Neutral sphingomyelinase, NADPH oxidase and reactive oxygen species. Role in acute hypoxic pulmonary vasoconstriction

Sphingolipids and Atherosclerosis: The Dual Role of Ceramide and Sphingosine-1-Phosphate

Sphingolipids are bioactive molecules that play either pro- and anti-atherogenic roles in the formation and maturation of atherosclerotic plaques. Among SLs, ceramide and sphingosine-1-phosphate showed antithetic properties in regulating various molecular mechanisms and have emerged as novel potential targets for regulating the development of atherosclerosis. In particular, maintaining the balance of the so-called ceramide/S1P rheostat is important to prevent the occurrence of endothelial dysfunction, which is the trigger for the entire atherosclerotic process and is strongly associated with increased oxidative stress. In addition, these two sphingolipids, together with many other sphingolipid mediators, are directly involved in the progression of atherogenesis and the formation of atherosclerotic plaques by promoting the oxidation of low-density lipoproteins (LDL) and influencing the vascular smooth muscle cell phenotype. The modulation of ceramide and S1P levels may therefore allow the development of new antioxidant therapies that can prevent or at least impair the onset of atherogenesis, which would ultimately improve the quality of life of patients with coronary artery disease and significantly reduce their mortality.

Mitochondrial oxygen sensing of acute hypoxia in specialized cells - Is there a unifying mechanism?

Acclimation to acute hypoxia through cardiorespiratory responses is mediated by specialized cells in the carotid body and pulmonary vasculature to optimize systemic arterial oxygenation and thus oxygen supply to the tissues. Acute oxygen sensing by these cells triggers hyperventilation and hypoxic pulmonary vasoconstriction which limits pulmonary blood flow through areas of low alveolar oxygen content. Oxygen sensing of acute hypoxia by specialized cells thus is a fundamental pre-requisite for aerobic life and maintains systemic oxygen supply. However, the primary oxygen sensing mechanism and the question of a common mechanism in different specialized oxygen sensing cells remains unresolved. Recent studies unraveled basic oxygen sensing mechanisms involving the mitochondrial cytochrome c oxidase subunit 4 isoform 2 that is essential for the hypoxia-induced release of mitochondrial reactive oxygen species and subsequent acute hypoxic responses in both, the carotid body and pulmonary vasculature. This review compares basic mitochondrial oxygen sensing mechanisms in the pulmonary vasculature and the carotid body.

Berberine reduces hepatic ceramide levels to improve insulin resistance in HFD-fed mice by inhibiting HIF-2α

Several studies have documented the effects of hypoxia and ceramides on lipid and glucose metabolism, resulting in insulin resistance. However, the roles of ceramide in hepatic hypoxia and hepatic insulin resistance remain to be clarified. This study aimed to explore the relationship between hypoxia, ceramide synthesis, and hepatic insulin resistance in high-fat diet (HFD)-fed mice. Given the interaction of hypoxia-inducible factors 2α(HIF-2α) and berberine determined using molecular docking, this study also assessed the pharmacological effects of berberine on the HIF-2α-ceramide-insulin resistance pathway. In the preliminary phase of the study, gradually aggravated hepatic hypoxia and varying levels of ceramides were observed with the development of type 2 diabetes mellitus (T2DM) due to increasing HIF-2α accumulation. Lipidomic analyses of animal and cell models revealed that berberine reduced hypoxia-induced ceramide production and attenuated ceramide-induced insulin resistance. This research provides timely and necessary evidence for the role of ceramide in hypoxia and insulin resistance in the liver. It also contributes to a better understanding of the pharmacological effects of berberine on ameliorating hypoxia and insulin resistance in T2DM therapy.

Cardiac Fibroblasts Promote Ferroptosis in Atrial Fibrillation by Secreting Exo-miR-23a-3p Targeting SLC7A11

The exact mechanism of atrial fibrillation (AF) has been not well elucidated. Ferroptosis is an iron-dependent cell death due to excessive accumulation of peroxidized polyunsaturated fatty acids. However, the molecular mechanism underlying AF and ferroptosis has never been reported. Here, we established the rapid pacing model in vivo and vitro to investigate the relationship between AF and ferroptosis. In canine model of rapid atrial pacing, the content of malondialdehyde and total ions in the atrial tissue of the Pacing group was significantly increased and the exosome inhibitor GW4869 reduced ferroptosis, fibrosis, and inflammation and improved histological and electrophysiological remodeling. In rapid pacing h9c2 cells, the expression of antioxidative stress genes associated with ferroptosis presented sequential changes and proteins involved in ferroptosis such as FTH1, SLC7A11, and GPX4 were gradually depleted. Furthermore, pacing cardiac fibroblast-derived exosomes (CF-exos) exacerbated ferroptosis in h9c2 cells and pretreated pacing-CF-exos with GW4869 alleviated injury to h9c2 cells. In mechanism, our results demonstrated that pacing-CF-exos highly expressed miR-23a-3p by informatics analysis and experimental verification. Inhibitor-miR-23a-3p protected h9c2 cells from ferroptosis accompanying with upregulation of SLC7A11. In addition, SLC7A11 was shown to be the target gene of miR-23a-3p. In conclusion, our results suggest that CF-exos-miR-23a-3p may promote ferroptosis. The development of AF in a persistent direction could be prevented by intervening with exosomal miRNAs to reduce oxidative stress injury and ferroptosis.

Acetazolamide prevents hypoxia-induced reactive oxygen species generation and calcium release in pulmonary arterial smooth muscle

Upon sensing a reduction in local oxygen partial pressure, pulmonary vessels constrict, a phenomenon known as hypoxic pulmonary vasoconstriction. Excessive hypoxic pulmonary vasoconstriction can occur with ascent to high altitude and is a contributing factor to the development of high-altitude pulmonary edema. The carbonic anhydrase inhibitor, acetazolamide, attenuates hypoxic pulmonary vasoconstriction through stimulation of alveolar ventilation via modulation of acid-base homeostasis and by direct effects on pulmonary vascular smooth muscle. In pulmonary arterial smooth muscle cells (PASMCs), acetazolamide prevents hypoxia-induced increases in intracellular calcium concentration ([Ca²⁺]_i), although the exact mechanism by which this occurs is unknown. In this study, we explored the effect of acetazolamide on various calcium-handling pathways in PASMCs. Using fluorescent microscopy, we tested whether acetazolamide directly inhibited store-operated calcium entry or calcium release from the sarcoplasmic reticulum, two well-documented sources of hypoxia-induced increases in [Ca²⁺]_i in PASMCs. Acetazolamide had no effect on calcium entry stimulated by store-depletion, nor on calcium release from the sarcoplasmic reticulum induced by either phenylephrine to activate inositol triphosphate receptors or caffeine to activate ryanodine receptors. In contrast, acetazolamide completely prevented Ca²⁺-release from the sarcoplasmic reticulum induced by hypoxia (4% O₂). Since these results suggest the acetazolamide interferes with a mechanism upstream of the inositol triphosphate and ryanodine receptors, we also determined whether acetazolamide might prevent hypoxia-induced changes in reactive oxygen species production. Using roGFP, a ratiometric reactive oxygen species-sensitive fluorescent probe, we found that hypoxia caused a significant increase in reactive oxygen species in PASMCs that was prevented by 100 μM acetazolamide. Together, these results suggest that acetazolamide prevents hypoxia-induced changes in [Ca²⁺]_i by attenuating reactive oxygen species production and subsequent activation of Ca²⁺-release from sarcoplasmic reticulum stores.

Potassium (K+) channels in the pulmonary vasculature: Implications in pulmonary hypertension Physiological, pathophysiological and pharmacological regulation

The large K⁺ channel functional diversity in the pulmonary vasculature results from the multitude of genes expressed encoding K⁺ channels, alternative RNA splicing, the post-transcriptional modifications, the presence of homomeric or heteromeric assemblies of the pore-forming α-subunits and the existence of accessory β-subunits modulating the functional properties of the channel. K⁺ channels can also be regulated at multiple levels by different factors controlling channel activity, trafficking, recycling and degradation. The activity of these channels is the primary determinant of membrane potential (Em) in pulmonary artery smooth muscle cells (PASMC), providing an essential regulatory mechanism to dilate or contract pulmonary arteries (PA). K⁺ channels are also expressed in pulmonary artery endothelial cells (PAEC) where they control resting Em, Ca²⁺ entry and the production of different vasoactive factors. The activity of K⁺ channels is also important in regulating the population and phenotype of PASMC in the pulmonary vasculature, since they are involved in cell apoptosis, survival and proliferation. Notably, K⁺ channels play a major role in the development of pulmonary hypertension (PH). Impaired K⁺ channel activity in PH results from: 1) loss of function mutations, 2) downregulation of its expression, which involves transcription factors and microRNAs, or 3) decreased channel current as a result of increased vasoactive factors (e.g., hypoxia, 5-HT, endothelin-1 or thromboxane), exposure to drugs with channel-blocking properties, or by a reduction in factors that positively regulate K⁺ channel activity (e.g., NO and prostacyclin). Restoring K⁺ channel expression, its intracellular trafficking and the channel activity is an attractive therapeutic strategy in PH.

The imbalance in the aortic ceramide/sphingosine-1-phosphate rheostat in ovariectomized rats and the preventive effect of estrogen

Background

The prevalence of hypertension in young women is lower than that in age-matched men while the prevalence of hypertension in women is significantly increased after the age of 50 (menopause) and is greater than that in men. It is already known that sphingosine-1-phosphate (S1P) and ceramide regulate vascular tone with opposing effects. This study aimed to explore the effects of ovariectomy and estrogen supplementation on the ceramide/S1P rheostat of the aorta in rats, and to explore a potential mechanism for perimenopausal hypertension and a brand-new target for menopausal hormone therapy to protect vessels.

Methods

In total, 30 female adult SD rats were randomly divided into three groups: The sham operation group (SHAM), ovariectomy group (OVX) and ovariectomy plus estrogen group (OVX + E). After 4 weeks of treatment, the blood pressure (BP) of the rats was monitored by a noninvasive system; the sphingolipid content (e.g., ceramide and S1P) was detected by liquid chromatography-mass spectrometry (LC-MS); the expression of the key enzymes involved in ceramide anabolism and catabolism was measured by real-time fluorescence quantitative polymerase chain reaction (qPCR); and the expression of key enzymes and proteins in the sphingosine kinase 1/2 (SphK1/2)-S1P-S1P receptor 1/2/3 (S1P1/2/3) signaling pathway was detected by qPCR and western blotting.

Results

In the OVX group compared with the SHAM group, the systolic BP (SBP), diastolic BP (DBP) and pulse pressure (PP) increased significantly, especially the SBP and PP (P < 0.001). For aortic ceramide metabolism, the mRNA level of key enzymes involved in anabolism and catabolism decreased in parallel 2–3 times, while the contents of total ceramide and certain long-chain subtypes increased significantly (P < 0.05). As for the S1P signaling pathway, SphK1/2, the key enzymes involved in S1P synthesis, decreased significantly, and the content of S1P decreased accordingly (P < 0.01). The S1P receptors showed various trends: S1P1 was significantly down-regulated, S1P2 was significantly up-regulated, and S1P3 showed no significant difference. No significant difference existed between the SHAM and OVX + E groups for most of the above parameters (P > 0.05).

Conclusions

Ovariectomy resulted in the imbalance of the aortic ceramide/S1P rheostat in rats, which may be a potential mechanism underlying the increase in SBP and PP among perimenopausal women. Besides, the ceramide/S1P rheostat may be a novel mechanism by which estrogen protects vessels.

New role for ceramide in hypoxia and insulin resistance

Ceramides are significant metabolic products of sphingolipids in lipid metabolism and are associated with insulin resistance and hepatic steatosis. In chronic inflammatory pathological conditions, hypoxia occurs, the metabolism of ceramide changes, and insulin resistance arises. Hypoxia-inducible factors (HIFs) are a family of transcription factors activated by hypoxia. In hypoxic adipocytes, HIF-1α upregulates pla2g16 (a novel HIF-1α target gene) gene expression to activate the NLRP3 inflammasome pathway and stimulate insulin resistance, and adipocyte-specific Hif1a knockout can ameliorate homocysteine-induced insulin resistance in mice. The study on the HIF-2α—NEU3—ceramide pathway also reveals the role of ceramide in hypoxia and insulin resistance in obese mice. Under obesity-induced intestinal hypoxia, HIF-2α increases the production of ceramide by promoting the expression of the gene Neu3 encoding sialidase 3, which is a key enzyme in ceramide synthesis, resulting in insulin resistance in high-fat diet-induced obese mice. Moreover, genetic and pathophysiologic inhibition of the HIF-2α—NEU3—ceramide pathway can alleviate insulin resistance, suggesting that these could be potential drug targets for the treatment of metabolic diseases. Herein, the effects of hypoxia and ceramide, especially in the intestine, on metabolic diseases are summarized.

Neutral Sphingomyelinase Modulation in the Protective/Preventive Role of rMnSOD from Radiation-Induced Damage in the Brain

Studies on the relationship between reactive oxygen species (ROS)/manganese superoxide dismutase (MnSOD) and sphingomyelinase (SMase) are controversial. It has been demonstrated that SMase increases the intracellular ROS level and induces gene expression for MnSOD protein. On the other hand, some authors showed that ROS modulate the activation of SMase. The human recombinant manganese superoxide dismutase (rMnSOD) exerting a radioprotective effect on normal cells, qualifies as a possible pharmaceutical tool to prevent and/or cure damages derived from accidental exposure to ionizing radiation. This study aimed to identify neutral SMase (nSMase) as novel molecule connecting rMnSOD to its radiation protective effects. We used a new, and to this date, unique, experimental model to assess the effect of both radiation and rMnSOD in the brain of mice, within a collaborative project among Italian research groups and the Joint Institute for Nuclear Research, Dubna (Russia). Mice were exposed to a set of minor γ radiation and neutrons and a spectrum of neutrons, simulating the radiation levels to which cosmonauts will be exposed during deep-space, long-term missions. Groups of mice were treated or not-treated (controls) with daily subcutaneous injections of rMnSOD during a period of 10 days. An additional group of mice was also pretreated with rMnSOD for three days before irradiation, as a model for preventive measures. We demonstrate that rMnSOD significantly protects the midbrain cells from radiation-induced damage, inducing a strong upregulation of nSMase gene and protein expression. Pretreatment with rMnSOD before irradiation protects the brain with a value of very high nSMase activity, indicating that high levels of activity might be sufficient to exert the rMnSOD preventive role. In conclusion, the protective effect of rMnSOD from radiation-induced brain damage may require nSMase enzyme.

Reactive oxygen species as mediators of oxygen signaling during fetal-to-neonatal circulatory transition

Reactive oxygen species (ROS) are frequently seen as pathological agents of oxidative stress. However, ROS are not always deleterious and can also act as cell signaling molecules. Vascular oxygen sensing and signaling during fetal-to-neonatal circulatory transition is a remarkable example of the physiological regulatory actions of ROS. The fetal relative hypoxic environment induces hypoxic pulmonary vasoconstriction (HPV) and ductus arteriosus (DA) relaxation favoring the presence of high pulmonary vascular resistance and right-to-left ductal shunt. At birth, the increase in oxygen tension causes relaxation of pulmonary arteries (PAs) and normoxic DA vasoconstriction (NDAV), thus diverting blood flow to the lungs. Although the response to changes in oxygen tension is diametrically opposite, the mechanisms responsible for HPV and NDAV appear to be the result of a similar interaction between triggering and modulating factors that lead to an increase in cytosolic Ca²⁺ concentration and Ca²⁺ sensitization of the contractile apparatus. Growing evidence points to an increase in ROS (mitochondria- and/or NADPH-derived superoxide and/or H₂O₂), leading to inhibition of voltage-gated K⁺ channels, membrane depolarization, and activation of voltage-gated L-type Ca²⁺ channels as critical events in the signaling pathway of both HPV and NDAV. Several groups of investigators have completed this pathway adding other elements such as neutral sphingomyelinase-derived ceramide, the sarcoplasmic/endoplasmic reticulum (through ryanodine and inositol 1,4,5-trisphosphate receptors), Rho kinase-mediated Ca²⁺ sensitization, or transient receptor potential channels. The present review focus on the role of ROS as mediators of the homeostatic oxygen sensing system during fetal and neonatal life not only in the PAs and DA but also in systemic arteries.

Ceramide and Regulation of Vascular Tone

In addition to playing a role as a structural component of cellular membranes, ceramide is now clearly recognized as a bioactive lipid implicated in a variety of physiological functions. This review aims to provide updated information on the role of ceramide in the regulation of vascular tone. Ceramide may induce vasodilator or vasoconstrictor effects by interacting with several signaling pathways in endothelial and smooth muscle cells. There is a clear, albeit complex, interaction between ceramide and redox signaling. In fact, reactive oxygen species (ROS) activate different ceramide generating pathways and, conversely, ceramide is known to increase ROS production. In recent years, ceramide has emerged as a novel key player in oxygen sensing in vascular cells and mediating vascular responses of crucial physiological relevance such as hypoxic pulmonary vasoconstriction (HPV) or normoxic ductus arteriosus constriction. Likewise, a growing body of evidence over the last years suggests that exaggerated production of vascular ceramide may have detrimental effects in a number of pathological processes including cardiovascular and lung diseases.

Riociguat versus sildenafil on hypoxic pulmonary vasoconstriction and ventilation/perfusion matching

INTRODUCTION

Current treatment with vasodilators for pulmonary hypertension associated with respiratory diseases is limited by their inhibitory effect on hypoxic pulmonary vasoconstriction (HPV) and uncoupling effects on ventilation-perfusion (V'/Q'). Hypoxia is also a well-known modulator of the nitric oxide (NO) pathway, and may therefore differentially affect the responses to phosphodiesterase 5 (PDE5) inhibitors and soluble guanylyl cyclase (sGC) stimulators. So far, the effects of the sGC stimulator riociguat on HPV have been poorly characterized.

MATERIALS AND METHODS

Contraction was recorded in pulmonary arteries (PA) in a wire myograph. Anesthetized rats were catheterized to record PA pressure. Ventilation and perfusion were analyzed by micro-CT-SPECT images in rats with pulmonary fibrosis induced by bleomycin.

RESULTS

The PDE5 inhibitor sildenafil and the sGC stimulator riociguat similarly inhibited HPV in vitro and in vivo. Riociguat was more effective as vasodilator in isolated rat and human PA than sildenafil. Riociguat was ≈3-fold more potent under hypoxic conditions and it markedly inhibited HPV in vivo at a dose that barely affected the thromboxane A2 (TXA2) mimetic U46619-induced pressor responses. Pulmonary fibrosis was associated with V'/Q' uncoupling and riociguat did not affect the V'/Q' ratio.

CONCLUSION

PDE5 inhibitors and sGC stimulators show a different vasodilator profile. Riociguat was highly effective and potentiated by hypoxia in rat and human PA. In vivo, riociguat preferentially inhibited hypoxic than non-hypoxic vasoconstriction. However, it did not worsen V'/Q' coupling in a rat model of pulmonary fibrosis.

The endothelium in hypoxic pulmonary vasoconstriction

Hypoxic pulmonary vasoconstriction (HPV) in combination with hypercapnic pulmonary vasoconstriction redistributes pulmonary blood flow from poorly aerated to better ventilated lung regions by an active process of local vasoconstriction. Impairment of HPV results in ventilation-perfusion mismatch and is commonly associated with various lung diseases including pneumonia, sepsis, or cystic fibrosis. Although several regulatory pathways have been identified, considerable knowledge gaps persist, and a unifying concept of the signaling pathways that underlie HPV and their impairment in lung diseases has not yet emerged. In the past, conceptual models of HPV have focused on pulmonary arterial smooth muscle cells (PASMC) acting as sensor and effector of hypoxia in the pulmonary vasculature. In contrast, the endothelium was considered a modulating bystander in this scenario. For an ideal design, however, the oxygen sensor in HPV should be located in the region of gas exchange, i.e., in the alveolar capillary network. This concept requires the retrograde propagation of the hypoxic signal along the endothelial layer of the vascular wall and subsequent contraction of PASMC in upstream arterioles that is elicited via temporospatially tightly controlled endothelial-smooth muscle cell crosstalk. The present review summarizes recent work that provides proof-of-principle for the existence and functional relevance of such signaling pathway in HPV that involves important roles for connexin 40, epoxyeicosatrienoic acids, sphingolipids, and cystic fibrosis transmembrane conductance regulator. Of translational relevance, implication of these molecules provides for novel mechanistic explanations for impaired ventilation/perfusion matching in patients with pneumonia, sepsis, cystic fibrosis, and presumably various other lung diseases.

Recent advances in oxygen sensing and signal transduction in hypoxic pulmonary vasoconstriction

Hypoxic pulmonary vasoconstriction (HPV) is a physiological reaction, which adapts lung perfusion to regional ventilation and optimizes gas exchange. Impaired HPV may cause systemic hypoxemia, while generalized HPV contributes to the development of pulmonary hypertension. The triggering mechanisms underlying HPV are still not fully elucidated. Several hypotheses are currently under debate, including a possible decrease as well as an increase in reactive oxygen species as a triggering event. Recent findings suggest an increase in the production of reactive oxygen species in pulmonary artery smooth muscle cells by complex III of the mitochondrial electron transport chain and occurrence of oxygen sensing at complex IV. Other essential components are voltage-dependent potassium and possibly L-type, transient receptor potential channel 6, and transient receptor potential vanilloid 4 channels. The release of arachidonic acid metabolites appears also to be involved in HPV regulation. Further investigation of the HPV mechanisms will facilitate the development of novel therapeutic strategies for the treatment of HPV-related disorders.

Guinea Pig Oxygen-Sensing and Carotid Body Functional Properties

Mammals have developed different mechanisms to maintain oxygen supply to cells in response to hypoxia. One of those mechanisms, the carotid body (CB) chemoreceptors, is able to detect physiological hypoxia and generate homeostatic reflex responses, mainly ventilatory and cardiovascular. It has been reported that guinea pigs, originally from the Andes, have a reduced ventilatory response to hypoxia compared to other mammals, implying that CB are not completely functional, which has been related to genetically/epigenetically determined poor hypoxia-driven CB reflex. This study was performed to check the guinea pig CB response to hypoxia compared to the well-known rat hypoxic response. These experiments have explored ventilatory parameters breathing different gases mixtures, cardiovascular responses to acute hypoxia, in vitro CB response to hypoxia and other stimuli and isolated guinea pig chemoreceptor cells properties. Our findings show that guinea pigs are hypotensive and have lower arterial pO₂ than rats, probably related to a low sympathetic tone and high hemoglobin affinity. Those characteristics could represent a higher tolerance to hypoxic environment than other rodents. We also find that although CB are hypo-functional not showing chronic hypoxia sensitization, a small percentage of isolated carotid body chemoreceptor cells contain tyrosine hydroxylase enzyme and voltage-dependent K⁺ currents and therefore can be depolarized. However hypoxia does not modify intracellular Ca²⁺ levels or catecholamine secretion. Guinea pigs are able to hyperventilate only in response to intense acute hypoxic stimulus, but hypercapnic response is similar to rats. Whether other brain areas are also activated by hypoxia in guinea pigs remains to be studied.

Temporal Cross Talk Between Endoplasmic Reticulum and Mitochondria Regulates Oxidative Stress and Mediates Microparticle-Induced Endothelial Dysfunction

AIMS

Circulating microparticles (MPs) from metabolic syndrome patients and those generated from apoptotic T cells induce endothelial dysfunction; however, the molecular and cellular mechanism(s) underlying in the effects of MPs remain to be elucidated.

RESULTS

Here, we show that both types of MPs increased expression of endoplasmic reticulum (ER) stress markers, X-box binding protein 1, p-eukaryotic translation initiation factor 2 α, and CHOP, and nuclear translocation of activating transcription factor 6 on human aortic endothelial cells (HAoECs). MPs decreased in vitro nitric oxide release by HAoECs, whereas in vivo MP injection into mice impaired the endothelium-dependent relaxation induced by acetylcholine. These effects were prevented when ER stress was inhibited, suggesting that ER stress is implicated in the endothelial effects induced by MPs. MPs affected mitochondrial function and evoked sequential increase of cytosolic and mitochondrial reactive oxygen species (ROS). Pharmacological inhibition of ER stress and silencing of neutral sphingomyelinase (SMase) with siRNA abrogated all MP-mediated effects. Neutralization of Fas ligand carried by MPs abolished effects induced by both MP types, whereas neutralization of low-density lipoprotein receptor on endothelial cells prevented T-lymphocyte MP-mediated effects. Innovation and Conclusion: Collectively, endothelial dysfunction triggered by MPs involves temporal cross talk between ER and mitochondria with respect to spatial regulation of ROS via the neutral SMase and interaction of MPs with Fas and/or low-density lipoprotein receptor. These results provide a novel molecular insight into the manner MPs mediate vascular dysfunction and allow identification of potential therapeutic targets to treat vascular complications associated with metabolic syndrome. Antioxid. Redox Signal. 26, 15-27.

Effects of Quercetin in a Rat Model of Hemorrhagic Traumatic Shock and Reperfusion

Background: We hypothesized that treatment with quercetin could result in improved hemodynamics, lung inflammatory parameters and mortality in a rat model of hemorrhagic shock. Methods: Rats were anesthetized (80 mg/kg ketamine plus 8 mg/kg xylazine i.p.). The protocol included laparotomy for 15 min (trauma), hemorrhagic shock (blood withdrawal to reduce the mean arterial pressure to 35 mmHg) for 75 min and resuscitation by re-infusion of all the shed blood plus lactate Ringer for 90 min. Intravenous quercetin (50 mg/kg) or vehicle were administered during resuscitation. Results: There was a trend for increased survival 84.6% (11/13) in the treated group vs. the shock group 68.4% (13/19, p > 0.05 Kaplan–Meier). Quercetin fully prevented the development of lung edema. The activity of aSMase was increased in the shock group compared to the sham group and the quercetin prevented this effect. However, other inflammatory markers such as myeloperoxidase activity, interleukin-6 in plasma or bronchoalveolar fluid were similar in the sham and shock groups. We found no bacterial DNA in plasma in these animals. Conclusions: Quercetin partially prevented the changes in blood pressure and lung injury in shock associated to hemorrhage and reperfusion.

Hypoxia-induced contraction of chicken embryo mesenteric arteries: mechanisms and developmental changes

The fetal cardiovascular responses to acute hypoxia include a redistribution of the cardiac output toward the heart and the brain at the expense of other organs, such as the intestine. We hypothesized that hypoxia exerts a direct effect on the mesenteric artery (MA) that may contribute to this response. Using wire myography, we investigated the response to hypoxia (Po2 ~2.5 kPa for 20 min) of isolated MAs from 15- to 21-day chicken embryos (E15, E19, E21), and 1- to 45-day-old chickens (P1, P3, P14, P45). Agonist-induced pretone or an intact endothelium were not required to obtain a consistent and reproducible response to hypoxia, which showed a pattern of initial rapid phasic contraction followed by a sustained tonic contraction. Phasic contraction was reduced by elimination of extracellular Ca2+ or by presence of the neurotoxin tetrodotoxin, the α1-adrenoceptor antagonist prazosin, or inhibitors of L-type voltage-gated Ca2+ channels (nifedipine), mitochondrial electron transport chain (rotenone and antimycin A), and NADPH oxidase (VAS2870). The Rho-kinase inhibitor Y27632 impaired both phasic and tonic contraction and, when combined with elimination of extracellular Ca2+, hypoxia-induced contraction was virtually abolished. Hypoxic MA contraction was absent at E15 but present from E19 and increased toward the first days posthatching. It then decreased during the first weeks of life and P45 MAs were unable to sustain hypoxia-induced contraction over time. In conclusion, the results of the present study demonstrate that hypoxic vasoconstriction is an intrinsic feature of chicken MA vascular smooth muscle cells during late embryogenesis and the perinatal period.

Activation of PPARβ/δ prevents hyperglycaemia-induced impairment of Kv7 channels and cAMP-mediated relaxation in rat coronary arteries

PPARβ/δ activation protects against endothelial dysfunction in diabetic models. Elevated glucose is known to impair cAMP-induced relaxation and Kv channel function in coronary arteries (CA). Herein, we aimed to analyse the possible protective effects of the PPARβ/δ agonist GW0742 on the hyperglycaemic-induced impairment of cAMP-induced relaxation and Kv channel function in rat CA. As compared with low glucose (LG), incubation under high glucose (HG) conditions attenuated the relaxation induced by the adenylate cyclase activator forskolin in CA and this was prevented by GW0742. The protective effect of GW0742 was supressed by a PPARβ/δ antagonist. In myocytes isolated from CA under LG, forskolin enhanced Kv currents and induced hyperpolarization. In contrast, when CA were incubated with HG, Kv currents were diminished and the electrophysiological effects of forskolin were abolished. These deleterious effects were prevented by GW0742. The protective effects of GW0742 on forskolin-induced relaxation and Kv channel function were confirmed in CA from type-1 diabetic rats. In addition, the differences in the relaxation induced by forskolin in CA incubated under LG, HG or HG + GW0742 were abolished by the Kv7 channel inhibitor XE991. Accordingly, GW0742 prevented the down-regulation of Kv7 channels induced by HG. Finally, the preventive effect of GW0742 on oxidative stress and cAMP-induced relaxation were overcome by the pyruvate dehydrogenase kinase 4 (PDK4) inhibitor dichloroacetate (DCA). Our results reveal that the PPARβ/δ agonist GW0742 prevents the impairment of the cAMP-mediated relaxation in CA under HG. This protective effect was associated with induction of PDK4, attenuation of oxidative stress and preservation of Kv7 channel function.

Oxygen sensing and signal transduction in hypoxic pulmonary vasoconstriction

Hypoxic pulmonary vasoconstriction (HPV), also known as the von Euler-Liljestrand mechanism, is an essential response of the pulmonary vasculature to acute and sustained alveolar hypoxia. During local alveolar hypoxia, HPV matches perfusion to ventilation to maintain optimal arterial oxygenation. In contrast, during global alveolar hypoxia, HPV leads to pulmonary hypertension. The oxygen sensing and signal transduction machinery is located in the pulmonary arterial smooth muscle cells (PASMCs) of the pre-capillary vessels, albeit the physiological response may be modulated in vivo by the endothelium. While factors such as nitric oxide modulate HPV, reactive oxygen species (ROS) have been suggested to act as essential mediators in HPV. ROS may originate from mitochondria and/or NADPH oxidases but the exact oxygen sensing mechanisms, as well as the question of whether increased or decreased ROS cause HPV, are under debate. ROS may induce intracellular calcium increase and subsequent contraction of PASMCs via direct or indirect interactions with protein kinases, phospholipases, sarcoplasmic calcium channels, transient receptor potential channels, voltage-dependent potassium channels and L-type calcium channels, whose relevance may vary under different experimental conditions. Successful identification of factors regulating HPV may allow development of novel therapeutic approaches for conditions of disturbed HPV.

Hypoxic pulmonary vasoconstriction, carotid body function and erythropoietin production in adult rats perinatally exposed to hyperoxia

KEY POINTS

Adult animals that have been perinatally exposed to oxygen-rich atmospheres (hyperoxia), recalling those used for oxygen therapy in infants, exhibit a loss of hypoxic pulmonary vasoconstriction, whereas vasoconstriction elicited by depolarizing agents is maintained. Loss of pulmonary hypoxic vasoconstriction is not linked to alterations in oxygen-sensitive K(+) currents in pulmonary artery smooth muscle cells. Loss of hypoxic vasoconstriction is associated with early postnatal oxidative damage and corrected by an antioxidant diet. Perinatal hyperoxia damages carotid body chemoreceptor cell function and the antioxidant diet does not reverse it. The hypoxia-elicited increase in erythropoietin plasma levels is not affected by perinatal hyperoxia. The potential clinical significance of the findings in clinical situations such as pneumonia, chronic obstructive pulmonary disease or general anaesthesia is considered.

ABSTRACT

Adult mammalians possess three cell systems that are activated by acute bodily hypoxia: pulmonary artery smooth muscle cells (PASMC), carotid body chemoreceptor cells (CBCC) and erythropoietin (EPO)-producing cells. In rats, chronic perinatal hyperoxia causes permanent carotid body (CB) atrophy and functional alterations of surviving CBCC. There are no studies on PASMC or EPO-producing cells. Our aim is to define possible long-lasting functional changes in PASMC or EPO-producing cells (measured as EPO plasma levels) and, further, to analyse CBCC functional alterations. We used 3- to 4-month-old rats born and reared in a normal atmosphere or exposed to perinatal hyperoxia (55-60% O2 for the last 5-6 days of pregnancy and 4 weeks after birth). Perinatal hyperoxia causes an almost complete loss of hypoxic pulmonary vasoconstriction (HPV), which was correlated with lung oxidative status in early postnatal life and prevented by antioxidant supplementation in the diet. O2 -sensitivity of K(+) currents in the PASMC of hyperoxic animals is normal, indicating that their inhibition is not sufficient to trigger HPV. Perinatal hyperoxia also abrogated responses elicited by hypoxia on catecholamine and cAMP metabolism in the CB. An increase in EPO plasma levels elicited by hypoxia was identical in hyperoxic and control animals, implying a normal functioning of EPO-producing cells. The loss of HPV observed in adult rats and caused by perinatal hyperoxia, comparable to oxygen therapy in premature infants, might represent a previously unrecognized complication of such a medical intervention capable of aggravating medical conditions such as regional pneumonias, atelectases or general anaesthesia in adult life.

CFTR and sphingolipids mediate hypoxic pulmonary vasoconstriction

Hypoxic pulmonary vasoconstriction (HPV) optimizes pulmonary ventilation-perfusion matching in regional hypoxia, but promotes pulmonary hypertension in global hypoxia. Ventilation-perfusion mismatch is a major cause of hypoxemia in cystic fibrosis. We hypothesized that cystic fibrosis transmembrane conductance regulator (CFTR) may be critical in HPV, potentially by modulating the response to sphingolipids as mediators of HPV. HPV and ventilation-perfusion mismatch were analyzed in isolated mouse lungs or in vivo. Ca(2+) mobilization and transient receptor potential canonical 6 (TRPC6) translocation were studied in human pulmonary (PASMCs) or coronary (CASMCs) artery smooth muscle cells. CFTR inhibition or deficiency diminished HPV and aggravated ventilation-perfusion mismatch. In PASMCs, hypoxia caused CFTR to interact with TRPC6, whereas CFTR inhibition attenuated hypoxia-induced TRPC6 translocation to caveolae and Ca(2+) mobilization. Ca(2+) mobilization by sphingosine-1-phosphate (S1P) was also attenuated by CFTR inhibition in PASMCs, but amplified in CASMCs. Inhibition of neutral sphingomyelinase (nSMase) blocked HPV, whereas exogenous nSMase caused TRPC6 translocation and vasoconstriction that were blocked by CFTR inhibition. nSMase- and hypoxia-induced vasoconstriction, yet not TRPC6 translocation, were blocked by inhibition or deficiency of sphingosine kinase 1 (SphK1) or antagonism of S1P receptors 2 and 4 (S1P2/4). S1P and nSMase had synergistic effects on pulmonary vasoconstriction that involved TRPC6, phospholipase C, and rho kinase. Our findings demonstrate a central role of CFTR and sphingolipids in HPV. Upon hypoxia, nSMase triggers TRPC6 translocation, which requires its interaction with CFTR. Concomitant SphK1-dependent formation of S1P and activation of S1P2/4 result in phospholipase C-mediated TRPC6 and rho kinase activation, which conjointly trigger vasoconstriction.

Roles and regulation of neutral sphingomyelinase-2 in cellular and pathological processes

Our understanding of the functions of ceramide signaling has advanced tremendously over the past decade. In this review, we focus on the roles and regulation of neutral sphingomyelinase 2 (nSMase2), an enzyme that generates the bioactive lipid ceramide through the hydrolysis of the membrane lipid sphingomyelin. A large body of work has now implicated nSMase2 in a diverse set of cellular functions, physiological processes, and disease pathologies. We discuss different aspects of this enzyme's regulation from transcriptional, post-translational, and biochemical. Furthermore, we highlight nSMase2 involvement in cellular processes including inflammatory signaling, exosome generation, cell growth, and apoptosis, which in turn play important roles in pathologies such as cancer metastasis, Alzheimer's disease, and other organ systems disorders. Lastly, we examine avenues where targeted nSMase2-inhibition may be clinically beneficial in disease scenarios.

Ceramide mediates acute oxygen sensing in vascular tissues

AIMS

A variety of vessels, such as resistance pulmonary arteries (PA) and fetoplacental arteries and the ductus arteriosus (DA) are specialized in sensing and responding to changes in oxygen tension. Despite opposite stimuli, normoxic DA contraction and hypoxic fetoplacental and PA vasoconstriction share some mechanistic features. Activation of neutral sphingomyelinase (nSMase) and subsequent ceramide production has been involved in hypoxic pulmonary vasoconstriction (HPV). Herein we aimed to study the possible role of nSMase-derived ceramide as a common factor in the acute oxygen-sensing function of specialized vascular tissues.

RESULTS

The nSMase inhibitor GW4869 and an anticeramide antibody reduced the hypoxic vasoconstriction in chicken PA and chorioallantoic arteries (CA) and the normoxic contraction of chicken DA. Incubation with interference RNA targeted to SMPD3 also inhibited HPV. Moreover, ceramide and reactive oxygen species production were increased by hypoxia in PA and by normoxia in DA. Either bacterial sphingomyelinase or ceramide mimicked the contractile responses of hypoxia in PA and CA and those of normoxia in the DA. Furthermore, ceramide inhibited voltage-gated potassium currents present in smooth muscle cells from PA and DA. Finally, the role of nSMase in acute oxygen sensing was also observed in human PA and DA.

INNOVATION

These data provide evidence for the proposal that nSMase-derived ceramide is a critical player in acute oxygen-sensing in specialized vascular tissues.

CONCLUSION

Our results indicate that an increase in ceramide generation is involved in the vasoconstrictor responses induced by two opposite stimuli, such as hypoxia (in PA and CA) and normoxia (in DA).

Sequestosome1/p62: a regulator of redox-sensitive voltage-activated potassium channels, arterial remodeling, inflammation, and neurite outgrowth

Sequestosome1/p62 (SQSTM1) is an oxidative stress-inducible protein regulated by the redox-sensitive transcription factor Nrf2. It is not an antioxidant but known as a multifunctional regulator of cell signaling with an ability to modulate targeted or selective degradation of proteins through autophagy. SQSTM1 implements these functions through physical interactions with different types of proteins including atypical PKCs, nonreceptor-type tyrosine kinase p56(Lck) (Lck), polyubiquitin, and autophagosomal factor LC3. One of the notable physiological functions of SQSTM1 is the regulation of redox-sensitive voltage-gated potassium (Kv) channels which are composed of α and β subunits: (Kvα)4 (Kvβ)4. Previous studies have established that SQSTM1 scaffolds PKCζ, enhancing phosphorylation of Kvβ which induces inhibition of pulmonary arterial Kv1.5 channels under acute hypoxia. Recent studies reveal that Lck indirectly interacts with Kv1.3 α subunits and plays a key role in acute hypoxia-induced Kv1.3 channel inhibition in T lymphocytes. Kv1.3 channels provide a signaling platform to modulate the migration and proliferation of arterial smooth muscle cells and activation of T lymphocytes, and hence have been recognized as a therapeutic target for treatment of restenosis and autoimmune diseases. In this review, we focus on the functional interactions of SQSTM1 with Kv channels through two key partners aPKCs and Lck. Furthermore, we provide molecular insights into the functions of SQSTM1 in suppression of proliferation of arterial smooth muscle cells and neointimal hyperplasia following carotid artery ligation, in T lymphocyte differentiation and activation, and in NGF-induced neurite outgrowth in PC12 cells.

The effects of intermittent hypoxia on redox status, NF-κB activation, and plasma lipid levels are dependent on the lowest oxygen saturation

Obstructive sleep apnea syndrome (OSAS) is described as repetitive obstructions of the upper airways during sleep, causing concomitant episodes of systemic hypoxia and associated cardiovascular and metabolic pathologies. The mechanisms generating these pathologies are controversial. Because recurrent hypoxia is the element of inadequate respiration that leads to the pathology, experimental models of OSAS consist in the exposure of the animals to intermittent hypoxia (IH) by cycling O2 percentages in their habitats. A proposed mechanism linking the IH of OSAS to pathologies is the increased production of reactive oxygen species (ROS). However, it has been argued that many patients seem to lack oxidative stress and that, to augment ROS in IH animals, intense hypoxia, seldom encountered in patients, has to be applied. To solve the controversy, we have exposed rats to two intensities of IH (cycles of 10 or 5% O2, 40s, and then 21% O2, 80s; 8h/day, 15 days). We then measured reduced and oxidized glutathione and lipid peroxide levels, aconitase and fumarase activities, and ROS-disposal enzyme activity in liver, brain, and lung. Liver levels of nuclear NF-κB-p65 and plasma C-reactive protein (CRP), as well as lipid levels, were also assessed. Lowest hemoglobin saturations were 91.7 ± 0.8 and 73.5 ± 1.4%. IH caused tissue-specific oxidative stress related to hypoxic intensity. Nuclear NF-κB-p65 and lipid content in the liver and CRP in the plasma all increased with IH intensity, as did both plasma triglycerides and cholesterol. We conclude that IH, even of moderate intensity, causes oxidative stress probably related to the pathologies encountered in OSAS patients.

Regulation of rat intrapulmonary arterial tone by arachidonic acid and prostaglandin E2 during hypoxia

AIMS

Arachidonic acid (AA) and its metabolites, prostaglandins (PG) are known to be involved in regulation of vascular homeostasis including vascular tone and vessel wall tension, but their potential role in Hypoxic pulmonary vasoconstriction (HPV) remains unclear. In this study, we examined the effects of AA and PGE2 on the hypoxic response in isolated rat intrapulmonary arteries (IPAs).

METHODS AND RESULTS

We carried out the investigation on IPAs by vessel tension measurement. Isotetrandrine (20 µM) significantly inhibited phase I, phase IIb and phase IIc of hypoxic vasoconstriction. Both indomethacin (100 µM) and NS398 attenuated KPSS-induced vessel contraction and phase I, phase IIb and phase IIc of HPV, implying that COX-2 plays a primary role in the hypoxic response of rat IPAs. PGE2 alone caused a significant vasoconstriction in isolated rat IPAs. This constriction is mediated by EP4. Blockage of EP4 by L-161982 (1 µM) significantly inhibited phase I, phase IIb and phase IIc of hypoxic vasoconstriction. However, AH6809 (3 µM), an antagonist of EP1, EP2, EP3 and DP1 receptors, exerted no effect on KPSS or hypoxia induced vessel contraction. Increase of cellular cAMP by forskolin could significantly reduce KPSS-induced vessel contraction and abolish phase I, phase II b and phase II c of HPV.

CONCLUSION

Our results demonstrated a vasoconstrictive effect of PGE2 on rat IPAs and this effect is via activation of EP4. Furthermore, our results suggest that intracellular cAMP plays dual roles in regulation of vascular tone, depending on the spatial distribution of cAMP and its coupling with EP receptor and Ca(2+) channels.

Predominant role of vasoconstrictors over dilatators derived from arachidonic acid in hypoxic pulmonary vasoconstriction

Prostanoids derived from arachidonic acid (AA) have been shown to play a permissive role in the regulation of vascular tone and wall tension. Conventionally, epoxyeicosatrienoic acids (EETs) and prostacyclin have been considered as dilatators, whereas thromboxane (TX) and hydroxyeicosatetraenoic acid (HETE) were considered as vasoconstrictors. However, the role of these prostanoids in the mediation of acute hypoxic pulmonary vasoconstriction is not yet clearly understood. In the present study, the role of prostanoids in the acute hypoxic response in rat isolated intrapulmonary arteries (IPAs) was investigated. Exogenous AA directly caused vasoconstriction, but exerted a significant inhibition on hypoxic vasoconstriction. The vasoconstriction by AA was mediated by the endothelium. AA metabolites from lipoxygenase (LOX) had no effect on vascular tone or hypoxic vasoconstriction. Consistent results from the blockage of cytochrome P450 (CYP) or CYP epoxide hydrolase showed that HETE contributed to endothelium‑independent hypoxic vasoconstriction. EET via epoxygenase exerted no effect on 80 mM KPSS‑induced vessel contraction or hypoxic vasoconstriction. In addition, prostacyclin also failed to inhibit hypoxic pulmonary vasoconstriction (HPV). However, blockage of thromboxane A2/prostanoid (TP) receptors almost eliminated hypoxic vasoconstriction, suggesting the primary role of TP receptors in the regulation of the hypoxic response in rat IPAs. In conclusion, the current data indicate the predominant role of vasoconstrictors instead of dilatators in mediating HPV. These data also highlight a pivotal role for voltage‑independent Ca2+ entry in pulmonary hypoxic response and suggest that modulation of these channels by prostanoids underlies their regulatory mechanisms.

Inducible cyclooxygenase expression mediating hypoxia/reoxygenation-induced pulmonary vasoconstriction is attenuated by a cyclooxygenase inhibitor in rats

OBJECTIVE

Hypoxic pulmonary vasoconstriction (HPV) is a well known phenomenon to temporarily offset a ventilation-perfusion mismatch. Sustained HPV may lead to pulmonary hypertension. In this protocol, we studied the relationships between the HPV response and inducible cyclooxygenase II (COX II) activation after hypoxia-reoxygenation (H-R) challenge in an isolated perfused lung model.

METHODS

An in situ isolated perfused rat lung model underwent inaction of hypoxia by ventilation with 5% CO(2)-95% N(2) for 10 minutes instead of 5% CO(2)-95% air; they were then reoxygenated with 5% CO(2)-95% air. We measured pulmonary arterial pressure (PAP) changes before, during, and after H-R challenge. We also estimated changes in blood concentrations of hydroxyl radicals, nitric oxide (NO) and thromboxane B(2) (TxB(2)) before and after H-R as well as mRNA expressions of COX II in lung tissue thereafter. A COX II inhibitor, celecoxib (10 mg/kg), was administered between 2 consecutive challenges.

RESULTS

Hypoxia induced pulmonary vasoconstriction by increasing PAP (4.1 ± 0.8 mm Hg). Consecutive hypoxic challenges did not show tachyphylaxis (P > .05). H-R of lung tissues induced significant increases in blood concentrations of hydroxyl radicals (48.5 ± 7.6 vs 75.8 ± 11.5 mmol/L; P < .01), NO (54.3 ± 12.3 vs 77.7 ± 15.7 pmol; P < .05), and TxB(2) (42.3 ± 6.9 vs 58.7 ± 8.6 pg/mL; P < .05). Lung tissue H-R also significantly increased COX II mRNA expression compared with sham tissues (1 ± 0 vs 4.0 ± 2.8; P < .001). The COX II inhibitor celecoxib significantly attenuated HPV responses (P < .05) and attenuated the elevated blood concentrations of TxB(2) (P < .05), hydroxyl radicals (P < .01), nitric oxide (P < .05), and COX II mRNA expression (P < .05) after H-R challenge.

CONCLUSIONS

Lung tissue H-R induced significant increases blood concentrations of inflammatory mediators and tissue mRNA expression of COX related to elevation of HPV responses. COX II inhibitor celecoxib attenuated the HPV responses by reducing TxB(2) release.

NADPH oxidase-derived ROS and the regulation of pulmonary vessel tone

Pulmonary vessel constriction results from an imbalance between vasodilator and vasoconstrictor factors released by the endothelium including nitric oxide, endothelin, prostanoids, and reactive oxygen species (ROS). ROS, generated by a variety of enzymatic sources (such as mitochondria and NADPH oxidases, a.k.a. Nox), appear to play a pivotal role in vascular homeostasis, whereas elevated levels effect vascular disease. The pulmonary circulation is very sensitive to changes in the partial pressure of oxygen and differs from the systemic circulation in its response to this change. In fact, the pulmonary vessels contract in response to low oxygen tension, whereas systemic vessels dilate. Growing evidence suggests that ROS production and ROS-related pathways may be key factors that underlie this differential response to oxygen tension. A major emphasis of our laboratory is the role of Nox isozymes in cardiovascular disease. In this review, we will focus our attention on the role of Nox-derived ROS in the control of pulmonary vascular tone.