Activation of Hsp90-eNOS and increased NO generation attenuate respiration of hypoxia-treated endothelial cells

Subcellular Localization Guides eNOS Function

Nitric oxide synthases (NOS) are enzymes responsible for the cellular production of nitric oxide (NO), a highly reactive signaling molecule involved in important physiological and pathological processes. Given its remarkable capacity to diffuse across membranes, NO cannot be stored inside cells and thus requires multiple controlling mechanisms to regulate its biological functions. In particular, the regulation of endothelial nitric oxide synthase (eNOS) activity has been shown to be crucial in vascular homeostasis, primarily affecting cardiovascular disease and other pathophysiological processes of importance for human health. Among other factors, the subcellular localization of eNOS plays an important role in regulating its enzymatic activity and the bioavailability of NO. The aim of this review is to summarize pioneering studies and more recent publications, unveiling some of the factors that influence the subcellular compartmentalization of eNOS and discussing their functional implications in health and disease.

Lyophilized Extracellular Vesicles from Adipose-Derived Stem Cells Increase Muscle Reperfusion but Degrade Muscle Structural Proteins in a Mouse Model of Hindlimb Ischemia-Reperfusion Injury

Ischemia-reperfusion (I/R) injury is a complication impacting multiple organs and tissues in clinical conditions ranging from peripheral arterial disease to musculoskeletal trauma and myocardial infarction. Stem cell-derived extracellular vesicles (EVs) may represent one therapeutic resource for preventing the tissue damage associated with I/R injury. Here we tested the hypothesis that lyophilized extracellular vesicles derived from adipose stem cells could serve as an "off-the-shelf" treatment modality for I/R injury in a mouse hindlimb ischemia model. Ischemia was induced for 90 min using a rubber band tourniquet and extracellular vesicles (0, 50, or 100 µg) administered via tail vein injection immediately prior to reperfusion. Perfusion was measured prior to, during, and after ischemia using laser Doppler imaging. Serum and tissue were collected 24 h after reperfusion. Mass spectrometry (MS)-based proteomics was used to characterize the EV cargo and proteins from the ischemic and non-ischemic hindlimb. Inflammatory cytokines were measured in muscle and serum using a multiplex array. Results indicate that EVs significantly increase reperfusion and significantly increase expression of the anti-inflammatory factor annexin a1 in skeletal muscle; however, the increased reperfusion was also associated with a marked decrease in muscle structural proteins such as dystrophin, plectin, and obscurin. Circulating inflammatory cytokines TNF-alpha and IL-6 were increased with EV treatment, and serum TNF-alpha showed a significant, positive correlation with reperfusion level. These findings suggest that, while EVs may enhance reperfusion, the increased reperfusion can negatively impact muscle tissue and possibly remote organs. Alternative approaches, such as targeting mitochondrial permeability, may be more effective at mitigating I/R injury.

Exercise intervention under hypoxic condition as a new therapeutic paradigm for type 2 diabetes mellitus: A narrative review

This review aims to summarize the health benefits of exposure to hypoxic conditions during exercise in patients with type 2 diabetes mellitus (T2DM). Exposure to hypoxic conditions during exercise training positively changes the physiological response in healthy subjects. Exposure to hypoxic conditions during exercise could markedly increase skeletal muscle glucose uptake compared to that in normoxic conditions. Furthermore, post-exercise insulin sensitivity of T2DM patients increases more when exercising under hypoxic than under normoxic conditions. Regular exercise under short-term hypoxic conditions can improve blood glucose control at lower workloads than in normoxic conditions. Additionally, exercise training under short-term hypoxic conditions can maximize weight loss in overweight and obese patients. Previous studies on healthy subjects have reported that regular exercise under hypoxic conditions had a more positive effect on vascular health than exercising under normoxic conditions. However, currently, evidence indicating that exposure to hypoxic conditions could positively affect T2DM patients in the long-term is lacking. Therefore, further evaluations of the beneficial effects of exercise under hypoxic conditions on the human body, considering different cycle lengths, duration of exposures, sessions per day, and the number of days, are necessary. In this review, we conclude that there is evidence that exercise under hypoxic conditions can yield health benefits, which is potentially valuable in terms of clinical care as a new intervention for T2DM patients.

Arginine recycling in endothelial cells is regulated BY HSP90 and the ubiquitin proteasome system

Despite the saturating concentrations of intracellular l-arginine, nitric oxide (NO) production in endothelial cells (EC) can be stimulated by exogenous arginine. This phenomenon, termed the "arginine paradox" led to the discovery of an arginine recycling pathway in which l-citrulline is recycled to l-arginine by utilizing two important urea cycle enzymes argininosuccinate synthetase (ASS) and argininosuccinate lyase (ASL). Prior work has shown that ASL is present in a NO synthetic complex containing hsp90 and endothelial NO synthase (eNOS). However, it is unclear whether hsp90 forms functional complexes with ASS and ASL and if it is involved regulating their activity. Thus, elucidating the role of hsp90 in the arginine recycling pathway was the goal of this study. Our data indicate that both ASS and ASL are chaperoned by hsp90. Inhibiting hsp90 activity with geldanamycin (GA), decreased the activity of both ASS and ASL and decreased cellular l-arginine levels in bovine aortic endothelial cells (BAEC). hsp90 inhibition led to a time-dependent decrease in ASS and ASL protein, despite no changes in mRNA levels. We further linked this protein loss to a proteasome dependent degradation of ASS and ASL via the E3 ubiquitin ligase, C-terminus of Hsc70-interacting protein (CHIP) and the heat shock protein, hsp70. Transient over-expression of CHIP was sufficient to stimulate ASS and ASL degradation while the over-expression of CHIP mutant proteins identified both TPR- and U-box-domain as essential for ASS and ASL degradation. This study provides a novel insight into the molecular regulation l-arginine recycling in EC and implicates the proteasome pathway as a possible therapeutic target to stimulate NO signaling.

Direct Current Electric Field Stimulates Nitric Oxide Production and Promotes NO-Dependent Angiogenesis: Involvement of the PI3K/Akt Signaling Pathway

Electric fields (EFs) promote angiogenesis in vitro and in vivo. These results indicate the feasibility of the application of EFs to modulate angiogenesis. Nitric oxide (NO) derived from endothelial nitric oxide synthase (eNOS) is an important regulator of angiogenesis. However, the role of direct current EFs in eNOS activity and expression in association with angiogenesis of endothelial cells has not been investigated. In the present study, we stimulated human umbilical vein endothelial cells (HUVECs) with EFs and evaluated the activity and expression of eNOS. EFs induced significant phosphorylation of eNOS, upregulation of the expression of eNOS protein, and an increase in NO production from HUVECs. L-NAME, a specific inhibitor of eNOS, abolished EF-induced HUVEC angiogenesis. EFs stimulated Akt activation. Inhibition of PI3K activity inhibited EF-mediated Akt and eNOS activation and inhibited NO production in the endothelial cells. Moreover, EFs stimulated HUVEC proliferation and enhanced the S phase cell population of the cell cycle. We conclude that EFs stimulate eNOS activation and NO production via a PI3K/Akt-dependent pathway. Thus, activation of eNOS appears to be one of the key signaling pathways necessary for EF-mediated angiogenesis. These novel findings suggest that NO signaling may have an important role in EF-mediated endothelial cell function.

Hereditary angioedema: Assessing the hypothesis for underlying autonomic dysfunction

Background

Attacks of Hereditary Angioedema due to C1-inhibitor deficiency (C1-INH-HAE)are often triggered by stressful events/hormonal changes.

Objective

Our study evaluates the relationship between autonomic nervous system (ANS) and contact/complement system activation.

Methods

Twenty-three HAE patients (6 males, mean age 47.5±11.4 years) during remission and 24 healthy controls (8 males, mean age 45.3±10.6 years) were studied. ECG, beat-by-beat blood pressure, respiratory activity were continuously recorded during rest (10’) and 75-degrees-head-up tilt (10’). C1-INH, C4, cleaved high molecular weight kininogen (cHK) were assessed; in 16 patients and 11 controls plasma catecholamines were also evaluated. Spectral analysis of heart rate variability allowed extraction of low-(LF) and high-(HF) frequency components, markers of sympathetic and vagal modulation respectively.

Results

HAE patients showed higher mean systolic arterial pressure (SAP) than controls during both rest and tilt. Tilt induced a significant increase in SAP and its variability only in controls. Although sympathetic modulation (LFnu) increased significantly with tilt in both groups, LF/HF ratio, index of sympathovagal balance, increased significantly only in controls.

At rest HAE patients showed higher noradrenaline values (301.4±132.9 pg/ml vs 210.5±89.6pg/ml, p = 0.05). Moreover, in patients tilt was associated with a significant increase in cHK, marker of contact system activation (49.5 ± 7.5% after T vs 47.1 ± 7.8% at R, p = 0.01).

Conclusions

Our data are consistent with altered ANS modulation in HAE patients, i.e. increased sympathetic activation at rest and blunted response to orthostatic challenge. Tilt test-induced increased HK cleavage suggests a link between stress and bradykinin production.

Exercise during short-term exposure to hypoxia or hyperoxia - novel treatment strategies for type 2 diabetic patients?!

Both hypoxia (decreased oxygen availability) and hyperoxia (increased oxygen availability) have been shown to alter exercise adaptations in healthy subjects. This review aims to clarify the possible benefits of exercise during short-term exposure to hypoxia or hyperoxia for patients with type 2 diabetes mellitus (T2DM). There is evidence that exercise during short-term exposure to hypoxia can acutely increase skeletal muscle glucose uptake more than exercise in normoxia, and that post-exercise insulin sensitivity in T2DM patients is more increased when exercise is performed under hypoxic conditions. Furthermore, interventional studies show that glycemic control can be improved through regular physical exercise in short-term hypoxia at a lower workload than in normoxia, and that exercise training in short-term hypoxia can contribute to increased weight loss in overweight/obese (insulin-resistant) subjects. While numerous studies involving healthy subjects report that regular exercise in hypoxia can increase vascular health (skeletal muscle capillarization and vascular dilator function) to a higher extent than exercise training in normoxia, there is no convincing evidence yet that hypoxia has such additive effects in T2DM patients in the long term. Some studies indicate that the use of hyperoxia during exercise can decrease lactate concentrations and subjective ratings of perceived exertion. Thus, there are interesting starting points for future studies to further evaluate possible beneficial effects of exercise in short-term hypoxia or hyperoxia at different oxygen concentrations and exposure durations. In general, exposure to hypoxia/hyperoxia should be considered with caution. Possible health risks-especially for T2DM patients-are also analyzed in this review.

Nitric oxide and hypoxia stimulate erythropoietin receptor via MAPK kinase in endothelial cells

Erythropoietin receptor (EPOR) expression level determines the extent of erythropoietin (EPO) response. Previously we showed that EPOR expression in endothelial cells is increased at low oxygen tension and that EPO stimulation of endothelial cells during hypoxia can increase endothelial nitric oxide (NO) synthase (eNOS) expression and activation as well as NO production. We now observe that while EPO can stimulate NO production, NO in turn can regulate EPOR expression. Human umbilical vein endothelial cells (HUVEC) treated with 10-50 μM of NO donor diethylenetriamine NONOate (DETANO) for 24h showed significant induction of EPOR gene expression at 5% and 2% of oxygen. Also human bone marrow microvascular endothelial cell line (TrHBMEC) cultured at 21 and 2% oxygen with 50 μM DETANO demonstrated a time and oxygen dependent induction of EPOR mRNA expression after 24 and 48 h, particularly at low oxygen tension. EPOR protein was also induced by DETANO at 2% oxygen in TrHBMEC and HUVEC. The activation of signaling pathways by NO donor stimulation appeared to be distinct from EPO stimulation. In reporter gene assays, DETANO treatment of HeLa cells at 2% oxygen increased EPOR promoter activity indicated by a 48% increase in luciferase activity with a 2 kb EPOR promoter fragment and a 71% increase in activity with a minimal EPOR promoter fragment containing 0.2 kb 5'. We found that DETANO activated MAPK kinase in TrHBMEC both in normoxia and hypoxia, while MAPK kinase inhibition showed significant reduction of EPOR mRNA gene expression at low oxygen tension, suggesting MAPK involvement in NO mediated induction of EPOR. Furthermore, DETANO stimulated Akt anti-apoptotic activity after 30 min in normoxia, whereas it inhibited Akt phosphorylation in hypoxia. In contrast, EPO did not significantly increase MAPK activity while EPO stimulated Akt phosphorylation in TrHBMEC in normoxia and hypoxia. These observations provide a new effect of NO on EPOR expression to enhance EPO response in endothelial cells, particularly at low oxygen tensions.

Angiotensin II impairs endothelial nitric-oxide synthase bioavailability under free cholesterol-enriched conditions via intracellular free cholesterol-rich membrane microdomains

Vascular endothelial function is impaired in hypercholesterolemia partly because of injury by modified LDL. In addition to modified LDL, free cholesterol (FC) is thought to play an important role in the development of endothelial dysfunction, although the precise mechanisms remain to be elucidated. The aim of this study was to clarify the mechanisms of endothelial dysfunction induced by an FC-rich environment. Loading cultured human aortic endothelial cells with FC induced the formation of vesicular structures composed of FC-rich membranes. Raft proteins such as phospho-caveolin-1 (Tyr-14) and small GTPase Rac were accumulated toward FC-rich membranes around vesicular structures. In the presence of these vesicles, angiotensin II-induced production of reactive oxygen species (ROS) was considerably enhanced. This ROS shifted endothelial NOS (eNOS) toward vesicle membranes and vesicles with a FC-rich domain trafficked toward perinuclear late endosomes/lysosomes, which resulted in the deterioration of eNOS Ser-1177 phosphorylation and NO production. Angiotensin II-induced ROS decreased the bioavailability of eNOS under the FC-enriched condition.

Mechanism of reversal of high glucose-induced endothelial nitric oxide synthase uncoupling by tanshinone IIA in human endothelial cell line EA.hy926

Endothelial nitric oxide synthase (eNOS) uncoupling plays a causal role in endothelial dysfunction in many cardiovascular and metabolic diseases. Tanshinone IIA (Tan IIA), an active compound from Salvia miltiorrhiza, has been used to treat cardiovascular and metabolic diseases. However, the effects of Tan IIA on eNOS uncoupling have not been reported. We hypothesize that Tan IIA can regulate eNOS uncoupling in endothelium cells under oxidative stress. The results showed that eNOS-mediated NO generation was significantly decreased, accompanied by increased superoxide production and NOX4 expression. The ratio of eNOS dimer to monomer and NOS cofactor tetrahydrobiopterin (BH4) to 7,8-dihydrobiopterin (BH2) as well as expressions of heat-shock protein of 90kDa (HSP90), GTP cyclohydrolase-1 (GTPCH1) and dihydrofolate reductase (DHFR) were significantly decreased. Tan IIA significantly inhibited superoxide production and expression of NOX4, and increased NO generation and eNOS homodimerization, as well as expressions of HSP90, GTPCH1 and DHFR in a concentration-dependent manner. The ratio of BH4 to BH2 was also elevated by Tan IIA. In addition, Tan IIA significantly inhibited the increase in expression of PI3K in high glucose treated cells. Wortmannin, a PI3K inhibitor, significantly inhibited the high glucose induced NOX4 expression. The results demonstrated that Tan IIA restored eNOS uncoupling induced by high glucose by targeting NADPH oxidase, HSP90, GTPCH1 and DHFR, and PI3K pathway, which leads to reduced intracellular oxidative stress and increased NO generation. Tan IIA may be used as a prototype agent to restore eNOS coupling under certain cardiovascular and metabolic diseases.

Hypoxic upregulation of arginase II in human lung endothelial cells

Activated arginase has been implicated in many diseases including cancer, immune cell dysfunction, infections, and vascular disease. Enhanced arginase activity has been reported in lungs of patients with pulmonary artery hypertension. We used hypoxia as a model for pulmonary hypertension and studied the effect of exposure to hypoxia on arginase activity in human lung microvascular endothelial cells (HMVEC). Hypoxia induces upregulation of arginase activity as well as mRNA and protein levels of arginase II (Arg II), the only arginase isoform we were able to identify in HMVEC. In endothelial cells, arginase shares and competes for the substrate l-arginine with nitric oxide (NO) synthase (NOS). Through regulation of substrate availability for NOS, arginase is able to modulate NO production. To evaluate the role of Arg II in regulation of NO production under hypoxia, we compared NO output (RFL-6 reporter assay) in cells with normal and silenced Arg II. Exposure to hypoxia led to an increase in NO levels produced by HMVEC. Inhibition of Arg II by specific small interfering RNA or by the pharmacological inhibitor BEC additionally enhanced the levels of NO. Another possible role for activated arginase is involvement in regulation of cell proliferation. However, we showed that hypoxia decreased cell proliferation and upregulated Arg II did not have an effect on cell proliferation. Since hypoxia-inducible factors (HIF) are a family of transcriptional factors activated by hypoxia, we tested the possibility of involvement of HIF-1 and HIF-2 in regulation of Arg II under hypoxia. The silencing of HIF-2 but not HIF-1 prevented the activation of Arg II by hypoxia.

Hyperthermia-induced Hsp90·eNOS preserves mitochondrial respiration in hyperglycemic endothelial cells by down-regulating Glut-1 and up-regulating G6PD activity

Uncoupling of NO production from NADPH oxidation by endothelial nitric-oxide synthase (eNOS) is enhanced in hyperglycemic endothelium, potentially due to dissociation of heat shock proteins 90 (Hsp90), and cellular glucose homeostasis is enhanced by a ROS-induced positive feed back mechanism. In this study we investigated how such an uncoupling impacts oxygen metabolism and how the oxidative phosphorylation can be preserved by heat shock (42 °C for 2 h, hyperthermia) in bovine aortic endothelial cells. Normal and heat-shocked bovine aortic endothelial cells were exposed to normoglycemia (NG, 5.0 mM) or hyperglycemia (30 mM). With hyperglycemia treatment, O(2) consumption rate was reduced (from V(O(2)max) = 7.51 ± 0.54 to 2.35 ± 0.27 mm Hg/min/10(6) cells), whereas in heat-shocked cells, O(2) consumption rate remained unaltered (8.19 ± 1.01 mm Hg/min/10 × 10(6) cells). Heat shock was found to enhance Hsp90/endothelial NOS interactions and produce higher NO. Moreover, ROS generation in the hyperglycemic condition was also reduced in heat-shocked cells. Interestingly, glucose uptake was reduced in heat-shocked cells as a result of decrease in Glut-1 protein level. Glucose phosphate dehydrogenase activity that gives rise to NADPH generation was increased by hyperthermia, and mitochondrial oxidative metabolism was preserved. In conclusion, the present study provides a novel mechanism wherein the reduced oxidative stress in heat-shocked hyperglycemic cells down-regulates Glut-1 and glucose uptake, and fine-tuning of this pathway may be a potential approach to use for therapeutic benefit of diabetes mellitus.

Characterization of the L-arginine-NO-cGMP pathway in spontaneously hypertensive rat platelets: the effects of pregnancy

Nitric oxide (NO) is a short-lived intercellular messenger that provides an efficient vascular regulatory mechanism to support homeostasis and prevent thrombosis. Endothelial dysfunction and reduced NO bioavailability have a central role in hypertension associated with pregnancy. The purpose of this study was to investigate the impact of pregnancy on the L-arginine-NO-cGMP pathway in platelets and its correlation to platelet function and blood pressure in normotensive rats and spontaneously hypertensive rats (SHRs). Platelets were obtained from blood on the 20th day of pregnancy from female SHRs (SHR-P) and normotensive controls (P) or age-matched nonpregnant rats (SHR-NP and NP). Intraplatelet NO synthase (NOS) activity was reduced in P compared to NP, despite unchanged L-arginine influx. The expression levels of endothelial NOS (eNOS) and inducible NOS (iNOS) were diminished during pregnancy in normotensive rats. Paradoxically, cyclic guanosine monophosphate (cGMP) levels were similar between NP and P, as were phosphodiesterase type 5 (PDE5) expression and platelet aggregation induced by adenosine diphosphate. In SHRs, L-arginine influx was reduced in SHR-P compared to SHR-NP. SHR-P exhibited impaired NOS activity and reduced iNOS expression compared with SHR-NP. Soluble guanylyl cyclase and PDE5 expression in platelets were lower in SHR-P than in SHR-NP, whereas no differences were noted between groups with respect to cGMP levels. However, increased levels of cGMP were observed in SHR-P compared to normotensive groups and platelet aggregability remained unaltered. In conclusion, these observations prompted the hypothesis that normal platelet aggregation in pregnant SHRs may be related to a reduction in PDE5 expression and consequently the maintenance of cGMP levels, independently of reduced platelet NO bioavailability.

Activation of Hsp90/NOS and increased NO generation does not impair mitochondrial respiratory chain by competitive binding at cytochrome c oxidase in low oxygen concentrations

Nitric oxide (NO) is known to regulate mitochondrial respiration, especially during metabolic stress and disease, by nitrosation of the mitochondrial electron transport chain (ETC) complexes (irreversible) and by a competitive binding at O2 binding site of cytochrome c oxidase (CcO) in complex IV (reversible). In this study, by using bovine aortic endothelial cells, we demonstrate that the inhibitory effect of endogenously generated NO by nitric oxide synthase (NOS) activation, by either NOS stimulators or association with heat shock protein 90 (Hsp90), is significant only at high prevailing pO2 through nitrosation of mitochondrial ETC complexes, but it does not inhibit the respiration by competitive binding at CcO at very low pO2. ETC complexes activity measurements confirmed that significant reduction in complex IV activity was noticed at higher pO2, but it was unaffected at low pO2 in these cells. This was further extended to heat-shocked cells, where NOS was activated by the induction/activation of (Hsp90) through heat shock at an elevated temperature of 42 degrees C. From these results, we conclude that the entire attenuation of respiration by endogenous NO is due to irreversible inhibition by nitrosation of ETC complexes but not through reversible inhibition by competing with O2 binding at CcO at complex IV.

Vitagenes, cellular stress response, and acetylcarnitine: relevance to hormesis

Modulation of endogenous cellular defense mechanisms via the stress response signaling represents an innovative approach to therapeutic intervention in diseases causing chronic damage, such as neurodegeneration and cancer. Protein thiols play a key role in redox sensing, and regulation of cellular redox state is crucial mediator of multiple metabolic, signaling, and transcriptional processes. Maintenance of optimal long-term health conditions is accomplished by a complex network of longevity assurance processes that are controlled by vitagenes, a group of genes involved in preserving cellular homeostasis during stressful conditions. Vitagenes encode for heat shock proteins (Hsp) Hsp32, Hsp70, the thioredoxin, and the sirtuin protein systems. Dietary antioxidants, such as polyphenols and L-carnitine/acetyl-L-carnitine, have recently been demonstrated to be neuroprotective through the activation of hormetic pathways, including vitagenes. The hormetic dose-response, challenges long-standing beliefs about the nature of the dose-response in a low dose zone, having the potential to affect significantly the design of pre-clinical studies and clinical trials as well as strategies for optimal patient dosing in the treatment of numerous diseases. Given the broad cytoprotective properties of the heat shock response, there is now strong interest in discovering and developing pharmacological agents capable of inducing these responses. In this review we discuss the most current and up-to-date understanding of the possible signaling mechanisms by which acetylcarnitine by activating vitagenes can differentially modulate signal transduction cascades inducing apoptosis/cell death in abnormal cancer cells but at the same time enhancing defensive enzymes to protect against carcinogenesis and neurodegeneration in normal cells. (c) 2009 International Union of Biochemistry and Molecular Biology, Inc.