Bovine endothelial cells in culture produce thromboxane as well as prostacyclin

Neurovascular dysfunction in glaucoma

Retinal ganglion cells, the neurons that die in glaucoma, are endowed with a high metabolism requiring optimal provision of oxygen and nutrients to sustain their activity. The timely regulation of blood flow is, therefore, essential to supply firing neurons in active areas with the oxygen and glucose they need for energy. Many glaucoma patients suffer from vascular deficits including reduced blood flow, impaired autoregulation, neurovascular coupling dysfunction, and blood-retina/brain-barrier breakdown. These processes are tightly regulated by a community of cells known as the neurovascular unit comprising neurons, endothelial cells, pericytes, Müller cells, astrocytes, and microglia. In this review, the neurovascular unit takes center stage as we examine the ability of its members to regulate neurovascular interactions and how their function might be altered during glaucomatous stress. Pericytes receive special attention based on recent data demonstrating their key role in the regulation of neurovascular coupling in physiological and pathological conditions. Of particular interest is the discovery and characterization of tunneling nanotubes, thin actin-based conduits that connect distal pericytes, which play essential roles in the complex spatial and temporal distribution of blood within the retinal capillary network. We discuss cellular and molecular mechanisms of neurovascular interactions and their pathophysiological implications, while highlighting opportunities to develop strategies for vascular protection and regeneration to improve functional outcomes in glaucoma.

Inhibition of neutrophil extracellular trap formation alleviates vascular dysfunction in type 1 diabetic mice

While neutrophil extracellular traps (NETs) have previously been linked to some diabetes-associated complications, such as dysfunctional wound healing, their potential role in diabetic vascular dysfunction has not been studied. Diabetic Akita mice were crossed with either Elane-/- or Pad4-/- mice to generate NET-deficient diabetic mice. By 24 weeks of age, Akita aortae showed markedly impaired relaxation in response to acetylcholine, indicative of vascular dysfunction. Both Akita-Elane-/- mice and Akita-Pad4-/- mice had reduced levels of circulating NETs and improved acetylcholine-mediated aortic relaxation. Compared with wild-type aortae, the thromboxane metabolite TXB2 was roughly 10-fold higher in both intact and endothelium-denuded aortae of Akita mice. In contrast, Akita-Elane-/- and Akita-Pad4-/- aortae had TXB2 levels similar to wild type. In summary, inhibition of NETosis by two independent strategies prevented the development of vascular dysfunction in diabetic Akita mice. Thromboxane was up-regulated in the vessel walls of NETosis-competent diabetic mice, suggesting a role for neutrophils in driving the production of this vasoconstrictive and atherogenic prostanoid.

Platelet Inhibition by Low-Dose Acetylsalicylic Acid Reduces Neuroinflammation in an Animal Model of Multiple Sclerosis

Aside from the established immune-mediated etiology of multiple sclerosis (MS), compelling evidence implicates platelets as important players in disease pathogenesis. Specifically, numerous studies have highlighted that activated platelets promote the central nervous system (CNS)-directed adaptive immune response early in the disease course. Platelets, therefore, present a novel opportunity for modulating the neuroinflammatory process that characterizes MS. We hypothesized that the well-known antiplatelet agent acetylsalicylic acid (ASA) could inhibit neuroinflammation by affecting platelets if applied at low-dose and investigated its effect during experimental autoimmune encephalomyelitis (EAE) as a model to study MS. We found that oral administration of low-dose ASA alleviates symptoms of EAE accompanied by reduced inflammatory infiltrates and less extensive demyelination. Remarkably, the percentage of CNS-infiltrated CD4+ T cells, the major drivers of neuroinflammation, was decreased to 40.98 ± 3.28% in ASA-treated mice compared to 56.11 ± 1.46% in control animals at the disease maximum as revealed by flow cytometry. More interestingly, plasma levels of thromboxane A2 were decreased, while concentrations of platelet factor 4 and glycoprotein VI were not affected by low-dose ASA treatment. Overall, we demonstrate that low-dose ASA could ameliorate the platelet-dependent neuroinflammatory response in vivo, thus indicating a potential treatment approach for MS.

GPCR-mediated EGF receptor transactivation regulates TRPV4 action in the vasculature

BACKGROUND AND PURPOSE

Transient receptor potential vanilloid-4 (TRPV4) is a calcium-permeant ion channel that is known to affect vascular function. The ability of TRPV4 to cause a vasoconstriction in blood vessels has not yet been mechanistically examined. Further in neuronal cells, TRPV4 signalling can be potentiated by GPCR activation. Thus, we studied the mechanisms underlying the vascular contractile action of TRPV4 and the GPCR-mediated potentiation of such vasoconstriction, both of which are as yet unappreciated aspects of TRPV4 function.

EXPERIMENTAL APPROACH

The mechanisms of TRPV4-dependent regulation of vascular tone in isolated mouse aortae were studied using wire myography. TRPV4-dependent calcium signalling and prostanoid production was studied in cultured human umbilical vein endothelial cells (HUVECs).

KEY RESULTS

In addition to the well-documented vasorelaxation response triggered by TRPV4 activation, we report here a TRPV4-triggered vasoconstriction in the mouse aorta that involves a COX-generated Tx receptor (TP) agonist that acts in a MAPK and Src kinase signalling dependent manner. This constriction is potentiated by activation of the GPCRs for angiotensin (AT1 receptors) or proteinases (PAR1 and PAR2) via transactivation of the EGF receptor and a process involving PKC. TRPV4-dependent vascular contraction can be blocked by COX inhibitors or with TP antagonists. Further, TRPV4 activation in HUVECs stimulated Tx release as detected by an elisa.

CONCLUSION AND IMPLICATIONS

We conclude that the GPCR potentiation of TRPV4 action and TRPV4-dependent Tx receptor activation are important regulators of vascular function and could be therapeutically targeted in vascular diseases.

COX-2-10aa-PGIS gene therapy improves erectile function in rats after cavernous nerve injury

INTRODUCTION

Erectile dysfunction (ED) is a very common complication after radical prostatectomy. COX-2-10aa-PGIS is a newly engineered protein with COX-2 and prostacyclin synthase activities that converts arachidonic acid directly to prostacyclin (prostaglandin I2 [PGI2]). PGI2 is a potent smooth muscle relaxant.

AIM

The purpose of this study was to explore the effect and mechanism of COX-2-10aa-PGIS gene therapy in penile rehabilitation.

METHODS

Bilateral cavernous nerve crush (BCNC) in adult Sprague-Dawley rats was used to mimic radical prostatectomy-induced ED. Sprague-Dawley rats were randomly assigned into four groups: 1. sham surgery; 2. BCNC; 3. BCNC + null control recombinant adenovirus intracavernous injection; and 4. BCNC + Ad-COX2-10aa-PGIS intracavernous injection. Twenty-eight days later, intracavernosal pressure (ICP) was recorded under cavernous nerve stimulation; in the meantime, the mean arterial pressure (MAP) was monitored. At the end of the measurement, the penis was harvested and processed for (i) immunohistochemistry analysis of endothelial nitric oxide synthase (eNOS), alpha-smooth muscle actin (α-SMA), and transforming growth factor beta-1 (TGF-β1); (ii) Masson's trichrome stain for smooth muscle/collagen ratios; (iii) Western blot of eNOS, α-SMA, TGF-β1, and COX2-10aa-PGIS; and (iv) terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) assay for apoptosis.

MAIN OUTCOME MEASURES

Erectile function was evaluated by ICP/MAP. Smooth muscle and endothelium functions in corpora cavernosum were assessed by Masson's trichrome stain, immunohistochemistry, and Western blot. Apoptosis was identified by TUNEL assay.

RESULTS

The results were the following: 1. COX2-10aa-PGIS gene therapy improved erectile function (82%, compared with control) in the BCNC rat model; 2. COX2-10aa-PGIS gene therapy increased eNOS (121%) and α-SMA (118%) expression and decreased TGF-β1 (45%) expression; 3. COX2-10aa-PGIS gene therapy reduced cell apoptosis after cavernous nerve injury (64%); and 4. COX2-10aa-PGIS gene therapy improved smooth muscle/collagen ratios (81%).

CONCLUSION

Our data demonstrated that COX2-10aa-PGIS improved erectile function after cavernous nerve injury through antifibrotic and anti-apoptotic mechanisms.

The endothelium: influencing vascular smooth muscle in many ways

The endothelium, although only a single layer of cells lining the vascular and lymphatic systems, contributes in multiple ways to vascular homeostasis. Subsequent to the 1980 report by Robert Furchgott and John Zawadzki, there has been a phenomenal increase in our knowledge concerning the signalling molecules and pathways that regulate endothelial - vascular smooth muscle communication. It is now recognised that the endothelium is not only an important source of nitric oxide (NO), but also numerous other signalling molecules, including the putative endothelium-derived hyperpolarizing factor (EDHF), prostacyclin (PGI(2)), and hydrogen peroxide (H(2)O(2)), which have both vasodilator and vasoconstrictor properties. In addition, the endothelium, either via transferred chemical mediators, such as NO and PGI(2), and (or) low-resistance electrical coupling through myoendothelial gap junctions, modulates flow-mediated vasodilatation as well as influencing mitogenic activity, platelet aggregation, and neutrophil adhesion. Disruption of endothelial function is an early indicator of the development of vascular disease, and thus an important area for further research and identification of potentially new therapeutic targets. This review focuses on the signalling pathways that regulate endothelial - vascular smooth muscle communication and the mechanisms that initiate endothelial dysfunction, particularly with respect to diabetic vascular disease.

Endogenous inhibitory mechanisms and the regulation of platelet function

The response of platelets to changes in the immediate environment is always a balance between activatory and inhibitory signals, the cumulative effect of which is either activation or quiescence. This is true of platelets in free flowing blood and of their regulation of haemostasis and thrombosis. In this review, we consider the endogenous inhibitory mechanisms that combine to regulate platelet activation. These include those derived from the endothelium (nitric oxide, prostacyclin, CD39), inhibitory receptors on the surface of platelets (platelet endothelial cell adhesion molecule-1, carcinoembryonic antigen cell adhesion molecule 1, G6b-B - including evidence for the role of Ig-ITIM superfamily members in the negative regulation of ITAM-associated GPVI platelet-collagen interactions and GPCR-mediated signalling and in positive regulation of "outside-in" integrin α(IIb)β(3)-mediated signalling), intracellular inhibitory receptors (retinoic X receptor, glucocorticoid receptor, peroxisome proliferator-activated receptors, liver X receptor), and emerging inhibitory pathways (canonical Wnt signalling, Semaphorin 3A, endothelial cell specific adhesion molecule, and junctional adhesion molecule-A).

Effects of postprandial plasma and chylomicrons on endothelial cells. Differences between dietary cream and cod liver oil

The acute effects of fatty meals (900 kcal) rich in saturated (cream) or n-3 polyunsaturated (cod liver oil, CLO) fatty acids on human umbilical vein endothelial cells (ECM) and platelet behavior were studied. The ECM were incubated for 24 hours at 37 degrees C with either plasma or chylomicrons (CM) obtained 3 hours after the meals. The ability of the ECM to inhibit platelet aggregation (PIA) and the release of prostaglandin I2 measured as 6-keto-prostaglandin F1 alpha (6-keto-PGF1 alpha) were measured after 24 hours of incubation, after stimulation and after freezing and thawing. Similar studies were done with CM from a patient with type V hyperlipoproteinemia. The release of 6-keto-PGF1 alpha was increased by postprandial plasma and by CM obtained after both meals. Plasma collected after CLO, but not after cream, increased PIA, whereas CM derived from all sources studied stimulated the PIA of ECM. No consistent correlation could be established between the release of 6-keto-PGF1 alpha and PIA. Increased platelet aggregation in platelet-rich plasma was always observed during postprandial hyperlipidemia.

Prostaglandins and platelet aggregation

Prostaglandins may induce or inhibit platelet aggregation and constrict ro dilate blood vessels. Recent interest has focused on prostaglandins which are derivatives of arachidonic acid including prostaglandin, endoperoxides, thromboxane A2, prostaglandin E2, prostaglandin D2 and prostacyclin. Prostacyclin (PGI2) is a potent vasodilator and inhibitor of platelet aggregation whose enhanced production by vessel walls should be beneficial. It now appears that the circulating levels of PGI2 in man are extremely low and little is known about the manner in which to increase them. Furthermore, aspirin, in doses of as little as 4 mg/kg inhibits prostacyclin as well as thromboxane formation. Thromboxane A2 may be involved in coronary ischemia because it is a potent vasoconstrictor that is biosynthesized during platelet aggregation. Although thromboxane A2 is very unstable indirect evidence obtained by using thromboxane A generating systems or a stable analogue called carbocyclic thromboxane A2 (CTA2) suggests that it exacerbates ischaemic damage because of a selective increase in vascular resistance due to coronary vasospasm and platelet aggregation which acts to decrease myocardial blood flow. The stable prostaglandins PGD2 and PGE2 are also of interest as both are formed during platelet aggregation. Like PGI2, PGD2 inhibits platelet aggregation.

Large-scale expression, purification, and characterization of an engineered prostacyclin-synthesizing enzyme with therapeutic potential

Recently, we reported that a novel hybrid enzyme (TriCat enzyme), engineered by linking human cyclooxygenase-2 (COX-2) with prostacyclin (PGI(2)) synthase (PGIS) together through a transmembrane domain, was able to directly integrate the triple catalytic (TripCat) functions of COX-2 and PGIS and effectively convert arachidonic acid (AA) into the vascular protector, PGI(2) [K.H. Ruan, H. Deng, S.P. So, Biochemistry 45 (2006) 14003-14011]. In order to confirm the important biological activity and evaluate its therapeutic potential, it is critical to characterize the properties of the enzyme using the purified protein. The TriCat enzyme cDNA was subcloned into a baculovirus vector and its protein was expressed in Sf-9 cells in large-scale with a high-yield ( approximately 4% of the total membrane protein), as confirmed by Western blot and protein staining. The Sf-9 cells' membrane fraction, rich in TriCat enzyme, exhibited strong TriCat functions (K(m)=3 microM and K(cat)=100 molecules/min) for the TriCat enzyme and was 3-folds faster in converting AA to PGI(2) than the combination of the individual COX-2 and PGIS. Another superiority of the TriCat enzyme is its dual effect on platelet aggregation: it completely inhibited platelet aggregation at the low concentration of 2 microg/ml and then displayed the ability to reverse the initially aggregated platelets to their non-aggregated state. Furthermore, multiple substrate-binding sites were confirmed in the single protein by high-resolution NMR spectroscopy, using partially purified TriCat enzyme. These studies have clearly demonstrated that the isolated TriCat enzyme protein functions in the selective biosynthesis of the vascular protector, PGI(2), and revealed its potential for anti-thrombosis therapeutics.

Characterization of the prostaglandin H2 mimic: binding to the purified human thromboxane A2 receptor in solution

For decades, the binding of prostaglandin H(2) (PGH(2)) to multiple target proteins of unrelated protein structures which mediate diverse biological functions has remained a real mystery in the field of eicosanoid biology. Here, we report that the structure of a PGH(2) mimic, U46619, bound to the purified human TP, was determined and compared with that of its conformation bound to the COX-downstream synthases, prostacyclin synthase (PGIS) and thromboxane A(2) synthase (TXAS). Active human TP protein, glycosylated and in full length, was expressed in Sf-9 cells using a baculovirus (BV) expression system and then purified to near homogeneity. The binding of U46619 to the purified receptor in a nonionic detergent-mimicked lipid environment was characterized by high-resolution NMR spectroscopy. The conformational change of U46619, upon binding to the active TP, was evidenced by the significant perturbation of the chemical shifts of its protons at H3 and H4 in a concentration-dependent manner. The detailed conformational changes and 3D structure of U46619 from the free form to the TP-bound form were further solved by 2D (1)H NMR experiments using a transferred NOE (trNOE) technique. The distances between the protons of H11 and H18, H11 and H19, H15 and H18, and H15 and H19 in U46619 were shorter following their binding to the TP in solution, down to within 5A, which were different than that of the U46619 bound to PGIS and U44069 (another PGH(2) mimic) bound to TXAS. These shorter distances led to further separation of the U46619 alpha and omega chains, forming a unique "rectangular" shape. This enabled the molecule to fit into the ligand-binding site pocket of a TP model, in which homology modeling was used for the transmembrane (TM) domain, and NMR structures were used for the extramembrane loops. The proton perturbations and 3D conformations in the TP-bound U46619 were different with that of the PGH(2) mimics bound to PGIS and TXAS. The studies indicated that PGH(2) can adopt multiple conformations in solution to satisfy the specific and unique shapes to fit the different binding pockets in the TP receptor and COX-downstream enzymes. The results also provided sufficient information for speculating the molecular basis of how PGH(2) binds to multiple target proteins even though unrelated in their protein sequences.

A profile of the residues in the second extracellular loop that are critical for ligand recognition of human prostacyclin receptor

The residues in the second extracellular loop (eLP2) of the prostanoid receptors, which are important for specific ligand recognition, were previously predicted in our earlier studies of the thromboxane A2 receptor (TP) using a combination of NMR spectroscopy and recombinant protein approaches. To further test this hypothesis, another prostanoid receptor, the prostacyclin receptor (IP), which has opposite biological characteristics to that of TP, was used as a model for these studies. A set of recombinant human IPs with site-directed mutations at the nonconserved eLP2 residues were constructed using an Ala-scanning approach, and then expressed in HEK293 and COS-7 cells. The expression levels of the recombinant receptors were six-fold higher in HEK293 cells than in COS-7 cells. The residues important for ligand recognition and binding within the N-terminal segment (G159, Q162, and C165) and the C-terminal segment (L172, R173, M174, and P179) of IP eLP2 were identified by mutagenesis analyses. The molecular mechanisms for the specific ligand recognition of IP were further demonstrated by specific site-directed mutagenesis using different amino acid residues with unique chemical properties for the key residues Q162, L172, R173, and M174. A comparison with the corresponding functional residues identified in TP eLP2 revealed that three (Q162, R173, and M174) of the four residues are nonconserved, and these are proposed to be involved in specific ligand recognition. We discuss the importance of G159 and P179 in ligand recognition through configuration of the loop conformation is discussed. These studies have further indicated that characterization of the residues in the eLP2 regions for all eight prostanoid receptors could be an effective approach for uncovering the molecular mechanisms of the ligand selectivities of the G-protein-coupled receptors.

Cytochrome P450 systems—biological variations of electron transport chains

Cytochromes P450 (P450) are hemoproteins encoded by a superfamily of genes nearly ubiquitously distributed in different organisms from all biological kingdoms. The reactions carried out by P450s are extremely diverse and contribute to the biotransformation of drugs, the bioconversion of xenobiotics, the bioactivation of chemical carcinogens, the biosynthesis of physiologically important compounds such as steroids, fatty acids, eicosanoids, fat-soluble vitamins and bile acids, the conversion of alkanes, terpenes and aromatic compounds as well as the degradation of herbicides and insecticides. Cytochromes P450 belong to the group of external monooxygenases and thus receive the necessary electrons for oxygen cleavage and substrate hydroxylation from different redox partners. The classical as well as the recently discovered P450 redox systems are compiled in this paper and classified according to their composition.

The calcium ionophore A23187 induces endothelium-dependent contractions in femoral arteries from rats with streptozotocin-induced diabetes

BACKGROUND AND PURPOSE

To study the importance of endothelium-derived contracting factors (EDCFs) in arteries of rats with type I diabetes.

EXPERIMENTAL APPROACH

Rat femoral arteries were collected four or twelve weeks after induction of diabetes with streptozotocin. Rings, with or without endothelium, were suspended in organ chambers for isometric tension measurement. COX protein levels were determined by Western blotting.

KEY RESULTS

Four weeks after the injection of streptozotocin, the endothelium-dependent relaxations (during contractions to phenylephrine) to A23817 were attenuated, but the endothelium-dependent contractions (quiescent preparations) to the ionophore were augmented. Indomethacin and S18886 prevented the endothelium-dependent contractions, while dazoxiben reduced them in rings from streptozotocin-treated rats, suggesting that thromboxane A2, activating TP- receptors, is involved. Twelve weeks after the injection of streptozotocin, the changes in endothelium-dependent relaxations and contractions to A23187 were even more noticeable. The protein expression of COX-1 was increased in femoral arteries of the diabetic rats. Valeryl salicylate and SC560 inhibited the contractions, suggesting that the EDCFs are produced by COX-1. At that time, a combination of S18886 with EP1-blockers was required to abolish the contractions, suggesting that the EDCFs involved act at both TP- and EP-receptors. Rings without endothelium from streptozotocin-treated rats exhibited a reduced maximal contraction to potassium chloride and U46619, combined with hyper-responsiveness to the latter, suggesting that more prolonged diabetes also alters the responsiveness of vascular smooth muscle.

CONCLUSION AND IMPLICATIONS

The production of EDCFs is progressively increased in the course of type I diabetes. Eventually, the disease also damages vascular smooth muscle.

Engineering of a protein with cyclooxygenase and prostacyclin synthase activities that converts arachidonic acid to prostacyclin

Prostacyclin (PGI2), a vascular protector with vasodilation and antithrombotic properties, is synthesized by coupling reactions of cyclooxygenase (COX, the first enzyme) with PGI2 synthase (PGIS, the second enzyme) using arachidonic acid (AA) as an initial substrate. The first COX product, prostaglandin H2 (PGH2) is also a command substrate for other prostanoid enzymes that produce distinct eicosanoids, such as thromboxane A2 (TXA2). The actions of TXA2 to cause vasoconstriction and platelet aggregation oppose the vasodilatory and anti-aggregatory effects of PGI2. Specifically upregulating PGI2 biosynthesis is an ideal model for the prevention and treatment of the TXA2-mediated thrombosis involved in strokes and myocardial infarctions. Here, we report that a single protein was constructed by linking COX-2 and PGIS together to form a new fusion enzyme through a transmembrane domain with 10 or 22 residues. The engineered protein expressed in HEK293 and COS-7 cells was able to continually convert AA to prostaglandin (PG) G2 (catalytic step 1), PGH2 (catalytic step 2), and PGI2 (catalytic step 3). The studies first demonstrate that a single protein with three catalytic functions could directly synthesize PGI2 from AA with a Km of approximately 3.2 microM. Specific upregulation of PGI2 biosynthesis through expression of the engineered single protein in the cells has shown strong activity in inhibiting platelet aggregation induced by AA in vitro, which creates a great potential for the fusion enzyme to be used as one of the new therapeutic interventions for strokes and heart attacks. The studies have also provided a model linking COX with its downstream enzymes to specifically regulate biosynthesis of eicosanoids which have potent biological functions.

Cytochrome P450 enzymes: Central players in cardiovascular health and disease

Cardiovascular disease (CVD) is a human health crisis that remains the leading cause of death worldwide. The cytochrome P450 (CYP) class of enzymes are key metabolizers of both xenobiotics and endobiotics. Many CYP enzyme families have been identified in the heart, endothelium and smooth muscle of blood vessels. Furthermore, mounting evidence points to the role of endogenous CYP metabolites, such as epoxyeicosatrienoic acids (EETs), hydroxyeicosatetraenoic acids (HETEs), prostacyclin (PGI(2)), aldosterone, and sex hormones, in the maintenance of cardiovascular health. Emerging science and the development of genetic screening have provided us with information on the differences in CYP expression among populations and groups of individuals. With this information, a link between CYP expression and activity and CVD, such as hypertension, coronary artery disease (CAD), myocardial infarction, heart failure, stroke, and cardiomyopathy and arrhythmias, has been established. In fact many currently used therapeutic modalities in CVD owe their therapeutic efficacy to their effect on CYP metabolites. Thus, the evidence for the involvement of CYP in CVD is numerous. Concentrating on treatment modalities that target the CYP pathway makes ethical sense for the affected individuals and decreases the socioeconomic burden of this disease. However, more research is needed to allow the integration of this information into a clinical setting.

Cellular and molecular biology of prostacyclin synthase

Prostacyclin synthase (PGIS) cDNA comprises 1500 nucleotides coding for a 500 amino acid protein. It is a heme protein with spectral characteristics of cytochrome p450 (CYP). It does not possess the typical CYP mono-oxygenase activity but catalyzes the rearrangement of prostaglandin H2 to form PGI2. Analysis of its structure-function by molecular modeling and site-directed mutagenesis reveals a long substrate channel lined by hydrophobic residues. Cys-441 has been identified as the proximal axial ligand of heme. Tyr-430 is nitrated by peroxynitrite which results in reduced PGIS catalytic activity, suggesting that Tyr-430 is located close to the heme pocket. PGIS is constitutively expressed and may be upregulated by cytokines, reproductive hormones, and growth factors. It is physically colocalized with cyclooxygenases and phospholipases, and functionally coupled with these enzymes. PGIS coupling with COX-2 has been shown to play an important role in vascular protection, embryo development and implantation, and cancer growth.

The N-terminal membrane anchor domain of the membrane-bound prostacyclin synthase involved in the substrate presentation of the coupling reaction with cyclooxygenase

To mimic the native conditions, the cyclooxygenase (COX)/prostaglandin I(2) synthase (PGIS) coupling reaction system was used to determine the coordination of PGIS with COX for the biosynthesis of prostacyclin (PGI(2)) using arachidonic acid (AA) as a substrate in a membrane-bound environment. The membrane-bound PGIS exhibited a faster isomerization of PGH(2) produced by COX to PGI(2) than the detergent-solubilized PGIS. To determine whether the N-terminal domain of PGIS responds to the facilitation of PGH(2) movement (presentation) from COX to the active site of PGIS, the first 20 residues of PGIS (Delta20-PGIS) were deleted and expressed in COS-7 cells. Delta20-PGIS retained membrane-bound properties and exhibited a slower substrate presentation property. Furthermore, a chimeric molecule (PGIS/TXAS(8-27)) with the replacement of the first 20 residues of PGIS by the corresponding membrane anchor region (residues 8-27) of thromboxane A(2) synthase was created to evaluate the mechanism influencing the biosynthesis of PGI(2) in coordination with COX. The chimera revealed a multiple fold delay in the PGH(2) presentation in low range concentrations of AA (0.3-3muM) at 30s reactions. However, the delay could be recovered by a longer incubation time in high range concentrations of AA (>10muM), but not in low range concentrations of AA. These results demonstrated that the N-terminal domain of PGIS plays a role in the facilitation of the substrate presentation to the PGIS active site in low concentrations of AA, which may be a physiological condition. The TXAS N-terminal domain could not replace the function of the corresponding domain of PGIS, indicating that the facilitation of the substrate presentation is specific.

Mammary uptake and excretion of prostanoids in relation to mammary blood flow and milk yield during pregnancy-lactation and somatotropin treatment in dairy goats

Mammary arterious-venous differences (A-V) and excretion into milk of four prostanoids were related to changes in milk yield and milk vein blood velocity (MBV) in goats at different stages of pregnancy and lactation, and during somatotropin (ST) treatment in mid-lactation. Arterial concentrations and mammary A-V for the vasodilators prostacyclin (PGI(2)) and prostaglandin (PG) E(2) (measured as 6-keto-PGF(1 alpha) and bicyclic PGE(2), respectively) decreased from late pregnancy to lactation. A-V were negatively correlated to MBV (r = -0.32 to -0.34). Arterial concentrations of the vasoconstrictors PGF(2 alpha) and TXA(2) (measured as TXB(2)) changed similarly, but no A-V across the mammary gland were found. The vasodilator to vasoconstrictor ratio in plasma was around 1:1, and in skimmed milk around 0.29-0.49 due to significantly higher TXB(2) levels in milk compared to plasma. Close linear correlations were established between milk yield and excretion of TXB(2) into milk (r = 0.80, P < 0.001), and between MBV and PGE(2) excretion into milk (r = 0.69, P < 0.001). ST treatment stimulated MBV and mammary prostanoid supply, and decreased prostanoid concentration in milk vein plasma. The high arterial levels of prostaglandins during pregnancy most likely reflected uterine synthesis. Our results support a role for PGI(2) and PGE(2) in local mammary blood flow regulation during lactation. Increased mammary uptake of these two prostanoids may be involved in the mammary blood flow response to ST. TXA(2) may be synthesized by mammary epithelial as well as vascular cells, and TXA(2) may be an important factor in regulation of mammary function.

Sexual dimorphism in prostanoid-potentiated vascular contraction: roles of endothelium and ovarian steroids

The effects of constrictor prostanoid (CP) pathway inhibitors on vascular reactivity to vasopressin (VP) and phenylephrine (PE) were examined in thoracic aortas of male, female, and ovariectomized (OVX) female Sprague-Dawley rats. Maximal contractile response of control (Cont) aortas to VP was markedly higher in females (3,885 +/- 332 mg/mg ring wt) than in males (810 +/- 148 mg). Indomethacin (Indo; 10 microM) attenuated maximal response to VP in females (3,043 +/- 277 mg) but not in males. SQ-29,548 (SQ; 1 microM) attenuated maximal response to VP in females (3,042 +/- 290 mg) to a similar extent as Indo. Dazoxiben (Daz; 10 microM) alone had no effect, but Daz + SQ attenuated maximal contractile response to VP to a similar extent as SQ alone. Removal of the endothelium in female aortas attenuated contractile responses to VP in Cont aortas. OVX attenuated maximal contractile response to VP in Cont aortas (2,093 +/- 329 mg) and abolished the attenuating effects of Indo. Indo, SQ, and Daz exerted identical effects on contractile responses of male, female, and OVX female aortas to PE. These findings establish the following in the rat aorta: 1) CP, probably thromboxane and/or endoperoxide, is responsible for approximately 25-30% of contractile responses of females, but not males, to VP and PE; 2) CP production by the female aorta is primarily endothelial in origin; and 3) ovarian steroids modulate production and/or actions of CP in female aortas.

Role of contaminating platelets in thromboxane synthesis in primary cultures of human umbilical vein endothelial cells

Previous studies suggested that cultured human endothelial cells metabolize arachidonic acid to thromboxane A2. When primary cultures of human umbilical vein endothelial cells were incubated with 14C-arachidonic acid and the 14C-metabolites resolved by reverse phase high pressure liquid chromatography, radioactive products were observed that comigrated with 6-keto-prostaglandin F1alpha and thromboxane B2, the degradation products of prostacyclin and thromboxane A2, respectively. Since platelets synthesize thromboxane A2, the present study examined the hypothesis that adherent platelets may contaminate the primary cultures of human umbilical vein endothelial cells and be responsible for thromboxane B2 production. Confluent primary cultures or passaged cells were stimulated with histamine (10(-5) M). Incubation buffer was analyzed by specific radioimmunoassays for 6-keto-prostaglandin F1alpha and thromboxane B2. The production of thromboxane B2 decreased in the passaged cells (207 +/- 44 pg/ml versus 65 +/- 12 pg/ml; primary versus passaged cells). A moderate decrease in the yield of 6-keto-prostaglandin F1alpha was measured in the passaged cells compared to the primary cultures (3159 +/- 356 pg/ml versus 1678 +/- 224 pg/ml, primary versus passaged cells). If the primary cultures were incubated with human platelet-rich plasma for 30 min prior to stimulation with histamine, the amount of thromboxane B2 increased approximately 10-fold. In an additional experiment, sub-confluent primary cells were incubated with platelet-rich plasma for 30 min, washed to remove non-adherent platelets, and allowed to reach confluency. Confluent cells were then passaged and stimulated with histamine. The amount of thromboxane B2 was not significantly different from that obtained with passaged cells that had not been incubated with platelet-rich plasma during the primary culture (83 +/- 15 pg/ml versus 65 +/- 12 pg/ml, respectively). If the cyclooxygenase inhibitor indomethacin was included in the incubations, the amounts of both thromboxane B2 and 6-keto-prostaglandin F1alpha decreased. In contrast, the thromboxane A2 synthase inhibitor dazoxiben blocked thromboxane production and had no effect on the amount of 6-keto-prostaglandin F1alpha. Light microscopy revealed the presence of adherent platelets in primary cultures with and without platelet-rich plasma but no platelets were observed in any group of passaged cells. Histofluorescence for platelet serotonin indicated the presence of platelets only in primary cultures of human umbilical vein endothelial cells or in cultures pre-incubated with platelet-rich plasma. These studies suggest that primary cultures of human umbilical vein endothelial cells contain adherent platelets that contribute to thromboxane synthesis.

Determinants of the cellular specificity of acetaminophen as an inhibitor of prostaglandin H(2) synthases

Acetaminophen has antipyretic and analgesic properties yet differs from the nonsteroidal antiinflammatory drugs and inhibitors of prostaglandin H synthase (PGHS)-2 by exhibiting little effect on platelets or inflammation. We find parallel selectivity at a cellular level; acetaminophen inhibits PGHS activity with an IC(50) of 4.3 microM in interleukin (IL)-1 alpha-stimulated human umbilical vein endothelial cells, in contrast with an IC(50) of 1,870 microM for the platelet, with 2 microM arachidonic acid as substrate. This difference is not caused by isoform selectivity, because acetaminophen inhibits purified ovine PGHS-1 and murine recombinant PGHS-2 equally. We explored the hypothesis that this difference in cellular responsiveness results from antagonism of the reductant action of acetaminophen on the PGHSs by cellular peroxides. Increasing the peroxide product of the PGHS-cyclooxygenase, prostaglandin G(2) (PGG(2)), by elevating the concentration of either enzyme or substrate reverses the inhibitory action of acetaminophen, as does the addition of PGG(2) itself. 12-Hydroperoxyeicosatetraenoic acid (0.3 microM), a major product of the platelet, completely reverses the action of acetaminophen on PGHS-1. Inhibition of PGHS activity by acetaminophen in human umbilical vein endothelial cells is abrogated by t-butyl hydroperoxide. Together these findings support the hypothesis that the clinical action of acetaminophen is mediated by inhibition of PGHS activity, and that hydroperoxide concentration contributes to its cellular selectivity.

Identification of the substrate interaction site in the N-terminal membrane anchor segment of thromboxane A2 synthase by determination of its substrate analog conformational changes using high resolution NMR technique

The present studies describe an investigation for the interaction of N-terminal membrane anchor domain of thromboxane A(2) synthase (TXAS) with its substrate analog in a membrane-bound environment using the two-dimensional NMR technique. TXAS and prostaglandin I(2) synthase (PGIS), respectively, convert the same substrate, prostaglandin H(2) (PGH(2)), to thromboxane A(2) and prostaglandin I(2), which have opposite biological functions. Our topology studies have indicated that the N-terminal region of TXAS has a longer N-terminal endoplasmic reticulum (ER) membrane anchor region compared with the same segment proposed for PGIS. The differences in their interaction with the ER membrane may have an important impact to facilitate their common substrate, PGH(2), across the membrane into their active sites from the luminal to the cytoplasmic side of the ER. To test this hypothesis, we first investigated the interaction of the TXAS N-terminal membrane anchor domain with its substrate analog. A synthetic peptide corresponding to the N-terminal membrane anchor domain (residues 1-35) of TXAS, which adopted a stable helical structure and exhibited a membrane anchor function in the membrane-bound environment, was used to interact with a stable PGH(2) analog,. High resolution two-dimensional NMR experiments, NOESY and TOCSY, were performed to solve the solution structures of in a membrane-mimicking environment using dodecylphosphocholine micelles. Different conformations were clearly observed in the presence and absence of the TXAS N-terminal membrane anchor domain. Through combination of the two-dimensional NMR experiments, completed (1)H NMR assignments of were obtained, and the data were used to construct three-dimensional structures of in H(2)O and dodecylphosphocholine micelles, showing the detailed conformation change upon the interaction with the membrane anchor domain. The observation supported the presence of a substrate interaction site in the N-terminal region. The combination of the structural information of and was able to simulate a solution structure of the unstable TXAS and PGIS substrate, PGH(2).

Thromboxanes: synthase and receptors

Thromboxane A2 is a biologically potent arachidonate metabolite through the cyclooxygenase pathway. It induces platelet aggregation and smooth muscle contraction and may promote mitogenesis and apoptosis of other cells. Its roles in physiological and pathological conditions have been widely documented. The enzyme that catalyzes its synthesis, thromboxane A2 synthase, and the receptors that mediate its actions, thromboxane A2 receptors, are the two key components critical for the functioning of this potent autacoid. Recent molecular biological studies have revealed the structure-function relationship and gene organizations of these proteins as well as genetic and epigenetic factors modulating their gene expression. Future investigation should shed light on detailed molecular signaling events specifying thromboxane A2 actions, and the genetic underpinning of the enzyme and the receptors in health and disease.

Involvement of thromboxane A2 in the endothelium-dependent contractions induced by myricetin in rat isolated aorta

1. The present study was undertaken to analyse the mechanism of the contractile response induced by the bioflavonoid myricetin in isolated rat aortic rings. 2. Myricetin induced endothelium-dependent contractile responses (maximal value=21+/-2% of the response induced by 80 mM KCl and pD2=5.12+/-0.03). This effect developed slowly, reached a peak within 6 min and then declined progressively. 3. Myricetin-induced contractions were almost abolished by the phospholipase A2 (PLA2) inhibitor, quinacrine (10 microM), the cyclo-oxygenase inhibitor, indomethacin (10 microM), the thromboxane synthase inhibitor, dazoxiben (100 microM), the putative thromboxane A2 (TXA2)/prostaglandin endoperoxide receptor antagonist, ifetroban (3 microM). These contractions were abolished in Ca2+-free medium but were not affected by the Ca2+ channel blocker verapamil (10 microM). 4. In cultured bovine endothelial cells (BAEC), myricetin (50 microM) produced an increase in cytosolic free calcium ([Ca2+]i) which peaked within 1 min and remained sustained for 6 min, as determined by the fluorescent probe fura 2. This rise in [Ca2+]i was abolished after removal of extracellular Ca2+ in the medium. 5. Myricetin (50 microM) significantly increased TXB2 production both in aortic rings with and without endothelium and in BAEC. These increases were abolished both by Ca2+-free media and by indomethacin. 6. Taken together, these results suggests that myricetin stimulates Ca2+ influx and subsequently triggers the activation of the PLA2 and cyclo-oxygenase pathways releasing TXA2 from the endothelium to contract rat aortic rings. The latter response occurs via the activation of Tp receptors on vascular smooth muscle cells.

Endothelial dysfunction in DOCA-salt hypertension: possible involvement of prostaglandin endoperoxides

The effects of the arachidonic acid metabolism inhibitors on the acetylcholine responses of aortae from control (CR) and deoxycorticosterone acetate (DOCA)-salt hypertensive (HR) rats were investigated. The acetylcholine decreased response observed in HR [relaxation (%): CR 95.5+/-2.7, n = 4; HR 52.0+/-6.3, n = 5, p < 0.05] was restored by the cyclooxygenase inhibitor piroxicam [relaxation (%): CR 99.8+/-0.2, n = 4; HR 86.0+/-4.0, n = 5] and by the thromboxane synthetase inhibitor and the thromboxane A2/prostaglandin H2 receptor antagonist ridogrel [relaxation (%): CR 92.1+/-4.4, n = 7; HR 93.1+/-2.0, n = 7] but not by the inhibitors of thromboxane synthetase, prostacyclin synthetase, cytochrome P-450 monooxygenase, and lipoxygenase. So, endoperoxide intermediates seem to be involved in the decreased endothelium-dependent relaxation to acetylcholine in DOCA-salt hypertension.

Characterization of the secondary structure and membrane interaction of the putative membrane anchor domains of prostaglandin I2 synthase and cytochrome P450 2C1

Prostaglandin I2 synthase (PGIS) produces prostaglandin I2 (PGI2) which has opposite actions on platelet aggregatory and vasoconstrictive properties compared to thromboxane A2 (TXA2) produced from the same substrate by another P450 enzyme, thromboxane A2 synthase (TXAS). PGIS and TXAS have only 16% amino acid sequence identity. Hydropathy analysis suggests that the putative NH2-terminal membrane anchor domain of PGIS is similar to many other membrane-bound microsomal P450s, which are believed to be anchored by a single transmembrane segment, and thus different from the TXAS anchor, which appears to have two transmembrane segments. To characterize the membrane anchor function of the PGIS NH2-terminal region, we have used the peptidoliposome reconstitution assay to identify the membrane anchor segment in the PGIS NH2-terminal domain and compared it with the anchor segment of P450 2C1. Four peptides, mimicking putative NH2-terminal membrane anchor segments of PGIS and P450 2C1, containing residues 1-28 (PGIS-LP1 and P450 2C1-LP1) or residues 25-54 (PGIS-LP2 and P450 2C1-LP2), were synthesized and their ability to insert in a lipid bilayer was evaluated. The results indicated that both LP1 peptides of PGIS and P450 2C1 became bound to the lipid bilayer, whereas both LP2 peptides did not bind the lipid. The two LP1 peptides were further characterized as to their conformation using CD spectroscopy. Helical structure induced in these peptides by addition of trifluoroethanol, dodecylphosphocholine, or incorporation into liposomes indicated that these segments tend to adopt a helical structure in a hydrophobic environment and thus could function as membrane anchor segments. These results support the hypothesis that PGIS and TXAS interact with the endoplasmic reticulum membrane in different ways, in which the NH2-terminal anchor domain of PGIS, as with P450 2C1, appears to have a single transmembrane segment.

Effects of high glucose concentration and a thromboxane synthase inhibitor on the production of thromboxane A2 and prostaglandin I2 and E2 by retinal endothelial cells

To clarify the involvement of the prostaglandin (PG)-thromboxane (TX) system in diabetic retinopathy, the production of thromboxane A2 (TXA2), prostaglandin I2 (PGI2) and prostaglandin E2 (PGE2) and the effects of a thromboxane synthase inhibitor (TXSI; KDI-792; 5Z-6[(2S,4R)-4-(4-chlorophenylsulfonylamino)-1-(3-pyridyl methyl)-2 pyrrolidinyl]-5-hexenoic acid hydrochloride) were examined under high glucose concentration using bovine retinal endothelial cells. TXB2 was used as an index of TXA2, and 6-keto-PGF1 alpha as an index of PGI2. The levels of TXA2 and PGI2 were 182.3 +/- 34.3 pg/mg and 336.7 +/- 36.1 pg/mg protein at 5.5 mM of glucose, and both increased linearly with the glucose concentration to reach 430.1 +/_ 29.7 pg/mg and 511.4 +/- 65.8 pg/mg protein at 33 mM glucose (mean +/- SD, P<0.01). Neither TXA2 nor PGI2 changed significantly under elevated osmolarity. The production of PGE2 was affected only slightly by high glucose concentrations or by TXSI. Normalization of the PGI2/TXA2 ratio by TXSI at high glucose concentrations was marked.

In vitro evaluation of electrostatic endothelial cell transplantation onto 4 mm interior diameter expanded polytetrafluoroethylene grafts

PURPOSE

To perform an in vitro evaluation of electrostatic endothelial cell transplantation of human umbilical vein endothelial cells (HUVEC) onto segments of 4 mm internal diameter expanded polytetrafluoroethylene (ePTFE) vascular prostheses.

METHODS

This evaluation consisted of exposing vascular graft segments that had been subjected to either electrostatic or gravitation transplantation with HUVEC to a physiologic shear stress (15 dynes/cm2) under steady flow conditions within a flow loop system. Biochemical assays were performed on freshly transplanted grafts by means of radioimmunoassay for prostacyclin and thromboxane A2.

RESULTS

There was a 30% loss of HUVEC after 30 minutes of shear stress exposure from the grafts subjected to gravitational transplantation with no additional significant (alpha = 0.05) loss after 120 minutes. Grafts subjected to electrostatic transplantation had no significant (alpha = 0.05) loss of HUVEC during exposure to physiologic shear stress. Furthermore, after 120 minutes of shear-stress exposure, the grafts subjected to electrostatic transplantation (78,420 +/- 6274 HUVEC/cm2) retained 2.3 times more HUVEC than the counterparts subjected to gravitational transplantation (34,427 +/- 4637 HUVEC/cm2). The biochemical assay results indicated no significant (alpha = 0.05) production of prostacyclin or thromboxane A2 regardless of the method of cell transplantation.

CONCLUSIONS

(1) The electrostatic transplantation technique was superior to the gravitational transplantation technique in terms of cellular retention when the ePTFE grafts were exposed to physiologic shear stress. (2) Production of prostacyclin and thromboxane A2 did not differ between transplanted HUVEC subjected to gravitational or electrostatic procedures.

Methacholine-induced contraction of rabbit pulmonary artery: role of platelet-endothelial transcellular thromboxane synthesis

Arachidonic acid- and methacholine-induced contractions of rabbit pulmonary arteries are mediated by thromboxane (TX) A2. Although removal of the endothelium abolishes the contractions, endothelial cells isolated from pulmonary arteries do not synthesize TXA2. Further studies described here showed that the expression of TX synthase was evident in platelets and intact pulmonary artery but not in endothelial cells. These studies examined the role of platelet TXA2 production in the vasoconstrictor response to methacholine. Endothelial cells were incubated with platelets in the presence or absence of methacholine. Methacholine caused an increase in TXB2 production. Pretreatment of endothelial cells with aspirin (100 micromol/L) before the addition of platelets did not impair the ability of methacholine to increase TXB2 synthesis. Conversely, if platelets were pretreated with aspirin, methacholine failed to stimulate TXB2. Using endothelial cells with their cellular lipids labeled with [3H]arachidonic acid, methacholine did not stimulate the production of [3H]TXB2. When the endothelial cells were incubated with methacholine and control platelets, [3H]TXB2 was detected. If aspirin-treated platelets were incubated with endothelial cells, methacholine did not increase the production of [3H]TXB2. However, pretreatment of the endothelial cells with aspirin did not affect the ability of methacholine to induce [3H]TXB2 release. This suggests that methacholine stimulated the endothelial cell to release arachidonic acid, which was transferred to the platelets and metabolized to TXA2. To test whether this cell-cell interaction is necessary for methacholine-induced contractions, rabbits were administered aspirin (20 mg/kg) for 2 days. On day 4, methacholine-induced contractions of pulmonary arteries were depressed in aspirin-treated compared with control subjects. Control arteries synthesized 6-keto-prostaglandin F1alpha and TXB2. Aspirin treatment inhibited both pulmonary artery and platelet TXB2 production but had no effect on vessel 6-keto-prostaglandin F1alpha. These studies implicate platelets as a vascular source of TXA2 and indicate that both endothelial cells and platelets may be required for methacholine-induced TXA2 synthesis and vasoconstriction.

Role of fatty acids and eicosanoids in modulating proteoglycan metabolism in endothelial cells

Endothelial cell dysfunction is considered to be a critical event in the etiology of atherosclerosis. Thus, the preservation of endothelial structure and function are a prerequisite for normal control of vascular permeability properties, mediation of both inflammatory and immunologic responses and the general 'communication' between blood-borne cells and abluminal tissues. Many of these properties can be influenced by proteoglycans present in vascular tissues. There is evidence that selected lipids can be atherogenic by altering endothelial proteoglycan metabolism. Little is known about the role of fatty acids in modulating proteoglycan composition in endothelial cells. Data suggest, however, that linoleic acid in particular can adversely alter proteoglycan metabolism, which may be related to an imbalance in eicosanoid synthesis patterns. These events could be sufficient to disrupt normal endothelial barrier function, initiate smooth muscle migration and proliferation, and result in other metabolic dysfunctions associated with the etiology of vascular diseases such as atherosclerosis. Thus, the focus of this review is on fatty acids and eicosanoids as they may alter proteoglycan metabolism of vascular tissues and in particular of the endothelium.

In vitro evaluation of antithrombogenicity of hybrid-type vascular vessel models based on analysis of the mechanism of blood coagulation

In vitro evaluation of antithrombogenicity of materials of artificial vascular vessels was carried out based on an analysis of the coagulation mechanism of blood. Coagulation of blood in tubes coated with endothelial cells (ECs) or segmented polyurethane (SPU) was measured by a rheological technique. Coagulation of blood in hybrid-type vascular vessel model tubes consisting of cultured ECs proliferated on carbon-deposited SPU was also examined. Coagulation of blood in these tubes occurred within 40 min, and appears to be due to the interaction between erythrocytes and a coagulant factor in plasma. From the in vitro evaluation test, when the time of onset of the coagulation of blood in contact with a surface of the artificial materials is longer than 20-39 min, the materials can be considered to provide an inert surface against thrombus formation and blood coagulation.

Effect of constant and modulated electrical charges applied to the culture material on PGI2 and TXA2 secretion by endothelial cells

Electrical stimulation of an endothelial cell culture applied through the culture support biomaterial induced strong secretion of prostaglandin I2 (PGI2) and, to a lesser extent, thromboxane A2 (TXA2), which varied with time, the sign of the stationary electrical charge and the amplitude and frequency of a superimposed sine wave. In stationary conditions, the response is more rapid at positive electrical charges (+ some tens of microC cm-2) than at negative ones (-some tens of microC cm-2). In sine wave conditions, the ratio [PGI2]/[TXA2] is strongly increased and is maximum for an amplitude of 50 mV and a frequency of 1 Hz. These results suggest that the extracellular matrix is not only involved in cellular anchoring but also participates actively in secretion of thrombomodulatory substances, an effect apparently related to the piezoelectric properties of its components.

Modulation of endothelial cell permeability by lung carcinoma cells: a potential mechanism of malignant pleural effusion formation

This study examined the hypothesis that tumor cells metastatic to the pleura secrete a soluble factor(s) that directly increases endothelial cell permeability. Nitrocellulose filters were endothelialized with bovine pulmonary artery endothelial cells and exposed to conditioned media from either human lung adenocarcinoma (Calu-3), human lung squamous cell carcinoma (SK-MES-1), or control media for 16 h. The diffusional permeability (Pd x 10(-5) cm/sec) to [14C]albumin was then determined for each monolayer with Ussing-type chambers. Both adenocarcinoma conditioned media (ACCM) and squamous cell carcinoma conditioned media (SCCM) caused a two- to threefold increase in endothelial monolayer permeability. The addition of indomethacin (10 micrograms/ml) blocked the observed permeability increase in ACCM but not in SCCM, suggesting that the increase in permeability by ACCM was secondary to the production of prostaglandins. To confirm this, a variety of prostanoids previously shown to be produced by the Calu-3 cell line were added directly to the endothelial monolayer. Prostaglandin F2 alpha (PGF2 alpha) in both low (10 ng/ml) and high (100 ng/ml) concentrations for 16 h resulted in a three- to fourfold increase in permeability. Prostaglandin E2 (PGE2) resulted in a small increase in [14C]albumin permeability but only at high concentrations (100 ng/ml). PGF2 alpha production by the two tumor cell lines was measured using radioimmunoassay. Baseline adenocarcinoma production of PGF2 alpha was 117.5 pmol/10(6) cells and fell to 24.2 pmol/10(6) cells hours following incubation with indomethacin. The decrease in PGF2 alpha occurred in parallel with the changes in permeability. Concomitant, reversible changes in cell shape and F-actin distribution were detected in endothelial cells exposed to ACCM. No significant production of PGF2 alpha by the squamous cell carcinoma cell line was detected. These results suggest that both adenocarcinoma and squamous cell carcinoma secrete a soluble factor(s) that directly increases endothelial cell permeability to albumin and that in the case of adenocarcinoma this soluble factor may be a prostanoid such as PGF2 alpha.

Role of thromboxane A2 in muscle injury following ischaemia

The effect of a thromboxane A2 receptor antagonist (GR32191) on gastrocnemius muscle blood flow, oedema and viability was assessed in a rodent model of 6-h unilateral hindlimb ischaemia and 4-h reperfusion, and the results compared with those in control and normal groups, and in animals undergoing 6-h ischaemia alone. Control animals demonstrated reduced muscle blood flow throughout reperfusion (at 10 min, P < 0.01 versus normal, P not significant versus ischaemia; at 120 min, P < 0.05 versus normal and ischaemia; at 240 min, P < 0.01 versus normal, P not significant versus ischaemia), and the development of muscle oedema (P < 0.01 versus normal and ischaemia) and muscle necrosis (P < 0.01 versus normal and ischaemia). In contrast, the thromboxane A2 receptor antagonist enhanced muscle blood flow (at 10 min, P < 0.01 versus control; at 120 min, P < 0.05 versus control; at 240 min, P < 0.01 versus control) and preserved muscle viability (P < 0.01 versus control; P not significant versus normal and ischaemia). These results indicate that thromboxane A2 is an important mediator of skeletal muscle reperfusion injury and suggest that administration of a thromboxane A2 receptor antagonist may improve limb salvage rates after surgery for acute ischaemia.

Characterization of monoclonal antibodies generated against bovine and porcine prostacyclin synthase and quantitation of bovine prostacyclin synthase

Monoclonal antibodies were raised against prostacyclin synthases purified from bovine and porcine aortae, respectively. Two monoclonal antibodies, RS1 and RS2, were purified and characterized. As shown by enzyme activity precipitation and Western blot analysis, in solubilized bovine and porcine aortae microsomes the monoclonal antibodies reacted only with prostacyclin synthase. The monoclonal antibody RS1 cross-reacts with partially purified prostacyclin synthase from human umbilical veins in an ELISA-based assay. None of the antibodies inhibited the enzyme activity. By combination of the monoclonal antibody RS2 with a polyclonal antibody we established an enzyme-linked immunosorbent assay (ELISA) for quantitation of bovine prostacyclin synthase. ELISA data were confirmed by Western blot analysis. Among different bovine tissues, aortae with 1665 +/- 200 ng/mg microsomal protein showed the highest content of PGIS. Significant lower concentrations were observed in tongue, lung, kidney and thymus ranging from 49 +/- 13.4 to 2.7 +/- 0.9 ng/mg protein. The monoclonal antibody RS1 binds to endothelial cells and vascular smooth muscle cells in human liver tissue.

Thromboxane A2 and prostacyclin biosynthesis in children and adolescents with pulmonary vascular disease

BACKGROUND

The pathogenesis of pulmonary vascular disease in children with congenital heart disease is incompletely understood. Thromboxane (TX) A2 and prostacyclin (PGI2) have opposing effects on platelet aggregation and pulmonary vascular smooth muscle. An imbalance in their biosynthesis could contribute to the progressive increase in pulmonary vascular resistance seen in older untreated patients with pulmonary hypertensive congenital heart disease and the thrombotic complications they may develop.

METHODS AND RESULTS

We investigated TXA2 and PGI2 biosynthesis in 15 young children (0.2 to 2.25 years old) with congenital heart disease with increased pulmonary blood flow and potentially reversible pulmonary vascular disease by measuring urinary excretion of 2,3-dinor-TXB2 and 2,3-dinor-6-oxoprostaglandin (PG) F1 alpha and compared the findings with those in 16 healthy children (0.5 to 2.8 years old). 2,3-Dinor-TXB2 excretion was greater in the patients than in control subjects (1253 +/- 161 versus 592 +/- 122 ng/g creatinine; P < .001). Excretion of 2,3-dinor-6-oxo-PGF1 alpha was 452 +/- 54 compared with 589 +/- 95 ng/g creatinine in control subjects. In 5 patients who underwent successful cardiac surgery > or = 1 year later excretion of 2,3-dinor-TXB2 decreased from 1100 +/- 298 to 609 +/- 131 ng/g creatinine (P < .05), a value comparable to those in 5 healthy children of similar age (749 +/- 226 ng/g creatinine). We also compared 15 patients (11 to 23 years old) with advanced irreversible pulmonary vascular disease with 19 healthy control subjects (10 to 23 years old). The ratio of TX to PGI2 metabolite excretion was greater in the patients than in control subjects (3.5 +/- 0.6 versus 2.0 +/- 0.3; P < .05).

CONCLUSIONS

There is increased 2,3-dinor-TXB2 excretion in children with congenital heart disease and a high pulmonary blood flow that may reflect an imbalance in biosynthesis of TXA2 and PGI2 in the pulmonary vascular bed. The imbalance may contribute to the progressive development of increased pulmonary vascular resistance and persists in older patients whose heart defects are uncorrected.

Chronic hypoxic pulmonary hypertension. Is thrombin involved?

Thrombin contracts vascular smooth muscle and stimulates its proliferation. Using a specific thrombin inhibitor, hirudin, we studied whether thrombin contributes to the pulmonary vasoconstriction and vascular proliferation that occurs in pulmonary hypertension. Hirudin was infused intravenously (0.2 mg/h/kg) by minipumps in nine rats during a 3-wk exposure to hypobaric hypoxia (HH). Vehicle (normal saline) was infused in eight hypoxic control (HC) and seven normoxic control (NC) rats. Sufficient hirudin delivery was confirmed by a failure of undiluted plasma from HH, but not from NC and HC, to clot in response to thrombin. When the plasma samples were diluted 1:10, the thrombin time was significantly prolonged in HH when compared with that in both NC and HC. Although hirudin slightly reduced mean pulmonary arterial pressure in open-chest rats, there was no significant difference between the hypoxic groups in total pulmonary resistance, right ventricle weight, morphologic remodeling of lung vessels, or the perfusion pressure-flow relationship in isolated lungs. Vasoconstrictor responses of isolated lungs to angiotensin II and acute hypoxic challenges were not affected by hirudin treatment. We conclude that hirudin, in a dose sufficient to reduce thrombin's catalytic effect on fibrinogen, does not significantly prevent the development of chronic hypoxic pulmonary hypertension.

Endothelium-dependent contractions in rabbit pulmonary artery are mediated by thromboxane A2

This study was designed to characterize the endothelium-dependent contracting factor (EDCF) released by arachidonic acid (AA) and methacholine (MeCH) in the rabbit pulmonary artery. AA and MeCH contract the rabbit pulmonary artery; however, the effects of both are blocked by denuding the vessels and by administration of indomethacin (a cyclooxygenase inhibitor), dazoxiben (a thromboxane [TX] synthase inhibitor), and SQ29548 (a TXA2/prostaglandin [PG] H2 receptor antagonist). When segments of rabbit pulmonary artery were incubated with [14C]AA and the [14C] metabolites were resolved by reverse-phase high-performance liquid chromatography (HPLC), radioactive products were observed that comigrated with 6-keto-PGF1 alpha and TXB2, the stable metabolites of prostacyclin and TXA2. The TXB2 radioactive peak was rechromatographed on normal-phase HPLC and again migrated with TXB2. Finally, the structures of derivatized [14C]6-keto-PGF1 alpha and [14C]TXB2 peaks were confirmed by gas chromatography/mass spectrometry. The synthesis of [14C]6-keto-PGF1 alpha and [14C]TXB2 was inhibited by removal of the endothelium and by indomethacin. Dazoxiben inhibited the synthesis of [14C]TXB2 but not [14C]6-keto-PGF1 alpha. Using specific radioimmunoassays, AA and MeCH stimulated 6-keto-PGF1 alpha and TXB2 release. Indomethacin blocked the production of both 6-keto-PGF1 alpha and TXB2, whereas dazoxiben only blocked TXB2. In a superfusion/bioassay system, AA stimulated an endothelium-intact donor vessel to release a labile substance that contracted an indomethacin-treated endothelium-denuded recipient vessel. The EDCF released by AA had an approximate half-life of 30 seconds. Cultured rabbit pulmonary arterial endothelial cells synthesized 6-keto-PGF1 alpha but not TXB2. Immunohistochemical studies indicated the presence of cyclooxygenase, but not TX synthase, in pulmonary artery endothelial cells. TXA2 appears to be the EDCF released by AA and MeCH in rabbit pulmonary artery; however, TXA2 is not produced by endothelial cells but may arise from cells that adhere to the luminal surfaces, such as platelets or macrophages.