Like insulin, leucine stimulates the mammalian target of rapamycin (mTOR)/p70 ribosomal S6 kinase (p70S6K) axis in various organs. Insulin proceeds via the canonical association of phosphatidylinositol 3-kinase (PI3K), phosphoinositide-dependent protein kinase-1 (PDK1), and protein kinase B (PKB/Akt). The signaling involved in leucine effect, although known to implicate a PI3K mechanism independent of PKB/Akt, is more poorly understood. In this study, we investigated whether PDK1 could also participate in the events leading to mTOR/p70S6K activation in response to leucine in the heart. In wild-type hearts, both leucine and insulin increased p70S6K activity whereas, in contrast to insulin, leucine was unable to activate PKB/Akt. The changes in p70S6K activity induced by insulin and leucine correlated with changes in phosphorylation of Thr389, the mTOR phosphorylation site on p70S6K, and of Ser2448 on mTOR, both related to mTOR activity. Leucine also triggered phosphorylation of the proline-rich Akt/PKB substrate of 40 kDa (PRAS40), a new pivotal mTOR regulator. In PDK1 knockout hearts, leucine, similarly to insulin, failed to induce the phosphorylation of mTOR and p70S6K, leading to the absence of p70S6K activation. The loss of leucine effect in absence of PDK1 correlated with the lack of PRAS40 phosphorylation. Moreover, the introduction in PDK1 of the L155E mutation, which is known to preserve the insulin-induced and PKB/Akt-dependent phosphorylation of mTOR/p70S6K, suppressed all leucine effects, including phosphorylation of mTOR, PRAS40, and p70S6K. We conclude that the leucine-induced stimulation of the cardiac PRAS40/mTOR/p70S6K pathway requires PDK1 in a way that differs from that of insulin.
The survival pathways phosphatidylinositol-3 kinase (PI3-K)/phosphoinositide-dependent protein kinase 1 (PDK1)/Akt modulate liver regeneration through hepatocyte size rather than proliferation