Regulation of microphthalmia-associated transcription factor MITF protein levels by association with the ubiquitin-conjugating enzyme hUBC9

Ubiquitination in melanoma pathogenesis and treatment

Melanoma is one of the most aggressive skin cancers with fiercely increasing incidence and mortality. Since the progressive understanding of the mutational landscape and immunologic pathogenic factors in melanoma, the targeted therapy and immunotherapy have been recently established and gained unprecedented improvements for melanoma treatment. However, the prognosis of melanoma patients remains unoptimistic mainly due to the resistance and nonresponse to current available drugs. Ubiquitination is a posttranslational modification which plays crucial roles in diverse cellular biological activities and participates in the pathogenesis of various cancers, including melanoma. Through the regulation of multiple tumor promoters and suppressors, ubiquitination is emerging as the key contributor and therefore a potential therapeutic target for melanoma. Herein, we summarize the current understanding of ubiquitination in melanoma, from mechanistic insights to clinical progress, and discuss the prospect of ubiquitination modification in melanoma treatment.

MC1R signaling. Intracellular partners and pathophysiological implications

The melanocortin-1 receptor (MC1R) preferentially expressed in melanocytes is best known as a key regulator of the synthesis of epidermal melanin pigments. Its paracrine stimulation by keratinocyte-derived melanocortins also activates DNA repair pathways and antioxidant defenses to build a complex, multifaceted photoprotective response. Many MC1R actions rely on cAMP-dependent activation of two transcription factors, MITF and PGC1α, but pleiotropic MC1R signaling also involves activation of mitogen-activated kinases and AKT. MC1R partners such as β-arrestins, PTEN and the E3 ubiquitin ligase MGRN1 differentially regulate these pathways. The MC1R gene is complex and polymorphic, with frequent variants associated with skin phenotypes and increased cancer risk. We review current knowledge of signaling from canonical MC1R, its splice isoforms and natural polymorphic variants. Recently discovered intracellular targets and partners are also discussed, to highlight the diversity of mechanisms that may contribute to normal and pathological variation of pigmentation and sensitivity to solar radiation-induced damage. This article is part of a Special Issue entitled: Melanocortin Receptors - edited by Ya-Xiong Tao.

Gomisin N Inhibits Melanogenesis through Regulating the PI3K/Akt and MAPK/ERK Signaling Pathways in Melanocytes

Gomisin N, one of the lignan compounds found in Schisandra chinensis has been shown to possess anti-oxidative, anti-tumorigenic, and anti-inflammatory activities in various studies. Here we report, for the first time, the anti-melenogenic efficacy of Gomisin N in mammalian cells as well as in zebrafish embryos. Gomisin N significantly reduced the melanin content without cellular toxicity. Although it was not capable of modulating the catalytic activity of mushroom tyrosinase in vitro, Gomisin N downregulated the expression levels of key proteins that function in melanogenesis. Gomisin N downregulated melanocortin 1 receptor (MC1R), adenylyl cyclase 2, microphthalmia-associated transcription factor (MITF), tyrosinase, tyrosinase-related protein-1 (TRP-1), and tyrosinase-related protein-2 (TRP-2). In addition, Gomisin N-treated Melan-A cells exhibited increased p-Akt and p-ERK levels, which implies that the activation of the PI3K/Akt and MAPK/ERK pathways may function to inhibit melanogenesis. We also validated that Gomisin N reduced melanin production by repressing the expression of MITF, tyrosinase, TRP-1, and TRP-2 in mouse and human cells as well as in developing zebrafish embryos. Collectively, we conclude that Gomisin N inhibits melanin synthesis by repressing the expression of MITF and melanogenic enzymes, probably through modulating the PI3K/Akt and MAPK/ERK pathways.

Parthenolide induces MITF-M downregulation and senescence in patient-derived MITF-M(high) melanoma cell populations

The activity of the M isoform of microphthalmia-associated transcription factor (MITF-M) has been attributed to regulation of differentiation, proliferation, survival and senescence of melanoma cells. MITF expression was shown to be antagonized by the activation of transcription factor NF-κB. Parthenolide, an inhibitor of NF-κB, has not been yet reported to affect MITF-M expression. Our results obtained in patient-derived melanoma cell populations indicate that parthenolide efficiently decreases the MITF-M level. This is neither dependent on p65/NF-κB signaling nor RAF/MEK/ERK pathway activity as inhibition of MEK by GSK1120212 (trametinib) and induction of ERK1/2 activity by parthenolide itself do not interfere with parthenolide-triggered depletion of MITF-M in both wild-type BRAF and BRAF^V600E melanoma populations. Parthenolide activity is not prevented by inhibitors of caspases, proteasomal and lysosomal pathways. As parthenolide reduces MITF-M transcript level and HDAC1 protein level, parthenolide-activated depletion of MITF-M protein may be considered as a result of transcriptional regulation, however, the influence of parthenolide on other elements of a dynamic control over MITF-M cannot be ruled out. Parthenolide induces diverse effects in melanoma cells, from death to senescence. The mode of the response to parthenolide is bound to the molecular characteristics of melanoma cells, particularly to the basal MITF-M expression level but other cell-autonomous differences such as NF-κB activity and MCL-1 level might also contribute. Our data suggest that parthenolide can be developed as a drug used in combination therapy against melanoma when simultaneous inhibition of MITF-M, NF-κB and HDAC1 is needed.

Absence of germline CDKN2A mutation in Sicilian patients with familial malignant melanoma: Could it be a population-specific genetic signature?

Germline CDKN2A mutations have been described in 25% to 40% of melanoma families from several countries. Sicilian population is genetically different from the people of Europe and Northern Italy because of its historical background, therefore familial melanoma could be due to genes different from high-penetrance CDKN2A gene. Four hundred patients with cutaneous melanoma were observed in a 6-years period at the Plastic Surgery Unit of the University of Palermo. Forty-eight patients have met the criteria of the Italian Society of Human Genetics (SIGU) for the diagnosis of familial melanoma and were screened for CDKN2A and CDK4 mutations. Mutation testing revealed that none of the families carried mutations in CDK4 and only one patient harboured the rare CDKN2A p.R87W mutation. Unlike other studies, we have not found high mutation rate of CDKN2A in patients affected by familial melanoma or multiple melanoma. This difference could be attributed to different factors, including the genetic heterogeneity of the Sicilian population. It is likely that, as in the Australian people, the inheritance of familial melanoma in this island of the Mediterranean Sea is due to intermediate/low-penetrance susceptibility genes, which, together with environmental factors (as latitude and sun exposure), could determine the occurrence of melanoma.

2-Ethoxybenzamide stimulates melanin synthesis in B16F1 melanoma cells via the CREB signaling pathway

Non-steroidal anti-inflammatory drugs are frequently used for the treatment of inflammation, pain, and fever. In this study, we found that 2-ethoxybenzamide (ETZ) significantly enhanced melanin synthesis in B16F1 melanoma cells, and also induced melanosome formation. Therefore, we investigated the mechanism by which ETZ up-regulated melanin synthesis. Western blot analysis demonstrated that ETZ increased melanogenic protein levels, except that for TRP-2. Moreover, semi-quantitative reverse transcription-polymerase chain reaction (RT-PCR) and real-time RT-PCR analyses showed that ETZ enhanced the mRNA levels of melanogenic genes, including microphthalmia-associated transcription factor and melanocortin 1 receptor. We also observed phosphorylation of cAMP response element-binding protein (CREB) following ETZ treatment. However, ETZ did not affect intracellular cAMP levels. ERK was also activated by ETZ treatment, and melanin content was enhanced upon treatment with the specific ERK inhibitor PD98059. Together, our results indicate that ETZ induces melanin synthesis via CREB phosphorylation.

Lebein, a Snake Venom Disintegrin, Induces Apoptosis in Human Melanoma Cells

Melanoma, the most threatening form of skin cancer, has a very poor prognosis and is characterized by its very invasive and chemoresistant properties. Despite the recent promising news from the field of immunotherapy, there is an urgent need for new therapeutic approaches that are free of resistance mechanisms and side effects. Anti-neoplasic properties have been highlighted for different disintegrins from snake venom including Lebein; however, the exact effect of Lebein on melanoma has not yet been defined. In this study, we showed that Lebein blocks melanoma cell proliferation and induces a more differentiated phenotype with inhibition of extracellular signal-regulated kinase (ERK) phosphorylation and microphthalmia-associated transcription factor (MITF) overexpression. Melanoma cells became detached but were less invasive with upregulation of E-cadherin after Lebein exposure. Lebein induced a caspase-independent apoptotic program with apoptosis inducing factor (AIF), BCL-2-associated X protein (BAX) and Bim overexpression together with downregulation of B-cell lymphoma-2 (BCL-2). It generated a distinct response in reactive oxygen species (ROS) generation and p53 levels depending on the p53 cell line status (wild type or mutant). Therefore, we propose Lebein as a new candidate for development of potential therapies for melanoma.

MiR-137 affects melanin synthesis in mouse melanocyte by repressing the expression of c-Kit and Tyrp2 in SCF/c-Kit signaling pathway

Previously, we created miR-137 overexpressing transgenic mice that produced lighten color phenotypes including gray mice phenotype. However, the miR-137 functional role in coat color regulation is still not well understood. In this study, the quantity of melanin granule and the relative expression of TYRP2 in gray miR-137 overexpression transgenic mouse skin were significantly lower than that in C57BL/6J black mouse skin. The mRNA and protein expression level of c-Kit and c-Kit downstream gene Tyrp2 in miR-137 expression plasmid-transfected melanocytes were significantly down-regulated comparing with that of the control melanocytes. In melanocytes, miR-137 overexpression could decrease the enhanced expression of c-Kit and Tyrp2 and the increased melanin production caused by UV treatment. The target relationship of miR-137 and c-Kit was identified by luciferase assay. The results suggest that miR-137 could inhibit melanogenesis in mouse skin melanocytes by repressing the expression of c-Kit and Tyrp2 in SCF/c-Kit signaling pathway.

Microphthalmia-associated transcription factor expression levels in melanoma cells contribute to cell invasion and proliferation

Microphthalmia-associated transcription factor (MITF) is a nodal point in melanoma transcriptional network that regulates dozens of genes with critical functions in cell differentiation, proliferation and survival. Highly variable MITF expression levels exist in tumor cell subpopulations conferring marked heterogeneity and plasticity in the tumor tissue. A model has been postulated whereby lower MITF levels favour cell invasion and suppress proliferation, whereas high levels stimulate differentiation and proliferation. Additionally, MITF is considered to be a prosurvival gene and a lineage addiction oncogene in melanoma. Herein, we review how MITF expression may affect the melanoma phenotype with consequences on the survival, invasion and metastasis of melanoma cells, and we discuss the research challenges.

Anti-melanogenic effects of resveratryl triglycolate, a novel hybrid compound derived by esterification of resveratrol with glycolic acid

Resveratrol is known to inhibit cellular melanin synthesis by multiple mechanisms. Glycolic acid (GA) is used in skin care products for its excellent skin penetration. The purpose of this study was to examine the anti-melanogenic effects of resveratryl triglycolate (RTG), a novel hybrid compound of resveratrol and GA, in comparison with resveratrol, GA, resveratryl triacetate (RTA) and arbutin. Resveratrol, RTG, and RTA inhibited the catalytic activity human tyrosinase (TYR) more potently than arbutin or GA did. Their cytotoxic and anti-melanogenic effects were examined using murine melanoma B16/F10 cells and human epidermal melanocytes (HEMs). The cytotoxicity of RTG was similar to that of resveratrol and RTA. RTG at 3-10 μM decreased melanin levels and cellular TYR activities in α-melanocyte-stimulating hormone-stimulated B16/F10 cells, and L-tyrosine-stimulated HEMs. RTG also suppressed mRNA and protein expression of TYR, tyrosinase-related protein 1, L-3,4-dihydroxyphenylalanine chrome tautomerase, and microphthalmia-associated transcription factor (MITF) in HEMs stimulated with L-tyrosine. This study suggests that, like resveratrol and RTA, RTG can attenuate cellular melanin synthesis effectively through the suppression of MITF-dependent expression of melanogenic enzymes and the inhibition of catalytic activity of TYR enzyme. RTG therefore has potential for use as a cosmeceutical ingredient for skin whitening.

Dual effects of acetylsalicylic acid on ERK signaling and Mitf transcription lead to inhibition of melanogenesis

Acetylsalicylic acid (ASA) is widely used as an analgesic/antipyretic drug. It exhibits a wide range of biological effects, including preventative effects against heart attack and stroke, and the induction of apoptosis in various cancer cells. We previously found that ASA inhibits melanogenesis in B16 melanoma cells. However, the mechanisms of how ASA down-regulates melanin synthesis remain unclear. Here, we investigated the effect of ASA on melanogenic pathways, such as extracellular signal-regulated kinase (ERK) and microphthalmia-associated transcription factor (Mitf) transcription. ASA significantly inhibited melanin synthesis in a dose-dependent manner without oxidative stress and cell death. Semi-quantitative reverse transcription-polymerase chain reaction analysis showed that the inhibitory effect of ASA might be due to the inhibition of Mitf gene transcription. Interestingly, ASA also induced ERK phosphorylation. Additionally, treatment with PD98059, a specific ERK phosphorylation inhibitor, abolished the anti-melanogenic effect of ASA. These results suggest that the depigmenting effect of ASA results from down-regulation of Mitf, which is induced by both the induction of ERK phosphorylation and the inhibition of Mitf transcription.

Sesamol decreases melanin biosynthesis in melanocyte cells and zebrafish: Possible involvement of MITF via the intracellular cAMP and p38/JNK signalling pathways

The development of antimelanogenic agents is important for the prevention of serious aesthetic problems such as melasma, freckles, age spots and chloasma. The aim of this study was to investigate the antimelanogenic effect of sesamol, an active lignan isolated from Sesamum indicum, in melan-a cells. Sesamol strongly inhibited melanin biosynthesis and the activity of intracellular tyrosinase by decreasing cyclic adenosine monophosphate (cAMP) accumulation. Sesamol significantly decreased the expression of melanogenesis-related genes, such as tyrosinase, tyrosinase-related protein-1,2 (TRP-1,2), microphthalmia-associated transcription factor (MITF) and melanocortin 1 receptor (MC1R). In addition, sesamol also induces phosphorylation of p38 mitogen-activated protein kinase (p38 MAPK) and c-Jun N-terminal kinase (JNK). Moreover, sesamol dose-dependently decreased zebrafish pigment formation, tyrosinase activity and expression of melanogenesis-related genes. These findings indicate that sesamol inhibited melanin biosynthesis by down-regulating tyrosinase activity and melanin production via regulation of gene expression of melanogenesis-related proteins through modulation of MITF activity, which promoted phosphorylation of p38 and JNK in melan-a cells. Together, these results suggest that sesamol strongly inhibits melanin biosynthesis, and therefore, sesamol represents a new skin-whitening agent for use in cosmetics.

A novel adamantyl benzylbenzamide derivative, AP736, inhibits melanogenesis in B16F10 mouse melanoma cells via glycogen synthase kinase 3β phosphorylation

Recently, much effort has been made to develop effective dermatological depigmenting compounds. In this study, we investigated the novel candidate compound, AP736 (an adamantyl benzylbenzamide derivative), and its effects on melanogenesis in B16F10 melanoma cells, as well as the mechanisms involved. AP736 has been reported to exert anti-melanogenic effects in melanocytes in vitro and in artificial skin equivalents through the inhibition of key melanogenic enzymes and the suppression of the cAMP-protein kinase A (PKA)-cAMP response element‑binding protein (CREB) signaling pathway. Thus, we examined another pathway of melanogenesis involving the effects of AP736 on the glycogen synthesis kinase 3β (GSK3β) pathway. Melanin content and tyrosinase activity were measured using a spectrophotometer after the cells were treated with AP736. The AP736-induced activation of signaling pathways was examined by western blot analysis. We confirmed that AP736 decreased melanin production in a dose-dependent manner; however, it did not directly inhibit tyrosinase, the rate-limiting melanogenic enzyme. The expression of microphthalmia-associated transcription factor, tyrosinase, and related signal transduction pathways was also investigated. The Wnt signaling pathway is deeply involved in melanogenesis; therefore, phosphorylation by GSK3β was assessed following treatment with AP736. AP736 induced GSK3β phosphorylation (inactivation), but it did not alter the level of β-catenin. Furthermore, the expression of α-melanocyte-stimulating hormone-induced tyrosinase was downregulated by AP736. Our data suggest that AP736 exerts hypopigmentary effects through the downregulation of tyrosinase via GSK3β phosphorylation.

Microphthalmia-associated transcription factor in melanoma development and MAP-kinase pathway targeted therapy

Malignant melanoma is a neoplasm of melanocytes, and the microphthalmia-associated transcription factor (MITF) is essential for the existence of melanocytes. MITF's relevance for this cell lineage is maintained in melanoma, where it is an important regulator of survival and balances melanoma cell proliferation with terminal differentiation (pigmentation). The MITF gene is amplified in ~20% of melanomas and MITF mutation can predispose to melanoma development. Furthermore, the regulation of MITF expression and function is strongly linked to the BRAF/MEK/ERK/MAP-kinase (MAPK) pathway, which is deregulated in >90% of melanomas and central target of current therapies. MITF expression in melanoma is heterogeneous, and recent findings highlight the relevance of this heterogeneity for the response of melanoma to MAPK pathway targeting drugs, as well as for MITF's role in melanoma progression. This review aims to provide an updated overview on the regulation of MITF function and plasticity in melanoma with a focus on its link to MAPK signaling.

N-Nicotinoyl tyramine, a novel niacinamide derivative, inhibits melanogenesis by suppressing MITF gene expression

We synthesized and investigated the inhibitory effects of a novel niacinamide derivative, N-nicotinoyltyramine (NNT) on melanogenesis. NNT inhibited melanin production in B16F10 murine melanoma cells stimulated with α-melanocyte stimulating hormone (α-MSH), in human melanocyte and in three-dimensional cultured human skin model. NNT did not affect the catalytic activity of tyrosinase, but acted as an inhibitor of microphthalmia-associated transcription factor (MITF) and tyrosinase expressions in B16F10 cells. These findings suggest that the hypopigmentary effect of NNT results from the down-regulation of MITF and subsequently of tyrosinase, although NNT did not directly inhibit tyrosinase activity. In addition, safety of NNT was verified through performing neural stem cell morphology assay and Human repeated insult patch test as whitening agent. Our findings indicate that NNT may be a potential and non-skin irritant whitening agent for use in cosmetics and in the medical treatment of pigmentary disorders.

Conditioned Media from Human Umbilical Cord Blood-Derived Mesenchymal Stem Cells Inhibits Melanogenesis by Promoting Proteasomal Degradation of MITF

Human umbilical cord blood-derived mesenchymal stem cells (hUCB-MSCs) secrete various beneficial molecules, which have anti-apoptotic activity and cell proliferation. However, the effect of hUCB-MSCs in melanogenesis is largely unclear. In this study, we show that conditioned media (CM) derived from hUCB-MSCs inhibit melanogenesis by regulating microphthalmia-associated transcription factor (MITF) expression via the ERK signalling pathway. Treatment of hUCB-MSC-CM strongly inhibited the alpha-melanocyte stimulating hormone-induced hyperpigmentation in melanoma cells as well as melanocytes. Treatment of hUCB-MSC-CM induced ERK1/2 activation in melanocytes. In addition, inhibition of ERK1/2 suppressed the anti-pigmentation activity of the hUCB-MSC-CM in melanocytes and in vitro artificial skin models. We also found that the expression of MITF was appreciably diminished while expression of phosphorylated MITF, which leads to its proteasomal degradation, was increased in cells treated with hUCB-MSC-CM. These results suggested that hUCB-MSC-CM significantly suppresses melanin synthesis via MITF degradation by the ERK pathway activation.

Lipoteichoic acid isolated from Lactobacillus plantarum inhibits melanogenesis in B16F10 mouse melanoma cells

Lipoteichoic acid (LTA) is a major component of the cell wall of Gram-positive bacteria. Its effects on living organisms are different from those of lipopolysaccharide (LPS) found in Gram-negative bacteria. LTA contributes to immune regulatory effects including anti-aging. In this study, we showed that LTA isolated from Lactobacillus plantarum (pLTA) inhibited melanogenesis in B16F10 mouse melanoma cells. pLTA reduced the cellular activity of tyrosinase and the expression of tyrosinase family members in a dose-dependent manner. The expression of microphthalmia-associated transcription factor (MITF), a key factor in the synthesis of melanin, was also decreased by pLTA. Further, we showed that pLTA activated melanogenesis signaling, such as extracellular signal-regulated kinase (ERK) and phosphatidylinositol 3-kinse (PI3K)/AKT. In addition, the expression of heterogeneous nuclear ribonucleoprotein A1 (hnRNP A1) and HuR, which are important RNA-binding proteins (RBPs), was reduced. pLTA likely degrades MITF via regulation of melanogenic signaling and RNA stability of melanogenic proteins, resulting in the reduction of melanin. Thus, our data suggest that pLTA has therapeutic potential for treating hyperpigmentation disorders and can also be used as a cosmetic whitening agent.

Efficient delivery and functional expression of transfected modified mRNA in human embryonic stem cell-derived retinal pigmented epithelial cells

Gene- and cell-based therapies are promising strategies for the treatment of degenerative retinal diseases such as age-related macular degeneration, Stargardt disease, and retinitis pigmentosa. Cellular engineering before transplantation may allow the delivery of cellular factors that can promote functional improvements, such as increased engraftment or survival of transplanted cells. A current challenge in traditional DNA-based vector transfection is to find a delivery system that is both safe and efficient, but using mRNA as an alternative to DNA can circumvent these major roadblocks. In this study, we show that both unmodified and modified mRNA can be delivered to retinal pigmented epithelial (RPE) cells with a high efficiency compared with conventional plasmid delivery systems. On the other hand, administration of unmodified mRNA induced a strong innate immune response that was almost absent when using modified mRNA. Importantly, transfection of mRNA encoding a key regulator of RPE gene expression, microphthalmia-associated transcription factor (MITF), confirmed the functionality of the delivered mRNA. Immunostaining showed that transfection with either type of mRNA led to the expression of roughly equal levels of MITF, primarily localized in the nucleus. Despite these findings, quantitative RT-PCR analyses showed that the activation of the expression of MITF target genes was higher following transfection with modified mRNA compared with unmodified mRNA. Our findings, therefore, show that modified mRNA transfection can be applied to human embryonic stem cell-derived RPE cells and that the method is safe, efficient, and functional.

MITF in melanoma: mechanisms behind its expression and activity

MITF (microphthalmia-associated transcription factor) represents a melanocytic lineage-specific transcription factor whose role is profoundly extended in malignant melanoma. Over the last few years, the function of MITF has been tightly connected to plasticity of melanoma cells. MITF participates in executing diverse melanoma phenotypes defined by distinct gene expression profiles. Mutation-dependent alterations in MITF expression and activity have been found in a relatively small subset of melanomas. MITF activity is rather modulated by its upstream activators and suppressors operating on transcriptional, post-transcriptional and post-translational levels. These regulatory mechanisms also include epigenetic and microenvironmental signals. Several transcription factors and signaling pathways involved in the regulation of MITF expression and/or activity such as the Wnt/β-catenin pathway are broadly utilized by various types of tumors, whereas others, e.g., BRAF^V600E/ERK1/2 are more specific for melanoma. Furthermore, the MITF activity can be affected by the availability of transcriptional co-partners that are often redirected by MITF from their own canonical signaling pathways. In this review, we discuss the complexity of a multilevel regulation of MITF expression and activity that underlies distinct context-related phenotypes of melanoma and might explain diverse responses of melanoma patients to currently used therapeutics.

Mitochondrial dynamics regulate melanogenesis through proteasomal degradation of MITF via ROS-ERK activation

Mitochondrial dynamics control mitochondrial functions as well as their morphology. However, the role of mitochondrial dynamics in melanogenesis is largely unknown. Here, we show that mitochondrial dynamics regulate melanogenesis by modulating the ROS-ERK signaling pathway. Genetic and chemical inhibition of Drp1, a mitochondrial fission protein, increased melanin production and mitochondrial elongation in melanocytes and melanoma cells. In contrast, down-regulation of OPA1, a mitochondria fusion regulator, suppressed melanogensis but induced massive mitochondrial fragmentation in hyperpigmented cells. Consistently, treatment with CCCP, a mitochondrial fission chemical inducer, also efficiently repressed melanogenesis. Furthermore, we found that ROS production and ERK phosphorylation were increased in cells with fragmented mitochondria. And inhibition of ROS or ERK suppressed the antimelanogenic effect of mitochondrial fission in α-MSH-treated cells. In addition, the activation of ROS-ERK pathway by mitochondrial fission induced phosphorylation of serine73 on MITF accelerating its proteasomal degradation. In conclusion, mitochondrial dynamics may regulate melanogenesis by modulating ROS-ERK signaling pathway.

Mechanistic studies of anti-hyperpigmentary compounds: elucidating their inhibitory and regulatory actions

Searching for depigmenting agents from natural sources has become a new direction in the cosmetic industry as natural products are generally perceived as relatively safer. In our previous study, selected Chinese medicines traditionally used to treat hyperpigmentation were tested for anti-hyperpigmentary effects using a melan-a cell culture model. Among the tested chemical compounds, 4-ethylresorcinol, 4-ethylphenol and 1-tetradecanol were found to possess hypopigmentary effects. Western blot analysis, reverse transcriptase polymerase chain reaction (RT-PCR), cyclic adenosine monophosphate (cAMP) assay, protein kinase A (PKA) activity assay, tyrosinase inhibition assay and lipid peroxidation inhibition assay were performed to reveal the underlying cellular and molecular mechanisms of the hypopigmentary effects. 4-Ethylresorcinol and 4-ethylphenol attenuated mRNA and protein expression of tyrosinase-related protein (TRP)-2, and possessed antioxidative effect by inhibiting lipid peroxidation. 1-Tetradecanol was able to attenuate protein expression of tyrosinase. The hypopigmentary actions of 4-ethylresorcinol, 4-ethylphenol and 1-tetradecanol were associated with regulating downstream proteins along the PKA pathway. 4-Ethylresorcinol was more effective in inhibiting melanin synthesis when compared to 4-ethylphenol and 1-tetradecanol.

Sumo E2 enzyme UBC9 is required for efficient protein quality control in cardiomyocytes

RATIONALE

Impairment of proteasomal function is pathogenic in several cardiac proteinopathies and can eventually lead to heart failure. Loss of proteasomal activity often results in the accumulation of large protein aggregates. The ubiquitin proteasome system (UPS) is primarily responsible for cellular protein degradation, and although the role of ubiquitination in this process is well studied, the function of an ancillary post-translational modification, SUMOylation, in protein quality control is not fully understood.

OBJECTIVE

To determine the role of ubiquitin-conjugating enzyme 9 (UBC9), a small ubiquitin-like modifier-conjugating enzyme, in cardiomyocyte protein quality control.

METHODS AND RESULTS

Gain- and loss-of-function approaches were used to determine the importance of UBC9. Overexpression of UBC9 enhanced UPS function in cardiomyocytes, whereas knockdown of UBC9 by small interfering RNA caused significant accumulations of aggregated protein. UPS function and relative activity was analyzed using a UPS reporter protein consisting of a short degron, CL1, fused to the COOH-terminus of green fluorescent protein (GFPu). Subsequently, the effects of UBC9 on UPS function were tested in a proteotoxic model of desmin-related cardiomyopathy, caused by cardiomyocyte-specific expression of a mutated αB crystallin, CryAB(R120G). CryAB(R120G) expression leads to aggregate formation and decreased proteasomal function. Coinfection of UBC9-adenovirus with CryAB(R120G) virus reduced the proteotoxic sequelae, decreasing overall aggregate concentrations. Conversely, knockdown of UBC9 significantly decreased UPS function in the model and resulted in increased aggregate levels.

CONCLUSIONS

UBC9 plays a significant role in cardiomyocyte protein quality control, and its activity can be exploited to reduce toxic levels of misfolded or aggregated proteins in cardiomyopathy.

Anti-melanogenic effects of δ-tocotrienol are associated with tyrosinase-related proteins and MAPK signaling pathway in B16 melanoma cells

Tocotrienols are known to possess potent antioxidant, anticancer, and cholesterol lowering activities. Being able to rapidly penetrate the skin, these vitamin E isoforms have been explored for potential treatment against melanoma. This study aimed to elucidate the mechanism involved in the anti-melanogenic effects of δ-tocotrienol (δT3) in B16 melanoma cells. Results showed that at 20 μM of δT3 significantly inhibited melanin formation and ROS generation. Treatment with δT3 also effectively suppressed the expression of melanogenesis-related proteins, including MC1R, MITF, TYRP-1, and TYRP-2. More importantly, we observed that the mitogen-activated protein kinase (MAPK) pathway was involved in mediating δT3's inhibitory effect against melanin production. Specifically, δT3 treatment markedly induced the activation of extracellular signal-regulated kinases (ERK). The use of ERK activation inhibitor (PD98059) abrogated the δT3-mediated downregulation expression melanogenesis-related proteins and restored melanin production. Furthermore, siRNA targeting ERK effectively blocked the δT3-induced repression of tyrosinase and TYRP-1 expression. These results suggest that δT3's inhibitory effect against melanogenesis is mediated by the activation of ERK signaling, thereby resulting in downstream repression of melanogenesis-related proteins and the subsequent melanin production. These data provide insight to δT3's effect and the targeting of ERK signaling for treatment against melanogenesis.

Melanoma: from melanocyte to genetic alterations and clinical options

Metastatic melanoma remained for decades without any effective treatment and was thus considered as a paradigm of cancer resistance. Recent progress with understanding of the molecular mechanisms underlying melanoma initiation and progression revealed that melanomas are genetically and phenotypically heterogeneous tumors. This recent progress has allowed for the development of treatment able to improve for the first time the overall disease-free survival of metastatic melanoma patients. However, clinical responses are still either too transient or limited to restricted patient subsets. The complete cure of metastatic melanoma therefore remains a challenge in the clinic. This review aims to present the recent knowledge and discoveries of the molecular mechanisms involved in melanoma pathogenesis and their exploitation into clinic that have recently facilitated bench to bedside advances.

Cinnamomum cassia essential oil inhibits α-MSH-induced melanin production and oxidative stress in murine B16 melanoma cells

Essential oils extracted from aromatic plants exhibit important biological activities and have become increasingly important for the development of aromatherapy for complementary and alternative medicine. The essential oil extracted from Cinnamomum cassia Presl (CC-EO) has various functional properties; however, little information is available regarding its anti-tyrosinase and anti-melanogenic activities. In this study, 16 compounds in the CC-EO have been identified; the major components of this oil are cis-2-methoxycinnamic acid (43.06%) and cinnamaldehyde (42.37%). CC-EO and cinnamaldehyde exhibited anti-tyrosinase activities; however, cis-2-methoxycinnamic acid did not demonstrate tyrosinase inhibitory activity. In murine B16 melanoma cells stimulated with α-melanocyte-stimulating hormone (α-MSH), CC-EO and cinnamaldehyde not only reduced the melanin content and tyrosinase activity of the cells but also down-regulated tyrosinase expression without exhibiting cytotoxicity. Moreover, CC-EO and cinnamaldehyde decreased thiobarbituric acid-reactive substance (TBARS) levels and restored glutathione (GSH) and catalase activity in the α-MSH-stimulated B16 cells. These results demonstrate that CC-EO and its major component, cinnamaldehyde, possess potent anti-tyrosinase and anti-melanogenic activities that are coupled with antioxidant properties. Therefore, CC-EO may be a good source of skin-whitening agents and may have potential as an antioxidant in the future development of complementary and alternative medicine-based aromatherapy.

The use of human tumour cell lines in the discovery of new cancer chemotherapeutic drugs

BACKGROUND

Human tumour cell lines have played a major role in anticancer drug discovery, but cell lines may model only some aspects of tumour behaviour in cancer patients. Growing evidence supports a theory that stem cells with self-renewing properties sustain tumours.

OBJECTIVE

This review considers the extent to which a deeper understanding of the origin and properties of tumour cell lines might lead to new strategies for anticancer drug discovery.

METHODS

Recent literature on normal and tumour stem cells is reviewed and placed in the context of a discussion on the derivation and properties of tumour cell lines.

RESULTS/CONCLUSION

Early-passage cell lines may model the more rapidly proliferating cells in human tumours and, thus, retain some of the properties of tumour stem cells. The effects of anticancer drugs on cell lines should be considered not only with regards to the induction of apoptosis, but also to the induction of senescence or other pathways that lead to host immune and inflammatory responses.

The ubiquitin-proteasome system in retinal health and disease

The ubiquitin-proteasome system (UPS) is the main intracellular pathway for modulated protein turnover, playing an important role in the maintenance of cellular homeostasis. It also exerts a protein quality control through degradation of oxidized, mutant, denatured, or misfolded proteins and is involved in many biological processes where protein level regulation is necessary. This system allows the cell to modulate its protein expression pattern in response to changing physiological conditions and provides a critical protective role in health and disease. Impairments of UPS function in the central nervous system (CNS) underlie an increasing number of genetic and idiopathic diseases, many of which affect the retina. Current knowledge on the UPS composition and function in this tissue, however, is scarce and dispersed. This review focuses on UPS elements reported in the retina, including ubiquitinating and deubiquitinating enzymes (DUBs), and alternative proteasome assemblies. Known and inferred roles of protein ubiquitination, and of the related, SUMO conjugation (SUMOylation) process, in normal retinal development and adult homeostasis are addressed, including modulation of the visual cycle and response to retinal stress and injury. Additionally, the relationship between UPS dysfunction and human neurodegenerative disorders affecting the retina, including Alzheimer's, Parkinson's, and Huntington's diseases, are dealt with, together with numerous instances of retina-specific illnesses with UPS involvement, such as retinitis pigmentosa, macular degenerations, glaucoma, diabetic retinopathy (DR), and aging-related impairments. This information, though still basic and limited, constitutes a suitable framework to be expanded in incoming years and should prove orientative toward future therapy design targeting sight-affecting diseases with a UPS underlying basis.

Liver X receptor activation inhibits melanogenesis through the acceleration of ERK-mediated MITF degradation

Liver X receptors (LXRs) are nuclear receptors that act as ligand-activated transcription factors regulating lipid metabolism and inflammation. In the skin, activation of LXRs stimulates differentiation of keratinocytes and augments lipid synthesis in sebocytes. However, the function of LXRs in melanocytes remains largely unknown. We investigated whether LXR activation would affect melanogenesis. In human primary melanocytes, MNT-1, and B16 melanoma cells, TO901317, a synthetic LXR ligand, inhibited melanogenesis. Small interfering RNA (siRNA) experiments revealed the dominant role of LXRβ in TO901317-mediated antimelanogenesis. Enzymatic activities of tyrosinase were unaffected, but the expression of tyrosinase, tyrosinase-related protein-1 (TRP-1), and TRP-2 was suppressed by TO901317. Expressions of microphthalmia-associated transcription factor (MITF), a master transcriptional regulator of melanogenesis, and cAMP-responsive element-binding activation were not affected. It is noteworthy that the degradation of MITF was accelerated by TO901317. Extracellular signal-regulated kinase (ERK) contributed to TO901317-induced antimelanogenesis, which was evidenced by recovery of melanogenesis with ERK inhibitor. Other LXR ligands, 22(R)-hydroxycholesterol (22(R)HC) and GW3965, also activated ERK and suppressed melanogenesis. The intermediary role of Ras was confirmed in TO901317-induced ERK phosphorylation. Finally, antimelanogenic effects of TO901317 were confirmed in vivo in UVB-tanning model in brown guinea pigs, providing a previously unreported line of evidence that LXRs may be important targets for antimelanogenesis.

Ceramide PC102 inhibits melanin synthesis via proteasomal degradation of microphthalmia-associated transcription factor and tyrosinase

A few types of ceramide are reported to decrease melanin synthesis. In the present study, we examined the effects of an artificial ceramide analog, PC102, on melanogenesis using a spontaneously immortalized melanocyte cell line (Mel-Ab). PC102 is currently used as a moisturizing additive in a variety of cosmetics. Our data showed that PC102 inhibited melanin production and tyrosinase activity in a dose-dependent manner, but did not directly affect tyrosinase activity. Microphthalmia-associated transcription factor (MITF), tyrosinase, and β-catenin protein levels decreased after 48 h of PC102 treatment. In contrast, PC102 did not decrease MITF, tyrosinase, and β-catenin mRNA levels. Therefore, we investigated whether the decrease in MITF and tyrosinase by PC102 is due to proteasomal degradation. MG132, a proteasomal inhibitor, completely abolished tyrosinase downregulation due to PC102 and partially reduced the downregulation of MITF and β-catenin due to PC102. Moreover, MG132 abrogated the inhibition of melanin synthesis by PC102. Taken together, our data suggest that PC102 may inhibit melanin synthesis through MITF and tyrosinase degradation.

Natural Melanogenesis Inhibitors Acting Through the Down-Regulation of Tyrosinase Activity

Melanogenesis is a biosynthetic pathway for the formation of the pigment melanin in human skin. A key enzyme, tyrosinase, catalyzes the first and only rate-limiting steps in melanogenesis, and the down-regulation of enzyme activity is the most reported method for the inhibition of melanogenesis. Because of the cosmetically important issue of hyperpigmentation, there is a big demand for melanogenesis inhibitors. This encourages researchers to seek potent melanogenesis inhibitors for cosmetic uses. This article reviews melanogenesis inhibitors that have been recently discovered from natural sources. The reaction mechanisms of the inhibitors on tyrosinase activity are also discussed.

Roles for the ubiquitin-proteasome pathway in protein quality control and signaling in the retina: implications in the pathogenesis of age-related macular degeneration

The accumulation of damaged or postsynthetically modified proteins and dysregulation of inflammatory responses and angiogenesis in the retina/RPE are thought be etiologically related to formation of drusen and choroidal neovascularization (CNV), hallmarks of age-related macular degeneration (AMD). The ubiquitin-proteasome pathway (UPP) plays crucial roles in protein quality control, cell cycle control and signal transduction. Selective degradation of aberrant proteins by the UPP is essential for timely removal of potentially cytotoxic damaged or otherwise abnormal proteins. Proper function of the UPP is thought to be required for cellular function. In contrast, age--or stress induced--impairment the UPP or insufficient UPP capacity may contribute to the accumulation of abnormal proteins, cytotoxicity in the retina, and AMD. Crucial roles for the UPP in eye development, regulation of signal transduction, and antioxidant responses are also established. Insufficient UPP capacity in retina and RPE can result in dysregulation of signal transduction, abnormal inflammatory responses and CNV. There are also interactions between the UPP and lysosomal proteolytic pathways (LPPs). Means that modulate the proteolytic capacity are making their way into new generation of pharmacotherapies for delaying age-related diseases and may augment the benefits of adequate nutrition, with regard to diminishing the burden of AMD.

Melanoma: from mutations to medicine

Melanoma is often considered one of the most aggressive and treatment-resistant human cancers. It is a disease that, due to the presence of melanin pigment, was accurately diagnosed earlier than most other malignancies and that has been subjected to countless therapeutic strategies. Aside from early surgical resection, no therapeutic modality has been found to afford a high likelihood of curative outcome. However, discoveries reported in recent years have revealed a near avalanche of breakthroughs in the melanoma field-breakthroughs that span fundamental understanding of the molecular basis of the disease all the way to new therapeutic strategies that produce unquestionable clinical benefit. These discoveries have been born from the successful fruits of numerous researchers working in many-sometimes-related, although also distinct-biomedical disciplines. Discoveries of frequent mutations involving BRAF(V600E), developmental and oncogenic roles for the microphthalmia-associated transcription factor (MITF) pathway, clinical efficacy of BRAF-targeted small molecules, and emerging mechanisms underlying resistance to targeted therapeutics represent just a sample of the findings that have created a striking inflection in the quest for clinically meaningful progress in the melanoma field.

Endocannabinoids stimulate human melanogenesis via type-1 cannabinoid receptor

We show that a fully functional endocannabinoid system is present in primary human melanocytes (normal human epidermal melanocyte cells), including anandamide (AEA), 2-arachidonoylglycerol, the respective target receptors (CB(1), CB(2), and TRPV1), and their metabolic enzymes. We also show that at higher concentrations AEA induces normal human epidermal melanocyte apoptosis (∼3-fold over controls at 5 μM) through a TRPV1-mediated pathway that increases DNA fragmentation and p53 expression. However, at lower concentrations, AEA and other CB(1)-binding endocannabinoids dose-dependently stimulate melanin synthesis and enhance tyrosinase gene expression and activity (∼3- and ∼2-fold over controls at 1 μM). This CB(1)-dependent activity was fully abolished by the selective CB(1) antagonist SR141716 or by RNA interference of the receptor. CB(1) signaling engaged p38 and p42/44 mitogen-activated protein kinases, which in turn activated the cyclic AMP response element-binding protein and the microphthalmia-associated transcription factor. Silencing of tyrosinase or microphthalmia-associated transcription factor further demonstrated the involvement of these proteins in AEA-induced melanogenesis. In addition, CB(1) activation did not engage the key regulator of skin pigmentation, cyclic AMP, showing a major difference compared with the regulation of melanogenesis by α-melanocyte-stimulating hormone through melanocortin 1 receptor.

In vivo role of alternative splicing and serine phosphorylation of the microphthalmia-associated transcription factor

The microphthalmia-associated transcription factor (MITF) is a basic helix-loop-helix leucine zipper protein that plays major roles in the development and physiology of vertebrate melanocytes and melanoma cells. It is regulated by post-translational modifications, including phosphorylation at serine 73, which based on in vitro experiments imparts on MITF an increased transcriptional activity paired with a decreased stability. Serine 73 is encoded by the alternatively spliced exon 2B, which is preferentially skipped in mice carrying a targeted serine-73-to-alanine mutation. Here, we measured the relative abundance of exon 2B+ and exon 2B- RNAs in freshly isolated and FACS-sorted wild-type melanoblasts and melanocytes and generated a series of knock-in mice allowing forced incorporation of either alanine, aspartate, or wild-type serine at position 73. None of these knock-in alleles, however, creates a striking pigmentation phenotype on its own, but differences between them can be revealed either by a general reduction of Mitf transcript levels or in heteroallelic combinations with extant Mitf mutations. In fact, compared with straight serine-73 knock-in mice with their relative reduction of 2B+ Mitf, forced incorporation of alanine 73 leads to greater increases in MITF protein levels, melanoblast and melanocyte numbers, and extent of pigmentation in particular allelic combinations. These results underscore, in vivo, the importance of the link between alternative splicing and post-translational modifications and may bear on the recent observation that exon 2B skipping can be found in metastatic melanoma.

Transcription factor E3, a major regulator of mast cell-mediated allergic response

BACKGROUND

Microphthalmia transcription factor, an MiT transcription family member closely related to transcription factor E3 (TFE3), is essential for mast cell development and survival. TFE3 was previously reported to play a role in the functions of B and T cells; however, its role in mast cells has not yet been explored.

OBJECTIVE

We sought to explore the role played by TFE3 in mast cell function.

METHODS

Mast cell numbers were evaluated by using toluidine blue staining. FACS analysis was used to determine percentages of Kit and FcεRI double-positive cells in the peritoneum of wild-type (WT) and TFE3 knockout (TFE3(-/-)) mice. Cytokine and inflammatory mediator secretion were measured in immunologically activated cultured mast cells derived from either knockout or WT mice. In vivo plasma histamine levels were measured after immunologic triggering of these mice.

RESULTS

No significant differences in mast cell numbers between WT and TFE3(-/-) mice were observed in the peritoneum, lung, and skin. However, TFE3(-/-) mice showed a marked decrease in the number of Kit(+) and FcεRI(+) peritoneal and cultured mast cells. Surface expression levels of FcεRI in TFE3(-/-) peritoneal mast cells was significantly lower than in control cells. Cultured mast cells derived from TFE3(-/-) mice showed a marked decrease in degranulation and mediator secretion. In vivo experiments showed that the level of plasma histamine in TFE3(-/-) mice after an allergic trigger was substantially less than that seen in WT mice.

CONCLUSION

TFE3 is a novel regulator of mast cell functions and as such could emerge as a new target for the manipulation of allergic diseases.

Role of the ubiquitin-proteasome in protein quality control and signaling: implication in the pathogenesis of eye diseases

The ubiquitin-proteasome pathway (UPP) plays important roles in many cellular functions, such as protein quality control, cell cycle control, and signal transduction. The selective degradation of aberrant proteins by the UPP is essential for the timely removal of potential cytotoxic damaged or otherwise abnormal proteins. Conversely, accumulation of the cytotoxic abnormal proteins in eye tissues is etiologically associated with many age-related eye diseases such as retina degeneration, cataract, and certain types of glaucoma. Age- or stress-induced impairment or overburdening of the UPP appears to contribute to the accumulation of abnormal proteins in eye tissues. Cell cycle and signal transduction are regulated by the conditional UPP-dependent degradation of the regulators of these processes. Impairment or overburdening of the UPP could also result in dysregulation of cell cycle control and signal transduction. The consequences of the improper cell cycle and signal transduction include defects in ocular development, wound healing, angiogenesis, or inflammatory responses. Methods that enhance or preserve UPP function or reduce its burden may be useful strategies for preventing age-related eye diseases.

Identification of genomic regions associated with phenotypic variation between dog breeds using selection mapping

The extraordinary phenotypic diversity of dog breeds has been sculpted by a unique population history accompanied by selection for novel and desirable traits. Here we perform a comprehensive analysis using multiple test statistics to identify regions under selection in 509 dogs from 46 diverse breeds using a newly developed high-density genotyping array consisting of >170,000 evenly spaced SNPs. We first identify 44 genomic regions exhibiting extreme differentiation across multiple breeds. Genetic variation in these regions correlates with variation in several phenotypic traits that vary between breeds, and we identify novel associations with both morphological and behavioral traits. We next scan the genome for signatures of selective sweeps in single breeds, characterized by long regions of reduced heterozygosity and fixation of extended haplotypes. These scans identify hundreds of regions, including 22 blocks of homozygosity longer than one megabase in certain breeds. Candidate selection loci are strongly enriched for developmental genes. We chose one highly differentiated region, associated with body size and ear morphology, and characterized it using high-throughput sequencing to provide a list of variants that may directly affect these traits. This study provides a catalogue of genomic regions showing extreme reduction in genetic variation or population differentiation in dogs, including many linked to phenotypic variation. The many blocks of reduced haplotype diversity observed across the genome in dog breeds are the result of both selection and genetic drift, but extended blocks of homozygosity on a megabase scale appear to be best explained by selection. Further elucidation of the variants under selection will help to uncover the genetic basis of complex traits and disease.

There are hundreds of dog breeds that exhibit massive differences in appearance and behavior sculpted by tightly controlled selective breeding. This large-scale natural experiment has provided an ideal resource that geneticists can use to search for genetic variants that control these differences. With this goal, we developed a high-density array that surveys variable sites at more than 170,000 positions in the dog genome and used it to analyze genetic variation in 46 breeds. We identify 44 chromosomal regions that are extremely variable between breeds and are likely to control many of the traits that vary between them, including curly tails and sociality. Many other regions also bear the signature of strong artificial selection. We characterize one such region, known to associate with body size and ear type, in detail using “next-generation” sequencing technology to identify candidate mutations that may control these traits. Our results suggest that artificial selection has targeted genes involved in development and metabolism and that it may have increased the incidence of disease in dog breeds. Knowledge of these regions will be of great importance for uncovering the genetic basis of variation between dog breeds and for finding mutations that cause disease.

C-KIT signaling depends on microphthalmia-associated transcription factor for effects on cell proliferation

The development of melanocytes is regulated by the tyrosine kinase receptor c-KIT and the basic-helix-loop-helix-leucine zipper transcription factor Mitf. These essential melanocyte survival regulators are also well known oncogenic factors in malignant melanoma. Despite their importance, not much is known about the regulatory mechanisms and signaling pathways involved. In this study, we therefore sought to identify the signaling pathways and mechanisms involved in c-KIT mediated regulation of Mitf. We report that c-KIT stimulation leads to the activation of Mitf specifically through the c-KIT phosphorylation sites Y721 (PI3 kinase binding site), Y568 and Y570 (Src binding site). Our study not only confirms the involvement of Ras-Erk signaling pathway in the activation of Mitf, but also establishes that Src kinase binding to Y568 and Y570 of c-KIT is required. Using specific inhibitors we observe and verify that c-KIT induced activation of Mitf is dependent on PI3-, Akt-, Src-, p38- or Mek kinases. Moreover, the proliferative effect of c-KIT is dependent on Mitf in HEK293T cells. In contrast, c-KIT Y568F and Y721F mutants are less effective in driving cell proliferation, compared to wild type c-KIT. Our results reveal novel mechanisms by which c-KIT signaling regulates Mitf, with implications for understanding both melanocyte development and melanoma.

Involvement of mTOR signaling in sphingosylphosphorylcholine-induced hypopigmentation effects

BACKGROUND

Sphingosylphosphorylcholine (SPC) acts as a potent lipid mediator and signaling molecule in various cell types. In the present study, we investigated the effects of SPC on melanogenesis and SPC-modulated signaling pathways related to melanin synthesis.

METHODS

Melanin production was measured in Mel-Ab cells. A luciferase assay was used to detect transcriptional activity of the MITF promoter. Western blot analysis was performed to examine SPC-induced signaling pathways.

RESULTS

SPC produced significant hypopigmentation effects in a dose-dependent manner. It was found that SPC induced not only activation of Akt but also stimulation of mTOR, a downstream mediator of the Akt signaling pathway. Moreover, SPC decreased the levels of LC3 II, which is known to be regulated by mTOR. Treatment with the mTOR inhibitor rapamycin eliminated decreases in melanin and LC3 II levels by SPC. Furthermore, we found that the Akt inhibitor LY294002 restored SPC-mediated downregulation of LC3 II and inhibited the activation of mTOR by SPC.

CONCLUSIONS

Our data suggest that the mTOR signaling pathway is involved in SPC-modulated melanin synthesis.

Regulation of MITF stability by the USP13 deubiquitinase

The microphthalmia-associated transcription factor (MITF) is essential for melanocyte development. Mutation-induced MAPK pathway activation is common in melanoma and induces MITF phosphorylation, ubiquitination, and proteolysis. Little is known about the enzymes involved in MITF ubiquitination/deubiquitination. Here we report the identification of a deubiquitinating enzyme, named ubiquitin-specific protease 13 (USP13) that appears to be responsible for MITF deubiquitination, utilizing a short hairpin RNA library against known deubiquitinating enzymes. Through deubiquitination, USP13 stabilizes and upregulates MITF protein levels. Conversely, suppression of USP13 (through knockdown) leads to dramatic loss of MITF protein, but not messenger RNA. Through its effects on MITF deubiquitination, USP13 was observed to modulate expression of MITF downstream target genes and, thereby, to be essential for melanoma growth in soft agar and in nude mice. These observations suggest that as a potentially drugable protease, USP13 might be a viable therapeutic target for melanoma.