Abstract 29: MEK and PI3K signaling cooperate through mTORC1/2 to promote PIK3CA mutant melanoma cell proliferation

Oncogenic transformation of mutationally activated BRAF or NRAS melanocytes often requires the cooperation of genetic aberrations in the phosphoinositide 3-kinase (PI3K) pathway to promote melanomagenesis. Activating point mutations in the p110α catalytic subunit of PI3K are detected at a low frequency in both BRAF and NRAS mutant melanomas, yet the principle mechanism of this cooperation remains elusive. Thus, using BRAFV600E/PIK3CAH1047R, NRASQ61H/PIK3CAH1047R, and PIK3CAE545K mutant melanoma cells derived from metastatic melanoma patients treated with pathway-targeted inhibitors, we examined the contribution of mutational activation of PIK3CA to melanoma maintenance and signaling and the consequential response of these PIK3CA mutant cells to α-specific PI3K inhibition. Combined MEK and isoform-selective PI3K inhibition elicited more potent anti-proliferative effects and greater suppression of S-phase progression of the cell cycle compared to single-agent inhibition of either pathway. Analysis of signaling downstream of MEK or PI3K revealed that these pathways cooperated to regulate PIK3CA cell proliferation through mTORC1-mediated effects on ribosomal protein S6 and 4E-BP1 phosphorylation in an AKT-dependent manner. Despite the profound anti-proliferative and biochemical effects of α-specific or PI3Kβ-sparing class I PI3K inhibition in vitro, these agents elicited largely cytostatic effects on PIK3CA xenograft tumors. However, combined inhibition of PI3Kβ-sparing class I PI3K and MEK did significantly cooperate to reduce tumor growth compared to the corresponding monotherapies. Furthermore, this study provides a biochemical mechanism to explain how mTORC1/2 moderates the cooperation of MEK and PI3K signaling for the maintenance of PIK3CA mutant melanoma cell proliferation. Citation Format: Jillian M. Silva, Marian M. Deuker, Bruce C. Baguley, Martin McMahon. MEK and PI3K signaling cooperate through mTORC1/2 to promote PIK3CA mutant melanoma cell proliferation. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 29.