A novel antioxidant 3-O-Caffeoyl-1-methylquinic acid enhances ultraviolet A-mediated apoptosis in immortalized HaCaT keratinocytes via Sp1-dependent transcriptional activation of p21(WAF1/Cip1)

It has become clear that ultraviolet A (UVA) radiation from the solar spectrum is a major environmental challenge to the skin. This necessitates developing novel mechanism-based agents capable of ameliorating UVA-induced effects in the skin. We recently described a novel antioxidant, 3-O-Caffeoyl-1-methylquinic acid (MCGA3) from leaves of bamboo. Here, we investigated the photochemopreventive effects of MCGA3 against UVA-mediated apoptosis in immortalized HaCaT keratinocytes. Pretreatment of MCGA3 rendered cells more sensitive to subsequent UVA irradiation-induced apoptosis as well as completely reversed UVA-induced sustained phosphorylation of extracellular signal-regulated kinase 1/2 and protein kinase Calpha, downregulation of p21, and reactive oxygen species generation. Interestingly, MCGA3 itself effectively induced p21 protein and mRNA levels. Silencing of p21 by RNA interference revealed a pivotal role of p21 in generating G(1)-S arrest and in enhancing UVA-mediated apoptosis. Transcriptional activation of p21 by MCGA3 was mediated through the proximal region of multiple Sp1 sites regardless of p53-binding site in p21 promoter, and this effect was augmented by desferroioxamine, an iron chelating agent. Additional studies suggested that iron chelation-driven hypoxia by MCGA3 may function in activation of p21. MCGA3 could be a useful agent to prevent photocarcinogenesis via apoptotic elimination of p53 mutant and DNA-repair defective cells caused by UVA radiation.

PDZ domain protein GIPC interacts with the cytoplasmic tail of melanosomal membrane protein gp75 (tyrosinase-related protein-1)

Tyrosinase and tyrosinase-related proteins (TRPs) are a family of melanosomal membrane proteins involved in mammalian pigmentation. Whereas the melanogenic functions of TRPs are localized in their amino-terminal domains that reside within the lumen of melanosomes, the sorting and targeting of these proteins to melanosomes is mediated by signals in their cytoplasmic domains. To identify proteins that interact with the cytoplasmic tail of gp75 (TRP-1), the most abundant melanosomal membrane protein, we performed yeast two-hybrid screening of a melanocyte cDNA library. Here, we show that the cytoplasmic domain of gp75 interacts with a PDZ domain-containing protein. The gp75-interacting protein is identical to GIPC, an RGS (regulator of G protein signaling)/GAIP-interacting protein, and to SEMCAP-1, a transmembrane semaphorin-binding protein. Carboxyl-terminal amino acid residues, Ser-Val-Val, of gp75 are necessary and sufficient for interaction of gp75 with the single PDZ domain in GIPC. Although endogenous and transfected GIPCs bind efficiently to transiently expressed gp75, only a small amount of GIPC is found associated with gp75 at steady state. Using a strategy to selectively synchronize the biosynthesis of endogenous gp75, we demonstrate that only newly synthesized gp75 associates with GIPC, primarily in the juxtanuclear Golgi region. Our data suggest that GIPC/SEMCAP-1 plays a role in biosynthetic sorting of proteins, specifically gp75, to melanosomes.

Diverse roles of conserved asparagine-linked glycan sites on tyrosinase family glycoproteins

The tyrosinase family of genes has been conserved throughout vertebrate evolution. The role of conserved N-glycan sites in sorting, stability, and activity of tyrosinase family proteins was investigated using two family members from two different species, mouse gp75/tyrosinase-related protein (TRP)-1/Tyrp1 and human tyrosinase. Potential N-linked glycosylation sites on the lumenal domains of mouse gp75/TRP-1/Tyrp1 and human tyrosinase were eliminated by site-directed mutagenesis (Asn to Gln substitutions). Our results show that selected conserved N-glycan sites on tyrosinase family members are crucial for stability in the secretory pathway and endocytic compartment and for enzymatic activity. Different glycan sites on the same tyrosinase family polypeptide can perform distinct functions, and conserved sites on tyrosinase family paralogues can perform different functions.

Expression of microtubule-associated protein 2 in benign and malignant melanocytes: implications for differentiation and progression of cutaneous melanoma

Cutaneous melanocytic neoplasms are known to acquire variable characteristics of neural crest differentiation. Melanocytic nevus cells in the dermis and desmoplastic melanomas often display characteristics of nerve sheath differentiation. The extent and nature of neuronal differentiation characteristics displayed by primary and metastatic melanoma cells are not well understood. Here, we describe induction of a juvenile isoform of microtubule-associated protein 2 (MAP-2c) in cultured metastatic melanoma cells by the differentiation inducer hexamethylene bisacetamide. Up-regulation of this MAP-2 isoform, a marker for immature neurons, is accompanied by extended dendritic morphology and down-regulation of tyrosinase-related protein 1 (TYRP1/gp75), a melanocyte differentiation marker. In a panel of cell lines that represent melanoma tumor progression, MAP-2c mRNA and the corresponding approximately 70-kd protein could be detected predominantly in primary melanomas. Immunohistochemical analysis of 61 benign and malignant melanocytic lesions showed abundant expression of MAP-2 protein in melanocytic nevi and in the in situ and invasive components of primary melanoma, but only focal heterogeneous expression in a few metastatic melanomas. In contrast, MAP-2-positive dermal nevus cells and the invasive cells of primary melanomas were TYRP1-negative. This reciprocal staining pattern in vivo is similar to the in vitro observation that induction of the neuronal marker MAP-2 in metastatic melanoma cells is accompanied by selective extinction of the melanocytic marker TYRP1. Our data show that neoplastic melanocytes, particularly at early stages, retain the plasticity to express the neuron-specific marker MAP-2. These observations are consistent with the premise that both benign and malignant melanocytes in the dermis can express markers of neuronal differentiation.

Regulation of tyrosinase-related protein-2 (TYRP2) in human melanocytes: relationship to growth and morphology

Treatment of human melanoma cells with the differentiation-inducing agent hexamethylene bisacetamide (HMBA) results in reciprocal changes in expression of melanocyte-specific genes tyrosinase-related proteins-1 and -2 (TYRP1 and TYRP2). In this study, we investigated the effects of HMBA on cultured neonatal human cutaneous melanocytes. Flow cytometric analysis showed that HMBA inhibited 12-O-tetradecanoylphorbol-13-acetate (TPA)-dependent growth of melanocytes by reducing the population of cells entering the DNA synthesis phase of cell cycle. Melanocyte growth inhibition was accompanied by an increase in the number of cells exhibiting polydendritic morphology. This morphologic change was less pronounced when HMBA was added to melanocytes in the absence of TPA. Northern blot analyses of total cellular RNA showed that expression of microphthalmia-associated transcription factor (MITF), tyrosinase (TYR), TYRP1, Silver (SILV/Pmel17) gene was down-regulated by HMBA, while TYRP2 mRNA was up-regulated (> 10-fold). When the inducer was added to cells in the absence of TPA, there was > 50-fold increase in TYRP2 mRNA with a moderate increase in MITF, tyrosinase and SILV gene mRNAs and complete repression of TYRP1 gene. Studies using inhibitors for protein kinases involved in cell signaling pathways suggested that stress-activated kinase p38 and mitogen-activated protein kinase kinase MEK are involved in TPA-independent regulation of TYRP2 expression in melanocytes. These data show that treatment of proliferating melanocytes with the differentiation inducer HMBA results in a distinct change in morphology and up-regulation of TYRP2, while quiescent melanocytes respond by a dramatic increase in expression of TYRP2 without change in morphology. These results suggest an inverse relationship of TYRP2 gene regulatory mechanisms to melanocyte growth regulatory pathways.

Expression and Up-regulation of alternatively spliced transcripts of melastatin, a melanoma metastasis-related gene, in human melanoma cells

Loss of expression of a novel suppressor of metastasis, melastatin (MLSN1), has recently been reported to correlate with metastatic potential of melanoma cells. Using differential display analysis, we identified MLSN1 among genes overexpressed in pigmented metastatic human melanoma cells treated with the differentiation inducer hexamethylene bisacetamide (HMBA). In this study, we show that multiple short transcripts of MLSN1 are present in melanocytes and pigmented metastatic melanoma cell lines while the full-length 5. 4-kb mRNA is detectable only in melanocytes. Treatment of pigmented melanoma cells with the differentiation-inducing agent, HMBA, results in up-regulation of the 5.4-kb MLSN1 mRNA as well as short RNAs. Analysis of a panel of nonpigmented primary and metastatic melanoma cell lines showed weak expression of a 1.8-kb mRNA in a few melanoma cell lines. Northern blot and RT-PCR analyses with DNA probes and oligonucleotide primers that correspond to distinct regions of full-length MLSN1 mRNA indicated that the short transcripts contained sequences corresponding primarily to either 5'- or 3'-end of the 5.4-kb mRNA. HMBA appears to up-regulate MLSN1 transcripts derived mainly from the 5'-end. Modulators of cAMP and protein kinase C pathways had no significant effect on MLSN1 expression. Our data show that multiple MLSN1 transcripts, both constitutively expressed and inducible, are present in cultured pigmented melanoma cells, and suggest that MLSN1 expression can be regulated at the level of both transcription and mRNA processing.

Topical corticosteroid in foam vehicle offers comparable coverage compared with traditional vehicles

BACKGROUND

A new preparation of betamethasone valerate in a novel foam vehicle is available for treatment of scalp dermatoses. The vehicle spreads between hair until it reaches the scalp, where it melts and delivers the active drug. Solids make up only a tiny fraction of the foam vehicle, leaving no apparent residue on the skin or hair. The uniqueness of this vehicle raises the question of how to compare it with other topical corticosteroid preparations.

OBJECTIVE

The purpose of this study was to determine the equivalency of a given quantity of the foam product to quantities of drugs in conventional vehicles.

METHODS

The number of fingertip units (FTUs) per gram and the area of coverage of an FTU of betamethasone valerate foam vehicle were determined and compared with those of cream, lotion, gel, and solution psoriasis treatments.

RESULTS

The weight of an FTU of foam vehicle was 52.5 +/- 5.7 microg. There were 9 to 12 times as many FTUs in 100 g of vehicle foam as in 100 g of cream or gel and 2.3 to 2.8 times as many as in 100 g of lotion or solution. The area covered by an FTU of foam vehicle was less than the area covered by an FTU of cream or gel.

CONCLUSION

The characteristics of foam vehicle are different from those of other vehicles. The greater number of FTUs in 100 g of foam vehicle made up for the lower coverage per FTU, such that total coverage area for 100 g of foam vehicle was comparable to the coverage area for 100 g of the drugs in traditional vehicles.

Sorting and targeting of melanosomal membrane proteins: signals, pathways, and mechanisms

Newly synthesized melanosomal proteins, like many other cellular proteins, traverse through a series of intracellular compartments en route to melanosomes. Entry and exit of proteins through these compartments is orchestrated by cellular sorting machinery that recognize specific sorting signals. Melanosomal membrane proteins begin their intracellular journey upon co-translational importation into the endoplasmic reticulum (ER). The biosynthetic output of tyrosinase, the key melanogenic enzyme, appears to be regulated by quality-control events at the ER, the 'port of entry' to the secretory pathway. Following maturation in the ER and through the Golgi, the sorting of these proteins in the trans-Golgi network for intracellular retention and transport along endosome/lysosome pathway requires cytoplasmically exposed signals. A di-leucine motif, present in the cytoplasmic tails of most melanosomal proteins, and its interaction with adaptor protein (AP) complexes, specifically AP-3, are critical for these events. Defects in sorting signals and the cytosolic components that interact with these signals result in a number of murine coat color phenotypes and cause human pigmentary disorders. Thus, missense or frame-shift mutations that produce truncated tyrosinase lacking the melanosomal sorting signal(s) appear to be responsible for murine platinum coat color phenotypes and a proportion of human oculocutaneous albinism-1; mutations in AP-3 appear to be responsible for the mocha phenotype in mice and Hermansky-Pudlak-like syndrome in man. Additional signals and sorting steps downstream of AP-3 appear to be required for endosomal sorting and targeting proteins to melanosomes. Signals and mechanisms that sequester melanosomal proteins from endosomes/lysosomes are not understood. Potential candidates that mediate such processes include proteins encoded by lyst and pallid genes. The common occurrence of abnormalities in melanosomes in many storage-pool disorders suggests that melanocytes utilize signals, pathways, and mechanisms shared by other proteins and cell types to assemble a number of specialized proteins and produce unique cell-type-specific organelles.

Role of microphthalmia transcription factor in regulation of melanocyte differentiation marker TRP-1

Tyrosinase and a family of tyrosinase-related proteins (TRPs) are melanocyte differentiation gene products involved in melanin pigmentation. Members of the tyrosinase family share upstream transcriptional regulatory elements suggesting that expression of these genes is regulated by shared mechanisms. Microphthalmia transcription factor MITF, a melanocyte-specific basic helix-loop-helix protein, has been shown to transactivate tyrosinase and TRP-1 genes in vitro by binding to a shared regulatory sequence known as M box. The role of MITF in concomitant regulation of these genes in vivo is not clear. We showed earlier that in human melanoma cells TRP-1 can be regulated independently of tyrosinase and pigmentation. To investigate the role of MITF in TRP-1 regulation, we studied the effect of pharmacological agents that modulate transcription of tyrosinase and TRP-1 on MITF. In melanoma cells treated with hexamethylene bisacetamide (HMBA), transcription of TRP-1 gene was selectively and completely inhibited while steady state levels of tyrosinase, TRP-2, MITF mRNA and melanin content showed a modest increase. HMBA caused no detectable change in cellular MITF or its nuclear localization. This MITF-independent regulation of TRP-1 required continued synthesis of RNA and protein. Selective down-regulation of TRP-1 by HMBA occurred even in the presence of cholera toxin which up-regulates TRP-1 by cAMP-mediated pathways. These data show that TRP-1 gene can be down-regulated independently of MITF by de novo activation of negative regulatory factors. Thus, both activation of positive factors such as MITF and inactivation of negative regulatory factors may be required for TRP-1 gene expression during melanocytic differentiation.

Sorting and secretion of a melanosome membrane protein, gp75/TRP1

The melanosome is an organelle specialized for melanin synthesis that is derived from the endocytic pathway. Several melanosome membrane proteins have been identified, forming a family of proteins known as tyrosinase-related proteins. Two members of this family, tyrosinase and gp75, are well-characterized melanocyte differentiation antigens. Our previous studies have shown that gp75, the mouse brown locus protein, is sorted to melanosomes along the endocytic pathway, directed by a hexapeptide sorting signal located in the cytoplasmic tail. In this study, we report the unexpected finding that a portion of gp75 is secreted. Substantial levels of secretory gp75 were detected in melanocytic cells. Cell surface expression of gp75 was also detected, representing 2% of cellular gp75. Characterization of secretory gp75 cells showed that it is: (i) a truncated form that lacks the transmembrane region, the cytoplasmic tail where the endosomal sorting signal is located, and a small portion of the lumenal domain; (ii) more extensively glycosylated than endocytic/melanosomal gp75, containing trans-Golgi processed sugar residues; and (iii) generated post-translationally in an acid sensitive compartment after processing in the trans-Golgi, and secreted rapidly after generation. Thus, these endocytic/melanosomal membrane proteins can be processed to abundant secretory forms, probably in an endocytic compartment through a potentially novel secretory pathway.

Aberrant retention of tyrosinase in the endoplasmic reticulum mediates accelerated degradation of the enzyme and contributes to the dedifferentiated phenotype of amelanotic melanoma cells

The loss of tyrosinase, the key enzyme in melanin synthesis, has been implicated in the dedifferentiation of malignant melanocytes. The presence of tyrosinase transcripts and antigenic peptides in melanoma tumors prompted us to investigate whether the basis for the loss of the enzyme was proteolytic degradation. Toward this aim, we followed the kinetics of synthesis, degradation, processing, chaperone binding, inhibitor sensitivity, and subcellular localization of tyrosinase in normal and malignant melanocytes. We found that, in amelanotic melanoma cell lines, tyrosinase failed to reach the melanosome, the organelle for melanin synthesis, because it was retained in the endoplasmic reticulum (ER) and then degraded. Tyrosinase appeared mostly as a 70-kDa core-glycosylated, endoglycosidase H-sensitive, immature form bound to the ER chaperone calnexin and had a life-span of only 25% of normal. Maturation and transit from the ER to the Golgi compartment was facilitated by lowering the temperature of incubation to 31 degrees C. Several proteasome inhibitors caused the accumulation of an approximately 60-kDa tyrosinase doublet that was more prominent in malignant than in normal melanocytes and promoted, to various degrees, the maturation of tyrosinase in melanoma cells and the translocation of the enzyme to melanosomes. The appearance of ubiquitinated tyrosinase after treatment of normal melanocytes with N-acetyl-L-leucinyl-L-leucinal-L-norleucinal reinforced our notion that some tyrosinase is normally degraded by proteasomes. Proteolysis of tyrosinase by proteasomes is consistent with the production of antigenic tyrosinase peptides that are presented to the immune system by major histocompatibility complex class I molecules.