DNA-damaging enediyne C-1027 inhibits initiation of intracellular SV40 DNA replication in trans

An extraction-free method by which a single slot blot can be used to quantify intracellular DNA damage (crosslinks or strand breaks) and changes in DNA damage response proteins or replication

We report an extraction-free assay in which the same slot blot membrane can be used to assess total genomic DNA damage (i.e., crosslinks or strand breaks) and DNA replication (i.e., bromodeoxyuridine incorporation) or protein levels (i.e., gamma-H2AX). 14C-thymidine radiolabeling of HCT116 cells loaded directly on a Hybond N+ membrane slot blot enables the quantitation of DNA interstrand crosslinks and DNA breaks, while bromodeoxyuridine incorporation or levels of gamma-H2AX can be assessed by incubating blots with primary monoclonal antibodies followed by detection with horseradish peroxidase (HRP) secondary antibodies. Uniform Ponceau staining of all samples on the membrane indicates that protein binding to the membrane is independent of DNA damage or elution. The use of a single membrane to assay levels of DNA damage and concomitant changes in damage response proteins or replication allows the direct quantitation of diverse parameters under identical conditions.

Pharmacokinetics of C-1027 in mice as determined by TCA-RA method

AIM

To validate a radioactivity assay, the TCA-RA method, for the measurement of C-1027 in serum and to evaluate its application in determination of pharmacokinetics of C-1027 in mice.

METHODS

(125)I-C-1027 was prepared by the Iodogen method and separated by HPLC. The radioactivity assay was established and used to determine (125)I-C-1027 in mice at doses of 10, 50 and 100 microg/kg after precipitation with 20% trichloroacetic acid (TCA-RA method). Several pharmacokinetic parameters were determined after intravenous injection of (125)I-C-1027 to mice.

RESULTS

After intravenous injection of (125)I-C-1027 to mice, at doses of 10, 50 and 100 microg/kg; the apparent distribution volumes (V(d)) were 0.26, 0.31 and 0.33 L/kg; the biological half-lives (T(1/2)) were 3.10, 3.40 and 3.90 h; the areas under curve (AUC) were 18.41, 103.69 and 202.74 ng/h/mL; the elimination rate constants (K) were 1.04, 1.26 and 0.58/h; and the total body clearance (Cl) were 0.54, 0.48 and 0.49 L/kg/h, respectively.

CONCLUSION

TCA-RA is a sensitive, reliable and suitable method for the determination of (125)I-C-1027 in mouse serum.

Regulation of cellular and SV40 virus origins of replication by Chk1-dependent intrinsic and UVC radiation-induced checkpoints

DNA replication is inhibited by DNA damage through cis effects on replication fork progression and trans effects associated with checkpoints. In this study, we employed a combined pulse labeling and neutral-neutral two-dimensional gel-based approach to compare the effects of a DNA damaging agent frequently employed to invoke checkpoints, UVC radiation, on the replication of cellular and simian virus 40 (SV40) chromosomes in intact cells. UVC radiation induced similar inhibitory effects on the initiation and elongation phases of cellular and SV40 DNA replication. The initiation-inhibitory effects occurred independently of p53 and were abrogated by the ATM and ATR kinase inhibitor caffeine, or the Chk1 kinase inhibitor UCN-01. Inhibition of cellular origins was also abrogated by the expression of a dominant-negative Chk1 mutant. These results indicate that UVC induces a Chk1- and ATR or ATM-dependent checkpoint that targets both cellular and SV40 viral replication origins. Loss of Chk1 and ATR or ATM function also stimulated initiation of cellular and viral DNA replication in the absence of UVC radiation, revealing the existence of a novel intrinsic checkpoint that targets both cellular and SV40 viral origins of replication in the absence of DNA damage or stalled DNA replication forks. This checkpoint inhibits the replication in early S phase cells of a region of the repetitive rDNA locus that replicates in late S phase. The ability to detect these checkpoints using the well characterized SV40 model system should facilitate analysis of the molecular basis for these effects.

Determination of the electron affinities of alpha- and beta-naphthyl radicals using the kinetic method with full entropy analysis. The C-H bond dissociation energies of naphthalene

The C - H bond dissociation energies for naphthalene were determined using a negative ion thermochemical cycle involving the gas-phase acidity (Delta H (acid)) and electron affinity (EA) for both the alpha- and beta-positions. The gas-phase acidity of the naphthalene alpha- and beta-positions and the EAs of the alpha- and beta-naphthyl radicals were measured in the gas phase in a flowing after glow-triple quadrupole apparatus. A variation of the Cooks kinetic method was used to measure the EAs of the naphthyl radicals by collision-induced dissociation of the corresponding alpha- and beta-naphthylsulfinate adducts formed by reactions in the flow tube portion of the instrument. Calibration references included both pi and sigma radicals, and full entropy analysis was performed over a series of calibration curves measured at collision energies ranging from 3.5 to 8 eV (center-of-mass). The measured EAs are 33.0 +/- 1.4 and 31.4 +/- 1.0 kcal mol(-1) (1 kcal = 4.184 kJ) for the alpha- and beta-naphthyl radicals, respectively. The gas-phase acidities for naphthalene were measured by the DePuy silane cleavage method, which utilizes the relative abundances of aryldimethylsiloxides and trimethylsiloxide that result from competitive cleavages from a proposed penta coordinate hydroxysiliconate intermediate. The measured acidities are 394.0 +/- 5.0 and 397.6 +/- 4.8 kcal mol(-1) for the alpha- and beta- positions, respectively. The C - H bond dissociation energies calculated from the thermochemical cycle are 113.4 +/- 5.2 and 115.4 +/- 4.9 kcal mol(-1) for the alpha- and beta-positions, respectively. These energies are, to within experimental error, indistinguishable and are approximately the same as the first bond dissociation energy for benzene.

DNA damage and replication checkpoints in the fission yeast, Schizosaccharomyces pombe

Eukaryotic organisms have developed an array of mechanisms for minimizing the consequences of damage to their DNA molecules and the consequences of interference with their DNA replication. Among these mechanisms are the DNA damage and replication checkpoints, which inhibit passage from one cell cycle stage to the next when DNA is damaged or replication is incomplete. Studies of these checkpoints in the fission yeast, Schizosaccharomyces pombe, complement studies in other organisms and provide valuable insight into the nature of the proteins responsible for these checkpoints and how such proteins may function.