Ku antigen is required to relieve G2 arrest caused by inhibition of DNA topoisomerase II activity by the bisdioxopiperazine ICRF-193

DNA Damage by an essential enzyme: A delicate balance act on the tightrope

DNA topoisomerases are essential for DNA metabolic processes such as replication and transcription. Since DNA is double stranded, the unwinding needed for these processes results in DNA supercoiling and catenation of replicated molecules. Changing the topology of DNA molecules to relieve supercoiling or resolve catenanes requires that DNA be transiently cut. While topoisomerases carry out these processes in ways that minimize the likelihood of genome instability, there are several ways that topoisomerases may fail. Topoisomerases can be induced to fail by therapeutic small molecules such as by fluoroquinolones that target bacterial topoisomerases, or a variety of anti-cancer agents that target the eukaryotic enzymes. Increasingly, there have been a large number of agents and processes, including natural products and their metabolites, DNA damage, and the intrinsic properties of the enzymes that can lead to long-lasting DNA breaks that subsequently lead to genome instability, cancer, and other diseases. Understanding the processes that can interfere with topoisomerases and how cells respond when topoisomerases fail will be important in minimizing the consequences when enzymes need to transiently interfere with DNA integrity.

Catalytic inhibitors of DNA topoisomerase II suppress the androgen receptor signaling and prostate cancer progression

Although the new generation of androgen receptor (AR) antagonists like enzalutamide (ENZ) prolong survival of metastatic castration-resistant prostate cancer (CRPC), AR-driven tumors eventually recur indicating that additional therapies are required to fully block AR function. Since DNA topoisomerase II (Topo II) was demonstrated to be essential for AR to initiate gene transcription, this study tested whether catalytic inhibitors of Topo II can block AR signaling and suppress ENZ-resistant CRPC growth. Using multiple prostate cancer cell lines, we showed that catalytic Topo II inhibitors, ICRF187 and ICRF193 inhibited transcription activities of the wild-type AR, mutant ARs (F876L and W741C) and the AR-V7 splice variant. ICRF187 and ICRF193 decreased AR recruitment to target promoters and reduced AR nuclear localization. Both ICRF187 and ICRF193 also inhibited cell proliferation and delayed cell cycling at the G2/M phase. ICRF187 inhibited tumor growth of castration-resistant LNCaP and 22RV1 xenografts as well as ENZ-resistant MR49F xenografts. We conclude that catalytic Topo II inhibitors can block AR signaling and inhibit tumor growth of CRPC xenografts, identifying a potential co-targeting approach using these inhibitors in combination with AR pathway inhibitors in CRPC.

A mitotic topoisomerase II checkpoint in budding yeast is required for genome stability but acts independently of Pds1/securin

Topoisomerase II (Topo II) performs topological modifications on double-stranded DNA molecules that are essential for chromosome condensation, resolution, and segregation. In mammals, G2 and metaphase cell cycle delays induced by Topo II poisons have been proposed to be the result of checkpoint activation in response to the catenation state of DNA. However, the apparent lack of such controls in model organisms has excluded genetic proof that Topo II checkpoints exist and are separable from the conventional DNA damage checkpoint controls. But here, we define a Topo II-dependent G2/M checkpoint in a genetically amenable eukaryote, budding yeast, and demonstrate that this checkpoint enhances cell survival. Conversely, a lack of the checkpoint results in aneuploidy. Neither DNA damage-responsive pathways nor Pds1/securin are needed for this checkpoint. Unusually, spindle assembly checkpoint components are required for the Topo II checkpoint, but checkpoint activation is not the result of failed chromosome biorientation or a lack of spindle tension. Thus, compromised Topo II function activates a yeast checkpoint system that operates by a novel mechanism.

Cell cycle-dependent DNA damage signaling induced by ICRF-193 involves ATM, ATR, CHK2, and BRCA1

Topoisomerase II is essential for cell proliferation and survival and has been a target of various anticancer drugs. ICRF-193 has long been used as a catalytic inhibitor to study the function of topoisomerase II. Here, we show that ICRF-193 treatment induces DNA damage signaling. Treatment with ICRF-193 induced G2 arrest and DNA damage signaling involving gamma-H2AX foci formation and CHK2 phosphorylation. DNA damage by ICRF-193 was further demonstrated by formation of the nuclear foci of 53BP1, NBS1, BRCA1, MDC1, and FANCD2 and increased comet tail moment. The DNA damage signaling induced by ICRF-193 was mediated by ATM and ATR and was restricted to cells in specific cell cycle stages such as S, G2, and mitosis including late and early G1 phases. Downstream signaling of ATM and ATR involved the phosphorylation of CHK2 and BRCA1. Altogether, our results demonstrate that ICRF-193 induces DNA damage signaling in a cell cycle-dependent manner and suggest that topoisomerase II might be essential for the progression of the cell cycle at several stages including DNA decondensation.

Small Molecule Modulation of the Human Chromatid Decatenation Checkpoint

After chromosome replication, the intertwined sister chromatids are disentangled by topoisomerases. The integrity of this process is monitored by the chromatid decatenation checkpoint. Here, we describe small molecule modulators of the human chromatid decatenation checkpoint identified using a cell-based, chemical genetic modifier screen. Similar to 1,2,7-trimethylyxanthine (caffeine), these small molecules suppress the G(2)-phase arrest caused by ICRF-193, a small molecule inhibitor of the enzymatic activity of topoisomerase II. Analysis of specific suppressors, here named suptopins for suppressor of Topoisomerase II inhibition, revealed distinct effects on cell cycle progression, microtubule stability, nucleocytoplasmic transport of cyclin B1, and no effect on the chromatin deacetylation checkpoint induced by trichostatin A. The suptopins provide new molecular tools for dissecting the role of topoisomerases in maintaining genomic stability and determining whether inhibiting the chromatid decatenation checkpoint sensitizes tumor cells to chemotherapeutics.

Analysis of the DNA replication competence of the xrs-5 mutant cells defective in Ku86

The radiosensitive mutant xrs-5, a derivative of the Chinese hamster ovary (CHO) K1 cell line, is defective in DNA double-strand break repair and V(D)J recombination. The defective phenotypes of xrs-5 cells are complemented by the 86 kDa subunit of Ku antigen. OBA is a protein, previously purified from HeLa cells, that binds in a sequence-specific manner to mammalian origins of DNA replication. The DNA-binding subunit of OBA has been identified as Ku86. We tested the xrs-5 cell line for its ability to replicate a mammalian origin-containing plasmid, p186, in vivo and in vitro. In vivo, the p186 episomal DNA replication in transfected xrs-5 cells was reduced by 45% when compared with the CHO K1 cells transfected with p186. In vitro, although total and cytoplasmic cell extracts from xrs-5 cells replicated the p186 with the same efficiency as the parental CHO K1 cell extracts, xrs-5 nuclear extracts did not possess any detectable replication activity. Addition of affinity-purified OBA/Ku restored replication in the xrs-5 nuclear extract reaction. Western blot analyses showed that the levels of other replication proteins (Orc2, PCNA, DNA polymerase epsilon and delta, Primase and Topoisomerase IIalpha) were comparable in both the xrs-5 mutant and CHO K1 wild-type cell lines. In addition, the in vivo association of Ku with the DHFR origin-containing sequence (oribeta) was examined in both the CHO K1 and xrs-5 cell lines by a chromatin immunoprecipitation (ChIP) assay. Anti-Ku antibodies did not immunoprecipitate a detectable amount of Ku from the xrs-5 cells in the origin-containing sequence, in contrast to the CHO K1 cells, wherein Ku was found to be associated with the oribeta origin. The data implicate Ku antigen in in vivo and in vitro DNA replication and suggest the existence of another protein with Ku-like functions in the xrs-5 cells.