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Monitoring Cell Cycle Progression in Cancer CellsDownload
July 03, 2018
Using Nuclear Staining to Assess Cellular DNA Content
Author: Paul Held, Ph.D., Laboratory Manager, Applications Department, BioTek Instruments, Inc., Winooski, VT
Cell cycle progression is a tightly regulated process that involves the duplication of nuclear DNA content prior to cell division. The control mechanisms that regulate this process are often disrupted in tumor cells and serve as viable targets for therapeutic compounds in the treatment of cancer. Cell cycle progression has historically been monitored using flow cytometry. Here we describe the use of a microplate reader to rapidly image and analyze nuclear stained tissue culture cells for nuclear content.
The progression through the cell cycle and cell division of an organism is a tightly regulated process associated with proliferation and differentiation. Generally, most cells are quiescent and do not undergo division unless signaled to enter the active segments of the cell cycle. In a number of disease states (e.g. cancer, psoriasis, hyperplasia), this regulation is diminished or disrupted. In these instances, it is important to identify the genetic basis and develop therapies to preferentially target those cells with abnormalities. One screening method for potential therapeutic drugs, or the effect of specific genes on cell cycle regulation, is to measure changes in cell cycle kinetics and DNA content using a nuclear stain.
Figure 1. Relationship between Cycle and DNA content histogram. As cells progress through the cell cycle, their DNA content doubles prior to mitosis. Cells treated with the nuclear stain Hoechst 33342 exhibit fluorescence proportional to their DNA content.
Cell cycle analysis by DNA content measurement is a method that until recently employed flow cytometry to distinguish cells in different phases of the cell cycle. Before analysis, the cells are treated with a fluorescent dye that stains DNA quantitatively. Propidium iodide is commonly used with flow cytometry due to its ability to be excited with a 488 nm laser common in many systems. The drawback for this dye is that it also binds RNA, necessitating the treatment of cells with RNase prior to analysis. Nuclear stains such as 4,6-diamino-2-phenylinoloe (DAPI) or Hoechst 33342 require UV excitation, but are specific to DNA. Regardless of the dye used, the fluorescence intensity of the stained cells correlates with the amount of DNA they contain.
As the DNA content doubles during the S phase, the intensity of fluorescence increases in proportion. Thus, cells in G0 and G1 phase (before S) have half the fluorescent signal as those in G2 or M phase (Figure 1). Here we use these elements in conjunction with image-based analysis rather than flow cytometry to assess cell cycle phase status and identify compounds that effectively stall cell cycle progression.
Materials and Methods
PC-3 cells were cultured in Hams F12K media supplemented with 10% fetal bovine serum and penicillin-streptomycin at 37 °C in 5% CO2. Cultures were routinely trypsinized (0.05% Trypsin-EDTA) at 80% confluency. For experiments, cells were plated into Corning 3904 black sided clear bottom 96-well microplates.
Thymidine Double Block
PC-3 cells were seeded into Corning 3904 plates at a density of 4000 cells per well in a volume of 100 μL and allowed to attach overnight at 37 °C, 5% CO2 in a humidified environment. The following day, thymidine was added to all the wells for a final concentration of 2 mM. Cells were treated for 16 hours then released by washing 1-time with fresh media, followed by 100 μL of media without thymidine. Cells were allowed to grow for 9 hours, after which thymidine was added to a final concentration of 2 mM for a second time. Cells were thymidine-treated for 16 hours and released as described previously. Plates were immediately loaded into a BioSpa™ 8 System for cell cycle progression.
Cell Cycle Progression
PC-3 cells synchronized using double thymidine block were released from blockade with fresh complete media. Cells were then fixed at various times following release. Using a BioSpa 8 System to control timing and maintain the necessary environmental control, individual strips of two plates were fixed with 4% PFA for 10 minutes at 1-hour intervals using a MultiFlo™ FX attached to the system. Fixed cells were maintained with 200 μL of PBS until the completion of the experiment, after which wells were stained with 10 μM Hoechst 33342 for 30 minutes followed by washing 3-times with PBS. Cells were maintained in a hydrated state using PBS.
Twenty-seven cytoactive-agents were obtained from R&D Systems/Tocris and were reconstituted to 10 mM with DMSO and stored at -80 °C. Working stocks of the compounds were adjusted to a concentration of 2 mM with DMSO and stored at -20 °C. On the day of drug induction, the compounds were thawed and further diluted to 20 μM in media. Negative controls (media only), and positive controls (nocodazole, vinblastine and mevinolin) were also assayed in the same plates. PC-3 cells were seeded into Corning 3904 plates at a density of 4000 cells per well in a volume of 100 μL and allowed to attach overnight at 37 °C, 5% CO2 in a humidified environment. The following day, 100 μL of drug treatment was added to wells (n=3) resulting in a final drug concentration of 10 μM. Compound exposure lasted for 24 hours after which cells were washed 3-times with PBS and then fixed for 10 minutes at RT with 4% paraformaldehyde in PBS. Fixed cells were washed 3-times with PBS and stained for 30 minutes with 10 μM Hoechst 33342 at RT. Excess stain was removed by washing 3x with PBS. Fixed and stained cells were kept hydrated during imaging with 200 μL per well of PBS.
Two compounds identified in the screening assay as possible hits in regards to their ability to affect PC-3 cell cycle progression were further analyzed with a dose titration. For these experiments, PC-3 cells were seeded at 4000 cells per well in 100 μL and allowed to attach overnight. The following day, 100 μL of compound serial titrations in media were added as 2x solutions in replicates of eight. Cells were exposed for 24 hours after which cells were fixed and stained as described previously.
Montage (6 x 6) images of each well were obtained with a Cytation™ 5 configured with a DAPI LED cube using a 10x objective. The DAPI cube is configured with a 377/50 excitation filter and a 447/60 emission filter in conjunction with a 409 nm cut off dichroic mirror. The imaging parameters, set automatically using Gen5™ Microplate Reader and Imager Software, used a LED intensity setting of 5, an integration time of 72 msec and a gain of 0.
Table 1. Image processing and Image-analysis parameters for nuclear content determination.
Images were automatically stitched into a single file using Gen5™ Microplate Reader and Imager Software. After stitching montage images were preprocessed to subtract background fluorescence prior to analysis (Table 1). Primary mask analysis identified objects using a threshold of 5000 and a lower and upper size limitation of 5 μm and 50 μm respectively. Histograms relating total (integral) object fluorescence (x-axis) to % count (y-axis) were generated using Gen5 software with the bin number set to 500. The histogram plots were subsequently used to identify G1 and G2 cells and set upper and lower signal thresholds from their respective count peaks. The intervening region between the G1 and G2 was used to identify S-phase cells.
Cell cultures exposed to a double thymidine block are enriched for cells in G1 phase of the cell cycle. The presence of high levels of thymidine results in the disruption of the deoxynucleotide metabolism pathway halting cell cycle progression at the G1/S border. As demonstrated in Figure 2, cells released from thymidine block quickly enter S-phase and begin replicating DNA content. This is observed by an increase in cellular fluorescence over time. Upon completion of DNA replication, the cells are in G2 phase with fluorescent staining double that of cells in G1. After mitosis, their nuclear staining returns to initial levels (Figure 2).
Histograms relating total fluorescence to object count from Figure 2 were used to define the G1 and G2 subpopulations. The initial (Time 0) histogram exhibited G1 and G2 peaks that allowed for min and max fluorescent gating values to be applied to identify these subpopulations. The region between the maximum G1 subpopulation value and the minimum G2 subpopulation value was used to define S-phase cells.
Figure 2. Cell Cycle progression of PC3 cells released from thymidine block. Histograms of cell population count vs. fluorescence intensity taken at various times after thymidine block release. Dashed lines represent gates defining G1, S, and G2/M subpopulations.
Using subpopulation analysis, the temporal relationship between DNA content and cycle progression becomes apparent. As seen in Figure 3, cells released from thymidine blockade are nearly synchronous with respect to their DNA content. Initially, about 70% of the cells have a 2N chromosome number. Within 6 hours, this percentage has dropped to 33%, while the percentage of cells in S-phase has increased from near 0 to approximately 33%. The number of cells exhibiting 4N chromosome number has also increased slightly by this time. By 12 hours, most cells have duplicated their DNA content and are in G2 phase of the cell cycle. During cellular mitosis, nuclear DNA is divided equally between the two daughter cells returning the cells to G1 phase. This is observed by the rapid decline in the G2 percentage after 15 hours concurrent with an increase in the percentage of cells with a G1 DNA content such that all subpopulation return to their original states.
Figure 3. Cell Cycle progression of Synchronized PC-3 Cells. PC-3 cells synchronized with double thymidine block were fixed and stained at timed intervals after release. The percentages of cells in G1, S and G2, determined by nuclear staining analysis, were plotted verse. time. Data points represent the mean of eight determinations.
Cell cycle phase determination made from image-based DNA content can be used to screen compounds for their ability to inhibit cell cycle progression. Threshold gates for G1, S and G2 phases based on nuclear fluorescence can be determined for histogram analysis (Figure 4) using known cell cycle inhibitors as controls to enrich cell populations in these specific cell cycle phases. Mevinolin has been demonstrated to stall cells in G1, while Nocodazole blocks mitosis and enriches cell populations in G2.
Using these data to define subpopulations, compounds with unknown efficacy can be tested for cell cycle blockade. As seen in Figure 5, PC-3 cells were exposed to 27 different compounds for 24 hours and the percentage of cells in G1, S, and G2 phases of the cell cycle calculated. Control wells contained compounds known to stall cells in either G1 or G2 phases of the cell cycle. Cutoff thresholds from these wells were used to define G1, S and G2 subpopulations. Deviation of greater than 2 standard deviations from untreated cells was used to identify compounds that stalled cell growth in G1, S or G2 phases of the cell cycle. Compounds effecting PC-3 progression were flagged using color-coding to quickly identify the cell cycle phase. Of the compounds tested, 1 compound, Mevinolin, was shown to enrich cells in G1, while 7 compounds caused an increase in the percentage of G2 cells. Note that Mevinolin was also used as a G1 control compound. When it was treated as an unknown compound, it was independently identified as blocking cells in G1, which confirms the hit criteria employed. Some of the G2 positive hits also increased the percentage of cells deemed to be in S-phase.
Figure 4. Histogram analysis of nuclear staining from Mevinolin and Nocodazole treated PC-3 cells. Mevinolin and Nocodazole treatments were used as controls to define G1 (RED) and G2 (BLUE) subpopulations. S-phase (GREEN) was defined as the region between G1 and G2.
Figure 5. Gen5 screenshot of Cell Cycle phase screen of compounds in a 96-well plate. PC-3 cells were exposed to 30 different compounds, and controls in triplicate. Histograms from control compounds were used to define minimum and maximum fluorescence gates for subpopulation analysis. Each well lists percentages of cells in G1, S and G2. Deviations of percentages of cells in different cell cycle phases of greater than two standard deviations from untreated control wells was used as cut off to identify potential compounds. G1 enriching compounds were identified with BLUE, while G2 was outlined with RED and S phase with GREEN.
One of the identified positive hits was examined in detail. PC-3 cells were exposed to half-log titrations of Mechlorethamine for 24 hours, which resulted in a dose dependent increase in the percentage of G2 cells after a 24-hour exposure (Figures 6).
Figure 6. Effect of mechlorethamine concentration on cell cycle progression. PC3 cells were treated with various concentrations of mechlorethamine for 24 hours. After compound exposure, cells were fixed and stained for nuclear content. Image based analysis was used to determine the percentage of cells in G1, S, and G2 phases of the cell cycle. Data points represent the mean of eight determinations.
These data indicate that using a nuclear stain to quantify nuclear DNA content along with image- based analysis can be used to assess cell cycle progression in adherent cells. Cell cycle analysis has traditionally been performed using flow cytometry. While convenient for nonadherent cell lines such as lymphocytes, its use with adherent lines, requires trypsinization or scaping of the cells in order to suspend them. This leads to a bottleneck in terms of throughput. The use of imagebased analysis of adherent cells fixed and stained in place in microplates allows for the high throughput necessary for compound screening.
Several of the compounds tested significant enrichment of cells in one or more phases of the cell cycle as compared to untreated control cells. Mevinolin, nocodazole and vinblastine were used as assay controls to define subpopulation thresholds. Under these criteria, these same compounds were also identified correctly as potential hits in assay screen. Because exposure to a single high concentration of a compound can be cytotoxic without necessarily stalling cells in any particular cell cycle phase, drug titrations on potential screen hits is a necessary confirmatory experiment.
Mechlorethamine was further investigated due to screening results which suggest that it is a good candidate for a cell cycle inhibitor. This compound is a nitrogen mustard-alkylating agent that works by binding and crosslinking DNA strands and preventing cell duplication. As such, this drug will halt cell cycle progression at the S/G2 border. With high doses, cell cycle progression will be halted during S-phase as highly cross-linked DNA cannot be repaired or replicated.
Cytation™ 5 and Gen5™ Microplate Reader and Imager Software are an ideal combination of value and performance. The system is capable of making multiple images to form a montage, which allow for a larger cell sampling for each sample. The multiple files are stitched into a single image file prior to analysis. Preprocessing of the image subtracts background, eliminating any well to well differences prior to object identification. Gen5 histogram analysis of object information allows the researcher to visually apply threshold gates to define G1 and G2 populations. A quick check of multiple wells can then be used to confirm goodness of the fit. The percentages of each subpopulation is then calculated and reported. Long term live cell experiments can be carried out using the BioSpa system to maintain environmental control of temperature, CO2 and humidity levels.
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