N tau-methylhistidine turnover in skeletal muscle cells measured by GC-MS

Quantification of Nτ -Methylhistidine and Nπ-Methylhistidine in Chicken Plasma by Liquid Chromatography-Tandem Mass Spectrometry

The concentration of N^τ-methylhistidine in plasma provides an index of skeletal muscle protein breakdown. This study aimed to establish a quantitative method for measuring the concentrations of N^τ-methylhistidine and its isomer N^π-methylhistidine in chicken plasma, using liquid chromatography-tandem mass spectrometry with stable isotope dilution analysis. The acceptable linear ranges of detection were 1.56-50.00 μmol/L for N^τ-methylhistidine and 0.78-25.00 μmol/L for N^π-methylhistidine. The proposed method detected changes in the plasma levels of N^τ-methylhistidine and N^π-methylhistidine in response to fasting and re-feeding. These results suggest that the method developed in this study can be used for the simultaneous measurement of N^τ-methylhistidine and N^π-methylhistidine in chicken plasma.

Assessment of skeletal muscle proteolysis and the regulatory response to nutrition and exercise

Skeletal muscle proteolysis is highly regulated, involving complex intramuscular proteolytic systems that recognize and degrade muscle proteins, and recycle free amino acid precursors for protein synthesis and energy production. Autophagy-lysosomal, calpain, and caspase systems are contributors to muscle proteolysis, although the ubiquitin proteasome system (UPS) is the primary mechanism by which actomyosin fragments are degraded in healthy muscle. The UPS is sensitive to mechanical force and nutritional deprivation, as recent reports have demonstrated increased proteolytic gene expression and activity of the UPS in response to resistance and endurance exercise, and short-term negative energy balance. However, consuming dietary protein alone (or free amino acids), or as a primary component of a mixed meal, may attenuate intramuscular protein loss by down-regulating proteolytic gene expression and the catabolic activity of the UPS. Although these studies provide novel insight regarding the intramuscular regulation of skeletal muscle mass, the role of proteolysis in the regulation of skeletal muscle protein turnover in healthy human muscle is not well described. This article provides a contemporary review of the intramuscular regulation of skeletal muscle proteolysis in healthy muscle, methodological approaches to assess proteolysis, and highlights the effects of nutrition and exercise on skeletal muscle proteolysis.

Hyperammonaemia in V1a vasopressin receptor knockout mice caused by the promoted proteolysis and reduced intrahepatic blood volume

An analysis of arginine-vasopressin (AVP) V1a receptor-deficient (V1aR-/-) mice revealed that glucose homeostasis and lipid metabolism were altered in the mutant mice. Here, we used V1aR-/- mice to investigate whether the deficiency of the V1a receptor, which led to altered insulin sensitivity, affected protein metabolism. The serum 3-methylhistidine levels were increased in V1aR-/- mice under feeding conditions, indicating that proteolysis was enhanced in muscle tissue from V1aR-/- mice. Furthermore, serum amino acid profiling revealed that the amino acid levels, including glycogenic and branched-chain amino acids, were reduced in V1aR-/- mice. In addition, an alanine-loading test showed that gluconeogenesis was enhanced in V1aR-/- mice. Blood ammonia, which is a by-product of amino acid catabolism, was two times higher in V1aR-/- mice without hepatopathy under the feeding and fasting conditions than in wild-type mice. Amino acid profiling also revealed that the amino acid pattern was not typical of a urea-cycle enzymatic disorder. An ammonia tolerance test and an indocyanine green elimination test showed that V1aR-/- mice had lower ammonia clearance due to a decreased intrahepatic circulating blood volume. Metabolic acidosis, including lactic- and keto-acidosis, was not observed in V1aR-/- mice. These results provide evidence that proteolysis promotes the production of glucose in the muscles of V1aR-/- mice and that hyperammonaemia is caused by promoted protein catabolism and reduced intrahepatic blood volume. Thus, our study with V1aR-/- mice indicates that AVP plays a physiological role via the V1a receptor in regulating both protein catabolism and glucose homeostasis.

The effects of breed and level of nutrition on whole-body and muscle protein metabolism in pure-bred Aberdeen Angus and Charolais beef steers

Eighteen pure-bred steers (live weight 350 kg) from each of two breeds, Aberdeen Angus (AA) and Charolais (CH), were split into three equal groups (six animals each) and offered three planes of nutrition during a 20-week period. The same ration formulation was offered to all animals with amounts adjusted at 3-week intervals to give predicted average weight gains of either 1·0 kg/d (M/M group) or 1·4 kg/d (H/H group). The remaining group (M/H) were offered the same amount of ration as the M/M group until 10 weeks before slaughter when the ration was increased to H. Data on animal performance, carcass characteristics and fibre-type composition in skeletal muscle are presented elsewhere (; ). On three occasions (17, 10 and 2 weeks before slaughter) the animals were transferred to metabolism stalls for 1 week, during which total urine collection for quantification of Nτ-methylhistidine (Nτ-MeH) elimination was performed for 4 d. On the last day, animals were infused for 11 h with [2H5] phenylalanine with frequent blood sampling (to allow determination of whole-body phenylalanine flux) followed by biopsies from m. longissimus lumborum and m. vastus lateralis to determine the fractional synthesis rate of mixed muscle protein. For both breeds, the absolute amount of Nτ-MeH eliminated increased with animal age or weight (P < 0·001) and was significantly greater for CH steers, at all intake comparisons, than for AA (P < 0·001). Estimates of fractional muscle breakdown rate (FBR; calculated from Nτ-MeH elimination and based on skeletal muscle as a fixed fraction of live weight) showed an age (or weight) decline for M/M and H/H groups of both breeds (P < 0·001). FBR was greater for the H/H group (P = 0·044). The M/H group also showed a lower FBR for the first two measurement periods (both at M intake) but increased when intake was raised to H. When allowance was made for differences in lean content (calculated from fat scores and eye muscle area in carcasses at the end of period 3), there were significant differences in muscle FBR with intake (P = 0·012) but not between breed. Whole-body protein flux (WBPF; g/d) based on plasma phenylalanine kinetics increased with age or weight (P < 0·001) and was similar between breeds. The WBPF was lower for M/M compared with H/H (P < 0·001) based on either total or per kg live weight0·75. Muscle protein fractional synthesis rate (FSR) declined with age for both breeds and tended to be higher at H/H compared with M intakes (intake × period effects, P < 0·05). Changing intake from M to H caused a significant increase (P < 0·001) in FSR. The FSR values for AA were significantly greater than for CH at comparable ages (P = 0·044). Although FSR and FBR responded to nutrition, these changes in protein metabolism were not reflected in differences in meat eating quality (Sinclair et al. 2000).

Induction of protein degradation in skeletal muscle by a phorbol ester involves upregulation of the ubiquitin-proteasome proteolytic pathway

Although muscle atrophy is common to a number of disease states there is incomplete knowledge of the cellular mechanisms involved. In this study murine myotubes were treated with the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) to evaluate the role of protein kinase C (PKC) as an upstream intermediate in protein degradation. TPA showed a parabolic dose-response curve for the induction of total protein degradation, with an optimal effect at a concentration of 25 nM, and an optimal incubation time of 3 h. Protein degradation was attenuated by co-incubation with the proteasome inhibitor lactacystin (5 microM), suggesting that it was mediated through the ubiquitin-proteasome proteolytic pathway. TPA induced an increased expression and activity of the ubiquitin-proteasome pathway, as evidenced by an increased functional activity, and increased expression of the 20S proteasome alpha-subunits, the 19S subunits MSS1 and p42, as well as the ubiquitin conjugating enzyme E2(14k), also with a maximal effect at a concentration of 25 nM and with a 3 h incubation time. There was also a reciprocal decrease in the cellular content of the myofibrillar protein myosin. TPA induced activation of PKC maximally at a concentration of 25 nM and this effect was attenuated by the PKC inhibitor calphostin C (300 nM), as was also total protein degradation. These results suggest that stimulation of PKC in muscle cells initiates protein degradation through the ubiquitin-proteasome pathway. TPA also induced degradation of the inhibitory protein, I-kappaBalpha, and increased nuclear accumulation of nuclear factor-kappaB (NF-kappaB) at the same time and concentrations as those inducing proteasome expression. In addition inhibition of NF-kappaB activation by resveratrol (30 microM) attenuated protein degradation induced by TPA. These results suggest that the induction of proteasome expression by TPA may involve the transcription factor NF-kappaB.

Amelioration of denervation-induced atrophy by clenbuterol is associated with increased PKC-alpha activity

Rat soleus muscle was denervated for 3 or 7 days, and total membrane protein kinase C (PKC) activity and translocation and immunocytochemical localization of PKC isoforms were examined. Dietary administration of clenbuterol concomitant with denervation ameliorated the atrophic response and was associated with increased membrane PKC activity at both 3 (140%) and 7 (190%) days. Of the five PKC isoforms (alpha, epsilon, theta, zeta, and mu) detected in soleus muscle by Western immunoblotting, clenbuterol treatment affected only the PKC-alpha and PKC-theta forms. PKC-alpha was translocated to the membrane fraction upon denervation, and the presence of clenbuterol increased membrane-bound PKC-alpha and active PKC-alpha as assayed by Ser(657) phosphorylation. PKC-theta protein was downregulated upon denervation, and treatment with clenbuterol further decreased both cytosolic and membrane levels. Immunolocalization of PKC-theta showed differences for regulatory and catalytic domains, with the latter showing fast-fiber type specificity. The results suggest potential roles of PKC-alpha and PKC-theta in the mechanism of action of clenbuterol in alleviating denervation-induced atrophy.

Stimulation of myofibrillar protein degradation and expression of mRNA encoding the ubiquitin-proteasome system in C(2)C(12) myotubes by dexamethasone: effect of the proteasome inhibitor MG-132

Addition of the synthetic glucocorticoid, dexamethasone (Dex) to serum-deprived C(2)C(12) myotubes elicited time- and concentration-dependent changes in N(tau)-methylhistidine (3-MH), a marker of myofibrillar protein degradation. Within 24 h, 100 nM Dex significantly decreased the cell content of 3-MH and increased release into the medium. Both of these responses had increased in magnitude by 48 h and then declined toward basal values by 72 h. The increase in the release of 3-MH closely paralleled its loss from the cell protein. Furthermore, Dex also decreased the 3-MH:total cell protein ratio, suggesting that myofibrillar proteins were being preferentially degraded. Incubation of myotubes with the peptide aldehyde, MG-132, an inhibitor of proteolysis by the (ATP)-ubiquitin (Ub)-dependent proteasome, prevented both the basal release of 3-MH (>95%) and the increased release of 3-MH into the medium in response to Dex (>95%). Northern hybridization studies demonstrated that Dex also elicited similar time- and concentration-dependent increases in the expression of mRNA encoding two components (14 kDa E(2) Ub-conjugating enzyme and Ub) of the ATP-Ub-dependent pathway. The data demonstrate that Dex stimulates preferential hydrolysis of myofibrillar proteins in C(2)C(12) myotubes and suggests that the ATP-Ub-dependent pathway is involved in this response.

Resistance exercise training increases mixed muscle protein synthesis rate in frail women and men >/=76 yr old

Muscle atrophy (sarcopenia) in the elderly is associated with a reduced rate of muscle protein synthesis. The purpose of this study was to determine if weight-lifting exercise increases the rate of muscle protein synthesis in physically frail 76- to 92-yr-old women and men. Eight women and 4 men with mild to moderate physical frailty were enrolled in a 3-mo physical therapy program that was followed by 3 mo of supervised weight-lifting exercise. Supervised weight-lifting exercise was performed 3 days/wk at 65-100% of initial 1-repetition maximum on five upper and three lower body exercises. Compared with before resistance training, the in vivo incorporation rate of [(13)C]leucine into vastus lateralis muscle protein was increased after resistance training in women and men (P < 0.01), although it was unchanged in five 82 +/- 2-yr-old control subjects studied two times in 3 mo. Maximum voluntary knee extensor muscle torque production increased in the supervised resistance exercise group. These findings suggest that muscle contractile protein synthetic pathways in physically frail 76- to 92-yr-old women and men respond and adapt to the increased contractile activity associated with progressive resistance exercise training.

Quantitation of blood and plasma amino acids using isotope dilution electron impact gas chromatography/mass spectrometry with U-(13)C amino acids as internal standards

A method to quantitate blood and plasma amino acids by isotope dilution gas chromatography/mass spectrometry (GC/mS) is described. Samples were spiked with U-(13)C amino acids as internal standards and the tert-butyldimethylsilyl derivatives (tBDMS) separated by capillary column gas chromatography. Linear regression curves, generated for individual amino acids, gave correlation coefficients of 0.9999. The reproducibility of the method was assessed from the analysis of 10 replicate blood and plasma samples. For most amino acids a coefficient of variance (CV) of </=1% was obtained with the exception of aspartate which gave a value of 1.8%. This was probably due to the low concentration of this amino acid in the samples analysed. Recovery of amino acids added to plasma was between 96 and 103%. The use of electron impact ionization (EI) allows the method to be used in laboratories where only the more basic GC/mS is available and reduces the time spent on instrument maintenance. The method should prove useful in areas of work where accurate and precise amino acid concentrations are required.

Signalling pathways regulating protein turnover in skeletal muscle

The protein content of skeletal muscle is determined by the relative rates of synthesis and degradation which must be regulated coordinately to maintain equilibrium. However, in conditions such as fasting where amino acids are required for gluconeogenesis, or in cancer cachexia, this equilibrium is disrupted and a net loss of protein ensues. This review, utilising studies performed in several situations, summarizes the current state of knowledge on the possible signalling pathways regulating protein turnover in skeletal muscle and highlights areas for future work.

Regulation of phospholipase D in L6 skeletal muscle myoblasts. Role of protein kinase c and relationship to protein synthesis

The addition of vasopressin or 12-O-tetradecanoylphorbol-13-acetate (TPA) to prelabeled L6 myoblasts elicited increases in [14C]ethanolamine release, suggesting the activation of phospholipase D activity or activities. While the effects of both agonists on intracellular release were rapid and transient, when extracellular release of [14C]ethanolamine was measured, the effect of vasopressin was again rapid and transient, whereas that of TPA was delayed but sustained. Effects of both agonists on intra- and extracellular release were inhibited by the protein kinase C (PKC) inhibitor, Ro-31-8220, and PKC down-regulation by preincubation with TPA. The formation of phosphatidylbutanol elicited by vasopressin and TPA mirrored their effects on extracellular [14C]ethanolamine release in that the former was transient, whereas the latter was sustained. Responses to both agonists were abolished by PKC down-regulation. When protein synthesis was examined, the stimulation of translation by TPA and transcription by vasopressin were inhibited by Ro-31-8220. In contrast, down-regulation of PKC inhibited the synthesis response to TPA but not vasopressin. Furthermore, following down-regulation, the effect of vasopressin was still blocked by the PKC inhibitors, Ro-31-8220 and bisindolylmaleimide. Analysis of PKC isoforms in L6 cells showed the presence of alpha, epsilon, delta, mu, iota, and zeta. Down-regulation removed both cytosolic (alpha) and membrane-bound (epsilon and delta) isoforms. Thus, the elevation of phospholipase D activity or activities induced by both TPA and vasopressin and the stimulation of translation by TPA involves PKC-alpha, -epsilon, and/or -delta. In contrast, the increase in transcription elicited by vasopressin involves mu, iota, and/or zeta. Hence, although phospholipase D may be linked to increases in translation elicited by TPA, it is not involved in the stimulation of transcription by vasopressin.