Cyclic AMP-dependent regulation of lipid mediators in white cells. A unifying concept for explaining the efficacy of theophylline in asthma.

Activation of white cells, including the neutrophil, eosinophil, basophil, and mast cell, has long been known to be suppressed by high, nonphysiological levels of E-prostaglandins (PGE). In contrast, PGE at levels consistent with an interaction with the PGE receptor (5 X 10(-9) M) have recently been shown to suppress leukotriene (LT) and prostaglandin (PG) production by neutrophils and eosinophils. This occurs by cyclic AMP-dependent inhibition of release of substrate arachidonic acid (AA) from phospholipid pools. The additional observation that indomethacin (10(-9) M) enhances release of eicosanoids by suppressing endogenous PGE2 acting to increase cAMP levels in these cells. Theophylline and other phosphodiesterase inhibitors precisely duplicate the action of PGE2, and the combined effects of such phosphodiesterase inhibitors and adenylate cyclase stimulators are synergistic. The mechanism of action of theophylline in asthma is not know, although it is generally agreed that its effect is a direct one on the bronchial smooth muscle. The findings described in this report now permit the bronchial smooth muscle, but is primarily one of suppressing mediator release from relevant white cells by inhibition of cAMP phosphodiesterase, an action that is enhanced by the presence of inflammatory prostaglandins in the lung.

Endothelial cell prostacyclin production induced by activated neutrophils.

A bovine aortic endothelial cell (EC) line released prostacyclin (greater than 1 pmol/10(+5) EC cells) when incubated with fMet-Leu-Phe (FMLP)-stimulated rat and human neutrophils (PMNs). This prostaglandin (PG) I2 was shown to come from the ECs and not from the PMNs by radioactive, high-performance liquid chromatography, and immunochemical criteria. Both FMLP-stimulated rat peritoneal and human peripheral PMNs as well as their stimulated cell-free supernatants and unstimulated sonicates could elicit the release of PGI2 from ECs. Since phorbol myristate acetate stimulated PMN adherence but elicited little PGI2 release from ECs, the PGI2 stimulation in ECs is unrelated to PMN adhesion. The addition of catalase and superoxide dismutase to FMLP-stimulated PMNs enhanced rather than reduced PGI2 formation, indicating that activated oxygen products of the PMN are not responsible for the induction of PGI2. Incubation of ECs with leukotriene (LT) B4, LTC4, or LTD4 did not trigger PGI2 release nor did aspirin pretreatment of the PMNs reduce the PGI2 induction. These data suggest that arachidonic acid metabolites of the PMNs were not responsible for the PGI2 induction. Available data indicates that the PMN factor that stimulates PGI2 from ECs is either released concomitantly with the azurophilic granules or is closely related to this event.

Lung tissue receptors for sulfidopeptide leukotrienes.

Studies of the binding of tritiated sulfidopeptide leukotrienes (LTs) to a membrane preparation from rat lung tissue revealed a site specific for LTC4 with a dissociation constant of 4.1 X 10(-8)M. Similar experiments with a guinea pig lung preparation demonstrated binding specific for LTD4 with a dissociation constant of 2.1 X 10(-10)M. The divalent cations Ca++, Mg++, and Mn++ significantly enhanced the affinity of binding of the respective LTs to both sites. The binding of LTC4 to guinea pig lung and rat lung exhibited similar characteristics, but the former was observed only in the presence of the gamma-glutamyl transpeptidase inhibitor, serineborate complex. The binding affinities of various isomers of both sulfidopeptide LTs paralleled the potency of their pharmacologic effects, which supports the contention that the sites are receptors specific for LTC4 and LTD4. The slow-reacting substance of anaphylaxis antagonist, FPL 55712, had a higher affinity for the LTD4 receptor, which is consistent with its more effective antagonism of the LTD4-induced contractile response of guinea pig lung parenchymal strips. The ability of Na+ and guanosine triphosphate to inhibit the binding of LTD4 suggests that the action of LTD4 is associated with a depression of intracellular concentrations of cyclic adenosine monophosphate.

Pharmacological effects of non-steroidal antiinflammatory agents on prostaglandin and leukotriene synthesis in mouse peritoneal macrophages.

Resident mouse peritoneal macrophages, exposed to zymosan, synthesized and released products of both the cyclooxygenase and lipoxygenase pathways. The effects of various non-steroidal antiinflammatory agents were evaluated for their abilities to inhibit zymosan-stimulated prostaglandin E2 (PGE2) and leukotriene C4 (LTC4) synthesis. The order of potencies to inhibit PGE2 synthesis and release was: indomethacin greater than or equal to sulindac sulfide greater than ibuprofen greater than or equal to aspirin greater than 3-amino-1-[3-(trifluoromethyl)-phenyl]-2-pyrazoline (BW755C) greater than benoxaprofen greater than or equal to nordihydroguaiaretic acid (NDGA) greater than 5,8,11-eicosatriynoic acid (ETYA). BW755C and ETYA also inhibited zymosan-stimulated LTC4 production. None of the compounds tested showed selective inhibition of lipoxygenase products.

Leukotriene C4 binding to rat lung membranes.

A high affinity binding site for leukotriene C4 (LTC4), one component of slow reacting substance of anaphylaxis, has been identified in a membrane preparation from rat lung. As measured by a filtration technique, [3H]LTC4 binding was saturable, specific, reversible, and heat-sensitive. In the presence of 20 mM CaCl2, the dissociation constant (KD) was 41 +/- 9 nM and the maximum number of binding sites (Bmax) was 31 +/- 10 pmol/mg of protein. Specificity was demonstrated by competition studies in which LTC4 had a Ki of 40 nM against specifically bound [3H]LTC4, whereas leukotriene D4 (LTD4) had a Ki of 4 microM. The stereoisomers (5R, 6R) LTC4, (5S, 6S) LTC4, and (5R, 6S) LTC4 had Ki values 3-, 15-, and 25-fold higher than that of natural (5S, 6R) LTC4. Leukotrienes E4 and B4, several prostaglandins and fatty acids, glutathione, and platelet activating factor were even less effective with Ki values above 10 microM. A slow reacting substance of anaphylaxis antagonist, FPL 55712, which, in some systems, distinguishes LTC4- from LTD4-induced contractions, was a weak competitor with a Ki of 16 microM. Serine-borate complex which inhibits gamma-glutamyl transpeptidase, an enzyme responsible for LTC4 metabolism, did not alter binding. In addition, 100 microM FPL 55712 did not reduce metabolism. These observations suggest that the binding observed for LTC4 may represent association with a physiological receptor for this molecule which has a relatively low affinity for LTD4.

Inhibition by prostaglandins of leukotriene B4 release from activated neutrophils.

Chemoattractant N-formylmethionylleucylphenylalanine (fMet-Leu-Phe) in the presence of cytochalasin B stimulates the release of leukotriene B4 (LTB4), superoxide (O2-), and N-acetylglucosaminidase from elicited rat peritoneal and human peripheral neutrophils [PMN (polymorphonuclear leukocytes)]. Prostaglandins E1 and E2 (PGE1 and PGE2) inhibit LTB4 release from PMN in a dose-related manner with an IC50 of 1 X 10(-8) M. This action is associated with increased levels of cyclic AMP. The inhibitory activity of a variety of PGs on LTB4 production by rat peritoneal PMN parallels their affinity for PGE receptors in other tissues. O2- release is also suppressed by low levels of PGE1 and PGE2 in a dose-related manner and this inhibition is enhanced by theophylline. In contrast, lysosomal enzyme release is only minimally affected by physiological levels of PGs. These data are consistent with an action of PGs at the level of the PG receptor on LTB4 and O2- release from the fMet-Leu-Phe-stimulated rat peritoneal PMN. In addition, the fMet-Leu-Phe-induced adherence of PMN to endothelial cells and inhibition of this phenomenon by PGs may now be explained by PG-mediated inhibition of LTB4 formation.

Leukotriene A4: preparation and enzymatic conversion in a cell-free system to leukotriene B4.

Treatment of leukotriene A4 (LTA4) methyl ester with sodium hydroxide in aqueous methanol at 4 degrees C afforded LTA4, the presence of which was inferred from the UV spectrum of the compound, its rate of reaction with water, and the identity of the hydration products obtained. The half-life of LTA4 in water (pH 7.4, room temperature) was increased from 14 to 500 s by 1 mg/ml of bovine serum albumin. This stabilized (chiral) LTA4 was converted to LTB4 by an epoxide hydrolase activity in the 100,000 x g supernatant fraction from sonified rat basophilic leukemia cells. Neither the ester of LTA4 nor the biologically incorrect enantiomer of LTA4 was metabolized to LTB4 under these conditions.

Evidence for two sources of arachidonic acid for oxidative metabolism by mouse peritoneal macrophages.

The products of arachidonic acid oxygenations by resident mouse peritoneal macrophages have been found to depend upon the nature of the stimulus. For example, soluble membrane-mediated inflammatory stimuli such as phorbol myristate acetate and lipopolysaccharide stimulated the formation of prostaglandin E2 via the cyclooxygenase pathway. In contrast, zymosan, a particulate, phagocytozable inflammatory mediator stimulated leukotrienes C4 and B4 synthesis via the lipoxygenase pathway in addition to stimulating prostaglandin E2 synthesis. Thus, the release of leukotrienes is not necessarily linked to the release of prostaglandins in a cell that has the enzymatic capability of producing both mediators. This suggests that the prostaglandin synthetase system can obtain substrate arachidonic acid from a source different from that for leukotriene synthesis.

Oxidation reactions by prostaglandin cyclooxygenase-hydroperoxidase.

oxidations of organic sulfides, amines, and even enzymes catalyzed by purified and microsomal forms of prostaglandin cyclooxygenase-hydroperoxidase have been studied using O2 incorporation into arachidonic acid to monitor oxygenase and [14C]15-hydroperoxyprostaglandin E2 reduction to prostaglandin E2 to measure hydroperoxidase. The oxygenase was protected by phenol against the irreversible deactivation induced by low levels of hydroperoxides. Furthermore, the EPR signal noted during reactions with the microsomal enzyme probably reflected the adventitious oxidation of endogenous materials. As described previously for phenol and other reducing cosubstrates, methyl phenyl sulfide (MPS) increased hydroperoxidase activity at all concentrations studied, while stimulating oxygenase at low levels and inhibiting it at 5-10 mM. In stoichiometric equivalence with 15-hydroperoxyprostaglandin E2 reduction, MPS was enzymatically oxidized to its analogous sulfoxide, methylphenyl sulfoxide, acquiring an oxygen atom exclusively from the hydroperoxide and demonstrating some chiral character. In contrast, other oxidizable compounds such as N,N-dimethylphenylenediamine and aminopyrine reacted via radical intermediates. Phenylbutazone, which is oxidized using dissolved molecular oxygen, did not compete with MPS oxidation. Hence, MPS was oxidized while bound to the enzyme, whereas the amine oxidation occurred in solution via an enzyme-formed oxidant. The Soret peak noted with cyclooxygenase-hydroperoxidase was examined as a possible measure of this binding, but was also noted in denatured and deactivated enzyme, suggesting that its relevance should be reconsidered. Despite the similarities in their drug-metabolizing profiles, cyclooxygenase-hydroperoxidase is clearly distinct from cytochrome P-450. The mechanism of this hydroperoxidase is considered in the context of other more extensively studied peroxidases.

Multiple sites on prostaglandin cyclooxygenase are determinants in the action of nonsteroidal antiinflammatory agents.

Evidence is presented to show that nonsteroidal antiinflammatory drugs react with two sites on the cyclooxygenase (8,11,14-eicosatrienoate, hydrogen-donor:oxygen oxidoreductase, EC 1.14.99.1). Although the degree of interaction with the catalytic site determines the potency of such compounds, interaction with the supplementary site is also obligatory for efficacy as cyclooxygenase inhibitors and may explain the selectivity of such drugs in inhibiting the cyclooxygenase but not the lipoxygenase pathway. Drugs that interact more effectively with the supplementary site than with the catalytic site--i.e., those of weak to moderate activity as cyclooxygenase inhibitors--are shown to prevent inhibition of the enzyme by indomethacin. Compounds in this class are also capable of blocking the ulcerogenic action of indomethacin, which suggests that this antiulcerogenic property stems from a direct action at the level of the cyclooxygenase in the stomach.

Effect of RNA and protein synthesis inhibitors on the release of inflammatory mediators by macrophages responding to phorbol myristate acetate.

The interaction of phorbol myristate acetate with resident populations of mouse peritoneal macrophages causes an increased release of arachidonic acid followed by increased synthesis and secretion of prostaglandin E2 and 6-keto-prostaglandin F1 alpha. In addition, phorbol myristate acetate causes the selective release of lysosomal acid hydrolases from resident and elicited macrophages. These effects of phorbol myristate acetate on macrophages do not cause lactate dehydrogenase to leak into the culture media. The phorbol myristate acetate-induced release of arachidonic acid and increased synthesis and secretion of prostaglandins by macrophages can be inhibited by RNA and protein synthesis inhibitors, whereas the release of lysosomal hydrolases is unaffected. 0.1 microgram/ml actinomycin D blocked the increased prostaglandin production due to this inflammatory agent by more than 80%, and 3 microgram/ml cycloheximide blocked prostaglandin production by 78%. Similar results with these metabolic inhibitors were found with another stimulator of prostaglandin production, zymosan. However, these inhibitors do not interfere with lysosomal hydrolase releases caused by zymosan or phorbol myristate acetate. It appears that one of the results of the interaction of macrophages with inflammatory stimuli is the synthesis of a rapidly turning-over protein which regulates the production of prostaglandins. It is also clear that the secretion of prostaglandins and lysosomal hydrolases are independently regulated.

Prostaglandins, arachidonic acid, and inflammation.

The enzymatic oxidation of arachidonic acid has been shown to yield potent pathological agents by two major pathways. Those of the prostaglandin (PG) pathway, particularly PGE2, have been implicated as inflammatory mediators for many years. The discovery and biological activities of thromboxane A2 and prostacyclin as well as a destructive oxygen-centered radical as additional products of this biosynthetic pathway now require these to be considered as potential inflammatory mediators. Like PGE2, their biosynthesis is prevented by nonsteroidal anti-inflammatory agents. More recently, the alternative metabolic route, the lipoxygenase pathway, has been shown to yield a new class of arachidonic acid oxygenation products, called the leukotrienes, which also appear to be important inflammatory mediators. Unlike the prostaglandins, some of which play important roles as biological regulators, the actions of the lipoxygenase products appear to be exclusively of a pathological nature.

The role of macrophage secretory products in chronic inflammatory processes.

Mononuclear phagocytes participate in various stages of chronic inflammatory responses and associated diseases. Such participation is mediated by (a) direct interaction with pericellular interstitial tissue components as well as with other cell types present at sites of inflammation and (b) by secretion of soluble mediators. Several of these mediators are synthesized and secreted in increased amounts after macrophages interact with inflammatory stimuli. In this paper we pay particular attention to neutral proteinases and prostaglandins. It is shown that these 2 classes of mediators are released in significant amounts under different conditions. Prostaglandins are synthesized most readily by resident populations of mouse peritoneal macrophages responding to various model inflammatory stimuli. Mouse peritoneal macrophage populations elicited in vivo by inflammatory stimuli are less responsive in this respect. In contrast neutral proteinase secretion does not occur in resident cell populations but is observed on a continuous basis in elicited populations. Such secretion can be increased further by addition of phagocytic stimuli and initiated in resident populations by model inflammatory stimuli such as phorbol myritate acetate. Other secretory products of macrophages with possible relevance to inflammation are discussed briefly. Finally some of the effects of antiinflammatory glucocorticoids, cyclooxygenase inhibitors and dapsone on the secretory activity of macrophages are briefly summarized.