Effects of the calpain proteases on the ubiquitin-proteasome system and protein synthesis signaling in rat skeletal muscle

Skeletal muscle atrophy can be a debilitating problem, and in extreme cases life threatening. Muscle wasting is a consequence of reduced protein synthesis and/or elevated protein degradation. While the ubiquitin-proteasome pathway (UPP) accounts for most of the elevated protein degradation, proteases acting upstream of the UPP may initiate the degradative process. Since the calpain proteases have been implicated in the initial events leading to elevated protein breakdown, one goal of the present investigation was to determine whether the calpain proteases act upstream of the UPP in skeletal muscle. Calpain activation had no discernible effect on the UPP (i.e., the level of myofibrillar/cytosolic ubiquitin-conjugated proteins or proteasome enzyme activity). However, the calpain-mediated rise in total protein degradation was prevented by proteasome inhibition, indicating that the proteasome is necessary for the processing of calpain cleavage products. In fact, acute calpain activation increased proteasome-dependent protein degradation by 45%. Our data indicate that calpain acts upstream of the proteasome system; thus calpain may be an initial step in elevated protein degradation.; Akt plays a critical role in protein synthesis, and the molecular chaperone heat shock protein 90 (HSP90) appears to regulate Akt activity. Since HSP90 is a calpain substrate, calpain activation may subsequently inactivate Akt and its immediate downstream elements. Calpain activation significantly reduced HSP90beta content and Akt activity. Calpain also inhibited Akt's downstream component mammalian target of rapamycin (mTOR). While activated Akt inhibits glycogen synthase kinase-3beta (GSK-3beta), a negative regulator of protein synthesis, calpain activation increased GSK-3beta activation.; Our data implicate calpain as an initial factor in elevated muscle proteolysis since these proteases appear to act upstream of the proteasome. The novel finding that calpain adversely affects the Akt/mTOR/GSK-3beta signaling pathway suggests that calpain also contributes to reduced protein synthesis. Finally, given the multitude of functions carried out by molecular chaperones, the finding that calpain reduces the level HSP90beta suggests that calpain activation may affect an assortment of cellular functions. Taken together, these data indicate that calpain likely plays a prominent role in the loss of muscle proteins during catabolic conditions and thus offers a practical target for intervention.