**10. The JNK/c-Jun pathway**

adhesion protein MelCAM that plays critical roles in melanoma development was increased upon active Akt expression [73,74]. PI3K and Akt is known to increase the expression of MMP2 and MMP9 by a mechanism involving Akt activation of NF-kappaB binding to the MMP promoter [75,76]. Akt overexpression led to upregulation of VEGF, increased production of superoxide ROS. Akt can suppress apoptosis by phosphorylating and inactivating many proapoptotic proteins like caspase 9 and Bad [77,78]. PI3K pathway emerges as the central axis that is deregulated in melanoma and along with constitutively active MAPK pathway makes an important role in melanoma development progression. Thus targeting PI3K is expected to

Βeta-catenin (β-catenin) is a key component of the Wnt signaling pathway. Signaling through this pathway controls a wide range of cellular functions and aberrant Wnt/β-catenin signaling can lead to cancer development and progression [79]. Wnts are glycoproteins that act as ligands to stimulate receptor-mediated signal transduction pathways involved in cell survival, proliferation, behavior and fate. Wnt proteins are known to activate 3 different extracellular pathways – Wnt/β-catenin, Wnt/planar-polarity and Wnt/Ca2+ pathways [80]. The Wnt/βcatenin also known as the canonical Wnt pathway plays an important role in melanoma development. In the absence of Wnt ligands, free β-catenin binds to the destructive complex of Axin, adenomatous polyposis coli (APC) and glycogen synthase kinase-3β (GSK-3β). GSK-3β mediates the phosphorylation of β-catenin at specific regulatory sites on the Nterminal side marking β-catenin for ubiquitination and subsequent proteosomal degradation. Upon the binding of Wnt ligand, GSK--catenin for ubiquitination and subsequent proteosomal degradation. Upon the binding of Wnt ligand, GSK-3β activity is inhibited resulting in accumulation of β-catenin in the cytoplasm and shuttles into the nucleus where it serves as an essential co-activator of the Tcf/Lef (T-cell factor / lymphoid enhancer factor) family [81]. Numerous genes implicated in the tumorigenic process like c-myc and cyclinD1 have been

Increased nuclear localization of β-catenin – an important indication of activated Wnt signaling pathway is observed in over a third of melanoma specimens [82,83,84]. Mutations in β-catenin have been observed in about 23% of melanoma cancer cell lines and these mutations affect phosphorylation sites at Ser33, Ser37, Thr41 and Ser45 [85] at the N-terminal domain. These mutations render β-catenin resistant to phosphorylation and subsequent degradation. Low rates of β-catenin mutation have been observed in primary melanomas and metastasis indicating that activating mutations is a rare event in melanoma tumorigenesis [82,83,84,86,87,88]. Mutations in APC were observed sporadically in primary melanomas [82, 85,88]. While APC promoter 1A hypermethylation was observed in 17% of melanoma biopsies and 13% of melanoma cell lines. Wnt signaling pathways is activated in tumors through aberration in other genes. ICAT (inhibition of β-catenin & T-cell factor), a gene that negatively regulates the association of β-catenin with TCF4 thus repressing the transactivation of βcatenin-Tcf4 target genes [89]. A study by Reifenberger J et al suggests that loss of ICAT

be an important therapeutic target modality for melanoma treatment.

**9. Wnt/β-catenin pathway**

50 Highlights in Skin Cancer

identified as targets of the canonical Wnt signaling.

Activation of Jnks is usually in response to diverse stresses. These kinases play an important role in the regulation of cell proliferation, cell survival, cell death, DNA repair and metabolism. A variety of extracellular stimuli by cytokines, growth factors, hormones, UV radiation and tumor promoters are known to activate Jnks [101]. Sequential protein phorphorylation through a MAP-kinase module (MAP3K-MAP2K-MAPK) is responsible for Jnk activation [102]. Depending upon the cellular context, Jnk has been shown to elicit both positive and negative effects on tumor development [103]. Activation of Jnk is required for Ras-mediated transfor‐ mation and mediate proliferation and tumor growth [104,105]. These observations are consistent with the findings of constitutively active Jnk in tumor samples and cell lines [103,106]. Jnk mediated the phosphorylation at serine 63 & 73 residues enhancing the ability of transcription factor c-Jun, a component of the AP-1 transcription complex [107]. The activation of Jnk leas to the induction of AP-1 dependent target genes that play important roles in cell proliferation, cell death and inflammation. Other members of the AP-1 transcription complex include c-Jun, Jun B, Jun D, c-Fos, Fra1 and Fra 2. The role of Jnk in oncogenesis is emerging; however c-Jun is a well defined oncogene in cancer. c-Jun is amplified and overex‐ pressed in undifferentiated and aggressive sarcomas [108], breast and lung cancer [109,110]. Since the 1990s, the role of Jnk pathway in melanomas was recognized [111,112]. c-Jun, Jun B, c-fos genes play a role in the transformation of melanocytes into malignant melanomas [111]. 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 strategy for antitumor intervention [115].

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

An Overview of Important Genetic Aspects in Melanoma

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

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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

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

and differentiate into melanocytes.

be used as a potential target for melanoma treatment [127].
