The mechanism of dNTP-unbalanced cell death induced by 5-fluorouracil and its derivatives

The mechanism of 5-fluorouracil (5-FU) and 5-fluorodeoxyuridine (FdUR)-induced death of mouse mammary tumor FM3A cells was studied. When the cells were exposed to 5-FU or FdUR, an unbalance of intracellular dNTP pool resulted. The unbalance was followed by breakage of mature DNA. DNA double strand breaks were observed in the FdUR (1 microM) treated cells 16 hrs after the administration. We assume that the double strand breaks play an important role in the mechanism of the FdUR-mediated cell death. In addition, the activity that can induce DNA double strand breaks was detected in the lysate of FdUR treated FM3A cells. Since intracellular dNTP pool unbalance seems to be the trigger of these events, this phenomenon may be termed as dNTP-unbalanced cell death.     

Deoxyribonucleoside triphosphate imbalance. 5-Fluorodeoxyuridine-induced DNA double strand breaks in mouse FM3A cells and the mechanism of cell death

The mechanism of cytotoxic action of 5-fluorodeoxyuridine (FdUrd) in mouse FM3A cells was investigated. We observed the FdUrd-induced imbalance of intracellular deoxyribonucleoside triphosphate (dNTP) pools and subsequent double strand breaks in mature DNA, accompanied by cell death. The imbalance of dNTP pools was maximal at 8 h after 1 microM FdUrd treatment; a depletion of dTTP and dGTP pools and an increase in the dATP pool were observed. The addition of FdUrd in culture medium induced strand breaks in DNA, giving rise to a 90 S peak by alkaline sucrose gradient sedimentation. The loss of cell viability and colony-forming ability occurred at about 10 h. DNA double strand breaks as measured by the neutral elution method were also observed in FdUrd-treated cells about 10 h after the addition. These results lead us to propose that DNA double strand breaks play an important role in the mechanism of FdUrd-mediated cell death. A comparison of the ratio of single and double strand breaks induced by FdUrd to that observed following radiation suggested that FdUrd produced double strand breaks exclusively. Cycloheximide inhibited both the production of DNA double strand breaks and the FdUrd-induced cell death. An activity that can induce DNA double strand breaks was detected in the lysate of FdUrd-treated FM3A cells but not in the untreated cells. This suggests that FdUrd induces the cellular DNA double strand breaking activity. The FdUrd-induced DNA strand breaks and cell death appear to occur in the S phase. Our results indicate that imbalance of the dNTP pools is a trigger for double strand DNA break and cell death.     

The relationship between cAMP, Ca(2)+, and transport of CFTR to the plasma membrane

The mechanism whereby cAMP stimulates Cl(-) flux through CFTR ion channels in secretory epithelia remains controversial. It is generally accepted that phosphorylation by cAMP-dependent protein kinase increases the open probability of the CFTR channel. A more controversial hypothesis is that cAMP triggers the translocation of CFTR from an intracellular pool to the cell surface. We have monitored membrane turnover in Calu-3 cells, a cell line derived from human airway submucosal glands that expresses high levels of CFTR using membrane capacitance and FM1-43 fluorescence measurements. Using a conventional capacitance measurement technique, we observe an apparent increase in membrane capacitance in most cells that exhibit an increase in Cl(-) current. However, after we carefully correct our recordings for changes in membrane conductance, the apparent changes in capacitance are eliminated. Measurements using the fluorescent membrane marker FM1-43 also indicate that no changes in membrane turnover accompany the activation of CFTR. Robust membrane insertion can be triggered with photorelease of caged Ca(2)+ in Calu-3 cells. However, no increase in Cl(-) current accompanies Ca(2)+-evoked membrane fusion. We conclude that neither increases in cAMP or Ca(2)+ lead to transport of CFTR to the plasma membrane in Calu-3 cells. In addition, we conclude that membrane capacitance measurements must be interpreted with caution when large changes in membrane conductance occur.     

The dNTP imbalance death.

The mechanism of intracellular deoxyribonucleoside-triphosphates (dNTP) pool imbalance-induced cell death in mouse FM3A (F28-7) cells was studied. When the cells were treated with 5-fluorodeoxyuridine (FdUrd), deoxyadenosine, 2-chlorodeoxyadenosine, or alpha,alpha-bis(2-hydroxy-6-isopropyltropon-3-yl)-4-methoxytolu ene, an imbalance in the cellular dNTP pool was induced. The imbalance was followed by DNA double-strand breaks and subsequent cell death. Fragmented DNA appeared to be approximately 100-200 kbp in size. The base of 5'-termini in the DNA were adenine and thymine. The endonuclease toward double stranded DNA has been found in a fraction of FdUrd treated cell lysate, and isolated using column chromatography. We propose the new mechanism dNTP pool imbalance induced cell death named; dNTP Imbalance Death.     

Deoxyribonucleoside-triphosphate imbalance death: deoxyadenosine-induced dNTP imbalance and DNA double strand breaks in mouse FM3A cells and the mechanism of cell death

The mechanism of deoxyadenosine (dAdo)-induced death of mouse mammary tumor FM3A cells was studied. When the cells were exposed to dAdo at 3 mM, an imbalance of intracellular dNTP pool resulted: dATP concentration was elevated 100-fold and the dGTP concentration was reduced to less than 1% of the control values. The imbalance was followed by breakage of mature DNA. DNA double strand breaks were observed in the dAdo treated cells 12 hr after the administration. We assume that the double strand breaks play an important role in the process of the dAdo-mediated cell death, and that the intracellular dNTP imbalance is the trigger of these events.     

Downregulation of JNK/SAPK activity is associated with the cross-resistance to P-glycoprotein-unrelated drugs in multidrug-resistant FM3A/M cells overexpressing P-glycoprotein

In the present study, cross-drug resistance in multidrug-resistant (MDR) cells, which overexpress P-glycoprotein (Pgp), a mdr1 gene product, against Pgp-unrelated drugs, and its relevance to c-Jun N-terminal kinase (JNK)/stress-activated protein kinase (SAPK) activity were examined. The multidrug-resistant FM3A/M cells overexpressing Pgp were resistant to apoptotic cell death induced either by Pgp-related drugs including vincristine and vinblastine, which are pumped out by Pgp, or by the Pgp-unrelated drugs including 5'-fluorouracil (5-FU) and bleomycin, which are not targets for Pgp, compared with the parental FM3A cells. Verapamil reversed the resistance of FM3A/M cells to apoptosis induced by the Pgp-related drugs but not that induced by the Pgp-unrelated drugs. Interestingly, FM3A/M cells have shown significantly lower basal and drug-stimulated JNK/SAPK activities than FM3A cells. After transfection with pEBG-SEK or pEBG-SAPK constructs, FM3A/M cells recovered the basal and Pgp-unrelated drug-stimulated activities of JNK/SAPK and the susceptibility to Pgp-unrelated drug-induced apoptotic cell death comparable to those of FM3A cells. Furthermore, FM3A cells became resistant to apoptotic cell death induced by vincristine and 5-FU after transfection with pEBG-SEK(K --> R), a dominant negative inhibitory mutant of SEK. These results suggest that downregulation of JNK/SAPK activity appears to confer on Pgp-associated FM3A/M cells a cross-resistance to Pgp-unrelated drugs.     

Regulation of cholesterol synthesis in cultured mouse mammary carcinoma FM3A cells

Mouse mammary carcinoma FM3A cells, which are able to grow in a serum-free medium, have novel characteristics that could be valuable in biochemical and somatic cell genetic studies. In FM3A cells grown in the presence of serum, both sterol synthesis and the activity of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, the major rate-limiting enzyme in the cholesterol biosynthetic pathway, were strongly suppressed by human low density lipoprotein (LDL). The addition of LDL (50 micrograms protein/ml) resulted in a 50% decrease in the reductase activity within 3 h and a 95% reduction after 24 h. Similarly, over 90% suppression of the reductase activity was obtained by the addition of LDL or mevalonolactone when the cells were grown on a serum-free medium. ML-236B (compactin), a specific inhibitor of HMG-CoA reductase, inhibited sterol synthesis from [14C]acetate by 80% at 1 microM. Reductase activity in FM3A cells was increased by 2.5- to 5-fold when the cells were treated with ML-236B (at 0.26-2.6 microM for 24 h). Thus, in FM3A cells, HMG-CoA reductase activity responded well to LDL, as is observed in human skin fibroblasts. Along with other novel features of this cell line, the present observations indicate that FM3A cells should be useful in biochemical and somatic cell genetic analysis of cholesterol metabolism, especially as regards the regulation of HMG-CoA reductase activity.     

Methotrexate differentially affects growth of suspension and adherent cells

The effects of low concentrations of methotrexate (MTX) on the growth of suspension (FM3A, 2B4 and THP-1) and adherent (NIH3T3 and V79) cells were compared. The concentration of methotrexate to cause the inhibition of cell growth was lower in suspension cells than in adherent cells. The IC(50) for FM3A, 2B4, THP-1, NIH3T3 and V79 cells were 3.5, 5, 9, 30 and 50 nM, respectively. The inhibition of cell growth was reversed completely by tetrahydrofolate and was fully or significantly reversed by adenosine and thymidine, suggesting that the effects of low concentrations of methotrexate result from the inhibition of biosynthesis of purines and pyrimidines. In suspension cells but not in adherent cells there was a decrease in the levels of S-adenosylmethionine and polyamines after methotrexate treatment. Growth of suspension but not adherent cells was significantly recovered by treatment with S-adenosylmethionine. However, treatment with spermidine did not reverse the effects of methotrexate in any of the cell lines. The preferential inhibitory effect of methotrexate in suspension cells versus adherent cells was due mainly to a more rapid uptake of methotrexate. This may be relevant to the in vivo effects of low doses of methotrexate, which have immunosuppressive and anti-inflammatory effects, because lymphocytes are suspension cells.     

Detection of deoxyribonucleoside-triphosphate imbalance death-induced DNA double strand breakage in FM3A cells by orthogonal-field-alternation gel electrophoresis (OFAGE)

The mechanism of intracellular deoxyribonucleoside-triphosphate (dNTP) imbalance death of mouse mammary tumor FM3A cells was studied. When the cells were exposed to 5-fluorodeoxyuridine, deoxyadenosine, or 2-chlorodeoxyadenosine, dNTP pool imbalance resulted. The imbalance was followed by DNA double strand breaks and subsequent cell death. The DNA double strand breaks have been directly examined by means of orthogonal-field-alternation gel electrophoresis (OFAGE). Fragmented DNA band appeared to be approximately 100-200 kb in size.     

dNTP pool imbalance induced endonuclease: 5-fluorodeoxyuridine induced DNA double strand break in mouse FM3A cells and the mechanism of cell death.

The mechanism of intracellular deoxyribonucleotide triphosphates (dNTP) pool imbalance-induced cell death in mouse FM3A cells was studied. When the cells were treated with 1 microM 5-fluorodeoxyuridine (FdUrd), the imbalance of the cellular dNTP pool was induced. The imbalance was followed by DNA double stranded breaks and subsequent cell death. The endonuclease toward double stranded DNA has been found in a fraction of FdUrd treated cell lysate, and isolated using column chromatography. SDS-polyacrylamide gel electrophoresis showed a major protein species of approximate 45 kDa. The endonuclease was revealed, using electrophoretic separation in SDS-polyacrylamide gels containing DNA, by incubating the gels in buffer to remove SDS and to allow renaturation and enzyme activity.     

Apoptosis and appearance of Trp53-positive micronuclei in murine tumors with different radioresponses in vivo.

The purpose of this paper is to determine the relationship between the response to radiation and the appearance of apoptosis and micronuclei with Trp53 protein in murine tumors after irradiation. Two murine tumors, EL4, which was derived from a mouse lymphoma, and FM3A, which was derived from a mouse mammary carcinoma, were locally irradiated with 15 Gy and sections were stained with H&E and an anti-Trp53 antibody. The response to radiation was greater in EL4 tumors than in FM3A tumors. The frequency of apoptotic cells in EL4 tumors was 6.1 +/- 1.2% at time zero, reached a peak of 36.3 +/- 3. 8% at 6 h, and then decreased with time through 72 h to 2.5 +/- 1.5% after 15 Gy irradiation. In FM3A tumors, no apoptotic cells were detected at 0, 1, 3, 6 or 24 h after exposure. At 48 and 72 h, the frequency was only 3.0 +/- 0.6% and 1.3 +/- 0.3%. Apoptotic cells increased significantly at 3, 6 and 24 h after irradiation in EL4 tumors (P < 0.008) and at 48 and 72 h in FM3A tumors (P < 0.006). The frequency of Trp53-positive cells was 17.9 +/- 2.2 and 15.2 +/- 2.3% at time zero in EL4 and FM3A tumors, respectively, increased to 74.5 +/- 4.5% in EL4 cells (P = 0.001), and increased to 33.9 +/- 1. 1% in FM3A cells (P = 0.005) 1 h after irradiation. Trp53-positive micronuclei appeared in cells in both tumors from 24 to 72 h after irradiation. The frequency of Trp53-positive micronuclei was 3.8 +/- 0.5 and 13.5 +/- 1.3% at 24 h in EL4 and FM3A tumors, respectively, and gradually decreased by 72 h. After exposure to 15 Gy, Trp53-positive micronuclei increased significantly in FM3A tumors compared to EL4 tumors at both 24 and 48 h (P < 0.02). The frequency of these micronuclei increased with increasing dose in FM3A tumors, and the difference between these percentages after 3 Gy and after 5, 10 and 15 Gy was significant (P < 0.02). Many apoptotic cells were observed in the radiosensitive EL4 tumor after irradiation. Death by apoptosis may be related to an early response to radiation in these tumors. The appearance of micronuclei may be an important mechanism of cell death in FM3A tumors in which no apoptosis was induced.     

Growth factor stimulated cell proliferation is accompanied by an elevated labile intracellular pool of zinc in 3T3 cells

Growth factors such as platelet-derived growth factor (PDGF), epidermal growth factor (EGF), and insulin-like growth factor-I (IGF-I) are required for quiescent 3T3 cells to proliferate, but zinc deprivation impairs IGF-I-induced DNA synthesis. We recently showed that labile intracellular pool of zinc is involved in cell proliferation. Our objective was to determine whether the labile intracellular pool of zinc plays a role in growth factor (PDGF, EGF, and IGF-I)-stimulated proliferation of 3T3 cells. Quiescent 3T3 cells were cultured in DMEM with or without growth factors. Labile intracellular pool of zinc, DNA synthesis, and cell proliferation were assessed using fluorescence microscopy, 3H-thymidine incorporation, and total cell number counts, respectively. After 24 h, growth factors stimulated DNA synthesis (24%) but not cell proliferation. After 48 h, growth factors stimulated both DNA synthesis (37%) and cell proliferation (89%). In response to growth factor stimulation, the labile intracellular pool of zinc was also elevated after 24 or 48 h of treatment. In summary, growth factor (PDGF, EGF, and IGF-I)-stimulated increase in DNA synthesis and cell proliferation were accompanied by an elevated labile intracellular pool of zinc in 3T3 cells. Since elevation of the labile intracellular pool of zinc occurred along with increased DNA synthesis, but cell proliferation remained unchanged, the elevation of the labile intracellular pool of zinc likely occurred during the S phase to provide the zinc needed to support DNA synthesis and ultimately cell proliferation.     

Glycolipid composition of a mutant cell line of mouse FM3A cells, and the effect of exogenous glycolipids on cell growth

Had-1 isolated from mouse mammary tumour FM3A cells as a non-permissive cell line to Newcastle disease virus infection is deficient in NDV receptors, and galactosylation of the complex type sugar chains of the glycoproteins is extensively reduced compared to FM3A cells. It is also deficient in UDP-galactose transport into Golgi vesicles. The major neutral glycolipids in FM3A is Lac-Cer, whereas, in Had-1 cell, Glc-Cer is the major glycolipid and the concentration of neutral glycolipids is one-tenth as low as that in FM3A. GM3, GD3 and sialyl i- and I-type lactosaminylceramide are the gangliosides present in both FM3A and Had-1, although their presence in both cells is only in traces. Had-1 contains relatively high N-glycolyl-neuraminic acid. Among the several glycolipids tested, Lac-Cer, Gg-4-Cer and Glc-Cer showed inhibitory effect on proliferation of Had-1 cells, but did not show any appreciable effect on that of FM3A cells. Lac-Cer had the most potent inhibitory effect and this inhibitory effect was completely reversible. While mice injected with 5 x 10(6) cells of FM3A died in one month, those injected of Had-1 cells at the same dose survived for more than 6 months. Thus glycolipids on the cell surface play an essential role during cell growth both in vivo and in vitro.     

dNTP imbalance and DNA double strand breaks in mouse FM3A cells and the mechanism of cell death

The mechanism of intracellular deoxyribonucleoside-triphosphate (dNTP) imbalance death of mouse mammary tumor FM3A cells was studied. When the cells were exposed to 5-fluorodeoxyuridine, deoxyadenosine, or 2-chlorodeoxyadenosine, dNTP pool imbalance resulted. The imbalance was followed by DNA double-strand breaks and subsequent cell death. The DNA double strand breaks were directly examined by means of orthogonal-field-alternation gel electrophoresis (OFAGE). Fragmented DNA band appeared to be approximately 100-200 kbp in size. The bases of 5'-termini in the DNA were cytosine and thymine. The imbalance induced endonuclease has been isolated by DEAE-agarose column chromatography.     

Thymidylate stress induces homologous recombination activity in mammalian cells.

We studied whether homologous recombination activity in mammalian cells could be induced by thymidylate stress (thymidylate deprivation). In vitro recombination activity in cell extracts was measured with pSV2neo-derived plasmids. When prior to the preparation of extracts, mouse FM3A cells were grown in 5-fluorodeoxyuridine (FdUrd), an inducer of thymidylate stress, the homologous recombination activity was significantly induced, as judged from an increase in the number of neomycin-resistant bacterial colonies. Maximum induction was observed in cells treated with 1 microM FUdR for 16 h. However, 3-8 h of treatment of FM3A cells with the drug followed by an additional 8-16-h incubation in its absence was sufficient to induce the recombination activity while slightly reducing their growth rates. These results indicate that thymidylate stress induces homologous recombination activity in mammalian cells as observed in Escherichia coli and in yeast.     

Radioprotective effect of 5-hydroxytryptamine and 5-hydroxytryptophan on mammalian cells irradiated in vitro.

The radioprotective effect in vitro of 5-hydroxytryptamine (5-HT) and 5-hydroxytryptophan (k-HTP) was studied with cultured mammalian cells of three cell lines: 5-HT-synthesizing FMA and 5-HT-non-synthesizing FM3A and B16-C2W. In these cells, the addition of 5-HT to the suspending medium induced only a weak protection or no protection at all. The increase in the 5-HT content of these cells at the time of irradiation was negligible. Pre-incubation of cells for 40 min in a 5-HTP-containing medium resulted in an elevation of the 5-HT content concomitantly with an increase in the radioresistance of FMA cells, where the DRF at 1 per cent was 1 x 8. In FM3A and B16-C2W cells such an effect was not observed. The same relationship between 5-HT content and radioresistance was also observed in FMA cells which were cultured in different densities or with reserpine. These results strongly suggest that the substance playing the main role in the induction of radioresistance in cells in vitro is the 5-HT that exists in the cells.     

Relationship between cellular diadenosine 5',5'-P1,P4-tetraphosphate level, cell density, cell growth stimulation and toxic stresses

In order to elucidate the postulated role of diadenosine 5',5'-P1,P4-tetraphosphate (Ap4A) in cell growth regulation, the Ap4A cellular content was measured in cells submitted to various treatments affecting the cell growth. Ap4A level was found to increase ten times when cells reached confluence, whereas no significant variation of the ATP pool was observed. Cell growth arrest after serum depletion did not cause any variation in the Ap4A pool. A limited increase in the Ap4A pool was observed when growth of arrested cells was reinitiated but this variation reflected only the increase of cell density. No significant variation in the Ap4A intracellular level was observed after submitting two eukaryotic cell lines to various stresses (cytotoxic drugs, ethanol and heat-shock treatments). These results suggest that, in eukaryotic cells, Ap4A is not involved in cell growth stimulation but rather is associated with cell contact growth inhibition. They also suggest that Ap4A is not an 'alarmone', contrary to what has been proposed for bacteria.     

Time-dependent restoration of the trigger pool of calcium after termination of angiotensin II action in adrenal glomerulosa cells

In adrenal glomerulosa cells, angiotensin II causes an immediate release of calcium from an intracellular trigger pool (Kojima, I., Kojima, K., and Rasmussen, H. (1985) Am. J. Physiol. 247, E36-E43). The present study was conducted to determine how the trigger pool of calcium is restored after cessation of the agonist action. Upon termination of angiotensin II action, calcium influx rate decreased immediately while total cell calcium increased rapidly. The increase in total cell calcium is not affected by 1 microM nitrendipine, which blocks angiotensin II-stimulated calcium influx without inhibiting basal influx of calcium. In contrast, total cell calcium did not increase in medium containing 1 microM calcium, in which basal calcium influx is negligible. A rapid increase in total cell calcium after an addition of the antagonist was not accompanied by changes in cytoplasmic free calcium concentration. A second stimulation of cells with either angiotensin II or carbachol did not cause calcium release when the interval of two stimulations was shorter than 20 min. The longer the interval, the greater the magnitude of calcium release in response to the second stimulator. The maximum response was obtained when the interval was 40 min or more. When exogenous arachidonic acid, which mobilized calcium by acting directly on the inositol trisphosphate-sensitive pool, was employed as a second stimulator, the magnitude of the decrease in total cell calcium was also dependent on the interval. These results suggest that, upon termination of angiotensin II action, calcium is rapidly accumulated first in an intracellular pool which is insensitive to either inositol 1,4,5-trisphosphate or arachidonic acid and that the trigger pool is restored gradually thereafter.