Effects of foscarnet on the cell kinetics of Madin-Darby canine kidney cells

The antiherpes compound, foscarnet (trisodium phosphonoformate), showed concentration-dependent effects on the cell kinetics of Madin-Darby canine kidney cells. At 1 mM, only minor effects could be seen on cell proliferation and cell cycle distribution, as measured by flow cytometry DNA analysis. Treatment with 5 mM foscarnet resulted in an accumulation of cells in the S-phase although no complete cell cycle block was evident. At 10 mM foscarnet, cells accumulated earlier in the S phase, probably at the G1/S border. However, at both 5 and 10 mM foscarnet the block was not established until after 15 h incubation. Upon removing 10 mM foscarnet after 24 h incubation, G1 cells rapidly entered the S phase, whereas the progression through S and G2 + M was delayed considerably. The DNA synthesizing S phase seems, therefore, to be the main cell cycle phase affected by foscarnet.     

Effect of etoposide and amsacrine on mitotic progression of GM-130 and Hep-2 cell lines. The flow cytometry assay

It has been shown that inhibitors of topoisomerase II (topo II) etoposide and amsacrine results in accumulation of GM-130 and Hep-2 cells with 4c DNA amount. The differential analysis based on flow cytometry (Zenin et al., 2001) and enabled us to discriminate cells with 4c DNA--G2, M, including metaphase and anaphase cells and cells in pseudo-G1. 1 microM etoposide evoked cell accumulation in G2 phase, while 40 microM etoposide blocked cell proliferation, which was confirmed by a complete absence of both mitotic cells and 4c DNA cell accumulation. GM-130 and Hep-2 cells that were first blocked and then washed from nocodazole, and after that treated with 50 microM etoposide or 20 microM amsacrine, were shown to enter pseudo-G1 with 4c DNA amount per cell. In the presence of nocodazole, 4 and 40 microM amsacrine evoked transition of all mitotic cells to pseudo-G1 within 1 h. 15 or 30 minutes pulse treatments of GM-130 cells with 40 microM amsacrine in the presence of nocodazole, followed by incubation in drug-free medium, resulted in the similar transition of cells to pseudo-G1.     

Antiproliferative effects of some novel synthetic solanidine analogs on HL-60 human leukemia cells in vitro

There is increasing evidence of the direct antiproliferative effects of various steroidal structures, including cardenolides, steroidal alkaloids and sexual hormones. The aim of the present study was to characterize the antiproliferative effects of three synthetic solanidine analogs (1-3) on HL-60 human leukemia cells. The three compounds exerted similar cytostatic effects (IC(50) values: 1.27-2.94 μM after a 72-h exposure) and the most effective (2) was selected for further investigations. Incubation with compound 2 resulted in a marked chromatin condensation followed by a gradual increase in cell membrane permeability detected by Hoechst dye 33258-propidium iodide double staining. A flow cytometric analysis revealed a marked decrease in the G1 phase and substantial increases in the S and G2/M phases after 24-h incubation, while after 48 h the proportion of cells in the subG1 phase was increased significantly with a concomitant decrease in cells in the G1 and G2/M phases. Compound 2 at 6.0 μM significantly decreased the activity of ribonucleotide reductase and proved to be a potent antioxidant in the lipid peroxidation and DPPH assays (IC(50) values: 2.0 and 13.1 μM, respectively). The antiproliferative effect of the test compound on the non-cancerous human lung fibroblast cell line (MRC-5) was significantly weaker than that on the leukemia cells. These results lead to the conclusion that compound 2 induces a marked disturbance in the cell cycle, which is, at least partially, a consequence of the inhibition of DNA synthesis.     

The effect of lycopene on cell growth and oxidative DNA damage of Hep3B human hepatoma cells

Lycopene, the predominant carotenoid in tomatoes and tomato-based foods, is reported to protect against various cancers, especially prostate cancer. We investigated the effect of lycopene on DNA damage and cell growth inhibition in the Hep3B human hepatoma cell line. Lycopene was analyzed by HPLC, and cell proliferation was determined by 3-(4,5-dimethylthiazolyl-2)-2,5-diphenyltetrazolium bromide (MTT) assay. A final lycopene concentration of 0.1-50 microM was added to cells plated in 96-well plates. After a 24-hr incubation, cell viability was measured as absorbance at 570 nm after the MTT assay. The effects of lycopene on cell cycle progression were investigated with flow cytometry. Lycopene induced G0/G1 arrest and S phase block. Oxidative DNA damage was determined by the Comet (single-cell gel electrophoresis) assay. Lycopene inhibited cell growth in a dose-dependent manner. Cell growth was inhibited 20% at 0.2 microM lycopene and 40% at 50 microM lycopene after a 24-hr incubation. In the Comet assay, lycopene-treated cells showed less DNA damage than did placebo-treated cells. The inhibition of Hep3B cell growth in this study demonstrates the antitumor properties of lycopene.     

Accelerated entry of aortic smooth muscle cells from spontaneously hypertensive rats into the S phase of the cell cycle.

The present study was designed to characterize the growth kinetics of the exaggerated proliferative response to mitogens of vascular smooth muscle cells from spontaneously hypertensive rats compared with cells from normotensive Wistar-Kyoto controls. Cellular DNA content, analyzed by flow cytometry, demonstrated a 4-h accelerated entry into the S phase of the cell cycle of vascular smooth muscle cells from spontaneously hypertensive rats; the significant (4.5-fold) increase in the percentage of cells in the S phase occurred between 8 and 12 h after calf serum stimulation. A 3.9-fold increase of cells in the S phase was seen in the normotensive controls only between 12 and 16 h. Transit through the cell cycle was quantitated by flow cytometry using the Hoechst 33,342--bromodeoxyuridine substitution technique. Vascular smooth muscle cells from spontaneously hypertensive rats went through the cell cycle 4 h ahead of cells from normotensive Wistar-Kyoto rats. This accelerated transit of spontaneously hypertensive rat cells was mostly due to an earlier entry into the S phase. Persistence of this new intermediate phenotype in cell culture suggests its primary pathogenetic role in spontaneous hypertension.     

Low pH leads to sister-chromatid exchanges and chromosomal aberrations,and its clastogenicity is S-dependent

The effect of low pH on sister-chromatid exchanges (SCE), chromosomal aberrations (CA), and the cell cycle were investigated in Chinese hamster cells. The cells were treated in media over the pH range 7.2–5.4 during 24-h continuous or 3-h pulse treatments. In Chinese hamster ovary K1 cells, slight increases in SCE frequency were induced by 3-h pulse treatment with a 28-h recovery time. In Chinese hamster V79 379A cells, similar slight increases in SCE frequency were observed with both treatments. A severe delay in the cell cycle was noted in both cell types. DNA analysis with flow cytometry indicated that the cell cycle delay occured in S phase. CA were observed in the first metaphase. Multiple fixation times over a 27-h period were used to determine whether or not CA could be induced in cells exposed to low pH medium in more than one part of the cell cycle. Only a few chromatid gaps were induced when the cells were fixed at 0–9 h after the 3-h treatment, most probably representing cells that were treated in their G2 or late S phase. CA were induced in cells fixed between 12 and 27 h after the 3-h treatment. These cells were most probably treated in early S phase, in G1, or in the previous G2/M. These results suggest that low pH clastogenicity is S-dependent.     

Dilazep, a nucleoside transporter inhibitor, modulates cell cycle progression and DNA synthesis in rat mesangial cells in vitro

The direct effects of the nucleoside transporter inhibitor dilazep on the cell cycle of mesangial cells have not before been investigated. The purpose of this study was to elucidate whether dilazep can inhibit the proliferation of mesangial cells and how it interferes with the cell cycle of these cells. DNA histograms were used and BrdUrd uptake rate was measured by flow cytometry. There was no significant difference in the cell numbers among the untreated group and the 10⁻⁵M, 10⁻⁶M or 10⁻⁷M dilazep-treated groups at 24 h of incubation. However, at 48 and 72 h, the cell numbers in the dilazep-treated groups were significantly lower compared with that of the untreated group (P0.005). The DNA histograms of cultured rat mesangial cells at 12, 24, and 48 h of incubation with 10⁻⁵ M dilazep showed that the ratio of the S phase population in the dilazep-treated group decreased by 2.2% at 12 h, by 9.6% at 24 h, and by 18.9% at 48 h compared with the untreated group. The ratio of the G₀/G₁ phase population in the dilazep-treated group significantly increased: 6.8% at 12h (P 0.05), 13.9% at 24 h (P 0.001), and 76.5% at 48 h (P 0.001) compared with the untreated group. A flow cytometric measurement of bivariate DNA/BrdUrd distribution demonstrated that the DNA synthesis rate in the S phase decreased after 6 h (P 0.005) and 12 h (P 0.05) of incubation compared with the untreated group. These results suggest that dilazep inhibits the proliferation of cultured rat mesangial cells by suppressing the G₁/S transition by prolonging G₂/M and through decreasing the DNA synthesis rate     

Is there an accumulation of cells during G2 in some acute lymphoblastic leukaemias?

In some cases of acute lymphoblastic leukaemia (ALL) the percentage of cells in G2 + M is higher than anticipated when compared with the percentage in S phase. This increase in G2 + M, as detected by flow cytometry measurement of DNA content, may be due to an accumulation of cells, either in G2 or during the end of S phase; it may also be related to the existence of small tetraploid clones generally ignored by cytogeneticists. In order to identify possible subpopulations of cells with a DNA index greater than or equal to 2.0, we have compared the results of a cytogenetic analysis to the G2 + M values. We have also studied the distribution of S phase cells in 24 cases of ALL by incorporating 5-bromodeoxyuridine, labelling the cells by indirect immunofluorescence, and analysing them by flow cytometry after propidium iodide staining. The distribution of cells during S phase was quantified: no accumulation of cells was ever observed at the end of S phase. The question of the existence of small tetraploid clones, G2 arrested cells or cells with a G2 elongation remains open. However, we feel that it is more probable that, in this pathology, an elongation of the duration of G2 occurs.     

Artelastin is a cytotoxic prenylated flavone that disturbs microtubules and interferes with DNA replication in MCF-7 human breast cancer cells

Artelastin, a novel prenylated flavone, previously isolated from the wood bark of Artocarpus elasticus, was evaluated for its capacity to inhibit the growth of fifty-two human tumor cell lines, representing nine different tumor types. A pronounced dose-dependent growth inhibitory effect was detected in all the cell lines, with GI50 values ranging from 0.8-20.8 microM. Studies to elucidate the basis of the growth inhibitory activity of artelastin were performed in the MCF-7 human breast cancer cell line (GI50 = 6.0 microM). We show that artelastin exerts a biphasic effect in the DNA synthesis of MCF-7 cells, a stimulatory effect at low concentrations (below GI50) for short times of exposition (6 h and 24 h), and an inhibitory effect at high concentrations (above GI50). Remarkably, treated cells that have DNA synthesis affected could be viable and metabolically active. Furthermore, artelastin acts as a cytotoxic rather than a cytostatic compound. Massive cytoplasmatic vacuoles were detected in cells after artelastin treatment. Together with these morphological alterations, cells show the presence of abnormal nuclear morphologies, and occasionally nuclear condensation, which were identified as apoptotic by TUNEL assay. Moreover, artelastin was shown to disturb the microtubule network while no effect was observed on the kinetochores. Flow cytometry analysis of cells treated with artelastin reveal an accumulation in S phase that interferes with the cell cycle progression. Additionally, according to BrdU patterns, studies with synchronized cells at G0 and at G1/S transition also suggest that artelastin delays DNA replication since progression of cells trough S-phase is perturbed.     

A two-parameter flow cytometry protocol for the detection and characterization of the clastogenic, cytostatic and cytotoxic activities of chemicals

Cultured, freshly-isolated rat fibroblasts were exposed in vitro to vincristine sulphate (VC), amethopterin (AM), bleomycin (BL), benomyl (BE) and practolol (PR). Cells treated for 5 h were subjected 24 h later to a two-parameter (DNA/protein) flow cytometry analysis. The fluorochromes used were sulphorhodamin 101 and DAPI. From DNA and protein histograms, alterations in cell-cycle kinetics, variations in the amount of DNA in individual G1-phase cells and the enhancement of or increased variation in the protein content of the exposed cells were determined. Each of the 5 chemicals induced a specific dose-dependent pattern of changes in the DNA and protein histograms. DNA dispersion was enhanced with VC, AM, BL and BE but not with PR. The cell cycle was blocked in the G2 phase with VC, at early S phase with amethopterin and, depending on the dose, at the G1 or G2 phase with bleomycin or at the S phase or G2 phase with benomyl. Practolol inhibited cells slightly in the S phase at the highest exposure level. Protein analysis allows cytotoxic activity (loss of proteins) or induced unbalanced growth (protein accumulation) of test compounds to be recognized. The results obtained imply that the proposed two-parameter DNA/protein analysis by flow cytometry is a suitable method for prospective testing of chemicals for their induction of structural or numerical chromosome aberrations. Simultaneously, a broad range of cytotoxic, cytostatic and cell-cycle perturbing activities of the test agents can be recognized.     

The effect of the plant cell inhibitor propachlor (alpha-chloro-N-isopropyl-acetanilide) on the cell cycle of L1210 cells as evaluated by flow cytometry

Previously it was reported that the herbicide propachlor (alpha-chloro-N-isopropyl-acetanilide) has a strong inhibitory effect on the proliferation of L1210 mouse leukemia cells. It is now demonstrated that propachlor treatment causes L1210 cells to accumulate in the G1 phase as determined by flow cytometric analysis. This effect of propachlor is dose-dependent with more than 90% of G1 cells accumulating at 10 microM. Kinetic experiments demonstrated that the accumulation of cells in G1 starts in about 10 hours, and increased for up to about 44 hours of incubation with 10 microM propachlor. Treated cells can be revised to a normal DNA distribution by removing propachlor.     

Flow cytometric cell cycle analysis of cultured brown bear fibroblast cells

The aim of this study was to assess by flow cytometry the cell cycle of brown bear fibroblast cells cultured under different growth conditions. Skin biopsies were taken in Cantabria (Spain) from a live, anaesthetized brown bear. DNA analysis was performed by flow cytometry following cell DNA staining with propidium iodide. Serum starvation increased (P<0.01) the percentage of G0/G1 phase cells (92.7+/-0.86) as compared to cycling cells (39.7+/-0.86) or cells cultured to confluency (87.3+/-0.86). DMSO included for 48h in the culture significantly increased (P<0.01) the percentage of G0/G1 phase of the cell cycle at all concentrations used and decreased percentages of S phase in a dose-dependent fashion. Roscovitine increased the G0/G1 phase of the cell cycle (P<0.01) at 15microM concentration. Interestingly, the G2/M stage significantly increased at 30 and 50microM compared to the control and 15microM (P<0.02). The cell cycle of brown bear adult fibroblast cells can be successfully synchronized under a variety of culture conditions.     

3-Deazauridine triggers dose-dependent apoptosis in myeloid leukemia cells and enhances retinoic acid-induced granulocytic differentiation of HL-60 cells

Therapeutic nucleoside analogue 3-deazauridine (DU) exerts cytotoxic activity against cancer cells by disruption of DNA synthesis resulting in cell death. The present study evaluates whether DU alone at doses 2.5-15 microM or in combination with all trans retinoic acid (RA) or dibutyryl cAMP (dbcAMP) is effective against myelogenous leukemia. The data of this study indicate that DU induces dose-dependent cell death by apoptosis in myeloid leukemia cell lines HL-60, NB4, HEL and K562 as demonstrated by cell staining or flow cytometry and agarose gel electrophoresis. 24h-treatment with DU produced dose-dependent HL-60 cell growth inhibition and dose-independent S phase arrest that was not reversed upon removal of higher doses of DU (10-15 microM). Exposition to nontoxic dose of DU (2.5 microM) for 24h followed by RA or dbcAMP and 96 h-cotreatment with DU significantly enhanced RA- but not dbcAMP-mediated granulocytic differentiation. Cell maturation was paralleled with an increase in the proportion of cells in G1 or G2+M phase. We conclude that, depending on the dose or the sequence of administration with RA, an inhibitor of DNA replication, DU triggers a process of either differentiation or apoptosis in myeloid leukemia cells.     

Mimosine reversibly arrests cell cycle progression at the G1-S phase border

It has previously been demonstrated that the compound mimosine inhibits cell cycle traverse in late G1 phase prior to the onset of DNA synthesis (Hoffman BD, Hanauske-Abel HM, Flint A, Lalande M: Cytometry 12:26-32, 1991; Lalande M: Exp Cell Res 186:332-339, 1990). These results were obtained by using flow cytometric analysis of DNA content to compare the effects of mimosine on cell cycle traverse with those of aphidicolin, an inhibitor of DNA polymerase alpha activity. We have now measured the incorporation of bromodeoxyuridine into lymphoblastoid cells by flow cytometry to determine precisely where the two inhibitors act relative to the initiation of DNA synthesis. It is demonstrated here that mimosine arrests cell cycle progression at the G1-S phase border. The onset of DNA replication occurs within 15 min of releasing the cells from the mimosine block. In contrast, treatment with aphidicolin results in the accumulation of cells in early S phase. These results indicate that mimosine is a suitable compound for affecting the synchronous release of cells from G1 into S phase and for analyzing the biochemical events associated with this cell cycle phase transition.     

A small-volume technique for simultaneous immunophenotyping and apoptosis detection in rat whole blood by four-color flow cytometry

BACKGROUND: Cell permeabilization for the detection of intracellular molecules by flow cytometry is usually incompatible with whole blood. This article describes a new technique for the simultaneous detection of surface antigens and DNA content in rat whole blood. METHODS: In 20 microl of rat whole blood, DNA staining is obtained by permeabilization of cells using a standard red blood cell lysing reagent (Erythrolyse). Immunophenotyping and apoptosis detection by flow cytometry are achieved by using a combination of three surface markers (CD3, CD4, and CD8alpha) and a DNA binding dye (TO-PRO-3). RESULTS: After a 24-h incubation of whole blood with 1 microM dexamethasone, apoptotic lymphocytes were clearly distinguishable from normal lymphocytes by their reduced size and DNA content. The dexamethasone-induced percentage of apoptotic cells was 58.9 +/- 4.6 for CD4+ and 77.4 +/- 2.9 for CD8+ T cells, compared with 12.6 +/- 2.7 for CD4+ and 17.2 +/- 3.5 for CD8+ T cells in the absence of dexamethasone (data from 10 animals with duplicate samples). CONCLUSIONS: We have developed a new technique to permeabilize nucleated cells in microsamples of rat whole blood. The methodology allows simultaneous immunophenotyping and apoptosis detection in rat whole blood.     

Cyanopyrazoles as analogs of purine precursors--II. Effects on macromolecular synthesis and cell cycle progression

The cytotoxic and cytokinetic effects, and in vitro inhibition of macromolecular synthesis by cyanopyrazoles were studied using Friend leukemia and Ehrlich ascites tumor cells. At concentrations in the range of 2.5 mM to 50 microM analog 3(5)-amino-4-cyano-5(3)-trichloromethylpyrazole (I) was highly cytotoxic and completely inhibited thymidine, uridine and leucine incorporation into macromolecular material. 24 hr incubation of FL cells with cytostatic concentrations of compound I (in the range of 2 to 0.5 microM) resulted in an accumulation of cells in the G2 + M phase. Analogs N-hydroxyethyl-3(5)-amino-4-cyano-5(3)-trichloromethylpyrazole (II) and 3(5)-amino-4-cyanopyrazole (III) were not cytotoxic at concentrations up to 5 mM and did not substantially inhibit precursor incorporation into macromolecules but exhibited a cytostatic activity. These compounds caused a decrease of FL cells in the G2 + M phase and an accumulation in the S phase. Analogs I and II displayed a similar in vivo inhibitory effect on thymidine incorporation into DNA in EAT cells. The results indicate that the cytotoxicity of cyanopyrazoles correlates with their ability to inhibit precursor incorporation into macromolecular material. On the other hand, the cytostatic action of compound I is not coupled to a block of nucleic acid synthesis.     

Unbalanced growth and cell death in HeLa S3 cultures treated with DNA synthesis inhibitors

Flow cytometry indicated that significant amounts of dsRNA were accumulated in HeLa S3 cells blocked at or near G₁/S boundary by hydroxyurea (HU) or excess thymidine (TdR). The dsRNA/DNA ratio increased in these cells in a manner characteristic of unbalanced cell growth. In HU-treated cells, dsRNA content was maximal 16 hours after addition of the drug and did not change significantly during the next 24 hours. The DNA content in blocked cells increased by 10%. Cell viability assessed by colony formation in soft agar decreased exponentially in HU-treated cultures after 16 hours of incubation. Correlation between loss of cell viability and rate of cell proliferation after removal of HU was observed, as determined by cell count and analysis of cell cycle progression. In TdR-treated cultures cells slowly progressed into mid S-phase during 40 hours and dsRNA accumulation continued during this period. Cell viability was not significantly affected by treatment with excess TdR, indicating that unbalanced growth per se, as measured by dsRNA accumulation, is not lethal for the cells. After reversal of DNA synthesis inhibition by removal of the drug, cells treated with HU for 16 hours or TdR for 16–24 hours promptly progressed through the cell cycle. This progression was accompanied by accumulation of significant amounts of dsRNA. As a result, cells in G₂ phase had a very high dsRNA content leading to retention of the unbalanced condition (increased dsRNA/DNA ratio) in the daughter cells. It is suggested that dsRNA accumulation in the cell is controlled to a certain degree by cell progression through the S phase. This type of control, evidently, was reflected in limited dsRNA accumulation in the cells blocked at or near G₁/S border, in continuous dsRNA accumulation in the cells slowly progressing through S phase, and in accumulation of large amounts of dsRNA after renewal of progression through the S phase.     

Is there an accumulation of cells during G2 in some acute lymphoblastic leukaemias?

Abstract. In some cases of acute lymphoblastic leukaemia (ALL) the percentage of cells in G₂+ M is higher than anticipated when compared with the percentage in S phase. This increase in G₂+ M, as detected by flow cytometry measurement of DNA content, may be due to an accumulation of cells, either in G ₂ or during the end of S phase; it may also be related to the existence of small tetraploid clones generally ignored by cytogeneticists. In order to identify possible subpopulations of cells with a DNA index ≥ 2-0, we have compared the results of a cytogenetic analysis to the G₂+ M values. We have also studied the distribution of S phase cells in 24 cases of ALL by incorporating 5-bromodeoxyuridine, labelling the cells by indirect immunofluorescence, and analysing them by flow cytometry after propidium iodide staining. The distribution of cells during S phase was quantified: no accumulation of cells was ever observed at the end of S phase. The question of the existence of small tetraploid clones, G₂ arrested cells or cells with a G₂ elongation remains open. However, we feel that it is more probable that, in this pathology, an elongation of the duration of G₂ occurs.     

Studies on antitumor effects and mechanism of action of l-hexylcarbamoyl-5-fluorouracil by DNA/BrdU double staining using flow cytometry.

An investigation was carried out to elucidate the mechanism of action of the oral antitumor agent 1-hexylcarbamoyl-5-fluorouracil (HCFU) by determining its effects on the growth and cell cycle of epipharyngeal carcinoma cells (KB cell) by DNA/BrdU double staining using flow cytometry (FCM). As a result, it was found that HCFU stimulates KB cells in the S phase to proliferate for the first 3 days of treatment in a low concentration (8 micrograms/ml) and caused cell accumulation in the later G2M phase. On the other hand, when administered in the concentration (20 micrograms/ml) that produces a 50 per cent cell kill, as determined from the cell growth curve, HCFU appeared to exhibit a cytocidal effect by blocking cells in S and G2M for the first 3 days after exposure. It was revealed by FCM for the first time that HCFU operates by a similar mechanism to that of 5-FU. This method seems to be of significance to therapeutic schemes that take into consideration the mechanism of action of antitumor drugs.     

Deoxyadenosine toxicity in an adenosine deaminase-inhibited human CCRF-CEM T-lymphoblastoid cell line causes cell swelling.

The human T-lymphoblastoid cell line CCRF-CEM, pre-treated with 2'-deoxycoformycin, was used as a model for adenosine deaminase deficiency to investigate how 2'-deoxyadenosine exerts its cytotoxic effects. Incubation of these cells with 1 microM or 5 microM deoxyadenosine for 24 and 48 h caused an increase of up to 50% in their modal cell volume as measured by a Coulter Size Distribution Analyzer and this increase in cell volume was accompanied by an increase in their fragility and deformability. The swelling of cells was concomitant with the phosphorylation of deoxyadenosine and its intracellular accumulation as dATP. There was no evidence of osmotic imbalance or of inhibition of the Na+/K(+)-dependent ATPase activity as the intracellular concentrations (and the intracellular:extracellular ratios) of Na+, K+ and Ca2+ were essentially unchanged. Cytochalasin B (20 microM) also caused lymphoblasts to swell over a 6-h period and its effect on cell size was similar to that of either 1 microM or 5 microM deoxyadenosine over 24 or 48 h. Longer time-courses of incubation with cytochalasin B caused severe toxicity leading to the death and lysis of a significant proportion of the cells. Other drugs, such as colchicine, vincristine and vinblastine that are known to affect various components of the cytoskeleton also caused swelling of cells in a concentration- and time-dependent manner but there was no evidence that these effects were additive or synergistic with those of deoxyadenosine. Inhibition of DNA synthesis, either directly by aphidicolin or indirectly by hydroxyurea, was less cytotoxic than the effect caused by deoxyadenosine. We conclude that one of the toxic effects resulting from the excessive phosphorylation of deoxyadenosine and its accumulation as dATP in human T-lymphoblasts is not dependent on inhibition of DNA synthesis but may be caused by the disruption of the cytoskeleton in these cells.