Simultaneous Measurement of Cell Cycle Phase Position and Ionizing Radiation-induced DNA Strand Breakage in Single Human Tumour Cells Using Laser Scanning Confocal Imaging

A discrete cell survival model including repair after high dose-rate of ionizing radiation

A discrete cell survival model has been developed that is represented by six parameters (gamma, delta, alpha, epsilon, kappa, and t0). It is assumed that, linearly with the dose, two types of lesions are generated, with the number per unit dose described by the parameters gamma and delta. The two types of lesions, irreparable (lethal damage abbreviated as LD) damage and reparable (potentially lethal damage abbreviated as PLD) damage, follow a Poisson statistic. The PLD can be either repaired by an enzymatic process or converted into a lethal damage by a time- and dose-dependent process. For repair of PLD a Michaelis-Menten kinetics has been assumed, described by the maximum velocity alpha of the process and the Michaelis-Menten constant, kappa. The unrepaired PLD are fixed and become lethal after replating. The applicability of the model was tested by fitting 11 experimental data sets obtained with different cell lines and variable repair times simulated by delayed plating recovery or inhibition of repair processes by different agents. It has been concluded from the results that the repair process is rather totally than partially saturated. Therefore, the model was adapted by assuming a zero-order reaction for the repair process. Good agreement between model assumptions and molecular mechanisms is obtained by assuming PLD and LD to be double-strand breaks. The model and the results obtained are discussed and compared with published results and experimental data.

Cell cycle related behavior of a chromosomal scaffold protein in MDCK epithelial cells

Because the mechanisms that govern mitosis are a key to the understanding of cell growth, the proteins associated with chromosomes specifically during this phase have received thorough attention. In the present work we report an Mr58000 protein in MDCK epithelial cells, recognized by a monoclonal antibody (LFM-1) that decorates chromosomes during M-phase. Cell fractionation methods followed by immunoblotting and immunofluorescence showed that this protein is associated with the nuclear fraction. Biochemical extraction procedures on isolated metaphase chromosomes from nocodazole-synchronized cells indicated that the Mr58000 protein behaves as a chromosomal scaffold protein, that is, it remains in the pellets after high salt (2M NaCl) or 3'-5' diiodosalicylic acid treatments, even in DNAse pre-digested samples. In addition, confocal microscopy of those chromosomes revealed the LFM-1 epitopes distributed on the external surface and the axis of chromatids. Parallel analysis of interphase nuclei revealed LFM-1 epitopes inside G1-, but excluded from G2-phase nuclei. These results were independently confirmed on nuclei sorted by flow cytometry and in cell populations synchronized by release of G1-/S-phase hydroxyurea arrest. The Mr58000 and a minor Mr38000 protein (which was enriched only in mitotic chromosomes of synchronized cells) were analyzed by Edman degradation. They shared the sequence at the amino-terminal end but failed to show total homology with known proteins. These results suggest that LFM-1 antigens fit some of the predictions of the licensing factor model, and may have a role in cell cycle dependent events.

The comet assay: a comprehensive review

The comet assay is a sensitive and rapid method for DNA strand break detection in individual cells. Its use has increased significantly in the past few years. This paper is a review of the studies published to date that have made use of the comet assay. The principles of strand break detection using both the alkaline and neutral versions of the technique are discussed, and a basic methodology with currently used variations is presented. Applications in different fields are reviewed and possible future directions of the technique are briefly explored.

Etoposide-induced cell cycle delay and arrest-dependent modulation of DNA topoisomerase II in small-cell lung cancer cells

As an approach to the rational design of combination chemotherapy involving the anti-cancer DNA topoisomerase II poison etoposide (VP-16), we have studied the dynamic changes occurring in small-cell lung cancer (SCLC) cell populations during protracted VP-16 exposure. Cytometric methods were used to analyse changes in target enzyme availability and cell cycle progression in a SCLC cell line, mutant for the tumour-suppressor gene p53 and defective in the ability to arrest at the G1/S phase boundary. At concentrations up to 0.25 microM VP-16, cells became arrested in G2 by 24 h exposure, whereas at concentrations 0.25-2 microM G2 arrest was preceded by a dose-dependent early S-phase delay, confirmed by bromodeoxyuridine incorporation. Recovery potential was determined by stathmokinetic analysis and was studied further in aphidicolin-synchronised cultures released from G1/S and subsequently exposed to VP-16 in early S-phase. Cells not experiencing a VP-16-induced S-phase delay entered G2 delay dependent upon the continued presence of VP-16. These cells could progress to mitosis during a 6-24 h period after drug removal. Cells experiencing an early S-phase delay remained in long-term G2 arrest with greatly reducing ability to enter mitosis up to 24 h after removal of VP-16. Irreversible G2 arrest was delimited by the induction of significant levels of DNA cleavage or fragmentation, not associated with overt apoptosis, in the majority of cells. Western blotting of whole-cell preparations showed increases in topoisomerase II levels (up to 4-fold) attributable to cell cycle redistribution, while nuclei from cells recovering from S-phase delay showed enhanced immunoreactivity with an anti-topoisomerase II alpha antibody. The results imply that traverse of G1/S and early S-phase in the presence of a specific topoisomerase II poison gives rise to progressive low-level trapping of topoisomerase II alpha, enhanced topoisomerase II alpha availability and the subsequent irreversible arrest in G2 of cells showing limited DNA fragmentation. We suggest that protracted, low-dose chemotherapeutic regimens incorporating VP-16 are preferentially active towards cells attempting G1/S transition and have the potential for increasing the subsequent action of other topoisomerase II-targeted agents through target enzyme modulation. Combination modalities which prevent such dynamic changes occurring would act to reduce the effectiveness of the VP-16 component.

Alkali-labile sites are prevalent in kidney tissue DNA

Inherent DNA damage in the form of single strand breaks and alkali labile sites is generated by essential intracellular processes, such as DNA replication and repair. Utilizing the in situ DNA unwinding assay and the comet assay (single cell gel), we have observed high levels of alkali induced DNA strand breaks in cells isolated from mouse kidney tissue homogenate. Kidney cell DNA demonstrated a 7.4 x increase in nucleoid expansion and a 7 x increase in comet length compared to negative control cells (thymocytes and splenocytes) in the two assay systems, comparable to epididymal sperm cells which have previously been demonstrated to contain abundant alkali-labile sites. These results argue for the existence of prevalent alkali-labile sites in kidney cell DNA.