In vitro phenotypic alteration of human melanoma cells induced by differentiating agents: heterogeneous effects on cellular growth and morphology, enzymatic activity, and antigenic expression

Exploring the effect of butyric acid, a metabolite from periodontopathic bacteria, on primary human melanocytes: An in vitro study

Effects of butyric acid, a bacterial metabolite implicated in periodontitis progression, have never been examined on oral melanocytes. Herein, primary human epidermal melanocytes were used as a model for oral melanocytes. Results show the adverse effects of butyric acid (sodium butyrate; NaB) on them, which comprise marked cytotoxicity at higher concentrations (>1 mM) and robust differentiation at lower nontoxic concentrations. NaB did not alter MITF protein levels; however, it stimulated tyrosinase protein synthesis and inhibited tyrosinase activity, with no changes in cellular melanin. NaB did not affect oxidative stress, although it induced significant levels of the pro-inflammatory cytokine IL-6.

Oxidative Stress Induces Skin Pigmentation in Melasma by Inhibiting Hedgehog Signaling

There is growing evidence that oxidative stress plays a role in melasma and disrupts primary cilia formation. Additionally, primary cilia have been suggested to have an inhibitory role in melanogenesis. This study examined the potential link between oxidative stress, skin hyperpigmentation, and primary cilia. We compared the expression levels of the nuclear factor E2-related factor 2 (NRF2), intraflagellar transport 88 (IFT88), and glioma-associated oncogene homologs (GLIs) in skin samples from patients with melasma, both in affected and unaffected areas. We also explored the roles of NRF2, IFT88, and GLIs in ciliogenesis and pigmentation using cultured adult human keratinocytes, with or without melanocytes. Our findings revealed decreased levels of NRF2, heme oxygenase-1, IFT88, and GLIs in lesional skin from melasma patients. The knockdown of NRF2 resulted in reduced expressions of IFT88 and GLI1, along with fewer ciliated cells. Furthermore, NRF2, IFT88, or GLI1 knockdown led to increased expressions in protease-activated receptor-2 (PAR2), K10, involucrin, tyrosinase, and/or melanin. These effects were reversed by the smoothened agonist 1.1. Calcium also upregulated these proteins, but not NRF2. The upregulation of involucrin and PAR2 after NRF2 knockdown was mitigated with a calcium chelator. In summary, our study suggests that oxidative stress in NRF2-downregulated melasma keratinocytes impedes ciliogenesis and related molecular processes. This inhibition stimulates keratinocyte differentiation, resulting in melanin synthesis and melanosome transfer, ultimately leading to skin hyperpigmentation.

Melanoma mutations modify melanocyte dynamics in co-culture with keratinocytes or fibroblasts

Melanocytic cell interactions are integral to skin homeostasis, and affect the outcome of multiple diseases, including cutaneous pigmentation disorders and melanoma. By using automated-microscopy and machine-learning-assisted morphology analysis of primary human melanocytes in co-culture, we performed combinatorial interrogation of melanocyte genotypic variants and functional assessment of lentivirus-introduced mutations. Keratinocyte-induced melanocyte dendricity, an indicator of melanocyte differentiation, was reduced in the melanocortin 1 receptor (MC1R) R/R variant strain and by NRAS.Q61K and BRAF.V600E expression, while expression of CDK4.R24C and RAC1.P29S had no detectable effect. Time-lapse tracking of melanocytes in co-culture revealed dynamic interaction phenotypes and hyper-motile cell states that indicated that, in addition to the known role in activating mitogenic signalling, MEK-pathway-activating mutations may also allow melanocytes to escape keratinocyte control and increase their invasive potential. Expanding this combinatorial platform will identify other therapeutic target mutations and melanocyte genetic variants, as well as increase understanding of skin cell interactions.

Sarsasapogenin Increases Melanin Synthesis via Induction of Tyrosinase and Microphthalmia-Associated Transcription Factor Expression in Melan-a Cells

Sarsasapogenin (SAR) is a steroidal sapogenin that is used as starting material for the industrial synthesis of steroids. It has various pharmacological benefits, such as antitumor and antidepressant activities. Since its effect on melanin biosynthesis has not been reported, we used murine melanocyte melan-a cells to investigate whether SAR influences melanogenesis. In this study, SAR significantly increased the melanin content of the melan-a cells from 1 to 10 μM. Based on an enzymatic activity assay using melan-a cell lysate, SAR had no effect on tyrosinase and DOPAchrome tautomerase activities. It also did not affect the protein expression of tyrosinase-related protein 1 and DOPAchrome tautomerase. However, protein levels of tyrosinase and microphthalmia-associated transcription factor were strongly stimulated by treatment with SAR. Therefore, our reports suggest that SAR treatment may induce melanogenesis through the stimulation of tyrosinase and microphthalmia-associated transcription factor expression in melan-a cells.

Evaluation of melanogenesis in A-375 melanoma cells treated with 5,7-dimethoxycoumarin and valproic acid

Malignant melanoma (melanoma malignum) is one of the most dangerous types of tumor. It is very difficult to cure. In recent years, a lot of attention has been given to chemoprevention. This method uses natural and synthetic compounds to interfere with and inhibit the process of carcinogenesis. In this study, a new treatment strategy was proposed consisting of a combination of 5,7-dimethoxycoumarin (DMC), an activator of melanogenesis, and valproic acid (VPA), a well-known drug that is one of the histone deacetylase inhibitors (HDACis). In conjunction with 1 mM VPA, all of the tested concentrations of DMC (10-150 μM) significantly decreased the proliferation of A-375 cells. VPA and DMC also induced the synthesis of melanin and the formation of dendrite and star-shaped cells. Tyrosinase gene expression and tyrosinase activity significantly increased in response to VPA treatment. Pyrolysis with gas chromatography and mass spectrometry (Py-GC/MS) was used to investigate the structure of the isolated melanin. This showed that the quantitative and qualitative components of melanin degradation products are dependent on the type of applied melanogenesis inductor. Products derived from eumelanin were detected in the pyrolytic profile of melanin isolated from A-375 cells stimulated with DMC. Thermal degradation of melanin isolated from melanoma cells after exposure to VPA or a mixture of VPA and DMC revealed the additional presence of products derived from pheomelanin.

The recycling endosome protein Rab17 regulates melanocytic filopodia formation and melanosome trafficking

Rab GTPases including Rab27a, Rab38 and Rab32 function in melanosome maturation or trafficking in melanocytes. A screen to identify additional Rabs involved in these processes revealed the localization of GFP-Rab17 on recycling endosomes (REs) and melanosomes in melanocytic cells. Rab17 mRNA expression is regulated by microphthalmia transcription factor (MITF), a characteristic of known pigmentation genes. Rab17 siRNA knockdown in melanoma cells quantitatively increased melanosome concentration at the cell periphery. Rab17 knockdown did not inhibit melanosome maturation nor movement, but it caused accumulation of melanin inside cells. Double knockdown of Rab17 and Rab27a indicated that Rab17 acts on melanosomes downstream of Rab27a. Filopodia are known to play a role in melanosome transfer, and in Rab17 knockdown cells filopodia formation was inhibited. Furthermore, we show that stimulation of melanoma cells with α-melanocyte-stimulating hormone induces filopodia formation, supporting a role for filopodia in melanosome release. Cell stimulation also caused redistribution of REs to the periphery, and knockdown of additional RE-associated Rabs 11a and 11b produced a similar accumulation of melanosomes and melanin to that seen after loss of Rab17. Our findings reveal new functions for RE and Rab17 in pigmentation through a distal step in the process of melanosome release via filopodia.

Melanocytes expressing MC1R polymorphisms associated with red hair color have altered MSH-ligand activated pigmentary responses in coculture with keratinocytes

The occurrence of red hair and pale skin in individuals, which is associated with UV-radiation sensitivity and increased skin cancer risk, is mainly due to polymorphisms in the melanocortin-1 receptor (MC1R) expressed in melanocytes. We have established a serum free human melanocyte-keratinocyte coculture system to study the behavior and functional abilities of melanocytes expressing MC1R red hair color (RHC) variants in order to identify differences from their wild type (WT) counterparts. This model revealed the importance of elevated calcium levels in promoting strong melanocyte interaction with the surrounding keratinocytes and resulted in a dendritic melanocyte morphology similar to that in skin. However, the dendricity response following agonist activation of the MC1R receptor by NDP-MSH peptide, was markedly enhanced in WT melanocytes in comparison to RHC strains. Analysis of mRNA expression and protein levels of the major pigmentation markers following NDP-MSH treatment distinguished the enzyme dopachrome tautomerase as preferentially upregulated in cocultures of WT strains, with negligible or a much reduced response in melanocytes with RHC variant alleles. These results highlight the use of the coculture system in determining fundamental differences in the physiology of melanocytes expressing RHC MC1R receptors and those of WT genotype, which are likely to contribute to the increased skin cancer risk for individuals that carry these variants.

Post-transcriptional regulation of melanin biosynthetic enzymes by cAMP and resveratrol in human melanocytes

Upregulation of microphthalmia-associated transcription factor (MITF) expression has been proposed to mediate melanogenesis stimulated by cAMP, whereas downregulation of MITF has been suggested to underlie the depigmentary effects of resveratrol, a promising chemotherapeutic found in red wine. We have assessed the contribution of MITF to pigmentation regulation by treating primary cultures of normal human melanocytes with the adenylate cyclase activator forskolin and/or resveratrol, then quantifying mRNA and protein levels for MITF, tyrosinase, tyrosinase-related protein-1, and dopachrome tautomerase (DCT). The inhibition of tyrosinase activity by resveratrol was not due to alterations in MITF, but instead was explained by both direct tyrosinase inhibition and a post-transcriptional effect that reduced the amount of fully processed tyrosinase. Glycosidase digestion revealed that the basis for the tyrosinase decrease was the retention of an immature form in the ER and subsequent loss of the mature, Golgi-processed enzyme. Elevation of intracellular cAMP by forskolin markedly increased protein levels for MITF, tyrosinase and DCT, however there was no concomitant increase in tyrosinase or DCT mRNA. This indicated that elevated levels of MITF were not sufficient to promote transcription of these melanogenic genes and that the increase in their protein abundance appeared to be predominantly mediated through post-transcriptional processing events.

Human melanoblasts in culture: expression of BRN2 and synergistic regulation by fibroblast growth factor-2, stem cell factor, and endothelin-3

The BRN2 transcription factor (POU3F2, N-Oct-3) has been implicated in development of the melanocytic lineage and in melanoma. Using a low calcium medium supplemented with stem cell factor, fibroblast growth factor-2, endothelin-3 and cholera toxin, we have established and partially characterised human melanocyte precursor cells, which are unpigmented, contain immature melanosomes and lack L-dihydroxyphenylalanine reactivity. Melanoblast cultures expressed high levels of BRN2 compared to melanocytes, which decreased to a level similar to that of melanocytes when cultured in medium that contained phorbol ester but lacked endothelin-3, stem cell factor and fibroblast growth factor-2. This decrease in BRN2 accompanied a positive L-dihydroxyphenylalanine reaction and induction of melanosome maturation consistent with melanoblast differentiation seen during development. Culture of primary melanocytes in low calcium medium supplemented with stem cell factor, fibroblast growth factor-2 and endothelin-3 caused an increase in BRN2 protein levels with a concomitant change to a melanoblast-like morphology. Synergism between any two of these growth factors was required for BRN2 protein induction, whereas all three factors were required to alter melanocyte morphology and for maximal BRN2 protein expression. These finding implicate BRN2 as an early marker of melanoblasts that may contribute to the hierarchy of melanocytic gene control.

Screening of human primary melanocytes of defined melanocortin-1 receptor genotype: pigmentation marker, ultrastructural and UV-survival studies

Recent population studies have demonstrated an association with the red-hair and fair-skin phenotype with variant alleles of the melanocortin-1 receptor (MC1R) which result in amino acid substitutions within the coding region leading to an altered receptor activity. In particular, Arg151Cys, Arg160Trp and Asp294His were the most commonly associated variants seen in the south-east Queensland population with at least one of these alleles found in 93% of those with red hair. In order to study the individual effects of these variants on melanocyte biology and melanocytic pigmentation, we established a series of human melanocyte strains genotyped for the MC1R receptor which included wild-type consensus, variant heterozygotes, compound heterozygotes and homozygotes for Arg151Cys, Arg160Trp, Val60Leu and Val92Met alleles. These strains ranged from darkly pigmented to amelanotic, with all strains of consensus sequence having dark pigmentation. UV sensitivity was found not to be associated with either MC1R genotype or the level of pigmentation with a range of sensitivities seen across all genotypes. Ultrastructural analysis demonstrated that while consensus strains contained stage IV melanosomes in their terminal dendrites, Arg151Cys and Arg160Trp homozygote strains contained only stage II melanosomes. This was despite being able to show expression of tyrosinase and tyrosinase-related protein-1 markers, although at reduced levels and an ability to convert exogenous 3,4-dihydroxyphenyl-alanine (DOPA) to melanin in these strains.

Agouti signal protein regulation in human melanoma cells

Production of the pigment eumelanin is controlled by alpha-melanocyte stimulating hormone (alpha-MSH) stimulation of melanocortin 1 receptor (Mc1r), whereas production of pheomelanin results from agouti antagonism of alpha-MSH signalling through Mc1r. The role of agouti in mouse pigmentation has been extensively investigated but a role for agouti signalling protein (ASIP) in human pigmentation has not been determined. To determine whether ASIP regulates known melanogenic genes in humans, ASIP was over-expressed in a human melanoma cell line. Levels of mRNA and protein were measured in genes known to be up or down-regulated by agouti in the mouse, namely microphthalmia (Mitf), tyrosinase (Tyr), tyrosinase-related protein 1 (Tyrp1), dopachrome tautomerase (Dct), Mc1r, silver, initiation transcription factor 2 (Itf2) and mini chromosome maintenance protein 6 (Mcm6). These melanogenic genes were not found to be significantly up or down-regulated by ASIP at the transcriptional level in human melanoma cells. However, ASIP down-regulation of tyrp1 was observed at the translational level. To identify novel genes that may be regulated by ASIP in melanoma cells, microarrays were used to determine differences in gene expression between the control and ASIP transfected melanoma cells. The expression level of human RNAs were determined by microarray analysis using a 19,200 cDNA and a 19,200 oligonucleotide array representing 13,000 and 18,864 individual genes, respectively. Genes observed to be modulated by ASIP were confirmed by quantitative real-time polymerase chain reaction. Results identify five genes, namely PPARbeta, eIF-4B, RRM2, MINOR and EVI2B that are down-regulated by ASIP, indicating a likely role for ASIP in human melanogenesis.

Modulation of microphthalmia-associated transcription factor gene expression alters skin pigmentation

The microphthalmia-associated transcription factor is implicated in melanocyte development and in the regulation of melanogenesis. Microphthalmia-associated transcription factor is thought to bind to the M-box promoter elements of tyrosinase, tyrosinase-related protein-1 and dopachrome tautomerase/tyrosinase-related protein-2 and transactivate these genes, resulting in increased pigmentation. Using a luciferase reporter construct driven by the microphthalmia-associated transcription factor promoter, we identified agents that modulate microphthalmia-associated transcription factor promoter activity. Changes in endogenous microphthalmia-associated transcription factor expression levels upon treatment with these agents were confirmed using northern and western blots, and their pigmentary modulating activities were demonstrated. Ultraviolet B irradiation and traditional Chinese medicine-1, a natural extract used in traditional Chinese medicine, upregulated microphthalmia-associated transcription factor gene expression and enhanced tyrosinase activity in vitro. Dihydrolipoic acid, lipoic acid, and resveratrol reduced microphthalmia-associated transcription factor and tyrosinase promoter activities. These agents also inhibited the forskolin- and ultraviolet B-stimulated promoter activities of these genes and significantly reduced tyrosinase activity in melanocyte cultures, resulting in depigmentation. Overexpressed microphthalmia-associated transcription factor was capable of rescuing the repressive effects of these compounds on the cotransfected tyrosinase promoter. Dark-skinned Yucatan swine treated with these agents showed visible skin lightening, which was confirmed histologically, whereas ultraviolet B-induced tanning of light-skinned swine was inhibited using these agents. Our findings suggest that modulation of microphthalmia-associated transcription factor expression can alter skin pigmentation and further confirm the central role of microphthalmia-associated transcription factor in melanogenesis.

Sodium butyrate modulates cell cycle-related proteins in HT29 human colonic adenocarcinoma cells

Sodium butyrate (NaB), a product of colonic fermentation of dietary fibre, has been shown to inhibit cell proliferation by blocking cells in the G0/G1 phase of the cell cycle through a mechanism of action still not completely understood. We investigated the effect of NaB on the level of some G1 phase-related proteins in a colon carcinoma cell line (HT29). In particular, we addressed our attention to cyclin D1 (the key regulator of G1S progression), p21waf1/cip1 (the main inactivator of the cyclin D/cdk complex), and p53 (the most important regulator of p21waf1/cip1 gene transcription). At inhibitory concentrations (higher than 1 mM) NaB reduced cyclin D1 and p53 level in a dose-dependent manner and sustained the synthesis of p21waf1/cip1, probably in a p53-independent way, accounting for the G0/G1 block observed by flow cytometry. Present results provide further evidence on the molecular mechanism at the basis of the physiological role of NaB and support the hypothesis that an unbalanced diet, poor in carbohydrates and therefore in NaB, could result in functional alterations with clinical and carcinogenic implications.

Novobiocin is a novel inducer of CD38 on cells of the myelomonocytic lineage

KG-1a, HL-60 and U-937 cells, which represent different stages of myelopoiesis, showed growth retardation in response to the coumarin antibiotic novobiocin. Novobiocin was found to increase CD38 expression (in all three cell lines) and to induce differentiation along the monocytic path in HL-60 and U-937 cells but not in KG-1a cells. The increase in surface expression of CD38 was matched by NAD glycohydrolase activity and by increases in the level of specific mRNA, indicating that the gene product is active and that regulation occurs at the level of transcription or mRNA stability. Of the three cell lines tested, only the early KG-1a expressed N-cadherin, a member of Ca(2+)-dependent adhesion molecules involved in embryonic differentiation processes. In contrast to CD38, N-cadherin was slightly down-regulated pointing to a specific role of novobiocin in gene regulation.

Critical targets of protein kinase C in differentiation of tumour cells

The ultimate target of pharmacological research is to find new drugs for treating human diseases such as cancer. Agents causing differentiation and thus growth arrest should be particularly useful in this regard. A potential target for such anticancer therapy is the enzyme family protein kinase C (PKC), which is involved in the transduction of signals for cell proliferation, differentiation, and apoptosis. Our recent work showing the induction of differentiation in melanoma cells by an activator of one PKC isoform, PKCdelta, touches on several important areas of investigation, which will form the basis of this review: the role of individual isoforms of PKC, their downstream targets and their specific substrates, the mechanism of activation of specific genes involved in the differentiation process, and the molecular basis for the morphological changes associated with differentiation. The central role that PKC plays in these processes points to the need for a greater understanding of the signalling pathways utilized by individual isoforms of this family of enzymes.

Antiproliferative and differentiating effects of benzodiazepine receptor ligands on B16 melanoma cells

In this study, we evaluated the effect of several ligands active at the central-type and peripheral-type benzodiazepine receptor (BzR) (clonazepam, diazepam, PK11195 and Ro5-4864) on the growth and differentiation of B16 melanoma cells. All tested BzR ligands were able to suppress proliferation of the cells at the micromolar range and in a concentration-dependent manner. However, agents selectively active at the peripheral-type BzR (PK11195 and Ro5-4864) exhibited more potent antiproliferative activity. In addition, the BzR ligands were demonstrated to affect the cell cycle by reducing the percent of cells in the S phase and increasing the percent in the G2/M phase. BzR ligands induced cellular phenotypic alterations, which have been previously shown to be associated with melanoma cell differentiation. These alterations included: marked morphological changes, enhancement of melanogenesis, lipid accumulation and increase in the activity of gamma glutamyl transpeptidase. All BzR ligands induced a marked reduction in the concentration of UTP and most of them did the same in GTP and CTP, while ATP levels were not significantly altered. In summary, BzR ligands (clonazepam, diazepam, PK11195 and Ro5-4864) were found to exert antitumor effects in B16 melanoma cells. These findings encourage further studies of a possible therapeutic potential of BzR ligands in treatment of melanoma.

Stimulation of melanogenesis in a human melanoma cell line by bistratene A

The polyether toxin, bistratene A, induced morphological and functional differentiation of a human melanoma cell line (MM96E). The cells became blocked at the G2/M transition and elaborated a number of processes. Tyrosinase activity and melanin content were substantially increased. Northern blot analysis showed up-regulation of mRNA for several genes known to be involved in melanin biosynthesis (pmel17, pmel34, and tyrosinase related proteins, TRP-1 and TRP-2). Bistratene A induced the phosphorylation of several proteins as assessed by 2D gel electrophoresis and one of these was identified as stathmin (oncoprotein 18), a cell-cycle regulated phosphoprotein. Bistratene A specifically induced the translocation of protein kinase Cdelta (PKCdelta) from a soluble to a particulate fraction without affecting other isoforms. These results implicate a role for protein kinase Cdelta in the induction of differentiation of this human melanoma cell line.

Different cis-acting elements are involved in the regulation of TRP1 and TRP2 promoter activities by cyclic AMP: pivotal role of M boxes (GTCATGTGCT) and of microphthalmia

In melanocytes and in melanoma cells, cyclic AMP (cAMP)-elevating agents stimulate melanogenesis and increase the transcription of tyrosinase, the rate-limiting enzyme in melanin synthesis. However, two other enzymes, tyrosinase-related protein 1 (TRP1) and TRP2, are required for a normal melanization process leading to eumelanin synthesis. In B16 melanoma cells, we demonstrated that stimulation of melanogenesis by cAMP-elevating agents results in an increase in tyrosinase, TRP1, and TRP2 expression. cAMP, through a cAMP-dependent protein kinase pathway, stimulates TRP1 and TRP2 promoter activities in both B16 mouse melanoma cells and normal human melanocytes. Regulation of the TRP1 and TRP2 promoters by cAMP involves a M box and an E box. Further, a classical cAMP response element-like motif participates in the cAMP responsiveness of the TRP2 promoter, demonstrating that the TRP2 gene is subjected to different regulatory processes, which could account for its different expression patterns during embryonic development or under specific physiological and pathological conditions. We also found that microphthalmia, a basic helix-loop-helix transcription factor, strongly stimulates the transcriptional activities of the TRP1 and TRP2 promoters, mainly through binding to the M boxes. Additionally, we demonstrated that cAMP increases microphthalmia expression and thereby its binding to TRP1 and TRP2 M boxes. These convergent and compelling results disclose at least a part of the molecular mechanism involved in the regulation of melanogenic gene expression by cAMP and emphasize the pivotal role of microphthalmia in this process.

Molecular characterization of glioblastoma cell differentiation

OBJECTIVE

Induction of cellular differentiation continues to be an attractive therapeutic strategy for malignant glioma. The purpose of this study was to develop a convenient in vitro model system for glioblastoma differentiation and to then characterize it using conventional techniques and flow cytometry.

METHODS

A subline of U138 MG cells ("U138B") was treated with 0 to 4 mmol/L sodium butyrate (or serum deprivation) for up to 96 hours. Cells were initially studied for effects on proliferation, morphology, and glial fibrillary acidic protein (GFAP) staining. Northern blot and immunoblot analyses of c-myc expression were performed. Multiparameter flow cytometry was then used to analyze GFAP, c-myc protein, and total cellular protein fluorescence and to relate them to changes in cell cycle distribution.

RESULTS

Butyrate treatment produced a dose-dependent inhibition of cellular proliferation and changes in morphology, GFAP staining, and c-myc expression consistent with a differentiation response. Detailed flow cytometric studies, including subpopulation analysis, showed that during 72 hours of treatment with 2 mmol/L butyrate, mean GFAP fluorescence increased to 420%, whereas c-myc protein decreased to 45 +/- 13% and total cellular protein increased to 181 +/- 17%. The effects of butyrate were distinct from those of serum deprivation and were not simply the result of cells shifting into Gzero/G1.

CONCLUSION

The butyrate-induced responses of the U138B cell line provide a convenient model system for studying the molecular events accompanying the differentiation of glioblastoma cells. Multiparameter flow cytometry is a useful technique for characterizing such differentiation.

Tumor selectivity and transcriptional activation by azelaic bishydroxamic acid in human melanocytic cells

Azelaic bishydroxamic acid (ABHA), a potent differentiating agent for lymphoid cells, was selectively toxic for 5 human tumor cell lines and transformed human melanocytes and keratinocytes (dose for 37% survival, D37, 30-100 microg/mL) compared with normal cells (melanocytes, fibroblasts; D37 > 300 microg/mL). Dendritic morphology was the only indicator found for increased differentiation, markers for the pigmentation pathway being unchanged or inhibited by ABHA. In contrast to hexamethylene bisacetamide and azelaic acid, ABHA significantly increased the HIV LTR, SV40 and c-fos promoter activities during a 24 hr treatment. Metallothionein promoter activity was enhanced by 5 hr treatment with ABHA in a sensitive melanoma cell line (MM96L) but was inhibited in a more resistant line (HeLa); c-fos promoter activity was inhibited in HeLa during this time. Transcription from a p53 binding response element was inhibited in MM96L by a 24 hr ABHA treatment but enhanced in HeLa. ABHA may represent a structural prototype for designing more potent and selective anti-melanoma agents.

Cinnamic acid: a natural product with potential use in cancer intervention

Cinnamic acid, a naturally occurring aromatic fatty acid of low toxicity, has a long history of human exposure. We now show that cinnamic acid induces cytostasis and a reversal of malignant properties of human tumor cells in vitro. The concentration causing a 50% reduction of cell proliferation (IC50) ranged from 1 to 4.5 mM in glioblastoma, melanoma, prostate and lung carcinoma cells. Using melanoma cells as a model, we found that cinnamic acid induces cell differentiation as evidenced by morphological changes and increased melanin production. Moreover, treated cells had reduced invasive capacity associated with modulation of expression of genes implicated in tumor metastasis (collagenase type IV, and tissue inhibitor metalloproteinase 2) and immunogenicity (HLA-A3, class-I major histocompatibility antigen). Further molecular analysis indicated that the anti-tumor activity of cinnamic acid may be due in part to the inhibition of protein isoprenylation known to block mitogenic signal transduction. The results presented here identify cinnamic acid as a new member of the aromatic fatty acid class of differentiation-inducers with potential use in cancer intervention.

Detection of the c-met proto-oncogene product in normal skin and tumours of melanocytic origin

The proto-oncogene c-met product (c-MET) is a receptor tyrosine kinase and functions as a receptor for hepatocyte growth factor (HGF). Although the function of c-MET has yet to be fully clarified, HGF stimulates the phosphorylation of tyrosyl residues on c-MET and triggers the signal transduction pathways, resulting in a contribution to the malignant progression of melanonocytes with synergic factors such as basic fibroblast growth factor and mast cell growth factor. Using immunohistochemical methods, we have studied the localization of c-MET in normal skin and various melanocytic tumours. c-MET was detected in keratinocytes, melanocytes, sebaceous cells, and other cells of the skin. In particular, basal melanocytes almost always showed nuclear labelling. Melanocytic naevi generally revealed predominantly nuclear staining of cells in the epidermis, whereas only a few cases showed a distinct cytoplasmic localization of c-MET in dermal naevus cells. The distribution pattern of c-MET in melanoma cells was basically similar to that of benign lesions, although the numbers tested were small. Cultured human melanoma cells also showed predominantly nuclear labelling, but were unresponsive to exogenous c-MET ligand HGF. Treatment with the glucosidase inhibitor castanospermine caused accumulation of protein at 220 kD, without diminishing the amount of normally-processed 190-kD c-MET. Although there was no significant difference in c-MET distribution between benign and malignant melanocytic lesions, it is suggested that malignant transformation of melanocytes may be associated with loss of response to HGF or other growth-regulating factors.

Differentiation of cultured human melanoma cells induced by the aromatic fatty acids phenylacetate and phenylbutyrate

The increasing incidence of melanoma and the poor responsiveness of disseminated disease to conventional treatments call for the development of new therapeutic approaches. Phenylacetate, a nontoxic differentiation inducer, can suppress the growth of other neuroectodermal tumors, i.e., gliomas, in laboratory models and in humans. This finding led us to explore the efficacy of phenylacetate and related aromatic fatty acids in melanoma. Phenylacetate and phenylbutyrate were found to a) induce selective cytostasis and maturation of cultured human melanoma cells, b) modulate the expression of genes implicated in tumor metastasis (type IV collagenase and tissue inhibitor of metalloproteinases-2) and immunogenicity (HLA class I); and c) enhance the efficacy of other agents of clinical interest, including retinoids, interferon-alpha, suramin, and 5-aza-2'-deoxycytidine. Reflecting on the phenotypic heterogeneity of melanoma, the degree of biologic alterations induced by phenylacetate/phenylbutyrate varied significantly among the tumor cell lines tested. Although losing invasive capacity and tumorigenicity in athymic mice, poorly differentiated cells exhibited only a marginal change in morphology, remained amelanotic, and resumed growth after treatment was discontinued. By contrast, treatment of melanoma cells that were in a more advanced stage of maturation resulted in profound alterations in cell growth, morphology, and pigmentation consistent with terminal differentiation. The in vitro antitumor activity was observed with nontoxic, pharmacologic concentrations of phenylacetate and phenylbutyrate, suggesting potential clinical use of these drugs in the treatment of melanomas.

Expression studies of pigmentation and POU-domain genes in human melanoma cells

Human melanoma cell lines have been used to examine the regulation of the tyrosinase (TYR) and tyrosinase-related protein genes TRP-1 and TRP-2 in response to differentiating chemicals and UV irradiation. TRP-1 mRNA levels can be repressed by treatment with the differentiating chemicals DMSO and HMBA. There is little effect of UV irradiation on pigment synthesis by human melanoma cell lines or tyrosinase activity, with variable effects on the levels of the TYR, TRP-1, and TRP-2 gene transcripts. The human TRP-1 gene promoter has been isolated and its activity tested by transient cell transfection to begin an examination of signal transduction mechanisms operating in response to pigmenting and differentiating agents. To identify transcription factors that may be involved in melanocytic gene expression, we studied the N-Oct-3 and N-Oct-5 octamer-binding activities normally expressed in the neuroectodermal cell lineage and which are expressed at high levels in melanoma cells. POU-domain-containing cDNA have been isolated from the A2058 human melanoma cell line that are homologous to the brn-2 gene that encodes N-Oct-3 and N-Oct-5.

Transcriptional regulation of differentiation, selective toxicity and ATGCAAAT binding of bisbenzimidazole derivatives in human melanoma cells

To study the relationship between the structure of minor groove ligands and their affinity for specific DNA sequences that regulate gene transcription, three analogues of the A-T-specific DNA minor groove ligands Hoechst 33258 and Hoechst 33342 were synthesized with 5, 8 or 12 carbons in an aliphatic chain attached to the phenolic oxygen of the molecule. There was a striking bimodal relationship between toxicity to HeLa cells and the lipophilicity of the five analogues, toxicity being low for the compounds with a free hydroxyl (Hoechst 33258) or a 12-carbon substituent, yet high for the 5-carbon analogue. Selective killing of human melanoma cells compared with normal fibroblasts was observed for the Hoechst analogue with a 12-carbon chain attached. Hoechst 33258 itself was selectively toxic for the MM96E melanoma cell line compared with other cell lines, induced a highly dendritic morphology, increased tyrosinase activity and tyrosinase mRNA but decreased the level of gp75 (TRP-1) mRNA; message for a third pigment gene, Pmel-17, was unchanged. Tyrosinase activity was decreased in the resistant A2058 melanoma cell line and transcription was affected to a lesser extent than in MM96E. Expression of gp75 protein and two intermediate filament proteins was inhibited by Hoechst 33258 in MM96E cells. There was no major difference in the amount of 125I-Hoechst 33258 taken up by sensitive and resistant cells. Of the five derivatives studied, the parent drug Hoechst 33258 and the 2-carbon analogue (Hoechst 33342) were found to have the most inhibitory effect on affinity of octamer binding proteins for the ATGCAAAT consensus sequence found in the promoter region of certain genes associated with proliferation and differentiation. In contrast to Distamycin A (also an A-T-specific minor groove ligand), Hoechst 33258 displaced proteins already bound to the octamer motif. The G-C ligand chromomycin A3 exhibited a different spectrum of cell toxicity and tyrosinase stimulation compared with Hoechst 33258. Chromomycin A3 but not Hoechst 33258, strongly inhibited the zinc-dependent transcriptional activity of the sheep metallothionein-Ia promoter in reporter gene assays of transfected cells. Since the six metal-responsive elements of the promoter are GC-rich, this provides independent evidence for the sequence-specificity of transcriptional inactivation by one of these drugs in melanoma cells. Overall, the results suggest that Hoechst 33258 acts by inhibiting the transcription of specific genes, cell lines evidently differing in the accessibility to drugs of certain A-T-rich sequences.

Inhibition of melanogenesis in human melanoma cells by novel analogues of the partial histamine (H2) agonist nordimaprit

A series of analogues of the weak histamine (H2) agonist S-[2-(N,N-dimethylamine)ethyl] isothiourea (nordimaprit) was produced to investigate the possibility that bulky substituents on the tertiary amine of nordimaprit would enhance potency for depigmentation and killing of melanoma cells. Cell survival studies showed that neither an increase in lipophilicity nor an increase in size of these groups produced selective toxicity, with only the N,N-diisobutyl derivative being more effective than the N,N-diisopropyl derivative in killing the constitutively pigmented human melanoma cell line MM418. The most hydrophobic analogue, the N,N-dibenzyl derivative, was also the most toxic to all cell lines tested. The association of toxicity with lipophilicity was confirmed by the piperidine derivative having greater toxicity than the less hydrophobic morpholine analogue. The ability to decrease tyrosinase activity was lost when lipophilicity and size of the N-terminal groups were increased, but these analogues produced marked depigmentation, even greater than that found with either nordimaprit or the diisopropyl derivative. Surprisingly, an increase in tyrosinase activity was achieved, the most potent agent being the N-ethyl-N-anilino analogue which caused complete depigmentation (0.6% of control) and elevated tyrosinase activity (148%) in MM418 cells after 1 month of treatment. This indicates that nordimaprit and its derivatives possess two different mechanisms of depigmentation, the first being tyrosinase dependent and the second being tyrosinase independent. The latter pathway is yet to be elucidated but appears to require a high degree of hydrophobicity.

Amino acid alcohols: growth inhibition and induction of differentiated features in melanoma cells

The effects of a series of D- and L-amino acid alcohols on the proliferation and phenotypic expression of B16 mouse melanoma cells were evaluated. B16 melanoma cells were incubated for different time intervals in the presence of D- or L-phenylalaninol (PHE), D- or L-alaninol (AL), D- or L-leucinol (LE), L-histidinol (HIS), L-tyrosinol (TYR) and L-methioninol (MET). All agents, including the D or L configuration, induced an anti-proliferative effect, although of considerably different magnitude. D-PHE was the most active growth inhibitor. The growth inhibitory effects were accompanied by phenotypic alterations, which included morphological changes and enhancement in the activities of NADPH cytochrome c reductase and tau-glutamyl transpeptidase. These phenotypic alterations correlated with the growth inhibitory effects of the different agents and seem to reflect a higher differentiated state.

Immunohistochemical detection of the p53 oncoprotein in tumours of melanocytic origin

We employed the polyclonal anti-p53 antibody NCL-CM1 to cultured cells and pathological tissues in order to investigate the expression of p53 oncoprotein in human malignant melanomas. The results in the cultured cells showed that the antigenic determinant was sensitive to formalin fixation, resulting in a lower reactivity than with fixation by alcohol. In pathological tissues, the expression of p53 oncoprotein increased with progression of the tumour. Among 79 melanomas 37 (47%) showed distinct nuclear labelling and the highest proportion of reactive cells was observed in metastatic melanomas (mean 4.8%). An immunocytochemical study also revealed the presence of mutant-type oncoprotein in human melanoma cell lines, which was recognized by monoclonal antibody P240, and we confirmed that the molecular weight of the antigens recognized by both antibodies was 53 kDa by Western blot analysis. Therefore, although the presence of point mutations in human melanomas is yet to be confirmed our data suggest that the antigen detected by NCL-CM1 is a mutant-type or a complex of mutant and wild-type p53 oncoproteins. This antibody may be useful in retrospective studies of tumours of melanocytic origin.

Alteration of melanoma melanogenesis by phenotypic modifiers

Human melanoma cells (MM96E) were incubated with a phenotypic modifier (L-ethionine) to compare its effects on phenotypic expression with those induced by sodium butyrate and dimethyl sulfoxide. In contrast to the latter agents, L-ethionine (8mM) failed to arrest the cell cycle at the G1 phase or to inhibit colony formation ability after 48 hr incubation. Tyrosinase activity changed in parallel with 5-S-cysteinyldopa (5-S-CD) content during treatment with sodium butyrate or dimethyl sulfoxide. Tyrosinase was inhibited in L-ethionine-treated cells, probably because of metabolism of L-ethionine to sulfhydryl compounds; this remains to be clarified. Gamma-glutamyl transpeptidase activity changed inversely with tyrosinase activity after sodium butyrate or dimethyl sulfoxide incubation, whereas L-ethionine did not significantly alter the enzyme activity. In addition, only sodium butyrate induced alkaline phosphatase activity. L-ethionine was less effective than sodium butyrate or dimethyl sulfoxide in inhibiting expression of the B8G3 melanosomal antigen, as determined by Western blotting. These results suggest that phenotypic modifiers (differentiation inducers) affect melanoma cells in various ways and that melanogenesis therefore reflects only one aspect of differentiation in pigment cells.

Induction of H1(0)-gene expression in B16 murine melanoma cells

Histone H1(0) accumulation is associated with the terminal stage of differentiation. Unlike other H1 histones, it is able to accumulate in the absence of DNA synthesis, however the transcription of its gene is cell-cycle dependent. The regulation of H1(0)-gene expression has been studied during the induced differentiation of B16 cells and during reversion of the process, which may be achieved when induced cells are released into an inducing-agent-free medium. During the earlier period of induced differentiation, H1(0) mRNA showed over-expression when the cells were still proliferating. Then the amount of H1(0) mRNA decreased as the cells became arrested in G0-G1. H1(0) mRNA half-life measurements and run-on experiments demonstrated that such modulation of the amount of mRNA originated from a transcriptional control of H1(0)-gene expression. When induced cells reverted to a proliferative undifferentiated state, H1(0) mRNA decreased very rapidly, indicating that an active process was involved in this decay. This behavior differed from that observed in rat liver hepatocytes allowed to proliferate and de-differentiate after partial hepatectomy, or in murine erythroleukemia cells when the inducing agent was removed from the culture.

Antiproliferative and depigmenting effects of the histamine (H2) agonist dimaprit and its derivatives on human melanoma cells

Human melanoma cells were treated in culture with the histamine (H2) agonist S-(3-(N-N-dimethylamino)propyl)isothiourea (dimaprit), a partial agonist, S-(2-(N,N-dimethylamino)ethyl)-isothiourea (nordimaprit), and two analogues of nordimaprit, S-(2-(N,N-diethylamino)ethyl)isothiourea (DENOR) and S-(2-(N,N-diisopropylamino)ethyl)isothiourea (DINOR), to investigate the effects on toxicity and tyrosinase activity. Cell survival studies showed highest toxicity in the constitutively pigmented human melanoma cell line MM418, DINOR being the most effective agent. Toxicity was not blocked by the H2 antagonist cimetidine. Dimaprit and its derivatives decreased tyrosinase activity in the amelanotic human melanoma cell line MM96E and inhibited expression of a melanosomal antigen. Loss of tyrosinase activity could be prevented by cimetidine and ranitidine, an H2 antagonist. Although the tyrosinase activity in MM418 cells was much more resistant to inhibition by these agents compared with that in MM96E cells, prolonged growth in the presence of non-toxic levels of DINOR caused a decrease in tyrosinase activity and subsequent depigmentation. Ultrastructural examination of the depigmented cells showed a decrease in the number of melanized melanosomes and the appearance of premelanosomes. These results indicate that bulky substituents on the tertiary amine group in nordimaprit significantly enhance potency for depigmentation and cell killing but only the former effect is mediated by the H2 receptor.