The MAPK and Akt pathways are nearly universally dysregulated in human melanomas, with specific occurrence of Pten loss together with Braf activation in at least 20% of tumors (Tsao et al., 2004
; Curtin et al., 2005
). The precise role of Wnt/β-catenin signaling in human melanoma has remained elusive despite extensive in vitro
study, mouse modeling efforts, and immunohistochemical analyses of human melanoma specimens. Here, by characterizing mouse models based on genetic alterations commonly observed in human melanoma, a definitive role for β-catenin as a mediator of tumor progression and metastasis in Pten inactivated, Braf activated melanomas has been established. In the Pten/Braf
mouse models, metastasis can be either enhanced or repressed by either increasing or decreasing β-catenin levels, respectively.
In recent analytical studies of human melanoma specimens, it has been proposed that reduced β-catenin levels are associated with a relatively worse prognosis (Chien et al., 2009
). However, when β-catenin is inactivated in Pten/Braf
melanomas, tumor formation is delayed, survival is extended, and metastasis is nearly eliminated. These data demonstrate the requirement for endogenous β-catenin in melanoma formation and progression. The phenotypic alterations in Pten/Braf/Bcat-KO
tumors seem to be related to decreased Wnt transcriptional output, as well as decreased Mitf levels and Mitf target gene expression. In a similar vein, when β-catenin is stabilized in the Pten/Braf
melanomas, survival is reduced and metastasis to lymph node, lung, bowel, and spleen is enhanced, some of the most common sites of metastasis in human melanoma patients. In addition to being more metastatic, Pten/Braf/Bcat-STA
melanomas are also characterized by increased Wnt-related transcription, primarily as an increase in Mitf and Mitf target gene expression. Thus, in the Pten/Braf
model, more metastatic tumors are characterized by enhanced melanocyte differentiation, while less metastatic tumors exhibit reduced melanocytic differentiation.
The relationship between melanoma differentiation/pigmentation and metastasis has long been of research interest. 25 years ago it was observed that more pigmented mouse melanoma cell lines had an enhanced ability to metastasize (Bennett et al., 1986
). Since this time, other work has suggested that the melanocytic differentiation program may intrinsically predispose melanomas to metastasis after oncogenic transformation (Gupta et al., 2005
). The finding of Mitf amplification in >20% of metastatic melanomas, also suggests a powerful oncogenic potential for melanocyte differentiation programs (Garraway et al., 2005
). These observations are also supported clinically, as melanomas are known to frequently metastasize early in disease progression, even from relatively thin primary tumors (Bedrosian et al., 2000
). These observations are contrasted with observations suggesting that in general melanomas with increased MITF staining (and β-catenin staining) are associated with relatively improved survival in melanoma (Chien et al., 2009
; Nazarian et al., 2010
An improved understanding of human melanoma genetics has begun to allow for classification of melanomas based on fundamental driving mutations, such as mutant NRAS or BRAF. Here, specific evaluation of the consequences of β-catenin stabilization or inactivation in the context of precisely defined genetic changes was possible. When β-catenin is stabilized in the context of Pten loss and Braf activation, melanomas are very metastatic, grow rapidly, and are highly differentiated. However, in the presence of wildtype Pten, Braf activation and β-catenin stabilization have very different effects. Although highly differentiated tumors form, they do not metastasize. Further, Braf/Bcat-STA melanomas grow very slowly and it is only in association with relative loss of melanocytic differentiation, that tumors exhibit more rapid growth. Comparison of Pten/Braf/Bcat-STA melanomas to Braf/Bcat-STA revealed that activation of Akt in the context of Braf activation and β-catenin stabilization is strongly associated with metastasis in the highly differentiated melanomas. Further, when one considers the amelanotic Braf/Bcat-STA melanoma variant, in which accelerated tumor growth occurred only in the context of relative loss of differentiation markers, it becomes clear that in some contexts melanocytic differentiation may actually restrict exuberant melanoma growth. It is notable that in this model loss of Pten has the potential to switch the melanocyte differentiation program from a transforming, but relatively growth restrictive pathway to a pathway that promotes growth and metastasis. It is also notable that increased differentiation, which includes increased E-cadherin expression, rather than preventing metastasis, is associated with an increase in this process.
These findings raise the possibility that factors promoting metastasis in one melanoma may not in related, but genetically distinct melanomas. These considerations suggest that broadly defining the biological effects of pathways such as Wnt signaling and MITF in melanoma may not be possible out of context of other mutational changes, and help to explain the relative inconsistency of previous studies in this regard. Such context-specific roles of mutated proteins in cancer are likely to emerge as a paradigm as we learn better ways to classify melanomas into biologically meaningful groups.
In the Pten/Braf
melanomas, simultaneous activation of core signaling pathways mediating growth and survival correlate very closely with the ability to metastasize. Concurrent activation of MAPK, PI3K/Akt, and Wnt/Mitf pathways are all required for the full metastatic melanoma phenotype, as loss of any component abrogates the effect (summarized in ). Further, changes to Wnt signaling not only alter Wnt output and the melanoma phenotype, but also impact signaling through other core pathways regulating cellular growth and survival. Pten/Braf/Bcat-STA
melanomas show enhanced MAPK and PI3K/Akt activation, while Pten/Braf/Bcat-KO
melanomas show decreased PI3K/Akt signaling. It is likely that MAPK pathway activation in Pten/Braf/Bcat-STA
tumors is at least in part related to increased Kit and/or Mitf levels, especially given the growth inhibitory effect of imatinib treatment, however other possible mediators of this effect are not excluded. Additionally, increased Mitf may also play a role in the elevated proliferation rates observed in Pten/Braf/Bcat-STA
melanomas, as Mitf has been shown to be an important integrator of upstream signaling pathways in regulating proliferation and differentiation the melanocytic lineage. Increased Akt activity, however, is likely due to transcriptional upregulation of Pin1, which enhances Akt stability, leading to an increase in Akt protein levels, and ultimately activated Akt. This link between Wnt signaling and Akt in melanoma is likely also important in human melanomas, where increased PIN1 transcription is associated with increased Wnt/MITF. As PIN1 has also been shown to potentiate the effects of additional oncogenes such as Jun (Han et al., 2011
), mutant p53 (Girardini et al., 2011
), and Cyclin D1 (Li et al., 2006
), PIN1-dependent effects are likely to play a broad role in cancer.
Signaling and phenotypic overview of murine melanoma models
Components of the MAPK and also the PI3K/Akt pathways are central targets for new cancer therapeutics. For example, vemurafenib (PLX4032), a mutant Braf inhibitor has been shown to improve survival in late stage melanoma patients with Braf mutations (Chapman et al., 2011
). Along these lines, understanding how simultaneous activation of additional signaling pathways in a given tumor can impact inhibitor-targeted pathways is essential and may assist in understanding why some patients with a given driver mutation respond differentially to treatment with the same inhibitor. In this context, it is particularly noteworthy that changes to β-catenin can have such profound effects to both the MAPK and PI3K/Akt signaling pathways.
Here we have described several mouse models of melanoma that have provided fundamental insight into mechanisms by which β-catenin/Wnt signaling alterations can impact melanoma formation and progression in vivo. Extensive analyses were conducted in order to ensure that these models closely resemble human melanomas with analogous genetic changes and cellular phenotypes. Faithful recapitulation of human melanoma, combined with features such as rapid tumor growth and reproducible metastasis to lymph nodes and lung, make this model attractive for future studies in melanoma.