Inflammation is a complex phenomenon involving multiple cellular and molecular interactions which must be tightly regulated. Cyclooxygenase-2 (COX) is the key enzyme that catalyzes the two sequential steps in the biosynthesis of PGs from arachidonic acid. The inducible isoform of COX, namely COX-2, plays a critical role in the inflammatory response and its over-expression has been associated with several pathologies including neurodegenerative diseases and cancer. Melatonin is the main product of the pineal gland with well documented antioxidant and immunomodulatory effects. Since the action of the indole on COX-2 has not been previously described, the goal of the present report was to test the effect of melatonin on the activities of COX-2 and inducible nitric oxide synthase (iNOS), using lipopolysaccharide (LPS)-activated RAW 264.7 macrophages as a model. Melatonin and its metabolites, N1-acetyl-N2-formyl-5-methoxykynuramine (AFMK) and N1-acetyl-5-methoxykynuramine (AMK), prevented COX-2 activation induced by LPS, without
affecting COX-1 protein levels. The structurally related compound 6-methoxy-melatonin only partially prevented the increase in COX-2 protein levels induced by the toxin. Likewise melatonin prevented iNOS activation and reduced the concentration of products from both enzymes, PGE2 and nitric oxide. Another endogenous antioxidant like N-acetyl-cysteine (NAC) did not reduced COX-2 significantly. The current finding corroborates a role of melatonin as an anti-inflammatory agent and, for the first time, COX-2 and iNOS as molecular targets for either melatonin or its metabolites AFMK and AMK. These anti-inflammatory actions seem not to be exclusively mediated by the free radical scavenging properties of melatonin. As a consequence, the present work suggests these substances as a new class of potential antiinflammatory agents without the classical side effects due to COX-1 inhibition.