Melanocyte proliferation (2006)

Melanocytes originate in the neural crest from pluripotent precursor cells and migrate as unpigmented melanoblasts to their final destinations in the epidermis and hair follicles. The ability of melanoblasts to undertake this long migration is thought to be related to the high propensity of melanoma cells to metastasize. However, what controls melanoblast migration and melanoma metastasis is poorly understood. Our recent work indicates that the Microphthalmia-associated transcription factor (Mitf) that plays a key role in the origin and survival of melanoblasts as well as in melanocyte proliferation and differentiation also regulates the invasive ability of melanoma cells via the regulation of factors downstream of Rho that control actin polymerization. The same pathway also allows Mitf to control cell cycle entry and may be important for melanoblast migration as well as in the differentiation functions of melanocytes including melanosome transport. The results emphasize the critical importance of Mitf as a major coordinator of melanocyte/melanoma behaviour. Carreira S, Goodall J, Aksan I, et al. Mitf cooperates with Rb1 and activates p21Cip1 expression to regulate cell cycle progression. Nature 2005: 433: 764–769. In this study an inducible version of Mitf, in which Mitf is expressed as a fusion to the tamoxifen-responsive oestrogen-receptor ligand binding domain, is used to demonstrate that Mitf can play a major anti-proliferative role. The effects of Mitf in inducing a G1 cell cycle arrest were attributed to its ability to bind and up-regulate the promoter of the gene encoding the p21Cip1 cyclin-dependent kinase inhibitor. In addition Mitf was shown to interact and cooperate with Rb1 in activation of the Tyrosinase promoter. These results, together with those of Loercher et al. and Wellbrock et al. provide a strong argument for an anti-proliferative role for Mitf. Garraway L A, Widlund H R, Rubin M A, et al. Integrative genomic analyses identify MITF as a lineage survival oncogene amplified in malignant melanoma. Nature 2005: 436: 117–122. In contrast to the results from other groups showing Mitf acts as an anti-proliferative factor, here this study provides compelling evidence that Mitf can also play a pro-proliferative role. The paper demonstrates that the Mitf gene can be amplified over 100-fold (although protein levels increase only around twofold) and importantly demonstrates that in engineered melanocytes ectopic Mitf expression together with activated BRAF resulted in transformation. Thus, MITF is identified as ‘oncogene’ able to facilitate proliferation in cells expressing activated BRAF. Hodgkinson C A, Moore K J, Nakayama A, Steingrimsson E, Copeland N G, Jenkins N A, Arnheiter H. Mutations at the mouse microphthalmia locus are associated with defects in a gene encoding a novel basic-helix-loop-helix-zipper protein. Cell 1993: 74: 395–404. Previous work had identified a conserved CATGTG element termed the M-box in the promoters of genes implicated in melanogenesis. These data implied the existence of a melanocyte-specific basic-Helix-loop-Helix leucine zipper factor able to recognize this motif. In this paper, a chance transgene insertion led to a white mouse with small, malformed eyes, a phenotype consistent with inactivation of a gene important for melanocyte survival and the differentiation of the retinal pigment epithelium (RPE). The disrupted gene was termed the microphthalmia-associated transcription factor (MITF) and encodes a bHLH-LZ important for both melanoblast and post-natal melanocyte survival, melanocyte and RPE differentiation, and regulation of proliferation. Ishizaki T, Morishima Y, Okamoto M, Furuyashiki T, Kato T, Narumiya S. Coordination of microtubules and the actin cytoskeleton by the Rho effector mDia1. Nat Cell Biol 2001: 3: 8–14. The results presented in this paper suggest that Dia1, a member of the formin homology domain family and a downstream effector of Rho, acts to coordinate the actin cytoskeleton with that of microtubules. Although not covered in this paper, actin-microtubule co-ordination is especially important for transport of melanosomes to the cell periphery. Loercher A E, Tank E M, Delston R B, Harbour J W. MITF links differentiation with cell cycle arrest in melanocytes by transcriptional activation of INK4A. J Cell Biol 2005: 168: 35–40. Here a negative role for Mitf in regulating proliferation is further substantiated by the demonstration that Mitf regulates the expression of the p16 INK4a cyclin-dependent kinase inhibitor in melanocytes. Although this mechanism is unlikely to operate in melanoma cells where p16INK4a is frequently inactivated by mutation, the results may be important for regulation of cell cycle exit during melanocyte differentiation in vivo. Miralles F, Posern G, Zaromytidou A I, Treisman R. Actin dynamics control SRF activity by regulation of its coactivator MAL. Cell 2003: 113: 329–342. This paper revels how signalling by Rho GTPase leads to activation of the nuclear transcription factor SRF (serum response factor, that activates a range of immediate early genes such as Fos). Mal is a cytoplasmic protein found associated with unpolymerized actin in serum starved cells. On addition of serum, actin polymerization mediated by signals arising from Rho leads to release of MAL that then translocates to the nucleus where it interacts with SRF and functions as an SRF transcriptional co-activator. The paper highlights the ability of the actin cytoskeleton to regulate transcription of a specific set of target genes. This is likely to be particularly important in understanding dendrite formation and retraction in melanocytes. Nishimura E K, Jordan S A, Oshima H, et al. Dominant role of the niche in melanocyte stem-cell fate determination. Nature 2002: 416: 854–860. Melanocytes exits in the epidermis of the skin and in the hair follicles. Yet each time a hair is lost or the skin damaged new melanocytes must be made to repopulate these areas. This paper revels that melanocyte stem cells are to be found in the bulge region of the hair follicle and that they are reactivated each hair cycle to both self renew and generate a transit amplifying population that then differentiates into mature pigment producing cells. Nishimura E K, Granter S R, Fisher D E. Mechanisms of hair graying: incomplete melanocyte stem cell maintenance in the niche. Science 2005: 307: 720–724. This paper demonstrates the loss of melanocyte stem cells in the hair follicle niche in bcl2 null and Mitfvit mice. For the Mitfvit allele the key finding is that differentiated melanocytes are found in the hair shaft whereas they would normally only be present in the hair matrix at the base of the hair follicle. Understanding the molecular mechanisms underlying survival of melanocyte stem cells will provide key insights into hair greying and most likely in melanoma proliferation also. Sahai E and Marshall C J. Differing modes of tumour cell invasion have distinct requirements for Rho/ROCK signalling and extracellular proteolysis. Nat Cell Biol 2003: 5: 711–719. Rho GTPAses regulate both the actin cytoskeleton as well as cell motility. This paper identified two modes of melanoma cell motility that differentially used Rho signalling. Importantly one mode, characterized by a rounded morphology and blebbing phenotype, operates through ROCK, a key Rho effector kinase. Inhibition of ROCK prevented melanoma cell invasiveness and places the ROCK signalling pathway firmly in the frame as a key regulator of melanoma metastasis. Steingrimsson E, Copeland N G, Jenkins N A. Melanocytes and the Microphthalmia Transcription Factor Network. Annu Rev Genet 2004: 38: 365–411. This is a comprehensive review of Mitf including the genetics, regulation of the promoter, isoforms, and post-translational modifications. Although the review does not propose any novel concepts and was published just before several papers appeared detailing the role of Mitf in proliferation, it nevertheless provides and excellent coverage of the background to this important protein. Wellbrock C, Marais R. Elevated expression of MITF counteracts B-RAF-stimulated melanocyte and melanoma cell proliferation. J Cell Biol 2005: 170: 703–708. Previous work from this group demonstrated that transformation of melanocytes led to a strong repression of MITF protein and mRNA levels. Here the authors go on to show that elevated MITF expression in melanoma cells expressing activated BRAF leads to decreased proliferation. The results suggest that one of the major role of oncogenic transformation by BRAF in melanoma is to down-regulate MITF expression and is consistent with Mitf playing an anti-proliferative role.