Kinetic studies of the localization of aminopeptidase N in monolayer and in follicle-associated cultures of porcine thyroid cells.

The fate of aminopeptidase N (APN) was investigated by means of immunofluorescence in porcine thyroid-cell cultures. At day 1, control cells cultured without TSH formed flat, cylinder-like aggregates in which APN was mainly located on the lateral surface. At day 2, the fluorescence completely disappeared. At days 3 and 4, APN appeared in the apical pole of monolayer cells directed toward the medium and in intracellular organelles. The cilium in each cell was intensely labelled and located in the center of the apical pole. In thyrotropin-stimulated cell cultures, the fluorescence at days 1 and 2 was located over the entire plasma membrane. At days 3 and 4, morphological polarization of cells, reorganized into follicles, occurs, APN being located either in the apical or basolateral regions and in intracellular vesicles. Between day 4 and 6, segregation of the enzyme to the apical membrane occurred. Between day 6 and 14, fluorescence was generally retained in the apical region and in an intracellular pool. In growing cell cultures, after 2 to 5 subcultures, the cells exposed APN in a pattern similar to primary monolayers. After conversion of monolayer cells into follicles, the antigen was recovered in the apical pole of cells facing the follicular lumen.