BRAF Cooperates with PI3K Signaling, Independent of AKT, to Regulate Melanoma Cell Proliferation

Mutationally activated BRAF 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 BRAF 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 BRAF ! 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 BRAF or PI3K signaling elicited antiproliferative effects, combinatorial inhibition was more potent. Analysis of signaling downstream of BRAF or PI3K revealed that these pathways cooperated to regulate protein synthesis through AKT-independent, mTOR complex 1 (mTORC1)-dependent effects on p70, ribosomal protein S6, and 4E-BP1 phosphorylation. Moreover, inhibition of mTORC1/2 inhibited cell proliferation as profoundly as single-agent inhibition of either BRAF or PI3K signaling. These data reveal a mechanism by which BRAF and PI3K signaling cooperate to regulate melanoma proliferation through AKTindependent effects on protein translation. Furthermore, this study provides a potential foundation for pathwaytargeted 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);