title: Uncoupled regulation of leukotriene C4 synthase in platelets from aspirin-intolerant asthmatics and healthy volunteers after aspirin treatment.
abstract: BACKGROUND
  We have reported that thromboxane A2 induces suppression of leukotriene (LT) C4 synthase activity in human platelets.
  
  
  OBJECTIVE
  In the present study, we describe a mechanism whereby aspirin treatment can lead to increased formation of LTC4, which is a potent bronchoconstrictor and inflammatory mediator. This mechanism is also demonstrated to be present in platelets from aspirin-intolerant asthmatics (AIA).
  
  
  METHODS
  The effect of arachidonic acid or platelet agonists on LTC4 synthase activity was investigated in platelets obtained from healthy volunteers, aspirin-intolerant asthmatics or aspirin-tolerant asthmatics after in vivo treatment or in vitro pre-incubation with aspirin.
  
  
  RESULTS
  Incubation of normal platelets with arachidonic acid or collagen provoked approximately 50% reduction of platelet LTC4 synthase activity, as determined by the conversion of LTA4 to LTC4. However, the inhibitory effect of arachidonic acid or collagen was not observed after oral administration of aspirin prior to collection of the platelets. Arachidonic acid-induced inhibition of LTC4 synthase activity was totally abolished in platelets collected from peripheral blood already 30 min after aspirin ingestion but was fully restored in platelets collected 3 to 7 days after the administration of aspirin. Treatment of platelet suspensions with aspirin in vitro dose-dependently counteracted the suppressive effect of arachidonic acid on LTC4 formation, with total reversal at approximately 40 microm. In contrast, the major aspirin metabolite, salicylic acid did not alter arachidonic acid-induced reduction of LTC4 synthase activity. Similarly, LTC4 synthase activity in platelets from AIA and aspirin-tolerant asthmatics (ATA) was reduced by approximately 50% after pre-treatment with arachidonic acid in vitro. Again the inhibitory effect was abolished when platelets were pre-incubated in the presence of aspirin.
  
  
  CONCLUSIONS
  The results indicate that oral aspirin administration can lead to uncoupling of thromboxane A2-dependent negative feedback mechanisms, which may normally restrict the production of cysteinyl leukotrienes. This mechanism can be of potential interest in aspirin-induced asthma.
authors.affiliation: Department of Medical Biochemistry and Biophysics, Division of Physiological Chemistry II, Karolinska Institutet, Huddinge University Hospital, Stockholm, Sweden. Susanne.Tornhamre@mbb.ki.se
authors.forename: S
authors.initials: S
authors.lastname: Tornhamre
authors.forename: A
authors.initials: A
authors.lastname: Ehnhage
authors.forename: K G
authors.initials: KG
authors.lastname: Kölbeck
authors.forename: C
authors.initials: C
authors.lastname: Edenius
authors.forename: J A
authors.initials: JA
authors.lastname: Lindgren
mesh_terms: D000328:Adult; D000894:Anti-Inflammatory Agents, Non-Steroidal / Q000009:adverse effects*; D016718:Arachidonic Acid / Q000494:pharmacology; D001241:Aspirin / Q000009:adverse effects*; D001249:Asthma / Q000097:blood / Q000139:chemically induced*; D001792:Blood Platelets / Q000187:drug effects / Q000378:metabolism*; D016022:Case-Control Studies; D003094:Collagen / Q000494:pharmacology; D003864:Depression, Chemical; D004305:Dose-Response Relationship, Drug; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005260:Female; D005982:Glutathione Transferase / Q000037:antagonists & inhibitors / Q000378:metabolism*; D006801:Humans; D008297:Male; D008875:Middle Aged; D013928:Thromboxane A2 / Q000378:metabolism
chemical_list: D000894:Anti-Inflammatory Agents, Non-Steroidal; D004791:Enzyme Inhibitors; D016718:Arachidonic Acid; D013928:Thromboxane A2; D003094:Collagen; D005982:Glutathione Transferase; C041116:leukotriene-C4 synthase; D001241:Aspirin