Killing Cells in Acidic Regions of Solid Tumors: Effects of Reduction of Intracellular pH as a Possible Mechanism for Updated Version

The environment of cells within solid tumors is known to be acidic relative to that in normal tissue, and the viability of tumor cells may depend on mechanisms which maintain intracellular pH (pi I,) above the extracellular pH (pH.). We have assessed therefore the toxicity in vitro of the proton ionophore carbonylcyanide-3-chlorophenylhydrazone (CCCP), since this agent has been reported to be capable of transporting IT equivalents through artificial lipid bilayers and mitochondria! mem branes. CCCP was toxic to the human bladder carcinoma cell line MGHUl and to the murine mammary sarcoma cell line EMT-6 only at pi I, < 6.5. CCCP transported I r equivalents through cell membranes at phys iological (7.35) and low pi I, (6.20). Cell lines were found to have steadystate pi li values approximately 0.1 to 0.2 pH units above pi I, at pii, < 6.50. Addition of CCCP led to a decrease in steady-state pi I, values as compared to untreated cells at pi I. < 6.50, whereas there was no apparent effect of CCCP on steady-state pH, values at pH, > 6.50. The CCCPinduced reduction in steady-state pi 1, combined with the uncoupling of oxidative phosphorylation by CCCP appeared to be the major mecha nisms leading to cell death at pi I, < 6.50. The toxicity of CCCP under acidic conditions was enhanced by amiloride and 4,4'-diisothiocyanostilbene-2,2-disulfonic acid, agents which are known to inhibit membranebased ion exchange mechanisms which regulate pi I, under acidic condi tions. When both agents were combined with CCCP, cell killing was observed at pi I, < 7.30. Our results suggest that mechanisms which regulate pi I, under acidic conditions which occur in solid tumors may represent targets for new forms of tumor-specific therapy.