Burn-induced stimulation of lysosomal enzyme activity in skeletal muscle

Burn-induced increase in atrogin-1 and MuRF-1 in skeletal muscle is glucocorticoid independent but downregulated by IGF-I

The present study determined whether thermal injury increases the expression of the ubiquitin (Ub) E3 ligases referred to as muscle ring finger (MuRF)-1 and muscle atrophy F-box (MAFbx; aka atrogin-1), which are muscle specific and responsible for the increased protein breakdown observed in other catabolic conditions. After 48 h of burn injury (40% total body surface area full-thickness scald burn) gastrocnemius weight was reduced, and this change was associated with an increased mRNA abundance for atrogin-1 and MuRF-1 (3.1- to 8-fold, respectively). Similarly, burn increased polyUb mRNA content in the gastrocnemius twofold. In contrast, there was no burn-induced atrophy of the soleus and no significant change in atrogin-1, MuRF-1, or polyUb mRNA. Burns also did not alter E3 ligase expression in heart. Four hours after administration of the anabolic agent insulin-like growth factor (IGF)-I to burned rats, the mRNA content of atrogin-1 and polyUb in gastrocnemius had returned to control values and the elevation in MuRF-1 was reduced 50%. In contrast, leucine did not alter E3 ligase expression. In a separate study, in vivo administration of the proteasome inhibitor Velcade prevented burn-induced loss of muscle mass determined at 48 h. Finally, administration of the glucocorticoid receptor antagonist RU-486 did not prevent burn-induced atrophy of the gastrocnemius or the associated elevation in atrogin-1, MuRF-1, or polyUb. In summary, the acute muscle wasting accompanying thermal injury is associated with a glucocorticoid-independent increase in the expression of several Ub E3 ligases that can be downregulated by IGF-I.

Antiproteolytic Action of Orally Delivered Insulin Using pH-Responsive Hydrogels in a Rat Burn Model

Subcutaneously delivered small doses of insulin has shown beneficial effects on burn injury-induced muscle wasting and wound healing. To improve the method of insulin treatment for clinical settings, this study investigated the effect of insulin delivered orally using pH-responsive poly(methacrylic-g-ethylene glycol) (P(MAA-g-EG)) hydrogels in a 20% total burn surface area rat burn injury model. P(MAA-g-EG) were synthesized in-house and insulin release characteristics were performed in vitro. Young rats weighing 80-150 g were subjected to 15-20% total body surface area burn injury and treated with insulin-containing hydrogels enclosed in gelatin capsules for 3 days. The dosage was adjusted to match 0.25 U (day 1), 0.5U (day 2), and 1.0 U (day 3) per 100 grams of body weight. All animals were housed in metabolic cages and their physical activity, body weight, food consumption, water uptake, circulating glucose levels, and urinary tyrosine content were monitored for 4 to 15 days after burn. Results show that the orally delivered insulin restored the body weight of burned rats and influenced wound healing, similar to subcutaneous delivery. Measured glucose levels showed significantly less perturbation, suggesting the possibility of increasing the dosage. In conclusion, muscle wasting can be significantly inhibited by the oral administration of insulin using pH-responsive hydrogels.

The effect of burn plasma on skeletal muscle proteolysis in rats

We studied the effects of plasma from burned rats on skeletal muscle proteolysis. Major burn injury (40% total body surface area (TBSA)) was produced in male Sprague-Dawley rats. Fluid resuscitation was given with intraperitoneal Ringer's solution (4cm(3)/(kg%) TBSA). Plasma was harvested daily for 5 days after burn injury from the tail blood vessel. This plasma was added in vitro to incubated soleus muscles from healthy animals. The incubation medium was assayed for amino acids by HPLC. Glutamine, glutamate, leucine and alanine were tested to monitor the amino acid profile in the medium. Results showed there was no significant change during the initial 4 days after injury, except that glutamine and alanine increased significantly on the first day. However, all of them had a tendency to increase on the fifth day after injury. Present results suggest that the humoral effect on muscle proteolysis did not exist during the initial days after burning. The humoral effect on skeletal muscle proteolysis may have been present 5 days post-burn.

Antiproteolytic action of insulin in burn-injured rats

BACKGROUND

Negative nitrogen balance is a typical metabolic response to burn injury resulting in decreased muscle mass and activity. Since insulin is an anabolic hormone, using insulin as a prophylactic agent in burned patients has received some attention. The present study was carried out to investigate the systemic effect of insulin on burn injury-induced muscle wasting.

PATIENTS AND METHODS

A 15-20% total body surface area (TBSA) scald burn injury was inflicted on the shaved dorsum of rats. Rats were treated with a daily subcutaneous insulin injection for 3 days (0.25-1.0 U/day). After the treatment, a variety of insulin-dependent physiological parameters were monitored. Overall body protein degradation rates were determined by measuring the urinary tyrosine. Also, protein degradations were measured in diaphragm muscles, splenocytes, and peripheral blood mononuclear cells to directly confirm the antiproteolytic activity of insulin.

RESULTS

Administration of insulin to burn-injured rats restored body weight primarily by reducing accelerated protein degradation and regaining the intracellular protein content in individual skeletal muscle. The measured physiological parameters showed no possible side effects. Protein degradation in immune cells was also suppressed after the therapy.

CONCLUSION

Results indicate that lower dose insulin particularly suppresses protein degradation without causing secondary effects. It may be a useful approach to preventing burn injury-induced muscle wasting and also has a potential to improve immune response.

Insulin suppresses the increased activities of lysosomal cathepsins and ubiquitin conjugation system in burn-injured rats

BACKGROUND

Burn injury results in increased rate of skeletal muscle protein degradation. In vitro studies on incubated muscles indicate that increased rate of protein degradation is due to activation of multiple proteolytic systems, but the supporting evidence is of an indirect nature. The present study was carried out to investigate the role of various lysosomal cathepsins, ubiquitin conjugation, and proteasome systems in accelerated proteolysis, and the effect of insulin in burn-induced muscle wasting syndrome.

MATERIALS AND METHODS

Fifteen to twenty percent total body surface area scald burn injury was inflicted on the shaved dorsum of young growing rats. Insulin-treated rats received a daily single subcutaneous injection for 3 days (0.25-1.0 U/day). The rate of ubiquitin conjugation to endogenous proteins and exogenously added (125)I-lysozyme and the activities of various proteases were measured in muscle homogenates.

RESULTS

Burn injury resulted in increased rate of ubiquitin conjugation to endogenous proteins and (125)I-lysozyme. Activities of cathepsins B, C, H, and L were also up-regulated following burn injury. When the burn-injured rats were treated with insulin, the increased rate of ubiquitin conjugation and cathepsin activities were suppressed to the control levels.

CONCLUSIONS

The increased ubiquitin conjugation and lysosomal cathepsins contribute to accelerated protein degradation in burn-injured rats and insulin suppresses the muscle protein degradation at least in part by suppressing the activities of lysosomal cathepsins and of ubiquitin conjugation system.

Burn injury-induced nicotinic acetylcholine receptor changes on muscle membrane

Patients with thermal injury show pharmacological responses akin to those seen following denervation of motor nerve where there is an increase in the number of muscle-type acetylcholine receptors (AChR). This study in the rat, examined the effect of 20%, 30%, and 50% body surface area burn injury on gastrocnemius and diaphragm AChR number, quantitated by [125I]alpha-bungarotoxin (alpha-BGT). Pharmacodynamic responses to D-tubocurarine were evaluated in the gastrocnemius muscle. AChR from gastrocnemius muscle were significantly increased in all burn groups at 14 days after burn. After 28 days, the AChR had returned to control in the 20% and 30% burn groups, but persisted in the 50% burn group. AChR from diaphragm did not show significant changes between groups. Resistance to D-tubocurarine was not seen in burn groups compared with controls. Burn trauma causes increases of AChR at sites distant from the area of injury. Diaphragm muscle may be less sensitive than gastrocnemius to mediators which upregulate AChR.

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.

Regulation of lysosomal proteolysis in burn injury

The regulation of the lysosomal pathway by burn injury was investigated using a local burn model in which one hindlimb of a rat is scaled at 85 degrees C for 3.6 s. By two days postinjury the rate of net protein breakdown in the incubated soleus muscle from the burned leg is doubled. The activity of cathepsin D increases 40% to 50% and that of cathepsins B and L increase 80% to 100%. The activity of a lysosomal nonprotease activity, N-acetylglucosaminidase, is not significantly increased. The latency of lysosomal enzymes (an estimate of cellular autophagy) did not change at any time postburn. Inhibitors of autophagy (ie, leucine and 3-methyladenine) inhibited net protein breakdown and increased latency to a similar extent in muscles from control and burned legs. Thus, there is no evidence that a change in cellular autophagy is responsible for the increased proteolysis seen in intact muscle. However, burn-induced changes in alternative routes of protein sequestration cannot be excluded. Burn did not increase either receptor-mediated or fluid phase endocytosis by incubated soleus muscle. Burn injury also did not reduce the inhibitory action of cytoplasmic inhibitors of cathepsins B and L in skeletal muscle. However, burn injury markedly stimulated the synthesis of glycoproteins in the microsomal fraction without affecting overall protein synthesis. This increase in synthesis preceded the rise in lysosomal protease activity. These results support the possibility that induction of lysosomal protease synthesis may underlie burn-induced increases in muscle proteolysis.