Protective Role Of Melatonin And Its Metabolites in Skin Aging Part 2

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3. Melatonin and Aging

3.1.An Overview of the Synthesis, Metabolism, and Function of Melatonin

The phylogenetically ancient molecule melatonin (N-acetyl-5-methoxytryptamine) is widely distributed in nature [130-132] and can be formed almost in all living organisms, including plants [133-136]. Melatonin was first isolated and identified in the bovine pineal gland by the dermatologist Aaron Lerner et al. in 1958 [137]. Lerner, together with his colleagues, was also the first to identify melatonin's chemical structure and its action as a lightening agent in melanophores counteracting the α-melanocyte-stimulating hormone (α-MSH)[138]. Historically, in mammals, this indolamine was thought to be uniquely released by the pineal gland, playing a major role in the regulation of circadian day-night rhythms and seasonal biorhythms [33,139]. Pineal-released melatonin can be measured at lower concentrations in the blood than in the cerebrospinal fluid (CSF) of the third ventricle of the brain, suggesting its role as a protector of the brain against oxidative stress[140,141]. Later, extraspinal sites of melatonin production were established. Thus, melatonin is also synthesized in numerous peripheral tissues such as the bone marrow, retina, lens, cochlea, lungs, liver, kidney, pancreas, thyroid gland, female reproductive organs, and finally the skin [14,15,22,142-146]. Indeed, the synthesis of melatonin is a multistep process that first starts with the hydroxylation of L-tryptophan to 5-hydroxy-tryptophan (5(OH)tryptophan, catalyzed by tryptophan hydroxylase [147-149]. Further 5(OH)tryptophan is decarboxylated to serotonin, which is subsequently transformed to N-acetylserotonin (NAS)by the enzyme arylalkylamine N-acetyltransferase(AANAT) [150,151]. Furthermore, it has been found that serotonin can be acetylated to NAS by alternative enzymes including arylamine N-acetyltransferase [152-156]. cistanche penis growth The last step in the synthesis is a conversion of NAS to melatonin by hydroxy indole-O-methyl transferase (HIOMT) [157].

The levels of melatonin are regulated by its rapid metabolism in the liver or directly at the site of its synthesis in peripheral organs [158]. In the classical hepatic metabolism, CYP450 enzymes (CYP1A1, CYP1A2, and CYP1B1)degrade circulating melatonin to 6-OH-melatonin [159,160]. Melatonin can also be demethylated in the liver to NAS by CYP2C19 or CYP1A, which represents a minor microsomal pathway [161,162]. Through the alternative indolic pathway, melatonin is deacetylated by liver aryl acrylamides to 5-OH-tryptamine, which is further deaminated by monoamine oxidase A [163]. The metabolism of melatonin through the kynurenic pathway begins with the formation of N'-acetyl-N--formyl-5-methoxykynuramine(AFMK) in a peroxidase-like reaction. Further AFMKis deformylated to N'-acetyl-5-methoxykynuramine(AMK)[164,165]. In mitochondria, an additional route of melatonin metabolism to AFMK by cytochrome Coxidation has also been described [166]. In the skin or skin cells, melatonin is metabolized rapidly through its 6-hydroxylation, through the indolic and kynurenic pathway, and through non-enzymatic processes including phototransformation induced by UVB, UVA, and reactive oxygen species [167-169]. The main products of melatonin metabolism in the epidermis are 6-hydroxymelatonin, AFMK, AMK, 5-methoxytryptamine, 5-methoxytryptophol, and 2-hydroxymelatonin. These products accumulate in the epidermis at detectable concentrations [170,171].

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The widespread melatonin distribution during evolution has made it as a vital multifunctional hormone, with remarkable essential functions [34,172]. The complex action of melatonin includes its work as a regulator of the circadian clock, a neurotransmitter and hormone, a metabolic modulator, and a modifier of cell response and cytokine release [173-177]. It also regulates the functions of many peripheral organs [174,178] and exerts oncostatin [179-184] and anti-aging capacity [48,185]. Many regulatory effects of melatonin on cardiovascular, endocrine, reproductive, and immune systems are mediated via specific melatonin 1 (MT1) and MT2 membrane receptors [19,186]. Melatonin, by interacting with MT1 and MT2, has been found to limit weight gain [176,187,188]. Melatonin can inhibit adipogenic differentiation and together with vitamin D, exhibits a negative regulation of adipogenesis in adipose-derived stem cells (ADSCs). It was recently found that melatonin significantly inhibited the transcription of specific adipogenesis-orchestrating genes， such as aP2 and peroxisome proliferator-activated receptor γ（PPAR-7）， as well as adipocyte-specific genes including lipoprotein lipase(LPL) and acyl-CoA thioesterase 2(ACOT2). Moreover, melatonin and vitamin D can modulate ADSCs through the up-regulation of epigenetic regulatory genes like histone deacetylase 1 (HDAC1), SIRT1, and SIRT2 [189].

Melatonin can also inhibit the effects of estrogens [190] and exhibits cardioprotective [191,192] and anticonvulsant activity [193]. cistanche salsa benefits MT1 and MT2 are also important for the protection of the skin against environmental stressors, aging, and cancerogenesis [179,194]. Moreover, often the melatonin level inversely correlates with an increased risk of cancer development. Of note, the blockage of melatonin receptors can impair the p53-dependent DNA damage response [195]. The antioxidant ability of melatonin relays the indirect receptor-mediated action, likely by the stimulation of antioxidant enzymes, SIRT3, and others [43,196]. Melatonin works also through non-receptor mediated mechanisms such as the direct scavenging of a variety of reactive species (both ROS and RNS)to counteract oxidative stress[39,41,130,197-199]. In addition to its high antioxidant potential, receptor-independently, melatonin serves as a mitochondrial protector [200] and anti-inflammatory agent [201]. Some of the protective properties of melatonin are shared with its kynurenic metabolites AFMK and AMK [178,202,203].

3.2. Protective Role of Melatonin in Systemic Aging

The"free radical theory of aging" has been discussed for over 50 years [204-206]. At the subcellular level, mitochondria are the major source for the generation of highly reactive and destructive species like peroxynitrite and the hydroxyl radical [207]. Their excessive production, resulting in enhanced mitochondrial oxidative stress and mtDNA mutations, occurs along with human aging and age-related pathologies [208-210]. Some intracellular enzymes outside the mitochondria (e.g., xanthine oxidase, monoamine oxidase, NADPH oxidases) also impact ROS production with advancing age [211-213]. Disturbances in mitochondrial redox balance promote cellular senescence and thus the mitochondria impairment determines the rate of aging [214]. Recently, it has been thought that most mtDNA mutations are caused by replication errors of mtDNA polymerase [215]. During aging, such defects in mtDNA replication machinery together with a failure of their repair might cause an accumulation of mutations with further mitochondrial dysfunction and augmentation of oxidative damage.

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Since free radicals abundantly are generated in mitochondria in aging, molecules that reduce their mitochondrial production or detoxify them may slow the rate of systemic aging. Melatonin is such a molecule, and its role in aging has been on the focus of many scientists in the last 20 years [42,216-218]. It was found that surgical pinealectomy of young rats resulted after time in accelerated oxidative damage in multiple tissues due to circadian disruption, and melatonin-deficient animals aged more rapidly [219].

While dysfunctional mitochondria contribute to the aging process [220], melatonin can maintain optimal mitochondrial physiology [42,221,222]. Melatonin concentrations are found at higher levels in mitochondria than in other cellular organelles, suggesting its significant role as a mitochondrial-targeted molecule involved in mitochondrial processes [42,200]. cistanche tubulosa dosage reddit The multiple beneficial protective actions of this indolic hormone at the mitochondrial level are well documented [223]. Melatonin can limit age-related oxidative stress directly by scavenging ROS/RNS [41,224] and by indirect activation of mitochondria-located superoxide dismutase (SOD2) [225]. Through the stimulation of mitochondria's localized SIRT3, melatonin prompts the deacetylation and activation of SOD2. The activation of antioxidant enzymes involved in the SIRT3/SOD2 signaling path-way by melatonin reduces mitochondrial oxidative damage and cytochrome C release, thus reducing mitochondria-related apoptosis [196,226]. Indeed, melatonin maintains the optimal mitochondrial membrane potential and preserves mitochondrial function not only by quenching free radicals[198] but also by inhibiting the mitochondrial permeability transition pore(MPTP)[227], activating uncoupling proteins (UCPs), and regulating mitochondrial biogenesis and dynamics [228].

Generally, melatonin may act as both pro-and anti-inflammatory molecules in a context-dependent fashion [201,229,230]. In aging, melatonin preferentially exerts anti-inflammatory actions on aging-related low-grade inflammation. Melatonin stimulates SIRT1, and their anti-inflammatory activities overlap during the process of aging [231]. SIRT1, functioning as an epigenetic aging regulator, alleviates the inflammation by down-regulating TLR4, which mediates pro-oxidant effects through the NF-kB signaling pathway [229]. Melatonin, by inhibition of either TLR-4 or the toll receptor-associated activator of interferon (TRIF), can suppress the release of several pro-inflammatory cytokines like TNFα, IL-1, IL-6, and IL-8 [232,233].

To summarize, melatonin, with its capacity to mitigate oxidative stress, protect mitochondrial functions, modulate the immune system, reduce inflammation, enhance circadian rhythm amplitudes, and exhibit neuroprotection, beneficially results in retarding the process of aging [174,216,234-240].

4. Melatonin, Its Metabolites and Skin Aging 4.1.Overview of Cutaneous Melatoninergic System

Melatonin is synthesized and metabolized in the skin. The ability of the mammalian skin to synthesize melatonin from serotonin through NAS was first published in 1996 [241]. Follow-up studies have provided evidence that human skin, as well as normal keratinocytes, melanocytes, and melanoma cells, can endogenously produce melatonin [13-15,22,242]. Moreover, the skin cells express the essential enzymes for transforming tryptophan to serotonin and eventually to melatonin, like tryptophan hydroxylase(TPH1—-all skin cells; TPH2—melanocytes and dermal fibroblasts)[13,14,23,243], AANAT/serotonin N-acetyltransferase (SNAT) and NAT [154,155],and HIOMT/N-acetylserotonin-methyltransferase (NASM) [13,14]. Cutaneous serotonin can be acetylated to NAS by both AANAT and NAT [13,152,156]. Hair follicles also generate melatonin and express its functional receptors [244]. Recently, the concentrations of melatonin and its metabolites in the human epidermis were quantified by liquid chromatography-mass spectrometry (LC-MS)[170,171]. cistanche แอ ม เว ย์ The level of epidermal melatonin varies depending on race, gender, and age. Kim et al.measured the highest concentrations of melatonin among African Americans and elderly Caucasians. The levels of its kynurenic metabolite AFMK were significantly higher in Caucasian males, whereas AMK demonstrated a higher concentration in African Americans than in Caucasians [171]. The accumulation of AMK in the epidermis suggests the cutaneous transformation of AFMK to AMK.

Melatonin in the skin undergoes rapid metabolism in vivo through either the indolic and kynurenic pathways, with 6-hydroxymelatonin being a major metabolite [168,169]. Indeed, all metabolites of melatonin, including the final kynurenic metabolites AFMK and AMK, are present in the epidermal cells and can potentially affect their mitochondrial functions [35,245]. Exposure of human skin to UVB can induce melatonin metabolism, leading to the generation of antioxidant metabolites AFMK and AMK in human keratinocytes [167,169]. The photo-induced melatonin metabolites further form a very potent anti-oxidative cascade. This cascade has been defined as the melatoninergic anti-oxidative system (MAS) of the skin [13,167]. Melatonin and its metabolites are essential for the regulation of many skin functions, including cutaneous pigmentary [13,246], adnexal [244,247,248], barrier [23,40,168], and immune [173| functions. They also protect the skin against external and internal insults (Figure 2) and possess an oncostatin potential in melanoma cells [180,249]. Unlike melatonin, AMK does not inhibit tyrosinase activity and has no significant effect on melanogenesis [170]. Some but not all the phenotypic effects of melatonin are mediated via interaction with membrane-bound G-protein-couple MT1 and MT2 receptors. MT1 has widespread localization, mainly in the epidermis (stratum granulosum, stratum spinosum, upper and inner root sheath of hair follicles)[19,22], whereas MT2 is often found in hair follicles and blood vessels, with lower expression or absence in the epidermal cells [13,244]. The expression of MT2 in hair follicles makes them a possible target for hair growth regulation by melatonin [248]." MT3 receptors" have been also detected in keratinocytes, melanocytes, and fibroblasts; however, their role requires clarification [179]. Nuclear retinoic orphan receptor α(Rora) has been found to be expressed in skin cells but it is not a receptor for melatonin, being identified as a receptor for sterols and secosteroids [250,251]. Melatonin regulation of mitochondrial functions is predominantly receptor-independent and requires high concentrations which can be achieved by efficient on-site production and/or topical melatonin application.

4.2. Role of Melatonin and Its Metabolites in Attenuation of Photoaging

Although skin has a well-equipped powerful antioxidant system to counteract oxidative stress, chronic exposure to UVR with its excessive ROS production can overcome the endogenous antioxidant defense of the skin, resulting in damage and premature aging in a process known as photoaging. Melatonin is one of the protective molecules biosynthesized at high concentrations in mitochondria of the skin cells to incapacitate ROS by electron donation and RNS by nitrosylation reactions [199,252,253]. Melatonin can prevent the formation of highly reactive free radicals by reducing the superoxide anion radical(O,·)in a process referred to as radical avoidance [228,254]. The positional advantage of melatonin increases its ability to immediately scavenge the toxic free radicals formed in abundance in mitochondria, mainly by UVA but also by UVB irradiation [198,245]. Melatonin may additionally stimulate enzymes that are able to degrade the weakly reactive ROS [130,255]It is important to note that the most harmful species(hydroxyl radicals and peroxynitrite) are not degraded by enzymes. They can only be removed by a direct highly efficient scavenger like melatonin [256-258]. The reaction of melatonin with hydroxyl radical initiates the formation of 2-OH-melatonin and 4-OH-melatonin, which are further metabolized to AFMK and by arylamine formamidase or catalase to AMK[196,202]. The effective toxic radical scavenging mediates the reduction of ROS-generated oxidative stress.

In normal and diabetic human dermal fibroblasts, melatonin can stimulate SOD, catalase (CAT),and glutathione peroxidase(GPx),and promote glutathione(GSH) produc-tion [259]. Indeed, through activation of MT1/MT2, melatonin up-regulates the expression of antioxidant genes in irradiated cells[43,245,260].

The molecular mechanism of the indirect antioxidant action of melatonin with regard to the activation of phase-2 antioxidant enzymes has recently been established in UV-exposed human keratinocytes [254] and UVB-treated melanocytes [194]. It was found that melatonin stimulated NRF2 expression and induced its translocation to the nucleus, leading to enhanced gene expression of its target enzymes including Y-glutamylcysteine synthetase (y-GCS), heme oxygenase-1 (HO-1), and NADPHquinone dehydrogenase-1 (NQO1) [254]. The up-regulation by the melatonin/NRF2-dependent pathway supports the elevated antioxidant response of both keratinocytes and melanocytes against UVB-induced oxidative stress.[37,194].Moreover, Nrf2 activation protects scalp hair growth against oxidative damage [261]. how much cistanche to take The ability of melatonin to attenuate UVA/UVB-induced alterations and prevent further photodamage has also been demonstrated in fibroblasts (Figure 3)[262,263]. In addition, it was found that melatonin can reduce the number of 8-hydroxy-2'-deoxyguanosine (8-OHdG)-positive cells, a marker of oxidative DNA damage [23,260]. Thus, by being a broad-spectrum antioxidant and amphiphilic molecule, melatonin can penetrate membranes and can also attenuate UVR-induced lipid peroxidation, protein oxidation, and mitochondrial and DNA oxidative damage [23,35,37,41, A47,264]. The other protective capability of melatonin is to counteract UVR-induced alterations in the mitochondrial ATP synthesis, plasma membrane potential, and pH in human keratinocytes [46,254,265.

Importantly, melatonin possesses an advantage when compared with other antioxidants, since melatonin exerts not only a potent antioxidant capacity but most of its metabolites are antioxidants as well [168,202]. Whereas classical antioxidants (vitamins C and E)scavenge a single radical, melatonin's antioxidant cascade detoxifies many toxic radicals. Moreover, accumulating evidence supports the reciprocal interaction between melatonin and NAS in mitochondria that would amplify the detoxification process [169,178,245]. In addition, melatonin activates cytochrome Cin mitochondria [159], which possibly mediates the formation of final kynurenic metabolites, which are even better free radical scavengers than melatonin itself [202,203,266]. AFMK and AMK generated non-enzymatically can accumulate in the skin [243]. However, AMK can disappear very quickly through oxidation and interactions with RNS [169].

Melatonin and its derivatives (6-hydroxymelatonin, NAS, AFMK, AMK, and 5-methoxytryptamine) have the capacity to protect keratinocytes and melanocytes against UVB-induced cell damage [23,37,194]. They not only reduce the formation of CPDs and 6-4 pyrimidine-pyrimidone photo products but also induce the repair of DNA damaged by UVB. It has been demonstrated that the topical application of melatonin and AFMK can prevent DNA damage and apoptosis in human and porcine skin ex vivo[47]. Furthermore, the pre-incubation of full-thickness skin and normal human keratinocytes with melatonin suppressed the UVB-mediated inflammatory and apoptotic effect, as measured by heat shock protein 70expression, expression of pro-inflammatory cytokines (IL-1β, I-6), and the pro-apoptotic protein caspase-3[267]. The photoprotective potential of topically administrated melatonin has been shown in many clinical studies. Thus, treatment of the skin with exogenous melatonin before and after sun exposure attenuates UVR-induced erythema and oxidative stress[268]. The effect is greater when the cutaneous application of melatonin cream occurs prior to UVB exposure [269]. Sunscreens supplemented with melatonin could be used to prevent skin photoaging and photocarcinogenesis [270]. One potential anti-wrinkle mechanism of melatonin was studied by Sung-Hoon Kim's group [44]. They found that melatonin, by reducing ROS production, diminished MMP-1 expression and increased collagen XVII expression in HaCaT keratinocytes exposed to UVB. Furthermore, in the same study melatonin was shown to reduce the transepidermal water loss(TEWL) on the skin of hairless mice8 weeks after UVB irradiation [44]. A clinical study also demonstrated a significant reduction of facial redness and wrinkles, and an improvement in the epidermal barrier function by using a night serum combination of melatonin, vitamin C (lipophilic and non-oxidizable form), and a polyphenol compound (bakuchiol)with retinol-like properties[271]. Additionally, the same night serum containing melatonin has been shown in vitro to increase filaggrin levels in keratinocytes, and collagen I and III in dermal fibroblasts, as well as to reduce the formation of apoptotic sunburn cells in UV-exposed skin ex vivo.[272]. The above findings confirm the clinical potential of melatonin as a broad-range photo-protector which can have a great impact on the attenuation of premature skin aging and the improvement of the hallmarks of photoaged skin [147,274].

4.3.Role of Melatonin and Its Metabolites in the Attenuation of Pollution-Induced Skin Aging

Environmental air pollutants promote mitochondrial dysfunction and oxidative damage due to excessive ROS generation, potentially resulting in prematurely aged skin and skin cancer [107,108]. Melatonin can restore mitochondrial function and maintain mitochondrial homeostasis [275]. It can reach the mitochondria by crossing the cell membranes, and it can also be synthesized in the mitochondrion. High concentrations of melatonin in mitochondria (endogenously produced or exogenously applied) can reduce oxidative damage, preserve mitochondrial respiration, limit mitochondria-related apoptosis, increase mitochondrial membrane potential and ATP production, and regulate mitochondrial biogenesis and mitophagy (removal of the damaged mitochondria). It has been proposed that SIRT1, which can be stimulated by melatonin as well, plays a crucial role against pollutant-related premature skin aging. The up-regulation of SIRT1 could downregulate the MMP-1 and MMP-3 involved in the collagen breakdown, and it could decrease inflammation through inhibition of NF-kβ signaling[127].

The use of creams containing melatonin, carnosine, and Helichrysum italicum extract on skin explants exposed to a mixture of polycyclic aromatic hydrocarbons and heavy metals leads to a reduction in skin damage and irritation [276]. The study demonstrated a significant decrease in pollution-activated transcription factor aryl hydrocarbon receptors (AhR) and type I collagen in melatonin-treated explants.

Therefore, a skin care product containing melatonin would be a real"weapon" in the prevention of premature skin aging caused by urban pollutants, heavy metals, and cigarette smoke [277].

4.4.Possible Role of Melatonin in Modifying Natural Process of Skin Aging

The healthy aging of the skin is a complex multifactorial process that can be aggravated by an oxidative environment. With advancing age, the capacity of the skin to produce melatonin, the main direct- and indirect-acting antioxidant, diminishes, thus contributing to a decline in the endogenous protective MAS. The decreased levels of melatonin with age are accompanied by dysregulation of the circadian rhythm. Additionally, an age-dependent decrease in MTL receptors is found in aged human fibroblasts [278]. The reduction in MT1 receptors along with a reduced melatonin level results in enhanced skin cellular damage and phenotypical signs of aging.

Therefore, the administration of exogenous melatonin would be a good anti-aging strategy. Orally supplemented melatonin appears in rather low levels in the blood due to prominent first-pass degradation in the liver, thus limiting skin access [14]. Topically applied melatonin may penetrate the stratum corneum and form a depot there due to its distinct lipophilic chemical structure [279]. The application of melatonin on the skin is a very good option for retarding the aging process and reducing the hallmarks of skin aging. The cutaneous application of melatonin is an efficacious and safe way to improve the clinical signs of aging (wrinkles, TEWL, hydration, skin roughness, sagging, etc.)[186]. Clinically, it is better to apply melatonin at nighttime when the skin permeability is higher because melatonin can mimic its endogenous production and effects.

With its pleiotropic protective function of the skin, melatonin, with its proven beneficial anti-aging properties, could be considered as a therapeutic candidate for retarding skin aging and reversing cutaneous aging signs. Therefore, endogenous intracutaneous melatonin production, together with topically applied exogenous melatonin, is expected to provide the most potent defense system against cutaneous photodamage and multiple other pathological conditions that produce oxidative stress(e.g., in chronic skin inflammation, such as atopic dermatitis)[280]. Additionally, topical melatonin can be used for the treatment of androgenic alopecia in women [281].

5. Conclusions and Perspectives

Since the discovery of the strong antioxidant properties that melatonin possesses [137], massive interest in terms of the biological effects of melatonin in human and animal biology has evolved. It was shown that this indoleamine is an important bioregulator as well as a pluripotent and essential protective agent in many cells, tissues, and compartments of unicells, animals, and humans [22,216,282]. Melatonin exerts protective effects on cell physiology and tissue homeostasis, particularly in cutaneous cells exposed to UVR. which induces severe skin damage accompanied by oxidative stress or DNA damage. These intracellular disturbances are significantly counteracted or modulated by melatonin in the context of a complex intracutaneous melatoninergic anti-oxidative system with UVR-enhanced or UVR-independent melatonin metabolites. Therefore, endogenous intra-cutaneous melatonin production, together with topically applied exogenous melatonin or its metabolites, may be expected to represent a promising anti-oxidative defense system against skin aging. Indeed, more research on appropriate in vitro, ex vivo, and in vivo models must be performed to substantiate the above idea. For example, we need to learn whether melatonin and its derivatives can affect the expression of senescence markers in the skin. It would be fascinating to explore the possibility of whether cutaneous melatonin production is altered during skin aging. Moreover, it is crucial to know whether the expression of functional MTs in cutaneous cell types is impaired in aged skin, which could eventually limit the anti-aging effects of any topically applied type of melatonin. In summary, the key question is whether melatonin can be exploited therapeutically as a protective agent, as "a skin survival factor" with anti-genotoxic capacities, or as"the neutralizer" of pathological changes including skin aging and cancerogenesis. The efficacy of topically applied melatonin and its derivatives needs further evaluation in future clinical trials. Another important point that needs further investigation is the use of nanotechnologies and nanomaterials for the topical delivery of melatonin and its metabolites for skin rejuvenation or to preserve the young skin phenotype.

This article is extracted from Int. J. Mol. Sci. 2022, 23, 1238. https://doi.org/10.3390/ijms23031238 https://www.mdpi.com/journal/ijms