DNA Damage Induced by DNA Topoisomerase I- and Topoisomerase II-Inhibitors Detected by Histone H2AXphosphorylation in Relation to the Cell Cycle Phase and Apoptosis

Histone H2AX is phosphorylated on Ser-139 by ATM kinase in response to damage that induces dsDNA breaks. Immunocytochemical detection of phosphorylated H2AX (gamma H2AX), thus, reveals the presence of dsDNA breaks in chromatin. Multiparameter cytometry was presently used to correlate the appearance of gamma H2AX with: a. cell cycle phase; b. caspase-3 activation; and c. apoptosis-associated DNA fragmentation in individual human leukemic HL-60 cells treated with the DNA topoisomerase (topo1) inhibitors topotecan (TPT) and camptothecin (CPT) or with the topo2 inhibitor mitoxantrone (MTX). In response to TPT or CPT maximal increase of gamma H2AX immunofluorescence was seen in S-phase cells by 90 min. In contrast, following MTX treatment the maximal rise of gamma H2AX was detected at 2 h in G(1) cells and the cell cycle phase specificity was much less apparent. A linear relationship between the drug concentration and increase of gamma H2AX immunofluorescence was seen only up to 200 nM TPT; a decline in gamma H2AX was apparent at a concentration range between 0.4 and 1.6 gamma M TPT. Thus, the intensity of gamma H2AX immunofluorescence, as a marker of cell survival following TPT treatment, can be used only within a limited range of drug concentration. Following treatment with TPT, CPT or MTX the peak of H2AX phosphorylation preceded caspase-3 activation and the appearance of apoptosis-associated DNA fragmentation, both selective to S-phase cells. Progression of apoptosis was paralleled by a decrease in gamma H2AX immunofluorescence. The data also indicate that regardless whether treated with inhibitors of topo1 or topo2, at comparable levels of dsDNA breaks, the cells replicating DNA have a higher proclivity to undergo apoptosis compared to G(1) or G(2)/M cells.