Vinblastine, leupeptin and propylamine (a lysosomotropic weak base) inhibit intracellular protein degradation by different mechanisms (impairment of microtubular function, inhibition of lysosomal proteases, and elevation of lysosomal pH, respectively). In isolated rat hepatocytes, a paradoxical accumulation of autophagosomes was seen after treatment with each of the inhibitors at concentrations which inhibited protein degradation strongly. Such accumulation might occur if formation of autophagosomes proceeded at a normal rate, while their subsequent processing (e.g. their fusion with lysosomes) were inhibited. Vinblastine could conceivably reduce the motility of both autophagosomes and lysosomes, thereby preventing contact between them, whereas leupeptin and propylamine might reduce the ability of lysosomes to fuse by causing lysosomal constipation and neutralization/swelling, respectively. Amino acids also inhibited hepatocytic protein degradation, but in contrast to the other inhibitors (and regardless of the presence of the latter) they caused a decrease rather than an increase in the contents of autophagosomes, in accordance with their accepted mechanism of action as primary inhibitors of autophagy. It is proposed that drug-induced accumulation of autophagosomes in most cases may be due to the inhibition of late steps in the autophagic/lysosomal pathway rather than to a stimulation of autophagy.
