Recovery Of Cyclooxygenase Activity Research Articles

Ozone inactivates cyclooxygenase in human tracheal epithelial cells without altering PGHS-2 mRNA or protein.

Exposure of human tracheal epithelial (TE) cells to ozone (0.1-0.5 ppm) leads to a transient increase followed by decreased production of prostaglandin (PG) E2 concomitant with dose-dependent loss and delayed recovery of cyclooxygenase (CO) activity [S.E. Alpert and R.W. Walenga. Am. J. Physiol. 269 (Lung Cell. Mol. Physiol. 13): L734-L743, 1995]. Formation of reactive oxygen species (ROS) in cultured tracheobronchial epithelial cells during ozone exposure was recently demonstrated (L.C. Chen and Q.Qu. Toxicol. Appl. Pharmacol. 143: 96-101, 1997). In the present study, we investigated if ROS generated by ozone-exposed human TE cells contribute to PGE2 production and/or CO inactivation and whether the delay in recovery of CO activity after ozone reflects impaired gene transcription and/or protein synthesis. Rapid, dose-dependent ROS generation, assessed by fluorescence of dihydrorhodamine 123, was detected in human TE monolayers exposed to 0.21-0.63 ppm ozone. In a different system, TE cells were exposed to air or 0.5 ppm ozone for 1 h by serial renewal/collection of an adherent film of media. Ozone-induced ROS formation, the transient increase and decline in PGE2, and CO inactivation were attenuated by an intracellular hydroxyl radical scavenger, 1,3-dimethyl-2-thiourea. Ibuprofen, a reversible CO inhibitor, prevented PGE2 release during ozone exposure (and hence autocatalytic CO inactivation) but not loss of CO activity. Although CO activity remained depressed for hours after ozone exposure, compared with air-exposed cultures, no differences were detected in mRNA and protein levels of prostaglandin endoperoxide G/H synthase 2 (PGHS-2), the only CO isoform present in human TE cells, or in the rate of de novo PGHS-2 synthesis. Our findings suggest that ozone-induced PGE2 production and CO inactivation are primarily the result of formation of intracellular oxidant molecules and that delayed recovery of CO activity in human TE cells after short-term ozone exposure is due to persistent inactivation of PGHS-2, rather than to interference with its synthesis.

Ozone exposure of human tracheal epithelial cells inactivates cyclooxygenase and increases 15-HETE production

We assessed the immediate and prolonged effects of ozone on arachidonic acid (AA) metabolism by primary cultured human tracheal epithelial (TE) cells. TE monolayers were exposed at a gas-fluid interface to air or 0.1, 0.25, or 0.5 ppm ozone (15 min air, then 45 min air/ozone), and serially collected effluents were analyzed by thin-layer chromatography (TLC) and/or high-performance liquid chromatography. Release of prostaglandin E2 (PGE2) and AA, but not 15-hydroxyeicosatetraenoic acid (15-HETE) or its metabolites, was detected from cultures prelabeled with [14C]AA. PGE2 production, measured by immunoassay, was nearly constant during air exposure. In contrast, PGE2 increased two- to threefold during the first 15-min exposure to all concentrations of ozone, but then progressively declined to 78 +/- 17, 57 +/- 12 (P < or = 0.05), and 45 +/- 15% (P < or = 0.05) of air controls after exposure to 0.1, 0.25, and 0.5 ppm ozone. Ozone did not induce a new spectrum of AA metabolites; only PGE2, lesser amounts of PGF2 alpha, and 15-HETE were present in media and cell extracts of air- or ozone-exposed cultures provided with 30 microM exogenous AA. However, cyclooxygenase (CO) activity (PGE2 produced from 30 microM AA) decreased to 82 +/- 9, 53 +/- 8 (P < or = 0.05), and 28 +/- 6% (P < or = 0.05) vs. controls after 0.1, 0.25, and 0.5 ppm ozone, whereas 15-HETE production was unimpaired. When cells exposed to 0.5 ppm ozone were maintained for up to 6 h in 5% CO2-air, spontaneous PGE2 production remained decreased and recovery of CO activity was extremely slow. TLC analysis of lipid extracts from [14C]AA-labeled cells revealed a nearly twofold increase in free intracellular 15-HETE, and hydrolysis of phospholipids demonstrated increased esterified 15-HETE. Exposure of human TE cells to ozone leads to a transient increase followed by prolonged decrease in PGE2 production and increased intracellular retention of 15-HETE. Loss of the bronchodilator and anti-inflammatory properties of epithelial PGE2, with or without increased 15-HETE, might contribute to ozone-induced airway dysfunction.

Corticosteroids suppress cyclooxygenase messenger RNA levels and prostanoid synthesis in cultured vascular cells

Prostacyclin synthesis by cultured vascular smooth muscle cells was inactivated by aspirin. Recovery required serum factors replaceable by EGF plus TGF-beta and was blocked by cycloheximide but not by actinomycin D. Recovery of cyclooxygenase activity was prevented by preincubation with dexamethasone (0.1 to 2 microM), which also suppressed basal enzyme activity by up to 70%. A full length 2.8 Kb cDNA hybridization probe for human cyclooxygenase identified a cyclooxygenase messenger RNA of approximately 2.8 Kb in these cells. Cyclooxygenase mRNA levels were enhanced by EGF/TGF-beta, but suppressed completely by corticosteroids. It is concluded that inhibition of prostanoid synthesis by corticosteroids is mediated by suppressing cyclooxygenase messenger RNA. These observations provide a new molecular mechanism for the anti-inflammatory activity of the corticosteroids.

Interactions of vasopressin, prostaglandins, and cAMP in rat renal papillary collecting tubule cells in culture.

To evaluate the hypothesis that prostaglandins (PGs) inhibit vasopressin stimulation of adenylate cyclase in the collecting tubule, we have studied the interactions of vasopressin, PGs, and intracellular cAMP in rat renal papillary collecting tubule (RPCT) cells in cell culture. Inhibition of PGE2 synthesis with acetylsalicylic acid (ASA) did not potentiate arginine vasopressin (AVP)-stimulated intracellular cAMP. Augmentation of prostanoid synthesis with arachidonic acid or exogenous addition of PGE2 did not decrease AVP-stimulated cAMP in the RPCT cells. Arachidonic acid or PGE2, used alone, increased cAMP and ASA reduced cAMP, consistent with the presence of a PG-sensitive adenylate cyclase. Six-hour incubations of RPCT cells produced clear evidence of homologous desensitization of the PGE2 receptor, after exposure to either PGE2 or arachidonic acid, but not heterologous desensitization of the AVP receptor linked to cAMP synthesis. Preincubation of the RPCT cells with AVP or 1-desamino-8-D-arginine vasopressin (dDAVP) induced homologous desensitization of AVP- or dDAVP-stimulated cAMP. Three-day incubations with dDAVP or AVP apparently induced cyclooxygenase activity since PGE2 synthesis increased in response to arachidonic acid and recovery of cyclooxygenase activity after ASA was enhanced by dDAVP or AVP. In conclusion, our data on AVP-stimulated cAMP in cultured RPCT cells do not support the hypothesis that PGE2 inhibits AVP-dependent collecting tubule cAMP.

Recovery of cyclooxygenase activity after aspirin in populations of platelets separated on stractan density gradients

Human platelets obtained before and at intervals up to one week following the ingestion of 650 mg acetylsalicylic acid (ASA) were subjected to fractionation on discontinuous gradients of stractan. Platelets were recovered from the density gradients and incubated with arachidonate. Thromboxane synthesis was estimated by radioimmunoassay (RIA). There were no differences in cyclooxygenase activity among the platelet fractions prior to ASA ingestion. ASA inhibited synthesis of thromboxane B 2(TXB 2) by 98% in all subfractions. Twenty-four hours after ASA ingestion the total platelet population exhibited 10% of the control cyclooxygenase activity suggesting that new platelets emerging from the bone marrow within 24 hours of an ASA dose have functional cyclooxygenase enzymes. The pattern of return of TXB 2 synthesis differed markedly among the platelet fractions, indicating differences in the distribution of hew platelets among these fractions. New platelets with functional cyclooxygenase enzyme were twice as concentrated in the less dense fractions as in the most dense fraction. By 72 hours up to 75% of the platelets in the less dense fractions synthesized thromboxanes, while only 27% of platelets in the most dense fraction were active. Newly formed platelets may increase in density as they age or platelets of low density have a more rapid turnover than platelets of high density.

Arachidonic acid metabolites and the gastro-intestinal toxicity of anti-inflammatory agents

The relationship between gastro-intestinal damage and the inhibition of cyclo-oxygenase by non-steorid anti-inflammatory drugs is discussed. In anti-inflammatory doses, aspirin and the newer substitutes, indomethacin, ketoprofen, naproxen and flurbiprofen, which reduce prostaglandin levels in inflammatory exudates likewise inhibit cyclo-oxygenase activity in the gastric mucosa. These compounds all induce gastric damage. In contrast, sodium salicylate and a novel compound BW755C inhibit prostaglandin production in the inflamamtory exudate, yet fail to inhibit gastric cyclo-oxygenase in the mucosa and do not form gastric erosions. Studies on the duration of cyclo-oxygenase inhibition with indomethacin indicate a close correlation between the rate of healing of gastric erosions and the recovery of cyclo-oxygenase activity in the gastric mucosa. No such relationship is seen in the small-intestine. The hydroperoxy metabolites of arachidonic acid, formed by the lipoxygenase enzymes, have vasodilator actions in the gastric circulation and the possible roles of lipoxygenase products in the gastric mucosa and their relationship to gastro-intestinal damage are discussed.

Impaired thromboxane production by newly formed platelets after aspirin administration to thrombocytopenic rats.

The in vivo inhibitory effect of aspirin on platelet cyclo-oxygenase is irreversible and lasts for the entire platelet life-span. Reappearance of cyclo-oxygenase activity in blood after aspirin has been proposed as a measure of the formation of new platelets and as an indirect indicator of platelet survival. A delay of 24--72 h in recovery, however, has been observed and it has been suggested that aspirin might also inhibit megakaryocyte cyclo-oxygenase. To test this possibility, aspirin (100 mg/kg) or saline were administered i.p. to rats made thrombocytopenic 2 h later (platelet count less than 5% of basal value) by a specific antiplatelet antiserum. Malondialdehyde (MDA) and thromboxane B2 (TxB2) production by platelets was measured by spectrophotometry and radioimmunoassay respectively, during the period of platelet count restoration. By 24 h after thrombocytopenia was induced, platelet count was about 15% of basal values in control and aspirin-treated rats. However, while in controls MDA and TxB2 production was restored to about 20% of basal values, in aspirin-treated rats less than 5% return of activity was detected. A marked difference between the two groups was still found 96 h after induction of thrombocytopenia, when platelet count restoration was similar. Since aspirin disappeared very rapidly from the circulation, the delay in recovery of cyclo-oxygenase activity supports the hypothesis of a megakaryocyte effect of this drug.