Stimulation of tyrosinase activity of cultured melanoma cells by lysosomotropic agents

Protein Stability and Functional Characterization of Intra-Melanosomal Domain of Human Recombinant Tyrosinase-Related Protein 1

Pigmentation is the result of a complex process by which the biopolymer melanin is synthesized and packed into melanosomes of melanocytes. Various types of oculocutaneous albinism (OCA), a series of autosomal recessive disorders, are associated with reduced pigmentation in the skin, eyes, and hair due to genetic mutations of proteins involved in melanogenesis. Human tyrosinase (Tyr) and tyrosinase-related protein 1 (Tyrp1) drives the enzymatic process of pigment bio-polymerization. However, within the melanogenic pathway, Tyrp1 has catalytic functions not clearly defined and distinct from Tyr. Here, we characterize the biochemical and biophysical properties of recombinant human Tyrp1. For this purpose, we purified and analyzed the intra-melanosomal domain (Tyrp1tr) for protein stability and enzymatic function in conditions mimicking the environment within melanosomes and the endoplasmic reticulum. The study suggests that Tyrp1tr is a monomeric molecule at ambient temperatures and below (<25 °C). At higher temperatures, >31 °C, higher protein aggregates form with a concurrent decrease of monomers in solution. Also, Tyrp1tr diphenol oxidase activity at pH 5.5 rises as both the pre-incubation temperature and the higher molecular weight protein aggregates formation increases. The enhanced protein activity is consistent with the volume exclusion change caused by protein aggregates.

The TYRP1-mediated protection of human tyrosinase activity does not involve stable interactions of tyrosinase domains

Tyrosinases are melanocyte-specific enzymes involved in melanin biosynthesis. Mutations in their genes cause oculocutaneous albinism associated with reduced or altered pigmentation of skin, hair, and eyes. Here, the recombinant human intra-melanosomal domains of tyrosinase, TYRtr (19-469), and tyrosinase-related protein 1, TYRP1tr (25-472), were studied in vitro to define their functional relationship. Proteins were expressed or coexpressed in whole Trichoplusia ni larvae and purified. Their associations were studied using gel filtration and sedimentation equilibrium methods. Protection of TYRtr was studied by measuring the kinetics of tyrosinase diphenol oxidase activity in the presence (1:1 and 1:20 molar ratios) or the absence of TYRP1tr for 10 hr under conditions mimicking melanosomal and ER pH values. Our data indicate that TYRtr incubation with excess TYRP1tr protects TYR, increasing its stability over time. However, this mechanism does not appear to involve the formation of stable hetero-oligomeric complexes to maintain the protective function.

Repeat domains of melanosome matrix protein Pmel17 orthologs form amyloid fibrils at the acidic melanosomal pH

Most amyloids are pathological, but fragments of Pmel17 form a functional amyloid in vertebrate melanosomes essential for melanin synthesis and deposition. We previously reported that only at the mildly acidic pH (4-5.5) typical of melanosomes, the repeat domain (RPT) of human Pmel17 can form amyloid in vitro. Combined with the known presence of RPT in the melanosome filaments and the requirement of this domain for filament formation, we proposed that RPT may be the core of the amyloid formed in vivo. Although most of Pmel17 is highly conserved across a broad range of vertebrates, the RPT domains vary dramatically, with no apparent homology in some cases. Here, we report that the RPT domains of mouse and zebrafish, as well as a small splice variant of human Pmel17, all form amyloid specifically at mildly acid pH (pH ∼5.0). Protease digestion, mass per unit length measurements, and solid-state NMR experiments suggest that amyloid of the mouse RPT has an in-register parallel β-sheet architecture with two RPT molecules per layer, similar to amyloid of the Aβ peptide. Although there is no sequence conservation between human and zebrafish RPT, amyloid formation at acid pH is conserved.

Vacuolar-type H(+)-ATPase with the a3 isoform is the proton pump on premature melanosomes

The melanosome, an organelle specialized for melanin synthesis, is one of the lysosome-related organelles. Its lumen is reported to be acidified by vacuolar-type H(+)-ATPase (V-ATPase). Mammalian V-ATPase exhibits structural diversity in its subunit isoforms; with regard to membrane intrinsic subunit a, four isoforms (a1-a4) have been found to be localized to distinct subcellular compartments. In this study, we have shown that the a3 isoform is co-localized with a melanosome marker protein, Pmel17, in mouse melanocytes. Acidotropic probes (LysoSensor and DAMP) accumulate in non-pigmented Pmel17-positive melanosomes, and DAMP accumulation is sensitive to bafilomycin A1, a specific inhibitor of V-ATPase. However, none of the subunit a isoforms is associated with highly pigmented mature melanosomes, in which the acidotropic probes are also not accumulated. oc/oc mice, which have a null mutation at the a3 locus, show no obvious defects in melanogenesis. In the mutant melanocytes, the expression of the a2 isoform is modestly elevated, and a considerable fraction of this isoform is localized to premature melanosomes. These observations suggest that the V-ATPase keeps the lumen of premature melanosomes acidic, whereas melanosomal acidification is less significant in mature melanosomes.

Tyrosinase maturation through the mammalian secretory pathway: bringing color to life

Tyrosinase has been extensively utilized as a model substrate to study the maturation of glycoproteins in the mammalian secretory pathway. The visual nature of its enzymatic activity (melanin production) has facilitated the identification and characterization of the proteins that assist it becoming a functional enzyme, localized to its proper cellular location. Here, we review the steps involved in the maturation of tyrosinase from when it is first synthesized by cytosolic ribosomes until the mature protein reaches its post-Golgi residence in the melanosomes. These steps include protein processing, covalent modifications, chaperone binding, oligomerization, and trafficking. The disruption of any of these steps can lead to a wide range of pigmentation disorders.

Skin pigmentation enhancers

The highest incidences of cancer are found in the skin, but endogenous pigmentation is associated with markedly reduced risk. Agents that enhance skin pigmentation have the potential to reduce both photodamage and skin cancer incidence. The purpose of this review is to evaluate agents that have the potential to increase skin pigmentation. These include topically applied substances that simulate natural pigmentation: dihydroxyacetone and melanins; and substances that stimulate the natural pigmentation process: psoralens with UVA (PUVA), dimethylsulfoxide (DMSO), L-tyrosine, L-Dopa, lysosomotropic agents, diacylglycerols, thymidine dinucleotides, DNA fragments, melanocyte stimulating hormone (MSH) analogs, 3-isobutyl-1-methylxanthine (IBMX), nitric oxide donors, and bicyclic monoterpene (BMT) diols. These agents are compared with regards to efficacy when administered to melanoma cells, normal human epidermal melanocytes, animal skin, and human skin. In addition, mechanisms of action are reviewed since these may reveal issues related to both efficacy and safety. Both dihydroxyacetone and topically applied melanins are presently available to the consumer, and both of these have been shown to provide some photoprotection. Of the pigmentation stimulators, only PUVA and MSH analogs have been tested extensively on humans, but there are concerns about the safety and side effects of both. At least some of the remaining pigmentation stimulators under development have the potential to safely induce a photoprotective tan.

Melanosomal pH controls rate of melanogenesis, eumelanin/phaeomelanin ratio and melanosome maturation in melanocytes and melanoma cells

The skin pigment melanin is produced in melanocytes in highly specialized organelles known as melanosomes. Melanosomes are related to the organelles of the endosomal/lysosomal pathway and can have a low internal pH. In the present study we have shown that melanin synthesis in human pigment cell lysates is maximal at pH 6.8. We therefore investigated the role of intramelanosomal pH as a possible control mechanism for melanogenesis. To do this we examined the effect of neutralizing melanosomal pH on tyrosinase activity and melanogenesis in 11 human melanocyte cultures and in 3 melanoma lines. All melanocyte cultures (9 of 9) from Caucasian skin as well as two melanoma cell lines with comparable melanogenic activity showed rapid (within 24 h) increases in melanogenesis in response to neutralization of melanosomal pH. Chemical analysis of total melanin indicated a preferential increase in eumelanin production. Electron microscopy revealed an accumulation of melanin and increased maturation of melanosomes in response to pH neutralization. In summary, our findings show that: (i) near neutral melanosomal pH is optimal for human tyrosinase activity and melanogenesis; (ii) melanin production in Caucasian melanocytes is suppressed by low melanosomal pH; (iii) the ratio of eumelanin/phaeomelanin production and maturation rate of melanosomes can be regulated by melanosomal pH. We conclude that melanosomal pH is an essential factor which regulates multiple stages of melanin production. Furthermore, since we have recently identified that pink locus product (P protein) mediates neutralization of melanosomal pH, we propose that P protein is a key control point for skin pigmentation. We would further propose that the wide variations in both constitutive and facultative skin pigmentation seen in the human population could be associated with the high degree of P-locus polymorphism.

Regulation of the catalytic activity of preexisting tyrosinase in black and Caucasian human melanocyte cell cultures

The activity of tyrosinase, the rate-limiting enzyme for melanin synthesis, is higher in Black skin melanocytes than in melanocytes derived from Caucasian skin. This variation in enzyme activity is not due to differences in tyrosinase abundance or tyrosinase gene activity, but, rather, is due to differences in the catalytic activity of preexisting tyrosinase. In melanocytes, tyrosinase is localized to the membrane of melanosomes and in Caucasian melanocytes the melanosome-bound enzyme is largely inactive. Conversely, in melanosomes of Black melanocytes, tyrosinase has high catalytic activity. Treatment of Caucasian melanocytes with the lysosomotropic compound ammonium chloride or with the ionophores nigericin and monensin results in a rapid and pronounced increase in tyrosinase activity. This increase occurs without any change in tyrosinase abundance, indicating that these compounds are increasing the catalytic activity of preexisting enzyme. Inhibition of the vacuolar proton pump V-ATPase by treatment of Caucasian melanocytes with bafilomycin also increases tyrosinase activity. In contrast to the 10-fold increase in tyrosinase observed in Caucasian melanocytes, neither ammonium chloride, monensin, nigericin, nor bafilomycin is able to increase the already high level of tyrosinase activity present in melanosomes of melanocytes derived from Black skin. Finally, staining of Caucasian melanocytes with the fluorescent weak base acridine orange shows that melanosomes of Caucasian, but not Black, melanocytes are acidic organelles. These data support a model for racial pigmentation that is based on differences in melanosome pH in Black and Caucasian skin types. The models suggests that melanosomes of Caucasian melanocytes are acidic, while those of Black individuals are more neutral. Since tyrosinase is inactive in an acid environment, the enzyme is largely inactive in Caucasian melanosomes but fully active in Black melanosomes.

Activation of melanogenesis by vacuolar type H(+)-ATPase inhibitors in amelanotic, tyrosinase positive human and mouse melanoma cells

In this study, we describe the activation of melanogenesis by selective vacuolar type H(+)-ATPase inhibitors (bafilomycin A1 and concanamycin A) in amelanotic human and mouse melanoma cells which express tyrosinase but show no melanogenesis. Addition of the inhibitors activated tyrosinase within 4 h, and by 24 h the cells contained measurable amounts of melanin. These effects were not inhibited by cycloheximide (2 microgram/ml) which is consistent with a post-translational mechanism of activation. Our findings suggest that melanosomal pH could be an important and dynamic factor in the control of melanogenesis in mammalian cells.

A pyradine-derivative from the mushroom Albatrellus confluens

A novel pyradine-derivative was isolated from the mushroom Albatrellus confluens. This compound promoted melanin synthesis by B16 melanoma cells.

Experimental metastasis and differentiation of murine melanoma cells: actions and interactions of factors affecting different intracellular signalling pathways

Various factors that modulate the differentiation of malignant cells are known to affect their experimental metastatic potential (EMP), or lung colonization after intravenous injection into syngeneic animals. However, some results and conclusions on the relation between cell differentiation and metastasis have appeared to conflict. We have reanalysed this by measurement of EMP of B16 melanoma sublines after culture with agents or conditions that acted on differentiation through various intracellular pathways. All tested agents did affect the EMP. EMP was usually positively correlated with differentiation under diverse conditions, but exceptions showed that there is no direct causal connection. Nor could all findings be explained in terms of cell proliferation or expression of major histocompatibility antigens. Some data helped to explain disparities between previous reports. Specific novel findings included the following. The stimulation of EMP by melanocyte-stimulating hormone (MSH) as well as all other tested effects of MSH were prevented by extended exposure to 12-O-tetradecanoyl phorbol acetate (TPA), suggesting a requirement for protein kinase C activity as well as G-protein coupling in MSH action. Cells grown with cholera toxin were always more differentiated than untreated cells, but the EMP could be either markedly increased or markedly decreased by cholera toxin under different conditions. The basic culture medium apparently determined this striking reversal. The EMP was also significantly affected by the extracellular pH.

High-molecular-weight forms of tyrosinase and the tyrosinase-related proteins: evidence for a melanogenic complex

Tyrosinase, tyrosinase-related protein-1 (TRP-1), and tyrosinase-related protein-2, (TRP-2, dopachrome tautomerase) were shown by immunoblotting and enzyme assays to copurify from extracts of Cloudman S91 melanoma cells. Antibodies to TRP-1 and TRP-2 immunoprecipitated tyrosinase activity, suggesting a stable interaction (complex) among these proteins. The tyrosine hydroxylase activity of tyrosinase was reduced in the complexed form; treatment with Triton X-100 dissociated the complex and activated the tyrosinase present within it. To further study this complex, we employed sucrose gradient density centrifugation of extracts from cultured murine melanocytes. Tyrosinase, TRP-1, and TRP-2 all existed in high molecular weight "multimers" of approximately 200 to > 700 kilodaltons. Extraction of cells with buffers containing the detergent CHAPS preserved the high molecular weight multimers; Triton X-100 caused their dissociation into monomers. Low pH, low ionic strength, and millimolar concentrations of calcium ions favored the maintenance of multimers. The results of this study demonstrate that the participation of tyrosinase, TRP-1, and TRP-2 in a multimeric complex could have important physiologic consequences, and raise the possibility that some of the well-known interactions between coat color genes may be explained by intermolecular interactions between the gene products.

The mouse brown (b) locus protein has dopachrome tautomerase activity and is located in lysosomes in transfected fibroblasts

Many genes mapping to pigmentation loci are involved in the regulation of melanin synthesis in the mouse. The brown (b) locus controls black/brown coat coloration, and its product has significant homology to the key melanogenic enzyme tyrosinase. This has led to suggestions that the b-protein is itself a melanogenic enzyme. In order to investigate its function, we have established lines of mouse fibroblasts stably expressing the b-protein by co-transfection of a b-protein expression vector and a plasmid conferring resistance to the antibiotic G418. The b-protein synthesised by these cells has the expected molecular mass of 75 kDa and reacts with three different anti-b-protein antibodies. We were unable to confirm previous reports that the b-protein has tyrosinase or catalase activity, but detected stereospecific dopachrome tautomerase activity in b-protein-expressing fibroblasts. This dopachrome tautomerase binds to Concanavalin A-Sepharose, and the major product of its action on L-dopachrome is 5,6-dihydroxyindole-2-carboxylic acid. Since this activity is not present in untransfected cells we conclude that the b-protein has dopachrome tautomerase activity. Fibroblasts do not contain melanosomes, the specialised organelles in which the b-protein is located in melanocytes. Nevertheless, indirect immunofluorescence localisation of the b-protein in transfected fibroblasts produces a distinctive pattern of intense juxtanuclear staining combined with punctate cytoplasmic staining. Double-labelling shows co-localisation of the b-protein with the late endosomal/lysosomal markers beta-glucuronidase and LAMP-1, both in transfected fibroblasts and in mouse melanoma cells. These findings are consistent with the hypothesis that melanosomes are closely related to lysosomes.

Effect of pH on the oxidation pathway of dopamine catalyzed by tyrosinase

The oxidation of 3,4-dihydroxyphenylethylamine (dopamine) by O2 catalyzed by tyrosinase yields 4-(2-aminoethyl)-1, 2-benzoquinone (o-dopaminequinone), which evolves nonenzymatically through two branches or sequences of reactions, whose respective operations are determined by the pH of the medium. The cyclization branch of o-dopaminequinone takes place in the entire range of pH and is the only significant branch at pH greater than or equal to 6. The hydroxylation branch of o-dopaminequinone only operates significantly at pH less than 6, and involves the accumulation of 2,4,5-trihydroxyphenylethylamine (6-hydroxydopamine) and 5-(2-aminoethyl)-2-hydroxy-1,4-benzoquinone (p-topaminequinone), identified from cyclic voltammetry assays. The kinetic characterization of the hydroxylation branch of o-dopaminequinone has been carried out by spectrophotometric and oxymetric assays. The successful fitting of data to the kinetic behavior predicted by the kinetic analysis at both pH greater than or equal to 6 and pH less than 6 confirms the overall oxidation pathway proposed for the dopamine oxidation catalyzed by tyrosinase. The antitumoral power of dopamine is possibly enhanced by the high cytotoxicity of 6-hydroxydopamine and p-topaminequinone, accumulated at the acidic pH characteristic of melanosomes and melanome cells.

Kinetic study on the effect of pH on the melanin biosynthesis pathway

This paper deals with the quantitative description of the regulatory effect of pH on the oxidation pathway of L-dopa to yield melanins. Tyrosinase catalyzes the oxidation by molecular oxygen of L-dopa to o-dopaquinone, which evolves non-enzymatically through a branched pathway with cyclization or hydroxylation reactions. The production of several quinones and semiquinones in the pathway has also been reported. The intermediates of the hydroxylation branch have been identified and the corresponding rate constants have been determined. These compounds, such as have been detected in melanosomes and in tumoral cells, have great cytotoxic power and could have physiological significance in acidic media.

Effect of pH on the oxidation pathway of alpha-methyldopa catalysed by tyrosinase

This paper deals with the quantitative description of the effect of pH on the oxidation pathway of alpha-methyldopa. Tyrosinase catalyses the oxidation by molecular oxygen of alpha-methyldopa to o-alpha-methyldopaquinone, which evolves non-enzymically through a branched pathway with cyclization or hydroxylation reactions. The intermediates of the hydroxylation branch have been identified, and the corresponding rate constants have been determined. These compounds, which have been detected in melanosomes and in tumour cells, have great cytotoxic power and could have physiological significance in acidic media.

Enhanced melanogenesis of murine melanoma cells cultured on or in collagen gel

To elucidate the interaction between melanoma and its matrix, we cultured B16 murine melanoma cells on and in type I collagen gel and evaluated specified functions of melanoma cells; tyrosinase activity and melanin-synthesizing capacity. Proliferation of cells cultured in these environments was markedly suppressed compared with that of cells cultured conventionally on plastic. On the other hand, the tyrosinase activity of cells cultured in or on collagen gel was two to three times higher than that of cells cultured on the plastics, while their melanin production was approximately double that achieved during conventional culture of cells. In conclusion, collagen gel influenced the growth and cell-specific functions of the melanoma cell. The culture system using collagen gel as substrate may be useful for the investigation of the interaction between melanoma and its matrix.

Citrate activates tyrosinase from B-16 murine melanoma and human skin

Citrate stimulates cresolase activity of tyrosinase from B-16 murine melanoma and human skin. Maximal stimulation by citrate was obtained at 2 mM, and stimulation was decreased at higher concentrations. Citrate stimulates tyrosinase not only from mammalian sources but also from mushroom. The stimulation was not due to reversal of inhibition of enzyme activity by excess tyrosine. On rapid decrease in pH of the enzyme solution from 6.8 to 5.0-5.2, the enzyme is no longer inhibited by excess tyrosine even when its activity was assayed at pH 6.8. Citrate also stimulates this form of enzyme. However, the stimulation is more at acidic pH than at pH 6.8. At higher concentrations of citrate the stimulatory effect decreases at both pH 5.0 and pH 6.8. Inhibition of this enzyme occurs at higher concentrations (22 mM) at pH 6.8. The physiological role of stimulation of cresolase activity of tyrosinase by citrate is yet to be unravelled.

pH-dependent interconversion of two forms of tyrosinase in human skin

1. We have shown that the characteristic lag in cresolase activity of human skin tyrosinase at inhibitory concentration of tyrosine was absent at all pH values studied, i.e. pH 5.2, 5.7, 6.2 and 6.8, if the enzyme solubilized at low pH was used as the source of enzyme, but the same enzyme when dialysed against buffers of various pH values showed linear activity only at pH 5.2 and was not inhibited by excess tyrosine, whereas at higher pH values it exhibited a lag and inhibition by excess tyrosine. 2. However, the enzyme solubilized in buffer/detergent, pH 6.8, when dialysed against buffer of the same pH showed linear activity at pH 5.2 and non-linear activity at pH 6.8. 3. The water/detergent-solubilized enzyme from human skin melanosomes showed linear activity even at inhibitory concentrations of tyrosine at pH 5.2 and 6.8 up to 2 h, but acceleration of rate was observed after 2 h for the enzyme measured at pH 6.8. 4. After dialysis of the water/detergent-solubilized enzyme against double-glass-distilled water, it still exhibits linear activity at inhibitory concentration of tyrosines at pH 6.8 for the first 2 h, but the same enzyme when dialysed against 0.02 M-sodium phosphate buffer, pH 6.8, exhibits negligible activity up to 1/2 h, in contrast with considerable activity before dialysis during the same interval of time, but without any loss of activity at later intervals of incubation time. 5. On the basis of these results, it is concluded that the enzyme exists in at least two interconvertible forms, one without lag and inhibition by excess tyrosine and the other with lag and inhibition by excess tyrosine. These two forms are interconvertible only by gradual change in pH over a period of hours.

Comparison of tyrosinase levels in amelanotic and melanotic melanoma cell cultures by a competitive enzyme-linked immunoadsorbent assay and by immunotitration analysis

Melanogenesis in mammalian pigment cells is regulated by changes in the activity of tyrosinase, the rate-limiting enzyme for melanin synthesis. Because recent evidence suggests that this enzyme may exist in pigment cells in both active and inactive stages, a competitive enzyme-linked immunoadsorbent assay (ELISA) was developed to compare tyrosinase levels in amelanotic and melanotic melanoma cell clones. The melanotic cell line used for this study, MEL-11A, had basal tyrosinase levels approximately 40 times that of the amelanotic cell line, AM-7. Both cell lines responded to melanocyte-stimulating hormone by demonstrating large increases in tyrosinase activity. For competitive ELISA analysis of tyrosinase levels in these two clones, microtiter plates were coated with purified tyrosinase, and trypsinized cell extracts were tested for their ability to compete with bound tyrosinase for antibody binding. Although tyrosinase activity in the amelanotic clone was 1/40 that of the melanotic clone, immunoreactive tyrosinase levels in AM-7 cells were found to be approximately one-half that present in the melanotic clone. Additional evidence for the presence of an inactive (or at least, catalytically less active) enzyme in AM-7 cells was obtained from immunotitration analysis of tyrosinase in cell extracts from both cell lines. These results suggest that at least some amelanotic melanoma cells may contain significant levels of catalytically inactive tyrosinase molecules and that the level of pigmentation in mammalian melanocytes may be regulated by a tyrosinase activation process.

Activation of tyrosinase in mouse melanoma cell cultures

Tyrosinase activity increased in Cloudman S-91 mouse melanoma cell homogenates incubated at 37 degrees C for a minimum of 8 h. Enzyme activity continued to increase for 48 h at which time the maximal level of activation was observed. Activation did not occur at 4 degrees C and did not occur in the cytosol fraction of the cell, suggesting that the response was localized to melanosomes. The activated enzyme was resistant to solubilization with the nonionic detergent, Triton X-100, and preparation of homogenates in this detergent did not inhibit the temperature-dependent activation of the melanosomal fraction of the cell. The activation process increased the Vmax of tyrosinase 10-fold and lowered the Km by a factor of 2 as determined by the tyrosine hydroxylase assay. The increase in tyrosinase activity was detectable by three assay methods: tyrosine hydroxylation, melanin synthesis, and by tyrosine decarboxylation. The formation of melanin, however, was found to be 1/20 that of either tyrosine hydroxylation or decarboxylation, a finding which suggests that the melanin pathway may be blocked at 5,6-dihydroxyindole. The "self-activation" response could not be mimicked by incubating cell homogenates with cyclic AMP-dependent protein kinase. Activated tyrosinase could be inhibited by the addition of fresh cell extracts, a finding which suggests that tyrosinase inhibitors may be present in these cells.

pH-dependent interconvertible allosteric forms of murine melanoma tyrosinase. Physiological implications

Murine melanoma melanosomal tyrosinase, solubilised at pH 6.8 and 1% Igepal, exhibits a lag in cresolase activity which increases with increasing concentration of tyrosine. The enzyme, solubilised at pH 5.0 and assayed at pH 5.0, does not exhibit lag even at inhibitory concentrations of tyrosine while the same enzyme when assayed at pH 6.8 exhibits characteristic lag. When the enzyme was solubilised from a melanosomal fraction with detergent/water without any buffer, significant linear activity for 2 h was seen at an inhibitory concentration of tyrosine, indicating for the first time the presence of a form of tyrosinase without lag and inhibition by excess tyrosine. Exposure of the enzyme solubilised in buffer/detergent at pH 6.8 to rapid decrease in pH to 5.0 or 4.7 makes the enzyme remain irreversibly in the form without characteristic lag, even at an inhibitory concentration of tyrosine and at pH 6.8. These results may be interpreted as follows. The enzyme at pH 6.8 exists in the E form with an allosteric site for tyrosine. Decrease of the pH of the enzyme solution from 6.8 to 5.0 or 4.7 by dialysis results in the reversible protonation of the enzyme, which no longer binds tyrosine at its allosteric site and consequently inhibition by excess tyrosine and lag were not observed at acidic pH. However, if the enzyme was rapidly brought to pH 5.0 from 6.8 it remains irreversibly in the protonated form even at pH 6.8. Ascorbic acid acts as an effective reductant for the hydroxylation of tyrosine by tyrosinase, while 3,4-dihydroxyphenylalanine is both an effective reductant and counteracts the inhibition by tyrosine at pH 6.8.

Electron microscopic evidence for stimulation of melanosomal maturation by lysosomotropic agents and monensin in cultured B16 mouse melanoma cells

Mouse melanoma cells, B16-C2M in monolayer culture were treated with either lysosomotropic agent, 10 mM NH4Cl, or 20 microM chloroquine, an ionophore, or 10 microns monensin for 3 h at 37 degrees C, and examined with regard to the site of melanin deposition and numbers of melanized (type 1) and unmelanized (type 2) melanosomes under a transmission electron microscope. The numbers of these two types of melanosomes were counted on electron micrographs of thin sections of 20 to 40 cells for each experimental group and expressed in terms of number per unit area of sectioned cytoplasm. Although most melanosomes were largely swollen in monensin-treated cells, melanin deposition was apparently confined in melanosomes in all experimental groups. The compound melanosomes were scarcely found. The mean population density (number per unit cytoplasmic area) of type 1 melanosomes was highest in the NH4Cl-treated cell group followed by monensin-treated, chloroquine-treated, and control cell groups. When the relative abundance of type 1 melanosomes was expressed as a fraction of total number of type 1 and 2 melanosomes (melanosomal maturation index, MMI), the differences were much more evident. Type 1 melanosomes were found in every cell (MMI not equal to 0) of the groups treated with NH4Cl and chloroquine only, which suggested the existence of a subpopulation of cells responsive to lysosomotropic agents but not to monensin in regard to melanosome maturation. All these findings indicate that the stimulation of melanogenesis proceeds mainly through maturation of preexisting melanosomes under these conditions.

Differentiation apparently repressed by the nucleus. Rapidly-induced pigmentation of enucleated melanoma cells

There is evidence for cytoplasmic control over gene expression in cell differentiation, but still very little is known of the intracellular mechanism, nuclear, cytoplasmic, or both, which actively initiates the differentiation of one cell type into another. Here the role of the cytoplasm was examined in the induction of differentiation of cultured mouse melanoma cells by melanocyte-stimulating hormone and alkaline medium. Intact cells were compared with cytoplasts, cells enucleated by centrifugation in the presence of cytochalasin D (CD). Surprisingly, early inductions of pigment (melanin) synthesis and of the principal melanin-synthesizing enzyme activity, tyrosinase, could be achieved in cytoplasts. Indeed these early changes were slower in nucleated cells and were accelerated by the inhibitor of protein synthesis, cycloheximide. Thus the initial activation of tyrosinase and melanin synthesis--although not necessarily any other or later aspects of melanoma cell differentiation--is apparently controlled through a labile, transcription- and translation-dependent repression. To our knowledge this is a novel mechanism for the initiation of differentiation; its generality remains to be tested.

Growth inhibition of murine melanoma by butyric acid and dimethylsulfoxide

Treatment of B16-F10 melanoma cells with dimethylsulfoxide (DMSO) or butyric acid (BA) inhibits cell growth and delays tumor appearance in syngeneic mice. Both agents induce morphological changes in these cells. Treatment of melanoma cells with DMSO results in a marked increase in tyrosinase activity and melanin content. BA, on the other hand, does not increase melanin content and decreases tyrosinase activity. The data show that there are marked differences in the effect of DMSO and BA on melanin biosynthesis, whereas both agents inhibit cell growth and cause a delay in tumor appearance. These findings indicate that decreased proliferation of melanoma cells and induction of melanin biosynthesis are not necessarily associated phenomena.

Stimulation by ionophores of tyrosinase activity of mouse melanoma cells in culture

The hypothesis that the melanosome is an acidic vesicle in which the tyrosinase action is suppressed under the ordinary culture conditions was examined with a variety of ionophores added in cultures of mouse melanoma cell line B16-C2M. In the presence of monensin or nigericin, which exchange H+ for Na+ or K+, respectively, through bio-membrane, the tyrosinase activity of cells in culture was more than 10 times that in the control culture. This stimulation was observed without delay after addition of the chemicals and was not inhibited by cycloheximide. The enzyme activity of sonicated cell-free extracts, in which melanosomes were disrupted, was not stimulated by these ionophores. The tyrosinase activity was stimulated to a lesser extent by a proton ionophore, p-trifluoromethoxyphenylhydrazone (FCCP). The activity was also stimulated by kryptofix 221, valinomycin (Na+ and K+ carrier, respectively), and tetraethylammonium ions (permeant cations) but only in the presence of a limited concentration of FCCP. N-Ethylmaleimide and N, N'-dicyclohexylcarbodiimide, inhibitors of lysosomal proton pump, stimulated tyrosinase activity of cells in the presence of FCCP. These facts are consistent with the hypothesis described above.