Regulation of muscle cathepsin B proteolytic activity in protein-depleted patients with chronic diseases

Background & aims: The lysosomal cathepsin system contributes to degrading cellular skeletal muscle proteins in many catabolic diseases. We have assessed the relationships between cathepsin B mRNA levels and the enzyme activity for this protease in the skeletal muscle of acutely ill patients with severe trauma (n=7) and in patients with a variety of chronic disease states (hemodialysis, n=3; nervous anorexia, n=1; type 2 diabetes, n=2; prolonged immobilization, n=1). Methods: Muscle biopsies were taken from the vastus lateralis muscle in patients and controls to assess tissue levels of cathepsin B mRNA by competitive–quantitative polymerase chain reaction, cathepsin B proteolytic activity and myofibrillar protein content as alkali-soluble protein to DNA ratio (ASP/DNA). In the trauma patients, muscle protein loss was assessed by the arteriovenous balance technique as rate of phenylalanine release from leg muscle. Results: The acute trauma patients exhibited a significant net phenylalanine release from leg muscle (33±4 nmol phenylalanine/min/100 ml leg volume) despite a continuous nutritional support. The muscle ASP/DNA ratio was lower (P<0.05) in the patients with chronic diseases (383±33) than in groups of healthy controls (554±41) or of uncomplicated, moderately obese subjects (525±26). Cathepsin B mRNA levels were 6–10 times greater (P<0.05) in the patients with acute trauma or chronic catabolic diseases than in the healthy subjects. Cathepsin B enzymatic activity were 2–3 times greater (P<0.05) in the chronic and acute patients than in the group of uncomplicated, moderately obese subjects. Regression analysis between cathepsin B mRNA and cathepsin B enzymatic activity indicates a significant direct correlation (r=0.84; P<0.05) in the chronic catabolic conditions, but not in the acute trauma patients (r=−0.05). Conclusions:In skeletal muscle of patients with stable chronic catabolic diseases, cathepsin B activity is directly related to cathepsin B mRNA levels, suggesting that in these patients this enzyme could be mainly regulated at the level of gene transcription.