Pim1 kinase promotes angiogenesis through phosphorylation of endothelial nitric oxide synthase at Ser-633

Protein phosphatase 4 mediates palmitic acid-induced endothelial dysfunction by decreasing eNOS phosphorylation at serine 633 in HUVECs

Plasma saturated free fatty acid (FFA)-induced endothelial dysfunction (ED) contributes to the pathogenesis of atherosclerosis and cardiovascular diseases. However, the mechanism underlying saturated FFA-induced ED remains unclear. This study demonstrated that palmitic acid (PA) induced ED by activating the NADPH oxidase (NOX)/ROS signaling pathway to activate protein phosphatase 4 (PP4) and protein phosphatase 2A (PP2A), thereby reducing endothelial nitric oxide synthase (eNOS) phosphorylation at Ser633 and Ser1177, respectively. Okadaic acid (OA) and fostriecin (FST), which are inhibitors of PP2A, inhibited the PA-induced decreases in eNOS phosphorylation at Ser633 and Ser1177. The antioxidants N-acetylcysteine (NAC) and apocynin (APO) or knockdown of gp91phox or p67phox (NOX subunits) restored PA-mediated downregulation of PP4R2 protein expression and eNOS Ser633 phosphorylation. Knockdown of the PP4 catalytic subunit (PP4c) specifically increased eNOS Ser633 phosphorylation, while silencing the PP2A catalytic subunit (PP2Ac) restored only eNOS Ser1177 phosphorylation. Furthermore, PA dramatically decreased the protein expression of the PP4 regulatory subunit R2 (PP4R2) but not the other regulatory subunits. PP4R2 overexpression increased eNOS Ser633 phosphorylation, nitric oxide (NO) production, cell migration and tube formation but did not change eNOS Ser1177 phosphorylation levels. Coimmunoprecipitation (Co-IP) suggested that PP4R2 and PP4c interacted with the PP4R3α and eNOS proteins. In summary, PA decreases PP4R2 protein expression through the Nox/ROS pathway to activate PP4, which contributes to ED by dephosphorylating eNOS at Ser633. The results of this study suggest that PP4 is a novel therapeutic target for ED and ED-associated vascular diseases.

HTK vs. HTK-N for Coronary Endothelial Protection during Hypothermic, Oxygenated Perfusion of Hearts Donated after Circulatory Death

Protection of the coronary arteries during donor heart maintenance is pivotal to improve results and prevent the development of coronary allograft vasculopathy. The effect of hypothermic, oxygenated perfusion (HOP) with the traditional HTK and the novel HTK-N solution on the coronary microvasculature of donation-after-circulatory-death (DCD) hearts is known. However, the effect on the coronary macrovasculature is unknown. Thus, we maintained porcine DCD hearts by HOP with HTK or HTK-N for 4 h, followed by transplantation-equivalent reperfusion with blood for 2 h. Then, we removed the left anterior descending coronary artery (LAD) and compared the endothelial-dependent and -independent vasomotor function of both groups using bradykinin and sodium-nitroprusside (SNP). We also determined the transcriptome of LAD samples using microarrays. The endothelial-dependent relaxation was significantly better after HOP with HTK-N. The endothelial-independent relaxation was comparable between both groups. In total, 257 genes were expressed higher, and 668 genes were expressed lower in the HTK-N group. Upregulated genes/pathways were involved in endothelial and vascular smooth muscle cell preservation and heart development. Downregulated genes were related to ischemia/reperfusion injury, oxidative stress, mitochondrion organization, and immune reaction. The novel HTK-N solution preserves the endothelial function of DCD heart coronary arteries more effectively than traditional HTK.

Pim Kinases: Important Regulators of Cardiovascular Disease

Pim Kinases; Pim-1, Pim-2, and Pim-3, are a family of constitutively active serine/threonine kinases, widely associated with cell survival, proliferation, and migration. Historically considered to be functionally redundant, independent roles for the individual isoforms have been described. Whilst most established for their role in cancer progression, there is increasing evidence for wider pathological roles of Pim kinases within the context of cardiovascular disease, including inflammation, thrombosis, and cardiac injury. The Pim kinase isoforms have widespread expression in cardiovascular tissues, including the heart, coronary artery, aorta, and blood, and have been demonstrated to be upregulated in several co-morbidities/risk factors for cardiovascular disease. Pim kinase inhibition may thus be a desirable therapeutic for a multi-targeted approach to treat cardiovascular disease and some of the associated risk factors. In this review, we discuss what is known about Pim kinase expression and activity in cells of the cardiovascular system, identify areas where the role of Pim kinase has yet to be fully explored and characterised and review the suitability of targeting Pim kinase for the prevention and treatment of cardiovascular events in high-risk individuals.

Diminazene aceturate uses different pathways to induce relaxation in healthy and atherogenic blood vessels

Diminazene aceturate (DIZE), a putative angiotensin-converting enzyme 2 (ACE2) activator, elicits relaxation in various animal models. This study aimed to determine the relaxing mechanisms in internal iliac arteries utilised by DIZE in healthy and atherogenic rabbit models. Studies were conducted on internal iliac artery rings retrieved from male New Zealand White rabbits fed a 4-week healthy control (n = 24) or atherogenic diet (n = 20). To investigate pathways utilised by DIZE to promote arterial relaxation, a DIZE dose response [10^-9.0 M - 10^-5.0 M] was performed on pre-contracted rings incubated with pharmaceuticals that target: components of the renin-angiotensin system; endothelial- and vascular smooth muscle-dependent mechanisms; protein kinases; and potassium channels. ACE2 expression was quantified by immunohistochemistry analysis following a 2 hr or 4 hr DIZE incubation. DIZE significantly enhanced vessel relaxation in atherogenic rings at doses [10^-5.5 M] (p < 0.01) and [10^-5.0 M] (p < 0.0001), when compared to healthy controls. Comprehensive results from functional isometric studies determined that DIZE causes relaxation via different mechanisms depending on pathology. For the first time, we report that in healthy blood vessels DIZE exerts its direct relaxing effect through ACE2/AT₂R and NO/sGC pathways; however, in atherogenesis this switches to MasR, arachidonic acid pathway (i.e., COX1/2, EET and DHET), MCLP, Ca²⁺ activated voltage channels, AMPK and ERK1/2. Moreover, quantitative immunohistochemical analysis demonstrated that DIZE increases artery ACE2 expression in a time dependent manner. We provide a detailed investigation of DIZE's mechanisms and demonstrate for the first time that in healthy and atherogenic arteries DIZE provides beneficial effects through directly inducing relaxation, albeit via different pathways.

Whole-Transcriptome Sequencing Analyses of Nuclear Antixoxidant-1 in Endothelial Cells: Role in Inflammation and Atherosclerosis

Inflammation, oxidative stress, and copper (Cu) play an important role in cardiovascular disease, including atherosclerosis. We previously reported that cytosolic Cu chaperone antioxidant-1 (Atox1) translocates to the nucleus in response to inflammatory cytokines or exogenous Cu and that Atox1 is localized at the nucleus in the endothelium of inflamed atherosclerotic aorta. However, the roles of nuclear Atox1 and their function are poorly understood. Here we showed that Atox1 deficiency in ApoE-/- mice with a Western diet exhibited a significant reduction of atherosclerotic lesion formation. In vitro, adenovirus-mediated overexpression of nuclear-targeted Atox1 (Ad-Atox1-NLS) in cultured human endothelial cells (ECs) increased monocyte adhesion and reactive oxygen species (ROS) production compared to control cells (Ad-null). To address the underlying mechanisms, we performed genome-wide mapping of Atox1-regulated targets in ECs, using an unbiased systemic approach integrating sequencing data. Combination of ChIP-Seq and RNA-Seq analyses in ECs transfected with Ad-Atox1-NLS or Ad-null identified 1387 differentially expressed genes (DEG). Motif enrichment assay and KEGG pathway enrichment analysis revealed that 248 differentially expressed genes, including inflammatory and angiogenic genes, were regulated by Atox1-NLS, which was then confirmed by real-time qPCR. Among these genes, functional analysis of inflammatory responses identified CD137, CSF1, and IL5RA as new nuclear Atox1-targeted inflammatory genes, while CD137 is also a key regulator of Atox1-NLS-induced ROS production. These findings uncover new nuclear Atox1 downstream targets involved in inflammation and ROS production and provide insights into the nuclear Atox1 as a potential therapeutic target for the treatment of inflammatory diseases such as atherosclerosis.

A systematic review on active sites and functions of PIM-1 protein

The Proviral Integration of Molony murine leukemia virus (PIM)-1 protein contributes to the solid cancers and hematologic malignancies, cell growth, proliferation, differentiation, migration, and other life activities. Many studies have related these functions to its molecular structure, subcellular localization and expression level. However, recognition of specific active sites and their effects on the activity of this constitutively active kinase is still a challenge. Based on the close relationship between its molecular structure and functional activity, this review covers the specific residues involved in the binding of ATP and different substrates in its catalytic domain. This review then elaborates on the relevant changes in protein conformation and cell functions after PIM-1 binds to different substrates. Therefore, this intensive study can improve the understanding of PIM-1-regulated signaling pathways by facilitating the discovery of its potential phosphorylation substrates.

The role of Pim kinase in immunomodulation

Pim kinase, which has three isozymes (Pim-1, Pim-2 and Pim-3), is a serine/threonine kinase abnormally expressed in many cancers. High Pim kinase expression has been recognized to be associated with disease progression and prognosis. It is well accepted that Pim kinase is considered a clinical biomarker and potential therapeutic target for tumor cell. In recent years, researches verified the role of Pim kinase in immunomodulation. The mechanisms by which Pim kinase modulates the immune microenvironment and regulates immune cells, as well as the effects of Pim kinase inhibitors on immunity, have not been systematically described. This review comprehensively focuses on the current research status of Pim kinase pathways and the immune regulation.

Inside-outside Ag nanoparticles-loaded polylactic acid electrospun fiber for long-term antibacterial and bone regeneration

Bone infections caused by bacteria during bone graft implantations can impair the ability of bone tissue repair, which is currently a clinical problem. In this study, the electrospinning technique was used to prepare a polylactic acid (PLLA)/silver (Ag) composite fiber, in which the silver nanoparticles (Ag-NPs) were uniformly distributed on the inner surface of PLLA fibers; dopamine (DA) was self-polymerized on the composite fiber surface to construct the adhesive polydopamine (PDA) film and chitosan (CS) was used to regulate Ag⁺ in situ through pulse electrochemical deposition for the construction of a stable Ag-NPs coating (CS/Ag), achieving the steady and slow release of Ag-NPs, therefore accomplishing the construction of a "inside-outside" Ag-NPs-loaded PLLA/Ag@PDA@CS/Ag composite fiber with dual functions of long-lasting antibacterial effect as well as bone regeneration promotion ability. The study results showed that the composite fiber has an excellent antibacterial effect against E. coli and S. aureus, and good osteoinductive and angiogenic properties. In summary, under the dual regulations of the strong adhesion of PDA and CS chelation, the "inside-outside" Ag-NPs-loaded composite fiber was endowed with good physiological stability, long-term antibacterial effect and bone infection inhibition ability, making it a promising bone implant material.

Cardiovascular bioimaging of nitric oxide: Achievements, challenges, and the future

Nitric oxide (NO) is a ubiquitous, volatile, cellular signaling molecule that operates across a wide physiological concentration range (pM-µM) in different tissues. It is a highly diffusible messenger and intermediate in various metabolic pathways. NO plays a pivotal role in maintaining optimum cardiovascular function, particularly by regulating vascular tone and blood flow. This review highlights the need for accurate, real-time bioimaging of NO in clinical diagnostic, therapeutic, monitoring, and theranostic applications within the cardiovascular system. We summarize electrochemical, optical, and nanoscale sensors that allow measurement and imaging of NO, both directly and indirectly via surrogate measurements. The physical properties of NO render it difficult to accurately measure in tissues using direct methods. There are also significant limitations associated with the NO metabolites used as surrogates to indirectly estimate NO levels. All these factors added to significant variability in the measurement of NO using available methodology have led to a lack of sensors and imaging techniques of clinical applicability in relevant vascular pathologies such as atherosclerosis and ischemic heart disease. Challenges in applying current methods to biomedical and clinical translational research, including the wide physiological range of NO and limitations due to the characteristics and toxicity of the sensors are discussed, as are potential targets and modifications for future studies. The development of biocompatible nanoscale sensors for use in combination with existing clinical imaging modalities provides a feasible opportunity for bioimaging NO within the cardiovascular system.

Downregulation of miRNA‑328 promotes the angiogenesis of HUVECs by regulating the PIM1 and AKT/mTOR signaling pathway under high glucose and low serum condition

Vascular complications are the primary reason for disability and mortality associated with diabetes mellitus (DM), and numerous microRNAs (miRNAs/miRs) are involved in the process, such as miR‑122, miR‑24 and miR‑423. It has been reported that miR‑328 regulates DM and cardiovascular disease; however, the role and mechanism of action underlying miR‑328 in HUVECs is not completely understood. The present study aimed to investigate the role and mechanism of action underlying the effects of miR‑328 on the functions of HUVECs. To simulate hyperglycemia combined with ischemia‑induced tissue starvation, HUVECs were cultured in endothelial cell medium with 25 mmol/l D‑glucose and 2% FBS for 24 h [high glucose (HG) + 2% FBS group]. HUVEC miR‑328 expression levels were detected by reverse transcription‑quantitative PCR. Cell migration, cytotoxicity and tube‑like structure formation were analyzed using wound healing, Cell Counting Kit‑8 and tube formation assays, respectively. Following transfection with miR‑328 inhibitor, miR‑328 expression was downregulated in HUVECs. Protein expression levels were determined by western blotting. Compared with the control group, the migration and tube‑like structure formation of HUVECs were decreased, and cell cytotoxicity was increased in the HG + 2% FBS group. The protein expression levels of vascular endothelial growth factor were also decreased, and the expression levels of miRNA‑328 in the HG + 2% FBS group were increased compared with the control group. However, miRNA‑328 downregulation reversed the aforementioned effects. Further experiments indicated that the AKT signaling pathway was inhibited in the HG + 2% FBS group; however, miR‑328 downregulation activated the AKT/mTOR signaling pathway, which was blocked by the AKT signaling pathway inhibitor, perifosine. Gene prediction and western blotting demonstrated that miR‑328 displayed a regulatory role via Pim‑1 proto‑oncogene, serine/threonine kinase (PIM1). In conclusion, miR‑328 expression was upregulated and angiogenesis was inhibited when HUVECs were subjected to high glucose and low serum conditions. miR‑328 downregulation enhanced angiogenesis by increasing PIM1 expression and activating the AKT/mTOR signaling pathway in HUVECs under high glucose and low serum conditions.

The histone deacetylase inhibitor tubacin mitigates endothelial dysfunction by up-regulating the expression of endothelial nitric oxide synthase

Endothelial nitric oxide (NO) synthase (eNOS) plays a critical role in the maintenance of blood vessel homeostasis. Recent findings suggest that cytoskeletal dynamics play an essential role in regulating eNOS expression and activation. Here, we sought to test whether modulation of cytoskeletal dynamics through pharmacological regulation of histone deacetylase 6 (HDAC6)-mediated tubulin deacetylation affects eNOS expression and endothelial function in vitro and in vivo We found that tubulin acetylation inducer (tubacin), a compound that appears to selectively inhibit HDAC6 activity, dramatically increased eNOS expression in several different endothelial cell lines, as determined by both immunoblotting and NO production assays. Mechanistically, we found that these effects were not mediated by tubacin's inhibitory effect on HDAC6 activity, but rather were due to its ability to stabilize eNOS mRNA transcripts. Consistent with these findings, tubacin also inhibited proinflammatory cytokine-induced degradation of eNOS transcripts and impairment of endothelium-dependent relaxation in the mouse aorta. Furthermore, we found that tubacin-induced up-regulation in eNOS expression in vivo is associated with improved endothelial function in diabetic db/db mice and with a marked attenuation of ischemic brain injury in a murine stroke model. Our findings indicate that tubacin exhibits potent eNOS-inducing effects and suggest that this compound might be useful for the prevention or management of endothelial dysfunction-associated cardiovascular diseases.

Reduced Hypoxia-Related Genes in Porcine Limbs in Ex Vivo Hypothermic Perfusion Versus Cold Storage

BACKGROUND

Ischemia-reperfusion injury remains the major limiting factor for limb replantation and transplantation. Static cold storage (SCS) on ice currently represents the standard mode of preservation but is limited to 6 h of duration. Ex vivo machine perfusion has evolved as a potential alternative to safely extend the duration of ex vivo preservation by providing continuous supply of oxygen and nutrients. This study aims to evaluate underlying molecular mechanisms of both preservation modalities.

METHODS

We assessed molecular changes in amputated porcine forelimbs stored on ice at 4°C for 2 h (n = 2) and limbs perfused with Perfadex solution at 10°C for 2 h (n = 3) or 12 h (n = 3) before replantation. Muscle biopsies were examined for histological changes and gene expression levels using H&E staining and a hypoxia-related PCR gene array, respectively.

RESULTS

Histology revealed only minor differences between the ice (SCS) and perfusion groups after 2 h of preservation, with decreased muscle fiber disruption in the perfusion groups compared with the ice (SCS) group. Perfused limbs demonstrated downregulation of genes coding for glycolytic pathways and glucose transporters after 2 h and 12 h when compared with SCS after 2 h. Similarly, genes that induce angiogenesis and those that are activated on DNA damage were downregulated in both perfusion groups as compared with SCS.

CONCLUSIONS

Perfusion of porcine limbs resulted in less activation of hypoxia-related gene families when compared with SCS. This may indicate a state more closely resembling physiological conditions during perfusion and potentially limiting ischemic injury. Our study confirms ex vivo perfusion for up to 12 h as a viable alternative for preservation of vascularized composite tissues.

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.

High glucose promotes vascular smooth muscle cell proliferation by upregulating proto-oncogene serine/threonine-protein kinase Pim-1 expression

Serine/threonine kinase proviral integration site for Moloney murine leukemia virus 1 (Pim-1) plays an essential role in arterial wall cell proliferation and associated vascular diseases, including pulmonary arterial hypertension and aortic wall neointima formation. Here we tested a role of Pim-1 in high-glucose (HG)-mediated vascular smooth muscle cell (VSMC) proliferation. Pim-1 and proliferating cell nuclear antigen (PCNA) expression levels in arterial samples from streptozotocin-induced hyperglycemia rats were increased, compared with their weak expression in normoglycemic groups. In cultured rat VSMCs, HG led to transient Pim-1 expression decline, followed by sustained expression increase at both transcriptional and translational levels. Immunoblot analysis demonstrated that HG increased the expression of the 33-kDa isoform of Pim-1, but at much less extent to its 44-kDa plasma membrane isoform. D-glucose at a concentration of 25 mmol/L showed highest activity in stimulating Pim-1 expression. Both Pim-1 inhibitor quercetagetin and STAT3 inhibitor stattic significantly attenuated HG-induced VSMC proliferation and arrested cell cycle progression at the G1 phase. Quercetagetin showed no effect on Pim-1 expression but decreased the phosphorylated-Bad (T112)/Bad ratio in HG-treated VSMCs. However, stattic decreased phosphorylated-STAT3 (Y705) levels and caused transcriptional and translational down-regulation of Pim-1 in HG-treated VSMCs. Our findings suggest HG-mediated Pim-1 expression contributes to VSMC proliferation, which may be partly due to the activation of STAT3/Pim-1 signaling.

Redox regulation of gasotransmission in the vascular system: A focus on angiogenesis

Reactive oxygen species have emerged as key participants in a broad range of physiological and pathophysiological processes, not least within the vascular system. Diverse cellular functions which have been attributed to some of these pro-oxidants within the vasculature include the regulation of blood pressure, neovascularisation and vascular inflammation. We here highlight the emerging roles of the enzymatically-generated reaction oxygen species, O₂^- and H₂O₂, in the regulation of the functions of the gaseous signalling molecules: nitric oxide (NO), carbon monoxide (CO), and hydrogen sulphide (H₂S). These gasotransmitters are produced on demand from distinct enzymatic sources and in recent years it has become apparent that they are capable of mediating a number of homeostatic processes within the cardiovascular system including enhanced vasodilation, angiogenesis, wound healing and improved cardiac function following myocardial infarction. In common with O₂^- and/or H₂O₂ they signal by altering the functions of target proteins, either by the covalent modification of thiol groups or by direct binding to metal centres within metalloproteins, most notably haem proteins. The regulation of the enzymes which generate NO, CO and H₂S have been shown to be influenced at both the transcriptional and post-translational levels by redox-dependent mechanisms, while the activity and bioavailability of the gasotransmitters themselves are also subject to oxidative modification. Within vascular cells, the family of nicotinamide adenine dinucleotide phosphate oxidases (NAPDH oxidases/Noxs) have emerged as functionally significant sources of regulated O₂^- and H₂O₂ production and accordingly, direct associations between Nox-generated oxidants and the functions of specific gasotransmitters are beginning to be identified. This review focuses on the current knowledge of the redox-dependent mechanisms which regulate the generation and activity of these gases, with particular reference to their roles in angiogenesis.

A novel modified physiologically relevant model for cardiac angiogenesis

Angiogenesis assays are important tools for studying both the mechanisms of cardiac angiogenesis and the potential development of therapeutic strategies to ischemic heart diseases. Currently, various assays have been used to quantitate cardiac tubule formation, yet no consensus has been reached regarding a suitable assay for evaluating the efficacy of angiogenic stimulants or inhibitors. Most in vivo angiogenesis assays are complex and difficult to interpret, whereas traditional in vitro angiogenesis models measure only one aspect of this process. To bridge the gap between in vivo and in vitro angiogenesis assays, here, we have developed a novel modified cardiac explants matrigel assay. We observed the morphology of vascular sprouts formed in three forms of cardiac angiogenesis assays then used quantitative image analyses to further compare the morphological features of vascular sprouts formed in two cardiac explants angiogenesis assays. Vascular sprouts formed in the fibronectin group were less and short, whereas those formed in the matrigel group were significantly longer, consisting of more area and branch points. Moreover, we found the benefits of this matrigel model by observing the ability of cardiac explants to form vascular sprouts under normoxia or hypoxia condition in the presence of angiogenic stimulant and inhibitor, VEGF and PEDF. In summary, the above analyses revealed that the morphology of vascular sprouts formed in this model appears more representative of myocardial capillary formation in vivo, and this accessible, reliable angiogenic assay is a more physiologically relevant assay which allows further assessment of pharmacologic compounds on cardiac angiogenesis.

MicroRNA-124-3p regulates cell proliferation, invasion, apoptosis, and bioenergetics by targeting PIM1 in astrocytoma

The PIM1 protein is an important regulator of cell proliferation, the cell cycle, apoptosis, and metabolism in various human cancers. MicroRNAs (miRNAs) are powerful post‐transcriptional gene regulators that function through translational repression or transcript destabilization. Therefore, we aimed to identify whether a close relationship exists between PIM1 and miRNAs. PIM1 protein levels and mRNA levels were significantly upregulated in astrocytoma tissues, indicating the oncogenic role of PIM1 in astrocytoma. Further bioinformatics analysis indicated that miR‐124‐3p targeted the 3′‐UTR of PIM1. We also observed an inverse correlation between the miR‐124‐3p levels and PIM1 protein or mRNA levels in astrocytoma samples. Next, we experimentally confirmed that miR‐124‐3p directly recognizes the 3′‐UTR of the PIM1 transcript and regulates PIM1 expression at both the protein and mRNA levels. Furthermore, we examined the biological consequences of miR‐124‐3p targeting PIM1 in vitro. We showed that the repression of PIM1 in astrocytoma cancer cells by miR‐124‐3p suppressed proliferation, invasion, and aerobic glycolysis and promoted apoptosis. We observed that the restoration or inhibition of PIM1 activity resulted in effects that were similar to those induced by miR‐124‐3p inhibitors or mimics in cancer cells. Finally, overexpression of PIM1 rescued the inhibitory effects of miR‐124‐3p. In summary, these findings aid in understanding the tumor‐suppressive role of miR‐124‐3p in astrocytoma pathogenesis through the inhibition of PIM1 translation.

Transcriptional and Posttranslational Regulation of eNOS in the Endothelium

Nitric oxide (NO) is a highly reactive free radical gas and these unique properties have been adapted for a surprising number of biological roles. In neurons, NO functions as a neurotransmitter; in immune cells, NO contributes to host defense; and in endothelial cells, NO is a major regulator of blood vessel homeostasis. In the vasculature, NO is synthesized on demand by a specific enzyme, endothelial nitric oxide synthase (eNOS) that is uniquely expressed in the endothelial cells that form the interface between the circulating blood and the various tissues of the body. NO regulates endothelial and blood vessel function via two distinct pathways, the activation of soluble guanylate cyclase and cGMP-dependent signaling and the S-nitrosylation of proteins with reactive thiols (S-nitrosylation). The chemical properties of NO also serve to reduce oxidation and regulate mitochondrial function. Reduced synthesis and/or compromised biological activity of NO precede the development of cardiovascular disease and this has generated a high level of interest in the mechanisms controlling the synthesis and fate of NO in the endothelium. The amount of NO produced results from the expression level of eNOS, which is regulated at the transcriptional and posttranscriptional levels as well as the acute posttranslational regulation of eNOS. The goal of this chapter is to highlight and integrate past and current knowledge of the mechanisms regulating eNOS expression in the endothelium and the posttranslational mechanisms regulating eNOS activity in both health and disease.