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 with a wide spectrum of action particularly at the level of mitochondria. » MiPNet article

Abbreviation: aMT

Reference: Acuña-Castroviejo 2014 Cell Mol Life Sci

Melatonin and protection from mitochondrial 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: ES Granada Acuna-Castroviejo D, AT Innsbruck Gnaiger E

Pineal and extrapineal melatonin

Melatonin (N-acetyl-5-methoxytryptamine, aMT) is a highly conserved molecule which is present in a broadrange of phylogenetic taxa, including bacteria, fungi, plants, algae, invertebrate and vertebrate organisms. Whereas pineal melatonin has been related with chronobiotic functions, extrapineal melatonin shows mainly antioxidant and antiinflammatory actions.

1. Pineal melatonin: Pineal 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 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 to circadian functions.
2. Extrapineal melatonin: Melatonin is produced in various tissues, fluids and organs other than the pineal gland. Extrapineal melatonin levels are in micromolar range and are thus much higher than the nanomolar pineal melatonin concentrations. The production of extrapineal melatonin is independent of the pineal synthesis and occurs in the tissues in a different functional context. Moreover, extrapineal melatonin differs from pineal melatonin in terms of its intracellular location and protection of the tissue.

Mechanisms of action

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.

Due to its lipophilic and hydrophilic nature, melatonin can cross biological membranes. Therefore, melatonin is present in all subcellular compartments, predominantly in the nucleus and mitochondria. Melatonin exerts highly relevant functions at the level of mitochondria, which are the main target of melatonin. Mitochondria are an important source of reactive oxygen and nitrogen species (ROS/RNS) in the cell, and 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 scavenging activity: Due to its structure and its high redox potential melatonin and its metabolites act as electron donors, 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 has anti-inflammatory properties

During inflammatory diseases (such as sepsis or fibromyalgia), an induction occurs in mitochondria of i-mtNOS (inducible mitochondrial isoform of nitric oxide synthase) which causes 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 producing finally an oxidative-nitrosative stress able to damage cellular structures, resulting in 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, inflammatory processes result in the activation of the nuclear factor NF-kB which acts in the nucleus triggering the expression of several proinflammatory genes. Melatonin inhibits the activation of the NF-kB pathway.

Melatonin exhibits oncostatic effects

Melatonin inhibits cell proliferation or induces apoptosis activation of tumoral cells by different mechanisms of action.

The 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 cardiolipin (CL) are involved in CI and CIV activities, mitochondrial supramolecular organization in supercomplexes (SC), the integrity of mitochondrial network and apoptotic processes. Therefore, alterations in cardiolipin structure, content and/or acyl chains compositions have significant implications on 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.

Conclusions

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 hyperoxidative damage mediated via various 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 of relevant components of mitochondrial membranes such as cardiolipin and polyunsaturated fatty acid (PUFAs), allowing to maintain an adequate structure and function and consequently preserving bioenergetic processes.

References

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