Altered pH gradient at the plasma membrane of osteosarcoma cells is a key mechanism of drug resistance

Abstract

Current therapy of osteosarcoma (OS), the most common primary bone malignancy, is based on a combination of surgery and chemotherapy. Multidrug resistance mediated by P-glycoprotein (P-gp) overexpression has been previously associated with treatment failure and progression of OS, although other mechanisms may also play a role. We considered the typical acidic extracellular pH (pHe) of sarcomas, and found that doxorubicin (DXR) cytotoxicity is reduced in P-gp negative OS cells cultured at pHe 6.5 compared to standard 7.4. Short-time (24–48 hours) exposure to low pHe significantly increased the number and acidity of lysosomes, and the combination of DXR with omeprazole, a proton pump inhibitor targeting lysosomal acidity, significantly enhanced DXR cytotoxicity. In OS xenografts, the combination treatment of DXR and omeprazole significantly reduced tumor volume and body weight loss. The impaired toxicity of DXR at low pHe was not associated with increased autophagy or lysosomal acidification, but rather, as shown by SNARF staining, with a reversal of the pH gradient at the plasma membrane (ΔpHcm), eventually leading to a reduced DXR intracellular accumulation. Finally, the reversal of ΔpHcm in OS cells promoted resistance not only to DXR, but also to cisplatin and methotrexate, and, to a lesser extent, to vincristine. Altogether, our findings show that, in OS cells, short-term acidosis induces resistance to different chemotherapeutic drugs by a reversal of ΔpHcm, suggesting that buffer therapies or regimens including proton pump inhibitors in combination to low concentrations of conventional anticancer agents may offer novel solutions to overcome drug resistance.

A Charge Reversible Self‐Delivery Chimeric Peptide with Cell Membrane‐Targeting Properties for Enhanced Photodynamic Therapy

“…Cell membrane is in charge of the nutrients exchange between the cells and external environment . What's more, many membrane proteins (such as ATP‐binding cassette transporter), and the reversal of the pH gradient at plasma membrane (∆pHcm), might promote resistance of cancer cells to many chemotherapy drugs . Considering the unique feature of cell membrane, direct destruction of tumor cell membrane would be a novel and efficient approach for tumor inhibition .…”

Spheroid‐based 3D cell cultures identify salinomycin as a promising drug for the treatment of chondrosarcoma

“…We found a significantly higher CA IX expression in spheroids as compared to monolayer cultures. CA IX expression is also often associated with acidosis and extracellular acidosis can also account for the induction of drug resistance to conventional anti‐mitotic drugs, like DXR, or to less conventional anti‐autophagic drugs, like CQ . CQ is an autophagy‐inhibiting agent currently under investigation in clinical trials in solid tumors that, however, revealed to be not effective on melanoma and colon cancer cells viability in acidic conditions.…”

“…Such 3D‐associated chemoresistance is very likely mediated by the tumor microenvironment that is recapitulated in the 3D structure, and that is typically characterized by hypoxic and acidic areas due to cancer cells tendency to glucose consumption with increased lactate production and a consequent high proton flow . Indeed, we and others have demonstrated that extracellular acidity contributes to chemoresistance of malignant cells . Spheroids develop radial gradients of intracellular and extracellular pH (pHi and pHe), with the lowest levels reached at the spheroid core, resulting in gene expression and phenotypic changes that possibly account also for the drug resistance observed in vivo .…”

Dimensions of Niche-Induced Anti-Apoptosis of Therapeutically Resistant Neoplasms in Radio-Chemo-Therapy

“…Osteosarcoma plasma membrane acidic pH induces chemo-resistance to different agents [20]. The further co-damage as relative to a rich angiogenesis would allow for resistance clones as illustrated by survival clonogenic assays.…”