Suppression of lysosomal proteolysis at three different steps in regenerating rat liver.

Decreased lysosomal proteolysis in regenerating liver after 70% hepatectomy was analyzed. The activities of cathepsins B and L increased transiently 4 h after hepatectomy, began to decrease gradually reaching about 30% of the control level at 24 h, then returned to near control level after 7 days. Immunoblot and RNA blot analyses confirmed that the changes in cathepsin activities coincided with changes in protein levels and mRNA levels. In parallel with the changes in cathepsins, we found that the amounts of LGP120, LGP110, and LGP85, three integral lysosomal membrane proteins, declined significantly after hepatectomy, suggesting that the lysosomal levels are also diminished in regenerating liver. We isolated dextran-loaded lysosomes and found that the protein content and marker enzyme activities of dextran-loaded lysosomes from partially hepatectomized livers are lower by 50 and 40%, respectively, compared with control livers. This indicates that there is a significant reduction in the cellular lysosomal level in regenerating liver. In addition, we used a sensitive biochemical assay to quantify leupeptin-induced autolysosomes and found that the autophagic activity is markedly suppressed in regenerating liver as compared with normal liver. Thus, the suppression of lysosomal proteolysis in regenerating liver is attained through three steps, i. e., decreased biosynthesis of cathepsins, decreased lysosomal biogenesis, and decreased cellular autophagy.