Topography of Prostaglandin H Synthase

The immunosuppressive capacity of human mesenchymal stromal cells derived from amnion and bone marrow

Mesenchymal stromal cells derived from amnion (AM-MSCs) can be easily obtained in large quantity by less invasive method in comparison to bone marrow-derived MSCs (BM-MSCs). However, the biological and immunosuppressive properties of AM-MSCs are still poorly characterized. Previous studies demonstrated that BM-MSCs expressed indoleamine 2,3-dioxygenase (IDO) to suppress T-cell responses. This study was designed to address whether IDO contributes to the immunosuppressive function of AM-MSCs. MSCs isolated from amnion were cultured in complete medium similar to BM-MSCs. After culture, AM-MSCs exhibited spindle shape morphology and expressed MSC markers similar to that of BM-MSCs. In addition, AM-MSCs were able to differentiate into adipocytes and osteoblasts. Fascinatingly, AM-MSCs and BM-MSCs exhibited comparable degree of immunosuppressive effect when they were co-cultured with activated T-cells. In addition, IDO secreted by AM-MSCs was responsible for induction of immunosuppressive activities in the same manner as BM-MSCs. Taken together; the results of the present study demonstrate that while AM-MSCs and BM-MSCs show similar immunosuppressive effect, AM-MSCs may have additional advantage over the BM-MSCs in terms of availability. Therefore, AM-MSCs might be considered a potential source for therapeutic applications especially for treatment of immune related diseases.

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Cells isolated from amnion possess MSC characteristics.

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AM-MSCs displayed a similar degree of immunosuppression to that of BM-MSCs.

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Immunosuppression of AM-MSCs relies on a highly complex mechanism.

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The key immunosuppressive mechanism of AM-MSCs centers on the IFNγ-mediated IDO expression.

Pre-existent asymmetry in the human cyclooxygenase-2 sequence homodimer

Prostaglandin endoperoxide H synthase-2 (PGHS-2), also known as cyclooxygenase-2 (COX-2), is a sequence homodimer. However, the enzyme exhibits half-site heme and inhibitor binding and functions as a conformational heterodimer having a catalytic subunit (Ecat) with heme bound and an allosteric subunit (Eallo) lacking heme. Some recombinant heterodimers composed of a COX-deficient mutant subunit and a native subunit (i.e. Mutant/Native PGHS-2) have COX activities similar to native PGHS-2. This suggests that the presence of heme plus substrate leads to the subunits becoming lodged in a semi-stable Eallo-mutant/Ecat-Native∼heme form during catalysis. We examined this concept using human PGHS-2 dimers composed of combinations of Y385F, R120Q, R120A, and S530A mutant or native subunits. With some heterodimers (e.g. Y385F/Native PGHS-2), heme binds with significantly higher affinity to the native subunit. This correlates with near native COX activity for the heterodimer. With other heterodimers (e.g. S530A/Native PGHS-2), heme binds with similar affinities to both subunits, and the COX activity approximates that expected for an enzyme in which each monomer contributes equally to the net COX activity. With or without heme, aspirin acetylates one-half of the subunits of the native PGHS-2 dimer, the Ecat subunits. Subunits having an S530A mutation are refractory to acetylation. Curiously, aspirin acetylates only one-quarter of the monomers of S530A/Native PGHS-2 with or without heme. This implies that there are comparable amounts of two noninterchangeable species of apoenzymes, Eallo-S530A/Ecat-Native and Eallo-Native/Ecat-S530A. These results suggest that native PGHS-2 assumes a reasonably stable, asymmetric Eallo/Ecat form during its folding and processing.

Physicochemical performances of indomethacin in cholesteryl cetyl carbonate liquid crystal as a transdermal dosage

A transdermal formulation of indomethacin (IMC) was developed by incorporation into cholesteryl cetyl carbonate (CCC). The liquid crystalline phase properties of the IMC-CCC mixture were detected by polarized light microscopy and differential scanning calorimetry. A low drug loading was obtained (1-5 %) similar to that used in conventional topical IMC in a clinical setting. A controlled release of IMC was found over 12 h. A low amount of IMC in 1 % IMC-CCC permeated the stratum corneum. Further formulation development has been carried out by the addition of lauryl alcohol into 5 % IMC-CCC mixture it was found that the permeation of IMC was significantly improved to 45 % within 24 h.

Cyclooxygenase reaction mechanism of PGHS--evidence for a reversible transition between a pentadienyl radical and a new tyrosyl radical by nitric oxide trapping

Incubation of prostaglandin H synthase-1 (PGHS-1) under anaerobic conditions with peroxide and arachidonic acid leads to two major radical species: a pentadienyl radical and a radical with a narrow EPR spectrum. The proportions of the two radicals are sensitive to temperature, favoring the narrow radical species at 22 °C. The EPR characteristics of this latter radical are somewhat similar to the previously reported narrow-singlet tyrosine radical NS1a and are insensitive to deuterium labeling of AA. To probe the origin and structure of this radical, we combined EPR analysis with nitric oxide (NO) trapping of tyrosine and substrate derived radicals for both PGHS-1 and -2. Formation of 3-nitrotyrosine in the proteins was analyzed by immunoblotting, whereas NO adducts to AA and AA metabolites were analyzed by mass spectrometry and by chromatography of (14)C-labeled products. The results indicate that both nitrated tyrosine residues and NO-AA adducts formed upon NO trapping. The NO-AA adduct was predominantly an oxime at C11 of AA with three conjugated double bonds, as indicated by absorption at 275 nm and by mass spectral analysis. This adduct amounted to 10% and 20% of the heme concentration of PGHS-1 and -2, respectively. For PGHS-1, the yield of NO-AA adduct matched the yield of the narrow radical signal obtained in parallel EPR experiments. High frequency EPR characterization of this narrow radical, reported in an accompanying paper, supports assignment to a new tyrosyl radical, NS1c, rather than an AA-based radical. To reconcile the results from EPR and NO-trapping studies, we propose that the NS1c is in equilibrium with an AA pentadienyl radical, and that the latter reacts preferentially with NO.

In Silico Screening of Nonsteroidal Anti-Inflammatory Drugs and Their Combined Action on Prostaglandin H Synthase-1

The detailed kinetic model of Prostaglandin H Synthase-1 (PGHS-1) was applied to in silico screening of dose-dependencies for the different types of nonsteroidal anti-inflammatory drugs (NSAIDs), such as: reversible/irreversible, nonselective/selective to PGHS-1/PGHS-2 and time dependent/independent inhibitors (aspirin, ibuprofen, celecoxib, etc.) The computational screening has shown a significant variability in the IC50s of the same drug, depending on different in vitro and in vivo experimental conditions. To study this high heterogeneity in the inhibitory effects of NSAIDs, we have developed an in silico approach to evaluate NSAID action on targets under different PGHS-1 microenvironmental conditions, such as arachidonic acid, reducing cofactor, and peroxide concentrations. The designed technique permits translating the drug IC₅₀, obtained in one experimental setting to another, and predicts in vivo inhibitory effects based on the relevant in vitro data. For the aspirin case, we elucidated the mechanism underlying the enhancement and reduction (aspirin resistance) of its efficacy, depending on PGHS-1 microenvironment in in vitro/in vivo experimental settings. We also present the results of the in silico screening of the combined action of sets of two NSAIDs (aspirin with ibuprofen, aspirin with celecoxib), and study the mechanism of the experimentally observed effect of the suppression of aspirin-mediated PGHS-1 inhibition by selective and nonselective NSAIDs. Furthermore, we discuss the applications of the obtained results to the problems of standardization of NSAID test assay, dependence of the NSAID efficacy on cellular environment of PGHS-1, drug resistance, and NSAID combination therapy.

Tumor-infiltrating, myeloid-derived suppressor cells inhibit T cell activity by nitric oxide production in an intracranial rat glioma + vaccination model

In rats bearing an intracranial T9 glioma, immunization with tumor antigens induces myeloid suppressor cells, which express neutrophil (His48) and monocyte (CD11bc) markers, to infiltrate the tumors. The His48(+)/CD11bc(+) cells were not derived from CNS microglia but were hematogenous; suppressed multiple T cell effector functions; and are myeloid-derived suppressor cells (MDSC). The glioma-infiltrating MDSC expressed arginase I, iNOS, indoleamine 2,3-dioxygenase and TGF-beta; however, inhibitor/blocking studies demonstrated that NO production was the primary mechanism of suppression which induced T cell apoptosis. These findings suggest that neuro-immunomodulation by MDSC in rat gliomas maybe mediated by a pathway requiring NO production.

Cyclooxygenase competitive inhibitors alter tyrosyl radical dynamics in prostaglandin H synthase-2

Reaction of prostaglandin H synthase (PGHS) isoforms 1 or 2 with peroxide forms a radical at Tyr385 that is required for cyclooxygenase catalysis and another radical at Tyr504, whose function is unknown. Both tyrosyl radicals are transient and rapidly dissipated by reductants, suggesting that cyclooxygenase catalysis might be vulnerable to suppression by intracellular antioxidants. Our initial hypothesis was that the two radicals are in equilibrium and that their proportions and stability are altered upon binding of fatty acid substrate. As a test, we examined the effects of three competitive inhibitors (nimesulide, flurbiprofen, and diclofenac) on the proportions and stability of the two radicals in PGHS-2 pretreated with peroxide. Adding nimesulide after ethyl peroxide led to some narrowing of the tyrosyl radical signal detected by EPR spectroscopy, consistent with a small increase in the proportion of the Tyr504 radical. Neither flurbiprofen nor diclofenac changed the EPR line width when added after peroxide. In contrast, the effects of cyclooxygenase inhibitors on the stability of the preformed tyrosyl radicals were dramatic. The half-life of total tyrosyl radical was 4.1 min in the control, >10 h with added nimesulide, 48 min with flurbiprofen, and 0.8 min with diclofenac. Stabilization of the tyrosyl radicals was evident even at substoichiometric levels of nimesulide. Thus, the inhibitors had potent, structure-dependent, effects on the stability of both tyrosyl radicals. This dramatic modulation of tyrosyl radical stability by cyclooxygenase site ligands suggests a mechanism for regulating the reactivity of PGHS tyrosyl radicals with cellular antioxidants.

Kinetic modelling of NSAID action on COX-1: focus on in vitro/in vivo aspects and drug combinations

The detailed kinetic model of Prostaglandin H Synthase-1 (COX-1) was developed to in silico test and predict inhibition effects of nonsteroidal anti-inflammatory drugs (NSAIDs) on target. The model takes into account key features of the complex catalytic mechanism of cyclooxygenase-1, converting arachidonic acid to prostaglandin PGH(2), and includes the description of the enzyme interaction with various types of NSAIDs (reversible/irreversible, non-selective and selective to COX-1/COX-2). Two different versions of the model were designed to simulate the inhibition of COX-1 by NSAIDs in two most popular experimental settings - in vitro studies with purified enzyme, and the experiments with platelets. The developed models were applied to calculate the dose-dependence of aspirin and celecoxib action on COX- 1 in vitro and in vivo conditions. The mechanism of the enhancement of aspirin efficiency in platelet as compared to its action on purified COX-1 was elucidated. The dose-dependence of celecoxib simulated with the use of the "in vivo" version of the model predicted potentially strong inhibitory effect of celecoxib on thromboxan production in platelets. Simulation of the combined effect of two NSAIDs, aspirin and celecoxib, on COX-1 allowed us to reveal the mechanism underlying the suppression of aspirin-mediated COX-1 inhibition by celecoxib. We discuss our modelling results in the context of the on-going debates on the potential cardio-vascular risks associated with co-administration of various types of NSAIDs.

Structural basis of enantioselective inhibition of cyclooxygenase-1 by S-alpha-substituted indomethacin ethanolamides

The modification of the nonselective nonsteroidal anti-inflammatory drug, indomethacin, by amidation presents a promising strategy for designing novel cyclooxygenase (COX)-2-selective inhibitors. A series of alpha-substituted indomethacin ethanolamides, which exist as R/S-enantiomeric pairs, provides a means to study the impact of stereochemistry on COX inhibition. Comparative studies revealed that the R- and S-enantiomers of the alpha-substituted analogs inhibit COX-2 with almost equal efficacy, whereas COX-1 is selectively inhibited by the S-enantiomers. Mutagenesis studies have not been able to identify residues that manifest the enantioselectivity in COX-1. In an effort to understand the structural impact of chirality on COX-1 selectivity, the crystal structures of ovine COX-1 in complexes with an enantiomeric pair of these indomethacin ethanolamides were determined at resolutions between 2.75 and 2.85 A. These structures reveal unique, enantiomer-selective interactions within the COX-1 side pocket region that stabilize drug binding and account for the chiral selectivity observed with the (S)-alpha-substituted indomethacin ethanolamides. Kinetic analysis of binding demonstrates that both inhibitors bind quickly utilizing a two-step mechanism. However, the second binding step is readily reversible for the R-enantiomer, whereas for the S-enantiomer, it is not. These studies establish for the first time the structural and kinetic basis of high affinity binding of a neutral inhibitor to COX-1 and demonstrate that the side pocket of COX-1, previously thought to be sterically inaccessible, can serve as a binding pocket for inhibitor association.

Interferon-gamma does not break, but promotes the immunosuppressive capacity of adult human mesenchymal stem cells

The ability of mesenchymal stem cells (MSC) to suppress alloresponsiveness is poorly understood. Herein, an allogeneic mixed lymphocyte response was used as a model to investigate the mechanisms of MSC-mediated immunomodulation. Human MSC are demonstrated to express the immunosuppressive cytokines hepatocyte growth factor (HGF), interleukin (IL)-10 and transforming growth factor (TGF)-beta1 at concentrations that suppress alloresponses in vitro. MSC also express cyclooxygenase 1 and 2 and produce prostaglandin E2 constitutively. Blocking studies with indomethacin confirmed that prostaglandins contribute to MSC-mediated allosuppression. The proinflammatory cytokine interferon (IFN)-gamma did not ablate MSC inhibition of alloantigen-driven proliferation but up-regulated HGF and TGF-beta1. IFN-gamma also induced expression of indoleamine 2,3, dioxygenase (IDO), involved in tryptophan catabolism. Use of an antagonist, 1-methyl-L-tryptophan, restored alloresponsiveness and confirmed an IDO contribution to IFN-gamma-induced immunomodulation by MSC. Addition of the tryptophan catabolite kynurenine to mixed lymphocyte reactions (MLR), blocked alloproliferation. These findings support a model where IDO exerts its effect through the local accumulation of tryptophan metabolites rather than through tryptophan depletion. Taken together, these data demonstrate that soluble factors, or products derived from MSC, modulate immune responses and suggest that MSC create an immunosuppressive microenvironment capable of modulating alloresponsiveness even in the presence of IFN-gamma.

Comparison of the properties of prostaglandin H synthase-1 and -2

Biosynthesis of prostanoid lipid signaling agents from arachidonic acid begins with prostaglandin H synthase (PGHS), a hemoprotein in the myeloperoxidase family. Vertebrates from humans to fish have two principal isoforms of PGHS, termed PGHS-1 and-2. These two isoforms are structurally quite similar, but they have very different pathophysiological roles and are regulated very differently at the level of catalysis. The focus of this review is on the structural and biochemical distinctions between PGHS-1 and-2, and how these differences relate to the functional divergence between the two isoforms.

Colocalization and interaction of cyclooxygenase-2 with caveolin-1 in human fibroblasts

Results from our previous study suggest that cyclooxygenase-2 (COX-2) induced by phorbol 12-myristate 13-acetate (PMA) may be localized to caveolae-like structures (Liou, J.-Y., Shyue, S.-K., Tsai, M.-J., Chung, C.-L., Chu, K.-Y., and Wu, K. K. (2000) J. Biol. Chem. 275, 15314-15320). In this study, we determined subcellular localization of COX-2 and caveolin-1 by confocal microscopy. COX-2 in human foreskin fibroblasts stimulated by PMA (100 nm) or interleukin-1beta (1 ng/ml) for 6 h was localized to plasma membrane in addition to endoplasmic reticulum and nuclear envelope. Caveolin-1 was localized to plasma membrane, and image overlay showed colocalization of COX-2 with caveolin-1. This was confirmed by the presence of COX-2 and caveolin-1 in the detergent-insoluble membrane fraction of cells stimulated by PMA. Immunoprecipitation showed complex formation of COX-2 with caveolin-1 in a time-dependent manner. A larger quantity of COX-2 was complexed with caveolin-1 in PMA-treated than in interleukin-1beta-treated cells. Purified COX-2 complexed with glutathione S-transferase-fused caveolin-1, which was not inhibited by the scaffolding domain peptide. Caveolin-1-bound COX-2 was catalytically active, and its activity was not inhibited by the scaffolding domain peptide. These results suggest that COX-2 induced by PMA and interleukin-1beta is colocalized with caveolin-1 in the segregated caveolae compartment. Because caveolae are rich in signaling molecules, this COX-2 compartment may play an important role in diverse pathophysiological processes.

Peroxidase self-inactivation in prostaglandin H synthase-1 pretreated with cyclooxygenase inhibitors or substituted with mangano protoporphyrin IX

Self-inactivation imposes an upper limit on bioactive prostanoid synthesis by prostaglandin H synthase (PGHS). Inactivation of PGHS peroxidase activity has been found to begin with Intermediate II, which contains a tyrosyl radical. The structure of this radical is altered by cyclooxygenase inhibitors, such as indomethacin and flurbiprofen, and by replacement of heme by manganese protoporphyrin IX (forming MnPGHS-1). Peroxidase self-inactivation in inhibitor-treated PGHS-1 and MnPGHS-1 was characterized by stopped-flow spectroscopic techniques and by chromatographic and mass spectrometric analysis of the metalloporphyrin. The rate of peroxidase inactivation was about 0.3 s(-)1 in inhibitor-treated PGHS-1 and much slower in MnPGHS-1 (0.05 s(-)1); as with PGHS-1 itself, the peroxidase inactivation rates were independent of peroxide concentration and structure, consistent with an inactivation process beginning with Intermediate II. The changes in metalloporphyrin absorbance spectra during inactivation of inhibitor-treated PGHS-1 were similar to those observed with PGHS-1 but were rather distinct in MnPGHS-1; the kinetics of the spectral transition from Intermediate II to the next species were comparable to the inactivation kinetics in each case. In contrast to the situation with PGHS-1 itself, significant amounts of heme degradation occurred during inactivation of inhibitor-treated PGHS-1, producing iron chlorin and heme-protein adduct species. Structural perturbations at the peroxidase site (MnPGHS-1) or at the cyclooxygenase site (inhibitor-treated PGHS-1) thus can influence markedly the kinetics and the chemistry of PGHS-1 peroxidase inactivation.

Structural analysis of NSAID binding by prostaglandin H2 synthase: time-dependent and time-independent inhibitors elicit identical enzyme conformations

Nonsteroidal antiinflammatory drugs (NSAIDs) block prostanoid biosynthesis by inhibiting prostaglandin H(2) synthase (EC 1.14.99.1). NSAIDs are either rapidly reversible competitive inhibitors or slow tight-binding inhibitors of this enzyme. These different modes of inhibition correlate with clinically important differences in isoform selectivity. Hypotheses have been advanced to explain the different inhibition kinetics, but no structural data have been available to test them. We present here crystal structures of prostaglandin H(2) synthase-1 in complex with the inhibitors ibuprofen, methyl flurbiprofen, flurbiprofen, and alclofenac at resolutions ranging from 2.6 to 2.75 A. These structures allow direct comparison of enzyme complexes with reversible competitive inhibitors (ibuprofen and methyl flurbiprofen) and slow tight-binding inhibitors (alclofenac and flurbiprofen). The four inhibitors bind to the same site and adopt similar conformations. In all four complexes, the enzyme structure is essentially unchanged, exhibiting only minimal differences in the inhibitor binding site. These results argue strongly against hypotheses that explain the difference between slow tight-binding and fast reversible competitive inhibition by invoking global conformational differences or different inhibitor binding sites. Instead, they suggest that the different apparent modes of NSAID binding may result from differences in the speed and efficiency with which inhibitors can perturb the hydrogen bonding network around Arg-120 and Tyr-355.

Arachidonic acid and nonsteroidal anti-inflammatory drugs induce conformational changes in the human prostaglandin endoperoxide H2 synthase-2 (cyclooxygenase-2)

By using the technique of site-directed spin labeling combined with EPR spectroscopy, we have observed that binding of arachidonic acid and nonsteroidal anti-inflammatory drugs induces conformational changes in the human prostaglandin endoperoxide H(2) synthase enzyme (PGHS-2). Line shape broadening resulting from spin-spin coupling of nitroxide pairs introduced into the membrane-binding helices of PGHS-2 was used to calculate the inter-helical distances and changes in these distances that occur in response to binding various ligands. The inter-residue distances determined for the PGHS-2 holoenzyme using EPR were 1-7.9 A shorter than those of the crystal structure of the PGHS-2 holoenzyme. However, inter-helical distances calculated and determined by EPR for PGHS-2 complexed with arachidonic acid, flurbiprofen, and SC-58125 were in close agreement with those obtained from the cognate crystal structures. These results indicate that the structure of the solubilized PGHS-2 holoenzyme measured in solution differs from the crystal structure of PGHS-2 holoenzyme obtained by x-ray analysis. Furthermore, binding of ligands induces a conformational change in the holo-PGHS-2, converting it to a structure similar to those obtained by x-ray analysis. Proteolysis protection assays had previously provided circumstantial evidence that binding of heme and non-steroidal anti-inflammatory drugs alters the conformation of PGHS, but the present experiments are the first to directly measure such changes. The finding that arachidonate can also induce a conformational change in PGHS-2 was unexpected, and the magnitude of changes suggests this structural flexibility may be integral to the cyclooxygenase catalytic mechanism.

Comparison of prostaglandin H synthase isoform structures using limited proteolytic digestion

Prostaglandin H synthase (PGHS) catalyzes a key step in the biosynthesis of a variety of bioactive lipid mediators. The two known isoforms (PGHS-1 and -2) share about 60% amino acid identity, but exhibit distinct interactions with substrates, activators, and inhibitors. Ovine PGHS-1 has previously been shown to have a distinctive protease-sensitive site near Arg277; cleavage by trypsin, chymotrypsin, or proteinase K produces fragments of 33 and 38 kDa and loss of activity. The ovine PGHS-1 crystal structure shows Arg277 located in an exposed loop structure; homology modeling predicts similar loop structures for both human isoforms (hPGHS-1 and -2). We have used limited proteolytic digestion of recombinant hPGHS-1 and hPGHS-2 to probe their structures. Incubation of hPGHS-1 with either trypsin or proteinase K produced 33- and 38-kDa fragments and loss of activity. In contrast, incubation of hPGHS-2 with the same proteases led to cleavage of only a 2- to 3-kDa fragment, with no decrease in activity. Immunoblotting with site-specific antibodies demonstrated that the cleaved fragment originated from the hPGHS-2 C-terminus. Similar immunoblotting experiments indicated that trypsin did not attack the ovine PGHS-1 C-terminus. Mutagenesis was used to replace Pro263 of hPGHS-2 (corresponds to Arg277 of ovine PGHS-1) with arginine, inserting a potential trypsin site. Incubation of this P263R hPGHS-2 mutant with either trypsin or proteinase K resulted in cleavage near the C-terminus and retention of activity, just as with wild-type hPGHS-2. A peptide containing residues 259-268 of the P263R mutant was cleaved by trypsin at the same rate as a peptide corresponding to hPGHS-1 residues 272-281, demonstrating that the sequence differences were not responsible for the lack of tryptic cleavage at residue 263 in the hPGHS-2 mutant. Preincubation of hPGHS-2 with graded levels of guanidinium HCl before incubation with proteinase K did not produce large proteolytic fragments, indicating that the hPGHS-2 loop region was not selectively unfolding. The results point to two regions of significant structural difference between PGHS-1 and -2: the Arg277 loop, which is protease-sensitive in PGHS-1 but protease-resistant in PGHS-2, and the C-terminus, which is protease-sensitive in PGHS-2 but not in PGHS-1.

Endotoxin priming of thromboxane-related vasoconstrictor responses in perfused rabbit lungs

In prior studies of perfused lungs, endotoxin priming markedly enhanced thromboxane (Tx) generation and Tx-mediated vasoconstriction in response to secondarily applied bacterial exotoxins. The present study addressed this aspect in more detail by employing precursor and intermediates of prostanoid synthesis and performing functional testing of vasoreactivity and measurement of product formation. Rabbit lungs were buffer perfused in the absence or presence of 10 ng/ml endotoxin. Repetitive intravascular bolus applications of free arachidonic acid provoked constant pulmonary arterial pressor responses and constant release reactions of TxA2 and prostaglandin (PG) I2 in nonprimed lungs. Within 60-90 min of endotoxin recirculation, which provoked progressive liberation of tumor necrosis factor-alpha but did not effect any hemodynamic changes by itself, both pressor responses and prostanoid release markedly increased, and both events were fully blocked by cyclooxygenase (Cyclo) inhibition with acetylsalicylic acid (ASA). The unstable intermediate PGG2 provoked moderate pressor responses, again enhanced by preceding endotoxin priming and fully suppressed by ASA. Vasoconstriction also occurred in response to the direct Cyclo product PGH2, again amplified after endotoxin pretreatment, together with markedly enhanced liberation of TxA2 and PGI2. In the presence of ASA, the priming-related increase in pressor responses and the prostanoid formation were blocked, but baseline vasoconstrictor responses corresponding to those in nonprimed lungs were maintained. Pressor responses to the stable Tx analog U-46619 were not significantly increased by endotoxin pretreatment, but some generation of TxA2 and PGI2 was also noted under these conditions. We conclude that endotoxin priming exerts profound effects on the lung vascular prostanoid metabolism, increasing the readiness to react with Tx-mediated vasoconstrictor responses to various stimuli, suggesting that enhanced Cyclo activity is an important underlying event.

Detection of an Fe2+-protoporphyrin-IX intermediate during aspirin-treated prostaglandin H2 synthase II catalysis of arachidonic acid to 15-HETE

Spectral intermediates associated with the dioxygenase and peroxidase activities of prostaglandin H2 (PGH2) synthase I and II were monitored by stopped-flow spectrometry. During reactions of PGH2 synthase I with arachidonic acid (AA) and ethyl hydrogen peroxide (EtOOH), compound I (Fe5+; formally (protoporphyrin-IX) x +Fe4+=O) and compound II (Fe4+; formally (protoporphyrin-IX)Fe4+=O) were detected. These intermediates were observed sooner with EtOOH (within 50 ms) than with AA (within 200 ms). Compound I and compound II were found to be kinetically competent with respect to AA-dependent O2 uptake. These findings are consistent with a mechanism in which peroxidative cleavage precedes AA dioxygenation. During reactions with PGH2 synthase II with AA, compound I and compound II were again observed within 200 ms and were kinetically competent to participate in dioxygenation. However, during reactions of PGH2 synthase II with EtOOH, compound I and compound II were detected much later (after 10 s). These findings would be inconsistent with a mechanism in which peroxidative cleavage precedes AA dioxygenation. When aspirin-treated PGH2 synthase II was reacted with EtOOH, a normal peroxidase cycle occurred with compound I and compound II formation occurring over 10 s. However, when aspirin-treated PGH2 synthase II was reacted with AA, a unique spectral intermediate with lambda(max) at 446 nm was detected within 3 ms and was strikingly similar to ferrous (Fe2+) protoporphyrin-IX. Aspirin-treated PGH2 synthase II was found to produce 15-HETE, and the appearance of the Fe2+ intermediate (within 3 ms) indicated that it was kinetically competent to participate in the 15-dioxygenation event. The detection of this Fe2+ intermediate and the slow formation of compound I and compound II observed with EtOOH in PGH2 synthase II suggest that peroxidative cleavage is not the initiating event in dioxygenation. Instead, it is proposed that the reduction of Fe3+ in heme to Fe2+ oxidizes a peroxide to yield an initiating peroxy radical. Since it is unlikely that 11- and 15-dioxygenation occurs via different mechanisms, our findings question mechanisms of catalysis in both PGH2 synthases.

Mechanism of selective inhibition of human prostaglandin G/H synthase-1 and -2 in intact cells

Selective inhibitors of prostaglandin synthase-2 (PGHS-2) possess potent anti-inflammatory, antipyretic, and analgesic properties but demonstrate reduced side-effects (e.g. gastrotoxicity) when compared with nonselective inhibitors of PGHS-1 and -2. We investigated the mechanism of the differential inhibition of human PGHS-1 (hPGHS-1) and -2 (hPGHS-2) in intact cells by nonsteroidal anti-inflammatory drugs (NSAIDs) and examined factors that contribute to the increased potency of PGHS inhibitors observed in intact cells versus cell-free systems. In intact Chinese hamster ovary (CHO) cell lines stably expressing the hPGHS isozymes, both PGHS isoforms exhibited the same affinity for arachidonic acid. Exogenous and endogenous arachidonic acid were used as substrates by both CHO [hPGHS-1] and CHO [hPGHS-2] cell lines. However, differences were observed in the ability of the hPGHS isoforms to utilize endogenous arachidonic acid released intracellularly following calcium ionophore stimulation or released by human cytosolic phospholipase A2 transiently expressed in the cells. Cell-based screening of PGHS inhibitors demonstrated that the selectivities and potencies of PGHS inhibitors determined using intact cells are affected by substrate concentration and differ from that determined in cell-free microsomal or purified enzyme preparations of PGHS isozymes. The mechanism of inhibition of PGHS isozymes by NSAIDs in intact cells involved difference in their time-dependent inhibition. Indomethacin displayed time-dependent inhibition of cellular hPGHS-1 and -2. In contrast, the selective PGHS-2 inhibitor NS-398 exhibited time-independent inhibition of hPGHS-1 but time-dependent inhibition of hPGHS-2 in intact cells. Reversible inhibition of cellular CHO [hPGHS-1] and CHO [hPGHS-2] was observed with the nonselective NSAIDs ibuprofen and indomethacin, whereas inhibition by the selective PGHS-2 inhibitor DuP-697 was reversible against hPGHS-1 but irreversible against hPGHS-2.

Inhibitor-induced changes in the intrinsic fluorescence of human cyclooxygenase-2

The steady state tryptophan fluorescence of apo-human cyclooxygenase-2 (hCox-2) is quenched approximately 40%-50% by the slow binding inhibitors diclofenac, indomethacin, ketoprofen, NS-398, and DuP-697. The effects of these inhibitors on tryptophan fluorescence are both time and concentration dependent. Addition of each inhibitor results in a rapid fluorescence decrease, followed by a slower time dependent quenching. The slow, time dependent loss of fluorescence follows first-order kinetics, the rate constants for the process increasing with inhibitor concentration in a saturation-type manner. The rapid fluorescence loss also increases with increasing inhibitor concentration in the same manner. These results are consistent with the initial formation of a rapid equilibrium complex of enzyme and inhibitor (EI), followed by the slower formation of a tightly bound enzyme-inhibitor complex (EI*). The fluorescence of the EI complex is not significantly different from that of the EI* complex. The kinetic parameters of each inhibitor derived for this process (Ki and kon) are close to those obtained by determination of the rate constants for the onset of enzyme inhibition, thereby linking the fluorescence changes with inhibitor binding. The reversible inhibitors ibuprofen and docosahexaenoic acid do not quench the protein fluorescence but do decrease both the rate of the slow fluorescence loss and the magnitude of the initial rapid fluorescence decrease caused by the slow binding inhibitors, consistent with their competitive behavior. ASA-acetylated apo-hCox-2 shows the same fluorescence-quenching behavior in the presence of most of the above inhibitors. However, acetylation apparently blocks the binding of diclofenac, whereas the affinity of ibuprofen is increased. The effects of the collisional quenching agents iodide and acrylamide on both the native and inhibited enzyme are small (< 20% quenching at 0.3 M), showing that inhibitor binding does not result in an increased solvent accessibility of protein tryptophans. The cause of the inhibitor-induced quenching of the intrinsic apo-hCox-2 fluorescence is likely energy transfer to the bound inhibitor. Calculations based on the inhibitor-tryptophan distances in ovine Cox-1 indicate that the distances are within the required range for significant quenching to occur.

Role of Val509 in time-dependent inhibition of human prostaglandin H synthase-2 cyclooxygenase activity by isoform-selective agents

Prostaglandin H synthase (PGHS), a key enzyme in prostanoid biosynthesis, exists as two isoforms. PGHS-1 is considered a basal enzyme; PGHS-2 is associated with inflammation and cell proliferation. A number of highly selective inhibitors for PGHS-2 cyclooxygenase activity are known. Inhibition by these agents involves an initial reversible binding, followed by a time-dependent transition to a much higher affinity enzyme-inhibitor complex, making these agents potent and poorly reversible PGHS-2 inhibitors. To investigate the PGHS-2 structural features that influence the time-dependent action of the selective inhibitors, we have constructed a three-dimensional model of human PGHS-2 by homologous modeling. Examination of the PGHS-2 model identified Val509 as a cyclooxygenase active site residue, that was not conserved in PGHS-1. Recombinant human PGHS-2 with Val509 mutated to either Ile (the corresponding residue in PGHS-1), Ala, Glu, or Lys was expressed by transient transfection of COS-1 cells to evaluate the effects of the mutations on cyclooxygenase activity and on inhibition by four agents reported to be selective for PGHS-2 (NS398, nimesulide, DuP697, and SC58125). All the recombinant proteins were of the expected mass. The mutants exhibited 45-210% of wild-type cyclooxygenase activity, with Km values for arachidonate of 2.1-7.6 microM (wild-type PGHS-2, 3.8 microM), indicating that changes in position 509 had modest effects on cyclooxygenase catalysis. Each of the agents inhibited wild-type PGHS-2 in a time-dependent fashion, and all but nimesulide did the same for the V509A mutant. In contrast, the V509E and V509I PGHS-2 mutants, like recombinant human PGHS-1, did not show time-dependent inhibition with any of the agents, and the V509K mutant responded in a time-dependent manner only to DuP697. Reversible inhibition was still observed with Val509 mutants that did not show time-dependent inhibition. Thus, the side chain structure at position 509 markedly influenced the ability of PGHS-2 to undergo the time-dependent transition without removing inhibitor or substrate binding. These results indicate that Val509 in PGHS-2 has a major role in the structural transition that underlies time-dependent inhibition by the isoform-selective agents.

Kinetics of the interaction of nonsteroidal antiinflammatory drugs with prostaglandin endoperoxide synthase-1 studied by limited proteolysis

Many nonsteroidal antiinflammatory agents (NSAIDs) bind to prostaglandin endoperoxide synthase (PGHS) and induce a conformational change in the PGHS apoprotein that renders it resistant to cleavage by trypsin at Arg277. In the present study, the trypsin protection assay was modified to permit detection of conformational changes at times as short as 5 s after the addition of inhibitor. The kinetics of the induction and reversal of trypsin resistance in apoPGHS-1 by a series of NSAIDs and isozyme-specific PGHS-1 and PGHS-2 inhibitors were determined. All compounds induced resistance to trypsin cleavage at a rapid rate. The conformational change induced by competitive inhibitors was reversed on prolonged incubation with trypsin (approximately 5 min). In contrast, the resistance induced by irreversible inhibitors was not lost during a 5 min incubation with trypsin. All of the selective PGHS-2 inhibitors protected against tryptic cleavage of apoPGHS-1 but did not inhibit the protein's cyclooxygenase activity. The results suggest that induction of trypsin resistance is a reflection of the initial association of reversible as well as irreversible inhibitors with the apoprotein.

Mechanistic studies on the selective inhibition of cyclooxygenase-2 by indanone derivatives

The cyclooxygenase step in the conversion of arachidonic acid is a key point in the biosynthesis of prostanoids, managed by two enzymatic isoforms. In the following study we focused on the mechanism of the inhibitory action of CGP 28238 and structurally-related indanone derivatives using purified enzymes. Consistent with our earlier studies on cell systems, CGP 28238 revealed selective inhibition of cyclooxygenase-2. The process affects the bisoxygenase subunit time-dependently, and is reversible in the early phase of inhibition. From structure-activity relationships, we propose the formation of a Schiff base between the oxo-groups of CGP 28238 and an amino group at the active site providing additional binding forces for an effective inhibition of cyclooxygenase-2.

Mechanism of action of anti-inflammatory drugs

Cyclooxygenase (COX) is the pivotal enzyme in prostaglandin biosynthesis. It exists in two isoforms, constitutive COX-1 (responsible for physiological functions) and inducible COX-2 (involved in inflammation). Inhibition of COX explains both the therapeutic effects (inhibition of COX-2) and side effects (inhibition of COX-1) of non-steroidal anti-inflammatory drugs (NSAIDs). A NSAID which selectively inhibits COX-2 is likely to retain maximal anti-inflammatory efficacy combined with less toxicity. The activity of a number of NSAIDs has been investigated in several test systems, showing that most of those marketed have higher activities against COX-1 or are equipotent against both isoforms. Adverse event data of marketed NSAIDs show a relationship between a poor safety profile and more potent inhibition of COX-1 relative to COX-2. There are several new non-steroidal COX-2 inhibitors in development. The most clinically advanced is meloxicam, which consistently demonstrates higher activity against COX-2 than COX-1 in several test systems.

Oxidation kinetics of caffeic acid by prostaglandin H synthase: potential role in regulation of prostaglandin biosynthesis

The naturally occurring catechol derivative caffeic acid is a moderate stimulator of prostaglandin H synthase cyclooxygenase activity and a good reducing substrate for prostaglandin H synthase-compounds I and II. The discrepancy between the two properties is explained by a specific peroxidative mechanism that includes the formation of an inhibitory complex of caffeic acid with native enzyme followed by a three-step irreversible ping-pong peroxidation. The concentration of caffeic acid necessary to produce 50% stimulation of 0.2 mM arachidonic acid oxidation is 0.8 +/- 0.1 mM. The rate constant for the reaction of prostaglandin H synthase with hydrogen peroxide, determined from steady-state results, is (5.68 +/- 0.1) x 10(5) M-1 s-1. The rate constant for the reaction of prostaglandin H synthase-compound II with caffeic acid is (1.25 +/- 0.1) x 10(6) M-1 s-1. The dissociation constant of caffeic acid from the inhibitory complex is 35 +/- 10 microM. In diluted enzyme solutions, caffeic acid binding is diminished and the enzyme exhibits higher peroxidase activity. Our results suggest that caffeic acid is not a O-demethylation product of ferulic acid degradation catalyzed by prostaglandin H synthase, nor a chelating agent for the heme iron. The oxidation of caffeic acid could be important in the regulation of both prostaglandin H synthase and lipoxygenase activities and hence prostaglandin and leukotriene biosynthesis.

The X-ray crystal structure of the membrane protein prostaglandin H2 synthase-1

The three-dimensional structure of prostaglandin H2 synthase-1, an integral membrane protein, has been determined at 3.5 A resolution by X-ray crystallography. This bifunctional enzyme comprises three independent folding units: an epidermal growth factor domain, a membrane-binding motif and an enzymatic domain. Two adjacent but spatially distinct active sites were found for its haem-dependent peroxidase and cyclooxygenase activities. The cyclooxygenase active site is created by a long, hydrophobic channel that is the site of non-steroidal anti-inflammatory drug binding. The conformation of the membrane-binding motif strongly suggests that the enzyme integrates into only one leaflet of the lipid bilayer and is thus a monotopic membrane protein.

QSAR and conformational analysis of the antiinflammatory agent amfenac and analogues

The new nonsteroidal antiinflammatory drug (NSAID) arylacetic amfenac (2-amino-3-benzoylphenylacetic acid) and 19 substituted derivatives were studied in order to correlate the biological activities with the structure-related parameters. The geometry of amfenac in neutral and anionic form was totally optimized, starting from standard geometries and crystallographic data, using semiempirical AM1 and MNDO quantum-mechanical methods. Conformational analysis shows the existence of a rigid structure for rotations of the acetic acid chain (alpha degrees) and the central carbonyl group (gamma degrees) around the bonds with the phenylamine ring, whereas the carboxyl group (beta degrees) and the phenyl ring of the benzoyl group (delta degrees) can rotate almost freely. Electrostatic potential maps were analyzed and showed that the electrostatic orientation effect seems to make an important contribution to the binding of the active compounds to prostaglandin synthase. An electrostatic orientation model of the binding site is proposed. The frontier orbital charge distribution was also described for each compound. On the other hand, steric, electronic and hydrophobic (log P) parameters were calculated and QSAR analysis showed that the most significant parameter for the antiinflammatory activity was the pi-electron density of the HOMO orbital in the second aromatic ring. These results suggest a possible electronic charge transfer between the aromatic fragments and the receptor.

Molecular evolution of cyclooxygenase and lipoxygenase

Four oxygenases of the arachidonic acid cascade (cyclooxygenase, 5-lipoxygenase, 12-lipoxygenase and 15-lipoxygenase) were investigated by the method of computer-assisted sequence comparison. From the calculations, some aspects of evolution and function of these enzymes were revealed. (1) The evolutionary origin of cyclooxygenase was different from that of lipoxygenases. (2) Cyclooxygenase was a distantly related member of a peroxidase family. (3) Enzymes with 12-lipoxygenase activity were created independently twice by gene duplication.

Chemical modification of prostaglandin endoperoxide synthase by N-acetylimidazole. Effect on enzymic activities and EPR spectroscopic properties

Prostaglandin H synthase apoprotein, without its prosthetic heme group, was inactivated by N-acetylimidazole under conditions typical for the O-acetylation of tyrosyl residues. A spontaneous reactivation occurred above pH 7.5 at 22 degrees C, which indicated spontaneous hydrolysis of acetylated residues. Below pH 7.5, where stable inactivation was observed, reactivation was achieved by reaction with hydroxylamine. Both enzymic activities of prostaglandin H synthase, cyclooxygenase and peroxidase, were inactivated and reactivated simultaneously and to the same extent. In contrast to the apoprotein, the holoenzyme with heme was not inactivated by N-acetylimidazole. The number of acetyl groups, as determined as hydroxamate after the reaction with hydroxylamine at pH 8.2, was 2.5 +/- 0.4 for the apoprotein and 1.0 +/- 0.24 for the holoenzyme. The specific binding of heme as the prosthetic group was no longer observed by EPR (signals at g = 6.7 and 5.3) when hemin was added to the N-acetylimidazole-reacted apoprotein. Treatment of N-acetylimidazole-reacted apoprotein with hydroxylamine restored the specific binding of heme. The N-acetylimidazole-reacted apoprotein supplemented with hemin and reacted with hydroperoxides, neither showed electronic absorption spectra of higher oxidation states nor an EPR doublet signal due to a tyrosyl radical. These results demonstrate that heme protects against the inactivating modification by N-acetylimidazole and that this modification prevents binding of the prosthetic heme group necessary for both enzymic activities. The absence of the prosthetic heme group explains the concomitant loss of cyclooxygenase and peroxidase activities, as well as the absence of higher oxidation states and the tyrosyl radical. We suggest that the acetylation of a residue in the heme pocket, most probably a tyrosine, although a histidine cannot be definitely disproved, exerts the inhibiting effects. This residue could be the axial ligand of the heme or in close contact to the heme. The results also show that the inhibition by N-acetylimidazole does not involve the acetylation of Ser530 which causes the inhibition by acetylsalicylic acid of cyclooxygenase. [The numbering of amino acids in ovine prostaglandin H synthase is according to DeWitt, D. L. and Smith, W. L. (1988) Proc. Natl Acad. Sci. USA 85, 1412-1416 including a signal peptide of 24 residues which is missing in the processed protein.

Prostaglandin and thromboxane biosynthesis

We describe the enzymological regulation of the formation of prostaglandin (PG) D2, PGE2, PGF2 alpha, 9 alpha, 11 beta-PGF2, PGI2 (prostacyclin), and thromboxane (Tx) A2 from arachidonic acid. We discuss the three major steps in prostanoid formation: (a) arachidonate mobilization from monophosphatidylinositol involving phospholipase C, diglyceride lipase, and monoglyceride lipase and from phosphatidylcholine involving phospholipase A2; (b) formation of prostaglandin endoperoxides (PGG2 and PGH2) catalyzed by the cyclooxygenase and peroxidase activities of PGH synthase; and (c) synthesis of PGD2, PGE2, PGF2 alpha, 9 alpha, 11 beta-PGF2, PGI2, and TxA2 from PGH2. We also include information on the roles of aspirin and other nonsteroidal anti-inflammatory drugs, dexamethasone and other anti-inflammatory steroids, platelet-derived growth factor (PDGF), and interleukin-1 in prostaglandin metabolism.

Inhibitory effect of aryl thienyl-ketones and -thioketones on arachidonic acid-induced malondialdehyde formation in human platelets: biological data and molecular modelling

A series of anti-thrombotic aryl thienyl-ketones and -thioketones was assayed in vitro for their inhibitory effect on malondialdehyde (MDA) production induced by arachidonic acid in human platelets. For several compounds MDA formation was strongly inhibited indicating that the anti-platelet target was situated on the cyclooxygenase pathway. A comparison between the inhibition constant Ki and the IC50 values revealed competitive inhibition kinetics. The molecular structure of one active compound was analysed by X-ray diffraction and theoretical calculations to provide information on its electronic and lipophilic properties.

Prostaglandin H synthase: spectroscopic studies of the interaction with hydroperoxides and with indomethacin

Prostaglandin H synthase has both a heme-dependent peroxidase activity and a cyclooxygenase activity. A current hypothesis considers the cyclooxygenase reaction to be a free radical chain reaction, initiated by an interaction of the synthase peroxidase with hydroperoxides leading to the production of a tyrosyl free radical [Stubbe, J. A. (1989) Annu. Rev. Biochem. 58, 257-285]. We have examined the kinetics of radical formation with both ethyl hydroperoxide (EtOOH) and 15-hydroperoxyeicosatetraenoic acid (15-HPETE) and have analyzed the effects of indomethacin (a selective cyclooxygenase inhibitor) and tetranitromethane (TNM; a selective agent for nitration of tyrosyl residues) on the synthase. At -14 degrees C both EtOOH and 15-HPETE generated within 5 s a free radical species whose electron paramagnetic resonance spectrum was dominated by a doublet centered at g = 2.005 (splitting of approximately 16 G; overall peak-to-trough width of 35 G) that has been attributed to tyrosyl radical. The doublet subsequently gave way to a singlet with a similar peak-to-trough width; the doublet-to-singlet transition was complete in 20-60 s. The intensity of the doublet/singlet combination peaked at 0.6 spins/heme after 120 s with EtOOH and at about 0.3 spins/heme after 20 s with 15-HPETE; the radical intensity declined slowly with EtOOH but more rapidly with 15-HPETE. Reaction of the indomethacin-synthase complex with EtOOH resulted in a narrower (peak-to-trough width of 24 G) singlet free radical signal, with no evidence of an earlier doublet; the intensity of the singlet peaked at 0.45 spins/heme after about 300 s. Reaction of TNM-treated synthase with EtOOH resulted in a singlet almost identical with that seen for the indomethacin-synthase complex. Reaction of the synthase holoenzyme with TNM at pH 8.0 led to inactivation of both cyclooxygenase and peroxidase activity, with the former being lost rapidly and completely while the latter was lost slowly and to about 50%. Ibuprofen, a competitive cyclooxygenase inhibitor, slowed the rate of inactivation of the cyclooxygenase by about 20-fold. The rate of inactivation of the cyclooxygenase activity in synthase apoenzyme by TNM was also about 20-fold less than that observed with the holoenzyme. Amino acid analyses revealed that TNM-reacted holoenzyme with less than 10% residual activity contained 1.8 nitrotyrosines/subunit; apoenzyme reacted under the same conditions had greater than 80% of the original activity and contained 0.7 nitrotyrosine/subunit.(ABSTRACT TRUNCATED AT 400 WORDS)

Arachidonic acid cascade and signal transduction

Since a review on this topic in this Journal appeared (Wolfe, 1982), the CNS has proved to be a major focus in eicosanoid research. Although our knowledge is limited at the moment, the research in this field is rapidly growing. In this short review, we summarize recent progress of research (1982-1989) in this field with special attention directed to eicosanoid metabolism, functions of eicosanoids in the neuroendocrine system and synaptic transmission, current information on eicosanoid receptors, and the link between eicosanoids and cerebral circulation. Knowledge of the eicosanoids has paved the way to a better understanding of intercellular signal transduction systems, including neuronal functions.