Preserved protein synthesis in the heart in response to acute fasting and chronic food restriction despite reductions in liver and skeletal muscle

Remodelling of the translatome controls diet and its impact on tumorigenesis

Fasting is associated with a range of health benefits1-6. How fasting signals elicit changes in the proteome to establish metabolic programmes remains poorly understood. Here we show that hepatocytes selectively remodel the translatome while global translation is paradoxically downregulated during fasting7,8. We discover that phosphorylation of eukaryotic translation initiation factor 4E (P-eIF4E) is induced during fasting. We show that P-eIF4E is responsible for controlling the translation of genes involved in lipid catabolism and the production of ketone bodies. Inhibiting P-eIF4E impairs ketogenesis in response to fasting and a ketogenic diet. P-eIF4E regulates those messenger RNAs through a specific translation regulatory element within their 5' untranslated regions (5' UTRs). Our findings reveal a new signalling property of fatty acids, which are elevated during fasting. We found that fatty acids bind and induce AMP-activated protein kinase (AMPK) kinase activity that in turn enhances the phosphorylation of MAP kinase-interacting protein kinase (MNK), the kinase that phosphorylates eIF4E. The AMPK-MNK-eIF4E axis controls ketogenesis, revealing a new lipid-mediated kinase signalling pathway that links ketogenesis to translation control. Certain types of cancer use ketone bodies as an energy source9,10 that may rely on P-eIF4E. Our findings reveal that on a ketogenic diet, treatment with eFT508 (also known as tomivosertib; a P-eIF4E inhibitor) restrains pancreatic tumour growth. Thus, our findings unveil a new fatty acid-induced signalling pathway that activates selective translation, which underlies ketogenesis and provides a tailored diet intervention therapy for cancer.

The Influence of Stress and Binge-Patterned Alcohol Drinking on Mouse Skeletal Muscle Protein Synthesis and Degradation Pathways

Adverse experiences (e.g., acute stress) and alcohol misuse can both impair skeletal muscle homeostasis, resulting in reduced protein synthesis and greater protein breakdown. Exposure to acute stress is a significant risk factor for engaging in alcohol misuse. However, little is known about how these factors together might further affect skeletal muscle health. To that end, this study investigated the effects of acute stress exposure followed by a period of binge-patterned alcohol drinking on signaling factors along mouse skeletal muscle protein synthesis (MPS) and degradation (MPD) pathways. Young adult male C57BL/6J mice participated in the Drinking in the Dark paradigm, where they received 2-4 h of access to 20% ethanol (alcohol group) or water (control group) for four days to establish baseline drinking levels. Three days later, half of the mice in each group were either exposed to a single episode of uncontrollable tail shocks (acute stress) or remained undisturbed in their home cages (no stress). Three days after stress exposure, mice received 4 h of access to 20% ethanol (alcohol) to model binge-patterned alcohol drinking or water for ten consecutive days. Immediately following the final episode of alcohol access, mouse gastrocnemius muscle was extracted to measure changes in relative protein levels along the Akt-mTOR MPS, as well as the ubiquitin-proteasome pathway (UPP) and autophagy MPD pathways via Western blotting. A single exposure to acute stress impaired Akt singling and reduced rates of MPS, independent of alcohol access. This observation was concurrent with a potent increase in heat shock protein seventy expression in the muscle of stressed mice. Alcohol drinking did not exacerbate stress-induced alterations in the MPS and MPD signaling pathways. Instead, changes in the MPS and MPD signaling factors due to alcohol access were primarily observed in non-stressed mice. Taken together, these data suggest that exposure to a stressor of sufficient intensity may cause prolonged disruptions to signaling factors that impact skeletal muscle health and function beyond what could be further induced by periods of alcohol misuse.

Protein turnover in pigs: A review of interacting factors

Protein turnover defines the balance between two continuous and complex processes of protein metabolism, synthesis and degradation, which determine their deposition in tissues. Although the liver and intestine have been studied extensively for their important roles in protein digestion, absorption and metabolism, the study of protein metabolism has focused mainly on skeletal muscle tissue to understand the basis for its growth. Due to the high adaptability of skeletal muscle, its protein turnover is greatly affected by different internal and external factors, contributing to carcass lean-yield and animal growth. Amino acid (AA) labelling and tracking using isotope tracer methodology, together with the study of myofiber type profiling, signal transduction pathways and gene expression, has allowed the analysis of these mechanisms from different perspectives. Positive stimuli such as increased nutrient availability in the diet (e.g., AA), physical activity, the presence of certain hormones (e.g., testosterone) or a more oxidative myofiber profile in certain muscles or pig genotypes promote increased upregulation of translation and transcription-related genes, activation of mTORC1 signalling mechanisms and increased abundance of satellite cells, allowing for more efficient protein synthesis. However, fasting, animal aging, inactivity and stress, inflammation or sepsis produce the opposite effect. Deepening the understanding of modifying factors and their possible interaction may contribute to the design of optimal strategies to better control tissue growth and nutrient use (i.e., protein and AA), and thus advance the precision feeding strategy.

Underfeeding Alters Brain Tissue Synthesis Rate in a Rat Brain Injury Model

Brain injuries (BI) are highly disruptive, often having long lasting effects. Inadequate standard of care (SOC) energy support in the hospital leads to dietary energy deficiencies in BI patients. However, it is unclear how underfeeding (UF) affects protein synthesis post-BI. Therefore, in a rat model, we addressed the issue of UF on the protein fractional synthesis rate (fSR) post-BI. Compared to ad libitum (AL)-fed animals, we found that UF decreased protein synthesis in hind-limb skeletal muscle and cortical mitochondrial and structural proteins (p ≤ 0.05). BI significantly increased protein synthesis in the left and right cortices (p ≤ 0.05), but suppressed protein synthesis in the cerebellum (p ≤ 0.05) as compared to non-injured sham animals. Compared to underfeeding alone, UF in conjunction with BI (UF+BI) caused increased protein synthesis rates in mitochondrial, cytosolic, and whole-tissue proteins of the cortical brain regions. The increased rates of protein synthesis found in the UF+BI group were mitigated by AL feeding, demonstrating that caloric adequacy alleviates the effects of BI on protein dynamics in cortical and cerebellar brain regions. This research provides evidence that underfeeding has a negative impact on brain healing post-BI and that protein reserves in uninjured tissues are mobilized to support cortical tissue repair following BI.

Metabolic deuterium oxide (D2O) labeling in quantitative omics studies: A tutorial review

Mass spectrometry (MS) is an invaluable tool for sensitive detection and characterization of individual biomolecules in omics studies. MS combined with stable isotope labeling enables the accurate and precise determination of quantitative changes occurring in biological samples. Metabolic isotope labeling, wherein isotopes are introduced into biomolecules through biosynthetic metabolism, is one of the main labeling strategies. Among the precursors employed in metabolic isotope labeling, deuterium oxide (D₂O) is cost-effective and easy to implement in any biological systems. This tutorial review aims to explain the basic principle of D₂O labeling and its applications in omics research. D₂O labeling incorporates D into stable C-H bonds in various biomolecules, including nucleotides, proteins, lipids, and carbohydrates. Typically, D₂O labeling is performed at low enrichment of 1%-10% D₂O, which causes subtle changes in the isotopic distribution of a biomolecule, instead of the complete separation between labeled and unlabeled samples in a mass spectrum. D₂O labeling has been employed in various omics studies to determine the metabolic flux, turnover rate, and relative quantification. Moreover, the advantages and challenges of D₂O labeling and its future prospects in quantitative omics are discussed. The economy, versatility, and convenience of D₂O labeling will be beneficial for the long-term omics studies for higher organisms.

A mitochondrial long-chain fatty acid oxidation defect leads to transfer RNA uncharging and activation of the integrated stress response in the mouse heart

AIMS

Cardiomyopathy and arrhythmias can be severe presentations in patients with inherited defects of mitochondrial long-chain fatty acid β-oxidation (FAO). The pathophysiological mechanisms that underlie these cardiac abnormalities remain largely unknown. We investigated the molecular adaptations to a FAO deficiency in the heart using the long-chain acyl-CoA dehydrogenase (LCAD) knockout (KO) mouse model.

METHODS AND RESULTS

We observed enrichment of amino acid metabolic pathways and of ATF4 target genes among the upregulated genes in the LCAD KO heart transcriptome. We also found a prominent activation of the eIF2α/ATF4 axis at the protein level that was independent of the feeding status, in addition to a reduction of cardiac protein synthesis during a short period of food withdrawal. These findings are consistent with an activation of the integrated stress response (ISR) in the LCAD KO mouse heart. Notably, charging of several transfer RNAs (tRNAs), such as tRNAGln was decreased in LCAD KO hearts, reflecting a reduced availability of cardiac amino acids, in particular, glutamine. We replicated the activation of the ISR in the hearts of mice with muscle-specific deletion of carnitine palmitoyltransferase 2.

CONCLUSIONS

Our results show that perturbations in amino acid metabolism caused by long-chain FAO deficiency impact cardiac metabolic signalling, in particular the ISR. These results may serve as a foundation for investigating the role of the ISR in the cardiac pathology associated with long-chain FAO defects.Translational Perspective: The heart relies mainly on mitochondrial fatty acid β-oxidation (FAO) for its high energy requirements. The heart disease observed in patients with a genetic defect in this pathway highlights the importance of FAO for cardiac health. We show that the consequences of a FAO defect extend beyond cardiac energy homeostasis and include amino acid metabolism and associated signalling pathways such as the integrated stress response.

Use of deuterium oxide (2H2O) to assess muscle protein synthesis in juvenile red drum (Sciaenops ocellatus) fed complete, and valine-deficient diets

The use of ²H₂O in tank water to assess protein synthesis rates in fish is a relatively novel methodology that could allow for a better understanding of the effects of particular nutritional and environmental variables on rates of protein accretion. As such, this study involved an assessment and comparison of protein synthesis rates in the muscle of juvenile red drum fed a control diet (nutritionally complete) versus a valine (Val)-deficient diet. Six groups of 12 juvenile red drum, initially weighing ~ 4.5 g/fish, were stocked in six separate 38-L aquaria operating as a recirculating system. Fish were acclimatized to experimental conditions for 2 weeks while being fed the control diet. Just prior to initiating the protein synthesis assay, one aquarium of fish was fed the control diet while a second aquarium of fish was fed the Val-deficient diet. Immediately after consuming the experimental diets, each group of fish was moved to an independent aquarium containing ²H₂O, and the fractional synthetic rate (FSR) of protein synthesis was obtained at 12, 24, 36 and 48 h after feeding by collecting two fish per treatment at each time point. This protein synthesis assay procedure was performed in three separate sessions, and considered as replicates over time (n = 3) for fish fed the control or Val-deficient diets immediately before initiating the session. Results indicated that a one-time feeding of a diet deficient in Val significantly reduced protein synthesis rates in the muscle of red drum. In addition, a significant effect of time after feeding was found, where observed FSR values peaked at 12 h after feeding and decreased as time progressed. In conclusion, deuterium methodologies were applicable to red drum, and this approach had the sensitivity to assess differences in protein synthesis rates when dietary perturbations were introduced.

Protein fractional synthesis rates within tissues of high- and low-active mice

With the rise in physical inactivity and its related diseases, it is necessary to understand the mechanisms involved in physical activity regulation. Biological factors regulating physical activity are studied to establish a possible target for improving the physical activity level. However, little is known about the role metabolism plays in physical activity regulation. Therefore, we studied protein fractional synthesis rate (FSR) of multiple organ tissues of 12-week-old male mice that were previously established as inherently low-active (n = 15, C3H/HeJ strain) and high-active (n = 15, C57L/J strain). Total body water of each mouse was enriched to 5% deuterium oxide (D₂O) via intraperitoneal injection and maintained with D₂O enriched drinking water for about 24 h. Blood samples from the jugular vein and tissues (kidney, heart, lung, muscle, fat, jejunum, ileum, liver, brain, skin, and bone) were collected for enrichment analysis of alanine by LC-MS/MS. Protein FSR was calculated as -ln(1-enrichment). Data are mean±SE as fraction/day (unpaired t-test). Kidney protein FSR in the low-active mice was 7.82% higher than in high-active mice (low-active: 0.1863±0.0018, high-active: 0.1754±0.0028, p = 0.0030). No differences were found in any of the other measured organ tissues. However, all tissues resulted in a generally higher protein FSR in the low-activity mice compared to the high-activity mice (e.g. lung LA: 0.0711±0.0015, HA: 0.0643±0.0020, heart LA: 0.0649± 0.0013 HA: 0.0712±0.0073). Our observations suggest that high-active mice in most organ tissues are no more inherently equipped for metabolic adaptation than low-active mice, but there may be a connection between protein metabolism of kidney tissue and physical activity level. In addition, low-active mice have higher organ-specific baseline protein FSR possibly contributing to the inability to achieve higher physical activity levels.

Ramadan Intermittent Fasting Affects Adipokines and Leptin/Adiponectin Ratio in Type 2 Diabetes Mellitus and Their First-Degree Relatives

Background

In view of the association of Ramadan intermittent fasting with profound changes in lifestyle both in nondiabetic and diabetic patients, the aim of this study was to investigate the effect of Ramadan fasting on adiponectin, leptin and leptin to adiponectin ratio (LAR), growth hormone (GH), human-sensitive C-reactive protein (hs-CRP), and diabetic and metabolic syndrome factors in patients with Type 2 Diabetes Mellitus (Type 2 DM), their first-degree relatives (FDRs), and healthy controls.

Methods

This cohort study involved 98 Yemeni male subjects aged 30-70 years old: 30 Type 2 DM, 37 FDRs of Type 2 diabetic patients, and 31 healthy control subjects. Subjects' body mass index (BMI), waist circumference (WC), and blood pressure (BP) were measured, and venous blood samples were collected twice: the first samples were collected a couple of days prior to Ramadan fasting (baseline) and the second samples after 3 weeks of fasting.

Results

Ramadan fasting did not affect BMI, WC, and BP in Type 2 DM and their FDRs with respect to the baseline levels prior to Ramadan, whereas triglyceride and cholesterol were borderline significantly decreased in Type 2 DM with no effect in FDRs. Fasting blood glucose was not affected in Type 2 DM but was significantly increased in FDRs and control groups, whereas glycated haemoglobin (HbA1c) was slightly decreased in Type 2 DM, FDRs, and healthy controls. C-peptide, insulin, and insulin resistance (HOMA-IR) were significantly increased in Type 2 DM and FDRs, with no effect in the control group, whereas β-cell function (HOMA-β) was significantly decreased in FDRs and controls with no change in Type 2 DM. Ramadan fasting significantly decreased GH in both FDRs and control groups, and significantly increased hs-CRP in the control with no effect in Type 2 DM and FDRs. Adiponectin was significantly decreased, and leptin and LAR were significantly increased in Type 2 DM, FDRs, and control groups.

Conclusion

Ramadan intermittent fasting decreased adiponectin and increased leptin, LAR, insulin, and insulin resistance in both Type 2 DM and FDRs as well as decreased GH in both FDRs and healthy controls and increased hs-CRP in healthy controls. Moreover, Ramadan intermittent fasting neither worsens a patient's glycemic parameters nor improves it, with the exception of a slight improvement in HbA1c in Type 2 DM, FDRs, and healthy controls.

Spatial and temporal studies of metabolic activity: contrasting biochemical kinetics in tissues and pathways during fasted and fed states

The regulation of nutrient homeostasis, i.e., the ability to transition between fasted and fed states, is fundamental in maintaining health. Since food is typically consumed over limited (anabolic) periods, dietary components must be processed and stored to counterbalance the catabolic stress that occurs between meals. Herein, we contrast tissue- and pathway-specific metabolic activity in fasted and fed states. We demonstrate that knowledge of biochemical kinetics that is obtained from opposite ends of the energetic spectrum can allow mechanism-based differentiation of healthy and disease phenotypes. Rat models of type 1 and type 2 diabetes serve as case studies for probing spatial and temporal patterns of metabolic activity via [²H]water labeling. Experimental designs that capture integrative whole body metabolism, including meal-induced substrate partitioning, can support an array of research surrounding metabolic disease; the relative simplicity of the approach that is discussed here should enable routine applications in preclinical models.

Fibroblast growth factor 21 controls mitophagy and muscle mass

BACKGROUND

Skeletal muscle is a plastic tissue that adapts to changes in exercise, nutrition, and stress by secreting myokines and myometabolites. These muscle-secreted factors have autocrine, paracrine, and endocrine effects, contributing to whole body homeostasis. Muscle dysfunction in aging sarcopenia, cancer cachexia, and diabetes is tightly correlated with the disruption of the physiological homeostasis at the whole body level. The expression levels of the myokine fibroblast growth factor 21 (FGF21) are very low in normal healthy muscles. However, fasting, ER stress, mitochondrial myopathies, and metabolic disorders induce its release from muscles. Although our understanding of the systemic effects of muscle-derived FGF21 is exponentially increasing, the direct contribution of FGF21 to muscle function has not been investigated yet.

METHODS

Muscle-specific FGF21 knockout mice were generated to investigate the consequences of FGF21 deletion concerning skeletal muscle mass and force. To identify the mechanisms underlying FGF21-dependent adaptations in skeletal muscle during starvation, the study was performed on muscles collected from both fed and fasted adult mice. In vivo overexpression of FGF21 was performed in skeletal muscle to assess whether FGF21 is sufficient per se to induce muscle atrophy.

RESULTS

We show that FGF21 does not contribute to muscle homeostasis in basal conditions in terms of fibre type distribution, fibre size, and muscle force. In contrast, FGF21 is required for fasting-induced muscle atrophy and weakness. The mass of isolated muscles from control-fasted mice was reduced by 15-25% (P < 0.05) compared with fed control mice. FGF21-null muscles, however, were significantly protected from muscle loss and weakness during fasting. Such important protection is due to the maintenance of protein synthesis rate in knockout muscles during fasting compared with a 70% reduction in control-fasted muscles (P < 0.01), together with a significant reduction of the mitophagy flux via the regulation of the mitochondrial protein Bnip3. The contribution of FGF21 to the atrophy programme was supported by in vivo FGF21 overexpression in muscles, which was sufficient to induce autophagy and muscle loss by 15% (P < 0.05). Bnip3 inhibition protected against FGF21-dependent muscle wasting in adult animals (P < 0.05).

CONCLUSIONS

FGF21 is a novel player in the regulation of muscle mass that requires the mitophagy protein Bnip3.

eIF2α phosphorylation is required to prevent hepatocyte death and liver fibrosis in mice challenged with a high fructose diet

BACKGROUND

Dietary fructose can rapidly cause fatty liver in animals through de novo lipogenesis (DNL) and contribute to the development and severity of nonalcoholic fatty liver disease (NAFLD). In response to diverse cellular insults including endoplasmic reticulum (ER) and oxidative stress, phosphorylation of the eukaryotic translation initiation factor 2 alpha subunit (eIF2α) attenuates general translation initiation, allowing cells to conserve resources and initiate adaptive gene expression to restore homeostasis. The present study aimed to investigate the role of eIF2α phosphorylation in protecting against NAFLD induced by high fructose ingestion in a hepatocyte-specific eIF2α-phosphorylation-deficient mouse model.

METHODS

Hepatocyte-specific non-phosphorylatable (S51A) eIF2α knock-in (A/A;fTg/0;Cre^Hep/0, A/A^Hep ) mice were generated by crossing A/A;fTg/fTg mice with the floxed WT eIF2α transgene (fTg) with Alfp-Cre recombinase transgenic S/A;Cre^Hep/0 (S/A-Cre^Hep ) mice. Hepatocyte-specific eIF2α-phosphorylation-deficient 3-month-old mice or 12-month-old mice were fed a 60% high fructose diet (HFrD) for 16 or 5 wks, and the effects of eIF2α-phosphorylation deficiency on NADP/NADPH and GSSG/GSH levels, ROS-defense gene expression, oxidative damage, cell death, and fibrosis were observed.

RESULTS

Prolonged fructose feeding to mice caused dysregulation of the unfolded protein response (UPR) sensor activation and UPR gene expression, and then led to decreased expression of several ROS defense genes including glutathione biogenesis genes. Nonetheless, these changes were not sufficient to induce the death of eIF2α phosphorylation-sufficient hepatocytes. However, there was a substantial increase in hepatocyte death and liver fibrosis in fructose-fed middle-aged mice deficient in hepatocyte-specific eIF2α phosphorylation because of diminished antioxidant capacity due to reduced expression of antioxidant enzymes (GPX1 and HO-1) and lower NADPH and glutathione levels, as well as a possible increase in ROS-induced damage from infiltrating NOX2-expressing leukocytes; all this led to a vicious cycle of hepatocyte death and leukocyte infiltration.

CONCLUSION

Our findings suggest that eIF2α phosphorylation maintains NADPH and GSH levels and controls the expression of ROS-defense genes, thereby protecting hepatocytes from oxidative stresses induced by fructose metabolism.

Ramadan fasting in Saudi Arabia is associated with altered expression of CLOCK, DUSP and IL-1alpha genes, as well as changes in cardiometabolic risk factors

BACKGROUND

During the fasting month of Ramadan, practicing Saudis develop severe disturbances in sleeping and feeding patterns. Concomitantly, cortisol circadian rhythm is abolished, diurnal cortisol levels are elevated and circulating levels of several adipokines are altered favouring insulin resistance.

AIM

To examine changes in the expression of CLOCK and glucocorticoid-controlled genes, such as DUSP1 and IL-1α in Saudi adults before and during Ramadan, and to investigate possible associations with selected cardiometabolic risk factors.

METHODS

Healthy young volunteers (5 females, 18 males; mean age +SEM = 23.2 +1.2 years) were evaluated before Ramadan and two weeks into it. Blood samples were collected at 9 am (±1 hour) and twelve hours later for determination of serum lipid profile, high sensitivity CRP (hsCRP), and adiponectin. The expression of CLOCK, DUSP1 and IL-1α was evaluated in circulating leukocytes.

RESULTS

Mean levels of GGT and morning adiponectin decreased, while those of LDL-c/ HDL-c and atherogenic index (AI) increased significantly in Ramadan compared to Shabaan. There was no significant difference between morning and evening adiponectin during Ramadan, while the diurnal rhythm of hsCRP was lost. CLOCK gene expression mean was significantly higher in morning than in evening during Shabaan. Mean morning and evening DUSP1 mRNA levels showed significant increase during Ramadan compared to Shabaan, however, its diurnal rhythm was maintained. Morning IL-1α mRNA expression remained significantly higher than in the evening during Ramadan, but was markedly decreased compared to Shabaan.

DISCUSSION

Ramadan fasting in Saudi Arabia is associated with improvements in some cardiometabolic risk factors, such as circulating GGT and hsCRP and leukocyte expression of IL-1α mRNA, suggesting that intermittent fasting might have a beneficial component. These benefits may be offset by the previously reported dysregulation in the circadian rhythm, excess glucocorticoid levels and action, and insulin resistance, explaining increased prevalence of cardiometabolic disorders and type 2 diabetes mellitus.

Cumulative Muscle Protein Synthesis and Protein Intake Requirements

Muscle protein synthesis (MPS) fluctuates widely over the course of a day and is influenced by many factors. The time course of MPS responses to exercise and the influence of training and nutrition can only be pieced together from several different investigations and methods, many of which create unnatural experimental conditions. Measurements of cumulative MPS, the sum synthesis over an extended period, using deuterium oxide have been shown to accurately reflect muscle responses and may allow investigations of the response to exercise, total protein intake requirements, and interaction with protein timing in free-living experimental conditions; these factors have yet to be carefully integrated. Such studies could include clinical and athletic populations to integrate nutritional and exercise recommendations and help guide their revisions to optimize the skeletal muscle function that is so important to overall health.

Mitochondrial protein turnover: methods to measure turnover rates on a large scale

Mitochondrial proteins carry out diverse cellular functions including ATP synthesis, ion homeostasis, cell death signaling, and fatty acid metabolism and biogenesis. Compromised mitochondrial quality control is implicated in various human disorders including cardiac diseases. Recently it has emerged that mitochondrial protein turnover can serve as an informative cellular parameter to characterize mitochondrial quality and uncover disease mechanisms. The turnover rate of a mitochondrial protein reflects its homeostasis and dynamics under the quality control systems acting on mitochondria at a particular cell state. This review article summarizes some recent advances and outstanding challenges for measuring the turnover rates of mitochondrial proteins in health and disease. This article is part of a Special Issue entitled "Mitochondria: From Basic Mitochondrial Biology to Cardiovascular Disease".

The measurement of protein synthesis for assessing proteostasis in studies of slowed aging

Slowing the aging process can reduce the risk for multiple chronic diseases simultaneously. It is increasingly recognized that maintaining protein homeostasis (or proteostasis) is important for slowing the aging process. Since proteostasis is a dynamic process, monitoring it is not a simple task and requires use of appropriate methods. This review will introduce methods to assess protein and DNA synthesis using deuterium oxide (D2O), and how protein and DNA synthesis outcomes provide insight into proteostatic mechanisms. Finally, we provide a discussion on how these assessments of protein and DNA synthesis are "mechanistic" investigations and provide an appropriate framework for the further development of slowed aging treatments.

A "PET" area of interest: myocardial metabolism in human systolic heart failure

Myocardial substrate metabolism provides the energy needed for cardiac contraction and relaxation. The normal adult heart uses predominantly fatty acids (FAs) as its primary fuel source. However, the heart can switch and use glucose (and to a lesser extent, ketones, lactate, as well as endogenous triglycerides and glycogen), depending on the metabolic milieu and superimposed conditions. FAs are not a wholly better fuel than glucose, but they do provide more energy per mole than glucose. Conversely, glucose is the more oxygen-efficient fuel. Studies in animal models of heart failure (HF) fairly consistently demonstrate a shift away from myocardial fatty acid metabolism and toward glucose metabolism. Studies in humans are less consistent. Some show the same metabolic switch away from FA metabolism but not all. This may be due to differences in the etiology of HF, sex-related differences, or other mitigating factors. For example, obesity, insulin resistance, and diabetes are all related to an increased risk of HF and may complicate or contribute to its development. However, these conditions are associated with increased FA metabolism. This review will discuss aspects of human heart metabolism in systolic dysfunction as measured by the noninvasive, quantitative method-positron emission tomography. Continued research in this area is vital if we are to ameliorate HF by manipulating heart metabolism with the aim of increasing energy production and/or efficiency.

Health impact of fasting in Saudi Arabia during Ramadan: association with disturbed circadian rhythm and metabolic and sleeping patterns

BACKGROUND

Muslims go through strict Ramadan fasting from dawn till sunset for one month yearly. These practices are associated with disturbed feeding and sleep patterns. We recently demonstrated that, during Ramadan, circadian cortisol rhythm of Saudis is abolished, exposing these subjects to continuously increased cortisol levels.

HYPOTHESIS

Secretory patterns of other hormones and metabolic parameters associated with cortisol, and insulin resistance, might be affected during Ramadan.

PROTOCOL

Ramadan practitioners (18 males, 5 females; mean age ±SEM = 23.16±1.2 years) were evaluated before and two weeks into Ramadan. Blood was collected for measurements of endocrine and metabolic parameters at 9 am (±1 hour) and again twelve hours later.

RESULTS

In Ramadan, glucose concentration was kept within normal range, with a significant increase in the morning. Mean morning concentration of leptin was significantly higher than pre-Ramadan values (p = 0.001), in contrast to that of adiponectin, which was significantly lower (p<0.001). These changes were associated with increased insulin resistance in morning and evening. Concentrations of hsCRP were lower during Ramadan than those during regular living conditions, however, normal circadian fluctuation was abolished (p = 0.49). Even though means of liver enzymes, total bilirubin, total protein and albumin were all decreased during Ramadan, statistically lower means were only noted for GGT, total protein, and albumin (p = 0.018, 0.002 and 0.001 respectively).

DISCUSSION

Saudi Ramadan practitioners have altered adipokine patterns, typical of insulin resistance. The noted decreases of hsCRP, liver enzymes, total protein, and albumin, are most likely a result of fasting, while loss of circadian rhythmicity of hsCRP is probably due to loss of circadian cortisol rhythm.

CONCLUSIONS

Modern Ramadan practices in Saudi Arabia, which are associated with evening hypercortisolism, are also characterized by altered adipokines patterns, and an abolished hsCRP circadian rhythm, all likely to increase cardiometabolic risk.

A validation of the application of D(2)O stable isotope tracer techniques for monitoring day-to-day changes in muscle protein subfraction synthesis in humans

Quantification of muscle protein synthesis (MPS) remains a cornerstone for understanding the control of muscle mass. Traditional [(13)C]amino acid tracer methodologies necessitate sustained bed rest and intravenous cannulation(s), restricting studies to ~12 h, and thus cannot holistically inform on diurnal MPS. This limits insight into the regulation of habitual muscle metabolism in health, aging, and disease while querying the utility of tracer techniques to predict the long-term efficacy of anabolic/anticatabolic interventions. We tested the efficacy of the D2O tracer for quantifying MPS over a period not feasible with (13)C tracers and too short to quantify changes in mass. Eight men (22 ± 3.5 yr) undertook one-legged resistance exercise over an 8-day period (4 × 8-10 repetitions, 80% 1RM every 2nd day, to yield "nonexercised" vs. "exercise" leg comparisons), with vastus lateralis biopsies taken bilaterally at 0, 2, 4, and 8 days. After day 0 biopsies, participants consumed a D2O bolus (150 ml, 70 atom%); saliva was collected daily. Fractional synthetic rates (FSRs) of myofibrillar (MyoPS), sarcoplasmic (SPS), and collagen (CPS) protein fractions were measured by GC-pyrolysis-IRMS and TC/EA-IRMS. Body water initially enriched at 0.16-0.24 APE decayed at ~0.009%/day. In the nonexercised leg, MyoPS was 1.45 ± 0.10, 1.47 ± 0.06, and 1.35 ± 0.07%/day at 0-2, 0-4, and 0-8 days, respectively (~0.05-0.06%/h). MyoPS was greater in the exercised leg (0-2 days: 1.97 ± 0.13%/day; 0-4 days: 1.96 ± 0.15%/day, P < 0.01; 0-8 days: 1.79 ± 0.12%/day, P < 0.05). CPS was slower than MyoPS but followed a similar pattern, with the exercised leg tending to yield greater FSRs (0-2 days: 1.14 ± 0.13 vs. 1.45 ± 0.15%/day; 0-4 days: 1.13 ± 0.07%/day vs. 1.47 ± 0.18%/day; 0-8 days: 1.03 ± 0.09%/day vs. 1.40 ± 0.11%/day). SPS remained unchanged. Therefore, D2O has unrivaled utility to quantify day-to-day MPS in humans and inform on short-term changes in anabolism and presumably catabolism alike.

Substantial Differences between Organ and Muscle Specific Tracer Incorporation Rates in a Lactating Dairy Cow

We aimed to produce intrinsically L-[1-(13)C]phenylalanine labeled milk and beef for subsequent use in human nutrition research. The collection of the various organ tissues after slaughter allowed for us to gain insight into the dynamics of tissue protein turnover in vivo in a lactating dairy cow. One lactating dairy cow received a constant infusion of L-[1-(13)C]phenylalanine (450 µmol/min) for 96 h. Plasma and milk were collected prior to, during, and after the stable isotope infusion. Twenty-four hours after cessation of the infusion the cow was slaughtered. The meat and samples of the various organ tissues (liver, heart, lung, udder, kidney, rumen, small intestine, and colon) were collected and stored. Approximately 210 kg of intrinsically labeled beef (bone and fat free) with an average L-[1-(13)C]phenylalanine enrichment of 1.8±0.1 mole percent excess (MPE) was obtained. The various organ tissues differed substantially in L-[1-(13)C]phenylalanine enrichments in the tissue protein bound pool, the highest enrichment levels were achieved in the kidney (11.7 MPE) and the lowest enrichment levels in the skeletal muscle tissue protein of the cow (between 1.5-2.4 MPE). The estimated protein synthesis rates of the various organ tissues should be regarded as underestimates, particularly for the organs with the higher turnover rates and high secretory activity, due to the lengthened (96 h) measurement period necessary for the production of the intrinsically labeled beef. Our data demonstrates that there are relatively small differences in L-[1-(13)C]phenylalanine enrichments between the various meat cuts, but substantial higher enrichment values are observed in the various organ tissues. We conclude that protein turnover rates of various organs are much higher when compared to skeletal muscle protein turnover rates in large lactating ruminants.

A self-defeating anabolic program leads to β-cell apoptosis in endoplasmic reticulum stress-induced diabetes via regulation of amino acid flux

Endoplasmic reticulum (ER) stress-induced responses are associated with the loss of insulin-producing β-cells in type 2 diabetes mellitus. β-Cell survival during ER stress is believed to depend on decreased protein synthesis rates that are mediated via phosphorylation of the translation initiation factor eIF2α. It is reported here that chronic ER stress correlated with increased islet protein synthesis and apoptosis in β-cells in vivo. Paradoxically, chronic ER stress in β-cells induced an anabolic transcription program to overcome translational repression by eIF2α phosphorylation. This program included expression of amino acid transporter and aminoacyl-tRNA synthetase genes downstream of the stress-induced ATF4-mediated transcription program. The anabolic response was associated with increased amino acid flux and charging of tRNAs for branched chain and aromatic amino acids (e.g. leucine and tryptophan), the levels of which are early serum indicators of diabetes. We conclude that regulation of amino acid transport in β-cells during ER stress involves responses leading to increased protein synthesis, which can be protective during acute stress but can lead to apoptosis during chronic stress. These studies suggest that the increased expression of amino acid transporters in islets can serve as early diagnostic biomarkers for the development of diabetes.

ER-stress-induced transcriptional regulation increases protein synthesis leading to cell death

Protein misfolding in the endoplasmic reticulum (ER) leads to cell death through PERK-mediated phosphorylation of eIF2α, although the mechanism is not understood. ChIP-seq and mRNA-seq of activating transcription factor 4 (ATF4) and C/EBP homologous protein (CHOP), key transcription factors downstream of p-eIF2α, demonstrated that they interact to directly induce genes encoding protein synthesis and the unfolded protein response, but not apoptosis. Forced expression of ATF4 and CHOP increased protein synthesis and caused ATP depletion, oxidative stress and cell death. The increased protein synthesis and oxidative stress were necessary signals for cell death. We show that eIF2α-phosphorylation-attenuated protein synthesis, and not Atf4 mRNA translation, promotes cell survival. These results show that transcriptional induction through ATF4 and CHOP increases protein synthesis leading to oxidative stress and cell death. The findings suggest that limiting protein synthesis will be therapeutic for diseases caused by protein misfolding in the ER.

Determination of steady-state protein breakdown rate in vivo by the disappearance of protein-bound tracer-labeled amino acids: a method applicable in humans

A method to determine the rate of protein breakdown in individual proteins was developed and tested in rats and confirmed in humans, using administration of deuterium oxide and incorporation of the deuterium into alanine that was subsequently incorporated into body proteins. Measurement of the fractional breakdown rate of proteins was determined from the rate of disappearance of deuterated alanine from the proteins. The rate of disappearance of deuterated alanine from the proteins was calculated using an exponential decay, giving the fractional breakdown rate (FBR) of the proteins. The applicability of this protein-specific FBR approach is suitable for human in vivo experimentation. The labeling period of deuterium oxide administration is dependent on the turnover rate of the protein of interest.

Calorie restriction does not increase short-term or long-term protein synthesis

Increased protein synthesis is proposed as a mechanism of life-span extension during caloric restriction (CR). We hypothesized that CR does not increase protein synthesis in all tissues and protein fractions and that any increased protein synthesis with CR would be due to an increased anabolic effect of feeding. We used short- (4 hours) and long-term (6 weeks) methods to measure in vivo protein synthesis in lifelong ad libitum (AL) and CR mice. We did not detect an acute effect of feeding on protein synthesis while liver mitochondrial protein synthesis was lower in CR mice versus AL mice. Mammalian target of rapamycin (mTOR) signaling was repressed in liver and heart from CR mice indicative of energetic stress and suppression of growth. Our main findings were that CR did not increase rates of mixed protein synthesis over the long term or in response to acute feeding, and protein synthesis was maintained despite decreased mTOR signaling.

Metabolic labeling reveals proteome dynamics of mouse mitochondria

Mitochondrial dysfunction is associated with many human diseases. Mitochondrial damage is exacerbated by inadequate protein quality control and often further contributes to pathogenesis. The maintenance of mitochondrial functions requires a delicate balance of continuous protein synthesis and degradation, i.e. protein turnover. To understand mitochondrial protein dynamics in vivo, we designed a metabolic heavy water ((2)H(2)O) labeling strategy customized to examine individual protein turnover in the mitochondria in a systematic fashion. Mice were fed with (2)H(2)O at a minimal level (<5% body water) without physiological impacts. Mitochondrial proteins were analyzed from 9 mice at each of the 13 time points between 0 and 90 days (d) of labeling. A novel multiparameter fitting approach computationally determined the normalized peak areas of peptide mass isotopomers at initial and steady-state time points and permitted the protein half-life to be determined without plateau-level (2)H incorporation. We characterized the turnover rates of 458 proteins in mouse cardiac and hepatic mitochondria and found median turnover rates of 0.0402 d(-1) and 0.163 d(-1), respectively, corresponding to median half-lives of 17.2 d and 4.26 d. Mitochondria in the heart and those in the liver exhibited distinct turnover kinetics, with limited synchronization within functional clusters. We observed considerable interprotein differences in turnover rates in both organs, with half-lives spanning from hours to months (≈ 60 d). Our proteomics platform demonstrates the first large-scale analysis of mitochondrial protein turnover rates in vivo, with potential applications in translational research.

In vivo up-regulation of the unfolded protein response after hypoxia

BACKGROUND

Low oxygen (O2) availability, a condition called hypoxia, has different and profound consequences in tissues and organs. Besides the hypoxia-inducible response, mammalian cells induce a coordinated cytoprotective pathway called Unfolded Protein Response (UPR). We studied the molecular basis of UPR and apoptosis in animal models exposed to different hypoxic stresses and assessed the ability of liver and myocardium to respond to low oxygen by activating different arms of the UPR according to the severity of the insults in a tissue specific manner.

METHODS

We assessed the levels of several UPR markers in hypoxic animals by Real Time PCR and Western blotting.

RESULTS

While the hepatocytes activate the apoptotic pathway mediated, in part, by CHOP and p-JNK, we could not detect an UPR-dependent apoptosis in myocytes. Moreover, severe hypoxia results in ATF4 translation, and induction of CHOP and GADD34 transcripts in liver, by contrast in the myocardium, the ATF4-CHOP-GADD34 signaling pathway is not detectably activated.

GENERAL SIGNIFICANCE

Comparison of several UPR markers in liver and myocardium enabled to underscore the ability of hepatocytes and myocites to selectively activate and fine tune the UPR signaling pathway during hypoxia in vivo.

A comprehensive assessment of mitochondrial protein synthesis and cellular proliferation with age and caloric restriction

It is proposed that caloric restriction (CR) increases mitochondrial biogenesis. However, it is not clear why CR increases an energetically costly biosynthetic process. We hypothesized that 40% CR would decrease mitochondrial protein synthesis and would be regulated by translational rather than transcriptional mechanisms. We assessed cumulative mitochondrial protein synthesis over 6 weeks and its transcriptional and translational regulation in the liver, heart, and skeletal muscle of young (6 month), middle (12 month), and old (24 month) male B6D2F1 mice that were lifelong CR or ad lib (AL) controls. Mitochondrial protein synthesis was not different between AL and CR (fractional synthesis over 6 weeks (range): liver, 91-100%; heart, 74-85%; skeletal muscle, 53-72%) despite a decreased cellular proliferation in liver and heart with CR. With CR, there was an increase in AMP-activated protein kinase phosphorylation/total (P:T) in heart and liver, and an increase in peroxisome proliferator-activated receptor gamma coactivator 1-α mRNA in all tissues, but not protein. Ribosomal protein S6 was decreased with CR. In conclusion, CR maintained mitochondrial protein synthesis while decreasing cellular proliferation during a time of energetic stress, which is consistent with the concept that CR increases somatic maintenance. Alternative mechanisms to global translation initiation may be responsible for selective translation of mitochondrial proteins.

The application of 2H2O to measure skeletal muscle protein synthesis

Skeletal muscle protein synthesis has generally been determined by the precursor:product labeling approach using labeled amino acids (e.g., [13C]leucine or [13C]-, [15N]-, or [2H]phenylalanine) as the tracers. Although reliable for determining rates of protein synthesis, this methodological approach requires experiments to be conducted in a controlled environment, and as a result, has limited our understanding of muscle protein renewal under free-living conditions over extended periods of time (i.e., integrative/cumulative assessments). An alternative tracer, 2H2O, has been successfully used to measure rates of muscle protein synthesis in mice, rats, fish and humans. Moreover, perturbations such as feeding and exercise have been included in these measurements without exclusion of common environmental and biological factors. In this review, we discuss the principle behind using 2H2O to measure muscle protein synthesis and highlight recent investigations that have examined the effects of feeding and exercise. The framework provided in this review should assist muscle biologists in designing experiments that advance our understanding of conditions in which anabolism is altered (e.g., exercise, feeding, growth, debilitating and metabolic pathologies).

The transcription factor ATF4 promotes skeletal myofiber atrophy during fasting

Prolonged fasting alters skeletal muscle gene expression in a manner that promotes myofiber atrophy, but the underlying mechanisms are not fully understood. Here, we examined the potential role of activating transcription factor 4 (ATF4), a transcription factor with an evolutionarily ancient role in the cellular response to starvation. In mouse skeletal muscle, fasting increases the level of ATF4 mRNA. To determine whether increased ATF4 expression was required for myofiber atrophy, we reduced ATF4 expression with an inhibitory RNA targeting ATF4 and found that it reduced myofiber atrophy during fasting. Likewise, reducing the fasting level of ATF4 mRNA with a phosphorylation-resistant form of eukaryotic initiation factor 2alpha decreased myofiber atrophy. To determine whether ATF4 was sufficient to reduce myofiber size, we overexpressed ATF4 and found that it reduced myofiber size in the absence of fasting. In contrast, a transcriptionally inactive ATF4 construct did not reduce myofiber size, suggesting a requirement for ATF4-mediated transcriptional regulation. To begin to determine the mechanism of ATF4-mediated myofiber atrophy, we compared the effects of fasting and ATF4 overexpression on global skeletal muscle mRNA expression. Interestingly, expression of ATF4 increased a small subset of five fasting-responsive mRNAs, including four of the 15 mRNAs most highly induced by fasting. These five mRNAs encode proteins previously implicated in growth suppression (p21(Cip1/Waf1), GADD45alpha, and PW1/Peg3) or titin-based stress signaling [muscle LIM protein (MLP) and cardiac ankyrin repeat protein (CARP)]. Taken together, these data identify ATF4 as a novel mediator of skeletal myofiber atrophy during starvation.

A comparison of 2H2O and phenylalanine flooding dose to investigate muscle protein synthesis with acute exercise in rats

The primary objective of this investigation was to determine whether (2)H(2)O and phenylalanine (Phe) flooding dose methods yield comparable fractional rates of protein synthesis (FSR) in skeletal muscle following a single bout of high-intensity resistance exercise (RE). Sprague-Dawley rats were assigned by body mass to either 4-h control (CON 4 h; n = 6), 4-h resistance exercise (RE 4 h; n = 6), 24-h control (CON 24 h; n = 6), or 24-h resistance exercise (RE 24 h; n = 6). The RE groups were operantly conditioned to engage in a single bout of high-intensity, "squat-like" RE. All rats were given an intraperitoneal injection of 99.9% (2)H(2)O and provided 4.0% (2)H(2)O drinking water for either 24 (n = 12) or 4 h (n = 12) prior to receiving a flooding dose of l-[2,3,4,5,6-(3)H]Phe 16 h post-RE. Neither method detected an effect of RE on FSR in the mixed gastrocnemius, plantaris, or soleus muscle. Aside from the qualitative similarities between methods, the 4-h (2)H(2)O FSR measurements, when expressed in percent per hour, were quantitatively greater than the 24-h (2)H(2)O and Phe flooding in all muscles (P < 0.001), and the 24-h (2)H(2)O was greater than the Phe flooding dose in the mixed gastrocnemius and plantaris (P < 0.05). In contrast, the actual percentage of newly synthesized protein was significantly higher in the 24- vs. 4-h (2)H(2)O and Phe flooding dose groups (P < 0.001). These results suggest that the methodologies provide "qualitatively" similar results when a perturbation such as RE is studied. However, due to potential quantitative differences between methods, the experimental question should determine what approach should be used.

A novel approach for assessing protein synthesis in channel catfish, Ictalurus punctatus

A comprehensive understanding of animal growth requires adequate knowledge of protein synthesis (PS), which in fish, has traditionally been determined by the flooding dose method. However, this procedure is limited to short-term assessments and may not accurately describe fish growth over extended periods of time. Since deuterium oxide ((2)H(2)O) has been used to non-invasively quantify PS in mammals over short- and long-term periods, we aimed at determining if (2)H(2)O could also be used to measure PS in channel catfish. Fish were stocked in a 40-L aquarium with approximately 4% (2)H(2)O and sampled at 4, 8 and 24h (n=6 at each time period) to determine (2)H-labeling of body water (plasma), as well as protein-free and protein-bound (2)H-labeled alanine. The labeling of body water reflected that of aquarium water and the labeling of protein-free alanine remained constant over 24h and was approximately 3.8 times greater than that of body water. By measuring (2)H-labeled alanine incorporation after 24h of (2)H(2)O exposure we were able to calculate a rate of PS: 0.04+/-0.01% h(-1). These results demonstrate that PS in fish can be effectively measured using (2)H(2)O and, because this method yields integrative measures of PS, is relatively inexpensive and accounts for perturbations such as feeding, it is a novel and practical assessment option.

Overexpression of FOXO1 in skeletal muscle does not alter longevity in mice

Caloric restriction (CR) is the most robust and reproducible intervention that can extend lifespan in rodents. Studies in invertebrates have led to the identification of genes that regulate lifespan, some of which encode components of the insulin or insulin-like signaling pathway, including DAF-16 (C. elegans) and dFOXO (Drosophila). Mice subjected to CR for 8 weeks showed an increase in FOXO1 mRNA and other longevity-related genes: Gadd 45alpha, glutamine synthase, and catalase in skeletal muscle. To investigate whether FOXO1 expression affects longevity in mammals, transgenic mice were studied that over-express FOXO1 in their skeletal muscle (FOXO1 mice), and in which muscle atrophy occurs. FOXO1 mice showed increases in Gadd 45alpha, and glutamine synthase proteins in skeletal muscle. In FOXO1 mice, the phosphorylation/dephosphorylation state of the p70 S6K and 4E-BP1 proteins were not altered, suggesting that translation initiation of protein synthesis might not be suppressed. The lifespan of FOXO1 mice was similar to their wild-type littermates. FOXO1 overexpression could not prevent aging-induced reduction in catalase, CuZu-SOD, and Mn-SOD mRNA in skeletal muscle. These data suggest that an increase in FOXO1 protein and its activation in skeletal muscle does not extend lifespan in mice.