Standardization of organoid culture for evaluation of melanogenesis induced by UVB, UVA and visible light

Abstract Background: Organoid cultures are primary cultures that maintain architectural characteristics and the relationships between cells, as well as the extracellular matrix. They are alternatives for pathophysiological or therapeutic investigation rather than animal and in vitro tests. Objective: Development of a cutaneous organoid culture model, aiming at the study of radiation-induced melanogenesis. Method: A validation study, which involved biopsies of the skin of the back of the adult ear. One sample was irradiated with different doses of UVB, UVA, or visible light (VL); the other was maintained in the dark for 72 h. The viability of the tissues was evaluated from the morphological and architectural parameters of the histology, and the expression of the glyceraldehyde-3-phosphate dehydrogenase (GAPDH) gene, by real-time polymerase chain reaction (PCR). The radiation-induced melanin pigmentation was standardized according to the doses of each radiation and evaluated by digital image analysis (Fontana-Masson). Results: The primary skin culture was standardized at room temperature using DMEM medium. The doses of UVB, UVA, and VL (blue light) that induced differential melanogenesis were: 166 mJ/cm2, 1.524 J/cm2, and 40 J/cm2. The expression of the GAPHD constitutional gene did not differ between the sample of skin processed immediately after tissue collection and the sample cultured for 72 h in the standardized protocol. Study limitations: This was a preliminary study that evaluated only the viability and integrity of the melanogenic system, and the effect of the radiation alone. Conclusions: The standardized model maintained viable melanocytic function for 72 h at room temperature, allowing the investigation of melanogenesis induced by different forms of radiation.

Critical role of metabotropic glutamate receptor 4 in bone marrow-derived dendritic cells in the Th17 cell differentiation and the melanogenesis of B16 cells

Vitiligo is an acquired pigmentary disorder resulting from selective destruction of melanocytes. Emerging studies have suggested that T helper cell 17 (Th17) is potentially implicated in vitiligo development and progression. It was recently discovered that metabotropic glutamate receptor 4 (mGluR4) can modulate Th17-mediated adaptive immunity. However, the influence of mGluR4 on melanogenesis of melanocytes has yet to be elucidated. In the present study, we primarily cultured mouse bone marrow-derived dendritic cells (BMDC) and then knocked down and over-expressed mGluR4 using transfection. Transduced BMDC were co-cultured with CD4+ T cells and the expression of Th17-related cytokines were measured. The morphology and melanogenesis of B16 cells were observed after being treated with co-culture medium of CD4+ T cells and transduced BMDC. We found that mGluR4 knockdown did not affect the co-stimulatory CD80 and CD86 upregulation after lipopolysaccharide stimulation but did increase the expression of Th17-related cytokines, and further down-regulated the expression of microphthalmia-associated transcription factor (MITF) and the downstream genes, decreased melanin production, and destroyed the morphology of B16 cells. Conversely, over-expression of mGluR4 reduced the expression of CD80 and CD86, suppressed the production of Th17-related cytokines, increased the expression of MITF, and did not destroy the morphology of B16 cells. Our study confirmed that mGluR4 modulated the Th17 cell polarization and resulted in the alteration of melanogenesis and morphology of B16 cells. Collectively, these findings suggest mGluR4 might be a potent target involved in the immune pathogenesis of vitiligo.

Evaluation of melanin production by Sporothrix luriei

There is a paucity of studies on the cell biology of Sporothrix luriei, the less common of the pathogenic Sporothrix species worldwide. The production of DHN-melanin, eumelanin, and pyomelanin were evaluated on the mycelial and yeast forms of the S. luriei ATCC 18616 strain. The mycelial form of this species produced only pyomelanin, which protected the fungus against environmental stressors such as ultraviolet light, heat, and cold. The yeast form was unable to produce any of the tested melanin types. The lack of melanin in the parasitic form of S. luriei may be an explanation for its low frequency in human infections.

Melanin particles isolated from the fungus Fonsecaea pedrosoi activates the human complement system

BACKGROUND Melanin production has been associated with virulence in various pathogenic fungi, including Fonsecaea pedrosoi, the major etiological agent for chromoblastomycosis, a subcutaneous fungal disease that occurs in South America. OBJECTIVE The aim of this study was to evaluate the effects of acid-basic extracted F. pedrosoi melanin particles and fungal cell ghosts obtained by Novozym 234 treatment on their ability to activate the human complement system. METHODS The ability of melanin particles and fungal cell ghosts to activate the human complement system was evaluated by complement consumption, immunofluorescence, and enzyme-linked immunosorbent assay (ELISA). FINDINGS Unsensitised melanin particles and melanin ghosts presented complement consumption of 82.67 ± 2.08% and 96.04 ± 1.13%, respectively. Immunofluorescence assays revealed intense deposition of the C3 and C4 fragments on the surface of melanin particles and ghosts extracted from F. pedrosoi. Deposition of the C3, C4, and C5 fragments onto melanin samples and zymosan was confirmed by ELISA. Deposition of small amounts of C1q and C9 onto melanin samples and zymosan was detected by ELISA. CONCLUSION Fonsecaea pedrosoi melanin particles and fungal cell ghosts activated the complement system mainly through an alternative pathway.

Characterization and biological activities of extracellular melanin produced by Schizophyllum commune (Fries).

Melanins are enigmatic pigments produced by a wide variety of microorganisms including bacteria and fungi. Here, we have isolated and characterized extracellular melanin from mushroom fungus, Schizophyllum commune. The extracellular dark pigment produced by the broth culture of S. commune, after 21 days of incubation was recovered by hot acid-alkali treatment. The melanin nature of the pigment was characterized by biochemical tests and further, confirmed by UV, IR, EPR, NMR and MALDI-TOF Mass Spectra. Extracellular melanin, at 100 µg/ml, showed significant antibacterial activity against Escherichia coli, Bacillus subtilis, Klebsiella pneumoniae and Pseudomonas fluorescens and antifungal activity against Trichophyton simii and T. rubrum. At a concentration of 50 µg/ml, melanin showed high free radical scavenging activity of DPPH (2,2-diphenyl-1-picrylhydrazyl) indicating its antioxidant potential. It showed concentration dependent inhibition of cell proliferation of Human Epidermoid Larynx Carcinoma Cell Line (HEP-2). This study has demonstrated characterization of melanin from basidiomycetes mushroom fungus, Schizophyllum commune and its applications.

Alkaptonuria: A very rare metabolic disorder.

Alkaptonuria (AKU) is a very rare autosomal recessive disorder of tyrosine metabolism in the liver due to deficiency of homogentisate 1,2 dioxygenase (HGD) activity, resulting in the accumulation of homogentisic acid (HGA). Circulating HGA pass into various tissues through-out the body, mainly in cartilage and connective tissues, where its oxidation products polymerize and deposit as a melanin-like pigment. Gram quantities of HGA are excreted in the urine. AKU is a progressive disease and the three main features, according the chronology of appearance, are: darkening of the urine at birth, then ochronosis (blue-dark pigmentation of the connective tissue) clinically visible at around 30 yrs in the ear and eye, and finally a severe ochronotic arthropathy at around 50 yrs with spine and large joints involvements. Cardiovascular and renal complications have been described in numerous case report studies. A treatment now is available in the form of a drug nitisinone, which decreases the production of HGA. The enzymatic defect in AKU is caused by the homozygous or compound heterozygous mutations within the HGD gene. This disease has a very low prevalence (1:100,000-250,000) in most of the ethnic groups, except Slovakia and Dominican Republic, where the incidence has shown increase up to 1:19,000. This review highlights classical and recent findings on this very rare disease.

THE COPPER INTERFERENCE WITH THE MELANOGENESIS OF Cryptococcus neoformans / A interferência de cobre na melanogênese de Cryptococcus neoformans

Melanin is a pigment produced by laccase, a phenoloxydase enzyme, and is related to the virulence of Cryptococcus neoformans as it is also considered an adaption mechanism to environmental conditions and protection against UV radiation, phagocytic system attack and antifungal drugs. Laccase synthesis is stimulated by several factors, including copper metabolism. The current study shows C. neoformans strains with higher melanization intensity when grown in L-dopa medium supplemented with different concentrations of copper sulfate. This increase shows that melanization rates may be enhanced in the presence of copper ions and may also enhance the virulence of C. neoformans in infected patients that present increasing copper concentrations in serum, such as those with HIV. The virulence of these strains may also be increased in the environment, where this metal is available as CuSO4 in algicidal and fungicidal compounds.

A melanina é um pigmento produzido pela enzima lacase, uma fenoloxidase, e está associada à virulência de Cryptococcus neoformans sendo considerada mecanismo de adaptação às condições ambientais e proteção contra a radiação UV, ataque do sistema fagocítico e antifúngicos. A lacase tem sua síntese estimulada por diversos fatores, incluindo o metabolismo de cobre. Este estudo mostra linhagens de C. neoformans com maior intensidade de melanização quando cultivadas em meio L-dopa suplementado com diferentes concentrações de sulfato de cobre. Este aumento demonstra que as taxas de melanização podem ser aumentadas na presença de íons cobre e também aumentar a virulência de C. neoformans em pacientes infectados que apresentam aumento nas concentrações séricas de íons cobre tais como pacientes com HIV. A virulência destas linhagens também pode ser incrementada no meio ambiente, onde este metal está disponível como CuSO4 em compostos algicidas e fungicidas.

Mechanisms regulating melanogenesis* / Mecanismos reguladores da melanogênese

Skin pigmentation is an important human phenotypic trait whose regulation, in spite of recent advances, has not yet been fully understood. The pigment melanin is produced in melanosomes by melanocytes in a complex process called melanogenesis. The melanocyte interacts with endocrine, immune, inflammatory and central nervous systems, and its activity is also regulated by extrinsic factors such as ultraviolet radiation and drugs. We have carried out a review of the current understanding of intrinsic and extrinsic factors regulating skin pigmentation, the melanogenesis stages and related gene defects. We focused on melanocyte-keratinocyte interaction, activation of melanocortin type 1 receptor (MC1-R) by peptides (melanocyte-stimulating hormone and adrenocorticotropic hormone) resulting from proopiomelanocortin (POMC) cleavage, and mechanisms of ultraviolet-induced skin pigmentation. The identification and comprehension of the melanogenesis mechanism facilitate the understanding of the pathogenesis of pigmentation disorders and the development of potential therapeutic options.

A pigmentação da pele é um importante traço fenotípico do ser humano mas apesar dos recentes avanços a sua regulação não está ainda totalmente esclarecida. O pigmento melanina é produzido nos melanossomas pelos melanócitos, num processo complexo designado por melanogénese. O melanócito interatua com os sistemas endócrino, imunitário, inflamatório e nervoso central e a sua atividade é também regulada por fatores extrínsecos como a radiação ultravioleta e fármacos. Fizemos uma revisão do conhecimento atual sobre os fatores intrínsecos e extrínsecos reguladores da pigmentação cutânea, etapas da melanogénese e defeitos genéticos relacionados. Fizemos enfoque na interação melanócito-keratinócito, na ativação do receptor da melanocortina tipo 1 (MC1-R) pelos péptidos (hormona estimuladora do melanócito e hormona adrenocorticotrófica) resultantes da clivagem da proopiomelanocortina (POMC) e mecanismos da pigmentação induzida pela radiação ultravioleta. A identificação e compreensão dos mecanismos reguladores da pigmentação cutânea facilitam o conhecimento dos mecanismos patogénicos dos distúrbios da pigmentação e o desenvolvimento de potenciais opções terapêuticas.

Glyceollins, a novel class of soybean phytoalexins, inhibit SCF-induced melanogenesis through attenuation of SCF/c-kit downstream signaling pathways

The anti-melanogenesis effect of glyceollins was examined by melanin synthesis, tyrosinase activity assay in zebrafish embryos and in B16F10 melanoma cells. When developing zebrafish embryos were treated with glyceollins, pigmentation of the embryos, melanin synthesis and tyrosinase activity were all decreased compared with control zebrafish embryos. In situ expression of a pigment cell-specific gene, Sox10, was dramatically decreased by glyceollin treatment in the neural tubes of the trunk region of the embryos. Stem cell factor (SCF)/c-kit signaling pathways as well as expression of microphthalmia-associated transcription factor (MITF) were determined by western blot analysis. Glyceollins inhibited melanin synthesis, as well as the expression and activity of tyrosinase induced by SCF, in a dose-dependent manner in B16F10 melanoma cells. Pretreatment of B16F10 cells with glyceollins dose-dependently inhibited SCF-induced c-kit and Akt phosphorylation. Glyceollins significantly impaired the expression and activity of MITF. An additional inhibitory function of glyceollins was to effectively downregulate intracellular cyclic AMP levels stimulated by SCF in B16F10 cells. Glyceollins have a depigmentation/whitening activity in vitro and in vivo, and that this effect may be due to the inhibition of SCF-induced c-kit and tyrosinase activity through the blockade of downstream signaling pathway.

Different culture media containing methyldopa for melanin production by Cryptococcus species / Avaliação de diferentes meios de cultura contendo metildopa para a produção de melanina por espécies de Cryptococcus

INTRODUCTION: Melanin production by species of Cryptococcus is widely used to characterize C. neoformans complex in mycology laboratories. This study aims to test the efficacy of methyldopa from pharmaceutical tablet as a substrate for melanin production, to compare the production of melanin using different agar base added with methyldopa, and to compare the melanin produced in those media with that produced in Niger seed agar and sunflower seed agar by C. neoformans, C. laurentii, and C. albidus. Two isolates of each species, C. neoformans, C. laurentii, and C. albidus, and one of Candida albicans were used to experimentally detect conditions for melanin production. METHODS: The following media were tested: Mueller-Hinton agar (MHA), brain and heart infusion agar (BHIA), blood agar base (BAB), and minimal medium agar (MMA), all added with methyldopa, and the media Niger seed agar (NSA) and sunflower seed agar (SSA). RESULTS: All isolates grew in most of the culture media after 24h. Strains planted on media BAB and BHIA showed growth only after 48h. All isolates produced melanin in MMA, MHA, SSA, and NSA media. CONCLUSIONS: Methyldopa in the form pharmaceutical tablet can be used as a substrate for melanin production by Cryptococcus species; minimal medium plus methyldopa was more efficient than the BAB, MHA, and BHIA in the melanin production; and NSA and SSA, followed by MMA added with methyldopa, were more efficient than other media studied for melanin production by all strains studied.

INTRODUÇÃO: A produção de melanina por espécies de Cryptococcus é uma característica amplamente utilizada em laboratórios de micologia para caracterização do complexoC. neoformans. O objetivo deste estudo foi verificar a eficácia da metildopa na forma farmacêutica de comprimido, como substrato para a produção de melanina por Cryptococcus, comparar diferentes bases de meios de cultura acrescidas de metildopa para produção de melanina e comparar o pigmento produzido nestes meios com o produzido em ágar Níger e ágar girassol por C. neoformans, C. laurentii e C. albidus. MÉTODOS: Foram testados dois isolados de cada uma das espécies, C. neoformans, C.laurentii e C.albidus, e um de C. albicans para avaliar a produção de melanina nos meios de cultura ágar Müeller-Hinton (MH), ágar brain heart infusion (BHI), ágar base sangue (BS), meio mínimo (MM), todos acrescidos de metildopa, e ainda ágar girassol e ágar Níger. RESULTADOS: Todos os isolados cresceram na maioria dos meios após 24h. O crescimento nos meios BS e BHI somente ocorreu após 48h. Todos os isolados produziram melanina nos meios MM, MH, girassol e Niger. CONCLUSÕES: A metildopa de origem farmacêutica pode ser utilizada como substrato para a produção de melanina por espécies de Cryptococcus; o MM acrescido de metildopa mostrou-se mais eficiente na produção de melanina do que os meios BS, MH e BHI; ágar girassol e ágar Níger seguidos de MM acrescido de metildopa foram os mais eficientes na produção de melanina pelos isolados estudados.

Resistance of melanized yeast cells of Paracoccidioides brasiliensis to antimicrobial oxidants and inhibition of phagocytosis using carbohydrates and monoclonal antibody to CD18

Paracoccidioides brasiliensis, a thermal dimorphic fungal pathogen, produces a melanin-like pigment in vitro and in vivo. We investigated the involvement of carbohydrates and monoclonal antibody to CD18, on phagocytosis inhibition, involving macrophage receptors and the resistance of melanized fungal cells to chemically generated nitric oxide (NO), reactive oxygen species (ROS), hypochlorite and H2O2. Our results demonstrate that melanized yeast cells were more resistant than nonmelanized yeast cells to chemically generated NO, ROS, hypochlorite and H2O2, in vitro. Phagocytosis of melanized yeast cells was virtually abolished when mannan, N-acetyl glucosamine and anti-CD18 antibody were added together in this system. Intratracheal infection of BALB/c mice, with melanized yeast cells, resulted in higher lung colony forming units, when compared to nonmelanized yeast cells. Therefore, melanin is a virulence factor of P. brasiliensis.

Lotus (Nelumbo nuficera) flower essential oil increased melanogenesis in normal human melanocytes

In this study, the essential oil from lotus flower extract, including petals and stamens, was assessed with regard to its effects on melanogenesis in human melanocytes. The lotus flower essential oil was shown to stimulate melanin synthesis and tyrosinase activity in a dose-dependent manner. The lotus flower essential oil induced the expression of tyrosinase, microphthalmia-associated transcription factor M (MITF-M), and tyrosinase-related proten-2 (TRP-2) proteins, but not tyrosinase mRNA. Moreover, it increased the phosphorylation of ERK and cAMP response element binding protein (CREB). In order to verify the effective components of the lotus flower oil, its lipid composition was assessed. It was found to be comprised of palmitic acid methyl ester (22.66%), linoleic acid methyl ester (11.16%), palmitoleic acid methyl ester (7.55%) and linolenic acid methyl ester (5.16%). Among these components, palmitic acid methyl ester clearly induced melanogenesis as the result of increased tyrosinase expression, thereby indicating that it may play a role in the regulation of melanin content. Thus, our results indicate that lotus flower oil may prove useful in the development of gray hair prevention agents or tanning reagents.

Clinical and mycological profile of cryptococcosis in a tertiary care hospital.

This study examined the extent of cryptococcosis in clinically diagnosed cases of meningitis in HIV-1 seropositive and apparently immunocompetent patients. One hundred and forty-six samples, obtained from 126 chronic meningitis patients comprised of cerebrospinal fluid (CSF), blood, sputum and urine. The samples were processed by standard microbiological procedures. Cryptococcal isolates were identified by microscopy, cultural characteristics, melanin production on niger seed agar and hydrolysis of urea. The isolates were further speciated on cannavanine glycine bromothymol blue (CGB) media. Cryptococcal antigen detection of CSF samples was performed by latex agglutination test (LAT). Minimum inhibitory concentration (MIC) of amphotericin B for the isolates was also tested. Cryptococcosis was diagnosed in 13 patients (eight HIV-1 seropositive and five apparently immunocompetent). Cryptococcus neoformans var. neoformans was the predominant isolate. Cryptococcal antigen was detected in all, whereas microscopy could detect yeast cells in nine patients. The isolates were sensitive to amphotericin B. CD4 cell counts ranged from 8 to 96/cu mm. The study concludes that all CSF samples with clinical diagnosis of subacute and chronic meningitis should be subjected to tests for detection of Cryptococcus in clinical laboratory irrespective of the immune status.

Avaliação da produção de melanina por espécies de Cryptococcus em quatro diferentes meios de cultura / Evaluation of melanin production by Cryptococcus species in four different culture media

A capacidade de Cryptococcus spp produzir melanina em meios contendo compostos fenólicos é amplamente utilizada na identificação destas espécies no laboratório. O objetivo do presente trabalho foi comparar a produção desse pigmento em quatro meios de cultura por Cryptococcus sp. Foram testadas 16 cepas de Cryptococcus neoformans, 17 de Cryptococcus albidus, 13 de Cryptococcus laurentii, e 2 de Cryptococcus uniguttulatus nos meios: ágar batata e cenoura, ágar alpiste, ágar semente de girassol e ágar L-dopa. A produção de melanina foi avaliada com base na pigmentação das colônias, e demonstrada em 5 dias de incubação por 93,8 por cento das cepas de Cryptococcus neoformans nos meios ágar batata e cenoura, ágar semente de girassol e ágar L-dopa. Dos isolados de Cryptococcus albidus, 29,4 por cento produziram o pigmento em ágar batata e cenoura e L-dopa, 11,8 por cento em ágar alpiste, e 36 por cento em ágar girassol. De Cryptococcus laurentii, 53,8 por cento produziram em batata e cenoura e em semente de girassol, 61,5 por cento em L-dopa, 84,6 por cento em ágar alpiste. Somente uma cepa de Cryptococcus uniguttulatus produziu fracamente o pigmento em ágar batata e cenoura.

The capacity of Cryptococcus spp to produce melanin in media containing phenol compounds is widely used for identifying these species in the laboratory. The aim of the present study was to compare the production of this pigment by Cryptococcus spp. in four culture media. Sixteen strains of Cryptococcus neoformans, 17 of Cryptococcus albidus, 13 of Cryptococcus laurentii and two of Cryptococcus uniguttulatus were tested in the following media: potato-carrot agar, Niger seed agar, sunflower seed agar and L-dopa agar. The melanin production was evaluated on the basis of colony pigmentation. Its production after five days of incubation was demonstrated by 93.8 percent of the strains of Cryptococcus neoformans in the media of potato-carrot agar, sunflower seed agar and L-dopa agar. From the isolates of Cryptococcus albidus, 29.4 percent produced the pigment in potato-carrot agar and L-dopa agar, 11.8 percent in Niger seed agar and 36 percent in sunflower seed agar. From Cryptococcus laurentii, 53.8 percent produced the pigment in potato-carrot agar and sunflower seed agar, 61.5 percent in L-dopa agar and 84.6 percent in Niger seed agar. Only one strain of Cryptococcus uniguttulatus presented slight production of the pigment, in potato-carrot agar.

Bee venom stimulates human melanocyte proliferation, melanogenesis, dendricity and migration

Pigmentation may result from melanocyte proliferation, melanogenesis, migration or increases in dendricity. Recently, it has been reported that secreted phospholipase A2(sPLA2) known as a component of bee venom (BV), stimulates melanocyte dendricity and pigmentation. BV has been used clinically to control rheumatoid arthritis and to ameliorate pain via its anti-inflammatory and antinociceptive properties. Moreover, after treatment with BV, pigmentation around the injection sites was occasionally observed and the pigmentation lasted a few months. However, no study has been done about the effect of BV on melanocytes. Thus, in the present study, we examined the effect of BV on the proliferation, melanogenesis, dendricity and migration in normal human melanocytes and its signal transduction. BV increased the number of melanocytes dose and time dependently through PKA, ERK, and PI3K/Akt activation. The level of cAMP was also increased by BV treatment. Moreover, BV induced melanogenesis through increased tyrosinase expression. Furthermore, BV induced melanocyte dendricity and migration through PLA2activation. Overall, in this study, we demonstrated that BV may have an effect on the melanocyte proliferation, melanogenesis, dendricity and migration through complex signaling pathways in vitro, responsible for the pigmentation. Thus, our study suggests a possibility that BV may be developed as a therapeutic drug for inducing repigmentation in vitiligo skin.

Human placental lipid induces mitogenesis and melanogenesis in B16F10 melanoma cells.

A hydroalcoholic extract of fresh term human placenta was found to be mitogenic as well as melanogenic on B16F10 mouse melanoma in an in vitro culture. The extract, a reservoir of a large number of bioactive molecules, was resolved to get the lipid fraction. Its activity was evaluated on B16F10 mouse melanoma by assessing the change in cellular morphology, growth and melanin induction. The lipid fraction, placental total lipid fraction (PTLF) tested in the study employed doses of 0 01 to 200 microg/ml; optimum growth and melanization accompanied by morphological changes were recorded at 10 and 100 microg/ml respectively. At intermediate doses growth and melanization were found to show a pattern of change over between growth and melanization and finally reached at an inverse relation at the respective optimal dose of response. Compared with defined sphingolipids, C(2) ceramide and sphingosine-1-phosphate, the results were mostly corroborative. The duality of biological response of sphingolipids as reported in numerous studies was comparable for the PTLF suggesting that its active component is a sphingolipid and showing its use for pigment recovery in vitiligo.

Stimulation of melanogenesis by glycyrrhizin in B16 melanoma cells

Glycyrrhizin (GR), triterpenoid saponin composed of one glycyrrhetinic acid (GA) and two glucuronic acids, is a main constituent of the hydrophilic fraction of licorice (Glycyrrhiza glabra) extracts and is known to have a wide range of pharmacological actions. In this study, we investigated the mechanism of GR effect on melanogenesis in B16 murine melanoma cells. The cellular levels of tyrosinase mRNA, protein, enzyme activities and melanin contents were increased by GR in a dose dependent manner. Expression of tyrosinase-related protein-2 (TRP-2) mRNA was also increased by GR, however, no significant change was observed on TRP-1. No cytotoxicity was observed at the effective concentration range of GR. GA showed no effect on melanogenesis at the equivalent nontoxic concentrations, indicating that glycoside structure is important in the stimulatory effect of GR on melanogenesis. These results indicate that GR-induced stimulation of melanogenesis is likely to occur through the transcriptional activation.

Medullary thyroid carcinoma with melanin production--a case report.

Melanin production in medullary thyroid carcinomas is rare. The present case illustrates melanin and other atypical features of medullary carcinoma of the thyroid in a fifty year old female. The diagnosis was suggested on the cytomorphological features seen on fine needle aspiration cytology smears. On histo-pathological examination the tumor was extensively pigmented with frequent mitosis. Amyloid was conspicuously scarce. Confirmation of diagnosis was done by immunohistochemical positivity for calcitonin and HMB-45 on tissue sections. The case is being presented in view of its rarity and distinct immunoreactivity. Review of literature is done and the implications of such dual positivity in the histogenesis and divergent phenotype of this tumor are discussed.

Lag kinetics of tyrosinase: its physiological implications.

The various theories put forward to explain the characteristic lag kinetics of oxidation of L-tyrosine by tyrosinase a rate regulatory step in the biosynthesis of melanin are reviewed Examination of the evidence in the literature and from experiments in the author's laboratory indicate that one of the hypotheses, that is, competition of tyrosine and dopa for met-tyrosinase and the formation of a dead-end complex of met-enzyme with tyrosine as explanation for lag kinetics is not consistent with available information. The alternative hypothesis that tyrosinase is an allosteric enzyme with tyrosine having negative effector site on the enzyme and dopa competing for it as an explanation for lag kinetics of tyrosinase is not yet disproved. Irrespective of the actual explanation for the lag kinetics of tyrosinase, it is suggested that the highly conserved lag kinetics may serve a physiological function. It is suggested that this function is to keep the enzyme essentially inactive during its transport to the specific organelle, namely the melanosome, in which an acidic environment exists. Only at acidic pH is the enzyme able to catalyze the biosynthesis of melanin.