Activity and expression of the 20S proteasome are increased in skeletal muscle during sepsis

Recent studies suggest that sepsis stimulates ubiquitin-dependent protein b reakdown in skeletal muscle. In this proteolytic pathway ubiquitinated prot eins are recognized, unfolded, and degraded by the multicatalytic 26S prote ase complex. The 20S proteasome is the catalytic core of the 26S protease c omplex. The role of the 20S proteasome in the regulation of sepsis-induced muscle proteolysis is not known. We tested the hypothesis that sepsis incre ases 20S proteasome activity and the expression of mRNA for various subunit s of this complex. Proteolytic activity of isolated 20S proteasomes, assess ed as activity against fluorogenic peptide substrates, was increased in ext ensor digitorum longus muscles from septic rats. The proteolytic activity w as inhibited by specific proteasome blockers. Northern blot analysis reveal ed an approximately twofold increase in the relative abundance of mRNA for the 20S alpha-subunits RC3 and RC9 and the beta-subunit RC7. However, Weste rn. blot analysis did not show any difference in RC9 protein content betwee n sham-operated and septic rats. The increased activity and expression of t he 20S proteasome in muscles from septic rats lend further support for a ro le of the ubiquitin-proteasome-pathway in the regulation of sepsis-induced muscle proteolysis.