Melatonin induces neural SOD2 expression independent of the NF-kappaB pathway and improves the mitochondrial population and function in old mice

The Vital Role of Melatonin and Its Metabolites in the Neuroprotection and Retardation of Brain Aging

While primarily produced in the pineal gland, melatonin's influence goes beyond its well-known role in regulating sleep, nighttime metabolism, and circadian rhythms, in the field of chronobiology. A plethora of new data demonstrates melatonin to be a very powerful molecule, being a potent ROS/RNS scavenger with anti-inflammatory, immunoregulatory, and oncostatic properties. Melatonin and its metabolites exert multiple beneficial effects in cutaneous and systemic aging. This review is focused on the neuroprotective role of melatonin during aging. Melatonin has an anti-aging capacity, retarding the rate of healthy brain aging and the development of age-related neurodegenerative diseases, such as Alzheimer's disease, Parkinson's disease, Huntington's disease, multiple sclerosis, amyotrophic lateral sclerosis, etc. Melatonin, as well as its metabolites, N1-acetyl-N2-formyl-5-methoxykynuramine (AFMK) and N1-acetyl-5-methoxykynuramine (AMK), can reduce oxidative brain damage by shielding mitochondria from dysfunction during the aging process. Melatonin could also be implicated in the treatment of neurodegenerative conditions, by modifying their characteristic low-grade neuroinflammation. It can either prevent the initiation of inflammatory responses or attenuate the ongoing inflammation. Drawing on the current knowledge, this review discusses the potential benefits of melatonin supplementation in preventing and managing cognitive impairment and neurodegenerative diseases.

Melatonin Ameliorates Autophagy Impairment in a Metabolic Syndrome Model

Metabolic syndrome is a global health problem in adults and its prevalence among children and adolescents is rising. It is strongly linked to a lifestyle with high-caloric food, which causes obesity and lipid metabolism anomalies. Molecular damage due to excessive oxidative stress plays a major role during the development of metabolic syndrome complications. Among the different hormones, melatonin presents strong antioxidant properties, and it is used to treat metabolic diseases. However, there is not a consensus about its use as a metabolic syndrome treatment. The aim of this study was to identify melatonin effects in a metabolic syndrome model. Golden hamsters were fed with 60% fructose-enriched food to induce metabolic syndrome and were compared to hamsters fed with regular chow diet. Both groups were also treated with melatonin. Fructose-fed hamsters showed altered blood lipid levels (increased cholesterol and LDL) and phenotypes restored with the melatonin treatment. The Harderian gland (HG), which is an ideal model to study autophagy modulation through oxidative stress, was the organ that was most affected by a fructose diet. Redox balance was altered in fructose-fed HG, inducing autophagic activation. However, since LC3-II was not increased, the impairment must be in the last steps of autophagy. Lipophagy HG markers were also disturbed, contributing to the dyslipidemia. Melatonin treatment improved possible oxidative homeostasis through autophagic induction. All these results point to melatonin as a possible treatment of the metabolic syndrome.

Melatonin Rescues the Ti Particle-Impaired Osteogenic Potential of Bone Marrow Mesenchymal Stem Cells via the SIRT1/SOD2 Signaling Pathway

Wear particles released by joint implants are a major cause of osteolysis around the prosthesis by negatively affecting bone reconstruction. Bone marrow mesenchymal stem cells (BMMSCs) stimulated by wear particles showed an impaired osteogenic potential. Melatonin has been shown beneficial effects on intracellular antioxidant functions and bone formation; however, whether it could restore the osteogenic potential of BMMSCs inhibited by wear particles was unknown. This study aimed to evaluate the protective effect of melatonin on the osteogenic capacity of BMMSCs exposed to titanium (Ti) wear particles and to investigated the underlying mechanisms involving intracellular antioxidant properties. When BMMSCs were exposed to Ti particles in vitro, melatonin treatment successfully improved the matrix mineralization and expression of osteogenic markers in BMMSCs, while decreasing the levels of intracellular reactive oxygen species (ROS) and mitochondrial superoxide. The protective effect of melatonin on osteolysis was validated in a Ti particle-exposed murine calvarial model. Meanwhile, silent information regulator type 1 (SIRT1) and intracellular antioxidant enzymes were significantly up-regulated, particularly superoxide dismutase 2 (SOD2), in melatonin-treated BMMSCs. Furthermore, inhibition of SIRT1 by EX527 completely counteracted the protective effect of melatonin on Ti particle-treated BMMSCs, evidenced by the reduced expression of SOD2, increased ROS and superoxide, and decreased osteogenic differentiation. These results demonstrated that melatonin restored the osteogenic potential and improved the antioxidant properties of BMMSCs through the SIRT1 signaling pathway. Our findings suggest that melatonin is a promising candidate for treating osteolysis induced by wear particles.

Selective autophagy, lipophagy and mitophagy, in the Harderian gland along the oestrous cycle: a potential retrieval effect of melatonin

Sexual dimorphism has been reported in many processes. However, sexual bias in favour of the use of males is very present in science. One of the main reasons is that the impact of hormones in diverse pathways and processes such as autophagy have not been properly addressed in vivo. The Harderian gland is a perfect model to study autophagic modulation as it exhibits important changes during the oestrous cycle. The aim of this study is to identify the main processes behind Harderian gland differences under oestrous cycle and their modulator. In the present study we show that redox-sensitive transcription factors have an essential role: NF-κB may activate SQSTM1/p62 in oestrus, promoting selective types of autophagy: mitophagy and lipophagy. Nrf2 activation in dioestrus, leads the retrieval phase and restoration of mitochondrial homeostasis. Melatonin’s receptors show higher expression in dioestrus, leading to decreases in pro-inflammatory mediators and enhanced Nrf2 expression. Consequently, autophagy is blocked, and porphyrin release is reduced. All these results point to melatonin as one of the main modulators of the changes in autophagy during the oestrous cycle.

Dietary melatonin attenuates chromium-induced lung injury via activating the Sirt1/Pgc-1α/Nrf2 pathway

Exposure to chromium (Cr) causes a number of respiratory diseases, including lung cancer and pulmonary fibrosis. However, there is currently no safe treatment for Cr-induced lung damage. Here, we used in vivo and in vitro approaches to examine the protective effects of melatonin (MEL) on Cr-induced lung injury and to identify the underlying molecular mechanisms. We found that treatment of rats or a mouse lung epithelial cell MLE-12 with MEL attenuated K₂Cr₂O₇-induced lung injury by reducing the production of oxidative stress and inflammatory mediators and inhibiting cell apoptosis. MEL treatment upregulated the expression of silent information regulator 1 (Sirt1), which deacetylated the transcriptional coactivator peroxisome proliferator-activated receptor-γ coactivator-1α (Pgc-1α). In turn, this increased the expression of the transcription factor nuclear factor erythroid 2-related factor 2 (Nrf2) and key anti-oxidant target genes. These results suggest that melatonin attenuates chromium-induced lung injury via activating the Sirt1/Pgc-1α/Nrf2 pathway. Dietary MEL supplement may be a potential new strategy for the treatment of Cr poisoning.

Mitochondria and Reactive Oxygen Species in Aging and Age-Related Diseases

Aging has been linked to several degenerative processes that, through the accumulation of molecular and cellular damage, can progressively lead to cell dysfunction and organ failure. Human aging is linked with a higher risk for individuals to develop cancer, neurodegenerative, cardiovascular, and metabolic disorders. The understanding of the molecular basis of aging and associated diseases has been one major challenge of scientific research over the last decades. Mitochondria, the center of oxidative metabolism and principal site of reactive oxygen species (ROS) production, are crucial both in health and in pathogenesis of many diseases. Redox signaling is important for the modulation of cell functions and several studies indicate a dual role for ROS in cell physiology. In fact, high concentrations of ROS are pathogenic and can cause severe damage to cell and organelle membranes, DNA, and proteins. On the other hand, moderate amounts of ROS are essential for the maintenance of several biological processes, including gene expression. In this review, we provide an update regarding the key roles of ROS-mitochondria cross talk in different fundamental physiological or pathological situations accompanying aging and highlighting that mitochondrial ROS may be a decisive target in clinical practice.

Melatonin and the electron transport chain

Melatonin protects the electron transport chain (ETC) in multiple ways. It reduces levels of ·NO by downregulating inducible and inhibiting neuronal nitric oxide synthases (iNOS, nNOS), thereby preventing excessive levels of peroxynitrite. Both ·NO and peroxynitrite-derived free radicals, such as ·NO2, hydroxyl (·OH) and carbonate radicals (CO3·-) cause blockades or bottlenecks in the ETC, by ·NO binding to irons, protein nitrosation, nitration and oxidation, changes that lead to electron overflow or even backflow and, thus, increased formation of superoxide anions (O2·-). Melatonin improves the intramitochondrial antioxidative defense by enhancing reduced glutathione levels and inducing glutathione peroxidase and Mn-superoxide dismutase (Mn-SOD) in the matrix and Cu,Zn-SOD in the intermembrane space. An additional action concerns the inhibition of cardiolipin peroxidation. This oxidative change in the membrane does not only initiate apoptosis or mitophagy, as usually considered, but also seems to occur at low rate, e.g., in aging, and impairs the structural integrity of Complexes III and IV. Moreover, elevated levels of melatonin inhibit the opening of the mitochondrial permeability transition pore and shorten its duration. Additionally, high-affinity binding sites in mitochondria have been described. The assumption of direct binding to the amphipathic ramp of Complex I would require further substantiation. The mitochondrial presence of the melatonin receptor MT1 offers the possibility that melatonin acts via an inhibitory G protein, soluble adenylyl cyclase, decreased cAMP and lowered protein kinase A activity, a signaling pathway shown to reduce Complex I activity in the case of a mitochondrial cannabinoid receptor.

Plant mitochondria synthesize melatonin and enhance the tolerance of plants to drought stress

Synthesis of melatonin in mitochondria was reported in animals. However, there is no report on whether plant mitochondria also produce melatonin. Herein, we show that plant mitochondria are a major site for melatonin synthesis. In an in vitro study, isolated apple mitochondria had the capacity to generate melatonin. Subcellular localization analysis documented that an apple SNAT isoform, MzSNAT5, was localized in the mitochondria of both Arabidopsis protoplasts and apple callus cells. The kinetic analysis revealed that the recombinant MzSNAT5 protein exhibited high enzymatic activity to catalyze serotonin to N-acetylserotonin with the K_m and V_max of 55 μmol/L and 0.909 pmol/min/mg protein at 35°C, respectively; this pathway functioned over a wide range of temperatures from 5 to 75°C. In an in vivo study, MzSNAT5 was drought inducible. The transgenic Arabidopsis ectopically expressing MzSNAT5 elevated the melatonin level and, hence, enhanced drought tolerance. The mechanistic study indicated that the ectopically expressing MzSNAT5 allows plant mitochondria to increase melatonin synthesis. As a potent free radical scavenger, melatonin reduces the oxidative stress caused by the elevated reactive oxygen species which are generated under drought stress in plants. Our findings provide evidence that engineered melatonin-enriched plants exhibit enhanced oxidative tolerance.

EphB2 signaling-mediated Sirt3 expression reduces MSC senescence by maintaining mitochondrial ROS homeostasis

Disruption of mitochondrial reactive oxygen species (mtROS) homeostasis is a key factor inducing UCB-MSC senescence. Accordingly, preventing mtROS accumulation will help in suppressing the UCB-MSC senescence. In this study, we observed that the expressions of EphrinB2 and EphB2 were inversely regulated by UCB-MSC passage-dependent manner. EphB2 signaling induced mitochondrial translocation of Sirt3. The knockdown of SIRT3 inhibited the effect of EphB2 signaling in UCB-MSCs. Subsequently, EphrinB2-Fc induced the nuclear translocation of Nrf-2 via c-Src phosphorylation dependent manner, and Sirt3 expression was regulated by Nrf-2. Among Sirt3 target genes, EphB2 signaling increased MnSOD and reduced the mtROS level in UCB-MSCs. Furthermore, the deacetylase effect of Sirt3 enhanced the MnSOD activity by deacetylation at the lysine 68 residue and therapeutic effect of UCB-MSCs on skin-wound healing was increased by EphB2 activation. In conclusion, the EphB2 can serve as a novel target for the optimizing the therapeutic use of UCB-MSCs in wound repair by MnSOD-mediated mtROS scavenging through EphB2/c-Src signaling pathway and Nrf-2-dependent Sirt3 expression.

Melatonin as a Potential Agent in the Treatment of Sarcopenia

Considering the increased speed at which the world population is aging, sarcopenia could become an epidemic in this century. This condition currently has no means of prevention or treatment. Melatonin is a highly effective and ubiquitously acting antioxidant and free radical scavenger that is normally produced in all organisms. This molecule has been implicated in a huge number of biological processes, from anticonvulsant properties in children to protective effects on the lung in chronic obstructive pulmonary disease. In this review, we summarize the data which suggest that melatonin may be beneficial in attenuating, reducing or preventing each of the symptoms that characterize sarcopenia. The findings are not limited to sarcopenia, but also apply to osteoporosis-related sarcopenia and to age-related neuromuscular junction dysfunction. Since melatonin has a high safety profile and is drastically reduced in advanced age, its potential utility in the treatment of sarcopenic patients and related dysfunctions should be considered.

Melatonin prevents cell death and mitochondrial dysfunction via a SIRT1-dependent mechanism during ischemic-stroke in mice

Silent information regulator 1 (SIRT1), a type of histone deacetylase, is a highly effective therapeutic target for protection against ischemia reperfusion (IR) injury (IRI). Previous studies showed that melatonin preserves SIRT1 expression in neuronal cells of newborn rats after hypoxia-ischemia. However, the definite role of SIRT1 in the protective effect of melatonin against cerebral IRI in adult has not been explored. In this study, the brain of adult mice was subjected to IRI. Prior to this procedure, the mice were given intraperitoneal with or without the SIRT1 inhibitor, EX527. Melatonin conferred a cerebral-protective effect, as shown by reduced infarct volume, lowered brain edema, and increased neurological scores. The melatonin-induced upregulation of SIRT1 was also associated with an increase in the anti-apoptotic factor, Bcl2, and a reduction in the pro-apoptotic factor Bax. Moreover, melatonin resulted in a well-preserved mitochondrial membrane potential, mitochondrial Complex I activity, and mitochondrial cytochrome c level while it reduced cytosolic cytochrome c level. However, the melatonin-elevated mitochondrial function was reversed by EX527 treatment. In summary, our results demonstrate that melatonin treatment attenuates cerebral IRI by reducing IR-induced mitochondrial dysfunction through the activation of SIRT1 signaling.

Age-related changes in mitochondrial function of mouse colonic smooth muscle: beneficial effects of melatonin

Aging is a multifactorial process that involves biochemical, structural, and functional changes in mitochondria. The ability of melatonin to palliate the alterations induced by aging is based on its chronobiologic, antioxidant, and mitochondrial effects. There is little information about the effects of melatonin on the in situ mitochondrial network of aging cells and its physiological implications. We have studied the ability of melatonin to prevent the functional alterations of in situ mitochondria of smooth muscle cells and its impact on contractility. Mitochondrial membrane potential was recorded in isolated colonic smooth muscle cells from young mice (3 month old), aged mice (22-24-month old), and aged mice treated with melatonin (starting at 14-month age). Aging induced a partial mitochondrial depolarization in resting conditions and reduced the depolarizing response to cellular stimulation. Use of oligomycin indicated that aging enhanced the resting activity of the mitochondrial ATP synthase, whereas in young cells, the enzyme operated mainly in reverse mode. Melatonin treatment prevented all these changes. Aging reduced both spontaneous and stimulated contraction of colonic strips and shifted the metabolic dependence of contraction from mitochondria to glycolysis, as indicated the use of mitochondrial and glycolysis inhibitors. These functional alterations were also palliated by melatonin treatment. Aging effects were not related to a decrease in Ca2+ store mobilization, because this was enhanced in aged cells and restored by melatonin. In conclusion, melatonin prevents the age induced in situ mitochondrial potential alterations in smooth muscle cells and the associated changes in contractility and metabolism.

Melatonin prevents pancreatic β-cell loss due to glucotoxicity: the relationship between oxidative stress and endoplasmic reticulum stress

Prolonged hyperglycemia results in pancreatic β-cell dysfunction and apoptosis, referred to as glucotoxicity. Although both oxidative and endoplasmic reticulum (ER) stresses have been implicated as major causative mechanisms of β-cell glucotoxicity, the reciprocal importance between the two remains to be elucidated. The aim of this study was to evaluate the differential effect of oxidative stress and ER stress on β-cell glucotoxicity, by employing melatonin which has free radical-scavenging and antioxidant properties. As expected, in β-cells exposed to prolonged high glucose levels, cell viability and glucose-stimulated insulin secretion (GSIS) were significantly impaired. Melatonin treatment markedly attenuated cellular apoptosis by scavenging reactive oxygen species via its plasmalemmal receptor-independent increase in antioxidant enzyme activity. However, treatments with antioxidants alone were insufficient to recover the impaired GSIS. Interestingly, 4-phenylbutyric acid (4-PBA), a chemical chaperone that attenuate ER stress by stabilizing protein structure, alleviated the impaired GSIS, but not apoptosis, suggesting that glucotoxicity induces oxidative and ER stress independently. We found that cotreatment of glucotoxic β-cells with melatonin and 4-PBA dramatically improved both their survival and insulin secretion. Taken together, these results suggest that ER stress may be the more critical mechanism for prolonged high-glucose-induced GSIS impairment, whereas oxidative stress appears to be more critical for the impaired β-cell viability. Therefore, combinatorial therapy of melatonin with an ER stress modifier may help recover pancreatic β-cells under glucotoxic conditions in type 2 diabetes.

Age-related differences in the response of the brain to dietary melatonin

The aged brain is prone to excessive levels of immune activity, not initiated by an acute response to an extrinsic agent. While dietary melatonin is reported to attenuate the extent of expression of proinflammatory genes, little is known about the extent to which these changes can be translated into altered levels of corresponding proteins. The baseline levels of the proinflammatory cytokines, tumor necrosis factor alpha (TNF-α) and interleukin-1 alpha, were greater in older (~29 months old) compared to younger (~7 months old) mouse brains. Acute (3 h) exposure to lipopolysaccharide (LPS) induced activation of nuclear factor kappa B (NF-κB), but not inflammatory cytokines in the brain. The serum level of TNF-α was increased after LPS injection, indicating a systemic immune response to the bacterial cell wall component. Dietary melatonin (40 ppm for 9.3 weeks) did not prevent LPS-induced changes in younger animals but caused an increased systemic TNF-α response in older mice. Melatonin did reduce markers of carbonyl formation in brain proteins of young animals and nitrosylative damage to peptide-bound amino acid residues, in the brains of older animals. Acute LPS challenge did not significantly affect these oxidative markers. Thus, despite lack of clear evidence of attenuation of the NF-κB-cytokine inflammatory trajectory within the CNS by melatonin, this agent did show a protective effect against free radical-initiated injury to amino acid residues within proteins. The results illustrate that previously reported changes in gene expression following melatonin treatment need not be closely paralleled by corresponding changes in protein content.

Autophagic and proteolytic processes in the Harderian gland are modulated during the estrous cycle

The Syrian hamster Harderian gland (HG) is an organ that undergoes physiological autophagy in response to oxidative stress induced by porphyrin production. Porphyrin production in the HG has marked sex differences and is closely linked to reproductive function. In the present study, we observed that the estrous cycle and associated estrogen variations may affect oxidative-stress-induced proteolytic processes. In particular, significant changes in autophagic activity were detected during the estrous cycle. Notably, increased activation of macroautophagy as well as chaperone-mediated autophagy in the estrus phase coincided with a minimal antioxidant capability and the highest protein damage levels. By contrast, autophagic machinery was found to be blocked in the diestrus phase, likely due to mammalian target of rapamycin activation, which could be corroborated by the subsequent pS6K activation. Analogous results were observed regarding proteasome activity, which also showed maximal activity in the estrus phase. Interestingly, all these mechanisms were associated with important morphological changes in the HG during the estrous cycle. We observed statistically significant increases in Type II cells, which may be related to extensive autophagy in the estrus phase. Physiologically, this would result in a significant release of porphyrins specifically when females are more receptive. These data support the role of porphyrins as pheromones, as other authors have previously suggested, thus making the HG a scent organ. In addition, these results suggest a porphyrin-based approach to the treatment of porphyria during pregnancy, a condition for which no treatment is currently known.

Adaptations of the aging animal to exercise: role of daily supplementation with melatonin

The pineal gland, through melatonin, seems to be of fundamental importance in determining the metabolic adaptations of adipose and muscle tissues to physical training. Evidence shows that pinealectomized animals fail to develop adaptive metabolic changes in response to aerobic exercise and therefore do not exhibit the same performance as control-trained animals. The known prominent reduction in melatonin synthesis in aging animals led us to investigate the metabolic adaptations to physical training in aged animals with and without daily melatonin replacement. Male Wistar rats were assigned to four groups: sedentary control (SC), trained control (TC), sedentary treated with melatonin (SM), and trained treated with melatonin (TM). Melatonin supplementation lasted 16 wk, and the animals were subjected to exercise during the last 8 wk of the experiment. After euthanasia, samples of liver, muscle, and adipose tissues were collected for analysis. Trained animals treated with melatonin presented better results in the following parameters: glucose tolerance, physical capacity, citrate synthase activity, hepatic and muscular glycogen content, body weight, protein expression of phosphatidylinositol 3-kinase (PI3K), mitogen-activated protein kinase (MAPK), and protein kinase activated by adenosine monophosphate (AMPK) in the liver, as well as the protein expression of the glucose transporter type 4 (GLUT4) and AMPK in the muscle. In conclusion, these results demonstrate that melatonin supplementation in aging animals is of great importance for the required metabolic adaptations induced by aerobic exercise. Adequate levels of circulating melatonin are, therefore, necessary to improve energetic metabolism efficiency, reducing body weight and increasing insulin sensitivity.

Protective effect of melatonin on TNF-α-induced muscle atrophy in L6 myotubes

Muscle atrophy, characterized by decreased cell number and size, is a serious concern for patients afflicted with inflammatory diseases. Mounting evidence indicates that tumor necrosis factor alpha (TNF-α) plays a critical role in muscle atrophy in a number of clinical settings. We hypothesize that reactive oxygen species (ROS) mediate TNF-α-induced muscle cell death and hypotrophy. Recently, melatonin has attracted attention because of its free-radical scavenging and antioxidant properties. The aim of the current study was to evaluate the possible protective role of melatonin in TNF-α-induced muscle cell death and hypotrophy in rat L6 myotubes. To examine this possible role, L6 myotubes were exposed to various concentrations of recombinant TNF-α for 24 hr. We found that TNF-α at a concentration of 100 ng/mL induced ROS generation and decreased cell viability. Further analysis revealed that apoptosis, but not autophagy, may be important for TNF-α-induced cell death. Melatonin significantly attenuated TNF-α-induced ROS generation and apoptosis. In addition, decreased muscle fiber diameter and increased muscle cell proteolysis by TNF-α was highly attenuated by treatment with melatonin. The effects of melatonin were mediated neither through its plasmalemmal receptors nor by modulating the nuclear factor kappa B pathway activated by TNF-α. Taken together, these results suggest that TNF-α may mediate ROS-induced muscle cell death and hypotrophy and that melatonin may be a useful tool for protecting against muscle atrophy stemming from inflammatory diseases.

Melatonin enhances antioxidative enzyme gene expression (CAT, GPx, SOD), prevents their UVR-induced depletion, and protects against the formation of DNA damage (8-hydroxy-2'-deoxyguanosine) in ex vivo human skin

UV radiation (UVR) induces serious structural and functional alterations in human skin leading to skin aging and carcinogenesis. Reactive oxygen species are key players in UVR-mediated photodamage and induce the DNA-base-oxidized, intermediate 8-hydroxy-2'-deoxyguanosine (8-OHdG). Herein, we report the protective action of melatonin against UVR-induced 8-OHdG formation and depletion of antioxidative enzymes using ex vivo human full-thickness skin exposed to UVR in a dose (0, 100, 300 mJ/cm(2))- and time-dependent manner (0, 24, 48 hr post-UVR). Dynamics of depletion of antioxidative enzymes including catalase (CAT), glutathione peroxidase (GPx), and superoxide dismutase (SOD), or 8-OHdG formation were studied by real-time PCR and immunofluorescence/immunohistochemical staining. UVR-treated skin revealed significant and immediate (0 hr 300 mJ/cm(2)) reduction of gene expression, and this effect intensified within 24 hr post-UVR. Simultaneous increase in 8-OHdG-positive keratinocytes occurred already after 0 hr post-UVR reaching 71% and 99% up-regulation at 100 and 300 mJ/cm(2), respectively (P < 0.001). Preincubation with melatonin (10(-3) M) led to 32% and 29% significant reductions in 8-OHdG-positive cells and the prevention of antioxidative enzyme gene and protein suppression. Thus, melatonin was shown to play a crucial role as a potent antioxidant and DNA protectant against UVR-induced oxidative damage in human skin.

Mitochondria and chloroplasts as the original sites of melatonin synthesis: a hypothesis related to melatonin's primary function and evolution in eukaryotes

Mitochondria and chloroplasts are major sources of free radical generation in living organisms. Because of this, these organelles require strong protection from free radicals and associated oxidative stress. Melatonin is a potent free radical scavenger and antioxidant. It meets the criteria as a mitochondrial and chloroplast antioxidant. Evidence has emerged to show that both mitochondria and chloroplasts may have the capacity to synthesize and metabolize melatonin. The activity of arylalkylamine N-acetyltransferase (AANAT), the reported rate-limiting enzyme in melatonin synthesis, has been identified in mitochondria, and high levels of melatonin have also been found in this organelle. From an evolutionary point of view, the precursor of mitochondria probably is the purple nonsulfur bacterium, particularly, Rhodospirillum rubrum, and chloroplasts are probably the descendents of cyanobacteria. These bacterial species were endosymbionts of host proto-eukaryotes and gradually transformed into cellular organelles, that is, mitochondria and chloroplasts, respectively, thereby giving rise to eukaryotic cells. Of special importance, both purple nonsulfur bacteria (R. rubrum) and cyanobacteria synthesize melatonin. The enzyme activities required for melatonin synthesis have also been detected in these primitive species. It is our hypothesis that mitochondria and chloroplasts are the original sites of melatonin synthesis in the early stage of endosymbiotic organisms; this synthetic capacity was carried into host eukaryotes by the above-mentioned bacteria. Moreover, their melatonin biosynthetic capacities have been preserved during evolution. In most, if not in all cells, mitochondria and chloroplasts may continue to be the primary sites of melatonin generation. Melatonin production in other cellular compartments may have derived from mitochondria and chloroplasts. On the basis of this hypothesis, it is also possible to explain why plants typically have higher melatonin levels than do animals. In plants, both chloroplasts and mitochondria likely synthesize melatonin, while animal cells contain only mitochondria. The high levels of melatonin produced by mitochondria and chloroplasts are used to protect these important cellular organelles against oxidative stress and preserve their physiological functions. The superior beneficial effects of melatonin in both mitochondria and chloroplasts have been frequently reported.

Melatonin modulates TLR4-mediated inflammatory genes through MyD88- and TRIF-dependent signaling pathways in lipopolysaccharide-stimulated RAW264.7 cells

Increasing evidence demonstrates that melatonin has an anti-inflammatory effect. Nevertheless, the molecular mechanisms remain obscure. In this study, we investigated the effect of melatonin on toll-like receptor 4 (TLR4)-mediated molecule myeloid differentiation factor 88 (MyD88)-dependent and TRIF-dependent signaling pathways in lipopolysaccharide (LPS)-stimulated macrophages. RAW264.7 cells were incubated with LPS (2.0 μg/mL) in the absence or presence of melatonin (10, 100, 1000 μm). As expected, melatonin inhibited TLR4-mediated tumor necrosis factor alpha (TNF-α), interleukin (IL)-1β, IL-6, IL-8, and IL-10 in LPS-stimulated macrophages. In addition, melatonin significantly attenuated LPS-induced upregulation of cyclooxygenase (COX)-2 and inducible nitric oxide synthase (iNOS) in macrophages. Further analysis showed that melatonin inhibited the expression of MyD88 in LPS-stimulated macrophages. Although it had no effect on TLR4-mediated phosphorylation of c-Jun N-terminal kinase (JNK), p38, and extracellular regulated protein kinase (ERK), melatonin significantly attenuated the activation of nuclear factor kappa B (NF-κB) in LPS-stimulated macrophages. In addition, melatonin inhibited TLR4-mediated Akt phosphorylation in LPS-stimulated macrophages. Moreover, melatonin significantly attenuated the elevation of interferon (IFN)-regulated factor-3 (IRF3), which was involved in TLR4-mediated TRIF-dependent signaling pathway, in LPS-stimulated macrophages. Correspondingly, melatonin significantly alleviated LPS-induced IFN-β in macrophages. In conclusion, melatonin modulates TLR4-mediated inflammatory genes through MyD88-dependent and TRIF-dependent signaling pathways.

Overexpression of manganese superoxide dismutase ameliorates high-fat diet-induced insulin resistance in rat skeletal muscle

Elevated mitochondrial reactive oxygen species have been suggested to play a causative role in some forms of muscle insulin resistance. However, the extent of their involvement in the development of diet-induced insulin resistance remains unclear. To investigate, manganese superoxide dismutase (MnSOD), a key mitochondrial-specific enzyme with antioxidant modality, was overexpressed, and the effect on in vivo muscle insulin resistance induced by a high-fat (HF) diet in rats was evaluated. Male Wistar rats were maintained on chow or HF diet. After 3 wk, in vivo electroporation (IVE) of MnSOD expression and empty vectors was undertaken in right and left tibialis cranialis (TC) muscles, respectively. After one more week, insulin action was evaluated using hyperinsulinemic euglycemic clamp, and tissues were subsequently analyzed for antioxidant enzyme capacity and markers of oxidative stress. MnSOD mRNA was overexpressed 4.5-fold, and protein levels were increased by 70%, with protein detected primarily in the mitochondrial fraction of muscle fibers. This was associated with elevated MnSOD and glutathione peroxidase activity, indicating that the overexpressed MnSOD was functionally active. The HF diet significantly reduced whole body and TC muscle insulin action, whereas overexpression of MnSOD in HF diet animals ameliorated this reduction in TC muscle glucose uptake by 50% (P < 0.05). Decreased protein carbonylation was seen in MnSOD overexpressing TC muscle in HF-treated animals (20% vs. contralateral control leg, P < 0.05), suggesting that this effect was mediated through an altered redox state. Thus interventions causing elevation of mitochondrial antioxidant activity may offer protection against diet-induced insulin resistance in skeletal muscle.

Neuroimmunomodulation in unipolar depression: a focus on chronobiology and chronotherapeutics

The rising burden of unipolar depression along with its often related sleep disturbances, as well as increasing rates of sleep restriction in modern society, make the search for an extended understanding of the aetiology and pathophysiology of depression necessary. Accumulating evidence suggests an important role for the immune system in mediating disrupted neurobiological and chronobiological processes in depression. This review aims to provide an overview of the neuroimmunomodulatory processes involved with depression and antidepressant treatments with a special focus on chronobiology, chronotherapeutics and the emerging field of immune-circadian bi-directional crosstalk. Increasing evidence suggests that chronobiological disruption can mediate immune changes in depression, and likewise, immune processes can mediate chronobiological disruption. This may suggest a bi-directional relationship in immune-circadian crosstalk. Furthermore, given the immunomodulatory effects of antidepressants and chronotherapeutics, as well as their associated beneficial effects on circadian disturbance, we--and others--suggest that these therapeutic agents may exert their chronobiotic effects partially via the neuroimmune system. Further research is required to better elucidate the mechanisms of immune involvement in the chronobiology of depression.

Neurons from senescence-accelerated SAMP8 mice are protected against frailty by the sirtuin 1 promoting agents melatonin and resveratrol

The senescence-accelerated prone 8 (SAMP8) mouse strain shows early cognitive loss that mimics the deterioration of learning and memory in the elderly and is widely used as an animal model of aging. SAMP8 mouse brain suffers oxidative stress, as well as tau- and amyloid-related pathology. Mitochondrial dysfunction and the subsequent increase in cellular oxidative stress are central to the aging processes of the organism. Here, we examined the mitochondrial status of neocortical neurons cultured from SAMP8 and senescence-accelerated-resistant (SAMR1) mice. SAMP8 mouse mitochondria showed a reduced membrane potential and higher vulnerability to inhibitors and uncouplers than SAMR1 mitochondria. DL-buthionine-[S,R]-sulfoximine (BSO) caused greater oxidative damage in neurons from SAMP8 mice than in those from SAMR1 mice. This increased vulnerability, indicative of frailty-associated senescence, was protected by the anti-aging agents melatonin and resveratrol. The sirtuin 1 inhibitor, sirtinol, demonstrated that the neuroprotection against BSO was partially mediated by increased sirtuin 1 expression. Melatonin, like resveratrol, enhanced sirtuin 1 expression in neuron cultures of SAMR1 and SAMP8 mice. Therefore, a deficiency in the neuroprotection and longevity of the sirtuin 1 pathway in SAMP8 neurons may contribute to the early age-related brain damage in these mice. This supports the therapeutic use of sirtuin 1-enhancing agents against age-related nerve cell dysfunction and brain frailty.

Melatonin modulates autophagy through a redox-mediated action in female Syrian hamster Harderian gland controlling cell types and gland activity

The Syrian hamster Harderian gland exhibits sexually dimorphic porphyrin biosynthesis, wherein the female glands display an extraordinarily high concentration of porphyrins. Damage derived from this production of porphyrins, mediated by reactive oxygen species, causes the glands to develop autophagic processes, which culminate in detachment-derived cell death; these cells normally play a central role in the secretory activity of the gland. The main aim of this study was to analyze how a change in the redox state impacts autophagy. Female Syrian hamsters were treated daily with melatonin (25 μg, subcutaneously) at ZT 10 for 1-2 months (N-acetyl-5-methoxytryptamine), an endogenous antioxidant that ameliorates the deleterious effects of free radicals via a variety of mechanisms. The length of treatment affected the redox balance, the autophagy machinery, and the activation of p53 and NF-κB. One-month treatment displaces redox balance to the antioxidant side, promotes autophagy through a p53-mediated mechanism, and increases cell detachment. Meanwhile, 2-month treatment restores redox balance to the oxidant side, activates NF-κB reducing autophagy to basal levels, increases number of type II cells, and reduces number of detached cells. Our results conclude that the redox state can modulate autophagy through redox-sensitive transcriptions factors. Additionally, these findings support a hypothesis that ascribes differences in the autophagic-lysosomal pathway to epithelial cell types, thereby restricting detachment-induced autophagic cell death to epithelial cell type I.

Melatonin: neuritogenesis and neuroprotective effects in crustacean x-organ cells

Melatonin has both neuritogenic and neuroprotective effects in mammalian cell lines such as neuroblastoma cells. The mechanisms of action include receptor-coupled processes, direct binding and modulation of calmodulin and protein kinase C, and direct scavenging of free radicals. While melatonin is produced in invertebrates and has influences on their physiology and behavior, little is known about its mechanisms of action. We studied the influence of melatonin on neuritogenesis in well-differentiated, extensively-arborized crustacean x-organ neurosecretory neurons. Melatonin significantly increased neurite area in the first 24h of culture. The more physiological concentrations, 1 nM and 1 pM, increased area at 48 h also, whereas the pharmacological 1 μM concentration appeared to have desensitizing effects by this time. Luzindole, a vertebrate melatonin receptor antagonist, had surprising and significant agonist-like effects in these invertebrate cells. Melatonin receptors have not yet been studied in invertebrates. However, the presence of membrane-bound receptors in this population of crustacean neurons is indicated by this study. Melatonin also has significant neuroprotective effects, reversing the inhibition of neuritogenesis by 200 and 500 μM hydrogen peroxide. Because this is at least in part a direct action not requiring a receptor, melatonin's protection from oxidative stress is not surprisingly phylogenetically-conserved.

Melatonin, and to a lesser extent growth hormone, restores colonic smooth muscle physiology in old rats

There is increasing evidence that aging is associated with oxidative damage, inflammation, and apoptosis in different cell types. However, there is limited information regarding aging mechanisms in colon smooth muscle. Old male Wistar rats (22 months) were treated for 10 wks with melatonin or growth hormone (GH). Animals were sacrificed at 24 months of age by decapitation. The colon was dissected and the smooth muscle homogenized. H(2)O(2) and malonyl dialdehyde (MDA) content and catalase and glutathione peroxidase (GPX) activities were determined using colorimetric kits. Expression of nuclear factor kappa B (NF-κB), cyclooxygenase 2 (COX-2), caspase-3, and caspase-9 were determined by Western blot. Aging of colon smooth muscle correlated with an increase in H(2)O(2) and MDA levels when compared with young animals in both proximal and distal segments; these changes were associated with a decrease in the catalase activity in the distal colon. Oxidative stress correlated with an increase in COX-2 and NF-κB expression, which were accompanied by an enhanced expression of the pro-apoptotic enzyme caspase-3 and its upstream enzyme, caspase-9. Melatonin treatment normalized the oxidative, inflammatory, and apoptotic patterns, whereas GH replacement, although effective in reducing oxidative stress in distal colon, did not reverse the age-related inflammation or apoptosis. These results suggest that melatonin should be the treatment of choice to most effectively recover physiological functions in aged colonic smooth muscle.