Effect of cycloheximide on development of methotrexate resistance of Chinese hamster ovary cells treated with inhibitors of DNA synthesis

Inhibition of apoptosis by overexpressing Bcl-2 enhances gene amplification by a mechanism independent of aphidicolin pretreatment

To study the effect of apoptosis on gene amplification, we have constructed HeLa S3 cell lines in which the expression of bcl-2 (BCL2) can be controlled by tetracycline in the growth medium. Induction of Bcl-2 expression caused a temporary delay of apoptosis and resulted in roughly a 3-fold increase in the frequency of resistant colonies when cells were selected with trimetrexate. This resistance was due to amplification of the dihydrofolate reductase gene. Cells grown out of the pooled resistant colonies retained the same level of resistance to trimetrexate whether Bcl-2 was induced or repressed, consistent with the theory that Bcl-2 functions by facilitating gene amplification, rather than being the resistance mechanism per se. Pretreating cells with aphidicolin is another method to increase gene amplification frequency. When Bcl-2-expressing cells were pretreated with aphidicolin, the resulting increase in gene amplification frequency was approximately the product of the increases caused by aphidicolin pretreatment or Bcl-2 expression alone, indicating that Bcl-2 increases gene amplification through a mechanism independent of that of aphidicolin pretreatment. These results are consistent with the concept that gene amplification occurs at a higher frequency during drug-induced cell cycle perturbation. Bcl-2 evidently increases the number of selected amplified colonies by prolonging cell survival during the perturbation.

Stimulation of dihydrofolate reductase promoter activity by antimetabolic drugs

Dihydrofolate reductase (DHFR; EC 1.5.1.3) is required in folate metabolism for the synthesis of purines, thymidine, and glycine. Although there have been several reports of induction of DHFR enzyme by methotrexate (MTX), a drug that competitively inhibits DHFR, there are no studies reported that examine the effect of MTX on DHFR gene transcription. We have examined the effect of MTX and other inhibitors of DNA synthesis on DHFR transcription using a transient expression assay. MTX stimulates transient expression in a concentration-dependent manner from a hamster DHFR promoter construct containing 150 base pairs 5' to the start of transcription. Addition of either tetrahydrofolate or hypoxanthine plus thymidine prevents the promoter induction in response to MTX, suggesting that stimulation by MTX results from inhibition of these metabolites. Furthermore, two other antimetabolic drugs--fluorodeoxyuridine and hydroxyurea--also stimulate the DHFR promoter in a concentration-dependent manner. In contrast, aphidicolin, which blocks cell growth through inhibition of DNA polymerase alpha, has no effect on the DHFR promoter. The potential relevance of these results to cross-resistance to chemotherapeutic agents and to the process of gene amplification is discussed.

Aphidicolin-induced topological and recombinational events in simian virus 40

Highly compacted (40S) SV40 DNA replication intermediates formed in vivo during aphidicolin exposure and immediately broke down in two stages. In the rapid initial stage, single strand DNA breaks caused loss of superhelicity in the 40S replication intermediates. This DNA breakage was accompanied by the formation of strong, permanent protein-DNA crosslinks which reached a maximum as nicking of the aberrant DNA replication intermediates was completed. These protein-associated DNA strand breaks were not repaired. In the slower second stage of breakdown, the aberrant DNA replication intermediates remained nicked and strongly associated with protein as they underwent DNA replication fork breakage and recombinational changes to produce high molecular weight forms.

Amplification of the ribonucleotide reductase small subunit gene: analysis of novel joints and the mechanism of gene duplication in vaccinia virus

Amplification of the M2 gene encoding the small subunit of ribonucleotide reductase (EC 1.17.4.1) was analyzed in a collection of vaccinia virus (VV) isolates selected for resistance to 5 mM hydroxyurea (HU). Most of the mutants harbored tandem direct repeat arrays of the M2 gene, but several had duplicated M2 as an inverted repeat by genomic rearrangements involving the chromosomal termini. Novel joints formed by direct repeats were mapped, amplified in vitro, and sequenced. The junctions were simple fusions between DNA downstream and upstream of the M2 gene. Lack of sequence homology at the breakpoints indicated that the initial genomic rearrangements leading to gene amplification were due to nonhomologous recombination events.

Selective killing of transformed fibroblasts by combined treatment with cycloheximide and aphidicolin

The possibility of selective killing of transformed cells in a mixed population of untransformed and transformed cells was examined using a cell culture system of rat 3Y1 fibroblasts (parental 3Y1 cells, 3Y1 cells transformed with either SV40, polyoma virus, Rous avian sarcoma virus, E1A gene of adenovirus type 12, or H-v-ras oncogene). The principle of the selective killing is as follows. Under suboptimal culture conditions, untransformed cells are inhibited from progressing through G1 phase and retain viability, while transformed cells are not arrested. When DNA synthesis is inhibited for a long period, both types of cells in S phase die. Therefore, if we administer inhibitors of G1 progression and of DNA synthesis simultaneously to a cell population consisting of untransformed and transformed cells, most untransformed cells are arrested in G1 phase, retaining viability, while transformed cells leak from the G1 phase, cease DNA synthesis, and gradually die The present study shows that all types of transformants in stationary-phase cultures (consisting of cells mainly with a G1 DNA content) were killed to higher extents compared with untransformed cells, during incubation at lower cell densities with a combination of cycloheximide (G1 inhibitor) and aphidicolin (DNA-synthesis inhibitor). However, cycloheximide reduced the killing effect of aphidicolin by changing the irreversible DNA-synthesis inhibition to a reversible inhibition. The availability of G1 inhibitors that do not interfere with the irreversibility of inhibition of DNA synthesis is required for the treatment of cancer based on this idea.

Defining cellular senescence in IMR-90 cells: a flow cytometric analysis

Using multiparameter flow cytometric analysis, we find that senescent cells accumulate in a unique cell-cycle compartment characterized in cell-cycle arrest in G1 and a significantly reduced nucleocytoplasmic ratio (genome size/cell mass) relative to cycling cells. With respect to gross cellular phenotype, the quiescent state of senescent cells differs from quiescence induced by density inhibition; the former is associated with a reduction in the nucleocytoplasmic ratio, while the latter is associated with an increase in the nucleocytoplasmic ratio. Senescent cells were present at all passages examined. The frequency of senescent cells was low in early-passage cultures and increased with passage number. Senescence of populations of IMR-90 cells reflects change in the relative frequency of these cells. The frequency of cells with karyotypic changes increased with the progressive accumulation of out-of-cycle cells.