Activation of the p70 S6 kinase by leucine is partly regulated by the activity of branched‐chain α‐keto acid dehydrogenase (BCKDH) complex

Abstract

Branched-chain amino acid leucine has been shown to activate the translational regulators. However, the leucine′ s effects are self-limiting because leucine promotes its own disposal by an oxidative pathway. The irreversible and rate-limiting step in the leucine oxidation pathway is catalyzed by the BCKDH complex. The complex contains E1 (α2β2), E2, and E3 subunits and its activity is abolished by phosphorylation of the E1α subunit by BCKDH kinase. In the present study, the relationship between the activity of BCKDH complex and leucine-mediated activation of the protein translation was investigated using the technique of RNA interference. The activity of BCKDH complex in C2C12 cell was modulated by transfection of siRNA for BCKDH kinase or BCKDH E2 subunit. We found that suppression of either kinase or E2 subunit produced opposite effects on the cell proliferation and the activation of translational regulators by leucine. Kinase suppression for 48 h resulted in a decrease cell proliferation. By contrast, E2 suppression led to increase a total cellular protein amount. The phosphorylation of p70 S6 kinase by leucine was increased in E2-siRNA transfected C2C12 cells, whereas the leucine′ s effect was diminished in kinase-siRNA transfected cells. These results suggest that the activation of the translational regulators by leucine was partly regulated by the activity of BCKDH complex.