CROSS-TALK BETWEEN CYCLOOXYGENASE AND NITRIC-OXIDE PATHWAYS - PROSTAGLANDIN E(2) NEGATIVELY MODULATES INDUCTION OF NITRIC-OXIDE SYNTHASE BY INTERLEUKIN-1

The inflammatory cytokine interleukin 1 beta (IL-1 beta) induces both cyclooxygenase (COX) and nitric oxide synthase (NOS) with increases in the release of prostaglandin (PG) and nitric oxide (NO) by mesangial cells. Recently, activation of the COX enzyme by NO has been described. However, the effects of COX products (PGs) on the NO pathway have not been fully clarified. Thus we determined the effect of COX inhibition and exogenous PGs on NO production and NOS induction in rat mesangial cells. A COX inhibitor, indomethacin, enhanced IL-1 beta-induced steady-state level of the inducible NOS (iNOS) mRNA and nitrite production. The effect of indomethacin was dose dependently reversed by the replacement of endogenous PGE(2) with exogenous PGE(2), which is the predominant product of the COX pathway in rat mesangial cells. In contrast to PGE(2), a stable analog of PGI(2), carba prostacyclin, enhanced IL-1 beta-induced iNOS mRNA levels and nitrite production. Forskolin, an activator of the adenylate cyclase, mimicked the effect of carba prostacyclin but not PGE(2). These data suggest that (i) endogenous PGE(2) downregulates iNOS induction, (ii) this inhibitory effect of PGE(2) on iNOS induction is not mediated by activation of adenylate cyclase, and (iii) exogenous PGI(2) stimulates COX induction possibly by activation of adenylate cyclase.