AMPK exerts dual regulatory effects on the PI3K pathway

In the present study we further investigated the mechanism of AMPK-regulated insulin signaling. Our results showed that 5-aminoimidazole-4-carboxamide-1 ribonucleoside (AICAR) greatly enhanced the ability of insulin to stimulate the insulin receptor substrate-1 (IRS1)-associated PI3K activity in differentiated 3T3-F442a adipocytes, leading to increased Akt phosphorylation at S473, whereas insulin-stimulated activation of mTOR was diminished. In 3T3-F442a preadipocytes, these effects were attenuated by expression of a dominant negative mutant of AMPK α1 subunit. The enhancing effect of ACIAR on Akt phosphorylation was also observed when the cells were treated with EGF, suggesting that it is regulated at a step beyond IR/IRS1. Indeed, when the cells were chronically treated with AICAR in the absence of insulin, Akt phosphorylation was progressively increased. This event was associated with an increase in levels of phosphatidylinositol -3,4,5-trisphosphate (PIP3) and blocked by Wortmannin. We then expressed the dominant negative mutant of PTEN (C124S) and found that the inhibition of endogenous PTEN per se did not affect phosphorylation of Akt at basal levels or upon treatment with AICAR or insulin. Thus, this result suggests that AMPK activation of Akt is not mediated by regulating phosphatase and tensin homologue (PTEN).Our present study demonstrates that AMPK exerts dual effects on the PI3K pathway, stimulating PI3K/Akt and inhibiting mTOR/S6K.AMP-activated protein kinase (AMPK) is a heterotrimeric enzyme consisting of an α catalytic subunit (α1, α2), and β (β1, β2) and γ (γ1, γ2, γ3) regulatory subunits [1]. The activation of AMPK occurs by binding of 5' AMP to the γ subunit and phosphorylation of T172 in the activation loop of the α catalytic subunit by upstream kinases such as LKB1 and CaMKK [1]. AMPK is activated in response to hypoxia, glucose deprivation, and muscle exercise, under which the AMP to ATP ratio is increased. In addition, AMPK activity is increase