Induction of cyclo-oxygenase-2 by cytokines in human pulmonary epithelial cells: regulation by dexamethasone

Sodium salicylate inhibits cyclo-oxygenase-2 activity independently of transcription factor (nuclear factor kappaB) activation: role of arachidonic acid

Acetylsalicylic acid (aspirin) is the drug most commonly self-administered to reduce inflammation, swelling, and pain. The established mechanism of action of aspirin is inhibition of the enzyme cyclo-oxygenase (COX). Once taken, aspirin is rapidly deacetylated to form salicylic acid, which may account, at least in part, for the therapeutic actions of aspirin. However, where tested, salicylic acid has been found to be a relatively inactive inhibitor of COX activity in vitro, despite being an effective inhibitor of prostanoids formed at the site of inflammation in vivo. Recently, the identification of a cytokine-inducible isoform of COX, COX-2, has led to the suggestion that salicylate produces its anti-inflammatory actions by inhibiting COX-2 induction through actions on nuclear factor kappaB (NF-kappaB). We have used interleukin 1beta-induced COX-2 in human A549 cells to investigate the mechanism of action of salicylate on COX-2 activity. Sodium salicylate inhibited prostaglandin E2 release when added together with interleukin 1beta for 24 hr with an IC50 value of 5 microg/ml, an effect that was independent of NF-kappaB activation or COX-2 transcription or translation. Sodium salicylate acutely (30 min) also caused a concentration-dependent inhibition of COX-2 activity measured in the presence of 0, 1, or 10 microM exogenous arachidonic acid. In contrast, when exogenous arachidonic acid was increased to 30 microM, sodium salicylate was a very weak inhibitor of COX-2 activity with an IC50 of >100 microg/ml. Thus, sodium salicylate is an effective inhibitor of COX-2 activity at concentrations far below those required to inhibit NF-kappaB (20 mg/ml) activation and is easily displaced by arachidonic acid.

Isoenzyme-specific cyclooxygenase inhibitors: a whole cell assay system using the human erythroleukemic cell line HEL and the human monocytic cell line Mono Mac 6

NSAIDs inhibit the conversion of arachidonic acid into Prostaglandin G2 and Prostaglandin H2 which is catalyzed by the enzyme cyclooxygenase (COX). Two genetically distinct isoforms have been discovered, COX-1 and COX-2. While COX-1 is thought to account for homeostatic amounts of eicosanoids, COX-2 is induced during inflammation leading to pathologic amounts of eicosanoids. Since NSAIDs inhibit both COX isoforms, antiinflammatory drug research has refocused to discovering COX-2 inhibitors that do not inhibit COX-1. For this purpose, we have developed a whole cell assay system using the human erythroleukemic cell line HEL as a source for COX-1 and the human monocytic cell line Mono Mac 6 as a source for COX-2. Mono Mac 6 cells express high amounts of COX-2 upon stimulation with lipopolysaccharide (LPS) in the absence of any detectable COX-1 protein. On the other hand, we find HEL cells to naturally express COX-1 protein, but not COX-2. Testing of a panel of NSAIDs as well as some COX-2 specific inhibitors showed that this assay system is suitable for identifying compounds that selectively inhibit either COX-1 or COX-2. This test system offers the advantage of assessing COX-1 and COX-2 inhibitors within the human species, within a similar test set-up, and circumvents the need for tedious purification of either platelets or peripheral blood monocytes.

Vaso-reactivity of isolated bovine intra-mammary artery to endogenous prostanoids and nitric oxide

The modulatory role of locally produced cyclooxygenase products and endothelium-derived nitric oxide in controlling vascular tone was investigated in bovine intra-mammary artery. Vascular reactivity initiated by vasoactive compounds, endothelin-1 (ET-1), bradykinin (BK), and substance P (SP) was measured isometrically in an isolated tissue bath. The effects of a cyclooxygenase inhibitor, indomethacin (10(-5) M) and an inhibitor of nitric oxide production, N omega-Nitro L-Arginine (L-NNA: 3 x 10(-4) M) were determined during agonist-mediated responses. Indomethacin alone markedly enhanced vascular contraction produced by ET-1, while L-NNA did not. Inhibition of endothelium-derived nitric oxide synthesis by L-NNA, however, significantly attenuated BK- and SP-induced vascular relaxations, whereas indomethacin had slight influence. The potentiation between indomethacin and L-NNA in regulating vasomotor tone was not observed in this vascular bed. Thus, it appeared that both the cyclooxygenase and endothelium-derived nitric oxide pathways participated in modifying vascular reactivity. Domination of one pathway over the other depended upon the agonist used to stimulate vascular tissue.

Alternate COX-2 transcripts are differentially regulated: implications for post-transcriptional control

Prostaglandin (PG) synthesis during inflammation occurs mainly via the transcriptionally regulated cyclooxygenase, COX-2. In pulmonary type II A549 cells, Northern analysis identified multiple IL-1 beta-inducible COX-2 mRNA transcripts. Amplification of 3'-cDNA ends by anchored PCR revealed products corresponding to the predominant 4.5 and 2.7 kb transcripts. Sequence analysis of amplification products indicated that these transcripts arose by alternate consensus and non-consensus polyadenylation site usage. The predominant 4.5 kb transcript showed a half-life in excess of two hours that was further stabilized by IL-1 beta. In addition, the COX-2 3'-untranslated region (UTR), which contains 22 copies of the putative RNA instability motif, AUUUA, when cloned downstream of a constitutively expressed luciferase gene, was found to confer partial IL-1 beta responsiveness in LA-4 cells. Finally, in vivo in LPS-treated rats, differential expression of similar COX-2 mRNA isoforms was also observed. Taken together these data suggest a functional role for post-transcriptional mechanisms, including alternate polyadenylation, in the control of COX-2.

Effect of interleukin-1 beta, tumour necrosis factor-alpha and interferon-gamma on the induction of cyclo-oxygenase-2 in cultured human airway smooth muscle cells

1. Increased levels of several pro-inflammatory cytokines including interleukin-1 beta (IL-1 beta) and tumour necrosis factor-alpha (TNF alpha) have been found in bronchoalveolar lavage fluid from symptomatic asthmatic patients. IL-1 beta, TNF alpha and interferon-gamma (IFN gamma) are known to stimulate a number of cells to produce inflammatory mediators such as prostaglandins. Although airway smooth muscle (ASM) is known to be a rich source of prostaglandins, the regulation of cyclo-oxygenase (COX) isoforms and prostanoid production by proinflammatory cytokines have not been studied in human airway smooth muscle. 2. We studied the effects of IL-1 beta, TNF alpha and IFN gamma on the induction of two isoforms of cyclo-oxygenase and its relation to prostaglandin E2 (PGE2) release and COX activity (reflected by PGE2 synthesis from exogenous arachidonic acid) in human cultured airway smooth muscle cells. 3. IL-1 beta, but not TNF alpha or IFN gamma, caused a time- and concentration-dependent enhancement in PGE2 and other prostanoid (6-keto-PGF1 alpha, PGF2 alpha, thromboxane B2 (TXB2) and PGD2) production, with PGE2 and 6-keto-PGF1 alpha as the principal products. This stimulation was accompanied by a corresponding increase in COX activity. 4. COX-2 protein measured by Western blot analysis was not detectable in untreated cells, but was increased in a time- and concentration-dependent manner by IL-1 beta, but not TNF alpha or IFN gamma. In contrast, no variation in the expression of COX-1 protein was observed. 5. Pretreatment with the conventional non-steroidal anti-inflammatory drugs (NSAIDs), indomethacin and ibuprofen, and the selective COX-2 inhibitors, NS-398 and nimesulide, completely blocked IL-1 beta-induced PGE2 release and COX activity. The glucocorticosteroid dexamethasone and protein synthesis inhibitors, cycloheximide and actinomycin D, not only markedly inhibited IL-1 beta-stimulated PGE2 release and COX activity but also suppressed IL-1 beta-induced COX-2 induction. 6. This study demonstrates that human cultured ASM cells release prostanoids in response to IL-1 beta stimulation and that the response is mostly mediated by the induction of COX-2 rather than COX-1 isoenzyme, implying that airway smooth muscle may be an important source of prostaglandins in human airways and that COX-2 may play an important role in the regulation of the inflammatory process in asthma.

Characterization of the induction of nitric oxide synthase and cyclo-oxygenase in rat aorta in organ culture

1. Within vessels, the formation of nitric oxide (NO) or prostaglandins is normally catalysed in the endothelium by constitutive isoforms of NO synthase (eNOS) and cyclo-oxygenase (COX-1), respectively. However, during inflammatory conditions, the underlying smooth muscle acquires the ability to release NO and prostaglandins after the expression of inducible isoforms of NOS (iNOS) and COX (COX-2). The co-induction of iNOS and COX-2 has been studied over 24 h in isolated vascular smooth muscle cells in vitro. However, due to the limitation of using cultured cells, the relationship between the activities of iNOS and COX over longer periods has not been addressed. Moreover, the relative contribution of the endothelium to the production of NO and prostaglandins under inflammatory conditions is not completely understood. 2. Here using an organ culture system, we have determined the profile of COX (6-keto prostaglandin F1 alpha (6-keto PGF1 alpha), PGE2, thromboxane B2 (TXB2) and NOS (nitrite and nitrate) metabolites released over a period of 10 days from segments of rat aorta. In each case, segments from the same animal were left untreated or treated with bacterial lipopolysaccharide (LPS; 10 micrograms ml-1) in order to induce iNOS and COX-2. Prostaglandins were measured by radioimmunoassay whilst nitrite and nitrate were measured, respectively, by Greiss reaction alone, or following a nitrate reductase step. The isoforms of NOS and COX responsible for metabolite release were characterized pharmacologically by use of inhibitors and at the molecular level by reverse transcription polymerase chain reaction with specific primers for iNOS, eNOS, COX-1 and COX-2. In separate experiments the role of the endothelium in the release of nitrite, nitrate and prostaglandins and in the expression of iNOS, eNOS, COX-1 and COX-2 was determined by comparing responses in endothelium denuded and endothelium-intact segments of rat aorta. 3. Under control culture conditions vessels released prostaglandins in the following rank order 6-keto PGF1 alpha = PGE2 > > TXB2. LPS increased the release of 6-keto PGF1 alpha and PGE2 but not of TXB2, an effect that was inhibited by the protein synthesis inhibitor cycloheximide (1 microM), the anti-inflammatory steroid dexamethason (1 microM), the nonsteroidal anti-inflammatory drug indomethacin (30 microM) and, where tested, the selective COX-2 inhibitor NS-398 (30 microM). Similarly, segments of rat aorta released detectable levels of nitrite and nitrate, which were reduced by NG-nitro-L-arginine methyl ester (L-NAME, 1 mM), which inhibits all isoforms of NOS, and by dexamethasone (1 microM), which inhibits the induction of iNOS. The proportion of nitrate to nitrite released over the 10 day period varied greatly from approximately 1:1 on days 5 to 8 to 5:1 on day 9. However, the sum of nitrite and nitrate (NOx) as well as PGE2 remained elevated over the whole 10 day period. The formation of 6-keto PGF1 alpha peaked on days 1 and 2. 4. In freshly prepared tissue, mRNAs for eNOS, COX-1, iNOS and COX-2 were detected. After 24 h in culture, there was an apparent increase in the level of mRNAs for iNOS and COX-2 but not for eNOS or COX-1, an effect that was further enhanced when LPS was included in the culture medium. The expressions of mRNA for eNOS, COX-1, iNOS or COX-2 were not greatly different in vessels with intact or disrupted endothelium. Similarly the release of NOx or PGE2 by vessels after the 1st or 9th day in culture were not significantly different from vessels prepared with or without endothelium. 5. Thus, COX-2 and iNOS are co-induced in intact vessels in culture, with the vascular smooth muscle being the main site of mediator generation. In contrast to data from isolated cells in culture (observed usually over 1 day), both COX and NOS activities in cultured blood vessels were elevated for at least 10 days. Also, unlike isolated cells in culture, the COX and NOS pathways were active independently; L-NAME had little effect on the activity of COX and indomethacin had little effect on the activity of NOS.

Interferon gamma induces prostaglandin G/H synthase-2 through an autocrine loop via the epidermal growth factor receptor in human bronchial epithelial cells

The induction of prostaglandin G/H synthase (PGHS; prostaglandin endoperoxide synthase, cyclooxygenase) by proinflammatory cytokines accounts, at least in part, for the altered eicosanoid biosynthesis in inflammatory diseases. In secondary cultures of normal human bronchial epithelial cells (NHBECs), interferon-gamma (IFN-gamma, 10 ng/ml for 24 h) increased the amount of prostaglandin E2 (PGE2) released in response to stimulation with exogenous arachidonic acid (5 microM). The enhanced production of PGE2 reflected the upregulation of PGHS-2 as indicated by enhanced expression of PGHS-2 RNA and increased recovery of PGHS-2 protein in NHBECs. IFN-gamma did not alter the production of PGE2 in A549 cells (a human lung adenocarcinoma cell line) or 6-keto-PGF1alpha in human umbilical vein endothelial cells (HUVECs), although prostaglandin release and/or the expression of PGHS-2 RNA in these cell lines was upregulated by other proinflammatory cytokines. Induction of PGHS-2 RNA in IFN-gamma-treated NHBECs, which peaked at 24 h, suggested the presence of an intermediary substance regulating the expression of PGHS-2. When the binding between the epidermal growth factor (EGF) receptor and its ligands was disrupted by a neutralizing antibody (LA-1), IFN-gamma failed to upregulate the release of PGE2 and the expression of PGHS-2 RNA in NHBECs. Furthermore, IFN-gamma induced the expression of RNAs for a number of ligands at the EGF receptor TGF-alpha; heparin-binding EGF-like growth factor (HB-EGF); and amphiregulin in NHBECs, and when administered exogenously, these ligands increased PGE2 release from NHBECs. Heparin at the concentration that neutralized the function of amphiregulin, or antibodies against TGFalpha or HB-EGF also reduced the release of PGE2 from IFN-gamma-stimulated NHBECs. These data are consistent with the presence of an autocrine growth factor/EGF receptor loop regulating PGHS-2 expression and PGE2 synthesis in bronchial epithelial cells.

Repression of inducible nitric oxide synthase and cyclooxygenase-2 by prostaglandin E2 and other cyclic AMP stimulants in J774 macrophages

The enhanced nitric oxide (NO) and prostaglandin (PG) generation of activated macrophages is controlled by glucocorticoid-sensitive inducible nitric oxide synthase (iNOS) and cyclooxygenase-2 (COX-2), respectively. Negative feedback regulation of iNOS expression by the products of both pathways has been suggested, but their effects on COX-2 expression have not been examined. We hae investigated the effect of E- and l-series prostaglandins that activate adenylate cyclase (AC), forskolin (a direct activator of AC), and other agents that influence the cyclicAMP/cyclicGMP systems on the ability of E. coli endotoxin (lipopolysaccharide, LPS) to induce iNOS and COX-2 in the murine macrophage cell line J774. After a 2-hr pretreatment before adding endotoxin, PGE2, PGI2, forskolin, IBMX (isobutylmethylxanthine, a cyclicAMP/cyclicGMP phosphodiesterase inhibitor), 8-bromo cyclicAMP, and arachidonic acid itself all inhibited the expression of both iNOS and COX-2 (as shown by Western blotting) and reduced NO release and COX activity, whereas PGF2 alpha and 8-bromo cyclic GMP were only weakly effective. The effects of PGE2, PGI2, and forskolin were enhanced by cotreatment with IBMX. The suppression of LPS-induced iNOS induction by PGE2 was functionally significant, in that it protected against the mild cytotoxicity of the NO generated in response to endotoxin. These results provide the first direct evidence for the feedback regulatory suppression of COX-2 induction by a PG-driven cAMP-mediated process, and show that the modulation of iNOS and COX-2 induction shares common features. They also suggest that such modulation is normally held in check by high phosphodiesterase activity within these cells.

Possible participation of cyclooxygenase-2 in the recurrence of allergic inflammation in rats

In the recurrence of allergic inflammation in a rat air pouch model, pouch fluid volume, prostaglandin E2 concentration in the pouch fluid, leukocyte infiltration into the pouch fluid, and granulation tissue weight were markedly increased by the antigen challenge. To clarify the role of cyclooxygenase-2 in the recurrence of allergic inflammation, the time-course of changes in protein levels of cyclooxygenase-1 and cyclooxygenase-2 in the granulation tissue and in the infiltrated leukocytes was examined by Western blot analysis. It was shown that cyclooxygenase-1 levels in the granulation tissue and in the infiltrated leukocytes were not changed by the antigen challenge, but cyclooxygenase-2 levels were increased. Furthermore, treatment with the selective cyclooxygenase-2 inhibitor, NS-398 ([N-2(cyclohexyloxy-4-nitrophenyl]-methanesulfonamide), suppressed the recurrence of allergic inflammation as did the non-selective cyclooxygenase-1/cyclooxygenase-2 inhibitor, indomethacin. The steroidal anti-inflammatory drug, dexamethasone, inhibited the induction of cyclooxygenase-2, and suppressed the allergic inflammation. These findings strongly suggested that cyclooxygenase-2 induced by the antigen challenge plays a role in the recurrence of inflammation induced by the allergic mechanism.

Current therapies for asthma. Promise and limitations

Effective treatments for asthma exist, but morbidity and mortality have continued to climb. Many attempts have been made to refine rather than change therapy over the past 20 years. Drugs currently used to treat asthma include beta 2-agonists, glucocorticoids, theophylline, cromones, and anticholinergic agents. For acute, severe asthma, the inhaled beta 2-agonists are the most effective bronchodilators. Short-acting forms give rapid relief; long-acting agents provide sustained relief and help nocturnal asthma; and serious adverse effects are rare when these drugs are used properly. First-line therapy for chronic asthma is inhaled glucocorticoids, the only currently available agents that reduce airway inflammation. Their side effects can be reduced by rinsing the mouth or by using large-volume spacers. Theophylline is a bronchodilator that is useful for severe and nocturnal asthma, but recent studies suggest that it may also have an immunomodulatory effect. Although theophylline is inexpensive, monitoring its plasma concentrations is both expensive and inconvenient. Cromones work best for patients who have mild asthma: they have few adverse effects, but their activity is brief, so they must be given four times daily. The anticholinergic bronchodilators are more useful for treating COPD than for chronic asthma. These drugs have virtually no side effects, and their onset is slower and their action longer than inhaled beta 2-agonists. The new direction in treating asthma will be orally administered medication that has few side effects and is targeted specifically to the pathogenesis of asthma.

Induction of cyclo-oxygenase-2 by cytokines in human cultured airway smooth muscle cells: novel inflammatory role of this cell type

1. Cyclo-oxygenase (COX) is the enzyme that converts arachidonic acid to prostaglandin H2 (PGH2) which can then be further metabolized to prostanoids which modulate various airway functions. COX exists in at least two isoforms. COX-1 is expressed constitutively, whereas COX-2 is expressed in response to pro-inflammatory stimuli. Prostanoids are produced under physiological and pathophysiological conditions by many cell types in the lung. However, the regulation of the different COX isoforms in human airway smooth muscle (HASM) cells has not yet been determined. 2. COX-1 and COX-2 protein were measured by Western blot analysis with specific antibodies for COX-1 and COX-2. COX-2 mRNA levels were assessed by Northern blot analysis by use of a COX-2 cDNA probe. COX activity was determined by measuring conversion of either endogenous or exogenous arachidonic acid to three metabolites, PGE2, thromboxane B2 or 6-ketoPGF1 alpha by radioimmunoassay. 3. Under control culture conditions HASM cells expressed COX-1, but not COX-2, protein. However, a mixture of cytokines (interleukin-1 beta (IL-1 beta), tumour necrosis factor alpha (TNF alpha) and interferon gamma (IFN gamma) each at 10 ng ml-1) induced COX-2 mRNA expression, which was maximal at 12 h and inhibited by dexamethasone (1 microM; added 30 min before the cytokines). Furthermore, COX-2 protein was detected 24 h after the cytokine treatment and the expression of this protein was also inhibited by dexamethasone (1 microM) and cyclohexamide (10 micrograms ml-1; added 30 min before the cytokines). 4. Untreated HASM cells released low or undetectable amounts of all COX metabolites measured over a 24 h period. Incubation of the cells with the cytokine mixture (IL-1 beta, TNF alpha, IFN gamma each at 10 ng ml-1 for 24 h) caused the accumulation of PGE2 and 6-keto-PGF1 alpha. 5. In experiments where COX-2 metabolized endogenous stores of arachidonic acid, treatment of HASM cells with IL-1 beta in combination with TNF alpha caused a similar release of PGE2 to that when the three cytokines were given in combination. 6. In other experiments designed to measure COX-2 activity directly, cells were treated with cytokines for 24 h before fresh culture medium was added containing exogenous arachidonic acid (30 microM for 15 min) after which PGE2 was measured. IL-1 beta and TNF alpha increased COX-2 activity and an additional small increase was produced by the three cytokines in combination. 7. These findings suggest that the increased expression of COX-2 is intimately involved in the exaggerated release of prostanoids from HASM cells exposed to pro-inflammatory cytokines. These data indicate a role for airway smooth muscle cells, in addition to their contractile function, as inflammatory cells involved in the production of mediators which may contribute to the inflammatory response seen in diseases such as asthma.

Asthma therapy with aerosols: clinical relevance for the next decade

Inhaled therapy is the mainstay of modern asthma management, as this optimizes the therapeutic ratio. Short-acting beta 2-agonists are the most effective bronchodilators and when given by inhalation give rapid relief of symptoms, without adverse effects, although there are concerns about overuse of these drugs. Inhaled long-acting beta 2-agonists are useful in some patients. Inhaled anticholinergics are particularly useful in patients with COPD and in the future long-acting drugs, such as tiotropium bromide, will be available. Inhaled glucocorticoids are the most effective therapy in controlling chronic asthma symptoms, and systemic effects are not a problem in the vast majority of patients. Improved inhalation devices and steroids with reduced oral bioavailability have resulted in reduced systemic side effects, which now arise largely from absorption from the lungs. In the future it is likely that new classes of drug will be developed, but whether they will be used by inhalation or given by mouth will depend on the frequency of side effects and the mode of action of the drugs. There are likely to be several improvements in inhaler delivery systems, so that the inhaled route will remain predominant for many years to come.

Differential response to dexamethasone on the TXB2 release in guinea-pig alveolar macrophages induced by zymosan and cytokines

Glucocorticosteroids reduce the production of inflammatory mediators but this effect may depend on the stimulus. We have compared the time course of the effect of dexamethasone on the thromboxane B₂ (TXB₂) release induced by cytokine stimulation and zymosan in guinea-pig alveolar macrophages. Interleukin-1β (IL-1β), tumour necrosis factor-α (TNF-α) and opsonized zymosan (OZ), all stimulate TXB₂ release. High concentrations of dexamethasone (1–10 μM) inhibit the TXB₂ production induced by both cytokines and OZ, but the time course of this response is different. Four hours of incubation with dexamethasone reduce the basal TXB₂ release and that induced by IL-1β and TNF-α, but do not modify the TXB₂ release induced by OZ. However, this stimulus was reduced after 24 h incubation. Our results suggest that the antiinflammatory activity of glucocorticosteroids shows some dependence on stimulus and, therefore, may have more than one mechanism involved.

The human bronchus model in vitro. Pharmacological approach of various components involved in the functional response

Studying the human bronchi in vitro, and therefore sheltered from the toxicity problems inherent in human experiments, makes it possible to conduct a monofactorial analysis, disregarding the perturbations engendered by reflex phenomena, hemodynamic changes, etc. Analysing the effects of mediators on tissues may be less simple that it looks, due to the multiplicity of the cell types that are present. For example, in studying the effects of bradykinin we have shown that bradykinin is a potent contractile agent of small-diameter isolated bronchi, whereas it has no significant contractile effect on larger bronchi. The bradykinin-induced contraction results from a contractile component due to stimulation of the TP receptor, and of a relaxant component due to relaxant prostanoids. The two components of the bradykinin effects are produced by stimulation of B2 receptors. In vitro stimulation of bronchi by LPS or interleukin-1 beta permits us to obtain hyperreactivity to bradykinin due to induction of thromboxane synthetase or isomerase rather than to induction of B2 receptors or cyclooxygenase. Involvement of the nervous system may persist in the in vitro bronchial model, and indeed we have shown, for example, that pentamidine, well known for its tussigenic effect, is an indirect parasympathomimetic compound. Thus, study of the isolated bronchus permits an approach to the mechanisms of action of medicinal drugs. Despite the simplification provided compared to the in vivo study, analysis of bronchoreactivity on the isolated bronchus must take into account numerous parameters which interfere with the proper effects of the substances.

Cytokine induction of cytosolic phospholipase A2 and cyclooxygenase-2 mRNA is suppressed by glucocorticoids in human epithelial cells

Prostaglandin (PG) release, which is increased in vivo by inflammatory conditions and in vitro by pro-inflammatory cytokines, is decreased by glucocorticoids. Two phospholipase A2 isoforms, secretory (sPLA2) and cytosolic (cPLA2,), have been implicated in inflammation. These enzymes catalyse the release of arachidonic acid which is then converted to prostaglandins by the cyclooxygenases (COX-1 and COX-2). Regulation of these events at the mRNA level is poorly characterised in epithelial cells. We have used a human epithelial-like cell line (A549) as a model system to study mRNA expression of sPLA2, cPLA2, COX-1 and COX-2. Following treatment of cells and extraction of RNA, semi-quantitative reverse transcription polymerase chain reaction (RT-PCR) was used to examine expression of these genes. We show a coordinate induction of both cPLA2 and COX-2 mRNA by pro-inflammatory cytokines which correlated with increased PGE2 release. By contrast, sPLA2 mRNA was undetectable and COX-1 was found to be expressed at a constant low level. In addition dexamethasone pretreatment significantly reduced both cPLA2 and COX-2 mRNA levels as well as PGE2 release following cytokine stimulation. These data indicate a major role for control of prostaglandin synthesis at the mRNA level of key synthetic genes in epithelial cells. Furthermore we show that a major mechanism of glucocorticoid action in preventing prostaglandin release occurs by suppression of cPLA2 and COX-2 mRNA levels.

Molecular mechanisms of anti-inflammatory action of glucocorticoids

Glucocorticoid hormones are effective in controlling inflammation, but the mechanisms that confer this action are largely unknown. Recent advances in this field have shown that both positive and negative regulation of gene expression are necessary for this process. The genes whose activity are modulated in the anti-inflammatory process code for several cytokines, adhesion molecules and enzymes. Most of them do not carry a classical binding site for regulation by a glucocorticoid receptor, but have instead regulatory sequences for transcription factors such as AP-1 or NF-kappa B. This makes them unusual targets for glucocorticoid action and emphasizes the need for novel regulatory mechanisms. Recent studies describe an important contribution by protein-protein interactions, in which several domains of the receptor participate; these studies provide a better understanding of the action of the receptor and offer opportunities for the design of steroidal compounds that could function more effectively as anti-inflammatory drugs.

Cyclooxygenase-2-dependent bronchoconstriction in perfused rat lungs exposed to endotoxin

BACKGROUND

Lipopolysaccharides (LPS), widely used to study the mechanisms of gram-negative sepsis, increase airway resistance by constriction of terminal bronchioles. The role of the cyclooxygenase (COX) isoenzymes and their prostanoid metabolites in this process was studied.

MATERIALS AND METHODS

Pulmonary resistance, the release of thromboxane (TX) and the expression of COX-2 mRNA were measured in isolated blood-free perfused rat lungs exposed to LPS.

RESULTS

LPS induced the release of TX and caused increased airway resistance after about 30 min. Both TX formation and LPS-induced bronchoconstriction were prevented by treatment with the unspecific COX inhibitor acetyl salicylic acid, the specific COX-2 inhibitor CGP-28238, dexamethasone, actinomycin D, or cycloheximide. LPS-induced bronchoconstriction was also inhibited by the TX receptor antagonist BM-13177. The TX-mimetic compound, U-46619, increased airway resistance predominantly by constricting terminal bronchioles. COX-2-specific mRNA in lung tissue was elevated after LPS exposure, and this increase was attenuated by addition of dexamethasone or of actinomycin D. In contrast to LPS, platelet-activating factor (PAF) induced immediate TX release and bronchoconstriction that was prevented by acetyl salicylic acid, but not by CGP-28238.

CONCLUSIONS

LPS elicits the following biochemical and functional changes in rat lungs: (i) induction of COX-2; (ii) formation of prostaglandins and TX; (iii) activation of the TX receptor on airway smooth muscle cells; (iv) constriction of terminal bronchioles; and (v) increased airway resistance. In contrast to LPS, the PAF-induced TX release is likely to depend on COX-1.

Prostaglandin production in cultured cerebral microvascular smooth muscle is serum dependent

To expand the understanding of cerebrovascular eicosanoid metabolism, the ability of smooth muscle isolated from murine cerebral microvessels to produce prostaglandins (PGs) was studied in vitro. Cultures from SJL and BALB/c mice produced primarily prostaglandin E2 (PGE2) and I2 (PGI2) in response to exogenous arachidonate and calcium ionophore as well as the agonists acetylcholine and epinephrine. Subconfluent smooth muscle cultures demonstrated a two- to threefold increased capacity to produce PG compared with confluent cultures. In contrast, serum deprivation of smooth muscle caused an 80-90% diminution in both PGE2 and PGI2 production but had no effect on PG release in cerebromicrovascular endothelium. Reintroduction of serum to smooth muscle restored PG production within 6h, and the restoration was inhibited by 1 microM dexamethasone. Message for both prostaglandin H synthase (PGHS)-1 and -2 was detectable in smooth muscle grown in the presence of serum, but PGHS-2 message was not present in serum-deprived cultures. Furthermore, readdition of serum induced a massive increase in PGHS-2 mRNA with only a small increase in PGHS-1 message. The serum induction of PGHS-2 was corroborated by immunohistochemistry and Western blotting. Thus cerebromicrovascular smooth muscle may contribute significantly to the formation of PG under circumstances likely to be present during central nervous system pathologies. The induction of PGHS, particularly PGHS-2, may play a key role in this process.

Distinct isoforms (COX-1 and COX-2) of cyclooxygenase: possible physiological and therapeutic implications

The discovery of an inducible isoform of cyclooxygenase (COX-2) requires a refinement of the theory that inhibition of cyclooxygenase activity explains both therapeutic and side effects of non-steroidal anti-inflammatory drugs (NSAIDs). Indeed, new pharmacological results suggest that COX-2 inhibition provides the therapeutic (ie, anti-inflammatory) activity of NSAIDs, whereas inhibition of constitutive COX-1 is responsible for their gastric and renal side effects as well as for their antithrombotic activity. However, a role of COX-1 in inflammation cannot be excluded. Furthermore, the functional relevance of COX-2 expression and induction in various tissues warrants further investigation. These studies should help in predicting potential adverse effects as well as new indications for selective COX-2 inhibitors.

Prostaglandins play a role in memory consolidation in the chick

Previous studies showed that inhibitors of cyclooxygenases have amnesic effects in chicks in a passive avoidance task. The onset of amnesia has a delay of 2 h post-training. To investigate if this effect is due to the inhibition of induction of the enzyme during learning, the release of cyclooxygenase products into the extracellular fluid was measured at 1, 2 and 3 h post-training. A cyclooxygenase inhibitor, ibuprofen, inhibited the training-dependent increase of cyclooxygenase products only 2 h and 3 h after learning when injected pre-training, as did dexamethasone which prevents cyclooxygenase induction, and SC58125 (1,2-diarylcyclopentene), an inhibitor of inducible cyclooxygenase. Injections 30 min post-training showed the same effect with the exception of dexamethasone. Injecting SC58125, ibuprofen, indomethacin, or dexamethasone i.c. before training showed amnesic effects for training on a one-trial passive avoidance task at 2 h but not 1 h after training. Injections 30 min post-training produced the same effects with the exception of dexamethasone. I conclude that cyclooxygenases are induced during training and that cyclooxygenase products are of importance in memory formation of the chick.

Prostaglandin synthase 1 gene disruption in mice reduces arachidonic acid-induced inflammation and indomethacin-induced gastric ulceration

Cyclooxygenases 1 and 2 (COX-1 and COX-2) are key enzymes in prostaglandin biosynthesis and the target enzymes for the widely used nonsteroidal anti-inflammatory drugs. To study the physiological roles of the individual isoforms, we have disrupted the mouse Ptgs1 gene encoding COX-1. Homozygous Ptgs1 mutant mice survive well, have no gastric pathology, and show less indomethacin-induced gastric ulceration than wild-type mice, even though their gastric prostaglandin E2 levels are about 1% of wild type. The homozygous mutant mice have reduced platelet aggregation and a decreased inflammatory response to arachidonic acid, but not to tetradecanoyl phorbol acetate. Ptgs1 homozygous mutant females mated to homozygous mutant males produce few live offspring. COX-1-deficient mice provide a useful model to distinguish the physiological roles of COX-1 and COX-2.

Molecular mechanisms of antiasthma therapy

Recently there has been a much greater understanding of the molecular mechanisms involved in the actions of antiasthma therapy. beta 2-agonists are the most effective bronchodilators and act predominantly on airway smooth muscle. Recent evidence suggests that beta 2-receptors in airway smooth muscle are coupled directly to maxi-K channels and may thereby bronchodilate without an increase in cyclic AMP. The issue of beta-receptor tolerance has been reawakened by the recognition that the protective effects of beta 2-agonists against bronchoconstrictor stimuli may become tolerant. Inhaled glucocorticoids are the mainstay of treatment in patients with chronic asthma. They suppress asthmatic inflammation predominantly by reducing transcription of genes coding for inflammatory mediators (particularly cytokines) and enzymes (inducible NO synthase, inducible cyclo-oxygenase). The inhibition of gene transcription is mediated predominantly by inhibition of transcription factors, such as activator protein-1 (AP-1) and nuclear factor-kappa B (NF-kappa B). There may be an abnormal activation of AP-1 in steroid-resistant asthma, and high concentrations of beta 2-agonists may induce a secondary resistance by an interaction between the transcription factor CREB and the glucocorticoid receptor. Theophylline may have immunomodulatory effects that are more important than its bronchodilator action. Some effects of theophylline are mediated via inhibition of phosphodiesterases and several PDE IV inhibitors are currently undergoing evaluation in asthma.