Evidence for Activation of Cyclooxygenase-1/-2 by Endogenous Nitric Oxide in Adjuvant Arthritic Lewis Rats

Abstract

Increased enzyme activities of cytosolic phospholipase A₂ (cPLA₂) and nitric oxide synthase (NOS) have been implicated in the pathophysiology of rheumatoid arthritis(RA)^1,2,3,4 and the alteration of immunological responses^5,6. As determined on messenger RNA (mRNA) and protein level, in human rheumatoid synoviocytes, both, cPLA₂ and growth factor-responsive prostaglandin H synthase-2 (PGHS-2 or cyclooxygenase-2, COX-2) activity were induced by interleukin-lβ, while secretory PLA₂ (sPLA₂) and constitutive prostaglandin H synthase-1 (PGHS-1 or cyclooxygenase-1, COX-1) remained unaffected. Reflecting the enhanced activity of this pathway, its induction was associated with high levels of prostaglandin-2 (PGE₂), a mediator of pain and inflammation in the joints of patients with RA². In addition, patients with RA showed high levels of nitrite (NO⁻ ₂, breakdown product of nitric oxide) and 3-nitrotyrosine, indicating elevated NOS activity and nitric oxide (NO^•) dependent oxidative damage^3,7. In order to evaluate a physiological link between cPLA₂, COX-1/-2 and NOS pathway in vivo, we investigated the effect of L-nitro-arginine-methyl-ester (L-NAME), an inhibitor of NOS, in adjuvant-induced arthritis in the rat, while dexamethasone served as control drug, This model exhibits several pathological features similar to those occurring in autoimmune reactive RA in humans, characterized by chronic inflammation of the joints. In our studies following parameters have been determined: i) PGE₂ as a product of the PLA₂ and COX-1/-2 pathway, ii) platelet-activating-factor (PAF, PAF-acether) as product of the PLA₂ and acetyltransferase pathway, iii) NO⁻ ₂ as product of the NOS pathway, iv) paw swelling as an indicator of in vivo response.