Inhibition of p70S6K does not mimic the enhancement of Akt phosphorylation by rapamycin

It has been suggested that the mTOR complex 1 (mTORC1)/p70S6K axis represses upstream PI3K/Akt signaling through phosphorylation of IRS-1 and its subsequent degradation. One potential and current model that explains Akt activation induced by the mTOR inhibitor rapamycin is the relief of mTORC1/p70S6K-mediated feedback inhibition of IRS-1/PI3K/Akt signaling, although this has not been experimentally proven. In this study, we found that chemical inhibition of p70S6K did not increase Akt phosphorylation. Surprisingly, knockdown of p70S6K even substantially inhibited Akt phosphorylation. Hence, p70S6K inhibition clearly does not mimic the activation of Akt by rapamycin. Inhibition or enforced activation of p70S6K did not affect the ability of rapamycin to increase Akt phosphorylation. Moreover, inhibition of mTORC1 with either rapamycin or raptor knockdown did not elevate IRS-1 levels, despite potently increasing Akt phosphorylation. Critically, knockdown or knockout of IRS-1 or IRS-2 failed to abolish the ability of rapamycin to increase Akt phosphorylation. Therefore, IRS-1 and IRS-2 are not essential for mediating rapamycin-induced Akt activation. Collectively, our findings suggest that Akt activation by rapamycin or mTORC1 inhibition is unlikely due to relief of p70S6K-mediated feedback inhibition of IRS-1/PI3K/Akt signaling.