THOUGHTS FROM DISNEYLAND
Report on 16th International Pigment Cell Conference
29 October - 3 November 1996
Sheila Mac Neil
(with help from John Haycock and Alain Taieb)
A difficult choice of venue for this international meeting on the melanocyte considering the gravity of the subject and the number of the conference participants (can we come back again please?).
With around 90 talks and 170 posters plus excellent symposium overview from the Chairs of the various symposia, this was a well balanced meeting with plenty of time for participants to discuss their current obsessions, the melanocyte.
Reflecting on the meeting, there was a real feeling of progress on many fronts, underscored by frustration with how little we yet know about the full range of functions of the melanocyte in man. Also in melanoma, as with many other cancers, there is an enormous gap between where we stand on the investigative molecular and cellular biology of the melanocyte and melanoma cell and how little of that as yet translated through to melanoma specific therapies for the patient. But, first the good news and there is plenty of it.
Work on the origin of the higher vertebrate melanocyte from the developmental biology perspective shows that lower vertebrates have a choice of three specialised pigmentary cells: the melanophores, the iridophores and the xenophores. Higher vertebrates, however, have only one - somewhere along the line these three quite different structures changed into the one (possibly two?) melanocytes we know today. It is possible that we retain structures related to the pigmentary vesicles possibly in a vestigial form. The three tyrosinase genes that we are aware of are almost certainly a result of gene duplication. With respect to regulation of mammalian melanocytes, several independent groups now find expression of pro-opiomelanocortin peptides (a-MSH and ACTH) in normal adult melanocytes and keratinocytes. Further, there is evidence that levels of ACTH and MSH INCREASE FOLLOWING UV irradiation of skin. It is highly unlikely that these peptides will be the only regulators influenced by UV radiation - excellent data on the production and release of endothelins from keratinocytes following UV stimulation showed that these will enhance pigmentation of melanocytes. Further, positional signals, extracellular matrix proteins, are also capable of directly elevating intracellular calcium in melanocytes and cells cultured on ECM proteins show changes in proliferation and tyrosinase activity and are more resistant to apoptosis. Against these factors which enhance pigment production, the agouti factor can prevent the melanogenic response to a-MSH. Its action seems to be downstream of the MSH receptor (and cyclic AMP?) and in the presence of the agouti factor, the ratio of eumelanin to phaeomelanin will decrease. Practical progress in the measurement of eumelanin and phaeomelanin has also greatly assisted understanding of the regulation these two differently coloured melanins. In vitro, the ratios of eu- to phaeomelanin can be manipulated experimentally by varying the concentrations of L-tyrosine and cysteine in the media.
With respect to the role of pigment, this remains a vexed area - there is broad consensus that melanin pigments will bind metals and other xenobiotics and, as such, may act as a cutaneous detoxification mechanism. The role of melanins in coping with/responding to oxidative stress remains challenging. Melanin is a mixture of pigments which have been arrived at by various routes, some under hormonal control and others possibly more under the control of the redox status of the cells. Some melanin pigments appear to act as very effective anti-oxidants - others do not. Some soluble melanin precursors can influence cells of the immune system in vitro. It is clear that this is not going to be a simple area and despite the best efforts of melanin chemists and cell biologists, there is still some way to correlate the production of particular melanins to changes in cell biology, immune responses, etc. Despite the problems, this is an area which is and must be tackled primarily because of the potential it holds to improve development of melanoma specific therapies if we can understand the role of the particular melanin precursors and melanins which are formed in melanocytes and melanoma tumours.
The number of patents in the area of commercial uses of melanins has mushroomed over the last decade - it is encouraging to note that oncologists and cosmetic companies share the need to know about the biology of the melanins. One particular lead of potential value to understanding the neoplastic transformation of the melanocyte comes from work in recent years on examining the ability of the melanoma cell versus the melanocyte to cope with oxidative stress. Melanocytes are, like all cells, vulnerable to oxidative stress - there is indeed some evidence that they produce reactive radicals and have a low level of anti-oxidant enzymes, responding to UV with a large but transient increase in superoxide anion whereas keratinocytes and fibroblasts do not. Indeed in some normal melanocytes cultured from melanoma patients, there are differences in the levels of some anti-oxidants compared to melanocytes from non-melanoma patients, consistent with these cells being more susceptible to any external peroxidising stress. Similar findings have been detected in melanoma cells giving rise to a hypothesis that such cells may experience a continuous peroxidising process. Consistent with this recent work shows that cells synthesising melanin, such as melanoma cells, are more vulnerable to the actions of BSO which inhibits glutathione synthesis suggesting that the synthesis of melanin puts particular demands on the melanocyte/melanoma cell which render it particularly vulnerable to further oxidative stress. Also, the comparative resting levels of the subunits forming the transcription factor NFkB (known to play a major role in cell response to oxidative stress) are altered between melanocytes and melanoma cells both under resting and UVB stimulated conditions suggesting that the transcriptional control of melanoma cells by NFkB is disregulated. Not only was the basal level of the proteins comprising the transcription factor complex altered in melanoma cells but the response of the complex to UVB stimulation was very exaggerated in the transformed cells.
Another very promising area understanding the difference between the melanoma cell and the melanocyte lies in the cell cycle regulatory proteins. Several groups are reporting mutations in the genes relevant to cell cycle control. Studies have found mutations in such genes in almost half of the patients with melanoma. For example, passage of cells through the G1 checkpoint occurs through phosphorylation of the retinoblastoma protein. A family of low molecular weight proteins normally inhibit these particular kinases - mutations in these inhibitors will allow inappropriate cell cycling. Possibly, such mutations occur through UV induced damage. Conversely, when melanocytes are deliberately induced to differentiate, changes in some of these cell cycle regulatory proteins are seen associated with induction of differentiation. The tumour suppressor protein P53 normally inhibits cell growth and is involved in DNA-induced apoptosis. Studies using knock-out mice showed that functional wild type P53 is necessary for UVB-induced cell cycle arrest and apopotosis. Defective P53 in melanomas creates resistance to cell cycle arrest and to induction of apoptosis. Such a resistance to cell cycle arrest would also favour melanoma resistance to drug therapy.
With respect to vitiligo, vitiligo melanocytes introduced into epidermal reconstructs failed to show any defect at the cellular level both in the absence and presence of UVB. The question of wether vitiligo melanocytes contain an inherent defect remains unanswered - possibly the defect is one which requires an extrinsic factor or sustained damage to be demonstrable. Other theories of vitiligo including the autoimmune hypothesis remain viable. A cautionary note that we may be trying to simplify vitiligo too much and that it may indeed be a collection of disorders which all result in depigmentation and eventual destruction of the melanocyte was delivered as a timely reminder to investigators in the field.
As an example of the latter, a fascinating study showing spread of pigment following blister roof grafting also revealed that pigment spread was seen in piebald patients but pigment loss was prominent in patients with generalised type vitiligo (but not in patients with segmental vitiligo), suggesting that there may indeed be different aetiologies of these forms of vitiligo. In a study of acquired dermal melanocytosis (ADM), melanocytes were found in an inactive, non-pigmented form in the lower dermis of acquired dermal melanocytosis lesions. Possibly UV may stimulate their multiplication in adulthood via endothelin-1 released by keratinocytes. The question of normal dermal melanocytes in humans is controversial. In ADM, it appears that they are engulfing elastic fibres essentially exhibiting macrophagic activity. In terms of inducing a loss of pigment, an antiviral drug (vidarabine) used for herpes simplex was found to cause loss of pigment in man without apparent effects on tyrosinase activity or melanin producing ability of cultured cells.
In summary, a lively and interesting meeting. The rate of progress in pigmentation research is now truly encouraging. I apologise to all of those authors whose work I have not referred to (however briefly or inaccurately). Obviously, the way to get a more accurate view of these meetings is to make sure you attend yourself next time - you won't regret it.