Curbing Breast Cancer by Altering V-ATPase Action on F-Actin, Heterochromatin, ETV7 and mTORC2 Signaling

Motivated by the vacuolar proton pump's importance in cancer, we investigate the effects of proton pump inhibition on breast cancer cell migration and proliferation, F-actin polymerization, lamin A/C, heterochromatin, and ETV7 expressions, nuclear size and shape, and AKT/mTOR signaling.

Lowly metastatic MCF7 and highly metastatic MDA-MB-231 breast cancer cells were treated with 120 nM of proton pump inhibitor Bafilomycin A1 for 24 hours. Cell migration was studied with wound- scratch assays, ATP levels with a chemiluminescent assay; cell proliferation was quantified by a cell area expansion assay. Nuclear size and shape were determined using DAPI nuclear stain and fluorescence microscopy. The levels of F-actin, lamin A/C, heterochromatin, and ETV7 were quantified using both immunocytochemistry and western blots; p-mTORC1, p-mTORC2, mTOR, p-AKT, and AKT were measured by western blots.

We reveal that proton pump inhibition reduces F-actin polymerization, cell migration, proliferation, and increases heterochromatin in both lowly and highly metastatic cells. Surprisingly, Bafilomycin decreases lamin A/C in both cell lines. Inhibition has different effects on ETV7 expression in lowly and highly metastatic cells, as well as nuclear area, perimeter, and circularity. Bafilomycin also significantly decreases p-mTORC1, p-MTORC2, and MTOR expression in both cell lines, whereas it significantly decreases p-AKT in lowly metastatic cells and surprisingly significantly increases p-AKT in highly metastatic cells. Our proton pump inhibition protocol reduces V-ATPase levels (~25%) within three hours. V-ATPase levels vary in time for both control and inhibited cells, and inhibition reduces cellular ATP.

Proton pumps promote F-actin polymerization and decrease heterochromatin, facilitating invasion. These pumps also upregulate both mTORC1 and mTORC2, thus highlighting the relevance of vacuolar proton pumps as metastatic cancer targets.