphorylation on Ser377. As a result, the sulfhydration of Prx6 was partially restored in samples from plaque containing arteries from statin-treated donors. Taken together, the regulation of CSE expression by shear stress/disturbed flow is dependent on KLF2 and miR-27b. Moreover, in murine and human arteries CSE acts to maintain endothelial redox balance at least partly by targeting Prx6 to prevent its decamerization and inhibition of its peroxidase activity.

Regulation of Respiration and Apoptosis by Cytochrome c Threonine 58 Phosphorylation

Cytochrome c (Cytc) is a multifunctional protein, acting as an electron carrier in the electron transport chain (ETC), where it shuttles electrons from bc₁ complex to cytochrome c oxidase (COX), and as a trigger of type II apoptosis when released from the mitochondria. We previously showed that Cytc is regulated in a highly tissue-specific manner: Cytc isolated from heart, liver, and kidney is phosphorylated on Y97, Y48, and T28, respectively. Here, we have analyzed the effect of a new Cytc phosphorylation site, threonine 58, which we mapped in rat kidney Cytc by mass spectrometry. We generated and overexpressed wild-type, phosphomimetic T58E, and two controls, T58A and T58I Cytc; the latter replacement is found in human and testis-specific Cytc. In vitro, COX activity, caspase-3 activity, and heme degradation in the presence of H₂O₂ were decreased with phosphomimetic Cytc compared to wild-type. Cytc-knockout cells expressing T58E or T58I Cytc showed a reduction in intact cell respiration, mitochondrial membrane potential (∆Ψ_m), ROS production, and apoptotic activity compared to wild-type. We propose that, under physiological conditions, Cytc is phosphorylated, which controls mitochondrial respiration and apoptosis. Under conditions of stress Cytc phosphorylations are lost leading to maximal respiration rates, ∆Ψ_m hyperpolarization, ROS production, and apoptosis.

Nitric oxide maintains endothelial redox homeostasis through PKM2 inhibition

Decreased nitric oxide (NO) bioavailability and oxidative stress are hallmarks of endothelial dysfunction and cardiovascular diseases. Although numerous proteins are S-nitrosated, whether and how changes in protein S-nitrosation influence endothelial function under pathophysiological conditions remains unknown. We report that active endothelial NO synthase (eNOS) interacts with and S-nitrosates pyruvate kinase M2 (PKM2), which reduces PKM2 activity. PKM2 inhibition increases substrate flux through the pentose phosphate pathway to generate reducing equivalents (NADPH and GSH) and protect against oxidative stress. In mice, the Tyr656 to Phe mutation renders eNOS insensitive to inactivation by oxidative stress and prevents the decrease in PKM2 S-nitrosation and reducing equivalents, thereby delaying cardiovascular disease development. These findings highlight a novel mechanism linking NO bioavailability to antioxidant responses in endothelial cells through S-nitrosation and inhibition of PKM2.

Regulatory mechanisms of Robo4 and their effects on angiogenesis

Roundabout4 (Robo4) is a transmembrane receptor that belongs to the Roundabout (Robo) family of axon guidance molecules. Robo4 is an endothelial-specific receptor that participates in endothelial cell migration, proliferation, and angiogenesis and the maintenance of vasculature homeostasis. The purpose of this review is to summarize and analyze three main mechanisms related to the expression and function of Robo4 during developmental and pathological angiogenesis. In this review, static shear stress and the binding of transcription factors such as E26 transformation-specific variant 2 (ETV2) and Slit3 induce Robo4 expression and activate Robo4 during tissue and organ development. Robo4 interacts with Slit2 or UNC5B to maintain vascular integrity, while a disturbed flow and the expression of transcription factors in inflammatory or neoplastic environments alter Robo4 expression levels, although these changes have uncertain functions. Based on the mechanisms described above, we discuss the aberrant expression of Robo4 in angiogenesis-related diseases and propose antiangiogenic therapies targeting the Robo4 signaling pathway for the treatment of ocular neovascularization lesions and tumors. Finally, although many problems related to Robo4 signaling pathways remain to be resolved, Robo4 is a promising and potentially valuable therapeutic target for treating pathological angiogenesis and developmental defects in angiogenesis.

Bupivacaine-induced contraction is attenuated by endothelial nitric oxide release modulated by activation of both stimulatory and inhibitory phosphorylation (Ser1177 and Thr495) of endothelial nitric oxide synthase

This study examined the mechanism associated with the endothelium-dependent attenuation of vasoconstriction induced by bupivacaine (BPV), with a particular focus on the upstream cellular signaling pathway of endothelial nitric oxide synthase (eNOS) phosphorylation induced by BPV in human umbilical vein endothelial cells (HUVECs). BPV concentration-response curves were investigated in the isolated rat aorta. The effects of Nω-nitro-L-arginine methyl ester (L-NAME), 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), methylene blue, calmidazolium, the Src kinase inhibitor 4-amino-3-(4-chlorophenyl)-1-(t-butyl)-1H-pyrazolo[3,4-d]pyrimidine, 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine (PP2) and the combination of L-arginine and L-NAME on BPV-induced contraction in endothelium-intact aorta preparations were examined. The effects of BPV alone and in combination with PP2 on the phosphorylation of eNOS (at Ser1177 or Thr495), caveolin-1 and Src kinase were examined in HUVECs. BPV-induced contraction was lower in endothelium-intact aortae than in endothelium-denuded aortae. L-NAME, ODQ, methylene blue and calmidazolium increased BPV-induced contraction in endothelium-intact aortae, whereas PP2 alone and combined treatment with L-arginine and L-NAME inhibited BPV-induced contraction. Low-concentration BPV (30 µM) induced both stimulatory (Ser1177) and inhibitory (Thr495) phosphorylation of eNOS in HUVECs. However, high-concentration BPV (150 µM) induced only stimulatory (Ser1177) eNOS phosphorylation. Additionally, phosphorylation of Src kinase, caveolin-1 and inhibitory eNOS (Thr495) induced by low-concentration BPV was inhibited by PP2. These results suggest that contraction induced by low-concentration BPV is attenuated by endothelial nitric oxide release, which is modulated both stimulatory (Ser1177) and inhibitory eNOS phosphorylation (Thr495). BPV-induced phosphorylation of eNOS (Thr495) is indirectly mediated by an upstream cellular signaling pathway involving Src kinase (Tyr416) and caveolin-1 (Tyr14).

Sources of Vascular Nitric Oxide and Reactive Oxygen Species and Their Regulation

Nitric oxide (NO) is a small free radical with critical signaling roles in physiology and pathophysiology. The generation of sufficient NO levels to regulate the resistance of the blood vessels and hence the maintenance of adequate blood flow is critical to the healthy performance of the vasculature. A novel paradigm indicates that classical NO synthesis by dedicated NO synthases is supplemented by nitrite reduction pathways under hypoxia. At the same time, reactive oxygen species (ROS), which include superoxide and hydrogen peroxide, are produced in the vascular system for signaling purposes, as effectors of the immune response, or as byproducts of cellular metabolism. NO and ROS can be generated by distinct enzymes or by the same enzyme through alternate reduction and oxidation processes. The latter oxidoreductase systems include NO synthases, molybdopterin enzymes, and hemoglobins, which can form superoxide by reduction of molecular oxygen or NO by reduction of inorganic nitrite. Enzymatic uncoupling, changes in oxygen tension, and the concentration of coenzymes and reductants can modulate the NO/ROS production from these oxidoreductases and determine the redox balance in health and disease. The dysregulation of the mechanisms involved in the generation of NO and ROS is an important cause of cardiovascular disease and target for therapy. In this review we will present the biology of NO and ROS in the cardiovascular system, with special emphasis on their routes of formation and regulation, as well as the therapeutic challenges and opportunities for the management of NO and ROS in cardiovascular disease.

Chronic Insulin Infusion Down-Regulates Circulating and Urinary Nitric Oxide (NO) Levels Despite Molecular Changes in the Kidney Predicting Greater Endothelial NO Synthase Activity in Mice

Insulin therapy is often needed to overcome insulin receptor resistance in type 2 diabetes; however, the impact of providing additional insulin to already hyperinsulinemic subjects is not clear. We infused male TALLYHO/Jng (TH) mice (insulin resistant) with insulin (50 U/kg·bw/d) or vehicle (control) by osmotic minipump for 14 days. One group of insulin-infused mice was switched to 4% NaCl diet (high-sodium diet, HSD) in the second week. Blood chemistry revealed a significantly higher anion gap and blood sodium concentrations with insulin infusion, i.e., relative metabolic acidosis. Systolic BP and heart rate were slightly (~5 mm Hg) higher in insulin-infused versus control mice. HSD resulted in a modest and transient rise in mean arterial blood pressure (BP), relative to control or insulin-infused, normal-NaCl-fed mice. In kidney, insulin infusion: (1) increased total and phosphorylated (serine-1177) endothelial nitric oxide synthase (eNOS) band densities; (2) reduced band density of the uncoupled form of eNOS; and (3) increased renal homogenate nitric oxide synthase (NOS) activity. Despite this, plasma and urine levels of nitrates plus nitrites (NOx) fell with insulin infusion, by day 14 (40–50%) suggesting worsening of resistance. Overall, insulin infusion ramps up the cellular means in kidney to increase vasodilatory and natriuretic NO, but in the long term may be associated with worsening of insulin receptor resistance.

Adenylate kinase potentiates the capsular polysaccharide by modulating Cps2D in Streptococcus pneumoniae D39

Streptococcus pneumoniae is a polysaccharide-encapsulated bacterium. The capsule thickens during blood invasion compared with the thinner capsules observed in asymptomatic nasopharyngeal colonization. However, the underlying mechanism regulating differential CPS expression remains unclear. CPS synthesis requires energy that is supplied by ATP. Previously, we demonstrated a correlation between ATP levels and adenylate kinase in S. pneumoniae (SpAdK). A dose-dependent induction of SpAdK in serum was also reported. To meet medical needs, this study aimed to elucidate the role of SpAdK in the regulation of CPS production. CPS levels in S. pneumoniae type 2 (D39) increased proportionally with SpAdK levels, but they were not related to pneumococcal autolysis. Moreover, increased SpAdK levels resulted in increased total tyrosine kinase Cps2D levels and phosphorylated Cps2D, which is a regulator of CPS synthesis in the D39 strain. Our results also indicated that the SpAdK and Cps2D proteins interact in the presence of Mg-ATP. In addition, in silico analysis uncovered the mechanism behind this protein–protein interaction, suggesting that SpAdK binds with the Cps2D dimer. This established the importance of the ATP-binding domain of Cps2D. Taken together, the biophysical interaction between SpAdK and Cps2D plays an important role in enhancing Cps2D phosphorylation, which results in increased CPS synthesis.

A physical interaction between two key enzymes explains how the bacterium responsible for causing pneumococcal disease thickens its external capsule during infection of the bloodstream. A team led by Dong-Kwon Rhee from Sungkyunkwan University in Suwon, South Korea, studied strains of Streptococcus pneumoniae expressing varying levels of an enzyme that helps maintain the proper balance of cellular energy. They found that this enzyme stimulated the production of sugar chains that coat the outside of the bacterial capsule by binding and activating an intermediary enzyme involved in the synthesis of these sugar chains. Since the capsule is critical in warding off the human immune response, the findings suggest that drugs designed to disrupt the enzyme-mediated induction of capsule formation could help prevent or treat pneumococcal disease.

The α2 isoform Na,K-ATPase modulates contraction of rat mesenteric small artery via cSrc-dependent Ca2+ sensitization

AIMS

The Na,K-ATPase is involved in a large number of regulatory activities including cSrc-dependent signalling. Upon inhibition of the Na,K-ATPase with ouabain, cSrc activation is shown to occur in many cell types. This study tests the hypothesis that acute potentiation of agonist-induced contraction by ouabain is mediated through Na,K-ATPase-cSrc signalling-dependent sensitization of vascular smooth muscle cells to Ca²⁺ .

METHODS

Agonist-induced rat mesenteric small artery contraction was examined in vitro under isometric conditions and in vivo in anaesthetized rats. Arterial wall tension and [Ca²⁺ ]_i in vascular smooth muscle cells were measured simultaneously. Changes in cSrc and myosin phosphatase targeting protein 1 (MYPT1) phosphorylation were analysed by Western blot. Protein expression was examined with immunohistochemistry. The α1 and α2 isoforms of the Na,K-ATPase were transiently downregulated by siRNA transfection in vivo.

RESULTS

Ten micromolar ouabain, but not digoxin, potentiated contraction to noradrenaline. This effect was not endothelium-dependent. Ouabain sensitized smooth muscle cells to Ca²⁺ , and this was associated with increased phosphorylation of cSrc and MYPT1. Inhibition of tyrosine kinase by genistein, PP2 or pNaKtide abolished the potentiating effect of ouabain on arterial contraction and Ca²⁺ sensitization. Downregulation of the Na,K-ATPase α2 isoform made arterial contraction insensitive to ouabain and tyrosine kinase inhibition.

CONCLUSION

Data suggest that micromolar ouabain potentiates agonist-induced contraction of rat mesenteric small artery via Na,K-ATPase-dependent cSrc activation, which increases Ca²⁺ sensitization of vascular smooth muscle cells by MYPT1 phosphorylation. This mechanism may be critical for acute control of vascular tone.

Role of hemodynamics in initiation/growth of intracranial aneurysms

BACKGROUND

Intracranial aneurysm (IA) is a disease of the vascular wall resulting in abnormal enlargement of the vessel lumen. It is a common pathology with a prevalence of 2%-3% in the adult population. IAs are mostly small, quiescent and asymptomatic; yet, upon rupture, severe brain damage or even death is frequently encountered. In addition to clinical factors, hemodynamic forces, mainly wall shear stress (WSS), have been associated with the initiation of IAs and possibly with their risk of rupture. However, the mechanism by which WSS contributes to aneurysm growth and rupture is not completely understood.

DESIGN

PubMed and Ovid MEDLINE databases were searched. In addition, key review articles were screened for relevant original publications.

RESULTS

Current knowledge about the relation between WSS and IA has been obtained from both computational fluid dynamic studies in patients and experimental models of IA formation and growth. It is increasingly recognized that a high wall shear stress (gradient) participates to IA formation and that both low and high WSS can drive IA growth. Primary cilia (PC) play an important role as mechanosensors as patients with polycystic kidney disease, which is characterized by the absence or dysfunction of PC, have increased risk to develop IAs as well as increased risk of rupture.

CONCLUSION

Wall shear stress is a key player in IA initiation and progression. It is involved in vascular wall remodelling and inflammation, processes underlying aneurysm pathophysiology.

Tyrosine phosphorylation of eNOS regulates myocardial survival after an ischaemic insult: role of PYK2

Aims

Endothelial nitric oxide (NO) synthase (eNOS) is known to play a cardioprotective protective. However, the molecular mechanisms regulating eNOS activity during ischaemia/reperfusion (I/R) injury are incompletely understood. eNOS is a substrate for several kinases that positively or negatively affect its enzymatic activity. Herein, we sought to correlate eNOS phosphorylation status with cardiomyocyte survival and we investigated the contribution of the proline-rich tyrosine kinase 2 (PYK2)/eNOS axis to the regulation of myocardial infarct size in vivo.

Methods and results

Exposure of H9c2 cardiomyocytes to H2O2 lead to PYK2 phosphorylation on its activator site (Y402) and eNOS phosphorylation on the inhibitor site Y656 and the activator site S1176. Both H2O2-induced eNOS phosphorylation events were abolished by PYK2 pharmacological inhibition or gene knockdown. Activity assays demonstrated that phosphorylation of the tyrosine inhibitory site exerts a dominant effect over S1176. In cardiomyocytes subjected to oxidative stress or oxygen-glucose deprivation, inhibition of PYK2 limited cell injury; this effect was prevented by inhibition of NO production. In vivo, ischaemia-reperfusion induced an early activation of PYK2, leading to eNOS phosphorylation on Y656, which, in turn, reduced NO output, as judged by the low tissue levels of its downstream effector cGMP. Moreover, pharmacological blockade of PYK2 alleviated eNOS inhibition and prevented cardiac damage following I/R injury in wild-type, but not in eNOS KO mice.

Conclusion

The current studies demonstrate that PYK2 is a pivotal regulator of eNOS function in myocardial infarction and identify PYK2 as a novel therapeutic target for cardioprotection.

Nitric oxide signalling in cardiovascular health and disease

Nitric oxide (NO) signalling has pleiotropic roles in biology and a crucial function in cardiovascular homeostasis. Tremendous knowledge has been accumulated on the mechanisms of the nitric oxide synthase (NOS)-NO pathway, but how this highly reactive, free radical gas signals to specific targets for precise regulation of cardiovascular function remains the focus of much intense research. In this Review, we summarize the updated paradigms on NOS regulation, NO interaction with reactive oxidant species in specific subcellular compartments, and downstream effects of NO in target cardiovascular tissues, while emphasizing the latest developments of molecular tools and biomarkers to modulate and monitor NO production and bioavailability.

Hydrogen Sulfide Preserves Endothelial Nitric Oxide Synthase Function by Inhibiting Proline-Rich Kinase 2: Implications for Cardiomyocyte Survival and Cardioprotection

Hydrogen sulfide (H₂S) exhibits beneficial effects in the cardiovascular system, many of which depend on nitric oxide (NO). Proline-rich tyrosine kinase 2 (PYK2), a redox-sensitive tyrosine kinase, directly phosphorylates and inhibits endothelial NO synthase (eNOS). We investigated the ability of H₂S to relieve PYK2-mediated eNOS inhibition and evaluated the importance of the H₂S/PYK2/eNOS axis on cardiomyocyte injury in vitro and in vivo. Exposure of H9c2 cardiomyocytes to H₂O₂ or pharmacologic inhibition of H₂S production increased PYK2 (Y402) and eNOS (Y656) phosphorylation. These effects were blocked by treatment with Na₂S or by overexpression of cystathionine γ-lyase (CSE). In addition, PYK2 overexpression reduced eNOS activity in a H₂S-reversible manner. The viability of cardiomyocytes exposed to Η₂Ο₂ was reduced and declined further after the inhibition of H₂S production. PYK2 downregulation, l-cysteine supplementation, or CSE overexpression alleviated the effects of H₂O₂ on H9c2 cardiomyocyte survival. Moreover, H₂S promoted PYK2 sulfhydration and inhibited its activity. In vivo, H₂S administration reduced reactive oxygen species levels, as well as PYK2 (Y402) and eNOS (Y656) phosphorylation. Pharmacologic blockade of PYK2 or inhibition of PYK2 activation by Na₂S reduced myocardial infarct size in mice. Coadministration of a PYK2 inhibitor and Na₂S did not result in additive effects on infarct size. We conclude that H₂S relieves the inhibitory effect of PYK2 on eNOS, allowing the latter to produce greater amounts of NO, thereby affording cardioprotection. Our results unravel the existence of a novel H₂S-NO interaction and identify PYK2 as a crucial target for the protective effects of H₂S under conditions of oxidative stress.

Multiple therapeutic effect of endothelial progenitor cell regulated by drugs in diabetes and diabetes related disorder

BACKGROUND

Reduced levels of endothelial progenitor cells (EPCs) counts have been reported in diabetic mellitus (DM) patients and other diabetes-related disorder. EPCs are a circulating, bone marrow-derived cell population that appears to participate in vasculogenesis, angiogenesis and damage repair. These EPC may revert the damage caused in diabetic condition. We aim to identify several existing drugs and signaling molecule, which could alleviate or improve the diabetes condition via mobilizing and increasing EPC number as well as function.

MAIN BODY

Accumulated evidence suggests that dysregulation of EPC phenotype and function may be attributed to several signaling molecules and cytokines in DM patients. Hyperglycemia alone, through the overproduction of reactive oxygen species (ROS) via eNOS and NOX, can induce changes in gene expression and cellular behavior in diabetes. Furthermore, reports suggest that EPC telomere shortening via increased oxidative DNA damage may play an important role in the pathogenesis of coronary artery disease in diabetic patients. In this review, different type of EPC derived from different sources has been discussed along with cell-surface marker. The reduced number and immobilized EPC in diabetic condition have been mobilized for the therapeutic purpose via use of existing, and novel drugs have been discussed. Hence, evidence list of all types of drugs that have been reported to target the same pathway which affect EPC number and function in diabetes has been reviewed. Additionally, we highlight that proteins are critical in diabetes via polymorphism and inhibitor studies. Ultimately, a lucid pictorial explanation of diabetic and normal patient signaling pathways of the collected data have been presented in order to understand the complex signaling mystery underlying in the diseased and normal condition.

CONCLUSION

Finally, we conclude on eNOS-metformin-HSp90 signaling and its remedial effect for controlling the EPC to improve the diabetic condition for delaying diabetes-related complication. Altogether, the review gives a holistic overview about the elaborate therapeutic effect of EPC regulated by novel and existing drugs in diabetes and diabetes-related disorder.

Modulation of Local and Systemic Heterocellular Communication by Mechanical Forces: A Role of Endothelial Nitric Oxide Synthase

In this review, we discuss the role of nitric oxide (NO) as a key physiological mechanotransducer modulating both local and systemic heterocellular communication and contributing to the integrated (patho)physiology of the cardiovascular system. A deeper understanding of mechanotransduction-mediated local and systemic nodes controlling heterocellular communication between the endothelium, blood cells, and other cell types (e.g., cardiomyocytes) may suggest novel therapeutic strategies for endothelial dysfunction and cardiovascular disease. Recent Advances: Mechanical forces acting on mechanoreceptors on endothelial cells activate the endothelial NO synthase (eNOS) to produce NO. NO participates in (i) abluminal heterocellular communication, inducing vasorelaxation, and thereby regulating vascular tone and blood pressure; (ii) luminal heterocellular communication, inhibiting platelet aggregation, and controlling hemostasis; and (iii) systemic heterocellular communication, contributing to adaptive physiological processes in response to exercise and remote ischemic preconditioning. Interestingly, shear-induced eNOS-dependent activation of vascular heterocellular communication constitutes the molecular basis of all methods applied in the clinical routine for evaluation of endothelial function. Critical Issues and Future Directions: The integrated physiology of heterocellular communication is still not fully understood. Dedicated experimental models are needed to analyze messengers and mechanisms underpinning heterocellular communication in response to physical forces in the cardiovascular system (and elsewhere). Antioxid. Redox Signal. 26, 917-935.

Selective Enhancement of Insulin Sensitivity in the Endothelium In Vivo Reveals a Novel Proatherosclerotic Signaling Loop

Rationale:

In the endothelium, insulin stimulates endothelial NO synthase (eNOS) to generate the antiatherosclerotic signaling radical NO. Insulin-resistant type 2 diabetes mellitus is associated with reduced NO availability and accelerated atherosclerosis. The effect of enhancing endothelial insulin sensitivity on NO availability is unclear.

Objective:

To answer this question, we generated a mouse with endothelial cell (EC)–specific overexpression of the human insulin receptor (hIRECO) using the Tie2 promoter–enhancer.

Methods and Results:

hIRECO demonstrated significant endothelial dysfunction measured by blunted endothelium-dependent vasorelaxation to acetylcholine, which was normalized by a specific Nox2 NADPH oxidase inhibitor. Insulin-stimulated phosphorylation of protein kinase B was increased in hIRECO EC as was Nox2 NADPH oxidase–dependent generation of superoxide, whereas insulin-stimulated and shear stress–stimulated eNOS activations were blunted. Phosphorylation at the inhibitory residue Y657 of eNOS and expression of proline-rich tyrosine kinase 2 that phosphorylates this residue were significantly higher in hIRECO EC. Inhibition of proline-rich tyrosine kinase 2 improved insulin-induced and shear stress–induced eNOS activation in hIRECO EC.

Conclusions:

Enhancing insulin sensitivity specifically in EC leads to a paradoxical decline in endothelial function, mediated by increased tyrosine phosphorylation of eNOS and excess Nox2-derived superoxide. Increased EC insulin sensitivity leads to a proatherosclerotic imbalance between NO and superoxide. Inhibition of proline-rich tyrosine kinase 2 restores insulin-induced and shear stress–induced NO production. This study demonstrates for the first time that increased endothelial insulin sensitivity leads to a proatherosclerotic imbalance between NO and superoxide.

Regulation of DENND3, the exchange factor for the small GTPase Rab12 through an intramolecular interaction

The Rab family of small GTPases functions in multiple aspects of cellular membrane trafficking. Proteins bearing a differentially expressed in normal and neoplastic cells (DENN) domain have emerged as the largest family of Rab-activating guanine nucleotide exchange factors (GEFs). Rab12 functions in the initiation of starvation-induced autophagy, and our previous work revealed that its activator, DENN domain-containing protein 3 (DENND3), is phosphorylated and activated upon starvation. However, how the GEF activity of DENND3 toward Rab12 is regulated at the molecular level is still not understood. Here, we combine size-exclusion chromatography, Förster resonance energy transfer, pulldown, and in vitro GEF assays to demonstrate that regulation of GEF activity is achieved through an intramolecular interaction that is controlled by a key residue in DENND3, tyrosine 940. Our study sheds light on the regulation of Rab12 activation and lays the groundwork for characterizing the regulation of other DENN domain-containing proteins.

Effects of shear stress on endothelial cells: go with the flow

Haemodynamic forces influence the functional properties of vascular endothelium. Endothelial cells (ECs) have a variety of receptors, which sense flow and transmit mechanical signals through mechanosensitive signalling pathways to recipient molecules that lead to phenotypic and functional changes. Arterial architecture varies greatly exhibiting bifurcations, branch points and curved regions, which are exposed to various flow patterns. Clinical studies showed that atherosclerotic plaques develop preferentially at arterial branches and curvatures, that is in the regions exposed to disturbed flow and shear stress. In the atheroprone regions, the endothelium has a proinflammatory phenotype associated with low nitric oxide production, reduced barrier function and increased proadhesive, procoagulant and proproliferative properties. Atheroresistant regions are exposed to laminar flow and high shear stress that induce prosurvival antioxidant signals and maintain the quiescent phenotype in ECs. Indeed, various flow patterns contribute to phenotypic and functional heterogeneity of arterial endothelium whose response to proatherogenic stimuli is differentiated. This may explain the preferential development of endothelial dysfunction in arterial sites with disturbed flow.

Nitric oxide: what's new to NO?

Nitric oxide (NO) is one of the critical components of the vasculature, regulating key signaling pathways in health. In macrovessels, NO functions to suppress cell inflammation as well as adhesion. In this way, it inhibits thrombosis and promotes blood flow. It also functions to limit vessel constriction and vessel wall remodeling. In microvessels and particularly capillaries, NO, along with growth factors, is important in promoting new vessel formation, a process termed angiogenesis. With age and cardiovascular disease, animal and human studies confirm that NO is dysregulated at multiple levels including decreased production, decreased tissue half-life, and decreased potency. NO has also been implicated in diseases that are related to neurotransmission and cancer although it is likely that these processes involve NO at higher concentrations and from nonvascular cell sources. Conversely, NO and drugs that directly or indirectly increase NO signaling have found clinical applications in both age-related diseases and in younger individuals. This focused review considers recently reported advances being made in the field of NO signaling regulation at several levels including enzymatic production, receptor function, interacting partners, localization of signaling, matrix-cellular and cell-to-cell cross talk, as well as the possible impact these newly described mechanisms have on health and disease.

Proline-rich tyrosine kinase 2 via enhancing signal transducer and activator of transcription 3-dependent cJun expression mediates retinal neovascularization

Despite the involvement of proline-rich tyrosine kinase 2 (Pyk2) in endothelial cell angiogenic responses, its role in pathological retinal angiogenesis is not known. In the present study, we show that vascular endothelial growth factor A (VEGFA) induces Pyk2 activation in mediating human retinal microvascular endothelial cell (HRMVEC) migration, sprouting and tube formation. Downstream to Pyk2, VEGFA induced signal transducer and activator of transcription 3 (STAT3) activation and cJun expression in the modulation of HRMVEC migration, sprouting and tube formation. Consistent with these observations, hypoxia induced activation of Pyk2-STAT3-cJun signaling axis and siRNA-mediated downregulation of Pyk2, STAT3 or cJun levels substantially inhibited hypoxia-induced retinal endothelial cell proliferation, tip cell formation and neovascularization. Together, these observations suggest that activation of Pyk2-mediated STAT3-cJun signaling is required for VEGFA-induced HRMVEC migration, sprouting and tube formation in vitro and hypoxia-induced retinal endothelial cell proliferation, tip cell formation and neovascularization in vivo.

Endothelial nitric oxide synthase in the microcirculation

Endothelial nitric oxide synthase (eNOS, NOS3) is responsible for producing nitric oxide (NO)--a key molecule that can directly (or indirectly) act as a vasodilator and anti-inflammatory mediator. In this review, we examine the structural effects of regulation of the eNOS enzyme, including post-translational modifications and subcellular localization. After production, NO diffuses to surrounding cells with a variety of effects. We focus on the physiological role of NO and NO-derived molecules, including microvascular effects on vessel tone and immune response. Regulation of eNOS and NO action is complicated; we address endogenous and exogenous mechanisms of NO regulation with a discussion of pharmacological agents used in clinical and laboratory settings and a proposed role for eNOS in circulating red blood cells.

Critical regulators of endothelial cell functions: for a change being alternative

SIGNIFICANCE

The endothelium regulates vessel dilation and constriction, balances hemostasis, and inhibits thrombosis. In addition, pro- and anti-angiogenic molecules orchestrate proliferation, survival, and migration of endothelial cells. Regulation of all these processes requires fine-tuning of signaling pathways, which can easily be tricked into running the opposite direction when exogenous or endogenous signals get out of hand. Surprisingly, some critical regulators of physiological endothelial functions can turn malicious by mere alternative splicing, leading to the expression of protein isoforms with opposite functions.

RECENT ADVANCES

While reviewing the evidence of alternative splicing on cellular physiology, it became evident that expression of splice factors and their activities are regulated by externally triggered signaling cascades. Furthermore, genome-wide identification of RNA-binding sites of splicing regulatory proteins now offer a glimpse into the splicing code responsible for alternative splicing of molecules regulating endothelial functions.

CRITICAL ISSUES

Due to the constantly growing number of transcript and protein isoforms, it will become more and more important to identify and characterize all transcripts and proteins regulating endothelial cell functions. One critical issue will be a non-ambiguous nomenclature to keep consistency throughout different laboratories.

FUTURE DIRECTIONS

RNA-deep sequencing focusing on exon-exon junction needs to more reliably identify alternative splicing events combined with functional analyses that will uncover more splice variants contributing to or inhibiting proper endothelial functions. In addition, understanding the signals mediating alternative splicing and its regulation might allow us to derive new strategies to preserve endothelial function by suppressing or upregulating specific protein isoforms. Antioxid. Redox Signal. 22, 1212-1229.

Regulation of eNOS enzyme activity by posttranslational modification

The regulation of endothelial NO synthase (eNOS) employs multiple different cellular control mechanisms impinging on level and activity of the enzyme. This review aims at summarizing the current knowledge on the posttranslational modifications of eNOS, including acylation, nitrosylation, phosphorylation, acetylation, glycosylation and glutathionylation. Sites, mediators and impact on enzyme localization and activity of the single modifications will be discussed. Moreover, interdependence, cooperativity and competition between the different posttranslational modifications will be elaborated with special emphasis on the susceptibility of eNOS to metabolic cues.

Shear stress-initiated signaling and its regulation of endothelial function

Atherosclerosis develops preferentially at branches and curvatures of the arterial tree, where blood flow pattern is disturbed rather than being laminar, and wall shear stress has an irregular distribution without defined directions. The endothelium in the atherosusceptible regions, in comparison to that in atheroresistant regions, shows activation of proproliferative and proinflammatory gene expressions, reduced production of nitric oxide (NO), increased leukocyte adhesion, and permeability, as well as other atheroprone phenotypes. Differences in gene expressions and cell phenotypes have been detected in endothelia residing in native atherosusceptible and atheroresistant regions of the arteries, or in arteries from animal models with artificial creation of disturbed flow. Similar results have also been shown in in vitro systems that apply controlled shear stresses with or without clear directions to cultured endothelial cells in fluid-dynamically designed flow-loading devices. The available evidence indicates that the coordination of multiple signaling networks, rather than individual separate pathways, links the mechanical signals to specific genetic circuitries in orchestrating the mechanoresponsive networks to evoke comprehensive genetic and functional responses.

Regulation of cellular communication by signaling microdomains in the blood vessel wall

It has become increasingly clear that the accumulation of proteins in specific regions of the plasma membrane can facilitate cellular communication. These regions, termed signaling microdomains, are found throughout the blood vessel wall where cellular communication, both within and between cell types, must be tightly regulated to maintain proper vascular function. We will define a cellular signaling microdomain and apply this definition to the plethora of means by which cellular communication has been hypothesized to occur in the blood vessel wall. To that end, we make a case for three broad areas of cellular communication where signaling microdomains could play an important role: 1) paracrine release of free radicals and gaseous molecules such as nitric oxide and reactive oxygen species; 2) role of ion channels including gap junctions and potassium channels, especially those associated with the endothelium-derived hyperpolarization mediated signaling, and lastly, 3) mechanism of exocytosis that has considerable oversight by signaling microdomains, especially those associated with the release of von Willebrand factor. When summed, we believe that it is clear that the organization and regulation of signaling microdomains is an essential component to vessel wall function.