Time-dependent changes in H1 content, H1 turnover, DNA elongation, and the survival of cells blocked in early S phase by hydroxyurea, aphidicolin, or 5-fluorodeoxyuridine

High concentrations of NaCl induce cell swelling leading to senescence in human cells

Cell swelling and retardation in DNA replication are always observed in senescent cells. When DNA replication is slowed down with RNA and protein syntheses unchanged in proliferating cells, it causes a phenomenon known as unbalanced growth. The purpose of this study is to assess the role of cell swelling in unbalanced growth in terms of senescence and investigate the mechanism underlying this phenomenon. We tried to induce cell swelling with minimum damage to cells in this study. We perturbed the osmoregulatory functions to induce cell swelling under hypotonic and hypertonic conditions in normal human fibroblasts. Addition of excess NaCl was found to induce significant cell and nuclear swelling in dose- and time-dependent manners. Excess NaCl immediately retarded DNA replication, accumulated cells at G1 phase of the cell cycle, and eventually deprived division potential of the cells. Such cells showed typical senescent cell shape followed by expression of the typical senescence-associated genes. Excess NaCl also activated ERK1/2, p38, and JNK of the mitogen activated protein kinase family. Addition of U0126, an inhibitor of ERK1/2, prevented appearance of senescent features induced by excess NaCl. These results suggest that hypertonic conditions induce cell swelling due to unbalanced growth, thereby leading to cellular senescence.

Chromosome fragility and the abnormal replication of the FMR1 locus in fragile X syndrome

Fragile X Syndrome (FXS) is a learning disability seen in individuals who have >200 CGG•CCG repeats in the 5' untranslated region of the X-linked FMR1 gene. Such alleles are associated with a fragile site, FRAXA, a gap or constriction in the chromosome that is coincident with the repeat and is induced by folate stress or thymidylate synthase inhibitors like fluorodeoxyuridine (FdU). The molecular basis of the chromosome fragility is unknown. Previous work has suggested that the stable intrastrand structures formed by the repeat may be responsible, perhaps via their ability to block DNA synthesis. We have examined the replication dynamics of normal and FXS cells with and without FdU. We show here that an intrinsic problem with DNA replication exists in the FMR1 gene of individuals with FXS even in the absence of FdU. Our data suggest a model for chromosome fragility in FXS in which the repeat impairs replication from an origin of replication (ORI) immediately adjacent to the repeat. The fact that the replication problem occurs even in the absence of FdU suggests that this phenomenon may have in vivo consequences, including perhaps accounting for the loss of the X chromosome containing the fragile site that causes Turner syndrome (45, X0) in female carriers of such alleles. Our data on FRAXA may also be germane for the other FdU-inducible fragile sites in humans, that we show here share many common features with FRAXA.

Improved retroviral suicide gene transfer in colon cancer cell lines after cell synchronization with methotrexate

Background

Cancer gene therapy by retroviral vectors is mainly limited by the level of transduction. Retroviral gene transfer requires target cell division. Cell synchronization, obtained by drugs inducing a reversible inhibition of DNA synthesis, could therefore be proposed to precondition target cells to retroviral gene transfer. We tested whether drug-mediated cell synchronization could enhance the transfer efficiency of a retroviral-mediated gene encoding herpes simplex virus thymidine kinase (HSV-tk) in two colon cancer cell lines, DHDK12 and HT29.

Methods

Synchronization was induced by methotrexate (MTX), aracytin (ara-C) or aphidicolin. Gene transfer efficiency was assessed by the level of HSV-TK expression. Transduced cells were driven by ganciclovir (GCV) towards apoptosis that was assessed using annexin V labeling by quantitative flow cytometry.

Results

DHDK12 and HT29 cells were synchronized in S phase with MTX but not ara-C or aphidicolin. In synchronized DHDK12 and HT29 cells, the HSV-TK transduction rates were 2 and 1.5-fold higher than those obtained in control cells, respectively. Furthermore, the rate of apoptosis was increased two-fold in MTX-treated DHDK12 cells after treatment with GCV.

Conclusions

Our findings indicate that MTX-mediated synchronization of target cells allowed a significant improvement of retroviral HSV-tk gene transfer, resulting in an increased cell apoptosis in response to GCV. Pharmacological control of cell cycle may thus be a useful strategy to optimize the efficiency of retroviral-mediated cancer gene therapy.

G1 phase-dependent nucleolar accumulation of human histone H1x

BACKGROUND INFORMATION

H1 histones are a protein family comprising several subtypes. Although specific functions of the individual subtypes could not be determined so far, differential roles are indicated by varied nuclear distributions as well as differential expression patterns of the H1 subtypes. Although the group of replication-dependent H1 subtypes is synthesized during S phase, the replacement H1 subtype, H1 degrees , is also expressed in a replication-independent manner in non-proliferating cells. Recently we showed, by protein biochemical analysis, that the ubiquitously expressed subtype H1x is enriched in the micrococcal nuclease-resistant part of chromatin and that, although it shares common features with H1 degrees , its expression is differentially regulated, since, in contrast to H1 degrees , growth arrest or induction of differentiation did not induce an accumulation of H1x.

RESULTS

In the present study, we show that H1x exhibits a cell-cycle-dependent change of its nuclear distribution. This H1 subtype showed a nucleolar accumulation during the G(1) phase, and it was evenly distributed in the nucleus during S phase and G(2). Immunocytochemical analysis of the intranucleolar distribution of H1x indicated that it is located mainly in the condensed nucleolar chromatin. In addition, we demonstrate that the amount of H1x protein remained nearly unchanged during S phase progression, which is in contrast to the replication-dependent subtypes.

CONCLUSION

These results suggest that the differential localization of H1x provides a mechanism for a control of H1x activity by means of shuttling between nuclear subcompartments instead of a controlled turnover of the protein.

Functional interaction between fluorodeoxyuridine-induced cellular alterations and replication of a ribonucleotide reductase-negative herpes simplex virus

G207 is an oncolytic herpes simplex virus (HSV) which is attenuated by inactivation of viral ribonucleotide reductase (RR) and deletion of both gamma(1)34.5 genes. The cellular counterparts that can functionally substitute for viral RR and the carboxyl-terminal domain of ICP34.5 are cellular RR and the corresponding homologous domain of the growth arrest and DNA damage protein 34 (GADD34), respectively. Because the thymidylate synthetase (TS) inhibitor fluorodeoxyuridine (FUdR) can alter expression of cellular RR and GADD34, we examined the effect of FUdR on G207 bioactivity with the hypothesis that FUdR-induced cellular changes will alter viral proliferation and cytotoxicity. Replication of wild-type HSV-1 was impaired in the presence of 10 nM FUdR, whereas G207 demonstrated increased replication under the same conditions. Combined use of FUdR and G207 resulted in synergistic cytotoxicity. FUdR exposure caused elevation of RR activity at 10 and 100 nM, whereas GADD34 was induced only at 100 nM. The effect of enhanced viral replication by FUdR was suppressed by hydroxyurea, a known inhibitor of RR. These results demonstrate that the growth advantage of G207 in FUdR-treated cells is primarily based on an RR-dependent mechanism. Although our findings show that TS inhibition impairs viral replication, the FUdR-induced RR elevation may overcome this disadvantage, resulting in enhanced replication of G207. These data provide the cellular basis for the combined use of RR-negative HSV mutants and TS inhibitors in the treatment of cancer.

Cell cycle analysis and synchronization of the Xenopus laevis XL2 cell line: study of the kinesin related protein XlEg5

Cell free extracts prepared from Xenopus eggs are one of the most powerful in vitro systems to analyze cell cycle-regulated mechanisms such as DNA replication, nuclear assembly, chromosome condensation, or spindle formation. Xenopus embryos can complete several synchronous cell cycles in the absence of transcription, consequently Xenopus extracts are very helpful to study the molecular level of cellular mechanisms. Many key cell cycle regulators like p34cdc2 and cdk2 have been discovered and characterized using those extracts, but their regulation during somatic cell cycles have only been studied in mammalian cultured cells. In this paper, we describe optimized conditions to obtain cell cycle arrested Xenopus XL2 cultured cells. Synchronization of XL2 cells at different stages of the cell cycle was achieved by serum starvation and drug treatments such as aphidicolin, nocodazole, and ALLN. The degree of synchronization was assessed by indirect fluorescence microscopy and FACS analysis. This method was used to study the cell cycle expression of the Xenopus kinesin-related protein, XlEg5, a microtubule-based motor protein involved in movement and cell division in early development. We found that the expression of the protein was maximum in mitosis and minimum in G1, which correlated with the expression of its messenger RNA. XL2 cultured cells were also used to analyze the ultrastructural sub-cellular localization of XlEg5. During mitosis, the protein was found around the centrosome in prophase, on the spindle microtubules in metaphase, and, interestingly, around the minus end of the midbody microtubules in telophase.

Cell cycle analysis and synchronization of the Xenopus cell line XL2

We have determined the length of the cell cycle and its different phases in a permanent Xenopus tadpole cell line, XL2. Following BrdU labeling, the total length of the cell cycle was estimated as 28 h. The different phases of the cell cycle, G1, S, G2, and M were, respectively, 14 h, 10 h 45 min, 2 h 30 min, and 54 min. Knowing these parameters, we were able to develop methods that selectively enrich cells in different phases of the cycle. Treatment with aphidicolin resulted in a S phase block in which more than 85% of the cells showed S phase chromosomes. Almost 60% of the cells were arrested in mitosis after a double block with aphidicolin/nocodazole or aphidicolin/ALLN (acetyl-leucyl-leucyl-norleucinal) treatment. This synchronization protocol will greatly facilitate studies of biochemical events associated with specific gene regulation through the cell cycle. Our synchronization protocol does not disturb cell metabolism as the expression of cyclin B2 during the cell cycle is in agreement with the results obtained with mammalian cells.

Varied expression patterns of human H1 histone genes in different cell lines

Five main type H1 histones have been described in man (H1.1-H1.5) in addition to the testis specific type H1t and the replacement subtype H1 degrees, which is found mainly in highly differentiated cells. We have isolated this whole complement of H1 genes and have studied the expression of the seven human H1 subtype genes in several cell lines. The RNAase protection assay was used to discriminate between the very similar transcripts derived from the seven H1 subtype genes. With the exception of H1.2 and H1.4, we found substantial differences between the H1 mRNA levels in the different cell lines tested. No H1.1 mRNA was detected in most of the cell lines and just a low level of H1.1 mRNA was found in human testis. In contrast to the differential patterns of the other subtypes, H1.2 and H1.4 were in all cells expressed at a high level, indicating a basal function compared with the other H1 histones. Because differences in the timing of H1 protein subtype synthesis have been reported, we have analyzed the kinetics of accumulation of H1 subtypes in synchronized HeLa cells and observed that all H1 subtypes examined (H1 degrees, H1.2-H1.5) were expressed in a replication-dependent manner. The analysis showed a differential rise of mRNA levels during S-phase, from four-fold (H1 degrees) to 15-fold (H1.5). Our results may point at a specific function of each subtype and suggest that expression of the H1 histone subtype genes depends on common S-phase-depent factors as well as on individual regulatory systems. Thus, the data presented here provide a basis for further analysis of the regulation and function of the complex H1 gene and protein family.

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 new class of reversible cell cycle inhibitors

The effects of three compounds on the cell cycle of HL-60 promyeloid leukemia cells has been examined. Ciclopirox olamine, an antifungal agent, and the compound Hoechst 768159 reversibly block the cell cycle at a point occurring roughly 1 h before the arrest mediated by aphidicolin, an inhibitor of DNA polymerase alpha activity, which acts in early S phase. Similar results are also obtained with the compound mimosine, a plant amino acid. Based on these data, it is concluded that all three agents inhibit cell cycle traverse at or very near the G1/S phase boundary and identify a previously undefined reversible cell cycle arrest point.

Synchronisation of cancer cell lines of human origin using methotrexate

U937 human histiocytic lymphoma cell line and SW626 ovarian carcinoma line of human origin were synchronised using very low, nontoxic concentrations (0.04-0.08 microM for 16-24 h) of methotrexate (MTX) under standard culture conditions. Satisfactory synchrony was achieved to study S phase events. Various kinetic behaviours and biological properties of the synchronised cells are considered for characterisation of the system. MTX-synchronisation was compared with that induced by aphidicolin (APC) alone and by serum deprivation and APC. In some cancer cell lines MTX appears to be the best choice for obtaining highly synchronised cell populations without cytotoxicity or physiological perturbations.

Control of cell division in hepatoma cells by exogenous heparan sulfate proteoglycan

The effects of cell surface heparan sulfate proteoglycan (HSPG) prepared from log and confluent monolayers of a rat hepatoma cell line on hepatoma cell growth were studied. When HSPG isolated from confluent cells was added exogenously to log phase cells, it was internalized and free heparan sulfate (HS) chains appeared transiently in the nucleus. Concurrently, the growth of the treated cells was inhibited, but the cells resumed logarithmic growth as the level of nuclear HS fell, and the cells grew to confluence and became contact inhibited. When HSPG prepared from log-phase hepatoma cells was added exogenously to log phase cells, it was internalized but very little of the internalized HS appeared in the nucleus, and there was no change in the rate of cell growth. However, when the rate of cell growth was reduced by culture of the cells in serum- and insulin-deficient medium, HSPG prepared from log-phase cells stimulated the growth rate of these slow-growing cells. The cell cycle dependency of HSPG uptake and growth inhibition was studied in cultures synchronized by a thymidine/aphidicolin double block. When [35SO4]HSPG from confluent cells was added to synchronized cells just as they were released from the second block, a portion of the [35SO4]HSPG was internalized and [35SO4]HS appeared in the nucleus. However, at mitosis the [35SO4]HS disappeared almost completely from all of the cellular pools, and after mitosis, more of the [35SO4]HSPG was taken up and [35SO4]HS reappeared in the nucleus and remained in the nucleus until the cells divided again. When cultures were released from the aphidicolin block, both control and HSPG-treated cells progressed through the S, the G2, and the M phases of the cell cycle. However, the length of the G1 phase of the cycle was increased in the HSPG-treated cells. The treated cultures then progressed through the second S, G2, and M phases. Thus, the inhibition of cell division occurred in the G1 phase of the cell cycle, prior to the G1/S boundary. Addition of the HSPG to the synchronized cultures just after the first mitosis resulted in an immediate arrest of the cell cycle in G1.(ABSTRACT TRUNCATED AT 400 WORDS)

Aphidicolin synchronization of mouse L cells perturbs the relationship between cell killing and DNA double-strand breakage after X-irradiation

Mouse L cells were synchronized in early S-phase by two 12 h incubations in medium containing aphidicolin (2 micrograms/ml), separated by 8 h in drug-free medium. The relationship between X-ray-induced cell killing and DNA double-strand breakage was then examined for cells that had entered S-phase, G2-phase, mitosis, and G1-phase following release from aphidicolin and was compared to the response of asynchronous cultures. Aphidicolin-synchronized cells showed cycle phase-dependent changes in their dose-responses for both killing and DNA dsb. However, on the basis of the level of DNA dsb per unit length of DNA required to produce a lethal lesion, aphidicolin-synchronized cells were more sensitive to X-rays than were asynchronous cultures. This sensitivity peaked 2 h after release from aphidicolin treatment and then progressively declined towards the asynchronous culture value. It is argued that these results are due to deregulation of the temporal order of DNA replication following aphidicolin treatment, and can be incorporated into the critical DNA target size model (Radford, Hodgson, and Matthews, in preparation) by postulating that the targets for radiation action in mammalian cells are DNA-associated with potentially transcriptionally active proto-oncogenes or constitutive fragile sites.

Changes in H1 content, nucleosome repeat lengths and DNA elongation under conditions of hydroxyurea treatment that reportedly facilitate gene amplification

Depletion of histone H1, changes in nucleosome repeat lengths, and extents of DNA elongation were investigated in synchronized Chinese hamster (line CHO) cells using the general conditions of hydroxyurea treatment that appear to increase the frequency of gene amplification, i.e., synchronized cultures of G1 cells were allowed to begin to enter S phase before treatment with hydroxyurea was effected to retard DNA synthesis (Mariani, B.D. and Schimke, R.T. (1984) J. Biol. Chem. 259, 1901-1910). During the time that synchronized G1 cells begin to enter S phase, there occur considerable synchrony decay and accumulation of new DNA that increase with time before treatment with hydroxyurea is initiated. During exposure to hydroxyurea, there occur depletion of histone H1 and shortened repeat lengths for the DNA synthesized in the presence of hydroxyurea. In contrast, DNA synthesized in S phase before exposure to hydroxyurea has essentially the same repeat lengths as bulk chromatin at both the time that hydroxyurea treatment is effected and after 6 h in its presence. Sedimentation measurements indicate that the early replicating DNA undergoes considerable elongation both before and during 6 h of exposure to 0.3 mM hydroxyurea. Thus, nearly all of the early replicating DNA is elongated to greater than average replicon size under those conditions of hydroxyurea treatment that appear to favor gene amplification. Because the extents of DNA synthesis and cell cycle progression vary as functions of drug concentration, treatment times, and unknown factors (from experiment to experiment), it would appear that the parameters must be carefully monitored in each experiment if biochemical results are to be related to the position of cells in the growth cycle.

Variable effects of DNA-synthesis inhibitors upon DNA methylation in mammalian cells

Post-synthetic enzymatic hypermethylation of DNA was induced in hamster fibrosarcoma cells by the DNA synthesis inhibitors cytosine arabinoside, hydroxyurea and aphidicolin. This effect required direct inhibition of DNA polymerase alpha or reduction in deoxynucleotide pools and was not specific to a single cell type. At equivalently reduced levels of DNA synthesis, neither cycloheximide, actinomycin D nor serum deprivation affected DNA methylation in this way. The topoisomerase inhibitors nalidixic acid and novobiocin caused significant hypomethylation indicating that increased 5-mCyt content was not a necessary consequence of DNA synthesis inhibition. The induced hypermethylation occurred predominantly in that fraction of the DNA synthesized in the presence of inhibitor; was stable in the absence of drug; was most prominent in low molecular weight DNA representing sites of initiated but incomplete DNA synthesis; and occurred primarily within CpG dinucleotides, although other dinucleotides were overmethylated as well. Drug-induced CpG hypermethylation may be capable of silencing genes, an effect which may be relevant to the aberrantly expressed genes characteristic of neoplastic cells.