A growing body of evidence suggests that the pineal hormone, melatonin, has immunomodulatory properties, although very little is known about its effect on leukocytes. Therefore, we aimed to investigate the effect of melatonin and its oxidation product N1-acetyl-N2-formyl-5-methoxykynuramine (AFMK) on cytokine production by neutrophils and peripheral blood mononuclear cells (PBMCs). AFMK (0.001-1 mM) inhibits the lipopolysaccharide (LPS)-mediated production of tumor necrosis factor-alpha (TNF-alpha) and interleukin-8 (IL-8) more efficiently in neutrophils than PBMCs. Moreover, the inhibitory activity of AFMK is stronger than that of melatonin. Interestingly, monocytes efficiently oxidize melatonin to AFMK. We conclude that neutrophils are one of the main targets for melatonin and that at least part of the effects described for melatonin on immune cells may be due to its oxidation product, AFMK. We also consider that the oxidation of melatonin may be an important event in the cross-talking between neutrophils and monocytes.
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