Tissue-specific regulation of 4E-BP1 and S6K1 phosphorylation by alpha-ketoisocaproate.

The indispensable branched-chain amino acid leucine acts as a key regulator of mRNA translation by modulating the phosphorylation of proteins that represent important control points in translation initiation, including the translational repressor, eukaryotic initiation factor (eIF) 4E-binding protein 1 (4E-BP1) and ribosomal protein S6 kinase (S6K1). In the current study, we compared the effects of L- and D-enantiomers of leucine on the phosphorylation of 4E-BP1 and S6K1. We also assessed whether leucine itself or its metabolite, alpha-ketoisocaproate (alpha-KIC), mediates the effects of leucine. Food-deprived (18 h) rats were orally administered 135 mg/100 g body weight L-leucine, D-leucine or alpha-KIC and were sacrificed after 1 h. L-Leucine administration had an obvious stimulatory effect on the phosphorylation of 4E-BP1 and S6K1 in both skeletal muscle and liver while D-leucine was much less effective, indicating that the effect of leucine is stereospecific. Oral administration of alpha-KIC mimicked the stimulatory effect of L-leucine in skeletal muscle. In contrast to skeletal muscle, provision of alpha-KIC was significantly less effective than L-leucine in the liver. The results showing that the efficacy of L-leucine and alpha-KIC in stimulating phosphorylation of S6K1 and 4E-BP1 is equivalent in skeletal muscle, may be explained by the conversion of alpha-KIC to L-leucine.