Amino acids inhibit Agrp gene expression via an mTOR-dependent mechanism

Metabolic fuels act on hypothalamic neurons to regulate feeding behavior and energy homeostasis, but the signaling mechanisms mediating these effects are not fully clear. Rats placed on a low-protein diet (10% of calories) exhibited increased food intake (P < 0.05) and hypothalamic Agouti-related protein (Agrp) gene expression (P = 0.002). Direct intracerebroventricular injection of either an amino acid mixture (RPMI 1640) or leucine alone (1 mug) suppressed 24-h food intake (P < 0.05), indicating that increasing amino acid concentrations within the brain is sufficient to suppress food intake. To define a cellular mechanism for these direct effects, GT1-7 hypothalamic cells were exposed to low amino acids for 16 h. Decreasing amino acid availability increased Agrp mRNA levels in GT1-7 cells (P < 0.01), and this effect was attenuated by replacement of the amino acid leucine (P < 0.05). Acute exposure to elevated amino acid concentrations increased ribosomal protein S6 kinase phosphorylation via a rapamycin-sensitive mechanism, suggesting that amino acids directly stimulated mammalian target of rapamycin (mTOR) signaling. To test whether mTOR signaling contributes to amino acid inhibition of Agrp gene expression, GT1-7 cells cultured in either low or high amino acids for 16 h and were also treated with rapamcyin (50 nM). Rapamycin treatment increased Agrp mRNA levels in cells exposed to high amino acids (P = 0.01). Taken together, these observations indicate that amino acids can act within the brain to inhibit food intake and that a direct, mTOR-dependent inhibition of Agrp gene expression may contribute to this effect.