Abstract

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 activityin 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 κB (NF-κB). We have used interleukin 1β–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 1β for 24 hr with an IC₅₀ value of 5 μg/ml, an effect that was independent of NF-κB 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 μm exogenous arachidonic acid. In contrast, when exogenous arachidonic acid was increased to 30 μm, sodium salicylate was a very weak inhibitor of COX-2 activity with an IC₅₀ of >100 μg/ml. Thus, sodium salicylate is an effective inhibitor of COX-2 activity at concentrations far below those required to inhibit NF-κB (20 mg/ml) activation and is easily displaced by arachidonic acid.