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

Abstract

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 μg) 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.

FGF21 is an endocrine signal of protein restriction

“…Rats consuming low-protein (LP) diet exhibited increased food intake (control: 23.53 ± 0.9 g/d; LP: 28.06 ± 1.2 g/d, P = 0.01), consistent with previous work (31,32). Although there was no effect of diet on final body weight or body weight gain, 14 days of LP diet increased body adiposity (control: 18.2% ± 0.005%; LP: 20.3% ± 0.008% fat, P = 0.034).…”

“…To determine whether FGF21 is required for adaptive responses to protein restriction, WT and Fgf21-deficient mice were placed on a LP diet for 14 days. As expected (4,31,(39)(40)(41)(42), WT mice displayed both increased food intake ( Figure 5E) and a progressive increase in EE ( Figure 5A) following the switch to LP diet. Specifically, EE increased, by approximately 15% in WT mice by days 5 to 7 (P < 0.01; Figure 5, C and D), an effect detected during both the light and dark periods.…”

mTORC1 signaling in energy balance and metabolic disease

“…These reports further confirmed that leucine-induced anorexia is dependent on hypothalamic mTORC1 activity and associated with the inhibition of AMPK. 17,18 Consistent with a cross-regulation between AMPK and mTORC1, the activation of these enzymes occurs in the same specific neuronal subtypes. 17 Furthermore, i.c.v.…”

“…administration of leucine decreases mRNA levels of neuropeptide Y, [16][17][18] while increasing mRNA levels of POMC within the ARC. 17,18 Recently, Blouet et al 54 have reported that intra-MBH administration of leucine reduces meal size and meal number by engaging a forebrain/hindbrain neurocircuit that activates ARC POMC neurons, paraventricular nucleus (PVN) oxytocin neurons and the nucleus of solitary tract neurons. Apart from S6K1, the authors have also identified the ERK1/2 pathway as a mediator of leucine's action on food intake.…”

“…10,[13][14][15] The mammalian target of rapamycin (mTOR) pathway is the latest intracellular fuel-sensing mechanism to be implicated in the regulation of energy balance. [16][17][18] mTOR is a highly conserved serine-threonine kinase, which in peripheral cells is known to integrate nutrient signals with hormonal signals to control cell growth and proliferation. 19 Cellular adenosine triphosphate (ATP) levels increase mTOR activity, and the mTOR kinase itself serves as a cellular ATP sensor.…”