Browsing by Author "Kidson, Sue"

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    Open Access
    Cell type-specific regulation of the chicken tyrosinase promoter
    (2002) Clarke, Ruth Elsie; Kidson, Sue
    Melanin, the pigment found in the eyes and coats of vertebrates, is synthesised by two main cell types: melanocytes and retinal pigment epithelial (RPE) cells. These two cell populations. which arise from distinct embryological origins, differ with respect to the rate at which they produce melanin and the ways in which they respond to melanogenic stimuli. Tyrosinase is the rate-limiting enzyme in the melanin synthesis pathway, and the regulation of tyrosinase gene expression in mammalian melanocytes has been extensively studied. In contrast, regulation oftyrosinase gene expression in RPE cells has received little attention. In the present study, the chicken tyrosinase gene promoter was used to investigate possible differences in the regulation of tyrosinase expression in melanocytes and RPE cells. Transient transfection experiments were carried out in which reporter constructs, consisting oftyrosinase promoter deletion fragments linked to a luciferase reporter gene, were introduced into melanocytes, RPE cells and a non-pigmented cell line. The following results were obtained. (1) Reporter expression obtained with the longest (2.1kb) promoter fragment was significantly higher in pigmented cells (both melanocytes and RPE cells) than in non-pigmented cells, demonstrating the pigment cell-specificity of the chicken tyrosinase promoter. (2) Reporter expression obtained with a 0.5kb promoter fragment, containing conserved core regulatory elements ( an lnr, M-box and Sp 1 binding site), was higher in melanocytes than in RPE cells. This result suggests that the core elements are sufficient for high levels of tyrosinase expression in melanocytes, but not in RPE cells. (3) Reporter activity obtained with a 248bp promoter fragment containing no elements implicated in initiating tyrosinase transcription was strikingly high in RPE cells, and very low in melanocytes. This result suggested the presence of RPE-specific regulatory elements in the tyrosinase promoter. To determine which portion of the 248bp promoter fragment contained the element(s) responsible for this RPE-specific activity, three additional deletion constructs were cloned. Transient transfection experiments with these new constructs revealed that the RPE-effect observed with the 248bp construct was a serendipitous / unfortunate experimental artefact brought about by the ligation of 203bp of proximal promoter with 45bp of distal promoter. Examination of the sequence generated by this ligation revealed the presence of an element similar to PCE-1, an element recently implicated in RPEspecific gene regulation. Factors present in RPE cells, but not in melanocytes, may bind to this element to initiate transcription. Further investigation of the mechanism mediating this RPEspecific effect could contribute to the understanding ofRPE-specific gene regulation. In conclusion, the results of the present study strongly suggest that expression of the chicken tyrosinase gene is regulated differently in RPE cells and melanocytes, and begin to identify regions in the chicken tyrosinase promoter that might be responsible for mediating such differences.
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    Characterisation and functional analysis of the chicken microphthalmia gene
    (2002) Pinder, Karen Elizabeth; Kidson, Sue
    Despite the fact that the chicken embryo is a model system for developmental biologists, comparatively little is known about the regulation of avian melanogenesis. There has only been one report of the characterisation of the chicken tyrosinase gene promoter, and only one full-length chicken microphthalmia cDNA (cmi9) has been identified to date.
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    The cloning and characterisation of the chicken tyrosinase-related protein gene family
    (1998) April, Craig Stuart; Kidson, Sue
    Very little is known about the molecular and genetic mechanisms controlling pigmentation within the bird kingdom. The aim therefore, of this study was to contribute towards the understanding of the genetic regulation of avian pigmentation by the cloning and characterisation of the chicken Tyrosinase-related protein (TRP) gene family. To accomplish this goal, neural crest cells from 500 black chick embryos were cultured under conditions supportive of melanocyte differentiation and proliferation. Using RNA extracted from these pigmented melanocyte cultures, a novel embryonic chick cDNA library was constructed. Screening of this library for chicken equivalents of the mammalian TRP gene family yielded more than 200 cDNA clones. After sequencing, three of these clones, 88.3, pcTRP- 1.6 and pcTRP -2. 10, were found to encode chicken Tyrosinase (Tyr), Tyrosinase-related protein-1 (Tyrp1) and Tyrosinase-related protein-2 (Tyrp2), respectively. In addition, a chicken Microphthalmia (Mi) cDNA clone (M156) was isolated using a mouse Mi cDNA probe. Comparative analyses revealed that chicken Tyr, Tyrp1 and Tyrp2 share approximately 68%, 72% and 70% amino acid sequence identity with their vertebrate orthologues. Northern blot hybridisation analysis demonstrated that the chicken TRPs are expressed in RNA from cultured retinal pigment epithelial (RPE) cells as well as in whole eye RNA. The major transcript sizes for the chicken Tyr, Tyrp1 and Tyrp2 genes are 2.5 kb, 2.3 kb and 3.5 kb, respectively. In situ hybridisation studies confirmed that both chicken Tyr and Tyrp2 genes are expressed in a pigment cell-specific fashion with signals detected in both the skin and RPE of chick embryos. Genomic Southern blot hybridisation analyses strongly suggested that all three chicken TRP genes contain several introns that are likely to be conserved within the vertebrate TRP gene family. Furthermore, the chick Tyr, Tyrp1 and Tyrp2 genes were found to span approximately 5-19 kb, 5-11 kb and 15-30 kb, respectively of the chicken genome. Comparisons between a black and white chick breed at the Tyr and Tyrp1 loci revealed no gross rearrangements at either of these loci. However, 1-2 kb alterations were observed between the same breeds at the Tyrp2 locus. The nature and significance of this alteration is not known. The cloning of the chicken Tyr, Tyrp1 and Tyrp2 cDNAs constitutes the first molecular cloning and characterisation of any avian TRP gene family. Taken together therefore, this study contributes towards the further understanding of the molecular mechanisms regulating pigmentation as well as the evolution of gene families.
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    Cloning, sequencing and functional analysis of the chicken tyrosine gene promoter
    (1996) Ferguson, Christine Anne; Kidson, Sue
    The differentiation of melanocytes from multipotential neural crest cells is an ideal system for studying the processes underlying lineage determination in development. Tyrosinase is a key enzyme in melanin biosynthesis and the activation of the tyrosinase gene is characteristic of differentiated melanocytes. In order to study the mechanisms underlying activation of melanocyte-specific genes during differentiation in chick embryos, a chicken genomic DNA library was screened for tyrosinase-encoding sequences using a mouse tyrosinase cDNA probe. Two identical hybridising clones were identified. Restriction mapping and sequencing revealed that both clones contained a 4.3 kb genomic DNA fragment, CTYR4.3, that included 2125 nt of the 5' flanking region, the first exon and part of the first intron of the chicken tyrosinase gene. The 5' flanking sequence of CTYR4.3, which is the most extensive to be reported for a lower vertebrate tyrosinase gene to date, was analysed further using computer-aided homology searches and primer extension. Alignment of the promoter sequences of CTYR4.3 with those of the human, mouse, quail and turtle tyrosinase genes revealed two evolutionary conserved regions. These regions may be functionally significant as they contain regulatory elements previously reported to play a role in melanocyte-specific expression of the tyrosinase gene in mammals. These include an initiator region and an associated SP1-binding site, the M-box and an upstream enhancer element, TDE. In addition, other potential transcription factor binding motifs were identified, including an AP-1-binding site, a UV-responsive element and glucocorticoid-responsive elements. Although several TATA box motifs were identified, they were situated more than 200 bp upstream of the transcription start sites mapped by primer extension analysis and therefore are unlikely to function as TFIID-binding sites. Transcription initiation appears to occur at heterogeneous start sites, and given the absence of a functional TATA box, may be mediated via the conserved initiator region and SP1-binding site. To test the ability of the 5' flanking sequence of CTYR4.3 to drive transcription and to begin to assess the functional significance of the various conserved elements, transient transfection assays were carried out. Constructs were generated in which 2.1 kb, 1.1 kb, 0.5 kb and 0.2 kb fragments of the 5' flanking sequence were linked to a luciferase reporter gene. These constructs were introduced into cultures of chicken retinal pigment epithelial cells (RPE), immortalised quail neural crest cells (MQTNC), and human liver cells (Hep G2) by calcium phosphate-mediated transfection. Transfections with all constructs resulted in luciferase activities significantly greater than those that were observed with the promoterless luciferase construct, thus confirming that the 5' flanking sequence of CTYR4.3 does possess promoter activity. However, the level of expression from the various constructs differed markedly in the different cell types. In the tyrosinase-negative Hep G2 cells, low levels of expression were observed with all constructs. In the tyrosinase-positive RPE cells, a high level of luciferase activity was obtained specifically with the smallest (0.2 kb) promoter construct. Since the 0.2 kb promoter fragment does not include the conserved initiator region, SP1-binding site, or M-box, the role of these elements in tissue-specific transcription initiation of the chicken tyrosinase gene is now questionable. These results suggest the existence of transcription regulatory mechanisms that are unique to avians and possibly other lower vertebrates. In contrast to the results obtained for RPE cells, the highest luciferase activity was obtained with the full length 2.1 kb promoter construct in the immortalised quail neural crest-derived cells. These results may have developmental significance since they suggest that the chicken tyrosinase gene promoter is regulated differently in RPE cells and neural crest-derived cells.
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    Corticotropin-releasing factor and acute post-operative gut function in truamatic abdominal injury and elective abdominal surgery
    (2009) Hill, Lauren; Kidson, Sue
    Shocked trauma patients in the Intensive Care Unit undergo a powerful, neuro-endocrine stress response driven by cytokine release and the hypothalamic-pituitary-adrenal axis. The response is activated under stress by corticotropin-releasing factor (CRF), the well-known 41 amino acid peptide neuro-hormone. Evidence from animal and human studies suggests that peripheral CRF is present in the gastrointestinal tract and associated with inflammatory changes. Critically ill patients frequently display somewhat unexplained gastrointestinal dysfunction including delayed gastric emptying, ileus and increased bowel permeability. The aim of the study was to investigate the role of CRF in critically ill adults with traumatic abdominal injury compared with elective surgical patients, and describe any association of CRF levels with alterations in acute post-operative gastrointestinal function. Eight patients with haemorrhagic shock following penetrating abdominal injury and seventeen patients undergoing elective surgery for hepato-biliary disease were studied for serial plasma and intestinal tissue CRF levels using radio-immunoassay. A RT-PCR technique was used to detect mRNA for CRF in intestinal tissue. Light microscopy was used to determine the quantity and distribution of mast cells in intestinal tissue. Post-operative gastric emptying was assessed using the paracetamol absorption test and intestinal permeability by measuring urinary lactulose:mannitol ratios following a bolus of these sugars. The study was approved by University of Cape Town Human Research Ethics Committee. Informed consent (retrospectively in the case of the trauma patients), was obtained from all subjects.
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    Generating hair follicle inductive dermal papillae cells from adipose derived mesenchymal stem cells
    (2018) Brown, Alice Clare; Kidson, Sue; Ballo, Robea
    Current management options for cutaneous burn wounds, including split thickness skin grafts and cultured epithelial autografts, generate an epithelial barrier which lacks a dermal layer and skin adnexae including hair follicles and sebaceous glands. This results in a loss of pliability and contractures that cause functional and cosmetic impairment. Embryological hair follicle morphogenesis results from a complex series of mesenchymal-epithelial interactions and to date a method of generating de novo folliculogenesis from human cells has yet to be accomplished. Existing models rely on combining 'inductive’ dermal and 'receptive’ epithelial components and placing them within a suitable model. Epithelial cells are easily obtainable from skin biopsies therefore obtaining sufficient quantities of 'trichogenic’ dermal cells remains the most significant challenge of this approach. The main aim of this project is to contribute to the achievement of de novo folliculogenesis by generating dermal papillae (DP) like-spheroids using adipose derived mesenchymal stem cells (ASCs) that, when combined with responsive epithelial cells, would be capable of inducing hair follicle formation. ASCs were directed towards a hair follicle DP-like fate by culture using the hanging drop method and exposure to Wnt, mimicking signalling and mesenchymal condensation in embryological hair follicle induction. Gene expression analysis using RT-PCR showed that the DP-cell marker Versican is expressed at high levels in ASCs under routine culture conditions and the exposure of ASCs to Wnt results in a more than threefold increase in this expression. These results suggest that Wnt/β-catenin signalling may regulate DP cell aggregative growth through modifying versican expression possibly through binding of β-catenin to the TCF transcription factor complex. Culture of ASCs using the hanging drop method produces spheroids similar in size to human hair follicle DP. Histology of these spheroids demonstrates viable cells that flatten around the outside. The spheroids grow out when replated onto Matrigel in a 3D culture model and exhibit a morphology similar to that of primary hair follicle DP cells. Analysis of mRNA expression demonstrates that Versican expression is significantly upregulated in DP-like spheroids in the absence or presence of Wnt demonstrating that Versican may be responsible for both induction and maintenance of mesenchymal cell condensates. Alpha smooth muscle actin is expressed in low levels in ASC spheroids compared to ASCs in a monolayer and this may reflect a 'migratory’ myofibroblast like phenotype of ASCs in a monolayer similar to cells with the hair follicle dermal sheath. The addition of Wnt to ASC spheroids has no additional effect on Versican expression possibly reflecting a negative feedback loop resulting from high local concentrations of endogenous Wnt expression from ASCs. The results of this study show that spheroid cell culture and exposure to Wnt of ASCs results in cell clusters with similar morphology and gene expression to hair follicle DP cells. The novel method of DP-like cell generation described in this study makes use of cells that are readily obtainable from patients and require minimal time and manipulation in culture and therefore could potentially be rapidly translatable to clinical trials.
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    Induction of hair follicles using neonatal mouse dermis and human keratinocytes: relevance for improved burn wound treatments
    (2020) Malise, Thudzelani Takalani Austin; Kidson, Sue; Ballo, Robea
    Second degree burns result in the destruction of the skin as well as its adnexa. Following such burns the wound heals without the formation of skin appendages and hair follicles. In normal embryonic development hair follicle formation requires the interaction between epithelial (keratinocytes) and mesenchyme cells. Attempts to recapitulate the process of hair induction in wounded skin in vitrousing human cells have to date been unsuccessful. The aim of this project is to attempt to elicit the early steps of hair follicle formation (induction) by co-culturing primary human keratinocytes with embryonic murine mesenchyme cells and assessing changes in expression patterns of genes associated with or reflective of induction. Mesenchymal cells and keratinocytes cells were obtained by enzymatically digesting dorsal neonatal mouse skin and neonatal human foreskin using dispase and collagenase. Cells were cultured separately, and their growth dynamics measured. The isolated neonatal mouse mesenchymal cells were shown to have hair induction potential because they expressed dermal papilla signature genes, Alp, Sox2 and Vcan. However, this characteristic was lost during in vitro propagation suggesting that mesenchymal cells lose their hair inductive potential during culture. In contrast, when cultured at high densities or in spheroids in hanging drops, the dermal papilla signature genes were upregulated suggesting that this might be a way to maintain inductive potential. Primary foreskin keratinocytes expressed high levels of basal layer marker, keratin 5 (K5), and low levels of the early differentiation marker, K10, suggesting that the isolated keratinocytes have stem cell properties. When co-cultured with neonatal mouse mesenchymal cells using Transwells, the mesenchymal cells were able to elicit colony formation on keratinocytes in co-cultures, indicating that they support keratinocyte proliferation. It was not possible to do hair follicle induction marker analysis of human foreskin keratinocytes cocultures because of challenges and difficulties encountered during expansion. Therefore, Immortalised HaCaT keratinocytes were tested. HaCaT keratinocytes were shown to be induced during cocultures because they upregulated Wnt signalling genes, β-catenin and NF-KB. As an additional approach, human foreskin keratinocytes were cultured in medium containing Wnt signalling pathway ligand, Wnt3a. β-catenin and NF-KB were slightly reduced, and only Lef1 was upregulated in human foreskin keratinocytes cultured in Wnt3a conditioned medium. The results of this study show that neonatal mouse mesenchymal cells have hair inducing capabilities and it is lost by in vitro propagation and can be restored by spheroid cell culture. The results also demonstrate that human foreskin keratinocytes need to be expanded using efficient culture methods to maintain an undifferentiated state.
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    Induction of the retinal pigment epithelium of the chicken embryonic eye
    (1997) Franz, Tamara Anne; Kidson, Sue
    During development of the eye, invagination of the optic cup gives rise to a double layered neuroepithelium, part of which differentiates into the retinal pigment epithelium (RPE). The molecular mechanisms which control differentiation of the RPE are not known. The present study was undertaken to determine 1) when induction of the RPE has occurred in chicken embryos and 2) to investigate whether contact with the presumptive neural retina (NR) is required for RPE differentiation. In order to investigate when RPE induction has occurred, early expression of two genes involved in pigmentation were investigated. Digoxigenin-labeled tyrosinase and tyrosinaserelated protein-2 (TRP-2) riboprobes were synthesised and used in ISH reactions on embryonic eye tissue. Tyrosinase transcripts were first detected at stage 19.5 (70-71 hours) and TRP-2 transcripts were detected a few hours earlier at stage 18.5 (67-69 hours) of embryonic development. These results indicate that induction has occurred by stage 18.5, approximately ten hours before distinct granules are visible in the RPE. The tyrosinase and TRP-2 transcripts were always localised first in the optical axis of the eye in the region where pigment granules are first present. This indicates that differentiation of the RPE proceeds from the optical axis of the eye cup outwards towards the periphery and that induction of the RPE may also proceed in this direction. To determine whether the presumptive NR is required for RPE induction, synthetic barriers were inserted into the uninvaginated optic vesicle of chicken embryos at stage 11 (40-45 hours) of development. The embryos were cultured in vitro until the optic vesicle had invaginated and sectioned to locate the barrier. Results suggest that contact with the presumptive NR may not be necessary for RPE induction.
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    Molecular detection of melanoma nodal metastases
    (2002) Davids, Virginia; Hanekom, Gideon; Kidson, Sue
    The aim of this study was to develop a practical and reproducible multi-marker RT-PCR essay, with the emphasis on achieving maximum specificity for the detection of melanoma nodal metastases. A novel protocol for the efficient homogenisation of nodal tissue was developed, with clinical applicability as the objective.
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    Morphological investigations into the development of the mammalian corneal endothelium using the mouse model
    (2004) Mgwebi, Thandiswa; Kidson, Sue
    The corneal endothelium (CE), a mesenchyme-derived tissue, is a monolayer of squamous cells on the inner corneal surface. In Foxc1-1• mice, the CE fails to form. The understanding of the cause of this defect has implications for the study of human eye disorders that are related to FOXC1 mutations. To understand the basis of CE defects in Foxc1-1- mice, an analysis of normal CE development was performed using scanning electron microscopy. Results showed that in normal mice the transformation from mesenchyme to endothelium was initiated at embryonic day (E) 12.5 and was characterised by a change from stellate to cobblestone shape and the formation of junctions. In FoxcN- mice, the process was initiated but a cobblestone shape not attained. The expression of adherens (N-cadherin) and tight junction (Z0-1) proteins was investigated by immunoflouresence microscopy. In the normal embryo, the expression of N-cadherin was initially in cytoplasmic vesicles and later at the cell membranes. ZO-l was first detected at the cell peripheries at E13.5. In Foxct-I- mice, N-cadherin peripheral bands failed to form. ZO-l was not expressed. These results suggest that the failure to form a monolayered CE in Foxc1 mice is due to incomplete mesenchyme-endothelial conversion. Junction formation was further investigated in vitro. N-cadherin was cytoplasmic in pre-confluent cells and at cell edges in confluent cells. ZO-l was not detected. These results suggest that in vitro, these cells are either unable to form tight junctions or the culture medium does not contain the appropriate signalling molecules.
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    Regulation of melanogenesis in conditionally immortalised mouse melanocytes expressing a temperature-sensitive SV40 large T antigen
    (1999) Prince, Sharon; Kidson, Sue
    The transformation of a normal melanocyte to a malignant melanoma involves a series of poorly understood genotypic and phenotypic alterations. In vitro models of melanoma formation generated by transforming mouse melanocytes with exogenous oncogenes have revealed that this process is frequently accompanied by a loss of pigmentation. The aim of this study was to establish, and to make use of unique cell lines to gain further insight into the mechanism(s) by which oncoproteins alter melanocyte differentiation. Primary cultures of mouse epidermal and dermal melanocytes were infected with a retrovirus carrying a temperature-sensitive mutant SV40 large T antigen. Six immortalised cell lines thus generated were analysed by northern and western blots and by enzymatic assays at the permissive temperature of the oncoprotein. Three epidermal and two dermal melanocyte clones remained pigmented and expressed tyrosinase, TRP-1 and -2 genes and the proteins encoded by them. In addition they expressed the mi gene and the c-kit receptor. In contrast, one dermal melanocyte clone (DMEL-3) gradually depigmented: this was accompanied by enhanced growth and down-regulation of melanocyte-specific gene expression. At the non-permissive temperature of the oncoprotein, proliferation ceased and DMEL-3 cells repigmented with a time-dependent increase in melanocyte-specific gene expression. Moreover, mi gene expression was down-regulated in the DMEL-3 cell line at the permissive temperature and was re-expressed at the non-permissive temperature. These results provided direct evidence for the role of the SV40 large T antigen in melanocyte dedifferentiation and emphasized the pivotal role of Mi in this process. Northern blot analysis of DMEL-3 cells cultured at the permissive and non-permissive temperatures revealed that there were no detectable levels of Pax3 transcripts at either temperature. In addition, Pax3 expression was absent in the highly pigmented DMEL-2 and melan-a cell lines. These results suggest that Pax3 is not required for mi expression and that it is unlikely to be a target of the T antigen-mediated repression of mi. To explore the possibility that other melanocyte markers are also altered as a consequence of alterations in mi expression, the DMEL-3 cells were examined for changes in the α-MSH and c-kit receptors. Melanin synthesis and tyrosinase activity assays showed that alterations in mi expression did not correlate to responsiveness to α-MSH, suggesting that the MSH receptor gene is not regulated by Mi. Furthermore, northern blot analysis showed that DMEL-3 cells did not express c-kit at either the permissive or nonpermissive temperature, suggesting that Mi does not regulate c-kit expression. To address the possible role of RB family members in melanocyte differentiation, it was investigated whether melanocyte differentiation is accompanied by an increase in their mRNAs and protein levels. Northern blot analysis strongly suggested that expression of the RB1, p130 and p107 is not altered when DMEL-3 cells were induced to differentiate at the non-permissive temperature. The results from western blot anaysis were inconclusive and require further investigations. Finally, the pigmented cell lines established in the present study provided a unique opportunity to investigate the stimulatory effect of TPA on melanogenesis because growth curves showed that the cells become TPA-independent. The results showed that stimulation of melanogenesis by TPA in a pigmented melanocyte line, DMEL-2, resulted in an increase in tyrosinase, TRP-1 and TRP-2 proteins and mRNAs. Additionally, TPA increased mi gene expression which suggests that Mi is necessary for the TPA-triggered signalling cascade that induces expression of the tyrosinase gene family. These results disclose, for the first time, a mechanistic link between TPA and the transcriptional induction of pigmentation.
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    A study of the differentiation and dedifferentiation of three human melanoma cell lines
    (1997) Davids, Lester Merlin; Kidson, Sue
    Pigment formation in melanocytes is the end-point of a series of biochemical reactions involving numerous melanocyte-specific proteins including, inter alia, the enzymes tyrosinase, tyrosinase-related protein-1 (TRP-1 ), tyrosinase-related protein-2 (TRP-2) and the melanosomal protein encoded by the P gene. The function of tyrosinase and TRP-2 have recently been clarified, but the roles of TRP-1 and the P protein remain unknown. The first aim of this study was to examine the expression of these proteins at a transcriptional and translational level in order to provide more insight into possible mechanisms which may lead to changes in melanoma cell differentiation. Three human melanoma cell lines (Mel 1, Mel-2 and Mel-3) with varying levels of pigmentation (highly melanised to amelanotic) were examined by enzyme assays and RNA quantification methods. The results showed gene expression of all four genes in the highly melanised Mel-1 and amelanotic Mel-3 cell lines. TRP-1 and TRP-2 were not expressed in the melanised Mel-2 cell line. These results suggest that there is no correlation between tyrosinase gene expression and level of pigmentation in these cell lines. In addition, they show that the level of pigmentation of human melanoma cell does not necessarily correlate to the level or pattern expression of the tyrosinase gene family. Furthermore the results of the present study show that the P gene is expressed at high levels in all the melanoma cell lines, irrespective of their level of pigmentation . The second broad aim of this study was to determine the effect of melanocytestimulating hormone (a melanogenic stimulator) on melanogenesis in Mel-1 cells. Mel-1 cells, which were exposed to 10⁻⁷ M MSH for 6 days, showed no change in tyrosinase mRNA levels, but the mRNA levels of TRP-1, TRP-2 and the P gene were reduced. This suggested the presence of a possible co-ordinated down-regulatory mechanism in the Mel-1 cells under the influence of MSH.