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Umbelliferone Stimulated Melanogenesis and Increased Glutathione Level in B16F10 Cells Yunjung Lee1, Bonhee Ku2, Dongsoo Kim1 & Eun-Mi Choi1,2 1

Department of Chemistry, Incheon National University, Incheon 22012, Republic of Korea 2 Department of Cosmetic Science & Management, Graduate School, Incheon National University, Incheon 22012, Republic of Korea Correspondence and requests for materials should be addressed to E.-M. Choi ([email protected]) Received 10 April 2017 / Received in revised form 9 June 2017 Accepted 13 June 2017 DOI 10.1007/s13530-017-0316-2 ©The Korean Society of Environmental Risk Assessment and Health Science and Springer 2017 pISSN : 2005-9752 / eISSN : 2233-7784 Toxicol. Environ. Health. Sci. Vol. 9(2), 152-160, 2017

Abstract Umbelliferone (7-hydroxycoumarin) treatment caused an increase in melanin content in B16F10 melanoma cells in a dose-dependent manner, without causing toxicity. The increase in melanin content was correlated with increases in the activity of tyrosinase, the rate-limiting enzyme in melanin synthesis, and the expressions of melanogenic proteins, including tyrosinase, tyrosinase-related protein 1, and microphthalmia-associated transcription factor, the master transcriptional regulator for melanogenesis. Unlike α-melanocyte-stimulating hormone, umbelliferone did not cause melanogenesis-associated oxidation and depletion of glutathione. Conversely, umbelliferone treatment resulted in a significant and dose-dependent increase in glutathione. Umbelliferone caused activation of JNK, p38 MAPK, and GSK3β in a dosedependent manner, suggesting possible involvement of those protein kinases in umbelliferone-induced stimulations of melanogenesis and antioxidant system. Our results suggest that umbelliferone stimulates both melanogenesis and antioxidant defense, providing more effective protection against UV-induced photodamage. They also imply possible applications of umbelliferone in self-tanning and treatment of skin disorders related with melanin deficiency. Keywords: Umbelliferone, Melanogenesis, Tyrosinase, Glutathione, MITF, MAPKs, GSK3β

Introduction Melanin is a complex pigment synthesized from Ltyrosine by a series of enzyme reactions in the melanosomes of melanocytes, located in the basal layer of the epidermis. Melanosomes containing melanin are transferred to the surrounding keratinocytes and distributed in the upper layers of the skin, determining visible pigmentation of the skin, hair, and eyes1. In addition, melanin has important functions to protect skin cells from harmful effects of UV radiation by absorbing UV radiation and transforming the UV energy into harmless heat2. Therefore, a deficiency in melanin can be associated with increased risk of sunburn, photo-immunosuppression, premature skin aging, and skin cancer caused by excessive UV exposure. Therefore, stimulation of melanin synthesis can contribute to natural defenses of skin against the harmful effects of UV, parti­ cularly in fair-skinned individuals, and also to improvement of disorders associated with hypopigmentation in skin and hair. Tyrosinase, the key enzyme involved in the synthesis of all types of melanin, catalyzes hydroxylation of tyrosine to 3,4-dihydroxyphenylalanine (DOPA) and subsequent oxidation of DOPA to dopaquinone, which is then spontaneously converted to dopachrome3. Tyro­ sinase-related protein 2 (TRP2), which is dopachrome tautomerase that converts dopachrome to 5,6-dihydro­ xyindole-2-carboxylic acid (DHICA), and tyrosinaserelated protein 1 (TRP1), which oxidizes DHICA to indole-5,6-quinone-carboxylic acid, are also involved in the synthesis of melanin. Melanin synthesis is regulated by genetic, environmental, and endocrine factors4. UV radiation is the most important environmental factor to regulate melanogenesis in the skin. Signaling from melanocortin-1 receptor (MC1R) is known to be the main factor regulating melanogenesis5. Melanin synthesis is stimulated via activation of MC1R by binding of agonists, such as α-melanocyte-stimulating hormone (α-MSH) and adrenocorticotropic hormone produced by pituitary gland and epidermal keratinocytes, which increases intracellular level of cyclic AMP (cAMP)6. In addition to the MC1R, other melanocyte receptors, such as adrenergic receptors, are also known to be associated with cAMP and inositol triphosphate/diacylglycerol pathways to stimulate melanogenesis7. The increase in cAMP results in

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melanogenesis through activation of protein kinase A (PKA) that phosphorylates cAMP response elementbinding protein (CREB), which then leads to an increase in the expression of microphthalmia-associated transcription factor (MITF)8. MITF binds the M-box promoter elements and stimulates expression of melanogenic genes, including tyrosinase, TRP1, and TRP2. Mitogen-activated protein kinases (MAPKs), a family of serine/threonine kinases that play central roles in mediating a variety of biological processes, have been shown to be involved in the regulation of CREB and MITF activities thus affecting melanogenesis9. Along with the α-MSH-MC1R signaling pathway, stem cell factor-KIT receptor kinase signaling pathway5 and Wnt/ β-catenin signaling pathway are also known to play important regulatory roles in melanogenesis in cells10. Coumarin and its derivatives constitute a group of phenolic compounds and are considered promising natural products having pharmacological activities with low toxicity to humans11. Many plant species in Apiaceae (Umbelliferae) family, commonly known as parsley family, contain coumarins. Umbelliferone (7hydroxycoumarin) is a bioactive molecule naturally present in edible fruits, such as bitter orange, and is regarded as the parent compound of the more complex coumarins. Umbelliferone has been reported to show antioxidative, anti-inflammatory, and immunomodulatory effects12. Umbelliferone is used in some sunscreen products because of its strong UV absorption characteristics13, and it was also reported to show UV-protective effects in human fibroblasts14. However, there have been only few reports on the effects of hydroxycoumarins on melanogenesis, mostly focusing on inhibition of melanin synthesis15. In the present study, we investigated the effect of umbelliferone on melanogenesis and the molecular mechanisms involved, using B16F10 mouse melanoma cells as a model. We also investigated the effect of umbelliferone on antioxidant system by analyzing cellular glutathione, a primary low molecular weight antioxidant, and its possible relationship to melanogenesis.

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Figure 1. The effect of umbelliferone on the proliferation of B16F10 cells. B16F10 cells were incubated with indicated concentrations of umbelliferone or α-MSH (0.1 μM) for 24 h, and the cell proliferation was determined by MTT assay. The results are expressed as the means±standard deviations (n = 16). **P< 0.01; ***P< 0.001 vs. no-treatment control cells (Con).

Results

as a positive control. We first determined the effects of umbelliferone and α-MSH on proliferation of B16F10 cells. Both umbelliferone and α-MSH caused decreases in the cell proliferation (Figure 1). However, at the concentrations ranging from 100 to 300 μM, umbelliferone caused lower degrees of growth retardation compared to αMSH. Based on this result, we used umbelliferone at the concentrations of 100-300 μM to treat the cells. To assess the effect of umbelliferone on melanogenesis, we treated the cells with umbelliferone for 96 h, at which the accumulation of melanin reached its maximum, and then measured melanin content in cells. As shown in Figure 2, umbelliferone treatment resulted in a significant increase in melanin content (P<0.001) in a dose-dependent manner. At the concentrations of 200 and 300 μM, umbelliferone caused significantly larger increases in melanin content compared to α-MSH. Secretion of melanin into medium was not observed during the treatments.

Umbelliferone Increased Melanogenesis in B16F10 Cells B16 murine melanoma cell line provides a useful model system to study melanogenesis, because it shows characteristics of normal differentiating melanocytes, such as dendrite-like structure formation and melanin synthesis16. We examined the effects of umbelliferone on melanogenesis in B16F10 mouse melanoma cells, using α-MSH (0.1 μM), a potent melanogenic hormone,

Umbelliferone Increased Tyrosinase Activity in B16F10 Cells Having observed that umbelliferone increased melanin content, we investigated its effect on the activity of tyrosinase, the rate-limiting enzyme in the melanin synthesis pathway. Figure 3 shows that umbelliferone treatment for 96 h significantly increased tyrosinase activity in a dose-dependent manner (P<0.01). At 300 μM, umbelliferone caused about the same level of acti­

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Figure 2. The effect of umbelliferone on melanin content in B16F10 cells. B16F10 cells were treated with indicated concentrations of umbelliferone or α-MSH (0.1 μM) for 96 h. Melanin in the cell pellet (A) was solubilized, and the melanin content was determined by the absorbance at 405 nm and normalized to the protein content (B). The results are expressed as the means±standard deviations (n = 6). ***P < 0.001 vs. no-treatment control cells (Con).

Figure 3. The effect of umbelliferone on tyrosinase activity in B16F10 cells. B16F10 cells were treated with indicated concentrations of umbelliferone or α-MSH (0.1 μM) for 96 h. Tyrosinase activity in cell lysate was measured. The results are expressed as the means±standard deviations (n = 6). **P< 0.01; ***P<0.001 vs. no-treatment control cells (Con). (A)

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Umbelliferone Increased Glutathione Level in B16F10 Cells To investigate the effect of umbelliferone on the antioxidant capacity of B16F10 cells, we monitored

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Umbelliferone Increased Melanogenic Protein Levels in B16F10 Cells To investigate the mechanism underlying the umbelliferone-induced increases in melanin content and tyro­ sinase activity, we analyzed the levels of tyrosinase, TRP1, and MITF, the master transcriptional regulator stimulating expression of melanogenic genes, in cells treated with umbelliferone by Western blot (Figure 4). We measured the protein levels before the accumulation of melanin in cells reached its maximum, that is, after 72 h of the treatment, in order to measure the levels of both MITF and its target gene products tyrosinase and TRP1. Umbelliferone caused dose-dependent increases in the cellular content of the three proteins.

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Figure 4. The effect of umbelliferone on melanogenic protein levels in B16F10 cells. B16F10 cells were treated with indicated concentrations of umbelliferone or α-MSH (0.1 μM) for 72 h. Cell lysates were subjected to Western blot analysis using antibodies to tyrosinase, TRP-1, MITF, and β-actin (A), and the blot was densitometrically analyzed (B). The plots show the representative of three independent experiments.

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the content and redox status of cellular glutathione during the treatment. As shown in Figure 5, during the treatments up to 72 h, significant changes in the content and redox status of glutathione were not observed. However, after 96 h, the cells treated with α-MSH showed marked decreases in the total glutathione content and GSH/GSSG ratio, indicating severe oxidation and depletion of cellular glutathione pool. On the contrary, the cells treated with umbelliferone showed a significant (P<0.001) and dose-dependent increase in glutathione content and no change in GSH/GSSG ratio compared to the no-treatment control. The decrease in glutathione content shown in the no-treatment control cells at 96 h is a normal response of the cells in monolayer culture reaching the confluent state17.

Umbelliferone Induced Activation of JNK, p38 MAPK, and GSK3β in B16F10 Cells To assess the effect of umbelliferone on the activa-

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Figure 6. The effect of umbelliferone on the activation of JNK, p-38 MAPK, and GSK3β in B16F10 cells. B16F10 cells were treated with indicated concentrations of umbelliferone or α-MSH (0.1 μM) for 1 h. Cell lysates were subjected to Western blot analysis using antibodies to JNK, p-JNK, p38 MAPK, p-p38 MAPK, GSK3β, p-GSK3β, and GAPDH (A), and the blot was densitometrically analyzed (B). The plots show the representative of three independent experiments.

tion of MAPKs and GSK3β, which have been shown to affect the upstream signaling pathways for differentiation and melanogenesis, we analyzed phosphorylation of those proteins in cells after 1 h of treatment with umbelliferone or α-MSH. As shown in Figure 6, umbelliferone increased phosphorylation of JNK and p38 MAPK in a dose-dependent manner; α-MSH also increased phosphorylation of JNK and p38 MAPK. Phosphorylation of ERK was not observed in the cells treated with either umbelliferone or α-MSH (data not shown). Umbelliferone caused a dose-dependent decrease in the phosphorylation of GSK3β. These results indicate that umbelliferone induced activation of JNK, p38 MAPK, and GSK3β. In contrast, α-MSH induced activation of JNK and p38 MAPK but not that of GSK3β.

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Discussion Natural melanogenesis process occurs as a response to UV irradiation. However, UV-induced tanning can cause damage to cellular components. In addition, melanogenesis process itself generates oxidative stress to cells 18. Therefore, a melanogenesis-stimulating agent that can prevent oxidative stress would be highly beneficial for photoprotection and treatment of hypo­ pigmentation-related diseases such as vitiligo. In this study, we evaluated melanogenesis-stimulating effect of umbelliferone, and we also examined its effect on the oxidative stress in B16F10 cells by analyzing cellular glutathione. Our results demonstrated that umbelliferone stimulated melanogenesis in a dose-dependent manner (100300 μM). Umbelliferone treatment resulted in an increase in intracellular melanin content (Figure 2). Accordingly, umbelliferone increased tyrosinase activity (Figure 3) and the levels of melanogenesis-related proteins, including tyrosinase, TRP-1, and MITF (Figure 4). Compared to α-MSH positive control, umbelliferone at 300 μM was found to result in much larger increase in melanin content, while resulting in similar level of increases in tyrosinase activity and melanogenic protein expression. This difference may be associated with the stability of melanosome and melanin. Although there is relatively little known about the degradation of the melanosome and melanin, their degradation at certain conditions, such as oxidative stress, has been reported19,20. A positive correlation between cellular antioxidant enzyme activity and melanin content in human melanocytes was also reported21. Melanogenesis is a complex process, involving a generation of oxidative intermediates and reactive oxygen species18. It is also known that melanocytes are more susceptible to oxidative damage22. An accumulation of H2O2 in normal melanocytes was found to be in direct proportion with the synthesis of melanin23. In this regard, antioxidants have been explored as inhibitors of melanin synthesis. However, there are also conflicting reports about the role of melanin and its intermediates as pro-oxidants24 or antioxidants25. While melanin clearly has a photoprotective effect, its prooxidant and antioxidant activities appear to depend on the redox state of the melanocytes26. Our results showed that α-MSH-induced melanogenesis was accompanied by a severe oxidative stress, as demonstrated by the severe depletion and oxidation of cellular glutathione (Figure 5). On the other hand, umbelliferone-induced melanogenesis was not accompanied by such oxidative stress. Conversely, a significant increase in glutathione level was found in umbelliferone-treated cells compared to no-treatment control

and α-MSH-treated cells. The increase in glutathione may also contribute to the increase in the melanin content by inhibiting oxidative destabilization of melanin and melanosome. Considering that the UV-induced tanning can cause oxidative damage, which has crucial roles in the onset and progression of skin diseases such as vitiligo and skin cancer27, the antioxidative effect of umbelliferone, in addition to its melanogenic effect, would provide an additional protection. In the present study, JNK and p38 MAPK were activated (phosphorylated) by both umbelliferone and αMSH (Figure 6), but ERK was not (data not shown). Antioxidant molecules, such as polyphenols, have been shown to activate MAPKs and subsequently induce antioxidant defense system, which is mediated by antioxidant-responsive elements (ARE)28,29. Glutathione system is a well-known target of ARE activation30. MAPKs have also been shown to trigger differentiation of B16 melanoma cells31. Stimulation of p38 MAPK signaling pathway in melanogenesis has been demonstrated in previous studies32. In addition, MAPKs are known to regulate the transcriptional activity of MITF via direct phosphorylation. However, the role of MAPKs in mel­anogenesis has been controversial33,34. Phosphorylation enhances transcriptional activity of MITF, in general. For example, the phosphorylation at Ser73 promotes the interaction with a MITF cofactor, histone acetyl transferase p300/CBP within the transactivation domain of MITF35. However, inhibition of the activity by phosphorylation, for example, phosphorylation at Ser409, via stimulating the interaction with other regulatory proteins was also reported36. Regulation of MITF protein level by proteasome-dependent turnover, which requires phosphorylation of MITF at Ser73, was also reported 37 . Unlike JNK and p38 MAPK, ERK has been mostly implicated in the inhibition of melanogenesis by stimulating ubiquitin-dependent degradation of MITF35. Our result also showed that umbelliferone resulted in the activation (dephosphorylation) of GSK3β, whereas α-MSH did not. GSK3β is inactive in phosphorylated form (at Ser 9) and active in dephosphorylated form. Various protein kinases, such as PKA, Akt, and p90 ribosomal kinase, and protein phosphatases, such as protein phosphatase 2A and protein phosphatase 1, were reported to regulated GSK3β activity38,39. The activation mechanism of GSK3β was not explored further in the present study. Phosphorylation of MITF (Ser 298) by GSK3β is known to enhance transcriptional activity of MITF40. cAMP was shown to activate GSK3β via inhibition of phosphatidylinositol 3-kinase, leading to stimulation of melanogenesis41. In turn, GSK3β was shown to increase cAMP generation via activation of adenylate

Umbelliferone Increased Melanogenesis and Glutathione

cyclase42. Regulation of CREB activity by GSK3β has also been reported, but the findings are apparently contradictory43. In contrast, phosphorylation of β-catenin by GSK3β is known to cause subsequent degradation of β-catenin via proteasome system, eventually resulting in a reduced expression of MITF44. Since our result showed an increase in MITF expression by umbelliferone treatment, β-catenin pathway does not seem to be a target of the activated GSK3β in umbelliferoneinduced melanogenesis. The detailed roles of each MAPK and GSK3β in the antioxidative and melanogenic effects of umbelliferone need to be further explored by using additional means, for example, specific inhibitors for each protein kinase. In conclusion, our study demonstrated that umbelliferone treatment resulted in an increase in melanin content in B16F10 melanoma cells, by increasing the activity and expression of melanogenic proteins, including tyrosinase, TRP-1, and MITF. Umbelliferone also enhanced antioxidant capacity, as demonstrated by the increase in glutathione, which contributes to prevention of malanogenesis-associated oxidative stress. Umbelliferone activated JNK, p38 MAPK, and GSK3β, indicating a possible involvement of those protein kinases in the antioxidative and melanogenic effects of umbelliferone. Our results suggest that umbelliferone stimulates both melanogenesis and antioxidant defense, providing more effective protection against UV-induced photodamage. Our results also imply possible applications of umbelliferone in self-tanning and treatment of skin disorders related with melanin deficiency.

Materials and Methods Materials Phenol red-free Dulbecco’s modified Eagle’s medium (DMEM), penicillin-streptomycin solution, and fetal bovine serum (FBS) were purchased from Gibco® (Life Technologies Korea, Seoul, Korea). Umbelliferone, αMSH, L-DOPA, bicinchoninic acid (BCA) solution, 3(4,5-dimethylthiazolyl-2)-2, 5-diphenyltetrazolium bromide (MTT), phenylmethylsulfonyl fluoride (PMSF), glutathione (reduced form: GSH; oxidized form: GSSG), dithiothreitol (DTT), o-phthalaldehyde (OPA), N-ethylmaleimide (NEM), sodium dodecyl sulfate (SDS), and dimethyl sulfoxide (DMSO) were from Sigma-Aldrich (St. Louis, MO, USA). Antibodies to tyrosinase, TRP1, β-actin, and glyceraldehyde 3-phosphate dehydrogenase (GAPDH) were purchased from Santa Cruz Biotechnology (Santa Cruz, CA, USA); MITF were from Thermo Fisher Scientific (Waltham, MA, USA); JNK, p-JNK, ERK 1/2, p-ERK 1/2, p38 MAPK, p-p38 MAPK, GSK3β, p-GSK3β, and protease/phosphatase

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inhibitor cocktail were from Cell Signaling Technology (Danvers, MA, USA). Enhanced chemiluminescence (ECL) reagents were form GE Healthcare Bio-Sciences (Piscataway, NJ, USA).

Cell Culture B16F10 mouse melanoma cells were obtained from the Korean Cell Line Bank (Seoul, Korea). B16F10 cells were cultured in phenol red-free DMEM supplemented with 4 mM glutamine, 10% (v/v) FBS, 100 U/ mL penicillin, and 100 μg/mL streptomycin (full medium) at 37°C with 5% CO2. The cells were then incubated in the medium containing 2% FBS for 12 h before the treatment with umbelliferone or α-MSH. Umbelli­ ferone was dissolved in DMSO, and the concentration of DMSO in the treatment medium was 0.1% (v/v) in all experiments. α-MSH was used as a positive control for melanogenesis. Measurement of Cell Proliferation B16F10 cells were incubated with various concentrations of umbelliferone or α-MSH (0.1 μM) in DMEM containing 2% FBS for 24 h in 96-well plates. The cell proliferation was determined by MTT assay. Briefly, 10 μL of 5 mg/mL MTT solution in phosphate-buffered saline (PBS) was added to each well and incubated for 4 h at 37°C. Then the medium was removed, the formazan product was solubilized in 200 μL of DMSO, and the absorbance was measured at 560 nm using a microplate spectrophotometer. Measurement of Melanin Content in Cells B16F10 cells were plated at 5 × 104 cells per well in 6-well plates and incubated for 12 h in full medium and for another 12 h in DMEM containing 2% FBS. The cells were then treated with various concentrations of umbelliferone or α-MSH (0.1 μM) for 96 h. After washing with PBS, the cell pellet was resuspended in 300 μL of 1 N NaOH containing 10% DMSO and heated at 80°C for 1 h. The absorbance of solubilized melanin was measured at 405 nm45. The melanin content was normalized to protein content determined by Lowry method46. Measurement of Tyrosinase Activity in Cells Cells were washed with ice-cold PBS, resuspended in lysis buffer (1% Triton X-100, 0.1 mM PMSF, 100 mM Na-phosphate, pH 6.8), and subjected to two freeze-thaw cycles. The cell lysates were clarified by centrifugation (12,000 × g, 30 min, at 4°C). 100 μL of each lysate was mixed with 100 μL of 10 mM LDOPA dissolved in phosphate buffer, pH 6.8, and the rate of dopachrome formation in the reaction mixture was determined by monitoring absorbance at 475 nm.

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The tyrosinase activity was normalized to the amount of protein in the cell lysate determined using BCA reagent.

Measurement of Glutathione in Cells Cellular glutathione was analyzed by HPLC method using fluorescence detection after precolumn derivatization with OPA47. Briefly, to determine the total glutathione content, that is, the sum of GSH and GSSG, a 5% perchloric acid (PCA) extract of the cells was neutralized and then reduced by 2.5 mM DTT. The reduced sample was derivatized with OPA as described previously48 and injected onto a SupelcosilTM LC-18 column (particle size 5 μm, 25 cm × 4.6 mm; Supelco, Bellefonte, PA, USA). The GSH-OPA adduct was eluted using a methanol gradient in a 0.1 M sodium phosphate buffer, pH 5.5, and detected by monitoring fluorescence at an excitation wavelength of 350 nm and an emission wavelength of 420 nm. To determine GSSG content, a duplicate PCA extract which was reacted with 2 mM NEM to eliminate GSH was used. GSH content was calculated from the difference between the amount of total glutathione and GSSG. Total glutathione content was expressed as GSH equivalents in nmol/mg protein. The redox status of cellular glutathione was presented as GSH/GSSG ratio. The PCA-precipitated protein pellet was solubilized by 1 N NaOH, and protein content was measured by Lowry method46. Western Blot Analysis Whole cell lysates were prepared using radioimmunoprecipitation assay (RIPA) lysis buffer (150 mM NaCl, 1% Triton X-100, 0.5% sodium deoxycholate, 0.1% SDS, 50 mM Tris-HCl, pH 8.0) containing protease/ phosphatase inhibitor cocktail. The proteins in cell lysates, with equal amounts of total proteins (20 μg), were separated on a 10% SDS-polyacrylamide gel electrophoresis gel and blotted onto a polyvinylidene fluoride membrane. The membrane was incubated with 5% skim milk in Tris-buffered saline containing 0.1% Tween 20 to block nonspecific binding. The blot was incubated with appropriate primary antibodies for 12 h at 4°C and subsequently with horseradish peroxidase-conjugated secondary antibody for 1 h at room temperature. The immunoreactive protein bands were detected using ECL, and the bands obtained were quantified using ImageJ. Statistical Analysis The significance of the differences between the experimental and control groups was determined using Student’s t-test. P values <0.05 were considered significant. The results are expressed as the means±standard deviations.

Acknowledgements This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (Ministry of Education) (NRF-2014S1 A5A2A03065802) and the Incheon National University Research Grant in 2015.

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