Insulin-modulated Akt subcellular localization determines Akt isoform-specific signaling

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Abstract
The 3 Akt protein kinase isoforms have critical and distinct functions in the regulation of metabolism, cell growth, and apoptosis, yet the mechanisms by which their signaling specificity is achieved remain largely unclear. Here, we investigated potential mechanisms underlying Akt isoform functional specificity by using Akt2-specific regulation of glucose transport in insulin-stimulated adipocytes as a model system. We found that insulin activates both Akt1 and Akt2 in adipocytes, but differentially regulates the subcellular distribution of these Akt isoforms. The greater accumulation of Akt2 at the plasma membrane (PM) of insulin-stimulated adipocytes correlates with Akt2-specific regulation of the trafficking of the GLUT4 glucose transporter. Consistent with this pattern, Akt constructs that do not accumulate at the PM to the same degree as Akt2 fail to regulate GLUT4 translocation to the PM, whereas enhancement of Akt1 PM association through mutation in Akt1 PH domain is sufficient to overcome Akt-isoform specificity in GLUT4 regulation. Indeed, we found that this distinct insulin-induced PM accumulation of Akt kinases is translated into a differential regulation by the Akt isoforms of AS160, a RabGAP that regulates GLUT4 trafficking. Our data show that Akt2 specifically regulates AS160 phosphorylation and membrane association providing molecular basis for Akt2 specificity in the modulation of GLUT4 trafficking. Together, our findings reveal the stimulus-induced subcellular compartmentalization of Akt kinases as a mechanism contributing to specify Akt isoform functions.