Platinum anticancer drugs are widely used to manage a variety of tumors and are highly successful in combating testicular cancer. The mechanism of action of these compounds involves passage through the cell membrane, which in some cases is facilitated by specific transporters, activation by aquation, binding to DNA in nucleosomes, and inhibition of transcription with attendant induction of cell death pathways. The treatment of cells with cisplatin leads to specific phosphorylation and acetylation of histone proteins. These modifications alter the efficiency by which cells can remove platinum adducts by the nucleotide excision repair pathway. They do not, however, affect the ability of platinum-induced cross-links to determine the setting of DNA on nucleosomes containing site-specific cis-diammineplatinum(II) cross-links. With the use of hydroxyl radical footprinting (1), we demonstrate that nucleosomes constructed with either recombinant or native histones are similarly able to override the rotational setting predefined in a nucleosome positioning sequence. A detailed analysis of the data reveals that the diammineplatinum(II) fragment in a 1,3-d(GpTpG) cross-link, a major adduct formed by carboplatin, faces toward the histone core proteins. This result is obtained for two nucleosomes reconstituted with DNA duplexes in which the platinum cross-link differed in position by half a helical turn. The data further revealed global DNA unwinding of approximately 24° by the platinum adduct in a region of the nucleosome in which the DNA is locally overwound in undamaged reference nuclesomes. The ability of the platinum cross-link to influence the positioning of nucleosomes may be of physiological relevance to its mechanism of action because at least 50% of the nucleosome positions in vivo are determined by the underlying DNA sequence.



References and Footnotes

1. Danford, A. J., Wang, D., Wang, Q., Tullius, T. D., and Lippard, S. J. Proc Natl Acad Sci 102, 12311-12316 (2005).