The present review describes several methods to characterize and differentiate between two different mechanisms of cell death, apoptosis and necrosis. Most of these methods were applied to studies of apoptosis triggered in the human leukemic HL‐60 cell line by DNA topoisomerase I or II inhibitors, and in rat thymocytes by either topoisomerase inhibitors or prednisolone. In most cases, apoptosis was selective to cells in a particular phase of the cell cycle: only Sphase HL‐60 cells and G0 thymocytes were mainly affected. Necrosis was induced by excessively high concentrations of these drugs. The following cell features were found useful to characterize the mode of cell death: (a) Activation of an endonuclease in apoptotic cells resulted in extraction of the low molecular weight DNA following cell permeabilization, which, in turn, led to their decreased stainability with DNA‐specific fluorochromes. Measurements of DNA content made it possible to identify apoptotic cells and to recognize the cell cycle phase specificity oaf the apoptotic process. (b) Plasma membrane integrity, which is lost in necrotic but not in apoptotic cells, was probed by the exclusion of propidium iodide (PI). The combination of PI followed by Hoechst 33342 proved to be an excellent probe to distinguish live, necrotic, early‐ and late‐apoptotic cells. (c) Mitochondrial transmembrane potential, assayed by retention of rhodamine 123 was preserved in apoptotic but not necrotic cells. (d) The ATP‐dependent lysosomal proton pump, tested by the supravital uptake of acridine orange (AO) was also preserved in apoptotic but not necrotic cells. (e) Bivariate analysis of cells stained for DNA and protein revealed markedly diminished protein content in apoptotic cells, most likely due to activation of endogenous proteases. Necrotic cells, having leaky membranes, had minimal protein content. (f) Staining of RNA allowed for the discrimination of G0 from G1 cells and thus made it possible to reveal that apoptosis was selective to G0 thymocytes. (g) The decrease in forward light shatter, paralleled either by no change (HL‐60 cells) or an increase (thymocytes) of right angle scatter, were early changes during apoptosis. (h) The sensitivity of DNA in situ to denaturation, was increased in apoptotic and necrotic cells. This feature, probed by staining with AO at love pH, provided a sensitive and early assay to discriminate between live, apoptotic and necrotic cells, and to evaluate the cell cycle phase specificity of these processes. (i) The in situ nick translation assay employing labeled triphosphonucleotides can be used to reveal DNA strand breaks, to detect the very early stages of apoptosis. The data presented indicate that flow cytometry can be applied in basic research on molecular and biochemical mechanisms of apoptosis, as well as in the clinic, where the ability to monitor early signs of apoptosis in samples from patients' tumors may be predictive of the outcome of some treatment protocols. © 1992 Wiley‐Liss, Inc.

Flow cytometry of heated sperm nuclei revealed a significant decrease in resistance to in situ denaturation of spermatozoal DNA in samples from bulls, mice, and humans of low or questionable fertility when compared with others of high fertility. Since thermal denaturation of DNA in situ depends on chromatin structure, it is assumed that changes in sperm chromatin conformation may be related to the diminished fertility. Flow cytometry of heated sperm nuclei may provide a new and independent determinant of male fertility.

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A cell undergoing apoptosis demonstrates multitude of characteristic morphological and biochemical features, which vary depending on the inducer of apoptosis, cell type and the “time window” at which the process of apoptosis is observed. Because the gross majority of apoptotic hallmarks can be revealed by flow and image cytometry, the cytometric methods become a technology of choice in diverse studies of cellular demise. Variety of cytometric methods designed to identify apoptotic cells, detect particular events of apoptosis and probe mechanisms associated with this mode of cell death have been developed during the past two decades. In the present review, we outline commonly used methods that are based on the assessment of mitochondrial transmembrane potential, activation of caspases, DNA fragmentation, and plasma membrane alterations. We also present novel developments in the field such as the use of cyanine SYTO and TO-PRO family of probes. Strategies of selecting the optimal multiparameter approaches, as well as potential difficulties in the experimental procedures, are thoroughly summarized.

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The expression and stability of the proliferation-associated nuclear antigen detected by Ki-67 antibody have been investigated in human promyelocytic leukaemic HL-60 cells in relation to their progression through the cell cycle. Expression of this antigen was minimal in late G1 and early S phase cells. The antigen accumulated in the cells predominantly during S phase, and its rate of increase per cell accelerated during the second half of this phase. The accumulation of Ki-67 antigen during S exceeded the increase in DNA content, and thus the Ki-67/DNA ratio rose 80% from late G1 to G2 + M. This antigen rapidly disappeared from post-mitotic cells. The half-life of this protein estimated in post-mitotic cells during stathmokinesis induced by vinblastine appeared to be shorter than 1 h. This rapid turnover should be compared with the relatively long (6-8 h) duration of G1 of the studied cells. In cells in which de novo protein synthesis was inhibited by 0.1 microgram/ml cycloheximide, the half-life of the Ki-67 antigen was also found to be about 1 h regardless of the cell position in the cell cycle. Thus, the data suggest that variations in the level of this protein during the cell cycle are a consequence of its different synthesis rate rather than phase-specific changes in the rate of its degradation. Because the late G1 and very early S phase cells express the antigen at levels only slightly above background, it is possible that, when using Ki-67 antibody as a marker of the cell growth fraction, some late G1 cells can be erroneously classified as non-cycling cells.

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Reviewed are the methods aimed to detect DNA damage in individual cells, estimate its extent and relate it to cell cycle phase and induction of apoptosis. They include the assays that reveal DNA fragmentation during apoptosis, as well as DNA damage induced by genotoxic agents. DNA fragmentation that occurs in the course of apoptosis is detected by selective extraction of degraded DNA. DNA in chromatin of apoptotic cells shows also increased propensity to undergo denaturation. The most common assay of DNA fragmentation relies on labelling DNA strand breaks with fluorochrome-tagged deoxy-nucleotides. The induction of double-strand DNA breaks (DSBs) by genotoxic agents provides a signal for histone H2AX phosphorylation on Ser139; the phosphorylated H2AX is named γH2AX. Also, ATM-kinase is activated through its autophosphorylation on Ser1981. Immunocytochemical detection of γH2AX and/or ATM-Ser1981(P) are sensitive probes to reveal induction of DSBs. When used concurrently with analysis of cellular DNA content and caspase-3 activation, they allow one to correlate the extent of DNA damage with the cell cycle phase and with activation of the apoptotic pathway. The presented data reveal cell cycle phase-specific patterns of H2AX phosphorylation and ATM autophosphorylation in response to induction of DSBs by ionizing radiation, topoisomerase I and II inhibitors and carcinogens. Detection of DNA damage in tumour cells during radio-or chemotherapy may provide an early marker predictive of response to treatment. DNA FRAGMENTATION DURING APOPTOSIS Involvement of different nucleases in DNA fragmentationCondensation of chromatin and internucleosomal DNA fragmentation, together with cell shrinkage and shedding of apoptotic bodies ('blebbing'), are widely recognized hallmarks of apoptosis (Kerr et al. 1972;Arends et al. 1990;Nagata 2000;Nagata et al. 2003). Several nucleases have been identified as contributing towards DNA degradation; their activity is modulated by divalent cations. Depending on cation concentration, three distinct steps of DNA fragmentation, likely mediated by different enzymes, can be identified: (i) in the presence of Mg 2+ (2 mM, DNA is fragmented to about 0.05-1 megabase (Mb)-size sections (type-I, high molecular weight DNA fragmentation)); (ii) at low (nanomolar) Ca 2+ concentration, nuclear DNA is cleaved into intermediate (∼300 kb) fragments (type-II, intermediate DNA fragmentation); (iii) at micromolar levels of Ca 2+ , internucleosomal (type-III) DNA fragmentation takes place leading to formation of DNA sections of the size of mono and oligonucleosomes, which form a characteristic 'DNA-ladder' pattern during electrophoresis (Arends et al. 1990 NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author ManuscriptAmong the nucleases associated with DNA fragmentation during apoptosis, the best characterized is CAD (caspase-activated DNase) with its inhibitor ICAD (inhibitor of CAD) in mice, and its human homologue DFF40/DFF45 (DNA fragmentation factor) (Enari et al. 1998). CAD and ICAD (or ...