Inhibition of the vacuolar ATPase induces Bnip3-dependent death of cancer cells and a reduction in tumor burden and metastasis

Abstract

The pro-apoptotic protein Bnip3 is induced by hypoxia and is present in the core regions of most solid tumors. Bnip3 induces programmed necrosis by an intrinsic caspase independent mitochondrial pathway. Many tumor cells have evolved pathways to evade Bnip3-mediated death attesting to the physiological relevance of the survival threat imposed by Bnip3. We have reported that acidosis can trigger the Bnip3 death pathway in hypoxic cells therefore we hypothesized that manipulation of intracellular pH by pharmacological inhibition of the vacuolar (v)ATPase proton pump, a significant pH control pathway, may activate Bnip3 and promote death of hypoxic cells within the tumor. Here we confirm that bafilomycin A1 (BafA1), a selective vATPase inhibitor, significantly increased death of breast cancer cells in a hypoxia and Bnip3-dependent manner and significantly reduced tumor growth in MCF7 and MDA-MB-231 mouse xenografts. Combined treatment of cells with BafA1 and the ERK1/2 inhibitor U0126 further augmented cell death. Combined treatment of mice containing MDA-MB-231 xenografts with BafA1 and the ERK1/2 inhibitor sorafenib was superior to either treatment alone and supported tumor regression. BafA1 and sorafenib treatments alone reduced MDA-MB-231 cell metastasis and again the combination was significantly more effective than either treatment alone and was without apparent side effects. These results present a novel mechanism to destroy hypoxic tumor cells that may help reverse the resistance of hypoxic tumors to radiation and chemotherapy and perhaps target tumor stem cells.

Extracellular acidity strengthens mesenchymal stem cells to promote melanoma progression

“…Graham RM et al show that the inhibition of the vacuolar ATPase induces a reduction in tumor burden and metastasis in breast cancer. 47 Overall, our results stress that acidosis of tumor microenvironment potentiates the pro-tumoral activity of MSC, representing a reasonable target for melanoma.…”

Vacuolar ATPase as a potential therapeutic target and mediator of treatment resistance in cancer

“…Again, no toxic side effects were observed using baf‐A1 (1 mg/kg). Additionally, when combined with sorafenib, it was shown that V‐ATPase treatment could result in tumor regression in MDA‐MD‐231 xenograft mice 86. Another study demonstrated that growth of a HepG2 orthotopic HCC xenograft model in nude mice was retarded by baf‐A1 42…”

TM9SF4 is a novel V-ATPase-interacting protein that modulates tumor pH alterations associated with drug resistance and invasiveness of colon cancer cells

“…TM9SF4/V-ATPase interaction: a key event in tumor pH gradient F Lozupone et al TM9SF4 silencing inhibits tumor cell invasiveness Extracellular acidity increases local invasiveness of cancer cells, and a number of studies have suggested that a key role in cell invasion is played by V-ATPase expressed on the cell surface. 8,[28][29][30][31] We thus investigated the migration and invasiveness of SCR and TM9SF4-KD cells. Migration was assessed by scratch test and by 8 μm pore size Boyden chamber-based assay, whereas invasiveness was evaluated in an assay based on the utilization of Matrigel-coated Boyden chambers.…”