Abstract
Proton pumps like the vacuolar-type H+ ATPase (V-ATPase) are involved in the control of cellular pH in normal and tumor cells. Treatment with proton pump inhibitors (PPI) induces sensitization of cancer cells to chemotherapeutics via modifications of cellular pH gradients. It is also known that low pH is the most suitable condition for a full PPI activation. Here, we tested whether PPI treatment in unbuffered culture conditions could affect survival and proliferation of human B-cell tumors. First, we showed that PPI treatment increased the sensitivity to vinblastine of a pre-B acute lymphoblastic leukemia (ALL) cell line. PPI, per se, induced a dose-dependent inhibition of proliferation of tumor B cells, which was associated with a dose- and time-dependent apoptotic-like cytotoxicity in B-cell lines and leukemic cells from patients with pre-B ALL. The effect of PPI was mediated by a very early production of reactive oxygen species (ROS), that preceded alkalinization of lysosomal pH, lysosomal membrane permeabilization, and cytosol acidification, suggesting an early destabilization of the acidic vesicular compartment. Lysosomal alterations were followed by mitochondrial membrane depolarization, release of cytochrome c, chromatin condensation, and caspase activation. However, inhibition of caspase activity did not affect PPI-induced cell death, whereas specific inhibition of ROS by an antioxidant (N-acetylcysteine) significantly delayed cell death and protected both lysosomal and mitochondrial membranes. The proapoptotic activity of PPI was consistent with a clear inhibition of tumor growth following PPI treatment of B-cell lymphoma in severe combined immunodeficient mice. This study further supports the importance of acidity and pH gradients in tumor cell homeostasis and suggests new therapeutic approaches for human B-cell tumors based on PPI.
Highlights
The hypoxic conditions and consequent acidity of tumor microenvironment play a key role contributing to tumor progression, chemoresistance, and metastatic behaviour [1, 2]
Because pump inhibitors (PPI) need an acidic pH to be protonated and activated [33], we evaluated the effect of omeprazole and esomeprazole in unbuffered culture conditions to allow cells spontaneously acidify the culture medium
Whereas the pH of a buffered medium decreased from 7.7 F 0.1 to 7.4 F 0.2 after 24 h in culture (0.3 pH units), the pH of unbuffered medium changed from 7.2 F 0.1 to 6.5 F 0.2 (0.7 pH units), providing the acidic conditions for a better activation of the PPI
Summary
The hypoxic conditions and consequent acidity of tumor microenvironment play a key role contributing to tumor progression, chemoresistance, and metastatic behaviour [1, 2]. Doi:10.1158/0008-5472.CAN-06-4095 face acidity with an up-regulated proton extrusion activity that allows tumor cells to survive in this unfavorable condition [3,4,5]. Increasing evidence suggests that acidic vesicles play a role in tumor homeostasis and growth. The up-regulated V-ATPase activity [10,11,12] and increased membrane V-ATPase expression detected in some human tumors are associated with chemoresistance and metastatic behavior of cancer cells [13]. It has been shown that in vivo inhibition of V-ATPase activity by RNA interference significantly delays human cancer growth by decreased proton extrusion [15], suggesting that pH regulation may have a key role in tumor homeostasis
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