Importance of Melatonin in Assisted Reproductive Technology and Ovarian Aging

Abstract

Melatonin is probably produced in all cells but is only secreted by the pineal gland. The pineal secretion of melatonin is determined by the light-dark cycle, and it is only released at night. Melatonin regulates biological rhythms via its receptors located in the suprachiasmatic nuclei of the hypothalamus. Melatonin also has strong antioxidant activities to scavenge free radicals such as reactive oxygen species (ROS). The direct free radical scavenging actions are receptor independent. ROS play an important role in reproductive function including in the ovulatory process. However, excessive ROS can also have an adverse effect on oocytes because of oxidative stress, thereby causing infertility. It is becoming clear that melatonin is located in the ovarian follicular fluid and in the oocytes themselves, which protects these cells from oxidative damage as well as having other beneficial actions in oocyte maturation, fertilization, and embryo development. Trials on humans have investigated the improvement of outcomes of assisted reproductive technology (ART), such as in vitro fertilization and embryo transfer (IVF-ET), by way of administering melatonin to patients suffering from infertility. In addition, clinical research has examined melatonin as an anti-aging molecule via its antioxidative actions, and its relationship with the aging diseases, e.g., Alzheimer's and Parkinson's disease, is also underway. Melatonin may also reduce ovarian aging, which is a major issue in assisted reproductive technology. This review explains the relationship between melatonin and human reproductive function, as well as the clinical applications expected to improve the outcomes of assisted reproductive technology such as IVF, while also discussing possibilities for melatonin in preventing ovarian aging.

Keywords: infertility; melatonin; ovarian aging; oxidative stress; reactive oxygen.

Figure 1

Presumed action of melatonin in ovarian follicle. Melatonin, secreted by pineal gland, is taken up into the follicular fluid from the blood. Reactive oxygen species (ROS) produced within the follicles, especially during the ovulation process, are scavenged by melatonin. Excess amounts of ROS may be involved in oxidative stress of oocyte and granulosa cells. Melatonin reduces the oxidative-stress-induced DNA damage, mitochondrial dysfunction, lipid peroxidation, and apoptosis of granulosa cells, showing that melatonin protects these cells by reducing free radical damage of cellular components including nuclei, mitochondria, and plasma membranes. The balance between ROS and antioxidants (melatonin) within the follicle may be critical for oocyte maturation, meiosis, and luteinization of granulosa cells.

Figure 2

The potential applications of melatonin in human reproduction. Since the application of melatonin has antioxidant effects in reproductive medicine, there are two possibilities. One is that in vivo melatonin administration to patients before ovulation may improve the oocyte quality. Another possibility is melatonin supplementation added to in vitro culture media to enhance oocyte maturation, fertilization, and embryonic development. IVF-ET: in vitro fertilization and embryo transfer, ICSI: intra-cytoplasmic sperm injection.

Figure 3

The reported mechanisms by which melatonin improves oocyte quality. The actions of melatonin are to be expected as a direct antioxidant effect to alleviate reactive oxygen species (ROS) and oxidative stress. Another indirect action of melatonin via cell membrane receptors (MT1, MT2) and nuclear receptor (RORα) also is considered to be very important for oocyte maturation and embryonic development. It is reported that antioxidant enzyme activity in oocytes, the expression of apoptosis-related factors, expression of genes involved in oocyte maturation and embryonic development, and epigenome changes such as DNA methylation and histone acetylation can be regulated by melatonin supplementation. ROS: reactive oxygen species; AC: adenylyl cyclase; PLC: phospholipase C; ATP: adenosine triphosphate; PI3K: phosphatidylinositol-3 kinase; PKC: protein kinase C; MAPK: mitogen-activated protein kinase; ERK: extracellular signal-regulated kinase; SOD: superoxide dismutase; GSH: glutathione; CAT: catalase; GPX: glutathione peroxidase; Casp: caspase; Bcl-2: B-cell lymphoma-2; Bax: Bcl-2-accociated X protein; Bim: Bcl-2 interacting mediator of cell death; PTX3: pentraxin-3; HAS2: hyaluronan synthase 2; EGFR: epidermal growth factor receptors; BMP: bone morphogenic protein; GDF: growth differentiation factor; HSP: heat shock protein; PGR: progesterone receptor.

Figure 4

The possible mechanism of melatonin to prevent ovarian aging. Melatonin is likely to reduce ovarian oxidative stress not only by its direct action as a free radical scavenger but also by its indirect action of enhancing the antioxidant enzyme activity. Melatonin enhances eukaryotic initiation factor 2 (eIF2) signaling, which is essential for translation initiation and protein synthesis in ribosomes, and growth arrest and DNA-damage-inducible 45 (GADD45) signaling, which is involved in DNA repair and checkpoint functions. Melatonin also suppresses autophagy-related protein (light-chain 3a, 3b: LC3a, LC3b) by enhancing intracellular pathways including eIF2, GADD45, and alternative reading frame (ARF) pathways. The mRNA expression of sirtuin longevity genes (SIRT1, SIRT3) and telomere length were also enhanced due to melatonin treatment. Melatonin delays ovarian aging by multiple mechanisms including antioxidant action, DNA repair, maintaining telomeres, SIRT family activity, ribosome function, and autophagy. M: melatonin; O₂^•−: superoxide anion; OH: hydroxyl radical

Figure 5

The anti-aging effects of melatonin on ovaries depends on the age of initiation of melatonin treatment. Melatonin treatment was started from 23 weeks (M23 weeks; melatonin group: M23, control group: C23) or 33 weeks (M33 weeks; melatonin group: M33, control group: C33) of age in mice, and the results of IVF outcomes at 43 weeks of age were analyzed. In the 23-week group, the number of ovulated oocytes (8.5 ± 2.2, C23; 16.8 ± 3.0, M23), fertilization rate (32.3%, C23; 59.5%, M23), and blastocyst rate (17.6%, C23; 39.2%, M23) were all significantly higher in the melatonin group than the control animals, and melatonin was found to have an anti-aging effect on the ovaries. On the other hand, in the 33-week group, the number of ovulated oocytes (9.6 ± 1.8, C33; 9.4 ± 2.7, M33), fertilization rate (30.2%, C33; 37.6%, M33), and blastocyst rate (22.9%, C33; 36.4%, M33) showed no significant difference between the melatonin treated and control animals.