Repletion of branched chain amino acids reverses mTORC1 signaling but not improved metabolism during dietary protein dilution

Abstract

ObjectiveDietary protein dilution (PD) has been associated with metabolic advantages such as improved glucose homeostasis and increased energy expenditure. This phenotype involves liver-induced release of FGF21 in response to amino acid insufficiency; however, it has remained unclear whether dietary dilution of specific amino acids (AAs) is also required. Circulating branched chain amino acids (BCAAs) are sensitive to protein intake, elevated in the serum of obese humans and mice and thought to promote insulin resistance. We tested whether replenishment of dietary BCAAs to an AA-diluted (AAD) diet is sufficient to reverse the glucoregulatory benefits of dietary PD.MethodsWe conducted AA profiling of serum from healthy humans and lean and high fat-fed or New Zealand obese (NZO) mice following dietary PD. We fed wildtype and NZO mice one of three amino acid defined diets: control, total AAD, or the same diet with complete levels of BCAAs (AAD + BCAA). We quantified serum AAs and characterized mice in terms of metabolic efficiency, body composition, glucose homeostasis, serum FGF21, and tissue markers of the integrated stress response (ISR) and mTORC1 signaling.ResultsSerum BCAAs, while elevated in serum from hyperphagic NZO, were consistently reduced by dietary PD in humans and murine models. Repletion of dietary BCAAs modestly attenuated insulin sensitivity and metabolic efficiency in wildtype mice but did not restore hyperglycemia in NZO mice. While hepatic markers of the ISR such as P-eIF2α and FGF21 were unabated by dietary BCAA repletion, hepatic and peripheral mTORC1 signaling were fully or partially restored, independent of changes in circulating glucose or insulin.ConclusionsRepletion of BCAAs in dietary PD is sufficient to oppose changes in somatic mTORC1 signaling but does not reverse the hepatic ISR nor induce insulin resistance in type 2 diabetes during dietary PD.

Restriction of essential amino acids dictates the systemic response to dietary protein dilution

“…Lastly, as we have previously shown that dietary protein or AA dilution can retard the development of obesity-related metabolic dysfunction, we then tested whether selective Thr restriction (LT), without total AA restriction or altered dietary carbohydrate supply, can mimic these effects in a mouse model of obesity-related metabolic dysfunction, the New Zealand Obese (NZO) mouse. Of note, similar to our prior findings of NZO mice subjected to dietary protein/AA dilution 14,49 , there was no effects of LT on body mass development and liver and adipose tissue weights (Suppl. Fig.…”

“…We previously demonstrated that dietary protein dilution (DPD) improves glucose and lipid homeostasis in obesity and that dilution of amino acids was sufficient to mimic these effects 14,16,49 . However, it remained to be resolved whether the dilution of protein by substituting with other dietary nutrients (i.e.…”

“…1H). As increased blood FGF21 levels also confer the effects of DPD on improved glucose metabolism 14,49 , we also assessed this by measuring an index of fasting insulin sensitivity (ISI(f)) ( Fig. 1I) from both fasting glucose (Suppl.…”

“…To reinforce the findings that only certain EAA such as Thr and Trp are important for the effects of dietary EAA restriction, we then wanted to confirm the importance of these EAA in other situations. We re-examined the serum AA response in humans subjected to a natural low protein diet from freely available foodstuffs 14,49 , and could demonstrate that the Thr ( Fig 5A), but not Lys or Trp (data not shown), serum excursion was blunted in response to an acute low protein meal following a one week adaptation period. Importantly, of this low protein diet, Thr was identified as the most limiting AA ( Fig.…”

“…On one hand, restriction of EAA such as the branched chain amino acids 21,[38][39][40][41] and sulphur-containing AAs [42][43][44][45] , have been shown to be sufficient in conferring the systemic effects of DPD. On the other hand, others have demonstrated that altered somatic non-EAA metabolism is sufficient and necessary for DPD effects 14,[46][47][48][49] . Here we attempted to resolve this issue by formally testing which particular amino acids are required to induce the systemic response to DPD.…”