MUSCLE WASTING IN A RAT MODEL OF LONG-LASTING SEPSIS RESULTS FROM THEACTIVATION OF LYSOSOMAL, CA2-ACTIVATED, AND UBIQUITIN-PROTEASOME PROTEOLYTIC PATHWAYS()

We studied the alterations in skeletal muscle protein breakdown in lon g lasting sepsis using a rat model that reproduces a sustained and rev ersible catabolic state, as observed in humans, Rats were injected int ravenously with live Escherichia coli; control rats were pair-fed to t he 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 la te septic phase (day 10), The importance of the lysosomal, Ca2+-depend ent, and ubiquitin-proteasome proteolytic processes was investigated u sing proteolytic inhibitors in incubated epitrochlearis muscles and by measuring mRNA levels for critical components of these pathways. Prot ein breakdown was elevated during the acute and chronic septic phases (when significant muscle wasting occurred) and returned to control val ues in the late septic phase (when wasting was stopped), A nonlysosoma l and Ca2+-independent process accounted for the enhanced proteolysis, and only mRNA levels for ubiquitin and subunits of the 20 S proteasom e, the proteolytic core of the 26 S proteasome that degrades ubiquitin conjugates, paralleled the increased and decreased rates of proteolys is throughout, However, increased mRNA levels for the 14-kD ubiquitin conjugating enzyme E2, involved in substrate ubiquitylation, and for c athepsin B and m-calpain were observed in chronic sepsis, These data c learly support a major role for the ubiquitin-proteasome dependent pro teolytic process during sepsis but also suggest that the activation of lysosomal and Ca2+-dependent proteolysis may be important in the chro nic phase.