**7. The Ras-Raf-Mek-Erk**

In 2002, a breakthrough study found that Braf to be mutated in a large percentage of melanomas – triggering new studies that focus on MAPK (mitogen activated protein kinase) signaling in melanomas. Braf is mutated in upto 82% of cutaneous nevi [45,46], 66% of primary melanomas [44] and 40-68% of metastatic melanomas [47,48]. A specific muta‐ tion substitution of valine with glutamic acid at residue 600 (BRAF V600E), account for 90% + BRAF mutation. Raf, a downstream effector of RAS is a serine-threonine specific protein kinase that activates Mek, which inturn activates Erk. Humans have 3 Raf genes: A-raf, Braf and C-raf. The occurrence of mutation in Nras or Braf is 80-90% of all melanomas suggests that constitutive activation of extracellular signal regulated protein kinase (Ras-Raf-Mek-Erk). Most Ras mutations are present in codon 61 of N-Ras with K-Ras and H-

Ras mutations being relatively rare [49,50]. Constitutive activation of Ras-Raf-Mek-Erk cascade has been shown to contribute to tumorigenesis by inhibiting apoptosis and increasing cell proliferation, tumor invasion and metastasis. Activated Erk plays a pivotal role in cell proliferation by controlling the G1- to S-phase transition by negative regula‐ tion of p27 inhibition and upregulation of c-myc activity [51,52]. Inhibition of Erk activity is associated with G1 cell cycle arrest by upregulation of p21 and reduced phosphoryla‐ tion [52]. Activated Erk is also known to stimulate cell proliferation by increasing the transcription and stability of c-Jun which is mediated by CREB (cyclic adenosine monophos‐ phate responsive element-binding) and Gsk-3β (glycogen synthase kinase-3beta) respective‐ ly [53]. Erk is also believed to increase proliferation by inhibiting differentiation. Constitutively active Erk limits differentiation in melanoma by targeting MITF (micro‐ phthalmia-associated transcription factor) for degradation [54,55,56]. The activated Erk pathway enhances melanoma specific survival by differentially regulating RSK-mediated phosphorylation and inactivation of the pro-apoptotic protein Bad [57] and inhibiting Jak-Stat pathway [58].

Erk signaling also contributes towards tumor invasion and metastasis by regulating the expression of integrin and matrix metalloproteinases (MMPs). Activated Ras-Mek-Erk pathway drives the production of MMP1 [59,60,61].
