Background: 

The protein kinase AKT is positioned at the intersection of multiple afferent signals and a plethora of substrates which modulate cardiac growth, metabolism, gene expression and contractility. In the heart highly homologous AKT1 and AKT2 isoforms are expressed. However, isoform specific functions begin to emerge as demonstrated by knock out models.

Objective: 

Uncovering of AKT-isoform specific functions and targets is necessary to understand the multifaceted nature of AKT signaling.

Methods: 

HL-1 cardiomyocytes with AKT1- or AKT2-specific knockdown and control cells were stimulated with insulin (200 nM, 5 min) to activate AKT kinases. After trypsin digestion the resulting peptides were subjected to a triple stable isotope dimethyl labeling for quantitative proteomics. Differentially labeled peptides were mixed at equal ratios, phosphopeptides were enriched, identified and relatively quantified by MS on the basis of their specific mass shift.

Results: 

More than 3000 phophopeptides were identified and relative quantification revealed around 10% of these peptides to be changed due to the AKT knock down. Isoform specific regulation was observed for > 60 phosphopeptides belonging to proteins such as 90K RSK3, caspase-3, Bcl-2-associated athanogene 3, p53, EF1-beta, eIF5B, HSP-90, telomere length regulation protein and transcription termination factor (RNA polymerase II). Several phosphoproteins (e.g. Exo70, Raptor, Rictor, LKB1, RSK, eIF5B) belonging to the insulin and mTOR pathways were specifically regulated by AKT2.

Conclusions: 

We have identified numerous phosphoproteins regulated in an AKT-isoform dependent manner. Especially the insulin and mTOR signaling pathways seem to be predominantly targeted by AKT2-specific signal transduction in cardiomyocytes after insulin stimulation.