Relation between intracellular acidification and camptothecin-induced apoptosis in leukemia cells

Leukemia cells (HL-60 and P388) treated with the topoisomerase I inhibitor camptothecin (CPT) undergo rapid apoptosis as judged from internucleosomal degradation of genomic DNA, morphological changes and flow cytometry analys is. The intracellular free calcium concentration is not affected by the tre atment with a high dose of CPT. In contrast. fluorescence measurements of c ells loaded with the pH indicator BCECF-AM indicate that the intracellular pH decreases significantly. Incubation of the leukemia cells with a high dr ug concentration for 5 h or with lower drug concentrations for 15 h results in a pronounced intracellular acidification. Measurements with the whole c ell population show a decrease of 0.3-0.4 pH units. The extent of the acidi c shift is proportional to the drug concentration and the period of incubat ion. No such effects were observed with P388CPT5 cells resistant to CPT. Th e results support the hypothesis that apoptosis induced in leukemia cells b y CPT is associated with decreased intracellular pH. Modification of intrac ellular pH by topoisomerase inhibitors is viewed as an essential event resp onsible for the induction and/or propagation of apoptosis. The role of CPT- induced cellular acidification in the mechanism of action of the drug is di scussed. (C) 2000 Elsevier Science B.V. All rights reserved.