It has been postulated that increased skeletal muscle inactivity-induced increases in ROS production and the resulting oxidative stress accelerate muscle protein breakdown in three different ways. First, oxidative stress promotes the expression of proteins involved in at least three proteolytic systems, including autophagy, calpain and the ubiquitin–proteasome system of proteolysis. Second, inactivity-induced oxidative stress in skeletal muscle results in the activation of two important proteases, calpain and caspase-3. Finally, increased ROS production in muscle fibres can also promote proteolysis by oxidative modification of myofibrillar proteins, which enhances their susceptibility to proteolytic processing. A brief summary of these connections between oxidative stress and proteolysis is presented in the following sections.
4.3.1 Oxidative Stress Increases Synthesis of Proteolytic Proteins
The major proteolytic systems found in skeletal muscle can be categorized into four groups: (1) autophagy (i.e. lysosomal proteases); (2) the ubiquitin–proteasome system; (3) calpains; and (4) caspase-3. Growing evidence reveals that cellular oxidative stress can increase the expression of key proteins involved in autophagy and the ubiquitin–proteasome system, and can increase expression of both calpain 1 and calpain 2. Specific details of these processes are highlighted in the following sections.
ROS Increase Expression of Key Autophagy Proteins Autophagy is a highly regulated proteolytic pathway for the degradation of non-myofibril cytosolic proteins and organelles [
29]. During autophagy, cytosolic components (i.e. proteins and organelles) are sequestered into vesicles called autophagosomes. After formation, these autophagosomes fuse to lysosomes and the cytosolic constituents are degraded by lysosomal proteases (i.e. cathepsins) [
30]. Although it has been established that several lysosomal proteases (i.e. cathepsins B, D and L) are activated in skeletal muscle undergoing disuse atrophy [
31], the role that the autophagic proteolytic system plays in muscle atrophy has received limited research attention. Nonetheless, emerging studies have revealed that accelerated autophagy contributes to skeletal muscle atrophy in response to both fasting and denervation [
32,
33]. Furthermore, a recent study has demonstrated that autophagosomes are formed in diaphragm muscle during prolonged mechanical ventilation, suggesting that autophagy contributes to ventilator-induced protein breakdown in the diaphragm [
34].
Emerging evidence indicates that oxidative stress increases the expression of autophagy genes in skeletal muscles. Indeed, a recent study has revealed that increased cellular ROS production promotes the expression of the autophagy-related beclin 1 and cathepsin L genes in cultured cells [
35]. Importantly, another report has indicated that inactivity-induced oxidative stress also promotes the expression of autophagy-related proteins in human skeletal muscle [
34]. These findings have been confirmed in rodent locomotor muscles exposed to prolonged immobilization [
24]. Specifically, prevention of inactivity-induced increases in mitochondrial ROS emission in hindlimb muscles prevents the expression of cathepsin L [
24]. Together, these results suggest that oxidative stress increases gene expression of selected autophagy-related genes, which have the potential to increase the rate of autophagy-mediated protein breakdown in cells.
Oxidative Stress Increases Expression of Proteins Required for the Ubiquitin–
Proteasome System The ubiquitin–proteasome system comprises the total proteasome complex (26S), which includes a core proteasome subunit (20S) combined with a regulatory complex (19S) attached at the end of the 20S proteasome core [
36,
37]. The 26S proteasome degrades ubiquitinated proteins only. Hence, the 26S proteasome degradation pathway is active when ubiquitin binds to protein substrates and labels these molecules for breakdown. The binding of ubiquitin to protein substrates is a three-step process, which requires the participation of three families of ubiquitin-activating enzymes [
4]. In this regard, evidence indicates that the ubiquitin-conjugating enzyme E2
14k is an important regulator of ubiquitin–protein conjugation in skeletal muscle [
1]. Further, several skeletal muscle-specific ubiquitin E3 ligases (e.g. atrogin-1 and muscle ring finger-1) exist, and these proteins play important roles in skeletal muscle atrophy [
38,
39].
Abundant evidence confirms that oxidative stress promotes increased gene expression of proteins involved in the ubiquitin–proteasome system. For instance, in vitro experiments have revealed that exposure of C2C12 myotubes to H
2O
2 increases the expression of specific ubiquitin-activating enzymes that contribute to muscle protein breakdown, including E2
14k, atrogin-1 and muscle ring finger-1 [
1,
40]. Similarly, in vivo experiments have revealed that oxidative stress augments the expression of atrogin-1 and muscle ring finger-1 in rodent skeletal muscles [
24,
26]. Together, these results verify that ROS-induced oxidative stress promotes the expression of key components of the ubiquitin–proteasome system of proteolysis in skeletal muscle.
Oxidative Stress Increases Calpain Expression Calpains are Ca
2+-dependent cysteine proteases and are located in all mammalian cells [
41]. While several calpain isoforms exist, the two best characterized calpains located in skeletal muscle are calpains 1 and 2 [
41]. Active calpains promote the release of sarcomeric proteins by cleaving cytoskeletal proteins (e.g. titin and nebulin) that anchor contractile elements [
41‐
43]. Further, calpain can break down selected kinases and phosphatases, and can also degrade oxidized contractile proteins, such as actin and myosin [
41,
44].
Several reports have indicated that oxidative stress increases the expression of calpains in both C2C12 myotubes and human myoblasts. For example, exposure of C2C12 myotubes to H
2O
2 increases calpain 1 mRNA levels [
40]. Further, exposure of human myoblasts to H
2O
2 promotes the expression of both calpain 1 and calpain 2 [
45]. Together, these investigations suggest that oxidative stress increases calpain expression in cultured muscle cells. At present, it is unknown if oxidative stress can increase the expression of calpain in skeletal muscle fibres in vivo.
4.3.2 Oxidative Stress Increases Protease Activation
As discussed in the previous sections, oxidative stress increases the expression of several important proteolytic proteins. This section presents robust evidence demonstrating that oxidative stress can promote the activation of selected proteases (e.g. calpain and caspase-3) in skeletal muscles.
Elevated Cellular ROS Production Activates Calpain Numerous studies have concluded that oxidative stress increases calpain activity in muscle cells. For example, treatment of C2C12 myotubes with H
2O
2 activates calpain 1 and stimulates myotube atrophy [
40]. Similarly, exposure of human myoblasts to H
2O
2 increases the activities of both calpain 1 and calpain 2 [
45]. Moreover, prevention of oxidative stress via antioxidants can prevent calpain activation in inactive diaphragm muscle in vivo [
26,
46]. Similarly, mitochondrial-targeted antioxidants can prevent calpain activation in immobilized limb muscles [
24]. Collectively, these studies have confirmed that increased production of ROS in skeletal muscle promotes the activation of calpain.
The mechanism(s) responsible for ROS-mediated calpain activation appear(s) to be linked to oxidative stress-induced disturbances in calcium homeostasis. The two key factors that regulate calpain activity in cells are cytosolic calcium levels and the concentration of the endogenous calpain inhibitor, calpastatin [
41]. Specifically, calpain can be activated by a sustained elevation in cytosolic free calcium and/or a decrease in cytosolic levels of calpastatin [
41]. During prolonged periods of muscle inactivity, it is established that muscle inactivity is accompanied by elevated cytosolic calcium levels and increased calpain activation [
47]. Although the mechanism responsible for this disuse-induced increase in cellular calcium remains under investigation, it is possible that increased ROS production plays a key role in this event [
48]. A potential connection between oxidative stress and increased cytosolic calcium is ROS-driven formation of reactive aldehydes (i.e. 4-hydroxy-2,3-trans-nonenal), which can impede plasma membrane Ca
+2 ATPase activity [
49]. Logically, a decline in membrane Ca
+2 ATPase activity would hinder Ca
+2 removal from the cell, resulting in increased cytosolic Ca
+2. It is also possible that oxidation of the ryanodine receptor can increase Ca
+2 leakage from the sarcoplasmic reticulum, resulting in increased cytosolic Ca
+2 levels [
50]. Nonetheless, it is not clear which of these mechanisms is responsible for inactivity-mediated calcium overload within skeletal muscle, and this subject remains an active area of research.
Oxidative Stress Promotes Caspase-3 Activation Research has revealed that the activation of caspase-3 contributes to skeletal muscle protein degradation and fibre atrophy [
51‐
53]. Specifically, active caspase-3 results in the degradation of actomyosin complexes, and inhibition of caspase-3 activity suppresses the overall rate of proteolysis in diabetes-mediated cachexia and disuse-induced muscle atrophy [
51‐
53].
Numerous studies have confirmed that oxidative stress activates caspase-3 in skeletal muscle. For instance, exposing C2C12 myotubes to H
2O
2 activates caspase-3 [
54]. Further, several studies have confirmed that prevention of inactivity-induced oxidative stress in skeletal muscles prevents caspase-3 activation [
24,
26,
46].
Control of caspase-3 activity in cells is complex and involves numerous signalling pathways. Inactivity-induced caspase-3 activation in skeletal muscle can occur by activation of caspase-12 via a calcium release pathway and/or activation of caspase-9 via a mitochondrial signalling pathway [
7]. A potential interaction between these caspase-3 activation pathways is that both signalling pathways can be activated by ROS [
3,
55]. Finally, note that caspase-3 can also be activated by calpain activation via a cross-talk mechanism between these two proteases [
51,
56]. Regardless of which pathway is responsible for inactivity-induced caspase-3 activation in skeletal muscle, it is apparent that ROS can promote caspase-3 activation.
4.3.3 Protein Oxidation Accelerates Proteolysis
Another link between oxidative stress and increased protein turnover is that oxidation of skeletal muscle proteins increases their vulnerability to proteolytic breakdown. Indeed, Davies and Goldberg were the first to demonstrate that ROS accelerate proteolysis [
57]. This early work has been confirmed, and it is now clear that oxidized proteins are swiftly degraded by several proteases, including the ubiquitin–proteasome system [
36,
37,
44]. Further, evidence has revealed that oxidation increases the susceptibility of skeletal muscle myofibrillar proteins to degradation by both calpains and caspase-3. Indeed, oxidation of sarcomeric proteins (e.g. myosin heavy chain, α-actinin, actin and troponin I) increases their breakdown by both calpain and caspase-3 in a dose-dependent manner [
44].
The connection between high cellular levels of ROS and accelerated protein breakdown is due, in part, to the fact that oxidation of muscle proteins results in unfolding of the affected proteins, resulting in enhanced susceptibility to proteolysis [
57]—that is, oxidative modification of a protein results in a change in the molecular structure such that the formerly protected peptide bonds are now exposed to enzymatic breakdown [
15,
44].