pH-Dependent Effects of the Ionophore Nigericin on Response of Mammalian Cells to Radiation and Heat Treatment

The extracellular pH ( pH e) in many solid tumors is often lower than the pH of normal tissues. The ${\rm K}^{+}/{\rm H}^{+}$ ionophore nigericin is toxic to CHO cells when pH e is below but not above 6.5, and thus it has potential for selective killing of tumor cells in an acidic environment. This study examines the pH-dependent effects of nigericin on the response of CHO cells to radiation and heat treatment. Cells held for 4 h in Hank's balanced salt solution, after 9 Gy irradiation, exhibit potentially lethal damage recovery (PLDR) which is maximal at pH e 6.7-6.8. Addition of nigericin, postirradiation, not only inhibits PLDR when pH e is below 6.8, but interacts synergistically with radiation to reduce survival below that of cells plated immediately after irradiation when pH e is 6.4 or lower. Nigericin enhances heat killing of CHO cells preferentially under acidic conditions, and where neither heat nor drug treatment alone is significantly toxic. Survival of cells held for 30 min at 42.1°C in the presence of 1.0 μg/ml nigericin is 0.6, 0.08, 0.003, and 0.00003 at pH e 7.4, 6.8, 6.6, and 6.4, respectively, relative to survival of 1.0 in untreated cultures. The biochemical effects of nigericin at pH e 7.4 vs pH e 6.4 have been investigated. Nigericin inhibits respiration, stimulates glucose consumption, and causes dramatic changes in intracellular concentrations of Na+ and K+ at pH e 7.4 as well as 6.4. The drug reduces intracellular levels of ATP, GTP, and ADP but has more pronounced effects under acidic incubation conditions. Others have shown that nigericin equilibrates pH e and intracellular pH ( pH i) only when pH e is 6.5 or lower. Our observations and those of others have led us to conclude that lowering of pH i by nigericin is either the direct or indirect cause of enhancement of radiation and heat killing of cells in an acidic environment.