THE MODE OF ACTION OF ASPIRIN-LIKE DRUGS - EFFECT ON INDUCIBLE NITRIC-OXIDE SYNTHASE

Nitric oxide synthesized by inducible nitric oxide synthase (iNOS) has been implicated as a mediator of inflammation in rheumatic and autoim mune diseases. We report that exposure of lipopolysaccharide-stimulate d murine macrophages to therapeutic concentrations of aspirin (IC50 = 3 mM) and hydrocortisone (IC50 = 5 mu M) inhibited the expression of i NOS and production of nitrite. In contrast, sodium salicylate (1-3 mM) , indomethacin (5-20 mu M), and acetaminophen (60-120 mu M) had no sig nificant effect on the production of nitrite at pharmacological concen trations. At suprapharmacological concentrations, sodium salicylate (I C50 = 20 mM) significantly inhibited nitrite production. Immunoblot an alysis of iNOS expression in the presence of aspirin showed inhibition of iNOS expression (IC50 = 3 mM). Sodium salicylate variably inhibite d iNOS expression (0-35%), whereas indomethacin had no effect. Further more, there was no significant effect of these nonsteroidal antiinflam matory drugs on iNOS mRNA expression at pharmacological concentrations . The effect of aspirin was not due to inhibition of cyclooxygenase 2 because both aspirin and indomethacin inhibited prostaglandin E(2) syn thesis by >75%. Aspirin and N-acetylimidazole (an effective acetylatin g agent), but not sodium salicylate or indomethacin, also directly int erfered with the catalytic activity of iNOS in cell-free extracts. The se studies indicate that the inhibition of iNOS expression and functio n represents another mechanism of action for aspirin, if not for all a spirin-like drugs. The effects are exerted at the level of translation al/posttranslational modification and directly on the catalytic activi ty of iNOS.