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. 2017 Jun 24;6(8):873-881.
doi: 10.1016/j.molmet.2017.06.009. eCollection 2017 Aug.

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

Adriano Maida  1 , Jessica S K Chan  2 , Kim A Sjøberg  3 , Annika Zota  2 , Dieter Schmoll  4 , Bente Kiens  3 , Stephan Herzig  2 , Adam J Rose  5
Affiliations

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

Adriano Maida et al. Mol Metab. .

Abstract

Objective: Dietary 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.

Methods: We 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.

Results: Serum 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.

Conclusions: Repletion 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.

Keywords: AA, amino acid; AAD, amino acid diluted; BCAA; BCAA, branched chain amino acid; Diabetes; Dietary protein; FGF21; FGF21, fibroblast growth factor 21; HF, high fat; ISR, integrated stress response; NZB, New Zealand black; NZO, New Zealand obese; PD, protein dilution; T2D, type 2 diabetes; mTORC1; mTORC1, mammalian target of rapamycin complex 1.

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Figures

Image 1
Graphical abstract
Figure 1
Figure 1
A specific reduction of fed serum BCAAs in humans following dietary PD. Serum urea (A), and BCAAs leucine (Leu), Isoleucine (Ile), and Valine (Val) (B–D) were measured in a meal test from serum collected from healthy men before (normal diet, N) or following 7 days of a protein-diluted (PD) diet regimen. Subjects ate an isocaloric meal (i.e. 60 kJ/kg BM) containing 15% (N group) or 9% protein (PD group). n = 5 per group. Data are mean ± SEM. *p < 0.05, **p < 0.01, ***p < 0.001 for significant effect of diet.
Figure 2
Figure 2
Serum BCAAs are elevated in diabetes and reduced by dietary PD in mice. Total serum AAs (A), BCAAs only (B) or individual BCAAs (C–E) were measured in serum collected from random fed C57Bl/6N mice fed either control diet (“C”) containing 20% caloric energy from protein or a protein-diluted (“PD”) diet containing 5% caloric energy from protein, diluted by added carbohydrate, with either 10% (“LF”) or 60% (“HF”) calories from fat. n = 6–8/group. Total serum AAs (F), BCAAs only (G) or individual BCAAs (H–J) were measured in serum from New Zealand black (“NZB”) or New Zealand obese (“NZO”) mice fed C or PD diets. n = 6–8/group. Data are mean ± SEM. *p < 0.05, **p < 0.01, ***p < 0.001 for significant effect of PD. ##p < 0.01, ###p < 0.001 for significant effect of mouse strain.
Figure 3
Figure 3
Repletion of BCAA in the setting of dietary amino acid dilution attenuates improvements in metabolic health and restores mTORC1 signaling in wildtype mice. Serum BCAA (A), body mass accrual (B) and endpoint tissue weights (C) were measured in random fed mice fed control (“C”), amino acid diluted (“AAD”), or BCAA replete AAD (“AAD + BCAA”) diets. Food intake (D), used to calculate feed efficiency (E), was assessed in the same mice as in (A–C) over experiment days 57–70. An intraperitoneal glucose tolerance test (“IPGTT”, F) was conducted and insulin sensitivity assessed using insulin sensitivity index (G) and an insulin tolerance test (H). Endpoint (day ∼ 100) serum (I) and hepatic transcript (J) levels of FGF21 were measured. n = 6–8/group. Signaling downstream of mTORC1 was assessed by western blot in liver (K–L), abdominal white adipose tissue (epididymal “aWAT”), subcutaneous WAT (inguinal “scWAT”), brown adipose tissue (“BAT”) and gastrocnemius (“GC”) muscle (M–P). Western band intensities expressed relative to respective loading controls are shown below each blot, except 4EBP1 which was expressed using the indicated bands on the total blot as hypophosphorylated: total. n = 3–4/group. Data are mean ± SEM. *p < 0.05, **p < 0.01, ***p < 0.001 vs. C. #p < 0.05 ##p < 0.01, ###p < 0.001 vs. AAD.
Figure 4
Figure 4
Dietary repletion of BCAAs opposes somatic mTORC1 signaling, but not improvements in glucose homeostasis in NZO mice. Endpoint random fed serum BCAAs (A), body mass accrual (B), feed efficiency (C), random fed blood glucose (D) and hepatic FGF21 transcript (E) were assessed in New Zealand obese (NZO) mice fed control (“C”), amino acid diluted (“AAD”), or AAD with BCAA replete (“AAD + BCAA”) diets. n = 6/group. Signaling downstream of mTORC1 was assessed in liver (F and G), white adipose tissue from abdominal (epididymal “aWAT”) and subcutaneous (inguinal “scWAT”) depots, brown adipose tissue (“BAT”) and gastrocnemius (“GC”) muscle (H–K) were assessed by western blot in a separate cohort of mice adapted to diets for 3 weeks, fasted overnight and re-fed for 45 min. Western band intensities expressed relative to respective loading controls are shown below each blot, except 4EBP1 which was expressed using the indicated bands on the total blot as hypophosphorylated: total. n = 3–4. Data are mean ± SEM. *p < 0.05, **p < 0.01, ***p < 0.001 vs. C. #p < 0.05 vs. AAD.
None
Supplementary Figure 1Area under the curve (AUC) for serum AAs following a meal test reveals a BCAA-related profile following dietary PD. Healthy men were switched from their normal diet (“N”) to PD diet (A) for 7 days. Serum AAs were measured before and over 250 min following a meal test. Area under the 250 min AA concentration curve was calculated for each subject using the x-axis as the baseline (B). Subjects ate an isocaloric meal (i.e. 60 kJ/kg BM) containing 15% (N group) or 9% protein (PD group). n = 5 per group. Data are mean ± SEM. *p < 0.05, **p < 0.01, for significant effect of diet.
None
Supplementary Figure 2Dietary PD influences serum AA profiles in diet-induced or polygenic obesity/diabetes. Wildtype C57Bl/6N mice were fed either control diet (“C”) or a protein-diluted (“PD”) diet, diluted by added carbohydrate, with either low fat (“LF”) or high fat (“HF”) (A). After 16 weeks on diets, individual AAs were measured in serum from random fed mice (B). n = 6–8/group. New Zealand black (“NZB”) or New Zealand obese (“NZO”) mice were fed C or PD diets for 10 weeks (C) followed by serum AA assessment in the random fed state (D). n = 6–8/group. Data are mean ± SEM. *p < 0.05, **p < 0.01, ***p < 0.001 for significant effect of PD. #p < 0.05 ##p < 0.01, ###p < 0.001 for significant effect of dietary fat/strain.
None
Supplementary Figure 3Metabolic and signaling parameters in wildtype mice following repletion of BCAA in the setting of dietary amino acid dilution. Wildtype mice fed control (“C”), amino acid diluted (“AAD”), or AAD with BCAA replete (“AAD + BCAA”) diets (A) underwent metabolic characterization (B), which included the indicated tolerance tests as well as feed efficiency (“FE”) assessment. Random fed serum amino acids (C) and MRI quantification of lean mass (D) and fat mass (E) were assessed in the same mice as in Figure 3A–J. Delta body mass for days 57–70 (F) was used together with food intake (see Figure 3D) to calculate feed efficiency in Figure 3E. Fasting plasma insulin (G), together with corresponding fasting glucose (see Figure 3F) was used to calculate insulin sensitivity index (see Figure 3G). n = 6–8/group. The hormonal and signaling response to these diets was studied in a separate cohort of re-fed mice (H). Phosphorylation eIF2α (I) and AKT (J) were assessed in liver tissue by western and band intensities expressed relative to respective loading controls are shown below each blot. Levels of blood glucose (K), plasma insulin (L) and FGF21 (M) were quantified. n = 3–4/group. Data are mean ± SEM. p < 0.05, **p < 0.01, ***p < 0.001 vs. C. #p < 0.05 vs. AAD.
None
Supplementary Figure 4Metabolic and signaling parameters in NZO mice following repletion of BCAA in the setting of dietary amino acid dilution. NZO mice fed control (“C”), amino acid diluted (“AAD”), or AAD with BCAA replete (“AAD + BCAA”) diets underwent metabolic characterization (A), which included feed efficiency (“FE”) assessment. Random fed serum amino acids (B) and endpoints tissue weights (C) were assessed in the same NZO mice as in Figure 4A–E. Food intake (D) together with changes in body mass (E) were used to determine feed efficiency in Figure 4C. Fasted blood glucose (F) and plasma insulin (G) were measured on day 45 and used to calculate insulin sensitivity index (H). n = 6/group. The hormonal and signaling response to these diets was studied in a separate cohort of re-fed NZO mice (I). Liver eIF2α phosphorylation (J) was determined and western band intensities expressed relative to respective loading controls are shown below each blot. Levels of energy intake (K), blood glucose (L), plasma insulin (M) and FGF21 (N) were quantified. n = 3–4/group. Data are mean ± SEM. p < 0.05, **p < 0.01, ***p < 0.001 vs. C. #p < 0.05 vs. AAD.
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