Induction of karyotype instability in a murine tumor cell line by quercetin, 2-amino-1-methyl-6 phenylimidazo[4,5-b]pyridine, and okadaic acid, as revealed by transmission distortion of the inactive X chromosome

Characteristics of okadaic acid--induced cytotoxic effects in CHO K1 cells

This article reports the results of investigations into the process of cell death induced in the Chinese hamster ovary cell K1 subclone (CHO K1) by okadaic acid (OA), a hydrophobic polyether produced by marine dinoflagellates. The IC50 was about 13 nM OA after 24 h of treatment, as determined using neutral red. With the MTT assay, the IC50 was 25 nM, although in this case 25% of the initial staining was still observed at 100 nM. Hoechst staining showed that mitotic figures accumulated at 12 nM OA after a 24- or 48-h treatment. In experiments limited to a 3-day treatment without changing the medium, CHO K1 cells were engaged in the death process at 50 nM OA after about 20 h and at 10 nM OA after 48 h. In many cells nuclear fragmentation that resulted in the apparent appearance of vesicles correlated with increasing cellular volume. But additional cell fragmentation was not observed with any treatment, and the chromatin material seemed to progressively disappear inside the cells. DNA fragmentation was analyzed by electrophoresis and with the TUNEL technique. With both techniques, the DNA was fragmented by 48 h in both 25 and 50 nM OA. Electrophoresis showed that both adherent and nonadherent cells were affected. Annexin-positive/ propidium iodide (PI)-negative cells were rarely observed after OA treatment. Some were seen under the scanning cytometer after 20 h at 50 nM OA or after 48 h at 10 nM OA, but they were never detected by flow cytometry. Most of the time scanning cytometry showed either unstained cells or PI-positive (annexin-positive or -negative) cells (48 h, 50 nM, or 72 h, 10 nM). Flow cytometry cytograms showed two cell subpopulations: one composed of a majority of smaller cells, the other of larger cells. The larger cells markedly decreased with time and OA treatment (50 and 100 nM). Stained-cell counting showed that all cells that stained were both annexin- and PI positive and that most PI-positive cells were smaller. Ki67 antigen labeling showed the proliferative activity of CHO K1 cultures but also demonstrated the loss of this activity in smaller cells treated with 50 nM OA for 48 h. We concluded that in our culture conditions the main OA target within CHO K1 cultures was dividing cells. Our results suggest that cells with disturbed metaphase-anaphase enter apoptosis, leading to necrotic daughter cells.

Accumulation of recombinant chromosomes and low fidelity of transmission of chromosome X DNA markers in gamma-ray-induced lymphomas lacking p53

F1 offspring of male MSM male mice with a p53-deficient (knockout) allele and normal female BALB/c mice were backcrossed with MSM mice to produce N2 mice. Female F1 and N2 mice were irradiated with gamma-radiation, and thymic lymphomas were obtained from 69 F1 and 82 N2 mice heterozygous for X chromosome markers. Of these 151 mice, 91 carried a p53-deficient allele. These lymphomas were analyzed for allelic loss by using four marker loci distributed on X chromosome to assess the stability of the inactive X chromosome, which contributes little to cellular functions. Twenty lymphomas showed allelic loss of all four loci, suggesting loss of a whole inactive X chromosome due to mitotic nondisjunction, whereas 24 lost only a part of an X chromosome, as a result of somatic recombination. The p53 status of the lymphomas was determined by genotyping and allelic loss analysis: 53 had retained two wild-type p53 alleles, suggesting normal function; 69 had lost both alleles, indicating loss of function; and the remaining 29 had at least one wild-type p53 allele, so their p53 status was unclear. Compilation of these two data revealed one nondisjunction-type change and five recombination-type mutations on X chromosome in 53 lymphomas retaining functional p53. In contrast, 14 and 16 of these alterations, respectively, were observed in 69 lymphomas lacking p53 function. These results suggest that p53 loss significantly increases the accumulation of recombinant chromosomes and decreases the fidelity of mitotic chromosome transmission of the X chromosome in gamma-ray-induced lymphomas.

Difference in chromatin packaging between active and inactive X chromosomes by fractionation and allele-specific detection

Using a novel method consisting of chromatin fractionation and allele-specific detection, chromatin packaging is compared between active X (Xa) and inactive X (Xi) chromosomes for five tumor cell clones that were derived from inter-subspecific F1 female mice. Separation of heterochromatic (H) and euchromatic (E) fractions is monitored by hybridization with subtelomeric satellite DNA and ribosomal RNA gene and by PCR amplification of p53 gene/pseudogene with one primer set. The H fraction was enriched with satellite and p53 pseudogene probably existing in heterochromatic regions while the E fraction showed inverse, suggesting fair separation. Analysis with seven marker and three gene loci revealed concentration of alleles on Xi in the H fraction and those on Xa in the E fraction, though the concentration levels varied. This implies that the packaging level of Xi is higher than that of active or inactive euchromatin on Xa. Intriguingly, one cell line showed biallelic expression and chromatin relaxation of the Pgk-1 locus, suggesting that the relaxation occur regionally on X chromosome.

Induction of minisatellite mutation in NIH 3T3 cells by treatment with the tumor promoter okadaic acid

Okadaic acid (OA) is a strong tumor promoter of mouse skin carcinogenesis and also a potent inhibitor of serine/threonine protein phosphatases. OA induces various genetic alterations in cultured cells, such as diphtheria-toxin-resistance mutations, sister chromatid exchange, exclusion of exogenous transforming oncogenes, and gene amplification. The present study revealed that it caused minisatellite mutation (MSM) at a high frequency in NIH 3T3 cells, although no microsatellite mutation was found. Nine of 31 clones (29%) exhibited MSM after 6 days of OA treatment, as opposed to only 1 of 30 clones (3%) without OA exposure. Moreover, NIH 3T3 cells treated with OA acquired tumorigenicity in nude mice, giving rise to 7 tumors within 25 weeks in 20 sites where 3 x 10(6) cells were injected. In contrast, the same numbers of untreated cells gave rise to only one tumor, and the tumor grew much slower. All of three OA-induced tumors examined manifested the MSM. The findings thus point to a molecular mechanism by which OA could function as a tumor promoter, and also the biological relevance of the induction of MSM in the tumorigenic process by OA.