Quantification of apoptotic cells with fluorescein isothiocyanate‐labeled annexin V in chinese hamster ovary cell cultures treated with cisplatin

Plasma membrane binding of annexin V was used to detect and quantitate apoptotic cells induced by cytotoxic drug treatment in epithelial cell cultures. Chinese hamster ovary (CHO) cells were incubated for 2 h with the ID90 concentration of Cisplatin (20 μM), and 24, 48, 72, and 96 h later the unfixed cells were stained with fluorescein isothiocyanate (FITC)‐conjugated annexin V. The fluorescence signal was quantitated by flow cytometry (FCM). During the early phase of the apoptotic response, the annexin V‐binding frequency histograms showed two separate cell populations, a dimly and a brightly fluorescent one. At t = 96 h after drug incubation, when the process of apoptosis was completed, only the brightly fluorescent population was present. A dose‐effect relationship could be established between the Cisplatin concentration used in the 2 h incubation and the binding of annexin V on the cell membrane, as estimated by FITC fluorescence. The dimly and brightly fluorescent populations were sorted on the basis of annexin V binding, and assayed for (1) DNA breaks by in situ nick translation assay and DNA content by DNA‐propidium iodine fluorescence in a bivariate analysis, (2) membrane integrity by dye exclusion, and (3) morphological characteristics of apoptosis. The dimly fluorescent cell population appeared to represent apoptotic cells in the early phase of the death process, as demonstrated by intact cell membranes, normal DNA content, few DNA breaks, and chromatin condensation. The brightly fluorescent cells predominantly had sub‐G1 DNA content, nuclear fragmentation, leaky cell membranes, and probably represent late apoptotic cells. These results demonstrate that cytotoxic drug‐induced apoptosis can be quantitated by annexin V binding and that by using this assay early and late apoptotic cells can be identified. © 1996 Wiley‐Liss, Inc.