Cell cycle kinetics estimated by analysis of bromodeoxyuridine incorporation

Flow cytometry: Aspects and application in plant and biological science

Flow cytometry is a potent method that enables the quick and concurrent investigation of several characteristics of single cells in solution. Photodiodes or photomultiplier tubes are employed to detect the dispersed and fluorescent light signals that are produced by the laser beam as it passes through the cells. Photodetectors transform the light signals produced by the laser into electrical impulses. A computer then analyses these electrical impulses to identify and measure the various cell populations depending on their fluorescence or light scattering characteristics. Based on their fluorescence or light scattering properties, cell populations can be examined and/or isolated. This review covers the basic principle, components, working and specific biological applications of flow cytometry, including studies on plant, cell and molecular biology and methods employed for data processing and interpretation as well as the potential future relevance of this methodology in light of retrospective analysis and recent advancements in flow cytometry.

Cytometry of DNA replication and RNA synthesis: Historical perspective and recent advances based on "click chemistry"

This review covers progress in the development of cytometric methodologies designed to assess DNA replication and RNA synthesis. The early approaches utilizing autoradiography to detect incorporation of (3) H- or (14) C-labeled thymidine were able to identify the four fundamental phases of the cell cycle G(1) , S, G(2) , and M, and by analysis of the fraction of labeled mitosis (FLM), to precisely define the kinetics of cell progression through these phases. Analysis of (3) H-uridine incorporation and RNA content provided the means to distinguish quiescent G(0) from cycling G(1) cells. Subsequent progress in analysis of DNA replication was based on the use of BrdU as a DNA precursor and its detection by the quenching of the fluorescence intensity of DNA-bound fluorochromes such as Hoechst 33358 or acridine orange as measured by flow cytometry. Several variants of this methodology have been designed and used in studies to detect anticancer drug-induced perturbations of cell cycle kinetics. The next phase of method development, which was particularly useful in studies of the cell cycle in vivo, including clinical applications, relied on immunocytochemical detection of incorporated halogenated DNA or RNA precursors. This approach however was hampered by the need for DNA denaturation, which made it difficult to concurrently detect other cell constituents for multiparametric analysis. The recently introduced "click chemistry" approach has no such limitation and is the method of choice for analysis of DNA replication and RNA synthesis. This method is based on the use of 5-ethynyl-2'deoxyuridine (EdU) as a DNA precursor or 5-ethynyluridine (EU) as an RNA precursor and their detection with fluorochrome-tagged azides utilizing a copper (I) catalyzed [3+2] cycloaddition. Several examples are presented that illustrate incorporation of EdU or EU in cells subjected to DNA damage detected as histone H2AX phosphorylation that have been analyzed by flow or laser scanning cytometry.

Cytometry of ATM activation and histone H2AX phosphorylation to estimate extent of DNA damage induced by exogenous agents

This review covers the topic of cytometric assessment of activation of Ataxia telangiectasia mutated (ATM) protein kinase and histone H2AX phosphorylation on Ser139 in response to DNA damage, particularly the damage that involves formation of DNA double-strand breaks. Briefly described are molecular mechanisms associated with activation of ATM and the downstream events that lead to recruitment of DNA repair machinery, engagement of cell cycle checkpoints, and activation of apoptotic pathway. Examples of multiparameter analysis of ATM activation and H2AX phosphorylation vis-a-vis cell cycle phase position and induction of apoptosis that employ flow- and laser scanning-cytometry are provided. They include cells treated with a variety of exogenous genotoxic agents, such as ionizing and UV radiation, DNA topoisomerase I (topotecan) and II (mitoxantrone, etoposide) inhibitors, nitric oxide-releasing aspirin, DNA replication inhibitors (aphidicolin, hydroxyurea, thymidine), and complex environmental carcinogens such as present in tobacco smoke. Also presented is an approach to identify DNA replicating (BrdU incorporating) cells based on selective photolysis of DNA that triggers H2AX phosphorylation. Listed are strategies to distinguish ATM activation and H2AX phosphorylation induced by primary DNA damage by genotoxic agents from those effects triggered by DNA fragmentation that takes place during apoptosis. While we review most published data, recent new findings also are included. Examples of multivariate analysis of ATM activation and H2AX phosphorylation presented in this review illustrate the advantages of cytometric flow- and image-analysis of these events in terms of offering a sensitive and valuable tool in studies of factors that induce DNA damage and/or affect DNA repair and allow one to explore the linkage between DNA damage, cell cycle checkpoints and initiation of apoptosis.

The differential staurosporine-mediated G1 arrest in normal versus tumor cells is dependent on the retinoblastoma protein

Previously, we reported that breast cancer cells with retinoblastoma (pRb) pathway-defective checkpoints can be specifically targeted with chemotherapeutic agents, following staurosporine-mediated reversible growth inhibition in normal cells. Here we set out to determine if the kinetics of staurosporine-mediated growth inhibition is specifically targeted to the G(1) phase of cells, and if such G(1) arrest requires the activity of wild-type pRb. Normal human mammary epithelial and immortalized cells with intact pRb treated with low concentrations of staurosporine arrested in the G(1) phase of the cell cycle, whereas pRb-defective cells showed no response. The duration of G(1) and transition from G(1) to S phase entry were modulated by staurosporine in Rb-intact cells. In pRb(+) cells, but not in Rb(-) cells, low concentrations of staurosporine also resulted in a significant decrease in cyclin-dependent kinase 4 (CDK4) expression and activity. To directly assess the role of pRb in staurosporine-mediated G(1) arrest, we subjected wild-type (Rb(+/+)) and pRb(-/-) mouse embryo fibroblasts (MEFs) to staurosporine treatments. Our results show that whereas Rb(+/+) MEFs were particularly sensitive to G(1) arrest mediated by staurosporine, pRb(-/-) cells were refractory to such treatment. Additionally, CDK4 expression was also inhibited in response to staurosporine only in Rb(+/+) MEFs. These results were recapitulated in breast cancer cells treated with siRNA to pRb to down-regulate the pRb expression. Collectively, our data suggest that treatment of cells with nanomolar concentrations of staurosporine resulted in down-regulation of CDK4, which ultimately leads to G(1) arrest in normal human mammary epithelial and immortalized cells with an intact pRb pathway, but not in pRb-null/defective cells.

Construction and characterization of multiple human colon cancer cell lines for inducibly regulated gene expression

Validation of targets for cancer drug discovery requires robust experimental models. Systems based on inducible gene expression are well suited to this purpose but are difficult to establish in several epithelial cell types. Using the recently discovered transcriptional transactivator (rtTA2S-M2), we developed a strategy for fast and efficient generation of Tet On cells. Multiple clones of HCT116, SW480, and HT29 human colon cancer cells for doxycycline-regulated gene expression were constructed that constitutively express green fluorescent protein (GFP) for selection/maintenance purposes. The cell lines displayed good fold inducibility (49-124xHCT116; 178-621xSW480; 261-787xHT29) and minimal leakiness after transient transfection with a luciferase reporter or with vectors driving inducible expression of red fluorescent protein (dsRed2), constitutively active c-Src or dominant negative K-Ras4B. The clones preserved their transformed phenotype as demonstrated by comparing their properties to respective wild type cells, in terms of growth in vitro and in vivo (as tumor xenografts), cell cycle traverse, and sensitivity to drugs used in chemotherapy. These engineered cell lines enabled tightly controlled inducible gene expression both in vitro and in vivo, and proved well suited for construction of double-stable cell lines inducibly expressing a protein of interest. As such they represent a useful research tool for example, to dissect oncogene function(s) in colon cancer. Supplementary material for this article be found at http://www.mrw.interscience.wiley.com/suppmat/0730-2312/suppmat/94/suppmat_welman.doc.

Estimating cell death in G2M using bivariate BrdUrd/DNA flow cytometry

BACKGROUND

In an accompanying paper (Asmuth et al.) it was found necessary to include cell death explicitly to estimate parameters of cell proliferation. The use of bivariate flow cytometry to estimate the phase durations and the doubling times of cells labeled with thymidine analogues is well established. However, these methods of analysis do not consider the possibility of cell death. This report demonstrates that estimating cell death in G(2)/M is possible.

METHODS

Mathematical models for the experimental quantities, the fraction of labeled undivided cells, the fraction of labeled divided cells, and the relative movement were developed. These models include the possibility that, of the cells with G(2)/M DNA content, only a certain fraction will divide, with the remainder dying after some time T(R). Simulation studies were conducted to test the possibility of using simple methods to estimate phase durations and cell death rates.

RESULTS

Cell death alters the estimates of phase transit times in a rather complex manner that depends on the lifetime of the doomed cells. However, it is still possible to obtain estimates of the phase durations of cells in S and G(2)/M and the death rates of cells in G(2)/M.

CONCLUSIONS

The methods presented herein provide a new way to characterize cell populations that includes cell death rates and common measurements of cell proliferation.

Histone H2AX phosphorylation induced by selective photolysis of BrdU-labeled DNA with UV light: relation to cell cycle phase

BACKGROUND

The induction of DNA double-strand breaks (DSBs) in chromatin triggers histone H2AX phosphorylation (on Ser-139) by ATM-, ATR-, or DNA-dependent protein kinases (DNA-PK). Phosphorylated H2AX, denoted as gammaH2AX, can be detected immunocytochemically using an antibody that is specific to the Ser-139-phosphorylated epitope. We previously reported that the induction DSBs by DNA topoisomerase I or II inhibitors can be monitored in individual cells by measuring gammaH2AX immunofluorescence (IF) by cytometry. The present study explored whether the detection of gammaH2AX IF can serve as a marker of the presence of the DNA precursor bromodeoxyuridine (BrdU) that is incorporated into DNA.

METHODS

HeLa cells growing on microscope slides were incubated with BrdU for 1 h, rinsed free of the precursor, and incubated for different periods for up to 12 h. The cells were then briefly incubated with Hoechst 33342 (to sensitize BrdU-labeled DNA to ultraviolet [UV] light), irradiated with 300 nm UV light to photolyze BrdU-labeled DNA, transferred back into culture for an additional hour, and fixed. Cells were concurrently immunostained for gammaH2AX (Alexa Fluor 633) and cyclin A (fluorescein isothiocyanate); their DNA was counterstained with 4,6-diamidino-2-phenylindole. The intensities of cellular far red (gammaH2AX), green (cyclin A), and blue (DNA) fluorescences were measured by laser scanning cytometry.

RESULTS

After a 1-h pulse of BrdU followed by exposure to UV, nearly all cells with S-phase DNA content had many-fold higher gammaH2AX IF than G(1) or G(2)/M cells. The nonirradiated cells had minimal ("programmed") expression of gammaH2AX, whereas the irradiated cells incubated without BrdU had uniformly elevated levels of gammaH2AX IF independent of the cell cycle phase. Pulse-chase experiments showed that the cohort of BrdU-labeled (gammaH2AX-positive) cells progressed through G(2)/M and into G(1) phase after 8 and 12 h of growth in BrdU-free medium, respectively. Bivariate analysis of gammaH2AX versus cyclin A expression for the gated S-phase cells showed a correlation between these variables, suggesting that the rate of BrdU incorporation (DNA replication) correlates with expression of cyclin A.

CONCLUSIONS

Photolysis of BrdU-labeled DNA induces DSBs and leads to H2AX phosphorylation. Detection of gammaH2AX IF indicates the presence of the incorporated BrdU, is compatible with concurrent detection of other intracellular antigens, and can be used to demonstrate cell cycle kinetics.

Cellular kinetics of murine lung: model system to determine basis for radioprotection with keratinocyte growth factor

PURPOSE

Normal tissue toxicity remains a dose limitation for cancer radiotherapy and chemoradiotherapy. Growth factors offer a novel means of mitigating normal tissue radiotoxicity. In particular, keratinocyte growth factor (rHuKGF), whose proliferative activity is restricted to epithelial cells, holds promise on the basis of the findings of preclinical models of epithelial cytoprotection and the clinical developments to date. We report the radioprotection of murine lung by an increase in tissue cellularity after rHuKGF-induced proliferation.

METHODS AND MATERIALS

Flow cytometric and image analysis techniques after bromodeoxyuridine labeling were used to estimate proliferative parameters. Our specialized analytical methods measure not only labeling indexes, but also the durations of S and G(2)+M phases, potential doubling times, and the net cell production rate. Image analysis techniques were used to identify the specific cell types that were proliferating (type II pneumocytes).

RESULTS

Lung labeling index control values (0.5%) rose to a maximum (5.5%) at 3 days after intratracheal rHuKGF, returning to normal by Day 7. The potential doubling time fell from 66 days to 4.4 days. The net cell production rate rose from a control value of 1%/d to >15%/d by Day 3. This resulted in a nearly twofold increase in alveolar epithelial cellularity, which remained significantly elevated on Day 7. Saline-treated control animals exhibited no significant changes in the proliferative parameter values or cellularity. On the basis of these data, mice were irradiated, solely to the thorax, with ranges of single doses of 250 kVp X-rays 7 days after either intratracheal administration of 5 mg/kg rHuKGF or phospate-buffered saline. This interval was chosen because the proliferative response of the type II cells was finished but the cellularity of the lung remained increased. Pretreatment with rHuKGF extended the latent period before onset of pneumonitis after all radiation doses. rHuKGF treatment 7 days before thoracic irradiation significantly protected against pneumonitis (median effective dose 13.7 Gy, 95% confidence limit 13.4-14.0) compared with the control pretreatment with phosphate-buffered saline (median effective dose 12.8 Gy, 95% confidence limit 12.6-13.1).

CONCLUSION

The data showed that an increase in tissue cellularity, caused by rHuKGF treatment before irradiation, protected the lung from damage due to pneumonitis.

Real-time monitoring of immune responses

With the advent of cellular immunotherapy, the ability to monitor immune responses during treatment will be essential to evaluate the effectiveness of the new therapies. While the ultimate determinate of the success of immunotherapy trials will be clinical outcome, methods of monitoring immunity in real-time have become available that will assist in the development of immunotherapy strategies and in the prediction of individual patient prognosis during the course of treatment. The essentials of existing immune assays are described here with examples of how these techniques have been used previously. A perspective on which approaches will likely prove the most useful for monitoring immune responses in real-time during immunotherapy is also presented.

Role of Smac in human leukaemic cell apoptosis and proliferation

Smac (or DIABLO) is a recently identified, novel proapoptotic molecule, which is released from mitochondria into the cytosol during apoptosis. Smac functions by eliminating the caspase-inhibitory properties of the inhibitors of apoptosis proteins (IAP), particularly XIAP. In this study, we stably transfected both full-length (FL) and mature (MT) Smac genes into the K562 and CEM leukaemic cell lines. Both FL and MT Smac transfectants increased the sensitivity of leukaemic cells to UV light-induced apoptosis and the activation of caspase-9 and caspase-3. Purified cytosol from the mature Smac transfectants, or the addition of human recombinant Smac protein or N-7 peptide into nontransfected cytosol, showed an increased sensitivity to cytochrome c-induced activation of caspase-3. The mature Smac enhanced the susceptibility of both K562 and CEM cells to TRAIL-induced apoptosis. Overexpression of the mature Smac protein also inhibited proliferation, as detected by reduced colony formation and Ki-67 expression in leukaemic cells. Cell cycle analysis revealed that Smac transfectants displayed significant G0/G1 arrest and reduction in 5-bromo-2'-deoxyuridine (BrdU) incorporation. Smac sensitized human acute myeloid leukaemia blasts to cytochrome c-induced activation of caspase-3. However, Smac failed to overcome Apaf-1-deficiency-mediated resistance to cytochrome c in primary leukaemic blasts. In summary, this study reveals that Smac/DIABLO exhibits a potential role in increasing apoptosis and suppressing proliferation in human leukaemic cells. Importantly, it also indicates that it is crucial to evaluate the levels of Apaf-1 and XIAP proteins in patient samples before using Smac peptide therapy in the treatment of human leukaemia.

Flow cytometry in analysis of cell cycle and apoptosis

The capacity for multiparametric measurement of large cell populations rapidly and accurately offered by cytometry has made this methodology indispensable in studies of cell proliferation and cell death. The reviewed cell cycle applications include (1) the univariate analysis of cellular DNA content for identification of G(0/1) versus S versus G(2)/M cells; (2) discrimination between noncycling (G(0); quiescent) and proliferating cells, based on the presence of proliferation-associated proteins; (3) identification of mitotic cells by histone H3 phosphorylation; (4) bivariate analysis of expression of cyclins D, E, A, or B1 versus DNA content; and (5) detection of DNA replicating cells and analysis of cell kinetics from the bivariate distributions of 5-bromo-2'-deoxyuridine (BrdU) incorporation versus DNA content. For the identification of apoptotic cells and discrimination between apoptosis and necrosis, flow cytometry techniques are applied to evaluate for changes in cell morphology, the presence of phosphatidylserine on cell surface, collapse of mitochondrial transmembrane potential, DNA fragmentation, and evidence of caspase activation. Semin Hematol 38:179-193.