Figure 7. Proposed acid-adaptation mechanism.

(a) LAMP2 expression in normal and cancer cells in breast tumour samples. LAMP2 membrane overexpression is indicated by arrows in breast cancer tumours (DCIS) in the right-most image that is a zoom-in of the yellow box in its neighbour. (b) LAMP2 expression in chronically acid-adapted cells versus non-adapted by ICC. To make sure LAMP2 is at the cell surface and not just cell periphery, we added wheat germ agglutinin (WGA) as a membrane marker to the cells; co-registration of LAMP2 and WGA was measured (merge) and confirmed membrane expression of LAMP2. (c) Membrane expression of LAMP2. Western blot of membrane protein extracted from AA and NA MCF-7 cells shows higher expression of LAMP2 at the cell surface of AA cells. (d) Schematic presentation of LAMP2 role in acid adaptation of cancer cells. In chronic acidosis, a possible mechanism could involve the exocytosis of more acidic lysosomes. This strategy helps the cells to secrete large amounts of acid in one step, and also by presenting of LAMP2 at the cell membrane protects themselves from acid degradation. LAMP2 with hydrated hyper-branched carbohydrate chains can protect the membrane against acidity. (e) The cathepsin-B assay in the media of AA and NA MCF-7 cells; AA MCF-7 cells secrete significantly more cathepsin B into their extracellular environment, consistent with increased lysosomal turnover rates.