VD₃ mitigates breast cancer aggressiveness by targeting V-H⁺-ATPase

Low vitamin D levels increase the risk of developing several cancer types including breast cancer. Breast cancer is the most incident cancer among women worldwide and in the United States. Our previous study showed that vitamin D (VD₃) decreases breast cancer aggressiveness by inhibiting mammalian target of rapamycin (mTOR). However, the full mechanism underlying VD₃ effects in breast cancer, including some activators of mTORC1, is yet to be explored. Metastatic cancer cells overexpress the V-H⁺-ATPase proton pump at the plasma membrane to maintain the optimal pH to sustain cancer growth promoting their own invasion and metastasis by acidifying the extracellular environment. Among its other roles, V-H⁺-ATPase overexpression and activity are associated with high glycolytic flux, mTORC1 activation, and hypoxia. V-H⁺-ATPase's role in mTORC1 activation and glycolytic metabolism supports our hypothesis that VD₃, a nontoxic and widely used compound, inhibits the proton pump resulting in a significant decrease in cancer aggressiveness. VD₃ and the specific inhibitor bafilomycin A₁ (positive control) profoundly inhibit V-H⁺-ATPase function and expression. Highly metastatic MB231 has more pronounced effects (high extracellular pH, low migration speed and changes in cell mechanics) than lowly metastatic MCF-7 due to the higher expression of V-H⁺-ATPase, which drives the more aggressive phenotype. Our data show, for the first time, that VD₃ strongly inhibited V-H⁺-ATPase function and expression in breast cancer cells, thereby suggesting its use as a possible therapeutic agent.