Prelysosomal and lysosomal connections between autophagy and endocytosis

In isolated rat hepatocytes electroloaded with [14C]sucrose, autophaged sugar accumulated in lysosomes under control conditions, and in prelysosomal autophagic vacuoles (amphisomes) in the presence of asparagine, an inhibitor of autophagic-lysosomal fusion. Endocytic uptake of the sucrose-cleaving enzyme invertase resulted in rapid and complete degradation of autophaged sucrose in both amphisomes and lysosomes. Pre-accumulated sucrose was degraded equally well in both compartments, regardless of amphisomal-lysosomal flux inhibition by asparagine, suggesting that endocytic entry into the autophagic pathway can take place both at the lysosomal and at the amphisomal level. The completeness of sucrose degradation by endocytosed invertase furthermore indicates that all lysosomes involved in autophagy can also engage in endocytosis. Endocytosed invertase reached the amphisomes even when autophagy was blocked by 3-methyladenine, and autophaged sucrose reached this compartment even when endocytic influx was blocked by vinblastine, suggesting that amphisomes may exhibit some degree of permanence independently of either pathway.