Cell kinetic disturbances induced by treatment of human diploid fibroblasts with 5-azacytidine indicate a major role for DNA methylation in the regulation of the chromosome cycle

Aberrant DNA methylation as a cancer-inducing mechanism

Aberrant DNA methylation is the most common molecular lesion of the cancer cell. Neither gene mutations (nucleotide changes, deletions, recombinations) nor cytogenetic abnormalities are as common in human tumors as DNA methylation alterations. The most studied change of DNA methylation in neoplasms is the silencing of tumor suppressor genes by CpG island promoter hypermethylation, which targets genes such as p16(INK4a), BRCA1, and hMLH1. There is a profile of CpG island hypermethylation according to the tumor type, and genes silent by methylation represent all cellular pathways. The introduction of bisulfite-PCR methodologies combined with new genomic approaches provides a comprehensive spectrum of the genes undergoing this epigenetic change across all malignancies. However, we still know very little about how this aberrant DNA methylation "invades" the previously unmethylated CpG island and how it is maintained through cell divisions. Furthermore, we should remember that this methylation occurs in the context of a global genomic loss of 5-methylcytosine (5mC). Initial clues to understand this paradox should be revealed from the current studies of DNA methyltransferases and methyl CpG binding proteins. From the translational standpoint, we should make an effort to validate the use of some hypermethylated genes as biomarkers of the disease; for example, it may occur with MGMT and GSTP1 in brain and prostate tumors, respectively. Finally, we must expect the development of new and more specific DNA demethylating agents that awake these methyl-dormant tumor suppressor genes and prove their therapeutic values. The expectations are high.

Chromosomal aberrations induced by 5-azacytidine combined with VP-16 (etoposide) in CHO-K1 and XRS-5 cell lines

A cytogenetic study was carried out with 5-azacytidine (5-azaC) and etoposide (VP-16) in CHO-K1 and XRS-5 (mutant cells deficient for double-strand break rejoining) cell lines to verify the interaction effects of the drugs in terms of induction of chromosomal aberrations. 5-azaC is incorporated into DNA causing DNA hypomethylation, and VP-16 (inhibitor of topoisomerase II enzyme) is a potent clastogenic agent. Cells in exponential growth were treated with 5-azaC for 1 h, following incubation for 7 h, and posttreatment with VP16 for the last 3 h. In K1 cells, the combined treatments induced a significant reduction in the aberrations induced in the X and "A" (autosome) chromosomes, which are the main target for 5-azaC. However, in XRS-5 cells, the drug combination caused a significant increase in the aberrations induced in those chromosomes, but with a concomitant reduction in the randomly induced-aberrations. In addition, each cell line presented characteristic cell cycle kinetics; while the combined treatment induced an S-arrest in K1 cells, alterations in cell cycle progression were not found for XRS-5, although each drug alone caused a G2-arrest. The different cell responses presented by the cell lines may be explained on the basis of the evidence that alterations in chromatin structure caused by 5-aza-C probably occur to a different extent in K1 and XRS-5 cells, since the mutant cells present a typical hyper-condensed chromosome structure (especially the X- and "A" chromosomes), but, alternatively, 5-aza-C could induce reactivation of DNA repair genes in XRS-5 cells.

Biological effects of a bifunctional DNA cross-linker. II. Generation of micronuclei and attached micronuclear-like structures

Madin-Darby bovine kidney (MDBK) cells were treated with the bifunctional DNA cross-linker, L-7, to examine the generation of micronuclei and other nuclear abnormalities. The preceding paper demonstrates that L-7 treatment induces the formation of triradial and quadriradial chromosomes in MDBK cells. These chromosomes are believed to result from interduplex DNA cross-links formed between G-C rich centromeric satellite DNA regions on non-sister chromatids. Treatment produces a majority of centromere-positive micronuclei. In addition, many daughter cells remain attached by chromatin bridges which are sometimes beaded with micronuclei. Up to 15% of cell nuclei become lobular and fused with numerous micronuclear-like structures attached to their membranes. These attached structures are classified as attached micronuclear-like structures (AMNLS). Fluorescence in situ hybridization (FISH) using a centromeric satellite sequence was performed on treated cells. Hybridization reveals that intercellular bridges are composed of centromeric sequences and initiate at centromeric foci in daughter cells. Furthermore, the majority of junctions between AMNLS and nuclei contain an enhancement of centromeric signal. The frequency of AMNLS appears dependent on the concentration of L-7 and the duration of treatment. Similar results were found for the generation of cross-linked chromosome products in the previous paper. We suggest that AMNLS result from the abnormal mitotic segregation of cross-linked chromosome products.

The DNA methylation machinery as a target for anticancer therapy

DNA methylation is now recognized as an important mechanism regulating different functions of the genome; gene expression, replication, and cancer. Different factors control the formation and maintenance of DNA methylation patterns. The level of activity of DNA methyltransferase (MeTase) is one factor. Recent data suggest that some oncogenic pathways can induce DNA MeTase expression, that DNA MeTase activity is elevated in cancer, and that inhibition of DNA MeTase can reverse the transformed state. What are the pharmacological consequences of our current understanding of DNA methylation patterns formation? This review will discuss the possibility that DNA MeTase inhibitors can serve as important pharmacological and therapeutic tools in cancer and other genetic diseases.

The propensity for gene amplification: a comparison of protocols, cell lines, and selection agents

We have studied cell lines of rodent and human origin for their propensity to become resistant to antifolates (methotrexate, trimetrexate), phosphonacetyl-L-aspartate (PALA), and colcemid, resistances associated with amplification of the DHFR, CAD, and MDR1 genes, respectively. We have employed two different methods: (1) a shallow step-wise selection protocol, where time to attain specified resistance is the quantitative measure, (2) the frequency of resistant colonies at specified drug concentrations. Although there are advantages and disadvantages to both methods, the two methods gave the same relative ranking of cell lines. Striking differences in the propensity for gene amplification (resistance) were found: human cell lines were less prone to amplify genes than Chinese hamster ovary (CHO) cells. This ranking was similar with all of the agents employed. Additionally, we observed that whereas PALA resistance in CHO cells is associated with amplification of the CAD gene, PALA resistance in the two human cell lines studied (HeLaS3 and VA13) was not associated with amplification and/or overexpression of the CAD gene, and thus this resistance to PALA occurs by an unknown mechanism.

Cell-cycle dependent micronucleus formation and mitotic disturbances induced by 5-azacytidine in mammalian cells

5-Azacytidine was originally developed to treat human myelogenous leukemia. However, interest in this compound has expanded because of reports of its ability to affect cell differentiation and to alter eukaryotic gene expression. In an ongoing attempt to understand the biochemical effects of this compound, we examined the effects of 5-azacytidine on mitosis and on micronucleus formation in mammalian cells. In L5178Y mouse cells, 5-azacytidine induced micronuclei at concentrations at which we and others have already reported its mutagenicity at the tk locus. Using CREST staining and C-banding studies, we showed that the induced micronuclei contained mostly chromosomal fragments although some may have contained whole chromosomes. By incorporating BrdU into the DNA of SHE cells, we determined that micronuclei were induced only when the compound was added while the cells were in S phase. Microscopically visible effects due to 5-azacytidine treatment were not observed until anaphase of the mitosis following treatment or thereafter. 5-Azacytidine did not induce micronuclei via interference with formation of the metaphase chromosome arrangement in mitosis, a common mechanism leading to aneuploidy. Supravital UV microscopy revealed that chromatid bridges were observed in anaphase and, in some cases, were sustained into interphase. In the first mitosis after 5-azacytidine treatment we observed that many cells were unable to perform anaphase separation. All of these observations indicate that 5-azacytidine is predominantly a clastogen through its incorporation into DNA.

Induction of replicative senescence by 5-azacytidine: fundamental cell kinetic differences between human diploid fibroblasts and NIH-3T3 cells

To analyse the putative role of methylation of cytosine residues in the nuclear DNA as a regulatory step during cellular ageing, we incubated ageing human amniotic fluid derived fibroblast-like cells and non-ageing NIH-3T3 cells with 5-azacytidine. BrdUrd/Hoechst and acridine orange (AO) flow cytometry was used to compare the effects of the base analogue on cell proliferation and cell differentiation. In NIH-3T3 cultures, 96h exposures to 4 microM 5-azacytidine caused diminished cell proliferation due to cell arrest in the G1 compartments of the second and third cell cycles of serum stimulated cells. The exit from the G0/G1 compartment was not affected. The 5-azacytidine induced cell kinetic disturbances were unstable in NIH-3T3 cultures, such that pre-treated cells reverted to normal cell cycle transit within 2-3 days after termination of treatment. In contrast, 5-azacytidine pre-treated amniotic fluid derived fibroblast-like cell cultures showed persistently elevated G2 phase arrests and delayed G0/G1 phase exit kinetics, which explain the premature cessation of proliferation observed in these primary cultures. In both cell systems, 5-azacytidine exposed cultures showed elevated numbers of G1 phase cells with increased RNA content as revealed by AO flow cytometry. Again, this effect was reversible in NIH-3T3 cells but not in amniotic fluid derived fibroblast-like cells. These contrasting responses to 5-azacytidine are likely to reflect intrinsic differences in methylation patterns or de novo methylase activity between ageing cell strains and non-ageing cell lines.

5-azacytidine induces micronuclei in and morphological transformation of Syrian hamster embryo fibroblasts in the absence of unscheduled DNA synthesis

It is known that 5-azacytidine (5-AC) induces tumors in several organs of rats and mice. The mechanisms of these effects are still poorly understood although it is known that 5-AC can be incorporated into DNA. Furthermore, it can inhibit DNA methylation. The known data on its clastogenic and/or gene mutation-inducing potential are still controversial. Therefore, we have investigated the kinds of genotoxic effects caused by 5-AC in Syrian hamster embryo (SHE) fibroblasts. Three different endpoints (micronucleus formation, unscheduled DNA synthesis (UDS) and cell transformation) were assayed under similar conditions of metabolism and dose at target in this cell system. 5-AC induces morphological transformation of SHE cells, but not UDS. Therefore, 5-AC does not seem to cause repairable DNA lesions. Furthermore, our studies revealed that 5-AC is a potent inducer of micronuclei in the SHE system. Immunocytochemical analysis revealed that a certain percentage of these contain kinetochores indicating that 5-AC may induce both clastogenic events and numerical chromosome changes.

Oxidants and antioxidants in proliferative senescence

In terms of the amount of experimental research it has generated the free radical theory of ageing is one of the most popular hypotheses to explain this ubiquitous phenomenon. From the theory two postulates were derived: either cellular defence mechanisms against free radical-dependent oxidants deteriorate during ageing of cells, or essential, unrepairable damages are imparted to the cell by oxidants regardless of the activity of antioxidant defence systems. The many reports dealing with a putative breakdown in antioxidant defence systems failed to positively support this postulate. However, a minor depletion in cellular glutathione by exposure to a model lipophilic peroxide led to a significant decrement in DNA and protein synthesis. In other words, the glutathione redox cycle is intrinsically fallible with respect to defending the cellular DNA replication system against this model lipophilic peroxide. Interestingly, after ageing in culture cells a partial uncoupling of the NADPH-producing and -consuming systems tends to take place. Experiments involving the addition of antioxidants to the culture medium have failed to significantly extend the lifespan of cultured diploid somatic cells. The level of antioxidants appears to be a modulator rather than a primary determinant of cellular ageing in culture. Several lines of evidence suggest that DNA damages accumulate during ageing of the organism, but no oxidant-related DNA damage has been pinpointed in the cultured cell system. Human mutants with defects in antioxidant enzymes have not shown conclusive signs of accelerated ageing. Cells from patients with Werner's syndrome (progeria of the adult), on the other hand, do not suffer from a defect in their antioxidant defence system, nor do they accumulate more than normal amounts of autofluorescent products resulting from lipid peroxidation. The recent finding that Werner's syndrome constitutes a mutator phenotype may prompt the comparison of oxidant- and ageing-related mutation spectra in order to investigate a mutational theory of ageing as a new derivative from the free radical hypothesis.