BRAFV600E- and PI3'K-Activated Signaling Pathways Cooperate to Regulate Phosphorylation of Ribosomal Protein S6 in Melanoma Cells

Metastatic melanoma is known for its aggressive clinical behavior, striking therapeutic resistance, and proclivity for metastasis, however, a better understanding of the pathophysiology is needed to successfully treat this disease.  The most common genetic alteration in melanoma is a point mutation in the BRAF gene, which substitutes a glutamic acid for valine (BRAF^V600E).  This mutation results in constitutive activation of the ERK1/2 mitogen-activated protein kinase (MAPK) pathway, which promotes cell proliferation through effects on the cell cycle and apoptosis.  However, in many cases, BRAF^V600E-mediated conversion of normal melanocytes to melanoma cells also requires the activation of phosphoinositide 3’-kinase (PI3’K) signaling, which occurs following silencing of the PI3’-lipid phosphatase, PTEN, tumor suppressor gene.   To understand the cooperation between BRAF and PI3’-kinase signaling, we applied pharmacological inhibitors of each pathway to melanoma-derived cell lines expressing oncogenic BRAF^V600E in the absence or presence of PTEN.  As expected, blockade of MEK1/2, ERK1/2, or PI3’K inhibited both proliferation and phosphorylation of p70^S6K and ribosomal protein S6 (rpS6) in melanoma cells that did or did not express PTEN.  Moreover, pharmacological blockade of AKT had no effect on cell proliferation and failed to inhibit p70^S6K or rpS6 phosphorylation.  Thus, these data suggest that there is an ERK1/2 and PI3’K-dependent, AKT-independent regulation of the p70^S6K-rpS6 signaling axis in BRAF^V600E melanoma cells.