Oxidation enhances myofibrillar protein degradation via calpain and caspase-3

Oxidative stress has been linked to accelerated rates of proteolysis and muscle fiber atrophy during periods of prolonged skeletal muscle inactivity. However, the mechanism(s) that links oxidative stress to muscle protein degradation remains unclear. A potential connection between oxidants and accelerated proteolysis in muscle fibers is that oxidative modification of myofibrillar proteins may enhance their susceptibility to proteolytic processing. In this regard, it is established that protein oxidation promotes protein recognition and degradation by the 20S proteasome. However, it is unknown whether oxidation of myofibrillar proteins increases their recognition and degradation by calpains and/or caspase-3. Therefore, we tested the hypothesis that oxidative modification of myofibrillar proteins increases their susceptibility to degradation by both calpains and caspase-3. To test this postulate, myofibrillar proteins were isolated from rat skeletal muscle and exposed to in vitro oxidation to produce varying levels of protein modification. Modified proteins were then independently incubated with active calpain I, calpain II, or caspase-3 and the rates of protein degradation were assessed via peptide mapping. Our results reveal that increased protein oxidation results in a stepwise escalation in the degradation of myofibrillar proteins by calpain I, calpain II, and caspase-3. These findings provide a mechanistic link connecting oxidative stress with accelerated myofibrillar proteolysis during disuse muscle atrophy.