It is widely held that cells with metastatic properties such as invasiveness and expression of matrix metalloproteinases arise through the stepwise accumulation of genetic lesions arising from genetic instability and "clonal evolution." By contrast, we show here that in melanomas invasiveness can be regulated epigenetically by the microphthalmia-associated transcription factor, Mitf, via regulation of the DIAPH1 gene encoding the diaphanous-related formin Dia1 that promotes actin polymerization and coordinates the actin cytoskeleton and microtubule networks at the cell periphery. Low Mitf levels lead to down-regulation of Dia1, reorganization of the actin cytoskeleton, and increased ROCK-dependent invasiveness, whereas increased Mitf expression leads to decreased invasiveness. Significantly the regulation of Dia1 by Mitf also controls p27 The ability of cancer cells to acquire properties of invasiveness and the potential to metastasize is not entirely understood. In the traditional model, genetic instability within a tumor would lead within some cells to the accumulation of genetic and stable epigenetic lesions that are compatible with increased invasiveness and metastatic potential. These genetic lesions and stable epigenetic modifications, such as histone and DNA methylation, would be inherited by the metastatic progeny and dictate alterations in gene expression compatible with metastatic potential (for a review, see Baylin and Ohm 2006). Alternatively, invasive potential could represent a specific epigenetic state that is inherently unstable and nonheritable, being imposed by the microenvironment of the cell; in this model, properties associated with metastasis may be lost once the invasive cell has taken up residence in another location. While considerable resources have been expended on trying to pinpoint genetic mutations that correlate with metastatic potential and, more recently, the resulting stable epigenetic changes, the molecular mechanisms that would underpin a dynamic epigenetic model for cancer metastasis have been relatively little explored.Some cancer types, such as melanoma, are intrinsically more metastatic than others, and it has been postulated that the ability of melanomas to invade and proliferate may be related to the inherent ability of melanoblasts to migrate from the neural crest and proliferate to populate the epidermis and hair follicles Gupta et al. 2005). Some genes have been identified whose expression appears to correlate with the ability of melanomas to metastasize. The expression of SLUG, for example, can promote metastasis and facilitates the loss of E-cadherin (Gupta et al. 2005), a key molecule that mediates melanocyte adhesion to keratinocytes that is lost when melanomas acquire invasive potential (Haass et al. 2005), while NEDD9, an adaptor protein that interacts with focal adhesion kinase, is frequently overexpressed in metastatic melanoma and promotes invasiveness (Kim et al. 2006). However, despite these advances, and some preliminary investigations into the concept tha...

The controls that enable melanoblasts and melanoma cells to proliferate are likely to be related, but so far no key regulator of cell cycle progression specific to the melanocyte lineage has been identified. The microphthalmia-associated transcription factor Mitf has a crucial but poorly defined role in melanoblast and melanocyte survival and in differentiation. Here we show that Mitf can act as a novel anti-proliferative transcription factor able to induce a G1 cell-cycle arrest that is dependent on Mitf-mediated activation of the p21(Cip1) (CDKN1A) cyclin-dependent kinase inhibitor gene. Moreover, cooperation between Mitf and the retinoblastoma protein Rb1 potentiates the ability of Mitf to activate transcription. The results indicate that Mitf-mediated activation of p21Cip1 expression and consequent hypophosphorylation of Rb1 will contribute to cell cycle exit and activation of the differentiation programme. The mutation of genes associated with melanoma, such as INK4a or BRAF that would affect either Mitf cooperation with Rb1 or Mitf stability respectively, would impair Mitf-mediated cell cycle control.

Tumor progression is a multistep process in which proproliferation mutations must be accompanied by suppression of senescence. In melanoma, proproliferative signals are provided by activating mutations in NRAS and BRAF, whereas senescence is bypassed by inactivation of the p16 Ink4a gene. Melanomas also frequently exhibit constitutive activation of the Wnt/␤-catenin pathway that is presumed to induce proliferation, as it does in carcinomas. We show here that, contrary to expectations, stabilized ␤-catenin reduces the number of melanoblasts in vivo and immortalizes primary skin melanocytes by silencing the p16 Ink4a promoter. Significantly, in a novel mouse model for melanoma, stabilized ␤-catenin bypasses the requirement for p16 Ink4a mutations and, together with an activated N-Ras oncogene, leads to melanoma with high penetrance and short latency. The results reveal that synergy between the Wnt and mitogen-activated protein (MAP) kinase pathways may represent an important mechanism underpinning the genesis of melanoma, a highly aggressive and increasingly common disease.[Keywords: Mitf; Wnt; senescence; development; tumor suppressor; oncogene] Supplemental material is available at http://www.genesdev.org.

The origin of tumor heterogeneity is poorly understood, yet it represents a major barrier to effective therapy. In melanoma and in melanocyte development, the microphthalmia-associated transcription factor (Mitf) controls survival, differentiation, proliferation, and migration/metastasis. The Brn-2 (N-Oct-3, POU3F2) transcription factor also regulates melanoma proliferation and is up-regulated by BRAF and B-catenin, two key melanoma-associated signaling molecules. Here, we show that Brn-2 also regulates invasiveness and directly represses Mitf expression. Remarkably, in melanoma biopsies, Mitf and Brn-2 each mark a distinct subpopulation of melanoma cells, providing a striking illustration of melanoma tumor heterogeneity with implications for melanoma therapy.