Background
Melatonin (chemically named N-acetyl-5-methoxytryptamine) is indeed a great multi-tasker. Whereas it is well known for its role in regulating circadian rhythm, much interest has been generated for its possible role in the pathogenesis and treatment of other disorders, namely cancer. The deviation from its primary role in the control of circadian rhythms including sleep came about from the observation that not only is it synthesized in the pineal gland, but also in the retina, GI tract, bone marrow and leukocytes (Hardeland 2011). Furthermore, in humans, melatonin receptors have been identified in enterocytes, gallbladder epithelium, exocrine and endocrine pancreatic cells, breast epithelium, ovarian granulosa cells, cardiac ventricular cells, and platelets, just to name a few (Fernando 2014, Hardeland 2011).
Melatonin is secreted from the pineal gland in a diurnal rhythm, being higher at night in darkness than during the daylight hours. Another significant source of melatonin in the body is the gastrointestinal system, where gastrointestinal tract (GIT) cells synthesize melatonin to regulate digestive function (Bubenik 2008). Much research has been devoted to the oncological risk that night shift workers may experience due to the disruption of their circadian clock, and therefore disruption of melatonin secretion (Bracci 2013). It was proposed that this risk may be mediated by the loss of antioxidant and hormone modulating properties of melatonin.
In addition to its antioxidant properties, melatonin also acts as an immune modulator, anti-inflammatory, and possesses cytostatic as well as cytotoxic properties in vitro as well as in vivo. A PubMed search of “melatonin and cancer” reveals thousands of articles on the subject, and many excellent reviews have been published. This article will outline some of the mechanisms behind the powerful pleiotropic effects of melatonin, and how this translates clinically with respect to naturopathic oncology. The focus of this article will be elucidating melatonin’s mechanism of action, while clinical data can be found summarized elsewhere (Fritz 2009, Seely 2012, Wang 2012).
Mechanisms
Melatonin as antioxidant
Several simultaneous processes act harmoniously to grant melatonin its impressive antioxidant abilities. First, it directly scavenges free radicals, both reactive oxygen species (ROS) and reactive nitrogen species (RNS). Hydrogen peroxide, while not a free radical but still an oxidizing agent, is also neutralized by melatonin. Interestingly, several melatonin metabolites produced during the scavenging reactions, such as N1-acetyl-N2-formyl-5-
methoxykynuramine (AFMK) and N1-acetyl-5-methoxykynuramine (AMK) also have free radical scavenging abilities (Hardeland 2011). Melatonin indirectly acts an antioxidant by way of stimulating the production of glutathione, the most abundant intracellular antioxidant our cells possess. In one study, melatonin outperformed the hepatoprotective effects of the antioxidant N-acetylcysteine in rat models of methanol intoxication (Koksal 2012). In another study, a more dilute solution of melatonin was able to mimic the antioxidant effect of a much more concentrated vitamin C solution (Montilla-López 2002). In addition, melatonin up regulates the production of detoxifying enzymes such as glutathione peroxidase (GPx), glutathione reductase (GR) and superoxide dismutase (SOD) (Reiter 2003). Oxidation leading to cell and DNA damage is a major contributor to oncogenesis, and melatonin may therefore be beneficial as a chemopreventive agent.
Melatonin as immune modulator
Melatonin has immune enhancing effects on both the innate and adaptive immune system. Melatonin has been shown to increase the production of natural killer cells (NK), monocytes and leukocytes (Srinivasan 2008). NK cells are the primary innate immune cell responsible for killing cancer cells by releasing cytotoxic proteins such as perforin and granzyme (Lui 2012). Melatonin also enhances the production of IL-1, IL-6, TNF-α and IL-12 from monocytes, and enhances the production of IL-2, IFN-ψ and IL-6 from peripheral blood mononuclear cells (Srinivasan 2008). Together these cytokines activate and regulate the cytotoxic T cell response, which kill tumour cells. The immunosurveillance that the innate and adaptive immune system help provide may have more of a role in the prevention of tumours.
Melatonin as anti-inflammatory
It is well accepted that over expression of COX-2 in tissues is important in mediating cancer growth and metastasis (Generali 2014, Khan 2011). Further, cancer cells themselves over express COX-2, leading to a self perpetuating system. Several studies have shown that melatonin possesses COX-2 suppressing actions at a pharmacological dose of 1mM (Wu 2014). Additionally, the previously mentioned metabolites of melatonin, AMFK and AMK, in addition to having antioxidant potential, also inhibit COX-2 expression (Wu 2014). Other immune cells involved in the inflammatory process, such as macrophages, have been shown to secrete melatonin, suggesting that melatonin is a key endogenous molecule released in response to inflammation (Wu 2014). Melatonin has also been shown to inhibit pro-inflammatory cytokines IL-8 and TNF-α in neutrophils, suggesting it may help to reduce the effects of acute and chronic inflammation (Silva 2004).
Melatonin as cytostatic
Cytostasis refers to the inhibition of cellular growth and replication. There are numerous studies on the anti-proliferative effects of melatonin on various types of cancer cells both in vitro and in vivo, and a comprehensive review of these is beyond the scope of this article. However, one of the mechanisms suggested as responsible for such inhibition is activation of p21 and p53 tumour suppressor genes, which act by halting the cell cycle (Mediavilla 1999). A second cytostatic mechanism may be via melatonin’s antiangiogenic effects. Melatonin appears to inhibit hypoxia inducible factor (HIF-1) and vascular endothelial growth factor (VEGF), both of which drive the growth of new blood vessels in the tumour environment (Lissoni 2001). Thirdly, melatonin may have anti-metastatic effects via inhibition of matrix metalloproteinase-9 (MMP-9) activity, an enzyme associated with extracellular matrix remodelling and cancer metastasis (Rudra 2013). Melatonin also appears to increase expression of cell surface adhesion proteins, E-cadherin and beta1-integrin, which may decrease cancer cell migration and metastasis (Ortiz-Lopez 2009).
Melatonin as cytotoxic
Cytotoxicity refers to the ability of an agent to be toxic to cells, in other words, to klll them. Conventional therapies, such as chemotherapy, aim at being cytotoxic to cells. One such way an agent can be cytotoxic is by promoting apoptosis.
The studies on the apoptotic potential and mechanisms of melatonin are quite exciting, as studies are showing it has this ability in both hematological and solid tumour cell lines, such as breast cancer, colon cancer, hepatocarcinoma, glioma and neuroblastoma, lymphoma and leukemia (Bizzarri 2013). Studies have shown that Burkitt lymphoma cells, and both acute and myeloid leukaemia undergo apoptosis via activation of caspase-3, an increase in cytochrome c levels, and down regulation of the anti-apoptotic protein Bcl-2 (Trubiani 2005). Whereas not all studies in these cells lines confirm this exact mechanism, studies generally support the apoptotic effects of melatonin.
In solid tumours, many interesting findings have been published. One study published evidence with respect to the apoptotic effect in breast cancer cell lines, showing that not only was p53 upregulated, but also its transcriptional agents, Bax, p21 and p27 (el-Aziz 2005). This effect was evident in both hormone dependent and hormone independent cancers. Studies in highly aggressive pancreatic cell lines show that melatonin had apoptotic effects via stimulation of caspase proteins (Gonzalez 2011, Leja-Szpak 2010). Melatonin exerted apoptotic effects in hormone sensitive and insensitive prostate cancer cells via inhibition of Sirt1, a gene over expressed in many cancers that when inhibited is associated with increased levels of apoptosis (Jung Hynes 2011). The same inhibition of Sirt1 was found in osteosarcoma cell lines (Cheng 2013). Sirt1 activity in cancer cells is associated with silenced tumour suppressor genes and cancer resistance to chemotherapy and ionizing radiation (Gonzalez 2011), and therefore inhibition of same may prove to be a target for gene therapy regimens.
Melatonin and Radiochemotherapy
The basic tenet for the use of radiochemotherapy is that the ionizing radiation or drug will cause sufficient damage to kill cancer cells. However, this damage is extended to non-cancerous cells as well, resulting in often severe side effects. Recent studies have investigated the role of melatonin alongside radiochemotherapeutic regimens. Specifically, studies demonstrate that melatonin reproducibly decreases the toxic side effects of these treatments (Kucuktulu 2012, Mand 2009, Ortiz 2014, Seely 2012).
Radioprotective agents are those that are given prior to radiotherapy to reduce injuries. Rat studies have shown that melatonin given after irradiation provides no radioprotective benefit (Shirazi 2007). It appears that melatonin may need to be administered beforehand to be present inside the cell prior to irradiation (Manda 2009).
An ideal radioprotector should fulfill several criteria: 1) must provide significant protection against the effects of radiation; 2) must have a general protective effect on the majority of organs; 3) must have an acceptable route of administration, preferable orally or intramuscularly; 4) must have an acceptable toxicity profile; 5) must have an acceptable stability profile; and 6) must have compatibility with the wide range of other drugs that will be available to patients (Hosseinimehr 2007).
Vijayalaxmi et al. conducted the first in vivo/in vitro studies showing that melatonin could be used as a cytoprotective agent for human cells exposed to ionizing radiation. In a study of healthy human volunteers, blood was taken before, one hour and two hours after a single dose of 300 mg of melatonin was given orally. Immediately after the blood was taken, it was exposed to 150 cGy of radiation and was cultured. The exposure of the cells to the radiation caused chromosomal aberrations, and lymphocytes collected after melatonin administration exhibited 60-65% reduced incidence of damage, with the best protective effect after two hours (Vijayalaxmi 2002). As the typical dose of melatonin is 20 mg per day, 300 mg seems at first to be quite high. However, the author showed that in humans, a dose of 1 gram daily for 30 days resulted in no observable negative side effects (Vijayalaxmi 2004).
A recent meta-analysis looked at the efficacy of adjuvant melatonin on response rates, survival and reduction of side effects related to radiochemotherapy (Wang 2012). The pooled data showed remission rates of 32.6% for the melatonin group versus 16.5% for the groups without melatonin. Similarly, survival rates were 52.2% for the melatonin group and 28.4% for the groups without melatonin. Studies that reported side effects showed overall rates markedly reduced when melatonin was used as an adjuvant with radiochemotherapy, including thrombocytopenia (2.2% versus 19.7%), neurotoxicity (2.5% versus 15.2%), and fatigue (17.2% versus 49.1%).
Another recent meta-analysis of 19 studies compared the cancer response rates to melatonin plus a chemotherapeutic drug to chemotherapy without melatonin (Seely 2012). The overall result showed a significant benefit on complete response, partial response, and stable disease with the addition of melatonin. Pooled results from trials evaluating mortality showed that the addition of melatonin reduced mortality rates at one year, compared to patients who did not receive melatonin, relative risk (RR) 0.63, 95% confidence interval (CI) 0.53-0.74; p< .001 (Seely 2012). Regarding the results for the effect of adjuvant melatonin on toxicities associated with chemotherapy, positive outcomes were found for the reduction of asthenia, leucopenia, nausea and vomiting, hypotension and thrombocytopenia. Side effects not improved with the addition of melatonin included diarrhea, anemia and alopecia (Seely 2012). Dosages from the trials included in this meta analysis ranged from 10-40mg, with 20mg being most common, given at bedtime (Seely 2012).
Conclusion
As briefly reviewed above, melatonin truly is a great multi-tasker. Its many actions include immune modulator, antioxidant, anti-inflammatory, anti-angiogenic, anti-metastatic, anti-proliferative, and pro-apoptotic. In addition, it has the ability to modify gene expression, reduce side effects of radio chemotherapy, and improve response rates to radio chemotherapy. Whereas the molecular mechanisms reviewed in this article are by no means exhaustive, this article is meant to ignite a level of excitement about an impressive supplement for anyone interested in naturopathic oncology.
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