Pro-Oxidant Effect of Melatonin in Tumour Leucocytes: Relation with its Cytotoxic and Pro-Apoptotic Effects

Dose-dependent effect of melatonin on postwarming development of vitrified ovine embryos

After cryopreservation, embryos become sensitive to the oxidative stress, resulting in lipid peroxidation, membrane injury, and structural destruction. The present study aimed to assess the effect of increasing concentration of melatonin during postwarming culture on embryo's ability to restore its functions after cryopreservation. In vitro-produced blastocysts were vitrified, warmed, and cultured in vitro in TCM 199 with 5 different supplementations: control (CTR): 10% fetal calf serum; bovine serum albumin (BSA): 0.04% (wt/vol) BSA; and MEL(-3), MEL(-6), MEL(-9): BSA plus melatonin 10(-3), 10(-6), and 10(-9) M. The medium with the highest melatonin concentration had the highest trolox equivalent antioxidant capacity, whose values were comparable with those determined in plasma sampled from adult ewes (8.7 ± 2.4 mM). The other media had lower trolox equivalent antioxidant capacity values (P < 0.01), below the range of the plasma. At the same time, embryos cultured with the highest melatonin concentration reported a lower in vitro viability, as evaluated by lower re-expansion and hatching rates, and lower total cell number compared with the other groups (P < 0.05). Their metabolic status was also affected, as evidenced by higher oxidative and apoptotic index and lower ATP concentration. The beneficial effects of melatonin on embryo development during postwarming culture were observed only at low concentration (10(-9) M). These results suggest that melatonin at high concentration may exert some degree of toxic activity on pre-implantation embryos. Thus, the dose at which the embryos are exposed is pivotal to obtain the desiderate effect.

The antioxidants, vitamin A and E but not vitamin C and melatonin enhance the proapoptotic effects of irinotecan in cancer cells in vitro

Irinotecan is one of the camptothecin analog which has been shown to have a broad spectrum of antitumor activities against various malignancies. The aim of this study was to evaluate the effect of vitamin A, C, E and melatonin on proapoptotic activity of irinotecan in human cancer cells in vitro. We observed that irinotecan induced apoptosis in all types of analyzed cell lines when used as a single agent. Combination of selected antioxidants with various doses of irinotecan (7.5-60μM) resulted in significant increase in apoptotic cell death in A549 and HT29 cancer cell lines. The highest killing efficiency was observed after co-incubation of the cells with irinotecan and vitamin A (10μM), or vitamin E (25μM), respectively. The addition of vitamin C and melatonin to irinotecan treatment did not promote increase in killing of cancer cells. Our results indicate that some antioxidants can enhance the proapoptoic activity (properties) of irinotecan in human cancer cells in vitro. These findings may be supportive for the optimization of therapeutic efficacy of irinotecan treatment.

Melatonin and the theories of aging: a critical appraisal of melatonin's role in antiaging mechanisms

The classic theories of aging such as the free radical theory, including its mitochondria-related versions, have largely focused on a few specific processes of senescence. Meanwhile, numerous interconnections have become apparent between age-dependent changes previously thought to proceed more or less independently. Increased damage by free radicals is not only linked to impairments of mitochondrial function, but also to inflammaging as it occurs during immune remodeling and by release of proinflammatory cytokines from mitotically arrested, DNA-damaged cells that exhibit the senescence-associated secretory phenotype (SASP). Among other effects, SASP can cause mutations in stem cells that reduce the capacity for tissue regeneration or, in worst case, lead to cancer stem cells. Oxidative stress has also been shown to promote telomere attrition. Moreover, damage by free radicals is connected to impaired circadian rhythmicity. Another nexus exists between cellular oscillators and metabolic sensing, in particular to the aging-suppressor SIRT1, which acts as an accessory clock protein. Melatonin, being a highly pleiotropic regulator molecule, interacts directly or indirectly with all the processes mentioned. These influences are critically reviewed, with emphasis on data from aged organisms and senescence-accelerated animals. The sometimes-controversial findings obtained either in a nongerontological context or in comparisons of tumor with nontumor cells are discussed in light of evidence obtained in senescent organisms. Although, in mammals, lifetime extension by melatonin has been rarely documented in a fully conclusive way, a support of healthy aging has been observed in rodents and is highly likely in humans.

Melatonin induces apoptosis through a caspase-dependent but reactive oxygen species-independent mechanism in human leukemia Molt-3 cells

Melatonin is a naturally occurring indoleamine synthesized in the pineal gland that exhibits an extensive repertoire of biological activities. An increasing number of studies indicate that melatonin protects normal cells, while it reducing cancer cell proliferation. In this study, we investigated the effect of melatonin on the growth of the human leukemia cells and found that it efficiently reduced the number of cells in a concentration- and time-dependent manner. Thus, incubation with the indoleamine increased the percentage of cells with a hypodiploid DNA content, augmented the number of annexin V-positive cells, and also provoked ultrastructural changes that are features of apoptotic cell death. Evaluation of caspases revealed that caspase-3, caspase-6, caspase-7, and caspase-9, but not caspase-8 and caspase-2, were quickly activated (3-6 hr). The increase in the activity of these proteases was associated with up-regulation of the pro-apoptotic factor Bax and also with the release of cytochrome c from mitochondria. Pretreatment of the cells with the general caspase inhibitor z-VAD-fmk, reduced melatonin-induced apoptosis, but it did not block cell death suggesting that melatonin activates an alternative cell death modality in the absence of caspase activity. Thus, the activation of caspases was preceded by a fast (<30 min) increase in reactive oxygen species (ROS). Rotenone and antimycin A reduced the levels of ROS stimulated by melatonin, indicating that the complex I and the complex III of the mitochondrial electron transport chain are important sources of these chemical species. However, the role of ROS in melatonin-induced cell death remains elusive because anti-oxidants that were shown to decrease ROS levels (glutathione, N-acetyl-l-cysteine and Trolox) were unable to abrogate melatonin-induced cell death.

Molecular mechanisms of melatonin's inhibitory actions on breast cancers

Melatonin is involved in many physiological functions and it plays an important role in many pathological processes as well. Melatonin has been shown to reduce the incidence of experimentally induced cancers and can significantly inhibit the growth of some human tumors, namely hormone-dependent cancers. The anticancer effects of melatonin have been observed in breast cancer, both in in vivo with models of chemically induced rat mammary tumors, and in vitro studies on human breast cancer cell lines. Melatonin acts at different physiological levels and its antitumoral properties are supported by a set of complex, different mechanisms of action, involving apoptosis activation, inhibition of proliferation, and cell differentiation.

Mechanisms involved in the pro-apoptotic effect of melatonin in cancer cells

It is well established that melatonin exerts antitumoral effects in many cancer types, mostly decreasing cell proliferation at low concentrations. On the other hand, induction of apoptosis by melatonin has been described in the last few years in some particular cancer types. The cytotoxic effect occurs after its administration at high concentrations, and the molecular pathways involved have been only partially determined. Moreover, a synergistic effect has been found in several cancer types when it is administered in combination with chemotherapeutic agents. In the present review, we will summarize published work on the pro-apoptotic effect of melatonin in cancer cells and the reported mechanisms involved in such action. We will also construct a hypothesis on how different cell signaling pathways may relate each other on account for such effect.

A preclinical study on the protective effect of melatonin against methotrexate-induced small intestinal damage: effect mediated by attenuation of nitrosative stress, protein tyrosine nitration, and PARP activation

PURPOSE

One of the major toxic side effects of methotrexate (MTX) is enterocolitis. To date, there is no efficient standard treatment for this side effect. Nitrosative stress is reported to play a critical role in MTX-induced mucositis. The purpose of this study is to investigate whether pretreatment with melatonin, an inhibitor of nitro-oxidative stress, prevents MTX-induced mucositis in rats.

METHODS

Rats were pretreated with melatonin (20 and 40 mg/kg body weight) i.p. daily 1 h before MTX (7 mg/kg body weight) administration for three consecutive days. After the final dose of MTX, the rats were killed and the small intestines were used for analysis.

RESULTS

The small intestines of MTX-treated rats showed moderate to severe injury. The villi were distorted, blunted, and atrophied and focally absent in various segments of the small intestines. Crypt abscesses were also found, suggesting an inflammatory response. Pretreatment with melatonin had a dose-dependent protective effect on MTX-induced mucositis. Morphology was saved to a moderate extent with 20 mg melatonin pretreatment, and near-normal morphology was achieved with 40 mg melatonin pretreatment. Damage to the villi and crypt abscess was reduced. The villi/crypt ratio was almost restored. Melatonin pretreatment protected the small intestines from MTX-induced damage by attenuating nitrosative stress, protein tyrosine nitration and PARP expression.

CONCLUSION

Because of its versatility in protecting against nitro-oxidative stress and reducing inflammation, we suggest that melatonin could be beneficial in ameliorating MTX-induced enteritis in humans.

A Comparison of B16 Melanoma Cells and 3T3 Fibroblasts Concerning Cell Viability and ROS Production in the Presence of Melatonin, Tested Over a Wide Range of Concentrations

Melatonin is a pleiotropic molecule with many cellular and systemic actions, including chronobiotic effects. Beneficial effects are widely documented concerning the treatment of neoplastic diseases in vivo as well as reductions in viability of cultured cells from melanoma, one of the most aggressive cancers in humans. However, studies of its effects on non-tumor cells in vitro have not focused on viability, except for experiments aiming to protect against oxidotoxicity or other toxicological insults. Furthermore, there is no agreement on the range of effective melatonin concentrations in vitro, and the mechanisms that reduce cell viability have remained unclear. Tumor cell-specific increases in the production of reactive oxygen and nitrogen species (ROS/RNS) may provide a possible explanation. Our aim was to analyze the potential inhibition of tumor (B16 melanoma 4A5) and non-tumor cell (3T3 Swiss albino) viability using a wide range of melatonin concentrations (10⁻¹¹–10⁻² M), and to determine whether intracellular ROS enhancement was involved in this process. In the absence of fetal bovine serum (FBS), low melatonin concentrations (10⁻⁹–10⁻⁵ M) reduced the proliferation of melanoma cells with no effect in fibroblasts, whereas, in the presence of FBS, they had no effect or even increased the proliferation of both fibroblast and melanoma cells. Melatonin concentrations in the upper millimolar range increased ROS levels and reduced the viability of both cell types, but more markedly so in non-tumor cells. Thus, low melatonin concentrations reduce proliferation in this specific melanoma cell line, whereas high concentrations affect the viability of both tumor (B16 4A5 melanoma) and non-tumor (3T3 fibroblasts) cells. Increased ROS levels in both lines indicate a role for ROS production in the reduction of cell viability at high—but not low—melatonin concentrations, although the mechanism of action still remains to be elucidated.

Melatonin suppresses doxorubicin-induced premature senescence of A549 lung cancer cells by ameliorating mitochondrial dysfunction

Melatonin is an indolamine that is synthesized in the pineal gland and shows a wide range of physiological functions. Although the anti-aging properties of melatonin have been reported in a senescence-accelerated mouse model, whether melatonin modulates cellular senescence has not been determined. In this study, we examined the effect of melatonin on anticancer drug-induced cellular premature senescence. We found that the doxorubicin (DOX)-induced senescence of A549 human lung cancer cells and IMR90 normal lung cells was substantially inhibited by cotreatment with melatonin in a dose-dependent manner. Mechanistically, the DOX-induced G2/M phase cell cycle arrest and the decrease in cyclinB and cdc2 expression were not affected by melatonin. However, the DOX-induced increase in intracellular levels of ROS, which is necessary for premature senescence, was completely abolished upon melatonin cotreatment. In addition, the reduction in mitochondrial membrane potential that occurs upon DOX treatment was inhibited by melatonin. An aberrant increase in mitochondrial respiration was also significantly suppressed by melatonin, indicating that melatonin ameliorates the mitochondrial dysfunction induced by DOX treatment. The treatment of A549 cells with luzindole, a potent inhibitor of melatonin receptors, failed to prevent the effects of melatonin treatment on mitochondrial functions and premature senescence in cells also treated with DOX; this suggests that melatonin suppresses DOX-induced senescence in a melatonin receptor-independent manner. Together, these results reveal that melatonin has an inhibitory effect of melatonin on premature senescence at the cellular level and that melatonin protects A549 cells from DOX-induced senescence. Thus, melatonin might have the therapeutic potential to prevent the side effects of anticancer drug therapy.

Melatonin potentiates chemotherapy-induced cytotoxicity and apoptosis in rat pancreatic tumor cells

Melatonin has antitumor activity via several mechanisms including its antiproliferative and proapoptotic effects in addition to its potent antioxidant action. Thus, melatonin has proven useful in the treatment of tumors in association with chemotherapeutic drugs. This study was performed to evaluate the effect of melatonin on the cytotoxicity and apoptosis induced by three different chemotherapeutic agents, namely 5-fluorouracil (5-FU), cisplatin, and doxorubicin in the rat pancreatic tumor cell line AR42J. We found that both melatonin and the three chemotherapeutic drugs induce a time-dependent decrease in AR42J cell viability, reaching the highest cytotoxic effect after 48 hr of incubation. Furthermore, melatonin significantly augmented the cytotoxicity of the chemotherapeutic agents. Consistently, cotreatment of AR42J cells with each of the chemotherapeutic agents in the presence of melatonin increased the population of apoptotic cells, elevated mitochondrial membrane depolarization, and augmented intracellular reactive oxygen species (ROS) production compared to treatment with each chemotherapeutic agent alone. These results provide evidence that in vitro melatonin enhances chemotherapy-induced cytotoxicity and apoptosis in rat pancreatic tumor AR42J cells and, therefore, melatonin may be potentially applied to pancreatic tumor treatment as a powerful synergistic agent in combination with chemotherapeutic drugs.

Melatonin: the smart killer: the human trophoblast as a model

Melatonin has both the ability to induce intrinsic apoptosis in tumor cells while it inhibits it in non-tumor cells. Melatonin kills tumor cells through induction of reactive oxygen species generation and activation of pro-apoptotic pathways. In contrast, melatonin promotes the survival of non-tumor cells due to its antioxidant properties and the inhibition of pro-apoptotic pathways. In primary human villous trophoblast, a known pseudo-tumorigenic tissue, melatonin promotes the survival through inhibition of the Bax/Bcl-2 pathway while in BeWo choriocarcinoma cell line melatonin induces permeabilization of the mitochondrial membrane leading to cellular death. These findings suggest that the trophoblast is a good model to study the differential effects of melatonin on the intrinsic apoptosis pathway. This review describes the differential effects of melatonin on the intrinsic apoptosis pathway in tumor and non-tumor cells and presents the trophoblast as a novel model system in which to study these effects of melatonin.

Protective effect of melatonin against human leukocyte apoptosis induced by intracellular calcium overload: relation with its antioxidant actions

Apoptosis or programmed cell death plays a critical role in both inflammatory and immune responses. Recent evidence demonstrates that control of leukocyte apoptosis is one of the most striking immune system-related roles of melatonin. For this reason, this study evaluated the protective effects of melatonin on human leukocyte apoptosis induced by sustained cytosolic calcium increases. Such protective effects are likely mediated by melatonin's free-radical scavenging actions. Treatments with the specific inhibitor of cytosolic calcium re-uptake, thapsigargin (TG), and/or the calcium-mobilizing agonist, N-formyl-methionyl-leucyl-phenylalanine (FMLP), induced intracellular reactive oxygen species (ROS) production, caspase activation as well as DNA fragmentation in human leukocytes. Also, TG- and/or FMLP-induced apoptosis was dependent on both cytosolic calcium increases and calcium uptake into mitochondria, because when cells were preincubated with the cytosolic calcium chelator, dimethyl BAPTA, and the inhibitor of mitochondrial calcium uptake, Ru360, TG- and FMLP-induced apoptosis was largely inhibited. Importantly, melatonin treatment substantially prevented intracellular ROS production, reversed caspase activation, and forestalled DNA fragmentation induced by TG and FMLP. Similar results were obtained by preincubating the cells with another well-known antioxidant, i.e., N-acetyl-L-cysteine. To sum up, depletion of intracellular calcium stores induced by TG and/or FMLP triggers different apoptotic events in human leukocytes that are dependent on calcium signaling. The protective effects resulting from melatonin administration on leukocyte apoptosis likely depend on melatonin's antioxidant action because we proved that this protection is melatonin receptor independent. These findings help to understand how melatonin controls apoptosis in cells of immune/inflammatory relevance.

Melatonin enhances hydrogen peroxide-induced apoptosis in human promyelocytic leukaemia HL-60 cells

Melatonin is an indoleamine secreted by the pineal gland that shows multiple tasks. This ubiquitously acting free radical scavenger has recently been shown to stimulate the production of reactive oxygen species (ROS) in tumour cells, making them undergo apoptosis, whilst it prevents apoptosis in healthy cells. The mechanisms by which melatonin exerts these dual actions are, however, not yet clearly understood. Thus, the aim of this study was to further investigate how melatonin can enhance oxidative stress-induced apoptosis in a leukaemia cell line. The results show that melatonin increased the apoptotic effects of H(2)O(2) in human myeloid HL-60 cells as assessed by cellular viability, mitochondrial permeability transition induction, mitochondrial membrane depolarization, ROS generation, caspases 3, 8 and 9 activity, phosphatidylserine externalization, and DNA fragmentation techniques. When healthy leucocytes were exposed to H(2)O(2), melatonin increased the viability of the cells. Taken together, the findings indicate that melatonin is a potential physiological tool capable of protecting healthy cells from chemotherapy-induced ROS production as well as inducing tumour cell death. Because cancer cells manifest increased oxidative stress as a result of their elevated metabolism, the use of melatonin may be useful in impairing their ROS buffering capacity.