**12. Melanoma stem cells**

Stem cells are cells that can self-renew and the ability to differentiate into various cell lineages. These cells are located in the restrictive niche (environment). The interaction between stem cells and their microenvironment is important for the self renewal process. These cells are highly clonogenic and slow cycling (quiescent) in response to proliferation and survival stimuli. Stem cells divide asymmetrically giving rise to a daughter cell that remains a stem cell (capable of self renewal) and another daughter cell that can rapidly divide and differentiate. Melanocytes that are found in the skin and in the choroid layer of the eye is derived from the neural crest (NC). Neural crest cells undergo EMT to migrate along the definite pathways in the embryo. NC cells give rise to a large array of differentiated cells – melanocytes, peripheral neurons & glia, endocrine and cartilage cells [123]. Melanoblasts which are melanocytic precursors – unpigmented cells with the potential to produce melanin, invade the skin areas and differentiate into melanocytes.

The possible role Jnk pathway has led research teams to study the clinical relevance of interfering with this pathway. siRNA or chemical inhibitors of Jnk signaling inhibited proliferation in breast and non-small cell lung cancer (NSCLC) [106,113] and also induced apoptosis in prostate cancer cells [114]. A study by Gurzov E et al demonstrated that knock‐ down of c-Jun and Jun B in B16F10 melanoma cells by siRNA resulted in increased cell cycle arrest and apoptosis also resulting in extended survival of mice inoculated with these modified melanoma cells[115], suggesting that inactivation of c-Jun and Jun B could provide a valuable

The NFκB family in mammals contains 5 members – p105/p50 (NFκB1), p100/p52 (NFκB2), RelA (p65), RelB and c-Rel (J-206, 207). The canonical activation of NFκB pathway involves TNFα stimulation resulting in the subsequent phosphorylation/activation of IKK (IκB kinase). In turn, IKK-mediated phosphorylation of IκB leading to ubiquitination of IκB and its proteosomal degradation, releasing the NFκB complex which activates a host of target genes [116,117]. The type of genes that get trans-activated depends on the composition of activated NFκB complex. For instance, complexes containing c-Rel activates pro-apoptotic genes (Dr4/ Dr5, Bcl-x) and inhibits anti-apoptotic genes (cellular inhibitor of apoptosis (cIAP1, cIAP2), survivin). Complexes containing RelA inhibits the expression of DR4/DR5 and upregulates

NFκB is activated in various tumors including melanomas and distinct mechanisms have been proposed for the elevated levels of NFκB activity in melanomas. Activation of NFκB in melanomas is also linked to the loss of E-cadherin, a frequent event in melanoma transforma‐ tion [119]. NIK (NFκB interacting kinase), an activator of IKK is overexpressed in melanoma cells while compared to normal cells. The major contribution of NFκB in melanoma develop‐ ment and progression relates to its function as an important regulator of survival and apop‐ tosis. A study by Meyskens at al demonstrated that in metastatic melanoma cells, an increase in DNA binding activity of NFκB is associated with an increased expression of p50 and RelA resulting in increased expression of anti-apoptotic regulators. Also the expression of c-Rel, the transcriptional activator of pro-apoptotic genes is markedly in melanoma cells compared with normal melanocytes [120]. Strong p50 nuclear staining also correlated with poor prognosis in melanoma patients [121]. Besides eliciting anti-apoptotic activities NFκB mediates the transcription of MMP2 and MMP9 [121,122]. Overexpression of MMPs is associated with

Stem cells are cells that can self-renew and the ability to differentiate into various cell lineages. These cells are located in the restrictive niche (environment). The interaction between stem

strategy for antitumor intervention [115].

52 Highlights in Skin Cancer

caspase 8, cIAP1 and cIAP2 [118].

tumor invasion, metastasis and angiogenesis.

**12. Melanoma stem cells**

**11. The NFκB (NF-kappaB) pathway**

The cancer stem theory suggests that cancer originates from a small subpopulation of neo‐ plastic stem cells that have the potential to self renew and are primarily responsible for sustaining the tumor and giving rise to progressively differentiating cells that proliferate rapidly and contribute to the cellular heterogeneity of the tumor (F-194). Cancer stem cells arise either from undifferentiated stem cells or from cells that possess stem cell like character‐ istics. Evidence suggests that aggressive melanoma cells acquire characteristics of embryonic stem cells having a multipotent plastic phenotype [124]. Studies by Bittner MP et al demon‐ strated that melanoma cells express genes associated with different cell types like endothelial, epithelial, fibroblastic, neuronal, hematopoietic and progenitor cells [125]. Strangely genes specific for melanocytes are downregulated in metastatic melanomas. Tyrosinase & MLANA (melan A), genes associated with pigmentation are greatly downregulated in aggressive melanomas [124]. Aggressive melanoma cells express endothelial-associated genes and form extravascular fluid-conducting networks which allow melanomas to greatly adapt to the hypoxic microenvironment of rapidly proliferating tumors, a phenomenon called as "vascular mimicry" [124,126]. From different melanoma cell lines, cells with stem cell-like features which have the ability to grow as non-adherent cell aggregates known as spheroids/spheres have been isolated (F-196). These cells have the ability to differentiate into various lineages – adipogenic, osteogenic, chondrogenic and melanogenic. A study by Bittner M et al demon‐ strated a subset of these spheroid cells express the cell surface marker CD20, a unique molecular signature of aggressive melanomas [125]. For the treatment of non-Hodkin's lymphoma, CD20 is a standard therapeutic target which raises the possibility that CD20 could be used as a potential target for melanoma treatment [127].

Several studies have demonstrated that aggressive melanoma cells share characteristics with embryonic progenitors. Evidence suggests a major role for stromal components in all stages of tumorigenesis (initiation, progression and metastasis) [128]. Noted scientist Stephen Paget had coined the term "seed & soil" hypothesis predicting that metastatic cells only colonize soils (organs) that are permissive to their growth[129,130]. Studies show embryonic microen‐ vironment has the capacity to reverse the metastatic phenotype of cancer cells. The microen‐ vironment of human embryonic stem cells reprograms aggressive melanoma cells towards a less aggressive phenotype [124]. Nodal, an embryonic morphogen of the TGFβ family is important for sustaining melanoma aggressiveness and plasticity. Nodal is regained in highly aggressive melanoma cell lines, invasive VGP (vertical growth phase)-stage melanoma and metastatic melanoma [131], implicating Nodal as a novel diagnostic marker in melanoma progression and could be a therapeutic target for metastatic melanoma treatment [124].

SCC - Squamous cell carcinoma

UVR - ultraviolet radiation VGP - vertical growth phase

WIF1 - Wnt-inhibitory factor 1

University of Pennsylvania School of Medicine, Philadelphia, PA, USA

tics. CA Cancer J Clin , 61, 69-90.

carcinoma. J Invest Dermatol , 122, 1180-1187.

peutics. J Am Acad Dermatol 66: e, 167-178.

cer. Int J Dermatol , 49, 978-986.

Neoplasia , 10, 207-216.

Surg , 29, 210-217.

base. Jpn J Clin Oncol 40: 710.

[1] Jemal, A, Bray, F, Center, M. M, Ferlay, J, Ward, E, et al. (2011). Global cancer statis‐

An Overview of Important Genetic Aspects in Melanoma

http://dx.doi.org/10.5772/55257

55

[2] Marghoob, A. A. (2011). Skin cancers and their etiologies. Semin Cutan Med Surg 30:

[3] Eichberger, T, Regl, G, Ikram, M. S, Neill, G. W, Philpott, M. P, et al. (2004). FOXE1, a new transcriptional target of GLI2 is expressed in human epidermis and basal cell

[4] Iwasaki, J. K, Srivastava, D, Moy, R. L, Lin, H. J, & Kouba, D. J. (2012). The molecular genetics underlying basal cell carcinoma pathogenesis and links to targeted thera‐

[5] Narayanan, D. L, Saladi, R. N, & Fox, J. L. (2010). Ultraviolet radiation and skin can‐

[6] Foster, P. J, Dunn, E. A, Karl, K. E, Snir, J. A, Nycz, C. M, et al. (2008). Cellular mag‐ netic resonance imaging: in vivo imaging of melanoma cells in lymph nodes of mice.

[7] Marugame, T, & Zhang, M. J. (2010). Comparison of time trends in melanoma of skin cancer mortality (1990-2006) between countries based on the WHO mortality data‐

[8] Ko, J. M, Velez, N. F, & Tsao, H. (2010). Pathways to melanoma. Semin Cutan Med

Tcf4 - T-cell factor-4

**Author details**

**References**

S, 1-5.

Rohinton S. Tarapore

### **13. Conclusion**

Our understanding of melanoma development and progression has evolved tremendously over the past three decades. Unfortunately our understanding of the molecular biology of melanoma is still far from complete despite extensive research and knowledge gained in chromosomal alterations, mutations in important melanoma-associated genes, epigenetic modifications and melanoma microenvironment. Even to this day, the best prognostic significance of primary melanoma is the thickness of the tumor (i.e. RGP → VGP transition) and the presence/absence of ulcerations. Melanoma still remains as a tumor that is refractory to current chemotherapeutic treatments. A further study of the interaction between various signaling pathways will help researchers decipher the complexity of the genetic and epigenetic changes which eventually would lead to better therapeutic modalities for the treatment of primary and metastatic melanomas.

### **Glossary**

AK - actinic keratosis


IKK - Inhibitor of IκB kinase


metastatic melanoma [131], implicating Nodal as a novel diagnostic marker in melanoma progression and could be a therapeutic target for metastatic melanoma treatment [124].

Our understanding of melanoma development and progression has evolved tremendously over the past three decades. Unfortunately our understanding of the molecular biology of melanoma is still far from complete despite extensive research and knowledge gained in chromosomal alterations, mutations in important melanoma-associated genes, epigenetic modifications and melanoma microenvironment. Even to this day, the best prognostic significance of primary melanoma is the thickness of the tumor (i.e. RGP → VGP transition) and the presence/absence of ulcerations. Melanoma still remains as a tumor that is refractory to current chemotherapeutic treatments. A further study of the interaction between various signaling pathways will help researchers decipher the complexity of the genetic and epigenetic changes which eventually would lead to better therapeutic modalities for the treatment of

**13. Conclusion**

54 Highlights in Skin Cancer

**Glossary**

AK - actinic keratosis

BCC - basal cell carcinoma

IKK - Inhibitor of IκB kinase

MMP - matrix metalloproteinase NF-κB - nuclear factor kappa-B NIK - NFκB-interacting kinase

Rb - retinoblastoma

RGP - radial growth phase

Cdks - cyclin-dependent kinases

CREB - cyclic adenosine monophosphate responsive element-binding

FAMS - familial atypical mole-melanoma syndrome

MITF - microphthalmia-associated transcription factor

GSK3β - glycogen synthase kinase-3beta

MAPK - mitogen activated protein kinase

Pten - phosphatase and tensin homolog

primary and metastatic melanomas.

