Interaction of an ultimate carcinogen, benzo[a]pyrene diol epoxide, with nucleosomal core particles: apparent lack of protection of DNA by histone proteins

Ellipticine increases the superhelical density of intracellular SV40 DNA by intercalation

We investigated the in vivo effect of ellipticine, a mammalian topoisomeraseII(topoII) inhibitor, on SV40 DNA topology. In contrast to epipodophyllotoxins, ellipticine did not cause significant double stranded cleavage of intracellular SV40 DNA. Furthermore, ellipticine reduced cleavage induced by epipodophyllotoxins, VP16 and VM26. Unexpectedly, ellipticine dramatically increased the superhelical density of a fraction of intracellular SV40 DNA. Several lines of evidence suggest that the formation of this highly supercoiled DNA species (Ih form DNA) is not due to the inhibition of topoII per se, but is the result of intercalation by ellipticine in a subfraction of the intracellular SV40 chromatin followed by the fixation of DNA linking number by a topoisomerase activity. Based on the linking number change and the known unwinding angle of ellipticine, the intercalation density was calculated as one ellipticine molecule per 10-20 bp in the Ih DNA. This result suggests the existence of different populations of intracellular SV40 chromatin with respect to the accessibility to ellipticine intercalation.

Efficient protection against oxidative DNA damage in chromatin

The role of histones and higher order chromatin structures in protecting against oxidative DNA damage was investigated using an in vitro system consisting of nuclear and nucleoid monolayers as model chromatin substrates. These substrates, derived from human skin fibroblasts, were challenged with hydroxyl radicals produced via a Fenton reaction involving Fe(II)-ethylenediaminetetraacetic acid and ascorbic acid. The resulting DNA strand breaks were measured using the alkaline unwinding technique. The sequential removal of chromosomal proteins from the DNA by pretreating nuclear monolayers with increasing concentrations of salt dramatically increased the frequency of hydroxyl radical-induced DNA strand breaks. Furthermore, the DNA in decondensed chromatin was found to contain 14-fold fewer DNA strand breaks than naked, supercoiled DNA, whereas the DNA of "native" chromatin and "condensed" chromatin contained 100-fold and 300-fold fewer breaks, respectively. We conclude that the binding of histones to the DNA and its organization into higher order chromatin structures dramatically protects the DNA against hydroxyl radical-induced DNA strand breaks and thus should be considered part of the cellular defense against the induction of oxidative DNA damage.

The importance of intercalation in the covalent binding of benzo(a)pyrene diol epoxide to DNA

The primary mode of non-covalent interaction of the strong carcinogen, benzo(a)pyrene diol epoxide, with DNA is through intercalation. It has variously been suggested that intercalative complexes may be prerequisite for either covalent binding or DNA-catalysed hydrolysis of the epoxide or both. Geacintov [Geacintov, N. E. (1986). Carcinogenesis 7, 589.] has recently argued that intercalation is important in covalent binding and presented theoretical constructs consistent with this proposal. A more general theoretical model is presented here which includes the possibilities that either catalysis of hydrolysis or covalent binding of benzo(a)pyrene diol epoxide DNA can occur (a) in an intercalation complex, or (b) without formation of a detectable, physically bound complex. It is shown that a variety of possible mechanisms formulated under this general theory lead to equations for overall reaction rates and covalent binding fractions which are all of the same form with respect to DNA concentration dependence. A consequence of this is that experimental studies of the dependence of hydrolysis rates and covalent binding fractions on DNA concentration do not distinguish between the various possible mechanisms. These findings are discussed in relation to the interactions of benzo(a)pyrene diol epoxide with chromatin in cells.