The impact of immobilisation and inflammation on the regulation of muscle mass and insulin resistance: different routes to similar end points

Loss of muscle mass and insulin sensitivity are common phenotypic traits of immobilisation and increased inflammatory burden. The suppression of muscle protein synthesis is the primary driver of muscle mass loss in human immobilisation, and includes blunting of post‐prandial increases in muscle protein synthesis. However, the mechanistic drivers of this suppression are unresolved. Immobilisation also induces limb insulin resistance in humans, which appears to be attributable to the reduction in muscle contraction per se. Again mechanistic insight is missing however, such that we do not know how muscle senses its “inactivity status” or whether the proposed drivers of muscle insulin resistance are simply arising as a consequence of immobilisation. An heightened inflammatory state is associated with major and rapid changes in muscle protein turnover and mass, and dampened insulin‐stimulated glucose disposal and oxidation in both rodents and humans. A limited amount of research has attempted to elucidate molecular regulators of muscle mass loss and insulin resistance during increased inflammatory burden, but rarely concurrently. Nevertheless, there is evidence that Akt (protein kinase B) signalling and FOXO transcription factors form part of a common signalling pathway in this scenario, such that molecular cross‐talk between atrophy and insulin signalling during heightened inflammation is believed to be possible (Fig. 1). To conclude, whilst muscle mass loss and insulin resistance are common end‐points of immobilisation and increased inflammatory burden, a lack of understanding of the mechanisms responsible for these traits exists such that a substantial gap in understanding of the pathophysiology in humans endures

Increasing cardiac pyruvate dehydrogenase flux during chronic hypoxia improves acute hypoxic tolerance

The pattern of metabolic reprogramming in chronic hypoxia shares similarities with that following myocardial infarction or hypertrophy, however the response of the chronically hypoxic heart to subsequent acute injury, and the role of metabolism is not well understood. Here, we determined the myocardial tolerance of the chronically hypoxic heart to subsequent acute injury, and hypothesised that activation of a key regulator of myocardial metabolism, the pyruvate dehydrogenase complex (PDC), could improve hypoxic tolerance. Mouse hearts, perfused in Langendorff mode, were exposed to 30min of hypoxia, and lost 80% of prehypoxic function (p=0.001), with only 27% recovery of pre-hypoxic function with 30min of re-oxygenation (p=0.046). Activation of the PDC with infusion of 1mM dicholorocacetate (DCA) during hypoxia and re-oxygenation did not alter function. Acute hypoxic tolerance was assessed in hearts of mice housed in hypoxia for 3wks. Chronic hypoxia reduced cardiac tolerance to subsequent acute hypoxia, with recovery of function 22% of pre-acute hypoxic levels, vs 39% in normoxic control hearts (p=0.012). DCA feeding in chronic hypoxia (per os, 70mg/kg/day) doubled cardiac acetylcarnitine content, and this fell following acute hypoxia. This acetylcarnitine use maintained cardiac ATP and glycogen content during acute hypoxia, with hypoxic tolerance normalised. In summary, chronic hypoxia renders the heart more susceptible to acute hypoxic injury, which can be improved by activation of the PDC and pooling of acetylcarnitine. This is the first study showing functional improvement of the chronically hypoxic heart with activation of the PDC, and offers therapeutic potential in cardiac disease with a hypoxic component

Maximal-intensity exercise does not fully restore muscle pyruvate dehydrogenase complex activation after 3 days of high-fat dietary intake

Background & aims: Exercise activates muscle pyruvate dehydrogenase complex (PDC), but moderate intensity exercise fails to fully activate muscle PDC after high-fat diet [1]. We investigated whether maximal intensity exercise overcomes this inhibition. Methods: Quadriceps femoris muscle biopsy samples were obtained from healthy males at rest, and after 46 and 92 electrically-evoked maximal intermittent isometric contractions, which were preceded by 3 days of either low- (18%) or high- (69%) isocaloric dietary fat intake (LFD and HFD, respectively). Results: The ratio of PDCa (active form) to total PDCt (fully activated) at rest was 50% less after HFD (0.32 ± 0.01 vs 0.15 ± 0.01; P<0.05). This ratio increased to 0.77 ± 0.06 after 46 contractions (P<0.001) and to 0.98 ± 0.07 after 92 contractions (P<0.001) in LFD. The corresponding values after HFD were less (0.54 ± 0.06; P<0.01 and 0.70 ± 0.07; P<0.01, respectively). Resting muscle acetyl-CoA and acetylcarnitine content was greater after HFD than LFD (both P<0.05), but their rate of accumulation in the former was reduced during contraction. Muscle lactate content after 92 contractions was 30% greater after HFD (P<0.05). Muscle force generation during contraction was no different between interventions, but HFD lengthened muscle relaxation time (P<0.05). Daily urinary total carnitine excretion after HFD was 2.5-fold greater than after LFD (P<0.01). Conclusions: A bout of maximal intense exercise did not overcome dietary fat-mediated inhibition of muscle pyruvate dehydrogenase complex activation, and was associated with greater muscle lactate accumulation, as a result of lower PDC flux, and increased muscle relaxation time

The effect of age and unilateral leg immobilisation for 2 weeks on substrate ulilisation during moderate-intensity exercise in human skeletal muscle

Age and inactivity have been associated with intramuscular triglyceride (IMTG) accumulation. Here, we attempt to disentangle these factors by studying the effect of 2 weeks of unilateral leg immobilization on substrate utilization across the legs during moderate-intensity exercise in young (n = 17; 23 ± 1 years old) and older men (n = 15; 68 ± 1 years old), while the contralateral leg served as the control. After immobilization, the participants performed two-legged isolated knee-extensor exercise at 20±1W(_50% maximalwork capacity) for 45 min with catheters inserted in the brachial artery and both femoral veins.Biopsy samples obtained from vastus lateralis muscles of both legs before and after exercise were used for analysis of substrates, protein content and enzyme activities. During exercise, leg substrate utilization (respiratoryquotient) did not differ between groups or legs. Leg fatty acid uptake was greater in older than in young men, and although young men demonstrated net leg glycerol release during exercise, older men showed net glycerol uptake. At baseline, IMTG, muscle pyruvate dehydrogenase complex activity and the protein content of adipose triglyceride lipase, acetyl-CoA carboxylase 2 and AMP-activated protein kinase (AMPK)γ3 were higher in young than in older men. Furthermore, adipose triglyceride lipase, plasma membrane-associated fatty acid binding protein and AMPKγ3 subunit protein contents were lower and IMTG was higher in the immobilized than the contralateral leg in young and older men. Thus, immobilization and age did not affect substrate choice (respiratory quotient) during moderate exercise, but the whole-leg and molecular differences in fatty acid mobilization could explain the age- and immobilization-induced IMTG accumulation

Mitochondrial DNA copy number associates with insulin sensitivity and aerobic capacity, and differs between sedentary, overweight middle-aged males with and without type 2 diabetes

Background/objectives: Increased risk of type 2 diabetes mellitus (T2DM) is linked to impaired muscle mitochondrial function and reduced mitochondrial DNA copy number (mtDNAnum). However, studies have failed to control for habitual physical activity levels, which directly influences both mtDNA copy number and insulin sensitivity. We, therefore, examined whether physical conditioning status (maximal oxygen uptake, V̇O2max) was associated with skeletal muscle mitochondrial volume and mtDNAnum, and was predictive of T2DM in overweight, middle-aged men.Methods: Whole-body physiological (ISI-insulin sensitivity index, HOMA-IR, V̇O2max) and muscle biochemical/molecular (vastus lateralis; mtDNAnum, mitochondrial and glycolytic enzymes activity, lipid content and markers of lipid peroxidation) measurements were performed in 3 groups of overweight, middle-aged male volunteers (n=10 per group): sedentary T2DM (ST2DM); sedentary control (SC) and non-sedentary control (NSC), who differed in aerobic capacity (ST2D

The Regulatory Roles of PPARs in Skeletal Muscle Fuel Metabolism and Inflammation: Impact of PPAR Agonism on Muscle in Chronic Disease, Contraction and Sepsis

The peroxisome proliferator-activated receptor (PPAR) family of transcription factors has been demonstrated to play critical roles in regulating fuel selection, energy expenditure and inflammation in skeletal muscle and other tissues. Activation of PPARs, through endogenous fatty acids and fatty acid metabolites or synthetic compounds, has been demonstrated to have lipid-lowering and anti-diabetic actions. This review will aim to provide a comprehensive overview of the functions of PPARs in energy homeostasis, with a focus on the impacts of PPAR agonism on muscle metabolism and function. The dysregulation of energy homeostasis in skeletal muscle is a frequent underlying characteristic of inflammation-related conditions such as sepsis. However, the potential benefits of PPAR agonism on skeletal muscle protein and fuel metabolism under these conditions remains under-investigated and is an area of research opportunity. Thus, the effects of PPARγ agonism on muscle inflammation and protein and carbohydrate metabolism will be highlighted, particularly with its potential relevance in sepsis-related metabolic dysfunction. The impact of PPARδ agonism on muscle mitochondrial function, substrate metabolism and contractile function will also be described

Peroxisome proliferator-activated receptor γ agonism attenuates endotoxaemia-induced muscle protein loss and lactate accumulation in rats

The peroxisome proliferator-activated receptor γ (PPARγ) agonist rosiglitazone (Rosi) appears to provide protection against organ dysfunction during endotoxaemia. We examined the potential benefits of Rosi on skeletal muscle protein maintenance and carbohydrate metabolism during lipopolysaccharide (LPS)-induced endotoxaemia. Sprague-Dawley rats were fed either standard chow (control) or standard chow containing Rosi (8.5±0.1 mg.kg-1.day-1) for two weeks before and during 24 h continuous intravenous infusion of LPS (15 μg.kg-1.h-1) or saline. Rosi blunted LPS-induced increases in muscle tumor necrosis factor-α (TNF-α) and interleukin-6 (IL-6) mRNA by 70% (P<0.05) and 64% (P<0.01), respectively. Furthermore, Rosi suppressed the LPS-induced reduction in phosphorylated AKT and phosphorylated Forkhead box O (FOXO) 1 protein, as well as the upregulation of muscle RING finger 1 (MuRF1; P<0.01) mRNA, and the LPS-induced increase in 20S proteasome activity (P<0.05). Accordingly, LPS reduced the muscle protein:DNA ratio (~30%, P<0.001), which Rosi offset. Increased muscle pyruvate dehydrogenase kinase 4 (PDK4) mRNA (P<0.001) and muscle lactate accumulation (P<0.001) during endotoxaemia were suppressed by Rosi. Thus, pre-treatment with Rosi reduced muscle cytokine accumulation and blunted muscle protein loss and lactate accumulation during endotoxaemia, and at least in part by reducing activation of molecular events known to increase muscle protein breakdown and mitochondrial pyruvate use

Do Antiretroviral Drugs Protect From Multiple Sclerosis By Inhibiting Expression Of MS-Associated Retrovirus?

The expression of human endogenous retroviruses (HERVs) has been associated with Multiple Sclerosis (MS). The MS-related retrovirus (MSRV/HERV-W) has the potential to activate inflammatory immunity, which could promote both susceptibility and progression towards MS. A connection between HERVs and MS is also supported by the observation that people infected with the human immunodeficiency virus (HIV) may have a lower risk of developing MS than the HIV non-infected, healthy population. This may be due to suppression of HERV expression by antiretroviral therapies (ART) used to treat HIV infection.In this pilot study, we compared RNA expression of the envelope gene of MSRV/HERV-W, as well as Toll-like receptors (TLR) 2 and 4, in a small cohort of HIV+ patients with MS patients and healthy controls (HC). An increased expression of MSRV/HERV-Wenv and TLR2 RNA was detected in blood of MS patients compared with HIV patients and HC, while TLR4 was increased in both MS and HIV patients. There was, however, no difference in MSRV/HERV-Wenv, TLR2 and TLR4 expression between ART-treated and -untreated HIV patients. The viral protein Env was expressed mainly by B cells and monocytes, but not by T cells and EBV infection could induce the expression of MSRV/HERV-Wenv in Lymphoblastoid cell lines (LCLs). LCLs were therefore used as an in vitro system to test the efficacy of ART in inhibiting the expression of MSRV/HERV-Wenv. Efavirenz (a non-nucleoside reverse transcriptase inhibitor) alone or different combined drugs could reduce MSRV/HERV-Wenv expression in vitro. Further experiments are needed to clarify the potential role of ART in protection from MS

Major elective abdominal surgery acutely impairs lower limb muscle pyruvate dehydrogenase complex activity and mitochondrial function

© 2020 The Author(s) Background & aims: This post hoc study aimed to determine whether major elective abdominal surgery had any acute impact on mitochondrial pyruvate dehydrogenase complex (PDC) activity and maximal mitochondrial ATP production rates (MAPR) in a large muscle group (vastus lateralis -VL) distant to the site of surgical trauma. Methods: Fifteen patients undergoing major elective open abdominal surgery were studied. Muscle biopsies were obtained after the induction of anesthesia from the VL immediately before and after surgery for the determination of PDC and maximal MAPR (utilizing a variety of energy substrates). Results: Muscle PDC activity was reduced by >50% at the end of surgery compared with pre-surgery (p < 0.05). Muscle MAPR were comprehensively suppressed by surgery for the substrate combinations: glutamate + succinate; glutamate + malate; palmitoylcarnitine + malate; and pyruvate + malate (all p < 0.05), and could not be explained by a lower mitochondrial yield. Conclusions: PDC activity and mitochondrial ATP production capacity were acutely impaired in muscle distant to the site of surgical trauma. In keeping with the limited data available, we surmise these events resulted from the general anesthesia procedures employed and the surgery related trauma. These findings further the understanding of the acute dysregulation of mitochondrial function in muscle distant to the site of major surgical trauma in patients, and point to the combination of general anesthesia and trauma related inflammation as being drivers of muscle metabolic insult that warrants further investigation. Clinical trial registration: Registered at (NCT01134809)

Effects of Endotoxaemia on Protein Metabolism in Rat Fast-Twitch Skeletal Muscle and Myocardium

It is unclear if the rat myocardium undergoes the same rapid reductions in protein content that are classically observed in fast-twitch skeletal muscle during endotoxaemia.To investigate this further, and to determine if there is any divergence in the response of skeletal muscle and myocardium in the mechanisms that are thought to be largely responsible for eliciting changes in protein content, Sprague Dawley rats were implanted with vascular catheters and administered lipopolysaccharide (LPS; 150 microg kg(-1) h(-1)) intravenously for 2 h, 6 h or 24 h (saline administered control animals were also included), after which the extensor digitorum longus (EDL) and myocardium were removed under terminal anaesthesia. The protein-to-DNA ratio, a marker of protein content, was significantly reduced in the EDL following 24 h LPS administration (23%; P<0.05), but was no different from controls in the myocardium. At the same time point, a significant increase in MAFbx/atrogin-1 and MuRF1 mRNA (3.7+/-0.7- and 19.5+/-1.9-fold increase vs. controls, respectively; P<0.05), in addition to protein levels of alpha1-3, 5-7 subunits of the 20S proteasome, were observed in EDL but not myocardium. In contrast, elevations in phosphorylation of p70 S6K residues Thr(421)/Ser(424), and 4E-BP1 residues Thr(37)/Thr(46) (P<0.05), consistent with an elevation in translation initiation, were seen exclusively in the myocardium of LPS-treated animals.In summary, these findings suggest that the myocardium does not undergo the same catabolic response as skeletal muscle during early endotoxaemia, partly due to the absence of transcriptional and signalling events in the myocardium typically associated with increased muscle proteolysis and the suppression of protein synthesis