Dynamic modelling of the PI3K/MTOR signalling network uncovers biphasic dependence of mTORC1 activity on the mTORC2 subunit SIN1

摘要

The PI3K/MTOR signalling network regulates a broad array of critical cellular processes,including cell growth, metabolism and autophagy. The mechanistic target of rapamycin(MTOR) kinase functions as a core catalytic subunit in two physically and functionally dis tinct complexes mTORC1 and mTORC2, which also share other common componentsincluding MLST8 (also known as GβL) and DEPTOR. Despite intensive research, howmTORC1 and 2 assembly and activity are coordinated, and how they are functionally linkedremain to be fully characterized. This is due in part to the complex network wiring, featuringmultiple feedback loops and intricate post-translational modifications. Here, we integratepredictive network modelling, in vitro experiments and -omics data analysis to elucidate theemergent dynamic behaviour of the PI3K/MTOR network. We construct new mechanisticmodels that encapsulate critical mechanistic details, including mTORC1/2 coordination byMLST8 (de)ubiquitination and the Akt-to-mTORC2 positive feedback loop. Model simulations validated by experimental studies revealed a previously unknown biphasic, thresholdgated dependence of mTORC1 activity on the key mTORC2 subunit SIN1, which is robustagainst cell-to-cell variation in protein expression. In addition, our integrative analysis demonstrates that ubiquitination of MLST8, which is reversed by OTUD7B, is regulated by IRS1/2. Our results further support the essential role of MLST8 in enabling both mTORC1 and 2’sactivity and suggest MLST8 as a viable therapeutic target in breast cancer. Overall, ourstudy reports a new mechanistic model of PI3K/MTOR signalling incorporating MLST8-mediated mTORC1/2 formation and unveils a novel regulatory linkage between mTORC1and mTORC2.