Cellular senescence, the irreversible proliferative arrest seen in somatic cells after a limited number of divisions, is considered a crucial barrier to cancer, but direct evidence for this in vivo was lacking until recently. The best-known form of human cell senescence is attributed to telomere shortening and a DNA-damage response through p53 and p21. There is also a more rapid form of senescence, dependent on the p16-retinoblastoma pathway. p16 (CDKN2A) is a known melanoma susceptibility gene. Here, we use retrovirally mediated gene transfer to confirm that the normal form of senescence in cultured human melanocytes involves p16, since disruption of the p16/retinoblastoma pathway is required as well as telomerase activation for immortalisation. Expression (immunostaining) patterns of senescence mediators and markers in melanocytic lesions provide strong evidence that cell senescence occurs in benign melanocytic naevi (moles) in vivo and does not involve p53 or p21 upregulation, although p16 is widely expressed. In comparison, dysplastic naevi and early (radial growth-phase, RGP) melanomas show less p16 and some p53 and p21 immunostaining. All RGP melanomas expressed p21, suggesting areas of p53-mediated senescence, while most areas of advanced (vertical growthphase) melanomas lacked both p16 and p21, implying escape from both forms of senescence (immortalisation). Moreover, nuclear p16 but not p21 expression can be induced in human melanocytes by oncogenic BRAF, as found in around 80% of naevi. We conclude that cell senescence can form a barrier to melanoma development. This also provides a potential explanation of why p16 is a melanoma suppressor gene.

UV radiation is an important etiologic factor for skin cancer, including melanoma. Constitutive pigmentation and the ability to tan are considered the main photoprotective mechanism against sun-induced carcinogenesis. Pigmentation in the skin is conferred by epidermal melanocytes that synthesize and transfer melanin to keratinocytes. Therefore, insuring the survival and genomic stability of epidermal melanocytes is critical for inhibiting photocarcinogenesis, particularly melanoma, the most deadly form of skin cancer. The paracrine factors A-melanocortin and endothelin-1 are critical for the melanogenic response of cultured human melanocytes to UV radiation. We report that A-melanocortin and endothelin-1 rescued human melanocytes from UV radiation-induced apoptosis and reduced DNA photoproducts and oxidative stress. The survival effects of A-melanocortin and endothelin-1 were mediated by activation of the melanocortin 1 and endothelin receptors, respectively. Treatment of melanocytes with A-melanocortin and/or endothelin-1 before exposure to UV radiation activated the inositol triphosphate kinase-Akt pathway and increased the phosphorylation and expression of the microphthalmia-related transcription factor. Treatment with A-melanocortin and/or endothelin-1 enhanced the repair of cyclobutane pyrimidine dimers and reduced the levels of hydrogen peroxide induced by UV radiation. These effects are expected to reduce genomic instability and mutagenesis. (Cancer Res 2005; 65(10): 4292-9)

Epidermal melanocytes are particularly vulnerable to oxidative stress due to the pro-oxidant state generated during melanin synthesis, and to intrinsic antioxidant defences that are compromised in pathologic conditions. Melanoma is thought to be oxidative stress-driven, and melanocyte death in vitiligo is thought to be instigated by a highly pro-oxidant state in the epidermis. We review the current knowledge about melanin and the redox state of melanocytes, how paracrine factors help counteract oxidative stress, the role of oxidative stress in melanoma initiation and progression and in melanocyte death in vitiligo, and how this knowledge can be harnessed for melanoma and vitiligo treatment.

The significance of melanotropic hormones as physiologic regulators of cutaneous pigmentation in humans is still controversial. Until recently, no direct effect for melanotropins could be demonstrated on human melanocytes.Here we present conclusive evidence that a!-melanotropin (a-melanocyte-stimulating hormone, ai-MSH) and the related hormone corticotropin (adrenocorticotropic hormone, ACTH) stimulate the proliferation and melanogenesis of human melanocytes maintained in culture in a growth medium lacking any AMP inducer. The minimal effective dose of either hormone is 0.1 nM. In time-course experiments, the increase in cell number and tyrosinase activity became evident after one treatment of the melanocytes with 100 nM a-MSH for 48 hr. The mitogenic effect gradually increased to 50-270S% above control, depending on the individual melanocyte strain, with continuous treatment with 100 nM a-MSH for 8 days, whereas the melanogenic effect became maximal (70-450% increase above control) after 4 days oftreatment. Western blot analysis of tyrosinase and the tyrosinase-related proteins TRP-1 and TRP-2 revealed that a-MSH increased the expression of those three melanogenic proteins. This was not accompanied by any change in their mRNA levels after brief (1.5-24 hr) or prolonged (6 days) treatment with 100 nM a-MSH, suggesting that the increased expression of these melanogenic proteins was due to posttranscriptional events. These results demonstrate both mitogenic and melanogenic effects of a-MSH and ACTH on human melanocytes. That both hormones are effective at subnanomolar concentrations, combined with the presence of melanotropin receptors on human melanocytes, strongly suggests that these melanotropins play a physiologic role in regulating human cutaneous pigmentation.a-Melanotropin (a melanocyte-stimulating hormone, a-MSH) is the physiologic hormone that regulates integumental pigmentation of many vertebrate species. For example, a-MSH induces rapid skin darkening in amphibians and reptiles and stimulates follicular eumelanogenesis in the mouse (1,2). In addition to the pigmentary effects, other functions for a-MSH and related melanotropins have been described, such as the antagonistic interaction with interleukin 1 (3, 4) and trophic effects on neurons (5, 6

Everyone knows and seems to agree that melanocytes are there to generate melanin -an intriguing, but underestimated multipurpose molecule that is capable of doing far more than providing pigment and UV protection to skin (1). What about the cell that generates melanin, then? Is this dendritic, neural crestderived cell still serving useful (or even important) functions when no-one looks at the pigmentation of our skin and its appendages and when there is essentially no UV exposure? In other words, what do epidermal and hair follicle melanocytes do in their spare time -at night, under your bedcover? How much of the full portfolio of physiological melanocyte functions in mammalian skin has really been elucidated already? Does the presence or absence of melanoctyes matter for normal epidermal and ⁄ or hair follicle functions (beyond pigmentation and UV protection), and for skin immune responses? Do melanocytes even deserve as much credit for UV protection as conventional wisdom attributes to them? In which interactions do these promiscuous cells engage with their immediate epithelial environment and who is controlling whom? What lessons might be distilled from looking at lower vertebrate melanophores and at extracutaneous melanocytes in the endeavour to reveal the 'secret identity' of melanocytes? The current Controversies feature explores these far too infrequently posed, biologically and clinically important questions. Complementing a companion viewpoint essay on malignant melanocytes (2), this critical re-examination of melanocyte biology provides a cornucopia of old, but underappreciated concepts and novel ideas on the slowly emerging complexity of physiological melanocyte functions, and delineates important, thought-provoking questions that remain to be definitively answered by future research. Praeludium pigmentosumFor those uninformed, the skin is an inert plastic wrap nature provides to keep us in and everything else out. How mistaken they are! The skin, in particular the epidermis, is one of the most active of all tissues ⁄ organs.Nature wisely placed the capillary circulation in the dermis. The epidermis has no vascular circulation thereby minimizing the probability that toxic chemicals, bacteria or fungi that penetrate through the stratum corneum can diffuse into the blood stream. That does not leave the epidermis defenseless. The epidermis has proteins called defensins that have anti-microbial properties. There are Toll-like receptors that recognize invading organisms and incite a host response. Even more interesting, it is well known that keratinocytes are avidly phagocytic. They have the capacity to phagocytize the wandering, invasive fungi or bacteria and digest them. It is both interesting and important that a-MSH stimulates the ingestion of candida by keratinocytes. a-MSH has a wide array of activities, only one of which is to stimulate the synthesis of melanin. There are receptors for a-MSH on Langerhans cells and keratinocytes as well as melanocytes. It has the ability to suppress infla...