Phosphorylation of eIF2α Is Involved in the Signaling of Indispensable Amino Acid Deficiency in the Anterior Piriform Cortex of the Brain in Rats

Abstract

Sensing of indispensable amino acid (IAA) deficiency, an acute challenge to protein homeostasis, is demonstrated by rats as rejection of IAA-deficient diets within 20 min. The anterior piriform cortex (APC) of the brain in rats and birds is essential for this nutrient sensing, and is activated by IAA deficiency. Yet the mechanisms that sense and transduce IAA reduction to signaling in the APC, or indeed in any animal cells, are unknown. Because rejection of a deficient diet within 20 min is too rapid to be explained by transcription-derived signals, brain tissue was taken from rats after 20 min access to either a threonine-basal, -devoid, or -corrected diet and examined for proteins associated with early signaling of IAA deficiency in the yeast model. Western blots and immunohistochemistry showed that the phosphorylation of eukaryotic initiation factor 2-alpha (p-eIF2alpha[Ser51]) and translation of its downstream product, c-Jun, were increased (47%, P < 0.005, and 55%, P < 0.025, respectively) in APC from rats offered devoid, but not corrected diets, compared with those offered basal diets. This was not seen in other brain areas. In cells intensely labeled for cytoplasmic p-eIF2alpha, there was intense fluorescence for c-Jun in the nucleus. Thus, p-eIF2alpha, which is pivotal in the initiation of global protein translation, and its downstream product, the leucine zipper protein, c-Jun, are increased in the mammalian APC within the time frame necessary for the behavioral response. We suggest that p-eIF2alpha and c-Jun participate in signaling nutrient deficiency in the IAA-sensitive neurons of the APC.

Insulin fails to enhance mTOR phosphorylation, mitochondrial protein synthesis, and ATP production in human skeletal muscle without amino acid replacement

“…Importantly, mTOR signaling has been reported to upregulate mitochondrial function and to positively modulate PGC-1␣ expression through mechanisms at least partly independent of the p70 S6K -4EBP1 protein synthetic pathway (5, 24). In the current study, we also investigated the activation of the eIF2 complex that can stimulate protein synthesis in response to amino acids (10,14). We did not detect changes to eIF2␣ phosphorylation, which is consistent with the lack of stimulation of other major amino aciddependent protein-anabolic pathways.…”

“…Importantly, acute insulin infusion invariably lowers plasma amino acid (AA) concentrations by suppressing whole body and muscle protein breakdown (4,7,9), and acute mitochondrial effects of insulin were observed when the insulin-induced decline in plasma AA was prevented by exogenous infusion (25). Stimulation of mitochondrial protein synthesis is a potential important contributor to mitochondrial insulin effects (25), and a key role of AA to stimulate muscle protein synthesis both independently (10,13,14,17,27) and synergistically with insulin (8,18,28) has also been well elucidated. Independent effects of dietary AA supplementation to enhance skeletal muscle mitochondrial biogenesis have also been recently demonstrated in vivo in rodents, particularly with branched-chain amino acid (BCAA)-enriched mixtures (6).…”

“…We further hypothesized there would be no changes to key signaling proteins of the mTOR pathway that regulate protein synthesis in response to amino acids (10,13,14,17,27) or insulin (21).…”

Ablation of C/EBP Homologous Protein Attenuates Endoplasmic Reticulum–Mediated Apoptosis and Cardiac Dysfunction Induced by Pressure Overload

“…Enhanced phosphorylation of eIF2␣ reduces protein translation 30 and has been reported to mediate the inhibition of protein synthesis in the rat liver by vasopressin and the rat brain by essential amino acid deficiency. 31,32 Under CHOP-deficient conditions, decreased expression of GADD34 in pressure-overloaded hearts may lead to enhanced phosphorylation of eIF2␣ and 33 It will be important to clarify the selective activation of CHOP in future investigations. Overexpression of CHOP leads to a decrease in Bcl2 protein, whereas overexpression of Bcl2 blocks CHOPinduced apoptosis.…”

Preservation of Liver Protein Synthesis during Dietary Leucine Deprivation Occurs at the Expense of Skeletal Muscle Mass in Mice Deleted for eIF2 Kinase GCN2

“…At the start of the feeding experiment, mice were acclimated to a nutritionally complete, control diet for a minimum of 3 days, and then randomly assigned to one of four dietary treatments as follows: AA, continued free access to the nutritionally complete, control diet; ϪLEU, free access to a diet that was devoid of the essential amino acid leucine; ϪGLY, free access to a diet that was devoid of the non-essential amino acid glycine; PF, restricted access to the AA diet that equaled the intake of mice freely consuming the ϪLEU diet as previously detailed (4). Rodents, when fed a diet deficient in an essential amino acid, will reduce their intake (26,27). Consequently, the PF group was used to compare the effects of amino acid versus calorie deprivation on signaling events regulating protein synthesis.…”