Degradation of intracellular protein in muscle. Lysosomal response to modified proteins and chloroquine

Reduced clearance of proteins labeled with diisopropylfluorophosphate in portacaval-shunted rats

Portacaval shunting is a model for hepatic encephalopathy that causes chronic hyperammonemia, disruption of metabolic, signaling, and neurotransmitter systems, and progressive morphological changes. Exposure of cultured cells to ammonia raises intralysosomal pH and inhibits proteolysis, and the present study tested the hypothesis that proteolytic capacity is diminished in portacaval-shunted rats. Proteins were labeled in vivo with tracer doses of diisopropylfluorophosphate (DFP) and clearance of label was assayed. This approach labeled proteins independent of protein synthesis, which is reported to be altered in shunted rats, and avoided complications arising from re-utilization of labeled amino acids that causes underestimation of degradation rate. Characterization of DFP labeling showed that protein labeling was fast, about 50% of the label was released during a 24 h interval, labeling by DFP metabolites was negligible, inhibition of brain acetylcholinesterase was not detectable, and labeling by [(3)H]- and [(14)C]DFP was equivalent. To assay degradative capacity, proteins were first labeled with [(3)H]DFP, followed by labeling with [(14)C]DFP that was given 24 or 72 h later. The (3)H/(14)C ratio in each animal was used as a relative measure of removal of (3)H-labeled proteins. (3)H/(14)C ratios were generally significantly higher in portacaval-shunted rats than in controls, consistent with reduced proteolytic capacity. Assays of amino acid incorporation into brain protein generally replicated literature reports, supporting the conclusion that protein synthesis unlikely to be markedly inhibited and amino acid recycling influences calculated protein synthesis rates in shunted rats. Therapeutic strategies to reduce ammonia level would help normalize lysosomal functions and protein and lipid turnover.

Release of free amino-acids during ageing in bovine meat

The amounts of free amino-acids in Longissimus dorsi (LD), Triceps brachii (TB) and Rectus femoris (RF) from 8 Friesian 20-month old bulls were determinated using gas chromatography. The amounts of free amino-acids released during storage from 3 to 14 days post mortem at 4 °C depended on the muscle. Total free hydroxyproline in LD increased from 3% at 3 days to 11% at 14 days post mortem. Free amino-acids seemed to increase more after 10 days than between 3 and 10 days post mortem. The myofibrillar fragmentation index was highly correlated with leucine, isoleucine, threonine and methionine contents in TB and RF. MFI was correlated with hydroxyproline, leucine and isoleucine in LD.

Prophylactic effects of swimming exercise on autophagy-induced muscle atrophy in diabetic rats

Diabetes decreases skeletal muscle mass and induces atrophy. However, the mechanisms by which hyperglycemia and insulin deficiency modify muscle mass are not well defined. In this study, we evaluated the effects of swimming exercise on muscle mass and intracellular protein degradation in diabetic rats, and proposed that autophagy inhibition induced by swimming exercise serves as a hypercatabolic mechanism in the skeletal muscles of diabetic rats, supporting a notion that swimming exercise could efficiently reverse the reduced skeletal muscle mass caused by diabetes. Adult male Sprague-Dawley rats were injected intraperitoneally with streptozotocin (60 mg/kg body weight) to induce diabetes and then submitted to 1 hr per day of forced swimming exercise, 5 days per week for 4 weeks. We conducted an intraperitoneal glucose tolerance test on the animals and measured body weight, skeletal muscle mass, and protein degradation and examined the level of autophagy in the isolated extensor digitorum longus, plantaris, and soleus muscles. Body weight and muscle tissue mass were higher in the exercising diabetic rats than in control diabetic rats that remained sedentary. Compared to control rats, exercising diabetic rats had lower blood glucose levels, increased intracellular contractile protein expression, and decreased autophagic protein expression. We conclude that swimming exercise improves muscle mass in diabetes-induced skeletal muscle atrophy, suggesting the activation of autophagy in diabetes contributes to muscle atrophy through hypercatabolic metabolism and that aerobic exercise, by suppressing autophagy, may modify or reverse skeletal muscle wasting in diabetic patients.

Metabolic stress induces the lysosomal degradation of neuropilin-1 but not neuropilin-2

The neuropilins-1 and -2 (NRP1 and NRP2) function as receptors for both the semaphorins and vascular endothelial growth factor. In addition to their contribution to the development of the nervous system, NRP1 and NRP2 have been implicated in angiogenesis and tumor progression. Given their importance to cancer and endothelial biology and their potential as therapeutic targets, an important issue that has not been addressed is the impact of metabolic stress conditions characteristic of the tumor microenvironment on their expression and function. Here, we demonstrate that hypoxia and nutrient deprivation stimulate the rapid loss of NRP1 expression in both endothelial and carcinoma cells. NRP2 expression, in contrast, is maintained under these conditions. The lysosomal inhibitors chloroquine and bafilomycin A1 prevented the loss of NRP1 expression, but proteasomal inhibitors had no effect. The hypothesis that NRP1 is degraded by autophagy is supported by the findings that its expression is lost rapidly in response to metabolic stress, prevented with 3-methyladenine and induced by rapamycin. Targeted depletion of NRP2 using small hairpin RNA revealed that NRP2 can function in the absence of NRP1 to mediate endothelial tube formation in hypoxia. Studies aimed at assessing NRP function and targeted therapy in cancer and angiogenesis should consider the impact of metabolic stress.

Expression of autophagy-associated genes in skeletal muscle: an experimental model of chloroquine-induced myopathy

OBJECTIVE

Chloroquine modulates autophagocytic protein degradation in the lysosome system, thereby inducing the formation of rimmed vacuoles consisting of autophagosomes and autolysosomes in skeletal muscle. The goal of this study was to investigate the contribution of the lysosomal system, particularly autophagosome formation (an autophagic process) at the molecular level, to the abnormal accumulation of vacuoles in an experimental model of chloroquine-induced myopathy.

METHODS

Histological, immunohistochemical and semiquantitative reverse transcriptase-polymerase chain reaction studies were performed on innervated and denervated rat soleus muscles after treatment with either saline or chloroquine.

RESULTS

Accumulation of rimmed vacuoles was observed only in chloroquine-treated denervated muscles. Microtubule-associated protein-1 light chain-3 (LC3) protein and mRNA levels were significantly increased exclusively in denervated muscles from chloroquine-treated rats, whereas Apg5 and Apg12 mRNA levels did not change significantly. Further, the mRNA levels of UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase (GNE), which are associated with distal myopathy with rimmed vacuoles showing numerous rimmed vacuoles in its skeletal muscle, were not decreased in denervatedmuscles treated with chloroquine.

CONCLUSIONS

LC3 mRNA may increase in association with rimmed vacuole formation in denervated muscles from chloroquine-treated rats, suggesting an increase in autophagy at the molecular level. Abnormal accumulation of rimmed vacuoles in this myopathy does not appear to be mediated by inhibition of autophagosome-related genes or GNE gene.

Skeletal muscle expression of clathrin and mannose 6-phosphate receptor in experimental chloroquine-induced myopathy

Previous studies suggest that the muscle fiber lysosome system plays a central role in the increased formation of autophagosomes and autolysosomes that occurs in the context of chloroquine-induced myopathy. The goal of this study was to characterize the contribution of receptor-mediated intracellular transport, particularly the endosomal pathway, to the abnormal accumulation of vacuoles in experimental chloroquine myopathy. Expression of the mannose 6-phosphate receptor (M6PR) and clathrin were analyzed in innervated and denervated rat soleus muscles after treatment with either saline or chloroquine. Accumulation of vacuoles was observed only in chloroquine-treated denervated muscles. Further, clathrin immunostaining and M6PR messenger ribonucleic acid (mRNA) were significantly increased in denervated soleus muscle from saline- and chloroquine-treated rats compared to contralateral, innervated muscles. However, there was no difference in clathrin levels when comparing saline- and chloroquine-treated denervated muscles. These data suggest that chloroquine activates the transport of newly synthesized lysosomal enzymes from the secretory pathway via the trans-Golgi network of the Golgi apparatus (an endosomal pathway) as well as autophagosome formation (an autophagic process) in skeletal muscles. Vacuoles may subsequently accumulate secondary to abnormal formation or turnover of autolysosomes at or after fusion of autophagosomes with early endosomes.

Effect of treatment with nifedipine, an L-type calcium channel blocker, on muscular atrophy induced by hindlimb immobilization

The purpose of this study was to investigate whether the prevention of calcium influx through L-type calcium channels contributed to the attenuation of muscular atrophy induced by hindlimb immobilization (HI) in a shortened position. Mice were divided into four groups (8 mice/group): control; nifedipine; HI; and HI with nifedipine. Mice received nifedipine at a dose of 5 mg/kg one day before and during the 8 days of HI. Quantitative alterations in the amount of myosin heavy chain (MyHC) and actin proteins in the soleus muscle were analyzed using SDS-PAGE. The weight of the soleus muscle decreased significantly by 40.8% (P<0.05) and 27.0% (P<0.05) after the hindlimb immobilization in the HI and HI with nifedipine groups, respectively, when compared to that of the control or nifedipine groups. Treatment with nifedipine alone appeared to have no effect on muscle mass or the amount of myofibrillar proteins. The level of MyHC proteins decreased significantly by 25.1% (P<0.001) and 17.4% (P<0.001) in the HI and HI with nifedipine groups, respectively. The level of MyHC protein in the HI with nifedipine group was significantly greater than that of the HI group (P<0.05), although there were no significant differences in the amount of actin protein. These findings suggest that nifedipine treatment may have a beneficial effect on muscular atrophy.

Effects of dexamethasone on protein degradation and protease gene expression in rat L8 myotube cultures

We examined the effects of a synthetic glucocorticoid (dexamethasone; Dex) on protoeolysis and on protease messenger RNA (mRNA) concentrations in rat L8 skeletal myotube cultures. Protein degradation was measured as release of radioactive trichloroacetic acid-soluble material from intracellular proteins pre-labelled with [3H]tyrosine. Dex (1 microM) stimulated protein degradation (P < 0.01). This effect was entirely blocked by the glucocorticoid antagonist, RU38486 (mifepristone; P < 0.01). Hence, actions of Dex on muscle protein degradation are mediated via intracellular glucocorticoid receptors. Molecular mechanisms by which glucocorticoids stimulate protein degradation in skeletal muscle are not known. Here, we investigated the regulation of protease (cathepsin B, cathepsin D, proteasome C2 subunit and m-calpain) mRNA concentrations by Dex in cultured L8 muscle cells. Cathepsin B mRNA concentration was enhanced 3.3-fold by Dex. This effect was blocked by RU38486. RU38486 alone did not affect cathepsin B mRNA concentration or mRNAs of other proteases. Concentrations of cathepsin D and m-calpain mRNAs were also increased by Dex. These effects were also abolished by RU38486. Proteasome C2 mRNA was unaffected by Dex and Dex reduced alpha-tubulin mRNA. Thus, glucocorticoids specifically regulate the concentrations of mRNAs encoding some proteases in muscle cells. The regulation of protease mRNA concentration is mediated via interaction between Dex with glucocorticoid receptors and is independent of the actions of Dex on mRNA encoding house-keeping proteins. These changes may underlie glucocorticoid-dependent control of proteolysis in muscle.

Chloroquine accumulation and alterations of proteolysis and pinocytosis in the rat conceptus in vitro

The teratogenicity of chloroquine (CQ) has been hypothesized to result from its effects on lysosomal function, specifically the ability of the visceral yolk sac (VYS) to capture and degrade external macromolecules. Using the rat whole embryo culture system, we evaluated the ability of CQ to accumulate in conceptual tissues and its effects on aspects of VYS function known to be important in the uptake and processing of nutrients. When CQ was added directly to the culture medium, it was found to accumulate rapidly in conceptual tissues, particularly the VYS. Tissue concentrations of CQ in the embryo proper reached approximately 10-fold those in the medium, whereas concentrations in the VYS exceeded by 100-fold the medium concentration within a 4-hr exposure on gestational day (GD) 10. Embryotoxic concentrations of CQ (10-30 microM) enhanced the activity of lysosomal cysteine proteinases measured in vitro under optimum pH conditions in both embryonic and VYS homogenates after a 26-hr treatment from GD 10-11. A different pattern of response in enzyme activity was observed between embryos and VYSs that could be attributed to the preferential accumulation of CQ in the VYS. Nonembryotoxic concentrations of CQ (1-7.5 microM) induced a concentration-dependent increase in VYS enzyme activity that peaked in conceptuses exposed to 20 microM CQ (an intermediate embryotoxic concentration). The enhanced cysteine proteinase activity was time dependent and appeared to increase gradually in conceptuses exposed to 10-20 microM CQ during the 26-hr culture period. This was in contrast to the rapid accumulation of CQ in conceptual tissues seen on gestational day 10. Protein content in the VYS was increased significantly after a 9-hr exposure of whole conceptuses to CQ (20 microM), indicating an inhibition of VYS proteolytic activity in situ. After 24 hr of exposure to 20 microM CQ, VYS protein content was not significantly different from control, but embryonic protein was reduced significantly by 20%. These observations are consistent with a model of reversible inhibition of VYS proteolysis by CQ followed by a compensatory increase in lysosomal proteinase activity. VYS fluid-phase pinocytosis was also assessed after CQ exposure and found to be inhibited only in the highest CQ concentration tested (30 microM). Lower concentrations of CQ that were still embryotoxic (10-20 microM) did not affect VYS fluid-phase pinocytosis, suggesting that inhibition of this activity is not primarily responsible for CQ embryotoxicity.

Muscle fiber degradation in distal myopathy with rimmed vacuoles

Late-onset distal myopathy showed numerous rimmed vacuoles with the same properties as autophagic vacuoles. Electron microscopy showed numerous degenerated mitochondria, glycogen, or cell membranes in rimmed vacuoles, but no evidence that these vacuoles engulfed and contained intact or partially disrupted myofibrils. Immunostaining for myosin, alpha-actinin, and actin, however, was sometimes positive within the vacuoles. Compared to the control muscle, there was increased staining activity by calpain around the rimmed vacuoles or in the cytoplasm of mainly atrophic fibers. The result seems to indicate an increase of calpain activity in these muscle fibers. We hypothesize that the myofibrils as well as mitochondria, glycogen, or cell membranes in this myopathy are degraded finally through a lysosomal autophagic process. However, the breakdown of the myofibrils may be not initiated by lysosomal activation; rather it may be the result of extralysosomal processes such as the calpain system.

Effect of denervation on overdevelopment of chloroquine-induced autophagic vacuoles in skeletal muscles

We studied how denervation affects the overdevelopment of autophagic vacuoles in muscles of chloroquine-treated rats. The number of autophagic vacuoles increased significantly in the chloroquine-treated soleus muscles after denervation as compared to similarly treated contralateral, innervated muscles. No vacuoles were present in the denervated and innervated muscles of saline-treated rats. After denervation, the autophagic vacuoles in chloroquine-treated muscle contained numerous glycogen particles and various heterogeneous materials. A biochemical study showed no significant difference in the activities of lysosomal proteases and hydrolases in the chloroquine- and saline-treated muscles after denervation, although these activities were markedly increased in comparison to the same activities in the contralateral, innervated muscles. Chloroquine treatment by itself did not, but denervation with or without chloroquine treatment did enhance the biochemical activities of lysosomal enzymes in the animals. We speculate that denervation induces the marked accumulation of autophagic vacuoles in chloroquine-induced myopathy.

Different mechanisms of increased proteolysis in atrophy induced by denervation or unweighting of rat soleus muscle

Mechanisms of accelerated proteolysis were compared in denervated and unweighted (by tail-cast suspension) soleus muscles. In vitro and in vivo proteolysis were more rapid and lysosomal latency was lower in denervated than in unweighted muscle. In vitro, lysosomotropic agents (eg, chloroquine, methylamine) did not lessen the increase in proteolysis caused by unweighting, but abolished the difference in proteolysis between denervated and unweighted muscle. Leucine methylester, an indicator of lysosome fragility, lowered latency more in denervated than in unweighted muscle. 3-Methyladenine, which inhibits phagosome formation, increased latency similarly in all muscles tested. Mersalyl, a thiol protease inhibitor, and 8-(diethylamino)octyl-3,4,5-trimethoxybenzoate hydrochloride (TMB-8), which antagonizes sarcoplasmic reticulum release of Ca2+, reduced accelerated proteolysis caused by unweighting without diminishing the faster proteolysis due to denervation. Calcium ionophore (A23187) increased proteolysis more so in unweighted than control muscles whether or not Ca2+ was present. Different mechanisms of accelerated proteolysis were studied further by treating muscles in vivo for 24 hours with chloroquine or mersalyl. Chloroquine diminished atrophy of the denervated but not the unweighted muscle, whereas mersalyl prevented atrophy of the unweighted but not of the denervated muscle, both by inhibiting in vivo proteolysis. These results suggest that (1) atrophy of denervated, but not of unweighted, soleus muscle involves increased lysosomal proteolysis, possibly caused by greater permeability of the lysosome, and (2) cytosolic proteolysis is important in unweighting atrophy, involving some role of Ca2(+)-dependent proteolysis and/or thiol proteases.

Effects of cimaterol on muscle protein metabolism and its actions in hypothyroid and hyperthyroid rats

Objectives were to examine mechanisms underlying anabolic actions of cimaterol in skeletal muscle and to evaluate cimaterol's actions in hypothyroid and hyperthyroid rats. In the first study growing rats were fed either a control diet or a diet containing cimaterol for 10 days. In a second study sham-thyroidectomized and thyroidectomized (Tx) rats were assigned to one of 5 treatments: control (sham-Tx), Tx, Tx supplemented with cimaterol, Tx injected with triiodothyronine (T3), and Tx rats injected with T3 and supplemented with cimaterol. Effects of treatments on growth, muscle weights and urinary NT-methylhistidine (NMH) excretion were evaluated in both trials. Muscle was also collected for determinations of DNA, RNA, protein and activities of several proteolytic enzymes. Cimaterol caused muscle hypertrophy and increased urinary NMH excretion. Hence, anabolic actions of cimaterol may result from an increase in myofibrillar protein synthesis which exceeds changes in myofibrillar protein degradation. Urinary NMH excretion was reduced by thyroidectomy and increased in hyperthyroid rats and both hypothyroidism and hyperthyroidism were characterized by myopathy. Cimaterol increased muscle weights in hypothyroid but not in hyperthyroid rats. Therefore, cimaterol's anabolic properties are thyroid hormone-independent and antagonized by excess thyroid hormone. Anabolic actions of cimaterol in hypothyroid rat muscle were attributed to an action on protein synthesis because urinary NMH excretion was not affected by cimaterol but muscle RNA concentration was increased. Activities of cathepsins B, D and L and neutral proteinase were dose-related to thyroid status, however, were unrelated to cimaterol-dependent perturbations in NMH excretion. Control of muscle protein balance by thyroid hormones may involve regulation of these enzymes; however, control of muscle protein degradation by cimaterol is likely directed towards other proteolytic mechanisms or to mechanisms which alter susceptibility of myofibrillar proteins to degradation.

Variation among lines selected for body size in the fractional rate of degradation of protein and acid protease activity in the muscle of quail (Coturnix coturnix japonica)

Fractional rates (%/day) of degradation of muscle protein were determined by measuring the output of NT-methylhistidine (NT-MH) in the excreta at 2 and 10 weeks of age in three lines of quail, a random-bred line and two lines selected for body size, one for increased and the other for decreased size. In all lines, fractional rates of degradation of muscle protein at 2 weeks of age were higher than those at 10 weeks of age. The fractional rate of degradation at 2 weeks of age was highest for the RR line, 9.1-9.2%/day. However, at 10 weeks of age, the rank order changed, and the RR line showed the lowest rate, 1.8-1.9%/day. The SS line (5.8-6.2%/day at 2 weeks and 5.8-5.9%/day at 10 weeks of age) was significantly higher than the LL line (4.1-4.2%/day at 2 weeks and 2.1-2.2%/day at 10 weeks of age). Acid protease activities in supernatants of homogenized muscle of the three lines of quail at 2 and 10 weeks of age were measured. In all lines, the acid protease activities in supernatant of homogenized muscle decreased from 2 to 10 weeks of age. At 2 weeks, the protease activity of the RR line was significantly higher than that of the LL and SS lines, which did not differ significantly. However, at 10 weeks of age, the SS line had higher activity in both sexes than the LL and RR lines. The results suggest that selection for body size brings about significant changes in both fractional degradation rate and acid protease activity in the muscle.