Muscle wasting in a rat model of long-lasting sepsis results from the activation of lysosomal, Ca2+ -activated, and ubiquitin-proteasome proteolytic pathways.

Abstract

We studied the alterations in skeletal muscle protein breakdown in long lasting sepsis using a rat model that reproduces a sustained and reversible catabolic state, as observed in humans. Rats were injected intravenously with live Escherichia coli ; control rats were pair-fed to the intake of infected rats. Rats were studied in an acute septic phase (day 2 postinfection), in a chronic septic phase (day 6), and in a late septic phase (day 10). The importance of the lysosomal, Ca 2 ϩ -dependent, and ubiquitin-proteasome proteolytic processes was investigated using proteolytic inhibitors in incubated epitrochlearis muscles and by measuring mRNA levels for critical components of these pathways. Protein breakdown was elevated during the acute and chronic septic phases (when significant muscle wasting occurred) and returned to control values in the late septic phase (when wasting was stopped). A nonlysosomal and Ca 2 ϩ -independent process accounted for the enhanced proteolysis, and only mRNA levels for ubiquitin and subunits of the 20 S proteasome, the proteolytic core of the 26 S proteasome that degrades ubiquitin conjugates, paralleled the increased and decreased rates of proteolysis throughout. However, increased mRNA levels for the 14-kD ubiquitin conjugating enzyme E2, involved in substrate ubiquitylation, and for cathepsin B and m-calpain were observed in chronic sepsis. These data clearly support a major role for the ubiquitinproteasome dependent proteolytic process during sepsis but also suggest that the activation of lysosomal and Ca 2 ϩ -dependent proteolysis may be important in the chronic phase. (

Atrophic Remodeling of the Heart In Vivo Simultaneously Activates Pathways of Protein Synthesis and Degradation

“…In a rat model of sepsis-induced atrophy of skeletal muscle, transcript levels of ubiquitin and a proteosome subunit increased after 2 days but normalized after 10 days of sepsis. 3 In contrast, gene expression of two key regulatory enzymes (E2-14k, MAFbx1/Atrogin1) that increase during skeletal muscle atrophy 11 did not significantly increase in the atrophied rat heart.…”

“…1 Previous studies in skeletal muscle atrophy have shown that activation of the UPP is associated with an increase in mRNA levels of ubiquitin, ubiquitin conjugating enzymes, ubiquitin ligases, and components of the proteasome. [2][3][4][5] The mTOR pathway is thought to be the main signaling cascade that activates protein translation, thereby regulating protein synthesis. 6 Atrophy in skeletal muscle leads to the decreased phosphorylation of downstream proteins of mTOR, such as p70S6K.…”

Proteolysis in illness-associated skeletal muscle atrophy: from pathways to networks

“…Increased mRNA expression of many subunits of the 20S proteasome has been observed in atrophying skeletal muscle (Table 1) [44][45][46][47] . Increased expression of some 19S cap subunits have also been reported but is not as consistent a finding 44,[48][49][50] .…”

Ubiquitin-proteasome-dependent proteolysis in skeletal muscle

“…The 14-kDa E2 has two transcripts (1.8 and 1.2 kb) arising from different polyadenylation sites [48]. The 1.8-kb transcript, which corresponds to the major mRNA species in rat muscle, is barely detectable in humans [26] and goats [22] [33] or acutely septic [45] rats, although in both conditions the activation of a ubiquitin-dependent proteolytic process was responsible for increased proteolysis. Furthermore, increased expression of the 14-kDa E2 also does not always correlate with enhanced protein breakdown [40].…”