Tyrosinase degradation via two pathways during reverse translocation to the cytosol

Inhibition of glycoprotein synthesis in the endoplasmic reticulum as a novel anticancer mechanism of (-)-epigallocatechin-3-gallate

(-)-Epigallocatechin-3-gallate (EGCG) has been found to trigger the unfolded protein response (UPR) likely due to the inhibition of glucosidase II, a key enzyme of glycoprotein processing and quality control in the endoplasmic reticulum (ER). These findings strongly suggest that EGCG interferes with glycoprotein maturation and sorting in the ER. This hypothesis was tested in SK-Mel28 human melanoma cells by assessing the effect of EGCG and deoxynojirimycin (DNJ) on the synthesis of two endogenous glycoproteins. Both tyrosinase and vascular endothelial growth factor (VEGF) protein levels were remarkably reduced despite unaltered mRNA expression in EGCG- or DNJ-treated cells compared to control. The hindrance of tyrosinase and VEGF protein synthesis could be prevented by proteasome inhibitor, lactacystine. Collectively, our results support that glucosidase II inhibitor EGCG interferes with protein processing and quality control in the ER, which diverts tyrosinase, VEGF, and likely other glycoproteins towards proteasomal degradation. This mechanism provides a novel therapeutic approach in dermatology and might play an important role in the antitumor effect or hepatotoxicity of EGCG.

Insights into the processing of MHC class I ligands gained from the study of human tumor epitopes

The molecular definition of tumor antigens recognized by cytolytic T lymphocytes (CTL) started in the late 1980s, at a time when the MHC class I antigen processing field was in its infancy. Born together, these two fields of science evolved together and provided each other with critical insights. Over the years, stimulated by the potential interest of tumor antigens for cancer immunotherapy, scientists have identified and characterized numerous antigens recognized by CTL on human tumors. These studies have provided a wealth of information relevant to the mode of production of antigenic peptides presented by MHC class I molecules. A number of tumor antigenic peptides were found to result from unusual mechanisms occurring at the level of transcription, translation or processing. Although many of these mechanisms occur in the cell at very low level, they are relevant to the immune system as they determine the killing of tumor cells by CTL, which are sensitive to low levels of peptide/MHC complexes. Moreover, these unusual mechanisms were found to occur not only in tumor cells but also in normal cells. Thereby, the study of tumor antigens has illuminated many aspects of MHC class I processing. We review here those insights into the MHC I antigen processing pathway that result from the characterization of human tumor antigens recognized by CTL.

Insights into MHC class I antigen processing gained from large-scale analysis of class I ligands

Short peptides derived from intracellular proteins and presented on MHC class I molecules on the cell surface serve as a showcase for the immune system to detect pathogenic or malignant alterations inside the cell, and the sequencing and analysis of the presented peptide pool has received considerable attention over the last two decades. In this review, we give a comprehensive presentation of the methods employed for the large-scale qualitative and quantitative analysis of the MHC class I ligandome. Furthermore, we focus on insights gained into the underlying processing pathway, especially involving the roles of the proteasome, the TAP complex, and the peptide specificities and motifs of MHC molecules. The identification of post-translational modifications in MHC ligands and their implications for processing are also considered. Finally, we review the correlations of the ligandome to the proteome and the transcriptome.

Role of the ubiquitin proteasome system in regulating skin pigmentation

Pigmentation of the skin, hair and eyes is regulated by tyrosinase, the critical rate-limiting enzyme in melanin synthesis by melanocytes. Tyrosinase is degraded endogenously, at least in part, by the ubiquitin proteasome system (UPS). Several types of inherited hypopigmentary diseases, such as oculocutaneous albinism and Hermansky-Pudlak syndrome, involve the aberrant processing and/or trafficking of tyrosinase and its subsequent degradation which can occur due to the quality-control machinery. Studies on carbohydrate modifications have revealed that tyrosinase in the endoplasmic reticulum (ER) is proteolyzed via ER-associated protein degradation and that tyrosinase degradation can also occur following its complete maturation in the Golgi. Among intrinsic factors that regulate the UPS, fatty acids have been shown to modulate tyrosinase degradation in contrasting manners through increased or decreased amounts of ubiquitinated tyrosinase that leads to its accelerated or decelerated degradation by proteasomes.

Long-term suppression of tyrosinase by terrein via tyrosinase degradation and its decreased expression

Previously, we reported that a fungal metabolite, terrein, decreases melanin synthesis via downregulation of microphthalmia-associated transcription factor (MITF). In the present study, we further investigated the long-term hypopigmenting action of terrein in a spontaneously immortalized mouse melanocyte cell line, Mel-Ab. Treatment with terrein at a concentration of 50 mum strongly decreased melanogenesis in a time-dependent manner. Interestingly, the decreased tyrosinase protein levels lasted for at least 7 days, even though the MITF protein levels were restored after 3 days of treatment. In accordance with the results of Western blot analyses, the tyrosinase mRNA levels were found to be continuously decreased for at least 7 days, even though recovery of the MITF mRNA levels began after 3 days of terrein treatment. Therefore, we evaluated tyrosinase downregulation to determine if it is caused by proteasomal degradation. We found that the reduction in tyrosinase levels that was induced by terrein was clearly recovered by MG-132, a proteasome inhibitor. Moreover, ubiquitination of tyrosinase increased following treatment with terrein in the presence of MG-132. Taken together, these results suggest that terrein decreases melanogenesis through ubiquitin-dependent proteasomal degradation as well as via decreased expression of its mRNA.

Evidence for tyrosinase as a beta1,6 branch containing glycoprotein: substrate of GnT-V

Tyrosinase is a rate-limiting enzyme in mammalian melanogenesis, and is known as a glycoprotein. Post-translational processing of mammalian tyrosinase is required for its folding, sorting, and for enzymatic activity. Here we show for the first time that the mammalian tyrosinase has beta1,6-branched N-glycan structure that can be recognized by binding with specific lectin Leukoagglutinating phytohematoagglutinin (L-PHA). Further, this specific glycoconjugate structure has been shown to have a function relationship in melanin synthesis.

Tyrosinase and ocular diseases: Some novel thoughts on the molecular basis of oculocutaneous albinism type 1

Tyrosinase (TYR) is a multifunctional copper-containing glycoenzyme (approximately 80 kDa), which plays a key role in the rate-limiting steps of the melanin biosynthetic pathway. This membrane-bound protein, possibly evolved by the fusion of two different copper-binding proteins, is mainly expressed in epidermal, ocular and follicular melanocytes. In the melanocytes, TYR functions as an integrated unit with other TYR-related proteins (TYRP1, TYRP2), lysosome-associated membrane protein 1 (LAMP1) and melanocyte-stimulating hormone receptors; thus forming a melanogenic complex. Mutations in the TYR gene (TYR, 11q14-21, MIM 606933) cause oculocutaneous albinism type 1 (OCA1, MIM 203100), a developmental disorder having an autosomal recessive mode of inheritance. In addition, TYR can act as a modifier locus for primary congenital glaucoma (PCG) and it also contributes significantly in the eye developmental process. Expression of TYR during neuroblast division helps in later pathfinding by retinal ganglion cells from retina to the dorsal lateral geniculate nucleus. However, mutation screening of TYR is complicated by the presence of a pseudogene-TYR like segment (TYRL, 11p11.2, MIM 191270), sharing approximately 98% sequence identity with the 3' region of TYR. Thus, in absence of a full-proof strategy, any nucleotide variants identified in the 3' region of TYR could actually be present in TYRL. Interestingly, despite extensive search, the second TYR mutation in 15% of the OCA1 cases remains unidentified. Several possible locations of these "uncharacterized mutations" (UCMs) have been speculated so far. Based on the structure of TYR gene, its sequence context and some experimental evidences, we propose two additional possibilities, which on further investigations might shed light on the molecular basis of UCMs in TYR of OCA1 patients; (i) partial deletion of the exons 4 and 5 region of TYR that is homologous with TYRL and (ii) variations in the polymorphic GA complex repeat located between distal and proximal elements of the human TYR promoter that can modulate the expression of the gene leading to disease pathogenesis.

Beta1,6-branched oligosaccharides regulate melanin content and motility in macrophage-melanoma fusion hybrids

In previous studies, fusion of peritoneal macrophages or blood monocytes with mouse melanoma cells produced hybrids with upregulated expression of the glycosyltransferase beta1,6-N-acetylglucosaminyltransferase V (GnT-V) and its enzymatic product, beta1,6-branched oligosaccharides. This correlated with marked increases in motility, metastatic potential and, surprisingly, melanin content. This study was designed to establish direct roles for beta1,6-branched oligosaccharides in melanogenesis and motility. The levels of beta1,6-branched oligosaccharides were lowered by transfecting beta1,4-N-acetylglucosaminyltransferase III, a competitive inhibitor of GnT-V. beta1,4-N-acetylglucosaminyltransferase III transfection virtually eliminated melanin production and markedly decreased chemotactic motility. This implied that the metastatic and melanogenic phenotypes in hybrids were each upregulated by beta1,6-branched oligosaccharides. Although roles for beta1,6-branched oligosaccharides in motility and metastasis have been reported previously, this is the first study to directly implicate these structures in melanogenesis. Although drawn from experimental models, the findings might explain the well known hypermelanotic regions of human cutaneous malignant melanoma as hypermelanotic cutaneous malignant melanoma cells are rich in beta1,6-branched oligosaccharides. They might also explain why melanogenesis pathways differ between malignant and normal melanocytes as GnT-V is a myeloid-associated enzyme that is aberrantly expressed in melanoma cells but not in normal melanocytes.

Approaches to Identify Inhibitors of Melanin Biosynthesis via the Quality Control of Tyrosinase

Tyrosinase, a copper-containing glycoprotein, is the rate-limiting enzyme critical for melanin biosynthesis in specialized organelles termed melanosomes that are produced only by melanocytic cells. Inhibitors of tyrosinase activity have long been sought as therapeutic means to treat cutaneous hyperpigmentary disorders. Multiple potential approaches exist that could control pigmentation via the regulation of tyrosinase activity, for example: the transcription of its messenger RNA, its maturation via glycosylation, its trafficking to melanosomes, as well as modulation of its catalytic activity and/or stability. However, relatively little attention has been paid to regulating pigmentation via the stability of tyrosinase, which depends on its processing and maturation in the endoplasmic reticulum and Golgi, its delivery to melanosomes and its degradation via the ubiquitin-proteasome pathway and/or the endosomal/lysosomal system. Recently, it has been shown that carbohydrate modification, molecular chaperone engagement, and ubiquitylation all play pivotal roles in regulating the degradation/stability of tyrosinase. While such processes affect virtually all proteins, such effects on tyrosinase have immediate and dramatic consequences on pigmentation. In this review, we classify melanogenic inhibitory factors in terms of their modulation of tyrosinase function and we summarize current understanding of how the quality control of tyrosinase processing impacts its stability and melanogenic activity.

Intracellular composition of fatty acid affects the processing and function of tyrosinase through the ubiquitin-proteasome pathway

Proteasomes are multicatalytic proteinase complexes within cells that selectively degrade ubiquitinated proteins. We have recently demonstrated that fatty acids, major components of cell membranes, are able to regulate the proteasomal degradation of tyrosinase, a critical enzyme required for melanin biosynthesis, in contrasting manners by relative increases or decreases in the ubiquitinated tyrosinase. In the present study, we show that altering the intracellular composition of fatty acids affects the post-Golgi degradation of tyrosinase. Incubation with linoleic acid (C18:2) dramatically changed the fatty acid composition of cultured B16 melanoma cells, i.e. the remarkable increase in polyunsaturated fatty acids such as linoleic acid and arachidonic acid (C20:4) was compensated by the decrease in monounsaturated fatty acids such as oleic acid (C18:1) and palmitoleic acid (C16:1), with little effect on the proportion of saturated to unsaturated fatty acid. When the composition of intracellular fatty acids was altered, tyrosinase was rapidly processed to the Golgi apparatus from the ER (endoplasmic reticulum) and the degradation of tyrosinase was increased after its maturation in the Golgi. Retention of tyrosinase in the ER was observed when cells were treated with linoleic acid in the presence of proteasome inhibitors, explaining why melanin synthesis was decreased in cells treated with linoleic acid and a proteasome inhibitor despite the abrogation of tyrosinase degradation. These results suggest that the intracellular composition of fatty acid affects the processing and function of tyrosinase in connection with the ubiquitin-proteasome pathway and suggest that this might be a common physiological approach to regulate protein degradation.

Immunity to melanoma antigens: from self-tolerance to immunotherapy

The development of effective immune therapy for cancer is a central goal of immunologists in the 21st century. Our laboratories have been deeply involved in characterization of the immune response to melanoma and translation of laboratory discoveries into clinical trials. We have identified a cohort of peptide antigens presented by Major Histocompatibility Complex (MHC) molecules on melanoma cells and widely recognized by T cells from melanoma patients. These have been incorporated into peptide-based vaccines that induce CD8(+) and CD4(+) T-cell responses in 80-100% of patients. Major objective clinical tumor regressions have been observed in some patients, and overall survival in vaccinated patients exceeds expected stage-specific survival. New clinical trials will determine the value of combination of melanoma helper peptides (MHP) into multipeptide vaccines targeting CD8 cells. New trials will also evaluate new approaches to modulating the host-tumor relationship and will develop new combination therapies. Parallel investigations in murine models are elucidating the immunobiology of the melanoma-host relationship and addressing issues that are not feasible to approach in human trials. Based on the fact that the largest cohort of melanoma antigens are derived from normal proteins concerned with pigment production, we have evaluated the mechanisms of self-tolerance to tyrosinase (Tyr) and have determined how T cells in an environment of self-tolerance are impacted by immunization. Using peptide-pulsed dendritic cells as immunogens, we have also used the mouse model to establish strategies for quantitative and qualitative enhancement of antitumor immunity. This information creates opportunities for a new generation of therapeutic interventions using cancer vaccines.

Post-translational modifications of naturally processed MHC-binding epitopes

A variety of different post-translational modifications of peptides displayed by class I and II MHC molecules have now been described. Some modifications promote the binding of peptides to MHC molecules, and might also influence the ability of the peptide to be produced by antigen processing pathways. In some instances, the antigen processing components themselves are actually responsible for generating post-translational modifications. Finally, evidence is accumulating that modifications can be altered as a consequence of inflammation, transformation, apoptosis and aging. This leads to altered repertories of MHC-associated peptides, which may be important in immune responses associated with autoimmune diseases, infection and cancer.

Regulated folding of tyrosinase in the endoplasmic reticulum demonstrates that misfolded full-length proteins are efficient substrates for class I processing and presentation

Short-lived protein translation products have been proposed to be the principal substrates that enter the class I MHC processing and presentation pathway. However, the biochemical nature of these substrates is poorly defined. Whether the major processing substrates are misfolded full-length proteins, or alternatively, aberrantly initiated or truncated polypeptides still remains to be addressed. To examine this, we used melanoma in which one-third of wild-type tyrosinase molecules were correctly folded and localized beyond the Golgi, while the remainder were present in the endoplasmic reticulum in an unfolded/misfolded state. Increasing the efficiency of tyrosinase folding using chemical chaperones led to a reduction in the level of substrate available to the proteasome and decreased the expression of a tyrosinase-derived epitope. Conversely, in transfectants expressing tyrosinase mutants that are completely misfolded, both proteasome substrate and epitope presentation were significantly enhanced. Proteasome substrate availability was a consequence of misfolding and not simply due to retention in the endoplasmic reticulum. Thus, the extent of folding/misfolding of a full-length protein is an important determinant of the level of epitope presentation.

25-Hydroxycholesterol Acts in the Golgi Compartment to Induce Degradation of Tyrosinase

Oxysterols play a significant role in cholesterol homeostasis. 25-Hydroxycholesterol (25HC) in particular has been demonstrated to regulate cholesterol homeostasis via oxysterol-binding protein and oxysterol-related proteins, the sterol regulatory element binding protein, and the rate-limiting enzyme of cholesterol biosynthesis, hydroxymethylglutaryl coenzyme A reductase. We have examined the effect of 25HC on pigmentation of cultured murine melanocytes and demonstrated a decrease in pigmentation with an IC(50) of 0.34 microM and a significant diminution in levels of melanogenic protein tyrosinase. Pulse-chase studies of 25HC-treated cells demonstrated enhanced degradation of tyrosinase, the rate-limiting enzyme of melanin synthesis, following endoplasmic reticulum (ER) and Golgi maturation. Protein levels of GS28, a member of an ER/cis-Golgi SNARE protein complex, were also diminished in 25HC-treated melanocytes, however levels of the ER chaperone calnexin and the cis-Golgi matrix protein GM130 were unaffected. Effects of 25HC on tyrosinase were completely reversed by 4 alpha-allylcholestan-3 alpha-ol, a sterol identified by its ability to reverse effects of 25HC on cholesterol homeostasis. Finally, the addition of 25HC to lipid deficient serum inhibited correct processing of tyrosinase. We conclude that 25HC acts in the Golgi compartment to regulate pigmentation by a mechanism shared with cholesterol homeostasis.

Fatty acids regulate pigmentation via proteasomal degradation of tyrosinase: a new aspect of ubiquitin-proteasome function

Fatty acids are common components of biological membranes that are known to play important roles in intracellular signaling. We report here a novel mechanism by which fatty acids regulate the degradation of tyrosinase, a critical enzyme associated with melanin biosynthesis in melanocytes and melanoma cells. Linoleic acid (unsaturated fatty acid, C18:2) accelerated the spontaneous degradation of tyrosinase, whereas palmitic acid (saturated fatty acid, C16:0) retarded the proteolysis. The linoleic acid-induced acceleration of tyrosinase degradation could be abrogated by inhibitors of proteasomes, the multicatalytic proteinase complexes that selectively degrade intracellular ubiquitinated proteins. Linoleic acid increased the ubiquitination of many cellular proteins, whereas palmitic acid decreased such ubiquitination, as compared with untreated controls, when a proteasome inhibitor was used to stabilize ubiquitinated proteins. Immunoprecipitation analysis also revealed that treatment with fatty acids modulated the ubiquitination of tyrosinase, i.e. linoleic acid increased the amount of ubiquitinated tyrosinase whereas, in contrast, palmitic acid decreased it. Furthermore, confocal immunomicroscopy showed that the colocalization of ubiquitin and tyrosinase was facilitated by linoleic acid and diminished by palmitic acid. Taken together, these data support the view that fatty acids regulate the ubiquitination of tyrosinase and are responsible for modulating the proteasomal degradation of tyrosinase. In broader terms, the function of the ubiquitin-proteasome pathway might be regulated physiologically, at least in part, by fatty acids within cellular membranes.

Insights into antigen processing gained by direct analysis of the naturally processed class I MHC associated peptide repertoire

MHC class I molecules are responsible for the presentation of antigenic peptides to CD8+ T lymphocytes. Based on their relatively promiscuous binding of peptides, these molecules display information derived from a large fraction of proteins that are made inside the cell. This review describes our characterization of the peptides comprising this repertoire, with particular attention given to their complexity and quantities, their post-translational modification, and the pathways leading to their expression.

Molecular anatomy of tyrosinase and its related proteins: beyond the histidine-bound metal catalytic center

The structure of tyrosinase (Tyr) is reviewed from a double point of view. On the one hand, by comparison of all Tyr found throughout nature, from prokaryotic organisms to mammals and on the other, by comparison with the tyrosinase related proteins (Tyrps) that appeared late in evolution, and are only found in higher animals. Their structures are reviewed as a whole rather than focused on the histidine (His)-bound metal active site, which is the part of the molecule common to all these proteins. The availability of crystallographic data of hemocyanins and recently of sweet potato catechol oxidase has improved the model of the three-dimensional structure of the Tyr family. Accordingly, Tyr has a higher structural disorder than hemocyanins, particularly at the CuA site. The active site seems to be characterized by the formation of a hydrophobic pocket with a number of conserved aromatic residues sited close to the well-known His. Other regions specific of the mammalian enzymes, such as the cytosolic C-terminal tail, the cysteine clusters, and the N-glycosylation sequons, are also discussed. The complete understanding of the Tyr copper-binding domain and the characterization of the residues determinant of the relative substrate affinities of the Tyrps will improve the design of targeted mutagenesis experiments to understand the different catalytic capabilities of Tyr and Tyrps. This may assist future aims, from the design of more efficient bacterial Tyr for biotechnological applications to the design of inhibitors of undesirable fruit browning in vegetables or of color skin modulators in animals.