BRAFV600E and PI3'-Kinase Signaling Pathways Cooperate to Regulate Protein Translation in Human Melanoma Cells

The most common genetic alteration in metastatic melanoma is a T1799A transversion encoding BRAF^V600E that results in the constitutive activation of the BRAF®MEK®ERK MAP kinase pathway.  In many cases, conversion of BRAF^V600E expressing melanocytes to melanoma cells also requires activation of PI3’K signaling, usually through silencing of the tumor suppressor PTEN.  To explore the mechanisms of cooperation between the BRAF^V600E and PI3’K signaling pathways, we applied pharmacological inhibitors of each pathway to BRAF^V600E expressing melanoma-derived cell lines.  Our data indicates that blockade of BRAF^V600E®MEK®ERK signaling inhibited p70^S6K and ribosomal protein S6 phosphorylation, whereas inhibition of PI3’K suppressed S6 and 4E-BP1 phosphorylation.  In addition, inhibition of BRAF^V600E®MEK®ERK or PI3’K blockade also led to the inhibition of melanoma cell proliferation.  Moreover, simultaneous inhibition of both the BRAF^V600E and PI3’K pathways abrogated the phosphorylation of the cap-dependent translational regulators, p70^S6K, S6, and 4E-BP1.  However, pharmacological inhibition of the PI3’K effector, AKT, had little effect on cell growth and failed to influence p70^S6K, S6 or 4E-BP1 phosphorylation.  Thus, these results suggest that the regulation of S6 may serve as a downstream integrator of cooperation between the BRAF^V600E and PI3’K pathways independent of AKT activation in melanoma cells