In vitro and in vivo anti-pigmentation effects of 2-mercaptobenzimidazoles as nanomolar tyrosinase inhibitors on mammalian cells and zebrafish embryos: Preparation of pigment-free zebrafish embryos

Recently, 10 2-mercaptobenzo[d]imidazole (2-MBI) compounds (1-10) were synthesized. Although all 2-MBI compounds are tyrosinase inhibitors that inhibit mushroom tyrosinase at extremely low concentrations (IC50 values: 20-740 nM) and effectively inhibit the browning of apples, to our knowledge, no studies have determined whether 2-MBI compounds inhibit mammalian tyrosinase. Mammalian tyrosinase is different from mushroom tyrosinase in its distribution within the cell and has structural characteristics that are different from mushroom tyrosinase in amino acid sequence and in the presence of a quaternary structure. Thus, the effect of the 10 2-MBI compounds on mammalian tyrosinase activity was investigated in B16F10 cells. Six compounds (1-6) exhibited stronger intracellular tyrosinase inhibition than that of kojic acid and phenylthiourea (PTU), which are known to be the most potent tyrosinase inhibitors; their strong tyrosinase inhibitory activity robustly inhibited intracellular melanin production in B16F10 cells. None of the tested 2-MBI compounds exhibited appreciable cytotoxicity in HaCaT and B16F10 cells. To confirm the anti-melanogenic efficacy of the 2-MBI compounds in vivo, a zebrafish embryo model was used. At concentrations 100 times lower than kojic acid, most 2-MBI compounds demonstrated much stronger depigmentation efficacy than that of kojic acid, and three 2-MBI compounds (2-4) showed depigmentation activity similar to or more potent than that of PTU, resulting in nearly pigment-free zebrafish embryos. These results suggest that 2-MBI compounds may be potential therapeutic agents for hyperpigmentation-related disorders.

8-Methoxybutin inhibits α-MSH induced melanogenesis and proliferation of skin melanoma by suppression of the transactivation activity of microphthalmia-associated transcription factor

Microphthalmia-associated transcription factor (MITF) is highly expressed in melanocytes and is the main regulator of melanogenesis and melanocyte cell fate. Although MITF is important for the differentiation and development of melanocytes, it is also considered an oncogene of skin melanoma. Based on these findings, MITF could be an attractive therapeutic target for skin cancer intervention. This study identified 8-methoxybutin as an inhibitor of MITF and investigated the underlying mechanism. 8-Methoxybutin inhibited α-MSH-induced melanogenesis in murine melanoma cells (B16F10) and skin melanoma proliferation by reducing melanogenic gene expression via blockade of the transactivation activity of MITF. In silico docking analysis and pull-down analysis suggested that 8-methoxybutin binds to the DNA-binding domain of MITF and further inhibits its binding to the E-box in the promoter of target genes, including tyrosinase. In addition, 8-methoxybutin suppressed growth of skin melanoma in a xenograft mouse model. These results indicate that 8-methoxybutin has potential as a therapeutic agent for hyperpigmentation disorder and skin cancer. SIGNIFICANCE STATEMENT: 8-Methoxybutin inhibits MITF transactivation activity resulting suppression of melanogenesis and skin melanoma growth.

In vitro anti-melanogenic effects of chimeric compounds, 2-(substituted benzylidene)-1,3-indanedione derivatives with a β-phenyl-α, β -unsaturated dicarbonyl scaffold

Tyrosinase is considered a key contributor to melanogenesis, and safe, potent tyrosinase inhibitors are needed for medical and cosmetic purposes to treat skin hyperpigmentation and prevent fruit and vegetable browning. According to our accumulated SAR data on tyrosinase inhibitors, the β-phenyl-α,β-unsaturated carbonyl scaffold in either E or Z configurations, can confer potent tyrosinase inhibitory activity. In this study, twelve indanedione derivatives were synthesized as chimeric compounds with a β-phenyl-α,β-unsaturated dicarbonyl scaffold. Two of these derivatives, that is, compounds 2 and 3 (85% and 96% inhibition, respectively), at 50 μM inhibited mushroom tyrosinase markedly more potently than kojic acid (49% inhibition). Docking studies predicted that compounds 2 and 3 both inhibited tyrosinase competitively, and these findings were supported by Lineweaver-Burk plots. In addition, both compounds inhibited tyrosinase activity and reduced melanin contents in B16F10 cells more than kojic acid without perceptible cytotoxicity. These results support the notion that chimeric compounds with the β-phenyl-α,β-unsaturated dicarbonyl scaffold represent promising starting points for the development of potent tyrosinase inhibitors.

Synthesis of cinnamic amide derivatives and their anti-melanogenic effect in α-MSH-stimulated B16F10 melanoma cells

Of the three enzymes that regulate the biosynthesis of melanin, tyrosinase and its related proteins TYRP-1 and TYRP-2, tyrosinase is the most important because of its ability to limit the rate of melanin production in melanocytes. For treating skin pigmentation disorders caused by an excess of melanin, the inhibition of tyrosinase enzyme is by far the most established strategy. Cinnamic acid is a safe natural product with an (E)-β-phenyl-α,β-unsaturated carbonyl motif that we have previously shown to play an important role in high tyrosinase inhibition. Since cinnamic acid is relatively hydrophilic, which hinders its absorption on the skin, fifteen less hydrophilic cinnamic amide derivatives (1-15) were designed as safe and more potent tyrosinase inhibitors and were synthesized through a Horner-Wadsworth-Emmons reaction. The use of conc-HCl and acetic acid for debenzylation of the O-benzyl-protected cinnamic amides 40-54 produced the following three results. 1) Cinnamic amides 43, 48, and 53 with a 2,4-dibenzyloxyphenyl group, irrespective of the amine type of the amides, produced complex compounds with high polarity. 2) Cinnamic amides 40-42, 44, 50-52, and 54 with a benzylamino, or diethylamino group produced the desired debenzylated cinnamic amides 1-3, 5, 10-13, and 15. 3) Cinnamic amides 45-47, and 49 with an anilino moiety provided 3,4-dihydroquinolinones 16-19 through intramolecular Michael addition of the anilide group. Notably, the use of BBr₃ as an alternative debenzylating agent for debenzylation of cinnamic amides 45-49 with the anilino moiety provided our desired cinnamic amides 6-10 without inducing the intramolecular Michael addition. Debenzylation of cinnamic amides 43, 48, and 53 with a 2,4-dibenzyloxyphenyl group was also successfully accomplished using BBr₃ to give 4, 9, and 14. Among the nine compounds that inhibited mushroom tyrosinase more potently at 25 μM than kojic acid, four cinnamic amides 4, 5, 9, and 14 showed 3-fold greater tyrosinase inhibitory activity than kojic acid. The docking simulation using tyrosinase indicated that these four cinnamic amides (-6.2 to -7.9 kcal/mol) bind to the active site of tyrosinase with stronger binding affinity than kojic acid (-5.7 kcal/mol). All four cinnamic amides inhibited melanogenesis and tyrosinase activity more potently than kojic acid in α-MSH-stimulated B16F10 melanoma cells in a dose-dependent manner without cytotoxicity. The strong correlation between tyrosinase activity and melanin content suggests that the anti-melanogenic effect of cinnamic amides is due to tyrosinase inhibitory activity. Considering that the cinnamic amides 4, 9, and 14, which exhibited strong inhibition on mushroom tyrosinase and potent anti-melanogenic effect in B16F10 cells, commonly have a 2,4-dihydroxyphenyl substituent, the 2,4-dihydroxyphenyl substituent appears to be essential for high anti-melanogenesis. These results support the potential of these four cinnamic amides as novel and potent tyrosinase inhibitors for use as therapeutic agents with safe skin-lightening efficiency.

Anti-Melanogenic Effect of Oenothera laciniata Methanol Extract in Melan-a Cells

We evaluated the antioxidant activity and anti-melanogenic effects of Oenothera laciniata methanol extract (OLME) in vitro by using melan-a cells. The total polyphenol and flavonoid content of OLME was 66.3 and 19.0 mg/g, respectively. The electron-donating ability, 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) radical-scavenging activity, and superoxide dismutase (SOD)-like activity of OLME (500 μg/mL) were 94.5%, 95.6%, and 63.6%, respectively. OLME and arbutin treatment at 50 μg/mL significantly decreased melanin content by 35.5% and 14.2%, respectively, compared to control (p < 0.05). OLME and arbutin treatment at 50 μg/mL significantly inhibited intra-cellular tyrosinase activity by 22.6% and 12.6%, respectively, compared to control (p < 0.05). OLME (50 μg/mL) significantly decreased tyrosinase, tyrosinase-related protein-1 (TRP-1), TRP-2, and microphthalmia-associated transcription factor-M (MITF-M) mRNA expression by 57.1%, 67.3%, 99.0%, and 77.0%, respectively, compared to control (p < 0.05). Arbutin (50 μg/mL) significantly decreased tyrosinase, TRP-1, and TRP-2 mRNA expression by 24.2%, 42.9%, and 48.5%, respectively, compared to control (p < 0.05). However, arbutin (50 μg/mL) did not affect MITF-M mRNA expression. Taken together, OLME showed a good antioxidant activity and anti-melanogenic effect in melan-a cells that was superior to that of arbutin, a well-known skin-whitening agent. The potential mechanism underlying the anti-melanogenic effect of OLME was inhibition of tyrosinase activity and down-regulation of tyrosinase, TRP-1, TRP-2, and MITF-M mRNA expression.

Effect of the tyrosinase inhibitor (S)-N-trans-feruloyloctopamine from garlic skin on tyrosinase gene expression and melanine accumulation in melanoma cells

In our searching for novel tyrosinase inhibitors from natural sources, (S)-N-trans-feruloyloctopamine isolated from garlic skin was found to be a potential mushroom tyrosinase inhibitor. Here, we examined the effects of the potential tyrosinase inhibitor in B16F10 cells on intracellular melanin contents, cytotoxicity, and the signaling mechanism involved in the expression of tyrosinase. The results showed the inhibitor displayed little or no cytotoxicity at all concentrations examined and decreased the relative melanin contents in a dose-dependent manner in the α-MSH-stimulated B16F10 cells. Real-time PCR and Western blot analysis showed that it inhibits melanogenesis signaling by down-regulates mRNA and protein expression levels of tyrosinase, which leads to a lower melanin contents. These results suggested that (S)-N-trans-feruloyloctopamine was an ideal tyrosinase inhibitor, and could be used in food and medical industry.