ARTICLE ABSTRACT
Mutationally activated BRAFV600E cooperates with PTEN silencing in the conversion of normal melanocytes to metastatic melanoma cells, but the mechanism underlying this cooperation is poorly understood. Here, the consequences of pharmacologic blockade of BRAFV600E or phosphoinositide 3-kinase (PI3K) signaling were explored using pathway-targeted inhibitors and a panel of human BRAF-mutated melanoma-derived cell lines. Blockade of BRAFV600E → MEK1/2 → ERK1/2 or class I PI3K inhibited melanoma proliferation, whereas inhibition of AKT had only modest effects, even in cells with mutated or amplified AKT. Although single-agent inhibition of either BRAFV600E or PI3K signaling elicited antiproliferative effects, combinatorial inhibition was more potent. Analysis of signaling downstream of BRAFV600E or PI3K revealed that these pathways cooperated to regulate protein synthesis through AKT-independent, mTOR complex 1 (mTORC1)-dependent effects on p70S6K, ribosomal protein S6, and 4E-BP1 phosphorylation. Moreover, inhibition of mTORC1/2 inhibited cell proliferation as profoundly as single-agent inhibition of either BRAFV600E or PI3K signaling. These data reveal a mechanism by which BRAFV600E and PI3K signaling cooperate to regulate melanoma proliferation through AKT-independent effects on protein translation. Furthermore, this study provides a potential foundation for pathway-targeted combination therapy designed to enhance the therapeutic benefit to patients with melanoma that contain combined alterations in BRAF and PI3K signaling.Implications: PI3K, but not AKT, represent potential targets for melanoma therapy. Mol Cancer Res; 12(3); 447–63. ©2014 AACR.
