Melatonin, the circadian multioscillator system and health: the need for detailed analyses of peripheral melatonin signaling

LINE-1 activity as molecular basis for genomic instability associated with light exposure at night

The original hypothesis that exposure to light at night increases risk of breast cancer via suppression of nocturnal melatonin production was proposed over 2 decades ago. In 2007, shift work that involves circadian disruption has been recognized by the World Health Organization as a probable human carcinogen. Our discovery of melatonin-dependent regulation of LINE-1 retrotransposon expression and mobilization is the latest addition to the list of cellular genes and processes that are affected by light exposure at night. This finding establishes an unexpected health relevant connection between this endogenous DNA damaging agent and environmental light exposure. It also offers an appealing hypothesis pertaining to the origin of genomic instability in the genomes of individuals with light at night- or age-associated disruption of melatonin signaling.

Day-night variation in heart rate variability changes induced by endotoxaemia in healthy volunteers

BACKGROUND

Morbidity and mortality in response to sepsis may be dependent on clock time for the initiation of sepsis. Endotoxaemia, an experimental model for systemic inflammation, induces alterations in sympatico-vagal balance in the autonomic nervous system (ANS). The activity of sympathetic and parasympathetic activity can be estimated by measuring heart rate variability (HRV). Based on the intimate link between ANS and the inflammatory response, we hypothesized, that HRV changes seen during endotoxaemia would be different based on time of the day the endotoxaemia is initiated. We investigated day/night variation in endotoxaemia-induced changes in HRV.

METHODS

A randomized, crossover study with 12 healthy men (age 18-31) was conducted. Endotoxaemia were induced by lipopolysaccharide (LPS) endotoxin 0.3 ng/kg b.w. in two visits (day visit and night visit). At the day visit, endotoxaemia were induced at 12:00 h, and at the night visit it was induced at 24:00 h. Holter recordings were started 1 h before administration of LPS, and continued for 10 h. Time-domain and frequency-domain parameters of HRV were analysed.

RESULTS

A total of nine persons finished the study with valid recordings. Endotoxaemia at both night and day resulted in a significant depression in HRV parameters high-frequency power (HF), low-frequency power (LF), standard deviation of normal-to-normal (NN) intervals, root mean square of successive differences and proportion of NN50 divided by total number of NNs (P<0.001). The ratio LF/HF and mean heart rate significantly increased by endotoxaemia (P<0.001). At night-time endotoxaemia, a more pronounced depression of LF, HF and SDNN (P<0.01) and a more pronounced increase in the ratio of LF/HF and mean heart rate (P<0.01) occurred compared with day-time endotoxaemia.

CONCLUSION

Endotoxaemia induced changes in HRV exhibit a day-night difference. This difference may have clinical consequences in patients with sepsis.

Melatonin-mediated inhibition of Purkinje neuron P-type Ca²⁺ channels in vitro induces neuronal hyperexcitability through the phosphatidylinositol 3-kinase-dependent protein kinase C delta pathway

Although melatonin receptors are widely expressed in the mammalian central nervous system and peripheral tissues, there are limited data regarding the functions of melatonin in cerebellar Purkinje cells. Here, we identified a novel functional role of melatonin in modulating P-type Ca(2+) channels and action-potential firing in rat Purkinje neurons. Melatonin at 0.1 μm reversibly decreased peak currents (I(Ba)) by 32.9%. This effect was melatonin receptor 1 (MT(R1)) dependent and was associated with a hyperpolarizing shift in the voltage dependence of inactivation. Pertussis toxin pretreatment, intracellular application of QEHA peptide, and a selective antibody raised against the Gβ subunit prevented the inhibitory effects of melatonin. Pretreatment with phosphatidylinositol 3-kinase (PI3K) inhibitors abolished the melatonin-induced decrease in I(Ba). Surprisingly, melatonin responses were not regulated by Akt, a common downstream target of PI3K. Melatonin treatment significantly increased protein kinase C (PKC) activity 2.1-fold. Antagonists of PKC, but not of protein kinase A, abolished the melatonin-induced decrease in I(Ba). Melatonin application increased the membrane abundance of PKCδ, and PKCδ inhibition (either pharmacologically or genetically) abolished the melatonin-induced IBa response. Functionally, melatonin increased spontaneous action-potential firing by 53.0%; knockdown of MT(R1) and blockade of P-type channels abolished this effect. Thus, our results suggest that melatonin inhibits P-type channels through MT(R1) activation, which is coupled sequentially to the βγ subunits of G(i/o)-protein and to downstream PI3K-dependent PKCδ signaling. This likely contributes to its physiological functions, including spontaneous firing of cerebellar Purkinje neurons.

Melatonin-mediated Bim up-regulation and cyclooxygenase-2 (COX-2) down-regulation enhances tunicamycin-induced apoptosis in MDA-MB-231 cells

Melatonin is involved in many physiological functions, and it has differential effects on apoptosis in normal and cancer cells. However, the mechanism of its antitumor roles is not well understood. In this study, we show that melatonin enhances tunicamycin-induced apoptosis in human breast carcinoma MDA-MB-231 cells. Melatonin up-regulates pro-apoptotic protein Bim expression at the transcriptional levels in the presence of tunicamycin. Melatonin inhibits tunicamycin-induced COX-2 expression in MDA-MB-231 cells. Furthermore, inhibition of COX-2 activity using the COX-2 inhibitor, NS398, increases tunicamycin-induced apoptosis. Interestingly, these effects were not associated with melatonin receptor signal pathways. Pertussis toxin (a general Gi protein inhibitor) or luzindole (a nonspecific melatonin receptor antagonist) did not reverse the effect of melatonin. In addition, melatonin blocked tunicamycin-induced NF-κB transcriptional activity, p65 nuclear translocation, and p38 MAPK activation. Melatonin-mediated p38 MAPK inhibition contributed to decreased COX-2 mRNA stability. Taken together, our results suggest that melatonin enhances antitumor function through up-regulation of Bim expression and down-regulation of COX-2 expression in tunicamycin-treated MDA-MB-231 cells.

Caloric restriction, resveratrol and melatonin: Role of SIRT1 and implications for aging and related-diseases

Aging is an inevitable and multifactorial biological process. Free radicals have been implicated in aging processes; it is hypothesized that they cause cumulative oxidative damage to crucial macromolecules and are responsible for failure of multiple physiological mechanisms. However, recent investigations have also suggested that free radicals can act as modulators of several signaling pathways such as those related to sirtuins. Caloric restriction is a non-genetic manipulation that extends lifespan of several species and improves healthspan; the belief that many of these benefits are due to the induction of sirtuins has led to the search for sirtuin activators, especially sirtuin 1, the most studied. Resveratrol, a polyphenol found in red grapes, was first known for its antioxidant and antifungal properties, and subsequently has been reported several biological effects, including the activation of sirtuins. Endogenously-produced melatonin, a powerful free radical scavenger, declines with age and its loss contributes to degenerative conditions of aging. Recently, it was reported that melatonin also activates sirtuins, in addition to other functions, such as regulator of circadian rhythms or anti-inflammatory properties. The fact that melatonin and resveratrol are present in various foods, exhibiting possible synergistic effects, suggests the use of dietary ingredients to promote health and longevity.

Melatonin potentiates cisplatin-induced apoptosis and cell cycle arrest in human lung adenocarcinoma cells

OBJECTIVES

Melatonin produces anti-cancer effects via several mechanisms, including by induction of apoptosis. In this way, it has been shown to be of use, in combination with chemotherapeutic drugs, for cancer treatment. The study described here has evaluated effects of melatonin on cytotoxicity, apoptosis and cell cycle arrest induced with the chemotherapeutic agent cisplatin, in human lung adenocarcinoma cisplatin-sensitive cell line (SK-LU-1), which previously had only limit data.

MATERIALS AND METHODS

Cells of the SK-LU-1 line were treated with melatonin alone at 1-5 mM concentration or cisplatin alone 10-200 μM, for 48 h in culture. Cytotoxicity was measured by MTT reduction assay. Apoptosis induction was detected by annexin V/PI staining using flow cytometric analysis and DAPI nuclear staining. Change in mitochondrial membrane potential (ΔΨm) was quantified using DiOC6(3) reagent and activities of caspases-3/7 were also investigated. DNA fractions were measured using propidium iodide (PI) staining.

RESULTS

Melatonin or cisplatin alone had 50% (IC50 ) cytotoxicity at 5 mM or 34 μM concentrations respectively. Combination of 1 or 2 mM melatonin and cisplatin significantly augmented cytotoxicity of cisplatin by reducing its IC50 to 11 and 4 μM, respectively. Consistently, combined treatment increased population of apoptotic cells by elevating mitochondrial membrane depolarization, activating caspases-3/7 and inducing cell cycle arrest in the S phase, compared to treatment with cisplatin alone.

CONCLUSION

These data demonstrate that melatonin enhanced cisplatin-induced cytotoxicity and apoptosis in SK-LU-1 lung cancer cells. SK-LU-1 cell population growth inhibition was mediated by cell cycle arrest in the S phase. These findings suggest that melatonin has the potential to be used for NSCLC treatment in combination with a chemotherapeutic agent such as cisplatin.

Melatonin uptake through glucose transporters: a new target for melatonin inhibition of cancer

Melatonin is present in a multitude of taxa and it has a broad range of biological functions, from synchronizing circadian rhythms to detoxifying free radicals. Some functions of melatonin are mediated by its membrane receptors but others are receptor-independent. For the latter, melatonin must enter into the cell. Melatonin is a derivative of the amino acid tryptophan and reportedly easily crosses biological membranes due to its amphipathic nature. However, the mechanism by which melatonin enters into cells remains unknown. Changes in redox state, endocytosis pathways, multidrug resistance, glycoproteins or a variety of strategies have no effect on melatonin uptake. Herein, it is demonstrated that members of the SLC2/GLUT family glucose transporters have a central role in melatonin uptake. When studied by docking simulation, it is found that melatonin interacts at the same location in GLUT1 where glucose does. Furthermore, glucose concentration and the presence of competitive ligands of GLUT1 affect the concentration of melatonin into cells. As a regulatory mechanism, melatonin reduces the uptake of glucose and modifies the expression of GLUT1 transporter in prostate cancer cells. More importantly, glucose supplementation promotes prostate cancer progression in TRAMP mice, while melatonin attenuated glucose-induced tumor progression and prolonged the lifespan of tumor-bearing mice. This is the first time that a facilitated transport of melatonin is suggested. In fact, the important role of glucose transporters and glucose metabolism in cell fate might explain some of the diverse functions described for melatonin.

Circadian and melatonin disruption by exposure to light at night drives intrinsic resistance to tamoxifen therapy in breast cancer

Resistance to endocrine therapy is a major impediment to successful treatment of breast cancer. Preclinical and clinical evidence links resistance to antiestrogen drugs in breast cancer cells with the overexpression and/or activation of various pro-oncogenic tyrosine kinases. Disruption of circadian rhythms by night shift work or disturbed sleep-wake cycles may lead to an increased risk of breast cancer and other diseases. Moreover, light exposure at night (LEN) suppresses the nocturnal production of melatonin that inhibits breast cancer growth. In this study, we used a rat model of estrogen receptor (ERα(+)) MCF-7 tumor xenografts to demonstrate how altering light/dark cycles with dim LEN (dLEN) speed the development of breast tumors, increasing their metabolism and growth and conferring an intrinsic resistance to tamoxifen therapy. These characteristics were not observed in animals in which the circadian melatonin rhythm was not disrupted, or in animals subjected to dLEN if they received nocturnal melatonin replacement. Strikingly, our results also showed that melatonin acted both as a tumor metabolic inhibitor and a circadian-regulated kinase inhibitor to reestablish the sensitivity of breast tumors to tamoxifen and tumor regression. Together, our findings show how dLEN-mediated disturbances in nocturnal melatonin production can render tumors insensitive to tamoxifen.

Circadian rhythmicity of antioxidant markers in rats exposed to 1.8 GHz radiofrequency fields

Background: The potential health risks of exposure to Radiofrequency Fields (RF) emitted by mobile phones are currently of considerable public interest, such as the adverse effects on the circadian rhythmicities of biological systems. To determine whether circadian rhythms of the plasma antioxidants (Mel, GSH-Px and SOD) are affected by RF, we performed a study on male Sprague Dawley rats exposed to the 1.8 GHz RF. Methods: All animals were divided into seven groups. The animals in six groups were exposed to 1.8 GHz RF (201.7 μW/cm² power density, 0.05653 W/kg specific absorption rate) at a specific period of the day (3, 7, 11, 15, 19 and 23 h GMT, respectively), for 2 h/day for 32 consecutive days. The rats in the seventh group were used as sham-exposed controls. At the end of last RF exposure, blood samples were collected from each rat every 4 h (total period of 24 h) and also at similar times from sham-exposed animals. The concentrations of three antioxidants (Mel, GSH-Px and SOD) were determined. The data in RF-exposed rats were compared with those in sham-exposed animals. Results: circadian rhythms in the synthesis of Mel and antioxidant enzymes, GSH-Px and SOD, were shifted in RF-exposed rats compared to sham-exposed animals: the Mel, GSH-Px and SOD levels were significantly decreased when RF exposure was given at 23 and 3 h GMT. Conclusion: The overall results indicate that there may be adverse effects of RF exposure on antioxidant function, in terms of both the daily antioxidative levels, as well as the circadian rhythmicity.

Melatonin-sulforaphane hybrid ITH12674 induces neuroprotection in oxidative stress conditions by a 'drug-prodrug' mechanism of action

BACKGROUND AND PURPOSE

Neurodegenerative diseases are a major problem afflicting ageing populations; however, there are no effective treatments to stop their progression. Oxidative stress and neuroinflammation are common factors in their pathogenesis. Nuclear factor (erythroid-derived 2)-like 2 (Nrf2) is the master regulator of oxidative stress, and melatonin is an endogenous hormone with antioxidative properties that reduces its levels with ageing. We have designed a new compound that combines the effects of melatonin with Nrf2 induction properties, with the idea of achieving improved neuroprotective properties.

EXPERIMENTAL APPROACH

Compound ITH12674 is a hybrid of melatonin and sulforaphane designed to exert a dual drug-prodrug mechanism of action. We obtained the proposed hybrid in a single step. To test its neuroprotective properties, we used different in vitro models of oxidative stress related to neurodegenerative diseases and brain ischaemia.

KEY RESULTS

ITH12674 showed an improved neuroprotective profile compared to that of melatonin and sulforaphane. ITH12674 (i) mediated a concentration-dependent protective effect in cortical neurons subjected to oxidative stress; (ii) decreased reactive oxygen species production; (iii) augmented GSH concentrations in cortical neurons; (iv) enhanced the Nrf2-antioxidant response element transcriptional response in transfected HEK293T cells; and (v) protected organotypic cultures of hippocampal slices subjected to oxygen and glucose deprivation and re-oxygenation from stress by increasing the expression of haem oxygenase-1 and reducing free radical production.

CONCLUSION AND IMPLICATIONS

ITH12674 combines the signalling pathways of the parent compounds to improve its neuroprotective properties. This opens a new line of research for such hybrid compounds to treat neurodegenerative diseases.

Melatonin enhances plant growth and abiotic stress tolerance in soybean plants

Melatonin is a well-known agent that plays multiple roles in animals. Its possible function in plants is less clear. In the present study, we tested the effect of melatonin (N-acetyl-5-methoxytryptamine) on soybean growth and development. Coating seeds with melatonin significantly promoted soybean growth as judged from leaf size and plant height. This enhancement was also observed in soybean production and their fatty acid content. Melatonin increased pod number and seed number, but not 100-seed weight. Melatonin also improved soybean tolerance to salt and drought stresses. Transcriptome analysis revealed that salt stress inhibited expressions of genes related to binding, oxidoreductase activity/process, and secondary metabolic processes. Melatonin up-regulated expressions of the genes inhibited by salt stress, and hence alleviated the inhibitory effects of salt stress on gene expressions. Further detailed analysis of the affected pathways documents that melatonin probably achieved its promotional roles in soybean through enhancement of genes involved in cell division, photosynthesis, carbohydrate metabolism, fatty acid biosynthesis, and ascorbate metabolism. Our results demonstrate that melatonin has significant potential for improvement of soybean growth and seed production. Further study should uncover more about the molecular mechanisms of melatonin's function in soybeans and other crops.

Protecting the melatonin rhythm through circadian healthy light exposure

Currently, in developed countries, nights are excessively illuminated (light at night), whereas daytime is mainly spent indoors, and thus people are exposed to much lower light intensities than under natural conditions. In spite of the positive impact of artificial light, we pay a price for the easy access to light during the night: disorganization of our circadian system or chronodisruption (CD), including perturbations in melatonin rhythm. Epidemiological studies show that CD is associated with an increased incidence of diabetes, obesity, heart disease, cognitive and affective impairment, premature aging and some types of cancer. Knowledge of retinal photoreceptors and the discovery of melanopsin in some ganglion cells demonstrate that light intensity, timing and spectrum must be considered to keep the biological clock properly entrained. Importantly, not all wavelengths of light are equally chronodisrupting. Blue light, which is particularly beneficial during the daytime, seems to be more disruptive at night, and induces the strongest melatonin inhibition. Nocturnal blue light exposure is currently increasing, due to the proliferation of energy-efficient lighting (LEDs) and electronic devices. Thus, the development of lighting systems that preserve the melatonin rhythm could reduce the health risks induced by chronodisruption. This review addresses the state of the art regarding the crosstalk between light and the circadian system.

Hormonal protection in acute pancreatitis by ghrelin, leptin and melatonin

Acute pancreatitis is a nonbacterial disease of the pancreas. The severe form of this ailment is characterized by high mortality. Whether acute pancreatitis develops as the severe type or resolves depends on the intensity of the inflammatory process which is counteracted by the recruitment of innate defense mechanisms. It has been shown that the hormones ghrelin, leptin and melatonin are able to modulate the immune function of the organism and to protect the pancreas against inflammatory damage. Experimental studies have demonstrated that the application of these substances prior to the induction of acute pancreatitis significantly attenuated the intensity of the inflammation and reduced pancreatic tissue damage. The pancreatic protective mechanisms of the above hormones have been related to the mobilization of non-specific immune defense, to the inhibition of nuclear factor kappa B and modulation of cytokine production, to the stimulation of heat shock proteins and changes of apoptotic processes in the acinar cells, as well as to the activation of antioxidant system of the pancreatic tissue. The protective effect of ghrelin seems to be indirect and perhaps dependent on the release of growth hormone and insulin-like growth factor 1. Leptin and ghrelin, but not melatonin, employ sensory nerves in their beneficial action on acute pancreatitis. It is very likely that ghrelin, leptin and melatonin could be implicated in the natural protection of the pancreatic gland against inflammatory damage because the blood levels of these substances increase in the initial phase of pancreatic inflammation. The above hormones could be a part of the innate resistance system which might remove noxious factors and could suppress or attenuate the inflammatory process in the pancreas.

Melatonin-induced temporal up-regulation of gene expression related to ubiquitin/proteasome system (UPS) in the human malaria parasite Plasmodium falciparum

There is an increasing understanding that melatonin and the ubiquitin/ proteasome system (UPS) interact to regulate multiple cellular functions. Post-translational modifications such as ubiquitination are important modulators of signaling processes, cell cycle and many other cellular functions. Previously, we reported a melatonin-induced upregulation of gene expression related to ubiquitin/proteasome system (UPS) in Plasmodium falciparum, the human malaria parasite, and that P. falciparum protein kinase 7 influences this process. This implies a role of melatonin, an indolamine, in modulating intraerythrocytic development of the parasite. In this report we demonstrate by qPCR analysis, that melatonin induces gene upregulation in nine out of fourteen genes of the UPS, consisting of the same set of genes previously reported, between 4 to 5 h after melatonin treatment. We demonstrate that melatonin causes a temporally controlled gene expression of UPS members.

Melatonin: plant growth regulator and/or biostimulator during stress?

Melatonin regulates the growth of roots, shoots, and explants, to activate seed germination and rhizogenesis and to delay induced leaf senescence. The antioxidant properties of melatonin would seem to explain, at least partially, its ability to fortify plants subjected to abiotic stress. In this Review we examine recent data on the gene-regulation capacity of melatonin that point to many interesting features, such as the upregulation of anti-stress genes and recent aspects of the auxin-independent effects of melatonin as a plant growth regulator. This, together with the recent data on endogenous melatonin biosynthesis induction by environmental factors, makes melatonin an interesting candidate for use as a natural biostimulating treatment for field crops.

Attenuation of ultraviolet A-induced alterations in NIH3T3 dermal fibroblasts by melatonin

BACKGROUND

Sun exposure is responsible for long-term clinical skin changes such as photoageing, photodamage and photocancers. Ultraviolet (UV)A wavelengths stimulate the production of reactive oxygen species (ROS) that may contribute to photoageing. To protect against oxidative stress, skin cells have developed several defence systems, including ROS and metal ion scavengers and a battery of detoxifying, haem-degrading and repair enzymes. Melatonin's antioxidant activity is the result of three different but complementary actions: (i) a direct action due to its ability to act as a free radical scavenger; (ii) an indirect action that is a consequence of melatonin's ability to reduce free radical generation (radical avoidance); and (iii) its ability to upregulate antioxidant enzymes.

OBJECTIVES

In this study, we focused our attention on the prevention of photodamage, choosing melatonin as an antioxidant agent.

METHODS

In the present study we analysed the effects of pretreatment of murine fibroblasts cells (NIH3T3) with melatonin (1 mmol L(-1) ) followed by UVA irradiation (15 J cm(-2) ). Thereafter, changes in components of the extracellular matrix and in some antioxidant enzymes (inducible and constitutive haem oxygenase) were evaluated.

RESULTS

We observed that UVA radiation caused altered expression of extracellular matrix proteins and induced the expression of inducible haem oxygenase. This increase was not sufficient to protect the cells from damage. Instead, melatonin pretreatment led to increased expression of haem-degrading enzymes and suppression of UVA-induced photodamage.

CONCLUSIONS

These results suggest that melatonin, as a modifier of the dermatoendocrine system, may have utility in reducing the effects of skin ageing.

Melatonin induces apoptosis through biomolecular changes, in SK-LU-1 human lung adenocarcinoma cells

OBJECTIVES

Anti-cancer effects of melatonin (N-acetyl-5-methoxytryptamine, an indole-amine), have been widely reported, however, little has been known, regarding its mechanism(s) of action in lung cancer. Thus, we investigated its induction of apoptosis through biomolecular changes (lipid, protein and nucleic acid/DNA) in the SK-LU-1 human lung cancer cell line.

MATERIALS AND METHODS

We used Fourier transform infrared (FTIR) microspectroscopy, and conventional methods, to confirm changes in lipid (annexin V/PI staining for membrane alteration), protein (caspase-3/7 protein activity) and DNA (DAPI staining for DNA fragmentation).

RESULTS

We observed from FTIR data that melatonin increased lipid content and reduced intensity of nucleic acid/DNA, confirmed by annexin V/PI and DAPI respectively. Secondary protein structure at 1656 cm(-1) (α-helix) was reduced and peak position of β-sheet structure (1637 cm(-1) ) was shifted to lower frequency. Alteration in apoptotic proteins was demonstrated via caspase-3/7 activity induction.

CONCLUSIONS

High melatonin concentration exerted anti-cancer effects by changing biomolecular structure of lipids, nucleic acids and proteins, supporting its enhancement of apoptotic induction.

Melatonin, noncoding RNAs, messenger RNA stability and epigenetics--evidence, hints, gaps and perspectives

Melatonin is a highly pleiotropic regulator molecule, which influences numerous functions in almost every organ and, thus, up- or down-regulates many genes, frequently in a circadian manner. Our understanding of the mechanisms controlling gene expression is actually now expanding to a previously unforeseen extent. In addition to classic actions of transcription factors, gene expression is induced, suppressed or modulated by a number of RNAs and proteins, such as miRNAs, lncRNAs, piRNAs, antisense transcripts, deadenylases, DNA methyltransferases, histone methylation complexes, histone demethylases, histone acetyltransferases and histone deacetylases. Direct or indirect evidence for involvement of melatonin in this network of players has originated in different fields, including studies on central and peripheral circadian oscillators, shift work, cancer, inflammation, oxidative stress, aging, energy expenditure/obesity, diabetes type 2, neuropsychiatric disorders, and neurogenesis. Some of the novel modulators have also been shown to participate in the control of melatonin biosynthesis and melatonin receptor expression. Future work will need to augment the body of evidence on direct epigenetic actions of melatonin and to systematically investigate its role within the network of oscillating epigenetic factors. Moreover, it will be necessary to discriminate between effects observed under conditions of well-operating and deregulated circadian clocks, and to explore the possibilities of correcting epigenetic malprogramming by melatonin.

Melatonin as a signal molecule triggering defense responses against pathogen attack in Arabidopsis and tobacco

Melatonin plays pleiotropic roles in both animals and plants. The possible role of melatonin in plant innate immune responses was recently discovered. As an initial study, we employed Arabidopsis to determine whether melatonin is involved in defense against the virulent bacterial pathogen Pseudomonas syringae DC3000. The application of a 10 μM concentration of melatonin on Arabidopsis and tobacco leaves induced various pathogenesis-related (PR) genes, as well as a series of defense genes activated by salicylic acid (SA) and ethylene (ET), two key factors involved in plant defense response, compared to mock-treated leaves. The induction of these defense-related genes in melatonin-treated Arabidopsis matched an increase in resistance against the bacterium by suppressing its multiplication about ten-fold relative to the mock-treated Arabidopsis. Like melatonin, N-acetylserotonin also plays a role in inducing a series of defense genes, although serotonin does not. Furthermore, melatonin-induced PR genes were almost completely or partially suppressed in the npr1, ein2, and mpk6 Arabidopsis mutants, indicative of SA and ET dependency in melatonin-induced plant defense signaling. This suggests that melatonin may be a novel defense signaling molecule in plant-pathogen interactions.

Melatonin promotes seed germination under high salinity by regulating antioxidant systems, ABA and GA₄ interaction in cucumber (Cucumis sativus L.)

Although previous studies have found that melatonin can promote seed germination, the mechanisms involved in perceiving and signaling melatonin remain poorly understood. In this study, it was found that melatonin was synthesized during cucumber seed germination with a peak in melatonin levels occurring 14 hr into germination. This is indicative of a correlation between melatonin synthesis and seed germination. Meanwhile, seeds pretreated with exogenous melatonin (1 μM) showed enhanced germination rates under 150 mM NaCl stress compared to water-pretreated seeds under salinity stress. There are two apparent mechanisms by which melatonin alleviated salinity-induced inhibition of seed germination. Exogenous melatonin decreased oxidative damage induced by NaCl stress by enhancing gene expression of antioxidants. Under NaCl stress, compared to untreated control, the activities of antioxidant enzymes including superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD) were significantly increased by approximately 1.3-5.0-fold, with a concomitant 1.4-2.0-fold increase of CsCu-ZnSOD, CsFe-ZnSOD, CsCAT, and CsPOD in melatonin-pretreated seeds. Melatonin also alleviated salinity stress by affecting abscisic acid (ABA) and gibberellin acid (GA) biosynthesis and catabolism during seed germination. Compared to NaCl treatment, melatonin significantly up-regulated ABA catabolism genes (e.g., CsCYP707A1 and CsCYP707A2, 3.5 and 105-fold higher than NaCl treatment at 16 hr, respectively) and down-regulated ABA biosynthesis genes (e.g., CsNECD2, 0.29-fold of CK2 at 16 hr), resulting in a rapid decrease of ABA content during the early stage of germination. At the same time, melatonin positively up-regulated GA biosynthesis genes (e.g., GA20ox and GA3ox, 2.3 and 3.9-fold higher than NaCl treatment at 0 and 12 hr, respectively), contributing to a significant increase of GA (especially GA4) content. In this study, we provide new evidence suggesting that melatonin alleviates the inhibitory effects of NaCl stress on germination mainly by regulating the biosynthesis and catabolism of ABA and GA4.

Melatonin Reverses Fas, E2F-1 and Endoplasmic Reticulum Stress Mediated Apoptosis and Dysregulation of Autophagy Induced by the Herbicide Atrazine in Murine Splenocytes

Exposure to the herbicide Atrazine (ATR) can cause immunotoxicity, apart from other adverse consequences for animal and human health. We aimed at elucidating the apoptotic mechanisms involved in immunotoxicity of ATR and their attenuation by Melatonin (MEL). Young Swiss mice were divided into control, ATR and MEL+ATR groups based on daily (x14) intraperitoneal administration of the vehicle (normal saline), ATR (100 mg/kg body weight) and MEL (20 mg/kg body weight) with ATR. Isolated splenocytes were processed for detection of apoptosis by Annexin V-FITC and TUNEL assays, and endoplasmic reticulum (ER) stress by immunostaining. Key proteins involved in apoptosis, ER stress and autophagy were quantified by immunoblotting. ATR treatment resulted in Fas-mediated activation of caspases 8 and 3 and inactivation of PARP1 which were inhibited significantly by co-treatment with MEL. MEL also attenuated the ATR-induced, p53 independent mitochondrial apoptosis through upregulation of E2F-1 and PUMA and suppression of their downstream target Bax. An excessive ER stress triggered by ATR through overexpression of ATF-6α, spliced XBP-1, CREB-2 and GADD153 signals was reversed by MEL. MEL also reversed the ATR-induced impairment of autophagy which was indicated by a decline in BECN-1, along with significant enhancement in LC3B-II and p62 expressions. Induction of mitochondrial apoptosis, ER stress and autophagy dysregulation provide a new insight into the mechanism of ATR immunotoxicity. The cytoprotective role of MEL, on the other hand, was defined by attenuation of ER stress, Fas-mediated and p53 independent mitochondria-mediated apoptosis as well as autophagy signals.

Acute melatonin treatment alters dendritic morphology and circadian clock gene expression in the hippocampus of Siberian hamsters

In the hippocampus of Siberian hamsters, dendritic length and dendritic complexity increase in the CA1 region whereas dendritic spine density decreases in the dentate gyrus region at night. However, the underlying mechanism of the diurnal rhythmicity in hippocampal neuronal remodeling is unknown. In mammals, most daily rhythms in physiology and behaviors are regulated by a network of circadian clocks. The central clock, located in the hypothalamus, controls melatonin secretion at night and melatonin modifies peripheral clocks by altering expression of circadian clock genes. In this study, we examined the effects of acute melatonin treatment on the circadian clock system as well as on morphological changes of hippocampal neurons. Male Siberian hamsters were injected with melatonin in the afternoon; 4 h later, mRNA levels of hypothalamic and hippocampal circadian clock genes and hippocampal neuron dendritic morphology were assessed. In the hypothalamus, melatonin treatment did not alter Period1 and Bmal1 expression. However, melatonin treatment increased both Period1 and Bmal1 expression in the hippocampus, suggesting that melatonin affected molecular oscillations in the hippocampus. Melatonin treatment also induced rapid remodeling of hippocampal neurons; melatonin increased apical dendritic length and dendritic complexity in the CA1 region and reduced the dendritic spine density in the dentate gyrus region. These data suggest that structural changes in hippocampal neurons are regulated by a circadian clock and that melatonin functions as a nighttime signal to coordinate the diurnal rhythm in neuronal remodeling.

Methylphenidate ameliorates depressive comorbidity in ADHD children without any modification on differences in serum melatonin concentration between ADHD subtypes

The vast majority of Attention-deficit/hyperactivity disorder (ADHD) patients have other associated pathologies, with depressive symptoms as one of the most prevalent. Among the mediators that may participate in ADHD, melatonin is thought to regulate circadian rhythms, neurological function and stress response. To determine (1) the serum baseline daily variations and nocturnal excretion of melatonin in ADHD subtypes and (2) the effect of chronic administration of methylphenidate, as well as the effects on symptomatology, 136 children with ADHD (Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition, Text Revision: DSM-IV-TR criteria) were divided into subgroups using the “Children’s Depression Inventory” (CDI). Blood samples were drawn at 20:00 and 09:00 h, and urine was collected between 21:00 and 09:00 h, at inclusion and after 4.61 ± 2.29 months of treatment. Melatonin and its urine metabolite were measured by radioimmunoassay RIA. Factorial analysis was performed using STATA 12.0. Melatonin was higher predominantly in hyperactive-impulsive/conduct disordered children (PHI/CD) of the ADHD subtype, without the influence of comorbid depressive symptoms. Methylphenidate ameliorated this comorbidity without induction of any changes in the serum melatonin profile, but treatment with it was associated with a decrease in 6-s-melatonin excretion in both ADHD subtypes. Conclusions: In untreated children, partial homeostatic restoration of disrupted neuroendocrine equilibrium most likely led to an increased serum melatonin in PHI/CD children. A differential cerebral melatonin metabolization after methylphenidate may underlie some of the clinical benefit.

Fundamental issues related to the origin of melatonin and melatonin isomers during evolution: relation to their biological functions

Melatonin and melatonin isomers exist and/or coexist in living organisms including yeasts, bacteria and plants. The levels of melatonin isomers are significantly higher than that of melatonin in some plants and in several fermented products such as in wine and bread. Currently, there are no reports documenting the presence of melatonin isomers in vertebrates. From an evolutionary point of view, it is unlikely that melatonin isomers do not exist in vertebrates. On the other hand, large quantities of the microbial flora exist in the gut of the vertebrates. These microorganisms frequently exchange materials with the host. Melatonin isomers, which are produced by these organisms inevitably enter the host's system. The origins of melatonin and its isomers can be traced back to photosynthetic bacteria and other primitive unicellular organisms. Since some of these bacteria are believed to be the precursors of mitochondria and chloroplasts these cellular organelles may be the primary sites of melatonin production in animals or in plants, respectively. Phylogenic analysis based on its rate-limiting synthetic enzyme, serotonin N-acetyltransferase (SNAT), indicates its multiple origins during evolution. Therefore, it is likely that melatonin and its isomer are also present in the domain of archaea, which perhaps require these molecules to protect them against hostile environments including extremely high or low temperature. Evidence indicates that the initial and primary function of melatonin and its isomers was to serve as the first-line of defence against oxidative stress and all other functions were acquired during evolution either by the process of adoption or by the extension of its antioxidative capacity.

Sleep deprivation aggravates median nerve injury-induced neuropathic pain and enhances microglial activation by suppressing melatonin secretion

STUDY OBJECTIVES

Sleep deprivation is common in patients with neuropathic pain, but the effect of sleep deprivation on pathological pain remains uncertain. This study investigated whether sleep deprivation aggravates neuropathic symptoms and enhances microglial activation in the cuneate nucleus (CN) in a median nerve chronic constriction injury (CCI) model. Also, we assessed if melatonin supplements during the sleep deprived period attenuates these effects.

DESIGN

Rats were subjected to sleep deprivation for 3 days by the disc-on-water method either before or after CCI. In the melatonin treatment group, CCI rats received melatonin supplements at doses of 37.5, 75, 150, or 300 mg/kg during sleep deprivation. Melatonin was administered at 23:00 once a day.

PARTICIPANTS

Male Sprague-Dawley rats, weighing 180-250 g (n = 190), were used.

MEASUREMENTS

Seven days after CCI, behavioral testing was conducted, and immunohistochemistry, immunoblotting, and enzyme-linked immunosorbent assay were used for qualitative and quantitative analyses of microglial activation and measurements of proinflammatory cytokines.

RESULTS

In rats who underwent post-CCI sleep deprivation, microglia were more profoundly activated and neuropathic pain was worse than those receiving pre-CCI sleep deprivation. During the sleep deprived period, serum melatonin levels were low over the 24-h period. Administration of melatonin to CCI rats with sleep deprivation significantly attenuated activation of microglia and development of neuropathic pain, and markedly decreased concentrations of proinflammatory cytokines.

CONCLUSIONS

Sleep deprivation makes rats more vulnerable to nerve injury-induced neuropathic pain, probably because of associated lower melatonin levels. Melatonin supplements to restore a circadian variation in melatonin concentrations during the sleep deprived period could alleviate nerve injury-induced behavioral hypersensitivity.

Melatonin promotes the acquisition of neural identity through extracellular-signal-regulated kinases 1/2 activation

Melatonin, a major pineal secretory product, exerts a range of physiological and neuroprotective effects. However, the functional significance of melatonin in determining neural identity, and the mechanisms by which this may occur, is unknown. In this study, P19 cells were used as a model system and cell behavior was monitored. Our data show that melatonin plays an important role in determining cell fate during neural commitment and promoting the differentiation of pluripotent P19 cells (Oct4(+) Sox2(+) ) into neural stem cells (Oct4(-) Sox2(+) ). This promotion appears to coincide with the activation of the MT1 receptor and phosphorylation of extracellular-signal-regulated kinases 1/2 (ERK1/2). Furthermore, our results show that melatonin regulates neural fate specification of P19 cells through two distinct mechanisms: the promotion of nuclear localization of ERK1/2 and upregulation of Sox2 transcription, and suppression of Smad1-induced expression of mesodermal-specific genes, such as Bra.

Theoretical insight into the antioxidant properties of melatonin and derivatives

Density functional theory calculations on melatonin, metabolites and synthetic derivatives thereof, and a range of other biological antioxidant molecules are presented, with a view to understanding the antioxidant ability of these molecules. After testing of the necessary calculations, we show that melatonin lies close to vitamin E on a donor-acceptor map, indicating that it should be an excellent electron donor but a poor acceptor. The neutral radical metabolite of melatonin is predicted to be an even better donor, whereas other metabolites and synthetic derivatives should retain antioxidant ability but are less powerful than the parent. QSAR models of antioxidant activity, measured in two different assays, are presented. We show that octanol-water partition coefficient is an excellent predictor of activity in lipophilic media, while properties related to electron donor/acceptor power give good fits against activity in aqueous media.

Melatonin and atopy: role in atopic dermatitis and asthma

Melatonin may have important immunostimulatory actions in allergic diseases, in addition to its well-known antioxidant and cytoprotective effects in several inflammatory conditions. The activation of the immune system leads to free radical production associated with decreased melatonin levels and depressed antioxidant enzyme activities in several inflammatory diseases. Many skin disorders, including atopic dermatitis, are accompanied by infiltration and activation of mast cells, which release vasoactive and proinflammatory mediators. Experimental data suggest that melatonin inhibits development of atopic eczema and reduces serum total IgE and IL-4. Allergic asthma is a condition characterized by bronchial hyperresponsiveness and the presence of IgE antibodies in response to inhaled allergens; often there is also enhanced total serum IgE levels. Melatonin regulates smooth muscle tone and influences the immune response. Melatonin may, however, act as a pro-inflammatory agent in asthma leading to bronchial constriction. The safety of melatonin as a sleep-inducing agent has been confirmed in asthmatic subjects, but its routine use is not recommended in bronchial asthma. This review summarizes what is known about the role of melatonin as an immunomodulatory agent in asthma and atopic eczema.

Melatonin improves mitochondrial function in inguinal white adipose tissue of Zücker diabetic fatty rats

Mitochondrial dysfunction in adipose tissue may contribute to obesity-related metabolic derangements such as type 2 diabetes mellitus (T2DM). Because mitochondria are a target for melatonin action, the goal of this study was to investigate the effects of melatonin on mitochondrial function in white (WAT) and beige inguinal adipose tissue of Zücker diabetic fatty (ZDF) rats, a model of obesity-related T2DM. In this experimental model, melatonin reduces obesity and improves the metabolic profile. At 6 wk of age, ZDF rats and lean littermates (ZL) were subdivided into two groups, each composed of four rats: control (C-ZDF and C-ZL) and treated with oral melatonin in the drinking water (10 mg/kg/day) for 6 wk (M-ZDF and M-ZL). After the treatment period, animals were sacrificed, tissues dissected, and mitochondrial function assessed in isolated organelles. Melatonin increased the respiratory control ratio (RCR) in mitochondria from white fat of both lean (by 26.5%, P < 0.01) and obese (by 34.5%, P < 0.01) rats mainly through a reduction of proton leaking component of respiration (state 4) (28% decrease in ZL, P < 0.01 and 35% in ZDF, P < 0.01). However, melatonin treatment lowered the RCR in beige mitochondria of both lean (by 7%, P < 0.05) and obese (by 13%, P < 0.05) rats by maintaining high rates of uncoupled respiration. Melatonin also lowered mitochondrial oxidative status by reducing nitrite levels and by increasing superoxide dismutase activity. Moreover, melatonin treatment also caused a profound inhibition of Ca-induced opening of mPTP in isolated mitochondria from both types of fat, white and beige, in both lean and obese rats. These results demonstrate that chronic oral melatonin improves mitochondrial respiration and reduces the oxidative status and susceptibility to apoptosis in white and beige adipocytes. These melatonin effects help to prevent mitochondrial dysfunction and thereby to improve obesity-related metabolic disorders such as diabetes and dyslipidemia of ZDF rats.

Extrapineal melatonin: sources, regulation, and potential functions

Endogenous melatonin is synthesized from tryptophan via 5-hydroxytryptamine. It is considered an indoleamine from a biochemical point of view because the melatonin molecule contains a substituted indolic ring with an amino group. The circadian production of melatonin by the pineal gland explains its chronobiotic influence on organismal activity, including the endocrine and non-endocrine rhythms. Other functions of melatonin, including its antioxidant and anti-inflammatory properties, its genomic effects, and its capacity to modulate mitochondrial homeostasis, are linked to the redox status of cells and tissues. With the aid of specific melatonin antibodies, the presence of melatonin has been detected in multiple extrapineal tissues including the brain, retina, lens, cochlea, Harderian gland, airway epithelium, skin, gastrointestinal tract, liver, kidney, thyroid, pancreas, thymus, spleen, immune system cells, carotid body, reproductive tract, and endothelial cells. In most of these tissues, the melatonin-synthesizing enzymes have been identified. Melatonin is present in essentially all biological fluids including cerebrospinal fluid, saliva, bile, synovial fluid, amniotic fluid, and breast milk. In several of these fluids, melatonin concentrations exceed those in the blood. The importance of the continual availability of melatonin at the cellular level is important for its physiological regulation of cell homeostasis, and may be relevant to its therapeutic applications. Because of this, it is essential to compile information related to its peripheral production and regulation of this ubiquitously acting indoleamine. Thus, this review emphasizes the presence of melatonin in extrapineal organs, tissues, and fluids of mammals including humans.

Immunomodulatory effect of melatonin in SK-LU-1 human lung adenocarcinoma cells co-cultured with peripheral blood mononuclear cells

OBJECTIVES

The anti-cancer potential of melatonin has been examined using a variety of experimental approaches. Melatonin immunomodulatory action was evaluated against the lung cancer cell line SK-LU-1, in co-culture with human peripheral blood mononuclear cells (PBMC).

MATERIALS AND METHODS

Melatonin was tested on the cell line only after 24 h incubation (direct effect), and on the co-culture system of SK-LU-1 and PBMC to investigate any indirect effect. Apoptotic induction of the cancer cells was assessed using annexin V/PI staining with flow cytometric analysis for membrane alteration. Intracellular superoxide anion (O2 (•-) ) and hydrogen peroxide (H2 O2 ) for intracellular oxidative stress and glutathione (GSH) for intracellular anti-oxidation were measured with specific fluorescence probes. DNA fractions were measured employing propidium iodide (PI) fluorescence staining.

RESULTS

High doses of melatonin were directly toxic to SK-LU-1 cells, while PBMC-mediated indirect effect occurred after moderate doses (1 μm). Under co-culture conditions, increases in apoptotic cell death, increase in oxidative stress by reduction of GSH and cell cycle arrest in G0 /G1 in SK-LU-1 cells, were observed as the immunomodulatory effect of melatonin.

CONCLUSION

Melatonin had indirect effects on lung cancer cells by enhancement of immunomodulatory effects, but further studies of mechanism(s) involved are needed.

The melatonin agonist ramelteon induces duration-dependent clock gene expression through cAMP signaling in pancreatic INS-1 β-cells

Prolonged exposure to melatonin improves glycemic control in animals. Although glucose metabolism is controlled by circadian clock genes, little is known about the role of melatonin signaling and its duration in the regulation of clock gene expression in pancreatic β-cells. Activation of MT1 and MT2 melatonin receptors inhibits cAMP signaling, which mediates clock gene expression. Therefore, this study investigated exposure duration-dependent alterations in cAMP element-binding protein (CREB) phosphorylation and clock gene expression that occur during and after exposure to ramelteon, a selective melatonin agonist used to treat insomnia. In rat INS-1 cells, a pancreatic β-cell line endogenously expressing melatonin receptors, ramelteon persistently decreased CREB phosphorylation during the treatment period (2-14 h), whereas the subsequent washout induced an enhancement of forskolin-stimulated CREB phosphorylation in a duration- and concentration-dependent manner. This augmentation was blocked by forskolin or the melatonin receptor antagonist luzindole. Similarly, gene expression analyses of 7 clock genes revealed the duration dependency of the effects of ramelteon on Rev-erbα and Bmal1 expression through melatonin receptor-mediated cAMP signaling; longer exposure times (14 h) resulted in greater increases in the expression and signaling of Rev-erbα, which is related to β-cell functions. Interestingly, this led to amplified oscillatory Rev-erbα and Bmal1 expression after agonist washout and forskolin stimulation. These results provide new insights into the duration-dependent effects of ramelteon on clock gene expression in INS-1 cells and may improve the understanding of its effect in vivo. The applicability of these results to pancreatic islets awaits further investigation.

Regulation of L1 expression and retrotransposition by melatonin and its receptor: implications for cancer risk associated with light exposure at night

Expression of long interspersed element-1 (L1) is upregulated in many human malignancies. L1 can introduce genomic instability via insertional mutagenesis and DNA double-strand breaks, both of which may promote cancer. Light exposure at night, a recently recognized carcinogen, is associated with an increased risk of cancer in shift workers. We report that melatonin receptor 1 inhibits mobilization of L1 in cultured cells through downregulation of L1 mRNA and ORF1 protein. The addition of melatonin receptor antagonists abolishes the MT1 effect on retrotransposition in a dose-dependent manner. Furthermore, melatonin-rich, but not melatonin-poor, human blood collected at different times during the circadian cycle suppresses endogenous L1 mRNA during in situ perfusion of tissue-isolated xenografts of human cancer. Supplementation of human blood with exogenous melatonin or melatonin receptor antagonist during the in situ perfusion establishes a receptor-mediated action of melatonin on L1 expression. Combined tissue culture and in vivo data support that environmental light exposure of the host regulates expression of L1 elements in tumors. Our data imply that light-induced suppression of melatonin production in shift workers may increase L1-induced genomic instability in their genomes and suggest a possible connection between L1 activity and increased incidence of cancer associated with circadian disruption.

Melatonin and the circadian system: contributions to successful female reproduction

OBJECTIVE

To summarize the role of melatonin and circadian rhythms in determining optimal female reproductive physiology, especially at the peripheral level.

DESIGN

Databases were searched for the related English-language literature published up to March 1, 2014. Only papers in peer-reviewed journals are cited.

SETTING

Not applicable.

PATIENT(S)

Not applicable.

INTERVENTION(S)

Melatonin treatment, alterations of the normal light:dark cycle and light exposure at night.

MAIN OUTCOME MEASURE(S)

Melatonin levels in the blood and in the ovarian follicular fluid and melatonin synthesis, oxidative damage and circadian rhythm disturbances in peripheral reproductive organs.

RESULT(S)

The central circadian regulatory system is located in the suprachiasmatic nucleus (SCN). The output of this master clock is synchronized to 24 hours by the prevailing light-dark cycle. The SCN regulates rhythms in peripheral cells via the autonomic nervous system and it sends a neural message to the pineal gland where it controls the cyclic production of melatonin; after its release, the melatonin rhythm strengthens peripheral oscillators. Melatonin is also produced in the peripheral reproductive organs, including granulosa cells, the cumulus oophorus, and the oocyte. These cells, along with the blood, may contribute melatonin to the follicular fluid, which has melatonin levels higher than those in the blood. Melatonin is a powerful free radical scavenger and protects the oocyte from oxidative stress, especially at the time of ovulation. The cyclic levels of melatonin in the blood pass through the placenta and aid in the organization of the fetal SCN. In the absence of this synchronizing effect, the offspring may exhibit neurobehavioral deficits. Also, melatonin protects the developing fetus from oxidative stress. Melatonin produced in the placenta likewise may preserve the optimal function of this organ.

CONCLUSION(S)

Both stable circadian rhythms and cyclic melatonin availability are critical for optimal ovarian physiology and placental function. Because light exposure after darkness onset at night disrupts the master circadian clock and suppresses elevated nocturnal melatonin levels, light at night should be avoided.

Transcriptional control of antioxidant defense by the circadian clock

SIGNIFICANCE

The circadian clock, an internal timekeeping system, is implicated in the regulation of metabolism and physiology, and circadian dysfunctions are associated with pathological changes in model organisms and increased risk of some diseases in humans.

RECENT ADVANCES

Data obtained in different organisms, including humans, have established a tight connection between the clock and cellular redox signaling making it among the major candidates for a link between the circadian system and physiological processes.

CRITICAL ISSUES

In spite of the recent progress in understanding the importance of the circadian clock in the regulation of reactive oxygen species homeostasis, molecular mechanisms and key regulators are mostly unknown.

FUTURE DIRECTIONS

Here we review, with an emphasis on transcriptional control, the circadian-clock-dependent control of oxidative stress response system as a potential mechanism in age-associated diseases. We will discuss the roles of the core clock components such as brain and muscle ARNT-like 1, Circadian Locomotor Output Cycles Kaput, the circadian-clock-controlled transcriptional factors such as nuclear factor erythroid-2-related factor, and peroxisome proliferator-activated receptor and circadian clock control chromatin modifying enzymes from sirtuin family in the regulation of cellular and organism antioxidant defense.

Molecular mechanisms of the circadian clockwork in mammals

Circadian rhythms enable organisms to co-ordinate biological processes with the predictable 24 h cycle of day and night. Given that molecular clocks that coordinate such biological timing have evolved in almost all organisms, it is clear that being synchronous with the external environment confers competitive advantage. Conversely, it is apparent that being out of phase is detrimental, resulting in a number of clinical conditions, many of which are linked to metabolic dysfunction. The canonical clockwork involves a core set of genes that negatively regulate themselves through a so-called transcription translation feedback loop. However, recent studies describing evolutionarily conserved oscillations in redox reactions link circadian rhythms to metabolic processes, and in particular, redox pathways. In this review we describe the evidence for the interaction between transcriptional loops, redox and metabolism in mammals and suggest the clock may be potential target for the treatment of disease.

Effect of melatonin and analogues on corneal wound healing: involvement of Mt2 melatonin receptor

PURPOSE

We have investigated the effect of melatonin and its analogues on rabbit corneal epithelial wound healing.

METHODS

New Zealand rabbits were anaesthetised and wounds were made by placing Whatman paper discs soaked in n-heptanol on the cornea. Melatonin and analogues (all 10 nmol) were instilled. Wound diameter was measured every 2 hours by means of fluorescein application with a Topcon SL-8Z slit lamp. Melatonin antagonists (all 10 nmol) were applied 2 hours before the application of the n-heptanol-soaked disc and then every 6 hours together with melatonin. To confirm the presence of MT2 receptors in corneal epithelial cells immunohistochemistry, Western blot and RT-PCR assays in native tissue and in rabbit corneal epithelial cells were performed. The tear components were extracted then processed by HPLC to quantify melatonin in tears.

RESULTS

Migration assays revealed that melatonin and particularly the treatment with the MT2 agonist IIK7, accelerated the rate of healing (p < 0.001). The application of the non-selective melatonin receptor antagonist luzindole and the MT2 antagonist DH97 (but not prazosin), prevented the effect of melatonin on wound healing (both p < 0.001). Immunohistochemistry, Western blot and RT-PCR assays showed the presence of MT2 melatonin receptor in corneal epithelial cells. In addition, we have identified melatonin in tears and determined its daily variations.

CONCLUSIONS

These data suggest that MT2 receptors are implicated in the effect of melatonin on corneal wound healing regulating migration rate. This suggests the potential use of melatonin and its analogues to enhance epithelial wound healing in ocular surface disease.

An increase in melatonin in transgenic rice causes pleiotropic phenotypes, including enhanced seedling growth, delayed flowering, and low grain yield

No previous reports have described the effects of an increase in endogenous melatonin levels on plant yield and reproduction. Here, the phenotypes of melatonin-rich transgenic rice plants overexpressing sheep serotonin N-acetyltransferase were investigated under field conditions. Early seedling growth of melatonin-rich transgenic rice was greatly accelerated, with enhanced biomass relative to the wild type (WT). However, flowering was delayed by 1 wk in the transgenic lines compared with the WT. Grain yields of the melatonin-rich transgenic lines were reduced by 33% on average. Other phenotypes also varied among the transgenic lines. For example, the transgenic line S1 exhibited greater height and biomass than the WT, while the S10 transgenic line showed diminished height and an increase in panicle numbers per plant. The expression levels of Oryza sativa homeobox1 (OSH1) and TEOSINTE BRANCHED1 (TB1) genes, two key regulators of meristem initiation and maintenance, were not altered in the transgenic lines. These data demonstrate that an alteration of endogenous melatonin levels leads to pleiotropic effects such as height, biomass, panicle number, flowering time, and grain yield, indicating that melatonin behaves as a signaling molecule in plant growth and reproduction.

Melatonin regulates mesenchymal stem cell differentiation: a review

Among the numerous functions of melatonin, the control of survival and differentiation of mesenchymal stem cells (MSCs) has been recently proposed. MSCs are a heterogeneous population of multipotent elements resident in tissues such as bone marrow, muscle, and adipose tissue, which are primarily involved in developmental and regeneration processes, gaining thus increasing interest for tissue repair and restoration therapeutic protocols. Receptor-dependent and receptor-independent responses to melatonin are suggested to occur in these cells. These involve antioxidant or redox-dependent functions of this indolamine as well as secondary effects resulting from autocrine and paracrine responses. Inflammatory cytokines and adipokines, proangiogenic/mitogenic stimuli, and other mediators that influence the differentiation processes may affect the survival and functional integrity of these mesenchymal precursor cells. In this scenario, melatonin seems to regulate signaling pathways that drive commitment and differentiation of MSC into osteogenic, chondrogenic, adipogenic, or myogenic lineages. Common pathways suggested to be involved as master regulators of these processes are the Wnt/β-catenin pathway, the MAPKs and the, TGF-β signaling. In this respect melatonin emerges a novel and potential modulator of MSC lineage commitment and adipogenic differentiation. These and other aspects of the physiological and pharmacological effects of melatonin as regulator of MSC are discussed in this review.

A review of metal-catalyzed molecular damage: protection by melatonin

Metal exposure is associated with several toxic effects; herein, we review the toxicity mechanisms of cadmium, mercury, arsenic, lead, aluminum, chromium, iron, copper, nickel, cobalt, vanadium, and molybdenum as these processes relate to free radical generation. Free radicals can be generated in cells due to a wide variety of exogenous and endogenous processes, causing modifications in DNA bases, enhancing lipid peroxidation, and altering calcium and sulfhydryl homeostasis. Melatonin, an ubiquitous and pleiotropic molecule, exerts efficient protection against oxidative stress and ameliorates oxidative/nitrosative damage by a variety of mechanisms. Also, melatonin has a chelating property which may contribute in reducing metal-induced toxicity as we postulate here. The aim of this review was to highlight the protective role of melatonin in counteracting metal-induced free radical generation. Understanding the physicochemical insights of melatonin related to the free radical scavenging activity and the stimulation of antioxidative enzymes is of critical importance for the development of novel therapeutic strategies against the toxic action of these metals.

Melatonin alleviates Echis carinatus venom-induced toxicities by modulating inflammatory mediators and oxidative stress

Viper bites cause high morbidity and mortality worldwide and regarded as a neglected tropical disease affecting a large healthy population. Classical antivenom therapy has appreciably reduced the snakebite mortality rate; it apparently fails to tackle viper venom-induced local manifestations that persist even after the administration of antivenom. Recently, viper venom-induced oxidative stress and vital organ damage is deemed as yet another reason for concern; these are considered as postmedicated complications of viper bite. Thus, treating viper bite has become a challenge demanding new treatment strategies, auxiliary to antivenin therapy. In the last decade, several studies have reported the use of plant products and clinically approved drugs to neutralize venom-induced pharmacology. However, very few attempts were undertaken to study oxidative stress and vital organ damage. Based on this background, the present study evaluated the protective efficacy of melatonin in Echis carinatus (EC) venom-induced tissue necrosis, oxidative stress, and organ toxicity. The results demonstrated that melatonin efficiently alleviated EC venom-induced hemorrhage and myonecrosis. It also mitigated the altered levels of inflammatory mediators and oxidative stress markers of blood components in liver and kidney homogenates, and documented renal and hepatoprotective action of melatonin. The histopathology of skin, muscle, liver, and kidney tissues further substantiated the overall protection offered by melatonin against viper bite toxicities. Besides the inability of antivenoms to block local effects and the fact that melatonin is already a widely used drug promulgating a multitude of therapeutic functionalities, its use in viper bite management is of high interest and should be seriously considered.

Protective effects of melatonin in reducing oxidative stress and in preserving the fluidity of biological membranes: a review

Free radicals generated within subcellular compartments damage macromolecules which lead to severe structural changes and functional alterations of cellular organelles. A manifestation of free radical injury to biological membranes is the process of lipid peroxidation, an autooxidative chain reaction in which polyunsaturated fatty acids in the membrane are the substrate. There is considerable evidence that damage to polyunsaturated fatty acids tends to reduce membrane fluidity. However, adequate levels of fluidity are essential for the proper functioning of biological membranes. Thus, there is considerable interest in antioxidant molecules which are able to stabilize membranes because of their protective effects against lipid peroxidation. Melatonin is an indoleamine that modulates a wide variety of endocrine, neural and immune functions. Over the last two decades, intensive research has proven this molecule, as well as its metabolites, to possess substantial antioxidant activity. In addition to their ability to scavenge several reactive oxygen and nitrogen species, melatonin increases the activity of the glutathione redox enzymes, that is, glutathione peroxidase and reductase, as well as other antioxidant enzymes. These beneficial effects of melatonin are more significant because of its small molecular size and its amphipathic behaviour, which facilitates ease of melatonin penetration into every subcellular compartment. In the present work, we review the current information related to the beneficial effects of melatonin in maintaining the fluidity of biological membranes against free radical attack, and further, we discuss its implications for ageing and disease.

Molecular deficiency (ies) in MT₁ melatonin signaling pathway underlies the melatonin-unresponsive phenotype in MDA-MB-231 human breast cancer cells

Melatonin has been shown repeatedly to inhibit the growth of human breast tumor cells in vitro and in vivo. Its antiproliferative effects have been well studied in MCF-7 human breast cancer cells and several other estrogen receptor α (ERα)-positive human breast cancer cell lines. However, the MDA-MB-231 breast cancer cell line, an ERα-negative cell line widely used in breast cancer research, has been shown to be unresponsive to melatonin's growth-suppressive effect in vitro. Here, we examined the effect of melatonin on the cell proliferation of several ERα-negative breast cancer cell lines including MDA-MB-231, BT-20, and SK-BR-3 cells. Although the MT1 G-protein-coupled receptor is expressed in all three cell lines, melatonin significantly suppressed the proliferation of SK-BR-3 cells without having any significant effect on the growth of MDA-MB-231 and BT-20 cells. We confirmed that the MT1-associated Gα proteins are expressed in MDA-MB-231 cells. Further studies demonstrated that the melatonin unresponsiveness in MDA-MB-231 cells may be caused by aberrant signaling downstream of the Gαi proteins, resulting in differential regulation of ERK1/2 activity.

Proliferative effects of melatonin on Schwann cells: implication for nerve regeneration following peripheral nerve injury

Activation of proliferation of Schwann cells is crucial for axonal guidance and successful nerve regeneration following peripheral nerve injury (PNI). Considering melatonin plays an important role in proliferative regulation of central glial cells, the present study determined whether melatonin can effectively promote Schwann cell proliferation and improve nerve regeneration after PNI. The spontaneous immortalized rat Schwann cell line (RSC 96 cells) was first analyzed by quantitative polymerase chain reaction (QPCR) to detect the potential existence of melatonin receptors. The melatonin receptor-mediated signaling responsible for proliferation was examined by measuring the phosphorylation of extracellular signal-regulated kinases (ERK1/2) pathway. The in vivo model of PNI was performed by the end-to-side neurorrhaphy. The quantity of Schwann cells as well as the number of re-innervated motor end plates (MEP) on target muscles was examined to represent the functional recovery of injured nerves. QPCR results indicated that MT1 is the dominant receptor in Schwann cells. Immunoblotting and proliferation assay revealed an enhanced phosphorylation of ERK1/2 and increased number of RSC 96 cells following melatonin administration. Nonselective melatonin receptor antagonist (luzindole) treatment significantly suppressed all the above findings, suggesting that the proliferative effects of melatonin were mediated by a receptor-dependent pathway. In vivo results corresponded well with in vitro findings in which melatonin effectively increased the amount of proliferated Schwann cells and re-innervated MEP on target muscles following PNI. As melatonin successfully improves nerve regeneration by promoting Schwann cell proliferation, therapeutic use of melatonin may thus serve as a promising strategy to counteract the PNI-induced neuronal disability.

Pineal calcification is a novel risk factor for symptomatic intracerebral hemorrhage

BACKGROUND

Pineal calcification is associated with symptomatic cerebral infarction in humans. However, there are limited data on the association of pineal calcification and intracerebral hemorrhage. We evaluated this association of symptomatic intracerebral hemorrhage and pineal calcification by computed tomography of the brain.

METHODS

We reviewed all computed tomographic (CT) scans of the brains of patients over 15 years of age during the year 2011 at a university teaching hospital. Symptomatic intracerebral hemorrhage was identified by having clinical syndrome of stroke and acute intracerebral hemorrhage from brain CT scans. Pineal calcification was also evidenced by brain CT scans. Other stroke risk factors were recorded. The association of various risk factors including pineal calcification and intracerebral hemorrhage was calculated using logistic regression analysis.

RESULTS

There were 2140 CT scans of the brains during the study period. Of those, 1071 scans (50.05%) met the study criteria. Intracerebral hemorrhage and pineal calcification were found in 77 (7.2%) and 689 (64.3%) patients, respectively. Pineal calcification was a significant risk factor for intracerebral hemorrhage with an adjusted odds ratio of 2.36 (95% confidence interval of 1.22-4.54). Other significant factors were age>50 years, hypertension, and diabetes.

CONCLUSION

Pineal calcification is associated with symptomatic intracerebral hemorrhage.

Melatonin synthesis in rice seedlings in vivo is enhanced at high temperatures and under dark conditions due to increased serotonin N-acetyltransferase and N-acetylserotonin methyltransferase activities

Temperature and light are important environmental factors for plant growth and development. The final two enzymes in the melatonin synthesis pathway in plants are serotonin N-acetyltransferase (SNAT) and N-acetylserotonin methyltransferase (ASMT), which have thermophilic characteristics. Thus, the effects of temperature and light on melatonin synthesis in rice seedlings were investigated. Here, we demonstrated that melatonin levels increased as temperature increased when rice seedlings were exposed to various temperatures for 1 hr. Moreover, the relative melatonin levels were higher in the dark. For example, exposure of rice seedlings to 1-hr darkness at 55°C resulted in a melatonin yield of 4.9 ng/g fresh weight (fw), compared with 2.95 ng/g fw under light conditions. Temperature-dependent melatonin synthesis was closely associated with an increase in both SNAT and ASMT activities, but not with transcript levels of melatonin biosynthetic genes. The daily melatonin levels in field-grown rice plants were unaffected as the positive effect of the relatively high temperature during the day was counteracted by the negative effect of the high light. The opposite effect occurred during the night, in which the positive effect of darkness on melatonin synthesis was counteracted by the negative effect of a low temperature.