Variations of endocytic and of lysosomal functions during the cell cycle have been investigated in synchronized hepatoma cells (derived from Morris hepatoma 7288c) by following the cellular uptake of horseradish peroxidase, dextran (mol wt. 70,000), and chloroquine. Cell fractionation and cytochemistry show that in asynchronously growing cells exposed for 1 h to 5 mg/ml peroxidase, the bulk of the enzyme taken up by the cells is found in phagosomes. By using the same experimental system with synchronized HTC cells, large variations of endocytosis are observed during the cell cycle. Peroxidase uptake is lowest during mitosis, increases 5--10 times during G1 phase, reaches a plateau, and finally decreases at the end of S phase and during G2 phase. A similar evolution is observed for the uptake of dextran (0.5 or 1 mg/ml), but it is likely that a significant part of the polysaccharide is still associated with the pericellular surface after 1 h. Moreover, dextran is transferred more slowly than peroxidase to lysosomes. Cellular accumulation of chloroquine is related to intralysosomal pH or to the buffering capacity of lysosomes. Our results show that this drug is taken up more rapidly during G1 and S phases while the rate of accumulation is lowest in mitotic cells. The results are discussed in relation to the modifications of the physical properties of lysosomes during the cell cycle observed previously by cell fractionation and electron microsocopy, and to the possible role of lysosomes in the initiation of mitosis. Cyclic changes of endocytosis in actively dividing cells are demonstrated by our observations and may induce large differences in the uptake rate of extracellular substances.

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