Inhibition of protein degradation in mouse hearts by agents that cause lysosomal dysfunction.

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Abstract
Although the heart contains lysosomes, it has been uncertain whether these organelles and their proteolytic enzymes can play an important role in cardiac protein degradation. Recent studies have demonstrated that fetal mouse hearts in organ culture sustain selective derangements in lysosomal structure and function during exposure to chloroquine or nonmetabolizable sugars. Accordingly, we tested the effects of these agents on cardiac proteolysis under controlled conditions in vitro using two techniques (measurement of loss of radioactivity from trichoroacetic acid-precipitable protein after prelabeling with tritiated phenylalanine and measurement of loss of cold phenylalanine after blockade of protein synthesis with cycloheximide). Chloroquine (0.1 mM) reduced the average rate of protein breakdown in hearts of matched littermates from 45%/24 hours to 32%/24 hours (P less than 0.01) and decreased the release of cold phenylalanine by 31 +/- 5% (0.108 vs 0.075 nmol/mg per hour, P less than 0.01). Exposure to 100 mM sucrose for 24-48 hours reduced the rate of breakdown from 44%/24 hours to 33%/24 hours (P less than 0.01) and decreased the release of cold phenylalanine by 35 +/- 9% (0.092 vs. 0.060 nmol/mg per hour, P less than 0.01). The results suggest that interference with lysosomal function in cultured fetal mouse hearts causes a significant reduction in the cardiac capacity to degrade proteins.