Macrophage protein turnover. Evidence for lysosomal participation in basal proteolysis

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Abstract
Turnover of intracellular proteins in cultured mouse macrophages was slightly accelerated by the omission of serum from the culture medium. Media containing 10% (v[vol]/v) or more of serum established basal degradation rates in the cultures. Basal degradation rates varied considerably between experiments, probably as a result of variable activation in vivo of the macrophages. The selective carboxyl proteinase inhibitor pepstatin, which appeared to enter the lysosomes of the cells by pinocytosis, gave a progressive inhibition of basal proteolysis up to a maximum of about 40%. Cellular cathepsin D was largely inhibited after 48 h of cultivation with pepstatin (100 .mu.g/ml). Leupeptin and 7-amino-1-chloro-3-tosylamidoheptan-2-one are less selective proteinase inhibitors. They induced 25-35% inhibition of degradation, but their actions may not have been restricted to lysosomes. Several solutes and particles that were endocytosed by macrophages and stored in lysosomes induce some inhibition of basal proteolysis, whether or not they themselves are substrates for proteolysis. Colchicine was without effect on protein degradation, but cytochalasin B and the local anesthetics lidocaine and procaine, which have effects on microfilaments, were significantly inhibitory. This inhibition may result from a decrease in the rate of autophagy, and of lysosomal proteolysis, due to prevention of microfilament action.