Melatonin

high-resolution terminology - matching measurements at high-resolution

Melatonin

Description

Melatonin (N-acetyl-5-methoxytryptamine, aMT) is a highly conserved molecule present in unicellular to vertebrate organisms. Melatonin is synthesized from tryptophan in the pinealocytes by the pineal gland and also is produced in other organs, tissues and fluids (extrapineal melatonin). Melatonin has lipophilic and hydrophilic nature which allows it to cross biological membranes. Therefore, melatonin is present in all subcellular compartments predominantly in the nucleus and mitochondria. Melatonin has pleiotropic functions with powerful antioxidant, anti-inflammatory and oncostatic effects at the level of mitochondria. » MiPNet article

Abbreviation: aMT

Reference: Reiter 2003 Acta Biochim Pol; Acuña-Castroviejo 2014 Cell Mol Life Sci

Melatonin and attenuation of mitochondrial oxidative damage

Doerrier C (2015) Melatonin and attenuation of mitochondrial oxidative damage. Mitochondr Physiol Network 2015-03-03.

Doerrier C (2015) MiPNet

Abstract: Melatonin (aMT) is a potent antioxidant and anti-inflammatory molecule able to attenuate mitochondrial oxidative damage, preserving mitochondrial function and organization.

• O2k-Network Lab: AT Innsbruck Gnaiger E

Pineal and extrapineal melatonin

Melatonin (N-acetyl-5-methoxytryptamine, aMT) is a highly conserved molecule which is present in a broad phylogenetic taxa, including bacteria, fungi, plants, algae, invertebrate and vertebrate organisms.

Melatonin is synthesized from tryptophan in the pinealocytes by the pineal gland. Its production is controlled by a circadian signal from suprachiasmatic nucleus (SCN). At night photoreceptors of the retina generate a potential action which finally triggers an increment in the levels and activity of arylalkylamine N-acetyltransferase (AANAT) protein. AANAT is the penultimate enzyme in melatonin synthesis. However, during the day the light maintains these photoreceptors hyperpolarized, blocking melatonin synthesis. Therefore, melatonin presents the maximum levels in plasma between 2-3 am (which are 10 times higher than diurnal levels). Once synthesized, melatonin is released into the bloodstream, accessing to cellular tissues and corporal fluids. Pineal melatonin is related with circadian functions.

However, melatonin is also produced in other organs, tissues and fluids (extrapineal melatonin). Extrapineal melatonin levels occur in much higher levels than pineal melatonin (micromolar range versus nanomolar range). The production of extrapineal melatonin occurs in the tissue when it is necessary and independently of the pineal synthesis. Moreover, extrapineal melatonin differs from pineal melatonin due to stays into the cell protecting the tissue.

Whereas pineal melatonin has been related with chronobiotic functions, extrapineal melatonin shows mainly antioxidant and antiinflammatory actions.

Mechanisms of action

Currently, two different mechanisms of action of melatonin have been described:

1) Receptor-mediated mechanism: Melatonin binds to membrane receptors (such as MT1 and MT2), nuclear receptors (RZR/ROR) and cytosolic proteins (such calmodulin and calreticulin).

2) Non receptor-mediated mechanism.

Melatonin has lipophilic and hydrophilic nature which allows it to cross biological membranes. Therefore, melatonin is present in all subcellular compartments predominantly in the nucleus and mitochondria. Precisely, melatonin exerts relevant functions at the level of mitochondria. Mitochondria are an important source of reactive oxygen and nitrogen species (ROS/RNS) into the cell. Mitochondria are the main target of melatonin. Here, melatonin exerts important actions protecting against mitochondrial damage.

Main functions of extrapineal melatonin

Melatonin shows pleiotropic functions with a wide spectrum of properties.

• Melatonin is a powerful antioxidant:

1. Melatonin presents direct free radical scavenger activity. Due to its structure and its high-potential redox melatonin and its metabolites have been able to act as electron donor, scavenging ROS.

2. Indirect antioxidant activity. Melatonin decreases ROS/RNS production, increases the expression and the activity of antioxidant systems (such as glutathione peroxidase, glutathione reductase, superoxide dismutase and catalase).

• Melatonin displays an important role as anti-inflammatory molecule:

During inflammatory diseases (such as sepsis or fibromyalgia), in mitochondria occur an induction of i-mtNOS (inducible mitochondrial isoform of nitric oxide synthase) which generates a significant rise in nitric oxide (NO●) production and consequently an increment in peroxinitrite anion (ONOO–) levels. Both NO● and ONOO– inhibit respiratory complexes, favoring electron leak and produce finally an oxidative-nitrosative stress able to damage cellular structures, resulting mitochondrial failure and cell death. Melatonin inhibits iNOS (cytosolic isoform of nitric oxide synthase) and i-mtNOS expression, restoring NO● levels. Accordingly, melatonin decrease RNS and ROS production, maintaining an optimal mitochondrial function.

On the other hand, in the inflammatory process results the activation of the nuclear factor NF-kB which acts in nucleus triggering the expression of several proinflammatory genes. Melatonin inhibits the activation of NF-kB pathway.

• Melatonin exhibits oncostatic effects by different mechanisms of action. Melatonin is able to inhibit proliferation or induce apoptosis activation of tumoral cells.

Lipid composition of mitochondrial membranes is relevant to maintain an adequate fluidity and consequently the organization and function of mitochondria. Important phospholipids present in mitochondrial membranes are very susceptible to the ROS attack and to the damage by lipid peroxidation (LPO). Moreover, phospholipids such as cardiolipid (CL) are involved in CI and CIV activities, mitochondrial supramolecular organization in supercomplexes (SC), the integrity of mitochondrial network and apoptotic process. Therefore, alterations in CL structure, content and/or acyl chains compositions show significant implications in mitochondrial function. Melatonin is able to protect these mitochondrial components against oxidative and nitrosative related-damage, providing and optimal membrane fluidity which is necessary for a proper mitochondrial function.

Mitochondrial dysfunction plays a key role in several pathologies such as neurodegenerative, cardiovascular and inflammatory diseases, metabolic disorders, ischemia-reperfusion, hypoxia, mucositis as well as in aging. Usually, mitochondrial dysfunction in these pathophysiological conditions is caused, at least in part, by an increment in oxidative and nitrosative stress. A large body of studies support that melatonin treatment protects against hiperoxidative damage mediated via different mechanisms. Melatonin allows an optimal mitochondrial function by their direct and indirect actions.

In summary, melatonin administration can counteract mitochondrial impairment mainly by decreasing ROS/RNS production, preventing LPO and hence reducing oxidative damage in relevant component of mitochondrial membranes such as CL and polyunsaturated fatty acid (PUFAs), allowing to maintain an adequate structure and function and consequently preserving bioenergetic process.