Pharmacologic inhibition of vacuolar H+ ATPase reduces physiologic and oncogenic Notch signaling

Notch signaling in prominently involved in growth regulation in metazoan tissues. Because of this, Notch is often upregulated in cancer and current efforts point to developing drugs that block its activation. Notch receptor endocytosis towards acidic compartments is a recently appreciated determinant of signaling activation. Vacuolar H+ ATPase (V‐ATPase) is responsible for acidification of endocytic organelles and mutants in V‐ATPase subunit encoding genes in model organisms have been recently shown to display loss of Notch signaling. Here, we show that administration of BafilomycinA1 (BafA1), a highly specific V‐ATPase inhibitor decreases Notch signaling during Drosophila and Zebrafish development, and in human cells in culture. In normal breast cells, we find that BafA1 treatment leads to accumulation of Notch in the endo‐lysosomal system, and reduces its processing and signaling activity. In Notch‐addicted breast cancer cells, BafA1 treatment reduces growth in cells expressing membrane tethered forms of Notch, while sparing cells expressing cytoplasmic forms. In contrast, we find that V‐ATPase inhibition reduces growth of leukemia cells, without affecting Notch activatory cleavage. However, consistent with the emerging roles of V‐ATPase in controlling multiple signaling pathways, in these cells Akt activation is reduced, as it is also the case in BafA1‐treated breast cancer cells. Our data support V‐ATPase inhibition as a novel therapeutic approach to counteract tumor growth via signaling pathways regulated at the endo‐lysosomal level.