Abstract 324: Unbiased proteomic and phosphoproteomic analysis identifies response signatures and novel susceptibilities after combined MEK and mTOR inhibition in BRAFV600E mutant glioma
Authors: Maxwell Micah J. , Arnold Antje , Sweeney Heather , Chen Ljun , Lih Tung-Shing M. , Schnaubelt Michael , Eberhart Charles G. , Rubens Jeffrey A. , Zhang Hui , Clark David J. , Raabe Eric H. 
Published: July 2021

Abstract
The mitogen-activated protein kinase (MAPK) pathway is one of the most frequently altered pathways in cancer. It is involved in the control of cell proliferation, invasion, metabolism, and can cause resistance to therapy. A number of aggressive malignancies including melanoma, colon cancer, and glioma, are driven by a constitutively activating missense mutation (V600E) in the BRAF component of the pathway. MEK inhibition is initially effective in targeting these cancers, but reflexive activation of mTOR signaling contributes to frequent therapy resistance. We have previously demonstrated that combination treatment with the MEK inhibitor trametinib and the dual mTORC1/2 inhibitor TAK228 improves survival and decreases vascularization in a BRAFV600E mutant glioma model. To elucidate the mechanism of action of this combination therapy and understand the ensuing tumor response, we performed comprehensive unbiased proteomic and phosphoproteomic characterization of BRAFV600E mutant glioma xenografts after short-course treatment with trametinib and TAK228. We identified 13,313 proteins and 30,928 localized phosphosites, of which 12,526 proteins and 17,444 phosphosites were quantified across all samples (data available via ProteomeXchange; identifier PXD022329). We identified distinct response signatures for each monotherapy and combination therapy and validated that combination treatment inhibited activation of the MAPK and mTOR pathways. Combination therapy also increased apoptotic signaling, suppressed angiogenesis signaling, and broadly suppressed the activity of the cyclin-dependent kinases. In addition, combination therapy had a profound impact on cancer cell metabolic pathways, increasing the expression of proteins (and their activating phosphorylations) involved in glycolysis, the tricarboxylic acid (TCA) cycle, nucleotide biosynthesis, and DNA replication. In response to combination therapy, both receptor tyrosine kinase and histone deacetylase proteins were activated. This study reports a detailed (phospho)proteomic analysis of the response of BRAFV600E mutant glioma to combined MEK and mTOR pathway inhibition and identifies new targets for the development of rational combination therapies for aggressive BRAF-driven tumors.
Citation Format: Micah J. Maxwell, Antje Arnold, Heather Sweeney, Ljun Chen, Tung-Shing M. Lih, Michael Schnaubelt, Charles G. Eberhart, Jeffrey A. Rubens, Hui Zhang, David J. Clark, Eric H. Raabe. Unbiased proteomic and phosphoproteomic analysis identifies response signatures and novel susceptibilities after combined MEK and mTOR inhibition in BRAFV600E mutant glioma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 324.