Not All Injury‐induced Muscle Proteolysis Is Due to Increased Activity of the Ubiquitin/Proteasome System: Evidence for Up‐Regulation of Macrophage‐associated Lysosomal Proteolysis in a Model of Local Trauma

A characteristic response to injury is a dramatic loss of skeletal muscle protein owing to increased muscle protein breakdown. Over the past decade, numerous studies have indicated that up‐regulaton of the ubiquitin‐proteasome system is a common mechanism underlying such injury‐induced muscle proteolysis. However, a recent study using a single‐impact trauma to the gastrocnemius muscle found that, although the rate of muscle proteolysis was dramatically increased, the ubiquitin‐proteasome system was not involved. Rather, an increase in lysosomal activity, through infiltration of the damaged tissue by mononuclear macrophages, is responsible for the high rates of protein breakdown.