Abstract
Melanin is the chief pigment responsible for the pigmentation of human skin. Increasing evidence indicates that traditional Chinese drugs with skin-whitening effects are attracting the attention of consumers and researchers because they are perceived to be milder, safer, and healthier than synthetic alternatives. This commentary summarizes the current research on Chinese herbal medicines that inhibit melanin and their biological activities. The findings presented in this study suggest that these traditional Chinese herbal medicines might be potential candidates for novel skin-whitening agents.
Melanin is the pigment responsible for the color of human skin and hair. Melanin serves as a double-edged sword that imposes both protective and spot-causing effects on the skin. Melanin plays a critical role in protecting the skin against ultraviolet (UV) damage, but high or uneven melanin production can cause freckles and age spots. 1 Melanogenesis is a critical pathway that regulates skin pigmentation and the development of skin-lightening/whitening drugs or cosmetics. 2 Traditional Chinese herbal medicine extracts are effective at inhibiting skin pigmentation in recent years and are highly efficient, low cost, and have few side effects, indicating they could have broad application prospects leading to large social and economic benefits.
Current Mechanisms for Inhibiting Melanin Production
There are several melanin production mechanisms, as shown in Figure 1. The alpha-melanocyte stimulating hormone (α-MSH) specifically binded to the G protein-coupled melanocortin type Ι receptor (MC1R), which resulted in a stimulation of adenylate cyclase enhancing the concentration of intracellular cyclic adenosine monophosphate (cAMP). 3 Intracellular cAMP-activated protein kinase A (PKA) that phosphorylates Ser-133 residue of the cAMP responsive element-binding protein (CREB). 4 Conversely, the expression of microphthalmia-associated transcription factor (MITF) which was a primary helix loop-helix protein essential for melanocyte development and differentiation was increased by phosphor (p)-CREB. 5 At last, MITF bound to the M box of tyrosinase promoter for the activation of transcription, thus resulting in the promotion of melanin biosynthesis. Phosphorylation of glycogen synthase 3β (GSK-3β) was inhibited by the activation of the Wnt pathway, which resulted in the accumulation of β-catenin. The cumulate β-catenin was carried into the nucleus and shaped a complex with the lymphoid-enhancing factor/T-cell factor transcription factor, after which caused the upregulation of MITF expression. 6 Binding of the stem cell factor to the extracellular domain of c-kit, a tyrosine kinase receptor, prompted dimerization of the receptor, and then a downstream of phosphatidylinositol 3′a-kinase and Ras-mitogen-activated protein kinase (Ras-MAPK) was activated via the Shc and Grb2 adaptor proteins. 7 -9 The c-kit receptor also phosphorylated itself by the MAPK pathway. 10,11 The activation of the tyrosinase enzyme was at the endpoint of this cascade. The production and secretion of endothelin-1 (ET-1) were induced by exposure of the skin to UVB. 12,13 Then the ET-1 stimulated melanocytes located in the vicinity of keratinocytes by binding to endothelin B receptor that activated intracellular signaling cascades chiefly composed of the protein kinase C pathways, which resulted in a synergistic increase of proliferation and melanin production by melanocytes. 14
The mechanism of melanin production. cAMP, cyclic adenosine monophosphate; CREB, cAMP responsive element-binding protein; ET-1, endothelin-1; ETB-R, endothelin B receptor; ERK, extracellular signal-regulated protein kinase; GSK-3β, glycogen synthase 3β; LEF, lymphoid-enhancing factor; α-MSH, alpha-melanocyte-stimulating hormone; MITF, microphthalmia-associated transcription factor; PKA, protein kinase A; PKC, protein kinase C; SCF, stem cell factor; TCF, T-cell factor; TRP, tyrosinase-related protein; TYR, tyrosinase.
Therefore, the key to inhibiting melanin is to prevent the formation and block the release of melanin.
Inhibition of Melanin Synthesis Through Multiple Signaling Pathways
AMP-activated protein kinase/MAPK
Kazinol U has been shown to have antimelanogenesis activity through inhibiting the expression of MITF; inactivating its downstream target genes, tyrosinase, tyrosinase-related protein (TRP)1 and TRP2; and AMP-activated protein kinase and MAPK proteins. 15 The members of the MAPK family, extracellular signal-regulated protein kinase (ERK) and c-Jun N-terminal kinase (JNK), play important roles in regulating melanogenesis. 16 Heracleum moellendorffii Hance (Umbelliferae) 17 and sphingosine-1-phosphate (S1P) 18 in melan-a cells and phytol in B16F10 cells effectively activate the phosphorylation of ERK to decrease MITF expression and, ultimately, melanin content. 19 Inhibition of protein kinase (Akt) and activation of phospho-ERK (pERK) or p38 MAPK may suppress melanogenesis in eupafolin-treated B16F10 mouse melanoma cells. 20 Not only does inhibiting melanin synthesis by regulating cAMP, p38 MAPK, and JNK signaling suppress MITF, but sesamol induces the destruction of tyrosinase via proteasomal and lysosomal degradation in melan-a cells. 21
cAMP responsive element-binding protein
α-MSH is produced by UV radiation in mammals, which stimulates adenylyl cyclase to increase the level of cAMP. Then, cAMP binds to melanocortin receptor 1 (MC1R) in the cell membrane and increases the activity of cAMP-dependent PKA. Then, PKA activates the phosphorylation of cAMP response element-binding protein (CREB) as a regulator, which leads to the expression of the MITF gene. 22,23 Dehydroglyasperin C (DGC) decreases the phosphorylation of CREB and increases cAMP activity in B16F1 melanoma cells, both of which are stimulated by α-MSH. These results support the depigmenting mechanism of DGC in the cAMP-CREB signaling pathway. 22,24 On the contrary, the whitening activity of luteolin may be the result of inhibiting cAMP in the α-MSH signaling pathway of B16 melanoma cells. 25 Studies have shown that oyster hydrolysate possesses antimelanogenesis activity in B16F10 cells through downregulation of the cAMP signaling pathway by reducing the number of active melanocytes and melanin granules. 26 Through inhibition of the PKA/CREB signaling pathway, the ethyl acetate fraction (PN3) of a Phyllostachys nigra stem extract degrades MITF, leading to the suppression of melanogenic enzymes and melanin production. 27
Wnt/β-catenin
β-catenin, which accumulates with the activation of Wnt/β-catenin signaling, is related to melanocyte differentiation and forms a complex with lymphocyte enhancer factor-1 to upregulate the expression of the MITF gene. 28 Moreover, β-catenin directly interacts with the MITF protein itself and then activates MITF-specific target genes. 29 Cardamonin is a chalcone from Alpinia katsumadai Hayata that inhibits pigmentation in melanocytes by suppressing the Wnt/β-catenin signaling pathway. 30 Recombinant Wnt5a adenoviruses infect melan-a cells and then make use of noncanonical Wnt/Ror2 pathway activation to inhibit the canonical Wnt pathway, leading to inhibition of melanin synthesis via downregulation of pigment cell-specific genes in melanocytes. 31
Inhibition of Tyrosinase Activity
The formation of dopaquinone is catalyzed by tyrosinase, which is a precursor of melanin. 32 -34 Therefore, the development of agents that can regulate the enzymatic activity of tyrosinase could have significant value in controlling the melanin content in the skin. 35 Nature has a myriad of sources of tyrosinase inhibitors, such as flavonoids, 36 -38 β-arbutin, 39 chalcones, 40 resveratrol, 41 and others, and natural sources usually attract more attention than chemically synthesized compounds for using cosmetic products. 42
Inhibition of Melanin Transport
Keratinocyte-secreted substances activate melanocytes to promote melanin synthesis, which is catalyzed by tyrosinase, the rate-limiting enzyme, and TRP-1 and TRP-2 in melanosomes. 43,44 Then, mature melanosomes including melanin are transported from the perinuclear area to the tips of melanocyte dendrites. 45 Kinesin, a motor protein, delivers melanosomes on microtubules to the perinuclear region. 46,47 Furthermore, the Rab27a and MyosinVa compound transport melanosomes associated with actin located at the tips of dendrites. 46,48 Finally, melanosomes are combined with surrounding keratinocytes in globules and are scattered throughout the skin. 49 In previous research, Manassantin B has been shown to be an inhibitor of the interaction between MyosinVa and melanophilin, which inhibits melanosome transport and decreases the melanin content when melanocytes are stimulated by α-MSH. 50 Although it suppresses melanosome transfer, niacinamide does not affect tyrosinase activity, melanin synthesis, or the melanocyte number in a monolayer culture system. 51 Ebselen is a nonprotein cell-permeable glutathione peroxidase mimic that seems to be a new depigmenting compound that can inhibit melanin synthesis and melanosome transfer to keratinocytes. 52 In addition to the above findings, melanosomal pH greatly affects melanogenesis. 53 A polymethoxyflavone mixture inhibits melanogenesis and the localization of tyrosinase in melanosomes through acidification of cell organelles, including melanosomes. 54 2-Methyl-naphtho[1,2,3-de]quinolin-8-one inhibits melanosome transport by reducing the expression of the Rab27a, Mlph, and MyoVa genes, and it does not affect melanin synthesis in melanocytes. 55 The above results indicate that obstructing melanosome transport in melanocytes is a valid strategy for skin whitening.
Active Autophagy
Autophagy is an intracellular process by which autophagosomes are formed by sequestering cytosol and organelles in double-membrane-bound structures that later deliver their contents to lysosomes/vacuoles for degradation. 56 -59 Recent studies have shown that autophagy may also be related to the biogenesis of melanin and degradation of melanosomes, suggesting that its activation is involved in skin color by reducing the production of melanin pigments. 60 3-MA, an autophagy inhibitor, also increases tyrosinase protein levels. 61 LED photo modulation at a 585 nm wavelength reduces the melanin content of inhuman epidermal melanocytes (HEMs), via dose-dependent inhibition of melanogenesis and induction of HEM autophagy. 62 It has been shown that β-mangostin from seedcases of Garcinia mangostana control α-MSH-mediated melanogenesis by inhibiting autophagy, which clearly recovers the premelanosome protein and tyrosinase degraded in B16F10 melanoma cells and a 3-dimensional human skin model. 2 Tranexamic acid (TXA) has been frequently used to decrease melanin synthesis in patients with melisma and as a raw material for functional whitening cosmetics. TXA can decrease melanin synthesis in melanoma B16F1 cells via activating the ERK signaling pathway and the autophagy system. 60 3-O-glyceryl-2-O-hexyl ascorbate (VC-HG) suppresses melanogenesis by activating the autophagy system. 63 In Rhizoma arisaematis extract-treated B16F1 cells, autophagy is activated, which inhibits α-MSH-stimulated growth of melanogenesis and downregulates the expression of TRP1 proteins in cells. 64 Shufeng Huoxue Fumula regulates melanin metabolism and enhances tyrosinase activity and melanogenesis through the autophagy pathway to inhibit the proliferation of B16 cells in vitro. 65 According to research that interrupting intracellular melanosome transport by knocking down MyosinVa degrades melanosomes through activating the autophagy system and then reduces the accumulation of melanosomes in cells, but these phenomena are only found in M-KD cells activated by theophylline. 66 Resveratrol is a type of natural phenol, and its antimelanogenesis activity is suppressed by the inhibition of autophagy. 67 3′-Hydroxydaidzein (3′-ODI), as an autophagy inducer, significantly reduces α-MSH-mediated melanogenesis in melanoma cells and melanocytes. Additionally, the inhibition of autophagy notably decreases the antimelanogenic effects of 3′-ODI in α-MSH-stimulated melanoma cells. 68 ARP101, which is a matrix metallopeptidase -2 inhibitor, strongly induces autophagy and autophagy-associated cell death in various cancer cells. 69,70 ARP101 inhibits melanogenesis and suppresses the expression of tyrosinase and TRP1 by regulating autophagy. 71
Inhibition of Oxidative Stress
Meyer has shown that the main components of polyphenolic compounds from Panax ginseng C.A are antioxidants and inhibit melanogenesis. 72 Metallothionein expressed in melanocytes acts as an inducible intracellular antioxidant, 73 -81 and its induction may be an effective method to suppress melanogenesis induced by nitric oxide and other melanogens. 82
Traditional Chinese Herbal Medicine for Whitening
Ginseng
Panax ginseng C. A. Meyer is a very well-known medical herb in Asian countries. Like Kwangmi Kim, extracts, powders, or some constituents of ginseng inhibit melanogenesis in vivo or in vitro.
83
We prefer to concentrate on the updated additions, novelties, and particular mechanisms that lead to hypopigmentation and/or depigmentation. Panax ginseng berry extract (Gb-AuNPs), which produces gold nanoparticles with versatile properties for cosmetic applications, can effectively scavenge and gradually suppress cellular tyrosinase and melanin in α-MSH-stimulated B16 cells.
84,85
Ginsenoside Rh23, Rb2, and Rh6; vina-ginsenoside R13; vina-ginsenoside R4; picrionoside A; 20-O-β-
Scutellaria baicalensis
According to componential analyses, baicalin, wogonoside, baicalein, wogonin, and oroxylin A are the main components of S. baicalensis. The effects of every fraction of antimelanogenesis have been investigated. The results showed that among these 5 flavones, wogonin and wogonoside exhibit high resistance to melanin production in both B16F10 melanoma cells and primary melanocytes. Wogonin clearly inhibits melanin production and evidently lightens the color of skin equivalents, possibly due to the calpain/proteasomal pathway promoting proteolytic degradation of melanophilin. SOX9 is a potential target for the effect of these S. baicalensis flavones. Additionally, wogonin and 2 wogonin analogs, mono-O-methyl flavones, considerably suppress melanosome transport. The structural specificities of the mono-O-methyl group in the flavone A-ring and the aglycone form play important roles in decreasing melanosome. It has been reported that Rab27A, synaptotagmin-like protein (SLP) 2A/synaptotagmin 2, a melanophilin (MLPH)/SLP homolog lacking C2 domains-A, and myosin Va are involved in the regulation of melanosome transport. 96,97 The o-methyl-positioned flavones inhibit melanosome transport through downregulating the level of MLPH. 98 Baicalin decreases MITF protein levels and also decreases the protein level of tyrosinase, which is transcriptionally regulated by MITF. Furthermore, the baicalin-induced hypopigmenting effect is related to the PI3K/Akt signaling pathway. However, in a study by Xiaohong Li et al, baicalin was shown to lead to phosphorylation of ERK and decrease the MITF protein level, tyrosinase activity and melanin level. 99,100
Ssanghwa-tang
Herbal cocktails containing a myriad of phytochemicals simultaneously affect multiple biological and pathological processes via synergistic and reciprocal actions. Appropriately formulated herbal cocktails may act in concert to amplify the therapeutic efficacy of their components while minimizing adverse effects. 101,102 These combined actions are known as pharmacological or pharmaceutical combinatorial effects. Ssanghwa-tang (SHT) is a traditional herbal medicine and has been widely used for years in Korea, China, and Japan. In a study by Aeyung Kim et al, the ability of SHT to inhibit melanin synthesis was evaluated. SHT significantly inhibits cAMP-induced melanin synthesis in B16F10 cells via repression of the PKA and p38 MAPK signaling pathways and subsequently reduces the level of CREB phosphorylation, MITF, and melanogenic enzymes. 103 Some single herbs in SHT have already been shown to suppress melanogenesis through inhibiting tyrosinase activity, but the effective doses are much higher and potentially cytotoxic compared with the dose used in SHT. 104
Ganoderma lucidum
Ganoderma lucidum polysaccharide (GLP), one of the chief active ingredients in G. lucidum, is a chronic traditional Chinese medicine that is widely used in China. 103 GLP can downregulate the expression of genes related to UVB-induced melanogenesis, which inhibits the UVB-activated PKA, MAPK, and cAMP signaling pathways. It has been shown that GLP is capable of inhibiting UVB-induced skin pigmentation in experiments with zebrafish. GLP can greatly relieve erythema reactions in guinea pig skin caused by high-dosage UVB irradiation. 88
Schisandra chinensis (Turcz.) Baillon
The fruit of Schisandra chinensis (Turcz.) Baillon is one of the main traditional Chinese herbal medicines. More than 30 types of lignans have been isolated from S. chinensis fruits, including schisandrin, gomisin A, gomisin N, and others. A previous study suggested that Gomisin N is involved in tyrosinase-derived eumelanin synthesis. Gomisin N inhibits the expression of tyrosinase and MITF in cultured mammalian cell lines and zebrafish embryos. Moreover, gomisin N seems to be more effective than the positive control at inhibiting melanin production. Gomisin N inhibits melanogenesis through the MC1R pathway and is likely to be related to the activation of the PI3K/Akt and MAPK/ERK pathways. 105 Furthermore, gomisin N and 1-O-methyl-fructofuranose have been shown to reduce melanin production by reducing the expression of MITF, tyrosinase, TRP-1, and TRP-2, probably through modulating the PI3k/Akt and MAPK/ERK pathways. 106,107
Others
The Chinese herb Paeonia suffruticosa Andrews, commonly called Cortex Moutan, significantly reduces not only cellular tyrosinase activity but also melanin formation in B16 cells, which may result in the downregulation of the protein levels of MC1R, MITF, and TRP-1. 108,109 Angiopoietin-like protein (ANGPTL) 2 is an inflammatory mediator produced in sun-exposed skin areas that can accelerate pigment production in keratinocytes and melanin-producing cells. Chrysanthemum indicum×Erigeron annuus suppresses ANGPTL 2 expression, thereby inhibiting tyrosinase activity in melanocytes. 110 The yields and components of essential oils extracted from Chrysanthemum boreale MANKINO (CBM) (CBMEOs) are different at each stage, but CBMEOs have antimelanogenic activities in all CBM harvesting stages, resulting in skin-whitening biological activities though phosphorylation of ERK 1/2 and p38 MAPK. 111 Euphorbia supina (ES) is an annual herbaceous plant and is largely used in traditional herbal formulations. ES extract weakens α-MSH-stimulated melanin synthesis by regulating the expression of tyrosinase and MITF. These activities might be due to gallic acid and protocatechuic acid, which have been detected in ES extract. 112 Rab27a is essential for melanosome transport to the dendrite tips in human melanocytes. 113 Sophora flavescens extract and kurarinone have been shown to have a strong inhibitory influence on the Rab27a protein. Compared with vehicle, Rab27a is reported to have significant effects on skin whitening, as determined by a visual assessment by dermatologists and chromameter measurements using a formulation containing 0.05% S. flavescens extract for 8 weeks, implying that it is an effective hypopigmentation agent. 114 Heracleum moellendorffii-treated melan-a cells exhibit increased pERK levels and subsequently decrease the expression of MITF, leading to the inhibition of melanogenic enzymes and melanin. 17 Table 1 summarizes the key properties and activities in relation to the botanical extracts described in this section.
The Active Components of Chinese Herbal Medicine and Their Biological Activities.
CBM, Chrysanthemum boreale MANKINO; CM, Cortex Moutan; ES, Euphorbia supina; CE, Chrysanthemum indicum×Erigeron annuus; GLP, Ganoderma lucidum polysaccharide; SHT, Ssanghwa-tang; MITF, microphthalmia-associated transcription factor; ANGPTL, angiopoietin-like protein; GF1, ginsenoside F1; A-Rh4, aglycones of Rh4; IL-13, interleukin 13; cAMP, cyclic adenosine monophosphate; 1-O-MFF, 1-O-methyl-fructofuranose.
Conclusions
The biological activities of Chinese herbal medicines that are potentially useful for treating skin hyperpigmentation are summarized in this text. The active components of Chinese herbal medicines and their biological activities are provided in Table 1. In recent years, herbal medicines have become an important approach in drug discovery programs for developing potent melanogenesis inhibitors. This method has several advantages, including being milder, safer, and less irritating than traditional methods. However, the skin-whitening effects of a single Chinese herbal medicine with a skin-whitening active ingredient are relatively limited, and they do not meet the needs of the majority of women. The development of new skin-whitening agents should search for more effective compound formulas with multiple orientations, multiple targets, and multiple levels, from reducing melanocyte formation to inhibiting tyrosinase activity to the process of migration to the epidermis to the reduction of melanin formation at the genetic level and others. Different skin-whitening active ingredients produce synergistic effects without a mutual reaction and exert a stronger whitening effect.
Footnotes
Acknowledgments
This work was supported by grants from the Student's Training Program for Innovation and Entrepreneurship of Jilin Province (201811923144) and a project of the innovation plan for graduate students of Beihua University (2018041, 2019021, 2019027), Jilin Province Scientific Technology Project of Traditional Chinese Medicine (2019129).
Declaration of Conflicting Interests
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding
The author(s) received no financial support for the research, authorship, and/or publication of this article.
