Abstract
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
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
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
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
Inhibition of Oxidative Stress
Meyer has shown that the main components of polyphenolic compounds from
Traditional Chinese Herbal Medicine for Whitening
Ginseng
Scutellaria baicalensis
According to componential analyses, baicalin, wogonoside, baicalein, wogonin, and oroxylin A are the main components of
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
Schisandra chinensis (Turcz.) Baillon
The fruit of
Others
The Chinese herb
The Active Components of Chinese Herbal Medicine and Their Biological Activities.
CBM,
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.
