B-RAF and PI-3 kinase signaling protect melanoma cells from anoikis

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Abstract
A hallmark feature of cancer is resistance to anoikis, apoptosis induced when cells either lose contact with or encounter an inappropriate extracellular matrix. Melanoma is inherently associated with a high degree of resistance to apoptosis. Mutations in B-RAF are prevalent in melanoma and promote constitutive MEK-ERK1/2 signaling and cell transformation. Acquisition of B-RAF mutations correlates with vertical phase growth when melanoma cells invade into the dermis, a collagen-rich environment that also contains fibronectin matrix. In addition, alterations in phosphoinositide-3 kinase (PI-3 kinase) signaling that lead to activation of AKT are detected in advanced melanomas. Here we show that knockdown of B-RAF expression by siRNA or pharmacological inhibition of MEK rendered melanoma cells susceptible to anoikis. Furthermore, adhesion to fibronectin but not collagen protected melanoma cells from anoikis through a PI-3 kinase-dependent pathway. Therefore, melanoma cells require either B-RAF or PI-3 kinase activation for protection from anoikis. Notably, AKT signaling in melanoma cells is substrate specific. These findings demonstrate that melanoma cells utilize multiple signaling pathways to provide resistance to apoptosis.