Synchronization of 9L rat brain tumor cells by centrifugal elutriation

The molecular basis of cell memory in mammals: The epigenetic cycle

Cell memory refers to the capacity of cells to maintain their gene expression program once the initiating environmental signal has ceased. This exceptional feature is key during the formation of mammalian organisms, and it is believed to be in part mediated by epigenetic factors that can endorse cells with the landmarks required to maintain transcriptional programs upon cell duplication. Here, we review current literature analyzing the molecular basis of epigenetic memory in mammals, with a focus on the mechanisms by which transcriptionally repressive chromatin modifications such as methylation of DNA and histone H3 are propagated through mitotic cell divisions. The emerging picture suggests that cellular memory is supported by an epigenetic cycle in which reversible activities carried out by epigenetic regulators in coordination with cell cycle transition create a multiphasic system that can accommodate both maintenance of cell identity and cell differentiation in proliferating stem cell populations.

Advances and enabling technologies for phase-specific cell cycle synchronisation

Cell cycle synchronisation is the process of isolating cell populations at specific phases of the cell cycle from heterogeneous, asynchronous cell cultures. The process has important implications in targeted gene-editing and drug efficacy of cells and in studying cell cycle events and regulatory mechanisms involved in the cell cycle progression of multiple cell species. Ideally, cell cycle synchrony techniques should be applicable for all cell types, maintain synchrony across multiple cell cycle events, maintain cell viability and be robust against metabolic and physiological perturbations. In this review, we categorize cell cycle synchronisation approaches and discuss their operational principles and performance efficiencies. We highlight the advances and technological development trends from conventional methods to the more recent microfluidics-based systems. Furthermore, we discuss the opportunities and challenges for implementing high throughput cell synchronisation and provide future perspectives on synchronisation platforms, specifically hybrid cell synchrony modalities, to allow the highest level of phase-specific synchrony possible with minimal alterations in diverse types of cell cultures.

Cell Cycle Synchronization of the Murine EO771 Cell Line Using Double Thymidine Block Treatment

This study shows that double thymidine block treatment efficiently arrests the EO771 cells in the S-phase without altering cell growth or survival. A long-term analysis of cell behavior, using 5(6)-carboxyfluorescein diacetate N-succinimidyl ester (CFSE) staining, show synchronization to be stable and consistent over time. The EO771 cell line is a medullary breast-adenocarcinoma cell line isolated from a spontaneous murine mammary tumor, and can be used to generate murine tumor implantation models. Different biological (serum or amino acid deprivation), physical (elutriation, mitotic shake-off), or chemical (colchicine, nocodazole, thymidine) treatments are widely used for cell synchronization. Of the different methods tested, the double thymidine block is the most efficient for synchronization of murine EO771 cells if a large quantity of highly synchronized cells is recommended to study functional and biochemical events occurring in specific points of cell cycle progression.

Quantitative temporal diffusion spectroscopy as an early imaging biomarker of radiation therapeutic response in gliomas: A preclinical proof of concept

PURPOSE

This study aims to test the ability of quantitative temporal diffusion spectroscopy (qTDS) to assess cellular changes associated with radiation-induced cell death in a rat glioma model.

METHODS AND MATERIALS

Tumor response to a single fraction of 20 Gy of x-ray radiation was investigated in a rat glioma (9L) model. Tumor response was monitored longitudinally at postinoculation days 21, 23, and 25, using a specific implementation of qTDS with acronym IMPULSED (Imaging Microstructural Parameters Using Limited Spectrally Edited Diffusion), as well as conventional diffusion and high-resolution anatomic imaging. IMPULSED method combines diffusion-weighted signals acquired over a range of diffusion times that are then analyzed and interpreted using a theoretical model of water diffusion in tissues, which generates parametric maps depicting cellular and subcellular structural information on a voxel-wise basis. Results from different metrics were compared statistically.

RESULTS

A single dose of 20 Gy x-ray radiation significantly prolonged survival of 9L-bearing rats. The mean cell sizes of irradiated tumors decreased (P < .005) after radiation treatment, which we associate with cell shrinkage and the formation of small cellular bodies during apoptosis and necrosis. A combination of IMPULSED-derived parameters (mean cell size d and extracellular structural parameter β ex ) separated 90% of irradiated tumors from the nonirradiated cases at post inoculation day 23, whereas a combination of tumor growth and conventional apparent diffusion coefficient did not differentiate irradiated tumors from nonirradiated tumors.

CONCLUSIONS

This proof-of-concept study demonstrates the IMPULSED method to be a new method for deriving quantitative microstructural parameters in a preclinical tumor model. The method provides unique information based on the diffusion time dependency of diffusion magnetic resonance imaging, which cannot be obtained by conventional diffusion weighted imaging methods, and the results have a close correlation with primary biologic markers of treatment efficacy, such as cell death and survival.

Overview of Cell Synchronization

The widespread interest in cell synchronization is maintained by the studies of control mechanism involved in cell cycle regulation. During the synchronization distinct subpopulations of cells are obtained representing different stages of the cell cycle. These subpopulations are then used to study regulatory mechanisms of the cycle at the level of macromolecular biosynthesis (DNA synthesis, gene expression, protein synthesis), protein phosphorylation, development of new drugs, etc. Although several synchronization methods have been described, it is of general interest that scientists get a compilation and an updated view of these synchronization techniques. This introductory chapter summarizes: (1) the basic concepts and principal criteria of cell cycle synchronizations, (2) the most frequently used synchronization methods, such as physical fractionation (flow cytometry, dielectrophoresis, cytofluorometric purification), chemical blockade, (3) synchronization of embryonic cells, (4) synchronization at low temperature, (5) comparison of cell synchrony techniques, (6) synchronization of unicellular organisms, and (7) the effect of synchronization on transfection.

Synchronization of Mammalian Cells and Nuclei by Centrifugal Elutriation

Synchronized populations of large numbers of cells can be obtained by centrifugal elutriation on the basis of sedimentation properties of small round particles, with minimal perturbation of cellular functions. The physical characteristics of cell size and sedimentation velocity are operative in the technique of centrifugal elutriation also known as counterstreaming centrifugation. The elutriator is an advanced device for increasing the sedimentation rate to yield enhanced resolution of cell separation. A random population of cells is introduced into the elutriation chamber of an elutriator rotor running in a specially designed centrifuge. By increasing step-by-step the flow rate of the elutriation fluid, successive populations of relatively homogeneous cell size can be removed from the elutriation chamber and used as synchronized subpopulations. For cell synchronization by centrifugal elutriation, early log S phase cell populations are most suitable where most of the cells are in G1 and S phase (>80 %). Apoptotic cells can be found in the early elutriation fractions belonging to the sub-Go window. Protocols for the synchronization of nuclei of murine pre-B cells and high-resolution centrifugal elutriation of CHO cells are given. The verification of purity and cell cycle positions of cells in elutriated fractions includes the measurement of DNA synthesis by [³H]-thymidine incorporation and DNA content by propidium iodide flow cytometry.

Synchronization of mammalian cells and nuclei by centrifugal elutriation

Synchronized populations of large numbers of cells can be obtained by centrifugal elutriation on the basis of sedimentation properties of small round particles, with minimal perturbation of cellular functions. The physical characteristics of cell size and sedimentation velocity are operative in the technique of centrifugal elutriation also known as counterstreaming centrifugation. The elutriator is an advanced device for increasing the sedimentation rate to yield enhanced resolution of cell separation. A random population of cells is introduced into the elutriation chamber of an elutriator rotor running in a specially designed centrifuge. By increasing step by step the flow rate of the elutriation fluid, successive populations of relatively homogeneous cell size can be removed from the elutriation chamber and used as synchronized subpopulations. For cell synchronization by centrifugal elutriation early log S phase cell populations are most suitable where most of the cells are in G1 and S phase (>80%). Protocols for the synchronization of nuclei of murine pre-B cells and high-resolution centrifugal elutriation of CHO cells are given. The verification of purity and cell cycle positions of cells in elutriated fractions includes the measurement of DNA synthesis by [(3)H]-thymidine incorporation and DNA content by propidium iodide flow cytometry.

Overview of cell synchronization

Widespread interest in cell synchronization is maintained by the studies of control mechanisms involved in cell cycle regulation. During the synchronization distinct subpopulations of cells are obtained representing different stages of the cell cycle. These subpopulations are then used to study regulatory mechanisms of the cycle at the level of macromolecular biosynthesis (DNA synthesis, gene expression, protein synthesis), protein phosphorylation, development of new drugs, etc. Although several synchronization methods have been described, it is of general interest that scientists get a compilation and an updated view of these synchronization techniques. This introductory chapter summarizes: (1) the basic concepts and principal criteria of cell cycle synchronizations, (2) the most frequently used synchronization methods, such as physical fractionation (flow cytometry, dielectrophoresis, cytofluorometric purification), chemical blockade, (3) synchronization of embryonic cells, (4) synchronization at low temperature, (5) comparison of cell synchrony techniques, (6) synchronization of unicellular organisms, and (7) the effect of synchronization on transfection.

Flavopiridol induces apoptosis and caspase-3 activation of a newly characterized Burkitt's lymphoma cell line containing mutant p53 genes

Burkitt's lymphoma cell lines have been important in vitro models for studying the pathogenesis of Burkitt's lymphoma (BL) and for exploring new treatment strategies. A new EBV(-) Burkitt's lymphoma cell line (GA-10) was established from a patient with a clinically aggressive, chemorefractory BL and characterized. Although functional p-glycoprotein could not be demonstrated by dye-efflux assays, both p53 genes were mutated in the GA-10 cells, perhaps contributing to the resistant phenotype of the original neoplasm. Two properties of BL cells which may be useful targets for novel cytotoxic therapeutics are their surface expression of CD77, the receptor for Shiga toxin (Stx), and their high rate of proliferation. Expression of CD77 on the GA-10 cells was heterogeneous in that certain subclones expressed high levels of CD77 and correspondingly exhibited strong growth inhibition by Stx while others showed low levels of CD77 expression and weak Stx-induced growth inhibition. Flavopiridol, a potent inhibitor of cell cycle progression through G1 and G2, induced cytotoxicity of the GA-10 cells with an LC(50) of approximately 40 nM vs 70 nM for HL-60 cells (P < 0.05). The concentrations of flavopiridol at which only 10% of the cells were viable (LC(10)) were approximately 280 nM for the GA-10 cells and 520 nM for the HL-60 cells (P < 0.05). Dose-related induction of apoptosis in response to flavopiridol was demonstrated in the GA-10 cells by morphology, TUNEL assay, and activation of caspase-3. Flavopiridol was also cytotoxic to seven other BL cell lines tested. These data suggest that flavopiridol may have therapeutic value in the treatment of Burkitt's lymphoma.

Cell cycle dependency of 67gallium uptake and cytotoxicity in human cell lines of hematological malignancies

67Gallium (67Ga) is a radionuclide which accumulates in hematological malignancies and is used for diagnostic imaging. We investigated in this in vitro study the cell cycle dependency of cellular uptake and cytotoxicity of 67Ga. Cell cycle synchronization of cells was achieved by counterflow centrifugal elutriation and the use of cytostatic drugs. The human lymphoma cell lines U-937 and U-715 were used and in elutriation experiments we also used the leukemic cell line HL-60. The transferrin receptor (CD71) expression, 67Ga uptake and cell proliferation inhibition were the parameters measured. We also studied cytotoxicity in various schedules for combination of 67Ga and drugs and the residual proliferative capacity was measured. The CD71 expression in the three cell lines increased from 106-177% on S phase cells and from 118-233% on G2M cells, as compared to the G0/G1 cell fraction. The 67Ga uptake varied from 108-127% for S cells and 128-139% for G2M cells. The drugs chosen induced cell cycle phase accumulation in S and/or G2M phase during preincubation. 67Ga preincubation induced accumulation in the G2M phase. Almost all combinations of 67Ga and drugs resulted in a non-interactive effect, except for methotrexate which resulted in an antagonistic effect. No preferential effect of any of the incubation schemes was seen. CD71 expression and 67Ga uptake were increased in S and G2M cells. Combination of 67Ga with drugs which arrest cells in these cell cycle phases did not result in a change in cytotoxicity. However, these results implicate that 67Ga and the cytostatic drugs tested except for methotrexate might be used together or sequentially in therapy.

Cell cycle-dependence of HL-60 cell deformability

In this study, the role of cytoskeleton in HL-60 deformability during the cell cycle was investigated. G1, S, and G2/M cell fractions were separated by centrifugal elutriation. Cell deformability was evaluated by pipette aspiration. Tested at the same aspiration pressures, S cells were found to be less deformable than G1 cells. Moreover, HL-60 cells exhibited power-law fluid behavior: mu = mu c(gamma m/ gamma c)-b, where mu is cytoplasmic viscosity, gamma m is mean shear rate, mu c is the characteristic viscosity at the characteristic shear rate gamma c, and b is a material constant. At a given shear rate, S cells (mu c = 276 +/- 14 Pa.s, b = 0.51 +/- 0.03) were more viscous than G1 cells (mu c = 197 +/- 25, b = 0.53 +/- 0.02). To evaluate the relative importance of different cytoskeletal components in these cell cycle-dependent properties, HL-60 cells were treated with 30 microM dihydrocytochalasin B (DHB) to disrupt F-actin or 100 microM colchicine to collapse microtubules. DHB dramatically softened both G1 and S cells, which reduced the material constants mu c by approximately 65% and b by 20-30%. Colchicine had a limited effect on G1 cells but significantly reduced mu c of S cells (approximately 25%). Thus, F-actin plays the predominate role in determining cell mechanical properties, but disruption of microtubules may also influence the behavior of proliferating cells in a cell cycle-dependent fashion.

In vitro radiation sensitivity of mouse lung fibroblasts isolated by flow cytometry

PURPOSE

Recently, we have isolated two major fibroblast cells (Thy-1+, Thy-1-) from mouse LAF1 lung tissue using the anti-Thy-1 antibody expression and fluorescence activated cell sorter. To examine the possibility that x- or gamma-ray-induced pulmonary fibrosis at the late stage of injury could arise from radioresistant cell subpopulations, the radiation sensitivities of Thy-1+ and Thy-1- cells were evaluated by the colony forming assay.

METHODS AND MATERIALS

Cell survival curves, repair of potentially lethal damage (PLD) and sublethal damage (SLD), and cell-age response curves were obtained after Cs-137 gamma-ray irradiation.

RESULTS

The cell survival curves measured after 0-10 Gy gamma-ray showed that Thy-1+ cells were slightly more radioresistant than Thy-1- cells. The D0, n, alpha, and beta values measured from the survival curves also confirmed this observation. After a single dose of 10 Gy, a small amount of PLD repair was observed in Thy-1- cells, while no PLD repair was found in Thy-1+ cells. Although the initial cell survival level of Thy-1- cells was lower, the final survival levels of Thy-1+ and Thy-1- cells became identical at 8 h after irradiation due to the PLD repair. After split-dose irradiation of 4 Gy followed by 4 Gy, a similar extent and rate of SLD repair was found in Thy-1+ and Thy-1- cells. Cell-age response curves were obtained from irradiated G0/G1, S, and G2M cells separated by centrifugal elutriation and irradiated with 8 Gy gamma-ray. The results indicated that Thy-1+ and Thy-1- cells had a similar S resistant, and G1, G2M-sensitive radiation cell-age response curve.

CONCLUSIONS

This study suggests that the selection of radioresistant lung fibroblast may not be responsible for the development of lung fibrosis in irradiated LAF1 mouse.

Evaluation of cell subpopulations isolated from human tumor xenografts by centrifugal elutriation

Human epidermoid tumor cells (Coll2, ME180, A431, HEp3) grown as xenografts in nude mice, were dissociated into single cell suspensions using an enzyme cocktail containing 0.025% collagenase, 0.05% pronase, and 0.04% DNase. The dissociated cell suspensions were separated by centrifugal elutriation into fractions containing homogeneous cell subpopulations primarily based on the differences in the rates of sedimentation. The quality of separation was evaluated by several techniques including flow cytometry, cell volume distributions, in vitro colony forming assay and morphological examination of Wright-Giemsa stained cells. In each separated fractions, the host to neoplastic cell ratio, the DNA ploidy, the plating efficiency and the cell cycle distribution were determined. After an initial separation of non-neoplastic host cells from malignant cells, a purity of greater than 95% host cells was obtained from the four xenografts studied. DNA analysis of tumor suspensions showed that neoplastic cells of different xenografts contained aneuploid cells with a DNA index of 1.51 to 1.95. The neoplastic cells were further separated into fractions according to their positions in the cell cycle. Fractions containing greater than 95% G1, 65% S, and 72% G2M cells were obtained from HEp3 xenografts. Less efficient separation with respect to cell cycle was attained with cells derived from Coll2, ME180, and A431 xenografts. Colony forming abilities of the neoplastic cells were determined at different phases of the cell cycle and found to be similar to those of the unseparated cell suspensions after corrections for non-neoplastic host cells were made. These investigations indicate that centrifugal elutriation is an effective technique of obtaining homogeneous subpopulations of cells from human tumor xenografts for various tumor biology and cell kinetics studies.

Changes in growth characteristics and macromolecular synthesis on recovery from severe hypoxia

Chinese hamster ovary cells subjected to severe hypoxia stop growing. When oxygen was reintroduced growth resumed, but at a slower rate. The longer the hypoxic stress, the slower the recovery growth rate. Six hours of hypoxia caused very little decrease in growth rate while a 24 h period almost halved the rate. Short hypoxic periods resulted in almost no growth lag, while longer periods caused significant lag. Clonogenic survival was 60% after 12 h of hypoxia and rose slowly during recovery, reaching control levels after 60 h. Following 24 h of hypoxia, survival remained around 60% throughout recovery. The cell cycle distribution after hypoxia was similar to that of aerobic cultures. After 4-6 h of recovery, a subpopulation of cells entered S phase, and reached G2 by 12 h. During this time few G2-M cells divided. With longer recovery, cells much larger than aerobic cells emerged, containing greater than 4C DNA content and enhanced amounts of RNA. When these cells were isolated, they exhibited slightly slower growth kinetics, greatly lengthened lag time and decreased survival when compared to aerobic cells or the smaller cells. Most of the extra DNA and RNA was lost within one cell cycle.

Early identification and retrieval or deletion of human lymphocyte subpopulations responding to influenza virus or respiratory syncytial virus challenge

Differences in immune responses of human mononuclear leukocytes (MNL) have been demonstrated following exposure in vitro to influenza virus or respiratory syncytial virus (RSV). In the current studies, we sought to identify early differences in reactive subpopulations that emerge from within the heterogeneous resting MNL pool after challenge. MNL were sham-exposed or exposed to influenza virus or RSV, separated, and retrieved by countercurrent centrifugal elutriation after 3 d. Exposure to influenza virus caused a relative decline in the number of large MNL, but an increase in small lymphocytes. Large cells that remained included primitive lymphoblasts, rare plasma cells, and typical lymphocytes of progressively greater volume. Exposure to RSV increased the number of large MNL, but diminished the number of small lymphocytes. The subpopulation of large cells consisted of atypical and large granular lymphocytes. Furthermore, deletion of the latter large, reactive lymphocytes led to abrogation of an RSV-specific proliferative response upon subsequent challenge. Thus, the specific and different subpopulations reactive after infectious virus challenge could be identified, retrieved, and manipulated without dependence on a priori, phenotypic markers.

A simple electronic volume cell sorter for clonogenicity assays

A single-parameter electronic volume flow cell sorter that can be easily and inexpensively constructed using existing technology is described. The instrument is designed for ease and flexibility of operation, including such features as a large open area for recovering sorted cells into a variety of dishes or vessels; a remote, electrically activated fluidics system; a mechanism for heating or cooling samples during sorting; a simple arrangement for monitoring and adjusting the sorting control parameters; and an interface to a standard IBM personal computer for data acquisition, analysis, and control of the sorting windows. Several researchers in our laboratory now routinely use this sorter for plating precise numbers of cells directly into culture dishes in an aseptic manner for clonogenicity assays. The instrument can sort cells at rates of up to approximately 2,000 per second with greater than 80% sorting efficiency and no cytotoxicity. An advantage of this system is that the sorting windows can be set to exclude acellular debris and include either the entire cell volume distribution or a subset thereof. Applications of the instrument are detailed, including 1) precise cell plating for low-dose survival studies, 2) separation of cells into age compartments, and 3) rapid inoculation of single cells into multiwell dishes for cloning studies. Advantages of this technology for cell survival studies are detailed, along with some limitations to its applicability.

Growth phase related variation in the radiation sensitivity of human colon adenocarcinoma cells

Changes in radiation sensitivity with length of time in culture are described for an early passage human colon adenocarcinoma cell line (WiDr). The cells were most radioresistant at the end of lag-phase, 2 days after subculture (Do = 1.7 Gy; n = 10). Radiation sensitivity then increased with time reaching a maximum during plateau-phase, between days 8 and 10 (Do = 1.0 Gy; n = 13). The oxygen enhancement ratio remained constant across the different growth phases of the culture. Cell volume decreased with time in culture as did the proportion of S- and G2M-phase cells. Flow cytometric analyses revealed an increase in the proportion of G1 type cells with a plateau between days 8 and 12 of around 75%. The cell age response measured from synchronized cells following 8 Gy showed that WiDr cells were most radiation resistant in mid S-phase with maximal sensitivity during G1. These cells did not show repair of potentially-lethal radiation damage but were efficient in the repair of sub-lethal damage. The ability to repair sub-lethal damage did not change with culture age.

Separation of spontaneously differentiating and cell cycle-specific populations of HL-60 cells

The human leukemia cell line HL-60 consists predominantly of abnormal promyelocytes. When grown in RPMI-1640 and 10% FCS between 5 and 10% of these cells spontaneously differentiate into more mature myeloid cells, becoming smaller in size and developing the ability to generate superoxide. Centrifugal elutriation was used to separate these G0 cells from the bulk of the cycling G1, S and G2M cells. These isolated differentiating cells are shown to be similar in size, DNA content, RNA content and NBT positivity not only to granulocyte induced HL-60 cells but also to human peripheral blood granulocytes. This methodology allows the study of differentiative vs proliferative processes through the quick one-step generation of homogeneous subpopulations of G0, G1, S and G2M cells.

Cell cycle responses of heterogeneous human colon adenocarcinoma subpopulations to X-irradiation

The cell cycle responses of two exponentially growing subpopulations of cells (clones A and D), originally obtained from a human colon adenocarcinoma to X-irradiation, were studied using centrifugal elutriation. Cell suspensions were separated by changing counter-current flow rate while keeping the rotor speed constant (1600 rpm) and the composition of eluted fractions was determined using flow cytometry. The X-ray sensitivity of unseparated clone D cells was somewhat greater than that of clone A cells (e.g. 10% greater at the 10% level of survival). This difference appeared to be due to a greater value of the alpha parameter (one-hit cell killing), using the linear-quadratic equation in which the relative survival S/S0 = exp - (alpha D + beta D2) with dose (D) in Gy. This finding was confirmed in the cell cycle studies where the alpha parameter was always greater for the clone D cells than for the clone A cells. The beta parameter was essentially the same for both cell lines through the cell cycle.

Characterization of radiation sensitivity of human squamous carcinoma A431 cells

Studies have been performed to investigate the radiosensitivity of human squamous carcinoma cells. A431 cells were grown in vitro as exponential and fed-plateau monolayer cultures or as multicellular spheroids. Radiobiological studies of various cultures showed that fed-plateau phase cells were more sensitive (D0 = 1.3 Gy) than exponentially growing cells (D0 = 1.5 Gy). After a single dose of 12 Gy or two doses of 6 Gy irradiation, A431 cultures exhibited a large capacity for potentially lethal damage (PLD) repair (PLD repair factor = 17), but a relatively small sublethal damage (SLD) repair. In order to measure the radiation sensitivity of proliferating (P) and quiescent (Q) cells, enriched populations of P- and Q-cells were isolated from A431 spheroids. Flow cytometric analysis with acridine orange (AO) staining demonstrated that there was a shift of the RNA histograms in fed-plateau and spheroid cultures towards lower values, suggesting the presence of a subpopulation of Q-cells. Centrifugal elutriation was used to isolate the Q-cells from dissociated spheroid cells. Coulter cell volume distributions and flow cytometric analysis showed that Q-cells had a small cell volume (approximately 1380 microns3), low RNA content and a G1-like DNA content. Continuous labelling experiments with tritiated thymidine confirmed the non-proliferating nature of the Q-cells. Irradiation of the Q-cells after isolation from spheroids with between 0 to 10 Gy showed that they were more radiosensitive (decreased D0) than the P-cells isolated from these spheroids. The latter were, however, similar in radiosensitivity to exponential G1 cells.

Rapid assay for cell age response to radiation by electronic volume flow cell sorting

A new technique is described for measuring cell survival as a function of cell cycle position using flow cytometric cell sorting on the basis of electronic volume signals. The sorting of cells into different cell age compartments is demonstrated for three different cell lines commonly used in radiobiological research. Using flow cytometric DNA content analysis and [3H]thymidine autoradiography of the sorted cell populations, we demonstrate that the resolution of the age compartment separation is as good as or better than that reported for other cell synchronizing techniques. The variation in cell survival as a function of position in the cell cycle after a single dose of radiation as measured by volume cell sorting is similar to that determined by other cell synchrony techniques. This new method has several advantages, including: no treatment of the cells is required, thus, this method is noncytotoxic; no cell cycle progression is needed to obtain different cell age compartments; the cell population can be held in complete growth medium at any desired temperature during sorting; and a complete radiation age-response assay can be plated in 2 h. The application of this method to problems in radiobiology and chemotherapy is discussed, along with some of the technical limitations.

Radiation response of proliferating and quiescent subpopulations isolated from multicellular spheroids

Two subpopulations enriched in cells with a G1-like content of DNA were isolated from EMT6/Ro spheroids using centrifugal elutriation. The techniques of two-step acridine orange staining followed by flow cytometry, and continuous [3H]-thymidine labelling agreed qualitatively that one of these subpopulations predominantly consisted of proliferating G1 cells, while the other contained about four times more quiescent G0/G1 cells. These two subpopulations had similar median cell volumes and DNA contents, but the cell volume distributions were different. The clonogenicity was greater in the 'proliferating' subpopulation than the 'quiescent' subpopulation. When cell number seeded was corrected for viability, regrowth studies showed that there was a longer time (25 h) for the 'quiescent' subpopulation than the 'proliferating' subpopulation (10 h) before any increase in cell number was observed. In addition, relative to the 'proliferating' cells, the 'quiescent' cells were more sensitive when exposed to 137Cs gamma-ray radiation. The D0's were similar between the two subpopulations (D0 = 1.6 Gy and 1.8 Gy for the 'proliferating' G1 and 'quiescent' G0/G1 subpopulation, respectively), but the width of the shoulder of the radiation survival curve was reduced in the 'quiescent' subpopulation (Dq = 2.3 Gy vs. 5.3 Gy).

Lymphoma models for B cell activation and tolerance. III. Cell cycle dependence for negative signalling of WEHI-231 B lymphoma cells by anti-mu

WEHI-231 B lymphoma cells have proven to be a useful model for the regulation of growth of normal B cells by anti-Ig reagents. We previously reported that the growth of these lymphoma cells is inhibited by heterologous or monoclonal anti-mu or anti-kappa reagents. Such cells cease to incorporate thymidine within 24-48 h of exposure to anti-Ig reagents, but are not adversely affected by antibodies directed at either class I or class II histocompatibility antigens. In fact, cell cycle analysis revealed that anti-mu causes a block in the transition of these cells from G1 to S phase. To further study the mechanism of growth inhibition, we have purified lymphoma cells in G1 by centrifugal elutriation, or enriched WEHI-231 cells at the G1/S interface by treatment with hydroxyurea, and followed their progression through the cell cycle in the presence or absence of anti-mu. Our data show that WEHI-231 B lymphoma cells receive a negative signal early in G1, since delayed addition of anti-mu (to late G1 cells) leads to no alteration in cell cycle progression at 24 h, and exposure to anti-mu during S does not alter progress through DNA synthesis and mitosis. Moreover, exposure to anti-mu for only 2 h prevents purified G1 cells from entering their first S phase. The nature of the relevant processes in early G1 is discussed in terms of models of B cell activation and tolerance induction.

A reduction in the in situ rates of oxygen and glucose consumption of cells in EMT6/Ro spheroids during growth

The rates of consumption of oxygen and glucose by EMT6/Ro cells in multicellular spheroids were measured at various times during normal growth. In situ spheroid cellular consumption rates were similar to those of exponentially growing single cells up to a spheroid diameter of 150 micron. Further growth resulted in decreases in the rates of both oxygen and glucose consumption which were correlated with the increase in spheroid diameter and cell number. At a diameter of 1300 micron, both rates of cellular consumption had decreased by a factor of 2.5. The rates of consumption per unit of nonnecrotic spheroid volume decreased in a similar manner. Measurements with single cells demonstrated that the rate of oxygen consumption was coupled with glucose concentration, and vice versa. The rates of consumption for cells dissociated from small spheroids indicated that there was some effect of the spheroid environment. As the spheroids grew, however, association in the spheroid structure accounted for a smaller proportion of the total observed reduction in the rates of nutrient consumption. The presence of central necrosis also appeared to have no effect on the rates of consumption of these nutrients. Spheroid-derived cells showed a decrease in cell volume with growth as the cells accumulated in a quiescent state. Measurements with single cells demonstrated that oxygen and glucose consumption were correlated with cell volume and with the development of nonproliferating cells. We conclude that the observed decrease in oxygen and glucose consumption with growth in spheroids is largely due to the progressive accumulation of cells in a quiescent state characterized by an inherently lower cellular rate of nutrient utilization.

Comparison of tumour age response to radiation for cells derived from tissue culture or solid tumours

Direct comparison of the cell age response of 9L and KHT tumour cells derived either from tissue culture or solid tumours was achieved. Cells from dissociated KHT and 9L tumours (the latter implanted either subcutaneously or intracerebrally) and cells from tissue culture were separated into homogeneous sized populations by centrifugal elutriation. In both tumour models these homogeneous sized populations correspond to populations enriched at different stages of the cell cycle. The survival of these elutriated cell populations was measured after a single dose of Cs-137 gamma rays. For cells isolated from 9L solid tumours, there was little variation in radiosensitivity throughout the cell cycle; however, a very small but significant increase in resistance was found in late G1 cells. This lack of a large variation in radiosensitivity through the cell cycle for 9L cells from solid tumours also was seen in 9L cells growing in monolayer tissue culture. When similar experiments were performed using the KHT sarcoma tumour model, the results showed that KHT cells in vitro exhibited a fairly conventional increase in radioresistance in both mid G1 and late S. However, the cell age response of KHT cells from solid tumours was different; particularly in the late S and G2 + M phases. These data demonstrate that direct extrapolation of in vitro cell age responses to the in situ situation may not always be valid.

Progressive increase in polyamine levels in 9L cells in vitro during the cell cycle: comparison between cells isolated by centrifugal elutriation and cells grown in synchrony

Centrifugal elutriation was used to separate 9L rat brain tumour cells into fractions enriched in the G1, S, or G2/M phases of the cell cycle. Cells enriched in early G1 phase were recultured, grown in synchrony, and harvested periodically for analysis of their DNA distribution and polyamine content. Mathematical analysis of the DNA distributions indicated that excellent synchrony was obtained with low dispersion throughout the cell cycle. Polyamine accumulation began at the time of seeding, and intracellular levels of putrescine, spermidine, and spermine increased continuously during the cell cycle. In cells in the G2/M phase of the cell cycle, putrescine and spermidine levels were twice as high as in cells in the G1 phase. DNA distribution and polyamine levels were also analysed in cells taken directly from the various elutriation fractions enriched in G1, S, or G2/M. Because we did not obtain pure S or G2/M populations by elutriation or by harvesting synchronized cells, a mathematical procedure--which assumed that the measured polyamine levels for any population were linearly related to the fraction of cells in the G1, S, and G2/M phases times the polyamine levels in these phases and that polyamine levels did not vary within these phases--was used to estimate 'true' phase-specific polyamine levels (levels to be expected if perfect synchrony were achieved). Estimated 'true' phase-specific polyamine levels calculated from the data obtained from cells either sorted by elutriation or obtained from synchronously growing cultures were very similar.

An ultra-pure in vitro phase synchrony method employing centrifugal elutriation and viable flow cytometric cell sorting

We present a method of synchronizing cells in G1-, S-, and G2M-phases employing sequential centrifugal elutriation and viable flow cytometric cell sorting of Hoechst-33342 stained Chinese hamster ovary cells. G1- and S-phase cells can be separated to greater than 99% homogeneity and G2-M to 70% purity. Most of the 30% contamination in the G2-M fraction was due to S-phase cells, whose reproductive integrity could be eliminated through the use of high specific activity 3H-TdR. There were minimal toxic effects or perturbations to growth following the selection procedures. The most significant limitation of this technique appears to be the rate of cell sorting, which, with current equipment, is approximately 3,000 cells per second.

Growth and cellular characteristics of multicell spheroids

The data reviewed here demonstrate that there are many similarities in growth and cellular characteristics for different types of tumor cells grown as multicell spheroids. Furthermore, where comparisons have been made many of the features of spheroids also occur in tumors in vivo. However, as for tumors, there are also many characteristics of individual types of spheroids which are relatively specific and cannot be generalized as properties of all spheroid model systems. The results also demonstrate the marked influence which cellular microenvironments regulated by a supply of oxygen and nutrients may have on the development of cellular heterogeneity. Furthermore, using spheroids it was shown that dynamic cellular and metabolic interactions exist in regulating the development of cellular subpopulations and microenvironments. Spheroids are more sensitive to alterations in culture environment than are monolayer or single-cell suspension cultures. Consequently, researchers who use this model system must characterize, optimize, and standardize the growth conditions for the spheroid cell type being investigated. This information then provides a base from which to undertake detailed studies, which are not possible in experimental tumors, of controlled manipulation of microenvironments in spheroids. The ranges of cellular microenvironments and cellular heterogeneity which exist at different stages of spheroid growth provide a model, at least in part, for coexisting size ranges of microregions in many solid tumors. Thus, spheroids provide a model, which at different stages of growth is readily manipulated and controlled experimentally, to facilitate studies of contributions of individual environmental factors, or concomitant changes in these, on cellular phenotypic expression. It is probable that the cellular changes which can be demonstrated to occur during spheroid growth, also occur in vivo. Modulation of cellular characteristics revealed by research with spheroids requires much more study to determine the mechanisms and effects on tumor cell behavior, as well as response to therapeutic agents and their relevance to tumors in vivo.

Treatment of artificially-induced pulmonary metastases with fractionated doses of vincristine and/or radiation therapy

The cytotoxic effects in vivo of vincristine (VC), radiation, or both modalities in combination on murine fibrosarcoma (FSa) cells grown as pulmonary tumors were determined. Fourteen days following the i.v. injection of viable FSa cells, recipient mice developed between 100 and 150 visible pulmonary nodules. At that time, tumor-bearing animals were exposed to either single or combined modality treatments, as well as single and fractionated dose regimens. Animals were sacrificed 1 hour after the last treatment. Tumor nodules were excised and made into a single cell suspension and separated on the basis of cell size by centrifugal elutriation. Flow microfluorometry (FMF) was used to determine the cell-cycle parameters and the relative synchrony of the separated populations, as well as the percentage contamination by normal diploid cells in each of the tumor cell populations. Known numbers of viable cells from each elutriator fraction were injected into recipient mice to determine their colony-forming efficiency (CFE). Surviving fractions were determined by comparing the CFEs of treated FSa cells from each of the separated elutriator fractions with those of appropriate untreated controls. Following a single dose of VC (1 mg/kg), populations of cells enriched in late S and G2 + M were the most sensitive. However, following the administration of five doses (0.25 mg/kg each) over a 24 hour period, populations of cells most enriched in G1 cells exhibited the lowest percentage of survivors. A single dose of radiation (1000 rad) was most effective in killing S-phase cells. When administered in five fractions (250 rad per fraction), each separated by a 6 hour interval, no phase-specific sensitivity was observed. The combination of both modalities exhibited a marked schedule dependence, with a single dose (1000 rad) of radiation followed by five doses of VC (0.25 mg/kg) being the most effective treatment protocol observed.

Inhibition of cell growth in synchronous human hypernephroma cells by recombinant interferon alpha-D and irradiation

It has been reported that interferon (IFN) can inhibit cell division on both normal and malignant cells. However, the effect of IFN on the cells at different phases of the cell cycle is still uncertain. In this report, we have studied the cytostatic properties of interferon in each phase of cell cycle by treating synchronous cell population with IFN alone and/or 60Co x-irradiation. Human hypernephroma cells ACHN (ATCC no. CRL 1611) originally initiated by Dr. Chang and Dr. Hogan were used in the experiments reported here. The cells were grown in monolayer cultures in MEM supplemented with 10% fetal calf serum. The biological properties of exponential growing cells showed an abnormal karyotype, large vacuoles in the cytoplasm, and a doubling time of 24 h. Cells were synchronized into G1, S, and G2 + M phases by centrifugal elutriation. Autoradiography and flow cytometry data indicated that greater than or equal to 95% G1 cells, greater than or equal to 80% S cells, and greater than or equal to 70% G2 + M cells were obtained by this method. A single dose of 10(3) U human interferon alpha (HuIFN-alpha) (Hoffman-LaRoche, recombinant leukocyte D interferon (IFN-alpha D), RO 22-9859) was given to the synchronous cell populations. The cell cycle delay, growth inhibition and cell viability were measured with the Coulter Counter and Channelyzer system, flow cytometry, and colony-forming assay. Preliminary studies showed that IFN inhibited ACHN cell growth to the same extent at each phase of the cell cycle. However, there was no reduction in the clonogenecity of synchronous cells after IFN treatment. The cell age response after single doses of 600 rad 137CS r-irradiation showed that S phase cells were resistant while both G1 and G2 + M cells were sensitive to radiation. The results of combining treatment of IFN and irradiation showed additional cell-killing effect as compared with irradiation alone.

Demonstration of recovery from the potentially mutagenic effects of ultraviolet light by replication-inhibited Chinese hamster V79 cells

The effects of replication-inhibiting conditions on the ability of Chinese hamster (V79) cells to recover from the potentially mutagenic effects of ultraviolet (UV) light were investigated. V79 cells were synchronized by a new technique using a low concentration of hydroxyurea (0.2 mM), which provided a mildly-toxic, nonmutagenic method for producing large quantities of synchronized cells needed for these studies. The protocol developed for this study involved UV-irradiation of synchronized V79 cells which were blocked at the Gl/S boundary. Following UV-irradiation, the cells were either allowed to enter S phase immediately or were blocked for increasing periods of time by the addition of more hydroxyurea. The former cells contained the highest frequencies of ouabain-resistant mutants, while cells whose replication was blocked following UV-irradiation showed decreasing mutation frequencies with respect to time. Therefore, V79 cells are able to demonstrate liquid holding recovery from potentially mutagenic UV-lesions. Since the UV-induced mutation frequency was reduced by almost 50% following 6 h of 'liquid holding', the mutagenic lesions seem to be removed at a faster rate than has previously been reported for the removal of pyrimidine dimers from these cells.

Cell subpopulations dispersed from solid tumours and separated by centrifugal elutriation

The degree of non-neoplastic host-cell infiltration was assessed in 3 in vivo-in vitro tumour models commonly used in radiobiological studies: EMT6/Ro mammary carcinoma, 9L/Ro tumour and KHT sarcoma. While the 2 former tumour models have been shown to be moderately to highly immunogenic when grown s.c., the KHT sarcoma is apparently non-immunogenic. Using differential staining on single-cell suspensions from enzymatically dissociated solid tumours, all 3 tumour types were found to contain large proportions (30-60%) of non-neoplastic host cells. The actual host-cell component found in the cell suspensions differed both in type and percentage for the 3 tumours studied. These host and neoplastic cells in the cell suspensions prepared from the solid tumours could be readily separated by centrifugal elutriation. After separation the clonogenic potential of the neoplastic cells was assessed, and was found to be higher than the clonogenic capacity of the unseparated cell suspension by a factor directly related to the host/neoplastic cell ratio. Even after the removal of the host cells, the clonogenic capacities of the neoplastic EMT6 and 9L tumour cells were lower than that of the corresponding in vitro sublines (approximately 30 vs 75%). However, in the KHT sarcoma the removal of the host cell component raised the plating efficiency to approximately 60%, which was similar to the value for the in vitro cell subline of this tumour.

Characterization of the separation properties of the Beckman elutriator system

The role of fluid flow in the elutriation process was visualized by pumping dye solution through the Beckman JE-6 elutriator rotor. Three major fluid flow disturbances were observed in the separation chambers, namely; jet-streaming, ripple flow, and whirl flow. In order to evaluate the effects of these non-ideal fluid flow patterns on the separation of homogeneous populations of particles or cells, 12--35 micron diameter latex spheres and 9L rat brain tumor cells were fractionated with the Beckman elutriator system. The elutriator system was evaluated on the basis of: (1) recovery, (2) elution loss during loading, (3) homogeneity of the size distributions, and (4) the relationship of the median volume of eluted particles or cells to the rotor speed and the collection fluid velocity. Both a conventional collection method (two 40-mL fractions at ech collection rotor speed) and a long collection method (10--15 40-mL fractions at several collection rotor speeds) were compared to determine if collection procedures could compensate for some of the difficulties caused by the non-ideal fluid flow patterns. Although more than 90% of the particles or cells were always recovered, about 5% eluted during the loading procedure. Neither collection method altered this phenomenon. The long collection method significantly improved the homogeneity of the collected populations, but this was accompanied by a reduction in cell yield. The median particle or cell volume of each fraction agreed with that expected under ideal fluid flow conditions except at high and low rotor speeds when the conventional collection method was used.