Lipoxygenase-mediated pro-radical effect of melatonin via stimulation of arachidonic acid metabolism

Melatonin, a neuro-hormone principally produced by the pineal glan, is being receiving much attention as a regulator of organism homeostasis; its potential therapeutic use in the treatment of many pathological conditions is a main goal of pharmacological research, since its natural origin would ensure lack of important side effects. Many evidence suggest that melatonin may exert an anti-inflammatory effect; it has been proposed that melatonin anti-oxidant ability may contrast the onset and progression of inflammation, phenomenon that implies production of reactive oxygen species and activation of pro-oxidant enzymes. The inflammatory response begins with an insult that activates and recruits leukocytes to the inflamed tissue through the fine tuning of a huge set of molecular mediators; an important role in the early events is played by phospholipase A2 (PLA2), an enzyme that cleaves membrane phospholipids liberating membrane-bound arachidonic acid (AA), which is processed by cyclooxygenases (COX) and lipoxygenases (LOX) to produce important inflammatory mediators such as prostaglandins and leukotrienes, respectively. We have shown that melatonin immediately and transiently stimulates intracellular free radical production on a set of leukocytes, possibly as a consequence of calmodulin binding. We show here that melatonininduced ROS are produced by lipoxygenase (LOX), since they are prevented by a set of LOX inhibitors, and are accompanied by increase of the 5-LOX product 5-HETE. LOX activation is accompanied by strong liberation of AA; inhibition of Ca2+-independent, but not Ca2+-dependent, phospholipase A2 (PLA2), prevents both melatonin-induced arachidonic acid and ROS production, whereas LOX inhibition only prevents ROS, indicating that PLA2 is upstream with respect to LOX, as occurs in many signaling pathways. Chlorpromazine, an inhibitor of melatonin–calmodulin interaction, inhibits both ROS and arachidonic acid production, thus possibly placing calmodulin at the origin of a melatonin-induced pro-radical pathway. Interestingly, it is known that Ca2+-independent PLA2 binds to calmodulin: our results are compatible with PLA2 being liberated by melatonin from a steady-state calmodulin sequestration, thus initiating an arachidonate signal transduction. These results delineate a novel molecular pathway through which melatonin may participate to the inflammatory response. The overlapping of the many effects exerted by melatonin, i.e., radical scavenging, ROS promotion, stimulation of one or another of the intracellular targets, dose and persistence of melatonin around target cells, combined with different cell sensitivities, produce a huge variety of effects in different compartments of the organism, suggesting a highly sophisticated fine tuning. This deserves much attention for focusing on possible therapeutic effects of melatonin as such, and for the design of specific derivates that may maintain only selected actions of melatonin, according to the requirement of the specific pathologies/disturbances to be treated.