Proteasome blockers inhibit protein breakdown in skeletal muscle after burn injury in rats

1. Burn injury stimulates ubiquitin-dependent protein breakdown in skeletal muscle. The 20S proteasome is the proteolytic core of the 26S proteasome that degrades ubiquitin conjugates. We examined the effects of the proteasome inhibitors N-acetyl-L-leucinyl-L-leucinal-L-norleucinal (LLnL), lactacystin and beta-lactone on protein breakdown in muscles from burned rats. 2. A full-thickness burn of 30% total body surface area was inflicted on the back of rats. Control rats underwent a sham procedure. After 24 h, extensor digitorum longus muscles were incubated in the absence or presence of 20S proteasome blocker and protein turnover rates and ubiquitin mRNA levels were determined. 3. LLnL resulted in a dose- and time-dependent inhibition of total protein breakdown in incubated muscles from burned rats. Lactacystin and beta-lactone blocked both total and myofibrillar muscle protein breakdown. In addition to inhibiting protein breakdown, LLnL increased ubiquitin mRNA levels, possibly reflecting inhibited proteasome-associated RNase activity. 4. Inhibited muscle protein breakdown caused by LLnL, lactacystin and beta-lactone supports the concept that the ubiquitin-proteasome pathway plays a central role in burn-induced muscle proteolysis. Because the proteasome has multiple important functions in the cell, in addition to regulating general protein breakdown, further studies are needed to test the role of proteasome blockers in the treatment or prevention of muscle catabolism.