Cross-talks via mTORC2 can explain enhanced activation in response to insulin in diabetic patients

Rasmus Magnusson, Mika Gustafsson, Gunnar Cedersund, Peter Strålfors, Elin Nyman

The molecular mechanisms of insulin resistance in type 2 diabetes have been extensively studied in primary human adipocytes, and mathematical modelling has clarified the central role of attenuation of mTORC1 activity in the diabetic state. Attenuation of mTORC1 in diabetes quells insulin signalling network-wide, except for the mTORC2-catalysed phosphorylation of PKB at serine-473, which is increased. This unique increase could potentially be explained by feedback and inter-branch crosstalk signals. To examine if such mechanisms operate in adipocytes, we herein analysed data from an un-biased phosphoproteomic screen in 3T3-L1 adipocytes. Using a mathematical modelling approach, we show that a negative signal from mTORC1-S6K to rictor-mTORC2 in combination with a positive signal from PKB to SIN1-mTORC2 are compatible with the experimental data. This combined cross-branch signalling predicted an increased phosphorylation of PKB at serine-473 in response to attenuation of mTORC1 - a distinguishing feature of the insulin resistant state in human adipocytes. This aspect of insulin signalling was then verified for our comprehensive model of insulin signalling in human adipocytes. Introduction of the cross-branch signals was compatible with all data for insulin signalling in human adipocytes, and the resulting model can explain all data network-wide, including the increased phosphorylation of PKB at serine-473 in the diabetic state. Our approach to first identify potential mechanisms in data from a phosphoproteomic screen in a cell line, and then verify such mechanisms in primary human cells, demonstrates how an un-biased approach can support a direct knowledge-based study.