Glucagon represses signaling through the mammalian target of rapamycin in rat liver by activating AMP-activated protein kinase

The opposing actions of glucagon and insulin on glucose metabolism within the liver are essential mechanisms for maintaining plasma glucose concentrations within narrow limits. Less well studied are the counter-regulatory actions of glucagon on protein metabolism. In the present study, the effect of glucagon on amino acid-induced signaling through the mammalian target of rapamycin (mTOR), an important controller of the mRNA binding step in translation initiation, was examined using the perfused rat liver as an experimental model. The results show that amino acids enhance signaling through mTOR resulting in phosphorylation of eukaryotic initiation factor 4E-binding protein (4E-BP)1, the 70-kDa ribosomal protein (rp)S6 kinase, S6K1, and rpS6. In contrast, glucagon repressed both basal and amino acid-induced signaling through mTOR, as assessed by changes in the phosphorylation of 4E-BP1 and S6K1. The repression was associated with the activation of protein kinase A and enhanced phosphorylation of LKB1 and the AMP-activated protein kinase (AMPK). Surprisingly, the phosphorylation of two S6K1 substrates, rpS6 and eukaryotic initiation factor 4B, was not repressed but instead was increased by glucagon treatment, regardless of the amino acid concentration. The latter finding could be explained by the glucagon-induced phosphorylation of the ERK1 and the 90-kDa rpS6 kinase p90^rsk. Thus, glucagon represses phosphorylation of 4E-BP1 and S6K1 through the activation of a protein kinase A-LKB-AMPK-mTOR signaling pathway, while simultaneously enhancing phosphorylation of other downstream effectors of mTOR through the activation of the extracellular signal-regulated protein kinase 1-p90 ^rsk signaling pathway. Amino acids also enhance AMPK phosphorylation, although to a lesser extent than glucagon and amino acids combined.