Cytosine Arabinoside Lesions Are Position-specific Topoisomerase II Poisons and Stimulate DNA Cleavage Mediated by the Human Type II Enzymes

Abstract

Cytosine arabinoside (araC) is an important drug used for the treatment of human leukemias. In order to exert its cytotoxic effects, araC must be incorporated into chromosomal DNA. Although specific DNA lesions that involve base loss or modification stimulate nucleic acid cleavage mediated by type II topoisomerases, the effects of deoxyribose sugar ring modification on enzyme activity have not been examined. Therefore, the effects of incorporated araC residues on the DNA cleavage/religation equilibrium of human topoisomerase II␣ and ␤ were characterized. AraC lesions were positionspecific topoisomerase II poisons and stimulated DNA scission mediated by both human type II enzymes. However, the positional specificity of araC residues differed from that previously reported for other cleavage-enhancing DNA lesions. Finally, additive or synergistic increases in DNA cleavage were observed in the presence of araC lesions and etoposide. These findings broaden the range of DNA lesions known to alter topoisomerase II function and raise the possibility that this enzyme may mediate some of the cellular effects of araC.Cytosine arabinoside (1-␤-D-arabinofuranosylcytosine, araC) 1 is one of the most important chemotherapeutic drugs used for the treatment of adult and pediatric leukemias (1, 2). The cytotoxicity of araC correlates with its incorporation into newly replicated DNA (1-7). As a prelude to this incorporation, the drug is converted to its "active" nucleoside triphosphate form by pyrimidine salvage pathways (1,2,8). Once integrated into chromosomes, the arabinoside inhibits chain elongation and bypass synthesis and in some cases can induce DNA chain termination or duplication of DNA sequences (1, 2, 7, 9 -19). Although the precise mechanism of araC-induced cell death has not been established, drug treatment leads to reduced rates of DNA replication, DNA strand breaks, and chromosome fragmentation (1, 2, 6, 16, 20 -23).Beyond their established mutagenic or cytotoxic effects, physiologically relevant DNA lesions such as apurinic/apyrimidinic sites and deaminated cytosine residues profoundly influence the activity of eukaryotic type II topoisomerases (24). These lesions act as position-specific topoisomerase II "poisons" and stimulate enzyme-mediated DNA scission when they are located within the 4-base stagger that separates the two phosphodiester bonds cleaved by the enzyme (24 -28). Some nucleoside alterations, such as abasic sites, enhance doublestranded DNA scission with a potency that is 1,000-fold greater than that of etoposide (24 -26, 28).Although DNA lesions that result from base loss or modification have been shown to alter the function of topoisomerase II (24 -28), the effects of sugar ring modification on enzyme activity have not been established. Therefore, the effects of incorporated araC residues on the DNA cleavage activity of human topoisomerase II␣ and -␤ were characterized. Substitution of a ␤-hydroxyl for a hydrogen at the 2Ј-position of deoxycytosine (converting the deoxyribose ring to an arabinose...

An Inhibitor of DNA Recombination Blocks Memory Consolidation, But Not Reconsolidation, in Context Fear Conditioning

“…ara-CTP can also be incorporated into newly synthesized DNA and thereby interrupt replication (Gandhi et al, 1997;Ohno et al, 1998;Abdel-Aziz et al, 2000;Han et al, 2000;Wills et al, 2000). DNA-incorporated ara-CTP interferes with the religation activity of topoisomerases (Cline and Osheroff, 1999;Pourquier et al, 2000;Chrencik et al, 2003;Gmeiner et al, 2003) or with transcription factor binding (Zhang and Kiechle, 2004). Importantly, however, ara-CTP does not act as a direct inhibitor of transcription (Wang et al, 2003).…”

DNA topoisomerase II, genotoxicity, and cancer

“…The position of the DNA damage relative to the scissile bond cleaved by the type II enzyme is critical to the actions of lesions as topoisomerase II poisons [173][174][175][176][177][178][179]. As discussed earlier, the two scissile bonds on the opposite strands of the double helix are separated by 4 base pairs.…”

“…In this regard, a number of naturally occurring DNA lesions that result from endogenous or environmental stress increase levels of topoisomerase II-DNA cleavage complexes [173][174][175][176][177][178][179]. Both the α and β isoforms of the enzyme are affected similarly in vitro [177,179].…”

Induction of Apoptosis of Human Tumor Cells by a 4,9-Diazapyrenium Derivative and Its Affects on Topoisomerase-II Action

“…Numerous natural and synthetic antitumor drugs act by trapping the covalent topoisomerase-II-DNA intermediate and converting the topoisomerase-II to a DNAdamaging agent [22][23][24] . Some of these drugs are DNA intercalators [22,25,26] .…”