Biochemical Mechanism of Rhododendrol-Induced Leukoderma

Copper chelation by d-penicillamine alleviates melanocyte death induced by rhododendrol without inhibiting tyrosinase

Oxidative metabolism of rhododendrol (RD), a skin-whitening ingredient, by tyrosinase has caused leukoderma in a certain population of Japanese consumers. Toxic RD metabolites and reactive oxygen species are proposed causes for the melanocyte death. However, the mechanism by which reactive oxygen species are produced during RD metabolism remains elusive. Some phenolic compounds are known to act as suicide substrates for tyrosinase, resulting in release of a copper atom and hydrogen peroxide during its inactivation. We hypothesized that RD may be a suicide substrate for tyrosinase and that the released copper atom may be responsible for the melanocyte death through hydroxyl radical production. In line with this hypothesis, human melanocytes incubated with RD showed an irreversible decrease in tyrosinase activity and underwent cell death. A copper chelator, d-penicillamine, markedly suppressed the RD-dependent cell death without significantly affecting the tyrosinase activity. Peroxide levels in RD-treated cells were not affected by d-penicillamine. Given the unique enzymatic properties of tyrosinase, we conclude that RD acted as a suicide substrate and resulted in release of a copper atom and hydrogen peroxide, which would collectively impair melanocyte viability. These observations further imply that copper chelation may alleviate chemical leukoderma caused by other compounds.

A novel dehydroabietic acid-based multifunctional fluorescent probe for the detection and bioimaging of Cu2+/Zn2+/ClO. Analyst 2023;148:1867-1876. [PMID: 36942689 DOI: 10.1039/d3an00001j]

A multifunctional dehydroabietic acid-based fluorescent probe (CPS) was designed and synthesized by introducing the 2,6-bis(1H-benzo[d]imidazol-2-yl)phenol fluorophore. The probe CPS could selectively recognize Cu²⁺, Zn²⁺ and ClO^- ions from other analytes, and it showed fluorescence quenching behavior toward Cu²⁺ and a ratiometric response to Zn²⁺ and ClO^- by changing from green fluorescence to blue and cyan, respectively. The detection limits toward Cu²⁺, Zn²⁺ and ClO^- ions were 3.8 nM, 0.253 μM and 0.452 μM, respectively. In addition, CPS presented many fascinating merits, such as high selectivity, a short response time (15-20 s), a wide pH range (3-10) and high photostability. The sensing mechanisms of CPS were verified by ¹H-NMR, ESI-MS, FT-IR and Job's plot methods. Meanwhile, CPS exhibited satisfactory detection performance in water samples. More importantly, the probe could be applied as a promising tool for visual bioimaging of three ions in living cells and zebrafishes.

Chemical- and Drug-Induced Allergic, Inflammatory, and Autoimmune Diseases Via Haptenation

Haptens are small molecules that only elicit an immune response when bound to proteins. Haptens initially bind to self-proteins and activate innate immune responses by complex mechanisms via inflammatory cytokines and damage-associated molecular patterns and the subsequent upregulation of costimulatory signals such as cluster of differentiation 86 (CD86) on dendritic cells. Subsequent interactions between CD86 and CD28 on T cells are critically important for properly activating naive T cells and inducing interleukin 2 production, leading to the establishment of adaptive immunity via effector and memory T cells. Accumulating evidence revealed the involvement of haptens in the development of various autoimmune-like diseases such as allergic, inflammatory, and autoimmune diseases including allergic contact dermatitis, atopy, asthma, food allergy, inflammatory bowel diseases, hemolytic anemia, liver injury, leukoderma, and even antitumor immunity. Therefore, the development of in vitro testing alternatives to evaluate in advance whether a substance might lead to the development of these diseases is highly desirable. This review summarizes and discusses recent advances in chemical- and drug-induced allergic, inflammatory, and autoimmune diseases via haptenation and the possible molecular underlying mechanisms, as well as in vitro testing alternatives to evaluate in advance whether a substance might cause the development of these diseases.

Metabolic gene signature in white adipose tissue of oral doses raspberry ketone [4-(4-hydroxyphenyl)-2-butanone] that prevent diet-induced weight gain and induce loss of righting reflex

Raspberry ketone (RK; [4-(4-hydroxyphenyl)-2-butanone]) is a synthetic flavoring agent and dietary supplement for weight control. This study investigated the metabolic signature of oral doses of RK that prevent weight gain or promote loss of righting reflex (LORR) in C57Bl/6J mice. Daily RK 200 mg/kg prevented high-fat diet (HFD; 45% Kcal fat) fed weight gain (∼8% reduction) over 35 days. RNA-seq of inguinal white adipose tissue (WAT) performed in males revealed 12 differentially expressed genes. Apelin (Apln) and potassium voltage-gated channel subfamily C member (Kcnc3) expression were elevated with HFD and normalized with RK dosing, which was confirmed by qPCR. Acute RK 640 mg/kg produced a LORR with a <5 min onset with a >30 min duration. Acute RK 200 mg/kg increased gene expression of Apln, Kcnc3, and nuclear factor erythroid 2-related factor 2 (Nrf2), but reduced acetyl-COA carboxylase (Acc1) and NAD(P)H quinone dehydrogenase 1 (Nqo1) in inguinal WAT. Acute RK 640 mg/kg elevated interleukin 6 (Il 6) and heme oxygenase 1 (Hmox1) expression, but reduced Nrf2 in inguinal and epididymal WAT. Our findings suggest that RK has a dose-dependent metabolic signature in WAT associated with either weight control or LORR.

Upregulation of CD86 and IL-12 by rhododendrol in THP-1 cells cocultured with melanocytes through ROS and ATP

BACKGROUND

The tyrosinase inhibitor rhododendrol (RD), used as a skin whitening agent, reportedly has the potential to induce leukoderma.

OBJECTIVE

Although an immune response toward melanocytes was demonstrated to be involved in leukoderma, the molecular mechanism is not fully understood.

METHODS

We hypothesized that if RD is a pro-hapten and tyrosinase-oxidized RD metabolites are melanocyte-specific sensitizers, the sensitizing process could be reproduced by the human cell line activation test (h-CLAT) cocultured with melanocytes (h-CLATw/M) composed of human DC THP-1 cells and melanoma SK-MEL-37 cells. Cell surface expression, ROS generation and ATP release, mRNA expression, and the effects of several inhibitors were examined.

RESULTS

When RD was added to the h-CLATw/M, the expression of cell-surface CD86 and IL-12 mRNA was greatly enhanced in THP-1 cells compared with those in the h-CLAT. The rapid death of melanoma cells was induced, with ROS generation and ATP release subsequently being greatly enhanced, resulting in the cooperative upregulation of CD86 and IL-12. Consistent with those observations, an ROS inhibitor, ATP receptor P2X7 antagonist, or PERK inhibitor antagonized the upregulation. CD86 upregulation was similarly observed with another leukoderma-inducible tyrosinase inhibitor, raspberry ketone, but not with the leukoderma noninducible skin-whitening agents ascorbic acid and tranexamic acid.

CONCLUSION

RD is a pro-hapten sensitizer dependent on tyrosinase that induces ROS generation and ATP release from melanocytes for CD86 and IL-12 upregulation in DCs, possibly leading to the generation of tyrosinase-specific cytotoxic T lymphocytes. The coculture system h-CLATw/M may be useful for predicting the sensitizing potential to induce leukoderma.

A cell-based evaluation of human tyrosinase-mediated metabolic activation of leukoderma-inducing phenolic compounds

BACKGROUND

Chemical leukoderma is a skin depigmentation disorder induced through contact with certain chemicals, most of which have a p-substituted phenol structure similar to the melanin precursor tyrosine. The tyrosinase-catalyzed oxidation of phenols to highly reactive o-quinone metabolites is a critical step in inducing leukoderma through the production of melanocyte-specific damage and immunological responses.

OBJECTIVE

Our aim was to find an effective method to evaluate the formation of o-quinone by human tyrosinase and subsequent cellular reactions.

METHODS

Human tyrosinase-expressing 293T cells were exposed to various phenolic compounds, after which the reactive o-quinones generated were identified as adducts of cellular thiols. We further examined whether the o-quinone formation induces reductions in cellular GSH or viability.

RESULTS

Among the chemicals tested, all 7 leukoderma-inducing phenols/catechol (rhododendrol, raspberry ketone, monobenzone, 4-tert-butylphenol, 4-tert-butylcatechol, 4-S-cysteaminylphenol and p-cresol) were oxidized to o-quinone metabolites and were detected as adducts of cellular glutathione and cysteine, leading to cellular glutathione reduction, whereas 2-S-cysteaminylphenol and 4-n-butylresorcinol were not. In vitro analysis using a soluble variant of human tyrosinase revealed a similar substrate-specificity. Some leukoderma-inducing phenols exhibited tyrosinase-dependent cytotoxicity in this cell model and in B16BL6 melanoma cells where tyrosinase expression was effectively modulated by siRNA knockdown.

CONCLUSION

We developed a cell-based metabolite analytical method to detect human tyrosinase-catalyzed formation of o-quinone from phenolic compounds by analyzing their thiol-adducts. The detailed analysis of each metabolite was superior in sensitivity and specificity compared to cytotoxicity assays for detecting known leukoderma-inducing phenols, providing an effective strategy for safety evaluation of chemicals.

The delicate relation between melanocytes and skin immunity: A game of hide and seek

Melanocytes exhibit a complex and intriguing relationship with the skin immune response, leading to several clinical conditions. In some disorders, inappropriate melanocyte destruction (e.g., vitiligo, halo naevi) is problematic, while in others, immune tolerance should be broken (melanoma). Important parts of the dysregulated pathways have been unraveled in pigment disorders, ranging from upregulated interferon (IFN)-γ signaling to memory T cells, regulatory T cells, and immune checkpoints. Although a network of many factors is involved, targeting key players such as IFN-γ or checkpoint inhibitors (e.g., programmed death-ligand 1 (PD-L1)] can shift the balance and lead to impressive outcomes. In this review, we focus on the immunological mechanisms of the most common inflammatory disorders where the interaction of the immune system with melanocytes plays a crucial role. This can provide new insights into the current state of melanocyte research.

Jelly Fig (Ficus awkeotsang Makino) Exhibits Antioxidative and Anti-Inflammatory Activities by Regulating Reactive Oxygen Species Production via NFκB Signaling Pathway

Antioxidant and anti-inflammatory activities of Ficus awkeotsang Makino extract (FAE) on Hs68 fibroblasts and BALB/c nude-mouse models are evaluated in this study. FAE was found to be non-toxic and showed high levels of DPPH, H₂O₂, and hydroxyl radical scavenging abilities; a ferrous chelating capacity; as well as ferric-reducing antioxidant capability. The antioxidant activity of FAE was strongly associated with polyphenolic content (flavonoids at 10.3 mg QE g⁻¹ and total phenol at 107.6 mg GAE g⁻¹). The anti-inflammatory activity of FAE and the underlying molecular mechanisms were also investigated. The a* value of the mouse dorsal skin after treatment with FAE at 1.5 mg/mL in addition to chronic UVB exposure was found to decrease by 19.2% during a ten-week period. The anti-inflammatory effect of FAE was evidenced by the decreased accumulation of inflammatory cells and skin thickness. Expression levels of UVB-induced inflammatory proteins, including ROS, NF-κB, iNOS, COX-2, and IL-6, were significantly reduced upon FAE treatment in vitro and in vivo. Collectively, our results suggest that the inhibition of ROS and UVB-induced activation of the NF-κB downstream signaling pathway by FAE, indicating considerable potential as a versatile adjuvant against free radical damage in pharmaceutical applications.

Potential metabolic activities of raspberry ketone

Novel food and food compounds interventions have attracted a lot of attention nowadays for the prevention and treatment of metabolic diseases. Raspberry ketone (RK) is aromatic compound found within red fruits and berries, has been used as an over-the-counter product for weight loss. However, actually, the effect of RK on weight loss is still controversial, and the mechanism is largely unknown. Besides, in vivo and in vitro studies have demonstrated the beneficial effect of RK on the development of other metabolic diseases. In this review, we comprehensively highlighted the synthesis, bioavailability, and metabolism of RK, and summarized the progress made in our understanding of the potential biological activities of RK, including antiobesity, antidiabetes, cardioprotection, and hepatoprotection, as well as their underlying mechanisms. This paper provides a critical overview about the current findings and proposes the future studies in the area of RK on human health. PRACTICAL APPLICATIONS: Raspberry ketone (RK) has been used for weight control for years, but this effect is controversial considering food intake. Additionally, RK is beneficial for T2DM, liver and heart injury. The underlying mechanisms of the protective effect of RK including accelerating fatty acid oxidation, balancing serum glucose level, anti-inflammation, antioxidant process, and so on. In this context, we provide a comprehensive analysis of the benefits of RK against many metabolic diseases and discuss the underlying molecular mechanisms. We hope our work will be helpful for further researches on RK and improve its public recognition.

GPNMB Extracellular Fragment Protects Melanocytes from Oxidative Stress by Inhibiting AKT Phosphorylation Independent of CD44

Glycoprotein non-metastatic melanoma protein B (GPNMB) is a type I transmembrane glycoprotein that plays an important role in cancer metastasis and osteoblast differentiation. In the skin epidermis, GPNMB is mainly expressed in melanocytes and plays a critical role in melanosome formation. In our previous study, GPNMB was also found to be expressed in skin epidermal keratinocytes. In addition, decreased GPNMB expression was observed in the epidermis of lesional skin of patients with vitiligo. However, the exact role of keratinocyte-derived GPNMB and its effect on vitiligo is still unknown. In this study, we demonstrated that GPNMB expression was also decreased in rhododendrol-induced leukoderma, as seen in vitiligo. The extracellular soluble form of GPNMB (sGPNMB) was found to protect melanocytes from cytotoxicity and the impairment of melanogenesis induced by oxidative stress. Furthermore, the effect of rGPNMB was not altered by the knockdown of CD44, which is a well-known receptor of GPNMB, but accompanied by the suppressed phosphorylation of AKT but not ERK, p38, or JNK. In addition, we found that oxidative stress decreased both transcriptional GPNMB expression and sGPNMB protein expression in human keratinocytes. Our results suggest that GPNMB might provide novel insights into the mechanisms related to the pathogenesis of vitiligo and leukoderma.

The Oxidation of Equol by Tyrosinase Produces a Unique Di-ortho-Quinone: Possible Implications for Melanocyte Toxicity

Equol (7-hydroxy-3-(4′-hydroxyphenyl)-chroman, EQ), one of the major intestinally derived metabolites of daidzein, the principal isoflavane found in soybeans and most soy foods, has recently attracted increased interest as a health-beneficial compound for estrogen-dependent diseases. However, based on its structure with two p-substituted phenols, this study aimed to examine whether EQ is a substrate for tyrosinase and whether it produces o-quinone metabolites that are highly cytotoxic to melanocyte. First, the tyrosinase-catalyzed oxidation of EQ was performed, which yielded three EQ-quinones. They were identified after being reduced to their corresponding catechols with NaBH₄ or L-ascorbic acid. The binding of the EQ-quinones to N-acetyl-L-cysteine (NAC), glutathione (GSH), and bovine serum albumin via their cysteine residues was then examined. NAC and GSH afforded two mono-adducts and one di-adduct, which were identified by NMR and MS analysis. It was also found that EQ was oxidized to EQ-di-quinone in cells expressing human tyrosinase. Finally, it was confirmed that the EQ-oligomer, the EQ oxidation product, exerted potent pro-oxidant activity by oxidizing GSH to the oxidized GSSG and concomitantly producing H₂O₂. These results suggest that EQ-quinones could be cytotoxic to melanocytes due to their binding to cellular proteins.

Herb Sanqi-Derived Compound K Alleviates Oxidative Stress in Cultured Human Melanocytes and Improves Oxidative-Stress-Related Leukoderma in Guinea Pigs

Sanqi, a traditional Chinese herb, is widely used for cardiovascular diseases, and its neuroprotective effects against oxidative stress were recently discovered. The purpose of this study was to investigate whether Sanqi-derived compound K (Sanqi-CK), an active metabolite of Sanqi, could protect melanocytes from oxidative stress. Cultured human primary skin epidermal melanocytes (HEMn-MPs) were treated with hydrogen peroxide (H₂O₂) in the presence or absence of Sanqi-CK. Sanqi-CK exhibited protective effects against H₂O₂-induced cell death by reducing oxidative stress. In addition, treatment with Sanqi-CK reversed the decreased glutathione reductase activity and decreased ratio of reduced glutathione (GSH)/oxidized glutathione (GSSG) seen in H₂O₂-treated melanocytes. Furthermore, topical application of Sanqi-CK alleviated leukoderma in guinea pigs, a disorder characterized by melanocyte cell death resulting from rhododendrol-induced oxidative stress. Taken together, these data suggest that Sanqi-CK protects melanocytes against oxidative stress, and its protective effects are associated with modulating the redox balance between GSH and GSSG and activating glutathione reductase. Thus, Sanqi-CK may be a good candidate for preventing melanocyte loss in oxidative-stress-associated pigmentary disorders.

Regioselective Hydroxylation of Rhododendrol by CYP102A1 and Tyrosinase

Rhododendrol (RD) is a naturally occurring phenolic compound found in many plants. Tyrosinase (Ty) converts RD to RD-catechol and subsequently RD-quinone via two-step oxidation reactions, after which RD-melanin forms spontaneously from RD-quinone. RD is cytotoxic in melanocytes and lung cancer cells, but not in keratinocytes and fibroblasts. However, the function of RD metabolites has not been possible to investigate due to the lack of available high purity metabolites. In this study, an enzymatic strategy for RD-catechol production was devised using engineered cytochrome P450 102A1 (CYP102A1) and Ty, and the product was analyzed using high-performance liquid chromatography (HPLC), LC-MS, and NMR spectroscopy. Engineered CYP102A1 regioselectively produced RD-catechol via hydroxylation at the ortho position of RD. Although RD-quinone was subsequently formed by two step oxidation in Ty catalyzed reactions, L-ascorbic acid (LAA) inhibited RD-quinone formation and contributed to regioselective production of RD-catechol. When LAA was present, the productivity of RD-catechol by Ty was 5.3-fold higher than that by engineered CYP102A1. These results indicate that engineered CYP102A1 and Ty can be used as effective biocatalysts to produce hydroxylated products, and Ty is a more cost-effective biocatalyst for industrial applications than engineered CYP102A1.

Oxidative Oligomerization of DBL Catechol, a potential Cytotoxic Compound for Melanocytes, Reveals the Occurrence of Novel Ionic Diels-Alder Type Additions

The exposure of human skin to 4-(4-hydroxyphenyl)-2-butanone (raspberry ketone, RK) is known to cause chemical/occupational leukoderma. RK is a carbonyl derivative of 4-(4-hydroxyphenyl)-2-butanol (rhododendrol), a skin whitening agent that was found to cause leukoderma in skin of many consumers. These two phenolic compounds are oxidized by tyrosinase and the resultant products seem to cause cytotoxicity to melanocytes by producing reactive oxygen species and depleting cellular thiols through o-quinone oxidation products. Therefore, it is important to understand the biochemical mechanism of the oxidative transformation of these compounds. Earlier studies indicate that RK is initially oxidized to RK quinone by tyrosinase and subsequently converted to a side chain desaturated catechol called 3,4-dihydroxybenzalacetone (DBL catechol). In the present study, we report the oxidation chemistry of DBL catechol. Using UV–visible spectroscopic studies and liquid chromatography mass spectrometry, we have examined the reaction of DBL catechol with tyrosinase and sodium periodate. Our results indicate that DBL quinone formed in the reaction is extremely reactive and undergoes facile dimerization and trimerization reactions to produce multiple isomeric products by novel ionic Diels-Alder type condensation reactions. The production of a wide variety of complex quinonoid products from such reactions would be potentially more toxic to cells by causing not only oxidative stress, but also melanotoxicity through exhibiting reactions with cellular macromolecules and thiols.

Chemical Reactivities of ortho-Quinones Produced in Living Organisms: Fate of Quinonoid Products Formed by Tyrosinase and Phenoloxidase Action on Phenols and Catechols

Tyrosinase catalyzes the oxidation of phenols and catechols (o-diphenols) to o-quinones. The reactivities of o-quinones thus generated are responsible for oxidative browning of plant products, sclerotization of insect cuticle, defense reaction in arthropods, tunichrome biochemistry in tunicates, production of mussel glue, and most importantly melanin biosynthesis in all organisms. These reactions also form a set of major reactions that are of nonenzymatic origin in nature. In this review, we summarized the chemical fates of o-quinones. Many of the reactions of o-quinones proceed extremely fast with a half-life of less than a second. As a result, the corresponding quinone production can only be detected through rapid scanning spectrophotometry. Michael-1,6-addition with thiols, intramolecular cyclization reaction with side chain amino groups, and the redox regeneration to original catechol represent some of the fast reactions exhibited by o-quinones, while, nucleophilic addition of carboxyl group, alcoholic group, and water are mostly slow reactions. A variety of catecholamines also exhibit side chain desaturation through tautomeric quinone methide formation. Therefore, quinone methide tautomers also play a pivotal role in the fate of numerous o-quinones. Armed with such wide and dangerous reactivity, o-quinones are capable of modifying the structure of important cellular components especially proteins and DNA and causing severe cytotoxicity and carcinogenic effects. The reactivities of different o-quinones involved in these processes along with special emphasis on mechanism of melanogenesis are discussed.

Synthesis and Studies of the Inhibitory Effect of Hydroxylated Phenylpropanoids and Biphenols Derivatives on Tyrosinase and Laccase Enzymes

The impaired activity of tyrosinase and laccase can provoke serious concerns in the life cycles of mammals, insects and microorganisms. Investigation of inhibitors of these two enzymes may lead to the discovery of whitening agents, medicinal products, anti-browning substances and compounds for controlling harmful insects and bacteria. A small collection of novel reversible tyrosinase and laccase inhibitors with a phenylpropanoid and hydroxylated biphenyl core was prepared using naturally occurring compounds and their activity was measured by spectrophotometric and electrochemical assays. Biosensors based on tyrosinase and laccase enzymes were constructed and used to detect the type of protein-ligand interaction and half maximal inhibitory concentration (IC50). Most of the inhibitors showed an IC50 in a range of 20-423 nM for tyrosinase and 23-2619 nM for laccase. Due to the safety concerns of conventional tyrosinase and laccase inhibitors, the viability of the new compounds was assayed on PC12 cells, four of which showed a viability of roughly 80% at 40 µM. In silico studies on the crystal structure of laccase enzyme identified a hydroxylated biphenyl bearing a prenylated chain as the lead structure, which activated strong and effective interactions at the active site of the enzyme. These data were confirmed by in vivo experiments performed on the insect model Tenebrio molitur.

Rhododenol Activates Melanocytes and Induces Morphological Alteration at Sub-Cytotoxic Levels

Rhododenol (RD), a whitening cosmetic ingredient, was withdrawn from the market due to RD-induced leukoderma (RIL). While many attempts have been made to clarify the mechanism underlying RIL, RIL has not been fully understood yet. Indeed, affected subjects showed uneven skin pigmentation, but the features are different from vitiligo, a skin hypopigmentary disorder, alluding to events more complex than simple melanocyte cytotoxicity. Here, we discovered that rhododenol treatment reduced the number of melanocytes in a pigmented 3D human skin model, Melanoderm™, confirming the melanocyte toxicity of RD. Of note, melanocytes that survived in the RD treated tissues exhibited altered morphology, such as extended dendrites and increased cell sizes. Consistently with this, sub-cytotoxic level of RD increased cell size and elongated dendrites in B16 melanoma cells. Morphological changes of B16 cells were further confirmed in the immunocytochemistry of treated cells for actin and tubulin. Even more provoking, RD up-regulated the expression of tyrosinase and TRP1 in the survived B16 cells. Evaluation of mRNA expression of cytoskeletal proteins suggests that RD altered the cytoskeletal dynamic favoring cell size expansion and melanosome maturation. Collectively, these results suggest that RD not only induces cytotoxicity in melanocytes but also can lead to a profound perturbation of melanocyte integrity even at sub-cytotoxic levels.

Natural and Bioinspired Phenolic Compounds as Tyrosinase Inhibitors for the Treatment of Skin Hyperpigmentation: Recent Advances

One of the most common approaches for control of skin pigmentation involves the inhibition of tyrosinase, a copper-containing enzyme which catalyzes the key steps of melanogenesis. This review focuses on the tyrosinase inhibition properties of a series of natural and synthetic, bioinspired phenolic compounds that have appeared in the literature in the last five years. Both mushroom and human tyrosinase inhibitors have been considered. Among the first class, flavonoids, in particular chalcones, occupy a prominent role as natural inhibitors, followed by hydroxystilbenes (mainly resveratrol derivatives). A series of more complex phenolic compounds from a variety of sources, first of all belonging to the Moraceae family, have also been described as potent tyrosinase inhibitors. As to the synthetic compounds, hydroxycinnamic acids and chalcones again appear as the most exploited scaffolds. Several inhibition mechanisms have been reported for the described inhibitors, pointing to copper chelating and/or hydrophobic moieties as key structural requirements to achieve good inhibition properties. Emerging trends in the search for novel skin depigmenting agents, including the development of assays that could distinguish between inhibitors and potentially toxic substrates of the enzyme as well as of formulations aimed at improving the bioavailability and hence the effectiveness of well-known inhibitors, have also been addressed.

Keratinocyte-derived IL-36γ plays a role in hydroquinone-induced chemical leukoderma through inhibition of melanogenesis in human epidermal melanocytes

Chemical leukoderma is an acquired type of vitiligo that can be initiated by various exogenous chemicals such as hydroquinone (HQ), rhododendrol (RD), or 4-tertiary butyl phenol (4-TBP). Despite the importance of epidermal keratinocytes in diverse dermatological conditions, their toxicological role in chemical leukoderma is poorly understood. To elucidate their role in the pathogenesis of chemical leukoderma, genome-scale transcriptional analysis was performed in human epidermal keratinocytes (HEKs) treated with a sub-cytotoxic HQ concentration (10 µM). The Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway-based functional enrichment analysis of HQ-induced differentially expressed genes (DEGs) revealed that HQ significantly upregulated DEGs related to the IL-17 signaling pathway and significantly downregulated DEGs associated with melanogenesis in HEKs. The meta-analysis between the HQ-induced and cytokine-induced transcriptional data (GSE53751) showed that 58 DEGs were commonly upregulated between HQ- and IL-17A-treated HEKs. Notably, the expression of IL36G was significantly increased in HEKs in response to both HQ and IL-17A. IL-36γ (2 µg/ml) directly inhibits melanin biosynthesis in cultured human epidermal melanocytes (HEMs) and downregulates the gene transcription of key enzymes in the melanogenesis pathway including TYR, DCT, and TYRP1. Moreover, IL-36γ autocrinally regulated keratinocyte function to produce the proinflammatory cytokines IL-36γ, IL-6, and CXCL8/IL-8 in a concentration-dependent manner, suggesting that IL-36γ may stimulate the amplification cycle of cutaneous inflammation. In this regard, hydroquinone-induced IL-36γ from human keratinocytes plays a pivotal role in the development of chemical leukoderma by autocrinally or paracrinally modulating the crosstalk between keratinocytes and melanocytes.

Resveratrol as a Multifunctional Topical Hypopigmenting Agent

Melanin is produced in melanocytes and stored in melanosomes, after which it is transferred to keratinocytes and, thus, determines skin color. Despite its beneficial sun-protective effects, abnormal accumulation of melanin results in esthetic problems. A range of topical hypopigmenting agents have been evaluated for their use in the treatment of pigmentary disorders with varying degrees of success. Hydroquinone (HQ), which competes with tyrosine, is the main ingredient in topical pharmacological agents. However, frequent occurrence of adverse reactions is an important factor that limits its use. Thus, efforts to discover effective topical hypopigmenting agents with less adverse effects continue. Here, we describe the potential of resveratrol to function as an effective hypopigmenting agent based on its mechanism of action. Resveratrol is not only a direct tyrosinase inhibitor but an indirect inhibitor as well. Additionally, it can affect keratinocytes, which regulate the function of melanocytes. Resveratrol regulates the inflammatory process of keratinocytes and protects them from oxidative damage. In this way, it prevents keratinocyte-induced melanocyte stimulation. Furthermore, it has a rescuing effect on the stemness of interfollicular epidermal cells that can repair signs of photoaging in the melasma, a typical pigmentary skin disorder. Overall, resveratrol is a promising potent hypopigmenting agent.