Activation of caspase-3 is an initial step triggering accelerated muscle proteolysis in catabolic conditions

Abstract

With trauma, sepsis, cancer, or uremia, animals or patients experience accelerated degradation of muscle protein in the ATP-ubiquitin-proteasome (Ub-P'some) system. The initial step in myofibrillar proteolysis is unknown because this proteolytic system does not break down actomyosin complexes or myofibrils, even though it degrades monomeric actin or myosin. Since cytokines or insulin resistance are common in catabolic states and will activate caspases, we examined whether caspase-3 would break down actomyosin. We found that recombinant caspase-3 cleaves actomyosin, producing a characteristic, approximately 14-kDa actin fragment and other proteins that are degraded by the Ub-P'some. In fact, limited actomyosin cleavage by caspase-3 yields a 125% increase in protein degradation by the Ub-P'some system. Serum deprivation of L6 muscle cells stimulates actin cleavage and proteolysis; insulin blocks these responses by a mechanism requiring PI3K. Cleaved actin fragments are present in muscles of rats with muscle atrophy from diabetes or chronic uremia. Accumulation of actin fragments and the rate of proteolysis in muscle stimulated by diabetes are suppressed by a caspase-3 inhibitor. Thus, in catabolic conditions, an initial step resulting in loss of muscle protein is activation of caspase-3, yielding proteins that are degraded by the Ub-P'some system. Therapeutic strategies could be designed to prevent these events.

Combined walking exercise and alkali therapy in patients with CKD4–5 regulates intramuscular free amino acid pools and ubiquitin E3 ligase expression

“…There was also no significant depletion of the 14kDa Actin fragment in the biopsies ( Figure 6) (a marker of caspase-dependent myofibrillar proteolysis) (Workeneh et al 2006) (Du et al 2004), which has previously been reported to decrease in exercising CKD patients (Workeneh et al 2006). This did show a tendency to decline relative to baseline in exercising STD patients, but this was not statistically significant (KruskalWallis ANOVA, P=0.096).…”

“…Insoluble pellets were retained for determination of the 14kDa actin fragment (Du et al 2004;Workeneh et al 2006). Lysates were subjected to SDS-PAGE using 12.5% gels on a MiniProtean3 system (Bio-Rad, UK).…”

“…Consequently expression of these genes has been used as a marker of proteolysis through the UPP. Caspase-3 cleaves actomyosin into fragments which can be degraded by the proteasome (Du et al 2004) leaving a characteristic 14kDa…”

TNF-like weak inducer of apoptosis (TWEAK) : not so weak after all.

“…MuRF1 of the UPS can ubiquitinate MyHC, but it cannot complete this task unless the proteins are removed from the actomyosin complex. This would implicate other proteolytic systems such as the caspase system in the process of muscle atrophy, which has been demonstrated in catabolic conditions such as diabetes 24 . Activation of caspase-3 was found to be imperative for this process and facilitated a 125% increase in protein degradation by the UPS 24 .…”

“…Proteases such as caspases and calpains may also be required for initial cleavage of the myofibrillar components, thereby accelerating disassembly and degradation by the UPS 24,25 . The activity of UPS is increased in response to many extracellular stimuli such as inflammatory cytokines, tumorderived factors, and oxidative stress.…”