The most evident metabolic explanation for muscle decline is an imbalance between protein catabolism and anabolism. At least four major proteolytic pathways (lysosomal, Ca
2+-dependent, caspase-dependent and ubiquitin–proteasome-dependent) operate in skeletal muscle, and may be altered in the process of sarcopenia and muscle cachexia. Aside from these four distinct pathways, the autophagic/lysosomal pathway also has to be considered. In this pathway, portions of the cytoplasm and cell organelles are sequestered into autophagosomes, which subsequently fuse with lysosomes, where the proteins are digested [
7]. Dissecting the molecular regulation of the ubiquitin–proteasome-dependent system (UPS) and autophagy it became evident that forkhead box O (FoxO) transcription factors take a central position. FoxO transcription factors, normally phosphorylated and inactivated by PI3K-Akt/PKB, translocate into the cell nucleus and induce the transcription of the skeletal muscle-specific E3 ubiquitin ligases, MuRF1 and MAFbx/atrogin [
8], as well as autophagy-related genes like
LC3 and
Bnip3 [
9]. Upstream of PI3K-Akt, several factors like reactive oxygen species (ROS), tumor necrosis factor α (TNF-α), the tumor-released proteolysis-inducing factor (PIF), the peroxisome proliferator-activated receptor gamma coactivator 1alpha (PGC-1α) or insulin-like growth factor 1 (IGF-1) have been shown to influence this regulatory system [
8,
10‐
12].
On the other hand, protein anabolic factors like IGF-1 are counteracting muscle atrophy. Besides inhibiting autophagy and the UPS, IGF-1 activates via Akt–mTOR (mammalian target of rapamycin)–p70
S6K (p70 S6 kinase) protein synthesis [
13,
14].
2.1 Sarcopenia
Experimental and human studies in the last decade clearly demonstrated that UPS is activated in several muscle wasting conditions (reviewed by Mitch and Goldberg [
15]). However, data on muscle wasting by the ubiquitin–proteasome system (UPS) in aging are conflicting. Several authors described an up-regulation of components of the UPS in sarcopenia [
16‐
18], whereas others found a down-regulation [
19‐
21] or no change [
22]. Therefore, at least in sarcopenia, the UPS seems not to be the major pathway responsible for muscle loss.
Calpains belong to a large family of calcium-dependent cystein proteases, and demonstrate a ubiquitous or tissue-specific expression [
23]. Besides its regulation by calcium, calpain activity is tightly controlled by its inhibitor calpastatin [
24]. In an animal study comparing the mRNA expression of calpains and calpastatin in the skeletal muscle of 3- and 24-month-old rats, a 38% increase in μ-calpain and a 28% decrease in calpastatin in the old specimens was evident [
25]. In addition, these changes at the expression levels were confirmed by calpain activity measurements. At least, these animal data point to a possible involvement of the calpains in muscle loss during ageing. Nevertheless, this has to be confirmed in human muscle biopsies, and further experiments have to elucidate the physiological targets of calpains in sarcopenia.
Lysosomes are responsible for the degradation of long-lived proteins and for the enhanced protein degradation observed under starvation conditions. Using a gene expression profile analysis from young (3–4 months) and old (30–31 months) rats, Pattison and colleagues [
26] described a slight up-regulation of cathepsin L in the old soleus muscle. Nevertheless, this result could not be confirmed in a later study by O’Connell et al. [
27], who screened for differentially expressed protein in the gastrocnemius muscle of 30- and 3-month-old rats. Data for the direct analysis of lysosomal components in young vs. old skeletal muscle are missing.
At least in transgenic mice overexpressing IGF-1 specifically in the skeletal muscle, it was evident that the age-related sarcopenia was prevented [
28]. Furthermore, it is well known that post-maturational aging is associated with reduced serum IGF-1 concentration. This finding was supported by detecting a reduced expression level in the skeletal muscle of older men when compared to younger ones [
29,
30]. Unfortunately, no correlation between muscle mass or protein synthesis rate was found [
30], whereby the functional significance of the alterations are uncertain.
2.2 Cachexia
The UPS is the major proteolytic machinery systematically activated in cachexia. To assess the role of the UPS in cancer cachexia, Williams and colleagues [
31] took biopsies of cancer and non-cancer patients undergoing laparotomy for various reasons. The mRNA levels for ubiquitin and the 20 S proteasome subunits were two to four times higher in muscle from patients with cancer than in muscle from control patients.
In a report by Lecker et al. [
32], muscles atrophying from different causes (cancer cachexia, streptozotocin-induced diabetes mellitus, uremia induced by subtotal nephrectomy, and from pair-fed control rats) were investigated. Proteins involved in protein degradation, including polyubiquitins, Ub fusion proteins, the Ub ligases atrogin-1/MAFbx and MuRF-1, multiple but not all subunits of the 20 S proteasome and its 19 S regulator, and cathepsin L were up-regulated [
32].
In a cancer cachexia animal study by Acharyya and coworkers [
33] it was demonstrated that myosin heavy chain (MyHC) is a selective target associated with a wasting state compared to other myofibrillar proteins [
33]. MyHC protein was significantly reduced, whereas MyHC mRNA levels were unchanged. Results showed that the mature form of MyHC could readily be immunoprecipitated with ubiquitin, which supports the involvement of this proteolytic pathway in the basal turnover of myosin.
The important role of the FoxO transcription factors was underlined in a study by Liu et al. [
34] when they targeted Foxo-1 in a cancer cachexia mice model by an oligonucleotide. It could be demonstrated that the RNA oligonucleotide can reduce the expression of Foxo-1 in normal and cachectic mice, leading to an increase in skeletal muscle mass of the mice. In the search for downstream target genes of Foxo-1, increased levels of MyoD and decreased concentrations of myostatin were found.
Investigating the UPS and the lysosomal proteolytic pathway in lung cancer patients, Jagoe and colleagues reported that mRNA levels for cathepsin B, but not for components of the ubiquitin–proteasome pathway, were higher in patients with cancer compared with controls suggesting that cathepsin B may have a role in inducing muscle wasting in the early stages of lung cancer [
35].
Besides an increased catabolism, there is reduced anabolism, which has been shown, for example in cancer-related cachexia. Although the underlying mechanism remains unknown, it could be demonstrated that the IGF-1 system is down-regulated in an animal model of cancer cachexia [
36]. Interestingly, the transgenic overexpression of locally acting IGF-1 in skeletal muscle inhibits ubiquitin-mediated muscle atrophy in chronic left-ventricular dysfunction [
37].
In cancer cachexia, in particular, the decrease in skeletal muscle protein synthesis is partly related to the increased serum level of the PIF. Intravenous administration of PIF to normal mice produced a rapid decrease in body weight that was accompanied by increased mRNA levels for ubiquitin in the gastrocnemius muscle [
11]. There were also increased protein levels of the 20 S proteasome core and 19 S regulatory subunit, suggesting activation of the ATP–ubiquitin-dependent proteolytic pathway. Recent evidence proposes that PIF decreases protein synthesis by inhibiting protein translation initiation through phosphorylation of the eukaryotic initiation factor 2 (eIF2-alpha) [
38].
Another factor that may contribute to a decreased anabolism is angiotensin II. In an animal model of continuously administered angiotensin II, a markedly reduced plasma IGF I levels occurred [
39]. Compared with sham, angiotensin II-infused hypertensive rats lost 18–26% of body weight by 1 week, which was completely reversible by losartan, an AT1 receptor antagonist.