Visualization of focal nuclear sites of DNA repair synthesis induced by bleomycin in human cells

In this study, we examined DNA repair synthesis in human cells treated with the radiomimetic drug bleomycin, which efficiently induces double-strand breaks (DSBs). Using tyramide-biotin to amplify fluorescent signals, discrete nuclear foci from the incorporation of 5-iododeoxyuridine (IdU) were detected in proliferating human cells treated with bleomycin. We believe this comes from the repair of DSBs. An increase in the number of foci (>5 per nucleus) was detected in a major fraction (75%) of non-S-phase cells labeled for 30 min with IdU 1 h after the end of bleomycin treatment. The fraction of cells with multiple IdU-containing foci was found to decrease 18 h after treatment. The average number of foci per nucleus detected 1 h after bleomycin treatment was found to decrease twofold between 1 and 3.5 h, indicating that the foci may be associated with the slow component of DSB repair. The presence of DSBs in bleomycin-treated cells was confirmed using antibodies against phosphorylated histone H2AX (gamma-H2AX), which is strictly associated with this type of DNA damage. After treatment with bleomycin, non-S-phase cells also displayed heterogeneous nuclear foci containing tightly bound proliferating cell nuclear antigen (PCNA), suggesting an ongoing process of unscheduled DNA synthesis. PCNA is known to be involved in base excision repair, but a fraction of the PCNA foci may also be associated with DNA synthesis occurring during the repair of DSBs.     

Organization of early and late replicating DNA in human chromosome territories

It has been suggested that DNA organized into replication foci during S-phase remains stably aggregated in non-S-phase cells and that these stable aggregates provide fundamental units of nuclear or chromosome architecture [C. Meng and R. Berezney (1991) J. Cell Biol. 115, 95a; E. Sparvoli et al. (1994) J. Cell Sci. 107, 3097-3103; D. A. Jackson and A. Pombo (1998) J. Cell Biol. 140, 1285-1295; D. Zink et al. (1998) Hum. Genet. 112, 241-251]. To test this hypothesis, early and late replicating DNA of human diploid fibroblasts was labeled specifically by incorporating two different thymidine analogs [J. Aten (1992) Histochem. J. 24, 251-259; A. E. Visser (1998) Exp. Cell Res. 243, 398-407], during distinct time segments of S-phase. On mitotic chromosomes the amount and spatial distribution of early and late replicating DNA corresponded to R/G-banding patterns. After labeling cells were grown for several cell cycles. During this growth period individual replication labeled chromosomes were distributed into an environment of unlabeled chromosomes. The nuclear territories of chromosomes 13 and 15 were identified by additional chromosome painting. The distribution of early and late replicating DNA was analyzed for both chromosomes in quiescent (G0) cells or at G1. Early and late replicating DNA occupied distinct foci within chromosome territories, displaying a median overlap of only 5-10%. There was no difference in this regard between G1 and G0 cells. Chromosome 13 and 15 territories displayed a similar structural rearrangement in G1 cells compared to G0 cells resulting in the compaction of the territories. The findings demonstrate that early and late replicating foci are maintained during subsequent cell cycles as distinctly separated units of chromosome organization. These findings are compatible with the hypothesis that DNA organized into replicon clusters remains stably aggregated in non-S-phase cells.     

Initiation of DNA synthesis in the prematurely condensed chromosomes of G1 cells

The objective of this study was to investigate whether G1 cells could enter S phase after premature chromosome condensation resulting from fusion with mitotic cells. HeLa cell synchronized in early G1, mid-G1, late G1, and G2 and human diploid fibroblasts synchronized in G0 and G1 phases were separately fused by use of UV-inactivated Sendai virus with mitotic HeLa cells. After cell fusion and premature chromosome condensation, the fused cells were incubated in culture medium containing Colcemid (0.05 micrograms/ml) and [3H]thymidine ([3H]ThdR) (0.5 microCi/ml; sp act, 6.7 Ci/mM). At 0, 2, 4, and 6 h after fusion, cell samples were taken to determine the initation of DNA synthesis in the prematurely condensed chromosomes (PCC) on the basis of their morphology and labeling index. The results of this study indicate that PCC from G0, G1, and G2 cells reach the maximum degree of compaction or condensation at 2 h after PCC induction. In addition, the G1-PCC from normal and transformed cells initiated DNA synthesis, as indicated by their "pulverized" appearance and incorporation of [3H]ThdR. Further, the initiation of DNA synthesis in G1-PCC occurred significantly earlier than in the mononucleate G1 cells. Neither pulverization nor incorporation of label was observed in the PCC of G0 and G2 cells. These findings suggest that chromosome decondensation, although not controlling the timing of a cell's entry into S phase, is an important step for the initiation of DNA synthesis. These data also suggest that the entry of a S phase may be regulated by cell cycle phase-specific changes in the permeability of the nuclear envelope to the inducers of DNA synthesis present in the cytoplasm.     

Inhibition of DNA polymerase activity by methyl methanesulfonate

Methyl methanesulfonate (MMS) inhibits both thymidine incorporation into DNA in mitogen-activated human lymphocytes and deoxythymidine triphosphate incorporation into template DNA by DNA polymerase-alpha in a cell-free system. When MMS-modified DNA was used as the template for DNA synthesis utilizing unmodified DNA polymerase-alpha, nucleotide incorporation into template DNA was not inhibited. When unmodified DNA was used as the template for DNA synthesis utilizing MMS-modified DNA polymerase-alpha, nucleotide incorporation was differentially inhibited dependent on the MMS concentration. An analysis of the kinetics of DNA polymerase-alpha inhibition showed that incorporation of all 4 deoxynucleoside triphosphates into DNA template was noncompetitively inhibited by MMS, which is consistent with nonspecific MMS modification of the enzyme. These data indicate that MMS modification of DNA polymerase-alpha alone is sufficient to inhibit the incorporation of deoxynucleoside triphosphates into template DNA in vitro. The data further indicate that alkylation of both DNA polymerase-alpha and DNA template synergistically increases inhibition of DNA synthesis.     

EGF-dependent growth inhibition in MDA-468 human breast cancer cells is characterized by late G1 arrest and altered gene expression.

The MDA-468 human breast cancer cell line displays the unusual phenomenon of growth inhibition in response to pharmacological concentrations of EGF. This study was initiated with the objective of elucidating the cellular mechanisms involved in EGF-induced growth inhibition. Following EGF treatment the percentage of MDA-468 cells in G1 phase increased, together with a concomitant depletion in S and G2/M phase populations, as revealed by flow cytometry of DNA content. The apparent G1 block in the cell cycle was confirmed by treating the cells with vinblastine. DNA synthesis was reduced to about 35% of that measured in control, untreated cells after 48 h of EGF treatment, as measured by the incorporation of [3H]thymidine. DNA synthesis returned to normal following the removal of EGF from the growth-arrested cells. In order to locate the EGF-induced event responsible for the G1 arrest more precisely, we examined the expression of certain cell cycle-dependent genes by Northern blot analysis. EGF treatment did not alter either the induction of the early G1 marker, c-myc, or the expression of the late G1 markers, proliferating cell nuclear antigen, and thymidine kinase. However, EGF-treated cells revealed down regulation of p53 and histone 3.2 expression, which are expressed at the G1/S boundary and in S phase, respectively. These results indicate that EGF-induced growth inhibition in MDA-468 human breast cancer cells is characterized by a reversible cell cycle block at the G1/S boundary.     

Effect of N-trifluoroacetyladriamycin-14-valerate on [3H]thymidine uptake and DNA synthesis of human lymphoma cells

The effects of N-trifluoroacetyladriamycin-14-valerate on the uptake of [3H]thymidine and its incorporation into DNA of human P3HR-1 lymphoma cells were studied. In the absence of the drug, at 0 degrees C, [3H]thymidine was transported into the cells but not incorporated into DNA, as determined by both the trichloroacetic acid-soluble and -precipitable counts obtained with the cells. At 37 degrees C, [3H]thymidine was readily transported into the cells and incorporated into DNA. In the presence of the drug, both [3H]thymidine uptake (as shown by acid-soluble counts) and the amount of its incorporation into acid-precipitable materials were markedly reduced. However, the uptake of [3H]thymidine at 0 degrees C was found to be equally sensitive to drug inhibition as at 37 degrees C. The incorporation at 37 degrees C of [3H]thymidine into acid-precipitable materials of the cells, which had been prelabeled at 0 degrees C with [3H]thymidine, was found to be insensitive to inhibition by the drug. The in vitro activities of DNA polymerases alpha and beta purified from human P3HR-1 cells were also found not to be susceptible to inhibition. Nuclei purified from cells pretreated with the drug continued to synthesize DNA. The cytofluorograms of the cells treated with the drug indicated that the treated cells accumulated at the G2/M phase, whereas the S phase of the cells was not arrested. These results suggest that N-trifluoroacetyladriamycin-14-valerate inhibits [3H]thymidine uptake but not cellular DNA synthesis in human P3HR-1 lymphoma cells.     

Cell cycle synchronization of porcine granulosa cells in G1 stage with mimosine

The success of somatic cell nuclear transfer depends critically on the cell cycle stage of the donor nucleus and the recipient cytoplast. Karyoplasts in the G0 or G1 stages are considered to be the most suitable for nuclear transfer. In the present study, we used a reversible cell cycle inhibitor, mimosine, to synchronize porcine granulosa cells (GCs) in G1 phase of the cell cycle. Porcine GCs were obtained from 3 to 5mm ovarian follicles of slaughtered gilts. The effect of mimosine on the proliferation, DNA synthesis and cell cycle stage of cultured cells was examined by incorporation of radiochemical 3H-thymidine, immunocytochemical detection of incorporated thymidine analogue 5-bromo-2-deoxyuridine (BrdU) and flow cytometry analyses. Mimosine treatment of pig GCs for 24h resulted in proliferation arrest in vitro. Treatment with 0.5mM mimosine significantly (P<0.05) inhibited 3H-thymidine incorporation after 24h of culture (4.6% +/- 0.1) and after 24h of culture in serum deprived medium (41.3% +/- 3.8), in comparison to controls (100%). Inhibition of DNA synthesis was further confirmed by immunocytochemical and flow cytometry analyses. Compared with controls (78.2%), mimosine treatment for 24h increased the proportion of G0/G1 cells in the culture (85.7%) more effectively than serum starvation (SS; 81.2%). Mimosine-caused G1 arrest of porcine GCs was fully reversible and cells continued to proliferate after removing the drug, especially when they were stimulated by EGF.     

Cytofluorometry of lymphocytes infected with Epstein-Barr virus: effect of phosphonoacetic acid on nucleic acid.

DNA synthesis in Epstein-Barr virus (EBV)-infected lymphocytes was inhibited by phosphonoacetic acid (PAA) as measured by [3H]thymidine incorporation. PAA, at a concentration of 200 microgram/ml, inhibited [3H]thymidine incorporation by human umbilical cord lymphocytes infected with EBV strain P94 but had little effect on DNA synthesis in mitogen-stimulated cells. Transformed cell lines did not develop from infected cord cell cultures treated with 100 microgram of PAA per ml. Cytofluorometric analysis showed marked increases in cellular nucleic acid content (RNA plus DNA) as early as 9 days after infection of cord cells in the absence of PAA and before significant enhancement of [3H]thymidine incorporation became apparent. Moreover, EBV led to increases in cellular nucleic acid even when 200 microgram of PAA per ml was added to cell cultures before infection. The apparent discrepancy between results obtained by [3H]thymidine incorporation and cytofluorometry is explained either by significant inhibition of cellular DNA polymerases by PAA or by a block at the G2 + M phase of the cell cycle. The data suggest that EBV initiates alterations in cellular nucleic acid synthesis or cell division without prior replication of viral DNA by virus-induced DNA polymerases.     

ANG II stimulates PKC-dependent ERK activation,DNA synthesis,and cell division in intestinal epithelial cells

PKC, a major target for the tumor-promoting phorbol esters, has been implicated in the signal transduction pathways that mediate important functions in intestinal epithelial cells, including proliferation and carcinogenesis. With the use of IEC-18 cells arrested in G0/G1, addition of phorbol esters resulted in a modest increase in [3H]thymidine incorporation and a slight shift toward the S and G2/M phases of the cell cycle, whereas the combination of EGF and phorbol 12,13-dibutyrate (PDB) synergistically stimulated DNA synthesis. To investigate the effects of receptor-mediated PKC activation on mitogenesis, we demonstrated that ANG II induced ERK activation, a response completely blocked by pretreatment with mitogen/extracellular signal-regulated kinase inhibitors or specific PKC inhibitors. Furthermore, ANG II stimulated an over threefold increase in [3H]thymidine incorporation that was corroborated by flow cytometric analysis of the cell cycle to levels comparable to that achieved by the combination of EGF and PDB. Taken together, our results indicate that receptor-mediated PKC activation, as induced by ANG II, transduces mitogenic signals leading to DNA synthesis and cell proliferation in IEC-18 cells.     

Glucose-stimulated DNA synthesis through mammalian target of rapamycin (mTOR) is regulated by KATP channels: effects on cell cycle progression in rodent islets

The aim of this study was to define metabolic signaling pathways that mediate DNA synthesis and cell cycle progression in adult rodent islets to devise strategies to enhance survival, growth, and proliferation. Since previous studies indicated that glucose-stimulated activation of mammalian target of rapamycin (mTOR) leads to [3H]thymidine incorporation and that mTOR activation is mediated, in part, through the K(ATP) channel and changes in cytosolic Ca2+, we determined whether glyburide, an inhibitor of K(ATP) channels that stimulates Ca2+ influx, modulates [3H]thymidine incorporation. Glyburide (10-100 nm) at basal glucose stimulated [3H]thymidine incorporation to the same magnitude as elevated glucose and further enhanced the ability of elevated glucose to increase [3H]thymidine incorporation. Diazoxide (250 microm), an activator of KATP channels, paradoxically potentiated glucose-stimulated [3H]thymidine incorporation 2-4-fold above elevated glucose alone. Cell cycle analysis demonstrated that chronic exposure of islets to basal glucose resulted in a typical cell cycle progression pattern that is consistent with a low level of proliferation. In contrast, chronic exposure to elevated glucose or glyburide resulted in progression from G0/G1 to an accumulation in S phase and a reduction in G2/M phase. Rapamycin (100 nm) resulted in an approximately 62% reduction of S phase accumulation. The enhanced [3H]thymidine incorporation with chronic elevated glucose or glyburide therefore appears to be associated with S phase accumulation. Since diazoxide significantly enhanced [3H]thymidine incorporation without altering S phase accumulation under chronic elevated glucose, this increase in DNA synthesis also appears to be primarily related to an arrest in S phase and not cell proliferation.     

低氧对肺动脉内皮细胞PDGF—B链释放及平滑肌细胞生长的影响

应用蛋白ｄｏｔｂｌｏｔ技术检测了低氧内皮细胞条件培养液（ＨＥＣＣＭ）和常氧内皮细胞条件培养液（ＮＥＣＣＭ）内ＰＤＧＦ相对含量,并利用［３Ｈ］－ＴｄＲ掺入法和流式细胞术观察了ＨＥＣＣＭ和ＮＥＣＣＭ及加入特异ＰＤＧＦ抗体对肺动脉平滑肌细胞（ＰＡＳＭＣ）生长的影响。结果表明,ＨＥＣＣＭ中的ＰＤＧＦ含量明显高于ＮＥＣＣＭ;ＨＥＣＣＭ能明显增强ＰＡＳＭＣ内ＤＮＡ合成,促进ＰＡＳＭＣ从Ｇｏ／Ｇ１期进入Ｓ期;当预先加入ＰＤＧＦ－Ｂ链抗体时,则会明显地抑制ＨＥＣＣＭ对ＰＡＳＭＣ的ＤＮＡ合成,阻止ＰＡＳＭＣ从Ｇｏ／Ｇ１期进入Ｓ期。结果提示,低氧时ＰＡＳＭＣ增殖与肺动脉内皮细胞分泌释放ＰＤＧＦ增加有关     

Evidence that microtubule depolymerization early in the cell cycle is sufficient to initiate DNA synthesis

Microtubule disrupting drugs initiated DNA synthesis in serum-free cultures of nonproliferating fibroblast-like cells. The addition of colchicine to chick, mouse and human fibroblasts in serum-free medium stimulated thymidine incorporation at least twofold, with a half-maximal concentration of 1 X 10(-7) M. This stimulation represented up to 75% of the maximal stimulation by thrombin and was paralleled by an increase in the percentage of labeled nuclei. Other microtubule disrupting drugs showed similar stimulation, whereas lumicolchicine had no effect. Indirect immunofluorescent staining of tubulin showed a correlation between microtubule depolymerization and initiation of DNA synthesis by these drugs. A 2 hr treatment with 10(-6) M colchicine caused complete disruption of the microtubular network and stimulated thymidine incorporation (measured 28 hr later) to an even greater extent than continuous colchicine exposure. A similar 2 hr exposure to 10(-6) M colcemid also stimulated thymidine incorporation and led to a 50% increase in cell number. Taxol, a drug which stabilizes cytoplasmic microtubules, blocks initiation of DNA synthesis by colchicine, indicating that microtubule depolymerization is necessary for this initiation. To determine if microtubule depolymerization is involved in stimulation of DNA synthesis by other growth factors, highly purified human thrombin was added to cells with or without colchicine. In no case did colchicine plus thrombin increase DNA synthesis above that of the maximal stimulation by thrombin alone. Furthermore, pretreatment of cultures with taxol (5 micrograms/ml) inhibited approximately 30% of the stimulation of thymidine incorporation by thrombin. Together, these studies demonstrate that microtubule depolymerization is sufficient to initiate both DNA synthesis and events leading to cell division and suggest that microtubule depolymerization may be a required step in initiation of cell proliferation by growth factors such as highly purified human thrombin.     

Effect of glucocorticoids and stress on 14C-thymidine incorporation into DNA and on mitosis in regenerating rat liver.

Hydrocortisone sodium succinate (65 mg/kg body weight), Zn-ACTH (40 U/kg) and the intraperitoneal injection of Celite (200 mg/kg) decrease the incorporation of 14C-thymidine into the DNA of regenerating rat liver by about 60%. This decrease is not followed by a corresponding inhibition of cell division. The agents applied at the end of G1 phase or in the S phase of the cell cycle probably change thymidine metabolism and the observed decrease of thymidine incorporation does not represent true inhibition of DNA synthesis. The experiment with regenerating liver slices has shown that this disproportion is partly caused by decreased 14C-thymidine transport into the cells.     

Inorganic lead stimulates DNA synthesis in human astrocytoma cells: role of protein kinase Calpha

As lead has been shown to activate protein kinase C (PKC), and gliomas are reported to be highly dependent on PKC for their proliferation, this study was undertaken to investigate whether lead may act as a mitogen in human astrocytoma cells, and to determine the role of PKC in this effect. Lead acetate (from 100 nM to 100 microM) induced a concentration- and time-dependent increase in DNA synthesis, as measured by incorporation of [methyl-3H]thymidine into cell DNA, without causing any cytotoxicity. Flow cytometric analysis showed that lead was able to stimulate the cell cycle transition from the G0/G1 phase to the S/G2 phase, resulting in increased percentage of cells in the latter phase. Western blot analyses showed that lead induced translocation of PKCalpha, but not of PKCepsilon or PKCzeta, from the cytosolic to the particulate fraction, with a concomitant increase in PKC enzyme activity. Prolonged exposure to lead caused down-regulation of PKCalpha, but not of PKCepsilon. The effect of lead on DNA synthesis was mediated through PKC as evidenced by the finding that two PKC inhibitors, GF 109203X and staurosporine, as well as down-regulation of PKC through prolonged treatment with 12-O-tetradecanoylphorbol 13-acetate, blocked lead-induced DNA synthesis. Further experiments using a pseudosubstrate peptide targeting classical PKCs and selective down-regulation of specific PKC isoforms indicated that the effect of lead on DNA synthesis was mediated by PKCalpha. Altogether, these results suggest that lead stimulates DNA synthesis in human astrocytoma cells by a mechanism that involves activation of PKCalpha.     

Cytophotometric determination of unscheduled DNA synthesis

We describe a cytophotometric assay for unscheduled DNA synthesis (UDS) in asynchronously growing cells. Monolayer cultures of human HEp-2 and mouse MCa-11 cells were incubated with the carcinogen methyl-methane sulfonate (MMS), as well as with hydroxyurea and (3H)thymidine. Slides were prepared, and the DNA contents and areas of nuclei were measured by absorption cytophotometry. The labeling of the nuclei, determined on the basis of their DNA content to be in G0/G1, was selectively measured after the preparation of autoradiographs. The labeling of the G0/G1 cells increased with increasing doses of MMS. We also found that the increased nuclear labeling after MMS treatment was not due to induction of replicative DNA synthesis or selective destruction of G0/G1 cells. The results of this assay compared favorably with a standard biochemical method for measuring unscheduled DNA synthesis by benzoylated naphthoylated DEAE cellulose chromatography.     

G2 arrest and differentiation in the petal of Tradescantia clone 4430

The floral organs of Tradescantia clone 4430 were used to investigate, in terms of cell cycle parameters, cellular behaviour during the maturation of a terminally differentiating system. Petals were sampled at different stages of development for (a) cell number; (b) nuclear DNA content by cytophotometry; (c) [³H]thymidine incorporation into nuclei by autoradiography; and (d) pigment production by spectrophotometry. DNA synthesis was confirmed by measurement of [³H]thymidine incorporation into TCA-insoluble material and changes in DNA content by colorimetric estimation of DNA extracts by diphenylamine. The development of the petal involved four sequential steps. First, there was an increase in cell number, an event characterized by mitoses, DNA synthesis, a few cells in G2 and a predominance in G1. Second, there was a cessation of cell division and DNA synthesis when all the cells accumulated in G1. Third, there was a shift of a large proportion of the total cell population from G1 to the G2 stage of the cell cycle and finally, there was pigment production. In addition, cytophotometric analysis of individual tissues in the mature petal revealed tissue specific differences in the proportion of cells in G2.     

Human epidermal growth factor and the proliferation of human fibroblasts

The effect of human epidermal growth factor (hEGF), a 5,400 molecular weight polypeptide isolated from human urine, on the growth of human foreskin fibroblasts (HF cells) was studied by measuring cell numbers and the incorporation of labeled thymidine. The addition of hEGF to HF cells growing in a medium containing 10% calf serum resulted in a 4-fold increase in the final density. The presence of hEGF also promoted the growth of HF cells in media containing either 1% calf serum or 10% gamma globulin-free serum. The addition of hEGF to quiescent confluent monolayers of HF cells, maintained in a medium with 1% calf serum for 48 hours, resulted in a 10- to 20-fold increase in the amount of ³H-thymidine incorporation after 20–24 hours. The stimulation of thymidine incorporation was maximal at an hEGF concentration of 2 ng/ml, was dependent on the presence of serum, and was enhanced by the addition of ascorbic acid. In confluent cultures of HF cells, subject to density dependent inhibition of growth, hEGF was able to stimulate DNA synthesis more effectively than fresh calf serum. Human EGF stimulated DNA synthesis in quiescent cultures, however, regardless of cell density. The addition of rabbit anti-hEGF inhibited all effects of this growth factor on HF cells.     

Lymphokine regulation of activated (G1) lymphocytes. II. Glucocorticoid and anti-Tac-induced inhibition of human T lymphocyte proliferation

The regulation of the first cell cycle of human, activated (G1) PBL was analyzed by flow cytometry and [3H]thymidine incorporation. Endogenous IL 2 production was blocked in situ by pharmacologic concentration of DEX (100 to 1000 nM), resulting in an 80 to 90% reduction of thymidine uptake. Although T lymphocyte activation (G0-G1a transition) by PHA was unaltered, cells remained in the G1a phase of the cell cycle due to insufficient RNA synthesis for proliferation. The addition of IL 2-containing supernatants reversed this inhibitory effect of DEX by allowing the cells to synthesize more RNA (G1a-G1b transition). Such cells could enter the S phase and proliferate. Similar studies were performed on cells treated with a monoclonal antibody (anti-Tac) against the IL 2 receptor. In these studies, IL 2-induced RNA synthesis, and subsequent proliferation of DEX-treated and PHA-stimulated cells was inhibited by anti-Tac. Anti-Tac did not, however, inhibit the effect of endogenous IL 2 (PHA-stimulated PBL without DEX treatment), although it did bind equally well to such cells. Thus, IL 2 directly or indirectly regulates human T cell proliferation at the level of RNA synthesis. Furthermore, anti-Tac can inhibit the mitogenic signal given by endogenous IL 2, but not by in situ produced IL 2, an observation of importance to further investigations of the mechanisms by which IL 2 interacts with specific receptors to elicit proliferation.     

Inhibition by 2-deoxy-D-ribose of DNA synthesis and growth in Raji cells

When Raji cells were cultured for 3 days in serum-free medium, addition of 2-deoxy-D-ribose at the start of culture inhibited incorporation of [3H]thymidine and cell division. At deoxyribose concentrations between 1 and 5 mM, viability was 80% or greater after 3 days of culture even though 5 mM deoxyribose inhibited thymidine incorporation 95-99%. Inhibition by deoxyribose could be completely reversed if the culture medium was replaced with fresh medium up to 8 hr after the start of culture. The inhibition was specific for deoxyribose since other monosaccharides had no effect. Inhibition of DNA synthesis did not appear to be due to depletion of essential nutrients in the medium since the percentage inhibition of thymidine incorporation by cells cultured either in suboptimal serum-free media or in media supplemented with 0.025-5% human AB serum was similar. When DNA repair synthesis was measured as hydroxyurea-resistant thymidine incorporation, addition of deoxyribose to Raji cultures caused increased thymidine incorporation. These results, together with data from others, suggest that deoxyribose damages DNA.