The mTOR pathway controls phosphorylation of BRAF at T401

• Published in: Cell communication and signaling Vol. 22; no. 1; pp. 428 - 13
• Main Authors: Christen, Daniel, Lauinger, Manuel, Brunner, Melanie, Dengjel, Jörn, Brummer, Tilman
• Format: Journal Article
• Language: English
• Published: England BioMed Central Ltd 02.09.2024; BioMed Central; BMC
• Subjects: 1-Phosphatidylinositol 3-kinase; AKT protein; Animals; Antibodies; Binding sites; Cancer; Cell activation; Development and progression; Drug therapy; Growth factors; Health aspects; HEK293 Cells; Homeostasis; Humans; Kinases; Mass spectroscopy; Mechanistic Target of Rapamycin Complex 1 - metabolism; Medical research; Medicine, Experimental; MEK inhibitors; Mice; Mutagenesis; Naphthyridines; Penicillin; Phosphorylation; Phosphorylation - drug effects; Physiological aspects; Protein kinases; Proteins; Proto-Oncogene Proteins B-raf - genetics; Proto-Oncogene Proteins B-raf - metabolism; Pyridones - pharmacology; Pyrimidinones - pharmacology; Raf protein; Signal transduction; Signal Transduction - drug effects; TOR protein; TOR Serine-Threonine Kinases - metabolism; Germany
• ISSN: 1478-811X; 1478-811X
• DOI: 10.1186/s12964-024-01808-2

BRAF serves as a gatekeeper of the RAS/RAF/MEK/ERK pathway, which plays a crucial role in homeostasis. Since aberrant signalling of this axis contributes to cancer and other diseases, it is tightly regulated by crosstalk with the PI3K/AKT/mTOR pathway and ERK mediated feedback loops. For example, ERK limits BRAF signalling through phosphorylation of multiple residues. One of these, T401, is widely considered as an ERK substrate following acute pathway activation by growth factors. Here, we demonstrate that prominent T401 phosphorylation (pT401) of endogenous BRAF is already observed in the absence of acute stimulation in various cell lines of murine and human origin. Importantly, the BRAF/RAF1 inhibitor naporafenib, the MEK inhibitor trametinib and the ERK inhibitor ulixertinib failed to reduce pT401 levels in these settings, supporting an alternative ERK-independent pathway to T401 phosphorylation. In contrast, the mTOR inhibitor torin1 and the dual-specific PI3K/mTOR inhibitor dactolisib significantly suppressed pT401 levels in all investigated cell types, in both a time and concentration dependent manner. Conversely, genetic mTOR pathway activation by oncogenic RHEB (Q64L) and mTOR (S2215Y and R2505P) mutants substantially increased pT401, an effect that was reverted by dactolisib and torin1 but not by trametinib. We also show that shRNAmir mediated depletion of the mTORC1 complex subunit Raptor significantly enhanced the suppression of T401 phosphorylation by a low torin1 dose, while knockdown of the mTORC2 complex subunit Rictor was less effective. Using mass spectrometry, we provide further evidence that torin1 suppresses the phosphorylation of T401, S405 and S409 but not of other important regulatory phosphorylation sites such as S446, S729 and S750. In summary, our data identify the mTOR axis and its inhibitors of (pre)clinical relevance as novel modulators of BRAF phosphorylation at T401.