Differential effects of 5,6-EET on segmental pulmonary vasoactivity in the rabbit

EETs Elicit Direct Increases in Pulmonary Arterial Pressure in Mice

OBJECTIVE

The biological role of epoxyeicosatrienoic acids (EETs) in the regulation of pulmonary circulation is currently under debate. We hypothesized that EETs initiate increases in right ventricular systolic pressure (RVSP) via perhaps, pulmonary vasoconstriction.

METHODS

Mice were anesthetized with isoflurane. Three catheters, inserted into the left jugular vein, the left carotid artery, and the right jugular vein, were used for infusing EETs, monitoring blood pressure (BP), and RVSP respectively. BP and RVSP were continuously recorded at basal conditions, in response to administration of 4 regioisomeric EETs (5,6-EET; 8,9-EET; 11,12-EET, and 14,15-EET; 1, 2, 5 and 10 ng/g body weight (BW) for each EET), and during exposure of mice to hypoxia.

RESULTS

All 4 EETs initiated dose-dependent increases in RVSP, though reduced BP. 11,12-EET elicited the greatest increment in RVSP among all EET isoforms. To clarify the direct elevation of RVSP in a systemic BP-independent manner, equivalent amounts of 14,15-EET were injected over 1 and 2 minutes respectively. One-minute injection of 14,15-EET elicited significantly faster and greater increases in RVSP than the 2-minute injection, whereas their BP changes were comparable. Additionally, direct injection of low doses of 14,15-EET (0.1, 0.2, 0.5, and 1 ng/g BW) into the right ventricle caused significant increases in RVSP without effects on BP, confirming that systemic vasodilation-induced increases in venous return are not the main cause for the increased RVSP. Acute exposure of mice to hypoxia significantly elevated RVSP, as well as 14,15-EET-induced increases in RVSP.

CONCLUSIONS

EETs directly elevate RVSP, a response that may play an important role in the development of hypoxia-induced pulmonary hypertension (PH).

Lung Circulation

The circulation of the lung is unique both in volume and function. For example, it is the only organ with two circulations: the pulmonary circulation, the main function of which is gas exchange, and the bronchial circulation, a systemic vascular supply that provides oxygenated blood to the walls of the conducting airways, pulmonary arteries and veins. The pulmonary circulation accommodates the entire cardiac output, maintaining high blood flow at low intravascular arterial pressure. As compared with the systemic circulation, pulmonary arteries have thinner walls with much less vascular smooth muscle and a relative lack of basal tone. Factors controlling pulmonary blood flow include vascular structure, gravity, mechanical effects of breathing, and the influence of neural and humoral factors. Pulmonary vascular tone is also altered by hypoxia, which causes pulmonary vasoconstriction. If the hypoxic stimulus persists for a prolonged period, contraction is accompanied by remodeling of the vasculature, resulting in pulmonary hypertension. In addition, genetic and environmental factors can also confer susceptibility to development of pulmonary hypertension. Under normal conditions, the endothelium forms a tight barrier, actively regulating interstitial fluid homeostasis. Infection and inflammation compromise normal barrier homeostasis, resulting in increased permeability and edema formation. This article focuses on reviewing the basics of the lung circulation (pulmonary and bronchial), normal development and transition at birth and vasoregulation. Mechanisms contributing to pathological conditions in the pulmonary circulation, in particular when barrier function is disrupted and during development of pulmonary hypertension, will also be discussed.

Cytochrome P450 2J2: distribution, function, regulation, genetic polymorphisms and clinical significance

Cytochrome P450 2J2 (CYP2J2) is an enzyme mainly found in human extrahepatic tissues, with predominant expression in the cardiovascular systems and lower levels in the intestine, kidney, lung, pancreas, brain, liver, etc. During the past 15 years, CYP2J2 has attracted much attention for its epoxygenase activity in arachidonic acid (AA) metabolism. It converts AA to four epoxyeicosatrienoic acids (EETs) that have various biological effects, especially in the cardiovascular systems. In recent publications, CYP2J2 is shown highly expressed in various human tumor cells, and its EET metabolites are demonstrated to implicate in the pathologic development of human cancers. CYP2J2 is also a human CYP that involved in phase I xenobiotics metabolism. Antihistamine drugs and many other compounds were identified as the substrates of CYP2J2, and studies have demonstrated that these substrates have a broad structural diversity. CYP2J2 is found not readily induced by known P450 inducers; however, its expression could be regulated in some pathological conditions, might through the activator protein-1(AP-1), the AP-1-like element and microRNA let-7b. Several genetic mutations in the CYP2J2 gene have been identified in humans, and some of them have been shown to have potential associations with some diseases. With the increasing awareness of its roles in cancer disease and drug metabolism, studies about CYP2J2 are still going on, and various inhibitors of CYP2J2 have been determined. Further studies are needed to delineate the roles of CYP2J2 in disease pathology, drug development and clinical practice.

Interactions between thromboxane A₂, thromboxane/prostaglandin (TP) receptors, and endothelium-derived hyperpolarization

Endothelium-dependent smooth muscle hyperpolarization (EDH) increasingly predominates over endothelium-derived nitric oxide (NO) as a participant in vasodilation as vessel size decreases. Its underlying nature is highly variable between vessel types, species, disease states, and exact experimental conditions, and is variably mediated by one or more transferable endothelium-derived hyperpolarizing factors and/or the electrotonic spread of endothelial hyperpolarization into the media via gap junctions. Although generally regarded (and studied) as a mechanism that is independent of NO and prostanoids, evidence has emerged that the endothelium-derived contracting factor and prostanoid thromboxane A2 can modulate several signalling components central to EDH, and therefore potentially curtail vasodilation through mechanisms that are distinct from those putatively involved in direct smooth muscle contraction. Notably, vascular production of thromboxane A2 is elevated in a number of cardiovascular disease states that promote endothelial dysfunction. This review will therefore discuss the mechanisms through which thromboxane A2 interacts with and modulates EDH, and will also consider the implications of such cross-talk in vasodilator control in health and disease.

Epoxides and soluble epoxide hydrolase in cardiovascular physiology

Epoxyeicosatrienoic acids (EETs) are arachidonic acid metabolites that importantly contribute to vascular and cardiac physiology. The contribution of EETs to vascular and cardiac function is further influenced by soluble epoxide hydrolase (sEH) that degrades EETs to diols. Vascular actions of EETs include dilation and angiogenesis. EETs also decrease inflammation and platelet aggregation and in general act to maintain vascular homeostasis. Myocyte contraction and increased coronary blood flow are the two primary EET actions in the heart. EET cell signaling mechanisms are tissue and organ specific and provide significant evidence for the existence of EET receptors. Additionally, pharmacological and genetic manipulations of EETs and sEH have demonstrated a contribution for this metabolic pathway to cardiovascular diseases. Given the impact of EETs to cardiovascular physiology, there is emerging evidence that development of EET-based therapeutics will be beneficial for cardiovascular diseases.

Cytochrome P450-derived epoxyeicosatrienoic acids and pulmonary hypertension: central role of transient receptor potential C6 channels

Hypoxia induces the constriction of pulmonary resistance arteries, which results in the redistribution of blood from poor to better ventilated areas, thus optimizing its oxygenation. Many different oxygen-sensing mechanisms have been proposed to regulate this process, including cytochrome P450 enzymes. These enzymes, which convert substrates such as arachidonic acid into bioactive epoxides (the epoxyeicosatrienoic acids [EETs]), are highly expressed in the lung as is the soluble epoxide hydrolase which metabolizes the epoxides to their less active diols. The EETs play a well-documented role as endothelium-derived vasodilators in the systemic vasculature, but in the pulmonary circulation, they are generated in vascular smooth muscle cells and potentiate vasoconstriction. Preventing the breakdown of 11,12-EET by the inhibition or genetic deletion of the soluble epoxide hydrolase strongly augments the response to hypoxia. Mechanistically, 11,12-EET potentiates the contractile response by recruiting transient receptor potential C6 channels to caveolae. Indeed, neither 11,12-EET nor hypoxia is able to elicit pulmonary vasoconstriction in TRPC6 knockout mice. The cytochrome and soluble epoxide hydrolase enzymes are also implicated in the vascular remodeling associated with chronic hypoxia and pulmonary hypertension. Thus, targeting this pathway may be in an attractive new therapeutic approach to treat this incapacitating disease.

Hypoxic pulmonary vasoconstriction

It has been known for more than 60 years, and suspected for over 100, that alveolar hypoxia causes pulmonary vasoconstriction by means of mechanisms local to the lung. For the last 20 years, it has been clear that the essential sensor, transduction, and effector mechanisms responsible for hypoxic pulmonary vasoconstriction (HPV) reside in the pulmonary arterial smooth muscle cell. The main focus of this review is the cellular and molecular work performed to clarify these intrinsic mechanisms and to determine how they are facilitated and inhibited by the extrinsic influences of other cells. Because the interaction of intrinsic and extrinsic mechanisms is likely to shape expression of HPV in vivo, we relate results obtained in cells to HPV in more intact preparations, such as intact and isolated lungs and isolated pulmonary vessels. Finally, we evaluate evidence regarding the contribution of HPV to the physiological and pathophysiological processes involved in the transition from fetal to neonatal life, pulmonary gas exchange, high-altitude pulmonary edema, and pulmonary hypertension. Although understanding of HPV has advanced significantly, major areas of ignorance and uncertainty await resolution.

Improved bioavailability of epoxyeicosatrienoic acids reduces TP-receptor agonist-induced tension in human bronchi

Epoxyeicosatrienoic acid (EET) and thromboxane A2are arachidonic acid derivatives. The former has initially been defined as an epithelium-derived hyperpolarizing factor displaying broncho-relaxing ( 4 ) and anti-inflammatory properties, as recently demonstrated ( 25 ), whereas thromboxane A2induces vaso- and bronchoconstriction upon binding to thromboxane-prostanoid (TP)-receptor. EETs, however, are quickly degraded by the soluble epoxide hydrolase (sEH) into inactive diol compounds ( 25 ). The aim of this study was to investigate the effects of 14,15-EET on TP-receptor activation in human bronchi. Tension measurements performed on native bronchi from various species, acutely treated with increasing 14,15-EET concentrations, revealed specific and concentration-dependent relationships as well as a decrease in the tension induced by 30 nM U-46619, used as a synthetic TP-receptor agonist. Interestingly, acute treatments with 3 μM N-(methylsulfonyl)-2-(2-propynyloxy)-benzenehexanamide, an epoxygenase inhibitor, which minimizes endogenous production of EET, resulted in an increased reactivity to U-46619. Furthermore, we demonstrated that chronic treatments with trans-4-[4-(3-adamantan-1-yl-ureido)-cyclohexyloxy]-benzoic acid (t-AUCB), a sEH inhibitor, reduced human bronchi reactivity to U-46619. During our tension measurements, we also observed that human bronchi generated small-amplitude contractions; these spontaneous activities were reduced upon acute 14,15-EET treatments in the presence of t-AUCB. Altogether, these data demonstrate that endogenous and exogenous 14,15-EET could interfere with the activation of TP-receptors as well as with spontaneous oscillations in human airway smooth muscle tissues.

Insight in modulation of inflammation in response to diclofenac intervention: a human intervention study

Background

Chronic systemic low-grade inflammation in obese subjects is associated with health complications including cardiovascular diseases, insulin resistance and diabetes. Reducing inflammatory responses may reduce these risks. However, available markers of inflammatory status inadequately describe the complexity of metabolic responses to mild anti-inflammatory therapy.

Methods

To address this limitation, we used an integrative omics approach to characterize modulation of inflammation in overweight men during an intervention with the non-steroidal anti-inflammatory drug diclofenac. Measured parameters included 80 plasma proteins, >300 plasma metabolites (lipids, free fatty acids, oxylipids and polar compounds) and an array of peripheral blood mononuclear cells (PBMC) gene expression products. These measures were submitted to multivariate and correlation analysis and were used for construction of biological response networks.

Results

A panel of genes, proteins and metabolites, including PGE₂ and TNF-alpha, were identified that describe a diclofenac-response network (68 genes in PBMC, 1 plasma protein and 4 plasma metabolites). Novel candidate markers of inflammatory modulation included PBMC expression of annexin A1 and caspase 8, and the arachidonic acid metabolite 5,6-DHET.

Conclusion

In this study the integrated analysis of a wide range of parameters allowed the development of a network of markers responding to inflammatory modulation, thereby providing insight into the complex process of inflammation and ways to assess changes in inflammatory status associated with obesity.

Trial registration

The study is registered as NCT00221052 in clinicaltrials.gov database.

Epoxyeicosatrienoic acids and the soluble epoxide hydrolase are determinants of pulmonary artery pressure and the acute hypoxic pulmonary vasoconstrictor response

Recent findings have indicated a role for cytochrome P-450 (CYP) epoxygenase-derived epoxyeicosatrienoic acids (EETs) in acute hypoxic pulmonary vasoconstriction (HPV). Given that the intracellular concentration of EETs is determined by the soluble epoxide hydrolase (sEH), we assessed the influence of the sEH and 11,12-EET on pulmonary artery pressure and HPV in the isolated mouse lung. In lungs from wild-type mice, HPV was significantly increased by sEH inhibition, an effect abolished by pretreatment with CYP epoxygenase inhibitors and the EET antagonist 14,15-EEZE. HPV and EET production were greater in lungs from sEH(-/-) mice than from wild-type mice and sEH inhibition had no further effect on HPV, while MSPPOH and 14,15-EEZE decreased the response. 11,12-EET increased pulmonary artery pressure in a concentration-dependent manner and enhanced HPV via a Rho-dependent mechanism. Both 11,12-EET and hypoxia elicited the membrane translocation of a transient receptor potential (TRP) C6-V5 fusion protein, the latter effect was sensitive to 14,15-EEZE. Moreover, while acute hypoxia and 11,12-EET increased pulmonary pressure in lungs from TRPC6(+/-) mice, lungs from TRPC6(-/-) mice did not respond to either stimuli. These data demonstrate that CYP-derived EETs are involved in HPV and that EET-induced pulmonary contraction under normoxic and hypoxic conditions involves a TRPC6-dependent pathway.

Beyond vasodilatation: non-vasomotor roles of epoxyeicosatrienoic acids in the cardiovascular system

Epoxyeicosatrienoic acids (EETs), derived from arachidonic acid by cytochrome P450 epoxygenases, are potent vasodilators that function as endothelium-derived hyperpolarizing factors in some vascular beds. EETs are rapidly metabolized by soluble epoxide hydrolase to form dihydroxyeicosatrienoic acids (DHETs). Recent reports indicate that EETs have several important non-vasomotor regulatory roles in the cardiovascular system. EETs are potent anti-inflammatory agents and might function as endogenous anti-atherogenic compounds. In addition, EETs and DHETs might stimulate lipid metabolism and regulate insulin sensitivity. Thus, pharmacological inhibition of soluble epoxide hydrolase might be useful not only for hypertension but also for abating atherosclerosis, diabetes mellitus and the metabolic syndrome. Finally, although usually protective in the systemic circulation, EETs might adversely affect the pulmonary circulation.

Emerging role of epoxyeicosatrienoic acids in coronary vascular function

The importance of endothelium-derived nitric oxide in coronary vascular regulation is well-established and the loss of this vasodilator compound is associated with endothelial dysfunction, tissue hypoperfusion and atherosclerosis. Numerous studies indicate that the endothelium produces another class of compounds, the epoxyeicosatrienoic acids (EETs), which may partially compensate for the loss of nitric oxide in cardiovascular disease. The EETs are endogenous lipids which are derived through the metabolism of arachidonic acid by cytochrome P450 epoxygenase enzymes. Also, EETs hyperpolarize vascular smooth muscle and induce dilation of coronary arteries and arterioles, and therefore may be endogenous mediators of coronary vasomotor tone and myocardial perfusion. In addition, EETs have been shown to inhibit vascular smooth muscle migration, decrease inflammation, inhibit platelet aggregation and decrease adhesion molecule expression, therefore representing an endogenous protective mechanism against atherosclerosis. Endogenous EETs are degraded to less active dihydroxyeicosatrienoic acids by soluble epoxide hydrolase. Pharmacological inhibition of soluble epoxide hydrolase has received considerable attention as a potential approach to enhance EET-mediated vascular protection, and several compounds have appeared promising in recent animal studies. The present review discusses the emerging role of EETs in coronary vascular function, as well as recent advancements in the development of pharmacological agents to enhance EET bioavailability.

5,6-EET-induced contraction of intralobar pulmonary arteries depends on the activation of Rho-kinase

The mechanism mediating epoxyeicosatrienoic acid (EET)-induced contraction of intralobar pulmonary arteries (PA) is currently unknown. EET-induced contraction of PA has been reported to require intact endothelium and activation of the thromboxane/endoperoxide (TP) receptor. Because TP receptor occupation with the thromboxane mimetic U-46619 contracts pulmonary artery via Rho-kinase activation, we examined the hypothesis that 5,6-EET-induced contraction of intralobar rabbit pulmonary arteries is mediated by a Rho-kinase-dependent signaling pathway. In isolated rings of second-order intralobar PA (1–2 mm OD) at basal tension, 5,6-EET (0.3–10 μM) induced increases in active tension that were inhibited by Y-27632 (1 μM) and HA-1077 (10 μM), selective inhibitors of Rho-kinase activity. In PA in which smooth muscle intracellular Ca2+ concentration ([Ca2+]i) was increased with KCl (25 mM) to produce a submaximal contraction, 5,6-EET (1 μM) induced a contraction that was 7.0 ± 1.6 times greater than without KCl. 5,6-EET (10 μM) also contracted β-escin permeabilized PA in which [Ca2+]i was clamped at a concentration resulting in a submaximal contraction. Y-27632 inhibited the 5,6-EET-induced contraction in permeabilized PA. 5,6-EET (10 μM) increased phosphorylation of myosin light chain (MLC), increasing the ratio of phosphorylated MLC/total MLC from 0.10 ± 0.03 to 0.30 ± 0.02. Y-27632 prevented this increase in MLC phosphorylation. These data suggest that 5,6-EET induces contraction in intralobar PA by increasing Rho-kinase activity, phosphorylating MLC, and increasing the Ca2+ sensitivity of the contractile apparatus.

Role of EETs in regulation of endothelial permeability in rat lung

This study tested the hypothesis that epoxyeicosatrienoic acids (EETs) derived from arachidonic acid via P-450 epoxygenases are soluble factors linking depletion of endoplasmic reticulum Ca2+ stores and store-dependent regulation of endothelial cell (EC) permeability in rat lung. EC permeability was measured via the capillary filtration coefficient ( Kf,c) in isolated, perfused rat lungs. 14,15-EET and 5,6-EET increased EC permeability, a response that was significantly different from that of 8,9-EET, 11,12-EET, and vehicle control. The permeability response to 14,15-EET was not significantly attenuated by the nonspecific Ca2+ channel blocker Gd3+ ( P = 0.068). In lungs perfused with low [Ca2+], 14,15-EET tended to increase EC permeability, although a significant increase in Kf,c was observed only following Ca2+ add-back. As positive control, we showed that the 3.7-fold increase in Kf,c evoked by thapsigargin (TG), a known activator of store depletion-induced Ca2+ entry, was blocked by both Gd3+ and low [Ca2+] buffer. Nonetheless, the permeability response to TG could not be blocked by the phospholipase A2 inhibitors mepacrine or methyl arachidonyl fluorophosphonate or the P-450 epoxygenase inhibitors 17-octadecynoic acid or propargyloxyphenyl hexanoic acid. Similarly, combined pretreatment with ibuprofen and dicyclohexylurea to block EET metabolism had no effect on the permeability response to TG. We conclude that EETs have a heterogeneous impact on EC permeability. Despite a requirement for Ca2+ entry with both TG and 14,15-EET, our data suggest that distinct signaling pathways or heterogeneity in EC responsiveness is responsible for the observed EC injury evoked by EETs and store depletion in the isolated rat lung.