Mapping the insulin signalling network (#7)
Binding of insulin to its cognate receptor in adipocytes initiates a diverse range of cellular signalling events. These initiating signals rapidly diverge to regulate a multitude of cellular processes including glucose disposal, lipolysis, protein synthesis and transcription. Many of these processes are ultimately regulated via Akt and mTOR which act as key signalling nodes in the cell. Utilizing mass spectrometry-based proteomics we have comprehensively mapped the dynamics of these signals in 3T3-L1 adipocytes in response to acute insulin stimuli. We further dissect the role of the PI3K-Akt pathway using selective inhibitory compounds targeting these nodes. These studies reveal 37,000 phosphorylation sites in the adipocyte with approximately 15% of these sites being modulated by insulin by >2-fold. More than 50% of the entire insulin responsive phosphoproteome was blocked in the presence of a PI3K inhibitor and of this the large majority was transmitted via the Akt node. The time course data revealed a surprising degree of temporal resolution within the data. In many cases kinases were clustered temporally with their substrates and different kinase nodes such as Akt and mTOR were segregated within the dynamic signalling network. Using a machine learning approach we developed a computational method for predicting kinase-substrate relationships. This was used to reveal several novel Akt substrates such as the mTORC2 subunit SIN1. Further analysis revealed that Akt-dependent phosphorylation of SIN1 enhanced mTORC2 kinase activity facilitating its ability to phosphorylate Akt at Ser473. This leads to a model whereby mTORC2 and Akt positively regulate each other in a positive feedback loop thus providing new insights into this pathway. These data identify a diverse range of cellular functions regulated by insulin and emphasise the enormous potential of mass spectrometry-based proteomics for the study of signalling.