Characterization of very acidic phagosomes in breast cancer cells and their association with invasion

Abstract

Human metastatic breast cancer cells in culture contain large acidic vesicles (diameter 5-10 μm) in which endocy-tosed extracellular matrix can be digested by activated lysosomal proteinases such as cathepsin D (P. Montcour-rier et al. (1990). Cancer Res. 50, 6045-6054). We examined these large compartments by transmission electron microscopy, measured their pH by video-enhanced epiflu-orescence using FITC-dextran, and studied their functional significance. Their presence in metastatic MDA-MB231 cells was found to be correlated with an increased ability of cells to migrate through Matrigel and a high cathepsin D concentration. These cells were able to phagocytose 1.24 μm diameter latex beads and fluorescent Matrigel and incorporate this extracellular material into large acidic vesicles. This indicated that large acidic vesicles were associated with both phagocytosis and invasion, and are het-erophagolysosomes rather than autophagosomes. Large acidic vesicles were actively acidified with a H+-ATPase vacuolar pump specifically inhibited by bafilomycin A1, and reached pH values (<4), lower than the lysosomal value (pH ∼5) in the same cells and in specialized phagocytotic cells such as macrophages. We conclude that the phagocytotic activity of breast cancer cells, associated with high cathepsin D expression, and high acidification potential, characterize cancer cells that have migrated through Matrigel.