Leucine deprivation increases hepatic insulin sensitivity via GCN2/mTOR/S6K1 and AMPK pathways

Effects of essential amino acids on lipid metabolism in mice and humans

Eight amino acids are considered essential for human nutrition, and three of them, including leucine, isoleucine and valine, are called as branched-chain amino acids (BCAAs). We recently discovered that dietary deficiency of any BCAA for 7 days rapidly reduces the abdominal fat mass in mice. The goal of this study was to investigate (1) whether dietary deficiency of the other five essential amino acids (EAAs), including phenylalanine, threonine, tryptophan, methionine and lysine, would produce similar effects and (2) whether an association between serum AAs and obesity was observed in humans in Chinese Han population. Similar to BCAAs deprivation, dietary deficiency of any of these five EAAs for 7 days significantly reduced abdominal fat mass, which is likely caused by increased energy expenditure. Expression of genes and proteins related to lipolysis, however, were differentially regulated by different EAAs. These results suggest a crucial role of EAAs deprivation on lipid metabolism in mice. Our human studies revealed that levels of four EAAs (leucine, isoleucine, valine and phenylalanine) were elevated in obese humans compared with those in lean controls in Chinese Han population. Based on the results obtained from mice, we speculate that these four EAAs might play important roles in human obesity.

Calorie Restricted High Protein Diets Downregulate Lipogenesis and Lower Intrahepatic Triglyceride Concentrations in Male Rats

The purpose of this investigation was to assess the influence of calorie restriction (CR) alone, higher-protein/lower-carbohydrate intake alone, and combined CR higher-protein/lower-carbohydrate intake on glucose homeostasis, hepatic de novo lipogenesis (DNL), and intrahepatic triglycerides. Twelve-week old male Sprague Dawley rats consumed ad libitum (AL) or CR (40% restriction), adequate (10%), or high (32%) protein (PRO) milk-based diets for 16 weeks. Metabolic profiles were assessed in serum, and intrahepatic triglyceride concentrations and molecular markers of de novo lipogenesis were determined in liver. Independent of calorie intake, 32% PRO tended to result in lower homeostatic model assessment of insulin resistance (HOMA-IR) values compared to 10% PRO, while insulin and homeostatic model assessment of β-cell function (HOMA-β) values were lower in CR than AL, regardless of protein intake. Intrahepatic triglyceride concentrations were 27.4 ± 4.5 and 11.7 ± 4.5 µmol·g⁻¹ lower (p < 0.05) in CR and 32% PRO compared to AL and 10% PRO, respectively. Gene expression of fatty acid synthase (FASN), stearoyl-CoA destaurase-1 (SCD1) and pyruvate dehydrogenase kinase, isozyme 4 (PDK4) were 45% ± 1%, 23% ± 1%, and 57% ± 1% lower (p < 0.05), respectively, in CR than AL, regardless of protein intake. Total protein of FASN and SCD were 50% ± 1% and 26% ± 1% lower (p < 0.05) in 32% PRO compared to 10% PRO, independent of calorie intake. Results from this investigation provide evidence that the metabolic health benefits associated with CR—specifically reduction in intrahepatic triglyceride content—may be enhanced by consuming a higher-protein/lower-carbohydrate diet.

Perspective: The Potential Role of Essential Amino Acids and the Mechanistic Target of Rapamycin Complex 1 (mTORC1) Pathway in the Pathogenesis of Child Stunting

Stunting is the best summary measure of chronic malnutrition in children. Approximately one-quarter of children under age 5 worldwide are stunted. Lipid-based or micronutrient supplementation has little to no impact in reducing stunting, which suggests that other critical dietary nutrients are missing. A dietary pattern of poor-quality protein is associated with stunting. Stunted children have significantly lower circulating essential amino acids than do nonstunted children. Inadequate dietary intakes of essential amino acids could adversely affect growth, because amino acids are required for synthesis of proteins. The master growth regulation pathway, the mechanistic target of rapamycin complex 1 (mTORC1) pathway, is exquisitely sensitive to amino acid availability. mTORC1 integrates cues such as nutrients, growth factors, oxygen, and energy to regulate growth of bone, skeletal muscle, nervous system, gastrointestinal tract, hematopoietic cells, immune effector cells, organ size, and whole-body energy balance. mTORC1 represses protein and lipid synthesis and cell and organismal growth when amino acids are deficient. Over the past 4 decades, the main paradigm for child nutrition in developing countries has been micronutrient malnutrition, with relatively less attention paid to protein. In this Perspective, we present the view that essential amino acids and the mTORC1 pathway play a key role in child growth. The current assumption that total dietary protein intake is adequate for growth among most children in developing countries needs re-evaluation.

The positive association of branched-chain amino acids and metabolic dyslipidemia in Chinese Han population

Background

It has been suggested that serum branched-chain amino acids (BCAAs) are associated with the incident, progression and prognostic of type 2 diabetes. However, the role of BCAAs in metabolic dyslipidemia (raised triglycerides (TG) and reduced high-density lipoprotein cholesterol (HDL-C)) remains poorly understood. This study aims to investigate 1) the association of serum BCAAs with total cholesterol (TC), TG, HDL-C and low-density lipoprotein cholesterol (LDL-C) and 2) the association between serum BCAAs levels and risk of metabolic dyslipidemia in a community population with different glucose homeostasis.

Methods

Demographics data and blood samples were collected from 2251 Chinese subjects from the Huaian Diabetes Protective Program (HADPP) study. After exclusion for cardiovascular disease (CVD), serious hepatic or nephritic diseases and others, 1320 subjects remained for analysis (789 subjects with hemoglobin A1c (HbA1c) > 5.7, 521 with HbA1c ≤ 5.7). Serum BCAAs level was measured by liquid chromatography-tandem mass spectrometry (LC MS/MS). The association of BCAAs with lipids or with the risk of metabolic dyslipidemia was analyzed.

Results

Elevated serum BCAAs (both total and individual BCAA) were positively associated with TG and inversely associated with HDL-C in the whole population. These correlations were still significant even after adjustment for confounding factors (r = 0.165, p < 0.001 for TG; and r = -0.126, p < 0.001 for HDL-C). For reduced HDL-C, we found higher odds risk (OR) of Valine (Val) in high HbA1c group than in the low one (OR = 1.055, p < 0.001 vs OR = 1.032, p = 0.059). Compared with that in the first quartile, the multivariable-adjusted OR (95 % CI) of the 4^th quartile of serum total BCAAs level for reduced HDL-C was 3.689 (2.325, 5.854) in high HbA1c group and 2.329 (1.284, 4.227) in low group, for raised TG was 3.305 (2.120, 5.152) and 2.972 (1.706, 5.176), and for metabolic dyslipidemia was 3.703 (2.261, 6.065) and 3.702 (1.877, 7.304), respectively (all p < 0.01).

Conclusion

Elevated serum BCAAs level are positively associated with incident metabolic dyslipidemia. In addition, glucose homeostasis could play a certain role in BCAAs-related dyslipidemia.

Melatonin-Mediated Intracellular Insulin during 2-Deoxy-d-glucose Treatment Is Reduced through Autophagy and EDC3 Protein in Insulinoma INS-1E Cells

2-DG triggers glucose deprivation without altering other nutrients or metabolic pathways and then activates autophagy via activation of AMPK and endoplasmic reticulum (ER) stress. We investigated whether 2-DG reduced intracellular insulin increased by melatonin via autophagy/EDC3 in insulinoma INS-1E cells. p-AMPK and GRP78/BiP level were significantly increased by 2-DG in the presence/absence of melatonin, but IRE1α level was reduced in 2-DG treatment. Levels of p85α, p110, p-Akt (Ser473, Thr308), and p-mTOR (Ser2481) were also significantly reduced by 2-DG in the presence/absence of melatonin. Mn-SOD increased with 2-DG plus melatonin compared to groups treated with/without melatonin alone. Bcl-2 was decreased and Bax increased with 2-DG plus melatonin. LC3II level increased with 2-DG treatment in the presence/absence of melatonin. Intracellular insulin production increased in melatonin plus 2-DG but reduced in treatment with 2-DG with/without melatonin. EDC3 was increased by 2-DG in the presence/absence of melatonin. Rapamycin, an mTOR inhibitor, increased GRP78/BiP and EDC3 levels in a dose-dependent manner and subsequently resulted in a decrease in intracellular production of insulin. These results suggest that melatonin-mediated insulin synthesis during 2-DG treatment involves autophagy and EDC3 protein in rat insulinoma INS-1E cells and subsequently results in a decrease in intracellular production of insulin.

Cumulative consumption of branched-chain amino acids and incidence of type 2 diabetes

BACKGROUND

Plasma branched-chain amino acids (BCAAs, including leucine, isoleucine and valine) were recently related to risk of type 2 diabetes (T2D). Dietary intake is the only source of BCAAs; however, little is known about whether habitual dietary intake of BCAAs affects risk of T2D.

METHODS

We assessed associations between cumulative consumption of BCAAs and risk of T2D among participants from three prospective cohorts: the Nurses' Health Study (NHS; followed from 1980 to 2012); NHS II (followed from 1991 to 2011); and the Health Professionals Follow-up Study (HPFS; followed from 1986 to 2010).

RESULTS

We documented 16 097 incident T2D events during up to 32 years of follow-up. After adjustment for demographics and traditional risk factors, higher total BCAA intake was associated with an increased risk of T2D in men and women. In the meta-analysis of all cohorts, comparing participants in the highest quintile with those in the lowest quintile of intake, hazard ratios (95%confidence intervals) were for leucine 1.13 (1.07-1.19), for isoleucine 1.13 (1.07-1.19) and for valine 1.11 (1.05-1.17) (all P for trend < 0.001). In a healthy subsample, higher dietary BCAAs were significantly associated with higher plasma levels of these amino acids (P for trend = 0.01).

CONCLUSIONS

Our data suggest that high consumption of BCAAs is associated with an increased risk of T2D.

Metabolic Responses to Dietary Protein Restriction Require an Increase in FGF21 that Is Delayed by the Absence of GCN2

FGF21 contributes to the metabolic response to dietary protein restriction, and prior data implicate GCN2 as the amino acid sensor linking protein restriction to FGF21 induction. Here, we demonstrate the persistent and essential role of FGF21 in the metabolic response to protein restriction. We show that Fgf21 KO mice are fully resistant to low protein (LP)-induced changes in food intake, energy expenditure (EE), body weight gain, and metabolic gene expression for 6 months. Gcn2 KO mice recapitulate this phenotype, but LP-induced effects on food intake, EE, and body weight subsequently begin to appear after 14 days on diet. We show that this delayed emergence of LP-induced metabolic effects in Gcn2 KO mice coincides with a delayed but progressive increase of hepatic Fgf21 expression and blood FGF21 concentrations over time. These data indicate that FGF21 is essential for the metabolic response to protein restriction but that GCN2 is only transiently required for LP-induced FGF21.

Decreased Consumption of Branched-Chain Amino Acids Improves Metabolic Health

Protein-restricted (PR), high-carbohydrate diets improve metabolic health in rodents, yet the precise dietary components that are responsible for these effects have not been identified. Furthermore, the applicability of these studies to humans is unclear. Here, we demonstrate in a randomized controlled trial that a moderate PR diet also improves markers of metabolic health in humans. Intriguingly, we find that feeding mice a diet specifically reduced in branched-chain amino acids (BCAAs) is sufficient to improve glucose tolerance and body composition equivalently to a PR diet via metabolically distinct pathways. Our results highlight a critical role for dietary quality at the level of amino acids in the maintenance of metabolic health and suggest that diets specifically reduced in BCAAs, or pharmacological interventions in this pathway, may offer a translatable way to achieve many of the metabolic benefits of a PR diet.

GCN2 contributes to mTORC1 inhibition by leucine deprivation through an ATF4 independent mechanism

It is well known that the GCN2 and mTORC1 signaling pathways are regulated by amino acids and share common functions, in particular the control of translation. The regulation of GCN2 activity by amino acid availability relies on the capacity of GCN2 to sense the increased levels of uncharged tRNAs upon amino acid scarcity. In contrast, despite recent progress in the understanding of the regulation of mTORC1 by amino acids, key aspects of this process remain unsolved. In particular, while leucine is well known to be a potent regulator of mTORC1, the mechanisms by which this amino acid is sensed and control mTORC1 activity are not well defined. Our data establish that GCN2 is involved in the inhibition of mTORC1 upon leucine or arginine deprivation. However, the activation of GCN2 alone is not sufficient to inhibit mTORC1 activity, indicating that leucine and arginine exert regulation via additional mechanisms. While the mechanism by which GCN2 contributes to the initial step of mTORC1 inhibition involves the phosphorylation of eIF2α, we show that it is independent of the downstream transcription factor ATF4. These data point to a novel role for GCN2 and phosphorylation of eIF2α in the control of mTORC1 by certain amino acids.

A rule-based model of insulin signalling pathway

BACKGROUND

The insulin signalling pathway (ISP) is an important biochemical pathway, which regulates some fundamental biological functions such as glucose and lipid metabolism, protein synthesis, cell proliferation, cell differentiation and apoptosis. In the last years, different mathematical models based on ordinary differential equations have been proposed in the literature to describe specific features of the ISP, thus providing a description of the behaviour of the system and its emerging properties. However, protein-protein interactions potentially generate a multiplicity of distinct chemical species, an issue referred to as "combinatorial complexity", which results in defining a high number of state variables equal to the number of possible protein modifications. This often leads to complex, error prone and difficult to handle model definitions.

RESULTS

In this work, we present a comprehensive model of the ISP, which integrates three models previously available in the literature by using the rule-based modelling (RBM) approach. RBM allows for a simple description of a number of signalling pathway characteristics, such as the phosphorylation of signalling proteins at multiple sites with different effects, the simultaneous interaction of many molecules of the signalling pathways with several binding partners, and the information about subcellular localization where reactions take place. Thanks to its modularity, it also allows an easy integration of different pathways. After RBM specification, we simulated the dynamic behaviour of the ISP model and validated it using experimental data. We the examined the predicted profiles of all the active species and clustered them in four clusters according to their dynamic behaviour. Finally, we used parametric sensitivity analysis to show the role of negative feedback loops in controlling the robustness of the system.

CONCLUSIONS

The presented ISP model is a powerful tool for data simulation and can be used in combination with experimental approaches to guide the experimental design. The model is available at http://sysbiobig.dei.unipd.it/ was submitted to Biomodels Database ( https://www.ebi.ac.uk/biomodels-main/ # MODEL 1604100005).

Hepatic autophagy contributes to the metabolic response to dietary protein restriction

Autophagy is an essential cellular response which acts to release stored cellular substrates during nutrient restriction, and particularly plays a key role in the cellular response to amino acid restriction. However, there has been limited work testing whether the induction of autophagy is required for adaptive metabolic responses to dietary protein restriction in the whole animal. Here, we found that moderate dietary protein restriction led to a series of metabolic changes in rats, including increases in food intake and energy expenditure, the downregulation of hepatic fatty acid synthesis gene expression and reduced markers of hepatic mitochondrial number. Importantly, these effects were also associated with an induction of hepatic autophagy. To determine if the induction of autophagy contributes to these metabolic effects, we tested the metabolic response to dietary protein restriction in BCL2-AAA mice, which bear a genetic mutation that impairs autophagy induction. Interestingly, BCL2-AAA mice exhibit exaggerated responses in terms of both food intake and energy expenditure, whereas the effects of protein restriction on hepatic metabolism were significantly blunted. These data demonstrate that restriction of dietary protein is sufficient to trigger hepatic autophagy, and that disruption of autophagy significantly alters both hepatic and whole animal metabolic response to dietary protein restriction.

Role of GCN2-Independent Signaling Through a Noncanonical PERK/NRF2 Pathway in the Physiological Responses to Dietary Methionine Restriction

Restricting availability of essential amino acids (EAAs) limits aminoacylation of tRNAs by their cognate EAAs and activates the nutrient-sensing kinase, general control nonderepressible 2 (GCN2). Activated GCN2 phosphorylates eukaryotic initiation factor 2 (eIF2), altering gene-specific translation and initiating a transcriptional program collectively described as the integrated stress response (ISR). Central GCN2 activation by EAA deprivation is also linked to an acute aversive feeding response. Dietary methionine restriction (MR) produces a well-documented series of physiological responses (increased energy intake and expenditure, decreased adiposity, and increased insulin sensitivity), but the role of GCN2 in mediating them is unknown. Using Gcn2(-/-) mice, we found that the absence of GCN2 had no effect on the ability of MR to reduce body weight or adiposity, increase energy intake and expenditure, increase hepatic transcription and release of fibroblast growth factor 21, or improve insulin sensitivity. Interestingly, hepatic eIF2 phosphorylation by MR was uncompromised in Gcn2(-/-) mice. Instead, protein kinase R-like endoplasmic reticulum (ER) kinase (PERK) was activated in both intact and Gcn2(-/-) mice. PERK activation corresponded with induction of the ISR and the nuclear respiratory factor 2 antioxidant program but not ER stress. These data uncover a novel glutathione-sensing mechanism that functions independently of GCN2 to link dietary MR to its metabolic phenotype.

BTG1 ameliorates liver steatosis by decreasing stearoyl-CoA desaturase 1 (SCD1) abundance and altering hepatic lipid metabolism

Liver steatosis, a condition in which lipid accumulates in liver cells, is a leading cause of many liver diseases. The livers of patients with hepatocellular carcinoma, a cancer characterized by liver steatosis, have decreased abundance of the transcription cofactor BTG1 (B cell translocation gene 1). We showed that the livers of db/db mice, which are a genetic model of obesity, had decreased BTG1 mRNA and protein abundance. BTG1 overexpression ameliorated liver steatosis in db/db mice, whereas knockdown of BTG1 induced liver steatosis in wild-type mice. Consistent with these changes, we found that BTG1 decreased triglyceride accumulation in cultured hepatocytes. BTG1 overexpression inhibited the expression of the gene encoding stearoyl-CoA desaturase 1 (SCD1), an enzyme involved in the synthesis of fatty acids, by suppressing the activity of activating transcription factor 4 (ATF4). Knockdown of SCD1 prevented liver steatosis in wild-type mice induced by knockdown of BTG1. Conversely, the ability of BTG1 overexpression to ameliorate liver steatosis in db/db mice was negated by ATF4 overexpression. Moreover, BTG1 transgenic mice were resistant to liver steatosis induced by a high-carbohydrate diet. BTG1 abundance was decreased by this diet through a pathway that involved mammalian target of rapamycin (mTOR), ribosomal protein S6 kinase 1 (S6K1), and cAMP response element-binding protein (CREB). Together, our study identifies a role of BTG1 in regulating hepatic lipid metabolism and specifically in preventing ATF4 and SCD1 from inducing liver steatosis.

Inhibition of the Mechanistic Target of Rapamycin (mTOR)-Rapamycin and Beyond

Rapamycin is a Food and Drug Administration (FDA)-approved immunosuppressant and anticancer agent discovered in the soil of Easter Island in the early 1970s. Rapamycin is a potent and selective inhibitor of the mechanistic target of rapamycin (mTOR) protein kinase, which acts as a central integrator of nutrient signaling pathways. During the last decade, genetic and pharmaceutical inhibition of mTOR pathway signaling has been found to promote longevity in yeast, worms, flies, and mice. In this article, we will discuss the molecular biology underlying the effects of rapamycin and its physiological effects, evidence for rapamycin as an antiaging compound, mechanisms by which rapamycin may extend life span, and the potential limitations of rapamycin as an antiaging molecule. Finally, we will discuss possible strategies that may allow us to inhibit mTOR signaling safely while minimizing side effects, and reap the health, social, and economic benefits from slowing the aging process.

Mechanisms Linking the Gut Microbiome and Glucose Metabolism

This review details potential mechanisms linking gut dysbiosis to metabolic dysfunction, including lipopolysaccharide, bile acids, short chain fatty acids, gut hormones, and branched-chain amino acids.

β-aminoisobutyric acid attenuates hepatic endoplasmic reticulum stress and glucose/lipid metabolic disturbance in mice with type 2 diabetes

β-aminoisobutyric acid (BAIBA) is a nature thymine catabolite, and contributes to exercise-induced protection from metabolic diseases. Here we show the therapeutical effects of BAIBA on hepatic endoplasmic reticulum (ER) stress and glucose/lipid metabolic disturbance in diabetes. Type 2 diabetes was induced by combined streptozotocin (STZ) and high-fat diet (HFD) in mice. Oral administration of BAIBA for 4 weeks reduced blood glucose and lipids levels, hepatic key enzymes of gluconeogenesis and lipogenesis expressions, attenuated hepatic insulin resistance and lipid accumulation, and improved insulin signaling in type 2 diabetic mice. BAIBA reduced hepatic ER stress and apoptosis in type 2 diabetic mice. Furthermore, BAIBA alleviated ER stress in human hepatocellular carcinoma (HepG2) cells with glucosamine-induced insulin resistance. Hepatic AMPK phosphorylation was reduced in STZ/HFD mice and glucosamine-treated HepG2 cells, which were restored by BAIBA treatment. The suppressive effects of BAIBA on glucosamine-induced ER stress were reversed by knockdown of AMPK with siRNA. In addition, BAIBA prevented thapsigargin- or tunicamycin-induced ER stress, and tunicamycin–induced apoptosis in HepG2 cells. These results indicate that BAIBA attenuates hepatic ER stress, apoptosis and glucose/lipid metabolic disturbance in mice with type 2 diabetes. AMPK signaling is involved to the role of BAIBA in attenuating ER stress.

ATF4 induction through an atypical integrated stress response to ONC201 triggers p53-independent apoptosis in hematological malignancies

The clinical challenge posed by p53 abnormalities in hematological malignancies requires therapeutic strategies other than standard genotoxic chemotherapies. ONC201 is a first-in-class small molecule that activates p53-independent apoptosis, has a benign safety profile, and is in early clinical trials. We found that ONC201 caused p53-independent apoptosis and cell cycle arrest in cell lines and in mantle cell lymphoma (MCL) and acute myeloid leukemia (AML) samples from patients; these included samples from patients with genetic abnormalities associated with poor prognosis or cells that had developed resistance to the nongenotoxic agents ibrutinib and bortezomib. Moreover, ONC201 caused apoptosis in stem and progenitor AML cells and abrogated the engraftment of leukemic stem cells in mice while sparing normal bone marrow cells. ONC201 caused changes in gene expression similar to those caused by the unfolded protein response (UPR) and integrated stress responses (ISRs), which increase the translation of the transcription factor ATF4 through an increase in the phosphorylation of the translation initiation factor eIF2α. However, unlike the UPR and ISR, the increase in ATF4 abundance in ONC201-treated hematopoietic cells promoted apoptosis and did not depend on increased phosphorylation of eIF2α. ONC201 also inhibited mammalian target of rapamycin complex 1 (mTORC1) signaling, likely through ATF4-mediated induction of the mTORC1 inhibitor DDIT4. Overexpression of BCL-2 protected against ONC201-induced apoptosis, and the combination of ONC201 and the BCL-2 antagonist ABT-199 synergistically increased apoptosis. Thus, our results suggest that by inducing an atypical ISR and p53-independent apoptosis, ONC201 has clinical potential in hematological malignancies.

Leucine supplementation via drinking water reduces atherosclerotic lesions in apoE null mice

AIM

Recent evidence suggests that the essential amino acid leucine may be involved in systemic cholesterol metabolism. In this study, we investigated the effects of leucine supplementation on the development of atherosclerosis in apoE null mice.

METHODS

ApoE null mice were fed with chow supplemented with leucine (1.5% w/v) in drinking water for 8 week. Aortic atherosclerotic lesions were examined using Oil Red O staining. Plasma lipoprotein-cholesterol levels were measured with fast protein liquid chromatography. Hepatic gene expression was detected using real-time PCR and Western blot analyses.

RESULTS

Leucine supplementation resulted in 57.6% reduction of aortic atherosclerotic lesion area in apoE null mice, accompanied by 41.2% decrease of serum LDL-C levels and 40.2% increase of serum HDL-C levels. The body weight, food intake and blood glucose level were not affected by leucine supplementation. Furthermore, leucine supplementation increased the expression of Abcg5 and Abcg8 (that were involved in hepatic cholesterol efflux) by 1.28- and 0.86-fold, respectively, and significantly increased their protein levels. Leucine supplementation also increased the expression of Srebf1, Scd1 and Pgc1b (that were involved in hepatic triglyceride metabolism) by 3.73-, 1.35- and 1.71-fold, respectively. Consequently, leucine supplementation resulted in 51.77% reduction of liver cholesterol content and 2.2-fold increase of liver triglyceride content. Additionally, leucine supplementation did not affect the serum levels of IL-6, IFN-γ, TNF-α, IL-10 and IL-12, but markedly decreased the serum level of MCP-1.

CONCLUSION

Leucine supplementation effectively attenuates atherosclerosis in apoE null mice by improving the plasma lipid profile and reducing systemic inflammation.

The conserved role of protein restriction in aging and disease

PURPOSE OF REVIEW

Dietary interventions are effective strategies for preventing disease and promoting health span. Many of the effects of dietary restriction are linked to amino acid and protein levels and their regulation of nutrient-signaling pathways. Thus, protein restriction is a promising therapeutic strategy for preventing aging-related diseases and extending life span.

RECENT FINDINGS

Studies in yeast and flies have shown that amino acid restriction promotes longevity and protection. In rodents, protein restriction extends life span and alleviates detrimental aging phenotypes. Finally, clinical trials in middle-aged adults have demonstrated the role of a protein-restricted diet in promoting health span. Interestingly, the population over the age of 65 may not benefit from severe protein restriction potentially because of the increased physiological decline that leads to decreased amino acid absorption and altered protein synthesis.

SUMMARY

Protein restriction can have profound effects on health and longevity, but excessive restriction is detrimental, particularly in the very old. The investigation of the mechanisms that modulate nutrient-sensing pathways is important to understand how regulation of protein intake can optimize health span and longevity.

Metabolic responses to dietary leucine restriction involve remodeling of adipose tissue and enhanced hepatic insulin signaling

Dietary leucine was incrementally restricted to test whether limiting this essential amino acid (EAA) would fully reproduce the beneficial responses produced by dietary methionine restriction. Restricting leucine by 85% increased energy intake and expenditure within 5 to 7 days of its introduction and reduced overall accumulation of adipose tissue. Leucine restriction (LR) also improved glucose tolerance, increased hepatic release of fibroblast growth factor 21 into the blood stream, and enhanced insulin-dependent activation of Akt in liver. However, LR had no effect on hepatic lipid levels and failed to lower lipogenic gene expression in the liver. LR did affect remodeling of white and brown adipose tissues, increasing expression of both thermogenic and lipogenic genes. These findings illustrate that dietary LR reproduces many but not all of the physiological responses of methionine restriction. The primary differences occur in the liver, where methionine and LR cause opposite effects on tissue lipid levels and expression of lipogenic genes. Altogether, these findings suggest that the sensing systems which detect and respond to dietary restriction of EAAs act through mechanisms that both leucine and methionine are able to engage, and in the case of hepatic lipid metabolism, may be unique to specific EAAs such as methionine.

Regulation of autophagy by amino acids and MTOR-dependent signal transduction

Amino acids not only participate in intermediary metabolism but also stimulate insulin-mechanistic target of rapamycin (MTOR)-mediated signal transduction which controls the major metabolic pathways. Among these is the pathway of autophagy which takes care of the degradation of long-lived proteins and of the elimination of damaged or functionally redundant organelles. Proper functioning of this process is essential for cell survival. Dysregulation of autophagy has been implicated in the etiology of several pathologies. The history of the studies on the interrelationship between amino acids, MTOR signaling and autophagy is the subject of this review. The mechanisms responsible for the stimulation of MTOR-mediated signaling, and the inhibition of autophagy, by amino acids have been studied intensively in the past but are still not completely clarified. Recent developments in this field are discussed.

High dietary protein decreases fat deposition induced by high-fat and high-sucrose diet in rats

High-protein diets are known to reduce adiposity in the context of high carbohydrate and Western diets. However, few studies have investigated the specific high-protein effect on lipogenesis induced by a high-sucrose (HS) diet or fat deposition induced by high-fat feeding. We aimed to determine the effects of high protein intake on the development of fat deposition and partitioning in response to high-fat and/or HS feeding. A total of thirty adult male Wistar rats were assigned to one of the six dietary regimens with low and high protein, sucrose and fat contents for 5 weeks. Body weight (BW) and food intake were measured weekly. Oral glucose tolerance tests and meal tolerance tests were performed after 4th and 5th weeks of the regimen, respectively. At the end of the study, the rats were killed 2 h after ingestion of a calibrated meal. Blood, tissues and organs were collected for analysis of circulating metabolites and hormones, body composition and mRNA expression in the liver and adipose tissues. No changes were observed in cumulative energy intake and BW gain after 5 weeks of dietary treatment. However, high-protein diets reduced by 20 % the adiposity gain induced by HS and high-sucrose high-fat (HS-HF) diets. Gene expression and transcriptomic analysis suggested that high protein intake reduced liver capacity for lipogenesis by reducing mRNA expressions of fatty acid synthase (fasn), acetyl-CoA carboxylase a and b (Acaca and Acacb) and sterol regulatory element binding transcription factor 1c (Srebf-1c). Moreover, ketogenesis, as indicated by plasma β-hydroxybutyrate levels, was higher in HS-HF-fed mice that were also fed high protein levels. Taken together, these results suggest that high-protein diets may reduce adiposity by inhibiting lipogenesis and stimulating ketogenesis in the liver.

Adaptive changes in amino acid metabolism permit normal longevity in mice consuming a low-carbohydrate ketogenic diet

Ingestion of very low-carbohydrate ketogenic diets (KD) is associated with weight loss, lowering of glucose and insulin levels and improved systemic insulin sensitivity. However, the beneficial effects of long-term feeding have been the subject of debate. We therefore studied the effects of lifelong consumption of this diet in mice. Complete metabolic analyses were performed after 8 and 80weeks on the diet. In addition we performed a serum metabolomic analysis and examined hepatic gene expression. Lifelong consumption of KD had no effect on morbidity or mortality (KD vs. Chow, 676 vs. 630days) despite hepatic steatosis and inflammation in KD mice. The KD fed mice lost weight initially as previously reported (Kennnedy et al., 2007) and remained lighter and had less fat mass; KD consuming mice had higher levels of energy expenditure, improved glucose homeostasis and higher circulating levels of β-hydroxybutyrate and triglycerides than chow-fed controls. Hepatic expression of the critical metabolic regulators including fibroblast growth factor 21 were also higher in KD-fed mice while expression levels of lipogenic enzymes such as stearoyl-CoA desaturase-1 was reduced. Metabolomic analysis revealed compensatory changes in amino acid metabolism, primarily involving down-regulation of catabolic processes, demonstrating that mice eating KD can shift amino acid metabolism to conserve amino acid levels. Long-term KD feeding caused profound and persistent metabolic changes, the majority of which are seen as health promoting, and had no adverse effects on survival in mice.

AMPK binds to Sestrins and mediates the effect of exercise to increase insulin-sensitivity through autophagy

OBJECTIVE

Exercise has beneficial effects on human health, including protection against metabolic disorders such as diabetes. However, the cellular mechanisms underlying the metabolic effects of exercise are not completely understood. We know that autophagy plays an important role in maintaining cellular homeostasis. In this study, we aimed to better understand the metabolic effects of autophagy in skeletal muscle and the role of Sestrins/AMPK in mediating the beneficial effects of physical exercise through autophagy.

MATERIAL/METHODS

We used wild type and AMPKα2(-/-) C57BL/6 mice as animal models to elucidate the role of AMPK in autophagy activation and the metabolism-promoting effects of acute and regular exercise. C2C12 myotubes were used to study the metabolic effects of autophagy in vitro.

RESULTS

Autophagy promotes glucose uptake in skeletal muscle. A single bout of exercise increased the activity of autophagy in the skeletal muscle of wild type mice but not of AMPKα2(-/-) mice. This difference was associated with increased amounts of both Sestrin2 and Sestrin3 coimmunoprecipitated with AMPKα2. Long-term physical exercise significantly increased the basal level of muscle autophagy and protein expression of Sestrin2 and Sestrin3 in both normal chow and high-fat diet-fed mice.

CONCLUSION

We believe that exercise-induced AMPK and Sestrins interaction may be involved in the beneficial metabolic effects of exercise by activating autophagy. This interaction provides a molecular mechanism that is a potential target in metabolic syndromes.

Reviewing the Effects of L-Leucine Supplementation in the Regulation of Food Intake, Energy Balance, and Glucose Homeostasis

Leucine is a well-known activator of the mammalian target of rapamycin (mTOR). Because mTOR signaling regulates several aspects of metabolism, the potential of leucine as a dietary supplement for treating obesity and diabetes mellitus has been investigated. The objective of the present review was to summarize and discuss the available evidence regarding the mechanisms and the effects of leucine supplementation on the regulation of food intake, energy balance, and glucose homeostasis. Based on the available evidence, we conclude that although central leucine injection decreases food intake, this effect is not well reproduced when leucine is provided as a dietary supplement. Consequently, no robust evidence indicates that oral leucine supplementation significantly affects food intake, although several studies have shown that leucine supplementation may help to decrease body adiposity in specific conditions. However, more studies are necessary to assess the effects of leucine supplementation in already-obese subjects. Finally, although several studies have found that leucine supplementation improves glucose homeostasis, the underlying mechanisms involved in these potential beneficial effects remain unknown and may be partially dependent on weight loss.

Hepatic Phosphoserine Aminotransferase 1 Regulates Insulin Sensitivity in Mice via Tribbles Homolog 3

Phosphoserine aminotransferase 1 (PSAT1) is an enzyme participating in serine synthesis. A role of PSAT1 in the regulation of insulin sensitivity, however, is unknown. In this study, we showed that hepatic PSAT1 expression and liver serine levels are reduced in genetically engineered leptin receptor-deficient (db/db) mice and high-fat diet (HFD)-induced diabetic mice. Additionally, overexpression of PSAT1 by adenovirus expressing PSAT1 improved insulin signaling and insulin sensitivity in vitro and in vivo under normal conditions. Opposite effects were observed when PSAT1 was knocked down by adenovirus expressing small hairpin RNA specific for PSAT1 (Ad-shPSAT1). Importantly, overexpression of PSAT1 also significantly ameliorated insulin resistance in diabetic mice. In addition, PSAT1 inhibited the expression of hepatic tribbles homolog 3 (TRB3) in vitro and in vivo, and adenoviruses expressing small hairpin RNA against TRB3-mediated inhibition of TRB3 reversed the attenuated insulin sensitivity in Ad-shPSAT1 mice. Interestingly, we found that serine mediates PSAT1 regulation of TRB3 expression and insulin signaling in vitro. These results identify a novel function for hepatic PSAT1 in regulating insulin sensitivity and provide important insights in targeting PSAT1 for treating insulin resistance and type 2 diabetes. Our results also suggest that nonessential amino acid serine may play an important role in regulating insulin sensitivity.

Mice lacking neutral amino acid transporter B(0)AT1 (Slc6a19) have elevated levels of FGF21 and GLP-1 and improved glycaemic control

OBJECTIVE

Type 2 diabetes arises from insulin resistance of peripheral tissues followed by dysfunction of β-cells in the pancreas due to metabolic stress. Both depletion and supplementation of neutral amino acids have been discussed as strategies to improve insulin sensitivity. Here we characterise mice lacking the intestinal and renal neutral amino acid transporter B(0)AT1 (Slc6a19) as a model to study the consequences of selective depletion of neutral amino acids.

METHODS

Metabolic tests, analysis of metabolite levels and signalling pathways were used to characterise mice lacking the intestinal and renal neutral amino acid transporter B(0)AT1 (Slc6a19).

RESULTS

Reduced uptake of neutral amino acids in the intestine and loss of neutral amino acids in the urine causes an overload of amino acids in the lumen of the intestine and reduced systemic amino acid availability. As a result, higher levels of glucagon-like peptide 1 (GLP-1) are produced by the intestine after a meal, while the liver releases the starvation hormone fibroblast growth factor 21 (FGF21). The combination of these hormones generates a metabolic phenotype that is characterised by efficient removal of glucose, particularly by the heart, reduced adipose tissue mass, browning of subcutaneous white adipose tissue, enhanced production of ketone bodies and reduced hepatic glucose output.

CONCLUSIONS

Reduced neutral amino acid availability improves glycaemic control. The epithelial neutral amino acid transporter B(0)AT1 could be a suitable target to treat type 2 diabetes.

The role of microbial amino acid metabolism in host metabolism

Disruptions in gut microbiota composition and function are increasingly implicated in the pathogenesis of obesity, insulin resistance, and type 2 diabetes mellitus. The functional output of the gut microbiota, including short-chain fatty acids and amino acids, are thought to be important modulators underlying the development of these disorders. Gut bacteria can alter the bioavailability of amino acids by utilization of several amino acids originating from both alimentary and endogenous proteins. In turn, gut bacteria also provide amino acids to the host. This could have significant implications in the context of insulin resistance and type 2 diabetes mellitus, conditions associated with elevated systemic concentrations of certain amino acids, in particular the aromatic and branched-chain amino acids. Moreover, several amino acids released by gut bacteria can serve as precursors for the synthesis of short-chain fatty acids, which also play a role in the development of obesity. In this review, we aim to compile the available evidence on the contribution of microbial amino acids to host amino acid homeostasis, and to assess the role of the gut microbiota as a determinant of amino acid and short-chain fatty acid perturbations in human obesity and type 2 diabetes mellitus.

Wnt-Lrp5 signaling regulates fatty acid metabolism in the osteoblast

The Wnt coreceptors Lrp5 and Lrp6 are essential for normal postnatal bone accrual and osteoblast function. In this study, we identify a previously unrecognized skeletal function unique to Lrp5 that enables osteoblasts to oxidize fatty acids. Mice lacking the Lrp5 coreceptor specifically in osteoblasts and osteocytes exhibit the expected reductions in postnatal bone mass but also exhibit an increase in body fat with corresponding reductions in energy expenditure. Conversely, mice expressing a high bone mass mutant Lrp5 allele are leaner with reduced plasma triglyceride and free fatty acid levels. In this context, Wnt-initiated signals downstream of Lrp5, but not the closely related Lrp6 coreceptor, regulate the activation of β-catenin and thereby induce the expression of key enzymes required for fatty acid β-oxidation. These results suggest that Wnt-Lrp5 signaling regulates basic cellular activities beyond those associated with fate specification and differentiation in bone and that the skeleton influences global energy homeostasis via mechanisms independent of osteocalcin and glucose metabolism.

Hepatic serum- and glucocorticoid-regulated protein kinase 1 (SGK1) regulates insulin sensitivity in mice via extracellular-signal-regulated kinase 1/2 (ERK1/2)

Insulin resistance is a major hallmark of metabolic syndromes, including Type 2 diabetes. Although numerous functions of SGK1 (serum- and glucocorticoid-regulated kinase 1) have been identified, a direct effect of SGK1 on insulin sensitivity has not been previously reported. In the present study, we generated liver-specific SGK1-knockout mice and found that these mice developed glucose intolerance and insulin resistance. We also found that insulin signalling is enhanced or impaired in Hep1-6 cells infected with adenoviruses expressing SGK1 (Ad-SGK1) or shRNA directed against the coding region of SGK1 (Ad-shSGK1) respectively. In addition, we determined that SGK1 inhibits ERK1/2 (extracellular-signal-regulated kinase 1/2) activity in liver and Ad-shERK1/2-mediated inhibition of ERK1/2 reverses the attenuated insulin sensitivity in Ad-shSGK1 mice. Finally, we found that SGK1 functions are compromised under insulin-resistant conditions and overexpression of SGK1 by Ad-SGK1 significantly ameliorates insulin resistance in both glucosamine-treated HepG2 cells and livers of db/db mice, a genetic model of insulin resistance.

Preoperative oral carbohydrate improved postoperative insulin resistance in rats through the PI3K/AKT/mTOR pathway

Background

Preoperative oral carbohydrate (OCH) improves postoperative insulin resistance (PIR) and insulin sensitivity. However, the exact mechanisms involved in the improvement of PIR with respect to preoperative OCH are still not clear. The aim of this study was to investigate the involvement of preoperative OCH and PI3K/AKT/mTOR pathway in reducing PIR in rats.

Material/Methods

Forty male Sprague-Dawley rats were randomly assigned to PreOp, glucose, saline, and fasting groups. Rats in the PreOp, glucose, and saline groups received OCH, 5% glucose solution, and saline, respectively. Rats in the fasting group did not receive anything but were fasted 3 h before surgery. Blood glucose, insulin and leucine levels, and insulin resistance, secretion, and sensitivity indexes were measured before and after surgery. mRNA and protein (total and phosphorylated) levels of mTOR, IRS-1, PI3K, PKB/AKT, and GlUT4 were measured using real-time polymerase chain reaction and Western blot in skeletal muscles.

Results

In the PIR experiment, blood glucose, serum insulin, insulin resistance, and serum leucine levels were all significantly lower in the PreOp group than in the other 3 groups (P<0.05) after surgery. HOMA-ISI were higher in the PreOp group vs the other 3 groups after surgery (P<0.05), and HOMA-β in the PreOp group was higher than that in the other 3 groups at 30 and 120 min after surgery. Additionally, post-operative phosphorylated IRS-1, PI3K, and AKT protein levels were significantly higher in the PreOp group than in the other 3 groups (P<0.05), but no significant differences were observed in their respective protein levels (P>0.05).

Conclusions

OCH decreases postoperative insulin resistance and improves postoperative insulin sensitivity in skeletal muscles through the PI3K/AKT/mTOR pathway.

[Adaptation to the availability of essential amino-acids: role of GCN2/eIF2α/ATF4 pathway]

In mammals, metabolic adaptations are required to overcome nutritional deprivation in amino-acids/proteins as well as episodes of malnutrition. GCN2 protein kinase, which phosphorylates the α subunit of the translation initiation factor eIF2, is a sensor of amino-acid(s) deficiencies. On one hand, this review briefly describes the main features of amino-acid metabolism. On the other hand, it describes the role of GCN2 in regulating numerous physiological functions.

Roles of the translation initiation factor eIF2α serine 51 phosphorylation in cancer formation and treatment

Cells respond to various forms of stress by activating anti-proliferative pathways, which allow them to correct the damage caused by stress before re-entering proliferation. If the damage, however, is beyond repair, stressed cells are eliminated from the host by undergoing death. The balance between cell survival and death is essential for cancer formation and is determined by several key pathways that impact on different stages of gene expression. In recent years, it has become evident that phosphorylation of the alpha (α) subunit of the translation initiation factor eIF2 at serine 51 (eIF2αS51P) is an important determinant of cell fate in response to stress. Induction of eIF2αS51P is mediated by a family of four kinases namely, HRI, PKR, PERK and GCN2, each of which responds to distinct forms of stress. Increased eIF2αS51P results in a global inhibition of protein synthesis but at the same time enhances the translation of select mRNAs encoding for proteins that control cell adaptation to stress. Short-term induction of eIF2αS51P has been associated with cell survival whereas long-term induction with cell death. Studies in mouse and human models of cancer have provided compelling evidence that eIF2αS51P plays an essential role in stress-induced tumorigenesis. Increased eIF2αS51P exhibits cell autonomous as well as immune regulatory effects, which can influence tumor growth and the efficacy of anti-tumor therapies. The findings suggest that eIF2αS51P may be of prognostic value and a suitable target for the design and implementation of effective anti-tumor therapies. This article is part of a Special Issue entitled: Translation and Cancer.

Mitochondrial target of thiazolidinediones

Insulin-sensitizing thiazolidinediones exert a pleiotropic pharmacology with therapeutic potential in a number of disease states ranging from metabolic syndrome and diabetes to neurodegeneration and cancer. A growing understanding of their mechanism of action, working from the site of their binding in the mitochondrion, provides insight into the mechanism of action of the insulin sensitizers and the reasons for their pleiotropic pharmacology. This review helps to frame the direction of future work that should be helpful in setting a new direction for the discovery and development of new, more useful therapeutic agents for metabolic disease.

Protein and amino acid restriction, aging and disease: from yeast to humans

Many of the effects of dietary restriction (DR) on longevity and health span in model organisms have been linked to reduced protein and amino acid (AA) intake and the stimulation of specific nutrient signaling pathways. Studies in yeast have shown that addition of serine, threonine, and valine in media promotes cellular sensitization and aging by activating different but connected pathways. Protein or essential AA restriction extends both lifespan and healthspan in rodent models. In humans, protein restriction (PR) has been associated with reduced cancer, diabetes, and overall mortality. Thus, interventions aimed at lowering the intake of proteins or specific AAs can be beneficial and have the potential to be widely adopted and effective in optimizing healthspan.

Central prolactin receptors (PRLRs) regulate hepatic insulin sensitivity in mice via signal transducer and activator of transcription 5 (STAT5) and the vagus nerve

AIMS/HYPOTHESIS

Recent studies have revealed the crucial role of the central nervous system (CNS), especially the hypothalamus, in the regulation of insulin sensitivity in peripheral tissues. The aim of our current study was to investigate the possible involvement of hypothalamic prolactin receptors (PRLRs) in the regulation of hepatic insulin sensitivity.

METHODS

We employed overexpression of PRLRs in mouse hypothalamus via intracerebroventricular injection of adenovirus expressing PRLR and inhibition of PRLRs via adenovirus expressing short-hairpin RNA (shRNA) specific for PRLRs in vivo. Selective hepatic vagotomy was employed to verify the important role of the vagus nerve in mediating signals from the brain to peripheral organs. In addition, a genetic insulin-resistant animal model, the db/db mouse, was used in our study to investigate the role of hypothalamic PRLRs in regulating whole-body insulin sensitivity.

RESULTS

Overexpression of PRLRs in the hypothalamus improved hepatic insulin sensitivity in mice and inhibition of hypothalamic PRLRs had the opposite effect. In addition, we demonstrated that hypothalamic PRLR-improved insulin sensitivity was significantly attenuated by inhibiting the activity of signal transducer and activator of transcription 5 (STAT5) in the CNS and by selective hepatic vagotomy. Finally, overexpression of PRLRs significantly ameliorated insulin resistance in db/db mice.

CONCLUSIONS/INTERPRETATION

Our study identifies a novel central pathway involved in the regulation of hepatic insulin sensitivity, mediated by hypothalamic PRLR/STAT5 signalling and the vagus nerve, thus demonstrating an important role for hypothalamic PRLRs under conditions of insulin resistance.

The TSC complex is required for the benefits of dietary protein restriction on stress resistance in vivo

Protein restriction (PR) is important for the benefits of dietary restriction on longevity and stress resistance, but relevant nutrient sensors and downstream effectors in mammals remain poorly defined. We used PR-mediated protection from hepatic ischemia reperfusion injury to probe genetic requirements for the evolutionarily conserved nutrient sensors GCN2 and mTORC1 in stress resistance. One week of PR reduced free amino acids and circulating growth factors, activating GCN2 and mTORC1 repressor tuberous sclerosis complex (TSC). However, although GCN2 was dispensable for PR-induced protection, hepatic TSC1 was required. PR improved hepatic insulin sensitivity in a TSC1-dependent manner prior to ischemia, facilitating increased prosurvival signaling and reduced apoptosis after reperfusion. These benefits were partially abrogated by pharmacological PI3K inhibition or genetic deletion of the insulin receptor in hepatocytes. In conclusion, improved insulin sensitivity upon short-term PR required TSC1, facilitated increased prosurvival signaling after injury, and contributed partially to PR-mediated resistance to clinically relevant ischemia reperfusion injury.

FGF21 is an endocrine signal of protein restriction

Enhanced fibroblast growth factor 21 (FGF21) production and circulation has been linked to the metabolic adaptation to starvation. Here, we demonstrated that hepatic FGF21 expression is induced by dietary protein restriction, but not energy restriction. Circulating FGF21 was increased 10-fold in mice and rats fed a low-protein (LP) diet. In these animals, liver Fgf21 expression was increased within 24 hours of reduced protein intake. In humans, circulating FGF21 levels increased dramatically following 28 days on a LP diet. LP-induced increases in FGF21 were associated with increased phosphorylation of eukaryotic initiation factor 2α (eIF2α) in the liver, and both baseline and LP-induced serum FGF21 levels were reduced in mice lacking the eIF2α kinase general control nonderepressible 2 (GCN2). Finally, while protein restriction altered food intake, energy expenditure, and body weight gain in WT mice, FGF21-deficient animals did not exhibit these changes in response to a LP diet. These and other data demonstrate that reduced protein intake underlies the increase in circulating FGF21 in response to starvation and a ketogenic diet and that FGF21 is required for behavioral and metabolic responses to protein restriction. FGF21 therefore represents an endocrine signal of protein restriction, which acts to coordinate metabolism and growth during periods of reduced protein intake.

Systems analysis and the prediction and prevention of Type 2 diabetes mellitus

Prevalence of Type 2 diabetes has increased at an alarming rate, highlighting the need to correctly predict the development of this disease in order to allow intervention and thus, slow progression of the disease and resulting metabolic derangement. There have been many recent 'advances' geared toward the detection of pre-diabetes, including genome wide association studies and metabolomics. Although these approaches generate a large amount of data with a single blood sample, studies have indicated limited success using genetic and metabolomics information alone for identification of disease risk. Clinical assessment of the disposition index (DI), based on the hyperbolic law of glucose tolerance, is a powerful predictor of Type 2 diabetes, but is not easily assessed in the clinical setting. Thus, it is evident that combining genetic or metabolomic approaches for a more simple assessment of DI may provide a useful tool to identify those at highest risk for Type 2 diabetes, allowing for intervention and prevention.

A novel function of microRNA 130a-3p in hepatic insulin sensitivity and liver steatosis

MicroRNAs (miRNAs) are endogenous, noncoding, short, single-stranded RNAs that are evolutionarily conserved and believed to play a role in controlling a variety of biological processes. The roles of miRNAs in insulin resistance and liver steatosis, however, are largely unknown. The objective of this study was to evaluate the roles of miR-130a in the regulation of insulin sensitivity and liver steatosis. In our current study, we observed that overexpression of miR-130a-3p increases insulin signaling in both HepG2 cells and primary mouse hepatocytes, and silencing of miR-130a-3p has the opposite effects. However, miR-130a-5p has no effect in the regulation of insulin signaling. Consistently, whole-body and hepatic insulin sensitivity are improved in mice injected with adenoviruses that overexpress miR-130a-3p. Furthermore, we provided evidence showing that growth factor receptor-bound protein 10 is required for miR-130a-3p-regulated insulin sensitivity. On the other hand, we observed that expression of miR-130a-3p is decreased in the livers of db/db mice and that adenovirus-mediated overexpression of miR-130a-3p reverses insulin resistance and liver steatosis, the latter of which is achieved via suppressing fatty acid synthase expression in these mice. This study identifies a novel function for hepatic miR-130a-3p in the regulation of insulin sensitivity and liver steatosis.

Dietary protein and blood glucose control

PURPOSE OF REVIEW

This review presents the different pathways by which protein and amino acid impact glucose control. The review more particularly discusses the contradictory effects reported in the literature on the involvement of amino acid on glucose production and in insulin secretion and sensitivity.

RECENT FINDINGS

Some recent findings allow a better understanding of the direct and indirect mechanisms involved in the insulinotropic activity of some amino acids in pancreatic β-cell and in the production of glucose through liver gluconeogenesis that participates to improve the control of glycemia. In contrast, the potential deleterious effects of branched chain amino acid, and particularly leucine, hypothesized in previous publications, have been discussed in some recent publications.

SUMMARY

These processes are of high clinical relevance since the role of protein and amino acid have been repeatedly discussed to improve insulin secretion in type 2 diabetes patients or in weight management strategy in overweight and obese individuals. In addition, whether blood amino acid could be used as biomarkers for the risk of type 2 diabetes needs to be discussed.

Insulin sensitivity is inversely related to cellular energy status, as revealed by biotin deprivation

We have reported an early decrease in glycemia in rats fed a biotin-deficient diet with reduced cellular ATP levels, suggesting increased insulin sensitivity. Here, we show that biotin-deprived rats are more tolerant of glucose, as shown by both oral and intraperitoneal glucose tolerance tests, during which insulin plasma levels were significantly diminished in deficient rats compared with controls. Biotin-deficient rats had lower blood glucose concentrations during intraperitoneal insulin sensitivity tests than controls. Furthermore, more glucose was infused to maintain euglycemia in the biotin-deficient rats during hyperinsulinemic euglycemic clamp studies. These results demonstrate augmented sensitivity to insulin in biotin-deprived rats. They are most likely the consequence of an insulin-independent effect of AMPK activation on GLUT4 membrane translocation with increased glucose uptake. In biotin-deficient cultured L6 muscle cells, there was increased phosphorylation of the energy sensor AMPK. We have now confirmed the augmented AMPK activation in both biotin-deprived in vivo muscle and cultured muscle cells. In these cells, glucose uptake is increased by AMPK activation by AICAR and diminished by its knockdown by the specific siRNAs directed against its α1- and α2-catalytic subunits, with all of these effects being largely independent of the activity of the insulin-signaling pathway that was inhibited with wortmannin. The enhanced insulin sensitivity in biotin deficiency likely has adaptive value for organisms due to the hormone promotion of uptake and utilization of not only glucose but other nutrients such as branched-chain amino acids, whose deficiency has been reported to increase insulin tolerance.

Metformin promotes lifespan through mitohormesis via the peroxiredoxin PRDX-2

The antiglycemic drug metformin, widely prescribed as first-line treatment of type II diabetes mellitus, has lifespan-extending properties. Precisely how this is achieved remains unclear. Via a quantitative proteomics approach using the model organism Caenorhabditis elegans, we gained molecular understanding of the physiological changes elicited by metformin exposure, including changes in branched-chain amino acid catabolism and cuticle maintenance. We show that metformin extends lifespan through the process of mitohormesis and propose a signaling cascade in which metformin-induced production of reactive oxygen species increases overall life expectancy. We further address an important issue in aging research, wherein so far, the key molecular link that translates the reactive oxygen species signal into a prolongevity cue remained elusive. We show that this beneficial signal of the mitohormetic pathway is propagated by the peroxiredoxin PRDX-2. Because of its evolutionary conservation, peroxiredoxin signaling might underlie a general principle of prolongevity signaling.

Effects of individual branched-chain amino acids deprivation on insulin sensitivity and glucose metabolism in mice

OBJECTIVE

We recently discovered that leucine deprivation increases hepatic insulin sensitivity via general control nondepressible (GCN) 2/mammalian target of rapamycin (mTOR) and AMP-activated protein kinase (AMPK) pathways. The goal of the present study was to investigate whether the above effects were leucine specific or were also induced by deficiency of other branched chain amino acids including valine and isoleucine.

METHODS

Following depletion of BCAAs, changes in metabolic parameters and the expression of genes and proteins involved in regulation of insulin sensitivity and glucose metabolism were analyzed in mice and cell lines including human HepG2 cells, primary mouse hepatocytes and a mouse myoblast cell line C2C12.

RESULTS

Valine or isoleucine deprivation for 7 days has similar effect on improving insulin sensitivity as leucine, in wild type and insulin-resistant mice models. These effects are possibly mediated by decreased mTOR/S6K1 and increased AMPK signaling pathways, in a GCN2-dependent manner. Similar observations were obtained in in vitro studies. In contrast to leucine withdrawal, valine or isoleucine deprivation for 7 days significantly decreased fed blood glucose levels, possibly due to reduced expression of a key gluconeogenesis gene, glucose-6-phosphatase. Finally, insulin sensitivity was rapidly improved in mice 1 day following maintenance on a diet deficient for any individual BCAAs.

CONCLUSIONS

Our results show that while improvement on insulin sensitivity is a general feature of BCAAs depletion, individual BCAAs have specific effects on metabolic pathways, including those that regulate glucose level. These observations provide a conceptual framework for delineating the molecular mechanisms that underlie amino acid regulation of insulin sensitivity.

Keeping the eIF2 alpha kinase Gcn2 in check

The protein kinase Gcn2 is present in virtually all eukaryotes and is of increasing interest due to its involvement in a large array of crucial biological processes. Some of these are universally conserved from yeast to humans, such as coping with nutrient starvation and oxidative stress. In mammals, Gcn2 is important for e.g. long-term memory formation, feeding behaviour and immune system regulation. Gcn2 has been also implicated in diseases such as cancer and Alzheimer's disease. Studies on Gcn2 have been conducted most extensively in Saccharomyces cerevisiae, where the mechanism of its activation by amino acid starvation has been revealed in most detail. Uncharged tRNAs stimulate Gcn2 which subsequently phosphorylates its substrate, eIF2α, leading to reduced global protein synthesis and simultaneously to increased translation of specific mRNAs, e.g. those coding for Gcn4 in yeast and ATF4 in mammals. Both proteins are transcription factors that regulate the expression of a myriad of genes, thereby enabling the cell to initiate a survival response to the initial activating cue. Given that Gcn2 participates in many diverse processes, Gcn2 itself must be tightly controlled. Indeed, Gcn2 is regulated by a vast network of proteins and RNAs, the list of which is still growing. Deciphering molecular mechanisms underlying Gcn2 regulation by effectors and inhibitors is fundamental for understanding how the cell keeps Gcn2 in check ensuring normal organismal function, and how Gcn2-associated diseases may develop or may be treated. This review provides a critical evaluation of the current knowledge on mechanisms controlling Gcn2 activation or activity.

Leucine modulates dynamic phosphorylation events in insulin signaling pathway and enhances insulin-dependent glycogen synthesis in human skeletal muscle cells

BACKGROUND

Branched-chain amino acids, especially leucine, are known to interact with insulin signaling pathway and glucose metabolism. However, the mechanism by which this is exerted, remain to be clearly defined. In order to examine the effect of leucine on muscle insulin signaling, a set of experiments was carried out to quantitate phosphorylation events along the insulin signaling pathway in human skeletal muscle cell cultures. Cells were exposed to insulin, leucine or both, and phosphorylation events of key insulin signaling molecules were tracked over time so as to monitor time-related responses that characterize the signaling events and could be missed by a single sampling strategy limited to pre/post stimulus events.

RESULTS

Leucine is shown to increase the magnitude of insulin-dependent phosphorylation of protein kinase B (AKT) at Ser473 and glycogen synthase kinase (GSK3β) at Ser21-9. Glycogen synthesis follows the same pattern of GSK3β, with a significant increase at 100 μM leucine plus insulin stimulus. Moreover, data do not show any statistically significant increase of pGSK3β and glycogen synthesis at higher leucine concentrations. Leucine is also shown to increase the magnitude of insulin-mediated extracellularly regulated kinase (ERK) phosphorylation; however, differently from AKT and GSK3β, ERK shows a transient behavior, with an early peak response, followed by a return to the baseline condition.

CONCLUSIONS

These experiments demonstrate a complementary effect of leucine on insulin signaling in a human skeletal muscle cell culture, promoting insulin-activated GSK3β phosphorylation and glycogen synthesis.

An Intimate Relationship between ROS and Insulin Signalling: Implications for Antioxidant Treatment of Fatty Liver Disease

Oxidative stress damages multiple cellular components including DNA, lipids, and proteins and has been linked to pathological alterations in nonalcoholic fatty liver disease (NAFLD). Reactive oxygen species (ROS) emission, resulting from nutrient overload and mitochondrial dysfunction, is thought to be a principal mediator in NAFLD progression, particularly toward the development of hepatic insulin resistance. In the context of insulin signalling, ROS has a dual role, as both a facilitator and inhibitor of the insulin signalling cascade. ROS mediate these effects through redox modifications of cysteine residues affecting phosphatase enzyme activity, stress-sensitive kinases, and metabolic sensors. This review highlights the intricate relationship between redox-sensitive proteins and insulin signalling in the context of fatty liver disease, and to a larger extent, the importance of reactive oxygen species as primary signalling molecules in metabolically active cells.