Further study of the mechanism of thyroid hormone secretion in an in vitro model system: direct evidence for fusion of lysosomes with thyroglobulin liposomes.
The interaction between thyroglobulin liposomes, as a model of colloid droplets, and thyroidal lysosomes was studied in an in vitro system. On sucrose density gradient centrifugation analysis, particulate lysosomes were distributed in a high density region and were apparently clearly separated from the liposomal peak. However, when the liposomes and lysosomes were incubated together and then analyzed, the liposomes were found to migrate with the lysosomal peak. The extent of this translocation of liposomes depended on the incubation temperature: after incubations at 37 C and 0 C, 56% and 35%, respectively, of the radioactivity of liposomes was found in the lysosomal peak. This phenomenon was inhibited by 50% in the presence of 10(-3)M chlorpromazine. Matrix-labeled liposomes [125I]thyroglobulin liposomes), and membrane labeled lipolomes ([14C]-cholesterol-labeled thyroglobulin liposomes) showed similar distribution patterns in the lysosomal peak on gradient centrifugation. The liposomal thyroglobulin was hydrolyzed in a time-dependent manner in the fraction showing both radioactivity and acid phosphatase activity. These results provide direct evidence of liposome-lysosome fusion, or formation of phagolysosomes.