The ability of the telomeric DNA-binding protein, TRF2, to stimulate t-loop formation while preventing t-loop deletion is believed to be crucial to maintain telomere integrity in mammals. However, little is known on the molecular mechanisms behind these properties of TRF2. In this report, we show that TRF2 greatly increases the rate of Holliday junction (HJ) formation and blocks the cleavage by various types of HJ resolving activities, including the newly identified human GEN1 protein. By using potassium permanganate probing and differential scanning calorimetry, we reveal that the basic domain of TRF2 induces structural changes to the junction. We propose that TRF2 contributes to t-loop stabilisation by stimulating HJ formation and by preventing resolvase cleavage. These findings provide novel insights into the interplay between telomere protection and homologous recombination and suggest a general model in which TRF2 maintains telomere integrity by controlling the turnover of HJ at t-loops and at regressed replication forks.
Acridine derivatives are interesting chemotherapeutic agents that were first used as antibacterial and antiparasite agents. In this review we wish to concentrate our attention on the anticancer properties of acridines used in clinics since the 1970's. Based on recent results, an outlook on antitumour acridine chemotherapy will be proposed. The biological activity of acridines is mainly attributed to the planarity of these aromatic structures, which can intercalate within the double-stranded DNA structure, thus interfering with the cellular machinery. Recent understanding of the mode of action of acridines leads to continuous and exciting research in this heterocyclic family. Indeed, biological targets such as topoisomerases I and II, telomerase/telomere and protein kinases emerge and allow the design of novel acridine-based patterns. This review further pinpoints the latest progress in the development of anticancer agents based on naturally occurring and synthetic acridines (e.g. acridones, pyridoacridines); for this matter in vitro/in vivo studies and clinical trial results will be discussed. The DNA-affinic property of acridine is also useful to vectorise drugs into cell nuclei and some applications in hypoxia-selective treatment, platinum or N-mustard derived conjugates will be reported. Some other properties including inhibition of multidrug resistance or potential impact on Alzheimer disease will be treated. It is noteworthy that the position and the nature of the substituent on the heterocyclic core are determinants for the biological property and selectivity observed. So, we wish also to disclose a summary of recent synthetic methodologies developed for acridine synthesis.
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