Sequential processing of epidermal growth factor in early and late endosomes of rat liver

We have used isolated perfused rat livers to examine the intracellular processing of ¹²⁵I-epidermal growth factor (EGF) and to determine where in the endocytic pathway the hydrolases which degrade EGF are acting. Following uptake of ¹²⁵I-EGF at 37 or 16°C, subcellular fractions enriched in endosomes and lysosomes were isolated, and their ¹²⁵I-EGF content was examined by reverse-phase high performance liquid chromatography. Three forms of EGF processed at their carboxyl termini are generated in endosomes. At 37°C, EGF is first processed in early endosomes by a carboxypeptidase B-like protease and is further processed in late endosomes by a trypsin-like protease and then a carboxypeptidase B-like protease. At 16°C, entry of EGF into late endosomes is slowed, and only the first processed form is generated over 60 min. Longer perfusions (180 min) at 16°C result in some processing (7%) by proteases found in late endosomes. EGF-horseradish peroxidase cytochemistry confirmed that the additional processing detected at 180 min correlated with movement of EGF from tubulovesicular to multivesicular endosomes. These results, combined with in vitro incubations of EGF in isolated endosomal and lysosomal fractions, suggest that different proteases are active at selective points in the endocytic pathway and that the full complement of proteases needed for complete degradation of EGF is active only in lysosomes.