• melanoma;
  • ephrinA2;
  • FGFR2;
  • PTEN;
  • BRAF;
  • TGF-β;
  • p16INK4A;
  • p14ARF;
  • IGFBP7;
  • p53;
  • stem cell marker;
  • ABCB5;
  • Nodal;
  • Snail;
  • CYLD;
  • MITF;
  • mesenchymal-elongated-type and rounded-amoeboid mode of movement;
  • LKB1;
  • AMPK


  1. Top of page
  2. Abstract
  3. Introduction
  4. Recent novel findings in melanoma research
  5. Conclusion
  6. Acknowledgements
  7. Teaching Materials
  8. References
  9. Supporting Information

Recently, several important findings on melanoma development and progression have accelerated progress towards a molecular understanding of melanoma biology. Furthermore, the development of experimental tools, such as a wide range of cell lines and animal models of metastasis, has turned melanoma into a model for general tumour research. However, it has also become evident that melanoma is distinct from other tumours with regard to its cellular origin and pathophysiological mechanisms. This review focuses on important new findings that have contributed to a greater understanding of the molecular processes leading to melanoma. Translating these innovative new results into potent therapeutic approaches will require even more effort. Copyright © 2009 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.


  1. Top of page
  2. Abstract
  3. Introduction
  4. Recent novel findings in melanoma research
  5. Conclusion
  6. Acknowledgements
  7. Teaching Materials
  8. References
  9. Supporting Information

Malignant melanoma is the most aggressive form of skin cancer; it accounts for approximately 4% of skin cancer cases but for 80% of all skin cancer deaths. According to the World Health Organisation (WHO), the number of melanoma cases worldwide is increasing faster than that of any other type of cancer. Recent estimates suggest a doubling of melanoma incidence every 10–20 years.

While there is a good chance of recovery for patients suffering from melanoma if the primary lesion is detected very early, the 5-year survival rate of patients with metastasized melanomas (stages III and IV) is less than 10%. This is due to the aggressiveness of the tumour but also to the current deficiencies of therapeutic approaches for treating advanced melanoma. There is great hope that a detailed understanding of the molecular mechanisms leading to melanoma development and progression will result in efficient therapies.

In this review, we focus on selected new findings in the last 2–3 years which have added important pieces to the so far incomplete jigsaw of molecular changes in malignant melanoma. The molecules and mechanisms presented are grouped into their biological context. Details about the involvement of H-Ras, N-Ras, C-Ras, MITF, Cdk4, HGF/Met, c-Kit, MIA, MMPs, cadherins, integrins, and some other molecules in melanoma development are relevant, but we suggest that the reviews in refs 1–7 set out the roles of these molecules in melanoma, and in addition we recommend a recent review on melanoma mouse models 8.

Recent novel findings in melanoma research

  1. Top of page
  2. Abstract
  3. Introduction
  4. Recent novel findings in melanoma research
  5. Conclusion
  6. Acknowledgements
  7. Teaching Materials
  8. References
  9. Supporting Information

Changes in melanoma cells linked to UV irradiation

The causative relationship between exposure to ultraviolet (UV) irradiation and non-melanoma skin cancer is well documented, in contrast to the role of sun exposure for the development of malignant melanoma. However, it is generally accepted that melanocytes transform into malignant melanomas through the interplay of genetic factors/genetic mutations and the UV spectrum of sunlight.

In a global screen of UV radiation-responsive genes, Zhang et al identified the receptor tyrosine kinase EphA2 (ephrinA2) in melanocytes to be regulated UV dependently 9. EphA2 protein levels are induced dose dependently by UV irradiation in melanocytes, keratinocytes, and fibroblasts. In the study, EphA2 was shown to be an essential p53-independent, but MEK (Ras/MAPK signalling)-dependent mediator of UV irradiation-induced apoptosis.

The role of EphA2 in tumour physiology is highly complex and differs from tissue type to tissue type. The analysis of the role of Eph2A during melanoma development revealed that EphA2 expression correlates with a more aggressive phenotype characterized by ‘vasculogenic mimicry’, which is consistent with the known role of EphA2 in tumour angiogenesis. Margaryan et al performed experiments in which EphA2 was down-regulated in human melanoma cell lines and noticed a significant decrease in invasion, proliferation, clonogenicity, and ‘vasculogenic mimicry’ in vitro10. To summarize, in melanocytes, EphA2 expression led to UV-dependent apoptosis, whereas during melanoma development EphA2 expression forced higher invasive capacity. This provides a possible mechanistic link between excessive UV exposure and melanoma risk. The involvement of EphA2 in invasion processes as melanoma cells shift from a mesenchymal to an amoeboid-like motility style is discussed in the section entitled ‘Cellular migration and polarity’.

Normal human melanocytes do not express bFGF (basic fibroblast growth factor, FGF-2) 11, 12, whereas some naevi 13–15 and virtually all melanomas produce bFGF, with expression levels increasing with tumour progression 16–18. Recently, Gartside et al reported that 10% of melanoma tumours and cell lines harbour mutations in one of the receptors for bFGF, the FGF2 receptor 2 (FGFR2) gene 19. The novel mutations include three truncating mutations and 20 missense mutations occurring at evolutionarily conserved residues in FGFR2. The mutations cause loss of function of the receptor through several distinct mechanisms, including loss of ligand binding. Furthermore, the mutation spectrum is characteristic for UV irradiation (ie 70% of the mutations were C : G to T : A transitions). The discovery of loss-of-function mutations in a receptor tyrosine kinase is unexpected, given the general expectation that activation of receptor tyrosine kinases drives tumourigenesis. The feature loss-of-function mutation of FGFR2 is also contrary to the previous report of Pollock et al, which documented the presence of activating mutations in the FGFR2 gene in endometrial cancer 20. FGFR2 may be one of the first genes for which both loss-of-function and gain-of-function mutations have been reported in different tissue types. Up to now, this controversy has not been fully explained and the biological consequences of loss-of-function mutations in malignant melanoma in FGFR2 are not clear.

A distinct association between UV irradiation-induced DNA damage and melanoma development is exemplified by a drastically increased risk of melanoma in patients with xeroderma pigmentosum, a syndrome characterized by severe defects in nucleotide excision repair (NER) genes. This disease was used as a model to determine the role of UV-linked mutations in melanoma. PTEN (phosphatase and tensin homologue deleted on chromosome 10) is frequently mutated in the advanced stages of several human cancers. Loss of tumour suppressor genes on chromosome 10 has been reported to contribute to the development of 30–60% of sporadic melanomas. Recent evidence suggests that PTEN is one of the genes whose loss may play an important role in melanoma development. However, mutations or deletions of PTEN are detected mainly in melanoma cell lines 21–24. While many prior studies focused on deletions, promoter methylation or immunohistochemical evidence for PTEN inactivation, Wang et al identified base substitution mutations of PTEN as an indicator of UV damage in 59 melanoma samples from eight patients with xeroderma pigmentosum 25. Their analyses revealed UV-dependent PTEN tumour suppressor gene mutation in 56% of the melanomas, supporting the concept that UV protection is relevant for the prevention of melanoma. Further analysis of the correlation between UV irradiation and loss of PTEN expression was performed in a mouse model with PTEN deleted in melanocytes 26. Neurological defects cause premature lethality in approximately 50% of these mice. The surviving mice had an increased number of melanocytes and were resistant to hair greying. Melanomas did not form spontaneously in the conditional PTEN knock-out mice, but exposure to carcinogens (DMBA and TPA) induced their development. It would be interesting to analyse the influence of UV irradiation also in these PTEN knock-out animals.

BRAF and interacting molecules in the progression of melanoma

Several studies have revealed BRAF mutations in 50–70% of all primary and metastatic melanomas, making it the most frequently mutated oncogene in this disease. The most common BRAF mutation creates a glutamic acid-for-valine substitution at the hotspot position 600 (V600E) 27. To clearly define the role of BRAF in melanoma, the focus of several groups has now moved to the detection of interacting molecules.

Evidence of cooperative interactions between two signature mutations, BRAF V600E mutation and loss of PTEN, was presented by Dankort et al28. The group generated mice with conditional melanocyte-specific expression of BRAF V600E. Upon induction of BRAF V600E expression with tamoxifen, the mice developed benign melanocytic hyperplasia (naevi) that failed to progress to melanoma over 20 months. To model the genetic profile of the BRAF V600E mutation and loss of PTEN expression, they combined the BRAF mutation with gene silencing of the tumour suppressor PTEN. Remarkably, they observed the development of melanoma with 100% penetrance, as well as metastasis into the lymph nodes and lungs. The combination of mutation of BRAF and silencing of PTEN is common in human melanoma; however, the cooperation of these two molecules for development of melanoma had not been clearly proven previously.

Increased activity of the phosphatidylinositol-3-OH kinase (PI3K) pathway occurs in approximately 70% of sporadic melanomas, due to loss of PTEN and increased expression of the serine/threonine kinase AKT3 resulting from elevated gene copy number. Deregulated AKT3 activity promotes the development of malignant melanoma 29. A new study also described novel AKT3 mutations (AKT E17K) in melanoma in situ and in cell lines 30. The authors concluded that the BRAF V600E mutation and activation of the PI3K pathway represent two of the main events facilitating melanocytic transformation to melanoma. The kinase AKT functions as a central integrator of PI3K signalling to modulate downstream effectors, most notably the TSC1/2–mTOR complexes. mTOR, a serine/threonine protein kinase and ‘target of rapamycin’, acts as a primary regulator of protein synthesis and cell growth. Recently published papers address the importance of the PI3K–mTOR pathway. Scott et al analysed genome-wide copy number amplifications of solid tumours including melanoma 31. GOLPH3, directing expression of a Golgi protein, was identified as a new oncogene targeted for gene amplification at 5p13. At the molecular level, GOLPH3 enhanced growth factor-induced mTOR signalling and altered the response and sensitivity to the mTOR inhibitor rapamycin.

In addition to BRAF, PTEN, and PI3K, TGF-β (transforming growth factor-β) seems to be an important factor for melanoma development. Lo and Witte stated that PTEN deficiency combined with BRAF activation induced a melanoma in situ-like phenotype without dermal invasion 32. In contrast to Dankort et al28, this group used an organotypic human skin culture model based on human dermal fibroblast-contracted collagen I matrix, keratinocytes, and melanocytes which show PTEN deficiency and BRAF activation. Interestingly, addition of TGF-β in the context of PTEN deficiency and BRAF activation led to dermal invasion in skin cultures of the immortalized melanocytes, without promoting proliferation of melanocytes in vitro or in vivo32. Immunohistochemistry revealed TGF-β-dependent hyperactivation and phosphorylation of Smad2 in naevi and melanomas. Based on these findings, the group considered that in the premalignant, naevoid stage, the autonomous TGF-β activation (phosphor-Smad2 signalling) may be rewired into a pro-invasive pathway by the acquisition of other genetic alterations, such as PTEN deficiency and BRAF activation. They further assumed that tumour cell autonomous stimulation of TGF-β may, at least in part, be responsible for the critical switch from radial to vertical growth during human melanoma development. One can speculate that the melanocytes in the mouse model of Dankort et al28 endogenously express TGF-β, resulting in the strong effects found in these mice.

Cellular senescence

Benign naevi consist of a clonal population of hyperplastic melanocytes that cannot progress because they are ‘locked’ in a state of senescence 33. In naevi, the genomic oncogenic-stress locus, responsible for the control of oncogene-induced cellular senescence (OIS), is prevalently the CDKN2A gene. Oncogenic-induced stress often correlates with the accumulation of p16INK4A and p14ARF followed by retinoblastoma (pRb1) protein-dependent G0/G1-like cell-cycle arrest. Alternatively, the OIS cell cycle is regulated by the p53 target p21. The CDKN2A locus is mutated and inactivated frequently in naevi and melanoma. Recently published studies broaden the narrow perspective of melanocytic senescence and also revealed mutated BRAF as an inductor of oncogenic senescence 34.

However, it remains unclear how an activated BRAF oncogene induces uncontrolled proliferation in melanoma but senescence in benign naevi. One hypothesis is that melanomas harbour a second oncogenic lesion that inactivates or overwhelms the BRAF-mediated senescence pathway and therefore mutated BRAF alone is incapable of transforming primary melanocytes into melanomas.

The mouse ‘senescence’ model as described by Dankort et al28 (already mentioned in the previous section entitled ‘BRAF and interacting molecules in the progression of melanoma’) is based on a BRAF V600E mutation. These mutant mice develop benign melanocytic hyperplasia but do not develop malignancies. The results gained in this model have been confirmed by a second mouse model, that of Dhomen et al35. Here, tamoxifen-inducible BRAF V600E expression specifically in melanocytes was used. After tamoxifen application, the mice developed skin hyperpigmentation and similar to the study of Dankort et al, the naevi harboured senescent melanocytes. Interestingly, these mice developed melanomas in approximately 70% of cases, without the obvious need for additional manipulation of the genome. Melanomas developed in 11% of the mice within 6 months, rising to 54% within a year and 64% within 14 months. An additional ‘BRAF V600E mouse’ was generated by Goel et al36. Here, mice carrying the BRAF mutation did develop melanomas. The genetic features of these melanomas suggest that senescence is overcome concurrently with the loss of p16INK4A expression and activation of AKT. Both publications are contradictory to the publication of Dankort et al, who had to combine BRAF V600E expression with PTEN knock-out to achieve melanoma development. Additional studies are necessary to clarify whether the background of the mice resulted in these differences; for example, general pigmentation could be the reason for this discrepancy.

Wajapeyee et al transduced melanocytes with a genome-wide shRNA library pool and additionally infected the cells with a BRAF V600E retrovirus. They then selected cell colonies that had bypassed the BRAF V600E-mediated cellular proliferation block (senescence) and identified the responsible shRNA by sequencing. Seventeen interesting genes were found which still led to proliferation in spite of the BRAF V600E mutation. One of the most interesting genes was IGFBP7 (insulin growth factor binding protein 7, IGFBP-rP1, MAC25). BRAF V600E expression in primary cells led to the synthesis and secretion of IGFBP7, which acts through autocrine/paracrine pathways to inhibit BRAF–MEK–ERK signalling and induces senescence and apoptosis, as seen in naevi 37. Interestingly, molecular studies and immunohistochemical analysis of human skin, naevi, and melanoma samples implicate loss of IGFBP7 expression as a critical step in the development of a BRAF V600E-positive melanoma. Generally, loss of IGFBP7 can be important for overriding oncogene-induced cellular senescence (OIS) in malignant melanoma, but how melanoma cells lose IGFBP7 expression is not yet understood.

Yu et al38 allude to the paper of Wajapeyee et al, and they also tried to explain why a minor subpopulation of primary human melanocytes may survive sustained expression of BRAF V600E and can still proliferate. They believe that primary human melanocytes with BRAF V600E mutation circumvent OIS by inactivation of p53. This inactivation results from a substantial deletion of the Rb1 gene located on the chromosome which leads to increased H2AX phosphorylation and p53-induced genomic instability. Although the role of p53 is controversial in the melanoma field (due to its low mutation frequency of only 10%), this group revealed that inactivation of p53 in melanocytes nevertheless has an important role. The group could show alterations in p53-target gene expression during the progression from naevi to melanomas using two independent gene expression profile data sets of human melanocytes. They speculated that despite the lack of genetic mutations, the p53 pathway becomes dysfunctional in malignant melanoma in vivo.

In summary, recent advances have convincingly demonstrated that senescence represents a true barrier for the progression of many cancers, including melanoma. The previously discussed papers deal with inactivating molecular aberrations in signalling cascades responsible for overcoming the oncogene-induced senescence of melanocytes. Once senescence is overcome, the naevus can exhibit dysplastic features and readily progress to melanoma.

Melanoma stem cells

During embryonic development, the precursors of melanocytes are non-pigmented melanoblasts derived from the neural crest. Once located in the hair follicle, melanoblasts can either differentiate into mature melanocytes or, alternatively, give rise to melanocytic stem cells responsible for maintaining the melanocytic system. Melanocytic stem cells reside within a specialized stem cell niche, called the hair bulge, which is located in the lower permanent portion of the hair follicle known as the outer root sheath. Yet on the basis of clinical and pathological evidence, these melanocyte stem cells of the hair bulge of mice and humans are not the targets of melanoma transformation 39, 40, suggesting that an alternative source of melanocyte-producing cells may exist. Whether melanoma stem cells, or cells perhaps better named as malignant melanoma-initiating cells (MMICs), are derived directly from melanocyte stem cells, melanocyte progenitors (melanoblasts or neural crest cells), or more mature melanocytes that have de-differentiated remains unclear at this point. It is speculated that a subpopulation of cancer stem cells is responsible for the chemo-resistance of melanoma and for the high recurrence and aggressive progression of this type of cancer.

New findings by Quintana et al deal with the frequency of tumour stem cells or MMICs in malignant melanomas. These cells were thought to be extremely rare on the basis of experiments showing that only a small fraction (0.1–0.0001%) of cells in a tumour can seed a tumour in immuno-compromised mice 41. However, using highly immuno-compromised NOD/SCID Il2rg−/− mice (which lack the interleukin-2 gamma receptor), Quintana et al demonstrated that about 27% of melanoma cells from human subjects are capable of seeding a tumour in this mouse model. To assess this, they sorted live human melanoma cells by flow cytometry from xenografted tumours obtained from different patients and then deposited one cell per well in Terasaki plates. After mixing with matrigel and injection into NOD/SCID Il2rg−/− mice, 12–65% of single cells formed tumours, depending on the patient. These findings highlight the need for more sensitive and reliable bioassays to define the true tumourigenic potential of individual human cancer-initiating cells. The critical question is whether optimization of xenotransplantation assays could reveal that some human cancers actually have more cells with tumourigenic potential than currently expected.

Klein et al found a correlation in the expression of the stem cell markers CD133, CD166, and nestin in primary and metastatic melanomas 42. However, CD133, commonly used as a human stem cell marker, did not accurately identify the cells initiating melanoma, as shown in the study of Quintana et al41.

Frank et al have identified ABCB5 [a P-glycoprotein family member, ATP-binding cassette (ABC) transporter] in 11% of primary melanocytes and 3% of melanoma cells (tissue samples and in cell lines) as preferentially marking a subset of CD133-expressing cells in melanomas 43, indicating that ABCB5-positive/CD133-positive cells may mark melanoma stem cells.

A recent study by the same group supported the hypothesis that exclusively ABCB5-positive melanoma cells, which compromise 1.6–20.4% of the total population, were able to induce tumours in NOD/SCID (non-obese diabetic/severe combined immunodeficiency) mice, while the main population of ABCB5-negative cells were not tumourigenic 44. Furthermore, it was shown in transplantation assays using a mixture of ABCB5-positive and -negative cells that were differently fluorescently labelled, that the ABCB5-positive cells overgrew the negative cells in vivo.

Hoek et al shed light on cellular characteristics important for cells to form the heterogeneous melanoma. They described a high endogenous capacity of tumour cells to display a stem cell-like phenotype. The group studied the in vivo tumourigenic behaviour of melanoma cell lines with different transcript signatures through selecting pairs of proliferative and invasive signature melanoma cell lines based on previous genome-wide transcription profiling experiments 45. The two transcript signatures determined correspond to a proliferative and an invasive cellular phenotype. In vivo melanoma cells may switch between these states. The proliferative signature comprised MITF (microphthalmia-associated transcription factor) and other melanocytic genes (eg Tyrosinase, Dct, Melan-A) that were up-regulated along with a number of additional neural crest-related factors (eg Sox10 and EDNRB). This signature is associated with high rates of proliferation, low motility, and sensitivity to growth inhibition by TGF-β. In the invasive signature, these genes are down-regulated, whereas others are up-regulated, eg secreted products (eg INHBA, COL5A1, and SERPINE1) which are involved in the modification of the extracellular environment. This invasive signature is associated with lower rates of proliferation, high motility, and resistance to growth inhibition by TGF-β. Many of the genes of the proliferative signature were frequent responders to Wnt signalling, and those of the invasive signature were commonly TGF-β signal-driven. Furthermore, proliferative signature cell types were detected most frequently in the peripheral margin of growing tumours. These data indicate that melanoma cells undergo transcript signature switching in vivo likely regulated by local micro-environmental conditions. To conclude, the data suggest that different transcriptional states are interchangeable programmes in between which melanoma cells oscillate during progression in response to changing micro-environmental cues (eg hypoxia and inflammation).

Details of the involvement of MITF in melanoma development remain controversial because it is postulated that low levels of MITF activity can promote proliferation 46, while high levels inhibit cell division 46.

Tumour-initiating cells are highly interesting and detailed knowledge could potentially lead to innovative new therapies. However, stem cell markers still have to be clearly defined. In addition, the new findings by Quintana et al again emphasize the role of the immune system in melanoma progression.

Genes involved in embryonic melanocyte development and malignant melanoma

During vertebrate embryogenesis, melanocytic precursors—the melanoblasts—migrate extensively on their pathways through the embryo; they bypass natural tissue barriers and basement membranes of the epidermis and hair follicle, and adjust to new environments by pro-survival strategies. These properties are unique for this cell type and metastatic melanoma cells potentially accomplish a molecular ‘reprogramming’ and adopt a neural crest or melanoblast-like phenotype.

In particular, the genes involved in neural crest induction and specification of neural crest cells to the point of melanocytic differentiation repeatedly appear in events during melanoma development. Thus, by determining the molecular mechanisms by which these factors regulate the normal development of neural crest cells, important insight into their related roles in carcinogenesis will be gained.

The transforming growth factor-β (TGF-β) family of growth factors (also mentioned in the section entitled ‘BRAF and interacting molecules in the progression of melanoma’) comprises more than 40 members, including, but not restricted to, TGF-β, Activins, bone morphogenetic proteins (BMPs), and Nodal. Nodal has been shown to be particularly important in ectopic formation of the embryonic axis and determining left–right asymmetries in vertebrate embryos 47. Topczewska et al's group showed that Nodal is present in human metastatic melanoma but not in normal skin and thus may be involved in later stages of melanoma pathogenesis 48. Remarkably, TGF-β may also exert tumour promoter activities at later stages of carcinogenesis. Postovit et al analysed the molecular mechanism behind the Nodal expression pattern. They correlated the embryonic stem cell micro-environment with the tumourigenic phenotype of aggressive cancer cells 49 and stated that the cancer-associated milieu lacks the appropriate regulatory mechanisms found during embryonic development to maintain a normal and healthy cellular phenotype. Metastatic tumour cells do not express Lefty, the inhibitor of the embryonic morphogen Nodal. This allows the tumour cells to overexpress Nodal in a non-regulated manner, leading to the maintenance of a dedifferentiated, multipotent, highly metastatic phenotype. However, exposure of the tumour cells to a human embryonic stem cell micro-environment (containing Lefty) leads to a dramatic down-regulation in their Nodal expression, concomitant with a reduction in clonogenicity and tumourigenesis. It has to be clarified whether Nodal and TGF-β act together for malignant melanoma progression and invasion or independently of each other.

Snail gene family members (Snail1 and Slug) are widely used as the earliest indicators of neural crest formation because they mark cells undergoing epithelial-to-mesenchymal transformation (EMT). It was shown that Snail1 is highly up-regulated in melanoma cells and represses E-cadherin (CDH1) expression 50. However, E-cadherin is not the only target gene of Snail1; further transcriptional targets were described, especially during cancer development. Recently, we showed that Snail1 inhibits expression of the tumour suppressor CYLD in melanoma 51. CYLD was originally identified as a tumour suppressor mutated in familial cylindromatosis, an autosomal-dominant predisposition to multiple tumours of skin appendages. As a direct consequence of CYLD repression by Snail1, the proto-oncogene BCL-3 translocates into the nucleus and activates Cyclin D1 and N-cadherin expression, resulting in proliferation and invasion of melanoma cells. Rescue of CYLD expression in melanoma cells reduced tumour growth and metastasis in vivo.

Interestingly, Kudo-Saito et al revealed that Snail1 accelerates not only cancer metastasis through enhanced invasion, but also induction of immunosuppression 52.

Wnts are cytosine-rich glycoproteins that bind to cellular receptors on the surface of cells and control transcriptional regulation through the transcriptional co-factor β-catenin. It was shown that Wnt signalling is involved in the differentiation of melanocytes during embryogenesis 53, 54. The signalling function of β-catenin in the so-called canonical Wnt/β-catenin pathway is conferred by a soluble cytoplasmic pool whose stability is tightly regulated. Wnt signals lead to the accumulation of stabilized β-catenin in the nucleus of melanoma cells that forms active transcriptional complexes with LEF/TCF transcription factors and leads to the activation of various genes involved in tumour development.

However, the observed presence of nuclear β-catenin in the majority of benign naevi, along with the loss of nuclear β-catenin seen with melanoma progression 55, 56, supports the hypothesis that activation of Wnt/β-catenin signalling is important for healthy cellular homeostasis. Chien et al found that elevated levels of nuclear β-catenin in both primary tumours and metastases correlate with reduced expression of markers of proliferation and with improved survival from melanoma 57. In particular, Chien et al showed that B16 melanoma cells expressing Wnt3a exhibit decreased tumour size and decreased metastasis when implanted into mice. Genome-wide transcriptional profiling revealed that Wnt3a up-regulates genes implicated in melanocyte differentiation, several of which are down-regulated with melanoma progression, and this paper offers a new point of view regarding the involvement of Wnt signalling and nuclear β-catenin in melanoma development and is in clear contrast to several publications describing the canonical β-catenin pathway to be activated in malignant melanoma through nuclear localization of β-catenin 58. To summarize, it is crucially important to differentiate between canonical and non-canonical Wnt activation (see the section entitled ‘Cellular migration and polarity’) and nuclear versus cytoplasmic β-catenin signalling in further analysis of this signalling pathway in melanoma.

Interestingly, although activating mutations in the β-catenin pathway are rare in melanoma 59–61, the non-canonical pathway in particular, often activated by Wnt5a, Wnt5b, Wnt4, and Wnt11 triggering intracellular calcium release and activation of PKC (protein kinase C) and CamKII (calmodulin kinase II), has been implicated in melanoma metastasis. Non-canonical Wnt5a signalling, which is not involved in an increase of β-catenin expression or nuclear translocation 56, can down-regulate the expression of melanocyte differentiation markers (MART-1, GP100, and tyrosinase) and leads to an increase of melanoma metastasis in vivo through STAT3 activation. Kageshita et al. claimed that targeting Wnt5a in melanoma before immunotherapy may lead to the enhancement of current targeted immunotherapy for patients with metastatic melanoma. Data on β-catenin signalling in malignant melanoma are not all concordant.

Cellular migration and polarity

The most basic process underlying cancer invasion and metastasis is a dynamic actomyosin cytoskeleton that in response to external factors drives cell polarity, turnover of cell–matrix interactions, and migration of cancer cells away from the primary lesion into surrounding tissues, blood, and lymph vasculature 62.

Expression of Wnt5a in human melanomas correlates with high grade and invasiveness, and it promotes the invasiveness and metastasis of melanoma cell lines in vivo63. Witze et al reported that the acute response to Wnt5a involves the recruitment of actin, myosinIIB, Frizzled3, and MCAM (also known as MUC18) into an intracellular structure in a melanoma cell line 64. Signalling molecules, cell adhesion molecules, and molecules for cell architecture are recruited simultaneously. Additionally, following a gradient of secreted factors (chemokines), these Wnt-mediated molecules are distributed in a polarized manner on the cell periphery, whereby the direction of cell movement, membrane contractility, and nuclear movement is controlled. This allows Wnt5a to control cell polarity and directional orientation, even in cells lacking positional information from cell–cell contacts (which is common for cancer cells). The process requires endosome trafficking; is associated with multi-vesicular bodies; and is regulated by Wnt5a through the small guanosine triphosphatases Rab4 and RhoB. These findings add new insights into the understanding of acute intracellular events mediated by Wnt signalling.

Friedl and Wolf have presented a model in which tumour cell movement occurs in one of two modes: a mesenchymal-elongated-type and a rounded-amoeboid mode of movement which allow invasive tumour cells to adapt to varying micro-environments 65. These two modes of cell movement are inter-convertible. In 2008, Sanz-Moreno et al identified DOCK3, NEDD9, WAVE2, and ARHGAP22 as key molecules regulating Rac and Rho signalling which determines the described mode of actomyosin movement driving melanoma cell metastasis 66. Rac signalling through WAVE2 is responsible for mesenchymal-type movement and suppresses rounded-amoeboid movement. Also involved in this signalling cascade are NEDD9 and DOCK3. Conversely, in amoeboid movement, Rho-kinase signalling activates ROCK and ARHGAP22 (a Rac GAP), and this suppresses mesenchymal-elongated movement by inactivating Rac. This study confirmed on a molecular level that melanoma cells flexibly adapt their invasive movement to different micro-environmental structures.

Ephrin tyrosine kinases have been studied regarding their roles in embryonic development, where they can transduce key signals for directional motility for neuronal growth cones, neuronal crest cells, as well as endothelial cells during vasculogenesis through repulsive (repellent factor) behaviour 67, 68. Emerging evidence also implicates Eph family proteins in cancer progression, as in melanoma 69. Data presented by Parri et al revealed that the re-expression of EphA2 in EphA2-deficient low metastatic F10-M3 melanoma cells increased their capacity to migrate through matrigel in vitro as well as to colonize host organs through a RhoA-dependent amoeboid-like strategy in vivo70. Again the cells switch from a mesenchymal to an amoeboid-like motility style in EphA2 dependency. EphA2 re-expression in melanoma cells activates a non-proteolytic invasive programme that proceeds through the activation of cytoskeleton motility.

Metabolic stress

Two recent articles 71, 72 explored the involvement of mutated BRAF linked to the tumour suppressor LKB1/AMPK signalling pathway in metabolic stress of malignant melanoma. LKB1 (STK11) is a multi-tasking serine/threonine kinase that is a primary upstream kinase of AMPK (AMP-activated protein kinase), the key sensor of metabolic stress in eukaryotic cells. AMPK is activated in conditions of ‘low energy’, when the AMP : ATP ratio increases, to conserve energy by promoting catabolism (eg increased glucose uptake and glycolysis) and blocking anabolism (eg protein and lipid synthesis). It also inhibits protein translation by inhibiting mammalian target of rapamycin (mTOR). Usually, AMPK is active in healthy cells under metabolic stress and regulates cell growth and apoptosis. Furthermore, resistance to stress conditions is essential for melanoma cell survival. The signalling pathway leading to AMPK activation generally involves p90RSK followed by LKB1, which associates directly with AMPK and activates it by phosphorylation at Thr172. The complex of LKB1 and AMPK stops protein synthesis after accumulation of AMP in the cell.

The two studies may explain how oncogenic BRAF V600E overwhelms the metabolic stress signals in melanoma that are normally responsible for inhibiting cell growth.

Both groups showed that constitutively activated BRAF V600E can lead to a ternary complex between BRAF V600E, LKB1, and ERK, resulting in phosphorylation of LKB1 on S325 and S428 and uncoupling of LKB1 and AMPK, which in turn blocked the activation of AMPK even under conditions of elevated AMP levels. The detailed function of S325 and S428 in LKB1 is still unresolved, as previous studies have failed to show clear roles for these sites in LKB1 regulation and effects 73, 74. Taken together, the data suggest that BRAF V600E cells have a limited response to low energy conditions and that BRAF V600E directly regulates metabolic signalling. The BRAF V600E–AMPK ‘axis’ allows protein synthesis and proliferation under metabolic stress.


One main aim of understanding the genes involved in melanoma is the development of targeted therapies. In this regard, several recent studies have explored candidate target molecules. For example, HLM006474 is a small molecule that inhibits E2F4 during melanocytic proliferation and subsequent invasion in a three-dimensional tissue culture model system 75. Another example is the inhibitor GDC-0879, with which Hoeflich et al targeted BRAF in mice that harboured cell line- and patient-derived BRAF V600E tumours and achieved improved survival rates 76. Augustine et al postulated that the novel pentapeptide ADH-1, which disrupts N-cadherin adhesion, could sensitize melanoma tumours to the cytotoxic effects of chemotherapy in xenograft animal models 77.

Despite success in in vitro experiments or in mouse models, therapeutic trials will be necessary to evaluate the potential of these new strategies in humans.

Immunotherapies which are developed for melanoma patients are described in detail in the review by Eggermont and Schadendorf 78.


  1. Top of page
  2. Abstract
  3. Introduction
  4. Recent novel findings in melanoma research
  5. Conclusion
  6. Acknowledgements
  7. Teaching Materials
  8. References
  9. Supporting Information

In this review, we have sought to draw attention to new insights into core aspects of melanoma research (Figure 1) and to highlight recent articles that have made this possible. However, we do not claim to have painted a complete picture, and almost daily, new interesting and relevant articles are published.

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Figure 1. Schematic overview presenting the molecules and pathways identified in this review in relation to melanoma formation, modified from Michaloglou et al79. The main molecules important for embryogenesis and recurring in melanoma development and progression are described: Nodal/Lefty; TGF-β (transforming growth factor β); Snail; Wnt5a; EphA2 (ephrinA2); bFGF (basic fibroblast growth factor); CYLD; IGFBP7 (insulin growth factor binding protein 7, IGFBP-rP1, MAC25); LKB1 (multi-tasking serine/threonine kinase)/AMPK (AMP activated protein kinase). BRAF V600E-induced senescence and overriding of senescence through p16INK4A pathway inactivation or PTEN and p53 mutation leading to melanoma formation is shown. Escape from senescence requires one (or more) additional, yet to be identified, hits (denoted ‘X’), which might collaborate with loss of p16INK4A activity. To gain immortality, the cells have to maintain a minimal telomere length, which can be achieved by activation of hTERT. Full oncogenic transformation may require an additional hit (denoted ‘Y’); this is probably the activation of TGF-β during the invasion of malignant melanoma. Some important molecules not mentioned in the review have been added to the figure to make the picture more complete

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The heterogeneity of melanoma and also the diversity of the molecules responsible for the aggressiveness of this barely treatable skin cancer are topics that have been of interest for many years; yet it is still necessary to learn more about the molecular networks driving malignant melanoma and to understand the links between different cellular signalling pathways. Hopefully this approach will help to identify the ‘weak points’ of melanoma behaviour that could be crucial for melanoma therapy.


  1. Top of page
  2. Abstract
  3. Introduction
  4. Recent novel findings in melanoma research
  5. Conclusion
  6. Acknowledgements
  7. Teaching Materials
  8. References
  9. Supporting Information

The authors receive support for research from the DFG, the German Cancer Foundation (German Melanoma Network), and the Wilhelm Sander Foundation.

Teaching Materials

  1. Top of page
  2. Abstract
  3. Introduction
  4. Recent novel findings in melanoma research
  5. Conclusion
  6. Acknowledgements
  7. Teaching Materials
  8. References
  9. Supporting Information

Power Point slides of the figures from this Review may be found in the supporting information.


  1. Top of page
  2. Abstract
  3. Introduction
  4. Recent novel findings in melanoma research
  5. Conclusion
  6. Acknowledgements
  7. Teaching Materials
  8. References
  9. Supporting Information
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Supporting Information

  1. Top of page
  2. Abstract
  3. Introduction
  4. Recent novel findings in melanoma research
  5. Conclusion
  6. Acknowledgements
  7. Teaching Materials
  8. References
  9. Supporting Information
PATH2617_fig1.ppt164KSupporting information; Teaching Materials: Fig 1 as Power Point slide

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