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.