Research Article | Open Access
Loratadine, an H1 Antihistamine, Inhibits Melanogenesis in Human Melanocytes
It has long been believed that histamine is associated with cutaneous melanogenesis. Specifically, H2-receptor antagonists reportedly inhibit melanogenesis, but H1-receptor antagonists, which are some of the most commonly prescribed medicines in dermatology, have not been studied to determine whether and how they regulate melanogenesis. Therefore, we screened H1-receptor antagonists to determine whether they inhibit melanogenesis and found that loratadine was particularly effective, in this regard without compromising cellular viability. Loratadine downregulated microphthalmia-associated transcription factor (MITF) and tyrosinase in melanocytes. To determine the intracellular signaling pathways, Akt was consistently activated by loratadine. PI3K/Akt pathway inhibitor, LY294002, restored the reduced melanin content that was induced by loratadine. In addition, phospho-GSK-3β also was found to be increased following loratadine treatment. Loratadine reduced the amount of PKC-βII in the membrane fraction, thereby decreasing its activity. Taken together, our data indicate that loratadine regulates melanogenesis via Akt/MITF and PKC-βII signaling, thereby leading to the inhibition of melanogenic proteins. The antimelanogenic effects of loratadine have potentially significant and useful roles in dermatologic practice, although further clinical studies will be required to test this.
Hyperpigmentary skin disorders are a major issue of concern for people with dark skin. Until now, clinicians have attempted to treat hyperpigmentary disorders using various therapeutic modalities, including hypopigmenting agents, but the results have been discouraging.
Diverse types of immune cells have been thought to play an important role in melanogenesis. Various inflammatory dermatoses can induce postinflammatory hyperpigmentation by affecting the morphologic change and functional activity of melanocyte. We recently reported the inflammatory features of melasma in Korean women and that mast cells are frequently found in the dermis of melasma lesions . In addition, we proposed the association between increased dermal mast cells and clinical features of hyperpigmentation and telangiectasia in photodamaging process . Therefore, inflammatory mediators such as histamine, prostaglandins, and nitric oxide have been identified as possible therapeutic approaches for hyperpigmentary skin disorders .
Histamine is a ubiquitously distributed inflammatory mediator that is released from tissue mast cells and blood basophils, thereby inducing various inflammatory skin disorders. Since the cutaneous actions of histamine as a melanogen were first reported , there have been studies on the role of histamine in skin hyperpigmentation [3, 5, 6]. Histamine-induced morphologic changes in human melanocytes and increased tyrosinase activity, resulting in increased melanogenesis and H2 antihistamines specifically, suppressed the stimulatory effects of histamine [5, 6]. However, the involvement of histamine in melanogenesis via other receptors such as H1, H3, and H4 receptors remains unclear. Especially considering common use of H1 antihistamines in dermatologic practice, it is remarkable that little has been reported about the effects of H1 antihistamines on melanogenesis. Therefore, we tried to focus for defining the effects of H1 antihistamines on melanogenesis.
2. Materials and Methods
Loratadine, 3,4-dihydroxy-L-phenylalanine (L-DOPA), cholera toxin (CT), 12-O-tetradecanoylphorbol-13-acetate (TPA), and antibodies specific to β-actin were purchased from Sigma-Aldrich Co. (St. Louis, MO, USA). Antibody specific for Akt (#2920S), phosphor-Akt (ser473, #4058S), β-catenin (#9562L), GSK-3β (27C10, #9315), phospho-GSK-3β (Ser9, #9336), p44/42 MAPK (Erk1/2) (#9102S), and p44/42 MAPK (Erk1/2) (Thr202/Tyr204, #9101S) were purchased from Cell Signaling Technology (Beverly, MA, USA). Antibodies specific for tyrosinase (C-19) and PKC-βII (C-18) were purchased from Santa Cruz Biotechnology, Inc. (Santa Cruz, CA, USA). Microphthalmia (MITF) Ab-1 (C5, MS-771-P0) was obtained from Neomarkers (Fremont, CA, USA).
2.2. Cell Culture
Normal human melanocytes (NHM) were obtained from Invitrogen and maintained in Medium 254 (Cascade Biologics, Portland, OR, USA), which contained human melanocyte growth supplement at 37°C in a 5% CO2 incubation . The Mel-Ab cell line is a mouse-derived, spontaneously immortalized melanocyte cell line that synthesizes large quantities of melanin. Mel-Ab cells were maintained in Dulbecco’s modified Eagle’s medium (DMEM) supplemented with 10% fetal bovine serum (FBS), 100 nM TPA, 1 nM CT, 1% antibiotic-antimycotic solution (100X) at 37°C in 5% CO2. B16F10 cells were maintained in DMEM supplemented with 10% FBS and 1% antibiotic-antimycotic solution (100X) at 37°C in 5% CO2 .
2.3. Cell Viability Assay
Cell viability of NHM and Mel-Ab cells was measured using MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide; Duchefa, Netherlands) assay. NHM and Mel-Ab cells were seeded at the same density in 24-well plate. After incubation with loratadine for 24 hr. MTT was prepared as 2.5 mg/ml stock solution in phosphate buffered saline (PBS) and stored at 4°C. Then, stock MTT solution was added at 200 μl/well and the plates incubated at 37°C for 30 min to 4 hr. Thereafter, MTT solution was removed. After addition of 3 ml of DMSO the plates were incubated for 15 min at 37°C to dissolve the formazan crystals. Absorbance was measured at 570 nm using an enzyme immunosorbent assay (ELISA) reader (Molecular Devices Co., Sunnyvale, CA). The reference wavelength was 560-650 nm.
2.4. Melanin Contents and Microscopy
NHM and Mel-Ab cells were treated with specific agonists and for 5 days or 3 days. Cells were dissolved in 550 μl of 1 N NaOH at 100°C for 30 min and centrifuged at 13,000 rpm for 5 min. The optical density (OD) of the supernatants was measured at 405 nm using a microplate reader. Before measuring the melanin content, the cells were observed under a phase contrast and photographed for microscope (Olympus, Tokyo, Japan).
2.5. Tyrosinase Activity
NHM and Mel-Ab cells were seeded in 6-well plates and incubated with loratadine for 5 days or 3 days. The cells were washed with ice-cold PBS and lysed with phosphate buffer (pH 6.8) containing 1% Triton X-100. The cells were then disrupted by freezing and thawing, and the lysates were clarified by centrifugation at 15000 rpm for 10 minutes. After quantifying the protein levels of the lysate and adjusting the protein concentrations with lysis buffer, 90 μl of each lysate containing the same amount of protein was placed in each well of a 96-well plate, and 10 μl of 10 mM L-DOPA was then added to each well. The control wells contained 90 μl of lysis buffer and 10 μl of 10 mM L-DOPA. Following incubation at 37°C, absorbance was measured every 10 min for at least 1 hr at 475 nm using a microplate reader.
2.6. Cell Fractionation
We used a cell fractionation kit from cell signaling. Cell fractionation was performed according to the manufacturer’s instructions.
2.7. Western Blot Analysis
Cells were lysed in protein lysis buffer and centrifuged at 13,000 rpm for 30 min. The protein concentration was determined using a Bradford protein assay. 20 μg of protein per lane was separated by SDS-polyacrylamide gel electrophoresis and blotted onto nitrocellulose membranes, which were then saturated with 5% skim milk in Tris-buffered saline containing 0.5% Tween 20. Blots were incubated with the appropriate primary antibodies following the manufacturer’s data sheet. Image analysis was used to determine the relative band densities, which was performed using Image J software (https://imagej.nih.gov/ij/).
2.8. Total RNA Extraction and cDNA Synthesis
Total cellular RNA was extracted from the NHM using the Favor Prep™ Total RNA purification mini according to the manufacturer’s instructions (Favorgen, Ping Tung, Taiwan). Following isolation, the quantity and quality of the RNA were determined using a NanoDrop ND-1000 spectrophotometer (ND-1000, NanoDrop Technologies, Inc. Wilmington, DE, USA). Single-stranded complementary DNA (cDNA) was synthesized from 1 μg of total RNA using RevertAid First Strand cDNA Synthesis kit according to the manufacturer’s instructions (Thermo Scientific, Rockford, IL, USA).
2.9. Real-Time RT-PCR
qRT-PCR was performed using the LightCycler® 480II machine coupled with SYBR Green chemistry (Roche Applied Science, Indianapolis, IN, USA). In terms of qRT-PCR settings, initial denaturation was performed at 95°C for 5 min, followed by amplification at 95°C for 10 sec, 60°C for 10 sec, and 72°C for 10 sec for 45 cycles. The cDNA obtained was amplified with the following primers: RPLP0, Forward 5′-GGCGACCTGGAAGTCCAACT-3′, Reverse 5′-CCATCAGCACCACAGCCTTC-3′. MITF, Forward 5′-ACTTTCCCTTATCCCATCCACC-3′, Reverse 5′-TGAGATCCAGAGTTGTCGTACA-3′. Primers specific for RPLP0 were used for loading control amplifications.
The statistical significance of the differences between groups was assessed using analysis of variance (ANOVA), followed by the Student’s t-test. In this study, p <0.05 is considered significant.
3.1. Loratadine, an H1- Receptor Antagonist, Suppresses Melanogenesis in NHM and Mel-Ab Cells
Even though H2-receptor agonists and antagonists have been extensively studied previously, the effect of H1-receptor antagonists on melanogenesis has not been fully understood. First, we explored whether H1-receptor antagonists influenced the melanogenesis in B16F10 cells. Among the H1-receptor antagonists screened, ebastine, clemisole, terfenadine, and loratadine significantly decreased the melanin content (Table 1 and Supplementary Fig. 1A). We selected ebastine and loratadine as they decreased the melanin content in a dose-dependent manner. While ebastine affected cellular viability in NHM and Mel-Ab cells (data was not shown), loratadine showed a dose-dependent response without affecting cellular viability in NHM and Mel-Ab cells (Figures 1(a) and 1(c)). Also, loratadine treatment decreased the tyrosinase activity in a dose-dependent manner (Figure 1(b)).
To determine the involvement of H1-receptor agonist on melanogenesis, we evaluated the effects of 2-pyridylethylamine, an H1-receptor agonist. The H1 receptor agonist did not affect the melanin content in Mel-Ab cells or reverse the loratadine induced melanin reduction (Supplementary Figure 1B and 1C).
3.2. Loratadine Inhibits Expression Levels of MITF in NHM
As loratadine decreased melanin synthesis and tyrosinase activity, we next determined whether loratadine affected the expression of MITF, which plays crucial role in tyrosinase gene expression in melanogenesis. Loratadine significantly decreased the protein level of MITF and tyrosinase at 12 hr (Figure 2(a)). Furthermore, MITF and tyrosinase mRNA level also decreased after loratadine treatment (Figure 2(b)). These results indicated that loratadine reduced melanogenesis through the downregulation of MITF signaling pathway.
3.3. Inhibitory Effects of Loratadine on Melanogenesis Were Partially Associated with Akt Phosphorylation in NHM
To investigate the intracellular signaling pathways, expression levels of β-catenin, phospho-GSK-3β, phospho-Akt, and phospho-ERK were detected in NHM (Figure 3(a)). To determine the effect of loratadine on PI3K/Akt/GSK-3β signaling pathway, phosphorylation of Akt and GSK-3β was found to be increased following loratadine treatment, markedly at 30 min. β-catenin and phospho-ERK were not altered.
We next evaluated whether LY294002, a selective inhibitor of PI3K, affected the inhibitory effect of loratadine on melanogenesis. As shown in Figure 3(b), loratadine monotreatment significantly reduced the melanin content, whereas cotreatment with LY294002 and loratadine reversely increased the melanin content in NHM. Our results revealed that antimelanogenic effect of loratadine in NHM is associated with activation of PI3K/Akt/GSK-3β signaling pathway.
3.4. Antimelanogenic Effects of Loratadine Were Associated with Membrane PKC-βII in NHM
H1-receptor is one of the G protein-coupled receptors that couples to Gαq/11 proteins, leading to the activation of the calcium/protein kinase C (PKC) pathway. Since, specifically, PKC-βII is activator of tyrosinase, we investigated whether loratadine could decrease activity of PKC-βII. As shown in Figure 3(c), loratadine treatment reduced the amount of PKC-βII in the membrane fraction, thereby decreasing its activity. The molecular mechanisms of antimelanogenic effect of loratadine demonstrated above are summarized in Figure 4.
Histamine acts through 4 different classes of receptors (H1, H2, H3, and H4 receptors) on effector cells. Among these 4 receptors, H1 and H2 receptors have been extensively studied and demonstrated well-known proallergic properties . Since the first synthesized in 1937, H1 antihistamines have been some of the most commonly prescribed medicine in dermatologic conditions, including allergic reactions, urticaria, atopic dermatitis, and pruritus. Antihistamines were considered histamine receptor antagonists, but they actually work as inverse agonists by binding to histamine receptors, thereby returning cellular equilibrium and reducing allergic property .
Moreover, H1 and H2 receptors exist on the surface of human melanocytes and melanoma cells . Yoshida et al.  have reported that histamine induces melanogenetic effects on human melanocytes by accumulating cyclic adenosine monophosphate (cAMP) and subsequently activating protein kinase A (PKA), especially via the H2 receptor. Kim et al.  investigated the signaling pathways involved in histamine-induced melanocyte proliferation and melanogenesis. They showed that an H2 antihistamine, famotidine, suppressed the effects induced by the H2 agonist, amthamine, and histamine itself. Histamine stimulated melanocyte proliferation and melanogenesis via the H2 receptor and Erk, CREB, or Akt activation . More recently, it was reported that the H2 receptor mediated growth-differentiation factor-15 (GDF-15) could be involved in histamine-induced melanogenesis . In an in vivo study, the UVB-induced hyperpigmentation of guinea pig skin was suppressed by topically applying an H2 antihistamine .
Although the H1 receptor is a major therapeutic target of inflammatory skin disorders, there have been few studies about melanogenesis of H1 antihistamine [6, 15]. For example, mepyramine, an H1 antihistamine, did not inhibit melanogenesis that is induced by histamine . Therefore, first we screened antimelanogenic effects by H1 antihistamines using LOPAC chemical library (Table 1). Among them, ebastine, clemisole, terfenadine, and loratadine significantly decreased the melanin content, but loratadine was ultimately selected as its dose-dependent linear hit without affecting cellular viability. Our study found that H1 antihistamine, especially loratadine, demonstrates obvious antimelanogenic effects in NHM. Loratadine led to the significant inhibition of mRNA and protein expression level of MITF, which in turn suppressed tyrosinase, a key enzyme that controls melanogenesis.
Akt activation has been reported to reduce melanogenesis via transcriptional downregulation of MITF gene expression . Furthermore, in other mechanism, PI3K/Akt/GSK-3β signaling pathway regulates posttranslational modification and proteasomal degradation of MITF protein . In our present study, loratadine suppressed the MITF mRNA expression in NHM, which reversely increased after inhibition of Akt pathway by the selective inhibitor of PI3K, LY294002. Therefore, antimelanogenic effects of loratadine in NHM are shown to be related to activation of PI3K/Akt/GSK-3β signaling and the subsequent decrease in the MITF mRNA level.
Unlike H2 receptor, which is bound to protein and regulates melanogenesis via cAMP/PKA/CREB signaling pathway, H1 receptor mainly acts by coupling proteins, which in turn activate inositol trisphosphate (IP3)/diacylglycerol (DAG) pathway and subsequently localizing PKC enzymes to membrane . PKC-βII, a regulator of tyrosinase activity, especially, is known to increase melanogenesis and the activity of PKC-βII is determined by the membrane localization . As expected, loratadine did not affect the phosphorylation of CREB, but reduced activity of PKC-βII.
Our study had several limitations. Although loratadine showed the antimelanogenic effect at the cellular level, these results do not always provide the same outcomes as a clinical manner. Therefore, for practical application of the results, further clinical studies will be required to determine the therapeutic regimen of loratadine for treatment of hyperpigmentary disorders in humans.
Taken together, we demonstrated strong antimelanogenic effect of H1 antihistamine, loratadine via modulating Akt/MITF, and PKC-βII signaling. Considering common use of H1 antihistamines in dermatologic practice, the antimelanogenic effects of loratadine may have potentially significant and useful roles. Although further clinical studies will be needed, loratadine may give benefits for treatment of hyperpigmentary changes accompanied by various types of dermatitis.
The datasets generated during and/or analyzed during the current study are available from the corresponding author upon reasonable request.
Part of this work has previously been presented at European Society for Pigment Cell Research 2016 Meeting (Poster session) and the Ninth Korean Society for Investigative Dermatology Research Camp 2017 (Free communications II).
Conflicts of Interest
The authors declare that they have no conflicts of interest.
Supplementary Figure 1: effects of H1-receptor agonist and antagonists on melanogenesis. Supplementary Figure 2: effects of loratadine on cellular viability in normal human melanocytes. (Supplementary Materials)
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