Indications for an active process underlying spontaneous and radiation-induced micronucleation in L929 cells.

PURPOSE: To investigate the mechanism of micronucleus formation in irradiated L929 cells. MATERIALS AND METHODS: Radiation-induced micronuclei (MN) of L929 cells isolated at 48 and 72 h after irradiation were processed for detection of DNA-laddering and higher-order chromatin fragments using conventional gel electrophoresis and pulse-field gel electrophoresis. Quantification of double-strand breaks in micronuclei and nuclei was performed with the TdT assay and quantified using image analysis. The number of binucleated cells containing micronuclei (cytochalasin B method) was counted after application of three unspecific endonuclease inhibitors (aurin, ATA, spermine), a topoisomerase II inhibitor (VM-26), administration of two PKC inhibitors (H-7, Gö6983) and after addition of N-acetylsphingosine (C2-ceramide). PKC activity was determined by measuring the incorporation of [gamma-32P]ATP into a suitable specific substrate. Proliferation was measured by detection of PCNA, RFP-A and BrdU (30-min pulse labelling) using both conventional immunofluorescence and laser scanning microscopy. RESULTS: (1) Higher chromatin fragments accumulated in MN with a size as they occur during early stages of apoptosis; (2) the frequency of MN was influenced by drugs known to play an important role in signalling and execution of apoptosis (endonucleases, topoisomerase II, protein kinases, ceramide); (3) MN are characterized by a reduced transcription ability (PCNA, RFP-A). CONCLUSIONS: A proportion of L929 MN may be formed by an active process comparable with the early stages of apoptosis; it may play a role in the re-organization of the damaged genome.

Isolation of micronuclei: high yield by sucrose gradient versus maximum purity by cell sorting.

Micronuclei (MNC) of L929 cells were isolated 72 h after irradiation with 6 Gy for characterization of their DNA content, using gel electrophoresis. A novel method for isolation of MNC based on a sucrose gradient ultracentrifugation was developed. With this efficient method (80% recovery) more than 150 x 10(6) MNC per day could be isolated. The purity was > 99%. However, the low number of main nuclei in the MNC isolate ( < 1%) resulted in a contamination of MNC DNA with about 15% main nucleus DNA, due to the several times higher DNA content of main nuclei. Cell sorting was utilized to maximize the purity by choosing the recommended sorting mode for highest purity. Isolation of MNC with the cell sorter was successful (100% purity), but also time-consuming (1-2 x 10(6) MNC per working day could be isolated) and insufficient (10% recovery). Extraction of DNA of these isolated MNC resulted in less than 1 ng/day. Hence, at least 1 week of cell sorting would be necessary for one electrophoretic run. When employing the sucrose gradient method, 2000 times more DNA of MNC have been isolated. We therefore consider this method as the most efficient way for rapid and low cost isolation of large amounts of purified ( > 99%) MNC without the employment of sophisticated and expensive techniques (cell sorting) and the accompanied knowhow. In contrast, maximum purity but low yields of MNC can be obtained by cell sorting.