Akt and mutant V600EB-Raf cooperate to promote early melanoma development

B-Raf is the most mutated gene in melanoma; however, the mechanism through which it promotes early melanomas remains uncertain. Most nevi contain activated B-V600E-Raf but few develop into melanoma, and expression in melanocytes is inhibitory with low protein levels present in surviving cells, suggesting unknown cooperative oncogenic events are necessary for melanoma development. Because many melanomas have B-V600F-Raf and active Akt3, it is possible that these proteins cooperatively facilitate melanocyte transformation. In this study, Akt3 is shown to phosphorylate B-V600E-Raf to lower its activity as well as that of the downstream mitogen-activated protein kinase (MAPK) pathway to levels promoting early melanoma development. Expression of active Akt3 in early melanoma cells containing B-V600-Raf reduced MAPK signaling and promoted anchorage-in dependent growth. Furthermore, expression of both B-V600E-Raf and active AW in melanocytes promoted a transformed phenotype. Mechanistically, aberrant AW activity in early melanomas serves to phosphorylate Ser(364) and Ser(428) on B-V600E-Raf to reduce activity of B-V600F-Raf to levels that promote rather than inhibit proliferation, which aids melanocytic transformation. Inhibition of B-V600E-Raf or Akt in advanced melanoma cells in which both pathways were active reduced anchorage-independent growth and tumor development in a cooperatively acting manner. Inhibition of Akt alone in these cells led to increased MAPK signaling. In summary, these results suggest that activating B-Raf mutations initially promote nevi development, but the resulting high, intense activation of the MAPK pathway inhibits further tumor progression requiring AW activation to bypass this barrier and aid melanoma development.