The relationship between skeletal muscle proteolysis and ubiquitin–proteasome proteolytic pathway in burned rats

Abstract

Negative nitrogen balance and accelerated muscle protein breakdown are characteristics of burn injury. The mechanism by which muscle proteolysis occurs may be activation of the ubiquitin–proteasome pathway, but needs to be further elucidated. The aim of this study was to gain more insight into the role of ubiquitin–proteasome pathway in muscle proteolysis, after burn injury in a rat burn injury model. The proteolytic rates and mRNA expression of ubiquitin, E2-14K, and subunit RC2 in extensor digital longus (EDL) and soleus (SOL) muscle were determined by amino acid analyzer and Northern blot, respectively. The results were as follows: the total and myofibrillar proteolytic rate of EDL muscle increased markedly, especially at 12 and 24 h post-burn. The levels of 2.4 kb mRNA for ubiquitin, 1.2 kb mRNA for E2-14K (a rate-limiting and regulated enzyme for conjugation of ubiquitin with protein substrate) and mRNA for subunit RC2 (the largest subunit of 20S proteasome) predominantly increased in EDL muscle after the stimulation of burn injury. No significant changes in proteolytic rate and transcription of ubiquitin, E2-14K, and subunit RC2 in SOL muscle were observed. There was a significantly positive correlation between the proteolytic rate and the levels of 2.4 kb mRNA for ubiquitin, 1.2 kb mRNA for E2-14K, or mRNA for subunit RC2. The results indicated that muscle wasting after burn injury was mainly due to the accelerated breakdown of myofibrils, and EDL muscle was more sensitive to burn injury than SOL muscle. The activation of ubiquitin–proteasome pathway was one reason for the enhanced protein catabolism in skeletal muscle. This is the first demonstration of upregulated expression of E2-14K and subunit RC2 in muscle, in response to burn injury, and it provides a clue to reduce muscle wasting by specifically inhibiting the specific enzymes or subunits involved in the enhancement of the activity of ubiquitin–proteasome pathway after burn injury.