Dissecting the mechanism of insulin resistance using a novel heterodimerization strategy to activate AKT

Insulin resistance can occur in response to many different external insults including chronic exposure to insulin itself as well as other agonists such as dexamethasone. It is generally thought that such defects arise due to a defect(s) at an early stage in the insulin signalling cascade. One model suggests that this involves activation of the mTOR/S6 kinase pathway which inactivates IRS via Ser/Thr phosphorylation. However, we have recently shown that IRS is not a major node for insulin resistance defects. To explore the mechanism of insulin resistance, we have developed a novel system to activate Akt independently of its upstream effectors as well as other insulin responsive pathways such as MAP kinase. 3T3-L1 adipocytes were rendered insulin resistant either with chonic insulin or dexamethasone treatment, but conditional activation of Akt2 stimulated HA-GLUT4 translocation to the same extent in these insulin resistant and control cells. However, addition of insulin to cells in which Akt was conditionally activated resulted in a reversion to the insulin resistant state, indicating a feed-forward inhibitory mechanism activated by insulin itself. This effect was overcome with wortmannin, implicating a role for PI3-kinase in this inhibitory process. We conclude that in chronic insulin and dexamethasone treated cells, acute activation with insulin itself is required to activate a feed-forward inhibitory pathway likely emanating from PI3-kinase that converges on a target downstream of Akt to cause insulin resistance.